Synergistic Effect of Ag and Pd Ions on Shape-Selective Growth of Polyhedral Au Nanocrystals with High-Index Facets

We report the synergistic effect of two foreign metal ions, Pd(II) and Ag(I), on the shape-selective growth of polyhedral gold nanocrystals in a facile one-pot polyol synthesis. By varying the concentrations of Ag(I) and Pd(II) in a growth solution containing HAuCl4, gold nanocrystals were formed in high yields (> 90%) with tailored shapes including truncated tetrahexahedra (THHs) bounded by {310} and {111} facets, truncated ditetragonal prisms (DTPs) enclosed by 12 {310} facets, and multiple-twinned bipyramids. We found that when Ag(I) and Pd(II) are introduced separately, Ag underpotential deposition (UPD) induces the formation of Au {110} facets, while Pd deposition facilitates the development of Au {100} facets. The combination of the two foreign metal ions leads to a completely different nanocrystal growth mode. At low concentrations of Ag(I), the addition of Pd(II) causes the appearance of Au {310} facets. Depending on the ratio of Ag(I)/Pd(II), single-crystalline Au nanocrystals either partially or entirely enclosed by {310} facets are formed. At high concentrations of Ag(I), multiple-twinned bipyramids are obtained. Elemental analyses indicated that these nanocrystals are mainly composed of Au (> 96 at. %) with a small amount of Ag and Pd.