Impact of particle size, oxidation state and capping agent of different cerium dioxide nanoparticles on the phosphate-induced transformations at different pH and concentration

by Isabella Römer, Sophie Marie Briffa, Yadira Arroyo Rojas Dasilva, Dimitri Hapiuk, Vanessa Trouillet, Richard E. Palmer, Eugenia Valsami-Jones

The potential hazard posed by nanomaterials can be significantly influenced by transformations which these materials undergo during their lifecycle, from manufacturing through to disposal. The transformations may depend on the nanomaterials’ own physicochemical properties as well as the environment they are exposed to. This study focuses on the mechanisms of transformation of cerium oxide nanoparticles (CeO₂ NPs) in laboratory experiments which simulate potential scenarios in which the NPs are exposed to phosphate-bearing media. We have experimented with the transformation of four different kinds of CeO₂ NPs, in order to investigate the effects of nanoparticle size, capping agent (three were uncapped and one was PVP capped) and oxidation state (two consisted mostly of Ce⁴⁺ and two were a mix of Ce^3+/Ce⁴⁺). They were exposed to a reaction solution containing KH₂PO₄, citric acid and ascorbic acid at pH values of 2.3, 5.5 and 12.3, and concentrations of 1mM and 5mM. The transformations were followed by UV-vis, zeta potential and XRD measurements, which were taken after 7 and 21 days, and by transmission electron microscopy after 21 days. X-ray photoelectron spectroscopy was measured at 5mM concentration after 21 days for some samples. Results show that for pH 5 and 5mM phosphate concentration, CePO₄ NPs were formed. Nanoparticles that were mostly Ce⁴⁺ did not dissolve at 1mM reagent concentration, and did not produce CePO₄ NPs. When PVP was present as a capping agent it proved to be an extra reducing agent, and CePO₄ was found under all conditions used. This is the first paper where the transformation of CeO₂ NPs in the presence of phosphate has been studied for particles with different size, shapes and capping agents, in a range of different conditions and using many different characterisation methods.