Multifaceted Gold-Palladium Bimetallic Nanorods and Their Geometric, Compositional, and Catalytic Tunabilities

Kinetically controlled, seed-mediated co-reduction provides a robust and versatile synthetic approach to multimetallic nanoparticles with precisely controlled geometries and compositions. Here, we demonstrate that single-crystalline cylindrical Au nanorods selectively transform into a series of structurally distinct Au@Au-Pd alloy core-shell bimetallic nanorods with exotic multifaceted geometries enclosed by specific types of facets upon seed-mediated Au-Pd co-reduction under diffusion-controlled conditions. By adjusting several key synthetic parameters, such as the Pd/Au precursor ratio, the reducing agent concentration, the capping surfactant concentration, and foreign metal ion additives, we have been able to simultaneously fine-tailor the atomic-level surface structures and fine-tune the compositional stoichiometries of the multifaceted Au-Pd bimetallic nanorods. Using the catalytic hydrogenation of 4-nitrophenol by ammonia borane as a model reaction obeying the Langmuir-Hinshelwood kinetics, we further show that the relative surface binding affinities of the reactants and the rates of interfacial charge transfers, both of which play key roles in determining the overall reaction kinetics, strongly depend upon the surface atomic coordinations and the compositional stoichiometries of the colloidal Au-Pd alloy nanocatalysts. The insights gained from this work not only shed light on the underlying mechanisms dictating the intriguing geometric evolution of multimetallic nanocrystals during seed-mediated co-reduction but also provide an important knowledge framework that guides the rational design of architecturally sophisticated multimetallic nanostructures toward optimization of catalytic molecular transformations.