Hydroquinone-Based Synthesis of Pd Nanostructures and the Interplay of Surface Capping, Reduction Kinetics, Attachment, Diffusion, and Fusion

The rational synthesis of nanostructured materials with desired properties calls for a thorough understanding of the growth mechanism. Here we report a mechanistic case study of Pd nanostructures synthesized by reducing a Pd(II) precursor with hydroquinone in the presence of KBr. As the reaction temperature was decreased from 100 to 20 °C, sub-10 nm cubes, concave nanocubes, and cube-like aggregates of much smaller particles were sequentially obtained. Our time-lapse experiments and a set of controls indicated that primary particles of 1–4 nm in size were formed during the initial stage of the synthesis, followed by their aggregation into cube-like structures through an attachment process. In addition to the influence of surface capping, the reaction temperature played a vital role in determining the exact shape or morphology of the final products by affecting the reduction kinetics, fusion of the attached particles, and surface diffusion of atoms. At 100 °C, corresponding to a quick depletion of the Pd(II) precursor, the primary particles in each aggregate could easily fuse together to form nanocubes with flat faces owing to adequate surface diffusion. At 60 °C, the constituent particles also fused into cubes and then evolved into concave cubes through atomic deposition at the corners. At 20 °C, although fusion did not occur due to the substantially decreased diffusion rate, the primary particles in each aggregate still grew through atomic deposition for the formation of larger, cube-like aggregates. Explicating the growth mechanism, this work offers a mechanistic understanding of the nonclassical growth mode involved in the formation of various metal nanostructures.