STABILIZATION OF NIO(111) THIN-FILMS BY SURFACE HYDROXYLS

The stabilization of NiO(111)-oriented thin films by surface hydroxylation has been studied by XPS, LEED, AES, and HREELS as a function of substrate temperature. The Ni2+ polar, crystalline films were grown on Ni(100) under low pressures of oxygen gas to a limiting thickness of approximately 3 monolayer, and a substantial fraction of the surface nickel sites were found to be terminated by adsorbed hydroxyls. The hydroxyl concentration was estimated to be 0.63-0.84 monolayers relative to the surface nickel sites, with the concentration somewhat variable from film to film. The hydroxyls stabilize the polar NiO(111) surface by partial compensation of repulsive cation-cation interactions, and their removal triggers a phase transition to the more stable NiO(100) orientation. While some hydroxyls are displaced at lower temperatures, the majority desorb at 593 K, leaving behind no detectable OHads in either XPS or HREELS and creating a concurrent transition from (111) to (100) symmetry in LEED.