Seed-Mediated Growth of Au Nanospheres into Hexagonal Stars and the Emergence of a Hexagonal Close-Packed Phase

Gold (Au) typically crystallizes in a cubic close-packed (ccp) structure to present a face-centered cubic (fcc) lattice or crystal phase. Herein, we demonstrate that Au nanoscale hexagonal stars featuring a hexagonal close-packed (hcp) structure can be synthesized in an aqueous system in the presence of fcc-Au nanospheres as the seeds. The success of this synthesis critically relies on the use of ethylenediaminetetraacetic acid to complex with Au3+ ions (the precursor) and the introduction of 2-phospho-l-ascorbic acid trisodium salt (Asc-2P) as a novel reducing agent to maneuver the reduction kinetics. The use of Asc-2P favorably promotes the formation of hexagonal stars with uneven surfaces at the top and bottom faces, together with concave side faces around the edges. By varying the amount of Asc-2P to fine-tune the reduction kinetics, we can adjust the concaveness of the side faces, with a faster reduction rate favoring greater concaveness and a red shift of the plasmon resonance peak to the near-infrared. For the first time, our results suggest that the phosphate and hydroxyl groups can act synergistically in controlling the morphology of Au nanocrystals. Most significantly, the newly deposited Au atoms can also crystallize in an hcp structure, leading to the observation of a phase transition from fcc to hcp along the growth direction. This new protocol based upon kinetic control can be potentially extended to other noble metals for the facile synthesis of nanocrystals featuring unprecedented structures or phases.