Atomic‐Scale Valence State Distribution inside Ultrafine CeO 2 Nanocubes and Its Size Dependence

Atomic‐scale analysis of the cation valence state distribution will help to understand intrinsic features of oxygen vacancies ( V O ) inside metal oxide nanocrystals, which, however, remains a great challenge. In this work, the distribution of cerium valence states across the ultrafine CeO 2 nanocubes (NCs) perpendicular to the {100} exposed facet is investigated layer‐by‐layer using state‐of‐the‐art scanning transmission electron microscopy‐electron energy loss spectroscopy. The effect of size on the distribution of Ce valence states inside CeO 2 NCs is demonstrated as the size changed from 11.8 to 5.4 nm, showing that a large number of Ce 3+ cations exist not only in the surface layers, but also in the center layers of smaller CeO 2 NCs, which is in contrast to those in larger NCs. Combining with the atomic‐scale analysis of the local structure inside the CeO 2 NCs and theoretical calculation on the V O forming energy, the mechanism of size effect on the Ce valence states distribution and lattice expansion are elaborated: nano‐size effect induces the overall lattice expansion as the size decreased to ≈5 nm; the expanded lattice facilitates the formation of V O due to the lower formation energy required for the smaller size, which, in principle, provides a fundamental understanding of the formation and distribution of Ce 3+ inside ultrafine CeO 2 NCs.