Nonhydrolytic sol-gel technology

Abstract

Since the early 1990s alternative nonhydrolytic sol-gel routes have emerged, aiming at solving the problems faced in conventional aqueous sol-gel by a drastic change of reactions and of reaction medium. Nonhydrolytic sol-gel (or nonaqueous sol-gel) is now a well-established method for the preparation of nanostructured silica-based or metal oxide-based materials. This chapter focuses on truly nonhydrolytic processes under strict water-free conditions. In contrast to hydrolytic sol-gel, in which water molecules supply the oxygen needed to build the oxide network, nonhydrolytic sol-gel involves an organic oxygen donor, for instance, an ether, an alcohol, or an alkoxide. The reactions are typically much slower than in aqueous media, as they generally involve the cleavage of O-C bonds rather than O-H bonds, providing improved control over the composition, structure, and texture of the resulting materials. For instance, nonhydrolytic sol-gel has been highly successful in the synthesis of crystalline oxide nanoparticles; it also offers simple, one-step routes to mixed oxides with wellcontrolled compositions and nonordered mesoporous textures, avoiding the use of supercritical drying or templates.