Understanding shape control in gold nanoparticles from molecular dynamics simulations

Gold nanoparticles are widely used in many areas such as photothermal cancer therapy, biochemical sensing and medical imaging due to their size and shape-dependent optical properties. Directly manipulating and control- ling the size and shape of gold nanoparticles is, therefore, a key step for their tailored applications. We use molecular dynamics simulations in order to understand the mi- croscopic origin of the asymmetric growth mechanism in gold nanorods. The different factors influencing the growth are selectively included in the models in order to unravel the role of the surfactants and ions. In particular, both infinite planes models, representing the mature stage of the growth, and nanoseed models in the size of a few nanometers are used to under- stand how asymmetry between the different facets of the nanorods builds up. We find that on all the investigated surfaces, cetyltrimethylammonium bromide forms a layer of distorted cylindrical micelles where channels among micelles provide direct ion access to the surface. When the low index facets are examined, a lower surface density of surfactant is found on the Au(111) facets, with respect to the Au(100) and Au(110) facets. In addition, a higher electrostatic potential difference is measured between the gold surface and the bulk solution at the Au(111) interface, which would provide a stronger driving force for the diffusion of negatively charged AuCl2 - species, which are reduced at the gold surface. The two factors together would result into higher diffusion flux of the gold reactant toward the Au(111) facets and could result into a preferential growth of the Au(111) surfaces. In order to investigate if the anisotropy is preserved at the nanoscale, we also investigate penta-twinned decahedral seeds and a cuboctahedral seed, whose dimensions are comparable to those of the cetyltrimethylammoniumbromide micelle. We find that the asymmetry in adsorption behavior be- tween the different low index facets, which characterized the infinite planes, shows up even more dramatically on the nanoseeds. Indeed, the Au(100) and Au(110) facets show structures similar to the ones observed on the in- finite planes in both the cuboctahedral as well as penta-twinned seeds. The (111) facets, which e.g. form the tips of the penta-twinned nanoseeds, on the other hand show basically no micellar adsorption. This huge difference in the coverage of the early stage seeds would then promote a symmetry breaking in the penta-twinned seeds and, therefore, an anisotropic growth of nanocrystals. Simulations also provides a microscopic understanding of the role of halides in controlling the anisotropic growth. In particular, we find that bro- mide adsorption on the gold nanorods is not only responsible for surface passivation, but also acts as the driving force for micelle adsorption and stabilization on the gold surface in a facet-dependent way. Partial replace- ment of bromide by chloride decreases the difference between facets and the surfactant density. Finally, while only chloride is present in the growing so- lution, no halides or micellar structure protect the gold surface and further gold reduction should be uniformly possible. Finally, we also address the role of the silver ions. We find that silver ions have a strong propensity to adsorb on the gold surface where they form AgBr islands with different specific geometry depending on the sur- face plane. Although the structure of the micellar layer is not qualitatively modified by the addition of silver, silver substantially increases the Br- con- centration at the interface, resulting into an increased surface passivation. Overall the asymmetry between facets is maintained, with a lower Br- sur- face density at the Au(111) interface with respect to that at the Au(100) and Au(110) interfaces