Changes in valence, coordination and reactivity that occur upon oxidation of fresh metal surfaces

To promote a greater understanding of the process and nature of metal passivation, we have performed a first-principles analysis of partially oxidized Ni(111) and Ni(311) surface and ultra-thin film NiO layers on Ni(111). We have adopted a bimodal theoretical strategy that considers the oxidation process using either a fixed generalized gradient approximation (GGA) functional for the description of all atoms in the system, or a perturbation approach, that perturbs the electronic structure of various Ni atoms in contact with oxygen by application of the GGA+U technique. This strategy allows us to assess the relative merits of the two approaches, and whether or not the two approaches are at variance with one another as concerns the process of metal passivation. We consider oxygen binding in the cases of isolated atomic adsorption, the development of an oxygen monolayer, and epitaxial NiO(111) monolayers and bilayers with various terminations. Selective application of GGA+U drives structural and charge-transfer processes at the interface, in particular, the octopolar reconstruction of high oxygen coverage pre-passive systems, which, in fact, template an epitaxial NiO(111)-oriented film. These outputs are observable through the development of cationic states in the nickel atoms at the interface, the emergence of a band gap in the projected density of states and in oxygen binding energies that approach the energy of oxide formation.