carbon nanotube and its possible applications -...

Indian Journal of Engineering & Materials Sciences Vol. 12, December 2005, pp. 529-551

Carbon nanotube and its possible applications

Zishan Husain Khana & M Husainb* aDepartment of Applied Sciences & Humanities, bDepartment of Physics,

Jamia Millia Islamia (Central University), New Delhi110 025, India

Received 13 January 2005; accepted 15 September 2005

Carbon nanotubes are the closed tabular structures consisting of nested cylindrical graphitic layers capped by fullerene-like ends with a hollow internal cavity, which were first discovered by Iijima in 1991. It consists of either one cylindrical graphene sheet, i.e., single-wall nanotube (SWNT)) or of several nested cylinders with an interlayer spacing of 0.34-0.36 nm, i.e., multiwall nanotube (MWNT)). The lengths of SWNTs and MWNTs are usually well over 1 μm and diameters range from ~1 nm (for SWNTs) to ~50 nm (for MWNTs). SWNTs are usually closed at both ends by fullerene-like half spheres that contain both pentagons and hexagons. Carbon nanotubes show exceptional electronic and mechanical properties. They are flexible but very hard to stretch and have extremely low turn-on fields and high current densities ranking them among the best electron field emitters for future field emission displays. This article presents an overview of current state of research on carbon nanotubes. In this review, we have focused on different synthesis routes for carbon nanotubes growth, used during last 12 years and possible future applications of carbon nanotubes especially in fuel cell and field emission displays. We have also discussed various parametric studies reported by several groups and extracted from their observations the common factors, which seem to be important towards a controlled production of carbon nanotubes. The limitations of these approaches, compatibility between an up-scaled production, the quality of the grown nanotube materials as well as the question of an economic production, have also been addressed.

IPC Code: B82B Carbon is an element with unique properties and is the lightest among all the elements of IV group of the periodic table. It differs in many ways from other elements like Si, Ge, Sn and Pb of the same group, which all have sp3 bonding in their cubic solid ground states, while carbon in the condensed phase has a hexagonal ground state graphite with sp2 bonding and is highly anisotropic, nearly two dimensional semimetal. Lying close in energy to graphite is diamond, a three-dimensional material with nearly isotropic properties. Recent discoveries of fullerene, a zero dimensional form of carbon and carbon nanotube, which is a one-dimensional form, have stimulated great interest in carbon materials overall. The most influential recent discovery was the identification of C60 as a molecule having the shape of a regular truncated icosahedron by Kroto et al.1, which stimulated theoretical as well as experimental work in this field. In the mid 1980’s, they developed the chemistry of fullerences1. Fullerenes are geometric cage-like structures of carbon atoms that are composed of hexagonal and pentagonal faces. The

first closed, convex structure formed was the C60 molecule (Fig. 1). Named after the architect known for designing geodesic domes, R. Buckminster Fuller, buckminster-fullerene is a closed cage of 60 carbon atoms where each side of a pentagon is the adjacent side of a hexagon similar to a soccer ball. It is often referred as a bucky ball. The structure of the bucky ball comprises 60 carbon atoms arranged by 20 hexagonal and 12 pentagonal faces to form a sphere, when a bucky ball is elongated to form a long and narrow tube of few nanometers diameter approxi-mately, which is the basic form of carbon nanotube. A

____________ *For correspondence

Fig. 1—Three dimensional view of fullerene (C60).

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synthesis route for preparing gram quantities of fullerenes2 is also discovered, thereby making possible a wide range of properties measurements and opening the new field for researchers. The observa-tion of relatively high Tc superconductivity in alkali metal (M) doped M3C60 compounds (highest observed at early stage Tc=33 K) was also an important breakthrough3. This stimulated a frenzy of activities in properties measurements of doped fullerenes. The discovery of fullerenes led to the discovery of carbon nanotubes by Iijima4 in 1991. The discovery of carbon nanotubes created much excitement and stimulated extensive research into the properties of nanometer-scale cylindrical carbon networks. Nanotubes are long, slender fullerenes where the walls of the tubes are hexagonal carbon (graphite structure) and often capped at each end. These cage-like forms of carbon have been shown to exhibit exceptional material properties that are a consequence of their symmetric structure. Many researchers have reported mechanical properties of carbon nanotubes that exceed those of any previously existing materials. Although there are varying reports in the literature on the exact properties of carbon nanotubes, theoretical and experimental results have shown extremely high modulus, greater than 1 TPa (the elastic modulus of diamond is 1.2 TPa) and reported strengths 10-100 times higher than the strongest steel at a fraction of the weight. Indeed, if the reported mechanical properties are accurate, carbon nanotubes may result in an entire new class of advanced materials. To unlock the potential of carbon nanotubes for application in polymer nanocomposites, one must fully understand the elastic and fracture properties of carbon nanotubes as well as the inter-actions at the nanotube/matrix interface. Although this requirement is no different from that for convention fiber-reinforced composites5, the scale of the reinforcement phase diameter has changed from micrometers (e.g. glass and carbon fibers) to nano-meters. In addition to the exceptional mechanical properties associated with carbon nanotubes, they also possess superior thermal and electric properties such as thermally stable up to 2800°C in vacuum, thermal conductivity about twice as high as diamond, electric current carrying capacity 1000 times higher than copper wires6.

These exceptional properties of carbon nanotubes have been investigated for devices such as field-emission displays7, scanning probe microscopy tips8 and micro-electronic devices9-13. Carbon nanotubes

present significant opportunities to basic science and nanotechnology, and pose significant challenge for future work in this field. The approach of direct growth of nanowires into ordered structures on surfaces is a promising route to approach nanoscale problem and create novel molecular-scale devices with advanced electrical, electromechanical and chemical functions. Gaining further control in nano-tube growth continues to open new possibilities in basic science and real-world applications. Carbon nanostructures have an edge over other materials for a wide range of applications because carbon can form sp3, sp2 and sp1 hybrids as well as stable πn→πn bonding (Fig. 2). This gives the ability to build various three, two, one and zero dimension structures with a broad variety of attainable physical, chemical, electronic and optical properties. In particular, carbon nanotubes have enormous potential applications in nanoelectronics for single electron transistors, high gigabit memories and flat panel display devices.

Structure of Carbon Nanotubes

The multilayered nanotubes were found in the cathode tip deposits that form when a DC arc is sustained between the graphite electrodes of a fullerene generator. They are typically composed of 2 to 5 concentric cylindrical shells, with outer diameter typically a few tens of nanometer and lengths of the order of micrometer. Each shell has the structure of a rolled-up graphene sheet⎯with the sp2 carbons forming a hexagonal lattice. The discovery of nano-tubes has revolutionized researches in different directions. A light and high strength nanotubes would be an ideal structural member for designing nano-structural instruments. Port14 has reported that nanotubes could become as familiar as silicon in this century and the full development of the nanotubes would be around 2010. In order to familiarize the uses and applications of the nanotubes and their related

Fig. 2—The sp3, sp2, sp1 hybridized bonding.

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products, an understanding of the structure, charac-terization and properties of the nanotubes is essential. The nanotubes possess conducting properties ranging from metallic to moderate band gap semiconductor15. In general, the nanotubes could be specified in terms of the tube diameter (d) and the chiral angle (θ) as shown in Fig. 3. The chiral vector (Ch) is defined as a line connected from two crystallographically equi-valent sites on a two-dimensional graphene structure. The chiral vector can be defined in terms of the lattice translation indices (n, m) and the basic vectors a1 and a2 of the hexagonal lattice (a layer of grapheme sheet)16, i.e.

Ch = na1 + ma2 …(1)

The chiral angle (θ) is measured as an angle between the chiral vector Ch with respect to the zigzag direction (n, 0), where θ = 0° and the unit vectors of a1 and a2. The armchair nanotube is defined as the θ = 30° and the translation indices is (n, n). All other types of nanotubes could be identified as a pair of indices (n, m) where n ≠ m (Fig. 4). The electronic conductivity is highly sensitive to a slight change of

these parameters, which cause a changing of materials between metallic and semiconductor statues.

Recently, it has been reported that the scanning tunneling microscopy (STM) and spectroscopy could be used to observe the electronic properties and atomic arrangement of SWNTs17. The models of different types of SWNTs are shown in Fig. 5a, which gives a more clear understanding of the structure of the nanotubes18. An image of zigzag (15, 0) SWNT19 is shown in Fig. 5b.

Multi- and Single-Walled Nanotubes

Multi-walled carbon nanotubes were first reported by Iijima in 1991 in carbon soot made by an arc-discahrge method4. About two years later, he made the observation of single-walled nanotubes (SWNTs)20. A SWNT is a graphene sheet rolled-over into a cylinder with typical diameter of the order of 1.4 nm (Figs 6 and 8), similar to that of a C60 bucky-ball. A MWNT consists of concentric cylinders with an interlayer spacing of 3.4Å and a diameter typically of the order of 10-20 nm (Fig. 7).

The lengths of the two types of tubes can be up to hundreds of microns or even centimeters. A SWNT is a molecular scale wire that has two key structural parameters (Fig. 9). By folding a graphene sheet into a cylinder so that the beginning and end of a (m, n) lattice vector in the graphene plane join together, one obtains an (m, n) nanotube. The (m, n) indices determine the diameter of the nanotube, and also the so-called chirality. (m, m) tubes are arm chair tubes, since the atoms around the circumference are in an

Fig. 3—Schematic diagram showing how a hexagonal sheet of graphite is rolled to form a carbon nanotube.

Fig. 4—Atomic structure of (a) an armchair and (b) a zig-zag nanotube.

Fig. 5—(a) By rolling a graphene sheet into cylinder and capping both ends to form nanotubes. A systematic theoretical model for SWNT with different chirality; (top) armchair, (middle) zigzag and (bottom) chiral tubules (b) Atomic structure and spectroscopy of metallic zigzag SWNTs captured by STM.


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Fig. 6—Different types of nanotubes.

Fig. 7—Carbon nanotubes imaged by transmission electron microscopy(TEM). TEM images of multi-walled carbon nanotube with closed caps. The parallel lines are the cross-sections of the sidewalls of concentric cylinders. The diameters of MWNTs are typical of the order of 10-20 nm.

Fig. 8—TEM images of the cross-section of bundle of single-walled carbon nanotubes. Each circle represents the cross-section of SWNT with a diameter ~1.4 nm.

Fig. 9—Schematic honeycomb structure of a graphene sheet. Carbon atoms are vertices and SWNT can be formed by folding the sheet along lattice vectors. The two basis vectors a1 and a2, and several examples of lattice vectors are shown.

arm-chair pattern (Fig. 10a). (m, 0) nanotubes are termed as zigzag in view of the atomic configuration along the circumference (Figs 10b and 10c). The other two types of nanotubes are chiral, with the rows of hexagons spiraling along the nanotube axis (Fig. 10d). A SWNT can be either a metal, semiconductor or small-gap semiconductor depending on the (m, n) structural parameters18. For a graphene sheet, the conduction and valence bands touch each other at the six corner points of the first Brillouin zone. These states are filled with electrons that have the highest energy (Fermi energy). A graphene sheet is therefore semi-metallic with a zero band gap. The electronic states of an infinitely long nanotube are parallel lines in k-space, continuous along the tube axis and quantized along the circumference (Figs 10a-10d). For (m, m) armchair tubes, their area always states crossing the corner points of the first Brillouin zone (Fig. 10a), suggesting that arm-chair tubes should always be metallic. For (m, n) nanotubes with m-n ≠ 3 x integer, the electronic states (lines) miss the corner points (Figs 10c and 10d) and the nanotubes are semiconducting. The energy gap scales with the tube diameter as 1/d and is of the order of 0.5 eV for a SWNT with typical diameter d = 1.4 nm. For m-n = 3 x integer, certain electronic states of the nanotube land on the corner points of the first Brillouin zone

Fig.10⎯Schematic structures of SWNTs (a) A (10,10) arm-chair nanotube. Bottom panel; the hexagon represents the first Brillouin zone of a graphene sheet in reciprocal space. The vertical lines represent the electronic states of nanotube. The center line crosses two corners of hexagon, resulting in a metallic nanotube. (b) A (12,0) zigzag tube. The electronic states crossing the hexagon corners, but a small band gap can develop due to the curvature of the nanotube. (c) The (14,0) zigzag tube is semiconducing because the states on the vertical lines miss the corner points of the hexagon. (d) A (7,16) tube is semiconducting. This figure illustrates the extreme sensitivity of nanotube electronic structures to the diameter and chirality of nanotubes.


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(Fig. 10b). These types of tubes would be semimetals but become small-gap semiconductors (band gap scales with 1/d2 ~10 meV for d~1.4 nm) due to a curvature induced orbital re-hybridization effect21. The extreme sensitivity of electronic property on structural parameters is unique for carbon nanotubes. This uniqueness leads to rich physical phenomena in nanotube systems, and possesses a significant challenge to chemical synthesis in terms of control-ling the nanotube diameter and chirality.

Synthesis of Carbon Nanotubes Since the discovery of carbon nanotubes, a variety

of techniques for the production of carbon nanotubes have been developed. It was Iijima4, who first observed multi-walled nanotubes in the arc discharge of carbon soot. Few years later, Iijima et al.20 and Bethune et al.22 reported the synthesis of single walled nanotubes. Primary synthesis methods for single and multi-walled carbon nanotubes include arc discharge4,23, laser ablation24, gas phase catalytic growth from carbon monooxide25 and chemical vapour deposition from hydrocarbons methods26-28. For application of carbon nanotubes, large quantities of nanotubes are required, and scale-up limitations of the arc discharge and laser ablation techniques would make the cost of nanotubes based devices prohibitive. During nanotube synthesis, impurities in the form of catalyst particles, amorphous carbon and non-tubular fullerenes are also produced. Thus, subsequent purifi-cation steps are needed to separate the carbon nano-tubes. The gas phase processes tend to produce nano-tubes with fewer impurities and are more amenable to large scale processing. It is believed that the gas phase techniques, such as CVD, for nanotube growth offer greater potential for the scaling up of nanotubes production for applications.

Initially, electric-arc discharge technique was the most popular technique to prepare the SWNTs as well as MWNTs4. In this technique, the carbon arc provides a simple and traditional tool for generating the high temperatures needed for the vapourization of carbon atoms into a plasma (>3000°C)29-31. Typical conditions for operating a carbon arc for the synthesis of carbon nanotubes include the use of carbon rod electrodes of 5-20 mm diameter separated by ~1 mm with a voltage of 20-25 V across the electrodes and a dc electric current of 50-120 A flowing between the electrodes. The arc is typically operated in ~500 torr He with a flow rate of 5-15 mL/s for cooling purposes. As the nanotubes form, the length of the

positive electrode (anode) decreases. Once the arc is in operation, a carbon deposits on the negative electrode. For multi-walled carbon nanotube synthesis, no catalyst is needed and the nanotubes are found in bundles in the inner region of the cathode deposit where the temperature is maximum. To achieve single-walled nanotubes, the electrodes are doped with a small amount of metallic catalyst impurity20, 22,23,32,33. Laser ablation is also an efficient route for the synthesis of bundles of single-walled carbon nanotubes with a narrow diameter distribution. It employs the laser vapourization of a graphite target. In early reports of the laser synthesis technique34, high yields with >70-90% conversion of graphite to single-walled nanotubes, were reported in the condensing vapour of the heated flow tube (operating at 1200°C). A Co-Ni/graphite composite laser vapourization target was used, consisting of 1.2 atom% Co-Ni alloy with equal amounts of Co and Ni added to the graphite (98.8 atom%)34. Two sequenced laser pulses were used to evaporate a target containing carbon mixed with a small amount of transition metal from the target. Flowing argon gas sweeps the entrained nano-tubes from the high temperature zone to the water-cooled copper collector downstream, just outside the furnace34,35.

Arc-discharge and laser ablation methods described above, have the problem of scaling-up the carbon nanotubes as they can produce the CNTs according to size of the carbon source (anode in arc-discharge and graphite target in laser ablation). It also requires subsequent purification steps necessary to separate the tubes from undesirable impurities. These limitations have motivated the development of gas-phase techniques, such as chemical vapour deposition. In these methods, the nanotubes are formed by the decomposition of carbon containing gases. These techniques are good for the scale-up production of carbon nanotubes since the carbon source is continuously replaced by flowing gas. In addition, the final purity of the as-produced nanotubes can be quite high, minimizing subsequent purification steps. The gas-phase growth of single-walled nanotubes by using carbon monoxide as the carbon source has already been reported25. They found the highest yields of single walled nanotubes (Fig. 12) occurred at the highest accessible temperature and pressure (1200°C, 10 atm). Smalley and his co-workers have modified this process to produce large quantities of single walled nanotubes with remarkable purity. The so-


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called HiPco nanotubes (high-pressure conversion of carbon monoxide) have received considerable atten-tion as the technology has been commercialized for large scale production of high-purity single-walled carbon nanotubes. Some gas-phase techniques utilize hydrocarbons as the carbon source for the production of single-walled as well as multi-walled carbon nanotubes by using chemical vapour deposition36-39. It was pointed out that hydrocarbons pyrolize readily on surfaces heated above 600-700°C and the nanotubes grown by using hydrocarbons, contain amorphous carbon, which requires further purification25.

Although the dissociation of hydrocarbons at lower temperatures affects the purity of the as-processed nanotubes. The lower processing temperatures also enable the growth of carbon nanotubes on a wide variety of substrates. CVD method has been successful to produce the CNTs in large quantity, and also to obtain the vertically aligned CNTs at relatively low temperature (Fig. 11). In particular, growth of vertically aligned CNTs on large substrate area at low temperature, for instance, softening temperature of the glass is an important factor for the practical applica-tion of the electron emitters to the field emission displays7. A lot of reports on the synthesis of single-walled as well as multi-walled carbon nanotubes using the plasma enhanced chemical vapour depo-sition and microwave plasma enhanced chemical vapour deposition techniques, are available in the literature. Here, we shall present an overview about the current state of the art in the production of carbon nanotubes.

Jeong et al.40 successfully synthesized vertically aligned carbon nanotubes (CNTs) at 550°C on Ni-coated Si substrate placed parallel to Pd plate as a dual catalyst and a tungsten wire filament (Fig. 13). The CNT length increased with increasing gas flow rates even at such a low growth temperature, sugges-ting that the hydrocarbon gases were sufficiently decomposed in the presence of tungsten wire filament

Fig. 11—SEM images of nanotube towers synthesized on 38×38 micron catalyst patterns and the well grown nanotubes by using plasma induced alignment method7.

Fig. 12—TEM micrograph of single layer nanotube growing from platinum group particles25.

Fig. 13—SEM images of CNTs grown at 600°C (a) without Pd plate (b) Pd plate as a dual catalyst and HRTEM images of CNTs grown using Pd dual catalyst with (c) low and (d) high magnification40.


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and dual Pd catalyst. The difference of growth behaviour from that at high temperature was explained by introducing the Ni–C solid solution, which enables the carbon atoms to diffuse more easily even at low temperature. Li et al.41 synthesized carbon nanostructures with nanocrystalline Ni catalyst from decomposition of methane using a CVD technique. Transmission electron microscopy (TEM) investi-gations suggested two kinds of carbon nanostructures, carbon fibers and bamboo shaped carbon nanotubes. The preferential growth direction of graphene sheets depends on the reaction conditions. The bamboo-shaped carbon nanotubes can be obtained only if the reaction temperature is higher than 1000 K, and carbon fibers can be obtained at lower temperatures. They have also discussed the role and state of the catalyst particles. They have found that a plug-shaped Ni particle always plunged at the top end of a nanotube. Experimental results indicated that the catalytic particles exist in a liquid state during the synthesis procedure. After having crystallized, the orientations of plunged Ni particles randomly distributed around the ⟨011⟩ axis. Zhang et al.42 have successfully grown vertically aligned CNTs on Fe, Co and Ni deposited quartz plates. All three metals deposited on the quartz plates are found to be efficient catalysts for the growth of CNTs in good alignment by CVD using ethylenediamine. The diameter and density of the aligned CNTs are determined by the thickness of the metal film while the length of the tubes can be controlled by varying the CVD reaction time. The CNTs are multiwalled with a bamboo-like graphitic structure. The hollow compartments of the tubes are separated by the graphitic interlink layers. This is a simple and efficient way for the production of carbon nanotubes with good order using ethylenediamine by CVD without plasma aid. The produced CNTs may be used in the electronic devices, hydrogen storage and nanotube-polymer composites. Yokomichi et al.43 produced CNTs by hot-wire CVD using a mixture of C2H2 and NH3 gases. They used crystalline c-Si and a-SiO2 substrates coated with Ni films of 30 nm in thickness as a catalyst for nanotube formation. SEM images showed that micron-sized grains were present in the deposit on the c-Si substrate, whereas nano-sized grains were evident in the deposit on the a-SiO2 substrate. Nanotube formation could not be confirmed by SEM, but there was evidence of the possible formation of nanotubes on the Ni-coated a-SiO2 substrate. Wang et al.44

employed a microwave PECVD to synthesize carbon nanostructures by using Fe (or Co, Ni)/Al2O3 as catalyst and a mixture of benzene, hydrogen and argon as precursors. By regulating the types of catalyst, the microwave power, the ratio and the flux of the precursors, many morphologies such as ordinary geometric, helix-shaped and planar spiral carbon nanotubes with aspect ratios of 100-1000 have been observed. Two novel nanostructures, which are probably the missing link between onion-like carbon particles and nanotubes have also been obtained. Low synthesis temperature <520°C due to the non-equilibrium characteristics of microwave plasma operated at low pressure is also reported, which is crucial for some fascinating applications. Chung et al.45 employed a high density plasma chemical vapour deposition (PECVD) to grow high quality carbon nanotubes at low temperatures. High density, aligned CNTs can be grown on Si and glass substrate. The CNTs were selectively-deposited on the patterned Ni catalyst layer, which was sputtered on Si substrate. The CNTs exhibited a turn-on field of 0.9 V/μm and an emission current of 480 μA/cm2 at a field of 3 V/ μm. Choi et al.46 synthesized pure carbon nanotubes at very low temperature using MW-PECVD with methane/hydrogen gas. The ratio of a gas mixture (CH4/H2) and the deposition time at a substrate temperature below 520°C, are optimized. Pure and dense carbon nanotubes are grown uniformly over a large area of Ni-coated silicon substrates without any pretreatment of substrates. The diameters and lengths of carbon nanotubes could be controlled by changing the ratio of gas mixture and the growth time. Raman spectrum clearly shows the peak at 1592 cm-1 (G-band), indicating the formation of graphitized carbon nanotubes. Zhang et al.47 prepared the massive carbon nanotubes on silicon, quartz and ceramic substrates using MW-PECVD. The nanotubes, ranging from 10 to 120 nm in diameter and a few tens of micron in length, were formed under hydrocarbon plasma at 720°C with the aid of iron-oxide particles. Morpho-logy of the nanotubes is strongly influenced by the flow ratio of methane to hydrogen. Defect-less nano-tubes with small diameters are favourably produced under a small flow ratio. In contrast, defect-rich nanotubes with large diameters and highly disordered directions are formed at a large flow ratio.

In the past few years, carbon nanotubes have attracted increasing interest due to their high mechanical strength48, peculiar electronics49, high


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hydrogen storage50 and electron field emitting7. In this context, the primary issue is the scale-up synthesis and the control of carbon nanostructures. To date, many methods for synthesizing carbon nanotubes have been developed and most of which operated at high temperature over 4000°C, e.g., for graphite arc discharge4 and laser ablation51. These temperatures are too high and unsuitable for the fabrication of electronic devices because most electric connections are made of aluminum with the melting point below 700°C. For the promising applications, the carbon nanotube arrays should be grown perpendicular to the surface of display glass, while the strain point of the best display glass is only 666°C27. These challenges have promoted the current exploration of low-tem-perature synthesis of carbon nanotubes such as CVD, PECVD and MWPECVD. Recently, these low-temperature methods have been successful in growing highly aligned and large quantities of carbon nanotubes. It is also possible to control over length, diameter and structure of carbon nanotubes grown by CVD techniques. Therefore, CVD techniques are the most popular methods to synthesize the carbon nanotubes.

Current Status of Research

The research on carbon nanotube was greatly stimulated by the initial report of observation of carbon tubules of nanometer dimensions and the subsequent report on the observation of conditions for the synthesis of large quantities of nanotubes29,30. Since these early reports, much work has been done, and the results show basically that carbon nanotubes behave like rolled-up cylinders of graphene sheets of sp2 bonded carbon atoms, except that the tubule diameters in some cases are small enough to exhibit the effects of one-dimensional (1D) periodicity. It is a unique material which can behave either as a semiconductor or as a metal, depending on its diameter and chirality21. In addition, it possesses excellent physical properties, such as effective field emission characteristics7,52 capability for the storage of large amount of hydrogen53, and high modulus54,55. These superb properties open a possibility for various applications, such as field emission displays56, vehicles for large hydrogen storage57, and nanoscale devices58. Several methods have been developed to synthesize carbon nanotubes including arc discharge, laser ablation and CVD. The growth of carbon nanotubes depended strongly upon the presence of

catalysts. The catalysts usually selected from Ni, Co, Fe, various metallic particles, Ni-Co, Fe-Ni and LaNi5.

Han et al.59 successfully obtained vertically aligned multiwall carbon nanotubes on different nickel-coated substrates by plasma enhanced hot filament chemical vapour deposition at low temperatures below 650°C. Acetylene and ammonia gas were used as the carbon source and a catalyst. The surface roughness of the nickel layer increased as NH3 etching time increased. The diameters of the nanotubes decreased and the density of nanotubes increased as NH3 etching time increased. The diameter of the nanotubes was 30-70 nm. A nickel cap was observed on the top of the grown nanotube and a very thin amorphous-carbon-like layer was found on the nickel cap. The morphology and microstructure of carbon nanotubes were measured using scanning electron microscopy and transmission electron microscopy. Yuan et al.60 reported the investigations of the electrical transport property for single pure or Au-doped multi-walled carbon nanotubes and super-long (millimetric scale) carbon nanotubes grown by CVD method. The structure of carbon nanotubes strongly dominate their electrical properties. Doping can modulate the electrical transport property of carbon nanotubes and the doping effect on the transport of the carbon nanotubes is discussed. The transport measurements on pure or doped carbon nanotubes revealed that the resistance of the CVD-grown carbon nanotubes has negative temperature dependence. Minami et al.61 reported that the optical absorption spectra of single wall carbon nanotubes (SWNT) changes drastically by the doping of halogens and alkali metals. From disappearance of absorption peaks attributable to interband optical transitions, it was established that the semiconducting phase of SWNT can be doped amphoterically. Emergence of new absorption peaks induced by heavy doping is explained by invoking the involvement of low-lying valence states (or high-lying conduction states) in the optical transition. It is also found that the interband absorption peaks considerably and reversibly broadened and shifted under high pressure up to 4.1 GPa. Good corres-pondence of the present results to recent theoretical works suggests that this change may reflect a semiconductor-metal induced by intertube interac-tions and/or by symmetry breaking. Kurt et al.62 successfully synthesized the crystalline nitrogenated carbon (C:N) films by hot filament chemical vapour


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deposition (HF-CVD). Nitrogen gas and ammonia were used as sources of atomic nitrogen, whereas methane acted as carbon precursor. Structure analysis reveals the growth of a new type of C:N films characterized by various polymorphs including worm like features and foil like structures at the nanometer scale. The effects of bias voltage and filament temperature on the film morphology are investigated in detail. The dependence of nucleation on the substrates, including pure Si- and Ni-coated wafers, was studied. The presence of Ni was found to initiate the growth of arrays of nanotubes. Field emission in vacuum was observed on C:N films deposited on pure Si above applied field of 20 V/μm. The onset field could be decreased below 3 V/μm with Ni-coated substrates due to the presence of well-separated nano-tubular structures. Cia et al.63 reported the crystal-lization behaviour of the amorphous carbon nanotubes prepared by CVD at high temperature. Transmission electron microscopy and X-ray diffraction were performed to characterize the structures of the carbon nanotubes. The results reveal that the microstructure of the as-grown carbon nanotubes, prepared by the floating catalyst method, is roughly amorphous. The as-grown carbon nanotubes were annealed at high temperatures, and the crystallization behaviour of the amorphous carbon nanotubes was investigated systematically. Because the carbon nanotubes have finite dimensions and tube-like shape, their crystallization behaviour is completely different from the bulk amorphous carbons. The results reveal that the graphene layers in the annealed carbon nanotubes will form into a uniform two-dimensional turbostratic stack, a configuration of carbon nanotubes with the lower Gibbs free energy. A thermodynamic model is presented to explain the crystallization behaviour of the amorphous carbon nanotube. Shyu et al.64 have reported the growth carbon nanotubes on silicon substrates coated with metal catalysts containing Ni and Fe at low temperatures by chemical vapour deposition using mixtures of acetylene and nitrogen at 90 Torr. At a growth temperature of 400°C, all nanotubes are well aligned. However, these nanotubes are not made of perfect graphite as characterized by transmission electron microscopy and Raman spectra. The amount of carbon soot in the film increases apparently with increase of the concentration of acetylene and the growth temperature. But, as the growth temperature is raised to 500 or 600°C, the graphitic-sheets begin to appear in the tube shell

although the alignment of nanotubes becomes worse due to various growth rates for different sizes of nanotubes. Methane and carbon monoxide reactants are studied and found to be unable to grow carbon nanotubes at the conditions employed for acetylene to grow nanotubes successfully. Field emission of the carbon nanotube film grown at 400°C is also measured. The turn on voltage is 11 V/μm. Zhang et al.65 developed a simple and reliable method for the controlled growth of well-aligned carbon nanotubes patterns with different sizes and shapes on a silicon substrate. A patterned film of sputtered cobalt on the silicon was prepared by a lift-off process, and the carbon nanotubes were grown via chemical vapour deposition using ethylenediamine as a precursor. The carbon nanotubes are vertically aligned with high density within the micro patterns. The diameter of carbon nanotubes is determined by the thickness of cobalt film while, the length of the nanotubes can be controlled by varying the reaction time. The bamboo-like multi-walled carbon nanotubes have been observed by high-resolution transmission electron microscopy. The micro-Raman spectrum further confirmed the graphitic structure of the nanotubes. The potential application of the carbon nanotubes to flat panel displays is demonstrated by the ability to provide stable high field-emission current densities. Zheng et al.66 have grown well-aligned carbon nanotubes (CNTs) on mesoporous silica films by chemical vapour deposition (CVD). Ethylene was used as the carbon source and CVD was performed at 1023 K and atmospheric pressure. The films were doped with Fe during sol-gel synthesis, and three different structure directing agents were used for mesoporous silica preparation: polyoxyethylene (10) cetyl ether (C16EO10), Pluronic tri-block copolymer (P123), and cetyltriethylammonium chloride (CTAC). A high degree of CNT alignment on C16EO10 mesoporous silica films was produced at Fe:Si molar ratio of 1:80. Similar alignment of CNTs was achieved in the other preparations, but on CTAC derived films CNTs only grew parallel to the substrate surface because the in-plane arrangement of the pore structure limited CNT growth to crack domains. They have also shown that the diameter of the CNTs can be controlled by changing the Fe concentration in the mesoporous silica substrate. Singh et al.67 have grown high purity, aligned multi-wall carbon nanotube films on quartz substrates by injecting a solution of ferrocene in toluene into a suitable reaction furnace


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(Fig. 14). The injection CVD method allows excellent control of the catalyst to carbon ratio. The detailed study demonstrates, how such a system can be used to control the nanotube diameter, length, alignment and yield by manipulating the experimental parameters. Primary growth was found to occur via a base growth mechanism, although overgrowths of single wall carbon nanotubes were obtained under certain condi-tions. Such a method also allows nanotubes of various packing densities to be produced which may be useful for specific applications such as electrodes. Rao et al.68 have prepared aligned carbon nanotube bundles and Y-junction nanotubes by the pyrolysis of appro-priate organic precursors. They have made it possible to synthesize nanotubes and nanowires of metal chal-cogenides by employing different strategies. Carbon nanotubes grown on ophthalmic glass substrate at room temperature, without any preconditioning and intentional heating of substrate, were found to be multiwalled with their diameters varying from 23-30 nm at optimized parameters. Biro et al.69 have grown straight carbon nanotubes, carbon nanotubes “knees,” Y-branches of carbon nanotubes and coiled

carbon nanotubes on a graphite substrate held at room temperature by the decomposition of fullerene under moderate heating at 450°C in the presence of 200-nm Ni particles. The grown structures were investigated without any further manipulation by STM. The formation of the carbon nanostructures containing non-hexagonal rings is attributed partly to the templating effect of the high pyrolitic graphite (HOPG), partly to the growth at room temperature, which enhances the probability of quenching-in for non-hexagonal rings. Similar coiled nanotubes were found after several steps of chemical treatment in a catalytically grown carbon nanotube sample. This observation suggests that in the coiled structures, the pentagons and heptagons do not play the role of defects, but that of regular building blocks. Coiled carbon nanotubes may be regarded as being built from a theoretically predicted material, the haeckelite, in which pentagons, hexagons and heptagons are equally considered as regular building elements. Chen et al.70 prepared a Ni–P–carbon nanotube (CNT) composite coating and carbon nanotube/ copper matrix composites by electroless plating and powder metallurgy techniques, respectively. The effects of CNTs on the tribological properties of these composites were evaluated. The results demonstrated that the Ni–P–CNT electroless composite coating exhibited higher wear resistance and lower friction coefficient than Ni–P–SiC and Ni–P–graphite composite coatings. After annealing at 673 K for 2 h, the wear resistance of the Ni–P–CNT composite coating was improved. Carbon nanotube/copper matrix composites revealed a lower wear rate and friction coefficient compared with pure copper, and their wear rates and friction coefficients showed a decreasing trend with increasing volume fraction of CNTs within the range from 0 to 12 vol.% due to the effects of the reinforcement and reduced friction of CNTs. The favourable effects of CNTs on the tribological properties are attributed to improved mechanical properties and unique topological structure of the hollow nanotubes. Cui et al.71 prepared carbon nanotubes by a classical CVD method with a nickel catalyst. In a first part, these nanotubes are structurally characterized before and after heat treatments (HTT=1500, 2000, 2500°C). Diffusion Raman experiments and diamagnetic susceptibility experiments demonstrated their limited graphitized structures. Then, in a second step, a well defined processing way to prepare nanocomposites

Fig. 14—SEM images of nanotube films grown at different temperatures, (a) 590°C, (b) 740°C, (c) 850°C and (d) 940°C for 60 min, 750 ml/minAr/H2 and at feed rate of 1.2 mL/h of the solution67.


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with a standard epoxy resin is presented. In particular, the use of a non-ionic surfactant (Tergitol) to disperse these nanotubes is analyzed. The influence of nano-tube contents is examined on the bulk nanocomposite density, the glass transition temperature of the nanocomposites, and the d.c. electrical conductivity behaviour. These results demonstrated that the interfacial properties are playing a fundamental role. On one hand, the glass transition temperature is increasing with the nanotube content, and on the other hand, the percolation threshold is found for a rather high critical volumetric concentration. Finally, it is demonstrated that a pure geometrical model is not sufficient to explain these behaviours and that a wrapping effect of the organic matrix around the nanotubes has to be considered. Kim et al.72 have examined growth of CNTs on a porous alumina template in order to improve the selectivity and uniformity of carbon nanotubes (CNTs). They have fabricated well-ordered, nano-sized pores on a Si substrate using an anodic oxidation method. Anodic aluminium oxide (AAO) template with pore diameter of 30-60 nm was formed on Si substrate through electro polishing, two step anodization, and pore widening. Irregularity in the cell configuration was attributed to the short period of the first and second anodization process. CNTs were grown within a nano-channel of the alumina template at 550°C. To grow CNTs in the pores, the pores should be widened after the second anodization. CNTs were grown on AAO/Si substrate with/without catalyst metal using PECVD. CNTs grew on AAO/Si without catalyst, while there was no overgrowth of CNTs on AAO/Si with catalyst. In fabrication of CNTs using AAO template without catalyst metal, alumina can work as catalyst to acetylene. In synthesis of CNTs using AAO template with Co layer, both alumina and Co can work as catalyst with flowing acetylene. Smith Jr. et al.73 developed a simple process for selective removal of carbon from single-walled carbon nano-tube samples based on a mild oxidation by carbon dioxide. The reactivity profiles of as-prepared and purified nanotube samples were determined using both thermogravimetry (TG) and a related analytical technique, controlled atmosphere programmed temperature oxidation (CAPTO). The complex differential rate curves for weight loss (DTG) or carbon dioxide evolution (CAPTO) could be resolved by a series of Gaussian peaks each associated with carbonaceous species of different reactivity. Compari-

sons were made between samples as received after preparation by the laser ablation method, after purification by nitric acid oxidation, and both of these after reaction with CO. Derivatic thermogravimetry (DTG) of as-prepared tubes had a broad major peak centered about 410°C. Mild oxidation of as-prepared nanotubes under flowing carbon dioxide at 600°C preferentially removed more reactive carbon species leaving behind a narrower distribution about the major peak in DTG. In contrast to the as-prepared material, the sample that had been purified using nitric acid had a more distinct separation of the major DTG peaks between more and less readily oxidized materials. Oxidation of this sample with CO selec-tively removed the peak associated with the most readily oxidized material. The original CO oxidation experiments performed on the analytical scale were successfully scaled up to a small preparative scale. Xiao et al.74 studied the performance of super-capacitors with multi-walled carbon nanotubes deposited with conducting polymer as activate materials. They found that the performance of super-capacitors was greatly enhanced in contrast with electric double-layer super-capacitor with carbon nanotubes due to the conducting polymer’s faradaic effect. They are promising as the secondary power sources in electric vehicles propulsion. Polypyrrole and poly(3-methylthiophene) were uniformly deposited onto multi-walled carbon nanotubes in organic system by chemical methods. A carbon nanotubes-polypyrrole composite-carbon nanotubes-poly(3-methyl-thiophene) composite based super-capacitor prototype (CNTspPy-CNTs-pMeT SCP), a carbon nanotubes-carbon nanotubes-polypyrrole based hybrid SCP (CNTs-CNTs-pPy SCP), a carbon nanotubes-carbon nanotubes-poly(3-methylthiophene) based hybrid SCP (CNTs-CNTs-pMeT SCP) as well as a CNTs-CNTs corresponding SCP were assembled in 1 M LiClO4 acetonitrile solution. Their voltam-metry characteristics, galvanostatic discharge and AC impedance spectra were carried out in two-electrode mode. Pseudocapacitance effects are found out from those SCPs with composite electrodes and their measured capacitances are 87, 45 and 72 Fg-1 for CNTs-pPy-CNTs-pMeT SCP, CNTs-CNTs-pMeT SCP and CNTs-CNTs-pPy SCP, respectively. They are much larger than that of 21 Fg-1 for the CNTs-CNTs corresponding SCP, which is a double-layer SCP. Their measured specific energy is 1.82, 0.88 and 1.33 Whkg-1 for those SCPs with composite


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electrodes. They are also much higher than that of 0.58 Whkg-1 for the CNTs-CNTs corresponding SCP. Alexandrescu et al.75 have explored the suitability of laser-assisted chemical vapour deposition for the formation and growth of carbon nanotubes. A medium-power continuous-wave CO2 laser was employed to irradiate a sensitized mixture of Fe(CO)5 vapour and acetylene and to simultaneously heat a silicon substrate on which the carbon nanotubes were grown. Scanning and transmission electron micro-scopy (TEM and HRTEM) as well as atomic force microscopy (AFM) were used to analyze the as-grown films and samples specially prepared on TEM grids and AFM substrates. Carbon nanotubes with different structures (straight, curved and even branched), including single- and multi-walled nanotubes were observed. Some nanotubes were found to be partially filled with a solid material (probably metallic iron) that seems to catalyze the nanotube growth. Some regions of the deposit also revealed the presence of nanoparticles. The present experimental conditions should be suitable to produce locally structured deposits of carbon nanotubes for various applications. Liming et al.76 synthesized straight and bamboo-like carbon nanotubes in a methane diffusion flame using a Ni–Cr–Fe wire as a substrate. The catalyst particles were nickel and iron oxides formed on the wire surface inside the flame. When the wire was oxidized using nitric acid more nanotubes could be produced. Most nanotubes grew by a base growth mode. The formation of bamboo-like nanotubes was related to the shape of the catalyst particles. They have pro-posed a segregation growth mechanism of bamboo-like nanotubes. Bittner et al.77 used a pulse mass analyzer to study the vapour phase adsorption of organic compounds on single-walled carbon nano-tubes and chemically modified/oxidized SWNTs. The change in mass of a packed bed of adsorbent held at 200°C was observed following the injection of a pulse of an organic compound from the series: ethanol, isopropanol, cyclohexane, cyclohexene, benzene, or n-hexane. The relative strength of adsorption was obtained by the mass increase resulting from injection of the pulse and by the time required for desorption. This time was broken into the transit time to reach the end of the bed and the half-time for return from peak to baseline. Hexane was the most strongly held compound of the organic sequence. Oxidative purifi-cation of a raw nanotube sample produced a less hydrophobic surface. The effect of the purification

was reversed by thermolysis at 700°C, which removed oxygenated functional groups and increased the affinity for hydrocarbons. The amorphous carbon associated with the raw nanotube sample is a strong adsorbent for hydrocarbons. By comparison, it was found that an activated carbon had a greater affinity for hydrocarbons than any of the nanotube samples. Jeong et al.78 have grown single-walled carbon nanotubes (SWNTs) over MgO supported Ni catalyst particles at 800°C with a mixture of acetylene and hydrogen gases. The diameters of SWNTs grown on 3 wt% Ni–MgO particles ranged from 0.7 to 1.0 nm, having a narrow diameter distribution compared to the previous works. This was attributed to the preparation procedure of Ni–MgO templates. Ni catalyst was co- precipitated with MgO, where an appropriate amount of Ni catalyst with uniform grain size remained on MgO surface after forming a solid solution during reaction. This method should be useful for a selective growth that is applicable to future electronic devices. Lopez et al.79 have synthesized the multi-walled carbon nanotubes using a pulsed catalytic method. Acetylene was used as carbon generation reactant and silica or alumina supported iron as catalysts. Different metal loadings and various reaction temperatures were evaluated. The carbon growth over the catalysts has been followed gravimetrically in situ. The reaction was stopped after different pulse numbers in an attempt to control both the diameters and the lengths of the carbon nanotubes, which were characterised by transmission electron microscopy (TEM). A consecutive step growing mechanism is used to describe their experiments. Peng80 reported the synthesis of carbon nanotube cables—a form of compound single- and multi-walled carbon nanotubes which could have the superior properties of both the SWCNTs and MWCNTs. This compound form of carbon nanotubes consists of a bundle of SWCNTs formed into a MWCNT, and the diameter of the inner most shell of the MWCNT ranges from few to tens nanometer. The growth of these compound carbon nanotubes cannot be explained readily via existing modes of carbon nanotube growth, but promises a new way for improving and controlling the physical properties of either single- or multi-walled carbon nanotubes. Jung et al.81 prepared two kinds of catalytic layers onto n-typed silicon substrate—nickel by r.f.-magnetron sputtering and iron (III) nitrate metal oxide by spin coating. For iron (III) nitrate metal oxide 0.5 mol of ferric nitrate nonahydrate


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(Fe2(NO3)2.9H2O] ethanol solution was coated onto silicon by spin coater at different rotation speeds (rev./min). They have synthesized the carbon nano-tubes on both Ni and iron (III) nitrate metal oxide layers by the HFPECVD (hot filament plasma-enhanced chemical vapour deposition) method. They used ammonia (NH3) and acetylene (C2H2) for the dilution gas and a carbon precursor for the growth of the carbon nanotubes, respectively. They observed the relationship between the catalytic cluster density and the nanotube density with scanning electron micro-scopy (SEM) images. The density of carbon nano-tubes on iron (III) nitrate metal oxide was controlled by the rev./min of the spin coater. Transmission elec-tron microscopy (TEM) image shows multi-walled carbon nanotube where the catalyst was found in the tip of the carbon nanotube. Electron dispersive X-ray spectrometry (EDS) peaks for CNTs tip show that it was constituted with nickel and iron, respectively. Raman spectroscopy of nanotubes shows D-band and G-band peaks approximately 1370 and 1590 cm-1. Lu et al.82 prepared coiled carbon nanotubes by catalytic chemical vapour deposition (CCVD) on finely divided Co nano-particles supported on silica gel under reduced pressure and relatively low gas flow rates. The morphology and the graphitization of the coil tube, coil bend, and coil node of the coiled carbon nanotubes were examined by transmission electron microscope (TEM). The influence of pH value, reaction pressure, and flow rate of C2H2 on the growth of the coiled carbon nanotubes were also discussed. With the drastic reduction in the consumption of C2H2 and lower required pressure with the modified CCVD approach, the amount of amorphous carbon coated on the carbon nanotubes was shown to be greatly reduced. Most importantly, this method offers a preferable alternative for the efficient, environment-friendly and safer growth of coiled carbon nanotubes. Zhu et al.83 synthesized close-packed bundles of multi-wall carbon nanotubes (MWNTs) in two-stage horizontal floating catalyst system. The MWNTs were assembled into bundles by controlling appropriate parameters in floating catalyst system. Common amorphous carbon layers, which resulted from the pyro-carbon deposition on small diameter carbon nanotubes, were found to be the key reason for the formation of close-packed MWNT bundles after small-diameter MWNTs were attracted together by weak Van der Waals attraction.

Purification of Carbon Nanotubes Carbon nanotubes grown by different methods, are

found to contain impurities such as amorphous carbon and carbon nanoparticles. Purification generally refers to the isolation of carbon nanotubes from the impurities. Three basic methods, namely; gas phase, liquid phase and intercalation, have been used with limited success for the purification of carbon nano-tubes. The classical chemical techniques for purifi-cation such as filtering, chromatography and centri-fugation have also been tried. These chemical techniques are also not found effective in removing the carbon nanoparticles, amorphous carbons and other unwanted species. Heating preferentially decreases the amount of disordered carbon relative to carbon nanotubes. Heating thus could be useful for purification, except that it results in an increase in nanotube diameter due to the accretion of epitaxial carbon layers from the carbon in the vapour phase resulting from heating.

The gas phase method removes nanoparticles and amorphous carbon in the presence of nanotubes by an oxidation or oxygen-burning process84-86. Much slower layer-by-layer removal of the cylindrical layers of multi-walled nanotubes occurs because of greater stability of a perfect graphene layer to oxygen than disordered or amorphous carbon or material with pentagonal defects85,86. This method was in fact used to synthesize a single-walled carbon nanotube. The oxidation reaction for carbon nanotubes is thermally activated with an energy barrier of 225 kJ/mol in air86. The gas phase purification process also tends to burn off many of the nanotubes. The carbon nanotubes obtained by gas phase purification are generally multi-walled nanotubes with diameters in the range 20-200Å and 10-1 micrometer in length84, since the smaller diameter tubes tend to be oxidized with the nanoparticles.

Liquid phase removal of nanoparticles and other unwanted carbons has been carried out with some success using a potassium permanganate KMnO4 treatment method which tends to give higher yields than the gas phase method, but results in nanotubes of shorter length87,88. Finally, the intercalation of unpurified nanotube samples with CuCl2-KCl results in intercalation of the nanoparticles and other carbon species, but not the nanotubes which have closed cage structures. Thus, subsequent chemical removal of the intercalated species can be carried out89. A method for


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the purification of samples containing single-walled nanotube ropes in the presence of carbon nano-particles, fullerenes and other contaminants has also been reported90. Therefore, the purification of carbon nanotubes is still a difficult work, in spite of several methods available in the literature. The impurities in the carbon nanotubes remain a problem, as there is no any particular method to remove all impurities such as carbon nanoparticles, amorphous carbon and other unwanted species.

Applications of Carbon Nanotubes Hydrogen storage in carbon nanotubes

The start of a new millennium presents us with countless opportunities and challenges. On one hand, the development of social economy and culture brings many of us high standards of living; on the other hand, our world is facing a rapid depletion of natural resources and serious global environmental problems. All of these are connected with the overuse of fossil fuels. Nowadays, public concern about the global environmental problem caused by the utilization of fossil fuels and the over-reliance of the economy on them is increasing. Therefore, there is a search for possible alternative sources of energy to replace fossil fuels. A number of primary energy sources are available, such as thermonuclear energy, nuclear reactors, solar energy, wind energy, hydropower and geothermal energy. In contrast to the fossil fuels, in most cases, these new primary energy sources cannot be used directly (e.g. used as fuels for transportation), and thus they must be converted into fuels, that is to say, a new energy carrier is needed. Among the many choices, hydrogen is one of the best candidates. Hydrogen is an ideal fuel and versatile energy carrier, and it is easy to produce, convenient fuel for trans-portation, converts easily to other energy forms at the user end, high utilization efficiency and environmen-tally compatible (zero- or low-emission)91,92. Therefore, hydrogen provides the best route to a sustainable energy for the transportation sector and some other uses, since it can be produced not only from the fossil fuels, such as coal and natural gas, but also from wind, solar, thermal, hydroelectric, biomass or municipal solid wastes with no consumption of non-renewable resources and no pollution of any kind.

Like conventional fossil fuels, the most important and the most urgent application of hydrogen is connected with transportation and vehicles. Hydrogen used as a fuel in vehicles is mainly divided into three kinds: In nickel-hydride battery, in which hydrogen is

combined as a metal hydride; in a spark-ignition engine powered car; and in a fuel cell. In the last case, hydrogen can be converted to electricity with emission of only water with very high efficiency because the process is not subjected to the limitations of the carnot cycles. Thus, true zero-emission vehicles can be produced. The most significant news has come from the worldwide vehicle corporations such as Daimler Chrysler, Ford, GM, Toyota, Honda, etc. that the development of proton exchange membrane fuel cell electric vehicles has made great progress. Recently, the Daimler Chrysler Corp. announced that it would be the first automaker worldwide to offer fuel cell vehicles on the market during next several years93. However, several barriers have to be overcome before hydrogen electric vehicles can be put into large-scale practical utilization. One of the most important challenges is the lack of a safe and efficient onboard storage technology, which may dramatically influence the vehicle’s cost, range, performance and fuel economy, as well as shape the scale, investment requirements, energy use, and potential emissions of a hydrogen-refueling infra-structure. That is to say, the development of onboard storage technology will directly determine the schedule of hydrogen-powered vehicles into market. Three technologies namely; cryogenic liquid hydrogen (LH2), compressed gas storage and metal hydride storage, for storing hydrogen fuel are under consideration. However, these three technologies either cannot reach the benchmarks just mentioned, or have significant disadvantages, e.g., liquefying hydro-gen wastes at least 1/3 of the stored energy and LH2 storage suffers from potential hydrogen losses due to evaporation. The hydride-based approach suffers from weight and cost concerns, and the crucial issue connected with compressed gas storage may be tank volume and safety. Recently, a tremendous interest has been aroused due to discovery94 and reproduc-tion57,95-98 of high hydrogen adsorption capacity in carbon nanotubes and other low-dimensional carbon materials. If these encouraging experimental results can be reproduced easily and the large-scale production of carbon nanotubes made available in near future, it will be possible to use fuel cell in vehicles in near future.

Carbon nanotubes have been known for more than 10 years. It is a challenge to fill their unique tubular structure with metals and gases. Especially, the absorption of hydrogen in single wall nanotubes has


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attracted many research groups worldwide. Carbon nanotubes show very surprising hydrogen storage capacity, in spite of their relatively small surface area and pore volume. The values published for the quantity of hydrogen absorbed in nano-structured carbon materials varies between 0.4 and 67 mass%. For nanotubes, one important issue currently being debated is whether hydrogen adsorption also occurs in the interstitial channels between adjacent nanotubes in a rope of SWNTs. Dresselhaus et al.99 presented two geometrical estimates for the filling of a rope (crystalline lattice) of SWNTs. One assumes that hydrogen is a completely deformable fluid that fills the space not occupied by the carbon nanotubes, and the other is the packing of hydrogen molecule of kinetic diameter 0.29 nm on inner walls and in the interstitial volume of the nanotubes, as shown in Fig. 15. Using geometrical model with close-packing of hydrogen molecules within the core of a (10,10) tube leads to 3.3 wt% hydrogen adsorption within the tube and 0.7 wt% adsorption within the interstitial space, or a total of 4.0 wt% hydrogen adsorption99. Moreover, Dresselhaus thought that a hydrogen molecule adsorbed in the interstitial space undergoes much stronger surface attraction than on a single planar graphene surface, since it is in close proximity to three graphenes. Therefore, the hydrogen adsorbed in the space would be expected to be denser than on the single graphene surface. Therefore, it is concluded that for SWNTs hydrogen is stored in both the pores formed by the inner tube cavities and the inter-tube space, and the storage density is possibly higher than that on a planar graphene surface. Accordingly, the hydrogen adsorption amount may be higher than

4.0 wt%, consistent with the experimental results. Dillon et al.57 first claimed that SWNTs have a

high reversible hydrogen storage capacity. They showed that hydrogen can condense to high density (estimated to 5~10 wt%) inside narrow SWNTs of 12 Å, and predicted that SWNTs with diameters of 16.3 and 20 Å would come close to the target H2 uptake density of 6.5 wt%. The adsorption of H2 in SWNT soots was probed with temperature prog-rammed desorption (TPD) spectroscopy and the TPD experiment suggested that physico-adsorption of hydrogen mainly occurs within cavities of SWNTs. The activation energy for hydrogen desorption was found to be 19.6 kJ/mol, which is much higher than the theoretically predicted value or approximately five time higher than that for a planar graphite surface, thereby, promoting hydrogen storage capacity at higher temperature. They have recently developed a method to produce samples with a high concentration of short SWNTs with open ends that are accessible to the entry of hydrogen molecules, and these purified cut SWNTs adsorbed 3.5~4.5 wt% hydrogen under ambient conditions in several times. After this finding, many groups started research on hydrogen storage in carbon nanotubes and have made some progress. Chen et al.95 reported that in the TPD experiment, a high H2 uptake of 20 and 14 wt% can be achieved in milligram quantities of Li-doped and K-doped multi-walled carbon nanotubes, respectively, under ambient conditions. The K-doped MWNTs can adsorb H2 at room temperature, but they are chemically unstable, whereas the Li-doped MWNTs are chemically stable, but require elevated tempera-tures (473 to 673K) for maximum adsorption and desorption of H2. Recently, Yang et al. repeated the same experiment with dry hydrogen and they found that K-doped MWNTs can only adsorb nearly 2 wt% hydrogen, but the experiment with wet hydrogen showed a value of 21 wt% hydrogen. They concluded that it was moisture in hydrogen that drastically increased the weights gain by reaction with the alkali species on carbon. Yang et al.100 reported that the hydrogen storage on multi-walled nanotubes (MWNTs) was dependent on the degree of catalyst removal. At atmospheric pressure, removal of the catalyst decreased the uptake from 0.6% to below detection limits. Hydrogen uptake of the metal oxide catalyst ranged from 0.25 to 0.98%, depending on surface area. Normalization by metal content and

Fig. 15—A typical configuration of H2 molecules adsorbed on a triangular array of carbon nanotubes. This configuration resulted from a classical Monte Carlo calculation in which the stimulated storage pressure was 10 MPa and the stimulated temperature was 50K99.


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temperature-programmed desorption studies suggest hydrogen dissociation and subsequent spillover to the MWNT. Metal support interactions were key to the spillover; dry mixing of the MWNT and catalyst did not enhance storage, whereas in situ production increased storage by 40%. The moderate temperature range of this material suggests a novel material for hydrogen storage applications. Ci et al.101 reported the mass production of carbon nanotubes with poor crystallization by the floating catalyst method. They found that the crystallization behaviour is different from that of bulk amorphous carbons due to the finite dimensions and tube like shape of the as-grown carbon nanotubes. Their hydrogen desorption experi-ments showed that the as-grown carbon nanotubes have poor hydrogen up-take capacity at room tempe-rature under modest pressure (10 MPa), however, the hydrogen storage amount was improved remarkably in the annealed carbon nanotubes at temperature range 1700-2200°C. The reason for the improved hydrogen storage capacity was discussed according to the change in their surface feature and microstructure during the annealing procedure.

More and more experimental and theoretical results continue to appear, and more and more reproducible evidence proves that carbon nanotubes are a potential hydrogen storage carrier. Thus, in order to use carbon nanotubes as a practical hydrogen storage medium, the mass production, purification, surface functioni-zation of carbon nanotubes, development and optimi-zation of pretreatment methods for opening the caps at tube ends of carbon nanotubes are needed. There-fore, still, scientists have to go a long way to use carbon nanotubes for hydrogen storage.

Field Emission Mechanism

Since the flat-panel display technique became a very promising in late 90’s, new techniques and materials for field emission have been widely explored. In the field emission, carbon nanotubes have become very popular candidate for electron emitter. Besides, other nano-scaled materials, such as nanowires and nanorods102-110 have been used as electron emitters as well. In thermionic emission and photoemission, electrons are given energy to overcome the potential barrier at the metal surface. While in the field emission, the potential barrier is deformed so that unexcited electrons may also leak through it.

Field enhancement factor (β) A lot of work102-110 is dedicated to the applications

of nanowires and nanotubes in field emission. It is found that extremely sharp tips can improve the field emission performance due to the enhancement electric field (F).

The local electrical field in a plane-to-plane anode-cathode configuration is defined as:

oVF Fd

= β = β … (2)

where Fo is the applied field, β is the field enhancement factor, V is the applied voltage, and d is the distance between the anode and cathode. The field enhancement factor is given as;

2 3 1/ 2 3 / 23ln ( / ) 3.79 106.83 10

(1/ )FNd J F FS s

d F⎛ ⎞ ⎛ ⎞× φ

= = − × ⎜ ⎟ ⎜ ⎟φ β⎝ ⎠ ⎝ ⎠

… (3)

The field enhancement factor is regarded as the aspect ratio r/h, where r is the radius and h is the height of the tip, respectively.

Field emission properties of carbon nanotubes

Carbon nanotubes are composed of graphene sheets rolled into seamless hollow cylinders with diameters ranging from 1 nm to about 50 nm. Several methods have been used to produce single-walled as well as multi-walled nanotubes. The diameter of the MWNTs typically ranges from 10 to 50 nm, while the length exceeds 10 micron. For SWNTs, the diameter is only 1 nm and the length is up to 100 micron. In 1991, MWNTs were discovered in a carbonaceous stalagmite-like deposit by Iijima4, which was left on an electrode after the recovery of fullerene soot produced by a carbon arc. SWNTs were discovered in 1993 during the course of synthesizing carbon nanocapsules filled with magnetic fine metal particles (Fe, Co, Ni)20,22,111-113. Since then, a lot of research has been carried out on this novel carbon nanomaterial with much success. Nanotubes exhibit unique physical and chemical properties as being a quasi-one dimensional material. These nanotubes especially for SWNTs are either metallic or semi-metallic, depending on the geometry of a graphene sheet rolled up into a tube (diameter and chiral angle)21,114,115. Nanotubes present one-dimensional confinement effects and behave as coherent quantum wires116-119. Mechanically nanotubes have high perfection in their structures and have the highest modulus of all known materials120. Due to these extreme properties, nano-


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tubes are under investigation towards several appli-cations, including electron field emitters121, probes of scanning-type microscopes122, hydrogen storage materials123, electrode materials of secondary batteries and capacitors57. Among these proposed applications, field emission electron sources would be industrially the most promising and are nearly within reach of practical use.

Field emission involves the extraction of electrons from a solid by tunneling through the surface potential barrier (Fig. 16). The emitted current depends directly on the local electric field at the emitting surface E, and on its work function (φ). Field emission is important in several areas of industry, including lighting and displays. The relatively low voltages needed for field emission in nanotubes could be an advantage in many applications. Fowler-Nordheim model shows that the dependence of the emitted current on the local electric field F and the work function φ, is exponential like. As a conse-quence, a small variation of the slope or surrounding of the emitter and/or the chemical state of the surface has a strong impact on the emitted current. The small diameter of carbon nanotubes is very favourable for field emission, the process by which a device emits electrons when an electric field or voltage is applied to it. The field F has to be very high, of the order of 107 V/cm. When a high electric field of the order of 107 V/cm is applied on a solid surface with a negative electrical potential, electrons from the solid are emitted into vacuum by the quantum mechanical tunneling effect. Such an extremely high field can be obtained on a sharp tip of very thin needle, because electric fields concentrate at the sharp points. The carbon nanotubes possess high aspect ratio, a sharp tip, high chemical stability, and high mechanical strength, which makes it a good candidate for field emitters. In 1995, field emission from an isolated

single MWNT was first reported by Rinzler et al.124, and field emission from a MWNT film was reported by de Heer et al.125. Subsequently, many studies on field emission from MWNTs126-128 and SWNTs129,130

were reported. The following section presents a review of the recent work on field emission of carbon nanotubes by different research groups. Figs 17a-c represent the TEM picture of the ends of different nanotubes, standard field emission model from a metallic emitter and typical set-up for field emission respectively.

Tzeng et al.131 reported that the carbon nanotube coated non-planar cold cathodes emit electrons of high current densities at low applied electric fields. Multi-walled carbon nanotubes were chemically vapour grown inside while pre-synthesized single walled carbon nanotubes were used to fill holes drilled into electrically conductive electrodes. Thermal chemical vapour deposition was carried out in mixtures of argon and acetylene using iron as the catalyst for the growth of carbon nanotubes. These non-planar cold cathodes emit electrons of high current density at low electric fields. Electron field emission current density approximately 700 mA/cm2 at 2.75 V/μm was measured. Wang et al.132 have synthesized aligned carbon nanotubes (CNTs) array (Fig. 18) using r.f. plasma-enhanced chemical vapour deposition on iron-coated silicon substrates from an acetylene and hydrogen mixture. The macroscopic field emission properties of the aligned CNTs array and randomly oriented CNTs layer were studied by means of a vacuum field emission system, which permits integrated field emission using an ITO/glass

Fig. 16 —Systematic diagram for field emission measurements.

Fig.17—(a) TEM picture of the ends of different nanotubes. Each black line corresponds to one graphene sheet view edge on (b) standard field emission model from a metallic emitter, showing the potential barrier and the corresponding energy distribution (energy on the vertical axis, current on the horizontal axis). (c) Typical set-up for field emission; a potential difference is applied between a nanotube (or an assembly of nanotubes) and a counter electrode.


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or a phosphor/ITO/glass screen. After field emission measurements with the applied electric field of <8 V/μm (Fig. 19), there was no obvious change in

Fig. 18—SEM images of as-synthesized aligned CNTs array before field emission measurements; (a) cross-sectional image; (b) high magnification image; and (c) top view image132.

Fig.19—Field emission current density vs. electric field for the aligned CNTs array (SEM images are shown in Fig.18). Inset is the Flower-Nordheim plot132. the alignment of the CNTs array, but the formation of tower-like structure on the top of CNTs was observed, indicating that nearly all the CNTs of aligned CNTs array were involved in field emission, and every field

emission site (tower-like structure) actually consists of some carbon nanotube tips. At high electric field, some of the aligned CNTs were pulled off due to the weak adhesion between the CNTs and the substrate. They also found that randomly oriented CNTs layer could have similar field emission property as aligned CNTs array. An inhomogeneous field emission and enhanced emission effect from the array sides were observed by using a phosphor screen. For a macro-scopic scale area, a homogenous and stable emission was difficult to obtain from the directly synthesized aligned CNTs array or randomly oriented CNTs layer on catalyst coated substrates. Kim et al.133 reported that the field emission behaviour of photolithography-based patterned carbon nanotubes (PP-CNTs) was irreversibly degraded, showing a gradual turn-on voltage shift to the high voltage region as a result of field emission measurements. The PP-CNTs show a flat region in Fowler–Nordheim (F–N) plots, which can be attributed to residue-induced emission suppres-sion. The observed degradation in field emission can be attributed to a subsequent degradation of carbon nanotubes emitters by a combination of the high electric-field-induced straightening out effect and the high current induced burning of PP-CNTs. The local emission current and the stability of electron emission were observed in an attempt to investigate the residue effect of the PP-CNTs on the field emission behaviour. Jung et al.134 reported the large-area and low-temperature synthesis of carbon nanotubes on glass substrates with transformer coupled plasma (TCP) type radio-frequency plasma-enhanced chemical vapour deposition (rf PE-CVD) system was performed, and their surface structures and field emission property were analyzed. By varying process conditions such as Ni layer thickness, gas flow rate, NH3 etching time, plasma power, and the distance between rf coil and substrate, the optimum conditions for growing carbon nanotubes were found. Their SEM and TEM images showed that their diameters are ranged from 40 to 80 nm, showing hollow tube structures. From the FTIR spectrum of carbon nanotubes synthesized in this study, large amount of –CH2 and –CH3 functional groups were detected. The XPS peak of C1s was split into one main peak and several small peaks, which means the chemical state of carbon atoms is divided. Their surface structures were proposed from the FTIR spectrum and the XPS results, and they are somewhat different from the samples grown by arc-discharge. The measured field


547

emission current density seems to be sufficient to be used for field emission display and increased with the growth time of carbon nanotubes. With these results and equivalent circuit modeling, body-emission was confirmed to be a dominant emission mechanism. Feng et al.135 studied the effect of structural parameters, i.e., tube diameter and density, on the field electron emission characteristics of carbon

nanotubes. Thermal chemical vapour deposition system was employed to synthesize carbon nanotubes. Nanotubes with different diameters and densities were obtained by adjusting the thickness of the iron (Fe) catalyst film. The morphologies of the Fe and carbon nanotube film were characterized by scanning electron microscopy respectively (Fig. 20). Further field emission measurement (Figs 21 and 22)

Fig. 20—High magnification SEM images of the Fe thin film after H2 treatment corresponding to the predeposited catalyst film thickness of (a) 24 nm, (b) 48 nm and (c) 72 nm respectively and morphologies of corresponding as-grown CNT films135.

Fig. 21—I-E curves of CNT films with different structural parameters127.

Fig. 22—F-N plots of CNT films with different structural parameters135.


548

confirmed that the tube diameter and density could significantly affect the electron emission properties of the carbon nanotube. They have also discussed the possible physical reasons for this effect. Chen et al.136 observed that the change in field emission currents of multi-walled nanotube (MWNT) is not sensitive but detectable to temperature variation within the range from 300 K to 20 K. However, the characteristic curve cannot be congruently fitted using Fowler–Nordheim (FN) theory. Assuming a semiconducting amorphous carbon dot grown on the tube end, the dominated field emission mechanism will be the thermal electron tunneling caused by the high aspect ratio of the tube. This analysis conclusively addresses that the electron affinity and the energy gap of the caps are not changed while the interface barrier height increases as temperature increases. Jonge et al.137 prepared individual multi-walled carbon nanotube field emitters in a scanning electron microscope. The angular current density, energy spectra, and the emission stability of the field-emitted electrons were measured. An estimate of the electron source brightness was extracted from the measurements. The results show that carbon nanotubes are promising candidates to replace existing sources in high-resolution electron beam instruments.

Thus, carbon nanotubes have a strong potential for applications in cold electron emission sources due to their excellent electron emission properties, which can be extended further to flat panel displays such as field emission displays (FEDs). FEDs are characteristic of superior display performances such as fast response time, wide viewing angles, wide temperature range of operation, cathode ray tube-like colours, ultra slim features, low cost and low power consumption. The application of CNTs to FEDs necessitates their vertical alignment on cathode electrodes for better electron emission. Therefore, it is needed to grow well-aligned carbon nanotubes at low temperatures and in large quantity for using them in field emission displays. The other applications of carbon nanotubes include their use in electrochemical applications, energy storage and chemical sensors. The nanotubes can be used as mechanical reinforcements in high performance composites, and as nanoprobes in metrology and biological and chemical investigations. The nanotubes can also used as templates for the fabrication of other nanostructures.

Conclusions

Carbon nanotubes have been utilized either individually or as an ensemble to build functional device prototypes, as has been demonstrated by many research groups. Ensembles of nanotubes have been used for field emission based flat-panel displays, composite materials with improved mechanical properties and electromechanical actuators. Bulk quantities of nanotubes have also been suggested as high capacity hydrogen storage media. Individual nanotubes have been used for field emission sources, tips for scanning probe microscopy, nanotweezers and chemical sensors. Nanotubes are also promising as the central elements for future miniaturized electronic devices. The success in nanotube growth has led to the wide availability of nanotube materials, which is a main catalyst behind recent leaps and bounds in basic physics studies and applications of nanotubes. The full potential of nanotubes for applications will not be realized until the growth of nanotubes can be further optimized and controlled. Real-world applications of nanotubes require either large quantities of bulk materials or device integration in a scale-up fashion. For applications such as composites and hydrogen storage, it is desired to obtain high quality nanotubes at the kilogram or ton level using growth methods that are simple, efficient and inexpensive. For devices such as nanotube based electronics, scale-up will unavoidably rely on self-assembly techniques or controlled growth strategies on surface combined with micro-fabrication techniques. Significant work has been carried out to tackle these issues. Nevertheless, there are still many challenges in this field. An efficient growth approach to structurally perfect nanotubes at large scale is not yet in hand. Growing defect-free nanotubes continuously to macroscopic lengths is still difficult.

Acknowledgement One of the authors (Z H Khan) is thankful to Jamia

Millia Islamia (Central University), New Delhi, for granting him study leave to pursue post doctoral research on carbon nanotubes at Department of Materials Science and Engineering, National Tsing Huo University, Hsinchu, Taiwan.

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