Insight into the structure-property relationship of UO$_{2}$ nanoparticles

Highly crystalline UO$_{2}$ nanoparticles (NPs) with sizes of 2–3 nm were produced by fast chemical deposition of uranium(IV) under reducing conditions at pH 8–11. The particles were then characterized by microscopy and spectroscopy techniques including high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), high-energy resolution fluorescence detection (HERFD) X-ray absorption spectroscopy at the U M$_{4}$ edge and extended X-ray absorption fine structure (EXAFS) spectroscopy at the U L$_{3}$ edge. The results of this investigation show that despite U(IV) being the dominant oxidation state of the freshly prepared UO$_{2}$ NPs, they oxidize to U$_{4}$O$_{9}$ with time and under the X-ray beam, indicating the high reactivity of U(IV) under these conditions. Moreover, it was found that the oxidation process of NPs is accompanied by their growth in size to 6 nm. We highlight here the major differences and similarities of the UO$_{2}$ NP properties to PuO$_{2}$, ThO$_{2}$ and CeO$_{2}$ NPs.