Unveiling the Stability and Kinetic Pathway from Plutonium Clusters to Colloidal PuO₂ Nanoparticles

The solution stability of plutonium(IV) oxo-hydroxo clusters is a central issue in actinide chemistry, with broad implications for fundamental coordination chemistry, the nuclear fuel cycle, and environmental migration. In this work, we investigate the long-term evolution of the hexanuclear [Pu₆O₄(OH)₄]¹²⁺ cluster stabilized by acetate ligands. Using ultraviolet-visible (UV-vis) absorption spectroscopy coupled with X-ray absorption (XAS) and small-angle X-ray scattering (SAXS) synchrotron techniques, we probed its structural transformation over nearly one year of aging. UV-vis and XAS analyses revealed a progressive loss of the molecular cluster signature accompanied by the emergence of low Pu-Pu coordination environments (CN = 5-6), characteristic of PuO₂ colloidal nanoparticles. In contrast, SAXS measurements indicated a gradual increase in the apparent particle diameter of up to 10-15 nm, which is inconsistent with the formation of compact 2-3 nm PuO₂ nanoparticles usually observed in aqueous solutions. The combined results most probably support a mechanism in which the hexameric cluster first converts into PuO₂ primary colloids, which subsequently assemble into larger aggregates without crystallographic coalescence.