Valence-band electronic structure ofCo3O4epitaxy on CoO(100)

Valence-band photoemission studies have been performed on a $\mathrm{CoO}\mathrm{}\left(100\right)\mathrm{}$ single crystal as it is slowly oxidized under $1\times {10}^{-4}\mathrm{Torr}$ ${\mathrm{O}}_2$ at 623 K, eventually forming a ${\mathrm{Co}}_3{\mathrm{O}}_4$ epitaxial film. The most significant changes occur in $3d$-related features, with the peak located at the top of the valence band sharpening and shifting to lower binding energies as the spinel oxide forms. Constant initial-state measurements indicate that ${\mathrm{Co}}_3{\mathrm{O}}_4$ contains admixture from neighboring O $2p$ in its $3d$ band, as observed for CoO and other monoxide charge-transfer insulators. Unlike the rocksalt monoxides, which have only a single type of cobalt $\left({\mathrm{Co}}^{2+}\right)$ located in octahedral lattice sites, the spinel ${\mathrm{Co}}_3{\mathrm{O}}_4$ has both octahedral ${\mathrm{Co}}^{3+}$ and tetrahedral ${\mathrm{Co}}^{2+}$ sublattices. The peak at the top of the ${\mathrm{Co}}_3{\mathrm{O}}_4$ valence band results from the ${3d}^{6}Ḻ$ final state of the octahedral ${\mathrm{Co}}^{3+}$ $3d$ band. Although some states derived from tetrahedral ${\mathrm{Co}}^{2+}$ may be present at the top of the valence band, the greatest contribution from the tetrahedral ${\mathrm{Co}}^{2+}$ sublattice appears at approximately -3.8 eV, overlapping with O $2p$ derived features of the spectrum. The ${\mathrm{Co}}^{3+}$ ${3d}^{n-1}$ satellite is much less intense in ${\mathrm{Co}}_3{\mathrm{O}}_4$ than in CoO, as is observed for the analogous structure in the cobalt $2p$ core spectra. An oxygen $2p$-derived structure remains fairly constant throughout the oxidation process, with the exception of an intermediate species, which imparts a broad, humplike appearance centered at -5.3 eV to the O $2p$ region and disappears as oxidation to ${\mathrm{Co}}_3{\mathrm{O}}_4$ is completed. The origin of the feature is not clear; however it is most likely due either to an adsorbate or to a defectlike intermediate in the oxidation process.