Nature, growth, and stability of vanadium oxides on Pd(111)

Abstract

Thin films of vanadium oxides grown on a Pd(111) single crystal surface have been studied using high resolution x-ray photoelectron spectroscopy (XPS), near edge x-ray absorption fine structure (NEXAFS), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). The vanadium oxides have been prepared by reactive evaporation of vanadium in pO2=2×10−7 mbar at 250 °C from submonolayer to 5 monolayer (ML) coverages. As observed on other substrates, the stoichiometry of the oxide phase varies as a function of the coverage, VO/VO2-like at low coverages to V2O3 for thicker oxide layers as indicated by XPS V 2p core level spectra and the characteristic NEXAFS fingerprints at both V 2p and O 1s edges. The V2O3 oxide phase grows epitaxially on the Pd(111) surface in the form of small three-dimensional (3D) islands as revealed by LEED and STM. The thermal stability of the oxides is also coverage dependent: the decomposition onset temperatures range from 300 °C for submonolayer coverage to ⩾500 °C for 5 ML. The V 2p XPS data indicate that the 1 ML V2O3-like oxide disproportionates into higher (4+) and lower (2+) oxidation state phases on heating to 300 °C. This surface shows a complex LEED pattern, due to the coexistence of ordered 3D V oxide islands and bare Pd patches as revealed by STM. On heating to 350 °C a simple p(2×2) LEED structure evolves, originating from a well ordered two-dimensional overlayer, which entirely wets the Pd(111) surface. At higher temperatures (400–500 °C), the vanadium oxide reduces to metallic vanadium and V/Pd alloy phases, as demonstrated by XPS.