Role of Solvent in the Shape-Controlled Synthesis of Anisotropic Colloidal Nanostructures

We use molecular dynamics simulations to study the role played by solvent in promoting anisotropic growth of colloidal nanostructures. Considering the growth of Ag nanowires and nanoplates in organic solvent, we study how solvent influences the aggregation of a small and relatively isotropic nanocrystal with a larger nanowire or a square nanoplate. We observe that when the two nanocrystals approach one another they almost always adopt a mesocrystal configuration, a free-energy minimum in which the two particles hover next to each other with their facets parallel and one or two layers of solvent between them-analogous to experimentally observed mesocrystal structures. Nanocrystal aggregation occurs from the mesocrystal state, and the free-energy barrier for aggregation is smallest on the smallest facets, which perpetuates anisotropic growth. By characterizing solvent ordering around the nanocrystal surfaces, as well as aggregation mechanisms, we find that solvent ordering is disrupted at the edges of the crystals, and this is where initial contact between the two nanoparticles is most likely to occur. Because the small nanocrystal is in close proximity to edges of the large nanostructure at its smallest facets, the free-energy barriers for aggregation are smaller there. Our general model contains features that are observed in a wide variety of systems that exhibit mesocrystal states and oscillatory solvation forces. These studies indicate that solvent can play a key role in promoting the anisotropic growth of colloidal nanostructures.