Understanding the Shape-Control Mechanism in the Solution-Phase Growth of Ag Nanocrystals

Nanostructures with well-defined shapes can be synthesized using the solution-phase polyol process in the presence of polyvinylpyrrolidone (PVP), which is known to play a key role in directing shape control. However, the mechanism by which PVP actuates shape control remains elusive. We hypothesize that the influence of PVP on the Ag atom deposition kinetics dominates the shape-control mechanism. To test this hypothesis, 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. These profiles are used to calculate the mean-first passage times and implement extensive Brownian dynamics simulations, which allows quantitative evaluation of the process kinetics. Our results show that PVP films can regulate the flux of Ag atoms to be greater towards Ag(111) than Ag(100) facets. 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 confirms our hypothesis.