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

The aim of this study is to synthesize PuO₂ nanoparticles (NPs) at low pH values and characterize the materials using laboratory and synchrotron-based methods. Properties of the PuO₂ NPs formed under acidic conditions (pH 1-4) are explored here at the atomic scale. High-resolution transmission electron microscopy (HRTEM) is applied to characterize the crystallinity, morphology and size of the particles. It is found that 2 nm crystalline NPs are formed with a PuO₂ crystal structure. 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 analysed using the theory based on the Anderson impurity model (AIM). The experimental data obtained on NPs show that the Pu(iv) oxidation state dominates in all NPs formed at pH 1-4. However, the suspension at pH 1 demonstrates the presence of Pu(iii) and Pu(vi) in addition to the Pu(iv), which is associated with redox dissolution of PuO₂ NPs under acidic conditions. We discuss in detail the mechanism that affects the PuO₂ NPs synthesis under acidic conditions and compare it with one in neutral and alkaline conditions. Hence, the results shown here, together with the first Pu M₄ HERFD data on PuF₃ and PuF₄ compounds, are significant for the colloid facilitated transport governing the migration of plutonium in a subsurface environment.