(55g) Thermodynamics and Kinetics of Ag Nanocrystals

Metal nanocrystals are useful in a host of applications, ranging from catalysis to plasmonics to electronics to various personal and health applications, and many studies show there are optimal nanocrystal sizes and morphologies for these applications. In the interest of reliably synthesizing optimal nanocrystals, it is important to understand the shape trajectories nanocrystals follow as they grow. In a self-seeding synthesis, the small (likely single-nm) nanocrystal seeds that form after nucleation are fluxional and we expect them to assume an equilibrium shape distribution as they grow. As the nanocrystals grow larger, they may become locked into kinetic shapes. It is important to understand the thermodynamic-to-kinetic shape transition to design processing routes by which nanocrystals with specific sizes and shapes could be synthesized. The time scales over which shape transitions occur as nanocrystals grow are poorly understood at present and it is important to first understand the thermodynamics of nanocrystal shapes to make progress toward this goal.

I will recount our efforts to understand both equilibrium and kinetic shapes of Ag nanocrystals. To resolve equilibrium shapes, we use parallel tempering molecular dynamics (MD) simulations to probe the minimum free-energy shapes of Ag nanocrystals containing 100−200 atoms in a vacuum. Time permitting, I will discuss our simulations in a solution environment. Our simulations reveal a shape intermediate between a Dh and an Ih, a Dh-Ih, that has distinct structural signatures and magic sizes. At certain critical sizes, nanocrystal shapes can change dramatically with the addition and removal of a single atom or with a change in temperature at a fixed size. We use accelerated MD simulations (hyperdynamics) to study the deposition of Ag atoms onto nanocrystals at various temperatures and experimentally viable deposition rates. Our studies show that even the smallest (100-atom) nanocrystals assume kinetic shapes as they grow at room temperature. As the temperature increases, the nanocrystals can assume equilibrium or kinetic shapes, depending on the temperature, their size, and the initial shape from which they evolved. The information in our study could be useful in efforts to devise processing routes to achieve selective nanocrystal shapes.

T. Yan, H. Zhang, and K. A. Fichthorn, ACS Nano 17, 19288-19304 (2023).

J. Cui and K. A. Fichthorn (In progress).