Pencil-like Ag Nanorods Asymmetrically Capped by Pd

We report a solution-phase method for the fabrication of Pd-Ag nanorods featuring an asymmetric, pencil-like structure with a sharpened Ag tip at one end and a Pd decahedron at the other end. The fabrication started from the synthesis of penta-twinned Pd-Ag nanorods via asymmetric growth of Ag from one side of the Pd decahedral seeds. When subjected to aqueous ammonia, etching of Ag was preferentially initiated from the Ag end lined with twin defects while the Pd end was kept almost intact. The selective etching of Ag generated a pencil-like nanostructure, with the sharpened Ag tip enclosed by high-index facets. We systematically analyzed the evolution of morphology, surface structure, and plasmonic properties and also proposed a plausible mechanism to account for the etching process. The asymmetric, pencil-like Pd–Ag nanorods showed tunable plasmon resonance, sharp Ag tips, and catalytically active surfaces (high-index facets of Ag and a Pd surface equivalent to that on a decahedral nanocrystal), offering a unique system for the investigation of plasmon- and tip-enhanced spectroscopies, as well as the exploration of catalytic applications.