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Binary Strengthening and Toughening of MXene/Cellulose Nanofibers Composite Paper with Nacre-Inspired Structure and Superior Electromagnetic Interference Shielding Properties Wen-Tao Cao, Fei-Fei Chen, Ying-Jie Zhu, Yong-Gang Zhang, Ying-Ying Jiang, Ming-Guo Ma, and Feng Chen ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00997 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on April 30, 2018

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Binary Strengthening and Toughening of MXene/Cellulose Nanofibers Composite Paper with Nacre-Inspired Structure and Superior Electromagnetic Interference Shielding Properties Wen-Tao Cao,†,‡ Fei-Fei Chen,‡ Ying-Jie Zhu,*,‡ Yong-Gang Zhang,‡ Ying-Ying Jiang,‡ Ming-Guo Ma,* ,† and Feng Chen*,‡

†

Engineering Research Center of Forestry Biomass Materials and Bioenergy, Beijing

Key Laboratory of Lignocellulosic Chemistry, College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, PR China ‡

State Key Laboratory of High Performance Ceramics and Superfine Microstructure,

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China

1 ACS Paragon Plus Environment

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ABSTRACT:

With

the

growing

popularity

of

electrical

communication

equipment,

high-performance electromagnetic interference (EMI) shielding materials are widely used to deal with radiation pollution. However, the large thickness and poor mechanical properties of many EMI shielding materials usually limit their applications. In this study, ultrathin and highly flexible Ti3C2Tx (d-Ti3C2Tx, MXene)/cellulose nanofibers (CNFs) composite paper with a nacre-like lamellar structure is fabricated via a vacuum filtration induced self-assembly process. By the interaction between one-dimensional (1D) CNFs and two-dimensional (2D) d-Ti3C2Tx MXene, the binary strengthening and toughening of the nacre-like d-Ti3C2Tx/CNFs composite paper has been successfully achieved, leading to high tensile strength (up to 135.4 MPa) and fracture strain (up to 16.7%), as well as excellent folding endurance (up to 14260 times). Moreover, the d-Ti3C2Tx/CNFs composite paper exhibits high electrical conductivity (up to 739.4 S m-1) and excellent specific EMI shielding efficiency (up to 2647 dB cm2g-1) at an ultrathin thickness (minimum thickness 47 µm). The nacre-inspired strategy in this study offers a promising approach for the design and preparation of the strong integrated and flexible MXene/CNFs composite paper, which may be applied in various fields such as flexible wearable devices, weapon equipments and robot joints.

KEYWORDS: MXene, Cellulose nanofibers; Paper; Mechanical properties; Electromagnetic interference shielding


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In the past decades, the use of wireless communication and electronic devices has become an increasingly common phenomenon with the development of economy. However, electromagnetic radiations are emitted inevitably by the electronic devices, which not only affect the normal function and the lifetime of the electronic instruments, but also are harmful to human health. Thus, electromagnetic interference (EMI) shielding materials with good performances are urgently required to resolve above-mentioned problems.1-3 Traditional metal shielding materials exhibit excellent EMI shielding performance. Nevertheless, the intrinsic high density and susceptible to corrosion in the harsh environment limit their applications.4,5 Recently, many carbon-based materials (e.g. graphene, reduced graphene oxide, carbon nanotubes) were reported and regarded as alternatives for the EMI-shielding application owing to their low density, and corrosion resistance.6,7 MXenes, two-dimensional (2D) nanomaterials, are produced by selectively etching the A-element from the ternary transition metal carbides with a general formula of Mn+1AXn, where M is an early transition metal, A represents a group of XIII and XIV elements, and X is C and/or N. Due to the large specific surface area and high electrical conductivity, MXenes exhibit light-weight and superior EMI shielding performance.8,9 It should be mentioned that Gogotsi and co-workers reported the free-standing MXene film (thickness 45 µm) and MXene-sodium alginate composite film (30 wt% Ti3C2Tx, thickness 8–9 µm), which showed an outstanding EMI shielding performance of >95 dB and >25 dB, respectively.1 Han et al.10 prepared surface modified Ti3C2 MXenes with a EMI shielding efficiency of 76.1 dB by a


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post-etching heat treatment method. Liu et al.11 prepared a hydrophobic MXene foam by using hydrazine to treat the MXene film at 90 oC, the as-prepared MXene foam exhibited a high EMI shielding effectiveness of ~70 dB at a thickness of 60 µm. The MXene materials possess a better EMI shielding performance than many metal and carbon-based materials.12,13 However, the unsatisfied mechanical properties of MXene materials limits their applications in wearable electronic devices, weapon equipments, and robot joints, which need a strong strength to endure mechanical deformation.14,15 The MXene such as Ti3C2Tx, fabricated by wet chemical etching possesses plenty of active terminal groups (e.g., F, O and OH) on its surface,16 which may interact with polymers by hydrogen bonding to make up for the low mechanical strength of the pure MXene material.17,18 As the most abundant and renewable natural polymer on the earth, cellulose seems like inexhaustible for all time.19 Cellulose nanofibers (CNFs), which can be derived from cellulose, are often used as a reinforcement material for composites owing to their high mechanical strength and great flexibility.20,21 More importantly, compared with other polymers, CNFs have a characteristic one-dimensional (1D) nanofiber structure, which will lead to less insulating contacts between 2D conductive nanosheets.22,23 In recent years, it was reported that CNFs could be integrated with graphene, reduced graphene oxide, and carbon nanotubes for applications in energy storage such as Li-ion batteries, Na-ion batteries, and supercapacitors.24-26 However, to the best of our knowledge, there is no relevant report on the d-Ti3C2Tx/CNFs composite paper for the EMI shielding application. Therefore, it is of great significance to prepare the d-Ti3C2Tx/CNFs


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composite paper with enhanced mechanical properties and EMI shielding performance. In this study, an ultrathin and flexible d-Ti3C2Tx/CNFs composite paper constructed with 1D CNFs and 2D delaminated Ti3C2Tx nanosheets has been successfully prepared by a facile vacuum-assisted filtration self-assembly method. The d-Ti3C2Tx/CNFs composite paper exhibits a nacre-like microstructure and excellent mechanical properties. The ultimate tensile strength and strain are up to 135.4 MPa and 16.7 %, respectively, and the bending number in the folding endurance test is up to 14260 times. Moreover, the d-Ti3C2Tx/CNFs composite paper also exhibits superior electrical conductivity and EMI shielding efficiency. The d-Ti3C2Tx/CNFs composite paper with a nacre-like structure and excellent mechanical properties has promising applications in the flexible electronics such as wearable electric devices, weapon equipments and robot joints.

RESULTS AND DISCUSSION Figure 1a shows the strategy for the preparation of the d-Ti3C2Tx/CNFs composite paper. The d-Ti3C2Tx was synthesized by selective etching Ti3AlC2 (MAX) in a mixture of hydrochloric acid (HCl) and lithium ﬂuoride (LiF), and delaminated the etched powder by manual shaking to form a stabilized dispersion (Figure 1b). Scanning electron microscopy (SEM) images exhibit that after etching the Al layers, the cohesive bulk Ti3AlC2 (MAX) changes to accordion-like multilayered Ti3C2Tx (m-Ti3C2Tx), and further turns into delaminated Ti3C2Tx (d-Ti3C2Tx) nanosheets by


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vigorous shaking. The shift of the characteristic peak (from 9.3o to 7.2o) of MXene in X-ray diffraction (XRD) patterns (Figure S1, Supporting Information) is in good agreement with the previous report, indicating the successful preparation of d-Ti3C2Tx nanosheets.11 The cellulose nanofibers (CNFs) were extracted from microcrystalline cellulose (MCC), which disintegrated from the agricultural waste of garlic husk. The transmission electron microscopy (TEM) image of CNFs suspension (Figure S2, Supporting Information) shows that the obtained CNFs are about 20 ~ 50 nm in diameter and several microns in length. After mixing the delaminated Ti3C2Tx stable dispersion and CNFs colloidal dispersion for 24 h under continuous stirring, a large number of aggregates form (Figure S3a-c, Supporting Information), indicating that there are interactions between 1D CNFs and 2D d-Ti3C2Tx nanosheets. After the exfoliation by wet chemical etching, active terminal groups (e.g., F, O and OH) are formed on the surface of d-Ti3C2Tx nanosheets. As a result, CNFs can easily adsorb on the surface of d-Ti3C2Tx nanosheets. On the basis of the experimental results, we propose that there are abundant hydrogen bonds formed between the CNFs and d-Ti3C2Tx nanosheets (Figure S3d, Supporting Information). The d-Ti3C2Tx/CNFs composite paper was obtained by a vacuum-assisted filtration process using the suspension containing d-Ti3C2Tx and CNFs. The pure d-Ti3C2Tx paper pure or CNFs paper as the control sample was also prepared by a similar method (Figure 1c). The mechanical strength of the pristine d-Ti3C2Tx paper is too low to be peeled off from the filtration filter paper, and cracks are observed on the d-Ti3C2Tx paper. The pure CNFs paper and d-Ti3C2Tx/CNFs composite paper sheets display a good mechanical


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property. The poor mechanical properties of the pure d-Ti3C2Tx paper are mainly attributed to the weak interconnection between adjacent nanosheets. CNFs with a long 1D structure are inclined to form a connected meshwork structure. Therefore, the integration of CNFs enhances the paper-forming ability of the d-Ti3C2Tx/CNFs (50% d-Ti3C2Tx) composite paper with superior mechanical properties (Movie S1, Supporting Information).

Figure 1. Illustration of the preparation process of the d-Ti3C2Tx/CNFs composite paper. (a) Ti3AlC2 (MAX) is etched by hydrochloric acid (HCl) and lithium ﬂuoride (LiF) to obtain the multilayered Ti3C2Tx (m-Ti3C2Tx) sediment. Then, the m-Ti3C2Tx sediment is dispersed in deionized water under vigorous shaking to obtain delaminated Ti3C2Tx (d-Ti3C2Tx) nanosheets. Then, the dispersion containing CNFs is added into the d-Ti3C2Tx dispersion. After stirring for 24 h, the resulting suspension is filtrated to form the d-Ti3C2Tx/CNFs composite paper. (b) SEM micrographs of


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Ti3AlC2 (MAX), m-Ti3C2Tx, and d-Ti3C2Tx. (c) Digital images of the as-prepared pure d-Ti3C2Tx paper, pure CNFs paper, and d-Ti3C2Tx/CNFs composite paper.

We have characterized the microstructure of the as-prepared d-Ti3C2Tx/CNFs composite paper, and found that the composite paper has an ordered lamellar structure which is similar to the “brick-and-mortar” structure of nacre (Figure 2a). The excellent mechanical performance of nacre originates from its hierarchically organized hard and soft building blocks. This biological assembly process in nacre can inspire us to design the synthesis strategy of complex materials with high strength and toughness. In this study, a simple vacuum-filtration method (Figure 2b) is used to mimic the biological assembly process, and displays a powerful potential in preparing the d-Ti3C2Tx/CNFs composite paper with a nacre-like microstructure. Different from viscous polymers which weaken the layered structure, 1D CNFs can promote the formation of the layered structure of the d-Ti3C2Tx MXene. The SEM micrographs of the d-Ti3C2Tx/CNFs composite paper indicate an ordered lamellar structure when the weight content of d-Ti3C2Tx in the composite paper varies from 20 % to 90 % (Figure 2c). When the d-Ti3C2Tx content is 20 wt%, the d-Ti3C2Tx/CNFs composite paper exhibits a tightly stacking layered structure. The oriented alignment of d-Ti3C2Tx nonosheets along the planar direction are surrounded by plenty of CNFs. With increasing d-Ti3C2Tx content to 90 wt%, excess d-Ti3C2Tx nanosheets form an undulating layered structure. In this biomimetic structure, the d-Ti3C2Tx nanosheets with a 2D structure display the role of the inorganic “brick”, and the CNFs with a 1D


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structure display the role of the organic “mortar”. This nacre-inspired structure not only guarantees the excellent mechanical properties of the d-Ti3C2Tx/CNFs composite paper, but also is beneficial to the electromagnetic wave reflection and absorption.

Figure 2. (a) Schematic illustration of the “brick-and-mortar” structure in nacre. (b) Schematic representation of the preparation of the d-Ti3C2Tx/CNFs composite paper with a nacre-like structure via a vacuum filtration method. (c) SEM images of the d-Ti3C2Tx/CNFs composite paper, exhibiting a nacre-like compact lamellar structure. The weight content of d-Ti3C2Tx in the d-Ti3C2Tx/CNFs composite paper varies from 20 % to 90 wt %.


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The contents of d-Ti3C2Tx in the d-Ti3C2Tx/CNFs composite paper sheets are estimated by thermogravimetric analysis (TGA), as shown in Figure S4 and Table S1 in the Supporting Information. Because of the loss of a certain amount of CNFs or d-Ti3C2Tx in the process of vacuum filtration, the content of d-Ti3C2Tx in the d-Ti3C2Tx/CNFs composite paper changes compared with the nominal content of d-Ti3C2Tx. Figure 3 shows the X-ray diffraction (XRD) patterns and Fourier transform infrared (FTIR) spectra of the d-Ti3C2Tx/CNFs composite paper. The FTIR spectra of the pure d-Ti3C2Tx, pure CNFs, and d-Ti3C2Tx/CNFs (50 wt %) composite paper are shown in Figure 3a. d-Ti3C2Tx has two typical peaks at 1387 and 584 cm-1, corresponding to the surface terminal group of C–F and –OH, respectively. After mixing with CNFs, the cellulose characteristic absorption bands at 2920 (C–H stretching), 1639 (–OH bending), and 663 cm-1 (–OH out-of-plane bending) are observed in the FTIR spectrum of the d-Ti3C2Tx/CNFs composite paper. X-ray photoelectron spectroscopy (XPS) patterns are shown in Figure 3b. The d-Ti3C2Tx/CNFs sample has higher C/Ti atomic ratio (14.66) and O/Ti atomic ratio (11.45) than the C/Ti atomic ratio (1.86) and O/Ti atomic ratio (2.56) of the pure d-Ti3C2Tx sample. Moreover, after the introduction of CNFs, the ratio of the terminal group of –F to Ti in the d-Ti3C2Tx/CNFs composite paper decreases from 2.16 to 0.36. The XRD patterns of the d-Ti3C2Tx and d-Ti3C2Tx/CNFs composite paper with different d-Ti3C2Tx contents are shown in Figure 3c. The two overlapping peaks at 2θ = 14.0–17.8o in the d-Ti3C2Tx/CNFs composite paper samples correspond to (101) _

and (101 ) crystal planes of cellulose I, and the diffraction peak at 2θ = 22.5o


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corresponds to the (002) crystal plane of cellulose I crystalline structure.

27

With

increasing content of CNFs from 20 wt % to 90 wt % in the composite paper, the two representative peaks of CNFs become increasingly distinct. In addition, the characteristic (002) peak shifts from 2θ = 7.2° to 5.9°, implying that the d-spacing of d-Ti3C2Tx nanosheets increases from approximately 12.3 Å for pristine d-Ti3C2Tx to ~15 Å for the 20 wt% d-Ti3C2Tx/CNFs composite paper. The change of the d-spacing is a strong evidence for the successful intercalation of CNFs into d-Ti3C2Tx nanosheets. The experimental results indicate that the CNFs have been successfully introduced into the as-prepared composite paper.

Figure 3. (a) FTIR spectra of the pure d-Ti3C2Tx, pure CNFs, and d-Ti3C2Tx/CNFs (50 wt%) composite paper. (b) XPS survey spectrum of the pure d-Ti3C2Tx paper and


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d-Ti3C2Tx/CNFs (50 wt%) composite paper. (c) XRD patterns of the d-Ti3C2Tx/CNFs composite paper with different d-Ti3C2Tx contents.

The tensile stress-strain curves of the d-Ti3C2Tx/CNFs composite paper sheets with different d-Ti3C2Tx contents are shown in Figure 4a, and the detailed mechanical properties are listed in Table S2 in the Supporting Information. The experiments indicate that the d-Ti3C2Tx/CNFs composite paper sheets exhibit excellent integration of the mechanical strength and toughness (Figure 4b,d). The pure CNFs paper shows a tensile strength of 49.3 ± 4.8 MPa, a fracture strain of 10.0 ± 0.5 %, a toughness of 3.2 ± 0.3 MJ/m3, and a Young’s modulus of 1.4 ± 0.1 GPa. With increasing d-Ti3C2Tx content from 20 to 90 wt%, the tensile strength of the composite paper increases from 69.1 ± 6.1 MPa to 135.4 ± 6.9 MPa, and then decreases to 44.2 ± 5.1 MPa; the tensile fracture strain increases from 11.9 ± 0.6 % to 16.7 ± 0.7 %, and then decreases to 3.1 ± 0.5 %; the toughness increases from 5.3 ± 0.4 MJ/m3 to 14.8 ± 0.4 MJ/m3, and then decreases to 1.2 ± 0.3 MJ/m3; and the Young’s modulus increases from 1.7 ± 0.2 GPa to 3.8 ± 0.3 GPa, and then decreases to 1.0 ± 0.1 GPa. The optimal mechanical properties of the d-Ti3C2Tx/CNFs composite paper are achieved when the d-Ti3C2Tx nominal content is 50 wt% (the measured d-Ti3C2Tx content is 59.3 wt%), and the corresponding tensile strength, fracture strain, toughness, and Young’s modulus are 135.4 ± 6.9 MPa, 16.7 ± 0.7 %, 14.8 ± 0.4 MJ/m3, and 3.8 ± 0.3 GPa, respectively. The d-Ti3C2Tx/CNFs composite paper (50 wt% d-Ti3C2Tx) with sizes of 40 mm × 15 mm can withstand a weight of ~500 g without breaking (Figure 4c).


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Figure 4. Mechanical properties of the d-Ti3C2Tx/CNFs composite paper with different d-Ti3C2Tx contents. (a) Tensile stress-strain curves of the d-Ti3C2Tx/CNFs composite paper with different d-Ti3C2Tx contents (0 wt%, curve 1; 20 wt%, curve 2; 40 wt%, curve 3; 50 wt%, curve 4; 60 wt%, curve 5; 80 wt%, curve 6; 90 wt%, curve 7). (b) Tensile strengths and tensile strains of the d-Ti3C2Tx/CNFs composite paper with different d-Ti3C2Tx contents. (c) Digital image of the d-Ti3C2Tx/CNFs composite paper which can withstand a weight of 500 g. (d) Toughness and young’s modulus of the d-Ti3C2Tx/CNFs composite paper with different d-Ti3C2Tx contents. (e) Folding times of the d-Ti3C2Tx/CNFs composite paper with different d-Ti3C2Tx contents under a loading of 4.9 N. (f) Digital image of the paper folding endurance instrument.

To further investigate the folding endurance of the d-Ti3C2Tx/CNFs composite paper, a paper folding endurance instrument was used under a 4.9 N pulling load (Figure 4e,f). All the d-Ti3C2Tx/CNFs composite paper sheets can be folded for more


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than 100 times. Significantly, when the d-Ti3C2Tx content is 50 wt % in the composite paper, the folding number can reach as high as 14260 times (Movie S2, Supporting Information). The folding number of the commercial printing paper (Golden Flagship, China) with a A4 size is also tested, which is ~400 times in the longitudinal direction and ~130 times in the transverse direction. The d-Ti3C2Tx/CNFs composite paper (50 wt%) exhibits excellent folding endurance that is even >35 or >109 times that of the commercial printing paper. The experimental results indicate that the appropriate integration of the CNFs can effectively enhance the mechanical properties of the d-Ti3C2Tx/CNFs composite paper. The mechanical properties of the d-Ti3C2Tx/CNFs composite paper are better than the pure d-Ti3C2Tx paper and pure CNFs paper, indicating that there is a strong interaction force between 1D CNFs and 2D d-Ti3C2Tx nanosheets in the nacre-like structure. The d-Ti3C2Tx nanosheets have abundant terminal groups (e.g., F, O and OH), and the CNFs possess abundant active hydroxyl groups (–OH). On the basis of the experimental results, we propose that there are abundant hydrogen bonds formed between the CNFs and d-Ti3C2Tx nanosheets (Figure S3d, Supporting Information). The SEM images (Figure 5a,b) of the fracture surface in the d-Ti3C2Tx/CNFs composite paper after tensile tests show that the fracture surface still maintains the closely stacked layered structure (nacre-like structure), which is formed by the interaction between 2D d-Ti3C2Tx nanosheets and 1D CNFs in the vacuum filtration induced self-assembly process. Unlike other polymers such as polyvinyl alcohol, polymethyl methacrylate, and polyimide, CNFs with the 1D nanofiber structure not


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only act as the binding agent to connect d-Ti3C2Tx nanosheets to form the layered structure, but also have less insulating contacts between 2D Ti3C2Tx nanosheets.28-30 The existence of CNFs between d-Ti3C2Tx nanosheets is observed in the SEM micrographs (Figure 5a). Moreover, the rough fracture surface of the d-Ti3C2Tx/CNFs composite paper exhibits obvious crack deflection and “zigzag” crack paths. To investigate the synergistic toughening effect from d-Ti3C2Tx nanosheets and CNFs, a fracture mechanism model is proposed and shown in Figure 5c. When the stretching starts, the hydrogen bonds between d-Ti3C2Tx nanosheets and CNFs are broken at first, and the d-Ti3C2Tx nanosheets tend to slide over each other with increasing tensile force. Meanwhile, the CNFs provide additional friction energy dissipation between adjacent d-Ti3C2Tx nanosheets to resist the sliding effect and toughen the d-Ti3C2Tx/CNFs composite paper until the formation of cracks. Subsequently, the CNFs molecular chains are stretched to avoid the crack propagation, which will dissipate more energy in the stretching process. During the fracture process, the anisotropic interconnection networks formed by 1D CNFs and 2D d-Ti3C2Tx nanosheets also play an important role, which will generate crack deflection and result in the “zigzag” crack paths. The main factors in preventing the fracture of the d-Ti3C2Tx/CNFs composite paper are hydrogen bonds, flexible CNFs, and anisotropic interconnection networks. The fracture morphology and the proposed mechanism indicate that the d-Ti3C2Tx/CNFs composite paper seems like a nacre structure, which achieves the best integration of strength and toughness by the “brick-and-mortar” layered structure. The 2D d-Ti3C2Tx nanosheets function as the


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“brick” and offer the frame for the d-Ti3C2Tx/CNFs composite paper. While the 1D CNFs act as the “mortar”, which not only connect the adjacent d-Ti3C2Tx nanosheets, but also promote the stress transfer and enhance the frictional energy dissipation. Owing to the synergistic toughening effect from d-Ti3C2Tx nanosheets and CNFs, the d-Ti3C2Tx/CNFs composite paper displays favorable strength and toughness, no crack or fracture is observed when the composite paper is folded into a complex shape (for example, a small boat) (Figure 5d).


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Figure 5. (a, b) SEM micrographs of the fracture surface of the d-Ti3C2Tx/CNFs composite paper. (c) Schematic illustration of the fracture mechanism of the d-Ti3C2Tx/CNFs composite paper. (d) Digital image of a small boat prepared by folding the d-Ti3C2Tx/CNFs composite paper which exhibits good flexibility. (e) Comparison of tensile strength and tensile strain of the d-Ti3C2Tx/CNFs composite paper with natural nacre and other EMI shielding materials. The numbers in (e) are the sample numbers listed in Table S3 in the Supporting Information.

As discussed above, the as-prepared d-Ti3C2Tx/CNFs composite paper has a “brick-and-mortar” layered structure and exhibits superior mechanical properties. The d-Ti3C2Tx/CNFs composite paper has a much larger tensile strain than that of the natural nacre.31 Some previously reported EMI shielding materials show unsatisfactory mechanical properties with low strength or strain. For example, the MXene foam with an excellent EMI shielding performance exhibited a low tensile stress (4.0 MPa) and strain (0.45 %).11 Graphene paper with double-layered EMI attenuators achieved a tensile stress of 246 MPa, however, its strain at fracture was only 0.8 %.32 The poor mechanical properties of EMI shielding materials limit their applications in various fields. Significantly, the d-Ti3C2Tx/CNFs composite paper reported in this study exhibits superior mechanical properties compared with many EMI shielding materials reported in the literature, which are shown in Figure 5e and Table S3 in the Supporting Information. Therefore, the as-prepared d-Ti3C2Tx/CNFs


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composite paper has promising applications in the EMI shielding and electronic devices. Electrical conductivity is of great significance for the EMI shielding materials, which can reflect the EMI shielding effectiveness (SE) to some extent. With increasing d-Ti3C2Tx content in the d-Ti3C2Tx/CNFs composite paper, the electrical conductivity of the composite paper markedly increases, as shown in Figure 6a. The d-Ti3C2Tx/CNFs composite paper with 40 wt% d-Ti3C2Tx content has an electrical conductivity of 0.6188 S m-1, which is more than 15 times that of the Ti3C2Tx/PVA composite film (0.04 S m-1).12,14 The d-Ti3C2Tx/CNFs composite paper with 50 wt% d-Ti3C2Tx has an electrical conductivity of 9.691 S m-1, which is much higher than 1 S m-1 required for practical applications of EMI shielding materials.23 The electrical conductivity of the d-Ti3C2Tx/CNFs composite paper reaches 739.4 S m-1 when the content of d-Ti3C2Tx is 90 wt%. In addition, the electrical conductivity of the d-Ti3C2Tx sample is 1142.5 S/m. Because CNFs are a kind of insulating polymer, the electrical conductivity of the d-Ti3C2Tx/CNFs composite paper decreases compared with the d-Ti3C2Tx sample. Fortunately, compared with other dielectric polymers, CNFs have a characteristic one-dimensional nanofiber structure, leading to less insulating contacts with two-dimensional conductive d-Ti3C2Tx nanosheets. Moreover, with the cohesive action of CNFs, the directional alignment of 2D d-Ti3C2Tx nanosheets along the planar direction can form a conductive network, which ensure the good connection of d-Ti3C2Tx nanosheets.


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Figure 6. (a) Plot of electrical conductivity versus d-Ti3C2Tx content for the d-Ti3C2Tx/CNFs composite paper sheets with different d-Ti3C2Tx contents. (b) Effect of d-Ti3C2Tx content on the EMI SE of the d-Ti3C2Tx/CNFs composite paper sheets with different d-Ti3C2Tx contents and different thicknesses in the X-band region. (c) Effect of d-Ti3C2Tx content on the SSE/t of the d-Ti3C2Tx/CNFs composite paper sheets with different d-Ti3C2Tx contents in the X-band region. (d) Comparison of total EMI shielding effectiveness (SETotal), microwave absorption (SEA), and microwave reflection (SER) at a frequency of 12.4 GHz of the d-Ti3C2Tx/CNFs composite paper sheets with different d-Ti3C2Tx contents.

The as-prepared d-Ti3C2Tx/CNFs composite paper exhibits a superior EMI shielding performance. As shown in Figure 6b, the d-Ti3C2Tx/CNFs composite paper sheets with different d-Ti3C2Tx contents (50, 80, and 90 wt%) exhibit excellent EMI


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shielding effectiveness (EMI SE) of > 20 dB over the whole X-band, and satisfy the requirement for the commercial EMI shielding applications. In consideration of the specimen thickness which is of equal importance to electrical conductivity for determining the EMI SE, a specific SE value of SSE/t (SE divided by the product of sample density and thickness) is used to evaluate the EMI shielding performance of the d-Ti3C2Tx/CNFs composite paper sheets with different d-Ti3C2Tx contents. (c) Figure 6c shows the effect of d-Ti3C2Tx content on the SSE/t of the d-Ti3C2Tx/CNFs composite paper sheets with different d-Ti3C2Tx contents in the X-band. Increasing the content of d-Ti3C2Tx can effectively enhance the SSE/t of the d-Ti3C2Tx/CNFs composite paper. For example, when the d-Ti3C2Tx content increases to 90 wt %, the SSE/t of the d-Ti3C2Tx/CNFs composite paper reach as high as 2647 dB cm2g-1 at 12.4 GHz. Additionally, the maximum shielding efficiencies of the d-Ti3C2Tx/CNFs composite paper sheets with different d-Ti3C2Tx contents are higher than 99.5 %, and the highest value is 99.74 % at 12.4 GHz. To explore the EMI shielding mechanisms of the d-Ti3C2Tx/CNFs composite paper, the total EMI shielding effectiveness (SETotal), microwave absorption (SEA) and microwave reflection (SER) of the d-Ti3C2Tx/CNFs composite paper at a frequency of 12.4 GHz were investigated. Figure 6d shows the comparison of SETotal, SEA, and SER of the d-Ti3C2Tx/CNFs composite paper sheets with different d-Ti3C2Tx contents at a frequency of 12.4 GHz. Both SER and SEA contribute to the SETotal. Regardless of the d-Ti3C2Tx content or sample thickness, the SEA makes more contribution to the shielding efficiency than SER. For example, the SETotal, SEA, and SER of the


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d-Ti3C2Tx/CNFs composite paper with 80 wt% of d-Ti3C2Tx are ~25.8, ~20.5, and ~5.3 dB at 12.4 GHz, respectively. The high microwave absorption and relatively low microwave reflection indicate an absorption-dominant EMI shielding mechanism of the d-Ti3C2Tx/CNFs composite paper. Specifically, there are three possible decay mechanisms during the electromagnetic shielding process, as shown in Figure 7a. When the incident electromagnetic waves are exposed to the d-Ti3C2Tx/CNFs composite paper, a part of electromagnetic waves is reflected back. The remaining electromagnetic waves interact with the high charge density d-Ti3C2Tx, leading to energy loss of the electromagnetic waves. At the same time, the nacre-like lamellar structure of the d-Ti3C2Tx/CNFs composite paper will facilitate the multiple internal reflection, resulting in absorption and energy dissipation of the electromagnetic waves. The metal-based materials, for example, open-cell foam of a Cu–Ni alloy integrated with carbon nanotubes,33 nickel filament polymer-matrix composites,34 and polypropylene/stainless-steel fiber composite foams35 exhibit relatively high EMI SE. However, some metal-based materials have relatively low absorption for electromagnetic waves and poor anti-corrosive performance, thus, their applications are limited. The carbon-based EMI shielding materials generally possess high EMI SE and low density, for example, reduced graphene oxide,36,37 carbon foam,38 multiwall carbon nanotubes,39-41 carbon black,40,42 and others.6,7,43-50 However, few EMI shielding materials can combine ultrathin thickness, high flexibility, and superior EMI SE properties. The specific SE value (SSE/t), which correlate three significant factors


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(EMI SE, thickness, and density), is used to compare the EMI shielding performance. As shown in Figure 7b and Table S4 in the Supporting Information, the d-Ti3C2Tx/CNFs composite paper with ultrathin thickness exhibits better EMI shielding performance compared with millimeter-thick metal-based and carbon-based shielding materials.

Figure 7. (a) Schematic illustration of the electromagnetic wave transfer across the d-Ti3C2Tx/CNFs composite paper. (b) Comparison of the SSE/t as a function of the sample thickness, and the numbers in (b) are sample numbers listed in Table S4 in the Supporting Information.


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CONCLUSION We have successfully prepared ultrathin and flexible d-Ti3C2Tx/CNFs composite paper with a nacre-like microstructure via a vacuum filtration induced self-assembly process. By adjusting the weight ratio of d-Ti3C2Tx to CNFs in the composite paper, the ultimate tensile strength and strain at fracture of the d-Ti3C2Tx/CNFs composite paper reach 135.4 MPa and 16.7 %, respectively. Significantly, the d-Ti3C2Tx/CNFs composite paper (50 wt% d-Ti3C2Tx) exhibits excellent folding endurance, and the folding number can reach as high as 14260 times. The nacre-like layered structure forms in the d-Ti3C2Tx/CNFs composite paper by the strong interaction between 2D d-Ti3C2Tx nanosheets and 1D CNFs, indicating a “brick-and-mortar” toughening mechanism. Moreover, the as-prepared d-Ti3C2Tx/CNFs composite paper also exhibits a high electrical conductivity and excellent EMI shielding performance. The as-prepared d-Ti3C2Tx/CNFs composite paper (90 wt% d-Ti3C2Tx) with a thickness of 47 µm has an electrical conductivity ~739.4 S m-1. The EMI SE of the d-Ti3C2Tx/CNFs composite paper with 80 wt% of d-Ti3C2Tx is ~25.8 dB at 12.4 GHz. The ultrathin, flexible d-Ti3C2Tx/CNFs composite paper with the excellent EMI shielding performance has promising applications in vrious fields such as flexible wearable devices, weapon equipments and robot joints.

EXPERIMENTAL SECTION Synthesis of delaminated Ti3C2Tx. The delaminated Ti3C2Tx was prepared according to a reported method.51,52 Specifically, 1 g of Ti3AlC2 was added in a


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mixture of 1 g of LiF (Sigma Aldrich, USA) and 20 mL of HCl solution (37 wt%, Sinopharm, China) at 35 oC under constant stirring for 24 h to extract Al. The obtained suspension was washed with deionized water for several times and centrifuged to obtain the multilayered Ti3C2Tx (m-Ti3C2Tx) sediment. Then, the m-Ti3C2Tx sediment was mixed with 10 mL deionized water under vigorous shaking for 5 min. After centrifugation, the dark-green supernatant containing delaminated Ti3C2Tx (d-Ti3C2Tx) nanosheets was collected and freeze dried. Extraction of CNFs from garlic husk. 10 g of dried garlic husk was immersed in 400 g of NaOH solution (2 wt%) at 140 oC for 5 h, and then washed with deionized water to the neutral pH value. Then, the microcrystalline cellulose (MCC) was obtained by respectively treating the alkali-treated garlic skin with 200 g of H2SO4 solution (2 wt%) and 300 g sodium chlorite (1.5 wt%) at 80 oC under constant stirring for 6 h to remove the lignin. After that, the MCC slurry was treated by ultrasonic at 800 W for 1 h, and subsequent high-speed stirring for 5 min to obtain the colloidal solution. Finally, the cellulose nanofibrils (CNFs) were obtained after the dialysis of the colloidal solution for 6 h. Preparation of the d-Ti3C2Tx/CNFs composite paper. The dried d-Ti3C2Tx was re-dispersed in deionized water with different contents, and then dropwise added to the CNFs suspension (0.2 wt%) under continuous stirring. The mixture of CNFs and d-Ti3C2Tx was sonicated for 15 min and stirred for 24 h to form a uniform suspension. The uniform suspension was filtered to form the d-Ti3C2Tx/CNFs composite paper by the vacuum-assisted filtration. The weight ratio of d-Ti3C2Tx to CNFs was 20:80,


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40:60, 50:50, 60:40, 80:20, and 90:10, respectively. For comparison, pure d-Ti3C2Tx and pure CNFs paper sheets were also fabricated under the same conditions. Characterization. Scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) elemental mappings were obtained by field-emission scanning electron microscope (Hitachi S-4800). Transmission electron microscopy (TEM) images were obtained by a transmission electron microscope (Hitachi H-800, Japan). Thermogravimetric analysis (TGA) was performed on a simultaneous thermal analyzer (STA 409PC, Netzsch, Germany) under flowing air with a heating rate of 10 °C min-1. Fourier transform infrared (FTIR) spectroscopy was obtained using a FTIR spectrometer (FTIR-7600, Lambda Scientific, Australia). X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (Rigaku D/max 2550 V, Cu Kα radiation, λ = 1.54178 Å). X-ray photoelectron spectroscopy (XPS) measurements were performed using an X-ray photoelectron spectrometer (ESCALAB 250Xi, Thermo Scientific). The mechanical properties of the d-Ti3C2Tx/CNFs composite paper were measured by a universal testing machine (Drick, China). Each sample was cut into a strip of 10 mm × 30 mm, and the loading rate was 0.2 mm/min. The folding endurance of the d-Ti3C2Tx/CNFs composite paper was tested by a paper folding endurance meter (DCP-MIT135, Sichuan Changjiang Paper Equipment Co.) under a load of 4.9 N. At least three samples were tested for each measurement to guarantee the accuracy of the data. The electrical conductivity measurements were performed at room temperature using the standard four-probe method on a Physical Property Measurement System


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(PPMS, Quantum Design). All samples were cut into a rectangle shape with sizes of 2.5 mm × 2.0 mm by the stainless steel cutter for measurements. Four-pin probe was tightly contacted with the d-Ti3C2Tx/CNFs composite paper. Then, the resistance was recorded, and the electrical conductivity was calculated. The EMI shielding properties were measured by an Agilent PNA-N5244A vector network analyzer using the wave-guide method within 8.2–12.4 GHz. All the samples were cut into a rectangle shape with sizes of 22.9 mm × 10.2 mm for measurements. The thicknesses of the d-Ti3C2Tx/CNFs composite paper ranged from 47 to 167 µm. The vector network analyzer was calibrated before measurement. Samples were tightly fixed with the copper sample holder by the transparent silicone film, and connected between the waveguide flanges. The power coefficients of reflection (R), transmission (T), and absorption (A) are calculated using the scattering parameters (S11 and S21). The total electromagnetic interference shielding effectiveness (EMI SET) of the d-Ti3C2Tx/CNFs composite paper is calculated as follows: SET = SEA + SER + SEM

(1)

SEA =

(2)

=

SER = –10 log (1–R) = –10 log (1–|S11|2)

(3)

Where SEA is the microwave absorption, SER is the microwave reflection, and SEM is the microwave multiple internal reflection. When SET ≥ 15 dB, the SEM can be negligible.40,53 EMI shielding efficiency (%) is obtained by using the following equation:


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Shielding efficiency (%) =

(4)

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1021/acsmi.xxxxx. XRD patterns of Ti3AlC2, m-Ti3C2Tx and the d-Ti3C2Tx; the schematic illustration of the preparation process of CNFs; digital image and TEM micrographs of CNFs; digital image of d-Ti3C2Tx nanosheets dispersed in deionized water; digital image of CNFs in deionized water; digital image of a aqueous mixture of d-Ti3C2Tx and CNFs after stirring for 24 h; schematic illustration of the interaction between CNFs and d-Ti3C2Tx by hydrogen bonds; TG curves of the d-Ti3C2Tx/CNFs composite paper with different d-Ti3C2Tx contents; d-Ti3C2Tx contents of the d-Ti3C2Tx/CNFs composite paper sheets determined by TGA; mechanical properties of the d-Ti3C2Tx/CNFs composite paper sheets with different d-Ti3C2Tx contents; comparison of mechanical properties of the natural nacre, the d-Ti3C2Tx/CNFs composite paper, and other EMI shielding materials; comparison of EMI shielding performance of the d-Ti3C2Tx/CNFs composite paper and other materials. (PDF) Movie S1. The d-Ti3C2Tx-CNFs composite paper has a high flexibility. Movie S2. Folding endurance test for the 50 wt% d-Ti3C2Tx-CNFs composite paper.


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected] * E-mail: [email protected] ORCID Ming-Guo Ma: 0000-0001-6319-9254 Ying-Jie Zhu: 0000-0002-5044-5046 Feng Chen: 0000-0002-1162-1684

ACKNOWLEDGEMENTS The financial support from the National Natural Science Foundation of China (31771081, 51472259), the Science and Technology Commission of Shanghai (15JC1491001), and the Youth Innovation Promotion Association of Chinese Academy of Sciences (2015203) is gratefully acknowledged.

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35. Ameli,

A.;

Nofar,

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S.;

Park,

C.

B.

Lightweight

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41. Arjmand, M.; Apperley, T.; Okoniewski, M.; Sundararaj, U. Comparative Study of Electromagnetic Interference Shielding Properties of Injection Molded Versus Compression Molded Multi-Walled Carbon Nanotube/Polystyrene Composites. Carbon 2012, 50, 5126-5134. 42. Ghosh, P.; Chakrabarti, A. Conducting Carbon Black Filled EDPM Vulcanizates: Assessment of Dependence of Physical and Mechanical Properties and Conducting Character on Variation of Filler Loading. Eur. Polym. J. 2000, 36, 1043-1054. 43. Yan, D.-X.; Pang, H.; Li, B.; Vajtai, R.; Xu, L.; Ren, P.-G.; Wang, J.-H.; Li, Z.-M. Structured Reduced Graphene Oxide/Polymer Composites for Ultra-Efficient Electromagnetic Interference Shielding. Adv. Funct. Mater. 2015, 25, 559-566. 44. Zhang, L.; Liu, M.; Roy, S.; Chu, E. K.; See, K. Y.; Hu, X. Phthalonitrile-Based Carbon Foam with High Specific Mechanical Strength and Superior Electromagnetic Interference Shielding Performance. ACS Appl. Mater. Interfaces 2016, 8, 7422-7430. 45. Yang, Y. L.; Gupta, M. C. Novel Carbon Nanotube−Polystyrene Foam Composites for Electromagnetic Interference Shielding. Nano Lett. 2005, 5, 2131-2134. 46. Agnihotri, N.; Chakrabarti, K.; De, A. Highly Efficient Electromagnetic Interference

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Nanoplatelet/Poly(3,4-Ethylenedioxythiophene)–Poly(Styrenesulfonate) Composites with Enhanced Thermal Conductivity. RSC Adv. 2015, 5, 43765-43771. 47. Kong, L.; Yin, X.; Han, M.; Yuan, X.; Hou, Z.; Ye, F.; Zhang, L.; Cheng, L.; Xu, Z.; Huang, J. Macroscopic Bioinspired Graphene Sponge Modified with in-Situ


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Grown Carbon Nanowires and Its Electromagnetic Properties. Carbon 2017, 111, 94-102. 48. Song, W.-L.; Guan, X.-T.; Fan, L.-Z.; Cao, W.-Q.; Wang, C.-Y.; Cao, M.-S. Tuning Three-Dimensional Textures with Graphene Aerogels for Ultra-Light Flexible Graphene/Texture Composites of Effective Electromagnetic Shielding. Carbon 2015, 93, 151-160. 49. Chen, Z.; Xu, C.; Ma, C.; Ren, W.; Cheng, H. M. Lightweight and Flexible Graphene Foam Composites for High-Performance Electromagnetic Interference Shielding. Adv. Mater. 2013, 25, 1296-1300. 50. Hong, X.; Chung, D. D. L. Carbon Nanofiber Mats for Electromagnetic Interference Shielding. Carbon 2017, 111, 529-537. 51. Zhang, C. J.; Anasori, B.; Seral-Ascaso, A.; Park, S. H.; McEvoy, N.; Shmeliov, A.; Duesberg, G. S.; Coleman, J. N.; Gogotsi, Y.; Nicolosi, V. Transparent, Flexible, and Conductive 2D Titanium Carbide (MXene) Films with High Volumetric Capacitance. Adv. Mater. 2017, 29, 1702678. 52. Alhabeb, M.; Maleski, K.; Anasori, B.; Lelyukh, P.; Clark, L.; Sin, S.; Gogotsi, Y., Guidelines for Synthesis and Processing of Two-Dimensional Titanium Carbide (Ti3C2Tx Mxene). Chem. Mater. 2017, 29, 7633-7644. 53. Kumar, P.; Shahzad, F.; Yu, S.; Hong, S. M.; Kim, Y.-H.; Koo, C. M., Large-Area Reduced Graphene Oxide Thin Film with Excellent Thermal Conductivity and Electromagnetic Interference Shielding Effectiveness. Carbon 2015, 94, 494-500.


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