Advanced Carbon Materials and Technology (Tiwari/Advanced) || Carbon Nanotubes and Their Applications

173

Ashutosh Tiwari and S.K. Shukla (eds.) Advanced Carbon Materials and Technology, (173–192) 2014 © Scrivener Publishing LLC

5

Carbon Nanotubes and Their Applications

Mohan Raja1,* and J. Subha2

1Amity Institute of Nanotechnology, Amity University, Noida, India2Central Institute of Plastics Engineering & Technology (CIPET), Bhopal, India

AbstractCarbon nanotubes (CNTs), which consist of rolled graphene sheets built from sp2-hybridized carbon atoms, are nowadays attracting scientists from various disciplines due to their attractive physical and chemical proper-ties. In this account, we will describe the recent progress in the advance of synthetic techniques for the large-scale production of carbon nanotubes, purifi cation, and chemical modifi cation that are enabling the integration of CNTs in thin-fi lm electronics and large-area coatings. Although providing strong mechanical strength in polymer composites and electrical and ther-mal conductivities for many electronics applications, CNTs sheets have already shown promising performance for use in applications including supercapacitors, actuators, lightweight electromagnetic shields,in bio-medical areas ranging from biosensing, etc. We predict that carbon nano-tubes will fi nd numerous applications and continue to take an important place in the development of emerging technologies in the near future.

Keywords: Carbon nanotubes, synthesis, composite, microelectronics, coatings, energy storage, biosensing

5.1 Introduction

Elemental carbon can form a diversity of remarkable structures. Apart from the renowned graphite, carbon can construct closed and open cages with honeycomb atomic arrangement. Kroto et al [1]

*Corresponding author: [email protected]

174 Advanced Carbon Materials and Technology

discovered the structure of C60 molecules. Carbon nanotubes can be thought of as rolled-up graphene sheets with no overlapping edges and were fi rst isolated by Iijima in 1991 [2]. Their diameters typically vary from 1 to 100 nm and their lengths can be several orders of magnitude larger, up to millimeters, even centimeters long [3]. The nanotubes consist of several types such as single-wall carbon nano-tubes (SWCNTs) [4], and multi-wall carbon nanotubes (MWCNTs) [2]. The beginning of widespread CNTs research in the early 1990s was preceded in the 1980s by the fi rst industrial synthesis of what are now known as MCWNTs, and documented observations of hollow carbon nanofi bers as early as the 1950s.

However, CNTs-related commercial activity has grown most substantially during the past decade. Since 2006, worldwide CNTs production capacity has increased at least ten-fold, and the annual number of CNTs-related journal publications and issued patents con-tinues to grow. Most CNTs production today is used in bulk composite materials and thin fi lms, which rely on unorganized CNTs architec-tures having limited properties. Organized architectures of CNTs such as vertically aligned forms and sheets show promise to scale up the properties of individual CNTs and also to realize new functional-ities. However, presently realized mechanical, thermal and electrical properties of CNTs macrostructures such as yarns and sheets remain signifi cantly lower than those of individual CNTs. Worldwide com-mercial interest in carbon nanotubes (CNTs) is refl ected in a produc-tion capacity that presently exceeds several thousand tons per year. Currently, bulk CNTs powders are incorporated in diverse commer-cial products ranging from rechargeable batteries, automotive parts, boat hulls, sporting goods, water fi lters and antimicrobial coatings. Advances in CNTs synthesis, purifi cation, and chemical modifi cation are enabling integration of CNTs in thin-fi lm electronics, large-area coatings and biosensors, etc. Although not yet providing compelling mechanical strength or electrical or thermal conductivities for many applications, CNTs yarns and sheets already show promising perfor-mance for use in applications including supercapacitors, actuators, and lightweight electromagnetic shields.

5.2 Carbon Nanotubes Structure

The binding in carbon nanotubes is sp2 with each carbon atom joined to three neighbor carbon atoms, as in graphite. Graphite

Carbon Nanotubes and Their Applications 175

has a sheet-like structure where carbon atoms all lie at the corners of hexagons in a plane and are only weakly bonded to the graph-ite sheets above and below with 0.34 nm of interlayer distance. Carbon nanotubes can be considered as rolled-up graphene sheets (graphene is the term to describe an individual graphite layer) [5]. Three types of nanotubes are possible, namely armchair, zig-zag and chiral nanotubes, depending on how the two-dimensional graphene sheet is rolled up, as shown in Figure 5.1. The primary symmetry classifi cation of a carbon nanotube is either achiral or chiral. Both armchair and zigzag nanotubes are achiral since their mirror image has an identical structure to the original one. Chiral nanotubes exhibit a spiral symmetry whose mirror image cannot be superimposed on the original one.

The different structure can be most easily explained in terms of the unit cell of carbon nanotubes in Figure 5.2. The so-called chiral vector of the nanotube, Ch, is defi ned by Ch = na1 + ma2, where a1 and a2 are unit vectors in the two-dimensional hexagonal lattice, and n and m are integers.

Another important parameter is the chiral angle, which is the angle between Ch and a1. An armchair nanotube corresponds to the case of m = n and a zigzag nanotube corresponds to the case of m = 0. All other (n, m) chiral vectors correspond to chiral nanotubes.

Armchair (5, 5)

Zigzag (9, 0)

Chiral (10, 5)

Figure 5.1 Carbon nanotube structures.


The diameter of carbon nanotube, Dt, can be calculated by L/π, in which L is the circumference of the carbon nanotube;

2 2

t

LD a n m nm

p= = + +

(5.1)

Where the lattice constant a = 0.249 nm. For example, the diameter of the zigzag nanotube (9, 0) in the Figure 5.1 is Dt = 0.882 nm.

The chiral angle θ denotes the tilt angle of the hexagons with respect to the direction of carbon nanotube axis. It can be calculated by the following equation:

3tan

2m

n mq =

+ (5.2)

5.3 Carbon Nanotube Physical Properties

The special nature of carbon combines with the molecular perfec-tion of nanotube structures to endow them with exceptional mate-rial properties, such as very high strength, stiffness, toughness, and electrical and thermal conductivity. There is no other element in the periodic table that bonds to itself in an extended network struc-ture with the strength of the carbon-carbon bond. The delocalized π-electron donated by each carbon atom is free to move along the complete structure, moderately, than stays with its donor atom,

Figure 5.2 Carbon nanotube lattice.


giving rise to the fi rst identifi ed molecule with metallic-type elec-trical conductivity. In addition, the high-frequency carbon-carbon bond vibrations provide an intrinsic thermal conductivity higher than even diamond. Carbon nanotubes represent a mixture of mol-ecules with various diameters, length and chirality.

The reported Young’s modulus of CNTs is in the order of 1–1.2 TPa, tensile strength in the order of 36GPa at a failure strain of 6% [6,7]. However, there are discrepancies in the values of reported ten-sile modulus and strength for CNTs as a result of different testing methods, calculations and system errors. Electronic conductivity of CNTs was predicted to depend sensitively on tube diameter and chiral angle, with only a slight difference in one parameter causing the change from a metallic to semiconductor state [8–10]

5.4 Carbon Nanotube Synthesis and Processing

In order to use carbon nanotubes in many devices, it is essential to produce these materials with a high crystallinity economically on a large scale. In this context, the catalytic chemical vapor deposition (CVD) method is considered optimum for producing large amounts of carbon nanotubes, mostly with the use of a fl oating-catalyst method [11–13]. This technique is more controllable and cost effi -cient when compared with arc-discharge and laser ablation meth-ods [14–17]. Various groups have been able to produce SWCNTs for laboratory experiments [18,19]. More recently, we reported an alter-native route for the large-scale synthesis of SWCNTs by combining the use of catalytic substrates and fl oating methods [20]. The tem-plate (substrate) prevents metal particle aggregation, thus result-ing in the formation of high-purity SWCNTs. The method is able to produce either individual nanotubes or nanotube bundles, exhibit-ing a wide range of diameters (0.4–4 nm). In particular, nanosized zeolites were impregnated with Fe-containing compounds (seed-ing method) and placed inside a furnace (ca. 1000°C) together with vapor of benzene, and H2 as the carrier gas. The resulting material was purifi ed via immersion in hydrofl uoric acid (HF).

Recently, some new synthetic methods were introduced for the production of carbon nanotubes at low temperature [21,22] along with the aligned growth of single-walled carbon nanotubes (SWCNTs) [23] which opened a possibility for the cost-effective syn-thesis of carbon nanotubes with controlled structures for practical applications. The CNTs were produced from the mixture solution


of dichlorobenzene along with ZnCl2 particles, which act as catalyst and nucleation site for CNTs growth. This solution is sonicated under ambient condition using ultrasonic water bath [24]. Alternatively, the synthesis of aligned CNTs that can be processed without the need for dispersion in a liquid offers the promise for cost-effective realization of compelling large properties. These methods include self-aligned growth of horizontal [25] and vertical [26] CNTs on sub-strates coated with catalyst particles, and production of CNTs sheets and yarns directly from fl oating-catalyst CVD systems [27]. CNTs forests can be manipulated into dense solids [28], aligned thin fi lms [29], and intricate three-dimensional (3D) microarchitectures [30], and can be directly spun or drawn into long yarns and sheets [31,32].

5.5 Carbon Nanotube Surface Modifi cation

Carbon nanotubes are observed in bundles due to the substantial van der Walls attraction. In order to manipulate the methods for CNTs, it is attractive to functionalize the sidewall of CNTs, thereby generating CNTs derivatives that are compatible with solvent as well as organic matrix materials. Both surface modifi cation tech-niques and non-covalent wrapping methods have been reported [33]. In chemical surface modifi cation, functional groups are cova-lently linked to the CNTs surface; this is also referred to as the covalent functionalization method [34] Based on the reaction, two approaches have been explored: the fi rst approach involves direct attaching of functional groups to the graphitic surface, and in the second approach, the functional groups are linked to the CNTs-bound carboxylic acids, which are created during the CNTs synthe-sis, or during post treatment of CNTs for the purifi cation purpose.

These carboxylic acids are considered the defect sites on the CNTs’ surface and the method is also known as “defect chemistry” [35,36]. The advantage of the chemical functionalization method is that the functional groups are covalently linked on the CNTs sur-face; the linkage is permanent and mechanically stable. However, reaction with the graphitic sheet also results in breaking of the sp2 conformation of the carbon atom. The conjugation of the CNTs wall is therefore disrupted, and it was observed that the electrical and mechanical properties of the chemically functionalized CNTs decreased dramatically as compared to the pristine tubes [37,38]. The surface of single-wall carbon nanotubes was modifi ed by the


addition of photoinitiator under UV irradiation with hexylamine. The effects of the CNTs surface modifi cation by ionic functionaliza-tion enhance their degree of dispersion in polymer matrices [39]. The larger surface area of CNTs can be used as templates to prepare nanoparticulate hybrid systems consisting of silver and copper nanoparticles for metal-functionalized CNTs [40].

5.6 Applications of Carbon Nanotubes

5.6.1 Composite Materials

Carbon nanotubes were initially used as electrically conductive fi llers in plastics, due to the attractive benefi t of their high aspect ratio to form a percolation network at concentrations as low as 0.01 weight percent (wt%). Disordered CNTs/polymer composites reach conductivities as high as 10,000 S m–1 at 10 wt% loading [41]. The inclusion of CNTs into polymer holds the potential to improve the mechanical, electrical, or thermal properties compared to tradi-tional fi llers [42], and signifi cant efforts have gone into fabricating CNTs/polymer composites for high performance [43–45] and mul-tifunction [46–50]. It is known that the homogeneous dispersion of CNTs in polymer matrix is diffi cult due to the tendency for forma-tion of CNTs bundles. The chemical functionalization of CNTs is considered as an effective way to achieve homogenous dispersion of CNTs in polymer matrices [51–53]. Surfactants have also been used to improve the dispersion and strengthen the interactions between CNTs and polymer matrix [54]. Several methods have been devel-oped to prepare CNTs/polymer composites, for example, solution casting [55–58], melt mixing [36], and in situ polymerization [59,60]. In the automotive industries, CNTs-fi lled plastics have enabled electrostatic-assisted painting of mirror housings, in addition to fuel lines and fi lters that dissipate electrostatic charge. Further products consist of electromagnetic interference (EMI)-shielding packages and wafer carriers for the microelectronics industry. For load- bearing applications, CNTs powders mixed with polymers or precursor resins can enhance stiffness, strength, and toughness [61]. Adding ~1 wt% MWCNTs to epoxy resin improves stiffness and fracture toughness by 6 and 23%, respectively, without com-promising other mechanical properties [62]. These enhancements depend on CNTs diameter, aspect ratio, dispersion, alignment and


interfacial interaction with the matrix. Many manufacturers of CNTs sell premixed resins and master batches with CNTs loadings from 0.1 to 20 wt%. Additionally, engineering nanoscale stick-slip among CNTs and CNTs/polymer contacts can increase material damping [63], which is used to enhance sporting goods, such as baseball bats, tennis racquets and bicycle frames (Figure 5.3).

Carbon nanotube resins are also used to enhance fi ber composites [64]. The latest good examples such as strong, lightweight wind tur-bine blades and hulls for maritime security boats are made by using carbon fi ber composite with CNTs-enhanced resin (Figure 5.4) and wind turbine composite blades. The CNTs can also be deployed as additives in the organic precursors used to form carbon fi bers.

Figure 5.3 CNTs composite bicycle frame (Photo courtesy of BMC Switzerla nd AG).

Figure 5.4 Carbon fi ber laminate with CNTs dispersed in the epoxy resin (inset), and a lightweight CNT-fi ber composite boat hull for maritime security boats. (Images courtesy of Zyvex Technologies).


The carbon nanotubes infl uence the arrangement of carbon in the pyrolyzed fi ber, making possible the fabrication of 1-mm diam-eter carbon fi bers with over 35% increase in strength (4.5 GPa) and stiffness (463 GPa) compared with control samples without CNTs [65]. The electroactive shape memory behavior was observed in polymer nanocomposites based on PU/M-CNT nanocomposites prepared using polyurethane and metal nanoparticles-decorated MWCNTs through the melt mixing process [66]. The mechanical properties such as Young’s modulus and yield stress increased signifi cantly for PU/metal nanoparticles-decorated CNT com-posites in comparison to the pristine CNT nanocomposites. The PU/M-CNT composites have excellent shape recoverability com-pared to PU/pristine CNT composites. The PU/metal-decorated CNTs showed high strain recovery ability after several cycles of training compared to PU/pristine carbon nanotube composites (Figure 5.5).

10

10

8

6

4

2

00 20 40 60 80

Strain, %100 120

8

(a) (b)

(c)

6

4 1 1

22

5

12

5

52

00 20 40 60

Strain, %

Str

ess,

MP

a

Str

ess,

MP

a

PU/P-CNT-5%

80 100 120

10

8

6

4

2

00 20 40 60

Strain, %

Str

ess,

MP

a

PU/Cu-CNT-5%

PU/Ag-CNT-5%

80 100 120

Figure 5.5 Stress vs strain plots of (a) PU/pristine CNT nanocomposites, (b) PU/Cu-CNT nanocomposites, and (c) PU/Ag-CNT nanocomposites. (Reprinted with permission from Elsevier).


Besides polymer composites, the addition of small amounts of CNTs to metals has provided amplifi ed tensile strength and mod-ulus [67] that may fi nd application in aerospace and automotive structures. Commercial Al-CNTs composites have strengths com-parable to stainless steel (0.7 to 1 GPa) at one-third the density (2.6 g cm–3). This strength is also comparable to Al-Li alloys, yet the Al-CNTs composites are reportedly less expensive

Carbon nanotubes can also be used as a fl ame retardant addi-tive in plastics; this effect is mainly predictable to changes in rheol-ogy by CNTs loading [68]. These CNTs additives are commercially smart as alternatives to halogenated fl ame retardants, which have restricted use due to environmental regulations. Last, DuPont intends to launch new CNTs-based materials for armor that improve the durability, strength and performance of body and vehicle armor and helmets (Figure 5.6). Additionally, this technology platform will enable DuPont to provide new materials for other industrial uses such as ropes and cables that offer strong and durable support for off-shore oil and gas platforms.

5.6.2 Nano Coatings – Antimicrobials and Microelectronics

Carbon nanotubes are promising as a multifunctional coating material. For example, the metal nanoparticle-decorated CNTs hybrid systems can be synthesized using CNTs consisting of car-boxyl groups in the backbone, which can bind ions of transition metals easily (Ag+ and Cu2+). These ions are well known for their

Figure 5.6 Carbon nanotubes ballistic coupon. (Image courtesy of DuPont).


broad-spectrum antimicrobial activity against bacterial and fungal agents together with their lack of cross resistance with antibiotics [40]. The MWCNTs-containing paints reduce biofouling of ship hulls (Figure 5.7) by discouraging attachment of algae and barna-cles [69]. They are a possible alternative to environmentally hazard-ous biocide-containing paints. Inclusion of CNTs in anticorrosion coatings for metals can enhance coating stiffness and strength while providing an electric pathway for cathodic protection.

Widespread development continues on CNTs-based fl exible transparent conducting fi lms [70] as an alternative to indium tin oxide (ITO). A concern is that ITO is becoming more expensive because of the shortage of indium, compounded by mounting demand for displays, touchscreen devices, and photovoltaics. Besides cost, the fl exibility of CNTs transparent conductors is a major advantage over brittle ITO coatings for fl exible displays. Further, transparent CNTs conductors can be deposited from solution (e.g., slot-die coating, ultrasonic spraying) and patterned by cost-effective nonlithographic methods (e.g., screen print-ing, microplotting). The latest profi table development effort has resulted in CNTs fi lms with 90% transparency and a sheet resis-tivity of 100 ohm per square (Figure 5.8). This surface resistiv-ity is adequate for some applications but still substantially higher than for equally transparent, optimally doped ITO coatings [71]. Related applications that have less stringent requirements include CNTs thin-fi lm heaters, such as for use in defrosting

Figure 5.7 Antifouling “Green Ocean Coating Heavy Duty” coatings used on the hull of a ship (Image courtesy of Bayer Material Science).


automobile windows or sidewalks. All of the above coatings are being employed industrially.

In recent years, the nanothickness CNTs fi lms used are transpar-ent, fl exible and stretchable, can be tailored into many shapes and sizes, and are freestanding or placed on a variety of rigid or fl ex-ible insulating surfaces. A piece of carbon nanotube (CNTs) thin fi lm can be a practical magnet-free loudspeaker simply by apply-ing an audio frequency current through it (Figure 5.9). This CNTs fi lm loudspeaker can produce sound with wide frequency range, high sound pressure level and low total harmonic distortion [72]. The CNTs thin-fi lm transistors (TFTs) are particularly attractive for driving organic light-emitting diode (OLED) displays, for the rea-son that they have shown higher mobility than amorphous silicon (~1 cm2 V−1 s−1) (56) and can be deposited by low temperature, non-vacuum methods. Recently, fl exible CNTs TFTs with a mobility of 35 cm2 V–1 s–1 and an on/off ratio of 6 × 106 were demonstrated (Fig. 5.9a) [73].

5.6.3 Biosensors

Sensors are devices that detect a change in physical quantity or event. There are many studies that have reported the use of CNTs-based pressure, fl ow, thermal, gas, chemical and biological sensors. Carbon nanotubes can be used as fl ow sensors [74,75]. The fl ow of a liquid on bundles of SWCNTs induces a voltage in the direction of

Figure 5.8 Carbon nanotubes fl exible transparent conducting fi lm. (Image courtesy of Printed Electronics World).


the fl ow. In the future, this fi nding can be used in micro machines that work in a fl uid environment, such as heart pacemakers, that need neither heavy battery packs nor recharging [74]. Piezoresistive pressure sensors can be made with the help of CNTs. Single-wall carbon nanotubes were grown on suspended square polysili-con membranes [76]. When uniform pressure was applied to the membranes, a change in resistance in the SWCNTs was observed. According to Caldwell et al. [77], fabrication of piezoresistive pres-sure sensors that incorporate CNTs can bring dramatic changes to the biomedical industry, as many piezoresistance-based diagnostic and therapeutic devices are currently in use there. The CNTs-based nanobiosensors may be used to detect deoxyribonucleic acid (DNA) sequences in the body [78,79]. These instruments detect a very spe-cifi c piece of DNA that may be related to a particular disease [80]. Such sensors enable detection of only a few DNA molecules that contain specifi c sequences, and thus possibly diagnose patients as having specifi c sequences related to a cancer gene. Biosensors can also be used for glucose sensing. Carbon nanotube chemical sensors for liquids can be used for blood analysis, for example, to detect sodium or fi nd pH value [81].

5.6.4 Energy Storages

Carbon nanotubes are being conceived for energy production and storage. Graphite, carbonaceous-based electrodes have been used for decades in fuel cells, batteries and several electrochemical

(a) (b)

Figure 5.9 Carbon nanotubes thin fi lm loudspeakers. (a) A4 paper size CNTs thin fi lm loudspeaker . (b) The cylindrical cage shape CNTs thin fi lm loudspeaker can emit sounds in all directions; diameter 9 cm, height 8.5 cm. (Reprinted by permission from the American Chemical Society).


storage applications (Figure 5.10) [82]. Carbon nanotubes are spe-cial because they have small dimensions, perfect surface speci-fi city and smooth surface topology, since only the basal graphite planes are exposed in their structure. The rate of electron transfer in carbon-based electrodes ultimately determines the effi ciency of fuel cells and this depends on various factors, such as the struc-tural morphology of the carbon-based material used as electrodes. Pure MWCNTs and MWCNTs deposited with metal catalysts (Pd, Pt, Ag) have been used to electrocatalyze an oxygen reduction reac-tion, which is important for fuel cells [82]. It is seen from several studies that nanotubes could be excellent replacements for conven-tional carbon-based electrodes. Similarly, the improved selectivity of carbon nanotube-based catalysts has been demonstrated in het-erogeneous catalysis. Ru-supported carbon nanotubes were found to be superior to the same metal on graphite and on other carbons in the liquid phase hydrogenation reaction of cinnamaldehyde [83]. The properties of catalytically grown carbon nanofi bers (which are basically imperfect carbon nanotubes) have been found to be desir-able for high power electrochemical capacitors [84].

Porous, high-surfacearea carbon

SeparatorMetal-foilelectrode

Electrolyte

Separator

Separator

Porous carbon electrode

Current flowduring

charging

Figure 5.10 Concept for CNTs-based supercapacitors. (Image courtesy of YEG Ultracapacitors).


5.7 Conclusion

This chapter has described several possible applications of carbon nanotubes, with an emphasis on materials science and electronics applications. What we would like to express through this chapter is that the unique structure, topology and dimensions of carbon nanotubes have created an excellent carbon material, which can be considered as the most perfect fi ber that has ever been fabricated. The extraordinary physical and chemical properties of carbon nanotubes create a host of application possibilities, some derived as an extension of traditional carbon fi ber applications, but many are new possibilities, based on the novel electronic and mechanical behavior of carbon nanotubes. It needs to be said that the excite-ment in this fi eld arises due to the versatility of this material and the possibility to predict properties based on its well-defi ned per-fect crystal lattice. Nanotubes truly bridge the gap between the molecular realm and the macro world, and are destined to be a star in future technology. According to reports, many companies are investing in diverse applications of CNTs such as transparent conductors, thermal interfaces, antiballistic vests, and wind turbine blades. However, often few technical details are released, and com-panies are likely to keep technical details hidden for a very long time after commercialization, which makes it challenging to pre-dict market success. Hence, the increases in carbon nanotube pro-duction capacity and sales are an especially important metric for emerging CNTs applications.

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Advanced Carbon Materials and Technology (Tiwari/Advanced) || Carbon Nanotubes and Their Applications