Bromide-Mediated Reduction Kinetics and Oxidative Etching for Manipulating the Twin Structure and Facet of Pd Nanocrystals for Catalysis

With Pd as an example, a set of quantitative analyses is designed to shed light on the bromide-mediated reduction kinetics and oxidative etching in determining the twin structure and facet of Pd nanocrystals. The success of this work relies on the kinetic measurements of Pd(II) precursor reduction and the close examinations of resultant Pd seeds and nanocrystals at different stages of synthesis. We observe there is a clear trend where low, moderate, and high initial Pd(II) reduction rates regulated by Br- ions correspond to the formation of Pd seeds with singly-twinned, multiply-twinned, and single-crystal structures in the nucleation stage, respectively. Our quantitative analyses also suggest the oxidative etching induced by oxygen/Br- pair can selectively remove the multiply-twinned Pd seeds from the products in the growth stage while leaving behind singly-twinned or single-crystal Pd seeds for the evolution into Pd nanocrystals with well-defined facets in high purity. The mechanistic insights obtained in this work can be extended to the synthesis of Pd@Pd-Pt core-shell nanocubes with high-index facets, which can be used as excellent electrocatalysts and photocatalysts for hydrogen generation.