ИСТИНА

Uranium dioxide remains one of the most essential uranium compounds due to its application as a nuclear fuel [1]. While bulk UO2 has been intensively studied, it is still not clear if the investigated properties persist the same at the nanoscale [2]. Nanoscale UO2 is readily oxidized with the formation of UO2+x, while the crystal structure does not significantly alter. For U–O systems, the number of stoichiometric binary oxides and solid solutions with various compositions are known. Uranium upon oxidation may form various oxides with mixed oxidation states of U (like U2O5, U3O7, U4O9, U3O8)[2]. The fluorite structure of UO2 can accommodate a large amount of excess oxygen up to UO2.25, therefore XRD, giving information about coherent scattering domains, is generally less sensitive to this kind of alterations. However, even the tiniest oxidation state impurities, which are present in many uranium oxides, may be detected by synchrotron-based high-energy resolution fluorescence detection (HERFD) X-ray absorption spectroscopy at the U M4 edge thanks to the high sensitivity of this technique [3-4]. This contribution will give an overview on the results of U oxide nanoparticle research. The UO2 NPs were synthesized from U(IV) aqueous solution by adding ammonia under reducing conditions. Due to the highly sensitive nature of U(IV) towards oxidation, all synthetic procedures, including the preparation of the samples for the following characterization methods were done in a glovebox under nitrogen atmosphere. The particles were then characterized by microscopic and spectroscopic techniques including high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), HERFD X-ray absorption spectroscopy at the U M4 edge and extended X-ray absorption fine structure (EXAFS) spectroscopy at the U L3 edge. It was found that 2-3 nm crystalline UO2-like NPs are formed and U(IV) is the dominant oxidation state of the freshly prepared UO2 NPs, while they readily oxidized with time and under the X-ray beam [5].