Ultratough Bioinspired Graphene Fiber via Sequential Toughening of

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Ultratough Bioinspired Graphene Fiber via Sequentially Toughening of Hydrogen and Ionic Bonding Xiaohui Wang, Jingsong Peng, Yuanyuan Zhang, Mingzhu Li, Eduardo Saiz, Antoni P. Tomsia, and Qunfeng Cheng ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07392 • Publication Date (Web): 21 Nov 2018 Downloaded from http://pubs.acs.org on November 22, 2018

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Ultratough Bioinspired Graphene Fiber via Sequentially Toughening of Hydrogen and Ionic Bonding Xiaohui Wang,†, ⊥ Jingsong Peng,†, ⊥ Yuanyuan Zhang,†, ⊥ Mingzhu Li,‡⊥ Eduardo Saiz,§

Antoni P. Tomsia,† and Qunfeng Cheng†,Δ,#* †Key

Laboratory of Bio-inspired Smart Interfacial Science and Technology of Ministry of

Education, School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing 100191, P. R. China ‡Key

Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences

(ICCAS), Beijing 100190, P. R. China §Department

of Materials, Centre for Advanced Structural Ceramics, Imperial College

London, London SW7 2AZ, U.K. ΔState

Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua

University, Shanghai 201620, P. R. China #State

Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical

Technology, Beijing 1000029, P. R. China ⊥These

four authors contributed equally to this work

*Correspondence should be addressed to Qunfeng Cheng, E-mail: [email protected]

Abstract: Graphene-based fibers synthesized under ambient temperature have not achieved excellent mechanical properties of high toughness or tensile strength compared with those 1 ACS Paragon Plus Environment

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synthesized by hydrothermal strategy or graphitization and annealing treatment. Inspired by relationship between organic/inorganic hierarchical structure, interfacial interactions and moderate growth temperature of natural nacre, we fabricate ultratough graphene fiber via sequentially toughening of hydrogen and ionic bonding through wet-spinning method under ambient temperature. A slight amount of chitosan is introduced to form hydrogen bonding with graphene oxide nanosheets, and the ionic bonding is formed between graphene oxide nanosheets and divalent calcium ions. The optimized sequential toughening of hydrogen and ionic bonding results in ultra-tough graphene fiber with toughness of 26.3 MJ/m3 and ultimate tensile strength of 743.6 MPa. Meanwhile, the electrical conductivity of resultant graphene fiber shows as high as 179.0 S/cm. This kind of multifunctional graphene fiber shows promising applications in photovoltaic wires, flexible supercapacitor electrodes, wearable electronic textiles and fiber motors etc. Furthermore, the strategy of sequentially toughening of hydrogen and ionic bonding interactions also offers an avenue for constructing high performance graphene-based fiber in the near future. Graphic Abstract:

Keywords: ultratough, bioinspired, graphene fiber, sequentially toughening

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Tough, strong, and conductive graphene-based fibers (GFs)1,2 have various applications in many fields. It has been researched widely since the graphene fiber reported in 2011.3 In order to achieve high performance, a series of GFs are synthesized through heating strategies, including hydrothermal strategy, graphitization and thermal annealing treatment. GFs synthesized through hydrothermal strategy developed relatively low toughness and tensile strength early.4,5 Graphitization and thermal annealing treated strategies greatly improve tensile strength or electrical conductivity of GFs, but the toughness is still inferior. For example, Xu et al. fabricated graphitization and thermal annealing treated GFs with tensile strength of 1450 MPa and electrical conductivity of 8000 S/cm respectively, but the toughness was only 3.9 MJ/m3.6 An ultrastrong GF synthesized by thermal annealing method showed tensile strength of 1150 MPa, but the toughness was only 8.3 MJ/m3.7 Ma et al. developed annealing treatment to achieve GFs with tensile strength of 724 MPa and toughness of 9.44 MJ/m3 respectively.8 Mussel-inspired GFs treated under 1000 °C achieved excellent electrical conductivity, but the tensile strength and toughness were only 650 MPa and 2.4 MJ/m3 respectively.9 Heating treated methods take the functional groups of graphene oxide (GO) away from GFs, so the fibers become fragile and the toughness is low. Recently, an ultra-tough GF was developed through hydrothermal method at 230°C with ultra-high toughness of 30 MJ/m3, but the ultra-high toughness was derived from the synergetic effect of interconnected graphene ribbons and graphene sheets, and the tensile strength was only 223 MPa.10 As we know, the high temperature of hydrothermal, graphitization and annealing strategy is harsh, energy-consuming,


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and incapable of large-scale production, and these disadvantages limit the various applications of fibers in different fields.11-16 Natural nacre exhibits outstanding integrated mechanical property benefited from the organic/inorganic hierarchical structure, interfacial interactions and moderate growth temperature.17-20 Usually, there are mainly four types of interfacial interactions existing between GO and added reinforcement materials: hydrogen bonding,21-23 covalent bonding,24-28 ionic bonding29-31 and π-π interactions.32-34 For instance, Zhao et al. grafted poly(glycidyl methacrylate) (PGMA) to GO by covalent bonding and synthesized GO-PGMA fiber through wet-spinning method, and the fiber showed tensile strength of 500 MPa, but the toughness was only 7.5 MJ/m3.35 In the prior work of our group, ionic bonding and covalent bonding synergistically toughened rGO-Ca2+-PCDO fiber achieved high tensile strength of 842.6 MPa, but the toughness was 15.8 MJ/m3.36 It still remains a great challenge to improve the toughness simultaneously with strength of GFs through interfacial interactions under ambient temperature. Herein, an ultratough bioinspired graphene fiber is demonstrated through sequentially toughening of hydrogen and ionic bonding by introducing a tiny amount of chitosan (CS) and divalent calcium ions (Ca2+) through wet-spinning method under ambient temperature. The functional groups of GO and CS are reserved to form hydrogen and ionic bonding interactions and the fibers are sequentially toughened. The toughness of resultant GFs reaches as high as 26.3 MJ/m3, and the tensile strength also reaches up to743.6 MPa. Meanwhile, the electrical conductivity is about 179.0 S/cm. This multifunctional GF shows promising applications in


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flexible supercapacitor electrodes,4 wearable electronic textiles,37,38 fiber motors39 and other applications.40-48 RESULTS AND DISCUSSION The preparation process of ultratough GFs through wet-spinning method is shown in Figure 1a. The CS solution was firstly added into GO solution with different GO/CS ratios, then GO nanosheets and CS molecules were mixed homogeneous by stirring for 24 hours (h) and sonication for 30 minutes (min). The CS molecules were adsorbed onto GO nanosheets through hydrogen bonding during stirring and sonication process, which was confirmed by atom force microscopy (AFM) images in Figure S1. The thickness of a single layer GO nanosheet is about 0.8 nm. After adding 4.95 wt% CS, the thickness of GO/CS hybrid building block is about 1.0 nm. Then the disordered GO nanosheets and CS molecules were assembled to macroscopic fibers through wet-spinning process by the extrusion forces of the syringe. After coagulating in CaCl2 coagulating bath and washing in ethanol/water solution, the graphene oxide-chitosanCa2+ (designated as GO-CS-Ca2+) fibers were obtained. The fibers were chemical reduced by hydroiodic (HI) acid solution, and some unreacted functional groups of GO nanosheets disappeared. Finally, the interlayer spacing of reduced GO (rGO) nanosheets decreased, and tightly hierarchical reduced GO-CS-Ca2+ (rGO-CS-Ca2+) fibers were obtained. The hierarchical structure was confirmed by the scanning electron microscopy (SEM) images in Figure 1b, and the magnification image shows regular layer structure in cross-section of a rGOCS-Ca2+fiber. Figure 1c shows the surface morphology of a single rGO-CS-Ca2+ fiber.


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Figure 1. a) Schematic illustration of the preparation process of the rGO-CS-Ca2+ fibers. The CS molecules are adsorbed onto the GO nanosheets through hydrogen bonding interactions after mixing (the magnification section shows the hydrogen bonding interactions between functional groups of GO and CS). Then the mixture of disordered GO nanosheets and CS molecules are assembled to macroscopic fibers in CaCl2 coagulating bath. Finally, the fibers are chemical reduced by HI solution. The interlayer spacing of rGO nanosheets decreaseand tightly hierarchical rGO-CS-Ca2+ fibers are obtained. b) SEM images of the cross-section of a


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single rGO-CS-Ca2+ fiber, and the magnification image shows regular layer structure of the fiber. c) Surface morphology of a single rGO-CS-Ca2+ fiber. The CS content has a great impact on the mechanical properties of resultant GFs. We fabricated a series of GO-CS-Ca2+ fibers with different contents of CS, and exact contents of CS were determined by thermogravimetric analysis (TGA), as shown in Figure S2 and Table S1. The ultimate CS content was 1.12 wt%, 4.95 wt%, 9.18 wt%, 12.91 wt% in GO-CS-I-Ca2+ fiber, GO-CS-II-Ca2+ fiber, GO-CS-III-Ca2+ fiber and GO-CS-IV-Ca2+ fiber respectively. The TGA curves of rGO-Ca2+ and rGO-CS-Ca2+ fibers were also shown in Figure S3. GO fiber was fabricated in acetone coagulating bath for comparing with GO-Ca2+ fiber in Fourier Transform infrared spectroscopy (FTIR). FTIR was conducted to test the changes of characteristic peaks of functional groups verifying the reactions between GO and CS and Ca2+ in GO-CS-Ca2+ fibers, as shown in Figure 2a. In our previous work,49 the flow force in the vacuum-filtration process accelerated spreading of the CS molecular chains on the surface of GO nanosheets at low CS content and reduced the sterichindrance of CS, then the exposed reaction sites (amine) on the CS chemically reacted with carboxyl groups on GO nanosheets. However, during the assemble process of the GFs, the GO/CS building blocks and hydrosolvent of the homogeneous mixture were simultaneously extruded to coagulating bath, and there was no flow force acted on GO/CS building blocks, so there was no chemical reaction occurred between CS and GO, which is confirmed by the FTIR spectra in Figure 2a. Compared with GO fiber, the carboxyl C=O stretch intensities at 1730 cm-1 in GO-Ca2+ and GO-CS-Ca2+ fibers decrease as carboxyl C-O stretch intensities at 1400 cm-1 increase and shift to lower wavenumbers of 1380 cm-1, suggesting the 7 ACS Paragon Plus Environment

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coordination between carboxyls of GO and Ca2+ ions. The intensity of epoxy/ether C-O stretch at 1222 cm-1 significantly decrease and even vanish in GO-Ca2+ and GO-CS-Ca2+ fibers, accompanied by the increase of alkoxide C-O stretches at 1050 cm-1, indicating that the epoxy groups of GO have ring-opening reaction and crosslink with Ca2+ ions.30 Energy dispersive spectroscopy (EDS) shows that Ca2+ ions distribute evenly in GO-CS-II-Ca2+ and rGO-CS-IICa2+ fibers respectively (Figure S4 and S5). X-ray diffraction (XRD) was conducted to test the interlayer distances of GO-CS-Ca2+ and rGO-CS-Ca2+ fibers, as shown in Figure S6 and Figure 2c. The detailed results are listed in Table S2. The interlayer distances of the rGO-CS-Ca2+ fibers increase as the loading of CS increasing, indicating that the CS molecules are successfully embedded into GO nanosheets. But the peaks become wide as the CS content increasing and there is no exact peak in fiber of rGO-CS-IV-Ca2+ when CS content reaches to 12.91 wt%, indicating that when a small amount of CS are added, CS distrbute evenly on the layer structure of GO nanosheets, and as the CS content increases, too much CS stacking together may influence the layer structure of resultant GFs. This also explaines why the subsequent mechanical properties of the rGO-CS-Ca2+ fibers increase firstly and then decrease with the CS content further increasing. Raman spectra of GOCS-Ca2+and rGO-CS-Ca2+ fibers are shown in Figure 2c and 2d respectively and the detailed data are listed in Table S3. The ID/IG ratios of GO-CS-Ca2+ and rGO-CS-Ca2+ fibers increase as CS content increasing, indicating that the defect (caused by hydrogen bonding crosslinking of CS and GO) content of GO and rGO increase as CS content increasing. Compared with GO-


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CS-Ca2+ fibers, the ID/IG ratios of corresponding proportional rGO-CS-Ca2+ fibers increase, indicating that the sp2 hybridized carbons are restored after HI reduction.36

Figure 2. a) FTIR spectra of GO-CS-Ca2+ fibers. The changes of the characteristic peaks at marked sections indicate the formation of ionic bonding between Ca2+ ions and functional groups on GO. b) XRD patterns of rGO-CS-Ca2+ fibers. The interlayer distances of the rGOCS-Ca2+ fibers increase as the loading of CS increasing. c, d) Raman spectra of GO-CS-Ca2+ and rGO-CS-Ca2+ fibers. The ID/IG ratios of GO-CS-Ca2+ and rGO-CS-Ca2+ fibers increase as CS content increasing, indicating that the defects of GO increase as CS content increasing. Compared with GO-CS-Ca2+ fibers, the ID/IG ratios of corresponding proportional rGO-CSCa2+ fibers increase, indicating that the sp2 hybridized carbons are restored after HI reduction.


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The typical stress-strain curves of GO-Ca2+ fiber (Curve 1), rGO-Ca2+ fiber (Curve 2), GO-CSII-Ca2+ fiber (Curve 3) and rGO-CS-II-Ca2+ fiber (Curve 4) are shown in Figure 3a. The tensile strength of GO-CS-II-Ca2+ fiber is 562.6 MPa, which is 1.2 and 2.3 times that of GO-Ca2+ fiber (466.4 MPa) and GO-CS fiber (241.1 MPa) (Figure S7), respectively. After reduction, the tensile strength of rGO-CS-II-Ca2+ fiber increase to 743.6 MPa. What’s more, the sequentially toughened rGO-CS-II-Ca2+ fiber shows a superior toughness up to 26.3 MJ/m3. This value reaches 2.9 and 3.5 times that of GO-Ca2+ fiber with a toughness of 9.2 MJ/m3 (Figure 3b) and GO-Ca2+ fiber with a toughness of 7.5 MJ/m3 (Figure S7), illustrating the advantage of sequential toughening. The promotion on tensile strength and toughening after HI reduction is mainly attributed to the more compact and regular layered structure. Because of the low content of CS, only part of the oxygen-containing functional groups of GO interacts with CS. Although some oxygen-containing groups on GO will be partially removed after HI reduction, the interfacial interactions between GO and CS could be still reserved due to the retained groups on rGO. It has been proved that the C/O ratio of rGO just increases to around 15:1 from the ratio of 2:1 to 4:1 of GO,50 indicating the enough retained oxygen-containing groups. So, it is helpful to the promotion of mechanical properties of the reduced fiber. The typical stress-strain curves of GO-CS-Ca2+ and rGO-CS-Ca2+ fibers with other content of CS are shown in Figure S8-S10 and the detailed data are listed in Table S4. When CS content is 4.95 wt%, the GFs show maximum toughness and tensile strength (Figure 3b and 3c), and this is similar to the ratio of organic/inorganic components in nature nacre.17 When adding a small amount of CS, CS molecules distribute evenly on GO nanosheets and hydrogen bonding was formed between functional groups on GO surface and CS chains, preventing the crack propagation in the stretching process. As the CS content increasing, excess CS molecules may change the orderly layer structure and promote chaos of the internal structure of GFs, and the toughness and strength of GFs decrease. The proposed fracture mechanism of rGO-CS-II-Ca2+ fiber in tensile stress loading process is shown in Figure 3b. The laminated structures of rGO-CS-II-Ca2+ fiber slip into loose layer structures when stretching, and the process is accompanied by resistances of hydrogen bonding and ionic bonding and displacement blocking of CS molecular chains. 10 ACS Paragon Plus Environment

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According to the previous reports,30 the interlayered ionic bonding did not resist tensile force well. At the beginning of stretching (Stage I), hydrogen bonding and interlayered ionic bonding are broken firstly. The coiled CS molecular chains are stretched and rGO nanosheets slip each other, dissipating energy. Further loading (Stage II), the CS chains are further stretched, accompanied by hydrogen bonding re-forming51 and breaking at locations with rGO nanosheets further slipping. As the loading increasing again (stage III), ionic bonding between adjacent rGO nanosheets are broken, and hydrogen bonding break up again. Then rGO nanosheets get a big slippage, dissipating more fracture energy.36 The rGO-CS-II-Ca2+ fiber achieves large ultimate elongation and fractures at last. So the excellent mechanical property is owing to the sequentially toughening52,53 of hydrogen and ionic bonding. The fractured cross-sections of rGO-Ca2+ and rGO-CS-II-Ca2+ fibers are shown in Figure 3e and 3f respectively. Compared with the relatively smooth fracture morphology of rGO-Ca2+ fiber, the layer structures of rGOCS-II-Ca2+ fiber are not regular any more. Some pieces of layers adhere together, and some pieces of layers are irregular pulled out and curled up, verifying that the hydrogen bonding between GO and CS play a role of resistance during the fracture process. The fracture morphologies (front and side views) of other GO-CS-Ca2+ and rGO-CS-Ca2+ fibers are shown in Figure S11 and S12 respectively.


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Figure 3. a) Typical stress–strain curves of GO-Ca2+ fiber (Curve 1), rGO-Ca2+ fiber (Curve 2), GO-CS-II-Ca2+ fiber (Curve 3) and rGO-CS-II-Ca2+ fiber (Curve 4). b, c) The toughness and strength of rGO-CS-Ca2+ fibers with different CS contents. When CS content is 4.95 wt%, the fibers show best mechanical properties. d) The proposed fracture mechanism of rGO-CSII-Ca2+ fiber under stretching detailed in three stages. Stage I, hydrogen bonding and interlayered ionic bonding are broken. Stage II, hydrogen bonding re-forming and breaking at locations with rGO nanosheets further slipping. Stage III, ionic bonding

between adjacent

rGO nanosheets and hydrogen bonding break up.e, f) The fractured cross-sections of rGOCa2+and rGO-CS-II-Ca2+ fibers. The comparison of toughness and strength of the rGO-CS-II-Ca2+ fiber with other GFs fabricated under ambient temperature is shown in Figure 4 with the detailed data shown in the Table S5. The sequentially toughened GFs demonstrate advanced toughness (26.3 MJ/m3) than pure GFs54,55 and is 6.4 times of the first reported GF.3 The dramatic increase is attributed to 12 ACS Paragon Plus Environment

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the synergy of hydrogen bonding between GO nanosheets and CS and ionic bonding between GO and Ca2+ ions. Compared with GFs with hydrogen bonding (GFs-hydrogen), including rGO-poly(vinyl alcohol) (rGO-PVA) fiber,56 hyperbranched polyglycerol (HPG) enveloped graphene sheets (HPG-e-Gs) fiber,57 GO-CS fiber58 and giant graphene oxide-HPG (GGOHPG) fiber,51 GFs with covalent bonding (GFs-covalent), including polyacrylonitrile-grafted GO sheets (GO-PAN) fiber59 and GO-PGMA fiber35 and GFs with ionic bonding (GFs-ionic), including GO-Ca2+ fiber58 and reduced giant graphene oxide-Ca2+ fiber (RGGF-Ca2+),60 the rGO-CS-II-Ca2+ fiber achieves higher toughness. This is because the sequentially toughened effect of hydrogen and ionic bonding further improves the toughness than single interaction. For example, GO fiber is coagulated by chitosan in GO-CS fiber,58 and the hydrogen bonding only exist on the surface of GO and CS. The absence of hydrogen bonding in inner structure of the fiber results in lower toughness. There is only single ionic bonding existing in GO-Ca2+ fiber58 and RGGF-Ca2+,60 and the hydrogen bonding in rGO-CS-Ca2+ fiber further improves the toughness. In addition, the toughness of rGO-CS-II-Ca2+ fiber is still higher than other sequentially toughened GFs with synergistic bonding (GFs-sequential) such as HPG and glutaraldehyde (GA) crosslinked giant graphene oxide (GGO-HPG-GA) fiber,51 10,12pentacosadiyn-1-ol (PCDO) and Ca2+ ions bonded reduced graphene oxide (rGO-Ca2+-PCDO) fiber36 and bioinspired graphene-based nanocomposite fiber (BGNF).53 Herein, the toughness is calculated by the area under the stress-strain curve, and ultimate elongation of GFs is another key factoras well as tensile strength. In GGO-HPG-GA fiber, the covalent acetal bridges between HPG and GA limit the regeneration of hydrogen bonding between HPG and GO in 13 ACS Paragon Plus Environment

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the stretching process and decrease the slippage of rGO nanosheets, so the ultimate elongation (4.1%) of GGO-HPG-GA fiber is lower than that (6.2%) of rGO-CS-II-Ca2+ fiber. The tensile strength (652 MPa) is also lower than that (743.6 MPa) of rGO-CS-II-Ca2+ fiber, so the toughness of GGO-HPG-GA fiber (13.0 MJ/m3) is lower. In rGO-Ca2+-PCDO fiber (ultimate elongation of 3.5%), the covalent bonding between PCDO and rGO in rGO-Ca2+-PCDO fiber are stronger than the hydrogen bonding between CS and rGO. The strong covalent bonding in rGO-Ca2+-PCDO fiber enhance the tensile strength but limit the slippage of rGO nanosheets under loading, absorbing lower fracture energy in the stretching process. Furthermore, the regeneration of hydrogen bonding of rGO-CS-II-Ca2+ fiber in the stretching process facilitates the slippage of rGO nanosheets and absorbs more fracture energy, so the toughness of rGOCS-II-Ca2+ fiber (ultimate elongation of 6.2%) is higher. In BGNF (tensile strength of 740.1 MPa, only a little lower than 743.6 MPa of rGO-CS-II-Ca2+ fiber), noncovalent π-π interactions dissipating loading energy are also restorable, but the quantity of π-π interactions in BGNF is less than that of hydrogen bonding in rGO-CS-II-Ca2+ fiber and it cause less slippage of rGO nanosheets. The ultimate elongation of BGNF is 4.2%, so the toughness is lower than that of rGO-CS-II-Ca2+ fiber.


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Figure 4. Comparison of toughness and strength of the rGO-CS-II-Ca2+ fiber with other GFs fabricated under ambient temperature. The rGO-CS-II-Ca2+ fiber achieves maximum toughness of 26.3 MJ/m3, higher than other GFs including pure GFs, GFs-hydrogen such as rGO-PVA fiber, HPG-e-Gs fiber, GO-CS fiber and GGO-HPG fiber, GFs-covalent such as GO-PAN fiber and GO-PGMA fiber, GFs-ionic such as GO-Ca2+ fiber and RGGF-Ca2+ and GFs-sequential such as GGO-HPG-GA fiber, rGO-Ca2+-PCDO fiber and BGNF. In addition, the rGO-CS-Ca2+ fibers have considerable electrical conductivity as shown in Figure S13 and the detailed data are listed in Table S6. It can be seen that the conductivities of rGO-CS-Ca2+ fibers are gradually decreased with the increase of CS content and the conductivity of resultant rGO-CS-II-Ca2+ fiber is 179.0 S/cm. This is due to the decreased order degree of the resultant GF caused by the excess CS, proved by the fact that the XRD peaks of rGO-CS-Ca2+ fibers become wider as the content of CS increasing. When CS content reaches 12.91 wt%, the width of XRD peak at half height is even too wide to see exact peak. But the layered structure of resultant GF is still kept, which is confirmed in the SEM images (Figure 15 ACS Paragon Plus Environment

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S12). Given that the key factor of conductivity is the electron transfer between graphene nanosheets, the resultant GFs with layered structure can still show relatively high electrical conductivity. It is helpful for the electrical applications of the resultant ultratough GFs. A light emitting diode is lighted by electric circuit composed with wire of rGO-CS-II-Ca2+ fiber with a knotted bend, showing in Figure S14. CONCLUSION In conclusion, inspired by the relationship between organic/inorganic hierarchical structure, interfacial interactions and moderate formation temperature of natural nacre, we have fabricated ultratough graphene fibers equentially toughened by hydrogen and ionic bonding interactions through wet-spinning method under ambient temperature. The resultant fiber shows ultrahigh toughness of 26.3 MJ/m3 and high tensile strength of 743.6 MPa. Meanwhile, the resultant fiber shows favorable electrical conductivity of 179.0 S/cm. This ultratough, strong and conductive graphene-based fiber has promising applications in mechanical and electrical fields such as flexible supercapacitor electrodes, wearable electronic textiles and fiber motors. The sequentially toughening strategy of hydrogen and ionic bonding interactions offers an avenue for constructing high performance graphene fibers in the near future.


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METHODS Materials: Graphite powder and calcium chloride were purchased from Qingdao Jing Ri Lai graphite co., Ltd and West long chemical co., Ltd, respectively. 57 wt% hydroiodic acid and chitosan (medium molecular weight, 75-85% deacetylated) were purchased from SigmaAldrich. Fabrication of rGO-CS-Ca2+, rGO-Ca2+ and rGO Fibers: GO was synthesized by modified Hummers method. Graphite powder (0.6 g) and sodium nitrate (1.0 g) were slightly added into concentrated sulfuric acid with ice-bath, and the mixture was stirred for 1 h. Then potassium permanganate (3 g) was slowly added to the mixture with stirring under ice-bath. After stirring for 4 h, the ice-bath was removed and the mixture was kept in 35 °C for 30 min with stirring. Then deionized water (150 mL) was slightly added to the mixture with stirring and the mixture was kept in 98 °C for 15 min with stirring. Then the mixture was slowly decanted to 200 mL deionized water (60 °C) with stirring and 10 mL aqueous hydrogen peroxide solution was added into the mixture. Then the mixture was standing and supernatant liquid was removed, and 250 mL hydrochloric acid solution was added into the mixture to remove the metal ions. Finally, the mixture was washed by deionized water and centrifuged with 8000 revolutions per minute for 5 min for 3 times and 11900 revolutions per minute for 15 min for 5 times, and GO was obtained. For rGO-CS-Ca2+ fibers, GO was dispersed in deionized water with concentration of 5 mg/mL and stirred for 24h, and CS was dispersed in 5% acetic acid solution with concentration of 20 mg/mL and stirred for 24 h. The CS solution was added to GO solution with different GO/CS ratios while stirring, and the mixture generated gel and precipitation. 17 ACS Paragon Plus Environment

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After continuing stirring for 24 h and sonicating for 30 min, the precipitation vanished and the mixture became homogeneous. Then the homogeneous mixture was injected to coagulation bath through capillary of 0.13 mm to form GO-CS-Ca2+ fibers, and the coagulation bath was ethanol-water (1:3 v:v) solution containing 5 wt% CaCl2. Then the fibers were washed with ethanol and deionized water to remove the salt and dried for 24 h under room temperature. The GO-CS-Ca2+ fibers were chemically reduced with aqueous solution of hydroiodic acid (57 wt%) under room temperature for 6 h. After washing with ethanol and deionized water and dried for 24 h under room temperature, the rGO-CS-Ca2+ fibers were finally obtained. For rGOCa2+ fiber, no CS solution was added into GO solution, other process was same with rGO-CSCa2+ fibers. For rGO fiber, GO solution was injected to acetone coagulating bath, other process was same with rGO-Ca2+ fiber. Characterization: FTIR spectra were tested by a Thermo Nicolet nexus-470 FTIR instrument. X-ray diffraction (XRD) profiles were taken with Cu-Kα radiation (λ=1.54 nm). Raman spectroscopy measurements were taken using LabRAM HR800 with 2.54 eV excitation energy and 488 nm wavelength. Scanning electron microscopy (SEM) images were obtained by the field-emission scanning electron microscope (JEOL-7500F). AFM images were obtained by a Leica TCSSP5. The GO/CS mixture was diluted in deionized water with a pretty low concentration and dropped on freshly cleaved mica to dry at room temperature for the AFM measurement. The thermogravimetric analysis (TGA) was tested on TG/DTA6300, NSK with a temperature rising rate of 10 K/min under nitrogen. Mechanical properties were received from a Shimadzu AGS-X Tester with a loading rate of 0.3 mm/min, and the fibers were cut 18 ACS Paragon Plus Environment

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into 20 mm long. The electrical conductivities were measured by a standard two-probe method using a source meter (Agilent E4980A). Acknowledgement: This work was supported by the Excellent Young Scientist Foundation of NSFC (51522301), the National Natural Science Foundation of China (21875010, 21273017, 51103004), the Program for New Century Excellent Talents in University (NCET-12-0034), the Fok Ying-Tong Education Foundation (141045), the 111 Project (B14009), the Aeronautical Science Foundation of China (20145251035, 2015ZF21009), State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology (oic-201701007), the State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University (LK1710), the Fundamental Research Funds for the Central Universities (YWF-16-BJ-J-09, YWF-17-BJ-J-33, YWF-18-BJ-J-13) and the Academic Excellence Foundation of BUAA (20170666) for Ph.D. Students. Supporting Information Available: The characterizations, such as AFM, TGA, EDS, XRD, SEM, and the stress-strain curves of GO fiber and GO-based composite fibers have been listed in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1) Wan, S.; Peng, J.; Jiang, L.; Cheng, Q. Bioinspired Graphene-Based Nanocomposites and Their Application in Flexible Energy Devices. Adv. Mater. 2016, 28, 7862-7898. (2) Liu, Y.; Xu, Z.; Gao, W.; Cheng, Z.; Gao, C. Graphene and Other 2D Colloids: Liquid Crystals and Macroscopic Fibers. Adv. Mater. 2017, 29, 1606794. 19 ACS Paragon Plus Environment

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