- Email: [email protected]
Growing carbon nanotubes
Growing
carbon nanotubes by Yoshinori Ando†*, Xinluo Zhao†, Toshiki Sugai‡, and Mukul Kumar†
The discovery of ‘fullerenes’ added a new dimension to the knowledge of carbon science1; and the subsequent discovery of ‘carbon nanotubes’ (CNTs, the elongated fullerene) added a new dimension to the knowledge of technology2. Today, ‘nanotechnology’ is a hot topic attracting scientists, industrialists, journalists, governments, and even the general public. Nanotechnology is the creation of functional materials, devices, and systems through control of matter on the nanometer scale and the exploitation of novel phenomena and properties of matter (physical, chemical, biological, electrical, etc.) at that length scale. CNTs are supposed to be a key component of nanotechnology. Almost every week a new potential application of CNTs is identified, stimulating scientists to peep into this tiny tube with ever increasing curiosity.
†Department of Materials Science and Engineering, Meijo University, 1-501 Tempaku-ku, Nagoya 468-8502, Japan *E-mail: [email protected] ‡Department of Chemistry, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan
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CNTs, born in the Ando lab2,3 and introduced to the world by Iijima, are now 13 years old and much has been said, written, and discussed about this teenage hero of the scientific Hollywood. The immediate challenge is to produce CNTs of desired properties in large quantities. The essence of this challenge lies in understanding the growth mechanism of CNTs, which is not yet clear. In this review, therefore, we focus on the synthesis aspect only, pointing out the products, problems, and limitations of the available CNT growth techniques. Although there have been many interesting and successful attempts to grow CNTs by various methods, we restrict our discussion to the three most widely used techniques: arc discharge, laser furnace, and chemical vapor deposition (CVD).
Arc-discharge method The arc-discharge method is the one by which CNTs were first produced and recognized. The history of CNTs is closely related to the mass production of fullerenes developed by Krätschmer et al.4 in 1990. They evaporated graphite rods in contact by applying an ac voltage in an inert gas to produce fullerenes. Soon after, we applied a dc arc voltage3 between two separated graphite rods (Fig. 1) by modifying our SiC powder production apparatus5. The evaporated anode generates fullerenes6 in the form of soot in the chamber, and a part of the evaporated anode is deposited on the cathode. In that cathode deposit, Iijima found the treasure, CNTs2. The schematic diagram of CNT production apparatus7 is shown in Fig. 1. After evacuating the chamber with a vacuum
ISSN:1369 7021 © Elsevier Ltd 2004
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Graphite rod
C-soot Graphite rod +
Deposit
+ DC arc discharge power source
Gas
Vacuum pump
Fig. 1 Schematic diagram of CNT formation apparatus by the arc-discharge method. (Reprinted with permission from7. © 1996 Meijo University.)
pump, an appropriate ambient gas is introduced at the desired pressure, and then a dc arc voltage is applied between the two graphite rods. When pure graphite rods are used, the anode evaporates to form fullerenes, which are deposited in the form of soot in the chamber6. However, a small part of the evaporated anode is deposited on the cathode, which includes CNTs2. These CNTs, made of coaxial graphene sheets and called multi-walled carbon nanotubes (MWNTs), are found not only on the top surface of the cathode deposit8 but also deep inside the deposit3. Large-scale synthesis of MWNTs by arc discharge has been achieved9,10 in He gas, and their thermal purification has also been successful11. When a graphite rod containing metal catalyst (Fe, Co, etc.) is used as the anode with a pure graphite cathode, single-walled carbon nanotubes (SWNTs)12,13 are generated in the form of soot. The first author of this review, Ando, has been carrying out the arc-discharge evaporation of pure graphite rods in various kinds of ambient gases (He, Ar, and CH4)14 since CNTs were first discovered2,3. It was found that CH4 gas was the best for forming MWNTs with high crystallinity and few coexisting carbon nanoparticles14. On the contrary, fullerenes cannot be produced in gas including hydrogen atoms15. This is the
essential difference between CNT and fullerene production. In order to clarify the effect of gas including hydrogen atoms in MWNT production, ambient CH4 gas was analyzed before and after arc discharge by mass spectroscopy16, which revealed that the thermal decomposition of CH4 gas is as follows: 2CH4 → C2H2 + 3H2. Therefore, pure graphite rods were arc evaporated in pure hydrogen gas17,18. The effectiveness of hydrogen arc discharge in producing MWNTs with high crystallinity was confirmed18, and a new morphology of carbon, the ‘carbon rose’, was also found17. A similar effect of ambient hydrogen gas was also reported by another group19. In fact, for MWNT production, a gas that includes hydrogen atoms is more effective than an inert gas, such as He or Ar. The reason might be the high temperature and high activity of the hydrogen arc. The MWNTs produced by hydrogen arc discharge (H2-arc MWNTs) contain very few coexisting carbon nanoparticles20, as seen in the scanning electron micrograph of Fig. 2a. These nanoparticles are easily removed by infrared irradiation or heating in air at 500°C. For instance, thermally purified MWNTs20 are shown in Fig. 2b. High-resolution transmission electron microscopy observation of the purified H2-arc MWNTs reveals that their crystallinity is very high (having regular graphene sheets at an interlayer spacing of 3.4 Å) and an inner diameter typically as low as 7 Å, which is equal to the diameter of C60. Until the beginning of this year, the smallest nanotube21 known was 4 Å in diameter (Fig. 3a). However, very recently 3 Å tubes have been found to exist inside H2-arc MWNTs22 (Fig. 3b). Moreover, a linear carbon chain has also been observed23 inserted into an MWNT with an innermost tube diameter of 7 Å. Surprisingly, both the 3 Å tube and the carbon chain exist in the same H2-arc MWNTs.
Fig. 2 Scanning electron micrographs of H2-arc MWNTs: (a) as grown; and (b) purified by infrared radiation. (Reprinted with permission from20. © 1998 Institute of Pure and Applied Physics.)
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(a)
(b)
Fig. 3 High-resolution transmission electron micrographs of H2-arc MWNTs: (a) 4 Å innermost diameter MWNT; and (b) 3 Å innermost diameter MWNT. (Part (a) reprinted with permission from21 © 2000 Macmillan Publishers Ltd. Part (b) reprinted with permission from22. © 2004 American Physical Society.)
Corresponding to these thin innermost tubes in H2-arc MWNTs, radial breathing modes (RBM) in Raman spectra are observed24,25. It is well known26 that the wave number, ω, of RBM is inversely proportional to the tube diameter, d. From the Raman spectra, it is clear that H2-arc MWNTs have an innermost tube diameter that is smaller than independently produced SWNTs. In the high wave number region (~1580 cm-1) of the Raman spectra, isolated H2-arc MWNTs exhibit splitting of the G-band27, which is usually only found in SWNTs. This is characteristic of the quantum confinement effect27 because of the very thin diameter of CNTs. The experimental history of SWNTs12,13 starts two years after the discovery of MWNTs2. SWNTs were also first produced by arc discharge using a graphite anode containing a metal catalyst (Fe or Co); however, SWNTs were obtained from the soot in the gas phase not from the cathode deposit. Of course, evaporated metal nanoparticles also coexist with the SWNTs, which have to be purified later. Mass production of SWNTs by arc discharge was achieved by Journet et al.28 using a bimetallic Ni-Y catalyst in He ambient gas. The method was effectively modified29 by using two graphite electrodes inclined at an angle of 30° instead of the conventional 180° alignment. This is known as the arcplasma-jet method29 and yields SWNTs at a rate of ~1 g/min. Another method30, using an Fe catalyst instead of
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Ni-Y and an H2-Ar gas mixture in place of He, produces a partly aligned macroscopic net of SWNTs as long as 30 cm (Fig. 4). Because the Fe nanoparticles attached to the SWNT net are covered with a very thin layer of amorphous carbon, the SWNTs are easily purified by heating in air at 420°C and then rinsing in mild HCl.
Fig. 4 A macroscopic net of SWNTs produced by arc evaporation in a H2-Ar gas mixture. (Reprinted with permission from30. © 2003 Elsevier Ltd.)
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In order to produce SWNTs, the use of metal catalysts is a necessary condition. The activities of various kinds of metal catalysts (Ni, Co, Fe, Pt, Pd, etc.) have been investigated31 and the maximum yield is achieved with Ni-Co. It was also observed that the addition of S increases the catalytic effect of other metal catalysts32,33. Double-walled nanotubes (DWNTs) produced by arc discharge have also been reported34-36 under conditions similar to those of largediameter SWNT formation. It may be noted that SWNTs are not found in the cathode deposit like MWNTs but in the chamber soot. Therefore, an ac arc discharge, in which both the electrodes evaporate alternately, can produce a higher yield of SWNTs37. Generally, it is hard to grow aligned CNTs (SWNTs, DWNTs, or MWNTs) by arc discharge, although partial alignment of SWNTs can be achieved by convection30 or directed arc plasma38. On the other hand, the growth temperature of the arc-discharge method is higher than that of other CNT production methods. As a result, the crystallinity and perfection of arc-produced CNTs are generally high, and the yield per unit time is also higher than other methods.
Laser-furnace method The laser vaporization method, which had been originally used as a source of clusters39 and ultrafine particles40,41, was developed for fullerene and CNT production by Smalley’s group42,43. Since the energy density of lasers is much higher than that of other vaporization devices, the laser is suitable for materials with a high boiling temperature such as carbon. When applied to carbon, fullerenes were discovered by mass spectrometry1 but their structural identification was impossible at that time because their quantities were too small. To produce large quantities of fullerenes and other nanomaterials, Smalley’s group developed the laser-furnace method42 together with an annealing system in 1992. Fullerenes with a soccer ball structure are produced only at higher furnace temperatures, underlining the importance of annealing for nanostructures42. These discoveries were applied to produce CNTs43 in 1996, especially SWNTs. Fig. 5 shows the setup of the laser furnace, which consists of a furnace, a quartz tube with a window, a target carbon composite doped with catalytic metals, a water-cooled trap, and flow systems for the buffer gas to maintain constant pressures and flow rates42,43. A laser beam (typically a YAG or CO2 laser) is introduced through the window and focused
onto the target located in the center of the furnace. The target is vaporized in high-temperature Ar buffer gas and forms SWNTs. The Ar flow rate and pressure are typically 1 cm•s-1 and 500 torr, respectively. The SWNTs produced are conveyed by the buffer gas to the trap, where they are collected. The vaporization surface is kept as fresh as possible by changing the focus point or moving the target. The method has several advantages, such as high-quality SWNT production, diameter control, investigation of growth dynamics, and the production of new materials. High-quality SWNTs with minimal defects and contaminants, such as amorphous carbon and catalytic metals, have been produced using the laser-furnace method together with purification processes44-46. High crystallinity has been known to originate in high-power laser vaporization, homogeneous annealing conditions, and target materials without hydrogen31,47. The laser has sufficiently high energy density not to cleave the target into graphite particles but to vaporize it at the molecular level. The graphite vapor is converted into amorphous carbon as the starting material of SWNTs48-50. The annealing conditions of the amorphous carbon are more homogeneous than those of the arc-discharge method, in which the electrodes and the convection flow disturb the homogeneity of the temperature and flow rate31,50,51. To achieve homogeneous conditions in arc discharge, a method called high-temperature pulsed arc discharge has been developed, which uses a dc pulsed arc discharge inside a furnace36,52-54. The method provides high-quality DWNTs and SWNTs. It can be considered a hybrid of steady arc discharge and the laser-furnace method. The SWNTs still have contaminants, such as amorphous carbon and catalytic metals. To remove these contaminants, purification processes
Fig. 5 Schematic diagram of the laser-furnace apparatus.
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based on oxidation by hot air (400-500°C) or chemicals such as H2O2 are applied to the crude SWNTs45,46. Amorphous carbon has many dangling bonds, which are not present in SWNTs, leading to high activity. High-quality SWNTs with few defects are more resistant to oxidation treatments than those of low quality. The oxidation treatments remove the protective layer of catalytic metal particles so that they are easily eliminated by acid treatments44-46. SWNT diameter can be controlled by changing the furnace temperature, catalytic metals, and flow rate26,49,55. A higher furnace temperature results in SWNTs with larger diameters26. The use of a Ni-Y alloy catalyst also increases the SWNT diameter, whereas a Rh-Pd catalyst reduces it55,56. Flow rate affects the diameter distribution, which suggests that the growth process is fairly slow (on a timescale of seconds) compared with vaporization processes (nanosecond scale)49. SWNT growth is initiated by a short laser shot, so that the start time of the growth is defined. The growth processes have been traced by high-speed video imaging of light emission or scattering, from which the growth time is estimated to be more than several milliseconds48-50. Novel nanomaterials, such as peapods and single-walled carbon nanohorns (SWNHs), are also produced by the laserfurnace method. Peapods, fullerenes encapsulated by SWNTs, were discovered in the soot of the laser furnace57, and later synthesized from isolated fullerenes and laser-produced, purified SWNTs58,59. The fullerenes are sublimated and introduced into SWNTs through open ends or holes in the wall at a high vacuum (~10-6 torr) at 500-600°C58. Since these vacancies are easily covered with amorphous carbon because of their active edges, purified SWNTs grown by the laser-furnace method are indispensable for maximizing the peapod yield. SWNHs, small hollow carbon edging particles, have been discovered in the carbon soot when using a CO2 laser60. SWNHs have a large surface/volume ratio and are promising as a support material for fuel cell catalysts61. In SWNH production, a CO2 laser with a long pulse duration (a few milliseconds) is crucial; SWNHs cannot be produced with a short-pulsed YAG laser (pulse durations of nanoseconds)62,63. A longer pulse duration, which leads to a lower energy density and lower vaporization temperature, is essential for the production process. The tunable duration of laser vaporization is another advantage in producing and exploring these nanoscale carbon materials. That is why the laser-furnace method is being used for growing various
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nanomaterials, and is expected to play a powerful role in nanotechnology.
Chemical vapor deposition CVD is another popular method for producing CNTs in which a hydrocarbon vapor is thermally decomposed in the presence of a metal catalyst. The method is also known as thermal or catalytic CVD to distinguish it from the many other kinds of CVD used for various purposes. Compared with arc-discharge and laser methods, CVD is a simple and economic technique for synthesizing CNTs at low temperature and ambient pressure, at the cost of crystallinity. It is versatile in that it harnesses a variety of hydrocarbons in any state (solid, liquid, or gas), enables the use of various substrates, and allows CNT growth in a variety of forms, such as powder, thin or thick films, aligned or entangled, straight or coiled, or even a desired architecture of nanotubes at predefined sites on a patterned substrate. It also offers better control over growth parameters. In fact, CVD has been used for producing carbon filaments and fibers since 195964-66. Using the same technique, soon after the discovery of CNTs by Iijima2, Endo et al.67 reported CNT growth from pyrolysis of benzene at 1100°C, while JoséYacamán et al.68 formed clear helical MWNTs at 700°C from acetylene. In both cases, Fe nanoparticles were used as the catalyst. Later, MWNTs were also grown from ethylene69, methane70, and many other hydrocarbons. SWNTs were first produced by Dai et al.71 from disproportionation of CO at 1200°C, catalyzed by Mo particles. Later, SWNTs were also produced from benzene72, acetylene73, ethylene74, and methane75,76 using various catalysts. Fig. 6a shows a diagram of the setup used for CNT growth by CVD in its simplest form. The process involves passing a hydrocarbon vapor (typically for 15-60 minutes) through a tube furnace in which a catalyst material is present at sufficiently high temperature (600-1200°C) to decompose the hydrocarbon. CNTs grow over the catalyst and are collected upon cooling the system to room temperature. In the case of a liquid hydrocarbon (benzene, alcohol, etc.), the liquid is heated in a flask and an inert gas purged through it to carry the vapor into the reaction furnace. The vaporization of a solid hydrocarbon (camphor, naphthalene, etc.) can be conveniently achieved in another furnace at low temperature before the main, high-temperature reaction furnace shown in Fig. 6a. The catalyst material may also be solid, liquid, or gas
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(a)
(c)
(b)
Fig. 6 (a) Schematic diagram of a CVD setup. (b) Probable models for CNT growth. (c) An example of a SWNT and a MWNT grown from camphor via CVD.
and can be placed inside the furnace or fed in from outside. Pyrolysis of the catalyst vapor at a suitable temperature liberates metal nanoparticles in situ (the process is known as the floating catalyst method). Alternatively, catalyst-plated substrates can be placed in the hot zone of the furnace to catalyze CNT growth. Catalytically decomposed carbon species of the hydrocarbon are assumed to dissolve in the metal nanoparticles and, after reaching supersaturation, precipitate out in the form of a fullerene dome extending into a carbon cylinder (like the inverted test tube shown in Fig. 6b) with no dangling bonds and, hence, minimum energy77,78. When the substrate-catalyst interaction is strong, a CNT grows up with the catalyst particle rooted at its base (known as the ‘base growth model’). When the substrate-catalyst interaction is weak, the catalyst particle is lifted up by the growing CNT and continues to promote CNT growth at its tip (the ‘tip growth model’). Formation of SWNTs or MWNTs is governed by the size of the catalyst particle. Broadly speaking, when the particle size is a few nanometers, SWNTs form, whereas particles a few tens of nanometers wide favor MWNT formation. The three main parameters for CNT growth in CVD are the hydrocarbon, catalyst, and growth temperature. General experience is that low-temperature CVD (600-900°C) yields MWNTs, whereas a higher temperature (900-1200°C) reaction favors SWNT growth, indicating that SWNTs have a
higher energy of formation (presumably owing to their small diameters, which results in high curvature and high strain energy). This could explain why MWNTs are easier to grow from most hydrocarbons than SWNTs, which can only be grown from selected hydrocarbons (e.g. CO, CH4, etc., that have a reasonable stability in the temperature range of 900-1200°C). Common efficient precursors of MWNTs (e.g. acetylene, benzene, etc.) are unstable at higher temperatures and lead to the deposition of large amounts of carbonaceous compounds other than CNTs. Transition metals (Fe, Co, Ni) are the most commonly used catalysts for CNT growth, since the phase diagram of carbon and these metals suggests finite solubility of carbon in these transition metals at high temperatures. This leads to the formation of CNTs under the growth mechanism outlined above. Solid organometallocenes (ferrocene, cobaltocene, nickelocene) are widely used as catalyst materials because they liberate metal particles in situ that efficiently catalyze CNT growth. The catalyst particle size has been found to dictate the tube diameter. Hence, metal nanoparticles of controlled size can be used to grow CNTs of controlled diameter79. Thin films of catalyst coated onto various substrates have also proved successful in achieving uniform CNT deposits80. In addition, the material, morphology, and textural properties of the substrate greatly affect the yield and quality of the resulting CNTs. Zeolite supports with
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catalysts in their nanopores have resulted in significantly higher yields of CNTs with a narrow diameter distribution81. Alumina materials are reported to be better catalyst supports than silica owing to their strong metal-support interaction, which allows high metal dispersion and, thus, a high density of catalytic sites82. Such interactions prevent metal species from aggregating and forming unwanted large clusters that lead to graphite particles or defective MWNTs. The key to obtaining high yields of pure CNTs is achieving hydrocarbon decomposition on catalyst sites alone and avoiding spontaneous pyrolysis. It is remarkable that transition metals have proven to be efficient catalysts not only in CVD but also in arc-discharge and laser methods. This indicates that these apparently different methods might have a common growth mechanism for CNTs, which is not yet clear. CNTs have been successfully synthesized using organometallic compounds (nickel phthalocyanine83 and ferrocene84) as carbon-cum-catalyst precursors, though the as-grown CNTs were mostly metal encapsulated. The use of ethanol has drawn attention for synthesizing SWNTs at relatively low temperatures (~850°C) on Fe-Co impregnated zeolite supports and Mo-Co coated quartz substrates85-87. Recently, a tree product, camphor, has been used to produce high yields of high-purity MWNTs88-90. Because of the low catalyst requirement with camphor, as-grown MWNTs are the least contaminated with metal, while the oxygen atoms present in camphor help oxidize amorphous carbon in situ91. Fig. 6c shows an SWNT and MWNT grown from camphor. CVD is ideally suited to growing aligned CNTs on desired substrates for specific applications, which is not feasible by arc or laser methods. Li et al.92 have grown dense MWNT arrays on Fe-impregnated mesoporous silica prepared by a sol-gel process, Terrones et al.93 have produced CNTs on Co-coated quartz substrates via CVD of a triazene compound with nearly no byproducts, while Pan et al.94 have reported the growth of aligned CNTs of more than 2 mm in length over mesoporous substrates from acetylene. Highly aligned nanotubes for electronics have been grown from acetylene95 using a Co catalyst impregnated in alumina nanochannels at 650°C, while pillars of parallel CNTs have been grown from ethylene on Fe-patterned Si plates at 700°C for field emission applications80. Bearing in mind that pyrolysis of a xyleneferrocene mixture leads to the growth of vertical CNTs on quartz96, Ajayan and coworkers have produced organized assemblies of CNTs on thermally oxidized Si wafers97,98.
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Since CVD is a well known and well established industrial process, CNT production is easy to scale up. MWNTs of controlled diameter are being produced in large quantities (~100 g/day) from acetylene using nanoporous materials as the catalyst support99. Wang et al.100 have developed a nano-agglomerate fluidized-bed reactor (a quartz cylinder 1 m long and 0.25 m wide) in which the continuous decomposition of ethylene gas on an Fe/alumina catalyst at 700°C produces a few kilograms of MWNTs per hour with a reported purity of 70%. Dai’s group has scaled up SWNT production from methane using a Fe-Mo bimetallic catalyst supported on a sol-gel derived alumina-silica multicomponent material101. However, Smalley’s lab still leads the way in the mass production of SWNTs (~10 g/day) by the high pressure carbon monoxide (HiPco) technique102. In this method, a Fe pentacarbonyl catalyst liberates Fe particles in situ at high temperatures, while a high pressure of CO (~30 atm) enhances the carbon feedstock manifolds, which significantly speeds up the disproportionation of CO molecules into carbon atoms and accelerates SWNT growth. Apart from large-scale production, CVD also offers the possibility of growing single nanotubes for use as probe tips in atomic force microscopes (AFM) or as field emitters in electron microscopes. Hafner et al.103 have grown single SWNTs and MWNTs (1-3 nm in diameter) rooted in the pores of Si tips suitable for AFM imaging. In another approach, single SWNTs are grown directly onto pyramids of Si cantilever-tip assemblies104. In this case, a SWNT grown on the Si surface (controlled by the catalyst density on the surface) protrudes from the apex of the pyramid. As-grown CNT tips are smaller than mechanically assembled nanotube tips by a factor of three and enable significantly improved resolution. CVD-produced CNTs have great promise for the fabrication of sophisticated instruments and nanodevices.
Concluding remarks We have seen here that each method has its own merits and limitations. Overwhelming research is going on around the world to solve the problems involved and understand this ‘nano world’. We have presented only a glimpse of the journey that nanotubes have taken over the last 13 years, though we sincerely feel that each day of this journey has been interesting, inspiring, and challenging. The journey through this long, narrow tunnel continues, with more dramatic turns, surprising scenes, and exciting events still ahead. MT
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carbon nanotubes by Yoshinori Ando†*, Xinluo Zhao†, Toshiki Sugai‡, and Mukul Kumar†
The discovery of ‘fullerenes’ added a new dimension to the knowledge of carbon science1; and the subsequent discovery of ‘carbon nanotubes’ (CNTs, the elongated fullerene) added a new dimension to the knowledge of technology2. Today, ‘nanotechnology’ is a hot topic attracting scientists, industrialists, journalists, governments, and even the general public. Nanotechnology is the creation of functional materials, devices, and systems through control of matter on the nanometer scale and the exploitation of novel phenomena and properties of matter (physical, chemical, biological, electrical, etc.) at that length scale. CNTs are supposed to be a key component of nanotechnology. Almost every week a new potential application of CNTs is identified, stimulating scientists to peep into this tiny tube with ever increasing curiosity.
†Department of Materials Science and Engineering, Meijo University, 1-501 Tempaku-ku, Nagoya 468-8502, Japan *E-mail: [email protected] ‡Department of Chemistry, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan
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CNTs, born in the Ando lab2,3 and introduced to the world by Iijima, are now 13 years old and much has been said, written, and discussed about this teenage hero of the scientific Hollywood. The immediate challenge is to produce CNTs of desired properties in large quantities. The essence of this challenge lies in understanding the growth mechanism of CNTs, which is not yet clear. In this review, therefore, we focus on the synthesis aspect only, pointing out the products, problems, and limitations of the available CNT growth techniques. Although there have been many interesting and successful attempts to grow CNTs by various methods, we restrict our discussion to the three most widely used techniques: arc discharge, laser furnace, and chemical vapor deposition (CVD).
Arc-discharge method The arc-discharge method is the one by which CNTs were first produced and recognized. The history of CNTs is closely related to the mass production of fullerenes developed by Krätschmer et al.4 in 1990. They evaporated graphite rods in contact by applying an ac voltage in an inert gas to produce fullerenes. Soon after, we applied a dc arc voltage3 between two separated graphite rods (Fig. 1) by modifying our SiC powder production apparatus5. The evaporated anode generates fullerenes6 in the form of soot in the chamber, and a part of the evaporated anode is deposited on the cathode. In that cathode deposit, Iijima found the treasure, CNTs2. The schematic diagram of CNT production apparatus7 is shown in Fig. 1. After evacuating the chamber with a vacuum
ISSN:1369 7021 © Elsevier Ltd 2004
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Graphite rod
C-soot Graphite rod +
Deposit
+ DC arc discharge power source
Gas
Vacuum pump
Fig. 1 Schematic diagram of CNT formation apparatus by the arc-discharge method. (Reprinted with permission from7. © 1996 Meijo University.)
pump, an appropriate ambient gas is introduced at the desired pressure, and then a dc arc voltage is applied between the two graphite rods. When pure graphite rods are used, the anode evaporates to form fullerenes, which are deposited in the form of soot in the chamber6. However, a small part of the evaporated anode is deposited on the cathode, which includes CNTs2. These CNTs, made of coaxial graphene sheets and called multi-walled carbon nanotubes (MWNTs), are found not only on the top surface of the cathode deposit8 but also deep inside the deposit3. Large-scale synthesis of MWNTs by arc discharge has been achieved9,10 in He gas, and their thermal purification has also been successful11. When a graphite rod containing metal catalyst (Fe, Co, etc.) is used as the anode with a pure graphite cathode, single-walled carbon nanotubes (SWNTs)12,13 are generated in the form of soot. The first author of this review, Ando, has been carrying out the arc-discharge evaporation of pure graphite rods in various kinds of ambient gases (He, Ar, and CH4)14 since CNTs were first discovered2,3. It was found that CH4 gas was the best for forming MWNTs with high crystallinity and few coexisting carbon nanoparticles14. On the contrary, fullerenes cannot be produced in gas including hydrogen atoms15. This is the
essential difference between CNT and fullerene production. In order to clarify the effect of gas including hydrogen atoms in MWNT production, ambient CH4 gas was analyzed before and after arc discharge by mass spectroscopy16, which revealed that the thermal decomposition of CH4 gas is as follows: 2CH4 → C2H2 + 3H2. Therefore, pure graphite rods were arc evaporated in pure hydrogen gas17,18. The effectiveness of hydrogen arc discharge in producing MWNTs with high crystallinity was confirmed18, and a new morphology of carbon, the ‘carbon rose’, was also found17. A similar effect of ambient hydrogen gas was also reported by another group19. In fact, for MWNT production, a gas that includes hydrogen atoms is more effective than an inert gas, such as He or Ar. The reason might be the high temperature and high activity of the hydrogen arc. The MWNTs produced by hydrogen arc discharge (H2-arc MWNTs) contain very few coexisting carbon nanoparticles20, as seen in the scanning electron micrograph of Fig. 2a. These nanoparticles are easily removed by infrared irradiation or heating in air at 500°C. For instance, thermally purified MWNTs20 are shown in Fig. 2b. High-resolution transmission electron microscopy observation of the purified H2-arc MWNTs reveals that their crystallinity is very high (having regular graphene sheets at an interlayer spacing of 3.4 Å) and an inner diameter typically as low as 7 Å, which is equal to the diameter of C60. Until the beginning of this year, the smallest nanotube21 known was 4 Å in diameter (Fig. 3a). However, very recently 3 Å tubes have been found to exist inside H2-arc MWNTs22 (Fig. 3b). Moreover, a linear carbon chain has also been observed23 inserted into an MWNT with an innermost tube diameter of 7 Å. Surprisingly, both the 3 Å tube and the carbon chain exist in the same H2-arc MWNTs.
Fig. 2 Scanning electron micrographs of H2-arc MWNTs: (a) as grown; and (b) purified by infrared radiation. (Reprinted with permission from20. © 1998 Institute of Pure and Applied Physics.)
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(a)
(b)
Fig. 3 High-resolution transmission electron micrographs of H2-arc MWNTs: (a) 4 Å innermost diameter MWNT; and (b) 3 Å innermost diameter MWNT. (Part (a) reprinted with permission from21 © 2000 Macmillan Publishers Ltd. Part (b) reprinted with permission from22. © 2004 American Physical Society.)
Corresponding to these thin innermost tubes in H2-arc MWNTs, radial breathing modes (RBM) in Raman spectra are observed24,25. It is well known26 that the wave number, ω, of RBM is inversely proportional to the tube diameter, d. From the Raman spectra, it is clear that H2-arc MWNTs have an innermost tube diameter that is smaller than independently produced SWNTs. In the high wave number region (~1580 cm-1) of the Raman spectra, isolated H2-arc MWNTs exhibit splitting of the G-band27, which is usually only found in SWNTs. This is characteristic of the quantum confinement effect27 because of the very thin diameter of CNTs. The experimental history of SWNTs12,13 starts two years after the discovery of MWNTs2. SWNTs were also first produced by arc discharge using a graphite anode containing a metal catalyst (Fe or Co); however, SWNTs were obtained from the soot in the gas phase not from the cathode deposit. Of course, evaporated metal nanoparticles also coexist with the SWNTs, which have to be purified later. Mass production of SWNTs by arc discharge was achieved by Journet et al.28 using a bimetallic Ni-Y catalyst in He ambient gas. The method was effectively modified29 by using two graphite electrodes inclined at an angle of 30° instead of the conventional 180° alignment. This is known as the arcplasma-jet method29 and yields SWNTs at a rate of ~1 g/min. Another method30, using an Fe catalyst instead of
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Ni-Y and an H2-Ar gas mixture in place of He, produces a partly aligned macroscopic net of SWNTs as long as 30 cm (Fig. 4). Because the Fe nanoparticles attached to the SWNT net are covered with a very thin layer of amorphous carbon, the SWNTs are easily purified by heating in air at 420°C and then rinsing in mild HCl.
Fig. 4 A macroscopic net of SWNTs produced by arc evaporation in a H2-Ar gas mixture. (Reprinted with permission from30. © 2003 Elsevier Ltd.)
REVIEW FEATURE
In order to produce SWNTs, the use of metal catalysts is a necessary condition. The activities of various kinds of metal catalysts (Ni, Co, Fe, Pt, Pd, etc.) have been investigated31 and the maximum yield is achieved with Ni-Co. It was also observed that the addition of S increases the catalytic effect of other metal catalysts32,33. Double-walled nanotubes (DWNTs) produced by arc discharge have also been reported34-36 under conditions similar to those of largediameter SWNT formation. It may be noted that SWNTs are not found in the cathode deposit like MWNTs but in the chamber soot. Therefore, an ac arc discharge, in which both the electrodes evaporate alternately, can produce a higher yield of SWNTs37. Generally, it is hard to grow aligned CNTs (SWNTs, DWNTs, or MWNTs) by arc discharge, although partial alignment of SWNTs can be achieved by convection30 or directed arc plasma38. On the other hand, the growth temperature of the arc-discharge method is higher than that of other CNT production methods. As a result, the crystallinity and perfection of arc-produced CNTs are generally high, and the yield per unit time is also higher than other methods.
Laser-furnace method The laser vaporization method, which had been originally used as a source of clusters39 and ultrafine particles40,41, was developed for fullerene and CNT production by Smalley’s group42,43. Since the energy density of lasers is much higher than that of other vaporization devices, the laser is suitable for materials with a high boiling temperature such as carbon. When applied to carbon, fullerenes were discovered by mass spectrometry1 but their structural identification was impossible at that time because their quantities were too small. To produce large quantities of fullerenes and other nanomaterials, Smalley’s group developed the laser-furnace method42 together with an annealing system in 1992. Fullerenes with a soccer ball structure are produced only at higher furnace temperatures, underlining the importance of annealing for nanostructures42. These discoveries were applied to produce CNTs43 in 1996, especially SWNTs. Fig. 5 shows the setup of the laser furnace, which consists of a furnace, a quartz tube with a window, a target carbon composite doped with catalytic metals, a water-cooled trap, and flow systems for the buffer gas to maintain constant pressures and flow rates42,43. A laser beam (typically a YAG or CO2 laser) is introduced through the window and focused
onto the target located in the center of the furnace. The target is vaporized in high-temperature Ar buffer gas and forms SWNTs. The Ar flow rate and pressure are typically 1 cm•s-1 and 500 torr, respectively. The SWNTs produced are conveyed by the buffer gas to the trap, where they are collected. The vaporization surface is kept as fresh as possible by changing the focus point or moving the target. The method has several advantages, such as high-quality SWNT production, diameter control, investigation of growth dynamics, and the production of new materials. High-quality SWNTs with minimal defects and contaminants, such as amorphous carbon and catalytic metals, have been produced using the laser-furnace method together with purification processes44-46. High crystallinity has been known to originate in high-power laser vaporization, homogeneous annealing conditions, and target materials without hydrogen31,47. The laser has sufficiently high energy density not to cleave the target into graphite particles but to vaporize it at the molecular level. The graphite vapor is converted into amorphous carbon as the starting material of SWNTs48-50. The annealing conditions of the amorphous carbon are more homogeneous than those of the arc-discharge method, in which the electrodes and the convection flow disturb the homogeneity of the temperature and flow rate31,50,51. To achieve homogeneous conditions in arc discharge, a method called high-temperature pulsed arc discharge has been developed, which uses a dc pulsed arc discharge inside a furnace36,52-54. The method provides high-quality DWNTs and SWNTs. It can be considered a hybrid of steady arc discharge and the laser-furnace method. The SWNTs still have contaminants, such as amorphous carbon and catalytic metals. To remove these contaminants, purification processes
Fig. 5 Schematic diagram of the laser-furnace apparatus.
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based on oxidation by hot air (400-500°C) or chemicals such as H2O2 are applied to the crude SWNTs45,46. Amorphous carbon has many dangling bonds, which are not present in SWNTs, leading to high activity. High-quality SWNTs with few defects are more resistant to oxidation treatments than those of low quality. The oxidation treatments remove the protective layer of catalytic metal particles so that they are easily eliminated by acid treatments44-46. SWNT diameter can be controlled by changing the furnace temperature, catalytic metals, and flow rate26,49,55. A higher furnace temperature results in SWNTs with larger diameters26. The use of a Ni-Y alloy catalyst also increases the SWNT diameter, whereas a Rh-Pd catalyst reduces it55,56. Flow rate affects the diameter distribution, which suggests that the growth process is fairly slow (on a timescale of seconds) compared with vaporization processes (nanosecond scale)49. SWNT growth is initiated by a short laser shot, so that the start time of the growth is defined. The growth processes have been traced by high-speed video imaging of light emission or scattering, from which the growth time is estimated to be more than several milliseconds48-50. Novel nanomaterials, such as peapods and single-walled carbon nanohorns (SWNHs), are also produced by the laserfurnace method. Peapods, fullerenes encapsulated by SWNTs, were discovered in the soot of the laser furnace57, and later synthesized from isolated fullerenes and laser-produced, purified SWNTs58,59. The fullerenes are sublimated and introduced into SWNTs through open ends or holes in the wall at a high vacuum (~10-6 torr) at 500-600°C58. Since these vacancies are easily covered with amorphous carbon because of their active edges, purified SWNTs grown by the laser-furnace method are indispensable for maximizing the peapod yield. SWNHs, small hollow carbon edging particles, have been discovered in the carbon soot when using a CO2 laser60. SWNHs have a large surface/volume ratio and are promising as a support material for fuel cell catalysts61. In SWNH production, a CO2 laser with a long pulse duration (a few milliseconds) is crucial; SWNHs cannot be produced with a short-pulsed YAG laser (pulse durations of nanoseconds)62,63. A longer pulse duration, which leads to a lower energy density and lower vaporization temperature, is essential for the production process. The tunable duration of laser vaporization is another advantage in producing and exploring these nanoscale carbon materials. That is why the laser-furnace method is being used for growing various
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nanomaterials, and is expected to play a powerful role in nanotechnology.
Chemical vapor deposition CVD is another popular method for producing CNTs in which a hydrocarbon vapor is thermally decomposed in the presence of a metal catalyst. The method is also known as thermal or catalytic CVD to distinguish it from the many other kinds of CVD used for various purposes. Compared with arc-discharge and laser methods, CVD is a simple and economic technique for synthesizing CNTs at low temperature and ambient pressure, at the cost of crystallinity. It is versatile in that it harnesses a variety of hydrocarbons in any state (solid, liquid, or gas), enables the use of various substrates, and allows CNT growth in a variety of forms, such as powder, thin or thick films, aligned or entangled, straight or coiled, or even a desired architecture of nanotubes at predefined sites on a patterned substrate. It also offers better control over growth parameters. In fact, CVD has been used for producing carbon filaments and fibers since 195964-66. Using the same technique, soon after the discovery of CNTs by Iijima2, Endo et al.67 reported CNT growth from pyrolysis of benzene at 1100°C, while JoséYacamán et al.68 formed clear helical MWNTs at 700°C from acetylene. In both cases, Fe nanoparticles were used as the catalyst. Later, MWNTs were also grown from ethylene69, methane70, and many other hydrocarbons. SWNTs were first produced by Dai et al.71 from disproportionation of CO at 1200°C, catalyzed by Mo particles. Later, SWNTs were also produced from benzene72, acetylene73, ethylene74, and methane75,76 using various catalysts. Fig. 6a shows a diagram of the setup used for CNT growth by CVD in its simplest form. The process involves passing a hydrocarbon vapor (typically for 15-60 minutes) through a tube furnace in which a catalyst material is present at sufficiently high temperature (600-1200°C) to decompose the hydrocarbon. CNTs grow over the catalyst and are collected upon cooling the system to room temperature. In the case of a liquid hydrocarbon (benzene, alcohol, etc.), the liquid is heated in a flask and an inert gas purged through it to carry the vapor into the reaction furnace. The vaporization of a solid hydrocarbon (camphor, naphthalene, etc.) can be conveniently achieved in another furnace at low temperature before the main, high-temperature reaction furnace shown in Fig. 6a. The catalyst material may also be solid, liquid, or gas
REVIEW FEATURE
(a)
(c)
(b)
Fig. 6 (a) Schematic diagram of a CVD setup. (b) Probable models for CNT growth. (c) An example of a SWNT and a MWNT grown from camphor via CVD.
and can be placed inside the furnace or fed in from outside. Pyrolysis of the catalyst vapor at a suitable temperature liberates metal nanoparticles in situ (the process is known as the floating catalyst method). Alternatively, catalyst-plated substrates can be placed in the hot zone of the furnace to catalyze CNT growth. Catalytically decomposed carbon species of the hydrocarbon are assumed to dissolve in the metal nanoparticles and, after reaching supersaturation, precipitate out in the form of a fullerene dome extending into a carbon cylinder (like the inverted test tube shown in Fig. 6b) with no dangling bonds and, hence, minimum energy77,78. When the substrate-catalyst interaction is strong, a CNT grows up with the catalyst particle rooted at its base (known as the ‘base growth model’). When the substrate-catalyst interaction is weak, the catalyst particle is lifted up by the growing CNT and continues to promote CNT growth at its tip (the ‘tip growth model’). Formation of SWNTs or MWNTs is governed by the size of the catalyst particle. Broadly speaking, when the particle size is a few nanometers, SWNTs form, whereas particles a few tens of nanometers wide favor MWNT formation. The three main parameters for CNT growth in CVD are the hydrocarbon, catalyst, and growth temperature. General experience is that low-temperature CVD (600-900°C) yields MWNTs, whereas a higher temperature (900-1200°C) reaction favors SWNT growth, indicating that SWNTs have a
higher energy of formation (presumably owing to their small diameters, which results in high curvature and high strain energy). This could explain why MWNTs are easier to grow from most hydrocarbons than SWNTs, which can only be grown from selected hydrocarbons (e.g. CO, CH4, etc., that have a reasonable stability in the temperature range of 900-1200°C). Common efficient precursors of MWNTs (e.g. acetylene, benzene, etc.) are unstable at higher temperatures and lead to the deposition of large amounts of carbonaceous compounds other than CNTs. Transition metals (Fe, Co, Ni) are the most commonly used catalysts for CNT growth, since the phase diagram of carbon and these metals suggests finite solubility of carbon in these transition metals at high temperatures. This leads to the formation of CNTs under the growth mechanism outlined above. Solid organometallocenes (ferrocene, cobaltocene, nickelocene) are widely used as catalyst materials because they liberate metal particles in situ that efficiently catalyze CNT growth. The catalyst particle size has been found to dictate the tube diameter. Hence, metal nanoparticles of controlled size can be used to grow CNTs of controlled diameter79. Thin films of catalyst coated onto various substrates have also proved successful in achieving uniform CNT deposits80. In addition, the material, morphology, and textural properties of the substrate greatly affect the yield and quality of the resulting CNTs. Zeolite supports with
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catalysts in their nanopores have resulted in significantly higher yields of CNTs with a narrow diameter distribution81. Alumina materials are reported to be better catalyst supports than silica owing to their strong metal-support interaction, which allows high metal dispersion and, thus, a high density of catalytic sites82. Such interactions prevent metal species from aggregating and forming unwanted large clusters that lead to graphite particles or defective MWNTs. The key to obtaining high yields of pure CNTs is achieving hydrocarbon decomposition on catalyst sites alone and avoiding spontaneous pyrolysis. It is remarkable that transition metals have proven to be efficient catalysts not only in CVD but also in arc-discharge and laser methods. This indicates that these apparently different methods might have a common growth mechanism for CNTs, which is not yet clear. CNTs have been successfully synthesized using organometallic compounds (nickel phthalocyanine83 and ferrocene84) as carbon-cum-catalyst precursors, though the as-grown CNTs were mostly metal encapsulated. The use of ethanol has drawn attention for synthesizing SWNTs at relatively low temperatures (~850°C) on Fe-Co impregnated zeolite supports and Mo-Co coated quartz substrates85-87. Recently, a tree product, camphor, has been used to produce high yields of high-purity MWNTs88-90. Because of the low catalyst requirement with camphor, as-grown MWNTs are the least contaminated with metal, while the oxygen atoms present in camphor help oxidize amorphous carbon in situ91. Fig. 6c shows an SWNT and MWNT grown from camphor. CVD is ideally suited to growing aligned CNTs on desired substrates for specific applications, which is not feasible by arc or laser methods. Li et al.92 have grown dense MWNT arrays on Fe-impregnated mesoporous silica prepared by a sol-gel process, Terrones et al.93 have produced CNTs on Co-coated quartz substrates via CVD of a triazene compound with nearly no byproducts, while Pan et al.94 have reported the growth of aligned CNTs of more than 2 mm in length over mesoporous substrates from acetylene. Highly aligned nanotubes for electronics have been grown from acetylene95 using a Co catalyst impregnated in alumina nanochannels at 650°C, while pillars of parallel CNTs have been grown from ethylene on Fe-patterned Si plates at 700°C for field emission applications80. Bearing in mind that pyrolysis of a xyleneferrocene mixture leads to the growth of vertical CNTs on quartz96, Ajayan and coworkers have produced organized assemblies of CNTs on thermally oxidized Si wafers97,98.
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Since CVD is a well known and well established industrial process, CNT production is easy to scale up. MWNTs of controlled diameter are being produced in large quantities (~100 g/day) from acetylene using nanoporous materials as the catalyst support99. Wang et al.100 have developed a nano-agglomerate fluidized-bed reactor (a quartz cylinder 1 m long and 0.25 m wide) in which the continuous decomposition of ethylene gas on an Fe/alumina catalyst at 700°C produces a few kilograms of MWNTs per hour with a reported purity of 70%. Dai’s group has scaled up SWNT production from methane using a Fe-Mo bimetallic catalyst supported on a sol-gel derived alumina-silica multicomponent material101. However, Smalley’s lab still leads the way in the mass production of SWNTs (~10 g/day) by the high pressure carbon monoxide (HiPco) technique102. In this method, a Fe pentacarbonyl catalyst liberates Fe particles in situ at high temperatures, while a high pressure of CO (~30 atm) enhances the carbon feedstock manifolds, which significantly speeds up the disproportionation of CO molecules into carbon atoms and accelerates SWNT growth. Apart from large-scale production, CVD also offers the possibility of growing single nanotubes for use as probe tips in atomic force microscopes (AFM) or as field emitters in electron microscopes. Hafner et al.103 have grown single SWNTs and MWNTs (1-3 nm in diameter) rooted in the pores of Si tips suitable for AFM imaging. In another approach, single SWNTs are grown directly onto pyramids of Si cantilever-tip assemblies104. In this case, a SWNT grown on the Si surface (controlled by the catalyst density on the surface) protrudes from the apex of the pyramid. As-grown CNT tips are smaller than mechanically assembled nanotube tips by a factor of three and enable significantly improved resolution. CVD-produced CNTs have great promise for the fabrication of sophisticated instruments and nanodevices.
Concluding remarks We have seen here that each method has its own merits and limitations. Overwhelming research is going on around the world to solve the problems involved and understand this ‘nano world’. We have presented only a glimpse of the journey that nanotubes have taken over the last 13 years, though we sincerely feel that each day of this journey has been interesting, inspiring, and challenging. The journey through this long, narrow tunnel continues, with more dramatic turns, surprising scenes, and exciting events still ahead. MT
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