Interaction of oxygen with ZrC(001) and VC(001): Photoemission and first-principles studies

Abstract

High-resolution photoemission and first-principles density-functional calculations were used to study the interaction of oxygen with ZrC(001) and VC(001) surfaces. Atomic oxygen is present on the carbide substrates after small doses of O2 at room temperature. At 500K, the oxidation of the surfaces is fast and clear features for ZrOx or VOx are seen in the O(1s), Zr(3d), and V(2p32) core levels spectra, with an increase in the metal/carbon ratio of the samples. A big positive shift (1.3-1.6eV) was detected for the C 1s core level in O ZrC(001), indicating the existence of strong O↔C or C↔C interactions. A phenomenon corroborated by the results of first-principles calculations, which show a CZrZr hollow as the most stable site for the adsorption of O. Furthermore, the calculations also show that a C↔O exchange is exothermic on ZrC(001), and the displaced C atoms bond to CZrZr sites. In the O ZrC(001) interface, the surface C atoms play a major role in determining the behavior of the system. In contrast, the adsorption of oxygen induces very minor changes in the C(1s) spectrum of VC(001). The O↔V interactions are stronger than the O↔Zr interactions, and O↔C interactions do not play a dominant role in the O VC(001) interface. In this system, C↔O exchange is endothermic. VC(001) has a larger density of metal d states near the Fermi level than ZrC(001), but the rate of oxidation of VC(001) is slower. Therefore the O ZrC(001) and O VC(001) systems illustrate two different types of pathways for the oxidation of carbide surfaces. © 2005 The American Physical Society.