1. Introduction

Graphene has been extensively studied by scientific and engineering communities for more than 15 years since its first fabrication reported in 2004 ^[1]. Graphene is a single layer of two-dimensional carbon atoms in a hexagonal lattice structure and has been widely used in many applications such as electronics ^[2], energy storing batteries ^[3], super capacitors ^[4], fuel cells ^[5] and solar cells ^[6][7] owing to its unique high surface area, thermal ^[8] and electrical conductivity ^[9] and mechanical strength ^[10]. Graphene is one of the allotropes of carbon and it is made of hexagons. Other allotropes include fullerenes (0D), nanotubes (1D) and graphite (3D). Graphene-based nanomaterials include reduced graphene oxide, graphene quantum dots and graphene oxides. Graphene ideally consists of sp² carbon atoms but other family members comprise of sp² and sp³ due to the introduction of the functional groups such as hydroxyl, carboxyl, carbonyl and epoxy groups ^[11].

Graphene consists of two-dimensional layer of carbon atoms with sp² hybridization arising from mixing of s, p x and p y orbitals. The remaining p z orbital of each carbon atom forms π bonds with three neighboring carbon atoms, known as valence band and a band of empty π* orbitals known as conduction band ^[12]. Carbon has four valence electrons, and three of them form sigma bonds that are back bone of hexagonal structure. The remaining electron forms one third of π bond with the nearest neighbor carbon atom. These out of plane interactions are extremely weak leading to out of plane electrical and thermal conductivities which are ~10³ times lower than that of in plane analogues ^[13]. A single layer of graphene consists of hexagonal layer of carbon atoms. Bi-layer has 2 and few layer graphene has 3 to 10 layers of two-dimensional sheets. Graphene with more than 10 layers is known as thick graphene and has less scientific interest. In Bi- layer and few layer graphene, carbon atoms are stacked in different possible ways, hexagonal or AA stacking, Bernal or AB stacking and rhombohedral way or ABD stacking ^[14]. In a twisted bilayer graphene, layers are twisted at a small angle ^[15].

Graphene attracted great attention due to its excellent electronic properties. Graphene is a semiconductor with a small band gap ^[16]. The electron mobility of graphene at room temperature was found to be as high as 15,000 cm 2V^-1 s^-1 with small temperature dependency and zero effective mass for the charge carriers ^[17]. In most of the materials, the electron movement is hindered by phonon scattering. However, in the case of graphene, the movement of electrons is hindered only by defect scattering. As a result, the theoretical limit of resistivity of graphene is estimated to be 10^-6 Ω cm which is the lowest resistivity measured at room temperature ^[18]. Absence of interatomic plane coupling provides high thermal conductivity. Graphene with defects exhibits lower thermal conductivity. Average thermal conductivity of high-quality exfoliated graphene is 3000–4000 W/m K and the thermal conductivity of graphene prepared via CVD method is 2500 W/m K ^[19]. Due to its excellent thermal property, graphene is being used as a temperature sensor, a thermoelectric sensor and a thermal biosensor in energy management systems ^[20][21]. The graphene has been synthesized using chemical vapor deposition of hydrocarbon on transition metal surfaces ^{[22][23][24][25]}, thermal decomposition of silicon carbide wafer under ultrahigh vacuum ^[26][27], and chemical and thermal reduction of graphene oxide ^[28][29]. Among the methods listed above, reduction of graphene oxide is the most economical method ^[30]. Thus, for electrochemical analysis, graphene has been prepared using this method. In chemical vapor deposition method, there may be impurities of transition metals (Ni or Cu) but the reduction method provides graphene with high purity. Furthermore, this method provides an effective way of studying electrocatalytic effects.

Graphene oxide is a single layer of graphite oxide and is generally produced by the chemical treatment of graphite through oxidation ^[31]. Graphene oxide comprises various functional groups containing oxygen. These functional groups are mostly hydroxyl and epoxide groups in the basal planes and consist of trace amount of carbonyl, carboxyl, phenol, lactone and quinone groups at the edges of the sheet ^[32]. The wide range of these functional groups at the edges and the basal planes of graphene oxide make it functionalized and exfoliated to yield well dispersed solutions of separate graphene oxide sheets in polar and non-polar solutions and therefore it has a wide range of applications in nanocomposites ^[33] , photocatalysis ^[34], battery ^[35], capacitors ^[36] and sensors ^[37].

2. Synthesis

Geim and Novoselov carried out groundbreaking experiments on the two-dimensional graphene using scotch tape method in 2004 ^[38]. There are bottom-up and top- down approaches available for the synthesis of graphene. For example, mechanical cleavage is the process where graphite is broken down into graphene (top-down) and on the other hand, in chemical vapor deposition method, graphene is developed in silicon carbide (bottom-up) ^[39].

As previously mentioned, the graphene sheets are held together by weak van der Waals forces. If these forces are broken, high purity graphene can be obtained. In mechanical exfoliation method, mechanical energy is used to destroy these weak bonds and separate the individual sheets. Exfoliation is generally a peeling process repeatedly carried out in graphite to obtain layers of graphene. This method was first developed by Geim and Novoselov using highly oriented pyrolytic graphite (HOPG) as a precursor ^[17]. In this process HOPG sheet of thickness 1 mm is used to dry etching by oxygen plasma to create many 5 μm deep mesas (an isolated flat-topped surface). Then these were used to photoresist and baked to stick on the mesas. Thereafter, scotch tape was used to peel off the graphene layers from graphite. These thin flakes were then released to the acetone and transferred on to a Si substrate. Thus, pure graphene flakes are produced on a Si substrate. The disadvantage of this method is that large industrial production cannot be scaled.

CVD is one of the most promising method for the synthesis of graphene cost effectively and can produce large area of graphene. In this CVD approach, hydrocarbon gaseous species are injected into the reactor and travel through a hot zone where these molecules degrade to form carbon radicals and deposit on the metal surface as single / few layer graphene. During this process metal surface acts as a catalyst and also influences the deposition mechanism of graphene which plays a key role in the preparation of pure graphene ^[40]. Researchers have used metals such as Ru, Ir, Pt, Co, Pd and Re. The nickel (Ni) and copper (Cu) are low cost, have better control of layers of graphene and are easier to transfer graphene. Thus, they are widely used as substrates in CVD ^[40]. Using cold-wall and hot-wall reaction chambers, the CVD growth of graphene has been carried out ^[41]. In this technique, the growth of graphene is fast, high quality and takes low power consumption. Furthermore, there is an enhancement in charge carrier mobility.

Epitaxial growth can be made under vacuum or at atmospheric conditions. Different types of SiC such as single crystal SiC wafers, polycrystalline SiC and SiC thin films are used in this method. SiC is heated to 1200–1600 °C in ultra-high vacuum which sublimates silicon (melting point of Si = 1100 °C) leaving carbon atoms in the reaction vessel. Later these carbon atoms aggregate to form graphene ^[42]. This method produced 1 to 3 layers of graphene and the number of layers dependent on the decomposition temperature ^[14]. This synthetic technique is capable of producing wafer scale layers of graphene and therefore its potential interest is high in the semiconductor industry ^[14]. As a development of this method, graphene was synthesized from Ni coated SiC substrate at a lower temperature of 750 °C ^[43]. This method consists of technical problems such as high cost of SiC substrates (single-crystal SiC) and high temperature. These issues should be properly addressed to extend the practical and economic feasibility.

3. Applications

Addition of heteroatoms such as B and N into the graphene frameworks improved the surface wettability due to the presence of more hydrophilic sites and improved reactions between electrolyte and electrode ^[44]. N-doped graphene was studied for the reversible discharge capacity and it was found that nitrogen atoms improved the Li ion intercalation ^[45]. In a recent study, N-doped graphene nanosheets were synthesized in ammonia (NH₃) environment by heat treatment of graphene oxide. Reversible capacity of around 250 mA h g ^-1 at 2.1 mA g^-1 current density and capacity of 900 mA h g^-1 at 4.2 mA g^-1 were obtained ^[46]. Yang et al. reported the fabrication of N-doped porous graphene hybrid nanosheets with induced growth of zeolitic imidazolate framework on graphene ^[47]. This framework on graphene increases the specific area and the electron transfer within graphene network ^[47]. Hetero atomic defect increased the distance between sheets and electrolyte wettability enhanced the thermal stability and the electrical conductivity of doped graphene ^[48]. Graphene has been studied as mostly as anode material instead of cathode material because of its low electrical conductivity, slower electron and Li ion transport and low specific capacity and the agglomeration of the particles while applied as the cathode material ^[49]. Among the studies reported above, graphene intercalated with Co₃O₄ nanoparticles has high specific capacity and current density. Here the nanoparticles intercalate between graphene sheets and flexibility of graphene increased it performance. There is a strong interaction between graphene and Co₃O₄ preventing volume expansion/contraction and aggregation of nanoparticles during the battery performance. Yusuf et al. ^[50] reported that this composite effectively shows high surface area, good conductivity, and mechanical flexibility. Thus, this Co₃O₄ graphene composite exhibits a better performance.

Electrochemical sensing is one of the most important application of graphene. The adsorption of molecules on the surface of graphene influences its electrical conductivity ^[10]. Carrier concentration of the graphene is dependent on the type of dopant (donor or acceptor) adsorbed on the surface. There are many properties of graphene which increase its efficiency of detecting molecules. Since graphene is a 2D material, whole surface can be exposed to the analyte ^[51]. Graphene has high conductivity and low noise of distortion. Thus a small change in the concentration can alter electrical conductivity ^[52]. It consists of very low crystal defects ^[17][53] which ensure the low noise of distortion due to the thermal switching ^[54]. Schedin et al. ^[51] first reported the sensing of graphene in 2007. In this report sensing of gas molecules such as NO₂, NH₃, H₂O and CO is discussed. Re-usage of graphene sensors after the vacuum annealing at temperature of 150 °C or under UV radiation is possible for a short time ^[51].

Indium titanium oxide (ITO) and fluorine doped tin oxide (FTO) are commonly used as transparent coating for liquid crystal displays, solar cells and touch panels ^[55]. Due to the brittle nature and expensiveness of indium, graphene is being studied for the application in the transparent electrodes. The unique chemical, physical and mechanical stability makes graphene perfect material as a transparent electrode for solar cell and displays. Large surface area, inertness towards water and oxygen, high hole transparent mobility make graphene as an ideal material for photovoltaics ^[14].

Owing to the low cost and oxidizing ability, TiO₂ is generally used as a semiconductor forming graphene based photocatalyst for the degradation of the organic and biological contaminants. Reduced graphene oxides with 10% of Titanium nanotubes (TNT) showed the photocatalytic degradation of three times higher than the free TNT. TNTs were preferred due to their larger surface area when compared with that of the spherical nanoparticles ^[56]. As the organic dyes are aromatic, their adsorption on the graphene surface is enhanced by the π-π stacking interaction between sp² orbitals of the both systems ^[57]. Due to the combination of the graphene and photocatalyst, the band gap of the photocatalyst is reduced thus increasing the efficiency of the degradation ^[57][58]. Graphene improves the electron-hole recombination through the sp² hybridized network. Here the graphene acts as an electron acceptor and gives a conductive platform to transport electrons which involves in the oxidation and reduction reaction during degradation ^[57].

4. Conclusions

Graphene is a two-dimensional carbon network with a considerable research interest. Owing to its unique chemical and physical properties graphene has been investigated and used in many applications such as electronics, energy storing batteries, supercapacitors, solar cells and photocatalysts. Several synthetic methods have been applied to produce graphene such as mechanical exfoliation, liquid phase exfoliation, unzipping of carbon nanotubes, chemical vapor deposition and oxidation and reduction. Among these methods, chemical vapor deposition is the most promising method as this method is cost effective and can produce a large amount of graphene. N-doped graphene nanosheets and nanoparticles incorporated graphene have been investigated to improve the performance of the lithium ion batteries. The incorporation of nitrogen with graphene increases specific area and electron transfer within the network of graphene. Metal oxide nanoparticles such as RuO₂, NiO₂, MnO₂, Co₃O₄, ZnO and SnO₂ and CNT-graphene have been incorporated with graphene to boost the performance of the super capacitors. Among these Co₃O₄ nanoparticles exhibit the best performance. Both graphene and Co₃O₄ have strong interaction and this helps to prevents volume expansion/contraction and the aggregation of nanoparticles. Several electrochemical sensors such as gas sensors, biomolecular sensors and heavy metal ion sensors have been under research and in application as well. Sensors have been developed by the incorporation of metal nanoparticle and used to track NO₂, NH₃, DNT, CO, glucose level, DNA sequences, and metal ions (Pb²⁺, Cd²⁺). Biosensors have been developed with chitosan protein. Chitosan protein provides functional groups to make the sensors hydrophilic. Nanoparticles play an important role in these sensors by increasing the performance. Theses biosensors have been utilized for the detection of blood glucose level. Graphene has been studied for the transparent electrode in DSSC and several studies have been undergoing to improve the light conversion efficiencies. Graphene has been developed on Ni (1.01 mA cm^-2) and Cu to use as transparent electrodes. Performance of the transparent electrode was depended on the thickness of the metal and the weight percentage of the graphene. As Cu is a low cost and high electrical conductivity material, has an excellent performance of 12.64 mA/cm², when compared with the Ni. Unique chemical property of the graphene paved the way for the environmental applications such as adsorption of heavy metals, organic contaminants, gases and applications, where graphene functions as photocatalyst for the degradation of the organic pollutants. Compared to the adsorption of the gases CO₂, H₂O, CH₄, and N₂, CO₂ was preferentially adsorbed by functionalized graphene. As levels of CO₂ is drastically increasing this might be a solution to reduce the levels of CO₂ and to tackle the climate change. Graphene remains a unique material with exceptional properties that could lead to promising applications.