Abstract
The nanoscience field often produces results more mystifying than any other discipline. It has been argued that changes in the plutonium dioxide (PuO2) particle size from bulk to nano can have a drastic effect on PuO2 properties. Here we report a full characterization of PuO2 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 L3 edge, X-ray absorption near edge structure (XANES) in high energy resolution fluorescence detection (HERFD) mode at the Pu L3 and M4 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 PuO2 NPs is very similar to that of the bulk PuO2. All methods give complementary information and show that investigated fundamental properties of PuO2 NPs, rather than being exotic, are very similar to those of the bulk PuO2.
Highlights
Plutonium (Pu) is one of the most complex and fascinating chemical elements in the periodic table.[1]
We report a full characterization of PuO2 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 L3 edge, X-ray absorption near edge structure (XANES) in high energy resolution fluorescence detection (HERFD) mode at the Pu L3 and M4 edges, high energy X-ray scattering (HEXS) and X-ray diffraction (XRD)
The synthesized PuO2 NPs are characterized by a variety of experimental methods: high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES) in high energy resolution fluorescence detection (HERFD) mode, high energy X-ray scattering (HEXS), X-ray diffraction (XRD)
Summary
Pu has at least six oxidation states – from Pu(II)[3] to Pu(VII) – under certain conditions. In environmentally relevant conditions Pu may exist and even co-exist in four of them, from Pu (III) to Pu(VI).[4,5] Pu(IV) strongly hydrolyses in aqueous solutions resulting in the formation of polynuclear species, intrinsic colloids that may form a stable suspension or precipitates.[6,7] The speciation of Pu in aqueous solutions due to the complexity of its chemical behaviour (solubility and redox reactions8,9) combined with a great variety of environmental substrates (ligands, mineral interfaces, bacteria, solids, etc.) makes Pu. Nanoscale geochemistry very complicated. While pH 8 is more typical for far-field of nuclear waste disposal, an alkaline pH (>10) is possible in alkaline nuclear waste tanks and can be reached in cement nearfield environments.[20] The synthesized PuO2 NPs are characterized by a variety of experimental methods: high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES) in high energy resolution fluorescence detection (HERFD) mode, high energy X-ray scattering (HEXS), X-ray diffraction (XRD).
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