Graphite to Graphene via Graphene Oxide: An Overview on Synthesis, Properties, and Applications

This work represents a state-of-the-art technique developed for the preparation of graphene from graphite–metal electrodes by the arc-discharge method carried out in a continuous flow of water. Because of continuous arcing of graphite-metal electrodes, the graphene sheets were observed in water with uniformity and little damage. These nanosheets were subjected to various purification steps such as acid treatment, oxidation, water washing, centrifugation, and drying. The pure graphene sheets were analyzed using Raman spectrophotometry, x-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), and tunneling electron microscopy (TEM). Peaks of Raman spectra were recorded at (1300–1400 cm−1) and (1500–1600 cm−1) for weak D-band and strong G-band, respectively. The XRD pattern showed 85.6% crystallinity of pure graphite, whereas pure graphene was 66.4% crystalline. TEM and FE-SEM micrographs revealed that graphene sheets were overlapped to each other and layer-by-layer formation was also observed. Beside this research work, we also reviewed recent developments of graphene and related nanomaterials along with their preparations, properties, functionalizations, and potential applications.


INTRODUCTION
Nanotechnology has been blossoming for more than two decades as its importance increases. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Nowadays, graphene emerges as a rising star and new paradigm of relativistic condensed-matter physics and material science. The discovery of graphene is an important addition as a world's thinnest material. Its derivatives play a significant key role in modern scientific life. 19 The fundamental breakthroughs toward the physical understanding of graphene and graphite were routed in the 1940s and 1950s. 20,21 Discovery and History of Carbon Nanomaterials The word ''graphene'' originates from the Greek word graphein, which means ''to write.'' Earlier research on preparation and fundamental properties of nanocarbons (e.g., epitaxial graphene films, nanoribbons, and nanopatches) provides a basic knowledge of the topic. Research on graphene has been successfully capitalized after its discovery in 2004 by Novoselov et al. 19 (see also Refs. 20,22,23). Graphene is an intriguing material for highly controlled systems, e.g., evolution from two-dimensional (2-D) to three-dimensional (3-D) topology having new properties. These 3-D materials can be engendered by the substituting the carbon atoms with selected heteroatoms or entire functional groups. 19,21,[24][25][26][27][28] Graphene is composed of a single-layer hybrid nanosheet of sp 2 or multiple layers of carbon atoms. These carbon atoms are densely packed into benzene rings stripped of their hydrogen atoms. This 2-D material has exceptional characteristics like electronic and high crystal quality. Its short history has already shown a cornucopia of new physics and potential applications. Graphene is the basic structural parent of all carbon allotropes, as shown in nanotubes (DWCNTs), and multiwalled carbon nanotubes (MWCNTs). [19][20][21] There are mainly three types of graphene sheets: single layer, bilayer, and few layers (<10). Kuilla et al. 25 reported about single-layer graphene sheets and other 2-D crystals. Graphene is considered as a conducting nanofiller with a one-atom-thick planar sheet of sp 2 bonded carbon atoms packed in a honeycomb crystal lattice. 29 Graphene has been researched due to its exceptional physical properties, chemical tenability, and potential for various applications. 30

Types of Graphene-Related Nanomaterials
Graphene-related carbon nanomaterials 28,31,32 are (I) doped graphene and derived graphene nanoribbons (GNRs), (II) graphene oxide (GO), (III) graphane, (IV) florographene, (V) graphyne and graphdiyne, (VI) graphone, and (VII) porous graphene. Graphene is characterized as a zero band gap semimetal in which conduction and valence bands meet at the Dirac point. The band gap can be tuned by doping and cutting the 2-D graphene into one-dimensional GNRs. 20,33 GO is an oxide-functionalized derivative of graphene, which has received tremendous interest of scientists. GO is a hydrophilic material because it possesses excellent water dispersity. It can adhere to interfaces because of lower interfacial energy. GO is used as a surfactant for the emulsification of organic solvents in water. It can be used for the dispersion of insoluble graphite and CNTs in water. This ability opens opportunities for developing functional hybrid materials of graphene and other P-conjugated systems. 33,34 Graphane is hydrogenated derivative of graphene and a theoretical nonmagnetic semiconductor with an energy gap formed by hydrogenation. Graphane is extended 2D polymer of carbon. It is newly invented hydrocarbon with stoichiometry formula unit of CH as shown in Fig. 2a. Graphane adds wealth to the carbon-based materials, showing its great potential in nanoelectronics and hydrogen storage applications. The hydrogen atoms alternate directions along with the graphane sheet and transform the carbon lattice from sp 2 to sp 3 hybridization. Graphane can also be converted back into a graphene sheet through an annealing process. Florographene is another important structural derivative of graphene with a stoichiometric formula unit of CF. Florographene has a similar geometric structure and sp 3 bonding configuration to graphane with each carbon bonded to one fluorine atom. Florographene can be used as a superior solid lubricant for batteries under extreme conditions. Graphyne and graphdiyne ( Fig. 2b and c) are new forms of non-natural carbon allotropes. These nanocarbon allotropes have shown interest better than graphene due to their unique structures, and electronic and intriguing properties. Graphyne is a one-atom-thick planar sheet of sp and sp 2 bonded carbon atoms arranged in a crystal lattice. Graphyne is another form of graphene, and it has acetylenic linkages connecting the hexagons of graphene. Graphone (Fig. 2d) is predicted as a semihydrogenated (50%) derivative of graphene with C 2 H as stoichiometric formula. 32 Hydrogen atoms are attached to one side of the carbon sheet. It is also a mixture of hybridized sp 2 and sp 3 carbon atoms. Porous graphene is a new class of lightweight material. It can be described as a distributed inherent structure within the covalent p-electronic framework of graphene.

Present Scenario of Graphene-Related Carbon Materials
The focus of scientists has moved toward more complex systems like modified graphene and 3-D systems based on the assembly of graphene sheets. Graphene is the latest sensation with unusual properties such as the half-integer quantum Hall effect and ballistic electron transport. 19 This 2-D material is expected to consist of single layer, but considerable interest has been observed in investigating two-layer and few-layer graphenes. The research has led to next-generation graphene-based nanomaterials. These next-generation graphene nanomaterials include modified graphene (carbon atoms replaced by N, B, S, or P) and architectures of 3-D graphenes (e.g., nanospheres, nanocapsules, nanopapers, and nanolayers). 24,29 Graphene has attracted both academic and industrial interests because it can improve properties of composites at low filler content.
In this article, we describe the continuous synthesis of graphene from graphite-metal electrodes by arc-discharge method carried out in continuous flow of water. These graphene sheets were purified before their characterization and testing. This article also reviews about types of graphene-related nanomaterials, various synthesis methods, functionalization, properties and applications of graphene, and its derivatives.

Materials
Research-grade (turbostratic and pyrolytic grade) graphite electrodes with high orientation and carbon electrodes were purchased from M/s Schurtz Carbon Electrodes Pvt. Ltd. (Gandhinagar, Gujarat, India). Different metal (tungsten, carbon, and copper) electrodes were used from scrap materials. Demineralized (DM) water was used as a continu-ous flowing medium for uniform dispersion of graphitic nanostructured sheets. An ice-bath was used for cooling purpose. Hydrochloric acid (HCl) was received from S.D. Fine-Chem Limited (Mumbai, India) and used as such for purification of graphene sheets.

Synthesis Method
Graphene has been synthesized by various methods like (I) catalytic chemical vapor deposition (CCVD) or microwave CVD, [20][21][22][23][24][25][28][29][30]33,[35][36][37][38][39] (II) arcdischarge method, 20,28,29 (III) micromechanical exfoliation, 19,20,23,25,29,30,33,37,[40][41][42][43] (IV) epitaxial growth on SiC, [20][21][22][23][24][25]28,29,33,37,39 (V) chemical reduction, 20,25,33,34,37,41,43,44 (VI) thermal reduction, 20,22,28 (VII) liquid phase exfoliation, 20,22,29,30,[40][41][42][43] and (VIII) unzipping of CNTs, 38 and (IX) gas phase microwave plasma reactor. 20 GO can be synthesized from oxidation of graphite by various methods like Brodie, Staudenmaier, and Hummers. The Brodie and Staudenmaier methods involve combining the oxidants like KClO 3 with HNO 3 , whereas the Hummers method involves combination of KMnO 4 and H 2 SO 4 . GO was prepared by a chemical reduction method using reducing agents like hydrazine, hydroquinone, sodium borohydride, and ascorbic acid. 30,31,34,44 Graphyne was synthesized using dehydrobenzo annulene and graphdyne (graphyne with acetylene group) was synthesized on copper substrates via a cross-coupling reaction using hexaethynyl benzene. Graphone was synthesized by applying pressure on to boron, nitrogen and hydrogenated graphene sheets so that nitrogen could pick hydrogen from graphene sheet. On releasing of pressure, the dehydrogenated graphene sheet was formed with all hydrogen atoms on the one side. Graphane was synthesized for the first time by annealing of graphene crystals at 573 K in an argon atmosphere for duration of 4 h. These crystals were exposed to cold hydrogen plasma and then a hydrogen and argon mixture was allowed to pass at low pressure for 2 h. A scalable method for hydrogenation of graphene is also reported. 32 This method includes thermal exfoliation of GO without a plasma source and in the presence of hydrogen atmosphere at 493-823 K and 6000-15,000 kPa pressure. This process produced quantity in gram of graphane. Thus, it can be a potential candidate for mass production. 31,32,45 Figure 3 shows synthesis methods of graphene and their derivatives along with their applications. Graphene sheets are classified according to the synthesis techniques used as per the available data of patents filed, published, and granted. 36 Figure 4 illustrates a pie chart that shows emerging techniques such as ion implantation, electrochemical deposition, arc-discharge, self-assembly, and laser irradiation. Based on this analysis, it can be said that substantial patenting steps have been directed toward the development of various methods. These methods include CVD, mechanical, and liquid phase exfoliation of graphite and chemical exfoliation of GO. Other dominant techniques are epitaxial growth, chemical synthesis and unzipping of CNTs. All above mentioned methods have significant potential for large scale production of graphene at an affordable cost. 36 The state-of-the-art technique for the production of graphene nanosheets is described in this article. Graphene sheets were developed by a continuous arc-discharge method in flowing water, which is shown in Fig. 5a-d.

Construction and Working of Arc-Discharge Setup
The arc-discharge setup was designed using various parts with provision of continuous flow of water. 35,36,[40][41][42] These parts include a sealed chamber of a horizontal tubular reactor, an ice-cooling bath, a direct current (DC) dual-power supply circuit, cathode-anode electrodes, a cathode-anode junction, and an interelectrode gap variation assembly. A cylindrical graphite rod was used as a cathode, while metal (carbon, copper, and tungsten) rods were used as an anode individually. The diameter and length measurements of anode-cathode electrodes were kept to be (0.01 m 9 0.005 m). These anode-cathodes were connected to DC power supply.
A DC power supply was designed with current range of 0-150 A and voltage range of 0-40 V. This range was sufficient for continuous arc-struck with inter electrode gap of 1-3 mm between the anode and cathode. A horizontal tubular reactor was made from transparent acrylic tube to visualize the actual arc-struck process between electrodes. This tube has dimensions of 0.3 m diameter and 1.2 m length. A provision of inlet and outlet was kept for water circulation. Figure 5a shows a schematic diagram of arc-discharge setup. Figure 5b shows arc-struck process and carbon deposition between cathode and anode. Figure 5c shows photographic image of interelectrode gap assembly and ice bath, while Fig. 5d shows a photograph of an actual arc-struck process, graphene deposition, and a DC supply machine.

Synthesis Procedure
As mentioned above, Fig. 5d shows an arc-struck process between anode-cathode with the interelectrode gap of 1-3 mm. The cathode was kept stationary, whereas the anode was adjustable and kept moving toward the cathode by adjusting slide of interelectrode gap variation assembly. The electrodes were brought continuously into contact by maintaining gaps of 1-3 mm during the arc-discharge process. The intensity of the arc was stable and no twinkling was observed. The temperature at the electrodes' edge was higher than that in the central region during the arc-discharge process. Initial values of current and voltage were kept same throughout experiment. The time of soot generation was kept at 2 h. This can also be made continuous for 24 h production in a single day. The water level was maintained (2 L) by controlling the inlet and outlet flow of water. The arc-discharge setup was positioned accurately to keep a steady and continuous flow of water at rate of 1 mL min À1 . The temperature of the viscous carbon clustered solution was controlled using a cooling bath. There is less damage in liquid environments owing to the better cooling capacity of water. 46,47 The carbon powder was developed continuously during the arc-struck process, which gets dispersed in flowing water. After completion of the arc-discharge process, carbon nanostructured materials were collected three ways: (I) soot was dissolved in crude continuously coming out from reactor, (II) from the cathode surface, and (III) most of the soot was observed to be settled down and deposited in the inner wall of tubular reactor. After the completion of arc-discharge, most of the soot was condensed near the wall of chamber. The settled and deposited soot was in the form of a continuous, thick, cloth-like film. The discharge produced in cake-shape graphitic carbonaceous material deposited on the cathode surface. 48 The crude was collected continuously from the outlet flow of water. It took 1 h to spend a 0.05 m anode rod to yield 0.002 kg of soot, so 0.048 kg of soot can be produced in 1 day. The crude sample containing graphitic carbon nanostructured material and metallic impurities was subjected to purification steps.

Purification Procedure
Obviously, it was necessary to purify the crude solution because it was a mixture of cotton-like soot and impurities. These materials were observed to have a web-like appearance of amorphous graphitic carbon nanoparticles, graphene sheets, tubular structures, and metallic particles if any. Purification of carbonaceous materials can be done by various steps like high-temperature hydrogen treatment, hydrothermal treatment, microfiltration, solvent extraction, acid treatment, and air oxidation. Gas-phase purification, microwave acid digestion, and high-temperature annealing are well-known purification methods, but these could affect the structural integrity of the carbon nanostructured materials. [49][50][51] Graphene sheets were purified by a novel method. Graphitic nanosheets (GS) base viscous crude was first sonicated in 35% HCl at 323 K for a duration of 30 min. Then, the sample was washed with distilled water and then dispersed in ethanol. The pure and metal-free graphitic nanosheets were filtered using Millipore (0.28 lm) filter paper and collected on the surface of it. The overall purification steps are illustrated in schematic cum photographic representation as shown in Fig. 6. Graphitic nanosheets were observed in agglomerated form before sonication. A cross-sectional view of this crude droplet shows that carbon soot was atomized uniformly after ultrasonication. The ultrasonication is a simple way to de-agglomerate the chemically reacted carbon nanostructures with organic materials. 52 This crude was again sonicated in concentrated HCl and rinsed with distilled water. Pure crude solution was mixed with DM water and then proceeded to centrifugation step. This separated graphene was oxidized in air at 453 K for a 12-h drying cycle. Then, oxidation and drying steps were carried out to remove the amorphous carbon.

Characterization
A Raman spectrophotometer is a useful tool for the analysis of chemical bonding, nature of disorder, diameter of nanotube, and defects in graphitic nanosheets materials. Raman spectra were recorded on Horiba JY Lab RAM HR800 micro-Raman spectrophotometer (Horiba, Japan) with 632.8 nm laser excitation (He-Ne laser). An x-ray diffraction analysis of pure graphene sheets was conducted on D8 Advance x-ray diffractometer (Bruker AXS, Karlsruhe, Germany) with CuKa 1 radiation (k = 1.5404 Å ) within the 2h range of 20°to 80 o . The surface morphology of pure graphene sheets was studied by S-4800, field emission-scanning electron microscopy (FE-SEM) (Hitachi, Tokyo, Japan). Pure graphene was dispersed in acetone by ultrasound treatment of 10 min to form a holey carbon film. Pure graphene sheets were subjected to a gold coating and mounted on specimen tub before viewing in SEM. The exact size and shape of graphene sheets were studied by CM200, tunneling electron microscopy (TEM) (Philips, Amsterdam, the Netherlands) at a resolution of 2.4 Å and operating voltage of 20-200 kV. Pure graphene sheets were dispersed in water and kept in a conventional ultrasonic bath for 10 min. This sonicated crude droplet of colloidal suspension was dried and then put on a holey electron microcopper grid of the microscope specimen.

Raman Spectra
Raman spectroscopy was used to study optical phonon spectrum and Raman spectrum of pure graphene. Raman spectra can be used to determine the ''quality'' of graphene. Raman spectra are also used to determine the number of layers for n-layer graphene (for n up to 5) from the shape, width, and position of the 2-D peak. The shifting and splitting of Raman modes can be useful for analyzing a mechanical strain in graphene. The intensity ratio of the D-and G-peaks is used to study metrics of disorder in graphene. The disorder in graphene could be in the form of charged impurities arising from ripples, edges, presence of domain boundaries, and others. 22,30 Figure 7a-c shows that the peaks are distributed in two bands, i.e., a very weak D-band (1300-1400 cm À1 ) and a strong G-band (1500-1600 cm À1 ). A Raman spectrum of graphene indicates a peak at 1580 cm À1 for a G-band, which shows the evidence of single-layered graphene. 22,29 A peak is observed at 1350 cm À1 for D-band indicating the presence of amorphous carbon. 22,50 This is because of the first-order zone boundary phonons and defect-free graphene. But defected graphene still exists. 30,40 A 2-D peak is observed at 2650-2700 cm À1 due to optical vibration of carbongraphite electrode. The 2-D peak at 2700 cm À1 is caused by in-plane optical vibration and second-order zone boundary phonons. Defects in graphene may occur due to the oxidation step during purification strategy. 46 X-ray Diffraction (XRD) Figure 8a and b shows XRD patterns of graphitic raw material and graphitic carbon nanosheets. The peaks are indexed to the reflections of hexagonal graphite. Figure 8a shows 2h at 26.3°, which indicates that the graphite is crystalline material and crystallinity was reported to be 85.6%. Figure 8b shows the XRD pattern, which indicates amorphous nature of graphitic carbon nanosheet and its crystallinity was reported to be 66.4%. A diffraction of pure graphite is clearly seen which confirms the presence of crystalline material.

FE-SEM
Typical SEM micrographs (Fig. 9) show the surface morphology of pure graphene sheets. Structural degradation is observed in the graphitic microstructure. The microstructured materials were found randomly in large form and long graphitic nanostructured sheets. In fact, several morphologies and different microstructures were also observed despite the small surface of the electrode.
A compact globular material was observed on the outer surface layer of cylindrical periphery. The temperature in this region was low because of the direct cooling liquid environment. 46 The damage was less in water medium, owing to the better cooling capacity of water. 49 TEM TEM micrographs in Fig. 10 show nanostructures of graphene sheet. The TEM study revealed that most multilayered graphene sheets 22,40,42 were overlapped to each other. Hence, the texture of sheets is highly crystalline with least defect due to effect of ultrasonication and centrifugation steps. 40 The morphologies of graphitic carbonaceous nanosheets look like a form of wrinkled sheets. The carbon was no longer deposited at the surface of cathode on applying voltage greater than 22 V, but fine a powder appeared to be dispersed in the water. For lower voltage, mainly amorphous carbon was deposited on the wall of reactor. So we can say that nanoporous carbon is the main resulting product of the process. 46

VARIOUS POSSIBLE FUNCTIONALIZATION
Forefront research on graphene is mainly pushed by active materials, which has progressed to nextgeneration graphene-related nanomaterials. These materials are divided into two main categories 24,30 : chemically modified graphene (CMG) and 3-D graphene architectures (3DG). In CMG materials, carbon atoms of graphene sheets are replaced by other atoms of N, B, S, or P, or by entire functional groups. In 3DG materials, graphene or CMG sheets were assembled together to form 3-D interconnected networks or highly complex nano-objects. Graphene sheets can be functionalized by chemical and electrochemical surface modification, 25,29,53 which give CMG and 3DG nanomaterials.

Chemical Modification of Surface
Pristine graphene materials are unsuitable for intercalation with polymer chains because bulk graphene has a pronounced tendency to agglomerate in a polymer matrix. Chemical functionalization of graphene related nanomaterial is an attractive target because it can tune its stability, electronic, and magnetic properties, and it can improve the solubility, processibility, and interactions with organic polymers. Doping of graphene-related nanomaterial is done for tuning their different properties. Doping can be done by various techniques. 31 Graphene can be organochemically modified by different approaches 25,28,29,45 such as the reduction of GO, covalent modification of graphene, noncovalent functionalization of graphene, nucleophilic substitution, diazonium salt coupling, and adsorption of metal. The reduction of GO is carried in a stabilized medium; e.g., KOH-treated GO can be modified with hydroxyl, epoxy, or carboxylic acid groups. Covalent modification of graphene is done by using lithium reagents, isocynates, and di-isocynates to reduce hydrophilic character of GO, e.g., chitosan functionalized GO and CNT 33,54,55 for biological and medical applications. GO nanoplatelets also can be functionalized by polysodium styrene sulfonate, which is an example of noncovalent functionalization. 33,45 Amine-modified GO is an example of nucleophilic substitution. GO can be reduced by hydrazine and treated by aryl diazonium salts, which is example of di-azonium salt coupling. Atoms of transition metals like Ti and Fe can be anchored to adsorb on the surface of GO. Derivatives of graphane can be fabricated by changing the substrate atoms (C, Si, Ge, and P) and the surface atoms (H, -OH, -NH 2 , He, Li, Fe, Mn, all VII A group elements). 45

Electrochemical Modification of Surface
A colloidal suspension of graphene can be prepared from electrochemically modified graphite. 25 In this 30-min electrochemical reaction, a commercial graphite electrode was used as a cathode and immersed in a phase-separated mixture of water and imidazolium-based ionic liquids. A constant potential of 10-20 V was applied across the electrodes so ionic liquid functionalized graphene sheets can be precipitated from the anode. A homogeneous dispersion of 1 mg mL À1 was prepared by ultrasonication process using functionalized and dried

p-p Interaction
It is reported that graphene can be modified by considering the p-p interaction between the p orbitals of graphene and poly (iso-propyl acrylamide) in the presence of water. 25 Water-dispersible graphene can be produced by ultrasonication in an ice bath. Another example of an p-p interaction is graphene with pyrene derivative soluble in DMF. 29 Graphene can be incorporated with metals nanoparticles. This may keep graphene sheets in deaggregated form during the reduction of GO. 29

VARIOUS CHARACTERISTIC PROPERTIES
Graphene is preferable as conductive nanofiller owing to remarkable and excellent properties. These characteristic properties include high surface area, aspect ratio, tensile strength, thermal conductivity and electrical conductivity, EMI shielding ability, flexibility, transparency, and low coefficient of thermal expansion. These intrinsic properties have generated enormous interest for the preparation of ''graphenium'' devices such as high speed and radio frequency logic devices, thermally and electrically conducting nanocomposite materials, ultrathin carbon films, electronic circuits, sensors, and transparent and flexible electrodes for displays and solar cells. Graphene sheets have shown characteristic properties, which are listed in the next sections.

Morphology and Structure
The graphene honeycomb lattice is composed of two equivalent sublattices of carbon atoms bonded together with r-bonds. Each carbon atom in the lattice has p orbital that contributes to a delocalized network of electrons. Apart from ''intrinsic'' corrugations, graphene in real 3-D space can have ''defects'' like topological shapes (pentagons, heptagons, or their combination), vacancies, adatoms, edges, cracks, and adsorbed impurities. 30 It is reported that when half of the carbon atoms are hydrogenated to form graphone, strong r bonds are formed between the carbon and hydrogen atoms. These r bonds disrupt the usual p bonding network of metallic and nonmagnetic 2-D graphene sheets and cause the electrons on the unbonded carbonhydrogen atoms, which make it localized and unpaired. 32 Electronic Graphene is a zero band gap 2-D semiconductor material. As a consequence of the graphene's structure, the first Brillouin zone has two equivalent points known as Dirac points where a band crossing occurs. A tight-binding interaction with a first neighbor provides the dispersion relation to the electrons near the Dirac points. 19,26,[28][29][30]33,56 Graphyne nanoribbons have band gaps in the semiconductor range of 0.59-1.25 eV and widths of one repeat unit to eight repeat units. Graphyne has a strain free value of 0.47 eV, which can act as very attractive semi conductive material. 32

Mechanical
Mechanical properties (young's modulus, elastic modulus, and fracture strength) can be studied from molecular dynamics. Young's modulus of defect-free graphene was reported to be 1 TPa and the fractural strength was reported to be 130 GPa. The elastic modulus of chemically modified graphene was reported of 0.25 TPa. The maximum elastic modulus and fracture strength of ''GO platelets paper'' were reported as $32 GPa and $120 GPa, respectively. Graphdiyne is a softer material than either graphyne or graphene, and it has plane stiffness of 120 N m À1 (=Young's modulus of 375 GPa). 22,30,32,33 Optical The high-frequency conductivity for Dirac fermions in graphene has been stated to be a constant that is shown in Eq. A1 of Appendix A. The optical transmittance (T) and reflectance (R) are shown in Eq. A2 of Appendix A. The expression of (T) and (R) in terms of fundamental constants that does not directly involve material parameters to determine the structure and electronic properties of graphene. The complex dielectric property of graphyne is a function of energy adsorbed for an electric field parallel and perpendicular to the graphyne sheet. 22,30,32 The optical properties of GNRs are independent of their edge shapes and widths. These unique properties make GNR a suitable candidate for various applications in optical and optoelectronic devices. 34 Thermal Thermal conductivity (j) of graphene is dominated by phonon transport, which is also known as diffusive conduction at high temperatures and ballistic conduction at low temperatures. A thermal conductivity of suspended monolayer graphene is reported to be $6000 W m À1 K À1 , which is higher than that of graphitic carbon. The thermal conductivity of CVD growth graphene is reported to be $2500 W m À1 K À1 at 900 K and 1400 W m À1 K À1 at 1046 K. The heat capacity of graphene is reported to be 29.32 ± 0.23 J mol À1 K À1 , which is 14.8% greater than the graphene. 22,30,33,45,57 Magnetic Magnetism in sp 2 hybridized carbon materials has been controversial because of the possible contamination with magnetic impurities. Paramagnetism and certain other magnetic features including spin-glass behavior and magnetic switching phenomena were observed in nanographite particles. 29

Electrical and Electrochemical
Graphene and carbon nanoparticles with a few layers showed semiconducting or insulating behavior with little change in their resistance in the temperature range of 100-373 K. The resistivity was found to increase sharply below 50 K. But this might decrease if the graphene is heated to high temperatures; e.g., GNRs and a palladium sheet sandwiched between graphene sheets can be used as an electrode material for supercapacitors because of its superconducting nature. 29,32 Surface and Sensing Single-layer graphene sheet has a large surface area $2600 m 2 g À1 . Surface areas of different few layer graphene samples were reported in the range of 270-1550 m 2 g À1 . GNRs are also useful for Toxicity A toxicological effect of CNTs and graphene nanomaterial is discussed in terms of cytotoxicity on the basis of their size. 22,54 Table I shows a chart on the mechanical, thermal, and electrical properties of graphene compared with CNT, steel, plastic, rubber, and fiber. The tensile strength of graphene is observed to be the same or slightly more than CNT but it is still much higher than steel, Kevlar, HDPE, and natural rubber. It is clearly seen from Table I that the thermal and electrical conductivity of graphene sheets are higher than all these materials. Polymer and graphene nanocomposites showed superior mechanical, thermal, gas barrier, electrical, and flame-retardant properties compared with the neat polymer. Table II, the fascinating of properties graphene and their derivatives enable them to be used in many potential applications, which are shown below based on their electrical resistance.

Graphene-Polymer Nanocomposites
Exfoliated graphene or graphite nanoplatelets and their functionalized derivatives can be incorporated in polymers like PMMA, PS, PVA, PET, PP, PVDF, PAni, PC, PU, polyester, silicone, and foam. Polymer-graphene nanocomposites have shown dramatic improvements in their thermal, electrical, and mechanical properties such as thermal stability, electrical conductivity, elastic modulus, and tensile strength. But these improvements are due to the low filler content, large interfacial area, and high aspect ratio of filler, which is required for achieving a percolation. 21,22,25,28,30,33,38 Graphene-Metal Hybrid Nanocomposites Graphene-metal hybrid nanocomposites have attractive properties that make them ideal templates. These graphene-based nanocomposites can be prepared by incorporating metal nanoparticles like Au, Ag, Pd, Pt, Ni, and Cu. Depending on type of the anchored nanoparticle, the graphene-metal nanocomposites are used in the applications of catalysis, electrochemical sensing, and surface-enhanced Raman scattering. Graphene-based templates have been synthesized using various kinds of semiconductor nanomaterials such as ZnO, NiO, Cu 2 O, TiO 2 , SnO 2 , MnO 2 , RuO 2 , Fe 2 O 3 , Fe 3 O 4 , Co 3 O 4 , CdS, and CdSe. These graphene nanocomposites can be useful in energy, electronics, and optics applications as Li-ion batteries, supercapacitors, and solar cells. 22,28,33

Field-Effect Transistors (FETs)
Graphene is a suitable material for metallic transistor applications. The graphene carriers are bipolar with electrons and holes that can be tuned by a gate electrical field due to a unique band structure. A graphene quantum-dots-based singleelectron transistor was made using the electron beam lithography technique. GNRs prepared from ''unzipped'' CNTs by plasma etching and chemical oxidization methods are potential production techniques for future graphene FET devices. Graphene derivatives are better materials, which can be used as super capacitors than silicon-based ones because graphene derivatives possess band gap and semiconductor properties. 19,23,28,30,32,33,45,57,58 Sensors A monolayer graphene sheet has the ability to sense a variety of gases and biomolecules. Its sensing ability is based on a large specific surface area and a change in conductance as a function of surface adsorption. As molecules adsorb into the graphene's surface, adsorption experiences a charge transfer with the graphene sheet as a donor or acceptor. This changes the fermi level, electrical resistance, and carrier density of graphene due to which chemical sensing occurs. A large body of research has suggested that monolayer and functionalized graphene are promising candidates to detect a variety of gas molecules of LPG, ammonia and CO, organic vapors, protein molecules, and DNA. The large elastic region of graphyne and graphdyne has the ability to strain and relax to its original shape by releasing strain without permanent deformation. This enables resilient electromechanical coupling, which is required in high-temperature sensing. Graphane is also used in the application of biosensing due to its electrochemical oxidation. [26][27][28]30,32,33,45,55,59  Graphene materials are promising candidates for TCFs because of their high carrier mobility, electrical conductivity, and optical transmittance in the visible range of the spectrum. The optical transmittance (transparency) of chemically modified graphene is reported to be 83% at a wavelength of 1000 nm. Disordered films of randomly stacked multilayered graphene platelets have shown $75% transmittance at wavelength of 550 nm. GO-based TCF show $87% transmittance at a wavelength of 550 nm. Graphene base TCFs have been used as electrodes for dye-sensitized solar cells, liquid crystal devices, and organic light-emitting diodes. 28,38 Clean Energy Devices Graphene is a promising electrode material due to its high theoretical surface area and electron transferability along its 2-D surface. Graphenebased electrodes are used as rechargeable lithiumion batteries and electrochemical double-layer capacitors. Graphene nanocomposites can form a conducting 3-D network due to the uniform dispersion of silicon particles and reconstitution of graphene platelets, which are important aspects for the high storage capacities. Chemically modified graphene sheets have the potential to act as an electrode material for ultracapacitors. Lithium-doped graphane was used for hydrogen adsorption and its storage applications. 30,32,45

Memory and Photovoltaic Devices
Graphene-related nanomaterials have also been used in memory devices, transparent electrodes, electron acceptors, and light adsorbers because of their good electronic properties, transparency, and large specific surface area. 33

CONCLUSION
Intensive investigations on graphene during the past several years have shown increasing efforts to search for new graphene-related functional materials. This paper endeavors to sum up state-of-the-art techniques and the current status on graphene and their derivatives which have been predominated in the field of functionalized graphene. Graphene has shown its potential as a technological material because of its good electronic conduction, easy functionalization and a versatile nanoscale design. These characteristics are useful in the applications of fuel cell electrodes, supercapacitors, and batteries. Finally, graphene has been proven as advanced functional material that can be used in many potential applications.
A considerable cost-effective yield of graphene sheets and carbon nanostructured materials can be obtained by facile, cheapest, simple, and continuous arc-discharge under the flow of water. This method also shows a possibility of mass production of carbon nanostructured material by arc-discharge setup. This requires a DC power supply and a metal-graphite electrode without any accessories such as pumps, seals, water-cooled vacuum chambers, and gas purging systems. The system is adaptable in a continuous manner without interruptions. The continuous arc-discharge technique was found to be very effective compared with other reported techniques. This method is more suitable, cheaper, and easier to implement for the large-scale production and efficient formation of graphitic nanosheets because of its superior controllability and optional conditions. Carbon nanostructures could be produced with high yields after effective control by optimizing the shape of graphite electrodes, catalyst concentration, and electric parameters. Based on this strategy, the continuous arc-discharge technique can be used for the efficient formation of carbon nanostructures with a high production rate and yield.

ACKNOWLEDGEMENT
The authors are thankful to Council of Scientific and Industrial Research (CSIR), New Delhi, India (Project No.: 02(0023)/11/EMR-II) for providing financial assistance to carry out this research work.

APPENDIX A
The high-frequency conductivity for Dirac fermions in graphene has been given by the following expression (Eq. A1).