Preparation, Purification and Characterization of CNT and CNT /CP Nanocomposite

## **Chapter 2**

## Preparation and Characterization of Electrically Conductive Polymer Nanocomposites with Different Carbon Nanoparticles

*Víctor J. Cruz-Delgado, Janett A. Valdez-Garza, José M. Mata-Padilla, Juan G. Martínez-Colunga and Carlos A. Ávila-Orta*

## **Abstract**

Carbon nanoparticles possess a combination of high electrical and thermal transport properties, as well as low density and different morphologies that make them a good choice to reinforce plastics. Polymer nanocomposites offer great expectations for new and unexpected applications due to the possibility of changing their electrical/thermal behavior by adding nanoparticles while retaining the flexibility and processability of plastics. The possibility of electrical and thermal conduction in a polymer matrix with low amounts of nanoparticles brings opportunity for high demanding applications such as electrical conductors, heat exchangers, sensors, and actuators. Polyolefin nanocomposites offer a significant challenge due to their insulative nature and low affinity for carbon nanoparticles; due to the latter, new production tendencies are proposed and investigated.

**Keywords:** carbon nanoparticles, polymer nanocomposites, electrically conductive, ultrasound-assisted melt extrusion, thermal properties

## **1. Introduction**

### **1.1 Carbon nanoparticles**

From the discovery of cylindrical nanometric structures composed of one or several layers of carbon atoms similar to graphite by Iijima in 1991 [1], the scientific community embarked on a fascinating multidisciplinary career in the study, synthesis, characterization, and possible applications of these new carbon nanostructures, excited by the unusual combination of properties that these nanomaterials possess, among which the conduction of electricity and heat, low density, high mechanical resistance and morphology stand out. These nanoparticles have diameters in the range of 1 to 100 nm, lengths of 10 to 1000 nm. They can contain one, two or up to 100 layers rolled on each other with an equidistant separation of 0.34 Å [2–4]. Later, Novoselov and Geim [5] made an enormous contribution to science with graphene discovery, whose laminar crystalline structure is composed entirely of carbon atoms with an sp2

hybridization, with a thickness of only one atom of carbon. Graphene has unusual properties between a metal and a superconductor and high mechanical, elastic, and chemical resistance. Therefore, graphene has been studied and proposed for various applications in electronic, aerospace, automotive, medical, and food industries [6–13].

Due to the ease of modifying its structure by incorporating other chemical elements, hybridization with functional groups, and decoration with organic molecules, carbon nanoparticle applications have been expanded enormously, leading to countless applications. For example, the miniaturization of electrical circuits composed of one or more carbon nanotubes, chemical and electromechanical sensors based on carbon nanotubes, the storage of hydrogen for fuel cells, the increase in charge capacity in batteries based on graphene or graphene nanoplatelets as well as the filtration capacity at the molecular level using graphene-based membranes, besides the reinforcement of polymeric matrices, to name only a few [4, 7, 11, 14, 15].

#### **1.2 Polymeric nanocomposites**

Materials science has been searching to generate new materials that possess a balance of properties, making them ideal for new and unexpected applications. Within this vast field are composite materials, which have a continuous phase (metallic, ceramic, or polymeric) and a discontinuous phase (filler or additive), which generally have high filler or additive contents of up to 70%, such as the case of titanium oxide (TiO2) or carbon black concentrates in a polyethylene matrix, since both additives are used as pigments in the plastics industry [16, 17]. With the beginning of nanotechnology and the growing supply of different carbon nanoparticles, a new class of materials has emerged called polymeric nanocomposites whose advantage lies in using a smaller quantity of particles to modify the behavior of the host matrix or continuous phase.

Electroconductive polymeric nanocomposites were originally based on graphite derivatives, later carbon nanofibers, carbon nanotubes (mono or multilayer), and recently on graphene or graphene nanoplatelets, as well as a wide variety of combinations between these and other nanoparticles with different nature and morphology [8, 17–20]. In order to improve the electrical properties of these materials, combinations of carbon nanotubes have been made with graphite, graphene, clays, copper oxide, titanium oxide, silver nanowires, etc.; in all cases, the aim is to generate three-dimensional networks interconnected to facilitate the passage of electrons or phonons, to generate an electro/thermo-conductive material [21, 22].

In addition to providing the ability to conduct heat and electricity since they can exhibit the Peltier and Seebeck effect, [23, 24] such effects are beneficial in the development of thermoelectric materials, polymeric nanocomposites have also exhibited a noticeable improvement in mechanical properties, a barrier to gases, thermal stability [6, 9, 25, 26] as well as the ability to modify the electrical properties of the host matrix to generate materials for capacitors, electromagnetic and/or radiofrequency shields, have even allowed the development of metamaterials capable of modifying their refractive index, dielectric constant and/or Seebeck effect [27–29].

#### **1.3 Polymeric nanocomposites preparation methods**

There are different methods for preparing polymeric nanocomposites, where the main objective up to now has been to achieve adequate dispersion and distribution of carbon nanoparticles that allow modulating the properties of the resulting material. Because carbon nanoparticles are held tightly together by van der Walls forces, different ways have been sought to separate them individually to combine them with a polymer later and obtain a homogeneous polymeric nanocomposite. The main methods employed to achieve this are briefly described below.

*Preparation and Characterization of Electrically Conductive Polymer Nanocomposites… DOI: http://dx.doi.org/10.5772/intechopen.95912*

#### *1.3.1 Mixed in solution*

In this method, the polymer is dissolved in a suitable solvent with the aid of magnetic, mechanical and/or heat stirring to facilitate complete dissolution of the polymer. The carbon nanoparticles are suspended in the same liquid (solvent) or a combination of them, and magnetic, mechanical, or ultrasonic stirring is applied to improve the dispersion of the nanoparticles. Subsequently, both solutions are mixed and kept under stirring, then the solvents are evaporated with heat or slowly in an extraction hood (the above will depend on the nature and reactivity of the solvent). Finally, the resulting material, usually a dark-colored powder, is compacted by applying pressure and heat to obtain a useful material. At the laboratory level, it is the most used method for research purposes; however, the large amount of solvents used makes its scaling at an industrial level unfeasible [30–32].

#### *1.3.2 Polymerization in situ*

In this method, one of the monomers or solvents used to obtain the polymer is mixed with the nanoparticles until a homogeneous dispersion is achieved; subsequently, the other reagents, including the corresponding catalysts, are added, and the polymerization reaction is carried out under the conditions of usual temperature and pressure. At the end of the reaction, the product obtained is purified, and the excess solvent is eliminated to recover the polymer formed with the incorporated nanoparticles. Given the complexity of this method, polyethylene's polymerization in the presence of carbon nanotubes at the laboratory level and of polyamide 6 with nanoclays at an industrial level has been successfully reported [20, 33, 34].

#### *1.3.3 Melt mixing*

This method is the most widely used at the laboratory level to obtain polymeric nanocomposites; it consists of passing the polymer and nanoparticles through a twin-screw extruder, whereby applying heat, the polymer melts and is transported by the screws that in turn impart shear forces to mix the components, in the different mixing zones that the extruder has. The mixture leaves the extruder, is cooled, and cut to obtain a polymeric nanocomposite. Due to its simplicity, this process can be easily scaled to an industrial level, in addition to the fact that it does not generate waste and does not use solvents [35].

#### *1.3.4 Ultrasound-assisted melt mixing*

Given the low affinity of polyolefins and in general of polymers for carbon nanoparticles, modifications have been made to the conventional melt mixing method by applying ultrasound waves in some specific sections of the extruder. It has been reported that this method can significantly improve the dispersion of nanoparticles of different nature and geometry, even with high nanoparticle content [36]. Different variants have evolved; the main difference being the mode of generation and application of ultrasound waves; conventionally fixed frequency ultrasound waves are generated, which are applied constantly or intermittently [37]. In another embodiment, the ultrasound waves are applied constantly, gaining a dynamic frequency sweep in a given interval [35, 38, 39].

There are other methods used for the production of polymeric nanocomposites, mainly at the laboratory level. Nevertheless, the choice of method will broadly define the level of dispersion and distribution of the nanoparticles within the polymeric matrix, and therefore the properties of the resulting material.

## **2. Methodology**

In **Table 1**, the most outstanding reports in electro/thermo-conductive polymer nanocomposites of the last five years are presented to have a broader outlook on the subject. By their nature, polyolefins are good electrical insulators exhibiting


*a SSWCNT small-bundle-diameter-single-walled CNTs.*

*b PP MFI = 34 g/10 min.*

*c Melt extruded without ultrasound.*

*d Melt extruded with ultrasound fixed frequency.*

*e Melt extruded with ultrasound variable frequency.*

*f Melt extruded previously dispersed in gas phase.*

*g Solid.*

*h Foam.*

*i PP MFI = 1200 g/10 min. j*

*SG-CNT supergrowth-CNT. k*

*CNT, NC700. l*

*CNT, CNS-PEG.*

*mCNT, Tuball. n*

*CNT, N-MWCNT A1, Nitrogen doped. o*

*CNT, N-MWCNT IFW, Nitrogen doped. p Boron doped SWCNT.*

#### **Table 1.**

*Electric/thermal parameters of the most relevant polymer nanocomposites with carbon nanoparticles.*

#### *Preparation and Characterization of Electrically Conductive Polymer Nanocomposites… DOI: http://dx.doi.org/10.5772/intechopen.95912*

electrical conductivity in the order of 10−12 to 10−15 S/cm. As can be seen, different techniques have been used for the preparation of polymeric nanocomposites, achieving fascinating results. It can also be seen that the most popular preparation method is melt mixing, which, as mentioned above, is a versatile and easily scalable method. Another variant that can be observed is that depending on the polymeric matrix; the result will change; even more important is the concentration of nanoparticles used. Another aspect that should be highlighted is the modification or doping of the carbon nanoparticles, which slightly increases this property. Finally, as is known, polyolefins are thermal insulators, and their thermal conductivity ranges between 0.1 to 0.4 W/mK. Thermal conductivity has also shown sharp increases, as shown in Aghelinejad and Leung's reports and Paszkiewicz et al. [45, 50], where the matrix used was polyethylene.

## **3. Case of study**

The motivation of present work was to perform a screening of several carbon nanoparticles to obtain polymeric nanocomposites with a better balance on properties such as electro/thermal conduction, mechanical and thermal stability. For this purpose, different carbon nanoparticles were selected. Their main differences lie in morphology (laminar versus fibrillar), structure (flat versus rolled layers), and functionalization (modified versus un-modified surface, i.e., CNT). Besides, the use of different polyolefins such as polyethylene and polypropylene, which bear significant differences in structure. On the one hand, polyethylene possesses a main chain almost free of pendant groups; meanwhile, polypropylene's main chain contains one methylene group each three carbon atoms. The best candidate is expected to be used to manufacture prototypes of thermistors (temperature sensors based on a change in electrical resistivity).

## **3.1 Materials and methods**

In the following section, the preparation of polymeric nanocomposites in high-density polyethylene (PE) and polypropylene (PP) and their combination with four types of carbon nanoparticles (CNP) are presented and discussed. In all cases, a content of 20% wt/wt of each nanoparticle was used. The characterization results by thermogravimetric analysis, mechanical properties in tension and bending, electrical resistivity, and dielectric constant as a function of frequency and thermal conductivity are also presented. The resins used to obtain the polymeric nanocomposites were the following: high-density polyethylene (PE) Alathon H4620 with MFI of 20 g/10 min and density of 0.940 g/cm3 provided by LyondellBasell (TX, USA), also polypropylene (PP) Formolene 4111 T with MFI of 35 g/10 min and density of 0.9 g/cm3 provided by Formosa Plastics, (Tamaulipas, Mexico). The carbon nanoparticles used and their main characteristics are listed in **Table 2**.

The materials' processing was carried out in a Thermo Scientific model PRISM 24MC twin-screw extruder; the diameter of the screws is 24 mm with a length/diameter ratio of 40:1. According to the formulation, a controlled feeder for powders and another for the resin were used, which were previously calibrated to dose the required amount. The addition of the nanoparticles and the resin was carried out simultaneously in the extruder. A screw rotational speed of 100 rpm was used, a flat temperature profile of 180 and 200°C for the nanocomposites with PE and PP, respectively. Under these conditions, a production speed of 3.2–3.5 Kg/h was obtained. To improve the nanoparticle's agglomerates' dispersion and distribution, a device specially designed


*\* SSA, Specific surface area.*

*1 CNT industrial grade.*

*2 MCNT, Industrial grade modified CNT with -COOH contain 1.2% of COOH groups.*

*3 GNP, industrial grade graphene nanoplatelets.*

*4 Carbon Black, Vulcan XC72 grade.*

#### **Table 2.**

*Characteristics and properties of the different carbon nanoparticles.*

to irradiate the extruded material with ultrasound waves was coupled at the extruder exit. The device consists of a chamber with controlled temperature; inside, there is a 12.5 mm diameter titanium catenoid sonotrode (Branson Corp.) connected to a homemade ultrasound wave generator, which can generate ultrasonic waves in the range of 10 to 50 kHz, with a 750 W power [35, 38]. Finally, the material was passed through a water bath and cutter. Subsequently, each material was compression-molded to obtain a 15 X 15 X 0.2 cm plate, and a PHI press was used, a pressure of 20 Tn, with temperatures of 180 and 200°C for the nanocomposites with PE and PP, respectively. Specimens were cut for the characterization of the polymeric nanocomposites.

The characterization of the polymeric nanocomposites was carried out using the following analytical techniques. The thermogravimetric analysis (TGA) was carried out using a thermogravimetric analyzer from TA Instruments model Q500, using a sample of approximately 8 mg, a temperature range of 25–600°C, with a heating rate of 10°C/min and an inert atmosphere with nitrogen gas with a flow of 50 ml/ min. The mechanical properties were evaluated in a universal testing machine, Instron model 1000, for tension tests in accordance with the ASTM D638 standard, using V-type specimens and a stretched speed of 50 mm/min and a load cell of 10 kN. The flexion tests were carried out according to the ASTM D790 standard using 12 X 1.25 X 0.2 cm specimens in 3-point bending mode; in both cases, five measurements were made, and the average value was reported. The electrical properties of resistance and capacitance were measured with an LCR analyzer in samples of 1 X 1 X 0.2 cm, both faces of the specimen were covered with silver paint, and a copper wire was placed as an electrode. The measurement was carried out at room temperature using a frequency range from 20 Hz to 2 kHz in increments of one decade; 5 measurements were made, and the average value was reported. The thermal diffusivity determination was carried out in a TA Instruments thermal diffusivity analyzer Discovery Xenon Laser Flash model (DXF-200). The analyzed specimen had circular geometry with 12.5 x 2 mm dimensions; both faces were coated with carbon paint and one of them with silver paint to ensure good contact with the temperature sensors; the measurement was carried out in triplicate at 25°C.

#### **3.2 Thermal stability**

The study of the thermal stability in electrically conductive materials is of great importance because when an electric current circulates through them, they can undergo heating and alter their behavior or ability to conduct electricity. On the other hand, this analysis makes it possible to determine the thermal stability of the materials and the amount of mass that they can lose due to the effect of temperature *Preparation and Characterization of Electrically Conductive Polymer Nanocomposites… DOI: http://dx.doi.org/10.5772/intechopen.95912*

in a controlled atmosphere. It should be mentioned that if the atmosphere is air, thermo-oxidative degradation will occur. In **Figure 1**, the corresponding thermograms to the nanocomposites based on PE and PP are presented. While in **Table 3**, the specific data for the mass loss of T5% and T50% are shown.

It can be observed that PE exhibits a loss of mass from 330°C, while polymeric nanocomposites exhibit this loss at a temperature around 411°C, regardless of the type of nanoparticle used. It is important to note that the nanocomposite containing CB exhibits the highest thermal stability. For PP, degradation begins at a temperature of 370°C, while for polymeric nanocomposites occurs around 420°C, regardless of the type of nanoparticle used. In this case, nanocomposites based on CNT and MCNT exhibit the highest thermal stability of all.

Various reports in the literature suggest that carbon nanoparticles provide greater thermal stability or heat resistance to polymers in general due to a mechanism based on the formation of a carbonaceous layer and a tortuous path similar to a labyrinth on the surface of the material that prevents the release of combustion gases [19, 26]. This analysis is of great importance for flame retardancy applications in aeronautics, automotive, and textile industries and to determine the safety temperature that the material can support before molten and inflamed by the passage of an electrical current.

#### **3.3 Mechanical properties**

The mechanical properties of polymeric nanocomposites are of great interest because, as mentioned above, the addition of carbon nanoparticles can improve

**Figure 1.**

*Thermal stability by TGA of polymeric nanocomposites with 20% wt/wt of different CNP, (A) PE base, and (B) PP base.*


**Table 3.**

*Degradation temperatures at T5%, T50%, of polymeric nanocomposites with different carbon nanoparticles.*


**Table 4.**

*Mechanical properties of polymeric nanocomposites with different carbon nanoparticles.*

their performance. In **Table 4**, the properties of the PE and PP-based nanocomposites with the different carbon nanoparticles are listed.

As expected, with the addition of nanoparticles, the different properties were modified; firstly, the PE exhibits a tensile modulus of 23.68 MPa, while the nanocomposites present a maximum increase of 180%, this increase in resistance to stress causes the elongation of the material to be markedly reduced, suggesting that the stiffness of the material has changed from a ductile to a brittle material, in which plastic deformation has been suppressed. For its part, the flexural modulus corroborates the above since PE has a value of 376 MPa, and in nanocomposites, this value has increased to 280%. A similar behavior occurs with PP, exhibiting an increase of 130% and 330% in the tensile and flexural modulus, respectively. In this sense, the greatest increase in mechanical properties for polyethylene is obtained with GNP > CNT > CB > MCNT, while for polypropylene, it is CB > CNT > GNP > MCNT. In this sense, it is worth mentioning that the surface modification made to the MCNTs did not improve by itself, the compatibility with the host matrix PE or PP.

In the literature, many reports can be found that mention the improvement in mechanical properties in polymeric nanocomposites reinforced with carbon nanoparticles. However, the addition of compatibilizing agents such as maleic anhydride grafted to the resin is required to achieve a substantial increase in the mechanical properties, even with low amounts of carbon nanoparticles [9, 26, 51, 52]. Due to the lightweight and high modulus obtained by the polymeric nanocomposites reinforced with carbon nanoparticles, aeronautics and automotive industries would be benefited from the development of these materials for different components, which can provide a reduction in weight and lower consumption of fuels.

#### **3.4 Electrical properties**

The evaluation of electrical properties was carried out using an LCR as a function of a frequency interval, as shown in **Figure 2**. First, the polyethylene-based system allows observing that the PE resin exhibits the highest electrical resistance values at low-frequency values; above 10 kHz, the material becomes polarized and shows a lower electrical resistance, which decreases three orders of magnitude when reaching 2 MHz. With the addition of GNP, the material exhibits a behavior similar to that of PE, one order of magnitude lower in terms of electrical resistance. Meanwhile, the materials that contain MCNT and CNT show a reduction of 7 and 8 orders of magnitude; however, the polarization effect occurs when reaching high frequencies of 100 kHz. The CB-based system exhibits the least electrical resistance with nine orders of magnitude reduction concerning PE alone. In addition to not

*Preparation and Characterization of Electrically Conductive Polymer Nanocomposites… DOI: http://dx.doi.org/10.5772/intechopen.95912*

**Figure 2.**

*Electrical resistance as a function of frequency, of polymeric nanocomposites with 20% wt/wt of different CNP, (A) PE base, and (B) PP base.*

showing polarization effects as a function of frequency, which suggests that it behaves as an excellent electrical conductor.

For materials based on PP, the behavior is slightly different PP only presents the highest values of electrical resistance at low-frequency values; above 10 kHz, the material is polarized and shows a lower electrical resistance, which decreases three orders of magnitude when reaching 2 MHz, in the same way as the PE. Surprisingly, the CB-based system exhibits an electrical resistance that is completely dependent on the frequency. When it increases, the electrical resistance decreases to four orders of magnitude concerning the PP, suggesting that the material behaves like a semiconductor. On the other hand, the materials that contain CNT and MCNT show a reduction of seven and eight orders of magnitude without presenting the polarization effect in the entire frequency range, which suggests that they behave like a good electrical conductor. Finally, the compound containing GNP shows the lowest electrical resistance with a reduction of nine orders of magnitude and a linear response throughout the entire frequency range used. Based on the above, it can be pointed out that the nature of the polymeric matrix and the type of carbon nanoparticle can notably modify the electrical behavior of the polymeric nanocomposite [8, 31, 53, 54].

The behavior of the dielectric constant of polymeric nanocomposites is presented in **Figure 3**. Analogously to the behavior of electrical resistance, the dielectric constant follows a similar trend with the addition of carbon nanoparticles. The PE has a value of 3 and a linear behavior in the entire frequency range, while the nanocomposite with GNP shows an increase of 1 order of magnitude and a linear behavior as a function of frequency. Materials containing CNT and MCNT show an increase of three orders of magnitude for PE, with a slight decrease at high frequencies. The material that contains CB exhibits a frequency-dependent behavior since, at 20 Hz, it shows an increase of four orders of magnitude and then it decreases two orders of magnitude from a frequency of 1 kHz; this behavior corresponds to that of a capacitor, capable of storing energy and releasing it suddenly when used in electrical/electronic circuits.

On the other hand, PP exhibits a dielectric constant of 3 and does not vary as a function of frequency; the nanocomposite with CB shows an increase of one order of magnitude with respect to pure PP, while the nanocomposites with CNT and MCNT show an increase in 3 orders of magnitude and a slight decrease at high-frequency values. Finally, the nanocomposite with GNP presents the highest value of dielectric constant, with an increase of up to four orders of magnitude at a frequency of 20 Hz, and decreases by one order of magnitude for the rest of the frequencies evaluated. Similar to the behavior of PE nanocomposites,

**Figure 3.**

*Dielectric constant of polymeric nanocomposites with 20% wt/wt of different CNP, (A) PE base, and (B) PP base.*

PP-based nanocomposites exhibit capacitor-like behavior throughout the evaluated frequency range.

The combination of properties for these new nanocomposite materials results in various applications that had not been previously conceived. For example, supercapacitors can be manufactured for systems that require a precise regulation of the supplied energy and a high energy storage capacity, and that in this way, the energy necessary to drive an electrical component can be supplied without the need to overload the electrical network of the circuit, besides not present a memory effect [25, 31]. Another field of interest for those materials would be the packaging industry, with the development of antistatic, static dissipative or semiconductive packages, for the protection of electronic components during their transportation, even for EMI or RF shielding for aerospace and defense to protect safety- and mission-critical systems from intentional and unintended electronics emissions [44]. The growing industry of electronic textile or smart textiles that develop wearable technology requires integrating textile fibers capable of conducting electrical signals. There are fabrics in which electrical and electronic elements such as microcontrollers, sensors, and actuators have been integrated that allow clothing to react, send information, or interact with the environment [55–57].

#### **3.5 Thermal conductivity**

The study of the thermal properties of polymeric nanocomposites intended for electronics applications is of great importance since, as mentioned above, the passage of electric current can induce a temperature gradient in electrical conductors, even in metals. The heat capacity was first determined, as well as the density and thermal diffusivity to determine the thermal conductivity of polymer nanocomposites. Values are shown in **Table 5**.

According to the data reported in **Table 5**, PE has the highest value of Cp; with the addition of the different nanoparticles, the Cp of the nanocomposites decreases significantly, the most notable case being the nanocomposite with CB. Meanwhile, PP exhibits an even higher Cp than PE, while the addition of the different nanoparticles promotes a decrease in this value, with graphene nanoplatelets being the material that most reduces this value. The decrease in Cp of the different nanocomposites can be associated with the ease they present for heat conduction, making the material less thermally insulating.

On the other hand, the thermal conductivity presents substantial improvements; in general, the PE-based nanocomposites exhibit the most significant increase in

*Preparation and Characterization of Electrically Conductive Polymer Nanocomposites… DOI: http://dx.doi.org/10.5772/intechopen.95912*


#### **Table 5.**

*Heat capacity (Cp, J/g K) and thermal conductivity (*κ*, W/m K) of polymeric nanocomposites with different carbon nanoparticles.*

thermal conductivity 79, 29, 16, and 4% for the nanoparticles in the following order CNT > GNP > CB > MCNT, suggesting that carbon nanotubes are the most effective additive to increase the thermal conductivity of the nanocomposite. The trend is reversed, with increases of 21, 14, 7, and − 11% for MCNT > CNT > CB > GNP for PP-based nanocomposites. Although the Cp of the nanocomposites follows a different trend towards thermal conductivity, it should be mentioned that the type of polymeric matrix, the morphology, distribution, and dispersion of the different nanoparticles play an important role in heat conduction. This phenomenon is carried out through phonons; therefore, if there are spaces in the material in which the nanoparticles are too far apart, the phonons' passage through the material will find a physical barrier for their passage.

Recent reports suggest that a polymeric nanocomposite's thermal conductivity can be affected by different factors, including the processing method, the number of defects in the carbon nanoparticles, and, finally, their dispersion within the polymeric matrix [21, 29, 45, 46, 58]. The capability to conduct heat in a polymeric nanocomposite makes an ideal candidate for different applications such as heat exchangers, solar water heaters, thermoelectric materials, electrical heaters, to mention a few [22]. These devices will take advance of the lightweight, mechanical strength, thermal and dimensional stability of these materials, in which automotive, construction, and green industries are interested.

#### **3.6 Thermistors**

The electrical resistivity of polymeric nanocomposites with carbon nanoparticles shows an anomalous increase near the melting point of the matrix; this effect is known as a positive temperature coefficient (PTC) of resistivity. On the other hand, the negative temperature coefficient (NTC) is a very sharp decrease in resistivity when the temperature is above the melting point of semicrystalline polymers. These kinds of materials have important industrial applications like overcurrent protectors and self-regulating heaters [59, 60].

The polymer nanocomposites obtained were evaluated for their potential use as a thermistor. For this purpose, a prototype will be constructed; it consists of a square piece with dimensions 1 X 1 X 0.2 cm; both sides were cover with silver paste as an electrode and a copper wire. Kapton tape was used to cover the prototype and isolate the wires during the heating cycle. A Mettler Toledo FP82 Hot Stage was used to supply heat in an interval from 40 to 160°C at a heating rate of 5°C/min, the Hot Stage was connected to a Mettler Toledo FP90 Central Processor, the electrical resistivity was measured with a Keithley Source Meter model 2400, in a 4-wire sense mode, to avoid the parasite signal in the circuit.

**Figure 4.** *Temperature versus electrical resistivity of PE base polymeric nanocomposites with 20% wt/wt of different CNP.*

As seen in **Figure 4**, all the polymer nanocomposites exhibit thermistor behavior, i.e., an increase of resistivity around 128°C. The intensity of the PTC (the electrical resistivity ratio at the melting point versus room temperature) depends on the type of carbon nanoparticle used. The interval of temperature at which this phenomenon occurs is between 127 and 131°C. In this sense, the intensity of the PTC is in the following order GNP > CNT > MCNT > CB. This behavior could be associated with the capability of the polymer chains to break apart the conductive pathway formed in the polymer nanocomposite, due to the semicrystalline nature of the polymer matrix and the reduction in viscosity, during the heating. It is worth mentioning that PE/CB nanocomposite exhibits the lowest PTC intensity, probably due to the high structure of the CB (CB possess the small average particle size) and could form new conductive pathways in the molten state as stated by Zeng et al. [61].

#### **4. Conclusions**

The polymer nanocomposites with carbon nanoparticles become an electrically conductive material whit the addition of a certain amount of carbon nanoparticles; this property is fundamental in electrical and electronic applications. For many years, carbon black has been chosen as the best candidate for this purpose; with other carbon nanoparticles such as CNF, CNT, GO, graphene, and their combination with other materials, significant improvements have been made for electrically conductive materials.

In this work, the preparation and characterization of electrically conductive polymeric nanocomposites with different carbon nanoparticles was addressed to screen the type of carbon nanoparticles that allows them to obtain polymeric nanocomposites with a better balance on properties such as electro/thermal conduction, mechanical, and thermal stability. A material with the desired properties for their application in electronics, such as low electrical resistivity, thermal stability, and mechanical strength, besides thermal conductivity, is PE/CB polymeric

*Preparation and Characterization of Electrically Conductive Polymer Nanocomposites… DOI: http://dx.doi.org/10.5772/intechopen.95912*

nanocomposite since it exhibits a better balance of properties. This set of properties makes them candidates for use in various applications. Besides thermistors, they may be candidates for use in electrical heaters, which are a kind of electrical resistor used to converts electrical energy into thermal energy, as thermoelectric materials for their use in the exploitation of renewable energies, in heat exchangers, as EMI and RFI shielding, and as a wearable textile for smart applications.

## **Acknowledgements**

The authors are grateful for the support of the CIQA technical staff for the preparation and characterization of materials: María G. Méndez Padilla, Gilberto F. Hurtado López, Rodrigo Cedillo García, Juan F. Zendejo Rodríguez and Jesús G. Rodríguez Velazquez. The financial support by SENER-CONACyT-CeMIE-SOL through the 207450-12 project is also appreciated.

## **Conflict of interest**

The authors declare no 'conflict of interest'.

## **Author details**

Víctor J. Cruz-Delgado1 \*, Janett A. Valdez-Garza1 , José M. Mata-Padilla2 , Juan G. Martínez-Colunga1 and Carlos A. Ávila-Orta1 \*

1 Centro de Investigación en Química Aplicada, Saltillo, Coahuila, Mexico

2 Consejo Nacional de Ciencia y Tecnología- Centro de Investigación en Química Aplicada, Saltillo, Coahuila, Mexico

\*Address all correspondence to: victor.cruz@ciqa.edu.mx; carlos.avila@ciqa.edu.mx

© 2021 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 3**

## Synthesis and Purification of Carbon Nanotubes

*Syed Awais Rouf, Zahid Usman, Hafiz Tariq Masood, Abdul Mannan Majeed, Mudassira Sarwar and Waseem Abbas*

## **Abstract**

In this chapter, we will evaluate the synthesis and purification of carbon nanotubes. Carbon nanotubes are cylindrical molecules that consists of graphene (rolled up of a single-layer carbon atom). A wide variety of synthesis techniques such as arc discharge synthesis, laser ablation of graphite/laser vaporization synthesis method, chemical vapor deposition (CVD), high pressure carbon monoxide synthesis and flame synthesis techniques, have been implemented to grow single and multi-walled carbon nanotubes for technological applications. All of the above methods exploit transition metals, like iron, cobalt, and nickel, as a catalyst. There are number of methods (filtering, chromatography and centrifugation) used to purify the carbon nanotubes, but the degree of purity remained questionable in these methods. In order to enhance the purification extent, alternate techniques such as Gas phase purification, Liquid phase purification and Purification by Intercalation are introduced. Here we will discuss the advantages and disadvantages of these purification routes. It will help researchers in selecting appropriate and effective method for synthesis and purification of carbon nanotubes.

**Keywords:** Graphene, Carbon Nanotubes, Synthesis, Purification, Laser Vaporization, Arc Discharge, Chemical Vapor Deposition, Gas Phase Purification, Liquid phase Purification, Intercalation

#### **1. Introduction**

Carbon atom contains six electrons with an electronic configuration of 2 2 1 ,2 *s s* and <sup>2</sup> 2 . *p* In its purest form, it crystallizes into graphite and diamond allotropes having different mechanical and optical properties. In former crystalline shape, the carbon atoms display <sup>2</sup> *sp* hybridization, where each carbon atom is covalently bonded with three other neighboring carbon atoms, making an angle of 1200 in x-y plane along with a pi (π) bond available in z-direction [1]. This makes honeycomb like hexagonal crystal structure of graphene and this structural pattern shows the basis for other materials like fullerenes [2]. In diamond allotrope of carbon, carbon atoms unveil −<sup>3</sup> sp type of hybridizing character, forming a regular tetrahedron [1]. Apart from existing allotropes of carbon (diamond, graphite and fullerenes).

With the emergence of the field of nanotechnology, the carbon material (graphene, fullerenes and carbon nanotubes) where <sup>2</sup> *sp* hybridization prevails have

attracted extreme focus from research community. Following the similar hierarchy, carbon nanotubes depict physical properties just like the graphene. Carbon nanotubes also offer <sup>2</sup> *sp* hybridization scheme and seems like a cylindrical coated graphene sheet in single and multiple wall patterns (**Figure 1a** and **b**). Nanotubes with single walls are named as single-wall carbon nanotubes (SWCNTs), firstly discovered in 1993 [5], while the nanotubes having multiple walls are termed as multi-wall carbon nanotubes (MWCNTs) discovered earlier in 1991 by Iijima et al. [6].

Immense interest in CNTs lies in their fascinating mechanical [7], electrical and optical properties [8] and hence are widely used in multiple applications such as field effect transistors [9], fuel cells [10], hydrogen energy storage applications [11], quantum computing [12] nanosensors [13–15] and battery electrodes [16]. The superior mechanical properties of CNTs are attributed to the higher values of tensile strengths and young modulus, thus revealing their potential use as a composite material to be used in futuristic Mars operation by NASA. Its use in such type of missions is subjected to its 50 times higher specific strength than the steel and hence creates exceptional load-bearing supports when integrated in composites. Field emission properties of CNTs have noticed enough attention from the research community, where the generation of electrons takes place under extreme conditions of electric field similar to thermionic emission. In addition, CNTs have also offered excellent chemical stability, higher electrical conductivity, nanosize and structural smoothness and are potentially used in flat display panels [8]. One can also attribute the use of CNTs in energy storage and energy production application to their smaller size, higher electron transfer rates, and superior surface topology in nanotubes.

As discussed above, CNTs have shown extremely smaller sizes, superior conductivity, greater mechanical strengths and elastic behavior, that is why these are used in other technological applications such as nanolithography, sensors, high resolution imaging and drug delivery systems also [17, 18].

Keeping in view the above-mentioned intriguing properties of CNTs, it is imperative to discuss the possible routes of their synthesis and the ways to

#### **Figure 1.**

*Schematic representation of SWCNT (A) and MWCNT(B) along with the transmission electron microscope (TEM) images of (C) SWCNT and (D) MWCNT respectively [3, 4].*

enhance purity of CNTs, as it will pave the way towards improved technological device applications.

## **2. Structural analysis of MWCNTs and SWNTs**

The type of bonding among carbon atoms plays crucial role in determining its different allotropes with distinct physical properties. When carbon constitutes SP2 hybridization, a layered structure is formed with weak van der Waal forces existing in out of plane carbon atoms, in contrast to stronger in-planes bonding among them. Ideal CNTs can be thought of nano-scaled graphene cylindrical shapes closing at each end via half fullerenes. In case of multi-walled carbon nanotubes, there exist at least two equicentered cylinders of graphene and theoretically, these numbers of cylinders can be infinitely large. It should be noticed that there must be regular spacing between any two concentric grapheme cylinders in MWCNTs. Previous studies have demonstrated a real spacing width of the order of 0.34 to 0.39 nm [19].

The real space analysis of multiwall nanotube images has shown a range of interlayer spacing (0.34 to 0.39 nm).it has been observed that the inner diameter of such nanotubes varies from 0.4 nm to roughly few nanometers, in comparison to its outer diameter ranging from 2 nm to 30 nm. MWCNTs are closed from both ends by pentagonal type of ring defect named as half-fullerenes, with significant axial size difference (1 μm- few cm) between both ends [19].

Previous studies on SWNTs has documented their length 109 times greater than their diameters [20]. SWCNTs can be combined together in the form of ropes, to give hexagonal crystalline structure [21]. SWCNTs can assume three different types of structures such as armchair, chiral, and zigzag (**Figure 2B**) depending upon their wrapping in cylindrical form. The structure of SWCNTs is categorized by a pair of indices (n, m) that define chiral vector, which has prominent effect on the electrical properties of carbon nanotubes. Unit vectors along both directions in the crystal lattice is determined by the integers *n* and *m* . When

*m* = 0 ; nanotubes having zigzag structure.

*n m*= ; nanotubes having armchair structure.

And other form is known as chiral structure.

The chiral vector can be defined as *C na ma* = +1 2 and it is used to measure the nanotube's diameter, where as *a*1 and *a*2 vectors explicate the grapheme base cell vectors. It is further stated that the chirality vector demonstrates the direction in which grapheme sheets are wrapped. One can estimate the diameter of a carbon nanotube is calculated by

$$d = a\sqrt{m^2 + mn + n^2} \text{ / } \pi \text{ \tag{1}$$

Where *a* = ×√ 1.43 3 shows lattice parameters of the grapheme sheet.

When m = 0, we get zigzag CNTs and if m = n, one ends up with armchair CNTs. For other values of m, chiral CNTs will be formed. If the difference of n-m is a number which is multiple of 3, then the nanotubes will show metallic behavior and will be of highly conducting nature, otherwise one will be dealing with semiconducting or semimetal types of nanotubes. Armchair type of SWCNTs are metallic in nature, while other structures can make the SWCNTs semiconductor also. The Russian model and Parchment model are two broadly used models to prepare the

#### **Figure 2.**

*Schematic of three different forms of SWNTs (A), where the chirality factor determines the diameter of carbon nanotubes and the (B) shows three different models of perfect SWCNT in atomic form [22].*

MWCNTs. In the **Russian Doll model,** carbon nanotubes confine another nanotube inside and the diameter of the outer carbon nanotube is greater than the inner tube. When a single graphene sheet is rolled up many times just like the scroll of paper, it is known as **Parchment model.** The properties of SWCNTs are identical with MWCNTs. The outer layer in MWCNTs protects the inner CNTs against the chemical activity. It is considered as the main cause of higher tensile strength, which is absent in SWCNTs [23]. SWCNTs display <sup>2</sup> *sp* bonding between two independent carbon atoms, and hence result in higher tensile strength even compared to steel, when used as composite material. This <sup>2</sup> *sp* bonding is stronger than <sup>3</sup> *sp* bonding, present in diamond. CNTs show elastic behavior upon the application of a strong force. It bends and twists without undergoing permanent deformation in carbon nanotubes. When external force is removed, it will come back to its original form. Its elasticity is measured by a quantity known as modulus of elasticity (**Table 1**).


#### **Table 1.**

*Difference between SWCNTs and MWCNTs [19].*

*Synthesis and Purification of Carbon Nanotubes DOI: http://dx.doi.org/10.5772/intechopen.98221*

## **3. Synthesis**

There are multiple methods to synthesis CNTs where gas phase processes are involved. These methods are mainly known as arc-discharge synthesis technique, laser-ablation method and Chemical Vapor Deposition (CVD). Laser ablation method involves the synthesis of CNTs under high temperatures, while in arc discharge technique, the synthesis of CNTs occurs at relatively low temperatures (<800°C). CVD method is currently in use, as it allows the control of nanotube's length, diameter, alignment, density and purity with maximum accuracy [24] during the synthesis.

### **3.1 Synthesis of CNTs via arc discharge method**

This method is implemented to synthesize the single and multi-walled carbon nanotubes (**Figure 3**) at a high temperature (above 1700°C).

The arc-discharge was initiated via applying a direct current of 200 A and a voltage of 20 V between the two electrodes. It was observed that the presence of iron, argon and methane was compulsory for the synthesis of SWNTs. The Arc discharge techniques is induced with the help of purest graphite electrodes having optical density of 6–10 mm and a diameter ranging from 6 to 12 mm. both of these electrodes were separated by 1–2 mm in a chamber containing helium gas at subatmospheric pressure. One can replace helium with hydrogen or methane gas. The working chamber consists of a graphitic anode and cathode, evaporated carbon [26] and minute amount of catalysts for example Ni, Co and Fe [27]. In arcing process is initiated by using direct current at pressure condition and the temperature of the chamber is raised up to 4000 K. In this procedure, half of the evaporated carbon is solidified on the tip of cathode. The rate at which evaporated carbon solidifies is set to be 1 mm/min and hence one gets "cigar like structure". During this process, the anode is also consumed. A remaining carbon is now a hard-gray shell structure, which is deposited on the edges and further condensates in the 'chamber soot' in nearby vicinity of the chamber's walls and 'cathode soot' on the negative graphite electrode (cathode). Furthermore, this inner material, anode soot and cathode soot (dark and soft materials) give rise to SWCNTS or MWCNTs along with nested graphene particles. Scanning electron microscopy (SEM) shows two different morphologies and surfaces are seen in the study of cathode deposited material. The

**Figure 3.** *The experimental set up of Arc discharge method [25].*

soft and dark inner core contains randomly oriented carbon nanotubes and the grey colored outer core is composed of grapheme layers.

In arc discharge synthesis technique, there are two different options to synthesized the carbon nanotubes; one with and other without using the catalyst precursors. Generally, the synthesis of MWCNTs is performed without using catalyst precursors. On the other hand, the synthesis of SWCNTs is subjected to the presence of different catalyst precursors. In order to expand the arc discharge, a complex anode [28], made of metal and graphite, is exploited. The metals used in complex anode range from Fe, Ni, Pt, Pd, Co-Pt, Ag, to a mixture of Ni-Ti Ni-Y, Co-Ni, Co-Cu. It is demonstrated to get highest yield (< 90%) of SWCNTs by using a complex anode, made up of a mixture of Ni-Y with an average diameter size of 1.4 nm [29] and this mixture is utilized worldwide to prepare SWCNTs on a large scale. This method is considered one of the most practiced method to synthesize SWCNTS in large quantities. But the main disadvantage of this method is least control over the chirality in the intended nanotubes.

#### **3.2 Laser ablation technique in the synthesis of CNTs**

A graphite block is heated in quartz tube via high power lasers in a furnace at a temperature of 1200°C in argon atmosphere [30]. Here the laser vaporizes the graphite target within the quartz tube and SWCNTs are formed in the presence of metallic catalysts. The diameter of prepared carbon nanotubes is manipulated as a function of laser power such as the diameter of the tube decreases upon increasing the power of laser pulses and vice versa. Some other studies have dictated that the ultrafast sub-picosecond lasers have the ability to produce single walled carbon nanotubes on a large scale too [31]. It is further reported to manufacture carbon nanotubes up to 1.5 g/h via laser ablation method.

To harness CNTS with desired structural and chemical features, one should monitor the effect of different properties of lasers (peak power, frequency, oscillation wavelength, cw versus pulse), chamber pressure, distance between graphite target and substrate, ambient temperature and the flow and pressure of the buffer gas. By using this process, one can achieve high quality (purity) SWCNTs in large quantities. The mechanism and principles of laser ablation is identical with the arcdischarge method. Here the required energy is provided by a laser which strike with pure graphite pellet holding catalyst material i.e. cobalt and nickel (**Figure 4**).

The primary advantages of this method are the presence of the smaller amounts of metallic impurities and higher yield of CNTs. On the other hand, the carbon nanotubes produced via laser ablation method are not perfectly straight and uniform. This is an expense method due to the requirement of high purity graphite rod and the availability of two laser beams to produce CNTs. By using this technique, the yield of nanotubes per day is relatively smaller than the arc discharge technique.

#### **3.3 Chemical vapor deposition for CNTs synthesis**

One of the best techniques for the production of CNTs is chemical vapor deposition (CVD). There are different CVD techniques such as catalytic chemical vapor deposition either thermal [33] and water assisted [6], plasma enhanced oxygen assisted CVD [34–36] or hot filament CVD (HFCVD) [37]. But most extensively implemented CVD method for the production of CNTs is known as catalytic chemical vapor deposition. This route involves the Chemical breakdown of hydrocarbon on a specified substrate and helps expand the CNTs on different type of materials. Carbon atoms remain intact with the metallic catalytic particles, as was the case for arc discharge technique. After that carbon atoms are enabled to come in contact

*Synthesis and Purification of Carbon Nanotubes DOI: http://dx.doi.org/10.5772/intechopen.98221*

**Figure 4.** *The laser ablation process [32].*

**Figure 5.** *Chemical vapor deposition [38].*

with metal particles and implanted with in the holes, initiating the production of carbon nanotubes (**Figure 5**).

This technique facilitates well aligned long carbon nanotubes and a layer of metallic catalyst particles are produced at 700°C. Most commonly catalyst metals are cobalt, nickel, iron and combination. The expansion of nanotubes carried out in fluidized bed reactor in the presence of a gas containing carbon such as ethylene, acetylene, methane, etc. and a process gas like H, Ne, or ammonia are used as well. The process gas reacts with the catalyst particles and disintegrates. Carbon atoms become prominent at the edges of nanoparticles where CNTs are created. CVD is very economical practical method for quite pure and large-scale production of carbon nanotubes as compare to laser ablation method. This method is easily controllable and give high purity of obtained materials, this is the main advantages of CVD [39].

## **4. Purification of carbon nanotubes**

Above mentioned as-synthesized methods of CNTs encounter certain impurities, such as smaller fullerenes, wrapped graphite sheets, metal catalyst particles, and amorphous carbon contaminations. It is observed that the percentage of these impurities generally enhances as long as the diameter of CNTs increases. Therefore, it is important to get rid of these impurities to obtain homogenously distributed CNTS in polymer or dispersion media due to their substantial effect on electro-mechanical properties of CNTs, interfering with the expected applications. It makes it unavoidable to apply certain purification techniques to get pure CNTs with better electrical and mechanical features [40, 41]. Due to the insoluble nature of CNTS, it is quite challenging to use liquid chromatography to get rid of these impurities. In addition, number of groups across the globe just characterize the commercially synthesized carbon nanotubes and do not have facilities to grow them. Due to the application of different analytical techniques such as Raman, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), even SWCNTs have shown doubled, triple and multi-walls of a single sample along with the presence of above-mentioned impurities. Hence, one cannot rely on the specification provided by different companies. Subjected to these various analytical characterization and impurities, researchers have applied various purification techniques, leading to significant loss of CNTs [42–44]. It is further observed that the use of acid treatment or surfactants might result in CNTs with activated surfaces, putting comprehensive changes in their desired properties [45].

Depending upon the nature of the structure (single-walled or multi-walled) in hands, growth process, and metal catalysts, various purification techniques such as mechanical, chemical and physical routes are to get dispersed carbon nanotubes with maximum possible exclusion of impurities [23]. The chemical methods allow the variation in surface energy by introducing functionalization of carbon nanotubes. It leads to better wettability and adhesion of CNTs to the polymer target media and hence the tendency of agglomeration decreases. But the use of acids might deteriorate the structural quality of CNTs, attributing un-desired physical properties.

The chemical route of purification produces highly pure CNTs but fragile to structural defects and product losses [46]. However, CNTs with higher purity can be achieved by removing the metal catalyst particles in controlled reaction. Physical methods are attractive due to the possibility of adsorption of variety of functional groups, leaving behind similar pi (π)- graphene structure and are implemented when higher weight fraction of CNTs is desired. Physical method separates the yield products on the bases of the size of CNTs [47]. Physical methods. These methods cause low damages and are more complex as well as less effective as compared to chemical methods. Here we will only explain the chemical methods for purification [48]. The most commonly used chemical purification method involves oxidation of synthesized CNTs in gas phase as well as in liquid phase. Most common purification methods with high success rates are


### **4.1 Gas phase**

Purification can be done in dry gas oxidation. Carbon dioxide, hydrogen gas and dry/wet air are commonly used oxidation gases for this method [49–51].

*Synthesis and Purification of Carbon Nanotubes DOI: http://dx.doi.org/10.5772/intechopen.98221*

#### *4.1.1 Air*

Air oxidation is one of the gas phase oxidation methods to purify CNTs. The impurities in CNTs are removed by the thermal air oxidation at moderate temperatures. The walls of the CNTs and the binding between the entangled CNTs are affected by the presence of oxygen. It is also known as a strengthening process which starts at 480°C and amorphous carbon usually decayed between 480 and 500°C [52, 53]. The reactivity rate is greater for structure and amorphous carbon than cylindrical wall of CNTs when oxidation is done in air. Due to this selective oxidation, the amorphous carbon can be bare-off from the cross-linked CNT collections. If the temperature is raised to 750°C during the annealing process, the loss rate of CNTs enhances to about 90% and the structure of CNTs is destroyed significantly.

#### *4.1.2 Other gases*

As in gas phase oxidation methods, controlled rate heating of CNTs is implemented for a longer period of time. Here the disordered amorphous carbon that is coming from the tip, destructs the purification on the base of oxidation by*CO*2 . Mild oxidation carried out with CO2 at 600°C [54].

The route of the reaction is shown:

$$\text{CO}\_2 + \text{C} \to 2\text{CO} \tag{2}$$

Amorphous carbon and metal catalyst particles coated with carbon may be removed by hydrogen gas treatment at high temperatures. Amorphous carbon is converted into the carbon dioxide in air and then transformed again into methane in the presence of hydrogen. Ammonia (NH3) is used to remove residual carbon impurities and repair the damaged in sidewalls of CNTs, instead of using hydrogen gas. Ammonia has advantages over hydrogen in terms of ease in handling. Only a small number of defects are observed in the CNTs when they were exposed to NH3 gas at high temperatures during purification process. In addition, strong van der wall forces are induced between CNTs after NH3 treatment, leading to a damage recovery of sidewall [52, 55] of CNTs.

#### *4.1.3 Effect of gas phase oxidation*

Oxidation of amorphous carbon in gas phase is easy to control as compared to liquid phase oxidation techniques. Higher activation energies are required in gas phase oxidation processes. Gas phase oxidation can better oxidize the CNTs than the liquid phase oxidation without introducing defects. This yield purified nanotubes, arranged in tight bundles without forming clusters. Moreover, there is no need to use complicated/sophisticated equipment, filtration and separation processes required after the purification [56, 57].

#### **4.2 Liquid phase**

Despite the fact that the advantages of gas phase oxidation are clear, it has some limitations. Metal particles cannot be eliminated straightaway, and further treatment is compulsory. In order to overcome this drawback, liquid phase purification treatments are developed to eliminate the amorphous carbon and metal catalysts [57, 58].

The oxidant and mineral acid, in the form of solution can uniformly react with the network of the raw CNTs samples. Therefore, processing with selective oxidizing agent with precise control can yield high-purity CNTs. The scientific community mostly uses HNO3, NaOH and H2O2 as an oxidizing agent for liquid phase oxidation.

#### *4.2.1 Nitric acid oxidation*

Nitric acid is commonly used for purification of CNTs due to its capability of removing metal catalysts, nontoxicity and economy. It can remove the amorphous carbon selectively because of its mild oxidizing ability. A concentrated nitric acid is used to produce SWCNTs through laser ablation in a single step. The synthesized SWCNTs were sonicated in concentrated nitric acid for a few minutes, following the refluxing for 4 h under magnetic stirring process carried at 120–130°C. The product reached 30–50 wt. % of its raw samples and the metal defects were reduced up to app. 1@ wt. %. The purity of SWCNTs and its production totally depends upon the concentration of nitric acid and reflux time in nitric acid treatment. The elimination of metallic impurities can be confirmed via XRD analysis of CNTs. During the purification, the nitric acid reacts with the defected parts and intercalate into the CNTs to unzip the tube walls by further oxidative etching, which in turn causes an increase in nanotubes inter-layer spacing. Normally, the reactive carbons were eliminated through the following chemical reaction:

$$\text{C} + 4\text{HNO}\_3 \rightarrow \text{CO}\_2 + 2\text{H}\_2\text{O} + 4\text{NO}\_2 \tag{3}$$

Most of the catalyst particles are removed in nitric acid's treatment at high temperatures for 24 h. The unwanted impurities are removed and melted effectively from CNTs, and some oxidative defects in the sidewall of CNTs are also induced in this process. The intensity of the D-band spectrum produced by the defects and carbon particles in the sample after acid treatment, can be used to determine the disorder degree of the sample [57, 59].

#### *4.2.2 Sodium hydroxide treatment*

It has been observed clearly from Scanning electron microscope (SEM) that silica and alumina support can be eliminated significantly after NaOH treatment. Based on the reaction between NaOH and carbon, a single-step method for simultaneous purification and opening of multi-walled carbon nanotubes has been formulated [48, 60]. The redox reactions between carbon and NaOH followed through the highly reactive sites of the material. As a result, NaOH only interacts with the carbon impurities and defects of the carbon nanotubes, that is with the tip while the uniform graphite layers of the nanotubes walls remain intact. This is because metallic sodium cannot be inserted into well-organized materials, and can only be carried out by highly disordered carbon impurities [61]. Therefore, in addition to the opening of tubes, NaOH treatment removed the catalytic support, amorphous carbon, and the catalyst metal particles. Its mild conditions removed the metal impurities without damaging the sidewalls of CNTs.

#### *4.2.3 Hydrogen peroxide oxidation*

Hydrogen peroxide (H2O2) attacks on the carbon surface and cannot eliminate metal particles due to its mild oxidization capability. It is inexpensive and green

*Synthesis and Purification of Carbon Nanotubes DOI: http://dx.doi.org/10.5772/intechopen.98221*

oxidizing agent and is commonly used with HCl. Generally, H2O2 can be transformed into a toxic salt. H2O2 with HCl has been examined to eliminate the metallic particles during purification of CNTs. Macro-scale purification consists of two parts such as refluxing treatment in H2O2 solution following the cleaning process performed with HCl. Particle size of Fe has a significant effect on the oxidation of amorphous carbon. The oxidation and removal of metal particles in this process is performed in a single container to make it simple. The purity and yield of the product in this treatment are better than NH3 treatment. Carbon coated iron impurities were liquified in an aqueous solution of H2O2 and HCl at 40-70°C for 4–8 h. The production of CNTs increased to approximately 50 wt. % and the purity raised up to 96 wt. % with this treatment [62].

#### **4.3 Intercalation method**

Halogen may be intercalated into carbon nanotubes for selective oxidation of carbonaceous impurities. Brominating is one of the effective procedures in CNTs purification process. Graphite intercalation compounds are formed by the attachment of atomic or molecular layers of a different chemical species between layers in graphite host materials. The intercalation of bromine (Br2) in CNTs is confirmed by using HR-TEM [48]. The mixing of raw CNTs with pure liquid bromine under nitrogen atmosphere yielded Br. Under these conditions, charge transfer between Br and carbon occurred, enabling the formation of complex C-Br2 on CNT surface and at deformed sites. It was observed that the orientation of Br on CNT surface is like a rod of a wheel, which is perpendicular to the curve of graphitic layers on CNTs. Intercalation of Br usually, happens on the surface of CNTs, where large numbers of defected sites are available. Br will be more reactive to those regions where different types of defects (amorphous carbon and other disorder carbonaceous materials) exist. When brominated, CNTs were passed through the air combustion at 550°C, and it was observed that the layers of the graphite were damaged along the line in which Br collected, showing the effect on the reactivity of the tubes toward oxygen upon adding Br. The amorphous carbon can be effectively oxidized due to the oxidation difference between brominated regions and CNTs. The catalyst particles, which was bounded, were opened and removed at the same time. Due to the tube action, Br diffused into the tubular CNTs and caused in the breakage of inner graphite layers during oxidation [63–65].

## **5. Conclusion**

A detailed overview of synthesis and purification of carbon nanotubes is presented in this chapter. Synthesis techniques (i.e., arc discharge synthesis, laser ablation of graphite/laser vaporization synthesis method, chemical vapor deposition (CVD), high pressure carbon monoxide synthesis and flame synthesis) have been described in detail to highlight their importance as well as drawbacks. Arc discharge synthesis method is one of the most used technique for carbon nanotubes in large quantities. Its main drawback is the lack of control over the chirality in the nanotubes. Laser ablation method has the ability to produce CNTs in large quantities having small impurities. However, it is an expensive method as compare to arc discharge method for the synthesis of CNTs. A high purity CNTs can be obtained by using Chemical vapor deposition method. It is most suitable for large-scale manufacturing of CNTs at economical cost than laser ablation method. Chemicalbased purification methods (i.e., gas phase, liquid phase and intercalation method) for CNTs are discussed comprehensively. These methods can efficiently eliminate

### *Carbon Nanotubes - Redefining the World of Electronics*

amorphous carbon, polyhedral carbon and metal impurities at the cost of decreasing a significant amount of CNTs or damaging structure of CNTs. Gas phase purification is considered for purifying CNTs because it does not significantly grow sidewall defects in CNTs. However, it has limitation that it does not remove metal particles straightforwardly. Liquid phase oxidation produces defects on CNTs sidewall and may break-down CNTs into shorter ones with different lengths. The intercalation is best suitable for purifying CNTs without destroying their alignment. These features of synthesis and purification methods of CNTs will help researchers to select between these different methods according to their requirements.

## **Author details**

Syed Awais Rouf<sup>1</sup> \*, Zahid Usman1 , Hafiz Tariq Masood2 , Abdul Mannan Majeed1 , Mudassira Sarwar1 and Waseem Abbas1

1 Division of Science and Technology, Department of Physics, University of Education, Lahore, Pakistan

2 Department of Physics, University of Sahiwal, Sahiwal, Pakistan

\*Address all correspondence to: awais.physicist@gmail.com

© 2021 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.

*Synthesis and Purification of Carbon Nanotubes DOI: http://dx.doi.org/10.5772/intechopen.98221*

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## Section 3
