Article pubs.acs.org/JPCC
Fluorine Patterning in Room-Temperature Fluorinated Graphite Determined by Solid-State NMR and DFT Anastasia Vyalikh,*,† Lyubov G. Bulusheva,‡ Galina N. Chekhova,‡ Dmitry V. Pinakov,‡ Alexander V. Okotrub,‡ and Ulrich Scheler† †
Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Strasse 6, 01069 Dresden, Germany Nikolaev Institute of Inorganic Chemistry, SB RAS, 3 Academician Lavrentiev Avenue, 630090 Novosibirsk, Russia
‡
S Supporting Information *
ABSTRACT: Fluorination of graphite at room temperature allows producing graphite fluoride compounds with a controlled content of fluorine. Here we combine solid-state NMR spectroscopy and DFT calculations to study the structure and reveal the fluorine patterning in graphite fluorides C2Fx intercalated with acetonitrile. Two major chemical states of carbon, namely the atoms covalently bound to fluorine and the bare atoms, are detected by 13C MAS NMR irrespective of the degree of fluorination. The data indicate that although all graphene sheets were subjected to fluorination, the near-planar configuration is preserved. The interaction between host C2Fx matrix and acetonitrile molecules is of van der Waals character. Decomposition of the 19F MAS NMR spectra reveals occurrence of at least six fluorine environments in each sample. By DFT calculations distinct 19F chemical shifts are attributed to isolated, end chain, “linked” (which include midchain, cyclic, and branched) CF groups and infinite CF arrays. The assignment is confirmed by 19F RFDR, which is sensitive to dipolar coupling. Analyzing the data for C2Fx samples with different degrees of fluorination x, an evolution of the fluorine pattern is proposed. The reported calculated 19F NMR shielding parameters provide classification criteria for assignment of 19F NMR chemical shifts in fluorinated carbon materials.
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INTRODUCTION 2D crystals have recently drawn considerable attention for both investigations of fundamental physical properties and for implementation in next-generation electronic devices.1,2 Of particular interest is graphene, a single-layer carbon compound, characterized by numerous spectacular properties. However, despite the advantages determined by its unique electronic properties, the absence of a band gap has limited its applicability in electronics. One of the possible approaches to open and control the band gap is the chemical modification of the surface of graphene, such as hydrogenation3−5 or fluorination.6 Covalent bonding changes the carbon hybridization from sp2 to sp3, preserving the two-dimensional structure of graphene. Unlike hydrogenation, fluorination proceeds more easily, and recent investigations have demonstrated that the fluorinated graphene is a more interesting electroactive wide band gap material7−9 than the pristine graphene. Fluorinated graphene can be produced by an exfoliation procedure from graphite fluoride, which is characterized by strong covalent in-plane bonding and weak van der Waals coupling between the layers.7,10 Two stoichiometric forms, poly(carbon monofluoride) (CF)n and poly(dicarbon monofluoride) (C2F) n, obtained using different experimental conditions are presently known. While direct fluorination of graphite to the (CF)n composition requires high temperatures (from 600 to 640 °C),11 semifluorinated graphite (C2F)n is © 2013 American Chemical Society
prepared at milder conditions. It is suggested that in (C2F)n produced at elevated temperatures 350−400 °C using elemental fluorine, both fluorinated and non-fluorinated carbon atoms are in the sp3-hybridized state so that two adjacent carbon sheets are linked by covalent C−C bonds forming a double-decked monolayer.12 In contrast, a room-temperature fluorination with a F2−HF mixture in the presence of a volatile fluoride preserves the planar configuration of the carbon sheets.13 Moreover, active molecules such as ClF3 or BrF3 can be used as a sole fluorinating agent to produce graphite fluoride of the C2F composition at ambient temperature.14,15 As a result of the synthesis, chemical species from the reaction medium are trapped between the fluorinated graphene layers, providing the final product to be in fact an intercalation compound of graphite fluoride (C2F)n. Replacing the trapped species by a new guest one can separate the layers by a distance depending on the intercalant molecule size, thus producing a stack of essentially noninteracting fluorinated graphenes. It has been demonstrated that the partially recovered upper layer in such a stack acts as highly sensitive gas sensor.16 Quantum-chemical calculations show that electronic, optical, and magnetic properties of fluorinated graphite and fluorinated graphene can be tuned by the fluorine content and the fluorination pattern.17−21 Indeed, the magnetic properties Received: March 21, 2013 Published: March 26, 2013 7940
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should emphasize that all graphite fluorides under study were synthesized under similar conditions (reagents, temperature) varying the concentration of fluorinating agent and duration of synthesis only.
measurements for a set of intercalation compounds of graphite fluorides (C2Fx)n (x ≤ 1) have revealed a decrease of the spin concentration with the increase of fluorine content.22 In the case of room temperature produced (C2F)n, in which only half of the carbon atoms are bound to fluorine atoms, different fluorination patterns could be expected. On the basis of the Xray diffraction (XRD) data, Yudanov et al. proposed that the fluorine atoms are attached with equal probability to both sides of a graphene sheet forming the strips of fully fluorinated hexagons separated by aromatic strips.23 Comparing the calculation results with the CKα spectrum of (C2F)n, an alternation of bare and fluorinated carbon chains was proposed as the most probable arrangement.24 The similar structure of the C2F layer was derived by Sato et al. from neutron diffraction analysis.25 Solid-state NMR is known as a powerful tool to identify and quantify different structural species present in inorganic materials.26−28 The NMR investigations on covalent graphite fluorides and fluorine-intercalated graphites have been critically reviewed by Panich.29 The difference between 19F chemical shifts for high-temperature and room-temperature synthesized compounds was attributed to the specific local atomic surrounding of CF groups, namely, to a state of neighboring carbon atoms in either sp3 or sp2 hybridization. In the following years graphite fluorides prepared using different approaches were examined by means of solid-state NMR.30−33 The 19F magic-angle spinning (MAS) NMR spectrum of high-temperature synthesized graphite fluorides (CF) n and (C 2 F) n commonly has two peaks. The most intense peak around −190 ppm corresponds to CF groups in graphite layers, while the second peak around −118 ppm is attributed to either CF2 groups located at the edge of graphite layers or associated with structural defects.31,34,35 The appearance of an additional peak around −147 ppm in the 19F MAS NMR spectrum of (C2.5F)n synthesized at room temperature in an atmosphere of F2 gas with an admixture of HF and ClF3 is related to hyperconjugation between C−F bonds and non-fluorinated carbon atoms.33 Moreover, using inverse cross-polarization (CP) MAS NMR, it has been shown that post-fluorination at elevated temperatures of the latter compounds results in a decrease of the averaged C−F bond lengths. The interpretation of the 19F data is supported by 13C MAS NMR. The 13C peaks at 82−90 and 111 ppm are associated with CF and CF2 species, respectively;34 the 13C signal of graphitic carbon in weak interaction with fluorine is observed in the range of 128−142 ppm,30,33 while the line at ca. 42 ppm is characteristic of sp3 aliphatic carbon atoms, providing a connection between the fluorocarbon sheet pairs.31 Here, for further clarification of the relationship between the chemical shift and structure of graphite fluoride we combine high-resolution ultrafast MAS solid-state NMR spectroscopy and density functional theory (DFT) calculations. To get insight into a mechanism of pattern formation upon roomtemperature fluorination, we study a set of compounds varying the fluorine content x in C2Fx matrix from ∼0.5 to ∼0.9 in C2Fx. Acetonitrile (CH3CN) is chosen as a guest molecule in the matrix for the following reasons: (1) graphite fluorides intercalated with this molecule are stable for long time at ambient conditions; (2) nitrogen is used as a standard for determination of guest content using elemental analysis; (3) the interactions between graphite fluoride matrix and intercalated molecules can be selectively probed applying 1 H−19F heteronuclear correlation NMR experiments. We
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MATERIALS AND METHODS Materials. Natural graphite from Zavalievo deposit (Ukraine) with a characteristic size of platelets of 0.4 × 0.3 × 0.02 mm3 was used as a starting material. Ash content after purification did not exceed 0.1 mass %. The XRD data (d002 = 3.357 Å and d004 = 1.678 Å), the absence of the band at 1330 cm−1 in the Raman spectrum, and a value of specific surface area of ∼0.5 m2/g determined by BET (N2 adsorption) demonstrated the characteristics reminiscent of those of wellcrystallized graphite. Acetonitrile-intercalated graphite fluorides were synthesized using a procedure described in refs 36 and 37. Briefly, at the first step graphite platelets were placed in a Teflon flask and held in the vapor over liquid Br2 for 1 day. The resulting sample, being a second-stage intercalation compound C8Br (i.e., two layers of a carbon matrix alternate with a layer of the intercalant), was transferred into another Teflon flask and located over a solution of BrF3 in Br2. The use of Br2 for preliminary graphite intercalation as well as for dilution of the fluorinating agent BrF3 results in a decrease of the amount of energy released with the fluorination, making the synthetic process safer. The fluorination of graphite layers proceeded at room temperature. The fluorine content in the product was controlled by the concentration of the fluorinating agent in Br2 and by a duration of the synthesis.36 As a result of the fluorination procedure, a first-stage intercalation compound, where the reaction medium components (BrF3, Br2) are included between adjacent fluorinated graphene layers, was formed.38 Washing the product by acetonitrile resulted in a replacement of BrF3 and Br2 by acetonitrile. At the final step, the drying of sample in nitrogen flow until a constant mass resulted in a second-stage compound, which can be schematically represented with a CH3CN/C2Fx/C2Fx/CH3CN sequence. Details of the characterization of the resulting materials by elemental analysis, XRD, and IR spectroscopy are given elsewhere.39,40 Data on the composition for four studied here samples C2FxBr0.01·yCH3CN with a different fluorination level x are listed in Table 1. Thereafter only the matrix composition C2Fx will be used for the sample notification. Table 1. Compositions of the Investigated Samples Described by the Formula C2FxBr0.01·yCH3CN y sample
x (±0.01)
elemental analysis (±0.009)
NMR (±0.01)
C2F0.92 C2F0.87 C2F0.69 C2F0.49
0.92 0.87 0.69 0.49
0.136 0.136 0.112 0.084
0.16 0.12 0.11 0.02
NMR Measurements. All NMR spectra were obtained on a (11.7 T) Bruker Avance 500 spectrometer operating at resonance frequencies of 500.1 MHz for 1H, 470 MHz for 19 F, and 126 MHz for 13C. The 19F ultrafast MAS experiment at 60 kHz was performed using a 1.3 mm probehead and a 90° pulse duration of 2.5 μs. For other experiments a 2.5 mm H/FX MAS probehead was used. 13C MAS NMR experiments were 7941
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acquired with 19F XiX decoupling at a spinning frequency of 10 kHz with a 90° pulse duration of 5 μs, a recycle delay of 100 s, and 4096 repetitions. For 13C{19F} CP measurements a 19F π/2 pulse duration of 5 μs, 19F decoupling with an XiX composite pulse of the rf field strength of 50 kHz, and the contact times of 0.3, 0.5, and 1 ms were used. 19F MAS NMR experiments were acquired at a spinning frequency of 30 kHz with a 90° pulse duration of 5 μs, 256 repetitions, and a recycle delay of 5 s. For 19 1 F{ H} CP NMR measurements a 1H 90° pulse duration of 3 μs was used with 16 384 repetitions. High-power two-pulse phase modulation (TPPM) 1H decoupling of 100 kHz was applied. Two-dimensional radio-frequency-driven recoupling (RFDR) experiment was performed at an MAS frequency of 31.25 kHz with a mixing time of 1 ms corresponding to 32 rotor cycles. Rotor-synchronized detection was used in both directions. 19F−1H CP and 19F RFDR simulations were carried out using SIMPSON.41 13C chemical shifts are calibrated relative to TMS using the upfield peak of adamantane at 29.5 ppm. The 19F chemical shifts were referenced relative to CFCl3 at 0 ppm using PTFE (δF = −122 ppm). The spectra were fitted using DMFit.42 Computational Details. The DFT calculations of absolute chemical shielding, which is related to NMR chemical shift, were performed using the CASTEP package based on the gauge-including projector augmented wave (GIPAW) method.43 The revised Perdew, Burke, and Erzerhof (PBE) functional44 was used in conjunction with ultrasoft pseudopotentials.45 The wave functions were expanded on a plane-wave basis set with a kinetic energy cutoff of 610 eV. The calculations were carried out on a periodically repeated one-layer supercell consisting of 4 unit cells in each in-plane direction of graphite and giving the supercell parameters a = b = 9.86 Å. Parameter c was adjusted to have a value of 15.31 Å, large enough (1) to prevent interactions between the carbon sheets and (2) to accommodate out-of-plane distortions. Fluorine atoms were deposited on the graphene fragment in an alternating fashion above and below the graphene plane in such a way that the fluorinated chains, cyclohexanes, or isolated C−F bonds were separated by corresponding non-fluorinated carbon units. The 2 × 2 × 1 Monkhorst−Pack grid46 was used for Brillouin zone integration. For NMR calculations all structures were fully geometry-optimized, keeping the unit cell parameters fixed. The NMR calculations were performed without any symmetry constraint, so the symmetry force on any atom did not exceed 0.01 eV/Å. The total energy calculations are converged to within 5 × 10−6 eV/atom.
Figure 1. (a) 13C MAS NMR spectra at 10 kHz for acetonitrileintercalated graphite fluorides. Asterisks denote rotational ssb related to the isotropic peak at 131 ppm. The dotted arrow beneath the spectrum of C2F0.49 shows the peak position at 119 ppm, where the signal from graphite is expected. (b) 13C{19F} CP MAS NMR spectra of C2F0.92 and C2F0.49 at the contact time (tCP) of 1 and 0.3 ms.
length variation. Regarding the latter, Deschamps et al.47 have demonstrated a linear correlation between the 13C chemical shift and the average ⟨CCC⟩ bond angles in C70 offering a way to understand the dispersion of 13C chemical shifts in graphitic materials in terms of local deviation from planarity. Comparing the 13C NMR shift for the nonfluorinated carbon atoms with the data for carbon atoms composing the adjacent hexagons in C70 (a belt at the fullerene equator), we estimate that the average ⟨CCC⟩ bond angle in the fluorinated graphenes is between 119° and 118°, whereas the angle of 120° corresponds to the pristine graphene. It has to be noted that the 13C shift at ca. 82 ppm is invariant on the fluorination degree of the investigated graphite fluorides. Additionally, we want to emphasize that no peak close to that of pure diamond carbon at 35 ppm, which would prove the connection between the fluorocarbon sheet pairs in fluorinated carbon materials,29 is observed in our work. This confirms the planar configuration of the carbon sheets in the roomtemperature synthesized fluorinated graphites. Positions of the peaks at 116.1 and −0.9 ppm are very close to those of the free CH3CN molecule, proving that the guest molecule in the systems under study does not form chemical bonds to the fluorinated graphitic matrix, and the host−guest interaction is weak and of van der Waals nature.40,48 The data presented in Table 2 demonstrate a systematic decrease of acetonitrile content from 14 to 2% as the fluorine content decreases. The fraction of the intercalated CH3CN y can be estimated with respect to graphite content giving the formula (C2Fx)·yCH3CN. Comparison with the acetonitrile fraction obtained by elemental analysis (Table 1) demonstrates excellent agreement for all samples except that with the lowest fluorine content. Figure 1a indicates that the C−F peak increases relative to the C sp2 peak when the fluorination degree increases. The
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RESULTS AND DISCUSSION C NMR. Single pulse 13C MAS NMR spectra of all samples showed two major peaks around 130 and 82 ppm (Figure 1a), which are characteristic of carbon in sp2 hybridization (i.e., graphite-like carbon) and carbon bonded to fluorine (C−F), respectively. The data of spectral deconvolution ignoring the signals with contribution less than 2% of the total intensity are collected in Table 2. The downfield shift of the NMR signal of the non-fluorinated carbon as compared to the graphite peak, which is appeared at ca. 119 ppm, is explained by a weak interaction of the carbon atoms with fluorine atoms from neighboring CF groups. The signal progressively moves from 127.8 to 131.5 ppm with the increase of the fluorine concentration. In general, a downfield shift of the NMR signal is caused by increased electronegativity of the atoms attached to carbon or proximity to nearby π-electron density or bond 13
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Table 2. 13C Chemical Shift Parameters (13δiso ±0.1, ppm, 13δani ±2, ppm) and Integrated Intensities (±2%) of NMR Peaks Corresponding to Carbon in sp2 Hybridization, Carbon Covalently Bound to Fluorine (C−F), and Carbon in Acetonitrile C sp2 δiso
sample
13
C2F0.92 C2F0.87 C2F0.69 C2F0.49
131.5 130.7 129.6 127.8 115b
13
δani
−124 −130 −114 −136
CH3CNa
C−F I 45 56 60 70 7
13
δiso
δani
13
82.8 82.8 82.8 82.3 66b
67 67 67 97
I
I
composition from NMR
41 33 30 19 2
14 11 10 2
C2F0.95 C2F0.74 C2F0.67 C2F0.43
Integrals of the 13C signals of acetonitrile resonating at 116.1 and −0.9 ppm are summed up including their spinning sidebands. bTypical measurement error of 13δiso for the peak at 115 and 66 ppm is ±1 ppm. a
poorly fluorinated graphite, i.e., in the structures corresponding to C4F1, C8F1, and C16F1 composition. Based on chemical shift variation, an estimate of the distance, which separates the fluorinated fragments, yields the size of the graphite-like domains to be around 1 nm. A scenario of graphite-like domains in fluorinated graphene layers is contradictory to the graphite fluoride formation mechanism proposed by Han et al.49 where one graphite layer is fully covered with fluorine while the other layers are still pristine. In that case the 13C NMR spectrum of C2F0.49 would give an integral intensity of the peak at 115 ppm of 50 ± 10% of the total spectral intensity, which is in a major disagreement to the value of 7 ± 2% observed in the experiment (Figure 1 and Table 2). 19 F NMR. The 19F MAS NMR spectra at 30 kHz of all samples studied in the present work are shown in Figure 2. Two dominating peaks around −145 and −178 ppm contribute with different weights to the spectra. In general, the increase in the F content occurs without a significant change in the spectral profile, indicating a similar environment for the fluorine atoms
chemical compositions based on the integrated intensities of the 13C NMR spectra presented in Table 2 are in a very good agreement with those obtained by elemental analysis and quantitative X-ray photoelectron spectroscopy (XPS) analysis.39 As compared to the 13C NMR spectra of the higher fluorination degree C2Fx compounds, the spectrum of C2F0.49 has a different shape characterized by significantly broadened lines and an appearance of two shoulder peaks at ca. 115 and 66 ppm (Figure 1a). Line broadening can be accounted for by inhomogeneous broadening resulting from a structural disorder caused by a low content of fluorine. The origin of the peak at 66 ppm, which comprises 2% of total intensity, is unclear. The appearance of the minor peak at 115 ppm can be attributed to either CF2 groups or to the presence of pure graphitic domains; that signal is expected at 119 ppm. The presence of some amount of CF2 groups has been identified in the XPS spectra of C2F0.49 and assigned to the “terminal” areas of the carbon layers, i.e., located at sheet edges, being the active centers of fluorine addition during the synthesis.39 Moreover, the XPS C 1s spectrum of C2F0.49 showed the peak with binding energies characteristic of pure graphite, and XRD indicates the presence of a broadened reflection corresponding to the interplanar d002 separation in graphite. An assignment problem in the 13C NMR data can be solved applying 19F to 13C cross-polarization (CP), where magnetization transfer is governed by the strength of the direct dipolar interactions, and therefore, 13C signal intensity depends on the distance to F. The 13C{19F} CP spectra are shown in Figure 1b. The signals from acetonitrile are no longer visible in the CP spectra due to mobility and to the remote distance of acetonitrile to fluorine atoms attached to the graphite sheet. Further, a comparison of the CP and single pulse (Figure 1a) spectra confirms our assignment for the two major kinds of carbon atoms. That is significant reduction of the sp2 carbon signal with respect to the C−F signal in the CP spectrum at tcp = 1 ms, even stronger pronounced at tcp = 0.3 ms, is caused by longer distances of non-fluorinated carbons to 19F as compared to carbons involved in covalent bond with fluorine. On the bottom of Figure 1b, the CP NMR spectrum of C2F0.49 is displayed. The low concentration of 19F as a polarization source results in a decreased signal-to-noise ratio. Nevertheless, a strong reduction of the spectral intensity at 115 ppm relative to the C−F peak at 82 ppm has been observed, ruling out the CF2 origin of the former. Thus, based on this observation, the signal at 115 ppm in the spectrum of C2F0.49 is attributed to the unreacted residual graphite (or areas of the graphitic structure), which are likely to be inaccessible for the fluorinating agent. This finding has been supported by DFT calculations (see Figure S1), which demonstrate appearance of the 13C chemical shifts around 115 ppm resulting from sp2 carbon atoms in
Figure 2. (a) 19F MAS NMR spectra at 30 kHz for acetonitrileintercalated graphite fluorides. Rotational sidebands of the −145 ppm signal are denoted by asterisks, while those of the −178 ppm signal by circles. (b) Experimental 19F MAS NMR spectrum at 60 kHz of C2F0.87 (top) and its decomposition (bottom) including the sum of the fit components. (c) 19F CP MAS NMR spectrum of C2F0.92 using cross-polarization from 1H of acetonitrile at the contact time of 1 ms and 1H decoupling. 7943
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resonances in the high-field region, whose positions at −174 and −185 ppm are very close to the isotropic chemical shifts of the spectral components found in the directly polarized spectra. Deconvolution of the CP spectrum revealed in addition the peaks at −142, −121, and −59 ppm, which are in very good agreement with the spectral components of the single pulse spectrum. It has to be noted that applying strong 1H heteronuclear decoupling does not result in resolution enhancement of the single pulse 19F spectra as expected. To conclude, the CP spectra do not show selective excitation of either 19F site, but their detection itself allows characterizing the host−guest interactions. Thus, a maximal 1H−19F distance is found to be within a range of 3−4 Å supporting the van der Waals character of the host−guest interaction. DFT. In order to understand the diversity of the 19F chemical shifts in terms of local structure, the NMR parameters have been calculated using DFT. Fluorine atoms were deposited on the graphene fragment forming either chain configurations (armchair, zigzag, or branched chains) with different numbers of F atoms or cyclic hexagonal patterns of fluorocarbons. The results have been analyzed with respect to fluorination patterning, number of fluorine atoms per cluster, and C−F bond length. First, overall calculated 19F absolute shielding parameters σF can be classified with respect to the fluorination patterning and fluorine concentration. The corresponding σF shielding ranges are schematically presented in Figure 3. The single (or isolated) fluorine atoms, which are separated by nonfluorinated carbon arrays from other fluorinated patterns, form the least shielded range (σF is within 162−185 ppm). Further, the 19F atoms located at the end of the chains irrespective of the chain length and configuration are always less shielded as compared to the middle chain atoms and occupy the range from 186 to 260 ppm. The middle-chain CF groups have been analyzed with respect to the local environment and the proximity to the next fluorinated chain. However, it has been shown that the σF values for midchain CF are insensitive to the distances to the next CF chain. All middle chain CF groups demonstrate σ19F in the range of 268−340 ppm. The CF configuration consisting of fluorinated cyclohexane rings shows a broad distribution of shielding ranging from 260 to 350 ppm covering the range of the midchain CF groups. Modeling of the
in the samples with significantly different C:F ratios. Although, here again, broadening of the spectrum of C2F0.49 is observed as compared to the samples with a higher fluorination degree. Variations of the sample MAS speed from 20 to 30 kHz and then to 60 kHz (Figure S2) exhibited no further line narrowing, reflecting the fact that the linewidth in C2Fx materials is caused by inhomogeneous broadening. Decomposition of the 19F MAS NMR spectra revealed at least six components for each sample. To avoid confusion caused by overlapping of the central lines with the spinning sidebands (ssb) of other components, the measurements have been carried out at ultrafast spinning speed of 60 kHz, which allows to place the ssb away from the spectral region of the central lines. The 19F spectrum of C2F0.87 at 60 kHz and the results of decomposition are shown in Figure 2b, confirming the presence of six signals, whose fit parameters are listed in Table 3. Table 4 summarizes the data used to model and quantitatively interpret the 30 kHz MAS spectra for all materials under study. Table 3. 19F MAS NMR Fit Data (60 kHz) for C2F0.87 19
F chemical shift (ppm) −69.0 −90.7 −116.4 −145.8 −178.5 −185.9
linewidth (ppm)
integration (%)
31 26 36 21 9 15
2 4 19 62 8 5
The 19F{1H} CP NMR experiments have been performed for two purposes: (1) to differentiate the 19F signals observed in the single-pulse 19F MAS NMR spectra and (2) to exploit the nature of host−guest interactions. Thus, magnetization has been transferred from 1H of acetonitrile, which is the only proton source in the sample, to fluorine nuclei attached to the carbon plane. Comparing to the directly polarized spectra (Figure 2a), the CP spectrum of C2F0.92 shown in Figure 2c has been detected with significantly lower signal-to-noise ratio that is caused by remote distances between intercalant and the host matrix as well as fast MAS and lower content of acetonitrile compared to 19F. However, one can see two distinguished
Table 4. 19F Isotropic Chemical Shifts δF and Relative Concentrations of Different CF Configurations Estimated from 19F MAS NMR at 30 kHz and Their Assignment
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Figure 3. Schematic diagram of the ranges of calculated 19F absolute shielding parameters σF in fluorinated graphene layer. Black spheres are fluorine atoms adjacent to carbons and contributing to the ranges shown beneath the structural fragments, while other fluorine atoms are colored in blue.
Figure 4. Calculated σF values versus experimentally measured δF chemical shifts. The solid line corresponds to a relationship δF = −0.70 × σF + 57 calculated over a set of fluorinated inorganic compounds in ref 50. The experimental 19F MAS NMR spectrum has been introduced on the left. The spectral components are colored according to the assigned correlation boxes.
infinite CF arrays, when F occupies a half of carbon matrix that corresponds to C2F1 composition, yields a very narrow σF range from 273 to 278 ppm for armchair configuration, and a single 19 F shielding value for zigzag CF chains at 340 ppm, indicating that all F atoms are equivalent. Thus, these classification criteria can be used to assign experimentally observed 19F chemical shifts in the C2Fx materials under study. To convert the calculated absolute shielding into the experimental chemical shifts, we applied a linear function reported in ref 50, which establishes the correlation between the experimental and calculated parameters for 12 fluorinated inorganic compounds with a good accuracy. Validation of this approach to the present work is based on the fact that the same level of theory has been used in both studies. Thus, the linear function given as δF = −0.70 × σF + 57 is used to translate σF values classified in Figure 3 into the corresponding chemical shifts δF. Therefore, the barycenter of each σF distribution is placed on the correlation line so that all data points within this distribution are translated into the chemical shift distribution yielding the characteristic boxes for a given 19F site (Figure 4). This plot is overlaid with the 19F MAS NMR spectrum (on the left), allowing to correlate the spectral components to the characteristic boxes of 19F chemical shifts. On the basis of such an approach, the least shielded peak (ca. −53 ppm with the linewidth of ca. 50 ppm) is attributed to single or isolated fluorine atoms, while the 19F signal observed at ca. −118 ppm is related to the end chain CF groups. Two signals at ca. −145 and −178 ppm can be assigned to either middle chain or cyclohexane or branched CF groups, which generally can be termed “linked” as they are connected to at least two fluorinated carbon atoms. It has to be emphasized that a fragment size and a level of calculation are the limiting factors for the accuracy of the calculated data leading to the high dispersion and overlapping of the shielding parameters for different structures. Although only two signals are experimentally observed within this region, which could arise from either of these structures, they can currently not be assigned unambiguously. Finally, an infinite array of CF zigzag configuration is most likely the origin of the 19F signal at ca. −185 ppm. The remaining signal at ca. −100 ppm can be assigned to small amounts of peripheral CF2 groups located at sheet edges or/and associated with other structural defects.
To support the assignment above, a 2D 19F−19F radiofrequency-driven recoupling (RFDR) experiment with a mixing time of 1 ms has been performed, which correlates neighboring fluorine atoms by homonuclear dipolar coupling. The RFDR spectrum shown in Figure 5a demonstrates the cross-peaks
Figure 5. 19F RFDR spectrum of C2F0.92 at 31.25 kHz MAS and mixing time of 1 ms (A) and SIMPSON simulations for a 19F−19F spin pair separated by a distance of 4 Å (B) and 5 Å (C). 7945
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between the signals at −145 and −178 ppm, pointing to the strong dipolar coupling occurring between the “linked” CF groups and, consequently, to a close spatial proximity between corresponding species. SIMPSON simulated 2D correlation spectra for a spin pair separated by a F−F distance of 4 and 5 Å are shown in Figures 5b and 5c, respectively. The appearance of the cross-peaks in the former spectrum and their absence in the latter one demonstrates that magnetization is transferred over the distance not longer than 4 Å, which corresponds to a width of two aromatic rings. The absence of the cross-peaks from the isolated CF groups (omitted in Figure 5a) can be explained by a small concentration of the corresponding species and/or their remote distances to other fragments. In contrast, a weak crosspeak between the signals at −145 and −118 ppm is an evidence of a close proximity between the end chain and the “linked” groups. The analysis of the nature of C−F bonds carried out on geometry-optimized structures with different CF configurations demonstrated a very narrow distribution of bond lengths around 1.43 Å regardless of the fluorination pattern. A slight variation has been observed with respect to the position within the fluorinated chain. So, the middle C−F groups are characterized by a bond length of ca. 1.42 Å, while for C−F terminating the chain the bond length achieves a value of 1.45 Å. The CF bond length within the cyclohexane ring has been found to be 1.43 Å. However, the isolated CF groups or those located at significant distances from other fluorinated patterns yielded considerably longer C−F distances (1.57 Å) and, consequently, remarkably distinct 19F shielding values (Figure 3). In general, the bond lengths calculated in this work are slightly larger than a value of 1.36 Å typical for covalent C−F bond in (CF)n and (C2F)n graphite fluorides obtained from neutron diffraction data,11 but closer to the values of 1.44−1.45 Å identified as so-called semi-ionic C−F bonding determined by NMR for thermally post-treated room-temperature synthesized fluorinated carbon materials.32 However, it has been shown that the C−F bond character in CxF is essentially covalent with the C−F bond length of 1.40 Å, and the results can be explained without involving semi-ionic bonding.25 Thus, we suggest that the difference in C−F bond lengths cannot be responsible for strong distinction of the observed chemical shifts, but the latter is most likely caused by variation of πelectron density formed in the vicinity of the fluorine atom and related to the local concentration of the fluorine distribution. Now, combining the quantitative information obtained from the single pulse 19F MAS NMR data and the spectral assignment provided by DFT calculations, one can get insight into the formation of fluorine patterns. Figure 6 represents a quantitative diagram of the different fluorinated patterns for four C2Fx samples, where the relative integrated intensity of each component was weighted with the corresponding fluorine content x to demonstrate the propagation of a given fluorinated segment with the increasing F content. Now we term the fluorinated segments on the base of the scenario graphically presented in Figure 4. Because of the fact that components at −145 and −178 ppm demonstrate similar qualitative behavior (Figure 6) and could be attributed to either of linked CF groups, no distinction between them will be done in following. Thus, the dominating concentration of the linked CF groups at the lowest F content suggests a clustering with a possible chain initiation. Further, when the F concentration increases from x = 0.49 to 0.87, the number of the linked groups increases. At a further increase x the fraction of the linked groups decreases
Figure 6. Quantification diagrams for graphite fluorides C2Fx obtained by integration of the 19F MAS NMR spectra weighted by the fluorine content x. The term “linked” includes all CF groups which are linked to at least two fluorinated carbon atoms (i.e., midchain, cyclic, and branched).
due to most likely transformation into the infinite CF chains. A steady increase of infinite CF arrays assumes propagating along a linear chain upon adding F atoms. A number of isolated CF groups rise up to a fluorine content of 0.87 and then remain constant possibly because of the limited space available for further attachment of fluorine atoms. So, it is reasonable to assume that at highest F concentration the single fluorine atoms will be attached to chain sides or to cyclohexanes contributing thereby to the addition of the end CF groups. This finding can account for a surprising rise of the concentration of the latter found in the present study. It has to be noted that the peripheral CF2 groups are formed at x > 0.49 and their concentration does not vary significantly at further adding fluorine. The proposed here formation of fluorine patterns confirms the scenario reported in ref 51, where the random direction of chain formation including the both zigzag and armchair configurations has been assumed.
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CONCLUSIONS The structure of room-temperature produced acetonitrileintercalated graphite fluoride compounds has been elucidated by means of solid-state NMR and DFT calculations. Based on the observations in the present study, the following conclusions are drawn regarding: (a) the character of the guest−host matrix interactions, (b) the structure of the carbon matrix, and (c) fluorine distribution in the layer. The interaction between the intercalated acetonitrile and the C2Fx matrix has been found to be of van der Waals nature. It has been confirmed that the room-temperature synthesis of graphite fluoride results in the planar configuration of the carbon sheets because only carbon atoms in sp2 hybridization (i.e., graphitic carbon) and carbon atoms bonded to fluorine have been detected in the 13C NMR spectra. Moreover, no signature of the presence of the pristine graphite sheets has been found, demonstrating that all carbon sheets were fluorinated. However the presence of graphite-like domains has been proven in the material with the lowest fluorination degree. A combination of 19F solid-state NMR and DFT calculations of 19F absolute shielding allowed classifying and quantifying the structural motives obtained upon fluorination. Thus, it has been established that all possible fluorination motives (including CF chains of both zigzag and armchair configurations as well as fluorinated cyclohexane rings and isolated CF pairs) may exist in the materials studied in the present work. Our data exclude the presence of uniform structural motives, such as e.g. chains, in low-temperature 7946
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fluorinated graphite. Based on the data of graphite fluorides C2Fx with different fluorination degree x, a process of the formation of fluorine patterns was proposed, involving CF chain formation and propagation toward the increasing chain length up to x = 0.87. Because of the limited size of the intact graphitic plane at the highest F concentration, the fluorine atoms are attached to the chain sides or to cyclohexane rings. The results obtained in the present work provide a basis for predicting the electronic properties, dynamics, and energetics of fluorinated graphene and graphite materials.
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ASSOCIATED CONTENT
S Supporting Information *
Calculated 13C absolute shielding for trans-linked chains of covalent CF groups versus the number of fluorine atoms per graphitic fragment; 19F MAS NMR spectra at different spinning speed values (60, 30, and 20 kHz). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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