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 ${\mathrm{O}}_{2}$ at room temperature. At $500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the oxidation of the surfaces is fast and clear features for $\mathrm{Zr}{\mathrm{O}}_{x}$ or $\mathrm{V}{\mathrm{O}}_{x}$ are seen in the $\mathrm{O}(1s)$, $\mathrm{Zr}(3d)$, and $\mathrm{V}(2{p}_{3∕2})$ core levels spectra, with an increase in the metal/carbon ratio of the samples. A big positive shift $(1.3--1.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ was detected for the C $1s$ core level in $\mathrm{O}∕\mathrm{Zr}\mathrm{C}(001)$, indicating the existence of strong $\mathrm{O}\ensuremath{\leftrightarrow}\mathrm{C}$ or $\mathrm{C}\ensuremath{\leftrightarrow}\mathrm{C}$ 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 $\mathrm{C}\ensuremath{\leftrightarrow}\mathrm{O}$ exchange is exothermic on ZrC(001), and the displaced C atoms bond to CZrZr sites. In the $\mathrm{O}∕\mathrm{Zr}\mathrm{C}(001)$ 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 $\mathrm{C}(1s)$ spectrum of VC(001). The $\mathrm{O}\ensuremath{\leftrightarrow}\mathrm{V}$ interactions are stronger than the $\mathrm{O}\ensuremath{\leftrightarrow}\mathrm{Zr}$ interactions, and $\mathrm{O}\ensuremath{\leftrightarrow}\mathrm{C}$ interactions do not play a dominant role in the $\mathrm{O}∕\mathrm{V}\mathrm{C}(001)$ interface. In this system, $\mathrm{C}\ensuremath{\leftrightarrow}\mathrm{O}$ 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 $\mathrm{O}∕\mathrm{Zr}\mathrm{C}(001)$ and $\mathrm{O}∕\mathrm{V}\mathrm{C}(001)$ systems illustrate two different types of pathways for the oxidation of carbide surfaces.