Direct Observation of the Transition Point Between Quasi-Spherical and Cubic Nanoparticles in a Two-Step Seed-Mediated Growth Method

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 25-27nm without any change in the nanoparticle size. The size of the obtained nanocubes remains constant (25-27nm) until most (> 90%) of the cuboctahedral nanoparticles are transformed to nanocubes. At this point, the (25-27nm) 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.