Post-Treatments for Multifunctional Property Enhancement of Carbon

Mar 11, 2016 - As a consequence, the CNT fibers had a large diameter (∼56 um) and low mechanical properties, with a tensile strength of only 0.052 G...
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Post-Treatments for Multifunctional Property Enhancement of Carbon Nanotube Fibers from the Floating Catalyst Method Thang Q. Tran,† Zeng Fan,† Anastasiia Mikhalchan, Peng Liu, and Hai M. Duong* Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, EA-07-05, Singapore 117575, Singapore S Supporting Information *

ABSTRACT: We investigated the effects of the synthesis conditions and condensation processes on the chemical compositions and multifunctional performance of the directly spun carbon nanotube (CNT) fibers. On the basis of the optimized synthesis conditions, a two-step post-treatment technique which involved acidification and epoxy infiltration was also developed to further enhance their mechanical and electrical properties. As a result, their tensile strength and Young’s modulus increased remarkably by 177% and 325%, respectively, while their electrical conductivity also reached 8235 S/cm. This work may provide a general strategy for the postprocessing optimization of the directly spun CNT fibers. The treated CNT fibers with superior properties are promising for a wide range of applications, such as structural reinforcements and lightweight electric cables. KEYWORDS: carbon nanotube fibers, acid treatment, epoxy infiltration, post-treatment, mechanical strength, electrical conductivity

1. INTRODUCTION In the interest of transferring the remarkable properties of individual carbon nanotubes (CNTs) to real-life applications, their assembly on a macroscopic scale has therefore attracted great attention in the past few decades.1 Among the various assemblies developed, such as CNT arrays,2 buckypapers3/ films,4−7 and aerogel,8 CNT fibers9−15 undoubtedly better preserve the 1D characteristic of individual nanotubes, and provide more design-friendly abilities for industrial utilization. The CNT fibers with enhanced multifunctional properties show great potential for a wide range of applications, such as flexible medical devices,1,16−19 structural reinforcements,20,21 and lightweight electric cables.22 By now, significant progress has been made in the fabrication of CNT fibers via wet chemistry,9,10 array-drawing,11,12 and direct growth13−15,23,24 approaches. In general, the wet spinning method can produce highperformance CNT fibers, but the involvement of a matrix phase (polymer or acid) is usually unavoidable.9,10 The array-drawing method, which provides strong and relatively clean fibers and films, has been reported by Jiang et al.25 and Zhang et al.26 Although the as-produced CNT products could be transparent and highly conductive, they still appear to be unfavorable for a large-scale fabrication, given the limited wafer size.1 Alternatively, the direct growth method, on the basis of the aerogel technique, has been considered a significant step toward scalable fiber production.27 While this method is widely used to synthesize aligned CNTs,28,29 it has been reported that the CNT fibers directly spun from this method commonly possess © 2016 American Chemical Society

high mechanical properties with good electrical conductivities.20 When the effects of gauge length (GL) are neglected, their strength and Young’s modulus could reach up to 3.24 and 357 GPa (at a 1 mm GL), respectively, after a simple acetone densification,15 whereas their electrical conductivities are on the order of ∼103 S/cm.13,24,30,31 Recently, Gspann et al.32 reported a comprehensive study of the floating chemical vapor deposition (CVD) process, in which purity issues and the roles of sulfur were highlighted. Nevertheless, a detailed investigation of the carrier gas flow may also be vital. Notably, the determination of the optimal synthesis parameters is a critical factor for the direct growth method, which can significantly alter the chemical composition and multifunctional properties of CNT fibers. CNT properties can be improved through the optimization of the synthesis process, to control their type and diameter,15,32 with minimum impurities,9,32 or through their annealing,33 to remove defects. In addition, post-treatments can be further conducted on the as-spun CNT fibers to maximize their fiber performance. Introducing twisting,12,34 stretching,27,35 or pressing27 can effectively compact the CNTs, thus potentially leading to enhancements in their macroscopic properties. However, these physical treatments are not capable of influencing the inter-CNT interactions. As the CNTs within the fiber are still contacted by the intrinsically weak van der Received: October 18, 2015 Accepted: March 11, 2016 Published: March 11, 2016 7948

DOI: 10.1021/acsami.5b09912 ACS Appl. Mater. Interfaces 2016, 8, 7948−7956

Research Article

ACS Applied Materials & Interfaces

while methane, ferrocene, and thiophene were injected at feeding rates of 160, 250, and 20 mL/min, respectively, and controlled to be similar across all syntheses. 2.3. Liquid Condensation of CNT Fibers. To investigate the effect of liquid condensation on the mechanical performance of the ascollected CNT fibers, both (i) on-line and (ii) off-line condensation processes were implemented and compared in this work. For the online condensation process, ethanol was mixed with N2 and directly sprayed onto the CNT fibers during fiber winding, while, for the offline condensation process, ethanol was manually sprayed onto the ascollected CNT fibers via a sprayer. 2.4. Post-Treatment of CNT Fibers. To further enhance the multifunctional properties of the as-spun CNT fibers, a two-step postprocessing technique, consisting of acid treatment and epoxy infiltration, was developed. First, the as-spun CNT fibers, after ethanol condensation, were immersed in 65 wt % HNO3 at room temperature, for purification and functionalization. A series of treatment times, 15 min and 0.5, 1, and 2 h, respectively, were performed to optimize the acid treatment process. After that, the CNT fibers were washed with deionized (DI) water three times, and dried in the ambient conditions overnight. For the epoxy infiltration step, Epicote 1004 resin and hardener were first mixed at a weight ratio of 5:2, as recommended by the supplier. To ensure smooth infiltration, the mixed epoxy system was diluted by acetone to form solutions of different weight fractions, namely, 10, 20, 30, and 50 wt %, respectively. For infiltration, the acidtreated CNT fibers were dipped into the solutions for 15 min, and then cured in air at room temperature for 24 h, to form cross-linked CNT fibers. 2.5. Characterization. The diameters of the as-spun, acid-treated, and epoxy-infiltrated CNT fibers were all measured by an optical microscope (Olympus LG-PS2). To determine the diameter of the fibers, we took measurements at 10 different positions along their lengths, which were then averaged. The structure and morphology of the CNT fibers were observed using a field emission scanning electron microscope (FE-SEM S4300, Hitachi) at 15 kV, and a transmission electron microscope (JEOL JEM-3010). To understand the consistency of the CNT fibers synthesized under different conditions, a thermogravimetric analysis (TGA; Shimadzu DTG60H) was done in synthetic air (oxygen 21.5%, water 5.00 ppm), from room temperature to 1000 °C, with a heating rate of 10 °C/min. The tensile behavior of the CNT fibers was evaluated by a fiber tensile tester (XS(08)X-15, Shanghai Xusai Co.) with a gauge length of 10 mm, at a tensile rate of 1.2 mm/min. The electrical properties of the as-spun and acid-treated CNT fibers were determined by an Agilent U1241B multimeter, based on the two-probe configuration.

Waals force, the enhancing effects of the aforementioned treatments on the CNT properties may not be so obvious.36 The surface modification of individual CNTs has shown its effectiveness in creating interfacial bonding and activating CNTs.37 Meng et al.38 reported surface modification of the CNT fibers by a concentrated acid, and demonstrated that such treatment could effectively enhance the strength, electrical conductivity, and volumetric capacitance of the array-drawn fibers by 52%, 128%, and 17%, respectively. Besides that, the existence of numerous functional groups is also critical for generating interfacial bonding between CNTs and various polymeric matrixes.39−41 On the basis of this point, the development of a posttreatment process that combines surface modification and polymer infiltration becomes vital. With an abundant amount of functional groups introduced on the CNT surfaces, a more comprehensive polymer infiltration and chemical bonding can therefore be expected.42,43 To the best of our knowledge, it still remains unclear how such a post-treatment can functionalize and impact the properties of the CNT fibers. As the directly spun CNT fibers have their own unique characteristics, a quantitative evaluation of the effects from each step of the posttreatment (i.e., acid treatment and polymer infiltration) would also be essential. In this work, a comprehensive study was first conducted to investigate the effects of the synthesis conditions, such as the flow rate of the carrier gas, collection speed, and condensation processes, on the multifunctional performance of the directly spun CNT fibers. Aimed at fulfilling their potentials, a two-step postprocessing technique, including acid treatment and epoxy infiltration, was then developed. For the acid treatment step, the optimal treatment time was determined to balance between functionalization and structural damage, whereas, for the epoxy infiltration step, the concentration of epoxy solutions was optimized for the best integration. To identify the respective roles of acid treatment and epoxy infiltration during the posttreatment, the electrical and mechanical properties of the CNT fibers were carefully quantified, and compared step by step to form a strategy for improving their multifunctional properties. A universal strategy was suggested here for improving the multifunctional properties of the CNT fibers, in particular those obtained from a CNT aerogel technique. Furthermore, this work may also provide a basis for realizing the potentials of various CNT assemblies for wide-ranging structural and functional applications.

3. RESULTS AND DISCUSSION 3.1. Liquid Condensation Effects on CNT Fiber Properties. Figure 1a compares the effects of different condensation processes (off-line and on-line ethanol spraying) on the electrical and mechanical properties of the as-prepared CNT fibers. While the off-line condensation process provided the CNT fibers with an electrical conductivity of 700 S/cm, a strength of 0.26 GPa, and a Young’s modulus of 2.3 GPa, the on-line-condensed CNT fibers apparently exhibited higher values, reaching 1325 S/cm in conductivity, a strength of 0.408 GPa, and a Young’s modulus of 14.6 GPa. Such a comparison may apparently verify the greater effectiveness of the on-line condensation process, compared to the off-line one. Accordingly, higher liquid penetration should be considered for a general densification purpose. Furthermore, the size of ethanol droplets also plays a significant role during the on-line spraying process. Both the diameter and strength of the as-obtained CNT fibers have been shown to correlate with the droplet diameter, as it varied from 0.93 to 2.23 mm under controlled conditions of N2 pressure. The larger droplets of ethanol, even resulting in a slight

2. EXPERIMENTS 2.1. Materials. Ferrocene, thiophene, ethanol, acetone, and concentrated nitric acid (HNO3; 65 wt %) were purchased from Sigma-Aldrich Co. Ltd. Methane (CH4), hydrogen (H2), nitrogen (N2), and helium (He) were purchased from Chem-Gas Pte. Ltd. Epicote 1004 epoxy resin and Epicote 1004 hardener were obtained from Polymer Technologies Pte. Ltd. (Singapore). All the chemicals were used as received. 2.2. Synthesis of CNT Fibers. The CNT fibers were synthesized via a floating catalyst−chemical vapor deposition (FC−CVD) method. A mixture of CH4, H2, ferrocene, and thiophene was first injected into the CVD reactor at 1200 °C, while the heated reactor was maintained under a N2 environment. Upon the reaction, a CNT “aerogel” was continuously formed at the heating region and blown out of the reactor by the carrier gas (H2), thereafter being wound onto a rotating spindle to collect the CNT fibers. To maintain synthesis stability and to investigate the formation of carbonaceous or other impurities at a certain synthesis, the H2 flow rate was varied from 1.0 to 2.0 L/min, 7949

DOI: 10.1021/acsami.5b09912 ACS Appl. Mater. Interfaces 2016, 8, 7948−7956

Research Article

ACS Applied Materials & Interfaces

diameter (∼56 um) and low mechanical properties, with a tensile strength of only 0.052 GPa (Table 1). Table 1. Properties of the CNT Fibers Synthesized at Different Carrier Gas Flow Rates processing parameter

characteristic

H2 flow rate, L/min

collection speed, m/min

diameter, μm

tensile strength, GPa

tensile modulus, GPa

1.0 1.0 2.0 2.0

28 34 28 34

55.96 57.70 8.44 8.22

0.052 0.082 0.347 0.354

1.90 2.80 12.90 17.10

electrical conductivity, S/cm 30 2130

The low performance of the fiber spun at 1.0 L/min of H2 may also stem from the poor alignment of their CNTs and CNT bundles along the fiber axis, as shown in Figure 2a. In

Figure 1. (a) Comparison of the electrical and mechanical properties of the off-line- and on-line-condensed CNT fibers, (b) fiber strength as a function of the droplet size, and (c) effect of the droplet sizes on the uniformity of the on-line-densified CNT fibers.

reduction of the fiber diameter, dramaticallyby nearly 50% as shown in Figure 1blowered the fiber strength, thus showing the less effective infiltration and weakened contacts between the CNT bundles in Figure 1b,c. This finding was quite in accordance with the fact that the finer the droplet size, the more uniform the liquid condensation. For the large droplet sizes, e.g., 2.23 mm in diameter as in Figure 1c, due to the droplet distribution being relatively scattered during spraying, the as-condensed CNT fibers evidently presented several inferior links in their structure, caused by their uncondensed parts. Therefore, fibers densified by the smaller sized droplets have a uniform structure with few fair-densified areas, while many poorly densified areas were observed along the fibers densified by large drop sizes (Figure 1c). At these areas, the van der Waals interactions between CNT bundles are weaker, leading to lower mechanical performance of the CNT fibers.1,20 3.2. Synthesis Condition Effects on the CNT Fiber Properties. For the floating catalyst synthesis process, as is commonly known, several processing parameters should be equally balanced to enable satisfactory continuous CNT production. One is the flow rate of hydrogen, used in the process as a reaction medium and a carrier gas.15,20,32 Its stream velocity can significantly determine the transfer of the reactants along the reactor and their retention time in the reaction zone.32,44 During that time, ferrocene, thiophene, and methane decompose in the gas stream, with in situ formation of iron catalyst nanoparticles. Successively, CNTs start nucleating and grow on their surface. Then the growing CNTs entangle and form a so-called “elastic aerogel”, which can be continuously drawn out of the reactor, and densified with spraying ethanol to produce CNT fibers. The experiments on a varied flow rate of H2 have shown hindered condensation for the CNT aerogel with the lowest H2 velocity (1.0 L/min). These synthesis conditions resulted in the abundant formation of a CNT material. Due to this fact, it was not possible to fully densify the CNT fibers with on-line ethanol spraying. As a consequence, the CNT fibers had a large

Figure 2. SEM images of the surface morphology of CNT fibers spun at (a) 1.0 L/min of H2 and a 28 m/min collection speed, (b) 2.0 L/ min of H2 and a 28 m/min collection speed, and (c) 2.0 L/min of H2 and a 34 m/min collection speed.

contrast, increasing the flow rate by 2-fold (up to 2.0 L/min) resulted in better alignments of their CNTs (Figure 2b), leading to a significant enhancement of the mechanical and electrical properties of the CNT fibers, with a 0.347 GPa strength, a 12.90 GPa stiffness, and an electrical conductivity of 2130 S/cm. However, further attempts to increase the collection speed of the fibers onto the roller, from 28 to 34 m/min, even as it showed a positive trend in fiber performance due to the slight increase of the CNT alignment (Figure 2c), revealed the instability of the spinning process for both types of fibers. According to the literature,32 the spinning stability may be hugely affected by the presence of cosynthesized impurities, which may also prevent the complete structural densification of the CNT aerogel into the CNT fiber during liquid condensation. Thus, a detailed morphology analysis of the CNT fibers was done to address their observed properties. 3.3. Morphology of the CNT Fibers. Under different synthesis conditions, a thermogravimetric analysis/differential thermal analysis (TGA/DTA) has shown a significant difference in CNT fiber composition, which may affect their mechanical and electrical performance. At the lowest H2 flow velocity (1.0 L/min), the observed residue in CNTs was at low 7950

DOI: 10.1021/acsami.5b09912 ACS Appl. Mater. Interfaces 2016, 8, 7948−7956

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ACS Applied Materials & Interfaces

observed at the CNT tips (Figure 3a−c), or kept between the inner cylindrical carbon layers in the nanotubes.50 Otherwise, if not yet reacted to form nanotubes, they can be trapped inside the fiber as carbon-encapsulated iron carbide particles.50−52 CNT fibers synthesized at a H2 flow rate of 1.5 L/min were mainly composed of multiwalled CNTs, with an outer diameter of ∼15 nm and approximately 15−20 walls (Figure 3d), with the outer layers formed from a less organized carbon, which was probably amorphous.46 The Raman spectrum showed the intensity ratio of the G (1580 cm−1) and D (1350 cm−1) bands to be approximately 2.4, as reported in our previous study.53 TGA/DTA tests revealed a residue content of 30 wt %, mainly composed of Fe2O3 particles, as confirmed with energydispersive X-ray spectroscopy (EDS) (Supporting Information, S1). This means that the CNT fibers have about 21% metallic iron by weight. Collected at the lowest rotational speed of 28 m/min, and having a diameter of ∼10 μm, the CNT fibers from 1.5 L/min of H2 showed a good balance of spinning stability, together with electrical and mechanical performance. The fibers exhibited a tensile strength and stiffness of 0.41 and 14.60 GPa, correspondingly, which were found to be within the range of those of other methane-based CNT fibers.32 As shown in Figure 4, the CNT fibers produced from the optimal conditions (1.5 L/min H2 flow rate) generally exhibit

levels (8 wt %). However, the CNTs contained much more cosynthesized carbonaceous impurities: 67 wt % versus 24 wt % of high-quality CNT bundles (the descriptive TGA/DTA data are provided in the Supporting Information, section S1). The term “carbonaceous impurities” here encompasses a broad range of short and highly deformed multiwalled carbon nanotubes (MWNTs), which we observed with a highresolution transmission electron microscopy (HRTEM) analysis (Figure 3).

Figure 3. HRTEM images showing (a, b) clusters of short and highly deformed MWNTs predominantly cosynthesized at a 1.0 L/min H2 flow rate, (c) an iron particle at the MWNT tip, and (d) a typical MWNT synthesized at 1.5 L/min of H2. The red dashed line in (d) highlights the outer layer formed by less organized (amorphous) carbon.

Figure 4. SEM image of the as-spun CNT fiber (a) and its surface morphology (b) spun at 1.5 L/min of H2.

good CNT alignment, being parallel to the fiber axis, and typically had a diameter of ∼10 μm after on-line condensation. Henceforth, the CNT fibers from such synthesis conditions will be referred to as the as-spun CNT fibers. The detected impurities confirmed the importance and necessity of posttreatments to fulfill the multifunctional properties of the CNT fibers, which we will discuss below. Also, the observed ability to collect CNT fibers (1.5 L/min of H2) at a dramatically high speed range (40−90 m/min) could also lead to alternative perspectives in process optimization through both methods, reducing the catalyst residue content and reaching better alignment of CNT bundles.49 By only regulating the spinning recipes and condensation processes, the multifunctional properties of the as-spun CNT fibers were still largely hindered by the weak van der Waals force; post-treatments are therefore required for further property enhancements. 3.4. Post-Treatments for Enhancing the Mechanical Properties of CNT Fibers. 3.4.1. Effect of Acid Treatment. In this work, concentrated HNO3 was applied as the first step to modify the as-spun CNT fibers. Figure 5 compares the surface morphology of CNT fibers before and after acid treatment, and the effect of different treatment times (ranging from 15 min to 2 h) on their tensile strength and Young’s modulus. Regarding the stacking of CNT bundles in the CNT fibers, the scanning electron microscopy (SEM) image in

Parts a and b of Figure 3a and b show the TEM images of various carbonaceous impurities abundantly cosynthesized at a 1.0 L/min H2 flow rate, together with CNTs of large diameter. It is known from the literature that the CNT diameter correlates well with the size of the catalyst particle.32,45−47 In addition, it was shown that the catalyst particles are able to grow by collision and coalescence during their transfer along the reactor.32,48 MWNTs of large diameter were observed by McKee et al.46 for syntheses with increased growth times. Moreover, Gspann et al.32 evidenced that, at longer retention times, large iron particles formed through collision were able to grow their own nanotubes to a length sufficient for them to entangle with a CNT aerogel, resulting in large amounts of carbonaceous impurities within the fiber.15,32,49 From this point of view, the presence of big clusters of impurities (up to several micrometers in size) may obstruct on-line densification, thus leading to a large diameter and low mechanical performance of the CNT fibers. In contrast, for a higher H2 flow rate (2.0 L/min), TGA/ DTA results have shown a significant reduction in the amount of carbonaceous impurities, from 67 to 14.5 wt %, at the same time competitively increasing the catalyst residue content by up to 38 wt % (Supporting Information, S1). Iron particles can be 7951

DOI: 10.1021/acsami.5b09912 ACS Appl. Mater. Interfaces 2016, 8, 7948−7956

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ACS Applied Materials & Interfaces

Figure 5. (a) SEM images of the surface morphology of the CNT fiber before and after acid treatment and (b) effect of the acid treatment time on the mechanical properties of the CNT fibers.

Figure 6. (a) SEM images of the surface morphology of CNT fibers before and after the epoxy infiltration treatment and (b) effect of the epoxy concentration on the mechanical properties of the CNT fibers.

readily formed during CNT acidification, and removed by the subsequent aqueous washing.37,55 However, as shown in Figure 5b, further prolonged treatment beyond 15 min could not offer further improvements, due to the destruction of the CNT structures.36,38 As discussed elsewhere,38,54 the competition between enhanced interfacial shearing and destroyed CNT structures would finally determine the mechanical behavior of the CNT fibers. The acid treatment hence needs to be carefully controlled, in terms of acid concentration and treatment time, for optimal purposes. 3.4.2. Effect of Cross-Linking. Functionalizing CNTs has been shown to be effective in improving the CNT dispersion in various epoxide-containing polymers.39,40 Herein, epoxy infiltration was applied as the second step of post-treatment, and performed on the CNT fibers from the best acidification condition (15 min). Given the high viscosity of the epoxy used, a set of diluted epoxy solutions (10, 20, 30, and 50 wt %) was prepared to ensure effective infiltration. As shown in Figure 6a, the epoxy was well-infiltrated into the CNT bundles, and fewer pores are observed, compared to the surface morphology of the fiber before the treatment. Figure 6b compares the tensile strength and Young’s modulus of these epoxy-infiltrated CNT fibers as a function of the epoxy fraction in solutions. Due to the cross-linking formed by the epoxy matrix,23 all the epoxy-infiltrated CNT fibers were found to be much stronger than the as-spun and acid-treated fibers. The highest tensile strength of 1.132 GPa and stiffness of 62.0 GPa were respectively obtained for the CNT fibers infiltrated by the 30

Figure 5a suggests that the acid treatment may increase the CNT bundle sizes and inter-CNT contacts, resulting in an increase in their bearing load, and improved mechanical properties. It can be noted that the as-spun CNT fibers only exhibited an average tensile strength and Young’s modulus of 0.408 ± 0.054 and 14.6 ± 2.7 GPa, respectively, while the acid treatment significantly increased their mechanical performance. The highest strength and Young’s modulus of CNT fibers were both obtained from a treatment time of 15 min, when their values reached 0.691 and 20.6 GPa, respectively, corresponding to 169% and 141% of those of the as-spun fibers. A possible reason for the improved load transfer may be attributed to the oxygen-containing groups induced by the acid treatment,36 where the dipole−dipole interactions and hydrogen bonding formed may enhance the intertube interactions. Additionally, a ∼10% decrease in fiber diameter could also be observed as a result of the acid treatment. This may indicate a synergetic role of this process, through integrating the CNT surface modification and further additional structural condensation of nanotubes into the high-performance CNT fibers.38,54 On the other hand, the mass density of the 15 min acidtreated fibers was measured to be 1.53 g/cm3, ∼15% lower than that of the as-spun fibers (1.80 g/cm3). From this point of view, we further noted that HNO3 may play an additional role similar to purification in this case. Due to the fact that amorphous carbon is reactive and invariably existed in the directly spun CNT fibers, oxidized debris from the amorphous sites would be 7952

DOI: 10.1021/acsami.5b09912 ACS Appl. Mater. Interfaces 2016, 8, 7948−7956

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ACS Applied Materials & Interfaces

Figure 7. SEM images of the fracture surfaces of the (a) as-spun and (b, c) epoxy-treated CNT fibers. The epoxy concentrations were (b) 30% and (c) 50%.

in Figure 7, and again indicated the decreased sliding distances and improved load transfer between CNT bundles. Nevertheless, the improvement in strength outweighed the decrease in elongation, so the CNT fibers eventually became tougher after such a two-step treatment. The tensile toughness (the area under the stress−strain curves) appeared to be 2 times that of the as-spun fibers. These results above demonstrate the effectiveness of this two-step post-treatment in enhancing the multiscaled mechanical performance of the directly spun CNT fibers. 3.5. Post-Treatments for Enhancing the Electrical Properties of CNT Fibers. The as-spun CNT fibers exhibited an electrical conductivity of 1325 S/cm. Figure 9 shows the

wt % epoxy solution, which corresponded to a further increase by 64% and 201%, respectively, compared with those of the 15 min acid-treated fibers. This significant increase in the strength and stiffness of the cross-linked CNT fibers can be attributed to the good infiltration of epoxy between the CNT bundles. The stronger intertube interaction minimizes intertube slippage, thus substantially improving stress transfer under loading. The higher density of the cross-linked fiber (2.5 g/cm3) again suggests good infiltration of epoxy into the CNT structures. The successful penetration of epoxy could be evidenced by their fracture morphologies, as shown in Figure 7. In contrast to the long pull-out distance of the as-spun CNT fibers (Figure 7a), the smoother fracture surfaces generated by the epoxy-infiltrated fibers (Figure 7b,c) apparently indicate the formation of a well-integrated polymeric network. Notably, the 50 wt % epoxy solution was still capable of penetrating and bonding the inter-CNT bundles. However, excessive matrixes in this case would be attached onto the fiber surfaces, thus negatively expressing a reduction in the overall performance of the treated fibers (as shown in Figure 6b). Figure 8 displays the representative stress−strain curves for the as-spun, acid-treated, and epoxy-infiltrated CNT fibers. The

Figure 9. Electrical conductivities of the as-spun and acid-treated CNT fibers. After acid treatment, all the fiber diameters were remeasured using an optical microscope.

electrical conductivities of the as-spun and acid-treated CNT fibers as a function of the treatment time. Due to the more compact structure and stronger intertube interactions achieved by acidification, the acid-treated CNT fibers exhibited enhancements in conductivity by at least 3-fold, compared to the asspun fibers. Among them, the 0.5 h acidified CNT fibers reached the peak value of 8235 S/cm, which is greater than most of the previously reported values,22 and even higher than those of the single-walled and double-walled CNT fibers in their as-spun state.22,30,61 Similar to the tendency observed for mechanical properties, the structural damage to individual CNTs would become the dominant factor that hinders electron hopping, as well as decreases fiber conductivity, when the treatment time was further prolonged. Interestingly, the optimal times for the best mechanical properties (15 min) and electrical conductivities (0.5 h) were found to be slightly different. This finding may provide a clue that the functional groups introduced also served as intertube electron channels in some way.62,63 A similar

Figure 8. Representative stress−strain curves for the as-spun, acidtreated, and epoxy-infiltrated CNT fibers.

figure revealed that the two-step post-treatment effectively improved the tensile performance of the directly spun CNT fibers, leading to their tensile strength and Young’s modulus increasing by 177% and 325%, respectively. It is worth mentioning that the highest tensile strength (1.132 GPa) and stiffness (62 GPa) achieved in this work were generally higher than those of most as-made CNT fibers (either by arraydrawing12,56 or direct spinning44,57), and even nearly 1 order greater than those of most wet-spun fibers.22,58−60 The epoxyinfiltrated CNT fibers also had a much smaller elongation than our as-spun fibers, which was consistent with the observations 7953

DOI: 10.1021/acsami.5b09912 ACS Appl. Mater. Interfaces 2016, 8, 7948−7956

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ACS Applied Materials & Interfaces phenomenon has also been detected for CNT films treated by other oxidative solutions, such as H2SO4.62

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4. CONCLUSIONS In conclusion, different synthesis conditions and condensation processes were found to significantly impact the multifunctional properties of the directly spun CNT fibers. It was found that a flow rate of carrier gas of 1.5 L/min and on-line densification using a droplet size of 0.93 mm are the optimum synthesis conditions to produce high-performance as-spun fibers. After a two-step postprocessing technique consisting of acid treatment and epoxy infiltration was applied, the tensile strength and Young’s modulus of the treated CNT fibers dramatically increased by 177% and 325%, respectively. Moreover, their electrical conductivity also reached up to 8235 S/cm with the 30 min acidification treatment, which is even higher than those of the single- and double-walled CNT fibers in their as-spun state. A universal strategy for optimizing the directly spun CNT fibers, from spinning to post-treatments, was comprehensively provided in this work.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09912. Detailed TGA/DTA analysis, evolved gas analysis, and contact information for the epoxy supplier (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +65-6516-1567. Author Contributions †

T.Q.T. and Z.F. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Defence Research and Technology Office (Grants R-394-001-077-232 and R-265-000-523-646) for their funding support. We are thankful to Dr. Markus Meyer and the NETZSCH Applications Laboratory (Dr. M. Schöneich and Dr. C. Fischer) for their help with the evolved gas analysis.

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DOI: 10.1021/acsami.5b09912 ACS Appl. Mater. Interfaces 2016, 8, 7948−7956