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.