XPS study of oxidation processes of CeOx defective layers

A CeO2 thin film has been subjected to Ar+ 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 Ar+ reduced film shows that it has a stoichiometry close to Ce2O3, being Ce3+ 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 O/Ce ratio and a progressive reoxidation of Ce3+ into Ce4+ 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, Ce3+ 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 CeOx (x<2) structure, while the remaining Ce3+ 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 Ce3+ species at the surface.