Post-Treatments for Multifunctional Property Enhancement ofCarbon Nanotube Fibers from the Floating Catalyst MethodThang 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 synthesisconditions and condensation processes on the chemicalcompositions and multifunctional performance of the directlyspun carbon nanotube (CNT) fibers. On the basis of theoptimized synthesis conditions, a two-step post-treatmenttechnique which involved acidification and epoxy infiltrationwas also developed to further enhance their mechanical andelectrical properties. As a result, their tensile strength andYoung’s modulus increased remarkably by 177% and 325%,respectively, while their electrical conductivity also reached8235 S/cm. This work may provide a general strategy for thepostprocessing optimization of the directly spun CNT fibers.The treated CNT fibers with superior properties are promisingfor 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 ofindividual carbon nanotubes (CNTs) to real-life applications,their assembly on a macroscopic scale has therefore attractedgreat attention in the past few decades.1 Among the variousassemblies developed, such as CNT arrays,2 buckypapers3/films,4−7 and aerogel,8 CNT fibers9−15 undoubtedly betterpreserve the 1D characteristic of individual nanotubes, andprovide more design-friendly abilities for industrial utilization.The CNT fibers with enhanced multifunctional propertiesshow great potential for a wide range of applications, such asflexible medical devices,1,16−19 structural reinforcements,20,21

and lightweight electric cables.22 By now, significant progresshas been made in the fabrication of CNT fibers via wetchemistry,9,10 array-drawing,11,12 and direct growth13−15,23,24

approaches.In general, the wet spinning method can produce high-

performance CNT fibers, but the involvement of a matrix phase(polymer or acid) is usually unavoidable.9,10 The array-drawingmethod, which provides strong and relatively clean fibers andfilms, has been reported by Jiang et al.25 and Zhang et al.26

Although the as-produced CNT products could be transparentand highly conductive, they still appear to be unfavorable for alarge-scale fabrication, given the limited wafer size.1

Alternatively, the direct growth method, on the basis of theaerogel technique, has been considered a significant step towardscalable fiber production.27 While this method is widely used tosynthesize aligned CNTs,28,29 it has been reported that theCNT fibers directly spun from this method commonly possess

high mechanical properties with good electrical conductiv-ities.20 When the effects of gauge length (GL) are neglected,their strength and Young’s modulus could reach up to 3.24 and357 GPa (at a 1 mm GL), respectively, after a simple acetonedensification,15 whereas their electrical conductivities are on theorder of ∼103 S/cm.13,24,30,31 Recently, Gspann et al.32 reporteda comprehensive study of the floating chemical vapordeposition (CVD) process, in which purity issues and theroles of sulfur were highlighted. Nevertheless, a detailedinvestigation of the carrier gas flow may also be vital. Notably,the determination of the optimal synthesis parameters is acritical factor for the direct growth method, which cansignificantly alter the chemical composition and multifunctionalproperties 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 toremove defects. In addition, post-treatments can be furtherconducted on the as-spun CNT fibers to maximize their fiberperformance. Introducing twisting,12,34 stretching,27,35 orpressing27 can effectively compact the CNTs, thus potentiallyleading to enhancements in their macroscopic properties.However, these physical treatments are not capable ofinfluencing the inter-CNT interactions. As the CNTs withinthe fiber are still contacted by the intrinsically weak van der

Received: October 18, 2015Accepted: March 11, 2016

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Waals force, the enhancing effects of the aforementionedtreatments on the CNT properties may not be so obvious.36

The surface modification of individual CNTs has shown itseffectiveness in creating interfacial bonding and activatingCNTs.37 Meng et al.38 reported surface modification of theCNT fibers by a concentrated acid, and demonstrated that suchtreatment could effectively enhance the strength, electricalconductivity, and volumetric capacitance of the array-drawnfibers by 52%, 128%, and 17%, respectively. Besides that, theexistence of numerous functional groups is also critical forgenerating interfacial bonding between CNTs and variouspolymeric matrixes.39−41

On the basis of this point, the development of a post-treatment process that combines surface modification andpolymer infiltration becomes vital. With an abundant amount offunctional groups introduced on the CNT surfaces, a morecomprehensive polymer infiltration and chemical bonding cantherefore be expected.42,43 To the best of our knowledge, it stillremains unclear how such a post-treatment can functionalizeand impact the properties of the CNT fibers. As the directlyspun CNT fibers have their own unique characteristics, aquantitative evaluation of the effects from each step of the post-treatment (i.e., acid treatment and polymer infiltration) wouldalso be essential.In this work, a comprehensive study was first conducted to

investigate the effects of the synthesis conditions, such as theflow rate of the carrier gas, collection speed, and condensationprocesses, on the multifunctional performance of the directlyspun CNT fibers. Aimed at fulfilling their potentials, a two-steppostprocessing technique, including acid treatment and epoxyinfiltration, was then developed. For the acid treatment step,the optimal treatment time was determined to balance betweenfunctionalization and structural damage, whereas, for the epoxyinfiltration step, the concentration of epoxy solutions wasoptimized for the best integration. To identify the respectiveroles of acid treatment and epoxy infiltration during the post-treatment, the electrical and mechanical properties of the CNTfibers were carefully quantified, and compared step by step toform a strategy for improving their multifunctional properties.A universal strategy was suggested here for improving themultifunctional properties of the CNT fibers, in particular thoseobtained from a CNT aerogel technique. Furthermore, thiswork may also provide a basis for realizing the potentials ofvarious CNT assemblies for wide-ranging structural andfunctional applications.

2. EXPERIMENTS2.1. Materials. Ferrocene, thiophene, ethanol, acetone, and

concentrated nitric acid (HNO3; 65 wt %) were purchased fromSigma-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 obtainedfrom Polymer Technologies Pte. Ltd. (Singapore). All the chemicalswere 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 intothe CVD reactor at 1200 °C, while the heated reactor was maintainedunder a N2 environment. Upon the reaction, a CNT “aerogel” wascontinuously formed at the heating region and blown out of thereactor by the carrier gas (H2), thereafter being wound onto a rotatingspindle to collect the CNT fibers. To maintain synthesis stability andto investigate the formation of carbonaceous or other impurities at acertain synthesis, the H2 flow rate was varied from 1.0 to 2.0 L/min,

while methane, ferrocene, and thiophene were injected at feeding ratesof 160, 250, and 20 mL/min, respectively, and controlled to be similaracross all syntheses.

2.3. Liquid Condensation of CNT Fibers. To investigate theeffect of liquid condensation on the mechanical performance of the as-collected CNT fibers, both (i) on-line and (ii) off-line condensationprocesses were implemented and compared in this work. For the on-line condensation process, ethanol was mixed with N2 and directlysprayed onto the CNT fibers during fiber winding, while, for the off-line condensation process, ethanol was manually sprayed onto the as-collected CNT fibers via a sprayer.

2.4. Post-Treatment of CNT Fibers. To further enhance themultifunctional properties of the as-spun CNT fibers, a two-steppostprocessing technique, consisting of acid treatment and epoxyinfiltration, was developed. First, the as-spun CNT fibers, after ethanolcondensation, were immersed in 65 wt % HNO3 at room temperature,for purification and functionalization. A series of treatment times, 15min and 0.5, 1, and 2 h, respectively, were performed to optimize theacid treatment process. After that, the CNT fibers were washed withdeionized (DI) water three times, and dried in the ambient conditionsovernight. For the epoxy infiltration step, Epicote 1004 resin andhardener were first mixed at a weight ratio of 5:2, as recommended bythe supplier. To ensure smooth infiltration, the mixed epoxy systemwas diluted by acetone to form solutions of different weight fractions,namely, 10, 20, 30, and 50 wt %, respectively. For infiltration, the acid-treated CNT fibers were dipped into the solutions for 15 min, andthen cured in air at room temperature for 24 h, to form cross-linkedCNT fibers.

2.5. Characterization. The diameters of the as-spun, acid-treated,and epoxy-infiltrated CNT fibers were all measured by an opticalmicroscope (Olympus LG-PS2). To determine the diameter of thefibers, we took measurements at 10 different positions along theirlengths, which were then averaged. The structure and morphology ofthe CNT fibers were observed using a field emission scanning electronmicroscope (FE-SEM S4300, Hitachi) at 15 kV, and a transmissionelectron microscope (JEOL JEM-3010).

To understand the consistency of the CNT fibers synthesized underdifferent conditions, a thermogravimetric analysis (TGA; ShimadzuDTG60H) 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 tensiletester (XS(08)X-15, Shanghai Xusai Co.) with a gauge length of 10mm, at a tensile rate of 1.2 mm/min. The electrical properties of theas-spun and acid-treated CNT fibers were determined by an AgilentU1241B multimeter, based on the two-probe configuration.

3. RESULTS AND DISCUSSION

3.1. Liquid Condensation Effects on CNT FiberProperties. Figure 1a compares the effects of differentcondensation processes (off-line and on-line ethanol spraying)on the electrical and mechanical properties of the as-preparedCNT fibers. While the off-line condensation process providedthe CNT fibers with an electrical conductivity of 700 S/cm, astrength of 0.26 GPa, and a Young’s modulus of 2.3 GPa, theon-line-condensed CNT fibers apparently exhibited highervalues, reaching 1325 S/cm in conductivity, a strength of 0.408GPa, and a Young’s modulus of 14.6 GPa. Such a comparisonmay apparently verify the greater effectiveness of the on-linecondensation process, compared to the off-line one. Accord-ingly, higher liquid penetration should be considered for ageneral densification purpose.Furthermore, the size of ethanol droplets also plays a

significant role during the on-line spraying process. Both thediameter and strength of the as-obtained CNT fibers have beenshown to correlate with the droplet diameter, as it varied from0.93 to 2.23 mm under controlled conditions of N2 pressure.The larger droplets of ethanol, even resulting in a slight

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reduction of the fiber diameter, dramaticallyby nearly 50% asshown in Figure 1blowered the fiber strength, thus showingthe less effective infiltration and weakened contacts betweenthe CNT bundles in Figure 1b,c. This finding was quite inaccordance with the fact that the finer the droplet size, themore uniform the liquid condensation. For the large dropletsizes, e.g., 2.23 mm in diameter as in Figure 1c, due to thedroplet distribution being relatively scattered during spraying,the as-condensed CNT fibers evidently presented severalinferior links in their structure, caused by their uncondensedparts. Therefore, fibers densified by the smaller sized dropletshave a uniform structure with few fair-densified areas, whilemany poorly densified areas were observed along the fibersdensified by large drop sizes (Figure 1c). At these areas, the vander 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 FiberProperties. For the floating catalyst synthesis process, as iscommonly known, several processing parameters should beequally balanced to enable satisfactory continuous CNTproduction. One is the flow rate of hydrogen, used in theprocess as a reaction medium and a carrier gas.15,20,32 Its streamvelocity can significantly determine the transfer of the reactantsalong the reactor and their retention time in the reactionzone.32,44 During that time, ferrocene, thiophene, and methanedecompose in the gas stream, with in situ formation of ironcatalyst nanoparticles. Successively, CNTs start nucleating andgrow on their surface. Then the growing CNTs entangle andform a so-called “elastic aerogel”, which can be continuouslydrawn out of the reactor, and densified with spraying ethanol toproduce CNT fibers.The experiments on a varied flow rate of H2 have shown

hindered condensation for the CNT aerogel with the lowest H2velocity (1.0 L/min). These synthesis conditions resulted in theabundant formation of a CNT material. Due to this fact, it wasnot possible to fully densify the CNT fibers with on-lineethanol spraying. As a consequence, the CNT fibers had a large

diameter (∼56 um) and low mechanical properties, with atensile strength of only 0.052 GPa (Table 1).

The low performance of the fiber spun at 1.0 L/min of H2may also stem from the poor alignment of their CNTs andCNT bundles along the fiber axis, as shown in Figure 2a. In

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 andelectrical properties of the CNT fibers, with a 0.347 GPastrength, a 12.90 GPa stiffness, and an electrical conductivity of2130 S/cm. However, further attempts to increase thecollection speed of the fibers onto the roller, from 28 to 34m/min, even as it showed a positive trend in fiber performancedue to the slight increase of the CNT alignment (Figure 2c),revealed the instability of the spinning process for both types offibers. According to the literature,32 the spinning stability maybe hugely affected by the presence of cosynthesized impurities,which may also prevent the complete structural densification ofthe CNT aerogel into the CNT fiber during liquidcondensation. Thus, a detailed morphology analysis of theCNT fibers was done to address their observed properties.

3.3. Morphology of the CNT Fibers. Under differentsynthesis conditions, a thermogravimetric analysis/differentialthermal analysis (TGA/DTA) has shown a significant differ-ence in CNT fiber composition, which may affect theirmechanical and electrical performance. At the lowest H2 flowvelocity (1.0 L/min), the observed residue in CNTs was at low

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

Table 1. Properties of the CNT Fibers Synthesized atDifferent Carrier Gas Flow Rates

processing parameter characteristic

H2 flowrate,L/min

collectionspeed,m/min

diameter,μm

tensilestrength,GPa

tensilemodulus,GPa

electricalconductivity,

S/cm

1.0 28 55.96 0.052 1.90 301.0 34 57.70 0.082 2.802.0 28 8.44 0.347 12.90 21302.0 34 8.22 0.354 17.10

Figure 2. SEM images of the surface morphology of CNT fibers spunat (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 H2and a 34 m/min collection speed.

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levels (8 wt %). However, the CNTs contained much morecosynthesized carbonaceous impurities: 67 wt % versus 24 wt %of high-quality CNT bundles (the descriptive TGA/DTA dataare provided in the Supporting Information, section S1). Theterm “carbonaceous impurities” here encompasses a broadrange of short and highly deformed multiwalled carbonnanotubes (MWNTs), which we observed with a high-resolution transmission electron microscopy (HRTEM)analysis (Figure 3).

Parts a and b of Figure 3a and b show the TEM images ofvarious carbonaceous impurities abundantly cosynthesized at a1.0 L/min H2 flow rate, together with CNTs of large diameter.It is known from the literature that the CNT diametercorrelates well with the size of the catalyst particle.32,45−47 Inaddition, it was shown that the catalyst particles are able togrow by collision and coalescence during their transfer alongthe reactor.32,48 MWNTs of large diameter were observed byMcKee et al.46 for syntheses with increased growth times.Moreover, Gspann et al.32 evidenced that, at longer retentiontimes, large iron particles formed through collision were able togrow their own nanotubes to a length sufficient for them toentangle with a CNT aerogel, resulting in large amounts ofcarbonaceous impurities within the fiber.15,32,49 From this pointof view, the presence of big clusters of impurities (up to severalmicrometers in size) may obstruct on-line densification, thusleading to a large diameter and low mechanical performance ofthe CNT fibers.In contrast, for a higher H2 flow rate (2.0 L/min), TGA/

DTA results have shown a significant reduction in the amountof carbonaceous impurities, from 67 to 14.5 wt %, at the sametime competitively increasing the catalyst residue content by upto 38 wt % (Supporting Information, S1). Iron particles can be

observed at the CNT tips (Figure 3a−c), or kept between theinner cylindrical carbon layers in the nanotubes.50 Otherwise, ifnot yet reacted to form nanotubes, they can be trapped insidethe fiber as carbon-encapsulated iron carbide particles.50−52

CNT fibers synthesized at a H2 flow rate of 1.5 L/min weremainly composed of multiwalled CNTs, with an outer diameterof ∼15 nm and approximately 15−20 walls (Figure 3d), withthe outer layers formed from a less organized carbon, whichwas probably amorphous.46 The Raman spectrum showed theintensity ratio of the G (1580 cm−1) and D (1350 cm−1) bandsto be approximately 2.4, as reported in our previous study.53

TGA/DTA tests revealed a residue content of 30 wt %, mainlycomposed of Fe2O3 particles, as confirmed with energy-dispersive X-ray spectroscopy (EDS) (Supporting Information,S1). This means that the CNT fibers have about 21% metalliciron 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/minof H2 showed a good balance of spinning stability, togetherwith electrical and mechanical performance. The fibersexhibited a tensile strength and stiffness of 0.41 and 14.60GPa, correspondingly, which were found to be within the rangeof those of other methane-based CNT fibers.32

As shown in Figure 4, the CNT fibers produced from theoptimal conditions (1.5 L/min H2 flow rate) generally exhibit

good CNT alignment, being parallel to the fiber axis, andtypically 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 detectedimpurities confirmed the importance and necessity of post-treatments to fulfill the multifunctional properties of the CNTfibers, which we will discuss below. Also, the observed ability tocollect CNT fibers (1.5 L/min of H2) at a dramatically highspeed range (40−90 m/min) could also lead to alternativeperspectives in process optimization through both methods,reducing the catalyst residue content and reaching betteralignment of CNT bundles.49 By only regulating the spinningrecipes and condensation processes, the multifunctionalproperties of the as-spun CNT fibers were still largely hinderedby the weak van der Waals force; post-treatments are thereforerequired for further property enhancements.

3.4. Post-Treatments for Enhancing the MechanicalProperties of CNT Fibers. 3.4.1. Effect of Acid Treatment.In this work, concentrated HNO3 was applied as the first stepto modify the as-spun CNT fibers. Figure 5 compares thesurface morphology of CNT fibers before and after acidtreatment, and the effect of different treatment times (rangingfrom 15 min to 2 h) on their tensile strength and Young’smodulus. Regarding the stacking of CNT bundles in the CNTfibers, the scanning electron microscopy (SEM) image in

Figure 3. HRTEM images showing (a, b) clusters of short and highlydeformed MWNTs predominantly cosynthesized at a 1.0 L/min H2flow rate, (c) an iron particle at the MWNT tip, and (d) a typicalMWNT 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 surfacemorphology (b) spun at 1.5 L/min of H2.

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Figure 5a suggests that the acid treatment may increase theCNT bundle sizes and inter-CNT contacts, resulting in anincrease in their bearing load, and improved mechanicalproperties. It can be noted that the as-spun CNT fibers onlyexhibited an average tensile strength and Young’s modulus of0.408 ± 0.054 and 14.6 ± 2.7 GPa, respectively, while the acidtreatment 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, whentheir values reached 0.691 and 20.6 GPa, respectively,corresponding to 169% and 141% of those of the as-spunfibers. A possible reason for the improved load transfer may beattributed to the oxygen-containing groups induced by the acidtreatment,36 where the dipole−dipole interactions and hydro-gen bonding formed may enhance the intertube interactions.Additionally, a ∼10% decrease in fiber diameter could also beobserved as a result of the acid treatment. This may indicate asynergetic role of this process, through integrating the CNTsurface modification and further additional structural con-densation of nanotubes into the high-performance CNTfibers.38,54

On the other hand, the mass density of the 15 min acid-treated fibers was measured to be 1.53 g/cm3, ∼15% lower thanthat of the as-spun fibers (1.80 g/cm3). From this point of view,we further noted that HNO3 may play an additional role similarto purification in this case. Due to the fact that amorphouscarbon is reactive and invariably existed in the directly spunCNT fibers, oxidized debris from the amorphous sites would be

readily formed during CNT acidification, and removed by thesubsequent aqueous washing.37,55 However, as shown in Figure5b, further prolonged treatment beyond 15 min could not offerfurther improvements, due to the destruction of the CNTstructures.36,38 As discussed elsewhere,38,54 the competitionbetween enhanced interfacial shearing and destroyed CNTstructures would finally determine the mechanical behavior ofthe CNT fibers. The acid treatment hence needs to be carefullycontrolled, in terms of acid concentration and treatment time,for optimal purposes.

3.4.2. Effect of Cross-Linking. Functionalizing CNTs hasbeen shown to be effective in improving the CNT dispersion invarious epoxide-containing polymers.39,40 Herein, epoxyinfiltration was applied as the second step of post-treatment,and performed on the CNT fibers from the best acidificationcondition (15 min). Given the high viscosity of the epoxy used,a set of diluted epoxy solutions (10, 20, 30, and 50 wt %) wasprepared to ensure effective infiltration. As shown in Figure 6a,the epoxy was well-infiltrated into the CNT bundles, and fewerpores are observed, compared to the surface morphology of thefiber before the treatment. Figure 6b compares the tensilestrength and Young’s modulus of these epoxy-infiltrated CNTfibers 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 strongerthan the as-spun and acid-treated fibers. The highest tensilestrength of 1.132 GPa and stiffness of 62.0 GPa wererespectively obtained for the CNT fibers infiltrated by the 30

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 onthe 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 epoxyconcentration on the mechanical properties of the CNT fibers.

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wt % epoxy solution, which corresponded to a further increaseby 64% and 201%, respectively, compared with those of the 15min acid-treated fibers. This significant increase in the strengthand stiffness of the cross-linked CNT fibers can be attributed tothe good infiltration of epoxy between the CNT bundles. Thestronger intertube interaction minimizes intertube slippage,thus substantially improving stress transfer under loading. Thehigher density of the cross-linked fiber (2.5 g/cm3) againsuggests good infiltration of epoxy into the CNT structures.The successful penetration of epoxy could be evidenced bytheir 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 bythe epoxy-infiltrated fibers (Figure 7b,c) apparently indicate theformation of a well-integrated polymeric network. Notably, the50 wt % epoxy solution was still capable of penetrating andbonding the inter-CNT bundles. However, excessive matrixesin this case would be attached onto the fiber surfaces, thusnegatively expressing a reduction in the overall performance ofthe 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 revealed that the two-step post-treatment effectivelyimproved the tensile performance of the directly spun CNTfibers, leading to their tensile strength and Young’s modulusincreasing by 177% and 325%, respectively. It is worthmentioning that the highest tensile strength (1.132 GPa) andstiffness (62 GPa) achieved in this work were generally higherthan those of most as-made CNT fibers (either by array-drawing12,56 or direct spinning44,57), and even nearly 1 ordergreater than those of most wet-spun fibers.22,58−60 The epoxy-infiltrated CNT fibers also had a much smaller elongation thanour as-spun fibers, which was consistent with the observations

in Figure 7, and again indicated the decreased sliding distancesand improved load transfer between CNT bundles.Nevertheless, the improvement in strength outweighed the

decrease in elongation, so the CNT fibers eventually becametougher after such a two-step treatment. The tensile toughness(the area under the stress−strain curves) appeared to be 2times that of the as-spun fibers. These results abovedemonstrate the effectiveness of this two-step post-treatmentin enhancing the multiscaled mechanical performance of thedirectly spun CNT fibers.

3.5. Post-Treatments for Enhancing the ElectricalProperties of CNT Fibers. The as-spun CNT fibers exhibitedan electrical conductivity of 1325 S/cm. Figure 9 shows the

electrical conductivities of the as-spun and acid-treated CNTfibers as a function of the treatment time. Due to the morecompact structure and stronger intertube interactions achievedby acidification, the acid-treated CNT fibers exhibited enhance-ments in conductivity by at least 3-fold, compared to the as-spun fibers. Among them, the 0.5 h acidified CNT fibersreached the peak value of 8235 S/cm, which is greater thanmost of the previously reported values,22 and even higher thanthose of the single-walled and double-walled CNT fibers intheir as-spun state.22,30,61

Similar to the tendency observed for mechanical properties,the structural damage to individual CNTs would become thedominant factor that hinders electron hopping, as well asdecreases fiber conductivity, when the treatment time wasfurther prolonged. Interestingly, the optimal times for the bestmechanical properties (15 min) and electrical conductivities(0.5 h) were found to be slightly different. This finding mayprovide a clue that the functional groups introduced also servedas intertube electron channels in some way.62,63 A similar

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%.

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

Figure 9. Electrical conductivities of the as-spun and acid-treated CNTfibers. After acid treatment, all the fiber diameters were remeasuredusing an optical microscope.

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phenomenon has also been detected for CNT films treated byother oxidative solutions, such as H2SO4.

62

4. CONCLUSIONSIn conclusion, different synthesis conditions and condensationprocesses were found to significantly impact the multifunctionalproperties of the directly spun CNT fibers. It was found that aflow rate of carrier gas of 1.5 L/min and on-line densificationusing a droplet size of 0.93 mm are the optimum synthesisconditions to produce high-performance as-spun fibers. After atwo-step postprocessing technique consisting of acid treatmentand epoxy infiltration was applied, the tensile strength andYoung’s modulus of the treated CNT fibers dramaticallyincreased by 177% and 325%, respectively. Moreover, theirelectrical conductivity also reached up to 8235 S/cm with the30 min acidification treatment, which is even higher than thoseof the single- and double-walled CNT fibers in their as-spunstate. A universal strategy for optimizing the directly spun CNTfibers, from spinning to post-treatments, was comprehensivelyprovided in this work.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.5b09912.

Detailed TGA/DTA analysis, evolved gas analysis, andcontact information for the epoxy supplier (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: mpedhm@nus.edu.sg. Phone: +65-6516-1567.Author Contributions†T.Q.T. and Z.F. contributed equally to this work.NotesThe authors declare no competing financial interest.

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

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