A photoelectron spectroscopy study of sub-monolayer V/TiO2(001) interfaces annealed from 300 up to 623 K

The temperature effects on sub-monolayers of V deposited at the TiO2(001) surface have been studied by ultraviolet and X-ray photoelectron spectroscopies, UPS and XPS, from similar to 300 up to 623 K. V coverages, Theta(V), between 0.2 and 0.7 monolayers (ML) were deposited by an e-beam evaporator at 300 K. The V 2p(3/2) core line region exhibits two well-defined components whose relative intensity depends on Theta(V). These two components, assigned to different oxidation states of V, are correlated with two features, with a dominant V 3d character, detected within the TiO2(001) band gap of the UPS valence band spectra. UPS and XPS measurements performed after in-situ thermal treatments show unambiguous and reproducible changes of these spectral components. After annealing at 623 K only the higher binding energy component is present in the V 2p(3/2) spectra; the Ti 2(p) core lines recover the typical symmetry of the clean and stoichiometric TiO2(001) surface and the higher binding energy feature only is detected in the TiO2 band gap. These data suggest that, within the volume probed by XPS and UPS, Ti ions have a mainly d(0) configuration, while V has a single and stable open-shell configuration, as revealed by the significant intensity detected within the TiO2 band gap. These annealing-induced changes are due, as suggested by the O 1s/Ti 2p core line intensity trend, to an oxygen diffusion from the TiO2 bulk to the surface. Finally, a detailed analysis of the data indicates that different V/TiO2(001) interfaces exhibit different behaviours after annealing treatments, depending on Theta(V). For Theta(V)=0.7 ML, V interdiffuses into the TiO2 sub-surface layers, whereas for Theta(V)=0.2 ML it remains at the surface. This finding is consistent with a rearrangement of V atoms, which under annealing occupy first the energetically most favorable surface sites (Theta(V) = 0.2 ML) before interdiffusing into the TiO2 lattice (Theta(V) = 0.7 ML). (C) 1997 Elsevier Science B.V.