Recent Advances in Methods for Synthesis of Carbon Nanotubes and Carbon Nanocomposite and their Emerging Applications: A Descriptive Review

Gacem, Amel; Modi, Shreya; Yadav, Virendra Kumar; Islam, Saiful; Patel, Aradhna; Dawane, Vinars; Jameel, Mohammed; Inwati, Gajendra Kumar; Piplode, Satish; Solanki, Vijendra Singh; Basnet, Anup

doi:https://doi.org/10.1155/2022/7238602

Journal of Nanomaterials

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Carbon-Based Nanomaterials in Pest and Environmental Management

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Review Article | Open Access

Volume 2022 | Article ID 7238602 | https://doi.org/10.1155/2022/7238602

Recent Advances in Methods for Synthesis of Carbon Nanotubes and Carbon Nanocomposite and their Emerging Applications: A Descriptive Review

Amel Gacem,¹Shreya Modi,²Virendra Kumar Yadav,³Saiful Islam,⁴Aradhna Patel,⁵Vinars Dawane,⁶Mohammed Jameel,⁴Gajendra Kumar Inwati,⁷Satish Piplode,⁸Vijendra Singh Solanki,⁹and Anup Basnet¹⁰

Academic Editor: Seema Ramniwas

Received20 Jun 2022

Accepted16 Sept 2022

Published29 Sept 2022

Abstract

Nanomaterials have gained huge applications ever since their discoveries, especially in the field of electronics, medicine, research, and environmental cleanup. Nanomaterials have a high surface area-to-volume ratio along with high surface energies making them suitable for such wide applications. Carbon nanotubes (CNTs) and carbon nanocomposite (CNC) materials are remarkable nanomaterials that have become the backbone of most industries these days. Both materials have gained huge attention in the last decade by the scientific community. CNTs come in two variants, i.e., single-walled CNTs (SW-CNTS) and multiwalled CNTs (MW-CNTs). Due to their wider applications, CNT synthesis is currently emerging with the advancement in technology. Currently, CNTs are being synthesized by chemical as well as physical approaches. The current review article focuses on the vital research and application for the synthesis of CNTs depending on the quality of the nanotube materials. Controlled routes to their organization and assembly are also discussed in detail over here. The aim is to provide recent advances in the synthesis methods, of CNTs, their current applications, future applications, and the potential of agrowaste and industrial waste for the synthesis of CNTs and nanomaterials.

1. Introduction

With the advances in nanotechnology and nanosciences, a drastic change is observed in the field of material sciences. Being small in size and having a high surface area-to-volume ratio (SVR), nanoparticles have gained huge attention in the field of electronics, medicine, and environmental cleanup [1, 2]. Among all the nanoparticles, carbon-based nanoparticles like graphene and carbon nanotubes are most widely exploited for industrial applications. Currently, CNTs have overpowered all carbon and metallic-based nanoparticles due to their unique and remarkable properties like high mechanical strength and lightweight [3]. Most of the synthesis techniques of CNTs are expensive due to the energy-intensive step. So, the scientific community is looking towards an alternate source of CNTs as a precursor material, for instance, coal fly ash (CFA), red mud, agricultural waste [4], plastic waste, and tires [5]. Most of these materials are waste of one industry and are easily available as a precursor material at a much more economical cost. So, the final product developed from such waste will also be cost-effective. The utilization of such waste-derived CNTs and carbon nanocomposites for environmental cleanup could prove to be much more economical.

Carbon nanotubes (CNTs) along with both single- and double-walled carbon nanotubes have left a notable impact. They have unique properties and were discovered in 1952 and 1976, respectively [6]. Since the discovery of CNTs in 1991 by the Japanese electron microscopist Sumino Iijima, a profound increase has been noted in the research and applications in this field [7]. CNTs are novel cylindrical hollow nanostructures that can be synthesized by both top-down and bottom-up approaches [8]. CNTs are made up of carbon atoms linked in a hexagonal shape, with each C-atom bonded to three other C-atoms by the covalent bond or a single sheet of pure graphite with a diameter of 0.7 to 50 nanometer (nm) [9]. The length of nanotubes may vary typically up to several microns [7]. With recent advancements in technology, the length of the nanotubes has been measured in centimeters also [10]. Research has been shifting towards the development of CNTs-based nanomaterials due to their advanced and unique chemical, electronic, mechanical, and structural features [11]. CNTs are built from graphene sheets having sp² carbon units, arranged in hexagonal networks [12]. CNTs are differing in length, thickness, structure, type of helicity, and number of layers [13]. CNTs show extraordinary properties with hardness almost equivalent to the diamond and thermal conductivity twice than that of pure diamond. According to their formation and types, nanotubes act either as metals or semiconductors [8]. According to their structure, CNTs are categorized as cylindrical shape and sometimes as pentagon structure with closed ends [5, 14], which are shown below in Table 1.

The authors have searched carbon nanotubes, carbon nanocomposites, and synthesis on science direct and found about more than 30000 articles, in the last five years, i.e., 2017-2022. There is continuous increase in the research on the CNTs which is evidenced by a huge number of articles in this field in recent years. In the year 2017, there were about 3283; in 2018, there were 3768; in 2019, there were 4453; in 2020, there were 5127; in 2021, there were 6546; and in year 2022 till August, there are about 6354 articles. CNTs has gained huge attention in the last few years. Out of all these 30000 articles in last five years, 16880 were alone research articles, followed by review articles which were 6431, 4256 as book chapters while rest were abstract, encyclopedia, and conferences. So, CNTs are one of the most widely focused material in the scientific community.

From the various articles, it has been found that due to high demand of CNTs and CNTs-based nanocomposites in the various fields like environmental cleanup, biomedicine, and electronics. The majority of synthesis are physical, like laser ablation, arc discharge method, and physical vapor deposition. Among chemical methods, chemical vapor deposition is most widely used. But most of these methods are expensive due to energy intensive process, i.e., reaction at high temperature. There are several reports where novel, innovative, ecofriendly, and economical approaches have been followed for the CNT synthesis. While among applications, it has been found that CNTs and their composites are widely used in the environmental cleanup like dye removal, heavy metal removal, antibiotic, and pesticide remediation from wastewater. Among medicines, it is widely used as a drug delivery agent for various anticancer drugs. After detailed investigation, all the applied techniques along with emerging ones are summarized over here.

The current review emphasizes on the current and emerging approaches for the synthesis of CNTs. Besides this, it also emphasizes on the potential of economical, biological, and industrial wastes as a source of raw material for the CNTs. The current and emerging applications of CNTs and nanocomposite are highlighted especially in the medicine, research, environmental clean-up, and electronics. The objective was to provide a summarized information about utilization of waste like agrowaste and industrial waste as an economical precursor source for the CNTs. Furthermore, utilization of waste for CNT synthesis will help in minimizing the solid wastes from the environment.

2. Recent Advances in the Synthesis Methods of CNTs

A number of physical, chemical, and electrochemical approaches have been used for nanotube synthesis [14]. Various types of precursors have been used for CNT development including acetylene, benzene, carbon monoxide, ethylene, and xylene [15]. CNTs have been synthesized by various physical and chemical methods for instance electrolysis, arc discharge, chemical vapor deposition (CVD) [15], sonochemical method, and laser ablation [16]. All these methods and their utilization for the synthesis of CNTs are given below in Figure 1.

2.1. Arc Discharge Method

Parkansky et al. synthesized MWCNT using single-pulse arc at room temperature. This method does not require any other kind of pretreatment. The sample is prepared using arc discharge between the target and counter electrode. Graphite plates, C-coated Cu grids, and Ni-coated glass slides are used in the reactor [17]. Wang et al. synthesized CNT using arc discharge in deionized water. A simple reactor is made using open vessel (1 l), graphite electrodes, and a DC power supply. Both anode and cathode are made of graphite and dipped in deionized water. After the arc discharge CNT is deposited on cathode. Black piece of CNT is collected for further examination. Within 60 seconds, the sample is prepared in the present study [18]. Wang et al. synthesized CNT using simple traditional direct current (DC) arc discharge reactor. Graphite tube, CuO powder, and graphite rod are used as consuming anode and cathode, respectively. Graphite tube is used as consuming anode and is filled with anthracite coal. The CuO-to-coal weight ratio is usually reported in the ration of 1 : 9. About 70 A and 20 V DC voltage is applied as experimental conditions, respectively, in the current investigation. The AD is conducted for 15 minutes, and the final sample is collected from cathode [19]. Tsai et al. synthesized CNT using single-pulse discharge. The synthesis mechanism is based on microelectrodischarge method and is comprised of microelectrodischarge system. The applied voltage was 80 V, 2.0 A, and 2 μm, respectively. The electric current and discharging distance were also considered experimental conditions. Graphite electrode and substrate were used for microelectrodischarge system. A thin layer of Ni was used as a catalyst in the system. After single-pulse discharge, CNT was collected for further study. The experiment was carried out completely in an open atmosphere [20]. Zhao et al. synthesized CNT using alternate current (AC) arc discharge plasma. The experiment was carried out at normal atmospheric pressure. Experimental conditions were voltage and current at 380 V and 6 A–20 A, respectively. Tungsten electrode was used for the reactor preparation. Argon and benzene have been used as the plasma source and carbon source, respectively. Benzene was entered in the system using argon flow; simultaneously, argon and benzene were also introduced into the system. Finally, the arc discharge sample was collected on the internal wall of the reactor [21].

Su synthesized CNT using the one-pulse discharge method. The microelectrodischarge system was used for the synthesis in present investigation. The – axis was used for position of discharging, and -axis was used to adjust the discharging distance. Peak currents (1.5, 2.0, and 2.5 A) and pulse durations (1200 and 1400 ms) were the experimental conditions used for the synthesis of CNT [22]. Maria and Mieno synthesized SWCNT using a low-frequency bipolar pulsed arc discharge method. They designed bipolar pulsed current circuit with constant current and constant pulse duration. Increased quantum efficiency was observed as compared to the unipolar pulsed arc discharge method. Two rectangular type C-Ni-Y electrodes were used in this study. He gas and 50 kPa pressure as experimental conditions were used for synthesis, and 0-5 Hz frequency was used for high quantum yield of synthesized material [23]. Berkmans et al. synthesized SWCNT using AD technique in open air. In the simple arc discharge setup, graphite was used as both the anode and cathode. A hole with measurement of 3 mm, and depth of 50 mm was made in the anode. This hole filled with 1 : 1 : 1 ratio of graphite, nickel, and yttria powders. Potential, direct current, power density, and current density were 36 V, 100 A, 3.8 kW/cm², and 105 A/cm² respective for experiment. After establishing, the arc discharge black fumes (CNT) was collected. The whole experiment was complete within 6 minute [24].

Arora and Sharma synthesized MWCNT from carbon black using AD technique. Two graphite electrodes were used for the construction of reactor. Anode having a hole and filled with carbon black was used. Graphite rod was used as cathode. After a 60-second arc discharge, the MWCNT was collected at the cathode [25]. Sun et al. synthesized CNT using AC arc and dielectric barrier discharge plasma. A simple AC arc plasma reactor with two sources were modified for the synthesis of CNTs. A mixture of propane and argon gas was injected into the preheated furnace. The furnace was made up of silicon nitride. In the simple procedure due to plasma generation, gaseous molecules were converted into positive and negative ions and neutral particles. During the synthesis, solid CNTs and gaseous byproducts were generated. CNT material was collected from the bottom of the container. Source can generate 20 kV and 20 to 40 kHz voltage and frequency range, respectively [26].

Riberio et al. reported the synthesis of CNTs by ADM, which was later on purified in order to remove the heavy metals, graphitized carbon, and amorphous carbon. Further analysis revealed that about 75% impurity was removed by adopting current techniques [27].

2.2. Laser Ablation (LA)

Muñoz et al. worked on the role of gas and pressure on synthesis of single-walled carbon nanotubes. They used Ar, N, and He gases at 50-500 Torr pressure for conducting the experimental procedures. Continuous wave 10.6 μm CO₂–laser was applied for laser ablation. This laser was focused onto a graphite, Ni (4.2 at. %)/Y (1 at. %) composite target rod. The spot size and power density were measured to be 0.8 mm² and 0.8 mm², respectively [28]. Bolshakov et al. synthesized SWCNT using CW laser–powder method. A CW CO₂ laser (maximum output power of 2.1 kW, 2 aperture of 40 mm, and divergence of 4 mrad) was used for the ablation of graphite (carbon source). Laser focused on the mixture of graphite and metallic catalyst powders with the effect of argon and nitrogen gas. The gas sample was simultaneously collected from the lens using pure buffer [29]. Zhang et al. synthesized SWCNT using LA method at room temperature. Co/Ni (0.6/0.6 at %) as a catalyst and graphite powder were converted into plate target by heat treatment. Target was placed used 200-400 Torr pressure created by argon gas. Laser beam was focused on the target to obtain the CNT material [30].

Radhakrishnan et al. synthesized MWCNT using excimer laser ablation method. KrF laser (Lambda Physik 210 I, 248 nm, FWHM 25 ns, and laser intensity of W/cm²) has been used for synthesis, and 2 Torr gas pressure of oxygen and argon was controlled during preparation. The sample was collected at Si substrate [31]. Chrzanowskas et al. observed the effect of laser wavelength during the formation of CNTs by the LA method where laser-furnace reactor was used for synthesis with Pa pressure under argon background was used in a quartz tube. Here, 1% graphite decorated with Ni and Co nanoparticles were placed in the quartz tube, and laser has been used for ablation. A quartz tube was placed in a preheated (1000°C) furnace. Double-pulse Nd:YAG Ekspla 303 D laser system was used for the irradiation purpose. This system generates two pulses with same wavelength. Effect of laser wavelength has been done using 355, 532, and 1064 nm wavelength [32]. Yuge et al. used CO₂ laser ablation method for synthesis of SWCNT at following operating conditions, i.e., 3.5 kW CO₂ laser was operated at mm target rotating at 2 rpm for 30 sec. The sample was collected after some time. Boron- and nitrogen-doped SWCNTs were also prepared in this method. Gas pressure and flow rate have been controlled at 760 Torr and 10 l/min. The sample was prepared using argon and nitrogen atmosphere [33].

Alkallas et al. reported the synthesis of CNTs, decorated with NiO NPs by laser ablation technique. The synthesized materials were analyzed by the sophisticated instruments followed by its application for the remediation of methyl orange dye from wastewater. The developed nanocomposite have high adsorption capacity due to large surface area [34].

Mostafa et al. synthesized multiwalled CNTs decorated with Ag NPs by using laser ablation method in a liquid media. As per the investigators, the synthesis method was green as well as the one-pot method. The synthesized nanocomposite was used as a catalytic degradable agent for the nitrogen compounds and dyes. The synthesized NC has good catalytic activity and high distribution without utilizing any surfactant [3].

2.3. Chemical Vapor Deposition (CVD)

Colomer et al. synthesized SWCNT using CCVD in a fixed-bed reactor. In this investigation MgO, methane, and mixture of methane/hydrogen were used as catalyst, carbon source, and carrier gas, respectively. After synthesis catalyst was removed using a concentration of hydrochloric acid and after purification, the sample was collected. The sample was prepared within 10 minutes using this method [35]. Scott et al. synthesized SWCNT using LA. In this method, graphite and cobalt/nickel system were used as the target and catalyst. Two Nd:YAG pulsed lasers were for ablation purpose operated at 532 nm and 1064 nm. The laser was operated on the target placed in the preheated furnace under argon environment created at 67 kPa [36]. Braidy et al. synthesized SWCNT using Nd:YAG laser vaporization method. In this method, three configurations of pulse at 532 nm, 1064 nm, and 532 nm with 1064 nm have been used for investigation. Target was prepared using graphite powder (2325 mesh), a C-cement, and Co–Ni powder catalyst (0.6% at.). The target was treated at 35 MPa and 800-1150°C in the presence of argon. The target mounted at quartz tube in preheated furnace was vaporized using pulse. After cooling, the sample was collected from the exit of the furnace [37].

Kokai et al. synthesized SWCNT using CO₂ laser vaporization. The target was prepared using graphite-Co/Ni (1.2 at. %). CO₂ laser beams (10.6 mm and 1 kW peak power) was used for the vaporization of the target material. The target was place in quartz glass situated at center of the preheated furnace. Simultaneously, argon gas was introduced in the furnace. After vaporization using CO₂ laser beam, the CNT was collected from the quartz glass plate [38]. Kokai et al. synthesized DWCNT using alumina-catalyzed CVD method. Methanol solution was used deposition of iron salt on an alumina support material. Flow of argon at 900°C temperature has initiated the process of CNT. After 10-minute, the sample was collected after cooling the reactor [38]. Qiu et al. synthesized SWCNT from coal gas using CVD. A horizontal quartz tube reactor was used for the synthesis. Two furnaces were also used in the system. In the first furnace, quartz boat having ferrocene catalyst was preheated (200°C) with coal gas. The second furnace was slowly heated and cooled after some time. After experiment, the black fiber-like materials deposited on quartz tube collected for further study [39]. Li et al. synthesized MWCNT using MgMoO₄-catalyzed CVD. Mg (NO₃)₂·6H₂O and (NH4)6Mo₇O₂₄·H₂O were used for the synthesis of MgMoO₄ catalyst. 1 g of MgMoO₄ catalyst was sprayed in a quartz boat put in a tube furnace at 1000°C. A mixture of methane/hydrogen gas was also used for synthesis [40]. Eftekhari et al. worked on the synthesis of MWCNT with cone-like heads using catalytic chemical vapor deposition (CCVD). Cobalt was used as the catalyst, and 1 : 1 ratio of barium chloride and calcium chloride was used as the catalyst support in the present investigation. Citric acid was also used as a foaming agent. All the chemicals were dissolved in water and dried at room temperature. The resultant product was a MWCNT with cone-like heads [41]. Terrado et al. synthesized MWCNT using thermal CVD. Quartz substrate was used to grow the CNT, and Co was used as a catalyst in the study. Different concentrations of cobalt nitrate in ethanol were sprayed on cleaned and sonicated quartz substrate under 1 bar pressure. At last, the sample was dried at room temperature. After that, the substrate was treated at 750–900°C in ambient condition and acetylene. Samples were collected after cooling the reactor [42]. Mckee et al. synthesized MWCNT using floating catalyst CVD method. Pyrolytic decomposition of a benzene-ferrocene has been responsible for the synthesis of CNT. In a typical reactor, two furnace systems were used. The furnace was heated up to 750°C temperature. Argon and ferrocene–benzene was introduced into the preheated furnace. After some time, the CNT sample was collected on the substrate. Various operational parameters including catalyst concentration, growth time, and substrate type were also evaluated in the present investigation [43].

Shirazi et al. examined the role of various C-sources on the formation of MWCNT. The roles of cyclohexanol and xylene were evaluated on the synthesis method. Horizontal stainless-steel reactor was used in the study. About 1 : 20 molar ration of ferrocene and cyclohexanol or xylene was used as reagent. Evaporation of all the reagents has been done using oil bath [44]. Mubarak et al. synthesized CNTs using two-stage CVD by using ferrocene was used as the catalyst. Ceramic boat and two furnaces were used in the reactor where C₂H₂ was used as the carbon source. A catalyst was placed in a ceramic boat near the first furnace. The CNT was collected from the second furnace. A mixture of hydrogen/argon gas was also used in synthesis [45]. Hata et al. synthesized 99.98% pure SWCNT using water as the solvent. Ethylene and Ar or He with H₂ were also used in the synthesis. Fe nanoparticles and thin film were used as a catalyst on Si metal foil, quartz, and wafers in the present investigation. The synthesis method was completed in 10 min of reaction time [46].

2.4. Flame Method for the Synthesis of CNTs

Vander Wal and Ticich synthesized SWCNT and MWCNT using flame and furnace and performed a comparative study. In this study, Fe or Ni was used as the catalyst along with mixture of CO/H₂ and C₂H₂/H₂ with Fe to produce SWCNT and MWCNT. The CO/H₂ was used as a pyrolysis flame which was established after the addition of nanosized (catalyst) precursor particles as an aerosol. The aerosol was generated by drying a nebulized Fe or Fe colloid solution [47]. Yuan et al. synthesized MWCNT using ethylene flame. Stainless steel and cobalt–electrodeposited stainless-steel grid was used for synthesis. The stainless grid was heated with propane–air flame. Nitrogen was introduced to maintain the temperature of the system. The final product was obtained using nitrogen diluted–ethylene diffusion flame [48]. Yuan et al. synthesized MWCNT using methane flame. The stainless grid with Ni-Cr wire was used in this method. The final product was obtained using laminar co-flow methane-air diffusion flame [48]. Vander Wal et al. synthesized SWCNT using multistage flame configuration. The first-stage flame produces Fe catalyst, and CO, H₂, and He with air were used as a fuel-rich mixture in the first-stage flame. This fuel-rich mixture mixed with acetylene–air mixture comprised the second-stage flame. The Fe catalyst was used in the second-stage flame for the formation of SWCNT [49]. Saveliev et al. synthesized CNT using oxygen flame. 96%CH₄ with 4%C₂H₂, 50%O₂ with 50%N₂, and Ni-alloy (73%Ni+17%Cu+10%Fe) have been used as fuel, oxidizer, and catalyst. Fuel and oxidizer were mixed in the burner. Nitrogen gas was also introduced in the reactor during the burning process. After 10 minutes of reaction, CNT was collected [50]. Lee et al. synthesized MWCNT using an ethylene inverse diffusion flame. Air and C₂H₂ were utilized as an oxidizer and fuel, respectively. Pyrex glass chimney was used as the reactor. Stainless steel (304) plates and nickel nitrate were used as the target and catalyst. The substrate was kept near the flame, parallel to the flow [51].

Gore and Sane synthesized MWCNT using premixed flames. In this study, stainless steel (SS304) chimney was used for synthesis. Both the catalyst and target were made up of cobalt used. Both the catalyst and target were placed on molybdenum mesh holder at 1100°C temperature. Ethylene was used as a fuel [52]. Woo et al. synthesized CNT using double-faced wall stagnation flow burner. Mixture of ethylene (fuel), air, and nitrogen (diluent) gas were used for flame generation. Nickel-coated stainless steel target was used for synthesis. The target was heated for 3-10 minutes for CNT production [53].

Memon et al. synthesized MWCNT and SWCNT using counterflow diffusion flame (CDF) and multiple-inverse diffusion flames (m-IDFs). In the CDF method, NiAl₂O₄, CoAl₂O₄, and ZnFe₂O₄ spinels were used as the catalyst for synthesis. In this study, nitrogen-diluted methane was used as the fuel. Both nitrogen and methane are in 1 : 1 ratio. The copper substrate was inserted in to the system. After 10 minutes, CNT was collected. In m-IDF, same spinels were used as CDF. Methane or ethylene with hydrogen having 1 : 10 ratio was used as the fuel. The substrate temperature was maintained between 500 and 700°C [54].

2.5. Electrolysis Method for Synthesis of CNTs

Zhou et al. synthesized CNT using electrochemical deposition. The construction of the electrochemical reactor was done using an electrochemical cell with three chambers. For standard reference, they used calomel electrode and the anode and cathode were built using Fe/Ni alloy-coated nanoparticles. The electrode comprised of a mixture of 40 vol% methanol (CH₃OH) and 60 vol% benzyl alcohol (C₆H₅CH₂OH). About 1000 V potential difference has been maintained in the cathode and anode, and the CNT was obtained at cathode [55]. Johnson et al. synthesized CNT using from CO₂ by molten electrolysis. Molten lithium carbonate has been used for synthesis. Reaction was completed in pure alumina crucible. Different metals and nickel were used as the cathode and anode, respectively. During electrolysis, CNT and oxygen were formed on the cathode and anode, respectively. The synthesis procedure was completed in two steps—dissolution and electrolysis. In the dissolution process, lithium carbonate was synthesized. After this, lithium carbonate was converted in CNT [56]. Li et al. synthesized CNT using CO₂-based molten salt electrolysis. In this method, Li₂CO₃, CaCO₃, SrCO₃, and BaCO₃ were used for CNT preparation. Various combinations of these chemicals have been placed in a crucible and used for the synthesis. Galvanized iron wire and nickel wire were employed as the cathode and anode, respectively. Crucible was heated at 750°C temperature. Slowly, current density was increased from 6 mA/cm² to 200 mA/cm²; as a result, CNT was deposited with increased current density [57].

2.6. Hydrothemal/Sonochemical Method for Synthesis of CNTs

Jeong et al. synthesize SWCNT using sonochemical method. Ferrocene (catalyst), p-xylene (carbon source), and silica powder (nucleation site) were mixed together. After ultrasonication at ambient condition, CNT was collected [58]. Raja and Ryu synthesized CNT using sonochemical method. In a simple procedure, ZnCl₂ particles and dichlorobenzene were sonicated in ultrasonic water bath. 50 W/40 KHz is the maximum output of the system [59]. Wang et al. synthesized CNT using hydrothermal synthesis by utilizing precursor materials like ethyl alcohol, distilled water, sodium hydoxides (NaOH), and polyethylene glycol (PEG). These materials were mixed in a 250 ml flask. The reaction mixtures were continuously stirred for 30 minutes on a magnetic stirrer. After completion of 30 minutes, the precursor was transfected in a reactor for further treatment at 160°C for 20 h. After cooling, the sample was collected [60]. Razali et al. synthesized CNT using hydrothermal method. Ferrocene (2 g), sulfur (4 g), and NaOH solution (10 M) were used as carbon sources, catalyst, and solvent, respectively. The precursor was kept in Teflon stainless steel autoclave reactor at 200°C for 24 hours. After cooling, the sample was collected for further study [61].

2.7. Catalysis Method for Synthesis of CNTs

Kitiyanan et al. synthesized SWCNT using Co–Mo/SiO₂ catalyst. The investigators have used 1 : 2 molar ratio of Co–Mo. Cobalt nitrate and ammonium heptamolybdate used as a starting material in the present investigation. The starting material calcinated at 500°C placed in a horizontal quartz tubular reactor for further process. Precursor was then heated at 500°C and 700°C in H₂ and He atmospheres, respectively. After that, CO was introduced for the formation of the SWCNT material [62]. Liu et al. synthesized MWCNT using catalytic pyrolysis. In this method, polypropylene was converted into MWCNT which was accomplished in two steps. In the first step, catalytic pyrolysis and, second step, decomposition reaction have been carried out. HZSM-5 zeolite and nickel catalysts were used for synthesis. Screw kiln reactors were used for the first and second steps, respectively [63].

3. Agrowaste and Biochars as a Source of CNTs

Everyday, a huge number of agrowastes are generated around the whole globe, out of which some fractions are used as a fuel for burning, and some fractions are used as biomanures but major fractions are dumped into environment. Since these agrowastes small amount of water content which attracts the insects and microorganisms which causes pollution in the environment [64]. These agrowastes are rich in carbon which could act as an economical and ecofriendly material for the formation of CNTs by applying chemical, biological, and physical approaches. The utilization such agrowaste for this purpose will not only minimize the solid waste but also act an economical source of CNTs; hence, the final CNTs will be economical and ecofriendly [65]. There are several reports in the literature where agrowaste have been used for the generation of CNTs. Mugadza et al. synthesized CNTs from the sugarcane bagasse or cellulose in combination with ferrocene and ionic liquid. The diameter of CNTs 63 nm and 38 nm for sugarcane bagasse and cellulose, respectively, and used as a charged storage and numerous other applications [66]. The use of yeast fermented wheat flour for making CNT’s was reported by Gao et al. [67]. He even used this to store energy. The CNTs produced using this method had a curved filamentous tabular structure with diameter between 100 and 200 nm. Another report suggested the use of agroindustrial waste such as wheat straw, hazelnut shell [68], coconut shell [69, 70], rapeseed cake, and oat hulls for synthesis of CNTs [71]. Here, the diameter of CNTs was inversely proportional to the energy band gap [72]. The following synthesis methods exhibit good potential for their applications in construction industries. Similarly, the use of poultry litter as raw material for synthesis of CNTs was studied by Haleem et al. [73]. The investigators reported the size of nanoparticles ranging between 26 and 200 nm in MWCNTs with well-aligned graphene walls. This method was further employed in the removal of chromium from wastewater during the tertiary treatment process [73].

Recently, Aboul-Enein et al. reported the synthesis of MW-CNTs from sugarcane bagasse. Here, the investigators used pyrolysis method for sugarcane bagasse (SCB), which was catalyzed by using zeolites. The investigators have used three different types of catalyst, namely, HZSM-5, HMOR, and HY. The temperature for pyrolysis was 450−700°C, while the SCB/ZSM-5 ratios were 3–12 [4].

3.1. Synthesis of CNTs from Biochar

Biochars are highly porous carbon-rich materials. They are obtained by the pyrolysis of substances of biological origin like coconut shell ash, corn cob, citrus peel waste, and rice husk ash [74]. Being rich in carbon, all these materials are the most suitable candidate for the synthesis of CNTs. Though there are several approaches for transformation of biological waste into biochars, the most preferred one is pyrolysis. Pyrolysis is considered advantageous over the biomass valorization processes. In this process, the waste can be handled in a much easier way and it also adds up to the sustainability of production processes.

There are numerous literatures which indicates the wider utility of these biochars especially in the soil. Since biochars are made up of organic carbon so even after burning, there is plenty of unburned carbon which could increase the organic carbon content in the soil. Moreover, due to highly porous nature, it could increase the water holding capacity of the soil, air aeration, and cation exchange capacity. It has also been found that these biochars could affect the soil pH based on their mineral content, i.e., could make the soil acidic or alkaline. Based on the mineral content, it could also affect the microbial community of the soil. Since biochars have various minerals which could act as a manure which could provide the nutrition to the soil for the growing plants, biochars has been used widely increase a sustainable alternative in the biomass valorization for value added product for instance adsorption or uptake of NH₄, NO₃^2-, Cd, phenols, production of Si compounds, and fulvic acids. Due to all these beneficial features, biochars has gained a huge response in the daily application in the field of agronomy and environmental purposes. Nowadays, it is widely used as a precursor material for the synthesis of CNTs by microwave-assisted method, where it has numerous superiority due to low cost and ecofriendly nature. Such methods have numerous advantageous points as it minimizes the overall utilization of raw materials used for the CNT synthesis [72].

Recently, a novel technique for biochar-based CNTs with self-ignition solvent was designed. This solvent-based method gave high concentrations of CNTs when synthesized using horizontal furnace. It resulted in the release of high amount of energy from the autoignition solvent in the furnace. A higher degree of wall graphitization was also observed in the residual material. This method gave workable amount of lignocellulosic residual biomass and thus is a suitable applicator for agroindustries and construction industries. This new suggested technique for utilizing biochar as a source of black carbon for CNT production gives large quantities of purified CNTs [72].

Salama et al. synthesized environment friendly nanocompoosite fertilizer for common beans. The developed nanocomposite was based on CNTs which was developed from agricultural biochar [75].

4. Industrial Waste as a Source of CNTs

The conservationists all over the world are worried over the excessive accumulation of plastic waste in oceans, lakes, landfills, and rivers. The biggest concern relating to plastic waste is the lack of proper collection and recycling methods. This specific concern was acknowledged with novel approaches of utilizing CNTs in managing plastic waste.

Orbaek White et al. synthesized the CNTs from the chemical recycling of waste composite carbon source. The diameter of CNTs varied from 43 to 49.2 nm. The number of walls between 18 and 52 and diameter range between 18 and 45 nm. The synthesized CNTs used as a CNT cable for fuel savings because CNT cables are lightweight than the cable made from copper [76]. Li et al. synthesized CNTs from the polyethylene waste, and diameter and length was 20-30 nm and few tens of micrometers, respectively. Used as a field of composite materials were solar cells and optical devices [77]. Singh et al. synthesized CNTs from industrial waste, namely, fly ash, red mud, and rock sample, and used for the removal of methyl orange from aqueous medium. The diameter and length of CNTs was 20-30 nm and 683.8 nm, respectively [78]. Smagulova et al. synthesized CNTs from polymer waste and diameter range between 40 and 100 nm and used for obtaining various types of composite materials [79]. Yao et al. used the postconsumer waste plastics as a raw material for the production of CNTs and synthesized CNTs, mostly multiwalled and outer diameter from 12 to 25 nm. The synthesized CNTs have unique properties and number of applications in a wide variety of industries.

5. Synthesis of CNTs from Plastic Wastes

Plastic being nonbiodegradable is a major source of air, water, and soil pollution. It severely affects the natural environment. The use of plastic waste to synthesis CNTs using catalytic pyrolysis method is a novel as well as environment efficient method [80]. Here, the bimetallic Ni-Fe catalyst are used at a reaction temperature of 800°C to convert plastic waste into hydrogen and high-quality CNTs. A two-step method is used to fix the bed reactor where the plastic waste is first pyrolyzed and then is mixed along with various chemicals for further reactions. There are few factors that determines the quality of yield of CNTs from plastic waste like the development of reactor, quality of catalyst, and the conditions at which the operations are carried.

There are several examples in the literatures where plastic waste has been used for the synthesis of CNTs. Bazargan and McKay reported the synthesis of CNTS, single-walled, and multiwalled from various plastic wastes like polyethylene (HDPE or LDPE), polypropylene (PP), and polyethylene terephthalate (PET). They further said that such synthesis could be done in the autoclaves, furnaces reactors, and fluidized beds [81]. Zhuo reported the synthesis of CNTs from the waste polyethylene plastics. The yield of source of nanocarbon was up to 13.6%, and such approach will not only minimize the solid waste arising from the plastics but also provide an economical source of CNTs [82]. Wu et al. reported the synthesis of CNTs from the plastic waste by catalytic pyrolysis method by using NiMnAl catalysts. They observed that the higher the content of Mn in the NiMnAl catalyst, the more the yield of carbon (57.7 wt. %) from the plastic waste. Further, they prepared composite materials by adding 2% LPDE [82, 83].

The method followed most popularly after catalytic pyrolysis is microwave treatment for transforming the plastic waste into H and CNTs [84]. Recently, investigators from parts of the globe have developed catalytic method for converting plastic waste to better-quality fuel. Their methods comprise of pulverization of the plastic, where plastics were blended with tiny bits. This was followed by addition of ferrous oxides and aluminum oxide catalysts. Further, this mixture was a compedition to microwave treatment, where plastic catalysts were heated. This resulted in a catalyst which creates several hot spots in the plastic due to which almost 97% of the plastic’s H, and high-quality CNTs was extracted. This novel process was reported superior over previously available techniques. These are less energy exhaustive, cheap, and single step process for fast production, i.e., within 30 to 90 seconds [85].

Smagulova et al. reported the synthesis of CNTs by CVD method where the polyethylene waste was thermally decomposed and were used were used as a carbon source. They observed the effect of decomposition temperature of polyethylene on the CNTs synthesis. Here, they impregnated the cenospheres with the aqueous solutions of nickel and cobalt nitrates. The latter was used as a catalyst for the synthesis of CNTs. They further concluded that for the synthesis of CNTs, optimum temperature for polyethylene decomposition was 450°C along with abovementioned catalytic conditions [84]. There are several reports where MWCNTs were also synthesized from the plastic waste which can eliminate or minimize the plastics from the environment. The plastic-based synthesized gelatin/MWCNT nanocomposite films was found water and oil resistant. Besides this, it also exhibited antimicrobial activity, due to which it could be applied as a food packaging material. The coating of these nanocomposite materials with the garlic microparticles increases their antimicrobial activity. This resolves the toxicity-associated problem of MWCNTs during the process of migration. In the process of migration, an unintentional transfer of packaging material is carried on to the food. The use of garlic microparticles coated on the nanocomposite films helps in the avoidance of food interactions with the MWCNTs [85].

Pattanshetti et al. reported the synthesis of MWCNTs from waste plastic bottles by applying thermo-CVD method, and later on, the 30-45 nm diameter MWCNTs were analyzed by the X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron diffraction X-ray (EDX) [86]. Further, a gelatin/MWCNT nanocomposite films was developed where garlic microparticles were synthesized by using the planetary ball milling method. Here, the percentage sof MWCNTs were varied from 1% to 3% to obtain different nanocomposites [86]. In a study, Tripathi et al. synthesized MWCNTs in the laboratory using used polypropylene centrifuge tubes [87]. The process of synthesis involved the use of double-layered stainless steel 316 (SS 316) metal tube. This dual layer acts both as the vessel for the reactor and as catalyst. The steal reactor is mixed up with Fe and sometimes Ni. This helps in the conversion of carbon that will be used for analysis by spectroscopic, microscopic, and thermogravimetric method. With the following experimentation, the optimum condition for reaction was concluded at 900°C under normal reaction conditions [87].

Most recently, Ramzan et al. synthesized CNTs in single step by using plastic waste as a precursor. The method adopted by investigators was microwave assisted by using various catalyst like NiFe₂O₄, Al₂O₃, and Fe₂O₃ which was synthesized by sol-gel technique. The investigation focused on the valorization of plastic and its transformation into CNTs [88].

6. Current and Emerging Applications of CNTs

Due to the light weight, high mechanical strength, unique electronic, and thermal features, CNTs have gained attention in the field of electronics, sensors, defense, biomedical, drug delivery, and environmental cleanup [89, 90]. Among electronics, it is widely used in batteries for efficient energy storage applications [91].

6.1. CNT for Energy Storage Applications

Whereas the durability and specific energy distribution of double-layered capacitive substances such as CNTs have been established and appreciated, the power output constraint has drawn a lot of interest. When superior electron capacitive components like graphene, activated carbon, metal oxide, transition sulfide, or conducting polymers are combined with carbon nanotube-oriented fibers, the conductivity of the native fibers can be greatly increased by maintaining the periodic features [92]. In this context, Peng et al. constructed a CNT/graphene composite fiber by covering the as-prepared MWNT assemblies with GO colloidal and spinning them into fibers. The transfer tendency of electrons in the composite fiber is noticeably enhanced because of contact of the pi-pi bond between the graphene oxide sheet and the carbon nanotube, and the graphene oxide layer can reduce carbon nanotube packing and enhance ion pathways. In comparison to naked CNT fiber (630 MPa), the chemical-reduced blended fiber seems to have a mechanical property of 500 MPa, whereas the conductance of the fiber reinforcement can be as strong as 450 20 S/cm. In comparison to the 5.83 F/g of the pure CNT fiber, the mixed fiber’s computed specific capacity was 31.5 F/g, while Foroughi et al. investigated a unique type of inductive carbon nanotube-graphene hybrid fiber under electrospinning chemical-reduced graphene within and covering the face of MWNT fiber during the drawing phase, based on a similar principle. The functionalized fiber had electrical properties of 900 50 S/cm, whereas its impact resistance, stiffness, and modulus of rupture were all about 140 MPa, 2.58 GPa, and 6%, correspondingly. At a scanning speed of 2 mV/s, the specific capacity was considerably enhanced to 111 F/g. The cospun CNT-based fiber could also improve the fundamental electrocatalytic activity as a comparison to the naked CNT fiber with the use of nanocrystals or nanoflakes [93]. Di et al. used a spinnable CNT array to paint a suspension of ordered microporous carbon (OMG) over 10 sheets of ordered CNT layers, which were then wrapped together into fiber concrete as one electrode [94]. Even at a scanning speed of 200 mV/s, the electrochemical behavior of the OMC/CNT hybrid fiber was tested in a three-electrode system, which revealed a rectangular-shaped CV curve. The particular volumetric capacitance of the produced negative electrode was up to 121.4 F/cm³ (or 116.3 F/g) at 0.43 A/cm³, which was regulated mostly by the amount of OMC particles in the hybrid fiber composite (76 wt%). Apart from conducting polyaniline, polypyrrole (PPy) was been found to have higher superior catalytic properties. Using two or more pieces of stainless-steel mesh as the working electrode, Di et al. successfully deposited a light coating of PPy on an aligned CNT sheet [94]. Two motors with direction opposite coiled the layered coatings into fibers in a wet condition. The as-prepared fiber electrode then demonstrated high capacitance (350 F/g) and remarkable durability in cyclic measurement, even when bent and twisted.

6.2. CNTs for Environmental Applications

Active atomic surfaces of polymeric structure CNTs have been already emerged to improve the adsorption and catalytic performance with respect to eliminate heavy ions and heterocyclic complexes. However, the hybrid structural characteristics of the CNTs materials were found more efficient towards environmental issues like dye degradation, metal adsorption, and water purifications from the wastewater contaminants [95]. In this regard, Adam et al. prepared novel metal–carbon-based catalysts for the adsorption of (Hg(II), Pb(II), Cd(II), and Sn(II) ions) heavy ions from an aqueous media. Here, the interfacial bond structures between the heavy metals and the ZnFe₂O₄ combination boost the adsorption ability of free fullerene CNT towards the examined toxic metal ions by 25% when homogenized with the ZnFe₂O₄ composition. The removal efficiency of various heavy metals is shown below in Figure 2. It is noted that the fullerenes are effective substances for the recovery of pollution from wastewater because of their flaws, reduced agglomeration propensity, and extensive surface sites [96].

ZnO/NiO supported MWCNTs were magnificently made up by coprecipitation technique by Khan et al. [97], and the degradation of methyl orange (azo dye) was examined under UV and visible illumination to estimate the photocatalytic performance of the photocatalyst as produced. Once NiO was deposited on the MWCNTs with a ZnO-coated layer, the reactivity increased even further, peaking at 3% NiO concentration. The decay behavior of ZnO-encased MWNTs and ZnO/NiO-covered MWCNTs differed, implying that comprehensive degradation of organic azo dyes can be obtained under rapid catalyzed decomposition processes when ZnO/NiO-treated MWCNTs are used. By hydrolyzing titanium isopropoxide in saturated alcohol, Jiang and Gao [98] had shaped TiO₂- (anatase-) loaded MWCNT nanocatalysts. On visible spectrum, phenol was decomposed, and the composites had greater potency (92.4 percent in 8 hours) than undoped TiO₂ and a mixed combination of MWCNTs and TiO₂, which is shown in Figure 3.

6.3. Applications of CNTs in Biomedical

It is generally understood that the CNTs have a fundamental atomic chemistry and phase purity influence interface attributes including polarity, wettability or high porosity, dispersion, and catalytic activities. This will influence the contacts itself with surroundings, particularly biomolecule adhesion. In the imaging subject area, there are a variety of CNT-based approaches. Photoluminescent imaging, for example, uses the fluorescence of activated SWCNT on near infrared-I and NIR-II spectrum, which are practically opaque to biomolecules and fluids, allowing for depth penetration properties [70, 99]. Conductive detectors, and more specifically differential resistive pulse devices constructed on MWCNT, have successfully demonstrated their usefulness in achieving single molecule information [100]. These refer to possible medical impacts wherein CNTs are used to optimize the functionality of established hospital devices rather than to generate new nano-based technologies.

7. Nanocomposite Material and Their Applications

Nanocomposite is a medium where the nanoparticles adhere to and improve the property of material [101]. The solid multiphase material has three dimensions with an average size lesser than 100 nm. Nanocomposite has become the backbone of all the scientific industries due to their advanced mechanical, electrical, thermal, optical, catalytic, and electrochemical properties, which make them remarkably differ from that of component materials [102]. Because of these properties, CNT metal matrix composites are growing as new materials and are also applied to solve environmental problems. Nowadays, nanocomposites are widely used in environmental cleanup, biomedicine, ceramics, defense, and aerospace [103].

7.1. Energy Storage and Electrical Conductivity

Energy storage nanocomposite has been produced by Chanchal et al. using Mn₃O₄ nanoparticles having the size of about 60 nm diameter and conductive MWCNTs. Thus, developed nanocomposite material shows the high value of specific capacitance, power density, and energy density. MWCNT/Mn₃O₄ nanocomposite was synthesized using modified hydrothermal (MHT) route. MWCNTs has increased conductivity and reduction in the charge transfer process because of its natural porous and conductive nature. According to GCD test, researchers had concluded that 8.5 wt% MWCNT/Mn₃O₄ nanocomposite represents the highest capacitance of 396 F g^-1 at 1 A g^-1 current density. This is 1.4 times higher than bare Mn₃O₄ with specific capacitor of 264 F g^-1. Researchers had also stated that nanocomposite shows enhanced cyclic stability, which makes a resourceful supercapacitor electrode material [104]. Another study based on electrical conductivity was carried out by Patole and Lubineau [105]. In their, they have used silver nanoparticles decorated ethylenediamine (EDA) functionalized MWCNT-EDA. The conductive layer of ethylene glycol treated poly(3, 4-ethelenedioxythiophene): poly(styrenesulfonate) (PED-OT: PSS) was used for Ag/MWCNT-EDA coating [105]. Researchers found that EP coating enhances the electrical conductivity of developed a nanocomposite material in two ways. The EP coating acts as an effectual scattering agent that enhances the equal dispersion of the Ag/MWCNT-EDA. The use of EP coating is also seen as a mutual bridge among Ag and MWCNT-EDA.

Kiani et al. have developed and characterized polythiophene/SWCNTs nanocomposite which shows advanced improved electrical conductivity and thermal stability. They have taken thiophene (Th) and 2-(2-thienyl) pyrrole (TP) as an interfacial modifier for the development of SWNT-poly (Th-TP) nanocomposite [106]. The characterization of nanocomposite materials was carried with sophisticated instruments such as UV-visible spectroscopy, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), thermogravimetric analysis (TGA), XRD, transmission electron microscopy (TEM), and scanning tunneling microscopy (STM). According to characterization date, thiophene and TP molecules are adsorbed onto the surface of hybrid SWNTs where SWNT constitutes the core and poly (Th-TP) acts as the shell. The developed nanocomposite material shows improved thermogravimetric stability along with electrical conductivity as large as 28 Scm^-1 which is much larger than the usual value of bulk poly (Th-TP).

Another work on enhanced electrical conductivity is based on nanocomposite developed using few-walled carbon nanotubes (FW-CNTs) decorated with palladium nanoparticles as fillers. During the nanocomposite synthesis, the FW-CNTs were initially adhered by the Pd and PdCl₂ particles, after the reduction process in dihydrogen, all the decorated particles on the surface of FW-CNTs were transferred to spherical Pd nanoparticles with average diameter of 5 nm. Thus, the developed nanocomposite material provide additional channels for electron transfer and also decrease the contact resistance due to p-type doping effect and the decrease in the SBH, which subsequently increase the electrical conductivity of nanocomposite material [107].

In recent times, electronic devices such as communication devices, touch screens, surgical, nd diagnostic implements are used widely. Designing a more flexible electrochemical energy storage is tremendously important for the growing demand of flexible devices. These days’ lithium-ion batteries (LiBs) and supercapacitors (SCs) are growing as typical energy devices. Incorporation of nanomaterials in such devices enhances their capacity and efficiency, due to advanced properties of nanomaterials. Yin et al. have used nano-based approach to assembling hybrid energy storage devices. In their study, the flexible, ultrathin, and low weight carbon nanotubes-polyaniline (CNT/PANI) composite films were developed and used as supercapacitor electrodes. Developed nanocomposite materials were directly introduced into a lead acid battery in series or parallel. The hybrid devices on the other hand showed better properties. There was an increase in 19% specific capacity and 21% in specific energy [108]. The wearable electronic gadgets are in high demand these days and to fulfil the demand. Wang et al. have designed fiber-shaped energy storage device. These devices can be woven into electronic textiles. The study showed that a coaxial fiber lithium-ion battery can be done by adding aligned carbon nanotube composite yarn. Designed novel nano-based material show good results with a linear energy density of 0.75 mWh cm^-1. A wearable energy storage textile has been produced by researchers with an aerial energy density of 4.5 0.75 mWh cm^-2 [109].

7.2. Thermal Conductivity

Carbon nanotubes incorporated with polyester/vinyl ester resins and infusion with glass fiber lead to the development of carbon nanotubes/glass fiber/polymer multiscale composite using ultrasonic process and shearing. In this study, the addition of just 3% of carbon nanotubes has resulted in 1.5-fold enhancement of thermal conductivity [110]. Researched had stated that the ultralow thermal conductivity of bulk polymer such as polythene can be enhanced by combining them with nanomaterials which are having high thermal conductivity. They have developed aligned carbon nanotube polythene composites (ACPCs). The most interesting and significant finding is that developed ACPCs nanocomposite material twice thermal conductivity as large as a suspended PE chain which is well known by its high K. As compared to reported CNT-polymer composites, the K ACPC is also at least 30 times higher, and due to these advanced properties, Liao et al. has predicted that aligned polymer-based composites may be used for wide variety of applications [111]. Another research study on carbon nanotube copper (CNT/Cu) has been carried by Chu et al. They have synthesized CNT/Cu nanocomposite using a novel particles-compositing process followed by spark plasma sintering (SPS) technique. They reported that sintering condition affects the thermal conductivity, i.e., the composite material sintered at 600°C for 5 min under 50 MPa pressure showed maximum thermal conductivity, while sintering at below 600°C and below 50 MPa pressure, thermal conductivity decreases. According to experimental data, they have concluded that the increases in both temperature and pressure enhanced the thermal conductivity of CNT/Cu composite [112]. To characterize the thermal conductivity, Kong et al. had incorporated CNTs into the Cu-Cr matrix and fabricated CNT/Cu-Cr composite. They had used powder metallurgy method which consists of ball milling of CNTs with matrix powder followed by hot processing during their study. High-resolution transmission electron microscopy was used to detect Cr₃C₂ carbide layer. The carbide layer increases the interfacial bonding because of its strong chemical bond. The heat transfer between the CNT and the Cu-Cr matrix is also improved. A significant increase of nearly 12-17% in thermal conductivity was reported in the study when compared to the Cu-Cr matrix with the CNT loading of 5 vol. % and 10 vol. %, respectively [113].

7.3. CNTs as Sensor

CNTs incorporated with different materials such as polymers, nanoparticles, and the bulk material shows advanced sensing properties. Indium oxide was grown on the surface of oxidized SWCNT to develop nanocomposite material using sol-gel synthesis method. Synthesized hybrid nanocomposite reveals high conductivity and sensitivity toward certain organic vapors at room temperature. To check the sensing properties of indium oxide/SWCNT hybrid nanomaterials, an electrical conductance experiment was carried out for dilute ethanol and acetone vapors. They have also observed that as the degree of annealing greatly affects the material’s response to acetone and ethanol, the intermediate calcinations condition yields the best sensitivity [114]. Qualitative and quantitative monitoring of harmful and hazardous gasses such as carbon monoxide (CO), nitrous oxide (NO), and nitrogen dioxide (NO₂) is of great importance in several application areas such as environmental pollution [114]. To overcome this problem, Sekhaneh and Dahmani have synthesized SWCNTs/ZNO nanocomposite for nitrogen dioxide (NO₂) detection at room temperature. As compared to SWCNT sensor, SWCNTs/ZNO nanocomposite sensor shows much interesting and impressive response at the temperature higher than 100°C and they are also having high sensitivity to NO₂ concentration as low as 1 ppm. In another study, Mendoza et al. have developed tin oxide carbon nanotubes (SnO₂-CNTs) composite by chemical vapor deposition method (CVD), which shows sensitivity against methanol, ethanol, and hydrogen sulfide (H₂S). As compared to SnO₂ and CNTs separately, SnO₂-CNTs composite shows advanced and efficient performance to ppm level of gas. Researched have stated that the developed nanocomposite material may be used as road alcoholmeters and industrial safety [115].

7.4. CNTs as Catalyst

The science of catalysis and their technology is very significant to a national economy. Today, about 90% of all the technical chemicals are manufactured by the use of catalysts. Due to a higher surface area-to-volume ratio, metal nanoparticles are highly important material as catalysts [116]. Nanosized titanium dioxide has gained considerable attention due to its novel properties, which include chemical stability, nontoxicity, large specific surface area, and high photocatalytic activity. Due to its exceptional properties, they show a broad range of applications, such as dye-synthesized solar cells, water splitting, and environmental applications. Researchers have reported that incorporation of CNTs with titanium dioxide enhanced its photocatalytic activity [117]. During their work, titania nanotubes (TNTs) were prepared by electrochemical anodization technique. CNTs were synthesized inside the titania nanotubes (TNTs) templates by a catalyst-free chemical vapor deposition (CVD) method. The synthesized CNT-TNT nanocomposite was characterized using Raman spectroscopy, SEM, transmission electron microscope (TEM), XRD, and X-ray positron spectroscopy (XPS). Characterization data reveals that the size of TNTs was found ~100 nm pore diameter and the thickness of CNTs were nm. Developed nanocomposite was used to remediate Rhodamine B dye. Researchers concluded that CNTs induce synergetic effects on the photocatalytic activity of TNTs, which enhanced Rhodamine B degradation [118]. In another study, researchers have developed nitrogen-doped carbon nanotube/nanoparticle (N-Fe-CNT/CNP) nanocomposite which possesses electrocatalytic activity for oxygen reduction reaction (ORR) in alkaline medium. The developed nanocomposite shows higher efficiency than platinum- (Pt-) based catalyst [119].

7.5. CNTs as Heavy Metal Adsorbent

Heavy metals are widely used in industrial purpose which includes painting and battery manufacturing [120]. Industrial processing results in the generation of large quantities of waste effluent that contains exceed the level of heavy metals [121]. Most of the heavy metals are very toxic which cause cancer. They are also nonbiodegradable and persist in the aquatic ecosystem and environment which affect living beings. To solve such problems, Sankararamakrishan et al. have carried out to study the adsorption properties of carbon nanotubes and activated alumina nanocomposite. Chemical vapor deposition technique was used to grow carbon nanotubes over Fe- and Ni-doped activated alumina. The next step involved washing of nanofloral clusters with acid. This nanoparticles can be used for absorption of Cr(VI) and Cd(II) in the pH of 2 and 9, respectively. The adsorption equilibrium data were best fitted by the Langmuir model. According to Van’t Hoff equation, the adsorption process is irreversible, stable, and feasible. As the developed NCs shows respective properties, researchers have concluded it as the unique adsorbent for cleaning up the environment. Generally, alginate and carbon nanotubes both are used as an adsorbent material [122]. Jeon et al. have taken this advantage and prepared a nanocomposite material using alginate, carbon nanotubes, and iron (III) oxide for the removal Cu(II) [123]. Iron oxide shows the respectable property for recovery after the removal of heavy metal to eliminate the secondary pollution. SEM and TEM data reveals that addition of CNTs enhanced the surface area for heavy metal adsorption. Developed nanocomposite showed about 60% enhanced recovery of Cu(II) which shows very impressive properties of the nanocomposite material [123]. In another study, iron oxide magnetic nanoparticles were incorporated along with multiwalled carbon nanotubes to prepare multiwalled carbon nanotube/iron oxide magnetic (MWCNTs/Fe₂O₃) nanocomposite for the removal of Ni(II) and Sr(II) contamination. Formation of the nanocomposite was confirmed by SEM analysis. Researchers had also concluded that pH and ionic strength affect the activity of nanocomposite for heavy metal adsorption [124].

8. Conclusion

Agrowastes could act as a potential candidate for the synthesis of economical and ecofriendly CNTs and nanocomposites. The utilization of agrowastes has resulted in the minimization of the solid waste and diseases arising from the disposal of wet agrowastes. Till yet, all the three approaches, i.e., biological, chemical, and physical, have been successfully applied for the synthesis of CNTs and nanocomposites. Since the CNTs are developed from agrowastes, these were biocompatible. Applications of CNTs have gained importance in the field of sensors, adsorbent, defense, and aerospace engineering. It has also gained wider applications in the biomedical field mainly prosthetics and stents which are very light and high durability. The utilization of agrowaste and development of CNTS will help in minimization of waste generated from agricultural activities around the globe every year.

Data Availability

The data sets used and analyzed during the current study are available within the article only.

Conflicts of Interest

All authors declare that there is no conflict of interest associated with this research work either in terms of financial or employment.

Authors’ Contributions

Amel Gacem, Shreya Modi, and Virendra Kumar Yadav contributed equally to this work and are considered co-first author.

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