Synchrotron photoemission characterization of TiO2 supported on SiO2

The system formed by TiO2 deposited on SiO2 has been studied by photoemission and X-ray absorption spectroscopies with synchrotron radiation. TiO2 spreads on the surface of SiO2 where it forms a layer (1-2 ML thick) prior to thickening. Extended X-ray absorption fine structure/X-ray absorption near-edge spectroscopy (EXAFS/XANES) analysis at the TiK edge shows that, whatever the coverage, the TiO2 films are amorphous with titanium ions in a B-fold coordination of oxygen ions. Photoemission with photons of 140 eV shows that for low coverages, the Ti3p binding energy increases by 0.5 eV with respect to the value in bulk TiO2. Under these conditions, a new feature appears in the O2s peak in the form of an extra shoulder around 24 eV. This new form of oxygen is attributed to oxygen ions at the interface acting as a bridge between TiO2 and SiO2 (i.e., formation of Si-O-Ti cross-linking bonds). The shift in the Ti3p peak is accompanied by a shift to higher binding energies in the valence band edge of the spectra at low TiO2 coverage. This shift would indicate that the band gap of the titanium oxide increases for low coverages of TiO2. A detailed analysis of the valence band region was carried out with photons of 35 < hv < 70 eV. The valence band spectra of TiO2 are narrower (3.6 eV fwhm) and less defined than that of bulk TiO2 (3.9 eV fwhm). Resonance photoemission of the valence band of a thin layer of TiO2 reproduces the pattern reported in the literature for bulk TiO2 characterized by a maximum enhancement of the intensity of the valence band spectrum for hv = 47 eV.