Understanding the nucleation and growth of nanoparticles in solution plays the key role to control the shape and morphology of the products. The growth mechanism of nanocrystals can be revealed from their microstructural details, such as the presence of pores, twins, and dislocations etc. However, conventional TEM images only show two-dimensional projections of three-dimensional (3D) particles, in which much information is lost in such 2D images. Electron tomography is the only technique which can provide a detailed analysis of the 3D shape and morphology at nanoscale. Here, we have firstly conducted a detailed analysis on CeO₂ single crystals using electron tomography to demonstrate the shape and internal structures, thus revealing the growth mechanism of the CeO₂ in solution. The CeO₂ nanocrystals were synthesized by the hydrothermal method. With the assistance of electron tomography, it is found that the porous CeO₂ nanocrystals have an irregular truncated octahedral shape with internal pores elongated along the <110> directions. Thus, it is concluded that in the hydrothermal process, the oriented attachment of nuclei through a lattice matched surface and subsequent Ostwald ripening results in the growth of CeO₂ nanocrystals with the pores inside. The dominant mechanism for the ripening of nuclei in hydrothermal reactions is the oriented attachment. It is believed that the crystallographic orientation of the pores and corresponding aggregation mechanism for the single crystalline and porous CeO₂ discussed here can also be applied to other oxide materials synthesized by self-assembly in solution.

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