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Cite This: Chem. Rev. 2018, 118, 3209−3250

Stability and Reactivity: Positive and Negative Aspects for Nanoparticle Processing Liang Xu, Hai-Wei Liang, Yuan Yang, and Shu-Hong Yu* Division of Nanomaterials and Chemistry, Hefei National Research Center for Physical Sciences at the Microscale, CAS Centre for Excellence in Nanoscience, Collaborative Innovation Center of Suzhou Nano Science and Technology, Hefei Science Centre of CAS, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China ABSTRACT: Nanoparticles exist far from the equilibrium state due to their high surface energy. Nanoparticles are therefore extremely unstable and easily change themselves or react with active substances to reach a relatively stable state in some cases. This causes desired changes or undesired changes to nanoparticles and thus makes them exhibit a high reactivity and a poor stability. Such dual nature (poor stability and high reactivity) of nanoparticles may result in both negative and positive effects for nanoparticle processing. However, the existing studies mainly focus on the high reactivity of nanoparticles, whereas their poor stability has been neglected or considered inconsequential. In fact, in some cases the unstable process, which is derived from the poor stability of nanoparticles, offers an opportunity to design and fabricate unique nanomaterials, such as by chemically transforming the “captured” intermediate nanostructures during a changing process, assembling destabilized nanoparticles into larger ordered assemblies, or shrinking/processing pristine materials into the desired size or shape via selective etching. In this review, we aim to present the stability and reactivity of nanoparticles on three levels: the foundation, concrete manifestations, and applications. We start with a brief introduction of dangling bonds and the surface chemistry of nanoparticles. Then, concrete manifestations of the poor stability and high reactivity of nanoparticles are presented from four perspectives: dispersion stability, thermal stability, structural stability, and chemical stability/reactivity. Next, we discuss some issues regarding the stability and reactivity of nanomaterials during applications. Finally, conclusions and perspectives on this field are presented.

CONTENTS 1. Introduction 2. Foundation of the Stability and Reactivity of Nanoparticles 2.1. Dangling Bonds of Nanoparticles 2.2. Surface Chemical Microenvironment 3. Stability and Reactivity of Nanoparticles 3.1. Dispersion Stability of Nanoparticles 3.1.1. Factor-Dependent Dispersion Stability of Nanoparticles 3.1.2. Heteroaggregation of Nanoparticles 3.2. Thermal Stability of Nanoparticles 3.3. Structural Stability of Nanoparticles 3.3.1. Ostwald Ripening and Digestive Ripening 3.3.2. Oriented Attachment Involved with the Surface Chemical Microenvironment 3.3.3. Transformation of Nanoparticles Induced by Stabilizer Depletion 3.3.4. Superlattices of Nanoparticles Induced by Destabilization 3.4. Chemical Reactivity and Stability of Nanoparticles 3.4.1. Chemical Transformation of Nanoparticles Based on High Reactivity 3.4.2. Oxidative Stability of Nanoparticles © 2018 American Chemical Society

4. Stability of Nanoparticles during Application 4.1. Stability of Nanoparticles in Devices 4.2. Stability of Nanoparticles in Biomedicine 4.3. Stability of Nanoparticles in the Environment 5. Conclusions and Remarks Author Information Corresponding Author ORCID Notes Biographies Acknowledgments Abbreviations Used References

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1. INTRODUCTION Nanoscale materials have attracted worldwide attention and emphasis since the end of the last century. Unlike bulk materials, by shrinking the size of materials to the nanoscale, the role of the surface becomes dominant, which can significantly change some properties of materials (such as the melting point and surface/volume ratio), even furnishing them

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Received: April 15, 2017 Published: March 8, 2018 3209

DOI: 10.1021/acs.chemrev.7b00208 Chem. Rev. 2018, 118, 3209−3250

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Figure 1. (a) The covalent bond classification of L, X, and Z ligands. (b) Classification of ligand binding motifs at the surface of a CdSe NP. Panel a reprinted with permission from ref 29. Copyright 2014 American Chemical Society. Panel b reprinted with permission from ref 1. Copyright 2016 Nature Publishing Group.

many functional nanomaterials can be prepared to better meet the requirements of practical applications, such as good stability, high sensitivity, or rapid dynamic response. (iii) The ultimate goal of the application of NPs is to realize a certain function. Processing NPs into complex entities to realize certain functions is a task at the boundary of chemistry, physics, material science, electronics, and engineering. Many explorations and cooperation from different fields are required to promote the rapid development of nanomaterials. From a chemistry perspective, NPs with high surface energy exhibit a dual nature: poor stability and high reactivity. On the one hand, due to the high reactivity of nanomaterials, they can be easily processed into desired functional nanomaterials by suitable chemical transformations, such as ion exchange and template synthesis;16,22 on the other hand, the high reactivity of NPs also makes them unstable; i.e., they can be easily oxidized or degraded during use.6 This binary cooperative complementary phenomenon can also be observed at many different levels in the universe and in nature,23 and it is regarded as a universal principle to design and construct novel functional materials.24,25 For NPs, the unstable process derived from their poor stability can also be used to design and fabricate unique nanomaterials by capturing intermediate nanostructures and then, based on their high reactivity, transforming them into special functional nanomaterials that are inaccessible via the existing direct synthesis methods. Therefore, the unstable process of NPs sometimes actually provides an opportunity for a new discovery. This phenomenon is similar to the intertransformation of the “Yin” and “Yang” in ancient Chinese philosophy and the dynamic coexistence of two opposite elements in dialectics.24,25 For example, the oxidation of NPs causes the degradation of their performance, but oxidative

with new effects (for example, the quantum confinement effect, surface plasmon resonance, and size-dependent physicochemical properties).1 These unique features at the nanoscale have prompted development of the research and commercialization of nanomaterials and nanoproducts.2 The number of nanomaterials produced annually is rapidly increasing,3 and these fascinating nanomaterials are widely used in many fields, such as sensing, energy storage, environmental remediation, catalysis, biological imaging and diagnostics, and nanoelectronics.4 The ultimate goal of nanomaterial research is to utilize nanoparticles (NPs) themselves or assemble them into complex entities to realize certain functions. To improve the performance of NPs and nanodevices, three main issues should be considered: stability, reactivity, and function. (i) As the foundation of the application of NPs, NPs must maintain a stable state during storage, processing, and utilization. Unfortunately, almost all NPs exist far from the equilibrium state and exhibit a high surface energy,5 meaning that NPs are unstable and that they require a stabilizing environment (such as a surfactant coating or the use of supports) to prevent aggregation and to maintain high reactivity during practical applications.6 (ii) The high surface energy not only makes NPs unstable but also makes their reactivity strongly size-dependent.3,7 Below a critical size, it is impossible to simply predict the properties of NPs by scaling the properties of bulk materials on the basis of the surface area.3 For instance, bulk noble metals are chemically inert and resistant to oxidation, but NPs of noble metals, such as Au NPs,4,8,9 Pt NPs,10−13 and Ag NPs,14,15 are chemically active and can be easily oxidized. Due to their high reactivity, NPs can be easily processed into functional materials via reasonable conversions under moderate conditions. These conversions include redox reactions, galvanic exchanges, ion exchanges, and so on.16−21 On the basis of these conversions, 3210


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Figure 2. Schematic illustration of the dangling bonds on a crystal surface.

surface atom and ligand headgroup can be rationalized using the classification of covalent bonds.1 Owen and co-workers30,31 proposed that interactions between NPs and surface ligands mainly included three classes of metal−ligand interaction:29,32 X-type, L-type, and Z-type (Figure 1). L-Type ligands are twoelectron donors and coordinate surface atoms with a lone electron pair; examples of these ligands include amines, phosphines, and phosphine oxides. X-Type ligands are oneelectron donors; thus, they require surface atoms to provide one electron to form a two-electron covalent bond. X-Type ligands typically include carbohydrates, thiolates, inorganic ions, and bound ion pairs. As zero-electron donors, Z-type ligands accept two electrons from surface atoms to bind with them. In view of this classification, for CdSe NPs coated with both Land X-type ligands, their composition and surface components can be precisely described as (CdSe)m(CdnXp·Lq), where m is determined by the size of the NP core, while the other parameters (n, p, and q) are dependent on the ligand shell composition.1 Unlike bulk materials, the ligand shell of NPs greatly affects their reactivity and stability. This topic will be discussed in detail in the next section. In addition, NP surfaces have many crystallographic defects that cause irregular arrangements of atoms or molecules at the crystal surface. In some cases, these irregular arrangements may create an incomplete bonding environment for the atoms around the defects, thus changing the reactivity of these atoms. This incomplete bonding environment mainly depends on two factors: the number of defects and the aggregation degree of the defects. To simplify the discussion of this issue, herein, we select vacancy defects to discuss the relationship between the reactivity and the defects. A vacancy defect occurs where an atom is missing from the normal crystalline array and results in unsaturation of the neighboring atoms. As shown in Figure 2, the saturation of the atoms around vacancy defects decreases as the number and aggregation degree of the defects increase. Compared with normal atoms in a crystalline array, these unsaturated atoms induced by vacancy defects may exhibit higher reactivity. Thus, for NPs, some defects are also a key factor that determines their reactivity.

etching is a useful tool for processing NPs into desired sizes and shapes.26,27 In this review, we systematically present the stability and reactivity (S&R) of NPs on three levels: their foundation, concrete manifestations, and applications. First, a brief introduction of dangling bonds on NP surfaces is given, followed by a discussion of the new viewpoint on the S&R of NPs: Nano@SCME (SCME: surface chemical microenvironment). Second, concrete manifestations of the S&R of NPs are presented in depth from four perspectives: dispersion stability, thermal stability, structural stability, and chemical stability/ reactivity. Next, the stability problems of nanomaterials in application are synoptically discussed, especially regarding NPs that are used in device, biomedicine, and environmental applications. Finally, conclusions and perspectives on this interesting field are presented.

2. FOUNDATION OF THE STABILITY AND REACTIVITY OF NANOPARTICLES 2.1. Dangling Bonds of Nanoparticles

Atoms in materials generally exhibit two different bonding environments: saturated internal atoms and unsaturated surface atoms. Different from the saturated internal atoms, the unsaturated surface atoms leave some chemical bonds “dangling” on the outside, i.e., dangling bonds. NPs obtained by any method all have a large number of surface atoms, leading to an increase in the number of unsaturated surface atoms. This increase can significantly enhance the density of dangling bonds on NP surfaces and therefore increase the surface energy of NPs. The surface energy determines how a surface interacts with the outside environment. A surface with a higher energy is more reactive, as it will tend to reduce its own energy by interacting with suitable substances from the outside environment.28 Hence, unsaturated surface atoms usually exhibit higher reactivity than saturated ones due to the higher surface energy derived from their incomplete bonding environments. As the size of materials decreases, the total surface area exponentially increases, along with the number of unsaturated surface atoms with high reactivity. Thus, compared with bulk materials, NPs exhibit much higher reactivity. However, NP surfaces generally are not “naked” but rather are coated with some ligands, which render the NPs more stable by saturating the dangling bonds and shielding them from the external environment. Interactions between the NP

2.2. Surface Chemical Microenvironment

It is well-known that “naked” NPs are thermodynamically and kinetically unstable in solution.33 NPs generally require a stabilizing environment to prevent aggregation and to maintain high reactivity. For instance, most NPs that are obtained by 3211


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“wet chemical” synthesis are coated with certain ligands, as illustrated in eq 1: MR + NR + ligands → MP@Ligands + NB

functionalizing components are more focused on the modulation of the chemical and physical properties, as well as the toxicity and the biocompatibility of NPs. In addition, in some cases the components coated on the surface of NPs can simultaneously play both roles: stabilization and functionalization.

(1)

where MR is the raw material, which corresponds to the precursor of the products. NR is the assisted reactant, which reacts with MR to generate the product. In some cases, NR is not necessary, such as in thermal decomposition reactions. Ligands are usually organic compounds that have a donor group with substituents of varying steric bulk, and they are used to prevent undesired aggregation to form well-dispersed NP suspensions. In addition, ligands also play an important role in controlling the size and shape of NPs by preferentially binding with a certain crystal facet to modulate the growth rate of NPs. MP@Ligands is the well-dispersed product coated with ligands. NB is the byproduct of the synthesis reaction. However, surface components of NPs are not static after synthesis; they develop, change, and respond to the surrounding environment during storage and application. On the one hand, NP surfaces can easily adsorb active molecules from the outside environment; on the other hand, they can be modified by functional ligands via ligand exchange to modulate their interfacial properties.34−37 Hence, the surfaces of most NPs are coated by not only original ligands but also adsorbed components and modified ligands. These chemical substances form a surface chemical microenvironment around the NP “core”, i.e., Nano@ SCME. The cores possess useful properties that are controlled by their composition, size, and shape.38 The external microenvironment ensures that the internal core is protected and stabilized; meanwhile, it greatly affects the S&R of the NPs. For example, the stability and catalytic activity of Pd and PdAg NPs can be modulated by changing the composition of the metal core and the monolayer of organic ligands on their surfaces.39 As an interface, by bridging the gap between the outside environment and NPs, the SCME has a wide range of impact on the application of NPs, ranging from biotechnology to advanced microelectronics.40 Biological labeling requires a water-soluble and nontoxic surface layer, and a polymerizable surface is beneficial for fabricating photoluminescent polymer composites, while a conductive layer is important for the preparation of electrical devices.41 The performance of NPs greatly depends on their SCME, because the constituent, the packing density, and the wetting properties of their SCME are closely related to their dispersibility, reactivity, and affinity. In general, the components of the SCME can be classified on the basis of function into two broad categories: components used to stabilize NPs and components used to functionalize NPs. Stabilizing components (surfactants, charged ions, and so on) are coated on the surface of NPs by complexation, electrostatic attraction, and encapsulation to reduce their surface energy and stabilize them.42 These components exhibit a higher affinity for the “core” of NPs and play an important role in NP stabilization. When excess stabilizing components are removed from the NP surfaces, the NPs become unstable43 and gradually decompose, resulting in a change in their properties. In addition, stabilizing components used to synthesize NPs are often neither suitable nor robust enough for many NP applications.37,44 Therefore, NPs must be functionalized with suitable components via a postprocessing step to enhance stability, reactivity, and affinity, along with other specific properties. The functionalization methods include ligand exchange,45−48 organic grafting,49−53 epitaxial growth,54−58 and so on.59−62 Compared with stabilizing components,

3. STABILITY AND REACTIVITY OF NANOPARTICLES Herein, we discuss the concrete manifestations of the S&R of NPs from four different aspects: dispersion stability, thermal stability, structural stability, and chemical stability/reactivity. 3.1. Dispersion Stability of Nanoparticles

In this section, Au NPs are used as a model to present their factor-dependent dispersion stability, which is influenced by their size, the polarity and the number of molecules coated on the surfaces, the solvent polarity, and so on. Then, the heteroaggregation of NPs is briefly discussed, and several cases on the preparation of functional nanocomposites via heteroaggregation are presented. 3.1.1. Factor-Dependent Dispersion Stability of Nanoparticles. As previously discussed, a NP is not a singlecomponent substance but rather a composite that has a solid “core” surrounded by a suitable SCME. The SCME plays a very important role in the various properties of NPs, especially the dispersion stability. The dispersion stability of NPs is primarily determined by several factors, including the NP size, the polarity and the number of chemical substances (such as surfactants, solvents, and other adsorbed molecules/ions) coated on the NP surface, and the polarity of the solvent being used to disperse the NPs.63 For a certain solvent, the polarity and the number of molecules coated on the surface determine the dispersion stability of NPs. Other factors also influence NP dispersion stability, such as pH, ion concentration, temperature, and storage time. In addition, NPs with the same “core” can exhibit large differences in dispersion stability because the SCME outside of the “core” may exhibit different behaviors regarding the van der Waals attraction, electrostatic repulsion, and solvent interactions, which are determined by the specific molecular structures of the components of the SCME. Herein, we selected Au NPs as a model to discuss this topic. Ligands coated on NP surfaces determine the dispersion stability of NPs. Although the “cores” of Au NPs are Au, Au NPs coated with water-soluble molecules [such as citric acid,64−67 cetyltrimethylammonium bromide (CTAB),68,69 and water-soluble polymers70−72] have a good dispersion stability in water, whereas this stability is difficult to achieve for those coated with oil-soluble molecules (such as alkanethiols73−75). However, the dispersion stability of NPs can be easily modulated by exchanging their native ligands with suitable ligands.44,76−79 The loose binding between citrate molecules and Au (6.7 kJ/mol) enables the exchanges between citrate molecules and other molecules with anchoring groups (thiolates, amines, carboxylates, phosphine moieties, etc.).9,79 Braunschweig80 and co-workers found that the coordinated number of Au atoms determined the rate of ligand exchange between citrate-covered Au NPs and 3-mercapto-1-propanesulfonate (MPS). For low-coordinated surface sites, the replacement of citrate with MPS molecules was completed within 100 s; in contrast, this replacement time increased to 23 min at terrace sites. On the basis of a modified Langmuir isotherm, the Gibbs free energy for the ligand exchange 3212


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between citrate and MPS was evaluated as approximately −46 kJ/mol. Thiols are the most commonly used surfactants in the synthesis of Au NPs. Compared with monodentate thiols, multidentate thiols are more effective for enhancing the dispersion stability of Au NPs due to the dual benefits of the strong chelate effect (i.e., multiple sites for binding) and the steric effect from the loose molecular structure of their solvophilic tails.81−84 In 2008, Lee and colleagues81 systematically investigated the dispersion stability of Au NPs modified with monodentate ligands (hexadecanethiol and 2,2-dimethylhexadecane-1-thiol), bidentate ligands (2-tetradecylpropane1,3-dithiol and 2-methyl-2-tetradecylpropane-1,3-dithiol), and even a tridentate ligand [1,1,1-tris(mercaptomethyl)pentadecane]. They found that the stability of Au NPs in an organic solution was enhanced with an increasing degree of chelation, meaning that the chelate effect was largely responsible for the enhancement of stabilization. In addition, imidazole functional groups also exhibit high affinity for the Au surface. Yoon and co-workers85 successfully transferred Au NPs into the aqueous phase by replacing oleylamine ligands with polymeric imidazole ligands (PILs) (Figure 3), which overcomes some disadvantages of multidentate thiol systems, such as oxidative cross-linking and disulfide bond formation.

capped Au NPs was a function of pH. During the cycle experiment, the color of the Au NP solution exhibited a reversible change (deep red at pH 6.7 and blue at pH 1.0), which was attributed to the aggregation/antiaggregation of the Au NPs. However, this reversible aggregation behavior was not observed for citrate-protected Au NPs, possibly because of the irreversible change of the binding conformation of citrate adsorbed on the Au NPs.64 3.1.2. Heteroaggregation of Nanoparticles. If NPs are dispersed into a liquid phase, in which they have a bad dispersion stability, they will spontaneously form irregular NP aggregates. The aggregation of NPs can be classified into two groups: homoaggregation and heteroaggregation.91,92 Unlike homoaggregation, which only involves the same NPs, heteroaggregation is the process of aggregation between different NPs, which is more important in various applications, such as filtration, flotation, medicine, and water purification.93 These NPs may differ in a variety of ways, such as in their composition, shape, size, or surface charge.94,95 Heteroaggregation involves mixing different materials together and presents a cost-effective method for fabricating functional composites. For example, the well-dispersed TiO2/SnO2 NP solution can form TiO2−SnO2 heterojunction particle networks through surface-charge-induced heteroaggregation by using formic acid to adjust their surface charge (Figure 4a).96 The pristine TiO2 and SnO2 NPs carry negative surface charges. After adjusting the surface charges of both oxides, TiO2 and SnO2 NPs exhibited opposite surface charges (Figure 4b). The selfassembly of these oppositely charged particles generated the desired TiO2−SnO2 mixed particle system. This blended

Figure 3. Ligand exchange of Au NPs with PIL40%-PEG (480). PILs: polymeric imidazole ligands. PEG: polyethylene glycol. Reprinted with permission from ref 85. Copyright 2016 Elsevier B.V.

As important parameters in solution, the ionic strength and pH exhibit significant effects on the dispersion stability of NPs.86−89 Compared with other high ionic strength media (artificial seawater, phosphate-buffered saline, and potassium phosphate buffer), cell culture medium is beneficial for maintaining the stability of Au NPs.86 In addition, the SCME of Au NPs greatly affects their dispersion stability in high ionic strength media. The colloidal stability of Au NPs was ranked as BSA−Au NPs (bovine serum albumin-coated Au NPs) > PEG−Au NPs (polyethylene glycol-coated Au NPs) > PVP− Au NPs (polyvinylpyrrolidone-coated Au NPs) > cAu NPs (citrate-coated Au NPs).86 Moreover, it is well-known that the pH value can influence the surface charge of NPs, which is closely related to their dispersion stability. Sharma and colleagues90 found that the reversible aggregation of PVP-

Figure 4. (a) Multiple particles mixed without (top) and with (bottom) adjusting their surface charges. (b) ζ-Potentials of TiO2 and SnO2 NPs as a function of formic acid concentration. Reprinted with permission from ref 96. Copyright 2012 American Chemical Society. 3213


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methods have been developed, such as coating NPs with suitable protective materials, loading NPs on supports, and forming alloys with other materials that have a high melting point.110−116 On the other hand, the low thermal stability is conducive to forming nanocomposites by fusing NPs or nanonetworks by welding metal nanowires (NWs) at very low temperatures.117−121 In 2008, Zhang and co-workers116 investigated the effect of the annealing temperature on the grain size and the twin domain thickness of sputtered Cu films with nanoscale growth twins. They found that the grain size rapidly increased from approximately 50 to 500 nm during annealing, but the average twin lamella thickness changed very slowly (from approximately 4 nm to slightly less than 20 nm) (Figure 5a). This result

particle system had an exceptionally high mixing quality and produced extremely high concentrations of heterojunctions at the interfaces of TiO2 and SnO2 after vacuum annealing. Compared with the directly mixed particle systems, surfacecharge-induced heteroaggregation particle networks showed an enhanced cross section for interparticle charge separation. In addition, Wiesner and co-workers97 found that NPs could heteroaggregate with activated sludge, and the order of the relative affinity was pristine CeO2 NPs, TiO2 NPs, and ZnO NPs > PVP-coated Ag NPs > citrate-coated CeO2 NPs > Ag NPs coated by gum arabic, which is important for predicting the environmental mobility and fate of NPs. Heteroaggregation is also a promising method to form composite catalysts with high activity. Due to its high conductivity, graphene is often regarded as an excellent catalytic support, which can dramatically enhance the activity of the catalyst.98−101 A common method is the direct growth of nanocatalysts on the surface of graphene,98,99,102,103 but a direct growth process may be incompatible with controlling the size and morphology of NPs, which are closely related to their catalytic activity. Compared with the direct growth process, heteroaggregation results in a mutually beneficial situation by combining graphene with presynthesized NPs that have a desired size and morphology. Recently, Sun co-workers104−106 developed a solution-phase self-assembly method to assemble preprepared FePt or Co NPs onto graphene to obtain highperformance catalysts. First, FePt or Co NPs were synthesized by an oil-phase method. Then, NPs dispersed in hexane were mixed with a dimethylformamide (DMF) solution of graphene, followed by ultrasonic treatment. Mixing two immiscible solutions (i.e., NPs in hexane and graphene in DMF) via ultrasonic treatment resulted in heteroaggregation between NPs and graphene. The obtained graphene−NP catalysts displayed higher oxygen reduction reaction (ORR) activity than that of Pt/C catalysts and NPs themselves. 3.2. Thermal Stability of Nanoparticles

Almost all NPs exist far from the equilibrium state and have an excess Gibbs free energy (ΔG),5 meaning that the thermal activation of NPs can stimulate and enhance the process of diffusion, relaxation, grain growth, and homogenization, resulting in partial or total structural damage of the NPs and performance degradation of the nanodevices.5,107 Reducing the size of bulk materials to the nanoscale causes a dramatic decrease in the melting point, resulting in poor thermal stability,108 which greatly limits the applications of NPs in some cases, especially for high-temperature processes. During heating, the thermal agitation of atoms leads to two typical processes: sintering (forming a solid mass of material by heating without melting it to the point of liquefaction) and fusion (forming a solid mass of material by melting the atoms together). Here, two temperatures must be noted: the Tammann temperature and the Hütting temperature. The Tammann temperature, at which lattices begin to be appreciably mobile, is approximately 0.5Tm; the Hütting temperature, at which surface atoms become significantly mobile, is approximately 0.3Tm, where Tm is the melting point in absolute units.109 The sintering of materials is strongly temperature-dependent and is closely related to the Hütting and Tammann temperatures.110 Generally, NPs have relatively low Hütting and Tammann temperatures, meaning that they easily aggregate under high temperatures through a sintering process. To improve the thermal stability of NPs, many

Figure 5. (a) Evolution of twin domains and grain size as a function of annealing temperature. The inlet shows the increase in the average lamellar thickness as a function of temperature. (b) Mean Ag particle size evolution with treatment temperature and time: (I) 8 wt % Ag/ mSiO2 and (II) 8 wt % Ag/SiO2. Panel a reprinted with permission from ref 116. Copyright 2008 American Institute of Physics. Panel b reprinted with permission from ref 110. Copyright 2006 American Chemical Society.

implies that a layered arrangement of low-angle coherent twin boundaries in nanostructures is beneficial for improving thermal stability. For monodisperse NPs, coating with hightemperature-stable materials is an effective method to improve their thermal stability. Somorjai and colleagues112 designed a core−shell configuration Pt@mSiO2 in which a Pt metal core was coated with a mesoporous silica shell to improve the stability of Pt NPs during high-temperature catalytic reactions. 3214


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Inorganic silica shells encage Pt cores to give them higher thermal stability; meanwhile, the mesopores in silica shells allow the reactant molecules to access Pt cores directly. Thus, these special Pt@mSiO2 NPs exhibit excellent catalytic activity and stability during high-temperature catalytic reactions. Moreover, NPs located inside the channels of mSiO2 (thus being confined) have better thermal stability than those that are unconfined. Bao and co-workers110 found that Ag NPs trapped inside the channels of ordered mSiO2 exhibited a long-term thermal stability without any observable coarsening at 773 K (Figure 5b), which is much higher than the Tammann temperature (i.e., 617 K) of bulk Ag. In contrast, the unconfined Ag NPs tended to be heavily aggregated when the temperature approached the Tammann temperature. In addition, Xia and co-workers122 successfully prepared silicadispersed copper NPs with high dispersion and unusual thermal stability by a simple precipitation−gel method. They found that the formation of copper phyllosilicate during the precipitation− gel process was critical for improving the structural stability and thermal stability of the samples. The average sizes of CuO and Cu0 in the samples were maintained below 9 nm after hightemperature calcination and reduction (1073 K for CuO and 773 K for Cu). Another very common method to improve the thermal stability of NPs is to form alloys with materials that have high melting points. The compositional ratio of alloys is critical to their thermal stability. For alloys of copper and zirconium, at a low Zr concentration of approximately 1%, the grains are maintained in the nanocrystalline state up to 0.85 of the Tammann temperature for pure Cu, but the grain size stability was not enhanced with higher Zr contents in copper (2% and 5%).111 Liu and Mucklich113 used an isothermal annealing approach at high temperatures to study the thermal stability of as-milled nano-RuAl. They found that the as-milled nano-RuAl showed three structural evolutions during annealing: reordering, strain relaxation, and grain growth. In 2012, Schuh and colleagues114 developed a theoretical framework to design stable nanocrystalline alloys by using two critical thermodynamic parameters: the enthalpy of mixing in the crystalline state ΔH

mix

Figure 6. (a) The nanostructure stability map for tungsten-based alloys at 1100 °C. (b) Nanostructure stability map, presenting delineated regions of stability (green), metastability (yellow), and no stability (red) in binary alloys as a function of their enthalpies of mixing and segregation. Panel a reprinted with permission from ref 114. Copyright 2012 AAAS. Panel b reprinted with permission from ref 115. Copyright 2012 Pergamon-Elsevier Science Ltd.

temperatures. Due to the low thermal stability of NPs, thermal decomposition can transform preprepared NPs into functional NPs with porous/hierarchical structures at relatively low temperatures (400−600 °C), such as NiO,123−125 Co3O4,126 MnO,127 SiO2,128 TiO2,129 and mixed metal oxides MxNyOz (M, N = Co, Ni, Zn, Mn).117,130−137 On the other hand, the poor thermal stability of NPs can easily cause the fusion between them at low temperatures. Recently, Wu and coworkers117 developed a low-temperature process to fuse core/ shell metal oxide NPs into porous ternary complex metal oxide NPs by thermal annealing. First, preprepared CoO NPs were coated with Mn3O4 NPs by decomposing Mn(acac)3 in oleylamine at 160 °C. Then, the obtained CoO/Mn3O4 core/ shell nanocomposites were annealed at 600 °C for 6 h in a 4% hydrogen-forming gas to create porous Co2MnO4 NPs without significant size changes (Figure 7). This method can also be extended to synthesize other ternary complex metal oxide NPs with well-controlled sizes and properties. Welding is a process of joining two pieces of metal together by thermal annealing. The welding of metallic nanomaterials plays an important role in the bottom-up fabrication of electrical and mechanical nanodevices.121 The thermal annealing process involved in nanowelding can be grouped into three broad types: conventional thermal annealing in an oven or on a hot plate,120 Jouleheating annealing,138,139 and light-driven annealing (such as

= zωcX(1 − X )

to represent the grain interior and the dilute-limit enthalpy of segregation ⎡ ωgb (ΩBγ B − Ω A γ A ) ⎤ ΔH seg = z⎢ωc − − ⎥ 2 2zt ⎦ ⎣

to capture the thermodynamics of the grain boundary environment. On the basis of this theory, choosing W−Ti alloys as a model (Figure 6a), the authors found that the W−20 atom % Ti alloy showed long-term thermal stability and could maintain a small grain size of approximately 20 nm at 1100 °C for a week. In addition, by calculating these thermodynamic parameters of alloys, they established a nanostructure stability map. This map includes three main regions, namely, stability, metastability, and no stability (Figure 6b).115 Different regions in the nanostructure stability map are determined by the enthalpies of mixing and segregation. This map can effectively aid in the selection of reasonable alloy systems for obtaining stable nanocrystalline structures in binary alloys. Although the poor thermal stability of NPs limits their applications, it also makes the thermal treatment of NPs (such as decomposition, fusion, and welding) accessible at very low 3215


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stability of the crystal structure of NPs is not reviewed. The discussion involves several topics: Ostwald ripening (OR) and digestive ripening (DR), oriented attachment (OA) of NPs, structural transformation of NPs induced by stabilizer depletion, and ordered assembly of NPs. 3.3.1. Ostwald Ripening and Digestive Ripening. The synthesis of nanomaterials is a dynamic process and is controlled by both thermodynamics and kinetics.143,144 Thermodynamics deals with the driving force of a system moving from the initial state to the product state, whereas kinetics is concerned with the energy barriers of the specific pathways in this process.143 Due to this thermodynamically and kinetically controlled scenario, NPs are constantly changing during synthesis to reach a relatively stable state. OR and DR are two typical phenomena that are used to illustrate this dynamic process. OR is a well-known concept as a typical phenomenon in the preparation of NPs. When the precursors of NPs are almost depleted, some smaller NPs may redissolve into the reaction solution. Then, these dissolved components are redeposited on the surface of larger NPs, resulting in an increase in the mean NP size and a compensating decrease in the number of NPs.145 In contrast, for DR, larger NPs are transformed into smaller ones with a uniform monodisperse state by refluxing NPs in a solution that contains DR ligands (long-chain thiols, amines, phosphines, and so on).146 In this process, DR ligands etch out surface atoms of NPs because of their strong affinity for NPs, and these released atoms then nucleate and grow into small NPs. In 2005, Klabunde and co-workers147 observed an interesting reversible morphology transformation of Au NPs by OR and DR. This phenomenon was induced by specific types of molecules that were coated on the Au NPs: (i) alkanethiols (RSHs) with a strong affinity on the surface of Au NPs and (ii) positively charged quaternary alkylammonium surfactants, such as didodecyldimethylammonium bromide (DDAB), which have a weaker interaction with Au NPs. As shown in Figure 9, in step

Figure 7. Porous Co2MnO4 NPs derived from CoO/Mn3O4 core/ shell NPs by thermal annealing. Reprinted with permission from ref 117. Copyright 2016 Tsinghua University Press and Springer-Verlag.

opto-thermal annealing using a broadband lamp or laser nanowelding).118,119 Owing to their large optical absorption cross section, metallic nanomaterials can serve as efficient lightdriven sources of heat.119,140,141 In addition, the grain growth in metals can occur at temperatures as low as 0.2Tm.119,142 Hence, light-induced nanowelding for metallic nanomaterials is feasible. In 2012, Ko and colleagues118 successfully fabricated superior transparent, flexible conductors by welding long Ag NW networks with a laser. Nanowelded spots between Ag NWs showed good fusion and crystalline characteristics (Figure 8).

Figure 8. Laser nanowelding of very long Ag NW network electrodes. (a) SEM (pseudocolored) and (b) HRTEM images of laser nanowelded spots between Ag NWs under optimum processing conditions. Reprinted with permission from ref 118. Copyright 2012 Royal Society of Chemistry.

Figure 9. Reversible change of Au NPs by Ostwald ripening and digestive ripening. (a) Large polyhedral Au NPs, (b) small Au NPs obtained under ambient conditions, and (c) uniform Au NPs formed by refluxing. Reprinted with permission from ref 147. Copyright 2005 American Chemical Society.

Compared with broadband lamp welding, this laser nanowelding method exhibited a fast processing speed (less than a few seconds) and good thermal controllability with minimal thermal damage to the substrate.

1, larger Au polyhedral NPs were broken up into polydisperse smaller Au NPs at room temperature, which was induced by RSHs. Then, these NPs were completely transformed into the nearly monodisperse Au NPs with a uniform size by refluxing them in toluene solvent with excess RSHs in step 2, i.e., DR. After refluxing was complete, the monodisperse Au NPs were collected by precipitating with ethanol, and the excess RSHs

3.3. Structural Stability of Nanoparticles

Herein, we discuss the structural stability of NPs from a physical perspective, which is mainly related to the shape/ morphology and the organization forms of NPs, while the 3216


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Figure 10. Experimental data and fitting results for the correlation between particle size and reaction time: (a) ZnS NPs in water at 140 °C, (b) mercaptoethanol-capped nano-ZnS NPs in water at 140 °C, and (c) ZnS NPs in a 4 M NaOH solution at 100 °C. (d) Concentration of Zn2+ ions in 4 M NaOH at 100 °C vs time. Panel a reprinted with permission from ref 150. Copyright 2003 American Chemical Society. Panel b reprinted with permission from ref 151. Copyright 2003 American Chemical Society. Panels c and d reprinted with permission from ref 152. Copyright 2006 American Chemical Society.

However, two distinct stages and a dividing knot were exhibited in the growth curve of mercaptoethanol-capped nano-ZnS during hydrothermal coarsening.151 In addition, unlike the hybrid crystal growth process of H2O−ZnS NPs, the first growth stage was controlled only by a pure OA process, which was primarily attributed to the presence of organic ligands (Figure 10b). Compared with organic ligand thiols, which can be decomposed or desorbed into water under hydrothermal conditions, resulting in a weakly controlled crystal growth process, inorganic hydroxyl ions are more stable and exhibit a stronger surface adsorption effect. When ZnS NPs were hydrothermally treated in a 4 M NaOH solution, the first growth stage corresponded to a long, pure OA process, resulting in a larger final size of approximately 7.2 nm, which was a 2-fold increase over the original size of approximately 2.4 nm (Figure 10c).152 Thus, the SCME of NPs plays an important role in their behaviors in solution. When ZnS NPs were coated with strong capping agents, such as thiols or hydroxyl ions, the strong surface adsorption effect decelerated the dissolution rate of ZnS NPs in solution. The low concentration of ZnS in solution postponed the OR process, which requires a saturation state. For this reason, the OA process was the only growth pathway in the early stage, and the growth time for the OA process was prolonged with an enhanced surface adsorption effect.153 The concentration change of the zinc ions in solution provided a powerful

were removed. In step 3, the obtained precipitate and a certain amount of DDAB were dispersed into toluene, and then, the obtained solution was refluxed again. Due to the weak binding of DAAB with Au surfaces, uniform Au NPs transformed back to larger polyhedral particles, i.e., OR. 3.3.2. Oriented Attachment Involved with the Surface Chemical Microenvironment. Unlike the OR process that produces large particles with low surface energy by consuming small particles, resulting in the formation of isotropic crystals with round shapes and smooth surfaces, the OA process is driven by reducing the surface energy via joining high-energy surfaces together, which finally leads to anisotropic crystals with complex hierarchical structures due to preferential orientation growth.148,149 Obviously, OA growth and OR growth are both driven by the tendency to minimize the surface energy. The surface energy of NPs is not only determined by the sizes and exposed crystal facets of NPs but is also closely related to their SCME. Thus, during a dynamic growth process, the SCME of NPs is also very important for controlling crystal growth. For the 2.0 nm ZnS NPs (H2O−ZnS) synthesized in water, the hydrothermal coarsening process involved both the OA process and the OR process.150 On the basis of the fitting curve of the experimental data, OA was the dominant process in the early growth stage, but OR became the only significant crystal growth pathway in the latter stage (Figure 10a). The boundary between these two processes of H2O−ZnS NPs was undefined. 3217


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(Figure 12a−c). First, the suspension of PbSe nanocrystals was placed on ethylene glycol (EG), which was a nearly immiscible liquid for the suspension. Then, the solvent of the suspension was evaporated under a nitrogen atmosphere at a given reaction temperature, and the solvent evaporation induced crystal attachment. The authors found that the detachment of the capping molecules from specific facets and their dissolution in EG drove crystal attachment.155 The addition of sufficient capping molecules into EG could stabilize all the facets of PbSe nanocrystals and prevent crystal attachments between PbSe nanocrystals, resulting in the formation of self-assembled hexagonal layers of stable PbSe nanocrystals (Figure 12d−f). Moreover, the obtained 2D honeycomb structures of PbSe were sufficiently robust to be transformed into 2D CdSe lattices through cation exchange while the nanoscale honeycomb geometry was maintained, thus presenting a route to access a new class of 2D semiconductors with a tunable composition.156 3.3.3. Transformation of Nanoparticles Induced by Stabilizer Depletion. On the basis of their function, the stabilizers of NPs can be divided into two main groups: surface stabilizers, which as a part of the SCME are the components located on the surface of NPs to stabilize them, and structural stabilizers, which as a part of NPs are the components embedded in NPs to form inorganic−organic hybrid materials and maintain their lamellar structure. When NPs experience a destabilization process, these stabilizers can be desorbed or released, resulting in a dramatic structural transformation of the NPs. For surface stabilizers, the NP stability greatly depends on their component, packing density, and hydrophilicity. Most discussions of surface stabilizers focus on their component and hydrophilicity, but the packing density is often overlooked. In fact, the packing density has an important effect on NP stability.160,161 Kotov and co-workers43,161−165 found that zerodimensional NPs could spontaneously assemble into onedimensional (1D), two-dimensional (2D), three-dimensional (3D), or other complex structures, i.e., dimensional transformation, by gradually depleting the stabilizers on their surfaces. These transformation processes involve the partial removal of the stabilizers on the surface of NPs (eq 2) and the reactions between the corresponding ions of NPs and dissolved oxygen (eq 3) or other ions in solution (eqs 4 and 5). On the basis of the reaction pathway of these dimensional transformation processes, they could be grouped into three main categories: (i) physical assembly, which maintains the composition of the original substance; (ii) transformation that involves oxidation (eq 3); and (iii) transformation that involves other ions (eqs 4 and 5).

evidence for the above discussion (Figure 10d). A relatively long period was required to reach the saturation of ZnS in the ZnS−NaOH system; thus, the crystal growth during this period was controlled only by the OA process. Moreover, the time point for ZnS saturation precisely matched the end point of the OA process, indicating that the saturation of ZnS triggered the OR process but terminated the OA process. In addition, after washing and redispersing the NPs, some ligands coated on the surface of the NPs can be partially removed due to dissolution and desorption. Because of the different binding energies between surface ligands and various crystal facets, ligands are preferentially desorbed from the crystal facets that have weaker interactions with them. These obtained “naked” crystal facets exhibit higher surface energy and cause the NPs to become unstable or more reactive. In thermodynamics, it is favorable to assemble these destabilized NPs into hierarchical crystals by joining these “naked” crystal facets with a high surface energy together, i.e., via the OA process. In 2013, Hanrath and co-workers154 successfully fabricated confined-but-connected PbSe quantum dot solids by a ligand-displacement-induced OA process. When the thin film of PbSe NPs on a substrate was submerged into a suitable solvent, oleic acid ligands preferentially desorbed from the {100} facets because of the stronger binding of oleic acid ligands to the {111} facets. The anisotropic surface ligand coverage rendered the {100} facets more reactive and directed the fusion of these destabilized facets with an orientational order by directional interactions. The solvent is very important for this ligand displacement because oleic acid exhibits various solubility in different solvents. The oriented fusion of PbSe NPs was achieved by treating the film with solvents that can solvate free oleic acid (such as acetone, ethanol, methanol, and dimethylaminoethanol). In solvents that do not solvate oleic acid (such as water, acetonitrile, or methylformamide), disordered aggregates were formed (Figure 11). Similarly, Vanmaekelbergh and colleagues155−159 also obtained superlattices of PdSe nanocrystals by interfacial self-assembly and OA

CdE@Stabilizers → CdE + stabilizers

(2)

2E2 − + O2 + 2H 2O → 2E + 4OH−

(3)

nE2 − + 2Mn + → 2MEn /2

(4)

pCd2 + + 2F p − → pCdF2/ p

(5)

Here, E is Se/Te, M is a cation, and F is an anion. In physical assembly, 1D CdTe NP chains were successfully organized from CdTe NPs by partly removing thioglycolic acid (TGA) molecules coated on their surfaces (Figure 13a).165 The main driving force in this process was dipole−dipole attraction due to the low reaction temperature (4 °C), which cannot offer enough thermal energy to recrystallize CdTe NPs into rods.

Figure 11. Quantum dot solids formed by treating the thin film of PbSe quantum dots with various solvents: DEG, diethylene glycol; DMK, dimethyl ketone; ACN, acetonitrile; EtOH, ethanol; MeOH, methanol; H 2 O, water; MF, methylformamide; and DMAE, dimethylaminoethanol. Reprinted with permission from ref 154. Copyright 2013 American Chemical Society. 3218


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Figure 12. Structures formed by the oriented attachment of PbSe nanocrystals: (a) linear structures, (b) ultrathin sheets with square nanostructuring, and (c) honeycomb superlattices. Effect of oleic acid on the oriented attachment of PbSe nanocrystals: (d) no additional oleic acid added, (e) 4 × 10−5 mol/L oleic acid, and (f) 4 × 10−4 mol/L oleic acid. All scale bars represent 50 nm. Reprinted with permission from ref 155. Copyright 2013 American Chemical Society.

NW networks (Figure 13k) and Hg1−xCdxTe NW networks.160,170 When suitable anions (such as EDTA) were added to the CdTe NP solution, they could promote the degradation of CdTe NPs by complexing with Cd2+ cations.163 For the structural stabilizers, inorganic−organic hybrid materials are selected to discuss their importance for the structural stability. Inorganic−organic hybrid materials with remarkable properties have attracted increasing attention, especially for II−VI semiconductor inorganic−organic hybrid NPs [MQ(L)0.5] (M = Cd, Mn, Zn; Q = S, Se, Te; L = amines).171−174 These unique semiconductor hybrids consist of two parts: II−VI semiconductor and organic amines. Organic amines are crucial for maintaining the structural stability because they, as structural stabilizers, bind different inorganic blocks together via coordination or covalent bonds to form a lamellar structure.175−178 When inorganic−organic hybrid materials encounter intense interactions from the outside environment (such as strong acid/alkali etching, ultrasound, extraction processes, ion-exchange, annealing operations, and hydrothermal processes), organic amines can be released from the hybrids, causing the deformation, reconstruction, or degradation of the lamellar structures of these inorganic− organic hybrid materials.179−199 In 2005, our group synthesized uniform and well-defined [ZnSe](DETA)0.5 (DETA: diethylenetriamine) nanobelts in a ternary solution (VN2H4·H2O/VDETA/VH2O = 5:14:16).200 As organic stabilizers, DETA molecules were bound to Zn2+ by coordination to maintain a certain distance between ZnSe layers. This large space allowed H+ ions to easily diffuse into the lamellar hybrid structures and attack DETA molecules, resulting in the release of protonated amines from the hybrids. Similar to previously discussed CdTe/CdSe NPs, stabilizerdepleted ZnSe layers were highly unstable and were quickly oxidized into porous Se nanosheets by dissolved oxygen (Figure 14).179 The porosity of the final products greatly depended on the pH of the solution. The low pH value was beneficial for increasing the porosity. Mesostructured wurtzite ZnS-nanowire-bundle/amine nanocomposites also exhibited this pH-dependent structural stability.181 When they were

When these destabilized CdTe NPs were aged in the dark at room temperature for several days, uniform CdTe NWs were obtained (Figure 13b).43 This assembly is also sensitive to the number and type of stabilizers and the reaction temperature. When CdTe NPs with a ratio of 2.4 (TGA to Cd2+) were replaced by CdTe NPs with a ratio of 1.0, twisting ribbons composed of individual CdS/CdTe nanocrystals were obtained within 72 h under ambient conditions (Figure 13c).166 The 2(dimethylamino)ethanethiol-stabilized CdTe NPs spontaneously assembled into free-floating particulate sheets after being stored for 1 month at 50 °C in a glovebox (Figure 13d,e).162 In addition, Gu and co-workers167 found that thiol-capped CdTe could assemble into 1D nanocrystals, 2D nanosheets, and even 3D aggregates, which was induced by L-cysteine. When EDTA2Na (EDTA: ethylenediaminetetraacetic acid) was added into the solution of destabilized CdTe NPs, EDTA could strongly bind to the destabilized CdTe NPs to form a Cd(EDTA)2− complex, and then, Te2− anions were gradually released into the solution.164 In this case, these released Te2− ions were subsequently oxidized to Te by dissolved oxygen because of the very low Te2−/Te redox potential (−1.143 V), resulting in transformation of CdTe NPs into Te nanocrystals with different shapes, such as nanorods, nanocheckmarks, nanomoths, and Y-shaped or X-shaped nanostructures (Figure 13f− h).164 By a similar process, CdTe NPs can be transformed into single-crystal double-directional tellurium nanoneedles with the help of an ammonia solution (Figure 13i).168 Due to the low Se2−/Se redox potential (−0.924 V), this transformation involving oxidation can be extended to CdSe NPs, resulting in Se NWs (Figure 13j) or other unusual, highly anisotropic Se nanostructures.161,169 The dissolution of ionic compounds is strongly influenced by the solubility product of the solute, the concentration of the corresponding ions of the solute, and the other ions in solution, which can interact with the solute or the corresponding ions of the solute. For transformations involving other ions, when some cations were added into the destabilized CdTe NP solution, the typical cation replacement reaction proceeded between CdTe NPs and the cations and produced the corresponding telluride nanowire networks, such as Ag2Te 3219


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Figure 13. TEM images of nanomaterials with special nanostructures obtained from CdTe/CdSe NPs through different reaction pathways: (a−d) physical assembly, (f−j) transformation involving oxidation, and (k) transformation involving Ag+ ions. (e) Fluorescence images of self-assembled sheets of CdTe NPs with different diameters. Panel a reprinted with permission from ref 165. Copyright 2004 American Chemical Society. Panel b reprinted with permission from ref 43. Copyright 2002 AAAS. Panel c reprinted with permission from ref 166. Copyright 2010 AAAS. Panels d and e reprinted with permission from ref 162. Copyright 2006 AAAS. Panels f−h reprinted with permission from ref 164. Copyright 2006 American Chemical Society. Panel i reprinted with permission from ref 168. Copyright 2009 American Chemical Society. Panel j reprinted with permission from ref 161. Copyright 2005 Wiley-VCH. Panel k reprinted with permission from ref 160. Copyright 2008 Wiley-VCH.

by a continuous outward flow of ZnSe from the core to the shell. Finally, iso-oriented ZnSe nanotubes formed through a complex process that involved the Kirkendall effect, recrystallization, OA, and OR.180 Pure hexagonal wurtzite ZnSe nanosheets, zinc-blended ZnSe NPs, and hexagonal wurtzite ZnS NWs were also obtained by extracting their corresponding hybrids.183,184 Interestingly, Zhang and colleagues186 found that the cation exchange of inorganic−organic hybrid materials could also lead to the release of organic stabilizers out of the hybrids and thus produced porous nanostructures. The pore size and composition of the corresponding products were controlled by changing the solvent or the ratio of the hybrid precursor to cations, respectively (Figure 15b). Because the solubility of DETA molecules varied with the solvents, at the same molar ratio of 1:2 (ZnSe−DETA to Cd2+), the final pore size of the porous CdS nanosheets was approximately 10−50 nm in water, but the pore size decreased to 10−20 nm in EG. The authors also found that hollow CdxZn1−xSe nanoframes could be derived from ZnSe-amine hybrid nanoflakes by a similar process.185 Because organic amines have a low boiling point of triangular plates and the order of Au ∼ Pt > glassy carbon > glass/ITO.246,252 The capping (either by citrate ions or glucose molecules) can substantially impede both the formation of Ag2O and the nucleation of AgO during the anodic sweep, whereas they poorly protect Ag NPs from oxidation in air.14 3.4.2.3. Pt NPs. Understanding and controlling the stability of Pt NPs are very important for improving their catalytic activity. Recently, Bukhtiyarov and co-workers253 used X-ray photoelectron spectroscopy (XPS) to study the S&R of Pt/ TiO2 catalysts. When interacting with NO2, Pt NPs on TiO2 gradually exhibit three different states: (1) the first state, [Pt− Osub], in which oxygen atoms are dissolved/incorporated into the Pt NPs; (2) platinum oxide, PtO; and (3) platinum dioxide, PtO2. The first state [Pt−Osub] is observed only for Pt NPs supported on oxide supports. These dissolved oxygen atoms in [Pt−Osub] exhibit a higher reaction ability to hydrogen than the 3227


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during storage. N2H4·H2O and ascorbic acid are beneficial for the protection of Cu NWs during storage. However, Cu NWs stored under the protection of citric acid or glucose are oxidized due to the low reducing capacity of these compounds. Capping components on the surface of NPs also affect their oxidation in different solvents.257 The synthesis of Cu NPs with surfactants or polymers leads to a tightly coated organic molecule shell on their surfaces. The interaction between organic ligands and solvent molecules can modulate the state of molecules in this organic shell. The higher solubility of organic ligands in the solvent results in a more unfolded and loose organic shell, which is beneficial to the diffusion and transport of ions or molecules (such as O2). For example, monodisperse Cu NPs coated with tetradecylphosphonic acid (TDPA) were slightly oxidized into a core/shell structure Cu@Cu2O in hexane at room temperature, while Cu NPs dispersed in chloroform transformed into hollow Cu2O NPs.258 The better solubility of TDPA in chloroform implied that the exposure of Cu NPs to dissolved oxygen was higher than that in hexane, and the greater exposure caused the heavier oxidation. The principal method to improve oxidation resistance for Cu NPs is minimizing the exposure of Cu NPs to oxygen or other oxidants by coating them with inert protective layers, such as long-chain surfactants, organic polymers, inorganic materials, or carbon-based materials.257 The type of coating (porous or nonporous) also affects the oxidation of Cu NPs.259 As shown in Figure 27a, the “naked” Cu NPs are directly oxidized to hollow CuO NPs due to the Kirkendall effect. Cu NPs coated with a nonporous coating exhibit superior oxidation resistance, while a porous coating is unsuccessful at preventing the oxidation.259 Qian and co-workers260 synthesized a Cu@carbon spherical core−shell structure by using vitamin C as both the reductant and carbon source in aqueous solution at 180 °C. After subsequent thermal treatment of the structures at 600 °C under an argon atmosphere, the carbonization of carbonaceous matrices resulted in embedding Cu NPs into the carbon shells, which effectively shielded the Cu NPs core from oxidation (Figure 27b). Trindade and colleagues261 investigated the chemical stability of copper nanomaterials by forming nanocomposites with cellulosic fibers. They found that Cu NWs had a better resistance to oxidation than Cu NPs, and bacterial cellulose (BC) fibers were an efficient substrate to delay surface oxidation of Cu nanomaterials (Figure 27c). This is because the 3D nanofiber network of BC forms interlaced “cages” that act as physical barriers to limit oxygen diffusion and thus protect Cu nanomaterials. However, the above methods only focus on the goal of protection but neglect the fact that the electrical conductivity is one of the most valuable property of copper nanomaterials. Recently, Wiley and co-workers262 reported a two-step approach to coat and alloy Cu NWs with nickel. The obtained Cu@Ni NWs were highly resistant to oxidation. To evaluate the resistance of Cu@Ni NWs to oxidation, the films of different samples with comparable transmittance (85−87%) were placed in an oven and kept at 85 °C. Compared with Cu NWs and Ag NWs, Cu@Ni NWs remained remarkably stable. The change of the sheet resistance was only 10 Ω sq−1 for a 10% Ni coating after 1 month. Due to this extreme oxidation resistance of Cu@Ni NWs and the abundance of Cu and Ni (1000 times more than Ag), Cu@Ni NWs can be regarded as an attractive alternative to Ag NWs for the fabrication of transparent conductive films. Other metals (such as Au, Ag, Pt,

Figure 25. Influence of the size (2/rm) of Pt NPs on the dissolution and oxidation potentials. The red points represent Pt NPs that disappeared within a single ECSTM image scan. The blue points correspond to Pt NPs that formed an oxide and followed a chemical dissolution route, 2/rm = 1/r1 + 1/r2, where r1 and r2 correspond to half the length of the major and minor axes of Pt NPs, respectively. ECSTM: electrochemical scanning tunneling microscopy. Reprinted with permission from ref 12. Copyright 2010 American Chemical Society.

Figure 26. (a) Schematic illustration of the oxidation of Cu NWs in solution. SEM images of (b) fresh Cu NWs and Cu NWs stored in (c) water, (d) isopropyl alcohol, and (e) cyclohexane after 1 month. All scale bars represent 500 nm. Reprinted with permission from ref 63. Copyright 2016 American Chemical Society.

solvents is different. The higher dissolved oxygen concentration, which is several times to hundreds of times higher than that in polar solvents, is responsible for the heavy oxidation in nonpolar organic solvents. In water, the oxidation is primarily driven by the increase of the O2/H2O potential due to the slight acidity from the dissolution of atmospheric carbon dioxide. In addition, we found that adding suitable reductants is an effective method to prevent the oxidation of fresh Cu NWs 3228


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Figure 27. (a) Oxidation of Cu NPs under various surface terminations: (i) uncoated Cu NPs, (ii) Cu NPs coated with nonporous Al2O3 films, and (iii) Cu NPs coated with porous Al2O3 films. (b) Scheme of the formation of Cu@carbon nanostructures and TEM images of Cu@carbon core− shell structures after annealing at 600 °C. (c) Digital photographs of BC/Cu NW composites during storage. Panel a reprinted with permission from ref 259. Copyright 2011 American Chemical Society. Panel b reprinted with permission from ref 260. Copyright 2010 Pergamon-Elsevier Science Ltd. Panel c reprinted with permission from ref 261. Copyright 2012 Wiley-VCH.

Figure 28. (a) Calculated phase diagrams for manganese oxides for bulk (left) versus 10 nm particles (right). (b) Calculated phase diagrams for cobalt oxides for bulk (left) versus 10 nm particles (right). Panel a reprinted with permission from ref 271. Copyright 2012 mineralogical Society of American. Panel (b) reprinted with permission from ref 269. Copyright 2010 AAAS.

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Zn, Sn, and In) are also good coatings for protecting Cu NWs from oxidation.263,264 3.4.2.5. Metal Oxides. Most metal oxide NPs exhibit good thermal stability and chemical stability.265−267 However, for metal oxides with different oxidation states, especially for transition-metal oxides, oxidation−reduction phase equilibria at the nanoscale are shifted to favor the phases with a lower surface energy.7,268,269 For manganese oxides, the surface energy follows the order of MnO2 pyrolusite > Mn2O3 bixbyite > Mn3O4 spinel. At the nanoscale, the phase diagram of manganese oxides changes: The phase field of Mn3O4 increases by occupying that of Mn2O3, while the phase field of Mn2O3 expands by decreasing that of MnO2 (Figure 28a).271 Due to the smaller difference in the surface energy between Mn2O3 and MnO2, the shift of the Mn3O4−Mn2O3 line is larger than that of the Mn2O3−MnO2 line. These redox shifts at the nanoscale may be a good strategy to design and understand efficient catalysts, such as using Co3O4 NPs as a catalyst for CO oxidation (eqs 6 and 7).270 Generally, bulk Co3O4 is reduced to CoO (eq 6) under typical catalytic conditions. However, the redox shifts at the nanoscale increase the phase field of Co3O4 by decreasing that of CoO (Figure 28b).269 In this case, nanoCo3O4 is the stable phase, while nano-CoO becomes unstable and can be easily oxidized into nano-Co3O4; i.e., it is a transient catalytic intermediate.268 Therefore, nano-Co3O4 is an excellent catalyst for low-temperature CO oxidation. Understanding nanoscale redox shifts of metal oxide NPs is very important for understanding their fascinating properties and for designing stable and functional metal oxide NPs. Co3O4 + CO = 3CoO + CO2

(6)

3CoO + 0.5O2 = Co3O4

(7)

Figure 29. (Top) Digital photos of samples stored in ethanol or water under ambient conditions: (a) a fresh sample and the sample after being stored for (b) 1 day, (c) 2 days, and (d) 6 days and (bottom) TEM images of Te NWs (center) and TeO2 obtained in ethanol (left) and water (right). Reprinted with permission from ref 274. Copyright 2007 American Chemical Society.

polymer coating, inert atmosphere protection, and thicknessinduced self-protection, were beneficial to greatly prolong the storage time of Te NW film (approximately 800 days). The standard electrode potentials of TeO2/Te, O2/H2O are 0.593 and 1.229 V, respectively. The difference between the redox potentials of two half-cell reactions, ΔE, is 0.636 V for the reaction between O2/H2O and TeO2/Te. The positive potential indicates that the oxidation of tellurium is thermodynamically easy, and Te NWs can be easily oxidized in either air or aqueous solution under ambient conditions. Many unsaturated Te atoms on the surfaces of Te NWs may further increase the reactivity of Te NWs and promote their oxidation.6 Oxygen-free conditions (such as polymer coating and inert atmosphere protection) and the addition of suitable reductants (such as N2H4·H2O) are effective methods to maintain the uniform morphology and high reactivity of ultrathin Te NWs during storage.6 3.4.2.7. II−VI Semiconductors. As important photoactive materials, II−VI semiconductor nanostructures have garnered extensive attention. Most of the scientific literature related to II−VI SNPs focus only on their applications, while their stability is somewhat overlooked. The robustness of semiconductor nanomaterials is not only a scientific question but also of great significance to their practical applications. In 2014, Vela and co-workers277 used CdSe and CdS as model systems to systematically investigate the stability of SNPs against different extreme chemical and physical conditions (acids, bases, oxidants, reductants, and heat) (Figure 30a). When CdSe nanorods were treated with 4 M strong acids, they quickly transformed into Se0 when using HCl, or SeO2 when using HNO3. For 4 M bases, axial partial etching was observed for both NaOH and tetramethylammonium hydroxide (Me4NOH). Me4NOH resulted in the formation of small amounts of Se 0 and CdO, whereas NaOH did not. Interestingly, CdSe nanorods could remain unchanged in the presence of strong oxidants (KMnO4 and H2O2) or at temperatures below 350 °C in air. When the temperature was above 350 °C, the CdSe nanorods were gradually oxidized into rock-salt CdO. Neither CdSe nor CdO was easily reduced by H2 gas. It was also difficult for n-butyllithium, a strong

3.4.2.6. Te NWs. In 2006, our group successfully synthesized ultrathin Te NWs with 4−9 nm diameters,272 which exhibited high chemical reactivity.22,232−238 However, this high reactivity also caused the Te NWs to have a metastable nature, meaning that they are easily degraded via oxidation during use.6,273−275 In 2007, our group systematically investigated the dispersion stability and chemical stability of Te NWs in different chemical environments.274 As a capping agent, PVP coated on the surface of Te NWs plays an important role in the dispersibility and the stabilization of Te NWs. The interactions between PVP and various solvents determine the final state of Te NWs in solution (aggregation or dispersion). Te NWs exhibited an excellent dispersity in water and ethanol but were heavily aggregated in acetone due to the different solubilities of PVP in various solvents. Fresh Te NWs were highly sensitive to oxygen, and they were oxidized into amorphous TeO2 NPs and single-crystalline TeO2 flakes at room temperature in ethanol and water, respectively (Figure 29). Due to the existence of oxygen and water vapor, Te NW film assembled by Te NWs via the Langmuir−Blodgett technique was also unstable in air but underwent a relatively slower oxidation rate.276 With increasing the exposed time at 25 °C (air relative humidity of 40%), numerous oxidized spots gradually emerged on the Te NW film, and the surface of Te NWs became rough. The wellcrystallized Te NWs were finally oxidized into small amorphous oxide particles. XPS analysis indicated that nearly 70% of the Te in the Te NW film was oxidized after a 60-day exposure to air, and the electrical resistance of the Te NW film increased to 513 MΩ (64 times the initial value). However, strategies that can efficiently hinder the contact of oxygen and water, such as 3230


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Figure 30. (a) Overall chemical stability of semiconducting CdSe nanorods against some common acids, bases, oxidants, and reductants (N.R. = no reaction). (b) Two separate mechanisms for the oxygen adsorption of II−IV semiconductors. (c) Stability comparison of PbSe nanocrystals derived from ZnSe NPs by cation exchange with PbX2 (X = I, Br, or Cl). (d) Stability comparison of PbSe nanocrystals synthesized from different reaction protocols. Absorption and PL spectra of (e) TOPO-HNRs and (f) OT-HNRs over 1 month of aging in air. Panel a reprinted with permission from ref 277. Copyright 2014 American Chemical Society. Panel b reprinted with permission from ref 278. Copyright 2014 American Chemical Society. Panels c and d reprinted with permission from ref 286. Copyright 2015 American Chemical Society. Panels e and f reprinted with permission from ref 287. Copyright 2015 American Chemical Society.

oxidized by air. Due to the different coordination ability with surface Pb2+ (I− > Br− > Cl−), I− ions exhibited the maximum effectiveness at protecting PbSe from oxidation. For PbSe nanocrystals obtained via other preparation pathways, all the samples terminated with halide ions showed better stability than the other samples (Figure 30d). In addition, the modulation of surface capping agents is likewise a useful method for obtaining air-stable SNPs.287 When the native ligands TOPO (TOPO: trioctylphosphine oxide) coated on the surface of CdSe/CdTe heterojunction nanorods (HNRs) were replaced by 1-octanethiol (OT) through a ligand exchange process, the CdTe band edge absorption of OT-HNRs remained unchanged after aging for 1 month, while that of TOPO-HNRs displayed a 4 nm blue-shift due to oxidation (Figure 30e,f). Interestingly, the photoluminescence (PL) spectra of CdSe/CdTe nanorods after recapping showed a 3fold increase over the original samples (Figure 30e,f). In addition, alkylselenide ligands or phosphonic acids can also effectively passivate SNPs.281,285 However, in some cases the unstable process of nanomaterials actually offers an opportunity to design and fabricate unique nanomaterials that are inaccessible through direct synthesis methods, such as by processing the initial materials into desired sizes or shapes by oxidative etching and chemically

reductant, to reduce CdSe nanorods to metallic Cd unless the temperature was above 300 °C. Compared with these extreme chemical and physical conditions, surface oxidation of SNPs caused by oxygen in air is the main factor that leads to the degradation of their performance during applications. The oxygen adsorption on the CdSe surface occurs via two different mechanisms: physisorption of oxygen and chemisorption of oxygen (Figure 30b).278,279 Chemisorption of oxygen can lead to the formation of an oxide shell on the surface and the etching of SNPs into smaller sizes. For example, up to 50% of the volume of PbSe nanocrystals was quickly oxidized in hexane under ambient conditions within 24 h; meanwhile, the oxidation-induced reduction in the size of the PbSe “core” caused the absorption onset to change to higher energies.280 Strategies that exist for improving the stability of SNPs can be classified as attempts to passivate either under-coordinated metallic sites or undercoordinated Se sites.281−284 Beard and and co-workers286 developed a direct cation-exchange process to produce airstable PbSe nanocrystals from ZnSe nanocrystals and PbX2 (X = Cl, Br, or I) precursors by both halide anion passivation and zinc cation passivation. With the prolonging of the storage time, the first exciton peak was gradually blue-shifted for all samples (Figure 30c), indicating that the PbSe nanocrystals were 3231


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Figure 31. (A) Schematic illustration of the oxidative etching process of Bi2Te3 nanoplates. (B) Atomic force microscope (AFM) images of Bi2Te3 nanoplates before (a1, a2) and during (b1−d1, b2−d2) oxidation, (a3, b3, c3, and d3) corresponding cross-sectional profiles for a2, b2, c2, and d2 along the blue lines, respectively. (C, D) HRTEM images of stairlike intermediate states during oxidation; the dotted lines are used to highlight the oxidation from outside to inside. (E) Elemental mapping of Bi2Te3 nanoplates before and after oxidation. Reprinted with permission from ref 27. Copyright 2016 Royal Society of Chemistry.

transforming the “captured” intermediate nanostructures during a changing process. Recently, Zhang and co-workers27 demonstrated that oxidative etching can serve as an efficient “top-down” technique for processing materials at the nanoscale (Figure 31A). Single-crystalline Bi2Te3 consists of quintuple layers that are coupled by weak van der Waals interactions along the c direction.288 By choosing suitable oxidizing agents [H2O2, Fe(NO3)3, and HNO3], the layered-crystalline-structured Bi2Te3 nanoplates could be corroded layer-by-layer into a single quintuplet Bi2Te3 layer. During this process, the thickness of Bi2Te3 became increasingly thinner and finally reached approximately 1.2 nm, which was identical to the thickness of a quintuplet layer. The edges of Bi2Te3 nanoplates were first etched by oxidation, and the intermediates exhibited an evident stairlike shape (Figure 31B). Thus, Bi 2 Te 3 nanoplates were etched layer-by-layer from the side into the center by oxidizing agents (Figure 31C,D). The lattice spacings of the two parts (i.e., the side and center) were consistent with each other (0.22 nm), matching with facet (110). Thus, the final Bi2Te3 nanosheets retained the initial crystallization (Figure 31D) and shape (Figure 31E). Similar to liquid-phase

exfoliation methods, this oxidative etching method also offers a new “top-down” approach to create a wide range of single semiconductor nanosheets, which is based on the high reactivity and the poor stability of their corresponding counterparts. After partial oxidation, ultrathin Te NWs evolve into special chain-dotted line structures, which still exhibit high reactivity and can act as an efficient template (chemical or physical) to synthesize other functional nanomaterials (Figure 32).6 First, as a chemical template, partially oxidized Te NWs (OTe NWs) could be used to synthesize telluride and Pt nanocrystals. Ag2Te NWs derived from OTe NWs inherit the one-dimensional chain-dotted structure of OTe NWs, and they have much rougher surfaces than those from normal Te NWs. For the galvanic replacement with H2PtCl6, the chemically transformed products show a dramatic morphological change: hollow nanotubes from the normal Te NWs and solid wirelike nanostructures from the OTe NWs. The inductively coupled plasma-atomic emission spectroscopy (ICP-AES) results indicate that the mass content of Te is maintained at approximately 20% in the products derived from normal Te 3232


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Figure 32. Schematic illustration of the partial oxidation of Te NWs and TEM images of the corresponding chemically transformed products derived from Te NWs and partially oxidized Te NWs. Reprinted with permission from ref 6. Copyright 2015 Springer.

NWs but rapidly decreases to 8% when Te NWs are oxidized for only 0.5 h, finally reaching a constant value of approximately 3%. Thus, moderate oxidation of Te NWs is beneficial for the reaction between Te NWs and H2PtCl6. Second, as a physical template, OTe NWs could be used to synthesize Te@carbon (Te@C) nanocables. Compared with the uniform and long Te@C nanocables derived from normal ultrathin Te NWs, those from OTe NWs are short and bent (Figure 32), providing a robust evidence for the fracture of ultrathin Te NWs during oxidation. Similarly, Se sheets derived from ZnSe(DETA)0.5 via stabilizer depletion and postoxidation can also serve as an excellent precursor for obtaining MxSey (M = Pt, Pd) and N2Se (N = Cu, Ag) nanosheets and even Se NWs.179

4. STABILITY OF NANOPARTICLES DURING APPLICATION In this section, we synoptically discuss the stability of nanomaterials during their application from three perspectives (Figure 33): devices, biomedicine, and environment. (i) With regard to devices, nanomaterials are incorporated into batteries, sensors, and solar cells, for example. The performance retention of these devices is greatly dependent on the stability of the nanomaterials, such as the chemical stability (oxidation and degradation) and structural stability (deformation). (ii) In biomedicine applications, nanomaterials enter into organisms through injection, ingestion, inhalation, and dermal exposure289 to realize certain goals, such as drug delivery, imaging, and diagnosis.290 Due to biorelevant conditions and biological metabolism, the stability of nanomaterials in organisms is very different from the stability of nanomaterials incorporated in devices and is closely related to biodissolution, biodegradation, and clearance.291 (iii) In the environment, nanomaterials are released for waste disposal or environmental remediation.292−294 Unlike the moderate conditions present in organisms, the rigorous natural conditions can easily cause

Figure 33. Schematic illustration of the stability and the reactivity of nanomaterials during application (NOM: natural organic matter).

the changes to the NPs, such as aggregation, redox, precipitation of secondary phases, sorption of (in)organic species from the environment, and chemical transformations.295 4.1. Stability of Nanoparticles in Devices

Due to their special physical and chemical characteristics, incorporating nanomaterials is a good method to improve the performance of devices, such as field-effect transistors, lithium ion batteries, and solar cells.296−305 Recently, because they can enable the combination of flexibility and transparency with an existing electronic band gap, many 2D materials “beyond graphene” have been used to fabricate field-effect transistors, such as atomically thin elemental materials (silicene and phosphorene), IV−VI compounds, and transition-metal dichalcogenides.306 The stability of the obtained field-effect transistors is greatly dependent on surface chemical reactivity of these 2D materials. Under ambient conditions, due to their high surface reactivity, silicene and phosphorene are oxidized by oxygen in air,307−309 resulting in a notable reduction in the 3233


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Figure 34. Three fundamental materials challenges for Si-based electrodes: (a) material pulverization, (b) morphology and volume changes of the whole Si electrode, and (c) continuous SEI growth. Reprinted with permission from ref 325. Copyright 2012 Elsevier Science Ltd.

Figure 35. (a) PCE variation of the solar cells based on MAPb(I1−xBrx)3 (x = 0, 0.06, 0.20, 0.29) with time stored in air at room temperature without encapsulation. (b) Normalized PCE of traditional PSCs (black) and metal oxide-based PSCs (red) as a function of storage time in an ambient environment. Panel (a) reprinted with permission from ref 332. Copyright 2013 American Chemical Society. Panel (b) reprinted with permission from ref 303. Copyright 2016 Nature Publishing Group.

based electrodes causes three fundamental materials challenges (Figure 34):325 (i) material pulverization, (ii) morphology and volume change of the whole Si electrode, and (iii) solid− electrolyte interphase. Concerning material pulverization, due to lithium insertion/extraction during cycling, Si-based electrodes can be cracked or pulverized by the stress induced by large volume expansion (Figure 34a), resulting in poor electrical contact and rapid capacity fading. Regarding the morphology and volume change of the whole Si electrode, at the level of the entire electrode, the expansion of Si nanomaterials in the electrode can also cause significant morphology and volume changes of the whole electrode, leading to a disorganization of the initial arrangement of Si nanomaterials (Figure 34b). This disorganization results in poor electrical connections between Si nanomaterials and further contributes to capacity fading. Considering the solid−electrolyte interphase, the decomposition of the organic electrolyte forms a layer on the electrode material surface, i.e., the “solid−electrolyte interphase” (SEI). A stable SEI layer is beneficial to obtain a long cycle life. However, the “expansion−shrink” process of Si nanomaterials during cycling causes the “growth−fracture” cyclic process for the SEI layer, resulting in continually increasing the thickness of the SEI layer (Figure 34c). Obviously, once nanomaterials are

mobility of charge carriers. The capping layer can efficiently prevent the oxidation of silicene and phosphorene, rendering them chemically stable.310,311 Unlike silicene and phosphorene, InSe, another suitable candidate for field-effect transistors, exhibits a good ambient stability. However, air-exposed indium selenide shows a p-type doping resulting from water decomposition at Se vacancies.312 Thus, when novel nanodevices are designed, both the performance of the nanomaterials and their S&R should be considered. In recent years, the nanostructured anode materials in lithium ion batteries have attracted much attention because of their special properties, such as high surface area, short lithium ion diffusion path length, and high electron transportation rate.313 Various nanomaterials have been used as anodes to improve the performance of Li+ ion batteries, for example, Sibased materials,296,314,315 metal alloys,316,317 transition-metal oxides,318,319 carbon-based materials,320 and layered transitionmetal chalcogenides.321,322 Among these materials, silicon exhibits a low discharge potential323 and a high capacity.296,313,324 However, Si-based electrodes generally suffer from poor capacity retention due to the large volume expansion (>400%) (i.e., poor structural stability) derived from the lithiation−delithiation process.313 This large deformation of Si3234


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Figure 36. Chemical fate of molecules and NPs in vivo. Reprinted with permission from ref 345. Copyright 2011 Wiley-VCH.

helpful for improving the stability of PSCs against water and oxygen degradation;303,335 examples of these inorganic transport layers are graphene oxide,336 reduced graphene oxide,337 and metal oxides.338−344 Yang and co-workers303 designed airstable PSCs via solution-processed metal oxide transport layers. As shown in Figure 35b, these PSCs have a p−i−n structure (glass/indium−tin oxide/NiOx/perovskite/ZnO/Al). Due to the good stability of ZnO in ambient air, the ZnO layer between the perovskite layer and the Al electrode can serve as a robust diffusion barrier against water to improve the stability of PSCs. These PSCs retain approximately 90% of their original efficiency after being stored in air at room temperature for 60 days, while the traditional PSCs fabricated with organic transport layers are completely degraded after only 5 days (Figure 35b). Thus, to obtain stable PSCs, the focus should not only be on the stability of the perovskite layer. The PSCs are a whole system consisting of a perovskite layer, charge transport layers, and electrodes. Other parts also play important roles in the stability of PSCs.

incorporated into a device, their stability not only influences their performance but also causally affects other aspects, such as the whole organized nanomaterial structure and the interactions with other components in the devices. Since the first report of perovskite solar cells (PSCs) by Miyasaka’s group in 2009,326 significant progress has been achieved, and the maximum power conversion efficiency (PCE) of PSCs has increased very fast by 5−6 times from 3.8% to above 20%.327 However, the poor stability of PSC devices greatly impedes their commercialization. Generally, the stability of PSC devices is related to several factors, such as moisture, oxygen, temperature, the solution process (solvents, solutes, and additives), and even the architecture, interface, and other components of the devices.328,329 Compared with the other factors, moisture is the major factor that causes the degradation of the perovskite layer.330,331 The most commonly used perovskite material is CH3NH3PbI3,330 which exhibits poor stability when in contact with moisture. However, when I− ions are partly substituted by Br− ions, the obtained mixed-halide perovskite materials (CH3NH3Pb[I1−xBrx]3, x = 0, 0.06, 0.2, and 0.29) show better stability.332 During stability testing, PSCs composed of these mixed-halide perovskite materials were intentionally exposed to a relatively high humidity (55%) on the fourth day while the humidity was kept at 35% on the other days (Figure 35a). CH3NH3PbI3 PSCs maintained good stability at a low humidity (35%) but began to decompose at a high humidity (55%). All of the PSCs composed of CH3NH3Pb[I1−xBrx]3 were more stable than CH3NH3PbI3 PSC. At the composition of x = 0.2, CH3NH3Pb[I1−xBrx]3 PSCs exhibited excellent stability and the highest efficiency. These results indicate that the stability of the PSCs can be greatly improved by adjusting the chemical composition of the perovskites. Similarly, the iodide−chloride mixed-halide perovskites also exhibit better stability.333 Karunadasa and coworkers334 found that the perovskites could also become more stable by changing hygroscopic CH3NH3 to more hydrophobic (C6H5(CH2)2NH3)2(CH3NH3)2. In addition, compared with organic charge transport layers, inorganic transport layers are

4.2. Stability of Nanoparticles in Biomedicine

Nanomaterials combine the mobile ability of molecules and the properties of classical solids.345 Once nanomaterials enter into a biological system, on the one hand, they can circulate within the body like molecules via the blood circulation to reach the targeted organs or cells; on the other hand, many bioactive substances or cells in organisms may react with the chemically active surface of nanomaterials to initiate complex biophysicochemical reactions, resulting in the aggregation, dissolution, degradation, accumulation, or clearance of nanomaterials (like solids).289 The chemical fate of traditional medicinal molecules in a biological system primarily includes degradation and metabolism. Unlike molecules, for the NPs without stabilizing ligands, their poor dispersion stability in biological media results in aggregation, which complicates their behaviors in vivo, such as their mobility, biodistribution, and clearance (Figure 36). Stabilized NPs can also become unstable due to the interactions between them and bioactive substances or cells 3235


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Figure 37. Timelines of the degradation and clearance of various siliceous nanomaterials. Reprinted with permission from ref 347. Copyright 2017 Wiley-VCH.

treatment of various diseases.347,350,351 The factors that influence the degradation and clearance of siliceous nanomaterials include (i) the NP characteristics, such as size, shape, porosity, chemical stability (e.g., oxidation), concentration, and aggregation; (ii) degradation media, for example, the pH of the media, bioactive molecules with redox ability or enzymatic catalytic ability in the media; and (iii) interfaces between the NPs and the degradation media, which are closely related to the surface coating, surface functionalization, and surface charge of NPs, i.e., the surface chemistry of the NPs.347,352,353 Figure 37 shows the detailed timelines of the degradation and clearance of various siliceous nanomaterials. Most siliceous nanomaterials can be quickly degraded and cleared within a short time, indicating that they exhibit a modest stability, which is beneficial to balance NP-induced activity and toxicity. In addition, the structural design, surface modification, and ion doping can significantly affect the degradation rate of NPs in vivo, thus presenting effective methods to regulate the stability, reactivity, and toxicity of NPs during biomedical applications.

(Figure 36). In addition, in vivo, NPs exhibit mass-related effects (dissolution and release) or catalytic effects (time- and mass-dependent),345 which can also alter their stability, reactivity, and toxicity. According to the basic safety principles, biomedical agents should be effectively cleared from the body to lower the accumulation in organs or tissues.289,346 The accumulation of nanomaterials and/or the release of ions (especially heavymetal ions) may pose potential health risks to humans.347 Although kidney filtration is a desirable pathway for the clearance of nanomaterials, it is ineffective for nanomaterials with a hydrodynamic diameter >8 nm unless these nanomaterials are degraded into smaller ones. Hence, these larger nanomaterials are required to have a modest stability to make them more degradable and clearable, resulting in lower bioaccumulations and favorable risk−benefit ratios.348,349 As typical representatives of biorelated degradable and clearable nanomaterials, siliceous nanomaterials (such as silicon NPs, silica NPs, and organosilica NPs) have been studied worldwide for biomedical applications, especially in the diagnosis and 3236


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4.3. Stability of Nanoparticles in the Environment

significantly change the surface properties of Ag NPs and inhibit the antibacterial effect of Ag NPs.359 On the other hand, the organic/inorganic sulfur present in natural systems can interact with Ag NPs, the Ag2O shell, and Ag+ cations to produce more thermodynamically stable Ag2S. Due to the lower solubility of Ag2S (Ksp = 5.92 × 10−51), the sulfidation of Ag NPs decreases their toxicity, potentially limiting their environmental impacts.295 Furthermore, these Ag2S NPs can be oxidized into Ag2SO4 by air or microbial transformation. Another concerning issue for the environmental behavior of NPs is understanding the interactions of engineered NPs with plants, such as biotransformation and accumulation. Many investigations have demonstrated that engineered NPs can accumulate or increase the concentration of their corresponding metal ions in the fruits and grains of agricultural crops.355,360−362 Figure 39 shows the uptake, translocation, and biotransformation of engineered NPs in a plant system. Engineered NPs or metal species released from NPs in soil can be taken up by plant roots and transported to stems or leaves through vascular systems, which is strongly dependent on the composition, shape, and size of NPs, as well as the plant anatomy.362 In addition, the dissolution behaviors of engineered NPs in various environmental media (e.g., soil, sewage sludge, aquatic environments) determine their final forms (such as NPs, metal ionic species, or both) that interact with the plants.355 Due to the poor solubility of CeO2 and TiO2 NPs in different media, these compounds mostly interact with plants in the nanoparticulate form. In contrast, ZnO, CuO, Ag, and Au NPs exhibit varying degrees of dissolution in soil, so they can affect plants both in particulate form and as metal ionic species. Carbon-based engineered NPs (fullerols and multiwalled carbon nanotubes) primarily interact with plants in nanoparticulate forms. Once these engineered NPs enter into plants, multiple charged cations and anions (e.g., Ca2+, PO43−, SO42−) in the nutrient solution may induce the aggregation of these NPs by compressing their electric double layer.363 In addition, the chemical substances in plants can also interact with the NPs and cause their chemical transformations. For example, very small CeO2 NPs can transform into Ce3+ species (e.g., cerium phosphates and carboxylates);363−366 ZnO NPs partially transform into Zn2+ species, such as Zn-citrate, Znphosphates, Zn-nitrate, Zn-histidine, and Zn-phytate;367−369 a portion of YbO3 NPs transforms into YbPO4;370 CuO NPs can partially transform into Cu2+ and Cu+ species;367,371 and a portion of Ag NPs transforms into Ag species, which chelate with S and O/N ligands in plants.372 The different dissolution behaviors of engineered NPs in various environmental media and their poor chemical stability in plants make their biotransformation and accumulation processes more diverse and complex. Thus, an in-depth understanding of the S&R of engineered NPs in various environment media and plants is beneficial to evaluate their potential risks to the environment and human health.

With the rapid development of nanoscience and nanotechnology, various engineered nanomaterials are being practically applied. Meanwhile, increasing numbers of commercial nanomaterials are released into the environment (air, water, soil, and sediment)354 for waste disposal, environmental remediation, or other specific purposes. This may have potentially negative impacts on the ecosystem and even the food chain due to the accumulation and biotransformation of NPs in plants.293,355 Herein, Ag NPs are selected as a model to discuss the environmental fate of NPs, i.e., their stability and reactivity in the environment. Then, the accumulation and the biotransformation of various NPs in plants are synoptically introduced. As an excellent antibacterial agent, Ag NPs have been used in many consumer products, such as clothes, cosmetics, food storage containers, household appliances, and children’s toys.292 With the use of these commercial nanoproducts, the contained Ag NPs can be released into the environment by washing, abrasion, and fracture. Figure 38 shows the transformations of

Figure 38. Environmental transformations of Ag NPs. Reprinted with permission from ref 295. Copyright 2012 American Chemical Society.

Ag NPs in the environment. In the short term, the initial SCME of Ag NPs is crucial for understanding the environmental behavior of Ag NPs, such as aggregation and oxidation. Uncoated Ag NPs are primarily electrostatically stabilized against aggregation due to their surface charge, while Ag NPs coated with organic ligands are stabilized by both steric stabilization and surface charge stabilization. The number, molecular conformation, and molecular weight of the coated organic ligands strongly affect their ability to stabilize Ag NPs. Kvitek et al. found that high molecular weight PVP can effectively stabilize Ag NPs compared with sodium dodecyl sulfate (SDS) and polyoxyethylene sorbitan monooleate (Tween 80).356 In addition, due to their high reactivity, these released Ag NPs can be quickly oxidized by oxygen in air, forming a silver oxide thin shell on surfaces. Once this oxide shell is dissolved (Ksp = 4 × 10−11),357 Ag+ cations are released into the environment, resulting in the toxicity of Ag NPs.358 This corrosion process (oxidation and dissolution) indicates that Ag NPs have a poor chemical stability. In the medium- and long-term, the pristine organic shell is gradually substituted by natural organic matter (NOM). These macromolecules can

5. CONCLUSIONS AND REMARKS In summary, the high surface energy of NPs causes them to easily react with surrounding active substances, thus bestowing on them a dual nature of poor stability and high reactivity, which results in both negative and positive effects for nanoparticle processing. The poor stability causes the degradation of NPs during storage and use, while the high reactivity of NPs makes them be commodiously transformed into other target functional materials. The ultimate goal of the 3237


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Figure 39. Uptake, translocation, and biotransformation of engineered NPs in plant tissues, including fruits and grains. The inset shows the transverse cross section of the root absorption zone. Reprinted with permission from ref 355 (modified from refs 373 and 364). Copyright 2014 American Chemical Society.

development of nanomaterials is to utilize NPs or assemble them into complex entities to realize certain applications that can potentially change our daily life and the world. Thus, two key issues arise that include (i) whether we can utilize these undesired processes of NPs derived from their poor stability and (ii) how to maintain NPs with a high reactivity for a long period during storage and application. On one hand, the undesired processes of NPs derived from their poor stability always cause problems, but they can offer new approaches to design and synthesize novel functional materials that are inaccessible via the existing direct synthesis methods. These approaches can be divided into three major categories: bottom-up, top-down, and chemical transformation of intermediates. For bottom-up, NPs can act as “nanoscale building blocks” to be assembled into larger structures in some unstable process, such as heteroaggregation, fusion, welding, oriented attachment, stabilizer-depleted assembly, and formation of supercrystals. In contrast, NPs in some unstable processes can be regarded as “raw materials” to process into smaller ones, i.e., top-down. These processes include digestive ripening, acidolysis, oxidative etching, and so on. In addition, by capturing intermediate nanostructures during unstable processes, on the basis of their high reactivity, these intermediate nanostructures can be easily chemically transformed into other functional nanostructures with sizes and shapes similar to those of the intermediates due to the template effect. On the other hand, from the viewpoint of practical applications, a major concern is how to keep highly reactive NPs relatively stable, and we believe that two aspects should be considered: the energy state and isolation of active substances. A stable chemical system is achieved at its lowest energy state. Thus, obtaining a lower energy state is an effective method to keep a system/material stable; examples of this are storing NPs at a low temperature or coating NPs with suitable substances to

reduce their surface energy. Another method to maintain the stability of NPs is to isolate NPs from active substances, for example, by storing NPs in a closed container, forming nonporous coatings on NP surfaces, or adding suitable chemical agents to preferentially react with active substances (such as adding suitable reductants to prevent the oxidation of NPs). Note that all the procedures that maintain NP stability should be compatible with the subsequent functionalization and application of NPs. In some cases, stabilizing procedures that completely segregate NPs from the external environment can actually render them inert and ineffective. Thus, a coating with a selective permeability is a better choice for stabilizing NPs, which can separate out undesired active substances to maintain the NP stability, while allowing target molecules or ions to penetrate through the coating and then react with internal NPs to produce the expected results. Furthermore, once nanomaterials are applied in real devices, investigating NP stability becomes more difficult, because the actual applications of nanomaterials are often related to many interactive chemical and physical processes. Hence, the stability of nanomaterials during application involves not only the micro level (their own) but also macro levels, such as their whole organized structure, the media, and even the environment (air, soil, water, and plants). These macro levels require the consideration and understanding of nanomaterial stability from a holistic perspective: nanomaterials during application are no longer isolated particles, but the particles are closely related with these macro levels. Hence, the stability of NPs during application strongly depends on the interactions between nanomaterials and these macro levels, such as chemical interactions (oxidation caused by oxygen in air, chemical adsorption, chemical etching, etc.), biological interactions (biodissolution, biodegradation, biochemical reactions, etc.), and physical interactions (dissolution, physical adsorption, 3238


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Biographies

deformation, fracture, etc.). Due to the interactions between nanomaterials and these macro levels, various factors are involved in the unstable processes of nanomaterials, which makes investigating the key factors for nanomaterial stability during application more difficult. Before a new strategy to maintain the good performance and long retention of nanomaterials is adopted, an in-depth understanding of the inter-relationships between nanomaterials and the macro levels should be obtained in which the key factor and its mechanism of action are determined, and also whether the nanomaterials can be stabilized at both the micro level and the macro level should be investigated. Nanomaterials or nanodevices with fascinating functions remain useless if they can maintain their good performance for only a short time. For practical applications, the performance and stability of nanomaterials must be balanced in most cases. Overall, although studies of nanomaterials have increased exponentially in recent decades, the works have focused on the synthesis (creating various NPs with different sizes and shapes) and function (improving the performance) of NPs, whereas the stability and reactivity of nanoparticles have been somewhat overlooked. As the foundation of the research and application of NPs, their stability and reactivity warrant more attention, because they are meaningful for establishing new design and preparation methods for nanomaterials, reinforcing the foundation of nanoscience and promoting the development of nanotechnology. To obtain an in-depth understanding of the stability and reactivity of NPs, we think that the following points should be seriously considered. (i) The intrinsic stability and reactivity of NPs are closely associated with their bulk counterparts, but the stability and reactivity of NPs cannot be simply evaluated by scaling the properties of bulk materials based on the surface area. The high surface energy of NPs and the abundant unsaturated surface atoms on the NP surface play important roles in determining their stability and reactivity. (ii) As an interface between NPs and the outside environment, the chemical substances that are coated on NP surfaces to some extent determine how they interact with the outside world and the strength of these interactions. Thus, the surface chemistry of NPs also greatly affects the stability and reactivity of NPs. (iii) The in situ and quantitative characterizations can help us more clearly and reasonably understand changes of NPs, which are very helpful for obtaining a quantitative and objective evaluation of the stability and reactivity of NPs. (iv) When nanomaterials are applied in practical situations, their stability and reactivity not only depend on themselves but also are strongly related to their whole organized structure, the substances used to disperse or load them, the practical conditions (such as temperature, atmosphere, and humidity), and other factors.

Liang Xu received his Bachelor’s Degree at Lanzhou University in nuclear chemical engineering and fuel cycle in 2011 and his Ph.D. in inorganic chemistry in 2016 from University of Science and Technology of China (USTC) under the supervision of Prof. ShuHong Yu. Now, he is a postdoctor at USTC, and his current research focuses on the stability and reactivity of nanomaterials. Hai-Wei Liang received his Ph.D. under the supervision of Prof. ShuHong Yu from USTC in 2011. From 2012 to 2015, he finished postdoctoral research under Prof. Xinliang Feng and Prof. Klaus Müllen at the Max Planck Institute of Polymer Research in Mainz, Germany. In 2016, he was appointed as a full professor at USTC. His current research interests include biomass-derived nanomaterials and carbon-based nonprecious-metal catalysts. Yuan Yang received his Bachelor’s Degree from USTC in chemistry in 2011 and his Ph.D. in inorganic chemistry in 2017 from USTC under the supervision of Prof. Shu-Hong Yu. Now, he is a postdoctor at USTC, and his current research focuses on the synthesis of onedimensional nanomaterials and their functions. Shu-Hong Yu received his Ph.D. in inorganic chemistry in 1998 from USTC. He worked in the Materials and Structures Laboratory of the Tokyo Institute of Technology, as a postdoctoral research fellow, and later as an Alexander von Humboldt Research Fellow at the Max Planck Institute of Colloids and Interfaces. He was appointed as a full professor in 2002 at USTC and was awarded the Cheung Kong Professorship in 2006 by the Ministry of Education of China. His research interests include bioinspired synthesis and self-assembly of nanoscale building blocks, macroscopic assemblies, and nanocomposites, and their related properties. He has authored more than 430 refereed journal publications and 17 invited book chapters. He serves as an associate editor for Langmuir, and is an editorial advisory board member of Accounts of Chemical Research, Chemistry of Materials, Chemical Science, Materials Horizons, Nano Research, ChemNanoMat, ChemPlusChem, CrystEngComm, and Crystals. He received the Chem Soc Rev Emerging Investigator Award (2010) and the Roy−Somiya Medal of the International Solvothermal and Hydrothermal Association (ISHA) (2010).

ACKNOWLEDGMENTS We acknowledge the funding support from the National Natural Science Foundation of China (Grants 51732011, 21431006, 21761132008), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant 21521001), Key Research Program of Frontier Sciences, CAS (Grant QYZDJ-SSW-SLH036), the National Basic Research Program of China (Grant 2014CB931800), and the Users with Excellence and Scientific Research Grant of Hefei Science Center of CAS (2015HSC-UE007). This work was partially carried out at the USTC Center for Micro and Nanoscale Research and Fabrication. ABBREVIATIONS USED 1D one-dimensional 2D two-dimensional 3D three-dimensional ACN acetonitrile AFM atomic force microscope BC bacterial cellulose BSA bovine serum albumin cAu NPs citrate-coated Au NPs CTAB cetyltrimethylammonium bromide

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Shu-Hong Yu: 0000-0003-3732-1011 Notes

The authors declare no competing financial interest. 3239


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DDAB DEG DETA DMAE DMF DMK DR DTAB ECSTM

didodecyldimethylammonium bromide diethylene glycol diethylenetriamine dimethylaminoethanol dimethylformamide dimethyl ketone digestive ripening dodecyltrimethylammonium bromide electrochemical scanning tunneling microscopy EDS energy dispersive spectroscopy EDTA ethylenediaminetetraacetic acid EG ethylene glycol Ep peak oxidation potential EtOH ethanol HAADF-STEM high-angle annular dark-field scanning transmission electron microscopy HNRs heterojunction nanorods ICP-AES inductively coupled plasma-atomic emission spectroscopy ITO indium−tin oxide Me4NOH tetramethylammonium hydroxide MeOH methanol MF methylformamide MPS 3-mercapto-1-propanesulfonate mSiO2 mesoporous silica N.R. no reaction NOM natural organic matter NPs nanoparticles NWs nanowires OA oriented attachment ODE 1-octadecene OR Ostwald ripening ORR oxygen reduction reaction OT 1-octanethiol OTe NWs oxidized Te nanowires PCE power conversion efficiency PEG polyethylene glycol PILs polymeric imidazole ligands PL photoluminescence PSCs perovskite solar cells PVP polyvinylpyrrolidone RSHs alkanethiols S&R stability and reactivity SCEM surface chemical microenvironment SDS sodium dodecyl sulfate SEI solid−electrolyte interphase SNPs semiconductor nanoparticles TA tartaric acid TDPA tetradecylphosphonic acid Te@C Te@carbon TETAH protonated triethylenetetramine TGA thioglycolic acid Tm melting point in absolute units TOP trioctylphosphine TOPO trioctylphosphine oxide Tween 80 polyoxyethylenesorbitane monooleate XAS X-ray absorption spectroscopy XPS X-ray photoelectron spectroscopy

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