Paper BI-MoM5
Challenges to Nanoparticle Preparation and Analysis: An Unexpected Phase Transformation of Ceria Nanoparticles

Monday, December 3, 2018, 9:20 am, Room Naupaka Salon 6-7

Nanoparticles in a variety of forms continue to grow in importance for fundamental research, technological and medical applications, and environmental or toxicology studies. Physical and chemical attributes that lead to multiple types of particle instabilities complicate the ability to produce, appropriately characterize, and consistently deliver well-defined particles, frequently leading to inconsistencies, and conflicts in the published literature. In previous work examining 3-5 nm cerium oxide crystallites that had formed ~10 nm soft agglomerates in aqueous media we had observed chemical state changes (the ratio of Ce⁺³/Ce⁺⁴) and related optical absorption changes during particle formation and in response to environmental changes. The transformations have been further examined using micro-X-ray diffraction and Raman spectroscopy. We observed that in response to the environmental changes – adding H₂O₂ to the solution – these particles transformed from a ceria structure to an amorphous complex and returned to the crystalline phase upon solution aging. For comparison, 40 nm ceria nanoparticles were not observed to undergo this transformation and particles made up of crystallites of ~ 8 nm appeared to partially transform (or transform more slowly). We note that ceria nanoparticles of smaller size frequently have beneficial biological effects in comparison to the larger particles. The chemical state changes observed in ceria nanoparticles are usually assumed to be particle size dependent and to involve a change from cubic fluorite-type dioxide (CeO₂) to a hexagonal cerium sesquioxide (Ce₂O₃) with a continuous range of partially reduced CeO_2-x phases, where oxygen vacancies can be rapidly formed, arranged or eliminated. Our XRD and Raman data suggest that a much more complex transformation can occur for smaller ceria crystallites. Such changes were not readily identified by macroscopic in situ measurement such optical measurements or ex situ examination using TEM and XPS but were discovered by examination of ceria nanoparticles with molecularly and structurally sensitive methods with the particles in wet conditions (near in situ). Considering cerium oxide’s useful abilities to scavenge radicals, control the oxygen environment and provide regenerative oxidation state switching, it appears that the ease of ceria nanoparticles to transform between Ce⁴⁺ and Ce³⁺ rich phases is facilitated by small size, but is not constrained to be a transformation between defected and non-defected ceria phases.