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In this paper the deposition of very thin films of TiO2 on different substrates (SiO2, MgO, Ag) is studied by XPS. Shifts in the Ti 2p BE and the Auger parameter of Ti (α‘ = Ti 2p BE + Ti L3M23V Auger KE) are observed on the three substrates. The magnitude and the sign of the shifts with respect to those of bulk TiO2 depend on coverage and on the type of substrate. In a parallel way, the magnitude of the energy gap of thin films of TiO2 changes depending on the substrate. This has been shown by UV−vis absorption spectroscopy for TiO2 deposited on SiO2 for a TiO2−Ag “cermet” (ceramic−metal thin film) and by photoemission with synchrotron radiation for TiO2 deposited on SiO2. It is proposed that the shift in the Auger parameter and the energy gap of TiO2 in these systems are two related parameters. Molecular orbital calculations (extended Hückel and INDO/1) with clusters simulating the TiO2−substrate interface explain qualitatively the variations in the Auger parameter and in the energy gap. The contribution of the polarization of the medium to the changes in the Auger parameter is approximated with an electrostatic model that accounts for the influence of the support in the observed shifts.

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 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 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 < hν < 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 hν ) 47 eV.

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A thick layer of SnO, equivalent in its electronic properties to the bulk material, has been investigated by means of resonant photoemission and the mathematical method of factor analysis. This study has shown that O2p, Sn5s, and Sn5p partial density of states are the main contributions to the valence band of this material. The distribution through the valence band of these partial contributions has been determined by spectral subtraction and factor analysis of the resonance photoemission spectra, as well as by band structure calculation. The resonance behavior ͑i.e., change in intensity with the photon energy͒ of these three contributions has been analyzed. The Sn5p levels present a typical Fano-like behavior with a minimum intensity at about 28 eV and a maximum at 40 eV. The Sn5s partial density distribution also depicts a change in intensity as a function of the photon energy with a minimum situated at 35 eV and a maximum at 55 eV. Tentatively, this behavior has been linked to the existence of a broad absorption feature detected by electron energy loss spectroscopy and constant final state spectra, both depicting a broad maximum at about 50 eV. ͓S0163-1829͑99͒07339-7͔