XPS study of oxidation processes of CeOx defective layers

A CeO2 thin film has been subjected to Ar+ bombardment at 298 K to induce t he reduction of its outmost layers by preferential removal of oxygen. An XP S study of the altered layer at normal and grazing angle has been carried o ut. The Factor Analysis (FA) of the XPS spectra of this Art reduced film sh ows that it has a stoichiometry close to Ce2O3, being Ce3+ the dominant spe cies at both collection angles. Simultaneously, the Ols spectra depict a la teral peak whose relative intensity is higher for those spectra recorded at grazing angle. Exposure to successive doses of O-2 at 298 K of the reduced layers produces the increase of the O/Ce ratio and a progressive reoxidati on of Ce3+ into Ce4+ as determined by FA of the Ce3d spectra. Simultaneousl y, the lateral component at the Ols peak also decreases, thus discarding th at it is due to surface contamination by -OH or similar species, as previou sly suggested in the literature. After exposure to a high pressure of oxyge n(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 sti ll detected. The lateral Ols component is tentatively attributed to oxygen ions with unusual coordinations in a defective CeOx (x < 2) structure, whil e the remaining Ce3+ ions might be due to fully coordinated species. Enrich ment 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. (C) 2000 Elsevier S cience B.V. All rights reserved.