Ultratough reduced graphene oxide composite films synergistically toughened and reinforced by polydopamine wrapped carbon nanotubes

Journal Pre-proof Ultratough reduced graphene oxide composite films synergistically toughened and reinforced by polydopamine wrapped carbon nanotubes Rui Zou, Feng Liu, Ning Hu, Huiming Ning, Shu Wang, Kaiyan Huang, Xiaoping Jiang, Chaohe Xu, Shaoyun Fu, Yuanqing Li, Cheng Yan PII:

S0008-6223(19)31279-5

DOI:

https://doi.org/10.1016/j.carbon.2019.12.044

Reference:

CARBON 14894

To appear in:

Carbon

Received Date: 10 October 2019 Revised Date:

9 December 2019

Accepted Date: 18 December 2019

Please cite this article as: R. Zou, F. Liu, N. Hu, H. Ning, S. Wang, K. Huang, X. Jiang, C. Xu, S. Fu, Y. Li, C. Yan, Ultratough reduced graphene oxide composite films synergistically toughened and reinforced by polydopamine wrapped carbon nanotubes, Carbon (2020), doi: https://doi.org/10.1016/ j.carbon.2019.12.044. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Rui Zou: Zou: Investigation, idea, data curation and validation, and writing. Feng Liu: Investigation and idea. Ning Hu: Supervision, Writing-Reviewing and Editing. Huiming Ning: Investigation, idea and validation. Shu Wang: Data curation and validation. Kaiyan Huang: Data curation and validation. Xiaoping Jiang: Data curation. Chaohe Xu: Data analysis. Shaoyun Fu: Supervision. Yuanqing Li: Data analysis. Cheng Yan: Data analysis and Supervision.

Ultratough reduced graphene oxide composite films synergistically toughened and reinforced by polydopamine wrapped carbon nanotubes

Ultratough reduced graphene oxide composite films synergistically toughened and reinforced by polydopamine wrapped carbon nanotubes

Rui Zou a, Feng Liu b,*, Ning Hu a,c,**, Huiming Ning b,***, Shu Wang b, Kaiyan Huang b, Xiaoping Jiang b, Chaohe Xu b, Shaoyun Fu b, Yuanqing Li b, and Cheng Yan d

a

School of Mechanical Engineering, Hebei University of Technology, Tianjin 300401, China

b

College of Aerospace Engineering, Chongqing University, Chongqing 400044, China

c

State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing 400044,

China d

School of Chemistry, Physics and Mechanical Engineering, Queensland University of

Technology (QUT), Brisbane, QLD 4001, Australia

Correspondence to: *Feng Liu (E-mail: [email protected]) **Ning Hu (E-mail: [email protected]; [email protected]) ***Huiming Ning (E-mail: [email protected])

Keywords: nacre; graphene film; polydopamine; carbon nanotube; reinforcing; toughening

Abstract: Nacre like graphene based composite films (GCFs) have been developed over the last decade. Although the progress in ultrastrong GCFs is remarkable, fabricating GCFs with excellent integrated properties of strength, flexibility, and toughness still remains a challenge. Here, a ternary GCF of reduced graphene oxide (rGO), polydopamine (PDA), and multiwalled carbon nanotubes (MWCNTs) was developed. The MWCNTs were modified by PDA 1

to form PDA@MWCNTs which were then compounded with GO to fabricate the rGOPDA@MWCNT films by evaporation-induced assembly and chemical reduction. The preparation was simple, cheap, and environmentally-friendly. The PDA increased the dispersion of MWCNTs in the rGO-PDA@MWCNT films, crosslinked the rGO sheets, and enhanced the entanglement of MWCNTs and the interactions between MWCNTs and rGO sheets. Benefiting from the synergistic effect of PDA and MWCNTs, the rGOPDA@MWCNT films exhibited excellent integrated properties by increasing strength, tensile fracture strain and toughness to 579±35 MPa, 12.03±0.56%, and 34.03±1.60 MJ/m3, which were 2.26, 2.02 and 4.94 times that of pure rGO films, respectively. The rGOPDA@MWCNT films also exhibited a high conductivity of 612±68 S/cm and excellent structural stability under extreme environments. These excellent integrated properties of the rGO-PDA@MWCNT films will promote their applications in aerospace engineering, flexible electrical devices and artificial muscle.

1. Introduction Nacre, which is strong and tough, is composed of 95 wt% 2-dimensional (2D) aragonite platelets and a small amount of 1-dimensional (1D) chitin nanofibers and soft protein. The aragonite platelets and chitin nanofibers stack into highly ordered layered structures which are held together by the soft protein “adhesive”. This so-called “brick and mortar” structure optimizes the load transfer between the rigid platelets and nanofibers, resulting in the excellent mechanical properties of nacre [1,2]. Inspired by the unique structure and excellent properties of nacre, graphene with a 2D molecular structure and extraordinary properties has been regarded as an ideal candidate to act as the role of “brick”, and graphene based composite films (GCFs) such as graphene oxide (GO) films or reduced graphene oxide (rGO) films with a binary or ternary “brick and mortar” structure have been developed to transform the extraordinary properties of microscopic graphene into macroscopic materials [3,4]. A 2

number of products have been developed, such as rGO-poly (vinyl alcohol) (PVA) [5], rGOpoly (10,12-pentacosadiyn-1-ol) (PCDO) [6], rGO-polydopamine (PDA) [7], rGO-poly (acrylic acid-co-(4-acrylamidophenyl) boronic acid) (PAPB) [8], rGO-chitosan (CS) [9], rGOpolyacrylic acid (PAA) [10], rGO-cellulose nanocrystals (CNC) [11,12], rGO-sulfonated styrene-ethylene/butylene-styrene (SSEBS) [13], and rGO-1-aminopyrene-disuccinimidyl suberate (AP-DSS) [14] composite films. In these binary GCFs, the soft phase, generally a polymer, links the rGO platelets, promoting the interaction and load transfer between the rGO platelets. Meanwhile, the structure and properties of the soft phase also affect the properties of GCFs. A soft phase with a flexible structure increases the ductility of GCFs but has limited benefit on the strength of GCFs. On the other hand, a soft phase with a rigid structure improves the strength of GCFs but also increases the brittleness of GCFs. Simultaneous improvement of strength, toughness and ductility of GCFs has been a challenge. To comprehensively increase the mechanical properties of GCFs, ternary GCFs such as rGOcarbon nanotubes (CNTs)-PCDO [15], rGO-molybdenum disulfide (MoS2)-thermoplastic polyurethane (TPU) [16], rGO-montmorillonite (MMT)-PVA [17], rGO-tungsten disulfide (WS2)-PCDO [18], rGO-nanofibrillar cellulose (NFC)-PCDO [19], and rGO-PCO-(AP-PSE) [20] have been developed. In these ternary GCFs, the third phase is normally a 1D/2D inorganic mineral or a polymer with a rigidity between those of rGO sheets and the soft phase. The third phase acts as the reinforcing phase in the soft phase and plays a role to transfer load between the rGO platelets and the soft phase. The synergistic effect of the soft phase and the reinforcing phase and the hierarchical microstructures help to increase the mechanical properties of GCFs. However, the mechanical properties of GCFs are still limited due to the poor dispersion of the reinforcing phase and the weak interfacial interactions between the rGO platelets and the reinforcing phase resulting from the lack of interlayer soft phase. In recent reports, ternary GCFs synergistically reinforced by metal ions and π-π interactions or covalent bonds, such as rGO-zinc ions-PCDO [21], rGO-nickel ions-PDA [22], rGO-copper ions-CS 3

[23], and rGO-chromium ions-(AP-PSE) [24], exhibit ultrahigh strength (400-945 MPa) due to the ionic bonds between the rGO platelets and the metal ions, overcoming the poor interactions between the rGO platelets and the reinforcing phase. However, the ductility of these GCFs is limited due to the rigid and short ion bonds, limiting the toughness of GCFs. Furthermore, the polymers PCDO and AP-PSE are very expensive, and the fabrication of the GCFs is tedious and detrimental to the environment due to the use of ultraviolet radiation. Therefore, despite the progress in ultra-strong GCFs, developing a simple, cheap, and environmentally-friendly method to fabricate GCFs with excellent integrated properties of strength, toughness, and ductility still remains a challenge. Here, based on the ternary structure and synergistic toughing and reinforcing effect of nacre, a ternary GCF was developed by using rGO as the “brick”, PDA as the soft “mortar”, and multi-walled CNTs (MWCNTs) as the reinforcing phase. Unlike the simple direct blending of the components in the previous approaches, the reinforcing phase MWCNTs were firstly surface modified by PDA to obtain PDA@MWCNTs, and the PDA@MWCNTs were further compounded with GO to prepare the rGO-PDA@MWCNT films by evaporation-induced assembly and chemical reduction. The fabrication was simple, cheap, and environmentallyfriendly. The surface modification of MWCNTs by PDA increased the dispersion of MWCNTs in the rGO-PDA@MWCNT films. Further, the PDA of PDA@MWCNT overcame the lack of the interlayer soft phase between rGO and MWCNTs, which enhanced the interactions between MWCNTs and rGO sheets. Furthermore, the cross-linking structure of PDA crosslinked the rGO sheets and MWCNTs and enhanced the entanglement of MWCNTs, restricting the slippage and fracture of the rGO sheets when the rGO-PDA@MWCNT films were under tension. Benefiting from the synergistic effect of the PDA and MWCNTs, the ternary rGO-PDA@MWCNT films exhibited excellent integrated properties of a strength of 579±35 MPa, a tensile fracture strain of 12.03±0.56%, and a toughness of 34.03±1.60 MJ/m3 which are 2.26, 2.02 and 4.94 times that of pure rGO films, respectively. Furthermore, the 4

rGO-PDA@MWCNT films also exhibited a high conductivity of 612±68 S/cm and excellent structural stability in deionized water, sulphuric acid (H2SO4) solution and sodium hydroxide (NaOH) solution. These excellent integrated properties of the GCFs will promote their applications in aerospace engineering, flexible electrical devices and artificial muscle.

2. Experimental 2.1 Materials Flake graphite (325 mesh), dopamine hydrochloride (DA-HCl), and tris (hydroxymethyl) aminomethane (Tris) were purchased from Aladdin Co., Ltd., China. MWCNTs with a mean diameter of 8 nm and a length of 0.5-2 µm were purchased from Chengdu Organic Chemicals Co., Ltd., China. H2SO4 (98%), NaOH, potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30%), hydrochloric acid (HCl, 37%), hydroiodic acid (HI, 57%) and ethanol were purchased from Chengdu Kelong Chemical Reagents Co., Ltd., China. All the reagents and solvents were used as received without further purification.

2.2 Synthesis of GO The GO (Figure S1) was fabricated according to a modified Hummer’s method [12]. In a typical preparation, flake graphite (3 g) was mixed with H2SO4 (70 mL) in an ice bath. After 1 h slight stirring, KMnO4 (9 g) was slowly added over 3 h. Then, the reaction system was kept at 0oC under stirring for 8 h. After the oxidation reaction was finished, ice deionized water (150 mL) was added by a peristaltic pump over 6 h while the system was kept at 0oC. Then, the mixture was poured into 1500 mL ice deionized water, and H2O2 (3 wt %) was added drop by drop until no more bubble was generated. After being settled down for 24 h, the product was divided into two liquid layers. The upper supernatant was poured out, and the lower mixture was dialyzed with deionized water until the pH approached 7. Then, the mixture was centrifuged at 5,000 rpm for 20 min, followed by another centrifugation step at 8,000 rpm for 5

20 min to remove the precipitates, and the GO aqueous solution was obtained. The GO aqueous solution was further concentrated by centrifugation at 12,000 rpm. The final GO aqueous solution with a specific concentration was prepared by diluting the concentrated GO solution with deionized water.

2.3 Preparation of PDA@MWCNTs DA-HCl (2 g) was dissolved in deionized water (1000 mL), and Tris (10 mmol) was added to adjust the pH of the solution to about 8.5. Then, MWCNT (1 g) was added into the DA-Tris buffer solution, and the resultant suspension was mixed by mechanical agitation and ultrasonication simultaneously for 30 min. The suspension was further mechanically agitated for 24 h at room temperature [25]. Finally, the PDA@MWCNT was obtained by filtration of the suspension. The PDA@MWCNT was further washed with deionized water until the solvent become colorless to remove the unreacted DA and dried in vacuum at 60oC for 24 h.

2.4 Preparation of GO-PDA@MWCNT films and rGO-PDA@MWCNT films A specific amount of PDA@MWCNT was dissolved in 50 mL deionized water by agitation and ultrasonication to obtain a PDA@MWCNT aqueous solution. Then, the PDA@MWCNT aqueous solution was slowly added into 50 mL GO aqueous solution (2 mg/mL) under continuous agitation and ultrasonication in 30 min, and the mixture was further agitated for 2 h to obtain a 100 mL GO-PDA@MWCNT solution. Then, the 100 ml GO-PDA@MWCNT solution was assembled into a 10 cm×10 cm GO-PDA@MWCNT film in a polystyrene petri dish by evaporation of the solvent. Finally, a rGO-PDA@MWCNT film was obtained after the reduction of the GO-PDA@MWCNT film in HI solution at room temperature for 12 h. The rGO-PDA@MWCNT film was washed with ethanol for several times to remove the residual HI and further dried in vacuum at 40oC for 6 h. According to the weight ratio of PDA@MWCNTs to the sum of GO and PDA@MWCNTs, the GO-PDA@MWCNT films 6

and rGO-PDA@MWCNT films can be marked as GO-PDA@MWCNT-x film and rGOPDA@MWCNT-x film, where x indicates the PDA@MWCNT weight ratio of 0%, 2.5%, 5%, 7.5%, 10%, and 15%, respectively.

2.5 Characterization Fourier transform infrared spectroscopy (FTIR) measurements were performed on a Nicolet iZ10 spectrometer (Thermo Electron). Thermogravimetric analysis (TGA) was performed on a NETZSCH TG209F3 Tarsus instrument in N2 atmosphere at a heating rate of 10°C/min from 40°C to 800°C. X-ray diffraction (XRD) patterns were obtained by a Bruker D8 Advance equipment using the Cu Kα (1.5406 Å) radiation. X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi, Thermo Electron) was employed to analyze the elemental compositions of the samples. Raman spectroscopy was used to characterize the structure and crystal quality of the composite films with the 532 nm laser source. The atomic force microscope (AFM) image of the prepared GO sheets was taken on a Multimode 8 model scanning probe microscope in the tapping mode. The morphologies of the composite films were characterized by scanning electron microscopy (SEM) (JEOL, JSM-7800). The morphologies of the MWCNTs and PDA@MWCNTs were characterized by transmission electron microscope (TEM) (Talos F200S, ThermoFisher Scientific). A Shimadzu EZ-LX equipment was used to test the mechanical properties of the composite films. The films were tailored into rectangular samples with a width of 3 mm and a length of about 20 mm. The gauge distance was 5 mm. The thicknesses of the samples were obtained from the SEM images by averaging the thicknesses of the samples at 5 different positions, and the loading rate was 1 mm/min. The electrical conductivity was measured according to the 4-probe method on a RTS-8 system.

7

Figure 1. Schematic illustration of the preparation of rGO-PDA@MWCNT films by evaporation-induced assembly and chemical reduction.

3. Results and discussion The preparation of the rGO-PDA@MWCNT films is illustrated in Figure 1. Firstly, the PDA@MWCNTs were prepared by the oxidative self-polymerization of DA to crosslinked PDA on MWCNTs in the DA-Tris buffer solution [26]. As shown in the TEM images (Figure S2), the PDA@MWCNTs exhibited obvious layered PDA on the surfaces of MWCNTs while the MWCNTs exhibited smooth surfaces. The EDS element distribution and mapping (Figure S3) indicated the uniform distribution of PDA on the MWCNT surfaces. Due to the polar groups such as hydroxyl groups and amine groups of PDA, PDA@MWCNTs exhibited better dispersibility in polar solvents such as water than that of MWCNTs (Figure S2d and Figure S2h), which can promote the dispersion of PDA@MWCNTs in GO solution and the resultant rGO-PDA@MWCNT films. The signal of N in the XPS spectrum of PDA@MWCNTs (Figure 2a), the signals of C-N, C-O, C=O, and O-C=O groups in the XPS C 1s spectrum of PDA@MWCNTs (Figures S4), and the element contents in Table S1 further verified the surface PDA structure of PDA@MWCNTs, which was also verified by the amine groups at 8

about 1535 cm-1 in the FTIR spectrum of PDA@MWCNTs as shown in Figure 2b. The thermal weight loss (Figure 2c, Figure 2d, and Table S2) of PDA@MWCNTs was about 23.66 wt% at 800oC while that of MWCNTs was 4.97 wt%, indicating that the content of PDA in PDA@MWCNTs was close to 20 wt%. The half peak width of the XRD pattern (Figure S5 and Table S3) of PDA@MWCNTs was slightly smaller than that of MWCNTs, indicating the slight increase of crystallinity, consistent with the smaller Raman ID/IG value (Figure 2e) of PDA@MWCNTs compared with that of MWCNTs, which can be attributed to the reduction effect of PDA on the trace oxygen containing groups (O-groups) of MWCNTs [27]. The PDA@MWCNTs were mixed with GO solution to prepare the GO-PDA@MWCNT films and rGO-PDA@MWCNT films by evaporation-induced assembly and HI reduction. In the XPS C 1s spectrum of [email protected]% film (Figure S4d), a new peak of 285.8 eV which is attributed to the C-N groups appeared, indicating the reaction between PDA@MWCNTs and GO sheets. After HI reduction, the [email protected]% film (Figure 2a, Figure S4f, Table S1) showed a decreased oxide content with the strong peak of C-N groups. The FTIR spectrum (Figure 2b) of the GO film showed the characteristic peaks of 1715 cm-1 (C=O), 1397 cm-1 (C-OH), and 1046 cm-1 (C-O-C). These O-group peaks redshifted and decreased in intensity in the spectrum of the [email protected]% film, attributes to the covalent cross-linking of PDA and GO sheets, while the red-shifting of 1595 cm-1 (C=C) peak indicates there is also a π-π interaction between the benzene ring structures of PDA and GO sheets [28,29]. Due to the low content of PDA, there was no obvious peak for amine groups in the spectrum of the [email protected]% film, while an obvious peak (1513 cm-1) of amine groups was shown in the spectrum of the [email protected]% film (Figure 2b) and the O-group peaks all decreased in intensity due to the HI reduction. The thermal weight loss of the [email protected]% film (Figure 2c, Table S2) was lower than that of the GO film due to the reduction effect of PDA on GO, consistent with the XPS 9

results (Table S1). Meanwhile, the decrease of the maximum weight loss temperature (Figure 2d) of 228oC for the GO film to 222oC for the [email protected]% film was also the result of the reduction effect of PDA on GO. The [email protected]% film showed a higher thermal weight loss than that of the rGO film due to the presence of PDA (Table S2). The Raman spectrum (Figure 2e) of the [email protected]% film showed a lower ID/IG value than that of the GO film, while the [email protected]% film showed a lower ID/IG value than that of the rGO film, due to the reduction effect of PDA on GO sheets. The XRD diffraction peak 2θ angle (Figure 2f, Table S4) of the GO-PDA@MWCNT films increased (d-spacing decreases), and the half peak width widened with the increase of the content of PDA@MWCNTs, indicating that the PDA@MWCNTs were successfully embedded into the GO sheets and filled the gaps between the GO sheets by strong covalent and π-π interactions between the GO sheets and PDA@MWCNTs to give the GOPDA@MWCNT films a much denser structure. After HI reduction, the rGO-PDA@MWCNT films showed XRD diffraction peaks (Figure 2g, Table S5) at the 2θ angles of about 25o which is the characteristic diffraction peak of the crystal structure of graphite, indicating the removal of the redundant O-groups and the restoration of the crystal structure of the GO sheets. The rGO-PDA@MWCNT films showed an increasing XRD diffraction peak 2θ angle (a decreasing d-spacing) and a narrowed half peak width with the increase of the content of PDA@MWCNTs, attributed to the strong covalent and π-π interactions between PDA@MWCNTs and rGO sheets and the increase of the crystallinity by the reduction effect of PDA on GO. However, there was a division of the XRD diffraction peak of the rGOPDA@MWCNT-15% film, indicating that a proper amount of PDA@MWCNTs can increase the interactions between rGO sheets and the compactness of rGO-PDA@MWCNT films, while an excessive amount of PDA@MWCNTs may enlarge the spacing between the rGO sheets and lead to poor interaction between rGO sheets. Due to the strong interactions

10

between PDA@MWCNTs and the rGO sheets, the [email protected]% film exhibited a dense wrinkled nacre-like layered structure (Figure 2h and Figure 2i).

Figure 2. (a) XPS spectra of MWCNTs, PDA@MWCNTs, GO film, [email protected]% film, rGO film, and [email protected]% film; (b) FTIR spectra of MWCNTs, PDA@MWCNTs, GO film, [email protected]% film, and [email protected]% film; (c) TGA curves, (d) DTG curves, and (e) Raman spectra of MWCNTs, PDA@MWCNTs, GO film, [email protected]% film, rGO film, and [email protected]% film; (f) XRD patterns of GO-PDA@MWCNT films with different contents of PDA@MWCNTs; (g) XRD patterns of rGO-PDA@MWCNT films with different contents of PDA@MWCNTs; (h) Photo image of a 10 cm×10 cm [email protected]% film; (i) SEM image of a [email protected]% film. 11

As indicated by the strength-strain curves (Figure 3a), the strength and toughness values (Figure 3b), and the detailed mechanical property data (Table S6), the strength, toughness and modulus of the GO-PDA@MWCNT films increased with the increase of the PDA@MWCNT content from 0% to 7.5% due to the toughing and reinforcing effect of PDA@MWCNTs and decreased with the further increase of the PDA@MWCNT content. On one hand, PDA@MWCNTs embedded into and filled in the gaps between the GO sheets, increase the compactness of the GO-PDA@MWCNT films. On the other hand, PDA@MWCNTs interacted with the GO sheets by the strong covalent and π-π interactions between PDA and the GO sheets, enhance the interaction between the GO sheets and crosslinking the GO sheets and MWCNTs together. Therefore, the tensile strength, modulus, and tensile fracture strain of the GO-PDA@MWCNT films increased with the increase of the PDA@MWCNT content from 0% to 7.5%. The further increase of the PDA@MWCNT content may inversely result in gaps between the GO sheets and decrease the compactness of GO-PDA@MWCNT films, leading to the decrease of tensile strength and modulus. However, due to the enhanced entanglement of MWCNTs by the crosslinked PDA, the tensile fracture strain increased with the further increase of the PDA@MWCNT content. The [email protected]% film exhibited the best integrated mechanical properties. After HI reduction, the rGOPDA@MWCNT films (Figure 3c, Figure 3d, and Table S7) showed further increased mechanical properties. The [email protected]% film showed a high tensile strength of 579±35 MPa, a tensile fracture strain of 12.03±0.56%, and an ultrahigh toughness of 34.03±1.60 MJ/m3, which are 2.26, 2.02 and 4.94 times that of the rGO films, respectively. Meanwhile, the tensile strength, modulus, and tensile fracture strain of the rGOPDA@MWCNT films showed the same trend with the increase of the PDA@MWCNT content, and 7.5% is the most proper content to endow the rGO-PDA@MWCNT film with an optimized structure and the most excellent mechanical properties. Note that all mechanical 12

and electrical properties in this work were obtained by averaging at least 5 samples. The standard deviations were carefully controlled within a small threshold (e.g., as shown Figure 3) by removing some extremely good or bad data.

Figure 3. (a) Strength-strain curves and (b) tensile strength and toughness of GOPDA@MWCNT films with different PDA@MWCNT contents; (c) Strength-strain curves and (d) tensile strength and toughness of rGO-PDA@MWCNT films with different PDA@MWCNT contents.

The integrated mechanical properties of the [email protected]% film were compared with those of the other nacre like GCFs with a binary or ternary structure. As shown in Figure 4 and Table 1, the GCFs with a binary structure have poor integrated mechanical properties. Specifically, the binary GCFs such as rGO-PVA [5], rGO-PCDO [6], 13

rGO-PDA [7], rGO-PAPB [8], and rGO-PAA [10] exhibited both low tensile strength and low tensile fracture strain (Figure 4 and Table 1) due to the poor mechanical properties of the polymers and the single interaction such as hydrogen bonds or covalent bonds between the polymers and the rGO sheets, resulting in low toughness. The rGO-SSEBS [13] exhibited ultrahigh tensile fracture strain of 14.2% due to the long chain structure of SSEBS while its tensile strength was limited to only 160 MPa due to the poor interaction between the nonpolar SSEBS molecular chains and the rGO sheets. The rGO-CNC [11,12] reinforced by the rigid CNC and rGO-(AP-DSS) [14] reinforced by the strong π-π interaction exhibited increased strength but still limited tensile fracture strain due to the single reinforcing effect [14]. On the other hand, ternary GCFs such as rGO-CNTs-PCDO [15], rGO-MoS2-TPU [16], rGO-MMTPVA [17], rGO-WS2-PCDO [18] and rGO-NFC-PCDO [19] showed relatively higher integrated mechanical properties (Figure 4 and Table 1) than those of binary GCFs due to the synergistic effect of the soft polymer and the third rigid phase such as CNTs, MoS2, MMT, WS2, and NFC. However, due to the poor dispersion of these rigid phases and the lack of interaction between the rGO sheets and the third reinforcing phase, their mechanical properties are limited. Recently, the GCFs such as rGO-PCO-(AP-PSE) [20], rGO-Zn+2PCDO [21], rGO-Ni+2-PDA [22], rGO-Cu+-CS [23], rGO-Cr+3-(AP-PSE) [24], and rGOBPDD [30] synergistically reinforced by two different bonds of ionic bonds, covalent bonds, or π–π bonds exhibited ultrahigh tensile strength up to 800-1054 MPa. However, due to the short bond length of ionic bonds and π–π bonds, the short chain structure of AP-PES, and the weak chain strength of PCO, the tensile fracture strain of these GCFs is also limited (Figure 4 and Table 1). Typically, the rGO-BPDD film [30] was fabricated by the π–π bonds between the long-chain π–π bonding agent BPDD and the rGO sheets, and the 1,4-addition polymerization of the diacetylene groups of BPDD further crosslinked the BPDD molecular chains and the rGO sheets. The synergistical reinforcing effects of π–π bonds and covalently crosslinking endowed the rGO-BPDD film with an amazing tensile strength of 1,054 MPa, an 14

ultrahigh toughness of 36 MJ/m3, and a tensile fracture strain of only 6.4% (Table 1). In our work, the [email protected]% film showed excellent balanced integrated mechanical properties. The tensile strength of 579±35 MPa is high. To the best of our knowledge, for the present [email protected]% film, its ultrahigh toughness of 34.03±1.60 MJ/m3 and tensile fracture strain of 12.03±0.56% are the second and the third highest ones in the published reports about GCFs (Figure 4 and Table 1). We further evaluate the overall mechanical performance (OMP) using the following index

σ i   i   i   + βλ  λ i =1Ln  + βε  ε T i =1Ln  OMPi = βσ  T i =1Ln  Max(σ i )   Max(λ i )   Max(ε i )   T     T 

(1)

where σ Ti , λ i , and ε Ti are the tensile strength, toughness and tensile fracture strain of the material i (e.g., No. 5 in Table 1), respectively. βσ , β λ and βε are the weighting factors of the tensile strength, toughness and tensile fracture strain, respectively. They are taken as 0.35, 0.35 and 0.3, respectively by considering the higher importance of the tensile strength and the toughness compared with the tensile fracture strain. n is the number of the all sampling i =1Ln

i =1Ln

i =1Ln

materials which is taken as 22 by seeing Table 1. Max(σ Ti ) , Max(λ i ) and Max(ε Ti ) represent the maximum tensile strength, toughness and tensile fracture strain which are 1054 MPa, 36 MJ/m3 and 14.3%, respectively as marked in boldface in Table 1. By employing Eq. (1) to evaluate the OMP, as shown in Table 1, we can find the present material is of the second highest value which is much higher than other values except that in Ref. [30]. The present material’s OMP is only slightly lower than that in Ref. [30].

15

Figure 4. Comparison of tensile strength and toughness of [email protected]% composite film with other GCFs.

Table 1. Comparison of tensile strength, tensile fracture strain, toughness, and electrical conductivity of [email protected]% composite film with other GCFs.

No.

Materials

Tensile strength (MPa) (σT )

Tensile fracture strain (%) ( εT )

Toughness (MJ/m3) (λ )

Electrical conductivity (S/cm)

OMP

Reference

1

rGOPDA@MWCN T-7.5%

579

12.0

34.0

612

0.7746

This work

2

rGO-PVA

189

2.7

2.5

53

0.1437

[5]

3

rGO-PCDO

157

7.9

3.9

232

0.2558

[6]

4

rGO-PDA

205

4.5

4.0

19

0.2014

[7]

5

rGO-PAPB

382

4.3

7.5

337

0.2900

[8]

6

rGO-CS

527

10.4

17.7

155

0.5653

[9]

7

rGO-PAA

310

7.9

8.9

109

0.3552

[10]

8

rGO-CNC

490

1.5

3.9

50

0.2321

[11]

9

rGO-CNC

765

6.2

15.6

1105

0.5358

[12]

10

rGO-SSEBS

160

14.2

15.3

--

0.4999

[13]

11

rGO-(AP-DSS)

539

6.6

16.1

430

0.4740

[14]

12

rGO-CNTsPCDO

374

7.9

9.2

394

0.3794

[15]

16

13

rGO-MoS2TPU

235

5.7

6.9

46

0.2647

[16]

14

rGO-MMTPVA

356

4.0

7.5

136

0.2750

[17]

15

rGO-WS2PCDO

414

9.3

17.7

243

0.5047

[18]

16

rGO-NFCPCDO

315

10.2

9.8

163

0.4139

[19]

17

rGO-PCO(AP-PSE)

945

5.6

21.0

512

0.6355

[20]

18

rGO-Zn+2PCDO

440

5.3

7.6

132

0.3312

[21]

19

rGO-Ni+2-PDA

417

14.3

19.5

188

0.6281

[22]

+

20

rGO-Cu -CS

869

4.4

14.0

235

0.5170

[23]

21

rGO-Cr+3-(APPSE)

821

7.0

20.0

416

0.6139

[24]

22

rGO-BPDD

1054

6.4

36.0

1192

0.8343

[30]

The rGO film exhibited an excellent electrical conductivity of 832±101 S/cm (Figure S6). Due to the electrical insulation of PDA, the electrical conductivity of the rGOPDA@MWCNT films decreased with the increase of the PDA@MWCNT content. However, the [email protected]% film still exhibited a good electrical conductivity of 612±69 S/cm which is the third highest one in the published reports about GCFs (Table 1). To clarify the synergistic effect of PDA@MWCNTs on the mechanical properties of rGOPDA@MWCNT films, the rGO film with 7.5 wt% PDA (rGO-PDA-7.5% film) and rGO film with 7.5 wt% MWCNTs (rGO-MWCNT-7.5% film) were prepared by the same method to compare with the [email protected]% film. As shown in Figure 5a, the [email protected]% film exhibited a lower Raman ID/IG value than that of the rGOMWCNT-7.5% film and rGO-PDA-7.5% film, indicating the higher regularity of the microstructures of [email protected]% film is due to the better dispersion of PDA@MWCNTs in the rGO-PDA-7.5% film and the stable interactions between the PDA@MWCNTs and the rGO sheets. The [email protected]% film showed a larger XRD diffraction 2θ (a smaller d spacing) (Figure 5b and Table S8) than that of the rGOMWCNT-7.5% film and rGO-PDA-7.5% film, indicating the denser layered structure of rGO17

MWCNT-7.5% film. The rGO film showed a relatively low tensile strength and strain (Figure 5c, Figure 5d, and Table S9) due to the lack of interactions between the rGO sheets. In the rGO films, there were only weak Van der Waals force and a small amount of hydrogen bonds. When the rGO films were under loading stress, the hydrogen bonds were easily damaged before being able to transfer stress between the rGO sheets, leading to the low mechanical properties of the rGO films. In the rGO-PDA-7.5% film, the PDA, crosslinked macromolecules with a large amount of benzene rings and pyrrole rings, interacted with the rGO sheets by strong covalent bonds and linked the rGO sheets together, promoting the stress transfer between the rGO sheets when the rGO-PDA-7.5% film was under loading stress, and leading to the higher tensile strength of the rGO-PDA-7.5% film (Figure 5c, Figure 5d, and Table S9). However, the rigid crosslinked structure of PDA is brittle, which has no benefit on dissipation of the loading energy, leading to the low tensile fracture strain and toughness of the rGO-PDA-7.5% film. As shown in Figure 6a, the fracture surfaces of the rGO-PDA-7.5% film were curly, and there were some small broken PDA aggregates on the fracture surfaces, attributed to that the PDA helped to transfer stress but the rigid crosslinked PDA structure restricted the slippage of the rGO sheets, and brittle fracture occurred at the PDA structure, which further led to the fracture of rGO sheets and resulted in the curling of the rGO sheet edges. In the rGO-MWCNT-7.5% film, the interactions between the MWCNTs and the rGO sheet were also weak Van der Waals force and hydrogen bonds. When the rGO-MWCNT7.5% film was under tensile loading, the rGO sheets slipped along the tensile direction, while the MWCNTs acted as surface protuberances to restrict the slippery of rGO sheets and adsorb energy by friction between MWCNTs and rGO sheets. Due to the high aspect ratio 1D structure of MWCNTs, the MWCNTs entangled with each other to further restrict the slippage of rGO sheets and absorb energy, endowing the rGO-MWCNT-7.5% film with a high tensile fracture strain close to 15% (Figure 5c). The bridging, slippage, entanglement, and pullout of the MWCNTs helped to transfer and absorb energy, toughing and reinforcing 18

the rGO-MWCNT-7.5% film. However, due to the weak interactions between the MWCNTs and rGO sheets, and the poor dispersion of MWCNTs in the rGO-MWCNT-7.5% film (Figure 6b), the tensile strength of the rGO-MWCNT-7.5% film was limited, therefore leading to a relatively limited toughness (Figure 5c, Figure 5d, and Table S9). As shown in Figure 6c and Figure 6d, the PDA@MWCNTs were uniformly dispersed in the [email protected]% film, in another word, PDA promoted the dispersion of MWCNTs in the [email protected]% film, which enhanced the reinforcing effect of MWCNTs. Also, the PDA interacted with the rGO sheets by the strong covalent bonds and π-π interaction, which enhanced the interaction between MWCNTs and the rGO sheets. Furthermore, PDA has crosslinked macromolecular structure which crosslinked the MWCNTs and rGO sheets together and enhanced the entanglement between MWCNTs. As shown in the proposed mechanism in Figure 6d, when the [email protected]% film was under tensile loading, the wrinkle structures of the rGO sheets were straightened, and the PDA@MWCNTs stood up acted as surface protuberances to restrict the slippage of the rGO sheets. The [email protected]% film adsorbed energy by the rGO sheets, PDA@MWCNTs and the friction between PDA@MWCNTs and rGO sheets. PDA promoted the stress transfer from the rigid rGO sheets to the rigid MWCNTs by its soft crosslinked macromolecular structure and the energy dispersion between rGO sheets, MWNCTs and PDA, leading to the high tensile strength of the [email protected]% film. When the [email protected]% film was under high tensile strength and tensile strain, the PDA@MWCNTs acted as linkage to prevent the slippage of the rGO sheets. The crosslinked macromolecular structure of PDA enhanced the entanglement of MWCNTs, promoting the stress and energy transfer between the MWCNTs, and conversely the nanosized MWCNTs reinforced the soft PDA. The mutual enhancement effect between the MWCNTs and PDA maximized the toughing and reinforcing effect of the PDA@MWCNTs. This synergistic effect of MWCNTs and PDA promoted the stress transfer between the rGO sheets and PDA@MWCNTs and shared the 19

energy, provided a high resistance toward crack propagation and generates high stress to attain high tensile strain (Figure 5c). This synergistic toughing and reinforcing effect of the PDA@MWCNTs endowed the [email protected]% film with excellent integrated mechanical properties.

Figure 5. (a) Raman spectra and (b) XRD patterns of rGO-PDA-7.5% film, rGO-MWCNT7.5% film, and [email protected]% film; (c) Strength-strain curves and (d) Tensile strength and toughness of rGO film, rGO-PDA-7.5% film, rGO-MWCNT-7.5% film, and [email protected]% film.

20

Figure 6. SEM images show the fracture morphology of (a) rGO-PDA-7.5% film, (b) rGOMWCNT-7.5% film, and (c) [email protected]% film, respectively; (d) The proposed synergistic reinforcing and toughening mechanism of rGO- [email protected]% film.

Except for mechanical properties and electrical conductivity, the structural stability under extreme environments is also very important for the practical applications of GCFs. As shown in Figure S7, the rGO film was totally decomposed after 4.5 h ultrasonication in deionized water due to the poor interaction between the rGO sheets and the residual O-groups of the rGO sheets. The structural stability of the rGO-PDA-7.5% film was also poor due to the hydrophilic PDA. The rGO-MWCNT-7.5% film exhibited better structural stability than that of the rGO film and rGO-PDA-7.5% film due to the entanglement of MWCNTs while most part of the rGO-MWCNT-7.5% film was also decomposed after 4.5 h ultrasonication in deionized water. Due to the improved interaction between PDA@MWCNTs and rGO sheets, and the entanglement of PDA@MWCNTs, the [email protected]% film exhibited better structural stability than that of the rGO film, rGO-PDA-7.5% film, and rGO-MWCNT7.5% film. As shown in Figure S7, there was no obvious structural damage of the rGOMWCNT-7.5% film even after 4.5 h ultrasonication in deionized water. As shown in Figure S8, the rGO film began to decompose after 3 h ultrasonication in H2SO4 solution and 21

decomposed completely after 4.5 h ultrasonication, while rGO-PDA-7.5% film, rGOMWCNT-7.5% film, and [email protected]% film were intact after 4.5 h ultrasonication in H2SO4 solution due to the stability of PDA and MWCNTs in H2SO4 solution. As shown in Figure S9, the rGO-PDA-7.5% film began to decompose after 3 h ultrasonication in NaOH solution due to the instability of PDA in NaOH solution, while the rGO film, rGO-MWCNT-7.5% film, and [email protected]% film all exhibited excellent structural stability in NaOH solution due to the reduction effect of NaOH on GO sheets. Therefore, the [email protected]% film exhibited improved structural stability under extreme environments.

4. Conclusions In conclusion, we provide a simple, cheap, and environmentally-friendly method to fabricate the ternary nacre like rGO-PDA@MWCNT films. Compared with previous approaches, the MWCNTs were firstly surface modified by PDA to increase the dispersion of MWCNTs in the rGO-PDA@MWCNT films and enhance the interactions between MWCNTs and rGO sheets. The cross-linking effect of PDA helped the crosslink between the rGO sheets and MWCNTs and enhanced the entanglement of MWCNTs. This synergistic toughing and reinforcing effect of PDA and MWCNTs in the PDA@MWCNTs endowed the [email protected]% films with a high tensile strength of 579±35 MPa, a high tensile fracture strain of 12.03±0.56%, and an ultrahigh toughness of 34.03±1.60 MJ/m3. Furthermore, the [email protected]% films exhibited a high conductivity of 612±68 S/cm and excellent structural stability under extreme environments. These excellent integrated properties will promote the applications of the rGO-PDA@MWCNT films in the areas of aerospace engineering, flexible supercapacitor electrodes, artificial muscles, and tissue engineering.

22

Acknowledgments This work was supported by the National Natural Science Foundation of China (No.11632004, No.U1864208, and No.11902056), the Key Project of Natural Science Foundation of CQ CSTC (No. cstc2017jcyjBX0063), the Key Program for International Science and Technology Cooperation Projects of the Chinese Ministry of Science and Technology (No. 2016YFE0125900), the Project Funded by Chongqing Special Postdoctoral Science Foundation (No. XmT2018052), and the China Postdoctoral Science Foundation (No.2017M622959, No.2019M653334).

References [1] F. Bouville, E. Marire, S. Meille, B. Van de Moortèle, A.J. Stevenson, S. Deville, Strong, tough and stiff bio-inspired ceramics from brittle constitutes, Nat. Mater. 13 (2014) 508−514. [2] J. Wang, Q. Cheng, Z. Tang, Layered nanocomposites inspired by the structure and mechanical properties of nacre, Chem. Soc. Rev. 41 (2012) 1111−1129. [3] S. Gong, H. Ni, L. Jiang, Q. Cheng, Learning from nature: constructing high performance graphene-based nanocomposites, Mater. Today 20 (2017) 210−219. [4] C. Huang, Q. Cheng, Learning from nacre: Constructing polymer nanocomposites, Compos. Sci. Technol. 150 (2017) 141−166. [5] Y. Li, T. Yu, T. Yang, L. Zheng, K. Liao, Bio-inspired nacre-like composite films based on graphene with superior mechanical, electrical, and biocompatible properties, Adv. Mater. 24 (2012) 3426–3431. [6] Q. Cheng, M. Wu, M. Li, L. Jiang, Z. Tang, Ultratough artificial nacre based on conjugated cross-linked graphene oxide, Angew. Chem. Int. Ed. 52 (2013) 3750–3755. [7] W. Cui, M. Li, J. Liu, B. Wang, C. Zhang, L. Jiang, Q. Cheng, A strong integrated strength and toughness artificial nacre based on dopamine cross-linked graphene oxide, ACS Nano 8 (2014) 9511–9517. 23

[8] M. Zhang, L. Huang, J. Chen, C. Li, G. Shi, Ultratough, ultrastrong, and highly conductive graphene films with arbitrary sizes, Adv. Mater. 26 (2014) 7588–7592. [9] S. Wan, J. Peng, Y. Li, H. Hu, L. Jiang, Q. Cheng, Use of synergistic interactions to fabricate strong, tough, and conductive artificial nacre based on graphene oxide and chitosan, ACS Nano 9 (2015) 9830–9836. [10] S. Wan, H. Hu, J. Peng, Y. Li, Y. Fan, L. Jiang, Q. Cheng, Nacre-inspired integrated strong and tough reduced graphene oxide–poly(acrylic acid) nanocomposites, Nanoscale 8 (2016) 5649–5656. [11] R. Xiong, K. Hu, A. M. Grant, R. Ma, W. Xu, C. Lu, X. Zhang, V. V. Tsukruk, Ultrarobust transparent cellulose nanocrystal-graphene membranes with high electrical conductivity, Adv. Mater. 28 (2016) 1501–1509. [12] Y. Wen, M. Wu, M. Zhang, C. Li, G. Shi, Topological design of ultrastrong and highly conductive graphene films, Adv. Mater. 29 (2017) 1702831. [13] P. Song, Z. Xu, Y. Wu, Q. Cheng, Q. Guo, H. Wang, Super-tough artificial nacre based on graphene oxide via synergistic interface interactions of p-p stacking and hydrogen bonding, Carbon 111 (2017) 807–812. [14] H. Ni, F. Xu, A. P. Tomsia, E. Saiz, L. Jiang, Q. Cheng, Robust bioinspired graphene film via π−π cross-linking, ACS Appl. Mater. Interfaces 9 (2017) 24987−24992. [15] S. Gong, W. Cui, Q. Zhang, A. Cao, L. Jiang, Q. Cheng, Integrated ternary bioinspired nanocomposites via synergistic toughening of reduced graphene oxide and double-walled carbon nanotubes, ACS Nano 9 (2015) 11568–11573. [16] S. Wan, Y. Li, J. Peng, H. Hu, Q. Cheng, L. Jiang, Synergistic toughening of graphene oxide-molybdenum disulfide-thermoplastic polyurethane ternary artificial nacre, ACS Nano 9 (2015) 708–714.

24

[17] P. Ming, Z. Song, S. Gong, Y. Zhang, J. Duan, Q. Zhang, L. Jiang, Q. Cheng, Nacreinspired integrated nanocomposites with fire retardant properties by graphene oxide and montmorillonite, J. Mater. Chem. A 3 (2015) 21194–21200. [18] S. Wan, Q. Zhang, X. Zhou, D. Li, B. Ji, L. Jiang, Q. Cheng, Fatigue resistant Bioinspired composite from synergistic two-dimensional anocomponents, ACS Nano 11 (2017) 7074−7083. [19] J. Duan, S. Gong, Y. Gao, X. Xie, L. Jiang, Q. Cheng, Bioinspired ternary artificial nacre nanocomposites based on reduced graphene oxide and nanofibrillar cellulose, ACS Appl. Mater. Interfaces 8 (2016) 10545−10550. [20] S. Wan, Y. Li, J. Mu, A. E. Aliev, S. Fang, N. A. Kotov, L. Jiang, Q. Cheng, R. H. Baughman, Sequentially bridged graphene sheets with high strength, toughness, and electrical conductivity, PNAS 115 (2018) 5359−5364. [21] S. Gong, L. Jiang, Q. Cheng, Robust bioinspired graphene-based nanocomposites via synergistic toughening of zinc ions and covalent bonding, J. Mater. Chem. A 4 (2016) 17073– 17079. [22] S. Wan, F. Xu, L. Jiang, Q. Cheng, Superior fatigue resistant bioinspired graphene-based nanocomposite via synergistic interfacial interactions, Adv. Funct. Mater. 27 (2017) 1605636. [23] Y. Cheng, J. Peng, H. Xu, Q. Cheng, Glycera-inspired synergistic interfacial interactions for constructing ultrastrong graphene-based nanocomposites, Adv. Funct. Mater. 28 (2018) 1800924. [24] S. Wan, S. Fang, L. Jiang, Q. Cheng, R. H. Baughman, Strong, conductive, foldable graphene sheets by sequential ionic and π bridging, Adv. Mater. 30 (2018) 1802733. [25] W. Zhao, L. Zhu, Y. Lu, L. Zhang, R. H. Schusterc, W. Wang, Magnetic nanoparticles decorated multi-walled carbon nanotubes by bio-inspired poly(dopamine) surface functionalization, Synthetic Met. 169 (2013) 59–63.

25

[26] H. Lee, Mussel-inspired surface chemistry for multifunctional coatings, Science 318 (2007) 426–430. [27] Y. Liu, K. Ai, L. Lu, Polydopamine and its derivative materials: synthesis and promising applications in energy, environmental, and biomedical fields, Chem. Rev. 114 (2014) 5057−5115. [28] J. He, P. Xiao, J. Zhang, Z. Liu, W. Wang, L. Qu, Q. Ouyang, X. Wang, Y. Chen, T. Chen, Highly efficient actuator of graphene/polydopamine uniform composite thin film driven by moisture gradients, Adv. Mater. Interfaces 3 (2016) 1600169. [29] R. Zou, F. Liu, N. Hu, H. Ning, X. Jiang, C. Xu, S. Fu, Y. Li, X. Zhou, Cheng Yan, Carbonized polydopamine nanoparticle reinforced graphene films with superior thermal conductivity, Carbon 149 (2019) 173–180. [30] S. Wan, Y. Chen, Y. Wang, G. Li, G. Wang, L. Liu, J. Zhang, Y. Liu, Z. Xu, A. P. Tomsia, L. Jiang, Q. Cheng, Ultrastrong graphene films via long-chain π-bridging, Matter 1 (2019) 1–13.

26

Highlights •

A ternary nacre like graphene based composite film (GCF) synergistically toughened and reinforced by polydopamine wrapped carbon nanotubes is developed.

•

This GCF exhibits excellent properties of a high tensile strength of 579±35 MPa, a high fracture strain of 12.03±0.56%, an ultrahigh toughness of 34.03±1.60 MJ/m3, a high electrical conductivity of 612±68 S/cm, and excellent structural stability under extreme environments.

•

These excellent integrated properties will promote the applications of GCFs in aerospace engineering, flexible electrical devices and artificial muscle.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: