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Parallel Alignment of Carbon Nanotubes Induced with Inorganic Molecules Tie Wang, Mingkui Wang, Xiaoge Hu, Xiaohu Qu, Feng Zhao, and Shaojun Dong* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, People’s Republic of China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China Received June 13, 2005. In Final Form: October 19, 2005 In this work, a new strategy is presented to form ordered multiwalled carbon nanotube (MWNT) arrays. The MWNTs are aligned horizontally and parallel in (3-aminopropyl)triethoxysilane (3-APTES) sol films on the surface of mica and glassy carbon (GC). 3-APTES is ready to form charged rodlike micelles, which play a key role in fabricating orderly MWNT arrays. Moreover, we prepare MWNT arrays on the surfaces of solid electrodes and detect the electrochemical response of the microarray electrodes for the Fe(CN)63-/ Fe(CN)64- couple.
Since the discovery of carbon nanotubes (CNTs) in 1991,1 intensive research has been conducted because of their unique structural, mechanical, electronic, and chemical properties.2 There has been growing interest in the fabrication of nanometer-sized devices, in which CNTs are used as building blocks, such as transistors,3 logic gates,4 and sensors.5,6 The key issue in the use of CNTs in nanofabrication is the need to prepare orderly CNT arrays that can be mass produced with a tractable procedure. However, it is difficult to form such arrays because of the large aspect ratio and strong intertube van der Waals interactions about CNTs. Many efforts to form ordered CNT arrays on substrates have just begun. Recently, CNTs have been directed to particular orientation by virtue of external forces, including the use of microfluidics,7-9 an electric field,10 a magnetic field,11 and the LangmuirBlodgett technique.12 The above-mentioned methods usually require complicated techniques. However, selfassembly methods that make use of the interactions between nanotubes and soft templates, including liquid crystals13,14 and biological molecules,15 to align CNTs have been proposed. For example, nanotubes were introduced * Corresponding author. E-mail:
[email protected]. Tel: +86431-5262101. Fax: +86-431-5689711. (1) Iijima, S. Nature 1991, 354, 56-58. (2) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787-792. (3) Keren, K.; Berman, R. S.; Buchstab, E.; Sivan, U.; Braun, E. Science 2003, 302, 1380-1382. (4) Derycke, V.; Martel, R.; Appenzeller, J.; Avouris, Ph. Nano Lett. 2001, 1, 453-456. (5) Qi, P.; Vermesh, O.; Grecu, M.; Javey, A.; Wang, Q.; Dai, H.; Peng, S.; Cho, K. J. Nano Lett. 2003, 3, 347-351. (6) Lin, Y.; Lu, F.; Tu, Y.; Ren, Z. Nano Lett. 2004, 4, 191-195. (7) Xin, H. J.; Wooley, A. T. Nano Lett. 2004, 4, 1481-1484. (8) Lay, M. D.; Novak, J. P.; Snow, E. S. Nano Lett. 2004, 4, 603-606. (9) Ko, H.; Peleshanko, S.; Tsukruk, V. V. J. Phys. Chem. B 2004, 108, 4385-4393. (10) Nagahara, L. A.; Amlani, I.; Lewenstein, J.; Tsui, R. K. Appl. Phys. Lett. 2002, 80, 3826-3828. (11) Walters, D. A.; Casavant, M. J.; Qin, X. C.; Huffman, C. B.; Boul, P. J.; Ericson, L. M.; Haroz, E. H.; O’Connell, M. J.; Smith, K.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 2001, 338, 14-20. (12) Whang, D.; Jin, S.; Wu, Y.; Lieber, C. M. Nano Lett. 2003, 3, 1255-1259. (13) Lynch, M. D.; Patrick, D. L. Nano Lett. 2002, 2, 1197-1201. (14) Dierking, I.; Scalia, G.; Morales, P.; LeClere, D. Adv. Mater. 2004, 16, 865-869. (15) Xin, H. J.; Wooley, A. T. J. Am. Chem. Soc. 2003, 125, 87108711.
onto a substrate involving aligned DNA. Nevertheless, it is still challenging to align CNTs with controlled morphology. The formation of siloxane sol involves the hydrolysis and condensation of suitable alkoxysilane precursors. Because of its remarkable facilitation of electrochemical studies and applications, many composite materials based on sol-gels have generated extensive interest over the past decade.16,17 Recently, there have been many challenges to incorporate CNTs with a siloxane sol.18-20 More interesting, some ionized siloxanes can form rodlike micelles, which are stacked by drying to form highly ordered structures.21 When the CNTs are dispersed in the siloxane sol, they can act as a template to control the orientation of CNTs during solvent evaporation. In this work, we describe a simple method for forming horizontal and parallel MWNT arrays incorporating (3aminopropyl)triethoxysilane (3-APTES). The charged rodlike micelles formed by ionized 3-APTES consist of hydrophilic amino groups outside and alkoxysilane groups inside during hydrolysis (Scheme 1).21 When multiwalled carbon nanotubes (MWNTs) are dispersed in charged micelles of 3-APTES solution, the amino groups of 3-APTES micelles and carboxylic acid groups of MWNT surfaces can form intermolecular hydrogen bonding. As a result, the self-assembly of 3-APTES induces large-scale MWNT arrays during the formation of highly ordered nanostructured polysiloxane materials by means of volatilization. The MWNTs with an average length of about 100-500 nm and a diameter of about 30-40 nm are assembled horizontally and parallel in siloxane sol films, demonstrating the proof of principle of our anticipated scheme. On the basis of the same strategy, we fabricated MWNT arrays on the surfaces of glassy carbon (GC) (16) Gavalas, V. G.; Law, S. A.; Ball, J. C.; Andrews, R.; Bachasa, L. G. Anal. Biochem. 2004, 329, 247-252. (17) Wang, B. Q.; Li, B.; Deng, Q.; Dong, S. J. Anal. Chem. 1998, 70, 3170-3174. (18) Luong, J. H. T.; Hrapovic, S.; Wang, D. S.; Bensebaa, F.; Simardc, B. Electroanalysis 2004, 16, 132-139. (19) Gavalas, V. G.; Andrews, R.; Bhattacharyya, D.; Bachas, L. G. Nano Lett. 2001, 1, 719-721. (20) Gong, K. P.; Zhang, M. N.; Yan, T. M.; Su, L.; Mao, L. Q.; Xiong, S. X.; Chen, Y. Anal. Chem. 2004, 76, 6500-6505. (21) Kaneko, Y.; Iyi, N.; Kurashima, K.; Matsumoto, T.; Fujita, T.; Kitamura, K. Chem. Mater. 2004, 16, 3417-3423.
10.1021/la051561c CCC: $30.25 © 2005 American Chemical Society Published on Web 11/11/2005
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Langmuir, Vol. 21, No. 26, 2005 12069 Scheme 1. Diagram of the Formation of Charged 3-APTES Micelles
electrodes and detected their electrochemical responses. This approach represents a simple route for organized configuration and provides an important tool in bottomup nanometer-sized fabrication. MWNTs (95% purity, Nanotech Port Ltd. Co., Shenzhen, China) were purified with the acid treatment previously reported.22 Carboxyl groups largely decorated the surfaces of MWNTs after the treatment.23 The MWNT arrays were fabricated as follows. A siloxane sol was prepared by mixing 3-APTES (0.35 mL, Sigma) with distilled water (1.38 mL), absolute ethanol (0.20 mL), and hydrochloric acid (0.06 mL, 0.10 M). The mixture was sonicated for 30 min until a clear, homogeneous suspension was obtained and was subsequently stored at room temperature for 2-3 h. Afterward, various amounts of MWNTs (0.50-0.01 mg) were added to 1.00 mL of the siloxane sol. The as-prepared solutions were sonicated to ensure a uniform suspension. A 10 µL drop of this suspension was used to coat a mica surface (1 × 1 cm) under rotation at 3000 r/m, and then the films were kept in the desiccator for 8 h. MWNT arrays on GC slides were prepared following a method similar to that described above. Tapping mode atomic force microscopy (AFM) image of siloxane sol films on the mica surface is shown in Figure 1. Some striped patterns that line up in a regular direction
The resulting 3D nano-ordered structure of 3-APTES can be used to arrange MWNTs. The parallel MWNT arrays were characterized by XL30 SESM FEG scanning electron microscopy (SEM). As shown in Figure 2A, the
Figure 2. SEM images of MWNT arrays on a mica surface. (A) MWNT (0.01 mg mL-1) arrays incorporating 3-APTES. (B and C) MWNT arrays prepared under the same conditions as for image A except for treatment with hydrofluoric acid before SEM measurements were made. Figure 1. Typical AFM height images of 3-APTES sol films by spin coating on a mica surface.
as indicated by white arrows in Figure 1 can be observed, and they correspond to rodlike micelles of 3-APTES. The average thickness of these micelles as determined from AFM is about 0.5 nm, which is consistent with the literature.21 Alkoxysilane aggregates and fabricates a 3D nano-ordered structure by stacking the rodlike micelles during solvent evaporation. (22) Patolsky, P.; Weizmann, Y.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 2113-2117. (23) Hirsch, A. Angew. Chem., Int. Ed. 2002, 41, 1853-1959.
SEM image of the 3-APTES sol films containing MWNTs (0.01 mg mL-1) on the mica surface indicates that the alignment of most MWNTs is orderly. The MWNT arrays can be observed on five repeated mica surfaces, and the majority of MWNTs are usually embedded in siloxane sol films. To remove the siloxane sol, we dipped the siloxane sol films containing MWNTs in hydrofluoric acid and then washed them with water. Figure 2B and C shows the presence of MWNTs in an ordered configuration. It should be noted that the number of MWNTs in a limited area is obviously decreased on the eroded mica surfaces, which is attributed to the fact that a portion of the MWNTs escape from the substrate during erosion with hydrofluoric
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Figure 3. SEM image of MWNT arrays obtained on the GC slides.
acid and rinsing with water. The above-mentioned results prove that the self-assembly of 3-APTES can induce parallel MWNT arrays. Moreover, the ordered alignment of MWNTs is correlated with the concentration of MWNTs used. When the films are prepared with 0.5 mg mL-1 MWNTs, we observe in the SEM image (data not shown) that MWNTs are aggregated in the siloxane sol films. The possible reason is that the intertube attraction is very steep, resulting in the difficulty in dispersing MWNTs up to a single nanotube level in the solvent.24 Having succeeded in aligning MWNTs in an orderly way on the mica surface, we tried to form MWNT arrays on GC slides in the same fashion. Figure 3 is the SEM image of the siloxane sol films on GC slides. The Figure shows that the ordered MWNT arrays are successfully obtained. It is clear that the substrate has less effect on the templating ability of the siloxane sol to align MWNTs. Because the homogeneous, well-dispersed suspension of MWNTs can be stable for several hours, we reasonably conclude that the carbon nanotubes are uniformly distributed throughout the siloxane sol films during desiccation. Therefore, we detected the electrochemical behavior of the MWNT arrays, which was performed in a standard single-compartment electrochemical cell that contained a working electrode, a Ag/AgCl reference electrode, and a platinum wire auxiliary electrode. Figure 4 depicts the cyclic voltammograms of the Fe(CN)63-/Fe(CN)64- couple at these modified electrodes. The apparent difference in the voltammetric responses is clearly observed. The MWNTs/GC modified electrode prepared with higher concentration MWNTs (0.5 mg mL-1) exhibits the typical response of the redox couple on a conventional dimension electrode (curve a), which is a pair of welldefined peak-shaped waves, indicating the semi-infinite linear diffusion-controlled redox process of the Fe(CN)63-/ Fe(CN)64- couple. The small peak separation (∆Ep ) 64 mV) and the near unity of the ratio of the cathodic-toanodic peak current (ipc/ipa ) 0.99) suggest a reversible redox process of the solution-phase Fe(CN)63-/Fe(CN)64couple at the MWNTs/GC modified electrode. However, when the content of MWNTs decreases to 0.01 mg mL-1, the MWNTs are horizontally aligned on the surface of the GC electrode, and microarray electrodes are obtained. Sigmoidal voltammetric behavior corresponding to the microelectrode feature is clearly observed (curve b). It indicates that there is no diffusion layer overlapping between the nanotube electrodes because most of the MWNTs are separated into parallel MWNT arrays with very low background current.6,20 For comparison, we (24) Shvartzman-Cohen, R.; Nativ-Roth, E.; Baskaran, E.; LeviKalisman, Y.; Szleifer, I.; Yerushalmi-Rozen, E. J. Am. Chem. Soc. 2004, 126, 14850-14857.
Figure 4. Typical CVs for 5.0 mM K3Fe(CN)6 at GC/MWNT3-APTES (curves a and b), GC/MWNT-PTES (curve c), and GC/3-APTES (curve d) film electrodes. The MWNT contents in the siloxane sol were 0.5 (a), 0.01 (b), 0.01 (c), and 0 (d) mg mL-1, respectively. The scan rate is 10 mV s-1.
prepared a 0.01 mg mL-1 MWNTs suspension in which 3-APTES was replaced by an equal content of propyltriethoxysilane (PTES), and its electrochemical response corresponds to curve c. Curve c is in agreement with the electrochemical behavior of a conventional-dimension electrode. The difference between curves c and b can be attributed to the different distribution of MWNTs in differently constituted sol films. The MWNTs generally aggregate together throughout the PTES sol films rather than form parallel array, which is demonstrated by the SEM image (data not shown). As reported by Iijima et al.,25 charged particles could be employed to disperse CNTs in water. Because the 3-APTES can form charged micelle, which is important for the stability of the MWNTs suspension, the MWNTs suspension of the 3-APTES sol can be stable for several hours. The PTES has no charged amino groups and cannot form micelles. Therefore, the MWNTs cannot be uniformly distributed throghout the PTES sol films even though the concentration about the MWNTs is the same as the situation in 3-APTES. It is worth noting that curve d shows a very small reduction/ oxidation current at the 3-APTES electrode without MWNTs, implying that the GC surface is fully covered by 3-APTES and the effective charge transfer is almost blocked. Thus, the effect of MWNTs in the films is evident, which is consistent with the conclusion by SEM measurements. Furthermore, the electrochemical response of these MWNTs/GC modified electrodes is affected by the concentration of the MWNTs, so the MWNTs/GC modified electrodes can be switched from conventional electrodes to microelectrodes by controlling the concentration of MWNTs. The result implies a potential application in biosensor and single enzymatic activity exploration. From our experiments, the spatial coverage of MWNTs in the films could be attributed to four factors. First, the formation of charged rodlike micelles of 3-APTES can produce repulsive barriers that are large enough to prevent MWNTs from approaching the aggregation when the concentration of MWNTs is lower (such as 0.01 mg mL-1).24 Second, the MWNTs in the 3-APTES sol are driven forward by the shear flow of the solutions when the films are spun onto the mica substrate,7-9 which may help the orientation (25) Zhu, J.; Yudasaka, Y.; Zhang, M.; Iijima, S. J. Phys. Chem. B 2004, 108, 11317-11320.
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of the MWNTs. Third, the aggregation of the siloxane sol forms a parallel nanostripe, which can act as a template to confine the orientation of MWNTs, just like liquid crystals inducing the parallel alignment of CNTs.13,14 Finally, when a self-assembly process takes place for 3-APTES during spinning and drying, the MWNTs are guided by 3-APTES to form parallel arrays by virtue of intermolecular hydrogen bonding between the amino groups of 3-APTES and the carboxylic acid groups of MWNTs. In summary, we report a simple method by which highly oriented MWNT arrays can be obtained on a large scale through the templating effect of 3-APTES. The MWNT arrays have been successfully obtained on various sub-
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strates. This technique is very valuable in the fabrication of devices with ordered MWNT arrays. We have also investigated the electrochemical characterization of the nanotube electrodes on GC. The microarray electrodes demonstrate a sigmoidal curve, and no array electrodes exhibit a conventional response. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20275037 and 20275036). We are grateful to Mr. Junpeng Gao and Weihuan Huang for their help with the AFM experiments. LA051561C
