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Large-Scale Synthesis of High-Quality Ultralong Copper Nanowires Yu Chang, Mei Ling Lye, and Hua Chun Zeng* Department of Chemical and Biomolecular Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Received January 25, 2005. In Final Form: March 11, 2005 The present difficulties in synthesis of one-dimensional copper are short length, nonlinear morphology, polydispersivity, poor crystallinity, low yield, and process complexity. In this work, we demonstrate that high-quality ultralong copper nanowires (90-120 nm in diameter, 40-50 µm in length; aspect ratio >350-450) can be synthesized in large scale with a facile aqueous reduction route at low cost. The prepared copper nanowires can also be used as starting solid precursor for fabrication of polycrystalline oxide nanotubes via direct oxidation in air.
Copper is one of the most important metals in modern technologies.1,2 For future bottom-up nanotechnology (e.g., nano-optoelectronic industry), as a first step, fabrication of one-dimensional (1D) nanomaterials of copper (wires/cables/rods) has received considerable attention in recent years, and a number of methods have been available, which include electrochemical reactions,3-8 vapor depositions,9,10 soft and hard template processes,11-13 reverse micellar systems,14-17 etc.18,19 Although the significant research endeavor has been devoted, there is still lack of effective methods for large-scale production of highquality nanostructured copper, or metal nanowires in general, with precise morphological control. Major synthetic difficulties encountered in this area are short length, nonlinear morphology, polydispersivity, poor crystallinity, low yield, and process complexity.3-19 Herein, we demonstrate for the first time that high-quality ultralong copper nanowires (all free-standing: 90-120 nm in diameter, 40-50 µm in length; aspect ratio >350-450) (1) Joseph, G. Copper: Its Trade, Manufacture, Use, and Environmental Status; ASM International: Materials Park, OH, 1999; pp 331371. (2) Yong, C.; Zhang, B. C.; Seet, C. S.; See, A.; Chan, L.; Sudijono, J.; Liew, S. L.; Tung, C.-H.; Zeng, H. C. J. Phys. Chem. B 2002, 106, 12366-12368. (3) Molares, M. E. T.; Buschmann, V.; Dobrev, D.; Neumann, R.; Scholz, R.; Schuchert, I. U.; Vetter, J. Adv. Mater. 2001, 13, 62-65. (4) Gao, T.; Meng, G.; Wang, Y.; Sun, S.; Zhang, L. J. Phys.: Condens. Matter 2002, 14, 355-363. (5) Gillingham, D. M.; Mu¨ller, C.; Bland, J. A. C. J. Phys.: Condens. Matter 2003, 15, L291-L296. (6) Pang, Y. T.; Meng, G. W.; Zhang, Y.; Fang, Q.; Zhang, L. D. Appl. Phys. A 2003, 76, 533-536. (7) Konishi, Y.; Motoyama, M.; Matsushima, H.; Fukunaka, Y.; Ishii, R.; Ito, Y. J. Electroanal. Chem. 2003, 559, 149-153. (8) Choi, H.; Park, S.-H. J. Am. Chem. Soc. 2004, 126, 6248-6249. (9) Liu, Z.; Bando, Y. Adv. Mater. 2003, 15, 303-305. (10) Liu, Z.; Bando, Y. Chem. Phys. Lett. 2003, 378, 85-88. (11) Monson, C. F.; Woolley, A. T. Nano Lett. 2003, 3, 359-363. (12) Liu, Z.; Yang, Y.; Liang, J.; Hu, Z.; Li, S.; Peng, S.; Qian, Y. J. Phys. Chem. B 2003, 107, 12658-12661. (13) Yen, M.-Y.; Chiu, C.-W.; Hsia, C.-H.; Chen, F.-R.; Kai, J.-J.; Lee, C.-Y.; Chiu, H.-T. Adv. Mater. 2003, 15, 235-237. (14) Pileni, M. P.; Gulik-Krzywicki, T.; Tanori, J.; Filankembo, A.; Dedieu, J. C. Langmuir 1998, 14, 7359-7363. (15) Lisiecki, I.; Sack-Kongehl, H.; Weiss, K.; Urban, J.; Pileni, M.-P. Langmuir 2000, 16, 8802-8806. (16) Lisiecki, I.; Sack-Kongehl, H.; Weiss, K.; Urban, J.; Pileni, M.-P. Langmuir 2000, 16, 8807-8808. (17) Lisiecki, I.; Filankembo, A.; Sack-Kongehl, H.; Weiss, K.; Pileni, M.-P.; Urban, J. Phys. Rev. B 2000, 61, 4968. (18) Li, Q.; Wang, C. Chem. Phys. Lett. 2003, 375, 525-531. (19) Wang, Z. L.; Kong, X. Y.; Wen, X.; Yang, S. J. Phys. Chem. B 2003, 107, 8275-8280.
can be synthesized in large scale with a facile aqueous reduction route at low temperatures. In each synthesis, 20-30 mL of NaOH (3.5-15 M) and 0.5-1.0 mL of Cu(NO3)2 (0.10 M) aqueous solution were added to a glass reactor (capacity 50 mL). Varying amounts of ethylenediamine (EDA; 0.050-2.0 mL; 99 wt %) and hydrazine (0.020-1.0 mL; 35 wt %) were also added sequentially, followed by a thorough mixing of all reagents. The reactor was then placed in a water bath with temperature control over 25-100 °C (optimized at 60 °C) for 15 min to 15 h; copper products were washed and harvested with centrifugation-redispersion cycles and stored in a water-hydrazine solution to prevent oxidation. Further details on the synthesis can be found in Supporting Information (SI-1). The crystallographic structure of products was determined with X-ray diffraction (XRD; Shimadzu XRD-6000, Cu KR). The spatial, morphological, and compositional investigations were carried out with scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDX; JSM-5600LV), field-emission SEM (FESEM/EDX; JSM-6700F), transmission electron microscopy and selected area electron diffraction (TEM/ SAED; JEM-2010F), high-resolution TEM (HRTEM/EDX; JEM 3010, 300 kV), and X-ray photoelectron spectroscopy (XPS; AXIS-HSi, Kratos Analytical).20 The formation of metallic copper in this work is based on the following redox reaction under the basic condition.
2Cu2+ + N2H4 + 4OH- f 2Cu + N2 + 4H2O (1) Figure 1 shows some FESEM and TEM images of copper nanowires. The reductive conversion of Cu2+ to metallic copper in this process is 100%, which was indicated in total disappearance of the light blue color (Cu2+; Supporting Information (SI)-1). Interestingly, the as-prepared nanowire cake is lifted up to the top of solution (Figure 1A) due to the high density of solution and entrapping of nitrogen bubbles (eq 1) among the nanowires after magnetic stirring. As shown in these images, the prepared nanowires are straight (Figure 1B,C), with constant diameters in the range of 60-160 nm (mostly in 90120 nm). The wires are ultralong, having lengths of more than 40 µm, which virtually corresponds to an aspect ratio of greater than 350! (20) Lou, X. W.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 26972704.
10.1021/la050220w CCC: $30.25 © 2005 American Chemical Society Published on Web 03/29/2005
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Figure 1. (A) As-prepared Cu nanowires in mother liquor. (B, C) FESEM images of general and detailed views of Cu nanowires. (D) TEM image of Cu nanowires.
Figure 2. (A) HRTEM image of a Cu nanowire. (B) Location of examined area (indicated with a frame) in (A). (C) A beadline model of the Cu (110) surface. (D) SAED pattern of a Cu nanowire (inset).
The nanowires obtained are single crystalline with high lattice perfection (e.g., the lattice fringe d111 ) 2.10 ( 0.05 Å), as shown in Figure 2A and B. The growth direction of these wires is along the 〈110〉 (Figure 2C), on the basis of selected area electron diffraction (SAED) analysis (Figure 2D; also SI-2). Our XRD study shows that all the prepared copper samples have a face-centered cubic (fcc) structure (SG: Fm3 h m; JCPDS card no. 03-1005), and the lattice constant of this cubic phase ao is equal to 3.615 Å (SI-3). The chemical composition of the nanowires was further confirmed with EDX spectroscopy technique (SI-4). Our experiments (SI-1) indicate that a high concentration of NaOH is essential to prevent the copper ions from forming copper hydroxide precipitates. On the other hand, a certain amount of EDA is also indispensable to control product morphology. The interplay between NaOH and EDA had been recognized in this work. For example, with a high concentration of NaOH, the needed amount of EDA is small, while for a lower concentration of NaOH, the amount of EDA has to be increased accordingly in order to obtain high regularity for the 1D product (SI-1). In
each case, furthermore, there is an optimal molar ratio between the two chemicals. With a moderate amount of EDA, both wire- and disklike morphologies could be obtained. When EDA was overused, however, the axial 1D growth of nanowires could be switched totally to a radial expansion (i.e., a 2D growth). Figure 3 reports XRD patterns and a disklike morphology of Cu at 100% morphological yield under such a condition (SI-5). The single-crystal disks also have 〈110〉 orientations, as determined by the SAED method (Figure 3B). Quite commonly, a seed area (holelike) in the center of the disk can be identified, revealing the above 2D growth mechanism. The observed growth preference can be attributed to steric hindrance and charge restriction of copper complexes in the starting solution, such as [Cu(OH)4]2-, [Cu(EDA)(OH)2], and [Cu(EDA)2]2+ etc.,21 on a growing crystal plane, and to synergetic effects of ligand (EDA) and electrolyte (NaOH) on adsorption and deactivation of the grown part of a product morphology. X-ray photoelectron spectroscopy (XPS) results for our air-dried Cu nanowires are reported in Figure 4A. Cuprite Cu2O (about 54% among total surface copper) was found on the wire surfaces, on the basis of an analysis for the binding energies of Cu 2p3/2 (932.4 eV for Cu0 and 932.2 eV for Cu+; reference C 1s was set at 284.7 eV; SI-6)22 and O 1s (530.2 eV for the anion O2- in the cuprite)23 photoelectrons and the HRTEM image of Figure 2B in which a rough surface region of Cu nanowires can be seen. Nonetheless, this surface oxidation can be prevented by storing the nanowires in dilute hydrazine solution. Apart from their electric conductor applications, the copper nanowires prepared can also be used as a solid precursor for fabrication of other nanostructures. As exampled in panels B and C of Figure 4, straight polycrystalline CuO nanotubes, which retain the original shape of Cu nanowires, have been fabricated from direct metal oxidation. A significant fraction of pristine Cu could be converted to CuO after heating in air at 300 °C in just 10 min (SI-7). It is thought that the preformed surface Cu2O may provide good starting points for metal out-diffusion, during which copper moves preferentially toward the surface region while oxygen anions on the surface are relatively immobile. The hollowing mechanism of copper can also be explained (21) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 4th ed.; John Willey & Sons: New York, 1980; Chapter 21, pp 689-821. (22) Espino´s, J. P.; Morales, J.; Barranco, A.; Caballero, A.; Holgado, J. P.; Gonza´lez-Elipe, A. R. J. Phys. Chem. B 2002, 106, 6921-6929. (23) McCafferty, E.; Wightman, J. P. Surf. Interface Anal. 1998, 26, 549-564.
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Figure 3. (A) XRD patterns of Cu nanowires and disks. (B) FESEM image of a large Cu disk; the white arrow indicates the seed area in the center. Inset of (B) shows the [110] zone diffraction spots of a Cu disk.
Figure 4. (A) XPS spectra of O 1s and Cu 2p3/2 photoelectrons of air-dried Cu nanowires. (B) TEM image of polycrystalline CuO nanotubes formed by oxidizing in air at 400 °C for 10 h. (C) SAED pattern of CuO nanotubes formed in air at 400 °C for 3 h (not shown).
with the Kirkendall-type diffusion process.24 Taking advantage of their interior space,24,25 the polycrystalline CuO (a p-type semiconducting oxide) nanotubes may find new applications in photocatalytic reactions such as water splitting with visible lights. In summary, using low-cost starting chemicals, largescale synthesis of high-quality ultralong copper nanowires can be achieved under mild conditions. The prepared copper nanowires can also be used as starting solid precursor for fabrication of polycrystalline oxide nanotubes via direct oxidation in air. (24) Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711-714. (25) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124-8125.
Acknowledgment. The authors gratefully acknowledge the financial support of the Ministry of Education, Singapore. Supporting Information Available: Tables of experimental conditions for selected samples and figures showing the color changing point at 30-45 min, the [001] zone diffraction spots of Cu nanowires, XRD patterns of some selected copper metal products, an EXD spectrum of copper nanowires, FESEM images of Cu disks, and surface analysis of air-dried copper nanowires. This material is available free of charge via the Internet at http://pubs.acs.org. LA050220W
