Synchrotron Photoemission Characterization of TiO₂ Supported on SiO₂

Abstract

The system formed by TiO 2 deposited on SiO 2 has been studied by photoemission and X-ray absorption spectroscopies with synchrotron radiation. TiO 2 spreads on the surface of SiO 2 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 TiO 2 films are amorphous with titanium ions in a 6-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 TiO 2 . 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 TiO 2 and SiO 2 (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 TiO 2 coverage. This shift would indicate that the band gap of the titanium oxide increases for low coverages of TiO 2 . A detailed analysis of the valence band region was carried out with photons of 35 < h ν < 70 eV. The valence band spectra of TiO 2 are narrower (3.6 eV fwhm) and less defined than that of bulk TiO 2 (3.9 eV fwhm). Resonance photoemission of the valence band of a thin layer of TiO 2 reproduces the pattern reported in the literature for bulk TiO 2 characterized by a maximum enhancement of the intensity of the valence band spectrum for h ν = 47 eV.