Alignment of Carbon Nanotubes in Carbon Nanotube Fibers Through

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Alignment of Carbon Nanotubes in Carbon Nanotube Fibers Through Nanoparticles: A Route for Controlling Mechanical and Electrical Properties Muhammad Mohsin Hossain, Md. Akherul Islam, Hossain Shima, Mudassir Hasan, and Moonyong Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12869 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 22, 2017

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ACS Applied Materials & Interfaces

Alignment of Carbon Nanotubes in Carbon Nanotube Fibers Through Nanoparticles: A Route for Controlling Mechanical and Electrical Properties Muhammad Mohsin Hossain,† Md. Akherul Islam,‡ Hossain Shima,§ Mudassir Hasan, and Moonyong Lee∥,* †

Clean Energy Priority Research Center and School of Chemical Engineering, Yeungnam University, Gyeongsan 712749, Republic of Korea ‡

Department of Pharmacy, Atish Dipankar University of Science & Technology, Banani, Dhaka 1213, Bangladesh

§

Department of Chemistry, Rajshahi Univesity, Rajshahi 6205, Bangladesh

Department ∥School

of Chemical Engineering, King Khalid University, Abha 61411, Kingdom of Saudi Arabia

of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea

ABSTRACT: This is the first study that describes how semiconducting ZnO can act as an alignment agent in carbon nanotubes (CNTs) fibers. Due to the alignment of CNTs through the ZnO nanoparticles linking groups, the CNTs inside the fibers were equally distributed by the attraction of bonding forces into sheet-like bunches, such that any applied mechanical breaking load was equally distributed to each CNT inside the fiber, making them mechanically robust against breaking loads. Although semi-conductive ZnO nanoparticles were used here, the electrical conductivity of the aligned CNT fiber was comparable to bare CNT fibers, suggesting that the total electron movement through the CNTs inside the aligned CNT fiber is not disrupted by the insulating behavior of ZnO nanoparticles. A high degree of control over the electrical conductivity was also demonstrated by the ZnO nanoparticles, working as electron movement bridges between CNTs in the longitudinal and crosswise directions. Well-organized surface interface chemistry was also observed, which supports the notion of CNT alignment inside the fibers. This research represents a new area of surface interface chemistry for interfacially linked CNTs and ZnO nanomaterials with improved mechanical properties and electrical conductivity within aligned CNT fibers. KEYWORDS: carbon nanotube fiber, ZnO nanoparticles, interface chemistry, electrical conductivity, mechanical properties

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Introduction

the nanostructure level. Crosslinking was performed in CNTs to improve the mechanical properties,11,12 but single CNTs and relatively small systems were used in these endeavors. For chemical treatment,11 the need for additional steps make these processes lengthy and hazardous to the environment. Graphene oxide (GO) was also used previously for chemical crosslinking,13–16 or for chemical bonding13,15 with the help of a polymer14 or pphenylenediamine16 (PPD). Unfortunately, the structural design and tensile load-bearing properties of GOcrosslinked CNT fibers is not yet clear.16

Lightweight materials with high mechanical and electrical performance are important for portable and flexible electronics, as well as in vehicle manufacturing. To that end, carbon nanotubes (CNTs) have attracted considerable attention as potential building blocks owing to their surprising mechanical, electrical properties, and potential applicaton.1,2,3 The alignment of CNTs in CNT fibers at the nanostructure level is a crucial issue in improving the mechanical properties and electrical conductivity of these systems.4 Several methods have been employed in recent years to attain structural links between CNTs in fibers and films.5–10 Even though these approaches have realized enriched load bearing properties, the lack of chemical linkage between individual CNTs hinders CNT alignment at

Exceptional load-bearing properties have been discovered in small fiber samples with submicron diameters and 1−2 mm device lengths.17–18 Measurement of high mechanical properties in small samples like these may not indica-

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tive of longer (101–103 m) fibers. A set precursor flow rate19 and liquid infiltration20 were used to align CNTs in CNT fibers to improve their mechanical properties. Due to a lack of information regarding the diameter of aligned fibers, no clear electrical conductivity was found in aligned CNT fibers.19 For other cases, the mechanical properties of the aligned CNT fibers were unclear.20 To clearly understand the effects of good alignment, the mechanical and electrical properties, as well as the interface chemistry of CNTs inside the aligned CNT fiber must be understood.

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Experimental

2.1 Instrumentation The surface morphology of the aligned CNT fibers was determined using field emission scanning electron microscopy (FESEM, S-4800, Hitachi, Japan) and crystallographic data were obtained by X-ray diffraction (XRD, thin film, MPD, PANalytical). Aligned CNT fiber crosssection samples were prepared by focused ion beam (FIB, HELIOS NANOLAB 650, FEI Company, USA) and geometrical arrangements of the CNTs inside the aligned CNT fibers were observed using the same instrument. ZnO and CNT fiber vibrational phonon modes were characterized by Raman spectroscopy (HORIBA JOBIN YVON, Lab RAM HR, laser wavelength 514.54). A Keithley four-point probe (2400 SourceMeter and 2000 MULTIMETER) was utilized to calculate the electrical conductivity of the aligned and bare CNT fibers. X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Scientific, USA) was used to measure the binding energies of C, various carbon functional groups, and Zn. Load barring properties of aligned and bare CNT fibers were determined by a universal testing machine (UTM 4464, INSTRON, USA).

The addition of nonmetal nanomaterials to CNT fibers to increase their mechanical properties through chemical bonding has been studied by other groups,21–24 with improvements of 2–4× compared to bare CNT fibers.21–23 In general, nonmetal nanomaterials or semiconductors decrease electrical conductivity of a system. For example, SiO2 semiconductors halve the electrical conductivity of a CNT fiber compared to the bare counterpart.23 A lack of distribution of these semiconductor nanomaterials within the CNT fiber does not help the alignment of CNTs inside the fiber. Aggregation was present in some places without well-distributed chemical bonds in the CNT fibers. Aggregated semiconductor nanoparticles form a barrier for electron movement, leading to decreased electrical conductivity. Since the CNTs were not aligned, the longer electron paths decreased the electrical conductivity. Aligned CNTs are beneficial for many different applications,25-26 but an accurate picture of the surface interface chemistry is not available to explain the interfacial attraction between CNTs for CNT alignment that improves the mechanical properties and electrical conductivity of the fiber.

2.2 Chemicals ZnO nanoparticles were prepared on the surface and inside of the CNT fiber from a precursor solution of zinc acetate dihydrate (Sigma Aldrich, 99% purity). Zinc acetate dihydrate was sonicated in ethanol (95%, Duksan, Korea) to prepare the precursor solution. A feed stock contained acetone (99.5%, Daejung Chemicals and Metals, Korea), ferrocene (98% Sigma Aldrich) and thiophene (99%, Sigma Aldrich) was used to produce the CNT fibers.42 The prepared CNT fiber was densified by dimethyl sulfoxide (DMSO, 99.5%, Sigma Aldrich).

In this study, aligned CNT fibers were prepared with semiconducting ZnO nanoparticles, for improved alignment of CNTs in within CNT fibers through the binding force of Zn atoms with C atoms of CNTs. The prepared CNT fibers showed excellent arrangement of CNTs into bunches with improved mechanical properties by longitudinal and crosswise linkage of CNTs. Although semiconducting oxides usually display very low or insignificant electrical conductivity, the deposited ZnO nanoparticles gave the aligned CNT fibers excellent electrical conductivity not unlike bare CNT fibers. By comparison, other bare CNT fibers and CNT fiber composites show less electrical conductivity27–41 and worse mechanical properties30–31,35–36 than that of our prepared aligned CNT fibers. Nullification of the semi-conductive or insulating behavior of the ZnO nanoparticles was obtained by crosswise and lengthwise linking of individual CNTs through the alignment, as sheets in which electron movement or conductive paths were well connected by linking bridges. This is the first report that uses semiconducting ZnO to arrange CNTs within CNT fibers by aligning them in bunched structure without further pretreatment or further functionalization of the CNTs.

2.3 Preparation process of aligned CNT fiber Aligned CNT fibers were prepared using a previously published method.42,43 In brief, zinc acetate dihydrate (21 g) was added to ethanol (300 mL) and sonicated for 80 min until a clear homogeneous solution was obtained. Three quartz glass beakers (1, 2, 3) were connected as shown in Figure 1(a, b, c) and 100 mL of this solution was added to each. A constant flow rate of solution was maintained across all three beakers. When the volume of zinc acetate solution was increased in beaker-3, some of it was transferred to beaker-1 from beaker-3. Here, beaker-1 was sonicated continuously to prevent the coagulation or precipitation of the zinc acetate solution. CNT fibers were mounted on a handmade steel wire frame (Figure 1b, in beaker-2). The steel frame was held for 10 min in beaker-2 with a constant zinc acetate flow rate across all beakers.

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After that, the zinc acetate flow from beaker-1 to beaker-2 and from beaker-2 to beaker-3 was stopped. All of the

to an accumulator at a rate of 5 m/min. The prepared CNT fibers were then densified by dimethyl sulfoxide

Figure 1. (a) Zinc acetate solution in beaker-1. (b) CNT fiber was mounted on a steel frame and immersed in zinc acetate solution of beaker-2, which was connected with beaker-1 to maintain continuously flow of zinc acetate solution. (c) The zinc acetate solution in beaker-3 was also collected with beaker-2 to maintain a continuous flow of zinc acetate solution from beaker-2 to beaker-3. (d) Zinc acetate solution was removed from beaker-2, at which point beaker-4 contained the zinc acetate-deposited CNT fiber on steel frame. (e) Beaker-4 was heated at 480 °C for 45 min to prepare aligned CNT fibers. zinc acetate solution was then removed from beaker-2 and the tubes between them were opened, as shown in Figure 1d (beaker-4). The quartz glass beaker containing the steel frame mounted with zinc acetate-soaked CNT fibers (Figure 1d, beaker-4) was positioned into a furnace and heated at 480 °C for 45 min to prepare aligned fibers (Figure 1e). Here, the furnace temperature was increased to 480 °C with a heating rate of 5°/min. The deposited zinc acetate layer on the CNT fiber surfaces decomposed into ZnO nanoparticles during the heating process.

(DMSO) and dried at 100 °C. 3. Results and Discussion 3.1 Aligned CNT fibers Figure 2(a, b) shows FESEM images of aligned CNT fibers, where Figure 1a shows low-resolution FESEM images of aligned CNT fiber. To clearly see the surface morphology of the aligned CNT fibers, higher resolution FESEM and SPM images were collected from Figure 2a as shown in Figure 2b and 2c, respectively. Figure 2c clearly shows ZnO nanoparticles deposited CNTs (few marked by dotted lines), whereas CNTs in bare CNT fibers (Figure 2e) featured smooth (few marked by arrows) individual CNTs. Here, high resolution image of bare CNT fiber (Figure 2e) was taken from the rectangular area of Figure 2d. To more clearly visualize the ZnO nanoparticles on surface of CNTs, the 3D SPM image (Figure 2f) from the rectangular area of Figure 2c was exhibited. It shows clearly ZnO nanoparticles (few marked by arrows) on

To make the CNT fibers for alignment, a liquid feedstock was made by mixing acetone, ferrocene (0.2 wt. %), and thiophene (0.8 wt.%).42 A chemical vapor deposition (CVD) furnace was heated at 1200 °C and the liquid feedstock was then injected. Here, acetone, ferrocene, and thiophene acted as the carbon source, catalyst, and promoter respectively.42 The carrier gas stream (1000 sccm) to the CVD furnace was maintained43 to synthesize a continuous CNT array. The prepared CNT fibers were wound

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CNTs surface. Actually, the rough surface (Figure 2f) is the deposited ZnO nanoparticles on the individual CNTs. Figure 2c and 2f were recorded by the WSXM program,44 which was in con-junction with the FESEM image.

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at 65–81° 2θ. The ZnO peaks were related to hexagonal wurtzite ZnO (JCPDS no. 01-075-0576).42,45 The peak at 19.8° 2θ on the bare CNT fiber (Figure 3b) shifted to 16.5° 2θ in the aligned CNT fiber (Figure 3a), suggesting that functional groups on the low crystalline carbon of the

Figure 2. FESEM images of aligned CNT fiber and bare CNT fiber. (a) Low resolution FESEM image of aligned CNT fiber, (b) high resolution FESEM image of aligned CNT fiber from the marked square area of (a), (c) SPM image of aligned CNT fiber, (d) low resolution FESEM image of bare CNT fiber, (e) high resolution FESEM image of bare CNT fiber shown from the square marked area of (d), (f) 3D image of ZnO nanoparticles deposited CNT, which is taken from the rectangular area of (c). XRD curves of the ZnO nanoparticles, bare CNT fiber, aligned CNT fiber, and bare silicone substrate are shown in Figure 3c, b, a, and d, respectively. All the materials were placed on silicone substrates to collect these spectra, and as such a few silicone peaks were present in all spectra. Figure 3b displayed peaks at 19.8° 2θ and 25.1° 2θ for the CNT fiber. The 19.8° 2θ peak was associated with lower crystalline carbon, whereas 25.1° 2θ indicates comparatively higher crystalline carbon in the CNT fiber. Due to the oxygen-containing functional groups in the CNT fiber, the peak of low crystalline carbon in oxygen-containing functional group was observed at a higher d-spacing (19.8° 2θ).42 The peaks denoted with square symbols in Figure 3b belonged to the silicon substrate, which were also present in Figure 3d. The XRD curve of the aligned CNT fiber (Figure 3a) featured peaks at 16.5° 2θ and 25.1° 2θ corresponding to carbon. The peaks for the ZnO nanoparticles in Figure 3a (100, 002, 101, 102, 110, 103, and 203) were similar to bare ZnO (Figure 3c). The 200, 112, 201, 004, 202, and 104 planar peaks of ZnO in the aligned CNT fibers were not present because of the intense Si substrate peak

CNTs in aligned CNT fibers strongly bonded with ZnO nanoparticles either by chemical or physical (electrostatic force) interactions. Possible chemical bonds are Zn-C, ZnO-CNT or CNT-COO-Zn (demonstrated by XPS, shown later). These chemical bonding interactions helped to increase the d-spacing of carbon in CNTs, leading to this peak shift. Fibers were split longitudinally (Figure 4a) to observe the CNTs inside the fibers, and the WSXM program43 was used to record images in conjunction with the FESEM image. As shown in Figure 4b, a bunch-like shape was present and some of the individual CNTs were closely crowded and attached (marked by arrows). This indicates that some parts of the individuals CNTs bonded to each other by physical or chemical bonding as previously discussed. Therefore, ZnO nanoparticles work as cross linkers among individual CNTs in the aligned CNT fiber by chemical or physical bonding. To clarify their bunch structures, a 3D image from the rectangular area of Figure

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Figure 3. XRD spectra of (a) aligned CNT fiber, (b) bare CNT fiber, (c) ZnO nanoparticles, and (d) silicon substrate. 4b is shown in Figure 4c, confirming an architecture in which CNTs were linked to each other. FIB was used to cut the aligned CNT fibers for cross-sectional imaging (Figure 5a), using gallium as an ion beam source. Figure 5b was taken from a rectangular area of the FESEM image (Figure 5a). A 3D image (Figure 5c) from the cross-section (Figure 5b) was obtained to determine the alignment of CNTs inside the aligned CNT fiber. A 3D image (Figure

5c) of the cross-section indicated many bunch-like structures (dotted marked lines) inside the aligned CNT fiber. Considering the evidence in Figure 4(b, c) and Figure 5c, bunches of CNTs (Figure 5d) inside the aligned CNT fiber were drawn. ZnO nanoparticles islands are marked by arrows in Figure 5c.

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3.2 Interface chemistry of aligned CNT fibers

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contributes to these two peaks. Based on these, two types of oxygen deficient environments (C-Zn-C and -O-Zn-C) of ZnO are available. For C-Zn-C, Zn is bonded directly to carbon via a chemical bond, leading to a peak at 532.9 eV peak. An -O-Zn-C motif features the more electronegative oxygen forming one bond with Zn and another with carbon, which creates the peak at 533.8 eV. Figure 6c shows the core level C1s de-convoluted curves of aligned CNT fibers at 284.8, 285.6, 286.1, 287.0, 287.8, 289.1, and 290.3 eV. The 284.8, 285.6, 286.1, and 287.0 peaks originate from the C=C bond of sp2 carbon, the C-C bond for sp3 amorphous carbon, from C-O-C, and from C=O respectively.51 The π-π* transition of π electrons in the sp2 (C=C) system here was also identified at 290.3 eV. The peaks at 287.8 and 289.1 eV indicate O-C=O functional groups, in which electropositive H bonds as HO-C=O for the peak at 289.1 eV, and the more electropositive Zn was chemically

XPS was used to determine the predicted chemical interactions and bonding properties of the CNT surfaces within the aligned CNT fibers (Figure 6). The 2p3/2 and 2p1/2 peaks (Figure 6a) at 1022 and 1045 eV, respectively indicate Zn in a ZnO oxidation state. The O1s core level peak (Figure 6b) disintegrated into peaks at 531.4, 532.9, 533.8, 534.9, and 536.9 eV. The 534.9 eV high energy peak arises from C=O functional groups connected chemically to Zn at the site of the lower crystalline carbon.46 The highest energy peak (536.9 eV) arises from C-O functional groups.47 Usually, more electronegative environments shift peaks to higher binding energies.48,49 Consequently, the peaks at 534.9 and 536.9 eV were also created in a more electronegative oxygen environment with Zn. O=CO-Zn-O-C=O is present at 534.9 eV, where C is bonded indirectly to Zn through oxygen atoms42 on C=O func-

Figure 4. SPM image, 3D image and internal geometry of the CNT fiber. (a) SPM image of split aligned CNT fiber, which was converted from FESEM image by WSXM program, (b) SPM image of bunched CNTs taken from the longitudinal split region, (c) 3D image of the rectangular area of (b). tional groups and C–O-Zn-O-C for 536.9 eV, where C is bonded through oxygen atoms on C-O functional groups. The 531.4 eV peak of O2- indicates a wurtzite structure of the ZnO lattice. The two peaks at 532.9 eV and 533.8 eV indicate oxygen deficiencies in the ZnO matrix,50 suggesting that the high electronegativity of C compared to Zn

bonded as Zn-O-C=O for the peak at 287.8 eV. The binding energy of the HO-C=O peak (289.1 eV) is higher than that of the Zn-O-C=O peak (287.8eV) due to H being less electropositive than Zn (or due to the higher electronegativity of H compared to Zn). The two peaks at 283.0 and 283.8 eV indicate carbide-type chemical bonds52,53 be-

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aligned through the deposition of ZnO nanoparticles on individual CNTs. Since different types of chemical bonds (Zn-C, Zn-O-C, and -O-Zn-C) were present based on XPS, the formation of these bonds is proposed in Figure 8. Figure 8a shows oxygen-containing functional groups available on the CNT surfaces inside the bare CNT fiber; these groups were also predicted by XRD (Figure 3) and Raman (Figure S1) analyses. When bare CNT fibers were immersed in zinc acetate (Figure 7b), zinc ions (Zn2+) react-

tween Zn and C (Zn-C). The peaks at 283.0 and 283.8 eV indicate two categories of carbide systems, namely C-ZnC and O-Zn-C, where C-Zn-C is less electronegative than he O-Zn-C system. Thus, the peak at 283.0 eV was attributed to C-Zn-C while the peak at 283.8 eV was attributed to O-Zn-C. Therefore, the alignment of CNTs in the CNT fibers was possible through the ZnO nanoparticles by the above chemical bonding motifs.

Figure 5. Cross-section cut aligned CNT fiber. (a) FESEM image taken from the cross-section cut area of a aligned CNT fiber, (b) high resolution SPM image of the rectangular area of (a), (c) 3D image of (b) in which bunches of CNTs were shown by dotted line, and (d) schematic of bunch-like CNTs inside the CNT fiber based on (c). On the basis of these analyses, a scheme of the CNT alignment in the fibers is proposed in Figure 7. Figure 7a shows CNTs inside the bare CNT fiber and Figure 7b shows CNTs of the bare CNT fiber in a zinc acetate solution. The aligned CNTs in the fiber had bunch-like arrangements as shown in Figure 7c, where CNTs were

ed with individual CNTs inside the bare CNT fiber (Figure 8b) and formed -CO-O-Zn-O-CO- and –O-Zn-O-CObonds with –COOH and –OH groups on the CNTs. Due to bond formation with zinc ions, CNTs were selectively aligned and brought closer through functional group attraction forces. After that, CNT fibers were removed from

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draw. Here CNT-Zn-CNT, CNT-O-Zn-CNT, and CNT/ZnO are indicated as products C, D and E, respectively. CNT-Zn-CNT, CNT-O-Zn-CNT, and CNT/ZnO also indicate C-Zn-C (C), C-O-Zn-C (D), and surface deposited ZnO nanoparticles, respectively. The reaction mechanism for the formation of product-C is shown in reaction-2, 3, 5, and 6. In reaction-2, zinc acetate was dissolved ethanol (B). This ethanol (B) was general grade, and therefore contained some water. Thus, zinc acetate was dissociated

the zinc acetate solution and heated at 480 for 45 min as described previously. During heating, CO2 was released from the CNT-CO-O-Zn-O-CO-CNT and CNT–O-Zn-OCO-CNT systems (Figure 8b,c). The CNT-CO-O-Zn-OCO-CNT and CNT–O-Zn-O-CO-CNT bonds were then converted to CNT-Zn-CNT (i.e., C-Zn-C) and CNT–O-ZnCNT (i.e., C-O-Zn-C) bonds (Figure 8c). The lengths of the bridging bonds of CNT-Zn-CNT (i.e., C-Zn-C) and CNT–O-Zn-CNT (i.e., C-O-Zn-C) were shorter than those

Figure 6. XPS spectra of (a) Zn 2p core level, (b) O 1s core level, and (c) C 1s core level for the aligned CNT fiber. in water as Zn2+ (G) and CH3COO- ions (F). When CNT fibers (containing –COOH group or CNT-COOH) were added to the solution, product-I was produced by reaction-3. Here, Zn2+ reacted with carboxylic groups (– COOH) on the CNTs to produce product-I. Product-J and product-K were produced from Product-I in the presence of product-F by reaction-4. Here, the positively-charged oxygen of product-I pulled an electron from the O-H bond to neutralize itself, and H released as H+ to produce

of CNT-CO-O-Zn-O-CO-CNT and CNT–O-Zn-O-COCNT. Therefore, CNTs were further bound to each other due to the decreasing bond length of these cross liking groups, resulting in CNTs compacted into sheet or bunch structures (Figure 8d). The aligned CNTs bunches were also connected to each other in an arrow-like direction (Figure 7c). The above bond formation (Figure 8) is further shown in Figure 9, where reactions-(1-12) are drawn by chemo-

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product-J. At the same time, the negatively-charged oxygen of product-F attracts the H+ ion to produce productK. Product-M was then produced from product-J via the L-product in reaction-5 at 480 °C for 45 min. It is possible that the oxygen on the carbonyl group in product-J pulls the electron of the C=O bond to form positively-charged carbon (product-L), which then pulls the lone pair electron of the neighboring neutral oxygen to form productM. After that, two molecules of CO2 are removed from product-M yielding product-C (CNT-Zn-CNT or C-Zn-C) via product-(M-1), followed by reaction-6. In this case, a negative charge was developed on the CNTs, due to its own electron withdrawing functional groups. The negatively-charged CNTs react with positively charge Zn ion and form product-C.

uct-K (reaction-8). During heating at 480 °C, one molecule of CO2 was released followed by reactions-9 and -10. In this case, the more electronegative oxygen of the C=O group (product-P, reaction-9), pulls an electron from the C=O bond, creating a negative charge on itself and a positive charge on carbon to yield product-Q. The positive charge of carbon pulls a lone pair electron from the neighboring oxygen to yield product-R (reaction-9). The positive charge of the oxygen pulls the electron from the Zn-O bond to neutralize its positive charge, while the negatively charged oxygen returns its extra electron to the O-C system to make an sp2 C=O bond (carbonyl). At the same time, a negative charge was developed on the CNT by taking a C-CNT bond electron on itself, and one CO2 molecule was released to produce the R-1 system of CNTO-Zn+ and ̶ CNT ion from product-R. After that, CNTs attached to the Zn+ of CNT-O-Zn+ by chemical bonding to produce product-D (CNT-O-Zn-CNT or C-O-Zn-C) by reaction-10.

For the case of product-D (C-O-Zn-C or CNT-O-ZnCNT) formation, CNT fibers (containing OH group like CNT-OH) were mixed with Zn2+ (G) and CH3COO- ions (F) according to reaction-7. The lone pair electron of oxygen of –OH functional groups in CNTs react with Zn2+

ZnO nanoparticles are also deposited onto the CNT sur-

Figure 7. Alignment of CNTs inside the CNT fiber. (a) Bare CNT fiber, (b) bare CNT fiber immersed in zinc acetate solution, and (c) aligned CNTs inside the aligned fiber. ions to produce product-O. Product–O has a positive charge on its oxygen atoms. Then, the positively-charged oxygen of product–O pulled a –OH bond electron to-

faces inside the aligned CNT fibers. CNTs with surfacedeposited ZnO nanoparticles are denoted as ZnO/CNT (product-E) in reaction-1. The mechanism of surface dep-

Table 1. Mechanical Properties of Aligned CNT Fiber and Bare CNT Fiber Materials Treatment Strain% Stress in MPa

Young Modulus in GPa

Bare CNT fiber

No treatment

14.7 ± 1.8

71.3 ± 4.1

4.7 ± 0.5

Bare CNT fiber

480˚C for 45 min

1.6 ± 0.8

49.1 ± 4.5

5.3 ± 2.3

Aligned CNT fiber

480˚C for 45 min

1.4 ± 0.21

112.5 ± 8.2

17.2 ± 3.6

wards itself and neutralized its positive charge by producing product-P and H+. At the same time, product-F received H+ by its negatively-charged oxygen to form prod-

osition (product-E) without any chemical reaction is shown in reaction-11 and -12. When zinc acetate was immersed in general grade alcohol, the zinc salt dissociated

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as Zn2+ and CH3COO- ion in water (H2O), as general grade alcohol contains some water. Oxygen atoms in water molecules carry two lone pair electrons. In reaction-11, the positive charge of Zn2+ pulls these lone pair electrons from water molecules to form product-S; this product produces Zn(OH)2 (product-T) by eliminating two protons. After that, product-T removed one water molecule and produced product-E (i.e. ZnO nanoparticles on CNTs surface) at 480 °C (reaction-12).

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112.5 MPa (Table 1), while the bare CNT fiber revealed a shorter value of tensile strength of 71 MPa. The corresponding Young’s moduli of the aligned and bare CNT fibers were measured from their tensile strength vs. strain curve, and were 17 GPa and 4.7 GPa respectively (Table 1). Therefore, alignment of bare CNT fibers with ZnO nanoparticles improves the tensile strength and Young modulus from71.3 to 121.5 MPa and 4.7 to 17.2 GPa respectively (Table 1), for improvements of 58 and 266%. A control experiment was also performed without ZnO nanoparticles. Here, only heat treatment (480 °C for 45 min) was performed on bare CNT fiber. The tensile strength of the heat-treated bare CNT fibers (Figure 10b,c and Table 1) decreased from 71.3 to 49.1 MPa, while the Young modulus (5.3 GPa) was similar to before heat treatment. Heat treatment alone is therefore not effective

3.3 Mechanical and electrical properties Mechanical properties (tensile strength and Young modulus) were measured using the scheme shown in Figure 10a. A tensile load was applied to break the fiber according the arrow direction using UTM. The measured

Figure 8. Surface and interface reactions of CNTs inside the aligned CNT fiber. (a) CNTs inside the bare CNT fiber, (b) zinc ion reacting with surface functional groups (–COOH and OH–) for alignment, (c) individual CNTs chemically bonded directly through the Zn at 480 °C for 45 min by removal of CO2, and (d) aligned CNTs in line-shape (bunch like) arrangements resulting from CO2 removal. for enhancing the mechanical properties of these systems, and ZnO nanoparticles are necessary. ZnO nanoparticles participate in the alignment of CNTs in aligned CNT fibers through chemical bonding, as shown previously.

tensile load and corresponding strain (%) are shown in Figure 10b. High-resolution mode of 2 strain (%) area is shown in Figure 10c to clearly see the load bearing properties. The aligned CNT fiber showed tensile strength of

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Figure 9. Reaction and mechanism are shown by chemodraw software. (1) Reaction of product-C (CNT-Zn-CNT), product-D (CNT-O-Zn-CNT), and product-E (CNT/ZnO) formation, (2-6) mechanism of product-C (CNT-Zn-CNT), (710) product-D (CNT-O-Zn-CNT), and (11-12) product-E (CNT/ZnO) formation, respectively. However, the way in which chemical bonds form between ZnO nanoparticles and the carbon of CNTs played an important role in boosting the mechanical properties, as shown in Figure S2. When tensile load (breaking load) was applied (Figure 10a), the breaking load (double head arrow, Figure S2) was active on individual CNTs inside the aligned CNT fiber to break the fiber. From XPS analysis, different types of chemical bonds are available inside the aligned CNT fibers. Among these, C-Zn-C (CNT-ZnCNT) and C-Zn-O-C (CNT-Zn-O-CNT) bonds enhance the tensile strength and Young’s modulus as shown in Figure S2. These bonds worked as cross linking groups (marked by arrow direction) between the individual CNTs inside of the aligned CNT fiber. Besides these cross linking groups, longitudinal linking (marked by arrows) is available, which chemically linked individual CNTs in a “head to tail’’ manner. When force is applied to break the aligned CNT fiber, the cross and longitudinal groups (the C-Zn-C or CNT-Zn-CNT and C-Zn-O-C or CNT-Zn-OCNT) create a force against breaking load like the marked “force against breaking load” through their chemical bond bridges. Thus these linking groups strongly hold the individual CNTs “side by side” (Figure 11a) and “head to tail” (Figure 11b). Subsequently, the force against breaking load (marked by arrows, Figure 11a,b) provided by the anchoring groups helped to increase the Young modulus (266%), and tensile strength (58%). The chemical-bonded interface with ZnO nanoparticles played a significant role in improving the mechanical properties, and these forces were not present in the bare CNT fiber as shown in Figure S3. However, the measured mechanical properties of the prepared aligned CNT fibers was better than the other CNT fiber.30–31,35–36 The improved mechanical properties suggested the effective alignment of CNTs inside of the aligned CNT fibers.

To measure the electrical conductivity of the aligned CNT fiber, a conductive sample (Figure 10d) was prepared for four point probe analysis. Gold wire was used for the 4-terminal connection (1, 2, 3, 4). Silver paste was used to connect the aligned CNT fiber with gold wire and also used to secure the entire circuit on a quartz glass substrate in conjunction with the gold wire. Current (I) was applied at point-1 and -4, and the corresponding output voltage (V) was measured between points-2 and -3.42 The calculated voltage (V) was transformed to resistance according to equation (i) and the calculated resistance was utilized to determine the resistivity by equations (ii, iii).42 Electrical conductivity (σ) was calculated by equation (iv) from the resistivity. Bare CNT fiber electrical conductivity was measured in the same manner. The electrical conductivity of the bare CNT fiber (975 S/cm) and aligned CNT fiber (912 S/cm) suggested that the electrical conductivity of the aligned CNT fiber was well-controlled by the cross and longitudinal linking groups. The electrical conductivity of ZnO is typically quite low, and even the incorporation of carbon materials does not much improve this conductivity (1 × 10–6 to 0.35 S/cm).54 The very high electrical conductivity (912 S/cm) of the aligned CNT fibers suggests minimal insulation by the semiconducting ZnO nanoparticles. Electron movement occurred through the CNT-Zn-CNT (C-Zn-C) bonds of the longitudinal and cross linking bridges between CNTs (Figure S4). The resistance developed due to the deposited ZnO nanoparticles was overcome due to the linking bridges (cross and longitudinal linking) through the ZnO nanoparticles. Here, Zn linked the CNTs by chemical bonding (indicated by dotted arrow) which was used to move electrons through the insulating layer (Figure 12 and 13). Thus, conduction of electron was not hampered by the insulating semiconductor ZnO layer. Even though semiconducting ZnO nanoparticles were present, the electrical conductivi-

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ty of the aligned CNT fibers (912 S/cm) is still higher than that of the bare CNT fiber composite and others reported elsewhere.27–41,55

However, further mechanical properties can be improved by functionalization of bare CNT fiber before using it for alignment. In our synthesis process, we did not

Figure 10. Mechanical property and electrical conductivity measurements. (a) Sample for mechanical property measurements, (b, c) tensile strength vs. strain (%) curve, and (d) electrical conductivity measurement setup for aligned CNT fibers.

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by bare CNT fiber, which has higher mechanical properties. Alignment can be done of the bare CNT fiber of higher mechanical properties to improve further mechanical properties.

functionalize the bare CNT fiber. Bare CNT fiber was used directly for the preparation of aligned CNT fiber. The intensity of D band of Raman spectrum suggested medium number of defect is available inside the CNT fiber, which is created possibly due to the existing functional groups in

Figure 11. Mechanism of mechanical properties enhancement through linking groups. Role of (a) cross linking group and (b) longitudinal linking group during breaking process under an applied breaking load.

CNTs inside the CNT fiber. Since, among the previous discussion, the –COOH and –OH were effective to link crosswise and length wise at interfaces of CNTs, hence if the number of –COOH and –OH group increase it will be helpful to make more linking groups at the interfaces of CNTs. Because, from the Figure 2c and 2f (3D image) shows a lot of ZnO nanoparticles on the surfaces of individual CNTs. It means many of the ZnO nanoparticles were not involved to link CNTs crosswise and lengthwise due to limited number of existing functional groups (COOH, and -OH). CNT fiber twisting can be done during or after deposition of ZnO nanoparticles. The twisting process can improve mechanical and electrical properties a lot. Additionally, mother bare CNT fiber can be replaced

………………………(i)

……………………(ii)

2 A= πr ……………………..(iii) ……………………..(iv)

R= Resistance, V= output voltage, I= applied current, ρ= resistivity, A=cross sectional area of CNT fiber, l= length of CNT fiber between points-1 and 2 (Figure 10d), r= radius of the CNT fiber, and (σ) = electrical conductivity. 4.

Conclusions

The mechanical properties of aligned CNT fiber were improved thanks to the geometric alignment of their in-

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Information. “This material is available free of charge via the Internet at http://pubs.acs.org.”

dividual CNTs promoted by Zn atoms in deposited ZnO nanoparticles. Here, ZnO nanoparticles worked to bind individual CNTs into liner bunches. The electrical conductivity of the aligned CNT fibers was controlled similar to bare CNT fibers. The very high electrical conductivity

AUTHOR INFORMATION Corresponding Author

Figure 12. Inner architecture for overcoming insulation behavior through cross linking (marked by dotted arrow) * [email protected]

of the aligned fiber suggested that the insulating behavior of ZnO was minimized due to the formation of longitudinal and crosswise linkages among the CNTs. Moreover, mechanisms of geometrical arrangement, mechanical properties improvements, and electrical conductivity were proposed. This work expands the field of CNT fibers owing to its use of chemical bond bridges with ZnO nanoparticles for high CNT alignment.

Funding Sources National Research Foundation of Korea (NRF) funded by the Ministry of Education (2014R1A6A1031189).

Notes The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting Information. Raman spectra, inner architecture of aligned and bare CNT are supplied as Supporting

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Figure 13. Inner architecture for overcoming insulation behavior by movement of electron through longitudinal linking. Dotted arrow indicated electron conduction bridge (head to tail) through longitudinal linking groups.

ACKNOWLEDGMENT 5.

This study was supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2014R1A6A1031189).

6.

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Alignment of Carbon Nanotubes in Carbon Nanotube Fibers Through

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