APPLIED PHYSICS LETTERS
VOLUME 80, NUMBER 22
3 JUNE 2002
X-ray photoemission spectroscopy study of fluorinated single-walled carbon nanotubes Kay Hyeok An, Jeong Goo Heo, Kwan Goo Jeon, Dong Jae Bae, Chulsu Jo, Cheol Woong Yang,a) Chong-Yun Park, and Young Hee Leeb) Department of Physics, Research Laboratory for Carbon Nanotubes, Center for Nanotubes and Nanostructured Composites, Sungkyunkwan University, Suwon 440-746, R. O. Korea
Young Seak Lee Department of Chemical Engineering, Nanotechnology Center, Sunchon National University, Chonnam 540-742, R. O. Korea
Young Su Chung Analytical Engineering Center, Samsung Advanced Institute of Technology, Suwon 440-600, R. O. Korea
共Received 14 January 2002; accepted for publication 15 April 2002兲 We have investigated the change of atomic and electronic structures of fluorinated single-walled carbon nanotubes 共SWCNTs兲 using x-ray photoemission spectroscopy 共XPS兲, electrical resistivity measurements, and transmission electron microscopy 共TEM兲. The fluorine content increases with increasing reaction temperature up to 300 °C. XPS indicated that the fluorinated SWCNT reveals an ionic-bonding character at low concentration and covalent-bonding character at high concentration. The resistivity increases with reaction temperatures, resulting from the band gap enlargement at high fluorine concentration. It is also observed from TEM that the fluorination at reaction temperature above 250 °C leads to the disintegration of the CNT structures and formation of various phases such as multiwall-like and turbostratic morphologies. © 2002 American Institute of Physics. 关DOI: 10.1063/1.1482801兴
In spite of intensive researches on carbon nanotubes 共CNTs兲, applications of CNTs to practical use of electronic and energy storage devices are still limited by a number of reasons. A chemical functionalization of the sidewalls can change the electronic properties of nanotubes and enhance the performance in hydrogen storage, secondary battery, and supercapacitor. However, the sidewall functionalization is not easily accessible, since the open ends of CNTs are more reactive than the sidewall due to the presence of dangling bonds. Recently there have been several reports on sidewall functionalization of single-walled carbon nanotubes 共SWCNTs兲 by fluorination. Theoretical calculations predict that the sidewall fluorination can yield either metallic or semiconducting nanotubes depending on the fluorination types and composition.1 Mickelson et al. have reported that the fluorinated nanotubes are highly resistive and can survive at temperatures up to 325 °C.2 Fluorination can also enhance the solvation of SWCNTs in various solvents by increasing the wettability in water.3 However, the bonding nature of fluorinated nanotubes and disintegration process upon fluorination are far from being clearly understood. In this report, we have done a systematic study on x-ray photoemission spectroscopy 共XPS兲 and resistivity measurements of SWCNT powder as a function of fluorination temperature. XPS data show a clear binding-energy shift towards the higher energy with increasing-fluorination temperatures. At low temperature of up to 200 °C, they show an ionica兲
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b兲
bonding character which implies the sidewall functionalization, whereas at high temperature the binding energy shifts to higher binding side, revealing a covalent-bonding character of CFn , which implies resistivity increase by enhancing sp 3 bonding and disintegration of nanotube walls, as confirmed by the four-point resistivity and the transmission electron microscopy 共TEM兲 observation. SWCNTs were prepared by conventional catalytic arc discharge. The chamber was pumped out to a base pressure of 100 mTorr and then helium gas was introduced to 100 Torr. The total amount of catalysts 共with the ratio of Ni:Co:FeS is 1:1:1兲 in a graphite powder was fixed at 5 wt %, where sulfur was added as a promoter. This significantly increased the yield of CNTs deposited in the chamber. The powder was collected from the chamber, collar, and cathode and then grinded together. The detail has been described elsewhere.4 This powder was brought into a Ni boat of the fluorine reaction chamber made by Ni and SUS-316 in order to prevent erosion. The reaction process is similar to that of carbon fibers.5 The chamber was pumped out to 10⫺2 Torr and purged by nitrogen gases to remove the residual oxygen gases and moisture. F2 gas was then introduced and the pressure was maintained at 0.2 bar for 10 min for a given reaction temperature. After the reaction, the chamber was pumped out again to 10⫺2 Torr and nitrogen gas was refilled prior to the extraction of the powder sample. The fluorinated CNTs were characterized by the measurement of resistivity, Fourier-transformed Raman spectroscopy 共BRUKER, RFS 100/S兲, XPS, and the observation of TEM 共JEOL, JEM-3011兲. The resistivity was measured by fourpoint probe method after pelletizing the powder. XPS measurements were carried out using PHI 5100 spectrometer us-
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Appl. Phys. Lett., Vol. 80, No. 22, 3 June 2002
An et al.
FIG. 1. 共a兲 C 1s XPS spectra of the undoped and fluorinated SWCNTs at various reaction temperatures, 共b兲 XPS C 1s curve for the fluorinated SWCNTs at 300 °C, 共c兲 F 1s XPS spectera of the undoped and fluorinated SWCNTs at various reaction temperatures, and 共d兲 F/C area ratio determined by XPS as a function of reaction temperature.
ing Mg K ␣ 共1253.6 eV兲 line. We have also performed density-functional calculations in order to know the electronic structures and stability.6 Figure 1共a兲 shows the C 1s spectra of CNTs fluorinated at different reaction temperatures. The spectrum of the undoped CNT shows a sharp peak at 284.3 eV, which is assigned to a s p 2 carbon. A tail near 285 eV indicates the presence of sp 3 carbon. The intensity of s p 2 peak significantly decreases and the width becomes broader with increasing reaction temperatures. It can be seen that the peaks at high binding-energy sides begin to appear above 150 °C and the peak shapes dramatically change at high temperature above 250 °C. The peaks at high binding-energy side indicate the existence of various carbon species bonded to fluorine. Figure 2共b兲 shows the XPS C 1s curve for the fluorinated SWCNTs at 300 °C. The fitting of the peaks with several gaussian peaks indicates s p 2 共A兲, s p 3 共B兲, oxygen related peaks 共C–E兲, and the formation of other types of CFx bonds 共F–H兲. The fitted peaks related CFx bonds of F at 291.2 eV, G at 292.9 eV, and H at 294.6 eV are ascribed to C–F, C–F2 , and C–F3 , respectively. Figure 1共c兲 shows that the intensity of F 1s peak increases and the binding energy shifts toward higher binding energy with reaction temperatures. The binding energy of F 1s jumps from 687 to 691 eV at temperature above 200 °C. The peak at 687 eV is assigned to semi-ionic fluorine and 691 eV to covalent fluorine. The binding energy of hexafluorobenzene with a strong covalent-bonding nature is about 691 eV. The F/C area ratio determined by the XPS is shown in Fig. 1共d兲. The value increases as a function of reaction temperatures and reaches close to 0.5 at 250 °C, in good agreement with the previous reports, although the reaction temperature is lower in our experiment.2,3 These indicate that the amount of adsorbed fluorine increase with increasing reaction temperature and the semi-ionic C–F bonding nature at a composition of CFx with x⬍0.5 changes to the covalent one at x⬎0.5. This is in good agreement with our IR results 共not shown here兲 that a band at 1100 cm⫺1 assigned to the semi-
FIG. 2. 共a兲 The resistivity measured by the four-point probe method from the pelletized fluorinated samples at room temperature. 共b兲 The XPS valance band spectra for the undoped and fluorinated CNTs at various reaction temperatures.
ionic C–F bonds decreases and a band at 1210 cm⫺1, a characteristic of the C–F covalent bonds, increases with reaction temperatures. The composition at 250 °C may correspond to C2 F. Our density-functional calculations show that a significant charge transfer of 0.19 e from carbon to fluorine atom is occurred at a C2 F composition, where the fluorine atoms are chemisorbed at the outer surface of the CNT wall.7 This will form a partial ionic bonding in addition to the strong covalent bonds. Formation of a stripe or quadra patterns in fluorinated tube results in a strong dipole layer on the CNT wall.7,8 This may be the cause of increased wettability in water and thus solvation capability. At 300 °C, F/C ratio reaches to 0.65. Since CFx composition with x⬎0.5 is prohibited in the sidewall fluorination due to electrostatic repulsion between fluorine atoms, we can only imagine this in terms of a phase formation. This will be discussed later. Figures 2共a兲 and 2共b兲 show the resistivity measured by the four-point probe method at room temperature and the XPS valence band spectra of the undoped and fluorinated CNTs, respectively. It can be seen that the resistivity increases with increasing reaction temperatures. Figure 2共b兲 shows that the valence band edge shifts to the higher binding-energy side with increasing reaction temperature, implying the increase of band gap. The electronic properties are altered by fluorination so as to decrease the conductivity of the individual CNTs. Although it is obvious that the resistivity increases with increasing F content, it is not obvious
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An et al.
Appl. Phys. Lett., Vol. 80, No. 22, 3 June 2002
FIG. 3. TEM images of 共a兲 the undoped and 共b兲–共d兲 fluorinated CNTs at a reaction temperature of 300 °C.
from the theoretical calculations. The most stable fluorinated CNT is a metallic one at a composition of C2 F. The discrepancy between theory and experiment is not clarified at this moment. Figure 3 shows the TEM images of 共a兲 the undoped and 共b兲–共d兲 the fluorinated CNTs at 300 °C. The undoped CNT bundles with an individual CNT diameter of 1.5 nm are clearly shown in Fig. 3共a兲, which is in good agreement with the value estimated from the Raman breathing mode. These bundles are transformed into multiwall-like phase after fluorination at 300 °C, as shown in Fig. 3共b兲. All these phases have needle-like closed ends. Mickelson et al. have also reported that the single-wall tubular structure fluorinated at 500 °C does not survive and instead some multiwalled nanotube-like structures are formed.2 We note that this phase keeps changing during the TEM observation, i.e., electron beam-induced phase transformation takes place, which was never observed from the undoped nanotubes. We also note some graphitic phases with a lattice parameter of 0.34 nm or larger, as shown in Fig. 3共c兲. In addition to the graphitically layered phases, we also observed an amorphous and turbostratic9 phases. Electron beam-induced deformation of the pure CNTs has been observed previously but at very high electron beam energy of 3 MeV.10 No structural deformation was observed from our undoped samples in electron energy to 300 keV. Fluorination will cause the different C–C bond lengths and thus heavy strain on the CNT wall 共0.8 eV/C–C bond in average兲. C–C bonds lengths are 1.35 Å in the puckered 共or buckled兲 tube wall, whereas FC–CF bond lengths are 1.53 Å, compared with a bond length of graphite of 1.42 Å. Therefore, we expect the FC–CF bond to have large strain energy. This may explain the disintegration of the fluorinated CNTs during the TEM observation at such a low electron
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beam energy. In addition, it is known that fluorine atoms are more susceptible to the electron beam and are therefore more likely to induce desorption and deformation of the tubes. Even at lower fluorine composition than C2 F, calculations show that the fluorine atoms favor certain adsorption sites. Otherwise the tubes are fragmented into several pieces.7 In real experimental situation, we can conjecture that fluorine molecules may attack randomly all different sites. There is always a chance to break the tube walls. The fragmented graphitic flakes may be recombined into various phases. Different composition of fluorination may lead to various intertube distances, as observed in the TEM images. The attack of fluorine may also create some local defects. These will induce a certain strain in the flake, which may result in the formation of closed shells, as shown in the TEM images.11 In summary, we have studied electronic and structural changes on fluorination of singlewalled carbon nanotubes. At a composition CFx with x⬍0.5, partially semi-ionic bonds are predominated by charge transfer from fluorine to carbon atoms. The resistivity increases with increasing the amount of adsorbed fluorine atoms, resulting from the increase of the band gap. At a composition CFx with x⬎0.5, sp 3 covalent bonds are predominated, starting fragmentation and the formation of various phases such as multiwall-like tubes with closed ends and turbostratic phases. The structural phase transformation induced by the electron beam is also observed. This work was supported in part by the MOST through the NRL program and through the CNNC at SKKU.
1
K. N. Kudin, H. F. Bettinger, and G. E. Scuseria, Phys. Rev. B 63, 045413 共2001兲. 2 E. T. Mickelson, C. B. Huffman, A. G. Rinzler, R. E. Smalley, R. H. Hauge, and J. L. Margrave, Chem. Phys. Lett. 296, 188 共1998兲. 3 E. T. Mickelson, I. W. Chiang, J. L. Zimmerman, P. J. Boul, J. Lozano, J. Liu, R. E. Smalley, R. H. Hauge, and J. L. Margrave, J. Phys. Chem. B 103, 4318 共1999兲. 4 Y. S. Park, K. S. Kim, H. J. Jeong, W. S. Kim, J. M. Moon, K. H. An, D. J. Bae, Y. S. Lee, G. S. Park, and Y. H. Lee, Synth. Met. 126, 245 共2002兲. 5 A. Bismark, R. Tahhan, J. Springer, A. Schulz, T. M. Klapo¨tke, H. Zell, and W. Michaeli, J. Fluorine Chem. 84, 127 共1997兲. 6 M. Elstner, D. Porezag, G. Jungnickel, J. Elsner, M. Haugk, Th. Frauenheim, S. Suhai, and G. Seifert, Phys. Rev. B 58, 7260 共2000兲. 7 We used 共5,5兲 armchair tube for fluorination stability test at a composition of C2 F. We found two stable geometries. One is so called a stripe type, where two adjacent fluorine atoms are attached alternatively along the tube axis. Another is a quadra, where four adjacent fluorine atoms alternate topologically. Both structures reveal metallic behaviors. The former is more stable than the latter only by 0.08 eV/C2 F bond. 8 G. Seifert, T. Kohler, and T. Frauenheim, Appl. Phys. Lett. 77, 1313 共2000兲. 9 J. C. Bokros, in Chemistry and Physics of Carbon, edited by P. L. Walker, Jr. 共Marcel Dekker, New York, 1969兲, Vol. 5, Chap. 1, pp. 7–23. 10 K. H. Chen, C. T. Wu, J. S. Wen, L. C. Chen, C. T. Wang, and K. J. Ma, J. Phys. Chem. Solids 62, 1561 共2001兲. 11 Y. H. Lee, S. G. Kim, and D. Tomanek, Phys. Rev. Lett. 78, 2393 共1997兲.
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