Selective Placement of Faceted Metal Tips on Semiconductor Nanorods

There is strong interest in the sequential synthesis of multicomponent nanostructures where there are segregated regions of different materials within one particle. To this end, there have been many reports on core–shell nanostructures as well as metal-semiconductor hybrids. Very recently, synthetic routes have even achieved the selective placement of one component onto another component of complex shape with a high degree of reproducibility. As an example, a metal (or semiconductor) can be selectively grown on the tips of semiconductor nanorods or onto semiconductor tetrapod arms. The potential utility of such structures was recently demonstrated in two different applications exhibiting the flexibility of such hybrid materials. In one study, the single particle conductivity was enhanced by many orders of magnitude when Au nanoparticles were selectively grown onto the ends of CdSe nanorods, compared with CdSe nanorods that did not have Au tips. In a second study, platinum tips were grown onto nanorods which had a CdSe seed embedded within a CdS nanorod. The catalytic activity of these particles with respect to hydrogen reduction in the overall water splitting reaction was then examined. It was demonstrated that the catalytic activity of the structure could be affected by either altering the length of the nanorod or by controlling the size of the CdSe seed. In prior work, the metal tips selectively deposited on the ends of semiconductor rods have been of undefined shape and faceting. Yet the catalytic activity and selectivity of the metal particles is strongly related to the crystal shape and specific surface facet on which the chemical reaction occurs. Herein, we demonstrate that it is possible to specifically control the faceting of metallic nanocrystals grown in colloidal solution on the tips of semiconductor nanorods by depositing cubeshaped Pt nanoparticles at the tips of both CdS and CdS/CdSe seeded nanorods. The cube-like particles were formed in two steps: an intitial deposition of Pt without shape control, followed by further growth in the presence of a molecule that favors defined facets. To initially grow platinum tips on the nanorods, a calculated quantity of nanorods was mixed with platinum acetylacetonate (Pt(acac)2), 1,2-hexadecanediol (HDD), oleic acid (OAC), and oleylamine (OAM) in a solution of diphenyl ether (see Supporting Information for experimental details). The procedure is similar to that reported by Habas et al., with slight modifications. This reaction yields small, Pt-metal particles of mostly undefined shapes on the rod tip. The metal particles were formed preferentially on either one or both tips ({0001} facets) of the nanorods (Figure 1, middle column), depending on the initial nanorod concentration. For our experiment we chose an initial rod concentration of 2.9 nmol (the amount of the Pt-precursor and a detailed description are given in the Supporting Information). This method achieved a high yield of nanorods having either one or two Pt-tips. HRTEM analysis of the as-synthesized samples did not show specific faceting of the small Pt tips (Supporting Information, Figure S4). To grow faceted, cube-like tips, we used carbon monoxide as both the metal-reducing and shape-directing agent. It has been well established that CO adsorbs preferentially to specific metal surfaces. In some cases, this selective binding can lead to directed growth of Pt atoms onto specific facets of the Pt nanoparticle. Carbon monoxide can also undergo oxidation to CO2 on selected Pt facets. [13] The behavior of CO molecules bound to Pt is also contingent on which facet the molecule is bound to. For example, CO molecules bound to the {100} surface of Pt are slightly more stretched and closer to the particle surface than COmolecules bound to the {111} facet of Pt. This is a result of the stronger binding of CO on the {100} Pt surface with respect to the {111} surface. In the case of platinum, these different binding [*] H. Schlicke, Dr. D. Ghosh, L.-K. Fong, Prof. A. P. Alivisatos Department of Chemistry, University of California Berkeley Berkeley, CA 94720 (USA) E-mail: alivis@berkeley.edu H. Schlicke, Dr. D. Ghosh, L.-K. Fong, Dr. H. L. Xin, Dr. H. Zheng, Prof. A. P. Alivisatos Materials Science Division, Lawrence Berkeley National Laboratory (LBNL) Berkeley, CA 94720 (USA) [] Present address: Department of Chemistry, Universit t Hamburg Martin-Luther-King-Platz 6, 20146 Hamburg (Germany)