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.