Calculations of oxide formation on low-index Cu surfaces.

Abstract

Density-functional theory is used to evaluate the mechanism of copper surface oxidation. Reaction pathways of O2 dissociation on the surface and oxidation of the sub-surface are found on the Cu(100), Cu(110), and Cu(111) facets. At low oxygen coverage, all three surfaces dissociate O2 spontaneously. As oxygen accumulates on the surfaces, O2 dissociation becomes more difficult. A bottleneck to further oxidation occurs when the surfaces are saturated with oxygen. The barriers for O2 dissociation on the O-saturated Cu(100)-c(2×2)-0.5 monolayer (ML) and Cu(100) missing-row structures are 0.97 eV and 0.75 eV, respectively; significantly lower than those have been reported previously. Oxidation of Cu(110)-c(6×2), the most stable (110) surface oxide, has a barrier of 0.72 eV. As the reconstructions grow from step edges, clean Cu(110) surfaces can dissociatively adsorb oxygen until the surface Cu atoms are saturated. After slight rearrangements, these surface areas form a "1 ML" oxide structure which has not been reported in the literature. The barrier for further oxidation of this "1 ML" phase is only 0.31 eV. Finally the oxidized Cu(111) surface has a relatively low reaction energy barrier for O2 dissociation, even at high oxygen coverage, and allows for facile oxidation of the subsurface by fast O diffusion through the surface oxide. The kinetic mechanisms found provide a qualitative explanation of the observed oxidation of the low-index Cu surfaces.