Formation of Diverse Supercrystals
from Self-Assembly
of a Variety of Polyhedral Gold Nanocrystals
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Abstract
Cubic, rhombic dodecahedral, octahedral, and corner-truncated
octahedral
gold nanocrystals with sizes of tens of nanometers have been used
as building blocks to form micrometer-sized supercrystals by slowly
evaporating a water droplet on a substrate placed in a moist environment.
Drying the droplet at 90 °C was found to yield the best supercrystals.
Supercrystals were evenly distributed throughout the entire substrate
surface originally covered by the droplet. Diverse supercrystal morphologies
have been observed. Nanocubes formed roughly cubic supercrystals.
Rhombic dodecahedra were assembled into truncated triangular pyramidal
supercrystals. Rhombic dodecahedral, octahedral, and hexapod-shaped
supercrystals were generated through the assembly of octahedra. Corner-truncated
octahedra formed tetrapod-shaped supercrystals at room temperature,
but octahedral, truncated triangular pyramidal, and square pyramidal
supercrystals at 90 °C. Nanocrystal assembly was found to be
strongly shape-guided. Expulsion of excess surfactant to the surfaces
of supercrystals suggests that responsive adjustment of surfactant
concentration during particle assembly mediates supercrystal formation.
Transmission X-ray microscopy and optical microscopy have been employed
to follow the supercrystal formation process. Surprising rotational
water current near the droplet perimeter carrying the initially formed
supercrystals has been observed. Supercrystals appear to grow from
the edge of the droplet toward the central region. Supercrystals assembled
from octahedra inherently contain void spaces and possibly connected
channels. The mesoporosity of these supercrystals was confirmed by
infiltrating H<sub>2</sub>PdCl<sub>4</sub> into the supercrystal interior
and reducing the precursor to form Pd nanoparticles. The embedded
Pd particles can still catalyze a Suzuki coupling reaction, demonstrating
the application of these supercrystals for molecular transport, sensing,
and catalysis