Article pubs.acs.org/JPCC
Preparing Highly Fluorinated Multiwall Carbon Nanotube by Direct Heating-Fluorination during the Elimination of Oxygen-Related Groups Xu Wang,† Yi Chen,† Yunyang Dai,† Qin Wang,† Jie Gao,† Jieyang Huang,† Jin Yang,‡ and Xiangyang Liu*,† †
State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University, Chengdu, Sichuan 610065, P.R. China ‡ State Key Lab of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, P.R. China S Supporting Information *
ABSTRACT: Pristine and oxidized multiwalled carbon nanotubes (MWCNTs) were separately prepared and directly fluorinated with F2 through two different routes: heating-fluorination and isothermal-fluorination. The amount of fluorine atoms (hereinafter referred to as “F-content”) bonding to the fluorinated samples was largely dependent on the modifing route and chemical bonding of MWCNTs. The F-content of heating-fluorinated pristine and oxidized MWCNTs was 3.2% and 9.2% respectively, which were about 8 times and 18 times that of the corresponding isothermal-fluorinated MWCNTs. According to structural analysis of samples before and after fluorination, it was found that thermal elimination of oxygen-related groups bonding to MWCNTs contributed to the formation of strongly covalent C−F bonds during heating-fluorination. It was considered that the oxygen-related groups provided reactive sites for the fluorination. The fluorination reaction took place at an sp3 carbon linking with the oxygen-related groups and did not increase the density of defect on MWCNTs. A radical-mediated mechanism is accepted for this reaction. Thus, MWCNTs could be first oxidized to increase the number of oxygen-related groups and then heating-fluorinated by F2 directly to get highly fluorinated MWCNTs with stable C−F bonds.
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DWCNTs.12−14 Thus, high temperatures (around 600 °C) are most often chosen to increase the amount and stability of the C−F bonds. Nevertheless, it leads to a serious cracking of the outmost graphene sheet of the MWCNTs due to the formation of C−F bonds, which are formed by breaking the carbon bonds and carbon lattice. It is observed that the curved aromatic layers of MWCNTs are distorted by fluorination at 500 °C, and they exhibit a chair configuration (sp3-hybridized carbon).6,7 This result seriously reduces the electrical properties of CNTs. Up to 600 °C, the whole tube structure is destroyed, which will seriously restrict their application in composite materials. Therefore, it is urgent that methods be found to fluorinate the carbon material with a high content of C−F
INTRODUCTION Among various techniques, fluorination has been considered a powerful tool for the chemical functionalization of single-walled carbon nanotubes (SWCNTs),1−4 double-walled carbon nanotubes (DWCNTs),5 and multiwalled carbon nanotubes (MWCNTs).6,7 It promotes their application in many areas such as advanced composites, hydrogen storage, lithium cell, secondary battery, and super capacitor, etc.8,9 Fluorination of carbon materials using molecular fluorine F2 is always affected by the graphitization degree, specific surface area, and curvature.1,8−11 Large specific surface area and curvature favor the fluorination at a low temperature. Besides, the higher the graphitization degree, the higher the reaction temperature is required. MWCNTs exhibit lower reactivity in fluorination due to their higher graphitization degree, lower specific surface area, and smaller curvature, compared with SWCNTs and © XXXX American Chemical Society
Received: December 28, 2012 Revised: May 20, 2013
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was raised to 250 °C. Residual F2 and byproducts in the chamber were removed after 45 min. The corresponding products of p-MWCNTs and o-MWCNTs were denoted as iFp-MWCNTs and iF-o-MWCNTs, respectively. 2.5. Heating Treatment of p-MWCNTs or o-MWCNTs under N2 Atmosphere. The p-MWCNT and o-MWCNT samples were heating treated under N2 atmosphere with temperature increasing from RT to 250 °C at a rate of 5 °C/ min. The corresponding products of p-MWCNTs and oMWCNTs were denoted as d-MWCNTs and d-o-MWCNTs, respectively. 2.6. Characterizations. The surface chemical composition of MWCNT samples was examined by XPS with monochromatized Al Ka rays (1486.6 eV) under the circumstance of 12 kV × 15 mA (Kratos, Inc.) at RT and at 2 × 10−7 Pa. Binding energies were referenced to the hydrocarbon peak at 284.8 eV. The takeoff angle was 20° with sampling depths of approximately 6−10 nm. FTIR was recorded by a Nicolet 560 Fourier transform spectrometer. TGA was performed on Netzsch 209 TG instruments. HR-TEM was performed on Tecnai G2 F20 S-TWIN. Raman spectroscopy measurements were performed on LabRamHR instruments. Raman spectroscopy was carried out using a backscattering geometry. The wavelength of the 441.6 nm line of the Kimmon Electric’s He− Cd laser was used to excite the sample.
bonds at low temperatures without distorting the curved aromatic layers of MWCNTs. In preparation of MWCNTs, some defects stemmed from the initial formation of the tubes.15 When the catalyst particles were removed by oxidation, the defects, forming both at the ends and on the side walls of the tube, are decorated with some oxygen-related groups. At around 200 °C, most of the residual oxygen-related groups on the CNTs are driven off. Thus, the bonds linking these groups to CNTs are very weak at around this temperature, which can be regarded as reactive sites for the fluorination using F2. In this work, pristine MWCNTs and oxidized MWCNTs (with more oxygen-related groups) were treated by direct F2 fluorination through two different routes. The F content of all the fluorinated samples was largely dependent on the modifying route and chemical bonding of the MWCNTs used as starting material. Chemical structures of the fluorinated and nonfluorinated samples were characterized by X-ray photoelectron spectroscopy (XPS), Raman, thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and high-resolution transmission electron microscopy (HR-TEM), etc. It was proved that the thermal elimination of oxygen-related groups bonding to the MWCNTs contributed to the formation of stronger covalent C−F bonds in the process of fluorination. The oxidation before fluorination created reactive sites for replacement by fluorine atoms, resulting in more F content bonding to the fluorinated MWCNTs. From the above, a novel idea is proposed to increase the number of fluorine atoms bonding to the final fluorinated MWCNTs without increasing density of defect.
3. RESULTS AND DISCUSSION 3.1. Characteristics of p-MWCNTs and o-MWCNTs. The chemical composition of MWCNT samples was studied by XPS before and after acid treatment with HNO3. As shown in Table 1, the O-content of p-MWCNTs, o-MWCNTs, d-
2. EXPERIMENT 2.1. Raw Materials. MWCNTs were purchased from Shenzhen Nanotech Port Co. Ltd., with 10−20 nm in outer diameter, 1−2 μm in length, and >95% purity. The pristine MWCNTs were denoted as p-MWCNTs. The F2/N2 mixed gas for direct fluorination consisted of 10 vol % F2 and 90 vol % N2. 2.2. Preparation of o-MWCNTs. MWCNTs (0.5 g) were put in a 500 mL three-neck flask that contained 380 mL of HNO3 aqueous solution (5 mol/L) and dispersed for 1 h in an ultrasonic bath. The solution was then stirred under reflux condition for 20 h at 60 °C. After strong centrifuging, the remaining solids were washed repeatedly with deionized water until reaching neutral pH and later dried at 120 °C for 10 h under a vacuum oven. The samples were denoted as oMWCNTs. 2.3. Direct Heating-Fluorination of p-MWCNTs and oMWCNTs. An amount of 0.5 g of MWCNTs (p-MWCNTs or o-MWCNTs) was put in a closed stainless steel (SUS316) chamber (20 L) equipped with a vacuum line. After exchanging nitrogen three times, F2/N2 mixed gas was introduced into the chamber at RT. The introduced amount of F2 was 3.4 g. Fluorination processed with temperature increasing from RT to 250 °C at rate of 5 °C/min. When the temperature reached 250 °C, residual F2 and byproducts in the chamber were removed at once by vacuum and absorbed by alkali aqueous solution. Then fluorinated MWCNTs were taken out and preserved in dry atmosphere. The corresponding products of p-MWCNTs and o-MWCNTs were denoted as hF-p-MWCNTs and hF-oMWCNTs, respectively. 2.4. Direct Isothermal-Fluorination of p-MWCNTs and o-MWCNTs. The isothermal fluorinated MWCNTs were prepared by using the same amount of F2, but the F2/N2 mixed gas was introduced into the chamber after temperature
Table 1. Chemical Composition of Fluorinated and Nonfluorinated MWCNTsa chemical composition measured by XPS sample notation
F (at %) (±0.2)
O (at %) (±0.2)
F/C
chemical formula
p-MWCNTs hF-p-MWCNTs o-MWCNTs hF-o-MWCNTs d-MWCNTs iF-p-MWCNTs d-o-MWCNTs iF-o-MWCNTs
0 3.2 0 9.2 0 0.4 0 0.5
4.7 2.5 12.5 3.4 2.2 2.4 2.9 3.2
0 0.033 0 0.105 0 0.004 0 0.005
C1O0.049 C1F0.033O0.025 C1O0.143 C1F0.105O0.038 C1O0.023 C1F0.004O0.025 C1O0.030 C1F0.005O0.033
a
Note: p, presents pristine; d, dissociated; o, oxidized; iF, isothermalfluorinated; hF, heating-fluorinated.
MWCNTs, and d-o-MWCNTs was 4.7%, 12.5%, 2.2%, and 3.1%, respectively. The O-content was reduced by 2.5% and 9.4%, respectively, for p-MWCNTs and o-MWCNTs after heating treatment. Figure 1 showed XPS spectra of electrons from the 1s orbital of oxygen atoms in p-MWCNTs and o-MWCNTs, which were curve-fitted with 35% Gaussian/65% Lorentzian functions of constrained identical width. The peaks at about 532 and 534 eV were assigned to a carbon oxygen single bond (C−O) and a carbon oxygen double bond (CO), respectively.16−19 The two components were both increased remarkably by oxidation, while the CO bonds were produced more favorably. After heating treatment, their corresponding peaks decreased obviously, showing the reduction of O-content, and the B
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Figure 1. XPS spectra of electrons from the 1s orbital of oxygen atoms in p-MWCNTs (a), o-MWCNTs (b), d-MWCNTs (c), and d-o-MWCNTs (d).
The XPS spectra of electrons from the 1s orbital of the fluorine atom of fluorinated samples were shown in Figure 3. Notably, the shape and intensity of the peak of the heatingfluorinated samples were very different from those of the isothermal-fluorinated samples. The peaks at around 688 eV of F 1s were obvious in spectra of hF-p-MWCNTs and hF-oMWCNTs, while there were almost no such peaks in the spectra of iF-p-MWCNTs and iF-o-MWCNTs. The F-content of iF-p-MWCNTs and iF-o-MWCNTs (isotherm-fluorinated samples) is only 0.4% and 0.5%, respectively, which reveals the low reactivity of MWCNTs with fluorine gas. This is consistent with the idea that the carbon materials with high graphitization degree are completely inert toward fluorine below 300 °C. Surprisingly, the F-content of hF-p-WNCNTs and hF-oMWCNTs (heating-fluorinated samples) reached 3.2% and 9.2%, respectively, which were about 8 times and 18 times that of the corresponding isothermal-fluorinated MWCNTs. It was remarkable that the F-content of hF-o-MWCNTs was much higher than that of hF-p-WNCNTs. F-content in the fluorinated MWCNTs was largely dependent on the modifying route and the chemical composition of starting materials. The surface compositions and assignments of F 1s components for fluorinated MWCNT samples are listed in Table 2. In the case of hF-p-MWCNTs, the first feature (F1) centered at 687.3 eV is assigned to a covalent C−F bond. The weaker peak (F2) at 688.9 eV has been attributed to CF2 appearing on edges or at local defect sites.18−21 The component concentration of CF and CF2 is 74.3% and 25.7%, respectively. No peak corresponding to CF3 is observed. It is worth noting that peak intensities of CF and CF2 for hFo-MWCNTs are both larger than that of hF-p-MWCNTs, as shown in Figure 3. For hF-p-MWCNTs, the two components were both increased significantly, while the concentration of CF2 shows a faster growth compared with CF, achieving at 42.8%, as shown in Table 2. A few of the CF3 groups, about 3.7%, were produced by fluorination of o-MWCNTs, which corresponds to the F3 peak in Figure 3. It is interesting that the relative intensities of the CO and C−O signals for the starting materials are about the same as the relative intensities of the CF2 and CF signals for the corresponding heatingfluorinated samples, as shown in Figure 1 and Figure 3. For o-
composition of O was similar for p-MWCNTs and oMWCNTs. The XPS spectra of C 1s for pristine and oxidized MWNCT samples have been provided as the supplementary data in the Supporting Information (Figure S1). The functional groups formed on MWCNTs by oxidation were investigated by FTIR as the supplementary data for the XPS analysis. The corresponding spectra of p-MWCNTs and oMWCNTs before and after evacuation under vacuum at 250 °C were presented in Figure 2. For p-MWCNTs (Figure 2a), the
Figure 2. IR spectra of p-MWCNTs and o-MWCNTs before and after evacuation under vacuum at 250 °C: (a) p-MWCNTs, (b) oMWCNTs, (c) d-MWCNTs, and (d) d-o-MWCNTs.
bands at about 1650 cm−1 corresponded to the streching vibration of carboxy groups (CO). The bands of 1373.6 and 3450.5 cm−1 confirmed the existence of hydroxy groups.16−19 For o-MWCNTs the peaks corresponding to the oxygenrelated groups were compared with that of p-MWCNTs (Figure 2b), indicating that the amount of oxygen-related groups (−OH, CO) has been increased after the oxidation process. In spectra of d-MWCNTs and d-o-MWCNTs (Figure 2c, 2d), these peaks weakened and even disappeared, showing the elimination of oxygen-related groups due to heating treatment under N2 atmosphere. 3.2. Direct F2 Gas Fluorination of MWCNTs. The pristine, oxidized, and corresponding fluorinated MWCNT samples have then been studied by XPS, which allows us to establish both the nominal composition and the nature of the functions created for each sample. C
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Figure 3. XPS spectra of electrons from the 1s orbital of fluorine atoms for heating fluorinated samples: hF-p-MWCNTs (a) and hF-o-MWCNTs (b); inset shows superimposed F 1s spectra of the heating fluorinated and isothermal fluorinated samples: hF-p-MWCNTs (hF in part (a)), iF-pMWCNTs (iF in part (a)), hF-o-MWCNTs (hF in part (b)), iF-o-MWCNTs (iF in part (b)).
Table 2. Surface Compositions and Assignments of F 1s Components for Fluorinated MWCNT Samples hF-p-MWCNTs
hF-o-MWCNTs
component
assignment
position (eV)
concentration (%)
position (eV)
concentration (%)
F1 F2 F3
CF1 CF2 CF3
687.3 688.9 ---
74.3 25.7 ---
687.8 689.4 692.7
53.5 42.8 3.7
MWCNTs, the O 1s ratio CO/C−O equals 49:52 before fluorination, and the F 1s ratio CF2/CF of hF-o-MWCNTs equals 43:53 after fluorination. This suggests that, upon fluorination, CO yields CF2 and C−O yields C−F. This resulted from the enhanced covalence of C−F bonds in hF-o-MWCNTs. It has been reported that hyperconjugation between C−C bonds in the nonfluorinated region that were close to C−F bonds resulted in the weakening of the C−F covalence.22 Thus, high F-content in surface would reduced the area of nonfluorinated region, further weakening the hyperconjugation. For hF-o-MWCNTs, the high F-content enhanced the covalence of their C−F bonds. Corresponding C 1s peaks were also investigated as the supplementary data for F1s peaks interpretation in Figure 4, and assignments are listed in Table 3. The C 1s curve fit spectrum of hF-p-MWCNTs exhibits two components corresponding to groups containing F atoms, C4 at 288.8 eV, and C5 at 290.3 eV, which are assigned to the covalent C−F bond and the perfluorinated C−F bond in CF2, respectively.18−22 Both intensities of these two components increased in the case of hF-o-MWCNTs. The concentration of CF2 increases about 94.4%, while that of CF increases 46.6%. The peak corresponding to CF3 is observed at 291.5 eV. The variation in the C 1s peaks corresponded well with the result of F 1s peak variation. In addition, the inset shows there are almost no peaks arising from C−F bonds in the C 1s spectra of iF-p-MWCNTs and iFo-MWCNTs in a range of 287−293 eV, which further confirms the low fluorine content of the isothermal fluorinated samples. As shown in Tables 1 and 3, the F-content in the fluorinated MWCNTs was largely dependent on modifing the route and chemical bonding of the starting materials. The chemical binding in the surface of MWCNTs was characterized by FTIR before and after fluorination, as shown in Figure 5. Bands at 1650 cm−1 decreased obviously, and bands at 1373.6 cm−1 almost disappeared in spectra of fluorinated samples (Figure 5a,
Figure 4. XPS spectra of electrons from the 1s orbital of carbon atoms for heating fluorinated samples: hF-p-MWCNTs (a) and hF-oMWCNTs (b); inset shows superimposed C 1s spectra of the heating fluorinated and isothermal fluorinated samples: hF-p-MWCNTs (hF in part (a)), iF-p-MWCNTs (iF in part (a)), hF-o-MWCNTs (hF in part (b)), iF-o-MWCNTs (iF in part (b)).
5b, 5e, and 5d) after heating-fluorination. This meant that the binding of carbon and oxygen (CO, C−O) was eliminated from the surface of MWCNTs. Meanwhile, some bands for D
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Table 3. Surface Compositions and Assignments of C 1s Components for Fluorinated MWCNT Samples concentration (%) component
assignment
position (eV)
hF-p-MWCNTs
oF-p-MWCNTs
C1 C2 C3 C4 C5 C6
C−C−C C−OH or C−CF or C−CO CO covalent C−F bond perfluorinated C−F bond in CF2 perfluorinated C−F bond in CF3
284.4 285.8 287.4 288.8 290.3 291.5
69.8 11.8 7.5 7.3 3.6 ---
54.1 18.5 7.2 10.7 7.0 2.6
without introducing more defects. Even for hF-o-MWCNTs with the highest content of fluorine, the outer tubular structure of MWCNTs was not destructed. It was legible in the HR-TEM picture in Figure 6 that the outer aromatic layers of hF-oMWCNTs aligned parallel to the tube axis, which evidenced that the fluorination reaction did not distort the aromatic layers of MWCNTs. After heating-fluorination, the dispersibility of MWCNTs in organic solvent was improved significantly without distorting their curved aromatic layers. This was very important for MWCNTs to retain their key properties on the preparation of the composite. 3.3. Effect of Oxygen-Related Groups on the Fluorination Process. For heating-fluorinated samples, the O-content of hF-p-MWCNTs and hF-o-MWCNTs was 2.5% and 3.4%. Their O-contents, respectively, were decreased 2.2% and 9.3% after heating treatment under F2 atmosphere compared with that of p-MWCNTs and o-MWCNTs, which was consistent with the amount of reduction of O-content under N2 atmosphere of 2.5% and 9.4%. After the heatingfluorination process, the O-content of hF-p-MWCNTs reduced 2.2% compared with that of p-MWCNTs, while its F-content increased 3.2%. For hF-o-MWCNTs, O-content reduced 9.3%, while F-content increased 9.4%. For iF-p-MWCNTs and iF-oMWCNTs, the oxygen-related groups had been eliminated before fluorination. In other words, very few oxygen-related groups were removed during the process of fluorination for iFp-MWCNTs and iF-o-MWCNTs. It was interesting that the F-content of products was quite close to the amounts of oxygen-related groups removed during fluorination. It seemed that the more oxygen-related groups were eliminated, the more fluorine atoms would be grafted on the MWCNTs. This surprising finding was further confirmed by the following analysis. TGA and differential thermal analyzer (DTA) data were provided in Figure 7. For p-MWCNTs and o-MWCNTs, the main losses of weight occurring before 250 °C corresponded to the elimination of oxygen-related groups (Figure 7a, 7c). Up to 250 °C, the total weight loss was 3.4 and 8.2 wt %, respectively.
Figure 5. FITR spectra of fluorinated and nonfluorinated MWCNT samples: (a) hF-p-MWCNTs, (b) hF-o-MWCNTs, (c) p-MWCNTs, (d) o-MWCNTs, (e) iF-p-MWCNTs, and (f) iF-o-MWCNTs.
fluorine-containing groups were observed at around 1200 cm−1, which was assigned to the C−F stretching vibration. Bands at 1252 cm−1 were assigned to the C−F stretching vibration of peripheral >CF2 groups, while bands at 1088 cm−1 were regarded as being due to their symmetric stretching vibrations.18−22 The spectra of iF-p-MWCNTs and iF-o-MWCNTs were similar to those of d-MWCNTs and d-o-MWCNTs. No obvious peaks of the C−F bond or hydroxy groups were there in the spectra (Figure 5e, 5d). It seems difficult for MWCNTs to be fluorinated at a low temperature (
