A PHOTOELECTRON-SPECTROSCOPY STUDY OF SUBMONOLAYER V TIO2(001) INTERFACES ANNEALED FROM 300 UP TO 623 K/

The temperature effects on sub-monolayers of V deposited at the TiO2(0 01) surface have been studied by ultraviolet and X-ray photoelectron s pectroscopies, UPS and XPS, from similar to 300 up to 623 K. V coverag es, 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 t wo 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, dete cted within the TiO2(001) band gap of the UPS valence band spectra. UP S and XPS measurements performed after in-situ thermal treatments show unambiguous and reproducible changes of these spectral components. Af ter annealing at 623 K only the higher binding energy component is pre sent in the V 2p(3/2) spectra; the Ti 2(p) core lines recover the typi cal 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 i ons have a mainly d(0) configuration, while V has a single and stable open-shell configuration, as revealed by the significant intensity det ected within the TiO2 band gap. These annealing-induced changes are du e, as suggested by the O 1s/Ti 2p core line intensity trend, to an oxy gen diffusion from the TiO2 bulk to the surface. Finally, a detailed a nalysis of the data indicates that different V/TiO2(001) interfaces ex hibit different behaviours after annealing treatments, depending on Th eta(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 f inding is consistent with a rearrangement of V atoms, which under anne aling occupy first the energetically most favorable surface sites (The ta(V) = 0.2 ML) before interdiffusing into the TiO2 lattice (Theta(V) = 0.7 ML). (C) 1997 Elsevier Science B.V.