Surface chemical structure of titania-silica nanocomposite powder

Titania-silica (TS) nanocomposite powder with three different composite structures, containing 10–30 mol% SiO₂ in each structure, have been prepared by sol-gel processes. The surface characteristics of these titania-silica samples have been investigated by X-ray photo-emission spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM). The study for all TS oxides annealed at 773 and 1173 K showed: an abnormal surface enrichment in Si increased with increasing annealing temperature; the Ti³⁺, Ti²⁺, Si³⁺ and Si²⁺ oxides coexisted with Ti⁴⁺ and Si⁴⁺ oxides, and the contents of these Ti/Si suboxides increased with increasing SiO₂ content and annealing temperature; there was a layerrich in O on the topmost surface and the excess O could be attributed to the chem-adsorption of H₂O; different composite structures could lead to different contents of Ti/Si suboxides. These results indicated that the surface of TS oxide powder derived by sol-gel process was a double layer with enriched O first and then SiO_x/TiO _y (x, y<2). Ti/Si suboxides could result from the thermal diffusion of Ti⁴⁺ and Si⁴⁺, which might be induced by the strong interaction between Ti⁴⁺ and Si⁴⁺. Supported by the National Natural Science Foundation of China (Grant Nos. 20776091 and 90306014)