**Table 1.** *A summary of electrochemical exfoliation and anodic oxidation of graphite.*

#### *Electrochemical Exfoliation of 2D Advanced Carbon Derivatives DOI: http://dx.doi.org/10.5772/intechopen.94892*

and further intercalation also occurred positively, thereby, also water decomposition and sonication steps were necessarily taken, achieving complete exfoliation (**Figure 13**) [44]. Liu et al. and Huang et al. [108, 109] have presented much effort, for accelerating the intercalation kinetics, by using molten (LiOH or LiCl) at 600°C. However, the intercalation was considered there so insufficient to be achieved perfect graphitic exfoliation, but sonication steps thus fallowed were still required to achieve remarkable production of graphene-based materials.

Swager and Zhong [78] suggested a synergetic method to be intercalated the graphite primarily with Li+ , by following tetra-alkyl-ammonium cations into two steps separately. Moreover, due to expanded nature of the cathode, the distance between electrodes was kept initially very large, exploring the high potential difference required to apply, to dominate the high Ohmic-drop, created by the electrolytic cell configuration. Resultantly, the organic electrolytic solvent was dissociated in that state, occurring later on, at all the stages of the procedure by disappearing slowly during intercalation process. That is why, additional steps were rendered through sonication mechanism again, need to be sufficient for achieving reasonable exfoliation proceedings.

Dimethylsulfoxide (DMSO) has shown a wide electrochemical window that is highly efficient solvent during the graphene solvent dispersion, reflecting typical dispersive qualities, by comparing those of NMP [110]. As a result; DMSO forms various solvated ions, containing both lithium and alkylammonium ions reasonably. The observed solvated ions are expected to be able to intercalate with graphite, via decomposition between the graphene layers making SO2 and/or along with amine-based apparent gases. The stress applied properly on the graphene sheets through the gaseous expulsion so occurred is evaluated enough to overcome the forces (van der Waals) that attracting the neighboring sheets, allowing separation of graphene sheets formed by the graphitic cathode, thereby, allowing dispersal occurring in the electrolytic solution. The authors of the literature [83] have applied the said principle to make many flakes, showing lateral dimension (upto 20 μm) of few-layer graphene towards DMSO-based electrolytic solution, containing triethylammonium and Lithium ions. Authors have adopted fashioned of electrochemical program, by applying a controlled Cathodic-potential towards the graphitic electrode, which presents complete intercalation prior to flakes formation spontaneously, so that exfoliation from the Cathodic end because of partial expansion occur consequently. It was greatly suggested that the triethylammonium ions, dissociated between the layers, give rise to triethylamine along with hydrogen gases, highly encouraging the exfoliation of flakes more prominently.

**Figure 12.** *Schematic and images of cathodic electrochemical expansion of graphite.*

*Electrochemical Exfoliation of 2D Advanced Carbon Derivatives DOI: http://dx.doi.org/10.5772/intechopen.94892*

#### **Figure 13.**

*(a) TEM images and electron diffraction pattern of cathodic exfoliated graphene, (b) electron diffraction patterns of (i) single and (ii) bilayer sheets, (c) AFM image of exfoliated graphene spin-coated onto a Si substrate. The thickness is* ∼*1.5 nm, corresponding to a bilayer. (d) (left) Raman spectra (532 nm laser) on Si substrates compared with the spectrum of graphite; (right) Lorentzian peak fitting of the 2D bands of the bilayer and trilayer [44].*

#### **Figure 14.**

*(a) SEM image, (b) AFM image of graphene flakes deposited on Si substrate, (c) TEM image, and (d) HR-TEM image of a graphene flake. The inset is an electron diffraction pattern and magnified portion of the edge of the graphene flake [111].*

#### **Figure 15.**

*(a) Photographs of as prepared HOPG, (b) HOPG expansion after 1000 s tetraethylammonium cation intercalation, (c) HOPG expansion after 1000 s tetrabutylammonium cation (TBA+ ) intercalation, (a–c scale in mm) (d) HOPG expansion after 10,000 s TBA<sup>+</sup> intercalation, (e) SEM image of HOPG expansion after 6000 s TBA+ intercalation, (f) SEM image showing micron-sized pores in HOPG after TBA+ intercalation, (g) SEM image showing selective exfoliation of HOPG electrode: The point on a HOPG electrode that was held by tweezers (left-hand side) whilst the rest of the electrode (right-hand side) was submerged [57].*

Zhou et al. [111] have efficiently presented, so far, the only familiar method followed to exfoliate graphitic cathodes into aqueous medium deliberately, using an electrolyte containing NaCl, DMSO, and thionin acetate salt. Sodium ions were chemically combined with (four or five) DMSO molecules, readily forming Na+ / DMSO complex-composite. Complexes so obtained were still intercalated in the form of graphene-galleries owing to graphite, clearly forming ternary graphiticintercalation compounds (Na<sup>+</sup> (DMSO)yCn− ). Further, interlayer spacing was systematically reported to be 1.246 nm, accordingly. However, perfect exfoliation was rather not achieved through only electrochemical-treatment, therefore the sample was necessarily subjected to sonication process in order to achieve more stable graphene dispersions (**Figure 14**). In addition, however, samples were observed as heavily contaminated (with sulfur, oxygen, and nitrogen impurities).

Cooper et al. have deliberately shown tetraalkylammonium salts to be cathodic intercalation into HOPG by using relatively low potentials (ca. −2 V) [112] and maybe systematically employed to produce purely cathodic-exfoliated materials, consisting clearly (2 or 5 layers) of graphenes (see **Figure 15**) [57]. More significantly, the materials were certainly found containing (no functionality or oxidation), rather inclusion of slightly 1% in atomic form oxygen, probably induced from the atmospheric exposure of the so obtained isolated materials.

Further, Yang et al. [113] employed a pure ionic-liquid, N-butyl, methylpyrrolidinium bis (trifluoromethylsulfonyl)-imide (BMP TF2N) towards cathodic-graphitic intercalation/exfoliation mechanism. In authors' view, [BMP]+ cations chemically intercalated between the highly negatively charged (graphene layers), causing the expanded interlayer spacing. The aforesaid expansion facilitates the bigger molecules insertion, such as the BMPTF2N ion-pair, subsequently, caused by higher expansion in graphite as well. The authors have certainly claimed that formation of graphene sheets was consisted of between (two and five layers), with 2.5% atomic-oxygen yet free defected materials. However, the authors, not suggested a reasonable explanation for the gel-like-phase, probably formed from the ionic- liquid during which (the cations or anions) are expected to be consumed in all irreversible reactions [114, 115].

## **4. Conclusions**

The process of electrochemical exfoliation has been confirmed to operate in a wide variety of layered materials; the majority of studies are conducted on largesized bulk single-crystals, which are costly and inefficient for industrial applications. Small-sized powders or flakes are readily produced from natural materials or industrial synthesis should be considered as an alternative for efficient and successive exfoliation. Both aqueous and non-aqueous electrolytes are employed to exfoliate layered materials, but the procedure is more often used in aqueous solutions and under anodic conditions for the exfoliation of graphite owing to better performance relative to cathodic scheme, in this technique most reliable and effective way is Li-ion insertion. Around the same time, a deeper understanding of process/mechanism of intercalation and exfoliation of powered by application of current is desperately required, which may encourage the use of electrochemical means to exfoliate more effectively a large number of layered materials.

#### **Conflict of interest**

Authors have declared no 'conflict of interest'.

## **Author details**

Muhammad Ikram1 \*, Ali Raza<sup>2</sup> , Sarfraz Ali2 and Salamat Ali<sup>2</sup>

1 Solar Cell Applications Research Lab, Department of Physics, Government College University Lahore, Lahore, Punjab, Pakistan

2 Department of Physics, Riphah Institute of Computing and Applied Sciences (RICAS), Riphah International University, Lahore, Pakistan

\*Address all correspondence to: dr.muhammadikram@gcu.edu.pk

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 6**

## Carbon Nanotubes

*Muhammad Ikram, Ali Raza, Atif Shahbaz, Haleema Ijaz, Sarfraz Ali, Ali Haider, Muhammad Tayyab Hussain, Junaid Haider, Arslan Ahmed Rafi and Salamat Ali*

#### **Abstract**

Carbon nanotubes (CNTs) are referred to as carbon nano-architecture allotropes, with wrapped graphene sheets forming a cylindrical structure. CNTs are either developed by metals or narrow-band semiconductors with rolling graphene sheets in various ways. Researchers have dedicated a great deal of attention to understanding the fascinating properties of CNTs over the years, and possess certain peculiar properties, such as a high degree of stiffness, a wide ratio of length to diameter, and remarkable toughness, and are employed in a number of applications. These properties can be enhanced by scheming the diameter, nature of walls, chirality, length of CNTs which is rolled up, and depending on the synthesis process. This chapter extensively covers the various properties of CNTs and how it influences to desired applications and also explains numerous methods of synthesis and processing of CNTs with advantages and some drawbacks.

**Keywords:** 2D materials, graphene oxide, carbon nanotube, arc discharge, laser ablation

#### **1. Introduction**

In recent decades, formation of nanowires and nanotubes has an attractive literature which emphasizes towards material growth. Amongst numerous materials containing organic and inorganic, nanotubes show versatile properties due to promising candidates such like carbon nanotubes, contributing a great part in potential applications relevant to disciplinary medicinal chemistry [1, 2]. Foundation of fullerenes [3] was extracted from carbon nanotubes (CNTs) that explore fabrication on a macroscopic level, thereby exhibiting continuous evolution [4]. The cylindrical shape of CNT is caused by rolling up of graphitic sheets; length is measured in micrometer scale while maximum diameter is taken as 100 nm. CNT also appears in bundle shape to form prominently complex nature structure [5]. Hexagon rings are in arranged form on which metallic nature or semiconducting behavior of CNT is evaluated. CNT belongs to the properties towards robust applications like fillers; bio-sensors are amongst nanotechnological pillars in exciting fields [6, 7]. However, some limitations such as insolubility and non-manipulation in solvents play role for creating hindrance to CNT use as solute in organic solvents as well as aqueous media. Dispersion of CNT may be carried out through sonication; however, precipitation is also occurred caused by the interruption of the process followed by the mechanism. Moreover, numerous studies also showed that CNT might react with a variety of chemical compounds [8–17].

Innovative nanodevices are greatly desired in research work and it may be met only by CNT best fabricating processing that is obtained by the synthesis of complex nature composites [18–20]. Furthermore, CNTs become highly reliable when chemical reactions are carried out to incorporate them in soluble activities into different systems such as organic or inorganic and biological accordingly. Thus, CNTs solubility approach in chemical reactions opens new routes for introducing promising materials [21, 22]. Unidirectional CNT structures may be prepared by modified approaches and their structural study is done by following group study containing three categories, first is that various chemical groups are incorporated on the surface of CNTs via covalent bonding, secondly non-covalent wrapping of functional groups and thirdly endohedral fulfillment of cavity. Many citations in this study have been appreciated due to which it is rapidly increasing by worth in literature, while this review presents a limited approach providing useful information in all citations followed in this study [23–26]. It has been systematically studied that CNTs may be prepared by employing synthesis methods containing arc discharge approach or chemical-vapor-deposition and laser-ablation technique [27, 28].

In arc discharge approach temperature is kept greater than 3000°C. This temperature is indispensable to evaporate carbon atoms to form a plasma state, in this way CNTs are shaped as single-walled as well as multi-walled structures. In this process, catalytic agent may or may not be involved during the formation of multi-walled carbon nanotubes (MWCNTs). However, inclusion of catalytic agent is mandatory to create individual single-walled carbon nanotubes (SWCNTs). The catalytic agents like Cobalt, Nickel, and Iron may be used as mandatory steps to complete the reactions reasonably [29–32]. In chemical-vapor-deposition (CVD) approach methane, ethylene, etc. are incorporated as hydrocarbon sources necessary to carry out reactions successfully. As far as laser-ablation approach is concerned, evaporation process of graphite occurs in a furnace at a temperature of 1200°C. Moreover, graphite appears as dominant material to produce species with converting ratio at maximum level. Moreover, biomaterial targets are achieved depending on degree of purity level, that is why macroscopic approach is carried out for the improved quality of carbon materials owing to achieve some characteristics like length and alignment [33]. It has been reported that MWCNTs were collected first time by Iijima (by employing arc-discharge approach), and this approach is too old that was adopted for carbon fibers synthesis [34, 35]. Subsequently, an in-situ emulsion of polymerization was presented by Khan et al. [36] in 2016 to synthesize carbon nanotubes (CNTs) in the form of composites, which was completed by employing a colloidal system to fabricate nanostructured brush.

#### **2. Classification of carbon nanotubes**

Nanotubes may be categorized into SWCNTs as well as MWCNTs (see **Figure 1**). A comparison between both SWCNT and MWCNT is demonstrated in **Table 1** [38, 41].

#### **3. Structure and morphology**

SWCNT comprised of carbon atoms from graphene sheet containing benzene rings in hexagonal shape as illustrated in **Figure 2a**. Cylindrical graphene sheets comprising honeycomb lattice are visualized in single-atomic graphitic-layer of crystalline nature, while MWCNT is in stacked form of graphene sheets that are rolled up into cylinders having same centers. The composition of nanotube

#### **Figure 1.**

*Molecular representations of SWCNT (top left) and MWCNT (top right) with typical transmission electron micrographs below [37].*


#### **Table 1.**

*Comparison between SWCNT and MWCNT [38–40].*

molecules contains a million atoms having length of tens micrometers and diameters are comparable with 0.7 nm value [41]. SWCNTs containing 10 atoms often lie along the circumference of tube-like structure with one-atom-thick thickness. A length to diameter ratio of carbon nanotubes is measured about 1000 (large aspect ratio), giving rise to be considered as unidirectional structures [43]. MWCNTs structure is formed by various single-walled tubes that are stacked in concentric cylinders inside each other. The MWCNTs are identified as nanostructures showing the outer diameter is (15 nm or less) while structures having a diameter more than 15 nm are considered as nanofibers, not nanotubes. CNTs are different from carbon-fibers owing to not a single (molecule) yet strand layers sheets of graphitic nature [43–46].

Depending upon the two aforesaid basic structures, carbon nanotubes may be categorized into three varieties as an armchair, zigzag, and chiral carbon nanotubes.

**Figure 2.**

*(a) Unrolled single-layer graphene sheet showing the geometry of the SWCNT, (b-d) Examples of the three types of nanotube sidewall; zigzag, armchair, and chiral (A color version of this figure can be viewed online) [42].*

The structure of a variety of carbon nanotubes depends on the formation of rolled up graphitic cylinders during synthesis process. The main focus is selection of rolling-axis relative to hexagonal graphitic network of sheets as well as radius of closing cylindrical network of nanotubes that are raised in various types of SWCNTs. In this structure, chiral vector contains and indices corresponding to two unit vectors directing along two-axis in graphene crystal lattice structure. In case of m = 0 zigzag-type nanotube, but when n = m the armchair nanotube is obtained while other configurations are attributed to chiral type nanotubes accordingly. In addition, SWCNTs with armchair structure, zigzag, and chiral structures have been illustrated in **Figure 2b**–**d**. Moreover, a further detailed structure may be visualized in literature reviews [7, 43, 47, 48].

#### **4. Properties**

Mechanical properties may drastically be raised, caused by the electrostatic forces between sp2 carbon–carbon-bonds. Previously no material has been yet found to display the collective mechanical, electronic, and thermal properties up till now. Densities of materials have been observed below 1.3 g/cm3 value (one-sixth stainless steel). Young's moduli measured material stiffness that was greater than 1 TPa and is considered approximately 5x higher than that of stainless steel [49, 50]. However, uniqueness of materials still depends upon strength that makes them apart from others. Furthermore, carbon nanotubes are those materials that showed the strongest stiffness in the history of mankind. The tensile strength of carbon nanotubes measured so far is up to 63 GPa that is considered about 50 times greater than that of stainless steel [51]. However, carbon nanotubes that are identified as the weakest one show only several GPa strength [52]. As far as chemical, environmental stability, thermal conductivity etc. are compared to diamond. Owing to such attractive properties along with lightness of carbon nanotubes opens new routes towards variety of applications particularly in the field of aerospace [40, 53–55].

Carbon nanotubes highly exhibit electronic properties as compared to other materials. On comparing with copper carbon nanotubes show an extraordinary electrical conductivity. The most notable fact here is metallic as well as semiconducting nature of carbon nanotubes. The rolled-up structure comes forward to break up symmetric shape of the planar system. In this way, different directions are observed attributing to hexagonal lattice of carbon material and also axial direction is disturbed. Axial direction and unit vectors describe hexagonal lattice,

#### *Carbon Nanotubes DOI: http://dx.doi.org/10.5772/intechopen.95442*

therefore, depending on electrical properties carbon nanotubes may have nature of metal or semiconducting material. Amongst other nanotubes, semiconducting nanotubes may have band gap inversely with diameter. Band gap range was found between (1.8–0.18 eV) relative to small diameter tubes as well as very wide SWCNT respectively [56, 57]. Consequently, various nanotubes may belong to higher conductivity as compared to copper metal, while some others relative to silicon have a more conducting nature. There is a still promising interest in fabrication of nanoscale electronic devices by active use of nanotubes. Various areas of technology need carbon nanotubes to prepare advanced materials. Thus carbon nanotubes are already frequently used in those areas of research. Some outcomes of nanotubes are flat-panel displays, fuel cells, scanning probe microscopes, and sensing fabricated devices [58].

#### **4.1 Optical properties**

Electronic properties owing to SWNTs have been theoretically studied in early decades. SWNTs may be predicted metallic or semiconductors based on parameters that are followed in structure formation of the nanotubes [35]. As far as metallic and semiconducting nature of nanotubes is concerned, one third belongs to metallic whereas two-third relates to semiconducting nanotubes concerning selected indices (n, m). The aforesaid model is identified as π tight-binding model related to zonefolding scheme. Tight-binding data is based on (σ and π) bands that produce the curvature of σ and π bands. This bending behavior indicates a very small gap lying between metallic and semiconducting nanotubes [59, 60].

#### **4.2 Electrical properties**

Electrical properties of carbon nanotubes show electrical transport impact that becomes an interesting area of various possible applications attributing to fabricate electronic devices at nanoscale basis. Nanotubes are classified as one-dimensional conductor owing to which attractive microscopic phenomena are observed at low temperatures. Phenomena are likewise single-electron charging, superconductivity, and resonant tunneling. On the other hand, high temperature based tunneling conductance expresses power-law suppression that is evaluated as a function of (temperature and bias voltage) consistent with one-dimensional Luttinger liquid. Scattering mechanism is raised by optical or zone-boundary-phonons in metal-like nanotubes. Scattering along with coherent-backscattering phenomena has resulted in the form of low-temperature phenomena. Probe measurements were two-type as well as four-type in transport experiments performed with respect to MWNTs [61], isolated SWNTs, and SWNT bundles respectively [62, 63].

Initially, electrical resistance was measured towards unique MWNT below T = 20 mK, Langer et al. determined [61], whereas magnetic field shows a logarithmic conductance trend at declining temperature whereas saturation level was identified at the temperature below T ~ 0.3 K. However, when magnetic field impact was measured and found perpendicular towards tube axis, at that time magneto-resistance measurements were also observed. Furthermore, temperature effect on conductance in magnetic field was also observed that was found inconsistent with two-dimensional weak-localization.

#### **4.3 Vibrational properties**

Atomic-vibrations into carbon nanotubes were successfully evaluated by employing force-constant models (zone-folding-approximation) [64], also for concrete structure of nanotubes [65], ranging (tight-binding-models) [66–69] and finally, ab-initio models were also observed [70]. To measure vibrational eigenfrequencies, experiments were performed by using light resonant Raman scattering in case of laser-light-energy when energy measurements are very close to available electronic transitions. Resonance limitations are entirely different for all types of nanotubes; therefore Raman spectroscopy presents results to display various nanotubes structures that exist in the nanotube specimen. Currently, Raman spectroscopy measured parallel polarized light relevant to MWNTs [71], SWNTs [72, 73] and cross-polarized-light on isolated SWNTs [74].

#### **4.4 Thermal properties**

Phonons were used to measure specific heat as well as thermal conductivity of carbon nanotube systems. When temperature was kept low enough, acoustic phonons were observed indicating dominant role of phonon contribution in the nanotube systems. Linear specific heat measurements and thermal conductivity yield at or above 1 K but below room-temperature [75, 76], whereas 0.62 T specific heat identifies temperature at or below 1 K [77]. Linear temperature was evaluated depending on linear k-vector and modes of vibration of acoustic phonons such as longitudinal and twist like vibrations [78]. Transverse acoustic phonons are relative to specific heat exhibits dependence behavior attributing to specific heat at or below 1 K along with quadratic k-vector trend [79]. Thermoelectric measurement power (TEMP) for nanotube systems presents active and direct information about carrier types along with conductivity mechanisms [80–83].

#### **5. Synthesis of CNTs**

High-quality carbon nanotubes are considered to be superior quality materials and proved to be main pillar towards promising and versatile applications, various synthesis routes are employed to achieve feasible application of CNTs as described in **Figure 3**. Superior quality indicates that density of structural defects is significantly less over length scale between 1 to 10 microns along tube-axes. Carbon nanotubes synthesis is rapidly increasing in research field but still, challenges are prevailing. Those challenges are required to resolve with respect to synthesis of CNT. The main challenges are of four types regarding nanotube synthesis [84]. First is mass-production scale, containing low-cost based synthesis with large-scale synthetic routes to produce high-quality SWCNTs nanotubes. Second is a selective production scale that raises control over structural defects and changes electronic properties relevant to produced nanotubes. Third is Organization level regarding control over location along with specific orientation towards produced nanotubes on specific substrate. Fourth is mechanism level that presents all procedures followed during growth of nanotubes in synthetic processes. But growth mechanism is considered still controversial because alternative mechanisms may be employed during fabrication of CNTs [27, 85, 86].

Different techniques have been systematically employed to develop and produce SWNTs as well as MWNTs showing various structural and morphological characters in laboratory quantities. Methods commonly followed are three in number to synthesize CNTs, first is arc discharge [87, 88], second is laser ablation [66, 89] and third is chemical vapor deposition [67–69, 90, 91]. Catalysts are considered basic elements that are selected as source of carbon towards nanotubes formation, having sufficient energy. A significant feature of all methods followed for CNTs

**Figure 3.** *Currently used methods for CNTs synthesis [84].*

fabrication is to enhance energy for carbon source producing fragments of carbon atoms that may recombine to yield SWNTs or MWNTs. The main goal is source of energy that is electricity and heat from an arc discharge and CVD respectively or high-intensity-light for laser ablation.

#### **5.1 Arc discharge and laser vaporization**

Amongst various methods that were allowed regarding SWNTs synthesis, arc-discharge or laser-ablation methods contributed relatively on large-scale basis (**Figure 4**). Subsequently, carbons atoms in a gaseous state are condensed caused by evaporation process of solid-state carbon atoms [92]. While growing single-wallnanotubes (SWCNTs) in arc-discharge system, metallic catalyst is mandatorily required to incorporate for speed-up desired chemical reactions [93]. On the other hand, superior-quality (SWCNTs) are successfully fabricated (1–10 g scale) by using a laser oven approach [94]. Besides aforementioned method wave, CO2-laser system was also employed regarding industrial-scale production of SWCNTs [95]. However, costly equipment as well as high energy consumption requirement makes them unfavorable approaches towards production of nanotube materials. Through employing arc ablation or laser methods only powder type specimens of carbon materials into bundle-shape form are controllably produced. The most common characteristic relevant to arc-discharge and laser-ablation approaches indicates higher energy need to induce carbon atoms to rearrange forming CNTs. Favorable temperature is prominently 3000 <sup>∘</sup> C (or higher value) that is considered more beneficial for fine crystallization growth of CNTs at this level since products are obtained with attractive graphite-alignment. Moreover basic needs of the systems such as vacuum-conditions, repeated graphite-target substitution create barriers towards production of CNTs on an industrial scale [96].

#### **Figure 4.**

*Schematic diagram showing the Arc discharge method [40].*

#### **5.2 Chemical vapor deposition (CVD)**

CVD approach presents carbon compounds decomposition in gaseous state where metallic nanoparticles are used as catalysts resulting in nucleation sites available for initial growth of carbon nanotubes (CNTs). Main drawback found in previous both methods was lack of large-scale fabrication of carbon materials, but CVD approach has presented preferred route towards carbon nanotubes production at large scale [97–99]. In this work, carbon is extracted from hydrocarbon source or some other carbon generating source. These chemical reactions are only successfully performed by using catalysts at or below 1200 <sup>∘</sup> C temperature. Resultantly, CNT structure involved parameters like wall number, length, alignment, and diameter that have proven controllable CVD process. In addition, CVD approach has greater scope and advantages over other methods showing mild operation with low cost and selective process. The previous twelve years period describe that various approaches have presented promising industrialscale synthesis of carbon nanotubes. All approaches indicate that CVD methods are main pillars of large scale production of nanotubes. Among various methods, main approaches are five in numbers that have proven to be successful large-scale yield [100].


#### *Carbon Nanotubes DOI: http://dx.doi.org/10.5772/intechopen.95442*

was considered an additional advancement in previously employed CO-CVD approach [105]. This approach incorporates Co/Mo bimetallic catalyst along with fluidized CVD-reactor during production of SWCNTs at large scale. The main benefit that arises from controllable use of fluidized CVD reactors was that they stop continuous addition as well as removal of solid-like particles during operation without stopping reactor work.


#### **5.3 Laser ablation method**

Both laser ablation and arc discharge approaches have the same principles with similar mechanisms. However, they are not similar with respect to energy sources that are adopted to complete reactions. A laser is main source of desired energy for laser ablation method and **Figure 5** is showing schematic experimental setup. The schematic structure contains quartz tube with graphitic block. Graphite block is heated at 1200°C temperature by using high-power-laser whereas metal particles are incorporated catalysts [110]. Argon gas is controllably used in stream form during reaction process. Graphite lying in quartz is systematically vaporized by functioning of laser. Argon present in chamber removes vapors of carbon by condensation process towards downstream cooler quartzwalls. Condensation process is completed in the presence of both SWCNTs and metallic nanoparticles (see **Figure 5**). Literature reveals that laser power may

**Figure 5.** *Schematic structure showing the laser ablation method.*

strongly affect CNTs diameter. On increasing laser pulse power rate a very thin diameter carbon nanotube is collected [40]. On other hand, some other reports give more information in favor of laser pulses that they behave like great potential, owing to be capable to provide SWCNTs in large amount [89, 111]. Reports indicate pure and superior quality production of SWCNTs in this case. Curved graphene sheets are observed showing position of carbon atoms in condense phase state caused by set up created by metal-catalyst to fabricate condensed carbon nanotubes.

In this case, carbon atoms rearrange them for formation of ring shape and in this way, electronegative properties become dominant to play role in preventing open edge from sealing [110]. Furthermore, there are main benefits relative to this method that indicate metallic impurities less in amount but high in yield owing to vaporization tendency creating at tube end of metallic atoms at closing position. However main drawback of technique is observed with respect to synthesis aspect of nanotubes that they are not regularly straight rather indicate degree of branching to some extent. In addition this technique involves high-quality graphitic rods with high-power laser rate. However, in this case, CNTs are produced but not greater than arc-discharge technique. Carbon-2019 for "PEER REVIEW" describes high-power-laser when metal particles are incorporated as mandatory catalysts in reaction process [110]. Argon gaseous stream is continuously used during reaction mechanism. Graphitic quartz is passed through vaporization process using a laser, argon media captures carbon vapors that result in condensed downstream towards cooler-walls of quartz but still SWCNTs with metallic-particles are located in condensation process. Laser power may also clearly affect CNTs diameter. Furthermore, diameter becomes comparatively narrowed on increasing laser pulse rate [30]. Other studies reported that ultrafast laser pulses are of great potential, and are capable to produce larger quantities of SWCNTs [112]. SWCNTs collected by this technique are observed owing to high-purity and superior-quality in nature. Location sites where carbons atoms initiate condensation process may set up curved shape graphene sheet along with metal-catalyst atoms. In this way condensed nanotubes are properly obtained showing peculiar properties. Moreover, carbon atoms merge to form specific rings, thereby raising electronegative properties relative to metallic atoms that become capable to prevent open-edge from closing [113].

The main benefit belongs to followed method, in this case, metallic impurities are observed relatively less in amount but with high yield that is caused by vapors formation tendency belonging to metallic atoms from tube end when closed once in a time. However main drawback relative to this technique indicates irregularity in straight shape for synthesized nanotubes whereas degree of branching occurs to some extent. Furthermore, pure graphitic rods are involved in this procedure along with high laser power rate. Resultantly production of CNTs was not in great amount as compared to arc- discharge method.

#### **6. Conclusions**

Carbon nanotubes have the ability to be more investigated, and it is possible to drive further advancements by using CNTs in different fields. The findings obtained in the synthesis, functionalization, and structure of CNTs have contributed significantly to promising developments in various fields. However, further perfections in synthesis protocols are needed to obtain highly durable CNTs for preferred applications. For an instant, catalyst size is directly influenced on diameter of CNT during CVD reaction. So, further analysis should also be undertaken

*Carbon Nanotubes DOI: http://dx.doi.org/10.5772/intechopen.95442*

to discover more effective methods of processing precisely uniform-sized catalyst particles in order to ensure the production desired diameter of SWCNTs; but CNTs are costly than other carbon nanomaterials. Efforts should be proceeded to look for modern, cost-effective, and plentiful carbon sources, so that cost of CNTs can be lowered to an acceptable amount.

### **Author details**

Muhammad Ikram1 \*, Ali Raza<sup>2</sup> , Atif Shahbaz1 , Haleema Ijaz3 , Sarfraz Ali2 , Ali Haider4 , Muhammad Tayyab Hussain2 , Junaid Haider<sup>5</sup> , Arslan Ahmed Rafi2 and Salamat Ali<sup>2</sup>

1 Department of Physics, Government College University Lahore, Punjab, Pakistan

2 Department of Physics, Riphah Institute of Computing and Applied Sciences (RICAS), Riphah International University, Lahore, Pakistan

3 Department of Allied and Health Sciences, Superior College University Campus, Lahore, Pakistan

4 Department of Clinical Medicine and Surgery, University of Veterinary and Animal Sciences, Lahore, Punjab, Pakistan

5 Tianjin Institute of Industrial Biotechnology, Chinese Academy of Sciences, Tianjin, China

\*Address all correspondence to: dr.muhammadikram@gcu.edu.pk

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## *Edited by Mujtaba Ikram and Asghari Maqsood*

Advanced carbon materials such as graphene, fullerenes, hierarchical carbon, and carbon nanotubes (CNTs) have exceptional physical properties, making them useful for several applications in fields ranging from energy and industry to electronics and drug delivery. This book includes comprehensive information on fabrication, emerging physical properties, and technological applications of advanced carbon materials. Over three sections, chapters cover such topics as advanced carbon materials in engineering, conjugation of graphene with other 2D materials, fabrication of CNTs and their use in tissue engineering and orthopaedics, and advanced carbon materials for sustainable applications, among others.

Published in London, UK © 2021 IntechOpen © undefined / iStock

21st Century Advanced Carbon Materials for Engineering Applications -

A Comprehensive Handbook

21st Century Advanced

Carbon Materials for

Engineering Applications

A Comprehensive Handbook

*Edited by Mujtaba Ikram and Asghari Maqsood*