A kinetic insight into the formation of neptunium(IV) dioxide NpO2 nanocrystals: a multi-technique study using UV-vis absorption spectroscopy and (high-resolution) transmission electron microscopy

UV-vis absorption spectroscopy proves itself as an convenient method for in-situ monitoring the formation and growth of waterborne An(IV) nanoparticles. Dilution of pure aqueous Np(CO3)56- species in ultrapure water leads to the dynamic self-assembling of NpO2 nanocrystals, monitored among others at absorbance 742 nm.

Nowadays, various state-of-the-art spectroscopic techniques (e.g., X-ray spectroscopies, laser-induced and vibrational spectroscopy, etc.) are available to investigate chemical interactions of actinides (An) on a molecular level. Old-fashioned (or conventional) UV-vis absorption spectroscopy is often underestimated as a powerful tool to investigate the chemistry of An, including An(IV) colloids. The present study spotlights the application of UV-vis absorption spectroscopy to studying the colloid system of An(IV). The chemistry of An(IV) in aqueous solution is typical of a small and highly-charged metal ion with a strong hydrolysis tendency leading low solubility, which often result in the underestimation of the migration behavior of An(IV) on the geological disposal of radioactive wastes. The formation mechanisms of An(IV) colloids?, especially under alkaline and near-neutral conditions relevant to the actual environment, are still unexplored even to date. One important issue to be addressed in An(IV) colloid chemistry is the chemical identification of An(IV) colloids. That is, are the colloids formed ill-defined hydroxide precipitate or hydrous oxides, or highly structured clusters/nanoparticles? In order to characterize An(IV) colloids, we investigated in-situ the aggregation of neptunium(IV) colloids formed under ambient aqueous alkaline conditions. The kinetics of the aggregation of Np(IV) colloids and the formation of Np(IV) nanoparticles were tracked by UV-vis absorption spectroscopy, and their morphology and internal structures were further investigated by transmission electron microscopy (TEM). In this study, we demonstrate that UV-vis absorption spectroscopy is an unique and powerful tool for in-situ monitoring of the hydrolysis reaction of Np(IV) and associated colloid/nanoparticle formation. The obtained results will be further discussed by combining with TEM and X-ray absorption spectroscopy.