Synergistic effect of two foreign metal ions on shape-selective synthesis of gold nanocrystals

Abstract

Shape-controlled synthesis of metal nanocrystals has been widely investigated for the last several decades because of its ability to tailor the morphology of metal nanocrystals, and therefore, their physical and chemical properties. These properties, which greatly differ from their bulk counterparts, are highly dependent on the size and the shape of the nanocrystals. Metal nanocrystals with many shapes such as cubes, octahedra, cubotahedra, icosahedra, plates, rods, and wires in various sizes have been synthesized. However, these nanocrystals are mainly enclosed by low Miller-index facets (i.e. {111}, {100}, and {110}). Recently, much focus has been given to metal nanocrystals with high-index facets due to their superior catalytic properties to those bounded by low-index facets. The metal nanocrystals with high-index facets are, however, difficult to be prepared due to the fact that high-index facets are not as stable as those low-index ones during the synthetic period. In this work, we present the facile PDDA-mediated polyol route for synthesis of a series of novel Au nanocrystals, namely, truncated octahedra bounded by both {111} and {310} facets, truncated ditetragonal prisms exclusively enclosed by {310} facets, and bipyramids with exposed {117} facets by simply varying the ratio of Ag and Pd ions. The synergistic effect of Ag and Pd ions on the formation of the novel Au nanocrystals was studied. In our experimental conditions, the underpotential deposition (UPD) of Ag on Au surface was believed to inhibit the growth along <110> directions, therefore lead to the formation of {110} facets on Au nanocrystals. Palladium ions could, on the other hand, also take part in the deposition on Au surface and stabilize {100} facets. Together, Ag and Pd ions enabled the growth of {310} facets on the Au nanocrystals as {310} facets are composed of {110} and {100} subfacets. Since the Au nanocrystals obtained in this report possess high-index facets, they are expected to be promising candidates for many catalytic applications.