Enhanced X-band Electromagnetic Interference Shielding

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Enhanced X-band Electromagnetic Interference Shielding Performance of Layer-structured Fabric-supported Polyaniline/Cobalt-Nickel Coatings Hang Zhao, Lei Hou, Siyi Bi, and Yinxiang Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07941 • Publication Date (Web): 31 Aug 2017 Downloaded from http://pubs.acs.org on September 3, 2017

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ACS Applied Materials & Interfaces

Enhanced X-band Electromagnetic Interference Shielding Performance of Layer-structured Fabric-supported Polyaniline/Cobalt-Nickel Coatings Hang Zhao, Lei Hou, Siyi Bi, Yinxiang Lu∗ Department of Materials science, Fudan University, Shanghai 200433, China ABSTRACT: Despite tremendous efforts, fabrication of lightweight conductive fabrics for high-performance X-band electromagnetic interference (EMI) shielding remains a daunting technical challenge. We herein report an ingenious and efficient strategy to deposit polyaniline/cobalt-nickel (PANI/Co-Ni) coatings onto lyocell fabrics that involved consecutive steps of in situ polymerization and electroless plating. The PANI-Co-Ni trinary-component system successfully induced a synergistic effect from EM wave-absorption and EM wave-reflection, and moreover upgraded the match level between magnetic loss and dielectric loss. By the judicious controlling of polymerization cycles and plating time, low-weight fabric-supported PANI/Co-Ni composites (with PANI and Co-Ni loading of 2.86 and 3.99 mg cm-2, respectively) were prepared, which displayed relatively high EMI SE (33.95–46.22 dB) when compared to their single peers (PANI-coated fabric and Co-Ni-coated fabric) or even the sum of them. Inspired by the so called “1+1>2” phenomenon, here we demonstrated that there was an EMI SE enhancement effect in this conductive polymer/metal system that may be associated with interphase chemical and/or physical interactions. Further analysis revealed that this EMI SE enhancement effect was evident under circumstances of relatively low metal content, and became weak with the increase of metal contents. The 1

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mechanisms involved were interpreted through a series of fundamental measurements, including Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), and vector network analysis (VNA). The linkage between PANI and Co-Ni coatings was in the form of Co-N/Ni-N, which mimics the atomic configuration occurring in cobalt porphyrins. The Co-N/Ni-N configuration strengthened the interphase adhesion and thus resulted in shielding fabrics with high durability for practical applications. Keywords: lyocell fabric, polyaniline, cobalt, nickel, electroless plating, electromagnetic interference, shielding effectiveness, X band ________________________________ *Corresponding author. Tel. & fax: +86 21 55665059; E-mail address: [email protected]

1. INTRODUCTION Currently, electromagnetic (EM) pollution has been exacerbating at a noticeable rate due to the proliferation of various electronics, such as cellular phones, tablet PCs and wireless local area networks.1, 2 EM pollution may cause operational malfunction within the electronic neighborhood as well as violate the intrinsic EM field of human beings.3, 4 The consequential hazards may be a loss of money, energy sources, time, or even precious human life.5 Hence, high-performance electromagnetic interference (EMI) shielding materials are required to isolate the internal electronics from the surroundings. Theoretically, when the incident EM radiations impinge upon shielding materials, they will be attenuated by three dissipation ways: reflection loss, absorption loss and internal multiple reflection loss.6 Shielding effectiveness 2


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(SE) is usually used to quantify the collective loss of EM radiation, which can be mathematically formulated as SEtot=SER+SEA+SEM. SER loss is the result of impedance mismatch between free space and shielding materials. In an extreme situation, zero-reflection can be achieved at the shielding material surface when the characteristic impedance is close to the value of 377 Ω sq-1 (standard impedance of the free space).7 SEA loss is generally regarded as the combined result from dielectric loss and magnetic loss. It is worth mentioning that the match level between magnetic loss and dielectric loss is a determinative factor in this loss mode. Mere high dielectric loss or high magnetic loss will certainly give rise to deficient SEA loss on account of the imbalance of the EM match.8 SEM loss is identified as the scattering effect of the inhomogeneous material, which can be neglected due to the tiny contribution in the collective loss when SEtot ≥ 15 decibels (dB).9 Therefore, SE for high-performance EMI shielding can be approximately simplified as SEtot ≈SER+SEA. Based on these well-established attenuation principles, shielding materials are classified into two categories: wave-reflection dominant materials and wave-absorption dominant materials. Metals are typical wave-reflection dominant materials owing to their abundance in mobile charge carriers that can interact with the electric vector of incident EM radiation. Wave-absorption dominant materials are composed of magnetic materials like carbonyl iron and ferrites (including Fe3O4 and α-Fe2O3),10-12 dielectric materials such as barium titanate,13 carbon-based materials (including carbon fibers, carbon black, graphite, graphene, single-/multi-wall carbon nanotubes and mesoporous carbon)14-17 and conductive polymers (including polyaniline (PANI), polyacetylene and polypyrrole)18, 19. However, their individual utilization usually encounters diverse problems. For example, metal-based shielding materials 3



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(including metal screen, metal foil, metal foam, and metal laminate) are always mechanically stiff, heavy and chemically oxidation/corrosion-susceptible; carbon-based materials are frequently used as nanoscale fillers into conductive composites because of their high mechanical strength, excellent electrical conductivity at low percolation, as reported by Liu et al,20, 21 however, incorporation of these carbon-based nanofillers into polymer matrix may require a relative complicated processing, which inevitably results in high production costs; the insoluble and infusible characteristics of conductive polymers cause them to exhibit poor processability and lack mechanical properties. Combining two or three of these materials together may be an effective way to bypass these inherent shortcomings. As an alternative, binary- or ternary-component shielding composites may have supplementary attenuation properties (not available from their single peer) toward EM waves, i.e., an improved match degree between magnetic loss and dielectric loss; a synergistic effect between wave absorption and wave reflection. In addition to the categories, the structure of shielding composites also has a significant influence on the EMI SE. In general, there are two kinds of structures for EMI shielding composites: layered and embedded. Layered structures are fabricated by the stepwise coating of shielding components. Embedded structures are formed by incorporation of fillers into non-/conductive polymeric matrices. Fang et al. investigated the effect of different composite structures on EMI SE. They prepared free-standing layer-structured and embedded-structured silver-nanowire/polyaniline films by a two-step casting process and a direct-mixing process, respectively. Comparatively, the layer-structured films exhibit a higher electrical conductivity (5,300 S cm-1) and a superior EMI shielding performance of above 50 dB over a bandwidth of 4


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1.2 GHz than their embedded-structured counterparts (electrical conductivity is 4,234 Scm-1 and a narrow bandwidth of 0.4 GHz above 50 dB).22 More importantly, the incorporation of fillers is likely to affect the pliability of composites, and moreover, fabrication of these embedded-structured composites generally requires indispensably sophisticated processing and prohibitive chemical additives, which inevitably increases costs. Conversely, layer-structured composites can be easily obtained by the direct coating of shielding components onto platforms without any specialized equipment. Therefore, in this research, layer-structured shielding composites were prepared by the successive coating of polyaniline (PANI) and magnetic alloy cobalt-nickel (Co-Ni), as shown in Scheme 1. Commercial lyocell fabrics were selected as the platforms to support the layer-structured trinary-component coatings. PANI coating and Co-Ni coating were deposited on lyocell fabrics by an in situ chemical polymerization reaction and an electroless plating approach, respectively. Another reason to choose the PANI-(Co-Ni) system is to imitate the Co-related atomic configurations commonly existing in Co-containing porphyrins (such as (Tetraphenylporphinato) cobalt) since it is presumed that Co atom bonds to PANI units, 23-26 thus allowing for the formation of Co–N and Ni–N sites in our case, which will certainly enhance the interphase bonding between the PANI coating and the Co-Ni coating. Furthermore, the interfacial chemical and/or physical reaction at the PANI/Co-Ni interface may have a positive impact on EMI SE. The mechanisms involved were interpreted through a series of fundamental measurements. Structures, morphologies, crystalline identities, chemical states and EMI SE of shielding fabrics were systematically investigated by using Fourier transform infrared spectroscopy (FTIR), field emission-scanning electron microscopy 5



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(FE-SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and vector network analysis (VNA).

2. EXPERIMENTAL SECTION 2.1 Chemicals and Materials The lyocell fabrics (128×78 count/cm2, 165 g/m2) were procured from Taicang Biqi Novel Material Co. Ltd., and were preliminarily cut into a specimen of 5 × 5 cm in size. Aniline monomer was distilled under vacuum before use. Ammonium peroxydisulfate (APS), cobalt sulfate heptahydrate and sodium hypophosphite were obtained from Sinopharm Chemical Reagent Co., Ltd. All other chemicals were of analytical grade and used as received. Deionized (DI) water (resistivity = 18.2 MΩ cm) was used in all experiments for synthesis and washings. 2.2 In situ Polymerization of PANI Pristine lyocell fabrics were mercerized in alkaline solution (20 g/L NaOH) under 90 °C for 30 min to remove lignin, hemicellulose, wax and oils covering on the fiber surfaces, and then rinsed with acetic acid and DI water until the pH value reached neutral (the chemical reaction of alkali with lyocell fabric is shown in Figure 1 (a)). As a typical method of preparing the PANI coating, 0.4 M of aniline was dissolved in 400 mL aqueous hydrochloric acid (0.5 M) solution to prepare the monomer solution. Similarly, 0.5 M of APS as an oxidant agent was dissolved in 400 mL aqueous hydrochloric acid (0.48 M) solution. The monomer solution and oxidant solution were stirred separately for 1 h. To diffuse monomers into the lyocell fibers, the fabric was soaked in the monomer solution for approximately 30 min. Thereafter, the 6


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oxidant solution was added gradually to the monomer solution containing fabric substrate under mild stirring. Then, the mixture was left for 1 h at rest to complete the polymerization. Herein, the molar ratio of APS/aniline was deliberately determined as 1.25, which would certainly result in emeraldine base/salts (the most stable form for PANI, as shown in Figure 1 (b)).27, 28 Finally, the PANI-coated fabric was brought out and drip washed using plentiful amounts of DI water to remove oligomers and other byproducts loosely bound on the fibers/yarns. 2.3 Electroless Deposition of Co-Ni In a typical route, the PANI-coated fabric was activated by seeding catalytic Co (0) nanoparticles (NPs) on the surfaces. The main reason for selecting Co (0) NPs as activator is to lessen the activation cost and generate bonds to PANI units beforehand. Co (0) NPs were immobilized on PANI-coated fabric surface by an in situ reduction method. Indeed, PANI (especially the emeraldine base/salts) is capable to reduce the metal ions that have relatively high reduction potential to their zero valent states, such as Au3+ (+1.00 eV), Pd2+ (+0.92 eV) and Ag+ (+0.80 eV). However, some metal ions, such as Cu2+ (+0.34 eV), Ni2+ (-0.22 eV) and Co2+ (-0.28 eV), have low reduction potentials, which make them difficult to reduce by using PANI alone. Therefore, an external electric current or additional reductant is usually required. A specific Co (0)-activation example was carried out as follows: i) Co2+ cation solution was preliminarily prepared by dissolving 2.8 g of cobalt sulfate heptahydrate into 200 mL 1.25% of sodium citrate solution; ii) PANI-coated fabric was introduced into the Co2+ aqueous solution at RT for 10 min to capture Co2+ ions; iii) Co2+ ions were then in situ reduced by immersing the Co2+-absorbed fabric into 0.7 M of KBH4 solution at RT for 10 min. The 7



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activated fabric was then rinsed with DI water to ensure the thorough removal of the physically adsorbed Co (0) NPs. Electroless Co-Ni deposition procedure was carried out immediately after Co activation. The electroless bath composition was cobalt sulfate heptahydrate (14 g/L), nickel sulfate hexahydrate (14 g/L), sodium hypophosphite (20 g/L), KNa-tartrate (140 g/L) and ammonium sulfate (65 g/L). The Co (0)-activated PANI-coated fabric was soaked into the one-pot bath under a constant temperature of 75 °C for 30-90 min (three levels). Upon removal from the one-pot plating bath, the fabric samples were rinsed with DI water and placed in the oven at 50 °C for 30 min to facilitate surface drying. Finally, the fabric-supported PANI/Co-Ni composites with layered structures were obtained. The resultant fabric can be repeatedly folded, easily deformed in all directions and arbitrarily tailored with scissors. The effects of repeatedly folding on electric/magnetic and EMI shielding properties were shown in S1 (see the supporting information). Indeed, coatings did not change the flexibility of the fabric substrates, so the handle and flexibility of the conductive fabrics after coating procedures were very close to that of the untreated fabrics.

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Scheme 1 Schematic illustration of the fabrication of layered fabric-supported PANI/Co-Ni composites (enlarged view of the interlaced fabric structures were shown as the round optical micrographs taken utilizing an optical microscope (BELONA, XSP-OO)).

Figure 1. Mercerization of the pristine lyocell fabric in alkaline (NaOH) solution (a); in situ polymerization of aniline monomers (b); interaction between PANI layer and fabric (c); chemical structure of (Tetraphenylporphinato) cobalt (d); presumed atomic configuration of PANI–Co and PANI–Ni (e). 2.4 Characterizations All the fabric samples were conditioned according to ASTM D 1776-04 before measurement. IR spectra were recorded by FT-IR spectrometer (Nicolet Nexus 470) in diffuse 9



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reflectance mode. XPS was measured on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg Kα X-ray source (1253.6 eV photons) at an accelerating voltage of 14 kV and current of 25 mA. Peak fitting was performed based on Shirley-type background subtraction method using XPSPEAK 4.1 software with a Gaussian/Lorentzian ratio of 20%. The morphological and compositional characterization was performed on a FE-SEM (S-4300SE, Hitachi, Japan) coupled with an energy-dispersive X-ray (EDX). A 5-nm platinum layer was sputtered on the fabric surface before the measurement. The crystalline identities of the fabric samples was investigated by XRD (Rigaku Dymax, Japan) using Cu Kα radiations (λ=1.54056 Å), and the diffraction patterns were documented with 2-Theta region of 10° to 80°. Magnetic properties were studied by vibrating sample magnetometry (VSM, LDJ Electronics Inc., Tory, MI. USA) at RT with the field ranging from -80 to 80 KOe. Potentiodynamic polarization corrosion tests were carried out on an electrochemical workstation (Versa STAT 3, METEK) to investigate the anti-corrosion resistance capacities of the fabric samples. The scattering (S11, S22, S12 and S21) parameters of the conductive fabrics were surveyed on a HP8510C VNA according to the waveguide method in the X-band (8.2-12.4 GHz) region. The fabric samples were cut into small rectangular pieces with dimensions of 22.9 mm × 10.2 mm to accommodate the X-band waveguide holders. The VNA setup was calibrated carefully before each measurement. SEA and SER were determined from the measured S parameters. The power coefficients including reflection coefficient (R) and transmission coefficient (T), were obtained by the equations R =[ER/EI]2=|S11|2= |S22|2 and T =[ET/EI]2=|S12|2=|S21|2, respectively.29-31 In particular, the absorption coefficient (A) was calculated based on the correlation of A+R+T =1. EMI SEtot refers to the logarithm ratio of 10


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the incident EM radiations (PI) to the transmitted EM radiations (PT), which is determined by the equation of SEtot = 10 log (PI/PT) dB.32 The experimental SEtot is the sum of the network attenuating by reflection mode (SER) and absorption mode (SEA), which can be expressed as SER = -10 log (1 - R) dB and SEA = -10 log (T/(1 - R))dB, respectively.33

3. RESULTS AND DISCUSSION Layer-structured fabric-supported PANI/Co-Ni composites were fabricated by consecutively loading PANI and Co-Ni through an in situ chemical polymerization reaction followed by an electroless plating approach. The loading of PANI could be easily tuned by repeating the in situ polymerization cycles (n = 1, 2, 3), and the content of Co-Ni could be simply tuned by adjusting the time (t = 30 min, 60 min, 90 min) of dips into the electroless bath. Herein, the PANI coatings obtained with different polymerization cycles were labeled as PANI-1, PANI-2 and PANI-3. Similarly, the alloy coatings plated with different plating time were labeled as Co-Ni-30, Co-Ni-60 and Co-Ni-90, respectively. According to the mass change before and after coatings, the loading of PANI and Co-Ni alloy is given by the following equations:

loading (mg cm-2) of PANI on fabric =

loading (mg cm-2) of Co-Ni on fabric =

(1) (2)

where A is fabric specimen area (25 cm2); and W0, WPANI-n and Walloy-t are the weights of mercerized fabric, PANI-coated fabric at n-cycle polymerization, and PANI-3/Co-Ni-coated fabric with t min electroless plating, respectively. The calculation results and corresponding surface resistance (Rs) are summarized in Table S1 (see the supporting information). Of note, 11



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PANI-3-coated fabric was used as the platform to support the following Co-Ni coatings. To investigate the role of PANI in the trinary-component shielding fabric, samples that were obtained in absence of PANI were used as the control groups. These samples were labeled as fabric-supported Co-Ni-30, Co-Ni-60 and Co-Ni-90 composites (relevant synthesis routes are shown as S2, see the supporting information). 3.1 Surface Physico-chemical Characterization of the PANI/Co-Ni Coatings. The FT-IR spectra obtained for the pristine, mercerized, PANI-coated and Co (0)-activated fabrics are shown in Figure 2. In the spectrum of pristine fabric, characteristic bands of cellulose occurred at the following locations: -OH stretching vibration at 3,501 cm−1, C–H stretching vibration at 2,900 cm−1, C=C stretching vibration of aromatic rings at 1,646 cm−1, and C–O stretching vibration at 1,136 cm−1. For mercerized fabric sample, the peak attributable to -OH stretching vibrations became broader due to increased number of -OH groups because of the cleavage of NaOH-sensitive bonds. In the spectrum of PANI-coated fabric (herein, PANI-3-coated fabric was selected as the representative), apart from cellulose-related bands, some bands arising from PANI were detected as well. The peaks at approximately 1,176 and 831 cm-1 were associated with in-plane and out-plane C–H bending, respectively. Aromatic C– N stretching vibration located at 1,347 cm-1 revealed the appearance of the secondary aromatic amine groups. The vibration peak in the vicinity of 752 cm-1 was characteristic of the para-disubstituted aromatic rings that indicated the formation of polymer chains. The bands at approximately 1,517 and 1,610 cm-1 were assigned to the benzenoid and quinoid ring units, respectively. It was observed that the PANI coating comprised both benzenoid and quinoid moieties (herein, the proportions of these moieties provides the compositional information of 12


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insulating and conducting phases in the polymer). Putatively, the interaction between PANI and lyocell fabric was by hydrogen bonding (Figure 1 (c)) as described previously in polypyrrole (-N-H)/cuprammonium (C-OH) system.34 In the case of Co (0)-activated fabric, a new vibration appeared at approximately 895 cm−1, which might be associated with Co–N atomic configuration. Nitrogen atoms in PANI chains were the main sites for cobalt attachment,23 which mimics the atomic configuration in Co-containing porphyrins (relevant chemical structures were illustrated in Figure 1 (d-e)).

Figure 2. FT-IR spectra obtained from the pristine, mercerized, PANI-3-coated and Co (0)-activated fabrics. Figure 3 (a) displayed XPS wide-scan spectra of the pristine, PANI-1-coated, Co2+-captured and Co (0)-activated fabrics. All fabric samples show dominant signals at binding energies (BEs) of 286.06 eV, 400.41 eV and 533.57 eV, attributable to C1s, N1s and 13



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O1s, respectively. Obviously, the relative atomic percentages of C/O (∼6.32) in PANI-coated fabric increased dramatically in comparison with that (∼3.05) in pristine fabric (as shown in Table S2, see the supporting information), further confirming the successfully loading of PANI coatings onto fabric during in situ polymerization. Narrow XPS spectra of the pristine and PANI-1-coated fabrics within the BE of 160 to 220 eV are shown in Figure 3 (b). Cl and S doping signals were detected at BEs of 198.39 eV and 169.62 eV, which existed in the mode of acid radical ions and emanated from hydrochloric acid and APS, respectively. Figure 3 (c-d) represented the core-level spectra of Co2p obtained from the fabric samples before and after in situ reduction. Two doublets were split from the Co2p scan spectrum, including divalent cobalt (Co 2p3/2 783.97eV and Co 2p1/2 799.41 eV) and shakeup satellite signals (BE=788.21 eV and 803.52 eV). After exposure to KBH4 solution, the Co2p core-level spectrum were curve-fitted into three doublets: divalent cobalt (Co 2p3/2 784.39 eV and Co 2p1/2 799.20 eV), shakeup satellite signals (BE=787.35 eV and 803.20 eV) and zero-valent cobalt (Co 2p3/2 783.10 eV and Co 2p1/2 790.46 eV). The newly appeared doublet was associated with Co (0) species, which revealed the reduction of segmental Co2+ ions to Co (0) NPs. In addition, to validate the existence of Co–N and Ni-N bonds inside the PANI/Co-Ni coatings, nitrogen binding configurations were investigated by the deconvolution of N1s core-level spectra. However, because of the technical difficulty in analyzing PANI/Co-Ni-30-coated fabric sample by using XPS measurement, we failed to get qualified N1s signals from the inner PANI/Co-Ni interface. It can be seen from the XPS wide-scan spectrum of PANI/Co-Ni-30-coated fabric that Co2p and Ni2p were the dominant signals, and limited signal of N1s was found (as shown in Figure S4, see the supporting information ). The 14


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absence of N1s signal could be ascribed to the outer Co-Ni-30 coating, which was too thick for the X-ray photoelectron (the detection depth is within 5-10 nm)) to permeate. With the aim to circumvent this problem, thin Co-Ni coating (labeled as Co-Ni-1) was plated on the PANI-3-coated fabric by shortening the electroless plating time to 1 min. Core-level spectra of N1s obtained from the PANI-3-coated (e) and PANI/Co-Ni-1-coated (f) fabrics were shown in Figure 3 (e-f). As for PANI-3-coated fabric, four peak components were split from the N1s scan spectrum, including imine (-N=) peak component at BE of 398.85 eV, amine (– NH–) peak component at BE of 399.38 eV, and positively charged nitrogen (N+) peak components at BE >400.69 eV. Whereas in the case of PANI/Co-Ni-1 fabric, both the -N= and –NH– absorption peaks moved to higher BEs with shifts of 0.62 eV (from 398.85 to 399.47 eV) and 0.23 eV (from 399.38 to 399.61 eV), respectively. The high-BE shifts suggested that Mχ (M=Co or Ni, 0≤χ≤+2) had adsorbed with nitrogen atom through coordinated bonds, which induced the electron density around the nitrogen atoms decreased and consequently BE became higher. Of note, the nitrogen here originated from imine/amine groups, and has a couple of unshared pair electrons. Therefore, it can be inferred that Co-N and Ni-N bonds have been formed at the PANI/Co-Ni interface. Notably, the R2 values of all peak-fitting results were close to 1, which revealed that these peak-fitting results were reasonable.

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Figure 3. (a) XPS wide-scan spectra of the pristine, PANI-1-coated, Co2+-captured and Co (0)-activated fabrics; (b) narrow XPS spectra of the pristine and PANI-1-coated fabrics within the BE of 160 eV to 220 eV; core-level spectra of Co2p obtained from the fabric samples before (c) and after (d) in situ reduction; core-level spectra of N1s obtained from the PANI-3-coated (e) and PANI/Co-Ni-1-coated (f) fabrics. The morphological evolution of the fabric samples was investigated by FE-SEM 16


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observation. The pristine fabric had a smooth surface with little micropits and grooves (Figure 4 (a)). These occasional micropits and grooves may be the outcome of tensile crystallization in the processing of lyocell fibers. After mercerization (Figure 4 (b)), the fabric surface became extremely clean due to the cleaning effect of the alkaline solution.35 It can be seen that some pits emerged as well, which may improve the mechanical adherence between fabric substrates and PANI coatings. After loading of PANI (Figure 4 (c)), the overall morphology of the fabric changed almost not at all, but the surfaces changing from smooth to rough suggesting that the PANI coatings had been successfully assembled around the fiber/yarn frameworks. As shown in Figure 4 (d), Co (0) NPs were immobilized on the PANI-coated fabric surface. Herein, PANI coating had ability to capture Co (0) NPs as well as improving their dispersion on the surfaces.36 To illuminate the Co element distribution on Co (0)-activated PANI-coated fabric, the elemental maps of a rectangular fiber region were represented in the Figure 5 (a). The results demonstrated a very homogeneous Co element distribution in the fabric sample, which implied that Co (0) NPs were uniformly anchored on the fabric surfaces. In order to further validate the uniformity of the Co (0) NPs distribution, linear EDX analysis was conducted as well. Continuous Co signals were detected by line scanning (as shown in Figure 5 (b)). The content vibration of the Co element was very slight, which indirectly revealed that Co (0) NPs dispersed uniformly on the fabric surface. After electroless plating (Figure 4 (e)), the fabric surface became rougher, and some globular alloy aggregations emerged. The whole co-deposited Co-Ni coating exhibited a continuous structure.

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Figure 4. FE-SEM images of the pristine (a), mercerized (b), PANI-coated (c), Co (0)-activated (d), PANI/Co-Ni-30-coated (e-f, before and after ultrasonic treatment) and Co-Ni-30-coated (g-h, before and after ultrasonic treatment) fabrics (a-h magnification =1,000, scale bar =10 µm; a-inset and b-inset magnification =10,000, scale bar =1 µm; c-inset and d-inset magnification =5,000, scale bar =1 µm; e-inset and g-inset magnification =3,000, scale bar =1 µm).

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Figure 5. Elemental maps (a) and linear EDX scanning spectrum (b) of Co (0)-activated PANI-coated-fabric sample. The characteristic crystalline identities of fabric samples were validated by XRD, and the patterns are shown in Figure 6. The XRD pattern for pristine lyocell fabric showed three peaks (located at 2θ = 12.31°, 20.18°, 21.78°), which were attributed to (1 0 0), (1 1 0) and (2 0 0) planes of cellulose II.37 For PANI-coated fabric, a weak peak located at 25.41° was detected, which was associated with the periodicity perpendicular to doped PANI chains. Similarly, there should be a diffraction peak belonging to the periodicity parallel to PANI chains. This diffraction signal may occur at approximately 21°, but overlaps with the strong characteristic diffraction signals from fabric substrate.13 After co-depositing a thin Co-Ni layer on the surface, the composite fabric exhibited Bragg reflections at 2θ =41.68°, 44.55°, 47.49° and 76.07°, which were indexed as Co (1 0 0), Co (0 0 2), Co (1 0 1) and Co (1 1 0) planes of hexagonal close-packed (HCP) crystalline phase as per JCPDS 05-0727. In addition, several peaks showed their presence at 2θ = 44.55°, 51.85°, 76.07°, which corresponded to Ni (1 1 1), Ni (2 0 0) and Ni (2 2 0) planes of faced-centered cubic (FCC) crystalline phase (JCPDS 04-0850). No signals of oxidation-state (such as CoO, Co2O3, NiO) or hydroxide-state (such as Co(OH)2 and Ni(OH)2) impurities were observed. Of note, peak intensity diffracted from the fabric substrate and embedded PANI layer decreased substantially (even vanished) after Co-Ni co-deposition. The phenomenon can be ascribed to the high covering degree of PANI/Co-Ni-30 coatings, indicating a compact loading of alloy coatings onto fabric. The average crystalline sizes of the alloy coatings were obtained from the Scherrer equation.37 19



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The calculated average grain size corresponding to the superimposed emission (i.e., located at 2θ = 44.55°, Co (0 0 2) and Ni (1 1 1) planes) was 19.41 nm, which was lower than that (21.76 nm) obtained from corresponding control group (the relevant XRD pattern was shown as Figure S5, see the supporting information). Obviously, the alteration in grain size was related to the embedded PANI layer, which induced formation of a compact Co-Ni coating.

Figure 6. (a) XRD patterns for pristine fabric, PANI-3-coated fabric and fabric-supported PANI/Co-Ni-30 composites; potentiodynamic polarization curves of fabric-supported PANI/Co-Ni-30 and Co-Ni-30 composites in (b) 3.5 wt% NaCl, (c) 10 wt% HCl and (d) 10 wt% NaOH solutions. 3.2 Stability of PANI/Co-Ni Coatings in Ultrasonic and Acid/Alkali/Salt Aqueous 20


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Environments To become commercially viable, shielding fabrics must overcome the barrier of dismal reliability in practical applications. In our present work, ultrasonic treatments were conducted to evaluate the adhesion of layered PANI/Co-Ni coatings to the fabric platform, and the results are shown in Figure 4 (e-f). Herein, ultrasonic treatments were carried out according to the guidelines described previously.37 It was revealed that the ultrasonic could shake away some redundant Co-Ni aggregations over the periphery alloy coating, but the inner Co-Ni coating still strongly adhered to the PANI layer. The satisfied adhesion performance was associated with both mercerization treatment and PANI attachment: i) mercerization produced some microscopic holes/cracks on the fabric surface, through which aniline monomers diffused into the fibers and then polymerized; as a result, the mechanical adherence of PANI coating to fabric substrate increased; meanwhile, the number of hydrogen bonding between fabric substrate and PANI increased as well; ii) attachment of PANI created rough topographies on the surfaces, through which PANI interlocked with Co-Ni coating and generated a stronger fabric/coatings interface; in addition to the mechanical anchoring, a more powerful bonding mechanism was achieved by the Co-N or Ni-N configuration between PANI and Co-Ni coating. As shown in Figure 4 (g-h), FE-SEM images regarding the control sample revealed signs of structural delamination in ultrasonic environment, as substantial cracks were observed. The result validated the importance of PANI in the preparation of high-performance EMI shielding fabric. Indeed, PANI served not only as the metal-binding sites for Co (0) NPs but also as an adhesion layer between the fabrics and the Co-Ni coating. The adhesion of coatings (layered PANI/Co-Ni coatings and Co-Ni alloy coating) to the fabric platforms was further evaluated by 21



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Scotch®-tape tests (Figure S6, see the supporting information) as reported previously37. It can be seen that there were no megascopic scraps on the removed tape after peeling, which indicated that the layered PANI/Co-Ni coatings were firmly adhered to the fabric surfaces. Comparatively, plentiful scraps were found on the removed tape regarding Co-Ni-30-coated fabric sample. The results of Scotch®-tape tests were in well agreement with that of ultrasonic washing treatment. Apart from the adhesion of coatings, another key for high reliability is the corrosion resistance of coatings in acid/alkali/salt aqueous environment. To investigate the corrosion resistance, potentiodynamic polarization curves (Figure 6(b-d)) were measured by immersing the fabric samples into 3.5 wt% NaCl, 10 wt% HCl and 10 wt% NaOH, respectively. Corrosion current density (Icorr) and corrosion potential (Ecorr) were documented to quantify the anti-corrosion performance, as summarized in Table 1. Generally, metallic coatings with low Icorr and high Ecorr simultaneously will exhibit a slow corrosion tendency. It appears that no matter the type of electrolyte solution, the fabric-supported PANI/Co-Ni composites always show superior corrosion resistance when compared with their corresponding control groups. The result further validated the importance of embedded PANI in high-performance EMI shielding fabric. The improvement in corrosion resistance might be explained as follows: embedded PANI layer induced formation of a compact Co-Ni coating which had relatively low grain size (as validated by XRD analysis); however, in general, grain refinement is beneficial for the improvement in anti-corrosion performance.38-40 Additionally, a comparison of anti-corrosion properties between the proposed PANI/Co-Ni-coated fabric and other metal-coated materials was represented in Table S3 (see the supporting information). It can be seen that the proposed EMI shielding fabrics had comparable to or even superior 22


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anti-corrosion properties than other reported metal-coated materials. Overall, the fabric-supported PANI/Co-Ni composites obtained here were suitable for long-term practical usage. Table 1 Corrosion current density (Icorr) and corrosion potential (Ecorr) of fabric-supported PANI/Co-Ni-30 and Co-Ni-30 composites. Icorr (A· cm-2) Sample

Ecorr (mV)

3.5 wt%

10 wt%

10 wt%

3.5 wt%

10 wt%

10 wt%

NaCl

HCl

NaOH

NaCl

HCl

NaOH

1.272×10-5

7.947×10-4

1.287×10-5

-333

-347

-283

3.627×10-5

1.800×10-3

5.511×10-5

-423

-322

-695

fabric-supported PANI/Co-Ni-30 fabric-supported Co-Ni-30 3.3 Synergistic Effect in PANI-Co-Ni Trinary-component System Multi-layers of PANI coatings were fabricated by repeating the in situ polymerization cycle to investigate the relationship between the thickness and the SER/SEA. As shown in Figure 7 (a), PANI-coated fabric samples with different thicknesses demonstrated very similar SER, which could be explained by the fact that SER is free of influence by material thickness and mainly dependent on the impedance match at the interfaces. The relationship is given as: SER(dB)＝20log |

valid)

( )

| ≈ 20log| |dB (for good conductor, the inequality 2” Enhancement Effect in PANI-Co-Ni Trinary-component System Obviously, the fabric-supported PANI/Co-Ni composites inherited EMI shielding capabilities from both PANI and Co-Ni components. As shown in Figure 9 (above), the PANI/Co-Ni-30-coated fabrics had higher EMI SE than either the PANI-coated fabric or the Co-Ni-coated fabric. The total experimental SE was in the range of 33.95-46.22 dB within the X-band frequency, indicating a capability of >99.9% EM-wave attenuation. As per document no. FTTS-FA-03 (Table S4, see the supporting information), the resultant EMI shielding fabric could be classified as attenuation levels of “AAAAA” for general civil use (such as maternity 27



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dress, apron, consumptive electronic products, and communication related products). Furthermore, we discovered a significant EMI SE enhancement phenomenon (so called “1+1>2” phenomenon in our present study) in layered PANI/Co-Ni coatings because the EMI SE of the PANI/Co-Ni-30-coated fabric was higher than the sum of their single peers. As shown in Figure 9 (below), the enhanced EMI SE showed a frequency-dependent change throughout the X-band region, and the maximum was 12.93 dB. Herein, the enhanced SE value was calculated by an equation of enhanced SE= SE3-(SE1+SE2), where SE1, SE2 and SE3 were the SE values of PANI-3-coated, Co-Ni-30-coated and PANI/Co-Ni-30-coated fabrics, respectively. Inspired by the analysis results in the surface physico-chemical characterization, the EMI SE “1+1>2” enhancement phenomenon can be explained in three aspects: i) As illustrated in Figure 10 (above), the loading of PANI created a surface with rough topography, which was comprised of two regions, namely, a flat region and a sunken region. In the electroless deposition process, Co-Ni NPs embedded into the sunken region and thereby increased the conductive paths inside the interphase; this provided a high-efficiency conductive network for converting EM energy into leaking current or heat. (ii) Metallic NPs are usually encapsulated by steric stabilizers (herein, citrate ions encapsulated the Co (0) NPs in the activation stage and tartrate ions encapsulated the Ni, Co and Ni-Co NPs in the electroless deposition stage) and then become negatively charged; when these negatively charged NPs embedded into PANI, they served as dopants and improved the conductivity, which in turn led to enhanced absorption of the EM radiation. Of note, there was slight coulomb interaction between the positively charged PANI chains and negatively charged metallic NPs, which would certainly enhanced the interfacial bonding. iii) The PANI coating had the ability to 28


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improve the dispersion of Co (0) NPs on the surface as well as strengthen their stability (such as preventing them from agglomerating and keeping them independent); additionally, the loose-structured PANI had large surface areas, which helped to form more active sites; all of these benefits resulted in a plated Co-Ni coating with compact structure (as validated by XRD analysis); a compact metallic coating tends to have high EMI SE due to its high electric conductivity. Further analysis revealed that EMI SE enhancement effect was only obvious at the low Co-Ni loading (3.99 mg cm-2) and weakened as the Co-Ni loading increased (Figure (S7-S8), see the supporting information). As illustrated in Figure 10 (below), interphase and inner PANI coating played a limited role in the attenuation of EMI radiation since the outer Co-Ni-60 or Co-Ni-90 coatings were thick enough to shield most of the incident energy. If we could take advantage of this EMI SE enhancement effect to the full extent by controlling the metal content, high-performance EMI shielding fabric with ultra-light weight would certainly be obtained. Generally, flexibility and light weight account for a considerable proportion in designing and evaluating the shielding fabrics. Hence, these findings may provide guidelines for preparing high-efficiency EMI shielding fabrics that have ultra-low weight. Fundamental studies are underway to understand the underlying mechanism of the EMI SE enhancement effect in layered PANI/Co-Ni coatings. Future research will be directed to investigate the universality of this EMI SE enhancement phenomenon in other layered conductive polymer/metal combinations.

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Figure 9. Frequency dependence of EMI SE of PANI-3-coated, Co-Ni-30-coated and PANI/Co-Ni-30-coated fabrics (above); quantitative EMI SE enhancement effect within the X-band frequency (below).

Figure 10. Schematic representation of presumed conduction paths (above) in PANI/Co-Ni 30


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interphase layer, and EMI shielding mechanism (below) displaying interaction of the shielding fabrics (PANI/Co-Ni-30, PANI/Co-Ni-60 and PANI/Co-Ni-90) with incident plane-wave radiations (of note, the dimension is not drawn to scale).

4. CONCLUSIONS The present work establishes a promising methodology to prepare flexible and lightweight X-band EMI shielding fabric based on consecutive steps of in situ polymerization and electroless plating. As a first attempt to fabricate X-band EMI shielding fabric with PANI-Co-Ni trinary-component system, its significance lies on the ingenious combination of wave-reflection dominant materials (conductive polymer) and wave-absorption dominant materials (metal). The fabric-supported PANI/Co-Ni composites inherited EMI shielding capabilities from single PANI and Co-Ni components and displayed higher EMI SE than the sum of their single peers (“1+1>2” phenomenon). The EMI SE “1+1>2” enhancement effect can be attributed to the interphase chemical as well as physical interactions. By carefully investigating the mechanisms involved, we disclosed that the EMI SE enhancement effect was merely observed when the metal content is relatively low. In addition, the significant improvement in reliability and shielding efficiency guarantee that the resultant shielding fabric meets requirements for commercial application. These findings may provide guidelines for preparing high-performance EMI shielding fabrics that simultaneously have ultra-low weight, excellent SE and favorable reliability. Future studies will be directed to investigate the universality of this EMI SE enhancement phenomenon in other layered conductive polymer/metal combinations. 31



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ASSOCIATED CONTENT Supporting Information The supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Synthesis routes of fabric-supported Co-Ni composites; Effects of repeatedly folding on properties (electric/magnetic and EMI shielding) of the proposed EMI shielding fabric; XPS wide-scan spectrum of PANI/Co-Ni-30 coated fabric; XRD pattern for fabric-supported Co-Ni composites; Schematic images of the Scotch®-tape test for PANI/Co-Ni-30-coated and Co-Ni-30-coated fabrics. Frequency dependence of total EMI SE of PANI-3-coated, Co-Ni-60-coated and PANI/Co-Ni-60-coated fabrics; Frequency dependence of total EMI SE of PANI-3-coated, Co-Ni-90-coated and PANI/Co-Ni-90-coated fabrics; Weight, loading and surface resistance of the fabric samples in each step; XPS atomic ratio (%) of fabric samples; Comparison of the anti-corrosion properties of different metal-coated materials; Classification of EM shielding textiles

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 61371019), the Shanghai Civil-military Integration Project (No. 140217), the Shanghai Technical Trade Solutions Project (16TBT011) and the research grant (No. 16DZ2260600) from Science and Technology Commission of Shanghai Municipality.

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Enhanced X-band Electromagnetic Interference Shielding

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