Microscopic Investigation of Chemoselectivity in Ag-Pt-Fe₃O₄ Heterotrimer Formation: Mechanistic Insights and Implications for Controlling High-Order Hybrid Nanoparticle Morphology

Three-component hybrid nanoparticle heterotrimers, which are important multifunctional constructs that underpin diverse applications, are commonly synthesized by growing a third domain off of a two-component heterodimer seed. However, because heterodimer seeds expose two distinct surfaces that often can both support nucleation and growth, selectively targeting one particular surface is critical for exclusively accessing a desired configuration. Understanding and controlling nucleation and growth therefore enables the rational formation of high-order hybrid nanoparticles. Here, we report an in-depth microscopic investigation that probes the chemoselective addition of Ag to Pt-Fe₃O₄ heterodimer seeds to form Ag-Pt-Fe₃O₄ heterotrimers. We find that the formation of the Ag-Pt-Fe₃O₄ heterotrimers initiates with indiscriminate Ag nucleation onto both the Pt and Fe₃O₄ surfaces of Pt-Fe₃O₄, followed by surface diffusion and coalescence of Ag onto the Pt surface to form the Ag-Pt-Fe₃O₄ product. Control experiments reveal that the size of the Ag domain of Ag-Pt-Fe₃O₄ correlates with the overall surface area of the Pt-Fe₃O₄ seeds, which is consistent with the coalescence of Ag through a surface-mediated process and can also be exploited to tune the size of the Ag domain. Additionally, we observe that small iron oxide islands on the Pt surface of the Pt-Fe₃O₄ seeds, deposited during the formation of Pt-Fe₃O₄, define the morphology of the Ag domain, which in turn influences its optical properties. These results provide unprecedented microscopic insights into the pathway by which Ag-Pt-Fe₃O₄ heterotrimer nanoparticles form and uncover new design guidelines for the synthesis of high-order hybrid nanoparticles with precisely targeted morphologies and properties.