Continuous-Flow and Scalable Synthesis of Pd@Pt Core-Shell Nanocrystals with Enhanced Activity toward Oxygen Reduction.

We report a scalable method based on continuous-flow reactors for conformally coating the surfaces of facet-controlled Pd nanocrystals with uniform, ultrathin shells made of Pt. The key to the success of such an approach is the identification of a proper polyol to generate the Pt atoms at a relatively slow rate to ensure adequate surface diffusion and thus the formation of uniform shells in a layer-by-layer fashion. We first demonstrate the concept using the production of Pd@Pt (n = 2-5) core-shell icosahedral nanocrystals and then have the strategy successfully extended to the syntheses of Pd@Pt cubic and octahedral nanocrystals. All these core-shell nanocrystals showed great enhancement in catalytic activity toward the oxygen reduction reaction. Our results suggest that seed-mediated growth can be combined with a continuous-flow reactor to achieve scalable production of bimetallic and even trimetallic nanocrystals with controlled sizes, shapes, compositions, and properties.