INTERPRETATION OF THE BINDING-ENERGY AND AUGER PARAMETER SHIFTS FOUNDBY XPS FOR TIO2 SUPPORTED ON DIFFERENT SURFACES

In this paper the deposition of very thin films of TiO2 on different s ubstrates (SiO2, MgO, Ag) is studied by XPS. Shifts in the Ti 2p BE an d the Auger parameter of Ti (alpha' = Ti 2p BE + Ti L(3)M(23)V Auger K E) 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 ener gy 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 photoe mission with synchrotron radiation for TiO2 deposited on SiO2. It is p roposed that the shift in the Auger parameter and the energy gap of Ti O2 in these systems are two related parameters. Molecular orbital calc ulations (extended Huckel and INDO/1) with clusters simulating the TiO 2-substrate interface explain qualitatively the variations in the Auge r parameter and in the energy gap, The contribution of the polarizatio n of the medium to the changes in the Auger parameter is approximated with an electrostatic model that accounts for the influence of the sup port in the observed shifts.