posted on 2014-08-26, 00:00authored byChih-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.