Flexible and ultrathin electrospun regenerate cellulose nanofibers and d-Ti3C2Tx (MXene) composite film for electromagnetic interference shielding

Flexible and ultrathin electrospun regenerate cellulose nanofibers and d-Ti3C2Tx (MXene) composite film for electromagnetic interference shielding

Journal of Alloys and Compounds 788 (2019) 1246e1255 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: htt...

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Journal of Alloys and Compounds 788 (2019) 1246e1255

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Flexible and ultrathin electrospun regenerate cellulose nanofibers and d-Ti3C2Tx (MXene) composite film for electromagnetic interference shielding Ce Cui a, Cheng Xiang a, Liang Geng a, Xiaoxu Lai a, Ronghui Guo a, *, Yong Zhang a, Hongyan Xiao a, Jianwu Lan a, Shaojian Lin a, Shouxiang Jiang b a b

College of Light Industry, Textile and Food Engineering, Sichuan University, Chengdu, 610065, China Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Kowloon, Hong Kong, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 October 2018 Received in revised form 22 February 2019 Accepted 25 February 2019 Available online 28 February 2019

Ultrathin and flexible electromagnetic interference (EMI) shielding materials are required to deal with increasingly serious radiation pollution. Herein, 2D ultrathin Ti3C2Tx nanosheets, a kind of MXenes, possess metallic electrical conductivity and hydrophilic surfaces. 2D Ti3C2Tx are considered as a promising alternative to graphene for providing excellent EMI shielding properties. Flexible delaminated Ti3C2Tx/electrospun regenerate cellulose nanofibers (d-Ti3C2Tx/r-CNFs) composite films with EMI shielding properties were fabricated via a facile vacuum-assisted filtration method. The obtained dTi3C2Tx/r-CNFs composite film with 15 mm of the thickness of d-Ti3C2Tx coating shows excellent EMI SE (up to 42.7 dB) at the frequency of 2e18 GHz. The d-Ti3C2Tx/r-CNFs composite films can be potentially used as flexible EMI shielding materials. © 2019 Elsevier B.V. All rights reserved.

Keywords: d-Ti3C2Tx Electrospun regenerate cellulose nanofibers Electromagnetic interference shielding

1. Introduction In recent years, the rapid development of electronic devices and wireless communication generates serious electromagnetic interference (EMI) and radiation, which has harmful impacts on the normal performance of electronic equipments as well as the health of human in their surroundings. Therefore, EMI shielding (EMI SE) materials are crucial to solve the above-mentioned problems [1e5]. Previously, metal materials and metal oxides have been developed for EMI shielding materials [6e10]. However, their use for small devices is less desirable due to their high density, narrow absorption bandwidth, poor dispersion or corrosion sensitivity [6,11]. Lightweight, low-cost, high-strength and easy-to-fabricate shielding materials are needed. Polymer-matrix composites composed of carbon-based fillers such as carbon nanotubes and graphene are common substitutes for EMI SE due to high processability and low density, however, there is no further breakthrough in EMI shielding efficiency [5,12]. MXenes, as a new two-dimensional (2D) early transition metal

* Corresponding author. E-mail address: [email protected] (R. Guo). https://doi.org/10.1016/j.jallcom.2019.02.294 0925-8388/© 2019 Elsevier B.V. All rights reserved.

carbides and carbonitrides, have been prepared by selectively etching out the A-element from the three-dimensional (3D) structure consisting of MAX phases [13e17]. The general formula of MXenes is Mnþ1XnTx, where M represents an early transition metal; X represents carbon and/or nitrogen element; n ¼ 1, 2 or 3 and Tx represents a surface termination such as a mixture of ¼ O, eOH and/or eF [18e20]. These 2D laminated nanocrystals exhibit large specific surface area and high electrical conductivity similar to graphene. Recently, Ti3C2Tx, a typical representative of 2D transition metal carbide, is one of the most significant members in MXenes group. Ti3C2Tx possesses the chemically active surfaces, coupled with the native defects and metallic character. These characteristics make Ti3C2Tx as a promising candidate for electromagnetic (EM) wave absorbing applications [21e23]. However, pure Ti3C2Tx material shows the disadvantage of poor mechanical properties and flexibility [24e26]. Polymer-matrix composites can solve the problems due to their high processability, low density and good flexibility. In addition, excellent hydrophilicity and metallic conductivity make Ti3C2Tx an alternative candidate for composite with polymer through the formation of hydrogen bonding with the termination groups of Ti3C2Tx, therefore, polymer is expected to improve the mechanical properties and flexibility of Ti3C2Tx [23,27]. In previous studies, the use of polymer as a filler material

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combined with Ti3C2Tx to improve its properties. Tong et al. [28] synthesized multilayer sandwich heterostructural Ti3C2Tx MXenes decorated with polypyrrole chains and found that the composites consisting of 25 wt% Ti3C2Tx/PPy hybrids in a paraffin matrix exhibit a minimum reflection loss of 49.2 dB (~99.99% absorption) with the thickness of 3.2 mm. Although the Ti3C2Tx/PPy hybrids possess excellent EMI SE efficiency, it loses flexibility due to its high thickness. Cao et al. [29]prepared a Ti3C2Tx/cellulose nanofiber composite paper with high tensile strength and fracture strain but the EMI shielding was unsatisfactory. Therefore, coating Ti3C2Tx on the porous and polymer film can maintain the EMI SE property of Ti3C2Tx and have certain mechanical properties and flexibility. Cellulose, the most abundant polymer, has attracted great attentions due to their hydrophilicity and biocompatiblity [30,31]. Cellulose possesses a number of hydroxyl groups, which can potentially facilitate the formation of hydrogen bonding with the terminating oxygen or fluorine on the Ti3C2Tx surface. The hydrogen bonding can assist incombination of cellulose with Ti3C2Tx. Recently, there is an increasing focus on nano-structured cellulose fibers by electrospinning because of their high specific surface area. The high specific surface area of cellulose nanofibers helps to increase contact with Ti3C2Tx and improve adhesive strength between them. However, it is very difficult to obtain cellulose nanofibers (CNFs) films directly via electrospinning and the unsatisfied mechanical properties of nano-structured cellulose by electrospinning limit their applications [32e34]. Alternatively, nanofibers of cellulose derivatives with excellent mechanical properties have been also obtained by electrospinning such as cellulose acetate (CA) [34e36], hydroxypropyl cellulose (HPC) [37], hydroxypropylmethy cellulose [38]. Among them, CA is the most common cellulose derivative for electrospinning because of its easy solubility in common solvents. However, there is no research on the preparation of Ti3C2Tx coated r-CNFs film by filtration. Herein, the r-CNFs by electrospinning CA were prepared. An ultrathin and flexible delaminated Ti3C2Tx/r-CNFs composite film (d-Ti3C2Tx/r-CNFs) was synthesized by a facile vacuum-assisted filtration method. EMI SE efficiency and mechanical properties of d-Ti3C2Tx/r-CNFs composite film were investigated. 2. Experimental 2.1. Materials CA (54.5%e56% acetyl content, Mw ¼ 30000e50000) was purchased from Sinopharm Chemical Reagent Co., Ltd. N,Ndimethylacetamide (DMAc), acetone, sodium hydroxide (NaOH), ethanol, HCl solution (37%) and Lithiumfluorid (LiF) were purchased from Kelong Reagent Co., Ltd. Ti3AlC2 (200 M) was purchased from Buhan (Shanghai) Chemical Technology Co., Ltd. All the chemicals are grade of analytical purity. 2.2. Preparation and deacetylation of CA electrospun film 12 wt% CA was dissolved in a solvent mixture of DMAc and acetone (2:1 w/w). A syringe pump was used to eject out the CA solution at a controllable feed rate (0.6 mL/h) while high voltage (20 kV) was used for establishing an electrical field between the needle and a grounded drum collector. The collected CA fiber film was deacetylated in 0.05 M NaOH/ethanol solution at room temperature for 48 h. After that, the fiber film was washed thoroughly in deionized water to remove sodium and acetate ions [39]. 2.3. Synthesis of delaminated Ti3C2Tx (d-Ti3C2Tx) MXene In order to obtain delaminated Ti3C2Tx (d-Ti3C2Tx), the

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multilayers Ti3C2Tx (m-Ti3C2Tx) firstly need to be prepared. 1 g of Ti3AlC2 powders were added in 20 mL solution of 3 mol/L LiF and 12 mol/L HCl, and the mixed solution was kept under continuous stirring to extract Al at 38  C for 48 h and then m-Ti3C2Tx was obtained after centrifugation [40]. The sediment, m-Ti3C2Tx, was then mixed with 250 mL of deionized water, followed by sonication for 1 h and then centrifugation to obtain the supernatant. Finally, the supernatant, d-Ti3C2Tx, was collected for further uses. 2.4. Preparation of the d-Ti3C2Tx/regenerate CNFs composite film The d-Ti3C2Tx/r-CNFs composite films with different thickness were prepared using vacuum-assisted filtration method by controlling the volumes of d-Ti3C2Tx dispersion during filtering and dried in vacuum oven at 60  C. The thickness of all the films was measured by a micrometer. The thickness of the r-CNFs film is 85 mm, and the thickness of d-Ti3C2Tx coating on r-CNFs is 6 mm,11 mm and 15 mm, respectively. 2.5. Characterization Chemical structure of CA film, r-CNFs film and d-Ti3C2Tx/r-CNFs composite film were investigated with a Nicolet 6700 FTIR spectrophotometer for each measurement over the spectra range of 650e4000 cm1 with a resolution of 4 cm1. XRD patterns were recorded on an X-ray diffractometer using Cu Ka radiation at 40 kV and 40 mA (l ¼ 1.54 Å). Diffraction angle (2q) scanned was changed from 5 to 70 . XPS measurements were performed using an X-ray photoelectron spectrometer (Escalab 250Xi). Morphology of Ti3AlC2 and Ti3C2Tx powders, r-CNFs and d-Ti3C2Tx/r-CNFs composite films were observed by SEM (JSM-5900LV). The lateral size of d-Ti3C2Tx were measured using dynamic light scattering (DLS) on the colloidal solution to confirm the particle size (Zetasizer Nano ZSP (ZEN5600)). TEM images were obtained by a transmission electron microscope (FEI Tecnai G2 F20 S-TWIN). Electrical resistance of d-Ti3C2Tx/r-CNFs composite film was measured using fourpoint probes equipment (RTS-9). The mechanical properties of dTi3C2Tx/r-CNFs composite film was measured on YM061 tensile tester at a speed of 0.1 mm/min. The EMI SE properties of d-Ti3C2Tx/r-CNFs composite film were evaluated at the frequencies ranging from 2 to 18 GHz on a vector network analyzer (Keysight E5063A ENA). 3. Result and discussion 3.1. Fabrication mechanism of the d-Ti3C2Tx/r-CNFs composite film The fabricated mechanism of the d-Ti3C2Tx/r-CNFs composite film is presented in Fig. 1. The as-spun CA nanofiber film transforms into r-CNFs film because acetyl groups of CA convert to hydroxyl groups of r-CNFs after CA is placed in NaOH/ethanol solution. Hydroxyl groups formed on the surface of the r-CNFs provides the possibility to combine with hydrophilic compound. In addition, Ti3AlC2 powders are etched in HCl/LiF mixtures and the m-Ti3C2Tx is obtained after centrifugation. The surface terminations on the surface of m-Ti3C2Tx such as F, O and OH are formed in the etching process [16,40]. The m-Ti3C2Tx is further sonicated and centrifuged to obtain d-Ti3C2Tx dispersion. The Van der Waals force among m-Ti3C2Tx is destroyed and d-Ti3C2Tx nanosheets are formed after sonication and centrifugation. The hydrophilic and dispersed d-Ti3C2Tx helps itself to be tightly stacked together during the filtration and can be bonded to the regenerated cellulose during drying to form a film through hydrogen bonds with certain mechanical properties. Therefore, the synthesized d-Ti3C2Tx/r-CNFs composite film possesses excellent EMI shielding, flexibility and

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Fig. 1. Fabrication schematic of the d-Ti3C2Tx/r-CNFs composite film.

mechanical properties. 3.2. Structure and morphology The XRD patterns of Ti3AlC2 powders before and after etching are shown in Fig. 2a. The XRD pattern of 2D m-Ti3C2Tx MXenes reveals that the most intense peak of Ti3AlC2 at 2q z 39 disappear after etching because of the elimination of Al and the formation of m-Ti3C2Tx MXenes. In addition, compared with (002) peak of Ti3AlC2, (002) peak of m-Ti3C2Tx shifts toward low 2q angle of 7.1 due to the significantly increased interlayer spacing in m-Ti3C2Tx MXenes [28,40]. The XRD patterns of the as-spun CA, r-CNFs, dTi3C2Tx and d-Ti3C2Tx/r-CNFs composite films are illustrated in Fig. 2b. The as-spun CA film has two broad amorphous halos in the ranges of 2q ¼ 5e14 and 15e30 , whereas the r-CNFs film characteristic peaks centered at 2q ¼ 13.1, 20.9, and 22.5 correspond to (101), (101) and (002) planes of the cellulose crystallites, respectively [33,39]. In addition, it is obvious that (002) peak of d-Ti3C2Tx film located at a lower angle (6.3 ) than that of m-Ti3C2Tx after sonicating, which is caused by the further enlargement of the interspace of the Ti3C2Tx flakes. Furthermore, compared with mTi3C2Tx powder (Fig. 2a), Ti3AlC2 peaks can be neglected, suggesting the d-Ti3C2Tx film has no impurity of Ti3AlC2. The d-Ti3C2Tx/r-CNFs composite film shows a diffraction pattern similar to d-Ti3C2Tx but a weak and broad peak can be observed at 2q ¼ 17.5e24 , which is resulted from r-CNFs. FTIR spectra of as-spun CA, r-CNFs film and d-Ti3C2Tx/r-CNFs composite film are shown in Fig. 2c. Absorption peaks of acetyl carbonyl is obviously observed at 1745 cm1 (nC]O) and 1375 cm1 (nC-CH3) from the as-spun CA film. In contrast, the peaks at 1745 cm1 and 1375 cm1 are not observed from the r-CNFs film, indicating the successful deacetylation of as-spun CA film [33]. The FTIR results together with the XRD data confirm that the CA is successfully transformed to r-CNFs by deacetylation in the aqueous NaOH solution. The absorption peak of hydroxyl at around 3400 cm1 of r-CNFs film becomes much broader than as-spun CA film because hydrogen bonds are formed among the regenerate cellulose molecule [39,41]. Compared with the r-CNFs film, the

characteristic peaks of d-Ti3C2Tx/r-CNFs composite films show no obvious change because the surface termination of d-Ti3C2Tx such as eO and eOH has the same characteristic peak as the hydroxyl group on r-CNFs film. The further details about surface chemical structures of dTi3C2Tx/r-CNFs composite film were characterized by X-ray photoelectron spectroscopy (XPS) as shown in Fig. 3aed. The XPS survey scan of d-Ti3C2Tx/r-CNFs composite film in Fig. 3a shows a typical XPS pattern of d-Ti3C2Tx [23,27]. The high-resolution XPS spectra (Fig. 3bed) represent Ti 2p, C 1s and O 1s regions of the d-Ti3C2Tx/rCNFs composite film, respectively. The XPS spectrum of the Ti 2p region could be fitted with four pairs of Gaussian-Lorentzian curves, where each pair is the 2p3/2 and the 2p1/2 component which is assigned to TieC, Ti (II) oxide, Ti (IV) oxide and TieF, respectively. The eight peaks centered at 454.4, 455.5, 458.0, 458.6, 460.5, 460.7, 462.3 and 464.3 eV are assigned to TieC 2p3, Ti (II) 2p3, TiO2 2p3, TieF 2p3, TieC 2p1, Ti (II) 2p1, TiO2 2p1 and TieF 2p1, respectively [23,42,43]. The five peaks centered at 282.0, 284.9, 285.3, 286.3 and 288.9 eV as shown in high resolution XPS spectrum of C 1s (Fig. 3c) corresponds to TieC, CeC, hydrocarbons (eCH2- & CH3-), CeO and eC]O, respectively [42]. The O 1s region of the d-Ti3C2Tx/r-CNFs composite film (Fig. 3d) could be fitted by two components assigned to titanium oxide: one component for stoichiometric TiO2 and one component for sub-stoichiometric TiOx. The O1s region indicates TieOH formation and H2O uptake [42]. The peak fitting of the XPS high-resolution spectra suggests that the OH, O and F surface functional groups of d-Ti3C2Tx. These surface functional groups contribute to the formation of hydrogen bonds between the r-CNFs film and the d-Ti3C2Tx. The microstructure of Ti3AlC2 and m-Ti3C2Tx powders was characterized by SEM. SEM images of Ti3AlC2 and m-Ti3C2Tx powders are shown in Fig. 4a and b, respectively. Typical accordion-like morphology and open interspace of m-Ti3C2Tx are observed in Fig. 4b, which shows the exfoliation of Ti3AlC2 powders. Transmission electron microscopy (TEM) analysis shows that a few layers of d-Ti3C2Tx possesses large lateral dimensions and does not contain the nanometer-size defects (Fig. S1a). The result indicates that the m-Ti3C2Tx turns into d-Ti3C2Tx nanosheets by sonication

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Fig. 2. (a) XRD patterns of Ti3AlC2 and m-Ti3C2Tx powders, (b) XRD patterns of CA, r-CNFs, d-Ti3C2Tx and d-Ti3C2Tx/r-CNFs composite film and (c) FTIR spectra of CA, r-CNFs and dTi3C2Tx/r-CNFs composite film.

and centrifugation. The inset of Fig. S1a shows Tyntall scattering effect in a colloidal solution of d-Ti3C2Tx flakes, indicating the good dispersion property of d-Ti3C2Tx, which is beneficial for preparing uniform films [44]. Furthermore, single layers with 1.1 nm thick were imaged using TEM (Fig. S1b) [14]. The d-Ti3C2Tx suspension was analyzed by dynamic light scattering (DLS) and the size of the flakes is in the range of 0.05e1.1 mm (Fig. S2). The SEM and digital images of the as-spun CA film, r-CNFs film and d-Ti3C2Tx/r-CNFs composite film are illustrated in Fig. 5. The diameter distribution statistics obtained from SEM images show that the CA film diameter is mostly ranged from 50 to 300 nm, while the r-CNFs film is mostly distributed between 100 and 250 nm (Fig. S3). The diameter of r-CNFs becomes more uniform and finer after deacetylation than CA fibers as shown in Fig. 5a and b, which is beneficial to increase the specific surface area of r-CNFs. However, it can be seen from SEM image of r-CNFs that some fiber doublets or fibers are partially combined together because of the formation of hydrogen bond [33]. The top-view SEM images of dTi3C2Tx/r-CNFs composite film at different magnifications are shown in Fig. 5c andd. The d-Ti3C2Tx deposited on the r-CNFs film forms wrinkled surfaces because the r-CNFs film swells in water during filtration and then shrinks during the drying. The crosssectional SEM image of d-Ti3C2Tx/r-CNFs composite film is illustrated in Fig. 5e. The d-Ti3C2Tx is densely stacked on the r-CNFs film due to the removal of water molecule among the d-Ti3C2Tx layers during filtration and drying [45]. In addition, the d-Ti3C2Tx possesses distinct 2D structure and d-Ti3C2Tx nanosheets are coated on the surface of r-CNFs. Digital images of r-CNFs film and d-Ti3C2Tx/rCNFs composite film are illustrated in Fig. 5f. The white r-CNFs film is covered with a layer of deep gray d-Ti3C2Tx after filtering. The d-

Ti3C2Tx and the d-Ti3C2Tx/r-CNFs composite film shows very flexible.

3.3. EMI SE The total EMI SE effectiveness (SET) of shielding material is the sum of reflection (SER), absorption (SEA) and multiple reflection (SEM). The SER is related to the impedance mismatch between air and absorber, while the SEA is resulted from the energy dissipation of electromagnetic radiation. The SEM is induced by the scattering effect of homogeneity inthe material and it can be neglected when SEA > 10 dB [46]. The total EMI SE (SET) is defined as the logarithmic ratio of incoming (Pi) to transmitted power (Pt) of radiation [47].

SET ¼ 10logðPi =Pt Þ ¼ SER þ SEA þ SEM zSER þ SEA

(1)

The S11 (or S22) and S12 (or S21) parameters of the two-port network system stand for the reflection and transmission coefficient. Transmittance (T), reflectance (R), and absorbance (A) of the shielding material are analyzed by the S parameters, which can be described in Equations (2)e(4).

T ¼ jS12 j2 ¼ jS21 j2

(2)

R ¼ jS11 j2 ¼ jS22 j2

(3)

A¼1TR

(4)

The effective absorbance can be expressed in Equation (5).

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Fig. 3. (a) Survey XPS spectrum of the d-Ti3C2Tx/r-CNFs composite film, high-resolution (b) Ti 2p spectra, (c) C 1s spectra and (d) O 1s spectra of sample.

Fig. 4. SEM images of (a) Ti3AlC2, (b) m-Ti3C2Tx.

Aeff ¼ ð1  T  RÞ=ð1  RÞ

(5)

As for the power of the effective incident electromagnetic radio inside the shielding material, the reflectance and effective absorbance can be expressed in Equations (6) and (7).

SER ¼ 10logð1  RÞ

(6)

SEA ¼ 10logð1  Aeff Þ ¼ 10logðT=ð1  RÞÞ

(7)

The EMI SE of r-CNFs film and d-Ti3C2Tx/r-CNFs composite films were measured at the frequency range of 2e18 GHz and the result is illustrated in Fig. 6. The EMI SE of shielding material is commonly expressed in decibels (dB). High SE values stands for the transmission of few electromagnetic waves that transmits through the shielding material [47]. SET, SEA and SER curves of r-CNFs film and dTi3C2Tx/r-CNFs composite films with different thickness of coated d-Ti3C2Tx are shown in Fig. 6aec, respectively, which were calculated according to Equations ((1)e(7)). The SET, SEA and SER values

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Fig. 5. (a) SEM image of as-spun CA film. (b) SEM image of r-CNFs film. (c)e(e) SEM images of d-Ti3C2Tx/r-CNFs composite film. (f) Digital images of r-CNFs film and d-Ti3C2Tx/r-CNFs composite film.

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Fig. 6. EMI SE of pure r-CNFs and d-Ti3C2Tx/r-CNFs films in frequency range of 2.0e18.0 GHz. (a) SET, (b)SEA, (c) SER, (d) average values of EMI SE of r-CNFs and d-Ti3C2Tx/r-CNFs films with different thickness of d-Ti3C2Tx coating, respectively, and (e) EMI SE stability after bending d-Ti3C2Tx/r-CNFs composite film.

of pure r-CNFs film are almost 0, indicating that the pure r-CNFs film is almost transparent to electromagnetic waves and exhibits no shielding ability because of its insulating feature. In contrast, the EMI SE values d-Ti3C2Tx/r-CNFs composite films with different thicknesses of coated d-Ti3C2Tx are 21.1 dB, 35.3 dB and 42.7 dB (Fig. 6a), respectively. The result indicates that the d-Ti3C2Tx/r-CNFs composite films exhibit excellent EMI SE, and the EMI SE values increase with the rise of thickness of d-Ti3C2Tx coating due to more d-Ti3C2Tx sheets are involved in EMI SE. In addition, the SEA values with various thicknesses are higher than SER values. The result suggests absorption-dominant EMI shielding of the d-Ti3C2Tx/rCNFs composite films. As EM waves strike the surface of a Ti3C2Tx

flake, some EM waves are immediately reflected because of abundant free electrons on the surface of the highly conductive Ti3C2Tx. The remaining waves pass through the Ti3C2Tx lattice structure where interaction with the high electron density of Ti3C2Tx induces currents that contribute to ohmic losses, resulting in a drop in energy of the EM waves [46]. Fig. 6d presents the average EMI SE of r-CNFs film and d-Ti3C2Tx/r-CNFs composite films with different thickness of d-Ti3C2Tx coating. It is obvious that the EMI SE increases with the rise of the thickness of the d-Ti3C2Tx coating, and the EMI wave absorption plays a dominated role. In addition, the corresponding electrical conductivity of the pure r-CNFs film and dTi3C2Tx/r-CNFs composite films are shown in Fig. S4. EMI shielding

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performance is related to the electrical conductivity, which is determined by the thickness of coated d-Ti3C2Tx. In order to illustrate the flexibility and the EMI SE stability of the d-Ti3C2Tx/r-CNFs composite film, the EMI SE was tested after the composite film was bent. The retention rate of EMI SE of the d-Ti3C2Tx/r-CNFs composite film after 500 bends is 92.9% as shown in Fig. 6e, showing excellent flexibility and stability. Compared with millimeter-thick metal-based and carbon-based shielding materials, the d-Ti3C2Tx/r-CNFs composite film with ultrathin thickness and flexibility shows excellent EMI shielding properties as shown in Fig. 7a and Table S1. Although their EMI SE is enough high, the high density, inflexibility and total reflection of metal-based materials limit their applications in emerging electromagnetic interference environments [6e9]. Low density is the most attractive advantage of carbon materials as high-performance

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EMI shielding materials such as graphene, reduced graphene oxides (rGO), carbon fibers (Cf) and carbon nanotubes (CNTs) [23]. In particular, foam composites of graphene and CNTs with polymer are favored by researchers because of their ultralight structure [1,2]. However, the filling amount of graphene and CNTs determines the EMI shielding properties of the foam composite. Therefore, few of them can combine ultrathin thickness, high flexibility and superior EMI SE properties. The unsatisfactory EMI SE and millimeter-sized thickness hinder their applications in portable and wearable smart electronics. In addition, the ultrathin and flexible d-Ti3C2Tx/r-CNFs composite films prepared in this work possess better EMI SE performance than other Ti3C2Tx-based composite. More importantly, the use of vacuum filtration to prepare composites for EMI SE can be extended to preparation of other MXene and hydrophilic nanofiber films.

Fig. 7. (a) Comparison of EMI SE as a function of the sample thickness with previous literature and (b) Scheme of the mechanism of EMI shielding of d-Ti3C2Tx/r-CNFs composite film.

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The mechanism of EMI SE of d-Ti3C2Tx/r-CNFs composite film prepared by filtration of d-Ti3C2Tx solution is presented in Fig. 7b. The d-Ti3C2Tx flakes continuously deposited on the surface of rCNFs film forms an overall electrical conductive network. The higher conductivity of d-Ti3C2Tx and larger specific surface area will produce more conductive paths for charge carriers, which is beneficial for EMI shielding. The dipolar polarization caused by the surface functional groups, localized defects and dangling bonds of d-Ti3C2Tx plays an important role in dielectric loss, which is comparable to that of reduced graphene oxides [5,48]. In addition, multilayered structure and excellent electrical conductivity of dTi3C2Tx can facilitate exciting hopping charge carriers by constructing conductive paths. Meanwhile, 2D stacked d-Ti3C2Tx sheets can contribute to formation of many interfaces, which increase the EM waves propagation path and the electrical current path discontinuities, thus the surviving EM waves will be dissipated as soon as possible [49,50]. A part of the EM waves is reflected back when the incident EM waves are exposed to the d-Ti3C2Tx/r-CNFs composite film as shown in Fig. 7b. The remaining EM waves interact with the high charge density d-Ti3C2Tx induces currents that contribute to ohmic losses, leading to energy loss of the EM waves. Additionally, when the surviving EM waves pass through the first layer of Ti3C2Tx, it encounters the next barrier layer, and the phenomenon of EM waves attenuation repeats. The multilayer structure of the d-Ti3C2Tx on the surface of r-CNF film will facilitate the multiple internal reflection and the EM waves can be reflected back and forth between the layers, resulting in completely absorption and energy dissipation of the EM waves in the structure [29,46]. Therefore, as the d-Ti3C2Tx coating on r-CNFs becomes thicker, the more EM waves can be absorbed, accordingly, the EMI SE efficiency is more excellent.

composite films is lower than that of r-CNFs due to the stiffening of the structure after filtrating d-Ti3C2Tx solution [51]. 4. Conclusion The flexible d-Ti3C2Tx/r-CNFs composite films were successfully prepared by vacuum-assisted filtration method. The d-Ti3C2Tx/rCNFs composite films possess high electrical conductivity (46.3 S cm1) and excellent EMI shielding properties (up to 42.7 dB) with 15 mm of the thickness of coated d-Ti3C2Tx. In addition, the dTi3C2Tx/r-CNFs composite film exhibits better mechanical properties than pure r-CNFs film. The composite film shows outstanding stability after being bent for 500 cycles. Therefore, the d-Ti3C2Tx/rCNFs composite films can be potentially used as flexible and ultrathin EMI shielding materials. Conflicts of interest There are no conflicts of interest to declare. Acknowledgements This work was financially supported by The National Natural Science Foundation of China and The Civil Aviation Administration of China (No. U1833118). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.02.294. References

3.4. Mechanical properties of the d-Ti3C2Tx/r-CNFs films The stress-strain curves of r-CNF and d-Ti3C2Tx/r-CNFs composite films are presented in Fig. 8. The pure r-CNFs film shows the poor tensile strength (16.3 cN) but relatively high breaking elongation (68%). However, the d-Ti3C2Tx/r-CNFs composite films exhibit high tensile strength and poor breaking elongation. The tensile strength of d-Ti3C2Tx/r-CNFs composite films increases from 16.8 cN to 29.1 cN with the rise of the thickness of the coated dTi3C2Tx as a result of the large interfacial interactions between the 2D d-Ti3C2Tx and the r-CNFs film which is formed by hydrogen bonding. The breaking elongation of all the d-Ti3C2Tx/r-CNFs

Fig. 8. Stress-strain curves ofr-CNFs and d-Ti3C2Tx/r-CNFs composite films with different thickness.

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