Growth Mechanism of Gold Nanorods: the Effect of Tip-Surface Curvature As Revealed by Molecular Dynamics Simulations.

An understanding of the anisotropic growth mechanism of gold nanorods (AuNRs) during colloidal synthesis is critical for controlling the nanocrystal size and shape and thus has implications in tuning the properties for applications in a wide range of research and technology fields. In order to investigate the role of the cetyltrimethylammonium bromide (CTAB) coating in the anisotropic growth mechanism of AuNRs, we used molecular dynamics (MD) simulations and built a computational model that considered explicitly the effect of the curvature of the gold surface on CTAB adsorption and therefore differentiated between the CTAB arrangements on flat and curved surfaces, representing the lateral and tip facets of growing AuNRs, respectively. We verified that on a curved surface, a lower CTAB coverage density and larger intermicellar channels are generated compared to those on a flat surface. Using umbrella sampling simulations, we measured the free energy profile and verified that the environment around a curved surface corresponds to an easier migration from the solution to the gold surface for the [AuBr2]- species than does a flat surface. Long unbiased molecular dynamics simulations also corroborated the umbrella sampling results. Therefore, the [AuBr2]- diffusion through the environment of the tips is much more favorable than that in the case of lateral facets. This shows that the surface curvature is an essential component of the anisotropic growth mechanism.