Probing Surface Oxidation of Reduced Uranium Dioxide Thin Film Using Synchrotron Radiation

The work presents a study of the formation of defects created by argon ion sputtering of UO2 thin film surfaces using high-resolution X-ray photoelectron spectroscopy with photon energy of 625 eV. Stable-defected surfaces of UO2-x were monitored by the uranium core level lines found at the low-binding energy side of the U+4 features. The lower end of these states gave binding energies at 377.1 and 387.7 eV attributed to U4f7/2 and U4f5/2 core levels peaks, respectively, for metallic uranium (U#circ#). In between both states (U#circ# and U+4), the clear presence of other electronic states is observed. These lines are attributed to intermediate, metastable states of uranium ions between U#circ# and U+4. Temperature-programmed X-ray photoelectron spectroscopy is performed to observe the oxidation process of these reduced states with increasing temperature. The increase in the U+4 intensity is not correlated with the decrease of U ions signal with lower oxidation states indicating that the limitation is the photoelectron escape depth. In other words, sputtering has created reduced clusters or ridges on the surface of average dimensions larger than the escape depth of the 4f photoelectron with kinetic energy of 225 eV. The activation energy for surface oxidation to U+4 is ∼20 kJ mol-1. This value is close to other values reported for vacancy-type diffusion mechanism in the fluorite structure