XANES and XPS investigations of (TiO2)x(SiO2)1-x: the contribution of final-state relaxation to shifts in absorption and binding energies

The (TiO2)(x)(SiO2)(1-x) system (0 <= x <= 0.33) was synthesized by the sol-gel method and investigated by X-ray absorption near-edge spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS). The use of both hard (Ti K-edge) and soft (Ti L-edge) X-rays provides a useful way to monitor changes in the bulk and surface, respectively, of these amorphous materials. The average CN of both bulk-Ti and surface-Ti increases with greater x in the chemical formula, due to the larger ionic radius of Ti. Comparison of Ti K-and L-edge spectra of annealed samples revealed that Ti atoms at the surface have a higher average CN than in the bulk, likely due to the presence of surface hydroxide and water groups that can coordinate to the Ti centres. The O K-edge, Ti L-edge, and Si L-edge XANES absorption energies showed little to no change with Ti content, while the O 1s, Ti 2p, and Si 2p XPS BEs were found to decrease with increasing Ti content due to nearest-neighbour and next-nearest-neighbour effects, which lead to increased final-state relaxation. The degree of final-state relaxation is more significant than previously believed for these amorphous powders.