(525h) Kinetic Influence of Polyvinylpyrrolidone in the Shape-Control Mechanism of Ag Nanocrystal Synthesis

In the shape-controlled synthesis of colloidal Ag nanocrystals, structure-directing agents, particularly polyvinylpyrrolidone (PVP), are known to be a key ingredient in making nanostructures with well-defined shapes. Although many Ag nanocrystals have been successfully synthesized using PVP, the mechanism by which PVP actuates shape control remains elusive. Surface-adsorbed structure-directing agents can influence both the surface free energies of the nanocrystalâ??s facets and the kinetics of the Ag atom deposition. To probe the kinetic influence of the adsorbed PVP film, we use umbrella sampling molecular dynamics simulations to calculate the potential of mean force (PMF) and diffusion coefficient profiles of Ag atom deposition onto Ag(100) and Ag(111) in ethylene glycol solution with PVP. The computed PMF and diffusion coefficient profiles are used to calculate the mean-first passage times and implement extensive Brownian dynamics simulations, which allows the kinetic effects to be quantitatively evaluated. Our results show that PVP films can regulate the flux of Ag atoms to be greater towards Ag(111) than Ag(100). Under kinetic control, {100}-faceted nanocrystals will be formed when the Ag flux is greater towards Ag(111). The predicted kinetic Wulff shapes are in agreement with the analogous experimental system, which supports our hypothesis that the kinetic influences of PVP dominate the mechanism of shape control for the seed-mediated growth of large nanocrystals.