**2. Graphene oxide and reduced graphene oxide**

**1. Introduction**

2582 Recent Advances in Graphene Research

[1–4].

crystalline hexagonal lattice, due to the sp<sup>2</sup>

graphite to reduce graphene oxide.

Carbon is one of the most interesting elements of nature and is one of the primary constitu‐ ents for the formation of all organic matter. Its capacity to bind to itself in different ways makes carbon a very versatile element, giving rise to a series of structures called allotropes. Some of these carbon forms have attracted great interest in the past few decades due to their small size, in the scale of nanometers, and their very particular shape or dimensionality, which directly affect their chemical and physical properties. Among these carbon nanostructures, graphene (GE) has become an outstanding material presenting a unique set of "superlative" properties such as mechanical, electrical, thermal and electronic characteristics, enlisted in many reports

Graphene is the name of a single layer of carbon atoms arranged in a two‐dimensional (2D)

in‐plane σ bonds, responsible for its high mechanical strength and flexibility, and it also has weak out‐of‐plane π bonds responsible for its thermal carrying, electrical charge, and trans‐ parency, and graphene is also impermeable. Nevertheless, all these properties are only observed in a single defect‐free graphene layer, which is costly to produce in a scalable degree. Alternatively, there are other ways to produce graphene with relative ease, such as the chemical phase exfoliation of graphite oxide. This method yields the synthesis of graphene oxide (GO), a highly oxidized version of graphene. The subsequent reduction in the oxygen content brings a partial restoration to graphenic state, producing reduced graphene oxide (rGO) or chemically converted graphene (CCG) (**Figure 1a**). These graphene‐based materials

**Figure 1.** (a) Schematic chemical structures of graphene, graphene oxide, and reduced graphene oxide. (b) Route of

hybridization of carbon. Thus, graphene has strong

As a result of the exceptional properties of graphene, different efforts have been made in order to scale up its mass production. In a brief historically account, one of the first approaches were conducted by Lang in 1975, where few layer graphite was obtained by chemical deposition method. Nevertheless, in those days, the characterization techniques were unable to show what Lang has achieved. More recently in 1999, was reported the mechanical cleavage of highly ordered pyrolytic graphite (HOPG) with atomic force microscopy (AFM) tips, in an attempt to exfoliate a single layer of graphene [1]. Finally, in 2004, Novoselov reported the isolate one layer of graphene through "peeling" many times natural graphite with "Scotch" tape. In this method, the graphite layers are sliced down by mechanical exfoliation until one layer is deposited in a substrate [2]. Despite the effectiveness of this method and the high‐quality obtained graphene, this process requires a great deal of time and the amount of the as‐produced graphene is not enough for practical applications [3, 4].

A possible solution, in order to obtain graphenic materials in larger amounts, has been found on the graphite oxide route, starting from the modified Hummers method [6], which is based on the introduction of functional groups in graphite layers using a mixture of sulfuric acid and potassium permanganate. The versatility of the method, the excellent dispersion acquired in different solvents [5], and the possibility of a high yield production, makes the graphite oxide route one of the most promising scalable methods. This process mainly consists of three stages (**Figure 1b**).

First, it is necessary to produce graphite oxide by the oxidative intercalation reaction. In this step, the sp2 carbon arrangement is disrupted by the introduction of oxygenated functional groups, such as hydroxyl and epoxy in the basal plane and carbonyl and carboxyl at the edges (surroundings). During this process, interlaminar space between graphene sheets is increased two or three times than that of pristine graphite, from 3.34 Å to 5.62 Å [1, 7]. Second, graphene oxide (GO) is produced through exfoliation and dispersion of oxidized graphite. It has been reported that cavitation produced by ultrasonic waves within a fluid (commonly distilled water) produces hot spots with temperatures approximately to 5000°C, high local pressures about 500 atm and rates of heating–cooling of 109  K/s [8]. Therefore, water molecules (or polar solvents) can be intercalated in graphite oxide, producing an interlaminar spacing of roughly 1.2 nm [9]. Finally, the third stage consists on reducing or diminishing the oxidation degree of GO, in other words the production of reduced graphene oxide. In terms of recovery thermal, electrical, and mechanical properties, GO has been subjected to a variety of treatments to diminish the oxidized state or the number of oxidized moieties on it. Treatments such as chemical, thermal, UV radiation, and electrochemical reduction have been applied in either individual or multi‐steps, partially recovering the sp<sup>2</sup> arrangement found in pristine graphene [6, 7].

#### **2.1. Synthesis of graphene oxide**

GO is a covalent carbon structure where at least 60% of its atoms show sp3 hybridization connected by σ bonds to oxygen atoms, whereas the structure of basal plane is preserved even though it has substantial deformations. Therefore, GO is a hybrid material considered as an insulator exhibiting a resistance about 1012 Ω/sq depending on sp<sup>2</sup> and sp3 formed during oxidation [10]. GO thickness is theoretically within 0.7–0.8 nm because of the oxygenated moieties present on its surface, that is, roughly twice as the thickness of single layer of pristine graphene; however, GO could be larger considering that the occasional presence of bulkier functionalities, organic adsorbates, even some contaminants and so on [11].

Inasmuch as GO is not an stoichiometric material, its structure depends on the oxidizing process, this produces an aromatic disrupted lattice rich in functional groups maintaining arranged zones sp2 (2–3 nm) within a matrix C–O sp3 [12], and, for this reason, its chemical structure is difficult to determine. Lerf–Klinowski model based on the evidence obtained through nuclear magnetic resonance (NMR) of graphite oxide describes this material such as randomized of oxidized areas. As a result of oxidation sp2 /sp<sup>3</sup> regions are conformed of aromatic or aliphatic six‐membered rings [13]. This model is one of the most accepted in order to describe GO structure. From Lerf–Klinowski model, Gao et al. [14], proposed a new model including the presence of lactole rings of five and six members on the edges besides of carboxyl and carbonyl and hydroxyl, while in the basal plane are found tertiary alcohol esters as well as epoxy and hydroxyl groups.

On the other hand, graphite is considering an inexpensive available material for scalable production of GO; additionally, it can be obtained from either natural or synthetic sources [15], which can be influential over the final properties of GO, as has been reported. Undoubtedly for GO is really important, though not unexpected that the final dimension of the GO sheets obtained is strongly influenced for both the incipient size of graphite flakes and its inherent defects in π‐structure in conjunction with the oxidation protocols and exfoliation procedure [16, 17].

different solvents [5], and the possibility of a high yield production, makes the graphite oxide route one of the most promising scalable methods. This process mainly consists of three stages

First, it is necessary to produce graphite oxide by the oxidative intercalation reaction. In this

groups, such as hydroxyl and epoxy in the basal plane and carbonyl and carboxyl at the edges (surroundings). During this process, interlaminar space between graphene sheets is increased two or three times than that of pristine graphite, from 3.34 Å to 5.62 Å [1, 7]. Second, graphene oxide (GO) is produced through exfoliation and dispersion of oxidized graphite. It has been reported that cavitation produced by ultrasonic waves within a fluid (commonly distilled water) produces hot spots with temperatures approximately to 5000°C, high local pressures

solvents) can be intercalated in graphite oxide, producing an interlaminar spacing of roughly 1.2 nm [9]. Finally, the third stage consists on reducing or diminishing the oxidation degree of GO, in other words the production of reduced graphene oxide. In terms of recovery thermal, electrical, and mechanical properties, GO has been subjected to a variety of treatments to diminish the oxidized state or the number of oxidized moieties on it. Treatments such as chemical, thermal, UV radiation, and electrochemical reduction have been applied in either

GO is a covalent carbon structure where at least 60% of its atoms show sp3 hybridization connected by σ bonds to oxygen atoms, whereas the structure of basal plane is preserved even though it has substantial deformations. Therefore, GO is a hybrid material considered as an

oxidation [10]. GO thickness is theoretically within 0.7–0.8 nm because of the oxygenated moieties present on its surface, that is, roughly twice as the thickness of single layer of pristine graphene; however, GO could be larger considering that the occasional presence of bulkier

Inasmuch as GO is not an stoichiometric material, its structure depends on the oxidizing process, this produces an aromatic disrupted lattice rich in functional groups maintaining arranged zones sp2 (2–3 nm) within a matrix C–O sp3 [12], and, for this reason, its chemical structure is difficult to determine. Lerf–Klinowski model based on the evidence obtained through nuclear magnetic resonance (NMR) of graphite oxide describes this material such as

aromatic or aliphatic six‐membered rings [13]. This model is one of the most accepted in order to describe GO structure. From Lerf–Klinowski model, Gao et al. [14], proposed a new model including the presence of lactole rings of five and six members on the edges besides of carboxyl and carbonyl and hydroxyl, while in the basal plane are found tertiary alcohol esters as well

carbon arrangement is disrupted by the introduction of oxygenated functional

 K/s [8]. Therefore, water molecules (or polar

arrangement found in pristine graphene

and sp3

/sp<sup>3</sup>

formed during

regions are conformed of

(**Figure 1b**).

2604 Recent Advances in Graphene Research

step, the sp2

[6, 7].

about 500 atm and rates of heating–cooling of 109

individual or multi‐steps, partially recovering the sp<sup>2</sup>

insulator exhibiting a resistance about 1012 Ω/sq depending on sp<sup>2</sup>

randomized of oxidized areas. As a result of oxidation sp2

functionalities, organic adsorbates, even some contaminants and so on [11].

**2.1. Synthesis of graphene oxide**

as epoxy and hydroxyl groups.

The fundamental procedure involved in the synthesis of graphene oxide had originally been developed to oxidize graphite by Brodie in 1859 which used fuming nitric acid (HNO3) with potassium chlorate KClO3, evolving to the Staudenmaier method where oxidation of graphite was carried out with the addition of sulfuric acid (H2SO4) and KClO3 in different parts of the oxidation reaction [18]. Finally, Hummers and Offeman developed the most commonly method derived from Staudenmaier's work, where GO is extensively produced from the oxidation of graphite and the process is completed by a subsequent exfoliation of graphite oxide generally made by ultrasound wave or rapid heating [19]. The reaction takes place when a mixture of potassium permanganate (KMnO4) as strong oxidizing agent is combined with a concentrated H2SO4 (Eq. (1)). In terms of the permanganate used as an oxidizing agent, it is generally accepted that the main reactive specie in oxidation of graphite is the dimanganese heptoxide (Mn2O7) as can been seen in Eq. (2), not taking into account the changes in the method. Mn2O7 can react explosively as long as the temperature raise above 55°C or to come into contact with organic compounds [3, 6].

$$\rm{KMnO\_4 + H\_2SO\_4 \to K^+ + MnO\_3^+ + H\_3O^+ + 3HSO\_4^-} \tag{1}$$

$$MnO\_3^+ + MnO\_4^- \to Mn\_2O\_7 \tag{2}$$

Chemical species involved in the synthesis of graphene oxide.

Few years ago, Stankovich et al. [20] reported to obtain chemically modified graphene sheets from complete exfoliation of graphite by Hummers method, due to the van der Waals forces are weakened by the oxygenated moieties formed during the oxidation and the change of graphite to hydrophilic character, the hydration and dispersion in aqueous media can be done to obtain stable colloidal dispersions of GO on water. Additionally, the oxygenated moieties on GO have a negative charge, which grant first a good dispersion as well as stability in some organic solvents, alcohol, and water throughout electrostatic repulsion [17]. Consequently, GO have been used in applications such as sensing, composites, electronic devices, it can not only get well dispersion but also offers a platform for functionalization through chemical reactions such as amidation, esterification, and so on [11, 21]. Additionally, another important feature of GO is its amphiphilicity caused by the heterogeneous distribution of functional groups producing both hydrophobic and hydrophilic domains, having great results in order to interact with other materials such as polymers [11] or creating Pickering emulsions with organic solvents acting as molecular dispersing agent of materials such as carbon nanotubes (CNT) tuning only the solution pH [22].

Generally, the dispersion of GO is made by ultrasonic waves because is an easy, quick and efficient technique in comparison with the stirring and rapid heating process; however, long periods of sonication can be damaged and reduced the size of GO layers impacting on its properties [6, 7]. Sonication can be followed by centrifugation in order to obtain different lateral sizes on graphene oxide sheets based on density‐gradient [17]. Furthermore, salts and ions formed, while the oxidation reaction was carried out are removed [3]. Finally, GO can be reduced in order to recover the initial structure present in graphite, after that, properties such as conductivity can be restored until four times as much as in GO.

#### **2.2. Synthesis of reduced graphene oxide**

The sp2 hybridization loss in GO can be partially restored through the removal of oxygen moieties present on its surface. In this regard, GO has been subjected to the different process of reduction resulting in reduced graphene oxide (rGO). In other words, in rGO, there occurs the formation of percolation pathways between nanometric sp2 domains disrupted in oxida‐ tion reaction of graphite [17]. Therefore, rGO is an important material which has captivated scientific attention for its properties alike to those of pristine graphene that has permitted its use in many potentials applications.

The chemical structure of rGO does not have a specific arrangement regarding oxygenated functionalities distribution and aromatic/aliphatic domains. The remaining oxygen atoms exist as a result of the formation of stable carbonyl and ether groups that cannot be removed without making damage to the basal plane. In addition, another kind of defects found in rGO is the so called Stone–Wales defects (heptagons and pentagons pairs) as well as holes caused by losses of carbon in the form of CO and CO2 in the reduction process [12]. Nonetheless, properties such as resistivity have important changes after the reduction; values of ≈28.6 kΩ/sq have been reported for rGO [3]. On the other hand, even with an incomplete reduction, rGO produced has some advantages in view, it can be both electrically better than GO and still keeps some functional group reactive sites where further functionalization can be made [23]. Therefore, it is possible that the properties on rGO can be tuned, according with the degree of reduction. Hu et al. highlighted the close relationship between oxidation degree of graphite oxide and defects of rGO. Experimentally, they found that it is possible to obtain rGO with a few amount of defects, first with a relative high oxidation degree and second with low addition of KMnO4 used like oxidizing agent (chemical route) in terms of low addition, defects appeared on edges of graphite [24].

As a result of the reduction process visible changes can be observed, for instance, the brown dispersion of GO turns into a black precipitate due to aromatic restoration and re‐agglomer‐ ation of the rGO sheets, respectively, as a result of the change from hydrophilic to hydrophobic character after removal of functional groups [16]. Additionally, remarkable differences between GO and rGO have been reported under optical microscopy, while GO is mostly transparent rGO has a very light contrast with the substrate (SiO2/Si wafer), this reflects the change from insulator GO to rGO an electrical conductive material [25]. In this regard, for UV– vis spectroscopy, GO shows an absorption peak near 230 nm due to π–π\* plasmon, and a shoulder around 300 nm associated with π–π\* transitions in C=O, while rGO exhibits only an absorption peak between 225 and 275 nm, attributed to a better structural order and more C=C bonds [17]. Another important parameter to take into account is the C/O atomic ratio which is obtained in most cases by X‐ray photoelectron spectroscopy, elemental analysis and/or energy dispersive spectroscopy [26]. In this regard, different values have been reported, from 2.2 to 2.7 typically for GO to 10–12 for rGO by chemical reduction, even C/O ratio of 50.2 was reported for reduction under acetylene (C2H2) atmosphere [10, 17, 27]. Besides, C/O ratio, electrical conductivity also demonstrates the restoration of sp2 domains through the formation of percolation pathways after reduction. For GO, values of 10-6 S cm-1, characteristic of an insulator material, have been reported, [17], whereas for rGO sheets functionalized with pyrene, a conductivity of 1314 S cm-1 has been reported [28]. In addition to these parameters, other characterization techniques, such as NMR, atomic force microscopy (AFM), Raman spectroscopy, transmission electron microscopy (TEM), are also used to reveal the structure and properties of rGO.

solvents acting as molecular dispersing agent of materials such as carbon nanotubes (CNT)

Generally, the dispersion of GO is made by ultrasonic waves because is an easy, quick and efficient technique in comparison with the stirring and rapid heating process; however, long periods of sonication can be damaged and reduced the size of GO layers impacting on its properties [6, 7]. Sonication can be followed by centrifugation in order to obtain different lateral sizes on graphene oxide sheets based on density‐gradient [17]. Furthermore, salts and ions formed, while the oxidation reaction was carried out are removed [3]. Finally, GO can be reduced in order to recover the initial structure present in graphite, after that, properties such

hybridization loss in GO can be partially restored through the removal of oxygen

moieties present on its surface. In this regard, GO has been subjected to the different process of reduction resulting in reduced graphene oxide (rGO). In other words, in rGO, there occurs the formation of percolation pathways between nanometric sp2 domains disrupted in oxida‐ tion reaction of graphite [17]. Therefore, rGO is an important material which has captivated scientific attention for its properties alike to those of pristine graphene that has permitted its

The chemical structure of rGO does not have a specific arrangement regarding oxygenated functionalities distribution and aromatic/aliphatic domains. The remaining oxygen atoms exist as a result of the formation of stable carbonyl and ether groups that cannot be removed without making damage to the basal plane. In addition, another kind of defects found in rGO is the so called Stone–Wales defects (heptagons and pentagons pairs) as well as holes caused by losses of carbon in the form of CO and CO2 in the reduction process [12]. Nonetheless, properties such as resistivity have important changes after the reduction; values of ≈28.6 kΩ/sq have been reported for rGO [3]. On the other hand, even with an incomplete reduction, rGO produced has some advantages in view, it can be both electrically better than GO and still keeps some functional group reactive sites where further functionalization can be made [23]. Therefore, it is possible that the properties on rGO can be tuned, according with the degree of reduction. Hu et al. highlighted the close relationship between oxidation degree of graphite oxide and defects of rGO. Experimentally, they found that it is possible to obtain rGO with a few amount of defects, first with a relative high oxidation degree and second with low addition of KMnO4 used like oxidizing agent (chemical route) in terms of low addition, defects appeared

As a result of the reduction process visible changes can be observed, for instance, the brown dispersion of GO turns into a black precipitate due to aromatic restoration and re‐agglomer‐ ation of the rGO sheets, respectively, as a result of the change from hydrophilic to hydrophobic character after removal of functional groups [16]. Additionally, remarkable differences between GO and rGO have been reported under optical microscopy, while GO is mostly transparent rGO has a very light contrast with the substrate (SiO2/Si wafer), this reflects the

as conductivity can be restored until four times as much as in GO.

tuning only the solution pH [22].

2626 Recent Advances in Graphene Research

**2.2. Synthesis of reduced graphene oxide**

use in many potentials applications.

on edges of graphite [24].

The sp2

One method of reduction of GO is through heating of the samples, also called thermal annealing. This approach is generally employed to exfoliate graphite oxide using gradients of temperature higher than 2000°C/min [29]; for example, it has been reported the use of arc discharge method under hydrogen atmosphere yielding C/O molar ratio of 15–18 and a conduction 10 times higher than the traditional arc discharge method, this can be attributable to the *in situ* elimination and healing during the exfoliation [30]. The exfoliation–reduction can be explained given the CO and CO2 found within layers of graphite oxide, suffering an abrupt expansion increasing the interlayer distance of GO sheets; moreover, the decomposition of oxygenated moieties also produces high pressure and both are able to overcome the van der Waals forces [10, 29]. McAllister et al. [31] reported the generation of pressures of 40 MPa and 130 MPa at 300 and 1000°C, respectively, based on state equation, this exceeds the value of 2.5 MPa, calculated from Hamaker constant as the pressure necessary to exfoliate GO platelets with interlayer distance approximately of 0.7 nm according to the X‐ray Diffraction (XRD) made during the oxidation [31]. However, the rGO sheets obtained are small and wrinkled as a result of both, the release of CO2 and the removal of functional groups; this can be the main difficulty to scale this method [10].

On the other hand, another important method to reduce GO has been the electrochemical reduction mainly caused by the electron exchange among GO and an electrolyte; this has been performed within a typical electrochemical cell through two different routes [10]. First, a one‐ step electrochemical approach consisting in a reduction from aqueous colloidal suspension of GO in the presence of buffer electrolyte to produce rGO on the electrode surface. The second, a one‐ or two‐step electrochemical approach performed with thin films of GO deposited on a substrate (electrode) in order to form a GO coated‐electrode, and subsequently, this is subjected to electrochemical reduction in a standard three electrode system [32]. C/O ratios from 3.57 to 5.57 have been reported with this technique in addition to be an ecofriendly and controllable method; however, it cannot produce rGO in mass [32, 33]. Other emerging methods to produce rGO such as solvothermal, photocatalyst, and multistep reduction have been reported [10];notwithstanding, they will not be discussed in this chapter.

In spite of the successful reduction with thermal and electrochemical routes, chemical reduc‐ tion of GO is the most promising method to scale the production of rGO due to the availability of graphite, the relating ease of processing and the possibility to make modifications to the method in order to obtain high‐quality graphene. In this regard, recently Chua et al. [26] make a possible classification of chemical reduction approaches in two categories. The first one called "well‐supported" mechanism which consist in chemical reduction through widely known agents used in synthetic chemistry such as aluminum hydride and sodium borohydride, which have a determined mechanism of reaction over specific oxygenated moieties. The second one or "proposed" mechanism consists in reducing agents that have not been used in synthetic chemistry in order to reduce GO; consequently, reaction mechanisms are not well determined.

One of the agents used in "proposed" mechanisms is hydrazine (N2H4). Hydroxyl, epoxide, and carbonyl groups are the dominant functionalities. Hydrazine is known to form hydrazone and hydrazides in the combination of carbonyl moieties; however, in GO reduction, only hydrazone is formed by the removal of oxygen. Even, when the mechanism of how reduction is carried out by hydrazine has been widely studied, one of the most accepted is the reduction via nucleophilic substitution proposed by Ruoff and co‐workers. In this mechanism, the initial resulting derivative from epoxide opening and hydrazine, an alcohol functional group that releases water, is aminoaziridine, which is finally thermally eliminated in form of diimide, forming at the end a double carbon bond on the graphene surface [20]. In spite of the success of hydrazine in order to yield rGO, its toxicity and dangerous handling have limited its use. This has originated different studies about new reduction agents which not only should be safe but also they need to be ecofriendly.

Owing to the extensive search to scale rGO in a safe and nontoxic manner, "green" and safe reduction agents, take ascorbic acid or plant extracts for instance, have been used. L‐ascorbic acid (L‐AA) or Vitamin C, as it is commonly known, is an essential nutrient which exhibits antioxidant properties. Gao et al. [34] produced rGO through L‐AA as reducing agent, while L‐tryptophan was added to stabilize the produced rGO aqueous dispersion by electrostatic repulsion. They proposed a two‐step reduction mechanism, first SN2 nucleophilic reaction and second thermal elimination. Due to electron withdrawing, five‐membered ring makes the hydroxyls more acidic that result in dissociation of two protons to form an oxygen anion of L‐ AA. Subsequently, the reduction could be continued with a back side SN2 nucleophilic as well as releasing H2O and formation of intermediate species. Finally, thermal elimination of those precedes the yield of rGO. During the reduction, the L‐AA pass through oxidation to dehy‐ droascorbic acid. Later was reported that dehydroascorbic acid can be converted into oxalic and gluluronic acid which can interact with the remaining carboxylic moieties in the periphery of rGO, disrupting π–π interactions between rGO sheets avoiding the restacking and conse‐ quent agglomerates [35].

**Table 1** (Adapted from Ref. [26]) shows a list of "green" agents for the reduction of GO. Additionally, some recent approaches made with antioxidant agent from plant extracts, high H2‐rich water, potassium carbonate (K2CO3), and caffeic acid are included.


rGO such as solvothermal, photocatalyst, and multistep reduction have been reported

In spite of the successful reduction with thermal and electrochemical routes, chemical reduc‐ tion of GO is the most promising method to scale the production of rGO due to the availability of graphite, the relating ease of processing and the possibility to make modifications to the method in order to obtain high‐quality graphene. In this regard, recently Chua et al. [26] make a possible classification of chemical reduction approaches in two categories. The first one called "well‐supported" mechanism which consist in chemical reduction through widely known agents used in synthetic chemistry such as aluminum hydride and sodium borohydride, which have a determined mechanism of reaction over specific oxygenated moieties. The second one or "proposed" mechanism consists in reducing agents that have not been used in synthetic chemistry in order to reduce GO; consequently, reaction mechanisms are not well determined. One of the agents used in "proposed" mechanisms is hydrazine (N2H4). Hydroxyl, epoxide, and carbonyl groups are the dominant functionalities. Hydrazine is known to form hydrazone and hydrazides in the combination of carbonyl moieties; however, in GO reduction, only hydrazone is formed by the removal of oxygen. Even, when the mechanism of how reduction is carried out by hydrazine has been widely studied, one of the most accepted is the reduction via nucleophilic substitution proposed by Ruoff and co‐workers. In this mechanism, the initial resulting derivative from epoxide opening and hydrazine, an alcohol functional group that releases water, is aminoaziridine, which is finally thermally eliminated in form of diimide, forming at the end a double carbon bond on the graphene surface [20]. In spite of the success of hydrazine in order to yield rGO, its toxicity and dangerous handling have limited its use. This has originated different studies about new reduction agents which not only should be

Owing to the extensive search to scale rGO in a safe and nontoxic manner, "green" and safe reduction agents, take ascorbic acid or plant extracts for instance, have been used. L‐ascorbic acid (L‐AA) or Vitamin C, as it is commonly known, is an essential nutrient which exhibits antioxidant properties. Gao et al. [34] produced rGO through L‐AA as reducing agent, while L‐tryptophan was added to stabilize the produced rGO aqueous dispersion by electrostatic repulsion. They proposed a two‐step reduction mechanism, first SN2 nucleophilic reaction and second thermal elimination. Due to electron withdrawing, five‐membered ring makes the hydroxyls more acidic that result in dissociation of two protons to form an oxygen anion of L‐ AA. Subsequently, the reduction could be continued with a back side SN2 nucleophilic as well as releasing H2O and formation of intermediate species. Finally, thermal elimination of those precedes the yield of rGO. During the reduction, the L‐AA pass through oxidation to dehy‐ droascorbic acid. Later was reported that dehydroascorbic acid can be converted into oxalic and gluluronic acid which can interact with the remaining carboxylic moieties in the periphery of rGO, disrupting π–π interactions between rGO sheets avoiding the restacking and conse‐

**Table 1** (Adapted from Ref. [26]) shows a list of "green" agents for the reduction of GO. Additionally, some recent approaches made with antioxidant agent from plant extracts, high

H2‐rich water, potassium carbonate (K2CO3), and caffeic acid are included.

[10];notwithstanding, they will not be discussed in this chapter.

2648 Recent Advances in Graphene Research

safe but also they need to be ecofriendly.

quent agglomerates [35].


*C/O ratio obtained by a X‐ray Photoelectron spectroscopy <sup>b</sup> Elemental analysis <sup>c</sup> Energy dispersive spectroscopy*

**Table 1.** Reducing agents for GO toward a green reduction.
