[5] A. Tracz, J. K. Jeszka, M. D. Watson, W. Pisula, K. Müllen, T. Pakula, J. Am. Chem. Soc. 2003, 125, 1682. [6] F. C. Grozema, T. J. Savenije, M. J. W. Vermeulen, L. D. A. Siebbeles, J. M. Warman, A. Meisel, D. Neher, H.-G. Nothofer, U. Scherf, Adv. Ma- ter. 2001, 13, 1627. [7] A. M. van de Craats,J. M. Warman, A. Fechtenkötter, J. D. Brand, M. A. Harbison, K. Müllen, Adv. Mater. 1999, 11, 1469. [8] a) H. Monobe, K. Awazu, Y. Shimizu, Adv. Mater. 2000, 12, 1495. b) H. Monobe, K. Awazu, Y. Shimizu, Mol. Cryst. Liq. Cryst. 2001, 364, 453. [9] J. C. Wittmann, P. Smith, Nature 1991, 352, 414. Direct Synthesis of Se@CdSe Nanocables and CdSe Nanotubes by Reacting Cadmium Salts with Se Nanowires** By Xuchuan Jiang, Brian Mayers , Thurston Herricks , and Younan Xia* Templating against currently existing nanowires (or rods, belts) provides a straightforward and powerful route to greatly expand the diversity of materials that can be processed as uniform, one-dimensional (1D) nanostructures. [1] In one approach, the surfaces of nanowires could be directly coated (using a range of different methods) with conformal sheaths made of a different material to generate coaxial nanocables. [2] Subsequent removal of the nanowires would lead to the for- mation of nanotubes with well-controlled dimensions. In an- other approach, it has been demonstrated that single crystal- line nanowires could serve as substrates for the epitaxial growth of another material to obtain coaxial, bilayer nano- tapes characterized by sharp structural and compositional in- terfaces. [3] By carefully modulating the composition of reac- tant in sequential steps, it is also possible to fabricate semiconductor multiple-sheath nanowire heterostructures via epitaxial growth. [4] In a third approach (or the so-called tem- plate-engaged process), nanowires had been partially (e.g., on the surface only) or completely converted to other materials without changing the 1D morphology when they were reacted with appropriate chemical reagents under carefully controlled conditions. [5] The concept of this method was originally dem- onstrated by Lieber and co-workers, where they found that highly crystalline nanorods of metal carbides could be formed by reacting carbon nanotubes with the vapors of metal oxides or halides at elevated temperatures. [5a] A similar procedure (including the use of both vapor- and solution-phase reac- tions) was later exploited by many research groups to gener- ate 1D nanostructures from a wealth of solid materials. These studies have also made it possible to incorporate a number of functions (e.g., luminescent, ferromagnetic, ferroelectric, piezoelectric, and superconducting) into an individual nano- wire that will find applications in various areas. Here, we would like to add another example to this list, where uniform nanowires of t-Se were employed as chemical templates to generate Se@CdSe nanocables and then CdSe nanotubes. Figure 1 shows a schematic outline of the approach. The first step involved the synthesis of single-crystalline nanowires of t-Se via a sonochemical process. [6] When refluxed in an aqueous medium containing Cd 2+ cations, elemental Se dis- proportionated into Se 2± and SeO 2± 3 anions. [7] The Se 2± anions then combined with the Cd 2+ cations to generate insoluble nanoparticles made of CdSe, which were deposited in situ as a conformal sheath around each t-Se template to produce a Se@CdSe nanocable structure. The major reactions involved in this process can be summarized as the following: 3Se (s) + 3H 2 O ® 2Se 2± (aq) + SeO 2± 3 (aq) + 6H + (aq) (1) Se 2± (aq) + SeO 2± 3 (aq) + 2Cd 2+ (aq) ® CdSe (s) + CdSeO 3 (s) (2) Note that the solubility of CdSeO 3 is sufficiently high at the refluxing temperature that it would not inhibit the formation of a dense CdSe sheath around each t-Se template via pro- cesses such as co-precipitation with CdSe. As the reaction was cooled down to room temperature, CdSeO 3 did precipitate out as nanoparticles decorating the surfaces of Se@CdSe nanocables. Fortunately, this unwanted byproduct could be ef- fectively removed by washing the as-synthesized sample with hot water in the setting of filtration. Due to the relatively low melting point of t-Se (~ 217 C) as compared with CdSe nano- particles, [8] the unreacted core of t-Se could be conveniently removed through evaporation (by heating at ~ 230 C for a few minutes) to obtain the nanotube made of pure CdSe. COMMUNICATIONS 1740 Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/adma.200305737 Adv. Mater. 2003, 15, No. 20, October 16 ± [*] Prof. Y. Xia, Dr. X. Jiang,Dr. B. Mayers Department of Chemistry University of Washington Seattle, WA 98195 (USA) E-mail: [email protected]T. Herricks Department of Materials Science and Engineering University of Washington Seattle, WA 98195 (USA) [**] This work has been supported in part by the STC Program of the National Science Foundation (NSF) under Agreement Number DMR-0120967, a Career Award from the NSF (DMR-9983893), and a Fellowship from the David and Lucile Packard Foundation. Y. X. is a Camille Dreyfus Teacher Scholar (2002) and an Alfred P. Sloan Research Fellow (2000). T. H. and B. M. thank the Center for Nanotechnology at the UW for the IGERT Fellowship Awards supported by the NSF (DGE-9987620). Fig. 1. Schematic illustration describing the formation of a Se@CdSe nanocable and then a CdSe nanotube when the t-Se nanowire was refluxed with Cd 2+ in an aqueous medium, followed by removal of the unreacted core of t-Se through thermal evaporation.
