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.