Copper-Based Nanowire Materials: Templated Syntheses

Mar 26, 2005 - ... 1D nanomaterials. This material is available free of charge via the Internet at http://pubs.acs.org. ...... Langmuir 2010 26 (3), 2...
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Copper-Based Nanowire Materials: Templated Syntheses, Characterizations, and Applications Xiaogang Wen, Yutao Xie, Chun Lung Choi, Kwun Chung Wan, Xiao-Yuan Li, and Shihe Yang* Department of Chemistry, Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Received January 6, 2005. In Final Form: February 17, 2005 Well-aligned Cu(OH)2 nanoribbon and CuO nanorod arrays have been prepared on copper substrates by liquid-solid reactions. The effects of temperature, reaction time, solvent, and pH value on the morphology and composition of the products are systematically studied. Using the Cu(OH)2 nanoribbons array as a reactive and sacrificial template, we have successfully synthesized Cu2O, Cu9S8, and Cu nanoribbon/ nanowire arrays, demonstrating the versatility of the template. The extensive series of copper-based onedimensional nanomaterials have been fully characterized by various structural, microscopic, and spectroscopic techniques. Moreover, the Cu nanowires are demonstrated to be an excellent surface-enhanced Raman scattering substrate with a sensitivity over an order of magnitude higher than that of a common roughened copper electrode.

Introduction Copper-based compounds are among the most important materials. The metallic Cu itself plays an irreplaceable role in modern electronic circuits due to its excellent electrical conductivity and low cost. Logically, Cu is expected to be an essential component in future nanodevices.1 On the other hand, due to its good biocompatibility and its surface-enhanced Raman scattering (SERS) activity, nanoscale copper may be used as nanoprobes in medicine and bioanalytical areas. CuO-based materials are well-known due to their relevance to high-temperature superconductivity2-4 and semiconducting antiferromagnetism. CuO is a potential field emission (FE) material,5 an important catalyst,6,7 and a gas-sensing medium.8,9 Cu2O is an important p-type semiconductor with a band gap of 2.0 eV, which makes it a promising solar cell material. Recent research indicated that the Cu2O could also be used as a negative electrode material for lithium ion batteries.10 Furthermore, Cu2O is a good catalyst for chemical refinement7 and for photochemical decomposition of water.11 CuxS is an interesting class of materials for their ability to form various stoichiometries, and they have been used for thermoelectric and photoelectric transformers and high-temperature thermistors,12 solar cells,13 superconductors,14 and gas sensors.15 * Corresponding author: [email protected]. (1) Monson, C. F.; Woolley, A. T. Nano. Lett. 2003, 3, 359. (2) Wu, M. K.; Ashburn, J. R.; Torng, C. J.; Hor, P. H.; Meng, R. L.; Gao, L.; Huang, Z. J.; Wang, Y. Q.; Chu, C. W. Phys. Rev. Lett. 1987, 58, 908. (3) He, H.; Bourges, P.; Sidis, Y.; Ulrich, C.; Regnault, L. P.; Pailhes, S.; Berzigiarova, N. S.; Kolesnikov, N. N.; Keimer, B. Science 2002, 295, 1045. (4) Schon, J. H.; Dorget, M.; Beuran, F. C.; Zu, X. Z.; Arushanov, E.; Deville Cavellin, C.; Lagues, M. Nature 2001, 414, 434. (5) Hsieh, C. T.; Chen, J. M.; Lin, H. H.; Shih, H. C. Appl. Phys. Lett. 2003, 83, 3383. (6) Reitz, J. B.; Solomon, E. I. J. Am. Chem. Soc. 1998, 120, 11467. (7) Ramirez-Ortiz, J.; Ogura, T.; Medina-Valtierra, J.; Acosta-Ortiz, S. E.; Bosch, P.; de los Reyes, J. A.; Lara, V. H. Appl. Surf. Sci. 2001, 174, 177. (8) Liao, B.; Wei, Q.; Wang, K. Y.; Liu, Y. X. Sens. Actuators B 2001, 80, 208. (9) Ishihara, T.; Higuchi, M.; Takagi, T.; Ito, M.; Nishiguchi, H.; Takita, Y. J. Mater. Chem. 1998, 8, 2037. (10) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Taracon, J. M. Nature 2000, 407, 496. (11) de Jongh, P. E.; Vanmaekelbergh, D.; Kelly, J. J. Chem. Commun. 1999, 1069.

It has been recognized that shrinking of metal and semiconductor materials to one dimension gives rise to new tunable properties and may be an engine for upcoming technological innovations.16,17 Although many methods have been developed for the synthesis of one-dimensional (1D) materials such as nanorods/nanowires, nanotubes, and nanobelts/nantoribbons,17,18 it remains a challenge to assemble the 1D materials into spatially regular patterns under template-free and mild conditions. Ultimate applications of the 1D nanomaterials will hinge on processing methods that can accomplish such assemblies in a predictive manner and with a low cost. There is an alternative, perhaps more advantageous, which combines synthesis and assembly of the 1D materials in situ on a predefined substrate. We have recently created Cu2S nanowire and Cu(OH)2 nanoribbon arrays vertically aligned on copper substrates using simple gas-solid and liquid-solid reactions, respectively, at room temperature.19,20 The successes were made possible by tweaking the interfacial transport and reaction kinetics aside from the favorable materials characteristics for 1D growth. This has opened avenue for further research and applications of 1D nanomaterials. For example, intriguing field emission properties have been discovered from devices derived from these materials.21 The well-aligned Cu(OH)2 nanoribbon array is an ideal sacrificial template for conversion to nanowire arrays of other Cu-based materials. The reason is that the Cu(OH)2 nanoribbon arrays are structurally and chemically adapt(12) Glazov, V. M.; Shchelikov, O. D.; Burkhanov, A. S. Inorg. Mater. 1989, 25, 633. (13) Reijnen, L.; Meester, B.; Goossens, A.; Schoonman, J. Chem. Vap. Deposition 2003, 9, 15. (14) Liang, W.; Whangbo, M. H. Solid State Commun. 1993, 85, 405. (15) Setkus, A.; Galdikas, A.; Mironas, A.; Simkiene, I.; Ancutiene, I.; Janickis, V.; Kaciulis, S.; Mattogno, G.; Ingo, G. M. Thin Solid Films 2001, 391, 275. (16) Lieber, C. M. MRS Bull. 2003, 486. (17) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. Adv. Mater. 2003, 15, 353. (18) Nanowires and Nanobelts: Materials, Properties and Devices; Wang, Z. L., Ed.; Kluwer Academic/Plenum Publishers: Boston, 2003. (19) Wang, N.; Fung, K. K.; Wang, S. H.; Yang, S. H. J. Cryst. Growth 2001, 233, 226. (20) Wen, X. G.; Zhang, W. X.; Yang, S. H. Langmuir 2003, 19, 5898. (21) Chen, J.; Deng, S. Z.; Xu, N. S.; Wang, S. H.; Wen, X. G.; Yang, S. H.; Yang, C. L.; Wang, J. N.; Ge, W. K. Appl. Phys. Lett. 2002, 80, 3620.

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able for replicating transformations. This has been demonstrated by our preliminary work on the successful conversion of a Cu(OH)2 nanoribbon array to a CuO nanoribbon array by heat treatment at 120-180 °C.20 In view of the importance of the Cu-based materials family, it is desirable to broaden the scope of the chemistry for creating the 1D versions of a more extensive series of these materials by taking full advantage of the versatility of the Cu(OH)2 nanoribbon array. Toward this end, we have conducted systematic studies on (1) the formation of Cu(OH)2 nanoribbon arrays under a broad range of reaction conditions as well as ultrathin CuO nanorod arrays and (2) the application of the Cu(OH)2 nanoribbon array as a reactive and sacrificial template for fabricating 1D nanostructures of Cu2O, Cu9S8, and Cu by dehydration, deoxygenation, sulfurization, and reduction. These persistent efforts lead to the creation of a number of novel Cu-based nanostructured arrays, which have been well characterized. It should be mentioned that some of the Cu-based nanowires mentioned above have been synthesized using different methods. For example, high-aspect-ratio Cu nanowires were grown along atomic step edge lines on Si(111) substrates by two wet process steps.22 Solution syntheses of CuO nanofibers, nanorods, nanowires, and nanoribbons were reported by a number of research groups.23-25 Cu2O nanowires were also synthesized by template methods.26-28 These methods for the syntheses of Cu-based nanowires often require special instruments or additional processing steps such as the removal of templates, or only produce powders with random wire orientations. The uniform Cu-based 1D nanostructures we prepared in the present work are vertically aligned and firmly adhered on copper substrates. Because of the inherent sacrificial conversion characteristics, our method does not need a posttreatment step to remove the hard template. This paper will give a full account of the syntheses and characterization of the extensive series of Cu-based 1D nanostructured arrays. In addition, as an application, we will demonstrate significantly enhanced SERS responses of the Cu nanoparticle wires array transformed from the Cu(OH)2 nanoribbon array. Experiments Materials and Methods. Chemicals used include aqueous solution of ammonia (NH3‚H2O, 25%, Riedel-de Haen), sodium hydroxide (NaOH, 99.0-100.5%, Riedel-de Haen), high-purity copper foil (Cu, 99.99%, Aldrich), hydrogen chloride (HCl, 37%, Scharlau), sodium borohydride (NaBH4, 96%, Merck), hydrogen peroxide (H2O2, 30%, Riedel-de Haen), oxygen gas (O2, 99.5%, Space Gas Ltd.), and hydrogen sulfide (H2S, 99.5%, Special Gas Products Inc.). The procedure for the synthesis of Cu(OH)2 nanoribbons was described previously.20 In brief, a carefully cleaned copper foil (using 4 M HCl and deionized (DI) water successively) was inserted into a diluted ammonia aqueous solution (or ammonia/ ethanol) (0.03 M) in a sealed glass reactor (600 mL). Three reactor temperatures were experimented, including ∼5 °C (obtained with a refrigerator), ∼25 °C (room temperature), and ∼40 °C (obtained with a water bath heater). After an appropriate reaction time, (22) Tokuda, N.; Watanabe, H.; Hojo, D.; Yamasaki, S.; Miki, K.; Yamabe, K. Appl. Surf. Sci. 2004, 237, 528. (23) Lu, C.; Qi, L.; Yang, J.; Zhang, D.; Wu, N.; Ma, J. J. Phys. Chem. B 2004, 108, 17825. (24) Chang, Y.; Zeng, H. C. Cryst. Growth Des. 2004, 4, 397. (25) Yang, R.; Gao, L. Chem. Lett. 2004, 33, 1194. (26) Wu, Y. Y.; Livneh, T.; Zhang, Y. X.; Cheng, G. S.; Wang, J. F.; Tang, J.; Moskovits, M.; Stucky, G. D. Nano Lett. 2004, 4, 2337. (27) Oh, J.; Tak, Y.; Lee, Y. Electrochem. Solid State Lett. 2004, 7, C27. (28) Wang, W. Z.; Varghese, O. K.; Ruan, C. M.; Paulose, M.; Grimes, C. A. J. Mater. Res. 2003, 18, 2756.

Wen et al. a blue layer was formed on the surface of the copper foil. Finally, the Cu foil coated with the product film was taken out, washed with deionized water three times, and dried in air for characterization. The preparation procedure of Cu2O nanoribbons is similar to that of CuO nanoribbons but with significant modifications.20 To start with, the Cu(OH)2 nanoribbon array obtained above was put into a sealed quartz tube (4 cm in diameter and 60 cm in length) and mounted into a horizontal tube furnace with a length of 40 cm. After the quartz tube was purged with N2 for 30 min to remove O2, the N2 flow was set at 80 sccm and the furnace was heated to 120 °C by a rate of 20 °C/min and kept at this temperature for 1 h. After that, the furnace temperature was raised to 180 °C for 1 h and maintained for 1 h and then to 700 °C and maintained for 3 h. Finally, the furnace was cooled to room temperature at the same rate (20 °C/min) and a uniform red film was collected. Cu8S9 nanoribbons were synthesized by reaction of H2S with the Cu(OH)2 nanoribbons. The Cu(OH)2 array sample was fixed on a glass rod and loaded into a sealed glass reactor (600 mL in volume). The reactor was pumped with a mechanical pump (4 L/s) for 10 min to evacuate the air and isolated. One hundred milliliters of H2S gas was injected into the reactor, and a prompt color change of the film from blue to green to black was observed. After reaction for 30 min, the sample was taken out and the residual H2S gas absorbed on the surface was removed in a fume cupboard. For the synthesis of Cu nanowires, the pristine Cu(OH)2 nanoribbon array was placed at the bottom of a small glass bottle (40 mL) and 20 mL of deionized water was added into the reactor. Then 100 µL of aqueous solution of 0.5 M NaBH4 was dropped into the water (100 µL/min). The film turned gradually from blue to green (∼5 min) to brown (∼10 min) to red-brown (∼20 min). Finally, the red-brown film was peeled off from the Cu substrate. After different reaction times (1 min to 2 h), the sample was taken out and washed with ethanol three times to remove the reducing agent. Characterizations. For X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) characterizations, the films on substrates were used directly except that of Cu nanoparticle wires, which was transferred to a silicon or glass substrate gently in absolute ethanol solution and dried in a vacuum. The nanostructures were characterized using JEOL 6300 (for EDX, energy-dispersive analysis X-ray emission) and a JEOL 6300F scanning electron microscope (SEM) at an accelerating voltage of 15 kV. XRD analysis was performed on a Phillips PW 1830 X-ray diffractometer with a 1.5405 Å Cu KR rotating anode point source operated at 40 kV and 40 mA. Transmission electron microscopy (TEM) measurements were conducted on Phillips CM20 and JEOL 2010F transmission electron microscopes using an accelerating voltage of 200 kV. For Cu(OH)2, CuO, Cu9S8, and Cu nanowires, the TEM samples were synthesized directly on the Cu grid. For Cu2O, the nanoribbons were transferred by gently scraping the products on the Cu foil with an amorphous carboncoated copper grid. A physical electronics PHI 5600 multitechnique system with a monochromatic Al KR X-ray source was used for XPS characterization. Thermal analysis was conducted on a TGA/DTA 92 setaram II thermal analyzer. The samples were heated from room temperature to 1100 °C at a rate of 5 °C/min in a steady flow of dry N2 (20 mL/min). UV-vis adsorption measurement was performed using a Perkin-Elmer UV-vis spectrometer in a reflection mode. SERS spectra were obtained on a micro-Raman instrument (Renishaw 1000, Gloucestershire, U.K.) equipped with a He/Ne laser (632.8 nm) and a CCD detector operated at room temperature. A 50× ultralong working distance objective was used in the present study through which a laser spot of ca. 3 µm in diameter was focused onto the surface of working electrode. For acquiring the SERS spectra, a three-electrode spectroelectrochemical cell with quartz window was used which contains a home-constructed working electrode (vide infra), a counter electrode (Pt wires, 99.99%, Aldrich), and a reference electrode (saturated calomel electrode, SCE). For the working electrode carrying Cu nanowires, the pretreated Cu foil (ca. 5 × 5 mm2) was glued onto a disk glassy carbon substrate (2 mm diameter) with Ag conductive epoxy, and the electrode was used directly

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Figure 1. (A) TEM image of Cu(OH)2 nanoribbons synthesized from Cu grid in water for 12 h. (B) SEM image of Cu(OH)2 nanoribbons synthesized from Cu foil in water for 4 days. (C) Cu(OH)2 nanoribbons synthesized from Cu foil in ethanol for 4 days. (D) EDX spectrum of Cu(OH)2 nanoribbons synthesized in ethanol. without any other treatment for the acquisition of SERS spectra. For the electrochemically roughened Cu working electrode, the procedure was similar to the first working electrode except that an untreated (flat) Cu foil of about same size was used and that the Cu foil was then electrochemically roughened with three oxidation-reduction cycles (-0.4 to +0.4 V at 100 mV/min and held at the anodic potential for 10 s in each cycle) in an electrolyte solution containing 0.1 M KCl. This working electrode was rinsed with distilled water before being transferred to the spectroelectrochemical cell for use. The electrolyte solution used for SERS spectra contains 0.1 M KCl and 0.05 M pyridine. A PAR model 273 A potentiostat was used to control the applied potential.

Results and Discussion Synthesis of Cu(OH)2 Nanoribbons. The synthesis and characterization of Cu(OH)2 from Cu foil have already been described previously.20 Only some new results will be reported here concerning the growth of Cu(OH)2 nanoribbons under different reaction conditions. Under appropriate conditions (e.g., T ) 5-25 °C, pH ≈ 9, reaction time ) 12-96 h), we could obtain large area, uniform arrays of long Cu(OH)2 nanoribbons as can be seen in Figure 1A,B. By variation of the reaction conditions such as solvent, temperature, pH, reaction time, etc., nanostructures with different morphologies, structures, and compositions were observed. Control experiments showed that the growth rate of the Cu(OH)2 nanoribbons increased with increasing O2 concentration in the solution. However, when we removed O2 in the solution by purging with highpurity Ar for 4-6 h with other conditions being the same (in ammonia solution, pH ) 9, T ) 25 °C, reaction time ) 96 h), no Cu(OH)2 nanoribbons could be formed on the substrate. Clearly, O2 is necessary for the growth of the Cu(OH)2 nanoribbons through the following reactions: Cu + O2 f Cu2+, Cu2+ + 2OH- f Cu(OH)2. Furthermore, when the water solvent was substituted by ethanol while keeping other conditions the same (in

air, T ) 25 °C, pH ) 9), only barely visible products were formed on the substrate with a faint light blue color. SEM image (Figure 1C) of the products displays a rodlike morphology mostly lying down on the substrate with a remarkable uniformity in both diameter and length. This is in striking contrast with the well-aligned Cu(OH)2 nanoribbons on the substrate synthesized in aqueous solution, which may be due to the more limited supply of reagents in the ethanol solution. To our surprise, we failed to obtain an XRD pattern of the Cu(OH)2 nanoribbons synthesized in ethanol. This may either indicate an amorphous structure or too weak XRD signals, which in turn may be due to either the lying-down configuration of the ultrathin nanoribbons or simply the too small amount of the nanoribbons available. In fact, the small amount of the nanoribbon sample did not allow it to be examined with HRTEM. Nevertheless, the color of the nanoribbons synthesized in ethanol is similar to that obtained in watersblue, which is characteristic of Cu(OH)2. The EDX pattern in Figure 1D indicates that the nanoribbons consist of Cu and O with a ratio of O/Cu being about 2:1. The Cu(OH)2 formula of the nanoribbons is also supported by XPS spectra shown in Supporting Information. It is likely that the growth mechanism of the Cu(OH)2 nanoribbons in ethanol is similar to that in aqueous solution except that the reaction is much slower here. The important steps for the nanoribbon growth involve the nucleation and propagation of the >CuCu< chain. The remarkable uniformity of the Cu(OH)2 nanoribbons obtained in ethanol suggests that the nucleation and growth stages are well separated. Formation of CuO Microballs and Nanorod Arrays. The Cu(OH)2 nanoribbons were susceptible to water loss to form CuO in basic solution. When the Cu(OH)2 nanoribbons (grown at pH ) 9) were kept in the reaction solution for an extended period of time, the film became

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Figure 2. (A) SEM image of the sample grown in the aqueous solution for 10 days at room temperature (pH ) 9). The microballs are made of CuO. (B) A magnified view of (A). (C) EDX spectrum of the microballs shown in (B) (inset: XRD pattern of the microballs). (D) SEM image of microballs synthesized in water for 2 days at 40 °C.

black from blue and was easy to peel off from the surface of the substrate and deposited on the bottom of the reactor. Figure 2A shows an SEM image of the sample grown in the aqueous solution for 10 days. Clearly, most of the Cu(OH)2 nanoribbons have been transformed to ball-like structures with smooth surfaces and diameters up to 10 µm (Figure 2B). The EDX spectrum in Figure 2C on a single microball gave the O/Cu ratio of about 1:1. The XRD pattern proved the monoclinic structure of the CuO microballs. Increased temperature enhanced the formation of CuO. For example, the sample synthesized in water for 2 days at 40 °C also produced abundant CuO microballs (Figure 2D), which are similar to the results of those synthesized at room temperature for 10 days. It is believed that the morphological transformation is due to the metastable nature of both the Cu(OH)2 and the CuO nanoribbons in the basic solution. When the pH value was increased to ∼10-11, no blue Cu(OH)2 nanoribbons were formed on the substrate. Instead, the color of the Cu foil surface became darker and darker gradually with the lapse of time. Eventually, it turned black after about 24 h. SEM images of the sample prepared at a pH ) 11 show a uniform film on the substrate surface (Figure 3A), which comprises vertically aligned ultrathin nanorods (Figure 3B,C). The XRD pattern in Figure 3D demonstrates the nanorods are monoclinic CuO crystals. Figure 3E is a TEM image of the CuO nanorods grown on the Cu grid directly, which shows an average diameter of only 5 nm and lengths of several hundred nanometers. Clear fringes were observed by HRTEM (Figure 3F) on a single CuO nanorod, running perpendicular to the nanorod axis. The fringe spacing measures 0.17 nm, which concurs well with the d value of monoclinic CuO (020) crystal plane, suggesting the CuO nanorod growth along the [010] direction. Another CuO nanorod in Figure 3G displayed fringes parallel to the nanorod axis with a spacing of 0.25 nm, which is close to the

interplanar spacing of CuO (002) and also consistent with [010] growth direction. Summing up, uniform CuO nanorod arrays can be fabricated in one step using the very simple method described above. Such nanorod arrays could be useful for sensors or hole-transport media of dyesensitized heterojunctions for solar energy conversion.29 Conversion from Cu(OH)2 Nanoribbon Arrays to Cu2O Nanoribbon Arrays. Figure 4 presents results from thermogravimetric analysis differential thermal analysis (TGA-DTA) of the Cu(OH)2 nanoribbons, which were peeled off from the Cu substrate using a stainless steel surgical blade. Two distinct endothermal peaks appear at 149 and 832 °C, respectively, in the DTA curve. Correspondingly, two weight-loss steps were detected around these temperatures in the TGA curve. According to the XRD analysis and weight-loss calculations, the lowtemperature peak (149 °C) is associated with the loss of water to form CuO and the high-temperature peak (832 °C) marks the conversion of CuO to Cu2O, which is also consistent with the color change from blue to black to red. Black CuO was reported to decompose to red Cu2O and O2 at 1030 °C at atmospheric pressure in air,30 but our CuO nanoribbons were transformed to Cu2O at a much lower temperature, which is attributed to the effect of the nanoscale size. In fact, we have succeeded in converting the Cu(OH)2 ribbons array to the Cu2O nanoribbons array at an even lower temperature (600 °C) in Ar at 1 atm. Shown in Figure 5A is an SEM image of the Cu2O nanoribbons array transformed from Cu(OH)2 (700 °C, 5 h). Clearly, the nanoribbon morphology is preserved. The higher magnification image (Figure 5B) indicates that some nanoribbon tips became bent and the edges became rough, compared to the original Cu(OH)2 nanoribbons (29) Anandan, S.; Wen, X. G.; Yang, S. H. Mater. Chem. Phys., in press. (30) Richardson, H. W. Handbook of Copper Compounds and Applications; Marcel Dekker: New York, 1997; P58.

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Figure 3. (A-C) SEM images of as-synthesized CuO nanorods array on a Cu substrate at different magnifications. (D) XRD pattern of the CuO nanorod arrays. (E) TEM image of as-synthesized CuO nanorods on a Cu grid. (F, G) HRTEM images of two selected single CuO nanorods.

shown in the inset of Figure 5B. A TEM image of a single Cu2O nanoribbon is shown in Figure 5C. Although the nanoribbon morphology is maintained, breakage points and missing parts at the edges can be frequently found due perhaps to the stress from high-temperature reactions that resulted in the release of H2O and O2 accompanying the formation of Cu2O. The HRTEM image in Figure 5D shows that the Cu2O nanoribbon is not a single crystal but comprises a myriad of little crystal particles, which are randomly oriented. As an example of two neighboring nanocrystals, different crystal orientations were observed: one is [110] with an interplanar spacing of 0.3 nm and the other is [111] with an interplanar spacing of 0.24 nm. This suggests that due to the large structural difference, the initial single-crystal structure is difficult to maintain for the transformation from Cu(OH)2 to CuO to Cu2O (orthorhombic to monoclinic to cubic) under our reaction conditions. Instead, small nanocrystal domains were formed so as to accommodate the stress arising from

Figure 4. DTA (left arrow) and TGA (right arrow) curves of Cu(OH)2 nanoribbons collected from Cu substrates (synthesized at room temperature, pH ) 9, reaction time ) 4 days).

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Figure 5. (A) SEM image of as-prepared Cu2O nanoribbons array on a Cu substrate. (B) Enlarged view of (A) (inset, as-synthesized Cu(OH)2 nanoribbons). (C) TEM image of a single Cu2O nanoribbon. (D) HRTEM image of a single Cu2O nanoribbon. (E) ED pattern of a single-crystal particle in a Cu2O nanoribbon. (F) ED pattern of a single polycrystalline Cu2O nanoribbon. (G) XRD pattern of the Cu2O nanoribbons.

the structural transformation. Electron diffraction studies indicate that the individual nanocrystals were single crystals (Figure 5E), but the whole nanoribbon was a polycrystal (Figure 5F). Taken together, the transformation of Cu(OH)2 to Cu2O consists of the following two steps

Cu(OH)2 f CuO + H2O 4CuO f 2Cu2O + O2

120 °C 600 °C

(1) (2)

At about 120 °C, Cu(OH)2 lost water and CuO was formed. The CuO nanoribbons started to evolve to Cu2O at about 600 °C by releasing O2. The temperature-dependent phase information was obtained from XRD investigations. Figure

5G presents the XRD patterns of the Cu2O products prepared under the same reaction conditions (5 h, Ar flow rate ) 80 sccm) except different temperatures. At 600 °C, a slow transformation to Cu2O has already started but the intense monoclinic CuO peaks could still be observed. When the temperature was increased to 650 °C, the transformation rate was increased significantly, although a small signal of CuO was still present. Finally, the transformation at 700 °C was complete with no CuO signal left. In a word, slow conversion of CuO nanoribbons to Cu2O nanoribbons can occur at about 600 °C and 1 atm of Ar, but transformation can proceed quickly at 700 °C, which is still much lower than that of the bulk material. Cu9S8 Nanoribbons from the Cu(OH)2 Nanoribbons. Figure 6A shows an SEM image of Cu9S8 nanor-

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Figure 6. (A, B) SEM images of as-synthesized Cu9S8 nanoribbons array on a Cu substrate at different magnifications. (C) TEM image of as-synthesized Cu9S8 nanoribbons on a Cu grid. (D) XRD pattern of the Cu9S8 nanoribbons. (E) HRTEM image of a single Cu9S8 nanoribbon (inset, ED pattern of the nanoribbon).

ibbons transformed from the Cu(OH)2 nanoribbons. The nanoribbons became less well aligned because of the reaction Cu(OH)2 + H2S f Cu9S8 + H2O. It can be seen from the higher magnification image in Figure 6B that some nanoribbons bundled together probably induced by the reaction between Cu(OH)2 and H2S. Figure 6C displays a TEM image of the Cu9S8 nanoribbons. The ribbon structure is evident from the image. Although the average width of the original Cu(OH)2 nanoribbons was only about 60 nm, that of Cu9S8 was about 250 nm. We believe that the width increase was mainly caused by the bundling of several nanoribbons during the transformation process. The XRD pattern of as-synthesized Cu9S8 sample matches the hexagonal Cu9S8 very well (see Figure 6D). As can be seen from the HRTEM image in Figure 6E, the nanoribbon consists of many small nanocrystals, and the clear fringes in different areas concur well with the d values of (1021) and (1013) crystal planes of hexagonal Cu9S8. The polycrystalline nature is also reflected in the SAED pattern in the inset of Figure 6E. It is worth mentioning that Cu9S8 nanowires were synthesized recently by using a hydrothermal method and their photoluminescence prop-

erties in the range from 300 to 500 nm have been revealed.31,32 Cu nanoparticle Wires from the Cu(OH)2 Nanoribbons. The conversion from the Cu(OH)2 nanoribbons to Cu nanowires was also realized by using NaBH4 as a reducing agent at the liquid/solid interface. The results are presented in Figure 7. Figure 7A is a typical SEM image of the Cu nanowires converted from the Cu(OH)2 nanoribbons. Although the surface became somewhat rough, the wirelike morphology is preserved after the reducing treatment. As shown in Figure 7B, the XRD analysis of the as-synthesized Cu nanowires peeled from the substrate indicates a complete conversion of Cu(OH)2 to cubic copper with no residual signal of Cu(OH)2 detected. This is confirmed by the SAED pattern from a single nanowire (see the insert of Figure 7B). Figure 7C shows TEM images of the sample after different reducing treatment times. The reducing reaction was very fast. After only 1 min, the nanoribbon surface began to be (31) Jiang, X. C.; Xie, Y.; Lu, J.; He, W.; Zhu, L. Y.; Qian, Y. T. J. Mater. Chem. 2000, 10, 2193. (32) Xu, C. Q.; Zhang, Z. C.; Ye, Q.; Liu, X. Chem. Lett. 2003, 32, 198.

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Figure 7. (A) SEM image of Cu nanowires converted from the Cu(OH)2 nanoribbons. (B) XRD pattern of the Cu nanowires (inset, ED pattern of a single Cu nanowire). (C) TEM images of Cu nanowires with different reaction times. (D) HRTEM image of a single Cu nanowire.

roughened. With longer reaction times, small nanoparticles appeared on the nanoribbon surfaces and grew larger with time. Eventually, the nanoribbons were changed to nanowires composed of Cu nanoparticles, which are 1020 nm in diameter. Figure 7D displays a HRTEM image near the side surface of a single Cu nanowire. Indeed, it is made up of many Cu nanoparticles with random crystal orientations. The redox reaction was 4Cu(OH)2 + NaBH4 f 4Cu + NaH2BO3 + 5H2O (E° ) 1.02 V). The Cu nanoparticles may be formed by the diffusion of BH4- on the Cu(OH)2 nanoribbon surface, followed by electron transfer to Cu2+. OH- was then released from the nanoribbon and Cu nanoparticles were formed. However, before the nanoribbon fully collapsed, the Cu nanoparticles aggregated together, and a nanoparticle-connected nanowire was eventually formed after the conversion from Cu(OH)2 to Cu was completed. Surface-Enhanced Raman Scattering from Molecules Adsorbed on Cu Nanoparticle Wires. Nanostructured Cu substrates such as electrochemically roughened Cu electrode and aggregated Cu colloids have been demonstrated to be effective SERS-active substrates.33-35 The nanoparticle aggregation structure of the Cu nanowires described above suggests that they may display excellent SERS activity because of the expected strong (33) Creighton, J. A.; Alvarez, M. S.; Woltz, D. A.; Garoff, S.; Kim, M. W. J. Phys. Chem. 1983, 87, 4793. (34) Pettinger, B.; Wenning, U.; Wetzel, H. Surf. Sci. 1980, 101, 409. (35) Temperini, M. L. A.; Chagas, H. C.; Sala, O. Chem. Phys. Lett. 1981, 79, 75.

field enhancement in the nanoparticle contact regions. Here we compare the SERS activity of our as-prepared Cu nanowires and that of a roughened copper foil using pyridine as the probe molecule. Figure 8 shows SERS spectra of pyridine adsorbed on the Cu nanowires (A) and the roughened Cu foil (B) obtained at different applied potentials. On both the Cu nanowires and the roughened Cu foil substrates, the typical SERS peaks of pyridine on Cu were observed clearly. When the applied potential was set to more negative, the peak intensities increased markedly. At ca. -0.6 V, a maximum intensity was reached for the peak at ∼1010 cm-1 (a1). Its peak intensity started to decrease when the applied potential became more negative. However, other peaks at 1596 cm-1 (a1), 1212 cm-1 (a1), and ∼631 cm-1 (a1) still kept increasing until ∼1.0 V. This pattern of change is more profound for SERS spectra from the Cu nanowires. Very weak peaks at ∼1641 cm-1, 1568 cm-1 (b2), 1479 cm-1 (a1), 1441 cm-1 (b2), 1234 cm-1 (b2), 1151 cm-1 (b2), 1061 cm-1 (a1, b2), 1035 cm-1 (a1), 939 cm-1 (b1), 751 cm-1 (b1), 694 cm-1 (b1), 417 cm-1 (b1), and 241 cm-1 (νCu-N) could be visualized for pyridine adsorbed on the Cu nanowires. These peaks are consistent with those reported in the literature.33 By comparison of the SERS results in parts A and B of Figure 8, it is found that the peak intensity from the Cu nanowires is in general higher than that from the electrochemicaly roughened Cu foil, presumably due to the significantly increased surface area in the former. At the open-circuit potential (OCP), the

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Langmuir, Vol. 21, No. 10, 2005 4737

Figure 8. (A) Potential-dependent SERS spectra of pyridine (0.05 M) on Cu nanowires/GC electrode. (B) Potential-dependent SERS spectra of pyridine (0.05 M) on a Cu plate roughened in HCl by ORC. The electrolyte used is 0.1 M KCl both for SERS measurements and for electrode roughening. Other SERS conditions: 632.8 nm; 3 mW; 10 s; four scans.

intensity ratio (ICu nanowires/ICu plate) is calculated to be ∼6. This indicates that the Cu nanowires have more SERSactive sites (hot sites) than the roughened Cu electrode.34 As the negative applied potential was increased, both the varied orientation of the adsorbed pyridine and the negative potential itself contributed to the SERS enhancement, so some of the peaks reached the maxima. When the negative applied potential was increased to -0.4 V, the ratio of ICu nanowires/ICu plate increased sharply to about 15. Upon further increase of the negative potential, the intensity ratio started to decrease, but it was generally g1. Our results demonstrate that the SERS spectra from Cu nanowires is over an order of magnitude higher than that from a roughened copper electrode. Most of the nanomaterials described above have also been subjected to XPS and UV-vis absorption measurements (see Supporting Information). These results further confirm the compositions and structures of the corresponding materials. Conclusions The work reported in this paper is developed from our previous success on the synthesis of well-aligned Cu(OH)2 nanoribbon arrays on Cu substrates by simple liquidsolid reactions at room temperature. More detailed studies have identified appropriate reaction conditions for the Cu(OH)2 nanoribbon growth including solvent, pH value of the solution, temperature, and reaction time. While the growth in water resulted in vertically aligned single-

crystal Cu(OH)2 nanoribbons tens of µm long and 20-150 nm wide, the Cu(OH)2 nanoribbons grown in ethanol were all lying down with an excellent size uniformity (4-5 µm long and 100-300 nm wide). Long reaction time, higher reaction temperature, and high pH value tend to accelerate the conversion to CuO. In particular, a thin film of aligned CuO nanorod array has been successfully fabricated by adjusting the above parameters. More importantly, with Cu(OH)2 nanoribbons as a reactive and sacrificial template, a series of Cu-based nanoribbons or nanowires have been created, which include Cu2O, Cu9S8, and Cu nanoribbons/nanowires. This demonstrates the versatile utility of the Cu(OH)2 nanoribbons as a precursor for the synthesis of novel Cu-based 1D materials. Finally, in light of the potential applications of these novel 1D materials, we have demonstrated the excellent SERS activity of the particulate Cu nanowires. Acknowledgment. We are grateful to the Research Grant Council of Hong Kong and the Chemistry Department of the Hong Kong University of Science and Technology for supporting the research. S.Y. wishes to thank the Hong Kong Young Scholar Cooperation Research Foundation of NSFC. Supporting Information Available: XPS and UVvis spectra of Cu-based 1D nanomaterials. This material is available free of charge via the Internet at http://pubs.acs.org. LA050038V