Microemulsion-based synthesis is found to be a versatile route to synthesize a variety of nanomaterials. The manipulation of various components involved in the formation of a microemulsion enables one to synthesize nanomaterials with varied size and shape. In this tutorial review several aspects of microemulsion based synthesis of nanocrystalline materials have been discussed which would be of interest to a cross-section of researchers working on colloids, physical chemistry, nanoscience and materials chemistry. The review focuses on the recent developments in the above area with current understanding on the various factors that control the structure and dynamics of microemulsions which can be effectively used to manipulate the size and shape of nanocrystalline materials.

The role of surfactant and solvent in the size and morphology of nickel oxalate particles synthesized from reverse micelles was investigated. Nanorods of nickel oxalate with aspect ratios of 5:1 and 6:1 were formed from n-hexane and cyclohexane, respectively. Our studies show that the bulkiness of the solvent molecules leads to larger dimensions of the nanorods. The surface charge on the nanorods also plays an important role in the anisotropic growth of the nanorods. Negative ζ potential values were observed for the nanorods, which may have a bearing on the growth of the rods along the cross-section, especially with surfactant molecules having positively charged headgroups (CTAB). The rodlike morphology could be modified by changing the surfactant. For example, we obtained nanoparticles ∼5 nm in size when the surfactant was changed from CTAB to TX-100, and nanocubes (∼50 nm in dimension) were formed with Tergitol as the surfactant. Our study shows that a larger headgroup of the surfactant (TX-100) provides a greater barrier to interdroplet exchange, leading to small sized particles. The nickel oxalate particles obtained above were decomposed to yield NiO nanoparticles. The size of the oxide nanoparticles depends on the aspect ratio of the precursor rods, which in turn appears to be dependent on the solvent chosen for synthesis.

This study focuses on understanding the growth and control of nanostructures using reverse micelles. It has been earlier realized that parameters like surfactant, cosurfactant, and aqueous content influence the size and shape of the nanostructures obtained using reverse micelles. However, a concerted effort to understand the role of these factors on the growth of a specific nanomaterial is missing. In this study we have focused on one nanomaterial (copper oxalate monohydrate) and determined how the above-mentioned factors control the size, shape, aspect ratio, and growth of these nanostructures. Our results show that cationic surfactants (CTAB, TTAB, and CPB) favor the formation of nanorods of copper oxalate. The aspect ratio of these rods could be controlled to obtain nanocubes (approximately 80-100 nm) and nanoparticles (approximately 8-10 nm) in the CTAB system using longer chain cosurfactants like 1-octanol and 1-decanol, respectively. Nanocubes of approximately 50-60 and approximately 60-80 nm were obtained using nonionic surfactants Triton X-100 and Tergitol, respectively. The size of the nanostructures could also be controlled by varying the molar ratio of water to surfactant (W0) by using a nonionic (Triton X-100)-based reverse micellar system. The study espouses the versatility of the microemulsion method to realize a variety of nanostructures of copper oxalate monohydrate. Our results will be of use in extending these ideas to other nanomaterials.

Nanorods of zinc oxalate dihydrate have been synthesized using the reverse micellar route. These nanorods were decomposed at 450 °C in air to obtain nanoparticles of zinc oxide. Transmission electron microscopy shows the nanorods to be 120 nm in diameter and 600 nm in length. The ZnO nanoparticles are 55 nm in diameter. The photoluminescence studies show two peaks at 370 and 403 nm which can be ascribed to free excitonic transition and donor–acceptor pair transition respectively. The temperature dependent PL intensity shows an anomalous non-monotonous temperature dependence probably due to two different optical processes.

Nanoparticles of several titanates and zirconates in the range of 20-60 nm have been obtained using the reverse micellar route. Important oxides like CeO 2 (mixture of nanorods; 7 nm diameter and 30 nm length and nanoparticles; 10 nm), ZrO 2 (3-4 nm) and SnO 2 (8 nm) have also been synthesized. Nanorods and nanoparticles of CaCO 3 in all three forms (aragonite, vaterite, and calcite) have been obtained using reverse micelles as nanoreactors. The specific reactions vary depending on the nature of the target nanomaterial. For synthesis of ternary oxides like BaTiO 3 , a modified and convenient route using microemulsions (avoiding Ba-alkoxide) has evolved. Monophasic tin dioxide (SnO 2 ) was obtained when liquid NH 3 was used as precipitating agent. Transmission electron microscopy (TEM) studies show that the SnO 2 nanoparticles are highly uniform and particle size was found to be 6-8 nm at 500 °C. The gas sensing characteristics of SnO 2 have also been investigated using n-butane, which shows high sensitivity and fast recovery time. Reverse micelles have been used, for the first time, to mimic the conditions suitable for the room-temperature synthesis of the high-temperature and -pressure orthorhombic phase of calcium carbonate (aragonite). Other forms of calcium carbonate (vaterite and calcite) could be obtained by varying the atmospheric conditions. At a lower temperature (5°C), homogeneous and monodisperse spheres of vaterite are obtained. The spherical particles aggregate after longer aging (168 h) to form nanorods, and the self-assembly is clearly seen at various stages by electron microscopy images. The samples were well characterized using powder X-ray diffraction (PXRD), line-broadening studies, TEM, variation in the dielectric properties with frequency and temperature, were measured on disks sintered at high temperature.

SAXS study for evaluating the effect of variation of co-surfactants on the shape of reverse micelles and growth of copper oxalate nanostructures.

The oxalate and carbonate precursors of cerium and zirconium have been prepared using reverse micelles as nanoreactors. Cerium oxalate precursor on thermal decomposition leads to a mixture of nanorods and nanoparticles of cerium oxide (nanoparticles of 10 nm and nanorods with 7 nm diameter and 30 nm length). Cerium oxide with crystallite size of 10 nm was obtained from cerium carbonate precursor. Monodispersed nanoparticles of zirconia with an average size of 3-5 and 12 nm were obtained from the oxalate and carbonate precursor, respectively. Detailed dielectric properties of sintered discs of nanocrystalline ceria and zirconia have been studied with variation of frequency and temperature.

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