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Thermal Recovery Behavior of Fluorinated Single-Walled Carbon Nanotubes Wei Zhao,*,† Chulho Song,† Bo Zheng,‡ Jie Liu,‡ and Tito Viswanathan† Department of Chemistry, UniVersity of Arkansas, Little Rock, Arkansas 72204, and Department of Chemistry, Duke UniVersity, Durham, North Carolina 27708 ReceiVed: August 27, 2001; In Final Form: October 29, 2001
The thermal recovery behavior of fluorinated single-walled carbon nanotubes has been systematically examined by UV-vis-near-IR absorption spectroscopy, IR absorption spectroscopy and electric resistance measurements under different annealing temperatures. The decrease in the IR intensity of C-F stretch bands centered at about 1207 cm-1 and the reduced resistance of the nanotube samples with heat annealing indicates that pristine nanotubes are recovered by the removal of sidewall groups. After annealing at 100 °C in noble gases, the IR spectrum shows nanotube features of A2u mode at 867 cm-1 and E1u mode at 1579 cm-1. Correspondingly, in the UV-vis-near-IR spectrum, only a new feature at about 640 nm (1.9 eV) is observed. At higher annealing temperatures g150 °C, the intensity of the absorption features at 640 nm (1.9 eV) of metallic nanotubes, and 900 nm (1.4 eV) and 1630 nm (0.8 eV) of semiconducting nanotubes increases simultaneously. Generalized two-dimensional UV-vis-near-IR correlation spectroscopy has revealed that the thermal recovery behaviors of fluorinated metallic and semiconducting nanotubes are very similar at annealing temperatures between 150 and 350 °C, indicating that differentiation of these nanotubes for purification is difficult under those annealing conditions.
Introduction The feasibility of sidewall functionalization of single-walled carbon nanotubes (SWNTs) has enable SWNT samples to be soluble in a wide range of organic solvents1-6 and thus opens a rich wet chemistry for SWNT nanotechnology applications such as molecular electronics,7 in particular, in combination with the reversibility of the sidewall groups4,5 which may provide a way to tune the electric and optical properties of nanotubes.8 The reversible behaviors of sidewall functionalized SWNTs have been demonstrated in fluorinated-SWNT (F-SWNT) samples with hydrazine as a defluorinating agent4 and in sidewallalkylated SWNT samples with heat annealing in air.5 Heat annealing has been used as an effective way to recover the pristine nanotubes.5,9 However, a detailed study on the recovery behaviors of sidewall functionalized SWNTs is still lacking. Importantly, since SWNT samples consist of about 66 mol % semiconducting nanotubes and 34 mol % metallic nanotubes,10 it is not clear how the thermal behaviors of these nanotubes change during sidewall defuctionalization. To clarify this subject is of great importance in directing sample purification. In this study, F-SWNT samples have been systematically examined by UV-vis-near-IR absorption spectroscopy, IR absorption spectroscopy and electric resistance measurements under different annealing temperatures. Generalized two-dimensional (2D) UV-vis-near-IR correlation spectroscopy has been used to differentiate different spectral features under temperature perturbation. 11-13 Experimental Section F-SWNT samples with C/F ratios between 2 and 2.3 were prepared by the method described in ref 1. The starting pristine * Corresponding author. E-mail:
[email protected]. † University of Arkansas, Little Rock. ‡ Duke University.
SWNTs had an average diameter of 1.35 nm. Two F-SWNT samples were used in the report. Sample A was a fresh powder sample without dissolution in solvents prior to use. Sample B was a gel sample formed in 2-propanol after dissolution about 12 months.4 Before use, about 0.5 mg of sample A and of sample B were dissolved into 4 mL of 2-propanol with aid of ultrasonification in an ultrasonic bath (Branson Model 1510). Sample A just needed about 0.5 min to be dissolved in the solvent, while 40 min were applied for sample B. Two corresponding film samples, film A and film B, with thicknesses about 1 µm were prepared by dropping a solution on a NaCl crystal substrate. Heat annealing of the film samples was conducted in a tube furnace under flowing Ar or He atmosphere (purity 99.999%) with temperatures at 100, 150, 200, 250, 300, and 350 °C. The annealing time was 1 h at each temperature. After heat annealing, the film samples were cooled to room temperature for various measurements. UV-vis-near-IR absorption spectra and IR absorption spectra of the film samples were measured by using a PerkinElmer UV-vis-near-IR spectrometer with a resolution of 2 nm and a Nicolet Magna-FT-IR 550 spectrometer at a resolution of 2 cm-1, respectively. The electric resistance of the film samples was measured using a four-probe method. All measurements were conducted in air. 2D UV-vis-near-IR correlation spectra were constructed using the generalized 2D correlation method developed by Noda under temperature perturbation.11-13 The Hilbert transformation of the dynamic spectra was used for calculations,13 and an averaged spectrum was used as a reference. Microcal Origin, Mathcad, and Matlab software packages were used to calculate and generate the 2D spectra. The negative cross-peaks in an asynchronous spectrum were shaded using the standard shading convention.11,12 In a synchronous spectrum, a positive crosspeak at (ν1, ν2) indicates that the intensity changes at these two wavelengths are in the same direction. A negative asynchronous
10.1021/jp0133135 CCC: $22.00 © 2002 American Chemical Society Published on Web 12/11/2001
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Figure 1. UV-vis-near-IR spectra (A) and IR spectra (B) of film A change as a function of annealing temperatures. The inset in Figure 1A shows the resistance dependence on the annealing temperatures. The insets in Figure 1B show E1u mode at 1579 cm-1 and A2u mode at 867 cm-1 of the nanotubes after annealing at 100 °C.
Figure 2. Difference spectra of UV-vis-near-IR absorption spectra of film A at different annealing temperatures after subtracting the preheat annealing spectrum.
cross-peak at (ν1, ν2) indicates that the spectral change at ν1 occurs later at a higher temperature compared to ν2. A positive asynchronous cross-peak indicates the opposite.11, 12 Results and Discussion F-SWNT samples used here are insulators.1 The resistance of film A is not measurable before heat annealing. After annealing at 100 °C, the resistance is measured on the order of 108 ohm. The resistance quickly drops to about 1000 ohm after annealing at 150 °C, and then decreases slowly at higher annealing temperatures, as shown in the inset of Figure 1A. Figure 1 also shows the UV-vis-near-IR absorption spectra and IR absorption spectra of film A measured at room temperature after heat annealing. Before heat annealing (25 ˚C), only one long-tailed feature at about 290 nm is observed for F-SWNTs, which is similar to the feature produced by π-plasmon observed in pristine nanotubes14 and will not be discussed here. After heat annealing, features characteristic of electronic transitions between 1D electronic density of states (DOS) singularities of pristine SWNTs show up at about 640, 900, and 1630 nm whose intensity increases with annealing temperature. By subtracting the long-tailed background absorption of the film measured before heating, features related to recovered SWNTs can be more easily identified as shown in Figure 2. It is interesting to note that, after 100 °C annealing, only an additional band appears at 640 nm where the absorption band of the metallic nanotubes is located.15-18 It is unclear whether this feature is related to partially defluorinated nanotubes or metallic nanotubes which may be preferentially recovered from F-SWNTs at low temperatures, and more work is needed to reveal its origin. No feature related to semiconducting nanotubes at about 900 and 1630 nm18 is observed at this annealing temperature.
The IR spectra in Figure 1B present several interesting features. The IR spectrum of preheat annealed film A has a broad band of C-F stretch centered at 1207 cm-1, with shoulders at 1152, 1241, and 1299 cm-1. A weak band at 737 cm-1 is also observed, indicating that some CF3 groups are involved,19 probably at the end of the nanotubes. The other two bands at 1630 and 3440 cm-1 could be assigned to the adsorbed water, as compared with the water bands in a moisture-contained KBr pellet. Additional bands at about 2300 cm-1 come from CO2 presented in air. After 100 °C annealing, the intensity of C-F bands decreases, indicating that some F groups are removed with the recovery of some nanotubes, consistent with the UV-vis-near-IR spectrum in Figure 2. The water bands become very weak. Additional new bands are observed at 1579 and 2900 cm-1. The bands at about 2900 cm-1 are the stretch bands of C-H groups that may come from organic contaminants during reaction or may be formed during heating. The 1579 cm-1 band could be assigned to an IR-active in-plane E1u mode of the nanotubes.20-22 A very weak band at 867 cm-1 is also present, which could be an IR-active out-of-plane A2u of nanotubes.20,21 The band at 867 cm-1 becomes stronger with annealing temperature and is more easily seen from the spectra. It is interesting to note that, after annealing at 150 °C, the bands at about 1200 and 1600 cm-1 have a reverse dispersive line shape. This result may arise from the strong interference effect of these vibrational bands with the electronic transition background of recovered nanotubes. In light of the UV-vis-nearIR spectrum at the same temperature in Figures 1 and 2, the intensity of absorption bands of semiconducting and metallic nanotubes become much stronger after 150 °C annealing, indicating more F-SWNTs are recovered at this temperature. The reverse dispersive C-F stretch bands consist of three bands at about 1193, 1218, and 1277 cm-1 that become weaker with the increase of annealing temperature. The reverse dispersive band at about 1600 cm-1 shows a doublet with positions at about 1566 and 1613 cm-1. The 1566 cm-1 band could be assigned to E1u of the nanotubes. Since the position of 1613 cm-1 band is very close to a band at 1606 cm-1 of acid-cut SWNT samples,23 and is almost the same as a band position observed in soluble SWNTs,6 one could assign this band to the stretching mode of CdO groups which may be formed by oxidization of adsorbed oxygen during heat annealing. For film B, as shown in the inset of Figure 3, its resistance is measurable with a value of about 1300 ohm before heat annealing. The resistance decreases with the increase of the annealing temperature and reaches a value of about 200 ohm after 350 °C annealing. The measurable resistance of preheat annealed film B indicates that some fluoronanotubes have changed back to pristine nanotubes by oxidization after pro-
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Figure 4. UV-vis-near-IR absorbance AT (annealing temperature T ) 150, 200, 250, 300, and 350 °C) in Figure 3 of film B divided by the absorbance A25 measured before heat annealing.
Figure 3. UV-vis-near-IR spectra (A) and IR spectra (B) of film B measured at room temperature after annealing at different temperatures. The inset shows the resistance dependence on the annealing temperatures.
longed exposure of sample B to air and by longer ultrasonification during dissolution.4 This conclusion can be supported by the UV-vis-near-IR spectrum and IR spectrum before heating shown in Figure 3. Some very weak features of pristine nanotubes between 500 and 1000 nm appear in the UV-visnear-IR spectrum before heat annealing, which may belong to metallic nanotubes. Correspondingly, the IR bands at 1184 and 1270 cm-1 related to C-F stretch and at 1604 cm-1 related to CdC and CdO stretches present a reverse dispersive line shape, an indication of interference between the vibrational bands of functionalized nanotubes and electronic transition background of recovered pristine nanotubes. With the increase of annealing temperature, the intensity of electronic absorption bands of pristine nanotubes increases with annealing temperature, indicating that more pristine nanotubes are recovered from fluoronanotubes. For IR absorption spectra, after heat annealing, the IR features of film B shown in Figure 3B are very similar to those of film A shown in Figure 1B. In agreement with UVvis-near-IR spectra, the vibrational bands of functionalized SWNTs become weaker, so more sidewall groups are removed with the increase of the annealing temperature. After annealing at 350 °C, weak features still can be seen at 864 cm-1 (A2u of nanotubes), 1188, 1216, and 1278 cm-1 (C-F stretches), and 1565 cm-1 (E1u or CdC stretch), and 1611 cm-1 (CdO stretch).6,22 Because the fluorinated samples contain SWNTs with different diameters between 1.13 and 1.53 nm,9 it is interesting to see how those nanotubes are recovered under heat annealing. It is observed that when the annealing temperature is at or above
150 °C, both metallic and semiconducting nanotubes are recovered simultaneously as shown in Figures 1-3, so additional information is difficult to get directly from these spectra. Generalized 2D correlation spectroscopy is a powerful tool to identify structural features that are not seen from highly overlapped conventional spectra. Here we construct the 2D synchronous and asynchronous spectra based on the UV-visnear-IR spectra of film B that have constantly changed background as shown in Figure 3. To eliminate the strong background absorption centered at about 290 nm which obstructs the observation of the semiconducting band at about 450 nm, the UV-vis-near-IR spectrum (AT) at each annealing temperature is divided by the UV-vis-near-IR spectrum before heat annealing (A25). The resulting spectra are shown in Figure 4. Four sets of bands are clearly seen in Figure 4, with positions centered at 1700 nm (0.7 eV), 960 nm (1.3 eV) with a shoulder at 1030 nm (1.2 eV), 650 nm (1.9 eV) with a shoulder at 700 nm (1.8 eV), and 450 nm (2.8 eV) with shoulders around 410 (3.0 eV), 470 (2.6 eV) and 520 nm (2.4 eV), respectively. They correspond to the first van Hove singularity in semiconducting nanotubes E11S, the second van Hove singularity E22S, the first van Hove singularity in metallic nanotubes E11M, and the third van Hove singularity in semiconducting nanotubes E33S, respectively.9,10,14-18 The synchronous and asynchronous 2D UV-vis-near-IR correlation spectra constructed from the data in Figure 4 are shown in Figure 5. Only positive cross-peaks are observed in the synchronous 2D correlation spectrum of heat annealed film B, with band positions centered at 460, 650, 960, and 1700 nm, respectively. The positive signs of these cross-peaks indicate that the temperature-induced changes in the spectral intensities of these bands are all in the same direction, i.e., increase with annealing temperature. The asynchronous 2D correlation spectrum in Figure 5B reveals four new bands centered at 570 (2.2 eV), 750 (1.7 eV), 1160 nm (1.1 eV), and 2490 nm (0.5 eV), respectively. These new bands form cross-peaks with the bands of recovered pristine nanotubes at 650, 960 and 1700 nm, also with a band at about 290 nm. The signs of asynchronous crosspeaks indicate that temperature-induced change of absorption intensity of 650, 960, and 1700 nm occurs before the intensity change of 570, 750, 1160, and 2490 nm at a lower temperature. One possible assignment for these new bands is that they may correspond to recovered nanotubes formed from more thermal stable fluoronanotubes.24 Further study is needed to identify their origin. One interesting observation in the asynchronous spectrum is that there are no cross-peaks between the band of metallic nanotube at 650 nm and semiconducting bands at 960 and 1700 nm. This result indicates that the thermal recovery behaviors
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Zhao et al. and F-SWNTs, a separation between metallic nanotubes and semiconducting nanotubes may be possible, which is of great importance for various applications of SWNTs.7,8,10 The examination on the thermal recovery behavior of fluorinated SWNTs indicates that F-SWNT samples could be recovered at an annealing temperature as low as 100 °C. After the samples are treated at this temperature, a UV-visble-near-IR absorption feature is observed at 640 nm whose origin needs to be addressed further. Generalized 2D UV-vis-near-IR correlation spectroscopy has indicated that, at the annealing temperature g150 °C, the thermal recovery behaviors of metallic and semiconducting nanotubes are very similar, suggesting that when treated at higher temperatures under present experimental conditions, differentiation of metallic nanotubes from semiconducting nanotubes for purification becomes difficult. References and Notes
Figure 5. Synchronous 2D correlation spectrum (A) and asynchronous 2D correlation spectrum (B) of film B.
of fluorinated metallic and semiconducting nanotubes are very similar at annealing temperature g150 °C. This similarity in the thermal recovery behaviors suggests that the difference in yield ratio between metallic nanotubes and semiconducting nanotubes at different annealing temperatures between 150 and 350 °C is very small, thus differentiation of those nanotubes for purification becomes difficult. This result may be comparable with the experimental results that growth of SWNTs under different temperatures yields no change in the ratio of metallic and semiconducting nanotubes.25 Conclusions It has been recently observed that the diameters of metallic nanotubes are smaller than those of semiconducting nanotubes in the nanotube samples. 6 Therefore, it is intuitive that the thermal recovery behavior of fluorinated metallic SWNTs and fluorinated semiconducting SWNTs may be different; thus, based on the difference in solubility of recovered nanotubes
(1) Mickelson, E. T.; Huffman, C. B.; Rinzler, A. G.; Smalley, R. E.; Hauge, R. H.; Margrave, J. L. Chem. Phys. Lett. 1998, 296, 188. (2) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95. (3) Hamon, M. A.; Chen, J.; Hu, H.; Chen, Y.; Itkis, M. E.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. AdV. Mater. 1999, 11, 834. (4) Mickelson, E. T.; Chiang, I. W.; Zimmerman, J. L.; Boul, P. J.; Lozano, J.; Liu, J.; Smalley, R. E.; Hauge, R. H.; Margrave, J. L. J. Phys. Chem. B 1999, 103, 4318. (5) Boul, P. J.; Liu, J.; Mickelson, E. T.; Huffman, C. B.; Ericson, L. M.; Chiang, I. W.; Smith, K. A.; Colbert, D. T.; Hauge, R. H.; Margrave, J. L.; Smalley, R. E. Chem. Phys. Lett. 1999, 310, 367. (6) Chen, J.; Rao, A. M.; Lyuksyutov, S.; Itkis, M. E.; Hamon, M. A.; Hu, H.; Cohn, R. W.; Eklund, P. C.; Colbert, D. T.; Smalley, R. E.; Haddon, R. C. J. Phys. Chem. B 2001, 105, 2525. (7) Service, R. F. Science 2001, 293, 782. (8) Dai, H. Phys. World 2000, 13, 43. (9) Chiang, I. W.; Brinson, B. E.; Smalley, R. E.; Margrave, J. L.; Hauge, R. H. J. Phys. Chem. B 2001, 105, 1157. (10) Dekker, C. Phys. Today 1999, 52, 22. (11) Noda, I. Appl. Spectrosc. 1993, 47, 1329. (12) Ozaki, Y.; Liu, Y.; Noda, I. Appl. Spectrosc. 1997, 51, 526. (13) Noda, I. Appl. Spectrosc. 2000, 54, 994. (14) Petit, P.; Mathis, C.; Journet, C.; Bernier, P. Chem. Phys. Lett. 1999, 305, 370. (15) Yu, Z.; Brus, L. E. J. Phys. Chem. B 2001, 105, 6831. (16) Pimenta, M. A.; Marucci, A.; Empedocles, S. A.; Bawendi, M. G.; Hanlon, E. B.; Rao, A. M.; Eklund, P. C.; Smalley, R. E.; Dresselhaus, G.; Dresselhaus, M. S. Phys. ReV. B 1998, 58, R16016. (17) Jorio, A.; Dresselhaus, G.; Dresselhaus, M. S.; Souza, M.; Dantas, M. S. S.; Pimenta, M. A.; Rao, A. M.; Saito, R.; Liu, C.; Cheng, H. M. Phys. ReV. Lett. 2000, 85, 2617. (18) Kim, P.; Odom, T. W.; Huang, J.; Lieber, C. M. Phys. ReV. Lett. 1999, 82, 1225. (19) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy; Academic Press: New York, 1990; p 380. (20) Kastner, J.; Pichler, T.; Kuzmany, H.; Curran, S.; Blau, W.; Weldon, D. N.; Delamesiere, M.; Draper, S.; Zandbergen, H. Chem. Phys. Lett. 1994, 221, 53. (21) Kahn, D.; Lu, J. P.; Phys. ReV. B 1999, 60, 6535. (22) Mawhinney, D. B.; Naumenko, V.; Kuznetsova, A.; Yates, J. T., Jr.; Liu, J.; Smalley, R. E. J. Am. Chem. Soc. 2000, 122, 2383. (23) Zhao, W.; Song, C. Unpublished. (24) Kudin, K. N.; Bettinger, H. F.; Scuseria, G. E. Phys. ReV. B 2001, 63, 045413. (25) Jost, O.; Gorbunov, A. A.; Pompe, W.; Pichler, T.; Friedlein, R.; Kunpfer, M.; Reibold, M.; Bauer, H.-D.; Dunsch, L.; Golden, M. S.; Fink, J. Appl. Phys. Lett. 1999, 75, 2217.
