SPECTROSCOPIC CHARACTERIZATION OF QUANTUM-SIZED TIO2 SUPPORTED ON SILICA - INFLUENCE OF SIZE AND TIO2-SIO2 INTERFACE COMPOSITION

In the present paper we study TiO2 thin films prepared by evaporation of Ti in the presence of oxygen onto silica substrates. Samples have been analyzed by UV-vis absorption spectroscopy, LEIS (low-energy ion scattering), XPS (X-ray photoelectron spectroscopy), and REELS (reflection electron energy loss spectroscopy). The supported titania phase shows a significant blue shift in its band gap UV-vis absorption spectra which is higher than the value expected based on the thickness of the TiO2 layer. XPS analysis of the samples shows that the TiO2-SiO2 interface is formed by cross-linking Ti-O-Si bonds, thus decreasing the positive charge of the Ti atoms at the interface, the mobility of the electrons in the titania phase, and the polarizability of the oxygen atoms at this interface. The influence of these effects on the observed band gap shift is expected to be significant and we have attributed the increase in the band gap to a combination of size quantization effects and interface phenomena! Comparison of the results obtained on silica with those obtained for TiO2 evaporated on graphite demonstrates the importance of support effects on the photoemission behavior and permits the characterization of the TiO2-SiO2 interface.