Time-Dependent Susceptibility of the Growth of Gold Nanorods to the Addition of a Cosurfactant

Gold nanorods have attracted more attention than other gold particle morphologies because of their tunable optical response, which is a product of simply changing the rod's aspect ratio. Advances in bottom-up synthetic methods of gold nanorods have greatly improved yields as well as allowed access to a range of aspect ratios. Despite great strides being made in synthetic methods, the growth mechanism of gold nanorods has been debated in recent years. A recent mechanism outlined by Park et al. shows that nanorods undergo five distinct stages of growth. Stages I and II of this growth mechanism are dominated by the epitaxial micellar adsorption of surfactant to the growing crystal and adatom migration during rod reconstruction, respectively. Both of these processes, occurring early in particle growth, clearly dominate the production of anisotropic species, as well as the resultant morphology. Therefore, our hypothesis is that nanorod growth should be most susceptible to crystal habit modification during early growth stages. In this work, we show that the addition of surfactants, having a structure similar to that of the primary morphology-guiding surfactant, cetyltrimethylammonium bromide, during stages I and II of nanorod growth drastically influences the final dimensionality and morphology of nanorods. In contrast, addition after longer periods of time has little or no influence on rod structure. Our results bolster the growth mechanism outlined by Park et al., proving nanorod growth is most susceptible to crystal habit modification during growth stages that are dominated by epitaxial micellar adsorption and adatom reorganization. Furthermore, this work emphasizes the sensitivity of nanorod growth to the existing micellar state of the growth solution.