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Synthesis of Cu(OH)2 and CuO Nanoribbon Arrays on a Copper Surface Xiaogang Wen, Weixin Zhang, 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 February 19, 2003. In Final Form: April 11, 2003 Cu(OH)2 and CuO nanoribbon arrays aligned approximately perpendicular to copper substrate surfaces are synthesized by the solution-treatment and subsequent heat-treatment processes. The Cu(OH)2 nanoribbons are fabricated by a simple coordination self-assembly method in an alkaline solution with Cu2+ ions being from the surface oxidation of copper. The CuO nanoribbons are formed by removing water from the Cu(OH)2 nanoribbons through heat treatment. The nanoribbons are ∼50-60 nm in average width and several nanometers in thickness, and the lengths can be well-controlled by varying the reaction temperature and time interval. Transmission electron microscopy, high-resolution TEM, scanning electron microscopy, electron diffraction, and X-ray diffraction techniques have been used to characterize the microstructures and morphologies of the nanoribbon materials.
Introduction One-dimensional (1D) semiconductor and metal nanomaterials are considered to be key structural components of electronic, magnetic, and photonic devices. Their unique properties could be harnessed for the design and fabrication of nanosensors,1,2 switches,2 nanolasers,3 and transistors.4 Many possible applications of 1D nanomaterials require the formation of well-aligned arrays to accentuate the anisotropy and satisfy the criterions of device design. The porous alumina-based template technique has been the workhorse for the fabrication of numerous metal and semiconductor nanowire arrays, including Co,5,6 Sb,7 FexAg1-x,8 C70,9 In2O3,10 and CdS.11,12 Other templates, such as nanoporous single-crystal mica,13 “track-etch” polycarbonate membranes,14 and self-assembled calix[4]hydroquinone nanotubes,15 have also been used to prepare nanowire arrays. On the other hand, the template-free synthesis of nanowire arrays has also been achieved but mostly involved vapor-phase processes. Xie et al.16 and Ren et al.17 synthesized carbon nanotube arrays by * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Cui, Y.; Wei, Q. Q.; Park, H. K.; Lieber, C. M. Science 2001, 293, 1289. (2) Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 293, 2227. (3) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (4) Lauhon, L. J.; Gudiksen, M. S.; Wang, D. L.; Lieber, C. M. Nature 2002, 420, 57. (5) Cao, H. Q.; Xu, Z.; Sang, H.; Sheng, D.; Tie, C. Y. Adv. Mater. 2001, 13, 121. (6) Garcia, J. M.; Asenjo, A.; Velazquez, J.; Garcia, D.; Vazquez, M.; Aranda, P.; Ruiz-Hitzky, E. J. Appl. Phys. 1999, 85, 5480. (7) Zhang, Y.; Li, G. H.; Wu, Y. C.; Zhang, B.; Song, W. H.; Zhang, L. D. Adv. Mater. 2002, 14, 1227. (8) Wang, Y. W.; Meng, G. W.; Liang, C. H.; Wang, G. Z.; Zhang, L. D. Chem. Phys. Lett. 2001, 339, 174. (9) Cao, H. Q.; Xu, Z.; Wei, X. W.; Ma, X.; Xue, Z. L. Chem. Commun. 2001, 541. (10) Zheng, M. J.; Zhang, L. D.; Zhang, X. Y.; Zhang, J.; Li, G. H. Chem. Phys. Lett. 2001, 334, 298. (11) Cao, H. Q.; Xu, Y.; Hong, J. M.; Liu, H. B.; Yin, C.; Li, B. L.; Tie, Z. Y.; Xu, Z. Adv. Mater. 2001, 13, 1393. (12) Routkevitch, D.; Bigioni, T.; Moskovits, M.; Xu, J. M. J. Phys. Chem. 1996, 100, 14037. (13) Sun, L.; Searson, P. C.; Chien, C. L. Phys. Rev. B 2000, 61, 6463. (14) Matin, C. R. Chem. Mater. 1996, 8, 1739. (15) Hong, B. H.; Bae, S. C.; Lee, C. W.; Jeong, S.; Kim, K. S. Science 2001, 294, 348.
chemical vapor deposition. Wang et al.18 prepared silica nanowire arrays by a vapor-liquid-solid (VLS) route. Ruhle et al.19 grew boron nanowire arrays on a Si substrate by a magnetron sputtering approach. Lee et al.20 fabricated silicon cone arrays by ion-beam sputtering. Our group also synthesized Cu2S nanowire arrays on copper through gas-solid reactions at room temperature.21,22 Until now, to our knowledge, no semiconductor nanoribbon arrays have been synthesized in solution by a template-free approach. Here, we report the use of a solutionbased method to fabricate well-aligned Cu(OH)2 nanoribbon arrays on a copper surface without using any template and surfactant at a low temperature. Through further heat treatment at 120-180 °C, we have achieved the transformation of the Cu(OH)2 nanoribbon arrays to CuO nanoribbon arrays without obvious morphological changes. Orthorhombic Cu(OH)2 is a well-known layered material. The magnetic properties of Cu(OH)2 are remarkably sensitive to the intercalation of molecular anions,23-25 making the material a candidate for sensor applications. Cu(OH)2 nanowires have been thought to be the precursors for the synthesis of Cu2O nanowires.26 Most recently, we have synthesized Cu(OH)2 nanoribbons with a high aspect ratio by coordination self-assembly in solution using Cu2S nanowires as precursors.27 The present work goes a step further to synthesize Cu(OH)2 nanoribbon arrays on a copper surface in a controlled fashion. Here, the synthesis (16) Li, W. Z.; Xie, S. S.; Qian, L. X.; Chang, B. H.; Zou, B. S.; Zhou, W. Y.; Zhao, R. A.; Wang, G. Science 1996, 274, 1701. (17) Ren, Z. F.; Huang, Z. P.; Xu, J. W.; Wang, J. H.; Bush, P.; Siegal, M. P.; Provencio, P. N. Science 1998, 282, 1105. (18) Pan, Z. W.; Dai, Z. R.; Ma, C.; Wang, Z. L. J. Am. Chem. Soc. 2002, 124, 1817. (19) Cao, L. M.; Hahn, K.; Wang, Y. Q.; Scheu, C.; Zhang, Z.; Gao, C. X.; Li, Y. C.; Zhang, X. Y.; Sun, L. L.; Wang, W. K.; Ruhle, M. Adv. Mater. 2002, 14, 1294. (20) Shang, N. G.; Meng, F. Y.; Au, F. C. K.; Li, Q.; Lee, C. S.; Bello, I.; Lee, S. T. Adv. Mater. 2002, 14, 1308. (21) Wang, S. H.; Yang, S. H. Chem. Phys. Lett. 2000, 322, 567. (22) Chen, J.; Deng, S. Z.; Xu, L. 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. (23) Fujita, W.; Awaga, K. Synth. Met. 2001, 122, 569. (24) Fujita, W.; Awaga, K. Inorg. Chem. 1996, 35, 1915. (25) Fujita, W.; Awaga, K. J. Am. Chem. Soc. 1997, 45633. (26) Wang, W. Z.; Wang, G. H.; Wang, X. S.; Zhan, R. J.; Liu, Y. K.; Zheng, C. L. Adv. Mater. 2002, 14, 67.
10.1021/la0342870 CCC: $25.00 © 2003 American Chemical Society Published on Web 06/04/2003
Cu(OH)2 and CuO Nanoribbon Arrays
Figure 1. SEM images of a Cu(OH)2 nanoribbon array. (A) Top view. (B) Side view.
is accomplished under a more basic condition and relies on the supply of O2. CuO is a p-type semiconductor with a narrow band gap (1.2 eV) and has been studied extensively as an important component of copper oxide superconductors.28-32 CuO has also been widely used as a catalyst33,34 and in sensors.35,36 In a new development of late, Kaito et al.37 succeeded in using CuO nanowires to template the growth of carbon nanotubes. So far, CuO nanowires have been mostly prepared by high-temperature approaches.37-39 The synthesis of CuO nanoribbon arrays on a metallic surface has not been reported as yet. Experimental Section Preparation. A typical synthesis of Cu(OH)2 nanoribbons was carried out as follows. First, a copper foil was washed with a 4.0 M HCl aqueous solution for ∼15 min and subsequently with deionized water three times to remove surface impurities. (27) Wen, X. G.; Zhang, W. X.; Yang, S. H.; Dai, Z. R.; Wang, Z. L. Nano Lett. 2002, 2, 1397. (28) 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. (29) Lang, K. M.; Madhavan, V.; Hoffman, J. E.; Hudson, E. W.; Eisaki, H.; Uchida, S.; Davis, J. C. Nature 2002, 415, 412. (30) Schon, J. H.; Dorget, M.; Beuran, F. C.; Zu, X. Z.; Arushanov, E.; Cavellin, C. D.; Lagues, M. Nature 2001, 414, 434. (31) Yoon, S.; Dai, H. J.; Liu, J.; Lieber, C. R. Science 1994, 265, 215. (32) Hodges, J. A.; Sidis, Y.; Bourges, P.; Mirebeau, I.; Hennion, M.; Chaud, X. Phys. Rev. B 2002, 66, 020501. (33) Ramirez-Ortiz, J.; Ogura, T.; Medina-Valtierra, J.; Acota-Ortiz, S. E.; Bosch, P.; de los Reyes, J. A.; Lara, V. H. Appl. Surf. Sci. 2001, 174, 177. (34) Reitz, J. B.; Solomon, E. I.; J. Am. Chem. Soc. 1998, 120, 11467. (35) Liao, B.; Wei, Q.; Wang, K. Y.; Liu, Y. X. Sens. Actuators, B 2001, 80, 208. (36) Ishihara, T.; Higuchi, M.; Takagi, T. Ito, M.; Nishiguchi, H.; Takita, Y. J. Mater. Chem. 1998, 8, 2037. (37) Suzuki, H.; Hukuzawa, N.; Tanigaki, T.; Sato, T.; Kido, O.; Kimura, Y.; Kaito, C. J. Cryst. Growth 2002, 244, 168.
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Figure 2. TEM images of Cu(OH)2 nanoribbons at different magnifications. Inset of part B: ED pattern of the single Cu(OH)2 nanoribbon. Inset of part C: HRTEM image at the boundary section of the nanoribbon. (The samples of A-C were all grown at 25 °C.) The washed copper foil was then immersed into a dilute solution of ammonia (Aldrich, 3.3 × 10-2 M, pH ≈ 10). After a given reaction time, the sample was taken out of the solution, washed with deionized water three times, and dried in air. A blue film was obtained, which covered uniformly on the Cu substrate. For the preparation of CuO nanoribbons, the Cu(OH)2 nanoribbon sample was loaded into a quartz boat and positioned at the end of a quartz tube. The quartz tube was then sealed and mounted inside a horizontal tube furnace (1.2 m in length and 10 cm in diameter). After purging with N2 for 15 min to remove O2 in the quartz tube, the furnace was heated to 120 °C, maintained at this temperature for 3 h to complete dehydration, and then set at 180 °C for 2 h to promote crystallization. Afterward, the furnace was naturally cooled to room temperature. The whole process was carried out with a constant flow of N2 (∼25 sccm), and the pressure inside the quartz tube was kept at 1 atm. Finally, the sample was collected for characterization. Characterization. The morphology of the nanoribbon materials obtained above was characterized using JEOL 6300 scanning electron microscopy (SEM) at an accelerating voltage of 15 kV. The phase identification was carried out using powder X-ray diffraction (XRD; Philips PW-1830). Further microstructure analysis was conducted using transmission electron microscopy (TEM; Philips CM20 and JEOL 2010 microscope operating at 200 kV). Two different methods were used to prepare the samples for the TEM characterization. One is the growth of Cu(OH)2 nanoribbons directly on TEM Cu grids, and the other involves the scraping of the Cu(OH)2 nanoribbons off of the Cu foils and onto the TEM grids.
Results and Discussion Shown in Figure 1 are SEM images of a Cu(OH)2 nanoribbon film, which was grown on a copper foil substrate after reaction for 96 h. As can be seen from the top view (38) Jiang, X. C.; Herricks, T.; Xia, Y. N. Nano Lett. 2002, 2, 1333. (39) Wang, S. H.; Huang, Q. J.; Wen, X. G.; Li, X. Y.; Yang, S. H. Phys. Chem. Chem. Phys. 2002, 4, 3425.
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Figure 3. SEM images of Cu(OH)2 nanoribbons synthsized at different temperatures and durations. (A) 25 °C, 12 h. (B) 25 °C, 24 h. (C) 25 °C, 96 h. (D) 5 °C, 12 h. (E) 5 °C, 24 h. (F) 5 °C, 96 h. Insets of parts A-C: the same SEM images with 10× magnification. Insets of parts D-F: the same SEM images with 5× magnification.
(part A), the nanoribbons cover the copper surface uniformly, smoothly, and compactly. In addition, the nanoribbons are roughly aligned perpendicular to the copper surface. Such an alignment can be better perceived from the side view in part B. The nanoribbons average ∼20-µm long and are approximately arrayed uprightly on the copper surface. Note that the nanoribbons are better aligned on the top than on the bottom. This indicates that the nanoribbon alignment is caused by the space-limited growth. Initially, the nanoribbon growth can explore all the directions above the copper surface. However, as the nanoribbons grow longer, the growth of more tilted nanoribbons is more likely to be hindered, whereas the upright nanoribbons grow unabated. More detailed discussion on this will follow. Figure 2A shows the low-magnification TEM image of the Cu(OH)2 nanoribbons grown directly from the copper grid at room temperature for 12 h. The thinning in the bend and wring sections, which would not occur for
cylindrical nanowires (see arrows in Figure 2A), demonstrates the ribbonlike morphology. These nanoribbons have widths of 20-130 nm (∼55 nm on average) and thicknesses of a few nanometers along the whole lengths of several tens of micrometers. Nanoribbons of widths as small as 8 nm can also be found. Figure 2B shows a lowmagnification TEM image of a typical single Cu(OH)2 nanoribbon with a width of 60 nm. Although some tiny cavities can be seen on the surface, the nanoribbon is actually a single crystal, as is evidenced by the corresponding electron diffraction (ED) pattern (inset of Figure 2B). The spotted selected area ED (SAED) pattern appears to be associated with the [010] zone axis of orthorhombic Cu(OH)2, and it shows that the growth direction of a single nanoribbon is [100]. A high-resolution transmission electron microscopy (HRTEM) image of the single nanoribbon is shown in Figure 2C. The fringe spacing of 0.28 nm matches well the distance between the (110) crystal planes. As the inset of Figure 2C shows, the [110] direction
Cu(OH)2 and CuO Nanoribbon Arrays
makes an angle (∼20°) with the axis of the nanoribbon, which is close to the angle between [100] and [110] of orthorhombic Cu(OH)2. This further confirms the nanoribbon growth direction of [100]. The single-crystal structure of the nanoribbon is evident from the clear fringe pattern in the HRTEM image, although some crystal defects such as edge dislocations have been spotted (see the rectangular dashed-line boxes). To understand the growth process of the Cu(OH)2 nanoribbons, we have studied the effects of reaction time and temperature. The results are presented in Figure 3, which consist of SEM images of Cu(OH)2 nanoribbons prepared at different reaction temperatures and for different time periods. Parts A-C of Figure 3 are SEM images of the samples synthesized at room temperature (25 °C) for 12, 24, and 96 h, respectively. When the reaction time is short (12 h), the Cu(OH)2 nanoribbons are sparse (∼30 µm in length) and lie down on the substrate (Figure 3A). As the reaction time is increased to 24 h, the nanoribbons grow longer (∼50 µm long) and their density higher, but the lying-down nanoribbon assemblies are still dominant (Figure 3B). For a reaction time up to 96 h, the Cu(OH)2 nanoribbons become much longer (∼100 µm) and almost completely cover the Cu substrate surface (Figure 3C). The density of the nanoribbons is so high that they start to grow vertically and form a honeycomblike structure. Parts D-F of Figure 3 show SEM images of the samples grown at 5 °C for 12, 24, and 96 h, respectively. The nanoribbons synthesized at the lower temperature appear to be quite different. First, the density of nanoribbons grown at the lower temperature is much higher than that at the higher temperature for the same reaction time. At the same time, the nanoribbons are more likely to grow uprightly. Similarly, with the increase of reaction time, the density of the nanoribbons becomes higher and higher. When the reaction time is 96 h, the nanoribbons cover the whole substrate uniformly and form a well-aligned Cu(OH)2 nanoribbon array on the copper substrate (Figure 3F). As another difference, the nanoribbons grown at the lower temperature are much shorter; even after growth for 96 h, the nanoribbons are still only ∼20 µm long (see Figure 1B). It appears that the nucleation of Cu(OH)2 crystals is faster at the lower temperature so that more nanoribbons grow at the same time, resulting in the higher nanoribbon densities than that at the higher temperature. These dense nanoribbons can then support each other, sustaining the vertical growth instead of the lying-down growth. On the other hand, the competitive growth of the much more abundant nanoribbons at the lower temperature inevitably slows down the growth rates and consequently the lengths of the individual nanoribbons. The TEM image in Figure 4 provides a side view of the Cu(OH)2 nanoribbons grown at 5 °C and with a reaction time of 12 h. Clearly, the nanoribbons grown at this lower temperature are more abundant and have a smaller width (∼30 nm), but they can easily bundle together from the bottom up and form a relatively aligned structure. The nanoribbons have an average length of 5 µm, which is much shorter than that grown at the higher temperature. The differences described above between the Cu(OH)2 nanoribbons prepared at the lower and higher temperatures is apparent when the TEM pictures in Figure 4 and Figure 2A are compared. XRD analysis was performed to further study the crystal structure and alignment of the Cu(OH)2 nanoribbons. Figure 5A shows XRD patterns of as-synthesized Cu(OH)2 nanoribbons on the Cu surface, which were prepared at different temperatures and reaction durations. In general, with the increase of the reaction time, the relative
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Figure 4. TEM image of a Cu(OH)2 nanoribbon sample grown at 5 °C and for 12 h.
Figure 5. XRD patterns of Cu(OH)2 nanoribbon samples grown at different temperatures and duractions. (A) As-synthesized on the Cu substrate. (B) Powder samples scraped from the Cu substrate.
intensities of the diffraction peaks (110), (111), (130), and (200) increase dramatically. On the other hand, these peaks are relatively weak in the corresponding powder XRD profiles of the nanoribbons (see Figure 5B) and the
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Figure 6. TEM image (A), XRD pattern (B), SEM image (C), and HRTEM image (D) of CuO nanoribbons converted from the Cu(OH)2 nanoribbons by heat treatment. Inset of part A: SAED pattern of an ensemble of CuO nanoribbons. Inset of part D: SAED pattern of a single CuO nanoribbon.
standard powder XRD data for the orthorhombic Cu(OH)2 [1998-JCPDS, 13-0420, wherein, taking the (021) diffraction peak intensity as 100, the relative intensities of the four peaks are 2, 16, 35, and 8, respectively]. Apparently, the deviation of the XRD diffraction pattern of the Cu(OH)2 nanoribbon film from the powder diffraction pattern is due to the rough alignment of the nanoribbons perpendicular to the copper substrate surface. This is consistent with our deduction above that the Cu(OH)2 nanoribbons grow along the [100] direction and are aligned vertically on the copper substrate surface. As a result of this nanoribbon alignment, the relative intensities of the (020), (021), and (002) diffraction peaks decrease markedly compared to those in the powder XRD data because these planes are orthogonal or nearly orthogonal to the (100) crystal plane. It is noticed that the enhancement of the diffraction peaks (110), (111), (130), and (200) seems to be more obvious at the lower nanoribbon growth temperature. This hints at a better nanoribbon alignment on the Cu surface at the lower temperature, which is consistent with the SEM observations shown in Figure 3. Furthermore, small monoclinic CuO diffraction peaks are recognized in the sample grown at the higher temperature after a long reaction time (96 h), whereas these peaks are not observed at the lower growth temperature. The CuO phase is a signature of the dehydration of Cu(OH)2, and it is expected to occur more readily at higher temperatures because of the activated nature of this process. The reactions that account for the formation of Cu(OH)2 probably proceed as follows:
Cu + O2 + NH3 f Cu[NH3]n2+ Cu[NH3]n2+ + OH- f Cu(OH)2
(nanoribbon)
This is consistent with our finding that the nanoribbon growth rate picks up when the O2 concentration is increased, which contrasts markedly with the Cu2S precursor-based nanoribbon synthesis.27 The oxidation step provides the Cu2+ ions necessary for the coordination self-assembly. Under the basic condition of the ammonia solution, OH- replaces NH3 in the Cu[NH3]n2+ complex, giving rise to square planar Cu(OH)42- units. As was discussed recently, the growth of Cu(OH)2 nanoribbons along the [100] direction can be understood on the basis of the assembly of olated chains >Cu(OH)2Cu
