Why Colloidal Syntheses of Bimetallic Nanoparticles Cannot be Generalized

Introducing one general synthesis to form bimetallic nanoparticles (NPs) could accelerate the discovery of NPs for promising energy applications. Although colloidal syntheses can provide precise structural and morphological control of bimetallic NPs, the complex chemical nature of multicomponent syntheses challenges the realization of such synthetic simplicity. Common synthetic issues are frequently ascribed to the variation in metal ion precursor reactivities and complex chemical interactions between the different metal surfaces and capping agents employed. However, no systematic studies have shown how these factors compete to ultimately assign the factor limiting the mixing and formation of bimetallic NPs. Here, we provide a parametric investigation of how the intrinsic standard reduction potentials (E⁰_red) of the metal ions and cocapping agents influence the formation of bimetallic AuCu, AuPd, and PdCu NPs. Using a combination of in situ X-ray total scattering along with transmission electron microscopy and nuclear magnetic resonance spectroscopy, we illustrate the multifunctional role of the cocapping agents through interactions with both the metal ion precursors and NP surfaces to stabilize metastable structures. Additionally, we demonstrate how system-specific side reactions and the local metal ion coordination environment can be used to selectively tune the formation kinetics, structure, and morphology of bimetallic NPs. Ultimately, these insights show that the chemical interactions rather than the intrinsic E⁰_red are responsible for the formation of bimetallic NPs. Broadly, these insights should aid the synthetic design of tailored multimetallic NPs.