Solution electrochemical approach to functionalized graphene: History, progress and challenges

Accepted Manuscript Solution Electrochemical Approach to Functionalized Graphene: History, Progress and Challenges

Hongwu Chen, Chun Li, Liangti Qu PII:

S0008-6223(18)30760-7

DOI:

10.1016/j.carbon.2018.08.027

Reference:

CARBON 13382

To appear in:

Carbon

Received Date:

26 June 2018

Accepted Date:

11 August 2018

Please cite this article as: Hongwu Chen, Chun Li, Liangti Qu, Solution Electrochemical Approach to Functionalized Graphene: History, Progress and Challenges, Carbon (2018), doi: 10.1016/j. carbon.2018.08.027

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ACCEPTED MANUSCRIPT

Solution Electrochemical Approach to Functionalized Graphene: History, Progress and Challenges Hongwu Chena, Chun Lia*1, Liangti Qua,b *2 a

Department of Chemistry, MOE Key Lab Bioorganic Phosphorus Chemistry & Chemical Biology, Tsinghua University, Beijing 100084, P. R. China b Key Laboratory for Advanced Materials Processing Technology, Ministry of Education of China; State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, P. R. China

Abstract Motivated by the successful fabrication of monolayer graphene and demonstration of its unique physical properties and potentials in materials science, immense efforts have been made to find the scalable, controllable and green methods for the production of graphene-based materials. Among them, traditional wet chemical pathway based on chemical oxidation and functionalization was proved to be energy-saving and highyield but yet not eco-friendly because of the involved strong acid media and hazardous oxidants. As an alternative, electrochemical method for the fabrication of solutionprocessable functionalized graphene (FG) was highly praised for its environmental benignity. Studied over decades, electrochemical approach is still challenged by the low yield of monolayer FG and the low functionalized degree. In this review, we will introduce the history of researches on electrochemical synthesis of FG, then address recent developments, and finally analyze the problems and potentials of the electrochemical approach through the dissection of fundamental physical process involved in it.

1 2

Corresponding author. E-mail: [email protected] (Chun Li) Corresponding author. E-mail: [email protected] (Liangti Qu) 1

ACCEPTED MANUSCRIPT Contents: 1. Introduction 2. Formation of graphite intercalation compound 2.1 History of intercalation chemistry of graphite 2.2 Basic process of the intercalation 2.3 Electrochemical synthesis of functionalized graphene based on intercalation-decomposition process of graphite 3. Functionalization and exfoliation of graphite intercalation compound 3.1 Basic process of the functionalization 3.2 Electrochemical synthesis of functionalized graphene based on functionalization of graphite intercalation compound 4. Chemical modification of functionalized graphene 4.1 Fabrication of graphene quantum dots through oxidation of functionalized graphene 4.2 Reduction and electrochemical deposition of graphene oxide 5. Applications of functionalized graphene and prospects on electrochemical approach 5.1 Applications of functionalized graphene synthesized by chemical oxidation and electrochemical approaches 5.2 Challenges and the future of electrochemical fabrication of functionalized graphene

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1． Introduction It spanned over 6,000 years from the first documented use of graphite [1] for decoration to the confirmation of its lamellar structure by single-crystal X-ray diffraction (XRD) in 1924 [2]. Studies of intercalation and solution chemistry of graphite were initiated from 1840s, when Schafhäutl reported the swelling behavior of graphite in concentrated sulfuric acid [3]. Through a hundred years from 1850s to 1950s, numerous researches on the solution chemistry of graphite laid the foundation for the synthesis of functionalized graphene (FG) materials, including the establishment of widely-used wet chemical approach to graphite oxide [4−7], first reports on the electrochemical intercalation and oxidation of graphite [8,9], and the groundbreaking works on the chemistry of graphite oxide and its reduced form [10,11]. Actually, in 1962, Boehm obtained transmission electronic microscope (TEM) image for atomically thin reduced graphite oxide sheets [12]. Restricted by the resolution of TEM at that time, it is still remaining skeptical for the number of layers they had observed. Besides, the years before 21st century also witnessed significant progress on the epitaxially grown ultra-thin graphite layers [13], nevertheless, without regarding the scale-up and special physical behavior of prepared materials. In 2004, Geim et al. provided the unambiguous evidence for the stable existence of monolayer graphene [14], which in turn boosted researches on the top-down method [14−17], bottom-up method [18−20], and conversion method [21,22] for the synthesis of pristine graphene (PG) or FG and massive explorations on their applications [23−28]. 3

ACCEPTED MANUSCRIPT The comparison between well-established methods for the fabrication of PG/FG was summarized in Fig. 1 in terms of the scalability/cost and quality of PG/FG. Generally, 1) mechanical exfoliation of high quality graphite (e.g., highly oriented pyrolytic graphite, HOPG) gives PG with the extraordinary physical properties approaching the limit of theoretically estimated value under certain conditions [14,29]; 2) chemical vapor deposition (CVD) method and epitaxial growth on crystalline SiC or metal surface also form structurally intact PG but with improved scalability and reduced cost compared with mechanical exfoliation [13,18]; 3) solution chemistry and other methods are scalable and cost-efficient; whereas the obtained PG/FG is more defective, which hinders their use in the applications that require graphene with intact structure [15].

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Fig. 1. Comparison among well-established methods for the preparation of pristine graphene (quadrant I and IV) or functionalized graphene (quadrant III) in respect of cost and quality of the products. Typically, an ideal method should locate at the marked area in quadrant II, namely, with the cost comparable to wet chemical approach but producing the defect-free graphene basal plane.

For conventional solution chemistry approaches, chemical oxidation of graphite in concentrated acid media results in the formation of graphite oxide, which could be further exfoliated to single layer oxygen functionalized FG (graphene oxide, GO). Generally, with the final product well-dispersed in aqueous media, the yield of GO was optimized to 150−200% [30,31]. Nonetheless, the toxic effluents such as NOx and ClO2 and carcinogenic Mn2+ waste were produced when performing the classic wet chemical method [4−7], in which the employment of irreplaceable oxidants—KMnO4 and KClO3, for their high efficiency in the oxidation of graphene layers—precludes any possibility to change the whole process completely to a truly eco-friendly one. Alternatively, wet 5

ACCEPTED MANUSCRIPT chemical method based on mild oxidation or non-oxidation strategies [32−35] did not make use of hazardous oxidants. For instance, Eigler presented the fabrication of graphite sulfate as the precursor for high quality graphene [32], and numerous works also prove the liquid exfoliation of graphite in stabilizer solution for the production of few-layer graphene [34,35]. However, these methods showed poor yield and incomplete exfoliation of graphite, severely obstructing the processability of FG dispersions and the downstream applications. Recently, because of the tremendous improvement on the efficiency, electrochemical functionalization [36−39] is becoming the panacea for industrial pursue for an environmentally benign, scalable and low-cost method for the fabrication of FG. The main characteristics of electrochemical method include: 1) the whole process is carried out in solution system, with a wide range of choices among aqueous media [40−42], organic solvents [43−45] and ionic liquids (IL) [46,47]; 2) the procedure is identified by the absence of chemical oxidants, and herein current acts as the green oxidants to initiate functionalization of graphene layers; 3) the functionalization degree and yield could be adjusted by the experimental set-ups and finally tuned to a level comparable to the chemical oxidation method [48,49]. Fundamentally, the solution electrochemical exfoliation of graphite and functionalization of graphene layers involves three distinct steps (Fig. 2) [49], which resemble the process in traditional chemical oxidation [50]: 1) the intercalation of graphite; 2) functionalization of graphene layers; 3) further modification on the chemical structure of FG. In this short review, we will present the story by analyzing critical issues in each reaction steps, then address the major 6

ACCEPTED MANUSCRIPT challenges and possible solutions based on our understandings.

Fig. 2. The solution chemistry method of the synthesis and modification of functionalized graphene, which is elaborated by three distinct steps: (a) intercalation of graphite precursor; (b) functionalization of graphene layers; (c) further modification on the chemical structure of functionalized graphene. Photos showing the different intermediates are adapted from [49] (scale bar: 1 mm) (Photos are reproduced with permission from Ren et al. [49], copyright @ 2018 Nature Publishing Group).

2． Formation of graphite intercalation compounds Intercalation of graphitic materials is the very first step for all solution chemical approaches to the fabrication of FG (Fig. 2a). Consider a graphite lattice, the adjacent layers of graphene are held together tightly by the π−π interaction. Therefore, the 7

ACCEPTED MANUSCRIPT weakening of this interaction, normally facilitated by the intercalation of solvent or solute molecules, cations or anions, is the prerequisite for further functionalization and exfoliation of graphite. We will review the fundamentals and recent progress on the intercalation chemistry of graphite in this part, and analyze the reactions in electrochemical intercalation operated in mild aqueous solutions and show the practices of electrochemical synthesis of FG based on intercalation-decomposition process of graphite.

2.1. History of intercalation chemistry of graphite At the dawn of modern chemistry, pioneers on the solution chemistry of graphite had pointed out the difference of graphite resource reacting with acid mixtures [3,4]. Brodie identified that, the graphite precursors with “lamellar” and “amorphous” structure give the different reaction products in the mixture of concentrated nitric and sulfuric acid [4], the former gently forming purple substance which is later known as intercalation compounds with sulfuric acid, whereas the latter being oxidized to yield small molecules. Hofmann and Boehm (Fig. 3a) established the basis of the intercalation chemistry of graphite [51,52]. They studied the intercalation product when crystalline graphite reacts with sulfuric acid [53]—blue graphite, also proposed its stoichiometric formula C24+·HSO4−·2H2SO4. The authors also studied the intercalation of graphite in a profusion of systems: HNO3, HClO4, H2SeO4 and H3PO4 as the intercalants; CrO3, HNO3 and KMnO4 as the oxidants, reaching the conclusion that sulfuric acid is the best 8

ACCEPTED MANUSCRIPT intercalant for graphite, and most importantly, they proved the basic rules in the intercalation chemistry: graphite intercalation compound (GIC) would only form in the concentrated acid media under the presence of oxidants (chemical substances or electrical potential). Furthermore, Hofmann for the first time introduced the concept of staging for GIC through detailed characterization by XRD (Fig. 3b and c) [53]. From 1930s to 1950s, the explorations on classic GIC spanned over abundant systems, as Rüdorff summarized in his review in 1959 [54], including 1) alkali metal GIC, binary product with formula Cx−M+ (x = 8, 24, 36, 48, 60, etc.), M = K, Rb, Cs) and ternary product Cx−M+Sy (S = NH3, 1,2-dimethoxyethane, dimethyl sulfoxide, etc.); 2) graphite salts like GIC, the anions usually include HSO4−, NO3−, ClO4−, CH3COO−, etc.; 3) Lewis acid (e.g., metal oxides and halides); 4) halogens (e.g., Cl2 and Br2). Electrochemical intercalation of graphite anode was also proved in 1930s. Thiele et al. obtained GICs by charging graphite anode in concentrated sulfuric acid and 30% perchloric acid [8,9]. They also studied the reaction of GIC with water, showing the first attempts to fabricate graphite oxide through electrochemical pathway [8]. Hofmann pointed out the intercalation could be also controlled by the potential applied to the anode [53]: the gradated potential during electrochemical intercalation reflects stage transition in GIC (Fig. 3d).

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Fig. 3. Early history of the intercalation chemistry of graphite. (a) Ulrich Hofmann (1903−1986) [51] and his protégé Hanns-Peter Boehm (1928−) [52]. They are considered as the pioneers of functionalized graphene research. (b) Concepts of staging in GIC and (c) the X-ray diffraction patterns collected for GIC and different stage level [53]. (d) Staging transition triggered by the potential applied to graphite anode in concentrated sulfuric acid electrolyte [53] (Photos and figures are reproduced with permission from (a) Lerf [51], copyright @ 2014 The Royal Society of Chemistry; Boehm [52], copyright @ 2010 John Wiley & Sons; (b−d) Rüdorff and Hofmann [53], copyright @ 1938 John Wiley & Sons).

Generally, the electrochemical intercalation of graphite takes place at anode. On the other hand, Besenhard presented systematic study on the intercalation of graphite cathode [55]. Immersed in organic solvents or liquid ammonium, ternary alkali metal or quaternary ammonium cation GIC was formed when applying the negative potential at graphite electrode. The resultant lithium ion GIC, intensively studied from 1980s [56,57], gradually became the system on which lithium-based secondary batteries 10

ACCEPTED MANUSCRIPT predicated [58]. Additionally, the electrochemical synthesis of FG by cathode intercalation of graphite, is also adopted in the case that requires high-quality and nonoxidized FG materials. Nowadays, intercalation chemistry of graphite is well-established through numerous pioneering works. However, what the intercalation mechanism down to the microscopic level should be and how far we could push the limit for molecular assembly between graphene layers, still remains terra incognita. Recently, Eigler et al. discussed the molecular mechanism of the difference in intercalation behavior of graphite with various stacking order along c-axis [59] (which was observed by Brodie and Hofmann et al.) from a completely new viewpoint based on friction between intercalant molecules and graphene basal plane (Fig. 4a). Regan et al. employed in situ STEM study on the intercalation process of high-quality graphite microcrystal [60], and they found that even for single crystal graphite, the intercalation dynamics is also governed by extrinsic factors, for instance, structural defects, rather than process predicted by thermodynamics (Fig. 4b). Conventional opinions on the limit for staging was also challenged by Swager et al., they showed “hyperstage GIC” which was prepared by cathodic intercalation with tetrabutylammonium ions (TBA+), displays a highly expanded graphite lattice with d-spacing over 15.3 Å [61] (Fig. 4c).

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Fig. 4. Examples of modern researches on the intercalation chemistry of graphite. (a) The influence of stacking order in graphite precursor on the intercalation process of sulfuric acid molecule and bisulfates into graphite gallery [59]. (b) Intercalation events visualized in single crystal graphite, the sharp peaks reflect the local staging transition influenced by structural defects [60]. (c) Hyperstage graphite formed in DMF/MeCN-TBA+ system [61] (Reproduced with permission from (a) Eigler et al. [59], copyright @ 2018 Nature Publishing Group; (b) Regan et al. [60], copyright @ 2017 Nature Publishing Group; (c) Swager et al. [61], copyright @ 2018 John Wiley & Sons).

The slow accretion of knowledge on intercalation chemistry and solution chemistry of graphite in 20th century, definitely, has turned into an avalanche of synthesis methodology in graphene era. In the next part, we will analyze the fundamental process of GIC formation based on the findings through the history.

2.2. Basic process of the intercalation 12

ACCEPTED MANUSCRIPT Definition. Intercalation is often to be confused with inclusion and interstitial process. The difference among them is well explained by Lerf [51]. To distinguish them laconically: 1) intercalation is the process of reversible uptake of guest molecules/ions into the array of host materials composed of layers/chains that held together by weak interaction. The process is usually characterized by the massive expansion in the crystal lattice and change in the apparent morphology of the bulk material. 2) inclusion or interstitial process involves the occupation of lattice empties by guest species and the whole reaction is slower and hardly reversible. Graphite lattice, the stacking of graphene layers by π−π interaction, then falls in the definition for intercalation when processing with concentrated sulfuric acid, in which the guest species reversibly get between electrically oxidized graphene sheets (Fig. 2a). Staging behavior of GIC was first described by Hofmann [53], in which the uptake of sulfuric acid molecules into graphite matrix results in the regular arrangement of intercalated and unoccupied interlayer spaces (Fig. 3b). However, unlike the ideal model proposed by Hofmann et al., Daumas and Hérold [62] developed a more accurate model based on the island-like intercalation of guest molecules into the host material, and the sliding of these islands results in the stage transition. Accordingly, the level of intercalation was determined across the whole system: the stage number k (in stage-k GIC) is defined as the average number of graphene layers between two layers of intercalants in the overall graphite matrix [62]. In general opinion, stage-1 GIC is the highest level of intercalation that is emblematic of the intrusion of intercalants between every adjacent graphene layers statistically. Daumas-Hérold model highlights the 13

ACCEPTED MANUSCRIPT kinetics in the intercalation process, whereas Rüdorff-Hofmann model [10] describes the thermodynamic stable state reached during the intercalation process (Fig. 5a and b). Nevertheless, the two models are in the macroscopic level. Regan et al. [60] pointed out that the structural defects on graphene layers determine the transient intercalation behavior, and the vigorous launching of the events contributes to the observed stage transition current (Fig. 5c). Their conclusion directly runs contrary to the RüdorffHofmann model [10] and Daumas-Hérold model [62], giving new understandings on the intercalation procedure.

Fig. 5. Schematic comparison between different stage-transition models from the charge and current change versus voltage during intercalation, which reflects the kinetics of intercalation process. (a) Rüdorff-Hofmann model [10]; (b) Daumas-Hérold model [62]; (c) The model proposed by Regan et al. [60] based on the stage-transition current response of high-quality graphite microcrystals.

Characterization of H2SO4 GIC. Because GIC formed in concentrated sulfuric acid is widely used as the precursor for the next step of functionalization and exfoliation to 14

ACCEPTED MANUSCRIPT obtain FG [7,15,30], in the following discussion we may focus on the interpretation of the intercalation process in H2SO4 GIC. The electrochemical intercalation of graphite in H2SO4 system was studied in 1980s (Fig. 6) [63,64]. Fisher et al. gave the quantitative results on the correlation between the stage of intercalation and the potential of HOPG anode [63]. Based on their results, the stoichiometric expression of the final products ranged from C58+·HSO4−·xH2SO4 and C12+·HSO4−·yH2SO4, which corresponds to the first appearance of pure stage-2 and the final stage-1 GIC. What is also intriguing was the change in resistivity of HOPG electrode during intercalation: the sharp drop in electrical conductivity of anode was observed from the pure stage-1 GIC formed. The authors explained this by confirming the formation of C−O bond by X-ray photoelectron spectroscopy characterization, what is we know today the functionalization of graphene layers in concentrated H2SO4. They also showed no difference was found in 96% H2SO4 and 10% oleum, precluding the influence of 4% water impurities on the final electrical behavior.

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Fig. 6. Voltammetry XRD characterization on the formation of GIC [63]. (a) Composition of GIC, identification of stage-transition, interlayer spacings in GIC and peak intensity derived from XRD patterns. (b) Relative crystal thickness and intercalation current versus potential applied during the formation of GIC (Reproduced with permission from Metrot and Fischer [63], copyright @ 1981 Elsevier).

Modern techniques are also implemented to explore the basic process during intercalation, such as Raman spectroscopy and optical microscope [50,65]. Tour et al. studied the in situ intercalation process of graphite flake in ammonium persulfatesulfuric acid system [65]. Stage-1 to stage-2 transition (deintercalation process) was visualized by the color change under optical microscope and confirmed by the changes in characteristic Raman E2g mode of graphitic materials. Typically, for stage-1 GIC the Raman G band is located around 1630 cm−1, and for stage-2 GIC the value is ~1620 cm−1 (Fig. 7a). They found that the intercalants enter and exit the graphite array at the 16

ACCEPTED MANUSCRIPT edges, resulting in the tidal wave-like structural deformation of the graphite flake. In addition to Raman spectroscopy, XRD is also a well-established utensil to reflect the intercalation stage in GIC [53, 65]: 2θ angle for the strongest diffraction lines of stage-1 H2SO4 GIC locates at 22.2° (002 signal) and stage-2 GIC at 24.2° (003 signal) (Fig. 7b), these characteristics serve as the evidence for the formation of ordered GIC.

Fig. 7. In situ Raman spectra and XRD characterization of the stage transition process of stage-1 via intermedia state to stage-2 GIC [65]. (a) Blueshift of Raman G band and (b) Change in XRD patterns when stage-1 GIC convert to stage-2 GIC (Reproduced with permission from Tour et al. [65], copyright @ 2013 The American Chemical Society).

2.3. Electrochemical synthesis of functionalized graphene based on intercalationdecomposition process of graphite We have mentioned that the formation of GIC marks the weakening interaction between graphene layers, therefore, facilitating the functionalization of graphene. On 17

ACCEPTED MANUSCRIPT the contrary, incomplete transformation of graphite to GIC may presumably lead to the low preparation yield of single or few layer FG in the practice of electrochemical synthesis. However, the methodology based on the exfoliation of graphite in dilute aqueous acid/salt solutions that are not suitable for the formation of stable stage-1 or stage-2 GIC is widely accepted for the production of FG materials [36,40‒42]. For instance, Feng and Müllen et al. [40] had presented excellent results for the FG synthesis via anode oxidation of graphite precursor in various systems based on mild aqueous solutions (e.g., 0.1 M H2SO4). Actually, the continuous intercalation process to GIC of in these systems is greatly surpassed and instead, accompanied by the oxidation/reduction reaction and in most cases vigorous bubble formation that decomposes the entire graphite electrode. Herein we denote the overall reaction as “intercalation-decomposition process (IDP)”, and the detailed analysis on the process is elaborated below. In 1980s, the electrochemical pretreatment of glassy carbon or HOPG electrode in mild aqueous acid were adopted extensively as the method for electron-transfer activation at that time [66,67], during which the “surface contaminants” were removed to make HOPG surface hydrophilic. McCreery et al. examined the process carefully [68], reaching the conclusions that in mild aqueous solutions the interplay between intercalation of anions and the oxidation reaction depends on both the thermodynamic and kinetic effects. They found that, in solutions that contain 1 M H3PO4, neither intercalation nor the oxidation was observed even applying potential to 2 V (versus SSCE); whereas in 1 M H2SO4, HNO3 and HClO4, the intercalation process happened 18

ACCEPTED MANUSCRIPT (judged from the change of Raman E2g mode) and the lattice was damaged quickly. This phenomenon is closely related to the reduction potential of different GIC: the potential of H3PO4 GIC formation is much higher than the stable GIC such as H2SO4 GIC. What was also highlighted is that the potential for H2SO4 GIC formation is higher than the redox potential of O2/H2O in mild acid solutions, henceforth the intercalation process might be obstructed by the reaction of graphene layers with electrolyte, leading to the simultaneous functionalization of graphene. The morphology change in graphite anode under electrochemical pretreatment in aqueous media was described as “surface blistering” [69,70]. In 1995, Murray et al. studied the electrochemical process in salt electrolytes (e.g., 1 M (NH4)2SO4, LiClO4 and K2HPO4) [70], with the observation of blister formation in sulfate, nitrate and perchlorate electrolyte but not for phosphate salts. The mechanism inferred from the voltammetry and morphology characterization (Fig. 8a) involved the following steps [70]: 1) oxidation of graphene layers at the defects and boundaries; 2) intercalation of anions; 3) gas formation via the rapid electrolysis of intercalants and electrolytes, which results in the decomposition of graphite electrode triggered by accumulated mechanical stress. Reaction steps above are closely related to the discharge of electrolytes or solvents: H2O → 2H+ + 2e− + 1/2 O2 H2O → H+ + e− + •OH Consequently, the interplay between oxidation of graphene layers by radical species and the gas formation at the interlayer spaces in IDP disintegrates the bulk graphite into 19

ACCEPTED MANUSCRIPT small pieces with reduced thickness, giving one of the strategies for the electrochemical synthesis of FG [40−42,46,47]. Feng et al. optimized the composition of electrolytes and the parameters of anode oxidation [40‒42,71], making great efforts in developing the IDP method to a fullfledged system that is perfect for the production of few layer high-quality FG materials. The traditional anode IDP operated under 10 V potential in 0.1 M H2SO4 received the considerable yield (> 80%, 1-3 layers) of FG [40]. Feng et al. improved it by changing the IDP in aqueous solution of inorganic salts ((NH4)2SO4, K2SO4, Na2SO4, etc.), which leads to the 85% yield production of 1-3 layers FG with large sheet size and good structural integrity, that is, hole mobility up to 310 cm2 V−1 s−1 (Fig 8b) [71]. Electrolytes

containing

(2,2,6,6-tetramethylpiperidin-1-yl)oxyl

(TEMPO)

was

employed to partially eliminate the oxidation damage effects of •OH radicals, resulting in the formation of FG with better quality (C/O ~ 25.3, hole mobility up to 405 cm2 V−1 s−1) (Fig. 8c) [41]. The scalability and exfoliation efficiency could also be improved by the implementation of alternating current (Fig. 8d) [42]. In this approach, both electrodes are constantly undergoing efficient IDP in TBA+HSO4− electrolyte system for they are alternately acting as anode and cathode, thus producing high-quality FG with carrier mobility up to 430 cm2 V−1 s−1.

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Fig. 8. Intercalation-decomposition process (IDP) and its applications in the fabrication of FG. (a) The mechanism of surface blistering phenomenon during the anode reaction of graphite electrode in mild aqueous solutions [70]; (b) Electrochemical synthesis of FG by IDP method, the electrolyte is 0.1 M (NH4)2SO4 solution [71]; (c) Improved synthesis of high-quality FG by IDP approach, TEMPO was employed for the elimination of oxygen-radicals [41]; (d) Highyield synthesis of FG by applying the alternating current [42] (Figures are reproduced with permission from (a) Murray et al. [70], copyright @ 1995 The American Chemical Society; (b) Feng et al. [71] copyright @ 2014 The American Chemical Society; (c) Feng et al. [41] copyright @ 2015 The American Chemical Society; (d) Feng et al. [42] copyright @ 2017 John Wiley & Sons). 21

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Ding et al. made a modification to the recipe for electrolytes by introducing H2O2 and oxalic acid for enhanced gas formation [72]. The final products, though primarily ranging from 3-5 layers, remains highly-undisturbed graphitic structure with Raman ID/IG = 0.022 and electrical conductivity of assembled films 267 S cm−1. They also proved the overwhelming power of radical cutting on graphite anode for the one-step synthesis of graphene quantum dots (GQD) in weak electrolyte (Fig. 9a) [73]. In this approach, the decomposition of graphene layer is more favored than intercalation process under the high-concentration of radicals, thus producing exclusively GQD. Zhong et al. presented an elegant approach to enhance the IDP via prolonging the contact time between graphite flakes and mixed metal oxide-coated titanium mesh anode (Fig. 9b) [74]. Therefore, the monolayer yield of GO was greatly improved to 66% due to the mechanically-assisted mass transfer process.

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Fig. 9. Optimization of electrolyte and mass transfer in the practice of IDP method to FG materials. (a) The high-concentration radicals enhance the oxidation cutting process of graphite anode, which results in the high-yield direct production of graphene quantum dots [73]. (b) Mechanical stir-enhanced contact between graphite flakes and titanium mesh anode for the improvement of functionalization degree [74] (Figures are reproduced with permission from (a) Ding et al. [73], copyright @ 2018 The American Chemical Society; (b) Zhong et al. [74] copyright @ 2016 The American Chemical Society).

IDP based on non-aqueous media served as another methodology to fabricate FG [43−45]. Slightly different from the reaction involved in anode IDP, the non-aqueous cathode IDP is dominated by the interplay between accumulated interlayer stress in graphite gallery during rapid intercalation and the simultaneous chemical functionalization. In early 21st century, the electrochemical expansion and fragmentation of graphite cathode in lithium ion batteries was the major challenge and everything scientists make efforts to circumvent [75,76]. Loh et al. re-considered the 23

ACCEPTED MANUSCRIPT issue in a diametrically opposite viewpoint: the destructive effects of graphite electrode in e.g., Li+ and propylene carbonate (PC) could also be used in the preparation of few layer FG (Fig. 10a) [43], and astonishingly, the exfoliation showed high efficiency with yield up to 70% (< 5 layers). The co-intercalation of Li+ and solvents resulted in the formation of ternary GIC, and the exfoliation starts with the accumulated interlayer stress at grain boundary. IL media was also employed in the electrochemical IDP approach: Chen et al. provided the early example [46], Loh et al. clarified the interplay between hydrophilic species (e.g., H2O) and intercalants (in IL, e.g., BF4−) and their influence on the oxidation degree of FG [47], concluding that for water content > 10%, the FG is dominated by oxygen functionalization, whereas for water content < 10%, FG is IL-functionalized (Fig. 10b). IDP approach was also proved to be highly scalable, eco-friendly and suitable for the preparation of few layer FG by many excellent works [40−47,71−74,77,78]. Nonetheless, the IDP limits the formation of high level GIC, thus lacking the sufficient functionalization degree necessary for the dispersion of FG at high concentration in aqueous system. Furthermore, the poor single-layer yield of IDP approach is also an obstruction for applications that requires delicate assembly of building blocks and specific surface area. Therefore, the exfoliation based on GIC with higher stage level is of undisputed importance to the increase of functionalization degree and monolayer yield, and we will discuss this process in the next section.

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Fig. 10. IDP approach to FG in non-aqueous system. (a) Fabrication of organic solution dispersible FG by co-intercalation of Li+ and PC [43]. (b) IDP in ionic liquid and the functionalization of graphene demonstrated by K. P. Loh et al. [47] (Reproduced with permission from (a) Loh et al. [43], copyright @ 2011 The American Chemical Society; (b) Loh et al. [47] copyright @ 2009 The American Chemical Society).

3.

Functionalization of graphite intercalation compounds Functionalization of GIC is the basic process in wet chemical oxidation of graphite.

In Fig. 2b, the GIC (e.g., formed in concentrated H2SO4) is functionalized in the presence of nucleophiles/electrophiles (e.g., water or diazonium salt), and the intrusion of functional groups modifies the solubility of resultant FG, making them solution processable. In the classic Charpy-Hummers-Offeman method [6,7], stage-1 GIC forms shortly after the addition of KMnO4 [50], followed by the oxidation effect and oxygen functionalization during hydrolysis, the graphite is transformed to bulk graphite oxide, 25

ACCEPTED MANUSCRIPT which can be easily exfoliated to single layer GO by mild sonication in deionized water. Recently, the paradigm of wet chemical oxidation was transplanted in the procedure of electrochemical synthesis of FG. We will review the attempts made on this methodology while place emphasis on the understanding and regulation of the reaction.

3.1. Basic process of the functionalization Understandings on the conversion of stage-1 GIC to functionalized graphite at molecular level is still in the black box even today. The overall issue was extensively but only poorly studied over the past years, and primarily with the researches on the transformation of GIC to graphite oxide [50,79,80]. Specifically, the insertion of oxidants and replacement of intercalants may follow the completely different patterns for every system [81]. And unfortunately, even for the structures of final products, which are recognized to be different in respect of the functional groups [81], also remain unknown in respect of their localized chemical structure. These challenges are still making scientists feel compelled to explore the system continuously from the phenomenal results, and the review of their findings are highlighted below. In wet chemical synthesis of FG, the reaction temperature during intercalation and oxidation process was showed to have a pronounced influence on the specific area and oxidation degree of final products, as Domínguez-Vega et al. discovered in the 0°C oxidation by Staudenmaier method [82]. Eigler et al. successfully tuned the functionalized degree of GO obtained by Charpy-Hummers-Offeman method with the same methodology (Fig. 11a) [83], and most importantly, they showed the formation 26

ACCEPTED MANUSCRIPT of structural defects was hindered by the low temperature. Difference in temperature in the functionalization step may change the thermodynamic stability of functional groups on graphene platform, hence leading to the desperate expression of the products.

Fig. 11. Understandings on the functionalization process of GIC from the chemical oxidation system. (a) Fabrication of GO with excellent structural integrity by the low-temperature functionalization process (< 10ºC) [83]. (b) The influence of water content in sulfuric acid on the chemical structure of GO [84]. (c) The underappreciated second step oxidation (hydrolysis procedure) in traditional Charpy-Hummers-Offeman method [85] (Reproduced with permission from (a) Eigler et al. [83], copyright @ 2013 John Wiley & Son; (b) Shi et al. [84] copyright @ 2016 The Royal Society of Chemistry; (c) Park et al. [85] copyright @ 2016 The American Chemical Society).

In addition to temperature, Shi et al. found that the water content in H2SO4 also plays an important role in the functionalized degree of GO (Fig. 11b) [84]. Park et al. pointed out the function of water in the chemical oxidation of graphite to GO [85,86]: the formation of structural defects and the lateral size of GO was closely correlated to the 27

ACCEPTED MANUSCRIPT conditions of the “second step oxidation” (Fig. 11c): during this step the addition of water decomposes Mn2O7 to MnO4− and facilitate the breakage of C−C bond.

3.2. Electrochemical synthesis of functionalized graphene based on functionalization of graphite intercalation compounds The functionalization process in electrochemical approach to FG is similar to that in the traditional wet chemical methods. The major difference lies on the oxidation pathway: for chemical oxidation, the redox reaction of graphene layers relies on the concentration of oxidizing agent, and the chemical species are omnipresent; but for electrochemical oxidation, the electrical contact between graphene layers controls the final oxidation degree, namely, the functionalization may terminate if GIC is adequately exfoliated to break the circuit in the conductive matrix. Furthermore, the functionalization mechanism for electrochemical oxidation is presumably different from chemical oxidation: in the former case the highly nucleophilic nature of water species (H2O, HO• and HO−) contributes to the functionalization of graphene [87], whereas oxidants are shown to be more crucial in the chemical oxidation. In fact, even though the swelled graphene gallery is prone to disintegrate with the aid of solvation effect, the functionalization degree contributed by the second oxidation step that equates to the hydrolysis procedure in chemical oxidation method is sufficient for the dispersion of FG in aqueous media [49]. Herein we denote the electrochemical synthesis of FG equipped with the functionalization step as “two-step method”, aiming to distinguish them from the “one-step IDP method” discussed previously. 28

ACCEPTED MANUSCRIPT Zhong et al. provided the prototype of two-step synthesis of GO followed by intercalation-functionalization procedure [48]. Their choice of the electrolyte is the 11.6 M perchloric acid, because this concentration satisfies both the criteria for intercalation and the following functionalization step. In their method, the lowest C/O (atomic ratio) for the GO is 3.0:1, which remarks a huge step towards the bulk functionalization of graphene layer that is shown in traditional chemical oxidation method (Fig. 12a). Kinloch et al. [88] and Ren et al. [49] successively reported the two-step synthesis of GO imitating the reaction stage happened in Charpy-Hummers-Offeman method: with the first intercalation step in concentrated H2SO4, and the second functionalization step in (NH4)2SO4 or dilute H2SO4 (Figure 12b and c). Kinloch highlighted the excellent structural integrity of GO fabricated through electrochemical method by demonstrating the highly conductive reduced GO film (546 S cm−1). Ren achieved the highest oxidation level for GO (C/O < 2) ever produced from electrochemical pathway, and they also showed the excellent processing performance for GO by the construction of transparent conductive films, paper-like materials, and elastic aerogels.

29

ACCEPTED MANUSCRIPT

Fig. 12. Two-step electrochemical synthesis of GO. (a) Two-step method in perchloric acid electrolyte [48]; Two-step synthesis that involves intercalation process in concentrated sulfuric acid and the second step functionalization process in (b) 0.1 M (NH4)2SO4 and (c) H2SO4 aqueous solution with different concentration [49,88] (Reproduced with permission from (a) Li et al. [48], copyright @ 2017 Elsevier; (b) Kinloch et al. [88] copyright @ 2017 The American Chemical Society; (c) Ren et al. [49], copyright @ 2018 Nature Publishing Group).

Apart from the anode oxidation, cathode process combined with functionalization step was also used to fabricate FG materials [61,89]. Swager et al. reported the intercalation of two guest cations, Li+ and the followed TBA+ (in PC media), for the enhancement of expansion of graphite gallery (Fig. 13a) [89]. In the second step, they functionalized graphene with aryldiazonium salts, making resultant FG could be well30

ACCEPTED MANUSCRIPT dispersed in DMF, which render FG samples with the ability to be processed to a bulk film material through filtration (Fig. 13b). Recently, Swager et al. proved the hyperstage GIC when applying a high negative potential (< 3.05 V, versus Ag/AgCl) to graphite cathode in TBA+/DMF/MeCN system (Fig. 4c and Fig. 13c) [61]. Moreover, the functionalization step featured the addition of 3,5-dinitrophenyl group onto graphene layers, and the formation of related Meisenheimer complexes endowed FG with improved dispersion stability in MeCN (Fig. 13d).

Fig. 13. Cathode functionalization of graphene. (a) The enhanced intercalation of graphite cathode in PC by Li+ and TBA+ [89]; (b) Functionalization of GIC by aryldiazonium salts and the demonstration of the dispersion in DMF and processability [89]. (c) Functionalization by 3,5-dinitrophenyl group onto graphene layers and (d) the formation of Meisenheimer complexes 2 [61] (Reproduced with permission from (a,b) Swager et al. [89], copyright @ 2012 The American Chemical Society; (c,d) Swager et al. [61], copyright @ 2018 John Wiley & Sons).

31

ACCEPTED MANUSCRIPT 4.

Chemical modification of functionalized graphene The final step deals with the most important part of the complete set of

electrochemical approaches to FG: modification on the chemical structure (Fig. 2c). Every specific FG with disparate chemistry fills its own position in reference to different application backgrounds [23−28,80]. Basically, for FG, especially the widely studied GO, oxidation coupled with flake cutting and reduction are the two major methods to modify the fraction of functional groups (Fig. 14), lateral size of FG, and proportion of conjugated area on FG sheets. Chemical approaches for the oxidation and reduction of GO were well-studied, whereas the electrochemical methods for these purposes are still need to be established. In this part, the general methods will be briefly discussed. Then we will evaluate the potential to achieve these processes using the electrochemical pathway.

Fig. 14. Further modification options of GO, including the reduction process and oxidation cutting process, the final product may be reduced GO and PG, or GQD.

32

ACCEPTED MANUSCRIPT 4.1. Fabrication of graphene quantum dots through oxidation of functionalized graphene GO could be further oxidized when treating with the same reaction system used for the transformation of graphite to GO. Schafhäutl described the complete oxidation of graphite to gas products in the mixture of sulfuric acid and nitric acid when repeating for several times [90]. Actually, the oxidation process of graphite is divided to three steps [91,92]: 1) formation of primary oxide; 2) formation of GO; 3) decomposition of GO to humic acid like molecule fragments. Generally, over-oxidation and cleavage of GO sheets are employed to obtain carboxyl-rich carbon nanomaterials and GO quantum dots [92−95]. The repeated oxidation of GO resulted in the formation of graphene acid, which contained a high proportion of carboxyl groups and O/C ratio up to 2.52 [92], providing the promising materials for large capacity sorption of metal ions and carbon dioxide. Oxidation unzipping of GO was also employed in the fabrication of GQD [95,96]. Wu et al. demonstrated a two-step oxidation cutting (first step in concentrated H2SO4 and HNO3, second step is hydrothermal cutting process) for the preparation of GQD with diameter ca. 10 nm that displayed blue photoluminescence [95]. Apart from the chemical method for oxidation cutting, Thiele in 1934 reported the first example of the electrochemical oxidation of graphite that featured the products of all oxidation steps, namely, from primary oxide to humic like species [8], but with relatively low transformation yield compared with the chemical oxidation method. Qu et al. achieved the electrochemical cutting of reduced GO film material in phosphate33

ACCEPTED MANUSCRIPT buffered saline electrolyte [96], and the GQD products exhibited green luminescence and could be well-dispersed in aqueous media because of the oxygen functionalization on basal plane. In the successive work. Qu et al. made use of the inherited structural defects on reduced GO flakes for the active sites for efficient N-doping (Fig. 15a) [97], producing GQD with comparable performance to the Pt/C catalysts for oxygen reduction reaction.

Fig. 15. Electrochemical methods for the chemical modification of functionalized graphene. (a) Electrochemical oxidation cutting of reduced graphene oxide films for the fabrication of Ndoped GQD as efficient oxygen reduction reaction catalyst [97]; (b) Electrochemical reduction deposition of graphene oxide on Au electrode for the preparation of high-rate supercapacitor [113]. (Reproduced with permission from (a) Qu et al. [97], copyright @ 2011 The American Chemical Society; (b) Shi et al. [113], copyright @ 2012 Nature Publishing Group).

4.2. Reduction and electrochemical deposition of graphene oxide Two paramount goals of reduction process (Fig. 14) involve 1) remove specific finds of oxygen-containing groups on the skeleton of GO, then the conjugated structure 34

ACCEPTED MANUSCRIPT intrinsic to PG could be partially restored; 2) repair the structural defects and remove the majority of oxygen moieties, and the electrical conductivity and other physical properties of reduced GO could be largely alternated to resemble PG. Generally, the accomplishment of the first goal comprises two major roads: the chemical/bio reduction [98−100] or thermal reduction [101,102]. Chemical reduction requires the development of efficient, cheap and eco-friendly reducing agents. Prevalent reduction recipes include hydrazine (N2H4) [103,104], hydroiodic acid (HI) [105,106], sodium borohydride (NaBH4) [107], and ascorbic acid [108], whereas none of these reducing agents satisfies all the criteria. In terms of thermal reduction, annealing of GO materials was also adopted to remove oxygen-containing groups [101,102]. The formation and enlargement of lattice defects under 200°C~800°C accompanied by the elimination of CO/CO2 poses obstacles in the fabrication of highquality graphene materials [102]. Photo-reduction and bio-reduction of GO were also proved [100,109], but the low efficiency and difficulty for scaling-up (from purification and cost) hindered the further development. The second goal—repair of topological defects on reduced GO, has been intensely studied because it served as the most important link for the bridging of FG to PG [15]. Similarly, the method to fulfill this goal also comes in two possible options: high temperature annealing (> 1500°C) [110] and CVD-assisted restoration of structural defects using e.g. ethylene as carbon source [111]. These methods rather run counter to the original intention of wet electrochemical method—the overall process must be costeffective. 35

ACCEPTED MANUSCRIPT Solution electrochemical reduction of GO still remains poorly studied. The major problems include 1) limited electrical contact for FG sheets and electrode in solution system block the functionalization process; 2) the functionalization product is not welldispersed in the given electrolytes. As a proof-of-concept, Dâna et al. observed that multi-layer GO films exposed in air could be reversibly oxidized or reduced by electrochemical method [112]. Shi et al. reported the electrochemical deposition of GO on cathode (Fig. 15b) [113]; the resulting highly porous 3D graphene matrix could be used as electrode material for the construction of high-rate electrochemical capacitors [114]. However, these practices are readily not scalable and they fail to transfer FG with one specific chemical identity to another with both samples dispersible [115−117]. The electrochemical pathway for the eco-friendly reduction of GO still need to be explored.

5.

Applications of functionalized graphene and prospects on electrochemical

approach Through the former sections all the key steps (Fig. 2) for the synthesis and modification of FG based on electrochemical strategies have been discussed. The paradigm and future for these processing strategies is to turn all the steps from hazardous chemical pathway or expensive physical exfoliation methods to eco-friendly and cost-efficient electrochemical avenue. In the last section of this review, the potential applications of the electrochemically synthesized FG is illustrated, and the challenges as well as the future of this methodology are discussed. 36

ACCEPTED MANUSCRIPT

5.1. Applications of functionalized graphene synthesized by chemical oxidation and electrochemical approaches The major explored applications of FG materials could be presented in four categories (Fig. 16): 1) energy storage and conversion, including the supercapacitors and batteries [27]; 2) structural materials for the enhancement of mechanical, electrical and thermal properties [25,28,118,119]; 3) Environmental science, such as the nanofiltration and adsorption of pollutants [120,121]; 4) Chemistry research, including the utilization of GO as carbocatalysts and electrochemical catalysts [23,26]. The material properties that ensure these applications stem directly from the interplay between intrinsic properties of FG and the micro-structure/stacking structure of the bulk FG assemblies. Therefore, the preparation method of FG and the related bulk materials is of vital importance to the applications. Compared with FG synthesized via traditional wet chemical method, FG fabricated through electrochemical approach generally showed lower oxidation degree, thus the poor

dispersibility

and

processability.

Consequently,

the

applications

of

electrochemically synthesized FG have been confined predominantly in the field of energy storage and conversion [122−124] (Fig. 16). For example, Wu et al. demonstrated the electrochemically produced FG/thiophene nanosheets based heterostructure supercapacitor with high rate capacity [122]. The excellent performance was attributed to the rapid electron transport through FG layers for the high electrical conductivity of intact FG. In other important applications for FG material, such as 37

ACCEPTED MANUSCRIPT structural materials, nano-filtration membranes and catalysts, the potentials of electrochemically synthesized FG are still poorly explored. These applications all require the high yield of single layer product and homogeneous functionalization of graphene, which is still the major challenges in electrochemical methods.

Fig. 16. Potential applications of FG. Energy storage and conversion is the major applications of FG: e.g., the lithium ions battery, all-solid supercapacitor and micro-supercapacitor; However, the applications in structural materials, nano-filtration and catalyst and sensor are still not explored in electrochemically synthesized FG because of the low yield of monolayer FG and homogeneous functionalization

5.2. Challenges and the future of electrochemical fabrication of functionalized graphene The challenges in electrochemical production of FG are: 1) Low fabrication yield to 38

ACCEPTED MANUSCRIPT FG with monolayer and considerable functionalized degree. This may stem from the fast deintercalation process when changing the reaction system directly into e.g., mild acid/salts solution, then the unattached few layer FG could not be further functionalized; Note that in the work of Ren et al. [49], the yield for GO with C/O ratio approaching 1.5~1.8 should not reach 96% because of the heterogeneous oxidation nature in this system; 2) Polydispersity in both the lateral size and structural integrity of the products; 3) The difficulty to modify the chemical structure of readily exfoliated and dispersed FG, namely, further oxidation or reduction based on the solution electrochemical approaches. In contrast, chemical oxidation system does not include the major problems displayed above, and the reason is attributed to the homogeneous distribution of oxidants in the whole reaction media and the powerful mass transfer process that ensure the complete and uniform functionalization of graphene. Therefore, a new set of remedies for these problems may include: 1) The enhancement of electrical contact between FG and the electrode by the fabrication of working electrode composed of graphite flakes and conductive binder, or making reversible contact between high specific area electrode and dispersed FG. The model described in [74] could be a solution, but still need to be optimized in terms of the efficiency; 2) In situ formation of oxidizing/reducing agents triggered by electrochemical reaction in the electrolyte that display high reaction activity on the functionalization of FG. This strategy may guarantee the continuous modification of FG in solution system. However, the graphene chemistry is still lacking the irreplaceable application 39

ACCEPTED MANUSCRIPT backgrounds because of the challenges in the synthesis of PG/FG with highly controllable quality at a large scale. To this end, electrochemical approaches not only rise to the occasion but also provide the solutions in an elegant way. Being developed through a century, in the field of functionalized graphene, as Lerf wrote [125], “most workers are just preparing footnotes to the seminal works of Kohlschütter, Hofmann and Boehm”, but we are not constrained in the great works by these pioneers, rather, there is still many things about synthesis and chemistry of FG should be studied by modern techniques with entirely new horizons. It is not the end of the story, on the contrary, the story just begins.

Acknowledgments This work was supported by National Key R&D Program of China (2016YFA0200200, 2017YFB1104300), and the National Natural Science Foundation of China (51673108, 21674056, 51433005, 51673026).

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