A HREELS AND NEXAFS CHARACTERIZATION OF THE ATOMIC AND MOLECULAR-OXYGEN SPECIES ON A VANADIUM (110) SURFACE

The interaction of oxygen with a V(110) surface, in the temperature ra nge of 80-1200 K, has been investigated by using high-resolution elect ron energy loss spectroscopy (HREELS) and near-edge X-ray absorption f ine structure (NEXAFS) techniques. At 80 K, the adsorption of oxygen o n V(110) occurs both dissociatively and molecularly. The dissociative adsorption of oxygen is identified by the observation of a nu(V-O) vib rational feature at 615 cm(-1), which is most likely related to atomic oxygen residing on the quasi-three-fold sites. The molecularly adsorb ed oxygen is characterized by a nu(O-O) mode at 1025 cm(-1) and by a N EXAFS sigma-resonance at 539.5 eV. Upon heating, the molecularly adso rbed O-2 dissociates on the surface at temperatures below 400 K. Oxyge n atoms start to diffuse into the bulk at higher temperatures, as indi cated by the observation of subsurface oxygen species in the temperatu re range of 500-1100 K. The onset of subsurface oxygen species is char acterized by a nu(V-O) mode at 1050 cm(-1). This 1050 cm(-1) feature i s differentiated from the 1025 cm(-1) nu(O-O) mode by a different angu lar-dependence in the off-specular HREELS measurements and by the diff erences in the NEXAFS spectra. Finally, a vanadium oxide layer can be produced by repeated cycles of dosing at 400 K followed by annealing t o 600 K. The stoichiometry of the oxide layer is estimated to be VO, b ased on the comparison of the NEXAFS spectrum of O/V(110) with those o f V2O3, V2O4 and V2O5 model compounds.