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Jun 20, 2017 - as crack bridging.40,41. In this study, inspired by nacre, we demonstrated WS2 ... and designated as rGO-WS2-PCDO-I (GO:WS2 = 99:1), rG...
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Fatigue Resistant Bioinspired Composite from Synergistic Two-Dimensional Nanocomponents Sijie Wan,† Qi Zhang,† Xiaohang Zhou,‡ Dechang Li,‡ Baohua Ji,‡ Lei Jiang,† and Qunfeng Cheng*,† †

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, PR China ‡ Biomechanics and Biomaterials Laboratory, Department of Applied Mechanics, Beijing Institute of Technology, Beijing 100081, PR China S Supporting Information *

ABSTRACT: Portable and wearable electronics require much more flexible graphene-based electrode with high fatigue life, which could repeatedly bend, fold, or stretch without sacrificing its mechanical properties and electrical conductivity. Herein, a kind of ultrahigh fatigue resistant graphene-based nanocomposite via tungsten disulfide (WS2) nanosheets is synthesized by introducing a synergistic effect with covalently cross-linking inspired by the orderly layered structure and abundant interfacial interactions of nacre. The fatigue life of resultant graphene-based nanocomposites is more than one million times at the stress level of 270 MPa, and the electrical conductivity can be kept as high as 197.1 S/cm after 1.0 × 105 tensile testing cycles. These outstanding properties are attributed to the synergistic effect from lubrication of WS2 nanosheets for deflecting crack propagation, and covalent bonding between adjacent GO nanosheets for bridging crack, which is verified by the molecular dynamics (MD) simulations. The WS2 induced synergistic effect with covalent bonding offers a guidance for constructing graphene-based nanocomposites with high fatigue life, which have great potential for applications in flexible and wearable electronic devices, etc. KEYWORDS: tungsten disulfide, synergistic effect, fatigue resistant, bioinspired, graphene enhanced in static tensile strength,18−23 stiffness24−26 and toughness.27−29 The fatigue behavior of graphene-based nanocomposites is poor and rarely investigated. According to the fatigue mechanisms of traditional polymer matrix composites,30−38 the fatigue resistance of graphenebased nanocomposites can be significantly enhanced by effectively suppressing crack propagation. Thus, there are two effective ways to achieve this goal: (i) introducing lubricating metal disulfide nanosheets into laminated structure of graphene-based nanocomposites, such as MoS2,39 etc., and (ii)

R

ecently, the miniaturized portable and wearable electronics have revolutionized our daily life, such as electronic papers, touch screens, roll-up displays, and wearable sensors.1 Graphene with integrated high mechanical properties and electrical conductivity has been utilized for fabricating flexible energy devices,2−6 for example, flexible supercapacitors,7−11 actuators,12−14 etc. To sustain the reliability of graphene-based flexible energy devices with repeatable deformation state, the fatigue crack growth should be effectively suppressed in the process of cyclic stretching, meaning that an excellent fatigue life is necessary along with the robust static mechanical properties. Recently a series of graphene-based nanocomposites with high mechanical performance15−17 have been achieved; however, the mechanical properties are only © 2017 American Chemical Society

Received: April 19, 2017 Accepted: June 20, 2017 Published: June 20, 2017 7074

DOI: 10.1021/acsnano.7b02706 ACS Nano 2017, 11, 7074−7083

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Figure 1. Illustration of the preparation process of the rGO-WS2-PCDO nanocomposites through vacuum filtration induced self-assembly. (a) The mixed homogeneous dispersion of WS2 and GO nanosheets is first filtrated into GO-WS2 hybrid film. Then, the GO-WS2 film is grafted by PCDO molecules via esterification. Under UV irradiation, PCDO molecules are cross-linked via 1,4-addition polymerization. The rGOWS2-PCDO nanocomposites are obtained by HI reduction of GO-WS2-PCDO nanocomposites. (b) Digital photograph of resultant rGO-WS2PCDO nanocomposites. (c) Cross-section morphology of rGO-WS2-PCDO-IV nanocomposite. (d) Corresponding W element EDS mapping, indicating that WS2 nanosheets are uniformly distributed in the rGO interlayers. (e) TEM image of the cross-section of rGO-WS2-PCDO-IV nanocomposite, indicating that WS2 nanosheets are sandwiched among rGO nanosheets.

WS2 nanosheets were exfoliated from bulk WS2 powders using sonication followed by centrifugation.50,51 The thickness and lateral size of GO and WS2 nanosheets are determined by atomic force microscopy (AFM) images (Figure S1) to be about 0.8 nm and 1−4 μm, and 1.0 nm and 0.1−0.8 μm, respectively. Additionally, the characterization of monolayer WS2 nanosheet is also confirmed by Raman spectrum (Figure S2) with E2g1 and A1g vibration modes at 356.5 and 417.7 cm−1 under the excitation wavenumber of 488 nm, respectively, which is in accordance with a previous report.52 The graphene-based nanocomposites were assembled via vacuum-assisted filtration (Figure 1a). First, the homogeneous dispersion of GO and WS2 nanosheets was filtrated into freestanding GO-WS2 hybrid film. Then the dried GO-WS2 hybrid film was grafted with PCDO via esterification reaction by soaking the GO-WS2 hybrid film into premixed tetrahydrofuran/PCDO solution.28 Under UV irradiation, the PCDO molecules were cross-linked via 1,4-addition polymerization.28 Finally, the GO-WS2-PCDO nanocomposites were chemically reduced into rGO-WS2-PCDO nanocomposites (Figure 1b) by hydriodic acid (HI). Five kinds of rGO-WS2-PCDO nanocomposites with different WS2 content have been fabricated and designated as rGO-WS2-PCDO-I (GO:WS2 = 99:1), rGOWS 2 -PCDO-II (GO:WS 2 = 97:3), rGO-WS 2 -PCDO-III (GO:WS2 = 95:5), rGO-WS2-PCDO-IV (GO:WS2 = 93:7) and rGO-WS2-PCDO-V (GO:WS2 = 91:9), respectively. In addition, the corresponding GO-WS2 and rGO-WS2 nanocomposites without covalently cross-linking were also fabricated for comparison. The exact WS2 content was measured by thermogravimetric analysis (TGA, Figure S3) and the detailed data are listed in Table S1. The resultant rGO-WS2PCDO-IV nanocomposite demonstrates orderly layered crosssection morphology (Figure 1c) with uniform dispersion of element W by energy-dispersive X-ray spectroscopy (EDS, Figure 1d). Other ternary rGO-WS2-PCDO nanocomposites with different WS2 content also present uniform distribution of

stitching adjacent graphene nanosheets of resultant nanocomposites by covalently cross-linking,28 similar to those in carbon fiber reinforced polymer composites.32,33,36 Although the crack propagation has been demonstrated to be suppressed through covalently cross-linking28 or metal disulfide nanosheets39 alone, to the best of our knowledge, simultaneously introducing these two mechanisms for restricting crack propagation to improve the fatigue life of graphene-based nanocomposites has not been reported yet. In fact, multisuppression of crack propagation has also existed in natural materials. For example, the nacre effectively suppresses crack growth in both the parallel and perpendicular directions of aragonite platelets through crack deflection and twisting, as well as crack bridging.40,41 In this study, inspired by nacre, we demonstrated WS2 induced synergistic effect with covalently cross-linking to achieve a kind of ultrahigh fatigue resistant graphene-based nanocomposite. Molecular dynamics (MD) simulation results reveal that the crack propagations were significantly restricted by synergistic deflection-bridging mode. As a result, the fatigue life is more than one million times at the stress level of 270 MPa, and the electrical conductivity can be well kept as high as 197.1 S/cm after 1.0 × 105 tensile testing cycles. Meanwhile, this kind of bioinspired graphene-based nanocomposite also shows integrated static mechanical properties with tensile strength of 413.6 MPa and toughness of 17.7 MJ/m3. The synergistic effect induced by WS2 with covalently cross-linking provides a concept for fabricating high fatigue resistant graphene-based nanocomposites with promising applications in flexible and wearable electronic devices in the near future.

RESULTS AND DISCUSSION The WS2 nanosheets, a typical two-dimensional (2D) transition metal dichalcogenides, have been intensively investigated recently due to extraordinary intrinsic lubricant, electronic and mechanical properties.42−49 In this study, the monolayer 7075

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Figure 2. (a) Static tensile stress−strain curves of pure GO film (curve 1), rGO film (curve 2), rGO-WS2-IV (curve 3), rGO-PCDO (curve 4) and rGO-WS2-PCDO-IV nanocomposites (curve 5). (b) The strength and toughness comparison of pure rGO film, rGO-WS2-IV, rGO-PCDO and rGO-WS2-PCDO-IV nanocomposites. (c) Model snapshots during the tensile process of simulated rGO-WS2-PCDO nanocomposites, indicating the unique crack suppression by deflection (elliptical dotted line) and bridging (rectangular dotted line). (d) Proposed synergistic fracture mechanism of rGO-WS2-PCDO nanocomposites: When stretching starts, the rGO nanosheets begin to slide each other and the crack is initiated, propelling WS2 nanosheets to move along with rGO nanosheets; With continuous stretching, the WS2 nanosheets slide mutually due to the lubrication to deflect the crack, accompanied by the stretching of coiled PCDO long chains and further bridging of crack; When further increasing the loading, the covalent bonding between PCDO and rGO nanosheets is broken, resulting in pull-out and curled edge morphology of rGO nanosheets. (e) Cross-section side view SEM images of pure rGO film, rGO-WS2-IV, rGO-PCDO and rGO-WS2-PCDOIV nanocomposites. The inset is the W element EDS mapping.

composites further confirm successful grafting of PCDO on the GO nanosheets. X-ray photoelectron spectroscopy (XPS, Figure S9) show that the peak intensity of C−O and CO of resultant nanocomposites is decreased and the atomic ratio of C1s to O1s is increased after HI reduction, as listed in Table S4, demonstrating the removal of unreacted functional groups and partial restoration of graphitic sp2 conjugated network after HI reduction.62 The typical tensile stress−strain curves of resultant nanocomposites are shown in Figure 2a and corresponding strength and toughness are compared in Figure 2b. Detailed mechanical properties are listed in Table S5. The rGO-WS2-PCDO-IV nanocomposite (curve 5) shows tensile strength of 413.6 ± 8.8 MPa and toughness of 17.7 ± 1.2 MJ/m3, which are 1.5 and 2.4 times higher than those of rGO-PCDO nanocomposite (curve 4), 2.0 and 3.5 times higher than those of rGO-WS2-IV nanocomposite (curve 3), 2.6 and 7.1 times higher than those of pure rGO film (curve 2), respectively. Moreover, compared with other graphene-based nanocomposites18,19,22,24−26,28,29,63−75 with different interfacial interactions (Table S6), this rGO-WS2-PCDO-IV nanocomposite shows the superiority of integrated high strength and toughness. Meanwhile, the mechanical properties are also much higher than previous reported ternary graphene-based nanocomposites.21,39,76−79 The unique mechanical properties of rGO-

element W and unique layered architecture, as shown in Figure S4. Furthermore, the cross-section of rGO-WS2-PCDO-IV nanocomposite is also characterized by transmission electron microscopy (TEM, Figure 1e), verifying that WS2 nanosheets are sandwiched among rGO nanosheets via alternatively layered structure, which is reminiscent of that of nacre-mimic hybrid nanocomposites prepared via layer-by-layer (LBL) assembly.53−61 X-ray diffraction (XRD) patterns (Figure S5−S6) show that the interlayer distance (d-spacing) of GO-PCDO and GO-WS2 nanocomposites is much larger than pure GO film, as listed in Table S2, which confirms the successful insertion of PCDO molecules and WS2 nanosheets into GO interlayers. On the other hand, the obvious 002 reflection at about 14.32° from the basal plane of WS2, accompanied by other weak peaks such as 004, 103, 006, 105, 008, indicates that WS2 nanosheets tend to restack during the process of vacuum-assisted filtration.50 Fourier transform infrared (FTIR) spectra (Figure S7) with blue-shifted CO stretching vibration (from 1730 to 1770 cm−1) and new peak of C−O−C stretching vibration (1135 cm−1) in ester groups demonstrate the covalently cross-linking of PCDO with GO nanosheets in GO-PCDO and GO-WS2PCDO nanocomposites, similar to a previous report.28 In addition, Raman spectra (Figure S8) with increased ID/IG ratio (Table S3) for GO-PCDO and GO-WS2-PCDO-IV nano7076

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Figure 3. VSP for strength and toughness of (a) rGO-WS2-PCDO and (b) GO-WS2-PCDO nanocomposites with different WS2 content.

Figure 4. (a) Tensile fatigue testing S−N curves and (b) corresponding dynamic stress−strain curves of pure rGO film, rGO-WS2-IV, rGOPCDO and rGO-WS2-PCDO-IV nanocomposites. (c) The fracture morphologies of pure rGO film, rGO-WS2-IV, rGO-PCDO and rGO-WS2PCDO-IV nanocomposites after tensile fatigue testing.

nanosheets due to the breakage of weak hydrogen bonding, resulting in the mutual slippage of rGO nanosheets. Subsequently, the WS2 nanosheets slide along with the surface of rGO nanosheets owing to the friction between WS2 and rGO nanosheets. But with continuous stretching, due to the lubrication of WS2 nanosheets, the crack propagation is deflected along the sulfur atoms layer of WS2 nanosheets. Meanwhile, the coiled cross-linked PCDO chains are stretched, further bridging the crack propagation. When further increasing the loading, the covalent bonding between rGO nanosheets and PCDO molecules is broken, resulting in pull-out of rGO nanosheets with highly curled edges (Figure 2e). The damaged WS2 nanosheets still lie on the surface of rGO nanosheets, as verified by the W element image from EDS result. The fracture morphologies of other nanocomposites are shown in Figure S11−S14. Compared to rGO-PCDO and rGO-WS2 nano-

WS2-PCDO-IV nanocomposite should be attributed to the synergistic effect induced by lubrication of WS2 nanosheets and covalently cross-linking. Steered molecular dynamics (SMD) simulations80,81 were applied to reveal the synergistic toughening mechanism. The model snapshots, as shown in Figure 2c, demonstrate that compared with rGO-PCDO and rGO-WS2 nanocomposites (Figure S10), the rGO-WS2-PCDO nanocomposites possess unique multisuppression of crack propagation, including crack deflection by the lubrication of WS2 nanosheets (elliptical dotted line) and bridging by covalent bonding between PCDO and rGO nanosheets (rectangular dotted line), during the tensile process. Furthermore, the corresponding synergistic fracture mode of rGO-WS 2 -PCDO nanocomposites is proposed, as shown in Figure 2d. At the beginning of stretching, the crack is initiated between adjacent rGO 7077

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Figure 5. Fatigue curves comparison of specific stress versus the number of cycles to failure. The results indicate the superiority of the bioinspired graphene-based nanocomposite by WS2 induced synergistic effect with covalent bonding.

rGO film. The fatigue crack propagation of rGO-WS2-IV and rGO-PCDO nanocomposites is impeded by the deflection from the lubrication of WS2 nanosheets and bridging of covalent bonding, respectively. Moreover, compared with the physical van der Waals’ force between graphene nanosheets and lubrication of WS2 nanosheets, the covalently cross-linking via the PCDO long chain could provide much stronger interfacial strength, dissipating much more energy during the fatigue testing process. Consequently, the rGO-PCDO nanocomposite demonstrates higher fatigue life than rGO-WS2-IV nanocomposite. Then, the rGO-WS2-PCDO-IV nanocomposite demonstrated higher fatigue life than rGO-PCDO and rGO-WS2-IV nanocomposites, due to the crack suppression by synergistic deflection-bridging mode. During a fatigue tensile cycle with sinusoidal dynamic stress, the sulfur atoms layer of WS2 nanosheets first deflects the crack propagation perpendicular to the surface of rGO nanosheets under low loading level. While the covalent bonding from PCDO long chain bridges the crack propagation parallel to the surface of rGO nanosheets, ensuring the structural integrity of rGO-WS2-PCDO-IV nanocomposite under high loading level. With continuously fatigue testing, the synergistic deflection-bridging mode for crack propagation suppression is sufficiently cycled through whole cross-section until the breakage of covalent bonding, dissipating considerable energy and providing ultrahigh fatigue life. Thus, the essence of synergistic effect is the sequential crack suppression in the directions both parallel and perpendicular to the surface of rGO nanosheets by deflection-bridging mode. As shown in Figure 4c, compared with the smooth fracture morphology of pure rGO film, the rGO-WS2-PCDO-IV nanocomposite shows substantial curling of rGO nanosheets after tensile fatigue testing, further confirming the WS2 induced synergistic effect with covalent bonding. Portable and wearable electronic devices require not only excellent fatigue resistance, but also lightweight performance. Thus, the density of the nanocomposites should also be considered when evaluating the fatigue properties. Figure 5 shows the fatigue curves comparison of specific stress versus the number of cycles to failure for different materials, such as traditional metal materials,83−85 graphene reinforced epoxy nanocomposites,30,86 montmorillonite (MMT)-nanofibrillar cellulose (NFC)-poly(vinyl alcohol) (PVA) nanocomposites,87 etc. The rGO-WS2-PCDO-IV nanocomposite shows much higher specific stress under the same number of cycles to failure. Moreover, the rGO-WS2-PCDO-IV nanocomposite

composites, the crack propagation of rGO-WS2-PCDO nanocomposites is significantly suppressed by the synergistic effect of crack deflection and bridging. Consequently, the maximum energy dissipation is achieved, which is much higher than that with sole deflection by WS2 or bridging by covalent bonding. Thus, the tensile strength and toughness of rGO-WS2-PCDO nanocomposites are simultaneously enhanced. Furthermore, the WS2 induced synergistic effect with covalent bonding could be quantificationally calculated by the following equation:82 VSP =

2M − (P + Q ) × 100% P+Q

In this equation, VSP represents synergy percentage; M is the measured mechanical properties of rGO-WS2-PCDO nanocomposites, while P and Q are those of rGO-PCDO and rGOWS2 nanocomposites, respectively. VSP values for strength and toughness gradually increase from 36.9% and 81.4% for rGOWS2-PCDO-I to 73.3% and 182.5% for rGO-WS2-PCDO-IV, and then dramatically decrease to 53.7% and 123.4% for rGOWS2-PCDO-V, respectively, as shown in Figure 3a. The relationship between VSP and WS2 content for GO-WS2PCDO nanocomposites (Figure 3b) shows the similar trend as rGO-WS2-PCDO nanocomposites. The detailed VSP values for strength and toughness of rGO-WS2-PCDO and GO-WS2PCDO nanocomposites are listed in Table S7. Thus, when the WS2 content is chosen as 6.4 wt %, the optimal synergistic effect is realized. If lower than 6.4 wt %, the crack deflection by the WS2 nanosheets is not sufficient. If higher than 6.4 wt %, the WS2 nanosheets would however heavily restack in rGOWS2-PCDO nanocomposites (Figure S15b) and lead to a poor loading transfer efficiency, which indicates that excess WS2 content is detrimental to inducing synergistic effect with covalent bonding. The strength and toughness of rGO-WS2PCDO and GO-WS2-PCDO nanocomposites with different WS2 content are compared in Figure S16. The WS2 induced synergistic effect with covalent bonding results in not only improvement in static strength and toughness of rGO-WS2-PCDO nanocomposites, but also greater enhancement in dynamic fatigue resistance. Figure 4a shows the typical fatigue curves of maximum tensile stress (S) versus the number of cycles to failure (N) (S−N) of resultant nanocomposites, according to previous report.30 The corresponding dynamic stress−strain curves are shown in Figure 4b. Under the same stress level, the rGO-WS2-IV and rGO-PCDO nanocomposites possess much higher fatigue life than pure 7078

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dispersion was mixed with GO solution, followed by stirring and sonication to form homogeneous solution. Then, the vacuum filtration was applied to assemble homogeneous solution into GO-WS2 nanocomposites. After HI reduction, the GO-WS2 nanocomposites were transferred into rGO-WS2 nanocomposites. On the basis of the input weight ratios of GO to WS 2 , five kinds of rGO-WS 2 nanocomposites have been fabricated: rGO-WS2-I (GO:WS2 = 99:1), rGO-WS2-II (GO:WS2 = 97:3), rGO-WS2-III (GO:WS2 = 95:5), rGO-WS2-IV (GO:WS2 = 93:7) and rGO-WS2-V (GO:WS2 = 91:9), respectively. Fabrication of rGO-WS2-PCDO Nanocomposites. Similar to the fabrication of rGO-PCDO nanocomposite, the as-prepared GOWS2 nanocomposites were immersed in PCDO solution, and then suffered from UV irradiation to form GO-WS2-PCDO nanocomposites. After HI reduction, the GO-WS2-PCDO nanocomposites were transferred into rGO-WS2-PCDO nanocomposites. Characterization. The static tensile mechanical properties were tested using a Shimadzu AGS-X Tester with a loading rate of 1 mm/ min under the environmental condition with the humidity of about 16% RH and temperature of 25 °C. All of the samples were cut into strips with the length of 10 mm and the width of 3 mm and the results for each sample are based on the average value of 3−5 specimens. The tensile fatigue tests were carried out at a frequency of 1 Hz using the Instron ElectroPulsE1000 test facility under the same environmental condition as static tensile testing and the ratio of the minimum stress to maximum stress was 0.1. All the tested samples were cut into strips with the length of 25 mm and the width of 5 mm. Atomic force microscopy (AFM) was performed on a Leica TCS SP5. Scanning electron microscopy (SEM) images were recorded by field-emission scanning electron microscope (JEOL-7500F). Transmission electron microscope (TEM) images were observed using a FEI Tecnai G20 instrument at 200 kV. Thermogravimetric analysis (TGA) was performed using TG/DTA6300, NSK under air with a temperature rising rate of 10 °C·min−1. X-ray photoelectron spectroscopy (XPS) was taken on an ESCALab220i-XL (ThermoScientific) using a monochromatic Al-Ka X-ray source. X-ray diffraction (XRD) profiles were recorded with Cu Kα radiation (λ = 1.54 nm). Raman spectra were performed on LabRAM HR800 (Horiba Jobin Yvon) with the excitation wavenumber of 488 nm. FTIR spectra were characterized by a Thermo Nicolet nexus-470 FTIR instrument. The electrical conductivities were tested by a standard two-probe method using a source meter (Agilent E4980A). The carrier concentration was tested using ET9000 Hall effect measurement system, which is shown in Table S8. Molecular Model and Simulation Method. Molecular dynamics (MD) simulations were performed using Large-scale Atomic/ Molecular Massively Parallel Simulator (LAMMPS) package.88 The reactive empirical bond order (REBO) potential parameters for C−H and C−H−O systems were used for rGO nanosheets, which has been validated by predicting binding energies of epoxy groups on graphene close to density functional theory based calculations.89−92 The WS2 nanosheets were described by a harmonic potential for stretching, a harmonic potential for angle bending, and a Lennard−Jones potential for nonbonded interaction as follows:

demonstrates 2−3 orders magnitude higher fatigue life than rGO-hydroxypropyl cellulose (HPC)-Cu2+ nanocomposite74 with synergistic effect from hydrogen and ionic bonding under the same specific stress level, indicating the much more effective improvement from WS2 induced synergistic effect with covalent bonding. In addition, the fatigue resistance of the rGO-WS2PCDO-IV nanocomposite is also superior to previously reported other ternary graphene-based nanocomposites, such as rGO-NFC-PCDO,79 rGO-double-walled carbon nanotube (DWNT)-PCDO77 and rGO-MMT-PVA.76 Furthermore, its excellent crack suppression property would result in little effect of dynamic loading on the electron transfer among adjacent rGO nanosheets. Consequently, the rGO-WS2-PCDO-IV nanocomposite still shows high electrical conductivity of 197.1 ± 15.3 S/cm even after fatigue testing for 1.0 × 105 cycles at the stress level of 270 MPa, which maintains ∼81% of original value at static state (Table S1). As a demonstration, the rGO-WS2-PCDO-IV nanocomposite after fatigue testing could serve as a conductive media in a circuit to light LED red bulb (Figure S17), suggesting a promising application in flexible electronic devices.

CONCLUSION In conclusion, this study demonstrated a kind of synergistic effect induced by WS2 with covalent bonding for achieving ultrahigh fatigue resistance in a bioinspired graphene-based nanocomposite. The crack propagation perpendicular and parallel to the surface of rGO nanosheets is substantially restricted by the sequential deflection-bridging mode, which is confirmed by the molecular dynamics (MD) simulations. In addition, the resultant bioinspired graphene-based nanocomposite also demonstrates integrated superior mechanical properties and high electrical conductivity. In comparison with other graphene-based nanocomposites, this bioinspired synergistic strategy has the following crucial advantages: (i) the lubrication of WS2 nanosheets is realized via orderly layered structure with GO nanosheets; (ii) the maximum synergistic effect is achieved via optimizing the content of WS2 nanosheets; (iii) a kind of high-performance multifunctional graphenebased nanocomposite is created, which shows promising applications in the field of flexible, wearable, portable electronic devices, intelligent devices, etc. MATERIALS AND METHODS Materials. Graphene oxide (GO) was prepared by modified hummers’ method. 10,12-Pentacosadiyn-1-ol (PCDO) was purchased from Tokyo Chemical Industry Co., Ltd. WS2 powders, Nmethylpyrrolidinone (NMP, anhydrous, 99.5%) and 57 wt % hydroiodic acid (HI) were purchased from Sigma-Aldrich. WS2 nanosheets were directly exfoliated from WS2 powders by liquid exfoliation based on the previous report.50,51 The typical exfoliation procedures are as following: 300 mg of WS2 powders were added to 100 mL of NMP. After sonication for 5 h under ice-bath, the dispersion was settled overnight. Then, the supernatant was centrifuged at 2000 rpm for 20 min and top 3/4 of dispersion was collected. Subsequently, the collected dispersion was centrifuged again at 3500 rpm for 30 min and top 3/4 of dispersion was collected as the WS2 nanosheets dispersion for following experiments. Fabrication of rGO-PCDO and rGO-WS2 Nanocomposites. The GO dispersion was filtered under vacuum to obtain pure GO film. The GO film was immersed in PCDO solution for 1 h, followed by UV irradiation for 2 h in N2 atmosphere to form GO-PCDO nanocomposite. Subsequently, the GO-PCDO nanocomposite was reduced by HI solution for 6 h, followed by washing with ethanol and drying to obtain rGO-PCDO nanocomposite. The as-prepared WS2

Etot = E bond + Eangle + E LJ

∑

=

Kbond(rij − r0)2 +

i∈W ,j∈S

+

∑ i,j∈W ,S

∑

Kα(α − α0)2

α∈φ,θ ,ψ ,ω

⎡⎛ ⎞12 ⎛ ⎞6 ⎤ σij σij 4ε⎢⎢⎜⎜ ⎟⎟ − ⎜⎜ ⎟⎟ ⎥⎥ r ⎝ rij ⎠ ⎦ ⎣⎝ ij ⎠

i≠j

where Kbond and Kα are the force constants for bond stretching and angle bending, respectively. The parameters r0 and α0 are the equilibrium values of bonds and angles, respectively, as shown in Figure S18. Since the mechanical properties of WS2 nanosheets are very similar to those of MoS2 nanosheets,93,94 the bonded interaction parameters of WS2 were taken from the force field of MoS2.95,96 The 7079

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ACS Nano nonbonded interaction parameters ε and σ for W and S were taken from the universal force field.97 The force field parameters for WS2 nanosheets are listed in Table S9−S10. Initially, the rGO and WS2 nanosheets were placed in parallel in a large distance, as shown in Figure S19a. The initial structure was first minimized in 100000 steps. After about 100 ps simulation, the equilibrium structure of rGO-WS2PCDO nanocomposites is obtained by the self-assembly of rGO and WS2 nanosheets, as shown in Figure S19b. The resultant equilibrium structure was then used for loading to reveal the crack suppression mechanism by steered molecular dynamics (SMD) simulations method. The time step was 0.5 fs. The van der Waals interaction was cut off beyond 10 Å. The temperature was held at 300 K during the process of tensile loading. All the model snapshots were created by visual molecular dynamics (VMD) package.98

ACKNOWLEDGMENTS This work was supported by the Excellent Young Scientist Foundation of NSFC (51522301), the National Natural Science Foundation of China (21273017, 51103004), the Program for New Century Excellent Talents in University (NCET-12-0034), the Fok Ying-Tong Education Foundation (141045), the Open Project of Beijing National Laboratory for Molecular Sciences, 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 Fundamental Research Funds for the Central Universities (YWF-16-BJ-J-09, YWF-17-BJ-J-33) and the Academic Excellence Foundation of BUAA for Ph.D. Students.

ASSOCIATED CONTENT S Supporting Information *

REFERENCES

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b02706. AFM images and corresponding height curves of exfoliated GO nanosheets and WS2 nanosheets; Raman spectrum of WS2 nanosheets; TGA curves of pure GO, WS2, GO-PCDO and GO-WS2-PCDO nanocomposites for measuring WS2 and PCDO content in the nanocomposites; Cross-section morphology and corresponding W element EDS results of rGO-WS2-PCDO nanocomposites; XRD patterns, side view and front view SEM images of fracture morphology of (r)GO films, (r)GO-PCDO, (r)GO-WS2, (r)GO-WS2-PCDO nanocomposites; FTIR spectra of (r)GO film, WS2, (r)GOPCDO and (r)GO-WS2-PCDO nanocomposites; Raman and XPS spectra of (r)GO film, (r)GO-PCDO and (r)GO-WS2-PCDO-IV nanocomposites; Model snapshots during the tensile process of simulated rGO film, rGO-WS2 and rGO-PCDO nanocomposites; TEM images of cross-section of rGO-PCDO and rGO-WS2PCDO-V nanocomposites; Tensile strength and toughness of (r)GO-WS2-PCDO nanocomposites with different WS2 content; A demonstration for rGO-WS2-PCDOIV nanocomposite as a conductive media to light LED red bulb after fatigue testing; Ball-and-stick model of WS2 nanosheet; Initial and equilibrium structure of simulated rGO-WS2-PCDO nanocomposites; Detailed data for WS2 and PCDO content, d-spacing, ID/IG ratio, C1s/O1s atomic ratio, thickness and mechanical properties, synergy percentage, electrical conductivity and carrier concentration of the resultant nanocomposites in our experiment; The bonded interaction parameters of WS2 nanosheets and nonbonded interaction parameters between rGO and WS2 nanosheets for MD simulations (PDF)

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AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Baohua Ji: 0000-0002-0092-6562 Qunfeng Cheng: 0000-0001-7753-4877 Notes

The authors declare no competing financial interest. 7080

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