A detailed morphological and structural analysis of CeO 2 nanoparticles has been performed using electron tomography in scanning transmission mode in high angle annular dark field. The nanoparticles have been prepared through a solvothermal synthesis assisted by microwave heating. An adequate choice of the synthesis parameters leads to particles with various well-defined morphologies: cubes, octahedrons, and nanorods. In the case of cubic CeO 2 nanoparticles, the three-dimensional analysis allowed us to precisely calculate the type and the proportion of the minor facets exposed at the nanoparticle surface. For the CeO 2 nanoparticles with an octahedron shape, it has been demonstrated that the ambiguous interpretation of the objects giving triangular views in classical transmission electron microscopy can be prevented; furthermore, precise assignments of their external shape, surface crystallography, and type of minor facets were realized. In the case of nanorods, it was shown that the external shape and the transversal symmetry are strongly dependent on the nanorod sizes. The presence of a well-defined porosity inside the rods was also evidenced thanks to the ability of the electron tomography to solve the internal structure of a nano-object.

The ability to control the size and morphology is crucial in optimizing nanoceria catalytic activity as this is governed by the atomistic arrangement of species and structural features at the surfaces. Here, we show that cuboidal cerium oxide nanoparticles can be obtained via microwave-assisted hydrothermal synthesis in highly alkaline media. High-resolution transmission electron microscopy (HRTEM) revealed that the cube edges were truncated by CeO 2 {110} surfaces and the cube corners were truncated by CeO 2 {111} surfaces. When adjusting synthesis conditions by increasing NaOH concentration, the average particle size increased. Although this was accompanied by an increase of the cube faces, CeO 2 {100}, the cube edges, CeO 2 {110}, and cube corners, CeO 2 {111}, remained of constant size. Molecular dynamics (MD) was used to rationalize this behavior and revealed that energetically, the corners and edges cannot be atomically sharp, rather they are truncated by {111} and {110} surfaces, respectively, to stabilize the nanocube; both the experiment and simulation showed agreement regarding the minimum size of ∼1.6 nm associated with this truncation. Moreover, HRTEM and MD revealed {111}/{110} faceting of the {110} edges, which balances the surface energy associated with the exposed surfaces, which follows {111} > {110} > {100}, although only the {110} surface facets because of the ease of extracting oxygen from its surface and follows {111} > {100} > {110}. Finally, MD revealed that the {100} surfaces are "liquid-like" with a surface oxygen mobility 5 orders of magnitude higher than that on the {111} surfaces; this arises from the flexibility of the surface species network that can access many different surface arrangements because of very small energy differences. This finding has implications for understanding the surface chemistry of nanoceria and provides avenues to rationalize the design of catalytically active materials at the nanoscale.

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Gold nanoparticles of various sizes, supported onto ceria, were synthesized using both deposition-precipitation and coprecipitation methods. Whatever the size, the study of the Au/CeO 2 interface confirms the existence of two preferential orientation relationships with a dislocation network which compensates the 25% interfacial lattice mismatch. Behaviors of supported gold nanostructures under the electron beam were examined by high-resolution transmission electron microscopy (HRTEM). The thermal stability of the gold nanostructures was found to be strongly affected by the particle size. For small nanostructures (<5 nm), reversible shrinkage of gold layer by layer onto ceria surface was observed. For larger ones, a progressive and irreversible encapsulation by a CeO 2-x layer was found and a four-step process was experimentally underlined.