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Acta Phys. -Chim. Sin.  2017, Vol. 33 Issue (10): 2106-2112    DOI: 10.3866/PKU.WHXB201705186
ARTICLE     
Investigation of the Growth Mechanism and Compositional Segregations of Monodispersed Ferrite Nanoparticles by Transmission Electron Microscopy
Wei-Yan LIU1,Ya-Dong LI1,2,Tian LIU1,Lin GAN1,2,*()
1 Division of Energy and Environment, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, Guangdong Province, P. R. China
2 Electron Microscopy Laboratory, Materials and Devices Testing Center, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, Guangdong Province, P. R. China
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Abstract  

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.



Key wordsNanocrystal growth mechanism      Ferrite nanocrystals      Surface segregation      Transmission electron microscopy      Electron energy loss spectroscopy     
Received: 30 March 2017      Published: 18 May 2017
MSC2000:  O642  
Fund:  the Guangdong Natural Science Foundation for Distinguished Young Scholars, China(2016A030306035);Shenzhen Basic Research Program, China(JCYJ20160531194754308)
Corresponding Authors: Lin GAN     E-mail: lgan@sz.tsinghua.edu.cn
Cite this article:

Wei-Yan LIU,Ya-Dong LI,Tian LIU,Lin GAN. Investigation of the Growth Mechanism and Compositional Segregations of Monodispersed Ferrite Nanoparticles by Transmission Electron Microscopy. Acta Phys. -Chim. Sin., 2017, 33(10): 2106-2112.

URL:

http://www.whxb.pku.edu.cn/10.3866/PKU.WHXB201705186     OR     http://www.whxb.pku.edu.cn/Y2017/V33/I10/2106

Fig 1 Morphology, size and compositional evolution of CFO nanoparticles prepared with nominal molar ratio of n(Co(acac)2) : n(Fe(acac)3) = 1 : 2 at different growth stages. (a-d) CFO nanoparticles prepared by thermal decomposition at 200 ℃ for 0.5, 1, 1.5 and 2 h, respectively; (e, f) CFO nanoparticles prepared by thermal decomposition at 200 ℃ for 1 and 2 h, respectively and subsequently heated at 265 ℃ for 1 h; (g) particle size distributions at different stages; (h) XRD patterns of CFO nanoparticles supported on carbon support, and (i) Co compositional evolution at different growth stages. A: atomic fraction; color online.
Fig 2 STEM-EELS elemental mapping of as-prepared CFO nanocrystals (a) 200 ℃ 2 h; (b) 200 ℃ 2 h + 265 ℃ 1 h
Fig 3 Schematic diagram of growth mechanism of ferrite nanocrystals prepared by thermal decomposition.
Fig 4 Preparation of MFO nanoparticles by changing the reaction time at low temperature stage. (a) 200 ℃ for 0.5 h, 265 ℃ for 1 h; (b) 200 ℃ for 1 h, 265 ℃ for 1 h; (c) 200 ℃ for 2 h, 265 ℃ for 1 h; (d) particle size distributions with the average sizes and standard deviations indicated; (e) XRD patterns; (f) high-resolution TEM image of a MFO nanocrystal in (a) oriented along < 111 > zone axis (inset shows the fast Fourier Transform pattern).
Fig 5 (a) STEM-EELS mapping of MFO nanocrystals and (b) EELS spectra of Mn and Fe. The background-subtracted Mn-L2, 3 edge and Fe-L2, 3 edge are modeled with Hartree-Slater30 and double arctan model31, respectively. L3: 2p3/2→3d3/23d5/2; L2: 2p1/2→3d3/2.
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