Surface chemical structure of SiO2-TiO2 sol-gel powders

Amorphous SiO2, TiO2 and x SiO2-(1-x) TiO2 powders, with nominal values of x = 0.9, 0.7 and 0.5, have been prepared via sol-gel, using silicon tetrahoxysilane (TEOS) and titanium tetraisopropoxide Ti(OPri)4. X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger electron spectroscopy (XAES) are used for studying the surface chemical structure of the powders as a function of the air thermal treatment temperature up to 1273 K. For the whole range of temperature, XPS and XAES signals indicate that silicon and titanium are present as Si4+ and Ti4+ oxides. From the line shape of the O 1s peak, it is possible to distinguish between the single O-Ti and O-Si bonds and to disclose also the presence of Si-O-Ti cross linking bonds that are supposed to act as bridges between SiO2 and TiO2 moieties. Starting from 873 K, the Si-O-Ti bonds are broken and formation of a low amount of new Ti-O and a higher amount of Si-O bonds takes place. Si/Ti atomic ratios obtained by curve fitting the O 1s peaks and from Ti 2p3/2 and Si 2p peaks, confirm the silicon oxide surface enrichment. Furthermore, with increasing temperature, XAES data indicate the formation of crystalline phases.