Ag-Au-Ag heterometallic nanorods formed through directed anisotropic growth.

Heterostructured nanocrystals containing multiple components are attracting attention due to not only their multifunctional properties but also new features arising from the effective coupling of different domains.1,2 Numerous semiconducting heterostructures have been synthesized by gas-phase deposition and are being used for the miniaturization of electronic and photonic circuits.3 Metallic heterostructures, in contrast, are generally grown on hard templates such as anodic aluminum oxides by electrochemical deposition.1,4 Synthesis without templates is relatively difficult owing to distinct reduction rates and lattice mismatch of the different components. Even when selecting materials with similar lattice constants, precise tuning of the reaction conditions is strictly required for the formation of multimetallic nanostructures. Xu et al. successfully generated heterodimer particles at a micelle liquid-liquid interface,5 and Yang et al. utilized polyhedral Pt nanocrystals to induce either isotropic growth of Pd nanopolyhedrons epitaxially or anisotropic growth of Au nanowires heterogeneously.6 There have been few reports of the synthesis of multisegmented metallic nanostructures without hard templates despite the fact that this approach is advantageous in terms of structural variety and controllability. We have recently demonstrated that single-crystalline polyhedral gold nanocrystals such as cubes, cuboctahedrons, and octahedrons could be readily formed by the preferential overgrowth of spherical particles.7 Using a similar strategy, we promoted anisotropic growth from multiply twinned particles (MTPs). In the present work, we report the synthesis of Ag-Au-Ag heterometallic nanorods through directed overgrowth from gold decahedrons and rods by adding silver ions and poly(vinyl pyrrolidone) (PVP) (Figure 1a). The silver component was readily converted to Ag2S by reaction with Na2S to generate a semiconductor-metal-semiconductor heterojunction on the nanorods. Briefly, multiply twinned gold decahedrons were used as a seed.8 The particle dispersion in diethylene glycol (DEG) was heated to the boiling temperature (260 °C), and 2 equiv of silver nitrate with respect to the gold concentration and PVP solutions in DEG were added periodically into the dispersion every 30 s over a period of 7.5 min. Refluxing the reaction mixture for 1 h yielded new nanorods in a high yield. As shown in the scanning electron microscopy (SEM) image in Figure 1b, each nanorod has a bright strip on the center, indicating that a heterometallic component with high reflectivity (Au) is included in the structure. The nanorods are estimated to be 194 ( 17 nm in length and 76 ( 9 nm in diameter, and the average aspect ratio is 2.6 ( 0.5. For an accurate structural analysis, gold nanorods with a 5-fold symmetry were used for the reaction. After the periodic addition of AgNO3 and PVP solutions, longer nanorods were formed in a high yield. Figure 1c exhibits nanorods with a thick bright strip on the center, indicating that the silver segments were grown symmetrically at both ends of the original gold nanorods. The average size of the product is estimated to be 440 ( 40 nm in length and 100 ( 11 nm in diameter with an average aspect ratio of 4.5 ( 0.7. The gold and silver segments in a nanorod are clearly distinguished in the transmission electron microscopy (TEM) image (Figure 2a). The average length (194 ( 24 nm) and diameter (93 ( 3 nm) of the gold segments are nearly unchanged from those of the original seeds after the silver overgrowth. The gold nanorods were successfully regenerated by selective dissolution of silver components with HNO3, and there was no indication of Ag-Au alloy formation in the interface (Figure S2 in Supporting Information). The selected area electron diffraction (SAED) exhibits a superposition of the diffractions along 〈111〉 and Figure 1. (a) Synthesis of Ag-Au-Au heterometallic nanorods from gold decahedrons and rods. SEM images of Ag-Au-Ag heterometallic nanorods grown from (b) gold decahedrons and (c) rods. The bar represents 500 nm.