A significant challenge in the development of functional nanomaterials is understanding the growth and transformations of colloidal metal nanocrystals. Despite the tremendous strides made in nanocrystal synthesis science, it is still difficult to achieve high, selective yields in most synthesis protocols. Many aspects of these complex syntheses remain poorly understood and fundamental studies can be beneficial. Since the shapes of metal nanocrystals are largely governed by phenomena occurring at their surfaces, studies based on principles in surface science are useful. 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 interaction of PVP with Ag(100) and Ag(111) surfaces. To scale our calculations to the solution phase, we develop a 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. This allows us to 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 and the Wulff shapes of Ag crystals, which are truncated octahedra. 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. Experimental studies demonstrate that 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.

Surface science of shape-selective metal nanocrystal synthesis from first-principles

ACS Fall 2020 Virtual Meeting

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