A HREELS and NEXAFS characterization of the atomic and molecular oxygen species on a vanadium (110) surface

Abstract

The interaction of oxygen with a V(110) surface, in the temperature range of 80–1200 K, has been investigated by using high-resolution electron energy loss spectroscopy (HREELS) and near-edge X-ray absorption fine structure (NEXAFS) techniques. At 80 K, the adsorption of oxygen on V(110) occurs both dissociatively and molecularly. The dissociative adsorption of oxygen is identified by the observation of a ν(VO) vibrational feature at 615 cm −1, which is most likely related to atomic oxygen residing on the quasi-three-fold sites. The molecularly adsorbed oxygen is characterized by a ν(OO) mode at 1025 cm −1 and by a NEXAFS σ∗-resonance at 539.5 eV. Upon heating, the molecularly adsorbed O 2 dissociates on the surface at temperatures below 400 K. Oxygen atoms start to diffuse into the bulk at higher temperatures, as indicated by the observation of subsurface oxygen species in the temperature range of 500–1100 K. The onset of subsurface oxygen species is characterized by a ν(VO) mode at 1050 cm −1. This 1050 cm −1 feature is differentiated from the 1025 cm −1 ν(OO) mode by a different angular-dependence in the off-specular HREELS measurements and by the differences in the NEXAFS spectra. Finally, a vanadium oxide layer can be produced by repeated cycles of dosing at 400 K followed by annealing to 600 K. The stoichiometry of the oxide layer is estimated to be VO, based on the comparison of the NEXAFS spectrum of O V(110) with those of V 2O 3, V 2O 4 and V 2O 5 model compounds.