Densification of hybrid silica-titania sol-gel films studied by ellipsometry and FTIR

Hybrid silica-titania films were prepared via soi-gel processing, by dip-coating on silicon wafer substrates, from organically-modified silane and titanium isopropoxide precursors. These hybrid sol-gel films were then subjected to selected heat treatments at temperatures up to 950 degrees C and their densification was Followed by ellipsometry and infrared absorption (FTIR) spectroscopy. The volume fraction of residual porosity was estimated, based on the refractive index data. A clear correlation was observed between the intensity of the infrared spectral shoulder near 1150 cm(-1) and the porosity values, for the different heat treatment temperatures. For any treatment up to 950 degrees C, the porosity of the hybrid silica-titania gel films was higher than that of the corresponding inorganic gel films, while both continuously decreased with increasing heat treatment temperature. The residual Si-CH3 groups, whose presence was identified in the hybrid silica-titania films until a temperature near 500 degrees C, tend to be transformed into Si-OH (silanol) groups as they are eliminated. Most silanol groups are eliminated at a temperature near 600 degrees C. The hybrid silica-titania films were found to have a more porous and more compliant gel network than the corresponding inorganic films, while their structure appears to exhibit a smaller spread in the Si-O-Si intertetrahedral angle distribution. (C) 2000 Published by Elsevier Science S.A. All rights reserved.