Acta Physico-Chimica Sinica ›› 2019, Vol. 35 ›› Issue (5): 539-545.doi: 10.3866/PKU.WHXB201806012

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In situ Liquid Environmental TEM Observation of Self-Assembly of Oxide Nanoparticles Driven by Electric Charge

Zhe ZHAO1,Yue LU1,Zhenhua ZHANG2,Manling SUI1,*()   

  1. 1 Beijing University of Technology, Institute of Microstructure and Property of Advanced Materials, Beijing 100124, P. R. China
    2 Hangzhou Dianzi University, Innovative Center for Advanced Materials, Hangzhou 310018, P. R. China
  • Received:2018-05-09 Published:2018-10-19
  • Contact: Manling SUI E-mail:mlsui@bjut.edu.cn
  • Supported by:
    the National Key Research and Development Program of China(2016YFB0700700);the National Natural Science Foundation of China(51621003);the Scientific Research Key Program of Beijing Municipal Commission of Education, China(KZ201310005002)

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Abstract:

The self-assembly of nanoparticles can effectively control morphology and surface exposure during materials processing and has promising potential applications in the synthesis and exploration of different types of materials. As the process often occurs in liquid solvents, it is difficult to observe and study the dynamic process and mechanism of the self-assembly of nanoparticles in situ. With the recent development of environmental transmission electron microscopy (TEM), researchers have been able to observe and study the dynamic processes of various chemical reactions in liquid environments in real time on the nanometer- and atomic scale. In this paper, we used advanced in situ liquid environmental TEM to characterize the self-assembly process of Co3O4 nanorods in water and study the mechanism of self-assembly. A self-assembly phenomenon of the Co3O4 nanorods in water was discovered under the influence of electron beam irradiation that caused a change in the dielectric constant and increased the electrical conductivity in the irradiated water. The movement of the nanorods was initiated and energized in the irradiated water. As the distance between the nanorods decreased, the drift velocity of the nanorods and the interaction force between them increased. By analyzing the nanorods' movement (including the distance and the rotation angle) as a function of time, the variation of the mean square displacement with respect to time was obtained. Based on the Stokes-Einstein equation, the driving force was estimated, and we found that the driving force increases with the decreasing distance and that the maximum value of the driving force is approximately 12 pN. By characterizing the morphology of the Co3O4 nanorods, we found that the exposed crystal facets of the nanorods are mostly in the {100}, {110}, and {111} planes. As Co3O4 is a polar metal oxide, these crystalline planes tend to carry certain electric charge according to the terminative Co or O atoms. In a conductive aqueous environment, it is because of these easily exposable surfaces with different surface charges that the Co3O4 nanorods will attract each other under the driving force of opposite electric charges. Furthermore, self-assembly of nanorods with a complementary morphology can be driven quickly. The polarity of the residual surface charge plays a decisive role in the effective assembly of nanorods. It is already known that surface charge compensation takes place on polar crystal surfaces; however, this work provides a deeper understanding of the incomplete nature of this surface charge compensation. The results presented here may provide important experimental data and theoretical reference for artificially regulating the metal oxide self-assembly process.

Key words: Cobalt oxide, Nanorod, Self-assembling, In situ, Liquid environmental TEM