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

Thin films of vanadium oxides grown on a Pd(111) single crystal surface hav e been studied using high resolution x-ray photoelectron spectroscopy (XPS) , near edge x-ray absorption fine structure (NEXAFS), low-energy electron d iffraction (LEED), and scanning tunneling microscopy (STM). The vanadium ox ides have been prepared by reactive evaporation of vanadium in p(o2) = 2 x 10(-7) mbar at 250 degrees C from submonolayer to 5 monolayer (ML) coverage s. As observed on other substrates, the stoichiometry of the oxide phase va ries as a function of the coverage, VO/VO2-like at low coverages to V2O3 fo r 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 ox ide phase grows epitaxially on the Pd(111) surface in the form of small thr ee-dimensional (3D) islands as revealed by LEED and STM. The thermal stabil ity of the brides is also coverage dependent: the decomposition onset tempe ratures range from 300 degrees C for submonolayer coverage to greater than or equal to 500 degrees C for 5 ML. The V 2p XPS data indicate that the 1 M L V2O3 Like oxide disproportionates into higher (4+) and lower (2+) oxidati on state phases on heating to 300 degrees C. This surface shows a complex L EED pattern, due to the coexistence of ordered 3D V oxide islands and bare Pd patches as revealed by STM. On heating to 350 degrees C a simple p(2X2) LEED structure evolves, originating from a well ordered two-dimensional ove rlayer, which entirely wets the Pd(111) surface. At higher temperatures (40 0-500 degrees C), the vanadium oxide reduces to metallic vanadium and V/Pd alloy phases, as demonstrated by XPS. (C) 1999 American Vacuum Society. [S0 734-2101(99)05704-8].