Theoretical study of O chemisorption on NiO. Perfect surfaces and cation vacancies

Self-consistent unrestricted Hartree-Fock calculations are presented for the adsorption of atomic oxygen onto the nickel oxide (100) surface. The perfect surface is shown to be nonreactive, but enhanced binding is found for vacancy sites in the first two atomic layers. Of particular interest is a second-layer anion-vacancy site. Various charge states are examined and charge transfer between adsorbate and surface are noted. A calculation for molecular oxygen in the neighborhood of a second-layer anion vacancy is also presented. ${\mathrm{O}}_2$ is found to bind to the surface less strongly than atomic oxygen, and accordingly a dissociation site is postulated. Analysis of the character of the electrons on the transition-metal atom indicates that bonding is localized in the $s$- and $p$-character electrons rather than in the $d$ electrons.