Atomic‐Scale Valence State Distribution inside Ultrafine CeO2 Nanocubes and Its Size Dependence

• Published in: Small (Weinheim an der Bergstrasse, Germany) Vol. 14; no. 42; pp. e1802915 - n/a
• Main Authors: Hao, Xiaodong, Yoko, Akira, Chen, Chunlin, Inoue, Kazutoshi, Saito, Mitsuhiro, Seong, Gimyeong, Takami, Seiichi, Adschiri, Tadafumi, Ikuhara, Yuichi
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 18.10.2018
• Subjects: Cations; Ce valence states distribution; cerium oxide; Cerium oxides; Dependence; Electron energy loss spectroscopy; Energy dissipation; Energy transmission; Free energy; Heat of formation; lattice expansion; Lattice vacancies; nanosize effect; Nanotechnology; Scanning electron microscopy; Scanning transmission electron microscopy; Size effects; STEM‐EELS; Surface layers; Transmission electron microscopy; Ultrafines
• ISSN: 1613-6810; 1613-6829
• DOI: 10.1002/smll.201802915

Atomic‐scale analysis of the cation valence state distribution will help to understand intrinsic features of oxygen vacancies (VO) inside metal oxide nanocrystals, which, however, remains a great challenge. In this work, the distribution of cerium valence states across the ultrafine CeO2 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 CeO2 NCs is demonstrated as the size changed from 11.8 to 5.4 nm, showing that a large number of Ce3+ cations exist not only in the surface layers, but also in the center layers of smaller CeO2 NCs, which is in contrast to those in larger NCs. Combining with the atomic‐scale analysis of the local structure inside the CeO2 NCs and theoretical calculation on the VO 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 VO due to the lower formation energy required for the smaller size, which, in principle, provides a fundamental understanding of the formation and distribution of Ce3+ inside ultrafine CeO2 NCs. Atomic‐scale cerium valence state distribution layer‐by‐layer in the surfactant‐modified CeO2 nanocubes is achieved using scanning transmission electron microscopy and electron energy loss spectroscopy. It is demonstrated that the increasing amount of Ce3+ in the center layers of CeO2 nanocube (as the size reduced to approximately 5 nm) is related to the nanosize‐effect induced local lattice expansion.