THE OXIDATION OF MO(100) STUDIED BY XPS AND SURFACE RAMAN-SPECTROSCOPY - THE ONSET OF MOO2 FORMATION AND THE FORMATION OF SURFACE POLYMOLYBDATE

Oxidation of the Mo(100) single crystal surface at 700-900 K with molecular oxygen has been studied in ultrahigh vacuum using X-ray photoelectron spectroscopy (XPS) and surface Raman spectroscopy. Whereas the onset of oxide formation was previously found to occur at oxygen coverages just over one monolayer, the nucleation of crystalline MoO2 requires a threshold oxide thickness between 6 and 8 angstrom, just over one unit cell thick, as indicated by the pronounced appearance of MoO2 features in the surface. Raman spectrum. After extensive oxidation at low oxygen pressures the formation of Mo6+ was observed by XPS. Contrary to previous conclusions the Mo6+ does not appear to be due to the formation of MoO3 but is associated with surface polymolybdate species on the oxidized surface.