Cerium Nanophases from Cerium Ammonium Nitrate

Abstract

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 CeO 2 nanoparticles (NC-CeO 2 ) 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 (NO 3 )•xH 2 O (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-CeO 2 , 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-CeO 2 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 17 O nuclear magnetic resonance spectroscopy. Formation of Ce−I and its transformation to NC-CeO 2 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-CeO 2 are identified. Sequentially, spherical NC-CeO 2 particles coexist with Ce−I, and asymmetric NC-CeO 2 with up to 35% Ce III forms at the expense of Ce−I, suggesting direct replacement.