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

Hybrid silica–titania films were prepared via sol–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°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°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°C, tend to be transformed into SiOH (silanol) groups as they are eliminated. Most silanol groups are eliminated at a temperature near 600°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.