Seed-mediated synthesis of palladium nanorods and branched nanocrystals and their use as recyclable Suzuki coupling reaction catalysts.

In this study, we have used a simple seed-mediated synthesis process to prepare uniform Pd nanorods with average lengths of approximately 200 and 300 nm through the addition of 50-100 microL of 0.004 M copper acetate solution into the growth solution for the first time. Because of their long lengths, they nicely settled to the bottom of the reaction vial and can easily be separated from the suspended faceted particles. The nanorods have an average diameter of approximately 20 nm, so they have high aspect ratios of 10-15 or more. They can readily self-assemble into high-density packing structures on substrates. By increasing the volume of the copper acetate solution added to 250 microL, extensively branched Pd nanocrystals were obtained. The crystal structures of both particle morphologies have been examined. The nanorods possess a penta-twinned structure. Different growth directions have been found for the branched nanocrystals. The growth mechanism of these nanostructures was studied in great detail. A mixture of short Pd rods and faceted particles was formed first and elongated into long rods or branched nanocrystals with the assistance of copper atom deposition. A reduction potential of copper lower than that of palladium leads to periodic deposition and reoxidation of copper atoms on the growing rods and faceted particles. Use of other metal ions did not form long nanorods and branched nanocrystals. UV-vis spectra of the 200 nm Pd nanorods showed a significant absorption band in the near-infrared region with band maximum at approximately 1800 nm, while the 300 nm rods have band maxima of 2000-2100 nm. Both the nanorods and branched nanocrystals were found to serve as highly efficient and recyclable catalysts for catalyzing a Suzuki coupling reaction between phenylboronic acid and iodobenzene. They are also thermally stable under the reflux condition and can be easily removed from the product solution. These novel Pd nanostructures should find use in other palladium-catalyzed reactions.