Electron spectroscopy of single crystal and polycrystalline cerium oxide surfaces

Valence band photoemission (XPS), valence band electron energy loss spectroscopy (EELS), Ce 3d and 4d XPS, O 1s XPS and O 1s X-ray absorption (XAS) have been investigated for oxidized and sputtered single crystal CeO2 films and for oxidized Ce foil. Features were identified that distinguish between the Ce4+ or Ce3+ oxidation states. Ce4+ was identified by the highest binding energy peaks in the Ce 4d spectrum at 122.8 eV and 126.0 eV and lowest energy peak in the O 1s XAS spectrum at 530 eV. Ce3+ was identified by the lowest binding energy peak in the Ce 4d spectrum at 105.4 eV, the Ce 4f valence level peak at 2.0 eV and excitations in the band gap in EELS. The Ce 4d spectra are similar to the Ce 3d spectra in that satellite peaks that are related to the final state occupation of the Ce 4f level dominate the spectra, The interpretation of the Ce 4d spectra is more complicated than that of the Ce 3d spectra, however, due to multiplet splitting which has a greater influence in the Ce 4d spectra. The O 1s XAS are much easier to interpret than XAS spectra from the Ce core levels. The O 1s XAS spectra reflect the unoccupied density of states of the cerium oxide without being significantly affected by final stare relaxation of the Ce energy levels. The O 1s xPS spectra show only a 0.3 eV difference in binding energy between Ce4+ and Ce3+ in the oxides provided the spectra are corrected for band bending. (C) 1998 Published by Elsevier Science B.V. All rights reserved.