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

The stabilization of NiO(111)-oriented thin films by surface hydroxyla tion has been studied by XPS, LEED, AES, and HREELS as a function of s ubstrate temperature. The Ni2+ polar, crystalline films were grown on Ni(100) under low pressures of oxygen gas to a limiting thickness of a pproximately 3 monolayer, and a substantial fraction of the surface ni ckel sites were found to be terminated by adsorbed hydroxyls. The hydr oxyl concentration was estimated to be 0.63-0.84 monolayers relative t o the surface nickel sites, with the concentration somewhat variable f rom film to film. The hydroxyls stabilize the polar NiO(111) surface b y partial compensation of repulsive cation-cation interactions, and th eir removal triggers a phase transition to the more stable NiO(100) or ientation. While some hydroxyls are displaced at lower temperatures, t he majority desorb at 593 K, leaving behind no detectable OHads in eit her XPS or HREELS and creating a concurrent transition from (111) to ( 100) symmetry in LEED.