Metal nanocrystals have gained tremendous attention due to their superior performance in various applications, ranging from selective catalysis to electronic devices to plasmonic applications, such as photovoltaics and sensing. The properties of nanocrystals are highly sensitive to their size and shape. To this end, solution-phase synthetic protocols have been highly successful at producing a variety of nanocrystal structures. However, great challenges remain in achieving high selectivity to particular nanostructures. A significant difficulty lies in understanding and controlling shape evolution in these systems.  A deep, fundamental understanding of the phenomena that promote selective growth in these syntheses would enable tight control of nanostructure morphologies in next-generation techniques.

I will discuss our efforts to understand the workings of PVP, a polymer capping molecule that facilitates the formation of selective Ag nanoparticle shapes. In these studies, we use first-principles density-functional theory (DFT) to characterize the binding of PVP repeat units to Ag(100) and Ag(111) surfaces.  To understand the solution-phase binding of PVP to these Ag surfaces, we develop a new metal-organic many-body force field with high fidelity to DFT.  We implement this force field into molecular-dynamics (MD) simulations to characterize the potential of mean force and the mean first-passage times for solution-phase Ag atoms to reach PVP-covered Ag facets.  Using these mean first-passage times, we predict kinetic shapes of large Ag nanocrystals (around 100 nm) and show that these should be {100}-faceted cubes.  We also use MD simulations to characterize the interfacial free energies of PVP-covered Ag facets in solution.  The thermodynamic shapes that we predict in these calculations are truncated octahedra, with a predominance of {111} facets.  These findings are consistent with experimental observations that sufficiently small Ag nanocrystals tend to have shapes with a predominance of {111} facets and larger nanocrystals become {100}-faceted during solution-phase growth in the presence of PVP.

Though our studies are consistent with experiments that demonstrate nanocube growth can be directed by PVP alone, many studies have demonstrated that more robust nanocube syntheses can be achieved in the presence of halide additives. We use DFT-based ab initio thermodynamics calculations to probe the influence of chloride on Ag nanoshapes.  Consistent with experiment, these calculations indicate that chloride adsorption alone can lead to truncated Ag cubes.  “Late breaking” calculations indicate there is a synergistic interaction between Cl and PVP, whereby the combination of these two additives can lead to “pointy” Ag cubes.