First-principles calculations for VxOy grown on Pd(111)

An approach to access the stability of oxides growing on top of a metal support is presented. In combination with first-principles calculations, it allows to predict the stable structures as a function of the thickness of the evaporated metal ad-layer and as a function of the oxygen pressure. The ideas are applied to thin vanadium oxide films growing on Pd(111). To investigate the stability of these oxide films, first-principles calculations for more than 50 thin films of V x O y on Pd were performed at varying stoichiometry and coverage. The general principles determining the growth of thin vanadium oxide films on Pd(111) are discussed, and the experimental results are interpreted in the light of the first-principles calculations. At 1 ML vanadium coverage, a complicated succession of structures is predicted by the calculations. At high oxygen pressure bulk like V2O3 phases are stable. At lower oxygen pressure, however, a surface stabilised (2×2) reconstruction with a formal stoichiometry of V2O3 is predicted, and rectangular and hexagonal vanadium-dioxide phases are expected to grow. At very low oxygen pressures, first the vanadium-dioxide phases and then the surface V2O3 phase decompose and the liberated V atoms move subsurface. These predictions are in good general agreement with experiment. An important result of the study is that the metal surface stabilises thin films which have no equivalent bulk phases.