Colloidal nanoparticle growth in liquid environments follows the mechanism of either addition of monomers to the existing nanoparticles or coalescence of the existing nanoparticles. With respect to the coalescence of nanoparticles, the details regarding the attachment and fusion between nanoparticles have not been studied due to the lack of effective in-situ probing technique.
Two research groups in the Lawrence Berkeley National Laboratory reported recently their pioneering research results, which have been published in the May 25 issue of Science. Both groups used the home-built liquid cells to directly observe the nanoparticle formation and coalescence in liquid solvents with high-resolution transmission electron microscopy. The atomic resolution of this technique helped them better understand the nanoparticle coalescence process. With the use of different model systems, two different mechanisms were concluded by these two groups. For example, the group led by Dr. James J. De Yoreo found that the ferrihydrite (5Fe2O3·9H2O) nanoparticles followed an oriented attachment mechanism in which the nanoparticles underwent continuous rotation and interaction until they find a perfect lattice match for intimate contact. Once the close contact occurred, the interfaces formed between adjacent particles would be eliminated through lateral atom-by-atom addition. In contrast, the group led by Dr. Haimei Zheng observed that the Pt3Fe nanoparticles first coalesced into winding polycrystalline nanoparticle chains through a shape-directed nanoparticle attachment process. The individual crystalline domains in the nanoparticle chains continued to rotate until all the domains oriented along the same direction, leading to the formation of single-crystalline, straight nanorods. Such insightful understandings on the kinetics of the nanoparticle interaction at the atomic resolution are very important for the design and synthesis of colloidal nanoparticles with precisely tailored properties.
Yugang Sun
Further reading:
H.-G. Liao, L. Cui, S. Whitelam, H. Zheng, “Real-time imaging of Pt3Fe nanorod growth in solution”, Science 2012, 336, 1011.
D. Li, M. H. Nielsen, J. R. I. Lee, C. Frandsen, J. F. Banfield, J. J. De Yoreo, “Direction-specific interactions control crystal growth by oriented attachment”, Science 2012, 336, 1014.