Detailed spectroscopic studies of oxygen on metal carbide surfaces

Transition metal carbides and nitrides possess remarkable physical properties due to the unusual chemical bonding phenomena that result from the inclusion of a nonmetal atom within the metallic matrix. It has been widely recognized that this unusual situation, which so profoundly affects the physical properties, should also influence the surface chemical behavior. This paper explores the surface chemistry of TiC(100) and VC(100) with oxygen, and the potential impact of surface electronic structure on this chemistry. X-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy (HREELS) were employed to observe the chemical state and specific bonds formed between resultant materials in the interfacial region. We find that oxygen adsorbed dissociatively on both surfaces, but discriminated between them by reacting with TiC to form an oxide while forming a metastable overlayer of VO species on VC. It is proposed that the reaction on TiC was initiated by the preference of oxygen for the carbon atom, due to the predominantly C 2p character of the highest occupied energy level, resulting in evolution of CO x and oxidation of the surface. This reaction was hindered on VC, where the additional electron resides in a predominantly V 3d level. Limited oxidation proceeded on VC with high temperature anneals following room temperature exposure.