Polyol Synthesis of Pd Icosahedral Nanocrystals: Insights into the Growth Mechanism and Size Control

摘要

We report a polyol method based on diethylene glycol for the synthesis of Pd icosahedral nanocrystals with uniform sizes controllable in the range of 7-20 nm. Our mechanistic study indicates that Ostwald ripening plays a vital role in the homogeneous nucleation and growth of Pd icosahedral nanocrystals with uniform sizes. As a result of the ''size focusing'' effect facilitated by Ostwald ripening, the broad distribution in size can be quickly narrowed to generate uniform particles with sizes up to 11 nm depending on the reduction kinetics and duration of reaction time. To obtain particles with larger sizes, one has to rely on seed-mediated growth, with the uniform icosahedral nanocrystals obtained through Ostwald ripening serving as the seeds. In such a synthesis, it is vital to suppress homogeneous nucleation while promoting heterogeneous nucleation and layer-by-layer growth. Experimentally, this condition can be accomplished by adding a PdBr_4~(2-) solution in one shot for the production of Pd icosahedral nanocrystals with uniform sizes up to 20 nm. The size can be tuned by simply controlling the growth time and/or the amount of the precursor involved in the growth step. These strategies built upon the mechanistic understanding are expected to be extendable to other noble metals, providing a robust and viable route to the synthesis of icosahedral nanocrystals with uniform sizes controllable over a broad range for fundamental studies and catalytic applications.