Interpretation of the Binding Energy and Auger Parameter Shifts Found by XPS for TiO₂Supported on Different Surfaces

In this paper the deposition of very thin films of TiO 2 on different substrates (SiO 2, MgO, Ag) is studied by XPS. Shifts in the Ti 2p BE and the Auger parameter of Ti (α‘ = Ti 2p BE + Ti L 3 M 23 V Auger KE) are observed on the three substrates. The magnitude and the sign of the shifts with respect to those of bulk TiO 2 depend on coverage and on the type of substrate. In a parallel way, the magnitude of the energy gap of thin films of TiO 2 changes depending on the substrate. This has been shown by UV−vis absorption spectroscopy for TiO 2 deposited on SiO 2 for a TiO 2 −Ag “cermet” (ceramic−metal thin film) and by photoemission with synchrotron radiation for TiO 2 deposited on SiO 2 . It is proposed that the shift in the Auger parameter and the energy gap of TiO 2 in these systems are two related parameters. Molecular orbital calculations (extended Hückel and INDO/1) with clusters simulating the TiO 2 −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.