carbon nanotubes: synthesis, properties, and applications

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Carbon Nanotubes: Synthesis,Properties, and ApplicationsEnkeleda Dervishi a , Zhongrui Li b , Yang Xu b , Viney Saini a ,Alexandru R. Biris c , Dan Lupu c & Alexandru S. Biris aa Applied Science Department, University of Arkansas at Little Rock,Little Rock, Arkansas, USAb Nanotechnology Center, University of Arkansas at Little Rock, LittleRock, Arkansas, USAc National Institute for Research and Development of Isotopic andMolecular Technologies, Cluj Napoca, RomaniaPublished online: 24 Mar 2009.

To cite this article: Enkeleda Dervishi , Zhongrui Li , Yang Xu , Viney Saini , Alexandru R.Biris , Dan Lupu & Alexandru S. Biris (2009) Carbon Nanotubes: Synthesis, Properties, andApplications, Particulate Science and Technology: An International Journal, 27:2, 107-125, DOI:10.1080/02726350902775962

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Particulate Science and Technology, 27: 107–125, 2009Copyright © Taylor & Francis Group, LLCISSN: 0272-6351 print/1548-0046 onlineDOI: 10.1080/02726350902775962

Carbon Nanotubes:Synthesis, Properties, and Applications

ENKELEDA DERVISHI1, ZHONGRUI LI2,YANG XU2, VINEY SAINI1, ALEXANDRU R. BIRIS3,DAN LUPU3, AND ALEXANDRU S. BIRIS1

1Applied Science Department, University of Arkansas at Little Rock,Little Rock, Arkansas, USA2Nanotechnology Center, University of Arkansas at Little Rock,Little Rock, Arkansas, USA3National Institute for Research and Development of Isotopic andMolecular Technologies, Cluj Napoca, Romania

This brief review presents a comprehensive outline of the present research statuson the fast moving carbon nanotube (CNT) field. It covers a short introduction tothe relation between carbon nanotubes, graphite, and other forms of carbon andexplains in detail the structure of CNTs. The electronic, electrical, and mechanicalproperties of CNTs, as well as the most widely used methods for CNT productionsuch as electric arc discharge, laser ablation, and chemical vapor deposition (CVD),are discussed. Some of the CNT applications covered in this review article are: fieldemission, hydrogen storage, carbon nanotube-based solar cells, and CNT compositematerials.

Keywords carbon nanotubes, electronic, electrical, and mechanical properties,synthesis and applications of nanotubes

Introduction

Diamond, graphite, and amorphous (noncrystalline) structures were the onlyallotropes of pure carbon that were known until 1985 (Wilson et al. 2002).In diamond structure, each carbon atom is tetrahedrally bonded to the other carbonatoms through sp3 hybrid bonds. Diamond is electrically insulating because itselectrons are strongly held within covalent (sigma (�)) bonds among the carbonatoms (Meyyappan 2005). Graphite has a layered structure, and each layer isformed by hexagons of carbon atoms bonded together through sp2 hybrid bonds(Poole & Owens 2003). The outer-shell electrons of each carbon atom in thegraphene layer form three in-plane � bonds and an out-of-plane � bond (orbital)(Meyyappan 2005). In addition, the out-of-plane � orbital or electron is delocalizedand distributed over the entire graphene plane, making graphite thermally andelectrically conductive (Meyyappan 2005). The carbon atoms in the hexagonal orgraphene plane are tightly bonded to each other covalently, whereas the graphene

Address correspondence to Enkeleda Dervishi or Alexandru S. Biris, Applied ScienceDepartment, University of Arkansas at Little Rock, Little Rock, AR 72204, USA. E-mail:[email protected] or [email protected]

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108 E. Dervishi et al.

layers are weakly held together by the van der Waals forces. The spacing betweenthe graphene layers is 3.35Å (Terrones 2003).

A novel form of carbon, called fullerene, was discovered in 1985 by Krotoet al. (1985). Since then a large family of new carbon materials has been discovered.The most well-known form of fullerene is the buckyball (C60), which is made of20 hexagons and 12 pentagons of carbon atoms (Meyyappan 2005). Carbon nanotubes(CNTs), which are considered a new form of fullerenes, were discovered by SumioIijima in 1991. Ijima found CNTs while he was looking for new carbon structures ona cathode surface used in an electric arc discharge method (Iijima 1991).

CNTs are often referred to as one-dimensional materials because of their highaspect ratio, a few tens of atoms in circumference and many microns in length(Dresselhaus & Eklund 2000). CNTs can be visualized as graphite sheets rolledinto seamless hollow cylinders. There are two types of CNTs, single-wall carbonnanotubes (SWCNTs), which consist of one tube of graphite, and multiwall carbonnanotubes (MWCNTs), which consist of a number of concentric tubes, cylindersinside the other cylinders. The interlayer spacing of the MWCNTs’ concentric tubesis approximately 3.4Å (Terrones 2003); the diameter of the innermost tube can beas small as 0.4nm, whereas the largest outermost tube in a MWCNT was found tobe hundreds of nm (Meyyappan 2005).

In 1992, Saito et al. (1992a,b) and other groups calculated the electronicstructure of individual SWCNTs. According to the calculations, SWCNTs mightbe metallic or semiconducting depending upon their chirality (helicity) anddiameter (Terrones 2003; Saito et al. 1992a,b). SWCNTs were discovered in 1993independently by two groups, Iijima & Ichihashi (1993) and Bethune et al. (1993).It is the structure, topology, and size of CNTs that make their electronic, electrical,and mechanical properties attractive when compared to planar graphite-relatedstructures (Dresselhaus et al. 2001; Ajayan & Zhou 2001).

Structure of Carbon Nanotubes

CNTs are formed when sheets of graphite, characterized by the hexagonal lattice,are coiled into seamless hollow cylinders. Figure 1 shows the unrolled honeycomblattice of an SWCNT with the chiral vector �Ch, the translational vector �T , the unitvectors a1 and a2, and the chiral angle �. The �Ch vector is called the circumferentialvector, and it is always perpendicular to the translational vector �T .

The circumference of an SWCNT is determined by the chiral vector �Ch.The circumference vector is defined by �Ch = ma1 + na2, where m and n are integersand a1 and a2 are unit vectors in the two-dimensional graphene sheet (Saito et al.1992a,b; Iijima & Ichihashi 1993; Bethune et al. 1993; Dresselhaus et al. 2001;Ajayan & Zhou 2001; Dresselhaus 2001). The value of the chiral vector �Ch isuniquely determined by the pair of integers (m�n).

Depending on the orientation of the graphite sheet (the hexagonal lattice ofcarbon atoms) relative to the tube axis (�T ), three types of SWCNTs are obtained:“armchair,” “chiral,” and “zigzag” nanotubes (Poole & Owens 2003; Dresselhauset al. 2001). When the vector �T is perpendicular to the C–C bonds, located onopposite sides of each hexagon in the honeycomb lattice, the structure referredto as “armchair” is obtained. These structures are named “armchair” because ofthe “\_/ \_/” shape perpendicular to the nanotube axis (Meyyappan 2005). In the“zigzag” configuration, the tube axis �T is parallel to the C–C bonds, which are

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Synthesis, Properties, and Applications 109

Figure 1. The two-dimensional hexagonal lattice of an SWCNT, with the chiral vector �Ch,translational vector �T , the unit vectors a1 and a2, and the chiral angle �. In this case thechiral vector is defined as �Ch = 5a1 + 2a2 for �m� n� = �5� 2� and the corresponding SWCNTis shown to the right of the unrolled graphene sheet.

located on opposite sides of the hexagons. The “zigzag” structures are named afterthe “/\/\/” shape perpendicular to the axis of the carbon nanotube (Meyyappan2005). For the rest of the arrangements, when the vector �T lies at an angle withrespect to the C–C bonds, structures known as “chiral” or helical are formed(Terrones 2003). Figure 2 shows the structural models of SWCNTs with threedifferent chiralities.

The SWCNT structure depends on the values of integers (m, n) and the chiralangle �. The angle between the chiral vector �Ch and the zigzag direction (� = 0�)is called the chiral angle (�), which is defined by the following equation (Poole &Owens 2003):

� = arctan[−

√3n

2m+ n

]

Figure 2. Structural models of SWCNTs presented at three different chiralities: (a) �5� 5�armchair, (b) �10� 0� zigzag, and (c) �6� 5� chiral configuration.

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The CNT diameter depends on the values of the integers (m, n� and is defined bythe following equation:

d = a√m2 +mn+ n2

�

where a is the C–C bond length in the graphene sheet and is given by: a = 1�42 ∗√3 = 0�246nm (Poole & Owens 2003). The “armchair” CNTs are formed when m =

n and the chiral angle is 30�, whereas “zigzag” nanotubes are formed when either nor m is zero and the chiral angle is 0�. The rest of the nanotubes (0 < � < 30�) areknown as “chiral” or “helical” nanotubes (Saito et al. 1992a,b; Dresselhaus 2001).

Properties of Carbon Nanotubes

The properties of CNTs are extremely sensitive to the degree of graphitization,chirality, and their diameter (Poole & Owens 2003). CNTs have a high aspect ratio,high tensile strength, low mass density, high heat conductivity, large surface area,and versatile electronic behavior (Terrones 2003; Saito et al. 1992b; Dresselhauset al. 2001). CNTs behave as ideal one-dimensional “quantum wires” with eithersemiconducting or metallic properties (Saito et al. 1992a,b; Dresselhaus et al. 2001;Ajayan & Zhou 2001; Dresselhaus 2001). Additionally, nanotubes are mechanicallystrong and excellent conductors of electricity. This section will briefly describe theelectronic, electrical, and mechanical properties of CNTs.

Electronic Properties of Carbon Nanotubes

The electronic properties of CNTs have been widely explored for variousapplications (Meyyappan 2005; Dresselhaus & Eklund 2000; Saito et al. 1992a,b;Dresselhaus et al. 2001; Ajayan & Zhou 2001; Dresselhaus 2001; Teo et al.2003). CNTs have very interesting electronic properties due to their high aspectratio and small dimensions. Depending on the diameter and the chirality (whichdescribes the way the graphene sheet is rolled), SWCNTs are either metallic orsemiconducting (Poole & Owens 2003). Therefore, depending on the values ofthe indices, m and n, SWCNTs are metallic when �2n+m� = 3i, where i is aninteger, and otherwise are semiconducting (Saito et al. 1992b; Dresselhaus 2001).All “armchair” carbon nanotubes are metallic since they satisfy the previousequation, while the other two configurations, “chiral” and “zigzag” nanotubes,can be either metallic or semiconducting. Based on the structural predictions,approximately one-third of the SWCNTs are metallic and two-thirds are semi-conducting. Figure 3 shows the graphene lattice of an SWCNT and the differenttypes of nanotubes formed, depending on the (m�n) values.

In the honeycomb lattice each carbon atom is covalently bonded to threeneighbor carbon atoms through sp2 bonds (Teo et al. 2003). There are two typesof covalent bonds between the carbon atoms: � bonds and � bonds. In graphenesheets the carbon atoms have an unhybridized � orbital, which is responsible fortransporting � electrons through the nanotube. The � bonds (located perpendicularto the plane of the graphene sheet) are delocalized and they are shared over theentire carbon nanotube. In the axial direction, the � electrons have no constrictionsmoving freely throughout the nanotube structure. They are sometimes comparedto the delocalized electrons in metals (Dresselhaus & Eklund 2000; Saito et al.

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Figure 3. The graphene sheet of an SWCNT, with the different (m�n) values and possiblechiral vectors. By rolling the graphene sheet in different ways, three different configurationsof SWCNTs are created: “armchair,” “zigzag,” and “chiral.”

1992a,b; Dresselhaus et al. 2001; Ajayan & Zhou 2001; Dresselhaus 2001). On theother hand, in the radial direction electrons are confined by the monolayer thicknessof the graphene sheet (Terrones 2003). If the wavelength of the electron is not amultiple of the circumference of the nanotube, it will destructively interfere withitself, and therefore only the wavelengths that are integer multiples of the nanotubecircumference will possibly exist (Poole & Owens 2003). Therefore, the dominantconduction path in a carbon nanotube is along its axis.

CNTs are often referred to as one-dimensional “quantum wires” due to thequantum confinement effect on the carbon nanotube circumference (Meyyappan2005; Bockrath et al. 1997; Frank et al. 1998). When the graphite sheet is rolled intoa cylinder, the � orbital becomes more delocalized outside the carbon nanotube,resulting in the � bonds being slightly out of plane (Meyyappan 2005). Dueto this � − � rehybridization the nanotubes become mechanically stronger andelectrically and thermally more conductive than graphite. The conductivity of CNTsincreases (when compared to the sheet of graphite) as the delocalization of the� orbital increases. Furthermore, the � − � rehybridization will lead to change inthe electronic properties of CNTs (Meyyappan 2005). The electronic states of theCNTs split into one-dimensional sub-bands, instead of a single wide electronicenergy band. Figure 4 shows the density of states (DOS) of a metallic (“armchair”)and a semiconducting (“zigzag”) carbon nanotube. The sharp intensities (spikes)observed in Figure 4 are known as van Hove singularities and are characteristicof one-dimensional quantum conduction that is not present in a three- (or two-)dimensional graphite structure (Terrones 2003).

If a gap exists between the valence band and the conduction band the materialbehaves as a semiconductor; otherwise, it behaves as a metal. In an insulator theband gap between the valence and the conduction band is larger, resulting inlower electrical conductivity. Metallic SWCNTs exhibit a band gap of 0 eV and thesemiconducting SWCNTs demonstrate a band gap of 0.4–0.7 eV (Terrones 2003).In metallic (“armchair”) CNTs, the valence and conduction bands cross each otherat the Fermi level (Ajayan 1999). In SWCNT ropes the wall-wall interactions induce

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Figure 4. Van Hove singularities in the density of states (DOS) for (a) “armchair” �10� 10�nanotube with metallic behavior and (b) “zigzag” �14� 0� nanotube with semiconductingbehavior.

a small band gap for the metallic CNTs and slightly reduce the band gap forthe semiconducting CNTs (Delaney et al. 1998). Additionally, in an MWCNT allthe semiconducting tubes behave as semimetallics due to their reduced band gap(Meyyappan 2005).

Electrical Properties of Carbon Nanotubes

The electrical properties of CNTs have been of great interest for many years. CNTspossess higher electrical conductivity than copper due to their low resistance andvery few defects along their structure. The electrical resistivity of CNTs was foundto be as low as 10−6 �-m and often can be altered by modifying the structure of thenanotube lattice (Meyyappan 2005). The electronic structure of CNTs will changewhen various molecules are introduced to the surface of the nanotube, resultingin an alteration of the electrical conductivity (Meyyappan 2005). The Fermi levelof CNTs is very sensitive to the type of dopant that comes into contact withthe nanotubes, and its control is very significant in the semiconductor industry(Meyyappan 2005).

The main methods of doping CNTs are interstitial doping and substitutionaldoping (Sze 1981). In interstitial doping, the CNT lattice remains the same (thedopant atoms do not substitute the carbon atoms) and the newly introduceddopants are adsorbed at the surface. Different gas molecules are adsorbed by theCNTs at the surface and between the tube bundles. Also, in many cases if the CNTsare open-ended the dopants reside inside the CNTs. In interstitial doping, dopantsand CNTs come together through non-covalent bonding. For example, O2 is foundto dominantly adsorb on the carbon nanotube surface, leading to p-type behavior,whereas C6H12 adsorption leads to n-type behavior (Krüger et al. 2003).

On the other hand, in substitutional doping, the dopant atoms replace thecarbon atoms and form sp2 bonding in the CNT structure. Some of the elementsexhibit a donor nature (group V) and transfer negative charge to the CNTs,while others demonstrate an acceptor behavior (group III) with negative chargeattained from the nanotube under covalent bonding (Meyyappan 2005). By doping

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MWCNTs or SWCNTs, for example with boron (B) or nitrogen (N), an orderof magnitude increase in the electrical conductivity is observed (Ajayan 1999).The B and N dopants make the CNTs either p- or n-types respectively throughsubstitutional doping (Zhao et al. 2000; Liu et al. 2001a,b; Czerw et al. 2001). It isthe intercalation of different metals in between the tubes and chemical doping thatsignificantly enhances the electrical conductivity of CNTs.

Mechanical Properties of Carbon Nanotubes

The mechanical properties of CNTs have been theoretically and experimentallystudied for several years (Ebbesen et al. 1996; Wong et al. 1997; Salvetat et al.1999; Poncharal et al. 1999). Since C–C bonds in the honeycomb lattice are oneof the strongest bonds in nature, it is prudent to explore the mechanical propertiesof CNTs. In addition, when the graphite sheet is rolled to form an SWCNT, theaxial component of the � bonding between carbon atoms increases significantly(Meyyappan 2005). This is the main reason why CNTs are stiffer than diamond andhave the highest Young’s modulus and tensile strength.

Young’s modulus is independent of tube chirality, but depends on tubediameter. SWCNTs with diameter between 1 and 2nm are found to have a veryhigh Young’s modulus of about 1TPa (Meyyappan 2005; Dresselhaus et al. 2001;Salvetat et al. 1999), whereas MWCNTs can have a Young’s modulus as high as1.2TPa (Teo et al. 2003). Young’s modulus of steel is only about 0.21TPa (Krügeret al. 2003). However, it is shown experimentally that the Young’s modulus of CNTsdecreases from 1TPa to 100GPa when the diameter of an SWCNT bundle increasesfrom 3nm to 20nm (Delaney et al. 1998). The Young’s modulus of MWCNTsis generally higher than that of SWCNTs due to different nanotube diameterscontained coaxially in the MWCNTs and also due to van der Waals forces actingbetween the tubes (Meyyappan 2005).

Since CNTs have only a few defects in the structure, they do not break orfracture even when they are bent severely. The CNT elastic response to deformationis also extraordinary (Meyyappan 2005). In addition, CNTs can be twisted andsustain large strains (40%) in tension before fracture (Yakobson et al. 1996), whereasmost materials fail with a strain of 1% or less due to propagation of dislocationsand defects (Meyyappan 2005; Lu & Han 1998). This is a unique result of the factthat the sp2 hybrids in the hexagonal lattice can rehybridize as the nanotubes arebent (Dresselhaus et al. 2001).

Young’s modulus is a measure of how stiff or flexible a material is, andtensile strength is a measure of the amount of stress needed to pull a materialapart. The tensile strength of individual MWCNTs was measured at about 50GPa(Ajayan & Zhou 2001; Yu et al. 2000). They are about 20 times stronger than steel(Dresselhaus et al. 2001). It was also found that the tensile strength of an individualnanotube can be as high as 150GPa, assuming that it has a Young’s modulus of1TPa (Meyyappan 2005).

Synthesis Techniques for Carbon Nanotubes

Ever since the discovery of CNTs, several techniques have been explored for large-scale and high-quality products. The most widely used methods to produce CNTsexplained in this section are: electric arc discharge (Ebbesen & Ajayan 1992; Journet

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et al. 1997), laser ablation (Thess et al. 1996), and chemical vapor deposition(Terrones et al. 1997; Li et al. 1996).

Electric Arc Discharge

Electric arc discharge is one of the first methods used to produce CNTs (Iijima &Ichihashi 1993; Bethune et al. 1993; Ebbesen & Ajayan 1992; Journet et al. 1997).The arc discharge method employs two graphite electrodes (6–12mm in diameter)that are separated by a short distance (1–4mm) inside a chamber filled with an inertgas (Meyyappan 2005; Poole & Owens 2003). A current of about 50–100A is passedthough the electrodes, and carbon atoms are ejected from the positive electrode(anode) and deposited on the negative electrode (cathode). As a result, the length ofthe anode decreases as the carbon nanotubes start forming on the cathode (Wilsonet al. 2002). Carbon is vaporized from the graphite anode in the form of crystallites,which generate small carbon clusters (mainly C3) (Meyyappan 2005). Next, thesecarbon clusters rearrange themselves into a tubular shape forming the MWCNTs,which drift toward the cathode and deposit on its surface. The limiting factor forhigh yield MWCNTs is the presence of “unwanted” graphite crystallites that do notform into nanotubes (Ajayan et al. 1997; Gamaly & Ebbessen 1995).

Metallic nanoparticles such as Fe, Co, Ni, Cu, Ag, Al, Pd, Pt, Fe/Co, Fe/Ni,Fe/Co/Ni, Co/Cu, Co/Ni, Co/Pt, and Ni/Cu (Ajayan et al. 1993; Lambert et al.1994; Kiang et al. 1995; Journet et al. 1997; Zhang & Iijima 1999; Guo et al. 1995b)are used to catalyze the SWCNT growth in the arc discharge method (Daenen et al.2003). This method can produce SWCNTs with diameters of 1–5nm and a lengthof 1m (usually arranged in bundles) and also MWCNTs with a large dispersionof their outer and inner diameters (Dresselhaus et al. 2001). In addition, MWCNTsproduced by the arc discharge method have fewer structural defects (due to highgrowth temperature) and better electrical, thermal, and mechanical properties thanthe nanotubes produced by other methods.

In the electric arc discharge method, production rates are pretty high: severalhundred mg of raw material is produced in about 10min (Dresselhaus et al. 2001).However, in order to obtain high-yield products, the reaction temperature has to beas high as 1500�C (Meyyappan 2005). In addition, it has been possible to producelarge amounts of MWCNTs by employing thick graphite rods and powerful arcreactors (Journet et al. 1997; Ebbesen et al. 1993).

Laser Ablation

In 1995, Smalley and his coworkers introduced a very promising approach toproduce CNTs, called the laser ablation method (Guo et al. 1995b). In a laserablation or evaporation method, a powerful laser is used to ablate a carbon target ina hot helium (He) or argon (Ar) atmosphere. As the graphite target inside a furnaceis heated up at about 1200�C, a pulsed laser beam incident on the target startsevaporating carbon from the graphite (Terrones 2003). The carrier gas sweeps thecarbon atoms from the high-temperature zone to a cold copper collector on whichthey condense into nanotubes (Poole & Owens 2003). In order to generate SWCNTsusing the laser ablation technique, it is necessary to impregnate the graphite targetwith transition metal catalysts (Terrones 2003). It is experimentally found that theSWCNT growth time in this technique is only a few milliseconds long (Yudasaka

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et al. 1999a; Sen et al. 2000; Zhang & Iijima 1999). Generally, along with SWCNTs(long bundles) (Saito et al. 1992b) and MWCNTs (closed-ended) (Guo et al. 1995b),fullerenes, amorphous carbon, and other carbon by-products are produced whenusing the laser ablation technique. MWCNTs produced by this method have anumber of layers varying from 4 to 24 and an inner diameter ranging between 1.5and 3.5nm (Meyyappan 2005).

The morphology and the properties of CNTs are highly influenced bymany different parameters such as light intensity, furnace temperature, type ofhydrocarbon and carrier gas, and the flow rate of different gases (Meyyappan 2005).For example, when the furnace temperature is below 800�C no carbon nanotubegrowth is observed, whereas a maximum SWCNT yield is obtained at about 1200�C(Puretzky et al. 2000; Yudasaka et al. 1999b). Unfortunately, the laser ablationtechnique is very expensive because it involves high-purity graphite rods and high-power lasers (Terrones 2003).

Chemical Vapor Deposition

Chemical vapor deposition (Ren et al. 1998; Kong et al. 1998) (CVD) is a relativelyslow method that produces long CNTs in large quantities. The hydrocarbonsource is heated at high temperatures, typically between 700� and 1000�C, insidea quartz tube in the presence of catalytic systems. CNTs are produced from thethermal decomposition of the carbon-containing gas molecules on desirable catalyticsystems. The latter present in the substrate provide nucleation sites for the nanotubegrowth (Dresselhaus et al. 2001). At high temperatures, once the hydrocarbondecomposes into hydrogen and carbon, carbon atoms dissolve and diffuse intothe metal surface and rearrange themselves into a network containing hexagonsof carbon atoms and finally precipitate out in the form of CNTs. Once the metalsurface is covered by amorphous carbon and its surface is “poisoned,” the carbonatoms cannot come into contact with the metal catalyst, which would result in thetermination of CNT growth. The hydrocarbon source exploited in the CVD methodcan be in a gas state such as acetylene, methane, and ethylene, a liquid state suchas benzene, alcohol, and hexane, or a solid state such as camphor, naphthalene, andmany more. Figure 5 shows a schematic diagram of the CVD method exploitingthe hydrocarbon source in any state (gas, liquid, solid). Transmission electronmicroscope (TEM) and scanning electron microscope (SEM) images of MWCNTsgrown on the Fe-Co/CaCO3 catalytic system by Biris et al. (2006) using the CVDmethod are shown in Figures 6(a) and 6(b) respectively.

Generally, in a thermal CVD setup, the quartz tube is first flushed with aninert gas such as He or Ar for 10min, and then usually the catalyst is reducedunder hydrogen at around 400�C for about 30min. It has been observed thathydrogen influences the particle size of the catalyst. Next, the hydrocarbon source isintroduced and the CNTs start growing. The catalytic system is composed of metalcatalysts such as Fe, Co, Ni, or Mo and metal supports such as MgO, CaCO3,Al2O3, or Si�

Higher yields of MWCNTs and SWCNTs are produced when the catalyticsystem is composed of two different metals (Seraphin et al. 1994; Seraphin & Zhou1994), and the ratio between these two metals in the catalytic mixture stronglyinfluences the CNT yield and morphology. For example, the highest yield ofMWCNTs was obtained when Fe:Co ratio was 2:1 in the Fe-Co/CaCO3 catalyst

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Figure 5. Schematic diagram of a CVD setup utilizing three different types of hydrocarbons:gas, liquid, and solid.

system (Li et al. 2008). One of the most well-known physical techniques used forpreparing a catalyst system in the form of small particles is thermal evaporation(Meyyappan 2005; Chhowalla et al. 2001). It has been shown theoretically andexperimentally that the diameter of the CNTs is closely related to the diameter ofthe metal nanoparticles (Lee et al. 2001; Sohn et al. 2001; Wei et al. 2001). As canbe seen from Figure 7(a), small metal catalysts of a particular catalytic system yieldCNTs with a small diameter, and larger diameter CNTs are grown on top of largemetal nanoparticles (as shown in Figure 7(b)) (Dervishi et al. 2007).

During CNT growth, two main scenarios are possible: base growth and tipgrowth. When the interaction between the nanoparticles and the support is strong,the carbon atoms precipitate from the metal nanoparticle (M) and the CNTs growon top of the nanoparticles, which are still attached to the support. This is calledthe root or base growth method, as shown in Figure 8(a). Conversely, when the

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Figure 6. (a) TEM and (b) SEM images of the MWCNTs grown on the Fe-Co/CaCO3

catalytic system using the CVD method.

Figure 7. (a) Small metal nanoparticles yield small nanotube diameters; (b) large metalnanoparticles yield larger nanotube diameters.

Figure 8. Base growth (a) and tip growth (b) mechanisms.

interaction between the metal nanoparticles and the support is weak, the carbonatoms start precipitating from the bottom of the metal surface. In this case theCNTs grow in between the metal nanoparticles and the support. This is called thetip growth model, as shown in Figure 8(b).

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Figure 9. Schematic of vertically aligned CNTs grown by CVD on a catalyst system.

There are two different CVD configurations used widely today: horizontalfurnace and vertical furnace. The horizontal furnace is currently the most popularconfiguration for production of CNTs (Teo et al. 2003). The length of CNTs ismostly controlled by the length of the hydrocarbon deposition time. The verticalfurnace is typically used for continuous mass production of CNTs. Usually, thehydrocarbon gas is injected at the top of the furnace and the produced CNTs arecollected at the bottom of the chamber (Teo et al. 2003).

One of the advantages of the CVD method is that it can be scaled up for large-scale and high-quality production of CNTs at a relatively low cost. In addition,the growth of CNTs can be controlled by adjusting the reaction parameters suchas the catalyst system, temperature, type of hydrocarbon, and the flow rate of thegases. Another advantage of the CVD method is that it enables the deposition ofCNTs on pre-designed lithographic structures, producing ordered arrays of CNTs(Dresselhaus et al. 2001). The CVD method is also used to grow vertically alignedCNTs (as shown in Figure 9), which can be used in applications such as flat paneldisplays.

Applications of Carbon Nanotubes

The amazing electrical, mechanical, chemical, and optical properties of CNTsenable a wide range of various applications such as electronic devices, fieldemission displays, energy storage devices, and drug-delivery devices (Dervishi et al.2007; Britto et al. 1999). CNTs are successfully used in fuel cells for energystorage (Britto et al. 1999; Che et al. 1998). In addition, CNTs are very goodshields of electromagnetic energy due to their high electrical conductive properties(Poole & Owens 2003). CNT composite polymer can be utilized in shieldingdifferent components from electromagnetic radiation. Due to their remarkablemechanical properties, CNTs are excellent candidates as reinforcing fibers fordifferent composite materials. The CNT applications that will be discussed in thissection are: field emission, hydrogen storage, carbon nanotube-based solar cells, andCNT composite materials.

Field Emission

CNTs possess a wide range of superior properties that make them very good fieldemitters. When a small electric field is applied parallel to the axis of a CNT,electrons are emitted at a very high rate from the ends of the tube (Poole & Owens2003). This phenomenon is called field emission. Field emission is a good techniquefor electron generation by extracting electrons from a conducting solid using anelectric field. A high electric field of several kV/m is required in order for theelectrons to tunnel through the potential barrier of the surface (Meyyappan 2005).CNTs were first discussed to be used as field emitters in 1995 (De Heer et al.

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1995). Both SWCNTs and MWCNTs are shown to be great candidates as fieldemitters (Bonard et al. 1999), but vertically aligned CNTs (Figure 9) have the largestpotential.

The most famous application of the field emission effect is in the developmentof flat panel displays. Today’s televisions use a controlled electron gun to impingeelectrons on the phosphor screen to locally induce light emission (Dresselhaus et al.2001). In field emission displays each individual pixel has its own electron source,and the brightness of each pixel is controlled by adjusting the current on thephosphor element (Meyyappan 2005). In Japan, cathode ray lighting elements werefabricated with CNTs and used as field emitters (Saito et al. 1998). In addition,diode flat panel displays have been fabricated using SWCNTs as the electronemission source (Wang et al. 1998).

Hydrogen Storage

In energy storage applications, materials with hydrogen storage capacity are veryattractive. CNTs have been explored for many years for liquid and gas storageor gas filtration applications due to their cylindrical geometry and their nanoscaledimensions (Meyyappan 2005; Pederson & Broughton 1992). During hydrogenphysisorption on CNTs, the hydrogen does not form any chemical bonds withthe nanotube but it adheres to the CNT surface through the van der Waal forces(Meyyappan 2005). Both academia and industry are investigating CNT utilizationfor hydrogen storage. SWCNTs are especially attractive because they demonstratea high and reversible hydrogen adsorption (Chambers et al. 1998). Dillon andcoworkers reported a molecular hydrogen storage capacity of SWCNTs as high as10 mass % of the total sample (Meyyappan 2005; Dillon et al. 1997). In addition,Chambers et al. (1998) reported a hydrogen storage capacity of 11.26 mass % forCNTs and 4.52 mass % for graphite (Meyyappan 2005). However, many of theresults in today’s literature have not yet been confirmed, and it is believed thatthey might not be reproducible. Consequently, it is not clear whether CNTs willhave significant impact in hydrogen storage applications, but this area is still beingexplored intensively (Ajayan & Zhou 2001).

Carbon Nanotube-Based Solar Cells

Superior properties of CNTs are also attracting a great deal of attention from thesolar cell field, since it is now very difficult to further improve the photovoltaicefficiency and lower the cost of Si-based solar cells. So far, CNTs are used in solarcell in three ways:

1. The photovoltaic effect can be achieved in ideal CNT diodes (Lee 2005).Individual SWCNTs can form ideal p-n junction diodes with perfect behavior,which is the theoretical limit for any diode performance (Lee 2005). Underillumination, SWCNT diodes show significant power conversion efficiencies dueto the enhanced properties of an ideal diode.

2. CNTs in organic solar cell applications were mainly confined to nanoscale fillers(providing transport path) to the polymer matrix or as transparent electrodesfor collecting charge carriers (Pradhan et al. 2006; Ago et al. 1999). Thehigh aspect ratios and large surface areas of nanotubes could be beneficial toexciton dissociation and charge carrier transport, thus improving the power

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conversion efficiency. As previously reported, nanotubes do not participatein photo-generation process; it is the conjugated polymers (e.g., P3HT) thatproduce excitons under optical excitation (Kymakis et al. 2008). Nanotubes,when embedded into the polymer matrix, only provide more interfacial area forexciton dissociation and charge transport path.

3. Very recently, double-walled carbon nanotubes were directly configured as energyconversion materials to fabricate thin-film solar cells, with nanotubes serving asboth photo-generation sites and charge carrier collecting/transport layers (Weiet al. 2007). The solar cells consist of a semitransparent thin film of nanotubescoated on an n-type crystalline silicon substrate to create high-density p-n hetero-junctions between nanotubes and n-Si in order to assist charge separation andextract electrons (through n-Si) and holes (through nanotubes) (Jia et al. 2008).Initial tests have shown a power conversion efficiency of > 1%, proving thatCNTs-on-Si is a potentially suitable configuration for making solar cells (Weiet al. 2007).

Carbon Nanotube Composite Materials

Carbon nanotube composites are used to replace several existing materials withproperties not as good as those of CNTs and are utilized in applications wherecomposites have not been used before (Meyyappan 2005; Ounaies et al. 2003).Due to their exceptional electronic, mechanical, and optical properties, CNTs areutilized in improving and altering the properties of different materials such aspolymers and ceramics. So far the most promising applications of CNTs aremechanical reinforcing of different types of materials and CNT polymer compositefilms (Dresselhaus et al. 2001).

Mechanical ReinforcementCNTs make excellent candidates for composite reinforcement due to theirextraordinary mechanical properties and their high aspect ratio. New compositematerials with improved mechanical properties are created by integrating CNTsinto various materials. A combination of the high strength and light weight makesnanotubes ideal for new and improved aircraft and car parts. As mentionedpreviously, due to the very strong C–C bond in the graphene sheet, CNTs areextremely robust (very high Young’s modulus and high tensile strength). Therefore,CNTs have been explored for altering the mechanical properties of differentmaterials such as polymers, ceramics, and metals.

When the CNTs are combined in the polymer matrix, using the in situpolymerization technique, the concentration of nanotubes strongly influences themechanical properties of the newly created polymer nanocomposite. For example,Jia et al. (1999) reported that when 1–10wt.% of CNTs were mixed in polymethylmethacrylate (PMMA), a 30% increase in the tensile strength was observed.To further improve the mechanical properties of CNT composites, an alignment ofCNTs should be performed (Fisher et al. 2002). Unfortunately, this still remains avery difficult challenge. Many CNT-composite products such as tennis rackets, golfclubs, and hockey sticks are already in the market today with improved mechanicalproperties (stronger and more durable).

Furthermore, by absorbing energy during their highly flexible elasticdeformation, CNTs increase the toughness of the composites such as ceramics

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(Ajayan & Zhou 2001). When MWCNTs are mixed with the SiC ceramic, a new andimproved ceramic composite with a 20% and 10% increase in strength and fracturetoughness, respectively, is obtained (Ma et al. 1998). In addition, the new ceramicnanocomposites show a significant improvement of 194% increase in fracturetoughness (compared to pure alumina) (Zhan et al. 2003). It is worth mentioningthat high-quality interfacial bonding between the CNT and the material is necessaryin order to improve the mechanical properties of the new nanocomposite.

Polymer Carbon Nanotube CompositesThe electrical, mechanical, and thermal properties of various polymers can beenhanced by incorporating CNTs into the matrix. CNTs interact with thepolymer chains and modify the surface characteristics of the polymer matrix(Ajayan & Zhou 2001). They alter the electrical properties of different compositematerials by significantly increasing their conductivity. However, the quality of thenew properties of CNT composite materials is dependent on the coupling betweenCNTs and the polymer matrix and on the transfer characteristics of such propertiesbetween the two materials. In addition, the most highly conductive polymers canbe acquired by combining only metallic CNTs (after separating them from thesemiconducting CNTs) into a polymer matrix. Conducting polymers have a widerange of applications from the automotive industry to military aircraft (used aselectromagnetic interference materials).

Furthermore, an interesting application is the CNT composite with photo-active polymers. MWCNTs and SWCNTs are mixed with a conjugated luminescentpolymer, poly (m-phenylenevinylene-co-2,5-dioctoxyp-phenylenevinylene) (PPV)(Curran et al. 1998). This composite shows a large increase in electrical conductivityand mechanical strength compared to the pristine polymer, with little lossin photoluminescence/electro-luminescence yield (Ajayan & Zhou 2001). CNT-composite polymers have been used in many different optical applications such asphotovoltaic devices and organic light-emitting diodes (Ago et al. 1999; Kymakis &Amaratunga 2002; Fournet et al. 2001).

Challenges in Carbon Nanotube CompositesThere are many challenges to overcome in synthesizing CNT polymer compositematerials. To further improve the mechanical properties (higher tensile strength andYoung’s modulus) of the composite materials, CNTs have to be uniformly dispersedand preferentially aligned or oriented in the materials. In order for a homogeneousdistribution of the mechanical load throughout the CNTs, cross-links between theshells of MWCNTs and between the individual SWCNTs in SWCNT bundles haveto be created (Dresselhaus et al. 2001). In addition, a strong bond between the CNTsand the material has to be achieved in order to maximize the mechanical propertiesof the new synthesized composites. Cross-linking or coupling of CNTs in differenthybrid and copolymer systems is created through two possible ways: (1) chemicalbonding (covalent or non-covalent bonds) and (2) physical non-bonded van derWaals interactions.

One of the problems that still remains difficult to resolve is homogeneouslydispersing CNTs in the polymer matrix. CNTs are very hydrophobic and do notdisperse in most aqueous solutions. They tend to form aggregates and clumptogether due to the van der Waal forces between the nanotube walls. Also, whenthe amount of added CNTs is too high, the dispersion in the polymer matrix is not

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very homogeneous. For example, in some cases when even less that 1 wt.% of CNTsare mixed in the polymer composite, a nonhomogeneous dispersion of CNTs in thematrix is seen (Safdi et al. 2002; Park et al. 2002).

In order to homogeneously disperse CNTs into the polymer matrix, CNTsshould be functionalized with various functional groups. CNTs are more active atthe ends than at the sidewalls, so functionalization at the tube ends is preferred.At the same time, additional functionalization on the sidewalls of CNTs (withoutdisturbing the nanotube structure) can increase bonding or linking of nanotubesin a nanotube-matrix composite material (Meyyappan 2005). Once the CNTsare functionalized, the tube ends or the sidewalls are terminated with differentfunctional groups such as carboxylic, carbonyl, or phenol (Meyyappan 2005).A good dispersion is achieved by using high-power ultrasound mixers and differentsurfactants on CNTs during the production of the new composites (Ajayan & Zhou2001).

Conclusions

In summary, CNTs have a high aspect ratio, high tensile strength, low mass density,high heat conductivity, large surface area, and versatile electronic behavior. Due tothese very interesting properties, CNTs are successfully used in various applicationssuch as production of new and improved composites. Even though there are stillmany challenges to resolve when synthesizing CNT-composite materials, CNTs stillremain an excellent candidate to alter and improve the mechanical and electricalproperties of the new nanocomposites. Chemical vapor deposition, unlike arcdischarge or laser ablation, can be scaled up for large-scale and high-quality CNTproduction at a relatively low cost. The excitement in this field arises due to theversatility of CNTs and their potential for various new applications.

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carbon nanotubes: synthesis, properties, and applications

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