Additive-Free\nSynthesis of (Chiral) Gold Bipyramids\nfrom Pentatwinned Nanorods

The production of colloidal metal nanostructures with\ncomplex geometries\nusually involves shape-directing additives, such as metal ions or\nthiols, which stabilize high-index facets. These additives may however\naffect the nanoparticles’ surface chemistry, hindering applications,\ne.g., in biology or catalysis. We report herein the preparation of\ngold bipyramids with no need for additives and shape yields up to\n99%, using pentatwinned Au nanorods as seeds and cetyltrimethylammonium\nchloride as surfactant. For high-growth solution:seed ratios, the\nbipyramids exhibit an unusual “belted” structure. Three-dimensional\nelectron microscopy revealed the presence of high-index {117}, {115},\nand {113} side facets, with {113} and {112} facets at the belt. Belted\nbipyramids exhibit strong near-field enhancement and high extinction\nin the near-infrared, in agreement with electromagnetic simulations.\nThese Ag-free bipyramids were used to seed chiral overgrowth using\n1,1′-binaphthyl-2,2′-diamine as a chiral inducer, with <i>g</i>-factor up to 0.02, likely the highest reported for bipyramid\nseeds so far.