Spray Solution Combustion Synthesis of Metallic Hollow Microspheres

Mar 16, 2016 - In this method, the combustion reaction in the liquid solution ... Before the measurements, the sample of 30–60 mg was degassed at 20...
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Spray Solution Combustion Synthesis of Metallic Hollow Microspheres G. V. Trusov,‡,§ A. B. Tarasov,†,§ E. A. Goodilin,† A. S. Rogachev,‡,§ S. I. Roslyakov,§ S. Rouvimov,∥ K. B. Podbolotov,⊥ and A. S. Mukasyan*,∥ †

Lomonosov Moscow State University, Moscow 119991, Russia Institute of Structural Macrokinetics and Materials Science, Russian Academy of Science, Chernogolovka 142432, Russia § National University of Science and Technology, “MISIS”, Moscow 119049, Russia ∥ Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States ⊥ Belarusian State Technological University, Minsk 220006, Belarus ‡

S Supporting Information *

ABSTRACT: A spray solution combustion synthesis method has been developed to produce hollow spherical metal nanostructured particles. In this approach, combustion reactions in the liquid solution contribute 100% of the overall energy released during the synthesis process without the involvement of an external gaseous flame. It has been shown that this method is effective for the synthesis of spherical hollow particles of metals (Ni, Cu) with an average diameter of about 3 μm and wall thicknesses of about 20 nm.

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more than 50% of the energy) it is called flame spray pyrolysis (FSP). If a noncombustible solution is fed into the flame, then LAFS is called flame-assisted spray pyrolysis (FASP). However, in all the above-mentioned cases, reactions occur within an external gaseous flame. The unique characteristic of spray-based combustion methods is that they allow the fabrication of spherical particles with relatively narrow size distributions, which is difficult to achieve by conventional SCS approaches. Binary and complex oxides, including perovskites, are typically produced by all the above-mentioned methods. The SCS methods been recently shown to be capable of producing pure metals such as nickel, copper, and Ni−Co−Fe alloys.6,17 In this work, a novel modification of solution combustion synthesis called spray solution combustion synthesis (SSCS) has been developed to produce spherical metal nanostructured particles. In this method, the combustion reaction in the liquid solution contributes 100% of the overall energy released during the synthesis process without the involvement of an external gaseous flame. It has been shown that this method is effective for the synthesis of spherical hollow particles of metals (Ni, Cu) with an average diameter of about 4 μm and wall thicknesses of about 10 nm. Some potential applications of these powders are also discussed.

INTRODUCTION A variety of combustion-based approaches, involving selfsustained chemical reactions, are used for the synthesis of advanced nanomaterials. They can be grouped depending on the physical nature of the reaction medium: (i) flame synthesis (FS), i.e., gas-phase combustion;1,2 (ii) self-propagating high temperature synthesis (SHS) in heterogeneous gasless systems;3,4 (iii) solution combustion synthesis (SCS).5−7 Among them, SCS is a self-sustained reaction in an aqueous solution of oxidizer (e.g., metal nitrate) and oxygen-containing fuel (e.g., glycine). Two basic modes of SCS are known: (a) traditional volume combustion synthesis, which is simple by set up but difficult to control processing, and (b) a well-controlled combustion wave propagation mode. The former includes a rapid reaction (thermal explosion) in a volume of a homogeneous aqueous solution, which can be uniformly heated by a variety of different heat sources (furnace, hot plate, microwave, etc.). In the latter case, the reactive media is preheated only locally, at one point (∼1 mm3), which results in reaction initiation, followed by combustion front propagation along the aqueous solution. Some features of solution combustion synthesis are present in liquid aerosol flame synthesis (LAFS). 8,9 In many modifications of this method, including liquid spray pyrolysis, emulsion CS, and thermally assisted reactions in aqueous sprays; initial precursors are in the liquid form.10−12 In particular, hollow metal and metal oxide particles can be obtained by ultrasonic spray pyrolysis.13−16 Furthermore, if the liquid precursor solution drives the flame process (contributes © 2016 American Chemical Society

Received: January 24, 2016 Revised: March 14, 2016 Published: March 16, 2016 7165

DOI: 10.1021/acs.jpcc.6b00788 J. Phys. Chem. C 2016, 120, 7165−7171

Article

The Journal of Physical Chemistry C

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EXPERIMENTAL SECTION Nickel nitrate hydrate (Ni(NO3)2·6H2O, Alfa Aesar, 98%) and copper nitrate hydrate (Cu(NO3)2·6H2O (Sigma-Aldrich, 99.99%) in the form of 1 M aqueous solutions were used as the metal (Me = Ni, Cu) precursors and glycine (C2H5NO2, Alfa Aesar, 98.5%) as a fuel component. For most of the experiments, the ratio (ϕ) between the fuel and the oxidizer was kept constant and equal to 2, which, as was previously shown,3,6 allows the synthesis of pure metals.6,17 Only one set of experiments was conducted with a more fuel rich composition (ϕ = 3). For comparison, two combustion-based methods were used for the synthesis of metal nanostructured powders. One is a conventional SCS approach in the thermal explosion mode. The other one is the spray solution combustion synthesis (SSCS) method. The synthesis of materials by SCS was performed by heating the reactive solution in a beaker placed on a ceramic hot plate in air. A temperature change of the reaction mixture was measured using a K-type thermocouple (127 μm; Omega Engineering Inc.). The output signal of the thermocouple was collected by a data acquisition system (Data Translation Inc.) and recorded with 1 kHz frequency using LGRAPH software package. A schematic representation of the SSCS experimental setup (Figure S1) with a corresponding brief description are presented in Supporting Information section. The precursors used in the SSCS experiments are similar to those described above for the conventional SCS. The fuel to oxidizer ratio (ϕ), temperature of the furnace, nature of the gas carrier and corresponding flow rates, which were used in experiments for nickel -based systems are summarized in Tables 1. For the

copper nitrate−glycine system, SSCS was performed at Tf = 600 °C with air and argon as the carrier gases, as well as at higher Tf = 750 °C in argon atmosphere. In all the experiments, the gas flow rate was 4 L/min. Typical consumption rate of the precursor and production rate of the particles were about 60 mL/h and 1 g/h, respectively. The structure of the obtained combustion products was characterized by different methods. The crystal lattice was examined using the diffractometer Rigaku D/MAX 2500 (Japan) with a rotating copper anode CuKα radiation, 5−80° 2θ range, and step size 0.02°. To determine the size of crystalline blocks the Scherrer formula was used. The reference data was used from the PDF2 database, cards [33-664], bunsenite (NiO); [4-850], nickel; [5-667], cuprite (Cu2O); and [4-836], copper. A study by electron microscopy was performed by means of the scanning electron microscope Leo Supra 50VP (Carl Zeiss, Germany), equipped with EDX/WDX system INCA Energy + (Oxford). The transmission electron microscope Titan 80−300 (FEI, USA), with a resolution of 0.136 nm in STEM mode and ∼0.1 nm information limit in HRTEM mode, was employed to characterize the local composition and morphology of the reaction products. The setup was also equipped with an energy dispersive X-ray spectroscopy (EDS, Oxford Inca) system with spectral energy resolution of 130 eV. Measurements of the specific surface area (SSA) of the powders were carried out by the low-temperature method of nitrogen adsorption using the analyzer of textural characteristics ATC-06 (Katakon, Novosibirsk). Helium (mark “A”) was used as the carrier gas. Before the measurements, the sample of 30− 60 mg was degassed at 200 °C in a stream of dry helium for 30 min. The specific surface area of the powder was calculated using a Brunauer−Emmett−Teller (BET) model and five points in the range of the partial pressure of nitrogen P/Po between 0.05 and 0.20 atm.

Table 1. Parameters for the SSCS in the Ni(NO3)2−Glycine System

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Ni(NO3)2−Glycine expt no.

Tf, °C

ϕ

carrier gas

flow rate, L/min

1 2 3 4 5 6 7 8

400 500 550 600 650 700 750 750

2 2 2 2 2 2 2 2 or 3

air argon argon argon argon argon air argon

1 1 or 2 2 4 4 4 1 or 4 4

RESULTS Conventional Solution Combustion Synthesis. Typical temperature−time profiles for SCS in these systems are shown in Figure 1a,b. When the metal nitrate and glycine aqueous solution is preheated to a temperature of ∼100 °C an intensive evaporation of water occurs leading to the formation of a viscous gel. Furthermore, at some specific temperature, i.e. ∼160 °C for both the systems, a rapid temperature change takes place, which indicates the initiation of the reaction. This process is associated with an intensive foaming of the gel, owing to release of a large amount of gaseous products. It can be seen

Figure 1. Time−temperature profile of SCS in different systems: (a) Ni(NO3)2 + glycine; (b) Cu(NO3)2 + glycine. 7166

DOI: 10.1021/acs.jpcc.6b00788 J. Phys. Chem. C 2016, 120, 7165−7171

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The Journal of Physical Chemistry C

Figure 2. XRD patterns of conventional SCS products in different systems: (a) Ni(NO3)2 + glycine; (b) Cu(NO3)2 + glycine.

that the maximum reaction temperature reaches ∼635 °C for nickel (Figure 1a) and ∼420 °C for copper (Figure 1b) based systems correspondingly. The XRD patterns for the Ni-based product are presented in Figure 2a. It can be seen that the produced powder includes nickel and traces of NiO. A calculated size of crystalline blocks for these phases are 10 ± 2 nm and 22 ± 4 nm, respectively. The corresponding data for Cu-based system (Figure 2b) also reveal the presence of two phases, i.e., copper(I) oxide (Cu2O) and metallic copper (Cu). The size of crystalline blocks for the Cu2O phase is 22 ± 4 nm and that for the Cu phase is 15 ± 3 nm. Typical microstructures of the products obtained in the Niand Cu-based systems are shown in Figure 3. It can be seen that

in both the cases, agglomerates have a porous structure. The high temperature achieved in the mixture during the formation of metals, promotes sintering, which explains the relatively low specific surface area, i.e., 4 ± 1 m2/g for Ni-based (Figure 3a,b) and 3 ± 1 m2/g for Cu-based products (Figure 3c,d). Spray Solution Combustion Synthesis. As demonstrated elsewhere5,6,18 and also confirmed in this work, the SCS products obtained in the thermal explosion mode are predominantly micrometer-size porous aggregates with a relatively high specific surface area. Such an irregular structure of the obtained materials affects their functional properties and thus limits their applications. To resolve this problem and produce a material with a fine controllable morphology, we suggest a novel modification of combustion synthesis, spray solution combustion synthesis (SSCS, Figure S1), which combines the concepts of solution combustion synthesis and aerosol spray pyrolysis. Typical XRD patterns for combustion products of the Nibased system synthesized in air and argon are shown in Figure 4, parts a and b, respectively. It can be seen that powder obtained with a furnace temperature of 400 °C in air (experiment no. 1, Table 1) is amorphous (Figure 4a). Experiments at higher temperature (750 °C) in air (experiment no. 7, Table 1) lead to the formation of a pure NiO phase. If an inert gas is used as a carrier, Ni and NiO phases were detected (experiment nos. 2−4, Table 1) at T ≤ 600 °C, while at higher temperatures the final product is a pure crystalline Ni (Fm3m) phase (experiment nos. 5, 6, and 8, Table 1). An increase of the furnace temperature leads to an increase of the product crystallinity. It is worth noting that the size of crystalline blocks of the material obtained in argon at 750 °C is much smaller than that synthesized in air flow (60−82 nm). Even a more striking difference is found when compare the microstructures of the powders synthesized under SCS (Figure 3) and SSCS (Figure 5) conditions. Crystalline NiO (Figure

Figure 3. Microstructure of SCS products: (a, b) Ni-based and (c, d) Cu-based systems.

Figure 4. XRD patterns of SSCS products in Ni (NO3)2−glycine system obtained at different furnace temperature and carrier gas: (a) air; (b) argon. 7167

DOI: 10.1021/acs.jpcc.6b00788 J. Phys. Chem. C 2016, 120, 7165−7171

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Figure 5. Microstructure of different SSCS products in the Ni-based system: (a−c) 750 °C, air flow; (d−f) 750 °C, argon flow.

Figure 6. XRD pattern of the SSCS products of mixture Cu(NO3)2 + glycine solution obtained at different temperatures of the furnace and different carrier gasses: (a) air; (b) argon.

Figure 7. Microstructure of the SSCS products obtained by using different carrier gas: (a) air at 600 °C; (b, c) argon at 750 °C.

temperature range (500−750 °C). However, by using inert gas carrier at Tf ≥ 650 °C we produced almost pure copper with some traces of Cu2O at Tf ≤ 600 °C. Microstructures of the corresponding products, i.e., pure Cu2O and pure Cu, are presented in Figure 7. It can be seen that all the products have a spherical morphology. In the case of metal oxide (Figure 7a), the size of the obtained microspheres varies from 0.5 to 3 μm. The surface of these microspheres contains nanoparticles with a crystallite block size of 16 ± 3 nm. For pure metals, sphere-like powder obtained at 750 °C (Figure 7b,c) are characterized by a bimodal distribution with maxima at 2 and 0.5 μm. However, the surfaces of the spheres consists of nanoparticles (Figure 7c). An estimation by the Scherrer formula gives the crystalline size 29 ± 4 nm. The specific surface area is 5 m2/g for copper spheres.

5a,b) and Ni (Figure 5d,e) powders synthesized at Tf ∼ 750 °C in air and argon have both a spherical morphology. It was revealed that the size of crystalline particles is in the range of 0.5−3 μm (Figure 5a,d), what correlates with a size of droplets, produced by used nebulizer. A TEM observation (Figure 5b,c,e,f) reveals that all crystalline spheres are hollow, with their wall thickness of about 25−50 nm. It is also evident that the surface of the obtained microspheres consists of 10−20 nm crystallites corresponding well to the calculated crystalline blocks size 21 ± 4 nm from the Scherrer formula. The specific surface area (BET) is found to be ∼10 m2/g for nickel spheres. Figure 6 shows the XRD patterns for products obtained in the Cu-based system by using air (Figure 6a) and argon (Figure 6b) atmospheres. It can be seen that primarily Cu2O phase is formed under air conditions for the whole investigated 7168

DOI: 10.1021/acs.jpcc.6b00788 J. Phys. Chem. C 2016, 120, 7165−7171

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DISCUSSION The method of aerosol spray pyrolysis (ASP) is characterized by a relatively predictable morphology and size of the produced powders, determined by the dispersion of the aerosol and the processes, inside the droplets during their transport through the furnace hot zone. In general, it is known that ASP allows the formation of spherical particles.8−12 It is also recognized that under certain conditions particles can be amorphous or crystalline, bulk or hollow with different wall thicknesses. The novelty of the suggested spray solution combustion synthesis (SSCS) method is that it permits the production of spherical hollow microspheres of pure metals in one step. Indeed, if one uses the aqueous solution of a metal nitrate as a precursor in the ASP scheme, in both air and inert atmosphere, decomposition of nitrate in the droplet results in the formation of a metal oxide based product. Our results demonstrate that in the case of SSCS, where the reactive solution of the oxidizer (metal nitrate) and the fuel (e.g., glycine) is utilized, a complex reaction in such droplets results in the formation of pure metal spherical particles. To understand the mechanism it is useful to briefly overview our recent results on the combustion mechanism in such reactive solution under the conventional SCS conditions.17,19 On the basis of a variety of in situ and post-mortem studies, it was concluded that the main reaction that controls the combustion process take place in the gas phase between the gaseous product of decomposition of both oxidizer and fuel reaction 1 and 2. For example, in the case of nickel nitrate− glycine system it is the extremely exothermic reaction between N2O and NH3 species reaction 3: Ni(NO3)2 → NiO + N2O + 2O2

(1)

C2H5NO2 + 3/2O2 → NH3 + 2CO2 + H 2O

(2)

3N2O + 2NH3 → 4N2 + 3H 2O + Q

(3)

However, it is important to understand how individual spherical particles form during the combustion of the droplet in a flow. In the considered case, the mist is not a continuous heterogeneous media along which the reaction may propagate in a self-sustaining fashion. The ultrasonic nebulizer with a working frequency of 2.64 MHz, used in this work, allows the formation of initial droplets with size range 0.5−5 μm. This aerosol at a controlled feeding of the carrier gas enters the quartz reactor located in the tube furnace. The temperature (Td) of the droplet that consists of a uniformly mixed aqueous solution of the oxidizer and the fuel starts to increase. At a some point, intensive water evaporation begins. Assuming a diameter of the droplet D ∼ 5 μm, let us consider several characteristic times, which are important for further analysis: (a) Thermal relaxation time along the droplet (trelax). This parameter indicates the time required for the temperature to become uniform along the droplet volume and can be estimated as follows: trelax = r 2/α

where r is droplet radius and α = the thermal diffusivity of the solution. Taking r = D/2 = 2.5 μm and α ∼ 2 × 10 −3 cm2/s (typical for aqueous solutions; e.g., for water α = 1.47 × 10−3 cm2/s), one can estimate that trelax ∼ 3 × 10 −5, s = 30 μs (b) Characteristic time for water evaporation (tev). Estimation of evaporation rate K = dD2/dt for water droplets of the diameter of 5 μm under low humidity conditions (also see eq S2 in the Supporting Information), sedimentation velocity (0.3 cm/s), and room temperature (20 °C) gives values ∼103μm2/s.20 It means that the droplet completely evaporates at tevapor ∼ 0.1 s. It is worth noting that all these characteristic times are much smaller than the average lifetime of droplets in the furnace, which, for all used flow rates, exceeds 10s (see eq S1). Next, it was experimentally shown that the amorphous gel and NiO powders were detected on the filter at Tf = 400 °C (experiment no. 1, Table 1). An increase of a flow rate at this temperature leads to the decrease of the amount of NiO powder. At higher temperature 450 °C < Tf < 700 °C well crystalline product is detected: it is NiO under airflow, while the product involves both Ni and NiO (111) phases (Figure 4) in argon. Good crystalline Ni is detected in argon at temperature above 700 °C. To explain the results, one may assume that the preheating of the droplets and the surrounding initially cold carrier gas from room temperature to the ignition temperature (Tig ∼ 250 °C) is a limiting stage of the SCS process. Experimental measurements show that the width of the zone where T exceeds Tig, at flow 1 L/min and, e.g., Tf = 400 °C is ∼20 cm, with a temperature difference along the tube cross section which does not exceed 30 °C. These parameters for 4 L/min are as follows: 17 cm and 50 °C. Thus, under all of the above conditions, the aqueous mist should be preheated to the ignition temperature.21 Another limiting stage for the self-sustained reaction, which could define the SCS process, is the kinetics of chemical reactions. During ignition a single droplet acts as a microreactor, conforming to the one-droplet-to-one-particle (ODOP) theory.22,23 As mentioned above (see Figure 2S), the overall reaction scheme in an inert gas atmosphere involves three main stages (i) decomposition of precursors, reactions 1

The adiabatic combustion temperature of the sum of reactions 1−3) is in the range of 1000−2000 °C and is a function of the amount of water in the initial solution. In the case of the excess of fuel (ϕ > 1), e.g., of glycine (C2H5NO2), we have the corresponding excess of NH3 species in the combustion wave. Ammonia reacts with the metal oxide phase formed during the decomposition of nitrate, reducing it to pure metal: 3NiO + 2NH3 → 3Ni + N2 + 3H 2O

(4)

The reaction mechanism in the SCS wave is presented schematically in Figure 2S. It is shown that the combustion wave can be subdivided in several characteristics regions: I, initial reactive solution (gel); II, preheating zone in which evaporation of water takes place; III, reaction zone with maximum temperature, where the gaseous exothermic reactions occur; IV, postcombustion zone with reduction of metal oxide to pure metal. Intensive gas evolving leads to the formation of porous structures with a high surface area (see Figure 3). It is a specific feature of SCS that the reactions, which govern the combustion process and the reaction, of the formation of final solid product, are separated in space. The combustion process occurs in a gas phase in the combustion zone while the reduction to form metal takes place on the surface of the oxide particle in the postcombustion region. One may expect that the similar sequence of reactions lead to the formation of pure metal during the SSCS conditions. 7169

DOI: 10.1021/acs.jpcc.6b00788 J. Phys. Chem. C 2016, 120, 7165−7171

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Figure 8. Scheme of reactions in aqueous aerosol droplets during synthesis.

surface, leading to a concentration gradient which is much higher than that in the central region of the droplet.26,27 So metal oxide crystallizes and grows on the surface of the droplet, followed by coalescence/sintering, and thus one droplet forms a hollow metal oxide sphere. On the third stage, in the case of excess fuel (ϕ > 1) and inert gas carrier, the existence of the reduction atmosphere around the spheres leads to the reduction of the oxide layer with formation of pure metal shell according to reaction 4. The above-mentioned scheme, which considers the critical conditions for the self-sustained reaction initiation in the thermal explosion mode, allows us to explain all the experimentally observed specifics of hollow spherical particle formation in the inert gas flow of the dispersed droplets. The furnace temperature (Tf) and temperature distribution in the tubular reactor as dependent on flow rate of gas carrier, are the main parameters that control the SSCS process. Optimization of these parameters allows for the synthesis of hollow spherical pure metal (Ni and Cu) particles in continuous fashion. Spherical hollow metal particles may have a variety of applications including catalysis, drug delivery and fabrication of highly porous structured materials. For example, Ni-spherical particles were used as a catalyst for hydrogen production via ethanol decomposition at relatively low temperature under conditions described elsewhere.28 It was shown in Figure 3S that hollow particles allow for higher hydrogen selectivity as compared to those powders produced by conventional SCS. Currently we are working on the direct synthesis spray combustion synthesis of supported Ni and Cu−based catalyst, which as shown in a current work28 should possess even higher activity and selectivity at temperatures below 200 °C.

and 2; (ii) gas phase combustion, reaction 3; and (iii) reduction of metal oxide, reaction 4. The decomposition reaction for metal nitrates and glycine start at ∼200 °C.17,19 These reactions are much less exothermic as compared to combustion and reduction reactions,17 which means that if the temperature of the droplet (e.g., D = 5 μm in diameter) is higher than 250 °C, both the precursors should decompose relatively quickly with the formation of reactive gel and gaseous products (reactions 1 and 2). Because the temperature of the droplet is either below or approximately equal to that of the environment, there is essentially no heat lost from the droplet. Combustion and reduction (reactions 3 and 4) are entirely different cases. These reactions are extremely exothermic,17 with adiabatic combustion temperature above 1000 °C. In order for these reactions to occur in a self-sustained manner, i.e. in the thermal explosion (TE) mode, the critical conditions between heat release and heat loss rates should be obeyed.24 Equation S5 indicates that for each diameter of the particle (D), the critical temperature of the furnace (Tfc) exists below which the reaction does not proceed in a self-sustained manner. It is worth noting that the size of the droplets in our experiments is small (2−5 μm), which suggests that, at relatively small Tf, reactions 3 and 4 do not significantly contribute to the overall process. The above hypothesis explains why at Tf < 500 °C, which is above the decomposition temperatures for all precursors, primarily oxide phases (NiO, Cu2O) are formed, while at higher Tf in an inert environment, the only reaction product is pure metal (Ni, Cu). Moreover, model experiments, which involve the heattreatment of the droplets on the surface of the hot plate with surface temperature 400 °C in argon flow, show that if the size of the droplet is less than 100 μm it does not combust in a selfsustained manner during 30 s of the experiments. Such small particles instead just decomposed to produce a metal oxide phase. Taking into account the above considerations, the overall scheme of the spray solution combustion synthesis process can be described as follows (also see Figure 8). On the first stage, the droplets with an average size of 3.6 μm, which consists of the reactive solution of metal nitrate and glycine, enter the inlet of the tubular furnace and begin preheating. When the temperature rises, hollow spheres of dried nitrate−glycine mixture form due to rapid water evaporation and high concentration of reagents.25 On the second stage, decomposition of the precursors takes place. If the temperature of the furnace is above the critical value, simultaneous with the precursor’s decomposition, the gas-phase exothermic reaction initiates in each droplets (microcombustors). Next, it is reasonable to assume that these reactions resulted in MeO nuclei formation first on the surface of the droplet, then growing to small oxide grains, thus forming the solid spherical skeleton. As suggested previously for the spray pyrolysis, the rate of chemical reaction on the surface is much faster than diffusion, causing rapid hydrolysis of the droplet

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CONCLUDING REMARKS

We reported a novel modification of the solution combustion synthesis: spray solution combustion synthesis, for production of nanostructured metal hollow microspheres. The furnace temperature, reactive solution composition, nature of carrier gas and its flow rate are critical parameters, which define the phase composition of the produced microspheres. By variation of these parameters one may produce pure metal (Ni,Cu), pure oxides (NiO, Cu2O) or mixture of these phases. It was shown that an outer diameter of the sphere is primarily defined by the diameter of the initial droplets formed by ultrasonic nebulizer. The observed wall thickness of the sphere was in the range 20− 50 nm. Additional studies are required to define parameters that control wall thickness. The suggested method can be used for production of particles. We believe that the developed SSCS method is pretty universal and allows synthesis of almost all known oxides and a variety of hollow microspheres, including pure metals, such as Cu, Ni, Co, and metal alloys (NiCu, NiCuFe, CoCu, etc.), except of highly reactive (Al, Mg) and alkali metals. 7170

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(15) Bang, J. H.; Didenko, Y. T.; Helmich, R. J.; Suslick, K. S. Nanostructured Materials through Ultrasonic Spray Pyrolysis. Aldrich Mater. Matter 2012, 15−20. (16) Helmich, R. J.; Suslick, K. S. Chemical Aerosol Flow Synthesis of Hollow Metallic Aluminum Particles. Chem. Mater. 2010, 22, 4835− 4837. (17) Manukyan, K. V.; Cross, A.; Roslyakov, S.; Rouvimov, S.; Rogachev, A. S.; Wolf, E. E.; Mukasyan, A. S. Solution Combustion Synthesis of Nano-Crystalline Metallic Materials: Mechanistic Studies. J. Phys. Chem. C 2013, 117, 24417−24427. (18) Patil, K. Chemistry of Nanocrystalline Oxide Materials: Combustion Synthesis, Properties and Applications; World Scientific Publishing Company: 2008. (19) Roslyakov, S. I.; Kovalev, D. Y.; Rogachev, A. S.; Manukyan, K. V.; Mukasyan, A. S. Solution Combustion: Dynamic of Phase Transformation during Synthesis of High Porous Nickel. Dokl. Phys. Chem. 2013, 449, 48−51. (20) Williamson, R.; Threadgill, E. A Simulation for the Dynamics of Evaporating Spray Droplets in Agricultural Spraying. Trans. ASAE 1974, 17, 254−261. (21) Widiyastuti, W.; Wang, W.-N.; Lenggoro, I. W.; Iskandar, F.; Okuyama, K. Simulation and Experimental Study of Spray Pyrolysis of Polydispersed Droplets. J. Mater. Res. 2007, 22, 1888−1898. (22) Kang, H. S.; Kang, Y. C.; Park, H. D.; Shul, Y. G. Morphology of Particle Prepared by Spray Pyrolysis from Organic Precursor Solution. Mater. Lett. 2003, 57, 1288−1294. (23) Shabde, V. S.; Emets, S. V.; Hoo, K. A.; Carlson, N. N.; Mann, U.; Gladysz, G. M. Modeling a Hollow Micro-Particle Production Process. Comput. Chem. Eng. 2005, 29, 2420−2428. (24) Rogachev, A. S.; Mukasyan, A. S. Combustion for Material Synthesis; CRC Press: 2015. (25) Nandiyanto, A. B. D.; Okuyama, K. Progress in Developing Spray-Drying Methods for the Production of Controlled Morphology Particles: From the Nanometer to Submicrometer Size Ranges. Adv. Powder Technol. 2011, 22, 1−19. (26) Chatterjee, M.; Enkhtuvshin, D.; Siladitya, B.; Ganguli, D. Hollow Alumina Microspheres from Boehmite Sols. J. Mater. Sci. 1998, 33, 4937−4942. (27) Che, S.; Sakurai, O.; Shinozaki, K.; Mizutani, N. Particle Structure Control Through Intraparticle Reactions by Spray Pyrolysis. J. Aerosol Sci. 1998, 29, 271−278. (28) Cross, A.; Roslyakov, S.; Manukyan, K. V.; Rouvimov, S.; Rogachev, A. S.; Kovalev, D.; Wolf, E. E.; Mukasyan, A. S. In Situ Preparation of Highly Stable Ni-Based Supported Catalysts by Solution Combustion Synthesis. J. Phys. Chem. C 2014, 118, 26191− 26198.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b00788. Scheme of the experimental setup, structure of the solution combustion wave, calculations of liquid droplet evaporation kinetics, thermal explosion criteria, catalytic properties of the obtained materials, and X-ray diffraction patters for samples obtained under different conditions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(A.M.) E-mail: [email protected]. Telephone: +1-574-6319825. Fax: +1-574-631-8366. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of MISiS and financial support of the RFBR from Grant Nos. 15-5304066 Bel_mol_a and BRFBR, researched Project No. X15PM022. The authors acknowledge partial support from the M. V. Lomonosov Moscow State University Program of Development for providing analytical research of the materials.

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REFERENCES

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DOI: 10.1021/acs.jpcc.6b00788 J. Phys. Chem. C 2016, 120, 7165−7171