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 O 2 at room temperature. At 500 K , the oxidation of the surfaces is fast and clear features for Zr O x or V O x are seen in the O ( 1 s ) , Zr ( 3 d ) , and V ( 2 p 3 ∕ 2 ) core levels spectra, with an increase in the metal/carbon ratio of the samples. A big positive shift ( 1.3 – 1.6 eV ) was detected for the C 1 s core level in O ∕ Zr C ( 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 ∕ Zr C ( 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 ( 1 s ) 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 ∕ V C ( 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 ∕ Zr C ( 001 ) and O ∕ V C ( 001 ) systems illustrate two different types of pathways for the oxidation of carbide surfaces.