Thermal Shape Stability of fcc Metal Nanocrystals Synthesized with Faceted Nonequilibrium Shapes

Abstract

Highly refined capabilities of the shape-controlled solution-phase synthesis of metal nanocrystals (NCs) allow the generation of NCs with faceted nonequilibrium shapes, which optimize properties for target applications such as catalysis and plasmonics. Often, for such applications and also for TEM analysis, the NCs are removed from the solution-phase environment. We explore the postsynthesis evolution of these metastable NCs in a high-vacuum TEM environment. Specifically, we analyze their reshaping toward the equilibrium Wulff shapes mediated by surface diffusion, where such reshaping degrades the above-mentioned optimized properties. Typical sizes for these NCs range from 5 to 30 nm or 10 3 −10 6 atoms, and reshaping often occurs on the time scale of minutes for temperatures around, say, 400 °C. We discuss the development of predictive stochastic atomistic-level models for NC evolution with a realistic description of surface diffusion. These models, in contrast to Molecular Dynamics, can naturally address the relevant time and length scales for these systems. KMC simulation results for the stochastic models are described, focusing on the reshaping of slightly elongated nanorods and of mildly truncated octahedra and nanocubes. In addition, we review appropriate theoretical formulations for reshaping, which involves the nucleation and growth on 2D islands or layers on outer facets of the NC. We note the limitations of classical nucleation theory in some scenarios and demonstrate the successes of a more fundamental and general master equation-based analysis.

Real-Time KMC Simulation of Vacancy-Mediated Intermixing in Au@Ag Octahedral Core–Cubic Shell Nanocrystals with Ab Initio-Guided Kinetics

“…The failure of these prescriptions to capture barriers for diverse surface diffusion processes (terrace vs step edge vs interlayer diffusion, etc. ), and to simultaneously describe diffusion on different facets, has been noted . Here, we also emphasize shortcomings in the description of vacancy-mediated bulk diffusion, especially with a constraint of simultaneously describing surface diffusion.…”

“…The Nanjing (and IVA) choices set E act = ε N + E i for these processes yielding barriers increasing from terrace to step edge to 2D vacancy diffusion to 3D bulk vacancy diffusion. While this is sometimes the case, step edge and 2D vacancy diffusion generally have lower barriers than terrace diffusion for fcc(100) facets. , See also Supporting Information S1. IVA and the other generic prescriptions fail since E act is not just controlled by interactions or coordination in the initial state (or the final state), but by distinct interactions in the transition state (TS) for hopping.…”

“…Limitations of the more generic formalisms described above (Metropolis, Nanjing, etc.) have been recognized for the description of surface diffusion, but these shortcomings also occur for bulk diffusion, as discussed below.…”