An aqueous seed-mediated growth method is adapted to explore the shape transformation of quasi-spherical Au seeds to nanocubes in a direct and continuous manner. Quenching the growth process at varied reaction-duration times and exploring the intermediate products by high-resolution transmission electron microscopy and UV/vis spectroscopy shows an abrupt cuboctahedral-to-nanocube transition at 25-27 nm without any change in the nanoparticle size. The size of the obtained nanocubes remains constant (25-27 nm) until most (>90%) of the cuboctahedral nanoparticles are transformed to nanocubes. At this point, the (25-27 nm) nanocubes initiate further continuous and homogeneous growth until they reach 50-nm Au cubes. These observations are ascribed to a scenario in which the kinetically controlled growth mode of the nanoparticle is significantly affected by the surface self-diffusion of metal adatoms, especially when the adatom's self-diffusion distance is comparable with the nanoparticle's size.
5 Figures and Tables
Figure 1. UV/Vis spectra of the dispersions obtained before (3.5-nm pre-formed Au seeds) and after the synthesis had proceeded for different times.
Table 1. Physical properties of metals.
Figure 2. TEM images of Au nanoparticles obtained by precipitation from their reaction dispersion after A) 1.5 min, B) 2.5 min, C) 3 min, D) 4 min, E) 6 min, F) 15 min, G) 1h, and H) overnight. The length of each image side is 110 nm.
Figure 4. Representative TEM images and their corresponding FFT patterns of 20-nm quasi-spherical Au nanoparticles with truncated (round pentagonal) (A, B) and regular (C, D) decahedral structure imaged along fivefold axis.
Figure 6. HRTEM image of A) a 27-nm nanocube, and B) its corresponding FFT, with cuboctahedral structure viewed along the  orientation.
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