XPS determination of the uranium valences in U3O8

Uranium oxides are tightly related to the whole nuclear fuel cycle in the nuclear industry. However, the understanding of the chemical states in mixed valence uranium oxides remains scattered and deserves more exploration. The current study aims to recognize the valences of uranium in U₃O₈ by a simple X-ray photoelectron spectroscopy (XPS) investigation. U₃O₈ mixed with TiO₂ was photocatalytically reduced under UV or X-ray irradiation, and the chemical states were in-situ determined by XPS. Under UV irradiation, the shoulder peaks at 380.9 eV (U 4f_7/2) and 392.7 eV (U 4f_5/2) gradually increased, and an enhancement in intensity was also observed at 399.6 eV, which is the characteristic of U(V) satellite. Therefore, it was proved that the shoulder peak originated from U(V), and U₃O₈ consists of U(V) and U(VI). Under the irradiation by X-ray, the main lines of U 4f shifted lower by ∼0.8 eV, and a new satellite at ∼397.6 eV appeared and grew with increasing irradiation time, proving the generation of U(IV). This in turn verified that the shoulder peaks in primary U₃O₈ should be U(V) rather than U(IV). Therefore, it was confirmed by the current study that there are two-U(V) and one-U(VI) in each U₃O₈ unit cell.