Dynamics of polar surfaces on ceria nanoparticles observed in situ with single-atom resolution

Atomic hopping processes on ceria nanoparticle surfaces are observed by in situ phase contrast high-resolution electron microscopy with an aberration-corrected imaging lens. It is shown that single-atom resolution is possible, and single-atom dynamics for cerium are observable. Discrete changes in contrast and discrete positional changes of contrast maxima can be safely interpreted as visual fingerprints of atomic displacements. Both single-atom movements and spontaneous sequential relocations of entire atomic rows are observed. Exclusive occurence of the effect on {100} type facets indicates polar dipole field mediated atomic rearrangements, while {111} facets are found to be stable. Molecular modelling confirms that the relocations follow genuine pathways involving partially occupied oxygen-terminated surfaces, by means of temperature induced fluctuations. A series of images tracks the detailed atomic motions over a time of 120 s and quantifies the ratio of reversible atom hopping versus atom ablation. The observation of single-atom movements at the surface of a solid is one of the ultimate goals of microscopy. Using atomic-resolution aberration corrected transmission electron microscopy and exploiting the inherent instability of CeO2 (100) nanoparticle surface facets, a series of images that demonstrate a hopping sequence of Ce atoms could be recorded in 2 s intervals. Important insights into the surface activity of ceria are gained, which are of high relevance for catalysis and other surface-activity related applications. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.