Facile and Effective Coloration of Dye-Inert Carbon Fiber Fabrics with

Sep 21, 2017 - Carbon fiber is a good candidate in various applications, including military, structural, sports equipment, energy storage, and infrast...
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Facile and Effective Coloration of Dye-Inert Carbon Fiber Fabrics with Tunable Colors and Excellent Laundering Durability Fengxiang Chen,†,‡,⊥ Huiyu Yang,†,⊥ Ke Li,†,⊥ Bo Deng,†,⊥ Qingsong Li,§,⊥ Xin Liu,†,⊥ Binhai Dong,‡,⊥ Xingfang Xiao,† Dong Wang,† Yong Qin,*,∥ Shi-Min Wang,*,‡ Ke-Qin Zhang,*,§ and Weilin Xu*,† †

State Key Laboratory of New Textile Materials & Advanced Processing Technologies and Hubei Key Laboratory of Advanced Textile Materials & Application, Wuhan Textile University, Wuhan, Hubei 430200, China ‡ Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials and Ministry of Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei University, Wuhan, Hubei 430062, China § National Engineering Laboratory for Modern Silk, College for Textile and Clothing Engineering, Soochow University, Suzhou, Jiangsu 215123, China ∥ State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China S Supporting Information *

ABSTRACT: Carbon fiber is a good candidate in various applications, including in the military, structural, sports equipment, energy storage, and infrastructure. Coloring of carbon fiber has been a big challenge for decades due to their high degrees of crystallization and insufficient chemical affinity to dyes. Here, multicolored carbon fiber fabrics are fabricated using a feasible and effective atomic layer deposition (ALD) technique. The vibrant and uniform structural colors originating from thin-film interference is simply regulated by controlling the thickness of conformal TiO2 coatings on the surface of black carbon fibers. Impressively, the colorful coatings show excellent laundering durability, which can endure 50 cycles of domestic launderings. Moreover, the mechanical properties only drop off slightly after coloring. Overall, these results open an alternative avenue for development of TiO2 nanostructured films with multifunctional features grown using ALD technologies. This technology is speculated to have potential applications in various fields such as color engineering and radiation-proof fabrics and will further guide material design for future innovations in functional optical and color-display devices. More importantly, this research demonstrates a route for the coloring of black carbon fiber-based materials with vibrant colors. KEYWORDS: carbon fiber fabrics, coloring, atomic layer deposition, titanium oxide, laundering durability arbon fiber (CF), produced from polymeric precursors or carbon allotrope building blocks through pyrolysis and carbonization in an inert environment, is defined as the fibrous carbon-based material with a graphite crystal structure consisting of at least 92.0 wt % carbon content.1,2 Excellent mechanical properties, good biological compatibility, high thermal resistance, electrical conductivity, and chemical inertness of carbon fibers make it a top candidate in various fields, including aerospace, sports, automotive, chemical industry, infrastructure, military, energy, and textile.3−6 Carbon fiber fabrics (CFFs) have been trapped inside a monochromatic black cage for decades due to their high degrees of crystallization and insufficient chemical affinity to dyes.2 Very little information could be dug out about colored carbon fabrics

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© 2017 American Chemical Society

except for the products via hybrid weaves with around 50% dyeable yarns such as glass, polyester, copper, and aramid, which will weaken its mechanical properties.7 In addition to the hybrid colored carbon fiber fabrics, patent EP0420655 A2 describes a colored carbon fiber fabric achieved using a polymer resin of a flaky colorant. Apple also patented a method (US7790637 B2) to color carbon fiber’s look by using an additional “scrim” layer to mask the carbon fibers and impart a color other than black. All in a word, nowadays, it is still a big challenge for scientists to color carbon fiber fabrics with lasting full colors directly while Received: July 20, 2017 Accepted: September 21, 2017 Published: September 21, 2017 10330

DOI: 10.1021/acsnano.7b05139 ACS Nano 2017, 11, 10330−10336

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Figure 1. Schematic illustrations of fabrication procedure of multicolored CFFs using the ALD method and the chemical reaction mechanism for deposition of TiO2 layer on CFFs.

maintaining their high mechanical properties.8 Even though projected use of carbon fiber continues to soar, real colored carbon fiber fabrics are still desired by markets. Structural color is a valuable gift that nature has given to us,9−13 which could be found in natural opal, butterfly wings, squid skin, peacock feathers, and sea mice spines, and has inspired a great deal of research devoted to mimicking this natural phenomenon.14,15 Structural color is caused by complex light interactions like thin-film interference, diffraction, coherent scattering, and spatially confined absorption from regular/irregular micro- and nanostructures.10,16,17 Generally, structural colors do not fade and are environmentally friendly, which is an advantage over coloring by traditional dying methods.8,13 Many construction techniques including sol−gel methods,18 self-assembly,19,20 holographic lithography,21 inkjet printing,22 anodization,23−25 electrophoretic deposition,8,26 electro-spinning,27 and atomic layer deposition (ALD)28−30 have been utilized to create structural color. Among these, ALD has been recognized as one of the most efficient and promising approaches due to its high efficiency, precise and simple thickness control, and excellent conformability to the sophisticated surfaces.31−35 Moreover, the ALD process consumes few chemicals and little water, thus minimizing environmental pollution,8 making ALD a promising alternative to traditional dying techniques. Furthermore, the oxygen-free characteristics of ALD make it particularly suitable to fabricate of multicolored (CFs) and CF-based fabrics (CFFs), which are thermally labile to oxygen. Building structural color via ALD allows us to both highlight the black cage of CFFs, with various colors, while maintaining the inherently excellent color brightness and color fastness of structural color and exploiting the environmental friendliness of ALD. Here, a strategy based on ALD is proposed to fabricate conformal TiO2 coatings on CFFs, to endow them with vibrant multiple colors. The reflective spectra are highly dependent on the thickness of the TiO2 layers, which are regulated by the number of ALD cycles (the CFF samples treated by different ALD cycles are denoted as CFF-ALD-n, where n is the number of cycles). Notably, the surface of CFFs is dominated by the chemically inert sp2-bonding, in contrast with the surface of silk fibers, which

has abundant out-of-plane functional groups.36−38 Thus, initiating the ALD reaction on CFFs is difficult.39 However, there are still some defects and oxygen-containing functional groups (such as −OH and −COOH) on the surface of the CFFs, which is confirmed by the following X-ray photoelectron spectroscopy (XPS) analysis (Figure S5a1,a2). The defects and oxygen-containing functional groups were reported to be effective to initiate the growth of TiO2 via ALD.31,36,40 The reaction mechanism shown in Figure 1 illustrates the deposition of TiO2 on the surface of CFFs via ALD. A detailed growth mechanism illustrating the ALD TiO2 on the CFFs is given in Figure S2. First, TIP was introduced onto the surface of the CFFs via a self-limiting chemical reaction with the active groups of the CFs, the −OH or −COOH, to form −OCH(CH3)2. Then, a monolayer TiO2 thin film with the outmost exposed −OH formed by reacting a second introduced H2O with the −OCH(CH3)2.40,41 Remaining species were removed by a nitrogen gas purge after each step. By repeating the above-mentioned ALD cycle to definite numbers, the desired thickness of TiO2 layers could be precisely regulated.

RESULTS AND DISCUSSION The field emission scanning electron microscope (FESEM) micrographs reveal that a bundle of CFF-ALD-4000 consists of many monofilament CFs with diameters of 7−8 μm. A few small irregular protuberances were identified as aggregates of TiO2 nanoparticles in the inset of Figure 2a.41 The FESEM micrographs in Figure 2a,b indicate that the monofilament CFs were fully covered by the uniform, compact TiO2 layers, which exhibit grooves that significantly resemble those of the pristine CFs (Figure S1), thus proving the conformability of ALD. To further investigate the interface between the TiO2 layer and CFs, the transmission electron microscopy (TEM) image of the fresh cross section of the CF-ALD-2000 monofilament treated by focused ion beam (FIB) is shown in Figure S3a,b. It can be clearly seen from Figure S3a that the TiO2 coating shows a uniform thickness. The vague interface (indicated by the red circle) between the CF core and the TiO2 shell, in particular, at the high magnification (Figure S3b), reveals the excellent interfacial adhesion between the CFs and the TiO2 coating. 10331

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Figure 2. FESEM and TEM images of the TiO2-coated CFs. (a) FESEM images of CFF-ALD-4000, shown at different magnifications. (b) FESEM images of a ruptured sample of CF-ALD-4000. The central part is the CF, and the surrounding portion is the TiO2 coating. (c) TEM images of a cross-sectional view of the interfacial region of a CF coated with TiO2, shown at different magnifications. (d) SEM images of TiO2 coating by removing the CFs under air-annealing at 550 °C for 240 min. The inset is the EDS of Ti and O of the isolated TiO2 shell. (e) X-ray diffraction spectra of CFFs, CFF-ALD-4000, and an individual TiO2 shell isolated from the CFF-ALD-4000, where the CFFs were removed by air-annealing at 550 °C for 240 min; the corresponding insets show the enlarged XRD patterns. (f) XPS spectra of the CFFs and CFF-ALD-4000.

tical XRD peaks, indicating the (101), (004), (200), (105), and (204) planes. However, before annealing, the coated TiO2 layers on the CFF-ALD-4000 exhibited the amorphous structure, as shown in the inset spectra of Figure 2e. The chemical composition and stoichiometry of the CFFs and CFF-ALD-4000 were compared using wide-scanning XPS spectra, as shown in Figure 2f. For the CFFs, only C 1s and O 1s peaks appeared (Figure S5a1,a2). The presence of O 1s reveals the existence of oxygen-containing functional groups on the surface of CFFs. However, the Ti peak of CFF-ALD-4000 indicates that TiO2 was successfully constructed on the surface of the CFFs. The CFFALD-4000 exhibited three carbon peaks (Figure S5b1), with binding energies at 284.35, 286.00, and 288.70 eV, which could be assigned to C−C, C−O, and COO− bonding, respectively.43 In addition, the CFF-ALD-4000 exhibited three deconvoluted oxygen peaks with binding energies at 530.19, 531.26, and 532.61 eV (Figure S5b2), which are associated with the lattice oxygen of TiO2, −OH, and C−O groups, respectively.44 Ti 2p1/2 and Ti 2p3/2 peaks with binding energies of 464.42 and 458.71 eV, respectively, can be clearly observed in the Ti spectrum of the CFF-ALD-4000 (Figure S5b3). The difference between the binding energy of the Ti 2p1/2 and Ti 2p3/2 peaks is 5.71 eV, which is in good agreement with that reported.34,36 The relationship between the color and thickness of the TiO2 coatings, which was regulated by the number of ALD cycles (n), is plotted linearly in Figure 3a. According to the calculation, the experimental growth rate of ∼0.086 nm/cycle is lower than the theoretical value of 0.1 nm/cycle.45 Many factors, such as the deposition temperature, surface properties of the substrate,

The complementary contours of energy-dispersive X-ray spectroscopy (EDS) mapping of C, O, and Ti (Figure S3c−g) focus on the TiO2/CF interface, further indicating that the TiO2 was coated firmly and conformally onto CFs. This good conformality of TiO2 coating is further proven by Figure S4. The thickness of the TiO2 layer on CF-ALD-4000 was measured from the corresponding TEM images of ultrathin samples prepared with the FIB. The actual average thickness of TiO2 coating of CF-ALD4000 was 350 ± 6.5 nm (averaged from 20 different positions), as shown in Figure 2c. Furthermore, a compact but unclear interface between the TiO2 coating and the CF was observed in the inset of the enlarged TEM micrograph. Figure 2d shows a hollow TiO2 coating, which was obtained by removing the CFs in an air atmosphere at 550 °C for 240 min. This proves that interfacial chemical reactions occur on the surface of the CFs form a conformal TiO2 coating through the ALD process. An EDS mapping (Figure S4) was performed using an energy-dispersive X-ray spectroscope, which demonstrates that the TiO2 coating was unequivocally formed on the surface of the CFFs (insets of Figure 2d). Figure 2e shows the X-ray diffraction (XRD) spectra of CFFs and CFF-ALD-4000. Both spectra exhibit peaks at 25.2 and 43.8°, which indicate the (002) and (004) crystallographic planes in turbostratic carbonaceous substances.42 The relatively lower intensity of the spectra of CFF-ALD-4000 compared with that of the CFFs could be attributed to the screening effects of the TiO2 coating, which limited the detection depth of the XRD. Annealing at 550 °C for 240 min transformed the amorphous TiO2 coatings into tetragonal rutile TiO2, which exhibited iden10332

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Figure 3. Optical properties of CFFs and CFF-ALD-n. (a) Thickness of TiO2 coatings on the CFFs as a function of ALD cycles. The insets show the corresponding optical images of multicolored CFFs. (b) Reflective spectra of CFF-ALD-0, CFF-ALD-2000, CFF-ALD-4000, and CFF-ALD6000. (c) Angle-resolved reflection spectra of CFF-ALD-6000. (d) Series of photographs of patterned WTU (Wuhan Textile University) logo (http://english.wtu.edu.cn/) with different colors on the CFFs taken at the normal angle by the digital camera, and their corresponding colors in CIE chromaticity. Logo was used with permission.

density of active sites, pulse length, and deposition pressure, could have cause this discrepancy.46−48 Meanwhile, a series of colors, such as blue, blue-white, gold, brown, green, and purple, were observed under natural light after various ALD cycles. Notably, a significant difference between the hue of the warp yarns (convex part) and weft yarns (concave part) of CFF-ALD can be distinguished by the naked eyes. Interestingly, the color brightness varied with the direction of incident light and observation. We speculated that this difference mainly originated from the smooth structures of fibers and their highly oriented arrangements in the CFFs. As depicted in Figure S6a, when the direction of incident light is parallel to the fiber axis, very little mirror-like reflected lights could be detected at the certain viewing angles. Meanwhile, when the incident light is perpendicular to the fiber axis, the light could be reflected in various directions by the curved surface, and thus, diffusely reflected light could be easily detected at various viewing angles. To demonstrate these differences, photos of CFF-ALD-3500 carbon fabrics under various illumination and viewing angles are presented in Figure S6b. The fibers that are perpendicular to the incident light can be seen from all viewing angles, whereas the fibers parallel to the incident light can be seen only in the directions when the combination of viewing and illuminating angles exhibits mirror symmetry. Thus, less reflected lights resulted in a dark purple appearance for warp yarns, and more reflected light led to brighter purple for weft yarns, which may present the specific color effects on the various fabric structures even with the conformal coating on the fibers, which coincides with the above results. The reflective spectra of CFF-ALD with 2000, 4000, and 6000 cycles were measured with an optical spectrometer, as

shown in Figure 3b. All spectra display multiple peaks of constructive thin-film interference over the entire spectra of incident light, which are similar to those of the spin-coated polymer films on smooth, highly reflective surfaces.49 For CFF-ALD-2000, the reflective peak at 419 nm is associated with a blue-white color. In contrast, the two reflective peaks at 374 and 763 nm of CFFALD-4000 together form the hue that the human eyes interpret as brown. In the curve of CFF-ALD-6000, two peaks at 374 and 546 nm represent green and gold colors, respectively, which are interpreted by human eyes as purple when mixed together. The angle-resolved reflective spectra of CFF-ALD-6000 with the incident angles of 0, 5, 10, 15, 20, 25, and 30° are shown in Figure 3c. The nearly angle-independent purple color of CFF-ALD-6000 can be demonstrated by negligible shifts around the reflective peak of 546 nm. This is because the tubular CFs in the bundles are only 7−8 μm in diameter. Thus, the incident light shifted minimally from the axis, and the light was averaged by adjacent fibers. Therefore, there are almost no peak shifts in the reflective spectra. Figure 3d demonstrates the structural colored CFFs with the logo of our university via a mask. Four colors, ranging from dark blue to purple, were realized by varying the ALD cycles. To straightforwardly sense the color variations with different ALD cycles, the reflective spectra were converted into Commission Internationale de l’Eclairage (CIE) chromaticity values, as shown in Figure 3d. Particularly when multicolored CFFs are used as clothing or personal protective equipment (PPE), their laundering durability and liquid repellency are important factors that should be considered. Here, the laundering durability of CFF-ALD-2500 was evaluated according to the American Association of Textile 10333

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Figure 4. (a) Laundering durability of CFF-ALD-2500. Washing of CFF-ALD-2500 was performed according to American Association of Textile Chemists and Colorists 61-2006 methods (AATCC 61−2006), tested with WOB detergent (0.15%, w/w) and 100 stainless steel balls in the water bath of 71 °C. (b) Stress−strain curves of CFF-ALD. 5 × 5 cm pieces and air-dried for further use after ultrasonic cleaning in the ethanol for 30 min. Titanium(IV) isopropoxide (TIP, 99.999% metals basis, analytical grade) was directly used as received from Aladdin Industrial Co., Ltd. Preparation of Multicolored Carbon Fiber Fabrics. ALD reaction was performed in a self-made hot-wall closed ALD reactor as previously reported.36−38 High-purity nitrogen (N2, 99.999%) was used as both purging and carrying gas at a steady flow rate of 50 sccm throughout the ALD process. Multilayer TiO2 was fabricated by the following procedure. First, the cleaned CFFs were placed into the ALD reactor and kept at 150 °C for 30 min in a vacuum (∼0.5 Torr) to reach an equilibrium. Second, H2O and TIP were alternatingly introduced into the ALD reactor. To provide sufficient vapor pressures for the ALD TiO2 process, the TIP was heated to 80 °C, while water was maintained at room temperature. One complete cycle of the ALD proceeded in the following order: H2O pulse/exposure/N2 purge/TIP pulse/exposure/ N2 purge, with a duration of 0.05/8/20/0.2/8/20 s, respectively. Finally, sample sets were thus prepared by varying the ALD cycle numbers as systematically set up and performed. All other reaction conditions were kept same without further noting. Morphological, Structural, and Composition Characterization. The surface morphologies and microstructures of multilayer TiO2coated CFFs were characterized by FESEM (S-4800, operated at 10 kV, Hitachi Ltd., Japan) after sputtered with platinum for 120 s. The diameter of the carbon fiber and the thickness of the TiO2 were determined by the FESEM and TEM images using ImageJ software. High-resolution TEM was performed using a JEOL JEM-2010 in bright-field mode. The cross sections of CF bundles were obtained using FIB (Helios Lab, operated at 30 kV, USA) technique. X-ray diffraction patterns were characterized using an X’Pert PRO XRD spectrometer (PANalytical, Holland) at a scanning rate of 10° min−1 in the 2θ range from 20 to 70°. The reflection spectra were collected by a reflective spectrometer, equipped with a Cu Kα radiation source (λ = 0.15405 nm) at a generator voltage of 40 kV and current of 50 mA. X-ray photoelectron spectroscopy (SPM-9700, SHIMADZU, Co.) measurements equipped with an Al Kα radiation source (1486.6 eV) were used in this study. Optical Measurement. Optical photographs of CFFs and CFFALD were taken by a digital camera (Nikon DSLR D5100) under ordinary white light.The normal reflective spectra of the CFFs and CFFALD with different ALD cycles were collected by PG2000-Pro spectrometer (Idea Optics Co., Ltd., China) equipped with a UV−vis−NIR light source. Angle-resolved reflective spectra were measured using an angle-resolved microspectroscopy system (ARM160, Ideaoptics, PR China). Water Repellency. The static water contact angles were measured using a Dataphysics OCA 30 (Germany) with a 5 μL deionized water droplet at ambient temperature. Photos of all liquid spheres (methylene blue aqueous solution (100 mg/L), oil red/ethanol solution (75 mg/L),

Chemists and Colorists test method 61-2006 (AATCC TM612006). The excellent laundering durability of the TiO2 coating on CFF-ALD-2500 was demonstrated by the K/S values (Table S1) and unoffset reflective peaks in Figure 4a, which both remained the same after 10 accelerated laundering cycles (equivalent to 50 commercial or domestic launderings). In addition to the laundering durability, liquid repellency is another factor that should be considered for textiles. The repellent effects of CFF-ALD-2500 against an aqueous solution of methylene blue, oil red/ethanol solution, coffee, milk, and ink are shown in Figure S7. All liquid droplets exhibit spherical shapes with high contact angles (CAs), thus confirming the excellent water-based liquid repellency of the CFF-ALD-2500. As we know, the excellent mechanical properties of carbon fibers make it a star materials in many fields. The surface modification and other additional treatments will largely sacrifice its mechanical properties. Therefore, the mechanical properties of CF-ALD-n should be evaluated. Figure 4b and Table S2 show the mechanical properties of the CF-ALD-n with different ALD cycles. The bare carbon fiber exhibits a tensile strength of 5.24 ± 1.28 GPa, modulus of 303.40 ± 26.09 GPa, and strain at break of 1.69 ± 0.32%. Corresponding values of CF-ALD-n drop off slightly after ALD treatment. This may be due to the weak oxidation resistance of CFFs under the synergy effects of temperature and residual oxygen as reported in literature.44

CONCLUSION In conclusion, a feasible, environmentally friendly TiO2 coating by ALD for producing color on dye-inert CFFs is proposed. Conformal coatings with vibrant colors are obtained by building up the TiO2 coating to the appropriate thickness on the surface of CFFs. Structural colors spanning different color categories with vibrant and uniform hues can be precisely regulated simply by varying the thickness of the TiO2 coatings. Moreover, the chemically bonded TiO2 demonstrated excellent laundering durability, which could endure machine washing 50 times without significant color fading. Applications of the colorful CFFs may extend into the consumer electronics and automobile industries. Furthermore, their excellent water repellency is another advantage, which may be applicable in water-proof fabrics. METHODS Materials. 3K plain weave PAN-based CFFs (1.72 g/cm3) were purchased from Toray group, Japan. The CFFs were cut into approximately 10334

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ACS Nano coffee, milk, and ink) on the surface of CFF-ALD were taken at 60 s after the dispensing needle detached from the droplet. Mechanical Properties. Mechanical properties of the carbon fibers and multicolored carbon fiber were evaluated using a XQ-1C tensile tester (Shanghai New Fiber Instrument Co., Ltd., China) at 25 °C and relative humidity of 63% at a gauge length of 20 mm and strain rate of 0.1%/s. The samples were obtained at least 20 times, and the average values were calculated.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b05139. Figures S1−S7 and Tables S1 and S2 (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yong Qin: 0000-0002-5567-1464 Weilin Xu: 0000-0003-2202-3476 Author Contributions ⊥

F.C., H.Y., K.L., B.D., Q.L., X.L., and B.D. contributed equally to this work. Funding

This research was financially supported by the National Science Foundation for Distinguished Young Scholars (Grant No. 51325306), the National Natural Science Foundation of China (Grant Nos. 51203124, 51373110, 51773158, and 51373110), and the Program for Middle-aged and Young Talents from Educational Commission of Hubei Province (Grant No. Q20120103). Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful to Dr. Ning Zhou, Wei Yuan (National Engineering Laboratory for Modern Silk, College for Textile and Clothing Engineering, Soochow University), Prof. Zhiguang Guo, Dr. Li Wan (School of Materials Science and Engineering, Hubei University), and Prof. Yaodong Liu (State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences) for their valuable advice. REFERENCES (1) Frank, E.; Steudle, L. M.; Ingildeev, D.; Spörl, J. M.; Buchmeiser, M. R. Carbon Fibers: Precursor Systems, Processing, Structure, and Properties. Angew. Chem., Int. Ed. 2014, 53, 5262−5298. (2) Kim, J. W.; Lee, J. S. Preparation of Carbon Fibers from Linear Low Density Polyethylene. Carbon 2015, 94, 524−530. (3) Frank, E.; Hermanutz, F.; Buchmeiser, M. R. Carbon Fibers: Precursors, Manufacturing, and Properties. Macromol. Mater. Eng. 2012, 297, 493−501. (4) Feldhoff, A.; Pippel, E.; Wolterdorf, J. Interface Engineering of Carbon Fiber Reinforced Mg−Al Alloys. Adv. Eng. Mater. 2000, 2, 471− 480. (5) Pimenta, S.; Pinho, S. T. Recycling Carbon Fibre Reinforced Polymers for Structural Applications: Technology Review and Market Outlook. Waste Manage. 2011, 31, 378−392. (6) Helmer, T.; Peterlik, H.; Kromp, K. Coating of Carbon Fibers The Strength of the Fibers. J. Am. Ceram. Soc. 1995, 78, 133−136. 10335

DOI: 10.1021/acsnano.7b05139 ACS Nano 2017, 11, 10330−10336

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DOI: 10.1021/acsnano.7b05139 ACS Nano 2017, 11, 10330−10336