XPS study of oxidation processes of CeO defective layers

A CeO2 thin film has been subjected to Arq bombardment at 298 K to induce the reduction of its outmost layers by preferential removal of oxygen. An XPS study of the altered layer at normal and grazing angle has been carried out. The Factor Analysis FA.of the XPS spectra of this Arq reduced film shows that it has a stoichiometry close to Ce2O3, being Ce3q the dominant species at both collection angles. Simultaneously, the O1s spectra depict a lateral peak whose relative intensity is higher for those spectra recorded at grazing angle. Exposure to successive doses of O2 at 298 K of the reduced layers produces the increase of the OrCe ratio and a progressive reoxidation of Ce3q into Ce4q as determined by FA of the Ce3d spectra. Simultaneously, the lateral component at the O1s peak also decreases, thus discarding that it is due to surface contamination by –OH or similar species, as previously suggested in the literature. After exposure to a high pressure of oxygen ca. 1 Torr., the XPS spectrum obtained at a normal collection angle shows an almost complete oxidation of the film to CeO2. However, in the spectrum at grazing angle, Ce3q species and the lateral component of oxygen are still detected. The lateral O1s component is tentatively attributed to oxygen ions with unusual coordinations in a defective CeO x-2. x structure, while the remaining Ce3q ions might be due to fully coordinated species. Enrichment of the surface of the defective cerium oxide with these oxygen species seems to be a result of the same structural rearrangements that favour the observed stabilization of Ce3q species at the surface. q2000 Elsevier Science B.V. All rights reserved.