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

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.