Precursor Identity and Surfactant Concentration Influence Shape of UO₂ Nanoparticles

A fundamental step toward studying the unique properties of actinide nanomaterials is control over the shape of actinide nanoparticles. Toward this goal, this work demonstrates the effects of precursor identity and surfactant concentration on the shape of uranium dioxide (UO₂) nanoparticles. UO₂ nanoparticles were synthesized by thermal decomposition of different precursors in the presence of oleylamine and oleic acid as surfactants. The size, shape, phase, and chemical composition of the nanoparticles was evaluated using transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), electron diffraction (ED), and U L₃-edge X-ray absorption fine-structure (XAFS) spectroscopy. Anisotropic UO₂ nanocubes of ∼ 4 nm were obtained only with low surfactant concentrations, while increasing the surfactant concentration resulted in formation of nanoparticles with an isotropic, sphere morphology. The importance of precursor identity was also investigated by employing U(hfa)₄, U(acac)₄, UO₂(acac)₂, and UO₂(hfa)₂·H₂O (where hfa = hexafluoracetylacetone and acac = acetylacetone) as precursors. The nanocube morphology was only observed when U(hfa)₄ was used as a precursor. XAS allowed for comparison of the disorder in anisotropic vs isotropic UO₂ nanoparticles.