Correction: Mixed H2O/H2 plasma-induced redox reactions of thin uranium oxide films under UHV conditions.

Correction for 'Mixed H2O/H2 plasma-induced redox reactions of thin uranium oxide films under UHV conditions' by Ghada El Jamal et al., Dalton Trans., 2021, DOI: 10.1039/d1dt01020d.

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 U4f7/2 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.

Time-dependent surface modification of uranium oxides exposed to water plasma.

Thin UO2 films exposed to water plasma under UHV conditions have been shown to be interesting models for radiation induced oxidative dissolution of spent nuclear fuel. This is partly attributed to the fact that several of the reactive oxidizing and reducing species in a water plasma are also identified as products of radiolysis of water. Exposure of UO2 films to water plasma has previously been shown to lead to oxidation from U(iv) to U(v) and (vi). In this work we have studied the dynamics of water plasma induced redox changes in UO2 films by monitoring UO2 films using X-Ray photoelectron Photoemission (XPS) and Ultra-Violet Photoemission (UPS) spectroscopy as a function of exposure time. The surface composition in terms of oxidation states obtained from U4f7/2 peak deconvolution could be retraced along the exposure time, and compared to the valence band. The spectral analysis showed that U(iv) is initially oxidized to U(v) which is subsequently oxidized to U(vi). For extended exposure times it was shown that U(vi) is slowly reduced back to U(v). UPS data show that, unlike the U(v) formed on the surface upon oxidation of U(iv), the U(v) formed upon reduction of U(vi) is localized in the bulk of the film. It also displays a different reactivity than the initially formed U(v). The experiments can be reproduced using a simple kinetic model describing the redox processes involved.

X-Ray and ultraviolet photoelectron spectroscopy studies of Uranium(IV),(V) and(VI) exposed to H2O-plasma under UHV conditions.

Thin films of UO2, U2O5, and UO3 were prepared in situ and exposed to reactive gas plasmas of O2, H2 and H2O vapour produced with an ECR plasma source (electron cyclotron resonance) under UHV conditions. The plasma constituents were analysed using a residual gas analyser mass spectrometer. For comparison, the thin films were also exposed to the plasma precursor gases under comparable conditions. Surface analysis was conducted using X-Ray and ultraviolet photoelectron spectroscopy before and after exposure, by measuring the U 4f, O 1s core levels and the valence band region. The evolution of the peaks was monitored as a function of temperature and time of exposure. After interacting with water plasma at 400 °C, the surface of UO2 was oxidized to a higher oxidation state compared to when starting with U2O5 while the UO3 film displayed weak surface reduction. When exposed to water plasma at ambient temperature, the outermost surface layer is composed of hexavalent uranium in all three cases.