The deposition of silica on titanium dioxide surfaces

Abstract

The deposition of amorphous silica from aqueous solution on to rutile particles has been studied with the aim of elucidating the nature of the silica-titania interactions occurring and of following the progressive build up of the silica coating.

The coating process, which involves the addition of an aqueous sodium silicate solution to an aqueous dispersion of titanium dioxide, has been investigated by performing a series of controlled preparations and using the technique of microelectrophoresis. Prepared silica-coated rutile samples ranging from partial to full silica coatings have been characterized using transmission electron microscopy, microelectrophoresis and nitrogen, argon and water sorption. Nitrogen and argon adsorption isotherms have been analysed using the equation of Brunauer, Emmett and Teller (BET). Differential energies of adsorption of nitrogen and argon have been determined calorimetrically. Water sorption isotherms have been analysed using the BET equation and the Frenkel - Halsey - Hill (FHH) equation. It has been shown that uniform silica coatings can be produced if adsorption of monomeric silica is followed by polymerization of silica at the solid/liquid interface. Surface cations on rutile may be hydroxylated or co-ordinately bound to water molecules and it appears that monomeric silica adsorbs preferentially by replacing ligand water molecules. Rutile particles with silica coatings thicker than approximately 2.5nm exhibit characteristics typical of silica and not of the base rutile. Silica coatings deposited at pH 10 contain narrow channels which are accessible to water molecules but not to nitrogen or argon. Neutralization to pH 7 reduces the volume in the coating accessible to water molecules.