Oxidation properties of uranium have a great influence on the stability of nuclear waste. U(VI) has a 1000 times higher solubility in water than U(IV), and so the dissolution of UO2 based spent fuel and release of matrix contained radionuclides strongly increases with the pre-oxidation of the surface. Incorporation of oxygen into surface UO2 has therefore been a subject of research for many years. Unfortunately the formation of off-stoichiometric phases (UO2+x) by corrosion is hard to differentiate from a non-completed reaction (thermodynamic versus kinetic effects). The strong composition gradient between surface corrosion layers and the unreacted bulk may lead to heterogeneous systems containing U in more than one oxidation state. When replacing bulk samples by thin films of a few atomic layers, diffusion paths are short, equilibrium easily attained and the reaction layer becomes homogeneous.
We present a thin film study of the electronic structure and reactivity of U2O5, which contains uranium at the oxidation state (V). It is well known that oxidation of UO2 leads to mixed valence compounds (U4O9, U3O8) containing U(V). U(V) appears as intermediate state in the oxidation of U(IV) to U(VI). But pure U(V) oxide has never been observed by surface spectroscopies. This may be due to its low stability and the easy of its transformation into one of the neighbouring stable oxides, UO2 or UO3. We will present an X-ray and Ultra-violet photoelectron spectroscopy (XPS and UPS, respectively) study of oxide thin films on Si and Au substrates. Films had a thickness of 2 to 50 monolayers. U2O5 was produced by exposing UO2 to atomic oxygen and UO3 to atomic hydrogen. These reactions mimic corrosion and reduction processes which may occur in nuclear waste repositories.
Determination of the oxidation states was based on the characteristic U-4f core level satellites, separated from the main lines by 6, 8 and 10 eV for U(IV), U(V) and U(VI), respectively. We succeeded in producing films with only one single 8 eV satellite, indicating the presence of pure U(V). U(V) formation was confirmed by the intensity evolution of the U5f valence emissions: the U5f/U4f ratio decreased by about 50% from UO2 and U2O5, which is consistent with the 5f2 (UO2) and 5f1 (U2O5) initial state configurations. Also the linewidth of the XPS 5f valence band spectra decreases from UO2, with the 5f1 final state multiplet configuration, to U2O5, with a 5f0 final state singlet. U-5d spectra show a multiplet structure due to interaction of the 5d9 final state with the localized 5f1 open shell. In UO3 this interaction is missing (the outer shell is empty) and the U-5d line is significantly narrower. Also the apparent spin-orbit splitting of the U-5d emission depends on the surface oxidation state. U2O5 formation will be compared with Np2O5 formation and the absence of Pu2O5 (or PuO2+x).