Cerium is one of the most fundamentally fascinating and technologically important lanthanides, due to its facile redox behavior and luminescence. Synthesis of Ce^IVO₂ nanoparticles with high Ce^III content on the surface plus accompanying oxygen vacancies is targeted for both catalytic and biomedical applications. Ce^IV is the primary driver for hydrolysis-promoted self-assembly due to higher acidity, challenging inclusion of Ce^III in nucleation processes and incorporation into growing frameworks. I will briefly present two fundamental studies; 1) understanding non-spherical growth mechanisms of Ce sulfate clusters, and 2) understanding spherical growth of CeO₂ nanoparticles from cerium ammonium nitrate (CAN).

In the presence of various charge-balancing cations, cerium sulfate rapid self-assembles into ring-shaped clusters containing 70 cerium polyhedra (Ce₇₀),. If the charge-balancing cation is ammonium, an incomplete ring (Ce₆₂) forms instead. We have used crystallography for structure determination to identify fragments of the Ce₇₀-ring, and small-angle X-ray scattering and pair distribution function (respectively SAXS and PDF) to track assembly pathways in cerium sulfate solids and solutions. Various intricate frameworks of Ce₇₀ have been isolated, and ion-exchange studies probe redox activity of Ce within these frameworks, as well determine access to the abundant pore space.

CAN is a unique Ce^IV-source, soluble in both aqueous and organic media. Prior studies suggested CAN is an oxo-linked dimer in solution, which also provides a starting point to its conversion to nano-CeO₂. We are investigating the influence of different solution electrolytes on its assembly pathways using SAXS, X-ray absorption near-edge spectroscopy (XANES) and transmission electron microscopy (TEM). SAXS suggests that alkalis inhibit growth of larger clusters, and also drive precipitation of the particles, providing one means of size control. While Li⁺ and Cs⁺ promote large particle growth followed by precipitation, K⁺ retains solubility of the nanoclusters; presenting a unique solubility trend. Alkalis added as chloride salts (LiCl) yields solutions containing CeO₂ nanoparticles and abundant Ce³⁺, via reduction by the chloride. Yet forced precipitation with CsCl shows the Ce³⁺ remains in solution, meaning there remains a challenge for simple aqueous routes to form nano-CeO₂ with strategically incorporated Ce³⁺. These and other effects of simple cations and anions will be presented.

Ian Colliard

Appearances