Monitoring the gradual change in oxidation state during surface oxidation or reduction of uranium oxides by photoemission spectroscopy of the 5f states

Abstract

Photoelectron spectroscopy study of the U5f emission gives valuable insight into the surface oxidation mechanism of uranium oxides. Its intensity is directly related to the electron count nf, which decreases with increasing oxidation number (U(IV): nf=2; U(V): nf=1; U(VI): nf=0). n5f can be quantified by analysing the U5f/U4f intensity ratio and using a standard of known composition. In addition, the 5f emission has a characteristic multiplet shape, directly related to n5f, which can be used to distinguish the 5f2 and 5f1 configuration of U(IV) and U(V), respectively. Three independent methods are used to determine the surface oxidation state: the U5f/U4f intensity ratio, the relative intensities of the U4f oxide shifted peaks, and the O1s/U4f intensity ratio. The first two reveal the concentration of the U in each oxide, the third indicates the total concentration of oxygen. These methods are applied to follow the surface modification of UO2 films when exposed to various oxidative conditions: molecular and atomic oxygen and water plasma at 400°C. In addition, the reduction of UO3 by atomic H is studied. Molecular oxygen oxidizes UO2 to UO2+x(x = 0.22), containing both U(IV) and U(V). Atomic oxygen also oxidizes U(IV) to U(V) at low dosages, but then continues oxidizing U(V) to U(VI) (UO3) at high dosages. Conversely, atomic hydrogen reduces UO3. In the early phase of reduction U(V) forms exclusively – no U(IV) is observed. Water plasma first transforms almost all UO2 (surface and subsurface) into U(V). With further plasma exposure the surface is oxidized to about 80% U(VI) and 20% U(V). Up to this point, a small fraction of U(IV) remains at the surface. Once it disappears, the surface oxidation stops and further water plasma exposure now leads to surface reduction into U(V) (the 5f1 peak increases again). Despite the reduction at high dosage, the O1s/U4f intensity ratio keeps increasing.