Na and K on Al(100) studied by low-energy electron diffraction and high-resolution core-level spectroscopy

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 300 up to 623 K.

V coverages, Θ_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 Θ_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 d⁰ 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 Θ_v. For Θ_v = 0.7 ML, V interdiffuses into the TiO₂ sub-surface layers, whereas for Θ_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 (Θ_v = 0.2 ML) before interdiffusing into the TiO₂ lattice (Θ_v = 0.7 ML).