Co 3 O 4 Epitaxial Formation on CoO(100)
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ABSTRACT Co 30 4 epitaxies grow readily on cobalt monoxide substrates under a wide range of oxidizing pretreatment conditions. The nucleation and growth of Co 30 4 on CoO(100) have been investigated by XPS, HREELS and LEED. Both rocksalt CoO and spinel Co 30 4 share the geometric characteristic of closest packed lattice 02- with oxygen to oxygen distances that match to within 5%. We propose a mechanism whereby the Co2 รท move from octahedral CoO(100) surface sites to bridging 0--0-2, positions that correspond to tetrahedral Co2" sites in the spinel. Oxygen added to the surface finds its way into the near-surface region both as lattice OU, chemically indistinguishable in CoO and Co 30 4 , and as excess surface oxygen. The excess oxygen, which gives an XPS binding energy of 531.6 eV and a HREELS signature at 137 meV (1100 cm-') identifies it as a superoxo species. This species can be added and removed reversibly from the surface by annealing under oxidizing/UHV reducing conditions.
INTRODUCTION CoO is a rocksalt transition metal monoxide with interesting surface properties. Its ability to readily exchange lattice oxygen presents potential applications in gas sensor technology and partial oxidation catalysis"'2 . At low temperatures, the monoxide is antiferromagnetic and a charge-transfer insulator 3, despite its incompletely filled valence band. Cobalt monoxide is often compared to the more widely studied NiO, with which it shares many structural, chemisorption and electronic properties4 . Unlike nickel oxide, however, solid state cobalt oxide has a readily available Co 3" oxidation state. Depending upon ambient oxygen partial pressure and substrate temperature, higher oxides can form at the CoO surface contaminating or otherwise giving mixed-phase oxides. Based on bulk thermodynamic considerations, Co 30 4 is particularly likely to form under ambient oxidizing conditions5-` and could potentially be trapped as a metastable phase on the CoO substrate when temperatures are quenched to lower ionic mobilities. In the present study, CoO(100) single crystal surfaces have been oxidized to produce thin film epitaxies of higher oxides. The films are ordered, as is observed by LEED, but facet upon annealing to produce complex coincidence LEED patterns. XPS indicates an increase in 0 Is lattice oxygen concentration in line with the production of Co30 4 stoichiometry. However an additional "excess oxygen" 0 is peak is also observed upon extended 02 exposure and can be reversibly added to and removed from the near-surface region by adjusting oxygen partial pressures and substrate annealing temperatures. HREEL spectra of the CoO(100) surface, taken as a function of oxygen annealing conditions, show additional phonon peaks attributable to CO30 4 , as well as oxygen adsorbate vibrations. In particular, the "excess oxygen" appears to correlate with a broad HREELS transition at 1100 cm-', attributed here to a superoxo 02-' species, in analogy with IR data9.
163 Mat. Res. Soc. Symp. Proc. Vol. 355 @1995 Materials Research Society
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