Radical Mechanism for Reduction of Graphene Derivatives Initiated by

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Radical Mechanism for Reduction of Graphene Derivatives Initiated by Electron Transfer Reaction Wenchuan Lai, Zaoming Wang, Yulong Li, Xu Wang, Yang Liu, and Xiangyang Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01941 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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The Journal of Physical Chemistry

Radical Mechanism for Reduction of Graphene Derivatives Initiated by Electron Transfer Reaction

Wenchuan Lai, Zaoming Wang, Yulong Li, Xu Wang*, Yang Liu and Xiangyang Liu*

State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University, Chengdu, Sichuan, 610065, P.R. China;

ABSTRACT: Electron paramagnetic resonance (EPR) spectroscopy was innovatively employed to reveal the particular mechanism for chemical reduction of graphene derivatives in this work. It was found that reduction of graphene oxide (GO) can occur in a radical mechanism consisted of the electron transfer (ET) and deoxygenation reaction, which was proved by detecting the relevant radical intermediates and change of radical centers on graphene nanosheet during reactions via EPR method. Further researches of fluorinated graphene (FG) also indicated that reduction of graphene derivatives was always involved with radical process initiated by ET reaction. The radical mechanism is further emphasized as the typical characteristic of the graphene chemistry.

INTRODUCTION Graphene is the most highly studied two-dimensional (2D) material over the years of

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21st century due to its exceptional electronic, optoelectronic, electrochemical and biomedical applications.1-3 Despite, it should be noted that the zero band gap, the inert reactivity and the poor dispersibility in other matrices have restricted its applications in many fields.4 The emerging of graphene oxide (GO), the most significant graphene derivative5, has largely overcome these restrictions.5-7 On the other hand, the preparation method of GO, namely the chemical oxidation of graphite8 has also paved the way to the large-scale production of graphene materials. To accurately synthesize graphene products with desired structures or properties, the chemistry of GO deserves further attention containing its reduction reactions and chemical modifications.6, 9 In the matter of the reduction, recently there have been numerous works investigating the reduction of GO by chemical, thermal, irradiation, electrochemical and photoexcited physical methods.10-17 The chemical method contains reduction caused by strong reducing agents such as hydrazine monohydrate, hydroquinone, sodium borohydride and hydrohalic acid, moderate reducing agents like metal/hydrochloric acid, non-toxic agents such as L-ascorbic acid, vitamin-C, D-glucose, wild carrot root and so on.11-12, 18 Accordingly, the mechanisms of these varieties of chemical reductions of GO have been partially proposed, while some of those are still assumptions without further experimental evidence.11-12, 19 For example, in the case of redcution by ethanediamine, a series of reactions happened step by step, leading to the final deoxygenatiton and formation of C=C bond.10 It seems that these proposed mechanisms are mainly deduced from the classical reactions of organic micromolecule without considering


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the peculiarity of 2D structure of GO material. The chemistry characteristic of GO possibly distinguished from micromolecule has not been revealed until now. Moreover, it should be noted that the fluorinated graphene (FG), another important member of graphene derivative family,20 has also shown the similar reduction behaviors as GO under similar conditions such as thermal treatment21, reductive or alkaline reagents.22-24 The potential common characteristic that could exist in reduction reactions of graphene derivatives as well as other reactions needs to be exposed. The purpose of this work is trying to reveal some particular mechanisms for chemical reduction of graphene derivatives especially GO, which is distinguished from the mechanism proposed in previous reports and can also enlighten us to understand the graphene chemistry more deeply. The main technique was chosen as electron paramagnetic resonance (EPR) spectroscopy, which can effectively observe the change of radicals centers (and paramagnetic defects, spin centers), and detect the radical intermediates during reactions.25-27 The reduction of GO by several reductive agents was investigated, including sodium hydroxide (NaOH), ethanediamine (EDA), sodium hydrogen sulfite (NaHSO3) and so on. It was demonstrated by EPR that the concentration of EPR active centers on graphene nanosheet evidently changed during reduction. Thus, the assumption that reduction of GO occurred in a radical mechanism was then proposed, further proved by successful detection of related radical intermediates via radical capturing method. The initiation effect of the electron transfer (ET) reaction on subsequent deoxygenation reaction was emphasized. Further



reduction researches of FG demonstrated that radical process initiated by ET could always be appropriate for reduction reactions of graphene derivative. In the end, the probable typical characteristic of the graphene chemistry was put forwarded as well.

EXPERIMENTAL SECTION

Preparations. Graphene oxide (GO) was prepared and purified according to the Hummers method.8 The reduction of GO by NaOH was performed via adding 5 mL NaOH aqueous solution (10M) into 50 mL GO suspension (1 mg/mL) with the aid of stirring for 2h. The solid reduced GO sample for characterizations was obtained by intensive centrifugation (1000 r/min, 30 min) of the final suspension, followed by washing with distilled water, and freeze-drying; the final reduced product was named as GO-NaOH. For detections of radical intermediate related to the reduction, 2 mL reacting suspension after 10 min of the addition of NaOH solution was fetched out and added into 10 mL 2-methyl-2-nitrosopropane (MNP) or 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) solution (50mM) for EPR measurement. Other reduced GO samples reduced by zinc powder (Zn, with the aid of hydrochloric acid HCl), or ethanediamine (EDA) and sodium hydrogen sulfite (NaHSO3) were performed in a similar way, and the final reduced products were name as GO-Zn/HCl, GO-EDA and GO-NaHSO3 respectively. The reduction by hydrazine hydrate (N2H4) vapor was performed in a drying tower for 5 days, obtaining reduced sample GO-N2H4 vapor. The capture of intermediate radical related to reduction by EDA or NaHSO3 was performed in a similar way as reduction


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by NaOH. The preparation of fluorinated graphene (FG) was referred to our early works28-29 and the reductions of FG by NaOH or EDA were performed in a similar way as GO, and the final reduced products were name as FG-NaOH and FG-EDA respectively.

Characterizations. X-ray photo-electron spectroscopy (XPS) which was carried out on a Kratos ASAM 800 spectrometer (Kratos Analytical Ltd, UK) at a base vacuum higher than 10-6 Pa under non-monochromatized Al Ka (1486.6 eV) X-ray source (a voltage of 15 kV and a wattage of 250 W) radiation. The Raman spectrum was obtained by using a LabRAM HR Raman spectrometer with an excitation wavelength of 532 nm. Thermo-gravimetric analysis (TGA) was performed on a Netzsch 209 TG instrument under N2 atmosphere at a heating rate of 10 °C/min from 30 to 800 °C. The fine microstructures of GO sample were characterized by using HR-TEM (Tecnai G2 F20 S-TWIN) with accelerating voltage of 200kV. Electron paramagnetic resonance (EPR) measurements were carried out on Bruker EPR EMX Plus spectrometer (Bruker Beijing Science and Technology Ltd, USA) with an ER4119HS resonator type, operating at frequency of 9.842 GHz. Spectra were recorded at 0.2 mW microwave power with 0.1 mT modulation amplitude and 100 kHz frequency modulation at room temperature. All samples were measured at 20 s sweep time, 20 mT sweep width for 8 times. Spectra processing and simulations were performed using Bruker WIN-EPR and Isotropicradicals software.



RESULTS AND DISCUSSION

Figure 1. XPS C 1s spectra of pristine GO (A) and reduced sample GO-NaOH (B) reduced by NaOH.

We firstly focused on the reduction of GO by sodium hydroxide (NaOH). In 2008, Zhang reported that the exfoliated GO can undergo quick deoxygenation reduction in strong alkali solutions at moderate temperatures30, providing a green route to the synthesis of graphene with excellent dispersibility in water. The mechanism of this reduction has been also investigated in some subsequent studies31-32, for example, the formation of chemical defects (the breaking of C-C bond and formation of -C=O groups on framework) on graphene nanosheet under attacking of hydroxide ion (OH-). However, as what follows in this paper shows, our work would reveal other possible pathway and mechanism of this reduction process. The comparison of X-ray photoelectron spectroscopy (XPS) spectrum between pristine GO and reduced sample


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GO-NaOH is depicted in Figure 1. The pristine GO obtained from Hummers method contained abundant oxygen related groups on graphene nanosheet (oxygen content 32.71%), including epoxide, hydroxyl and carboxyl groups. After reduction by NaOH, the oxygen content of GO-NaOH samples was reduced to 16.45%. By comparison of C 1s spectrum between GO and GO-NaOH, the different signature of GO-NaOH whereby significantly less intense peak of C-O group (epoxide or hydroxyl group) as well as the enhancing peak of C=C bond were observed, which was consistent with the previous report of Zhang. It was also suggested by detailed analysis of C 1s spectra that the content of carboxyl group increased a bit after reduction, in agreement with the previous reports31.

Figure 2. EPR spectra comparison between pristine GO and GO-NaOH sample.

The introducing of electron paramagnetic resonance (EPR) spectroscopy then showed another possible mechanism for GO reduction by OH-. The EPR spectra



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comparison between pristine GO and GO-NaOH samples is shown in Figure 2, and all EPR spectra holds a similar g-factor at around 2 with narrow linewidth about 0.30 mT, which are typical for radical centers (or spin centers) of graphene materials25, 27, 33. It was observed that compared to GO, the GO-NaOH behaved a more intense EPR signal, demonstrating that concentration of spin centers increased during reduction. The existence of those spin centers was suggested to be depended on the stabilizing effect of aromatic conjugation regions on graphene nanosheet. The interesting EPR spectra demonstrated that the C=C bonds of reduced GO were highly likely resulted from the coupling reaction of spin centers on nanosheet instead of the one-step generating. Therefore, it could be then concluded that the main reduction reaction (deoxygenation) of GO should occur in a radical mechanism bringing about spin centers on graphene nanosheet, and the coupling of adjacent spin centers would generate C=C bonds (insert 1 in Figure 2). This finding provided another pathway to reestablish unsaturated carbon bonds and aromatic regions during reduction of GO. Meanwhile, the key issues should be solved that why this radical deoxygenation reaction can happen and what other products were except for spin centers on graphene nanosheet during reductions.

OH OH-

OH OH ET e

·OH

OH OH - +

+

-

spin center

OH-

OH ET

Reduced GO

GO O e-

ET

O·OH

+

ET OH -, H 2O

2 OH -

+

OH -

Scheme 1 The proposed reduction mechanism of GO under attacking of sodium hydroxide. The


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“ET” means electron transfer reaction.

To shed light on the issues, the concept of electron transfer reaction34-36 (ET) was then introduced into this reduction study. Since deoxygenation reaction in radical pathway can not occur spontaneously, the deoxygenation should be drived by some driving force, which was supposed to be ET reaction between oxygen-containing group and reductive agents. Although the ET induced GO reduction have been investigated in reduction by solvated electrons or reducing free radicals37-38, the ET between GO and non-radical agent is still unreported. Based on this assumption, the particular mechanism for reduction by OH- was then proposed for the first time, as Scheme 1 exhibits. The discussed deoxygenation reaction in Scheme 1 mainly concentrated on hydroxyl and epoxide group, without considering the carboxyl group, because carboxyl existed in defect regions and their concentration even increased during reduction. Taking the removal of hydroxide group for explanation, the central reaction of reduction process was suggested to be the ET reaction between hydroxide ion and -C-OH group on nanosheet, producing a transition state -C-OH- structure and a hydroxide radical intermediate. The metastable structure was then dissociated into two parts, namely a new hydroxide ion and carbon radical on nanosheet (spin center) which has been confirmed by EPR spectra in Figure 2. Further ET reaction between hydroxide ion and -C-OH group adjacent to original spin center would generate another spin center, while the final C=C bonds of reduced GO were stemmed from the coupled reactions of these spin centers. The deoxygenation reaction of epoxide group



happened in a similar process, resulting in spin centers and C=C bond on nanosheet, as well as the hydroxide ions. It can be found that ET reaction played an important role in reduction of GO trigging the subsequent deoxygenation reaction. However, from the point of thermodynamics, it is still doubtful whether the above mentioned ET reaction between hydroxide ion and -C-OH group could happen. Actually, as Zhang reported, only beyond 80 °C can the reduction of GO (about 1 mg/mL, 150mL) happen under attacking of NaOH (1 mL, 8M). Whereas at temperatures as low as 15 °C, the reduction was able to occur only when the pH of the suspension was high enough. These results can be explained from the viewpoint of electrochemistry thermodynamics. Since the redox potential of GO14 and hydroxide ion are approximately 0.75 eV and -1.5 eV, the redox reaction or the ET reaction between GO and NaOH can not take place spontaneously under standard condition based on The Nernst Equation ( E = E Θ − ln J a RT ZF ). In comparison, when temperature rising highly enough or the concentration of hydroxide ion was large enough, the Gibbs free energy of the reaction would be transformed to be negative, thus leading to the occurring of the reduction. Anyway, the proposed mechanism in Scheme 1 firstly indicated the reduction of GO by NaOH can take place in a radical mechanism, which would generate spin centers and C=C bonds on graphene nanosheet, hydroxide ions as well as hydroxide radical intermediates.


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Figure 3. EPR spectra of trapped radical intermediate related to reduction by NaOH using 2-methyl-2-nitrosopropane (MNP) as trapping agent (due to the poor solubility of MNP in water, the mixed solvent ethanol/water was finally chosen). The black line was obtained from the experimental data, while the others come from the simulation (Isotropicradicals software). The olive line was the standard spectrum of ·OH radical captured by MNP.

To further prove the proposed mechanism in Scheme 1, it is significant to confirm the existence of intermediate hydroxide radical generated in ET reaction. Due to the short life of intermediate, the radical trapping method was then employed by choosing 2-methyl-2-nitrosopropane (MNP) as the trapper39, and the resulting EPR spectra is shown in Figure 3. It was observed that no evident EPR signal was detected in GO suspension without adding of NaOH (Figure S1). As a comparison, in the presence of NaOH the reduction of GO occurred, leading to the complex EPR spectrum of captured radical intermediates (black line, Figure 3). Further simulation of obtained



spectrum by Isotropicradicals software demonstrated the black line can not match the standard spectrum of ·OH radical captured by MNP (olive line). However, it was suggested that the black line was made up by two lines, blue line 1 attributed to radical with two α-protons magnetic nucleus and pink line 2 assigned to another radical without any magnetic nucleus in α-position. The hyperfine splitting constant of line 1 is Aα-N =1.71 mT and Aβ-H =1.13 mT (·CH2R), while Aα-N =1.72 mT for line 2 (·CR1R2R3). Considering the lively reactivity of ·OH and its complex reactions with ethanol, the line 1 is suggested to be the spectrum of MNP with ·CH2CH2OH radical formed by subtracting β-H of CH3CH2OH.40 Hence, the existence of ·OH has been indirectly confirmed and the proposed mechanism in Scheme 1 was preliminarily proved. Despite, it should be noted that meanwhile another radical ·CR1R2R3 also appeared in the trapping system, which can be attributed to adduct of spin center on graphene nanosheet with MNP trapper. This is consistent with the result of Figure 2 that new radical centers were produced on nanosheet during reduction of GO.


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Figure 4. EPR spectra of trapped radical intermediate related to reduction by NaOH using MNP as trapping agent. The mixed solvent DMF/water was chosen.

To avoid the influence of ethanol in radical intermediate during reduction by OH-, further trapping experiments were performed by using DMF/water mixture solvents to dissolve MNP, and the EPR spectra of captured radicals is depicted as Figure 4 (GO-NaOH-MNP, black line). The spectra line of GO-NaOH-MNP can also be divided into two lines. The splitting of simulation line 3 was caused by three kind of magnetic nucleus, resulting in hyperfine splitting constant of Aα-N =1.71 mT, Aβ-H =1.12 mT and Aγ-N = 0.23 mT. Similarly, taking the reaction of radical intermediate with DMF into consideration, the captures radical of fitting line 3 can be attributed to radical of DMF whose hydrogen in methyl position was abstracted by ·OH. Meanwhile, another radical ·CR1R2R3 captured by MNP also appeared in Figure 4 (line 4), which was belonged to spin centers on graphene nanosheet. The spectra in



Figure 4 further confirmed the existence of radical intermediate generated from ET reaction. However, the subsequent capturing experiments of radical intermediate by using 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) failed due to some reasons unknown (Figure S1). Despite, the EPR spectra of captured radicals in Figure 3 and Figure 4 have still demonstrated that the reduction of GO by NaOH can be closely involved with the ET reaction between OH- and oxygen-containing group of GO, which initiated the subsequent deoxygenation reaction just as Scheme 1 exhibits. Another pathway for reduction process of GO under attacking of OH- has been proved here, various from the proposed mechanism in previous reports.

Figure 5. The EPR spectra of reduced GO caused by various reductive agents.

Moreover, the above revelation of reduction mechanism of GO under attacking of NaOH incurred our further thought that the radical mechanism induced by ET might


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be appropriate for majority reduction reactions of GO and other graphene derivatives. The investigation about reduction of GO by other several agents was then performed. As Figure 5 indicates, compared to GO sample, the various reduced GO samples reduced by Zn powder, NaHSO3, EDA or N2H4 vapor were all showing the changed EPR signal, which implied that reduction of GO by these several kinds of agents can also be involved with radical process leading to the change of spin centers on graphene nanosheet. For GO-Zn/HCl and GO-N2H4 vapor samples, the EPR signal intensity was even weaker than GO sample, which can be explained that the originally existing spin centers on GO nanosheet were eliminated by coupling with the newly produced spin centers. The increasing or decreasing tendency of spin centers might be related to reduction degree or deoxygenation rate of GO during reduction process. It was found during the experiment that the reduction of GO by Zn was more complete than others, so was the reduction by N2H4 vapor for 5 days. For incomplete reduction by NaHSO3 or EDA, the concentration of spin centers of reduced samples was more than pristine GO similar as reduction by NaOH.

OH OH -

OH OH

ET

[·NH2-R]+

OH NH2-R OH-

+

+

e-

ET

spin center NH2-R OH OH-

OH OH

ET ·SO3H +

OH

-

SO3H

OH- +

ET

e-

SO3H

Scheme 2. The proposed mechanism for reduction of GO by EDA and NaSHO3 consisted of ET and deoxygenation reaction as well as the formation of C=C bond.



The deoxygenation of GO under attacking of other agents generating spin centers on graphene nanosheet could also be induced by ET reactions of these agents with oxygen-containing group of GO. The corresponding mechanism reduction by EDA or NaHSO3 is supposed as Scheme 2. Taking reaction of hydroxyl group as explanation, the electron can be transferred from amino group of EDA or -SO3H to hydroxyl group, resulting in formation of -C-OH- structure and radical intermediate related to EDA ([·NH2-R]+) or -SO3H (·SO3H). Further ET reaction of reductant with -C-OH group adjacent to original spin center would generate a new radical center, and the coupling reaction of two adjacent spin centers would produce the final C=C bond on nanosheet. For removal of epoxide group under attacking of these agents, the ET reaction and subsequent deoxygenation could occur in a similar way as Scheme 1 indicates. For reduction of GO by active metal with aid of HCl, the deoxygenation initiated by ET reaction between metal atom and oxygen-containing group might also happen, providing another pathway to reduced GO except for reduction by activated hydrogen stemmed from the reaction of metal with HCl11.


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Figure 6. EPR spectra of trapped radical intermediate related to reduction by EDA using MNP as trapping agent. The solvent ethanol/water was chosen.

Figure 7. EPR spectra of trapped radical intermediate related to reduction by NaHSO3 using MNP as trapping agent. The solvent ethanol/water was chosen.



To further prove the proposed mechanism in Scheme 2, more investigations using radical capturing method were then performed in order to confirm the reasonability of ET reaction between reductive agents with oxygen-containing group. The EPR spectra of capture radical intermediate related to reduction by EDA is shown as Figure 6. The fitting of obtained GO-EDA-MNP spectrum can be divided into two lines: line 5 and line 6. The splitting of simulation line 5 was caused by two kind of magnetic nucleus, resulting in hyperfine splitting constant of Aα-N =1.55 mT and Aβ-N = 0.18 mT. It was indicated that line 5 can be attributed to cationic radical of EDA captured by MNP, which was exactly arisen from the ET reaction of EDA with oxygen-containing group of GO. The EPR spectra of captured radical related to reduction by NaHSO3 is depicted in Figure 7, and its fitting was consisted of line 7 and line 8. The splitting of simulation line 7 was caused by one magnetic nucleus with hyperfine splitting constant of Aα-N =1.48 mT, attributed to adduct of MNP with ·HSO3 radical come from the ET reaction of -SO3H with GO. Meanwhile, it should be mentioned that another radical ·CR1R2R3 captured by MNP also appeared in Figure 6 (line 6) and Figure 7 (line 8), which was belonged to spin centers on graphene nanosheet generated during reductions, in agreement with the change of spin centers of GO samples as EPR spectra in Figure 5 suggests.


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Figure 8. The EPR spectra of FG as well as its derivatives after treating by NaOH, EDA or NaOH (A) and captured radical intermediate using MNP as trapping agent (ethanol solvent). The insert 1 in A was the proposed reduction mechanism of FG consisted of ET and defluorination reaction. The “Re” in insert 1 of A means reductant.

With the aforementioned results, the reduction mechanism proposed in Scheme 1 and Scheme 2 has been effectively proved, consisted of ET reaction of reductant with GO and subsequent deoxygenation reaction, which showed another pathway of reduction process. These findings about the reduction mechanism, which is distinguished from the proposed mechanism in previous reports, could update our understanding on chemistry of GO. The stable existence of spin centers on graphene nanosheet can influence the reaction process of GO materials to a great extent, including reduction reactions as well as other reactions. Moreover, our further researches demonstrated that reduction of fluorinated graphene (FG), another important graphene derivative, can also occur in a radical mechanism accompanied by the change of radical concentration on graphene frmework, just as Figure 8A shows. It was demonstrated that similar with deoxygenation of GO, the defluorination of FG



can also occur initiated by ET reaction of C-F bond with reductants. As our early work41 and other defluorination study of fluorinated single-walled carbon nanotubes42 suggested, the mechanism for chemical defluorination of FG was exhibited in insert 1 of Figure 8A. The radical of reductant, spin centers on graphene nanosheet as well as fluorine ions were generated in ET and defluorination reaction, while the coupling reaction of spin centers brought about the formation final C=C bond. Further experiments in Figure 8B using radical capturing method also demonstrated the existence of radical intermediates related to reductants (NaOH, EDA, and NaHSO3) stemmed from ET reaction, proving the proposed mechanism in insert 1 of Figure 8A. Based on these discussions, now it can be concluded that the reduction of graphene derivatives are always involved with the radical process initiated by ET reaction, and the generated spin centers on graphene nanosheet during reduction would further influence other subsequent reactions of graphene derivatives. As a result, the radical mechanism can be regarded as the typical characteristic of the chemistry of graphene derivatives.

CONCLUSIONS

In summary, we have innovatively employed EPR spectroscopy to explore the reduction mechanism of GO caused by several reductive agents. The change of radical centers on GO nanosheet after the reduction was observed. Thus, along with the successful detection of related radical intermediate by radical trapping method, it was first revealed that the reduction of GO can occur in a radical mechanism, consisted of


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the electron transfer (ET) and deoxygenation reaction. This showed another particular pathway of reduction process of GO except reduction mechanism proposed in previous researches. Further researches demonstrated that the reduction reactions of FG were always involved with radical process initiated by ET reaction. Based on these findings, the radical mechanism is further emphasized as the typical characteristic for reaction of graphene derivatives, which could refresh our understanding on the graphene chemistry.

ASSOCIATED CONTENT

Supporting Information Details of the experimental section and other supplementary explanations. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *To whom correspondence should be addressed. E-mail: [email protected], [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was financially supported by the National Natural Science Foundation of



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China (Grant No. 51633004 and Grant No. 51573105) and State Key Laboratory of Polymer

Materials

Engineering

(Grant

No.sklpme2017-2-03).

The

authors

acknowledge Analytical & Testing Centre of Sichuan University, College of Polymer Science and Engineering of Sichuan University and the State Key Laboratory of Polymer Materials Engineering (Sichuan University) for characterization.

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Radical Mechanism for Reduction of Graphene Derivatives Initiated by

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