Carbon nanotubes: Synthesis, characterization, and applications

Carbon nanotubes: Synthesis, characterization, and applications

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Abd El-Moez A. Mohameda,b, Mohamed A. Mohamedc a School of Metallurgy and Materials, University of Birmingham, Birmingham, United Kingdom, bDepartment of Physics, Faculty of Science, University of Sohag, Sohag, Egypt, c Plant Pathology Research Institute, Agricultural Research Center (ARC), Giza, Egypt

1 Introduction The carbon atom has six electrons distributed among 1s, 2s, and 2p orbitals. The 1s electrons are tightly bound, forming the core while the 2s and 2p electrons are the valence electrons. The wave function of 2s and 2p electrons overlaps due to the small energy difference between these orbitals than the chemical bond. This is known as hybridization (Saito et al., 1998). The type of hybridization is determined according to the number of overlapped p orbitals, which are sp1, sp2, and sp3. In the sp1 type, the 2s orbital hybrids with one p orbital with 180 degree. In sp2, the 2s is hybridized with two of p orbitals in the same plane with 1200. In sp3, the 2s is hybridized with three p orbitals separated by 109.50 forming a tetrahedral bond shape. Carbon atom shows different allotropes diamond 3D structure and carbon nanotubes (1D structure). Carbon nanotubes (CNTs) are a hexagonal rolled-up layer of graphene, forming a hollow seamless cylindrical tubular structure; the carbon atoms are connected via sp2 bonds. According to the number of these rolled graphene sheets, CNTs can be classified into single-walled carbon nanotubes or multiwalled carbon nanotubes (SWCNTs and MWCNTs) (see Fig. 1). In MWCNTs, layers are separated by a small distance due to the van der Waals force. CNTs show superior properties such as a specific strength that is higher than steel in addition to being lightweight with high elasticity. According to their various properties, CNTs are good candidates for different applications. For example, the large surface area and aspect ratio are ideal for biomedical applications such as drug and enzyme delivery. Thermal stability, high thermal conductivity, and large current capacity properties are good for interconnect applications. In this chapter, we focus on the physical and chemical synthesis methods of carbon CNTs, their characterizations, and their applications.

1.1  Electrical and thermal properties The graphene unit cell contains two carbon atoms with an even number of electrons. Therefore, it can be a semiconductor or a metal. However, the conductivity nature depends on the degree of graphene sheet twist. The graphene sheet conductivity is calculated via the σ = (2e2/h)·(2πleE/hν) equation where le is the scattering length and h is the Plank constant. Experimentally, the one-dimensional nature of Carbon Nanomaterials for Agri-food and Environmental Applications. https://doi.org/10.1016/B978-0-12-819786-8.00002-5 © 2020 Elsevier Inc. All rights reserved.

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Carbon Nanomaterials for Agri-food and Environmental Applications

Fig. 1  Graphene sheet, single-walled CNT, and multiwalled CNT.

an SWCNT makes it hard to measure the electrical resistance due to the additional contact because of the mismatch between the external wire contact and the tube (Datta, 1995). According to the Landauer formula and considering the perfect contact, the intrinsic resistance of SCNT is 10−4Ohm−1 cm−1 at 27°C. On the other hand, MWCNT conductivity is complex due to the interwall interactions. When a magnetic field is applied, the band gap oscillates with the increase in magnetic field intensity, so the metallic CNT will be a semiconductor first, then go back to metallic behavior (Kittel, 1976). Diamond and graphene show high thermal conductivity, which is controlled via phonons (Dresselhaus et  al., 2008). The MWCNTs show the same thermal conductivity behavior of bulk graphene, in contrast with SWCNTs. This difference in SWCNTs is attributed to the optical and acoustic phonons. The expected thermal conductivity by theoretical works is 6600 W mK−1 (Berber et al., 2000). However, the measured value was >200 W mK−1, which is very close to the good metal value. Unlike several materials, the coherence length in graphene is <1000 A, and the phonon free path is controlled via boundary scattering above 140 K. Also, the ­defect-free CNTs are suggested to be in long lengths that exceed several microns, which is longer than the crystallite diameter. In addition, the thermal expansion of CNTs is a little bit different from graphene. It has been shown that the thermal radial expansion coefficient of MWCNTs will be identical to the on-axis thermal expansion coefficient, despite the fact that they are separated by similar distances to the interplanar separation in graphene (Ruoff, 1992). This is because the nanotube sheet is wrapped onto itself, therefore the radial expansion is governed entirely by the carbon covalent bonding network; the van der Waals interaction between nested cylinders is, therefore, incidental to the radial thermal expansion. We, therefore, expect that the thermal coefficient of expansion will be isotropic in a defect-free SWNT or MWNT.

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1.2  Mechanical properties The strain energy of nanotubes can be predicted using the elastic properties of graphene sheets (Tersoff, 1992). The elastic strain energy that results from simple calculations based on continuum elastic deformation of a planar sheet compares very favorably with the more sophisticated ab initio results. The results have been confirmed by the ab initio calculations of Mintmire et al. (1994). This suggests that the mechanical properties of nanotubes can be predicted with some confidence from the known properties of single crystal graphene. In defect-free CNTs, the stiffness constant of SWCNTs can be calculated using the elastic modulus of graphite. Moreover, in spite of theoretical studies that predict a high tensile strength and a Young’s modulus that reaches 1 TPa for one CNT, the experimental results have shown results far from this predicted value, which refers to the existing defects in CNTs (Hyung et al., 2017). Hyung et al. (2017) designed an accurate technique of tensile testing for MWCNTs, where they found a tensile strength and Young’s modulus value of 0.85 and 34.65 GPa, respectively.

2  Synthesis methods 2.1  Arc discharge It is a traditional and common method in CNT production where carbon rods are put between the cathode and anode at a very close distance surrounded by an inert gas. Then, a high dc electric current is applied, leading to a high temperature resulting from the discharge. This evaporates the rod surface, forming small rods; however, the plasma arc uniformity controls the CNT formation (see Fig. 2).

Fig. 2  Schematic design for the arc discharge technique.

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Carbon Nanomaterials for Agri-food and Environmental Applications

2.2  Laser ablation This is a similar way of arc discharge, although here the laser is used in carbon evaporation (see Fig. 3), where the pulsed and contentious laser is used to heat the graphene surface. A graphene rod with nickel and cobalt (as a catalyst) is put in an oven at 1200°C in an inert gas atmosphere. Two pulses are used to break down the larger particles and produce CNTs. The dimension of the produced CNTs can vary according to the catalyst composition and temperature. In addition, it was found that a carbon dioxide laser could be used in producing SWCNTs, where it was found that the increase in laser power increases the CNT diameter. Also, there is a thermal synthesis process method that is similar to laser ablation and arc discharge. However, the only difference is that in this method, only heat is used without exceeding 1200°C. There are disadvantages to laser ablation and arc discharge. In spite of the high CNT quality produced by these two methods, they have some disadvantages that limit their use: 1. The large bulk graphene target needed for the process. 2. The huge energy needed in these processes to produce CNTs. 3. The necessary purification process for the CNTs to get rid of unwanted carbon and catalysts.

3  Ball milling An annealing process should follow this method to ensure the CNT formation. Graphene powder is put in an evacuated container with stainless steel balls filled with an inert gas for a long time, then this powder is annealed at 1400°C for 6 h. This method is effective in MWCNTs more than SWCNTs.

3.1  Chemical vapor deposition The advantage of this method is that it can produce a large number of CNTs with controlling the growth direction on a substrate (Monthioux and Kuznetsov, 2006). A hot substrate of 700–900°C is needed with hydrocarbon gas and process gas such as nitrogen. Due to the hydrocarbon gas decomposition, CNTs grow on the substrate that works as a catalyst. In fact, the substrate type is an important factor in the preparation processes, as it determines the type of the obtained CNTs. The porous silicon substrate

Fig. 3  Schematic plot of laser ablation system.

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is the best choice for growing self-oriented CNTs. In more details, CNTs are grown on the substrate and implanted with nanoparticles such as Fe, Ni, or Co. Afterward, the hydrocarbon will decompose and deposit on the substrate and carbon will connect to the metal particles in the holes, creating CNTs.

4  Flame method This method has a great potential to produce large amounts of CNTs at a low cost. Three main components are needed for this method: a metallic catalyst particle, a heat source, and a source of carbon. The flame parameters can be tuned to obtain different sizes of CNTs, where Lee et al. (2004) found that flame temperature has a great effect on CNT fabrication and alignment. SWCNT flame fabrication is not easy because it requires a gas phase catalyst. However, they can be formed in special conditions using a premixed acetylene, oxygen, and argon flame at 50 Torr with Fe vapor pentacarbonyl as the catalyst.

4.1  Saline solution method A carbon paper substrate containing electric current for heating is submerged in a Co:Ni saline solution catalyst at a 1:1 ratio, then a carbon gas source is passed over the substrate. This leads to an interaction between the gas and the catalyst, forming CNTs on the substrate (Bhaskar et al., 2013).

4.2  Spray pyrolysis method This is a modified technique from the CVD method to fabricate high-purity MWCNTs (Gul et al., 2019). The samples are mounted on a stainless substrate in a tube furnace, then an inert gas is introduced. When the furnace temperature reaches 170°C, the inert gas flow rate is increased. The mixture of ferrocene dissolved dinxylene is rapidly injected until the solution reaches the injector head, producing CNTs that are ready for collection and purification from the catalyst using nitric acid. The advantage of this method in comparison with CVD is that we can still contentiously introduce hydrocarbons and catalysts, decreasing the expense of CNT production (Wasel et al., 2007).

4.2.1 Characterization Optical characterization of CNTs is a good means to explore their internal properties. Several techniques use the optical response such as infrared spectroscopy, Fourier transform infrared, and Raman spectroscopy. In this part, we will present some of these techniques.

4.3  X-ray photoelectron spectroscopy When CNTs are exposed to X-rays, they release photoelectrons. The binding energy estimation of these electrons enables us to determine the surface composition. This

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Carbon Nanomaterials for Agri-food and Environmental Applications

refers to the inelastic photoelectron mean free path that allows the recognition of the elemental composition of the surface, except the hydrogen (Cano et  al., 2017). In addition, X-ray photoelectron spectroscopy (XPS) introduces information about other existing elements on the surface because the binding energy is easily affected by neighbor atoms. The only problem with XPS is the necessary large mass of the sample.

4.4  Infrared spectroscopy Infrared spectroscopy is a familiar technique in detecting the vibrational motion of the functional groups attached on the CNT surface. However, one disadvantage related to this technique is that it cannot give a quantitative analysis for the detected groups.

4.5  Raman spectroscopy Raman spectroscopy is a vibrational spectroscopic technique that provides information about molecular vibrations and crystal structures. When a material is exposed to light, almost all the photons are elastically scattered with the same energy. But a small number of photons are scattered with different energy and have inelastic collisions with the material; that is known as the Raman effect. The normal Raman spectra for an ordered sidewall CNT is the G-band, in contrast with the nonordered CNT side wall that shows a D-band. The defect density in CNTs can be easily determined from the ratio of these two bands (IG:ID), which introduces information about the structure change. In some cases, the radial breathing mode is used to estimate the CNT diameter according to their peaks (Rao et al., 2001). The advantages of the Raman spectroscopy characterization technique are a fast analysis tool, no sample preparation, nondestructive analysis, and all allotropic forms of carbon are active for this spectroscopy (Arepalli et al., 2004).

4.6  Transmission electron microscope Microscopy techniques play an important role in the easy detection of impurities with CNTs, which affects the structure and other properties. This is because most of the synthesis methods result in amorphous carbon that is adsorbed in the CNT wall side and residual metals, which are embedded between CNT layers. Using transmission electron microscopy (TEM) has several advantages, as it can distinguish these impurities based on the difference in transmission properties (Hyung et al., 2017), determine the length of CNTs using low magnification, measure the inner and outer diameter, and sometimes count the number of CNTs. Using high-magnification TEM, Kiang et al. found that the intershell spacing varies from 0.34 to 0.39 nm according to the CNT diameter (Belin and Epron, 2004), which is greater than the interplaner distance of graphite. This difference refers to the graphene sheet curvature, leading to an increase in the repulsive force and the size effect. However, in some cases, the electron used in TEM itself damages the CNT walls, so it was suggested to expose only the target area for short durations.

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4.7  X-ray diffraction In contrast with TEM, X-ray diffraction (XRD) is a nondestructive characterization technique used to identify impurity elements and determine some structural properties such as the interlayer distance and strain (Arunkumar et  al., 2018). XRD patterns of CNTs are quite similar to graphene; however, the carbon structure in graphene shows a random orientation (Rasel et al., 2015). The XRD peak intensity of CNTs is a morphology-dependent factor. In more detail, a (002) peak with (hkl) reflections is produced when X-rays hit a wall of the SWCNT. Meanwhile, when X-rays pass in the empty core of the CNT, an extra hexagonal (hko) reflection is produced (Rasel et al., 2015). It is worth mentioning that the (hko) results because of the asymmetric curvature on CNT while the (hkl) results from the regular arrangement of layers.

4.8  Carbon nanotube applications 4.8.1  Drug delivery, genetic engineering CNTs have great potential in drug delivery due to their large surface area. The drug can be integrated on walls and can cross the cell membrane, delivering drugs in a safe way (Chen et al., 2008). They also can be used in gene manipulation and atom tissue engineering (Kam et al., 2005). The nucleotides of DNA are connected with CNTs, changing their electrostatic properties, which increases the CNT potential application in diagnostics, therapy, and DNA analysis. In addition, CNTs can be used in gene therapy or cancer treatment due to the tabular nature, where it was found that CNTs imposed by DNA release the DNA before being destroyed by the self-defense system. Some in vitro studies have shown the destruction of cancer cells by hyperthermia using CNTs as a result of the high thermal conductivity (Kam et al., 2005).

4.8.2  Artificial implants and biomedical applications The body exhibits painful reactions against implants; however, CNTs with external implants can easily be attached with proteins to decrease this pain. CNTs can also be used as bone structures due to their strong tensile strength, and sometimes can be filled with calcium (Ding et al., 2001). In addition to the high sensitivity of CNTs in DNA detection, CNTs can also be used in pressure sensors, which are important for eye surgery and respiratory devices (Qiu et al., 2015). Moreover, polymer-based CNTs are used in artificial muscle devices because of their strength (Foroughi et al., 2016). CNTs also are used to detect cancer cells in their early stages, where they are hard to recognize by normal means such as X-ray and magnetic resonance imaging (MRI). Therefore, CNTs penetrating tissues enhances the diagnostic process in the early stages of cancer disease.

4.8.3  Field emission source in electronic devices Electrons are emitted from CNT tips when an electric field is applied between the CNT surface and the anode. Because of that, CNTs can be used in multiple electronic devices such as X-ray sources (Rinzler et al., 1995). The advantages of CNTs as a field

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emission source are a long life time, the stability of the emitted field does not need a high vacuum to work, and a high current density, where it can achieve 4 A cm−2.

4.8.4 Sensors Biosensor efficiency can be enhanced by CNTs attached on top, where they can attract functional groups (Wong et al., 1998). Therefore, it is possible to fabricate different sensors with CNTs that are sensitive to gases so they can detect leakage. Where there is a change in the CNT electrical resistance when the surrounding environment changes, for example, SWCNTs are very sensitive (Collins et  al., 2000) and MWCNTs are sensitive to NH2, H2O, CO, and CO2. In 2002, high-sensitivity microwave resonant sensors were fabricated using SWCNTs and MWCNTs for NH3 detection (Chopra et al., 2002).

4.9 Supercapacitor Several materials have been investigated for energy storage devices; however, there are some struggles to manipulate the efficiency with geometry, where the good devise should interest with flexibility and lightweight that are struggling with the high efficiency. Supercapacitors are interesting tools in energy storage in comparison with others due to faster charge discharge rates, higher power density, a superlong life cycle, and pollution-free operation (Sheila and Bahareh, 2018). Textile supercapacitors are novel energy storage devices designed by coating the fibers with functional thin film layers such as carbon nanotubes (Islam et al., 2017). Jiang et al. in 2015 found that MnO2 nanoflakes can be electrodeposited by wrapping MWCNTs on cotton textiles and treating with acid. This is an interesting conductive textile, exhibiting high flexibility and strong adhesion between the CNT and the textiles of interest. Moreover, conductive textile-based supercapacitors were found to show high specific and capacity retention of 94.7% that can be maintained at 2000 continuous charge-discharge cycles (Jiang et al., 2015).

4.10  Water treatment CNTs show a high sorption ability that can be affected by the attached functional group. Using that, CNTs can be used in water treatment to adsorb different types of heavy metals (Gul et  al., 2019). For example, CNTs with COO− functional groups can be used to adsorb mercury, which is a toxic heavy metal (Pokhrel et al., 2017). In a similar way, the different morphology CNTs works as adsorptive membrane for the Ni ions (Tofighy and Mohammadi, 2015). In addition, CNTs can remove harmful organic groups, where they are considered a good absorbent for toxic organic dyes (Santhosh et  al., 2016). For instance, methyl orange (MO) can be easily removed using MWCNTs supported with palladium nanocubes (Cano et al., 2017). In water treatment, CNTs are used as electrode materials, Table 1 shows CNTs and functional groups used in heavy metal detection. This table shows the direct detection of Zn, Cu, and Cd heavy metals even with simple CNTs; however, catalyst-free CNTs show some

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Table 1  Types of CNTs and functional groups used in heavy metal detection. Adsorbent CNTs MWCNTs CNT-COO CNT-OH SWCNT CNT-COO CNTs CNT fibers CNTs

Absorbed heavy metal ion 2+

Pb Ni2+ Hg2+ Cu2+ Zn2+ Cd2+ Cr6+ Ag1+ Pb2+

Ref. Mubarak et al. (2016) Lu and Liu (2006) Anitha et al. (2015) Anitha et al. (2015) Long and Yang (2001) Anitha et al. (2015) Atieh (2011) Jahandari et al. (2013) Pérez et al. (2016)

limitations. Another potential application of CNTs in water treatment is oil removal, which is an important area due to water pollution from some industrial waste or ship leakage. Two main kinds of CNTs are used in oil detection: P-CNTs and C-CNTs. It is worth mentioning that P-CNTs show higher efficiency in oil detection (97%) compared to C-CNTs (87%) (Fard et al., 2016).

4.10.1  Toxicity of CNTs In spite of the wide range of CNT applications, a toxic side effect is registered (Francis and Devasena, 2018). CNT toxicity is controlled and determined through factors such as morphology, size, shape, and purity (Luo et al., 2013). This is because CNTs penetrate cells, inducing a cytotoxic response where the contact between the CNT with a cell results in a strain on the cytoskeleton of the phagocyte. In accordance, some studies have shown that longer-length and larger-diameter CNTs have more of a toxicity effect than smaller ones (Lamberti et al., 2014). Moreover, CNT purity is an additional important factor, where metallic impurities lead to cell death (Meng et al., 2013).

5 Conclusion Carbon nanotubes are of wide interest due to their electrical and physical properties. They are produced in different synthesis methods such as arc discharge and laser ablation according to the required physical properties such as diameter and length. The large surface area has made them good drug delivery systems; therefore, a purification process is required after synthesis to avoid toxicity.

Acknowledgment The first author would like to thank Basma F. for the continuous support.

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Further Reading Baughman, R., 2016. Knitted carbon-nanotube-sheath/spandex-core elastomeric yarns for artificial muscles and strain sensing. ACS Nano 10, 9129–9135. Li, W., Xie, S., Qian, L., Chang, B., Zou, B., Zhou, W.Y., Zhao, R., Wang, G., 1996. Large–scale synthesis of aligned carbon nanotubes. Science 274 (5293), 1701–1703.