Unexpected nanoscale CeO₂ structural transformations induced by ecologically relevant phosphate species

In the present study, the dissolution and microstructural transformation of CeO₂ nanoparticles (NPs) in a phosphate-containing milieu were investigated. The dissolution behaviour of 2 nm and 5 nm CeO₂ NPs in phosphate buffer solutions was found to differ markedly from that observed in 0.01 M NaClO₄. Through synchrotron X-ray diffraction analysis and X-ray absorption spectroscopy, the interaction between CeO₂ NPs and phosphate species was examined, revealing the transformation of the oxide into sodium-cerium double phosphate, with cerium predominantly existing in the Ce(IV) state. According to scanning and transmission electron microscopy observations, thus formed Na-Ce(IV) phosphate consists of spindle-like aggregates of nanocrystalline rods, presumably formed during phosphate anions sorption on the initial CeO₂ surface. Pair distribution function analysis revealed that Na-Ce(IV) phosphate has a three-dimensional framework crystal structure, similar to NaTh₂(PO₄)₃, as reported earlier, with large channels along the c-axis containing disordered sodium atoms. This study represents the first detailed analysis of phosphate-induced speciation and microstructural transformation of CeO₂ NPs, resulting in the formation of Ce(IV) phosphate. Similar processes may occur in natural ecosystems upon the introduction of CeO₂ NPs.