Transformation of Pd Nanocubes into Octahedra with Controlled Sizes by Maneuvering the Rates of Etching and Regrowth

Palladium octahedra with controlled edge lengths were obtained from Pd cubes of a single size. The success of this synthesis relies on a transformation involving oxidative etching and regrowth. Because the {100} side faces of the Pd nanocubes were capped by Br- ions, Pd atoms were removed from the corners during oxidative etching, and the resultant Pd2+ ions could be reduced and deposited back onto the nanocubes, but preferentially on the {100} facets. We could control the ratio of the etching and regrowth rates (R-etching and R-regrowth) simply by varying the amount of HCl added to the reaction solution. With a large amount of HCl, etching dominated the process (R-etching >> R-regrowth), resulting in the formation of Pd octahedra with an edge length equal to 70% of that of the cubes. In contrast, with a small amount of HCl, all of the newly formed Pd2+ ions could be quickly reduced and deposited back onto the Pd cubes. In this case, R-etching approximate to R-regrowth, and the resultant Pd octahedra had roughly the same volume as the starting cubes, together with an edge length equal to 130% of that of the cubes. When the amount of HCl was between these two extremes, we obtained Pd octahedra with intermediate edge lengths. This work not only advances our understanding of oxidative etching in nanocrystal synthesis but also offers a powerful means for controlling the shape and size of metal nanocrystals simply by adjusting the rates of etching and regrowth.