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Formation of Free-Standing Supercrystals from the Assembly of Polyhedral Gold Nanocrystals by Surfactant Diffusion in the Solution

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journal contribution
posted on 26.08.2014 by Chih-Wen Yang, Chun-Ya Chiu, Michael H. Huang
Gold supercrystals with polyhedral morphologies can be prepared from the ordered packing of octahedral and rhombic dodecahedral nanocrystals in the presence of a sufficient amount of surfactant by slow water droplet evaporation. The whole supercrystal formation process has been video-recorded using a specially designed chamber to enclose a substrate containing the nanocrystal droplet in a moist environment. Supercrystal growth from the assembly of octahedra is completed within a shorter time. The presence of cetyltrimethylammonium chloride (CTAC) within the supercrystals has been confirmed by small-angle X-ray diffraction analysis. Transmission electron microscopy examination reveals the tendency of two gold octahedra with face contact to fuse, a process frequently observed in the formation of octahedron-assembled supercrystals. Remarkably, we have developed a diffusional surfactant transport approach to make free-standing supercrystals in bulk aqueous solution by adding a concentrated CTAC solution to a concentrated particle solution with a lower CTAC concentration in an Eppendorf tube. Gradual diffusion of CTAC to the lower nanocrystal solution promotes the growth of polyhedral supercrystals. A solution with a sufficiently high surfactant concentration has been shown to be necessary for particle aggregation and supercrystal formation. This method allows the deposition of dense but evenly distributed supercrystals on a substrate. Supercrystals were also used to make a modified electrode for electro-oxidation of glucose. This simple and organic solvent-free approach to making a large quantity of supercrystals allows an ample supply of supercrystals for studies of densely assembled nanocrystal systems and for biomedical applications.