Collagen Fiber Membrane as an Absorptive Substrate To Coat with

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Collagen fiber membrane as absorptive substrate to coat with carbon nanotubes-encapsulated metal nanoparticles for lightweight, wearable and absorption-dominated shielding membrane Chang Liu, Xiaoxia Ye, Xiaoling Wang, Xuepin Liao, Xin Huang, and Bi Shi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01930 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 16, 2017

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Industrial & Engineering Chemistry Research

Collagen fiber membrane as absorptive substrate to coat with carbon nanotubes-encapsulated metal nanoparticles

for

lightweight,

wearable

and

absorption-dominated shielding membrane Chang Liu,† Xiaoxia Ye,‡ Xiaoling Wang,‡ Xuepin Liao,‡ Xin Huang*,†,‡ and Bi Shi‡ †

Department of Biomass Chemistry and Engineering, Sichuan University, Chengdu 610065,

China; ‡

National Engineering Laboratory for Clean Technology of Leather Manufacture, Sichuan

University, Chengdu 610065, China.

ACS Paragon Plus Environment

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ABSTRACT: A lightweight, wearable and high-performance electromagnetic interference (EMI) shielding membrane was prepared by using collagen fiber membrane (CFM) as microwave absorptive substrate and multiple wall carbon nanotubes encapsulated metal nanoparticles (MNPs@MWCNTs) as efficient microwave absorbers coated on CFM. CFM is able to consume microwaves via dielectric loss besides the role as supporting matrix. Encapsulating of metal nanoparticles (MNPs) enhances the dissipation capability of MWCNTs and also improves the stability of encapsulated MNPs. A cooperative shielding mechanism between CFM and MNPs@MWCNTs is responsible for the absorption-dominated shielding performance. The asprepared MNPs@MWCNTs/CFM composite membrane exhibited a high shielding effectiveness (SE) of 30-60 dB with 4.0 wt% of MNPs@MWCNTs in a wide frequency range of 0.5-12.0 GHz, and the specific SE was high up to 162 dB cm-3 g-1. The MNPs@MWCNTs/CFM membrane also features to lightweight, good flexibility and wearability. This work provides a novel strategy for designing lightweight and high-performance shielding membrane.


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1. INTRODUCTION Electromagnetic interference (EMI) has been a newly emerging environmental pollution in recent decades.1-3 Current strategy to address this issue relies on the use of conductive polymer composites (CPCs) that dissipate microwaves via reflection and/or dielectric loss to achieve EMI shielding. CPCs are generally prepared by incorporating conductive metallic/carbonous fillers with polymer substrates. However, the metallic fillers-based CPCs are easily oxided, and their application is restricted by their high mass density and unsatisfactory flexibility.4-6 Consequently, great efforts have been dedicated to developing carbonous fillers-based CPCs owing to their distinct advantages of lightweight and good flexibility. In particular, CPCs consisted of carbon nanotubes (CNTs) have attracted considerable attention due to their high aspect ratio with remarkable mechanical and electronic properties.7-10 It has been demonstrated that the high aspect ratio of CNTs can help to promote the dissipation to EMI, thus showing substantially enhanced shielding effectiveness (SE) as compared with conventional carbonous fillers.11 Various CPCs have been developed by incorporating CNTs with different polymer substrates, including polyurethane (PU), polystyrene (PS), epoxy, etc.4-16 Although progressive achievements have been made in developing CNTs-based CPCs, the shielding performances of these CPCs still need further improvements since the conventionally used polymers simply serve as supporting substrates without the capability to dissipate microwaves. In spite that the SE could in principle be improved by increasing the loading of CNTs, this approach usually leads to serious agglomeration of CNTs in CPCs, inevitably resulting in a marginal enhancement on SE. For example, Kim et al.12 incorporated 40 wt% of multiple wall carbon nanotubes (MWCNTs) with polymethyl methacrylate (PMMA) for preparing CPCs, while the obtained highest SE was merely ~27 dB. Meanwhile, a high loading


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of conductive fillers inevitably leads to a low specific SE (16~75 dB cm-3 g-1), which damages the good processability of CPCs, leading to the nanocomposites brittle.6,13-20 More recently, the decoration of CNTs using metal nanoparticles (NPs) has been carried out to enhance the EMI shielding performances of CNTs-based CPCs. Conductive metal NPs, such as Ag, Cu, Fe and Co,21-24 are decorated on the outer surface of CNTs so as to increase the charge carrier density for improving the SE.25 However, the decorated metal NPs still have the risk of oxidation without sufficient stability during the application process. Moreover, a high content of metal NPs decorated CNTs (e.g. 40 wt%) is required for obtaining high SE, resulting in a low specific SE.24 Narrow shielding bandwidth is another persisting problem encountered by CNTs-based CPCs. Previously reported CNTs-based CPCs are exclusively used for EMI shielding in the X band (8.0-12.0 GHz),8-16 and such constrained shielding bandwidth is mainly originated from the intrinsic electromagnetic characteristics of CNTs. Although increasing the thickness of CPCs may change and/or broaden the bandwidth,26 this strategy would inevitably damage the flexibility of CPCs and limit their practical applications. To date, it is still challenging to prepare CNTs-based CPCs with the features of lightweight, high SE and wide bandwidth. In our previous work, we have found that collagen fiber membrane (CFM) can be used as microwave absorptive substrate to coat with Cu@Ag nanoflakes for preparing lightweight and high-performance CPCs that can shield EMI through the reflectivity of Cu@Ag nanoflakes to microwaves and the dielectric loss of CFM to microwaves.27 Although the as-prepared shielding CPCs can be used for EMI shielding in 0.01-3.0 GHz and 8.0-12.0 GHz, with specific SE high up to 120 dB cm-3 g-1, they are reflection-dominated EMI shielding materials. To radically solve the EMI pollution, absorption-based shielding materials are more preferred. Considering the unique properties of CFM, we therefore hypothesized that, if high efficient CNTs-based


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conductive fillers could be prepared and stably coated on CFM, a lightweight, absorptiondominated shielding membrane would be facilely prepared. In the present investigation, we simultaneously realize lightweight, high SE and ultra-wide bandwidth of absorption-dominated shielding membrane by using CFM as microwave absorptive substrate and MWCNTs-encapsulated metal nanoparticles (MNPs@MWCNTs) as efficient microwave absorber. CFM, processed from sheep skin, contains abundant acidic (-COOH) and basic (-NH2) functional groups that can act as dipoles to dissipate microwaves.28-31 The dielectric loss tangent (tanδe) of CFM is ~0.1 in the frequency range of 0.5-3.0 GHz (as shown in Figure S1), which suggest that CFM is capable of consuming microwaves via dielectric loss. CFM also features to a hierarchical fibrous network formed by the assembly of collagen nanofibers (Figure S2). Such hierarchical structure brings a high diffusion reflectivity (Figure S3), which can enlarge the transmission route of microwaves with enhanced dissipation capacity. As confirmed in Figure S1, the SE of CFM is high up to ~20 dB in 0.5-3.0 GHz, which is distinct from conventional polymer substrate without dissipation capacity. Our previous investigations have shown that the encapsulation of metal nanoparticles (MNPs) can enhance π electron density on MWCNTs surface due to the spatial-confined effect.32 Hence, we encapsulated conductive MNPs, including CuNPs and AgNPs, inside the channel of MWCNTs in order to improve the performance of MWCNTs as microwave absorber. In this work, a lightweight, wearable and high-performance shielding membrane was prepared by coating MNPs@MWCNTs on CFM. The influences of encapsulated metals species and concentration on the mechanical property, electric conductivity, EMI shielding performance and stability of MNPs(x)@MWCNTs/CFM membrane were investigated. The essential role of CFM as microwave absorptive substrate was demonstrated in comparison with other conventional substrates (PMMA membrane and


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nonwoven fabrics membrane), and the EMI shielding mechanisms were discussed in detail. The as-prepared MNPs(x)@MWCNTs/CFM membrane exhibited a high SE of 30-60 dB with just 4.0 wt% of MNPs@MWCNTs in a wide frequency range of 0.5-12.0 GHz, and the specific SE was high up to 162 dB cm-3 g-1. 2. EXPERIMENTAL SECTION 2.1. Materials. MWCNTs (TNM3, purity > 95 wt%, outer diameter = 15-80 nm, inner diameter = 5-20 nm and length = 10-20 µm) were purchased from Chengdu Organic Chemicals Company Ltd., Chinese Academy of Sciences, Chengdu, China. End-opened MWCNTs was obtained by ball-milling (QM-3SP04 planetary ball mill, Nanjing university instrument plant, Nanjing, China) the MWCNTs in absolute ethyl alcohol at the speed of 3000 r/min for 5.0 h, followed by drying at 40 °C. Water-borne PMMA (35 wt% solid content) was purchased from Sichuan Dowell Science & Technology Incorporation, Chengdu, China. CuCl2·2H2O, AgNO3 and other chemicals were of analytic grade. CFM was provided by the National Engineering Laboratory for Clean Technology of Leather Manufacture, Chengdu, China, which was tanned by a commercialized chrome tanning method using sheep skin as the starting material. The thickness of CFM was fixed at 0.6 mm, while the area of CFM was varied from 1.0 cm2 to ~0.85 m2. Nonwoven fabrics (NF) membrane was supplied by Jiangyin Jin Gang Non-woven Co., Ltd, Jiangyin, china, which was simply interwovened by ~10 µm polypropylene fibers. Appropriate amount of Cu2+ (10 mg, 20 mg, 30 mg, 40 mg and 80 mg) and Ag+ (40 mg) were dissolved in 2.0 mL of deionized water, respectively, and used as precursor solutions. 2.2. Synthesis of MNPs(x)@MWCNTs. CuNPs(x)@MWCNTs was prepared according to our previous work with a slight modification.32 Above prepared precursor solutions of Cu2+ were slowly dropwise added in the end-opened MWCNTs (200 mg) under stirring. This process


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allows the introduction of precursor solutions into the channels of end-opened MWCNTs via capillary forces. The resultant intermediates were dried at room temperature, and further thermally treated in Ar/H2 (H2, 5%) atmosphere at 340 °C for 2.0 h. The resultant products were denoted as CuNPs(x)@MWCNTs (x is the loading amount of Cu, x = 5%, 9%, 13%, 17%, 29%). AgNPs(17%)@MWCNTs was prepared by a similar method adopted for the synthesis of CuNPs(x)@MWCNTs except that the reduction temperature was 200 °C. As control, CuNPs(17%)-MWCNTs

and

AgNPs(17%)-MWCNTs

were

prepared

by

conventional

impregnation method using pristine MWCNTs as the supporting matrix. 2.3. Preparation of MNPs(x)@MWCNTs/CFM membrane. PMMA (~3 mg cm-2) was first spray-coated on CFM and then dried at 40 °C to form a PMMA base on CFM surface. After that, absolute ethyl alcohol containing CuNPs(x)@MWCNTs (1%, w/w) were spray-coated on PMMA base to form a coating layer of microwave absorber. Subsequently, top-coating of PMMA (~2 mg cm-2) was carried out to cover the coating layer of CuNPs(x)@MWCNTs. By controlling

the

coating

amount

of

CuNPs(x)@MWCNTs,

a

series

of

CuNPs(x)@MWCNTs/CFM membranes were prepared, where the coated amount of CuNPs(x)@MWCNTs was 0.67 wt%, 1.3 wt%, 2.0 wt%, 2.7 wt%, 3.3 wt%, and 4.0 wt%, respectively. AgNPs(17%)@MWCNTs/CFM membrane was prepared by above similar procedures, and the coating amount of AgNPs(17%)@MWCNTs was fixed at 4.0 wt%. For comparison, CuNPs(17%)-MWCNTs/CFM and AgNPs(17%)-MWCNTs/CFM membranes were also

prepared

by

the

same

procedures

adopted

for

the

synthesis

of

CuNPs(17%)@MWCNTs/CFM membrane except that 4.0 wt% of CuNPs(17%)-MWCNTs or AgNPs(17%)-MWCNTs

was

CuNPs(17%)@MWCNTs/PMMA,

coated

on

CFM.

CuNPs(17%)@MWCNTs/NF,

AgNPs(17%)@MWCNTs/NF


and

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AgNPs(17%)@MWCNTs/PMMA membranes were fabricated by spray-coating 4.0 wt% of CuNPs(17%)@MWCNTs or AgNPs(17%)@MWCNTs on NF or PMMA membranes, and these membranes still employed base coating and top-coating of PMMA to protect the coated absorber. For scaled-up synthesis, a CFM with the size of ~0.85 m2 was used as the substrate, and the coating amount of CuNPs(17%)@MWCNTs was fixed at 4.0 wt%. The obtained large size CuNPs(17%)@MWCNTs/CFM membrane was further tailored into wearable EMI shielding clothes. 2.4. Characterizations. The microstructure observations were carried out by using the field emission scanning electron microscopy (FESEM, Cambridge CamScanCS3400) and field emission transmission electron microscopy (FETEM, TECNAI G2 F20) operating at 200 kV. XRD patterns of samples were recorded by using a Cu Kα wide-angle X-ray diffractor (XRD, PANalytical, Holland). The X-Ray photoelectron spectrum (XPS, Shimadzu ESCA-850, Japan) of the MNPs@MWCNTs were recorded by employing Mg-Kα X-rays (hν 1/4 1253.6 eV) and a pass energy of 31.5 eV. The near-infrared diffuse reflectivity (NIR DR) spectra were recorded by a UV-3600 UV-Vis-NIR spectrophotometer (Shimadzu, Japan) equipped with an integrating sphere attachment (LISR-3100, Shimadzu, Japan) using BaSO4 as the standard sample. Raman spectra were obtained with a Renishaw inVia plus laser Raman spectrometer with a laser wavelength of 532 nm. The resistivity measurements of composites were carried out by a ST2253 four-probe meter using ammeter and voltmeter (Jingge Electronic Company, Suzhou, China). The tearing strength and tensile strength of CuNPs(17%)@MWCNTs/CFM membrane were measured by a universal testing machine (AI-7000 S, Gotech Testing Machines Co., Ltd., Taiwan, China). Bending tests were carried out by using a 2226 Bally Flexometer (Jinan Xing Hua Instruments Co., Ltd., Shandong, China), and the tested samples were in the size of 70.0


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mm × 45.0 mm. Weathering test was conducted by a weather resistance tester (QUV/SE, Gotech Testing Machines Co., Ltd., Dongguan, China). The samples were tested under a 2UV Twin Models using two different wavelengths (254 nm and 365 nm) at 50 °C for 12 h with the 0.68 watt UV lamps. 2.5. Electromagnetic measurements. A two-port Agilent N5230A vector network analyzer (VNA, Agilent N5230A, USA) was used to measure the scattering parameters (S-parameters) of samples in the frequency region of 0.5-12.0 GHz, the S-parameters of each sample were recorded and applied to calculate the electromagnetic shielding effectiveness and complex permittivity. For the electromagnetic shielding effectiveness tests, all the tested samples were tailored into a tablet-shaped mold (φ = 13.0 mm). For the complex permittivity measurements, all the tested samples were shaped into a coaxial circle with 3.0 mm of inner diameter and 7.0 mm of outer diameter. 3. RESULTS AND DISCUSSION 3.1. Material fabrication and characterization. We first encapsulated CuNPs inside the channels of end-opened MWCNTs to prepare a series of CuNPs(x)@MWCNTs with increasing Cu loading (x = 5%, 9%, 13%, 17%, 29%). Figure 1a and Figure S4 show the TEM images of CuNPs(x)@MWCNTs (x = 5%, 9%, 13%, 17%), where CuNPs are discretely encapsulated inside the channels of MWCNTs. However, an excessive high Cu loading (29%) leads to the formation of CuNPs on the outer surface of MWCNTs, which are easily oxidized (Figure S5). High resolution TEM (HRTEM) image in Figure 1b shows that the encapsulated CuNPs fully occupy the channels of MWCNTs, which is beneficial for electron donation from concave inner to the convex outer surface of MWCNTs. High crystallization of encapsulated CuNPs was confirmed by TEM-SADPs (inset in Figure 1b) and X-ray diffraction (XRD) patterns (Figure 1c).


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As shown in Figure 1d, the C 1s peak of end-opened MWCNTs shifts to a lower binding energy field after the encapsulation of CuNPs, and the shift is proportional to the increased amount of encapsulated CuNPs, from 284.8 eV to 284.6 eV. Accordingly, the Cu 2p3/2 peak of CuNPs(x)@MWCNTs exhibits a gradual increase of binding energy with the increased amount of encapsulated CuNPs, from 933.3 eV to 934.7 eV (Figure 1e). These results manifest that electrons are donated from encapsulated CuNPs to MWCNTs.

Figure 1. (a) TEM, (b) HRTEM images and TEM-selected area diffraction pattern (TEMSADPs, the inset in b) of CuNP(17%)MWCNTs; (c) X-ray diffraction (XRD) pattern of CuNP(17%)MWCNTs; (d) C 1s XPS spectra of CuNP(x)MWCNTs; (e) Cu 2p3/2 XPS spectra of CuNP(x)MWCNTs; (f) conductivity of CuNP(17%)MWCNTs with varied coated amount, and the corresponding log value of the conductivity with the threshold of CuNP(17%)MWCNTs, φc = 0.67 wt% (the inset in f).


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The CuNPs@MWCNTs/CFM membrane was facilely prepared by spray coating CuNPs(x)@MWCNTs on CFM surface. To enhance the mechanical stability, the coating of CuNPs(x)@MWCNTs was sandwiched between two PMMA layers. Figure S6 is the schematic illustration showing the preparation procedures of CuNPs@MWCNTs/CFM membrane. The coating thickness of CuNPs(17%)@MWCNTs is tunable from 20 to 130 µm by adjusting the coating amount of CuNPs(17%)@MWCNTs, from 0.67 wt% to 4.0 wt%. The conductivity of CuNPs(17%)@MWCNTs coating layer is increased from 1.8 S m-1 to 1070 S m-1 along with the increase of coated CuNPs(17%)@MWCNTs (Figure 1f), and the typical percolation (φc) was determined to be 0.67 wt% (the inset in Figure 1f). Figure S7a shows the typical cross-section view of CuNPs(17%)@MWCNTs/CFM membrane, where the coating of CuNPs(17%)@MWCNTs is protected between two PMMA layers, and these sandwich-structured layers are supported on the hierarchically fibrous structured CFM. The CuNPs(17%)@MWCNTs/CFM membrane is characterized by a low mass density of 0.375 g cm1

with 4.0 wt% of coated CuNPs(17%)@MWCNTs, which can be supported on a grass leaf

(Figure S7b) due to its lightweight feature. The CuNPs(17%)@MWCNTs/CFM membrane is also robust enough to be folded into different shapes (Figure S7c and Figure S8). The specific tear strength and tensile strength of CuNPs(17%)@MWCNTs/CFM membrane are as high as 95.7 N mm-1 per g cm-3 and 68.8 MPa per g cm-3, providing excellent processability. Figure S7d shows the photo of large-size CuNPs(17%)MWCNTs/CFM membrane (~0.85 m2), which can be tailored into shielding clothes (Figure S7e and Figure S9). As shown in Figure 2a, the SE of MWCNTs/CFM membrane is 30-43 dB in 0.5-3.0 GHz, and MWCNTs(without ball-milling)/CFM shows similar shielding performances. As for the CuNPs(x)@MWCNTs/CFM membrane, its SE is substantially higher than that of


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MWCNTs/CFM membrane, and the enhancement on SE is proportional to the amount of encapsulated CuNPs. However, the CuNPs(29%)@MWCNTs/CFM membrane exhibits similar shielding performances to the CuNPs(17%)@MWCNTs/CFM membrane (Figure 2a). TEM analysis (Figure S5) has shown that a considerable amount of CuNPs is formed on the outer surface of CuNPs(29%)@MWCNTs. These exposed CuNPs are easily oxided, showing a limited contribution to SE. Therefore, the CuNPs(17%)@MWCNTs is employed as the microwave absorber in the following investigations. The SE of CuNPs(17%)@MWCNTs/CFM membrane is influenced by the coating amount of CuNPs(17%)@MWCNTs (Figure S10). In order to keep lightweight feature, the coating amount of CuNPs(17%)@MWCNTs was fixed at 4.0 wt%.

Figure

2.

(a)

SEs

of

MWCNTs/CFM,

MWCNTs(without

ball-milling)/CFM

and

CuNPs(x)@MWCNTs/CFM membranes (x = 5%, 9%, 13%, 17%, 29%); (b) SET, SEA and SER of CuNPs(17%)@MWCNTs/CFM membrane in the frequency region of 0.5-3.0 GHz; (c) SEs of


12

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CuNPs(17%)@MWCNTs/CFM,

CuNPs(17%)@MWCNTs/NF

and

CuNPs(17%)@MWCNTs/PMMA membranes in the frequency region of 0.5-3.0 GHz; (d) SEs of


CuNPs(17%)@MWCNTs/NF

and

CuNPs(17%)@MWCNTs/PMMA membranes in the frequency region of 3.0-6.0 GHz.

3.2. Absorption-dominated shielding performances. In general, the total SE (SET) is the sum of reflection-based SE (SER), absorption-based SE (SEA) and multiple reflection-based SE (SEM), while SEM is normally neglected (when SET > 15 dB),33 since most of the incident microwaves will be absorbed within the shielding material. We calculated the SET, SEA and SER of CuNPs(17%)@MWCNTs/CFM membrane in 0.5-3.0 GHz. As shown in Figure 2b, SER has a limited contribution to SET, while SEA has substantially higher contribution to SET. We also investigated the SET, SEA and SER of CuNPs(17%)@MWCNTs/CFM membrane with a varied coated amount of CuNPs(17%)@MWCNTs (0.67 wt% and 2.0 wt%) (Figure S11). SEA still contributes the most to SET, and it is noted that the increase of coated CuNPs(17%)@MWCNTs has more profound influences on the enhancement on SEA. These results manifest that the CuNPs(17%)@MWCNTs/CFM membrane is a typical absorption-dominated shielding material. 3.3. Cooperative shielding mechanism. We also coated CuNPs(17%)@MWCNTs on the substrates of NF and PMMA membranes. As shown in Figure S3, NF and PMMA membranes have much lower diffusion reflectivity due to the absence of hierarchical fibrous network (Figure S12,S13), and they almost have no shielding ability (Figure S14). As shown in Figure 2c, the SEs of CuNPs(17%)@MWCNTs/PMMA and CuNPs(17%)@MWCNTs/NF membranes are far below those of CuNPs(17%)@MWCNTs/CFM membrane in 0.5-3.0 GHz. Similarly, the CuNPs(17%)@MWCNTs/CFM membrane still shows advantage of SE in 3.0-6.0 GHz (Figure


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2d). These results suggest that a cooperative mechanism exists between CFM and CuNPs(17%)@MWCNTs. To confirm this mechanism, we investigated the SEs of CFM and the coating layer of CuNPs(17%)@MWCNTs, separately, and compared them with that of CuNPs(17%)@MWCNTs/CFM membrane.

Figure 3. SEs of CFM, CuNPs(17%)@MWCNTs and CuNPs(17%)@MWCNTs/CFM membrane, and the summed SEs of CFM and CuNPs(17%)@MWCNTs in the frequency range of (a) 0.5-3.0 GHz and (b) 3.0-6.0 GHz; (c) SEs of CuNPs(17%)@MWCNTs/CFM and


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CuNPs(17%)-MWCNTs/CFM membranes in the frequency range of 0.5-6.0 GHz; (d) the real (ε') and (e) imaginary (ε'') parts of the complex permittivity of CuNPs(17%)@MWCNTs/CFM membrane; (f) dielectric loss (tanδe) of CuNPs(17%)@MWCNTs/CFM membrane in the frequency range of 0.5-6.0 GHz.

As shown in Figure 3a, the SE of CFM is kept at 5.5-20 dB in 0.5-3.0 GHz, while the SE of CuNPs(17%)@MWCNTs coating layer is in the range of 27-37 dB. We summed the SEs of CFM and CuNPs(17%)@MWCNTs together, and found that the resultant value is much lower than that of CuNPs(17%)@MWCNTs/CFM membrane. Obviously, the SE advantage of CuNPs(17%)@MWCNTs/CFM membrane becomes more obvious along with the increase of frequency from 0.5 to 3.0 GHz, that is, the coating layer of CuNPs(17%)@MWCNTs enables CFM to dissipate more higher-frequency microwaves. As shown in Figure 3b, CFM lost its shielding ability in 3.0-6.0 GHz. For the CuNPs(17%)@MWCNTs coating layer, its SE is ranged from 27 dB to 13 dB with the frequency increase from 3.0 to 6.0 GHz. When summed the SEs of CFM and CuNPs(17%)@MWCNTs together, the resultant value is never beyond ~67% of the SE obtained by the CuNPs(17%)@MWCNTs/CFM membrane. It is apparent that for the CuNPs(17%)@MWCNTs/CFM membrane, more than 33% of the SE is originated from the cooperative shielding mechanism between the coating layer of CuNPs(17%)@MWCNTs and the substrate of CFM. The enhanced dissipation capacity of CuNPs(17%)@MWCNTs to microwaves can be explained by the quantum optics theory. According to the quantum optics theory,34,35 the microwaves can be considered as photons. Hence, the interactions between microwaves and CuNPs(17%)@MWCNTs can be regarded as collisions occurred between photons and the free π


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electrons on the coating layer of CuNPs(17%)@MWCNTs surface. In this photon-electron system, the collisions obey the laws of momentum conservation and energy conservation, which can be expressed as follows: h/λ0 = h/λ0ʹ + meve

Eq. (1)

hν0 = hν0ʹ + meve2

Eq. (2)

where h is the Planck constant (h = 6.626176 ± 0.000036 × 10-34 J·s), λ0 and λ0ʹ are the wavelength of incident microwave before and after the collision, me is the mass of electron, ve is the velocity of electron obtained after the collision, ν0 and ν0ʹ are the frequency of photos before and after the collision. From above equations, it is deduced that after the collision, energy Ee (meve2) and momentum Pe (meve) are transferred from photons to the free π electrons of CuNPs(17%)@MWCNTs. Due to the gained Ee and Pe, the moving π electrons induce currents, and finally decay as heat. Based on this assumption, it is understandable that the encapsulation of CuNPs substantially increases the π electron density on the outer surface of MWCNTs, which allows MWCNTs to have more free electrons to collide with microwaves, showing stronger dissipation with higher SE. For comparison, CuNPs were directly supported on MWCNTs (Figure S15), and the resultant CuNPs(17%)-MWCNTs was coated on CFM to prepare CuNPs(17%)-MWCNTs/CFM

membrane.

As

shown

in

Figure

3c,

the

SE

of

CuNPs(17%)@MWCNTs/CFM membrane is always higher than that of CuNPs(17%)MWCNTs/CFM membrane in 0.5-6.0 GHz. Our previous DFT calculations have shown that the encapsulation of metal NPs can uniformly enhance the π electron density on the entire surface of MWCNTs through electron donation.32 But for the CuNPs(17%)-MWCNTs, a localized enrichment of surface electron density is engaged, which means that the CuNPs(17%)-MWCNTs would have less free π electrons to collide with microwaves, thus showing a lower SE.


16

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We ground the CuNPs(17%)@MWCNTs/CFM membrane into powder and compared their complex permittivities. As shown in Figure 3d, the CuNPs(17%)@MWCNTs/CFM membrane exhibits a lower ε' value. Normally, a lower ε' value corresponds to a lower reflection coefficient, which is beneficial for the permeability of microwaves. Hence, microwaves are much easier to transmit into the CuNPs(17%)@MWCNTs/CFM membrane. The imaginary part (ε'') of complex permittivity of the ground powder is much lower than that of the original membrane (Figure 3e), especially in 0.5-3.0 GHz. The ε'' value represents the ability of absorptive materials to dissipate the microwaves via electric loss.36-38 Apparently, the CuNPs(17%)@MWCNTs/CFM membrane exhibits a larger dielectric loss to microwaves in its membrane form. The dielectric loss tangent (tanδe = ε''/ε') is another critical parameter to evaluate the absorption capability to microwaves. The tanδe value of CuNPs(17%)@MWCNTs/CFM membrane is substantially higher than that of the powder counterpart (Figure 3f), thus indicating a much stronger ability to dissipate the microwaves. These results manifest that a cooperative shielding mechanism indeed exists between CFM and the coating layer of CuNPs@MWCNTs.

Figure

4.

Schematic

illustration

showing

the

proposed

shielding

mechanism

of

CuNPs(17%)@MWCNTs/CFM membrane. We proposed a shielding mechanism of CuNPs(17%)@MWCNTs/CFM membrane in Figure


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4.

When

the

incident

microwaves

(Mi)

transmit

into

the

Page 18 of 31

coating

layer

of

CuNPs(17%)@MWCNTs, the CuNPs(17%)@MWCNTs efficiently dissipates the microwaves via dielectric loss along with a limited reflection. Once the residual microwaves further pass through the CFM, the acidic and basic functional groups of CFM act as dipoles to successively dissipate the microwaves via dielectric loss.39-41 During this process, the hierarchical fibrous network of CFM could substantially enhance the dissipation to microwaves due to the enlarged transmission routes by the high diffusion reflectivity. In this way, the intensity of transmitted microwaves (Mt) is substantially decreased, showing enhanced absorption-based shielding performance.

Figure 5. (a) SEs of CuNPs(17%)@MWCNTs/CFM and CuNPs(17%)-MWCNTs/CFM membranes

before

and

after


the

weathering

tests;

(b)

XRD

CuNPs(17%)-MWCNTs/CFM


patterns

of and

18

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CuNPs(29%)@MWCNTs/CFM

membranes

after

the

weathering

tests;

(c)

SEs

of

CuNPs(17%)@MWCNTs/CFM membrane before and after bending 10,000 times; (d) the photo of CuNPs(17%)@MWCNTs/CFM membrane after bending 10,000 times.

3.4. High stability and flexibility of the shielding membranes. As shown in Figure 5a, the CuNPs(17%)@MWCNTs/CFM membrane exhibits a pretty good stability, showing a limited loss of SE after the weathering test. In contrast, an obvious loss of SE was observed on the CuNPs(17%)-MWCNTs/CFM membrane after the weathering test. Based on XRD analysis (Figure 5b), oxidation of CuNPs is the main reason responsible for the SE loss of CuNPs(17%)MWCNTs/CFM membrane. For the CuNPs(17%)@MWCNTs/CFM, the CuNPs are protected inside the MWCNTs during the weather test, which effectively prevents the loss of SE caused by oxidation of CuNPs. The mechanical stability of CuNPs(17%)@MWCNTs/CFM membrane was also evaluated by bending 10,000 times, which shows some loss of SE (Figure 5c). For the bent area (Figure 5d), and the fibrous structure of CFM is well preserved (Figure S21), while the repeative bending still leads to some fractures of PMMA coating layer as well as deformation of CuNPs coating, which results in a more obvious decrease of SE as compared with that of weathering test. 3.5. Further enhancement on shielding performances. AgNPs were encapsulated inside the channels of MWCNTs to further enhance the dissipation ability of MNPs@MWCNTs/CFM membrane since AgNPs donate more electrons to MWCNTs with significantly enhanced π electron density, which is beneficial for the collision with microwaves (photons), thus inducing currents to dissipate more electromagnetic energy. TEM (Figure 6a) and HRTEM (Figure 6b) confirm the successful encapsulation of AgNPs inside the channels of MWCNTs. TEM-SADPs


19


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Page 20 of 31

(the inset in Figure 6a) and XRD pattern (Figure S16) demonstrate the existence of metallic AgNPs. Based on XPS analysis (Figure 6c), an obvious shift to the lower binding energy field is observed in the C 1s peak of end-opened MWCNTs after the encapsulation of AgNPs, manifesting the electron transfer from AgNPs to MWCNTs. Raman spectra analysis (Figure S17) also indicates the electron donation from AgNPs to MWCNTs due to that the G band of end-opened MWCNTs has a red shift after the encapsulation of AgNPs.

Figure 6. (a) TEM image, (b) TEM-SADPs (the inset in a) and HRTEM image of


20

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AgNPs(17%)@MWCNTs; (c) C 1s XPS spectra of AgNPs(17%)@MWCNTs and end-opened MWCNTs; (d) SEs of AgNPs(17%)@MWCNTs/CFM and CuNPs(17%)@MWCNTs/CFM membranes in the frequency region of 0.5-12.0 GHz; (e) SEs of AgNPs(17%)@MWCNTs/CFM, AgNPs(17%)-MWCNTs/CFM,


and

AgNPs(17%)@MWCNTs/PMMA membranes in the frequency region of 0.5-12.0 GHz; (f) SET, SEA and SER of AgNPs(17%)@MWCNTs/CFM membranes in the frequency region of 0.5-12.0 GHz; (g) SEs of AgNPs(17%)@MWCNTs/CFM membrane before and after the weathering tests; (h)

SEs

of

AgNPs(17%)@MWCNTs/CFM

membrane,

CuNPs(17%)@MWCNTs/CFM

membrane and other already reported carbonous EMI shielding materials in 8.0-12.0 GHz; (i) Comparison of shielding performance among AgNPs(17%)@MWCNTs/CFM membrane, CuNPs(17%)@MWCNTs/CFM membrane and previously reported carbonous EMI shielding materials in terms of the relation between SE/thickness and the mass density.

As shown in Figure 6d, the SE of AgNPs(17%)@MWCNTs/CFM membrane (with 4 wt% of coated AgNPs(17%)@MWCNTs) is high up to 30-60 dB in an extremely wide frequency region of 0.5-12.0 GHz, showing an obvious advantage of SE as compared with the CuNPs(17%)@MWCNTs/CFM membrane, especially in the frequency region of 3.0-12.0 GHz. The SE of AgNPs(17%)@MWCNTs/CFM membrane is still superior to the AgNPs(17%)MWCNTs/CFM,


and

AgNPs(17%)@MWCNTs/PMMA

membranes (Figure 6e). We calculated the SET, SEA and SER of AgNPs(17%)@MWCNTs/CFM membrane. As shown in Figure 6f, SEA is always higher than SER, indicating that the AgNPs(17%)@MWCNTs/CFM membrane is still an absorption-dominated shielding material. Figure S18 shows the real (ε') and imaginary (ε'') parts of the complex permittivity and the


21


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Page 22 of 31

dielectric loss tangent (tanδe) of AgNPs(17%)@MWCNTs/CFM membrane. It is apparent that the AgNPs(17%)@MWCNTs/CFM membrane has strong ability to dissipate microwaves via dielectric loss. Cooperative shielding mechanism is still responsible for the high shielding performances of AgNPs(17%)@MWCNTs/CFM membrane due to that the summed SEs of individual AgNPs(17%)@MWCNTs and CFM are substantially lower than those of AgNPs(17%)@MWCNTs/CFM membrane (Figure S19). The AgNPs(17%)@MWCNTs/CFM membrane still shows an excellent mechanical stability, which exhibits only ~10 dB of SE loss after 10,000 times of bending (Figure S20). Moreover, the AgNPs(17%)@MWCNTs/CFM membrane show limited loss of SE after the weathering test (Figure 6g), and the encapsulated AgNPs exhibit a pretty good stability as no oxidation was found during the weathering test (Figure S16). The specific SE of AgNPs(17%)@MWCNTs/CFM membrane is high up to 82-162 dB

cm-3

g-1

in

the

frequency

region

of

0.5-12.0

GHz.

Notably,

the

AgNPs(17%)@MWCNTs/CFM and CuNPs(17%)@MWCNTs/CFM membranes show apparent advantages both in shielding effectiveness and mass density (Figure 6h,i) when compared with ever reported carbonous EMI shielding materials in the X band. As shown in Figure 6h, an outstanding EMI shielding performance of around 40 dB and 20 dB are achieved for the AgNPs(17%)@MWCNTs/CFM and CuNPs(17%)@MWCNTs/CFM membranes with 4 wt% of MNPs@MWCNTs. The SE divided by thickness of the shielding materials as a function of density can exhibit the EMI shielding performance more directly. Based on SEM observation (Figure

S22),

the

thickness

of

AgNPs(17%)@MWCNTs/CFM

and

CuNPs(17%)@MWCNTs/CFM membranes was determined to be 979 µm and 931 µm, respectively. Compared with previously reported carbonous EMI shielding materials, the AgNPs(17%)@MWCNTs/CFM and CuNPs(17%)@MWCNTs/CFM membranes have much


22

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lower mass density as well as larger normalized value in terms of SE divided by thickness (Figure 6i). The MNPs(17%)@MWCNTs/CFM membrane also shows substantially improved SE and mechanical strengthen (Table S1) than our previous developed lightweight shielding CPCs.27 4. CONCLUSIONS To address the newly emerging electromagnetic interference (EMI) pollution issue, we have successfully developed a lightweight and high-performance EMI shielding membrane by coating the multiple wall carbon nanotubes encapsulated metal nanoparticles (MNPs@MWCNTs) on the microwave absorptive substrate of collagen fiber membrane (CFM). The high shielding performance of MNPs@MWCNTs/CFM membrane lies on the cooperative shielding mechanism between CFM and MNPs@MWCNTs. The as-prepared MNPs@MWCNTs/CFM composite membrane exhibited a high shielding effectiveness of 30-60 dB in a broad frequency range of 0.5-12.0 GHz, and the specific SE was high up to 162 dB cm-3 g-1. Our strategy developed in the present investigation might be extended for the synthesis of other lightweight and highperformance absorption-dominated shielding materials. ASSOCIATED CONTENT Supporting Information. The following file is available free of charge. The electromagnetic parameters and SEs of CFM and EMI shielding membranes; the photos of EMI shielding membranes; the microstructure of CuNPs(x)@MWCNTs (x = 5%, 9%, 13%, 17%, 29%), CFM, NF, PMMA, and EMI shielding membranes; diffusion reflectivity of CFM, NF and PMMA membranes; XRD patterns of AgNPs(17%)@MWCNTs/CFM membrane before and


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

after

the

weathering

tests;

comparisons

of

basic

physichemical

Page 24 of 31

properties

of

MNPs(17%)@MWCNTs/CFM and Cu@Ag/CFM membranes. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We gratefully acknowledge the financial supports provided by the National Natural Science Foundation of China (51507107, 21176161) and the Science and Technology Fund for Distinguished Young Scholars of Sichuan Province (2016JQ0002). REFERENCES (1) Kumar, A.; Singh, A. P.; Kumari, S.; Dutta, P. K.; Dhawana, S. K.; Dhar, A. Polyaromatichydrocarbon-based carbon copper composites for the suppression of electromagnetic pollution. J. Mater. Chem. A 2014, 2, 216632. (2) Shen, B.; Li, Y.; Yi, D.; Zhai, W. T.; Wei, X. C.; Zheng, W. G. Strong Flexible polymer/graphene composite films with 3D saw-tooth folding for enhanced and tunable electromagnetic shielding. Carbon 2017, 113, 55.


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(45) Hong, X. H.; Chung, D. D. L. Carbon nanofiber mats for electromagnetic interference shielding. Carbon 2017, 111, 529. (46) Ameli, A.; Jung, P. U.; Park, C. B. Electrical properties and electromagnetic interference shielding effectiveness of polypropylene/carbon fiber composite foams. Carbon 2013, 60, 379.


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For Table of Contents Only


31

Collagen Fiber Membrane as an Absorptive Substrate To Coat with

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