XPS study of the surface chemistry of UO₂ (111) single crystal film

A (111) air-exposed surface of UO₂ thin film (150 nm) on (111) YSZ (yttria-stabilized zirconia) before and after the Ar⁺ etching and subsequent in situ annealing in the spectrometer analytic chamber was studied by XPS technique. The U 5f, U 4f and O 1s electron peak intensities were employed for determining the oxygen coefficient k_O = 2 + x of a UO_2+x oxide on the surface. It was found that initial surface (several nm) had k_O = 2.20. A 20 s Ar⁺ etching led to formation of oxide UO_2.12, whose composition does not depend significantly on the etching time (up to 180 s). Ar⁺ etching and subsequent annealing at temperatures 100–380 °C in vacuum was established to result in formation of stable well-organized structure UO_2.12 reflected in the U 4f XPS spectra as high intensity (∼28% of the basic peak) shake-up satellites 6.9 eV away from the basic peaks, and virtually did not change the oxygen coefficient of the sample surface. This agrees with the suggestion that a stable (self-assembling) phase with the oxygen coefficient k_O ≈ 2.12 forms on the UO₂ surface.