Nonaqueous and Surfactant-Free Synthesis Routes to Metal Oxide Nanoparticles Georg Garnweitner and Markus Niederberger w Max Planck Institute of Colloids and Interfaces, Colloid Chemistry, D-14424 Potsdam, Germany Nonaqueous sol–gel routes to metal oxide nanoparticles have become a promising alternative to aqueous methods, allowing the controlled synthesis of a variety of metal oxides as highly crystalline products at comparably low temperatures. The use of solvents like benzyl alcohol that also function as surface mod- ifiers makes the addition of surfactants superfluous, resulting in improved product purity. In addition to a short overview of such nonaqueous routes to binary and ternary metal oxides, the facile synthesis of sodium and potassium niobates as well as of sodium tantalate and barium stannate nanoparticles via straightfor- ward, surfactant-free pathways is reported. I. Introduction A MONG the large family of metal oxides, polar oxides play an outstanding role in science and technology owing to their dielectric, optic, piezoelectric, and pyroelectric properties. 1 The most important class of polar oxides are those possessing the perovskite structure, because distortion from the ideal ABO 3 cubic symmetry allows to change the physical properties on ap- plication of an external stimulus. 2 This ferroelectric behavior makes perovskites, in particular BaTiO 3 , SrTiO 3 , PbTiO 3 , PbZrO 3 , and solid solutions thereof, the materials of choice for applications in electroceramics. 3–5 As the physical properties generally arise from the crystal chemistry, the formation of pure, stoichiometric, homogeneous, and crystalline materials with controlled crystal size is crucial. 2 Although size effects on the nanometer scale have long been investigated in ceramics, it is a relatively new trend to utilize them in a controlled way to im- prove the properties of nanocrystalline materials. 6–8 The conventional synthesis of perovskite materials involves solid-state reactions between the individual metal oxide or car- bonate powders at temperatures between 6001 and 11001C. In order to bring the reaction partners sufficiently close together and to provide high mobility, these solid-state reactions require a high temperature and small particle sizes. However, these harsh conditions ruin any opportunity for subtle control of the reactions and often prevent the formation of thermally labile and metastable solids. 9 It is obvious that for the synthesis of nanoparticles, whose size and shape are crucial factors in deter- mining the properties, other preparation methodologies have to be developed. The most promising alternatives are soft-chemis- try routes, 10–12 where good control from the molecular precur- sor to the final product is achieved, offering high purity and homogeneity and low processing temperatures. It is not surpris- ing that aqueous sol–gel chemistry, which has a long and suc- cessful tradition in the synthesis of bulk metal oxides, 13,14 has been adapted for the preparation of nanoscale metal oxides. As aqueous sol–gel routes often lead to amorphous products, the use of nonaqueous synthesis approaches has become a valuable alternative, allowing the preparation of a large variety of nano- crystalline materials under better control over particle size, shape, crystallinity, and surface properties. 15,16 This is based on the fact that switching from aqueous sol–gel chemistry and its high reactivity of water to nonhydrolytic processes drastically decreases the reaction rates and leads to controlled crystalliza- tion. The chemistry of the oxygen–carbon bond is well known from organic chemistry and therefore, these routes provide the possibility to adapt reaction principles from organic chemistry to the synthesis of inorganic nanomaterials. In contrast to aque- ous systems, where smallest changes in the experimental condi- tions result in alteration in the products, nonaqueous procedures are very robust within one system. As a consequence, most of these processes are highly reproducible, easy to scale up to gram quantities, and applicable to a broad family of metal oxides. On the other hand, the morphology of the final product strongly depends on the precursor and solvent used, i.e., metal oxides with the same composition and crystal structure, however obtained from different precursors and/or solvents, are often char- acterized by different particle sizes and shapes. This observation highlights the crucial role of the organic side of the process, but also provides a precious tool to tailor the particle morphology. But nonaqueous reaction approaches also have some disad- vantages. First of all, many nonaqueous procedures are carried out in environmentally problematic organic solvents. However, this issue can be mitigated by the development of processes using non-toxic solvents like benzyl alcohol, which react readily with various metal oxide precursors and exhibit a high boiling point. Hazardous surfactants in the organic solvents, often required for stabilization of the nanoparticles and control of the crystal growth, constitute another disadvantage of these nonaqueous processes regarding purity and accessibility of the surface. In order to circumvent this problem, nonaqueous processes have been developed where organic solvents act as reactants as well as control agents for particle growth, enabling the synthesis of high-purity nanomaterials in surfactant-free media. In this pa- per, we will discuss the following reaction systems in more detail: (i) metal halides in benzyl alcohol, (ii) metal alkoxides in benzyl alcohol, and (iii) metal alkoxides in ketones. All these systems have some peculiarities with respect to each other, providing many possibilities to control and tailor the particle size and shape, as well as the surface and assembly properties. II. Surfactant-Free Synthesis of Binary Metal Oxide Nanoparticles The extension of nonhydrolytic reaction processes, which were very successful in the synthesis of various metal oxide gels, 17 to the preparation of zincite, 18 zirconia, 19 and titania 20–22 nano- crystals, were important steps in opening up new pathways to metal oxide nanoparticles. The most widely explored approach to synthesize metal oxide nanoparticles in nonaqueous media in the absence of any surfactants involves the reaction of metal halides with alcohols. The particularly low reaction temperature of these processes makes it possible to perform the synthesis in 1801 J ournal J. Am. Ceram. Soc., 89 [6] 1801–1808 (2006) DOI: 10.1111/j.1551-2916.2006.01005.x r 2006 The American Ceramic Society G. Messing—contributing editor Financial support by the Max Planck Society is gratefully acknowledged. Presented at the 9th International Ceramic Processing Science Symposium, Coral Springs, FL, Jan. 8–11, 2006. w Author to whom correspondence should be addressed. e-mail: markus.niederberger@ mpikg.mpg.de Manuscript No. 21333. Received January 6, 2006; approved February 13, 2006.