Comparative Study of SWCNT Fluorination by Atomic and Molecular

Article pubs.acs.org/cm

Comparative Study of SWCNT Fluorination by Atomic and Molecular Fluorine Wei Zhang,†,‡ Pierre Bonnet,*,†,‡ Marc Dubois,†,‡ Christopher P. Ewels,§ Katia Guérin,†,‡ Elodie Petit,†,‡ Jean-Yves Mevellec,§ Loïc Vidal,⊥ Dimitri A. Ivanov,⊥ and André Hamwi†,‡ †

Clermont Université, Université Blaise Pascal, Institut de Chimie de Clermont Ferrand, F-63171, Aubière Cedex CNRS, UMR 6296, ICCF, F-63177, Aubière, France § Institut des Matériaux Jean Rouxel, Université de Nantes−CNRS, UMR 6502 − France ⊥ Institut des Sciences des Matériaux of Mulhouse, CNRS, LRC 7228, France ‡

S Supporting Information *

ABSTRACT: Single-wall carbon nanotubes (SWCNTs) are fluorinated around 200 °C with molecular fluorine (F2) and xenon difluoride (XeF2) as fluorination agents. In this latter case, fluorination is carried out by atomic fluorine F• generated by the thermal decomposition of gaseous XeF2 on the nanotube surface. XeF2 treatment results in stoichiometries from CF0.05 to CF0.32, and F2 treatment gives compositions in the range CF0.04 and CF0.37. Transmission electronic microscopy (TEM), solid state Nuclear Magnetic Resonance (NMR), Raman scattering and Optical Absorption (AO) studies demonstrate that different fluorination mechanisms occur using molecular fluorine (F2) and atomic fluorine (F•). Atomic fluorine results in less sample damage and a more homogeneous fluorine distribution over the SWCNT surface than F2. This is explained via DFT calculations showing that HF catalyzed F2 deposition necessarily leads to highly fluorinated domain formation whereas F• addition occurs spontaneously at the initial species arrival site. KEYWORDS: carbon nanotubes, fluorination, 19F NMR, Raman diffusion, density functional theory

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tion experiments have been performed with molecular fluorine gas (F2)10,17−19 but several other methods have been also reported including CF4-plasma treatment,20−22 bromine trifluoride BrF323,24 and xenon difluoride XeF2.25 Direct fluorination using pure fluorine gas remains the most useful method to obtain fluorinated carbon materials thanks to the high reactivity of molecular fluorine. Fluorination close to C2F stoichiometry can be obtained.10 However, the reactivity becomes a disadvantage when a F/C fluorine content lower than one is required. It is then necessary to control the reactivity, using either dilution with an inert gas (N2, Ar) or by decreasing the reaction temperature. The use of a fluorinating agent (FA) appears to be an interesting route because the kinetics of fluorine addition to nanocarbons depends on the rate of decomposition of the FA. This method, called controlled fluorination, was successfully applied for the fluorination of fullerenes. Highly fluorinated fullerenes were obtained using different fluorinating agents such as MnF3, TbF4, and CeF4.26 Multiwall carbon nanotubes (MWNTs) have been successfully fluorinated at room temperature under UV irradiation using XeF2 as the fluorinating agent,25 although only low fluorination levels (CF0.04) were achieved.

INTRODUCTION Chemical functionalization (covalent and noncovalent) of carbon nanotubes (CNTs) and especially single-walled carbon nanotubes (SWCNTs) has been a subject of intense study over the past decade.1−4 Chemical modification of CNTs is necessary for many applications. Because individual SWCNTs are only carbonaceous surfaces, a functionalization treatment, even small, can drastically alter their physical and chemical properties. It is now well-established that covalent functionalization induces important modifications in CNT properties, notably their electronic behavior.2,5 Conversely, noncovalent functionalization (polymer wrapping, porphyrins, ...3) appears softer than covalent modification for the CNT properties.6−8 Hence, it is critical to select an appropriate CNT functionalization method, and to control this method as precisely as possible in order to tune the CNT properties for the expected applications. Among covalent chemistry of CNTs, fluorination is one of the most studied and important ways. Not only is fluorination a good starting point for further covalent sidewall modification of CNTs (alkylation, hydroxylation, amino-functionalization, ...),1,4,9 but fluorinated carbon nanotubes (F-CNT) also exhibit a large range of possible applications themselves. Fluorination allows dispersion of CNT in alcoholic solvents,10 and potential applications of F-CNTs include electric storage as cathodes in lithium batteries,11,12 in supercapacitor electrodes,13 sensors, and solid lubricants.14−16 Experimentally, most CNT fluorina© 2012 American Chemical Society

Received: November 16, 2011 Revised: April 10, 2012 Published: April 14, 2012 1744

dx.doi.org/10.1021/cm203415e | Chem. Mater. 2012, 24, 1744−1751

Chemistry of Materials

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probe (Bruker) with fluorine decoupling on a 4 mm rotor was used. The 19F → 13C match was optimized on polytetrafluoroethylene (PTFE). For 19F MAS spectra, a simple sequence was used with single π/2 pulse duration of 5.5 μs. The 13C spectra were recorded using a solid echo sequence (two 5.5 μs π/2 pulses separated by 25 μs). UV−vis−NIR optical absorption (OA) spectra of the samples in suspension were recorded with a Cary 5G spectrophotometer. The samples were also characterized in solid form by Raman spectroscopy at 514.5 nm (2.41 eV) and 1064 nm (1.16 eV) with Jobin Yvon T64000 and RFS 100 Bruker FT Raman spectrometers respectively (see the Supporting Information for experimental details).

In this paper, we describe a comparative study of the fluorination of SWCNTs by F2 and XeF2. XeF2 decomposes on the surface of carbonaceous materials into inert Xe gas and atomic fluorine F•, which immediately reacts with carbon; the recombination of two F• to form a F2 molecule is avoided because of this spontaneous reaction. The present work focuses then on the fluorination mechanisms involving F• and molecular fluorine F2. Experiments have been performed in similar conditions of temperature. We have produced samples with several fluorine contents by the two methods. The fluorinated nanotubes were studied by transmission electronic microscopy (TEM), and by magnetic, optical and vibrational spectroscopies (NMR, Optical Absorption spectroscopy and Raman scattering). We investigate particularly the homogeneity of the fluorination on the SWCNT sidewalls and the damage induced by the chemical functionalization. The experimental results have been interpreted with density functional theory (DFT) calculations.

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RESULTS AND DISCUSSION 1. TEM. We first compare TEM images of F-SWCNTs obtained by the two methods with similar fluorine contents (x = 0.32 by XeF2 thermal decomposition and x = 0.37 by direct fluorination). Because the two samples were produced at similar fluorination temperatures, the morphological differences observed in the images are mainly due to differences of reactivity between atomic and molecular fluorine. Images A and B in Figure 1 show TEM images of fluorinated nanotubes synthesized with the direct method (x = 0.37). The FSWCNT(F2) remain in bundles similar in appearance to the raw material,28 and nonhomogeneous expansion in the nanotube bundles can be seen. These observations confirm the aggressiveness of fluorination by molecular fluorine, in good agreement with results presented in the literature.17,29 This is clearly different in the case of nanotubes fluorinated by the controlled method. In Figure 1C we observe individualized F-SWCNT(XeF2). Figure 1D shows remarkably little damage on their surfaces, notably the sidewalls seem uniform suggesting a homogeneous functionalization. The individualization of the SWCNTs also underlines the homogeneity of the fluorination treatment by the controlled method, since fluorination along the entire surface of the SWCNT is necessary to induce complete debundling. Nonhomogeneous fluorination will only induce local expansion of the bundles, as observed in the case of the direct fluorination. Thus the TEM study highlights clearly that for similar fluorine contents, at similar fluorination temperatures, the morphology of the F-SWCNTs is very different according to the fluorination method. The direct method appears to be very aggressive, whereas the controlled method is less damaging and more homogeneous. 2-. Solid-State NMR. To facilitate the comparison, we show only the results for the two highly fluorinated samples presented in the TEM section (Figure 2a). 19F MAS NMR spectra are recorded with a spinning speed of 20 kHz. The spectrum for F-SWCNT(F2) prepared by direct fluorination exhibits an intense line at −163 ppm/CFCl3 assigned to covalent C−F bonds.30−34 The F-SWCNT(XeF2) spectrum, in addition to these covalent bonds, also features a line at −152 ppm indicating weaker carbon−fluorine bonding. In general the higher the covalence, the lower the 19F chemical shift. For comparison, covalent (C2F)n and (CF)n type graphite fluorides exhibit a value of −190 ppm independent of fluorine content, whereas for room temperature samples (synthesized using a gaseous catalytic mixture of F2, HF, and MFn, where MFn = IF5, BF3, or ClFx), δ19F lies between −150 and −170 ppm.35,36 In this latter case, hyper-conjugation between C−C bonds in the nonfluorinated region and closed C−F bonds results in the weakening of the C−F covalence, 35,37 symptomatic of a lower fluorine surface density.

EXPERIMENTAL METHOD

Materials and Sample Synthesis. SWCNTs synthesized by the HiPCO process were purchased from Unidym. Fluoro-nanotubes were prepared by two fluorination processes: direct fluorination under a F2 atmosphere and fluorination by the controlled thermal decomposition of XeF2. Fluorination using gaseous XeF2 was performed using the equilibrium

XeF2(s) ⇔ XeF2(g)

(1)

Solid XeF2 and SWCNTs are located in two separated parts of a closed reactor of 100 mL volume (for details, see the Supporting Information). As under UV irradiation,25 xenon difluoride is easily decomposed during heating according to the reaction:

XeF2(g) → Xe(g) + 2F•

(2)

The reactor was then placed at 180 °C for 12 h for each experiment. As fluorine is consumed during the process, four successive additions of XeF2 were then performed. Starting from a batch of 100 mg SWCNTs, successive additions of 50 mg of XeF2 resulted in samples with F/C fluorine content of 0.05 ; 0.10 ; 0.20 ; 0.28 and 0.32. For comparison, direct fluorination under a pure fluorine gas flow (1 atm.) was also carried out following the procedure described by Zhang et al.27 Two samples were synthesized at 100 and 200 °C with F/C values of 0.04 and 0.37, respectively. The chemical compositions were determined by weight uptake. The samples obtained using thermal decomposition of XeF2 (controlled method) are denoted F-SWCNT(XeF2) and those obtained using pure fluorine gas (direct method) denoted F-SWCNT(F2). In addition, we note F/C = x in the CFx composition. All the sample compositions are summarized in Table 1.

Table 1. Compositions Deduced by Weight Uptake of FSWCNT Synthesized by Controlled and Direct Methods sample and fluorination method F-SWCNT (XeF2) controlled method (XeF2) F-SWCNT (F2) direct method (F2)

fluorine contents (x=F/C) 0.05 0.04

0.1

0.2

0.28

0.32 0.37

Characterization. To determine the homogeneity of fluorination and damage caused by the functionalization, we characterized selected samples by transmission electron microscopy (TEM, FEI CM200 operated at 200 kV). NMR experiments with magic angle spinning (MAS) were performed at room temperature using a Tecmag spectrometer (working frequencies for 13C and 19F were 73.4 and 282.2 MHz respectively). A special cross-polarization/MAS NMR 1745



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Figure 1. TEM pictures of F-SWCNT elaborated by direct method (A and B) and controlled method (C and D). The arrow indicates an example of local inflation in F-SWCNT bundles (picture A) in the sample x = 0.37 obtained by the direct method.

Figure 2. NMR spectra of the SWCNT fluorinated with F2 and XeF2: (a) 19F MAS spectra at spinning speed of 20 kHz, (b) 19F → 13C CP-MAS at 20 kHz. (*) marks spinning sidebands.

The 19F chemical shift of −163 ppm for F-SWCNT(F2) is significantly lower than that of covalent graphite fluorides, since the nanotubes are only fluorinated on their external surface, which coupled with local curvature effects prevents formation of fully tetrahedral sp 3 -C. Such an effect has been experimentally found for highly fluorinated flullerenes and derivates, regarding their 19F chemical shifts.26 The residual sp2 character results in a weakening of the overlapping of the hybridized lobes of carbon and the fluorine atomic orbitals. The

−163 ppm value for both samples is explained by this main effect and represents high surface density fluorination. The F-SWCNT(XeF2) sample shows a second line at 152 ppm. If fluorine atoms are homogenously dispersed over the tube surface, C−F bonds must be separated by nonfluorinated carbon atoms for compositions lower than CF0.5. Hyperconjugation may occur, and coupled with mechanical lattice resistance to formation of fully sp3-C results in a further weakening of the covalence. This explains the 19F chemical shift 1746



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Figure 3. Optical and vibrational characterisations of raw SWCNTs. (A): Kataura plot of SWCNTs with data extracted from the literature.40,41 Lines indicate wavelengths 514.5 and 1064 nm respectively. Hatched red area symbolizes the range of SWCNTs diameters with the HiPCO synthesis method.42 (B) Optical absorption (OA) spectrum of SWCNTs in suspension. Arrows indicate the Eii transition domains. (C) Raman spectra in RBM region of raw SWCNTs at 514.5 and 1064 nm, respectively. For each wavelength, the RBM peaks are relative to individual SWCNTs in resonance conditions in the Kataura plot (cf. lines at 514.5 and 1064 nm in a).

increase from −163 to −152 ppm. In the case of controlled fluorination (XeF2), the two types of CF bonds coexist. XeF2 breaks down to give two F• and in some cases they stick to the surface next to each other, in other cases they are more spread out. We note that this sample also contains a few CF2 groups as revealed by the narrow line at −120 ppm. The CP-MAS (Figure 2b) spectra of both F-SWCNT(F2) and F-SWCNT(XeF2) indicate the simultaneous presence of covalent C−F bonds and graphitic parts, where carbon atoms are sp3 and sp2 hybridized respectively. The two typical lines are located at 86 and 130 ppm/TMS for C−F and sp2 C, respectively. Using 19F → 13C cross-polarization sequence, the presence of the nonfluorinated sp2 carbon line implies that these atoms are in interaction, even weakly, with fluorine involved in the C−F bonds. However these experiments cannot be interpreted quantitatively. Using a solid echo sequence the F/C molar ratio can be extracted (see Figure S1 in the Supporting Information). F/C ratios of x = 0.29 and x = 0.40 were obtained for F-SWCNT(XeF2) and F-SWCNT(F2), respectively. These values are not in perfect accordance with weight uptake presented in Table 1. A value of x = 0.29, determined by NMR, is lower than that obtained by weight uptake (x = 0.32) for F-SWCNT(XeF2). Indeed, the weight uptake considers only the formation of C−F bonds, mainly present in fluorinated carbon. The origin of this overestimation of F/C is the presence of CF2 groups, highlighted by the 19F NMR considering the peak at −120 ppm (Figure 2a). Taking into account the NMR data, the corrected composition of FSWCNT(XeF2) should be CF0.29(CF2)0.015, or CF0.32. In contrast with the direct fluorination, the F/C ratio of 0.40 is higher than 0.37 obtained by weight uptake. By analogy with others fluorinated carbons, when the fluorination level is underestimated by weight uptake, this means that partial decomposition occurred during fluorination and volatile CF4 and C2F6 groups have been formed. This phenomenon is in good concordance with the damage observed in the TEM pictures. The control of fluorination using XeF2 seems to avoid this decomposition but involves the formation of small amounts of CF2, presumably at edges and defect sites.

3-. UV−Vis−NIR and Raman Spectroscopies: Modification of the Electronic Properties. Because individual SWCNTs are only surfaces, a covalent functionalization, even small, will induce important changes in their physical and chemical properties.2,5 Electronic properties are especially sensitive to these modifications.5 The electronic density of states of SWCNTs exhibits sharp peaks called van Hove singularities (VHs).38 Optical transitions occur between symmetrical VHs of the valence and the conduction bands around the Fermi Level. Moreover, SWCNTs are either semiconducting, or metallic. In the following, we note Sii and Mii the transitions between the ith VHs for semiconducting and metallic SWCNTs, respectively, and EXii (X = S or M) the associated energies. The successive electronic transition energies as a function of the SWCNT diameters are given by the Kataura Plot39 as presented in Figure 3a. As seen in Figure 3b, the nanotube optical absorption (OA) spectrum consists of peaks which correspond to the successive electronic transitions for all the SWCNTs in the sample. Because the energies of the band gap transitions vary inversely with the SWCNT diameters, the peaks observed at high wavelengths correspond to large tubes. The black color of SWCNTs is due to the strong light absorption in all the visible range. Raman spectroscopy is a powerful technique to study the vibrational and electronic properties of carbon nanotubes (see the Supporting Information). Raman radial breathing modes (RBM) spectra of raw SWCNTs are given in Figure 3c for two excitation wavelengths (1064.0 nm i.e. 1.16 eV and 514.5 nm i.e. 2.41 eV). The spectrum with 1064 nm excitation exhibits a strong band at 268 cm−1 and two smaller peaks at 309 and 329 cm−1. In the case of measurements at 514.5 nm, two main groups of peaks are observed: an intense one between 220 and 300 cm−1 and a smaller between 160 and 220 cm−1, with an isolated band appearing at 309 cm−1. Because Raman scattering is a resonant process for SWCNTs, the RBM spectrum depends strongly on the laser excitation energy. The RBM intensity is greatly enhanced if the laser excitation energy corresponds to the energy of an electronic transition between VHs. As a consequence, only the RBM peaks of SWCNTs in resonant 1747



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modifications in the electronic structure of the SWCNT induced by functionalization,46 in this case fluorination. Similarly to fluorescence quenching, a drop in the total density of states and band gap opening cause a significant decrease of the global RBM intensities. The electronic structures of FSWCNTs have been calculated and the studies indicate an increase of the band gap in good agreement with our results (see the Supporting Information).47 That is why in the following, we will discuss the modifications of the Raman resonance conditions with a simple model of a rigid electronic band shift. The Raman spectra in the RBM range of the FSWCNTs obtained by the two methods are compared in Figure 5. The spectra obtained with a laser line at 1064 nm (Figure 5a−c) show the disappearance of all the RBM bands whatever the fluorination rate for F-SWCNT(XeF2) synthesized with the controlled method. In contrast the F-SWCNT(F2) produced by the direct method still show a peak at 265 cm−1 for a low fluorine content (x = 0.04). In Figure 5d−f RBM peaks observed with a laser excitation line at 514.5 nm are shown. In the case of the samples obtained with the controlled method, RBM are always seen and as well as global intensity changes, we can also observe individual peaks changing progressively with fluorine content. At low concentrations x = 0.05, a major peak at 200 cm−1 is the most intense, but at x = 0.1 and x = 0.2, a peak at 225 cm−1 has now comparable intensity to the peak at 200 cm−1. Between x = 0.2 and x = 0.28, the relative intensities of peaks at 248 and 253 cm−1 also increase. These peak intensity changes demonstrate a progressive shift toward more intense higher frequency peaks for higher fluorine concentrations. Conversely, for F-SWCNT(F2) elaborated by the direct method, peaks around 250 cm−1 are always the most intense. These changes can be explained by Raman resonance arguments. Fluorination induces a band gap opening corresponding to a progressive increase of the transition energies. This progressive change is very clear in the RBM features at 514.5 nm for FSWCNT(XeF2) produced by the controlled method. For raw SWCNTs, using the Kataura Plot (Figure 3a) the RBM bands can be attributed to nanotubes in resonance conditions with different electronic transitions: peaks in the ranges 220−309 cm−1 and 160−220 cm−1 are attributed to M11 transitions (small nanotubes) and S33 transitions (large nanotubes) respectively. When the fluorine content increases, these transitions come out of resonance conditions and in turn nanotubes with diameters about 1.0−1.1 nm with M 11 transitions, and later SWCNTs with smaller diameters for S22 transitions, move into resonance. This leads to main contributions to the RBM peaks at around 200 cm−1 at low fluorine contents, and 250 cm−1 at high concentrations. In the case of fluorination by the direct method, the Raman spectrum at low fluorine contents is very similar to that of raw nanotubes with intense modes at around 250 cm−1. This is attributed to a nonhomogeneous fluorination of the nanotube surfaces by this method. Indeed, if SWCNTs are not functionalized, or a large part of their surface is not functionalized, their Raman signal will be similar to the signal of the pristine SWCNTs. Nanotubes located in the outer part of the bundles may be more fluorinated than in the inner part, who may be less affected. At higher concentrations, there are no differences between the two methods because of a large coverage of the nanotube’s surface by the fluorine atoms. The measurements at 1064 nm can be explained with similar arguments. Here, the S11 transitions are

conditions are seen. The Raman shift (ωRBM) of these modes depends on the SWCNT diameter d as ωRBM = A/d + B, where A and B are constants. Many different values of A and B have been proposed,43,44 in the following we use those given by Bachilo et al.:41 A = 223.5 cm−1.nm and B = 12.5 cm−1. HiPCO nanotubes exhibit diameters typically in the 0.7−1.3 nm range.42 According to the Kataura plot (Figure 3a), measurements made at 1064 nm probe only semiconducting nanotubes with small diameters (∼0.9 nm), while measurements at 514.5 nm probe both semiconducting and metallic nanotubes for a large range of diameters (∼0.7−1.3 nm). Optical Characterization. UV−vis−NIR absorption measurements have been performed with suspensions of pristine and fluorinated SWCNTs between 1700 and 500 nm. The UV− vis−NIR spectra of raw and fluorinated SWCNTs are presented in Figure 4. Unlike pristine SWCNTs, the spectra of the F-

Figure 4. Optical absorption F-SWCNT(XeF2) (x = 0.05 and 0.32), FSWCNT(F2) (x = 0.37) and raw SWCNT (x = 0) in aqueous suspensions. Red hatched area is relative to the D2O absorption domain. The increase in absorption background for F-SWCNT at high fluorine content is due to the high level of sidewall functionalization.45.

SWCNTs (x = 0.05, 0.32 and 0.37) are structureless, without VH transition peaks. The features of the spectra are not very dependent on the fluorine content except the absorption backgrounds related to the sidewall functionalization rates.45 Drastic modifications are observed as soon as x = 0.05. We note that all the peaks are affected by the fluorination, showing there is no selectivity of the functionalization to nanotube diameter, nor the semiconducting or metallic nature of the SWCNT. No differences are observed between the spectrum of F-SWCNT(XeF2) (x = 0.32) obtained by XeF2 decomposition and the spectrum of F-SWNT(F2) (x = 0.37) obtained with F2. Resonant Raman Spectroscopy. Fluorination of SWCNTs strongly affects the intensity but not the absolute shift of the resonant Raman RBM peaks. The low signal-tonoise ratio and peak broadening is similar to that observed in RBM spectra for fluorinated tubes in the literature.18,19 This is because low laser power densities (0.05 mW/μm2) were used in order to avoid laser induced thermal effects, notably defluorination.18 Additionally, in Figure 5, a strong decrease of the overall intensity of these bands compared to the pristine SWCNTs is observed (Figure 3a). Different arguments are usually proposed to explain the fall of intensity of RBM peaks: a change of symmetry (the cylindrical shape is affected by the C−F covalent bonds), a decrease in the oscillatory motion in the radial direction or the increased mass of the vibrating unit.19,20 Instead, we propose that the changes in RBM band intensities are caused by 1748



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Figure 5. Raman spectra in RBM region of F-SWCNT at (A−C) 1064 nm and at (D−F) 514.5 nm. (A) Spectra of F-SWCNT(XeF2) at 1064 nm for x = 0.05, 0.1, 0.28, and 0.32. (B) Comparison at 1064 nm between raw SWCNTs, F-SWCNT(XeF2) x = 0.05 and F-SWCNT(F2) x = 0.04. The arrow indicates the position of the main RBM peak in raw nanotubes. (C) Comparison at 1064 nm between raw SWCNTs, F-SWCNT(XeF2) x = 0.32 and F-SWCNT(F2) x = 0.37. The arrow indicates the position of the main RBM peak in raw nanotubes. (D): spectra of F-SWCNT (XeF2) at 514.5 nm for x = 0.05, 0.1, 0.28, and 0.32. The arrow indicates the shift trend for the main RBM massif as a function of the fluorine content. (E) Comparison at 514.5 nm between raw SWCNTs, F-SWCNT(XeF2) x = 0.05 and F-SWCNT(F2) x = 0.04. Arrow indicates the position of the main RBM massif in raw nanotubes. (F) Comparison at 514.5 nm between raw SWCNTs, F-SWCNT(XeF2) x = 0.32 and F-SWCNT(F2) x = 0.37. Arrow indicates the position of the main RBM massif in raw nanotubes.

energy of 54.88 kcal/mol. As a consequence, fluorine atom addition will be spontaneous, and fluorine distribution on the nanotube surface will therefore be simply controlled by the electrostatic repulsion between an incoming fluorine atom and any fluorine already present on the nanotube surface. Given that addition will thus be a stochastic process, this agrees well with our observation that F• addition to SWCNTs is homogenously distributed. Van Lier et al.50 have shown with DFT calculations that fluorination of SWCNT surface by F2 leads to the formation of bands of fluorine atoms. In other words, the fluorination mechanism with F2 tends to aggregate fluorine atoms on the surface of SWCNTs. The existence of areas with high fluorine density separated by parts of pristine SWCNTs has been corroborated with STM observations.51 These results are in good agreement with our microscopic and spectroscopic observations. Notably our current calculations for F• addition and banded C2F fluorination give C−F bonds of 1.457 Å and 1.373−1.401 Å, respectively, consistent with the observed weak and strong C−F bonded signals seen in NMR. HF is commonly present in the reactors due to the reaction of F2 with trace H2O impurities. Recently Osuna et al.52 proposed a HF-catalyzed fluorination mechanism of SWCNTs by molecular fluorine. By this method a (1,4) addition pattern

probed. The absence of RBM peaks is due to the large band gap opening and there is then no SWCNTs in resonance conditions. The RBM peaks still observed for x = 0.04, with the direct method, are due to the remaining or only partially functionalized SWCNTs in the sample. 4-. Density Functional Calculations and Discussion. To understand the different addition processes better, we performed a series of density functional calculations under the local density approximation using the AIMPRO code.48 Calculations used a single k-point and HGH-pseudopotentials,49 using a localized Gaussian orbital basis set with 22 independent functions per carbon atom, 28 per fluorine, and 16 per hydrogen atom. All atoms were geometrically optimized at each stage with no symmetry constraints. We used a periodic supercell to simulate an infinite (8,8) SWCNT, using 320 carbon atom segments (25.35 Å length). Reaction barriers were calculated using the climbing nudged elastic band method. The TEM images along with the NMR and Raman characterizations clearly show that fluorinations of SWCNTs by molecular (F2) and atomic (F•) fluorine, for similar fluorine contents, lead to materials with significant differences of homogeneity on their functionalized surfaces. In the case of XeF2 thermal decomposition atomic fluorine F• is generated. When we simulated F• addition to the nanotube, no addition barrier was found, with a F• binding 1749



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formation of banded areas, even though nonfunctionalized tube segments are still present. This effect can also change the Raman diffusion intensity. This is similar to the effects observed here with fluorination. In the current case the effect is more pronounced since our functionalization level is at least three times higher.

is obtained which rearranges into a dense (1,2) spacing by fluorine diffusion, although very similar binding energies and barriers were found for these two configurations. Given that this study considered isolated F2+HF on the carbon surface, and the earlier DFT study showed energetic preference for banding, it seems likely that HF catalyzed F2 deposition in the presence of pre-existing fluorine banded regions will favor further (1,2) addition and extension of these fluorinated regions. The question then is why there might be preferential HF catalyzed F2 bonding in the proximity of pre-existing fluorinated regions, as compared to nonfluorinated regions. To address this question we fluorinated half of the tube segment in our calculation, giving an infinite tube with alternate bands of pristine nanotube (C160) and C2F fluorine banded regions (C160F80) (see Figure 6a). We then placed a HF

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SUMMARY AND CONCLUSIONS

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ASSOCIATED CONTENT

In the present study, we have presented a comparative study of the fluorination of SWCNTs by two chemical routes. Nanotubes have been functionalizated by the thermal decomposition of xenon difluoride (XeF2) and pure fluorine gas (F2). The two methods correspond to fluorination by atomic fluorine (F • ) and by molecular fluorine (F 2 ) respectively. The choice of the fluorination method allows the production of fluorinated carbon nanotubes with similar fluorine content but different morphologies, as demonstrated via TEM imaging. Indeed, the process using atomic fluorine, which is evolved during the thermal decomposition of the fluorinating agent, appears as a controlled method compared with direct fluorination. The fluorine atoms are homogeneously dispersed on the walls of the nanotubes, whatever their position in the bundle, as shown by our NMR and Raman results. On the contrary, the fluorinated parts formed by direct fluorination under F2 flow seem more concentrated. In this case our DFT calculations suggest that the reaction with F2 probably starts on the defects of SWCNTs and the fluorination process then proceeds only via HF catalysis neighboring these regions, resulting in fluorinated domains less homogeneous both in the bundles and along the tubes. This is consistent with STM observations of fluorine banding under similar treatment conditions.51 These results demonstrate that controlled fluorination via XeF2 appears an interesting and promising method for the functionalization of SWCNTs. TEM images indicate fluoronanotubes produced by this method appear isolated with relatively little surface damage, while NMR and Raman spectra indicate good homogeneity of the fluorination. These observations are in good agreement with quantum chemical simulations which show two different mechanisms between fluorination by atomic fluorine and by molecular fluorine leading to a more homogeneous sidewall functionalization by the controlled method. Since fluorination is often the first step of chemical covalent functionalizations of nanotubes and opens a large range of possible applications, detailed control of the SWCNTs sidewall fluorination is a key point. In this way, the use of a fluorinated agent such as XeF2, appears as a promising alternative to conventional fluorination method with pure F2 gas. This study also suggests that progress in the field of fluorinating agents is important for tailoring the properties of nanocarbon materials and their applications.

Figure 6. Relaxed structures from the DFT calculations of (a) infinitely repeating (8,8) nanotube length consisting of alternating bands of pristine nanotube segment (C160) and C2F segment (C160F80), repeated here for improved visibility; (b, c) close-up of the tube surface with HF (b) weakly physisorbed in the center of the pristine tube segment and (c) strongly chemisorbed next to the fluorinated band section. All lengths are in Angstroms, carbon atoms in gray, fluorine atoms in yellow, hydrogen in white.

molecule on the tube surface. When in the center of the pristine zone the molecule sits preferentially vertically above a carbon atom with the H atom nearer to the tube at a distance of 1.98 Å from the tube surface (Figure 6b). However in this arrangement the HF binding to the tube is extremely weak at only 3.46 kcal/mol, i.e. the molecule is only very weakly physisorbed, and at room temperature will have an extremely short lifetime on the tube surface in the pristine regions. However when we place the HF molecule in the pristine region at the very edge of the fluorinated section, its binding energy increases to 10.84 kcal/mol, i.e. it becomes chemisorbed. In this case it rotates so the H instead faces one of the fluorine atoms on the surface, at a distance of 1.69 Å (Figure 6c). Thus this result shows that HF will only be present on the nanotube surface in the presence of pre-existing fluorinated regions, and hence will only be able to catalyze further tube fluorination through F2 addition in these regions. This explains the heterogeneous fluorine distribution and growth of fluorinated bands. A similar confined propagation of covalently bonded polymeric [(CH2)5COOH]n on SWCNTs was recently reported.53 The authors reported a reduction of feature intensities in their optical absorption spectra due to the

S Supporting Information *

Experimental methods, 13C echo-solid MAS NMR, optical characterization and Raman spectroscopy of D and G bands of F-SWCNTs. This material is available free of charge via the Internet at http://pubs.acs.org. 1750



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.E. and J.Y.M. thank the ANR Project “SPRINT”, ANR-2010BLAN-0819-04 for financial support.

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