Initiation and Progression of Anisotropic Galvanic Replacement Reactions in a Single Ag Nanowire: Implications for Nanostructure Synthesis.

The galvanic replacement reaction (GRR) is a convenient method for synthesizing hollow/porous noble metal nanostructures with energy, health, and environmental applications. Understanding the reaction mechanism is important for optimizing the produced nanostructures' physicochemical properties. Using liquid-phase scanning transmission electron microscopy (LPSTEM), we quantitatively analyzed the GRR process in individual silver nanowires (AgNWs) reacting with an aqueous HAuCl₄ solution. The experiments and atomic-scale simulations show that GRR is a highly selective process with respect to the exposed surface facets, and we discover that the process progression is influenced by the internal crystal domains. We observe that the etching of AgNWs starts preferentially from facets with high energy sites while not favorable on low energy {111} facets, where even the internal twin facets within the nanostructures are found to be temporarily stable. The LPSTEM-observed etch rates in single or multiple crystal segments in AgNWs are shown to approach diffusion-limited conditions. These results provide intricate and detailed insights into the GRR process, which are difficult to achieve by other methods, and such studies will be beneficial for the understanding of how the surface energy and number of available surface sites influence the initiation probability, which will theoretically guide the synthesis of nanostructures, also supported with the deeper understanding of how the internal structure may influence the process. [ABSTRACT FROM AUTHOR]