Surface electronic structure of NiO: Defect states,O2andH2O interactions

The electronic structure of surface defects on NiO(100) and their interaction with ${\mathrm{O}}_2$ and ${\mathrm{H}}_2$O have been studied on single crystals cleaved in ultrahigh vacuum by using ultraviolet and x-ray photoemission and Auger spectroscopies, as well as low-energy-electron diffraction. Defects created by inert ion bombardment of the NiO(100) surface are associated with anion vacancies. These result in $F_s$ centers when isolated, and in the formation of a 1.5-eV-wide surface conduction band as the defect density increases. The satellites, which appear at approximately 7 eV from each of the Ni photoemission peaks in NiO, decrease in amplitude as the density of anion vacancies increases. This result is in agreement with the identification of these satellites as resulting from unscreened emission. Oxygen does not interact with the perfect surface, but for small exposures it chemisorbs dissociatively at defect sites, rapidly depopulating the band-gap states associated with the defects. This phase of adsorption is essentially complete after 10 L of exposure (1 L = 1 langmuir≡${10}^{\mathrm{-}6}$ Torr sec). A second phase having a small sticking coefficient continues to adsorb up to ${\mathrm{O}}_2$ exposures of ${10}^9$ L. The photoemission spectra of this second phase differ distinctly from those of both lattice oxygen and molecular ${\mathrm{O}}_2$, and we tentatively identify this phase as ${\mathrm{O}}_2$ ${\mathrm{}}^{2\mathrm{-}}$. This assignment is compatible with previous interpretations of intermediate phases of oxygen adsorption on metals at low temperatures and with electron-energy-loss data from other oxides. ${\mathrm{H}}_2$O adsorption on NiO takes place only in the presence of nonlattice oxygen. The resultant adsorption is dissociative, leading to the formation of ${\mathrm{OH}}^{\mathrm{-}}$ radicals on the surface.