Solvent Directed Anisotropic Growth of Uranium Dioxide Nanoparticles

Nanoparticles are of interest due to their unusual behavior as a result of having a high surface area to volume ratio. The nanoscale size regime results in increased surface energy due to steric strain and increased lattice vacancies. Still, certain surface facets are more stable than others, which are modulated as a function of particle size, leading nanospheres to be the preferred morphology. To stabilize higher energy surface facets and achieve anisotropic nanoparticle morphologies, either one of two approaches is typically employed: 1) a surface binding species is utilized that preferentially binds to certain surface facets over others or 2) trace elemental species are added to the species which cap high energy surface facets and slow their growth rate. While these two approaches have been widely observed for noble metal and to a certain extent transition metal nanoparticles, little research has considered anisotropic growth in underexplored materials such as uranium dioxide. Of particular interest, it is possible that solvent coordination may also play a role in anisotropic growth due to their differential charge distribution. To this end, our initial findings show that in fact solvents can play a role and control anisotropic growth in uranium dioxide. Specifically, we have investigated various high boiling point organic solvents, such as dibenzyl ether, dihexyl ether and dioctyl ether, which have all resulted in different nanoparticle morphologies. Additionally, we have explored the effects of mixed solvents and variable surfactant mixtures with different functional groups including ethers, amines, ketones, aliphatic and aromatic compounds, which therefore sample various surface binding possibilities. In order to probe anisotropic growth pathways, time course reactions using transmission electron microscopy along with small and wide-angle X-ray scattering have been conducted. Overall, this presentation will demonstrate initial and illuminating progress towards morphological control over actinide oxides.