A General Bioinspired, Metals-Based Synergic Cross-Linking Strategy

A General Bioinspired, Metals-Based Synergic Cross-Linking Strategy toward Mechanically Enhanced Materials Ke Chen, Jin Ding, Shuhao Zhang, Xuke Tang, Yonghai Yue,* and Lin Guo* Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing 100191, People’s Republic of China S Supporting Information *

ABSTRACT: Creating lightweight engineering materials combining high strength and great toughness remains a significant challenge. Despite possessing-enhanced strength and stiffness, bioinspired/polymeric materials usually suffer from clearly reduced extensibility and toughness when compared to corresponding bulk polymer materials. Herein, inspired by tiny amounts of various inorganic impurities for mechanical improvement in natural materials, we present a versatile and effective metal ion (Mn+)-based synergic crosslinking (MSC) strategy incorporating eight types of metal ions into material bulks that can drastically enhance the tensile strength (∼24.1−70.8%), toughness (∼18.6− 110.1%), modulus (∼21.6−66.7%), and hardness (∼6.4−176.5%) of multiple types of pristine materials (from hydrophilic to hydrophobic and from unary to binary). More importantly, we also explore the primarily elastic−plastic deformation mechanism and brittle fracture behavior (indentation strain of >5%) of the synergic cross-linked graphene oxide (Syn-GO) paper by means of in situ nanoindentation SEM. The MSC strategy for mechanically enhanced integration can be readily attributed to the formation of the complicated metals-based cross-linking/complex networks in the interfaces and intermolecules between functional groups of materials and various metal ions that give rise to efficient energy dissipation. This work suggests a promising MSC strategy for designing advanced materials with outstanding mechanical properties by adding low amounts ( 10)). (f) Comparison of tan δ of pure GO and Syn-GO paper dependent on vibration frequency (10−190 Hz) for nanoscale damping characteristics.

many natural materials.26−29 It was proposed that different types of Mn+ ions could react with a quantity of negative oxygen-containing/amine groups, which served as cross-linking/complex sites, to form complex synergic ionic bonding networks to enhance the load transfer capability of the material. The MSC strategy will open a great and high-potential technological space for the mechanical enhancement of advanced materials.

surprising MSC strategy in natural materials, mixed components of eight Mn+ ions (Figure 1b②) with the same molar ratio as nacre were incorporated into the interlamination (Figure 1b①) of GO nanosheets and the intermolecular voids (Figure 1b①) of these curly polymer chains to bond/cooperate with synergistically active sites (Figure 1b③,④). It is an effective strategy to adjust the mechanical properties via a higher density of the synergic cross-linking/coordinating networks, distinguished from previous cross-linking strategies using only one or two metal ions.30 Through fine-tuning the content of the mixed Mn+ ions occupying the voids between the GO nanosheets or the polymers, optimized mechanical properties can be obtained. We define these cross-linking/complex reactors into three types (Figure 1b④), as defined in our previous work: (i) divalent A2+ ions (e.g., Mg2+, Ni2+, and Ca2+); (ii) trivalent M3+ (Al3+) ions; or (iii) TiO2+ (−Ti−O−Ti−).31 To construct the bioinspired GO papers, GO was first oxidized from natural graphite flakes by a modified Hummers’ method,32 and then individual GO nanosheets were obtained in aqueous solution by ultrasonic exfoliation (see Supporting Information). By adding eight types of Mn+ ions into the GO system, brown synergic Mn+-cross-linked GO paper (SMGO), named Syn-GO with eight metal ions, was successfully

RESULTS AND DISCUSSION The existence of various inorganic metal ions/minerals was demonstrated in several natural materials (Figure 1a①); they were uniformly distributed in the bulk phases (see Figure S4a− e in the Supporting Information). Therefore, it was conjectured that these metal ions could easily cooperate with protein and polysaccharide chains, which form random coils and short bent helices, to form stable metal−protein/polysaccharide compounds. Namely, high-density MSC networks are generally formed in these organic matrixes (Figure 1a②).17,18 Natural materials use such an MSC strategy to wisely manage energydissipating deformations in their crossed-complex structures to prevent/delay catastrophic fracture.2,12,14,16 Inspired by the 2837

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ACS Nano fabricated by vacuum-assisted filtration, as shown in Figure 1c1. For comparison, single Mn+ cross-linking (Mn+-), bivalent synergy (A2+-), bitrivalent synergy (A2/3+-), and bi(oxygen)trivalent synergy (A2/3+-ZrO2+- and A2/3+-TiO2+-) cross-linked GO papers were also prepared by the same method. In addition, three types of synergic Mn+ cross-linked composite papers, Syn-M n+/CS (Syn-CS), Syn-M n+ /PMMA (SynPMMA), and Syn-Mn+/CS-MTM (Syn-CS/MTM), were also constructed via the bottom-up spin-coating layer-by-layer (BSC-LBL) assembly method (see Figure S2). Digital photos demonstrated their good light transmittance, as observed Figure 1d1−f1. Cross-sectional morphology images showed the close-packed GO nanosheet layers (Figure 1c3) in Syn-GO, which was distinguished from the incompact layered microstructure of pure GO (Figure 1c2), as well as clear differences between pure polymer-based papers (Figure 1d2−f2) and synergic cross-linked composite papers (Figure 1d3−f3). Within a 10 μm × 10 μm area, while the root-mean-square (RMS) roughness of the SMGOs was approximately 114.5−196.1 nm at the bottom surface, the RMS roughness of the other samples was approximately 63.8−91.5 nm at the upper surface (Figure S5). Scanning electron microscopy (SEM)−energy dispersive spectrometer (EDS) element mapping images showed that the eight types of Mn+ ions were dispersed uniformly in Syn-GO and the other samples (Figure S6), which was the same as the extant form of the minerals in natural materials. These SMGOs were used as ideal models for characterizing the cross-linking interactions. All of the SMGOs had a higher 2θ angle than the GO (Figure S7a), indicating a lower d-spacing (∼0.8 Å), suggesting that various Mn+ ions could be easily inserted into the space between layered GO nanosheets and might react immediately with oxygen-containing groups to reduce the dspacing.30 Fourier transform infrared spectroscopy (FTIR) showed the bonds at 1625, 1410, and 1050 cm−1 (CC, C−O (carboxyl), and C−O (alkoxyl/alkoxide)) for GO shifted to the higher wavenumbers of 1635, 1425, and 1070 cm −1 , respectively, and the peak centered at 971 cm−1 (−O−CO, carboxyl) disappeared in SMGO (Figure S7b). This could be interpreted as solid evidence for carboxylic acid coordinating to Mn+ ions, which could potentially cause chemical cross-linking reactions.33 In particular, the bond at 1225 cm−1 exhibited gradually decreasing/disappearing C−O (epoxy/ether) stretching intensity and shifted to higher wavenumbers at approximately 1260 cm−1, which may be due to the ring opening of the epoxides.30 Raman spectra (Figure S7c) demonstrated that all of the ID/IG ratios (1.31−1.08) in SMGOs were clearly lower than that (1.58) of GO, indicating that Mn+ ions could enhance the cross-linking behaviors between GO nanosheets.34 Moreover, the ID/IG ratios gradually decreased from 1.31 for A2+- to 1.16 for A2/3+-ZrO2+- to 1.08 for Syn-GO, which may be due to the different bonding capabilities between different types of Mn+ ions and GO nanosheets. X-ray photoelectron spectroscopy (XPS) analysis (Figure S7d) showed that compared with GO the peak intensity at 285.5 eV (C−OH) increased slightly and the peak intensity corresponding to the C−O−C group decreased clearly in Syn-GO, suggesting that these Mn+ ions caused the ring opening of the epoxides, resulting in the reduction of the fitting C 1s peak.30,35 Moreover, the peak positions of the C−O−C and CO groups were slightly shifted from 286.8 and 287.8 eV (GO) to 287.0 and 288.0 eV, respectively, which could be explained by the change of the binding energies caused by the interaction/

coordination between various Mn+ ions and the oxygencontaining groups.30,33,36 The macromechanical properties of SMGOs had been enhanced appreciably in comparison with those of GO,22,37 as shown in Figure 2a−c and Tables S1 and S2, but the toughness (∼1.5−2.0 MJ m−3) of these papers was slightly lower than that (∼2.4 MJ m−3) of GO (Figure 2c). Notably, both the ultimate stress (∼162.0 MPa) and excellent Young’s modulus (∼ 7.5 GPa) for Syn-GO could be obtained, which were approximately 70.8% and ∼66.7% higher than those of GO. In addition, the mechanical properties of Syn-GO were improved compared to those of other SMGOs or Mn+-GO papers (see Tables S1 and S2). On the basis of the above analysis (Figures S6 and S7), we could attribute such a reinforcing behavior to the synergic cross-linking networks as “load distributers” derived from many special structural units of various Mn+ ion- and oxygen-containing functional groups, in which these Mn+ ions mediated synergistically the interfacial interactions of GO nanosheets in response to the simultaneous enhancement of strength and Young’s modulus and restricted greatly the slipping between GO nanosheets to cause the reduction of toughness (energy dissipation). For other SMGOs (from A2+ to A2/3+-TiO2+), their ultimate stress changed generally from ∼125 MPa for A2+ to ∼120 MPa for A2/3+ to 150.4 MPa for A2/3+-TiO2+. The Young’s modulus oscillated faintly between ∼5.1 and ∼7.9 GPa, and toughness first decreased (∼1.5 MJ m−3 for A2+, ∼1.0 MJ m−3 for A2/3+) and then increased from ∼1.6 MJ m−3 for A2/3+-ZrO2+ to ∼2.0 MJ m−3 for A2/3+-TiO2+. Notably, the different MSC strategies reflected the diverse increases in mechanical properties. For Mn+-GO, our experimental results further showed that different Mn+ ions caused the diverse increases in mechanical properties (Table S1), analogous to our previous experimental results,31 which can be mainly attributed to the clear differences in the bonding styles and different bonding energies of the M−O bonds. For instance, A3+ ions could be generally suitable to improve greatly Young’s modulus and strength, and A2+ ions could generally achieve the improvement in tensile strength and Young’s modulus/toughness, while TiO2+ allows a stable mechanical improvement in strength, Young’s modulus, and toughness. Therefore, the MSC networks for enhancing the mechanical property of a material must maintain an optimized balance. In addition, Figure 2d indicated that the addition of synergic Mn+ ions greatly enhanced the micromechanical behaviors. From right to left, the contact depths became shallow from GO (∼450 nm) to Syn-GO (∼220 nm) gradually, and SMGOs exhibited typical E′ (from ∼2.4 GPa (A2+) to ∼1.8 GPa (A2/3+) to ∼5.1 GPa (Syn)) and H′g (from ∼333.1 MPa (A2+) to ∼237.7 MPa (A2/3+) to ∼468.1 MPa (Syn)) values, which were greatly higher than those of GO. In addition, nanoindentation mapping (Figure S8) revealed that Syn-GO had a more stable/ higher modulus and hardness, consistent with previous mechanical measurements (Figure 2a−e) made using a related species. Therefore, the MSC effects were further demonstrated to exist, and Syn-GO achieved excellent mechanical properties, superior to those of other SMGOs, caused by the optimized mechanical balance. Damping represents the energy dissipation ability of a material, which is of prime importance in dynamic systems.38 The nanodamping behavior (tan δ) (∼0.11−0.28) of Syn-GO was slightly lower than that (∼0.22−0.34) of GO (Figure 2f), suggesting a relatively lower viscoelasticity or an enhanced shear strength (modulus or hardness).39,40 Ob2838

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Figure 3. Comparison of the microdeformation analysis for GO and Syn-GO membrane. (a) Schematic of the suspended GO membrane over the holes of a copper grid for nanoindentation tests. (b, c) Digital photo and SEM image of the prepared membrane. Imperfect or broken suspended membranes often present on the copper grid can be observed. (d) Typical force−displacement curves by controlling the loading to 10 μN. (e, f) The histograms of statistical analyses of the nanoindenter results (d).

and more stable than that (0.6−3.4) of GO over all loading velocities (10−300 μN S−1) using a loading of 200 μN (Figure S10), indicating that Syn-GO could absorb relatively larger amounts of contact energy by the mode of elastic deformation within the indentation strain of 2.0%. However, by adjusting the indentation strain (Figure S10c), we further found that the dissipated energy ratio of Syn-GO was distinctly higher than that of GO (more than 5.0% strain), suggesting that the relatively large deformation caused the unrecoverable brittle collapses, which are discussed later. Figure 4d shows that Syn-GO achieved a higher loading slope and a smaller displacement (2548.2 ± 497.2 nm) than GO (4954.3 ± 165.7 nm). In contrast, the indentation response was distinctly anisotropic (Figure 4b−d). Here, both the slopes of the loading curves along the zz direction and the yy direction for pure GO were lower than those for Syn-GO (Syn-GO (zz): ∼4.1 μN nm−3, GO: ∼2.0 μN nm−3; Syn-GO (yy): 8.8 μN nm−3, GO: 4.2 μN nm−3). The in situ observations (Figure 4e,f (iii)) revealed that Syn-GO had a smaller displacement and deformation region located directly beneath/around the indenter compared with those of GO. After fully unloading, the disparities between the deformation areas were even more apparent (Figure 4e,f (iv)), with partial elastic recovery in SynGO, which was unambiguously distinct from the heavy and irreversible deformation behavior in GO during the high-load impact. Therefore, these observations corroborate the notion that the micro-nanoscale (elastic−plastic) deformation plays an important role in adjusting additional dissipation of impact energy.1,15,39 Notably, most of the elastic deformation in SynGO could absorb smaller relative amounts of contact energy. The loading force and displacement of Syn-GO have a nearly linear relationship, while GO was vulnerable to plastic deformation, where a plateau appeared after two cycles with

viously, the enhanced shear strength constricted the sliding of GO nanosheets, dissipating less absorbed energy on the macroscale. Therefore, it was concluded that although pure GO paper superficially achieved a relatively larger but unstable energy dissipation (toughness, 2.4 ± 1.5 MJ m−3), Syn-GO, with stable damping behavior, possessed only mild energy dissipation under complicated loading. A Syn-GO membrane with the eight uniformly dispersed metal elements and relatively small RMS roughness (54 nm, Figure S9), as the reinforcing structural unit, was carefully constructed on a copper grid to further verify the MSC strategy (Figure 3a). For statistical analysis of the variation in the displacement, we tested many specimens. The representative force−displacement curves demonstrated that the displacement (l ≈ 100−450 nm) of the Syn-GO membranes was much smaller than that (l ≈ 460−1800 nm) of the GO membranes (Figure 3b). Furthermore, the histograms (Figure 3c,d) of the derived displacements showed deformations (l) of 1073.3 ± 385.0 (GO) and 213.3 ± 97.3 nm (Syn-GO), respectively. These results suggested the existence of the MSC effect in the structural units. Nanoindentation tests with a blunt (flat, 5 μm) tip along the yy and zz directions (Figure 4a−l and Movies S1−4) have been conducted to reveal the differences in the deformation and fracture mechanisms of the materials. Figure 4b,c show that although the apparent morphological differences were not obvious (Figure 4b), there was a large disparity in the mechanical responses (Figure 4c). Syn-GO had a higher slope, indicating a higher Young’s modulus; the force loaded on Syn-GO (13 167.7 ± 1520.0 μN) was about 2 times as high as on GO (6258.7 ± 398.9 μN), which was consistent with the tensile testing (Figure 2a). Furthermore, we estimated that the dissipated energy ratio (0.3−0.8) of Syn-GO was clearly lower 2839

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Figure 4. continued

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Figure 4. Comparison of indentation response, contact deformation, and fracture mechanisms of GO and Syn-GO revealed by in situ nanoindentation ESEM observations of the plane and cross-section beneath the contact points. (a) Schematic plane and cross-section center of one sample, with the arrows representing the indentation location where indentation studies were carried out. (b, c) yy direction: typical loading−unloading curves while maintaining a displacement of 1500 nm (b) with micrographs, illustrating the deformation region (c, scale bar: 2.5 μm). (d−f) zz direction: typical loading−unloading curves while maintaining a 10 000 μN loading (d) with micrographs depicting the change of elastic−plastic deformation (e, f). (i) micrographs before indentation, illustrating no contact; (ii) micrographs showing initial contact, with small deformation regions; (iii) micrographs exhibiting maximum contact, representing a deformation region in GO (e) and a small deformation region in Syn-GO (f); (iv) micrographs after indentation revealing the large (eiv) and small (fiv) plastic region near the flat indenter edge and the surrounding small and large elastic regions (scale bar, 3 μm). (g−i) zz direction: typical loading−unloading cycles while maintaining a fixed displacement of 4500 nm (g), together with their micrographs depicting the change of the deformation. (h, i) Full unloading was performed to obtain the contact depth at given peak load: (h, i) (i) first unloading, (ii) second unloading, (iii) third unloading, (iv) fourth unloading (scale bar, 3 μm). (j−l) zz direction, indentation fracture: (j) schematic indentation location in cross-section edge of the sample; (k, l) micrographs illustrating a large plastic deformation (sunken region, k) and flexible fracture features (l); (l) micrographs revealing collapse fracture and rigid layered features.

no increase in the force, as shown in Figure 4g (see regions (ii)−(iii) and (iii)−(iv)). Furthermore, as exhibited in the indentation curve of GO, there was a large residual strain after fully unloading, which was opposed to an elastic mechanism that involved very small residual strains for Syn-GO.39 Meanwhile, such observations were also made in the immediate vicinity of the indenter (Figure 4h (i)), corresponding to their cycle curves. In the first cycle, when the displacement was set to 1000 nm (region (i)), there were no obvious difference in the morphologies, but the force loaded (2000.5 μN) to Syn-GO was more than 3 times higher than the force loaded (655.7 μN) to GO, indicating an enhanced mechanical behavior for SynGO and that plastic deformation had already occurred for GO in the first 1000 nm loading (region (i) in Figure 4h); in the second cycle, there was no obvious difference in the shape of the curves, but the plastic deformation region extended greatly for GO (region (ii) in Figure 4h), while there was no obvious plastic deformation for Syn-GO (region (ii) in Figure 4i), which was consistent with the quasi-linear load−displacement curve (Figure 4g). In the third and fourth cycles, for GO, there

was no force increase for GO, and a serious plastic deformation occurred, demonstrated by both the force−displacement curve (Figure 4g) and the SEM images (Figure 4h). However, the force−displacement curve for Syn-GO maintained its quasilinear feature, a large deformation occurred in the fourth cycle, and there was no obvious plastic deformation when we checked the sample (Figure 4i). After unloading, we observed a partial elastic recovery for Syn-GO and a serious and irreversible deformation for GO, which was in agreement with the analysis results of micrographs corresponding to the loading−unloading curve (Figure 4g). The postindentation SEM micrographs (Figure 4k,l and Movie S4) further revealed that while GO showed the large plastic deformation (Figure 4k (i), sunken region) and flexible fracture features (Figure 4k (i, ii)) of some flexible polymers,1,32 Syn-GO possessed a collapse damage region (Figure 4l (i)) and brittle fracture features (Figure 4l (ii, iii)), which had been demonstrated by the dissipated energy ratio. From the results mentioned above, it was strongly demonstrated that (i) both Young’s modulus and the hardness 2841

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Figure 5. Comparison of mechanical improvements of these materials after and before addition of various Mn+ ions and the related materials performance. (a) Stress−strain curves with schematic diagrams and cross-section morphologies. (b) From left to right: ultimate stresses, Young’s modulus, and toughness. (c) Partial loading−unloading curves. (d) Tensile strength versus toughness for metal-ion-related films/ papers.

high as ∼61.0 MPa and ∼5.0 MJ m−3, approximately 77.8% and ∼733.3% higher than that of PMMA (σ, ∼34.7 MPa; W, ∼0.6 MJ m−3), and the increase in Young’s modulus was up to ∼38.5%. Moreover, when compared with CS/MTM (σ, ∼137.2 MPa; W, ∼19.8 MJ m−3; E, ∼3.7 GPa), Syn-CS/MTM possessed a higher strength of ∼193.2 MPa (in some cases up to 212.8 MPa, Figure 5a), a higher toughness of ∼41.6 MJ m−3 (in some cases up to 45.4 MJ m−3, close to the toughness (∼50 MJ m−3) of Kevlar 49 fibers),32 and a modulus of ∼4.5 GPa, which were ∼40.8%, ∼ 110.1%, and ∼21.6% higher than those of CS/MTM. Their micromechanical enhancements were further proved by the nanoindentation measurement (Figure 5c and Table S2). Tensile strength versus toughness of the metal-ion-related films/papers has been summarized in Figure 5d. Our bioinspired composite papers possessed a good combination of strength and toughness when compared with certain GOrelated composites, regardless of their stiffness. Obviously, while the strength of Syn-GO was higher than that of all the single-metal-based cross-linking GO papers to date, referring to Mg2+-, Ca2+-, Fe3+-, and Al3+-GO and our Mn+-GO papers (Table S1), its toughness was also much higher than that of most of them.30,41,42 Furthermore, the toughness of our polymer composites (e.g., Syn-CS, ∼38.9 MJ m−3; Syn-CS/ MTM, ∼41.6 MJ m−3) was far higher than that of all of the metal-ion-related and most of the other related materials (Figure 5d and Table S3), such as ternary Cu2+-CMC/MTM (∼1.7 and ∼2.1 MJ m−3),43 Fe3+-GO/TA (∼0.35 MJ m−3),42 Ca2+-MTM/PDDA (∼0.96 MJ m−3), and GA-PVA/MTM (∼0.68 MJ m−3) and binary GO/CS (∼19.0 MJ m−3),44 GO/

of the pristine GO paper have been dramatically increased by the MSC strategy, the chemically strong synergic Mn+ crosslinking networks in the interlamination of GO nanosheets, and that (ii) the in situ deformation and fracture mechanism observation clearly demonstrated the distinct disparities of damage fractures before and after incorporation of metal ions, which will provide special insight for studying the interfacial reinforcing effect of other two-dimensional assembled materials. The macro-/micromechanical properties (Figure 5a−c and Tables S1 and S2) of other composite papers, including SynCS, Syn-PMMA, and Syn-CS/MTM, exhibited clearly higher strength, toughness, Young’s modulus, and hardness in comparison with the pristine paper. In addition, there was a large strain-hardening region, as demonstrated in metallic materials,40 which should originate from the high cross-linking/ coordinating effect. The strongly synergic cross-linking reactions between various Mn+ ions and active groups in these polymers or composite matrixes caused the enhanced yielding stress and pronounced plastic deformation. Because of load transfer to the ionic/coordination bonding among these intermolecular sites, the tensile stresses required for plastic yielding increased from ∼80, ∼34.3, and ∼115.0 MPa to ∼115.0, ∼70.0, and ∼185.0 MPa for Syn-CS, Syn-PMMA, and Syn-CS/MTM, respectively. For Syn-CS, the cross-linking strategy strongly reinforced the mechanical properties of the composite, improving the tensile strength by ∼24.1% to ∼132.4 MPa and the toughness by ∼18.6% to ∼38.9 MJ m−3; the enhancement of Young’s modulus was up to ∼25.0% when compared with CS (σ, ∼106.7 MPa; W, ∼32.4 MJ m−3; E, ∼2.4 GPa). The tensile strength and toughness of Syn-PMMA was as 2842

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ACS Nano PMMA (∼2.4 and ∼1.0 MJ m−3),23,36 CMC/MTM (∼4.0 MJ m−3),45 and MTM/UFOs (∼7.5 MJ m−3).46 Meanwhile, the tensile strength (∼193.2 MPa) of Syn-MTM/CS was distinctly higher than that of most of the MTM-related composite materials (e.g., ∼109 MPa for Ca2+-MTM/PDDA, ∼110 MPa for PVA/MTM, ∼140 MPa for UFOs/MTM). In general, as intrinsic toughening mechanisms are related to plasticity and thus tensile strength, a compromise is often reached in engineering materials where one of the properties is set aside.40,47 However, our designed MSC strategy could overcome this dilemma without compromise.

to guarantee the full dispersion of chitosan and MTM nanosheets in the aqueous solution. All these nanocomposite films were fabricated by the BSC-LBL technique, as shown in Figure S2. (1) Syn-CS and Syn-CS/MTM films: The specific mixed Mn+ ion (corresponding to the mole ratio of various minerals/ions except for Ca in nacre, and assuming a molar ratio of 1:1 for Mg2+ to Ca2+) aqueous solutions including A2/3+ (Mg2+, Ni2+, Ca2+, Cu2+, Co2+, and Al3+) ions, ZrO2+, TiO2+, and CS aqueous solution (0.3 wt %), or the mixed CS-MTM solution were spun on the glass substrate (500 rpm, 20 s), and the routine was repeated until the desired thickness (10 ± 2 μm) was reached. (2) Syn/PMMA film: The specific mixed Mn+ ion acetone solutions were spun on the glass substrate (500 rpm, 20 s), and the routine was repeated until the desired thickness (10 ± 2 μm) was reached. For comparison, pure CS, CS/MTM, and PMMA films were also prepared using the same method. Preparation of Pure GO and Syn-GO Membranes. A 100-μmdiameter and 25-μm-thick micro copper grid substrate (Zhong Jing Ke Yi, Beijing) was used for GO growth under ambient conditions (25 °C, 20% relative humidity (RH)) for the suspended GO membranes. A colloidal GO dispersion (0.2 mg mL−1) was utilized to assemble the suspended GO membranes (Figure S3a). Both the colloidal GO dispersion and the mixed Mn+ ions (5.0 × 1.0−4 mol L−1) aqueous solution were utilized to assemble the Syn-GO membrane (Figure S3b). To achieve a constant thickness of these assembled membranes, the four assemblies were created by repeated dipping into the colloidal GO dispersion. Characterization. The cross-sectional nacre from clams and other samples such as nutshells, shrimp shells, and natural silks were coated with ultrathin gold to reduce charging effects prior to SEM imaging. SEM images were obtained by a FEI Quanta 250 FEG at an acceleration voltage of 5 kV and a working distance of ∼6−9 mm. EDS mappings were generated using a field emission JEOL 7500F with an accelerating voltage of 20 kV. Atomic force microscopy (AFM) was performed by a Bruker Dimension Icon. All of the XPS measurements were made with an ESCALab 220i-XL (Thermo Scientific) using a monochromatic Al Kα X-ray source. The Raman spectroscopy measurements were performed using a LabRAM HR800 (Horiba Jobin Yvon) with a 514 nm wavelength incident laser. X-ray diffraction (XRD) experiments were carried out with a Shimadzu Lab XRD-6000 X-ray diffractometer using Cu Kα radiation. FTIR spectra were collected using a Thermo Nicolet Nexus-470 FTIR instrument. Tensile Tests. The tensile mechanical properties were measured using a Shimadzu AGS-X tester with a dynamic mechanical analyzer. Static tensile tests were evaluated at a load speed of 1 mm min−1 with a gauge length of 5 mm. All of the papers for the tensile test were cut into strips with a length of 20 mm and a width of 3 mm. For fracture tests, for each ligament length, at least five samples were fractured. The absorbed energy until failure was calculated by the integral area of the load−displacement. The specimen thickness was obtained from SEM imaging of the fracture edges. Micromechanical Testing. Nanoindentation studies were performed on the plane of the samples in ambient conditions (25 °C, 15% RH) by a commercial TI 950 triboindenter (Hysitron, Minneapolis, MN, USA) equipped with a Berkovich diamond tip (R ≈ 100 nm) using a continuous depth-sensing indentation technique at a peak force of 1000 μN. The load function consisted of a 5 s loading to 1000 μN, followed by a 5 s hold at that force and then a 5-s unloading. The modulus and hardness were calculated from the unloading curve of each sample, using the Oliver−Pharr method. Partial unloading/ cyclical tests were also performed with 33 and 25 cycles, peak force of 1000 μN, and unloading fraction of 0.5 and 1.0, respectively. Nanodynamic mechanical analysis (NanoDMA) was performed by a Hysitron Triboindenter TI950 with a 100 nm Berkovich tip to characterize the nanoscale damping behavior (see Supporting Information) of the Syn-Mn+-GO and Mn+-GO composite papers. NanoDMA experiments used the test type of variable dynamic load with a series set of parameters, including preloading of 2 μN,

CONCLUSION Inspired by minorities of various inorganic impurities for mechanical improvement in natural materials, we have developed a simple and general MSC strategy for generating bioinspired composite papers with excellent mechanical properties. While Syn-GO realized significant improvements in tensile strength, Young’s modulus, and hardness, other composites achieved an overall great enhancement in mechanical behaviors. The discrepancy can be mainly attributed to the difference in the ionic/coordination bonding sites (2D materials (e.g., GO): the interlamination of 2D nanosheets; polymers: the intermolecular active sites of these polymers). Notably, we directly observed the primary elastic−plastic deformation mechanism and brittle fracture behaviors in SynGO, strongly demonstrating the existence of the MSC effect. More importantly, our excellently developed strategy provided an effective approach to design large-scale advanced, bioinspired, polymeric materials with outstanding mechanical performance, which might be widely applicable in future aerospace, tissue engineering, protection, and electronics devices. Additionally, the MSC strategy provided an indirect method to understand the synergic effect derived from various metal ions for mechanical enhancement in natural materials. EXPERIMENTAL SECTION Preparation of Syn-Mn+-GO Papers. A class of SMGOs was successfully prepared by filtering diluted colloidal GO dispersions followed by different types of aqueous mixed Mn+ solutions through an anodic filter membrane (Figure S1). The wet GO paper could be obtained by colloidal GO dispersions (80 mL, 1 mg mL−1), followed by separate addition of these aqueous mixed solutions: type A (A2+): Mg2+, Ni2+, Ca2+, Cu2+, and Co2+; Ttype B (A2/3+): A2+ and Al3+; type C (A2/3+-ZrO2+): A2+, Al3+, and ZrO2+; type D (A2/3+-TiO2+): A2+, Al3+, and TiO2+; type E (Syn): all eight Mn+ ions; 20−80 μL of 0.05 mM solutions. For comparison, a series of single Mn+-reinforced GO papers were further prepared by introducing one type of Mn+ ion. Meanwhile, the pure GO paper was also prepared by the same method. After filtration, specimens were air-dried until they could be peeled off from the paper for analysis. A water-circulation multifunction vacuum pump with vacuum filter holder was utilized for vacuum filtration. (Note: the proportional concentration of these Mn+ ions corresponds to the percentage composition of various mineral components in nacre.) Preparation of Syn-Mn+/CS (Syn-CS), Syn-Mn+/PMMA (SynPMMA), and Syn-Mn+/CS-MTM (Syn-CS/MTM) Papers. A dispersion of Na-MTM in deionized water (0.3 wt %) was stirred thoroughly for 1 week and then centrifuged at 3000 rpm for 10 min to remove unexfoliated Na-MTM. Chitosan (0.3 wt %) was dissolved in an aqueous solution of 2 wt % glacial acetic acid 24 h before use. PMMA (0.3 wt %) was dissolved in acetone. The specific volume ratio of the exfoliated Na-MTM solution to CS solution (Na-MTM concentration of 2.5 wt %) was mixed under constant stirring for 2 h 2843

DOI: 10.1021/acsnano.6b07932 ACS Nano 2017, 11, 2835−2845

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ACS Nano beginning quasi-static loading of 10 μN, end quasi-static loading of 1200 μN, a frequency of 50 Hz, and a dynamic load of 5.00 μN. The in situ nanoindentation studies were performed on the planes/ transverse cross sections of the samples in ambient air using a PI-85 SEM picoindenter (Hysitron) equipped with a flattened tip (R = 5 μm) by the displacement/loading control. The loading−unloading cycles were measured while keeping a fixed displacement of 4500 nm. The load function consisted of a 5 s loading to 5 mN followed by a 5 s hold at that force and then a 5 s unloading.

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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.6b07932. Materials and some detailed experimental methods for the synthesis of GO, the preparation of the dispersions, the element components and microstructural features of natural materials, comparison of macro- and micromechanical data, contour maps, cross-sectional SEM images, AFM images, EDS element, XRD, FTIR, XPS, Raman spectra analysis, macro-/micromechanical properties, fracture morphologies, and theoretical methods (PDF) Movies showing the deformation behaviors of the samples using in situ nanoindentation tests. Movie S1 (AVI) Movie S2 (AVI) Movie S3 (AVI) Movie S4 (AVI)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Fax & Tel: +86-010-82338162. ORCID

Ke Chen: 0000-0001-7612-0226 Yonghai Yue: 0000-0002-8945-2032 Lin Guo: 0000-0002-6070-2384 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS J.D., S.H.Z., and X.K.T. contributed equally to this work. We thank R. Hao for designing the table of contents (TOC) graphic. This study was supported by the National Basic Research Program of China (2014CB931802), Research Fund for the Doctoral Program of Higher Education of China (20131102120053), and Basic Scientific Research Foundation of Beihang University (No. YWF-16-GJSYS-25). REFERENCES (1) Zhang, J.; Feng, W.; Zhang, H.; Wang, Z.; Calcaterra, H. A.; Yeom, B.; Hu, P. A.; Kotov, N. A. Multiscale Deformations Lead to High Toughness and Circularly Polarized Emission in Helical NacreLike Fibres. Nat. Commun. 2016, 7, 10701. (2) Studart, A. R. Biological and Bioinspired Composites with Spatially Tunable Heterogeneous Architectures. Adv. Funct. Mater. 2013, 23, 4423−4436. (3) Bonderer, L. J.; Studart, A. R.; Gauckler, L. J. Bioinspired Design and Assembly of Platelet Reinforced Polymer Films. Science 2008, 319, 1069−1073. 2844

DOI: 10.1021/acsnano.6b07932 ACS Nano 2017, 11, 2835−2845

Article

ACS Nano

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DOI: 10.1021/acsnano.6b07932 ACS Nano 2017, 11, 2835−2845