Template-Assisted in Situ Synthesis of Ag@Au Bimetallic Nanostructures Employing Liquid-Phase Transmission Electron Microscopy.

Noble metal nanostructure synthesis via seed-mediated route is a widely adopted strategy for a plethora of nanocrystal systems. Ag@Au core-shell nanostructures are radiolytically grown in real-time using in situ liquid-cell (scanning) transmission electron microscopy. Here we employ a capping agent, dimethyl-amine (DMA) and a coordinating complex, potassium iodide (KI) in an organic solvent (methanol) in order to (1) slow down the reaction kinetics to observe mechanistic insights into the overgrowth process and (2) shift the growth regime from galvanic-replacement mode to direct synthesis mode resulting in the conventional synthesis of Ag@Au core-shell structures. A theoretical approach based on classical simulations complements our experiments, providing further insight on the growth modes. In particular, we focus on the shape evolution and chemical ordering, as currently there is an insufficient understanding regarding mixed composition phases at interfaces of alloys even with well-known miscibilities. Furthermore, the comparison of theoretical and experimental data reveals that the final morphology of these nanoalloys is not simply a function of crystallinity of the underlying seed structure but instead is readily modified by extrinsic parameters such as additives, capping agent, and modulation of surface energies of exposed crystal surfaces by the encapsulating solvent. The impact of these additional parameters is systematically investigated using an empirical approach in light of ab initio simulations.