Facile synthesis of tadpole-like nanostructures consisting of Au heads and Pd tails.

Hybridization provides an effective strategy for enhancing the functionality of materials.1 It is also possible to tune the electronic and optical properties of nanoparticles by forming a hybrid system.2 Palladium serves as a major catalyst in many industrial applications.3 It has been shown that incorporation of Au into Pd nanoparticles can reduce the material cost while enhancing their catalytic activity, selectivity, and stability.4 Here we report the synthesis of Pd-Au hybrid nanostructures in a tadpole shape, with the head being a Au nanoparticle and the tail being a Pd nanorod. The synthesis is based upon the galvanic replacement reaction between single-crystal Pd nanorods and AuCl4 ions. Different from previous systems that involved Ag-based templates and AuCl4 ions,5 the Au atoms resulting from the galvanic reaction did not coat the entire surface of a Pd nanorod to generate a core-sheath or hollow structure. Instead, the nucleation and deposition of Au was localized to the end(s) of a Pd nanorod, leading to the formation of a Pd-Au tadpole. Figure 1A shows a TEM image of the initial Pd nanorods that had an octagonal cross section, with the side surface enclosed by a mix of {100} and {110} facets. They had an average diameter of 4.0 ( 0.3 nm and length of 17.4 ( 2.4 nm (78% of them were rods). The nanorods were synthesized by reducing PdCl4 ions with ethylene glycol in water in the presence of poly(vinyl pyrrolidone) (PVP) and Brions.6 According to our proposed mechanism, cuboctahedral seeds were formed in the nucleation stage, whose surfaces were passivated by Brions.7 Oxidative etching due to the presence of oxygen (from air) and chloride (from PdCl4) could break the cubic symmetry of a seed, leading to the formation of an octagonal nanorod.8 Panels B-D of Figure 1 show TEM images of the products after the Pd nanorods had reacted with different volumes of 0.4 mM HAuCl4. Specifically, after the addition of 1 mL of HAuCl4 (Figure 1B), both ends of each Pd nanorod started to be enlarged and became rounded due to the deposition of Au via the following reaction: