Fast and Efficient Purification for Highly Conductive Transparent

Oct 22, 2010 - nanotube (SWCNT) films with low sheet resistance. .... FE-SEM images of (a) AP, (b) TR, (c) TO, and (d) TO-TR SWCNTs. Scale bar is 500 ...
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J. Phys. Chem. C 2010, 114, 19169–19174

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Fast and Efficient Purification for Highly Conductive Transparent Carbon Nanotube Films Ju Yeon Woo,† Duckjong Kim,† Joondong Kim,† Jongkyoo Park,‡ and Chang-Soo Han*,† Department of Nano-Mechanics, Korea Institute of Machinery and Materials (KIMM), Daejeon 305-343, Korea, and Composite Laboratory, Agency for Defense DeVelopment (ADD), Daejeon 305-600, Korea ReceiVed: May 10, 2010; ReVised Manuscript ReceiVed: October 1, 2010

A combination of thermal oxidation and thermal recovery was used to obtain transparent single-walled carbon nanotube (SWCNT) films with low sheet resistance. The process resulted in the effective removal of impurities and healing of defects in the SWCNTs, thus markedly increasing their conductivity. SWCNT samples before and after the process were analyzed by Raman spectroscopy, scanning and transmission electron microscopy, thermogravimetric analysis, contact angle measurement, and transparent conductive film fabrication. Following purification, the sheet resistance decreased from 5380 to 187 Ω/sq at 80% optical transmittance and at a wavelength of 550 nm. The process requires a short time of about 3 h and produces 30% yield of the asproduced sample. Introduction Because of their unique physical, chemical, electrical, and mechanical properties, single-walled carbon nanotubes have attracted the attention of researchers for various potential applications such as display, energy devices, tools in nanotechnology, sensors, electronic devices, and composites.1–5 Thin, optically transparent, and highly conductive carbon nanotube films have been studied widely for replacing indium tin oxide.6 Among several types of CNT, transparent conducting film (TCF) made of arc-discharge SWNTs presents the best performance of any CNT, such as CVD grown SWCNT, double-walled CNT, and multiwalled CNT.7 Yet the arc-discharge SWCNT is expensive so that one should be careful not to lose too much SWCNT during the purification process before TCF fabrication. However, to obtain the optimal performance of SWCNTs in various applications or studies, the high yielding purification of SWCNTs is critical. The impurities found typically in as-produced SWCNTs by the arc discharge method include byproduct (fullerenes, amorphous carbon, and carbon shells) and residues of precursor materials (graphite flakes or metal catalysts).8 These structural defects and impurities on SWCNTs deteriorate their typical properties and have restricted practical application of SWCNT films.9 Although there are many reports about the purification of SWNT and its application to TCF, investigation of the effect of each purification step before TCF fabrication, such as thermal oxidation, acid treatment, and recovery, has not been reported. Important factors for purification include process time, weight loss, and performance of the film. One could sacrifice the process time and weight loss to obtain high performance of TCF through a severe purification process, but if the TCF already meets the performance requirements, for example, of a touch panel, then the next requirement would be to minimize process time and weight loss. Accordingly, in this study, we suggest a novel purification process considering all three factors. As shown in Figure 1, the typical purification process before film fabrica* Corresponding author. Phone: +82-42-868-7126. Fax: +82-42-8687123. E-mail: [email protected]. † KIMM. ‡ ADD.

Figure 1. Flowchart of the process for obtaining highly conductive transparent SWCNT film. Instead of acid treatment, we applied a thermal recovery process to improve the conductivity of SWCNTs. The total process time takes 3 h before making the film.

tion follows thermal oxidation, acid treatment, and then centrifugation. In this Article, we did not consider the doping process and acid treatment. A doping process could be added to enhance the conductivity of the TCF.10,11 However, the doped materials appear to be unstable for long-term exposure to air, so this process is problematic for industrial application.12 The usual purification method generally includes an acid treatment process to remove the metal catalysts, which might induce surface damage on the CNT wall. With respect to TCF fabrication, how the severe acid purification before film fabrication relates to the conductivity of TCF remains unclear. In our experiments, acid treatment before thermal recovery showed no effect on the sheet resistance of TCF. Herein, we suggest an effective purification and recovery of SWCNTs produced by the arc-discharge method, performed using a combination of thermal oxidation and high temperature annealing without acid treatment. Using only thermal oxidation and centrifugation, we could achieve 540 Ω/sq of sheet resistance at 80% transmittance. In addition, we introduced a

10.1021/jp107691q  2010 American Chemical Society Published on Web 10/22/2010

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TABLE 1: Process Time, Final Yield, and Carbon Content for Each Step

AP TR TO TO-TR

process time (h) (including dispersion and centrifugation time)

yield (%)

carbon content (at. %)

1.5 1 0.5 3

100 61 48 30

72.83 47.12 54.69 59.35

thermal recovery process to heal the defects on SWCNTs, which resulted in a marked reduction in the sheet resistance of the fabricated film by about 3-fold. The overall process time before film fabrication takes 3 h, which is not too long to be applied to a realistic manufacturing process. After the thermal processes, the remaining quantity of SWCNTs was about 30% of the asproduced (hereafter, AP) sample (Figure 1). We believe that the proposed processes are effective in view of the real production of TCF. Experimental Details Materials and Preparation of TCF Films. As-produced SWCNTs made by arc-discharge were provided by Topnanosys Inc. (Korea). We conducted preliminary experiments at various temperatures (350, 400, 800, 900, 1000, 1300, and 1400 °C), times (30 min, 1 h, 3 h), and with various gases (vacuum, ammonia flow, Ar flow) to find optimal conditions of oxidation and heat treatment. The best results were obtained at 400 °C for 30 min under an oxygen flow in case of oxidation and at 1000 °C under Ar flow for 1 h in case of heat treatment. First,

for the purification of SWCNTs, thermal oxidization (hereafter, TO) was conducted at 400 °C for 30 min under an oxygen flow to remove carbonaceous particles. For both AP-SWCNTs and TO-SWCNTs, thermal recovery (hereafter, TR) was then carried out at 1000 °C under Ar flow for 1 h to heal the defects on the SWCNTs, as well as to remove additional impurities. For all heat-treated samples, a mixture of 0.08 mg/mL SWCNTs and 10 mg/mL sodium dodecyl sulfate (SDS) dispersed in doubly deionized (DI) water was tip-sonicated for 30 min. The surfactant, SDS, was purchased from Sigma-Aldrich and used as received. The SWCNT solution was then centrifuged at 10 000 rpm for 1 h to remove large impurities, and the supernatant was decanted carefully. Finally, the suspension was collected by membrane filtration (pore size, 0.1 um), which has been applied widely to TCF fabrication.13,14 The fabricated film was washed with DI water several times. To remove the filter, a NaOH solution was used, and the floating film was then transferred onto a glass slide. Table 1 summarizes the process time, the final yield, and carbon content for each step. Measurement Methods. Raman spectra were measured using an inVia Raman system (Renishaw Co.) equipped with 633 nm excitation laser sources. Carbon nanotube morphology was characterized using FE-SEM (Sirion model, FEI Co.) and FETEM (Tecnai G2 F30 S-Twin model, FEI Co.). Energy dispersive spectroscopy (EX-200, Horiba) analysis was used to analyze carbon content and other impurities. The thermal behavior of SWCNTs was monitored by thermogravimetry (TG 209 F3, Netzsch Inc.). Sheet resistance of the thin conductive film was measured using the four-point probe method (CMT100MP, AIT Co.), and the optical transmittance was character-

Figure 2. FE-SEM images of (a) AP, (b) TR, (c) TO, and (d) TO-TR SWCNTs. Scale bar is 500 nm.

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Figure 3. FE-TEM images of (a) AP, (b) TR, (c) TO, and (d) TO-TR SWCNTs. Scale bar is 50 nm. The inset shows FE-TEM images with high resolution for SWCNTs, respectively. Scale bar is 10 nm.

ized at 550 nm using a UV-vis spectrophotometer (SD-1000, Scinco Co.). Water was dropped on the fabricated TCF film in air at room temperature using the sessile drop method. Still images of the droplets were captured with a CCD camera and used to measure the contact angle. Results and Discussion FE-SEM and TEM Characterization. Figure 2 shows the FE-SEM images of the sample of (a) AP, and after (b) TR, (c) TO, and (d) TO-TR. The images of these samples were taken in their original powder condition without dispersion and centrifugation. As shown in Figure 2a, AP SWCNTs are covered by numerous amorphous carbon and metal particles on their surfaces. From the FE-SEM images, we could hardly find pure SWCNTs because the impurities thoroughly covered the SWCNTs. However, after heat treatment, the amorphous carbon was well removed so that many nanotubes without impurities could be observed (Figure 2b,c). As predicted, metal particles remained due to the lack of chemical treatment. The structural morphology of TO SWNCTs appears to be straighter than that of TR SWCNTs. During removal of amorphous carbons by the TO process, oxygen and/or small amount of water might be associated with improving the straightness, as reported in the literature about high-quality CNT growth.15,16 Moreover, as compared to the image in Figure 2b, many SWCNTs without impurities can be observed in Figure 2c. As for the TO process, the FE-SEM images of TO-TR SWCNTs (Figure 2d) did not demonstrate distinct changes in terms of the impurities and structural morphology of SWCNTs.

To observe the morphological variation and the structural changes in nanoscale after heat treatment, FE-TEM observations were made for SWCNT samples after dispersion and centrifugation. Figure 3a presents an FE-TEM image of AP SWCNTs covered by large amounts of impurities including amorphous carbon, catalysts, and graphitic nanoparticles. As shown in Figure 3b, the FE-TEM image of only TR SWCNTs is very similar to that of AP SWCNT in terms of the entangled state of the bundles, amorphous carbon, and catalyst remains. This indicates that the TR process alone is not sufficient to effectively remove the impurities from the AP sample. In contrast, TO SWCNTs (Figure 3c) show outstanding change with respect to the purification. Most impurities, including amorphous carbon, appear to have been removed from the SWCNT bundles. We can understand why thermal oxidation is an essential process for the purification of SWCNTs by removing the amorphous carbon. Likewise, for the combination of TO and TR (Figure 3d), the TO-TR SWCNTs display a straighter appearance in structural shape and are more uniform in diameter as compared to the other samples. SWCNTs with very thin diameters are found at several sites in the image of Figure 3d. One can explain that the defects on the nanotubes recovered as the purification progressed. Note that TCF made of straight and thin SWCNTs can achieve lower sheet resistance, as reported previously.6 The FE-TEM images demonstrate that the combination of TO and TR is a very effective method to purify SWCNTs, as well as to apply to TCF. Raman Spectrum and Thermal Analysis. Figure 4 shows the Raman spectra excited at 633 nm for the AP, TR, TO, and

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Figure 5. TGA curves of the AP and postheat-treated SWCNTs.

Figure 4. Raman spectra of (a) low frequency and (b) high frequency for AP, TR, TO, and TO-TR SWCNTs, respectively.

TO-TR SWCNTs. Samples for the Raman spectrum were made after dispersion with SDS and centrifugation. From radial breathing mode spectra (Figure 4a), the diameter range of arcdischarge SWCNTs was about 1.26-1.56 nm.17 The peak intensities changed a little before and after heat treatments, and a significant change in the locations of several peaks was not observed, suggesting they are composed of similar SWCNT species (chirality and/or diameter) after heat treatments. In contrast, for the high-frequency range displayed in Figure 4b, the D-band (1350 cm-1) denotes the disorder-induced band of carbon material, and the D/G peak intensity ratio can determine the state of defects on SWCNTs. For the samples shown in Figure 4b, the D/G ratios for AP, TR, TO, and TO-TR SWCNTs were 0.10, 0.08, 0.04, and 0.03, respectively. It demonstrated that thermal oxidation markedly reduced the defects of pristine SWCNTs. As the plausible explanations, we considered two aspects. First, the nanotubes with many defects might be destroyed after thermal oxidation and be eliminated by centrifugation. Second, the defects on SWNTs might be recovered due to the heat annealing. In any way, the D/G peak ratio for the final nanotubes after TO-TR treatment was reduced to 25% of that of the as-produced sample. As for the G-band of the Raman spectrum, the peaks have a Breit-Wigner-Fano line shape for all samples, as expected. From the G- feature (around 1575 cm-1) in the G-band, metallic portions of the heat-treated samples showed greater increase than that of AP SWCNTs. It

Figure 6. Sheet resistance of the SWCNT films as a function of transmittance of the AP and postheat-treated SWCNTs.

seemed that this change related to the enhancement of TCF conductivity. Yet, further study was needed to reveal the exact reason. The TGA results are presented in Figure 5. Samples were taken in their original powder condition without dispersion and centrifugation. The experiments (sample amount, 10 mg) were performed in air (air flow rate, 20 mL/min) at a heating rate of 10 °C/min. The TGA profile of AP SWCNTs presents a residual weight of around 58%, attributable to catalyst nanoparticles that had not been removed.2 The AP SWCNTs began oxidization at 380 °C, and the slight weight loss is due mainly to the removal of amorphous carbon on SWCNTs. For the overall temperature range, SWCNTs treated with the TR process alone experienced much more weight loss than those treated with the TO process, indicating that much more impurities are removed by TO. Notably, the residual weight increased slightly in the range of 600-800 °C through the TR process after TO. The incorporation of oxygen into the formation of the metallic oxides at 500 °C seems to have had occurred. Moreover, it was noteworthy that no weight loss appeared in the temperature range below 500 °C, which was attributable to the preliminary elimination of most

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Figure 7. Optical image of water droplets on SWCNT TCF film. The contact angle increased with the following processes: (a) AP, (b) TR, (c) TO, and (d) TO-TR.

impurities, including carbonaceous materials. The weight loss of SWCNTs after TO-TR occurred at higher temperatures than other processes because no impurities existed to be burned away. Sheet Resistance and Contact Angle. Figure 6 shows the sheet resistance of the SWCNT films as a function of transmittance of the AP and postheat-treated SWCNTs. As expected, the sheet resistance of these films increased reasonably with increasing transmittance (decreasing film thickness) regardless of postheat treatment.18 The thin conductive SWCNT film prepared with AP SWCNTs exhibited a sheet resistance of 5380 Ω/sq at 80% optical transmittance. For the films prepared with only the TR process, TO, and the combined TO and TR process, the sheet resistance decreased drastically from 2465 to 540 and 187 Ω/sq, respectively, at 80% optical transmittance. As a major reason for the huge reduction in sheet resistance between AP and TO, we consider the removal of nonconductive impurities during TO and TO-TR. We believe that defect recovery, increase of metallic portion, and morphology change into straight shape would be other reasons to reduce the sheet resistance of TCF during the TO and TO-TR process. This result suggests the strong networks of individual nanotubes and nanotube bundles with well-defined and aligned channels were formed after postheat treatment, which improved the conductivity of SWCNT films significantly. Figure 7 displays images of water droplets on TCF film. Similar conditions were maintained for measuring the contact angles. The hydrophobicity of the sample increased sequentially in the order of (a) AP, (b) TR, (c) TO, and (d) TO-TR. Change in contact angle was attributed mainly to surface chemistry changes, with the progressive elimination of C-H bonds caused by heat treatment.19 In the respects of TCF, contact angle information of the surface would be closely related to the humidity effect and dispersion condition. The contact angle increased markedly after TO, which helped to recover the intrinsic property of the SWCNTs.

Conclusion Highly conductive TCF films were prepared with highly purified SWCNTs, and the purification effects were analyzed using Raman spectroscopy, FE-SEM, FE-TEM, TGA, and contact angle measurement. The comprehensive electrical and optical performance, especially the conductivity, could be improved significantly after a combination of TO and TR processes due to the effective removal of impurities and healing of the defects. As a result of the TO-TR process, the sheet resistance of TCF decreased dramatically from 5380 to 187 Ω/sq at 80% optical transmittance and a wavelength of 550 nm. These results led us to conclude that the TO-TR process can play an important role in achieving high-quality SWCNTs and high conductivity of TCF as well. Acknowledgment. This work was financially supported by the Center for Nanoscale Mechatronics & Manufacturing, one of the 21C Frontier Program. We thank Dr. Lim in KIMM for the measurement of the contact angle. References and Notes (1) Smalley, R. E.; Dresselhaus, M. S.; Dresselhaus, G.; Avouris, P. Carbon Nanotubes: Synthesis, Structure, Properties and Applications; Springer: Berlin, 2001. (2) Kaempgen, M.; Duesberg, G. S.; Roth, S. Appl. Surf. Sci. 2005, 252, 425. (3) Wang, W.; Fernando, K. A. S.; Lin, Y.; Meziani, M. J.; Veca, L. M.; Cao, L.; Zhang, P.; Matrin, M. K.; Sun, Y.-P. J. Am. Chem. Soc. 2008, 130, 1415. (4) Montoro, L. A.; Rosolen, J. M. Carbon 2006, 44, 3293. (5) Ma, W.; Song, L.; Yang, R.; Zhang, T.; Zhao, Y.; Sun, L.; Ren, Y.; Liu, D.; Liu, L.; Shen, J.; Zhang, Z.; Xiang, Y.; Zhou, W.; Xie, S. Nano Lett. 2007, 7, 2307. (6) Paula, S.; Kim, D. W. Carbon 2009, 47, 2436. (7) Geng, H.; Kim, K. K.; Lee, K.; Kim, G. Y.; Choi, H. K.; Lee, D. S.; An, K. H.; Lee, Y. H. Nano 2007, 2, 157. (8) Choi, S. K.; Lee, S. B. Curr. Appl. Phys. 2009, 9, 658.

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(9) Gong, Q. M.; Li, Z.; Wang, Y.; Wu, B.; Zhang, Z.; Liang, J. Mater. Res. Bull. 2007, 42, 474. (10) Bhavin, B.; Giovanni, F.; Goki, E.; Manish, C. Appl. Phys. Lett. 2007, 90, 121913. (11) Geng, H.; Kim, K. K.; So, K. P.; Lee, Y. S.; Chang, Y.; Lee, Y. H. J. Am. Chem. Soc. 2007, 129, 7758. (12) Jackson, R.; Domercq, B.; Jain, R.; Kippelen, B.; Graham, S. AdV. Funct. Mater. 2008, 18, 2548. (13) Tantang, H.; Ong, J. Y.; Loh, C. L.; Dong, X.; Chen, P.; Chen, Y.; Hu, X.; Tan, L. P.; Li, L.-J. Carbon 2009, 47, 1867. (14) Wang, J.; Sun, J.; Gao, L.; Liu, Y.; Wang, Y.; Zhang, J.; Kajiura, H.; Li, Y.; Noda, K. J. Alloys Compd. 2009, 485, 456.

Woo et al. (15) Hata, K.; Futaba, D. H.; Mizuno, K.; Namai, T.; Ymura, M.; Iijima, S. Science 2004, 306, 1362. (16) Zhang, G.; Mann, D.; Zhang, L.; Javey, A.; Li, Y.; Yenilmez, E.; Wang, Q.; McVittie, J. P.; Nishi, Y.; Gibbons, J.; Dai, H. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 16141. (17) Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Jorio, A. Phys. Rep. 2005, 409, 47. (18) Gruner, G. J. Mater. Chem. 2006, 16, 3533. (19) Mattia, D.; Rossi, M. P.; Kim, B. M.; Korneva, G.; Bau, H. H.; Gogotsi, Y. J. Phys. Chem. B 2006, 110, 9850.

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