Versatile graphene oxide nanosheets via covalent functionalization and their applications

Minju Park† ^a, Namhee Kim† ^a, Jiyoung Lee† ^a, Minsu Gu *^b and Byeong-Su Kim *^a
aDepartment of Chemistry, Yonsei University, 50 Yonsei-ro, Seoul 03722, Republic of Korea. E-mail: bskim19@yonsei.ac.kr
^bDepartment of Chemical Engineering (BK21 FOUR), Dong-A University, Busan 49315, Republic of Korea. E-mail: sbgms@dau.ac.kr

First published on 13th April 2021

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Minju Park

Minju Park received her BS from the Department of Chemical Engineering at Ulsan National Institute of Science and Technology (UNIST), Republic of Korea, in 2014 and her PhD from the Department of Chemical Engineering at UNIST in 2020 under the supervision of Prof. Byeong-Su Kim. She is currently a postdoctoral researcher in Dr Heesuk Kim's group at Korea Institute of Science and Technology (KIST). Her research interests include carbon-based nanomaterials and polymers for energy related applications.

Namhee Kim

Namhee Kim is currently an MS/PhD student in Prof. Byeong-Su Kim's group in the Department of Chemistry at Yonsei University. She received her BS from the Department of Chemistry at Chungnam National University, Republic of Korea, in 2020. Her research interests include nanomaterials and polymers for energy conversion.

Jiyoung Lee

Jiyoung Lee is currently a PhD student in Prof. Byeong-Su Kim's group in the Department of Chemistry at Yonsei University. She received her BS and MS from the Department of Chemistry at Soongsil University, Republic of Korea, in 2018 and 2020 under the supervision of Prof. Ik-Soo Shin. Her research interests include nanomaterials and polymers for energy conversion.

Minsu Gu

Minsu Gu is an Assistant Professor in the Department of Chemical Engineering at Dong-A University, Republic of Korea. He received his BS from the School of Nano-Bioscience and Chemical Engineering at Ulsan National Institute of Science and Technology (UNIST), Republic of Korea, in 2013, and his PhD from the Department of Energy Engineering at UNIST in 2018 under the supervision of Prof. Byeong-Su Kim. He was a postdoctoral researcher in Dr Allen J. Bard's group in the Department of Chemistry at the University of Texas at Austin. His research interests include the electrochemistry of functional nanomaterials and polymers for energy conversion and storage applications.

Byeong-Su Kim

Byeong-Su Kim is a Professor in the Department of Chemistry at Yonsei University, Republic of Korea. He received his BS and MS in Chemistry at Seoul National University and his PhD in Chemistry at the University of Minnesota, Twin Cities, in 2007. After his postdoctoral research at MIT, he started his independent career at UNIST in 2009 and recently moved to Yonsei University in 2018. His group investigated a broad range of topics in macromolecular chemistry for the study of novel polymer and hybrid nanomaterials, including the molecular design and synthesis of self-assembled polymers and carbon-based nanostructures.

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1. Introduction

Sustainability is becoming the most important aspect when developing new materials and applications. Carbon-based materials are considered paradigmatic examples of sustainability due to their high surface area, scalable production, tunable surface chemistry, and environmentally benign properties. In this regard, carbon-based materials have remained at the forefront of many fields from nanotechnology to devices and are receiving increasing attention as part of the imminent ‘Carbon Age’.^1–4

Among the allotropes of the mother element, such as graphite, diamond, and fullerenes, much interest has been focused on carbon nanotubes and graphene with anisotropic one- and two-dimensional structures, respectively, with sp²-conjugated carbon atoms densely packed in a honeycomb crystal lattice.^5,6 These unique chemical structures provide a broad spectrum of outstanding properties, such as extremely high mechanical strength, high thermal conductivity, high stability, and rich electronic properties.^7,8

To utilize carbon nanomaterials more efficiently, non-covalent functionalization has been widely exploited,⁹ based on the van der Waals forces^10–12 and π–π interactions^13–15 with organic molecules or polymers. Because of this relative weak non-covalent interaction, non-covalently functionalized carbon nanomaterials often suffer from poor dispersibility in common organic solvents and other materials, which poses critical challenges for practical applications.^16,17 On the other hand, covalent modification can offer a robust approach toward the preparation of GO derivatives with desired applications. To address these issues, a covalent chemistry approach can be employed to modify the surfaces of carbon nanomaterials for the reliable synthesis of carbon derivatives.^18,19 Furthermore, covalent functionalization of carbon nanomaterials can offer a powerful means to tailor the physical and chemical properties, providing them more practical opportunities for diverse applications. In this context, graphene oxide (GO) is an attractive alternative to pristine graphene due to its diverse functionalities, which can potentially provide great opportunity for further modifications. GO nanosheet with various reactive functional groups such as carboxylic acid, epoxide, or hydroxyl groups are prerequisites, which are generated by oxidation of graphite with strong oxidants and subsequent exfoliation of the resulting graphite oxide.^20–23 In 1859, Brodie first synthesized GO by adding potassium chlorate to graphite with fuming nitric acid.²⁴ After that, Hummers’ method has commonly used in the synthesis of GO with potassium permanganate and sodium nitrate in concentrated sulfuric acid.²⁰ The most widely accepted model for GO is based on Lerf and Klinowski's model, which contains an sp²-hybridized carbon region and oxygen-containing functional groups on the surface.²⁵

The high degree of functionalization of GO via its multitude of epoxide, hydroxyl, and carboxylic acid groups makes it suitable for further derivatization.^22,26 In particular, significant efforts have been focused on the covalent functionalization of GO via exploiting classical organic transformations to tune the material's physicochemical properties, comparing to modification of chemically inert graphene.^19,27 Unlike non-covalent functionalization, covalent chemistry can offer a more robust approach toward the preparation of GO derivatives with the desired molecules to tailor the properties of pristine GO for target applications. Moreover, the electrical properties and sp² lattice of GO could be partially recovered through either the thermal treatments or chemical reduction. Specifically, thermal annealing under reductive environments and chemical reduction with strong reducing agents such as hydrazine,^28,29 metal hydrides,³⁰ and HI³¹ are widely employed to provide a promising alternative toward the large-scale synthesis of GO nanosheets with improved electrical properties.^32,33

While the properties and application of GO have been extensively reviewed so far, in this review, we aim to cover the recent progress in the covalent functionalization of GO and its subsequent applications (Fig. 1). Specifically, we address how chemical modification can tune the properties of GO, such as dispersibility, electrical and thermal conductivity, and catalytic activity, toward realizing the desired application.

2. Covalent functionalization

Functionalization of GO is categorized into two parts: edge-group and basal-plane modification of GO nanosheets (Fig. 2).

2.1 Edge-group modification

In most approaches, carboxylic acid groups are transformed into active sites and used for conversion into esters or amides (Fig. 2b). In general, carboxylic acid groups on GO nanosheets can be activated by using thionyl chloride (SOCl₂) to produce the reactive intermediate, acyl chloride, for further condensation reactions.^34–36 Furthermore, the carboxylic acid group can be activated by coupling agents such as 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC),^37–39 or N,N′-dicyclohexylcarbodiimide (DCC),⁴⁰ followed by condensation with nucleophilic alcohols or amines, typically in excess, to proceed the reaction. In carbodiimide coupling chemistry, GO is exfoliated in a polar solvent (typically N,N′-dimethylformamide (DMF), 1-methyl-2-pyrrolidinone (NMP), or water) and then treated with an activator such as 4-dimethylaminopyridine (DMAP) or N-hydroxysuccinimide (NHS).

As a representative example, our group has widely employed EDC chemistry for the modification of carboxylic acid groups.^39,41–45 For example, the carboxylic acids in GO can be replaced with amine groups through the formation of amides using various amines such as ethylenediamine and ethanolamine. Furthermore, a multitude of macromolecules can be employed for the edge functionalization of GO nanosheet, including chitosan, poly(ethylene glycol) (PEG), polydiacetylenes (PDA), polyacrylamide (PAM), polyurethane (PU), and poly(methyl methacrylate) (PMMA).^46–48 Successful functionalization of the carboxylic acid groups on the GO nanosheets was confirmed via Fourier-transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) measurements. In the FT-IR spectra, the peak intensity of the carboxyl groups was considerably decreased after the coupling reaction with amines, whereby the peak at around 1640 cm⁻¹ signified the formation of amide bonds. In addition, XPS measurements confirmed that distinct peaks of heteroatoms such as N and S appeared after functionalization, indicating the presence of the desired molecules covalently incorporated on the GO nanosheets.

Besides esterification or amide formation of carboxylic acid groups, the hydroxyl groups can be functionalized using coupling chemistry.⁴⁹ For example, Zhang group demonstrated that the hydroxyl group of GO could serve as the nucleophile and condense with the carboxylic acid using coupling chemistry. At first, hydroxyl groups can be capped with S-1-dodecyl-S′-(α,α′-dimethyl-α′′-acetic acid)trithiocarbonate (DDAT) molecules through DCC coupling. After that, poly(N-vinylcarbazole) was grafted onto the DDAT-functionalized GO. Furthermore, the hydroxyl groups can react with trialkoxysilanes or alkyltrichlorosilanes to afford siloxy linkage.^50–52 It is also worth noting that Swager group reported the covalent functionalization of hydroxyl groups through a Claisen rearrangement (Fig. 2b).⁵³ Allylic alcohol groups are converted into vinyl allyl ethers by using N,N-dimethylamide groups that are then transformed into the corresponding carboxylate. As a result, these functionalized GO derivatives have dramatically increased solubility in aqueous media.

2.2 Basal plane modification

Another route for modifying GO is derivatization of the basal plane of GO nanosheets through ring-opening reactions involving epoxy functional groups with strong nucleophiles in polar solvents.³⁹ For example, Swager group demonstrated that malononitrile carbanions can form C–C bonds with the epoxy groups of GO (Fig. 2c).⁵⁴ Successful modification was confirmed via XPS analysis, which showed that N was incorporated at around 3.4% during the addition of malononitrile to GO and indicates that the alkyl nitrile groups were bound to the surfaces of the GO nanosheets. Similarly, ethylene glycol (EG) can be functionalized through the nucleophilic reaction between epoxy and hydroxyl groups under strongly basic conditions.³⁹ The functionalized GO derivatives exhibited two additional peaks at 2966 and 2865 cm⁻¹ in FT-IR spectra, indicating asymmetric and symmetric stretching modes of C–H bonds from the ethylene spacer group in EG, respectively. In addition, atomic force microscopy (AFM) images demonstrate that the thickness of GO nanosheet after chemical modification was similar to that of pristine GO, reflecting the mild nature of this synthetic protocol.

Furthermore, highly reactive intermediates such as aryl diazonium salts, carbenes, and nitrenes can be directly functionalized on the aromatic basal plane of GO.⁵⁵ Especially, diazonium chemistry has been widely employed to functionalize the surface of graphene with aryl diazonium salts. Under acidic conditions, the diazonium salt forms an aryl cation by releasing N₂ gas, while the graphene lattice provides an electron for the aryl diazonium ion that is readily incorporated to the sp²-carbon network.⁵⁶ Through this reaction, the sp² hybridization of basal plane could be changed to sp³ upon functionalization. Tour group was the first to demonstrate that various aromatic moieties can be introduced on chemically reduced GO (rGO) nanosheets.⁵⁷ Since diazonium salts can react with the graphitic lattice on the basal plane of GO, pristine GO is reduced with hydrazine to recover the sp²-hybridized graphitic lattice followed by chemical functionalization.⁵⁸ Functional groups of GO improved dispersion stability dramatically in DMF, NMP, and N,N′-dimethylacetamide (DMAC) even under centrifugation forces.³⁹

Click chemistry has been successfully applied in material modification due to its advantages such as excellent functional group tolerance and high selectivity, quantitative yield, and high fidelity under mild experimental conditions with a wide range of chemical species.^59,60 In 2010, Wu group introduced the click chemistry to functionalize graphene.⁶¹ First, azido-terminated polystyrene (PS) was prepared, and GO nanosheets were then functionalized by using propargyl alcohol through acylation. Then, the click reaction was proceeded between alkyne-functionalized GO and azido-terminated PS with copper bromide as the catalyst, which afforded PS-grafted GO nanosheets. After this early investigation, various molecules and polymers have since been anchored to the graphene nanosheets by exploiting click chemistry.^62–65

As we have described in this section, a variety of organic transformations can be readily applied to realize the covalent functionalization of GO nanosheets via the introduction of various molecules.

3. Dispersibility

The dispersibility of GO in various aqueous and organic solvents is critical for expanding its potential for use in a wide variety of applications.⁶⁶ Although the rich oxygen functionality of GO enables its innate dispersion stability in aqueous media, it limits its dispersity in most non-polar organic solvents. To address this challenge, the covalent modification of the surface of GO nanosheets can offer stable GO dispersion in various solvents. The interactions between the solvents and the functional groups introduced reduces the intersheet hydrogen bonding and van der Waals forces between GO nanosheets, thereby increasing the colloidal stability in solvents. For example, sulfanilic acid group was chemically incorporated into GO nanosheets, which resulted in a superior dispersibility in water due to ionic repulsion. On the other hand, polar and non-polar groups introduced into GO nanosheets can induce dipole–dipole and van der Waals interaction with polar and non-polar solvents, respectively. The primary purpose of this section is to present the dispersion behavior of GO in various solvents in the context of covalent functionalization. Specifically, the dispersion stability of GO in lubricating oils is also described, which is relevant to tribological applications of GO nanosheets, by exploiting the weak van der Waals interactions between adjacent graphene lamellae.

3.1 Aqueous suspension

GO prepared by using oxidizing graphite flakes with strong acid offers a layered morphology with various oxygen functionalities on the periphery of the graphitic planes that disrupt the stacking of multilayered GO nanosheets. Dispersions of the resulting highly functionalized GO are stable in aqueous solvents. However, when the oxygen functionalities are removed in the preparation of rGO, the graphene nanosheets lose their water solubility and eventually undergo irreversible aggregation. Therefore, additional processing steps are often required to produce aqueous dispersion with isolated graphene nanosheets.

Samulski and coworkers first reported chemical pathways toward producing isolated and sulfonated graphene that is soluble in aqueous media by reducing GO in two stages.⁶⁷ The pre-reduction of GO was carried out with sodium borohydride at 80 °C for 1 h to remove most of the oxygen-related functionality. After that, sulfonation of GO was then performed in an ice bath with an aryl diazonium salt of sulfanilic acid for 2 h, followed by post-reduction with hydrazine at 100 °C for 24 h to remove any remaining oxygen functionality. The highly negative charge of the sulfonate group was easily generated due to the very low pK_a (−6.62) of the sulfonic acid group inducing significant repulsion between the graphene nanosheets, thus preventing the graphene nanosheets from aggregating in aqueous media and improving aqueous dispersity.

Due to GO's good biocompatibility, excellent mechanical strength, and high electric conductivity, GO can be applied to biomedicine and clinical diagnostics such as biosensor^68–70 and bioenzyme electrodes.⁷¹ Furthermore, GO can be applied to water splitting application due to its superior electron mobility. Therefore, in order to extend the application range of GO in biological and photocatalytic applications, it is important to modify the surface of GO to further increase solubility in water.

3.2 Non-aqueous suspension

The dispersibility of exfoliated GO was initially investigated to understand the interactive forces in different solvents.⁶⁶ GO can be dispersed in only a few solvents such as water and NMP, and is slightly soluble in DMF, and EG. The poor solubility of GO in organic solvents is due to the combination of highly polar surface functional groups and lamellar morphology that gradually leads to aggregation and precipitation. To mitigate this, the covalent functionalization of GO nanosheets with model compounds that resemble the chemical nature of the desired solvent is generally performed to improve the dispersion of GO. As a representative example, GO nanosheets can be chemically functionalized with ethanolamine (EA), ethylene glycol (EG), and sulfanilic acid (SA) to bestow excellent dispersion stability in the respective organic solvent, especially EG as a model coolant (Fig. 3a).³⁹ Specifically, carboxylic acids groups on the GO nanosheets react with amine groups in EA through EDC-mediated surface modification. In parallel, GO functionalized with EG (GO-EG) can be synthesized by ring-opening of the epoxy groups with hydroxy groups under basic conditions. Sulfonated GO (GO-SA) was separately prepared by applying diazonium chemistry through directly anchoring sulfonic acid containing aryl radicals on the surface of the graphitic layer. The functionalized GO derivatives were successfully dispersed in EG at a concentration of 9.0 mg mL⁻¹ (0.50 vol%). Especially, GO-EG and GO-EA showed enhanced dispersibility approximately 96 and 48 times higher than unreacted GO nanosheets, respectively, in EG.

Alkyl and aryl isocyanate modification of GO can induce stable dispersion in polar aprotic solvents by making the hydroxyl and carboxyl groups on the GO nanosheets into carbamate and amide groups, respectively.⁷² In contrast to the parent GO, the isocyanate-treated GO is not dispersed in water at all. However, after a short ultrasonic treatment, stable colloidal dispersions can be readily formed in polar aprotic solvents such as DMF, NMP, dimethyl sulfoxide (DMSO), and hexamethylphosphoramide (HMPA). The phenyl isocyanate-treated GO in DMF forms the dark brown dispersion, which is stable for weeks without forming any visible precipitate.

Yang et al. modified graphite oxide with alkyne-terminated PS via click chemistry.⁷³ Azido-modified GO was synthesized via esterification and substitution of the hydroxyl groups on the surface of GO nanosheets. The PS-grafted graphene is obtained by grafting alkyne-functionalized PS onto the surface of the GO. The functionalized GO displayed excellent solubility in polar solvents such as THF, DMF, and chloroform (Fig. 3b). Besides these methods, the preparation paths for the target hydroxy-related functionalization of GO have been extended to enhance the dispersibility of GO in a variety of organic solvents.

As demonstrated by these examples, significant efforts have been directed toward the covalent functionalization of GO to afford the necessary solubility in organic solvents. These methods will open up further opportunities for a wide range of potential applications of GO nanosheets, such as organocatalysts,^74,75 polymer composites,⁷⁶ and lubricant additives.^77,78 The dispersion stability coupled with the high surface area of the GO nanosheets presents the high catalytic efficiency of organic compounds.⁷⁹ Furthermore, the homogenous dispersion of functionalized GO in solvents and matrix polymers allows the incorporation of GO nanosheets as a conducting filler into polymer composites, resulting in excellent composites possessing high mechanical strength, electrical conductivity, and flexibility.⁸⁰ In addition, the dispersibility of alkylated GO in hydrocarbon solvents is observed to induce long-term dispersion stability in lubricating oil.⁸¹ The weak van der Waals interactions between the atomic-thick lamellar structures of GO sheets reduce shear resistance and lower friction.⁸² The examples of the application of GO as the lubricant additives are described more in the following section.

3.3 Tribological properties

Liquid lubricants are placed to sliding interfaces through oil delivery mechanisms, whereas solid lubricants are applied as thin films by using physical and/or chemical vapor deposition methods.⁸³ The solid lubricant films eventually wear out and lose lubricating effectiveness due to their finite thickness. In addition, they are more sensitive to oxygen, and humidity in the surrounding air, whereas the liquid lubricants are environmentally less sensitive and highly durable, making them more desirable as robust lubricants. Graphene can be potentially used as a lubricant due to its high surface area, superior mechanical strength, and excellent thermal conductivity. In particular, the weak van der Waals interactions between the lamellar structures of graphene play an important role when graphene is used as an additive to lower shear resistance and reduce friction.^84–86 However, the poor long-term dispersion stability of graphene in lubricating oils has created a major challenge for its use in the lubricant industry. This is because graphene tends to become agglomerated in most lubricants due to highly cohesive interactions and its poor dispersibility. As an alternative, chemically reactive GO can be applied as an additive via chemical functionalization to improve its dispersion stability in lubricating oils. For example, the grafting of long alkyl chains onto GO increases the dispersibility of the resulting modified GO in mineral and synthetic lubricating oils. The compatibility of GO in lubricants is primarily driven by the weak van der Waals force between the long alkyl chains grafted on GO and the hydrocarbon moiety of the lubricating base oil.⁸⁷

The carboxyl groups in GO bestow tremendous potential for covalent functionalization with various oil-compatible organic moieties. Specifically, thionyl chloride used as a coupling agent with carboxylic groups can be selectively targeted for alkylamine grafting via amide linkage. The long alkyl chains of octadecylamine (ODA) modified GO (GO-ODA) provide sufficient cohesive interaction with hexadecane, leading to the uniform dispersion of GO-ODA in lubricating oil that, at a concentration of 0.06 mg mL⁻¹, decreased the coefficient of friction and wear scar diameter of hexadecane by 26% and 9%, respectively.⁸⁸ The enhanced tribological performance of GO-ODA was attributed to its uninterrupted presence between the steel ball and friction surface.

Khatri and coworkers also prepared GO-ODA that was well dispersible in lubricating oil.⁸⁹ Simultaneous reduction of the oxygen functional groups in GO restores the graphitic structure, and the resulting GO-ODA nanosheets at the contact interfaces not only protect the surface against tribo-damage from undesirable wear but also significantly reduce friction. In another approach, oleic diethanolamide borate (ODAB) has been selectively functionalized on GO through amide linkage (Fig. 3c).⁹⁰ The resultant GO-ODAB showed excellent dispersibility in the base oil, and using 0.02 wt% GO-ODAB between the steel ball and friction surface reduced the coefficient of friction and wear scar diameter by 38.4% and 42%, respectively (Fig. 3d).

Dodecyl chains were grafted on the surface of GO via click chemistry. Alkyne-functionalized GO is prepared via amide linkage with carboxylic acid groups of GO, followed by the click coupling between alkyne grafted GO and azidodecane. The resulting decane-functionalized GO as an additive for petroleum lube oil enhanced the tribological properties by reducing the friction and wear by 16% and 30%, respectively.⁹¹ Therefore, solid lubricant additives are very important for low friction and protection of contact surfaces, which can be achieved by dispersion of graphene nanosheets in the applicable lubrication oil.

Over the last decade, considerable efforts have been made for covalent functionalization of GO to achieve stable dispersion in lubrication oils, and the poor solubility of GO in lubricants has primarily been overcome by introducing hydrophobic chains like alkyl groups on the surface of the GO nanosheet. Thus, covalent chemical functionalization of GO can afford novel additives that are stably dispersible in lubricating oils.

4. Conductivity

Chemically oxidized GO nanosheets produced by oxidation and exfoliation has been difficult to apply in the field of electronic devices since GO itself exhibits low electrical properties due to the presence of various defect sites in the GO nanosheets.^48,92 As the oxygen content of GO increases, the sp² structure of GO is damaged, resulting in a decrease in electrical conductivity, particularly when the oxygen content exceeds 25 wt%.²¹ To improve the electrical properties of GO by restoring the sp² network, many approaches have been reported to thermally or chemically reduce GO toward energy applications.

Meanwhile, the hydrophilic functional groups on the surface and edges of GO together with its large specific surface area improve the ionic conductivity.⁹³ By functionalization of GO, more interconnected ion transport channels can be provided,^94,95 and the ion transport pathway can be shortened by reducing interlayer interaction to further improve ionic conductivity.⁹⁶ In this section, we describe the examples of improving the electrical and/or ionic conductivity of GO through covalent functionalization toward its use in various applications such as supercapacitors, sensors, batteries, and flexible devices.⁹⁷

4.1 Electrical conductivity

Kong and coworkers reported the covalent functionalization of reduced GO aerogel (RGOA) through diazotization and amidation to graft a conducting polymer, polyaniline (PANI) for capacitive material onto the surface of RGOA (Fig. 4a).⁹⁸ Covalent linkage between the RGOA and PANI enables not only to enhance electrical conductivity, but also to prevent the agglomeration of graphene nanosheets while increasing the surface area of the RGOA. As a result, the PANI-grafted RGOA exhibited higher capacitive performance (396 F g⁻¹ at 10 A g⁻¹) than that of RGOA (183 F g⁻¹ at 10 A g⁻¹). The PANI-grafted RGOA also demonstrated a high cycling stability of 56% capacitance retention after 4000 charge/discharge cycles at 10 A g⁻¹. These features showed that PANI-grafted RGOA is suitable for high-performance supercapacitor electrodes.

In another approach, Lu and coworkers fabricated polydopamine-reduced GO (pGO)-incorporated into a chitosan (CS) and silk fibroin (SF) scaffold (pGO-CS/SF) to develop an electroactive wound dressing that can respond to physiological electrical signals (Fig. 4b).⁹⁹ In this strategy, functionalized GO with the catechol moiety was reduced to pGO upon self-polymerization of dopamine, resulting in restoring the high conductivity of pGO. Inherently, CS/SF is non-conductive, but along with an increase in pGO content, the electrical conductivity of the CS/SF scaffold was increased significantly. Also, it is worth to note that pGO demonstrated higher conductivity than the control group using hydrazine-reduced GO (hGO), which only had the conductivity of 0.1 S cm⁻¹. Since pGO was well distributed in the scaffold along with the formation of electronic pathway, it resulted in improved electrical conductivity. The maximum conductivity of pGO-CS/SF reached 0.26 S cm⁻¹, which enables the transmittance of electric signals to regulate cellular activities.

Sheremet and co-workers produced single-layer diazonium-functionalized graphene through a strategy to improve electrical conductivity by cleaving C–aryl group bonds through laser irradiation (LMod-G) (Fig. 4c).¹⁰⁰ The strategy is unusual in that it differs from the conventional case of functionalized GO that becomes conductive owing to the removal of oxygen-containing groups and reduction. The cleavage of C–aryl group bonds via laser irradiation affected the transformation of sp³-hybridized carbon structures to sp²-hybridized network with the restoration of the conjugated carbon structure. As a result, diazonium-functionalized graphene with a low sp³-carbon content was formed, and the electrons can flow freely through the LMod-G film, resulting in excellent electrical conductivity. The conductance of LMod-G with 2.7 mm thickness was 275.8 × 10⁻⁹ S. Due to this improved electrical conductivity, LMod-G showed significantly improved sensitivity of up to 15-fold than that of GO to breath (water vapor and CO₂) and the detection of ethanol, thereby demonstrating its applicability as a breath sensor.

4.2 Ionic conductivity

Kim et al. has produced ionic-liquid modified reduced GO (rGO-IL) nanosheets anchoring manganese oxide (Mn₃O₄) (Fig. 5a).¹⁰¹ The introduction of the ionic-liquid moiety increased the solubility of GO in a wide range of solvents, which can facilitate the separation of ionic groups. Hence, the ionic conductivity of GO was enhanced and Mn₃O₄ nanoparticles could easily grow onto the rGO-IL nanosheets via electrostatic interactions and hydrolysis. Especially, rGO-IL/Mn₃O₄ (10:1) nanoparticles displayed a surface resistance of 61.1 ohm sq⁻¹, indicating higher conductivity than rGO-IL/Mn₃O₄ (2:1) with a higher Mn₃O₄ ratio (52.5%), in which the surface resistance was 120.3 ohm sq⁻¹. Furthermore, rGO-IL/Mn₃O₄ (10:1) had a maximum peak power density of 120 mW cm⁻², showing the possibility of its application as a cathode material in Zn–air batteries.

Kowsari and Chirani prepared covalent ionic-liquid-functionalized GO (GO-IL) and its derivatives as a highly effective electrolyte additive for dye-sensitized solar cells (Fig. 5b).¹⁰² They demonstrated that GO-IL formed a molecular bridge for electron transfer in the IL-based electrolyte due to the presence of polar alkyl chains improving the movement of free ions in an arranged pathway. As a result, the ionic conductivity improved by approximately two-fold to 3.14 × 10⁻⁴ S cm⁻¹ when the GO-IL additive was used in the standard electrolyte.

In another notable example, Jiang and coworkers reported that the ionic conductivity of GO was improved by sulfonation (Fig. 5c).¹⁰³ In this study, GO was sulfonated with 3-(trihydroxysilyl)-1-propanesulfonic acid and subsequently neutralized with an amino-terminated polyoxypropylene-b-polyoxyethylene block copolymer (NIMs-GO). The synthesized NIMS-GO formed a very efficient proton-hopping site and a wide range of hydrogen-bonding networks by the introduction of sulfonic acid/amine acid–base pairs. In addition, owing to the introduction of the polyoxyethylene group, its water uptake and retention ability was enhanced even under low humidity operating conditions, and proton transfer occurred smoothly. Consequently, the NIMs-GO membrane exhibited a 52% increase in proton conductivity compared to a pristine sulfonated polysulfone membrane at 75 °C under 100% relative humidity, suggesting its high potential in the proton exchange membrane of fuel cells.

5. Catalytic activity

5.1 Carbocatalysts

In general, most previous studies have reported using GO as a platform to provide an anchoring site for active materials and to enhance catalytic activity. In this context, covalent functionalization can be used to introduce active ligands that can bind to metals and metal ions to improve catalytic performance. For example, our group reported GO functionalized with the mussel-inspired chemical moiety, dopamine (GO-Dopa).¹⁰⁴ The catechol groups of dopamine offer electrons for reducing Ag ions to form Ag nanoparticles on the surface of GO-Dopa. The resulting hybrid Ag/GO-Dopa was utilized as an organocatalyst that exhibited excellent catalytic activity in the reduction of nitroarenes.

Alternatively, GO and its derivatives are attracting considerable attention as promising metal-free heterogeneous catalysts owing to their high surface area, easy recyclability, and the variety of functional groups. In the case of pristine GO, its acidic and oxidative nature allows it to serve as a heterogeneous acid or oxidant.¹⁰⁵ In a pioneering study, Bielawski group showed that GO could serve as an efficient catalyst for the aerobic oxidation of benzylic hydrocarbons with high conversion (>98%) and selectivity (>95%) under mild conditions.¹⁰⁶ Moreover, the catalytic reactivity could be sustained for up to 10 cycles, albeit with a modest reduction after the first run.

Loh group further developed the application of GO in various organic reactions (Fig. 6a).¹⁰⁷ As a notable example, they synthesized base- and acid-treated GO that displayed high catalytic activity for the aerobic oxidation of primary amines to their corresponding imines. Furthermore, the results from a mechanistic investigation performed on a model system suggest that carboxylic acid groups play a vital role in the catalytic reactions. These oxidation reactions with GO open the possibility of using it as the carbocatalyst for other aerobic oxidation reactions.

GO derivatives as heterogeneous catalysts can be engineered by covalent functionalization to anchor active sites (Table 1). Sulfonated GO (S-GO), which is functionalized with phenylsulfonic acid groups by diazonium chemistry, is a representative example of a heterogeneous acid catalyst due to its low pK_a value of −6.62 (Fig. 2).³⁹ It has been found that S-GO demonstrated high activity for the hydrolysis of ethyl acetate and could be recycled several times without chemical degradation (Fig. 6b).⁵⁸

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Li group first reported a GO-based acid–base bifunctional catalyst by using the intrinsic carboxylic acids on the edges and the amine groups post-grafted onto the GO basal surface (Fig. 6c).¹⁰⁸ Amine groups incorporated via silylation showed good catalytic activity in one-pot deacetalization-Knoevenagel cascade reactions. Since GO-based catalysts create isolated regions of acid–base functionality, they reacted independently with different reactants, resulting in high selectivity and yields. Further studies and the detailed utilization of GO derivatives as carbocatalysts are summarized in Table 1.

More recently, several groups have developed GO-based carbocatalysts for the multi-step conversion of biomass chemicals to value-added products.^109,110 Biomass feedstocks such as cellulose and biomass-derived substrates hold significant potential for future industrial applications. The application of efficient heterogeneous catalysts to transform biomass into valuable products is one of the main current challenges in green chemistry. In this context, reduced sulfonated GO obtained from the reaction of rGO using a sulfanilic acid diazonium salt intermediate was employed as an acid catalyst to produce furfural from xylose solution.¹¹¹ In addition, a reusable reduced sulfonated GO catalyst for the catalytic dehydration of xylose produced a high yield of furfural (∼66%). Similarly, it has been reported that sulfonated GO can serve as a solid catalyst for the conversion of 5-hydroxymethylfurfural into biofuels such as alkyl levulinate and 5-alkoxymethylfurfural.¹¹² Compared to commercial catalysts (Amberlyst-15), the sulfonated GO produced a high yield with the necessary stability over consecutive catalytic runs.

As covered in this section, GO-based heterogeneous catalysts possess extraordinary advantages in terms of sustainability and tunability when designing the active catalytic centers to promote the desired catalytic reaction.

5.2 Photocatalysts

Efficient photocatalysts can be designed by combining photo-responsive materials with GO to accelerate photoreactions with electron–hole pair generation. Since the carrier mobility and high specific area of GO enables the efficient charge transfer in the hybrid material, GO has been applied for donor–acceptor hybrid materials with photoactive moieties. Thus, the notable electron mobility of GO can enhance the photocatalytic activity of conventional materials that suffer from rapid recombination of electron–hole pairs. In this section, we describe the photo-responsive properties of GO derivatives and their applications in photocatalysis and photovoltaics.

Ru complexes have been widely used as a photosensitizer due to their excellent photostability, light-harvesting capability, and efficient charge transport behavior. As a representative example, Jain group synthesized a heterogeneous photocatalyst comprising a GO-based Ru composite for the photocatalytic CO₂ reduction to methanol under visible light irradiation (Fig. 7a).¹²⁰ Specifically, the trinuclear Ru complex based on phenanthroline ligands was grafted onto the carboxylic acid group on the GO surface. The Ru complex facilitates the transportation of photoinduced electrons from it to the conduction band of GO. As a result, the GO–Ru complex displayed a reduction in the CO₂ to methanol yield of 3977.57 ± 5.60 μmol g_cat⁻¹ under a 20 W white cold LED, which is a higher photocatalytic performance than GO without the Ru complex (Fig. 7b). In addition, these catalysts could be easily recovered and reused for 4 subsequent runs without a significant loss of catalytic activity.

In another approach, Yao et al. synthesized a photocatalyst consisting of rGO nanosheets as an electron acceptor and covalent organic frameworks (COFs) as a photoactive material (rGO-COF) via the facile grafting of phenylenediamine to rGO, which was then incorporated into the synthetic process for the COFs.¹²¹ The resulting rGO-COF showed an enhanced H₂ evolution rate of 11.98 mmol g⁻¹ h under visible-light irradiation, which was 4.85 and 2.50 times higher than pure COF and GO/COF without any covalent coupling between the two components, respectively. The enhanced photocatalytic property was attributed to the covalently grafted rGO in the composite that not only promotes the separation of photogenerated charges as an electron acceptor but also serves as an active electron transporter. Furthermore, the uniform morphology of the evenly distributed COFs on the layered rGO nanosheets can facilitate the migration of photoexcited electrons.

Fang and coworkers reported that rGO nanosheets grafted with polypyridyl-Ru-derivatized polystyrene (PS-Ru) exhibited enhanced photocurrent and power conversion efficiency over five-fold higher than those of devices without rGO (Fig. 7c and d).¹²² Ru(II) polypyridine derivatized polymer was formed from methyl bromoisobutyrate initiation units on the hydroxyl group of rGO by atom transfer radical polymerization. The graphene moiety in the hybrid material acted as an excellent supporting matrix for the photo-responsive Ru complexes as well as offering superior electron transfer to suppress the recombination of photoinduced charges.

Furthermore, Vinoth et al. synthesized a hybrid material consisting of rGO and Ru complex with polyaniline (PANI–Ru) that was grafted onto the hydroxyl groups of the rGO nanosheets via covalent functionalization to afford rGO/PANI–Ru.¹²³ The chemical connection between PANI–Ru and rGO facilitates the photogenerated electron transfer from the Ru complex to rGO through the backbone of the conjugated PANI polymer chains. Polymer solar cells fabricated with rGO/PANI–Ru attained an almost 6-fold and 2-fold improvement in open circuit potential (V_oc) and short circuit current density (J_sc) compared to the control device using PANI–Ru without rGO under the illumination of AM1.5G. The superior electron transfer and charge separation characteristics of rGO further promoted the electron injection from PANI–Ru to phenyl-C₆₁-butyric acid methyl ester (PC₆₀BM) acting as an electron acceptor polymer that consequently improved the overall performance of the polymer solar cells.

These several outstanding examples demonstrate that covalent functionalization chemistry on the surface of GO can enhance photocatalytic activity by combining it with photoactive materials, resulting in the synergistic effect of functional moieties and an efficient charge transfer capacity. Combining various photo-responsive materials with GO will possibly expand GO's potential for use in a wider range of applications (Table 2).

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5.3 Electrocatalysts

To overcome energy crises and environmental pollution, numerous efforts have been devoted to alternative and renewable energy that can be substituted for conventional fossil fuel-based energy system. One promising solution is the utilization of key electrochemical energy conversion reactions, including oxygen evolution reaction (OER), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO₂RR), and nitrogen reduction reaction (NRR).^127,128 Typically, electrocatalysts based on noble metals such as Pt, Ir, and Ru have been widely used for these electrochemical reactions. In particular, Pt/C catalyst is the most widely used commercially owing to its excellent adsorption and charge transfer.^129,130 However, there are still some challenges concerning the high cost, low selectivity, poor stability, and vulnerability to gas poisoning.^131,132 Therefore, significant research endeavor has been devoted to fabricating alternative electrocatalysts with high abundance, excellent efficiency, and good stability.

In this context, modified GO has stood as a new class of metal-free electrocatalyst. Various oxygen- and nitrogen-containing groups in GO can absorb the active reactant and transfer electrons at the electrode interface.^127,133 In this section, we will introduce examples of improving electrocatalytic performance by modification of GO. The quantitative comparison of catalyst performance through parameters such as overpotential, Tafel slope, and limiting current density with pristine GO or commercial Pt/C catalysts is also presented (Table 3).

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6. Summary and outlook

In this review, we presented controllable physicochemical characteristics of versatile GO scaffolds, such as dispersibility, mechanical strength, conductivity, and catalytic active sites, through covalent functionalization with various molecules and polymers via the oxygen-containing chemical functional groups on the edges and basal plane of GO. Without chemical functionalization and hybridization with other counterparts, the simple reduction of GO still poses limitations on acquiring the physical and chemical properties required for various practical applications. On the other hand, the introduction of covalently bonded chemical functionalities to GO can offer a convenient access to functionalized GO nanosheets by introducing desired molecules for target specific properties for various applications, which in turn, can result in high catalytic activity and the development of practical devices.

While there have been tremendous efforts and progress in the fundamental research and technological development of graphene-based materials in diverse fields, there are still some challenges to overcome in the field of functionalization of GO. For example, the preparation of homogeneous and highly reproducible GO samples and a method of accurately accessing the type and stoichiometry of surface functional groups as a single molecular entity remain considerable challenges. With the advances of the novel synthetic protocols and separation techniques such as a bottom-up approach¹⁶² using radical addition reactions and more accurate separation techniques beyond current size filtration^163,164 and centrifugation method¹⁶⁵ to prepare GO nanosheets with defined structures and compositions, we envision there will be enormous opportunities remains to be explored in the future.

Author contributions

All authors contributed to the discussion of contents and the editing of the manuscript prior to submission. M. P., M. G., and B.-S. K. conceptualized the article. M. P. researched data related to covalent functionalization and carbocatalysts. N. K. researched data related to dispersibility, and photocatalysts. J. L. researched data related to conductivity and electrocatalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF-2017M3A7B4052802 and NRF-2018R1A5A1025208).

References

1. M.-M. Titirici, R. J. White, N. Brun, V. L. Budarin, D. S. Su, F. del Monte, J. H. Clark and M. J. MacLachlan, Sustainable carbon materials, Chem. Soc. Rev., 2015, 44, 250–290 RSC .
2. A. H. Castro Neto, The carbon new age, Mater. Today, 2010, 13, 12–17 CrossRef .
3. X. Duan, H. Sun and S. Wang, Metal-free carbocatalysis in advanced oxidation reactions, Acc. Chem. Res., 2018, 51, 678–687 CrossRef CAS PubMed .
4. H. Wang, Y. Shao, S. Mei, Y. Lu, M. Zhang, J.-K. Sun, K. Matyjaszewski, M. Antonietti and J. Yuan, Polymer-derived heteroatom-doped porous carbon materials, Chem. Rev., 2020, 120, 9363–9419 CrossRef CAS PubMed .
5. A. K. Geim, Graphene: Status and prospects, Science, 2009, 324, 1530–1534 CrossRef CAS PubMed .
6. K. S. Novoselov, V. I. Fal′ko, L. Colombo, P. R. Gellert, M. G. Schwab and K. Kim, A roadmap for graphene, Nature, 2012, 490, 192–200 CrossRef CAS PubMed .
7. A. K. Geim and K. S. Novoselov, The rise of graphene, Nat. Mater., 2007, 6, 183–191 CrossRef CAS PubMed .
8. C. N. R. Rao, K. Biswas, K. S. Subrahmanyam and A. Govindaraj, Graphene, the new nanocarbon, J. Mater. Chem., 2009, 19, 2457–2469 RSC .
9. V. Georgakilas, J. N. Tiwari, K. C. Kemp, J. A. Perman, A. B. Bourlinos, K. S. Kim and R. Zboril, Noncovalent functionalization of graphene and graphene oxide for energy materials, biosensing, catalytic, and biomedical applications, Chem. Rev., 2016, 116, 5464–5519 CrossRef CAS PubMed .
10. J. A. Mann, J. Rodríguez-López, H. D. Abruña and W. R. Dichtel, Multivalent binding motifs for the noncovalent functionalization of graphene, J. Am. Chem. Soc., 2011, 133, 17614–17617 CrossRef CAS PubMed .
11. B. Long, M. Manning, M. Burke, B. N. Szafranek, G. Visimberga, D. Thompson, J. C. Greer, I. M. Povey, J. MacHale, G. Lejosne, D. Neumaier and A. J. Quinn, Non-covalent functionalization of graphene using self-assembly of alkane-amines, Adv. Funct. Mater., 2012, 22, 717–725 CrossRef CAS .
12. J. A. Mann and W. R. Dichtel, Noncovalent functionalization of graphene by molecular and polymeric adsorbates, J. Phys. Chem. Lett., 2013, 4, 2649–2657 CrossRef CAS .
13. Y. Xu, H. Bai, G. Lu, C. Li and G. Shi, Flexible graphene films via the filtration of water-soluble noncovalent functionalized graphene sheets, J. Am. Chem. Soc., 2008, 130, 5856–5857 CrossRef CAS PubMed .
14. H. Zhou, A. Uysal, D. M. Anjos, Y. Cai, S. H. Overbury, M. Neurock, J. K. McDonough, Y. Gogotsi and P. Fenter, Understanding defect-stabilized noncovalent functionalization of graphene, Adv. Mater. Interfaces, 2015, 2, 1500277 CrossRef .
15. A. Ghosh, K. V. Rao, S. J. George and C. N. R. Rao, Noncovalent functionalization, exfoliation, and solubilization of graphene in water by employing a fluorescent coronene carboxylate, Chem. – Eur. J., 2010, 16, 2700–2704 CrossRef CAS PubMed .
16. S. Gilje, S. Han, M. Wang, K. L. Wang and R. B. Kaner, A chemical route to graphene for device applications, Nano Lett., 2007, 7, 3394–3398 CrossRef CAS PubMed .
17. V. V. Neklyudov, N. R. Khafizov, I. A. Sedov and A. M. Dimiev, New insights into the solubility of graphene oxide in water and alcohols, Phys. Chem. Chem. Phys., 2017, 19, 17000–17008 RSC .
18. C. K. Chua and M. Pumera, Covalent chemistry on graphene, Chem. Soc. Rev., 2013, 42, 3222–3233 RSC .
19. S. Eigler and A. Hirsch, Chemistry with graphene and graphene oxide—challenges for synthetic chemists, Angew. Chem., Int. Ed., 2014, 53, 7720–7738 CrossRef CAS PubMed .
20. W. S. Hummers and R. E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc., 1958, 80, 1339 CrossRef CAS .
21. D. C. Marcano, D. V. Kosynkin, J. M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L. B. Alemany, W. Lu and J. M. Tour, Improved synthesis of graphene oxide, ACS Nano, 2010, 4, 4806–4814 CrossRef CAS PubMed .
22. D. R. Dreyer, S. Park, C. W. Bielawski and R. S. Ruoff, The chemistry of graphene oxide, Chem. Soc. Rev., 2010, 39, 228–240 RSC .
23. A. M. Dimiev and J. M. Tour, Mechanism of graphene oxide formation, ACS Nano, 2014, 8, 3060–3068 CrossRef CAS PubMed .
24. B. C. Brodie, XIII. On the atomic weight of graphite, Philos. Trans. R. Soc. London, 1859, 149, 249–259 CrossRef .
25. A. Lerf, H. He, M. Forster and J. Klinowski, Structure of graphite oxide revisited, J. Phys. Chem. B, 1998, 102, 4477–4482 CrossRef CAS .
26. W. Gao, Graphene oxide: Reduction recipes, spectroscopy, and applications, Springer International Publishing, 2015, pp. 61–95 Search PubMed .
27. D. R. Dreyer, A. D. Todd and C. W. Bielawski, Harnessing the chemistry of graphene oxide, Chem. Soc. Rev., 2014, 43, 5288–5301 RSC .
28. S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen and R. S. Ruoff, Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide, Carbon, 2007, 45, 1558–1565 CrossRef CAS .
29. S. Stankovich, D. A. Dikin, G. H. B. Dommett, K. M. Kohlhaas, E. J. Zimney, E. A. Stach, R. D. Piner, S. T. Nguyen and R. S. Ruoff, Graphene-based composite materials, Nature, 2006, 442, 282–286 CrossRef CAS PubMed .
30. H.-J. Shin, K. K. Kim, A. Benayad, S.-M. Yoon, H. K. Park, I.-S. Jung, M. H. Jin, H.-K. Jeong, J. M. Kim, J.-Y. Choi and Y. H. Lee, Efficient reduction of graphite oxide by sodium borohydride and its effect on electrical conductance, Adv. Funct. Mater., 2009, 19, 1987–1992 CrossRef CAS .
31. I. K. Moon, J. Lee, R. S. Ruoff and H. Lee, Reduced graphene oxide by chemical graphitization, Nat. Commun., 2010, 1, 73 CrossRef PubMed .
32. S. Pei and H.-M. Cheng, The reduction of graphene oxide, Carbon, 2012, 50, 3210–3228 CrossRef CAS .
33. C. K. Chua and M. Pumera, Chemical reduction of graphene oxide: A synthetic chemistry viewpoint, Chem. Soc. Rev., 2014, 43, 291–312 RSC .
34. D. Yu, Y. Yang, M. Durstock, J.-B. Baek and L. Dai, Soluble P3HT-grafted graphene for efficient bilayer-heterojunction photovoltaic devices, ACS Nano, 2010, 4, 5633–5640 CrossRef CAS PubMed .
35. X.-Z. Tang, W. Li, Z.-Z. Yu, M. A. Rafiee, J. Rafiee, F. Yavari and N. Koratkar, Enhanced thermal stability in graphene oxide covalently functionalized with 2-amino-4,6-didodecylamino-1,3,5-triazine, Carbon, 2011, 49, 1258–1265 CrossRef CAS .
36. Y. Xu, Z. Liu, X. Zhang, Y. Wang, J. Tian, Y. Huang, Y. Ma, X. Zhang and Y. Chen, A graphene hybrid material covalently functionalized with porphyrin: Synthesis and optical limiting property, Adv. Mater., 2009, 21, 1275–1279 CrossRef CAS .
37. H. Bao, Y. Pan, Y. Ping, N. G. Sahoo, T. Wu, L. Li, J. Li and L. H. Gan, Chitosan-functionalized graphene oxide as a nanocarrier for drug and gene delivery, Small, 2011, 7, 1569–1578 CrossRef CAS PubMed .
38. J. T. Robinson, S. M. Tabakman, Y. Liang, H. Wang, H. Sanchez Casalongue, D. Vinh and H. Dai, Ultrasmall reduced graphene oxide with high near-infrared absorbance for photothermal therapy, J. Am. Chem. Soc., 2011, 133, 6825–6831 CrossRef CAS PubMed .
39. M. Park, K. Song, T. Lee, J. Cha, I. Lyo and B.-S. Kim, Tailoring graphene nanosheets for highly improved dispersion stability and quantitative assessment in nonaqueous solvent, ACS Appl. Mater. Interfaces, 2016, 8, 21595–21602 CrossRef CAS PubMed .
40. T. T. Nguyen, P. Bandyopadhyay, X. Li, N. H. Kim and J. H. Lee, Effects of grafting methods for functionalization of graphene oxide by dodecylamine on the physical properties of its polyurethane nanocomposites, J. Membr. Sci., 2017, 540, 108–119 CrossRef CAS .
41. M. Park, T. Lee and B.-S. Kim, Covalent functionalization based heteroatom doped graphene nanosheet as a metal-free electrocatalyst for oxygen reduction reaction, Nanoscale, 2013, 5, 12255–12260 RSC .
42. H. Hwang, P. Joo, M. S. Kang, G. Ahn, J. T. Han, B.-S. Kim and J. H. Cho, Highly tunable charge transport in layer-by-layer assembled graphene transistors, ACS Nano, 2012, 6, 2432–2440 CrossRef CAS PubMed .
43. B. Park, W. Lee, E. Lee, S. H. Min and B.-S. Kim, Highly tunable interfacial adhesion of glass fiber by hybrid multilayers of graphene oxide and aramid nanofiber, ACS Appl. Mater. Interfaces, 2015, 7, 3329–3334 CrossRef CAS PubMed .
44. Y. Choi, D. Jeon, Y. Choi, D. Kim, N. Kim, M. Gu, S. Bae, T. Lee, H.-W. Lee, B.-S. Kim and J. Ryu, Interface engineering of hematite with nacre-like catalytic multilayers for solar water oxidation, ACS Nano, 2019, 13, 467–475 CrossRef CAS PubMed .
45. E. Ahn, T. Lee, M. Gu, M. Park, S. H. Min and B.-S. Kim, Layer-by-layer assembly for graphene-based multilayer nanocomposites: The field manual, Chem. Mater., 2017, 29, 69–79 CrossRef CAS .
46. Y. Lin, J. Jin and M. Song, Preparation and characterisation of covalent polymer functionalized graphene oxide, J. Mater. Chem., 2011, 21, 3455–3461 RSC .
47. K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J.-H. Ahn, P. Kim, J.-Y. Choi and B. H. Hong, Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature, 2009, 457, 706–710 CrossRef CAS PubMed .
48. X.-D. Zhuang, Y. Chen, G. Liu, P.-P. Li, C.-X. Zhu, E.-T. Kang, K.-G. Noeh, B. Zhang, J.-H. Zhu and Y.-X. Li, Conjugated-polymer-functionalized graphene oxide: Synthesis and nonvolatile rewritable memory effect, Adv. Mater., 2010, 22, 1731–1735 CrossRef CAS PubMed .
49. B. T. McGrail, B. J. Rodier and E. Pentzer, Rapid functionalization of graphene oxide in water, Chem. Mater., 2014, 26, 5806–5811 CrossRef CAS .
50. X. Ou, L. Jiang, P. Chen, M. Zhu, W. Hu, M. Liu, J. Zhu and H. Ju, Highly stable graphene-based multilayer films immobilized via covalent bonds and their applications in organic field-effect transistors, Adv. Funct. Mater., 2013, 23, 2422–2435 CrossRef CAS .
51. H. Yao, L. Jin, H.-J. Sue, Y. Sumi and R. Nishimura, Facile decoration of Au nanoparticles on reduced graphene oxide surfaces via a one-step chemical functionalization approach, J. Mater. Chem. A, 2013, 1, 10783–10789 RSC .
52. J. Ou, Y. Wang, J. Wang, S. Liu, Z. Li and S. Yang, Self-assembly of octadecyltrichlorosilane on graphene oxide and the tribological performances of the resultant film, J. Phys. Chem. C, 2011, 115, 10080–10086 CrossRef CAS .
53. W. R. Collins, W. Lewandowski, E. Schmois, J. Walish and T. M. Swager, Claisen rearrangement of graphite oxide: A route to covalently functionalized graphenes, Angew. Chem., Int. Ed., 2011, 50, 8848–8852 CrossRef CAS PubMed .
54. W. R. Collins, E. Schmois and T. M. Swager, Graphene oxide as an electrophile for carbon nucleophiles, Chem. Commun., 2011, 47, 8790–8792 RSC .
55. L. Dai, Functionalization of graphene for efficient energy conversion and storage, Acc. Chem. Res., 2013, 46, 31–42 CrossRef CAS PubMed .
56. C. E. Hamilton, J. R. Lomeda, Z. Sun, J. M. Tour and A. R. Barron, High-yield organic dispersions of unfunctionalized graphene, Nano Lett., 2009, 9, 3460–3462 CrossRef CAS PubMed .
57. J. R. Lomeda, C. D. Doyle, D. V. Kosynkin, W.-F. Hwang and J. M. Tour, Diazonium functionalization of surfactant-wrapped chemically converted graphene sheets, J. Am. Chem. Soc., 2008, 130, 16201–16206 CrossRef CAS PubMed .
58. J. Ji, G. Zhang, H. Chen, S. Wang, G. Zhang, F. Zhang and X. Fan, Sulfonated graphene as water-tolerant solid acid catalyst, Chem. Sci., 2011, 2, 484–487 RSC .
59. Z. Jin, T. P. McNicholas, C.-J. Shih, Q. H. Wang, G. L. C. Paulus, A. J. Hilmer, S. Shimizu and M. S. Strano, Click chemistry on solution-dispersed graphene and monolayer CVD graphene, Chem. Mater., 2011, 23, 3362–3370 CrossRef CAS .
60. N. D. Luong, L. H. Sinh, L.-S. Johansson, J. Campell and J. Seppälä, Functional graphene by thiol-ene click chemistry, Chem. – Eur. J., 2015, 21, 3183–3186 CrossRef PubMed .
61. S. Sun, Y. Cao, J. Feng and P. Wu, Click chemistry as a route for the immobilization of well-defined polystyrene onto graphene sheets, J. Mater. Chem., 2010, 20, 5605–5607 RSC .
62. Y. Cao, Z. Lai, J. Feng and P. Wu, Graphene oxide sheets covalently functionalized with block copolymers via click chemistry as reinforcing fillers, J. Mater. Chem., 2011, 21, 9271–9278 RSC .
63. F. Meng, H. Ishida and X. Liu, Introduction of benzoxazine onto the graphene oxide surface by click chemistry and the properties of graphene oxide reinforced polybenzoxazine nanohybrids, RSC Adv., 2014, 4, 9471–9475 RSC .
64. G. Xu, P. Xu, D. Shi and M. Chen, Modification of graphene oxide by a facile coprecipitation method and click chemistry for use as a drug carrier, RSC Adv., 2014, 4, 28807–28813 RSC .
65. Y. Oz, A. Barras, R. Sanyal, R. Boukherroub, S. Szunerits and A. Sanyal, Functionalization of reduced graphene oxide via thiol–maleimide “click” chemistry: Facile fabrication of targeted drug delivery vehicles, ACS Appl. Mater. Interfaces, 2017, 9, 34194–34203 CrossRef CAS PubMed .
66. J. I. Paredes, S. Villar-Rodil, A. Martínez-Alonso and J. M. D. Tascón, Graphene oxide dispersions in organic solvents, Langmuir, 2008, 24, 10560–10564 CrossRef CAS PubMed .
67. Y. Si and E. T. Samulski, Synthesis of water soluble graphene, Nano Lett., 2008, 8, 1679–1682 CrossRef CAS PubMed .
68. Y. Liu, D. Yu, C. Zeng, Z. Miao and L. Dai, Biocompatible graphene oxide-based glucose biosensors, Langmuir, 2010, 26, 6158–6160 CrossRef CAS PubMed .
69. F. Liu, J. Y. Choi and T. S. Seo, Graphene oxide arrays for detecting specific DNA hybridization by fluorescence resonance energy transfer, Biosens. Bioelectron., 2010, 25, 2361–2365 CrossRef CAS PubMed .
70. C.-H. Lu, H.-H. Yang, C.-L. Zhu, X. Chen and G.-N. Chen, A graphene platform for sensing biomolecules, Angew. Chem., Int. Ed., 2009, 48, 4785–4787 CrossRef CAS PubMed .
71. A. K. Yagati, J. Min and S. Cho, Electrosynthesis of ERGO-NP nanocomposite films for bioelectrocatalysis of horseradish peroxidase towards H₂O₂, J. Electrochem. Soc., 2014, 161, G133–G140 CrossRef CAS .
72. S. Stankovich, R. D. Piner, S. T. Nguyen and R. S. Ruoff, Synthesis and exfoliation of isocyanate-treated graphene oxide nanoplatelets, Carbon, 2006, 44, 3342–3347 CrossRef CAS .
73. X. Yang, L. Ma, S. Wang, Y. Li, Y. Tu and X. Zhu, “Clicking” graphite oxide sheets with well-defined polystyrenes: A new strategy to control the layer thickness, Polymer, 2011, 52, 3046–3052 CrossRef CAS .
74. W. Zhang, S. Wang, J. Ji, Y. Li, G. Zhang, F. Zhang and X. Fan, Primary and tertiary amines bifunctional graphene oxide for cooperative catalysis, Nanoscale, 2013, 5, 6030–6033 RSC .
75. N. Porahmad and R. Baharfar, Graphene oxide covalently functionalized with an organic superbase as highly efficient and durable nanocatalyst for green Michael addition reaction, Res. Chem. Intermed., 2018, 44, 305–323 CrossRef CAS .
76. Y. Hou, D. Wang, X.-M. Zhang, H. Zhao, J.-W. Zha and Z.-M. Dang, Positive piezoresistive behavior of electrically conductive alkyl-functionalized graphene/polydimethylsilicone nanocomposites, J. Mater. Chem. C, 2013, 1, 515–521 RSC .
77. N. A. Ismail, N. W. Mohd Zulkifli, Z. Z. Chowdhury and M. R. Johan, Grafting of straight alkyl chain improved the hydrophobicity and tribological performance of graphene oxide in oil as lubricant, J. Mol. Liq., 2020, 319, 114276 CrossRef CAS .
78. S. Kumari, O. P. Sharma, R. Gusain, H. P. Mungse, A. Kukrety, N. Kumar, H. Sugimura and O. P. Khatri, Alkyl-chain-grafted hexagonal boron nitride nanoplatelets as oil-dispersible additives for friction and wear reduction, ACS Appl. Mater. Interfaces, 2015, 7, 3708–3716 CrossRef CAS PubMed .
79. R. Tan, C. Li, J. Luo, Y. Kong, W. Zheng and D. Yin, An effective heterogeneous L-proline catalyst for the direct asymmetric aldol reaction using graphene oxide as support, J. Catal., 2013, 298, 138–147 CrossRef CAS .
80. W.-S. Ma, J. Li and X.-S. Zhao, Improving the thermal and mechanical properties of silicone polymer by incorporating functionalized graphene oxide, J. Mater. Sci., 2013, 48, 5287–5294 CrossRef CAS .
81. S. S. Rawat, A. P. Harsha, O. P. Khatri and R. Wäsche, Pristine, reduced, and alkylated graphene oxide as additives to paraffin grease for enhancement of tribological properties, J. Tribol., 2021, 143, 021903 CrossRef CAS .
82. B. Yu, K. Wang, Y. Hu, F. Nan, J. Pu, H. Zhao and P. Ju, Tribological properties of synthetic base oil containing polyhedral oligomeric silsesquioxane grafted graphene oxide, RSC Adv., 2018, 8, 23606–23614 RSC .
83. D. Berman, A. Erdemir and A. V. Sumant, Graphene: A new emerging lubricant, Mater. Today, 2014, 17, 31–42 CrossRef CAS .
84. M. D. Stoller, S. Park, Y. Zhu, J. An and R. S. Ruoff, Graphene-based ultracapacitors, Nano Lett., 2008, 8, 3498–3502 CrossRef CAS PubMed .
85. C. Lee, X. Wei, J. W. Kysar and J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science, 2008, 321, 385–388 CrossRef CAS PubMed .
86. A. A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao and C. N. Lau, Superior thermal conductivity of single-layer graphene, Nano Lett., 2008, 8, 902–907 CrossRef CAS PubMed .
87. H. P. Mungse and O. P. Khatri, Chemically functionalized reduced graphene oxide as a novel material for reduction of friction and wear, J. Phys. Chem. C, 2014, 118, 14394–14402 CrossRef CAS .
88. S. Choudhary, H. P. Mungse and O. P. Khatri, Dispersion of alkylated graphene in organic solvents and its potential for lubrication applications, J. Mater. Chem., 2012, 22, 21032–21039 RSC .
89. H. P. Mungse, N. Kumar and O. P. Khatri, Synthesis, dispersion and lubrication potential of basal plane functionalized alkylated graphene nanosheets, RSC Adv., 2015, 5, 25565–25571 RSC .
90. Z.-L. Cheng, W. Li, P.-R. Wu and Z. Liu, A strategy for preparing modified graphene oxide with good dispersibility and transparency in oil, Ind. Eng. Chem. Res., 2017, 56, 5527–5534 CrossRef CAS .
91. N. A. Ismail and S. Bagheri, Highly oil-dispersed functionalized reduced graphene oxide nanosheets as lube oil friction modifier, Mater. Sci. Eng., B, 2017, 222, 34–42 CrossRef CAS .
92. T. S. Sreeprasad and V. Berry, How do the electrical properties of graphene change with its functionalization?, Small, 2013, 9, 341–350 CrossRef CAS PubMed .
93. S. Alipoori, M. M. Torkzadeh, M. H. M. Moghadam, S. Mazinani, S. H. Aboutalebi and F. Sharif, Graphene oxide: An effective ionic conductivity promoter for phosphoric acid-doped poly(vinyl alcohol) gel electrolytes, Polymer, 2019, 184, 121908 CrossRef CAS .
94. P. Dai, Z.-H. Mo, R.-W. Xu, S. Zhang, X. Lin, W.-F. Lin and Y.-X. Wu, Development of a cross-linked quaternized poly(styrene-b-isobutylene-b-styrene)/graphene oxide composite anion exchange membrane for direct alkaline methanol fuel cell application, RSC Adv., 2016, 6, 52122–52130 RSC .
95. L. Liu, C. Tong, Y. He, Y. Zhao and C. Lü, Enhanced properties of quaternized graphenes reinforced polysulfone based composite anion exchange membranes for alkaline fuel cell, J. Membr. Sci., 2015, 487, 99–108 CrossRef CAS .
96. C. Long, C. Lu, Y. Li, Z. Wang and H. Zhu, N-spirocyclic ammonium-functionalized graphene oxide-based anion exchange membrane for fuel cells, Int. J. Hydrogen Energy, 2020, 45, 19778–19790 CrossRef CAS .
97. S. Thakur and N. Karak, Alternative methods and nature-based reagents for the reduction of graphene oxide: A review, Carbon, 2015, 94, 224–242 CrossRef CAS .
98. R. Li, Y. Yang, D. Wu, K. Li, Y. Qin, Y. Tao and Y. Kong, Covalent functionalization of reduced graphene oxide aerogels with polyaniline for high performance supercapacitors, Chem. Commun., 2019, 55, 1738–1741 RSC .
99. P. Tang, L. Han, P. Li, Z. Jia, K. Wang, H. Zhang, H. Tan, T. Guo and X. Lu, Mussel-inspired electroactive and antioxidative scaffolds with incorporation of polydopamine-reduced graphene oxide for enhancing skin wound healing, ACS Appl. Mater. Interfaces, 2019, 11, 7703–7714 CrossRef CAS PubMed .
100. R. D. Rodriguez, A. Khalelov, P. S. Postnikov, A. Lipovka, E. Dorozhko, I. Amin, G. V. Murastov, J.-J. Chen, W. Sheng, M. E. Trusova, M. M. Chehimi and E. Sheremet, Beyond graphene oxide: Laser engineering functionalized graphene for flexible electronics, Mater. Horiz., 2020, 7, 1030–1041 RSC .
101. J.-S. Lee, T. Lee, H.-K. Song, J. Cho and B.-S. Kim, Ionic liquid modified graphene nanosheets anchoring manganese oxide nanoparticles as efficient electrocatalysts for Zn–air batteries, Energy Environ. Sci., 2011, 4, 4148–4154 RSC .
102. E. Kowsari and M. R. Chirani, High efficiency dye-sensitized solar cells with tetra alkyl ammonium cation-based ionic liquid functionalized graphene oxide as a novel additive in nanocomposite electrolyte, Carbon, 2017, 118, 384–392 CrossRef CAS .
103. J. Li, H. Wu, L. Cao, X. He, B. Shi, Y. Li, M. Xu and Z. Jiang, Enhanced proton conductivity of sulfonated polysulfone membranes under low humidity via the incorporation of multifunctional graphene oxide, ACS Appl. Nano Mater., 2019, 2, 4734–4743 CrossRef CAS .
104. E. K. Jeon, E. Seo, E. Lee, W. Lee, M.-K. Um and B.-S. Kim, Mussel-inspired green synthesis of silver nanoparticles on graphene oxide nanosheets for enhanced catalytic applications, Chem. Commun., 2013, 49, 3392–3394 RSC .
105. D. R. Dreyer and C. W. Bielawski, Carbocatalysis: Heterogeneous carbons finding utility in synthetic chemistry, Chem. Sci., 2011, 2, 1233–1240 RSC .
106. D. R. Dreyer, H.-P. Jia and C. W. Bielawski, Graphene oxide: A convenient carbocatalyst for facilitating oxidation and hydration reactions, Angew. Chem., Int. Ed., 2010, 49, 6813–6816 CAS .
107. C. Su, M. Acik, K. Takai, J. Lu, S.-J. Hao, Y. Zheng, P. Wu, Q. Bao, T. Enoki, Y. J. Chabal and K. Ping Loh, Probing the catalytic activity of porous graphene oxide and the origin of this behaviour, Nat. Commun., 2012, 3, 1298 CrossRef PubMed .
108. F. Zhang, H. Jiang, X. Li, X. Wu and H. Li, Amine-functionalized GO as an active and reusable acid–base bifunctional catalyst for one-pot cascade reactions, ACS Catal., 2014, 4, 394–401 CrossRef CAS .
109. S. Navalon, A. Dhakshinamoorthy, M. Alvaro and H. Garcia, Carbocatalysis by graphene-based materials, Chem. Rev., 2014, 114, 6179–6212 CrossRef CAS PubMed .
110. A. Corma, S. Iborra and A. Velty, Chemical routes for the transformation of biomass into chemicals, Chem. Rev., 2007, 107, 2411–2502 CrossRef CAS PubMed .
111. E. Lam, J. H. Chong, E. Majid, Y. Liu, S. Hrapovic, A. C. W. Leung and J. H. T. Luong, Carbocatalytic dehydration of xylose to furfural in water, Carbon, 2012, 50, 1033–1043 CrossRef CAS .
112. M. M. Antunes, P. A. Russo, P. V. Wiper, J. M. Veiga, M. Pillinger, L. Mafra, D. V. Evtuguin, N. Pinna and A. A. Valente, Sulfonated graphene oxide as effective catalyst for conversion of 5-(hydroxymethyl)-2-furfural into biofuels, ChemSusChem, 2014, 7, 804–812 CrossRef CAS PubMed .
113. S. Verma, H. P. Mungse, N. Kumar, S. Choudhary, S. L. Jain, B. Sain and O. P. Khatri, Graphene oxide: An efficient and reusable carbocatalyst for aza-Michael addition of amines to activated alkenes, Chem. Commun., 2011, 47, 12673–12675 RSC .
114. M. R. Acocella, M. Mauro and G. Guerra, Regio- and enantioselective Friedel–Crafts reactions of indoles to epoxides catalyzed by graphene oxide: A green approach, ChemSusChem, 2014, 7, 3279–3283 CrossRef CAS PubMed .
115. A. Dhakshinamoorthy, M. Alvaro, P. Concepción, V. Fornés and H. Garcia, Graphene oxide as an acid catalyst for the room temperature ring opening of epoxides, Chem. Commun., 2012, 48, 5443–5445 RSC .
116. L. Wang, D. Wang, S. Zhang and H. Tian, Synthesis and characterization of sulfonated graphene as a highly active solid acid catalyst for the ester-exchange reaction, Catal. Sci. Technol., 2013, 3, 1194–1197 RSC .
117. K. Li, J. Chen, Y. Yan, Y. Min, H. Li, F. Xi, J. Liu and P. Chen, Quasi-homogeneous carbocatalysis for one-pot selective conversion of carbohydrates to 5-hydroxymethylfurfural using sulfonated graphene quantum dots, Carbon, 2018, 136, 224–233 CrossRef CAS .
118. A. Dhakshinamoorthy, M. Alvaro, M. Puche, V. Fornes and H. Garcia, Graphene oxide as catalyst for the acetalization of aldehydes at room temperature, ChemCatChem, 2012, 4, 2026–2030 CrossRef CAS .
119. Y. Li, Q. Zhao, J. Ji, G. Zhang, F. Zhang and X. Fan, Cooperative catalysis by acid–base bifunctional graphene, RSC Adv., 2013, 3, 13655–13658 RSC .
120. P. Kumar, B. Sain and S. L. Jain, Photocatalytic reduction of carbon dioxide to methanol using a ruthenium trinuclear polyazine complex immobilized on graphene oxide under visible light irradiation, J. Mater. Chem. A, 2014, 2, 11246–11253 RSC .
121. Y.-H. Yao, J. Li, H. Zhang, H.-L. Tang, L. Fang, G.-D. Niu, X.-J. Sun and F.-M. Zhang, Facile synthesis of a covalently connected RGO–COF hybrid material by in situ reaction for enhanced visible-light induced photocatalytic H₂ evolution, J. Mater. Chem. A, 2020, 8, 8949–8956 RSC .
122. Z. Fang, A. Ito, A. C. Stuart, H. Luo, Z. Chen, K. Vinodgopal, W. You, T. J. Meyer and D. K. Taylor, Soluble reduced graphene oxide sheets grafted with polypyridylruthenium-derivatized polystyrene brushes as light harvesting antenna for photovoltaic applications, ACS Nano, 2013, 7, 7992–8002 CrossRef CAS PubMed .
123. R. Vinoth, S. G. Babu, V. Bharti, V. Gupta, M. Navaneethan, S. V. Bhat, C. Muthamizhchelvan, P. C. Ramamurthy, C. Sharma, D. K. Aswal, Y. Hayakawa and B. Neppolian, Ruthenium based metallopolymer grafted reduced graphene oxide as a new hybrid solar light harvester in polymer solar cells, Sci. Rep., 2017, 7, 43133 CrossRef CAS PubMed .
124. P. Kumar, A. Kumar, B. Sreedhar, B. Sain, S. S. Ray and S. L. Jain, Cobalt phthalocyanine immobilized on graphene oxide: An efficient visible-active catalyst for the photoreduction of carbon dioxide, Chem. – Eur. J., 2014, 20, 6154–6161 CrossRef CAS PubMed .
125. P. Kumar, A. Bansiwal, N. Labhsetwar and S. L. Jai, Visible light assisted photocatalytic reduction of CO₂ using a graphene oxide supported heteroleptic ruthenium complex, Green Chem., 2015, 17, 1605–1609 RSC .
126. C.-X. Tian, S.-C. Cui, X.-Y. Liu and J.-G. Liu, A hybrid composite of rhenium complexes covalently grafted on reduced graphene oxide/hydrogenated TiO₂ as an efficient catalyst for CO₂ reduction under visible light, Res. Chem. Intermed., 2020, 46, 1–13 CrossRef CAS .
127. V. S. Sapner, B. B. Mulik, R. V. Digraskar, S. S. Narwade and B. R. Sathe, Enhanced oxygen evolution reaction on amine functionalized graphene oxide in alkaline medium, RSC Adv., 2019, 9, 6444–6451 RSC .
128. L. Dai, Y. Xue, L. Qu, H.-J. Choi and J.-B. Baek, Metal-free catalysts for oxygen reduction reaction, Chem. Rev., 2015, 115, 4823–4892 CrossRef CAS PubMed .
129. X. Ren, Y. Wang, A. Liu, Z. Zhang, Q. Lv and B. Liu, Current progress and performance improvement of Pt/C catalysts for fuel cells, J. Mater. Chem. A, 2020, 8, 24284–24306 RSC .
130. Z. Zhao, C. Chen, Z. Liu, J. Huang, M. Wu, H. Liu, Y. Li and Y. Huang, Pt-Based nanocrystal for electrocatalytic oxygen reduction, Adv. Mater., 2019, 31, 1808115 CrossRef PubMed .
131. X. Liu and L. Dai, Carbon-based metal-free catalysts, Nat. Rev. Mater., 2016, 1, 16064 CrossRef CAS .
132. J. Lai, A. Nsabimana, R. Luque and G. Xu, 3D porous carbonaceous electrodes for electrocatalytic applications, Joule, 2018, 2, 76–93 CrossRef CAS .
133. F. Li, X. Jiang, J. Zhao and S. Zhang, Graphene oxide: A promising nanomaterial for energy and environmental applications, Nano Energy, 2015, 16, 488–515 CrossRef CAS .
134. C. K. Chua and M. Pumera, Monothiolation and reduction of graphene oxide via one-pot synthesis: Hybrid catalyst for oxygen reduction, ACS Nano, 2015, 9, 4193–4199 CrossRef CAS PubMed .
135. J. Park and M. Yan, Covalent functionalization of graphene with reactive intermediates, Acc. Chem. Res., 2013, 46, 181–189 CrossRef CAS PubMed .
136. Y. Jin, Y. Zheng, S. G. Podkolzin and W. Lee, Band gap of reduced graphene oxide tuned by controlling functional groups, J. Mater. Chem. C, 2020, 8, 4885–4894 RSC .
137. A. Kaplan, Z. Yuan, J. D. Benck, A. Govind Rajan, X. S. Chu, Q. H. Wang and M. S. Strano, Current and future directions in electron transfer chemistry of graphene, Chem. Soc. Rev., 2017, 46, 4530–4571 RSC .
138. A. Navaee and A. Salimi, Efficient amine functionalization of graphene oxide through the bucherer reaction: An extraordinary metal-free electrocatalyst for the oxygen reduction reaction, RSC Adv., 2015, 5, 59874–59880 RSC .
139. V. S. Sapner, P. P. Chavan and B. R. Sathe, L-Lysine-functionalized reduced graphene oxide as a highly efficient electrocatalyst for enhanced oxygen evolution reaction, ACS Sustainable Chem. Eng., 2020, 8, 5524–5533 CrossRef CAS .
140. M. S. Ahmed and Y.-B. Kim, 3D graphene preparation via covalent amide functionalization for efficient metal-free electrocatalysis in oxygen reduction, Sci. Rep., 2017, 7, 43279 CrossRef PubMed .
141. J. Yuan, W.-Y. Zhi, L. Liu, M.-P. Yang, H. Wang and J.-X. Lu, Electrochemical reduction of CO₂ at metal-free N-functionalized graphene oxide electrodes, Electrochim. Acta, 2018, 282, 694–701 CrossRef CAS .
142. C. Hu and L. Dai, Doping of carbon materials for metal-free electrocatalysis, Adv. Mater., 2019, 31, 1804672 CrossRef PubMed .
143. J. Duan, S. Chen, M. Jaroniec and S. Z. Qiao, Heteroatom-doped graphene-based materials for energy-relevant electrocatalytic processes, ACS Catal., 2015, 5, 5207–5234 CrossRef CAS .
144. Y. Zheng, Y. Jiao, Y. Zhu, L. H. Li, Y. Han, Y. Chen, A. Du, M. Jaroniec and S. Z. Qiao, Hydrogen evolution by a metal-free electrocatalyst, Nat. Commun., 2014, 5, 3783 CrossRef PubMed .
145. R. Paul, Q. Dai, C. Hu and L. Dai, Ten years of carbon-based metal-free electrocatalysts, Carbon Energy, 2019, 1, 19–31 CrossRef .
146. S. Kaushal, M. Kaur, N. Kaur, V. Kumari and P. P. Singh, Heteroatom-doped graphene as sensing materials: A mini review, RSC Adv., 2020, 10, 28608–28629 RSC .
147. W. Zheng, Y. Zhang, K. Niu, T. Liu, K. Bustillo, P. Ercius, D. Nordlund, J. Wu, H. Zheng and X. Du, Selective nitrogen doping of graphene oxide by laser irradiation for enhanced hydrogen evolution activity, Chem. Commun., 2018, 54, 13726–13729 RSC .
148. S. Agnoli and M. Favaro, Doping graphene with boron: A review of synthesis methods, physicochemical characterization, and emerging applications, J. Mater. Chem. A, 2016, 4, 5002–5025 RSC .
149. J. Zhang and L. Dai, Heteroatom-doped graphitic carbon catalysts for efficient electrocatalysis of oxygen reduction reaction, ACS Catal., 2015, 5, 7244–7253 CrossRef CAS .
150. X. Yu, P. Han, Z. Wei, L. Huang, Z. Gu, S. Peng, J. Ma and G. Zheng, Boron-doped graphene for electrocatalytic N₂ reduction, Joule, 2018, 2, 1610–1622 CrossRef CAS .
151. R. Li, Z. Wei and X. Gou, Nitrogen and phosphorus dual-doped graphene/carbon nanosheets as bifunctional electrocatalysts for oxygen reduction and evolution, ACS Catal., 2015, 5, 4133–4142 CrossRef CAS .
152. A. Wang, C. Li, J. Zhang, X. Chen, L. Cheng and W. Zhu, Graphene-oxide-supported covalent organic polymers based on zinc phthalocyanine for efficient optical limiting and hydrogen evolution, J. Colloid Interface Sci., 2019, 556, 159–171 CrossRef CAS PubMed .
153. A. A. Ensafi, M. Jafari-Asl and B. Rezaei, Pyridine-functionalized graphene oxide, an efficient metal free electrocatalyst for oxygen reduction reaction, Electrochim. Acta, 2016, 194, 95–103 CrossRef CAS .
154. S. Song, Y. Xue, L. Feng, H. Elbatal, P. Wang, C. N. Moorefield, G. R. Newkome and L. Dai, Reversible self-assembly of terpyridine-functionalized graphene oxide for energy conversion, Angew. Chem., Int. Ed., 2014, 53, 1415–1419 CrossRef CAS PubMed .
155. M. Jahan, Q. Bao and K. P. Loh, Electrocatalytically active graphene–porphyrin MOF composite for oxygen reduction reaction, J. Am. Chem. Soc., 2012, 134, 6707–6713 CrossRef CAS PubMed .
156. K. Qu, Y. Zheng, S. Dai and S. Z. Qiao, Graphene oxide-polydopamine derived N,S-codoped carbon nanosheets as superior bifunctional electrocatalysts for oxygen reduction and evolution, Nano Energy, 2016, 19, 373–381 CrossRef CAS .
157. Y. Ito, W. Cong, T. Fujita, Z. Tang and M. Chen, High catalytic activity of nitrogen and sulfur co-doped nanoporous graphene in the hydrogen evolution reaction, Angew. Chem., Int. Ed., 2015, 54, 2131–2136 CrossRef CAS PubMed .
158. L. Jiang, L. Qiu, T. Cen, Y.-Y. Liu, X. Peng, Z. Ye and D. Yuan, Controllable Co@N-doped graphene anchored onto the NRGO toward electrocatalytic hydrogen evolution at all pH values, Chem. Commun., 2020, 56, 567–570 RSC .
159. J. H. Dumont, U. Martinez, K. Artyushkova, G. M. Purdy, A. M. Dattelbaum, P. Zelenay, A. Mohite, P. Atanassov and G. Gupta, Nitrogen-doped graphene oxide electrocatalysts for the oxygen reduction reaction, ACS Appl. Nano Mater., 2019, 2, 1675–1682 CrossRef CAS .
160. P. Su, K. Iwase, S. Nakanishi, K. Hashimoto and K. Kamiya, Nickel–nitrogen-modified graphene: An efficient electrocatalyst for the reduction of carbon dioxide to carbon monoxide, Small, 2016, 12, 6083–6089 CrossRef CAS PubMed .
161. Z. Lin, G. H. Waller, Y. Liu, M. Liu and C.-p. Wong, 3D nitrogen-doped graphene prepared by pyrolysis of graphene oxide with polypyrrole for electrocatalysis of oxygen reduction reaction, Nano Energy, 2013, 2, 241–248 CrossRef CAS .
162. J. Cai, P. Ruffieux, R. Jaafar, M. Bieri, T. Braun, S. Blankenburg, M. Muoth, A. P. Seitsonen, M. Saleh, X. Feng, K. Müllen and R. Fasel, Atomically precise bottom-up fabrication of graphene nanoribbons, Nature, 2010, 466, 470–473 CrossRef CAS PubMed .
163. J. Chen, Y. Li, L. Huang, N. Jia, C. Li and G. Shi, Size fractionation of graphene oxide sheets via filtration through track-etched membranes, Adv. Mater., 2015, 27, 3654–3660 CrossRef CAS PubMed .
164. B. Zhang, L. Fan, H. Zhong, Y. Liu and S. Chen, Graphene nanoelectrodes: Fabrication and size-dependent electrochemistry, J. Am. Chem. Soc., 2013, 135, 10073–10080 CrossRef CAS PubMed .
165. S. Li, F. Zhu, F. Meng, H. Li, L. Wang, J. Zhao, Q. Yue, J. Liu and J. Jia, Separation of graphene oxide by density gradient centrifugation and study on their morphology-dependent electrochemical properties, J. Electroanal. Chem., 2013, 703, 135–145 CrossRef CAS .

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Footnote

† These authors contributed equally to this work.

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