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Synthesis of Aligned Few-Walled Carbon Nanotubes on Conductive Substrates Hyung Seok Kim,†,| Byungwoo Kim,†,| Byeongdu Lee,§ Haegeun Chung,† Cheol Jin Lee,‡ Ho Gyu Yoon,*,† and Woong Kim*,† Department of Materials Science and Engineering, School of Electrical Engineering, Korea UniVersity, Seoul 136-713, South Korea, and X-ray Science DiVisions, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: August 12, 2009; ReVised Manuscript ReceiVed: September 19, 2009
We report a robust synthesis of densely aligned few-walled carbon nanotubes on industrially useful conductive substrates such as carbon fibers and carbon papers. This was achieved by water-assisted chemical vapor deposition using an Al/Fe bimetallic catalyst and a thin epoxy-based polymer layer beneath the catalyst. The carbon nanotubes had a mean diameter of 6 nm, mainly double walls, and lengths up to a couple of millimeters. Raman spectroscopy showed high G-band/D-band intensity ratio (>5) indicating high quality of the nanotubes. They were tightly bound and electrically connected to the substrates, as confirmed by ultrasonication test and enhanced cyclic voltammetry signals, respectively. High quality carbon nanotubes synthesized on conductive substrates may find applications in fuel cells, lithium-ion batteries, and field emitters. Introduction Owing to their excellent electrical properties, high specific surface area, and chemical inertness, carbon nanotubes (CNTs) have been exploited to improve properties of electrode materials in a variety of applications, such as fuel cells,1,2 Li ion batteries,3 field emission devices,4 supercapacitors,5 and biosensors.6 In order to fully take advantage of the excellent properties of CNTs, it is essential to establish high quality electrical contact between the CNTs and the electrode materials. The direct synthesis of CNTs on electrode materials is one of the most promising ways to achieve this, and may lead to significant improvements in overall device performance. Furthermore, the synthesis of fewwalled CNTs is preferable to that of multiwalled CNTs due to their higher specific surface area and fewer structural defects.7 Therefore, it is highly desirable to develop a robust method to grow high quality few-walled CNTs directly on conductive substrates. Nanotubes can be grown directly on substrates by chemical vapor deposition (CVD), but CVD synthesis has been limited mostly to insulating substrates8,9 and successful demonstration of CNT synthesis on conductive substrates is rare.10,11 The difficulty in growing CNTs on conductive substrates is mainly due to the degradation of substrates and catalytic nanoparticles under the reactive CNT growth conditions, such as the hydrogen environment at elevated temperatures. For example, hydrideforming metals, including Ti and Ta, are partially etched by the formation of volatile metal hydrides and become resistive, and metals with a low melting temperature, such as Au, ball up and become discontinuous after CNT growth.12 Moreover, the catalytic activity of the seed nanoparticles can deteriorate * To whom correspondence should be addressed. E-mail: (W.K.) woongkim@ korea.ac.kr; (H.G.Y.)
[email protected]. † Department of Materials Science and Engineering, Korea University. ‡ School of Electrical Engineering, Korea University. § Argonne National Laboratory. | These authors equally contributed to this work.
10.1021/jp9078162 CCC: $40.75
because the catalysts may react with the supporting conductive substrates at high growth temperatures. For example, catalytic iron nanoparticles react with silicon to form FeSi2 or Fe2SiO4,13 or with carbon-based substrates to produce unwanted phases of carbon materials14 and lose their catalytic activity. Catalyst poisoning caused by amorphous carbon deposition on the nanoparticle surface can also significantly reduce the catalytic activity and lower the efficiency of the synthesis.9,15 In addition, the aggregation of nanoparticles at elevated temperatures promotes the growth of multiwalled CNTs instead of few-walled CNTs.16,17 Therefore, in order to grow few-walled CNTs directly on conductive substrates, it is essential to employ nonreactive substrates, preserve the catalytic activity, and control the catalytic nanoparticle size during the CVD process. We demonstrate a successful direct synthesis of few-walled CNTs on conductive substrates using the following strategies. First, conductive substrates that are compatible with the reductive and high-temperature growth conditions, such as carbon fibers and carbon papers, are used. These substrates can survive through the harsh CNT growth conditions18-20 and have a variety of important applications. Composite materials made from carbon fibers are popular in industrial areas, such as aerospace, motor, and civil engineering, owing to their high tensile strength, low weight, and low thermal expansion coefficient. In addition, carbon papers are promising materials for use in a range of energy storage applications, including fuel cells and lithiumion battery cells.21 Second, a thin polymer layer is used to prevent any detrimental reactions between the catalysts and the carbon-based substrates. If the polymer layer does not interfere with CNT synthesis and mostly decomposes during the CVD process, then it will eventually lead to direct electrical contact between the CNTs and substrates. Third, a small amount of water is introduced as a mild oxidizing agent during growth. Water promotes or preserves the catalytic activity by counteracting the effects of amorphous carbon deposition.9 Fourth, an aluminum (Al) and iron (Fe) bilayer is used as a catalyst to 2009 American Chemical Society
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Figure 1. Synthesis of CNTs on carbon fibers. (a) Carbon fibers with a polymer coating. (b) E-beam evaporation of 10 nm of Al and 1 nm of Fe as a catalyst. (c) A schematic representation and (d) an SEM image of the “curtains” of CNTs grown on the carbon fibers.
preferentially grow few-walled CNTs (dFWCNT < 10 nm) over multiwalled CNTs (dMWCNT < 100 nm). The underlying Al layer supposedly prevents the aggregation of Fe nanoparticles and facilitates the growth of few-walled CNTs.16,17 Aligned few-walled CNTs were synthesized on carbon fibers and carbon papers via water-assisted CVD. Raman spectroscopy showed a high G-band (graphite-like) to D-band (disorderinduced) intensity ratio (G/D ∼ 6) which indicates that the CNTs are of high quality. Transmission electron microscopy (TEM) showed that the number of CNT walls was mainly 2, ranging from 1 to 5, and that the average diameter was ∼6 nm. CNTs are bound relatively strongly to the substrates and withstand mild ultrasonication. The CNT-bearing carbon paper electrode showed a cyclic voltammetry (CV) signal that is an order of magnitude higher compared to that acquired from the bare carbon paper electrode, which suggests that the CNTs are electrically connected to the carbon paper. Structural information, including diameter and alignment, was determined by the analysis of TEM, scanning electron microscopy (SEM), and grazing incidence small-angle X-ray spectroscopy (GISAXS). Finally, we demonstrate that our method of CNT synthesis can be applied to other conductive substrates such as doped-silicon wafers and stainless steel (SS) plates. Experimental Section For CVD synthesis, the catalyst-coated fibers were placed in the middle of a 1 in. diameter quartz tube. The sample was
heated in a tube furnace (Lindberg, TF55030A) with a flow of argon, hydrogen, and water vapor. Once the temperature reached 750-800 °C, ethylene as a carbon source was introduced to the quartz tube for 10 min. For water vapor transport, a small amount of argon (5-10 sccm) was flowed through a bubbler containing DI water. The ethylene, hydrogen, and argon flow rates were in the range of 5-20, 5-1000, and 500 sccm, respectively. Polymer coating on carbon papers were prepared as follows. Diglycidyl ether bisphenol A (DGEBA) type epoxy was mixed with an aromatic-amine based curing agent TH-432 (Kukdo Chemical Co.) at a 1:1.1 equivalent weight ratio. Two phr (parts per hundred of epoxy resin) of a 1-benzyl-2-methylimidazole catalyst was added. The solution mixture was spin-cast on carbon papers. It soaked into the carbon papers due to their highly porous nature and formed a coating that is roughly 200 nm in thickness after being cured at 160 °C for 1 h. CV measurement was performed using an electrochemistry analyzer (Ivium Technologies, CompactStat). A carbon paper (1.1 cm by 1.4 cm), an Ag/AgCl electrode (CH Instruments, CHI111, 3 M KCl), and a platinum gauze were used as a working, a reference, and a counter electrode, respectively. 1 M potassium nitrate (KNO3) and 2 mM potassium hexacyanoferrate (III) (K3Fe(CN6), purchased from Sigma-Aldrich, were used as electrolyte and redox species, respectively. For GISAXS measurement, samples were placed on the x-y plane with its surface normal vector parallel to the z axis, and
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Figure 2. (a) An SEM image of the CNTs on carbon fibers. (b) Alignment of the CNTs on a carbon fiber. (c) A magnified SEM image of the aligned CNTs. (d) A TEM image of the few-walled CNTs.
the X-ray beam was propagated along the x-axis in this setup. An X-ray beam with an energy of 12 keV and a size of 500 µm × 30 µm (y × z direction) was impinged on upper part of the CNT forest with incident angle 0°. A two-dimensional chargecoupled-device (CCD) camera was positioned approximately 2.3 m downstream from the sample to capture the scattering images. A rectangular beam-stop was used to protect the CCD camera by blocking direct X-ray beam. Results and Discussion Figure 1 illustrates the procedure to synthesize few-walled CNTs on carbon fibers. Fibers with a diameter of ∼7 µm and a resistivity of 1.6 × 10-3 Ω · m were purchased from Toray (T700S). The as-received fibers were coated with a sizing agent, which is an epoxy-based polymer applied for better handling and enhanced adhesion properties. In our experiments, the fibers were mostly used as they were (Figure 1a). The polymer coating was removed only for the control experiments by immersing the fibers in 40 wt % of HNO3 for 2 h, rinsing with deionized (DI) water, and drying at 90 °C for 24 h. On top of the polymercoated carbon fibers, a 10 nm Al layer and a 1 nm Fe layer were deposited by e-beam evaporation and used as the catalyst (Figure 1b). As a result of CVD, densely aligned CNTs were obtained on the carbon fibers. Figure 1c and d show a schematic diagram and the corresponding SEM image of the nanotubes, respectively. The CNTs had the appearance of a curtain because the nanotubes grew ∼500 µm along the entire length of the narrow carbon fibers (LCF ) 1-2 cm, dCF ∼7 µm). Figure 1d shows an SEM image of the “CNT curtains” grown along a collection of carbon fibers.
Figure 2 presents the morphology of the CNTs synthesized on carbon fibers. CNTs grew in one direction because the catalyst was e-beam evaporated on only one side of the carbon fibers (Figure 2a and b). No nanotubes were observed on some parts of the carbon fibers, where catalyst deposition was masked by other randomly oriented carbon fibers. Good alignment of the nanotubes can be seen in the SEM images (Figure 2b and c). The slight waviness of the nanotubes suggests that the nanotubes have small number of walls. This was confirmed by TEM, which showed that most of the CNTs have 1-5 walls and diameters ranging from 5 to 7 nm (Figure 2d). Our synthetic method was extended to carbon papers, which is a two-dimensional version of carbon fibers (Figure 3a). The papers were 280 µm thick with an in-plane electrical resistivity of 5.8 × 10-3 Ω · m (Toray, TGP-H-090). Since the as-received carbon papers were not coated with the epoxy-based polymer, the polymer coating was prepared as described in Experimental Section. After the polymer coating, a 10 nm Al layer and a 1 nm Fe layer were deposited and CVD was carried out. Figure 3b presents an angled view of the CNTs. The CNTs were bundled up in a micrometer scale domain rather than forming a continuous nanotube forest. This discontinuity originated from the underlying structure of the carbon fiber network. A magnified SEM image shows that the CNTs were well-aligned (inset in Figure 3b). Good quality CNTs were obtained as shown by Raman spectroscopy. A G-band to D-band ratio as high as 6-7 was observed (Figure 3c), which is the highest value reported for nanotubes grown on carbon fibers or papers thus far. Direct contact was established between the CNTs and the substrates most likely because the polymer layer decomposed
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Figure 3. (a) An SEM image of carbon papers. (b) An angled view of CNTs grown on the carbon paper. The inset shows a magnified SEM image of the aligned CNTs (scale bar ) 500 nm). (c) Raman spectrum of the CNTs. (d) Comparison of cyclic voltammogram measured using a CNTgrown carbon papers (CNT/CP) vs a bare carbon paper (CP) electrode.
during CVD growth. The nanotubes were bound relatively strongly to the carbon fibers and carbon papers. After 3 min of ultrasonication at 36 W in a methyl ethyl ketone (MEK) solvent, the nanotubes were still adhered to the fibers. Significant detachment was observed only when the power was increased to 52 W. To confirm that the CNTs were electrically connected to the carbon papers, CV measurement was carried out using the CNT-grown carbon paper and the bare carbon paper as the working electrode. The details are described in the Experimental Section. The CNT-grown carbon paper showed enhanced electrochemical signal by 1 order of magnitude compared to that obtained by the bare carbon paper due to the increased surface area by the CNTs. This suggests that the CNTs are electrically connected to the conductive carbon paper (Figure 3d). We found that the presence of the polymer layer was crucial in producing CNTs with high yield and of high quality on carbon-based substrates, but more thorough investigation is necessary to elucidate the interface chemistry between the substrates, the polymer, and the catalyst during CVD process. The successful synthesis of densely aligned and high quality few-walled CNTs on carbon fibers and papers can only be achieved by integrating all of the four aforementioned factors to the synthesis process, that is, (1) selection of a stable substrate, (2) insertion of a polymer layer to prevent deleterious reaction between the catalyst and the substrate, (3) addition of water to extend the catalytic activity, and (4) use of an Al under-layer to prevent the aggregation of Fe nanoparticles.
The same synthetic conditions were applied to other conductive substrates, such as doped-silicon and stainless steel (SS304). A precursor solution of the polymer was heated to 80 °C, spincast at 6000 rpm for 40 s and cured as described in Experimental Section. This process led to a ∼5 µm thick polymer layer. An Al and Fe catalyst layer was deposited on top of the polymer layer. A forest of vertically aligned CNTs grew on silicon via CVD synthesis, as shown in Figure 4a. Most of the polymer was decomposed during the CVD process and was not observed in cross-sectional SEM images, suggesting a direct contact between the CNTs and the substrates. The nanotube growth was not noticeably dependent on the thickness of the polymer layer. However, when the thickness of polymer layer was increased to 20 µm, 2 µm thick layer was left between the nanotubes and the substrates after the CVD process. Therefore, the polymer layer should be sufficiently thin for the CNTs and the substrates to make direct contact. Nanotubes as long as 1.5 mm were grown (Figure 4b) over the 10 min reaction time, which implies a growth rate of >2.5 µm/s. Vertically aligned CNTs were also successfully synthesized on the SS304. The quality of the CNTs was excellent as indicated by the high G-band/D-band ratio (∼5) in the Raman spectra (Figure 4c). The CNTs were bound relatively weakly to the silicon and SS304 substrates compared to those on the carbon fibers and carbon papers and were easily detached by sonication. Therefore, the adhesion needs to be improved for practical applications.
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Figure 4. (a) An SEM image of the aligned CNTs grown on a doped silicon substrate. (b) A photograph of a forest of 1.5 mm long CNTs grown on silicon. (c) Raman spectrum of the CNTs grown on SS304. The inset shows an SEM image of vertically aligned CNTs grown on SS304 (scale bar ) 100 µm). (d) A GISAXS scattering image (upper inset, intensity is color-coded) and a line-cut along qy at qz ) 0.
Additionally, the structure of the aligned CNTs was examined noninvasively using GISAXS. Information on diameter, orientation, and intertube distance was obtained without further sample preparation process. A sample of a CNT forest grown on a 1 cm × 1 cm silicon piece was mounted on the GISAXS stage. Vertical orientation of nanotubes was suggested by the presence and the shape of two peaks along the y-axis (or qy axis) in the scattering image (inset in Figure 4d), and the power-law slope of -3 at qy > 0.1 Å-1 in the line-cut scattering profile along the qy direction at qz ) 022 (Figure 4d). The position of the peak and the Guinier Knee at approximately qy ) (0.1 Å-1 indicate that the center to center distance of neighboring CNTs and the tube diameter are both approximately 6 nm,23 (calculated from the Bragg relation; 2π /0.1 Å-1), indicating that the CNTs are closely packed. The strong scattering at smaller q (or qy < 0.04 Å-1) indicates the presence of objects larger than 16 nm, and these objects are likely to be bundles of several CNTs. The CNT structure was stable upon heat treatment and did not show any noticeable changes when heated to 650 °C under the flow of 4% hydrogen in helium. Conclusions Vertically aligned few-walled CNTs were grown on conductive substrates, such as carbon fibers, carbon papers, doped
silicon, and SS. The CNTs had a mean diameter of 6 nm, mainly double walls (ranging from 1 to 5 walls), and lengths up to a couple of millimeters. Water-assisted CVD using an Al and Fe bimetallic catalyst was used for the efficient growth of CNTs. The presence of a thin epoxy-based polymer layer beneath the catalyst film improved the CNT yield and synthesis reproducibility on the carbon fibers and papers. The G-band and D-band ratio of 6-7 of Raman spectra indicates that CNTs of good quality were produced. The electrochemical signal measured on the CNT-grown carbon paper as a working electrode was enhanced by approximately 1 order of magnitude compared to that acquired from the bare carbon paper electrode. GISAXS confirmed that the CNTs were aligned vertically. High quality few-walled CNTs grown on conductive substrates will find a variety of applications in fuel cells, lithium-ion batteries, field emitters, and biosensors. Acknowledgment. The authors thank Dr. Myung Hwa Kim for his help with the GISAXS measurement. We acknowledge the supports from the Korea Research Council of Fundamental Science & Technology (KRCF) and Korea Institute of Science & Technology (KIST) for “National Agenda Project program”, and the National Research Foundation of Korea through the Pioneer Research Center Program (No. M10711160001-08M1116-
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00110), and World Class University Project (WCU, R32-2008000-10082-0) funded by the Ministry of Education, Science and Technology. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. References and Notes (1) Wang, C.; Waje, M.; Wang, X.; Tang, J. M.; Haddon, R. C.; Yan, Y. S. Nano Lett. 2004, 4, 345. (2) Kongkanand, A.; Vinodgopal, K.; Kuwabata, S.; Kamat, P. V. J. Phys. Chem. B 2006, 110, 16185. (3) Masarapu, C.; Subramanian, V.; Zhu, H. W.; Wei, B. Q. AdV. Funct. Mater. 2009, 19, 1008. (4) Zhang, J. H.; Wang, X.; Yang, W. W.; Yu, W. D.; Feng, T.; Li, Q.; Liu, X. H.; Yang, C. R. Carbon 2006, 44, 418. (5) Futaba, D. N.; Hata, K.; Yamada, T.; Hiraoka, T.; Hayamizu, Y.; Kakudate, Y.; Tanaike, O.; Hatori, H.; Yumura, M.; Iijima, S. Nat. Mater. 2006, 5, 987. (6) Lin, Y. H.; Lu, F.; Tu, Y.; Ren, Z. F. Nano Lett. 2004, 4, 191. (7) Hou, Y.; Tang, J.; Zhang, H. B.; Qian, C.; Feng, Y. Y.; Liu, J. ACS Nano 2009, 3, 1057. (8) Kong, J.; Soh, H. T.; Cassell, A. M.; Quate, C. F.; Dai, H. J. Nature 1998, 395, 878. (9) Hata, K.; Futaba, D. N.; Mizuno, K.; Namai, T.; Yumura, M.; Iijima, S. Science 2004, 306, 1362.
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