(67b) Shape-Selective Growth of Nanoscale Materials: Insights from Multi-Scale Theory and Simulation

A significant challenge in the development of functional nanomaterials is understanding the growth and transformations of colloidal metal nanocrystals. From a practical perspective, a knowledge of how to selectively synthesize desired metal nanocrystal sizes and shapes would benefit numerous applications. For example, nanocrystal synthesis science is playing an increasingly prominent role in electrocatalysis for fuel-cell applications, photocatalysis for the production of solar fuels, and the catalytic production of biofuels from biomass. Metal nanocrystals figure prominently in solar-cell technologies, for example as nanometallic plasmonic structures to enhance the light absorption and efficiency of photovoltaics, as non-plasmonic photosensitizers for light-energy conversion, and as nanowire-based transparent conducting films. Metal nanocrystals are also featured in emerging applications targeting energy efficiency, such as photothermic desalination based on plasmonic nanocrystals, wearable triboelectric generators, flexible energy storage devices, electrochromic smart windows, and life-inspired nanosystems. For these and many other applications, the science of shape-selective nanocrystal synthesis has been advancing at an increasingly rapid pace, with numerous recent reports on the synthesis of various beneficial nanocrystal morphologies.

Despite ample demonstrations that it can be highly beneficial to tune nanocrystal morphologies for specific applications and despite the tremendous strides made in nanocrystal synthesis science, it is still difficult to achieve high, selective yields in most synthesis protocols. Many fundamental aspects of these complex syntheses remain poorly understood and empiricism still runs rampant. Our research has focused on understanding shape-selective nanocrystal growth via multi-scale theory and simulations.

I will discuss our efforts to understand the thermodynamics and kinetics of shape evolution for Ag and Cu nanocrystal growth in solution. Our multi-scale theoretical studies based in quantum density-functional theory (DFT) highlight how various nanocrystal shapes can be governed by either thermodynamics or kinetics. We use a variety of classical molecular-dynamics (MD) simulation techniques based on our many-body force field to show that the growth of sufficiently large Ag nanocubes with PVP capping molecules is induced by the facet-selective deposition kinetics of solution-phase Ag atoms/ions. When chloride is introduced to the synthesis, Ag nanocubes grow with a thermodynamic driving force. Fivefold-twinned Ag nanowires grow by surface diffusion induced by the unique, strained structure of these fascinating objects. I will also discuss the growth of fivefold-twinned Cu nanowires in the presence of chloride and HDA, where DFT calculations indicate that growth is dominated by deposition kinetics. These scenarios indicate the various “knobs” that can be turned to achieve shape-selective syntheses in the future.