Electronic properties and structure of vanadia ultra-thin films grown on TiO2(110) in a water vapour ambient

This paper reports on the deposition of a vanadium oxide ultra-thin film on TiO2(1 1 0) by means of e-beam evaporation in an atmosphere of water vapour at room temperature. Photoelectron, X-ray excited Auger and valence band spectra have shown features very similar to those reported in the literature for vanadia ultra-thin films on TiO2(1 1 0) identified as V2O3. An X-ray photoelectron diffraction analysis has been performed as a function of the overlayer thickness and after annealing treatments. It demonstrates that the overlayer grows ordered on the short range, pseudomorphic to the substrate, with a defective rutile crystal structure. The epitaxial relationship is maintained, with an approximately linear decrease in anisotropy, up to at least a coverage of 16 monolayers (ML); short annealing treatments are sufficient to restore the ordered structure, even on 20 ML thick films. Although in the bulk phase V2O3 is characterised by the corundum structure, which is also found when depositing vanadium oxide under similar experimental conditions on other substrates, such as Al2O3(0 0 0 1) and Pd(1 1 1), it appears that TiO2(1 1 0) stabilises the isomorphic rutile lattice, which pertains to bulk VO2 in its metallic phase, as well as to Magnéli phases of general formula VnO(2n−1). This particular behaviour of vanadia ultra-thin films grown on TiO2 could be a key to understanding the catalytic activity and selectivity of the vanadia/titania systems in several chemical reactions.