Atomic-Scale Valence State Distribution inside Ultrafine CeO2 Nanocubes and Its Size Dependence
Atomic-Scale Valence State Distribution inside Ultrafine CeO2 Nanocubes and Its Size Dependence
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DOI：
10.1002/smll.201802915
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发表时间：
2018-10-18
期刊：
影响因子：
13.3
通讯作者：
Ikuhara, Yuichi
中科院分区：
文献类型：
--
作者：
Hao, Xiaodong;Yoko, Akira;Ikuhara, Yuichi
原子尺度上的阳离子价态分布分析将有助于理解金属氧化物纳米晶体中氧空位（V-O）的本征特征，然而，这仍然是一个巨大的挑战。在这项工作中，分布的铈价态的超细CeO 2纳米立方体（NC）垂直于{100}暴露的小平面逐层使用最先进的扫描透射电子显微镜-电子能量损失谱研究。结果表明，当粒径从11.8 nm变化到5.4 nm时，CeO 2纳米晶内Ce的价态分布随粒径的变化而变化，表明Ce 3+不仅存在于纳米晶的表面层，而且存在于纳米晶的中心层，这与较大的纳米晶形成了鲜明的对比.结合对CeO 2纳米晶内部局域结构的原子尺度分析和V-O形成能的理论计算，阐述了Ce价态分布和晶格膨胀的尺寸效应机理：当尺寸减小到约5 nm时，纳米尺寸效应导致整体晶格膨胀;由于较小尺寸所需的较低形成能，膨胀的晶格有利于V-O的形成，这在原理上提供了对超细CeO 2 NC内Ce 3+的形成和分布的基本理解。
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 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 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 approximate 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 Ce3+ inside ultrafine CeO2 NCs.