Electronic properties of CoO(100) surfaces: Defects and chemisorption

Ultraviolet photoemission spectroscopy, Auger-electron spectroscopy, and low-energy electron diffraction have been used to study the electronic structure of CoO(100) surfaces obtained by cleaving single crystals in ultrahigh vacuum. Two distinct types of surface defect states that can be produced by ${\mathrm{Ar}}^{+}$-ion bombardment are identified. Both types of defects result from oxygen vacancies near the surface. The first type of defect consists primarily of surface oxygen vacancies; it forms after any degree of reduction of the surface and probably exists in small concentration even on cleaved surfaces. Its electronic structure is interpreted in terms of the occupation of exchange- and ligand-field--split d orbitals of Co ions at defect sites. The second type of defect appears only after heavy reduction of the surface and presumably consists of interacting oxygen vacancies or clusters of Co atoms. The interaction of ${\mathrm{O}}_{2}$, CO, and ${\mathrm{H}}_{2}$O with the nearly perfect surface and with surfaces containing one or both types of defects at room temperature has also been investigated. The nearly perfect surface is relatively inert to all three molecules studied, exhibiting either no reaction or minimal chemisorption only at residual defect sites below at least ${10}^{4}$ langmuir exposure. ${\mathrm{O}}_{2}$ initially adsorbs dissociatively at both types of defect site; high ${\mathrm{O}}_{2}$ exposures result in a molecularly adsorbed species. Exposure to CO further reduces surfaces that contain predominantly only the first type of defect, but not the more heavily reduced surfaces that contain both types of defects. ${\mathrm{H}}_{2}$O adsorbs dissociatively on the heavily reduced surface, resulting in adsorbed ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ ions. The results for CoO(100) are compared and contrasted with those obtained previously for NiO(100) and MnO(100).