Grafting carbon nanotubes directly onto carbon fibers for superior mechanical stability: Towards next generation aerospace composites and energy storage applications

Accepted Manuscript Grafting Carbon Nanotubes Directly onto Carbon Fibers for Superior Mechanical Stability: Towards Next Generation Aerospace Composites and Energy Storage Applications Mohammad S. Islam, Yan Deng, Liyong Tong, Shaikh N. Faisal, Anup K. Roy, Andrew I. Minett, Vincent G. Gomes PII:

S0008-6223(15)30333-X

DOI:

10.1016/j.carbon.2015.10.002

Reference:

CARBON 10385

To appear in:

Carbon

Received Date: 10 July 2015 Revised Date:

2 September 2015

Accepted Date: 1 October 2015

Please cite this article as: M.S. Islam, Y. Deng, L. Tong, S.N. Faisal, A.K. Roy, A.I. Minett, V.G. Gomes, Grafting Carbon Nanotubes Directly onto Carbon Fibers for Superior Mechanical Stability: Towards Next Generation Aerospace Composites and Energy Storage Applications, Carbon (2015), doi: 10.1016/ j.carbon.2015.10.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Grafting Carbon Nanotubes Directly onto Carbon Fibers for Superior Mechanical Stability: Towards Next Generation Aerospace

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Composites and Energy Storage Applications Mohammad S. Islama*, Yan Denga, Liyong Tonga*, Shaikh N. Faisalb, Anup K. Royb, Andrew I. Minettb, Vincent G. Gomesc

School of Aerospace Mechanical and Mechatronic Engineering, The University of Sydney,

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a

Sydney, NSW 2006, Australia.

Laboratory for Sustainable Technology, School of Chemical and Biomolecular Engineering,

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The University of Sydney, NSW 2006, Australia c

Integrated Polymer and Systems Engineering Group, School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006, Australia.

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ABSTRACT. A novel chemical method was developed to graft carbon nanotubes (CNTs) onto carbon fiber (CF) by direct covalent bonding to form a CNT−CF hierarchical reinforcing structure. The grafting via ester linkage (formed at a low temperature of 70oC without using any contaminating catalyst or coupling agent) was evidenced by SEM, FTIR, RAMAN, XPS and

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XRD spectroscopy. The CNT failure stress obtained from in situ SEM pulling out experiments varied from 25 to 31 GPa, depending on the grafting reaction conditions. CNT fracture was the

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only breaking mechanism observed from the pulling out experiments indicating an existence of a strong carbon-carbon covalent bonding at the CNT−CF interface and the real grafting strength was actually higher than the measured failure stress. This high grafting strength can significantly increase the interfacial and impact properties desirable in next generation advanced aerospace composite structures. Further, the CNT attachments on CF led to increased electrochemical capacitance properties by rapid ion diffusion through active CNT sites and defects created during

*

Authors to whom correspondence should be addressed: Tel: 61 2 9351 2342. E-mail: [email protected]; Tel: 61 2 9351 6949. E-mail: [email protected]

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grafting. The fibrous film of CNT−CF exhibits a specific capacitance that is 3.5 times greater than that of CF, indicating substantial promise as a material for fabricating textile

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supercapacitors with superior strength, flexibility and performance.

1. Introduction

Carbon nanotubes (CNTs) can be used to hierarchically reinforce carbon fibers (CFs) to obtain the benefits of enhanced interfacial shear and impact strength since more energy is required to separate the CNTs from CF as well as pulling CNTs out from the matrix [1, 2]. Chemical vapor

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deposition (CVD) and electro-deposition methods are used to link CNTs with CF by physical adsorption (van der Waals interaction), which is weaker compared to chemical covalent bonding,

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limiting the reinforcing effect [3-5]. Alternatively, the catalyst needed for the growth of CNTs contaminate CF [6] and at the same time the use of high temperature degrades the mechanical properties of CF [3, 6].

Grafting CNTs with polymer materials via strong chemical (covalent) bond formation can be performed either by “grafting to” approach which involves pre-formed polymers reacting with

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the surface functionalized groups of CNTs or by “grafting from” approach which involves the polymerization of monomers from surface-derived initiators on CNTs [7]. Due to the advantage of the “grafting to” method’s use of pre-functional groups on the surface of CFs, the grafting of CNTs on CF by ester or amide linkage has been studied by several research groups [6, 8, 9]. The

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added advantage of ester linkage over amide linkage reactions is that the formed ester linkage lacks hydrogen bonding ability and therefore does not self-associate, allowing more carboxylic

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(−COOH) and hydroxyl (−OH) functional groups to diffuse and interact with each other, leading to increased grafting density.

A coupling agent or a combination of coupling agent and dendrimer are used for grafting CNT on CF by amide linkage [6, 8]. However, the grafting process using only a coupling agent suffers from long reaction times. The combined use of coupling agent and dendrimer suffers from large molecular structure that can limit the grafting reaction by added steric hindrance that affects diffusion of the coupling agent to the surface functional groups of CNTs and CF, leading to a low grafting density. Another method for grafting CNTs onto CF involves functionalizing the

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CNTs by carboxylic groups and CF by amine, carboxyl or hydroxyl groups [9]. However, this method requires high temperature for the functionalization of CF (600°C) and the grafting of CNTs on CF (200°C). Moreover, the grafting linkage between CNT and CF occurs in a random manner, i.e., via ester or amide or combination of both of the linkages. Thus all of the above

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methods suffer from either the use of high temperature which degrades the fiber properties and increases the processing cost or the use of coupling agent which may limit the reinforcing effect.

CNTs grafted on fibers have been shown to have increased interfacial adhesion to polymer

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matrix compared to their non-grafted counterparts, thus providing increased strength and

toughness to the composite as a whole [10]. In fact, for the CNT−grafted fiber/epoxy composites

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there exist three dominating interfacial failure mechanisms, namely, (i) detachment of CNTs from CF surface; (ii) pull−out of CNTs from matrix; and (iii) tensile failure (breaking) of CNTs. Therefore, characterization of pulling out behavior of CNTs and the corresponding failure mechanism is of significant importance for the design of high−performance hierarchical nanocomposites. Along with CNT−CF grafting strength, the density of CNT on CF also influences the interfacial shear strength (IFSS) and fracture toughness of fiber−polymer

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composites. Qian et al. [11] concluded that an increase in the CNT density along with other factors such as CNT length, volume fraction and IFSS can lead to a higher Mode-I delamination toughness of composite laminates. The increase in IFSS can be attributed to the increased fiber surface area, good wettability of the CNT-grafted fibers and mechanical interlocking between

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CNTs and the matrix. However, it should be noted that if the grafting density is above a certain point, the narrower gaps between CNTs may restrain the flow of liquid polymer matrix onto CF

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surfaces resulted in a weak interfacial bonding between CF and matrix [12]. For instance, Wang et al. [12] pointed out that the high density of CNTs obtained by CVD was not better than that with a lower density of CNTs obtained by chemical method for IFSS enhancement, even though the CNT grafting density was much higher in CVD than that of the chemical method.

In this work, a new method was used which chemically (covalent ester linkage) graft CNTs directly onto CF at low temperature without any use of dendrimer or coupling agent and in this way produces highly dense hierarchical composites of CNTs on CF. The pulling out behavior of CNTs from the synthesized hierarchical reinforcing structure and the corresponding failure

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mechanism was explored along with their electrochemical capacitance for fabricating flexible textile electronic devices.

2. Methods

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2.1 Oxidation of CFs and CNTs

50 mg CF (Toray's T300 with Young’s modulus of 230 GPa and tensile strength of 3530 MPa from Carbon Fibre Australia) was desized with acetone by refluxing at 60°C for 48 hrs and then dried overnight at 65°C. 100 mL of 1:1 (v/v) mixture of 70% HNO3 and deionised water was

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taken in a round bottom flask with 50 mg preweighed CFs to sonicate for 30 minutes. The mixture was then refluxed at 60°C for 6 hrs. For oxidizing CNTs, 50 mg of CNT (High purity

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multi-walled carbon nanotube powder was obtained from Nanomaterial Store, Sun Innovations, Inc. USA. Their average diameter varies from 30 to 50 nm and their length from 10 to 20 µm), 100 mL of 1:1 (v/v) mixture of 70% HNO3 and deionised water were taken in a round bottom flask to sonicate for 10 minutes. The mixture was then kept at room temperature for 2 hrs. After cooling to room temperature, each of the reaction mixture of CF and CNTs was filtrated and

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washed thoroughly with deionised water and dried overnight at 65°C.

2.2 Reduction of CFs

10 mg of oxidized CF was mixed with 5 mL BH3 in THF at 0 to −5°C for 10 minutes. The mixture was then warmed to 25°C and kept at that temperature for 48 hrs [13]. The reduced CF

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(hydroxyl functionalized) was then vacuum-filtrated and dried overnight at 65oC.

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2.3 Grafting of CNTs on CF

To graft CNT on CF, 2 mg of carboxyl (−COOH) functionalized CNT was mixed with 10 mL of DMF in a glass vial and sonicated for 15 minutes. The mixture was then heated to 70°C for 30 minutes. 2 mg of hydroxyl (−OH) functionalized CF was taken into another flask and a couple of drops of H2SO4 added into it. Afterwards, the heated mixture of CNT in DMF was added dropwise and refluxed at 70°C for 30 min. 4Å molecular sieves were used to remove water produced during reaction. After cooling to room temperature, the CNT grafted on CF was then vacuumfiltrated and dried overnight at 65°C.

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2.4 Characterization The morphology of the samples was examined by using a field emission scanning electron microscopy (FESEM, Zeiss ULTRA plus, Germany). FTIR spectroscopy was carried out using a Bruker Tensor 27 FTIR Spectrometer with Hyperion 3000 FPA microscope. The Raman spectra

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were measured using a Renishaw microRaman spectroscopy system with a 514.5 nm argon-ion laser under ambient conditions. X−ray photoelectron spectroscopy (XPS) analysis were carried out using an instrument (ESCALAB250Xi) with mono-chromated Al K alpha (energy 1486.68 eV) X−ray source of radiation at 90° electron take-off angle. The wide angle X−ray diffraction

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(XRD) patterns were obtained by using a Netherlands 1710 diffractometer with a Cu Ka

2.5 Grafting force measurement

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irradiation source (l ¼ 1.54 A).

For in situ scanning electron microscope (SEM) pulling out experiment, a micromanipulator fitted with a force measurement system (FMS) plug−in tool (Kleindiek Nanotechnik) was mounted in the SEM, allowing sufficient displacement and force resolution at the same time. Pulling out forces were measured by using a highly sensitive piezoresistive atomic force

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microscope (AFM) tip (FMT−400), for which the force resolution was 1 nN and the maximum tip force was 80 µN. Load was applied to the CNT through a displacement-controlled method with a nanometer resolution of 0.25 nm. Each in situ test was recorded on video and the displacement was later analyzed by converting the video to images, followed by relating the

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displacement to the corresponding force exported from the FMS.

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2.6 Supercapacitive performance measurement The carbon fibers and the fibrous film of CNT grafted on CF were directly used as working electrodes by clamping with clamp-type gold wire as current collector in a three-electrode cell with Pt wire as the counter electrode and Ag/AgCl as the reference electrode. CFs and CNTs grafted on CFs were pressed to attach together to a woven clothe like flexible structure. Cyclic voltammetetry (CV) and galvanostatic charge–discharge measurements were carried out using a Biologic SP300 digital electrochemical workstation with potential window from −0.0 to 0.8V in 1.0 M H2SO4 electrolyte.

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Results and Discussion

3.1 Grafting of CNTs on CF CNTs were grafted on CF via ester linkage to hierarchically reinforce CF and achieve superior grafting force between CNT and CF to be used in the development of interfacial shear or impact

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properties of aerospace composite structures. Figure 1 shows the schematic of grafting reaction between oxidized (carboxylic functionalized) CNTs and reduced (hydroxyl functionalized) CF via ester linkage. CNTs were first sonicated and oxidized with nitric acid to form carboxylic (−COOH) functional groups on their surface. As received CF were desized with acetone,

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sonicated and oxidized with nitric acid to form −COOH groups on their surface. The surface −COOH groups of the oxidized CF were then reduced with BF3 in THF to hydroxyl (−OH)

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functional groups. For grafting reaction, carboxylic functionalized CNTs were mixed with DMF and sonicated. The mixture was then added drop-wise to hydroxyl functionalized CF in the presence of concentrated H2SO4 (catalyst) and molecular sieves (water adsrober) to graft CNT on CF via ester linkages. This grafting reaction was carried out at a low temperature (70°C) and completed within 30 minutes to obtain densely populated CNTs on CF forming a CNT−CF

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hierarchical reinforcing structure.

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Figure 1 Schematic of grafting reaction between oxidized CNT and reduced CF.

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3.2 Characterization of CNT-CF Grafting The surface morphology of the untreated and treated CF (Figure S1) as well as CNT–CF hierarchical reinforcing structure (Figure 2) was studied using SEM. The as received CF shown in Figure S1a contains impurities of sizing materials as well as grooves with different width and

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depth along the axial direction. Desizing with acetone removes the impurities as evidenced by Figure S1b. The oxidized and reduced CFs are presented in Figure S1c,d respectively. After

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oxidizing with nitric acid, the fiber surface was cleaner, and on reducing the fiber with borane in THF, the fiber surface remained unchanged.

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Figure 2 SEM images of CNTs grafted with (a) oxidized CF and (b) reduced CF with a

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magnified portion showed in the inset.

Figure 2a,b represents the covalent grafting of carboxylic functionalized CNTs onto oxidized and reduced CF respectively. It is evident from the micrograph that comparatively higher degree of grafting was achieved for reduced CF than that of oxidized CF. This could be attributed to the fact that the reduced CF contains larger quantity of hydroxyl functional group (−OH) which is essential for forming ester linkage with carboxylic functional groups (−COOH) present in CNTs. Figure 2b inset shows a magnified portion of the hierarchical reinforcing structure of CF, in which the grafted CNTs are seen to stick densely onto the CF surface at different angles along

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the outer edges of the grooves present on the CF surface. Additional SEM images for grafting CNTs on CF obtained with reduced CF are given in Figure S2.

FTIR spectra (Figure 3) were obtained to explore the condition of functional groups upon

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oxidation, reduction and grafting reaction of oxidized CF, reduced CF and CNT grafted on CF samples. In the oxidized CF sample, a strong peak at 3360 cm−1 corresponds to the stretching mode of hydroxyl group (−OH) while the peak at 1712 cm−1 indicates carbonyl stretching in carboxylic functional group (−COOH). In the reduced CF sample, the peak for OH stretching is

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found to strengthen and broaden, and the peak for CO stretching in COOH is found to be lower in intensity and shift towards 1710 cm−1 indicating increase of –OH groups. In addition, due to

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the increase of –OH groups, the peak at 2889 cm−1 corresponding to hydrocarbon groups (–CHx) in the oxidized CF sample, is found to increase in intensity and shift to 2891 cm−1 in reduced CF sample. For CNT grafted on CF sample, two peaks at 2854 and 2921 cm−1 indicate sp3 carbonhydrogen stretching and the peak at 1742 cm−1 corresponds to carbonyl stretching [14-16]. These three main peaks indicate interaction between CNT and CF and the formation of ester linkage

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between them.

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33 71 1710

1712

O xidized C F

2889

3360

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Absorbance [a.u.]

R educed C F

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2891

2854

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1 742

2921

C N T gra fted on C F 1000

1 500

2 000

2 500

3 000

350 0

400 0

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W avenum ber (cm )

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Figure 3 FTIR spectra of oxidized CF, reduced CF and CNT grafted on CF.

The grafting of CNTs on CF was further assessed by analyzing the Raman spectra of the as received CF, oxidized CNT and CNT grafted on CF samples as shown in Figure S3. For CF, oxidized CNT and CNT grafted on CF samples, the signals corresponding to D and G bands

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occur at 1370, 1343 and 1348 cm−1 and 1587, 1604 and 1572 cm−1, respectively, which is possibly associated with disordered and graphitized carbon, respectively [17-19]. The relatively high intensity of the D band for the CNT−grafted carbon fiber sample than that of CF sample,

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indicates the presence of CNTs on CF [20].

In Raman spectra, the ratio of D and G bands (ID/IG) can be regarded as a measure of the crystalline order of carbon materials. The CF sample shows broad peaks with linewidths of approximately 200 cm−1 (D band) and 152 cm−1 (G band), indicating a rather low degree of ordering of the graphitic structures. Oxidized CNT sample shows narrower peaks with linewidths of approximately 149 cm−1 (D band) and 134 cm−1 (G band), indicating a higher degree of ordering of the graphitic structures. The bands significantly narrow down to intermediate

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linewidth of 166 cm−1 (D band) and 132 cm−1 (G band) for the CNTs grafted on CF sample further indicating interaction between CNTs and CFs and formation of hierarchical reinforcing structure. Also, the ratio of D and G bands (ID/IG) was found to decrease from 0.86 to 0.83 for CF sample (CF without CNTs) relative to the CNTs grafted on CF sample (CF with CNT) also

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indicative of the bonding between CNTs and CF. The radial breathing mode (RBM) below 300 cm−1 was not detected in the Raman spectra since the CNTs used for grafting were MWCNTs.

To further evaluate the change in chemical composition oxidized CF, reduced CF and CNT

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grafted on CF samples were characterized by X-ray photoemission (XPS) spectroscopy as shown in Figures S4 and Figure 4. To determine any difference in chemical composition for these

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samples, XPS C1s and O1s spectra were measured. The peaks for carbon and oxygen were centered at 284.5 eV and 531.5 eV, respectively. It can be seen from Figure S4a that the oxidized CF sample contained 77 at.% of carbon, 17.3 at.% of oxygen and insignificant amount of nitrogen and chloride. After reduction of oxidized CF using borane, the oxygen content of the reduced CF sample decreased to 16 at.%. For the CNTs grafted on CF sample, the atomic percentage of carbon and oxygen was 74.8 and 17.4%, respectively. The deconvolution of C1s

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peaks (Figure S4b−d) at 284.8 eV (peak 1) originates from the sp2 hybridized graphitic carbon [21] while the peaks at 286.3−286.5 eV (peak 2), 287.8 eV (peak 3), 288.8-288.9 eV (peak 4) and 290.4−290.8 (peak 5), can be associated with C–O (alcohol, ether, phenol) C=O (carbonyl), −COOH (carboxylic) groups and π−π*, respectively [22, 23]. From Figure S4b−d, it can be seen

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that the peak for C−O (alcohol) contained 10.2 at.% of carbon for oxidized CF sample which increased to 11.7 at.% of carbon for reduced CF sample as would be expected due to conversion

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of −COOH groups to −OH groups. The content of carbon decreased from 11.7 at.% for reduced CF sample to 8 at.% for CNTs grafted on CF sample indicating the use up of the −OH groups for the formation of ester linkage. Similarly, the peak for −COOH groups contained 4.3 at.% of carbon for oxidized CF sample which decreased to 3.4 at.% of carbon for reduced CF sample and 2.6 at.% of carbon for CNT grafted on CF sample. For all three samples, the deconvolution of O1s peaks (Figure 4a−c) at 531.1-531.7 eV (peak 1) and 533.1−533.6 eV (peak 2) originates from oxygen doubly bonded to carbon (O=C in quinones, ketones, and aldehydes) and oxygen singly bonded to carbon (O−C in ethers and

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phenols), respectively. Because oxygen atoms in carboxylic (−COOH) groups have both single and double bonds with carbon atoms, the oxygen atoms in −COOH groups can contribute to those two peaks [24, 25]. In addition to the above mentioned peaks, the deconvolution of O1s peaks for CNT grafted on CF sample (Figure 4c) produced an extra peak at 532.3 eV (peak 3)

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which originates from carbonyl oxygen atoms in ester (C−O−C=O) groups with 11.94 at.%

oxygen, further confirming the ester linkage between CNT and CF [26]. The peaks for −COOH groups from oxygen singly and doubly bonded to carbon decreased from 17.3 at.% of oxygen for oxidized CF sample (Figure 4a) to 16 at.% and 5.4 at.% of oxygen for reduced CF sample

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(Figure 4b) and CNT grafted on CF sample (Figure 4c), respectively. This indicates the

conversion of −COOH groups to hydroxyl (−OH) groups in the reduced CF sample upon

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bonds in the CNT grafted on CF sample.

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reduction of oxidized CF to reduced CF and the use of −COOH groups for the formation of ester

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538

536

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532

530

(a)

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532

530

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Bindingenergy (eV)

(b)

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Intensity [a.u.]

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Binding energy (eV)

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2

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Intensity [a.u.]

Intensity [a.u.]

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536

3

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2

532

530

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Binding energy (eV)

(c)

Figure 4 Curve fitting of O 1s peak area to surface functional groups of (a) oxidized CF (b) reduced CF and (c) CNT grafted on CF.

XRD was also used to evaluate the interaction between CNTs and CF for the formation of CNT−CF hierarchical reinforcing structure. Figure S5 represents the XRD patterns of CF and CNT grafted on CF samples. As can be seen, the CF has a diffraction peak at 2θ = 25.5° assigned to the (002) plane of graphite crystal which was found to be stronger and sharper at 2θ = 26.6°

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because of the graphitic structure being more ordered and arranged indicating the interaction between CNTs and CF upon grafting reaction [27, 28].

3.3 Measurement of Grafting Strength between CNT and CF

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In situ SEM pulling out experiment was carried out for assessing the grafting strength between chemically bonded (covalent ester linkage) CNTs and CF. Figure 5 shows the SEM observation of three pulling tests and the corresponding failure mode for the in situ testing, where a,b,c refers to CNT number #1, #2 and #3, respectively. The free end of nanotube was first bonded to the

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AFM tip through high-vacuum compatible SEM glue, which remains uncured under relatively low imaging currents and hardens when focusing the electron beam on a very small area for

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about 20s. Special care was taken to ensure enough curing time and contact length between CNT and AFM tip, to avoid debonding at the CNT/AFM tip interface during all of the tests. After successfully mounting and bonding a nanotube, the load was applied by moving the sharp AFM tip along the direction perpendicular to the CF surface. The videos for the in situ SEM pulling

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out experiments of the CNTs (#1, #2 and #3) are given in S6.

Figure 5 SEM images of the pulling process for three samples

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In order to characterize the effect of different manufacturing conditions such as reaction time and temperature on the resulting mechanical properties, CNT–CF hierarchical structures from two different batches (batch I and batch II) are employed for the pull out testing. Different grafting

Table 1.

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reaction conditions were used in batch I (described in SI) than that of batch II as shown in

Table 1 A comparison of batch I and batch II grafting reaction conditions Batch I

Batch II

Oxidation

CFs and CNTs were oxidized at

CF were oxidized at 60°C for 6 hrs and

60°C for 6 hrs

CNTs were oxidized at 25°C for 2 hrs

Reduction

CNTs were reduced

CFs were reduced

Grafting

Oxidized CFs and reduced CNTs

Oxidized CNTs and reduced CFs were

were refluxed at 90°C for 1 hr

refluxed at 70°C for 30 min

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Reaction Type

Since CNTs in batch 1 were subjected to harsher oxidation conditions, reduction reaction and higher grafting time and temperature than the CNTs in batch II, lower failure stress of CNTs

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would be expected in batch I than that of batch II, which is confirmed by the results shown in Table 2. Along with the maximum pulling forces, the geometrical parameters of each nanotube and failure mechanisms are also listed in Table 2, where Dout stands for the outer diameter of the

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nanotube, and Fmax refers to the measured maximum force.

Table 2 Experimental parameters and failure mechanisms of tested CNTs

#1 #2 #3

Batch

Dout (nm)

Fmax (nN)

Failure mechanism

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22.3

588

CNT breaking

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66.7

2141

CNT breaking

II

45.6

1499

CNT breaking

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CNT number

For a hierarchical reinforcement structure (hybrid composite) consisting of CNTs grafted onto a CF, as illustrated in Figure 6, the failure mode of pull out experiment actually depends on the competition between the bonding strength of CNT–CF and the tensile strength of the grafted

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CNTs. If the detachment of CNT from CF surface occurs first, it indicates that the grafting force between CNT–CF is relatively low, and CNTs will be pulled out from the CF surface. On the other hand, if the fracture of CNT happens first, then it demonstrates that there exists a strong bonding between CNT–CF and the real grafting force is actually higher than the maximum force

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measured or the CNT breaking force.

Figure 6 Illustration of two possible dominating failure modes for pull out testing.

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The maximum load carrying capacity of the CNT can be evaluated by the failure stress ߪ௙ , which is a key mechanical property for the CNT–CF hybrid composites. During the in situ pulling out

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testing, the axial stress carried out by a CNT can be defined as the nominal tensile stress at which the failure occurs. As it is believed that only the outmost layer of the MWCNTs carries the tensile load [29-31], the failure stress is calculated by ߪ௙ ൌ

‫ܨ‬௠௔௫ ‫ܨ‬௠௔௫ ൌ ‫ܣ‬ ߨ‫ܦ‬௢௨௧ ‫ݐ‬௦

where A is the cross-sectional area of the outer layer of the nanotube, and ‫ݐ‬௦ is the interlayer separation of graphite, 0.34 nm.

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Figure 7 shows the failure stress obtained from the present in situ testing. He et al. [32] also carried out the pulling out experiment to determine the grafting forces of CNT–CF hybrids

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fabricated by chemical reaction. Their results are also presented here for comparison.

Figure 7 A comparison between failure stress obtained from the current work and the available literature.

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As shown in Figure 7, failure stress obtained from Ref. [32] shows a relatively wide distribution, which ranges from 0.71 to 16.17 GPa. Also, the breaking mechanism observed from all their

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samples was the detachment of CNTs from CF surface. Compared with their experimental results, failure stress obtained from the present work are much higher. In fact, the failure stress of the present work is in good agreement with tensile failure strength of CNT reported in the literature. As reported by Yu et al. [31], the experimental failure strength of MWCNT is about 11-63 GPa, with a mean value of 28 GPa. For our tested samples, the average failure strength of CNT from batch I and batch II is around 25 GPa and 30 GPa, respectively. Therefore, it is demonstrated that the grafting reaction conditions play an important role in affecting the intrinsic mechanical properties of the grafted CNTs. Moreover, it should be noted that all CNTs break before the detachment occurs, which implies that the measured failure stress is actually the lower

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bound of the real grafting strength. This result indicates that there exists a strong carbon−carbon covalent bonding at the CNT–CF interface, due to the ester linkage formed between CNT and CF surface. It would be expected that the stronger CNT-CF grafting strength obtained in this study, would significantly increase the composites IFSS due to bridging effects introduced by the

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presence of CNTs on CF as reported by Tong et el. [33]. Before carrying out pulling out tests, CNT-CF grafted sample was sonicated to get rid of the presence of any CNTs on CF attached by other means such as van der Waals attraction force between CNT and CF as reported by He et al. [32]. In addition to that, for the pull-out tests CNTs on CF were chosen randomly and none of

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them showed detachment with a weaker pull-out force as seen by He et al. [32]. Therefore, it

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appears that all of the attachments of CNTs on CF were due to grafting.

3.4 Measurement of Supercapacitive Performances of CF and CNT-CF Hierarchical Structure

The supercapacitive performances of CF and CNT grafted on CF were evaluated with a three−electrode cell configuration where a clamp type working electrode with a gold wire was used as the current collector. Fibers were pressed to attach with each other to form a woven cloth

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type flexible structure (Figure S7). Cyclic voltammograms (CVs) of both the samples of carbon fiber and CNT−grafted carbon fiber were measured with a potential window from 0.0 V to +0.8 V at a scan rate of 100 mV s−1 in 1.0 M H2SO4 aqueous electrolyte (Figure 8a). In comparison to CF, the CNT grafted on CF exhibited high capacitive behaviour with increased areas of CV

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curves. The defects on the CF surface after the grafting with CNTs can lead to increase in surface area as well as rapid ion diffusion. The specific capacitance calculated from CV curves acquired

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at 100mV s−1 was able to deliver specific capacitance of 142 F g−1 for CNT grafted on CF and 40 F g−1 for CF which suggests that grafting CNT on CF can increase the capacitance 3.5 times over that of CF.

Figure 8b reveals the CV profiles of CNT grafted on CF at scan rates ranging from 50−500 mV s−1. Consistent proportionate CV profiles with increasing scan rates were observed that indicates good wetting and ease of access by ions [34]. To determine the specific capacitance by three electrode system, galvanostatic charge–discharge (C−D) measurements were carried out at a current density of 1 A g−1 as shown in Figure 8c. The charge curves were

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nearly symmetric with respect to their corresponding discharge curves for the potential range tested. This indicates a high degree of reversibility between the C−D processes. The specific capacitance calculated from the discharge curves of CNT grafted on CF was 143 F g−1, which is approximately 3.5 fold that obtained for CF (39 F g−1). The galvanostatic C–D curves of CNT

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grafted on CF at current densities of 1–10 A g−1 (Figure 8d) present consistent profile shapes, indicating that the CNT grafted on CF is stable over a wide range of current flows, and only 1.17% of the specific capacitance is lost even at a high current density of 10 A g−1.

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The findings for the current study indicate that the grafting of CNT on CF generate enhanced electrode capacitance due to improved accessibility of electrolyte ions through CNT active

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surface sites along with the defects which are introduced on the surface due to the grafting of

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CNTs providing the ability to accumulate more charges than CF.

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Figure 8 (a) Current density versus potential at 100mV s−1 for CF and CNT grafted on CF, (b) cyclic voltammetry of CNT grafted on CF at 50-500 mV s−1 scan rates, (c) comparison of CV

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based on charge-discharge curves at 1 A g−1 for CF and CNT grafted on CF and (d) charge discharge curves of CNT grafted on CF at 1−10 A g−1.

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Conclusions

In the current study, CNTs were directly covalently bonded with CF through ester linkage without using any catalyst or coupling agents. SEM micrographs confirmed the formation of denser hierarchical reinforcement of CNTs with reduced CF than with the oxidized CF. To confirm the oxidation and reduction of CF along with grafting of CNTs on CF through ester

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linkage, the samples were characterized with FTIR, Raman, XPS and XRD spectroscopy. The failure stress obtained from in situ SEM pulling out experiments varied from 25 to 31 GPa, depending on the reaction conditions. The breaking mechanism observed from all samples tested was the fracture of CNTs, indicating a strong carbon-carbon covalent bonding at the CNT–CF

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interface. Thus, the real grafting strength would actually be greater than the measured failure stress and significantly greater than the values reported in the literature (0.71−16.17 GPa). Thus the grafted fibers obtained from the current grafting method will be suitable for application in next generation advanced aerospace composite structures with large increase of interfacial and

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impact strengths. Furthermore, the attachment of CNTs on CF which are active surface sites along with the defects on the surface due to grafting can increase ion diffusion, leading to enhanced electrochemical capacitance properties. The fibrous film of CNT–CF exhibited 3.5

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times higher specific capacitance (143 F g−1 at a current density of 1 A g−1) than the CF, which can be a promising material for fabricating high performance flexible textile electronic devices. Thus, the hierarchical reinforcing structure produced by grafting of CNTs on CF can have multifunctional applications primarily as high performance composite structures and secondarily

Acknowledgments

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as flexible energy storage devices.

This research was supported under Australian Research Council's Discovery Projects funding scheme (project number DP 130103958). The authors would like to acknowledge Mr Mahbub

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Hassan of School of Chemical and Biomolecular Engineering, The University of Sydney, Australia for his advices. The authors would also like to thank Mr Steven Moody of Australian

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Centre for Microscope & Microanalysis (ACMM) of the University of Sydney for his assistance with the pull-out tests. The Authors are also thankful to ACMM and the Vibrational Spectroscopy Core Facility of the University of Sydney.

REFERENCES

[1] Wang, C, He X, Tong L, Peng Q, Wang R, Li Y, and Li Y. Theoretical prediction and experimental verification of pulling carbon nanotubes from carbon fiber prepared by chemical grafting method. Compos Part A- Appl Sci Manuf 2013; 50:1-10.

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[2] Wagner, HD, Lourie O, Feldman Y, and Tenne R. Stress-induced fragmentation of multiwall carbon nanotubes in a polymer matrix. Appl Phys Lett 1998; 72(2):188-190. [3] Thostenson, ET, and Chou T-W. On the elastic properties of carbon nanotube-based composites: Modelling and characterization. J Phys D: Appl Phys 2003; 36:573-582.

surface of carbon fibers. Carbon 2008; 46(2):380-383.

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[4] Zhao, J, Liu L, Guo Q, Shi J, Zhai G, Song J, and Liu Z. Growth of carbon nanotubes on the

[5] Bekyarova, E, Thostenson E, Yu A, Kim H, Gao J, Tang J, Hahn H, Chou T-W, Itkis M, and

composites. Langmuir 2007; 23(7):3970-3974.

SC

Haddon R. Multiscale carbon nanotube-carbon fiber reinforcement for advanced epoxy

[6] He, X, Zhang F, Wang R, and Liu W. Preparation of a carbon nanotube/carbon fiber multi-

M AN U

scale reinforcement by grafting multi-walled carbon nanotubes onto the fibers. Carbon 2007; 45(13):2559-2563.

[7] Spitalsky, Z, Tasis D, Papagelis K, and Galiotis C. Carbon nanotube–polymer composites: Chemistry, processing, mechanical and electrical properties. Prog Polym Sci 2010; 35(3):357401.

[8] Peng, Q, He X, Li Y, Wang C, Wang R, Hu P, Yan Y, and Sritharan T. Chemically and

TE D

uniformly grafting carbon nanotubes onto carbon fibers by poly (amidoamine) for enhancing interfacial strength in carbon fiber composites. J Mater Chem 2012; 22(13):5928-5931. [9] Laachachi, A, Vivet A, Nouet G, Doudou BB, Poilâne C, Chen J, and Ayachi MH. A chemical method to graft carbon nanotubes onto a carbon fiber. Mater Lett 2008; 62(3):394-397.

EP

[10] Lachman, N, Qian H, Houllé M, Amadou J, Shaffer MS, and Wagner HD. Fracture behavior of carbon nanotube/carbon microfiber hybrid polymer composites. J Mater Sci 2013;

AC C

48(16):5590-5595.

[11] Agnihotri, P, Basu S, and Kar, K. Effect of carbon nanotube length and density on the properties of carbon nanotube-coated carbon fiber/polyester composites. Carbon 2011; 49(9):3098-3106.

[12] Wang, C, Li Y, Tong L, Song Q, Li K, Li J, Peng Q, He X, Wang R, and Jiao, W. The role of grafting force and surface wettability in interfacial enhancement of carbon nanotube/carbon fiber hierarchical composites. Carbon 2014; 69:239-246. [13] McMurry, JE, and Melton J. New method for the conversion of nitro groups into carbonyls. J Org Chem 1973; 38(26):4367-4373.

22

ACCEPTED MANUSCRIPT

[14] Kim, W-J, Kang S-O, Ah CS, Lee Y-W, Ha DH, Choi IS, and Yun WS. Functionalization of shortened swcnts using esterification. Bull Korean Chem Soc 2004; 25(9):1301-1302. [15] Chaturvedi, A, Tiwari A, and Tiwari A. Spectroscopic and morphological analysis of graphene vinylester nanocomposites. Adv Mater Lett 2013; 4:656-661.

RI PT

[16] Lehman, JH, Terrones M, Mansfield E, Hurst KE, and Meunier V. Evaluating the characteristics of multiwall carbon nanotubes. Carbon 2011; 49(8):2581-2602.

[17] Dresselhaus, M, Jorio A, and Saito R. Characterizing graphene, graphite, and carbon nanotubes by Raman spectroscopy. Annu Rev Condens Matter Phys 2010; 1(1):89-108.

SC

[18] Antunes, E, Lobo A, Corat E, Trava-Airoldi V, Martin A, and Veríssimo C. Comparative study of first-and second-order raman spectra of mwcnt at visible and infrared laser excitation.

M AN U

Carbon 2006; 44(11):2202-2211.

[19] Zhu, J, Cao W, Yue M, Hou Y, Han J, and Yang M. Strong and stiff aramid nanofiber/carbon nanotube nanocomposites. ACS Nano 2015; 9(3):2489-2501. [20] De Greef, N, Zhang L, Magrez A, Forró L, Locquet J-P, Verpoest I, and Seo JW. Direct growth of carbon nanotubes on carbon fibers: Effect of the CVD parameters on the degradation of mechanical properties of carbon fibers. Diam Relat Mater 2015; 51:39-48.

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[21] Yu, H, Jin Y, Peng F, Wang H, and Yang J. Kinetically controlled side-wall functionalization of carbon nanotubes by nitric acid oxidation. J Phys Chem C 2008; 112(17):6758-6763.

[22] Moreno-Castilla, C, Lopez-Ramon M, and Carrasco-Marın F. Changes in surface chemistry

EP

of activated carbons by wet oxidation. Carbon 2000; 38(14):1995-2001. [23] Hassan, M, Haque E, Reddy KR, Minett AI, Chen J, and Gomes VG. Edge-enriched

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graphene quantum dots for enhanced photo-luminescence and supercapacitance. Nanoscale 2014; 6(20):11988-11994.

[24] Kundu, S, Wang Y, Xia W, and Muhler M. Thermal stability and reducibility of oxygencontaining functional groups on multiwalled carbon nanotube surfaces: A quantitative highresolution XPS and TPD/TPR study. J Phys Chem C 2008; 112(43):16869-16878. [25] Rosenthal, D, Ruta M, Schlögl R, and Kiwi-Minsker L. Combined XPS and TPD study of oxygen-functionalized carbon nanofibers grown on sintered metal fibers. Carbon 2010; 48(6):1835-1843.

23

ACCEPTED MANUSCRIPT

[26] Zielke, U, Hüttinger KJ, and Hoffman WP. Surface-oxidized carbon fibers: I. Surface structure and chemistry. Carbon 1996; 34(8):983-998. [27] Khani, H, and Moradi O. Influence of surface oxidation on the morphological and crystallographic structure of multi-walled carbon nanotubes via different oxidants. J

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Nanostructure Chem 2013; 3(1):73.

[28] Cheng, H, Dong Z, Hu C, Zhao Y, Hu Y, Qu L, Chen N, and Dai L. Textile electrodes woven by carbon nanotube–graphene hybrid fibers for flexible electrochemical capacitors. Nanoscale 2013; 5(8):3428-3434.

SC

[29] Peng, B, Locascio M, Zapol P, Li S, Mielke SL, Schatz GC, and Espinosa HD.

Measurements of near-ultimate strength for multiwalled carbon nanotubes and irradiation-

M AN U

induced crosslinking improvements. Nat Nanotechnol 2008; 3(10):626-631.

[30] Wei, X, Naraghi M, and Espinosa HD. Optimal length scales emerging from shear load transfer in natural materials: Application to carbon-based nanocomposite design. ACS Nano 2012; 6(3):2333-2344.

[31] Yu, M-F, Lourie O, Dyer MJ, Moloni K, Kelly TF, and Ruoff RS. Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load. Science 2000; 287(5453):637-

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640.

[32] He, X, Wang C, Tong L, Wang R, Cao A, Peng Q, Moody S, and Li Y. Direct measurement of grafting strength between an individual carbon nanotube and a carbon fiber. Carbon 2012; 50(10):3782-3788.

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[33] Tong, L, Sun X, and Tan P. Effect of long multi-walled carbon nanotubes on delamination toughness of laminated composites. J Compos Mater 2008; 42(1):5-23.

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[34] Lin, Z, Liu Y, Yao Y, Hildreth OJ, Li Z, Moon K, and Wong C-P. Superior capacitance of functionalized graphene. J Phys Chem C 2011; 115(14):7120-7125.

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Supplementary Information

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Grafting Carbon Nanotubes Directly onto Carbon Fibers for Superior Mechanical Stability: Towards Next Generation Aerospace Composites and Energy Storage Applications

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Mohammad S. Islama†, Yan Denga, Liyong Tonga*, Shaikh N. Faisalb, Anup K. Royb,

a

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Andrew I. Minettb, Vincent G. Gomesc

School of Aerospace Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia.

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Laboratory for Sustainable Technology, School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006, Australia

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Integrated Polymer and Systems Engineering Group, School of Chemical and Biomolecular

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Engineering, The University of Sydney, NSW 2006, Australia.

Lists of supplementary materials:

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Figure S1 SEM images of (a) as received CF, (b) desized CF, (c) oxidized CF, (d) reduced CF. Figure S2 SEM images of hierarchical reinforcement obtained using the method of batch II. Figure S3 Raman spectra of CF, oxidized CNT and CNT grafted on CF.

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Figure S4 (a) XPS wide scan spectra of oxidized CF, reduced CF and CNT grafted on CF; Curve fitting of C1s peak area to surface functional groups of (b) oxidized CF (c) reduced CF and (d) CNT grafted on CF.

Figure S5 XRD spectra of carbon fiber and CNT grafted on carbon fiber. Figure S6 Videos of the in situ SEM pulling out experiments of CNTs.

†

Authors to whom correspondence should be addressed: Tel: 61 2 9351 2342. E-mail: [email protected]; Tel: 61 2 9351 6949. E-mail: [email protected]

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Figure S7 (a) Woven cloth type structure of CF, (b) showing the flexibility of the cloth type

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structure and (c) A clamp type working electrode

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Figure S1 SEM images of (a) as received CF, (b) desized CF, (c) oxidized CF, (d) reduced CF.

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Figure S2 SEM images of hierarchical reinforcement obtained using the method of batch II.

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Figure S4 (a) XPS wide scan spectra of oxidized CF, reduced CF and CNT grafted on CF; Curve fitting of C 1s peak area to surface functional groups of (b) oxidized CF (c) reduced CF and (d)

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CNT grafted on CF.

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Figure S6 Videos of the in situ SEM pulling out experiments of CNTs.

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Figure S7 (a) Woven cloth type structure of CF, (b) showing the flexibility of the cloth type

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Grafting Steps and Conditions for Batch I Oxidation of CF and CNTs.

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50 mg CF was desized with acetone by refluxing at 60°C for 48 hrs and then dried overnight at 65°C. For oxidizing CNTs and CF, 100 mL of 1:1 (v/v) mixture of 70% HNO3 and deionised

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water were taken separately in two different round bottom flask and added 50 mg of CF in one flask and 50 mg of CNTs to another one to sonicate the mixtures for 30 minutes. The mixtures were then refluxed separately at 60°C for 6 hrs. After cooling to room temperature, reaction mixture of CNTs was filtrated and washed thoroughly with deionized water and dried overnight at 65°C.

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Reduction of CNTs. 10 mg of oxidized CNTs was mixed with 5 mL BH3 in THF at 0 to −5°C for 10 minutes. The mixture was then warmed to 25°C and kept at that temperature for 48 hrs [13]. The reduced CNT

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(hydroxyl functionalized) was then vacuum−filtrated and dried overnight at 65°C.

Grafting of CNTs on CF.

To graft CNT on CF, 2 mg of hydroxyl (−OH) functionalized CNT was mixed with 10 mL of DMF in a glass vial and sonicated for 15 minutes. The mixture was then heated to 70°C for 30

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minutes. 2 mg of carboxyl (−COOH) functionalized CF was taken into another flask and a

couple of drops of H2SO4 added into it. Afterwards, the heated mixture of CNT in DMF was

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added drop−wise and refluxed at 90°C for 1 hr. 4Å molecular sieves were used to remove water produced during reaction. After cooling to room temperature, the CNT grafted on CF was then

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vacuum−filtrated and dried overnight at 65oC.

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