GRAPHENE/GRAPHENE OXIDE/REDUCED GRAPHENE OXIDE
Graphene is a well-known allotrope of carbon.134 It was discovered by Andie Geim and Kostya Novoselov by scotch tape method in 2004, and they won Nobel prize for this discovery in 2010.135 Graphene has attracted an incredible interest in researchers in the modem-day generation of emerging nanochemistiy field due to its optical, electronic, thermal, and mechanical properties.136 Because of the unique physicochemical properties of graphene, it has many applications in science and technology. Graphene is the building block of other important allotropes because zerodimensional fullerenes are the wrapped form of graphene, nanotubes (ID) as the rolled form of graphene and stacked form of graphene gives the 3D structured graphite.137 GO is a biocompatible carbon-based nanomaterial. It is an alternative material for graphene due to the similarities in properties. GO is a layered material with a large specific surface area and graphite is the main precursor for the synthesis of GO. Hydroxyl, carbonyl groups, carboxylic, and epoxy group are major groups present in GO147 and all the groups present are hydrophilic in nature. Due to the presence of these groups, GO is easily dissolved in water. Here we discuss the different green routes for the synthesis of graphene and its oxides.
GENERAL CHARACTERISTICS OF GRAPHENE, GO, AND rGO
The single carbon layer of graphite is called graphene. The main precursor for the synthesis of graphene is GO. Complete reduction of all functional groups (hydroxyl, epoxy, etc.) present in GO results in high-quality graphene. But a partial reduction of GO leads to the formation of reduced graphene oxide (rGO) (Fig. 1.6). The rGO sheets are generally considered as one type of the chemically derived gr aphene.138 Graphite is the starting material for the synthesis of graphene, GO, and rGO.
FIGURE 1.6 Schematic representation of (a) graphene, (b) graphene oxide, and (c) reduced graphene oxide.
Large-scale synthesis of high-quality graphene, GO, and rGO in an eco- friendly and inexpensive maimer is a big challenge. Lots of chemical methods are used for the production of these carbon-based nanomaterials, like scotch tape method, chemical vapor deposition, vacuum thermal annealing, micro-mechanical exfoliation of graphite, liquid phase exfoliation, epitaxial graphene grown on SiC, chemical intercalation of gr aphite, CVD/PECVD, epitaxial gr owth, solvothermal, or chemical reduction of GO and electric arc discharge.141142 But intensive energy requirements, high cost, usage of toxic, corrosive chemicals, and formation of hazardous by-products are the major disadvantage of these techniques. Unlike these chemical methods, green or biological methods by using plants and microorganisms do not require any toxic chemicals, high cost, and large amount of energy. Green methods do not produce any hazardous w-aste and the products usually do not need purification. The green synthesis of graphene, GO, and rGO using plants extract, biomolecules, and microbes provide economic and environmental benefits.155 Therefore, the green method is an excellent way for the synthesis high-quality graphene, GO, and rGO without any harmful by products.143144
Graphene is an ultrathin material, with a plant sheet of sp2 hybridized carbon atoms with honey comb like structure. The carbon-carbon distance in graphene is 0.142 mn and the thickness of a single graphene layer is about 0.34 mil.137 A perfect graphene sheet is vety ordered and show's several extraordinary properties including very high electron mobility, strong chemical durability, excellent thermal conductivity, outstanding surface areas, high Young’s modulus, and anomalous quantum Hall effect.139 Due to the two-dimensional structure of graphene, every single atom is exposure to chemical reaction.155 These special characteristics of graphene opened a new w’ay of extensive range of applications.144 The band gap of graphene is 0 eV, suggesting that means there is no energy gap between valence band and conduction band. This special property of graphene is mainly applicable in electronic devices.165 The surface of graphene is highly hydrophobic, since graphene offers an extraordinary support to anchor oxygen-containing functional groups such as, carboxyl, epoxy, and hydroxyl group and forms rGO or GO. Compared to conventional commercial adsorbent, graphene show's better capacity for regeneration, low'-temperatme modification, and reusability properties. The presence of large and delocalized л- election system is the reason for the binding of target pollutants.163
Л.2.2 GRAPHENE OXIDE/REDUCED GRAPHENE OXIDE
The structure of GO is similar to graphene, but the GO sheets are thicker than pure graphene sheet because of the presence of carboxylic acid, hydroxyl, carbonyl, and other oxygen-containing functional groups.141 Due to the presence of these functional groups, GO can be used for the preparation of nanocomposites and GO is highly soluble than graphene. GO and rGO have almost similar properties. The only difference is the decrease in oxygen-containing functional groups in rGO. The UV-vis. absorption of GO shows the peak at 230 nm due to the 7г-я* transition, but after reduction the absorption peak of rGO at 270 nm undergo red shift due to the removal of oxygen-containing functional groups.166
GREEN SYNTHESIS OF GRAPHENE, GRAPHENE OXIDE, AND REDUCED GRAPHENE OXIDE
Two primary approaches for the synthesis of graphene are the top down and bottom up. In top-down rout graphene is synthesized by solution exfoliation of graphite, micromechanical cleavage, and graphitization of SiC, but poor yield is the major disadvantage of this method.164 Graphene sheets are mainly synthesized by reduction of GO and the GO is reduced to graphene by the help of strong reducing agents. Low cost and high yield are the advantages of this reduction method, but the chemical reduction of GO using toxic and explosive reducing agents like sodium borohydride, hydrazine hydrate, and dimethyl hydrazine cause many environmental issues due to the elimination of hazardous by-products.142 Therefore, reduction of GO without using hazardous reagents or expensive instruments are very essential for the synthesis of graphene. From the past decade, development of green reducing agents for GO such as plant extract, microbes, biomolecules, etc. are environmentally safe and hazardless reducing agents like sugar, ascorbic acid, heparin, wild carrot roots, aloe vera (AV), vancomycin, E. fergusoni, Escherichia coli, Pseudomonas aeruginosa, and bovine serum albumin highlight the green approach.156
,3.1 REDUCTION USING PLANT EXTRACTS
lit general, plants contain many biomolecules, such as polysaccharides amino acids, alkaloids, alcoholic compounds, chelating agents polyphenols, enzymes, vitamins, and proteins. These molecules play an important role in bioreduction.156 Saikumar Manchala et al. synthesized graphene by the reduction of GO using Eucalyptus polyphenol solution obtained from Eucalyptus bark extract. The polyphenols present in the Eucalyptus bark extract have strong reducing ability, which is responsible for the formation of graphene from GO.142 Coconut water (Cocos nucifera L.) is a natural reducing agent which is nontoxic and environment friendly. Kartickl et al. synthesized high-quality graphene from GO with the help of coconut water.155 Porous graphene-like nanosheets (PGNSs) is used as a sensor for hydrogen peroxide. PGNSs were fust facilely designed fr omficus-iacertiolia fruit with active functional groups for the sensing of H,Ov This innovative thinking was from Taotao Liang and coworkers.144 Another green-reducing agent for the synthesis of graphene is from pomegranate juice and anthocyanine is the main content in pomegranate juice. Anthocyanins are water soluble. Due to the hydrophilic nature it can easily convert GO into graphene .This green and economic method for the preparation of graphene nanosheet was introduced by Famosh Tavakoli et al.141 Yan Wang and coworkers proposed a method for the reduction of exfoliated GO using green tea solution Due to the presence of aromatic rings of tea polyphenol (TP) is the reason for its reducing capability. The characterization of the obtained graphene confirm the removal of the oxygen-containing functional groups in GO.148
One of the interesting route for the preparation of graphene by the deoxygenation of GO using Zante currants (ZC) extract. GO is effectively reduced by ZC. This method shows outstanding reproducibility and this novel synthetic method is proposed by Mohd Zaid Ansari and coworkers.146 Sangiliyandi Gunmathan et al. reported Ginkgo biloba extract (GbE) as stabilizing and reducing agent for the synthesis of cytocompatible graphene which is an efficient method for the preparation of graphene.158 EC process is a high efficient and relatively low cost method for the synthesis of GO. The pollution-free environment is the major advantage of this green method. Various graphitic materials such as graphite rode, pencile core, and graphite flakes are used for the synthesis of GO by EC oxidation. Songfeng Pei et al. reported a green method to synthesize high-quality GO by water electrolytic oxidation of graphite.152 The product of removal of oxygen functional group from GO gives rGO. Lot of green-reducing agents are used for the synthesis of rGO including leaf extracts of Colocasia esculent a and Mestia ferrea Linn, grape extract Citrus sinensis (orange peel).139
Gourav Bhattacharya reported a method for the synthesis of rGO from GO using AV as a green-reducing agent. AV extract contains the organic compounds like sugars anthaquinones and polysaccharides, etc. All these have the natural reducing ability.150 Azadirachta indica (AI) which belongs to the family Meliaceae and originates in tropical and semitropical regions, alkaloids, flavonoids, and terpenes are the major phytochemicals present in the AI leaves. The presence of these phytochemicals is responsible for the strong reducing tendencies. Using this reducing ability of AI leaves Guana kumar et al. developed a synthetic route for the preparation of rGO using AI leaves as a reducing agent.151 Synthesis of rGO from GO by the help of green tea extract is a veiy simple, efficient, and low cost method introduced by Melvin Jia-Yong Tai and team.153 Li Ganl et al. proposed a green approach for the synthesis of rGO sheet. The advantage of this method is the utilization of bagasse and it establishes a waste-to-resource supply chain.147 For the green synthesis of Pluronic stabilized rGO, ascorbic acid is used as a reducing agent for the reduction of GO to rGO. This is a facile green route was introduced by Cherian R.S et al. 136 Lots of plant extract are used for the synthesis of nanomaterials, among various options, the Opuntia ficus-indica (OFI) plant extract is a strong reducer and stabilizer, and using this rGO is synthesized from commercial graphite and also the reaction mixture was placed in a high energy wet ball milling.145
,3.2 REDUCTION USING MICROORGANISMS
Various scientific reports are evident for the synthesis of nanoparticles using microorganisms. The mechanism of the reduction reaction depends on the nature of bacterial cells that have the capacity to hydrolyzing the acid group present in GO.158 An environment friendly green approach for the synthesis of soluble graphene using Bacillus marisflavi biomass as a reducing and stabilizing agent under mild conditions in aqueous solution was carried out. The GO formed by this method is applicable for many biomedical applications and this novel idea was suggested by Sangili- yandi Gunmathan et al.143 Akhavan et al. reported that interactions of the chemically exfoliated GO sheets and E. coli bacteria living in mixed-acid fermentation with anaerobic conditions was investigated for the different exposure times of the sheets to the bacteria. The effects of the bacteria and their proliferation on the chemical state, carbon structure, and electrical characteristic of the GO sheets were examined by XPS, Raman spectroscopy, and current-voltage (I-V) measurement, for the different exposure tunes to the bacteria.157 Wang et al. reported that Shewcmella is used for the reduction of GO via external electron transport mediated by c-type cytochromes included heme group and this electron mediators are secreted from Shewanella}62 Bacillus subtilis is another type of microorganism used for the reduction of GO and the reduction product is used for the development of a supercapacitor and this excellent idea was developed by Zhang et al.163
REDUCTION USING BIOMOLECULES
Biomolecules are used as a strong green reducer. Gelatin is a natural reducer, its reducing capability is used for the synthesis of graphene from GO. This interesting idea was proposed by Kunping Liu and coworkers. Gelatin plays an important role for the prevention of aggregation of graphene nanosheet. The synthesized gelatin-GNS have lots of applications. A green and facile method for the preparation of gelatin functionalized graphene nanosheets (gelatin-GNS) was reported by using gelatin as a reducing reagent. Meanwhile, the gelatin also played an important role as a functionalized reagent to prevent the aggregation of the graphene nanosheets.154 Recently, Zhang et al. suggested that L ascorbic acid is used for the reduction of the GO sheets under mild conditions. This is an excellent method for the synthesis of water-soluble graphene.149 For the preparation of graphene, vitamin C is used as a secure reductant for the deoxygenation of GO. Vitamin C is a best alternative to the highly toxic reducing agent such as hydrazine and also this reduction reaction is carried out in different organic solvent.159 The reducing sugars such as fructose and glucose act as mild reducing agents for the synthesis of the graphene nanosheet from GO. In addition to the reducing capability, they also act as capping agent. This was reported by Zhu et al.160 Another important biomolecule, Melatonin was used for the synthesis of graphene by the reduction of GO with maximum efficiency. This is a biocompatible and very safe bioreducer for the large production of graphene with varity of bio applications.161
The green synthesis of graphene, GO, and rGO using plant extract, microorganisms, and biomolecules is efficient, eco-friendly, and cost effective method. Recently, researchers mainly focused on green rout for the synthesis of nanoparticle due its various advantages. Graphite is the main precursor of the synthesis of like graphene, GO, and rGO. The oxidation of graphite gives GO and graphene is a product of strong reduction of GO and partial reduction of GO gives rGO. Reduction using natural- reducing agents happens at mild conditions: atmospheric pressure and room temperature. Considering the tremendous applications of graphene, GO, and rGO, green technology is profoundly promising because of its advantages like nontoxicity, cheap cost, and eco-ffiendliness. Identification of more efficient green reducers is one of the future challenges of this research field.