Salt Catalysts Containing Basic Anions and Acidic Cations for the Sol−Gel Process of Titanium Alkoxide: Controlling the Kinetics and Dimensionality of the Resultant Titanium Oxide

Abstract

The sol−gel process from titanium alkoxide monomer to titanium oxide polymer was remarkably enhanced by salt catalysts containing basic anions and acidic cations. An ammonium carbonate catalyst promoted the precipitation of spherical titanium oxide particles at low concentrations of titanium alkoxide and water at room temperature. Dehydrative polycondensation in the sol−gel process involves both deprotonation and dehydroxylation, and the carbonate anion enhanced the deprotonation as a base catalyst while the ammonium cation enhanced the dehydroxylation as an acid catalyst. If the ammonium acetate catalyst was added, the monolithic titanium oxide gel which consists of sheetlike titanium oxide particles was formed. The acetate anion acted not only as the catalyst for the dehydrative polycondensation but also as a ligand coordinating to titanium for capping the polycondensation sites, resulting in planar titanium oxide particles.