A comparative study of the interaction of oxygen with clusters and single-crystal surfaces of nickel

The interaction of oxygen with Ni(100), (110) and (111) surfaces as well as with Ni clusters of varying sizes has been investigated by employing core-level spectroscopy. On single-crystal surfaces, the main oxygen species are O1− and O2− with characteristic O(1s) binding energies of 531 and 530 eV, the percentage of the former being ∼30. At relatively high oxygen exposures, Ni2+ and Ni3+ are found on these surfaces. On the surfaces of small clusters, O1− is the primary product (≥ 50%) and the proportion of the O2− species increases with the cluster size. The small clusters distinctly show that Ni2+ is formed first, followed by Ni3+ at higher exposures. A sequence of transformations occurring on Ni surfaces exposed to oxygen is suggested.