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Characterization of conformation and locations of CF bonds in graphene derivative by polarized ATR-FTIR Xu Wang, Weimiao Wang, Yang Liu, Mengmeng Ren, Huining Xiao, and Xiangyang Liu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b00115 • Publication Date (Web): 09 Mar 2016 Downloaded from http://pubs.acs.org on March 15, 2016

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Analytical Chemistry

Characterization of conformation and locations of C-F bonds in graphene derivative by polarized ATR-FTIR Xu Wang,a, b Weimiao Wang,a Yang Liu,a Mengmeng Ren,a Huining Xiao,*, b and Xiangyang Liu *, a a

State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and

Engineering, Sichuan University, Chengdu, Sichuan, 610065, P.R. China; b

Department of Chemical Engineering, University of New Brunswick, Fredericton, NB E3B

5A3, Canada. Supporting Information Placeholder ABSTRACT: It is still a challenge to explore the orientation and location of chemical groups in the twodimensional derivative of graphene. In this study, polarized attenuated total reflectance fourier transform infrared spectroscopy (Polarized ATR-FTIR) was employed to investigate the orientation and location of C-F groups in the corresponding graphene derivative sheets, which facilitates building a relationship between the bonding nature and fine structure. There were two types of C-F bonding, (C-F)I and (C-F)II, in fluorinated graphene sheets. It was found that (C-F)II bonds were linked at the coplanar carbon atoms in weakly fluorinated region (CxF, x ≥2), whereas the (C-F)I bonds cluster at the strongly deformed carbon framework with a F/C ratio of about 1. The thermostability of (C-F)II is lower than that of (C-F)I bonds. This is due to that the coplanar structure of weakly fluorinated region more tends to transform to the pla*Corresponding author: [email protected] (Xiangyang Liu); [email protected] (Huining Xiao)

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nar aromatic ring with the breaking of C-F bond as compared with the strong fluorinated nonplanar region.

1. INTRODUCTION Since the initial isolation of one-atom thick graphene sheets in 2004,

1

this carbon material

has been attracting much attention as precursors of two-dimensional (2D) materials for past several years due to its superior mechanical strength, remarkably high thermal and electronic conductivities, high surface area, and many other desirable properties.

2, 3

The appearance of

other properties in 2D crystals as compared with their three-dimensional counterparts encourages scientists to look for various 2D materials with numerous interesting properties and high attraction for more applications. The covalent functionalization of graphene offers an exciting direction to tailor its properties and creates some new derivatives attracting considerable interest for their potential applications, such as graphene oxide,

4-6

graphane,

7-10

graphene

chloride, 11-13 and fluorinated graphene (FG). 14-17 The introduction of chemical groups or heteroatom will greatly affect the structure and the corresponding physical and chemical properties of the resultant products. From both practical and fundamental perspectives, it is important to understand the nature of disorder and the distribution of chemical group in the modified graphene sheet. 18, 19 FG has one single type of functional groups and the higher stability as compared with other derivatives, which is suitable as the representative to be studied in this work to address the final structure of modified graphene sheets. Similar to graphene,

20

two-dimensional fluorinated graphene is regarded as a basic building

block of other carbon material fluorides (CMFs). Therefore, study of its chemical structures facilitates the accurately understanding and adjusting that of other CMFs. Due to the difference of pristine carbon materials and fluorination route, the C-F bonds always show different binding characters which are of great important for determining the physical and chemical

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properties of CMFs. The “semi-ionic” versus “covalent” the binding nature has been proposed to differentiate them for graphite 21 and carbon nanotubes 22 and extended to graphene 16, 23, 24 based on the different positions of their C 1s peaks in XPS data and IR absorption. Based on the interpretation, Zhou et al. proposed that C-F bonds in fluorinated graphene have a character intermediate between the semi-ionic and covalent according to the results of density functional theory calculations and XPS. 25 It was reported that semi-ionic C-F bonds had a higher discharge voltage and a more excellent rate capability by compared to covalent C-F bonds. 26 The semi-ionic C-F bond was introduced to fluorinated graphene by Lee et al., which can be selectively reduced and lead to the recovery of conductivity. 27 In comparison with semi-ionic C-F bond, covalent C-F bond is considered to possess more excellent stability and higher bond dissociation energy (460 kJ/mol).

28

But, the interpretation was called into question by

Sato et al. 29 on the basis of the X-ray diffraction data showing the absence of any such “semiionic”or “semi-covalent” bond, which is supported by Stine et al. They supposed that the order of the so-called “semi-ionic” C-F bond in CxF was slightly lower than those in poly(carbon monofluoride) ((CF)n) and poly(dicarbonmonofluoride) ((C2F)n), but it were essentially covalent with a larger bond length of 0.140 nm which was attributed to the hyperconjugation between the bare carbon atoms on the carbon sheets and carbon atom of C-F bond. 29, 30 Therefore, the exact nature of the bonding state is somewhat contentious. The exact bonding nature of C-F bonds is inevitably related to their chemical environment. This contains if the honeycomb carbon network of graphene remains planar conformation, if the C-F bonds are distributed randomly, if they tend to cluster and create patches of more strongly fluorinated graphene intermixed with patches of near-perfect graphene, and so on. To provide answers to some of these questions, polarized attenuated total reflectance fouriertransform infrared spectroscopy (Polarized ATR-FTIR) experiments have been performed for FGs to elucidate the C-F bonding characters in this work. It is found there are mainly two types of C-F bonding, (C-F) II bonding with a higher bonding energy and (C-F) I with a lower 3 ACS Paragon Plus Environment


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bonding energy, which are attributed to IR peaks at 1220 and 1150 cm-1, respectively. The (CF) II bonds are linked at the coplanar carbon atoms in weakly fluorinated region (CxF, x ≥2), while (C-F) I bonds cluster at the strongly deformed carbon framework with a F/C ratio of about 1. This provides a feasible method to investigate the fine structural characters of other derivatives, such as the location, conformation, and orientation of heterogroups in the graphene sheets, and promotes the exploration of the exact bonding nature of chemical groups in CMFs.

2. THEORETICAL BASIS 31 Figure 1 shows a scheme for the characterization using polarized ATR-FTIR spectroscopy. FG paper will be positioned on the surface of the internal reflection element (IRE). The IRE is made of highly refractive materials (ZnSe) with a incidence angle of 45° which is used to realized total reflection of the incident light. By varying the electric vector of polarized IR radiation, different spectra will be collected.

Figure 1. The scheme of ATR-FTIR, the left picture for 0° polarized angle, the right picture for 90° polarized angle cos θ ∙

(1)

sin θ ∙ ∙ sin 45 √2/2 ∙ ∙ sin θ

(2)

sin θ ∙ ∙ cos 45 √2/2 ∙ ∙ sin θ

(3)

Here, θ represents the angle between the X-axis direction and electric vector of IR incident light, which is called Angle of polarization. The intensity of polarized IR radiation is a con4 ACS Paragon Plus Environment

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stant I, and the X, Y, and Z axial components of light intensity (IX, IY, IZ) can be calculated by equation (1), (2), and (3). 32, 33 As shown in Figure 1(left), the electric vector of incident light after a polarizer is parallel to the X-axis direction, so θ is 0o and the corresponding IX gets the maximum value. In Figure 1(right), the electric vector of polarized IR radiation perpendicular to the X-axis, so θ is 90o. Therefore, the value of IX is zero, while IZ gets the maximum value. Aθ represents the IR absorption intensity collected under different Angle of polarization (θ). For example, the A0 is obtained when θ is 0o; the A90 is obtained when θ is 90°. The dichroic ratio (RATR) is defined as equation (4).

° °

(4)

3. EXPERIMENTAL SECTION 3.1 Preparation of FG Fluorinated graphene was synthesized on the basis of reported literature.

34

More details are

shown in Supporting Information. Products with different fluorination degree were obtained by adjusting the fluorination temperature. The corresponding product was denoted as FGfluorination temperature. For example, FG prepared at 120 ºC was denoted as FG-120. 3.2 Preparation of FG paper A certain amount of FG was dispersed into ethanol by ultrasonic treatment, obtaining a stable suspension of FG in ethanol. The FG papers were obtained by filtering the dispersions were filtered using a polytetrafluoroethylene microporous membrane (0.22 µm), and dried in a vacuum oven at room temperature for 48 h. 3.3 Characterization The surface chemical composition of FG paper was characterized by XPS with monochromatized Al Ka rays (1486.6 eV) under the circumstance of 12 kV×15 mA, Kratos, Inc., at RT and at 2×10-7 Pa with a take-off angle of 20º. ATR-FTIR and polarized ATR-FTIR spectra were 5 ACS Paragon Plus Environment


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collected on a Nicolet 560 Fourier transform spectrometer, and the polarized angle varied from 0º to 180º and a series of spectra were record by every 10 deg for the polarized ATRFTIR measurement. Scanning electron microscope (SEM) was carried out with FEI InspectF (FEIcompany, USA). AFM was operated with a NanoScopeMultiMole& Explore from Vecco Instruments, using tapping mode. TGA was performed on Netzsch 209 TG instrument.

4. RESULTS AND DISCUSSION 4.1 Preparation and characterization of FG In this work, FGs were prepared from SG by direct fluorination using F2 as a fluorinating agent. Products with different fluorination degree were prepared by changing the fluorinating temperature while the usage of fluorine and duration were fixed. The molar ratio of fluorine to carbon (F/C ratio) was measured by XPS, which reflects the fluorination degree of products.

Figure 2.Molar ratio of fluorine to carbon (F/C ratio) was measured by XPS for the FG samples with dif-

ferent fluorination degree.


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Figure 3. (a) Curve-fitting of C 1s (right) XPS spectra of fluorinated and nonfluorinated samples: SG, FG-

120, FG-130, FG-200, FG-220 (bottom to top); (b) optical images of non-fluorinated and fluorinated graphene: pristine SG, FG-130, FG-180 and FG-220 (left to right); (c) The evolution of FTIR spectra of fluorinated samples with fluorination temperature obtained at different temperature, bottom to top: 50, 100, 120, 130, 150, 180, 200, and 220 °C.

The function of the value of F/C ratio as the fluorination temperature is shown in Figure 2 in the manuscript. Fluorination of graphene sheets using F2 as flourinating reagent is a gas-solid reaction, controlled by the diffusion of gas and reactivity of raw materials. High temperature facilitates the generating of F radical with high reactivity and the diffusion of F2 during graphene layers. Thus, F/C ratio increased with temperature as shown in Figure 2. The value is around 0.2 when the temperature is lower than 100°C. Between 120 and 130 °C, F/C ratio increases rapidly to 0.5 and then remains. After the temperature exceeds 180 °C, the value starts to increase rapidly and nearly reaches at 1.0 at 200 °C. It is worth to notice that the value remains at around 0.5 over the temperature range of 130 to 170 °C. This is attributed to the fol7 ACS Paragon Plus Environment


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lowing two reasons: a. the formed C-F bonds decrease electron density of graphene sheet due to the strong electron withdraw ability of fluorine atoms, and thus weaken the reactivity of graphene for following fluorination; b. there is strong repulsive force between fluorine atoms, so fluorine atom bonded on the graphene sheets would restrain the contact between F2 or F radical and nonfluorinated carbon atom for further reaction. Thus, the progressing of fluorination needs a higher temperature after fluorinated degree reaching a certain level. In XPS spectra (Figure 3a), the peaks at around 289 eV are attributed to the C 1s binding energies of C-F bonds.21-23, 35 The enhanced intensity of these peaks with growth temperature confirms the increasing of F concentration of the corresponding products. There are two types of C-F bonding due to the difference of their binding energy according to the curve-fitting of C 1s XPS spectra in Figure 3a. All fitted peaks are kept in shape of 20 % Lorentzian-Gaussian after performing a linear background correction. Peak 1 at 284.5 eV and peak 2 at 286 eV respectively correspond to nonfluorinated carbon atom -C-C- and -C-CF-. CF2 and CF3 groups formed at the defects and edge fluorinated graphene sheet, and their content increased with progressing of fluorination, which are respectively assigned to peak 6 and peak 7 in the XPS spectra. The C 1s binding energy of (C-F)I has a larger value of about 290.0 eV (peak 4), as compared with that of (C-F) II at 288.5 eV (peak 3). With the increasing of temperature and corresponding fluorination degree, the intensities of these two peaks show different changing tendency. The intensity of peaks assigned to (C-F)I has been increasing with temperature while that of (C-F)II increases first and then decrease. In FTIR spectra, the vibration frequencies of the C-F bonding, ν(C-F), of carbon fluorides are at around 1200 cm-1.36, 37 Figure 3c shows that the evolution of the band shape of ν(C-F) is a function of the F/C ratio of FG. It is clear that the band is composed of two different peaks at 1220 and 1150 cm-1. At a low F/C ratio, peak at 1150 cm-1 takes the predominate role. The raised fluorination temperature and resultant increase of the F/C ratio are companied with the gradual enhancement of the peaks of C-F bonds in the spectra, among which the absorption 8 ACS Paragon Plus Environment

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intensity of the peak at 1220 cm-1is increased at a larger rate as compared with that of the peak at 1150 cm-1. The FTIR results are consistent with that of XPS. Peaks at 1220cm-1 correspond to the C-F bonding with a larger binding energy, (C-F)I, while peaks at 1150cm-1 are assigned to (C-F) II bonding. In order to differentiate C-F bonding in fluorinated graphene, the “semi-ionic” versus “covalent” the binding nature has been proposed based on the different positions of their C 1s peaks in XPS data and IR absorption. 22, 23, 35 Actually, scientists had reported these two kinds of IR absorption band for the study of fluorinated graphite many years ago.

36, 37

When fluorine at-

om was linked with the completely sp3-hybridized carbon atom, the corresponding C-F bond was regarded as covalent bonding, corresponding to (C-F) I bond with the peak at 1221 cm-1. It was reported that the intensity of C-F covalent bonding was enhanced with the increasing of fluorination degree, which is consistent with our results. For peak at 1150 cm-l, the shift of the absorption to the lower wavelength was attributed to the partial iconicity of the corresponding C-F bonds. In subsequent research, the C-F bonding was always called the “semi-ionic” or “semi-covalent” bonding, corresponding to (C-F) II bond. However, this interpretation of the “semi-ionic” versus “covalent” the binding nature for C-F bonds was called into question based on the X-ray diffraction data reported by Sato et al. which did not show any presence of so-called “semi-ionic” bond. (C-F)

II

29

Sato considered that the

bond order was slightly lower than the C-F bond in poly(carbon monofluoride)

((CF)n) and poly(dicarbonmonofluoride) ((C2F)n), which was attributed to the hyperconjugation between the bare carbon atoms on the carbon sheets and carbon atom of C-F bond.

29

In

the following section, polarized ATR-FTIR was employed to explore the difference of these two types of C-F bonding in the view of the structure of FG sheets. 4.2 Polarized ATR-FTIR Analysis According to the AFM image and the previous reports, few-layer fluorinated graphene sheets are obtained. 14 Similar to graphene oxide and reduced graphene oxide, FG nanosheets could 9 ACS Paragon Plus Environment


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be easily made into macroscopic, freestanding, robust, and flexible paper via a one-step vacuum filtration protocol, as shown in Figure 4. Meanwhile, the FG sheet takes the in-plane orientation in the paper due to its 2D lamellar structure (Figure 4d). The paper sample is well suited to be characterized by polarized ATR-FTIR.

Figure 4. Optical pictures of FG paper (a, b), AFM image (c) of FG sheets, and cross-section SEM image

(d) of FG-180 paper; (e) TEM and (f) HRTEM micrographs of the highly fluorinated sample FG-180; (g) Selected area electron diffraction pattern; (h) Magnified image of the chosen section in panel (f).

The FG sample fluorinated at 120° is chosen for ATR-FTIR characterization, which is denoted with FG-120. This sample has IR absorption at 1600, 1220, and 1150 cm-1, ascribing to the C-C stretching vibration in the aromatic region (ν(C=C)A), the F-C stretching vibration of (CF) I bond (ν(C-F) I), and the F-C stretching vibration of (C-F) II bond (ν(C-F) II), respectively. Figure 5 shows the polarized ATR-FTIR spectra of the paper of FG sample obtained by fluorination at 120 °C. With the increasing of polarization angle from 0° to 90°, the intensities and shapes at about of IR peak at 1600, 1220, and 1150 cm-1 present different tendency. The intensity of peak at 1600 cm-1 (ν(C=C)A) decreased clearly while that of peak at 1150 cm-1 (ν(CF) II) increased obviously.


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Figure 5. (a) linearly polarized ATR-FTIR spectra under different angle of polarization, top to bottom: 90°,

70°, 50°, 30°, 20°, 0°; Polar diagrams of IR absorbance at different positions as a function of angle of polarization, obtained using linearly polarized ATR-IR spectra: (b)1150 cm-1, (c) 1220 cm-1, and (d)1600 cm-1.

Figure 5 (b, c, d) shows the polar diagrams of IR absorbance at different positions as a function of angle of polarization: (b) 1150, (c) 1220, and (d) 1600 cm-1. The polar diagram of 1150 cm-1 exhibits a minimum value of IR absorbance along the direction of 0°←→180° (Figure 5b) and a maximum value along the direction of 90°←→270°. Meanwhile, the Z-axial component (IZ) of incident polarized light achieves a minimum intensity with an angle of polarization of 0° or 180° and a maximum intensity with an angle of polarization of 90° or 270°, as shown in Figure 1. Thus, in the FG sheets, the transition moment of (ν(C-F) II) orients parallel to the Z-axis. The polar diagram of 1600 cm-1 obtains a maximum value of IR absorbance 11 ACS Paragon Plus Environment


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along the direction of 0°←→180° and a minimum value along the direction of 90°←→270°, which shows that the transition moments of the ν(C-C)A in aromatic region favorably orient parallel to the plane of FG paper. However, the intensity of peaks at 1220 cm-1 (ν(C-F)I) did not show a significant change with the polarization angle. There is not a maximum or minimum value in its polar diagram along any direction. This indicates that the transition moment of (ν(C-F)I) is random. Table 1 contains the peak position and the corresponding dichroic ratios (RATR) for kinds of vibration of FG-120 characterized by polarized ATR-FTIR. The averaged direction of the transition moment of ν(C-C)A is in the plane of the aromatic region of FG sheets. Therefore, the dichroic ratio of 1.91 for the IR band of ν(C-C)A at 1600 cm-1 confirms that FG sheets are oriented in the plane of the tested paper, which results from the rigid and 2D lamellar structure of FG sheets. The transition moment of ν(C-F) is along C-F bond. RATR of ν(C-F)II is 0.71, reflecting that the (C-F)II bonds take the “upstanding conformation”, being normal to the layer planer of the FG sheet, while the (C-F)I bonds are random with the RATR of 1.01. Table 1.The peak positions of kinds of vibration of FG (bands assignment) and the corresponding dichroic ratios (RATR)

Positions

Mode

RATR

1600 cm-1

C=C stretching vibration in aromatic region; ν(C=C)A

1.90

1220 cm-1

F-C stretching vibration of (C-F)I bond;

ν(C-F)I

1.01

1150 cm-1

F-C stretching vibration of (C-F)II bond;

ν(C-F)II

0.71

In reported work about density functional theory calculations

25

, it was considered that F

chemisorption orderly occurs on both sides of the carbon plane in the highly fluorinated domain with a F/C ratio close to 1, and the carbon atoms in the fluorinated graphene sheet are still coplanar. This is an ideal structure for the highly fluorinated region, and we agree with that the F atoms are bonded on the both sides of the carbon plane. Covalent C-F bonds in the FG are formed by linking one fluorine atom with one sp3 carbon atom in the highly fluorinat12 ACS Paragon Plus Environment

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ed region, which results in the transformation of aromatic ring to cyclohexane with boat or chair conformations as shown in Figure 6. Actually, fluorination processes very fast due to the high reactivity of F2. so the boat and chair cyclohexane would distribute irregularly. The irregular distribution of boat and chair conformations destroys the planarity of the graphene sheet, and results in the waviness and polycrystalline structure of graphene sheet as shown in TEM picture (In Figure 4) and the random orientation of covalent C-F bonds.

Figure 6. Structural scheme of perfluorinated cyclohexane with boat or chair conformations in graphene sheets

The nonplanar conformation is based on the high F concentration being close to that of (CF)n. For the FG with a low F concentration, such as CxF (x ≥2), the C atom of C-F bond is surround by bare C atoms and hyperconjugates with them. This hinders the fluorinated C atom to form the regular tetrahedron conformation during the transformation from sp2 to sp3 hybridization, resulting in the C atoms in the FG sheets still are (or close to be) coplanar. The (C-F)II bond should be linked in this region, and the vertical orientation proves the coplanarity of the carbon atoms. The hyperconjugation reduces electron density within the C-F bonds being accompanied with the redshift of infrared absorption from 1220 to 1150 cm-1,

38, 39

and the

structure is more likely to transform to the planar aromatic ring with the breaking of C-F bond more easily as compared with the nonplanar region. On the surface, the different orientation of the C-F bonds seems results in different binding energy and stability, but the real reason is the difference of chemical environment. That is, (C-F)Iand (C-F)II located in the region with different fluorination degree, which leads to the different orientations and binding energy of 13 ACS Paragon Plus Environment


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the C-F bonds.

Figure 7. Linearly polarized ATR-FTIR spectroscopy under different polarized angle of incident polarized light, top to bottom: 90°, 70°, 50°, 30°, 20°, 0°, for FG samples prepared at different temperature, left to right: 120, 130, and 220 °C.

Linearly polarized ATR-FTIR spectra of FGs prepared at different temperature with different fluorination degree are shown in Figure 7. For FG-220 with F/C ratio of about 1, there mainly are the absorption peaks of (C-F)I, which is consistent with above discussion that the high F concentration is accompanied with the random orientation of C-F bonds. However, the (C-F)I also exists in the FG sheets with low fluorination degree, though the amount is small. This indicates C-F bonds tend to cluster in graphene sheets, creating patches of more strongly fluorinated graphene intermixed with patches of weakly fluorinated or near-perfect graphene, which is similar to graphite fluoride. Even in graphite fluoride with low fluorination degree, there are some islands of higher fluorine concentration in the sheet associated with the strong local deformation of the carbon framework. As shown in the spectra of FG-120, it is indicated that three kinds of regions simultaneously exist in the graphene sheet: strongly fluorinated, weakly fluorinated and aromatic regions. FG-220 is mainly composited by strongly fluorinated region as compared with FG-130 by weakly fluorinated region. Combining with the results in Figure 2, the F/C ratio of strongly fluorinated region is near to 1; whereas that of weakly fluorinated region is about 0.5. In the spectra of FG-130, the peak at 1600 cm-1, assigned to stretching vibration of C=C bond in ar14 ACS Paragon Plus Environment

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omatic region, almost disappeared. As the increasing of temperature of only10 oC from 120 to 130 oC, the F/C ratio increased to 0.5 rapidly with the rapid reduction of aromatic regions, while the value remains until to the temperature arrives at 180. Thus, it is supposed that the C2F is one favor compound during fluorination.

Figure 8. Several structural models of fluorine atoms for FG of C2F, (a) all carbon atoms have only sp3 hybridization due to C-F covalent bonds, filled circles symbolize F atoms; (b) (c) (d) open and fill circles symbolize non fluorinated and fluorinated carbon atoms.

The several structural models of fluorine atoms for FG of C2F are shown in Figure 8. An earlier model in the literature for the C2F (CF0.5) system of graphene fluoride indicated that all carbon atoms have only sp3 hybridization due to C-F covalent bonds (Figure 8a), which is energetically favorable in comparison with other structure models. 40, 41 For this structure, the CF bonds correspond to the similar binding energy and IR absorption position as compared with that of (CF)n. This does not match well with the experimental results in this study. The structure models b, c and d in Figure 8 have half of the carbon atoms with sp3 hybridization due to C-F covalent bonds and the other half of nonfluorinated with sp2 hybridization, and model b representing the axial addition for carbon nanotubes with the maximum C2F fluorine coverage which is more stable by minimizing strain in the host nanotube.

42

Models b and c

have the carbon-carbon double bonds respectively in the 2, 3-positions and 3, 4-positions while the fluorinated carbons at 1, 2-positions (ortho direction) and 1, 4 positions (paradirection). However, the conjugated π-bonds do not existent in the FG sheets with when the 15 ACS Paragon Plus Environment


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fluorination temperature exceeds 130 oC according to the polarized ATR-FTIR spectra of FG130. The fluorinated carbons arranges in the meta-positions in model d. That is, the bare carbon atoms are surrounded by fluorinated carbon and thus cannot form carbon-carbon double bonds. The hyperconjugation between fluorinated and nonfluorinated carbon atoms and the strong electron-withdrawing properties of C-F bond assure the stability of the structure. Meanwhile, the interaction among carbon atoms in the region largely keeps the local coplanarity of the graphene sheets. The structure more tends to transform to the planar aromatic ring with the breaking of C-F bond more easily as compared with the strong fluorinated nonplanar region, which is proved by the following study on the thermal performance of FG. In addition, the fluorination of carbon materials using elemental fluorine F2 is always affected by the graphitization degree, specific surface area and curvature.

43-45

Large specific surface

area and curvature favor the fluorination at a low temperature. Besides, the higher graphitization degree, the higher the reaction temperature is required. FG, prepared from the graphene oxide by chemical reduction, inevitability has some defects and amorphous sp3 carbons which can reactive with F2 is and form C-F bonds even at low temperature. This is the reason why the FG prepared at a low temperature also has (C-F)I. Meanwhile, it is proposed that the local deformation of fluorinated region is due to the increasing of F concentration originated at the edge and defect sites of graphene sheets. The non-uniform distribution of fluorine atoms in depth indeed can cause the anisotropic effects of ATR-FTIR absorption for C-F bonds, and the above XPS analysis shows that the distribution of fluorine atoms in fluorinated graphene sheet maybe non-uniform. However, the FTIR testing depth is in the range of 0.6 ~ 2 µm, and the thickness of FG sheets is less than 10 nm. 16, 34 That is, several dozens or even of hundreds of FG sheets were tested together by ATR-FTIR. Therefore, the distribution of fluorine atoms in the FG film can be considered to be uniform, even that in one FG sheet is non-uniform. 4.3 Thermostability of two types of C-F bonding 16 ACS Paragon Plus Environment

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According to the TGA and corresponding DTA lines of FG in Figure 9, there are clearly two weight loss stages around 250 and 400 °C. The thermal decompositions of CMFs in N2 atmosphere before 800 °C are mainly attributed to the breaking of C-F bonds. The two weight loss peaks indicate the existence of two different types of C-F bonding.

Figure 9. TGA (left) and DTA (right) lines of FG-120 under N2 atmosphere at a heating rate of 10 °C/min.

In order to assign each peak in the DTA line, the FG sample (FG-120) with isothermal heat treatment at 250 °C for 30 min was characterized by polarized ATR-FTIR. Figure 10 shows the resulting spectra. It is clear that the intensity of (C-F)II peak at 1150 cm-1 is lower than that of (C-F)I peak at 1220 cm-1 under any angle of polarization (Figure 10c), which is completely different from that in the spectra of pristine FG (Figure 10b). In other words, the intensity of (C-F)I peak is better maintained in compared with that of (C-F) II peak in the heating process, which suggests the thermostability of (C-F)I bond is superior to that of (C-F)II bond. It has been reported that the thermal stability of fluorinated carbon materials increased with the increasing of the temperature of fluorination. 36 This was attributed to the varying of C-F bonding nature and increasing content of C-F bond with temperature.

46

Meanwhile, the decrease

of (C-F) II peak and the increase of the peak at 1600 cm-1 in Figure 10a implies the rupture of (C-F) II bond with the formation of a new aromatic region in the FG sheet. We supposed that the large difference of thermal stability results from the different chemical environment of C17 ACS Paragon Plus Environment


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F bonds. (C-F)I bonds take the random orientation and the fluorine atom is linked with sp3 carbon atom, so the structure of highly fluorinated graphene is similar to that of perfluoroaliphatics which C-F bonds can bear 400°C. The lightly fluorinated region with coplanar structure can form hyperconjugation between carbon atom of C-F bond and surrounding non fluorinated carbon atoms. Its (C-F)II bonds take the vertical orientation, proving the coplanarity of the carbon atoms, in which the carbon atoms cannot form the regular tetrahedron conformation due to the hyperconjugation. Thus, the coplanar structure is more likely to transform to the planar aromatic ring with the breaking of C-F bond as compared with the highly fluorinated nonplanar region. That is, (C-F)I bonds should be more stable than (C-F)II bonds. Therefore, the rupture of (C-F)I was at a high temperature, while that of (C-F)II bonds happened at a lower temperature.

Figure 10.(a)FTIR spectra of FG sample with isothermal heat treatment at 250 °C for 30 min under different polarized angle of incident polarized light, bottom to top: 90°, 70°, 50°, 30°, 20°, 0°; (b) polar diagrams of absorbance of peaks at 1150 (black line) and 1220 cm-1 (red line) as a function of the angle of polarization of incident polarized light, obtained using linearly polarized IR spectroscopy in Figure 5a; (c) polar diagrams of absorbance of peaks at 1150 (black line) and 1220 cm-1 (red line) obtained using linearly polarized IR spectroscopy in Figure 7a.

CONCLUSIONS Based on the FTIR spectra of FG with different fluorination degree, it is proposed that there are two types of C-F bonds. The polarized ATR-FTIR spectroscopy confirms that C-F bonds 18 ACS Paragon Plus Environment

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show different IR absorption and thermostability due to the difference of their locations in the fluorinated graphene sheets. The (C-F)II bonds are linked at the coplanar carbon atoms in weakly fluorinated region (CxF, x ≥2), while (C-F) I bonds cluster at the strongly deformed carbon framework with a F/C ratio of about 1. The thermostability of the (C-F) II is lower than that of the (C-F)I bonds. It is considered that the coplanar structure of weakly fluorinated graphene sheets is more likely to transform to the planar aromatic ring with the breaking of C-F bond more easily as compared with the strong fluorinated nonplanar region.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author [email protected]

(Xiangyang Liu); [email protected] (Huining Xiao)

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by State Key Laboratory of Polymer Materials Engineering (Grant No. sklpme2014-2-04) , the National Natural Science Foundation of China (Grant No.51573105) and NSERC strategic network Sentinel-Bioactive Paper (Canada). We acknowledge Prof. Mingbo Yang for characterization. SUPPORTING INFORMATION Preparation of FG; SEM images of graphene oxide sheets; optical images of spongy graphene (SG) oxide and it SEM image; SEM pictures of SG; optical images and SEM of fluorinated graphene; TEM and HRTEM images of the FG. This material is available free of charge via the internet at http://pubs.acs.org. 19 ACS Paragon Plus Environment


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polarized attenuated total reflectance fourier transform infrared spectroscopy was employed to investigate the orientation and location of C-F groups in the corresponding graphene derivative sheets 156x80mm (150 x 150 DPI)

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Characterization of Conformation and Locations of ... - ACS Publications

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