Purification and Dissolution of Carbon Nanotube Fibers Spun from the

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Purification and Dissolution of Carbon Nanotube Fibers Spun from Floating Catalyst Method Thang Q. Tran, Robert James Headrick, E. Amram Bengio, Sandar Myo Myint, Hamed Khoshnevis, Vida Jamali, Hai Minh Duong, and Matteo Pasquali ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09287 • Publication Date (Web): 29 Sep 2017 Downloaded from http://pubs.acs.org on September 29, 2017

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ACS Applied Materials & Interfaces

Purification and Dissolution of Carbon Nanotube Fibers Spun from Floating Catalyst Method Thang Q. Tran,‡, † Robert J. Headrick,‡, § E. Amram Bengio,§ Sandar Myo Myint,† Hamed Khoshnevis,† Vida Jamali§, Hai M. Duong, *,† and Matteo Pasquali*,§ †Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, EA-07-05, Singapore 117575, Singapore §Department of Chemistry, Department of Chemical & Biomolecular Engineering, Department of Materials Science & NanoEngineering, The Smalley Institute for Nanoscale Science & Technology, Rice University, Houston, Texas 77005, United States KEYWORDS: carbon nanotube fibers, superacid, tactoids, mechanical strength, electrical conductivity, extensional viscosity.

ABSTRACT: In this study, we apply a simple but effective oxidative purification method to purify carbon nanotube (CNT) fibers synthesized via a floating-catalyst technique. After the purification treatment, the resulting CNT fibers exhibited significant improvements in mechanical and electrical properties with an increase in strength, Young’s modulus, and electrical conductivity by approximately 81%, 230%, and 100%, respectively. With the successful dissolution of the CNT fibers in superacid, an extensional viscosity method could be applied to measure the aspect ratio of the CNTs constituting the fibers while high purity CNT thin films could be produced with a low resistance of 720 (Ω/sq) at a transmittance of 85%. This work suggests that oxidative purification approach and dissolution process are promising methods to improve the purity and performance of CNT macroscopic structures.

many potential applications. Although these impurities can be reduced by controlling the concentrations of catalysts and chemicals used in the feedstock during the synthesis process4, the high throughput, which is the primary advantage of the floating catalyst method, can be impaired21. Alternatively, the impurities within the CNT fibers can be eliminated by purification6,7,21,22. Purification methods of unstructured CNT powders have been well studied and documented23. Several of these methods require ultrasonication, filtration, and centrifugation steps, and may not be applicable to the macroscopic CNT assemblies due to lack of scalability and because they can be detrimental to the CNT assembly morphology and integrity11. Oxidative purification has been widely used for purifying CNT powders, due to relative simplicity, cost efficiency, and scalability23. Liquid phase oxidation uses acids or bases or oxidation in the gas phase employing air, oxygen, or other gases, and may be followed by washing with non-oxidizing acids such as hydrochloric acid (HCl)11. While liquid phase oxidation methods have been applied successfully to purify and significantly enhance the CNT fibers’ performance6,7,21, research on the purification of CNT fibers employing gas phase oxidation has not yet been studied, especially for CNT fibers spun from the floating catalyst method. The dissolution of CNTs in superacids, of which chlorosulfonic acid (HSO3Cl) is a true solvent for CNTs24, has received increasing attention due to the ability to achieve wellcontrolled CNT morphologies24 and further fabricate many different macroscopic CNT structures1,9. While the dissolution of CNT powder in superacid has been widely studied24, there is no published research on the dissolution of CNT fibers spun from floating catalyst method. Moreover, characterization of CNT length and CNT aspect ratio of direct spun fibers is very

1. INTRODUCTION In order to leverage the superior mechanical, electrical, and thermal properties of individual carbon nanotubes (CNTs) for novel macroscale applications, many CNT structures with controlled morphologies such as continuous CNT fibers1-8, CNT films5,9-11, and CNT aerogels12,13, have been developed. With their 1D structures and aligned morphologies, CNT fibers better preserve the excellent anisotropic properties of individual CNTs6,14. Therefore, they possess desirable multifunctional properties and have great potential for a broad range of applications, such as structural reinforcements13, supercapacitors10, flexible heaters5, medical devices15,16, and lightweight electric cables6,17,18. Continuous CNT fibers can be produced from three main methods: spinning from CNT solutions1,8, dry-state spinning from a CNT array 3, and direct spinning via the floating catalyst method2,5,6,12,19. Compared to the first two methods, the floating catalyst method shows great advantages in terms of manufacturing cost, time consumption, and mass production of continuous high-quality CNT fibers with controlled morphology by a one-step process 20. However, while the first two methods can fabricate relatively clean CNT fibers, the fibers produced from the floating catalyst method contain impurities such as metallic catalyst particles from the synthesis process, and carbonaceous species such as amorphous carbon, fullerenes, multi-shell carbon nanocapsules, and nanocrystalline graphite4. This is due to the single-step fabrication procedure of the CNT fibers where all the feedstock passes through the reactor simultaneously along with the product. The presence of residual impurities within the CNT fibers not only lowers their properties but also limits their use in 1

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limited26, although such CNT properties play crucial roles on the fiber performance. In this paper, we apply an oven oxidative purification approach using air, followed by HCl washing, to purify CNT fibers spun from floating catalyst method and improve their electrical and mechanical performance. We also dissolve the CNT fibers in superacid to study the phase behavior of the constituent CNTs and characterize their length via extensional viscosity25. We also introduce a simple but effective method to transform CNT fibers into highly pure, transparent, and conductive CNT thin films. The experimental results suggest that direct-spun CNT fibers could be a useful material source for fabricating other high performance CNT macroscopic structures with high purity for many advanced applications. 2. EXPERIMENTAL SECTION 2.1 Materials Ferrocene, thiophene, ethanol, and acetone were purchased from Sigma-Aldrich Co. Ltd. (Singapore). Methane (CH4), hydrogen (H2), nitrogen (N2), and helium (He) were purchased from Chem-Gas Pte. Ltd. (Singapore). HCl 37% was obtained from EMD Millipore Corporation (USA). Diethyl ether and chlorosulfonic acid (CSA) 99% were purchased from Sigma Aldrich (USA). All the chemicals were used as received. 2.2. Synthesis of CNT fibers Continuous CNT fibers were fabricated with a horizontal CVD reactor at 1100–1200 oC using the floating-catalyst method7. In the fabrication process, a pre-heated feedstock consisting of toluene (carbon source), ferrocene (catalyst precursor), thiophene (promoter), and hydrogen (carrier gas) with flow rates of 1-2 g/h, 100–400 ml/min, 10–20 ml/min, and 1000–2000 ml/min, respectively, was injected into a reactor to form CNT aerogels continuously. The CNT aerogels were mechanically pulled out by a steel rod and spun continuously with a motorized winding system at a winding rate of 15 m/min. The pulling-out process oriented the CNTs into fibers with good alignment along the fiber axis. A sprayer was used to spray ethanol on the as-prepared fibers to enhance CNT packing density before the fibers were collected by the winder. 2.3 Purification of CNT fibers As-spun CNT fibers were first oxidized in air at 500oC for 12h to burn out carbonaceous impurities. The oxidized fibers were then immersed in HCl 37 % to dissolve iron catalyst impurities. This process was repeated several times until the acid remained clear after fiber immersion. The sample was finally neutralized with deionized water and dried in an oven at 120oC to remove all residual water. 2.4 CNT solution and CNT capillary preparation CNT fibers and chlorosulfonic acid (CSA) were mixed in a glass vial using a speed mixer (DAC 150.1 FV-K, Flack Tek Inc) for 2 hours at 2350 rpm. The CNT solutions were then flame sealed in rectangular glass capillaries (Electron Microscopy Sciences (USA)) with a height of 0.1 mm and a width of 1 mm for observation under a polarized microscope. 2.5 Aspect ratio characterization CNT aspect ratio was measured by an extensional viscosity method developed by Tsentalovich et al.25. The extensional

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viscosity of CNT solutions was measured using a Trimaster capillary thinning rheometer. The rheometer consists of two pistons with a diameter of 1.2 mm. During the test, they were moved apart axially from an initial separation of 0.6 mm to stretch CNT solutions. The solutions were loaded between the two pistons uniformly by using a stainless steel spatula. The stretching rate was 25 mm/s and the stretching distance was 1.0 mm for all samples. Videos of the capillary thinning tests were recorded using a Fastec Troubleshooter HR high-speed camera (1000 frames/s). To determine the midplane diameter of the thinning fluid filament as a function of time, Matlab software was employed to analyze the filament profile. Due to the hygroscopic nature of CSA, all the tests were performed in a drybox that was continuously purged with dry air to keep the relative humidity level below 10%25. 2.6 Fabrication of CNT film To fabricate CNT films, a CNT solution was first sandwiched between two glass slides. Then, one glass slide was slid over the other to well distribute the CNT solution over the glass slide surfaces and align the CNTs in the solution along the sliding direction. After that, the CNT glass slides were immersed into diethyl ether bath for 3 min for coagulation, followed by a drying process in an oven at 150oC. 2.7 Characterization and analysis The structures of CNT fibers were investigated and measured with a field emission scanning electron microscope (FE-SEM, Model S-4300, Hitachi) and a transmission electron microscope (TEM, JEOL2100F). The structures and surface roughness of CNT thin films were characterized using an atomic force microscope (AFM, Bruker Multimode 8 Instruments, Bruker Corporation). A polarized microscope (Zeiss Axioplan optical light microscope) was used to observe the CNT capillaries to determine the isotropic cloud point and phase behavior of the CNT solutions. Raman spectra were obtained with a Renishaw InVia Confocal Raman microscope at 514 nm excitation wavelength. A thermogravimetric analysis (TGA; Shimadzu DTG60H) was conducted in synthetic air (oxygen 21.5%, water 5.00 ppm), from room temperature to 1000 °C, with a heating rate of 10 °C/min. Tensile testing of CNT fibers was performed on a fiber tensile tester (XS(08)X-15, Shanghai Xusai Co.) with an extension rate of 1.2 mm/min and a gauge length of 10 mm. The electrical conductivity of the CNT fibers was measured by an Agilent U1241B multimeter, based on a two-probe method. The transmittance of CNT films at the wavelength of 550 nm was measured by a Uv-vis spectrometer (Shimadzu UV-1800) while the film resistance was measured by a linear four-point probe device (Jandel model RM3-AR). 3. RESULT AND DISCUSSION 3.1. Purity evaluation of CNT fibers Figure 1 compares TEM, Raman, and TGA results of the asspun and purified CNT fibers produced by the floating catalyst method. Analysis of the TEM images shown in Figure 1a and 1b suggests that the as-spun CNT fibers consist of about 81% double-walled carbon nanotubes (DWNTs) and 19% multiwalled nanotubes (MWNTs) with an average diameter of 5.5 nm. Many iron particles encapsulated by graphitic layers could be observed in the fiber structure. However, after the purification process was applied, the CNT fibers were much 2


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cleaner with almost no iron impurities observed in the TEM image (Figure 1c).

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5nm Figure 1. (a),(b) TEM images of as-spun CNT fibers, (c) TEM images of purified CNT fibers, (d) Damaged CNT due to purification, (e)TGA results, and (f) Raman results of as-spun and purified CNT fibers.

In TGA analysis of CNT samples, water and other adsorbed species are removed from CNT surfaces at temperatures below 3500C while the weight loss between 3500C and 4000C corresponds to the removal of amorphous and disordered carbon from the sample7,27. CNTs are generally oxidized at above 4000C7,27. According to the TGA result in Figure 1e, the amount of carbonaceous impurity in the purified CNT fibers was below 1%; HCl purification reduced iron impurities from 12% to 4%. This finding was supported by the significant temperature upshift (50°C) of the CNT oxidative decomposition after the purification treatment, as shown in Figure 1e. Since the presence of metal catalysts (Fe) lowered the activation energy and catalyzed the parasitic oxidation of CNTs27, reducing the iron inclusions improved the thermal stability of the CNT fibers after the purification process. These results are consistent with the improved thermal stability of purified CNT sheets reported by Lin et al.11. Both TEM and TGA suggest that the purification process using air and HCl washing effectively removed the iron and carbonaceous impurities from the CNT fibers spun from floating catalyst method. As the catalyst impurities in the asspun CNT fibers were encapsulated by carbon shells, they could not be removed by simple acidic washing. However, the oxidation by air of these protective multiple carbon shells was catalyzed by the metallic particles, lowering their oxidation resistance11,23. Due to the oxidation resistance difference between CNTs, amorphous carbon, and multishell carbon nanocapsules, the catalyst particles were exposed and could be dissolved in the hydrochloric acid. Thus, the purification process yielded CNT fibers with low impurity content. In the

Raman spectra of CNT fibers observed in Figure 1f, the G band at ~1590 cm-1 arises from the in-plane vibrations of sp2hybridized graphitic carbon27 while the disorder D band, located at ~ 1340 cm-1, corresponds to defects and functional groups on the walls or ends of the CNTs, or amorphous carbon. Therefore, the intensity ratio of the D-band to the Gband (ID/IG) is a measure of the extent of defects present on the CNTs27. The ID/IG ratio of the CNT fibers increased from 0.17 to 0.56 after the purification process (Figure 1f), indicating that purification may have introduced more defects into the CNT structures. A purification-induced increase in defects in CNT structures may occur when oxidation attacks defective sites on the CNTs themselves, although CNTs resist oxidation better than amorphous carbon27. This purification method also decreased the number of CNTs and left surviving CNTs with damage such as lacking part of the outer wall and caps at the ends (Figure 1d), leading to higher defect density23. These findings are in good agreement with the increasing defects on CNT structures of CNT sheets reported by Lin et al.11 and CNT powders reported by Osswald et al.27 after their air oxidative purification process was applied. Therefore, it is important to carefully design and optimize the oxidative treatment to balance impurity removal requirements, CNT structure preservation, and improved performance of CNT fibers for applications. 3.2 Morphologies, mechanical, and electrical properties of CNT fibers Figure 2 shows surface morphologies, mechanical, and electrical properties of the as-spun and purified CNT fibers. 3



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As can be seen, the as-spun CNT fibers had an average diameter of 8 ± 0.2 μm, consisting of aligned CNTs along the fibers axis (Figures 2a and c). After the purification process, the diameter of the CNT fibers decreased by nearly 19%, reaching 6.5 ± 0.18 μm (Figure 2b). This diameter reduction of the CNT fibers is attributed to the removal of CNT impurities. More interestingly, the good alignment of CNTs and CNT bundles of the purified CNT fibers shown in Figure 2d indicates that the purification process preserved fiber structure. Furthermore, the larger size of the CNT bundles observed in Figure 2d after the purification process evidently supports the positive effects of the impurity removal, resulting in better van der Waal’s interactions between CNTs and CNT (b) after the removal of the (a) bundles. Due to the volume shrinkage internal impurities, a strong condensation of CNTs inward manifested as a wrinkled surface morphology (Figure 2d).

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The tensile strength and Young’s modulus of the as-spun fibers were 0.32 GPa and 8.41 GPa, respectively (Figure 2e and 2f). While the purification process might damage CNTs by introducing defects, the significant reduction of the CNT fiber diameter after purification contributes positively to the fiber strength, leading to the improvement of their mechanical and electrical properties. Based on the diameter reduction and hence the cross-sectional area reduction by a factor of 1.5 after purification, we expect that both the conductivity and the mechanical properties would change by the same factor. The mechanical performance of the CNT fibers significantly improved with a tensile strength and Young’s modulus of 0.38 GPa and 20.66 GPa, respectively, corresponding to 119% and 246% of those of the as-spun CNT fibers. However, due to the reduction of the fiber diameter, the maximum tensile force before fiber breakage reduces slightly from 1.6 cN to 1.2 cN. More interestingly, the electrical conductivity of the CNT fibers also showed a significant enhancement of nearly 100%, from about 2,400 S/cm to 4,600 S/cm. These impressive improvements in CNT fibers properties might stem from the fact that the denser and cleaner structures of the CNT fibers after purification enhanced the interactions between CNTs and CNT bundles as well as their contact areas. Hence, the efficient purification of CNT fibers led to significant enhancements in their electrical and mechanical performance. These findings are consistent with the better morphologies and properties of the CNT sheets and twisted CNT fibers obtained by purification using air oxidation methods11,22. 3.3 Dissolution of CNT fibers in superacid 3.3.1 Phase behavior of CNT solution

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Figure 3. Transmitted light microscope images of CNT solutions of (a) as-spun CNT fibers and (b) purified CNT fibers.

Figure 3 shows optical micrographs of the solutions of the as-spun and purified CNT fibers in CSA. As shown in Figure 3a, the solution of as-spun CNT fibers was inhomogeneous with many black CNT bundles observed, indicating that the fibers were not fully dissolved in superacid. In contrast, the solution of purified CNT fibers (Figure 3b) showed almost no CNT bundles or aggregates, indicating that the CNT fibers were successfully dissolved in CSA. The improved solubility of the CNT fibers after purification might be due to the removal of significant impurities in the fibers. According to TGA and TEM, the as-spun CNT fibers contained 1% of carbonaceous impurities stuck on the CNTs and 12% of catalyst impurities encapsulated by graphitic shells. Carbonaceous impurities hinder CNT protonation by CSA, resulting in incomplete dissolution of the CNT fibers. Once these impurities were removed, the CNTs in the fibers were accessible to the acid, resulting in good solubility of the CNT fibers in superacid.

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Figure 2. SEM images of as-spun CNT fibers (a), purified CNT fibers (b), surface morphologies of (c) as-spun and (d) purified CNT fibers, stress-strain curves (e), and mechanical and electrical results of the as-spun and purified CNT fibers (f). 4


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Figure 4. Polarized light microscopy images of CNT solutions of purified CNT fibers at (a) 1390 ppm, (b) 1850 ppm, (c) 4700 ppm, (d) 1850 ppm, and the brightness of the 1 day old tactoid at (e) 45◦ and (f) 0◦ and the merging process of two tactoids (g). Crossed arrows show the orientation of crossed polarizers.

the CNTs28. The one-day-old tactoids mostly show a uniform change in brightness when rotated from 0◦ to 45◦ relative to either of the polarizers and uniformly dark at 0o indicative of a homogenous configuration (Figures 4e and 4f). The homogeneous to bipolar transition in this solution is expected to happen for larger tactoids (>100 μm) suggesting the CNTs are long, shifting the transition to larger length scale. 3.3.2 CNT length characterization Figure 5 shows the results obtained from capillary thinning experiments following the method developed by Tsentalovich et al.25 for measuring CNT length from extensional viscosity. The filament profiles in Figure 5a exhibited homogeneous and symmetric fluid about the mid-filament. With increasing concentrations from 2470 ppm to 5700 ppm, the solution extensional viscosity increased from ~0.2 to ~0.4 Pa.s (Figure 5b). The calculated CNT aspect ratio was ~400 and showed no dependence on solution concentration (Figure 5c), in good agreement with the results of Tsentalovich et al.25 The CNT length distribution was estimated by assuming a log-normal distribution and using the measured isotropic cloud point, viscosity-average aspect ratio, and diameter of the CNTs (measured by TEM). The average CNT length was ~ 2 μm, with ~90% of the CNTs shorter than 6 μm (Figure 5c), consistent with the formation of CNT tactoids and the length of the short F-Actin tactoids reported by Oakes et al.29. The length of the CNTs constituting the fibers was longer than many high purity commercial SWNTs and DWNTs such as HiPco 188.3 (0.29µm), HiPco 183.6 (1.51µm), SWeNT CG300 (0.71µm), and UniDym OE (1.92µm)25.

Figure 4 shows polarized light microscopy images of CNT solutions at different concentrations. At 1390 ppm, the CNT solutions were isotropic and they appeared dark (Figure 4a). At slightly higher concentrations (1540 ppm), the appearance of birefringence indicates that the solution is in the biphasic regime, where the isotropic phase coexists with the nematic liquid crystal phase (Figure 4b). The isotropic cloud point defined as the concentration below which the birefringence disappears is located at the midpoint between the most concentrated not birefringent solution (1390 ppm) and the least concentrated birefringent solution (1539 ppm)25 and is about 1430 ppm for our CNT solution. Figure 4c show the formation of a fully nematic liquid crystal phase at higher concentrations (4700 ppm).At the concentration of 1850 ppm, many small spindle-like nematic droplets in coexistence with the isotropic phase were observed in the CNT solution (Figure 4d). These droplets, known as tactoids, are generally formed in solutions of CNTs with a low aspect ratio at intermediate concentrations28. These tactoids grew larger with time and during the growth process, neighboring tactoids coalesce into a single larger tactoid with a similar characteristics of the initial tactoid29, as shown in Figure 4g. Theoretically tactoids are characterized with a bipolar or a homogeneous configuration based on the average orientation of the particles inside the droplet28. In the former configuration, the particles align parallel to the droplet interface while in the latter the particles align parallel to the long axis of the tactoid. The size-dependent transition from homogenous to bipolar configuration has been observed previously in CNT solutions and is dependent on the length of 5



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Figure 6. (a) Optical microscopic image and (b) AFM image of CNT film, and (c) resistance of CNT film at different transmittance value.

4. CONCLUSION In conclusion, CNT fibers directly spun from the floating catalyst method were successfully purified by a two-step approach using high temperature air oxidation and HCl washing. After purification, the resulting fibers exhibited high purity, with less than 5% metal catalyst impurities, leading to significant improvements in their electrical and mechanical properties. The purified fibers presented good dissolution in superacid with different phases observed in the solutions, allowing the successful characterization of CNT length by extensional viscosity. Additionally, highly pristine, transparent, and electrically conducting CNT thin films were fabricated successfully from the CNT solutions. This work demonstrates the potential of purification and dissolution processes to produce fibers and thin films with high purity and improved mechanical and electrical performance from CNTs grown in continuous synthesis reactors.

Figure 5. (a) Filament profiles from capillary thinning experiments for 3800 ppm CNT solution, (b) extensional viscosity and aspect ratio of CNT solution, and (c) probability density function for length data.

Notably, CNTs are likely shortened during the purification process by oxidation at their ends; the average CNT length in the as-spun fibers may be above 2 µm. The CNT length and CNT aspect ratio of the fibers can be improved by optimizing the synthesis process such as controlling the catalyst particle size to reduce the CNT diameters or using a different carbon source, synthesis temperature to increase the CNT growth time, and optimizing the oxidative purification procedure4. 3.3.3 Morphologies and properties of CNT films Figure 6 shows the morphologies and electrical conductivity of CNT films fabricated from the CNT solutions in CSA. The CNT films consisted of partially aligned CNT bundles (Figure 6b). This alignment stems from the shear field formed by the glass sliding in the fabrication process. During this process, the pre-existing liquid domains on the glass slides were stretched and aligned, resulting in the good alignment of the CNTs in the films. The thickness of the CNT films was controlled by the gap between the glass slides and the concentration of the CNT solutions used. The film resistance reached 720 Ω/sq at 85% transmittance (Figure 6c), better than the SWNT films produced by dip-coating30, 31 and linepatterning32, but not as good as earlier work on CSA-based dip coated films9. As these properties are expected to improve with CNT quality and aspect ratio, improved CNT growth in continuous CVD process reactors may lead to better CSAprocessed conductive CNT thin films for many practical applications, especially flexible electronic devices.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (H.M.D.), *E-mail: [email protected] (M.P.)

Author Contributions ‡T.Q.T and R.J.H contributed equally in this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors would like to thank Temasek Laboratory@NUSSingapore (R-394-001-077-232 and R-265-000-514-28), the Air Force Office of Scientific Research (AFOSR) grant FA9550-15-10370, and the Welch Foundation grant C-1668 for financial support. RJH was supported by a NASA Space Technology 6


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Research Fellowship (NSTRF14), grant number NNX14AL71H. We thank Oliver Dewey for taking AFM image of the CNT film.

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Purification and Dissolution of Carbon Nanotube Fibers Spun from the

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