Cerium Nanophases from Cerium Ammonium Nitrate

Ceria nanomaterials with facile Ce^III/IV redox behavior are used in sensing, catalytic, and therapeutic applications, where inclusion of Ce^III has been correlated with reactivity. Understanding assembly pathways of CeO₂ nanoparticles (NC-CeO₂) in water has been challenged by "blind" synthesis, including rapid assembly/precipitation promoted by heat or strong base. Here, we identify a layered phase denoted Ce-I with a proposed formula Ce^IV(OH)₃(NO₃)·xH₂O (x ≈ 2.5), obtained by adding electrolytes to aqueous cerium ammonium nitrate (CAN) to force precipitation. Ce-I represents intermediate hydrolysis species between dissolved CAN and NC-CeO₂, where CAN is a commonly used Ce^IV compound that exhibits unusual aqueous and organic solubility. Ce-I features Ce-(OH)₂-Ce units, representing the first step of hydrolysis toward NC-CeO₂ formation, challenging prior assertions about Ce^IV hydrolysis. Structure/composition of poorly crystalline Ce-I was corroborated by a pair distribution function, Ce-L3 XAS (X-ray absorption spectroscopy), compositional analysis, and ¹⁷O nuclear magnetic resonance spectroscopy. Formation of Ce-I and its transformation to NC-CeO₂ is documented in solution by small-angle X-ray scattering (SAXS) and in the solid-state by transmission electron microscopy (TEM) and powder X-ray diffraction. Morphologies identified by TEM support form factor models for SAXS analysis, evidencing the incipient assembly of Ce-I. Finally, two morphologies of NC-CeO₂ are identified. Sequentially, spherical NC-CeO₂ particles coexist with Ce-I, and asymmetric NC-CeO₂ with up to 35% Ce^III forms at the expense of Ce-I, suggesting direct replacement.