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Solution Combustion Synthesis of Nanoscale Materials Arvind Varma,*,† Alexander S. Mukasyan,‡ Alexander S. Rogachev,∥,⊥ and Khachatur V. Manukyan§ †

School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907, United States Department of Chemical & Biomolecular Engineering, and §Nuclear Science Laboratory, Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, United States ∥ Institute of Structural Macrokinetics and Materials Science, RAS, Chernogolovka 142432, Russia ⊥ National University of Science and Technology, MISiS, Moscow 119049, Russia ‡

ABSTRACT: Solution combustion is an exciting phenomenon, which involves propagation of self-sustained exothermic reactions along an aqueous or sol−gel media. This process allows for the synthesis of a variety of nanoscale materials, including oxides, metals, alloys, and sulfides. This Review focuses on the analysis of new approaches and results in the field of solution combustion synthesis (SCS) obtained during recent years. Thermodynamics and kinetics of reactive solutions used in different chemical routes are considered, and the role of process parameters is discussed, emphasizing the chemical mechanisms that are responsible for rapid self-sustained combustion reactions. The basic principles for controlling the composition, structure, and nanostructure of SCS products, and routes to regulate the size and morphology of the nanoscale materials are also reviewed. Recently developed systems that lead to the formation of novel materials and unique structures (e.g., thin films and two-dimensional crystals) with unusual properties are outlined. To demonstrate the versatility of the approach, several application categories of SCS produced materials, such as for energy conversion and storage, optical devices, catalysts, and various important nanoceramics (e.g., bio-, electro-, magnetic), are discussed.

CONTENTS 1. Introduction 2. Classification of SCS Reactions 3. Fundamentals of Solution Combustion Synthesis 3.1. Thermodynamics of SCS Processes 3.1.1. General Considerations 3.1.2. The Equilibrium Composition of Products and the Adiabatic Combustion Temperature 3.2. Kinetics of SCS Reactions 3.3. Mechanisms of SCS Reactions 3.3.1. Mechanism of Oxide Forming Systems 3.3.2. Mechanism of Metal Formation in SCS Reactions 3.3.3. Theoretical Models 4. Microstructural Characteristics of CombustionDerived Nanomaterials 4.1. Simple Oxides 4.2. Ferrites 4.3. Perovskites 4.4. Spinels 4.5. Garnets 4.6. Other Multicomponent Single-Phase Oxides 4.7. Mixed Oxides 4.8. Nanophosphors 4.9. Phosphates and Hydroxyphosphates 4.10. Metals and Metal−Ceramic Nanocomposites © 2016 American Chemical Society

4.11. Spheres 4.12. Metal Sulfides 5. Solution Combustion-Derived Materials 5.1. Advanced Materials for Energy Technologies 5.1.1. Rechargeable Batteries 5.1.2. Supercapacitors 5.1.3. Materials for Solid-Oxide Fuel Cells 5.1.4. New Trends in Combustion-Derived Materials for Energy Applications 5.2. Heterogeneous Catalysts 5.2.1. Emission Control Catalysts 5.2.2. Methane and Other Hydrocarbon Conversion Reactions to Fuel and Chemicals 5.2.3. Reforming Reactions of Alcohol toward Hydrogen 5.2.4. Catalysts for Liquid-Phase Organic Reactions 5.3. Semiconductors and Optical Materials 5.3.1. Semiconductors 5.3.2. Optical Materials 5.4. Thin Films 5.4.1. Crystalline Thin-Film Transistors 5.4.2. Amorphous Thin-Film Transistors 5.4.3. Combustion-Derived Thin Films for Other Applications

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Received: May 2, 2016 Published: September 9, 2016

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DOI: 10.1021/acs.chemrev.6b00279 Chem. Rev. 2016, 116, 14493−14586

Chemical Reviews 5.5. Nanoceramics 5.5.1. Conventional Oxide Ceramics 5.5.2. Electroceramics 5.5.3. Magnetic Oxides 5.5.4. Bioceramics 6. Conclusions and Prospects for the Future Author Information Corresponding Author Notes Biographies Acknowledgments Abbreviations References

Review

product composed of α-Al2O3, as well as a large amount of gases. The overall reaction was suggested as follows:

14553 14553 14555 14557 14560 14563 14565 14565 14565 14565 14565 14565 14566

2Al(NO3)3 · 9H 2O + 5CH4N2O = Al 2O3 + 8N2 + 5CO2 + 28H 2O

Note that reaction R2 occurs without the participation of oxygen from the air, because the initial aqueous solution contains a stoichiometric amount of oxidizer. The chemical mechanism of the process R2 is complicated (see section 3). For example, the reactions of thermal decomposition and combustion of the metal complex compounds can be part of this mechanism, along with formation and combustion of a hypergolic mixture of gases (HNCO, NH3, N2O4, NO2, N2O5) formed during the decomposition of process.3 Because SCS is a self-sustained thermal process where the main source of heat comes from combustion reactions, this process can be considered as a specific type of the more general process known as self-propagating high-temperature synthesis (SHS)7−9 or simply combustion synthesis (CS).10−12 However, SCS possesses three features that distinguish it from all other varieties of CS. First, the initial components for SCS are mixed in aqueous solution at the molecular level, while powders used for SHS are typically mixed on the microlevel. Indeed, the sizes of ions and ligands in the solution are commonly in the range 0.1−1 nm, while the sizes of solid powder particles are ∼102− 105 nm. Second, the reaction that is responsible for SCS solid product formation can be different from the self-sustained combustion process. In the classical SHS, the same reaction realizes both synthesis and heat generation functions, such as

1. INTRODUCTION An unusual approach for nanoscale materials synthesis, termed solution combustion synthesis (SCS), was invented in the mid1980s. The investigation of the low-temperature thermal decomposition of metal hydrazinecarboxylate hydrates (N2H5Me(N2H3COO)3·H2O, where Me = Fe, Co, Ni, or Zn) can be recognized as the starting point for this method.1,2 Typically, these compounds are in the solid state at room temperature and possess a complex crystal structure. The elemental crystalline cell contains atoms that function as a fuel (H, C), as well as atoms of oxidizer (e.g., O), which are separated by angstrom-scale distances. The metal hydrazinecarboxylate hydrates decompose in air at relatively low temperature (125−250 °C), yielding ultrafine solid oxides of the corresponding metal and a large amount of gaseous products. For example, the complete decomposition of the iron hydrazinecarboxylate compound can be represented as follows:

Ti + C = TiC + 209 kJ

(R3)

In the SCS processes, the majority of the heat evolves due to burning (oxidation) of organic fuel (e.g., carbon and hydrogen) components, while the target products are mainly metal oxides or metals. Third, the SCS process generates a large amount of gaseous byproducts, as shown in reaction R2. Such gasification leads to (i) a significant expansion of the solid product, and (ii) a rapid decrease of temperature after the reaction, which makes the solid product porous and finely dispersed. These features play a critical role in SCS of nanoscale powders. We can define SCS as a complex self-sustained chemical process, which takes place in a homogeneous solution of precursors. SCS starts with dehydration and thermal decomposition of the homogeneous solution and involves several thermally coupled exothermic reactions, which result in the formation of at least one solid product and a large amount of gas. It is important to note that this process allows for the synthesis of a variety of nanoscale materials, including oxides, metals, alloys, and sulfides, which are currently used in many important applications.12−18 This Review focuses on the analysis of new approaches and results in the field of SCS obtained during recent years (earlier results were described in prior reviews14,16,19 and books3,12). Thermodynamics and kinetics of reactive solutions used in different chemical routes are considered, and the role of process parameters is discussed, emphasizing the chemical mechanisms that are responsible for rapid self-sustained combustion reactions. The basic principles for controlling the composition, size, structure, and microstructure of SCS products, and routes to regulate the size and morphology of the nanoscale materials

4{N2H5Fe(N2H3COO)3 ·H 2O} + 5O2 = 2Fe2O3 + 12CO2 + 16NH3 + 8H 2O + 8N2

(R2)

(R1)

It can be seen that formation of large amount of gases is a distinctive feature of such reactions, and, for example, in reaction R1 1 mole of iron oxide product is accompanied by the generation of 22 mol of gases. According to differential thermal analysis (DTA), the low-temperature decomposition of N2H5Me(N2H3COO)3·H2O involves several stages, both endothermic and exothermic, while the overall heat release is positive due to oxidation.3 It is worth noting that some of the Fe or Co metals can be substituted by other metal atoms, such as N2H5Me1/3Fe2/3(N2H3COO)3·H2O or N2H5Me1/3Co2/3(N2H3COO)3·H2O, where Me = Mn, Mg, Ni, Zn, obtaining more complexes. Thermal decomposition of such compounds yields the formation of ultrafine ferrites MeFe2O4 or cobaltites MeCo2O4.4,5 Reactions of the type R1, due to their exothermic and autocatalytic features, can proceed in a self-sustained mode. This means that once initiated, they can continue to full conversion without any additional external energy input. A critical step toward the discovery of SCS was made when aluminum nitrate hydrate (Al(NO3)3·9H2O) as an oxidizer, and urea (CH4N2O) as a fuel, were dissolved in distilled water and placed in a muffle furnace preheated to 500 °C.6 It was unexpectedly discovered3 that the water solution boiled, foamed, ignited, and burned as a glowing flame with maximum temperature ∼1350 °C. The duration of the entire combustion process was ∼3 min, which yielded the fluffy, voluminous solid 14494


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the technique, along with an impressive variety of important product applications.

are also reviewed. Recently developed systems that lead to the formation of novel materials and unique structures (e.g., thin films and two-dimensional crystals) with unusual properties are outlined. To demonstrate the versatility of the approach, several application categories of SCS produced materials are discussed: • Advanced materials for energy technologies: Batteries, supercapacitors, fuel and solar cells, along with other devices for high-efficiency, low-cost energy conversion and storage technologies. • Heterogeneous catalysts: In-depth analysis of relations between combustion characteristics and performance of SCS catalysts are identified. Specific issues related to SCS-derived catalysts are addressed in the context of important catalytic reactions, including hydrocarbon reforming, oxidation and automotive/diesel exhaust emissions treatments. • Semiconductors and optical materials: Synthesis, characterization and applications of SCSmaterials, suitable for various photocatalytic and optical applications are reviewed. Strategies and principles to dope SCS semiconducting materials for tuning the light-emitting behavior are outlined. • Thin-f ilms: The use of SCS nanomaterials for development of large-area, low-cost thin film transistors and other devices is discussed. Specific features and prospects of low-temperature SCS in the development of thin films are considered. • Nano-ceramics: Recent developments in design, synthesis, characterization, and applications of nanostructured functional ceramic materials are summarized. Specifically, recent trends in combustion processing of electro-, magnetic- and bio-ceramics are outlined. Finally, the future prospects of SCS for processing of nanoscale materials and its potential commercialization pathways are summarized. The diversity of the SCS-based methods for nanoscale materials synthesis is broad, and new chemical and technical routes are being created regularly. Since the origin of the technique, significant growth has occurred in SCS related publications. Figure 1 shows that more then 5000 SCS related articles have appeared over the last 20 years, more than 650 in 2015 alone. This growth can be attributed to the simplicity of

2. CLASSIFICATION OF SCS REACTIONS SCS systems can be classified according to the chemical composition of fuel, oxidizer, and solvent (Table 1). Different Table 1. Most Frequently Used Components for the Solution Preparation16,19,22 oxidizer metal nitrates or nitrate hydrates: Meν(NO3)ν·nH2O ν - metal valence ammonium nitrate (NH4NO3)

nitric acid (HNO3)

fuel

solvent

urea (CH4N2O) glycine (C2H5NO2) sucrose (C12H22O11) glucose (C6H12O6) citric acid (C6H8O7) hydrazine-based fuels:

water (H2O) hydrocarbons: kerosene benzene (C6H6) alcohols: ethanol (C2H6O)

carbohydrazide (CH6N4O) oxalyldihydrazide (C2H6N4O2) hexamethylenetetramine (C6H12N4) acetylacetone (C5H8O2)

methanol (CH4O) furfuryl alcohol (C5H6O2) 2-methoxyethanol (C3H8O2) formaldehyde (CH2O)

types of organic fuels or their mixtures are typically dissolved in a solvent with metal nitrate hydrates. The prevalence of this class of oxidizer is probably explained by their good solubility in water and relatively low decomposition temperature that results in the formation of active oxygen. For example, aluminum nitrate hydrate, Al(NO3)3·9H2O, decomposes at ∼130 °C, while the onset temperature for decomposition of aluminum sulfate, Al2(SO4)3, is ∼600 °C. In addition, the solubility of nitrate in water at room temperature (∼64 wt %) is higher than that of the corresponding sulfate (∼27 wt %). In some cases, where nitrates are not available, other metal precursors (such as hydroxides) are dissolved in nitric acid, or ammonium nitrate can be used as oxidizer.20,21 Table 1 shows the most commonly used fuels in SCS reactions. These compounds are the source of carbon and hydrogen and, in many cases, form complexes with metal ions, thus facilitating homogeneous mixing of cations in solutions or gels. The ideal fuel should have high solubility in the solvent and low decomposition temperature (below 400 °C), yield no other residual mass, be compatible with metal nitrates (reaction should not lead to explosion), and be readily available or easy to prepare. Water is used as a solvent in most works related to SCS. However, occasionally kerosene,23,24 alcohols,22,25−28 or formaldehyde29 are also utilized. It is known that SCS, as well as other combustion synthesis techniques, can be performed in two modes.30−32 The first mode is called volume combustion or thermal explosion (Figure 2A), where the entire volume of the reactive mixture is preheated uniformly to the boiling point of the solvent (Figure 2B, stage I). This stage is followed by a relatively long constant temperature range (stage II) where free and a portion of the bound water evaporates. The next preheating stage (stage III) is characterized by a higher rate, and then at some temperature (ignition temperature, Tig) it suddenly increases to a maximum value, Tm (stage IV), followed by the cooling stage V. In the other case, a small volume (∼1 mm3) within the reactive solutions or gels is heated locally to initiate the exothermic reaction,33 which self-propagates along the rest of

Figure 1. Published articles in each year according to Web of Science, August, 2016. Keywords: solution combustion synthesis; sol−gel combustion. 14495


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Figure 2. Synthesis of Ca3Al2O6 in volume combustion mode (A) and time−temperature history of Fe2O3 synthesis in Fe(NO3)3+glycine system in volume combustion mode. Reprinted with permission from ref 31 (A) and ref 32 (B). Copyright 2009 Elsevier and 2004 American Chemical Society.

Figure 3. Reaction in nickel nitrite−glycine system in the selfpropagating combustion mode (A) and time−temperature history (B) of the process. Reprinted with permission from ref 33. Copyright 2013 American Chemical Society.

the volume in the form of a combustion wave (Figure 3A). This mode is called the self-propagating combustion mode.34 A time−temperature profile (Figure 3B) indicates that the preheating stage in this mode is relatively short as compared to the volume combustion case. There are several different methods for heating the initial reactive solution; each of them creates a specific route for SCS. The simplest method is placing a glass or ceramic container with the precursor solution on a hot plate35 (Figure 4) or inside a preheated furnace36 (Figure 5). Commonly, the temperature of the furnace or hot plate is about 300−500 °C. Heating, evaporation of the solvent, formation and decomposition of the gel, self-ignition, combustion, and formation of the solid product proceed in one technological step. This approach is the most commonly used heating method for SCS. The next route is the so-called sol−gel combustion synthesis,19,37 also termed gel CS.22,38 Occasionally, SCS and sol−gel combustion synthesis are used synonymously. However, there are some principle differences between these approaches. In the case of gel combustion, the initially prepared aqueous reactive solution is dried at a temperature below the boiling point of the solvent. As a result of the drying process, unbound water is evaporated, and a gel-type media is formed. In the limiting drying case, the gel is crystallized into solid, brittle reactive media. Taking advantage of the fact that reactive aqueous solution has low viscosity, one may impregnate this solution in a variety of porous media (layers). It was demonstrated that combustion reactions can also be accomplished in such impregnated layers, leading to the synthesis of

Figure 4. Solution combustion synthesis of nickel foam: The reaction ignites (A) and moves slowly through the reactive solution (B and C). Reprinted with permission from ref 35. Copyright 2008 John Wiley & Sons.

desired materials, the so-called impregnated layer CS.30,39,40 These layers may be inert or active from the reactivity standpoint. Thus, one may outline CS in porous inert media21,41,42 and CS in porous active media (e.g., celluloseassisted CS43,44). In the former case, typically nanostructured supported catalysts are produced (Figure 6). The latter approach is used to synthesize materials where the initial aqueous solutions possess relatively low heats of reaction, and thus combustion of the active layer facilitates the combustion front propagation. If one uses a porous media, such as porous silica or alumina with desired diameter, to control the size of the synthesized particles, then the method is called template14496


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One may also use a different method to preheat the precursor solutions to the self-ignition temperature; here we discuss microwave-assisted SCS.49−51 Microwave heating differs fundamentally from conventional heating mechanisms. In the former case, heat is directly generated in the materials by the interaction of the electromagnetic field with electric and magnetic dipoles, which accounts for the material’s dielectric, electric, and magnetic properties. Thus, the interaction of the applied alternating electromagnetic field with the reactive aqueous solution is uniquely related to the material’s dielectric properties. This process typically proceeds in the volume combustion mode. Many features of solution combustion synthesis are also present in aerosol flame synthesis.52−54 In many modifications of this method, such as spray pyrolysis55,56 (Figure 8), emulsion CS, or emulsion evaporation,54,57 the initial reaction medium is also an aqueous solution of the fuel and oxidizer. However, in most cases, reactions occur in the external gaseous flame. The aerosol solution combustion synthesis method was developed58 where the synthesis of hollow metal spheres occurs in the flux of inert gas solely due to the reaction between fuel and oxidizer, which are the precursors of the sprayed solution. Recently, a novel SCS-based method has been reported for the production of high-quality thin films for electronic devices (transistors) and solar cell applications.17,22,59 In this technique, reactive solution can be deposited on a hard (silicon wafer) or soft (flexible polymer) substrate using either spin coating or spraying techniques (Figure 9). Combustion of thin precursor layers produces uniform high-quality oxide films.

Figure 5. Aqueous solution of ceric ammonium nitrate, palladium nitrate, and oxalyldihydrazide (A), the solution combustion in a preheated furnace (B), and palladium doped cerium oxide product (C). Reprinted with permission from ref 36. Copyright 2009 American Chemical Society.

Figure 6. One-step synthesis of Ni or NiO catalysts supported on silica (SiO2) prepared by combustion of nickel nitrate−glycine−ammonium nitrate reactive gels impregnated onto porous SiO2. Reprinted with permission from ref 21. Copyright 2014 American Chemical Society.

3. FUNDAMENTALS OF SOLUTION COMBUSTION SYNTHESIS

assisted solution combustion synthesis45−48 (Figure 7). The porous template media is removed (dissolved) after synthesis, and nanoparticles with a narrow size distribution or nanotubes are extracted.

3.1. Thermodynamics of SCS Processes

3.1.1. General Considerations. Solution combustion synthesis can be accomplished in an aqueous solution of the oxidizer and fuel, which is sufficiently exothermic to maintain a self-sustained chemical reaction. As mentioned in section 2, typical oxidizers are hydrated metal nitrates, while fuels represent a broad range of compounds including urea, glycine, citric acid, etc. (see Table 1). It is worth noting that the role of the fuel is not only to provide sufficient heat for the system, but also to ensure the formation of stable complexes with the metal ions to increase their solubility and prevent selective precipitation of the metal ions during water removal.60,61 Thus, SCS involves a self-sustained redox exothermic reactions between hydrated metal nitrates and fuel(s) mixed on the molecular level, which obey all features of other combustible systems.

Figure 7. Schematic representation of the template-assisted combustion synthesis of α-Fe2O3: initial template (A) is impregnated by reactive solution (B) followed by initiation of reaction, which selfpropagates (C) forming nanoparticles inside the channels (D) followed by leaching of the template and obtaining α-Fe2 O3 nanoparticles (E). Reprinted with permission from ref 45. Copyright 2014 American Chemical Society.

Figure 8. Schematics formation of hollow Mn3O4 spheres by aerosol spray pyrolysis process. Reprinted with permission from ref 56. Copyright 2014 Royal Society of Chemistry. 14497


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Figure 9. Schematic of the SCS of metal oxide (MO) films and the corresponding bottom gate top contact thin-film transistor structure. Reprinted with permission from ref 22. Copyright 2015 United States National Academy of Sciences.

to a single crystal state of the same substance. This excess is generally small as compared to the original chemical energy, but when the product reaches high temperatures, the recrystallization process takes place, the grains grow, and the surface area of the boundaries decreases together with the specific energy density of these boundaries. When the temperature decreases, the process of grain growth slows and at some point essentially halts. This behavior is not a specific feature of the SCS products, because polycrystalline materials, from all processing techniques, behave similarly during the cooling stage. Thus, from a thermodynamic viewpoint, the SCS products are nearly at the equilibrium state; that is, the change in system’s Gibbs free energy, due to the microstructural transformations, is significantly smaller than that during combustion itself. Understanding these general specifics of SCS systems is critical for the accurate calculation of their thermodynamic characteristics. 3.1.2. The Equilibrium Composition of Products and the Adiabatic Combustion Temperature. For combustion processes, thermodynamics is used to calculate the equilibrium reaction products and the adiabatic combustion temperature.11 All software packages, which allow for the calculation of equilibrium products and the adiabatic combustion temperature, can be used for SCS systems as well. However, typically a specific approach, based on the calculations of the reducing/ oxidizing valences of the redox mixture, is used for thermodynamic analysis of SCS systems.63,64 We briefly discuss this approach using the example of the glycine (fuel)−metal nitrate (oxidizer) system. In this method, metals, carbon, and hydrogen are considered reducing elements with the corresponding valence for metal (v), carbon (+4), and hydrogen (+1). Oxygen, as an oxidizer, possesses the valence (−2), and nitrogen is assumed to have zero valence. For example, the reducing valence of glycine (NH2CH2COOH) is (0 + 2 + 4 + 2 + 4 − 2 − 2 + 1) = +9, whereas the valence of a metal nitrate, Mev(NO3)v, is (v + 3v· (−2·)) = −5v. Note that hydration water (e.g., m molecules) does not affect the overall compound valence. It is also assumed that H2O, CO2, and N2 are the gaseous products formed in the combustion reaction. The balance of the metal nitrate hydrate decomposition reaction R4 and fuel (glycine) oxidation reaction R5 can be presented as follows:

In general, the driving force for such redox reactions is the intention of the system to reduce its Gibbs free energy (G) by converting the chemical potential into heat. At first, we can consider the SCS system on a macroscopic scale, under the assumption that energy and mass exchanges with the surrounding environment can be neglected, that is, adiabatic conditions in an isolated system. At sufficiently low temperatures, the initial reactive solution can exist indefinitely, without noticeable changes in composition, temperature, or pressure. However, the initial stationary state of combustion systems is not stable. If the temperature of the system is increased to the reaction onset point, a chemical reaction with heat generation initiates. As a result of this exothermic reaction, the temperature of the solution increases, which in turn leads to an increase in the reaction rate, and the process becomes self-accelerating. This process ends when the reagents are completely transformed into the products and the system reaches a new stationary state. By definition, the equilibrium state of the system is one in which the energy G is minimum. Thus, in the above statements, the initial state of the SCS system can be described as a nonequilibrium quasi-stationary state. Indeed, it is nonequilibrium because the free energy G in this state is not minimal, and it is quasi-stationary because the system does not undergo significant changes within a reasonably long time. During combustion, the system free energy moves it toward an equilibrium state, which could last indefinitely (thus be stationary), if the sample is isolated from the environment. On long time scales (10−100 s), however, the sample is not isolated and exchanges heat with the environment. The combustion reaction leads to a large temperature difference between the sample and the surrounding atmosphere, and thus the sample temperature continuously changes. After being cooled to ambient temperatures, the sample reaches a new equilibrium steady state. From a materials viewpoint, several types of equilibria can be considered: chemical, thermal, phase, and structural. Whether these equilibria are reached depends on the kinetics of the processes (discussed in section 3.2) and on the external conditions. Clearly, at a slow cooling rate, one should expect the formation of products that are closer to equilibrium, while at rapid cooling the products may contain nonequilibrium and metastable phases.12,62 The SCS products are polycrystalline, sometimes multiphase nanoscale materials, which always contain grain and interphase boundaries. These boundaries have excess free energy relative

Me v(NO3)v ·mH 2O = MeOv /2 + mH 2O + v /2N2 + 5/2·vO2 14498

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2C2H5NO2 + (9/2)O2 = 5H 2O + 4CO2 + N2

Table 2. Standard Enthalpies of Formation for Some Metal Nitrates and Fuels

(R5)

The overall redox reaction is given by the following scheme: Mev(NO3)v ·mH 2O + 2nC2H5NO2 + 1/2·[9n − 5/2·v]O2 = MeOv /2 + (5n + m)H 2O + (n + v /2)N2 + 4nCO2 (R6)

If we assign 9n/(5/2·v) = n18/5v = φ and thus correspondingly n = (5/18)·φv, then we have the following overall reaction for the SCS system, which involves 1 mol of hydrated nitrate and n moles of fuel (glycine): Me v(NO3)v ·mH 2O + 5/9φvC2H5NO2 + (5/4)v[φ − 1] O2 = MeOv /2 + (25/18·φv + m)H 2O + v(5/18φ + 1/2)N2 + (10/9) ·vφCO2

Ni(NO3)2 ·6H 2O + (10/9)C2H5NO2 + (5/2)[φ − 1]O2 = NiO + (25/9φ + 6)H 2O + (5/9φ + 1)N2 + (20/9)φCO2

The difference in standard enthalpies (ΔHf0) of formation for combustion products and precursors provides the amount of heat released during the reaction, called the heat of reaction (Q):

∑ njΔf H j0 − ∑ niΔf Hi0 (1)

where i and j specify reactants and products, respectively, and ni and nj are the amounts of each compound. The ΔHf0 values for some metal nitrates and fuels are presented in Table 2.65,66 Under adiabatic conditions, all of this energy is used to heat the reaction product to a temperature, defined as the adiabatic combustion temperature (Tad), and can be determined by solving the following equation:

3 + Tad

∑j njBj 2

∑j njBj 2

T02 +

2 + Tad

⎢

∑j njCj

⎣

3

∑ njAjTad−⎢⎢Q + j

T03

⎥

∑ njAjT0⎥⎥ = 0 j

⎦

nitrate

ΔHf0, kJ/mol

Ni(NO3)2·6H2O glycine, solid NiO H2O(g) CO2(g) N2(g) O2(g)

−2211.7 −528.5 −239.7 −241.8 −393.5 0 0

Tad = T0 + Q / cp (2)

(4)

Cp, J/mol·K 464 95 44 30 + 43 + 27 + 25 +

0.015T 0.011T 0.004T 0.015T

(K) (K) (K) (K)

(5)

where cp = ∑jnjAj is the average specific heat capacity of the products at room temperature. Such estimation typically gives a higher value of Tad as compared to accounting for the temperature dependence of Cp and can be used only for systems where the heat capacities of the products depend weakly on the temperature. For example, Tad calculated using eq 5 for the nickel nitrate−glycine system has a value of ∼1280 K. The above method was used in many recent studies of SCS for different systems, including calcium and aluminum

where Cp,j(T) is the specific heat of the j-product as a function of temperature. Equation 2 assumes no phase transitions, such as melting, decomposition, where the enthalpies of the transitions also need to be accounted for.11 The temperature dependence of the specific heats of substances and compounds can be found in handbooks,66 and are usually presented in the form of polynomials: Cp = A + B ·T + C·T 2

−528.5 −333.5 −1543.8 50.63

It is worth noting that for rough estimation of Tad, the specific heat can be considered constant for each product (Bi,Ci = 0 in eq 3), and then from eq 4 one may obtain the expression for adiabatic temperature, which is often used in studies of combustion processes:

Tad

j

glycine, solid urea, solid citric acid, solid hydrazine, liquid

Table 3. Standard Enthalpies of Formation and Heat Capacities for Nickel Nitrate−Glycine System

(R9)

0

−2211.7 −1326.3 −425.1 −3285.3 −674.9 −2211.20 −1630.5 −1325.9 −1021.20 −471.79 −2110 −1217 −349.95

As an example, for reaction R9, which leads to the formation of NiO during combustion of the nickel nitrate−glycine system, calculations using eq 4 and data presented in Table 364,66 give, for a stoichiometric mixture (φ = 1) and at T0 = 300 K, an adiabatic temperature Tad = 1076 K.67

the overall redox reaction R6 takes the form:

∫T ∑ njCp,j dT

Ni(NO3)2·6H2O Ni(NO3)2·3H2O Ni(NO3)2 Fe(NO3)3·9H2O Fe(NO3)3 Co(NO3)2·6H2O Co(NO3)2·4H2O Co(NO3)2·3H2O Co(NO3)2·2H2O Co(NO3)2 Cu(NO3)2·6H2O Cu(NO3)2·3H2O Cu(NO3)2

+

(R8)

Q=

ΔHf0, kJ/mol

3

Ni(NO3)2 ·6H 2O = NiO + 6H 2O + N2 + (5/2)O2

i

fuel

∑j njCj

where φ = 1 means that the initial mixture does not require external (atmospheric) oxygen for complete oxidation of fuel, while φ > 1 ( q−. Critical conditions between mild reaction and thermal explosion correspond to the point C, where the heat loss line and heat release curve are tangent. Using this diagram (Figure 13A), Semenov derived an expression for the critical temperature Tcr: Tcr = T0 +

RTcr2 RT02 ≈ T0 + Ea Ea

d ⎛⎜ dT ⎞⎟ dT λ − cρU + QW = 0 ⎝ ⎠ dx dx dx

(9)

Equation 9 and Figure 13A indicate that the critical temperature of ignition is defined by the activation energy (Ea) of the chemical reaction, the ambient temperature (T0), and the heat exchange conditions. However, real kinetics of

(10)

with boundary conditions set at infinity: x = −∞ , T = T0 ; 14502

x = +∞ , T = Tad

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Figure 16. Thermal structure of the combustion wave. Reprinted with permission from ref 12. Copyright 2015 CRC Press Taylor & Francis Group.

The flux of the initial reactive mixture carries internal chemical energy that is released during the reaction and is converted into heat, which is carried out by the hot products of the reaction. The preheating-reaction zone exchanges both matter and energy (heat) with the environment, and can be considered a thermodynamically open system where irreversible physical and chemical processes occur. An analytical solution of the problem (eqs 10,11) with Arrhenius kinetics (eq 7) and in an approximation of the narrow reaction zone79 can be represented as follows: U≈

1 2Qλk 0 cρ(TC − T0)

∫T

TC

0

e−E / RT dT (12)

It can be seen that values of the activation energy of the reaction and pre-exponential function are critical to control the characteristics of the combustion wave propagation. It should be noted that the self-propagating mode is not widely used in SCS works. This combustion regime, however, is more controllable than the thermal explosion mode. We can conclude that for both thermal explosion or selfpropagating regimes, knowledge of the chemical reaction kinetics is an important factor for optimization of the temperature−time history of the process and control over both the nanostructure and the properties of the resulting materials. However, as is shown in section 3.3, the reaction mechanism in solution combustion is complex. It is difficult to obtain data on the intrinsic kinetics systems that describe the rate of each chemical reaction involved in the process. To describe the kinetics, the iso-conversional approach is typically used.80,81 Despite widespread use, the physical meaning of the activation parameters obtained by the approach is not entirely

Figure 14. TGA/DTA for iron nitrate in air and its combustion characteristics with glycine (A) and reaction ignition temperatures for the glycine−iron nitrate system as a function of fuel to oxidizer ratio (φ). Reprinted with permission from ref 32. Copyright 2004 American Chemical Society.

where λ, c, and ρ are the heat conductivity, heat capacity, and density of the reaction media, respectively. A typical temperature profile in the adiabatic combustion wave is shown in Figure 16.

Figure 15. TGA results (A) for glycine (curve 1) and nickel nitrate hexahydrate (curve 2) and TGA/DTA results of reaction in glycine−nickel nitrate hexahydrate (φ = 1.75) aqueous solution. Reprinted with permission from ref 67. Copyright 2011 John Wiley & Sons. 14503


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clear, and is still a matter of debate.82 The principle idea of this method is simple and is based on two main assumptions.80 First, the rate of the process is defined by two independent functions, one of temperature and the other degree of conversion (eq 7). Second, the effective activation energy is obtained on the basis of a set of kinetic trials from the dependencies of time versus temperature (for isothermal measurements), and temperature versus heating rate (for integral and incremental methods with linear heating rate). The evaluation is carried out at fixed conversion. After the separation of variables and integration, eq 7 gives

∫0

η

dη = k0 ϕ(η)

∫0

t

e−E / RT dt

profiles of a nickel nitrate−urea mixture, impregnated on alumina, obtained at different heating rates and the corresponding Arrhenius plot, following the Kissinger analysis, are shown in Figure 17A and B, respectively.64 It can be seen

(13) Figure 17. Temperature−time profiles for a combustion of nickel nitrate−urea−alumina mixture at different heating rates (A) and Arrhenius plot according to the Kissinger method (B). Reprinted with permission from ref 64. Copyright 2014 Elsevier.

One of the most widely used approaches is the iso-conversional method at linear heating rates (β): T = T0 + β ·t

(14)

From eqs 13 and 14, the folloing is obtained: β

∫0

η

dη = k0 ϕ(η)

∫0

t

e−E / RT dt

that the obtained apparent activation energy is on the order of 176 kJ/mol, which, as noted by the authors, is slightly higher than for the thermal decomposition of anhydrous nickel nitrate (153 kJ/mol). It is remarkable that DTA studies of a nickel nitrate−glycine system with φ = 1.75 showed two reactions (Figure 15B).67 Analysis of DTA results based on the Kissinger method revealed different activation energies for these reactions. The authors speculated that the first process with an activation energy of ∼123 kJ/mol corresponds to the reaction between NH3 and HNO3, which forms during decomposition of glycine and nickel nitrate, respectively. It was also suggested that the second reaction may be related to the reduction of NiO by hydrogen. The activation energy of this reaction is lower (∼111 kJ/mol) and within values for hydrogen reduction (85−110 kJ/ mol) reported elsewhere.89,90As shown in the next section, kinetic data are necessary to develop the mechanisms of SCS processes. Thus, to control the SCS process and to fabricate materials with desired properties, we hope that future research will include additional kinetic studies for combustible solutions.

(15)

or

∫0

η

t k kE dη e−E / RT dt = 0 a = g (η ) = 0 ϕ(η) β 0 βR ∞ e −x k 0Ea dx = p(x ) x βR x2

∫

∫

(16)

where g(η) is the integral conversion function (see Table 1 in ref 83 for g(η) of different reaction models applied to describe the reaction kinetics in heterogeneous solid-state systems), Ea is the apparent activation energy in the investigated range of degree of conversion η, x = Ea/RT, and p(x) is the temperature integral, which has no analytical solution. To overcome this difficulty, the temperature integral has been solved using approximation methods, series expansions, and numerical methods.84 The considered iso-conversional integral methods are based on an approximate form of the temperature integral that results from rearrangement and integration of eq 15. For example, the Kissinger−Akahira−Sunose (KAS) method85,86 uses the Coats−Redfern87 approximation of the temperature integral that leads to ⎛ kR ⎞ ⎛ β ⎞ Ea ⎟⎟ − ln⎜ 2 ⎟ = ln⎜⎜ 0 RT (η) ⎝ T (η) ⎠ ⎝ Ea, ηg (η) ⎠

3.3. Mechanisms of SCS Reactions

The mechanism of solution combustion synthesis implies knowledge on the elementary stages of the process, intrinsic kinetics of each elementary reaction, as well as determination of the formation sequence for the intermediate and final phases. Typically for complex phenomenon such as SCS, the mechanism means understanding the sequence of reactions that are responsible for combustion and product formation, as well as knowledge of the effective (apparent) kinetics, which account for heat and mass transfer processes. In this case, it is better to consider macrokinetic mechanisms, as originally proposed by Frank-Kamenetsky.91 It should be noted that, even in the macrokinetic sense, few SCS reaction mechanisms have been reported in the literature. Typically, they are based on the results of TGA/DTA and differential scanning calorimetry (DSC) analysis of the precursors. More sophisticated approaches also include the thermodynamic analysis of the combustion process and the results obtained by using in situ techniques, such as mass spectrometry and time-resolved X-ray diffraction (XRD). In this section, we present chemical mechanisms of combustion processes in reactive solutions and gels. 3.3.1. Mechanism of Oxide Forming Systems. Patil et al. proposed the combustion synthesis mechanism (Figure 18)

(17)

Thus, for η = const, a plot of ln(β/T (η)) versus 1/T(η), obtained from thermal curves recorded at several heating rates, is a straight line whose slope allows for evaluation of the apparent activation energy and intercept allows the value of the pre-exponential factor to be obtained for a known analytical form of the integral function of conversion. Accounting for the above-noted assumptions, one may extract useful information regarding the overall kinetics of the SCS process by using isoconversion methods. The obtained values of effective activation energy and pre-exponential factor permit prediction of parameters for both the volume combustion and the selfpropagating modes. Unfortunately, only a few works report data on reactive solution kinetics.64,67,88 For example, the temperature−time 2

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for combustion are allowed to be removed, combustion does not occur. Also, if the mixture is heated at a slow rate, the flame does not appear, because the time required to reach the ignition temperature is too long and all of the gases responsible for combustion escape from the foam. The mechanism of combustion reactions in iron nitrate−fuel (glycine, citric acid, and hydrazine) systems was suggested on the basis of TGA/DTA, direct measurements of the temperature−time history of the process, and thermodynamic considerations.32 Additionally, model experiments in an argon atmosphere were also reported. Figure 19 represents typical temperature−time profiles for combustion of the iron nitrate− glycine mixture with an excess of fuel (φ = 3) in air and argon. A comparison of such profiles allows one to conclude that the ignition temperature (Tig) does not depend on the composition of the ambient gas and the observed maximum temperature is lower in argon than in air. The absence of the second reaction stage in argon clearly indicates that a two-step reaction mechanism is responsible for the final product formation in air. First, a relatively rapid reaction marked stage I in Figure 19A initiates at 125−150 °C and proceeds primarily due to glycine oxidation by oxygen-containing species formed in the solution. A slower burning process (stage II) utilizing oxygen from air follows rapid stage I. Figure 14A shows data from TGA/DTA, indicating that the measured temperature region for the combustion reaction ignition (125−150 °C; see Figure 14B) fits well with the temperature interval for decomposition of iron nitrate. It is shown that decomposition of iron nitrate forms HNO3, which, at the relatively high temperature, either directly or with preliminary oxygen formation reacts with glycine.78 Figure 19A shows that the second stage of oxidation is slower because the oxygen is not derived from the ideally mixed (on the molecular level) solution, but from the ambient air. This gas−solid combustion is controlled by oxygen diffusion through the inert component of air, that is, nitrogen. The reaction pathway of the iron nitrate− hydrazine system is different.32 The ignition temperature for this system is lower and in the range 105−115 °C. It was shown that rapid interaction in this system is related to N2H4 boiling and not to iron nitrate decomposition. Additional experiments conducted in argon for a fuel-rich solution (φ = 3) showed that a mild and slow oscillating-type combustion occurred as compared to the vigorous reaction that was observed during combustion in air.

Figure 18. Schematic representation of the SCS mechanism in aluminum nitrate hydrate−urea system. Reprinted with permission from ref 3. Copyright 2008 World Scientific Publishing.

for Al2O3 powder in the stoichiometric solution of Al(NO3)3· 9H2O and urea introduced in a preheated (500 °C) furnace.3 They suggested that, during first 2 min of heat treatment, dehydration of the hydrated metal nitrate, as well as decomposition of the urea, take place with formation of Al(OH)(NO3)2, urea nitrate (CH5N3O4), biuret (C2H5N3O2), cyanuric acid ((HNCO)3), and ammonia (NH3). These compounds are close to what can be expected due to the decomposition of aluminum nitrate nonahydrate92 and urea.93 Indeed, it is shown that cyanuric acid is the primary decomposition product of urea at temperatures above 150 °C, followed by biuret (C2H5N3O2) formation, as a result of the reaction of cyanuric acid with intact urea.93 Meanwhile, HNO3, N 2 O 4 , NO 2 , N 2 O 5 , and H 2 O were identified during decomposition of aluminum nitrate at ∼100−150 °C.92 In the second stage (Figure 18), the viscous mixture foams due to the generation of gaseous decomposition products, which leads to swelling. It is important that these gaseous species (i.e., nitrogen oxides, NH3, and HNCO) are hypergolic in contact with each other. It was suggested that the foam, if made from cyanuric acid and nitrate, is also combustible. In the third minute, the combustion reaction was initiated to form αAl2O3 powder. The authors noted that, if the gases responsible

Figure 19. Temperature time profiles of iron nitrate−glycine reaction (with φ = 3) in air (A) and argon (B). Reprinted with permission from ref 32. Copyright 2004 American Chemical Society. 14505


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K. 67 The authors noted that the oxidizer and fuel simultaneously supply the gaseous products HNO3 and NH3, respectively. Kumar et al. also performed67 thermodynamic calculations to estimate the adiabatic combustion temperature and gas-phase compositions. These calculations showed that the HNO3 + NH3 mixture is highly exothermic. They also reported calculated adiabatic temperatures and equilibrium products of the HNO3 + NH3 + NiO + H2O system. The results showed that this system is characterized by a reasonably high adiabatic temperature (700−900 K). Also, three distinct regions of products have been found, depending on the NH3 content. At low NH3 amounts, NiO remains unreduced, while the increase in NH3 concentration results in the formation of NiO−Ni or Ni phases. Meanwhile, it was shown that the hydrogen concentration increases with the increase in the NH3. This analysis allowed the authors to conclude that increasing the glycine content in the reactive solution will form reductive gases, such as NH3 and H2, in the first stage of combustion. Such variation of the gaseous species composition, in turn, leads to a change in the process of the solid products from NiO through NiO−Ni mixture to pure Ni. More detailed studies were reported for combustion mechanism of nickel nitrate−glycine gels in self-propagating reaction mode.33,100 In these researches quenching techniques, as well as in situ methods, such as time-resolved X-ray diffraction, high-speed infrared imaging, and TGA/DTA with mass spectrometry, were applied to further validate the mechanism of metal formation in the SCS wave. High-speed video recording and an infrared temperature distribution map (Figure 20) of combustion wave for a gel with φ = 1.25 clearly suggest three distinct areas in the combustion wave. Zones 1 and 2 are the preheating zones. Intensive evaporation of bound water takes place in zone 1. The temperature−time profile shown in Figure 3B also indicates a local decrease in the combustion temperature at the preheating stage, which corresponds to water evaporation. The exothermic reaction begins on the boundary between zones 2 and 3. The estimated width of the chemical reaction zone is ∼500 μm, and the total reaction time is on the order of 0.1 s. Time-resolved X-ray diffraction of the combustion reaction in the nickel nitrate−glycine system with φ = 1 and φ = 1.25 is shown in Figure 21. These data allowed the authors to conclude that NiO forms in the reaction front for both compositions. However, in fuel-rich gels, the Ni phase also forms in the combustion front, and the intensities of the Ni peaks rapidly grow. Low intensity diffraction peaks of NiO can still be observed in Figure 21C and D. These results demonstrate that utilization of in situ diagnostic approaches provides more realistic data on the mechanism of combustion synthesis than do DTA and DSC analyses, both of which provide relatively slow heating rates (such as 20 °C/min) of the reactive gels. On the basis of these results, as well as in situ mass-spectroscopic studies and transmission electron microscopy (TEM) analysis of materials taken from the quenched combustion front, the mechanism of the SCS reaction in the nickel nitrate−glycine system was suggested (Figure 22). The main driving force of the combustion process is the highly exothermic reaction between N2O and NH3. N2O gas forms during decomposition of nickel nitrate at 250 °C. Meanwhile, decomposition of glycine yields mainly NH3, CO2, H2O, and solid glycylglycine (dipeptide) and 2,5-piperazinedione. The onset temperature (250 °C) of the reaction coincides with the

Considering these observations, the interaction mechanism in this system was suggested as follows. Because of the relatively low temperature of hydrazine evaporation, its interaction with oxygen in air becomes significant. Increasing φ increases the availability of fuel in the gas phase, which explains why the reaction is more vigorous for larger φ, contrary to the iron nitrate−glycine system. Further, as shown experimentally, while the solution is boiling in air, ignition of pure hydrazine does not occur. Thus, initiation of the reaction in solution at lower (than for the glycine case) temperature means that hydrazine molecules in the gas phase are sufficiently active to react with iron nitrate before its decomposition. Following the reaction initiation, evaporation of excessive fuel intensifies, and its gasphase oxidation in air becomes predominant. In the iron nitrate−citric acid system, the ignition temperature is shown to be in the range 120−140 °C, which coincides with the onset of nitrate decomposition.32 It should be noted that melting/decomposition of citric acid is higher than 175 °C.94 Identical ignition temperatures of glycine and citric acid assisted reactive solutions may indicate that the reaction could have similar characteristics. It was shown, however, that in the iron nitrate−citric acid system, the reaction is slower and does not depend as strongly on φ as compared to the glycineassisted reaction. While the two systems have similar adiabatic combustion temperatures, differences in their behavior can be explained by the reactivity of the two fuels. Erri et al. showed60 that the activity of a −NH2 type ligand appears to be higher than that of a −OH group, which in turn is more active than −COOH. This explains why the amino group containing glycine is a more reactive fuel than citric acid, which involves both −OH and −COOH. 3.3.2. Mechanism of Metal Formation in SCS Reactions. Recently, it was demonstrated that by using SCS, it is possible to produce not only oxides of different forms along with metal−oxide composites,95−98 pure metals,33,35,67,99−103 and alloys.35,104−106 It was shown that in several systems, SCS of reactive solutions with an excess of fuel (φ > 1) leads to the formation of pure metals. Investigations of the reaction mechanism in such metal-forming systems allow explanation of this interesting effect.33,67,104 Kumar and coauthors obtained TGA data for decomposition of glycine (curve 1) and nickel nitrate (curve 2) in air, and TGA/DTA profiles for the solution of these precursors are shown in Figure 15A and B, respectively.67 Analysis of these results, in conjunction with previously reported data,107 allowed the authors to conclude that glycine decomposition starts at ∼515 K; the main product of the first stage is NH3, while CO and HNCO are formed in the temperature range 585−730 K. The decomposition of nickel nitrate hexahydrate also involves several stages,108 which first takes place in the temperature range 310−350 K and involves water evaporation. In the second stage, at 420−460 K, partial decomposition of the nitrate may form Ni(NiO3)(OH)0.25·H2O, which decomposes during the third stage (530−570 K) according to the following reaction: Ni(NO3)(OH)1.5 O0.25 ·H 2O = 0.5Ni 2O3 + HNO3 + 1.25H 2O

(R11)

Comparative analysis of these results with TGA/DTA of the nickel nitrate−glycine reactive solution (Figure 15B) suggests that decomposition reactions, as well as combustion reactions, occur essentially in the same temperature range of 510−620 14506


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active layer combustion (IALC).109 The IALC process typically involves three steps (Figure 23), including the impregnation of the reactive aqueous solution into a porous layer (cellulose paper), subsequent drying, followed by initiation of the combustion reaction.43,111 This approach includes the main synthesis reaction and the burning of the porous cellulose matrix. The cellulose burning leads to the formation of gaseous products (CO, CO2), while combustion of the reactive solution results in the desired solid product, such as metal oxides in the form of hollow tubes. An example of a typical temperature profile for IALC is shown in Figure 23. It can be seen by deconvolution that there are two distinct temperature peaks, corresponding to the two exothermic reactions. The lowtemperature zone is heated by conduction and can involve evaporation of water and devolatilization of the cellulose paper. The scanning electron microscope (SEM) image of the noncombusted zone in Figure 23 shows that the reactive solution is uniformly distributed as thin layers on the surfaces of the cellulose fibers (Figure 23A). The SEM micrograph of the material collected after the passage of the low-temperature front (Figure 23B) shows some changes in the fiber structure, but there is no significant change in the overall microstructure. The material collected after the second front (Figures 23C−E) shows that, along with some carbon residue, the morphology of the as-synthesized product can be described as hollow tubes with thin walls. This approach is beneficial for the synthesis of metal oxide or metals with low caloric nitrate−fuel systems. The additional heat formed through burning of the cellulose makes the oxide formation reaction self-sustained. For example, the method was used to fabricate SrRuO3 perovskites for fuel cell applications.112 Deshpande et al. had previously shown that reactions between strontium/ruthenium nitrates and glycine were not sufficiently exothermic to combust under normal conditions.113 It should be noted that pure cellulose paper employed in the synthesis also may not exhibit a controlled low-temperature smoldering combustion wave. Thus, neither exothermic reaction can occur by itself in a low-temperature mode. Conversely, the impregnated system did display the desired smoldering mode resulting from heat exchange between the burning cellulose and nitrate reduction reactions. As a result, low-temperature synthesis of a nanoscale product was possible with the IALC process. Examples of typical temperature profiles are shown in Figure 24. It can be seen that there are two distinct temperature peaks, corresponding to the two exothermic reactions. Any model of the IALC process must account for the two primary exothermic reactions, corresponding to either the leading cellulose burning reaction or the trailing oxide synthesis reaction. It is also important to note that the synthesis reaction may be preceded by an endothermic stage, either due to decomposition of the reactants or due to gasification of adsorbed water molecules within the hydrate structure. Thus, in the model developed in ref 110, the following reaction scheme was considered:

Figure 20. High-speed video (A) and infrared (B) images of SCS for a nickel nitrate−glycine gel with φ = 1.25. Reprinted with permission from ref 33. Copyright 2013 American Chemical Society.

ignition temperature of the reactive gels. Therefore, it is proven that the gel combustion is triggered by an exothermic reaction between N2O and NH3 gases. However, two successive endothermic steps are responsible for metal formation during the combustion of fuel-rich compositions. In the first step, NiO nanoparticles form by decomposition of nickel nitrate hydrate at ∼250 °C. The second step occurs at higher temperatures (>450 °C) and involves the reduction of NiO by excess of NH3 to metallic nickel by the following reaction: 3NiO + 2NH3 = 3Ni + N2 + 3H 2O

(R12)

Jiang and co-workers reported TGA/DTA and massspectroscopic data of nickel nitrate−citric acid gels for the preparation of nickel.104 They also observed an abrupt weight loss from 220 to 245 °C, which indicates combustion of the gel. In situ mass spectroscopy showed that H2, H2O, CH4, NO, CO, NH3, and NO2 species form during the combustion of gels. The authors noted that the release of reducing H2 and CH4 gases might be responsible for the redox reaction for synthesizing metals from oxides. 3.3.3. Theoretical Models. The final logical step in the study of SCS reactions mechanism is the development of a theoretical model, which describes the specific features of the process and allows for prediction of synthesis conditions. However, only a few such models have been reported.109,110 Kumar et al. developed a model related to the impregnated

A → C, B → D → P

(R13)

where the first step in the second reaction, B → D, is expected to be endothermic. In R13, the leading reaction A → C represents burning of the cellulose matrix, forming a primarily gaseous product, which escapes from the system. B represents the reactive solution, D some intermediate, and P the product. Assuming a steady-state regime, when the combustion wave 14507


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Figure 21. Results of time-resolved X-ray diffraction analysis for SCS of nickel nitrate−glycine gels with φ = 1 (A and B) and φ = 1.25 (C and D). Reprinted with permission from ref 33. Copyright 2013 American Chemical Society.

Figure 22. Schematic representation of the combustion synthesis mechanism for metallic nickel. Reprinted with permission from ref 33. Copyright 2013 American Chemical Society.

moves with constant velocity u (in a coordinate system moving to the left with the same speed), the following equation describes the temperature−time history of the process:

where k is the sample thermal diffusivity, T is the temperature, a, b, and d are concentrations of the species A, B, and D in reaction R13, t is the time, and x is the spatial coordinate. The

Tt = kT ″ − uT ′ − α(Yp)(T − T0) + q1R1 − q2R 2 + q3R3

subscript 1 refers to the leading reaction A → C, the subscript 2

(18)

refers to the endothermic reaction B → D, and the subscript 3

at = − ua′ − R1 ,

represents the final synthesis reaction D → P. The heats of the

bt = − ub′ − R 2 ,

reactions are q1, q2, and q3. Ri are the reaction rate terms, which

dt = − ud′ + R 2 − R3

are taken as first-order Arrhenius kinetics:

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Figure 23. Structure of the combustion wave and a snapshot of a high-speed video recording. SEM images of reactive solution impregnated layer (A), material collected after the passage of the low-temperature front (B), and final product collected after the high-temperature front (C−E). Reprinted with permission from ref 111. Copyright 2010 Elsevier.

uref =

xref = tref

K 2k

(22)

The boundary conditions are x → −∞ : a → 1, b → 1, d → 0, T → T0 x → +∞ : Tx → 0

Up to this point, the specifics of the IALC process are reflected by the suggested kinetic scheme R13, which includes the endothermic reaction B → D. In addition, it was noted that the use of a constant heat loss coefficient α, while commonly employed in many combustion applications, does not describe IALC conditions. The nanostructure of the product results in a significant increase of the materials’s specific surface area as the product synthesis progresses, leading to a corresponding increase in heat losses. To account for this effect, modeling the heat loss coefficient was suggested, which changes as the reaction progresses as follows:

Figure 24. Temperature profiles for cellulose (carbon) oxidation reaction and synthesis of SrRuO3 (A) as well as Fe2O3 formation with the reversed reaction order (B). Reprinted with permission from ref 110. Copyright 2011 Elsevier.

R1(x , t ) = a(x , t )K1 e−E1/ T R 2(x , t ) = b(x , t )K 2 e−E2/ T R3(x , t ) = d(x , t )K3 e−E3/ T

α(Yp) = a1 + a 2Yp

(20)

Ei and Ki are the activation energy and frequency factor for reaction i, respectively. Finally, T0 is the ambient temperature, and α is a heat loss coefficient. The time and space are nondimensionalized using the following characteristic values tref and xref: tref = K 2−1 ,

xref =

k K2

(23)

(24)

where Yp is the ratio of the intrinsic conversion of the product P to the theoretical maximum (assuming complete conversion of B and D). While the model allows for analysis of any kinetics described in R13 and by eq 19, attention was paid to the case where the cellulose burning leads to oxide formation, as for example in the synthesis of SrRuO3.112 First, the solution behavior of the leading (cellulose burning) reaction was investigated with the goal of determining qualitative behavior driving the system toward more effective production of the nanoscale product. To

(21)

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Figure 25. Dimensionless combustion wave velocity (u), product yield, and maximum temperature (A) and distance between the fronts as a function of q1/q2 (B). Reprinted with permission from ref 110. Copyright 2011 Elsevier.

possible to keep a high final yield and allows for rapid cooling of the product, thus without giving much time for sintering. These factors make this process suitable for the production of nanoscale materials with a high surface area and large pore volume. In summary, we can suggest a classification of the mechanisms in solution combustion reactions. The type I mechanism, such as iron nitrate−glycine system in argon, involves the combustion reaction in the gas phase, formed due to decomposition of the precursors, and under high temperatures the initial metal nitrate completely decomposes to the corresponding oxide. In this case, the combustion front propagation is controlled solely by the gas-phase reaction, while condensed phase reactions, which lead to the final product formation, do not directly contribute to the combustion process. For this mechanism, the microstructure and properties of the synthesized materials should primarily depend on the temperature−time history in the postcombustion zone. In type II mechanisms (e.g., aluminum nitrate−urea or some IALC systems), the combustion front propagation is governed by both condensed and gas-phase reactions. The microstructure of the final product also depends on the processes, which take place in the reaction zone. Finally, type III mechanisms (e.g., pure metal formation in nickel nitrate−glycine system) are characterized by a sequence of gas and condensed phase reactions connected with each other. Clearly, an important task is to find general relations between the combustion mechanisms discussed above and specific product structure formation routes during SCS. The experimental and theoretical data available in the literature, however, do not permit us to elaborate a detailed description of the SCS mechanisms. An analysis of all published results allows one to distinguish the following stages during the process: Evaporation of water from solution takes place during the preheating stage of the process; that is, solution converts into a gel; dehydration, involving removal of bound water; pyrolysis (decomposition) of fuels and oxidizers with formation of NOx, NH3, CH4, or other reactive molecules, with simultaneous nucleation of solid oxide particles; high-temperature combustion reaction in the gas phase or on the surface of condensed phase; precipitation and growth of the solid product leading to rounded grains for the liquid-phase process, while plates, rods, and whiskers for the solid−liquid−gas processes; and possible reduction reaction(s) to form corresponding metal particles.

this end, it is clearly desirable to have the largest possible product yield. In addition, more rapid wave propagation is expected to inhibit the residence time of particles in the reaction region, which would lead to smaller particle size in the synthesized product. Similarly, higher temperatures may increase the size of the reaction zone, and thus, for a constant propagation velocity, low temperatures are desirable for promoting small particle size. Finally, the distance L between the two exothermic fronts is considered. Assuming that the leading reaction represents cellulose combustion, a large distance could promote deterioration of the carrier matrix, which would lessen the possibility of the synthesis sequential reaction pair following the cellulose burning. Figure 25A illustrates some of the design considerations for optimizing production. The final product yield is maximized by large values of q1/q2 which can be achieved by, for example, more complete drying of the impregnated sample. This is accompanied by an increased wave speed, which would also favor products with a smaller microstructure. However, the resulting increase in temperature may increase the average particle size in the product. Thus, a relatively small distance between the two exothermic reaction zones is desirable, and the peak in L shown in Figure 25B as well as larger q1/q2 values represent an undesirable parameter regime. In a similar way, the properties of the endothermic reaction were investigated. It was found that the wave speed is essentially insensitive to the absorbed heat, but results in a decrease of temperature and a significantly reduced front separation at the cost of a reduction in product yield. However, this reduction is not significant, and it appears that enhancing the heat absorption via the endothermic stage is an effective mechanism for enhancement of the small product particle size. Another model109 paid attention to other features of solution combustion, which are related to the gaseous products and porosity (specific surface area) of the formatted solid product and how they influence the combustion parameters of the system. For example, it was shown that the gas products evolved in the combustion front reduce the combustion temperature and slow the front velocity. The pores generated by these gases increase the total surface area and, in turn, further increase the heat loss to the environment. High thermal diffusivity of the product decreases the combustion velocity and increases the width of the combustion wave. In general, on the basis of simulations, it was concluded that optimization of heat losses due to different factors makes it 14510


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to faster cooling of solid product and forced separation of particles, thus preventing sintering. We hope that this discussion will inspire more fundamental studies on self-sustained reactions in solutions. It is clear that a fundamental understanding of the SCS reactions is critical for controlling the structure and properties of the synthesized materials. As demonstrated above, a variety of state-of-the-art techniques can be employed to obtain the necessary data to establish chemical mechanisms of SCS reactions.

The stages outlined above can overlap in time, and sequence of the stages can alter for different systems. In general, a lower combustion temperature (e.g., smoldering combustion mode) leads to finer particles for all structure formation mechanisms. One may control this temperature by changing the composition of the initial reactive solution (fuel/oxidizer ratio, inert dilution, adding gasifying oxidizer such as ammonium nitrate, etc.) or surrounding atmosphere (inert or reactive gas environment, gas pressure). Specific morphology (see Figure 26) of the product,

4. MICROSTRUCTURAL CHARACTERISTICS OF COMBUSTION-DERIVED NANOMATERIALS In this section, we consider important micro-/nanostructural aspects of SCS products, focusing on the influences of synthesis parameters such as composition of initial solutions and gels, ignition method, temperature−time regime, among others. The diversity of SCS products microstructures can be divided into five types, as illustrated in Figure 26. Types A, B, and C are formed by isotropic nanograins (spherical, polyhedral, irregularshaped, etc.) associated in different nano-, micro-, and macrostructures. Thin plates, flakes, and sheets represent type D. These elements can be characterized as two-dimensional (2D) nanostructures. Anisotropic nanostructure elements with two dimensions much smaller than the third make type E, which includes rods, whiskers, and fibers. Let us consider these typical nanostructures based on the simplest and most widespread compounds. 4.1. Simple Oxides

The synthesis routes and microstructures of simple oxides have been reviewed in several prior works.3,16,19 However, some aspects remained in the focus of recent research. Aluminum oxide can easily be produced by dissolving aluminum nitrate and urea in water, and placing the solution in a muffle furnace maintained at 500 °C3 until self-ignition. The flame temperature reaches ∼1350 °C; the synthesized product is a white powder composed of nearly hexagonal platelet particles of αAl2O3 with sizes ranging from 200−800 nm.3 Laishram et al. recently proposed an alternative method of combustion initiation by microwave heating for alumina synthesis.114 Combustion of the aluminum nitrate−urea solution was initiated in a microwave oven operating at 900 W, 2.45 GHz, resulting in agglomerates of ∼100 nm in size. The same solution was initiated in a muffle furnace, preheated to 700 °C for 5 min, and yielded particles ranging from 50−90 nm with polyhedral morphology. The microwave combusted nanoparticles were identified as α-Al2O3 with particle size ranging from 18 to 20 nm with nearspherical uniform morphology (type A nanostructure). These particles form small agglomerates. The authors concluded that uniform heating of the reaction mixture took place in the microwave, which resulted in finer, more uniform, and smaller agglomerates as compared to inhomogeneous heating in a furnace.114 However, the comparative data on the combustion temperatures in microwave and furnace routes were not presented. Zhuravlev and coauthors used thermocouples to measure the combustion temperature in near stoichiometric (φ = 0.8−1.2) aluminum nitrate−urea solutions initiated by a hot plate.115 The maximum temperature in the combustion zone was ∼750 °C, which decreased to 600 °C when φ = 0.6. These values are much lower than ∼1350 °C, reported in earlier work.3 The foamy product was α-Al2O3 with specific surface area 52−54

Figure 26. Classes of microstructures in SCS-derived materials: ZnO127 (type A), NiO−Ni119 (type B), CeO2135 (type C), CuO136 (type D), and W18O49138 (type E). Reprinted with permission from above references. Copyright 2012−2014 Elsevier and Copyright 2015 the Royal Society of Chemistry.

however, depends on the mechanism of structure formation, which is defined primarily by characteristic temperatures of the precursors (melting, dissociation, sublimation, etc.). The amount of gas phase is also a controlling parameter, which influences size and phase composition of the product. Typically, the release of a large amount of gases yields finer particles, due 14511


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Figure 27. Crystallite sizes and specific surface area of Co3O4 prepared by SCS of cobalt nitrate hexahydrate with glycine (A) and urea (B) as functions of the combustion temperature. Plotted on the basis of of experimental data presented in ref 70. Copyright 2010 Elsevier.

m2/g, which corresponds to an average particle size of 28−30 nm. Thus, even for the same composition, the product structure and combustion features depend strongly on the method of reaction initiation: muffle furnace, microwave oven, or hot plate. The use of glycine instead of urea in the solutions with aluminum nitrate results in a dramatic decrease of flame intensity; the combustion temperature drops to 570 °C.116 The reaction product is amorphous foamy agglomerates 50−250 nm in size (depending on fuel to oxidizer ratio); crystalline alumina forms only after a calcination step. Bai and co-workers reported on the synthesis of fine agglomerated grains of MgO (10 nm or less) possessing the type A structure, utilizing Mg(NO3)2·6H2O as an oxidizer and soluble starch ((C6H10O5)n) as the fuel, by microwave (800 W) ignition.117 Jegatha et al. produced pure NiO with type A nanostructure using nitrate−glycine and nitrate−citric acid solutions, ignited under similar conditions (furnace preheated to 300 °C).118 The products exhibited different nanoparticle morphology. The product of the nitrate−glycine solutions consists of rounded and irregularly shaped particles ∼100−200 nm in diameter, contained in agglomerates ranging from 500− 1000 nm. The nitrate−citric acid combustion product is composed of octahedral particles with face length ∼200 nm, with sizes ranging from 100 to 250 nm. The morphological difference was explained as a result of the more effective complexing ability of citric acid and higher heat generation in the nitrate−citric acid system, which promotes more uniform mixing and growth of crystalline grains with octahedral shape.118 Experimental data on combustion temperature, however, were not presented. Wen et al. used nickel nitrate and nickel acetate in combination with different fuels, such as hydrazine hydrate or glycine, to prepare NiO/Ni composites.119 It was shown that the amount of fuel influences the morphology of the porous network, while nickel acetate was found to be a key factor to achieve high porosity (Figure 26, type B). It was found also96 that addition of NaF to the reactive solution of nickel nitrate with citric acid sharply increased gas evolution rate during SCS. This resulted in a quick expansion of the product, so that part of powder was lifted off and naturally fell around the container, leaving fluffy product with large volume. A new name “eruption combustion synthesis” was suggested for this combustion mode.96 The NiO/Ni nanocomposite powders yielded by eruption combustion synthesis consist of rounded particles ∼20−30 nm in diameter (several times less than product

obtained without NaF addition), and exhibit excellent dyeabsorption ability as well as lithium storage capacity. A comparative microstructure study was reported by Mangalaraja et al. for the solution combustion-synthesized Y2O3 and Yb-dopped Y2O3 nanopowders.120 On the basis of SEM data, it was shown that the type of fuel exerts a strong influence on the morphology of the combustion products: platelet particles were found for the yttrium nitrate−urea mixture, sponge-like powders when citric acid was the fuel, and flaky thin platelets for glycine. A similar conclusion was reached for the microwave-assisted Y2O3 synthesis: brittle foam was obtained using urea; foamy, porous, and spongy product for citric acid; and soft porous foam for glycine.121 Toniolo et al. reported valuable results concerning the influence of experimental conditions on the product structure for cobalt oxide nanopowders.70 The cobalt nitrate−glycine and cobalt nitrate−urea water solutions were placed inside a stainless steel container, which was heated by a Bunsen burner. The solution turned into a transparent viscous gel, which autoignited in the temperature range 130−180 °C. Fuel to oxidizer ratio was varied over a wide range (φ = 0.14−2.5). The results suggest that the combustion temperature increases monotonically with increasing φ from 319 to 575 °C for glycine, and from 238 to 563 °C for urea as fuel, which is much lower than the calculated adiabatic temperatures even accounting for inaccuracies in the thermocouple measurements. The surface area and mean crystalline size as functions of the combustion temperature are shown in Figure 27. Decreasing specific surface area and increasing crystalline size with rising temperature are predictable general trends. On the other hand, the increase of surface area in the temperature range 300−575 °C for the urea-based mixture shows that temperature is not the only parameter that controls the nanostructure. The amount of gases generated during the reaction increases with increasing φ by several times, both for the glycine and for the urea-based solutions, which may result in the formation of a more porous, foamy product with higher free surface. Several methods of reaction initiation were applied for the SCS of Co3O4, which resulted in similar microstructures of the combustion products. Groven and co-workers initiated the reaction of the nitrate−glycine stoichiometric (φ = 1) solutions in a glass beaker122 or stainless steel vessel123 on a hot plate preheated at 300−400 °C to produce the foamy oxide with specific surface area 24−32 m2/g. Sahoo et al. placed stoichiometric reactive solutions of cobalt nitrate and urea in 14512


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a furnace preheated to different temperatures, from 300 to 800 °C.124 It was concluded that the size of Co3O4 nanoparticles increases from 5−18 nm (furnace temperature 300 °C) to 200−400 nm (furnace temperature 800 °C), with a corresponding decrease of the specific surface area from 39 to ∼2 m2/g. However, because the products were subjected to postsynthesis heat treatment inside the furnace for 30−60 min, this effect could be caused by the long-term annealing at different temperatures, rather than combustion conditions. Ai and Jiang also proposed microwave-assisted ignition of the nitrate-urea solutions to produce highly porous Co3O4 foams with nanocrystallite size ∼65 nm (estimated by XRD, Scherrer’s equation).125 Synthesis of porous Co3O4 nanostructures was also reported by transferring a cobalt nitrate−glycine solution to a preheated furnace maintained at 400 °C.126 After low temperature smoldering combustion, the resulting black amorphous precursor was heated in air at 355 °C for 1 h at a heating rate 5 °C/min to achieve the porous Co3O4 structure. The grain size was determined to be ∼12 nm by the Scherrer formula. TEM imaging suggested that product particle size varies between several nanometers to tens of nanometers, consistent with the values calculated by using the Scherrer formula. Microwave ignition of the zinc nitrate−urea water solution produced foamy porous nanopowder with type A microstructure127 (Figure 26). It was shown that the spherical ZnO nanoparticles consisted of small nanocrystals and nanopores, with average particle size ∼10 nm. The crystallite size estimated from XRD using Scherrer’s equation was ∼43 nm. Several manganese oxides,128 as well as MnO−carbon matrix composites,129 were obtained by SCS using manganese nitrate−glycine mixtures, initiated in heated furnaces. The crystallite size (XRD) was in the range 19−23 nm, and the specific surface area was 43−47 m2/g for the Mn2O3, Mn3O4, or MnO2 phases.128 Recently, green SCS of various oxide nanopowders has been reported using extracts of fresh plant leafs, such as Tinospola cordifolia,130 Calotropis gigantea,131 Artocarpus gomezianus,132 Cassia fistula,133 and Epigallocatechingallate,134 as fuels. In all cases, the leaf extracts were mixed with the appropriate metal nitrates and placed in a furnace preheated to 400 °C until selfignition. For example, spherical CuO nanoparticles were prepared130,131 with crystallite size ∼7 nm.130 However, the particle size determined by TEM was much larger at ∼100 nm.130 This approach made it possible to also produce agglomerated ZnO nanoparticles ∼10−30 nm, in the type A or type B microstructures.132−134 The type C microstructures (Figure 26), hollow spheres, are usually obtained in the processes of spray pyrolysis. For example, Shih et al. atomized solutions of cerium ammonium nitrate (NH4)2Ce(NO3)6 and glycine into small droplets, and transferred them by air flow into a heated tubular reactor with three heating zones of 250, 650, and 350 °C.135 Two-dimensional microstructures are usually in the form of thin nanoplates, sheets, or flakes. For example, Umadevi et al. reported SCS of flower-like CuO nanomaterial using a copper nitrate−glycine solution ignited in a furnace at 300 °C (Figure 28 and Figure 26, type D).136 Figure 28 indicated that the size of a single “flower” is ∼10 μm, and consisted of thousands of dense CuO nanosheets connected together on the basis, rooted in one center. The typical length of one nanosheet is in the range of 300−650 nm with thickness of 50 nm (Figure 26, type

Figure 28. SEM image of the CuO nanoflowers nanopetals. Reprinted with permission from ref 136. Copyright 2013 Elsevier.

D), while the widths of the bases and tips are in the range of 360 and 120 nm, respectively. A similar experimental method was applied to the copper nitrate−citric acid solution, and yielded CuO with nanorod morphology (type E).137 The diameter of the CuO nanorods was 20−25 nm, with length ranging from 100 to 250 nm. Recently, Chen and co-workers synthesized pure and doped W18O49 nanorods by combustion of ammonium paratungstate, (NH4)6W7O24·6H2O, ammonium nitrate, and glucose solutions.138 Figure 26 (type E) illustrated that pure W18O49 nanorods have average diameter of ∼50 nm with lengths longer than 10 μm. There is no clear explanation why, in some cases, the SCS reaction produces 2D structures, while in other cases small spherical particles or nanorods are produced. The role of fuel is evident. For example, as noted above, spherical nanoparticles of ZnO result from the nitrate−urea mixture.127 The SCS of the nitrate−glycine mixture, however, leads to the formation of 2D ZnO, possessing flake-like morphology with smooth surfaces, and sizes ranging from tens to hundreds of micrometers.139 Other work shows formation of particulate and nanorod structures in the same system, as for SCS of VO2 using combustion of ammonium metavanadate and maleic acid solutions, initiated in a muffle furnace at 470 °C.140 The approximate length of the nanorods was 600 nm, with width 60 nm. In some cases, the 2D structures appear at micrometer scale, while closer examination reveals that they are composed of uniform 3D nanoparticles. For example, Dong et al. reported on the formation of NiO sheets with thickness of several micrometers on the alumina substrate, due to nickel nitrate− ethylene glycol combustion.141 However, close SEM and TEM observations revealed that the sheets are assembled of irregularly shaped nanoparticles with smooth surfaces, from several to tens nanometers in diameter. Recently, Wen and Wu attempted to address the microstructure controllability of SCS products.61 A general and quite obvious conclusion was made that phase composition, morphology, particle size, and surface area of the SCS products can be altered to a certain extent by adjusting the fuel, fuel to oxidizer ratio, and pH of the solution. It is also noted61 that higher combustion temperatures during SCS usually result in improved crystallinity, larger grain size, more agglomeration, and lower specific surface area of the products. Certainly, chemical composition of fuel and oxidizer, fuel to oxidizer ratio, gas atmosphere, solution pH, heating mode, combustion temperature, and other experimental factors should affect the 14513


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structure formation in SCS, below we consider briefly other main classes of the SCS products.

SCS product microstructure. However, the mechanisms of such influence and quantitative correlations between properties of the initial solutions, process parameters, and microstructure characteristics of the products are not known. Clearly, further works are needed to address the exact formation mechanisms responsible for the various morphologies. Such understanding will lead to better control of product properties. On the basis of the considered examples of simple oxides, we can identify several conclusions on the structure of SCS products: The vast majority of the simple oxides obtained by SCS have microstructure of the type A or B (Figure 26). Rounded or irregular-shaped isotropic grains, observed in most products, apparently precipitated in the gel-like liquid matrix, as volatile components of the matrix released due to evaporation and decomposition processes, and concentrations of the nonvolatile residuals increase. The size of the individual grains usually lies in the nanometer range, around 5−50 nm. Particles larger than 100 nm, as a rule, are agglomerates composed of smaller grains. In some cases, larger particles appear due to calcination, if the product remains inside the furnace for long time. Here, we consider only the primary microstructure that forms directly during SCS, without any postprocess treatment. Two major physical factors that determine the product microstructure are reaction temperature and amount of gas product. Temperature accelerates reactions, grain growth, and sintering. The higher is the temperature, the larger are the solid grains and better sintered are the agglomerates of these grains. Gas products make solid materials fluffy and voluminous. The larger amount of gas released during combustion, the higher is the porosity of the product. Expanding gases form spongy or foamy microstructure. When the hot gas escapes the sample, it removes a large amount of heat (enthalpy), and solid product cools faster, with smaller grains. Therefore, the temperature and gas release may act as competitive factors in the microstructure formation. At the same time, if the temperature of the gas flame is higher than the temperature of the condensed phase, the heat of the gaseous exothermic reactions may promote reactions and structure formations in the condensed phase. The temperature and amount of gas released depend on many experimental conditions. Among them are composition of the fuel and oxidizer, fuel to oxidizer ratio, methods of heating, and ignition of the initial solutions, etc. We believe that understanding the influence of experimental conditions on the product microstructure will become easier if one can answer the questions of how these conditions affect two major physical factors, temperature and gas release in the reaction zone. The structure of hollow spheres, type C, is formed similarly to the A and B structures in cases when the initial solution is dispersed into droplets. The gas release distends the droplet converting it into bubbles; solid grains precipitate in the bubble envelope (wall) and form a solid hollow shell. Only a few examples of 2D and 1D microstructures (types D and E) were found for simple oxides. In some cases, the formation of anisotropic grains (crystallites) can be explained by strong anisotropy of the product crystal structure; however, in other cases, rounded and platelet grains form for the same phase (ZnO). We may speculate that anisotropic grains grow on the interface between liquid and gas. However, this hypothesis needs further investigation. The considered simple oxides represent only a small part of all compounds produced by SCS during recent years. For a better understanding of process parameters and mechanisms of

4.2. Ferrites

Ferrites are complex oxides formed by Fe2O3 with the oxides of other metals.142,143 They attract great interest due to their unique magnetic properties, and can be easily synthesized through SCS using solutions of different metal nitrates and fuels. Recently, a large number of publications related to the SCS of ferrites was published. However, most of the works involve calcination or sintering of the reaction products, and no data were provided about the microstructure of the primary product before calcination. In this section, we consider the primary product microstructures and, in some cases, compare the SCS-derived product with postsynthesis heat treatments. By use of metal nitrates and citric acid, Wu et al. prepared substituted cobalt ferrites with the formula CoFe2−xRxO4, where R is Ce144 or Y.145 Highly porous products were obtained, with sponge-like type B microstructure145 and complex microstructure combining types A and B.144 The presented SEM images144 do not allow us to distinguish finer microstructural details. Zhang et al. used solutions of yttrium and iron nitrates, citrate acid, ethylene glycol, ammonium nitrate, and ammonia to synthesize yttrium orthoferrite (YFeO3) nanocrystals.146 The as-burnt product also has sponge-like structure composed of rounded nanograins (Figure 29A); the size of the grains

Figure 29. SEM images of as-burnt powder (A) and its YFeO3 nanocrystals obtained after slight crushing and annealing at 500 °C for 10 min (B). Reprinted with permission from ref 146. Copyright 2013 Elsevier.

increases significantly after a short annealing period (Figure 29B). The crystallite size of the initial product calculated from XRD data was found to be 88 nm, which is in good agreement with the SEM images. Cubic spinel zinc-substituted ferrites with composition of Cs0.5−x/2ZnxMn0.05Fe2.45−x/2O (x = 0−0.5) were also prepared using metal nitrates and an ethylene glycol solution.147 Ignition was performed in the furnace at 600 °C, and the products were kept in the furnace for 30 min. Despite the relatively long annealing, the size of the rounded ferrite particle remained at 15−20 nm according to XRD estimation using the Scherrer formula, confirmed by direct TEM analysis. Finally, SCS of Pddoped YFeO3 and LaFeO3 water-ethanol based solutions of yttrium and lanthanum nitrates, iron and palladium acetilacetonates, and citric acid heated by microwaves, should be noted.148 The optimized reaction time was 150 s for La1−xPdxFeO3−δ and from 120 s to several minutes for Y1−xPdxFeO3−δ, depending on the microwave power. The products possessed type A microstructure and were composed of near spherical particles about 100 nm in diameter. The crystallite size determined by the Scherrer formula was in the range 30−75 nm. The specific surface area was near 25 m2/g 14514


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for LaFeO3 materials and ∼5 m2/g for YFeO3 materials. It should be noted that ortoferrites MFeO3 (M = metals) have a structure of perovskite, and ferrites with formula MFe2O4 have spinel structure; these structures are discussed below.

present only microstructures of products after postsynthesis heat treatment or sometimes sintering. Nagabhushana and co-workers studied the effects of the fuel to oxidizer ratio and postcombustion treatment on the microstructure of LaMnO3+δ nanopowders.153 They ignited aqueous solutions of lanthanum and manganese nitrates, and oxalyaldihidrazide on a hot plate. It was found that increasing the fuel/oxidizer ratio from 0.5 to 1.5 results in an increase of the particle and crystallite size up to 1−2 μm and 44 nm, respectively. Meanwhile, the surface area of the products decreased from 33 to 10 m2/g. An increase of the fuel to oxidizer ratio also causes a decrease in the porous product density from 0.11 to 0.03 g/cm3. The products possess type A microstructure for small φ and type B for larger φ values. It was mentioned that increasing the fuel content leads to increased gas generated during the reaction, 10−30 mol per mole of solid products. The authors also outlined that the heat of reaction increases without providing measured combustion temperatures. High-temperature (900−1200 °C) calcination of the product formed at φ = 1 resulted in a significant drop of the surface area from 24 to 6 m2/g. At the same time, the size of the rounded particles does not change dramatically (Figure 31).153 We may assume that the near-spherical particles shown in Figure 31A are porous agglomerates of smaller nanoparticles; therefore, the calcination process results in sintering and coalescence of clusters inside the micrometer-scale particle, making less effect on sintering between the particles. Tarragó et al. produced strontium-doped lanthanum manganites from water solutions of three nitrates, La(NO3)3· 6H2O, Sr(NO3)2, and Mn(NO3)2·4H2O, with urea and/or sucrose.154 SCS products had crystalline size of 18−34 nm. After calcination at 750 °C for 3 h, the product microstructure is spongy, similar to Figure 31C, with much smaller grains of ∼100 nm. A similar microstructure was found in the LaAlO3 produced from the aluminum and lanthanum nitrates, mixed with citric and oxalic acids, then calcined at 750 °C for 2 h.155 The crystallite size determined from XRD peak broadening was 24−35 nm, and the grain size observed from TEM images was ∼30−60 nm.

4.3. Perovskites

Perovskites are complex oxides with a specific crystal structure (Figure 30), having the classical formula ABC3, where A and B

Figure 30. Perovskite structure.

are metals and C is oxygen, named after a mineral perovskite CaTiO3.149 An ideal perovskite structure cell has cubic symmetry; however, many perovskite-type phases are known to possess slightly noncubic (orthorhombic or monoclinic) structure. Many metals can form complex perovskites, for example, cation A = Ca, Ba, Ce, Sr, La, Nd, Pr, Na, K, Th, U; cation B = Ti, Nb, Ta, Zr, Hf, Sn, W, Fe, Ni, etc. Moreover, two or more metals can take the positions of the A and B cations in the same phase, for example, (La1−xSrx) (Fe1−xNix)O3 with x = 0.0, 0.1, and 0.2.150 Solution combustion synthesis can be effectively used for the preparation of multielement perovskites, because this method allows mixing of different metal cations in the precursor solutions and gels, and, consequently, in the crystal structure of the products. Preparation of LaAlO3 by a long-term solid-state reaction between Al2O3 and La2O3 should be carried out at 1800−2000 K, while SCS of pure lanthanum aluminate involves heating the gels containing aluminum and lanthanum nitrate, and citric acid at 200 °C followed by a short annealing stage at 750 °C. 151 Multication nanopowder oxides, such as La0.75Sr0.25Cr0.5Mn0.5O3−δ, possessing uniform perovskite type crystal structure were also prepared in this manner.152 Despite numerous publications on perovskites, SCS data concerning the product microstructure that forms directly during combustion are limited. Most of the published works

4.4. Spinels

Spinels have a general formula of AB2O4 and a quite complicated crystal structure (Figure 32). The bivalent cation A is located in the middle of tetrahedrons surrounded by four oxygen ions. The trivalent B cations are inside octahedrons formed by six oxygen ions. Each oxygen ion is bonded with one bivalent and three trivalent cations, and all oxygen ions are close-packed in the planes parallel to the octahedrons facets. One may expect that ordering of such structure takes a long time (natural spinel crystals, MgAl2O4, grow during geologicalscale periods); however, many compounds with spinel structure were obtained by SCS. For example, nanoscale powders of

Figure 31. SEM micrographs of LaMnO3 powder: (A) as-formed and calcined at (B) 900 °C, 6 h and (C) at 1200 °C, 3 h. Reprinted with permission from ref 153. Copyright 2010 Taylor & Francis Group. 14515


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Figure 33. SEM images of ZnAl2O4 sample prepared by microwave (A) and by furnace ignition (B). Reprinted with permission from ref 160. Copyright 2014 Elsevier.

Figure 32. Spinel structure of MgAl2O4.

whether the morphology of the products can be changed for the same compound by simply varying the conditions of the SCS. These questions are topics for future research. Ianoş and coauthors prepared complex, four-component spinel solid solutions of Mg1−xNixAl2O4 in a heating mantle (300 °C) from solutions of nitrates using different organic fuels and subsequent heat treatment at 1000 °C for 3 h.162 The samples had surface areas ranging from 5.8−7.0 m2/g and crystallite size 35−39 nm estimated by XRD. Formation of micrometer-scale spinel titanate flakes was also reported.163 However, these flakes represent fragments of the walls of foamy microstructure (type B) and are composed of uniform nanoparticles.

nickel aluminate (NiAl2O4) were produced from solution combustion using nickel and aluminum nitrates as oxidizers and extracts of Aloe vera156 or Sesame indicum157 leaves as fuels. Two types of ignition were used and compared in these works. The first case involved conventional ignition in the muffle furnace heated to 600 °C with subsequent heat treatment in the same temperature for 3 h, while the second method involves placing the reactive solutions in the microwave oven at 950 W for 15 min. The authors noted that, in both cases, the cubic phase of NiAl2O4 was obtained. The products were composed of rounded nanoparticles ranging in size from 10−50 nm, agglomerated with types A and B microstructures.156,157 The crystallite size was found to be 19.9 nm for furnace ignition and 16.4 nm for microwave ignition. It is important to note that both types of ignition yielded very similar microstructures and identical crystal structures. A similar approach was used for the synthesis of CoAl2O4,158 CuAl2O4,159 and ZnAl2O4.160 The cobalt aluminide obtained using the Aloe vera extract consists of fine agglomerated particles, tens of nanometer in diameter observed by SEM and TEM. Microwave ignition provides smaller nanoparticles and a more uniform size distribution than the furnace method.158 XRD analysis showed that the crystallite size is 20 and 18 nm for furnace and microwave ignition, respectively. Synthesis of CoAl2O4 using glycine as fuel, hot plate ignition, and annealing at 1000 °C for 1 h yielded a crystallite size of 61−74 nm, with a specific surface area of 11−16 m2/g.161 The size of the nanoparticles ranged from 50−70 nm determined by TEM imaging. It was noted that the crystal size increases with the fuel to oxidizer ratio due to the increased combustion temperature. A significant difference in product morphology was obtained by furnace or microwave heating for the CuAl2O4 spinels: agglomerated “nanorices” with grain size of ∼50−60 nm for the former, and rounded particles, ∼100−300 nm, for latter.159 The crystallite sizes determined by XRD analysis were not very different: 28.6 and 22.8 nm, respectively. Ragupathi et al. reported interesting microstructures of type E and D for zinc aluminate with spinel structure (Figure 33).160 Well-faceted nanoplates (type D) were formed after microwave heating for a few minutes (Figure 33A), while some rounder rods (type E) were obtained during reaction in the furnace followed by annealing at 400 °C during 3 h (Figure 33B). Evidently, the temperature and duration of heating are the main factors influencing the microstructure formation. However, a specific mechanism of the microstructure formation is not known. The puzzling question remains regarding the formation of particles, plates, or rods for the compounds exhibiting similar spinel crystal structure. The dominant factors that govern formation of such crystals are unknown. It is also unknown

4.5. Garnets

Garnets represent three- or four-component complex oxides with formula A3B2(DO4)3, where A = Mg, Fe, Mn, Ca, Y, La, etc., B = Al, Fe, Cr, etc., D = Si, Al, Fe, etc., and require even more complicated ordering of the crystal structure, as compared to the ferrites, perovskites, and spinels. Natural garnets (minerals) are silicates, but some ferrites and aluminates can also possess garnet crystal structure. Despite the complicated crystal ordering, SCS of oxides with garnet structures was reported 25 years ago.3,164 The products of combustion are amorphous and transform into a single phase with cubic garnet structure during a postsynthesis calcination step. A precursor of cerium-doped yttrium aluminum garnet was obtained by microwave-induced combustion of a mixture of aluminum, yttrium, and cerium nitrates in glycerol and fructose.165 This bulky loose brown product transformed into crystalline garnet at temperatures above 900 °C. However, the well-faceted crystals of garnet appeared upon heating of the powders only at 1100 °C. Guo et al. pointed out that the garnet structure attracts special attention due to its possibility to encapsulate actinide elements and thus solve a problem of nuclear waste storage.166 They reported citrate−nitrate combustion for production of cerium-substituted yttrium iron garnet, where Ce atoms were considered as analogue of Th, Pu, or U. The self-propagating combustion of gels yielded brownish-black aggregates of loose powders that were converted into cubic yttrium iron garnet after calcination in air at 1300 °C for 24 h. Thus, the formation of garnet complex crystal structure requires a two-stage process, involving SCS and calcination. 4.6. Other Multicomponent Single-Phase Oxides

This class involves many types of crystal structures and chemical compositions. An example of such materials can be ceria-based complex oxides. Strontium-doped ceria solid solutions Ce1−xSrxO2−δ (x = 0−0.15), produced via glycinenitrate SCS, had type B microstructure (highly porous spongy 14516


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CuO-promoted CeO2−MxOy mixed oxides (MxOy = ZrO2, La2O3, Pr2O3, and Sm2O3), and the specific surface areas of materials were varied in the range 36−56 m2/g.173

ash) consisting of nearly spherical clusters.167 It was shown that postsynthesis calcination at different temperatures leads to significant microstructural transformations. For example, calcination at 600 °C destroys the skeleton and makes agglomerates of primary particles with less open structure, while calcination at 850 °C promotes the growth of primary particles and closes small pores. Calcination at 1550 °C for 4 h yields almost completely sintered material. It is asserted that fine doped ceria powder obtained by SCS can be easily sintered at temperature lower than that required for sintering of commercially available powders.167 A similar method was used for the synthesis of multicomponent oxide nanoscale powders based on Ce2O3 structure containing up to 5 substituting metal ions (for example, CeCo0.30Ni0.45Cu0.15V0.05Fe0.05O3).168 Very fine microstructures of type A were obtained in the solid solutions Ce0.65Zr0.25RE0.1O2−δ (RE = Tb, Gd, Eu, Sm, Nd, Pr, and La) produced by glycine−nitrate SCS.169 Even after calcinations at 600 °C for 2 h, the spherical nanoparticles had diameter of ∼10−15 nm (determined by TEM), crystallite size of 7.7−14.4 nm, and specific surface area of 40−65 m2/g. Kim et al. compared conventional solid-state reaction, hydrothermal, coprecipitation, and SCS for producing Ce0.8Ln0.2O2−δ (Ln = La, Nd, Sm, Eu, Gd, Y, Ho, Tm, and Yb) oxides.170 They noted that SCS and hydrothermal synthesis provides materials with similar product sizes, while materials obtained by coprecipitation show finer particles. Solid-state reactions yielded much larger particles. This trend was observed regardless of the dopant type and content. The SCS powders were porous fractals (type B) consisting of tightly agglomerated nanocrystallites with particle size of 5−8 nm (SEM/TEM) and specific surface area of 30−36 m2/g.

4.8. Nanophosphors

Inorganic phosphors are a general category of various compounds that exhibit luminescence properties.174−176 The phosphors are often simple or complex oxides doped with rareearth elements. The oxide phosphors are promising materials for field emission and plasma displays, light-emitting devices, and imaging applications. Fine tuning the wavelength of emitting light requires very precise predesigned distribution of all elements, including the doping agent, in the material structure. It also requires control of crystallite size, morphology, texture, and lattice defects as well as nanosized crystallites.176,177 It has been shown that SCS allows fulfillment of all of these requirements for a wide variety of systems. Among the recently studied nanophosphors produced by SCS, we can mention Tb-doped ZrO2,178 Eu-doped ZrO2,176 EuVO4,179 Y2SiO5,180 BaMgAl10O17,181 Sr2.91V2O8,182 Ba3Y2(BO3)4,183 Sr3Y2(BO3)4,183 Mn-activated ZnGa2O4,184 and Eu- and Dydoped BaAlxOy.185 The SCS-derived nanophosphors usually have microstructure of type A or B (Figure 35) and crystalline size from several nanometers to several tens of nanometers.

4.7. Mixed Oxides

In some cases, two or more immiscible oxide phases appear in the SCS process, which leads to the formation of mixed oxides. For example, nanoscale CeO2−MxOy mixed oxides (MxOy = SiO2, TiO2, ZrO2, and Al2O3) were obtained from corresponding nitrate−urea solutions using microwave initiation.171,172 Ceria−silica and ceria−titania mixed oxides were in amorphous form, and ceria−alumina was in crystalline form. Ceria−silica had the higher surface area of 125 m2/g, and a lower value of 38 m2/g was found for ceria−alumina.171 All products had type A microstructure, where particles of different oxides are mixed, as shown in Figure 34.172 Similar microstructures were repored for

Figure 35. TEM image of Dy3+/Li+-doped ZnO nanophosphor produced by nitrate−urea SCS with furnace ignition. Reprinted with permission from ref 177. Copyright 2004 American Chemical Society.

Figure 34. TEM image of CeO2−ZrO2 mixed oxide prepared by microwave ignition. Reprinted with permission from ref 172. Copyright 2009 Springer.

Figure 36. SEM image of EuVO4 phosphor produced by nitrate−citric acid SCS (A) and after annealing at 1150 °C for 5 h in air (B). Reprinted with permission from ref 179. Copyright 2014 Elsevier.

The microstructure of the synthesized nanophosphors, such as EuVO4 red phosphor, undergoes significant changes during postcombustion annealing, as shown in Figure 36.179 Intense growth of the particles and smoothening of the surface due to the congregation of crystallites was observed in the temperature range 850−1150 °C.

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hydroxyapatite nanotubes (Figure 38) were synthesized through nitrate-ethilene glycol SCS on an alumina template.46

Nanoscale particles of phosphors can serve as oxide semiconductor quantum dots for future light-emitting devices. Because the energy levels and frequency of the emitting photons depend on the size of the quantum dot, nanostructure control becomes very important. Blue light-emitting properties of surface and deep levels of the SnO2 quantum dots, synthesized by SCS using tin chloride pentahydrate (SnCl4· 5H2O) and urea water solution initiated in a muffle furnace, were recently studied.186 It was shown that the crystalline size of the SnO2 quantum dots increased from 2.2 to 3.6 nm with increasing combustion temperature (temperature of the furnace) from 350 to 550 °C. According to high-resolution TEM analysis, nanoparticles of ∼3 nm agglomerated in the type A structures. Nanorods (type E) of MAl2O4:Eu,Dy phosphors, where M = Ca, Sr, Ba, were prepared by nitrate−urea SCS.187 The average diameter of the nanorods was ∼40 nm, whereas the length was ∼1 μm. A comparative study of Nd2O3 nanocrystalline phosphors showed that combustion-derived products exhibited type A microstructure, versus type E microstructure of the product obtained by hydrothermal synthsis.188

Figure 38. SEM (A) and TEM (B) images of hydroxyapatite nanotubes. Reprinted with permission from ref 46. Copyright 2008 Elsevier.

The nanotubes exhibited uniform outer diameters of ∼100 nm, equal to the channel diameter of the alumina template, and length up to 60 μm. Microwave-assisted SCS of chlorapatite (Ca5(PO4)3Cl)195 and fluorapatite (Ca10(PO4)6F2)196 also resulted in the formation of nanowhiskers and nanotubes. The fluorapatite nanotubes have hexahedral cross-section and a “Y” shape (conical) channel inside (Figure 39). It was noted that the sizes

4.9. Phosphates and Hydroxyphosphates

Calcium phosphates and calcium hydroxyapatite Ca10(PO4)6(OH) attract attention due to their excellent biocompatibility, osteoconductive properties, and similarity to the inorganic component of natural bone.189,190 Microstructure control of the synthesized phosphates is a critical condition for most biomedical applications. Comparison of SCS with other methods led to the conclusion189 that the low cost is a main advantage, while the wide size distribution (from nano- to submicrometer irregular particles) and aggregation are disadvantages of the combustion method. However, it was stated that weekly agglomerated nanoscale hydroxyapatite powder can be prepared simply by optimized composition of reactive solutions and the combustion temperature.191,192 The addition of some metal ions, such as Sr2+, helps to modify the crystallinity, morphology, and lattice parameter and obtain nanorods of hydroxyapatite from 15 to 70 mn in length and 5 ± 1 nm diameter.193,194 An increase of the Sr content leads to an increase of the flame temperature and a consequent increase in the particle size (Figure 37). When the Sr content increased from 0 to 30%, crystalline sizes (XRD) increased from 16 to 48 nm, and the observed length of the nanorods increased from 15−20 to 40−75 nm.194 Earlier, much larger nanowhiskers of hydroxyapatite ∼1000 nm long and ∼100 nm thick were synthesized by microwave-assisted SCS.49 The high aspect ratio

Figure 39. SEM (A) and TEM (B) images of fluorapatite nanotubes. The arroy points to the “Y” shaped channel. Reprinted with permission from ref 196. Copyright 2015 Elsevier.

of the columnar crystals and a “Y” channels depend on microwave irradiation time and temperature.196 Under longer microwave irradiation, the size of the fluorapatite crystals is expected to increase via reaction with a molten phase. Hexahedral whiskers of strontium phosphate composed of Sr2P2O7 and Sr5(PO4)3Cl were obtained by microwave-assisted SCS.197 It was shown that the coexistence of Ca2+ and Sr2+ ions in the reaction can inhibit one-dimensional crystal growth. As opposed to calcium and strontium phosphate nanorods, lithium−iron phosphate forms uniform particles. The nitrate− glycine−malonic acid SCS followed by annealing at 750 °C in the furnace under an argon or hydrogen atmosphere led to the formation of a LiFePO4/C composite with type A microstructure and particle sizes of 50−100 nm198 or ∼20 nm.199 It should be noted that the same production route was used in both articles. However, the difference in particle size was not explained. 4.10. Metals and Metal−Ceramic Nanocomposites

Nanoscale rounded alumina particles distributed in copper matrix were obtained in one-step SCS from copper and aluminum nitrates with urea as the fuel.200 Roy et al. prepared Ni−Al2O3 catalysts by SCS and the sol−gel method. They showed that fine Ni particles, with a narrow size distribution of

Figure 37. TEM image of hydroxyapatite particles with 0% Sr (A) and 20% Sr (B) produced from solution of calcium acetate (Ca(C2H3O2)2), strontium chloride (SrCl2), diammonium hydrogen phosphate ((NH4)2HPO4), and urea in muffle furnace ignition. Reprinted with permission from ref 194. Copyright 2015 Elsevier. 14518


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solution of ferrocene (C10H10Fe) and cobalt acetylacetonate (C10H14CoO4) in a mixing solvent composed of xylene (C8H10) and tetrahydrofuran (C4H8O) was sprayed into H2/ air flame (Figure 41).204 This method is closely related to SCS; however, the difference is that the initial solution contains only fuel(s), without oxidants, and therefore combustion becomes possible only in the presence of external flame or oxidizing gas. A water-based aerosol composed of TiCl4 and urea was passed through a tubular furnace, preheated to 600−1000 °C, to produce nitrogen-doped TiO2 microspheres (Figure 42).205

5−30 nm (Figure 40A), were obtained by SCS, as compared to 10−80 nm particles produced by the sol−gel approach.201

Figure 40. TEM images of Ni−Al2O3 (A) and iron (B) nanostructures prepared by SCS: Bright spots in (A) are Ni nanoparticle. Reprinted with permission from refs 201 and 103. Copyright 2012 Elsevier and 2012 Springer.

Li and coauthors reported202 a new SCS method of Au/ZnO nanomaterials that includes dissolution of chlorauric acid (HAuCl4·2H2O) and zinc nitrate in ethanol, and consequent ignition of the solution in air in an open evaporating ceramic dish. The solid products exhibit granular morphology with an average diameter decreasing from 130 to 50 nm as the Au content increases from 0 to 8 at. %. Pure Ni nanoparticles and Ni−NiO composites with type B structure were synthesized by nitrate−glycine SCS.21,33 Highly porous iron (Figure 40B) with type B nanostructure was also synthesized by gel combustion method.103 These results indicate that an inert ceramic support (Al2O3, ZnO) allows for production of highly dispersed metal nanoparticles by SCS, while without support, the reactive system forms porous metal with type B microstructure.

Figure 42. SEM images of nitrogen-doped TiO2 microspheres produced from aqua TiCl4−urea solutions with 1 wt % urea at 600 °C (A) and 5 wt % urea 800 °C (B). Reprinted with permission from ref 205. Copyright 2014 Royal Society of Chemistry.

It was noted that an increase of urea content results in broken spheres and rougher cores, possibly due to a larger amount of gases produced by urea decomposition. Finally, some direct attempts to initiate SCS process in microdroplets have been reported recently.206 However, more experimental evidence is needed to prove the formation of microspheres by the SCS process in microdroplets. 4.12. Metal Sulfides

Combustion of metal nitrates and water-soluble fuels results in nanoscale oxides or metals. Preparation of other materials, such as metal sulfides (CdS, ZnS, NiS, NiS2, CoS2, Bi2S3, In2S3, Fe7S8, etc.), using two general schemes of SCS-based approaches have also been reported. In the first case, metal nitrate complexes with thiourea or thiosemicarbazide were prepared first, and then self-propagating decomposition of complexes was initiated under an inert atmosphere.207,208 The second synthesis formulation includes direct reaction of metal nitrates with sulfur-containing fuels.209−211 The metal sulfides find uses in photovoltaic cells and as photocatalysts.210,211 The combustion temperature, phase composition, particle size, and morphology of sulfides prepared by self-propagating decomposition were shown to be strongly dependent on inert gas pressure inside the synthesis reactor.208 For example, the combustion temperature of zinc nitrate−thiourea complex

4.11. Spheres

Formation of microspheres (type C microstructure) becomes possible when microdroplets serve as precursors.135 Different spraying methods, including gas jets, sprayers, and ultrasonic nebulizers, are used to obtain dispersions of microscopic droplets in gas (air, argon, etc.). The obtained sprays undergo high-temperature treatment when passing through a tubular furnace, microwave reactor, or gas flame. Most of the published works describe the processes of pyrolysis, drying, or calcination of the droplets, without the combustion reaction; therefore, those works remain outside our Review. Another approach is a flame spray pyrolysis (FSP) method,203 where the spray is injected into a gas flame. For example, core−shell spherical nanoparticles Co3Fe7−CoFe2O4 were synthesized when a

Figure 41. TEM image of spherical nanoparticles composed of metal core and oxide shell, produced by FSP method. Reprinted with permission from ref 204. Copyright 2012 American Chemical Society. 14519


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Figure 43. SEM images of zinc sulfide powders obtained by [Zn(thiosemicarbazide)2](NO3)2 combustion at inert gas pressure of 0.25 (A), 0.3 (B), and (C) 3 MPa. Reprinted with permission from ref 208. Copyright 2002 Springer.

changed from 480 to 1440 °C with an increase of inert gas pressure from 0.25 to 3.0 MPa. β-ZnS was the main product at lower temperature (480 °C), while α-ZnS formed at higher combustion temperatures (1290−1440 °C). The combustion temperature of zinc nitrate−thiosemicarbazide complex increased from 1300 to 1560 °C upon increase of gas pressure. The morphology of combustion products for zinc nitrate− thiosemicarbazide complex synthesized at different gas pressures is shown in Figure 43. α-ZnS crystals with ∼1 μm size formed at 0.25 MPa gas pressure. At 0.3 MPa, the crystals have a needle shape with up to 5 μm length and 2 μm diameter, while elevated pressure (3 MPa) resulted in the formation of rounded particles with sizes below 1 μm. The mechanism of microstructure formation was not discussed. The authors, however, conjectured that significantly high vapor pressure of product at elevated temperatures may be responsible for the formation of diverse morphologies. Amutha et al. reported microwave-assisted combustion synthesis of faceted CdS nanoparticles by using cadmium thiocyanate complex as a precursor.212 It was shown that KNO3 byproduct formed during the preparation of precursor complex, as well as microwave irradiation time, may significantly influence the synthesis conditions and yield CdS nanoparticles with mixtures of octahedral and hexagonal crystals. The authors suggested that during microwave combustion, the cadmium thiocyanate complex decomposes releasing Cd2+ and S2− anions, which react to generate the CdS nuclei. Because of the high surface energy, CdS nuclei can self-assemble followed by Ostwald ripening resulting in faceted CdS nanoparticles. Mani and co-workers recently reported direct solution combustion synthesis of nickel sulfide using nickel nitrate and thiourea as reactants.210,211 The aqueous reactive solutions were combusted in a preheated (300 °C) furnace in ambient atmosphere. The analysis of phase composition of products revealed that at a low fuel to oxidizer ratio (1), NiO is the main product of reaction. Upon gradual increase of the fuel to oxidizer ratio to 5, Ni3S2, NiS, and NiS2 phases may be formed. The average crystallite sizes calculated by using the Scherrer formula were found to be 8, 35, and 16 nm, respectively, for Ni3S2, NiS, and NiS2 phases. Summarizing this section of this Review, we can note the following: • Many works demonstrate results on the microstructure of SCS products, but there is no systematic study of the structure formation mechanisms during this process. • Among the wide variety of compounds obtained by SCS, oxides with type A or B microstructure and particle size in the range 10−100 nm form a large majority up to now. • The isotropic rounded particles precipitate typically from the liquid phase. When gas molecules (ammonia, nitrogen oxides, etc.) escape the solution, refractory oxide nuclei start to precipitate and grow. Depending on

the characteristics and microstructure of the liquid phase, different types of product microstructure may form. Bulk liquid layers and relatively large droplets generally yield microstructure of type A. If the parent liquid is foamed, because solid nanoparticles precipitate inside the thin films that form partitions of the foamy structure, microstructure of type B is expected. Finally, in case of dispersed small liquid droplets in a gas, type C microstructure appears. • Formation of type D microstructure composed of faceted plate-like 2D crystals depends on a variety of factors. Among these are anisotropy of crystal structure, wettability of the crystal by melt, influence of gas phase resulting in atomic layer-by layer growth in addition to the crystal growth from melt, and other factors influencing product morphology. It is worth noting that precipitation of nanoparticulate solid products inside the thin films (partitions of foamy gel) can result in the formation of various “sheets”, “belts”, and other pseudo2D structures.213,214 • Rod-like crystals and whiskers (type E microstructure, 1D) likely grow by a gas−liquid−solid mechanism, typical for such crystals. • Multicomponent oxides with complex crystal structure (spinels, garnets, etc.) can be produced by SCS due to intermixing of the metal anions at the atomic level in the precursor solutions. The variety of SCS product microstructures can be significantly expanded if we account for external influences, such as different templates,45−48 or postcombustion treatments such as calcination or leaching.213−215 In section 4, however, we consider solely as-formed products of the SCS process.

5. SOLUTION COMBUSTION-DERIVED MATERIALS 5.1. Advanced Materials for Energy Technologies

One of the major challenges of modern society is providing highly efficient, low cost, safe, and environmentally friendly electrochemical energy conversion and storage devices for applications, ranging from portable electronics to electric vehicles. The performance of these devices greatly depends on the properties of materials they are made from. Energy conversion and storage involve chemical reactions and physical interactions at the surface or interface of materials. As compared to microscale, nanostructured materials offer more favorable mass, heat, and charge transfer, as well as accommodate dimensional changes associated with chemical reactions and phase transitions. Nanostructured electrode materials for lithium ion batteries should have a set of properties that include large specific surface area for fast interfacial reaction, and small distances for mass and charge transport. Electrodes should also have a porous structure to provide sufficient space for guest ions and to allow 14520


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compounds such as lithium iron phosphates, lithium manganese oxides, and other complex oxides were implemented as cathode materials.224−226 On the other hand, to improve the electrochemical characteristics, simple (e.g., Co3O4, Fe3O4, NiO) and complex oxides (e.g., Li4Ti5O12) were proposed for use as anode materials.227,228 These oxides are capable of Li+ insertion/extraction, resulting in significantly larger reversible capacities than commercially used graphite. SCS offers great potential for the preparation of both cathode and anode materials with tailored structure and properties. Combustion synthesis enables one to tune the microstructure and material composition by changing the fuel to oxidizer ratio. These advantages allow for the production of a variety of layered complex Li-, Co-, Mg-based oxides,229−233 lithium vanadium oxides and phosphides,234,235 and sodium iron pyrophosphate,236 as cathode materials using inexpensive nitrates as metal precursor oxidizers. Sucrose, glucose, glycine, and citric acid were used as fuel. In contrast to solid-state synthesis methods, SCS significantly reduces the duration and temperature of the calcination step, which is often necessary to prepare highly crystalline products. 5.1.1.1. Co- and Mn-Based Cathode Materials for LithiumIon Batteries. Cathodes made from combustion-derived LiCoO2 exhibit appreciable capacity and high capacity retention over charge/discharge cycles. During SCS of LiCoO2, reactive solutions containing lithium and cobalt nitrates as oxidizer and diformyl hydrazine,237 starch, or urea238 as fuels have been introduced into preheated furnaces. These solutions can be ignited at temperatures as low as 250 °C. However, often postcombustion calcination was used to improve the crystallinity and control the phase composition of the products. It was shown that the synthesis/calcination temperature should be higher than 600 °C to produce phase pure rhombohedral layered LiCoO2.237,238 XRD and Raman spectroscopy studies showed that products formed at lower temperature consist of both cubic LiCoO2 (spinel) and rhombohedral LiCoO2 phases. The maximum energy capacity (136 mA·h/g) of the optimized LiCoO2 electrode was observed during the first cycle.238 Cyclic voltammetric tests of material prepared by starch-assisted combustion showed good reversibility with respect to Li+ ions.239 The electrochemical stability and performance of the cathodes were demonstrated over 30 cycles with a capacity fade of less than 10% of the initial capacity. Several studies240−243 showed that substitution of cobalt by Ni, Zn, Mn, or Mg enables modification of the layered LiCoO2 structure and enhances the cathode electrochemical performance. X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy studies suggest that the layered structure of LiCO2 transforms into a spinel structure at a certain critical amount of substitute metal. X-ray diffraction studies indicate that, below the critical content, doping favors the formation of highly refined crystals of layered structure with an expanded “c” lattice, thereby providing more space for lithium-ion intercalation/deintercalation. Such structural modifications increased the specific capacities of electrodes made from LiCo1−xMn(Ni,Mg)xO2 as compared to LiCoO2 synthesized by the same method.241,243 Solution combustion synthesis allows precise control over the ratio of metals in more complex cathode materials; this is done by changing the relative concentration of metal nitrates in reactive solutions.231,244−248 This feature makes it possible to prepare a number of multicomponent layered oxide nanoparticles such as LiNi1/3Mn1/3Co1/3O2.231,247 Electrochemical

for effective diffusion to achieve high energy and power density as well as long cycle life. High nanomaterial surface defect concentrations may promote interfacial reactions and phase transitions, while bulk defects can enlarge the lattice constants and generally enhance electrical conductivity. Thin carbon coatings on nanoparticles may enhance the overall electrical conductivity of the electrode, as well as introduce desired surface defects. The capacitance of supercapacitors is directly proportional to the total available surface area of porous nanomaterials. However, small pores may introduce increased electrolyte ion diffusion resistance, which leads to lower power density. In some cases, however, the micropores significantly increase the capacitance of supercapacitors.216 Impurities in the porous materials can also have ambiguous effects, as they may react with the electrolyte to degrade the cyclic stability or may enhance the surface charge density and thus increase the capacity of devices. In solid oxide fuel cells, the dense electrolyte should possess high oxygen ion mobility between electrodes. Additionally, the electrolyte must have low electronic conductivity and a stable pore-free structure. The electrodes, however, should have high electronic conductivity, high catalytic activity for oxygen reduction, sufficient porosity after sintering, and be compatible with the electrolyte. As a synthesis method for nanoscale materials, solution combustion is sufficiently flexible to meet the above requirements for energy conversion and storage devices.18 Most nanostructured materials that have been used in these devices, or possess high potential, are complex oxides. As described in this Review, SCS is an established technique for production of such oxides with a high level of chemical and phase uniformity. It is important, however, to tailor the synthesis conditions to produce materials with controlled nanostructural characteristics. In this section, we discuss important use of combustionderived materials for electrochemical energy storage and conversion devices, rechargeable batteries, supercapacitors, and solid-oxide fuel cells. We also highlight important parameters and synthesis conditions that govern the structure and electrochemical performance of the nanomaterials. 5.1.1. Rechargeable Batteries. Rechargeable lithium-ion batteries are widely used in portable electronics and electric/ hybrid vehicles. The most popular lithium-ion batteries are based on layered lithium cobalt oxide (LiCoO2) in the positive electrode (cathode) and graphite in the negative electrode (anode); the lithium ions inside the battery transfer between the positive and negative electrodes during charge and discharge.217,218 LiCoO2-based batteries offer high energy density, but present safety risks as well.219 These batteries contain flammable organic electrolytes, which are susceptible to intense smoking or even fire when improperly charged or stored. Isostructural LiNiO2 is attracting intense attention because of its lower redox potential, causing it to be less prone to electrolyte oxidation.218,220 LiMn2O4, with a spinel crystal structure, is also a prospective candidate for the cathode material due to its low cost, acceptable environmental impact, and excellent voltage profile characteristics.221,222 Lithium-ion batteries are relatively expensive due to the specially designed cell, requiring a perfectly dry environment during some manufacturing steps, and costly organic electrolytes.219 Therefore, an alternative rechargeable lithium-ion battery with an aqueous electrolyte was introduced to address the safety issues.223 To improve the stability of batteries, many other 14521


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Figure 44. SEM images of materials produced by sucrose and nitric acid-assisted combustion reaction (A) and by decomposition process (B). TEM (C) and high-resolution TEM (D) images; detailed crystal planes (E,F); and corresponding fast Fourier transform algorithm pattern (G,H) of SCSderived material. Reprinted with permission from ref 258. Copyright 2016 American Chemical Society.

50 cycles at a charge−discharge rate of 1 C.254 This material also exhibited good discharge capability, at 98 mA·h/g at a rate of 10 C. Recently, Chen et al. synthesized a composite nanomaterial consisting of a lithium-rich layered oxide and a spinel phase by the sucrose-assistant gel combustion approach.258 Detailed microstructure analysis (Figure 44A,C−H) suggested that combustion of gels containing metal acetates, sucrose, and nitric acid facilitates the formation of product with nonagglomerated uniform nanoparticles of about 200 nm, composed of the Li1.2Mn0.54Ni0.13Co0.13O2 layered oxide as the major phase with small amounts of the spinel phase (Li2MnO3) byproduct embedded in the nanoparticles. The product formed by thermal decomposition of the metal acetate mixture appears as agglomerates of larger particles (see Figure 44B). XRD analysis showed that thermal decomposition does not form the spinel byproduct phase. Electrochemical tests showed that, in the graphite anode-based cell configuration, combustion-derived materials with such unique microstructure and phase composition deliver a capacity as high as 253 mA·h/ g at 0.1 C, corresponding to a specific energy density of 801 W· h/kg. The authors outlined that the performance of SCSderived material is better than similar materials reported in the literature, indicating that it is a promising cathode candidate for a high-energy-density lithium-ion battery. 5.1.1.2. Phosphate-Based Cathode Materials for LithiumIon Batteries. SCS has become a popular synthesis technique for carbon-coated lithium iron phosphate (LiFePO4), a promising cathode material for rechargeable batteries.259,260 Rechargeable batteries based on this compound have somewhat lower energy density than the more common LiCoO2, but offer longer lifetimes and better power density. Also, batteries prepared using LiFePO4 cathodes are inherently safer.

characterization showed that these nanoparticles may react with standard electrolyte solutions due to their small particle sizes (below 50 nm).247 Such reactions form a surface protective layer on the nanoparticles. However, due to surface migration, the Ni/Mn ratio may be changed slightly in the bulk of nanoparticles, which leads to decrease in their electrochemical performance. Therefore, Sclar et al. used a short postsynthesis calcination step to decrease the surface area of nanoparticles, thus decreasing reaction between nanoparticles and the electrolyte.247 The SCS was also used to prepare cubic LiMn2O4 as a cathode material for lithium ion batteries.249−252 The use of LiMn2O4 offers benefits such as low cost, natural abundance, and environmental friendliness of manganese precursors, and thermal stability of the spinel.221 However, its practical application has been limited by a relatively low diffusion rate, low specific capacity, and severe capacity fading with cycling in contrast to LiCoO2. Lu et al. have reported a urea-assisted combustion method to prepare phase pure product at temperatures as low as 500 °C and subsequent calcination at 700 °C.250 As-prepared materials showed a specific capacity of 121 mA·h/g and sustained 229 cycles between 3.0 and 4.3 V at a charge−discharge rate of 0.1C before reaching an 80% charge retention critical value. Zhang et al. synthesized spherical LiMn2O4 nanoparticles with sizes of ∼100 nm.252 This material displayed an excellent capacity retention ratio of 95.1% for 100 cycles at constant current rate of 1 C. The current trend in this direction is to improve the electrochemical performance of LiMn2O4 by substituting some manganese with transition metals.248,253−257 For example, a high-voltage (∼4.7 V vs Li+/Li) LiNi0.5Mn1.5O4-based cathode material was synthesized, which showed an initial discharge capacity of 125 mA·h/g and retained 97.2% of the capacity after 14522


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gelation agent. Detailed parametric studies indicated that the particle size of the resulting powder could be tuned by adding ethanol into the precursor solution and changing the quantity of gelation agent. In the optimum conditions, uniform Li3Fe2(PO4)3 nanoparticles with an average diameter of 70 nm were prepared. Cyclic voltammetry studies showed that the synthesized compound has good reversibility and high cyclic stability. The material also has a high discharge capacity of 126 mA·h/g (theoretical capacity of 128.2 mA·h/g) and a long cycle life. 5.1.1.3. Cathode Materials for Sodium-Ion Batteries. Recent investigations demonstrated that solution combustion is a promising synthesis method of cathode materials for high capacity sodium-ion batteries.236,266−269 Sodium-ion batteries are emerging as a good choice due to the abundance and relatively low-cost raw material price used to fabricate the electrodes and electrolytes, as compared to the lithiumcontaining precursors. In addition, these materials also have high structural stability during charge/discharge cycles. For example, by using first-principles calculations, Jiang et al. showed that for Na2V6O15 materials the volume change between the fully discharged and charged states was only 6.4%, implying high structural stability of cathode material.266 They also reported combustion synthesis of NaV6O15 nanosheets with an average length 400 nm and width of 100 nm using an aqueous solution containing NaNO3, NH4VO3, and citric acid. These nanosheets exhibited a high initial discharge capacity of 149.48 mA·h/g at a current density of 20 mA/g. A new family of nanostructured cathode materials (Na2FeP2O7, Na2CoP2O7, Na2MnP2O7) for sodium ion batteries was also synthesized by combustion of solutions containing NaH2PO4 and corresponding metal nitrate and citric acid, involving the final calcination step at 600 °C for 1−6 h.236,267,268 From a synthesis viewpoint, these materials can be easily prepared both by solution combustion as well as by solidstate synthesis. Nevertheless, the need for prolonged heat treatment in the solid-state route leads to aggressive grain growth and agglomeration, thus forming large micrometric particles. In contrast, combustion synthesis can yield the final product rapidly, with smaller particle sizes (300−400 nm). Operating at an average voltage of 3−3.6 V (vs Na/Na+), SCSderived materials delivered a reversible capacity of 80−82 mA· h/g with good rate capability. Low cost, a theoretical capacity close to 100 mA·h/g, excellent reversibility, and rate capability make these cathode materials promising for future sodiumbased batteries. Recently, a NaTi2(PO4)3/C composite, containing up to 25 wt % carbon, was synthesized by gel combustion followed by heat treatment at 650−750 °C.270 Highly crystalline materials prepared after calcination were investigated by intercalation/ deintercalation kinetics in aqueous solutions, using cyclic voltammetry and galvanostatic charging/discharging measurements. The authors noted that the sample calcined at 700 °C showed improved stability for up to 50 cycles, as compared to the similar materials reported in the literature. 5.1.1.4. Anode Materials. Various transition metal oxides (e.g., Fe3O4, Co3O4, NiO, MnO) have been prepared using the solution combustion route as potential anode materials for rechargeable lithium-ion batteries.96,129,271−274 These compounds have diverse chemical and physical properties and can deliver high reversible capacities between 500 and 1000 mA·h/ g. To prepare nanostructured crystalline NiO and Co3O4 by SCS, an inert salt (e.g., NaF) is added to the reactive

Combustion synthesis of porous carbon-coated LiFePO4 nanostructured materials has been reported, which utilize reactive solutions containing lithium nitrate, iron(III) nitrate (or oxalate), ammonium dihydrogen phosphate (NH4H2PO4), and glycine.198,261,262 A small amount of carbon nanoparticles was also added to initial solutions to form a conductive carbon layer on the product particles. Postsynthesis heat treatment of highly porous combustion products in inert atmosphere facilitated formation of a crystalline carbon layer with 100 nm thickness. The synergistic effect of carbon layer and the nanoporous network of the LiFePO4 resulted in high capacities of 160 mA·h/g (theoretical value of 170 mA·h/g) at C/20 rate with only 3% capacity fade in 50 cycles. Malonic acid and sucrose were also suggested as carbon sources for in situ formation of conductive layer on the surface of LiFePO4 nanoparticles.198,262 Vujković and coauthors showed that the thickness of the carbon layer can be precisely controlled by varying the content of additive in the initial solutions.198 An interesting approach was employed to produce a lithium pyrophosphate (Li2FeP2O7) phase using inexpensive Fe(NO3)3 reactants.263 A small amount of ascorbic acid was used to transform Fe(III) into Fe(II) prior to adding fuel (citric acid) into the solution. The as-prepared solution then was preheated to form a gel, and it was placed in a furnace heated to 600 °C for 1 min. This process formed carbon-coated near spherical particles of monoclinic Li2FeP2O7 with narrow size distribution (70−90 nm). The presence of the Fe(II) species in the product was confirmed by Mössbauer spectroscopy. Contrary to SCS, conventional solid-state synthesis requires at least 12 h heating at 700 °C to produce phase pure Li2FeP2O7.264 The average surface area (45 m2/g) of combustion-derived material was 17 times higher than that of the solid-state synthesized one. Electrochemical characterization showed a near one-electron theoretical capacity of ∼100 mA·h/g (at C/20 rate) with a 3.5 V Fe3+/Fe2+ redox activity. The material also showed excellent reversibility over the first 60 cycles (Figure 45). The gel combustion approaches are the most recent in the synthesis of Fe-based cathode materials. For example, sol−gel combustion method was also reported for the production of Li3Fe2(PO4)3 nanopowders using iron(III) nitrate.265 In this case, poly(vinyl alcohol) was employed both as fuel and as

Figure 45. Galvanostatic charge−discharge curves for Li2FeP2O7 positive electrode cycled between 2 and 4.3 V (at 25 °C) at C/20 rate. Reprinted with permission from ref 263. Copyright 2012 Royal Society of Chemistry. 14523


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Figure 46. TEM images of a-Fe2O3 (A−C) and c-Fe2O3 (E−G) diffraction pattern of a-Fe2O3 (D) and c-Fe2O3 (H); a-Fe2O3 and c-Fe2O3 are denoted as amorphous and crystalline oxides, respectively. Reprinted with permission from ref 277. Copyright 2014 Elsevier.

Figure 47. Comparison of cycling performance of a-Fe2O3, c-Fe2O3, and commercial Fe2O3 nanoparticles (A). Cycling performance of c-Fe2O3 at 500 mA/g and 0−3 V after three cycles at 100 mA/g (B). Rate capability of c-Fe2O3 (C). Reprinted with permission from ref 277. Copyright 2014 Elsevier.

solution.96,271 This approach leads to a remarkably increased surface area of NiO that exhibits high charge/discharge capacity and Coulombic efficiency of 96% over 20 cycles.96 However, large volume changes during the lithium-ion insertion/extraction processes cause rapid loss of structural integrity and consequent capacity fading of oxide-based anodes. To preserve electrode integrity and accommodate large volume change, a layer of carbon was used.272,273 For example, while pure Co3O4 nanoparticle electrodes fade quickly, retaining only 60% (523.94 mA·h/g) after 30 cycles, combustion-derived composite of cobalt oxides with graphene showed high lithium storage capacity of 801.31 mA·h/g after 30 cycles.272 Zhu and co-workers produced MnO nanoparticles embedded in a carbon matrix by solution combustion and subsequent heat treatment in an inert atmosphere.129 The carbon content in the composite was controlled by changing the amount of glycine

fuel used during synthesis. A composite with carbon content of 28% delivered a reversible specific capacity of 437.6 mA·h/g at a high current density of 500 mA/g after 300 cycles. Recently, a combination of reactive solution and aerosol spray pyrolysis was used to prepare oxide-based nanostructured materials.275−277 Guo et al. reported the synthesis of amorphous carbon−MnO spherical nanoparticles by spraying an atomized solution of manganese nitrate and sucrose through a heated furnace.275 The uniform distribution of carbon and oxide in the product particles improved the electrochemical reaction kinetics, which result in superior rate capacity. In follow-up studies, Xu and co-workers applied the same concept to produce hollow carbon−CuO276 and Fe2O3-based277 spherical nanomaterials for lithium-ion battery anodes. Figure 46 shows that, depending on the furnace temperature (600 or 14524


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800 °C), either amorphous (denoted as a-Fe2O3) or crystalline (denoted as c-Fe2O3) can be synthesized. The cycling stability of both c-Fe2O3 and a-Fe2O3, as well as electrodes made from commercial Fe2O3 nanoparticles (particle size 50 nm), was investigated by charge/discharge processes at 0−3.0 V intervals, at a current density of 500 mA/g (Figure 47A). Although both c-Fe2O3 and a-Fe2O3 showed higher capacity as compared to commercial oxide-based electrode, crystalline iron oxide spheres showed superior cycling performance. Because of the high cycling stability of crystalline phase, the authors performed extended cycling at 500 mA/g after three cycles at 100 mA/g (Figure 47B). After 300 cycles, no capacity fading occurred. The Coulombic efficiency of materials is 100% over the 300 cycles. The rate capability of c-Fe2O3 anodes was also examined at different current densities. Excellent rate performance of the anode materials is demonstrated in Figure 47C. A high capacity of 300 mA·h/g was retained at a 10 C rate (1 C = 1006 mA/g), which is comparable to graphite used in commercial lithium-ion batteries. Wen and co-workers reported on macroporous NiO/Ni nanocomposites fabricated by reactive solutions containing nickel nitrate−glycine and hydrazine hydrate.119 They also utilized nickel acetate to tailor the structure of the materials, along with nickel nitrate, hydrazine hydrate, and glycine in water. Depending on the amounts of hydrazine hydrate and glycine in the initial solutions, products with a 3D or 2D porous network structure can be produced (Figure 48). It was shown

compared to materials prepared by the solid-state route using the same reactants.227,278,279 To this end, cellulose-assisted glycine−nitrate combustion using anatase (TiO2) as solid source of titanium was used,278 where cellulose served as support and also as a source of carbon that regulates combustion characteristics. However, such approach was insufficient to prepare pure phase Li4Ti5O12. Later, homogeneous aqueous solutions containing TiO(NO3)2 and LiNO3 as the oxidant precursors and glycine as fuel were applied.227 The particle size of resultant pure Li4Ti5O12 material ranged between 20 and 50 nm. During their galvanostatic charge/ discharge at varying rates, electrodes made from Li4Ti5O12 showed capacity close to the theoretical value of 175 mA·h/g at 0.5 C rate. The electrodes also exhibited significantly higher capacities than material produced via solid-state methods over 100 cycles (Figure 49).

Figure 49. Capacity versus cycle number plot for nanocrystalline Li4Ti5O12 synthesized by SCS at different discharge rates. Inset shows capacity versus cycle number for Li4Ti5O12 prepared by solid-state method. Reprinted with permission from ref 227. Copyright 2010 American Chemical Society.

Figure 48. Schematic illustration for NiO/Ni formation with various architectures. Reprinted with permission from ref 119. Copyright 2013 Elsevier.

Yuan et al. used sucrose during cellulose-assisted combustion synthesis to produce a thin (5 nm) layer of amorphous carbon on Li4Ti5O12 nanoparticles.280 The carbon-coated anode materials showed improved lithium insertion/extraction capacity, electrode kinetics, and stable cycling performance at various temperatures. A different in situ approach was used by Li and co-workers to coat the surface of Li4Ti5O12 by a 5 nm thick conductive TiN layer.281 In this case, a solution containing titanyl nitrate, lithium nitrate, and glycine was placed in an alumina crucible and heated in a muffle furnace. Another crucible with different amount of pure urea was also placed in the furnace and rapidly heated in an argon atmosphere to 800 °C. This approach allowed simultaneous formation of Li4Ti5O12, a TiN layer due to ammonia released from the decomposition of urea. The thickness of the TiN layer was tuned by varying the amount of urea used. The electrochemical characterization indicated that a product with ∼5 wt % of TiN exhibits the highest rate capability and stability for lithium storage. At charge/discharge rates of 1, 2, 8, and 15 C, the discharge capacities of the optimized electrode material were 159, 150, 128, and 108 mA·

that the porous structure can significantly influence the electrochemical performance of materials used as anodes in Li-ion. NiO/Ni material with 3D macroporous structure exhibited better electrochemical performances than the 2D macroporous one. Cyclic voltammetry studies revealed that, after 60 cycles at 4 C and 2000 cycles at a higher current of 20 C (14 360 mA/g), the reversible capacity of the 3D macroporous material stabilized at 707 and 230 mA·h/g, respectively. The authors suggested that the presence of metal and the continuous porous network in the materials with 3D structure improves the electrical conductivity flow of electrolytes; additionally the porous structure compensated for the large volume change of the active materials during cycling. SCS was also efficiently used to fabricate complex oxides, for example, spinel-type lithium titanate (Li4Ti5O12) anode materials, which showed superior stability and capacity as 14525


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specific capacitance of 123 F/g than 71 F/g of plate-shape εMnO2 nanoparticles, at the current density of 1 A/g in 1 M NaSO4 electrolyte solution. Jayalakshmi et al.288,289 used glucose as fuel to prepare a number of carbon-coated simple and mixed oxides for pseudocapacitor electrodes. Among these materials, Fe2O3/ carbon showed the highest specific capacitance of 255 F/g. Combustion-derived cobalt−nickel oxides/C/Ni ternary nanocomposites showed a much higher specific capacitance of 446 F/g at 1 A/g (or 280 F/g at 10 A/g).290 It is interesting that adding acid-treated carbon nanotubes into precursor combustion solution made it possible to significantly improve the specific capacitance up to 579 F/g at 1 A/g, or 350 F/g at 10 A/g, due to the high electrical conduction of the nanotubes. Recently, Tao et al. reported on amorphous NiO/carbon composite with mesoporous structures by ignition of nickel nitrate−citric acid solutions at 250 °C in air.291 They studied the electrochemical performance of the combustion-synthesized materials depending on the citric acid content in the reactive solution. In all cases, they used 2 g of nickel nitrate hexahydrate and 1.445 (sample I), 1.156 (sample II), 0.903 (sample III), and 0.826 (sample IV) g of citric acid. The combustion products of the first three samples were amorphous, whereas sample IV was shown to contain some metallic nickel. The specific capacitance values for these samples are shown in Figure 50A. Figure 50B further shows the cycling performance of the samples at a high current density of 10 A/g. For all samples, a slight increase in specific capacitance was observed during the early cycles. After 1000 cycles, the capacity retention of samples I, II, III and IV are 81%, 84%, 89%, and 92%, respectively. The authors reported a more detailed electrochemical performance of sample III. This material showed outstanding electrochemical performance (1272 F/g or 1.82 F/cm2) as a pseudocapacitor electrode. Moreover, it was shown that at a high mass loading of 9.38 mg/ cm2, an areal capacitance up to 5.52 F/cm2 can be achieved at a current density of 9.38 mA/cm2. Combustion-synthesized complex compounds (e.g., MnFe2O4, NiMoO4, Ni2Fe2O4) were also characterized for pseudocapacitor applications.292−294 Senthilkumar and coauthors used combustion of reactive solutions containing nickel nitrate, ammonium molybdate, and urea to synthesize highly crystalline α-NiMoO4 phase with a surface area as high as 80 m2/g.292 This material delivered 1517 F/g specific capacitance at a current density 1.2 A/g. It also showed superior energy density 52.7 W·h/kg at a power density 300 W/kg in 2 M NaOH as an electrolyte. Combustion-derived MnFe 2 O4 nanoparticles were also used to prepare graphene/polyaniline/MnFe2O4 hybrid electrodes for supercapacitors.294 It was shown that the capacitance of hybrid electrodes was 7.5 times higher than that of MnFe2O4. Several attempts have been made to use combustion-derived materials (LiMn2O4, and TiP2O7) as anodes in hybrid supercapacitors.295,296 Although these materials showed good current rate capability and stability, the specific energy density is still low and needs to be improved. 5.1.3. Materials for Solid-Oxide Fuel Cells. Solid-oxide fuel cells (SOFCs) are among the most important energy conversion technologies that generate electric power.297−299 These devices have low environmental impact, high energy conversion efficiency, and fuel flexibility. SOFCs use a dense solid oxide electrolyte (e.g., yttira-stabilized zirconia [YSZ], gadolinium doped ceria [GDC], etc.) to conduct negative

h/g, respectively. After 200 cycles at 1 C, its capacity retention was 98.5%. The same group of authors has also reported novel porous Li2ZnTi3O8 and Li2CoTi3O8 spinel structure by ignition of reactive solution in preheated furnace.163 The obtained Li2ZnTi3O8 and Li2CoTi3O8 electrodes showed discharge capacities of 192 and 201 mA·h/g (at current density 100 mA/g) with retentions of 81% and 89%, respectively. Both electrodes exhibited excellent capacity retentions independent of the rate used, even at a current density as high as 2000 mA/ g. This is especially true for the Li2CoTi3O8 electrode; its capacity of more than 100 mA·h/g can be retained after 200 cycles even at the current rate of 2000 mA/g. These results indicate that the prepared anodes with spinel particles have a large reversible capacity and high rate performance, which can be attributed to the 3D porous structures and their intrinsic characteristics. 5.1.2. Supercapacitors. A capacitor is an energy storage medium similar to an electrochemical battery. Applying a voltage differential on the positive and negative plates charges the capacitors. Recently, much attention has been devoted to develop supercapacitors for applications in many fields such as electric and hybrid electric vehicles, energy back-up systems, etc. On the basis of their energy storage mechanism, supercapacitors are classified into electric double-layer capacitors (EDLCs) and pseudocapacitors.282,283 EDLCs utilize an electrochemical double layer capacitance at the electrode/ electrolyte interface (non-Faradaic process) where electric charges are accumulated on the electrode surface and ions of opposite charge are arranged in the electrolyte side. Different carbon-based materials have been used as EDLC electrodes due to their high surface area, relatively low cost, and chemical stability.283 RuO2 or conducting polymers (e.g., polyaniline) have been employed as electrodes, which utilize the charge-transfer pseudocapacitance.282,284 In pseudocapacitors, charge storage is accomplished through transfer of charge between electrode and electrolyte by electrosorption, reduction−oxidation reactions, and intercalation processes. These Faradaic processes allow pseudocapacitors to achieve greater capacitance, energy density, and better cycle life as compared to EDLCs. RuO2 electrodes exhibit high energy density as compared to other active materials and a higher specific capacitance of 750−1300 F/g.282 However, the high cost and toxicity of RuO2 limits its commercialization. Alternatively, MnO2, Fe2O3, SnO2, and some hydroxides were proposed as inexpensive electrode materials.282,285 Despite significant improvements, the energy density of pseudocapacitors is still low as compared to lithiumion batteries (∼250 W·h/kg). To further improve the energy density, an asymmetric electrode configuration with battery-like electrode material (Faradaic process) along with EDLC electrode (non-Faradaic) has been applied.282,286 This configuration is termed “hybrid supercapacitor”. Recent reports have shown that SCS offers easy and inexpensive approaches to prepare simple and complex oxides as suitable electrode materials for pseudocapacitors.287−289 Yu et al. showed that ε-MnO2 nanostructures may be easily prepared using reactive mixtures containing manganese nitrate and glycine.287 It was shown that plate-shape nanostructure with thickness ∼10 nm could be obtained at a higher ratio of Mn(NO3)2/C2H5NO2, while spherical particles (10−20 nm) were obtained with a lower ratio. Electrochemical characterizations showed that the spherical particles have a higher 14526


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conductivity) of doped ceria powders prepared by SCS may be tailored by simple variation of the fuel/oxidizer ratio.300,304 In this way, the surface area of materials may be adjusted in the range 40−110 m2/g.304 Ceramic powders with higher surface area showed reduced sintering temperature, thus preventing intensive grain growth processes.305 As a result, SCS-derived dense (relative density ∼98%) electrolytes exhibit excellent total ionic conductivity ∼0.08 S/cm at 500−600 °C in air.305 Chen et al. noted that increased ionic conductivity in combustion-synthesized Ce0.8Sm0.2O1.9 materials may be related to the formation of a mixed Ce4+/Ce3+ valence state in the reducing atmosphere, allowing one to operate them at temperatures below 600 °C. An interesting solution combustion approach of Sm-doped ceria was reported by Liu et al., who used poly(vinyl alcohol) (PVA) and urea as fuels.306 Urea was initially evenly distributed inside the PVA binder together with the metal precursors. During slow heating, the urea gradually decomposed to NH3 and CO2, leading to dramatic expansion of elastic PVA-based gel. At higher temperatures, gel ignites, producing highly porous Sm-doped ceria materials with an average particle size of 3−5 nm. Such fine material showed improved sinterability and displayed high conductivity of 0.081 S/cm at 800 °C. The mixed fuel strategy was also used by Medvedev et al. to prepare Ce0.8Sm0.2O2 and BaCe0.8Sm0.2O3 oxides and their composites.307 It was found that nanostructured composites consist of two desired phases without any trace of other impurities. The developed composite materials showed enhanced stability, good total conductivity, and the lowest contribution of electron conductivity in both oxidizing and reducing atmospheres. SCS was also employed to prepare novel proton-conducting SOFC electrolytes.308,309 Khani et al. developed a unique core− shell nanostructure of yttrium-doped barium zirconate (BZY)− yttrium-doped barium cerate (BCY).308 They first prepared BZY core with average particle diameter 15 nm using combustion of solutions containing barium, zirconium, and yttrium nitrates as oxidizer and glycine as fuel. In the second stage, a thin layer (∼10 nm) of BCY was deposited on BZY using a hydrogel containing acrylates of yttrium, barium, and cerium. The resultant BZY−BCY core−shell nanoparticles showed ∼200 °C reduction of sintering temperature as compared to pure BZY. Dense BZY−BCY materials exhibited high proton conductivity of 4.1 × 10−4 to 9.5 × 10−3 S/cm at 300−600 °C, which is an order of magnitude higher than that of BZY. 5.1.3.2. Electrode Materials. In contrast to electrolyte materials, electrodes in SOFCs should have high electronic conductivity and suitable porosity. In anode materials, dispersed metals, such as Ni, should adsorb fuel molecules and then promote its electrochemical oxidation by oxygen ions diffusing though the electrolyte. Combustion-synthesized complex ceramic powders have suitable characteristics to be used as anode materials for SOFCs.310−313 For example, Gavrielatos et al. employed urea-assisted combustion to synthesize highly porous Au−Ni/YSZ cermet. 311 They prepared a gel by heating a nickel nitrate and urea solution that also contained a small amount of HAuCl4. The gel then was applied on the surface of YSZ pellet with a brush and placed into a furnace heated to 600 °C. At this temperature, the gel ignited in less than a minute, and a homogeneous NiO−Au layer with thickness of ∼30 μm was formed on the pellet. Finally, the film was sintered at 850 °C and reduced by hydrogen to form a porous NiAu-YSZ cermet. The prepared

Figure 50. Dependence of specific capacitances on applied currents (A) and the variation of specific capacitance during 1000 cycles (B) for samples I, II, III, and IV at a current density of 10 A/g. Reprinted with permission from ref 291. Copyright 2015 Elsevier.

oxygen ions from the cathode (e.g., lanthanum strontium manganite, LSM) to the anode (e.g., Ni-yttira-stabilized zirconia, Ni-YSZ). The cathode functions as the catalyst for electrocatalytic reduction of molecular oxygen to oxygen ions by providing electrons that are transferred from the anode, while the anode catalyzes the oxidation of fuel with oxygen ions that diffused from the cathode through the electrolyte with the simultaneous release of electrons, which are transported through an external circuit to the cathode. SOFCs operate at temperatures between 500 and 1000 °C. Recently, protonconducting SOFCs are being developed which transport protons instead of oxygen ions through the electrolyte with the advantage of being able to operate at lower temperatures than traditional SOFCs.298 5.1.3.1. Electrolyte Materials. Precursor ceramic powders of solid electrolytes should have high surface area to exhibit improved sintering behavior at low temperatures and thus provide high microstructural homogeneity and enhanced mechanical properties. The electrode materials in SOFCs should also have reduced porosity and as high ionic conductivity as possible. In ceria-based solid electrolyte nanomaterials, doping with rare earth metals (Y3+, Gd3+, Sm3+, Nd3+, and Pr3+) significantly increases the ionic conductivity of electrodes through the formation of a large number of oxygen vacancies.300−303 It was reported that important characteristics (surface area, sintering temperature, 14527


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Figure 51. Mechanism for the formation of nitrogen doped r-GO sheets functionalized with nanoparticles. Reprinted with permission from ref 323. Copyright 2012 Elsevier.

structure indicate that the highly dispersed active platinum sides distributed along the surface of combustion-derived perovskite make them sufficiently active. 5.1.4. New Trends in Combustion-Derived Materials for Energy Applications. Recently, several novel classes of materials, such as graphene-based hybrid structures or novel doped oxide phases, were prepared by solution combustion for energy applications. Graphene oxide (GO) is a two-dimensional water dispersible oxidized carbon layer containing hydroxyl, epoxy, and carboxyl-functionalized groups, obtained by treating graphite with strong oxidizers.320 Highly energetic and thermally unstable GO can readily undergo exothermic self-propagation disproportionation reactions to produce reduced GO (r-GO).321,322 Recently, several studies showed that glycine or urea can be used as fuels to reduce GO.323−325 In SCS of r-GO-based hybrids, GO is dispersed in solutions of metal nitrates and fuels and then uniformly heated to produce reactive gels. Mayavan et al. reported the synthesis of nitrogen doped r-GO composites with silver nanoparticles by heating gels of silver nitrate, glycine, and GO to ∼500 °C in an inert atmosphere.323 Thermal analysis of gels revealed that glycine reacts with silver nitrate at 100−200 °C, producing exfoliated graphene sheet decorated with silver nanoparticles (Figure 51). The reaction of glycine with nitrate is accompanied by the release of gases such as NH3 and NOX, which induce nitrogen doping most likely at the edges and defect sites of graphene. The removal of oxygen species from GO by thermal reduction leaves behind a large amount of vacancies, topological defects (edge defects), and holes on the graphene sheets, which provides active sites for nitrogen doping into the graphene framework (Figure 51). Indrawirawan and co-workers reported on the combustion synthesis of nitrogen-doped r-GO using ammonium nitrate as an oxidizer and nitrogen source.326 The authors show that the combustion product with nitrogen loading of 5−8 at. % can be produced. They also outlined that the microstructure and chemical compositions of the resultant materials depend on the calcination conditions. Li et al. prepared 3D structured graphene using reduction of GO and glycine.327 GO was prepared by oxidation of flaky graphite. The GO suspension was then sonicated with glycine for 30 min, and the resulting colloidal solution was heated to 150 °C for production of viscous colloidal solutions.

material was tested as SOFC anode under methane-rich steam reforming conditions and showed high tolerance to carbon deposits, even at methane to water ratios of 3, at temperatures ranging between 700 and 900 °C. Chen et al. studied the effect of nickel loading on electrical conduction, thermal expansion, and mechanical properties of Ni−samaria-doped ceria (Ni−SDC) materials prepared by SCS.312 It was shown that hydrogen reduction of sintered materials creates a porous microstructure consisting of uniformly distributed Ni on the surface of SDC phases. The Ni−SDC materials containing 50−60 vol % Ni were shown to be optimal. These compositions offer sufficient open porosity (∼30%), superior electronic conductivities of over 1000 S/cm (at 600−800 °C), as well as excellent mechanical properties. Electrode materials prepared by SCS include traditional lanthanum strontium manganites and their composites313−315 as well as novel doped perovskites (e.g., Sr2FeNiMo0.5O6).316−318 For example, Dai et al. reported novel Sr2FeNiMo0.5O6 complex materials prepared by SCS, which possess a 3D interconnected network microstructure composed of nanoparticles.316 It was shown that Ni2+ affects conductivity of materials, improving it by 60 S/cm at 450 °C, which is more than twice that of the material without Ni doping. These cathodes showed excellent electrochemical performance and the lowest interface polarization resistance. The maximum power density of a single cell prepared by this cathode was 1.27 W/cm2 at 750 °C, which suggests that the material is a good cathode candidate for intermediate temperature SOFCs. Solution combustion was also shown to be an effective approach for preparing a number of perovskite catalysts to be used in liquid feed direct alcohol fuel cell applications.112,113,319 The Sr-based perovskites showed much better performance comparable to the standard Pt−Ru catalyst. It was observed that in situ doping of SrRuO3 with Pt during synthesis yielded highly active catalysts. On the other hand, the presence of SrRuO3 significantly enhanced the catalytic activity of Pt, leading to superior performance even at lower Pt loadings. Combustion-derived catalysts, having on average 4 times lower Pt loading, showed current densities comparable to that of the Pt−Ru standard. Specific surface areas of the composite catalysts (∼10 m2/g) are much lower as compared to the Pt− Ru standard (∼60 m2/g). These remarkable differences in 14528


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Combustion was initiated by heating of the solutions at 250 °C. Figure 52 shows important characteristics of the combustion-

sheets turned into an interconnected porous network. The decomposition products of glycine can exothermically react with the epoxy, carboxyl, or hydroxyl groups on the graphene oxide surface. Gaseous compound formed during combustion create pores between the layers of the graphene sheets. Figure 52C shows the XRD patterns of GO and combustion-derived graphene termed as 3D-G. A typical broad peak near ∼10° of GO disappears after the reaction, indicating efficient exfoliation and deoxygenation. The FTIR spectrum of GO shows characteristic −OH, CC, and carbonyl peaks, which were mostly eliminated in the spectrum of the combustion product (Figure 52D). Figure 52E,F shows the Raman spectra of the material in different areas of the same sample, confirming formation of a complex multilayer graphene structure. Combustion-derived 3D graphene samples were decorated with platinum nanoparticles and tested for methanol electrooxidation. The catalyst shows remarkably high activity and stability. The authors showed that the catalyst exhibits higher tolerance to corrosion than carbon black. Several researchers reported a SCS-based process for r-GO/ TiO2 materials using combustion of GO, titanyl nitrate, and urea gels.324,328 Co3O4/r-GO272 and CoFe2O4/r-GO329 hybrids were also fabricated by SCS. Co3O4/r-GO hybrids were tested as an anode material for lithium-ion batteries. It was shown that combustion-derived Co3O4 nanoparticles (25−50 nm) were uniformly deposited on the graphene sheets and showed better electrochemical performance than pure Co3O4. Recently, another type of novel nanomaterial (Al-doped LiNiO2) was developed by SCS as catalyst in oxygen evolution reaction (OER).330 This reaction is critical for hydrogen generation by electrolysis of water as well as in charging of rechargeable batteries and fuel cells operating with an air cathode.331−334 Gupta et al. compared solid-state and solution combustion synthesis of Al-doped LiNiO2.330 To produce these materials by SCS, an aqueous solution containing a stoichiometric amount of LiNO3, Ni(NO3)2·6H2O, Al(NO3)3· 9H2O, and urea was mixed in a glass beaker and heated rapidly to 400−450 °C in a furnace. The products were then heated to 650 °C for 16 h to remove residual carbon and to obtain highly crystalline phases. The authors inferred that up to 40% Al could

Figure 52. SEM (A) and TEM (B) images of the 3D-graphen (3D-G) materials as well as XRD patterns for 3DG and GO (C). FTIR spectra for 3D-G and GO (D) and Raman spectra for 3D-G (E,F). Reprinted with permission from ref 327. Copyright 2015 Royal Society of Chemistry.

derived material. SEM and TEM images (Figure 52A,B) of the as-prepared material indicate that, after combustion, the GO

Figure 53. Bright-field (A) and high-resolution TEM (B) images of combustion-synthesized LiNi0.8Al0.2O2. (C) High-resolution TEM image of the same materials after 1300 repeated OER sweeps; vulcan carbon used for sample preparation for OER test can also be detected along with the LiNi0.8Al0.2O2. Crystallites: (D) scanning TEM image on the same crystallite as shown in panel (C), (E) Ni map, (F) Al map, (G) C map, and (H) C + Ni + Al mix map. Reprinted with permission from ref 330. Copyright 2015 John Wiley & Sons. 14529


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be substituted for Ni in LiNiO2, whereas the solid-state synthesis method allowed only 25% of Ni substitution. Microstructural analysis indicated that the materials obtained by combustion are agglomerates of small (50−100 nm) nanoparticles (Figure 53A,B), while the solid-state-synthesized samples had well-formed crystallites of size comparable to the agglomerates (size of 2−10 μm) of the combustion-synthesized samples. Detailed electrochemical tests indicated that the OER activity of combustion-derived LiNi0.8Al0.2O2 is comparable to that of commercial IrO2 catalysis and is significantly higher than materials produced by the solid-state reaction, as well as other types of catalysts reported in the literature. The material also exhibited a remarkable stability of OER cycles in alkaline medium. The results of TEM (Figure 53C) and scanning TEM and energy-dispersive X-ray spectroscopy (EDS) (Figure 53D− H) analysis on the sample after 1300 cycles show no signs of deterioration of the catalyst surface due to the formation of an amorphous layer after OER cycles. EDS mapping shows no changes in elemental distribution of combustion-synthesized materials. After OER cycling (Figure 53D−H), Ni and Al are still uniformly distributed in crystalline nanoparticles. The authors conclude that low cost of preparation, good stability, and an easy synthesis procedure makes combustion-derived LiNi0.8Al0.2O2 materials a viable candidate for use as an OER catalyst in a rechargeable battery with an air cathode.

catalytic reaction, as-prepared nanomaterials can be subjected to postcombustion treatment. For example, Pfeil et al. used 30 min heat treatment of combustion-synthesized nanostructured Co3O4 materials for hydrogen generation during hydrolysis of NaBH4.123 Another example of bulk SCS-derived catalyst was reported by Erri et al., who synthesized cerium- or nickelsubstituted LaFeO3 perovskites.336 After calcination, these catalysts had high activity and exhibited excellent stability for autothermal reforming of hydrocarbon mixtures (JP-8). Kumar et al. reported the synthesis of bulk metallic catalysts (e.g., Cu, Ni, NiCu, NiFeCu) for ethanol reforming reactions.337 As compared to oxides, these catalysts should be reduced by hydrogen-containing gas before catalytic run. They noted that the reduction temperature of monometallic catalysts was higher as compared to alloys. As an example of supported catalysts utilizing SCS, manganese oxides can be synthesized onto pore-free cordierte (2MgO·2Al2O3·5SiO2) monolith filters recently reported by Piumetti et al.128 They dipped the filters into the reactive solution of manganese nitrate and glycine and then placed them in a preheated furnace to initiate combustion. As-prepared Mn3O4 evenly coated the support, resulting in a change of filter color from white to black (Figure 54). The thickness of layer

5.2. Heterogeneous Catalysts

The preparation, characterization, and use of solid catalysts is recognized as an area of major interest in chemistry and chemical engineering. Catalysts exist in various forms, and their preparation involves different protocols with a multitude of possible schemes. Schwarz and coauthors reviewed catalyst preparation techniques and divided them into three general categories.335 In the first category, catalysts are generated as a solid phase by either precipitation or decomposition reactions. In the second route of catalyst preparation, the active phase is introduced onto a pre-existing solid phase (support). The two most commonly used supported catalyst preparation methods are impregnation and precipitation-deposition. In the impregnation method, an active phase (e.g., a metal precursor solution, an oxide, etc.) is deposited onto the support from a liquid phase, followed by a calcination step. In the precipitationdeposition method, the active phase, such as metal, metal oxide, or metal sulfide, is deposited onto the surface of the support during in situ precipitation process. Another widely used method to prepare solid catalysts is based on sol−gel chemistry.335 This method takes place as a homogeneous process that results in the continuous transformation (evaporation or sublimation) of a solution into a hydrated solid precursor. The precursor is then subjected to calcination to remove excess solvents and formation of the crystalline catalyst. Solution combustion synthesis is a new method for catalyst preparation. It uses elements of conventional preparation approaches and involves its own unique techniques. SCSderived catalysts can be categorized into two major groups: bulk and supported catalysts. In the first group, as-synthesized porous foam-like aggregates or powders can be directly used as a catalyst. In the second group, the active phase is deposited onto a solid support using a rapid high temperature reaction. In the first group, SCS-derived catalysts include simple and complex oxides as well as alloys. Depending on the specific

Figure 54. Mn3O4-based monolith catalyst obtained by SCS route. Reprinted with permission from ref 128. Copyright 2015 Elsevier.

can be controlled by regulating the concentration of metal nitrates and fuel in the mother solution. This procedure may be also repeated several times to achieve the desired amount of active phase on the monolith surface. Such preparation enables production of a porous coating of an active phase on the walls of the solid support. The adhesion test showed that the adherence between the deposited catalysts and the ceramic support was excellent. Specifically, after 1 h of ultrasonic treatment, the coated filters showed insignificant deposited catalyst losses. As-prepared catalysts showed excellent activity in total oxidation of volatile organic compounds.128 Cross and coauthors have reported on the synthesis of a Ni active phase on highly porous γ-Al2O3 supports with a surface area of ∼160 m2/g.338 They immersed the γ-Al2O3 pellets into reactive solutions containing nickel nitrate and glycine for 5 s to 14530


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seen in Figure 56. This simple process allowed the authors to produce high surface area catalysts (∼80 m2/g) with high pore

30 min. The dried pellets were then placed onto the preheated hot plate to initiate combustion. The impregnation of the reactive fluid was shown to be rapid; only a minute was required to obtain a uniform metal distribution across the combusted pellet. Highly active Co3O4/Al2O3 catalysts for catalytic oxidation of methane were prepared by Zavyalova et al. using incipient wetness impregnation of an γ-Al2O3 support by reactive solutions followed by their combustion.339 Cross et al. have also reported direct synthesis of high surface area (155 m2/g) Ni (or NiO) catalysts supported on fumed silica.21 They prepared a reactive solution of nickel nitrate, glycine, and ammonium nitrate and mixed them with fumed silica powder. Drying of the resultant slurry yielded a softened gel, which was then placed in a reactor with gas feed under air or argon. The reaction was initiated by local preheating (spot of ∼1 mm3) of the gel by a resistively heated tungsten wire. After ignition, the chemical reaction propagated through the sample in the form of a rapidly moving combustion wave. Combustion of gels in air resulted in highly dispersed nickel nanoparticles (5 nm), which instantly oxidized in air (Figure 55). Low concentrations of oxygen impurity (less than 0.001%) in argon caused passivation of the Ni nanoparticles through the formation of a thin amorphous oxide layer.

Figure 56. TEM images of mesoporous CeO2 catalysts prepared by colloidal solution combustion synthesis. Reprinted with permission from ref 48. Copyright 2016 American Chemical Society.

volume (0.6 cm3/g). Because of the unique mesoporous structure, this catalyst showed excellent activity for soot and CO oxidation. The evident advantages of combustion reactions to prepare solid catalysts include utilization of simple and rapid techniques, as well as the ability to control the catalyst microstructure by changing the combustion conditions. One of the most important advantages is, however, related to preparation of doped complex catalysts with highest possible structural homogeneity. For example, Murugan and Ramaswamy compared Mn-doped ceria catalysts prepared by wetimpregnation, coprecipitation, solid-state reaction, and solution combustion synthesis methods.341 They noted that the first three methods lead to clustered manganese oxide species on surface of ceria, while the solution combustion route yielded Ce1−xMnxO2 solid solutions. Ionic dispersion of the active phase provides a way to prepare Mn−CeO2 materials with reducible Mn3+ species along with a Mn2+ ceria matrix. Later, in this section, we will discuss advantages of these unique catalysts in specific reactions. Presently, the solution combustion approach enables the preparation of a number of active and selective catalysts for methane or alcohol reforming, oxidation, and hydrogenation reactions.64,342 SCS-derived catalysts were also used in emission control catalytic reactions,36 some specific catalysts used in carbon nanotube synthesis,343,344 hydrogen generation during hydrolysis reactions,122,123 burn rate modifiers in propellants,345 chemical looping combustion,346,347 etc. Combustion-synthesized catalysts are also extensively employed in liquid-phase organic reactions as well as mineralization of toxic organic wastes.348−350 Below we present and discuss the performance of SCS-derived catalysts in specific reactions. 5.2.1. Emission Control Catalysts. 5.2.1.1. Carbon Monoxide Oxidation Reaction. Combustion of fossil fuels remains the most important energy source for the global economy today. However, toxic and harmful combustion byproducts (CO, NO, residual hydrocarbons) have severe impacts on the global climate. The implementation of automobile exhaust gas after-treatment systems has provided remarkable results in the past decades in terms of reducing atmospheric pollutants. These systems contain three-way catalysts that are composed of precious metals (Pt and Rh) and Al2O3, CeO2 ceria oxides.351−353 Such catalysts operate in a closed-loop system near stoichiometry and can simultaneously oxidize CO and hydrocarbons to CO2 and water, while reducing NO to nitrogen as follows:

Figure 55. Combustion synthesis of supported Ni catalysts. Reprinted with permission from ref 21. Copyright 2014 American Chemical Society.

The impregnated active layer combustion (IALC) was reported by Kumar et al.109,111 and discussed in section 3.3.3. This simple approach allows for avoidance of calcination steps that are often needed to produce highly crystalline catalysts using SCS. Han and co-workers recently prepared Fe−Mg−K/ Al2O3 catalysts with unique porous structures by surface impregnation combustion for the synthesis of light olefins from syngas.340 Voskanyan et al. utilized template-assisted combustion synthesis to produce porous CeO2 catalysts.48 They added colloidal SiO2 nanoparticles with size ∼20 nm to cerium nitrate−glycine solutions. The as-prepared solutions were heated to 150 °C on a hot plate to initiate the combustion reaction. Combustion products were treated with a sodium hydroxide solution to remove the template. A highly porous CeO2 with uniform spherical pores with size ∼20 nm is clearly 14531


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2CO + O2 = 2CO2

(R14)

Cx Hy + (x + 0.25y)O2 = xCO2 + 0.5y H 2O

(R15)

2NO + 2CO = N2 + 2CO2

(R16)

(2x + 0.5y)NO + Cx Hy = (x + 0.25y)N2 + xCO2 + 0.5y H 2O

(R17)

The combination of Pt and Rh ensures a complete removal of NO, CO, and hydrocarbons. There is also a growing interest in partly replacing Pt with less-expensive Pd.353 SCS makes it possible to prepare highly active and stable catalysts for such applications. For example, Cu-, Co-, Pt-, Pd-, Au-, Rh-based catalysts were developed and showed excellent catalytic performance on exhaust gas oxidation/reduction reactions.354−363 Several research groups used combustion synthesis to prepare a number of Cu−CeO2, Pt−CeO2, Pd− CeO2, and Au−CeO2 catalysts with surface area ∼100 m2/g for the CO oxidation reaction. In a typical synthesis, they introduced aqueous solutions of ceria ammonium nitrate, other metal salts, and oxyalydihdrazide into a furnace preheated to 350 °C. Combusted materials were porous agglomerates of highly crystalline nanoparticles with sizes ranging from 10−30 nm. Detailed XRD, TEM, and X-ray photoelectron spectroscopy (XPS) analysis showed that Cu, Pd, and Pt were ionically dispersed on the surface of CeO2 nanoparticles.354,356,359 On the other hand, Au was dispersed as Au0 as well as Au3+ states on the CeO2 surface.355 Interestingly, metallic Pt or Pd nanoparticles with sizes of 5−10 nm can be formed during the combustion of solutions containing aluminum nitrate, noble metal salt, and oxyalydihdrazide.356,360 This difference may be attributed to the formation of Ce1−xMexO2−δ solid solution phases. The results of XPS analysis showed that in 1%Pd− CeO2 catalysts, the Pd2+ ion has a lower coordination as compared to PdO, indicating an oxygen ion vacancy around Pd2+.357 Because of this unique structure, that is, ionically distributed Pd and the presence of oxygen vacancies, combustion-derived 1%Pd−CeO2 catalysts showed high activity for the CO oxidation reaction at much lower temperatures (T100% = 150 °C) than either Pd (T100% = 350 °C) metal, PdO (T100% = 230 °C), or Pd/CeO2 (T100% = 270 °C) catalysts prepared by the conventional impregnation method. Later, Hegde et al. reported on the synthesis of Ti0.99Pd0.01O2−δ and Ce0.7Ti0.25Pd0.02O2−δ in a similar SCS scheme.36 These catalysts exhibited outstanding activity in the CO oxidation reaction. Figure 57 shows the typical CO oxidation profile over Pd ion-substituted ionic catalysts. The catalysts were stable, and no deactivation was observed during 25 h on stream. Recently, it was also shown364,365 that depending on the type of noble metal (e.g., Rh) and reduction conditions, highly dispersed CeO2-supported metal nanoparticles can be produced. Deshpande et al.366 and Reddy et al.173 prepared noble metal-free doped catalysts for CO oxidation. By utilizing the fuel mixing approach and tuning the ratio between glycine, urea, hexamine, and oxalyl dihydrazide, Deshpande et al. were able to selectively produce either mixed CeO2−Al2O3 oxides or a solid solution of CeO2 with Al substitution.366 However, the activity of catalyst was significantly lower as compared to noble metal-based catalysts and showed activity in the 250−400 °C range.

Figure 57. CO conversion as a function of temperature over different Pd ionic catalysts as compared to the Pd metal impregnated on CeO2. Reprinted with permission from ref 36. Copyright 2014 American Chemical Society.

Reddy et al. utilized microwave-assisted urea combustion synthesis to prepare CuO promoted CeO2−MxOy (M = Zr, La, Pr, and Sm) mixed oxides.173 Raman spectroscopy measurements and XPS analysis showed that defective structures (oxygen vacancies) in solid solutions formed due to the presence of Ce3+ and Ce4+ oxidation states. All doped oxides exhibited better CO oxidation activity than the undoped copper−ceria catalysts. ZrO2-doped copper−ceria catalysts, however, showed high activity (T50% ≈ 105 °C) followed by samarium, praseodymium, and lanthanum oxide doped samples, respectively. They also noted that SCS-derived catalysts have superior activity as compared to similar catalysts prepared by the coprecipitation method. Complete conversion of CO over the optimized combustion-derived catalysts occurred at 150 °C, while complete oxidation for catalysts prepared by coprecipitation route required 300 °C. Ziaei-Azad and co-workers tested Pd-doped LaBO3 (B = Mn, Fe, Co, Ni) perovskite catalysts in oxidation of CO and CH4 for natural gas fueled vehicles.367 PdO formed as a separate phase in contrast to Ce1−xPdxO2 catalyst prepared by Hegde et al.36 On the other hand, PdO significantly facilitated the reducibility of B site in the perovskite structure and promoted the mobility of lattice oxygen in the perovskite. Among different perovskites studied, the Pd effect was most pronounced for the LaFeO3 catalysts. This catalyst showed 100% and over 90% conversion of CO and methane at 560 °C, respectively. The authors proposed that it may be related to greater specific surface area as well as high stability of LaFeO3 structure as compared to other perovskites. 5.2.1.2. Conversion of Other Exhaust Pollutants. During the past decade, solution combustion synthesis has been used to develop multifunctional catalysts for clean combustion technologies of methane (deep oxidation) and diesel fuel. Diesel engines show both carbon monoxide and nonoxidized hydrocarbon outlet concentrations much lower than those produced by spark ignition engines. However, these engines generate nitrogen monoxide and particulate matter, often called soot, which is a complex aerosol system with diameters ranging 14532


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from 10−500 nm.368,369 These particles originate from the incomplete combustion of fuel; additionally, during their nucleation and aggregation stages, they adsorb gases and condensed volatile hydrocarbons. Such complex pollutants possess a serious threat to human health. The capture of soot particles is performed through physical entrapment, preventing its release into the atmosphere. These filters have a monolith shaped reactor with a honeycomb structure. The filters cannot accumulate particles indefinitely, so they need to be thermally regenerated, which involves in situ combustion of the trapped soot particles. Soot removal can be continuous (passive regeneration) or periodic (active regeneration). Most filters use catalysts to lower the combustion temperature of soot to limit the energy requirement associated with regeneration. In addition to soot combustion, the filters must be able to catalyze the oxidation of CO and reduction of NOx pollutants.368,369 Over the past decade, a research group in Politecnico di Torino (Italy) designed and developed new catalytic filter technologies for on-board collection and combustion of particulate matter, in addition to neutralization of harmful gases. They developed several approaches to incorporate Labased perovskites39,370−373 and ceria374−376 catalysts onto ceramic filters during combustion synthesis. For example, LaCrO3 catalysts were deposited onto silicon carbide and cordierite honeycomb filters.39 Russo et al. indicated that Labased perovskite catalysts were able to decrease the combustion temperature of soot from 650 to 400 °C.372,373 Perovskitecoated catalytic filters based on LaNiO3 and LaCoO3 can provide a temperature window of 300−450 °C, in which combustion of soot, oxidation of CO, reduction of NO, and decomposition of N2O may occur simultaneously.370,371 Moreover, regeneration of catalytic traps was shown to be 3 times faster than that of the noncatalytic ceramic filters. SCSderived catalysts have high stability and show no deactivation during soot combustion even after severe high-temperature (850 °C) aging in humid air for 16 h. Combustion-derived catalysts also showed high performance in NO catalytic reduction by NH3 or H2. Selective catalytic reduction of NOx flue gases under lean conditions (ammonia or hydrogen) has been proven to be an effective technique for removal of NOx in exhaust gases from both stationary and mobile sources.377,378 For such applications, complex manganese oxide-based catalysts,379,380 perovskite catalysts, and novel doped TiO2-based catalysts were prepared by the solution combustion route. Furfori and coauthors optimized the synthesis of Pd promoted La−Sr−FeO3 perovskite nanostructured catalysts, which were deposited on a ceramic filter.381,382 The results have demonstrated that the catalytic activity of these perovskites essentially depends on the “B site” reduction and on the presence of oxygen vacancies that are suitable for NO adsorption. The La0.8Sr0.2Fe0.9Pd0.1O3 perovskite-type oxide exhibited the highest activity as a consequence of its greater capability to be reduced at lower temperatures during the temperature-programmed reduction. However, the results obtained are not completely satisfactory yet, because the maximum achieved conversion of NO into N2 was only 60− 70%. Selective catalytic reduction of NO by NH3 has been studied using SCS-derived Cr, Mn, Fe, Co, Cu doped TiO2 catalysts. The stoichiometric reaction of NO reduction by ammonia in the presence of oxygen is the following: 4NH3 + 4NO + O2 = 4N2 + 6H 2O

Roy et al. showed383 that these catalysts have higher activity for NO reduction reaction than perovskite catalysts, indicating that the ionic state of the metals and oxide ion vacancies are the key factors for higher catalytic activity. The catalyst with composition Ti 0.9 Mn0.05 Fe 0.05 O 2−δ has a high nitrogen selectivity (80%) at 200−400 °C range. Later, Guan et al. prepared Ti0.9Ce0.05V0.05O2 catalysts.384 They found a direct correlation between the ignition temperature and surface area of the catalysts; that is, the surface area increased from 23 to 330 m2/g when the furnace temperature decreased in the 250−650 °C range. With the increase of the temperature, the well-crystallized compound and rutile TiO2 phase appeared in the catalyst. The catalyst prepared at 350 °C exhibited the best activity in a broad temperature window of 150−400 °C, in which more than 80% NOx was reduced with superior N2 selectivity above 95%. The results of temperature-programmed desorption indicated that the optimized catalyst has a higher ability to adsorb NOx and NH3 species, and simultaneously enhances the activation of both species, resulting in the activity improvement. Chen and coauthors have further improved catalytic activity toward 100% conversion of NO over the 200−350 °C range by using another anatase-based catalyst (Ti0.75Ce0.15Cu0.05W0.05O2−δ).385 The structure of the catalysts and their performance were strongly dependent on fuel used during synthesis. Urea forms a product containing rutile, anatase, ceria, CuO, and WO3. Glycine and citric acid allow the preparation of the desired doped anatase phase. The optimized catalysts showed selectivity toward N2 above 95%. Another promising catalyst, with a composition of Ti0.85Mn0.1W0.05O2−δ, was synthesized and tested by Kong and co-workers.386 Titanium substitution with manganese and tungsten as well as high surface area (280 m2/g) of the catalyst provided above 95% conversion of NO at 180−400 °C. 5.2.1.3. Catalytic Combustion of Natural Gas. Natural gas (hydrocarbon gas mixture consisting primarily of methane) is an ideal fuel for energy generation due to its abundance and ease of purification for removal of sulfur compounds. It also possesses the largest heat of combustion relative to other hydrocarbons. However, the release of unburned CH4 during homogeneous combustion is a serious problem, given that CH4 is a greenhouse gas with an effect that is 20 times larger than CO2. Catalytic combustion of natural gas has been extensively studied as an alternative option to conventional thermal combustion for the production of heat and energy in view of its capability to achieve effective combustion at much lower temperatures.352,387 Methane combustion over SCS-derived catalysts has been shown to be highly efficient in reducing the emitted pollutants such as CO, NO, and unburned CH4.388−398 Specchia and coauthors introduced highly active combustionsynthesized LaMnO3−ZrO2 catalysts for methane combustion.389,399 Catalytic activity was attributed to the presence of Mn4+ in addition to Mn3+ that leads to cationic defects in the lattice. In addition to the defective structure of perovskites, the presence of dispersed Pd cationic and metallic centers on the surface provided superior activity and stability of catalyst in low temperature methane oxidation (T50% = 450 °C and T90% = 550 °C). These catalysts were also continuously aged in hydrothermal conditions in an atmosphere that contains 200 ppmv SO2 for 3 weeks. It was shown that such treatment is equivalent to a utilization lifetime of approximately 3 years under real operation conditions.389 Interestingly, such severe aging led to an increase in methane combustion catalytic activity. The

(R18) 14533


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converted to CO2, and the single-pass combined yield of C2H4 and C2H6 is currently limited to 27% obtained by using solution combustion synthesis catalysts. A number of Ir, Ru, Pt, and Ni supported catalysts were synthesized by the efficient one-step combustion synthesis approach for methane (or biogas) reforming42,411−416 and water gas shift reactions.405,406,417,418 For example, Postole et al. prepared a unique and active Ir−CeO2 catalyst (with 0.1 wt % of Ir loading) for steam reforming of methane.414 It was shown that combustion-derived catalysts lose almost 20 wt % of their initial weight during heat treatment in an inert gas atmosphere. Mass-spectroscopic studies revealed that such weight loss is accompanied by the release of NH3, H2O, CO2, and NO, suggesting incomplete combustion. The authors developed a postcombustion thermal treatment protocol to eliminate residues in the combusted powder and boost its catalytic activity. Figure 58 shows the effect of inert (“inert”), oxidizing

unusual behavior of the catalysts was attributed to segregation of Mn oxide species upon aging. It was suggested that manganese oxides might have higher catalytic activity. Later, Piumetti et al. confirmed this suggestion by showing that combustion-synthesized Mn3O4 has superior activity during oxidation of many hydrocarbons (e.g., ethylene, propylene, and toluene) at temperatures below 300 °C.128 Specchia and coauthors have compared the catalytic activity of fresh and aged 2%Pd−LaMnO3−ZrO2, and 2%Pd−CeO2− ZrO2 catalysts. The latter showed higher catalytic activity at low temperatures (T50% = 380 °C);390,399 however, they progressively deactivated upon hydrothermal aging in sulfurcontaining media.390 Such difference in catalytic performance was attributed to the fact that the majority of the surface Pd clusters in 2%Pd−CeO2−ZrO2 catalysts oxidized after 2 weeks of aging. 5.2.2. Methane and Other Hydrocarbon Conversion Reactions to Fuel and Chemicals. In addition to energy generation, methane is also an important source for production of other liquid fuels and chemicals of industrial importance. Catalytic conversion of methane involves several direct and indirect methods.400−403 All indirect methods require carbon monoxide and hydrogen synthesis gas. The production of CO and H2 in the appropriate ratios is achieved through the combination of three principal processes: steam reforming (R19); carbon dioxide or dry reforming (R20); and partial oxidation (R21): CH4 + H 2O ↔ CO + 3H 2 ,

ΔH o r = 206 kJ/mol (R19)

CH4 + CO2 ↔ 2CO + 2H 2 ,

ΔH

o

ΔH

o

r

= 247 kJ/mol (R20)

CH4 + 0.5O2 ↔ CO + 2H 2 ,

r

= −35 kJ/mol

Figure 58. Hydrogen formation rate as a function of time during steam reforming of methane over CeO2 and Ir−CeO2 catalysts at 750 °C. Reprinted with permission from ref 414. Copyright 2015 Elsevier.

(R21)

The two reforming reactions are highly endothermic and require extensive heat input. The partial oxidation reaction, however, is slightly exothermic, but requires oxygen or air. This pathway involves oxidation of a part of CH4, followed by reforming of the remaining CH4 with CO2 and H2O. The two steps may be formally separated into a process known as autothermal reforming.400 As a complement to these three methods, the water gas shift reaction: CO + H 2O ↔ CO2 + H 2 ,

(“ox”), reducing (“red”), and oxidizing then reducing (“oxred”) treatments on the Ir−CeO2 catalytic performances in methane reforming. Interestingly, the activity of Ir−CeO2 oxidation followed by reduction cycle is higher as compared to other treatments. The results of high-resolution TEM analysis indicate no structural transformation before and after the catalytic run (Figure 59). Ir particles are dispersed in the size range 1−3 nm on a ceria surface (main image, Figure 59A) or at the grain boundaries (inset). Ceria supports exhibit a layered structure with defects (round holes). After exposure to the severe conditions employed in the reforming reaction, ceria exhibits atomic-scale morphology similar to that immediately after the oxidation−reduction treatment, and faceted Ir nanoparticles with essentially unchanged size are still visible (Figure 59B). Figure 59C and D shows TEM images of the Ir/ CeO2 catalyst prepared by the impregnation method, before and after methane reforming, respectively. The fresh sample shows the presence of many Ir particles of 1−3 nm. After a 15 h catalytic run, many particles became larger, which caused significant deactivation of the catalyst. The authors suggested that combustion synthesis and the heat treatment procedure utilized resulted in strong anchoring of the Ir particles to the ceria support and made the catalysts thermally stable over long reaction times.

ΔH o RT = − 41 kJ/mol (R22) 404−406

may be used to adjust the H2/CO ratio in synthesis gas. The synthesis gas is the primary source for production of many chemicals, such as methanol and formaldehyde.400 One of the most widely used technologies for the methane conversion process is the gas-to-liquids process, which involves conversion of methane into synthesis gas in the first step followed by conversion of syngas to liquid hydrocarbons in the second step via the Fischer−Tropsch process.401,407,408 A typical direct method for methane conversion is oxidative coupling, which circumvents the expensive syngas step.409,410 This method should have a distinct economic advantage over indirect methods, but the product yields are generally small. In this pathway, CH4 and O2 react over a catalyst at elevated temperatures. Methyl radicals, which are formed at the surface of the catalyst, enter the gas phase where they couple to form ethane C2H6 as a primary product, which yields C2H4 as a secondary product. Unfortunately, both CH4 and C2H4 may be 14534


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optimized Sr/Al ratio, the catalysts show 18% methane conversion and 12% yield of C2 hydrocarbons with an ethylene/ethane ratio of 7. Significantly higher yield of C2 hydrocarbons (25%) was reported for Na2WO4−Mn2O3−SiO2 at 750 °C, which is among the highest obtained to date for any catalyst.419,420 Although higher C2 yields of 29% were reported using a catalyst prepared by other methods, however, they deactivated within a few hours on-stream.419 Combustionsynthesized Na2WO4−Mn2O3−SiO2 was tested for 48 h, and showed no deactivation and no change in structure of spent catalyst.409 Some modification of the SCS-derived catalysts by adding small amount of La increased410 the C2 yield to 27% (Figure 60A), along with a C2H4/C2H6 ratio of 3.6 (Figure 60B). These catalysts were prepared using tetraethoxysilane (denoted as NWMS1) as the silica precursor. Two alternative SiO 2 precursors, aminopropyl silsesquioxane oligomer (NWMS2), and dimethylsiloxane-ethyl oxide block copolymer (NWMS3), were also used to prepare catalysts. Figure 60 indicates that silica precursor has a significant effect on C2 yield and C2H4/C2H6. SCS catalysts were also efficiently used in processing of hydrocarbons other than methane.41,336 For example, authothermal reforming of JP-8 fuel that consists of dodecane (34.7%), 1-decene (32.6%), methylcyclohexane (16.7%), and tert-butyl benzene (16.0%) was studied using cerium- or nickeldoped LaFeO3 catalysts.336 Ni-based catalysts provided higher conversion (95.2−98.5 mol %) of fuel toward hydrogen and CO2 (CO) as compared to cerium-based catalysts (91.8−94.5 mol %). However, cerium-based catalysts showed somewhat higher selectivity for hydrogen. The optimized LaFe0.6Ni0.4O3 catalysts exhibited no deactivation during a 12 h run (Figure 61). The authors also noted that, with the exception of methane, effluent gas concentrations approached the calculated thermodynamic values (horizontal lines in Figure 61). The nitrogen in Figure 61 comes from air used in feed, rather than oxygen. They also indicated that carbon content in nickel-based spent catalyst was slightly higher than that in ceria-based ones. 5.2.3. Reforming Reactions of Alcohol toward Hydrogen. Renewable fuels, such as hydrogen, ethanol, and biodiesel, are currently being exploited to promote sustainable development and reduce dependence on fossil fuels. Hydrogen is an attractive alternative fuel, and can be utilized in different applications. Renewable biomass is a promising source for

Figure 59. TEM images of Ir−CeO2 SCS catalysts before (A) and after (B) catalytic run, as well as Ir/CeO2 catalysts prepared by impregnation method before (C) and after run (D). The Ir nanoparticles are indicated by circles in (A). Inset: Atomic-scale resolution image of an Ir nanoparticle (same scale as main image). Reprinted with permission from ref 414. Copyright 2015 Elsevier.

Gayen and coauthors also reported a similar strong interaction between the metal nanoparticles and support in combustion-synthesized Pt/Ce0.56Zr0.44O2 catalysts.405 Moreover, in this case, Pt nanoparticles and the support maintained an epitaxial relationship. Such strong interactions in catalysts showed superior activity in water gas shift reactions. Rh−CeO2 and Pt−CeO2 catalysts were also reported to show high activity in the water gas shift reaction.406,417 In these catalysts, the metals were predominantly dispersed in the ionic state on the surface of ceria. Ghose et al. synthesized highly active monometallic and/or complex metal oxide catalysts (La2O3, Sr−Al, La−R−Al complex oxide, Na2WO4−Mn2O3−SiO2) for oxidative coupling of methane.409,410 For the Sr−Al series, it was shown that conversion of methane, yield of ethylene and ethane, as well as their ratio depend on the Sr/Al ratio in the catalysts. At the

Figure 60. Effect of using Si precursors on 5%La−10%Na2WO4−5%Mn/SiO2 for (A) C2 yield as a function of temperature, and (B) ethylene/ ethane ratio as a function of temperature. Reprinted with permission from ref 410. Copyright 2014 Elsevier. 14535


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Al2O3 and CuO−ZrO2−Al2O3 catalysts for methanol oxidative reforming with similar catalytic activity at 150−300 °C and hydrogen selectivity of 50%.428 In addition, they noted superior stability of catalysts over 60 h time on stream at 250 °C. Roy et al. investigated aqueous phase reforming of ethanol over the Ni−Al2O3201,429,430 and Ni−CeO2431 catalysts. They used combustion synthesis to prepare Al2O3, followed by NiO deposition using the wet impregnation method.430 The activity of the catalysts with different Ni loading was compared to catalysts derived from sol−gel synthesis approaches at different temperatures, pressures, flow rates, and feed composition. These two catalysts showed different performances due to differences in the metal particle size distribution governed by the preparation method. Combustion-synthesized Al2 O 3 showed better metal dispersion and superior ethanol conversion H2 and CO2. Sol−gel-derived catalysts form CO at higher temperatures. Detailed structural analysis revealed that the Ni particles on sol−gel derived alumina were subjected to sintering during heat treatment and catalytic reaction.201 Particle size growth for the SCS catalysts was less severe under similar conditions. In addition, the SCS-derived support contained a mesoporous structure with a higher pore volume. XPS analysis of the spent catalysts demonstrated less coke formation and less bulk spinel formation on the SCS sample as compared to sol−gel prepared catalysts. It is interesting that sol−gel Ni−CeO2 samples showed higher activity and selectivity for H2 and CO2 as compared to the SCS samples.431 Instead of using supported metal catalysts, Kumar et al. employed SCS-derived metal-based (Ni, Fe, and Cu) bulk catalysts for ethanol partial oxidative reforming and decomposition reactions.105,337,432 Pure bulk Ni was the most active and selective for hydrogen and methane at lower temperatures,337 Cu was selective for acetaldehyde, and Fe was selective for CO2 and ethane. Hydrogen selectivity was found to be highest for Fe at high temperatures. They also mixed metal nitrates to produce bi- and trimetallic catalysts. The Ni, Cu, and Fe metals, when present together, exhibit a synergistic effect to form a more reducible Ni1Fe0.5Cu1 catalyst than the individual metal oxides. The trimetallic catalysts contain NiCu and NiFe2O4 spinel phases.105 Cross et al. used a modified SCS process to prepare highly dispersed Ni nanoparticles on γ-SiO221 or Al2O3338 supports. Catalytic runs indicated that both γ-Al2O3 support and Ni-γAl2O3 were active in ethanol decomposition from 150 to 300 °C. However, alumina support was not selective to hydrogen, producing water and ethylene, while nickel loaded alumina catalysts showed high hydrogen selectivity. Ni-SiO2 catalysts exhibit high activity during ethanol decomposition toward hydrogen at low temperatures (200 °C) and excellent stability toward deactivation with essentially no change of catalyst activity over 100 h of operation.21 Nickel-based combustion-synthesized catalysts were also tested in catalytic reforming of glycerol toward hydrogen.433,434 Manfro and coauthors prepared Ni−CeO2 catalysts using combustion of nickel nitrate, cerium nitrates, and urea solutions followed by hydrogen reduction.433 In addition, wet impregnation and coprecipitation methodologies were also employed to produce Ni−CeO2 catalysts. Using a solution of 1% glycerol, a maximum glycerol conversion of 30% was achieved by the combustion-derived catalysts at 270 °C, while only up to 25% was achieved by the other catalysts. In the gas phase, the molar fraction of H2 was always higher than 70%, and the formation of CH4 was lower than 1%. SCS prepared complex oxides of

Figure 61. Effluent gas concentrations as a function of time for LaFe0.4Ni0.6O3 catalyst at 775 °C, 0.1 MPa. Reprinted with permission from ref 336. Copyright 2006 Elsevier.

hydrogen production due to its environmental benefits because the produced CO2 is consumed during biomass growth providing a carbon neutral cycle. Catalytic conversion of biomass-derived feedstocks, for example, glucose, sorbitol, glycerol, methanol, and ethanol to hydrogen using reforming processes, has become increasingly important.421−425 Typical routes for hydrogen production from alcohols include steam and partial oxidative reforming, as well as decomposition reactions.423,426 In these catalytic reactions, supported metal catalysts are used typically. The stability of catalysts, however, is among the most important challenges for hydrogen production from alcohol reforming reactions.426 Catalyst deactivation is generally attributed to the deposition of carbonaceous species, as well as sintering and/or oxidation of metallic nanoparticles. Recent advances in SCS address many challenging issues in the design and development of novel catalysts that can exhibit high catalytic activity at low temperatures, and prevent undesirable deactivation processes. Kumar109,111 et al. and Schuyten427 et al. studied oxidative reforming of methanol over combustion-synthesized Cu−Zn− Zr−Pd complex oxide catalysts. They showed that Cu−Zn−Zr catalysts synthesized with glycine-rich solutions have low activity because of low surface area, high carbon content, and larger crystallite sizes. Adding palladium to the catalyst significantly enhanced the overall catalytic activity. The combustion of reactive solutions impregnated on high surface area ZrO2 and Al2O3 showed promising results with methanol conversion over 90% at 235 °C for the ZrO2 support and at 300 °C for the Al2O3 support. They also used impregnated cellulose paper combustion to prepare Zu−Cu−Zr oxide catalysts first, and then the products were impregnated with Pd nitrate and glycine solutions. Deposition of Pd on the Zu−Cu−Zr oxide was also done by combustion. These catalysts were achieved for methanol oxidative reforming even at room temperature.111 However, hydrogen was formed at 150−300 °C, and complete conversion of methanol was achieved at 300 °C. An increase of Pd in catalysts above 1 wt % promotes methanol oxidation to carbon dioxide and water, and decreases hydrogen selectivity. Baneshi and coauthors also reported on the synthesis of CuO−CeO2− 14536


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general formula La1−xCexNiO3, with different Ce content, were also tested as precursors of Ni-supported catalysts for steam reforming of glycerol at 500 °C.434 The precursors led to active catalysts after reduction in hydrogen-containing gas streams. However, deactivation was seen to occur irrespective of catalyst composition. Substitution of 50% La by Ce provided more deactivation-resistant catalysts, allowing only minor amounts of carbon deposits. 5.2.4. Catalysts for Liquid-Phase Organic Reactions. Solution combustion-synthesized catalysts were traditionally utilized in gas-phase heterogeneous catalytic reactions. Recently, these catalysts were also applied in liquid-phase organic synthesis reactions such as condensation,348 dehydrogenation,435 oxidation,157,326 and Suzuki coupling436 reactions. For example, copper-substituted CeO2 prepared for CO oxidation was used in direct oxidation of benzene to phenol.437,438 The ionic doped nature of Cu in combustionderived Cu0.10Ce0.90O2−δ catalysts provided superior activity in direct oxidation of benzene to phenol (∼43% conversion and ∼100% selectivity) with H2O2 as oxidant at 70 °C under atmospheric pressure in acetonitrile. In addition, copper leaching during the catalytic run over combustion-derived Cu0.10Ce0.90O2−δ is lower as compared to catalysts prepared by other methods. Several bimetallic ion substituted ceria, Pd0.01Ru0.01Ce0.98O2−δ, catalysts were tested for hydrogenation of para-chloronitrobenzene at ambient conditions.349 These catalysts showed much higher activity and selectivity toward para-chloroaniline than the monometallic ion-substituted analogue Pd0.02Ce0.98O2−δ, while Ru0.02Ce0.98O2−δ was inactive. Nanosized SrFeO3−δ (SFO) was produced using solution combustion containing citric acid, oxalic acid, and glycine as fuels with corresponding metal nitrates as precursors.350 The use of fuel mixtures significantly affects the crystallite size of the resultant compound. The average particle size of the samples prepared from single fuels was ∼45 nm, whereas for samples obtained from fuel mixtures a size of ∼25 nm was obtained. Fine nanostructured products showed higher catalytic activity in the catalytic reduction of nitrobenzene to azoxybenzene (Figure 62). Combustion-synthesized ZrO2-based catalysts showed enhanced catalytic activity in condensation reactions for production of coumarin and its derivatives. For example, WO3−ZrO2 and MoO3−ZrO2 were prepared by placing reactive solutions containing zirconyl nitrate, ammonium molybdate, or tungstate, along with fuel, in a preheated furnace.439,440 It was shown that MoO3 promotes the formation of a stabilized tetragonal phase of ZrO2, while in the WO3− ZrO2 catalyst, both tetragonal and monoclinic phases were observed. These composites were found to be efficient catalysts for the condensation of phenols with ethylacetoacetate under solvent-free conditions and with microwave irradiation. A variety of structurally diverse coumarins were synthesized with excellent yield and purity of the product using the nanocomposites as catalysts. Yadav et al. have reported on the synthesis of high sulfurcontaining tetragonal ZrO2 catalysts by the solution combustion approach.348 The authors noted that such high sulfur content with preservation of the tetragonal phase has not been reported before. Preparation of these catalysts involves a stage of solution combustion synthesis followed by treatment of products with chlorosulfonic acid. As-prepared zirconia has a mesoporous structure with narrow pore size distribution, making it a shape-selective catalyst. The authors found a direct

Figure 62. Activity profiles of the SrFeO3−δ catalysts prepared with single or mixed fuels for catalytic reduction of nitrobenzene to azoxybenzene. SFO-1, citric acid; SFO-2, oxalic acid; SFO-3, glycine; SFO-4, oxalic acid + citric acid; SFO-5, citric acid + glycine; SFO-6, oxalic acid + glycine; SFO-7, oxalic acid + citric acid + glycine. Reprinted with permission from ref 350. Copyright 2014 American Chemical Society.

correlation between catalytic performance of zirconia and fuel/ oxidizer ratio used during synthesis. Fuel-lean sulfated zirconia was shown to be more active than fuel-rich sulfated zirconia. Moreover, fuel-lean sulfated zirconia catalysts were even more stable and active than sulfuric acid treated commercial catalysts. The main advantages of the as-prepared ZrO2-based catalyst are simple experimentation, solvent-free conditions to produce coumarin with high yield and purity, short reaction times, and catalyst reusability. Several studies have been reported using Co-based catalysts for liquid-phase degradation of phenol.441,442 This reaction is important for mineralization of industrial wastewaters. Both bulk and supported Co3O4 catalysts showed complete phenol degradation (oxidation) in the presence of dissolved inorganic oxidants (e.g., oxone). The activity of the supported (SiO2, TiO2, Al2O3) catalysts exhibited better performance than that of unsupported cobalt oxide due to the high dispersion on the support, presence of basic surface centers, and Co2+ species. The catalytic degradation of phenol on supported Co catalysts followed the order Co3O4−TiO2 > Co3O4−SiO2 > Co3O4− Al2O3. The Co3O4−SiO2 catalysts, however, exhibited better stability than the other two catalysts. The deactivation was mainly attributed to leaching of cobalt and changes in surface composition of catalysts. The same group recently proposed an innovative combustion-based approach for nitrogen doped reduced graphene oxide preparation as metal-free catalyst for oxidation of phenol by peroxymonosulfate as an oxidant.326 In this process, the graphene oxide precursor is blended with ammonium nitrate and introduced into the preheated furnace. The surface area and nitrogen content in the products were tuned by changing the ignition temperature. A combination of high surface area (above 100 m2/g) and presence of nitrogen dopant (5−7 at. %) makes this material a green and efficient 14537


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commercial TiO2 (e.g., Degussa P-25) used in solar-related applications is only 50−80 m2/g. By utilizing thermal desorption and thermogravimetric analyses, infrared spectroscopy and nuclear magnetic resonance, as well as surface acidity measurements, it was concluded that SCS anatase (prepared with glycine) has a large amount of surface hydroxide groups.448 In addition, this nanopowder exhibited two optical absorption thresholds at 562 and 468 nm that correspond to the band gap energies of 2.21 and 2.65 eV, respectively, while the commercial TiO2 showed a single absorption edge at 395 nm (3.14 eV).449 On the basis of XPS studies and elemental analysis, Nagaveni et al. suggested448 that the lower band gap energies of anatase can be attributed to the carbide ion substitution in the form of TiO2−2xCx[VO2••]x, where [VO2••]x is the oxide ion vacancy created for charge balance. Anatase prepared using hexamethylenetetramine and oxalyldihydrazide also showed a slight red shift in the UV absorption as compared to commercial anatase.448 Most likely, the amount of carbide ion in these materials was lower than that with glycine combustion. Because of a significantly lower band gap and unique structural and textural characteristics, combustion-synthesized TiO2 was employed in photocatalytic degradation of a number of organic compounds,448−454 inactivation of microorganisms,455 and photocatalytic reduction of metal ions456 using solar light and UV radiation. It was shown that photoactivity of the combustion-synthesized titania was always significantly higher than that of the commercial TiO2 for both UV and solar exposure. For example, Figure 63 shows that during the

catalyst. Electron paramagnetic resonance studies and quenching tests indicated that the catalyst activates peroxymonosulfate to produce both hydroxyl and sulfate radicals, which induce extensive phenol degradation. 5.3. Semiconductors and Optical Materials

Oxide semiconductors are eminently attractive candidates for solar photovoltaic, solar water splitting, and photocatalytic remediation and other applications.443−445 Nanostructured inorganic materials for these applications are typically required to have a large specific surface area and appropriate surface chemical composition. However, for every specific application, requirements to those semiconductors may be different. For example, in dye-sensitized solar cells and quantum dotsensitized solar cells, semiconductors should possess excellent charge mobility and long lifetime, and possibly have some light scattering or photon trapping capability as well. Perfect crystallinity and minimal surface and bulk defects are desired, and the grain boundaries connecting individual nanostructures should be controlled to be as low as possible. In photocatalytic applications, however, surface defects may be an advantage, and they may serve as active centers. The active semiconductor material should have an optimal combination of optical (bandgap energy, Eg) and electronic properties. The effectiveness of such solar-driven processes is dictated to a large extent by the semiconductor’s capability to absorb visible and infrared light, as well as its ability to suppress the rapid combination of photogenerated electrons and holes. Doping of semiconducting materials is the most important approach to tailor their light adsorbing (or emitting) characteristics. In this section, we outline progress in combustion-derived semiconducting oxides and phosphors. 5.3.1. Semiconductors. 5.3.1.1. Titanium Oxide. Nanoscale TiO2 is one of the most promising materials for solar energy utilization. However, the photoreaction efficiency of TiO2 is severely limited by its large intrinsic band gap (>3 eV), capable of absorbing only the ultraviolet portion of the solar spectrum.443 A crucial prerequisite for enhancement of the solar energy conversion efficiency is to enable the absorption of TiO2 for the more abundant visible light by reducing its band gap. To increase the limited optical absorption of TiO2 under sunlight, there have been persistent efforts to vary the chemical composition of TiO2 using metal or nonmetal impurities that generate donor or acceptor states.443,444 Through doping, the solar absorption characteristics of TiO2 have been improved to some extent. For example, when nonmetallic light-element (e.g., nitrogen) dopants are introduced, the optical absorption of TiO2 can be modified as the result of electronic transitions from the dopant 2p or 3p orbitals to the Ti 3d orbitals.446,447 The solution combustion method provides the nanocrystalline anatase phase TiO2 in a single step process. A typical synthesis of anatase involves rapid heating of a stoichiometric aqueous solution of TiO(NO3)2 and fuels such as glycine, hexamethylenetetramine, or oxalyldihydrazide.448−452 During combustion, the temperature reaches about 1000 °C for several seconds, ensuring formation of highly crystalline product. Because of the short synthesis duration, both TiO2 growth and the phase transition to rutile are hindered. The evolution of a large amount of gases during the process helps to dissipate the heat and inhibit sintering of the particles, resulting in a large surface area of the product. Surface areas measured for combustion-synthesized TiO2 vary from 140 to 250 m2/g, depending on the fuel used,448 while the surface area of

Figure 63. Degradation profiles of pentachlorophenol under UV exposure in the presence of combustion-synthesized TiO2 (open symbols) and Degussa P-25 (solid symbols). Initial concentrations of pentachlorophenol were 11 ppm (squares), 7 ppm (circles), and 5 ppm (triangles). Reprinted with permission from ref 450. Copyright 2004 Elsevier.

degradation of pentachlorophenol, the concentration rapidly decreased with time in the presence of SCS-derived TiO2 without formation of any intermediate compounds.450 However, when organic dye was degraded in the presence of commercial anatase, the concentration decreased significantly slower (Figure 63). Sivalingam et al. also noted that some toxic 14538


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Dye degradation occurs very fast under direct sunlight. The authors speculated that charge carrier recombination was low for optimized nanomaterials due to high crystallinity and lower diffusion length due to the mesoporous structure. Assynthesized nitrogen-doped materials were also tested in dyesensitized solar cell performance.463 Mesoporous and electrically interconnected nanocrystalline TiO2−xNx particle agglomerates lead to significantly high, 6.9%, photovoltaic efficiency higher by about 47% as compared to that of commercial TiO2. A different sulfur doping strategy was also developed by Pany et al., who utilized titanium oxysulfate sulfuric acid complex (TiOSO4·H2SO4) as a precursor for titanium and sulfur dopant and sucrose as fuel.462 FTIR, EDS, and XPS studies confirmed the presence of both nitrogen and sulfur in the combustion products. The optimized materials with the highest surface area of ∼75 m2/g and lowest band gap energy (2.7 eV) showed excellent catalytic performance for phenol degradation under solar light. Sulfur and molybdenum codoped titanium oxide nanomaterials were prepared by Rajeshwar and de Tacconi by using titanyl sulfate as titanium sources, urea, and/or thiurea as fuels and MoCl5 as molybdenum source.464 The authors noted that SCS-derived products were highly colored (yellow or gray), evidence of a strong visible light absorption tendency. The optical bandgap energies of S and Mo codoped products showed a low energy optical transition Eg ≈ 1.17 eV with the main ∼3.0 eV feature. Detailed XPS studies of TiO2 synthesized using titanyl sulfate precursor showed both S4+ and S6+ cationic sulfur species in the combustion-synthesized product. This analysis also showed evidence of the presence of nitrogen, carbon, as well as molybdenum with oxidation states ranging from Mo4+ through Mo6+ in the products. Simultaneous incorporation of both Mo and S into TiO2 results in high photocatalytic activity in the reduction of Cr6+ under visible light excitation. The reference P-25 TiO2 sample showed negligible visible light absorption in this range. Thind et al. also reported a strategy of titanium oxide codoping with nitrogen and tungsten.465 Their results demonstrate that doped TiO2 nanomaterials have high surface areas (120−150 m2/g) and mesoporous structures with an average pore size of 5 nm. In addition, the codoping significantly narrowed the band gap (∼2.7 eV). It was also found that such doped nanomaterials exhibit high visible light activity in the photodegradation of organic dyes, which was attributed to the synergistic effect of the red shift in absorption combined with a high surface area. 5.3.1.2. Zinc Oxide. Zinc oxide is a wide band gap (3.37 eV) semiconductor. It has been investigated as a short-wavelength light-emitting, transparent, conducting, and piezoelectric material.466,467 ZnO nanoclusters and thin films also exhibit roomtemperature UV lasing properties.468 Combustion-synthesized ZnO-based nanostructured materials have been tested in photocatalytic oxidation of organic wastes,26,469−472 gas sensing, 139,473,474 and destruction of cancer cells and bacteria134,475 among other applications. Unlike SCS anatase, ZnO (with hexagonal wurtzite structure) has a relatively low surface area (typically 20−50 m2/g) and near spherical particles with sizes ranging from 30−350 nm.127,134,474,476,477 Lee et al. synthesized ZnO using zinc nitrate (or zinc hydroxide) as the zinc source and carbohydrazide (or glycine) as the fuel.476 When using zinc hydroxide, it was dissolved in nitric acid first. Pure phase ZnO powders could be easily

intermediates were observed when dyes degraded in the presence of commercial TiO2.450 In general, the photocatalytic degradation mechanism of organic compound can be presented as three sequential steps.448,450 The first step is absorption of light of a wavelength higher than the bandgap energy of TiO2, resulting in the transition of an electron from the valence band to the conduction band, leaving a hole behind. In the next step, the adsorbed water or hydroxide ions trapped by holes produce •OH radicals, which are known to be the most oxidizing species: e−CB + O2 → O•− 2

(R23)

− 2O•− 2 + 2H 2O → 2 • OH + 2OH + O2

(R24)

Electrons are trapped at the surface by the reaction with adsorbed molecular oxygen to produce superoxide anion radicals, which then form more •OH radicals. The reaction of these •OH radicals with the organic pollutants leads to oxidation of the compounds.

h+VB + 2OH−(ads) → OH− + •OH

(R26)

The surface hydroxyl groups accept holes generated by solar light and UV irradiation to form hydroxyl radicals and prevent electron−hole recombination. Therefore, high photocatalytic activity of combustion-synthesized TiO2 can be attributed to a greater number of hydroxyl groups, which yield a higher reaction rate. Several studies have shown that the photocatalytic activity of combustion-synthesized TiO2 can be tuned by changing the concentration of surface hydroxyl groups though the substitution (or impregnation) of metals.97,457−460 In this case, appropriate amounts of the metal salt solution were added to the precursor solutions using glycine as fuel. Metal substitutions were selected with oxidation states being either lower valent (Cu2+, Fe3+, Sm3+), isovalent (Ce4+, Zr4+, Pt4+, Mn4+), or higher valent (W6+, V5+) than the Ti4+ host. XRD, TEM, and Raman studies of these metal-substituted nanomaterials showed no significant variations in crystallinity, and crystal structure relative to the undoped TiO2. However, metal substitutions have a significant effect on the optical absorption characteristics of the TiO2 host. For example, Fe- and Cumodified TiO2 had absorption thresholds corresponding to bandgap energies of 2.18 and 2.65 eV, respectively.460 Although degradation rates of 4-nitrophenol with these catalysts were lower than that of undoped TiO2, they exhibited higher activity as compared to commercial TiO2 under both UV exposure and solar radiation. Sivaranjani et al.461 and Pany et al.462 reported doping of combustion-synthesized TiO2 by lighter elements such as nitrogen and sulfur. Sivaranjani et al. synthesized mesoporous TiO2−xNx nanomaterials by using urea as both fuel and nitrogen source. Depending on the fuel to oxidizer ratio, either phase pure anatase or a mixture of anatase with rutile was produced. In the optimized synthesis conditions, mesoporous anatase with a high surface area (235 m2/g) was produced with 6.7 at. % nitrogen. These materials exhibited high activity for rhodamine-B degradation, despite a high band gap of 3.24 eV. 14539


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obtained regardless of the starting materials and fuels. However, the particle sizes of ZnO varied greatly with the selected pair of oxidants and fuels, with more than a 4-fold variation reported. Using Zn(OH)2 and glycine, the synthesized ZnO powder showed an average grain size of 75 nm and surface area of ∼94 m2/g. The zinc nitrate and glycine pair yielded materials with an average particle size of 125 nm and a surface area of ∼27 m2/g. The largest particle size (320 nm) and lowest surface area ZnO were prepared by the zinc nitrate−carbohydrazide pair.476 Regardless of the synthesis conditions, the optical energy gap of undoped ZnO was estimated to be 3.1−3.3 eV, enabling excellent photocatalytic activity under UV light.127,470,476,477 Simultaneous doping of ZnO with different metal ions was employed to tailor the optical characteristics and gas sensing properties.469,473,478−482 As a result, the optical band gap energy of the doped ZnO phases was shown to be in the range 2.9−3.3 eV. Several controversial studies reported the possibility of nitrogen doping of ZnO. Mapa and Gopinath reported on the formation of orange-colored ZnO nanocrystals with bulk nitrogen concentrations up to 15% using combustion of zinc nitrate and urea solutions.472 They reported only a minor lattice contraction of product associated with nitrogen incorporation even with the highest nitrogen content. Raman spectra of the as-prepared materials showed bands at 384, 414, 441, and 586 cm −1, which were assigned to local vibration modes of nitrogen and used as evidence to support the nitrogen incorporation into the ZnO lattice. Recently, Söllradl et al., however, argued such interpretation of these Raman bands.483 They pointed out another report484,485 showing that these Raman bands can also exist in metal doped ZnO thin films grown in nitrogen-free conditions. On the other hand, Mapa and Gopinath bombarded the doped ZnO samples with O and C ions and showed the release of nitrogen species using secondary-ion mass spectroscopy.472 As counter-evidence, Söllradl et al. used the Prompt Gamma Activation Analysis method to determine bulk concentration of nitrogen in SCS-derived samples, thus reporting very low nitrogen content.483 In addition, Söllradl et al. detected a coexisting organic crystalline compound (isocyanuric acid or cyanuric acid) within a narrow range of fuel to oxidizer ratio, different from that Mapa and Gopinath used. On the basis of these findings, Söllradl et al. suggested that nitrogen is located on the surface of ZnO nanoparticles in the form of an organic phase, which can form during the decomposition of urea. In a follow-up study,486 Mapa et al. used a SCS codoping strategy to incorporate both indium and nitrogen into the ZnO lattice. They report a minor lattice expansion with a small change in the hexagonal wurtzite structure to 10% incorporation (Zn1−zInz)(O1−xNx) materials. They also compared the optical absorption spectra of (Zn1−zInz)(O1−xNx) and pure In2O3, ZnO, as well as fuel (urea)-rich ZnO samples without indium dopant (Figure 64A). Pure ZnO showed a well-defined absorption cutoff edge at 380 nm. The pale greenish yellow In2O3 shows a gradual decrease in absorption from 370 to 440 nm. A new visible absorption band was observed at ∼480 nm on ZnO1−xNx materials, while this band shifted ∼450 nm for (Zn1−zInz)(O1−xNx) materials (Figure 64B). In- and N-doping helped form shallow donor and acceptor levels from N 2p and In 5s/5p and close to the valence and conduction band, respectively, and effectively decreases the bandgap energy to about 1 eV (from 2.3 eV).

Figure 64. UV−visible absorption spectra of (Zn1−zInz)(O1−xNx) materials prepared with urea/(Zn+In) = 5 ratio (UZ) for (A) different In-content and (B) after calcination at different temperatures for UZ5In5, and In2O3 and ZnO. Color change from the materials is shown in the inset photograph. Reprinted with permission from ref 486. Copyright 2010 American Chemical Society.

Recently, a single-step SCS-based method was reported to incorporate ZnO in a porous boron−carbon−nitrogen framework.487 This hybrid material was prepared by heating B2O3, glycine, and zinc nitrate mixture at 500 °C. The incorporation of ZnO onto BCN matrix has extended the photoresponse of ZnO in the visible region. As-prepared material was used to prepare self-cleaning coatings over aluminum substrate. 5.3.1.3. Other Simple Oxide Semiconductors. As compared to ZnO, tungsten trioxides (WO3) are more suitable semiconducting compounds for utilization of a portion of solar spectrum with optical band gap energy of 2.75 eV. By using SCS, Morales et al. shifted the light adsorption response of WO3 further toward the visible range with estimated optical band gap energy of 2.5 eV.488 XPS data on the SCS-derived WO3 using three different fuels, glycine (G), urea (U), or thiourea (T), showed uptake of carbon (G), nitrogen (G, U, or T), and sulfur (T) by the oxide host. Methylene blue was used as a probe of the surface characteristics and photocatalytic attributes of the WO3 samples relative to the standard semiconducting samples. The adsorption and subsequent decomposition of this dye can be monitored via its visiblelight absorption signature (at λmax = 660 nm). Figure 65A shows color change, concentrations of this dye upon exposure to WO3 (commercial WO3 and TiO2), and kinetics of decomposition in the dark or under visible-light illumination. The data suggest that ∼75%, ∼85%, ∼95% of the initial dye was neutralized from the aqueous solution by adsorption on the WO3-G, WO3-U, and WO3-T surfaces after 30 min. A 14540


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Figure 66. TEM (A) and high-resolution TEM (B) images of a single nanowire of W18O49 + 0.5 wt % Fe3+. The inset of (A) shows the selected area electron diffraction pattern. Reprinted with permission from ref 138 Copyright 2015 Royal Society of Chemistry.

In addition to tungsten oxide, combustion synthesis of different nanostructured semiconducting oxides such as MoO3,490−493 CuO,494 SnO2,495 and In2O3496 has been reported in gas sensing and organic pollutant degradation applications. For example, Bedi and Singh synthesized CuO nanostructures by introducing cetyltrimethylammonium bromide as a cationic surfactant into the reactive sol containing copper nitrate and citric acid.494 They suggest that surfactant molecules separate the sol particle due to the electrostatic stabilization. As a result, spherical CuO nanocrystallites of size ∼20 nm and with a minimum degree of agglomeration were prepared. A sensor prepared using surfactant-assisted CuO exhibited enhanced ∼12-fold response toward ammonia at room temperature as compared to sensor without the addition of surfactant. 5.3.1.4. Complex Oxide Semiconductors. Combustion synthesis was successfully applied in developing novel complex semiconducting nanostructured materials for wastewater purification, photocatalytic degradation of organic compounds, as well as water splitting applications.497−499 The novel visiblelight driven photocatalyst, BiVO4, with monoclinic scheelitestructured and a narrow bandgap (Eg = 2.40 eV) has attracted considerable interest for its good visible light-induced photocatalytic properties.498,500,501 Spherical shaped monoclinic BiVO4 crystallites were first synthesized using solution combustion by Jiang et al.497 They applied both urea and citric acid as fuels, bismuth nitrate as oxidizer, and ammonium metavanadate as a vanadium source. The particle size of the BiVO4 was about 400−600 nm, while the solid-state reaction yielded coarse particles (1.5 μm). They also found that as the fuel to oxidizer ratio is increased, the degree of crystallinity of the BiVO4 crystallites increases. The nanomaterial prepared with a fuel to oxidizer ratio of 5 showed the highest photocatalytic activity during methylene blue degradation studies. Regardless of the fuel to oxidizer ratio, SCS-derived nanocrystals exhibit significantly higher catalytic activity than products of the solid-state reaction. The bandgap absorption edges of the BiVO4 crystallites prepared by the SCS method and the solid-state reaction method were determined to be 523 and 540 nm, corresponding to the bandgap energies of 2.45 and 2.40 eV, respectively. Those results suggest that the high catalytic activity of SCS-derived samples may be related to greater surface area. Later, the same group reported on the synthesis of CuO/ BiVO4500 and V2O5/BiVO4501 nanostructured materials with improved photocatalytic properties in dye degradation. Garciá

Figure 65. Plot showing the remaining methylene blue in solution equilibrated with 2 g/L of the respective four WO3 samples and TiO2 in the dark for 30 min (A). Photos of corresponding dye solutions are inserted for each sample. Subsequent photocatalytic decoloration under visible light for WO3 samples (B). WO3-B, WO3-G, WO3-U, and WO3-T donate as commercial (WO3-B) and combustionsynthesized sample using glycine (G), urea (U), and (T) thiourea fuels. Reprinted with permission from ref 488. Copyright 2008 American Chemical Society.

commercial WO3 sample, denoted as WO3-B, shows only a 16% decrease in dye concentration. A commercial TiO2 sample showed very little proclivity for dye adsorption even after 24 h. After 30 min in the dark, the kinetics of dye degradation under visible light are shown in Figure 65B. These results indicate that tungsten oxide samples produced using urea and thiourea showed the best performance. The authors suggested that such results may be explained by a higher surface area and the presence of nitrogen and/or sulfur in the products. Later, Singh and Madras prepared WO3−TiO2 nanocomposites by SCS to improve the photocatalytic activity of commercial TiO2.489 They showed that the optimal loading of WO3 was 15 wt % based on decolorization experiments with methylene blue and orange G dyes under both UV and visible radiation. Chen et al. reported on the synthesis of highly crystalline iron-doped W18O49 nanowires with diameters ranging from 50 to 250 nm and lengths of 5−10 μm.138 The photoluminescence spectra indicated that iron doping increases the emission intensity of materials due to the increase of defect concentration. The optimized 0.5 wt % Fe3+-doped W18O49 nanowires (Figure 66) exhibited excellent photocatalytic efficiency toward the degradation of organic compounds in aqueous media under UV−visible light irradiation. 14541


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Figure 67. TEM image (A) of Gd2O3:Eu nanophosphor prepared with a citric acid−metal nitrate (C/M) ratio of 0.7, as well as the emission spectrum (B) of Gd2O3:Eu with different C/M ratios and Y2O3:Eu commercial phosphor. Reprinted with permission from ref 534. Copyright 2010 IOP Publishing.

Pérez et al.499 and Zhang et al.502 further tailored the synthesis conditions and the microstructure of BiVO4-based materials to tune the visible light-induced catalytic performance of dye degradation and phenol mineralization. Nagabhushana et al. showed that combustion-synthesized BiVO4 exhibits high activity for hydrogen evolution from water under UV-light irradiation in the absence of either coupling oxides or doping metals.498 The yield of hydrogen generated was ∼490 mmol per 2.5 h of reaction. The ultralight yellow crystalline combustion-derived nanopowder also exhibited a porous morphology with strong absorption in the visible light region. The powder showed highly visible photocatalytic activity toward methylene blue degradation under sunlight irradiation. Many different types of novel semiconducting complex oxides, such as LaFeO3,503 ZrV2O7,504 ZrMo2O8,505 ZnOFe2O3,506 Cd2SnO4,507 CuNb2O6,508 and ZnNb2O6508 nanostructures, were also prepared and characterized for different applications. A family of novel metal tungstate compounds, including Bi2WO4, Ag2WO4, CuWO4, ZnWO4, and BiAgW2O8, has been successfully synthesized using solutions of corresponding metal nitrates, Na2WO4 (or ammonium tungstate), and fuels (glycine or urea).509−511 Thomas et al. indicated that phase pure monophasic tungstates can be formed using Na2WO4 as a precursor, whereas biphasic products are observed for samples using (NH4)2WO4.511 Photocatalytic studies of all materials toward methyl orange degradation showed superior activity against benchmark obtained by solidstate synthesis. The authors noted that crystalline tungstate nanomaterials can be used even without additional heat treatment. 5.3.2. Optical Materials. Inorganic phosphors are luminescent materials that emit photons upon excitation by an electron or X-ray beam and light. Phosphors consist of an inert host lattice, and an activator, typically a 3d or 4f electron metal such as Eu2+/Eu3+, Ce3+, Tb3+, Gd3+, Yb3+, Dy3+, Sm3+, etc. 174,512−515 The process of luminescence occurs by absorption of energy at the activator site, relaxation, subsequent emission of a photon, and then a return to the ground state. The phosphors can be classified into broad band (d−f electronic transition) and narrow band (transition between the f levels) emitters. The absorption of energy takes place by either the host lattice or the rare earth ions, while the emission of energy originates mostly from the dopant ions. A second

type of dopant, often called sensitizers, can also be used to absorb the energy and transfer it to the main dopant. These luminescent materials are important for many applications including sensors and radiation detectors,516,517 fluorescent lamps,518,519 light-emitting diodes (LEDs),514,520−524 displays for computer and mobile phones,512,525,526 as well as in medical imaging and labeling applications.174,527,528 The most common methods for the preparation of inorganic phosphors involve solid-state reactions, requiring high temperatures and long reaction times, which result in micrometer-size crystals with a low level of structural homogeneity.174,512,519 Nanostructured phosphors may have higher packing densities, low light-scattering effects and sintering temperatures, and can be easily suspended in the liquid phase for biomedical applications. Nanosized phosphors, however, have a large number of defects due to the larger surface area, which decreases the emitting energy. SCS is one of the main synthesis methods for inorganic phosphors174,175,518 such as multielement complex oxides, with a high degree of dopant structural uniformity. In fact, these materials are among the most active current directions of SCS. The high luminescence emission intensity is correlated with the measured flame temperature, illustrating the importance of well-crystalline nanostructured phosphors. In some cases, SCS does not require further calcination and repeated heating. In most cases, however, short (1−5 h) postsynthesis hightemperature (600−1000 °C) annealing is used to improve the crystallinity of phosphors. Combustion synthesis was used for the preparation of many types of doped simple oxides and complex compounds, such as aluminides, silicates, borates, phosphates, and titanates. In this section, we consider recent developments in combustion-derived nanophosphors and their use in light-emitting devices applications. 5.3.2.1. Simple Oxides. Gd2O3-based nanophosphors with different rare earth dopants have been synthesized by SCS.529−533 Different complexing agents/fuels such as glycine, citric acid, and urea were utilized in these synthesis formulations. A monoclinic phase or a mixture of the cubic and monoclinic phases of Gd2O3 are typical products.529,532 It is known that Gd2O3 crystallizes into a cubic phase below 1200 °C or in a monoclinic phase at a higher temperature. The cubic phase of Gd2O3 is a more suitable host for rare-earth doping.534 Jacobsohn and coauthors showed that glycine-assisted combustion synthesis results in predominantly based-centered 14542


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Figure 68. Confocal images of HeLa cells after (A) 4 h, (B) 10 h, and (C) 24 h incubation of polyvinylpyrrolidone-coated doped Cd2O3 phosphors. The nanoparticles were excited with a 980 nm femtosecond laser (observed as red spots, insets). The autofluorescence of HeLa cells was excited with a wavelength of 488 nm argon laser (observed as green regions). The signal was collected in the range from 508 to 585 nm. Reprinted with permission from ref 538. Copyright 2015 Royal Society of Chemistry.

monoclinic Gd2O3 doped with 5 at. % of Eu.535 They indicated that monoclinic phase progressively transformed into bodycentered cubic phase during annealing at 1000 °C in air for ∼150 h. Moreover, luminescent behavior was significantly changed by annealing of combustion product, and an inversion of the main emission line from ∼621 to 609 nm was reported. Xia et al. reported on the synthesis of Gd2O3:Eu phosphors using citric acid as fuel.534 It was shown that at the citric acid− metal nitrate (C/M) ratio of 0.5, the product consists of both monoclinic and cubic phases, whereas solution with φ = 0.5 or 0.9 enabled synthesis of a highly crystalline cubic phase. The optimized product showed nearly spherical nanoparticles of Gd2O3:Eu in the range 20−40 nm (Figure 67A) and high luminescent intensity comparable with a commercial Y2O3:Eu phosphor (Figure 67B), even without postsynthesis annealing. Kumar and co-workers reported on the synthesis of cubic Gd2O3:Eu nanophosphors using citric acid as fuel with φ = 2.536 However, they used post-synthesis heat treatment at 800 °C to achieve high crystallinity. It was shown that an increase in Eu3+ content leads to a red shift in the charge transfer band attributed to the increase in covalency and Eu−O bond length. Moreover, at a critical content of dopant quenching, luminescent intensity starts to decrease, due to the increase of nonradiative energy transition. Experimental results of timeresolved decay and theoretical calculation suggested that the dipole−dipole interaction may be the major mechanism in concentration quenching of Eu3+ ion in oxide host lattice. SCS-derived Gd2O3-based phosphors with Er3+ or Yb3+ dopants were considered as up-converting nanoparticles, as a new generation of magnetic probes for magnetic resonance imaging (MRI).537,538 Up-conversion of photons is a process in which the absorption of two or more photons leads to the emission of light at shorter wavelength than the excitation wavelength.527,528 Kumar et al. showed that the luminescent properties of Gd 2 O 3 -based phosphors depend on Er 3+ concentration. The emitted color was tuned by determining the CIE (Commission internationale de l’éclairage) coordinates for different concentrations of Er3+ ion. Kaminska et al. prepared Cd2O3 nanoparticles doped with 1%Er3+ and 18%Yt3 as potential material for MRI imaging of cancer cells.538 They noted that optimal optical up-conversion quantum yield of materials can be achieved by codoping with

zinc ions using a microwave-assisted SCS. The doped Gd2O3 phosphors were coated with polyvinylpyrrolidone and introduced into HeLa cancer cells and then incubated for 4, 10, and 24 h. Next, the cells were fixed and exposed to a femtosecond pulse of near-infrared radiation. Confocal microscope images of incubated cells are shown in Figure 68. The increase of incubation time enhanced the number of phosphor nanoparticles in the cytoplasm or on the cell membrane. Photoluminescence (PL) of nanoparticles indicated the presence of electronic transition for Er3+ ions at ∼550 nm and ∼660 nm. These results show that nanoparticles are not toxic and can be used as labels for bioimaging and biodetection. Similar to Gd2O3-based materials, SCS was successfully utilized to produce Y2O3-based up-converting nanophosphors with both rare earth dopants and a sensitizing additive ion.539−541 For example, Mishra et al. prepared Y2O3:Yb3+ green light-emitting phosphors with Li+ sensitizer.540 Rai and co-workers reported on the SCS of Y2O3:Er3+−Eu3+−Yb3+ upconverting codoped phosphor.541 These materials with cubic crystal symmetry were excited by a laser with 980 nm wavelength. It was shown that energy transfer and cross relaxation between Er3+ and Eu3+ ions were responsible for increases of the luminescent efficiency, while codoping of Yb3+ enhances intensity of emission. Pure and doped Y2O3 prepared by SCS were also tested for thermo-luminescent as well as ionizing radiation detection applications.542−544 It was shown that the luminescent properties of cubic Y2O3 can be changed by use of different fuels during combustion synthesis. For instance, ethylene diamine tetracetic acid (EDTA) and its disodium derivative (Na2-EDTA) both can be used as efficient fuels for producing Y2O3. However, photoluminescent intensities of Na2-EDTAderived materials are much higher, while EDTA aids in the formation of Y2O3 with better thermoluminescent features. Several studies have recently reported on the use of another unconventional fuel, extracts of aloe vera plant, to produce Y2O3:Eu3+ and Y2O3:Dy3+ phosphors.545,546 Mukherjee and coauthors reported on the use of different amino acids as a fuel for preparation of Y2O3:(Eu, Dy, Tb).547 These authors inferred that the codoped product of the reaction with glycine as the fuel has a smoother surface, higher crystallinity, and better luminescent properties. 14543


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chromaticity diagram shows, however, that all materials have orange-red color coordinates (inset in Figure 69). The authors concluded that poly(vinyl alcohol) serves as the soft template, limiting the agglomeration, and produces fine spherical CeO2:Sm3+ nanophosphors. The suitable excitation wavelength (370 nm), regular shape, and intense luminescence make this material a superior candidate for the preparation of ultraviolet LEDs. Shi and coauthors prepared and characterized another ceriabased nanophosphor by SCS using urea as fuel.550 The effect of Eu dopant concentration in CeO 2 :Eu phosphors was investigated using photo- and X-ray luminescence. The nanophosphors were excited with UV (360 nm) and blue lights (466 nm). At low Eu3+ concentrations, the sample can be effectively excited with 360 nm. With an increase in the Eu3+ concentration from 2 to 8 mol %, the dominant excitation wavelength is changed to 466 nm. The quenching concentration of Eu3+ can be moved to 16 mol %. The authors suggested that this effect can be related to the interface effects of the nanophosphors. X-ray luminescence of produced phosphor materials showed intense red-light emission, indicating the potential of these materials to be used in radiation detection applications. Recently, many studies were reported for successful SCS and characterization of novel light-emitting phosphors with less common host simple oxide lattices such as MgO,551−554 Al2O3,555 and ZrO2.178,556 These new materials such as MgO:Fe3+ and ZrO2:Tb3+ showed tailored light emission characteristics, and also photocatalytic activity in some reactions due to decreased band gap and capability of reducing electron hole pair recombination.178,554 5.3.2.2. Complex Oxides. 5.3.2.2.1. Garnets and Aluminates. 5.3.2.2.1.1. Garnets. Double oxides of Y2O3 or Ga2O3 with Al2O3 or Ga2O3 belong to the garnet group with general formula of A3B5O12 cubic crystal structure. In the crystal lattice of Y3Ga5O12, Y3Al5O12, and Gd3Ga5O12, partial substitution of Y3+ or Gd3+ with Ce3+, Dy3+, Pr3+, Tm3+,Yb3+, Tb3+ forms a large family of optically active materials. Yttrium aluminum garnet activated with trivalent cerium (Ye3‑xAl5O12:Ce3+) is used in solid-state lighting devices due to its efficient yellow luminescence located at ∼550 nm.557,558 When excited by a shorter wavelength light from blue LEDs, yellow secondary light emitted garnet phosphor can complement the blue emission to yield white light.559,560 This phosphor was successfully prepared by Pan et al. utilizing SCS with urea as fuel and metal nitrates as oxidizer.557 It was shown that the emission band position could be changed from 532 to 563 nm by partial or full substitution of Y3+ by Gd3+. Pan and co-workers also revealed that SCS is the most efficient method for production of Ye3−xAl5O12:Ce3+ as compared to coprecipitation sol−gel and solid-state synthesis routes.558 They noted that solid-state reaction and SCS methods were preferred for the synthesis of materials with high luminescence intensity, although large irregular particles were obtained. However, the duration of solid-state reaction methods is much longer than that of SCS. Also, the solid-state reaction should be performed under continuous flow of CO gas to avoid formation of Ce4+ in the phosphor crystal lattice. Particles from coprecipitation and sol−gel methods were spherical with small size, but had severe aggregations and lower luminescence intensity. Zhang et al. proposed560 citric acid as fuel for SCS of Ye3−xAl5O12:Ce3+. The results of detailed parametric studies

Several doped simple oxide phosphors have been synthesized by polymer-assisted SCS.548,549 Krsmanovic et al. synthesized Lu2O3 doped with 3 at. % of Sm3+ and Tb3+ with polyethylene glycol as fuel.548 In their formulations, the metal nitrate− polyethylene glycol solutions were continuously stirred at 80 °C for a few hours to produce a polymer-based solid complex, which was combusted at 800 °C in a furnace and was sintered at the same temperature for 2 h. Microscopy and X-ray energy dispersive mapping confirmed that samarium and terbium ions are uniformly distributed in the lutetium oxide solid solution. The material doped with Sm3+ exhibited characteristic reddishorange emission, while Tb3+ ions resulted in a combination of blue, green, and red emission. Wu and co-workers have prepared Sm3+ activated CeO2 phosphors by poly(vinyl alcohol) assisted combustion reaction.549 In this process, the authors dissolved Sm2O3 in nitric acid to form a Sm(NO3)3 aqueous solution. The cerium nitrate, urea, and poly(vinyl alcohol) then were mixed according to the molar ratio Ce:Sm:urea:PVA = (1 − x):x:5:0.1. The aqueous solutions were transferred into a preheated furnace to initiate a combustion reaction at 550 °C. After the reaction, the samples were sintered at 600−900 °C for 2 h. A control sample was also prepared by the solid-state reaction method with CeO2 and Sm2O3 reactants at 1000 °C for 2 h. SEM analysis of the materials shows a very high morphological uniformity. The particles produced by combustion reaction have a spherical shape and size of ∼10 nm. It was shown that an increase in the sintering temperature from 600 to 900 °C leads to a significant increase in the grain size from 30 to 250 nm. The luminescence intensity and CIE chromaticity coordinates of sintered CeO2:Sm3+ (1.5 mol %), prepared with and without polymer, as well as solid-state reaction are shown in Figure 69. The luminescence of the phosphor prepared by solid-state route is weak. For the sample prepared by SCS without polymer, the luminescence intensity is 3 times stronger. The luminescence intensity of the sample prepared by polymerassisted combustion synthesis is enhanced nearly 10-fold in comparison with that of the solid-state method. The CIE

Figure 69. Emission spectra (370 nm wavelength) and corresponding images under a 365 nm UV lamp (left), and CIE chromaticity diagram (upper right) of CeO2:Sm3+ (1.5 mol %) prepared SCS with (a) and without (b) poly(vinyl alcohol) and subsequent sintering at 850 °C, as well as (c) solid-state route at 1000 °C. Reprinted with permission from ref 549. Copyright 2014 Royal Society of Chemistry. 14544


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Figure 70. Illustration of multimodal luminescence (A) through upconversion (UC), downshifting (DS), and quantum cutting (QC) processes and energy level diagram (B) of Tb3+ and Yb3+ showing the mechanisms of electronic transitions in Tb3+ and Yb3+ codoped Gd3Ga5O12 phosphor. Reprinted with permission from ref 562. Copyright 2014 American Chemical Society.

indicated that regardless of the fuel to oxidizer ratio, phase pure cubic garnet is the reaction product. The luminescent intensity is influenced by the fuel to oxidizer ratio, while the emission band position did not change. Other experimental parameters that can influence emission characteristics include concentration of cerium, pH of reactive solutions, as well as the sintering temperature. An increase in the Ce3+ amount from 1 mol % causes quenching to occur via reciprocity between electrical dipole and electrical quadrupole. Rabasovic and coauthors used Dy3+ to tune the optical characteristics of Y3Al5O12 garnet.559 Incorporation of Dy3+ ions in the garnet host originated several emissions in the blue, yellow, red, and infrared spectral regions upon excitation of 350 nm wavelength light. It was shown that materials with 2 mol % Dy3+ can be used as a source of white light due to mixing of emission with different colors. Several different doped garnet phosphors, including Gd3Ga5O12:Pr3+/Tm3+561 and Y3Ga5O12:Tb3+/Yb3+,562 were also prepared by SCS. Structural characterization revealed that the garnet is the major phase for both materials, while a secondary phase may also be formed. Excitation of Tb3+/Yb3+ codoped Y3Ga5O12 phosphors in a wide, 250−500 nm, wavelength region leads to green emission through downshifting and up-conversion processes (Figure 70A).562 Meanwhile, near-infrared (NIR) emission of ∼1000 nm was observed by the quantum cutting mechanism due to the Tb3+ → Yb3+ electronic transition (Figure 70B). Figure 71B shows the energy level diagram and mechanism of electronic transitions for multimodal Y3Ga5O12:Tb3+/Yb3+ phosphors. The cooperatively excited Tb3+ ions in the 5D4 level, due to the involvement of Yb3+ ion pair through cooperative energy transfer (CET), give strong green and relatively weak blue and red emissions. The authors noted that the 5D3 level can be populated when few Tb3+ ions in the 5D4 level reabsorb the incident 976 nm photons and were promoted to higher lying excited states (5D1) through a process known as excitedstate absorption. Another pathway to populate this level is the nonradiative energy transfer from one excited Yb3+ ion. After the 5D3 level is populated through a nonradiative process, emissions in UV/blue then were observed. Such materials with

Figure 71. Photos of as-prepared long-persistent luminescence nanoparticles. SrAl2O4:Eu2+, Dy3+ (A); BaAl2O4:Eu2+, Nd3+ (B), CaAl2O4: Eu2+, La3+ (C). Reprinted with permission from ref 567. Copyright 2007 Elsevier.

multimodal emission characteristics can be used in phosphorcoated silicon solar cells. 5.3.2.2.1.2. Eu2+ and Eu3+ Doped Alkali Earth Aluminates. The high chemical stability and low toxicity of Eu2+ doped aluminates make them an important class of luminescent phosphors for many applications.174 Eu2+ phosphors exhibit broad emission bands after excitation, caused by parity-allowed electron transitions from the 4f65d1 excited states into the 4f7 ground states. The emission wavelength varies from the UV to the red spectral range, depending on the host lattice and particularly on the coordination number of the cations, the symmetry, and the covalence of bonds, etc. The synthesis of these compounds is challenging as the long process duration by solid-state reaction yields Eu3+. Therefore, synthesis is usually performed in a flow of reductive gas mixtures. SCS enables in situ preparation of Eu2+ doped phosphors. Several researchers 14545


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reported on the synthesis of Eu2+ doped strontium aluminates by immersing reactive solutions into furnaces preheated to 600 °C.563−566 However, they used postsynthesis calcination or heat treatment in a reducing atmosphere to increase the crystallinity of phosphors or oxidation of carbon-containing residues. In many other studies,68,567−571 the combustion process was tailored in a way to avoid high temperature postsynthesis heat treatment. For instance, Qiu et al. synthesized nanoparticles of strontium, barium, and calcium aluminates codoped with Eu2+ and other rare earth elements (Dy, Nd, and La) at 600 °C without a post-synthesis annealing process.567 The authors noted that excessive amounts of urea fuel were used to produce a reductive atmosphere and avoid the formation of Eu3+. It was shown that as-synthesized SrAl2O4:Eu2+, Dy3+, BaAl2O4:Eu2+, Nd3+, and CaAl2O4:Eu2+, La3+ nanophosphors have perfect crystal structure, and the luminescence changes from yellowgreen to blue-green and then to blue-purple in the visible range, respectively. These materials exhibited a long afterglow time. Figure 71 shows the photos of as-prepared long-persistent luminescence samples. The afterglow of phosphor nanoparticles lasted for over 7 h after the excited source was cut off. Ianoş et al. utilized urea and glycine fuel mixtures for the preparation of BaAl2O4:Eu2+, Dy3+ phosphors by solution combustion.570 To ensure the formation of Eu2+, they used 50 wt % excess urea. Moreover, they added NH4NO3 to reactive solutions to enhance the combustion temperature and crystallinity of the products. The emission spectra of the BaAl2O4:Eu2+, Dy3+ phosphors show a band at 499 nm. Increasing the Eu2+ concentration led to some increase of the emission band intensity. Adding Dy 3+ as a sensitizer considerably improved the afterglow. They also noted that postsynthesis annealing of the samples resulted in significant deterioration of the optical properties of the phosphors. Recently, Krishna and co-workers prepared CaAl2O4:Eu2+ and CaAl2O4:Eu2+, Cr3+ phosphors using oxyalyl dihydrazide, urea, and urea + oxyalyl dihydrazide fuels to tune the combustion characteristics and optical properties of products.68,569 It was found that the fuel to oxidizer ratio is a key parameter for tailoring of the optical bend gap, excitation, and emission intensities of phosphors.569 When the fuel to oxidizer ratio was 1 or higher, highly crystalline phosphors were formed. The photoluminescence spectra of the materials formed at low fuel contents showed weak blue (440 nm, Eu2+) and red (612 nm, Eu3+) emission peaks. Samples with higher fuel to oxidizer ratios (1.5−2) exhibited only 440 nm emission. The authors also showed that the purity of blue emission may be improved by increasing the fuel to oxidizer ratio. The chromaticity diagram shown in Figure 72 illustrates that by simply changing fuel to oxidizer ratio, the color hue can be modified from impure blue (A, B points) to blue (C point) and eventually to pure blue (D point). A similar enhancement in PL intensity at 440 nm corresponding to Eu2+ blue emission of CaAl2O4:Eu2+, Cr3+ phosphors was tuned by blending urea + oxyalyl dihydrazide in a 5:1 ratio.68 In contrast to phosphors with Eu2+ activator, Eu3+ aluminate phosphors are not sensitive to the fuel to oxidizer ratio. Yoon et al. showed that postsynthesis annealing significantly increases the crystallinity and emission intensity of red light.572 They outlined that annealing temperature should be 1000 °C to provide the most pure red light with highest intensity. Singh et al. noted that postsynthesis treatment of GdSrAl3O7:Er3+ at 550 °C for 1 h is sufficient to produce high-quality red

Figure 72. Color coordinates of CaAl2O4:Eu plotted using the CIE1976 chromaticity diagram. A, B, C, and D stand for samples with fuel to oxidizer ratios of 0.5, 1.0, 1.5, and 2.0. Reprinted with permission from ref 569. Copyright 2014 Elsevier.

phosphors.573 The color purity in this case can be tuned by changing the Gd/Eu ratio. Shaat and co-workers prepared other types of Eu3+-based aluminate nanophosphor.574 Co-doping of CaxSr1−xAl2O4 with Eu3+ and Tb3+ enabled preparation of materials with white light PL (excited with 227 nm UV light), which was the result of a combination of blue and green emissions from Tb3+ and red emission from Eu3+. Recently, several other aluminate host lattices and Eu3+ activator were proposed to tune the red light emission characteristics.575,576 For example, Fu and Liu suggested that the intensity of red color may be tuned by changing the Eu3+ content in the SCS-derived LaAlO3. They also suggested that this material may be a promising candidate for scintillator in radiation detection devices. In addition to these phosphors, a number of novel aluminate luminescent compounds, such as ZnAl2O4:Er3+, Yb3+,577 ZnAl2O4:Mn2+,578 LaAlO3:Tb3+,579 YAlO3:Dy3+,580,581 YAlO3:Ni2+,582 and SrAl4O7:Cu2+583 were successfully synthesized and characterized for LEDs, radiation detection, and biomedical labeling applications. Among these new aluminates, spherical ZnAl2O4:Er3+, Yb3+ nanomaterials with sizes of ∼80−150 nm prepared by aerosol combustion synthesis technique are remarkable.577 In this approach, a reactive solution of metal nitrates and urea was injected into the tube furnace heated ∼1000 °C. The nanoparticles were optimized to emit in the red luminescence range (Er3+, 661 nm) when excited with nearinfrared light. As-synthesized nanoparticles were coated with a polymer and tested as biological markers. 5.3.2.2.2. Silicates. Silicate phosphors are interesting due to their high stability as compared to aluminates or sulfides. The preparation of different phosphors by SCS, such as yttrium,584,585 cadmium,586,587 or alkaline-earth588−592 metal silicates, is more difficult than other complex multielement compounds. Typically, the reactive solution that is utilized to prepare these nanophosphors involves metal nitrates, fuels, and high surface area silica. For example, Rakov and co-workers used fumed silica with particle size ∼14 nm, metal nitrates, and urea to produce precursor gels for the synthesis of doped yttrium silicates.593−595 The combustion of the reactive gels was initiated by immersion in a preheated furnace (500 °C). 14546


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Postsynthesis heat treatment at 1200 °C is needed to obtain phase pure crystalline nanophosphors. Shinde et al. prepared Cd2Si2O7:Ce nanophosphors with a triclinic crystal structure utilizing reactive gels of metal nitrates, glycine, and silica.587 It was shown that gels ignited at 580 °C yield amorphous materials, which could be crystallized during the calcination stage at 1000 °C for 2 h. However, pure phase product could only be obtained at 1200 °C. The authors also outline that the solid-state synthesis of Cd2Si2O7:Ce phosphors can be only achieved during 20 h heating of the simple oxide mixture at 1400 °C. Contrary to Cd2Si2O7, CdSiO3-based phosphors with monoclinic structure crystallize at remarkably lower temperatures.596−598 It should be noted that regardless of the used silica sources (tertraethyl orthosilicate or silica fumed), 2 h postsynthesis annealing at 800 °C is sufficient to produce highly crystalline nanomaterials. A similar trend was observed for the preparation of calcium,588,599,600 magnesium,601,602 and strontium589 silicate nanophosphors. For example, Lei and coauthors prepared Sr2SiO4:Eu2+ phosphors both by SCS and by solid-state reaction.589 Combustion synthesis of these phosphors includes preparation of heterogeneous reactive mixtures of strontium nitrate, urea, and fumed silica by rapid ultrasound mixing followed by ignition of mixtures in a preheated furnace at 600− 900 °C. The authors noted that single-phase α-Sr2SiO4:Eu2+ is obtained during ignition at 800 °C. Below this temperature, unreacted strontium nitrate is present as an impurity phase. It was also shown that an increase in temperature resulted in a mixture of α- and β-Sr2SiO4:Eu2+ phases. In conventional solidstate synthesis, SrCO3, SiO2, and Eu2O3 were calcined at 1350 °C for 4 h to produce a mixture of α- and β-Sr2SiO4:Eu2+ phases. The mean particle size of the product obtained by SCS was shown to be in the range of 30−50 nm, while solid-state synthesis yields coarse particles (1−3 μm). The authors also inferred that the samples produced by SCS are characterized by homogeneous distribution of Eu2+ ions in silicate lattice. Kulkarni et al. reported similar results for synthesis of CaSiO3 doped with Pb and Mn.600 The combustion product formed after ignition at 500 °C was amorphous, which crystallizes at 800 °C to the α-phase of the silicate. Higher calcination temperatures (900 °C) would produce the β-phase. The authors noted that the phase transition temperature for combustion-derived CaSiO3 was found to be lower than the powder obtained in a solid-state reaction. SCS of more complex silicate phosphors, such as Ba3MgSi2O8:Eu2+, Mn2+, Ba2ZnSi2O7:Eu2+, Ba2MgSi2O7:Eu2+, or Sr3MgSi2O8:Eu2+, Mn2+, Li2(Ba0.99Eu0.01SiO4:B3+, were also reported.603−606 The calcination stage of Eu2+ containing nanophosphors was performed in a reducing atmosphere to prevent formation of Eu3+. Yao and Chen used urea as fuel and H3BO3 as flux along with metal nitrites and Si(OC2H5)4 to obtain a bluish green emitting phosphor Li2(Ba0.99,Eu0.01)SiO4:B3+ without a calcination step.606 Properties of the phosphors were investigated by changing the urea and H3BO3 concentrations as well as ignition temperature. Products formed even at 400 °C were crystalline. However, at lower ignition temperatures, a SiO2 impurity was observed, while the synthesis at 600 °C forms phase pure phosphor with hexagonal crystal structure. 5.3.2.2.3. Borates. Borates are well-known hosts for luminescent materials due to their high transparency, excellent chemical and thermal stability, and high luminescence efficiency.607,608 Examples of borate-host phosphors produced

by SCS approach are Pb2+ activated ultraviolet emitting materials609,610 and Eu3+ doped nanophosphors.611−614 In addition to these traditional nanophosphors, several novel light-emitting materials were reported. For example, Wang et al. prepared Li2B4O7:Cu phosphors for temperature sensing applications.615 They used Ag, In, Dy, or Ti as codopants to sensitize thermoluminescence emissions. The authors showed that codoping with silver ions is beneficial as it increases the thermosluminescence intensity. It is important that the intensity of emitted light could be tuned by silver ion content in the phosphor lattice. Borisov and co-workers prepared a new thermographic phosphor based on Cr3+-doped yttrium aluminum borate using high-temperature single-crystal growth and SCS techniques.616 In the first method, the crystals were grown from the solution in high-temperature flux of potassium molybdate. The phosphors obtained by this technique had higher purity. On the other hand, this method is time-consuming, and the crystals should be ground for preparation of thin phosphor coatings. The combustion-derived phosphor could be excited both in the blue and in the red range of the spectrum and showed bright near-infrared emission. The brightness of the phosphor is comparable to that of well-known lamp phosphors. The materials were tested for thermographic phosphor coatings. The phosphors were dispersed in a hydrogel and then deposited on a surface to produce a ∼10 μm thick coating (Figure 73). A high-power LED with 405 nm wavelength was used as an excitation source. Figure 73 shows photos for the same sensor coating at different temperatures with homogeneous color distributions. The authors measured luminescent decay times from these images and concluded that the material is suitable for imaging of temperature distribution on different surfaces. 5.3.2.2.4. Phosphates. SCS of phosphate-based phosphors include preparation of water-based reactive slurries of metal nitrates, ammonium dihydrogen phosphate, and urea followed by ignition in preheated furnaces at temperatures between 500− 600 °C.617−619 For incorporation of fluorine into the crystal structure of phosphors, ammonium fluoride was also added into reactive solutions and slurries.620,621 In most cases, postsynthesis annealing in air at temperatures between 800− 1200 °C was applied. It is reported that for the preparation of Eu2+ activated phosphors, the annealing stage should be performed in reductive atmosphere.622−624 For example, Tang and Xue reported SCS of a series of orthophosphate solid solutions with a general formula of Sr8Mg1−mZnmY(PO4)7:Eu2+ by immersion of reactive solutions in preheated furnace for 5 min to initiate the combustion reaction.623 The products obtained were annealed at 1300 °C for 3 h under a 5%H2/95% N2 gas mixture. The lattice parameters of the prepared solid solutions were linearly dependent on the m value. The increase in Zn content in phosphors led to the emission color shift from yellow to green (under near-UV excitation). The authors explained this shift by competition of the crystal field strength and bond covalence factors. Several researchers indicated that proper selection of the fuel to oxidizer ratio may allow for avoidance of the postsynthesis reductive treatment stage.625,626 As an example, we can consider the synthesis of β-Ca1.95P2O7:Eu2+ reported by Ta and Chen.626 They suggested that 3 times more urea than the stoichiometric amount should be used to produce phosphors with the highest luminescent intensity. In their synthesis formulation, precursor slurries were introduced into a muffle 14547


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of γ-Ca(PO3)2 into the β-Ca2P2O7. The optimized materials exhibited broad excitation with the main band centered at 387 nm, while the emission spectrum showed a single intense band centered at 421 nm. 5.3.2.2.5. Other Phosphors. Many other types of complex compounds, such as zirconate,627−630 titanates,631,632 vanadates,633−636 cerates,637,638 and molybdates,639,640 were also prepared by SCS and characterized for different solid-state light-emitting applications. Citric acid was used as fuel to prepare reactive gels in these synthesis reactions. In some cases, to ensure complete chelation of citric acid with metal ions, stirring and refluxing of initial solutions at 100 °C under the infrared lamp was utilized.641 The combustion was initiated in preheated furnaces at 300−600 °C. The products were always calcined at higher temperatures. For example, Gupta et al. synthesized undoped and Tb3+ doped SrZrO3 perovskites using citric acid-assisted gel-combustion synthesis.641 They found broad host lattice related green (544 nm) emission along with strong terbium ion orange-red emissions (587 and 622 nm), indicating incomplete energy transfer from the host group to Tb3+. Photoluminescent decay analysis and X-ray adsorption spectroscopy suggested the presence of two different types of Tb3+ ions occupying either six-coordinated Zr4+ site without inversion symmetry or eight-coordinated Sr2+ with inversion symmetry. Recently, Bhat and co-workers prepared two polymorphs of KNdW2O8 (α-KNdW2O8 and β-KNdW2O8) by SCS.642 They noted that bulk α-KNdW2O8 was previously prepared by solidstate reaction at 1020 °C, while β-phase was synthesized at 770 °C for 48 h. In their combustion-based formulation, both phases were produced by changing the amount of urea in the reactive solutions and varying the annealing temperature. The duration of postsynthesis heat treatment was 2−6 h, indicating that the SCS technique is more efficient and needs much less time and lower calcination temperatures. The emission spectra for two different excitation wavelengths show peaks in the blue and red regions, respectively (Figure 74). The detailed structural analysis helped the authors explain the difference in the observed photoluminescent properties for both phases. The broader bandwidth for β-phase KNdW2O8 is correlated with a high degree of distortion in Nd−O polyhedral in the crystal lattice. This distortion also results in increased crystal field strength, contributing to the high intensity of photolumines-

Figure 73. Photographic images of the ∼10 μm thick coating (50% w/ w of Cr-doped YAl3(BO3)4 and in a hydrogel) and lifetime distribution for the same coating at different temperatures. Reprinted with permission from ref 616. Copyright 2014 American Chemical Society.

furnace maintained at 750−1050 °C. Five minutes later, the white voluminous foamy phosphors were obtained by rapid combustion. Excessive amounts of urea created a reductive atmosphere during the rapid ignition, and thus ensured formation of the Eu2+ dopant. It was found that at igniting temperatures of 700−1000 °C, the γ-Ca(PO3)2 phase formed and an increase in furnace temperature facilitated the transition

Figure 74. Emission spectra of α-KNdW2O8 (A) and β-KNdW2O8 (B) excited at 355 nm (curve a) and 545 nm (curve b). Reprinted with permission from ref 642. Copyright 2014 American Chemical Society. 14548


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cence peaks in comparison to the emission peaks of αKNdW2O8.

materials produced by conventional liquid-phase deposition approaches at the same temperatures. These important advantages allowed the authors to prepare oxide device on flexible substrate (Figure 75). In this case, they deposited an

5.4. Thin Films

The future of low-cost macroelectronic devices (e.g., large size flat-panel displays) depends on the development of new organic and inorganic materials, and on understanding fundamental details of their charge transport and thin-film nanostructure evolution. Metal−oxide semiconductors and dielectrics, especially in amorphous phases, represent appealing materials for next generation electronics due to their high carrier mobilities, good thermal stability, mechanical flexibility, and optical transparency.643−646 Commonly, sol−gel techniques are used for metal oxide film growth, including films for thin-film transistors.646 However, the required sol−gel condensation, densification, and impurity removal steps typically require >500 °C processing temperature, which is incompatible with inexpensive glasses and typical flexible plastic substrates.645 Recently, Marks and coauthors at Northwestern University originated a novel variation of the SCS method for the preparation of thin-film transistors for next generation electronics. A good number of studies have already been published aiming to optimize the synthesis conditions and characteristics of the resulting films. We expect that combustion-based deposition approaches may have a significant impact, not only for the preparation of electronic devices but also for the development of solar cells and electrochemical devices. 5.4.1. Crystalline Thin-Film Transistors. In pioneering work, Kim et al. reported a general strategy for the fabrication of metal oxide thin-film transistors, In2O3, Zn−Sn−O, In−Zn− O, and In−Sn−O (ITO), at relatively low temperatures (200 °C) using the solution combustion approach.17 To obtain the precursor solution, they used In(NO3)3, Zn(NO3)2, and SnCl2 as metal sources with NH4NO3 dissolved in 2-methoxyethanol, and an appropriate amount of fuel (acetylacetone or urea) was added to the solution. An ammonia solution was also added for regulation of the solution pH. Before film casting, precursor solutions were aged for 72 h. Solutions with a total metal concentration of 0.05 M were then spin coated on silicon wafers and heated at 150−400 °C for 30 min. Using grazing incidence angle X-ray diffraction analysis and XPS studies, they demonstrated that the combustion precursor was converted into desired crystalline oxides, such as In2O3 or ITO at far lower temperatures than for typical solution-based processes. Phase-pure oxides were formed after precursor ignition at ∼200 °C. Using XPS and time-of-flight secondary ion mass spectrometry, the authors showed low carbon contamination in the product. Atomic force microscopy (AFM) indicated that combustion yields smooth, dense, contiguous films. It was also shown that thin films, on the order of 30 nm thickness or less, have very smooth surfaces (root-mean-square roughness less than 1.0 nm) with continuous coverage and negligible porosity. However, films of thickness on the order of 70 nm exhibit evidence of porosity. It is reasonable to suggest that the short diffusion length for the removal of gas products in thin films enabled fast mass transport, thereby avoiding porous structures. To achieve thicker dense films, multiple depositions of 30 nm films may be easily performed. Saturation mobility and conductivity of thin films deposited on silicon wafers at lower ignition temperatures (below 300 °C) were higher by several orders of magnitude than similar

Figure 75. Flexible combustion-deposited In2O3 thin-film transistor on polyester substrate. Reprinted with permission from ref 17. Copyright 2011 Macmillan Publishers Limited.

In2O3 precursor solution by spin coating on a polyester substrate with a thin alumina dielectric layer, followed by heating at 200 °C. SCS-derived thin-film transistors exhibited good performance with saturation mobility of 6.0 cm2/V·s. The authors noted that this characteristic is well above the mobility of amorphous silicon (below 1 cm2/V·s), which is currently utilized on a large scale in large-area electronic applications. Subsequently, Kim et al. investigated the ignition and combustion of acetylacetone-nitrate-based precursors.647 They heated 10 mg of the vacuum-dried combustion precursor in a thermogravimetric analyzer to 125 and 150 °C with a heating rate of 10 °C/min followed by an isothermal stage at the given temperature. It was demonstrated that the precursors could not be ignited at 125 °C even with the longest isothermal duration (80 min), while at 150 °C all solution-derived precursors can be ignited. However, the ignition delay times varied from 30 to 60 min. Such a delayed ignition process allowed the authors to prepare thin films at 150 °C. The results of XRD analysis suggested that all products were highly crystalline and corresponded to In2O3, ZnO, or ITO. The important parameters to control film characteristics are precursor concentration, spin rate, and ignition temperature. Grazing incidence angle XRD patterns of ∼60 nm thick In2O3 films (ignited at 150 °C) shown in Figure 76A exhibited crystalline In2O3 reflections. The authors noted that the ignition temperature is far below that of the reported dehydration temperature of In(OH)3 at ∼270 °C648 and the crystallization temperature of amorphous In2O3 films at ∼180 °C.649 These results indicate that the precursors were converted to crystalline products due to the rapid high-temperature combustion process. These results (Figure 76A) demonstrate that the higher ignition (or annealing) temperature enhances the film crystallinity of 60 nm In2O3 films. Another important parameter is the effect of the cast precursor layer critical thickness on the crystallinity and composition of the final films. Figure 76B suggests that multiple thin-film depositions, yielding an overall ∼60 nm and or 105 nm thicknesses, consist of crystalline indium oxide, 14549


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Figure 77. SEM images of nanomaterial and composite films. (A and B) Cross-section and plane view (inset) SEM images of as-deposited ITO nanoparticles (A) and coated with four overlayers of combustionderived ITO nanocomposite film (B). (C and D) Cross-section and plane view (inset) SEM images of as-deposited ZnO nanorod film (C) and two-layer combustion ZnO-overlayer-coated ZnO nanorod film (D). Reprinted with permission from ref 647. Copyright 2012 American Chemical Society.

Similar films were also developed for ZnO. Figure 77C shows well-packed ZnO nanorods, and combustion-overcoated/ impregnated nanorods are shown in Figure 77D. The formation of more interconnected ZnO domains is evident. Thus, it is possible to achieve significantly greater structural interconnectivity of oxide nanoparticle films using combustion coating/ impregnation. To confirm that this approach may enhance charge-transport characteristics of films, the authors showed that ZnO nanorod-based (prepared from solution-based synthesis) devices afford electron mobilities of 10−2−10−3 cm2/V·s. However, when the nanorod films are overcoated/ impregnated with the ZnO combustion precursor, the electron mobility is increased to ∼0.2 cm2/V·s. Moreover, oxygen vacancy generation and dopant activation can be independently controlled at the oxide nanomaterial synthesis stage. Similarly, commercially available ITO nanoparticles with high charge carrier concentrations were successively combined with the combustion precursor to produce transparent conducting films. The conductivity achieved at 150 °C is ∼10 S/cm without a postreductive treatment that is usually required in conventional deposition methods. 5.4.2. Amorphous Thin-Film Transistors. Despite the high electron mobility of In2O3 combustion-derived crystalline thin-film transistors, they exhibit an unsatisfactory on/off current ratio (Ion/Ioff) and nonuniform threshold voltage (VT) over large areas. Hennek et al. suggested that application of amorphous thin films may enable more controllable carrier densities with minimal grain boundary effects.650 To achieve this goal, they implemented an yttrium doping strategy and tuned the ignition (annealing) temperature. Specifically, In and Y combustion precursor solutions were prepared from nitrates at a total metal concentration of 0.05 M with acetylacetone + ammonia as the fuel. The Y molar concentration was varied from 2.5% to 25% in solution. Thin (∼12 nm) indium−yttrium oxide (IYO) films were deposited onto Si substrates by spincoating. After ignition of the precursors at 250 or 300 °C and repetition of this process three times, the thermal analysis data

Figure 76. Grazing incidence angle XRD scans (A) of 60 nm thick In2O3 films ignited at different temperatures and 150 °C annealing films with thickness varied from 7.5 to 105 nm (B). The reflections at the bottom in black are from the XRD database. Reprinted with permission from ref 647. Copyright 2012 American Chemical Society.

while thinner films contain indium hydroxide. The authors used XPS analysis to confirm that the thinner films indeed contained indium hydroxide rather than In2O3. Kim et al. also developed a novel combustion coating method that allows conversion of porous and electrically disconnected nanomaterial films into dense, electrically connected films at remarkably low processing temperatures.647 For example, commercially available oxide nanomaterials with different structures (e.g., nanoparticles, nanorods, nanoflakes) can be deposited onto the substrate surface by spin coting. However, as-prepared films are highly porous, and high temperature annealing is required to produce electrically connected films. To avoid such a process, Kim et al. impregnated a reactive solution that, upon combustion, yields the same oxide composition.647 In the cross-sectional SEM images (Figure 77A,B), the initial ∼380 nm thick ITO porous film deposited from commercial nanoparticles transformed into a dense ∼440 nm thick ITO nanocomposite film after overcoating/impregnating combustion. It can be seen that the overcoated combustion ITO primarily fills voids between the ITO nanoparticles. This overlayer coating and void filling are also evident from the significant reduction of voids and particle features in the plane view SEM images in the insets of Figure 77A,B. Furthermore, the root-mean-square surface roughness changes from 15 nm (without an overlayer) to 5 nm (with 4 overlayers) measured from the AFM images. 14550


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revealed that the combustion precursor ignition occurs at ∼200 °C for the entire composition range. XRD data for the resulting IYO films ignited at 250 °C suggest that even for small yttrium concentrations (e.g., 2.5−5%), all films were amorphous, while Y doping at ≤2.5 mol % yields polycrystalline films upon ignition at 300 °C. Yttrium doping of ≥10 mol % significantly disrupted the In2O3 lattice, yielding amorphous films. Surface characterization by AFM revealed smooth, featureless films with root-mean-square roughness in the 0.19−0.28 nm range. Two types of thin-film transistors were fabricated on the basis of amorphous ITO, placed on different substrates, SiO2/Si or ZrO2/Si. The performance of ZrO2-based transistors was shown to be superior with electron mobilities of 5.0 and 7.3 cm2/V·s for 250 and 300 °C, respectively.647 In the following publications, the same group of authors developed several other strategies to obtain amorphous thin film transistors with tailored parameters.651−653 One such strategy was related to deposition of thin double layer films with In2O3 (1−5 nm) and indium gallium oxide (10 nm) layers.651 Figure 78 demonstrates the microstructure of bilayer

whereas the indium gallium oxide layer exhibits a disordered structure. This bilayer architecture addressed the low mobility of amorphous indium gallium oxide and the unsatisfactory low Ion/Ioff ratio of single-layer In2O3 thin-film transistors. The authors report that at an ignition temperature of 250 °C, the mobility and Ion/Ioff ratio of the bilayer devices are ∼2.56 cm2/ V·s and ∼1 × 108, respectively. Another strategy to balance the transistor parameters was directed to combustion synthesis of amorphous indium−zinc− oxide by controlled doping with gallium, scandium, yttrium, or lanthanum.652,653 Detailed XPS analysis showed that only Ga3+ at high doping concentrations significantly changes the structure of nanomaterials through oxygen binding. In contrast, having larger ionic radii as compared to In3+ and Zn2+, other ions act as effective amorphization agents, but not as strong oxygen binders.652 Optimized Ga-doped indium−zinc-oxide fabricated on SiO2 or HfO2 self-assembled dielectrics supported these conclusions.652,653 Kang et al.654 and Kim et al.655 conducted a detailed parametric studies to demonstrate the influence of different metal precursors, ignition temperature, and pH for thin-film transistors using SiO2-based dielectrics. Kang et al. used metal (e.g., In, Zn) acetates and metal chlorides as precursors to prepare the thin films.654 As expected, these precursors were unable to react exothermically with fuels, thus showing poor device performance. On the other hand, tuning the pH (by addition of NH4OH) and ignition temperature, combustionderived thin films of metal nitrate−acetoacetate systems showed high device performance. To prepare zinc−tin−oxide films, Kim et al. used zinc acetate, tin chloride, and ammonium nitrate.655 In this case, the heat released during the exothermic reaction could be varied by tuning the molar ratio between the acetate precursor and ammonia nitrate. Recent developments have shown that SCS can be used, not only to prepare active film, but also for other transistor components such as gate dielectrics.656−658 For example, Ha et al. incorporated combustion-derived amorphous Al2O3 films into Au nanoparticles embedded in pentacene (C22H14) thinfilm transistors on a poly(ether sulfone) substrate.657 Branquinho and coauthors, for the first time, prepared an allcombustion transistor using both a zinc−tin−oxide active layer and alumina dielectric nanolayers on a p-type silicon substrate.658 In the following study, Branquinho et al. exploited solution combustion synthesis of aluminum oxide thin films ignited at 350 °C and their application in gallium−zinc−tin oxide thin films.659 They conducted an interesting parametric study comparing water and 2-methoxyethanol solvents and their effects on the structure and performance of thin-film transistors. They first compared thermal analysis results of freshly prepared and aged (up to 30 days) Al(NO3)3−urea reactive precursor solutions based on water or 2-methoxyethanol as solvents. It is noteworthy that all water-based solutions behaved similarly during the thermal analysis, exhibiting a broad exothermic peak at ∼170 °C. In the case of 2-methoxyethanol, the 3 days aged solution showed the highest heat release at ∼200 °C. However, either fresh or longer (15−30 days) aged solutions showed two small endotherm peaks at 200−350 °C range. This indicates that 3 days aged solutions of 2-methoxyethanol should have some distinctive features. Kim et al. proposed that, when metal nitrates are dissolved in 2-methoxyethanol, a ligand exchange reaction from nitrate to solvent may occur, and such an

Figure 78. Microstructural characterization of bilayer transistors (A). High-resolution cross-sectional TEM image of In2O3/indium gallium oxide bilayer. (B,C) Diffraction patterns of In2O3 and indium gallium oxide films. Reprinted with permission from ref 651. Copyright 2013 American Chemical Society.

thin film transistors with optimized parameters. The inset in Figure 78A clearly shows a distinctive interface between the two oxide layers. Thinner In2O3 layers appear darker and are located close to the dielectric SiO2 surface. These results suggest that during combustion, deposition diffusion of Ga atoms does not occur in the In2O3 layer under the exothermic conditions. From the diffraction patterns of the bilayer shown in Figure 78B,C, it can be seen that the In2O3 exhibits the characteristic diffraction pattern of a polycrystalline film, 14551


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Figure 79. SEM images (scale bar = 1 μm) of the CH3NH3PbI3 films grown on NiO:Cu layers (A) and incident photon-to-current density efficiency (IPCE) spectra of the device based on NiO:Cu HTLs prepared through different methods (B). Reprinted with permission from ref 59. Copyright 2015 John Wiley & Sons.

while the precursor solutions were loaded into the spray gun and sprayed intermittently (60-s cycles) on the substrates until the desired thickness (20 or 50 nm) was obtained (see Figure 9). This new approach enables the production of thin films with desired thinness in a single-stage process within 10 min, thus eliminating the previously developed multiple spin-coting and annealing procedures. More importantly, a combination of short pulses with in situ “continuous” combustion allows effective removal of gas trapped in product, thus yielding dense, high-quality, macroscopically continuous films of both crystalline and amorphous metal oxides. In addition, the authors showed that single-layer (50 nm-thick) indium−gallium−tin− oxide transistors made by this novel method showed 102−104 greater carrier mobilities (7−20 cm2/V·s) than those achieved with spin-coated combustion or sol−gel processes. The authors also noted that the quality and characteristics of their devices are approaching those fabricated by optimized magnetron sputtering protocols. Wang and co-workers reported also the fabrication of alloxide thin film transistors utilizing spray combustion synthesis by an automated spray-coater with an ultrasonic nozzle.662 The solutions were sprayed on different substrates followed by annealing of at temperature below 350 °C. Step-by-step deposition and annealing allowed one to produce homogeneous multilayer coatings of high-quality ZrO2 or Al2O3 gate dielectrics, a high-mobility semiconductor layer. Detailed characterization showed that dielectric films exhibited low leakage current densities (10−7 A/cm2 at 2 MV/cm) and high areal capacitance (>600 nF/cm). SCS-derived transistors produced on a polymer substrate exhibited electron mobilities of 8−10 cm2/V·s at operating voltages of 1 the surface area of products increases from 2 to 33 m2/g. Iron nitrate−hydrazine solutions show the opposite trends in that the surface area of products decreases with an increase of φ. The authors also reported on the use of citric acid, combinations of glycine and hydrazine fuels, and iron oxalate as an iron source and ammonium nitrate as an oxidizer. They produced pure αFe2O3, γ-Fe2O3, and Fe3O4 nanopowders with maximum surface areas of 65, 120, and 45 m2/g, respectively. The combined use of iron nitrate and oxalate, as well as glycine and hydrazine, may also produce mixtures of α and γ phases with surface areas ranging from 75 to 175 m2/g. Deshpande et al. 14557


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varying the furnace temperature to ignite reactive solutions. The size of Co3O4 particles produced at 300 °C ranged from 5−8 nm. An increase in the furnace temperature to 800 °C results in coarse (200−400 nm) particles. Magnetic susceptibility measurements revealed that all materials showed lower Neel temperatures (25−30 K) as compared to bulk Co3O4 (above 40 K). The solution combustion approach was also used to prepare doped oxide materials, such as iron doped In2O3732 or CuO,733 Ca-doped CeO2,734 and iron−chromium maghemites.735 These studies enjoyed the advantage of solution combustion to make uniformly doped semiconducting oxides, such as CuO and In2O3, for potential applications in spintronics, such as spin transistors and nonvolatile storage devices, where weak magnetic fields can be controlled by spin injection. Le et al. synthesized Cu1−xFexO nanocrystals (x = 0−0.3) by using the urea−nitrate combustion. X-ray diffraction analysis demonstrated that single-phase structure was obtained for the 0−20% Fe-doped CuO. The increase in Fe content gradually increases the cation vacancies, and the Cu2+ sites were partly substituted by Fe3+ ions. The magnetic characterizations indicated that the samples combine both ferromagnetic and antiferromagnetic components. ́ Garcia-Guaderrama and co-workers reported novel Fe−Cr maghemite, γ-Fe2−xCrxO3 (0.75 ≤ x ≤ 1.25), with a comparable amount of Fe3+ and Cr3+ ions.735 This material exhibited maghemite-like magnetic properties, while being much more stable then γ-Fe 2 O 3 because their γ-Fe 2O 3 → α-Fe 2O 3 transformation takes place at ∼700 °C instead of ∼300 °C. The authors used X-ray absorption techniques to probe oxidation states, local, and overall crystal structure of materials. It is shown that the combustion-derived material with x = 1 is structurally most uniform. X-ray absorption spectroscopy indicated that the Cr3+ ions occupy octahedral sites, whereas Fe3+ ions occupy both octa- and tetrahedral positions. It is shown that low-temperature magnetic characteristics of the materials depend on their iron content. Increasing the iron content leads to narrowing of the hysteresis loop and growth in the saturation value of magnetization (Figure 85). 5.5.3.2. Complex Oxides. Solution combustion synthesis was successfully utilized to produce nanostructured complex oxidebased powders. Citric acid was used as a suitable fuel for preparation of magnetic oxides including ferrites.736−743 Yue et al.,736 Cannas et al.,738 as well as Prabhakaran and Hemalatha744 pointed out that the pH of the solution has an important role in

Figure 83. SEM image of γ-Fe2O3 nanoparticles coated with oleic acid. Reprinted with permission from ref 727. Copyright 2014 Elsevier.

84C) reveals a small hysteresis loop with a coercive field of 0.02 kOe and a weak remnant magnetization of ∼0.3 emu/g. Meanwhile, the hematite exhibited record high magnetization (21 emu/g) at 10 kOe. Such unusual magnetic properties of combustion-derived αFe2O3 nanoparticles were explained by considering nanoparticles as a core−shell system, where an antiferromagnetic core does not bring a significant contribution to the magnetization. The contribution from the shell of uncompensated surface spins tends to interact ferromagnetically. The magnetization of the shell randomly flips under the influence of thermal fluctuations, while exhibiting high magnetization values in an external magnetic field. The contribution of such spins to the total magnetization increases with the reduction of nanoparticle size, both because of the increased surface-tovolume ratio and because of the reduction of the size of the antiferromagnetic interior core. The superparamagnetic behavior and high magnetization of hematite hold promise for new applications, particularly where superparamagnetic properties (near-zero remnant magnetization and near-linear response to the external magnetic field) are advantageous over ferromagnetic properties.730 Sahoo et al. reported a different strategy to control the particle sizes of magnetic Co3O4.731 This was achieved by

Figure 84. Microstructure (A) and particle sized distribution (inset in A), temperature dependence of magnetization (B) for ultrasmall α-Fe2O3 nanoparticles cooled H = 0 (zero-field-cooled) and H = 1.0 kOe (field-cooled), and magnetic field dependence of magnetization at 300 K (C). Reprinted with permission from ref 45. Copyright 2014 American Chemical Society. 14558


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from 2 to 8 nm, by increasing the metals/tetraethoxysilane ratio (or content of ferrite phase) in the precursor gels (Figure 86). The surface area was shown to be a function of the ferrite/silica ratio in the composites. These products also showed high thermal stability, as high-temperature heat treatments did not affect much the surface area and porosity of the materials. The mean particle size of the ferrite phase, however, increased after calcination, ranging from 3 to 28 nm. The authors also report the magnetic properties of calcined materials with different ferrite/silica ratio.746 Magnetization measurements indicated that the materials show remarkable size-dependent magnetic properties. Specifically, composites with small sizes of ferrite particles (7 nm) exhibited superparamagnetic behavior at ∼50 to 300 K, and an increase in the size of particles to 28 nm leads to an increase in the blocking temperature from ∼50 to ∼210 K. Blocking temperatures of materials were also measured by Mössbauer spectroscopy. These two methods show similar results for blocking temperatures. The authors also noted that, unlike many other ferrites, the magnetic anisotropy constant of cobalt ferrite nanoparticles is smaller than that of the bulk material. Toksha et al. reported747 on the gel combustion synthesis of pure CoFe2O4 and CoCrxFe2−xO4 (x = 0−1) nanomaterials using citric acid as the fuel and gelation agent. In contrast to results obtained by Cannas et al.,745 the ferrite particles were much larger (50−100 nm). This can be attributed to the absence of a SiO2 matrix, which can prevent the growth of particles during the synthesis. Such differences in morphology change the magnetic properties of CoFe2O4. Room-temperature magnetic measurements suggest that pure CoFe2O4 with a larger particle size exhibited a large coercive field. The latter can be increased even more by partial iron replacement with chromium, indicating that a much stronger magnetic field should be used to overcome the anisotropy for flipping magnetic moments in CoCrxFe2−xO4 nanomaterials. Other examples of SCS of ferrites by using citric acid-based gel precursor are recent reports of Durmus et al.748,749 They successfully prepared graphene nanosheets decorated with SrFe12O19 and BaFe12O19 hard magnetic nanoparticles by citricassisted sol−gel combustion method. Magnetic characterization of these nanocomposites showed that the introduction of graphene layers into the nanocomposite did not decrease the magnetization value of ferrite nanoparticles. It was also shown that ferrite nanoparticles were formed between the graphene nanosheets, thus making these composites suitable for applications of microwave absorbing materials. Meng and co-workers also reported on glycine-based gelcombustion formulation of hexagonal BaFe12O19 ferrite.750 It

Figure 85. Magnetization versus field for the three different Fe−Cr maghemite samples. Reprinted with permission from ref 735. Copyright 2014 American Chemical Society.

tailoring the microstructure, composition, and magnetic properties of NiCuZn- and Co-ferrites. It was shown that increasing the pH by adding liquid ammonia significantly increases the overall heat of reaction through formation of ammonium nitrate in gels.736 Junliang and co-workers739 have studied the influence of the citric acid to metal ion ratio and pH on the phase compositions of the synthesized powders and their magnetic properties. They suggested that the phase compositions of the products of barium hexaferrite may be changed from a multiphased mixture to a single phase BaFe12O19 with the barium and iron ions complexing fully with citric acid as the pH, as well as citric acid/metal ratio, increases. Cannas and coauthors have combined sol−gel and combustion synthesis techniques to produce high surface area CoFe2O4−SiO2 composites with tunable magnetic properties.745,746 They used solutions of metal nitrates and citric acid and tetraethoxysilane as the silica source. Detailed parametric studies showed that the pH, temperature, amounts of tetraethoxysilane, and citric acid are key parameters that influence the preparation of uniform gels. Infrared spectroscopy investigation revealed that, in the precursor gels, metal ammonium carboxylate complexes form in a silica network. Ignition of these stable gels in a preheated furnace led to the formation of high surface area (200−820 m2/g) products. The average particle size of the ferrite component was regulated

Figure 86. TEM images of CoFe2O4−SiO2 composites with 15%, 30%, and 50% of ferrite phase (dark particles). Reprinted with permission from ref 746. Copyright 2006 AIP Publishing. 14559


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magnetic separation and reuse of the catalyst (see inset in Figure 87). Another viable strategy of tailoring the magnetic properties of ferrites is to use minor dopants.741,758 For example, the magnetic properties of CoFe2O4 ferrites mainly depend on the nature and distribution of their cations in the two tetrahedral and octahedral sublattices of a cubic structure.755,759 Cobalt ferrite belongs to the category of inverse spinel ferrites, where the octahedrally coordinated high spin Co2+ ions exhibit a strong spin−orbital coupling and introduce large magnetocrystalline anisotropy. The partial substitution of Fe3+ by dysprosium or samarium allowed Kambale et al.754 and Ahmad et al.755 to change the cation distribution and manipulate the magnetic coupling of the resulting materials. This approach made it possible to tailor the magnetic susceptibility, magnetization, and coercivity of the resulting materials. A similar strategy was utilized to tailor the structure and properties of multiferroic bismuth ferrite (BiFeO3) with perovskite structure.760,761 Iorgu and co-workers used reactive solutions containing α-alanine, bismuth, and iron nitrates as the principal reactants as well as nitrates of Pr, Sm, Eu, and Gd to prepare doped BiFeO3.761 They showed that incorporation 10 mol % of rare-earth ions on bismuth sites in the crystalline lattice of ferrite led to the partial transformation from rhombohedral symmetry of pure BiFeO3 to orthorhombic symmetry for all doped products. Such structural modifications changed the magnetic properties of the materials. Arya et al. partially substituted both Bi and Fe ions by In and Ti, respectively.762 Such codoping somewhat distorted the crystal structure of BiFeO3, and thus increased the lattice strain, reducing the crystalline sizes and increasing the Fe3+/Fe2+ ratio. As a result, the magnetization of the doped sample increases 12 times. BiFeO3 is currently considered as a potential biomarker for multiphoton imaging due to the combination of unique nonlinear optical properties with higher spontaneous polarization and interesting multiferroic properties.763−766 Recently, Schwung et al. published a new SCS formulation for pure BiFeO3 nanoparticles.763 They used reactive solutions of iron and bismuth nitrates in diluted nitric acid as well as 2-amino-2hydroxy-methylpropane-1,3-diol as a new fuel. This organic compound formed stable chelates with metal ions and produced stable reactive resins. Ignition of reactive resins yielded BiFeO3 nanoparticles with an irregular shape. The authors also developed an efficient disintegration technique for large agglomerates of produced powder and then prepared a colloidal suspension, which was then characterized by hyper Reyleigh scattering and nonlinear polarization microscopy. The result indicated that BiFeO3 is a promising material for frequency conversion applications. In addition, the moderate ferromagnetic response of material due to the presence of γFe2O3 impurity allowed the material to be separated magnetically from colloidal solutions. 5.5.4. Bioceramics. Bone and other calcified tissues can be considered as natural anisotropic composites consisting of biominerals embedded in a protein matrix, other organic materials, and water.190,192 The major structural constituent of bone tissue is calcium phosphate that is similar to hydroxyapatite (HAp) with a calcium to phosphate molar ratio of 5:3 (1.67) and a chemical formula of Ca10(PO4)6(OH)2 (Figure 88). The biomineral phase comprises 65−70% of bone, water accounts for 5−8%, and the organic phase, which is primarily in the form of collagen, accounts for the remaining

was shown that the magnetic properties of this ferrite depend on the Ba/Fe ratio in precursor gels. SCS of PbFe12O19, another important member of hexagonal ferrite family, was also reported by using oxalic, malonic, succinic, or maleic acids as fuels.751 The phase composition and morphology of the materials were regulated by the pH of the solution, fuel type, and fuel to oxidizer ratio. It was shown that maleic acid permits production of hexagonal-shaped flakes. The authors suggested that the product morphology is related to the formation of a stable supramolecular structure between carboxyl and hydroxyl groups and Fe3+ ions, which stabilizes the reactive gel. The magnetic properties of SCS-derived ferries have also been controlled by tailoring the particles sizes, crystallinity, and doping with different elements.752−756 One of the primary means to control the structure and properties of the products is the application of different fuels. For example, Kaur et al. reported the formation of various morphologies of CdFe2O4 ferrite with cubic spinel structure using urea, glycine, ethylene glycol, and oxalyl dihydrazid as fuels.757 The authors noted that urea was the best fuel to provide phase pure cadmium ferrite with nanorod morphology. Magnetic studies using Mössbauer spectroscopy reveal that the synthesized materials exhibit superparamagnetism with nonsaturation of magnetization loops and nearly zero remanence as well as very small coercivity at room temperature. Duraz used ammonium nitrate in reactive solutions to regulate the synthesis temperature of CoFe2O4 ferrite.752 NH4NO3 increased the reaction temperature and thus the crystalline sizes and crystallinity of the product, resulting in material with higher magnetization and coercivity. Manikandan et al. showed that microwave ignition of reactive solutions may form spherical nanoparticles of CoFe2O4 with sizes of 100−200 nm, whereas ignition in a furnace forms larger flaky nanoparticles.753 Such morphological differences significantly influenced the magnetic properties of the product (Figure 87). It was shown that the products prepared with microwaveassisted ignition process can be used as an efficient photocatalyst for oxidation of liquid organic pollutants. Good magnetic properties of the synthesized cobalt ferrite permitted

Figure 87. Magnetic hysteresis loops of CoFe2O4 spherical nanoparticles (blue) prepared by microwave combustion and flaky nanoparticles (purple) prepared by conventional combustion. Reprinted with permission from ref 753. Copyright 2014 Elsevier. 14560


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Figure 88. Hierarchical structure of a bone at various length scales. Reprinted with permission from ref 192. Copyright 2013 Elsevier.

Figure 89. Preparation of bioceramic powders by SCS. Reprinted with permission from ref 192. Copyright 2013 Elsevier.

portion.189,192 The collagen, which gives the bone its elastic resistance, acts as a matrix for the deposition and growth of minerals. Bones, which have been damaged or lost as a result of some disease or aging, can be successfully regenerated or repaired. Bioactive scaffolds, made from bioglasses or different calcium phosphate ceramics such as synthetic HAp, tricalcium phosphate (Ca3(PO4)2, TCP), and biphasic calcium phosphate (BCP, mix of HAp and TCP), fluorapatite (Ca5(PO4)3F), and chloreapatite (Ca5(PO4)3Cl), are considered as potential materials to repair and reconstruct diseased or damaged bone or dental tissue. In recent years, calcium phosphates, especially HAp and TCP, have also attracted significant interest in simultaneous use as a bone substitute and drug delivery material.190,767−769 For many drug delivery materials, biodegradation of the carrier at the target site can also pose a threat to the patient. Degradation products of calcium phosphates are Ca2+ and PO43−, which are already inherent in the body. This natural occurrence of calcium phosphates is one of the primary advantages over other synthetic drug delivery systems. The performance of bioceramics is highly dependent on its morphology. For example, spherical or rod-like nanostructures can better imitate the biomimetic features of human bone.769 Therefore, the design of bioceramic with desired shape and hierarchical structures is of high importance in tissue engineering and drug delivery devices. Several review papers189,192 have highlighted the different synthetic methods of bioceramic materials, among which SCS shows great potential as an inexpensive and simple method. As illustrated in Figure 89, solution combustion synthesis of bioceramics involves self-sustained exothermic reaction be-

tween calcium nitrate and a suitable fuel (glycine, urea, sucrose, citric acid, succinic acid) and ammonium hydrogen phosphate ((NH4)2HPO4) as phosphorus source. In typical synthesis, the aqueous stock solutions are first mixed, followed by adding a small amount of HNO3 to dissolve the resulting precipitate.193,770−773 The reaction can be initiated by heating the solution in a furnace at 300−500 °C. For example, Tas reported on the synthesis of HAp and TCP by adding calcium nitrate, ammonium hydrogen phosphate, and urea into simulated body fluid (SBF), and then the resulting solution was placed in a preheated (500 °C) furnace.774 SBF is a solution that mimics the chemical composition of human body fluid and contains seven cations (Na+, K+, Mg2+, Ca2+, Cu2+, Fe2+, Zn2+) and four anions (Cl−, HCO3−, HPO42−, and SO42−). The combustion product, however, consisted of undesired phases such as Ca8H2(PO4)5, Ca(OH)2, and CaO. Calcination of this product at 1000 °C provided the formation of pure phase hydroxyapatite powder. Inductively coupled plasma mass spectroscopy suggested that the combustion product contains small concentrations of Mg, Na, K, Cl, Fe, Zn, Cu in addition to primary Ca and P elements. The presence of these elements may be desirable to increase biocompatibility of HAp and TCP with the natural bones. Several works have been published by Ghosh et al.770−773 and Sasikumar and Vijayaraghavan775,776 to optimize the nature of fuel, fuel to oxidizer ratio, and furnace temperature for combustion synthesis bioceramic powders. Sasikumar and Vijayaraghavan claimed that an additional calcination step of the combustion product is required only when citric acid, tartaric acid, sucrose, or glucose were used as fuels, while urea 14561


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and then samples were added to protein solution and incubated for 24 h. Afterward, the loaded samples were separated from solutions washed and dried under vacuum for 48 h. The in vitro delivery of protein was studied by immersing samples in SBF solution at body temperature. The concentrations of released albumin were measured by UV−vis spectroscopy. The authors also used thermogravimetric analysis to quantify the maximum concentration of loaded albumin. This analysis showed that all combustion-derived materials adsorb significant amounts of albumin. The in vitro release experiments indicated that the combustion-derived materials exhibited a sustained release of albumin during 48 h. The authors modeled release behavior and suggested that surface diffusion via ion exchange and intraparticle diffusion are rate-limiting stages of protein desorption from bioceramics. These researchers suggest that significant progress in preparation of bioceramic powders by SCS was achieved. The microstructural control of the product, however, remains challenging. Several modifications of solution combustion show some promising results for tailoring the microstructure of bioceramics. One such result was reported by Yuan and coauthors who prepared HAp nanotubes by use of a sol−gel combustion process in an anodized aluminum oxide template.46 They used Ca(NO3)2·4H2O and PO(CH3O)3 as the Ca and P sources, and ethylene glycol as the polymeric matrix and ethanol as solvent. The alumina template was immersed in the sol containing reactants and ethylene glycol. The template was taken out from the sol, dried, and heated in a furnace to initiate combustion. The template was dissolved by concentrated sodium hydroxide solution. As-prepared HAp nanotubes have length of 10−50 μm and diameter of ∼100 nm (see Figure 38). Bhaduri and his team utilized microwave heating of solutions containing calcium nitrate, monopotassium phosphate (KH2PO4), and urea to produce bioceramic nanomaterials.49,784,785 They combine self-sustaining exothermic reactions with salt-assisted synthesis and microwave heating. In a typical synthesis process, a significant amount of salt (NaNO3 or NaCl) is added to reactive solutions. The solution then was placed in a microwave oven and irradiated for 5 min. The authors suggested that microwave irradiation helps in uniform molecular-level heating of reactive solutions. The molten NaNO3 formed during combustion dissolves the initial calcium phosphate precipitates, which form during the boiling and evaporation of water. These particles served as nucleation centers in molten salt. During the cooling stage, rapid crystallization of the phosphate phases occurs along the preferred growth axes resulting in nanowhiskers by a sequential dissolution−crystallization−growth mechanism. The solubility of the specific calcium phosphates in molten salts has a significant role in the crystallization process. The morphology of the whiskers was strongly dependent on soaking time at the peak temperature. The fuel to oxidizer ratio, pH of initial solutions, as well as cooling kinetics were other relevant experimental parameters for influencing the structure of products.785 For example, both phase composition and morphology of product may be controlled by pH and urea content of reactive solutions at constant Ca/P ratio of 1.5. Also, changing the pH by adding nitric acid enabled one to control the aspect ratio of whiskers. The biocompatibility of as-prepared whiskers was tested in vitro by immersing them into SBF solution at 37 °C for 7 days and subjecting them to osteoblast attachment and protein assay

and glycine directly produce crystalline product during combustion.776 They showed that different fuels lead to different ratios of calcium phosphate bioceramic phases. Citric acid or succinic acid resulted in carbonated HAp, whereas βTCP is only formed when a mixture of them was used.775 Ghosh et al. reported that combinations of urea and glycine, and the addition of small amounts of glucose, lead to different synthesis temperatures.771 A small quantity of glucose added to either urea or glycine significantly reduced the flame temperature. The specific surface area of the powder obtained from glucose−urea−glycine mixed fuels was higher than that obtained from the urea−glycine composition. They also demonstrated that a decrease in both batch size and furnace temperature and/or an increased fuel to oxidizer ratio results in a decrease in crystallite size.772 In most cases, the product consists of large (0.5−5 μm) agglomerates of calcium phosphates nanoparticles with sizes ranging from 20 to 100 nm. Ghosh et al. sintered combustion-derived HAp and BCP into porous samples (∼35 vol %) and subsequently tested their bone formation response in vivo.770 Sterilized samples were implanted into the lateral side of radius bone of 18 goats, in which a blank hole was left unfilled in a group of 6 specimens to act as control. The bone formation response of the materials was evaluated by microscopy and histological analysis and compared to biologically active glass containing SiO2, Na2O, CaO, P2O5, B2O3, and TiO2. It was observed that interfacial response was strongly dependent on combinations of Ca/P ratio and pore sizes of implants. The surface of β-TCP exhibited characteristics similar to those of bone and was distinct from other materials. The lowest bone ingrowth and reduced strength was observed with HAp implants. ́ Aghayan and Rodriguez optimized the phase composition of BCP produced by combustion synthesis.777 They indicated that mainly α-TCP could be produced using glycine as a fuel, while the β-TCP phase prevails using urea. It was also shown that HNO3 influences the HAp/TCP ratio in products. Volkmer et al. also showed that pH has a critical effect in regulating the combustion products in this system.778 Zhao et al. reported on the synthesis of HAp, β-TCP, and BCP by use of citric acid as fuel and ammonium nitrate as combustion aid.779 To achieve high crystallinity and pure HAp and β-TCP, they used subsequent calcination at 950−1050 °C. In a follow-up publication,780 they reported incorporation of fluorine and chlorine anions into the crystal structure of apatite by adding NH4F or NH4Cl into reactive solutions. It is known that fluorine substitution improves the acid resistance of apatite crystals. Also, a certain concentration of fluoride release from fluorapatite could inhibit the metabolism of bacteria.781 The significance of chlorapatite is related to its ability to activate the osteoclasts in the bone resorption process.782 Zhao et al. showed that with an increase of fluorine and chlorine anions, the hydroxyl groups in HAp were gradually substituted, and fluorapatite and chlorapatite were obtained. The calculated unit cell parameters of products revealed that the apatite structure has been retained. They also used FTIR spectroscopy to confirm the incorporation of fluorine and chlorine anions in the apatite structure. More recently, Zhao et al. prepared bioceramic powders with different Ca/P ratios using citric acid as the fuel, which were then tested as drug delivery materials.783 Bovine serum albumin was applied as a model protein to study its adsorption and release behavior. Before the in vitro tests, combustion-derived TCP, BCP, and HAp powders were sintered at 1150 °C for 3 h, 14562


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tests.784 Upon immersion in SBF solution, new Ca- and Pcontaining nanocrystals formed on the surface of SCS-derived bioceramics. These experiments also showed that the surface area of bioceramics has increased from single-digit values to about 45−115 m2/g. Such changes indicate that combustionderived bioceramic whiskers exhibited enhanced apatiteforming ability. Figure 90 illustrates secondary apatite nano-

Figure 90. SEM image of HAp whiskers coated with apatite phase after immersion in SBF. Reprinted with permission from ref 784. Copyright 2006 John Wiley & Sons.

crystals formed on the surface of combustion-synthesized HAp whiskers during 1 week. In vitro cell culture tests with the mouse, osteoblasts revealed that cells were attached and proliferated on the surface of all HAp, TCF, and BCP bioceramics (Figure 91). The microwave-assisted SCS coupled with molten salt processing is a viable technique for the preparation of biologically active and biocompatible calcium phosphates with tailored microstructure such as high aspect ratio nanowhiskers. It was also noted that high aspect ratio whiskers reinforce the pure HAp phase and increase the fracture toughness from ∼1 to 2−12 MPa m1/2.786−788 Such a dramatic enhancement in mechanical properties is advantageous for application of bioceramics in load-bearing dental and orthopedic tissue engineering. Recently, the concept of microwave-assisted molten-salt controlled SCS was also adopted to prepare flourapatite,196 europium-doped phosphates,195,789 strontium phosphate,790 and calcium sodium phosphate791 nanowhiskers. Nabiyouni and co-workers reviewed the synthesis methods of fluorapatite, indicating that the existing methods such as sol−gel, wet chemistry, solid-state, and hydrothermal synthesis procedures are sophisticated and time-consuming.196 In contrast, the combustion-based approach of fluorapatite synthesis requires only 5 min of irradiation in a household microwave oven. In their synthesis formulation, Nabiyouni et al. used NaF or CaF2 as fluorine source to produce large aspect ratio hexagonal nanotubes of fluorapatite (see Figure 39). They suggested that the fluoride salts can dissolve into molten sodium nitrite and thus supply F− ions, facilitating the growth of hexagonal crystals. The inner channels in hexagonal crystals were not continuous and uniform in diameter. The authors speculated that the shape of the channel formed inside the crystal was related to the slowing crystal growth rate during the cooling stage due to the gradually decreased ion diffusion flow through the molten phase. The synthesized nanotubes showed good cytocompatibility. The tests showed that ions released from the

Figure 91. Number of cells on whiskers (A) and protein assays for whiskers (B); control is Al2O3. Reprinted with permission from ref 784. Copyright 2006 John Wiley & Sons.

material promote the proliferation of cells and that the proliferation rate is correlated with the concentration of fluorapatite. Europium-doped HAp was also prepared by Wagner et al. using microwave-assisted molten salt bath SCS.789 Doping of HAp with such lanthanide elements could provide formation of luminescent bioceramics. Such multifunctional biomaterials can be used to monitor biological processes such as protein production and cellular trafficking. Wagner et al. showed that introduction of 5 mol % of Eu did not change the crystal structure and morphology of HAp. Moreover, as expected, the resulting Eu:HAp material was capable of emitting red (696 nm) wavelength associated with 5D0 → 7F4 transition upon UV excitation. Confocal and multiphoton microscopy confirmed that Eu:HAp nanowhiskers provide sufficient intensity for utilization in imaging applications. However, the authors noted that the material showed some cytotoxicity, and further studies are needed to address this issue.

6. CONCLUSIONS AND PROSPECTS FOR THE FUTURE As may be seen from this Review, SCS has become a popular rapid preparation method for numerous simple and complex oxides, metals, alloys, composites, and sulfides in the form of nanoscale powders, porous items, thin films, etc. SCS also allows for efficient doping of materials, even with a trace amount of elements. It provides easy formation of high-quality multielement compounds with complex crystal structures, such as perovskites, garnets, spinels, silicates, and phosphates. Several issues need to be addressed, however, before this 14563


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temperature is an efficient route to fabrication of fine but highly crystalline products. Other improvements in the properties of the synthesized product can be achieved by proper selection of the fuel. Sucrose, glucose, and cellulose are viable fuels for the production of carbon-coated nanostructured materials, due to their tendency to form carbonaceous intermediates during their pyrolysis. Carbon-coated nanostructures are greatly beneficial in electrochemical applications. On the other hand, urea or hydrazine-based fuels form relatively small amounts of carbon residues, which is important for catalysts, optical nanomaterials, and ceramic powders. The room-temperature liquid-phase postcombustion treatment with H2O2 is a reliable approach for reducing undesired carbon-containing residues in combustion products. In catalysis, the activity of catalysts often correlates with its surface area. Therefore, in most cases, researchers have to used excessive quantities of fuel to decrease the reaction temperature and thus obtain catalysts with higher surface area. This approach is not always correct because an excess of fuel may lead to carbon deposition over the catalyst surface. Hightemperature calcination is often used to remove the carbon, which typically decreases the surface area. Nagaveni et al. showed that surface areas measured for combustion-synthesized TiO2 particles were 246, 164, and 143 m2/g for glycine, hexamethylenetetramine, and oxalyl dihydrazide fuels, respectively.448 This example clearly demonstrates the importance of proper fuel selection. Moreover, Hegde et al. showed that properly prepared CeO2- and TiO2-based powders can be directly used as catalysts, without any postcombustion treatment.36 The subject of in situ doping of combustion-derived semiconductors (TiO2, ZnO) with light elements, such as nitrogen, remains a debated topic, and further studies are needed to improve the incorporation of nitrogen into the lattice of the semiconductor. To this end, using a nitrogen-containing gas (e.g., ammonia) during combustion might be helpful. Furthermore, simultaneous application of surface and bulk characterization techniques such as XPS, Prompt Gamma Activation Analysis, and secondary ion mass spectrometry will be valuable for reliable interpretation of the localization of the investigated light elements.472,483,486 Another key question is related to environmental impacts of hazardous (NO2, NO, N2O, N2O5) gases that may be released during large-scale applications of SCS processes. In most works, the authors assume that a stoichiometric reaction of metal nitrates with fuels yields nitrogen, water, and carbon dioxide. However, the real process is more complex, and nitrous oxides (NO2, NO, N2O) were detected by mass-spectroscopic analysis. In future studies, the quantities of these oxides should be evaluated and assessed for potential impact during largescale production. Depending on their quantities, several possible solutions may develop to prevent their formation. The fuel-rich reactive solution may lower the amount of harmful gases due to in situ reduction by fuel fragments (such as NH3 formed during decomposition of glycine, or methane in citric acid). Another possible solution to the problem would be the utilization of mixed fuel formulations with different quantities of reducing (C, H) or inert (N) elements. This may tune the oxygen content to reduce element ratios, and thus prevent the formation or reduce the quantities of hazardous gases.

combustion-based approach can be implemented in large-scale production of advanced nanoscale materials. Until recently, a limitation of SCS has been the relatively low level of control over the morphological uniformity of the fabricated materials. The preparation of regular, uniform, nonagglomerated nanostructures has been a challenging task. This is related to the fact that, partially due to the rapid and complex nature of the process, the mechanisms of structure formation for SCS products remain generally unknown. On the basis of the examples shown in this Review, however, we may expect that in the near future, thorough study of the structure formation mechanisms will allow one to establish new effective ways to control the morphology of the products. Indeed, a good number of new approaches, based on the solution combustion concept, were recently developed to tailor the morphology of the resultant materials. Incorporation of hard and soft templates, utilization of the reactive solutions in spray pyrolysis, use of microwave ignition, and molten salt-assisted methods have significantly enhanced the structural diversity of combustion products. The other fundamental issue is understanding the reaction mechanism and its relation to reaction kinetics. In this respect, application of thermal analysis techniques coupled with massspectrometric measurements and other dynamic analysis methods is very beneficial. Routine measurements of the temperature−time history of combustion by direct or indirect methods would provide relevant information on the thermal regime of reactions and would serve as an efficient tool for optimizing synthesis conditions. Following recent achievements in the preparation of thin films, fundamental mechanistic studies are needed to understand the combustion process in reactive systems with high surface/volume ratios. It is clear that reactive solutions form high-quality films that often outperform similar films prepared by other methods. Most likely, exothermic reactions in these systems may proceed in the volume combustion mode. It is also possible that the redox reactions may occur in novel and yet unknown regimes under unique conditions. Particular attention should be paid to address the issue of product dimensionality. Electrochemical performance of electrode materials for battery applications is not only related to high surface area but also to the shape of the nanocrystals. For example, due to the presence of preferential crystal orientations, the use of nanowires or nanosheets may have advantages over spherical particles. The hollow spherical particles synthesized by spray pyrolysis also possess more structural stability during charging/discharging cycles. A drawback of the current SCS-based pathway is use of a calcination step after the synthesis, which makes it a two-step technology. In many cases, combustion of reactive solutions occurs in a low-temperature regime, which leads to the formation of amorphous materials with large surface area. A prolonged calcination step is then often required to improve the crystallinity of the materials. Recent works have demonstrated different approaches, which allow for avoidance of the calcination step. For example, it can be achieved by use of an additional completely gasified oxidizer, such as ammonium nitrate with an appropriate amount of fuel. Variation of the reactive solution pH by use of ammonia is another viable approach for tailoring the heat release of the SCS process. These approaches, while increasing the combustion temperature due to rapid gasification of NH4NO3, prevent agglomeration of particles. Finally, controlling the ignition 14564


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It has been suggested that SCS reactions can be performed in continuous schemes.16 Additionally, in continuous SCS technologies, selective catalytic reduction (SCR) of NOx, CO, and SO2 by urea or ammonia may be applied.792 Commercial SCR systems are typically used in utility and industrial boilers, gas turbines, and diesel engines. This widely utilized approach shows high reduction rates for hazardous gases at temperatures of 200−400 °C and can be integrated into continuous SCS-based production schemes. Looking toward the future, we believe that most current limitations of the SCS method can be overcome by fundamental studies of combustion reactions, using advanced experimental diagnostic techniques and by developing new theoretical models to predict reaction conditions and product characteristics. We expect that, in the near future, SCS products will find wide applications in different industrial fields such as energy conversion, storage and optical devices, catalysis, electronics, and biomedical applications.

from the Institute of Chemical Physics, Dr. Sci. (1995) degree from the Institute of Structural Macrokinetics and Materials Science RAS (ISMAN), and was awarded Professor title in 2006. Since 1989 he has been Head of Laboratory for Dynamics of Microheterogeneous Processes in ISMAN. He is also a professor at National University of Science and Technology MISiS, Editor-in-Chief of International Journal of Self-Propagating High-Temperature Synthesis, and Academician of the World Academy of Ceramics. He studies micro- and nanoheterogeneous physical-chemical processes at high temperatures for various applications. Khachatur V. Manukyan received his Ph.D. in Chemistry from Yerevan State University (YSU, Armenia) in 2006. He remained at YSU for several years as a lecturer and was also visiting scientist at the Institute of Chemical Physics, National Academy of Sciences (Armenia), before moving to the Department of Chemical and Biomolecular Engineering, University of Notre Dame. In 2013 he joined the Department of Physics at the University of Notre Dame as Research Associate and is currently Research Assistant Professor. Dr. Manukyan’s research interests are in rapid transformations occurring in reactive solutions and nanocomposites that store and release large amounts of energy and produce nanoscale materials for energy-related applications. He is also interested in interactions of accelerated particles with solids and developing analytical methods for art forensics. He has published nearly 60 research articles in these areas. He has organized professional meetings, and currently serves as an editorial board member of International Journal of Self-Propagating High-Temperature Synthesis. Dr. Manukyan is a recipient of the Presidential Prize and Gold Medal of Republic of Armenia in the Field of Natural Sciences (2009), a Fulbright Scholarship Award (2010), and a Best Electron Microscopy Publication Award of Notre Dame Integrated Imaging Facility (2013).

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Arvind Varma is the R. Games Slayter Distinguished Professor and Jay & Cynthia Ihlenfeld Head, School of Chemical Engineering at Purdue University. His research interests are in chemical and catalytic reaction engineering, synthesis of new materials, and clean energy sources. He has published nearly 300 archival journal research papers in these areas, coauthored three books (Mathematical Methods in Chemical Engineering, Oxford University Press, 1997; Parametric Sensitivity in Chemical Systems, Cambridge University Press, 1999; and Catalyst Design: Optimal Distribution of Catalyst in Pellets, Reactors and Membranes, Cambridge University Press, 2001), and coedited two books. He is the founding Editor (1996−present) of the Cambridge Series in Chemical Engineering, a series of textbooks and monographs published by the Cambridge University Press. Professor Varma received his Ph.D. degree in Chemical Engineering from the University of Minnesota (1972). He has received a number of recognitions for his research and teaching, including AIChE’s Richard H. Wilhelm (1993) and Warren K. Lewis (2013) Awards, and the Chemical Engineering Lectureship Award of ASEE (2000).

ACKNOWLEDGMENTS We acknowledge support from by the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST “MISIS” Grants K2-2015-068 and K3-2016-018. Partial support from the Defense Threat Reduction Agency (DTRA), Grant HDTRA1-10-1-0119, and the R. Games Slayter Fund is also acknowledged. We are thankful to Mr. Christopher E. Shuck for helpful discussions. ABBREVIATIONS AFM atomic force microscopy BCP biphasic calcium phosphate CIE Commission internationale de l’éclairage DSC differential scanning calorimetry DTA differential thermal analysis EDLCs electric double-layer capacitors ETL electron transporting layers EDS energy-dispersive X-ray spectroscopy FSP flame spray pyrolysis GDC gadolinium doped ceria GO graphene oxide HAp hydroxyapatite IALC impregnated active layer combustion ITO In−Sn−O IYO indium−yttrium oxide KAS Kissinger−Akahira−Sunose LSM lanthanum strontium manganite LEDs light-emitting diodes MRI magnetic resonance imaging NIR near infrared emission

Alexander S. Mukasyan obtained his Ph.D. degree at the Institute of Chemical Physics Russian Academy of Sciences (RAS) in 1986 under Professor Alexander Merzhanov, and his Doctorate degree at the Institute of Structural Macrokinetics and Material Sciences (RAS) in 1994. In 1996 he moved to the University of Notre Dame, where from 2000 he has been Research Professor of Chemical and Biomolecular Engineering. From 2008 he has been Director of the Laboratory of Advanced Electron Microscopy of Notre Dame Integrated Imaging Facility. His main research interests are related to the fundamentals of heterogeneous combustion, nanotechnology, high energy density materials, and joining of refractory and dissimilar materials. He has published more than 250 archival journal research papers and patent letters in these areas, coauthored three books (among them Combustion for Material Synthesis, CRC Press, Taylor and Francis, 2015), and coedited one book. Alexander S. Rogachev graduated from the Moscow Institute for Physics and Technology in 1979, and received his Ph.D. degree (1986) 14565


Chemical Reviews Ni-SDC Ni-YSZ OER PL PVA r-GO SEM SCR SHS CS SBF SOFCs SCS TGA TCP 2D XRD XPS YSZ BCY BZY

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Ni-samaria-doped ceria Ni-yttira-stabilized zirconia oxygen evolution reaction photoluminescence poly(vinyl alcohol) reduced GO scanning electron microscope selective catalytic reduction self-propagating high-temperature synthesis combustion synthesis simulated body fluid solid-oxide fuel cells solution combustion synthesis thermogravimetric analysis tricalcium phosphate two-dimensional X-ray diffraction X-ray photoelectron spectroscopy yttira-stabilized zirconia yttrium-doped barium cerate yttrium-doped barium zirconate

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