To form or not to form: PuO2 nanoparticles at acidic pH

Properties of the PuO2 nanoparticles (NPs) formed under acidic conditions (pH 1-4) are explored in this study at the atomic scale. High-resolution transmission electron microscopy (HRTEM) was applied to characterize the crystallinity, morphology and size of the precipitates. It was found that 2 nm crystalline NPs are formed with a crystal structure similar to bulk PuO2. High energy resolution fluorescence detected (HERFD) X-ray absorption spectroscopy at the Pu M4 edge has been used to identify the Pu oxidation states and recorded data are analyzed using the theory based on the Anderson Impurity Model (AIM). The experimental data obtained on NPs show that Pu(IV) oxidation state dominates in all NPs formed at pH 1-4. However, the suspension at pH 1 demonstrate the presence of Pu(III) in addition to the Pu(IV), which is associated with redox dissolution of PuO2 NPs under acidic conditions. As shown by UV-Vis the contribution of Pu(III) in the HERFD spectrum for the sample at pH 1 originates from the solution rather than from NPs themselves. We discuss in detail the mechanism that affects the PuO2 NP synthesis under acidic conditions and compare it with one in neutral and alkaline conditions. Future investigations of plutonium chemistry will fundamentally benefit from the insights this work provides and further applied to various environmental and technological applications