© Copyright 2001 by the American Chemical Society
VOLUME 105, NUMBER 43, NOVEMBER 1, 2001
LETTERS Gas-Phase Synthesis of Single-wall Carbon Nanotubes from Colloidal Solution of Metal Nanoparticles Hiroki Ago,* Satoshi Ohshima, Kunio Uchida, and Motoo Yumura Research Center for AdVanced Carbon Materials, National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba, 305-8565, Japan ReceiVed: May 31, 2001; In Final Form: August 23, 2001
Metal nanoparticles play an important role in chemical vapor deposition (CVD) synthesis of carbon nanotubes because nanoparticles not only catalyze the nanotube growth but also determine the structural characteristics of the nanotubes. We report on the synthesis of single-wall carbon nanotubes (SWNTs) on the basis of the gas-phase reaction of colloidal solutions of metal nanoparticles containing Co and Mo. The colloidal solution of the nanoparticles is prepared by a reverse micelle method and injected into a furnace, where the solvent serves as the carbon source while the nanoparticles act as the catalyst. We have found that addition of a small amount of thiophene leads to formation of the SWNTs. The formation mechanism of the SWNTs is discussed by comparing the present CVD and laser-ablation methods.
I. Introduction Carbon nanotubes, in particular single-wall carbon nanotubes (SWNTs), have attracted a great deal of interest in terms of fundamental nanoscopic physics and various applications in industry.1 SWNTs were originally synthesized by an arcdischarge method2,3 using metal-filled graphite rods, although the yield is low and impurity concentration is high. Later, it was shown that a laser-ablation method produces SWNTs with a much higher yield and fewer impurities than the arc-discharge method.4 Despite the recent progress of synthetic techniques for SWNTs, there still remain two major problems in the SWNT synthesis for further progress of the nanotube research: largescale and chirality-selected synthesis. Recently a chemical vapor deposition (CVD) method has emerged as a new candidate for nanotube synthesis, because the CVD method is capable of controlling growth direction on a substrate5,6 and synthesizing a large quantity of nanotubes.7,8 The CVD method has also been used for growth of multiwall * Corresponding author. E-mail:
[email protected].
carbon nanotubes (MWNTs) aligned normal to the substrate, which is promising for field-emission displays.9-11 In the CVD method, it is widely known that metal nanoparticles play an essential role in the SWNT growth because carbon nanotubes are obtained only in the presence of nanoparticles of transition metals, such as Fe, Co, and Ni. There are two types of CVD syntheses of SWNTs, depending on the form of supplied catalyst. One is to use the catalyst embedded in porous material or supported on a substrate. In this method, the catalyst is placed at a fixed position of a furnace and heated in a flow of hydrocarbon gas. Another method is to use gas phase for introducing the catalyst, in which both the catalyst and reactant hydrocarbon gas are fed into a furnace followed by the catalytic reaction in a gas phase. The latter method is suitable for a largescale synthesis because the nanotubes are free from catalytic supports and the reaction can be operated continuously. Actually, Nikolaev et al. have developed a high-pressure CO reaction (HiPCO) method in which carbon monoxide (CO) gas reacts with iron pentacarbonyl (Fe(CO)5) to form SWNTs with a much smaller amount of carbon impurities.7 Cheng et al. have also
10.1021/jp012084j CCC: $20.00 © 2001 American Chemical Society Published on Web 10/03/2001
10454 J. Phys. Chem. B, Vol. 105, No. 43, 2001 synthesized SWNTs from a benzene and ferrocene (Fe(C5H5)2) mixture in a hydrogen gas flow.12 In both methods, catalyst nanoparticles are formed through thermal decomposition of organometal compounds such as iron pentacarbonyl7 and ferrocene.12 We have focused on a chemical synthesis of metal nanoparticles in order to realize more controlled size distribution of the catalyst compared with those formed through the thermal decomposition of organometal compounds. We employed a reverse micelle method to prepare the catalyst nanoparticles because it offers the nanoparticles with a relatively homogeneous size distribution as well as solubility due to their surrounding surfactant.11,13 It was shown that CVD reaction of a Co nanoparticle-cast film results in an aligned MWNT array with less nanotube density than other arrays reported, which is supposed to be suitable to field-emission application.11 However, in this method, the Co nanoparticles cast on a Si substrate aggregate to form much larger clusters and give thicker MWNTs (20-80 nm outer diameter) compared with those of the original nanoparticles (4 nm diameter on average).11 In this Letter, we have studied the gas-phase reaction of the colloidal solution of metal nanoparticles prepared by a reverse micelle method. This gas-phase reaction prevents nanoparticles from thermal aggregation and enables us to investigate the nanotube formation mechanism by separating the nanotube growth process from the nanoparticle formation process. It is shown that SWNT growth takes place from the nanoparticles during the gas-phase reaction.
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Figure 1. Setup of the gas-phase synthesis of carbon nanotubes, equipped with a solution-filled syringe, vertically arranged furnace, and nanotube collection part.
II. Experimental Section Nanoparticles containing Co and Mo were prepared by a reverse micelle method as described elsewhere.11,13 The reverse micelle method utilizes a nanoscale water pool surrounded by surfactants. Metal ions dissolved in the water pool are reduced chemically to form a nanoparticle whose surface is covered with surfactant molecules. The presence of surfactant makes the nanoparticle soluble in organic solvent, such as toluene and benzene. First, 0.5 g of cationic surfactant, didecyldimethylammonium bromide, was dissolved in 5 g of toluene and stirred. In this solution, cobalt chloride hexahydrate (CoCl2‚6H2O, 143 mg, 0.6 mmol) and molybdenum chloride (MoCl5, 164 mg, 0.6 mmol) were added and stirred. Reduction of metal ions was carried out by adding aqueous solution of sodium tetrahydroborate (NaBH4, 75 mg, 2 mmol) dissolved in 150 µL of H2O. The reduction changed the color of the solution from dark green to black, suggesting formation of colloid of nanoparticles. Finally, 50 mg of thiophene was added to the colloidal solution as a promoter of carbon nanotube growth. We injected the colloidal solution into a furnace directly by using a syringe as shown in Figure 1. A quartz tube with 26 mm inner diameter and 1000 mm length was employed for the reaction. The temperature of the furnace was kept at 1200 °C, and the top of the quartz tube where a needle tip is situated was cooled with water to around 100 °C. The solution fed by an automatic liquid feeder was mixed with hydrogen 100% carrier gas at the needle tip using a spray and injected into the furnace. The colloidal solution vaporizes simultaneously with the injection and reaction occurs to form carbon product, where toluene vapor and metal nanoparticles act as carbon source and catalyst, respectively. The carbon product was pushed away from a hot zone of the furnace by the gas stream and obtained at a collection part situated at the bottom of the quartz tube. Metal nanoparticles and carbon product were analyzed by a HITACHI S5000 scanning electron microscope (SEM), a JEOL 2000FX
Figure 2. TEM micrograph of CoMo nanoparticles prepared by a reverse micelle method (a) and histogram of diameter distribution of the nanoparticles (b).
transmission electron microscope (TEM) equipped with an energy-dispersive analysis of X-ray (EDAX) system, and an ULVAC-PHI 5800 X-ray photoelectron spectroscope (XPS). III. Results and Discussion A TEM image of CoMo metal nanoparticles prepared by a reverse micelle method and their diameter distribution are shown in Figure 2. Because their surface is covered with surfactant, there is a finite separation between each nanoparticle of around 2 nm. We suppose that this distance corresponds to two layers of surfactant molecules that cover two neighboring nanoparticles. The CoMo nanoparticle molecules showed a monodispersed size distribution, and their average diameter is determined to be 10.8 nm from a Gaussian fitting. This average diameter is larger than
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J. Phys. Chem. B, Vol. 105, No. 43, 2001 10455
Figure 3. SEM image of fibrous carbon product obtained at the collection part.
that which we observed previously (4 nm) because the ratio of metal chlorides to surfactant is much higher than that used before.11 As we will mention later, the present gas-phase reaction requires a higher concentration of nanoparticles than that used for nanotube array. An XPS analysis of the cast film of the colloidal solution showed that the concentrations of Co and Mo are almost equivalent (Co1Mo0.89). The colloidal solution of CoMo nanoparticles was subjected to the gas-phase reaction without any purification steps. The colloidal solution was injected into a furnace by using a syringe with a rate of 80 µL/min under a hydrogen gas flow of 1200 cm3/min. In this condition, the residence time of the reactant gas in a furnace heated at 1200 °C was 2.1 s. Soon after injection, black powder starts to fall off and organize into a weblike deposit at the collection part. We have obtained 0.9 mg of carbon product in the collection part (bottom of a quartz tube in Figure 1) from the 1.0 mL of colloidal solution. The carbon yield is estimated to be around 0.1%. The carbon product contains SWNTs, metal-encapsulated nanoparticles, and amorphous carbon, as described below. An amount of SWNTs contained in the pyrolytic product is small (roughly 10% from SEM observation) due to amorphous carbon coating and metalencapsulated nanoparticles, but the yield would be improved by increasing the hydrogen gas flow rate or optimizing the composition of the nanoparticle catalyst. The SEM image of the fibrous deposit is shown in Figure 3. We have observed long filamentous structures with diameters of 20-30 nm, along with narrower filaments with diameters less than 10 nm. These filaments are accompanied by particles with diameters of 10-100 nm. We have found that a small amount of thiophene (1 wt %) in the original solution results in the fibrous deposit, while the solution without thiophene gave only powdered amorphous carbon particles with no filaments. This is consistent with the result obtained by Cheng et al., where they used 0.5-5 wt % of thiophene as a promoter for carbon nanotube growth.12 Most of the filaments obtained at the bottom of the quartz tube were found to be SWNTs in a bundle form from TEM measurements, as shown in Figure 4. The SWNTs showed a broad diameter distribution ranging from 1.1 to 1.9 nm. It is interesting to note that the residence time (2.1 s) of toluene vapor in a hot furnace, which corresponds to the growth time of the SWNTs, is much shorter than that of supported catalyst (the reaction time is more than 30 min14,15). The SWNTs partially suffer from amorphous carbon coating, as seen from Figure 4. Self-decomposition of toluene molecules at high temperature is likely to be the origin of the amorphous carbon coating. We note this amorphous carbon coating can be easily removed
Figure 4. TEM micrographs of SWNT bundles (a) and the crosssectional image (b).
because it is more facile to oxidation due to its less-graphitized structure. For example, annealing of the raw SWNT material in the presence of oxygen at 455 °C for 20 min removes the coating. The growth of SWNTs from the colloid of 10.8 nm nanoparticles suggests that metal nanoparticles as small as the diameter of SWNTs are not necessary for the SWNT growth. We also note that the both Co and Mo peaks are observed in the EDS spectra of a SWNT sample, suggesting the nanoparticle catalyst is in a bimetallic state. There is still open a question whether SWNTs are formed by the tip-growth or base-growth models. In the laser-ablation method, it is observed that bundles of SWNTs grow from the surface of amorphous carbon-coated metal nanoparticles and tips of SWNTs are free from metal nanoparticles. It is proposed that precipitation of carbon atoms from the hot metal-carbon molten mixture leads to the SWNT formation.16 In the CVD method of supported catalysts, Dai et al. observed a SWNT with a metal nanoparticle inside the tip,14 while Colomer et al. observed that the SWNT tips are free from nanoparticles.15 The difference of the growth mechanism of the CVD method may depend on the degree of metal-support interaction.15 Interestingly, we have found that the condition suitable for the SWNT growth in our gas-phase CVD method is similar to that realized in the laser-ablation method. For example, the carbon concentration of our system is 5 × 1017 C-atoms/cm3 (∼1017 C-atoms/cm3 in the laser ablation4), the temperature is 1200 °C (1200 °C), and the metal concentration (i.e., [M]/[C] atomic ratio) is 0.3 at. % (1.2 at. %4). Although the local temperature of the laser ablation is much higher than 1200 °C (> 3000 °C) near the target, it has been shown that such a high temperature is used for vaporizing metal and carbon atoms and that SWNT nucleation and growth take place at a moderate temperature of 850-1250 °C.17 It is also interesting to note that SWNTs grow at a rate of a few micrometer lengths per second for the laser ablation17 and the SWNT growth of the present
10456 J. Phys. Chem. B, Vol. 105, No. 43, 2001 CVD occurs in a similar time scale (2.1 s). We find that higher metal concentration is favorable for the SWNT growth in the present CVD method but solubility of metal chlorides limited the metal concentration of the solution. Therefore, we suppose that our SWNTs grew on the basis of the somewhat similar mechanism to that of the laser-ablation method. We speculate that in the gas-phase CVD reaction, toluene molecules suffer dehydrogenation reaction on the nanoparticle surface, followed by diffusion of carbon atoms into the nanoparticle. The as-formed metal carbide is expected to give the SWNTs through precipitation of carbon atoms. The main difference of the gas-phase CVD and laser-ablation method is the presence of sulfur elements; our system requires 1 wt % thiophene, while the laser ablation does not require such additives. We have confirmed that addition of sulfur powder instead of thiophene also gives SWNT bundles. Hence, sulfur atoms in thiophene are found to promote the one-dimensional SWNT growth. Interestingly, it turns out that MWNTs are formed instead of SWNTs when 10 wt % thiophene is added in the colloidal solution. In that case, the MWNTs containing nanoparticles at their tips were frequently observed. These facts also suggest that sulfur is essential in the present catalytic CVD reaction. One possible explanation of a role of sulfur is the change of the surface state of nanoparticles because sulfur is known to block a catalytically active surface site by forming chemical bonds with metal atoms (i.e. a poisoning effect).18 This may change the precipitation processes of graphitic carbon from saturated carbon solution in a metal nanoparticle. We speculate that a nanoparticles whose surface is partially blocked by sulfur atoms (in the case of 1 wt % thiophene) gives SWNTs, while a nanoparticles highly covered with sulfur atoms (10 wt %) gives MWNTs. The fact that sulfur lowers the eutectic temperature of the metal-carbon molten mixture may also modify the carbon precipitation process and contribute to the nanotube growth.19,20 Further microscopic investigation on the catalyst is necessary for better understanding of a role of sulfur. IV. Conclusions Bundles of SWNTs are synthesized from the colloidal solution of metal nanoparticles containing Co and Mo. The reverse micelle method we employed has the advantage that it provides nanoparticles with a variety of compositions as well as solubilities. We propose that the present CVD synthesis is explained in terms of the precipitation mechanism, in which SWNTs grow from the surface of the metal nanoparticles. We have found
Letters sulfur atoms are responsible for nanotube growth and that metal nanoparticles as small as the diameter of SWNTs are not necessary for the nanotube growth. Acknowledgment. This work is partially supported by the Frontier Carbon Technology (FCT) project of Ministry of International Trade and Industry (MITI), Japan. H.A. acknowledges the support from Agency of Science and Technology of Japan (Research Fund for Young Scientists). We would like to thank Dr. S. Terauchi and A. Goto for the experimental help. References and Notes (1) Tanaka, K., Yamabe, T., Fukui K., Eds. In The Science and Technology of Carbon Nanotubes; Elsevier: Oxford, 1999. (2) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (3) Bethune, D. S.; Kiang, C. H.; de Vries, M. S.; Gorman, G.; Savoy, R.; Vazaquez, J.; Beyers, R. Nature 1993, 363, 605. (4) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y. H.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Toma´nek, D.; Fischer, J. E.; Smalley, R. E. Science 1996, 273, 483. (5) Cassell, A. M.; Franklin, N. R.; Tombler, T. W.; Chan, E. M.; Han, J.; Dai, H. J. Am. Chem. Soc. 1999, 121, 7975. (6) Su, M.; Li, Y.; Maynor, B.; Buldum, A.; Lu, J. P.; Liu, J. J. Phys. Chem. B 2000, 104, 6505. (7) Nikolaev, P.; Bronikowski, M. J.; Bradley, R. K.; Rohmund, F.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Chem. Phys. Lett. 1999, 313, 91. (8) Su, M.; Zheng, B.; Liu, J. Chem. Phys. Lett. 2000, 322, 321. (9) Fan, S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. Science 1999, 283, 512. (10) Dai, L.; Mau, A. W. H. J. Phys. Chem. B 2000, 104, 1891. (11) Ago, H.; Komatsu, T.; Ohshima, S.; Kuriki, Y.; Yumura, M. Appl. Phys. Lett. 2000, 77, 79. (12) Cheng, H. M.; Li, F.; Su, G.; Pan, H. Y.; He, L. L.; Sun, X.; Dresselhaus, M. S. Appl. Phys. Lett. 1998, 72, 3282. (13) Chen, J. P.; Lee, K. M.; Sorensen, C. M.; Klabunde, K. J.; Hadjipanayis, G. C. J. Appl. Phys. 1994, 75, 5876. (14) Dai, H.; Rinzler, A. G.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1996, 260, 471. (15) Colomer, J. F.; Bister, G.; Willems, I.; Ko´nya, Z.; Fonseca, A.; Tendeloo, G. V.; Nagy, J. B. Chem. Commun. 1999, 1343. (16) Yudasaka, M.; Komatsu, T.; Ichihashi, T.; Achiba, Y.; Iijima, S. J. Phys. Chem. B 1998, 102, 4892. (17) Gorbunov, A. A.; Friedlein, R.; Jost, O.; Golden, M. S.; Fink, J.; Pompe, W. Appl. Phys. A 1999, 69, S593. (18) For example: Somoraj, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley and Sons: New York, 1994. (19) Binary Alloy Phase Diagrams, 2nd ed.; Massalski, T. B., Ed.; ASM International; 1990. (20) Villars, P.; Prince, A.; Okamoto, H. Handbook of Ternary Alloy Phase Diagrams; ASM International; 1995.
