The green reduction of graphene oxide

Abstract

Graphene is an ultra-thin material, which has received broad interest in many areas of science and technology because of its unique physical, chemical, mechanical and thermal properties. Synthesis of high quality graphene in an inexpensive and eco-friendly manner is a big challenge. Among various methods, chemical synthesis is considered the best because it is easy, scalable, facile, and inexpensive. Different kinds of chemical reducers have been used to produce graphene sheets. However, some chemicals are toxic, corrosive, and hazardous. For this reason, researchers have been using different environmentally friendly substances (termed green reducers) to produce functional graphene sheets. This paper presents an overview and discussion of the green reduction of graphene oxide (GO) to graphene. It also reviews the characterization of GO and its oxide reduction through the analysis of different spectroscopic and microscopic techniques such as Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and atomic force microscopy.

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1. Introduction

Graphene promises to be the most exciting material of the decade. It is a plane sheet of sp² hybridized carbon atoms tightly confined into a honeycomb lattice. In 2004, the first discovery of graphene using a scotch tape peeling method brought a dramatic revolution, especially in the world of materials science.¹ Recently, this single sheet of carbon has attracted huge interest among the scientific community owing to the two dimensional structure.² It is proved that an ideal graphene sheet is highly ordered and shows several extraordinary behaviors including outstanding surface areas (2630 m² g⁻¹), high Young's modulus (1.0 TPa), high thermal conductivity (∼5000 W m⁻¹ K⁻¹) and strong chemical durability and high electron mobility (2.5 × 10⁵ cm² V⁻¹ s⁻¹).³ It also exhibits single-molecule gas detection sensitivity,⁴ quantum confinement in nano scale ribbon,⁵ long range ballistic transport at room temperature⁶ and high optical transmittance (∼97.7%).^7,8 These unique characteristics of graphene opened a new way for its wide range of application at different branches of technology such as: lithium-ion batteries,^9,10 catalyst engineering,^11–15 chemically derived sensor,^16–18 biosensor,^19,20 anti-bacterial activities,²¹ flexible thin film transistor,²² delivery of drugs,^23,24 solar cells,^18,25,26 photovoltaic devices,²⁷ imaging,²⁸ p–n junction materials,²⁹ super capacitor,³⁰ touch panel,^31–33 electromagnetic shielding application,³⁴ water purification,³⁵ absorption of different non aqueous liquid e.g., oils, alkenes, aromatic compounds, dyes, organic solvents, and ionic solutions.^36–41

Large quantities of graphene materials are required to fulfil the huge demand of the aforementioned applications. Production of the best quality graphene in an inexpensive manner and on the desired scale is a big challenge of the time. There are a number of approaches have been designed to synthesis graphene from pristine graphite such as: epitaxial growth,^42–44 vacuum thermal annealing,^45,46 non-catalytic synthesis,¹³ scotch tape method,⁴⁷ micromechanical cleavage,⁴⁸ chemical vapour deposition (CVD),^49,50 thermal exfoliation,⁵¹ carbon nanotubes cutting,⁵² liquid phase exfoliation,^53,54 direct ultrasound sonication⁵² and reduction of graphene oxide.⁵⁵ There are also some unconventional and hazardous methods of graphene production i.e. the chemical based detonation,⁵⁶ arc discharging⁵⁷ and carbon nano tube unzipping.⁵⁸ Epitaxial growth is able to produce good quality multilayer graphene but it cannot isolate conductive single or bilayer that are suitable for device applications.⁴⁸ A remarkable limitation of mechanical cleavage is repeated peeling during layer isolation and it is difficult to maintain the number of separated layers produced by this method.⁵⁹ On the other hand, thermal exfoliation results in high production costs.⁴⁸ In case of CVD, it is not easy to achieve layer separation without material damaging and successful exfoliation from the substrate has been a stumbling block. In contrast, graphene oxide (GO) reduction is considered a promising approach for the mass production of graphene. The product obtained by this method is commonly termed as reduced graphene oxide (RGO) or graphene nano sheet (GNS).

However, GO acts as a precursor to synthesis RGO or GNS. It is produced from graphite flakes by oxidative reaction, which heavily decorates the sheets with different oxygen moieties. The presence of abundant amounts of oxygen functional groups over the carbon basal plane not only makes GO electrically insulating but also provokes thermal instability.^54,60 Therefore, reduction of oxygen molecules is the only key to resettle the π lattice. It helps to produce thermally stable graphene and regain electronic conductivity. Several reduction approaches of GO e.g., the electrochemical reduction,^13,61–70 thermal reduction,^71,72 photocatalytic reduction,^73,74 and chemical reduction have been reported in recent time. Electrochemical reduction is unable to heal the defects and vacancies inherited from the precursor GO. Secondly, the oxygen content should be as low as possible in the GO precursor for exfoliation to single layer GO sheets. But the low oxygen content in GO may depress the interaction between oxygen functionalities. It could produce stable species like in-plane ether or out-of-plane carbonyl groups which are relatively difficult to remove by the electrochemical reduction method.⁷⁵ Meanwhile, thermal deoxygenation is expensive. It is also a complicated phenomenon because of the thermal-energy induced multistep removal process of intercalated H₂O and oxide groups of –OH (hydroxyl groups), –COOH (carboxyl groups) and [double bond splayed left]O (epoxy groups).⁷⁶ Photocatalytic reduction requires active performance of light sensitive materials under continuous ultraviolet radiation.^73,74 Hence, in comparison with the aforementioned reduction approaches, the chemical reduction is considered as the easiest way to produce ultrathin graphene sheet with large lateral dimension and area.⁷⁷ In addition, this method is relatively fast, economic, facile, and suitable for subsequent processing and chemical modifications.

Scientists have applied different types of reducing agents to convert GO to graphene. Hydrazine, sodium borohydride, hydroquinone, etc. are toxic reducers. As alternatives, different kinds of organic acids, biochemical substances, amino acids, bacteria, fungus, plant extract, cellulosic compounds or metal powders have been applied to prepare graphene sheets from the GO. These reducing agents are termed “green reducers”. Since they are free from corrosion, carcinogenicity, and toxicity. There are few review papers available about the chemical or other reduction of GO.^75,78 However, there is no compilation giving an overview of the green reduction of GO to graphene. Therefore, in this review, we summarize the reduction of GO using different environmentally friendly reducing agents in recent time. Herein, we also briefly discussed the selection of pristine graphite, synthesis of GO precursor and the exfoliation of GO. The results of different characterization techniques such as Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) have also been summarized in this study. Obviously, this review will show a guideline to the researchers interested in the green synthesis of graphene.

2. Synthesis of graphene nano sheets

This is a two step procedure. The first step is the production of GO precursor from graphite and the following step is the reduction of oxygen from the GO sheets. In order to synthesise GO the selection of graphite is also important. Brief descriptions of the processes are presented in the following subsections.

2.1. Selection of parent graphite

Parent graphite is the source for GO production, which is exfoliated and later reduced to synthesise functional graphene sheets. It is experimentally investigated⁷⁹ that the GO produced from natural graphite (NGrO) performs better than the synthetic graphite (SGrO) when exfoliating in aqueous solution. The probable reason is that NGrO possesses a larger crystalline structure. It also contains a large number of sp³ carbon to carbon bonds (NGrO ≈ 12.8% and SGrO ≈ 4.9%) and C–OH bonds (NGrO ≈ 36.2% and SGrO ≈ 1.8%). In contrast, GO produced from synthetic graphite (SGrO) possesses higher degree of carboxylic groups (NGrO ≈ 4.4% and SGrO ≈ 10.7%) and ring shaped epoxy groups (C–O–C) (NGrO ≈ 14.3% and SGrO ≈ 39.4%). This presence of large amounts of C–OH groups in the inward direction of the basal plane leads to easy exfoliation of NGrO. According to the same study the exfoliation yield of NGrO is nearly double that of SrGO.⁷⁹ The size of graphite flake also plays important role in the oxidation of graphite. McAllister et al.⁸⁰ observed that the small sized flakes (∼45 μm) are quickly oxidized and intercalated with a maximum reliability, while large sized flakes (∼400 μm) take more time to oxidize and it regularly fails to intercalate completely. It is noteworthy that the reduction of the size of starting flakes allows faster and more reliable oxidation without a significant difference in the final size of the GO sheets.⁸⁰

Different types of graphite powder can be used as a starting material such as natural flaky graphite, artificial graphite, kish graphite and highly oriented pyrolytic graphite (HOPG) (please refer to Table 1 from the cited article).⁸¹ This selection of starting graphite is an important factor to pre-determine the number of layer in functional graphene produced upon the reduction of GO.⁸² Table 1 discusses the possible layer number of functional graphene obtained from different types of parent graphite. Significantly, graphite with low crystallinity and small lateral size is suitable to produce high quality single layer graphene with good electron conductivity ∼ 1 × 10³ S cm⁻¹. On the other hand, the highly oriented pyrolytic graphite (HOPG) is appropriate to produce thick or multiple layer graphene (>10 sheets).

Table 1 The relationship between parent graphite and number of layers in graphene

Types of parent graphite	Number of layers in graphene	Possibility	Reference
Artificial graphite	Single layer	∼80%	82
Flake graphite	Single and double layer
Kish graphite	Double and triple layer
Natural flakes	Few layers (4–10)
Highly-oriented pyrolytic graphite	Thick graphene (>10)

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2.2. Synthesis of graphene oxide from graphite flakes

The Hummers method (HM)⁸³ and the modified Hummers method (MHM)⁸⁴ are the commonly used method of graphene oxide synthesis. In Hummers method the oxidation of graphite is conducted with the assistance of sodium nitrate (NaNO₃), potassium permanganate (KMnO₄) and sulphuric acid (H₂SO₄). Hydrogen peroxide (H₂O₂) is added to terminate the reaction and remove residual permanganate and manganese dioxide producing a bright-yellow colour suspension. This colour ensures the successful conversion of GO from pristine graphite. Here, H₂SO₄ helps to create acidic reaction condition.⁸⁵ And KMnO₄ acts as a strong oxidizing agent specially in the acidic media.⁸⁶ With the aid of this KMnO₄, a complete intercalation of graphite with concentrated H₂SO₄ can be occurred. It develops graphite bisulfate in which each mono-layer of graphene is sandwiched by the layer of bisulphate ions.^87,88 This successful intercalation confirms the efficacious penetration of KMnO₄ into graphene layers for the oxidation of graphite.

The modified Hummers method is either one step method or two step method. The one step modified Hummers method is developed by Chen et al.⁸⁹ It is also called as “Improved Hummers” method where the graphite was oxidized without using NaNO₃. In two step modified Hummers method the graphite powder is initially oxidized by H₂SO₄, K₂S₂O₈ and P₂O₅. Then the pre-oxidized graphite was subjected to oxidation by Hummers method. Marcano and co-workers⁹⁰ developed a new method which they called “Improved synthesis” of GO that can be seen in Fig. 1. They used KMnO₄ in a 9 : 1 weight ratio with H₂SO₄ and H₃PO₄ mixture. In addition to, Xu et al.⁹¹ developed another method by reducing KMnO₄/graphite ratio from 3 : 1 to 1 : 1. This approach is termed “mild oxidation”. A brief experimental procedure of all the aforementioned methods has been discussed below. In Table 2 a comparative study of all these methods has also been discussed.

Fig. 1 Representation of GO synthesis by different methods with the production of under-oxidized hydrophobic carbon material.⁹⁰

Table 2 A comparative study of GO synthesis

Method of oxidation	Advantages	Disadvantages
HM	• One step method	• Higher degrees of defect due to harsh oxidation
	• High efficiency	• Production of higher under-oxidized hydrophobic carbon materials
	• High yielding	• Generation of toxic gases (NO2, N2O4, ClO2)
	• Free from the emission of acid fog	• Production of residual Na^+ and NO3^− ions which are difficult to remove from the waste water
Two step modified HM	• High yielding	• Two step method
	• Penetration of more oxygen functional groups throughout the sheets	• Time consuming
		• Formation of higher defects or disorders
One step modified HM/improved HM	• Short oxidation period	N/A
	• Simple procedure
	• High yielding
	• Low cost
	• Free from the production of toxic gases and residual Na^+ and NO3^− ions
	• Production of very low under-oxidized hydrophobic carbon materials
Improved synthesis	• One step method	• Lengthy oxidation period
	• High yield	• Requires twice as much KMnO4 and 5.2 times as much H2SO4 as those applied by Hummers method
	• Fewer defects	• Introduction of H3PO4 as an additional oxidising agent in the reaction system
	• Free from the production of toxic gases
	• Production of lower under-oxidized hydrophobic carbon materials
Mild oxidation	• One step method	• Low yield
	• Good water solubility	• Production of higher unoxidized graphite in the form of aggregate
	• Large π-conjugated structure
	• Less lattice disorder
	• Act as a low defect precursor to prepare highly conductive graphene upon chemical reduction

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2.2.1. The original Hummers' method. The GO was synthesised by stirring 100 g of graphite flake and 50 g of NaNO₃ into 2.3 liters of H₂SO₄. The ingredients were mixed in 15 liter battery jar and cooled to 0 °C in an ice bath. During the conduction of vigorous agitation 300 g of KMnO₄ was added to the suspension. The rate of addition was managed carefully to keep the suspension temperature below 20 °C. After that the ice bath was removed and the solution was warmed at 35 ± 3 °C for 30 minutes. As the oxidation progressed, the mixture gradually thickened. At the end of 20 minutes the mixture became pasty with the release of small amounts of gas. The colour of the paste was brownish gray. At the end of 30 minutes, 4.6 liters of water was gradually added and stirred into the paste to dilute it. The dilute suspension was brown in color. During this addition of water a large exothermal heat was generated to 98 °C which is maintained for more 15 minutes through exothermal heating. The suspension was further diluted by pouring 14 liter of warm water and treated with 3% H₂O₂. After treating with peroxide, the suspension took brownish yellow color appearance. Later the suspension was filtered resulting a yellow-brown filter cake. The filter cake was washed 3 times and the obtaining residue was dispersed in 32 liter of water to yield 0.5% solids. The dry form of the product was obtained by centrifugation followed by vacuum dehydration at 40 °C.

2.2.2. The two step modified Hummers' method. The graphite flake (20 g) was added to a mixture of concentrated H₂SO₄ (30 mL), K₂S₂O₈ (10 g) and P₂O₅ (10 g). After the admixture a dark blue color suspension was produced with large exothermal heat. The resultant suspension allowed to cool at ambient temperature over a period of 6 h. Distilled water was carefully added to the mixture to dilute it. Several filtration and washing was carried out until the pH of the rinse water became neutral. The product was dried in the room temperature for overnight. Thus the pre-oxidized graphite was prepared which is later subjected to oxidation by the Hummers' method. The pre-oxidized graphite powder (20 g) was put into cold concentrated H₂SO₄ (460 mL). KMnO₄ (60 g) was slowly added while agitating and cooling the solution, during which the temperature of the solution was not allowed to exceed 20 °C. Then the mixture was stirred at 35 °C for 2 h with the addition distilled water (920 mL). Upon 15 min, the reaction was terminated by pouring a large amount of distilled water (2.8 L) and a 30% H₂O₂ solution (50 mL), after which the color of the mixture was turned into bright yellow. After that, the mixture was filtered and washed with a 1 : 10 HCl solution (5 L) to discard metal ions.

2.2.3. The improved Hummers' method/one step modified Hummers' method. The graphite flake (3 g) was added to a mixture of concentrated H₂SO₄ (70 mL) under the agitation in an ice bath. Under vigorous stirring, KMnO₄ (9 g) was slowly added to keep the suspension temperature lower than 20 °C. Successively, the reaction mixture was shifted to a 40 °C oil bath and vigorously stirred for about 30 minutes. After that, 150 mL distilled water was poured and the solution was stirred for more 15 min at 95 °C. The reaction was terminated by pouring additional water (500 mL) and a 30% H₂O₂ solution (15 mL), after which the colour of the suspension was turned into yellow from brown. The mixture was then filtered and washed with a 1 : 10 HCl solution (200 mL) to discard metal ions.

2.2.4. The method of “improved synthesis” of GO. A 9 : 1 mixture of concentrated H₂SO₄/H₃PO₄ (360 : 40 mL) was poured into a mixture of graphite powder (3 g, 1 wt equiv.) and KMnO₄ (18 g, 6 wt equiv.), producing a mild exotherm to 35–40 °C. Then the reaction mixture was warmed to 50 °C and stirred for 12 h. The mixture was cooled to room temperature and poured onto ice (∼400 mL) with 30% H₂O₂ (3 mL). For work-up, the mixture was sifted through a metal U.S. Standard testing sieve (W.S. Tyler, 300 μm) and then filtered through polyester fiber (Carpenter Co.) The filtrate was centrifuged at 4000 rpm for 4 h and the supernatant was removed. The remaining solid material was then washed in succession with 200 mL of water, 200 mL of 30% HCl and 200 mL of ethanol (2×). For each wash, the mixture was sifted through the U.S. standard testing sieve and then filtered through polyester fiber. Later the filtrate was centrifuged at 4000 rpm for 4 h and the supernatant was decanted away. The remaining material was removed by multiple-washing and the resulting suspension was filtered over a polytetrafluoroethylene membrane with a 0.45 μm pore size. The solid yield on the filter was vacuum-dried at room temperature for overnight to extract 5.8 g of GO.

2.2.5. The method of “mild oxidation”. Graphite (0.5 g, 1 wt equiv.) and KMnO₄ (0.5 g, 1 wt equiv.) were successively mixed with concentrated sulphuric acid (15 mL) under stirring at room temperature. Then the reaction mixture was transferred to a 50 °C water bath for about 3 h. Then the mixture was poured into 150 mL water and the solution was agitated for another 15 min. The oxidation was terminated by the addition of 10 mL H₂O₂ (30%). The mixture was then filtered and washed with 1 : 10 HCl aqueous solution (50 mL) to discard metal ions followed by washing with water to remove the acid. The resulting solid was dispersed in 150 mL water by ultrasonication for 1 h to make an aqueous dispersion. The obtained dispersion was then centrifuged at 3000 rpm for 30 min to get a black-brown supernatant. The sediments at the bottom were again dispersed in 50 mL water by ultrasonication for 1 h and the dispersion was treated by centrifugation as above. The two parts of supernatants were mixed to get the aqueous dispersion of mildly oxidised GO. The yield of as-synthesised GO was calculated to be about 20% (the weight of mildly oxidized graphene divided by the weight of graphite powder).

The physical properties of GO including electron conductivity, surface charge, transparency, band gap energy etc. can be determined by the type and quantity of oxygen functionalities over the GO sheets. The lateral size of GO is affected by the degree of oxidation, oxygen moieties and defect sites. It can be increased by a high degree oxidation which can rupture the GO sheets during the exfoliation step.⁹² Lerf et al.⁹³ proposed a new structural model of GO. They demonstrated that the hydroxyl and epoxy groups are the major or dominant oxygen functionalities of GO. These groups are located at the basal plane of the sheets. In contrast, the other oxygen moieties such as the carboxyl and carbonyl groups are typically found at the edge of the sheets.⁹⁴ The quality of graphene oxide is evaluated by the carbon and oxygen (C/O) atomic ratio and the average size of the GO sheets. The increment of oxidation temperature decreases the C/O atomic ratio and the size of the sheets. It is attributed to the formation of more functional groups and defect sites due to the extensive oxidation.⁹⁵ This relationship can be represented as follows in Fig. 2.

Fig. 2 The relationship of oxidation temperature with C/O atomic ratio and average size of the sheets.

The thickness of GO is another important parameter to evaluate the successful production of GO. Typically it is increased after oxidation owing to the attachment of covalently bonded oxygen. It is briefly discussed in the AFM section of the study. A list of C/O atomic ratio and thickness of GO produced by different procedure has been tabulated in Table 3.

Table 3 Summary of C/O atomic ratio and thickness of GO samples

Method of GO synthesis	Size of graphite flakes	Time of oxidation	Temperature of oxidation	Ratio of C/O	Thickness of GO (nm)	Reference
a Modified Hummer method.
Hummer method	<1 mm	∼2 hours	98 °C	2.30	∼1.0–1.4	96
Hummer method	30 μm	∼4 hours	35 °C	2.33	∼0.9–2.3	97
Hummer method	30 μm	∼2 hours	60 °C	2.28	∼1.0	97
Hummer method	30 μm	∼2 hours	45 °C	2.25	∼1.0	97
Two step MHM^a	74 μm	∼8.5 hours	Pre-oxi: 80 °C	1.18	∼0.80	95
			Oxi: 35 °C
Two step MHM^a	74 μm	∼8.5 hours	Pre-oxi: 80 °C	1.24	∼0.89	95
			Oxi: 27 °C
Two step MHM^a	74 μm	∼8.5 hours	Pre-oxi: 80 °C	1.26	∼0.82	95
			Oxi: 20 °C
One step MHM	325 mesh	∼1 hour	40 °C	2.36	∼0.80	89
Improved synthesis	150 μm	Over night	50 °C	N/A	∼1.1	90
Mild oxidation	325 mesh	3 hours	50 °C	3.1	∼0.90	91

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2.3. Exfoliation of graphene oxide

The oxidation reaction helps to penetrate oxygen functionalities on both sides of the sheets. It widens the inter sheet distance and disbands the tightly packed sp² hybridized atoms of carbon.^98,99 Botas et al.¹⁰⁰ pointed out a probable mechanism for the invasion of oxygen into the graphite flakes. They claimed that in the case of graphites with smaller crystal size (optical texture composed of mosaics) the invasion of oxygen occurs predominantly at the boundaries of the sheets, yielding small size GO sheets. In contrast, the more ordered graphites (optical texture composed of flow domains) are predominantly attacked in the basal plane where the epoxy groups are developed. It yields large size GO sheets. This study also revealed that GO derived from lower crystalline graphite contains a large number of carboxyl and hydroxyl groups and GO obtained from higher crystalline graphite contains abundant epoxy groups. As a result of the above mechanism, the material becomes hydrophilic facilitating the separation of individual GO sheet using sonication⁹⁹ or magnetic stirring.^95,101 This phenomenon is called exfoliation of GO. Water is the widely used solvent to disperse GO. Samulski et al.¹⁰² claimed, Water soluble graphene is similar to single graphene sheets peeled from pyrolytic graphite (0.9 nm thick). Besides water the dispersion of GO is also possible in several organic solvents such as: N-methyl pyrrolidine (NMP), ethylene glycol and N,N-dimethyl formamide (DMF).⁹⁶ It has been observed that graphite could utilize the surface energy of NMP to exfoliate and produce single layer GO sheets.^54,103 But solvents are expensive and their high boiling point limited their applications. Lotya et al.⁵³ demonstrated that graphene exfoliation is possible in a surfactant–water solution (sodium dodecyl benzene sulfonate). The major benefit of surfactant is the prevention of sheet aggregation. In another study, Green and Hersam⁵³ used sodium cholate to simultaneously extract single graphene sheets from the bulk graphite with the help of a density gradient centrifuge machine at different buoyant density.

Both ultrasonication¹⁰² and magnetic stirring¹⁰¹ can be used in graphene exfoliation. However, excessive sonication can reduce the sheet quality. Chhowalla et al.¹⁰⁴ found that GO dispersed in DMF solution could laterally degrade after 10 hours of sonication. Over sonication may increase the number of sheet to sheet junctions which limits electron mobility.¹⁰⁴ These types of sheets are not suitable for device applications. A higher amount of oxygen functionalities (epoxy and hydroxyl) can generate areas of weakness mainly consisting of cracks and fault lines. This defective zone leads to sheet breaking when ultrasonicated for a longer duration. It follows the mechanism of cooperative unzipping proposed by Li et al.¹⁰⁵ In addition, erosion of ultrasound tip is an unavoidable outcome of prolonged ultrasonication.¹⁰⁶ Furthermore, a recent investigation by Qi et al.¹⁰⁷ showed that the first hour of sonication is the critical point. Because the exfoliation and sheet break up mainly occur in the very first hour, while later sonication facilitates sheet restacking. This suggests that when GO sheets are degraded in size, they would trend to intercalate or restack together. Therefore in order to address poor quality or hole defects of graphene higher sonication time should be avoided.¹⁰⁸ It is better to continue until the whole solution becomes clear with no visible particulate matter and brilliant yellow in colour.⁹⁹

2.4. The green reduction of graphene oxide

In a typical reduction, dispersed GO suspension and environmentally friendly reducing agents are mixed together to produce reaction solution first. Secondly, the reaction solution is kept under controlled temperature for different duration. In most of the previous studies, the reaction temperature and the concentration of GO have been observed 80–95 °C and 0.5–1 mg mL⁻¹, respectively. In Table 4, the concentration of GO, temperature and time of GO reduction for different studies has been discussed. The adjustment of pH is an important parameter for the reaction solution. Because the reduction is possible in both acidic¹⁰⁹ or alkaline environment.^110,111 Notably, HCl,^{109,112–114} NH₄OH^115–118 and dilute NaOH^52,119 are used to regulate the pH of the solution. According to Merino et al.¹²⁰ alkaline condition is able to reduce the time of reduction. It can deoxygenate GO sheets without using any further reducer.¹¹⁰ It strongly stimulates the colloidal durability of GO with the help of electrostatic repulsion. Bosch et al.¹²¹ clearly illustrated the effects of different pH values on the GO sheets. According to their findings, acidic pH is responsible for a higher concentration of defects with sheet aggregation and size reduction. An acidic pH may turn the sheet like structure of graphene into other nano forms of graphite such as fullerene, nanoonion or multi-walled carbon nano tubes. This investigation suggests that the alkaline pH is more suitable to produce high quality or less defective graphene.¹²² After reduction the brownish yellow colour solution of GO turns into black. Graphene sheets are extracted from the black slurry by filtration^113,119,123 or centrifugation.¹¹⁸ After centrifugation, the product should be washed and recentrifuged several times with deionised water, alcohol or 5% ammonia to remove all the unwanted materials (unreduced graphene, excess reducers, and reaction by-products). After completing the washing procedure, the pH of the product should be neutral (pH = 7).^124,125

Table 4 A list of different green reducing agents used in graphene oxide reduction^a

Reducing agents of GO	Concentration of GO in aqueous solutions (mg mL^−1)	Temperature of reduction	Time of reduction	Ref.
a RT = room temperature.
Aluminum powder	1	RT	30 min	109
Aqueous phyto extracts	0.5	RT	5–8 h	130
Baking soda	0.1	90 °C	3 h	131
Benzylamine	0.5	90 °C	90 min	123
Benzyl alcohol	8	100 °C	5 days	132
Beta carotene	2	95 °C	24 h	133
Caffeic acid	0.1	95 °C		113
Carrot root	0.5	RT	48 h	134
Casein	1	90 °C	7 h	116
Cinnamon	1.6	N/A	45 min	135
Clove extract	1.6	100 °C	30 min	136
Coconut water	0.66	100 °C	12, 24, 36 h	137
Curcumin	N/A	85 °C	2 h	138
E. coli	5	37 °C	48 h	139
Enhanced green fluorescent protein	1	90 °C	1 h	140
Formic acid	2.5	100 °C	18 h	141
G. biloba	0.5	30 °C	12 h and 24 h	142
Grape juice	0.6	95 °C	1 h, 3 h and 6 h	143
Gallic acid	4	RT and 95 °C	24 h and 7 h	115
Green tea polyphenol	1	80 °C	8 h	128
Green tea polyphenol + iron	0.1	40, 60, 80 °C	10 min	144
Glucosamine	0.496	90 °C	7 h	52
Glycine	0.25	95 °C	24 h	145
H. sabdariffa	0.4	N/A	1 h	146
Humanin	1	40 °C	1 h	147
Iron powder	0.5	RT	30 min	114
L-Ascorbic acid	0.1	23 °C	N/A	148
L-Cysteine	0.5	26 ± 2 °C	12–72 h	124
L-Glutathione	0.1	50 °C	6 h	149
L-Lysine	1	90 °C	9 h	150
L-Valine	0.1	90 °C	Several hours	118
Low temperature aluminum	4	100–200 °C	3 h	151
Melatonin	0.1	40, 60, 80 °C	3 h	152
Metallic zinc	0.5	RT	6 h	119
Metal salts	0.1	RT	1 h	153
Manganese powder	0.5	RT	Till black sedimentation	154
Nascent hydrogen	1.0	RT	30 min, 20 min, 6 h	155
Natural cellulose + ionic liquid	0.8	80 °C	12 h	156
Oligothiophene	0.48	RT	6 h	126
Oxalic acid	2.5	75 °C	18 h	157
P. glutinosa	5	98 °C	24 h	158
PDDA	3	90 °C	5 h	159
Polyphenol	1	80 °C	8 h	128
Pomegranate juice	0.4	N/A	12 h, 18 h, 24 h	160
Potassium hydroxide	0.5–1	50–90 °C	Few minutes	122
Pyrrole	0.5	95 °C	12 h	161
Rose water	7	95 °C	5 h	162
Reducing sugar	0.1	95 °C	1 h	117
Spinach	1.6	N/A	30 min	163
Shewanella sp.	300	RT	48 h	164
Sodium acetate trihydrate	1	95 °C	24 h	165
Sodium carbonate	2	80 °C	4 h	110
Sodium hydroxide	0.5–1	50–90 °C	Few minutes	122
Supercritical alcohols	125	400 °C	2 h	127
T. chebula	1	90 °C	24 h	166
Tea solution	0.5	90 °C	N/M	167
Thiourea dioxide (TUD)	1	90 °C	Several hours	125
Tin powder	1	RT	0.5–3 h	112
Tannin	N/A	N/A	Several hours	168
Vitamin C	0.1	95 °C	0.5–4 h	120
Vitamin C + amino acid	0.1	80 °C	24 h	129
Yeast	0.5	35–40 °C	72 h	169
Zn powder + ultrasonication	N/A	RT	Till black sedimentation	170
Zn powder + acid	1	RT	30 min	171

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But in case of filtration, after each filtering the filter cake should be dissolved in deionised water and resonicated for 10 minutes prior to next filtering.¹²⁵ In different studies, 0.45 μm poly ether sulfonate paper,¹²⁶ poly vinylidene fluoride filter,¹²⁷ 0.22 μm nylon membrane^128,129 or 0.20 μm Whatman paper have been used to drain out the waste solution. Finally, the solid and black RGO sediment can be dried inside a vacuum oven at 50–100 °C.^109,124 Xiao et al.¹¹⁵ also dried the graphene at room temperature but it took 2 days. Thus the functional graphene sheets are obtained as final product. A pictorial image of green reduction of GO using L-valine is shown in Fig. 3. Moreover, the quality of the functional graphene highly dependents on the following factors: (i) the properties of pristine graphite, (ii) method of oxidation, and (iii) final deoxygenation procedure of GO to graphene. A flow chart of green reduction of GO is shown in Fig. 4.

Fig. 3 The green reduction of graphene oxide using L-valine.¹¹⁸

Fig. 4 A schematic representation of green reduction of graphene oxide.

3. Characterization techniques to evaluate GO and RGO

GO and RGO expose unique set of spectroscopic and microscopic characteristics that differentiate them. The coming section will discuss the commonly used spectroscopic and microscopic techniques to evaluate GO and RGO.

3.1. Raman spectroscopy characterization

Raman spectroscopy is a most important tool to characterise graphene and other carbon related materials such as carbon nano tube,¹⁷² graphene oxide,⁶⁴ and graphite.¹⁷³ Its main purpose is to study the electronic and compositional characteristics of graphene. The spectrum of Raman displays two bands: the D band and the G band together with a weak 2D band.¹⁷⁴ The D band is associated with the breathing mode of K point phonons of A_1g symmetry. It mainly indicates the surface disorder and defects of graphene.¹⁷⁵ And the G band is the result of first order scattering of E_2g phonon from sp² carbon. It expresses the graphitic composition of the materials.¹⁷⁶ In pristine graphite, a sharp and intense G band observes at 1575 cm⁻¹. It is related to the lattice structure of graphite. It also displays a weak D band at approximately 1355 cm⁻¹ caused by the graphite edges.¹⁷⁷ After the oxidation reaction the synthesized graphene oxide exhibits characteristic D band G band at around 1350 and 1580 cm⁻¹, respectively.¹⁷⁴ In GO the G band is broaden and blue shifted to higher wave number compared to the pristine graphite.¹⁷⁷ On the other hand, the D band of the Raman spectrum of GO becomes prominent. It ensures the degradation of the in-plane sp² domains in GO due to the extensive oxidation.⁹⁹ While GO is being reduced, both of the D band and G band is blue shifted to lower wave number.¹⁷⁸ It reveals the construction of new structure through reduction reaction. Structurally, the D band and G band of RGO are sharper and more intense in comparison with those of GO.^177,179 Additionally, the weak 2D band of the Raman spectrum of RGO centres within the range of 2700 cm⁻¹. It is ascribed to the second-order of the zone-boundary phonons generated by the double resonance Raman scattering with two-phonon emission. It can be used to see whether the RGO is single layer or multiple.¹⁷⁴ Generally, the Lorentzian peak for the 2D band of single layer graphene is observed at 2679 cm⁻¹. This peak is broadened and shifted to a higher wave number if multilayer graphene is formed.¹³⁰ For example, Akhavan found this band at 2699 cm⁻¹ for the bilayer graphene.¹⁸⁰ In Fig. 5 the Raman spectroscopy of graphite, GO and RGO has been given.

Fig. 5 Raman spectra of graphite, GO and RGO (here SRGO = baking soda reduced graphene oxide).¹³¹

The intensity ratio of band D and G ((I_D/I_G)) is used to investigate the degree of disorders in graphene. This ratio is unique to differentiate RGO from GO. Because it is either increase or decrease in RGO (Table 5). The value of both of the band is required to calculate this ratio through the Tuinstra–Koenig relation.¹⁷³ After reduction two different types of regime can arise in RGO: the high defect density regime and the low defect density regime.^59,181,182 In high defect density regime, the ratio starts to fall with the increment of defect density. This is ascribed to the larger amount of sp² amorphous texture of carbon. In this case the I_D/I_G ratio is proportional to the square of the size of nanocrystalline graphitic domains.¹⁸³

Table 5 I_D/I_G ratio of GO and RGO after different chemical reduction reactions^a

Reducing agents	GO	RGO	Reference
a *the decrease of ID/IG ratio from GO to RGO.
Aluminium powder	0.96	1.81	109
Amino acid (L-cysteine)	0.94	1.17	124
Baking soda	1.47*	1.37*	131
Benzyl alcohol	N/A	1.20	132
E. coli	1.37*	0.97*	139
Gallic acid	1.74	1.86	115
Carrot root	0.80	1.09	134
Glucose amine	1.43	1.50	52
Glycine	0.98	1.09	145
Humanin	1.4	2.3	147
Iron powder	0.96	1.02	114
Lithium aluminium hydride	0.96	1.30	191
Hydrazine hydrate	0.96	1.21	191
Metallic zinc	1.19	1.40	119
Manganese dioxide	1.44	1.68	194
Nascent hydrogen	1.01	1.36, 1.65, 1.88	155
Natural cellulose	1.32	1.53	156
Non aromatic amino acid	0.95	1.02, 1.05, 1.11	118
Oligothiophene*	1.19*	1.02*	126
Oxalic acid	1.2	1.3	157
Pyrrole*	1.3*	0.1*	161
Sodium acetate trihydrate	0.87	0.96	165
Sodium hydrosulphite	0.73	1	195
Sodium carbonate	0.81	1.04	110
Solvothermal	0.86	1	190
Sulfur coating compounds	N/A	0.95–1.22	196
Supercritical alcohols	1.11	1.30	127
Tanin	0.97	1.18	168
Thiourea dioxide	0.83	>1	125
Tin powder	0.94	≥1.30	112
Vitamin C + amino acid	1.56	1.75	129
Vitamin C/L-ascorbic acid	0.95	1.2	120
Yeast	0.80	1.44	169
Zinc powder + acid	0.89	1.61	171

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On the other hand, in the low defect density regime the I_D/I_G ratio starts to increase. Here defect density is assorted with the generation of higher elastic scattering. This regime is regarded as nanocrystalline graphite.^184–187 The reason of this increment is the degradation of overall size of sp² domains.^188,189 In this case the intensity I_D/I_G ratio is inversely proportional to the size of sp² domains.¹⁹⁰ Notably, in RGO the new graphitic domains are more numerous than those of GO.¹⁹¹ In order to explain this phenomenon Stankovich et al.⁹⁹ pointed out that the reduction of GO increases the number of aromatic domains of smaller average size in graphene, which could lead to an increase of the I_D/I_G ratio. Additionally, the increase of I_D/I_G ratio is a proven sign of crystal defects, which are of two types. The first one is the edge defect, which does not cause structural rapture. It plays the main role to the increment of I_D/I_G ratio in RGO. And the second one is the basal defect responsible for structural disruption.¹⁴⁵ Moreover, the I_D/I_G ratio of high temperature reduction is greater than that of room temperature, which indicates a lower degree of disorder at room temperature.¹¹⁹ But room temperature reduction is unable to increase the average size of the graphene domains.¹¹⁵

The Knights empirical equation is used to calculate the size of the sp² carbon domains (designated as L_a) using the I_D/I_G ratio of the samples.¹⁹²

L_a = 4.35(I_D/I_G)⁻¹ (1)

In an investigation, Khai et al.¹⁹³ calculated the size of sp² domains to be approximately 5 and 4 nm for GO and RGO, respectively. This achievement suggests that the chemical reduction can cause nucleation of sp² domains in the sp³ matrix. As a result, the density of small sized sp² nuclei increases and the average size of sp² domains decreases.

3.2. Fourier transforms infrared spectroscopy characterization

In graphene research, it is important to study what types of oxygen functional groups are attached to the basal plane. FTIR is the technique to identify the bonding configuration of various sorts of oxygen. It is also a complementary tool to Raman spectroscopy. The FTIR spectrum of pristine graphite exhibits no characteristic peak for the discernible functional groups.¹⁹⁷ It only displays two peaks at approximately 1610 and 450 cm⁻¹ attributed to the skeletal vibrations from graphite domains (the sp² aromatic C=C) and the vibration of adsorbed water molecules (the O–H stretching), respectively.⁵² After treating with oxidizing agents, the oxygenated graphene sheet could display a series of different absorption bands or characteristics peaks ranging from 900 to 3500 cm⁻¹. These includes carboxyl peaks (1700–1750 cm⁻¹) mainly present at the edges of the sheet, alkoxy stretching vibrations (1040–1170 cm⁻¹), O–H stretching vibrations (3300–3500 cm⁻¹), O–H deformation peaks (1300–1400 cm⁻¹) and the stretching vibration of epoxy C–O groups (1000–1280 cm⁻¹). Notably, the aromatic C=C peak could be seen between the range of 1600–1650 cm⁻¹. This peak is resulted from the sp² domains of the unoxidized graphite^125,165 and this vibration generated from the unoxidized region is termed skeletal vibration.^102,126 However, in different GO samples, the above characteristic peaks are found in slightly different positions (Table 6). This may be the consequence of different reaction system and conditions.

Table 6 FTIR characterization of GO in different studies^a

Peak positions of oxygen functionalities for GO samples					Agents of GO reduction	Ref.
C=O stretching	Epoxy C–O	Alkoxy C–O	O–H stretching	O–H deformation
a N/A = not available.
1746	1220	1053	3420	1395	Amino acid (L-cysteine)	124
1720	1204	1049	3400	N/M	Aquatic phyto extract	130
1706	1219	1052	3395	1386	Baking soda	131
1728	1223	1051	N/M	1395	Carrot root	134
1720	1224	1048	3424	N/M	Gallic acid	115
1727	1224	1065	3424	N/M	Glycine	145
N/M	1250	1120	N/M	N/M	Glucose amine	52
1720	1220	1030	3400	1400	Humanin	147
1726	1226	1052	3395	1410	L-Ascorbic acid	148
1730	1064	1123	3430	1410	Metallic zinc	119
1730	1220	1050	N/A	N/A	Natural cellulose	156
1730	1224, 985	N/M	N/A	1417	Oligothiophene	126
1740	N/M	N/M	N/A	N/A	Polyphenol	128
1739	N/M	1600	3403	1395	Reducing sugar	117
1730	1037	N/A	3420	1395	Sodium acetate trihydrate	165
1730	1222	1050	3387	N/A	Sodium carbonate	110
1730	1220	1050	3380	N/A	Super critical alcohol	127
1718	1050	N/A	3200	1380	Thiureadioxide	125
1725	1224	1054	3421	1398	Tin powder	198
1700	1600	N/A	3650	N/A	Urea ammonium nitrate	199
1720	1220, 980	N/A	3000–3500	1300–1350	Vitamin C	120
1732	1231	1051	3408	1394	Yeast	169

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While GO is being reduced by the chemical reducers, the abovementioned characteristic peaks are gradually weaker over the course of the reaction and some of them vanish completely. This indicates the successful removal of oxygen from the sheets. The spectra of RGO displays a sharp C=C band within the range of 1600 cm⁻¹.^124,165 Additionally, It displays two new peaks in the region of 2922 and 2853 cm⁻¹ attributed to the presence of –CH₂ and –CH stretching vibrations. It is also explained as the adsorption of reducing agent by the sheet surface.^126,134,169 These observations confirm that the atomic frame of sp² carbon has formed in RGO. In a previous study, the alkoxy and epoxy vibration peaks remained undistorted in RGO although their position has been changed. This is ascribed to the partial reduction of GO.¹³⁴ In another study, the absorption peaks of alkoxy and carboxyl groups are found to be weaker at higher temperature reduced graphene oxide (HT-RGO) than those of room temperature reduced graphene oxide (RT-RGO). This investigation suggests that higher degree of reduction is related to higher temperature (90–95 °C).¹¹⁵ Khai et al.¹⁹³ also found in completely disappeared carboxyl acid and hydroxyl peaks after the solvothermal treatment of GO. This is attributed to the mild reduction procedure. Herein, Fig. 6 shows the FTIR spectra of GO and RGO and Table 6 shows the FTIR characterization of different GO samples.

Fig. 6 FTIR spectra of GO and RGO (here, SRGO = baking soda reduced graphene oxide).¹³¹

3.3. X-ray photoelectron spectroscopy characterisation

It is very difficult to reduce hundred percent of the oxygen molecules from the sheets of GO.^{90,97,109,200} That is why elemental analysis is crucial to characterize the newly synthesised graphene. XPS is commonly used to determine the relative amounts of carbon, oxygen, and other functional groups present in graphene.²⁰¹ The XPS spectrum of GO and RGO exhibit two major peaks (C1s and O1s) at the wide regions.¹⁰⁹ The characteristic C1s peak is the result of sp² carbon–carbon bonding and the O1s peak is associated with different sp³ carbon–oxygen bonding. The relative intensity of these two peaks varies with sample to sample, depending on the conditions of the reaction system.⁹¹ Both of the O1s and C1s spectra can be used to calculate the ratio of peak intensity between O1s and C1s i.e. the C/O atomic ratio. This ratio can be used to determine the oxygen moieties present in the carbon skeleton of graphene.²⁰² Table 7 discusses the C/O atomic ratio of RGO produced using different reducing agents.

Table 7 C/O atomic ratio of RGO at different studies

Reducing agents	Reaction time	C/O atomic ratio	References
Aluminium powder	30 min	18.6	109
Caffeic acid	24 h	7.15	113
Carrot root	72 h	11.9	134
Glycine	24 h	11.14	145
Gallic acid	6 h	5.82	115
Hibiscus sabdariffa	1 h	3.1	146
Hydroxyl amine	1 h	9.7	208
Iron powder	6 h	7.9	114
L-Ascorbic acid	48 h	5.7	148
Metallic zinc	7 h	3.76	119
Oligothiophene	6 h	6.12	126
Pyrrole	2 h	7.7	161
Sodium citrate	3 h	5.6	209
Sodium carbonate	4 h	8.15	110
Tea solution	Several hours	3.1	167
Yeast	72 h	5.9	169
Zn powder + acid	30 min	8.2	171

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Typically, in the wide region the C1s and O1s peak of GO and RGO can be observed at ∼530 eV and ∼284 eV (Fig. 7a). In order to conduct the quantitative analysis, both of the C1s and O1s spectra are needed to be separated into their individual components. This is performed by fitting the total spectrum into a linear superposition for all elements. Gaussian–Lorentzian peak shape method is used to fit the C1s and O1s curve after performing the Shirley background correction.^112,202 This separation helps to determine the fractions of every single element present in the sheet. The software XPSPEAK41 is an useful tool for background subtraction, curve fitting, and peak separation.^119,203

Fig. 7 XPS pattern of graphene: (a) wide region (GO + RGO), (b) C1s region of GO, (c) C1s region of RGO¹⁰⁹ and (d) O1s region of GO.²⁰²

The high resolution C1s signal of GO and RGO represents five different elemental peaks: C–C ≈ (BE = 284.6 eV), C–OH ≈ (BE = 285.6 eV), C–(epoxy group) ≈ (BE = 286.8 eV), C=O ≈ (BE = 288.2 eV) and O–C=O ≈ (BE = 289.4 eV), as seen in Fig. 7b and c; although the accurate position of these peaks as well as their exact binding energy (BE) is difficult to specify.^{202,204–206} On the other hand, the O1s spectrum of GO displays several individual component peaks which represent C=O groups ≈ (BE = 531.20 eV), C–O groups ≈ (BE = 533 eV), O–C=O groups ≈ (BE = 534.4–535.6 eV), respectively, as seen in Fig. 7d. All of this information allows to determine the fraction of epoxy, hydroxyl or carboxyl groups present in the basal plane.^202,205 The information obtained by the analysis of O1s spectrum can complement the information obtained by the analysis of C1s spectrum. Because the photoelectron kinetic energies of O1s are lower than those of the C1s, the O1s sampling depth is smaller, and therefore the O1s spectra are slightly more surface specific.²⁰²

After the reduction of GO the intensity of the peak related to the C=O, C–O and O–C=O groups decreases to a much lower value (Fig. 7c and d). This phenomenon suggests that most of the oxygen functional groups have been disappeared after the reduction of GO.⁶⁶ Secondly, the pick intensity of the sp² carbon to carbon bonding considerably increases in the C1s spectrum of RGO. This result suggests the restoration of new sp² network of graphene.^66,207 Besides these two peaks some additional peaks might be observed in the spectra of RGO such as sodium peak,¹²⁴ nitrogen peak,¹²³ iron peak,¹¹⁴ sulphur peak,¹²⁶ tin peak,¹¹² phosphate peak.¹⁶⁹ These are probably the residual components of the reaction mixture caused by other side reactions. These components clearly show the involvement of the reducing agent in the reaction.¹⁴⁵

The elemental analysis on the basis of C1s spectrum indicated that GO contained approximately 69.63 at% carbon and 30.37 at% oxygen in the C–C/C=C and C–O bonds.¹⁴⁶ Upon reduction the atomic percentage of carbon is increased in RGO. According to several studies the better range of carbon percentage is found to be 90–95%.^109,134,198 Meanwhile, the C/O atomic ratio of GO is around 2.3, as mentioned in Table 3. After the reduction of GO the C/O atomic ratio is also increased in RGO, as discussed in Table 7. Temperature is an important factor to control the graphene C/O atomic ratio. Yang et al.¹¹⁹ and Zhang et al.¹²² demonstrated that the graphene produced by the room temperature reduction contains large amount of residual oxygen than that of higher temperature. Reduction time is another important factor. It is established that the increment of reduction time is proportional to the removal of oxygen groups from the plane of graphene.^113,123,203

3.4. X-ray diffraction characterization

The inter sheet gap is the most significant parameter to access the structural information of graphene. XRD is the technique which deals with the layer to layer space and crystalline structure of graphene. In this study, we highlighted the interlayer distance of GO before and after reduction. The XRD spectra of primitive graphite revels more intense and sharp peak in comparison to GO and RGO.¹²⁴ Typically, this graphitic peak is found at 2θ ≈ 26° corresponding to a d-spacing of 0.34 nm.⁹⁰ After oxidation, this sharp and strong graphitic peak becomes invisible and a new peak observes in the range of 2θ ≈ 9–11°.^210,211 It has been reported that the value of d-spacing is found within the range of 8–9 Å upon different method of oxidation such as: Hummer method (8 Å), modified Hummer method (9.5 Å), and improved synthesis (9 Å).⁹⁰ This result strongly indicates that the d-spacing of GO is directly increased by oxidation due to the penetration of epoxy, hydroxyl, and carboxyl groups on both side of the sheets.^212,213 From this result, it can be said that the inter layer spacing of graphene material is proportional to the degree of oxidation. After treating with the green reducing agents the XRD peak of GO vanishes and a broad, prominent and new pick can be seen near 2θ ≈ 26°. It is related to the opening of epoxy rings and removal of other oxygen moieties. This result confirms the successful restoration of new graphitic network in RGO. Table 8 shows the interlayer distance of GO and RGO after different reduction reactions (Fig. 8).

Table 8 Interlayer distance of different GO and RGO samples^a

Reducing agents	GO (2θ)	d-Spacing of GO (nm)	RGO (2θ)	d-Spacing of RGO (nm)	Ref.
a N/A – not available.
Aluminium powder	10.3°	0.80 nm	23.4°	0.375 nm	109
L-Cysteine	11.3°	0.78 nm	26.2°	0.370 nm	124
Phytoextract	9.75°	0.906 nm	25°	0.360 nm	130
Calcium carbonate	N/A	N/A	26.3°	N/A	214
Caffeic acid	10.02°	0.880 nm	24.79°	0.359 nm	113
Carrot root	11.2°	0.79 nm	23.96	N/A	134
Gallic acid	9.8°	0.90 nm	26°	N/A	115
Glucose amine	11.2°	0.79 nm	N/M	N/A	52
Glycine	11.05°	0.7997 nm	23.9°	0.3718 nm	145
Iron powder	10.3°	N/A	24.3°	N/A	114
Metallic zinc	11.6°	0.779 nm	24°	0.368 nm	119
Cellulose	11.9°	0.74 nm	22.5°	N/A	156
Oxalic acid	11.9°	0.743 nm	21.9°	0.405 nm	157
PDDA	11.4°	0.78 nm	20°	N/A	159
Pyrrole	10.1°	0.87 nm	20.1°	N/A	161
Sodium acetate	11.2°	0.790 nm	25.4°	0.350 nm	165
Supercritical alcohol	11.76°	0.752 nm	23–25°	0.358–0.381 nm	127
Tannin	9.7°	0.90 nm	26.3°	0.340 nm	168
Tin powder	11°	N/A	26.6°	N/A	112
Tea solution	9.6°	0.92 nm	24.6°	0.340 nm	167
Yeast	11.32°	0.7816 nm	23.5°	N/A	169

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Fig. 8 XRD spectra of (a) GO, (b–d) aquatic phytoextract reduced graphene oxides and (e) graphite.¹³⁰

3.5. Microscopy characterization

Microscopy is essential to study morphology of graphene. The TEM, SEM and AFM are commonly used in this purpose. A brief of the above techniques are discussed below:

3.5.1. Transmission electron microscopy. TEM acts on the basis of diffraction patterns emitted by the samples.^215,216 To generate this diffraction pattern the highly energetic electrons beams are transmitted through the sheet. The scattered electrons are captured and processed by an electromagnetic lens to display an image. The low magnification TEM image is quite useful in sheet visualization, but they are unable to provide complete information about the internal atomic structure of the sheets. For the structural characterization, the selected area electron diffraction (SAED) patterns are often obtained from the individual sheets.

The two dimensional (2D) GO and RGO nano sheets show transparent, corrugated or wrinkled structure under the TEM.^148,217 It is also explained as ultrathin silk veil morphology with fold and scroll on its edges.²¹⁸ It is attributed to the intrinsic nature of graphene.^169,219 Yu et al.²¹⁹ claimed that the corrugation can be present not only at the edge but also in the middle of the nanosheets. It is responsible for the sheets to look folded and coiled.^{119,124,134,219} Notably, high resolution transmission electron microscopy (HR-TEM) is able to show the number of layers at different locations of the sheets.^134,145 With this technique, the measured lattice spacing of single layer graphene is 0.345 nm.²¹⁴ Some TEM and HR-TEM images of RGO are portrayed in Fig. 9.

Fig. 9 TEM image of GO and RGO at lower magnification (a and b) over a TEM grid with lacey carbon support.¹³¹ HR-TEM image (c) of RGO is showing the formation of single layer graphene.²¹⁴

The structure of GO is different from RGO particularly in the SAED analysis. This illustrates the crystalline structure or hexagonal symmetry of the graphene layer. After chemical reduction, strong and well-defined diffraction spots are observed in the polycrystalline ring of SAED (Fig. 10b). This result clearly confirms the destruction of inter layer coherence and random orientation of oxygen free sheets.^{127,145,148,151,159,169,220} The SAED patterns of GO and RGO are depicted in Fig. 10.

Fig. 10 SAED pattern of (a) GO and (b) RGO displaying six fold symmetry.¹¹⁸

3.5.2. Scanning electron microscopy. The SEM is commonly used to study the surface morphology of graphene, GO and graphite. Generally, graphite sheets appears to be heaped up in thick cakes.²²¹ It also exhibits flaky appearance for the strong sp² carbon to carbon bonding in the plane.¹³¹ In contrast, the GO nanosheets show wavy wrinkled appearance under the SEM where the surface are hairy and coarse, and the edges of the sheets are blurry.²²¹ At higher concentration the surface of the GO displays soft carpet like morphology which may be the attachment of residual water molecules, carboxyl and hydroxyl groups with the sheets.¹⁴⁷ In RGO the nanosheets display thin and wrinkled texture. It is caused by the stacking of individual sheets by various self-assembly techniques. This is attributed to the intrinsic properties of graphene.⁷⁵ However, the average lateral size of giant GO sheets have been reported to be 18.5 μm with a relatively wide size distribution ranging from one to several micrometres.²²² Gurunathan et al.¹⁴⁷ also used the SEM image to count the average lateral size of GO and RGO. They found the average lateral size of GO and RGO sheets are 8 and 10 μm, respectively, although a wide range of sizes (between 2 and 20 μm) were analysed in the study. The SEM images of graphite, GO and RGO are portrayed in Fig. 11.

Fig. 11 SEM image of (a) graphite, (b) GO¹³¹ and (c) RGO.²¹⁴

3.6. Atomic force microscopy

AFM is a promising technique to measure the atomic steps present on a surface. It is used to determine the morphology and texture of the graphene materials. It also helps to determine the exact Z-height and number of layers in the specimen. The well-known van der Waals thickness of a pristine graphene sheet is ∼0.34 nm and it is atomically flat. After oxidation the graphene oxide sheets are expected to be thicker due to the presence of covalently bound oxygen on either side of the sheets.⁹⁹ The typical thickness of single layer GO sheets is ∼0.8 nm which is ∼0.44 nm thicker than that of pristine graphene.¹⁸⁰ In the case of bi-layer graphene oxide, the thickness was found to be 1.7 nm.¹³⁴

Upon the reduction of GO the smaller thickness of RGO sheets is a clear proof of oxygen elimination from the graphene oxide.^{122,145,148,165,194} A AFM image of GO and RGO is depicted in Fig. 12. But in several studies the thickness of RGO was found to be higher than that of GO. It is attributed to either the binding of capping agents with the sheets or restacking of sheets in the absence of stabilizer molecules.^134,168 For example, Akhavan et al.¹⁴⁴ observed an increased thickness of graphene caused by the attachment of oxidized green tea polyphenols (GTPs) on both sides of the RGO. Some of the similar types of results have been reported in Table 9. Therefore, it is reasonable to conclude that the stabilizer or capping reagent play an important role to increase the thickness of graphene sheet, although most of the oxygen-containing functional groups are vanished after the reduction.¹¹⁷ Notably, the stabilization mechanisms between the reducer and the graphene layer are proposed to be π–π interactions or hydrogen bonding.^128,138 Here, the capped molecules supply steric hindrance to stabilize the graphene sheets.^128,168 Beside thickness analysis AFM has also been applied to analyse the lateral dimension of GO and RGO. In one of our previous work Mehrali and co-workers measured the average lateral dimension of GO and RGO around 3.88 ± 0.99 μm and 2.37 ± 0.65 μm.¹⁷⁸ This finding also pointed out that the RGO sheets are laterally smaller than the GO sheet.

Fig. 12 AFM image of (a) GO and (b) RGO.¹³⁴

Table 9 A comparative study of the thickness of GO and RGO

Reducing agents	Thickness of GO (nm)	Number of layers	Thickness of RGO (nm)	Number of layers	Attachment of molecules on both side of the RGO sheets	References
Alkaline conditions	1.2	Single	0.881	Single	N/A	122
Caffeic acid	1.116	Single	0.846	Single	N/A	113
Calcium carbonate + magnesium	N/A	N/A	2.5	4–10	N/A	214
Carrot root	1.7	Bilayer	2.5	Restacked	N/A	134
Curcumin	0.8	Single	1.5	Single	Curcumin molecules	138
E. coli	1	Single	N/A	N/A	N/A	223
Gallic acid	1.0	Single	1.0 and 1.4	Single	N/A	115
Green tea polyphenol	1.19	Single	1.39	N/A	Oxidized tea polyphenol (TP)	128
Green tea polyphenol + iron	1.2	Single	Average 1.8, considered one layer as 0.80 nm	Single	Oxidized greentea polyphenol (GTP)	144
Humanin	N/A	N/A	Between 1 and 3	N/A	N/A	147
Iron powder	1	Single	1–5	Multiple	N/A	114
L-Ascorbic acid	1.2	Single	N/A	N/A	N/A	148
L-Cysteine	1	Single	0.8	Single		124
L-Glutathione	N/A	N/A	8	N/A	Capping agent	149
L-Valine	1	Single	1	Single	N/A	118
Melatonin	0.9	Single	1.8 (considered one layer as 0.80 nm)	Single	N/A	152
Metallic zinc	1.22	N/A	0.93	Single	N/A	119
Metal salts	1.50	N/A	0.86	Single	N/A	153
Natural cellulose + ionic liquids	1	Single	1.9	N/A	Cellulose chains	156
Pomegranate juice	N/A	N/A	0.67	Single	N/A	160
Pyrrole	1.3	Single	2.7	N/A	Stabilizer molecules	161
Reducing sugar	0.97	Single	1.1	Single	Capping agent	117
Sodium acetate trihydrate	0.97	Single	0.79	Single	N/A	165
Tannin	1.1	Single	1.5	Single	Tannic acid (TA)	168
Tea solution	1	Single	Average 2	N/A	Tea polyphenol (TP)	167
Vitamin C	1	Single	1	Single	N/A	120
Vitamin C + amino acid	N/A	N/A	Average 2	N/A	L-Tryptophan	129
Yeast	0.8	Single	1.2	Bilayer	NADPH	169
Zn powder	1	Single	0.55	Single	N/A	170
Zn powder + acid	1.2	Single	N/A	N/A	N/A	171

---

4. Conclusion and suggestions

The chemical reduction of GO is found to be very promising technique to produce large scale functional graphene. The use of eco-friendly reducing agents to synthesis graphene achieves a great interest of the scientific community, evidenced by the continuous increment of research publications on the topic. In this review, the authors described some of the recent achievements in the green reduction of GO. It is considered as one of the most versatile method. These types of reagents are seemed to be an alternate of hydrazine or other poisonous reducing agents. They are also safe to handle and the reaction coproduces are biocompatible. But green reducing agents are not out of limitations. Sometimes one agent cannot fully deoxygenate the GO alone, but it needs the aid of a supporting reducing agent or surface modifier. There is also a big possibility for the attachment of addition functional groups or capping agents with the sheets supplied by the reducers. This phenomenon can hamper the electron conductivity of the sheets. Notably, The irreversible aggregation, lower electron conductivity, increased thickness, substrate specificity, chance of product contamination e.g. mixing of metallic impurities when reduced with metal powders, limited solubility, several downstream processing e.g. repeated centrifugation or filtration to remove all the reaction by-products and maintenance of complex or light sensitive reaction system e.g. bacterial reduction of GO can be considered as the limitations of the approaches. These drawbacks limited their application to the laboratory and they are not yet encouraged in the industrial batch production of graphene. To overcome all the drawbacks is a challenging task for the researchers in the field.

The replacement of the toxic and harsh chemical agents with mild reagents will be a big achievement of this study. More studies can help to understand the exact reduction mechanism of GO using most of the reported green reducing agents. Theoretical calculations, modeling the interaction between GO and reducing molecules could offer some solutions.

Acknowledgements

“The authors are thankful to University of Malaya for financial support under the High Impact Research MoE Grant: UM.C/625/1/HIR/MoE/ENG/40 (D000040-16001) from the Ministry of Education Malaysia.”

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