Elucidating growth mechanism and shape evolution of highly branched PtNi alloy nanocrystals and their electrocatalytic performance

Due to the difficulty in controlling thermodynamics and kinetics of the growth for branched bimetallic metal alloy nanocrystals in a solution system, understanding the growth mechanisms of this class of nanostructures is critical for their shape-controlled synthesis. Herein, we report an approach to control over shape of branched PtNi alloy nanocrystals by tuning the molar ratio of Pt to Ni in an oleyamine solution. The detailed analyses confirmed that the morphologies of PtNi alloy nanocrystals evolve from dendrites (less than 5 at% Ni) to multipods (5–10 at% Ni) to star shapes (10–20 at% Ni) and finally to polyhedrons (100 at% Ni) as the nickel content in PtNi alloy nanocrystals increase. During the morphology evolution, the growth mechanism changes from oriented attachment growth to a new mechanism combining oriented attachment and anisotropic overgrowth, then to kinetically controlled overgrowth and finally to thermodynamic control growth. More importantly, we find that the growth of branched PtNi alloy nanocrystals is always along the [220] direction, and the growth direction determines the number of branches in nanocrystals. Electrochemical results show that highly branched PtNi alloy nanocrystals exhibit significantly enhanced electrocatalytic activity and durability compared with commercial Pt catalysts. These in-depth findings will provide new hints to precisely control synthesis of highly branched PtNi nanocrystals.