The oxidation of Mo(100) studied by XPS and surface Raman spectroscopy: the onset of MoO sub 2 formation and the formation of surface polymolybdate

Oxidation of the Mo(100) single crystal surface at 700-900K 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 MoO sub 2 requires a threshold oxide thickness between 6 and 8 A, just over one unit cell thick, as indicated by the pronounced appearance of MoO sub 2 features in the surface Raman spectrum. After extensive oxidation at low oxygen pressures the formation of Mo exp 6+ was observed by XPS. Contrary to previous conclusions the Mo exp 6+ does not appear to be due to the formation of MoO sub 3 but is associated with surface polymolybdate species on the oxidized surface. Copyright (c) 1994 Elsevier Science B.V. All rights reserved.