Ultraviolet, x-ray-photoelectron, and electron-energy-loss spectroscopy studies of LaCoO3and oxygen chemisorbed on LaCoO

We report ultraviolet and x-ray-photoelectron spectroscopy and electron-energy-loss spectroscopy results for clean LaCo${\mathrm{O}}_{3}$ and LaCo${\mathrm{O}}_{3}$ surfaces covered with chemisorbed oxygen at room temperature. Also the temperature dependence of the Hell spectrum ($h\ensuremath{\nu}=40.8$ eV) for clean LaCo${\mathrm{O}}_{3}$ was monitored between \ensuremath{\sim}140 and 1050 K. We found that chemisorbed oxygen caused significant changes in the surface electronic structure of LaCo${\mathrm{O}}_{3}$ near the Fermi energy, as did changing temperature. Temperature-dependent electronic properties are expected, based on the known bulk low-spin-high-spin conversion of the trivalent ${d}^{6}$ Co ions, as has been successfully addressed by Goodenough. The temperature dependences we observed, while consistent with Goodenough's bulk model, could also be understood if the ground state of the surface Co ions were reversed from that of the bulk. We suggest that such a reversal could be driven by a small positional (expansion) of the top atomic layers of the material. Finally we discuss the implications of the chemisorption results and the postulated surface relaxation in light of the Voorhoeve picture of the high catalytic activity of LaCo${\mathrm{O}}_{3}$ for CO oxidation.