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Apr 30, 2018 - MXene/Cellulose Nanofiber Composite Paper with Nacre-Inspired Structure and Superior. Electromagnetic Interference Shielding. Propertie...
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Binary Strengthening and Toughening of MXene/Cellulose Nanofiber 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, P.R. China ‡ State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P.R. China S Supporting Information *

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 nanofiber (CNF) 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) dTi3C2Tx MXene, the binary strengthening and toughening of the nacre-like dTi3C2Tx/CNF 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 14 260 times). Moreover, the d-Ti3C2Tx/CNF composite paper exhibits high electrical conductivity (up to 739.4 S m−1) and excellent specific EMI shielding efficiency (up to 2647 dB cm2 g−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/CNF composite paper, which may be applied in various fields such as flexible wearable devices, weapon equipment, and robot joints. KEYWORDS: MXene, cellulose nanofibers, paper, mechanical properties, electromagnetic interference shielding

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applications owing to their low density and corrosion resistance.6,7 MXenes, two-dimensional (2D) nanomaterials, are produced by selectively etching the A-element from 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 lightweight and superior EMI shielding performance.8,9 It should be mentioned that Gogotsi and co-workers reported the free-

n the past decades, the use of wireless communication and electronic devices has become an increasingly common phenomenon with economic development. However, electromagnetic radiation is emitted inevitably by the electronic devices, which not only affects the normal function and the lifetime of the electronic instruments but also is harmful to human health. Thus, electromagnetic interference (EMI) shielding materials with good performances are urgently required to resolve the above-mentioned problems.1−3 Traditional metal shielding materials exhibit excellent EMI shielding performance. Nevertheless, intrinsic high density and susceptibility 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 EMI shielding © 2018 American Chemical Society

Received: February 6, 2018 Accepted: April 30, 2018 Published: April 30, 2018 4583

DOI: 10.1021/acsnano.8b00997 ACS Nano 2018, 12, 4583−4593

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Figure 1. Illustration of the preparation process of the d-Ti3C2Tx/CNF composite paper. (a) Ti3AlC2 (MAX) is etched by HCl and 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 being stirred for 24 h, the resulting suspension is filtered to form the d-Ti3C2Tx/CNF composite paper. (b) SEM micrographs of Ti3AlC2 (MAX), m-Ti3C2Tx, and d-Ti3C2Tx. (c) Digital images of the as-prepared pure d-Ti3C2Tx paper, pure CNF paper, and d-Ti3C2Tx/CNF composite paper.

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 and >25 dB, respectively.1 Han et al.10 prepared surfacemodified Ti3C2 MXenes with a EMI shielding efficiency of 76.1 dB by a postetching heat treatment method. Liu et al.11 prepared a hydrophobic MXene foam by using hydrazine to treat the MXene film at 90 °C, and the as-prepared MXene foam exhibited a high EMI shielding effectiveness of ∼70 dB at a thickness of 60 μm. The MXene materials possess an EMI shielding performance better than that of many metal and carbon-based materials.12,13 However, the unsatisfied mechanical properties of MXene materials limit their applications in wearable electronic devices, weapon equipment, and robot joints, which need to be strong to endure mechanical deformation.14,15 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 Earth, cellulose seems to be 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/CNF composite paper for the EMI shielding application. Therefore, it is of great significance to prepare the d-Ti3C2Tx/CNF

composite paper with enhanced mechanical properties and EMI shielding performance. In this study, an ultrathin and flexible d-Ti3C2Tx/CNF composite paper constructed with 1D CNFs and 2D delaminated Ti3C 2Tx nanosheets has been successfully prepared by a facile vacuum-assisted filtration self-assembly method. The d-Ti3C2Tx/CNF 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 14 260 times. Moreover, the d-Ti3C2Tx/ CNF composite paper also exhibits superior electrical conductivity and EMI shielding efficiency. The d-Ti3C2Tx/ CNF composite paper with a nacre-like structure and excellent mechanical properties has promising applications in flexible electronics such as wearable electric devices, weapon equipment, and robot joints.

RESULTS AND DISCUSSION Figure 1a shows the strategy for the preparation of the dTi3C2Tx/CNF composite paper. The d-Ti3C2Tx was synthesized by selective etching Ti3AlC2 (MAX) in a mixture of hydrochloric acid (HCl) and lithium fluoride (LiF) and the etched powder was delaminated 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 vigorous shaking. The shift of the characteristic peak (from 9.3 to 7.2°) 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 dTi3C2Tx nanosheets.11 The CNFs were extracted from microcrystalline cellulose (MCC), which disintegrated from the agricultural waste of garlic husk. The transmission electron microscopy (TEM) image of a CNF suspension (Figure S2, 4584

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Figure 2. (a) Schematic illustration of the “brick-and-mortar” structure in nacre. (b) Schematic representation of the preparation of the dTi3C2Tx/CNF composite paper with a nacre-like structure via a vacuum filtration method. (c) SEM images of the d-Ti3C2Tx/CNF composite paper, exhibiting a nacre-like compact lamellar structure. The weight content of d-Ti3C2Tx in the d-Ti3C2Tx/CNF composite paper varies from 20 to 90 wt %.

the d-Ti3C2Tx/CNFs (50% d-Ti3C2Tx) composite paper with superior mechanical properties (Movie S1, Supporting Information). We have characterized the microstructure of the as-prepared d-Ti 3 C 2 T x/CNF 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/ CNF 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/CNF 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/CNF composite paper exhibits a tightly stacked layered structure. The oriented alignment of d-Ti3C2Tx nanosheets 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

Supporting Information) shows that the obtained CNFs are about 20−50 nm in diameter and several microns in length. After the delaminated Ti3C2Tx stable dispersion and CNF colloidal dispersion were mixed 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/CNF composite paper was obtained by a vacuum-assisted filtration process using the suspension containing d-Ti3C2Tx and CNFs. The pure d-Ti3C2Tx paper or CNF 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 paper, and cracks are observed on the dTi3C2Tx paper. The pure CNF paper and d-Ti3C2Tx/CNF composite paper sheets display a good mechanical 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 4585

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

Figure 4. Mechanical properties of the d-Ti3C2Tx/CNF composite paper with different d-Ti3C2Tx contents. (a) Tensile stress−strain curves of the d-Ti3C2Tx/CNF 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/CNF composite paper with different d-Ti3C2Tx contents. (c) Digital image of the d-Ti3C2Tx/CNF composite paper which can withstand a weight of 500 g. (d) Toughness and Young’s modulus of the d-Ti3C2Tx/CNF composite paper with different d-Ti3C2Tx contents. (e) Folding times of the d-Ti3C2Tx/CNF composite paper with different d-Ti3C2Tx contents under a loading of 4.9 N. (f) Digital image of the paper folding endurance instrument.

(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/CNF composite paper changes compared with the nominal content of d-Ti3C2Tx. Figure 3 shows the XRD patterns and Fourier transform infrared (FTIR) spectra of the d-Ti3C2Tx/CNF composite paper. The FTIR spectra of the pure d-Ti3C2Tx, pure CNFs, and d-

d-Ti3C2Tx nanosheets with a 2D structure display the role of the inorganic “brick”, and the CNFs with a 1D structure display the role of the organic “mortar”. This nacre-inspired structure not only guarantees the excellent mechanical properties of the d-Ti3C2Tx/CNF composite paper but also is beneficial to the electromagnetic wave reflection and absorption. The contents of d-Ti3C2Tx in the d-Ti3C2Tx/CNF composite paper sheets are estimated by thermogravimetric analysis 4586

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Figure 5. (a,b) SEM micrographs of the fracture surface of the d-Ti3C2Tx/CNF composite paper. (c) Schematic illustration of the fracture mechanism of the d-Ti3C2Tx/CNF composite paper. (d) Digital image of a small boat prepared by folding the d-Ti3C2Tx/CNF composite paper which exhibits good flexibility. (e) Comparison of tensile strength and tensile strain of the d-Ti3C2Tx/CNF 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.

cellulose I, and the diffraction peak at 2θ = 22.5° corresponds to the (002) crystal plane of cellulose I crystalline structure.27 With increasing content of CNFs from 20 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 dspacing of d-Ti3C2Tx nanosheets increases from approximately 12.3 Å for pristine d-Ti3C2Tx to ∼15 Å for the 20 wt % dTi3C2Tx/CNF composite paper. The change of the d-spacing is strong evidence for the successful intercalation of CNFs into dTi3C2Tx nanosheets. The experimental results indicate that the CNFs have been successfully introduced into the as-prepared composite paper. The tensile stress−strain curves of the d-Ti3C2Tx/CNF 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/CNF composite paper sheets exhibit excellent integration of the mechanical

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 being mixed 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/ CNF composite paper. X-ray photoelectron spectroscopy (XPS) patterns are shown in Figure 3b. The d-Ti3C2Tx/ CNFs sample has a C/Ti atomic ratio (14.66) and O/Ti atomic ratio (11.45) higher than that of 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/CNF composite paper decreases from 2.16 to 0.36. The XRD patterns of the dTi3C2Tx and d-Ti3C2Tx/CNF composite paper with different dTi3C2Tx contents are shown in Figure 3c. The two overlapping peaks at 2θ = 14.0−17.8° in the d-Ti3C2Tx/CNF composite paper samples correspond to (101) and (101̅) crystal planes of 4587

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Figure 6. (a) Plot of electrical conductivity versus d-Ti3C2Tx content for the d-Ti3C2Tx/CNF composite paper sheets with different d-Ti3C2Tx contents. (b) Effect of d-Ti3C2Tx content on the EMI SE of the d-Ti3C2Tx/CNF 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/CNF 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/CNF composite paper sheets with different d-Ti3C2Tx contents.

paper. The experimental results indicate that the appropriate integration of the CNFs can effectively enhance the mechanical properties of the d-Ti3C2Tx/CNF composite paper. The mechanical properties of the d-Ti3C2Tx/CNF composite paper are better than the pure d-Ti3C2Tx paper and pure CNF 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/ CNF 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 poly(vinyl alcohol), poly(methyl methacrylate), and polyimide, CNFs with the 1D nanofiber structure not 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/CNF composite paper exhibits obvious crack deflection and “zigzag” crack paths. To investigate the synergistic toughening effect from dTi3C2Tx 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

strength and toughness (Figure 4b,d). The pure CNF 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 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 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 to 3.8 ± 0.3 GPa and then decreases to 1.0 ± 0.1 GPa. The optimal mechanical properties of the d-Ti3C2Tx/CNF 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/CNF composite paper (50 wt % d-Ti3C2Tx) with sizes of 40 mm × 15 mm can withstand a weight of ∼500 g without breaking (Figure 4c). To further investigate the folding endurance of the dTi3C2Tx/CNF composite paper, a paper folding endurance instrument was used under a 4.9 N pulling load (Figure 4e,f). All the d-Ti3C2Tx/CNF composite paper sheets can be folded for more than 100 times. Significantly, when the d-Ti3C2Tx content is 50 wt % in the composite paper, the folding number can reach as high as 14 260 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/CNF composite paper (50 wt %) exhibits excellent folding endurance that is even >35 or >109 times that of the commercial printing 4588

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ACS Nano CNFs provide additional friction energy dissipation between adjacent d-Ti3C2Tx nanosheets to resist the sliding effect and toughen the d-Ti3C2Tx/CNF composite paper until the formation of cracks. Subsequently, the CNF 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/CNF composite paper are hydrogen bonds, flexible CNFs, and anisotropic interconnection networks. The fracture morphology and the proposed mechanism indicate that the d-Ti3C2Tx/CNF 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 “brick” and offer the frame for the d-Ti3C2Tx/CNF composite paper. The 1D CNFs act as the “mortar”, which not only connect the adjacent dTi3C2Tx 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/CNF composite paper displays favorable strength and toughness, and no crack or fracture is observed when the composite paper is folded into a complex shape (for example, a small boat) (Figure 5d). As discussed above, the as-prepared d-Ti3C2Tx/CNF composite paper has a “brick-and-mortar” layered structure and exhibits superior mechanical properties. The d-Ti3C2Tx/ CNF composite paper has a tensile strain much larger 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/CNF composite paper reported in this study exhibits mechanical properties superior to those with many EMI shielding materials reported in the literature, which are shown in Figure 5e and Table S3 in the Supporting Information. Therefore, the asprepared d-Ti3C2Tx/CNF 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/CNF composite paper, the electrical conductivity of the composite paper markedly increases, as shown in Figure 6a. The d-Ti3C2Tx/CNF 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/CNF 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/CNF 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/CNF 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 twodimensional 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 ensures the good connection of d-Ti3C2Tx nanosheets. The as-prepared d-Ti3C2Tx/CNF composite paper exhibits a superior EMI shielding performance. As shown in Figure 6b, the d-Ti3C2Tx/CNF composite paper sheets with different dTi3C2Tx contents (50, 80, and 90 wt %) exhibit excellent EMI shielding effectiveness 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/CNF composite paper sheets with different d-Ti3C2Tx contents. Figure 6c shows the effect of d-Ti3C2Tx content on the SSE/t of the d-Ti3C2Tx/ CNF 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-Ti 3 C 2 T x/CNF composite paper. For example, when the d-Ti3C2Tx content increases to 90 wt %, the SSE/t of the d-Ti3C2Tx/CNF composite paper reaches as high as 2647 dB cm2 g−1 at 12.4 GHz. Additionally, the maximum shielding efficiencies of the dTi3C2Tx/CNF 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/ CNF composite paper, the total EMI shielding effectiveness (SETotal), microwave absorption (SEA), and microwave reflection (SER) of the d-Ti3C2Tx/CNF 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/CNF 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 d-Ti3C2Tx/CNF composite paper with 80 wt % of dTi3C2Tx 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/CNF 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 dTi3C2Tx/CNF composite paper, a part of the 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/CNF 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 atrix composites,34 and polypropylene/ stainless teel fiber composite foams,35 exhibit relatively high EMI SE. However, some metal-based materials have relatively low absorption for electromagnetic waves and poor anti4589

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CNF composite paper with 80 wt % of d-Ti3C2Tx is ∼25.8 dB at 12.4 GHz. The ultrathin, flexible d-Ti3C2Tx/CNF composite paper with the excellent EMI shielding performance has promising applications in various fields such as flexible wearable devices, weapon equipment, 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 mixture of 1 g of LiF (Sigma-Aldrich, USA) and 20 mL of HCl solution (37 wt %, Sinopharm, China) at 35 °C under constant stirring for 24 h to extract Al. The obtained suspension was washed with deionized water several times and centrifuged to obtain the multilayered Ti3C2Tx (m-Ti3C2Tx) sediment. Then, the mTi3C2Tx sediment was mixed with 10 mL of 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. Ten grams of dried garlic husk was immersed in 400 g of NaOH solution (2 wt %) at 140 °C for 5 h and then washed with deionized water to the neutral pH value. Then, the MCC was obtained by treating the alkali-treated garlic skin with 200 g of H2SO4 solution (2 wt %) and 300 g of sodium chlorite (1.5 wt %) at 80 °C under constant stirring for 6 h to remove the lignin. After that, the MCC slurry was ultrasonicated at 800 W for 1 h and subsequent high-speed stirring for 5 min to obtain the colloidal solution. Finally, the cellulose nanofibrils were obtained after the dialysis of the colloidal solution for 6 h. Preparation of the d-Ti3C2Tx/CNF Composite Paper. The dried d-Ti3C2Tx was redispersed in deionized water with different contents and then dropwise added to the CNF 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 dTi3C2Tx/CNF composite paper by the vacuum-assisted filtration. The weight ratio of d-Ti3C2Tx to CNFs was 20:80, 40:60, 50:50, 60:40, 80:20, and 90:10. For comparison, pure d-Ti3C2Tx and pure CNF paper sheets were also fabricated under the same conditions. Characterization. Scanning electron microscopy images and energy-dispersive spectroscopy elemental mappings were obtained by a field-emission scanning electron microscope (Hitachi S-4800). TEM images were obtained by a transmission electron microscope (Hitachi H-800, Japan). TGA was performed on a simultaneous thermal analyzer (STA 409PC, Netzsch, Germany) under flowing air with a heating rate of 10 °C min−1. FTIR spectroscopy was obtained using a FTIR spectrometer (FTIR-7600, Lambda Scientific, Australia). XRD patterns were recorded on an X-ray diffractometer (Rigaku D/max 2550 V, Cu Kα radiation, λ = 1.54178 Å). XPS measurements were performed using an X-ray photoelectron spectrometer (ESCALAB 250Xi, Thermo Scientific). The mechanical properties of the dTi3C2Tx/CNF 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/CNF 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 (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. A four-pin probe was tightly contacted with the d-Ti3C2Tx/CNF composite paper. Then, the resistance was recorded, and the electrical conductivity was calculated. The EMI shielding properties were measured by an Agilent PNAN5244A vector network analyzer using the waveguide 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/CNF composite paper ranged from 47 to 167 μm. The

Figure 7. (a) Schematic illustration of the electromagnetic wave transfer across the d-Ti3C2Tx/CNF 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.

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 correlatesthree significant factors (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/CNF composite paper with ultrathin thickness exhibits better EMI shielding performance compared with millimeter-thick metal-based and carbon-based shielding materials.

CONCLUSION We have successfully prepared ultrathin and flexible d-Ti3C2Tx/ CNF 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 dTi3C2Tx/CNF composite paper reach 135.4 MPa and 16.7%, respectively. Significantly, the d-Ti3C2Tx/CNF composite paper (50 wt % d-Ti3C2Tx) exhibits excellent folding endurance, and the folding number can reach as high as 14 260 times. The nacre-like layered structure forms in the dTi3C2Tx/CNF composite paper by the strong interaction between 2D d-Ti3C2Tx nanosheets and 1D CNFs, indicating a “brick-and-mortar” toughening mechanism. Moreover, the asprepared d-Ti3C2Tx/CNF composite paper also exhibits a high electrical conductivity and excellent EMI shielding performance. The as-prepared d-Ti3C2Tx/CNF 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/ 4590

DOI: 10.1021/acsnano.8b00997 ACS Nano 2018, 12, 4583−4593

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ACS Nano Present Address

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/CNF composite paper is calculated as follows: SE T = SEA + SE R + SEM

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⎛ |S |2 ⎞ ⎛ T ⎞ 21 ⎟ = − 10 log⎜ ⎟ SEA = − 10 log⎜ 2 ⎝1 − R ⎠ ⎝ 1 − |S11| ⎠

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SE R = − 10 log(1 − R ) = − 10 log(1 − |S11|2 )

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∞

Laboratory of Biomimetic Materials and Translational Medicine, Department of Orthopedics, Shanghai Tenth People’s Hospital, Tongji University School of Medicine, Shanghai 200072, P.R. China. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The financial support from the National Natural Science Foundation of China (31771081, 51472259), the Fundamental Research Funds for the Central Universities (2015ZCQ-CL03), the Science and Technology Commission of Shanghai (15JC1491001), and the Youth Innovation Promotion Association of Chinese Academy of Sciences (2015203) is gratefully acknowledged.

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:

⎛ 1 ⎞ shielding efficiency (%) = 100 − ⎜ SE/10 ⎟ × 100 ⎝ 10 ⎠

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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.8b00997. XRD patterns of Ti3AlC2, m-Ti3C2Tx, and d-Ti3C2Tx; 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/CNF composite paper with different d-Ti3C2Tx contents; dTi3C2Tx contents of the d-Ti3C2Tx/CNF composite paper sheets determined by TGA; mechanical properties of the d-Ti3C2Tx/CNF composite paper sheets with different d-Ti3C2Tx contents; comparison of mechanical properties of the natural nacre, the d-Ti3C2Tx/CNF composite paper, and other EMI shielding materials; comparison of EMI shielding performance of the dTi3C2Tx/CNF composite paper and other materials (PDF) Movie S1: d-Ti3C2Tx/CNF composite paper with high flexibility (AVI) Movie S2: Folding endurance test for the 50 wt % dTi3C2Tx/CNF composite paper (AVI)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ying-Jie Zhu: 0000-0002-5044-5046 Ming-Guo Ma: 0000-0001-6319-9254 Feng Chen: 0000-0002-1162-1684 4591

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