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Synthesis of Inorganic Nanostructures in Reverse Micelles Limin Qi College of Chemistry and Molecular Engineering, Peking University, Beijing, P.R. China INTRODUCTION There is an immense interest in the synthesis of inorganic nanostructures, which have at least one dimension between 1 and 100 nm by deﬁnition, owing to their fascinating properties and tremendous potential for technological applications.[1–4] Because the mechanical, optical, electrical, magnetic, thermal, chemical, and biomedical properties of inorganic nanomaterials are largely dependent on the size, shape, composition, and architecture of the nanostructures, many efforts have been devoted to the manipulation of these factors. In this regard, different approaches, ranging from physical to chemical methods, have been exploited for the controlled fabrication of well-deﬁned inorganic nanostructures. Increasingly, colloidal chemists are contributing to the biomimetic synthesis of inorganic nanostructures with dimensional, morphological, and architectural speciﬁcity by using organized self-assemblies of surfactants as nanostructured reaction media or templates. This sophisticated approach is largely inspired by biomineralization, where organized aggregates of biomacromolecules exert exquisite control over the nucleation, growth, and patterning of inorganic minerals. In particular, as a typical membrane mimetic system, reverse micelles have been widely used as nanostructured reaction media or nanoreactors for the biomimetic synthesis of various inorganic nanostructures. This article gives an overview of the controlled synthesis of inorganic nanostructures in reverse micelles, with its attention focused on the latest advances in the synthesis mechanism, processing method, and control of the morphology and architecture of nanostructures. After a general discussion on the reverse micelle-based synthesis technique, the development in the synthesis of spherical nanoparticles, nonspherical nanostructures and their assemblies, and core–shell nanostructures is summarized. REVERSE MICELLES AS NANOREACTORS Surfactants normally contain a hydrophilic head and a hydrophobic chain, and these amphiphilic molecules can self-assemble into a rich variety of organized structures in solution, such as normal and reverse micelles, microemulsions, vesicles, and lyotropic liquid crystals. Speciﬁcally, reverse micelles are globular aggregates formed by the self-assembly of surfactants in apolar solvents, whereas normal micelles are globular aggregates formed by the self-assembly of surfactants in water. The structure of reverse micelles is characterized by a polar core formed by the hydrophilic heads and an apolar shell constituted by the hydrophobic chains. Water can be readily solubilized in the polar core to form water-in-oil (w/o) droplets, which are usually called reverse micelles at low water content and w/o microemulsions at high water content. Traditionally, microemulsions are deﬁned as thermodynamically stable, optically transparent dispersions of two immiscible liquids stabilized by an interfacial surfactant ﬁlm.[7,8] The w/o microemulsions normally consist of water droplets ranging from 5 to 100 nm in size, slightly larger than reverse micelles, which are typically smaller than 5 nm.[9,10] Although there are several arguments to strongly support a differentiation between reverse micelles and w/o microemulsions, the demarcation line is quite blurred and there is often no clear distinction between these two systems in the literature related to nanoparticle synthesis in nanostructured media.[10,11] Accordingly, in this entry, no such distinction is made and all systems consisting of nanosized water droplets stabilized by surfactants and dispersed in oil are termed as either reverse micelles or w/o microemulsions, regardless of the relative component proportions. Practically, a reverse micelle solution is often called a w/o microemulsion and a reverse micelle may correspond to a microemulsion droplet. The droplet size of reverse micelles can be readily modulated in the nanometer range by various parameters, in particular, the water–surfactant molar ratio, W [W ¼ (water)/(surfactant)]. On the other hand, the droplets are kinetically unstable and a dynamic exchange process occurs between colliding droplets. These unique properties together with the speciﬁc adsorption of surfactants on inorganic materials enable the use of reverse micelles as effective nanoreactors for the controlled synthesis of inorganic nanostructures with tailored size, shape, composition, and architecture. Encyclopedia of Surface and Colloid Science DOI: 10.1081/E-ESCS-120023694 Copyright # 2006 by Taylor & Francis. All rights reserved. 6183 6184 The synthesis of inorganic nanoparticles in reverse micelle systems was ﬁrst demonstrated for monodisperse metal particles (3–5 nm) by Boutonnet et al., in the early 1980s. Since then, reverse micelle- or microemulsion-based synthesis of inorganic nanostructures has expanded dramatically, and quite a few reviews dealing with various aspects of the technique are available.[3,13–28] Nanoparticle Synthesis in Reverse Micelles For a feasible nanoparticle synthesis in reverse micelles, water–surfactant–oil formulations that give stable microemulsions must be identiﬁed. Phase diagrams already available in the literature for the ternary mixtures of surfactant–oil–water or the quaternary mixtures of water–surfactant–cosurfactant–oil are useful for this purpose. However, the effects of the reactants and products on the domain of stable w/o microemulsions should be taken into account, which sometimes severely limits the highest reactant concentrations that can be used for precipitation reactions. Among the various surfactants exploited in formulating microemulsions for nanoparticle synthesis, the anionic double-chained surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and nonionic polyoxyethylated surfactants [e.g., poly (ethylene oxide)-(5)-nonylphenyl ether, (NP-5)] appear to be the most popular. The attraction of AOT and the nonionic surfactants is partly attributed to the fact that they can form reverse micelles without the need for cosurfactants. In addition, irrespective of the nature of the apolar solvent, the nearly spherical and monodisperse water-containing reverse micelles are formed by the AOT, the size of which is quite independent of the surfactant concentration and regulated mainly by the W value, e.g., at W > 10, the droplet radius, Rd, can be directly obtained from the W value by the linear relationship Rd/nm ¼ 0.17 W.[6,28] Generally, the size of reverse micelles increases with increasing the W value; however, the size and shape of reverse micelles and their dependence on the water and surfactant concentrations are actually system speciﬁc. A unique advantage of the nonionic surfactants is that their use does not involve the introduction of potentially undesirable counterions; moreover, the ability to alter the size of the hydrophilic oxyethylene groups and the hydrophobic alkyl groups provides additional ﬂexibility in surfactant selections. In general, a number of factors should be considered carefully when selecting a suitable surfactant for a speciﬁc synthesis reaction in reverse micelles. For example, the surfactant should be chemically inert with respect to all other components of the microemulsion, and the possible interference of the counterions of ionic surfactants should be taken into consideration. Synthesis of Inorganic Nanostructures in Reverse Micelles Notably, in some cases, functionalized surfactants, the counterions of which serve as one of the reactants, may be employed for a desirable control of the nanoparticle synthesis in reverse micelles. To illustrate, Ag bis(2-ethylhexyl) sulfosuccinate, Ag(AOT), can be used for the preparation of metallic Ag and semiconductor Ag2S nanoparticles in the Ag(AOT) reverse micelles. The synthesis of inorganic nanoparticles in reverse micelles has been achieved mainly by mixing two identical reverse micelle solutions (microemulsions) containing the appropriate hydrophilic reactants, i.e., a two-microemulsion method. The schematic picture shown in Fig. 1 illustrates the formation of the nanoparticles of the product (P) from the precipitation reaction between the two reactants A and B solubilized, respectively, in the water cores of two microemulsions. Upon mixing, the droplets collide, coalesce to form transient droplet dimers, exchange the water contents, and disintegrate into droplets, continuously. Fig. 1 Schematic representation of nanoparticle synthesis in reverse micelles by mixing two reverse micelle solutions (microemulsions). Synthesis of Inorganic Nanostructures in Reverse Micelles Then the precipitation reaction takes place inside the nanodroplets, which is followed by nucleation, growth, ripening, and coagulation of primary particles, resulting in the formation of the ﬁnal nanoparticles surrounded by water and/or stabilized by surfactants. In addition to the most commonly used two-microemulsion method, a single-microemulsion method that includes a number of variations has also been frequently used for the reverse micelle-based nanoparticle synthesis. For example, one of the reactants (e.g., precipitating or reducing agent, and alkoxide) can be added directly, in the form of a solution or a solid, liquid, or gas, to the microemulsion carrying the other reactant. Alternatively, a microemulsion containing all the reactants can be activated by a trigger (e.g., pulse radiolysis, laser photolysis, ultrasonication, and temperature elevation) to initiate the reactions that eventually lead to the nanoparticle formation. It should be pointed out that, in addition to the size of the reverse micelles, the dynamic nature of the reverse micelles plays a key role in the formation of the ﬁnal nanoparticles for both the two- and singlemicroemulsion methods. Because the exchange of aqueous contents between the microemulsion droplets or the intermicellar material 6185 exchange is closely related to the formation process of nanoparticles in reverse micelles, it is necessary to consider how the intermicellar exchange inﬂuences various aspects of the nanoparticle formation (Fig. 2). It is generally accepted that the water contents of microemulsion droplets are exchanged rapidly (on a microsecond scale) through droplet collision, fusion, and dimer ﬁssion, with the fusion step as the ratedetermining step. The exchange rate can be characterized by a second-order, intermicellar exchange rate constant, kex, expressed in terms of the droplet concentration in the continuous oil medium while the ﬁssion rate of the fused dimers can be characterized by a ﬁrst-order, ﬁssion rate constant, kﬁss. Around room temperature, kex is generally between 106 and 108 dm3/mol/s for the AOT reverse micelles, while it is between 108 and 109 dm3/mol/s for the reverse micelles formed by nonionic surfactants of the poly(ethylene oxide) (PEO) monoalkyl ether (CiEj) type. On the other hand, the droplet encounter rate constant, ken, is approximately 1010 dm3/mol/s at room temperature for microemulsion droplets dispersed in a typical oil like n-heptane, which indicates that only 0.01–1% of binary droplet collisions result in droplet fusion and intermicellar exchange for the AOT reverse Fig. 2 Mechanisms of nanoparticle formation in reverse micelles via intermicellar exchange. 6186 micelles, which correspond to relatively rigid droplets. Generally, chemical reactions of the hydrophilic reactants within the nanodroplets are very fast with a reactant rate constant, kr, much larger than kex, indicating that the intermicellar material exchange is the rate-determining step in the overall reaction leading to nanoparticle formation. It may be pointed out that many ideas based on bulk nucleation and growth models may not be valid any more for the particle formation in these dynamic compartmentalized media. However, if the involved reaction is very slow (e.g., the hydrolysis of tetraethoxysilane to produce silica), the intermicellar exchange does not play an important role in the particle formation, and the reaction proceeds in the same way as it does in the bulk. In this case, the most simple bulk ideas of nucleation and growth can be applied to explain the experimental results by using a pseudophase model. Autocatalysis and ripening are two typical ways of nanoparticle growth in microemulsion droplets, which also manifest the considerable effects of intermicellar exchange. As the reaction takes place in reverse micelles, more and more droplets could contain products and reactants simultaneously. The reaction inside nanodroplets can be catalyzed by the surface of an existing product particle, and a large particle has a greater probability of acting as a catalyst owing to its large surface. Therefore, if the reaction is autocatalytic, the exchange of reactants between the two colliding droplets containing a product particle would result in the reaction occurring on the existing particle or the growth of the particle. The Ostwald ripening is another typical growth process of nanoparticles formed in reverse micelles. The ripening theory assumes that the larger particles will grow by condensation of material, coming from the smaller particles that solubilize more than the large ones. In microemulsions, this growth may take place through the fused dimers, and a small particle formed inside a droplet can be transferred to another one carrying a larger particle if the size of the channel connecting the two coalescing droplets is larger than the small particle. This indicates that the surfactantﬁlm ﬂexibility plays a fundamental role in the ripening contribution to the growth process because the interdroplet exchange of the growing particles is inhibited by the inversion of the ﬁlm curvature in the fused dimers. It is interesting to point out that there is an important difference between the autocatalysis and ripening mechanisms: the autocatalysis depends on the reactant exchange whereas the ripening depends on the product exchange. Once the nanoparticles attain their ﬁnal size, they could be located within the water cores of the droplets that are stabilized by a surfactant ﬁlm. However, in most cases, the surfactant molecules may be attached Synthesis of Inorganic Nanostructures in Reverse Micelles directly to the particle surface, thus stabilizing the nanoparticles and protecting them from further growth. In this case, the whole surfactant ﬁlm can be adsorbed onto the particle by an ‘‘implosion’’ mechanism when a droplet containing a particle with a comparable size coalesces with an empty droplet. This stabilization mechanism is favored when the surfactant ﬁlm is relatively rigid. For a highly ﬂexible surfactant ﬁlm, the particle may approach the droplet interface and some of the surfactant molecules may be adsorbed onto the particle leaving the droplet, which is followed by an inversion of the ﬁlm curvature and the adsorption of a whole surfactant ﬁlm onto the particle. When the surfactant ﬁlm is very ﬂexible, the ﬁlm is able to adjust to the particle size during the particle growth, possibly leading to the formation of particles much larger than the droplet size. Finally, it must be emphasized that under speciﬁc conditions, the inorganic particles formed inside reverse micelles may undergo further growth or aggregation to form ﬁnal particles that are signiﬁcantly larger than their parent nanodroplets. In particular, inorganic nanowires with lengths up to tens of micrometers can be produced in reverse micelle solutions because of the complex interplay between the dynamic nature of the parent micelles, the growth habit of the inorganic materials, and the speciﬁc interfacial organic–inorganic interactions, which will be discussed in more detail in the part on the synthesis of nonspherical and complex nanostructures. Control of Nanoparticle Size In most cases, spherical nanoparticles are obtained in reverse micelles where the surfactant-stabilized water nanodroplets play the role of nanoreactors. It was initially assumed that the nanodroplets could be used as templates to control the ﬁnal size of the particles obtained in reverse micelles. However, later research has shown that there is no direct correlation between the droplet size and the particle size, although the droplet size does seem to have a great inﬂuence on the ﬁnal particle size in many cases. Nevertheless, the combination of the droplet size with other parameters including intermicellar exchange rate, surfactant ﬁlm ﬂexibility, and reactant concentration can exert a delicate control over the ﬁnal particle size. Basically, there are two different mechanisms for the particle-size control in the microemulsion-based synthesis, i.e., the microemulsion-controlled mechanism and the surfactant-controlled mechanism. In the microemulsion-controlled mechanism, the particle size varies as a function of various microemulsion parameters, such as the droplet size, intermicellar exchange rate, and reactant concentration. On the contrary, in the surfactant-controlled mechanism, the particle Synthesis of Inorganic Nanostructures in Reverse Micelles 1) Droplet size: In principle, the particle size increases with the droplet size. As the droplet size is approximately proportional to the water-to-surfactant ratio, W, for most reverse micelles, a nearly linear increase of the particle size with W has been observed for the microemulsion-based synthesis of a variety of inorganic nanoparticles. As an example, in Fig. 3, the variation of the size of Cu nanoparticles obtained in the AOT reverse micelles at various water contents has been shown. It suggests an increase in the particle size on increasing the ratio, W, from 1 to 10 or on increasing the droplet size, approximately, from 0.6 to 3 nm, i.e., the particle size varies from 2 to 13 nm when iso-octane is used as the bulk solvent, whereas the particle size varies from 1 to 7 nm when cyclohexane is used as the bulk solvent. It is worth noting that on increasing the water content, the particle size reaches a plateau at a certain water content, e.g., at W ¼ 10 for the iso-octane system. This has been originally explained in terms of the water structure in the droplet, as an increase in the water content induces a gradual change from ‘‘bound’’ to ‘‘free’’ water in the droplets. Recently, a plausible explanation based on the transition from the microemulsion-controlled particle size to the surfactant-controlled particle size 14 Average Diameter (nm) size is controlled by the adsorption or capping of surfactants on the particles inside the microemulsion droplets, and it is independent of the key microemulsion parameters. The size control by surfactant adsorption has been mostly observed in the preparation of metal catalyst nanoparticles because of the favored adsorption of surfactants on catalysts. For example, the ﬁrst report on the microemulsion-based nanoparticle synthesis has revealed that the particle size of Pt, Rh, Pd, and Ir remains almost constant (3–5 nm), independent of the water amount and reaction concentration. In this case, the particles are stabilized by the surfactants, and the role of microemulsions in the particle size control does not seem obvious because ligands or capping agents may be used directly in bulk solution to inhibit the particle growth and obtain similar results. Therefore, the following discussion will be focused on the microemulsioncontrolled mechanism for the particle-size control in the microemulsion-based technique. The microemulsion-controlled particle size and the particle size distribution (PSD) depend on the particular combination of all the relevant microemulsion parameters, and the effects of various factors on the ﬁnal particle size are actually system speciﬁc. However, according to a range of experimental results, the effects of several key parameters on the particle size can be generally summarized, especially for the spherical nanoparticle synthesis based on the simple precipitation or reduction reaction occurring within water droplets: 6187 12 10 8 6 4 2 0 0 2 4 6 8 10 12 14 16 W Fig. 3 Variation of the size of Cu nanoparticle synthesized in AOT reverse micelles formed by different bulk solvents with the W value. Bulk solvent: () iso-octane, (G) cyclohexane. (From Ref..) with increasing droplet size has been proposed for the tendency to reach a plateau. 2) Surfactant ﬁlm ﬂexibility: Generally, the particle size increases with the surfactant ﬁlm ﬂexibility. In reverse micelles, the intermicellar exchange process is governed by the dimer stability, which depends on the interdroplet attractive interaction, and by the surfactant ﬁlm ﬂexibility, which depends on the type of surfactant, cosurfactant, and oil used as well as on the droplet size. In principle, the ﬁlm ﬂexibility can be increased by several means: increasing oil molecular weight, increasing the cosurfactant concentration, approaching the instability boundary of microemulsions, etc. Particularly, in the water–AOT–iso-octane reverse micelles, the replacement of iso-octane by cyclohexane induces a considerable decrease in the ﬁlm ﬂexibility, resulting in a decrease in kex by one order of magnitude. Therefore, at a ﬁxed water content, the size of the Cu nanoparticles is smaller when the bulk solvent is cyclohexane instead of iso-octane (Fig. 3). However, the overall inﬂuence of kex on the particle size is dependent on the relative proportion of different growth mechanisms (e.g., autocatalysis and ripening), which is relevant to the ﬁlm ﬂexibility, and on the reactant nature and concentration. 3) Reactant concentration: It has been found that both absolute reactant concentrations and relative reactant concentrations show a considerable inﬂuence on the ﬁnal particle-size distribution. First, the particle size increases with reactant concentration for a ﬂexible surfactant ﬁlm, and the size distribution changes from unimodal to bimodal for a rigid ﬁlm. Second, the particle size decreases as the excess of one of the reactants increases, until a plateau is reached at high excesses. This phenomenon has been previously 6188 explained assuming that the reactant excess implies a faster nucleation, resulting in smaller particles. However, recent simulation results have shown that the reactant excess hardly inﬂuences the nucleation process but greatly inﬂuences the growth mechanism, i.e., the possibility of ripening growth increases with increasing reactant excess. It indicates that bulk nucleation and growth models might be invalid for the current dynamic compartmentalized media. Because many parameters have different inﬂuences on the ﬁnal particles size and PSD, and it is experimentally difﬁcult to ﬁx all the other parameters to study the effect of a given parameter; various computer simulations have been performed recently to gain more insight into the problem. In particular, the experimental results listed above correlate rather well with the Monte Carlo simulations carried out by the López– Quintela’s group, under the assumption that the synthesis is performed by mixing two identical microemulsions carrying the reactants, and that the chemical reaction occurring in the nanodroplets is much faster than the interdroplet material exchange.[24,25] The simulations took the following parameters into account: the ﬁlm ﬂexibility parameter f, the reactant exchange parameter k, the reactant concentration, the reactant excess ratio, the droplet size, the droplet volume fraction, and critical nucleus. In addition, the possibility of autocatalysis and the possibility of ripening were considered in the simulations. It should be pointed out that there have been various modeling efforts that used different combinations of assumptions regarding initial reactant distribution, intermicellar exchange rules, and the ﬁniteness of the nucleation/reaction process. There are recent efforts to organize all the possible scenarios into a coherent, uniﬁed framework by the use of a fairly general model. Developments in Processing Method It has been shown that reverse micelles can be used as ideal nanoreactors for the size-controlled synthesis of nanoparticles of a wide range of inorganic materials under rather mild conditions. However, the separation of the product nanoparticles from the microemulsion constituents, as well as the recovery and recycling of the organic solvents remains a challenge. Carbon dioxide is an attractive alternative to organic solvents because it is nontoxic, nonﬂammable, inexpensive, and environmentally benign, and it has low viscosity, high diffusion coefﬁcient, as well as tunable solvent properties (e.g., density, compressibility, diffusivities, and dielectric properties). Water-in-CO2 (w/c) microemulsions can be formed in supercritical (SC) CO2 by employing specially designed surfactants that contain a ‘‘CO2-philic’’ ﬂuorinated tail. As seen with many Synthesis of Inorganic Nanostructures in Reverse Micelles w/o microemulsions, certain w/c microemulsions exhibit a spherical droplet structure with a droplet size directly proportional to the W value. Similarly, reverse micelles in CO2 may serve as nanoreactors for the nanoparticle preparation. A major advantage with w/c microemulsions is the possibility of breaking the microemulsion by simply controlling the temperature and pressure of the system, leading to the direct separation of nanoparticles. In addition, the deposition of nanoparticles can be accomplished in situ on porous materials, by utilizing the low viscosity and high diffusivities of SC CO2 continuous phase. Moreover, the solvent properties of SC CO2 can be regulated by the adjusting temperature and pressure, thereby allowing for the regulation of the surfactant ﬁlm ﬂexibility, intermicellar exchange rate, and consequently, the ﬁnal particle size. Therefore, w/c microemulsions have shown tremendous implications for industrial applications, as an effective reaction media for the controlled synthesis of inorganic nanoparticles.[31–33] Johnston and coworkers ﬁrst reported the synthesis of CdS nanoparticles in w/c microemulsions formed by an ammonium carboxylate perﬂuoropolyether (PFPE–COONH4) surfactant and found that the particle size increased with the increasing W value. Subsequently, the PFPE–COONH4-based reverse micelles in SC CO2 were used for the preparation of TiO2[33,35] and Ag[32,36] nanoparticles. Similarly, Wai and coworkers prepared Ag,[37,38] Cu, Pd, Rh, and silver halide nanoparticles in w/c microemulsions formed by a surfactant mixture of AOT and perﬂuoropolyether-phosphate ether (PFPE–PO4), and they demonstrated that Pd and Rh nanoparticles produced in w/c microemulsions can be used as promising catalysts for the hydrogenation reaction in SC CO2.[39,40] In addition, they have used w/c microemulsions formed by a ﬂuorinated AOT to release the controlled synthesis of CdS and ZnS nanoparticles in SC CO2. After the preparation of nanoparticles in w/c microemulsions, their removal from the high-pressure reactor is essential. For the collection of nanoparticles prepared in w/c microemulsions, there are two basic methods reported so far, i.e., a reducing pressure method and a rapid expansion method. However, the collected nanoparticles are tightly aggregated in many cases, and so, new methods for better collection of nanoparticles with minimum agglomeration will be explored. For the traditional microemulsion technique, it is still a challenge to separate the produced nanoparticles and deposit them on a solid support. One common procedure to prepare supported catalysts is to add a solvent such as tetrahydrofuran (THF), which dissolves the surfactant and is miscible with both oil and water, to the microemulsion, thereby resulting in the destabilization of the microemulsion and sedimentation of the particles. Another commonly used method for the Synthesis of Inorganic Nanostructures in Reverse Micelles processing of nanoparticles synthesized in reverse micelles, such as CdS nanoparticles, is direct recovery and immobilization by using thiol-modiﬁed supports via chemical bonding. Yet, another deposition method involves the centrifugation of Pt nanoparticles followed by redispersion in water by a stabilizing surfactant, and the addition of alumina support. A new method has been developed for the deposition of a large amount of metal particles onto suitable supports, which is of great interest for industrial application. This method consists of spraying the microemulsion solution containing Au nanoparticles into an air/acetylene ﬂame, which allows for the preservation of the original structures of the nanoparticles. More recently, w/o microemulsions have been used for the preparation of oxide nanoparticles and their noncovalent attachment onto carbon nanotubes, which is important for preserving the mechanical and electrical properties of carbon nanotubes. It is worth noting that compressed CO2 has been successfully used as an antisolvent to recover ZnS and Ag nanoparticles from AOT-based reverse micelles, leading to the precipitation of well-dispersed nanoparticles with the surfactant remaining in the solution. Recently, Au nanoparticles were prepared in a CO2-induced w/o microemulsion, and the recovery of Au particles was easily accomplished by the venting of CO2, while the poly(ethylene oxide)-poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO– PEO) block copolymer surfactant remained in the organic phase. A novel method was recently reported for the recovery of Ag nanoparticles from reverse micelles of PEO–PPO–PEO surfactants in xylene by simply breaking the reverse micelles with an increase in temperature. Another development in the microemulsion technique is the microemulsion-mediated hydrothermal/ solvothermal synthesis that involves hydrothermal/ solvothermal treatment of microemulsions at temperatures higher than 100 C. Gan et al.[51,52] ﬁrst prepared Mn-, Cu-, and Eu-doped ZnS nanocrystallites in nonionic w/o microemulsions under hydrothermal treatment. It was shown that the hydrothermal treatment considerably enhanced the photoluminescence of the doped ZnS nanoparticles, which was attributed to the increased crystallinity, surface passivation of nanocrystals, and forced migration of the impurity species into the host lattice of ZnS. On the other hand, nanoparticles of both anatase and rutile types of TiO2 were prepared by hydrothermal treatment of nonionic microemulsions at 120 C. It had some advantages over the room-temperature microemulsion technique, in which amorphous TiO2 particles are usually yielded, and additional calcinations at much higher temperatures are required to produce crystalline TiO2 particles. Subsequent investigation suggested that the reaction 6189 temperature (120 C) was much above the existence region of the starting microemulsion; hence, the microemulsion could not work as a template during the hydrothermal treatment, although it could work as an initial nanoreactor during the mixing step. Recently, nanosized CdS-sensitized TiO2 crystalline photocatalyst, which exhibited high efﬁciency for the decomposition of methylene blue, was successfully prepared by a microemulsion-mediated solvothermal method. Furthermore, it has been demonstrated that under speciﬁc conditions, microemulsion-mediated hydrothermal synthesis can favor the one-dimensional (1-D) growth of inorganic nanocrystals in microemulsion media. For example, under hydrothermal treatment, CdS and BaF2 nanorods were fabricated in microemulsions formed by the cationic surfactant cetyltrimethylammonium bromide (CTAB), whereas SnO2 nanorods were produced in microemulsions formed by the anionic surfactant sodium dodecyl sulfate (SDS). SYNTHESIS OF SPHERICAL NANOPARTICLES It is generally accepted that reverse micelles are good nanoreactors for obtaining spherical nanoparticles. Until now, the overwhelming majority of literature concerning microemulsion-based synthesis reports the preparation of spherical nanoparticles of a variety of inorganic materials, such as metals, chalcogenides, oxides, and other precipitates. Metal and Metal Chalcogenide Nanoparticles Many of the earliest microemulsion-based nanoparticle syntheses reported in the literature involved the preparation of metal nanoparticles and metal chalcogenide semiconductor nanoparticles. One reason may be that metal and semiconductor nanoparticles, often called quantum dots (QDs) owing to the quantum size effects, exhibit novel optical, electronic, magnetic, and chemical properties, and great potential for promising applications. Another reason seems to be that metals and metal chalcogenides can be easily produced in reverse micelles by the simple reduction and coprecipitation reactions occurring in aqueous solution, respectively. Since the synthesis of metal nanoparticles in microemulsions was ﬁrst reported by Boutonnet et al. in the early 1980s, the microemulsion technique has been successfully used for the preparation of a number of metal nanoparticles, including Pt, Rh, Pd, Ir, Au, Ag, Cu, Fe, Co, Ni, and Bi nanoparticles as well as their alloy nanoparticles, mainly because of the potential application of the products as catalysts.[1,26,27] For the reduction reaction occurring in reverse micelles, the reducing agent must be stable in an aqueous 6190 environment and not react with the other components of the reverse micelle system. Speciﬁcally, both CTAB and AOT are stable against hydrogen, but stronger reducing agents such as borohydride ions can be problematic. As bubbled H2 gas results in slow reduction for many metals, particularly at room temperature, NaBH4 and hydrazine are most commonly employed in metal nanoparticle synthesis. The reduction reaction for most metals is usually straightforward, although metal borides (e.g., Ni2B and Co2B) may result from the reduction of metal ions with NaBH4 sometimes. The preparation of metal nanoparticles is usually implemented by mixing two microemulsions with one containing metal ions and the other containing the reducing agent. Similarly, nanoparticles of substitutional alloys can be prepared if different metal ions are dissolved in one reverse micelle solution prior to mixing, provided the two metals are miscible in the metallic state. Originally, stable dispersions of monodisperse Pt nanoparticles (3 nm) were produced by reducing H2PtCl6 with hydrazine in a w/o microemulsion consisting of penta(ethylene oxide) dodecyl ether (C12E5)-hexanewater. By a similar method, Pt nanoparticles also were prepared in a microemulsion formed by C12E4, and the evolution of their optical properties with reaction time was related to the mechanism of the formation and stabilization of Pt particles. Recently, in situ x-ray absorption spectroscopy (XAS) was successfully applied to explore the formation of Pt nanoparticles at the early stages within AOT reverse micelles, which conﬁrmed the Pt4þ ! Pt2þ ! Pt0 reduction sequence and provided a detailed insight into the mechanism of the nucleation and growth of Pt particles. Similarly, Ag nanoparticles were synthesized by reducing AgNO3 with NaBH4 in reverse micelles, whereas bimetallic Au/Pt and Pt/Ru alloy nanoparticles were produced by reducing appropriate mixed metal ions with N2H4. Notably, Bi nanocrystallites were fabricated by reducing BiOClO4 with NaBH4 in AOT reverse micelles under the Ar protection. In addition to the normal metals salts, functional surfactants, such as Cu(AOT)2 and Ag(AOT), were introduced as metal precursors for the synthesis of Cu and Ag nanoparticles in AOT reverse micelles, to avoid any potential interference from the counterions. It has been shown that water content, bulk solvent, droplet concentration, and the addition of cosurfactants largely inﬂuence the Cu nanoparticle formation from Cu(AOT)2 in reverse micelles.[68,69] However, in the case of Co nanoparticle synthesis from the Co(AOT)2 reverse micelles, the amount of the reducing agent NaBH4 was one of the key parameters in controlling the nanoparticle-size distribution, and Co nanoparticles with high monodispersity could be obtained when there was a high volume of reducing agent and the reverse micelles were destroyed. Synthesis of Inorganic Nanostructures in Reverse Micelles As a typical semiconductor material exhibiting an absorption spectrum in the visible range, CdS is the ﬁrst chalcogenide semiconductor prepared in reverse micelles;[71,72] subsequently, nanoparticles of a variety of chalcogenide semiconductors including metal sulﬁdes, selenides, and tellurides have been reported. For the preparation of CdS nanoparticles, Cd2þ ions can be supplied from normal cadmium salts or functional surfactants like Cd(AOT)2 whereas S2 ions can be supplied from Na2S or H2S. In general, it is desirable to deoxygenate by bubbling with N2 through reverse micelle solutions containing aqueous reactants to avoid colloidal sulfur precipitation. In most cases, AOT reverse micelles are employed for the CdS nanoparticle preparation; however, it was observed that in a quaternary CTAB microemulsion, the presence of cosurfactant (n-pentanol) allowed a facile modulation of the CdS particle size, size distribution, and crystallinity. Recently, an extreme sensitivity of the CdS nanoparticle formation in reverse micelles to the preparation procedure was observed, e.g., a seemly trivial change in the solution volume resulted in substantial changes in the crystal structure and optical absorption of the CdS nanoparticles. In a similar way, nanoparticles of other metal sulﬁdes (e.g., ZnS, PbS, and Ag2S) and mixed sulﬁdes (e.g., Cd1yZnyS and Cd1yMnyS) can also be synthesized. It is interesting to note that ultra-small ZnS nanoparticles (several angstroms) with a high nanoparticle concentration were synthesized recently by a novel solid–solid reaction between Na2S and ZnSO4 conﬁned in anhydrous AOT reverse micelles. Compared with the synthesis of sulﬁde nanoparticles, the synthesis of metal selenide/telluride nanoparticles generally requires more rigorous preparation conditions because the selenium/tellurium precursors are usually highly oxidizable or water-sensitive. For example, CdSe nanoparticles were prepared by adding a solution of [bis(trimethylsilyl) selenium (Me3Si)2Se] in heptane to a degassed, AOT reverse micelle solution containing Cd2þ ions, whereas MoSe2 and WSe2 nanoparticles were synthesized by adding H2Se to a degassed, anhydrous reverse micelle solution containing MoCl4 or WCl4. By mixing the two degassed AOT reverse micelles containing, respectively, Na2Te and Cd(AOT)2, CdTe nanoparticles were produced; on the other hand, Bi2Te3 nanoparticles were fabricated by adding a hexane solution of (Me3Si)2Te under N2 to a AOT reverse micelles containing BiOClO4. In recent years, the self-organization of colloidal nanoparticles into ordered nanoparticle self-assemblies, such as two-dimensional (2-D) and three-dimensional (3-D) nanoparticle superlattices, has attracted an intense interest because self-assembled nanoparticles combine the unique properties of individual nanoparticles with collective properties owing to interactions between the particles. The microemulsion technique Synthesis of Inorganic Nanostructures in Reverse Micelles has shown great potential for obtaining 2-D and 3-D nanoparticle superlattices as a controlled PSD is essential for the ordered self-assembly of nanoparticles, and reverse micelles provide effective nanoreactors for monodisperse nanoparticles. In particular, Pileni has prepared a variety of 2-D and 3-D superlattices from spherical metal and semiconductor nanocrystals produced in AOT reverse micelles. In all the cases, a capping agent like thiododecane is introduced to the reverse micelle solution containing the nanoparticles to passivate the particle surface and extract the particles from reverse micelles. A size-selected precipitation process is usually employed to further decrease the polydispersity, and hence favor the self-organization of the nanoparticles into well-deﬁned superlattices. In addition, the deposition procedure, nanoparticle concentration, substrate, and alkyl chain length of the capping agent play key roles in the formation of nanoparticle superlattices. The ﬁrst 2-D and 3-D superlattices were observed with Ag2S nanoparticles extracted from AOT reverse micelles by thiododecane addition. Fig. 4 shows typical TEM images of the 3-D superlattices with a fcc structure, which consisted of highly monodisperse Ag2S nanoparticles (14% size distribution) of different sizes. Formation of 2-D and 3-D superlattices of Ag metal nanoparticles has been achieved by the same technique. For the formation of large-scale 3-D superlattices of Co nanoparticles, the nanoparticles with a diameter of 8 nm were coated with lauric acid and deposited at an applied magnetic ﬁeld. Meanwhile, Martin et al.[85,86] have investigated the formation of 2-D and 3-D superlattices of Au and Pt nanoparticles synthesized in anhydrous, nonionic reverse micelles and capped with alkanethiols, and found that the gap between the particles in the coherent domains increased with the increasing thiol chain length. Silica and Metal Oxide Nanoparticles Metal oxides including silica represent a large class of inorganic materials that ﬁnd many scientiﬁc and technological applications. Nanoparticles of a wide variety of metal oxides, such as silica, titania, zirconia, iron oxide, and complex oxides, have been synthesized in reverse micelles through two basic reaction routes, i.e., precipitation of metal salts and hydrolysis of metal alkoxides.[18,21] In the ﬁrst route, precipitation of hydroxides is typically induced by adding aqueous alkaline solution (e.g., NH4OH) or a microemulsion containing the alkaline precipitating agent to a microemulsion containing metal ions, which is usually followed by centrifugation and heating to remove water and/or improve crystallinity. This technique can be extended to complex oxide systems by the simultaneous coprecipitation of several metal ions 6191 Fig. 4 Transmission electron microscopy (TEM) images of an island of Ag2S in a f100g plane of a close-packed facecentered cubic (fcc) structure made of 3 nm (A, B); 4 nm (C, D); and 5.8 nm (E, F) particles. (From Ref..) in water nanodroplets. Alternatively, precursor nanoparticles can be prepared ﬁrst by coprecipitation of several metal ions in water nanodroplets with a certain precipitating agent (e.g., oxalic acid), which are decomposed to the desired oxide phase by subsequent calcinations, although this will inevitably result in some degree of agglomeration. In the second route, an oilsoluble metal alkoxide, M(OR)n, or an anhydrous reverse micelle solution containing the alkoxide in the oil phase is added to a water-containing reverse micelle solution, which results in the hydrolysis and condensation of the alkoxide producing nanoparticles of metal hydroxides and oxides. Similarly, complex metal oxides can be synthesized by the simultaneous hydrolysis of several different metal alkoxides. In most cases, the products obtained by alkoxide hydrolysis in reverse micelles are amorphous or poorly crystallized, and subsequent calcinations are necessary for obtaining 6192 the desired crystalline nanoparticles of metal oxides except for silica. Silica nanoparticles can be prepared in microemulsions by the acidiﬁcation of aqueous silicate solutions; however, the available reports on microemulsionmediated synthesis of silica nanoparticles are based almost exclusively on the base-catalyzed alkoxide hydrolysis route. Osseo-Asare and Arriagada reported on the preparation of monodisperse silica nanoparticles in the size range of 50–70 nm, with standard deviation below 8.5%, by the ammonia-catalyzed hydrolysis of tetraethyl orthosilicate (TEOS) in a nonionic w/o microemulsion formed by NP-5. Their later work showed that the particle size went through a minimum as W increased at a relatively high ammonia concentration, which was attributed to an increased surfactant aggregation number, free water concentration, and intermicellar exchange rate, kex. As the hydrolysis reaction of TEOS is rather slow and the ﬁnal particle size is signiﬁcantly larger than the corresponding water droplet size, a reverse micellar pseudophase model has been proposed to interpret the growth kinetics of silica nanoparticles in the microemulsions. The effects of surfactant molecular structure and type of oil on the silica particle size were also investigated by Chang and Fogler. The obtained results suggested that the particle size decreased with decreasing surfactant-ﬁlm ﬂexibility, supporting the general effect of the ﬁlm ﬂexibility on the particle size. Recently, highly ﬂuorescent, photostable, and biocompatible organic-dye-doped silica nanoparticles have been successfully prepared by acid-catalyzed TEOS hydrolysis in nonionic reverse micelles. TiO2 nanoparticles, which exhibited aggregates of anatase or rutile nanoparticles ranging in size typically from 20 to 50 nm, were synthesized by the precipitation of TiCl4 with ammonium hydroxide in water droplets of a nonionic microemulsion and subsequent calcinations at different temperatures. Actually, the titanium alkoxide hydrolysis method has been more frequently used for the microemulsion-based synthesis of TiO2 nanoparticles; however, a post-treatment of hightemperature calcination is usually required to increase the crystallinity of the nanoparticles. To obtain crystalline TiO2 nanoparticles without the post-treatment of calcination, hydrothermal treatment of microemulsions has been applied.[53,54] Alternatively, a novel hot-ﬂuid annealing technique was developed to synthesize stable dispersion of monodisperse, crystalline TiO2 nanoparticles, which involved an in situ annealing process in the presence of reverse micelles. Similarly, ZrO2 nanoparticles can be produced in reverse micelles by the hydrolysis of zirconium alkoxide instead of titanium alkoxide. For example, sulfated titania nanoparticles, which showed a high catalytic activity in n-butane isomerization, were prepared by the addition of zirconium Synthesis of Inorganic Nanostructures in Reverse Micelles butoxide to a nonionic w/o microemulsion containing H2SO4, and subsequent calcination at 873 K. As another example, cerium oxide nanoparticles were fabricated by the precipitation of Ce(NO3) 3 with ammonia in cationic CTAB reverse micelles, followed by calcination. Extension of the microemulsion technique to complex oxides has been explored for a wide variety of materials. For example, crystalline barium hexaaluminate nanoparticles, which exhibited an ultrahigh surface area, superb thermal stability, and excellent catalytic performance in methane combustion, have been successfully synthesized through the hydrolysis and condensation of barium and aluminum alkoxides in nonionic w/o microemulsions and subsequent calcination leading to crystallization.[95,96] It was found that higher water-to-alkoxide ratio generally resulted in smaller particle size and increased surface area after calcination. The method of metal salt precipitation in reverse micelles has proven to be particularly useful in preparing mixed metal ferrites; speciﬁcally, Zhang and coworkers[97–100] have prepared nanoparticles of a series of spinel ferrites including MgFe2O4, MnFe2O4, CoCrFeO4, and CoCrxFe2xO4 in anionic reverse micelles formed by dodecylbenzenesulfonate and toluene. Nanoparticles of various mixed oxides, such as Fe–Al oxide, Ce–Te oxide, and In–Sn oxide, can likewise be prepared by coprecipitation of corresponding mixed metal ions in reverse micelles. Finally, nanoparticles of many complex oxides including BaFe12O19, YBa2Cu3O7x superconductor, and perovskite-type mixed oxides have been obtained by calcination of oxalate precursor nanoparticles produced through coprecipitation of corresponding mixed metal ions in w/o microemulsions.[16,21] Other Inorganic Nanoparticles In a similar way, microemulsion-based synthesis of nanoparticles of many other inorganic nanoparticles can be realized, especially by precipitation reaction occurring in water droplets. In particular, the synthesis of silver halide nanoparticles in reverse micelles has attracted much attention because this simple binary halide system provides a useful model system for the in-depth investigation of the microemulsion technique while silver halide nanoparticles are important for photographic and electronic industries. Typically, silver halide, (AgX) (X ¼ Br, Cl), nanoparticles are readily prepared by mixing two microemulsions containing solubilized Agþ ions and X ions. Recently, the formation mechanism of uniform AgCl nanoparticles in an NP-6 reverse micelle system has been investigated through a new approach that uses a double jet technique, in which microemulsions containing AgNO3 and Synthesis of Inorganic Nanostructures in Reverse Micelles KCl were continuously introduced at the same time. It has been revealed that the AgCl nanoparticles actually grow through Ostwald ripening, and not by the coagulation process. Recently, AgCl nanoparticles were prepared by the direct precipitation of silver ions with the surfactant counterions in the water droplets of microemulsions formed by dioctyldimethyl ammonium chloride. Furthermore, very small, lightfast AgCl and AgBr nanoparticles have been synthesized through a solid–solid reaction performed by mixing two water-free AOT reverse micelle dispersions containing AgNO3 and KX. In addition to silver halide, many other inorganic precipitate systems have been exploited in the microemulsion-based nanoparticle synthesis. To illustrate, Moulik et al. prepared Cu2[Fe(CN)6] and PbCrO4 nanoparticles by precipitation reaction in AOT reverse micelles, while they synthesized H2WO4 nanoparticles by the reaction of Na2WO4 with HCl in nonionic w/o microemulsions. They observed that the obtained nanoparticles were spherical on the whole. Sometimes both the starting materials and the surfactants must be carefully chosen; for example, NH4MnF3 nanoparticles were produced by the reaction between NH4F and manganese acetate in reverse micelles formed by an ammoniumsubstituted AOT (NH4-AOT), where the possible contamination of the desired product by impurity ions  such as Naþ and NO 3 ions was avoided. SYNTHESIS AND ASSEMBLY OF NONSPHERICAL NANOSTRUCTURES The morphological control of colloidal nanoparticles is a real challenge, because it is a complex process requiring a fundamental understanding of the solid state chemistry, solution chemistry, and interfacial adsorption and reaction that are involved in crystal growth or inorganic mineralization. Recently, there 6193 have been great advances in the controlled synthesis of nonspherical nanoparticles in reverse micelles although more data are still needed to ascertain the general principles that determine the particle shape. Another intriguing issue accompanying the particle-shape control is the hierarchical assembly of nonspherical nanostructures into ordered superstructures or complex architectures, which is relevant to the bottom–up approaches toward future nanodevices and offers opportunities to explore the novel collective properties. One-Dimensional Nanostructures One-dimensional nanostructures such as nanowires, nanorods, nanobelts, and nanotubes have stimulated intensive interest owing to their unique applications in mesoscopic physics and fabrication of nanodevices. The majority of the literature concerning 1-D nanostructure synthesis in reverse micelles deals with nanowires and nanorods. Essentially, there are two mechanisms proposed for the microemulsion-mediated synthesis of inorganic nanowires/nanorods: templatedirected growth and oriented aggregation (Fig. 5). In the template-directed growth mechanism, elongated water droplets or interconnected water channels play the role of template to induce the formation of elongated nuclei, which ﬁnally grow into nanorods with dimensions considerably larger than the templates. In the oriented aggregation mechanism, precipitation within spherical water droplets initially results in the formation of surfactant-encapsulated primary nanoparticles, which subsequently undergo oriented aggregation involving linear attachment, and coalescence owing to speciﬁc interactions of inorganic crystals with surfactants and/or certain additives, thereby leading to the growth of single-crystalline nanowires with high aspect ratios. Besides, the anisotropic growth habit determined by the inherent crystallographic structure of a solid material may partially contribute to the Fig. 5 Schematic illustration of proposed mechanisms for reverse micelle-based synthesis of inorganic nanowires/nanorods. 6194 growth of 1-D nanostructures in reverse micelle solutions for certain synthesis systems. Pileni’s work on preparation of copper nanoparticles from Cu(AOT)2 microemulsions with varying internal structures provides a rather persuasive support for the template-directed growth of 1-D nanostructures in microemulsions. It was shown that synthesis in a microemulsion consisting of interconnected water cylinders yielded cylindrical copper nanorods (6.5 nm in width and 19.8 nm in length) together with some spherical copper nanoparticles, while synthesis in a microemulsion consisting of spherical water droplets predominantly produced spherical copper nanoparticles (Fig. 6). The obtained results demonstrate that the shape of the water phase of microemulsions at least partially controls the shape of the produced nanoparticles, i.e., a template-directed growth exists. Another example of template-directed growth is the synthesis of CdS nanorods in reverse micelles formed by mixed surfactants of the anionic AOT and the zwitterionic phosphatidylcholine (lecithin). It is known that addition of lecithin to AOT reverse micelles induces a structural change from spherical reverse micelles to worm-like, ellipsoidal reverse micelles. Synthesis of CdS nanoparticles in the ellipsoidal reverse micelles results in the formation of CdS nanorods (4.1 nm in width and 50–150 nm in length) with a hexagonal structure of wurtzite (Fig. 7). The templating effect of elongated water droplets in the 1-D nanocrystal growth has been recently conﬁrmed by the ultrasonically induced formation of silver nanorods and nanoﬁbers in AOT reverse micelles. Results from small-angle x-ray scattering (SAXS) revealed that by ultrasonication, the reverse micelles Synthesis of Inorganic Nanostructures in Reverse Micelles changed from a spherical to an ellipsoidal structure, indicating that the reverse micelles played the role of a template in the formation of the rod- and wireshaped Ag particles. However, further investigation on the preparation of copper nanoparticles from Cu(AOT)2 microemulsions with addition of salts suggests that the nanocrystal growth markedly depends on the salt used, even if the reverse micelle structure does not change with various salt additions. For example, chloride ions enable the growth of Cu nanorods with an aspect ratio controlled by the chloride concentration, whereas bromide ions favor the formation of Cu nanocubes. From these data, it has been concluded that the 1-D nanocrystal growth is more related to selective adsorption of ions during the crystal growth than to the nature of the templates. In other words, the reverse micelle template is not the key parameter in the anisotropic nanocrystal growth, while additives and/or impurities inside the template play the major role in the 1-D nanocrystal growth. Nevertheless, a nice work on the synthesis of BaSO4 nanoparticles in cylindrical reverse micelles formed by polymerizable surfactants suggested that cylindrical nanoparticles were formed in the partially polymerized reverse micelles while spherical nanoparticles were formed in the unpolymerized reverse micelles. This result indicates that the templating effect of elongated nanodroplets for anisotropic nanoparticles could be considerably enhanced by employing partially polymerized or more rigid surfactant ﬁlms. On the other hand, there are many examples of 1-D nanostructure formation in spherical reverse micelles where there is no obvious templating effect. In this case, Fig. 6 Transmission electron microscopy images of copper nanoparticles synthesized in Cu(AOT)2 microemulsions of different shapes: (A) interconnected water cylinders and (B) spherical water droplets. (From Ref..) Synthesis of Inorganic Nanostructures in Reverse Micelles 6195 Fig. 7 (A) Transmission electron microscopy and (B) high-resolution transmission electron microscopy (HRTEM) images of CdS nanorods synthesized in ellipsoidal reverse micelles formed by AOT and lecithin. Inset shows the corresponding electron diffraction pattern. (From Ref..) the oriented aggregation mechanism is frequently adopted to explain the formation of nanowires with high aspect ratios in reverse micelle solutions. Qi et al. ﬁrst reported the synthesis of single-crystalline, c-axisoriented BaCO3 nanowires with aspect ratios as large as 10,000 in nonionic reverse micelles of C12E4 in cyclohexane. The obtained nanowires were 10–30 nm in diameter and up to 100 mm in length, which formed a rather regular 2-D nanowire array in a large area upon deposition on a Formvar-covered copper grid (Fig. 8). According to time-dependent TEM observations of the crystal growth process, a directional (oriented) aggregation mechanism was initially proposed for the nanowire formation in reverse micelles. Similarly, nanowires and nanorods of other water-insoluble salts including BaSO4, BaCrO4, BaWO4, CaSO4, AgI, and K3[PMo12O40] nH2O were prepared by simple precipitation reactions in reverse micelles around room temperature. Interestingly, remarkable helical BaSO4 nanowires together with bundles of BaSO4 nanoﬁbers with coiled terminus can be obtained in AOT reverse micelles, sometimes, as shown in Fig. 9. Recently, coaligned bundles of vaterite nanowires were prepared by water-induced crystallization of alkylbenzenesulfonate-coated amorphous calcium carbonate (ACC) nanoparticles in AOT reverse micelle solutions, indicating that hybrid 1-D nanostructures can be assembled by oriented aggregation and transformation of amorphous primary nanoparticles at the mesoscopic level. It is worth noting that microemulsion-mediated hydrothermal procedure has been shown to be powerful in the synthesis of a variety of 1-D nanostructures. Speciﬁcally, single-crystalline nanowires/nanorods of CdS, BaF2, SnO2, and Ca10(PO4)6(OH)2 have been successfully synthesized in reverse micelles under hydrothermal conditions. In these cases, an oriented aggregation mechanism is usually adopted because reverse micelles may be destroyed under hydrothermal conditions, and they just play the role of a template for the formation of primary nanoparticles during the mixing step, which undergo subsequent oriented aggregation to form the ﬁnal 1-D nanostructures. Notably, crystalline oxide nanorods can be prepared directly by alkoxide hydrolysis in microemulsions at room temperature. To illustrate, shuttle-like TiO2 nanoparticles of the tetragonal rutile structure were prepared by the HCl-catalyzed hydrolysis of titanium tetrabutoxide in NP-5 reverse micelles, while rod-like ZnO nanoparticles of the hexagonal wurtzite structure were produced by the ammonia-catalyzed hydrolysis of zinc dibutoxide in NP-6 reverse micelles. Moreover, g-V2O5 nanorods of the orthorhombic structure were obtained by direct hydrolysis of a vanadium alkoxide in AOT reverse micelles. In these cases, the mechanism for the nanorod formation is poorly understood; however, a continuous reaction and crystallization process seems more reasonable than an oriented aggregation process. The 1-D growth of nanoparticles may be largely attributed to the inherent crystal growth habit and/or speciﬁc adsorption of surfactants or ions. In addition, there are several reports on the crystalline nanorod synthesis involving the room-temperature synthesis of amorphous precursor nanoparticles in reverse micelles 6196 Synthesis of Inorganic Nanostructures in Reverse Micelles Fig. 9 TEM images of BaSO4 nanowires synthesized in AOT reverse micelles showing (A) closely packed bundle of nanowires and (B) helical nanowires. (From Ref..) Fig. 8 (A) Low- and (B) high-magniﬁcation TEM images of BaCO3 nanowires. (From Ref..) Hierarchical Assemblies of 1-D Nanostructures followed by crystallization through heat treatment. In particular, nanorods of rutile SnO2 and barium hexa-aluminate were prepared by the calcination of the corresponding precursor powders whereas those of hematite Fe2O3 were fabricated by reﬂux Fe2O3 gel powders in tetralin. Recently, the microemulsion-based procedure has been successfully applied for the synthesis of inorganic nanotubes, which represent another important type of 1-D nanostructures. In particular, silica nanotubes, which have lengths larger than 2 mm, diameters about 150 nm, and a wall thickness of 27 3 nm (Fig. 10), were fabricated by a reverse-microemulsion-mediated sol–gel (RMSG) technique. It was proposed that the addition of FeCl3 favored the formation of cylindrical AOT reverse micelles, which acted as templates for the hydrolysis and condensation of TEOS around the surface of the reverse micelles. After washing with ethanol, AOT was removed and silica nanotubes were obtained. The diameter of the silica nanotubes was tunable, which can be done by using different apolar solvents. More recently, unique titanium phosphate nanotubes were synthesized via microemulsion-based solvothermal synthesis in reverse micelles of H3PO4 acidiﬁed trioctylamine in benzene; however, the detailed formation process remained unclear. Many recent efforts have been focused on the integration of 1-D nanoscale building blocks into ordered superstructures or complex functional architectures, which is a crucial step toward the realization of functional nanosystems. It has been demonstrated that ordered nanorod superstructures can form by the self-assembly of uniform nanorods preformed in reverse micelles. Furthermore, reverse micelles can be used as nanostructured reaction media for the direct growth of hierarchical architectures assembled by primary 1-D nanostructures. A striking example is the formation of linear chains of nanorods by the coupled synthesis and self-assembly of BaCrO4 nanorods in AOT reverse micelles. As shown in Fig. 11, the chain-like arrays consist of uniform, prismatic nanoparticles (16 nm in length and 6 nm in width) that preferentially align side-by-side and are separated by a regular spacing of 2 nm, consistent with the thickness of an interdigitated layer of surfactant molecules. Furthermore, rectangular superlattice of nanorods can form by 2-D assembly of uniformly sized BaCrO4 nanoprisms. Subsequently, a novel Langmuir–Blodgett technique was developed to assemble uniform BaCrO4 nanorods, which were prepared similarly in AOT reverse micelles, at the Synthesis of Inorganic Nanostructures in Reverse Micelles 6197 Fig. 10 (A) Scanning electron microscopy; (B) TEM images of silica nanotubes synthesized in AOT reverse micelles; and (C) schematic representation of the nanotube formation. (From Ref..) water–air interface. The TEM images presented in Fig. 12 clearly show the pressure-induced isotropic-2-D, smectic-3-D nematic phase transitions as well as common textual defects in the 3-D nematics. As another example, remarkable doughnut-like aragonite particles showing sponge-like networks of branch-like nanorods were fabricated by mesoscale self-assembly and transformation of ACC precursor nanoparticles in AOT reverse micelles. Recently, the so-called catanionic reverse micelles formed by mixed cationic–anionic surfactants have turned out to be promising nanostructured media for the controlled synthesis and hierarchical assembly of 1-D nanostructures. Catanionic reverse micelles formed by an equimolar mixture of undecylic acid and decylamine were ﬁrst employed for the synthesis of c-axis-oriented, single-crystalline BaWO4 nanowires with diameters as small as 3.5 nm and lengths up to more than 50 mm. By addition of a double-hydrophilic block copolymer, poly(ethylene glycol)-block-poly (methacrylic acid) (PEG-b-PMAA), to the reverse micelles, novel penniform architectures assembled from BaWO4 nanowires were successfully synthesized. Fig. 13 presents typical TEM images of the obtained penniform BaWO4 superstructures at different magniﬁcations, which suggests that the obtained penniform BaWO4 nanostructures consist of c-axis-oriented, singlecrystalline BaWO4 nanowires about 3.5 nm in diameter and a nearly a-axis-oriented, crystalline BaWO4 shaft with widths 200–400 nm. It was revealed that the presence of PEG-b-PMAA favored the formation of BaWO4 shafts, and the formation of the penniform superstructures involved an initial formation of rod-like shaft followed by the gradual growth of nanowires on both sides of the shafts. For the unique catanionic reverse micelle system, the molar ratio (r) between the mixed anionic and cationic surfactants plays a key role in the synthesis 6198 Synthesis of Inorganic Nanostructures in Reverse Micelles Fig. 11 The TEM image of ordered chains of prismatic BaCrO4 nanorods synthesized in AOT reverse micelles. (From Ref..) and assembly of 1-D nanostructures. By simply changing the mixing ratio, r, a variety of novel BaCrO4 nanostructures, such as nanowires, nanobelts, and treelike superstructures consisting of nanobelts, have been synthesized in the catanionic reverse micelles formed by undecylic acid, decylamine, and decane. For example, bundles of BaCrO4 nanowires were obtained at r ¼ 1, whereas unusual bundles of BaCrO4 nanobelts were obtained at r ¼ 1.4. If the mixing ratio was further increased, i.e., r ¼ 1.7, hierarchical, treelike BaCrO4 superstructures (10 mm in length), which consisted of branches with numerous, nearly parallel nanoleaﬂets (100 nm in length, 20 nm in width, and 4 nm in thickness) grown on two opposite sides, were produced (Fig. 14). A two-stage growth mechanism involving preferential adsorption of undecylic acid on speciﬁc crystal surfaces has been proposed for the formation of the tree-like superstructures. Nanocubes, Nanoplates, and Their Assemblies In addition to 1-D nanostructures, nonspherical nanoparticles with other well-deﬁned morphologies, such as nanocubes and nanoplates, are also of interest, as the particle shape may have pronounced effects on the particle property and function. Therefore, the controlled synthesis of these nanoparticles in reverse micelles has attracted considerable attention, and some interesting results have been obtained. In principle, for crystalline solids with a cubic crystal structure, it is possible to obtain their nanoparticles with a cubic morphology in reverse micelles under suitable conditions. Monodisperse nanocubes of hydrophobic Prussian blue [ferric hexacyanoferrate(II)], Fig. 12 Transmission electron microscopy images of BaCrO4 nanorod assemblies at the water–air interface at different compression stages: (A) isotropic distribution at low pressure; (B) monolayer with smectic arrangement at medium pressure; (C) nanorod multilayer with nematic conﬁguration at high pressure; and (D) textual defects within the nanorod multilayer. Insets in panels (B) and (D) show the Fourier transformation of the corresponding image and the nanorod director orientation in the vicinity of disclinations, respectively. (From Ref..) a typical molecular magnet with a cubic structure, were ﬁrst prepared by the photoreduction of [Fe(C2O4)3]3 in the presence of [Fe(CN)6]3 ions in AOT reverse micelles. Moreover, the obtained nanocubes can self-assemble into highly ordered 2-D square superlattices. As shown in Fig. 15, after ageing for two days in the microemulsion reaction media, well-ordered superlattice structures were assembled from nanocubes with a mean length of 16 nm, while after two weeks of aging, the nanocubes were slightly larger (mean 18 nm) and the domain length of the superlattices was reduced. Subsequently, this method was adapted to a wide range of coordination polymer materials, by mixing two AOT microemulsions instead of the photoreduction process; for example, molecule-based magnetic nanocubes of cobalt hexacyanoferrate and chromium hexacyanochromate were routinely prepared. Similarly, uniform antiferromagnetic KMnF3 nanocubes were synthesized Synthesis of Inorganic Nanostructures in Reverse Micelles 6199 Fig. 14 Transmission electron microscopy images of treelike BaCrO4 superstructures formed in catanionic reverse micelles at r ¼ 1.7. Inset shows the corresponding electron diffraction pattern. (From Ref..) Fig. 13 (A)–(D) TEM; and (E) HRTEM images of penniform BaWO4 superstructures formed in catanionic reverse micelles with addition of PEG-b-PMAA. Insets show the corresponding electron diffraction patterns. (From Ref..) in CTAB reverse microemulsions and their selfassembled 2-D superlattices were observed. It is noted that polydisperse metallic copper nanocubes were obtained in Cu(AOT)2 reverse micelles in the presence of NaBr, indicating that the selective adsorption of Br ions on the copper f100g faces played a key role in the Cu nanocube formation. On bubbling H2S into Cd(AOT)2 reverse micelles, equilateral, triangular CdS nanoplates with a high crystallinity were produced. As shown in Fig. 16, most of the particles are characterized by an angular shape with an average size of 10 nm, and the triangular nanoplates are crystallized in a hexagonal wurtzite structure with a top face of the (001) plane. The thickness of the triangular nanoplates was estimated to be 3–6 nm from the related absorption spectroscopy study. In this case, there is no direct correlation between the particle shape and the shape of the parent water droplets, hence excluding the possibility of the templating effect. More recently, single-crystalline silver nanodisks in equilibrium with silver spheres were produced by the reduction of Ag(AOT) with N2H4 in AOT reverse micelles. In this case, the nanodisk size was tuned by the relative amount of reducing agent whereas their aspect ratios remained in the same order of magnitude. Interestingly, a recent work reported the spontaneous formation of novel, complex structures of calcite CaCO3 nanoplates by the addition of a carbonate-containing AOT microemulsion to a reverse micelle solution formed by calcium dodecylbenzenesulfonate under conditions of high alkalinity. Scanning electron microscopy (SEM) studies revealed that the CaCO3 superstructure consisted of stacked array of calcite nanoplates (20 nm in thickness) with a pseudohexagonal morphology (Fig. 17). The nanoplates varied in width from around 150 nm at each end of the stack to 1 mm in the central region, to produce a stacked arrangement with bilateral symmetry. In contrast to the microemulsion-based synthesis and assembly of doughnut-shaped particles of interlinked aragonite nanorods by mesoscale transformation of ACC nanoparticles, the calcite nanoplate superstructures were produced by the solution-mediated primary nucleation. 6200 Synthesis of Inorganic Nanostructures in Reverse Micelles Fig. 16 (A) TEM image of triangular CdS nanoplates formed in Cd(AOT)2 reverse micelles; (B) high resolution TEM image of a single nanoplate; and (C) Fourier transformation of the image in panel B. (From Ref..) Fig. 15 Transmission electron microscopy images of prussian blue nanocubes formed in AOT reverse micelles after ageing for (A) four days and (B) two weeks. (From Ref..) SYNTHESIS OF CORE–SHELL NANOSTRUCTURES The microemulsion-based synthesis technique is able to complicate nanostructures by surface coating and modiﬁcation to form nanocomposites exhibiting novel properties and improved functionalities. In particular, the controlled synthesis of different kinds of core–shell nanostructures can be realized in reverse micelles. The procedure usually consists of a two-step process, i.e., the formation of uniform core particles with sizes smaller than the droplet sizes, and the formation of a shell to coat a single core particle or a nanosized matrix to incorporate many core particles. Alternatively, core–shell nanostructures may be fabricated by simultaneous formation of the core and shell materials in reverse micelles or post-treatment of precursor nanoparticles formed in reverse micelles. and ZnS in AOT reverse micelles, and the ZnS–CdSe core–shell nanoparticles were prepared in a similar way with an interchange of the Cd and Se reagents with those of Zn and S. Luminescence spectra indicated that the growth of ZnS on the CdSe seed resulted in a signiﬁcant increase in the luminescence quantum yield Core–Shell Nanostructures with a Metal-Containing Shell Originally, CdSe–ZnS core–shell nanoparticles were prepared by the successive precipitation of CdSe Fig. 17 SEM image of stacked lamellar superstructures of calcite nanoplates formed in reverse micelles. (From Ref..) Synthesis of Inorganic Nanostructures in Reverse Micelles accompanying a blue shift to a narrow band near the exciton absorption. Subsequent studies have demonstrated that the surface passivation of a semiconductor core by an inorganic shell material of wider bandgap could lead to more efﬁcient and photostable luminescent nanocrystals. In this regard, highly luminescent and photostable yellow-light-emitting ZnS-passivated CdS : Mn (CdS : Mn/ZnS core–shell) nanocrystals were recently prepared in AOT reverse micelles.[145,146] Many metal nanoparticles (e.g., Fe) are susceptible to rapid oxidation, and this problem can be largely circumvented by coating the nanoparticles with gold or other inert metals. In particular, FePtx–Au and CoPtx–Au nanoparticles with magnetic alloy cores were prepared by successive reduction of appropriate metal ions in reverse micelles. Moreover, the synthesis can begin with Au nanoparticles, followed by a coating with Fe and then followed again by a coating of Au, creating an Au–Fe–Au onion type nanostructure. Similarly, Fe–Cu–Fe onion type nanoparticles with antiferromagnetic coupling between Fe layers were prepared in AOT reverse micelles, which provided a promising route for understanding the magnetic interactions at the nanoscope level. Furthermore, combinations of reduction, oxidation, precipitation, and hydrolysis reactions can be performed sequentially in reverse micelles to produce core–shell nanostructures of other systems including metals–oxides, oxides–metals, oxides–oxides, and so forth. For example, Fe–iron oxide core–shell nanoparticles with a metal-core diameter of approximately 6.1 nm and an oxide shell thickness of approximately 2.7 nm, which maintained the favorable magnetic properties of metallic iron and showed a very good resistance against oxidation, were successfully synthesized in nonionic reverse micelles. Core–Shell Nanostructures with a Silica Shell Incorporation of inorganic nanoparticles in silica can provide inorganic nanomaterials with several beneﬁts, such as high stability against oxidation, degradation, and coagulation, as well as biocompatibility and functionality. Considerable attention has been paid to the synthesis of the core–shell nanostructures with a silica shell, by using the microemulsion technique. In particular, Asher and coworkers[150,151] have made efforts to prepare semiconductor and metal quantum dots/silica nanocomposites in nonionic w/o microemulsions. The synthesis of monodisperse CdS–silica nanocomposite spheres (40–300 nm) containing incorporated CdS nanoparticles (approximately 2.5 nm) was achieved by simultaneous coprecipitation of CdS in the water droplets during the silica sphere synthesis through TEOS hydrolysis in the microemulsions. 6201 In a similar way, monodisperse Ag–silica nanocomposite spheres (approximately 100 nm) containing homogeneously dispersed Ag nanoparticles (2 5 nm) were fabricated by in situ photochemical reduction of silver ions during the hydrolysis of TEOS in w/o microemulsions. These Ag–silica particles had signiﬁcant surface charge and readily self-assembled into crystalline colloidal array photonic crystals that Braggdiffracted light in the visible region. Among various inorganic core–silica shell nanostructures, silica-coated magnetic nanoparticles have attracted particular interest owing to their great potential in biomedical and technological applications. For instance, ultrasmall silica-coated iron oxide nanoparticles (<5 nm) were prepared by the precipitation of iron oxides with NH4OH in nonionic reverse micelles, which was followed by NH4OH-catalyzed hydrolysis of TEOS, resulting in a uniform silica coating, as thin as 1 nm. Subsequently, tunable superparamagnetic g-Fe2O3–silica materials were produced by the thermal annealing of iron oxide–silica composites obtained in microemulsions with a high content of the iron oxide precursor. Recently, rod-like e-Fe2O3–silica nanocomposites, exhibiting a giant coercive ﬁeld (Hc) value of 2.0 T at room temperature, were successfully synthesized in CTAB reverse micelles by the addition of Ba2þ ions as a stabilizer for e-Fe2O3. In addition to iron oxides, other magnetic materials have also been employed as magnetic cores for the silica coating in reverse micelles. To illustrate, CoFe2O4 and MnFe2O4 spinel ferrites–silica nanocomposites with tunable magnetic cores have been synthesized by the microemulsion technique. As another interesting example, monodisperse, air-stable, superparamagnetic, a-Fe–silica composite nanospheres (50 nm) were prepared by the NH4OH-catalyzed hydrolysis of TEOS in the presence of Fe2þ ions in NP-5 reverse micelles, which was followed by reduction with H2 at 450 C (Fig. 18). Fig. 18 TEM images of Fe–silica composite nanospheres synthesized in nonionic reverse micelles. The outer shell is composed of silica and the dark spots correspond to a-Fe nanocrystals. (From Ref..) 6202 It is noteworthy that the polymer-coated inorganic nanoparticles or inorganic–organic nanocomposites, which offer interesting prospects in various areas because of their synergistic properties derived from several components, can also be readily prepared by using the microemulsion technique. For example, metal oxides– polymer core–shell nanoparticles were synthesized by the hydrolysis of modiﬁed metal alkoxides in w/o microemulsions and subsequent atom transfer radical polymerization initiated by the surface-functionalized metal oxide nanoparticles. Recently, novel polystyrene-encapsulated Ag nanorods and nanoﬁbers were synthesized by combination of reverse micelles, gas antisolvent, and ultrasound techniques. Furthermore, unusual CdS–polystyrene nanocomposite hollow spheres were fabricated in reverse micelles by g-radiationinitiated, simultaneous styrene polymerization and CdS formation. CONCLUSIONS Reverse micelles or w/o microemulsions have proved to be versatile nanostructured reaction media for the bio-inspired synthesis of a huge variety of inorganic nanostructures. In particular, reverse micelles can be employed as effective nanoreactors for the sizecontrolled synthesis of spherical nanoparticles of widely varied, technologically important, inorganic materials. Considerable attention has been paid to the precise control of the ﬁnal particle size and size distribution, by simply varying the experimental parameters. Recent computer simulations are fruitful in elucidating the effects of various parameters on the particle size; however, much effort is still needed to adapt this theoretical method to more complicated systems. On the other hand, recent studies have demonstrated that the microemulsion technique is also powerful for the shape- and architecture-controlled synthesis of inorganic nanostructures, which represents an exciting aspect in the microemulsion-based synthesis and opens new opportunities for the solution synthesis and assembly of inorganic nanostructures with complex hierarchy. The development in this aspect, at present, is still in its infancy and requires a much better understanding of the underlying mechanisms for the morphological control and hierarchical assembly. Moreover, commercial applications of the microemulsion technique call for further improvement in the processing methods involved in the large-scale preparation, separation, and deposition of different nanostructures. In this regard, w/c microemulsions are particularly attractive as they are environmentally benign and favorable for easy separation of the produced inorganic nanostructures. It is clear that much more work needs to be done to realize the full potential Synthesis of Inorganic Nanostructures in Reverse Micelles of the reverse micelle-based synthesis of inorganic nanostructures, and excitement involved in the development of this useful synthesis technique will continue. ACKNOWLEDGMENTS Support from the National Natural Science foundation of China (20325312, 20473003, and 20233010) and the Foundation for the Author of National Excellent Doctoral Dissertation of China (200020) is gratefully acknowledged. The author would like to thank Prof. Jiming Ma for helpful discussion. 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