Controlling the {111}/{110} Surface Ratio of Cuboidal Ceria Nanoparticles.

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 CeO2{110} surfaces and the cube corners were truncated by CeO2{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, CeO2{100}, the cube edges, CeO2{110}, and cube corners, CeO2{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.