Valence-band and core photoelectron spectroscopic studies of molybdenum aqueous oxidation and the influence of argon-ion etching

Abstract

The behaviour of a polished molydenum metal surface upon exposure to both aerated and deaerated water and 1.0 mol dm^–3 sodium chloride solution, in a previously described anaerobic cell, has been studied by X-ray photoelectron spectroscopy (XPS). No oxidation was found on metal samples exposed to deaerated solutions. Exposure to aerated sodium chloride solution also produced no oxide formation. However, exposing the polished metal surface to aerated water produced significant oxidation, in agreement with previous results. The valence band studies complement the core-level data, as well as providing information not available in the core spectra. We find that metal surfaces cleaned by argon-ion etching rather than by mechanical polishing of the molybdenum are inert to oxidation by aerated water. These etched surfaces show differences in the valenceband region that are consistent with the generation of a defect molybdenum structure, possibly containing oxygen. This defect structure, generated by argon-ion etching, appears to passivate the metal. The valenceband data were interpreted by calculated spectra generated by multiple-scattered-wave X-α cluster calculations.