理解纳米晶的生长机制对单分散纳米晶的可控合成至关重要.本文以热分解法制备的双金属铁氧体(钴铁氧和锰铁氧)纳米颗粒为例,利用透射电子显微镜(TEM)系统研究了铁氧体纳米晶的生长机制,揭示了由此造成的成分偏聚现象.对不同时间阶段的反应产物的分析结果表明,两步加热法(即先后在相对低的温度和相对高的温度下加热反应)是制备高质量的单分散铁氧体纳米晶的关键;通过控制低温反应阶段的时间可实现纳米晶的形核阶段和生长阶段的有效分离,从而有利于单分散纳米晶的合成.利用扫描透射电子显微镜(STEM)及电子能量损失谱(EELS)谱学成像技术分析,我们进一步发现了双金属铁氧体纳米晶中的成分偏聚现象,表明双金属铁氧体纳米晶在形核阶段主要形成富Fe的核芯,而在生长阶段则形成更富Co/Mn的双金属铁氧体壳层.这些结果对制备高质量的单分散铁氧体纳米晶具有重要的指导意义,同时也有助于正确理解热分解法制备的铁氧体纳米晶的表面成分和相关表面物理化学性质.%Understanding the growth mechanism of nanocrystals is crucial for the synthesis of high-quality monodispersed nanoparticles.In contrast to the widely studied growth mechanism of metal nanocrystals,the growth mechanism of metal oxide nanoparticles is still poorly understood.Exemplified by cobalt/manganese ferrite nanoparticles prepared by thermal decomposition,we reveal the growth mechanism and associated compositional segregations of bimetallic metal oxide nanoparticles by using transmission electron microscopy combined with electron energy loss spectroscopy (EELS).We found that a two-stage heating protocol,involving a first-stage heating at a relatively lower temperature followed by a second-stage heating at a relatively higher temperature,is crucial to synthesize monodispersed ferrite nanoparticles.Controlling the reaction time of the first-stage heating can effectively decouple the nucleation stage and growth stage of ferrite nanoparticles,leading to monodispersed nanoparticles with a narrow size distribution.EELS spectrum imaging further reveals previously unreported compositional segregations in the as-prepared ferrite nanoparticles,suggesting that an Fe-rich core formed at the nucleation stage and a Co-/Mn-rich shell formed at the growth stage.Our results provide useful insight into the synthesis of high-quality monodispersed metal oxide nanoparticles as well as a correct understanding of the surface chemistry and related physiochemical properties of spinel oxide nanocrystals prepared by thermal decomposition.
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