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Radical chain reaction mechanism of graphene fluorination
Accepted Manuscript Radical chain reaction mechanism of graphene fluorination Wenchuan Lai, Xu Wang, Jiemin Fu, Teng Chen, Kun Fan, Xiangyang Liu PII:
S0008-6223(18)30456-1
DOI:
10.1016/j.carbon.2018.05.005
Reference:
CARBON 13131
To appear in:
Carbon
Received Date: 28 February 2018 Revised Date:
17 April 2018
Accepted Date: 2 May 2018
Please cite this article as: W. Lai, X. Wang, J. Fu, T. Chen, K. Fan, X. Liu, Radical chain reaction mechanism of graphene fluorination, Carbon (2018), doi: 10.1016/j.carbon.2018.05.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Radical chain reaction mechanism of graphene fluorination
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Wenchuan Lai1, Xu Wang1, Jiemin Fu, Teng Chen, Kun Fan and Xiangyang Liu*
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Radical chain reaction mechanism of graphene fluorination
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Wenchuan Lai1, Xu Wang1, Jiemin Fu, Teng Chen, Kun Fan and Xiangyang Liu*
State Key Laboratory of Polymer Materials Engineering, College of Polymer Science
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and Engineering, Sichuan University, Chengdu, Sichuan, 610065, P.R. China;
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ABSTRACT: Electron paramagnetic resonance (EPR) spectroscopy was specially employed to investigate the mechanism for graphene fluorination by F2, which was mainly regarded as molecular addition of F2 into unsaturated carbons of graphene in previous reports. By observing the changing of spin centers on graphene nanosheet
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after fluorination, our work demonstrated that graphene fluorination by F2 was closely involved with radical reactions. The fluorination process comprised by a sequence of
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radical chain reactions was systematically proposed, and the initiation effect of chemical defects on fluorination reaction of raw graphene was emphasized. In
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comparison to molecular fluoridation of graphene, the chain radical fluorination can bring about much higher fluorinated degree of fluorinated product. The direct fluorination reactions were also influenced by purity such as oxygen or ferric compound.
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These authors contributed equally to this work.
* Corresponding author. E-mail: [email protected].:+86 28 85403948; Fax: +86 28 85405138.
ACCEPTED MANUSCRIPT 1. Introduction The functionalizations of graphene (covalent and noncovalent) have been a subject of intense study since 21th century.[1-4] Due to the the nature of pristine graphene with
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a zero band gap, inert reactivity and poor dispersibility, chemical modifications is very necessary which can effectively modulate structures/ properties of graphene and bring about various potential applications. [2, 4-7] Fluorinated graphene (FG) is the
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most significant member of functionalized graphene and graphene derivatives except
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for graphene oxide (GO).[8] It possesses majority of the excellent properties of graphene materials; on the other hand, the introducing of intriguing fluorine element endowes FG some peculiarities or functionalities, such as high functionalization density and great thermal stability, the surface, electromagnetic as well as biomedical
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properties. [8, 9]
The preparation of of FG can be achieved by exfoliation of graphite fluoride, or through the fluorination of graphene. The exfoliation strategy is generally low-cost
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and nondestructive, mainly containing thermal exfoliation[10], sonochemical
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exfoliation,[11] ionic-liquid exfoliation[12] and modifed Hummer’s exfoliation.[8, 13] Fluorination of graphene can be accomplished by employing reagents like xenon difluoride (XeF2)[14, 15], fluorine gas (F2)[16], hydrofluoric (HF)[17], sulfur hexafluoride (SF6)[18], bromine trifluoride (BrF3)[19, 20], tetrafluoromethane (CF4)[10], excite electrophilic fluorinating reagent[21] or fluoropolymer as fluorine source,[8, 9] or using plasma fluorination process[18, 22, 23]. The various methods are performed in different conditions with different reaction time, obtaining FG
ACCEPTED MANUSCRIPT product with various F/C ratios and nanostructures. Among them, the direct fluorination using pure F2 or mixture of F2 with inert gas (N2, Ar) remains the most useful method to obtain abundant FG material due to the high reactivity of molecular
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fluorine, and FG with adjustable F/C ratio (CFn stoichiometry, n=0~1) can be easily prepared.[8, 16] In the matter of reaction mechanism, yet there have been some works focusing on mechanism of graphene fluorination via experimental and computational
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simulation methods.[19, 20, 24-28] However, there are still few literatures concerning
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the in-depth mechanism for mechanism of graphene fluorination by F2. Due to the complexity of this reaction system, the already exsited studies about mechanism for fluorination by F2 are mainly depending on simulations method to calculate the optimizing fluoridating pathway. [29] The experimental studies are absent. Besides,
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the graphene fluorination by F2 is mainly regarded as the molecular addition of F2 into unsaturated carbons on graphene framework. In this paper, we specially employed electron paramagnetic resonance (EPR)
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spectroscopy[30, 31], which is an efficient technique for detection of unpaired
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electron in the substance (such as radical, or spin center), to shed light on in-depth reaction mechanism of graphene fluorination by F2. By measuring the change of spin centers (C-centered graphene radicals) of FG sample compared to pristine graphene, our work demonstrated that direct fluorination of graphene was closely related to radical process, which updated the previous viewpoint that direct fluorination was only involved with simple molecular addition of F2 into unsaturated carbons of graphene.[29] The comparative EPR study of fluorinated samples prepared from
ACCEPTED MANUSCRIPT graphene (G) and C6G6 graphene (labled as CG, with much fewer chemical defects) confirmed the initiation effect of chemical defects on the radical fluorination. The direct fluorination process of graphene material comprised by a sequence of radical
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chain reactions was systematically proposed as well. The influence of purity such as molecular oxygen or ferric compound adsorbate on fluorination reactions was
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discussed as well.
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2. Experimental 2.1 Materials
The graphene (G, O content of 16.45% and H/C ratio of 0.33) with many defects was purchased from The Sixth Elementary (Changzhou) Materials Technology Co., Ltd.
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The graphene oxide (GO) was prepared and purified according to the Hummers method.[32] The graphene with fewer defects was purchased from C6G6 Technology Co., Ltd, named as “CG” (O content of 1.94% and H/C ratio of 0.06). The XPS
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spectra, chemical composition, Raman spectra and TEM images of three kinds of raw
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graphene materials can be found in Supporting Information. The F2/N2 (10 vol% for F2) mixture with purity of 99.99% was obtained from Chengdu Kemeite Fluorine Industry Plastic Co., Ltd. The ferric chloride (FeCl3) was purchased from the Adamas Reagent, Ltd with commercially analytical grade. All other chemical reagents were commercially analytical grade and used without further purification. 2.2 Preparation of fluorinated samples The preparation method of fluorinated graphene (FG) was referred to our early
ACCEPTED MANUSCRIPT work.[16, 33] An amount of 0.5 g graphene (G) was put in a closed stainless steel (SUS316) chamber (20 L) equipped with a vacuum line. After removing air and exchanging nitrogen three times, F2/N2 mixed gas was introduced into the chamber at
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RT. To prepare FG with series of fluorine content, the pressure of introduced F2/N2 mixture were set as 5, 10, 25, 40, 50 and 80 kPa. Fluorination processed with temperature increasing from RT to 180 °C at heating rate of 5 °C/min, and keeping
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constant at 180 °C for 30min. The residual F2/N2 and gaseous by-products in the
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chamber were removed at once by vacuum and absorbed by alkali aqueous solution after the fluorination finished. The products were named as FG1, FG2, FG3, FG4, FG5 and FG6 corresponding to the F2/N2 mixture pressure of 5, 10, 25, 40, 50 and 80 kPa, respectively.
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The fluorination of GO (100mg) was performed in a similar way for 30min, and the products were named as FGO1 (fluorination conditions: 10kPa F2/N2, RT), FGO2 (50kPa F2/N2, RT) and FGO3 (50kPa F2/N2, 150 °C) respectively. The comparative
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fluorination of G and CG (50mg) was simultaneously performed in a similar method,
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and the products were labelled as FG-1/FCG-1 (10kPa F2/N2, RT), FG-2/FCG-2 (50kPa F2/N2, RT) and FG-3/FCG-3 (50kPa F2/N2, 180 °C). The comparative fluorination and oxyfluorination of graphene (100mg) were performed in a similar way, and the products were labelled as FG-RT (50kPa F2/N2, RT) or OFG-RT (50kPa F2/N2, 10kPa air, RT) and FG-180°C (50kPa F2/N2, 180 °C) or OFG-180°C (50kPa F2/N2, 10kPa air, 180 °C). The adsorption of FeCl3 for graphene was performed in solvent. 0.5g FeCl3 was
ACCEPTED MANUSCRIPT adequately dissolved in 50 mL mixed solvent water and N-methylpyrrolidone (NMP), and then 1g graphene was added. After 1h stirring, the suspension was filtered to remove superfluous FeCl3, and the graphene filter cake was dried at 80 °C, named as
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G-FeCl3. For a comparison, the graphene was also treated by NMP without FeCl3 and then dried. The subsequent fluorination of graphene materials (100mg) was simultaneously performed in a similar way, and the products were labelled as
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LFG/LFG-FeCl3 (10kPa F2/N2, RT) and HFG/HFG-FeCl3 (50kPa F2/N2, 180 °C).
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2.3 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
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a wattage of 250 W) radiation, with energy resolution of 0.05 eV. The Raman spectrum was obtained by using a LabRAM HR Raman spectrometer with an
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excitation wavelength of 532 nm, with energy resolution of 1.9 cm-1. Elemental analysis was performed with the ELEMENT vario EL cube (German) (All the
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measurements were performed on the powder samples). The fine microstructures of graphene materials 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 2 mW microwave power with 1 G modulation amplitude and 100 kHz
ACCEPTED MANUSCRIPT frequency modulation at room temperature. All samples were measured at 50 s sweep time, 1000 G sweep width for 4 times. Spectra processing and simulations were performed using Bruker WIN-EPR software package. All the measurements were
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performed on the powder samples.
3. Results and discussion
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Fluorinated graphene (FG) samples with F/C ratio ranging from 0~1.12 were
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prepared by direct heating fluorination of graphene (G) using F2/N2 as fluorine source (the detailed chemical component see Table S1). The electron paramagnetic resonance (EPR) spectra of FG samples are exhibited in Figure 1. It is observed that all samples except for pristine graphene behave evident EPR signal in the region of g-factor at
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around 2, which is typical spectrum lines of radical or paramagnetic defects (spin centers) for carbon materials. The EPR signal demonstrates the existence of spin centers (or dangling bonds, C-centered graphene radicals) on FG nanosheet, and these
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spin centers should be produced during direct fluorination process of graphene.
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Therefore, it appeared that fluorination by using F2 was also closely associated with radical reactions. This finding refreshes the present understanding that this fluorination method was only related to molecular addition of F2 into unsaturated carbon-carbon bond of graphene.[28, 29] The involved radical reaction can contain radical initiation, radical growth and radical termination reaction. More importantly, with the F/C ratio improving, the density of spin centers for FG samples firstly increases with fluorination starting and then roughly decreases after reaching medium
ACCEPTED MANUSCRIPT fluorine content (Figure 1B). Such a changing tendency can imply the reaction character of graphene fluorination, that is to say: the radical growth reaction mainly occurred at preliminary fluorination stage and radical termination reaction at deep
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fluorination stage, which could be responsible for the most spin centers of FG3 sample with medium fluorine content. Besides, from FG1 to FG6, the slight shift of middle field (Figure 1A) namely the g-factor, as well as the linewidth of EPR spectra
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are observed (Figure 1B), implying that the type of spin centers changed as
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fluorination proceeded. The phsical-chemical environment of spin centers have also changed, resulting in increasing spin relaxation intereaction corresponding to broad
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linewidth.
Figure 1. The change of EPR spectra for FG samples with various F/C ratio. A) EPR spectra, B) the linewidth and density of spin centers for corresponding EPR spectra. In (A), the spectrum of FG2 has been zoomed out for three times, and FG3 zoomed out for ten times. The F/C ratio here means the ratio between F content and C content.
EPR spectra of graphene oxide (GO) and fluorinated graphene oxide (FGO)
ACCEPTED MANUSCRIPT samples are exhibited in Figure 2A. It was different from the pristine graphene that pristine GO has behaved evident EPR signal with a narrow line (linewidth of 2.8 G), which can be assigned to original spin centers on GO nanosheet. These spin centers
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can be produced during the oxidation exfoliation process of graphite, namely the Hummers method.[32] Subsequently after the initial fluorination, FGO1 exhibits a different EPR signal, comprised of two kinds of lines: line 1 and line 2 (Figure 2B).
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The line 1 possesses sextet hyperfine splitting with constant of 97.6 G and linewidth
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of 51.2 G, which can be attributed to characteristic of Mn purity[34] contained in GO materials, since KMnO4 was used as an oxidizing agent during preparation of GO. The line 1 was almost absent in GO, FGO1 and FGO3, which was caused by the different valence state of contained Mn purities. Another line of FGO1 is line 2, with
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no hyperfine splitting (narrow linewidth of 5.1G). The line 2 can be assigned to the radicals of FGO1 which have existed in GO, or the GO radicals whose signal was brodened after the introducing of fluorine atoms. It was found that line 2 of FGO2
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was weaker than signal of GO, suggesting that the original spin centers of GO can
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participate in primary fluorination reaction, and be partly consumed. By comparing chemical component of GO and FGO1 (Table S2), it appeared that initial fluorination was also involved with reaction of oxygen-containing groups, including their elimination and substitution reaction. The increase of unsaturated bond implied by the color deepening of FGO1 confirmed the partial reduction of raw GO material (Figure S1). With fluorination proceeding continuously, the EPR linewidth of FGO2 and FGO3 gradually became broader, from 11.7 G to 81.7 G. The EPR change of GO and
ACCEPTED MANUSCRIPT FGO samples also demonstrates the significance of spin centers in direct fluorination
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reactions of graphene material.
Figure 2. EPR spectra of GO and fluorinated GO (FGO) samples (A). (B) is the peak fitting and simulation of EPR spectrum for FGO1 sample.
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The above results demonstrated that spin centers on graphene nanosheet can be produced or terminated during direct fluorination process. However, the formation
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mechanism of these spin centers is still ambiguous. Since the fluorination of graphene was performed at temperature 180 °C, the possible dissociation of F2 molecule
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resulting in fluorine radical can’t be ignored, and graphene fluorination by fluorine radicals was also a pathway to generate spin centers on nanosheet. Therefore, was fluorination by fluorine radical the real approach to produce spin centers of fluorinated samples? To answer the question, the fluorination at lower temperature was then performed in order to avoid the possible dissociation of F2 molecules into fluorine radicals. The EPR spectra of fluorinated samples prepared at different fluorination temperatures including room temperature (RT, about 10 °C), 100 °C and
ACCEPTED MANUSCRIPT 180 °C are shown in Figure 3 (chemical composition see Table S3). It is observed that even at RT where dissociation of F2 can almost be ignored, the obtained sample FG-RT still behaved evident EPR signal, more intense than that of FG-180°C sample.
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This result demonstrates that for fluorination at RT, the spin centers of fluorinated product were not stemmed from the fluorination by fluorine radical produced from the dissociation of F2. Moreover, the linewidth of EPR spectrum of FG-RT sample is
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relatively narrow (linewidth of 8 G), while that of FG-100°C and FG-180°C is
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broader (linewidth 38 G). The broadening of lines can be caused by the increasing spin relaxation intereaction, or the coexistence of various radical species with
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different g-factors.
Figure 3. EPR spectra of FG samples prepared at different fluorination temperatures.
Now that the spin centers of fluorinated sample were not generated from the
ACCEPTED MANUSCRIPT graphene fluorination by fluorine radical, what should be the formation mechanism of spin centers during direct fluorination process? As for this matter, it is suggested by reaction mechanism for direct fluorination of polymers[35-38] that the chain radical
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fluorination can take place by using F2 as reactant, initiated by reaction such as
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The initiation reaction is very advantageous in thermodynamics and can occur even at temperature 77K, followed by the subsequent fluorination including radial growth, transferring or termination reaction. After fluorination finishing, some alkyl radicals can be remained in the final fluorinated polymers. It appeared that spin centers of FG
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samples can originate from the similar approach. However, as previous works suggested, the initiation reaction (2) for isolated C=C bond is advantage in thermodynamics, while the reaction (2) for aromatic C=C double bond is an
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endothermic reaction, which is not so favorable in thermodynamics.[37] As
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consequence, it was still questionable whether reaction (2) of aromatic C=C bond of graphene can be responsible for the chain fluorination of graphene resulting in spin centers on nanosheet.
The question was then responded by comparison of fluorinated samples prepared
from graphene (G) and C6G6 graphene (CG). The pristine G had more chemical defects on nanosheet (ID/IG=1.20, Raman spectra see Figure S2), with more oxygen-containing groups (O content of 16.45% suggested by XPS calculation) and
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(H/C ratio of 0.06). It is demonstrated by F/C ratio comparison between FG and FCG in Figure 4 that CG was more difficult to be fluorinated than G under the same fluorination condition, thus resulting in much lower fluorine content. The detailed
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composition data of FG and FCG samples can be found in Table S4. For instance, for
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fluorination at 180 °C with 50kPa F2/N2, FG-3 obtained F/C ratio of 0.79, while that of FCG-3 was only 0.18. Therefore, the chemical defects of raw graphene including oxygenic groups (C-O, C=O) and hydrogen-containing groups (C-H) can promote the direct fluorination of graphene materials to a great extent. More importantly, as EPR
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spectra of FG and FCG samples in Figure 4 shows, all FG samples behaved evident EPR signal, whose lineshape transformed with fluorination deepening. As a contrast, it occurs that all FCG samples behave no EPR signal. The EPR difference
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demonstrates direct fluorination of CG was hardly involved with radical reactions,
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which can be the reason leading to the much lower fluorine content of its fluorinated products. Nevertheless, the fluorination of G promoted by its defects on nanosheet (oxygen-containing group can be regarded as chemical defect as well) was closely related to radical reactions, namely chain radical fluorination reactions. As a result, the chain fluorination of graphene by F2 should be initiated by those defects. The corresponding initiation reaction can be similar with reaction (1) and (2), namely reactions of F2 with C-O/C=O, C-H as or isolated C=C bonds which were preferable
ACCEPTED MANUSCRIPT in thermodynamics. Once being started, the fluorination of graphene would proceed spontaneously until the chain termination reaction. Besides, it was also indicated by absence of spin centers for FCG-3 prepared at 180 °C, that fluorination by fluorine
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radical (produced from direct dissociation of F2) wasn’t responsible for spin centers of FG-3 sample, because of the same fluorination conditions and high reactivity of
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fluorine radicals.
Figure 4. EPR spectra of fluorinated samples prepared from various material G (graphene) and CG (C6G6 graphenewith much fewer defects). The insert in (C) is the peak fitting for spectrum of FG-2.
It has been demonstrated above that direct fluorination of graphene was associated
ACCEPTED MANUSCRIPT with the radical chain reactions, while radical reaction are always affected by molecular oxygen purity, a typical radical trapper.[39-41] The comparative EPR study of samples prepared by fluorination and oxyfluorination was then carried out. EPR
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spectra in Figure 5 indicates that fluorination in the existence of oxygen purity led to the relatively weaker EPR signal, namely fewer spin centers of OFG samples (prepared by oxyfluorination) compared to corresponding FG samples prepared by
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pure fluorination at RT or 180 °C. It was explained that some spin centers on
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nanosheet can be combined by oxygen molecule during fluorination process, generating peroxy radicals which were terminated by air atmosphere eventually. As a consequence, there were fewer spin centers on nanosheet of OFG samples. Meanwhile, the combination of spin center with oxygen would bring about more
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oxygen-containing groups in OFG sample in comparison to FG sample prepared on the same fluorination conditions on one hand, which was proved by oxygen content of 9.23% for FG-180°C and 12.89% for OFG-180°C sample (detailed chemical
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composition see Table S5). On the other hand, those spin centers on nanosheet was
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the reaction centers of fluorination, and the consumption of those centers by oxygen would result in respectively weaker fluorination and fewer fluorinated content for OFG samples (F/C ratio of 0.82 for FG-180°C and 0.76 for OFG-180°C, for instance). The difference in oxygen and fluorine content of fluorinated multiwalled carbon nanotubes prepared by fluorination or oxyfluorination in our previous work can also support these discussions.[42]
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Figure 5. EPR spectra of fluorinated samples (FG) and oxyfluorinated samples (OFG). The O-G sample was obtained by heating graphene in air atmosphere (180 °C).
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Moreover, it is suggested that some compounds of transition metal like iron element can also act as radical trapper in radical reactions.[39, 41] The comparative
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EPR study of fluorinated samples prepared from graphene and graphene with FeCl3 adsorbate (FeCl3 content about 1.5%) under the same fluorination conditions was then
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performed. The EPR spectra in Figure 6 demonstrate that a small quantity of FeCl3 salt brought the evident difference in EPR signal intensity between FG and fluorinated graphene-FeCl3 (FG-FeCl3) samples. Analogous to oxyfluorination, the relatively fewer spin centers of FG-FeCl3 was caused by the similar radical trapping effect of FeCl3 during fluorination process, which would partly inhibit fluorination reaction and eventually resulted in fewer fluorine content of FG-FeCl3 samples (detailed chemical composition see Table S6). Nevertheless, the small difference for fluorine content
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reactions was fairly small.
Figure 6. The EPR spectra comparison of fluorinated samples prepared from the fluorination
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of graphene and graphene after adsorbing FeCl3.
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Based on the above discussions, the reaction mechanism for graphene fluorination
by F2 gas was then systematically proposed, shown as Figure 7. The main fluorination process was involved with a sequence of radical chain reactions, inlucding initiation (I1 to I4), propagation (P1 and P2) as well as termination (T1 to T4) reactions. The primary radicals were generated by the initiation reactions of F2 with chemical defects on graphene nanosheet such as C-H group, C-O/C=O group (on edge or defect regions) or isolated C=C bond, resulting in fluorine radicals and spin centers on
ACCEPTED MANUSCRIPT graphene nanosheet. The initiation reaction I1, I2 and I3 were advantageous in thermodynamics suggested by works of Kharitonov[35, 37] and Table S7. The thermodynamics evaluation of I4 was difficult due to undefined products; however; it
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is suggested by Figure S3 that thermal deoxygenation of GO (mainly for C-O groups) can produce radical centers on GO nanohseet, which can also happen for residual C-O groups of reduced graphene material G under attacking of F2. The preliminary
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fluoridated products were also obtained at initiation step (>C·-CF<). After these
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initiation reactions, the radical propagation took place via reaction of aromatic C=C bond with fluorine radical, or reaction of F2 with spin center on nanosheet, which would bring about the new radicals or spin centers as well as further fluorination reactions. The propagation reactions happened spontaneously due to the reactivity of
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fluorine radicals and radical centers on graphene nanohseet. The reactions shown in “termination” stage were likely candidates for a chain termination reaction of fluorination process, including random combination of two radical centers such as
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spin center with fluorine radical, fluorine radical with fluorine radical, or spin center
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with spin center.[35] It can appear that the coupling reaction of two spin centers on two different graphene nanosheets leading to formation of new C-C bond, which accomplished the chemical crosslinking of graphene nanosheets.[43] The spin centers on graphene nanosheet can also be affected by purity in reaction system such as molecular oxygen or compound of transition metal (FeCl3) and partly consumed by those radical trappers. The introduction of oxygen would eventually bring about the relatively more oxygen content of the fluorinated product. However, the chain radical
ACCEPTED MANUSCRIPT fluorination was not very sensitive to those trappers, distinguished from the high sensitivity of common radical reactions such as radical polymerization[41]. Besides, implied by the slight fluorination of CG material with few chemical defects on
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nanosheet and absence of spin centers of FCG samples in Figure 4, the fluorination of graphene related to molecular addition of F2 into unsaturated carbons can’t be excluded during direct fluorination process. In spite of that, the molecular fluoridation
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was fairly weak, while the chain radical fluorination can bring about much deeper
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fluorination of graphene and much higher fluorine content for fluorinated graphene
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products.
Figure 7. The proposed chain reaction mechanism for direct fluorination of graphene.
ACCEPTED MANUSCRIPT 4. Conclusions In summary, we have employed EPR spectroscopy to investigate the in-depth reaction mechanism of graphene fluorination by F2 gas. By comparison of EPR signal between
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pristine graphene and fluorinated graphene samples, our work demonstrated that direct fluorination of graphene was also closely involved with radical reactions, distinguished from the previous understanding that direct fluorination of graphene
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was the simple molecular addition of F2 into unsaturated carbons on graphene
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nanosheet. The chemical defects of raw graphene material were emphasized as active sites triggering the radical fluorination by means of their reactions with molecular fluorine. The fluorination process of graphene comprised by a sequence of radical chain reactions was systematically proposed as well. The fluorination reactions were
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also influenced by purity such as molecular oxygen or ferric compound. The work reveals the in-depth reaction mechanism of graphene fluorination and enriches the
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graphene chemistry.
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Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Grant No. 51573105 and Grant No. 51633004) 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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Appendix A. Supplementary data
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Supplementary data related to this article can be found at http://dx.doi.org/*****/.
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S0008-6223(18)30456-1
DOI:
10.1016/j.carbon.2018.05.005
Reference:
CARBON 13131
To appear in:
Carbon
Received Date: 28 February 2018 Revised Date:
17 April 2018
Accepted Date: 2 May 2018
Please cite this article as: W. Lai, X. Wang, J. Fu, T. Chen, K. Fan, X. Liu, Radical chain reaction mechanism of graphene fluorination, Carbon (2018), doi: 10.1016/j.carbon.2018.05.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Radical chain reaction mechanism of graphene fluorination
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Wenchuan Lai1, Xu Wang1, Jiemin Fu, Teng Chen, Kun Fan and Xiangyang Liu*
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Radical chain reaction mechanism of graphene fluorination
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Wenchuan Lai1, Xu Wang1, Jiemin Fu, Teng Chen, Kun Fan and Xiangyang Liu*
State Key Laboratory of Polymer Materials Engineering, College of Polymer Science
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and Engineering, Sichuan University, Chengdu, Sichuan, 610065, P.R. China;
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ABSTRACT: Electron paramagnetic resonance (EPR) spectroscopy was specially employed to investigate the mechanism for graphene fluorination by F2, which was mainly regarded as molecular addition of F2 into unsaturated carbons of graphene in previous reports. By observing the changing of spin centers on graphene nanosheet
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after fluorination, our work demonstrated that graphene fluorination by F2 was closely involved with radical reactions. The fluorination process comprised by a sequence of
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radical chain reactions was systematically proposed, and the initiation effect of chemical defects on fluorination reaction of raw graphene was emphasized. In
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comparison to molecular fluoridation of graphene, the chain radical fluorination can bring about much higher fluorinated degree of fluorinated product. The direct fluorination reactions were also influenced by purity such as oxygen or ferric compound.
1
These authors contributed equally to this work.
* Corresponding author. E-mail: [email protected].:+86 28 85403948; Fax: +86 28 85405138.
ACCEPTED MANUSCRIPT 1. Introduction The functionalizations of graphene (covalent and noncovalent) have been a subject of intense study since 21th century.[1-4] Due to the the nature of pristine graphene with
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a zero band gap, inert reactivity and poor dispersibility, chemical modifications is very necessary which can effectively modulate structures/ properties of graphene and bring about various potential applications. [2, 4-7] Fluorinated graphene (FG) is the
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most significant member of functionalized graphene and graphene derivatives except
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for graphene oxide (GO).[8] It possesses majority of the excellent properties of graphene materials; on the other hand, the introducing of intriguing fluorine element endowes FG some peculiarities or functionalities, such as high functionalization density and great thermal stability, the surface, electromagnetic as well as biomedical
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properties. [8, 9]
The preparation of of FG can be achieved by exfoliation of graphite fluoride, or through the fluorination of graphene. The exfoliation strategy is generally low-cost
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and nondestructive, mainly containing thermal exfoliation[10], sonochemical
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exfoliation,[11] ionic-liquid exfoliation[12] and modifed Hummer’s exfoliation.[8, 13] Fluorination of graphene can be accomplished by employing reagents like xenon difluoride (XeF2)[14, 15], fluorine gas (F2)[16], hydrofluoric (HF)[17], sulfur hexafluoride (SF6)[18], bromine trifluoride (BrF3)[19, 20], tetrafluoromethane (CF4)[10], excite electrophilic fluorinating reagent[21] or fluoropolymer as fluorine source,[8, 9] or using plasma fluorination process[18, 22, 23]. The various methods are performed in different conditions with different reaction time, obtaining FG
ACCEPTED MANUSCRIPT product with various F/C ratios and nanostructures. Among them, the direct fluorination using pure F2 or mixture of F2 with inert gas (N2, Ar) remains the most useful method to obtain abundant FG material due to the high reactivity of molecular
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fluorine, and FG with adjustable F/C ratio (CFn stoichiometry, n=0~1) can be easily prepared.[8, 16] In the matter of reaction mechanism, yet there have been some works focusing on mechanism of graphene fluorination via experimental and computational
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simulation methods.[19, 20, 24-28] However, there are still few literatures concerning
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the in-depth mechanism for mechanism of graphene fluorination by F2. Due to the complexity of this reaction system, the already exsited studies about mechanism for fluorination by F2 are mainly depending on simulations method to calculate the optimizing fluoridating pathway. [29] The experimental studies are absent. Besides,
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the graphene fluorination by F2 is mainly regarded as the molecular addition of F2 into unsaturated carbons on graphene framework. In this paper, we specially employed electron paramagnetic resonance (EPR)
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spectroscopy[30, 31], which is an efficient technique for detection of unpaired
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electron in the substance (such as radical, or spin center), to shed light on in-depth reaction mechanism of graphene fluorination by F2. By measuring the change of spin centers (C-centered graphene radicals) of FG sample compared to pristine graphene, our work demonstrated that direct fluorination of graphene was closely related to radical process, which updated the previous viewpoint that direct fluorination was only involved with simple molecular addition of F2 into unsaturated carbons of graphene.[29] The comparative EPR study of fluorinated samples prepared from
ACCEPTED MANUSCRIPT graphene (G) and C6G6 graphene (labled as CG, with much fewer chemical defects) confirmed the initiation effect of chemical defects on the radical fluorination. The direct fluorination process of graphene material comprised by a sequence of radical
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chain reactions was systematically proposed as well. The influence of purity such as molecular oxygen or ferric compound adsorbate on fluorination reactions was
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discussed as well.
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2. Experimental 2.1 Materials
The graphene (G, O content of 16.45% and H/C ratio of 0.33) with many defects was purchased from The Sixth Elementary (Changzhou) Materials Technology Co., Ltd.
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The graphene oxide (GO) was prepared and purified according to the Hummers method.[32] The graphene with fewer defects was purchased from C6G6 Technology Co., Ltd, named as “CG” (O content of 1.94% and H/C ratio of 0.06). The XPS
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spectra, chemical composition, Raman spectra and TEM images of three kinds of raw
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graphene materials can be found in Supporting Information. The F2/N2 (10 vol% for F2) mixture with purity of 99.99% was obtained from Chengdu Kemeite Fluorine Industry Plastic Co., Ltd. The ferric chloride (FeCl3) was purchased from the Adamas Reagent, Ltd with commercially analytical grade. All other chemical reagents were commercially analytical grade and used without further purification. 2.2 Preparation of fluorinated samples The preparation method of fluorinated graphene (FG) was referred to our early
ACCEPTED MANUSCRIPT work.[16, 33] An amount of 0.5 g graphene (G) was put in a closed stainless steel (SUS316) chamber (20 L) equipped with a vacuum line. After removing air and exchanging nitrogen three times, F2/N2 mixed gas was introduced into the chamber at
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RT. To prepare FG with series of fluorine content, the pressure of introduced F2/N2 mixture were set as 5, 10, 25, 40, 50 and 80 kPa. Fluorination processed with temperature increasing from RT to 180 °C at heating rate of 5 °C/min, and keeping
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constant at 180 °C for 30min. The residual F2/N2 and gaseous by-products in the
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chamber were removed at once by vacuum and absorbed by alkali aqueous solution after the fluorination finished. The products were named as FG1, FG2, FG3, FG4, FG5 and FG6 corresponding to the F2/N2 mixture pressure of 5, 10, 25, 40, 50 and 80 kPa, respectively.
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The fluorination of GO (100mg) was performed in a similar way for 30min, and the products were named as FGO1 (fluorination conditions: 10kPa F2/N2, RT), FGO2 (50kPa F2/N2, RT) and FGO3 (50kPa F2/N2, 150 °C) respectively. The comparative
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fluorination of G and CG (50mg) was simultaneously performed in a similar method,
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and the products were labelled as FG-1/FCG-1 (10kPa F2/N2, RT), FG-2/FCG-2 (50kPa F2/N2, RT) and FG-3/FCG-3 (50kPa F2/N2, 180 °C). The comparative fluorination and oxyfluorination of graphene (100mg) were performed in a similar way, and the products were labelled as FG-RT (50kPa F2/N2, RT) or OFG-RT (50kPa F2/N2, 10kPa air, RT) and FG-180°C (50kPa F2/N2, 180 °C) or OFG-180°C (50kPa F2/N2, 10kPa air, 180 °C). The adsorption of FeCl3 for graphene was performed in solvent. 0.5g FeCl3 was
ACCEPTED MANUSCRIPT adequately dissolved in 50 mL mixed solvent water and N-methylpyrrolidone (NMP), and then 1g graphene was added. After 1h stirring, the suspension was filtered to remove superfluous FeCl3, and the graphene filter cake was dried at 80 °C, named as
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G-FeCl3. For a comparison, the graphene was also treated by NMP without FeCl3 and then dried. The subsequent fluorination of graphene materials (100mg) was simultaneously performed in a similar way, and the products were labelled as
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LFG/LFG-FeCl3 (10kPa F2/N2, RT) and HFG/HFG-FeCl3 (50kPa F2/N2, 180 °C).
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2.3 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
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a wattage of 250 W) radiation, with energy resolution of 0.05 eV. The Raman spectrum was obtained by using a LabRAM HR Raman spectrometer with an
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excitation wavelength of 532 nm, with energy resolution of 1.9 cm-1. Elemental analysis was performed with the ELEMENT vario EL cube (German) (All the
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measurements were performed on the powder samples). The fine microstructures of graphene materials 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 2 mW microwave power with 1 G modulation amplitude and 100 kHz
ACCEPTED MANUSCRIPT frequency modulation at room temperature. All samples were measured at 50 s sweep time, 1000 G sweep width for 4 times. Spectra processing and simulations were performed using Bruker WIN-EPR software package. All the measurements were
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performed on the powder samples.
3. Results and discussion
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Fluorinated graphene (FG) samples with F/C ratio ranging from 0~1.12 were
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prepared by direct heating fluorination of graphene (G) using F2/N2 as fluorine source (the detailed chemical component see Table S1). The electron paramagnetic resonance (EPR) spectra of FG samples are exhibited in Figure 1. It is observed that all samples except for pristine graphene behave evident EPR signal in the region of g-factor at
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around 2, which is typical spectrum lines of radical or paramagnetic defects (spin centers) for carbon materials. The EPR signal demonstrates the existence of spin centers (or dangling bonds, C-centered graphene radicals) on FG nanosheet, and these
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spin centers should be produced during direct fluorination process of graphene.
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Therefore, it appeared that fluorination by using F2 was also closely associated with radical reactions. This finding refreshes the present understanding that this fluorination method was only related to molecular addition of F2 into unsaturated carbon-carbon bond of graphene.[28, 29] The involved radical reaction can contain radical initiation, radical growth and radical termination reaction. More importantly, with the F/C ratio improving, the density of spin centers for FG samples firstly increases with fluorination starting and then roughly decreases after reaching medium
ACCEPTED MANUSCRIPT fluorine content (Figure 1B). Such a changing tendency can imply the reaction character of graphene fluorination, that is to say: the radical growth reaction mainly occurred at preliminary fluorination stage and radical termination reaction at deep
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fluorination stage, which could be responsible for the most spin centers of FG3 sample with medium fluorine content. Besides, from FG1 to FG6, the slight shift of middle field (Figure 1A) namely the g-factor, as well as the linewidth of EPR spectra
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are observed (Figure 1B), implying that the type of spin centers changed as
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fluorination proceeded. The phsical-chemical environment of spin centers have also changed, resulting in increasing spin relaxation intereaction corresponding to broad
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linewidth.
Figure 1. The change of EPR spectra for FG samples with various F/C ratio. A) EPR spectra, B) the linewidth and density of spin centers for corresponding EPR spectra. In (A), the spectrum of FG2 has been zoomed out for three times, and FG3 zoomed out for ten times. The F/C ratio here means the ratio between F content and C content.
EPR spectra of graphene oxide (GO) and fluorinated graphene oxide (FGO)
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can be produced during the oxidation exfoliation process of graphite, namely the Hummers method.[32] Subsequently after the initial fluorination, FGO1 exhibits a different EPR signal, comprised of two kinds of lines: line 1 and line 2 (Figure 2B).
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The line 1 possesses sextet hyperfine splitting with constant of 97.6 G and linewidth
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of 51.2 G, which can be attributed to characteristic of Mn purity[34] contained in GO materials, since KMnO4 was used as an oxidizing agent during preparation of GO. The line 1 was almost absent in GO, FGO1 and FGO3, which was caused by the different valence state of contained Mn purities. Another line of FGO1 is line 2, with
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no hyperfine splitting (narrow linewidth of 5.1G). The line 2 can be assigned to the radicals of FGO1 which have existed in GO, or the GO radicals whose signal was brodened after the introducing of fluorine atoms. It was found that line 2 of FGO2
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was weaker than signal of GO, suggesting that the original spin centers of GO can
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participate in primary fluorination reaction, and be partly consumed. By comparing chemical component of GO and FGO1 (Table S2), it appeared that initial fluorination was also involved with reaction of oxygen-containing groups, including their elimination and substitution reaction. The increase of unsaturated bond implied by the color deepening of FGO1 confirmed the partial reduction of raw GO material (Figure S1). With fluorination proceeding continuously, the EPR linewidth of FGO2 and FGO3 gradually became broader, from 11.7 G to 81.7 G. The EPR change of GO and
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reactions of graphene material.
Figure 2. EPR spectra of GO and fluorinated GO (FGO) samples (A). (B) is the peak fitting and simulation of EPR spectrum for FGO1 sample.
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The above results demonstrated that spin centers on graphene nanosheet can be produced or terminated during direct fluorination process. However, the formation
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mechanism of these spin centers is still ambiguous. Since the fluorination of graphene was performed at temperature 180 °C, the possible dissociation of F2 molecule
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resulting in fluorine radical can’t be ignored, and graphene fluorination by fluorine radicals was also a pathway to generate spin centers on nanosheet. Therefore, was fluorination by fluorine radical the real approach to produce spin centers of fluorinated samples? To answer the question, the fluorination at lower temperature was then performed in order to avoid the possible dissociation of F2 molecules into fluorine radicals. The EPR spectra of fluorinated samples prepared at different fluorination temperatures including room temperature (RT, about 10 °C), 100 °C and
ACCEPTED MANUSCRIPT 180 °C are shown in Figure 3 (chemical composition see Table S3). It is observed that even at RT where dissociation of F2 can almost be ignored, the obtained sample FG-RT still behaved evident EPR signal, more intense than that of FG-180°C sample.
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This result demonstrates that for fluorination at RT, the spin centers of fluorinated product were not stemmed from the fluorination by fluorine radical produced from the dissociation of F2. Moreover, the linewidth of EPR spectrum of FG-RT sample is
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relatively narrow (linewidth of 8 G), while that of FG-100°C and FG-180°C is
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broader (linewidth 38 G). The broadening of lines can be caused by the increasing spin relaxation intereaction, or the coexistence of various radical species with
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different g-factors.
Figure 3. EPR spectra of FG samples prepared at different fluorination temperatures.
Now that the spin centers of fluorinated sample were not generated from the
ACCEPTED MANUSCRIPT graphene fluorination by fluorine radical, what should be the formation mechanism of spin centers during direct fluorination process? As for this matter, it is suggested by reaction mechanism for direct fluorination of polymers[35-38] that the chain radical
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fluorination can take place by using F2 as reactant, initiated by reaction such as
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The initiation reaction is very advantageous in thermodynamics and can occur even at temperature 77K, followed by the subsequent fluorination including radial growth, transferring or termination reaction. After fluorination finishing, some alkyl radicals can be remained in the final fluorinated polymers. It appeared that spin centers of FG
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samples can originate from the similar approach. However, as previous works suggested, the initiation reaction (2) for isolated C=C bond is advantage in thermodynamics, while the reaction (2) for aromatic C=C double bond is an
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endothermic reaction, which is not so favorable in thermodynamics.[37] As
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consequence, it was still questionable whether reaction (2) of aromatic C=C bond of graphene can be responsible for the chain fluorination of graphene resulting in spin centers on nanosheet.
The question was then responded by comparison of fluorinated samples prepared
from graphene (G) and C6G6 graphene (CG). The pristine G had more chemical defects on nanosheet (ID/IG=1.20, Raman spectra see Figure S2), with more oxygen-containing groups (O content of 16.45% suggested by XPS calculation) and
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(H/C ratio of 0.06). It is demonstrated by F/C ratio comparison between FG and FCG in Figure 4 that CG was more difficult to be fluorinated than G under the same fluorination condition, thus resulting in much lower fluorine content. The detailed
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composition data of FG and FCG samples can be found in Table S4. For instance, for
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fluorination at 180 °C with 50kPa F2/N2, FG-3 obtained F/C ratio of 0.79, while that of FCG-3 was only 0.18. Therefore, the chemical defects of raw graphene including oxygenic groups (C-O, C=O) and hydrogen-containing groups (C-H) can promote the direct fluorination of graphene materials to a great extent. More importantly, as EPR
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spectra of FG and FCG samples in Figure 4 shows, all FG samples behaved evident EPR signal, whose lineshape transformed with fluorination deepening. As a contrast, it occurs that all FCG samples behave no EPR signal. The EPR difference
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demonstrates direct fluorination of CG was hardly involved with radical reactions,
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which can be the reason leading to the much lower fluorine content of its fluorinated products. Nevertheless, the fluorination of G promoted by its defects on nanosheet (oxygen-containing group can be regarded as chemical defect as well) was closely related to radical reactions, namely chain radical fluorination reactions. As a result, the chain fluorination of graphene by F2 should be initiated by those defects. The corresponding initiation reaction can be similar with reaction (1) and (2), namely reactions of F2 with C-O/C=O, C-H as or isolated C=C bonds which were preferable
ACCEPTED MANUSCRIPT in thermodynamics. Once being started, the fluorination of graphene would proceed spontaneously until the chain termination reaction. Besides, it was also indicated by absence of spin centers for FCG-3 prepared at 180 °C, that fluorination by fluorine
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radical (produced from direct dissociation of F2) wasn’t responsible for spin centers of FG-3 sample, because of the same fluorination conditions and high reactivity of
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fluorine radicals.
Figure 4. EPR spectra of fluorinated samples prepared from various material G (graphene) and CG (C6G6 graphenewith much fewer defects). The insert in (C) is the peak fitting for spectrum of FG-2.
It has been demonstrated above that direct fluorination of graphene was associated
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spectra in Figure 5 indicates that fluorination in the existence of oxygen purity led to the relatively weaker EPR signal, namely fewer spin centers of OFG samples (prepared by oxyfluorination) compared to corresponding FG samples prepared by
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pure fluorination at RT or 180 °C. It was explained that some spin centers on
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nanosheet can be combined by oxygen molecule during fluorination process, generating peroxy radicals which were terminated by air atmosphere eventually. As a consequence, there were fewer spin centers on nanosheet of OFG samples. Meanwhile, the combination of spin center with oxygen would bring about more
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oxygen-containing groups in OFG sample in comparison to FG sample prepared on the same fluorination conditions on one hand, which was proved by oxygen content of 9.23% for FG-180°C and 12.89% for OFG-180°C sample (detailed chemical
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composition see Table S5). On the other hand, those spin centers on nanosheet was
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the reaction centers of fluorination, and the consumption of those centers by oxygen would result in respectively weaker fluorination and fewer fluorinated content for OFG samples (F/C ratio of 0.82 for FG-180°C and 0.76 for OFG-180°C, for instance). The difference in oxygen and fluorine content of fluorinated multiwalled carbon nanotubes prepared by fluorination or oxyfluorination in our previous work can also support these discussions.[42]
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Figure 5. EPR spectra of fluorinated samples (FG) and oxyfluorinated samples (OFG). The O-G sample was obtained by heating graphene in air atmosphere (180 °C).
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Moreover, it is suggested that some compounds of transition metal like iron element can also act as radical trapper in radical reactions.[39, 41] The comparative
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EPR study of fluorinated samples prepared from graphene and graphene with FeCl3 adsorbate (FeCl3 content about 1.5%) under the same fluorination conditions was then
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performed. The EPR spectra in Figure 6 demonstrate that a small quantity of FeCl3 salt brought the evident difference in EPR signal intensity between FG and fluorinated graphene-FeCl3 (FG-FeCl3) samples. Analogous to oxyfluorination, the relatively fewer spin centers of FG-FeCl3 was caused by the similar radical trapping effect of FeCl3 during fluorination process, which would partly inhibit fluorination reaction and eventually resulted in fewer fluorine content of FG-FeCl3 samples (detailed chemical composition see Table S6). Nevertheless, the small difference for fluorine content
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reactions was fairly small.
Figure 6. The EPR spectra comparison of fluorinated samples prepared from the fluorination
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of graphene and graphene after adsorbing FeCl3.
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Based on the above discussions, the reaction mechanism for graphene fluorination
by F2 gas was then systematically proposed, shown as Figure 7. The main fluorination process was involved with a sequence of radical chain reactions, inlucding initiation (I1 to I4), propagation (P1 and P2) as well as termination (T1 to T4) reactions. The primary radicals were generated by the initiation reactions of F2 with chemical defects on graphene nanosheet such as C-H group, C-O/C=O group (on edge or defect regions) or isolated C=C bond, resulting in fluorine radicals and spin centers on
ACCEPTED MANUSCRIPT graphene nanosheet. The initiation reaction I1, I2 and I3 were advantageous in thermodynamics suggested by works of Kharitonov[35, 37] and Table S7. The thermodynamics evaluation of I4 was difficult due to undefined products; however; it
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is suggested by Figure S3 that thermal deoxygenation of GO (mainly for C-O groups) can produce radical centers on GO nanohseet, which can also happen for residual C-O groups of reduced graphene material G under attacking of F2. The preliminary
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fluoridated products were also obtained at initiation step (>C·-CF<). After these
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initiation reactions, the radical propagation took place via reaction of aromatic C=C bond with fluorine radical, or reaction of F2 with spin center on nanosheet, which would bring about the new radicals or spin centers as well as further fluorination reactions. The propagation reactions happened spontaneously due to the reactivity of
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fluorine radicals and radical centers on graphene nanohseet. The reactions shown in “termination” stage were likely candidates for a chain termination reaction of fluorination process, including random combination of two radical centers such as
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spin center with fluorine radical, fluorine radical with fluorine radical, or spin center
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with spin center.[35] It can appear that the coupling reaction of two spin centers on two different graphene nanosheets leading to formation of new C-C bond, which accomplished the chemical crosslinking of graphene nanosheets.[43] The spin centers on graphene nanosheet can also be affected by purity in reaction system such as molecular oxygen or compound of transition metal (FeCl3) and partly consumed by those radical trappers. The introduction of oxygen would eventually bring about the relatively more oxygen content of the fluorinated product. However, the chain radical
ACCEPTED MANUSCRIPT fluorination was not very sensitive to those trappers, distinguished from the high sensitivity of common radical reactions such as radical polymerization[41]. Besides, implied by the slight fluorination of CG material with few chemical defects on
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nanosheet and absence of spin centers of FCG samples in Figure 4, the fluorination of graphene related to molecular addition of F2 into unsaturated carbons can’t be excluded during direct fluorination process. In spite of that, the molecular fluoridation
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was fairly weak, while the chain radical fluorination can bring about much deeper
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fluorination of graphene and much higher fluorine content for fluorinated graphene
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products.
Figure 7. The proposed chain reaction mechanism for direct fluorination of graphene.
ACCEPTED MANUSCRIPT 4. Conclusions In summary, we have employed EPR spectroscopy to investigate the in-depth reaction mechanism of graphene fluorination by F2 gas. By comparison of EPR signal between
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pristine graphene and fluorinated graphene samples, our work demonstrated that direct fluorination of graphene was also closely involved with radical reactions, distinguished from the previous understanding that direct fluorination of graphene
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was the simple molecular addition of F2 into unsaturated carbons on graphene
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nanosheet. The chemical defects of raw graphene material were emphasized as active sites triggering the radical fluorination by means of their reactions with molecular fluorine. The fluorination process of graphene comprised by a sequence of radical chain reactions was systematically proposed as well. The fluorination reactions were
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also influenced by purity such as molecular oxygen or ferric compound. The work reveals the in-depth reaction mechanism of graphene fluorination and enriches the
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graphene chemistry.
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Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Grant No. 51573105 and Grant No. 51633004) 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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Appendix A. Supplementary data
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Supplementary data related to this article can be found at http://dx.doi.org/*****/.
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