Oxygen transport in unreduced, reduced and Rh(III)-doped CeO2 nanocrystals

Ceria, CeO2, based materials are a major (active) component of exhaust catalysts and promising candidates for solid oxide fuel cells. In this capacity, oxygen transport through the material is pivotal. Here, we explore whether oxygen transport is influenced (desirably increased) compared with transport within the bulk parent material by traversing to the nanoscale. In particular, atomistic models for ceria nanocrystals, including perfect: CeO2; reduced: CeO1.95 and doped: Rh0.1Ce0.9O1.95, have been generated. The nanocrystals were about 8 nm in diameter and each comprised about 16,000 atoms. Oxygen transport can also be influenced, sometimes profoundly, by microstructural features such as dislocations and grain-boundaries. However, these are difficult to generate within an atomistic model using, for example, symmetry operations. Accordingly, we crystallised the nanocrystals from an amorphous precursor, which facilitated the evolution of a variety of microstructures including: twin-boundaries and more general grain-boundaries and grain-junctions, dislocations and epitaxy, isolated and associated point defects. The shapes of the nanocrystals are in accord with HRTEM data and comprise octahedral morphologies with {111} surfaces, truncated by (dipolar) {100} surfaces together with a complex array of steps, edges and corners. Oxygen transport data was then calculated using these models and compared with data calculated previously for CeO1.97/ YSZ thin films and the (bulk) parent material, CeO197. Oxygen transport was calculated to increase in the order: CeO2 nanocrystal < (reduced) CeO1.95 nanocrystal approximately Rh0.1Ce0.9O1.95 nanocrystal < CeO1.97/YSZ thin film < (reduced) CeO1.97 (bulk) parent material; the mechanism was determined to be primarily vacancy driven. Our findings indicate that reducing one- (thin film) or especially three- (nanocrystal) dimensions to the nanoscale may prove deleterious to oxygen transport. Conversely, we observed dynamic evolution and annihilation of surface vacancies via surface oxygens migrating to the bulk of the nanocrystal; the vacancies left are then filled by other oxygens moving to the surface. Coupled with previous simulation studies, in which we calculated that oxygen extraction from the surface of a ceria nanocrystal was energetically easier compared with the bulk surface, our calculations predict that ceria nanocrystals would facilitate effective oxidative catalysis. This study describes framework simulation procedures, which can be used in partnership with experiment, to explore transport in nanocrystalline ionic systems, which include complex microstructures. Such data can provide predictions for experiment or help reduce the number of experiments required.