Direct observation of pure pentavalent uranium in U2O5 thin films by high resolution photoemission spectroscopy

Abstract

Thin films of the elusive intermediate uranium oxide U2O5 have been prepared by exposing UO3 precursor multilayers to atomic hydrogen. Electron photoemission spectra measured about the uranium 4f core-level doublet contain sharp satellites separated by 7.9(1) eV from the 4f main lines, whilst satellites characteristics of the U(IV) and U(VI) oxidation states, expected respectively at 6.9(1) and 9.7(1) eV from the main 4f lines, are absent. This shows that uranium ions in the films are in a pure pentavalent oxidation state, in contrast to previous investigations of binary oxides claiming that U(V) occurs only as a metastable intermediate state coexisting with U(IV) and U(VI) species. The ratio between the 5f valence band and 4f core-level uranium photoemission intensities decreases by about 50% from UO2 to U2O5, which is consistent with the 5f 2 (UO2) and 5f 1 (U2O5) electronic configurations of the initial state. Our studies conclusively establish the stability of uranium pentoxide.

Insight into the structure–property relationship of UO2nanoparticles

“…Uranium upon oxidation may form various oxides with mixed oxidation states of U (like U 2 O 5 , U 3 O 7 , U 4 O 9 , U 3 O 8 ). 64,65 The fluorite structure of UO 2 can accommodate a large amount of excess oxygen up to UO 2.25 , therefore XRD, giving information about coherent scattering domains, is generally less sensitive to this kind of alterations. In other words, the XRD patterns of U 4 O 9 and UO 2 are very similar and the presence of those species in UO 2 NPs cannot be detected by To further complicate matters, there is a peak broadening effect for NPs in XRD, making reliable analysis nearly impossible, especially in the case of extremely small NPs.…”

X-Ray and ultraviolet photoelectron spectroscopy studies of Uranium(iv),(v) and(vi) exposed to H2O-plasma under UHV conditions

“…53 . In a previous paper, the differences in the fingerprints of the three oxides when it comes to the U4f main line BE, FWHM and satellites peak position have been identified 38 . Before U 2 O 5 was produced by exposing UO 3 to atomic hydrogen.…”

“…initial state configuration to a final state singlet, associated 5 0 with a initial state 38 . After O 2 -plasma exposure at 500°C 5 1 (green diamonds), the U5f peak intensity diminishes markedly.…”

The nature of the chemical bond in UO2

“…A lot of attention has been focused on theoretical studies of uranium dioxide's electronic structure . Studies of the valence electronic states in UO 2 employ photoelectron (PES) and X‐ray photoelectron spectroscopy (XPS) . Auger O KLL spectrum, O 4,5 (U)‐edge resonant emission spectra, nonresonant and resonant O 4,5 (U) emission spectra, valence electron conversion spectra (CES), X‐ray absorption spectra (XAS), and resonant inversion photoelectron spectra of UO 2 have been studied previously.…”

“…[2][3][4][5][6][7][8][9][10][11][12][13] Studies of the valence electronic states in UO 2 employ photoelectron (PES) [14][15][16][17][18][19][20] and X-ray photoelectron spectroscopy (XPS). [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] Auger O KLL spectrum, [39] O 4,5 (U)-edge resonant emission spectra, [40,41] nonresonant and resonant O 4,5 (U) emission spectra, [42] valence electron conversion spectra (CES), [42] X-ray absorption spectra (XAS), [43,44] and resonant inversion photoelectron spectra [20] of UO 2 have been studied previously.…”