2. Green methods to reduce GO

useful for theoretical studies for several technological applications. Current applications of graphene include flexible electronics, batteries, and so on [2]. Diverse methods have been proposed to produce high-quality single and few layer graphene films. Among them, graphite micromechanical cleavage, chemical vapor deposition, and graphitization of SiC have been the most utilized methods [3]. Although these methods produce high-quality graphene in a con-

In the past years, graphene-derived materials, such as graphene oxide (GO), graphane (the hydrogenated version of graphene), graphene fluoride, and so on [4, 5] have been paid special interest because of their potential applications. Particularly, GO and its reduced version, reducedgraphene oxide (rGO), have emerged as a technologically important material by its their own

GO is mainly prepared through chemical methods and therefore achieves unique and useful physiochemical properties to prepare a variety of functional materials for a range of advanced applications, such as rGO self-assembled microstructures [7, 8] and, rGO-based composites with inorganic nanoparticles (metals, semiconductors, metal oxides). These GO-derived materials have successfully been tested in the technological areas of nanomedicine, electronics,

The chemical methods to prepare single-layer GO use graphite as the raw material, which is exfoliated either using strong oxidants in aqueous medium (based on Hummers' method) or using organic solvents (based on the solution-phase technique), among others [10]. During the graphite oxidation process, oxidative species intercalate into graphite galleries provoking the

rich species. This results on the weakening of the interlayer attractive force, so that single-layer

From a structural point of view, GO is considered as a graphene sheet comprising in-plane undisturbed π-conjugated domains, and functionalized ones with covalently attached hydroxyl and epoxy groups, and additional carboxyl and carbonyl groups located at the sheet edge [11]. This chemical structure gives GO an amphiphilic character and then makes it dispersible in polar or nonpolar solvents [12]. This amphiphilic character preserves in rGO because it is obtained after the partial remotion of these functional groups by a reduction

Interestingly, rich oxygenated groups attached to the graphene structure makes GO and rGO highly hydrophilic and susceptible for further functionalization. Therefore, pristine or reduced GO can conveniently be functionalized to facilitate the interfacial interaction between GO and other materials including polymers, metal oxides, and inorganic nanoparticles to form GObased composite materials, or to link the sheets together and then lead to macroscopic GO-

Due to its multiple applications, GO is produced at an industrial level. Nowadays, worldwide research groups are looking for ways to find cost-effective and environment-friendly methods for graphene-derived materials' mass production. These include electrochemical, mechanical, and chemical exfoliation of graphite [15]. In general, these methods produce GO-like materials,

GO sheets are easily obtained upon application of low power sonication in water [8].


trolled way, they suffer from mass production scaling.

130 Graphene Materials - Structure, Properties and Modifications

environmental remediation, energy conversion, and others [7–9].

partial disruption of the graphene sp<sup>2</sup>

right [6].

process.

based materials [13, 14].

For applications where the exceptional electrical conductivity and transparency of graphene are demanded, GO can be subjected to an additional chemical treatment to detach the covalently oxygenated groups on graphene basal plane and restoring the sp<sup>2</sup> -hybridization. As previously mentioned, the detected drawbacks for the chemical reductants, such as hydrazine, hydroquinone, and sodium borohydride [19], have fuelled the search for both environmentfriendly methods and chemicals for GO reduction. The so-called "green technologies" satisfy both criteria, and the most reported green technologies may be classified, selected reducing agent, into four groups, as indicated in Table 1.


Table 1. Representative green technologies for the GO reduction.

#### 2.1. Green reducers and their effectiveness

#### 2.1.1. Bacterial reducers

Bacteria are living beings capable of surviving under the most extreme conditions, i.e., in severe temperature and chemical composition. Bacteria have been found in the most warmed underwater pools, where tectonic plates emanate pernicious gasses and incandescent material or in lakes of extreme saline composition, surrounded by an environment that is highly concentrated in arsenic, such as those found in Mono Lake, California [20]. To survive, bacteria can take organic and inorganic molecules from the surrounding environment and transform them into the substance required to start the cellular process in which oxidation-reduction mechanism is employed to obtain an energy source [21, 22]. The overall redox process carried out by bacteria has been used in GO reduction by means of Shewanella [23], Bacillus subtilis [24], Extremophiles bacteria [25], Escherichia coli [26], and Gluconacetobacter xylinus [27].

Note that the involved reaction mechanisms depend on the bacteria cell structure, which determines the capacity for directly or indirectly hydrolyzing the acidic groups attached to the GO molecular structure, particularly, the groups that comprise oxygen atoms. Wang et al. [23] used Shewanella for reducing GO (Figure 1) through a mechanism that consists segregating the heme group proteins such as c-type cytochromes, through the membrane and these proteins act as electron intercessor [28].

Zhang et al. [24] reported that depending on the bacteria type, it is possible to select a process to efficiently reduce GO for specific applications of the final nanomaterial. It was also proposed that, in the bacteria-based reduction processes, the parallel action of different bacteria could increase the effectiveness of reduction process. Based on Zhang's observations, Raveendran et al. [25] achieved reducing GO using extremophiles bacteria, obtaining graphene with excellent conductive properties.

Figure 1. As bacteria, Shewanella oneidensis utilizes terminal electron acceptors during its respiratory metabolism. It transfers electrons from cell surface to any extracellular acceptor such as metal oxides or graphene oxide. It was proposed that GO reduction by Shewanella procceds via an electron exchange among MtrB, MtrC and OmcA cytochromes to finally transfer an electron to GO and then reduce it.

#### 2.1.2. Biological reducers

The chemical compounds naturally existing in plants (phytochemicals) have been used for years as nutrients, drugs, etc. In the past few years, phytochemicals, such as vitamins, amino acids, saccharides, alkaloids, proteins, and enzymes [29, 30], have been studied as reductant precursors for metals and GO. The reported attempts for the GO reduction by using phytochemicals go into the employment of either laboratory-extracted (plant extracts) or commercial-purchased phytochemicals.

Herein, we present some relevant results emphasizing on the reductant chemical source.

#### 2.1.2.1. Plant extract reducing

2.1. Green reducers and their effectiveness

132 Graphene Materials - Structure, Properties and Modifications

teins act as electron intercessor [28].

lent conductive properties.

2.1.2. Biological reducers

transfer an electron to GO and then reduce it.

Bacteria are living beings capable of surviving under the most extreme conditions, i.e., in severe temperature and chemical composition. Bacteria have been found in the most warmed underwater pools, where tectonic plates emanate pernicious gasses and incandescent material or in lakes of extreme saline composition, surrounded by an environment that is highly concentrated in arsenic, such as those found in Mono Lake, California [20]. To survive, bacteria can take organic and inorganic molecules from the surrounding environment and transform them into the substance required to start the cellular process in which oxidation-reduction mechanism is employed to obtain an energy source [21, 22]. The overall redox process carried out by bacteria has been used in GO reduction by means of Shewanella [23], Bacillus subtilis [24], Extremophiles bacteria [25], Escherichia coli [26], and Gluconacetobacter xylinus [27].

Note that the involved reaction mechanisms depend on the bacteria cell structure, which determines the capacity for directly or indirectly hydrolyzing the acidic groups attached to the GO molecular structure, particularly, the groups that comprise oxygen atoms. Wang et al. [23] used Shewanella for reducing GO (Figure 1) through a mechanism that consists segregating the heme group proteins such as c-type cytochromes, through the membrane and these pro-

Zhang et al. [24] reported that depending on the bacteria type, it is possible to select a process to efficiently reduce GO for specific applications of the final nanomaterial. It was also proposed that, in the bacteria-based reduction processes, the parallel action of different bacteria could increase the effectiveness of reduction process. Based on Zhang's observations, Raveendran et al. [25] achieved reducing GO using extremophiles bacteria, obtaining graphene with excel-

The chemical compounds naturally existing in plants (phytochemicals) have been used for years as nutrients, drugs, etc. In the past few years, phytochemicals, such as vitamins, amino

Figure 1. As bacteria, Shewanella oneidensis utilizes terminal electron acceptors during its respiratory metabolism. It transfers electrons from cell surface to any extracellular acceptor such as metal oxides or graphene oxide. It was proposed that GO reduction by Shewanella procceds via an electron exchange among MtrB, MtrC and OmcA cytochromes to finally

2.1.1. Bacterial reducers

To date, the GO reduction by means of plant extract is intensively studied [31]. In this approach, the plant is chosen considering the antioxidant compound contents. For preparing the plant extract leaves, flowers, stems, and/or roots are refluxed in water, alcohol, or wateralcohol mixtures as solvents.

Green tea has proven to be an excellent source of antioxidant biomolecules. For example, it was successfully used for reduction of graphene oxide [32]. The reducing capacity of green tea is based on the antioxidant biomolecules extracted from emulsion, mainly polyphenols.

Extracts of chrysanthemum flower and lycium barbarum plants, used in the traditional Chinese medicine, were recently reported for GO reduction by Hou et al. [33, 34]. The extracts were obtained in aqueous media at boiling temperature and then filtered. Afterward, the extract was poured into the GO dispersion at the water boiling point for 24 h. The authors reported that the chemical composition of extracts, namely, chrysanthemum extract and flavonoids (diosmetin, luteolin, apigenin, and glucoside), were the predominant phytochemicals. Whereas the lycium barbarum extract comprised flavonoids, phenols, carotenoids, and polysaccharides as dominant phytochemicals.

The authors suggested that polyphenols present in chrysanthemum and lycium barbarum extracts transform to quinone releasing H+ ions that interact with GO for reducing it. Importantly, chrysanthemum and lyceum barbarum plants hold promise to effectively reduce GO, because the C/O ratio values obtained by X-ray photoelectron spectroscopy (XPS) were 1.35, 4.96, and 6.5 for pristine GO, rGO-chrysanthemum, and rGO-lycium barbarum, respectively.

#### 2.1.2.2. Commercial reductants

#### 2.1.2.2.1. Vitamins

Vitamin C (L-ascorbic acid) has been widely used in GO reduction because of its reducing effectivity and is comparable to that of hydrazine, besides promoting highly stabilized dispersions of rGO sheets in water. It has been observed that oxidized L-ascorbic acid is unreactive and stable and does not provoke damage to living cells [31].

In some GO reduction reactions, L-tryptophan (an aromatic amino acid) has been considered as a stabilizing agent to prepare highly stable rGO aqueous dispersions [35]. It effectively prevents against agglomeration of the rGO sheets because it readily adsorbs on undisturbed π–π domains at the basal plane of the rGO chain, which minimizes the attractive π–π interaction. Furthermore, the remaining terminal carboxylate anion of L-tryptophan has provided an electrostatic repulsion between the individual graphene sheets.

The L-tryptophan-stabilized rGO dispersion prepared with vitamin C exhibited good electrical conductivity of 14.1 S/m (pristine GO: 5.72 · 10<sup>10</sup> S/m). The mechanistic aspects for the GO chemical reduction remains unknown, but a plausible reduction mechanism was proposed as comprising two-step SN2 nucleophilic reactions. That is, L-ascorbic acid oxidizes into the dehydroascorbic anion releasing electrons and protons, which react with oxygenated groups on the GO sheet to reduce it.

The free-stabilizing agents including vitamin C-reduced rGO dispersions were also prepared by Zhang, who reported high stability for all the prepared samples. The electrical conductivity with a value of 800 S/m was obtained in the sample prepared for 48 h [36]. Fernandez-Merino et al. [37] reported that the reduction capability of vitamin C could be improved by increasing the alkali concentration into reducing solution; using this approach the reduction time was shortened to 15 min. Furthermore, rGO showed good dispersibility in polar organic solvents, with high C/O ratio (~12.5) as well as high electrical conductivity (7700 S/m). In addition, riboflavin (vitamin B2), phosphate salt of vitamin B2, and pyridoxine (vitamin B6) were used to reduce GO. These bioreductants have also been proven to successfully reduce GO [38].

#### 2.1.2.2.2. Saccharides

Saccharides are nutrients that may be used as reducing agents; these are classified into four chemical groups: mono/di/oligo/polysaccharides. Monosaccharides, glucose, and fructose have demonstrated mild reductive ability and nontoxic property in GO reduction experiments. In general, their potential for reduction is closely related to the ease to form open-chained structures [31]. In the GO reduction, it was found that their oxidized products play an important role to stabilize rGO sheets in aqueous dispersions, i.e., they may act as capping agents. Both saccharides and their oxidized products are environmental friendly. Zhu et al. [39] used glucose, fructose, and sucrose in aqueous ammonia solution for the reduction of GO. They determined that the ammonia solution is useful for both completion and enhancement of the GO deoxygenation reaction rate. In addition, they found that the reduction capability of sucrose was weaker than that for the glucose and fructose, under similar reaction conditions. The resulting rGO powder was biocompatible and highly dispersible in water. Likewise, Akhavan et al. [40] found that glucose increases its power to reduce GO in the presence of an iron catalyst under neutral condition.

On the other hand, dextran (a polysaccharide) was tested as a GO reducer in aqueous ammonia [41]. However, the as-reduced rGO exhibited a rather low electrical conductivity (1.1 S/m) that can be notably improved (10,000 S/m) upon thermal annealing (500C under Ar atmosphere).

#### 2.1.2.2.3. Amino acids

L-Cysteine is a thiol-containing amino acid that is liable to oxidate to cystine. It inhibits oxidative properties because thiol groups can suffer redox reactions. Chen et al. [42] synthesized rGO using L-cysteine as reducing agent under mild conditions. They proposed the reduction pathway for GO by L-Cysteine might be like that observed in the GO reduction by vitamin C. That is, at first, the reactions comprehend nucleophilic attack by thiol groups, which develop upon proton releasing during the L-Cysteine oxidation process. Afterward, the released protons react with the oxygenated groups producing water and byproducts, inducing the GO reduction. The rGO suspension conductivity increases by about 106 times in comparison to that of pristine GO. Bose et al. [43] used other amino acid such as glycine for reducing GO. They found that glycine not only reduces the GO but also functionalized it, as a result amine group can covalently bound to a GO network. In other work, L-Lysine was successfully used for reduction of graphene oxide in the presence of carboxymethyl starch (CMS) as stabilizing agent. The rGO suspension exhibited good dispersion stability in water [44]. Furthermore, L-aspartic acid has been employed for synthesizing rGO, the product obtained by this process also presents uniform separation in water as well as good electrical conductivity of ~700 S/m [45]. Other studies have revealed that some amino acids such as tryptophan, arginine, and histidine reduce the GO and also augmented the consolidation of rGO–metal nanoparticles [46]

#### 2.1.2.2.4. Gallic and citric acids

The L-tryptophan-stabilized rGO dispersion prepared with vitamin C exhibited good electrical conductivity of 14.1 S/m (pristine GO: 5.72 · 10<sup>10</sup> S/m). The mechanistic aspects for the GO chemical reduction remains unknown, but a plausible reduction mechanism was proposed as comprising two-step SN2 nucleophilic reactions. That is, L-ascorbic acid oxidizes into the dehydroascorbic anion releasing electrons and protons, which react with oxygenated groups

The free-stabilizing agents including vitamin C-reduced rGO dispersions were also prepared by Zhang, who reported high stability for all the prepared samples. The electrical conductivity with a value of 800 S/m was obtained in the sample prepared for 48 h [36]. Fernandez-Merino et al. [37] reported that the reduction capability of vitamin C could be improved by increasing the alkali concentration into reducing solution; using this approach the reduction time was shortened to 15 min. Furthermore, rGO showed good dispersibility in polar organic solvents, with high C/O ratio (~12.5) as well as high electrical conductivity (7700 S/m). In addition, riboflavin (vitamin B2), phosphate salt of vitamin B2, and pyridoxine (vitamin B6) were used to reduce GO. These bioreductants have also been proven to successfully reduce GO [38].

Saccharides are nutrients that may be used as reducing agents; these are classified into four chemical groups: mono/di/oligo/polysaccharides. Monosaccharides, glucose, and fructose have demonstrated mild reductive ability and nontoxic property in GO reduction experiments. In general, their potential for reduction is closely related to the ease to form open-chained structures [31]. In the GO reduction, it was found that their oxidized products play an important role to stabilize rGO sheets in aqueous dispersions, i.e., they may act as capping agents. Both saccharides and their oxidized products are environmental friendly. Zhu et al. [39] used glucose, fructose, and sucrose in aqueous ammonia solution for the reduction of GO. They determined that the ammonia solution is useful for both completion and enhancement of the GO deoxygenation reaction rate. In addition, they found that the reduction capability of sucrose was weaker than that for the glucose and fructose, under similar reaction conditions. The resulting rGO powder was biocompatible and highly dispersible in water. Likewise, Akhavan et al. [40] found that glucose increases its power to

On the other hand, dextran (a polysaccharide) was tested as a GO reducer in aqueous ammonia [41]. However, the as-reduced rGO exhibited a rather low electrical conductivity (1.1 S/m) that can be notably improved (10,000 S/m) upon thermal annealing (500C under Ar atmosphere).

L-Cysteine is a thiol-containing amino acid that is liable to oxidate to cystine. It inhibits oxidative properties because thiol groups can suffer redox reactions. Chen et al. [42] synthesized rGO using L-cysteine as reducing agent under mild conditions. They proposed the reduction pathway for GO by L-Cysteine might be like that observed in the GO reduction by vitamin C. That is, at first, the reactions comprehend nucleophilic attack by thiol groups, which develop upon proton releasing during the L-Cysteine oxidation process. Afterward, the released protons react with the oxygenated groups producing water and byproducts, inducing the GO reduction. The rGO suspension conductivity increases by about 106 times in

reduce GO in the presence of an iron catalyst under neutral condition.

on the GO sheet to reduce it.

134 Graphene Materials - Structure, Properties and Modifications

2.1.2.2.2. Saccharides

2.1.2.2.3. Amino acids

Gallic and citric acids are natural organic acids that have been tested as GO reductants. It was found that both acids could play the dual role as a reducing agent and a surfactant. Li et al. [47] found that the GO can be significantly reduced by gallic acid in aqueous ammonia, either at room temperature or under heating condition. Although, the reduction mechanism of GO by gallic acid has not been explored, it is expected that its three adjacent hydroxyl groups (pyrogallol moieties) interact with the GO in-plane oxygenated groups. The prepared rGO suspensions displayed excellent dispersibility in various solvents such as H2O, N-Methyl-2 pyrrolidone (NMP), dimethylsulfoxide (DMSO), dimethylformamide (DMF), and methanol, probably due to adsorbed oxidized gallic acid.

On the other hand, citric acid has extensively been studied for the synthesis of silver and gold nanoparticles. Recently, Ortega-Amaya et al. [18] used the one pot approach to produce highly dispersible functionalized rGO by using citric acid. This process was made in aqueous medium at room temperature, under Ar atmosphere. To explain the dual role of citric acid as a reducer and a stabilizer, the authors assumed that protons released by the citric acid dissociation bind to epoxy or hydroxyl groups to form water molecules and an active carbocation at the GO network. Afterward, a di-ionized citrate HCit<sup>2</sup> anion covalently binds to the carbocation to stabilize it. The whole effect was one of reduction by protons, and functionalization by HCi2 anion. Last one being the predominant specie in the aqueous solution at pH 4.

#### 2.1.3. Irradiation as reducer

UV, microwave, or ultrasound irradiation have been used for transforming colloidal GO to graphene with a similar quality as that produced by means of hydrazine. In acidic GO colloids, Lu et al. [48] obtained free contaminants rGO by microwave heating. First, an acidic GO colloid at pH 1, 3, or 5 was separately prepared by dropping a NaOH solution. Afterwards, each mixture was heated at 150C under microwave irradiation, employing a power of 80 W, for 10 min. They monitored the GO reduction advance by visual observation of the color changes from brownish-yellow to black [49, 50].

A different method for the GO reduction based on electromagnetic irradiation was reported by Ding et al. [51]. The authors reported clean reduction of colloidal GO using the strong UV absorption property of water [52]. The UV radiation dissociates the water molecule into three radicals (hydrogen H2, hydroperoxyl HO2, and hydrated electrons e), each one retaining one of the earlier bounded electrons [53]. Then, hydrated electrons behave as a reducer to form rGO (Figure 2). Although the reduction process takes a long reaction time, it is possible to monitor the formation of rGO dispersions through UV-vis spectroscopy [54].

Another green processing by irradiation was published by Nyangiwe et al. [53], which is a very simple method and is described for the reduction of GO solution. By irradiating a GO sample dispersed in water with sunlight, the most oxygenic functional groups in GO were removed. The authors considered that photoreduction of GO by sunlight can be explained by a model proposed by Ji et al. [55], where the absorbed UV radiation in solvent excites the water molecule near its photoionization threshold (6.5 eV), generating solvated electrons, which will act like reducers. The complete process is described by the following equations [56]:

$$GO + hv \to GO(hole + e^{-})\tag{2.1}$$

$$2\text{ holes} + 2\text{H}\_2\text{O} \rightarrow \text{O}\_2 + 4\text{H}^+\tag{2.2}$$

$$4e^- + GO + 4H^+ \to rGO + 2H\_2O \tag{2.3}$$

#### 2.1.4. Polymers as reducers

There are scarce reports on the GO reduction by polymers. Zang et al. [57] reported the GO reduction using poly(diallyldimethylammonium chloride) (PDDA) polyelectrolyte [57]. It has been reported that the addition of PDDA to a GO aqueous dispersion triggers a chemical reaction that promotes a color change in the GO dispersion, indicating that GO transforms to rGO. Although the mechanistic aspects of the GO reduction were not clearly explained, the PDDA-functionalized rGO exhibited an excellent dispersion in water. Therefore, polyelectrolyte might be used as a reducing agent as well as a stabilizer to prepare a colloidal suspension of graphene. This method is based on the Yang et al. report [56], where PDDA was adsorbed on the external surface of carbon nanotubes through π–π and electrostatic interactions [56, 58]. It was assumed that repulsive electrostatic interaction dominates to produce well dispersed PDDA-functionalized carbon nanotubes in water.

Figure 2. Schematic representation of the reduction of GO under UV irradiation.

#### 2.1.5. Transition metals as reducers

of the earlier bounded electrons [53]. Then, hydrated electrons behave as a reducer to form rGO (Figure 2). Although the reduction process takes a long reaction time, it is possible to

Another green processing by irradiation was published by Nyangiwe et al. [53], which is a very simple method and is described for the reduction of GO solution. By irradiating a GO sample dispersed in water with sunlight, the most oxygenic functional groups in GO were removed. The authors considered that photoreduction of GO by sunlight can be explained by a model proposed by Ji et al. [55], where the absorbed UV radiation in solvent excites the water molecule near its photoionization threshold (6.5 eV), generating solvated electrons, which will

monitor the formation of rGO dispersions through UV-vis spectroscopy [54].

136 Graphene Materials - Structure, Properties and Modifications

act like reducers. The complete process is described by the following equations [56]:

4e

PDDA-functionalized carbon nanotubes in water.

Figure 2. Schematic representation of the reduction of GO under UV irradiation.

2.1.4. Polymers as reducers

GO þ hv ! GOðhole þ e

There are scarce reports on the GO reduction by polymers. Zang et al. [57] reported the GO reduction using poly(diallyldimethylammonium chloride) (PDDA) polyelectrolyte [57]. It has been reported that the addition of PDDA to a GO aqueous dispersion triggers a chemical reaction that promotes a color change in the GO dispersion, indicating that GO transforms to rGO. Although the mechanistic aspects of the GO reduction were not clearly explained, the PDDA-functionalized rGO exhibited an excellent dispersion in water. Therefore, polyelectrolyte might be used as a reducing agent as well as a stabilizer to prepare a colloidal suspension of graphene. This method is based on the Yang et al. report [56], where PDDA was adsorbed on the external surface of carbon nanotubes through π–π and electrostatic interactions [56, 58]. It was assumed that repulsive electrostatic interaction dominates to produce well dispersed

�Þ (2.1)

4holes þ 2H2O ! O<sup>2</sup> þ 4H<sup>þ</sup> (2.2)

� þ GO þ 4H<sup>þ</sup> ! rGO þ 2H2O (2.3)

An interestingly eco-friendly approach toward the GO reduction consists of using transition metals (e.g., Fe, Zn, Cu, and Co) as GO reducing agents. In this case, the reduction mechanism strongly depends on the experimental conditions (mainly pH and temperature) and it follows a frequently complex pathway. Some examples are described below.

GO was reduced by iron in aqueous medium by Fan et al. [59]. They studied the GO reduction by powdered iron (10 µm average size) in an acidic HCl-water mixture at room temperature. They proposed that H+ interacts with the iron surface particle to bring forth the Fe/Fe2+ core/ shell structure (iron particle with a thin sheet of charged Fe2+ ions). These positively charged Fe/Fe2+ species interacts with the functional groups on the GO sheets and after electron transport from Fe/Fe2+ to GO, the reduction of GO was achieved.

Experiments on GO reduction using Zn powder were essayed by Yang et al. [60]. To evaluate the Zn reduction capability and how it is affected by the solution pH and temperature, they prepared aqueous GO colloids with and without sodium hydroxide at room temperature and 100C. They obtained lower reduction levels for all cases other than alkaline in 100C conditions. The proposed reduction mechanism consists of an electron exchanging between Zn and GO to produce rGO and by products. The GO reduction using Zn powder were also carried out by Mei and Ouyang [61] and Liu et al. [62] under acidic conditions.

The formation of Cu2O/rGO nanostructures during the GO reduction by Cu nanoparticles was described by Wu et al. [63]. They mixed polydisperse Cu nanoparticles with water-dispersed GO under neutral conditions. After sonication and heating at 95C a composite comprising of rGO sheet decorated by Cu2O nanoparticles was observed. Authors claimed that GO was reduced through a redox reaction between Cu an GO, in which Cu nanoparticles transformed to nanozised Cu2O and GO was reduced. In addition, they reported that the GO reduction strongly depended on the Cu particle size, because experiments involving fine grained Cu powder was unable to effectively reduce GO.

GO reduction experiments using metal foils as a substrate were done by Cao et al. [64]. A number of metal foils (Cu, Ni, Co, Fe, and Zn) were separately immersed in a GO aqueous dispersion at pH = 6. After taking out and drying at ambient temperature, the metal foil was coated with a rGO film. It was assumed that the rGO film was developed by a self-assembly process of rGO nanosheets and that the GO was spontaneously reduced by direct transference of electrons from metal ions to GO. Some metal ions were found in the GO layers' galleries.

Hu et al. [65] used a GO dispersion at pH = 4 to immerse metallic foils (Cu, Fe, Zn, Co, and Al) and also a nonmetallic (carbon) film supported by Cu, and after (1–12 h) immersion, the metal foil was covered by self-assembled rGO multilayers and was dried at ambient conditions or freeze dried. They found that there had been electron transfer between the metal and GO propitiated for the acid condition (Figure 3). A significant difference is that they found metal oxide nanoparticles decorating the rGO; another important result was that the electron exchange to reduce GO had taken place even when a conductive layer (carbon or Au, Pt, Ag) covers the Cu substrate.

Figure 3. Scheme of GO reduction and metal oxidation. A hydroxyl group present at GO was protonated in acid conditions and then an electron transfer between metal GO took place, rGO was obtained, and oxidized metal.

#### 2.1.6. Mechanochemical reducer: ball milling system

The mechanical reduction of GO into graphene was tested by a hydrogen-assisted ball-milling process [66]. The ball-milling process was carried out in a planetary micro ball-milling machine with a stainless steel chamber and stainless steel 5 mm diameter balls. First, 2.0 g of previously prepared GO powder by a modified Hummers' method [67] was loaded into the ball-milling chamber, and then filled with hydrogen gas. The chamber was rotated at 900 rpm for different times in the 30- to 240-min interval, to obtain a variety of ball-milled rGO samples. The GO reduction process with the milling time was visually verified by observing the GO color change from a brownish-yellow to black. The final powder, as analyzed by transmission electron microscopy, XPS, and infrared absorbance spectroscopy, consisted of well-exfoliated oxygen-free single-layer graphene [68, 69].

#### 2.1.7. Electrochemical reduction

The electrochemical technique is widely used in thin film deposition on conductive substrates. The electrochemical reduction of GO develops either during the film deposition process or a preformed film as described in the comprehensive review [70]. The one-step and two-step approaches are usually employed to produce GO, and the reduction level can be controlled by varying the processing time, electrode material, on-off cycles, electrolyte type, and potential values. A variety of nontoxic electrolytes, such as NaCl, KCl, NaHPO4, Na2SO4, KNO3, and phosphate buffer solution (PSB), have been used. Furthermore, glass carbon, Au, Pt, Ag, and 3 aminopropyltriethoxysilane (APTES) have been tested as electrode materials [70].

In the one-step approach, GO sheets are dispersed into a mixture of electrolyte and buffer solution, and the power source is turned on and the GO thin film deposition and reduction occur simultaneously at the cathode surface material.

In the two-step approach, a thin film is deposited by some technique (drop-casting, spray pyrolysis, layer-by-layer, etc.) on an electrode of a three-electrode system (reference, working, and auxiliary electrodes), and then immersed into the electrolyte solution. Under controlled conditions of electrolyte temperature and composition, as well as electrodes potential, a rGO thin film is obtained. Recently, Fang et al. [71] used the two-step system to produce large area rGO and rGO/silk fibroin composites. They used a reference electrode of Ag/AgCl, auxiliary electrodes of ITO, Ag wire, and titanium and were tested individually and the working electrode GO materials, electrolytes of NH4Cl, KCl, or [dmin][BF4] were used. After the reduction process, the large area GO were tested for electrical properties, having 28,200 S/m conductivity after the reduction.

#### 2.2. Summary of green reducing methods

2.1.6. Mechanochemical reducer: ball milling system

138 Graphene Materials - Structure, Properties and Modifications

oxygen-free single-layer graphene [68, 69].

occur simultaneously at the cathode surface material.

2.1.7. Electrochemical reduction

conductivity after the reduction.

The mechanical reduction of GO into graphene was tested by a hydrogen-assisted ball-milling process [66]. The ball-milling process was carried out in a planetary micro ball-milling machine with a stainless steel chamber and stainless steel 5 mm diameter balls. First, 2.0 g of previously prepared GO powder by a modified Hummers' method [67] was loaded into the ball-milling chamber, and then filled with hydrogen gas. The chamber was rotated at 900 rpm for different times in the 30- to 240-min interval, to obtain a variety of ball-milled rGO samples. The GO reduction process with the milling time was visually verified by observing the GO color change from a brownish-yellow to black. The final powder, as analyzed by transmission electron microscopy, XPS, and infrared absorbance spectroscopy, consisted of well-exfoliated

Figure 3. Scheme of GO reduction and metal oxidation. A hydroxyl group present at GO was protonated in acid conditions and then an electron transfer between metal GO took place, rGO was obtained, and oxidized metal.

The electrochemical technique is widely used in thin film deposition on conductive substrates. The electrochemical reduction of GO develops either during the film deposition process or a preformed film as described in the comprehensive review [70]. The one-step and two-step approaches are usually employed to produce GO, and the reduction level can be controlled by varying the processing time, electrode material, on-off cycles, electrolyte type, and potential values. A variety of nontoxic electrolytes, such as NaCl, KCl, NaHPO4, Na2SO4, KNO3, and phosphate buffer solution (PSB), have been used. Furthermore, glass carbon, Au, Pt, Ag, and 3-

In the one-step approach, GO sheets are dispersed into a mixture of electrolyte and buffer solution, and the power source is turned on and the GO thin film deposition and reduction

In the two-step approach, a thin film is deposited by some technique (drop-casting, spray pyrolysis, layer-by-layer, etc.) on an electrode of a three-electrode system (reference, working, and auxiliary electrodes), and then immersed into the electrolyte solution. Under controlled conditions of electrolyte temperature and composition, as well as electrodes potential, a rGO thin film is obtained. Recently, Fang et al. [71] used the two-step system to produce large area rGO and rGO/silk fibroin composites. They used a reference electrode of Ag/AgCl, auxiliary electrodes of ITO, Ag wire, and titanium and were tested individually and the working electrode GO materials, electrolytes of NH4Cl, KCl, or [dmin][BF4] were used. After the reduction process, the large area GO were tested for electrical properties, having 28,200 S/m

aminopropyltriethoxysilane (APTES) have been tested as electrode materials [70].


A summary of the above-mentioned green reducing methods and its reduction rate is presented in Table 2.

Table 2. Summary of reductive green methods.
