Theoretical and experimentally measured properties of carbon nanotubes (Schadler 2004).
Albeit initially reported by a Russian group and reported again by Oberlin and co-workers (Oberlin et al., 1976
2. Carbon nanotubes
CNT is one of carbon allotropes such as diamond, graphite, graphene, fullerene and amorphous carbon. But, it is the one-dimensional carbon form which can have an aspect ratio greater than 1000 makes it interesting (Figure 1). The bonding in carbon nanotubes is sp², with each atom joined to three neighbours, as in graphite. CNTs are rolled-up graphene sheets (graphene is an individual graphite layer). This type of structural bonding, which is stronger than the
The properties of nanotubes depend on atomic arrangement (on how the sheets of graphite are ‘rolled’), the diameter and length of the tubes, and morphology, or nano structure. Single walled nanotubes (SWNTs) consist of a single sheet of graphene rolled seamlessly to form a cylinder with diameter of order of 1 nm and length of up to centimeters (Bethune et al., 1993; Ijima et al., 1993), Multi-walled nanotubes (MWNTs) consist of an array of such cylinders formed concentrically and separated by 0.35 nm, similar to the basal plane separation in graphite (Ijima 1991). MWNTs can have diameters from 2 to 100 nm and lengths of tens of microns. Apart from these two main types, there was also the few-walled carbon nanotube (FWNTs) that was reported and found to have peculiar properties, often with much better structural integrity (Hutchinson et al. 2001).
SWNTs are formed by rolling a sheet of graphene into a cylinder along an (
It is clear that CNTs have many advantages over other materials in terms of mechanical, electrical and thermal properties although there is no consensus on the exact properties of CNTs (Table 1). Theoretical and experimental results have shown unusual mechanical properties with Young’ modulus as high as 1.2 TPa and tensile strength of 50–200 GPa (Qian et al.; 2002). The relatively high values of modulus and strength values have been measured on high quality SWNT and arc discharge MWNT. However, CVD-MWNT was measured to have a modulus of 0.45 TPa and tensile strength of ~4 GPa (Xie et al.; 2000). The much larger variation in modulus for CVD-MWNT compared to arc-MWNT strongly suggests that the modulus is very sensitive to defect concentration and type.
Depending on the relationship between this axial direction and the unit vectors describing the hexagonal lattice, CNTs can either be metallic or semi-metallic. Semi-metallic nanotubes have band gaps that scale inversely with diameter, ranging from approximately 1.8 eV for very small diameter nanotubes to 0.18 eV for the widest possible stable SWNT (Elliot et al.; 2004). In addition, carrier mobility as high as 105 cm2/Vs have been observed in semi-conducting nanotubes (Kim et al.; 2004). Superconductivity has also been observed in SWNTs at the transition temperatures of 5 K (Tang et al.; 2001). CNTs are also very conductive for phonons. Theory predicts a room temperature thermal conductivity of up to 6000 W/m K (Che et al.; 2000 & Osman et al.; 2001) but, the values around 200 W/m K have been measured (Hone et al.; 2002). These properties offer CNTs great potential for wide applications in field emission, conducting plastics, thermal conductors, energy storage, conductive adhesives, thermal interface materials, structural materials, fibers, catalyst supports, biological applications, air and water filtration, ceramics to name a few (Thostenson et al. 2001; Ajayan et al.; 2003; Coleman et al.; 2006 ).
|Lattice structure||(Cylindrical) hexagonal lattice helicity|
Nanotubes: ropes, tubes arranged in triangular lattice with lattice parameters of a =1.7 nm, tube-tube distance = 0.314
|Planar hexagonal, plane-to-plane distance c=0.335|
|Specific gravity||0.8-1.8 gcc-1 (theoretical)||2.26 gcc-1|
|Elastic modulus||~1 TPa for SWNT|
~0.3-1 TPa for MWNT
|1 TPa (in-plane)|
|Strength||50-500 GPa for SWNT|
10-50 GPa for MWNT
|Resistivity||~5-50 micro-ohm·cm||50 (in-plane)|
|Thermal conductivity||3000 W m-1K-1 (theoretical)||3000 Wm-1K-1 (in-plane)|
6 Wm-1K-1 (c axis)
|Thermal expansion||Negligible (theoretical)||-1 × 10-6 K-1 (in-plane)|
29 × 10-6K-1 (c axis)
|Oxidation in air||"/700º C||450-650º C|
The synthesis methods for CNTs include arc-discharge (Journet et al. 1997), laser ablation (Rinzler et al. 1998), gas-phase catalytic growth from carbon monoxide (Nikolaev et al. 1999) and chemical vapor deposition (CVD) from hydrocarbons (Ren et al 1999& 2007). The arc-discharge and laser ablation are well established in producing high-quality and nearly perfect nanotube structures. But, due to their scale-up limitations the production cost is too hight hence limiting its access. During the synthesis process, impurities in the form of catalyst particles, amorphous carbon and non-tubular fullerenes are also produced. Thus, subsequent purification steps are required to separate the tubes. The gas-phase catalytic growth with carbon monoxide as the carbon source yielded in quantity high-purity single walled nanotubes at the highest accessible temperature and pressure (1200º C, 10 atm). However, this method is not economically practical due to high temperature and pressure of the synthesis conditions. The CVD process tended to tend nanotubes with fewer impurities and was more amenable to large-scale processing. Therefore, the CVD method has the highest potential to produce a large quantity and high purity of CNTs.
3. Functionalization of carbon nanotubes
The main problem with the majority of popular synthetic methods is that they produce samples yielding a mixture of various diameters and chiralities of nanotubes that are normally contaminated with metallic and amorphous impurities. Thus, post-synthesis chemical processing protocols (Strano et al.; 2004 & Li et al.; 2002) that purify tubes that can also separate individual tubes according to diameter and chirality by taking advantage of their differential reactivities are often the only viable routes to rational and predictable manipulation of the favorable electronic and mechanical properties of these materials (Niyogi et al.; 2002& Tasis et al.; 2003).
The full potential of CNTs as reinforcements has been severely limited due to poor interfacial interaction, van der Waals interaction, between CNTs and polymer matrix. The nature of dispersion problem for CNTs is rather different from other conventional fillers, such as spherical particles and carbon fibers, because CNTs are characteristic of small diameter in nanometer scale with high aspect ratio (>1000) and thus possessing large surface area. In addition, the commercialized CNTs are supplied in the form of heavily entangled bundles, resulting in inherent difficulties in dispersion.
To resolve those problems, it has been directed towards developing methods to modify surface properties of CNTs. These approaches can be simply divided into chemical (covalent) and physical (noncovalent) functionalization as interactions between active materials and CNTs. The characterization corresponding to these methods are summarized in Figure 4.
3.1. Covalent functionalization
The end caps of nanotubes tend to be composed of highly curved fullerene-like hemispheres, which are therefore highly reactive, as compared with the side walls (Hirsch 2002& Sinnott 2002). The sidewalls themselves contain defect sites such as pentagon-heptagon pairs called Stone-Walls defects,
Chemical functionalization is based on the covalent bond of functional groups onto carbon form of CNTs. It can be performed at the end caps of nanotubes or at their sidewalls which have many defects. Direct covalent sidewall functionalization is associated with a change of hybridization from
Another method is defect functionalization of CNTs. These intrinsic defects are supplemented by oxidative damage to the nanotube framework by strong acids which leave holes functionalized with oxygenated functional groups (Chen et al.; 1998). In particular, treatment of CNTs with strong acid such as HNO3, H2SO4 or a mixture of them (Esumi et al.; 1996& Liu et al.; 1998), or with strong oxidants such as KMnO4 (Yu et al.; 1998), ozone (Sham et al.; 2006), reactive plasma (Wang et al.; 2009) tend to open these tubes and to subsequently generate oxygenated functional groups such as carboxylic acid, ketone, alcohol and ester groups, that serve to tether many different types of chemical moieties onto the ends and defect sites of these tubes. These functional groups have rich chemistry and the CNTs can be used as precursors for further chemical reactions, such as silanation (Ma et al.; 2006), polymer grafting Sano et al. 2001& Kong et al. 2003), esterification (Hamon et al. 2002), thiolation (Liu et al.; Science 1998), and even some biomolecules ( Coleman et al.; 2006 ). The CNTs functionalized by the covalent methods has good advantage which that was soluble in various organic solvents because the CNTs possess many functional groups such as polar or non-polar groups.
However these methods have two major drawbacks. First, during the functionalization reaction, especially along with damaging ultrasonication process (Figure 7), a large number of defects are inevitably created on the CNT sidewalls, and in some extreme cases, CNTs are fragmented into smaller pieces (Figure 8). Namely, the carbon hybridization of CNTs was changed from
3.2. Non-covalent functionalization
The advantage of non-covalent functionalization is that it does not destroy the conjugated system of the CNTs sidewalls, and therefore it does not affect the final structural properties of the material. The non-covalent functionalization is an alternative method for tuning the interfacial properties of nanotubes. The CNTs are functionalized non-covalently by aromatic compounds, surfactants, and polymers, employing
Aromatic molecules, such as pyrene, porphyrin, and their derivatives, can and do interact with the sidewalls of CNTs by means of
Polymers, especially conjugated polymers, have been shown to serve as excellent wrapping materials for the non-covalent functionalization of CNTs as a result of
In addition, surfactants polymers have also been employed to functionalize CNTs (Figure 11). The physical adsorption of surfactant on the CNTs surface lowered the surface tension of CNTs that effectively prevented the aggregation of CNTs. Furthermore, the surfactant-treated CNTs overcame the van der Waals attraction by electrostatic/steric repulsive forces. The efficiency of this method depended strongly on the properties of surfactants, medium chemistry and polymer matrix. The relation between surfactants and CNTs studied previously as followed: (i) non-ionic surfactants, such as polyoxyethylene 8 lauryl or C12EO8 (Gong et al.; 2000), polyoxyethylene octylphenylether (Triton X-100) (Vaisman et al.; 2006), (ii) anionic surfactants, such as sodium dodecylsulfate (SDS), sodium dodecylbenzenesulfonate (NaDDBS), poly(styrene sulfate) (PSS) (Islam et al.; 2003& Yu et al.; 2007) (iii) cationic surfactants, such as dodecyl tri-methyl ammoniumbromide (DTAB) (Whitsitt et al.; 2003), cetyltrimethylammounium 4-vinylbenzoate (Kim et al.; 2007). Although surfactants may be efficient in the solubilization of CNTs, they are known to be permeable plasma membranes. They are toxic for biological applications. Therefore, the use of surfactant-stabilized CNTs complexes is potentially limited for biomedical applications (Klumpp et al.; 2006).
The solubilization of CNTs with biological components is certainly more appropriate towards integration of this new type of material with living systems (Figure 12). The biomacromolecules for non-covalent functionalization of CNTs has included simple saccharides and polysaccharides (Barone et al.; 2006; Star et al.; 2002; Chambers et al.; 2003&
Ikeda et al.; 2007), proteins ( Chen et al.; 2001 ), enzymes, DNA, etc. Various biomaterials such as
3.3. Alternative routes for the functionalization of carbon nanotubes
Though various methods for enhancing the interaction between CNTs and polymers were adopted in the recent past, the two main routes; namely covalent and non-covalent functionalization generally provided. As discussed earlier, both methods had their own merits depending on the platform. Additionally, the traditional covalent functionalization strategy of CNT is most frequently initiated by chemical acid oxidation acid treatment. However, dramatic amounts of induced defects during functionalization hinder the intrinsic mobility of carriers along CNTs, which is not preferred in any case. This method not only functionalizes the nanotube surfaces with carboxylic acid groups but leaves behind detrimental structures, hence hampering their potential for practical applications and can also compromise the mechanical properties of the nanotubes. Therefore, as a common rule, and a now widespread approach to alleviate these problems is to find alternative routes such as an effective functionalization method that can not only introduce high density and homogenous surface functional groups, which enhance the compatibility between CNTs and the foreign matrix, but allow direct grafting and has little or no structural damage to the CNTs, thus, optimizing their properties for various applications.
To overcome this challenge, Baek et al. (2005; 2004 ; 2006; 2007; 2010) have reported an efficient route to covalently functionalize CNTs via direct Friedel-Crafts acylation technique (Figure 13). This kind of covalent grafting of the nanotubes is a promising strategy to not only improve nanotube dispersion but also provide a means for creating microscopic interlinks. On the whole, this kind of surface functionalization not only enhances the reactivity, but also improves the specificity and provides an avenue for further chemical modification of CNTs. Considerable achievements have been made in enhancing the various functionalities of CNT-polymer nanocomposites, generally not achievable for each of the components individually. The approach is conceptualized on the basis of our foray into the CNT chemistry using ‘direct’ Friedel-Crafts acylation technique which has superior operational simplicity. Not only a mild and an alternative route to functionalize CNTs, this strategy was previously shown to be a less-destructive and/or nondestructive reaction condition for the efficient dispersion and functionalization of carbon nanomaterials. As a result, CNT damage from severe chemical treatments including oxidation and sonication can be avoided to a larger extent. Thus, maximum enhanced properties can be expected from improved dispersion stability as well as chemical affinity with matrices.
4. Conclusion and outlook
In light of the continuous progress of nanotechnology and material science over the last two decades, CNT based materials have opened new pathways for developing novel functional materials. In summary, CNTs were always been regarded as new and interesting type of materials, especially for energy and electronic applications. All types of CNTs are being investigated with equal importance. While few strategies have been developed so that the handling and manipulation of CNTs be relatively easier, on the whole there is much room for investigations in areas such as supercapacitors. Very recent reports show CNT based supercapacitor devices with better capacitor performance. Precisely, the combination of CNTs with particular macromolecules that offer to enhance the conductivity of the material is one interesting research. Particularly, the large electrochemical window and the environmental stability draw critical importance on such materials. Furthermore, the extension of these functional methods to the 2D forms of carbon namely graphene based materials are also now a fast growing area. While the quest for new materials are always on, the research on CNT based materials in special fields such as doping are still open for investigations and discussion.
This research work was supported by World Class University (WCU), US-Korea NBIT, and Mid-Career Researcher (MCR) programs through the National Research Foundation (NRF) of Korea funded by the Ministry of Education, Science and Technology (MEST) and the U.S. Air Force Office of Scientific Research (AFOSR).