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

Exploring the Change in Redox Reactivity of UO2 Induced by Exposure to Oxidants in HCO3– Solution

“…Note that there is another inconspicuous satellite peak in the H 2 O 2 and radiation exposure reference samples with the distance toward the U 4f 5/2 peak close to 8.45 eV, indicating that U(V) is present. Moreover, the reported U 4f 5/2 -satellite distance value for pure U 2 O 5 is 7.9 eV . There are two satellite peaks observed in the O 2 , H 2 O 2 , and radiation exposed samples, with the U 4f 5/2 –satellite distance of approximately 5.6 ± 0.15 and 7.95 ± 0.2 eV.…”

“…The U 4f 5/2 −satellite distances for U(VI) are at 4.4 and 9.9 eV, respectively. 26 As can be seen, these satellite peaks are not present in any of the samples indicating the absence of U(VI) in all the measured samples. Full XPS scans of US 1−6 were performed, and the spectra are shown in the Supporting Information (Figures S1−S6).…”

“…As can be seen, the measured U 4f 7/2 and U 4f 5/2 in the reference samples are close to 380.0 and 391.0 eV, respectively, which is in line with previous results. ,, Both peaks shift to higher binding energy after exposure to O 2 , H 2 O 2 , or γ-radiation, indicating that the surface was oxidized. There are four methods that can indicate oxidation states of uranium from XPS spectra: (1) deconvolution of the U 4f 7/2 peak; (2) distance between the U 4f 5/2 peak and its corresponding satellite peaks; (3) peak center and FWHM of the O 1s peak; and (4) peak area ratio between O 2p 3/2 and U 5f. − The present work will utilize all four methods to investigate the oxidation state of uranium. Note that only method 1 was used for quantitative analysis.…”

“…The close peak position of the more intense satellite peaks between the oxidized samples indicates the close stoichiometric ratio of O/U. The U 4f 5/2 –satellite distances for U­(VI) are at 4.4 and 9.9 eV, respectively . As can be seen, these satellite peaks are not present in any of the samples indicating the absence of U­(VI) in all the measured samples.…”

“…For the surface analysis, specimens exposed to either O 2 , H 2 O 2 , or γ-radiation in aqueous solutions containing 10 mM HCO 3 – were used. The XPS and UPS results were analyzed using the following methods: (1) deconvolution of U 4f 7/2 peak; (2) energy difference between the U 4f 5/2 peak and its corresponding satellite peaks; (3) peak center and FWHM of the O 1s peak; (4) peak area ratio between O 2p 3/2 and U 5f. − The observed impact of exposure on the reactivity of UO 2 is discussed in view of the XPS and UPS results.…”