Simulation of Site-Selective Etching by Invoking Effects of Ligands and Reactant Diffusion

We explore the origin of inequivalent etching on equivalent crystal sites by developing a kinetic Monte Carlo model. This new model focuses on the effects of both the diffusion of reactants and ligands, factors that are ubiquitous in wet-chemistry experiments and yet often overlooked or oversimplified in conventional simulations, where the defaults are rapid reactant redistribution and rapid equilibration of ligand adsorption/desorption. Our results show the dramatic differences arising from non-equilibrium ligand control, which cause selective etching at the corners, edges, or facets of Au triangular nanoplates. Basically, the ligand effects provide a positive feedback when etching targets the ligand-deficient sites, and the newly exposed atoms are ligand-free. In contrast, slow reactant diffusion provides a negative feedback, as a reactive site cannot etch unrestrictedly due to the lack of reactants. The dynamic competitions are clearly manifested in the focused etching mode when ligand effects coincide with large diffusion rates. By appropriately setting these two crucial factors, we reproduce in silico a series of etching products that are fully consistent with previous experimental results, along with the details in the shape of notches at the edges and holes on the plane. We believe that our simulation model could be expanded to many other systems and provide detailed inner workings for the mechanisms of abnormal crystal morphologies.