The missing pieces of the PuO nanoparticle puzzle.

The nanoscience field often produces results more mystifying than any other discipline. It has been argued that changes in the plutonium dioxide (PuO) particle size from bulk to nano can have a drastic effect on PuO properties. Here we report a full characterization of PuO nanoparticles (NPs) at the atomic level and probe their local and electronic structures by a variety of methods available at the synchrotron, including extended X-ray absorption fine structure (EXAFS) at the Pu L edge, X-ray absorption near edge structure (XANES) in high energy resolution fluorescence detection (HERFD) mode at the Pu L and M edges, high energy X-ray scattering (HEXS) and X-ray diffraction (XRD). The particles were synthesized from precursors with different oxidation states of plutonium (III, IV, and V) under various environmentally and waste storage relevant conditions (pH 8 and pH > 10). Our experimental results analyzed with state-of-the-art theoretical approaches demonstrate that well dispersed, crystalline NPs with a size of ∼2.5 nm in diameter are always formed in spite of diverse chemical conditions. Identical crystal structures and the presence of only the Pu(iv) oxidation state in all NPs, reported here for the first time, indicate that the structure of PuO NPs is very similar to that of the bulk PuO. All methods give complementary information and show that investigated fundamental properties of PuO NPs, rather than being exotic, are very similar to those of the bulk PuO.