NMR and NEXAFS Study of Various Graphite Fluorides - American

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NMR and NEXAFS Study of Various Graphite Fluorides Y. Ahmad,†,‡ M. Dubois,*,†,‡ K. Guérin,†,‡ A. Hamwi,†,‡ Z. Fawal,§ A. P. Kharitonov,∥ A. V. Generalov,⊥ A. Yu. Klyushin,⊥ K. A. Simonov,⊥ N. A. Vinogradov,⊥,#,○ I. A. Zhdanov,⊥ A. B. Preobrajenski,⊥,# and A. S. Vinogradov⊥ †

Université Blaise Pascal, Clermont Université, ICCF, 63177 Aubière, France CNRS, UMR 6296, 63170 Aubière, France § Faculté Des Sciences, Université Libanaise, B.P. 826 Tripoli, Lebanon ∥ The Branch of the Institute for Energy Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow Region, 142432, Russia ⊥ V.A. Fock Institute of Physics, St. Petersburg State University, St. Petersburg 198504, Russia # MAX-lab, Lund University, Box 118, 22100 Lund, Sweden ○ Department of Physics and Astronomy, Uppsala University, Box 530, 75121 Uppsala, Sweden ‡

ABSTRACT: Graphite fluorides with different structural types (CyF)n (y = 2.5, 2, and 1) and room temperature graphite fluorides were studied by solid state NMR and NEXAFS. Data extracted from those two techniques are complementary, providing information about the C−F bonding and the hybridization character of the carbon atom valence states. The comparison of data obtained by different methods such as NMR, Raman, and X-ray absorption leads to similar conclusions regarding the chemical bonding in fluorographites. Several major configurations of fluorinated graphites are discussed, that is, planar sheets with mainly sp2 hybridization in room temperature graphite fluorides and corrugated sheets with sp3 hybridization in covalent high temperature graphite fluoride. Different references such as highly oriented pyrolytic graphite (HOPG), graphitized carbon nanodiscs (graph-CNDs) and nanodiamonds (NDs) have also been investigated for comparison.

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INTRODUCTION Due to their numerous applications, such as electrode material of primary lithium batteries, solid lubricants, reservoir for storage of strong oxidant, and fluorinating agents (BF3, ClF3), graphite fluorides are extensively studied during several decades.1−6 For all these uses, the chemical, electrochemical, and tribological properties depend strongly on the C−F bonding in the fluorocarbon matrix. The control of the covalence and the fluorine content allows the tuning of the properties, for example, the discharge potential and the capacity in lithium batteries.5,7 The higher the covalence, that is, the higher the strength of the bonding, the lower the discharge potential. The C−F bonding is highly versatile in fluorinated carbons. The strength of interaction between the fluorine and carbon atoms can considerably vary from a very weak (van der Waals) one for the case of fluorine adsorption on surfaces of carbonaceous materials to a strong covalent one in graphite fluorides with (C2F)n and (CF)n structural types, prepared by treatment of graphite with molecular fluorine at 350 and 600 °C, respectively.8,9 An intermediate state with a weakened covalence can also be synthesized.10 The covalence weakening can be achieved by three routes: (i) By intercalation of fluorine into graphite at temperature lower than 100 °C, resulting in the formation of fluorine-graphite intercalated compounds (F© XXXX American Chemical Society

GICs) with (CxF) composition. The planar configuration coordination of carbon atoms in graphite is preserved, and the nature of the C−F bond evolves from ionic to weakened covalent in going from lower to higher fluorine content. More recently, fluorinated graphites denoted RTGF were prepared using a synthesis at room temperature by treatment with gaseous mixture of F2, HF and volatile fluorides (BF3, IF5, ClFx, ...).10 (ii) By forming structures where fluorinated carbon atoms having sp3 hybridization coexist with nonfluorinated sp2bonded ones in the C-layers (hyperconjugation).11 (iii) By inducing a curvature of the carbon lattice which prevents a purely sp3 hybridization of carbon C atoms during fluorination. The residual sp2 character results in a reduced overlap between the lobes of carbon hybridized valence orbitals and the fluorine atomic orbitals. Thus, covalence is weakened due to this curvature.12 A thorough characterization of fluorinated carbons is necessary to enable full control over the structure and properties crucial for practical applications. 19F and 13C nuclear magnetic resonance (NMR) and infrared spectroscopy were extensively used for this purpose.13−20 The 19F NMR chemical Received: February 13, 2013 Revised: May 31, 2013

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dx.doi.org/10.1021/jp401579u | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. Schematic view of the stacking sequence and the carbon valence states hybridization (black circle, sp3 and gray circle, sp2) in conventional graphite fluorides (CF)n, (C2F)n,22 and (C2.5F)n.

Table 1. Synthesis Methods of Various Graphite Fluorides starting materials

a

producer of the raw material

synthesis method

graphite UF4 graphite UF4 RTGF

Mersen Mersen homemade (CF0.72)(IF5)0.02

(C2.5F)n

homemade (CF0.32)a

F2 F2

From 13C solid echo NMR

purity

TF2 (°C)

F/C

notation

refs

>99% >99%

600 380 0 400 550 250

1 0.6 0.72 0.72 0.92 0.39a

(CF)n (C2F)n RTGF RT-400 RT-550 (C2.5F)n-250

15−17 14 13, 15, 23

18

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EXPERIMENTAL SECTION Fluorination and Materials. Table 1 summarizes starting materials and applied fluorination conditions (by treatment with undiluted gaseous F2 at 1 atm pressure under dynamic condition in a flow reactor). The details of sample preparation are described in the corresponding papers (last column in Table 1). Fluorination at high temperature (between 300 and 600 °C) generates two types of covalent fluorinated graphites with (C2F)n and (CF)n structural types. On the other hand, the use of a catalytic gaseous mixture F2/MFn/anhydrous HF increases the reactivity of fluorine with graphite at room temperature (MFn = IF5, IF7, BrF5, ClF3, WF6, MoF6, BF3).10 The synthesized fluorinated graphites are noted as RTGF and (C2.5F)n when IF5 and ClFx were used as catalysts, respectively. To increase both the C−F bonding covalence character and the fluorine content, RTGF samples have been post-treated with pure F2 gas at temperatures TF2 ranged between 100 and 600 °C.13,15,18,23 Such a two-step process is called doublefluorination. The graphitized carbon nanodiscs (graph-CNDs) were supplied by NTec Norway. The sample consists of a mixture of carbon nanodiscs (70 wt %), carbon nanocones (20 wt %), and amorphous carbon (10 wt %) annealed in argon at 2700 °C for graphitization.24 CNDs used in this research were produced by pyrolysis of heavy oil using the Kvaerner Carbon Black and Hydrogen Process (CBH).25 Physical-Chemical Characterization. NMR experiments were performed at room temperature using a Tecmag spectrometer (working frequencies for 13C and 19F were equal to 73.4 and 282.2 MHz, respectively). For 19F NMR spectra, recorded with magic angle spinning (MAS), a simple pulse sequence was used with a single π/2 pulse duration of 5.5 μs. The 13C spectra were recorded using a solid echo sequence

shifts as well as the wavenumber of the C−F vibrations during FTIR experiments are relevant indicators of the C−F bonding.13,15−18,20 Higher IR wavenumber νIR relates to a higher covalence, the two limits being 1100 and 1220 cm−1 for the weakened and the pure covalence, respectively. The values of the 19F chemical shift δ are ranged between −136 and −190 ppm/CFCl3; the higher the covalence, the lower the δ. Considering 13C chemical shifts, values from 82 to 88 ppm/ TMS are recorded for weakened and pure covalence, respectively. As in the work by Seki et al.,21 our strategy here is to use complementary techniques, NMR, infrared spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) photoelectron spectroscopy, vacuum-ultraviolet optical spectroscopy, and NEXAFS in Seki’s work, and a large panel of samples and references for comparison. In the present paper graphite fluorides of various types, that is, with different character of the C−F bonding, were systematically investigated using solid state NMR, infrared spectroscopy and NEXAFS spectroscopy. The possible structural types F-GICs, (C2.5F)n, (C2F)n, and (CF)n graphite fluorides with different stacking structure22 (Figure 1) were synthesized. The (C2.5F)n structure derives from the (C4F)n one, the chemical composition was established by quantitative NMR.18 Moreover, intermediate structures were also synthesized by a post-treatment of the original (grown at room temperature) graphite fluorides with undiluted F2 gas. The overall purpose of this study is to use the complementarity of NEXAFS and NMR techniques for characterizing samples with different fluorine content and different nature of the C−F bonding. B



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(two 5.5 μs π/2 pulses separated by 25 μs); this sequence allows the acquisition of the whole signal without loss due to the electronic dead time, followed by a quantitative determination of different contributions. 13C and 19F chemical shifts refer to tetramethylsilane (TMS) and CFCl3, respectively. Fourier transform infrared spectroscopy (FT-IR) was carried out using a Thermo Nicolet 5700 in an Attenuated Total Reflectance (ATR) mode. NEXAFS. The C 1s and F 1s X-ray absorption spectra of the graphite fluorides as well as of reference systems were measured using monochromatic synchrotron radiation and the facilities of the Russian-German beamline (RGBL) at the BESSY II (Berlin)26 and the beamline D1011 of the MAX-IV laboratory (Lund)27 under the same experimental conditions. The samples for X-ray absorption measurements were prepared in air. The powders of the materials under study were rubbed into a scratched surface of a pure substrate (metallic indium or copper plate 5 × 5 mm2 in size) in order to ensure the substrate uniform surface coating without noticeable gaps. The near-edge X-ray absorption fine structure (NEXAFS) spectra at the C 1s and F 1s thresholds of the pristine and fluorinated carbon systems were obtained by recording the total electron yield (TEY) and the partial electron yield (PEY) of the X-ray photoemission as a function of the incident photon energy. The TEY spectra were acquired by measuring the drain current from the sample, while the PEY spectra were recorded by the multichannel plate detector with the retarding voltage Vret of −150 V. These techniques for measuring X-ray absorption are characterized by a probing depth of ∼5−20 and ∼0.5−1 nm, respectively.28 All X-ray absorption spectra were measured under ultrahigh vacuum conditions with residual gas pressure in the experimental chamber below 2 × 10−10 mbar. The samples were located at an angle of ∼45° with respect to the incident beam of monochromatic radiation. No sample charging was observed upon X-ray absorption spectra acquisition. The spectra were recorded several times from different points of the sample, and their structure showed usually a good reproducibility. The photon-energy resolution ΔE of the monochromator over the range of F 1s X-ray absorption edge (photon energy hν ∼ 680 eV) and the C 1s X-ray absorption edge (hν ∼ 285 eV) was around 150 and 70 meV, respectively. The X-ray absorption spectra were normalized to the incident photon flux, which was monitored by recording the TEY from the clean surface of a gold mesh mounted in the beamline (in front of the sample). The photon energy hν over the range of F and C 1s absorption spectra fine structure was calibrated against the energy position of the first narrow peak in the F 1s X-ray absorption spectrum of solid K2TiF6 (F 1s → t2g, 683.9 eV29) and the C 1s X-ray absorption spectrum of solid C60 (C 1s → LUMO, 284.5 eV30).

Figure 2. 19F MAS NMR spectra of various graphite fluorides (spinning speed of 14 kHz; * and × mark spinning sidebands for C−F and CF2 isotropic lines, respectively).

fluoride18 (it is to note that narrow lines over 0−50 ppm range are assigned to IFn intercalated species, that is, IF5 and IF6−). For the case of (C2.5F)n, the line at −147 ppm is assigned to the fluorocarbon matrix, the lower δ19F of the fluorocarbon matrix in (C2.5F)n indicates larger hyperconjugation between C−F bonds and unfluorinated carbon atoms, that is, more pronounced weakening of the covalence.11 In accordance with the works of Panich,20 who extensively studied the fluorineGIC compounds, the second line is ascribed to intercalated fluorine ions. In addition to 19F NMR, 1H NMR experiments, not shown here, confirm a very low amount of 1H nuclei; as a matter of fact, the corresponding resonance lines of FHF− and HClF− at +13.5 and +6.3 ppm/TMS, respectively, are very weak. Thus, (C 2.5 F) n compound represents mainly a fluorocarbon host matrix close to C4F with additionally intercalated fluorine anions F−. Similar to (C2.5F)n, RTGF exhibits a weakened covalence and similarities with fluoride-graphite intercalation compounds (FGICs) because few residual catalysts are still intercalated. RTGF was post-treated by pure F2 gas at 400 and 550 °C (samples RT-400 and RT-550).13,18,23 The shifts of the resonance line confirm the conversion of C−F bonding from weakened covalence to strong one. 19F line of the fluorocarbon matrix changes its position from −154 ppm for the raw compound to −189 ppm for RT-400 and RT-550 (Figure 2 and Table 2). It happens after the removal and conversion of the intercalated IF5, IF6-, and IF7, showing that these species play a key role as catalysts for room temperature graphite fluorides. Such a shift reveals the enhancement of the covalence, which results also in an increase of the δ of C−F on the 13C spectra (not shown) of RT-400 and RT-530, the value being close to 88 ppm, whereas for the raw sample and RTGF, δ = 82 ppm. The strengthening of the covalence is also related to a higher

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RESULTS AND DISCUSSION In order to average or to decrease the strong 19F−19F homonuclear dipolar coupling and possible chemical shift anisotropy, which result in a substantial broadening of the lines, 19 F NMR experiments were performed using magic angle spinning (MAS) with spinning speed of 14 kHz. 19F MAS NMR spectra measured for various graphite fluorides are shown in Figure 2. The higher the covalence, the lower the 19F chemical shift;13−20,23 δ19F of −190 ppm was measured for covalent (CF)n16,17,19 and (C2F)n13 types, whereas a value of −160 ppm was obtained for room temperature graphite C



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Table 2. NMR Chemical Shifts δ of C−F Bonds and IR Wavenumber (νIR) of the C−F Vibrations

(CF)n (C2F)n (C2.5F)n RTGF RT-400 RT-550 a

F/C

NMR chemical shifts δ19F/CFCl3 (ppm)

νIR (cm−1)

1 0.6 0.39a 0.72b 0.83 0.98

−190 −189 −151.3 −154 −189 −189

1215 1215 1082 1140 1215 1215

refs 16, 17 14 18 13, 15, 23

From 13C solid echo NMR. b(CF0.72)(IF5)0.02.

wavenumber of the C−F infrared band (Table 2): weakened covalence results in values close to 1100 cm−1, whereas 1215 cm−1 is measured for pure covalence. NMR investigations on 13C, 19F, and 1H of the structure and chemical bonding in raw and fluorinated detonation nanodiamonds underline the formation of different fluorocarbon groups on the nanodiamond surface, which substitutes for hydrocarbon and hydroxyl groups.31−33 The 13C line with chemical shift of 35 ± 1 ppm for the raw NDs, which are investigated using NEXAFS, was attributed to the sp3 carbons belonging to the nanodiamond core. 1H to 13C crosspolarization allows the determination of surface-hydrogenated groups (CH, CH2, and COH) and underlines the quasi-absence of a sp2 carbon fullerene-like shell on the diamond surface.33 Such a characteristics justifies the use of this sample as a reference for the pure sp3 hybridization in NEXAFS. NEXAFS spectra at the C 1s and F 1s absorption edges measured for various fluorinated graphites (CyF)n (y = 2.5, 2, and 1) simultaneously in the TEY and the PEY modes are shown in Figures 3 and 4, respectively. The TEY and PEY

Figure 4. F 1s NEXAFS spectra of graphite fluorides (CF)n, (C2F)n, and (C2.5F)n recorded in the TEY (red lines with symbols) and PEY (blue lines) modes. F 1s NEXAFS (TEY) spectrum of white graphite fluoride, WGF,32 is given for comparison with that of (CF)n.

certain errors in the relative intensities of absorption structures. The background intensity in the TEY spectra, as a rule, is significantly higher than that in the PEY ones. This is due to the fact that only high-energy Auger electrons directly generated by X-ray absorption are recorded in the partial spectra, whereas almost all the electrons, including the low-energy secondary electrons that result from inelastic scattering of Auger electrons during their travel to the sample surface, are collected in the TEY spectra.28 Using the universal curve of the electron mean free path in solids as a function of electron kinetic energy,34 one can estimate the thickness of the near-surface sample layer (probing depth) from which the electrons recorded in each spectrum escape into the vacuum. The range of kinetic energies of electrons detected in PEY is determined by the retarding potential Vret of −150 V and the kinetic energy of the C KVV and F KVV Auger electrons produced by the X-ray absorption of the carbon and fluorine atoms (∼250 and ∼650 eV, respectively). The probing depth of this technique is estimated to be about 0.8 nm for the C 1s spectra and slightly above 1 nm for the F 1s spectra. The probing depth increases substantially in the TEY mode due to the presence of low-energy secondary electrons. The minimum kinetic energy of these electrons is determined by the band gap value, which is around 3−5 eV for fluorinated graphites (CyF)n (y = 1, 2, ..., 6).35 As a result, the probing depth in the TEY spectra at the C1s and F 1s edges is close to 10 nm. The C 1s and F 1s spectra are further normalized to the same level of continuous carbon and fluorine 1s absorption at the photon energy of 320 eV (C 1s edge) and 725 eV (F 1s edge), respectively. It corresponds to the normalization of spectra per one absorbing C or F atom and allows us to compare the relative intensities of the absorption structures. It is clearly seen that the TEY and PEY spectra of each graphite fluoride consist of the same number of absorption bands (A−F*), which have close energy positions in both spectra. This finding points to a similar structure in the near-surface area and in the bulk of the studied graphite fluorides particles. The main differences between the TEY and PEY spectra of particular graphite fluoride at the C 1s and F 1s edges (Figures 3 and 4) are that the absorption bands in the PEY spectra have a lower contrast and are characterized by poorer counting statistics. The former is probably due to the presence of defects in the near-surface region of the sample, while the latter results from a lower

Figure 3. C 1s NEXAFS spectra of graphite fluorides (CF)n, (C2F)n, and (C2.5F)n recorded in the TEY (red lines with symbols) and PEY (blue lines) modes.

signals are first divided by the current measured from a gold mesh in the beamline to account for temporal and spectral changes in the X-ray flux incident on the sample under study. A linear fit based on the pre-NEXAFS region is then subtracted from the data to remove the background caused by lower energy electronic transitions from valence shells. Here it should be noted that such a simple procedure for separating and removing the background under the fine structure of the X-ray absorption spectrum is not unambiguous and can lead to D



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manifest themselves in the form of discrete transitions in the spectrum of the HOPG. The broad bands D−F are attributed to electron transitions to the empty σ electronic states in the graphite conduction band, which are associated with the interaction of carbon hexagons in the graphene layer.37 To exclude the role of crystal effects in the subsequent comparison with the spectra of graphite fluoride powders, we will also use the C 1s spectrum of a polycrystalline graphitic sample, the graphitized carbon nanodiscs (graph-CNDs). It is seen from Figure 5 that the polycrystalline graph-CNDs C 1s absorption spectrum is in a good agreement with that of the HOPG. Indeed, in going from the crystalline HOPGs to the graph-CNDs sample, only the resonance A shows a considerable decrease in relative intensity, whereas other structures in the C 1s absorption spectrum remain practically unchanged. It is well-known that the intensity of the π resonance A in the C 1s spectrum of HOPG depends on the orientation of the polarization vector of linearly polarized synchrotron radiation with respect to the normal to the graphene layer.37,40 Thus, the observed decrease of A in intensity probably results from averaging the intensity of this resonance over the randomly oriented graphite-like particles (nanodiscs) in the graph-CNDs sample. Clearly the origin of the main absorption bands in the spectra of the latter and HOPG has the same nature and is directly related to the characteristics of planar sp2 hybridization of carbon atoms valence states. Considerable changes in the spectral shape are observed in Figure 5 when comparing the C1s spectrum of graph-CNDs with those of graphite fluorides (CyF)n (y = 2.5, 2, 1). The most significant differences involve a drastic decrease of the π resonance A in intensity (which is reduced to zero with increasing degree of fluorination along the C2.5−C2F−CF series) and a formation of new σ absorption structures B1*−F* instead of the earlier ones B−F among which a low-energy narrow peak B1* and a high-energy isolated band E*−F* stand out. The designations of the (CyF)n spectra absorption structures were made taking into account the correlation of energy positions of these structures and the structures in spectra of graphite-like samples, HOPG and graph-CNDs. It is evident that the changes observed in the C 1s NEXAFS spectra when moving from graph-CNDs to (CyF)n are governed by the graphite fluorination. At first we consider the spectra of graphite monofluoride (CF)n and a reference sample, the white graphite fluoride (WGF) measured under similar experimental conditions previously.36 Powdered WGF was prepared by a direct fluorination of a coke at 300 °C. The latter was previously purified at a temperature of 2800 °C using the industrial thermochemical method. The product with a maximum fluorine content of 62.4 wt % was almost white in color. The WGF was found to be essentially graphite monofluoride (CF)n prepared under slightly different conditions than that studied in the present work. This statement is confirmed by direct comparison between the C 1s absorption spectra of (CF)n and WGF because those spectra are characterized by the almost identical fine structure. The presence of the low-intense band A in the WGF spectrum probably results from the conservation of a small number of carbon particles in the sample, which are not fully fluorinated due to insufficiently high fluorination temperature (300 °C). The fine structure of these spectra is determined by the dipole allowed transitions of C 1s electrons to unoccupied

intensity of the PEY spectrum due to the use of retarding voltage. Hereinafter, mainly the TEY spectra will be considered, because they demonstrate better defined and most contrast absorption structures. The C 1s absorption spectra of graphite fluorides (CyF)n (y = 1, 2, 2.5) are compared in Figure 5 with those of highly

Figure 5. Comparison of C 1s NEXAFS spectra of graphite fluorides (CF)n, (C2F)n, and (C2.5F)n with those of reference systems HOPG, graph-CNDs, WGF, and NDs. The vertical dashed lines indicate the approximate onset of the C 1s absorption spectrum of ND and graphite fluorides.

oriented pyrolytic graphite (HOPG), graphitized carbon nanodiscs (graph-CNDs), white graphite fluoride (WGF),36 and nanodiamonds (NDs). It should be emphasized that the spectra of all reference systems, including white graphite fluoride, were measured under the same experimental conditions and, thus, can be compared in detail with the graphite fluorides (CyF)n spectra. The present C 1s absorption spectrum of the HOPG crystal is in good agreement with the results of previous measurements performed with a similar high-energy resolution of about 100 meV.37,38 It is known that the most characteristic absorption peaks (resonances) A and B−C in the HOPG spectrum reflect the planar sp2 hybridization of valence 2s and 2px,y states of carbon atoms and, hence, are the fingerprints of the trigonal coordination of carbon atoms in graphite. The absorption structures under consideration are associated with the dipole-allowed transitions of the C 1s electrons to the empty π and σ states in the conduction band of the HOPG. These states (peaks A and B−C) are formed by the C π2pz and sp2-hybridized C σ2s,2px,y orbitals oriented perpendicular and parallel to the plane of the carbon layer (graphene), respectively. These states are quasimolecular in nature, that is, they are similar to the unoccupied electron states in the molecule of benzene C6H6.39 Therefore, these states are predominantly localized within one carbon hexagon and E



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its turn, only minor changes of other absorption structures B*− F* are indicative of the preserved tetrahedral surroundings for all C atoms in (C2F)n. Obviously, these are the C3F coordination for one-half of the carbon atoms and the C4 coordination for the second half. Thus, in both (CF)n and (C2F)n graphite fluorides all C atoms are tetrahedrally surrounded. This conclusion is in good agreement with the atomic spatial structures that are commonly accepted for these fluorine−carbon systems22 (Figure 1) and with the above NMR data. The conclusion about similar bonding between C and F in the graphite fluorides (CF)n and (C2F)n is also confirmed by their F 1s NEXAFS spectra (Figure 4), which are caused by the dipole allowed transitions of F 1s electrons to unoccupied electron states of the conduction band of graphite fluorides containing the contributions from the F 2p electron states. Indeed, it can be clearly seen that the F 1s absorption spectra of (CF)n and (C2F)n, as well as WGF exhibit the same structures B1*−F*. The main difference between these spectra is that B1*−F* show a slight decrease in their contrast when moving from WGF to (CF)n and (C2F)n. It should be also emphasized that the pronounced fine structure of these spectra indicates a well-defined structural order in these graphite fluorides. In other words, all F atoms, similar to C atoms, have identical and unique coordination. For additional characterization of chemical bonding in graphite fluorides, one can compare the C 1s and F 1s NEXAFS spectra aligned in energy taking into account energy separation ΔE between the F 1s and C 1s core levels. In Figure 6, the C and F 1s NEXAFS spectra of (CF)n are aligned in

electron states of the conduction band of (CF)n, which contain the contributions from the C 2p electron states. For the following discussion, it is important to emphasize that these states can be approximated by empty electronic states of a polyatomic group (quasi-molecule) formed by an absorbing atom and atoms of its nearest surroundings.29 The C 1s NEXAFS spectrum of graphite monofluoride is completely different from that of the HOPG but resembles the spectrum of nanodiamonds (NDs). Apparently, the spectrum of (CF)n is more structured than that of NDs, and its onset is shifted by approximately 1 eV to the high-energy side (vertical dashed lines in Figure 5). This observation clearly indicates that the chemical bonding between carbon atoms in (CF)n and in diamond is due to covalent σ bonds, while π bonds are absent. Hence, upon fluorination, F atoms attach to the basal plane of graphene sheets rather than substitute C in the plane. In this case, carbon atoms form new σ bonds with fluorine atoms through the participation of the C 2pz states and planar graphene layers become corrugated. This inference is confirmed by a direct comparison of the C 1s NEXAFS spectrum of (CF)n, with the corresponding spectra of fluorobenzenes C 6 H 6−n F n (n = 1−6) molecules, 41 where the planar coordination of carbon atoms is retained. Actually, the compared spectra differ substantially and the π resonances are observed in the spectra of all fluorobenzenes. Thus, the C atoms in graphite monofluoride have a spatial coordination rather than the planar one, and it is similar to the tetrahedral coordination in diamond. Appearance of a single high-energy σ band E*−F* in the C 1s absorption spectrum of (CF)n at the photon energy hν of about 305 eV can be regarded as an additional indirect evidence for the change from a graphitic to a diamond-like coordination caused by fluorination. Indeed, a similar isolated high-energy band, together with the absence of the π resonance, is a characteristic feature of the C 1s NEXAFS spectra from diamond.39 It should be noted that each carbon atom in (CF)n is surrounded by the C3F tetrahedron, while the C atom surroundings in diamond is the C4 one. The presence of fluorine atom in the nearest surroundings of carbon atom in (CF)n causes some electron transfer from the latter to the fluorine atom. As a consequence, the effective positive charge of the carbon atom in (CF)n in comparison with ND grows, thereby increasing the binding energy of the C 1s electrons and causing the high-energy shift of the C1s absorption onset. In going from the (CF)n spectrum to the (C2F)n one (Figure 5), the absorption structures B*−F* remain practically unchanged retaining their relative intensities and energy positions. The most significant differences between these spectra involve a considerable decrease in intensity of the characteristic narrow peak B1* and the appearance of a weak absorption band A. The appearance of a low-intense band A in the C 1s NEXAFS spectrum of (C2F)n probably reflects the presence of a small number of graphitic particles in the sample. As revealed by 13C NMR,13 they survived due to incomplete fluorination of the original graphitic sample at the fluorination temperature of 380 °C. Thus, the stoichiometry of graphite fluorides, (CF)n and (C2F)n is unambiguously characterized by the intensity of narrow absorption peaks B1*, which is different in the C 1s NEXAFS spectra of (CF)n and (C2F)n. In other words, it can be argued that the intensity of B1* reflects the number of σ bonds between the carbon and fluorine atoms in graphite fluorides. In

Figure 6. Comparison between the C 1s and F 1s NEXAFS spectra of graphite fluoride (CF)n. The spectra are aligned in energy using the energy separation ΔE (F 1s − C 1s) equal to ∼399.0 eV.11.

energy using ΔE (F 1s − C 1s) equal to ∼399.0 eV.11 Evidently, both spectra exhibit a fine structure that is similar in the number of main structures B1*−F* and their energy positions, implying that these structures are caused by the transitions of C 1s and F 1s electrons to the same set of unoccupied electron states formed by the C 2p and F 2p F



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valence electron states. All these results suggest a hybridized (covalent) character of empty electron states in (CF)n, in good agreement with the conclusion made above regarding the sp3 hybridization of the C valence electron states. Finally, the suggested tetrahedral coordination of C atoms due to the sp3 hybridization of the valence C 2s and C 2px,y,z electron states in CF is consistent with the results obtained from the investigation of the graphite fluorides (CyF)n formation mechanism by Raman spectroscopy.42 It should be noted here that a detailed comparison of our C 1s and F 1s NEXAFS spectra of (CF)n and (C2F)n with those measured earlier by Seki et al.21 is not possible, because they were obtained under different experimental conditions (samples studied, photon energy resolution, normalization, and energy calibration of the spectra). Indeed, an examination of C 1s absorption spectra shows the coincidence of energy positions for two important absorption structures B1* and E*− F* (B and H in ref 21), while a different spectral shape is observed between these structures. In its turn, the F 1s NEXAFS spectra are completely different in the spectral shape and energy positions of absorption structures. However, both works come to the same conclusion that upon fluorination of graphite the fluorine atoms are attached to the carbon atoms normally to the graphene planes. Further comparison of the C 1s NEXAFS spectrum of (C2.5F)n with those of (C2F)n and (CF)n (Figure 5) indicates significant differences between their absorption structures. First of all, an intense peak A characteristic for the spectra of graphite-like systems with the planar sp2 hybridization of the valence C 2s and C 2px,y electron states exists in the spectrum of (C2.5F)n. It is shifted by about 0.35 eV to the low-energy side in comparison with the spectra of graph-CNDs and HOPG. On the other hand, the overall spectral shape of the C 1s NEXAFS in (C2.5F)n is also characterized by low-contrast absorption structures B1*−F* observed in the C 1s absorption spectra of (C2F)n and (CF)n. As shown above, these structures indicate a tetrahedral sp3 hybridization of the valence C 2s and C 2px,y,z electron states. Therefore, regions with both trigonal and tetrahedral coordination of the absorbing C atoms are present in the (C2.5F)n structure. In this case, we can assume that the graphene sheets in (C2.5F)n are partially planar (sp2 hybridization) and partially corrugated (sp3 hybridization). In other words, the atomic structure of (C2.5F)n should be similar to that shown in Figure 1a. In this case, we can expect that the C−F bonds exhibit weakened covalence because of the hyperconjugation involving the C−C bonds on the carbon sheets and C−F bonds.11 Although fluorine content is equal to 0.39 fluorine atom per one carbon atom, the peak A is of high intensity and is only slightly shifted to lower energy in comparison with the pure graphitic compounds. A distortion of the carbon crystal lattice of graphene layers in (C2.5F)n results in an overlap of B1*, B*, and C* peaks in the X-ray absorption spectra of the latter. In Figure 7, the C 1s NEXAFS spectrum of (CF)n is compared with those of graphite fluorides RT-400 and RT-550, which were prepared by fluorination of graphitic sample with IF5 at room temperature and post-treated in pure F2 gas at 400 and 550 °C. The compared spectra show similar features B1*− F* characteristic for the (CF)n spectrum. The same similarity is also observed for the F 1s NEXAFS spectra of these compounds (not shown). The main difference between the C 1s spectra of RT-400 and RT-550 is the presence and intensity of peak A. It has a very low intensity in the spectrum of RT-400

Figure 7. C 1s NEXAFS spectra of (CF)n, RT-400, and RT-550 graphite fluorides.

and practically disappears in that of RT-550. This finding indicates that a small number of C atoms with the sp2 hybridization of their valence electron states still remains in these graphite fluorides, while most of the C atoms are the sp3 hybridized ones. It should be noted that a residual graphitic phase was also revealed by NMR for RT-400.13,17 The postfluorination of RTGF at 400 °C by F2 gas results in a deintercalation of iodine catalyst species and a change of the C hybridization from sp2 to sp3. Our NEXAFS data confirm that this conversion is efficiently achieved. An increase of the postfluorination temperature to 550 °C results in a total disappearance of the sp2 bonded C atoms. Such a two-step process (fluorination at room temperature followed by a postfluorination) allows to reduce disorder in covalent graphite fluorides. In (C2F)n, (CF)n, RT-400, and RT-500 the C−F bonds are covalent and involve sp3 carbon hybridization, as confirmed by NMR and NEXAFS spectroscopies.

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CONCLUSIONS Solid state NMR and NEXAFS are complementary techniques to investigate fluorinated graphites of any structural type. The covalence of the C−F bonds, as well as the fluorine content, can be extracted from the NMR spectra of 19F and 13C nuclei, regarding the chemical shifts and the integrated areas of various lines. A comparative analysis of the C 1s NEXAFS spectra of reference carbon compounds (HOPG, graph-CNDs, ND) with those of a large panel of fluorinated graphites with different structures and C−F bonding allows us to gain important information about the chemical bonding in graphite fluorides. The chemical bonding between carbon and fluorine atoms in the (CF)n and (C2F)n is due to covalent C−C and C−F σ bonds, but π bonds are absent, as in the case of diamond. Upon fluorination, fluorine atoms form σ bonds with the C atoms with the participation of the C 2pz states. The main absorption structures in the C 1s and F 1s NEXAFS spectra are caused by the transitions of C 1s and F 1s electrons to the same set of unoccupied electron states formed by the C 2p and F 2p valence electron states. This results from a clear correlation of absorption structures in the C 1s and F 1s NEXAFS spectra of (CF)n aligned in energy using the energy separation ΔE (F 1s − C 1s) between the F 1s and C 1s core levels. These findings suggest a hybridized (covalent) character of empty electron states in (CF)n and (C2F)n. When the fluorine content is low, as for the case of (C2.5F)n, sp2, and sp3 bonded C atoms coexist, that is, there coexist G



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regions with trigonal and tetrahedral coordination for the carbon atoms. The graphene sheets are still mainly planar and the C−F bonds exhibit weakened covalence because of the hyperconjugation. Finally, the postfluorination of RTGF results in a transformation from sp2- to sp3-bonded carbon and in a change in the C−F bonding from weakened to pure covalence, as verified by our NEXAFS study, in accordance with the NMR data.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +33 4 73 40 71 05. Fax: + 33 4 73 40 71 08. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the St. Petersburg State University (Grant No. 11.38.638.2013), the Russian Foundation for Basic Research (Grant Nos.12-02-00999 and 12-02-31415), the bilateral Program “Russian-German Laboratory at BESSY” and the Swedish Research Council.

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NMR and NEXAFS Study of Various Graphite Fluorides - American

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