Effect of vanadium admixing on the surface structure of TiO₂(110) under non-oxidizing conditions

Single crystalline Ti + V mixed oxide layers have been prepared by doping vanadium into TiO₂(110) thin films on TiO₂(110) single crystal substrates with a Ti + Ta mixed oxide interlayer between the film and the substrate. The interlayer prevents the diffusion of vanadium into the substrate and also the diffusion of Ti³⁺ between substrate and overlayer. Mixing vanadium into the TiO₂ lattice increases the reducibility of the host oxide as concluded from an appreciable degree of reduction produced by comparatively mild annealing. A high density of bridging oxygen vacancies was identified at the surface of films with a low vanadium content (2%) while a (1 × 2) reconstruction as also known for massively reduced TiO₂(110) was observed for layers with 8% of vanadium. Studies of methanol adsorption indicate that the vanadium atoms are mostly located below the surface since there is no indication of a vanadium-methanol interaction. We provide evidence that the reducibility of the vanadium ions in the thin film is higher than that of the titanium ions and we suggest that this is the origin of the increased reducibility of the mixed oxide