Mixed HO/H plasma-induced redox reactions of thin uranium oxide films under UHV conditions.

X-ray photoelectron spectroscopy (XPS) has been used to study the effect of mixed HO/H gas plasma on the surfaces of UO, UO and UO thin films at 400 °C. The experiments were performed under ultra-high vacuum conditions. Deconvolution of the U4f peaks into U(IV), U(V) and U(VI) components revealed the surface composition of the films after 10 min plasma exposure as a function of H concentration in the feed gas of the plasma. Some selected films (unexposed and exposed) were also analysed using grazing-incidence X-ray diffraction (GIXRD). The XPS results show that U(V) is formed as a major product upon 10 minutes exposure of UO by a mixed HO/H plasma in a fairly wide H concentration range. When starting with U(V) (UO), rather high H concentrations are needed to reduce U(V) to U(IV) in 10 minutes. In the plasma induced oxidation of UO, U(V) is never observed as a major product after 10 minutes and it would seem that once U(V) is formed in the oxidation of UO it is rapidly oxidized further to U(VI). The grazing incidence X-ray diffraction analysis shows that there is a considerable impact of the plasma and heating conditions on the crystal structure of the films in line with the change of the oxidation state. This structural difference is proposed to be the main kinetic barrier for plasma induced transfer between U(IV) and U(V) in both directions.