Core level spectroscopy of oxidized and reduced CexU1−xO2 materials

Study of the XPS U4f regions indicated the presence of three types of U cations (U6+, U5+, and U4+). U6+ cations are mainly present in the fresh material with high % independent of U concentration (XPS U4f7/2 binding energy at 381.1 eV and associated pairs of satellites at 4.5 and 10.0 eV above each split). Upon sputtering with Ar ions, a large fraction of these U6+ cations are reduced to U5+(XPS U4f7/2 binding energy at 380.4 eV and satellite at 8.2 eV above each split line) then to U4+ cations (XPS U4f7/2 at 379.9 eV and its typical satellite at 6.9 eV above). After five minutes Ar ion sputtering the CexU1−xO2 materials were mainly composed of U4+ cations. On the other hand, XPS Ce3d lines indicated the presence of some contribution of Ce3+ cation in addition to lines characteristic of Ce4+ cations in the as prepared materials. Upon reduction considerable amount of Ce4+ is reduced to Ce3+ (up to 75%) in the mixed oxides while CeO2 alone showed very mild reduction. This is interpreted as due to charge transfer from the U cations further reducing Ce4+ cations. Valence band analyses corroborated the core level results with the reduced U containing mixed oxide showing large contribution of U5f/Ce4f lines while CeO2 alone has small Ce4f contribution even upon Ar ion sputtering.