Bioinspired Single-Walled Carbon Nanotubes as a Spider Silk

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Bio-inspired Single-walled Carbon Nanotubes as A Spider Silk Structure for Ultra-high Mechanical Property Chengzhi Luo, Fangying Li, Delong Li, Qiang Fu, and Chun-Xu Pan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11678 • Publication Date (Web): 25 Oct 2016 Downloaded from http://pubs.acs.org on October 26, 2016

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ACS Applied Materials & Interfaces

Bio-inspired Single-walled Carbon Nanotubes as A Spider Silk Structure for Ultra-high Mechanical Property Chengzhi Luo a, Fangying Li a, Delong Li a, Qiang Fu a, b,, Chunxu Pan a, b, *

a

School of Physics and Technology, and MOE Key Laboratory of Artificial Micro- and

Nano-structures, Wuhan University, Wuhan 430072, China.

b

Center for Electron Microscopy, Wuhan University, Wuhan 430072, China.

*Author to whom correspondence should be addressed. E-mail: [email protected], Tel: +86-27-68752481 ext. 8168

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ACS Paragon Plus Environment


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Abstract: Due to its unique hierarchical structure, natural spider silk features exceptional mechanical properties such as high tensile strength and great extensibility, making it one of the toughest materials. Herein, we design a bio-inspired spider silk single-walled carbon nanotubes (BISS-SWCNTs) that combine the hierarchical structure of spider silk and the high strength and conductivity of SWCNTs. To imitate the hierarchical structure, Fe nanoparticles are embedded on the surface of directly synthesized SWCNTs skeleton, and then followed by coating an amorphous carbon layer. The carbon layer forms the spider silk-featured skin-core structure with SWCNTs, thus making the tube junction tougher. The embedded Fe nanoparticles act as glue spots for preventing interfacial slippages between the BISSSWCNTs and reinforced matrix. With only 2.1 wt% BISS-SWCNTs added, the tensile strength and Young’s modulus of the BISS-SWCNTs/PMMA composites can be improved by 300%. More importantly, the BISS-SWCNTs also retain the high conductivity and transmittance of the pristine SWCNTs film. This unique bio-inspired material will be of great importance in applications of multi-functional composite materials, and has important implications for the future of biomimetic materials.

Key Words: single-walled carbon nanotubes; spider silk; biomimetic materials; reinforcement; mechanical properties It is well-known that single-walled carbon nanotubes (SWCNTs) possess high strength, high toughness, high thermal stability, and superior electrical conductivity, and has been 2


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extensively studied as an addictive in multifunctional composites.1-4 Although single SWCNT has Young’s modulus of 1 TPa and tensile strength over 60 GPa,5,6 the SWCNTs reinforced composites only moderately enhanced in modulus and strength, even 2-3 orders of magnitude lower than that of the theoretical predictions. It has been known that two inherent problems of SWCNTs shadow their promise as efficient load-bearers. One is owing to the weak van der Waals interaction which holds SWCNTs together in the bundle, and results in the tubes slide on each other easily.7 The other is the limited interfacial shear strength between SWCNTs and reinforced matrix. Because of their atomically smooth surfaces, the SWCNTs’ bonds to the forced matrix are non-covalent if no chemical modification is applied to the walls of SWCNTs, which leads to ubiquitous interfacial slippages.8 To enhance the strength and toughness of the SWCNT-SWCNT or SWCNTs-matrix interactions, several strategies have been utilized, including chemical functionalization and electron-beam irradiation.7,9-11 However, chemical functionalization requires ultrasonic or stirring in aqueous solution, while electron-beam irradiation always introduces damages. It means that these two strategies frequently introduce defects or contaminants to SWCNTs, and thus decreasing its inherent mechanical properties.12 Therefore, it is desirable to develop procedures through which the mechanical properties of SWCNTs can be improved without disrupting the structural integrity. Recent studies showed that the macro-scale SWCNTs films with continuous reticulate structure have been directly synthesized by floating catalyst chemical vapor deposition (FCCVD) method.13,14 In these SWCNTs films, the intertube junctions could be formed among SWCNTs. Therefore, with increasing strain, this reticulate SWCNTs films extended continuously and homogeneously until they were completely aligned, which meant that the load on the tubes has been continuously transferred through the films. However, in order to

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maximize the efficiency of load transfer, the SWCNTs junctions must be firmly connected to prevent the interfacial slippages between SWCNTs and reinforced matrix. As known, spider web also has a reticulate structure. The spider silk, which constitutes spider web, possesses high strength, elasticity, flexibility, elongation, and fracture resistance. Its excellent comprehensive performance reinforces the unsurpassed superiority, when compared to natural and synthetic fibers.15-17 The mechanical behavior of the spider silk, like that of other biological materials, is determined by the nature of its constituent molecules and their hierarchical assembly into networks. That includes: 1) The spider silk is a kind of skin-core structure with highly organized protein surrounded by a semi-amorphous matrix. When exposed to stretch, the skin-core structure make the spider silk maintain high strength and good toughness at the same time.16 2) The junctions of the spider web are coated by special proteins that can reinforce the connection between silks so as to prevent slippage during stretch.17 3) The surface of the spider silk is coated by glue spots that function as a viscoelastic material to capture prey, thus greatly improving the efficiency of hunting.17,18 Therefore, it is desirable to imitate the structure of spider silk to enhance the mechanical property of SWCNTs. A. M. Beese et al.2 proposed an inspiration from the mechanical behavior in spider silk, which contains hydrogen bonds, that allowed for large deformation while maintaining structural stability. By imitating the structure of spider silk, they realized closely reproduced in carbon nanotubes (CNTs)-PVA (PVA, poly(vinylalcohol)) composite yarns with better mechanical properties. K.-Y. Chun et al.4 developed ultrafast hybrid carbon nanotube yarn muscles by using the strategy spiders deploy to eliminate uncontrolled spinning at the end of dragline silk. However, no research has been reported to imitate the spider silk from external morphology to internal structure.

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In the present work, we take advantage of the unique hierarchical structure of natural spider silk and the integrated reticulate structure of FCCVD-synthesized SWCNTs to design a kind of the bio-inspired spider silk single-walled carbon nanotubes (BISS-SWCNTs) films. These BISS-SWCNTs films were prepared by using the normal FCCVD-synthesizedSWCNTs (denoted as N-SWCNTs) network as skeleton, then the Fe nanoparticles were embedded on the surface of the SWCNTs films, and at last coated an amorphous carbon layer. The carbon layer can not only form the spider silk featured skin-core structure with SWCNTs, but also makes the tube junctions tougher. The embedded Fe nanoparticles acted as glue spots that prevent interfacial slippages between the BISS-SWCNTs and reinforced matrix. Experimental results revealed that the hierarchical structure and the strong interfacial strength resulted in high load-transfer efficiency in sandwiched nano-composites. Moreover, the BISS-SWCNTs films also retained the high conductivity and transmittance of the N-SWCNT films, which are superior to the solution based SWCNTs films.

Results and discussion Preparation and characterization of the BISS-SWCNTs films The BISS-SWCNTs films were prepared in a novel magnetic field-assisted FCCVD system (Figure S1, Supporting Information). Figure 1a illustrates the schematic diagram of the preparation procedure. It is worth noting that although the N-SWCNTs film has the similar reticulate structure as spider web, its internal structures such as smooth surface and non-skincore structure (Figure S2, Supporting Information), are different from the spider silk.19,20 In order to imitate the internal structures of spider silk, we firstly prepared a N-SWCNTs film in the FCCVD system by using ferrocene as catalyst and CH4 as carbon source. Then, we turned off the carbon source and introduced the magnetic field. Because of lack of carbon source, the pyrolyzed Fe particles would be transported to the downstream zone and deposited on the 5



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surface of SWCNTs due to the attraction of magnetic field. Finally, we removed catalyst and introduced carbon source again. Because of lack of catalyst, the carbon source would be pyrolyzed into C and H atoms in high temperature, and the C atoms formed an amorphous carbon layer on the surface of Fe particles and SWCNTs. Figure 1c, d show the scanning electron microscope (SEM) morphologies of the BISS-SWCNTs film, in which the SWCNTs were homogeneously distributed and entangled with each other. In comparison with the structure of spider silk (Figure 1b and S3), we find that not only the external morphology, but also the internal structure of the BISS-SWCNTs (Figure 1e, f) are similar to spider silk, i.e., the BISS-SWCNTs bundles are composed of skin-core structure with highly organized SWCNTs surrounded by an amorphous carbon layer, the surfaces of the BISS-SWCNTs bundles are embedded by Fe particles that function as the glue spots of the spider silk. It is generally acknowledged that reticulate SWCNTs film has its advantage on evenly delivering load over a large area than aligned SWCNTs film, because the load on the tubes can be continuously transferred through the network rather than merely through weak SWCNTs/reinforced matrix interfaces.8 At this point, two key factors must be concerned: 1) The intrinsic mechanical properties of the SWCNTs film. The hierarchical structure of the BISS-SWCNTs can further improve the strength and toughness of the SWCNTs film, therefore it is not easy to be torn during stretching. 2) The load-transfer efficiency between SWCNTs and reinforced matrix. The surfaces of the BISS-SWCNTs are embedded by Fe nano-particles that form a bump structure (Figure 1f). The bump structure makes the surfaces of SWCNTs no longer smooth, so as to achieve the same effect as the functional groups to prevent interfacial slippages between the BISS-SWCNTs and reinforced matrix.

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Raman tests (Figure 1g) revealed that although the intensity of D band in the BISSSWCNT film was slightly stronger than that of the N-SWCNT film due to the introduction of amorphous carbon layer, the intensity ratio of D band and G band (ID/IG) for the BISSSWCNT film was only 0.04, indicating a low content of defects.21 Figure 1h shows the X-ray photoelectron spectroscopy (XPS) results for both BISS-SWCNT and N-SWCNT films. Compared to the N-SWCNT film, the Fe content in the BISS-SWCNT film was increased, while O content was still low. It means that the magnetic field can effectively control the deposition of Fe nano-particles on the SWCNTs surface. On the other hand, it also shows that our approach does not introduce the oxygen-containing functional groups. Mechanical property of the BISS-SWCNTs films As mentioned above, the strength and toughness of the BISS-SWCNTs film would be further improved due to its hierarchical structure as natural spider silk. In order to evaluate the BISSSWCNTs film’s mechanical property, the in-situ SEM observation was performed during the process of tensile deformation and fracture. The changes of morphologies are shown in Figure 2a-f. As known, the network is easy to be torn in the process of tensile strain unless the junctions are firmly connected.13 In our experiment, we found that the BISS-SWCNTs network would not be torn with increasing strain but extended continuously and homogeneously until aligned arrangement (Figure 2b, e). When the strain was near to the stain-to-failure, the breaking point developed and the concentrated stress split the film rapidly (Figure 2c, f). The aligned arrangement of CNTs during stretch can significantly enhance their mechanical property.22,23 But for the normally aligned CNTs, the following two drawbacks limit their applications: 1) Aligned CNTs film exhibits significantly an anisotropic reinforced mechanical properties.22,23 In the direction parallel to the CNTs alignment, there is an 7



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obviously increase in tensile strength, while in the direction perpendicular to the CNTs alignment, the tensile strength increases slightly. 2) The millimeter-scale lengths disenable them from bearing tensile load, and there seems to be little chance that we can obtain CNTs arrays with lengths of tens of centimeters in the near future.24 In our work, the BISSSWCNTs can transform to aligned arrangement during stretching, and the direction of the alignment is along the tensile direction. Therefore, the reinforcement is isotropic, which means the tensile strength can increase obviously in all directions. Besides, the skin-core structure and firmly connected junctions make the BISS-SWCNTs film a continuous reticulate structure, which enable it from bearing a macro-scale tensile load. Although the NSWCNTs film can also transform to aligned arrangement during stretching, the weak van der Waals interaction among the tubes and the loose connection in junctions obviously cannot suffer a strong tensile load. Figure 2g showed a schematic diagram and illustrated the deformation process for the BISS-SWCNTs film during stretching. It vividly clarified the load transfer mechanism of the BISS-SWCNTs film. In order to further compare the mechanical property between the BISS-SWCNTs film and the N-SWCNTs film, tensile tests were performed, as shown in Figure 2h. Under strain, the reticulated SWCNTs can transform to aligned arrangement and the direction of the alignment is along the tensile direction. Therefore, the direction of stress-strain measurement was also parallel to the alignment. As expected, the BISS-SWCNTs film exhibited much higher tensile strength and better toughness. The tensile strength and Young’s modulus of the BISS-SWCNTs film were about 550 MPa and 6.5 GPa, respectively, which surpassed the NSWCNTs film (350 MPa and 3.0 GPa) and the FCCVD synthesized SWCNTs film from other literatures (360 MPa and 5 GPa).8,13,25 More importantly, there were four distinct regimes characteristic of the BISS-SWCNTs film in the strain-stress curve, i.e.: 1) The stiff initial response governed by homogeneous stretching. When the strain was far below the 8


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strain-to-failure, the networks deformed continuously and homogeneously along the direction of strain. 2) The stiffening regime as SWCNTs aligned and load was transferred to the SWCNTs. In this regime, firmly connected junctions can prevent the slippages between tubes. This was also the regime that exhibited a significant difference with the N-SWCNTs film. 3) The slip deformation of the SWCNTs bundles. The strain in this regime was near to failure, extension and slippages mainly occurred at the breaking point, and results in the decrease of strength. 4) The failure. The breaking point developed and the concentrated stress split the film rapidly. Coincidentally, previous research showed that the strain-stress curve of natural spider silk could also be divided into four regimes, in which the initial response regime, stiffening regime, and slip deformation regime were similar to that of the BISS-SWCNTs film.15 It demonstrated that not only the structure but also the mechanical behavior of the BISS-SWCNTs were similar to that of natural spider silk. Mechanical property of the BISS-SWCNTs reinforced composite films To further investigate the superiority of the BISS-SWCNTs film on reinforced composites, we adopted poly(methyl methacrylate) (PMMA) as polymer matrix to represent typical behavior of reinforcement effect. The prerequisite to composite BISS-SWCNTs film with PMMA is to completely transfer the large area BISS-SWCNTs film on PMMA. To solve this problem, we imitated the method that transfer graphene.26,27 The detailed transfer process is shown in Supporting Information (Figure S4). In comparison with the commonly used drypress transfer method,14,28 our method can not only transfer the BISS-SWCNTs film to arbitrary substrates, such as PMMA (inset Figure 3a), polyethylene terephthalate (PET) (Figure S5a, Supporting Information), even

suspended over 30 mm hole (Figure S5b,

Supporting Information), but also guarantee its integrated reticulate structure. The more important advantage is that it can effectively transfer ultra-thin (10 nm) BISS-SWCNTs film. 9



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To test the mechanical property, the PMMA/BISS-SWCNTs/PMMA film with a sandwich structure was fabricated (Figure 3a), i.e. a piece of as grown BISS-SWCNTs film (200 nm in thickness) was transferred on PMMA film followed by casting PMMA solution on the top of above two-layered film. The total thickness of the composite film was 5 µm. For comparison’s sake, we used the same-thickness N-SWCNTs film to conduct the same experiment. Thermogravimetric analysis (Figure S6, Supporting Information) showed that due to the introduction of Fe nano-particles and carbon layer, the mass fraction of the BISSSWNTs (2.1 wt%) inside the composite film was higher than that of the N-SWCNTs (0.25 wt%). Figure 3b illustrates the typical stress-strain curves for the composite films and pure PMMA film with the same total thickness (5 µm). Obviously, the addition of SWCNTs has significant effect on the mechanical property of PMMA matrix. The measured tensile strengths for the PMMA/N-SWCNTs/PMMA and the PMMA/BISS-SWCNTs/PMMA composite films were 21 and 69 MPa, respectively, which were 23% and 300% enhanced than that of pure PMMA film (17 MPa). Coincidently, the Young’s modulus for the PMMA/BISS-SWCNTs/PMMA composite film (1.8 GPa) was also 300% enhanced than that of pure PMMA film (0.43 GPa). It indicates that the mechanical enhancement of the BISSSWCNTs on PMMA is more obvious than that of the N-SWCNTs. Due to the sandwich structure of the composite films, one point should be noticed that since the mechanical properties of pure PMMA are much lower than the BISS-SWCNTs film, most of the stress loaded on the specimen is carried by the middle true effective composite zone. Therefore, while keep the total thickness constant, the mechanical property of the composites is expected to be controllable by adjusting the thickness of BISS-SWCNT film. In the present work, the BISS-SWCNTs film thickness (confirmed by SEM and AFM, Figure S7 and S8, Supporting Information) can easily be controlled within a range of 0-500 nm by changing the preparation time. Figure 3c illustrated the relationships between Young’s 10


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modulus of the composite films and the thicknesses of the BISS-SWCNT films. Approximately, the young’s modulus exhibited a linear relationship with the thicknesses of the BISS-SWCNT films. Therefore, if the thicknesses of the BISS-SWCNT films are adjusted within a range of 0-500 nm, the Young’s modulus of the PMMA/BISSSWCNTs/PMMA composite film can be easily changed in a quite wide range from 0.43 to 3.0 GPa. Here, we propose that the mechanical enhancement of the PMMA/BISSSWCNTs/PMMA than PMMA/N-SWCNTs/PMMA is based upon the following reasons: 1) The excellent intrinsic mechanical property of BISS-SWCNTs. This has been discussed above. 2) The strong interfacial bonding strength between the BISS-SWCNTs and PMMA matrix. In general, this phenomenon can be directly confirmed from the observation of the fracture surface.12 As shown in Figure 3d, the fracture surface of the PMMA/NSWCNTs/PMMA composite exhibited characteristic flat surface, indicating a brittle fracture behavior. The smooth surface of the pull-out N-SWCNTs revealed a poor interfacial strength between the N-SWCNTs and PMMA. On the contrary, for the PMMA/BISSSWCNTs/PMMA composite, as shown in Figure 3d, f, the fracture surface was rough and most of the BISS-SWCNTs bundles were pulled out, which implied that the composite underwent a plastic deformation before cracking. In addition, the pull-out BISS-SWCNTs bundles always sheathed with PMMA on the surface, as shown in Figure 3f inset, which revealed a strong interfacial strength between the BISS-SWCNTs and PMMA. This pull-out phenomenon also demonstrated the fact that the embedded Fe nanoparticles would form a bump structure to prevent the interfacial slippage between the BISS-SWCNTs and matrix. Conductivity and optical transmittance of the BISS-SWCNTs films

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Apart from the excellent mechanical property, high conductivity and high optical transmittance are also superiorities for CNTs utility in multifunctional composites,29,30 such as in transparent electrode.31,32 However, these CNTs films are generally post-treated via solution-based filtration processes, which frequently introduce defects or contaminants, and thus reducing the conductivity and transmittance.33 In the present work, the BISS-SWCNTs films don’t need to directly contact with solution in the preparation and transfer processes. This “dry” method can retain the excellent inherent characteristics of the BISS-SWCNTs. The experiments showed that when the BISS-SWCNTs film was transferred on the surface of PET, the BISS-SWCNTs/PET composite film exhibited a high transparency in the visible region, as shown Figure 4a. As a proof-of-principle demonstration for high conductivity, we used the BISS-SWCNTs/PET film as connecting wire to illuminate a commercial lightemitting diode, as shown in Figure 4b. Figure 4c illustrated the transmittance spectra in a range of 300-700 nm of the BISS-SWCNTs films with different thicknesses. The transmittance at 550 nm was as high as 90% for the 10 nm-thick BISS-SWCNTs film. With increasing of the BISS-SWCNTs film’s thickness from 10 nm to 500 nm, the transmittance at 550 nm decreased from 90 % to 10%. Figure 4d showed comparison of the present BISS-SWCNTs film with the reported results. It can be seen that the transmittance and sheet resistance of our BISS-SWCNTs films were coincident with that of normal FCCVD synthesized SWCNTs films,14 but superior to that of the other SWCNTs films.28,30,33,34 Nonetheless, different from normal SWCNTs films, to imitate the hierarchical structure of spider silk, we introduced Fe nanoparticles and amorphous carbon layer on the surface of the BISS-SWNTs. By comparing the transmittance and sheet resistance of the N-SWCNTs films with that of BISS-SWCNTs films (Figure S9, Supporting Information), we could find that the transmittance and sheet resistance were

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nearly unchanged. It demonstrated that the Fe nanoparticles and the amorphous carbon layer have no obvious impacts on conductivity and optical transmittance.

Conclusions In conclusion, a kind of BISS-SWCNTs with ultra-high strength have been successfully prepared by imitating natural spider silk structure. The preparation strategy is based on magnetic field assisted FCCVD method. The BISS-SWCNTs are composed of pure SWCNTs as skeleton, embedded Fe nanoparticles and coated amorphous carbon layer. The carbon layer can form the spider silk-featured skin-core structure with SWCNTs, thus make the tube junction tougher. The embedded Fe nanoparticles act as glue spots that prevent interfacial slippages between the BISS-SWCNTs and reinforced matrix. The experimental results reveal that the Young’s modulus of the composite can be adjusted in a range of 0.433.0 GPa via controlling the BISS-SWCNTs film thickness in a range of 0-500 nm. In addition to the excellent mechanical property, the BISS-SWCNTs films also possess superior conductivity and transmittance. It is expected that the BISS-SWCNTs will exhibit wide potential applications, such as electromagnetic shielding materials, film electrodes, electromechanical actuators and so on. The present nature-inspired work also opens up a new path to the advanced biomimetic materials.

Methods section

Preparation of BISS-SWCNTs films The BISS-SWCNTs were prepared in a self-made magnetic field-assisted floating catalyst chemical vapor deposition (FCCVD) system in which the distance between reaction zone and the substrate is 20 cm (for details, see the Supporting Information). Figure S1 shows the experimental setup. Different from the normal FCCVD method, our preparation procedure 13



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include three steps, i.e. synthesis of SWCNTs by normal FCCVD; magnetic field induced deposition of Fe particles on the surface of SWCNTs; coating an amorphous carbon layer on the surface of Fe particles and SWCNTs. Transfer of BISS-SWCNTs films

Because the BISS-SWCNTs films were directly deposited on Al foil, it was necessary to transfer the films on target substrate. Considering the thinnest film prepared by us was only 10 nm, in order to ensure the integrity of the hierarchical structure, we proposed a new transfer method that inspired by the transferring of graghene. Figure S3 summarized the steps of the transfer process. Typically, BISS-SWCNTs film was removed from the Al foil by etching in a 1 M HCl aqueous solution. After the Al foil was dissolved, the target substrate was brought into contact with the BISS-SWCNTs film and it was “pulled” from the solution. The transfer process is simple, and no dispersion or cleaning steps were needed prior to transfer. Preparation and mechanical measurement of PMMA/BISS-SWCNTs/PMMA composite sandwich films Poly(methyl methacrylate) (PMMA) was dissolved in dimethyl formamide (DMF) to give a 6 wt% solution, and then the PMMA solution was spin coated onto a glass slide. Then, the glass slide was dried at 60 ºC for 5 h to get PMMA film. Transfer the BISS-SWCNTs film on the PMMA film by using the transfer method mentioned above. Then a top PMMA layer was made by carefully casting PMMA solution on the top of above two-layered film, followed by drying at 60 ºC for 5 h. PMMA/BISS-SWCNTs/PMMA sandwich film was peeled from glass 14


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slide by a doctor blade. The total thickness of the composite film is 5 µm. For the mechanical measurement, the composite film was cut into dumbbell-shaped specimens. Mechanical properties were measured at room temperature with a universal testing machine (CMT6350 Shenzhen SANS, China) with a 200 N load cell at a cross-head speed of 2 mm/min. For comparison, we used the same-thickness N-SWCNTs film to conduct the same experiment. The pure PMMA film with same total thickness was also measured as comparison.

Characterizations The morphologies and microstructures of the samples were characterized using a scanning electron microscopy (SEM) (S-4800, HITACHI, Japan) and high resolution transmission electron microscopy (HRTEM) (JEM 2010FEFHRTEM, JEOL, Japan). For the HRTEM observations, the BISS-SWCNTs were directly deposited upon the ϕ3 mm TEM Cu grids. Raman spectra were obtained by using a Raman spectrometer (Join Yvon LabRam HR, HORIBA, France) with 488 nm laser. Chemical compositions of the samples were analyzed by using an X-ray photoelectron spectroscopy (XPS) (AXIS-Ultra instrument, Kratos Analytical, England) with monochromatic Al Kα radiation (225 W, 15 Ma, 15 kV). Light transmittances of the samples were measured by using a diffuse reflectance accessory of UV– vis spectrophotometer (UV-2550; Shimadzu, Kyoto, Japan) in transmission-mode. Sheet resistance was measured by using a four-point probe (RTS-9, 4 Probes Tech, China). To test the transmittance and the sheet resistance, the SWCNTs films with different thicknesses were transferred on the surface of PET. The electrical sheet resistance and optical transmittance were measured at 550 nm wavelength at 5 different locations on round SWCNTs/PET electrode of 10 cm in diameter.

ASSOCIATED CONTENT

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

Detailed methods; Schematic diagram of the magnetic field-assisted FCCVD system; HRTEM micrographs of the N-SWCNTs; SEM morphologies of spider silk; Schematic diagram of the transfer of BISS-SWCNTs film from Al foil to target substrate; Photographs of the BISS-SWCNT films that transferred on different substrates; Thermogravimetry curves of PMMA/BISS-SWCNTs/PMMA film, PMMA/N-SWCNTs/PMMA film, and pure PMMA; Side-view SEM morphologies of the BISS-SWCNTs films with different thicknesses; AFM images of the BISS-SWCNT films with different thicknesses; The transmittance and sheet resistance of the N-SWCNT films and that of BISS-SWCNT films.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], Tel: +86-27-68752481 ext. 8168

Author Contributions C.L. and C.P. conceived the idea and designed the experiments. C.L. and F.L. prepared the BISS-SWCNT samples. C.L. performed the mechanical property measurements. D.L. and Q.F. performed the physical characterizations. C.L. and C.P. analysed the data and wrote the

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manuscript. All authors contributed to discussions of the results. All authors reviewed the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We gratefully acknowledge financial support from the National Nature Science Foundation of China (No. 11174227). We also thank Dr. Y. Meng and Dr. W. Meng for their assistances. REFERENCES

(1) Yu, D.; Goh, K.; Wang, H.; Wei, L.; Jiang, W.; Zhang, Q.; Dai, L.;Yuan C. Scalable Synthesis of Hierarchically Structured Carbon Nanotube-Graphene Fibres for Capacitive Energy Storage. Nat. Nanotechnol. 2014, 9, 555-562. (2) Beese, A. M.; Sarkar, S.; Nair, A.; Naraghi, M.; An, Z.; Moravsky, A.; Loutfy, R. O.; Buehler, M. J.; Nguyen, S. T.; Espinosa, H. D. Bio-Inspired Carbon Nanotube Polymer Composite Yarns with Hydrogen Bond-Mediated Lateral Interactions. ACS Nano 2013, 7, 3434-3446. (3) Zhou, G.; Wang, Y. Q.; Byun, J. H.; Yi, J. W.; Yoon, S. S.; Cha, H. J.; Lee, J. U.; Oh, Y.; Jung, B. M.; Moon, H. J.; Chou, T. W. High-Strength Single-Walled Carbon

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 27

Nanotube/Permalloy Nanoparticle/Poly (vinyl alcohol) Multifunctional Nanocomposite Fiber. ACS Nano 2015, 9, 11414-11421. (4) Chun, K. Y.; Kim, S. H.; Shin, M. K.; Kwon, C. H.; Park, J.; Kim, Y. T.; Spinks, G. M.; Lima, M. D.; Haines, C. S.; Baughman, R. H.; Kim, S. J. Hybrid Carbon Nanotube Yarn Artificial Muscle Inspired by Spider Dragline Silk. Nat. Commun. 2014, 5, 3322. (5) Yu, M.-F.; Files, B. S.; Arepalli, S.; Ruoff, R. S. Tensile Loading of Ropes of Single Wall Carbon Nanotubes and their Mechanical Properties. Phys. Rev. Lett. 2000, 84, 5552-5555. (6) Yu, M. F.; Lourie, O.; Dyer, M. J.; Moloni, K.; Kelly, T. F.; Ruoff, R. S. Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes Under Tensile Load. Science 2000, 287, 637-640. (7) Kis, A.; Csanyi, G.; Salvetat, J. P.; Lee, T. N.; Couteau, E.; Kulik, A. J.; Benoit, W.; Brugger, J.; Forro, L. Reinforcement of Single-Walled Carbon Nanotube Bundles by Intertube Bridging. Nat. Mater. 2004, 3, 153-157. (8) Ma, W.; Liu, L.; Zhang, Z.; Yang, R.; Liu, G.; Zhang, T.; An, X.; Yi, X.; Ren, Y.; Niu, Z.; Li, J. High-Strength Composite Fibers: Realizing True Potential of Carbon Nanotubes in Polymer Matrix through Continuous Reticulate Architecture and Molecular Level Couplings. Nano Lett. 2009, 9, 2855-2861.

18


Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(9) Gulotty, R.; Castellino, M.; Jagdale, P.; Tagliaferro, A.; Balandin, A. A. Effects of Functionalization on Thermal Properties of Single-Wall and Multi-Wall Carbon NanotubePolymer Nanocomposites. ACS Nano 2013, 7, 5114-5121. (10) Cui, L.-J.; Geng, H.-Z.; Wang, W.-Y.; Chen, L.-T.; Gao, J. Functionalization of Multiwall Carbon Nanotubes to Reduce the Coefficient of the Friction and Improve the Wear Resistance of Multi-Wall Carbon Nanotube/epoxy Composites. Carbon 2013, 54, 277-282. (11) Jiang, C.; Saha, A.; Young, C. C.; Hashim, D. P.; Ramirez, C. E.; Ajayan, P. M.; Pasquali, M.; Martí, A. A. Macroscopic Nanotube Fibers Spun from Single-Walled Carbon Nanotube Polyelectrolytes. ACS Nano 2014, 8, 9107-9112. (12) Ajayan, P. M.; Tour, J. M. Nanotube Composites. Nature 2007, 447, 1066-1068. (13) Ma, W.; Song, L.; Yang, R.; Zhang, T.; Zhao, Y.; Sun, L.; Ren, Y.; Liu, D.; Liu, L.; Shen, J.; Zhang, Z. Directly Synthesized Strong, Highly Conducting, Transparent SingleWalled Carbon Nanotube Films. Nano Lett. 2007, 7, 2307-2311. (14) Nasibulin, A. G.; Kaskela, A.; Mustonen, K.; Anisimov, A. S.; Ruiz, V.; Kivisto, S.; Rackauskas, S.; Timmermans, M. Y.; Pudas, M.; Aitchison, B.; Kauppinen, M. Multifunctional Free-Standing Single-Walled Carbon Nanotube Films. ACS Nano 2011, 5, 3214-3221. (15) Cranford, S. W.; Tarakanova, A.; Pugno, N. M.; Buehler, M. J. Nonlinear Material Behaviour of Spider Silk Yields Robust Webs. Nature 2012, 482, 72-76. 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

(16) Keten, S.; Xu, Z.; Ihle, B.; Buehler, M. J. Nanoconfinement Controls Stiffness, Strength and Mechanical Toughness of β-Sheet Crystals in Silk. Nat. Mater. 2010, 9, 359-367. (17) Koski, K. J.; Akhenblit, P.; McKiernan, K.; Yarger, J. L. Non-invasive Determination of the Complete Elastic Moduli of Spider Silks. Nat. Mater. 2013, 12, 262-267. (18) Sahni, V.; Blackledge, T. A.; Dhinojwala, A. Viscoelastic Solids Explain Spider Web Stickiness. Nat. Commun. 2010, 1, 19. (19) Song, L.; Ci, L.; Lv, L.; Zhou, Z.; Yan, X.; Liu, D.; Yuan, H.; Gao, Y.; Wang, J.; Liu, L.; Zhao, X. Direct Synthesis of a Macroscale Single-Walled Carbon Nanotube Non-Woven Material. Adv. Mater. 2004, 16, 1529-1534. (20) Luo, C.; Wan, D.; Jia, J.; Li, D.; Pan, C.; Liao, L. A Rational Design for the Separation of Metallic and Semiconducting Single-Walled Carbon Nanotubes Using a Magnetic Field. Nanoscale 2016, 8, 13017-13024. (21) Luo, C.; Fu, Q.; Pan, C. Strong Magnetic Field-Assisted Growth of Carbon Nanofibers and Its Microstructural Transformation Mechanism. Sci. Rep. 2015, 5, 9062. (22) Wang, B.; Ma, Y.; Li, N.; Wu, Y.; Li, F.; Chen, Y. Facile and Scalable Fabrication of Well-Aligned and Closely Packed Single-Walled Carbon Nanotube Films on Various Substrates. Adv. Mater. 2010, 22, 3067-3070. (23) Xu, W.; Chen, Y.; Zhan, H.; Wang, J. N. High-Strength Carbon Nanotube Film from Improving Alignment and Densification. Nano Lett. 2016, 16, 946-952. 20


Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(24) Ci, L.; Suhr, J.; Pushparaj, V.; Zhang, X.; Ajayan, P. M. Continuous Carbon Nanotube Reinforced Composites. Nano Lett. 2008, 8, 2762-2766. (25) Ma, W.; Liu, L.; Yang, R.; Zhang, T.; Zhang, Z.; Song, L.; Ren, Y.; Shen, J.; Niu, Z.; Zhou, W.; Xie, S. Monitoring a Micromechanical Process in Macroscale Carbon Nanotube Films and Fibers. Adv. Mater. 2009, 21, 603-608. (26) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312-1314. (27) Reina, A.; Son, H.; Jiao, L.; Fan, B.; Dresselhaus, M. S.; Liu, Z.; Kong, J. Transferring and Identification of Single- and Few-Layer Graphene on Arbitrary Substrates. J. Phys. Chem. C 2008, 112, 17741-17744. (28) Kaskela, A.; Nasibulin, A. G.; Timmermans, M. Y.; Aitchison, B.; Papadimitratos, A.; Tian, Y.; Zhu, Z.; Jiang, H.; Brown, D. P.; Zakhidov, A.; Kauppinen, E. I. AerosolSynthesized SWCNT Networks with Tunable Conductivity and Transparency by a Dry Transfer Technique. Nano Lett. 2010, 10, 4349-4355. (29) Cai, L.; Li, J.; Luan, P.; Dong, H.; Zhao, D.; Zhang, Q.; Zhang, X.; Tu, M.; Zeng, Q.; Zhou, W.; Xie, S. Highly Transparent and Conductive Stretchable Conductors based on Hierarchical Reticulate Single-Walled Carbon Nanotube Architecture. Adv. Funct. Mater. 2012, 22, 5238-5244. 21



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Page 22 of 27

(30) Zhang, D.; Ryu, K.; Liu, X.; Polikarpov, E.; Ly, J.; Tompson, M .E.; Zhou, C. Transparent, Conductive, and Flexible Carbon Nanotube Films and Their Application in Organic Light-Emitting Diodes. Nano Lett. 2006, 6, 1880-1886. (31) Jeon, I.; Cui, K.; Chiba, T.; Anisimov, A.; Nasibulin, A. G.; Kauppinen, E. I.; Maruyama, S.; Matsuo, Y. Direct and Dry Deposited Single-Walled Carbon Nanotube Films Doped with MoOx as Electron-Blocking Transparent Electrodes for Flexible Organic Solar Cells. J. Am. Chem. Soc. 2015, 137, 7982-7985. (32) Kim, S. H.; Song, W.; Jung, M. W.; Kang, M. A.; Kim, K.; Chang, S. J.; Lee, S. S.; Lim, J.; Hwang, J.; Myung, S.; An, K. S. Carbon Nanotube and Graphene Hybrid Thin Film for Transparent Electrodes and Field Effect Transistors. Adv. Mater. 2014, 26, 4247-4252. (33) Green, A. A.; Hersam, M. C. Processing and Properties of Highly Enriched Double-Wall Carbon Nanotubes. Nat. Nanotechnol. 2009, 4, 64-70. (34) Feng, C.; Liu, K.; Wu, J. S.; Liu, L.; Cheng, J. S.; Zhang, Y.; Sun, Y.; Li, Q.; Fan, S.; Jiang, K. Flexible, Stretchable, Transparent Conducting Films Made from Super aligned Carbon Nanotubes. Adv. Funct. Mater. 2010, 20, 885-891.

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Figures:

Figure 1. Microstructures and morphologies of the BISS-SWCNTs. (a) Schematic diagram of the fabrication procedure; (b) Schematic diagram of the hierarchical structure of natural spider silk; (c) SEM morphology of the BISS-SWCNTs film; (d) The edge image; (e) TEM micrographs of the BISS-SWCNTs (inset: the junction covered by amorphous carbon layer); (f) HRTEM micrograph of the Fe nano-particles that tightly attached to the surface of SWCNTs; (g) Raman and (h) XPS spectra of the N-SWCNTs and the BISS-SWCNTs.

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Figure 2. Mechanical property of the BISS-SWCNTs film. (a, b, c) In-situ SEM observation of the deformation process under different strain; (d, e, f) Corresponding high magnification SEM morphologies; (g) Schematic diagram of the structure deformation during tensile test; (h) Stress-strain curves of the N-SWCNTs and the BISS-SWCNTs. The direction of stress-strain measurement was parallel to the alignment.

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Figure 3. Mechanical property of the composite films. (a) Schematic diagram of the sandwich structure of the PMMA/BISS-SWCNTs/PMMA composite film (inset: photo image); (b) Stress-strain curves of pure PMMA, N-SWCNTs and the BISS-SWCNTs composite films; (c) Relationship between Young’s modulus and thickness of the BISS-SWCNTs film, while the total thickness of the composite film keeps constant; (d) Side-view SEM morphology of the fracture section of normal SWCNTs composite; (e, f) Side-view SEM morphologies of the fracture section of the BISS-SWCNTs composite (inset: fracture tip).

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Figure 4. Electrical conductivity and transparency of the BISS-SWCNT film. (a) Image of the BISS-SWCNTs/PET film, which shows high transparency; (b) Image of a lighted LED by using the BISS-SWCNTs/PET strip as a connecting wire; (c) Transmittance spectra of the BISS-SWCNTs/PET films with different BISS-SWCNTs thicknesses; (d) Comparison of SWCNTs prepared by different methods (sheet resistance versus optical transparency at 550 nm).

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Table of Contents Graphic

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Bioinspired Single-Walled Carbon Nanotubes as a Spider Silk

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