Abstract
In this work, we have successfully used gold nanocubes, rhombic dodecahedra, octahedra, and corner-truncated octahedra as building blocks with different sizes for the fabrication of supercrystals under a controlled solvent evaporation process. The gold nanocrystals were synthesized using a seed-mediated growth approach developed in our laboratory. The nanocrystal droplets were placed on a substrate in a moist environment. Using a slow solvent evaporation rate, these nanocrystals can have sufficient time to contact each other and form many micron-sized supercrystals with cubic, triangular, octahedral, and rhombic dodecahedral shapes. Remarkably, the supercrystals were found to scatter everywhere within the evaporating droplet, rather than concentrate near the rim of the droplet. The packing arrangements in these supercrystals have been carefully examined and their packing models are presented. Unlike the typical packing by spherical nanoparticles, large-scale assembly by non-spherical polyhedral nanocrystals is strongly shape-guided. The formation process was also studied by using transmission X-ray microscopy, which records in-situ nanocrystal assembly process on the substrate with a nanocrystal droplet. Optical microscopic observation revealed swirling motion at the rim of the droplet that may facilitate the formation of perfect supercrystals. These supercrystals remain adherent to the substrate when subjected to sonication, suggesting their use as modified electrodes. Treatment with NaOH solution can remove the entire supercrystals from the substrate. The presence of ample void spaces, particularly for supercrystals constructed from octahedral particles, may endow the supercrystals with mesoporosity. Accessibility of the mesopores was demonstrated by infusion of palladium source into the supercrystals and the deposition of Pd nanoparticles within the pores. Thus, the supercrystals may be considered as mesoporous gold with ordered pore geometry and arrangement. The supercrystals may also find use as excellent substrates for surface-enhanced Raman spectroscopy (SERS).