Mixed H₂O/H₂ 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 H₂O/H₂ gas plasma on the surfaces of UO₂, U₂O₅ and UO₃ thin films at 400 °C. The experiments were performed in situ under ultra-high vacuum conditions. Deconvolution of the U4f_7/2 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 H₂ 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 UO₃ by a mixed H₂O/H₂ plasma in a fairly wide H₂ concentration range. When starting with U(V) (U₂O₅), rather high H₂ concentrations are needed to reduce U(V) to U(IV) in 10 minutes. In the plasma induced oxidation of UO₂, 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 UO₂ 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.