Selective growth of metal and binary metal tips on CdS nanorods.

Advances in the size1 and shape2 control of solution-grown nanocrystals have driven recent interest in the development of welldefined multicomponent nanostructures. Composition control, through the incorporation of two or more distinct nanostructured components, represents an effective approach to tailoring the chemical and physical properties of nanocrystals. The resulting hybrid structures often have multifunctional capabilities with tunable or enhanced properties. Alloy and core/shell structures are two of the most fundamental types of hybrid nanomaterials. These structures have been prepared for various materials with distinctive properties. For example, metal alloy and core/shell nanoparticles have been demonstrated for systems such as FePt and CoPt3 alloys3 and Pt/Pd core-shell particles.4 Analogous systems have also been shown for semiconductor nanocrystals, including PbSexS1-x alloys,5 CdSe/ZnS coreshell particles,6 and other compositions. Recently, one of the coauthors demonstrated a novel route to form heterostructures by the selective growth of metals on semiconductors.8 This type of metal-semiconductor composite combines two materials with different properties to yield a unique hybrid nanostructure with new properties and functionalities. Various approaches have been used to selectively grow metals on semiconductors through reduction, physical deposition, or photochemistry. Some of the materials that have been developed include Au-CdSe,7 Au-CdS,8 Au or Ag on ZnO,9 Co and Au on TiO2, and other systems.11 Recently, Weller and co-workers demonstrated an organometallic approach to prepare NixPt1-x nanocrystals12 with magnetic properties that are of interest for high-density data storage and other applications. Additionally, bimetallic materials often exhibit properties distinct from their constituent metals. For example, NiPt nanoparticles have potential application as catalysts, where they show enhanced activity for oxygen reduction relative to pure Pt.13 Herein, we demonstrate the synthesis of three systems: Pt-CdS, PtNi-CdS, and PtCo-CdS hybrid nanostructures with controllable size and composition. A mixture of Pt precursor and CdS rods was injected into diphenyl ether containing oleylamine and oleic acid at 200 °C. The reaction was quenched after several minutes, and the Pt-CdS composite separated from free metal particles by centrifugation. Ni or Co precursors could be added to the diphenyl ether prior to heating to promote bimetallic PtNi or PtCo formation (see Supporting Information for experimental details). Figure 1 shows the selective growth of Pt nanoparticles (4.3 nm) on CdS nanorods (120 nm × 4 nm). The anisotropic wurtzite crystalline structure of the CdS allows for selective chemistry on different facets. The reactivity of the nanorods is higher at the tips than along the body of the rod due to the increased surface energy, which also leads to preferential growth along the 〈001〉 axis of the CdS rods, as can be seen in the transmission electron microscopy (TEM) image in Figure 1A. During the early stages of Pt growth, or at a low Pt concentration, Pt deposition appears to occur preferentially on one tip of the rods (Figure 1B). This can be understood by considering the distribution of Cd and S along the 〈001〉 axis. The lattice planes along the axis are alternately composed of either Cd or S atoms, which makes growth at earlier stages more favorable on the S rich facets. Alternatively, due to the considerable length of the CdS rods relative to the metal particles, each tip may act as an isolated nucleation site independent of the opposite tip. Therefore, if the concentration of metal precursors is low enough that some of the rods do not experience metal growth on either tip, then, statistically, the remainder of metal nucleation will occur on only one of the tips. Increasing the amount of Pt relative to the concentration of CdS rods leads to two sided growth (5.7 nm), as shown in Figure 1C. The XRD patterns in Figure 1D for pure CdS rods and the PtCdS heterostructures agree well with the bulk CdS wurtzite crystal structure and face-centered cubic Pt. Selected area energy dispersive X-ray spectroscopy (EDS) performed on a single metallic tip (Figure † Department of Chemistry, University of California at Berkeley. ‡ Materials Science Division, Lawrence Berkeley National Laboratory. § The Molecular Foundry, Lawrence Berkeley National Laboratory. Figure 1. Selective growth of Pt nanoparticles with different sizes on CdS nanorods, (A) CdS rods (120 nm × 4 nm), (B) CdS with small single Pt tips (4.3 nm), (C) CdS with larger double Pt tips (5.7 nm), (D) XRD patterns of CdS rods and Pt-CdS hybrid structures with corresponding CdS and Pt bulk patterns (stick patterns shown above and below, respectively), and (E) selected area EDS spectrum of a single Pt tip, with inset HRTEM image of two Pt-CdS hybrids. Published on Web 02/27/2008