Basic Principles of Sol-Gel Chemistry

Abstract

The first sol-gel synthesis of silica was described by J.J. Ebelmen about 150 years ago. He noticed that silicic esters (or alkoxides) hydrolyze slowly in the presence of moisture to give hydrated silica [I]. The synthesis and chemical properties of metal alkoxides were extensively studied much later, in the seven-ties [2]. However this work was restricted to the community of molecular chemists and materials scientists became really interested by alkoxides precursors in the late eighties [3]. The sol-gel synthesis of oxide materials occurs at temperatures much lower than usual solid-state reactions. It allows the powderless processing of glasses and ceramics. Thin films or tibers can be produced directly from the solution by techniques such as dip-coating or spin-drawing [4]. More recently, hybrid organic inorganic materials have been made via the sol-gel route. They fill the gap between polymers and glasses and open new fields in materials science such as "sol-gel optics" or "molecular composites" [5]. 2. SOL-GEL CHEMISTRY OF SILICA 2.1 Hydrolysis and condensation The sol-gel synthesis of silica is based on the hydrolysis and condensation of silicon alkoxides M(OR)z where R is an alkyl group (R = Me, Et, Pr ...). Hydroly-sis gives reactive silanol groups whereas condensation leads to the formation of bridging oxygen. >Si-OR + H20 => >Si-OH + ROH (hydrolysis) >Si-OH + RO-Si< => >Si-O-Si< + ROH (condensation) M. A. Aegerter et al. (eds.), Sol-Gel Technologies for Glass Producers and Users