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

Kristen A. Fichthorn

Dept. of Chemical Engineering

Penn State University

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.  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 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.

Ag nanowires can also be grown in ethylene glycol solution with PVP.  Our calculations indicate that Ag nanowires with high aspect ratios, comparable to experiment, arise from surface diffusion.  On the other hand, a synergistic interaction between adsorbed halide and capping molecules leads to a higher flux of solution-phase cuprous ions to the ends of Cu nanowires and promotes their growth.

For Webinar information please contact Kyle Page (kmp265@cornell.edu)