Synthesis Methods for Carbon-Based Materials

Kumar, Pradip

doi:10.1007/978-981-16-1892-5_10

Pradip Kumar^25,26

Part of the book series: Indian Institute of Metals Series ((IIMS))

1799 Accesses

Abstract

Carbon is a most versatile element and its bonding and special nature have long been noted largely due to the variety and quantity of structures. Carbon can make different allotropes like graphite, diamond and fullerene due to its sp¹, sp² and sp³ possible hybridization nature. The development and understanding of carbon-based materials are topics of major interest in science and technology due to their excellent electrical, thermal, mechanical and optical properties. On the other hand, carbon is hardly considered to be toxic material, which makes it easily biocompatible. Carbon-based materials are synthesized using various top-down and bottom-up synthesis approaches. In this chapter, various conventional and more practical synthesis strategies, as well as their mechanism for diamond, fullerene, carbon nanotubes, carbon nanofibers, graphene and graphene oxide with the extracts from published investigations by numerous researchers, will be discussed.

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Technologies of Carbon Materials. Syntheses and Preparations

A Broad Family of Carbon Nanomaterials: Classification, Properties, Synthesis, and Emerging Applications

Keywords

10.1 Introduction

Carbon-based materials like graphite, fullerene, diamond, carbon fiber, carbon nanotube, graphene, activated carbon and soot particles have been well employed for various applications due to their variety of structure and properties. For example, graphite is well-known electrode material and moderator in nuclear reactors. Graphite is mostly used in composites including in pencil, wind turbine generators, pouring nozzles, thermocouples, crucibles, etc. [1]. In addition to graphite applications, it is being used as a precursor for the synthesis of most emerging carbon materials; graphene. Next to graphite, diamond is one of the well-known carbon materials for its hardness, high thermal conductivity, insulating and transparent properties [2]. Natural as well as synthetic, both types of diamonds are available in the market. Diamond is used in many potential applications including, optics, electronics, medical, and radiation detectors [3]. Due to their hardness, diamonds have been used for cutting, making wedding rings and other jewelry for a long time. Thus, the large-scale synthesis of diamonds with the desired size and shape is important. Another exciting form of carbon, fullerene, was discovered in 1985 at Rice University [4]. Fullerene invention won the Chemistry Novel prize in 1996. Now fullerenes have been greatly considered well-known carbon allotropes, which had previously been limited to graphite, diamond and amorphous carbon.

After the fullerene discovery, carbon nanotubes (CNTs) were first time identified in 1991 by Iijima et al. [5]. They found this new form of carbon materials during the synthesis of fullerene by an arc evaporation method. CNT is rolled from of single-layer carbon atoms. This new form of carbon-based materials has opened many opportunities for materials scientists in many fields. CNTs diameter may vary up to a few nm, while the length may be achieved in tens of micometers [6]. The most important properties of CNTs, which make them ultimate materials, are high electron mobility, thermal conductivity and mechanical strength. CNTs conducting properties highly depend on the way it constructs like armchair (metallic), zigzag or chiral (semiconductor). The ultimate properties of CNTs have been employed in many potential applications including supercapacitors, fuel cells, hydrogen energy storage, electromagnetic interference, sensors, etc. [7]. Along with, CNTs research, carbon nanofibers were also explored [8].

The major breakthrough in carbon-based materials was achieved by the identification of sp² hybridized single layer of carbon atoms; graphene in 2004 by Andrew Geim and Novoselov at Manchester University UK [9]. For this wonder material, they have been awarded a Physics Nobel Prize in 2010. Graphene was identified by a simple Scotch-tape method. This method produced very high-quality graphene. Graphene’s unique properties like ultrahigh electron mobility, high surface area (~2600 m²g⁻¹), large aspect ratios, the extreme thermal conductivity of ~5000 W/mK, excellent mechanical stiffness (Young’s modulus of 1 TPa) and optical properties make it emerging materials for high-speed electronics, optical devices, energy storage, chemical sensors, thermal management and electromagnetic interference shielding applications [10,11,12,13]. Since graphene discovery, it has been considered a wonder material among the scientific community. Thus, the synthesis of graphene and its large-scale production is important. Many top-down including exfoliation and bottom-up including chemical vapor deposition (CVD) approaches are being used for the synthesis of graphene and other carbon materials [14,15,16]. In this chapter, various synthesis mechanisms for graphite, diamond, CNTs, CNFs and graphene are discussed.

10.2 Synthesis of Graphite

Graphite is one of the most stable and natural crystalline forms of carbon. It is composed of stacked layers of sp²-hybridized carbon atoms. These stacked layers in graphite are weakly attached by weak Van der Waals force and π–π* interaction which gives its low hardness and perfect cleavage. Natural graphite can be formed by the reaction of carbon compounds in the rock during hydrothermal metamorphism. A small amount of graphite can be found in metamorphic and igneous rocks. Most of the graphite seen at earth's surface was formed at convergent plate boundaries where organic-rich limestones and shales were subjected to the pressure and heat of regional metamorphism. This process produced tiny crystals and flakes of graphite. However, synthetic graphite is required for industrial applications.

The first synthetic graphite was manufactured in the late 1800s by Edward Goodrich Acheson. He discovered it accidentally while attempting the manufacture of silicon carbide in an electric furnace. He used a combination of silica and amorphous carbon and found that silicon vaporized at about 4150 °C leaving the carbon behind in graphitic carbon. This discovery of graphite was extremely valuable and used for many potential applications. The commercial production of graphite was started in 1897 when Acheson formed a company in 1899. The synthesized graphite was found to have a purity of over 99% carbon and it is being used in manufacturing products where extremely pure material is required. Currently, many precursor materials are being used for synthetic graphite production. For example, in the USA, high-quality graphite is prepared by using petroleum coke as the primary material. Generally, the process of synthetic graphite manufacturing consists of powder preparation, shape-forming, baking and graphitization [17]. Graphite is also synthesized by the reaction of calcite with hydrogen at different temperatures, pressure and time. The synthetic graphite can be formed in various shapes and size, powders and granular materials.

Since its discovery, graphite has been employed in many industrial applications. The well-known application of graphite is in making pencils, which consist of a mixture of clay and microcrystalline graphite [1]. High purity graphite is being used for manufacturing carbon brushes for electrical motors. Composites of graphite (e.g., silver-graphite and copper-graphite) are used in wind turbine generators, railway technology and tacho generators. Composites of alumina-carbon are used in ladles for pouring nozzles, liquid steel and sliding gates. Thermocouples, heater tubes and crucible for heating are made of composites of clay-graphite and silicon carbide-graphite. Graphite is also used as electrode materials, e.g., in electric arc furnaces. Sliding bearings are made from bronze-graphite composites. Graphite powder is also used for making graphite foil which is used for manufacturing high temperature gaskets and packages. Lithium-ion and zinc-carbon batteries used high purity graphite powder as cathodic material. Graphite is also used in nuclear reactors as a moderator [18]. Not only industrial applications, now graphite is the main source for the top-down synthesis of other carbon materials like CNTs, graphene, graphene oxide and graphene nanoribbons, which will be discussed later in this chapter [19, 20].

10.3 Synthesis of Diamond

Diamond is a second naturally occurring allotrope of carbon. The name “Diamond” is derived from the ancient Greek–adámas meaning “unbreakable”. In diamond, each carbon atom is sp³-hybridized, arranged in a variation of the face-centered cubic crystal structure. This arrangement of carbon atoms makes the diamond very hard material. Diamond has been considered one of the best materials in the scientific community as well as commercial level due to its remarkable properties including electrically insulation, high thermal conductivity, lowest thermal expansion coefficient, chemical inertness and optical transparency. Most of the natural diamond's formation occurred at high pressure and temperature in the Earth's mantle at depths of 140–190 km Natural diamond is colorless but the color in a diamond can be originated by a small number of defects or impurities such as boron and nitrogen.

Synthetic diamond has been much attracted since its establishment as a crystalline form of carbon. Initially, the diamond was synthesized using a high pressure approach due to its highest density. The first synthesis of a diamond at high pressure was developed by “General Electrics” in 1955 [21]. The Bulk amount of diamonds can be synthesized in a thermodynamic stable region at high pressure and high temperature (HPHT). In contrast, metastable thermodynamic regions at low pressure and low-temperature methods can produce thin diamond coatings and powders. To date, various methods have been successfully developed for diamond synthesis including HPHT, chemical vapor deposition (CVD), thermal activation of graphite, hydrothermal synthesis and the reduction of carbides (HSRC) [22, 23]. However, CVD is the most studied method for diamond fabrication [24]. This section summarizes diamonds synthesis by using various synthetic routes.

10.3.1 High Pressure and High Temperature (HPHT)

By using this technique, gem-purity large diamond crystals can be grown up to several millimeters. This is a conventional and effective method used for diamond synthesis for research as well as commercial applications. The HPHT method can produce high-quality sheets shape large cubic diamonds, which are mostly used for the development of cutting and grinding tools. Nevertheless, these diamonds have disadvantages such as higher cost and size limit. Many companies and researchers are using a reaction cell in HPHT conditions for the synthesis of various types of diamond crystals for commercial applications. Although they used different designs for diamond growth, typical pressure and temperatures used in HPHT are 5–7 GPa and 1200–1800 °C, respectively [25]. Generally, in the growth process, diamond morphology changes from cubic crystal with (100) dominated faces and cuboctahedral crystal with (100) and (111) dominated faces, to octahedral crystals with (111) dominated crystal faces with the increase of reaction temperature (Fig. 10.1).

The obtained results and method proved that they were not good enough to synthesize diamonds in that early experiment. After that many researchers modified the experimental conditions and reported good results using the HPHT method. In the modified HPHT method, Pt-30% Rh/Pt-6% Rh thermocouple was used for temperature monitoring. Room temperature pressure calibration was done by changing the resistance of standard substances. But, high temperature pressure calibration was performed by the graphite-diamond equilibrium. Liu and co-workers used carbonyl iron powder as a catalyst for diamond crystallization in the range of 5–7 GPa pressure and 1200–1800 °C temperatures [26]. A high purity graphite rod was used as source material. In this growth process, the purpose of using carbonyl iron powders was to see the effect of nitrogen and oxygen on diamond growth. The diamond growth process was performed on the mixture of graphite rod and catalyst powders (7:3, weight ratio) for 4 h. After crystallization experiments, sample columns were first cracked. The remaining impurities of graphite and catalysis were removed by dissolving the products in a hot mixture of H₂SO₄ and HNO₃. Diamond growth was first confirmed by XRD measurements as shown in Fig. 10.2. From the XRD pattern, it is evident that the sample has a cubic diamond structure which is composed of {111}, {220} and {311} characteristic lines. In contrast, the SEM image showed an irregularly shaped diamond crystal grown at 1600–1650 °C temperature without a clear {100} or {111} crystal faces (Fig. 10.3a). Further, an increase in reaction time for 15 min, diamond crystals of 0.5–0.9 mm with lamellar and strip shapes were formed (Fig. 10.3b, c). With the further increase in temperature, it was found that {111} faces played a key role in the crystallization of diamond. Diamond crystals obtained at 1700–1750 °C temperature showed {111} dominant faces and minor {100} faces (Fig. 10.3d). The above experimental outcomes revealed that high temperature conditions are more suitable for the stable growth of octahedron diamond crystals.

Diamond crystals were further synthesized from the sulfur-carbon (S–C) mixture at high temperatures (1550–2000 °C) and pressures (6.3–7.5 GPa) [27]. Graphite rod (99.99% pure), sulfur powder and cuboctahedral synthetic diamond crystals of 500 µm size as seed particles were taken as starting materials. For the nucleation of diamond crystals, first, the graphite rod was placed into a capsule of 7.2 mm diameter and 7 mm high. Then, diamond seed crystals and sulfur powder together were placed into a cylindrical sample and then placed into the graphite capsule. Then, diamond crystal growth was investigated at different temperatures and pressures for the time of 3–40 h. Growth of diamond crystals in the above system exhibited significant dependency on temperature, pressure and nucleation time. For example, there was no diamond growth on seed crystal was found at 6.3 GPa pressure and 1550 °C temperature. In contrast, metastable graphite and slight diamond growth on the {100} and {111} faces of the seed crystals were obtained on higher temperatures (1650 °C). Even at this temperature, there was no spontaneous diamond nucleation was established. Spontaneous nucleated diamond crystals were formed at a high temperature of 1750 °C (Fig. 10.4). The size of nucleated diamond crystals was found to be ~50 μm with {111} dominant and {100} minor faces. The different surface morphology of diamond crystals was depending on the surface indices of seed crystals. For example, the flat growth of layers was found on {111} faces, while {100} and {110} faces result in pyramidal relief and striation structure, respectively. The above findings conclude that the seed crystals can form a stable octahedron diamond crystal at high temperatures. Also, metastable graphite of ~100 μm size was grown in the upper part of the capsule. Further experiment showed the regrown metastable graphite and diamond growth layers at relatively low temperatures (1550 and 1600 °C) and higher pressure (7.0 GPa). There was no sign of spontaneous nucleation of the diamond up to 40 h run. In contrast, intense diamond crystallization with more than 50% graphite to diamond conversion was found at the temperature of 1700 °C. Obtained diamond crystals show size up to 500–700 μm which can be further grown up to ~1 mm. Spontaneous nucleation of seed crystals and growth of diamond crystals were established at higher temperatures (1750, 1800 and 1900 °C) without any metastable graphite. Further, a rise in temperature up to 2000 °C gives diamond crystals of 400 μm along with metastable graphite crystals.

In recent work, Ge doped single-crystal diamond was synthesized from an Mg-Ge-C system at high pressure of 7.0 GPa and a high temperature of 1500–1900 °C [28]. The mixed powders of Mg_0.9Ge_0.1 and several seed crystals were used. This HPHT system produced relatively large size diamond crystals of 2–3 mm at 1900 °C. Further, they also synthesized diamond crystals using a phosphorous-carbon system at 6.3–7.5 GPa pressure and 1400–1850 °C temperature [29]. Hu et al. also developed high-quality cubic diamond crystals for commercial purposes by using a multiseed method [30]. The diamond crystals were synthesized using Fe–Ni alloy-carbon system under HPHT conditions by a temperature gradient method. Graphite powder as a carbon source and Fe–Ni alloy (64:36 by wt%) as catalyst/solvent were used, respectively. The growth temperature and pressure were set to 1250–1350 °C and 5.5–5.7 GPa, respectively. Pure and large size diamond crystals were successfully synthesized using this HPHT. But this method is extremely expensive for bulk production.

10.3.2 Chemical Vapor Deposition

The chemical vapor deposition (CVD) method is one of the most common synthesis techniques for all types of carbon-based materials. This method involves the decomposition of hydrocarbon gases and then deposition onto a substrate. In this process, a process gas, excitation source and a substrate on which sample will be deposited are required. Various gases including hydrogen, oxygen, methane, carbon dioxide, argon and nitrogen were used as process gas. To activate the chemical process, various energy sources including microwave, radiofrequency, direct current, hot filament, laser-induced and chemical activation have been used. In most of the cases, hydrogen and methane gas mixture were used for carbon-based materials. Methane or other hydrocarbons act as a carbon source, while the role of atomic hydrogen or oxygen is the most critical. The key role of hydrogen or oxygen in the CVD method is to terminate the dangling carbon bonds on the surface of the developed crystalline carbon materials. It is also reported that hydrogen atoms can cleave neutral hydrocarbons and create reactive radicals such as CH₂. The cleaved hydrocarbon can bond with exposed carbon atoms to form tetrahedral sp³ (ta-C) or trigonal sp² (a-C) bonded carbon [31]. Another benefit of using hydrogen in diamond synthesis is to avoid the growth of graphite because atomic hydrogen can etch sp² bonded graphite much faster than an sp³ bonded diamond.

For the first time, Eversole reported the gas-phase synthesis of diamond in 1958 [32]. This method uses a carbon-containing gas, which is passed to diamond seed crystals at a pressure of a few torr and temperature of ~1000 °C. Interestingly, it was observed that the diamond was grown when source gas has a methyl group (CH₃–) like methane, ethane, methyl mercaptane, propane, methyl chloride and acetone. However, the low growth rate of diamond crystals of ~1 Å/h was the major issue for applications of diamond in real life. In contrast, hydrocarbon gases like benzene, which do not have methyl groups, could not grow diamonds by this method. The systematic procedure of diamond synthesis from the CVD method is illustrated in Fig. 10.5. A few years later, Angus et al. [33] also reported diamond synthesis using the CVD method on diamond seeds crystals by using mixtures of hydrocarbons and hydrocarbon–hydrogen gases. So far, the diamond has been synthesized using various modified CVD methods (Fig. 10.5b). It has been noted that diamond growth in CVD depends on various parameters such as pre-treatment of the sample, range of temperature, type of activation source and substrate selection, which play an important role. Generally, molybdenum, tungsten carbide and silicon nitride are being used as a substrate in CVD growth. In the CVD method, the temperature distribution, internal energy and solubility clearly depend on the excitation source. Based on the excitation sources, CVD processes can be divided into heated gas and ionization plasma CVD techniques which can be further sub-divided into chemical, electrical, or electromagnetically and thermal activation processes.

In the thermal activation CVD process, the gas phase is activated by hot filaments or hot surfaces as shown in Fig. 10.6 [24]. In this approach, the first process gas flows into the chamber and then hot filament was used for heating. The distance between substrate and filament can vary from 5 to 20 mm and substrate temperature can be achieved around 1000 K [24, 34]. Eiichi and co-workers reported the diamond growth by an advanced hot filament CVD method. In this method, the substrate temperature is independently controlled [35]. A mixture of CH₄/H₂ gas was used as a source precursor and silicon as a substrate. The distance between substrate and filament was maintained to 6–10 mm. Filament temperature was measured to be ~2050 ± 10 °C by an optical pyrometer. The controlled pressure of 30 torr was applied by the MKS pressure control system. The gas flow rate was maintained to ~600 sccm and the ratio of methane to hydrogen gas was 1%. It was found that diamond growth mainly depends on substrate distance and its temperature. Maximum diamond growth was observed in the range of 900–1000 °C temperature. However, with the increase of substrate temperature, the growth rate was reduced. In the same year, Park et al. produced a synthetic diamond film using a hot filament CVD method [36]. They deposited diamond films on a polycrystalline titanium substrate. Direct radiation from filament was used to heat the substrate. The flow of reactant gases (CH₄ and H₂) into the reactor was controlled through a mass flowmeter. Their experimental findings revealed that surface morphology played a significant role in the nucleation and therefore in diamond film growth. The surface roughness increased the nucleation density of diamond and reduced the incubation time required for nucleation. As a result, the growth rate of individual diamond particles decreased. Further, Kobayashi and co-workers established a relationship between growth rate and flow time of source gases in the hot filament method [37]. They set the temperature of filament and substrate to 850 °C and 2100 °C, respectively. The pressure was varied from 5 to 106 kPa. In the pressure range of 5–50 kPa, all growth rates increased very fast and get maximum at a residence time of 1.1 ms. After that growth rate was abruptly deceased and saturated with residence time. Simultaneously, diamond and nanostructured graphite thin films were grown by hot filament CVD [38]. By changing the distance between substrate and filament, diamond and graphite growth can be established. At a distance of 5 mm, the diamond was obtained, while graphite deposition was observed at a distance of 15 mm.

Chemically activated growth of diamond using CVD on combustion flame was the first time described by Hirose et al. [39]. The temperature of the combusting flame can be reached in the range of 2000 to 3550 K. In chemically activated CVD processes, the heating was done by an exothermic conversion of the source gases. In general, acetylene and oxygen gases were applied for the combustion process. Generally, the substrate temperature was used from 770 to 1470 K. Figure 10.6b shows the typical setup for the combustion flame-based CVD method. The flame was generated by a commercially available oxyacetylene torch, which is vertically placed to the substrate. The whole process was controlled by changing the ratio of C₂H₂/O₂ with continuous monitoring of the substrate temperature by thermocouples and finally, diamond crystals were successfully deposited on the substrate. To increase the deposition area, multiple torches can be used. However, chemically induced diamond deposition is not a significant method for industrial applications.

Similar to the thermally and chemically activated CVD process, diamond growth also has been demonstrated by using mixed-gas plasma [40, 41]. The common growth process in microwave plasma CVD involves placing a substrate in a quartz tube which is perpendicular to the waveguide. Microwave plasma can be generated by a magnetron. The substrate can be fully exposed to the plasma by adjusting through a plunger. Typical gas pressure was adjusted to ~40 torr and the gas flow rate was set to 10–100 sccm. The successful synthesis of high-quality diamonds using plasma CVD was developed by Setaka and co-workers. However, this method has some limitations like large-area deposition because the plasma was limited to the quartz tube size. In this context, Hiroshi et al. developed a new type of microwave plasma deposition system in which they used a high magnetic field with improved plasma density [42]. In another work, a high-quality diamond was grown from microwave plasma CVD by using graphite as the carbon source [43]. In this work, carbon is etched from graphite and placed inside the plasma. They used a conventional bell jar system for plasma generation. The hydrogen flow was fixed around 300 sccm, growth time of 24 h and microwave power was around 940 W. Silicon was used as a substrate. Diamond was grown on four different hydrogen gas pressure of 80, 100, 120 and 140 torr. At low pressure, the growth was not uniform. However, the best quality diamond was obtained at 140 torr pressure. Further, it has also been reported that diamond can be grown using a pre-treated mixed-gas-system of carbon dioxide and hydrogen [44]. However, diamond synthesized in the above methods is not feasible for industrial applications in which large-area diamond films are required.

Scalable fabrication of large-area diamonds was considered as a key step for industrial applications. Tiwari et al. reported that enhanced nucleation and diamond growth by using a microwave plasma CVD method [45]. They have grown diamond film on Pt/SiO₂/Si substrate. In the deposition process, first, the substrate was cleaned by ultrasonication in acetone and alcohol for 10 min. After that, adamantine was seeded on the cleaned substrate by using ultrasonication. Diamond was grown using 1% CH₄ in H₂. The obtained thickness of the diamond film was about 900 nm for the deposition time of 120 min. However, further, the improvement was required for industrial production. Qi and co-workers reported a large area of single-crystal diamond using the microwave plasma CVD method [46]. The bulk production of diamond was achieved using 75 kW and 915 MHz microwave plasma-assisted CVD system. A mixture of H₂/CH₄/N₂ gases and pressures from 90 and 180 torr was used with recorded growth rates from 10 to 30 μm/h. A single-crystal diamond of 2.5 mm thickness was obtained in a single deposition run (Fig. 10.7). By using this technique, single-crystal diamonds production was achieved to 100 g/day and out of the 25% of the diamond, crystals were observed to be colorless.

10.3.3 Other Methods

Thermally activated graphite also can be used for diamond growth at low pressures. Palnichenko et al. reported diamond nucleation and growth using a short and intense heat pulse [47]. The diamond growth was obtained on different substrates including nickel, copper, aluminum, silicon and quartz. All the substrates were in the form of 10 cm² area plates and placed at a distance of 5 ± 1 mm from the electrode tip. The substrate temperature was maintained up to 10 K. As a result, well-shaped colorless diamond particles of 10 µm size were observed on the substrate as shown in Fig. 10.8. The maximum size of diamond particles can be reached up to 100 µm. The average density of diamond particles obtained by a single pulse was measured to be 10 ± 5 microparticles cm² with homogenously distributed irrespective of the nature of the substrate. The diamond formation was characterized using XRD, Raman and IR spectroscopic techniques.

Another alternative route for diamond fabrication is hydrothermal synthesis. Syzmanski and co-workers reported diamond synthesis by a hydrothermal process at different supercritical-fluid systems [48]. Gogotsi et al. reported diamond crystals synthesis using hydrothermal synthesis [22]. The sp³ bonded carbon was obtained in the temperature range of 300–800 °C and pressure within 500 MPa under hydrothermal conditions by selective leaching of silicon carbide (SiC). The formation of diamond upon hydrothermal treatment of SiC has been reported elsewhere [49]. Following the above work, few researchers successfully diamond synthesis without diamond seeds by hydrothermal process. Lou et al. synthesized diamond crystal up to a size of 250 µm by reduction of CO₂ and magnesium carbonate with metallic supercritical- fluid sodium at a temperature of 440 °C [50].

For a long time, diamond has been popular for a variety of applications including, medicals, optics, electronics and radiation detectors. Before the graphene research, the diamond was known for the best thermal conductivity and chemical inertness [2]. Single-crystal diamonds have applications in optical science [3]. It has been well noted that pure diamond revealed the widest spectral transmission range among all solid materials. Diamond windows have been used in the chemical industry for spectroscopic quality control of molten plastics, caustic alkalies and so on. In recent years, diamond coatings have been applied to many medical devices such as temporomandibular joint prostheses, microelectromechanical systems and heart valves [51]. Functionalized nanosized diamonds have been used for the immobilization of chemotherapeutic agents. Many researchers have demonstrated imaging of cell and tissues with nanodiamond particles. Another important application of diamond is in the making of the radiation detectors. Due to its hardness, diamonds have been used for cutting, making wedding rings and other jewelry for a long time.

10.4 Synthesis of Fullerene

The first fullerene molecule was discovered by Richard Smalley, Robert Curl, James Heath, Sean O'Brien and Harold Kroto at Rice University in 1985 [4]. For this new invention, they won the Chemistry Novel prize in 1996. This new form of carbon was found to have truncated icosahedron structure and called fullerene, named Buckminsterfullerene, on the name of architect Buckminster Fuller who designed geodesic domes in the 1960s. A fullerene molecule is made from sp² hybridized carbon atoms. Typically, a C₆₀ fullerene molecule has icosahedral symmetry closed cage structure with 20 hexagonal and 12 pentagonal rings. To date, fullerenes have been greatly considered a well-known carbon allotrope. Since its discovery, fullerenes have been significantly considered for fundamental research as well as for potential technological applications, especially in materials science and electronics. In this section, various synthetic routes for fullerene synthesis will be discussed.

Fullerenes can be found in nature like in flames but its first experimental identification was observed in carbon vapor as shown in Fig. 10.9a. This unusual fullerene molecule was detected from laser pulse vaporized carbon species obtained from the surface of graphite, toward high-density helium flow [4]. After that vaporized carbon species traveled toward the ionization region and C₆₀ can be formed at particular clustering conditions, which is about 40 times larger than the neighboring clusters. Time of flight mass spectra confirmed the most of the fullerene molecules consist of C₆₀ with few C₇₀ molecules (Fig. 10.9b). Figure 10.9c confirms the truncated icosahedral structure of featured C₆₀ fullerene molecules. This flame technique can produce good quality fullerene molecules. However, this method produced microscopic amounts of fullerenes, which limits its commercialization.

Since the first identification of fullerene by Kroto et al., W. Krätschmer and D. R. Huffman (K–H) synthesized C₆₀ fullerene using arc method. The arc was created between graphite rods and burned under the helium atmosphere [52]. The carbon radicals were produced by slow evaporation burned graphite rod. However, the rate of clustering is slow in comparison with pulsed laser vaporization. Also, in this K-H method, the cooling rate of condensing carbon vapor is much slower. But, C₆₀ clusters were successfully grown by adjusting the pressure of helium buffer gas around the evaporating graphite rods. Later, it was investigated that a simple AC or DC arc can produce C₆₀ and the other fullerene molecules in sufficient quantity for commercial applications [53]. The formation of carbon clusters in the size of the range of C₆₀ was easily achieved by adjusting the pressure of helium buffer gas around graphite rods. Following the above pioneering work, fullerene was synthesized using various methods like soot, vapor-phase, arc discharge, etc.

10.4.1 Soot Method

After microscopic production by Kroto et al., C₆₀ fullerenes growth was identified in flames. Howard et al. reported the fullerene growth in premixed laminar flames of benzene and oxygen with Ar diluent [54]. The flame was stabilized over a copper burner of 70 mm diameter, which was used for delivering the feeding mixture. The flames were produced under various conditions. Soot was deposited for 53–170 min on quartz probe for each flame. Figure 10.10 shows the electron-mass spectrum of the as-deposited soot materials. Based on the reported data, it is confirmed that the soot sample contained both C₇₀ and C₆₀ fullerene molecules, which was further confirmed by FTIR.

In most of the soot method, fullerene was extracted from bulk soot [55]. The first time, Reilly and co-workers demonstrated the direct observation of fullerene growth in flame-generated soot [56]. They used a real-time measurement experimental setup for fullerene detection. They used a burner consisting of four 1.1 mm inner diameter and 1.5 mm outer diameter. A brass tube of 65 mm long was used for creating a torch-like flame. The flow rate of pure acetylene without any oxygen or air was about to be 2.4 cm³/s through the burner tubes. Soot particles were collected onto exhaust snorkel of 4-inch diameter, which was placed over the flame. The sample tube was allowed to cool down automatically. Based on their experimental findings, it was concluded that the carbonization process of polycyclic aromatic hydrocarbons (PAHs) produced fullerene in the particle phase. This method also produced individual soot particles in low quantities. In summary, by this technique, we can track the composition of fullerene molecules.

10.4.2 Chemical Vapor Deposition

Similar to diamond growth, fullerene formation was demonstrated by CVD using gaseous species such as CH₄ or C₂H_2. Lee et al. reported the fullerene formation during the synthesis of diamond film using the CVD method [57]. The experiment was performed in a conventional hot filament CVD chamber having an 8 cm tungsten filament of 0.75 mm diameter. The current and filament temperature was maintained about 50–55 A and ~2200 °C, respectively. The pressure of the chamber was maintained from 30–35 torr. Methane (99.8% pure) and H₂ (99.99% pure) were used as precursor gases with a controlled flow rate. Silicon wafer placed on flat stainless-steel was used as a substrate and maintained at a temperature from 850 and 900 °C. Fullerene deposition was obtained between 5 to 24 h. In each run, 10–75 mg soot was collected soot. Mass spectrum measurements confirmed the fullerene growth in the hot filament CVD method. Further, it was found that changing the composition of precursor gases (2–5% CH₄, 95–98% H₂), substrate temperature (400–750 °C) and moving the substrate away from filament enhanced the growth of fullerene formation to 10–20 mg/h. There was a strong correlation between the diamond and the fullerene growth process. Figure 10.11 shows the diamond and fullerene formation by using the above process (solid arrow) and by other researchers with other routes (thin arrow).

Some other researchers have also developed other CVD methods for fullerene formation [58]. Fullerene was formed by low-temperature plasma CVD under atmospheric pressure [59]. Low-temperature plasma was generated by applying rf voltage under a constant flow of atmospheric pressure Ar or He gas. Benzene or naphthalene gases were used as a source for deposition. At He (70 sccm) and 70–100 rf powers, the gas plasma produced a tary deposit upon the flow of naphthalene or benzene (0.01–1 sccm). The products from naphthalene (0.3–0.6 sccm) formed soot when Ar (13 sccm) was introduced into He (47 sccm) plasma. It was found that product appearance depends on the plasma conditions. Fullerene formation was confirmed by UV–visible spectra. Further, Wang et al. produced the Fe-included onion-like fullerenes at a temperature of 1100 °C [60]. The diameter of the as-synthesized fullerenes was in the range of 15–40 nm which was confirmed through microscopic techniques. In this method, fullerene growth mainly depends on the substrate temperature and carbon source.

10.4.3 Arc Discharge

Since its development in the 1990s, the arc discharge method has been considered an effective and popular method for large-scale synthesis of fullerene [61, 62]. In this method, graphite electrodes are vaporized in a low pressure helium atmosphere. Arc is generated by passing the current through electrodes and fullerene containing soot was formed. Kratschmer’s group in 1990 developed the first arc discharge system which is the most popular technique for fullerene synthesis till now [52]. After that several authors demonstrated fullerene formation using the arc discharge method. It was found that fullerene formation strongly depends on the operation parameters of carbon plasma-like very high temperature and inert atmosphere at reduced pressure. The typical setup of the arc discharge method is mainly composed of a stainless-steel double-walled cylindrical chamber [63]. Both graphitic electrodes were mounted horizontally near the bottom of the reaction chamber. But anode is mounted in a controlled guiding mechanical system for maintaining a constant distance between both electrodes during arc discharge. For soot formation, the chamber is evacuated until a pressure of 10^–3 to 10^–4 torr. The chamber was filled with an Ar or He gases at the pressure 50–200 torr at 30 ± 2 °C during the process. From the obtained soot, C₆₀ fullerene was extracted with toluene and by vacuum evaporation. Fullerene formation was confirmed by XRD measurements. The yield of fullerene synthesis was up to 6–8% which was higher than previous reports. The best yield of fullerene synthesis (~8%) was achieved at an arc current of 150 A and 100 torr of chamber pressure.

It has been noted that synthesis yield depends on the thermal and photochemical stability of C₆₀; however, both have not been defined well. The experimentally low decomposition temperature of 1000 K was determined for C₆₀ in a vacuum [64]. Theoretical results showed the fullerene stability up to a temperature of 1800 K [65]. In general, C₆₀ has been considered a relatively stable molecule. However, it can degrade under a variety of conditions [66]. It was demonstrated that fullerene decomposition significantly depends on the intensity of emitted light energy [67, 68]. The arc gap also influenced fullerene yields. The maximum yield was reported for large electrode gaps (2–4 mm) [69, 70]. However, in contrast, in some studies maximum yield was reported at a smaller electrode gap [71]. This contrast in experimental findings is considered due to large experimental errors [72].

The first experimentally investigation of the arc gap on the fullerene formation is reported by Andrzej et al. [53]. They optimized the experimental parameters such as input power, pressure, buffer gas and AC or DC feeding mode. The electrode gap was set to 0.5 mm with 4 mm for DC arcing. The whole system is controlled by an optoelectronic system of 0.2 mm accuracy. Initially, the electrodes were positioned close together. In contrast to the conventional method of striking anode against cathode, high-voltage glow discharge was used for arc discharge intimation. A helium pressure of 13.3 kPa was used for tests. The quantity of fullerene resulting from soot obtained from electrode arcing was tested by conventional spectrophotometric techniques. Based on their experimental observation, a significant distinction was found at a smaller electrode gap, while larger gap distance gave a significant difference in the fullerene formation. The fullerene content decreases with the increase of the electrode gap. Further, they also found that not only the electrode gap but also complex vaporization and mass transport phenomena greatly influenced the fullerene yield. Further, fullerene synthesis was also demonstrated from DC discharge between carbon electrodes in the mixture of Ar and ferrocene gas [73]. Another alternative, the solar energy-based approach was also developed for fullerene synthesis [74].

Fullerene has received much attention for a wide range of applications due to its interesting features including nonlinear optical properties and superconductivity [75]. Fullerenes have potential applications in 2D/3D metal–organic frameworks, solar energy conversion, liquid crystals materials, thermoresponsive materials and C₆₀-polymer hybrid materials, etc. [76,77,78,79,80]. Fullerene's unique shape and hydrophobic nature make it very effective for medical applications including sensitizers for photodynamic therapy and as photoactive molecular devices [81, 82].

10.5 Synthesis of Carbon Nanotubes

Carbon nanotubes (CNTs) have received first recognition in 1991, but their history goes long back. Radushkevich and Lukyanovich [83] first observed and described CNTs in 1952, which was later confirmed by Oberlin et al. in 1976 [84]. However, CNTs received significant research interest after its discovery in 1991 by Iijima et al. [5]. MWCNTs were obtained, while they were developing a new arc evaporation method for the synthesis of C₆₀ fullerene. After two years later, Iijima et al. [85] described the growth mechanism of SWCNTs. CNTs have a tubular structure with a diameter as small as 1 nm and length can be varied from few nm to microns. The construction principle of SWNTs using a graphene sheet along the chiral vector C is shown in Fig. 10.12 [6]. CNT's structure can be zigzag, armchair, or chiral, totally depending on the chiral vector during construction, which determines its most of the properties. For example, Dresselhaus et al. investigated the electrical properties of CNTs constructed from different chiral vectors [86]. Thus, CNTs properties depend on its structure like chiral angle, length, diameter, etc., which can give them unique properties. Due to the exceptional properties of CNTs, they have been considered novel nanomaterials for a wide range of applications.

For fundamental study and commercialization of CNTs in the field of material science, various synthesis methods such as CVD, laser ablation and arc discharge have been developed. CNTs were first prepared using arc discharge techniques and then followed by a low-temperature CVD technique (<800 °C). Later, other non-standard methods including liquid pyrolysis and organic approach were also developed. Irrespective of techniques, CNTs can be synthesized in the form of single-wall carbon nanotubes (SWNTs), double-wall carbon nanotubes or multi-wall carbon nanotubes depending on the experimental parameters.

10.5.1 Arc Discharge

Since its first identification by Iijima et al., CNTs growth have been developed by many other groups. The arc discharge technique produced CNTs at higher temperatures (˃1700 °C). Arc discharge technique produced CNTs have shown some structural defects. Figure 10.13 showed a typical experimental setup of the arc discharge technique [87]. The arc chamber is composed of two electrodes (anode and cathode) which are mounted vertically or horizontally. The cathode is filled with a pure graphite rod and the anode is filled with powdered carbon precursor along with the catalyst. High temperature is achieved from created arc discharge. SWNTs can be produced by using graphite rods doped with metal catalysts like Fe and Co as the anode and pure graphite as a cathode [5].

The synthesis of SWNTs was further reported by carbon evaporation with cobalt [6]. Nickel has been considered the most widely used catalyst for SWNTs synthesis. The catalytic role of the growth of carbon nanoclusters was investigated using dc arc discharge. Nickel-filled anode improves the growth of SWNTs [88]. SWNT’s growth was further investigated with platinum-group metals (Ru, Rh, Pd, Ir, Pt) and it was found that only Rh, Pd and Pt were found to have catalytic activity for growing SWNTs of 1.3–1.7 nm diameters [89]. In another study, it was found that Mo improved the yield of soot [90]. However, the purity of SWNTs in collected soot was not changed significantly. The mass production of SWNTs can be achieved by using a bimetallic Ni-Y catalyst under He ambient gas [91].

Similar to SWNTs, double-walled nanotubes (DWNTs) were synthesized by the arc discharge method. However, DWNTs synthesis from the arc discharge method is much more complicated in comparison with SWNTs production. Many research groups reported DWNTs synthesis using the arc discharge method [92,93,94]. Hutchison et al. reported the production of DWNTs from the arc discharge technique by using a mixture of argon and hydrogen [94]. The obtained materials were confirmed as bundles of DWNTs along with SWNTs as a by-product. Further, high-quality DWNTs was synthesized from high temperature pulsed arc discharge method [95]. Later, Huang et al. reported the synthesis of super bundles of DWNTs in a hydrogen-free atmosphere [96]. Liu et al. synthesized selective DWNTs by hydrogen arc discharge technique with nickel formate dihydrate catalyst [97]. Graphite powders or MWNTs/carbon nanofibers were used as a carbon source (Fig. 10.14). The HRTEM results confirmed around 80% DWNTs and the rest of were SWNTs and ends of DWNTs were uncapped. In this work, it was also found that the growth of DWNTs mainly depended on the cobalt catalyst [98].

Arc discharge methods were also used for the production of MWNTs. The morphology of MWNTs depends on the synthesis conditions. Some groups have reported the use of methane or hydrogen atmosphere for MWNTs growth. Wang et al. reported MWNTs synthesis using DC arc discharge under the helium and methane atmosphere [99]. Thin and long MWNTs were obtained by using methane gas pressure of 50 torr and an arc current of 20 A for the anode. Further, long and fine MWNTs can be prepared under a hydrogen gas atmosphere. But a big difference was found with the use of He and methane gas atmosphere. It was found that methane and helium gases atmosphere give more carbon smoke in comparison with the hydrogen gas atmosphere [23]. In another work, it was found that DC arc discharge in the hydrogen gas atmosphere can produce long MWNTs and graphene sheets as a by-product [24]. MWNTs were also synthesized at various pressures ranged from 150 to 500 torr under acetone, ethanol and hexane atmosphere. It was demonstrated that the production of MWNTs was at least two times higher in ethanol, acetone and hexane atmospheres than that of MWNTs produced in the He atmosphere.

Further, the growth of MWNTs was also reported from the arc discharge deposition process with the use of a pulsed laser. Parkansky and co-workers used a single-pulse arc for the synthesis of MWNTs in ambient air [100]. MWNTs were deposited vertically oriented on Ni/glass substrate with a length of up to 3 mm and a diameter of about 10 nm with a single 0.2 ms pulse. Tsai et al. also demonstrated MWNTs with an inner and outer diameter of 5 and 17 mm, respectively by using single-pulse discharge in the air [101]. Moreover, there are several reports on MWNTs deposition by using a gas atmosphere with arc discharge in liquid solutions. Sornsuwit et al. reported the synthesis of high-quality SWNTs and MWNTs in H3VO4 aqueous solution from pure graphite electrodes through arc-discharge [102]. HRTEM analysis revealed the high-quality well-ordered MWNTs without any defects. The outer diameter of MWNTs was 10–20 nm and interlayer distance of ~ 0.35 nm between graphene layers [103]. Similarly, the synthesis of MWNTs in high yield can be obtained by arc discharge in liquid nitrogen [104]. Arc discharge in water can also produce high yield MWNTs production [105].

10.5.2 Laser Ablation

Laser ablation methods have been successfully developed for the production of SWNTs and MWNTs. For the first time, the Smalley group demonstrated the principles and mechanisms of CNTs growth by laser ablation technique [106]. Generally, Nd: YAG and CO₂ lasers are being used in the laser ablation method. In this method, the graphite target is vaporized by a laser beam at high temperatures under an inert atmosphere. The laser generates carbon species, which are swept by the flowing inert gas from the high temperature zone to a conical water-cooled copper collector. The reaction temperature is an important parameter in CNTs production quality and yield. It was reported that the size and average diameter of CNTs can be tuned by tuning the laser power, growth temperature, catalyst composition, type of gases and its pressure [107,108,109]. Maser and co-workers demonstrated the high-density SWNTs obtained from a simple CO₂ laser system [110]. Also, it was found that laser power, the wavelength of laser and pulse duration can play a key role in the properties of the final product [111]. For example, the average diameter of SWNTs increased with the increase of CO₂ laser power from 500 to 800 W. However, the excitation wavelength growth mechanism of CNTs has not been clearly understood. Chen et al. [112] reported that SWNTs diameter can be controlled by varying the furnace temperature, catalytic metals and flow rate. Higher furnace temperature produced SWNTs with larger diameters. It was found that the use of a Ni–Y alloy catalyst increases the SWNT diameter, whereas an Rh-Pd catalyst reduces it [113].

For commercial purposes, polystyrene-MWNTs thin films deposited on alumina substrates by PLD techniques with Nd: YAG laser ablation [114]. High-quality DWNTs and SWNTs were produced from a combined method of steady arc discharge and the laser-furnace methods [115, 116]. Impurities such as catalytic metals and amorphous carbon present in CNTs can be removed by purification processes based on chemicals such as H₂O₂ or oxidation by hot air (400–500 °C) [117]. Both arc discharge and laser ablation techniques can give high-quality growth of CNTs, but these are expensive techniques due to high power equipment. Also, the CNTs production yield is low in both methods.

10.5.3 Chemical Vapor Deposition

To date, the CVD technique has been established as one of the best methods for CNTs production. In this method, CNTs are formed by the decomposition of the carbon precursor on the catalytic substrate surface. Thermal or plasma CVD is considered as the standard method for CNTs synthesis. Also, other CVD techniques such as hot filament (HFCVD), water-assisted CVD, oxygen assisted CVD, radiofrequency CVD (RF-CVD) or microwave plasma (MPECVD) have been developed [118,119,120,121]. CVD techniques are very economical for industrial production of quite pure CNTs in comparison with laser ablation. In this technique, first, a catalyst is prepared and then the actual synthesis of nanotube was performed. The catalyst role in the CVD method is to improve the decomposition of the carbon precursor. When the catalytic substrate is heated up in a carbon-rich gaseous environment, CNTs were produced at 500 to 1000 °C. The advantage of CVD techniques over arc discharge or laser ablation is that it can produce CNTs in many forms like straight or coiled, films and vertically aligned structures.

The systematic of catalytic CVD reactor system for CNTs synthesis is shown in Fig. 10.15a [122]. A mixture of xylene-ferrocene was used as a carbon source into an inert gas medium. The reaction starts with the decomposition of the ferrocene-xylene mixtures at atmospheric pressure and temperatures of 625–775 °C. As obtained nanotubes were highly pure (Fig. 10.15b, c) with a maximum length of ~50 µm. Many substrates including Ni, Si, Cu, Cu/Ti/Si, SiO₂, etc. were used CNTs growth [123]. CNTs growth was also investigated on the mesoporous silica substrate as it can play as a template for the initial nanotube growth [124, 125].

In the CVD technique, the selection of catalysts is a very important parameter for CNTs growth. The effect of catalytic nanoparticle composition and morphology on CNTs growth in the CVD method has been demonstrated [126]. Also, CNTs growth was reported from catalysts derived from Co/Fe/Al layered double hydroxides (LDHs) [127]. It was found that Co content in precursors showed a significant effect on CNTs growth. CNTs with a smaller diameter and fewer defects were formed at higher Co content. In another work, Flahaut et al. investigated the catalytic effect on the preparation of CNTs [128]. The results showed a significant effect of catalyst composition on CNTs growth. Jiang et al. reported the in situ growth of CNTs on the pre-treated graphite electrode in the presence of Ni(NO₃)₂ catalyst via CCVD method. For a shorter growing time, MWNTs revealed the length of 200–1000 nm with the inner and outer diameter of 20 and 80 nm, respectively [129].

Not only catalysis, but carbon source also is a key parameter for CNTs growth. High-quality DWNTs can be obtained by decomposition of benzene at 900 °C in the presence of Fe-Mo/Al₂O₃ catalyst [130]. As-prepared DWNTs bundles were found without any amorphous carbon on the surface, with fewer structural defects. Further, the well-aligned MWNTs were obtained via pyrolysis of C₂H₂ on a large area of Ni deposited Si/SiO₂ substrate using the thermal CVD technique at 900 °C [131]. The studies showed the crucial effect of NH₃ pre-treatment on the surface morphology metal catalyst and therefore to get the vertical aligned CNTs. Highly aligned CNTs can be obtained at a higher density of Ni particles due to the effect of steric hindrance between neighboring CNTs. It was found that the degree of CNTs crystallization increased with the increase of NH₃ pre-treatment time. The detail of the direct growth of aligned CNTs, substrate effect, growth mechanism and advantages and shortcomings of CVD have been reviewed [132]. Some researchers used Nickel oxide-silica binary aerogels as the catalyst for the growth of MWNTs by decomposition of methane at 680 °C for 120 min. The diameters of DWNTs were observed to be 40–60 nm [133]. Also, CNTs can be grown using two carbon sources of ethanol/Co and benzene/Fe systems [134].

CNTs were further grown by plasma-enhanced CVD (PECVD). The use of the PECVD method for SWCNTs has been well summarized by Lim et al. [135]. PECVD method gives high-quality CNTs. Forest like MWNTs deposited on a solid substrate is shown in Fig. 10.16. PECVD methods can be used in many different modes: direct current (DC-PECVD), radio frequency (RF-PECVD), diffusion (DPECVD) or microwave (MWPECVD). Like thermal CVD, CNTs synthesis using DPECVD has been demonstrated on the various substrate (i.e., Ag, Pt, W, Ta, Ir and NiV) at low-temperature (480–612 °C) [136]. Vollebregt et al. [137] reported self-aligned vertically grown CNTs and CNFs by using Pd as catalyst material. Samples were fabricated using a different catalyst (Pd, Ni, Fe, Co) and at different temperatures such as 450–640 °C for atmospheric pressure chemical vapor deposition (APCVD) and 450–500 °C for PECVD. APCVD produced highly dense self-aligned CNTs when Pd acts as the catalyst, while the random growth of CNTs was found in the Co and Fe catalyst. Further, they also concluded that CNTs grown by PECVD in the presence of Pd catalyst, produced large bundles of tubes, while large-diameter CNFs were formed in the presence of Ni catalyst. Biocompatible supported CNTs electrodes also have been developed using the CVD method by using ferritin as the catalyst. This type of CNTs electrodes might be useful in neuro-implants [138].

Since its discovery in 1991, CNTs have stimulated intensive research in the area of nanotechnology due to their outstanding electronic, thermal and mechanical, properties. Their implementation in various potential applications forms a great research effort and is now in progress. CNTs application has been demonstrated in scanning tunneling microscopy tips. CNTs have also been demonstrated for solar cells, fuel cells, biosensor, hydrogen storage media and carbon–metal composites for strong and lightweight structures applications. In summary, low cost, rapid and readily scalable route for carbon nanotubes fabrication have been developed for all future applications.

10.6 Synthesis of Carbon Nanofibers

The first synthesis of carbon filaments was reported in the 1970s, but the significant research was carried along with the discovery of CNTs in 1991. These carbon filaments with the cylindrical structure are known to be as carbon fibers (CFs). They can be nanofibers or microfibers, depends on their diameter. Carbon nanofibers may have different microstructural configurations like platelet CNFs as shown in Fig. 10.17 [139]. Herringbone CNFs are formed when graphite sheets are inclined from the fiber axis with some angle. The next structure is tubular CNFs, wherein graphite sheets and fiber axis are parallel to each other. Based on their morphology, CFs can be produced in various shapes including planar, twisted, branched, coiled, spiral, helical and so on. Mainly chemical vapor deposition and electrospinning method have been adapted for significant synthesis of carbon fibers. However, other methods including template-assisted, hot-filament assisted sputtering and microwave-based synthesis have also been reported. In this section, the synthesis and mechanism of carbon fibers using the CVD and electrospinning method will be discussed.

10.6.1 Chemical Vapor Deposition

Again, the CVD technique has been considered as one of the most versatile approaches for carbon nanofibers synthesis. CVD techniques have been developed for scalable and reproducible synthesis of CNFs, in which gaseous precursors such as C₂H₂ and CH₄ are used on a metal catalyst support at elevated temperature. Typically, the use of catalysts, the chemical composition of precursor and reaction parameters define the final structure of the product. In this method, hydrocarbon decomposition of carbon precursor provides a continuous supply of carbon to produced well-organized tubular filaments of sp²-carbon in hexagonal form. However, this technique is expensive and also produces short fibers [140].

Catalytic thermal CVD (CTCVD) was mostly used for the fast growth of helical carbon fibers. In the early 1990s, Motojima et al. [141] first time used the CTCVD method for the synthesis of regular coiled shape carbon filaments. The CFs were synthesized by pyrolysis of acetylene on Ni-catalyzed substrate at a temperature of 350–750 °C. The obtained CFs were found to be composed of pair-coiled fibers. These fibers are elastic and can extend up to three times the original length of the coil. Figure 10.18 showed the perpendicular grown carbon coils on the graphitic substrate [142]. Catalyst Ni grain material was found on the tip of all carbon coils, which is shown by arrows. The diameter of the regular carbon coils was ~3 to 6 μm with the coil pitch of 0.5–0.7 μm without any gap. For the reaction time of 2-h, coil length was increased up to 5–8 mm with a growth rate of 7 μm/s. The high yield of carbon nanocoils was obtained by using acetylene as a carbon source and iron-coated indium tin oxide as catalytic material [143]. Similar to CNTs synthesis, CF synthesis has been developed by the decomposition of acetylene, methane, ethylene, etc. over various transition metals catalysts including Fe, Co, Ni, etc. in the temperature range of 500–1000 °C [144, 145].

Further, carbon nanocoils (CNC) were produced by the low-temperature decomposition of acetylene [131]. The obtained CNCs exhibited a yield of 11 g in each run and formed a nanospring. Minea et al. [146] prepared carbon nanofibers at room temperature by plasma-enhanced CVD. They used low pressure, high-density plasma technique and grown the fibers on a thick Ni film deposited upon a silicon substrate. The CNFs were scratched from the substrate by a diamond tip. The obtained CNFs were amorphous and have a short length of ~50 nm at a deposition rate of 2 nm/min. Carbon nanofibers are also synthesized without any catalyst by microwave pyrolysis CVD using N₂ as a carrier gas and CH₄ as a source at a temperature of 1050–1150 °C [147]. Carbon fibers are radially grown on the porous Al₂O₃ substrate. The prepared CFs were uniform in diameter in the same region. Simultaneously, it was also noted that the ends of fiber were hemispheroidal and some cauliflower-like fiber ends can also be observed in the as-prepared samples.

10.6.2 Electrospinning

Commercial synthesis of carbon fibers was obtained from the electrospinning method. The first patent on electrospinning was published by Cooley in 1902. After that, researchers produced carbon fibers through electrospinning [148]. However, this method got much attention in the last two decades and is optimized for carbon fibers fabrication. This technique is based on the processing of polymeric solutions or melts by using an electrically force fluid system [149]. In the electrospinning process, there are many controlling parameters such as solution properties and processing parameters as well as on surrounding temperature and humidity conditions [150]. The structure of CNFs strongly depends on the nature of the polymeric precursor. The CNFs fabrication has been reported using various polymeric precursor including polyacrylonitrile (PAN), pitch, cellulose, poly (amic acid) (PAA), polybenzimidazole (PBI), polyimide (PI), poly (p-xylenetetrahydrothiophenium chloride) (PXTC) and poly (vinyl alcohol) (PVA) [151, 152]. High-quality CNFs production can be obtained by selecting the right organic polymers, followed by thermal annealing under inert conditions.

Kong et al. [153] successfully synthesized CNFs by electrospinning of PAN/DMF polymer solution and then stabilization and carbonization. The stabilization was carried out in the air between temperature 200 to 300°C. The carbonization process was performed at 2800°C under an inert atmosphere (Fig. 10.19) [154,155,156,157]. CNFs can be also prepared with the addition of inorganic species including MWNTs, Ag, TiO2 and etc. Ra and co-workers first time demonstrated the CNFs preparation by embedding MWNTs in the PAN matrix. Further, electrospun Ag/PAN nanofibers were obtained at the Ag content of 0-5 wt% [158]. Recently, Inagaki et al. reviewed the systematic process for CNF fabrication [151]. Preparation and applications of CNFs prepared by electrospinning have also been reviewed elsewhere [152].

Indeed, carbon nanofibers are currently available on a large scale for a large number of potential applications like field emission sources, catalyst substrate, hydrogen storage, chemical sensors, electrode material, scanning probe tip, nanoelectronics and mechanical reinforcements.

10.7 Synthesis of Graphene

Since its discovery in 2004, graphene is being considered as a wonder material for a wide range of potential applications. The unique properties of graphene make it emerging materials for high-speed electronics, optical devices, energy storage, chemical sensors, thermal management and electromagnetic interference shielding applications [11, 12, 159,160,161,162,163,164,166]. Optimization of graphene synthesis using various top-down and bottom approach has led to an extraordinary amount of interest in both industry and academics. To date, graphene and graphene oxide has been synthesized from various top-down and bottom-up approaches. In this section, different methods will be discussed for the synthesis of graphene, including the recent developments.

10.7.1 Top-Down Approach

In this approach, graphite is being used as starting materials. Graphite is constructed from sp² hybridized carbon atoms arranged in honeycomb lattice structure with an interlayer distance of 3.35 Å along the c-axis. In graphite, all carbon layers bonded together with a weak Van der Waals force. Therefore, graphite can easily exfoliate by either chemical or mechanical exfoliation methods.

10.7.1.1 Mechanical Exfoliation of Graphite

The first single layer of graphene was identified using micromechanical cleavage of highly oriented pyrolytic graphite (HOPG) in 2004 [159]. This method involved an adhesive tape to peel off the graphene layers from HOPG. The peel-off process was repeated many times to achieve a single layer of graphene on tape, followed by transfer on a cleaned substrate [167]. Figure 10.20a showed the optical images of graphene and few-layer graphene deposited on a 300 nm SiO₂ substrate. The thick layer graphene appeared yellow. Upon reduction of the graphene layer thickness, it showed bluishly. The darker side showed a few layers graphene, while lighter shades showing the single-layer graphene [168]. This micromechanical cleavage process is known as the “Scotch-tape method”, which facilitated the initial experimental measurements on the exceptional electronic, thermal conductivity, mechanical and optical properties of graphene. These unique properties generated a huge interest in graphene in almost all branches of science. This Scotch-tape can produce single-layer graphene up to 1 mm with a minimal defect. The produced graphene is good in quality but bulk production is not possible. Therefore, for real-life applications, other synthesis methods are required which can have produced graphene on large scale.

10.7.1.2 Solution-Based Exfoliation of Graphite

Solution exfoliation of graphite has emerged with great interest because it can give producible results on a scalable large scale. Similar to the direct dispersion of carbon nanotubes, many authors directly dispersed the graphite into various organic solvents and then sonicated for exfoliation into a single-layer or multilayer graphene sheets. Coleman et al. obtained good quality graphene sheets by dispersion and sonication of graphite into the NMP solvent [169]. Graphene dispersion was further achieved by 3 h ultrasonication of HOPG in DMF [170]. Various polar and no-polar solvents were explored for graphene synthesis [171]. Bulk production can be achieved by this solution exfoliation method but the production yield is very low. A further attempt was made to ultrasonication-free exfoliation by intercalating graphite with alkaline metals [172]. A high yield of graphene up to 2 mg/mL was obtained by spontaneous exfoliation of HOPG in chlorosulfonic acid [173]. Graphene synthesis in aqueous was also explored by using various surfactants [174, 175]. The analysis confirmed the synthesis of single or few-layer graphene up to a size of a few mm. However, exfoliated graphene sheets are less stable in water in comparison with organic solvents. Graphene synthesis using the solvent exfoliation method depends on many factors like the size of graphite flakes, sonication time and centrifugation conditions and the solvents. From an industrial point of view, this method is appealing but this solution-based exfoliation approach has several drawbacks. Most of the exfoliated flakes possess a multilayer structure, which gives low yield and large size and thickness heterogeneity among the exfoliated flakes. Moreover, long-time sonication reduces the size of the graphene sheets, resulted in small size graphene sheets, which can significantly affect the properties of graphene sheets. Also, surfactant exfoliated graphene sheets can alter the resulting electronic properties of graphene.

10.7.1.3 Electrochemical Exfoliation of Graphite

After the development of the solution-based exfoliation method, there is a continuous demand for new approaches for the bulk production of high-quality graphene. Then, the researchers developed an electrochemical method for graphene synthesis. The purpose of electrochemical methods is to intercalate the ions within graphite and improve the yield of graphene production [176]. The use of intercalating agents provides an attractive way to exfoliate graphite while retaining the basic properties of graphene. This method involves the application of anodic (oxidation) or cathodic (reduction) potentials in an electrolytes solution, graphitic material as working electrode, reference electrode (SCE, Ag/AgCl, etc.) and a counter (usually Pt) electrodes. The most common and widely studied intercalated ions are Li⁺, Ni²⁺, F⁻, SO₄²⁻, NO³⁻, Cl⁻, etc. [177]. Several electrolytes and electrochemical conditions were applied to achieve the bulk exfoliation of graphene [178, 179]. It was found that applied potential and electrolytes strongly affected the quality of graphene. Few studies showed that the use of high voltages promotes the formation of oxygen groups on graphene sheets surface and also damages the structure. Graphite intercalation in aqueous SDS surfactant solution explored to drive the exfoliation process. The resulted graphene sheets were found to have a size of 500 and 1 nm thickness was successfully obtained [180]. To avoid the formation of oxygen functionalities on the graphene surface, the cathodic reduction/intercalation method was also attempted [181]. Recently, Adriano and co-workers prepared graphene sheets from commercially available graphite foil as shown in Fig. 10.21a [182]. Three different electrolytes; H₂SO₄, LiClO₄ and Na₂SO₄ were tested with a common exfoliation procedure. Optical images of graphite foil before exfoliation and after applying the +10 V potential for prolonged time are shown in Fig. 10.21b, c, respectively. As obtained graphene dispersion in NMP is shown in Fig. 10.21d. Figures 10.21e, f show the exfoliation process at the time of zero, 5 min and 20 min, respectively. As obtained exfoliated graphene was easily dispersible and formed stable dispersion in DMF at the concentration of 1 mg/mL. It was noticed that out of three different electrolytes used in the exfoliation process, either H₂SO₄ or Na₂SO₄ have shown more vigorous and efficient exfoliation.

In comparison with micromechanical cleavage and solvent exfoliation, graphene synthesis using electrochemical exfoliation is more environmentally friendly and generally fast. However, this method also not providing homogenous graphene sheets. Also, the use of anodic conditions for ions intercalation causes unwanted oxygen-containing functional groups, which disrupt the sp²-hybridized carbon structure.

10.7.1.4 Chemical Exfoliation and Reduction

In the chemical exfoliation method, chemical species intercalated within graphitic layers and oxidized the graphitic layers. A little energy in the form of sonication or shaking results as single-layer graphene oxide. Brodie (1859) reported the first successful oxidation of graphite by using potassium chlorate as an oxidizing agent in the presence of nitric acid (HNO₃) [183]. He attempted to establish an exact formula of graphite by producing highly oxidized graphite. He found that the overall mass of graphite flakes was increased upon oxidization due to the presence of hydrogen and oxygen on graphene sheets. This method was further improved by adding chlorate into fuming during the reaction [184]. Further, concentrated sulphuric acid was also added into the mixture to increase oxidization.

Later on, Hummers and Offeman developed another effective oxidation method in which graphite flakes react with the mixture of potassium permanganate (KMnO₄) and sodium nitrate in concentrated sulfuric acid (H₂SO₄) with improved experimental safety [19]. This method is considered the primary route for GO preparation and became more popular for the synthesis of graphene. Some researchers have developed slightly modified versions of Hummer’s method to avoid the poisonous nitrous gas, which evolves during the reaction in the presence of sodium nitrate in the mixture. After complete oxidation, the interlayer distance between graphitic layers increased to 6 Å or more which is due to the presence of oxygen-containing functional groups on its surface and edges. This increased interlayer distance in graphite oxide weakens the force between layers, can produce a separate layer upon sonication [185]. The separated layers are called graphene oxide (GO). By using this method, a bulk amount of GO can be produced, which is required for commercial applications [186]. Graphene oxide can be dispersed easily in aqueous as well as in organic solvents. But, the main problem with GO is its electrical insulator nature, which is due to the presence of oxygen-containing functional groups on its surface and edges. A lot of research has been performed on GO and its composite materials [187–192]. FESEM images of large size GO prepared by modified Hummer’s method is shown in Fig. 10.22a. Figure 10.22b showed its size distribution [188]. More than 50 μm graphene sheets can be synthesized. Size selections of GO sheets strongly depend on the lateral size of graphite flakes and separation conditions. The thickness of as-prepared GO sheets was measured to be ~1 nm (Fig. 10.22c) [189]. The increased thickness of GO sheets is because of oxygen functional groups on the graphene basal plane and edges.

The first serious mechanism of GO synthesis was investigated by Tour and co-workers [193]. In this study, they explained the step-by-step synthesis of graphite into GO as shown in Fig. 10.23. The first step is showing the intercalation of sulphuric acid into graphite, the second step is to covert the intercalated graphite into oxidized graphite and the last step is to exfoliate the graphite oxide into graphene oxide (GO). The study also showed the single-layer GO yield is depending on the oxidation degree of GO sheets. For example, Kim et al. obtained the three different sizes of GO sheets by using different sizes of graphite flakes with variable oxidation time and studied their size-dependent liquid crystalline properties [194]. Production of single-layer GO can be improved by using pre-oxidized graphite flakes. Single-layer GO yield also improved by exfoliation of graphite in presence of tetrabutylammonium ions. However, the process is very time taking [195]. A low-temperature approach was also established for the large production of single-layer GO [196]. Most of the established chemical exfoliation methods used hazardous chemicals, which release poisonous gases. By using the K₂FeO₄ oxidation agent, a 100% yield of GO synthesis was achieved within a short reaction time of 1 h [197]. Similar to other chemical exfoliation methods, this method also releases harmful gases.

The chemically exfoliated graphene oxide has oxygen-containing functional groups on its surfaces. The present functional groups can help for the dispersion of other inorganic nanomaterials and further functionalization of GO for composites and other potential applications. However, graphene oxide sheets are insulating in nature due to oxygen-containing functional groups. To make it conductive, which is required for most of the graphene applications, we need to remove or reduce these oxygens containing functional groups from the GO surface. There are mainly three common approaches that are being used for such reduction of GO, namely, thermal, chemical and electrochemical reduction methods.

In a thermally reduced approach, GO is annealed at high temperature (1000–2000 °C) under inert conditions. In this method, gaseous species are generated within the interlayers of the GO structures, thin-film/powder and consequently removal of oxygen functional groups and simultaneous graphitization at elevated temperatures, resulting in highly conducting products [198, 199]. Many researchers reported that thermally reduced graphene can improve its electrical and thermal conductivity, but reduced graphene oxide sheets have shown some defects. For example, Xin et al. produced free-standing graphene paper via thermal reduction and graphitization at a very high temperature ~2200 °C [200]. As prepared free-standing graphene paper showed high thermal conductivity of 1238 W/mK. This high temperature thermal annealing approach is straight forward. Thermally reduced graphene oxide has a great potential be used for composite materials, EMI shielding, electrochemical devices and conductive ink and paint. But this method has some drawbacks of long-time heating and annealing at high temperature.

Electrochemical reduction of GO is another approach, which can be performed at ambient conditions. Many authors have obtained the pristine graphene-like properties from electrochemically reduced GO [201–203]. Recently, Ambrosi et al. studied the reduction of GO using the electrochemical method, which gives precise control of C/O ratio between 3 to 10 [204]. Thermal and electrochemical reduction methods can reduce GO significantly, however, a chemical reduction is the most widely used method for GO reduction. A chemical reaction/reduction is performed on aqueous GO solution, powder and thin films to remove most of the functional groups. The most studied chemical reducing agent is hydrazine, which was already tested in past in 1937 by Hofmann and König [205]. Beceril et al. investigated GO film reduction by immersing it into a hot aqueous hydrazine solution. In other studies, hydrazine vapor is used for GO reduction even at room temperature. Another reducing agent, NaBH₄ was also explored, but all these methods give rigid and fragile rGO films with low electrical conductivity [208]. Experimental findings revealed that both hydrazine and NaBH₄ are not effective in reducing agents for GO powder or films. Also, the hydrazine reduction mechanism is not clearly understood.

Further, a more significant route was developed by using halogen hydrohalic acids, which can give highly conductive rGO films [209]. The reduced GO film maintains its flexibility and mechanical strength. Recently, many other reducing agents such as lithium aluminum hydride, hydroxylamine, eco-friendly L-ascorbic acid, saccharides, have been developed for GO reduction [210–213]. Other ways of reducing like a camera flash, UV light and laser scribes were also used for the reduction of GO films [214,215,216]. It was noticed that chemical reduction procedures can give sufficient electrical, thermal and mechanical properties, however, overall performance is poor compare to pristine graphene. This poor performance of chemically reduced GO is due to incomplete removal of functional groups and the creation of defects on the graphene sheet during the reduction reaction. However, with this poor performance of reduced GO, the chemical reduction method is most widely used for bulk synthesis of reduced GO, which has shown good results in energy storage, sensor and composite applications. For example, Kumar et al. reported the much-improved electrical conductivity of ~3000 S/m and ultrahigh in-plane thermal conductivity of ~19.5 W/mK from low-temperature HI reduced composite film [162].

10.7.1.5 Unzipping Carbon Nanotubes

Unzipping of CNTs can produce graphene. This approach mainly produced different kinds of graphene, called graphene nanoribbons (GNRs) [217, 218]. The typical width of the GNRs is ≤50 nm with an aspect ratio of more than 10. Dai et al. reported the GNRs synthesis using partially embedded multiwalled CNTs (MWCNTs) in a PMMA film under an Ar plasma treatment for the various duration [217]. As-prepared GNRs exhibit smooth edges with a narrow width of 10–20 nm (Fig. 10.24). The yield of GNRs is nearly 100% in this method. The resulting graphene nanoribbons contained oxygen functional groups, which can be removed either chemically or thermal annealing method. In contrast to the above work, Tour and co-workers proposed a more scalable approach to open the MWCNTs longitudinally by oxidizing them in presence of sulphuric acid and potassium permanganate [219]. The resulted GNRs exhibited functional groups on their surface. To reduce the defects and vacancies on the GNRs surface, H₃PO₄ was introduced in the oxidation process [218]. Heating of MWNTs in presence of potassium resulted in lower defects on GNRs surface.

Various other methods have been proposed for GNRs synthesis including metal catalyst assisted cutting, thermal exfoliation step, lithium insertion and mechanical sonication in organic solvents [221–225]. Li et al. demonstrated effective intercalation of MWCNTs for the scalable synthesis of GNRs [225]. GNRs have also synthesized the intercalation of Li-NH₃ into MWCNTs and thermal exfoliation [226]. Another interesting approach for GNRs synthesis is catalytic nano-cutting of CNTs [224]. This nano-cutting approach can produce partial or complete unzipping of carbon nanotubes. This technique has some advantages like smooth and sharp edges GNRs with specific orientations can be obtained on large scale. However, the major problem of this technique is that it does not ensure complete cutting in all MWCNTs, which limits its applicability. Synthesis of GNRs without any chemical or catalyst contamination was reported using high DC pulses by using MWCNTs as starting materials [227].

10.7.2 Bottom-Up Methods

This method is mainly based on small molecules to combine by a catalytic process to produce a high-quality graphene sheet [228]. In the first half of the twentieth century, Scholl and Clar pioneered the synthesis of polycyclic aromatic hydrocarbons. Clar et al. (in 1958) firstly synthesized a small graphene sheet with 42 carbon atoms [229]. After that, a series of polycyclic aromatic hydrocarbons with a much larger sp² system was proposed [230]. In this route, first branched oligophenylenes were prepared by Diels–Alder reactions and then subjected to oxidative cyclo-dehydrogenation to give planar graphene disks. Under specific conditions, carbon-containing molecules combined into an sp²-hybridized carbon network. Further, an attempt of graphene synthesis was made using chemical vapor deposition (CVD) and epitaxial growth [231]. These early efforts on the synthesis of monolayer graphene were followed by a large number of scientists [232]. In this section, synthesis aspects of the bottom-up methods for graphene, specifically, CVD and epitaxial growth approach will discuss.

10.7.2.1 Chemical Vapor Deposition

CVD is a well-established technique for controlled synthesis of various carbon nanomaterials including CNTs, fibers, fullerenes. This method involves the decomposition of carbon sources on transition metal catalyst deposited wafer at high temperature. The decomposed carbon atoms are deposited and then assembled at high temperatures. Large-area epitaxial graphene films (up to a size of few mm) can be obtained. CVD grown graphene was first reported in 2008 and 2009, using Ni and Cu substrates, which was followed by grown over several metal substrates such as iridium, ruthenium, platinum [233–237]. This technique demonstrated the synthesis of single-crystal graphene domains currently up to dimensions of the size in centimeters [238]. The graphene quality was comparable to the Scotch off method produced graphene.

Graphene grown on nickel and copper metal substrates is more popular in comparison with other metal substrates [231, 239, 240]. For graphene growth on a nickel substrate, first polycrystalline Ni films were annealed in Ar/H₂ atmosphere at 900–1000 °C to increase grain size and then the H₂/CH₄ gas mixture was exposed to the substrate. This step involves the decomposition precursor gas and then decomposed carbon atoms dissolve into the Ni film to form a solid solution. Finally, the samples were allowed to cool down in Ar gas. Figure 10.25a illustrates the growth process of graphene on the Ni surface [241]. Ni surface would be an excellent lattice-matched substrate for graphene growth because Ni (111) has a lattice similar to the densely packed hexagonal lattice of graphene (Fig. 10.25b) and they also have similar lattice constants [242]. As-prepared graphene on Ni substrate can be transferred to the required substrate for further study and applications. A low-magnified TEM image of graphene with step-shaped edges is shown in Fig. 10.25c. The inset figure showed the SAED pattern of graphene along the [001] direction. Graphene can main its original structure after transfer on another substrate as shown in Fig. 10.25d. Lewis et al. have also grown graphene on patterned Ni films for desired geometries at specific positions [243]. Wafer-scale graphene synthesis on evaporated Ni films was also demonstrated [239]. It was also demonstrated that the feasibility of transferring graphene on a flexible and transparent substrate has been employed for large-scale flexible transparent electronics applications (Fig. 10.25f) [244]. Tuning of experimental conditions such as deposition temperature, pressure, type and quality of metal substrates and cooling time can improve the graphene growth up to mm scale [245, 246].

Despite good progress in the growth of CVD graphene, there are still many challenges to be overcome for their practical applications. For example, removal of metal catalyst and transfer onto a different arbitrary surface. Thus, the research has been focused on the transfer process of CVD grown graphene on different surfaces. The transfer of CVD graphene onto arbitrary substrate was obtained using polymer-supported metal etching/transfer or mechanical exfoliation with special functionalized polymers [248]. However, the transfer process usually causes some wrinkles or structural damages, which degrade the quality of graphene [249]. One of the most common transfer methods is the etching of metal substrate while the graphene is supported by an inert polymer such as PMMA or PDMS [247, 250]. So far, several other substrates like PVC, PTFE, PC, CN/CA, PET, paper and cotton cloth have been investigated and applied via a hot/cold lamination process [251]. After complete etching of the metal substrate, the graphene is transferred to the desired substrate followed by the removal of the supporting inert polymer. However, this procedure is very sensitive because it can induce mechanical stress to cause structural damages and alterations. This process also gives some contamination by metallic impurities, which can dramatically alter the electronic properties of the transferred graphene [252, 253]. For example, nickel and copper metal substrates dissolved by etching agents such as FeCl₃, Fe(NO₃)₃ contaminate the transferred graphene with a significant amount of Fe impurities [254, 255]. Moreover, the incomplete etching process also can contaminate CVD graphene with an extremely large number of redox-active residuals, such as Cu or Ni metals. Recently, PMMA-graphene was separated from the growth substrates without etching by inducing O₂ bubble generation with a mixture of NH₄OH, H₂O₂ and H₂O [256]. Further, Gao et al. developed a face-to-face transfer of wafer-scale graphene films [257]. This novel approach was based on nascent gas bubbles and capillary bridges that provide high-quality graphene with fewer defects. Several improved techniques were developed such as plasma-enhanced chemical vapor deposition (PECVD) and microwave-assisted CVD for the high-quality fabrication of graphene [258]. Therefore, CVD has been considered as the most versatile and commercially viable technique for the manufacturing of continuous graphene films to meet the industrial demand for electronic-grade materials.

10.7.2.2 Epitaxial Growth

Similar to CVD, the epitaxial technique is also a substrate-based technique in which single-layer graphene is used to grow on a single-crystal carbide (SiC) by vacuum graphitization. The thermal treatment of SiC under vacuum results in the sublimation of the silicon atoms while the carbon-enriched surface rearranges to form graphitic layers (epitaxial graphene). Precise control of the sublimation may lead to the formation of very thin graphitic layers over the entire surface of SiC wafers, with an occasional monolayer of graphene. Recently, a wafer-scale coating of monolayer graphene was achieved by annealing at a higher temperature (1650 °C) under the Ar atmosphere rather than 1150 °C under UHV conditions [259]. The obtained wafer-scale monolayered graphene exhibited carrier mobility of 2000 cm²/Vs at room temperature for a carrier density of ~10¹³ cm²_. This value is only five times lesser than the mechanically exfoliated graphene. Some other researchers have also reported the mechanisms and kinetics of epitaxial graphene growth on SiC [260, 261]. A schematic of the synthesis process is shown in Fig. 10.26 [262]. Epitaxial growth on SiC can be achieved by an additional supply of carbon without subliming Si. This additional supply of carbon may be given by hydrocarbon gas decomposition or by sublimation of solid carbon source in molecular beam epitaxy [263]. Apart from SiC, other carbides have been exploited early on to fabricate supported graphene. Successful preparation of graphene monolayers has been achieved on the (100), (111) and (410) surfaces of titanium and the (111) faces of tantalum by decomposition of ethylene gas [264]. Interestingly the morphology of the TiC faces determines the graphene structure. In particular, monolayer graphene nanoribbons were obtained on the 0.886 nm-wide terraces of TiC(410), while large monolayer crystallites were formed on terrace-free TiC (111). It should be noted that SiC is a wide bandgap semiconductor and no transfer of graphene is required on other substrates, which allows very easy fabrication of the devices. Therefore, epitaxially grown graphene is very promising for the mainstream electronics industry. However, this technique has some limitations for large-scale fabrication due to the high cost of single-crystal SiC wafers. Another challenge in this technique is the uniform growth of large-area single-layer graphene.

Graphene has been considered as one of the most promising materials for a wide range of applications not only because of its excellent electronic, optical, thermal and mechanical properties but also due to its easy synthesis and handling for technology transfer. The unique properties of graphene make it useful in most areas including environment, sensors, solar cells, supercapacitor, batteries, fuel cell and catalysis [265–267]. Chemical exfoliation and reduction procedure can produce a single layer on a large scale which is mainly required for most industrial applications. Nevertheless, CVD produced graphene has shown promising results for electronic/device applications.

Conclusion

This chapter gives clear indications on the synthesis of various carbon materials Several synthesis methods like soot, arc discharge, chemical vapor deposition, mechanical and chemical exfoliation, electrospinning and epitaxial growth have been highlighted. The growth mechanism for all carbon materials has been discussed. Among carbon-based materials, graphene is a newly invented material with ultimate properties. Therefore, high-quality graphene synthesis by various routes in important for different kinds of applications.

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Integrated Approach for Design and Product Development Division, CSIR-Advanced Materials and Processes Research Institute, Hoshangabad Road, Bhopal, Madhya Pradesh, 462026, India
Pradip Kumar
Formely at Chemistry Division Bhabha Atomic Research Centre, Mumbai, Maharashtra, 400085, India
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Chemistry Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
A. K. Tyagi
Chemistry Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
Raghumani S. Ningthoujam

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Kumar, P. (2021). Synthesis Methods for Carbon-Based Materials. In: Tyagi, A.K., Ningthoujam, R.S. (eds) Handbook on Synthesis Strategies for Advanced Materials. Indian Institute of Metals Series. Springer, Singapore. https://doi.org/10.1007/978-981-16-1892-5_10

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DOI: https://doi.org/10.1007/978-981-16-1892-5_10
Published: 23 October 2021
Publisher Name: Springer, Singapore
Print ISBN: 978-981-16-1891-8
Online ISBN: 978-981-16-1892-5
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Synthesis Methods for Carbon-Based Materials

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Technologies of Carbon Materials. Syntheses and Preparations

A Broad Family of Carbon Nanomaterials: Classification, Properties, Synthesis, and Emerging Applications

A Broad Family of Carbon Nanomaterials: Classification, Properties, Synthesis, and Emerging Applications

Keywords

10.1 Introduction

10.2 Synthesis of Graphite

10.3 Synthesis of Diamond

10.3.1 High Pressure and High Temperature (HPHT)

10.3.2 Chemical Vapor Deposition

10.3.3 Other Methods

10.4 Synthesis of Fullerene

10.4.1 Soot Method

10.4.2 Chemical Vapor Deposition

10.4.3 Arc Discharge

10.5 Synthesis of Carbon Nanotubes

10.5.1 Arc Discharge

10.5.2 Laser Ablation

10.5.3 Chemical Vapor Deposition

10.6 Synthesis of Carbon Nanofibers

10.6.1 Chemical Vapor Deposition

10.6.2 Electrospinning

10.7 Synthesis of Graphene

10.7.1 Top-Down Approach

10.7.1.1 Mechanical Exfoliation of Graphite

10.7.1.2 Solution-Based Exfoliation of Graphite

10.7.1.3 Electrochemical Exfoliation of Graphite

10.7.1.4 Chemical Exfoliation and Reduction

10.7.1.5 Unzipping Carbon Nanotubes

10.7.2 Bottom-Up Methods

10.7.2.1 Chemical Vapor Deposition

10.7.2.2 Epitaxial Growth

Conclusion

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