On the origin of the substrate-induced oxidation of Ni/NiO(001) studied by X-ray Photoelectron Spectroscopy and Molecular Dynamics Simulations

Metallic Ni, vapor-deposited on NiO(001) near room temperature, could be gradually oxidised upon annealing between 800 K and 940 K in Ultra High Vacuum (UHV), as evidenced by X-ray Photoelectron Spectroscopy for initial Ni coverage of 1.6, 3.8 and 7.5 equivalent monolayers (ML). The time dependence of the oxidation process was consistent with a diffusion mechanism, supplying oxygen via the NiO crystal to a coalesced particulate deposit and resulting in an oxide shell, which grew over the entire surface and enclosed a shrinking metallic core. Similar to the well known behaviour upon gas phase oxidation, the process was fast within a depth of two atomic layers of Ni, limited by the diffusive supply of oxygen from the substrate. Molecular Dynamics Simulations for 0.06, 0.11 and 0.22 ML of Ni ions deposited on a model NiO(001) substrate indicated the formation of NiO islands via oxygen ions transferred from the surface and near-surface layers of the crystal. A significant atomic concentration of oxygen vacancies of the order of 10 to 20% could be created in each underneath layer, before the next one started donating lattice anions. This suggests a possible explanation for the aforementioned NiO-substrate-induced oxidation of deposited Ni, whereby the formation of oxygen vacancies inside the crystal supplies the necessary oxygen.