Mechanisms of initial oxidation of the Co(0001) and Cr(110) surfaces

Despite the fundamental importance of metal surface oxidation in a number of emerging fields such as catalysis, microelectronics, and biosensor technology, a comprehensive picture of the processes leading to the growth of thin oxide layers at low temperature is still lacking. In this work, we employ advanced first-principles molecular dynamics based on density-functional theory to investigate the early oxidation stages of the Co(0001) and Cr(110) surfaces and observe surprisingly different chemical mechanisms governed by a delicate balance between thermodynamical and kinetic effects On cobalt, oxide nucleation occurs via an early place-exchange of the metal atoms of the surface layer with oxygen atoms of dissociatively adsorbing O-2 molecules Further development of the oxide takes place according to a kinetically constrained reaction path, which leads to pseudoamorphous structures beating evident Co3O4-like magnetic, electronic and structural properties. These results help to rationalize a number of low-temperature experimental data for the (0001) and other Co surface orientations. In contrast, chromium oxidizes along a thermodynamically stable path involving the formation of perfect oxygen ad-layers, which strongly reduce the reactivity of the surface toward further oxygen adsorption. Only the thermally activated extraction of Cr atoms off the metal surface and the unexpected formation of chromate-like precursory structures initiates the experimentally observed layer-by-layer growth of epitaxial Cr2O3 oxide films.