Introduction

Graphene is a sp2 hybridized carbon atoms densely arranged in two-dimensional hexagonal lattice with a 0.142-nm carbon–carbon bond length (Singh et al. 2011). Since being exfoliated from highly oriented pyrolytic graphite by Novoselov et al. (2004), graphene nanosheets have been mass produced through several exfoliation processes, destroying Van der Waals’ force between layers via gas (Wu et al. 2018), liquid (Amiri et al. 2018), electrical discharge (Segundo et al. 2018) and ultrasound exfoliation (Navik et al. 2018; Balasubramaniam and Balakumar 2016). Graphene and its derivatives have attracted close attention due to their unique morphologies and extraordinary electrical, mechanical, optical, thermal and antimicrobial properties (Soldano et al. 2010). These fascinating materials have been proposed to be used in semiconductor, sensors, energy storage and medical areas (Huang et al. 2011; Traversi et al. 2013; Kamat 2009). For example, graphene is very promising in bone scaffold due to its biocompatibility and capability of differentiating human mesenchymal stem cells into bone cells without hampering their proliferation (Nayak et al. 2011). It is also an excellent adsorbent for removing heavy metals owing to its large specific surface area (Elgengehi et al. 2020). Graphene or its nanocomposites with metal, metal oxide, metal salt and nonmetallic elements play an important role in fuel cells, supercapacitors and solar cells areas (Tsang et al. 2020; Balasubramaniam and Balakumar 2017a, b). Optical devices based on graphene exhibit good refractive index and optical transparency in the visible light range (Zhang et al. 2010a, b). High photo-thermal conversion capacity makes graphene be promising responsive materials for phototherapy, flexible photonic and electronic devices, and smart windows photodetectors (Chi et al. 2019).

Graphene oxide (GO, Fig. 1), one derivative of graphene, is heavily decorated with a series of oxygen-containing groups such as hydroxyl, carboxyl, epoxy and ketone along the carbon atoms. The common methods for chemical preparation of GO and their features were listed in Table 1. Graphite was exfoliated chemically by potassium chlorate and fuming nitric acid via Brodie method (Brodie 1859). Staudenmaier (1898) used sulfuric acid instead of two-third of fuming nitric acid and multiple aliquot of potassium chlorate in the reaction, but the obtained graphite layers were often damaged. In Hummers method, graphite was oxidized by concentrated sodium nitrate and potassium permanganate (Hummers and Offeman 1958) in a few hours to produce GO. It is a very common method with various oxidization conditions (Wu et al. 2009; Zhao et al. 2010), but toxic gas such as NO2 and N2O4 was produced. Graphites oxidized by Hummers method had a high oxygen contents and large interlayer distances than the above methods (Muzyka et al. 2017). The oxygen moieties were also easier to remove upon thermal annealing than those by the other methods, showing a better electrochemical performance (Botas et al. 2013; Poh et al. 2012). The modified Hummers method being developed by Marcano et al. (2010) employed double amount of potassium permanganate and sulfuric acid/phosphoric acid to replace sodium nitrate. It produces more hydrophilic carbon material without toxic gas and large exotherm than Hummers method. The parameters of the oxidation reaction, such as potassium permanganate amount, oxidation temperature and time, influenced the oxygen contents and interlayer spacing of GO, providing different chemical or physical structure even after GO was reduced (Kavimani et al. 2017; Trenczek-Zajac et al. 2016).

Fig. 1
figure 1

Schematic reduction method of graphene oxide

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Table 1 Chemical methods for preparing GO
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GO can be converted into the reduced graphene oxide (rGO, Fig. 1) via removing oxygen-containing groups, showing quite different properties in some respects. For example, the hydrophobic rGO is prone to aggregation to change its characteristics such as shape and specific area, but the oxygen-containing groups enhance the hydrophilicity greatly and make GO more stable in water (Stankovich et al. 2006). It is still a controversial to conclude which one possesses better antibacterial capacity (Zou et al. 2016). However, rGO demonstrates better chemical stability and electronic conductivity, with the resistance ranges of (5–500) GΩ cm and (100–1000) Ω cm, respectively, for GO and rGO (Pei and Cheng 2012; Xu et al. 2018; Becerril et al. 2008). These features have appealed continuous studies on the performance of rGO and opened exciting new opportunities for applications of rGO and rGO-based device in many areas.

Many methods have recently been proposed to prepare rGO, with different residue groups and structural defects to ultimately affect their function. Low energy consumption and non-toxicity process have been two main focuses for the synthesis of rGO, especially for large-scale industrial production. This paper summarized the progress on the reduction method for GO and relevant reaction mechanism.

Reduction via reductant

Chemical reagents

Oxygen-containing groups of GO can be eliminated through a redox reduction with the reductive reagents. Reduction via common chemical reagents is the most traditional way to produce rGO, which has shown great potential in the large-scale production of graphene. Hydrazine hydrate possessed high reductivity to carbonyl groups, but nitrogen doping in the carbon lattice changed the conductive of rGO as well. The result proposed by Stankovich et al. (2007) showed that C/N ratio of rGO was 16.1 after the reduction by hydrazine hydrate for 24 h. Lin et al. fabricated N-dope few-layer mesoporous carbon with the conductive and specific capacitance of 360 S cm−1 and 790 F g−1 in 0.5 M H2SO4 electrolyte, respectively. The efficient reduction of GO and restoration of π–π conjugated network of sp2 hybridized carbon structure presented advantages in the field of electronics (Lin et al. 2015). Sodium borohydride was more effective than hydrazine hydrate because no extra elements would be doped on the carbon planes. It reduced ketones to alcohol groups more easily than carbonyl and epoxy groups (Shin et al. 2009). The mixture of sodium borohydride and calcium chloride was reported to promote the elimination of the hydroxyl groups compared with sodium borohydride alone. The resulting rGO with a C/O ratio of 19.0 presented lower electrical resistance and high capacitance of 162 F g−1 in alkaline media and 164 F g−1 in acidic media (Yang et al. 2015). A two-step reduction strategy using sodium borohydride and hydrazine successively was applied to produce more conductive rGO than the ones by sodium borohydride or hydrazine alone, showing potential application as supercapacitors, batteries or sensors (Wei et al. 2017). Krishna et al. (2015) developed a method via using hydrazine hydrate and citric acid in an alkaline condition, and obtained more exfoliated rGO with better electrical property than the reductive one by hydrazine hydrate.

Hydroxylamine was proven to be an effective chemical reductant due to less toxic and explosive than hydrazine hydrate. rGO being reduced by hydroxylamine at 90 °C for 1 h possessed a C/O ratio from 2.0 to 9.8, in comparable to hydrazine hydrate reduction for 24 h. However, it showed more defect of carbon lattice than the ones by hydrazine or sodium borohydride (Yaragalla et al. 2017). Hydroiodic acid was applied to prepare flexible rGO film that showed higher C/O ratio and conductive capacity than those reduced by hydrazine hydrate and sodium borohydride, because the latter two released gas by-product to destroy the films. However, a small amount of iodine remained on the rGO films. Other halogenation agents including SOCl2 and HBr were also reported to reduce GO in different levels (Pei et al. 2010), demonstrating the possibility of GO reduction in acidic condition.

Generally, the above chemical compounds are toxic and hazardous that will cause health risks or environmental pollutions. Therefore, non-toxic reductants including sodium oxalate (Hanifah et al. 2015), sodium acetate trihydrate (Zhang et al. 2014a, b) and dipotassium hydrogen phosphate (Zhang et al. 2013a, b) were employed to produce rGO in alkaline condition. Figure 2 illustrates the deoxygenation mechanism of various oxygen-containing groups of GO. Hydroxyl (Fig. 2a) was first protonated under sodium oxalate basic solution and converted into H2O to form the carbocation. The oxalate ions then abstracted the neighboring hydrogen of the carbocation to form double bonds. Carbonyl groups (Fig. 2b) can be protonated to form hydroxyl ions that were reduced in a similar mechanism mentioned above (Hanifah et al. 2015). Dehydration reaction was suggested as another solution to GO reduction. Sodium acetate trihydrate was hydrolyzed to produce alkaline environment, where hydroxyl groups could combine with its neighboring hydrogen atoms through dehydration reaction to produce rGO (Fig. 2c), or the epoxy groups (Fig. 2d) were hydrated and transformed into two hydroxyl groups that reduced further with the neighboring hydrogen to give H2O (Zhang et al. 2014a, b). Ethanol, ethylene glycol and glycerol were also proven as precisely selective reductants of epoxy groups, while hydroxyl and carboxyl groups remained (Xu et al. 2014). These reductants showed poor reducing degree though less hazardous than hydrazine hydrate, which would limit them from large-scale production.

Fig. 2
figure 2

Mechanism of chemical reduction: nucleophilic reaction of hydroxyl group (a) and carbonyl (b) by sodium oxalate (Hanifah et al. 2015), and dehydration reaction of hydroxyl group (c) and epoxy group (d) by sodium acetate trihydrate (Zhang et al. 2014a, b)

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GO can also be reduced by certain solvents in a sealed container above the boiling point without any additional reductants. N,N-dimethylformamide (solvent) was used to reduce GO at high temperature (120–200 °C) for 12 h and obtained rGO with the intensity ratio of D-Raman peak to G-Raman peak (ID/IG) increasing to 0.93. C/O ratio of rGO via solvothermal reduction at 180 °C was 5.52 compared to 2.02 of GO, and long reaction time facilitated to obtain rGO with high C/O atomic ratio (Zhou et al. 2011; Kim et al. 2016). The introduction of sulfuric acid (as catalysts) shortens the reaction time and enhanced the reaction efficiency. Only 1 h of reduction by solvent and catalysts produced rGO with high C/O ratio of 8.4 compared with 6.5 of solvent reduction (Tien et al. 2012). rGO via vinylpyrrolidone (solvent) reduction at 250 °C had the conductivity up to 1.38 × 103 S.m−1, showing potential applications in solar cells or electromagnetic interference shielding of signal cables (Dubin et al. 2010). Since N,N-dimethylformamide and vinylpyrrolidone introduced nitrogen in the carbon lattice, mixed solvent of acetone and sodium hypochlorite was employed as the substitution to reduce GO at 120 °C for 2 h and obtained rGO with a C/O ratio of 8.23 and ID/IG ratio of 1.65 (Kim et al. 2014a, b). Though the mechanism of solvothermal reduction is still questionable, it is generally accepted that high pressure and temperature promote the deoxygenation of GO, where the solvents can behave as strong electrolytes to cleave the oxygen-containing bonds (Zhou et al. 2009; Chen et al. 2013). A green method based on refluxing aqueous GO suspension was designed to control oxygen contents, but C/O ratio only reached 6 after 7 d of reflux. Such long reduction time would limit its application in large-scale commercial production (Park et al. 2014).

In summary, the reduction of GO by chemical reductants can be realized at room temperature or by moderate heating, making it a cheap and easy way to produce stable dispersion of rGO sheets. Reductants like hydrazine hydrate and sodium borohydride are most commonly used because of their high effectiveness, but most of them are corrosive, explosive or hazardous. Much effort has been made to explore non-toxic reducing reagents as the substitutes, though most of them present lower reduction efficiency than traditional ones and are likely to aggregate due to strong π–π stacking tendency and Van der Waals’ interaction among rGO sheets (Guo et al. 2011). Solvothermal reduction without extra chemical reductants is very promising; however, it often requires special equipment and rigorous condition.

Bio-reductants

Most chemical reagents are toxic and do not suitable for bio-related applications; therefore, economic and eco-friendly natural reductants have been considered. Amino acids contain many reactive groups on the main and/or side chains to react with GO, among which l-cysteine with the reductive thiol groups was the first amino acid for the reduction (Chen et al. 2011). Amine groups of alanine can also attack epoxy and hydroxyl groups of GO nucleophilically (Wang et al. 2017). Reducing sugars (glucose, fructose and sucrose) can form H-bond with oxygen-containing groups of GO and convert into lactone themselves (Zhu et al. 2010; Akhavan et al. 2012). l-Ascorbic acid was prone to be oxidized because of its strong reductivity (Zhang et al. 2010). During the reduction process (Fig. 3), ascorbic acid functioned as the nucleophile and attacked the hydroxyl (a) and epoxide (b) groups to release H2O. The resulted intermediate underwent thermal elimination reaction, forming the double bonds on rGO and dehydroascorbic acid (Gao et al. 2010Ascorbic acid was also proven to produce a more stable rGO in the alkaline condition through electrostatic repulsion to inhibit rGO agglomeration (Xu et al. 2015). Recently, Hou et al. (2018) synthesized rGO using artemisinin in ethanol with a C/O ratio as high as 11.7, probably owing to the formation of an endoperoxide bridge. In general, the reduction degree via bio-reductants was lower than those of traditional chemical reagents, and the defects were introduced on carbon plane meanwhile.

Fig. 3
figure 3

Mechanism of the reduction process by ascorbic acid: (a) nucleophilic reaction of double hydroxyl group; (b) nucleophilic reaction of epoxy group (Gao et al. 2010)

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Plant extraction has attracted much attention because it contains bioactive proteins, vitamins, amino acids, polyphenol and flavonoid that may act as the reducing or capping agents in the production of rGO. Therefore, reducing process via plant extract is very complicated. For example, bio-reduction via eucalyptus leaf involved 11 major biomolecules (Li et al. 2017; Jin et al. 2018). Hou et al. (2017) compared the plant extraction with common chemical reductant and found that rGO reduced by hydrazine hydrate displayed much smaller plates with obvious curled edges and the one by Lycium barbarum extract possessed more defects.

Polyphenols are a class of natural secondary metabolites characterized by the presence of polyatomic phenol, and existed largely in different parts of peels, roots, leaves and fruits. Seed extracts of Terminalia chebula were reported as green reductants to reduce the oxygen-containing groups and stabilized rGO solution by preventing aggregation as well (Maddinedi et al. 2015). Flavonoids are non-ketone polyhydroxy polyphenol compounds contained in different parts of plant, and also a class of plant secondary metabolites to perform many functions, such as flower coloration, chemical messengers, physiological regulators and cell cycle inhibitor (Galeotti et al. 2008). The degree of deoxygenation using chrysanthemum extract containing flavonoids (diosmetin, luteolin, apigenin and glucoside) was comparable to hydrazine hydrate (Hou et al. 2016). Polyphenols and flavonoids with oxygen anion and phenolic hydroxyl group can react with the epoxide moiety through a SN2 mechanism to induce the opening reaction of the oxirane ring, or attack carbonyl and hydroxyl groups through eliminating water molecules, resulting in the corresponding benzoquinone themselves (Thakur and Karak 2012; Sykam et al. 2018).

Nettle extract containing histamine and serotonin was an extremely effective reductant to reduce GO according to a SN2 nucleophilic reaction. The epoxy groups (Fig. 4a) were ring-opened by the nucleophilic attack of NH2 groups of histamine and serotonin, followed by the release of H2O molecules. C=C bonds were thus formed after exiting the intermediate compound. The elimination of carboxylic acid group on GO may occur due to the formation of salt bands (COO−–NH3+) (Fig. 4b). Nettle extract can also protect the resulting rGO from free radicals to decrease weathering and corrosion degradation of the substrates beneath rGO coating (Mahmudzadeh et al. 2019).

Fig. 4
figure 4

Mechanism of the reduction process of histamine and serotonin by a nucleophilic reaction of epoxy groups (a) and formation of salt band with carboxylic acid groups (b) (Mahmudzadeh et al. 2019)

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Diverse plants extraction has recently been used as bio-reductants to produce rGO for different applications. rGO being reduced by algal extract (Ahmad et al. 2019) and sugarcane bagasse extract (Li et al. 2018) was applied to remove Cu, Pb and other heavy metals. Reduction by Spinach (Suresh et al. 2015a), Cinnamomum zeylanicum (Suresh et al. 2015b) and Syzygium aromaticum (Suresh et al. 2015c) provided rGO with excellent dye elimination activity toward organic dye and antioxidant activity toward 2,2-diphenyl-1-picrylhydrazyl free radicals. Plant extraction capping was also proven as fine surfactants for rGO to prevent from aggregation and enhanced the biocompatibility as well. rGO being reduced by Ocimum sanctum (Shubha et al. 2017) and Platanus orientalis leaf extract (Xing et al. 2016) showed less cytotoxic than GO toward Balb 3T3 fibroblasts and cardiac cell lines of Catla, respectively. Extract-reduced rGO via Allium Cepa killed 90.5% of E. coli in 120 h of incubation (Khanam and Hasan 2019), and 500 μg of grape seed extract-reduced rGO was effective in killing nearly 88% colon cancer cells (Yaragalla et al. 2017).

Table 2 summarized some plant extracts that have been used to reduce GO. Generally, eco-friendly plant extractions are abundant in nature and easily isolated after reduction. More importantly, the reduction reaction could be performed at room temperature and atmospheric pressure, which are very crucial factors for industrial production. The obtained rGO exhibits good biocompatibility for further biomedical applications. Among those green reductants, l-ascorbic acid was proven to be a potential alternative to toxic yet powerful reductants like hydrazine hydrate (Fernández-Merino et al. 2010).

Table 2 Bio-reduction reaction of GO using various plant extracts
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Irradiation reduction

Photocatalytic reduction of GO was initially proposed by Williams and his coworker, where GO powders were reduced to rGO in TiO2-ethanol colloidal suspension under UV irradiation (Williams et al. 2008). Epoxy and carboxylate groups of GO were first attached to hydroxyl groups on the photocatalytic reductants such as TiO2, WO3 and LaNiO3, and the holes were then scavenged to produce ethoxy radicals, leaving the electrons to accumulate within the photocatalytic particles to reduce oxygen-containing groups (Choobtashani and Akhavan 2013). Photo-oxidation is regarded as another mechanism to reduce GO under UV or sun lights. Through TiO2-supported photolysis, ethanol was oxidized into acetic acid that eliminated carbonyl and epoxide groups of GO dominantly. The reduction time was only 10–15 min, much faster than the above photocatalysis (Lee et al. 2019). Photocatalytic reduction is a very useful method due to its characteristics of energy saving and high efficiency; however, the irradiation time should be controlled strictly because the photodegradation would decrease the carbon content of rGO (Akhavan et al. 2010). More interestingly, rGO improved the photocatalytic activity of its composites with ZnS (Chen et al. 2019), MoS2 (Li et al. 2019), Boron (Singh et al. 2018), TiO2 (Raghavan et al. 2018; Yang et al. 2016), V2O5 (Aawani et al. 2019) and NiO (Rahimi et al. 2018) because of efficient transfer and rapid separation of photogenerated electrons and holes at the heterojunction. These rGO-composites were considered as one of the most promising materials for photocatalytic degradation of organic pollutant released from leather, plastic, cosmetic and textile industries (Kim et al. 2014a, b).

Microwave reduction utilizes heat transformed from microwave, a form of electromagnetic radiation with frequencies between 300 MHz and 300 GHz. Free-defect basal plane and π electrons of graphene strongly absorb microwave, resulting in the internal and volumetric heating. However, microwave absorption was negatively related to the amount of oxygen-containing groups, and therefore, it was impossible to reduce GO directly. Graphene/GO co-reaction system was reported to successfully reduce GO into rGO, because heat released from graphene would superheat the adjacent GO and contribute to local reduction under the microwave irradiation. Once GO deoxygenated to a certain degree, it would act as another heat absorbent to accelerate the process (Menendez et al. 2010; Hu et al. 2012). Considering that residue graphene can be hardly removed from rGO, Tang et al. (2019) designed a layer of mildly reduced GO membrane as a microwave absorbent and obtained rGO with C/O ratio up to 17.84 under argon at 2000 W for only 30 s.

Laser irradiation causes local heat to trigger the deoxidation reaction of GO, and the reduction degree can be controlled by the irradiation parameters including wavelength, power density, scanning speed, beam energy profile or focusing and processing conditions (Zhang et al. 2014a, b; Yang et al. 2018a, b). During the reduction process, the local temperature raised to approximately 500 °C via a 663-nm continuous wave diode laser (Zhou et al. 2010) and was even higher up to 1400 °C under the irradiation of 1064-nm picosecond laser (Trusovas et al. 2013). Laser irradiation produced rGO in a tunable reduction degree, though the elimination of water molecules and oxygen-containing groups by evaporation might cause the surface defects. Different from most laser sources that required complex and high-cost instrument, continuous wave CO2 laser is an inexpensive technology, showing potential application for mass production of rGO. CO2 laser at the wavelength of 10.6 μm has a far more penetration depth than any kind of laser mentioned above; therefore, the pyrolysis can eliminate extremely fast the oxygen-containing groups of GO to produce rGO with different size of pores randomly arranged in the cross-sectional surface. rGO showed the equivalent series resistance and interface resistance, respectively, of 0.52 Ω and 0.93 Ω and the specific capacitance of 35 F g−1, illustrating excellent electrochemical performances for potential symmetric supercapacitor application (Bhattacharjya et al. 2018).

Plasma, one of the fundamental states of matter, can be artificially generated by heat, high-pressure discharge or strong electromagnetic field. Via activation by electron beam, gas molecules generate various radicals and ions that react with oxygen-containing groups to restore the defects and/or dope atoms on the carbon basal plane (Yu et al. 2013). Several plasma have already been employed in the reduction of GO (Table 3); however, oxygen plasma can even introduce oxygen-containing groups on rGO surface reversely (Kondratowicz et al. 2018). Hydrogen plasma produced atomic hydrogen to react and remove oxygen groups from GO films at low temperatures (< 150 °C) (Lee et al. 2012), much lower temperature than the thermal reduction. Hydrogen plasma removed as much oxygen as chemical reduction by NaBH4, but it was simpler, faster and cleaner, and the remaining oxygen formed labile groups (Alotaibi et al. 2018). However, hydrogen plasma treatment might lead to high defective rGO and was often limited by the etching effect of active hydrogen atoms (Li et al. 2015). A CH4 remote plasma for GO pre-treated via chemical vapor deposition was reported to reduce the defect greatly. rGO became thinner and smoother due to eliminating sp3 carbon domains as the treatment time increased (Cheng et al. 2012). Under the flux of Ar/CH4 plasma using Ar as a promoter, CH4 molecules were dissociated into CHx (X < 4) radicals and active H ions through the collision with high-energy electrons, which can react with oxygen-containing groups of GO. The remaining CHx radicals and amorphous carbon species were then deposited on the carbon lattice (Yang et al. 2018a, b). Ar/CH4 mixture plasma provided a potential method to produce thin film because of low impact on GO surface, tunable oxygen content and growth of sp2 cluster (Baraket et al. 2010). NH3 plasma was not only able to reduce GO, but also to introduce nitrogen element on the carbon lattice to alter the electrical conductivity and transmittance greatly. The conductivity of rGO films was reported to increase from 100 to 1666 S/m with plasma treatment time (Kim et al. 2013; Kim and Choi 2014). Plasma exfoliation under several gas (O2, N2, CH4) can prepare graphene from GO at room temperature efficiently and environment friendly. It was believed that oxygen was produced during the treatment due to the breaking of C–O bond and generating oxygen molecules under the ion bombardment. GO-CH4 based electrodes showed excellent power and energy density (597.2 W kg−1 at 14.93 Wh kg−1), demonstrating their potential applications as supercapacitors (Wang et al. 2015). Nevertheless, most plasma treatment required high sophisticated equipment, specific environment and pumping system and can use only small size of samples in batch mode that limited their scalability and application for large samples and mass production of films and devices.

Table 3 Properties and reaction condition of rGO via different plasma reduction
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More interestingly, plasma or laser irradiation has also demonstrated its capability in patterning rGO. Atmospheric plasma with high electron densities (> 1013 cm−3) and energetic electrons (> 20 eV) produced reactive gas species and radicals to ionize and remove the oxygen groups on GO. A short reduction time of 60 s at room temperature provided rGO films with a resistance of 160 Ω/sq without damaging their structure, which was impossible to achieve by conventional wet chemical and thermal reduction process. While combined with a x–y scanning plasma software, rGO films were also allowed to introduce various patterns on the surface of glass, plastic or textile with various shapes, showing patterning capability with low defects and high conductivity (Alotaibi et al. 2018). rGO lines being doped via a femtosecond laser (λ = 795 nm) exhibited 5 order-of-magnitude decrease in resistivity compared to a non-irradiated sample (Kang et al. 2018). Periodic patterns of rGO being obtained by a femtosecond laser at higher fluences than the reduction of GO can modify the surface chemical composition, and the ripples appeared either after or simultaneously with the reduction reaction. However, the authors did not entirely interpret the formation of laser-induced periodic patterns on rGO (Kasischke et al. 2018). A single technique to simultaneously reduce and periodically pattern GO can accelerate and simplify the production of graphene nanostructures, which will be prone to realize the complex integration of graphene-based devices.

Generally, irradiation reduction is superior to other methods in the aspect of extremely short reaction period and effectively repairing defects in the carbon plane, although relatively expensive equipment and special condition are usually needed.

Electrochemical reduction

Electrochemical reduction derived from the electron being transferred from anode to GO in a standard electrochemical cell, where reaction parameters including the applied voltage, electrical current, electrolyte solution and reduction time played important roles in GO reduction (Toh et al. 2014; Alanyalioglu et al. 2012). Theoretically, the increase in reaction time led to a large degree of GO reduction, and overpotential was often required to achieve more complete rGO (Kauppila et al. 2013). Since organic solvents had broader potential windows than aqueous solutions, more negative potentials would be applied to obtain rGO with a very high C/O ratio on the surface of Si substrate (Marrani et al. 2018). Acidic or near neutral electrochemical medium was also reported to be suitable for chemical reduction of GO (Dreyer et al. 2010).

The electrochemical reduction can be classified into one-step and two-step approaches. In the one-step electrochemical reduction, the negatively charged GO in electrolytic buffer was attracted by anode, and rGO thin film was eventually produced on the electrode surface due to the different solubility between GO and rGO (Liu et al. 2011). In the two-step electrochemical reduction, GO thin film was pre-assembled onto anode and then subjected to reduction using a standard three-electrode system in the presence of a buffer or supporting electrolyte to produce rGO films. Compared with the one-step electrochemical reduction, the latter can obtain desirable size, shape and thickness of rGO by controlling the deposition on electrode, which relied on the deposition techniques and parameters used (Eda and Chhowalla 2010; Paredes et al. 2008).

The oxygen-containing groups of GO were reported to be eliminated at different potentials during the electrochemical reduction. For example, the carbonyl groups were reduced at − 1.3 V, while hydroxy and epoxyl groups at − 1.5 V (Guo et al. 2009). More interestingly, the amount of hydroxyl groups reduced slightly or even increased during the elimination of epoxyl groups, perhaps due to the fact that the hydroxy bonds remained as carboxylate and epoxyl groups were reduced (Marrani et al. 2018; García-Argumánez et al. 2019). As illustrated in Fig. 5, carboxylate groups were eliminated and the resulting α-rGO was further reduced to β-rGO (Fig. 5a), where methyl groups were formed instead of the remanent oxygen-containing groups and a π-conjugated lattice was formed either (Kmen et al. 2017). On the other hand, carbonyl groups were converted into hydroxyl groups at the edge of rGO through one-electron approach (Fig. 5b), and epoxyl groups were reported to convert into C=C bonds through two-electron mechanism (Fig. 5c) (Marrani et al. 2019). In the electrochemical reducing process, the transfer of electrons to rGO was carried out by the proton transfer involving the solvent. Electrochemical reduction is extremely useful when directly fabricating symmetric rGO-based supercapacitor, biosensor, electrocatalysis without further treatment. It is also regarded as an eco-friendly, rapid method to produce rGO.

Fig. 5
figure 5

Mechanism of GO reduction via different process: a two-step reaction of reducing carboxylic acid groups (Kmen et al. 2017); b one-step reaction of reducing carbonyl groups via one-electron approach; c one-step reaction of reducing epoxy groups via two-electron approach (Marrani et al. 2019)

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Microbial reduction

Microorganisms that metabolize with energy release and electron transfer are able to utilize some molecules to displace the deficiency of the corresponding essential substance such as respiratory substrate. The possibility for GO to participate in the metabolism has inspired scientists to synthesis rGO by incubating with microorganisms. Shewanella species was proven as GO-reduced bacterial via respiration in an anaerobic or aerobic condition (Salas et al. 2010). Interior electrons being produced by the bacterial respiration were transferred to GO and ultimately led to the reduction. This process was mediated by outer membrane c-type cytochromes (Mtr/Omc) and self-secreted electron mediators, in which MtrA, MtrB and CymA comprised the transfer pathway, while MtrF was dispensable (Jiao et al. 2011). E. coli was also reported to reduce GO in mixed-acid fermentation under an anaerobic condition through electron transference in metabolic activity (Jiao et al. 2011). Interestingly, the proliferation of E. coil on rGO surface decreased significantly while compared with GO, in consistent with “self limiting” or “self killing” effect that the cytotoxicity of rGO was higher than GO (Zou et al. 2016; Jiao et al. 2011).

Bio-enzyme being produced by bacteria could remove the oxygen-containing groups directly or indirectly. Baker’s yeast containing NAPDH was utilized to reduce the epoxy ketones of GO, forming C-NH on the surface. Nitrogenase from Azotobacter chroococcum was an effective bio-enzyme when bound on femo-cofactor, where the oxygen-containing groups received electrons/protons and were eventually removed by dehydration reactions. However, C/O ratios of rGO reduced by baker’s Yeast and Azotobacter chroococcum were, respectively, 5.9 and 4.18, much lower than the reduction process by NaBH4 (15.1) and hydrazine (10.4) (Khanra et al. 2012; Chen et al. 2017). Other bacteria, such as Bacillus sibtilus (Zhang et al. 2013a, b), Extremophiles bacteria (Raveendran et al. 2013) and Gluconacetobacter xylinus (Nandgaonkar et al. 2014), have also been used to reduce GO. Microbial reduction is a potential method for large-scale rGO synthesis in an eco-friendly, cost effective and simple way, though it needs long reduction time and careful culture procedure.

Conclusions and major challenges

The reduction routes of GO and the possible mechanism were reviewed with an aim at promoting the application of graphene and its derivatives (Table 4), though no optimal method was settled especially for large-scale industrial production. Various oxygen-containing groups on GO sheets were removed via different routes chemically or physically, and these methods showed confining high activity toward certain oxygen functional groups. Because precise oxidation of GO via a reliable technique was still questionable, it was very difficult to control the reduction degree precisely and repeatedly. Therefore, multistep reduction strategies might allow us to eliminate specific functional groups more precisely. In addition, method combining reduction with pattern of rGO was a fast and simple direct treatment to produce conductive rGO films and print different patterns on GO device under environmentally friendly conditions without damaging the underlying substrate. It still needs a deeper understanding of graphene structure and fabrication technology for scalable production of these rGO-based devices.

Table 4 Four main reduction methods
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