Composite Foams for Electromagnetic Wave ... - ACS Publications

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Functional Nanostructured Materials (including low-D carbon)

Lightweight Ti2CTX MXene/Poly(vinyl alcohol) Composite Foams for Electromagnetic Wave Shielding with Absorption Dominated Feature Hailong Xu, Xiaowei Yin, Xinliang Li, Minghang Li, Shuang Liang, Litong Zhang, and Laifei Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21671 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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ACS Applied Materials & Interfaces

Lightweight Ti2CTX MXene/Poly(vinyl alcohol) Composite Foams for Electromagnetic Wave Shielding with Absorption Dominated Feature

Hailong Xu, [a] Xiaowei Yin,[a] Xinliang Li, [b] Minghang Li, [a] Shuang Liang, [a] Litong Zhang, [a] Laifei Cheng[a] [a] H. Xu, Prof. X. Yin, M. Zhu, M. Li, H. Wei, Prof. L. Zhang, Prof. L. Cheng Science and Technology on Thermostructural Composite Materials Laboratory Northwestern Polytechnical University, Xi’an, 710072, China E-mail: [email protected] [b] Department of Materials Science and Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, P. R. China

Keywords: Ti2CTX MXene, polymer, electromagnetic wave shielding, electromagnetic wave absorption, ultralight foam. Abstract: Lightweight absorption-dominated electromagnetic interference (EMI) shielding materials are more attractive than conventional reflection-dominated counterparts by minimizing the twice pollution of reflected electromagnetic (EM). Here, porous Ti2CTX MXene/poly(vinyl alcohol) composite foams constructed by few layered Ti2CTx (f-Ti2CTx) MXene and poly(vinyl alcohol) (PVA) are fabricated via a facile freezing-dry method. As a superior EMI shielding material, the calculated specific shielding effectiveness reaches up to 5136 dB cm2 g-1 with an ultralow filler content of only 0.15 vol% and a reflection effectiveness (SER) of less than 2 dB, representing the excellent absorption-dominated shielding performance. Contrast experiment reveals that the good impedance matching deriving from the multiple porous structure, internal reflection and polarization effect (dipole and interfacial

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polarization) play a synergistic role on the improved absorption efficiency and superior EMI shielding performance. Consequently, this work provides a promising MXene based EMI shielding candidate with lightweight and high strength features. 1. Introduction Along with the coming 5G cellular network communication technology using the gigahertz electromagnetic (EM) wave, the EM wave pollution has been a threat to communication modular of wearable device and portable electronics. The demand for high-efficiency EM wave shielding materials (ESMs) is more urgent than ever.1-5 An effective ESMs should defend the component from stray external signals with reducing undesirable emissions.6 Until now, three electromagnetic interference (EMI) shielding mechanisms have been proposed, which can be summarized as follow: (1) Reflection of EM wave generates by the direct interaction between the charge carriers and the electromagnetic fields.7 Therefore, this kind of ESMs always possess high electrical conductivity and results in twice pollution of the reflected EM wave. (2) Absorption of EM wave generates by the interaction between electric or magnetic dipoles inside materials and the electromagnetic fields.8-13 (3) Multiple internal reflections caused by scattering centers including interfaces or defect sites within the shielding material.14-15 Various kinds of ESMs accompanied with one or several shielding mechanisms have been investigated. Metals with high electrical conductivity were selected as EMI shielding materials,16 including aluminum foil, copper foil, et al. However, the large density and worse corrosion resistance make metal less desirable.17 High electrical conductive carbon materials with lightweight feature such as graphene, reduced


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graphene oxides (RGOs), carbon nanotubes (CNTs) or their hybrid have become popular alternatives for EMI shielding.18-23 Our group has prepared graphene sponge modified with in-situ grown carbon nanowires (CNWs), exhibiting an EMI shielding effectiveness of 36 dB in X-band.19 In addition, carbon nanotube-multilayered graphene edge plane core-shell hybrid foam prepared by chemical vapor deposition also exhibits excellent EMI shielding effectiveness which exceeds 47.5 dB in X-band.20 Even though the shielding mechanism is ascribed to the strong EM wave attenuation arising from the defects and interface or junction interface between CNWs(CNTs) and graphene, EM wave reflection always dominates their shielding mechanism. Other than reflection, EM wave absorption, which absorbs the incident EM wave energy effectively and then converts it into thermal energy is a more efficient way for protecting the component from stray external signals with reducing undesirable emissions as much as possible.14 To prepare an absorption-dominated EMI shielding material, the reflection effectiveness (SER) should be less than 3 dB (less than 50% EM wave energy reflected) as shown in Figure 1a (The calculation method is shown in detail in supporting information as S-1). However, almost all the previously reported typical EMI shielding materials with whether foam or film structure have a SER larger than 3 dB because of their high electrical conductivity, as shown in Figure 1b. Notably, the introduction of insulating polymers can decrease the SER of graphene foam to 1 dB, but inevitably weak the total EMI SE to 13 dB due to its poor shielding mechanism.24 Therefore, exploring efficient methods to prepare absorption-dominated shielding materials is necessary but still remains a big challenge.



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Figure 1. The relationship of SER and the proportion of reflected EM wave energy in the total EM wave energy, which is calculated with the equation 5 (a), the previously reported typical EMI shielding materials based on graphene, carbon nanotubes, and MXene with foam or film structure (b), the detailed data is shown in Table S1 in the supporting information. Recently, MXenes as a new family of two-dimensional materials have attracted lots of intention due to their outstanding performance in the EM wave absorption and shielding filed.1,

25

Especially, few-layered MXenes possess metallic conductivity and

hydrophilicity properties make MXenes a good “brick” for assembling functional materials.26-27 Typically, Gogotsi et al. reported the freestanding Ti3C2TX films with a best EMI shielding effectiveness of

about 68 dB (SER=23 dB, thickness =11μm)

among synthetic materials so far.6 Also, Zhang et al. reported that foaming treatment of Ti3C2TX film could result in the decrease of electrical conductivity and SER.

27

Inspired by the above work, assembling conductive few-layered MXenes and insulating polymers and forming pores are reasonable to generate a new kind of absorptiondominated EMI shielding candidate. Moreover, the total EMI shielding performance can be maintained or even improved by enlarging absorption and multiple internal reflection.


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Here, we reported a kind of absorption-dominated EMI shielding model and evidenced by assembling few-layered Ti2CTx (f-Ti2CTx) MXenes with poly(vinyl alcohol) (PVA) for the first time. The porous foam structure could not only reduce the EM wave reflection by decreasing the conductivity, but also enlarge multiple internal reflections and absorption. The EM wave absorption and shielding performance of f-Ti2CTx/PVA hybrid foams were explored in the frequency range of 8.2-12.4 GHz. By contrast, the EMI shielding performance of f-Ti2CTx/PVA film obtained by compressing the fTi2CTx/PVA hybrid foam were also investigated. This work provide an avenue for constructing absorption-dominated shielding materials. 2. Experiment 2.1 Materials The lithium fluoride (LiF; ≥ 99%), hydrochloric acid solution (HCl), and poly(vinyl alcohol) (PVA) were purchased from Aladdin biochemical polytron technologies Inc., Commercial Ti2AlC ( ≥ 98% purity) powders were obtained from Beijing Lianlixin Technology Co., Ltd., China. No further purification treatment in this work. 2.2 Synthesis of few layered Ti2CTX (f-Ti2CTX) MXene. The f-Ti2CTX was obtained via a gentle etching method using LiF and HCl as the etching agent.14 Typically, 1 g of LiF powder was dissolved in 20 ml 6M HCl solution in a Teflon container and stirred 20 min to obtain the homogeneous acid solution containing fluorine ion. And then 1 g of Ti2AlC was added into the acid solution at 40 ºC under stirring continuously for 24 h to extract Al. The obtained solution is a multilayer Ti2CTx (m-Ti2CTx) dispersion. To obtain f-Ti2CTX nanosheets solution, the



multilayer Ti2CTx dispersion was treated repeatedly with deionized water (DI) and centrifuged at 3500 rpm until the supernatant turned to be a stable black solution. 2.3 Preparation of f-Ti2CTX/PVA hydrosol. The precursor suspension of f-Ti2CTX/PVA solution was obtained using a blending method. Initially, the PVA solution (20 mg/ml) was prepared by dissolving a certain amount of PVA particles in hot deionized water. Then, a certain amount f-Ti2CTX was added into deionized water to obtain a concentration of 30 mg/ml f-Ti2CTX solution. A series of f-Ti2CTx/PVA hydrosol with gradient concentration were prepared by adding deionized water into the mixture of PVA solution and f-Ti2CTx dispersion solution with the corresponding volume of PVA solution. The total volume was set as 2.5 ml. The prepared samples were donated as foam-1 (10 mg/20 mg), foam-2 (20 mg/20 mg) and foam-3 (30 mg/20 mg) according to the different dosage of f-Ti2CTx (10 mg, 20 mg, 30 mg). 2.4 Preparation of f-Ti2CTX/PVA foam and film. The f-Ti2CTX/PVA foam was prepared by a freezing-dry method using ice as template. The above hydrosol was poured into a hollow square teflon tubes assembled with a copper plate at the bottom. The dimension of the teflon tubes was 22.86 mm × 10.16 mm × 40 mm. After that, the suspension was frozen entirely and free-dried for 48 h in a freeze-dryer under 0.1 Pa pressure. The f-Ti2CTX/PVA film-1 and film-2 was obtained by compressing the f-Ti2CTX/PVA foam-1 once and twice using a rolling machine. 2.5 Characterization.


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The morphology of samples was characterized by SEM (S-4700, Hitachi, 15 kV), TEM (F200X, FEI-Tecnai, 200 kV) and AFM (Dimension Icon Bruker). The solution of MXenes was diluted with deionized water and treated with ultrasonic for enough long time, then the TEM characterized sample was obtained by dropping the diluted MXenes solution on a grid copper mesh. The solution of MXenes were dropped onto mica plate and kept at room temperature for 24 h before the AFM measurements.28 The microstructure of samples were analyzed by Renishaw Raman spectroscopy (Raman; inVia, Renishaw, 532 nm He-Ne laser) and X-ray diffraction (XRD; Bruker, D8 Avance) with a scan range of 5-65°. The chemical bonding information was obtained with Xray photoelectron spectra (XPS) (Model K-Alpha, Thermo Scientific, Waltham,). Sparameters (S11 and S21) and dielectric data of samples were measured using waveguide method by a VNA (Anritsu, MS4644A) in the X-band. The size of the samples for the S-parameters and complex permittivity measurement is 22.86×10.16 mm with a certain thickness less than 10 mm. 3. Result and discussion 3.1 Microstructures of f-Ti2CTX/PVA foam and film. Figure 2a shows the strategy for the preparation of f-Ti2CTX/PVA foam and film. The few-layered Ti2CTX (f-Ti2CTX) were obtained by selective etching of Al atoms of Ti2AlC MAX phase in a mixture of lithium fluoride (LiF) and hydrochloric acid (HCl). Then the precursor hydrosol was obtained by mixing poly-(vinyl alcohol) (PVA) solution and f-Ti2CTX solution as shown in Figure 2b. After freeze drying in the homemade hollow square teflon tube equipped with a copper plate placed at the bottom,



a f-Ti2CTX/PVA foam with a size of 22.86 mm×10.16 mm×10.00 mm was prepared as shown in Figure 2c. The f-Ti2CTX/PVA foam can stand on a dandelion indicating its light density. The measured density is only ~10.9 mg/cm3 (yielding a porosity of about 99.3% of f-Ti2CTX/PVA-1, see S2 for the calculation method in supporting information) which is comparable to those other ultralight aerogels.29-30 Due to the strong hydrogen bonding between PVA molecular chains and surface moieties (Tx: a mixture of OH, =O, -F) of f-Ti2CTX, the obtained f-Ti2CTX/PVA composite foam shows strong enough mechanical property and can support a metal block of 140000 mg which is >5000 times larger than its own weight (Figure 2f). The f-Ti2CTX/PVA composite film can be prepared by compressing of the f-Ti2CTX/PVA composite foam as shown in Figure 2d and the composite film shows flexibility also originated from the strong hydrogen bonding (Figure 2g).

Figure 2. (a) Illustration of the preparation process of the f-Ti2CTX/PVA composite foam and film. (b) A bottle of PVA solution which is nearly transparent, a bottle of f-


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Ti2CTX solution with pure black color, and a bottle of their f-Ti2CTX/PVA solution. (c) Photograph of f-Ti2CTX/PVA foam. (d) Typical photograph of f-Ti2CTX/PVA film, (e) a piece of f-Ti2CTX/PVA foam standing on a dandelion indicates it ultralow density. (f) Typical photograph of f-Ti2CTX/PVA foam which supporting more than 5000 times than its own weight. (g) Photograph of flexible f-Ti2CTX/PVA film. Figure 3 show the morphology and structure information of the samples. Figure 3a is a typical scanning electron microscope (SEM) images of Ti2AlC MAX phase showing its dense structure. After selective etching of Al with LiF and HCl as etching agent, the cohesive bulk Ti2AlC MAX phase changes to multi-layered Ti2CTx MXene (m-Ti2CTx) with an accordion-like structure as shown in Figure 3b,31 and further shifts to few layered Ti2CTx MXene (f-Ti2CTx) after shaking and washing repeatedly. The SEM images of f-Ti2CTx film obtained by filtration of f-Ti2CTx dispersion show that the film is tightly stacked with very thin and nearly transparent nanosheets. The morphology of f-Ti2CTx nanosheets is further studied by TEM and AFM. As shown in Figure 3d, the f-Ti2CTx nanosheets show obvious two-dimensional lamellar structure, one piece of f-Ti2CTx nanosheet with folded-paper microstructure is shown in Figure 3e, and it is nearly transparent under electron beam. Besides, the HRTEM image in Figure 3f, indicates that the f-Ti2CTX nanosheet contains 2~4 stacked layers, and the measured thickness of the single Ti2CTX nanosheet is 1.96 nm. Moreover, the thickness of the f-Ti2CTX nanosheet is evidenced by AFM shown in Figure 3g-i. The heights of the marked region by line 1, 2, and 3 are ~2.0 nm, ~6.3 nm and ~13.1 nm, corresponding to the 1, 3, and 6 stacked layers of Ti2CTX.32 The results are consistent with TEM



images (Figure 3f). In addition, the thicknesses of ~6.3 nm and ~13.1 nm are measured from some folds of Ti2CTX nanosheet comes from the solvent evaporation in the preparation process of AFM characterized samples.

Figure 3. (a) Typical SEM images of Ti2AlC MAX phase, (b) multi-layered Ti2CTX (mTi2CTX), (c) few-layered Ti2CTX (f-Ti2CTX). (d) Typical TEM images of f-Ti2CTX nanosheet, (e) the edges of f-Ti2CTX nanosheet, (f) HRTEM images of the edges of fTi2CTX nanosheet. (g-i) AFM images of a single f-Ti2CTX nanosheet. Then, the morphology and structure of the prepared f-Ti2CTX/PVA foam and film are clearly shown in SEM image (Figure 4). Figure 4a clearly evidences the porous structure of f-Ti2CTX/PVA foam-1 after the evaporation of ice template. A typical ellipsoidal hole marked with white dotted ellipse shows that the pore diameter is about 90 μm (Figure 4b). The magnified SEM image in Figure 4c shows the flexible hole wall


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with a thin thickness. Further increasing the magnification, the wrinkling structure marked with white dotted line could be observed obviously as shown in Figure 4d, and the wrinkling structure is beneficial for the multiple internal reflection of EM

Figure 4. Typical images of f-Ti2CTX/PVA foam (a~f), and the f-Ti2CTX/PVA film (g~j), element mapping images of f-Ti2CTX/PVA foam (k). wave in the pores and dissipated by the related mechanism rather than reflected on the surface of the foam. The thickness of the hole wall is about ~90 nm as shown in Figure 4 e, f. Moreover, no obvious aggregation could be observed which indicates the strong



interaction between f-Ti2CTx lamellae and PVA molecules. The morphology of the fTi2CTX/PVA film was also studied by SEM, the thickness of the f-Ti2CTX/PVA film-1 obtained by once compression is about ~300 μm (with a porosity of 88.3%) as shown in Figure 4g, and the pores are squeezed away verified by the magnified SEM image (Figure 4h). After compression again, the thickness changes to ~100 μm (with a porosity of 64.9 %), and the SEM image of f-Ti2CTX/PVA film-2 shown in Figure 4j indicates an ordered lamellar structure. The shift from porous morphology of fTi2CTX/PVA foam-1 to ordered lamellar structure of f-Ti2CTX/PVA film may decrease the multi-reflection of EM wave in pores, and only reflection between the interlayer could be retained.6, 31 The element mapping analysis of f-Ti2CTX/PVA foam shown in Figure 4k suggests the well-proportioned distribution of C, Ti, O, F, Cl element, and the existed Cl element is from the residual of HCl etchant.1

Figure 5. Raman spectra (a) and XRD patterns (b) of Ti2AlC MAX phase, m-Ti2CTX, f-Ti2CTX and f-Ti2CTX/PVA composite, (c) XPS spectra of Ti2AlC MAX phase, fTi2CTX and f-Ti2CTX/PVA composite. Raman spectroscopy, XRD and XPS spectra were recorded of the samples to investigate their phase information. As shown in Figure 5a, the Raman peaks in Ti2AlC MAX phase are corresponding to Ti-Al and Ti-C peaks (black line).33 The upshifting in the


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range of 150 cm-1~500 cm-1 and broadening in 600 cm-1~750 cm-1 of peaks in the Raman spectra of Ti2CTx are owning to the introduced interlayer adsorbents during solution etching.26 Besides, the peak appeared at 1580 cm-1 is due to the stretching of sp3 carbon.34 The disappearance of peaks in the range of 150 cm-1~700 cm-1 of fTi2CTX/PVA composite, indicating the low filler loading of TiC2TX. The phase transition of samples was investigated by XRD as displayed in Figure 5b, the peaks at 2θ=39o of Ti2AlC MAX phase disappeared and (002) peak is kept and shifts from 2θ=13o to 7.65o of m-Ti2CTx and 7.25o of f-Ti2CTx after etching, indicating the successful etching of Ti2AlC MAX phase. And the d-spaces calculated from Bragg’s equation (2dsinθ = nλ) of (0 0 2) basal plane is 1.15 nm of m-Ti2CTx and 1.22 nm of f-Ti2CTx.35 The left shift of the (0 0 2) peak means that the interlayer spacing is increasing as compared with Ti2AlC MAX phase and m-Ti2CTX.27, 36 The high purity of the prepared m-Ti2CTX and f-Ti2CTX can be confirmed from the absence of impurity peaks. In addition, the downward shift of (0 0 2) peak with an increased peak intensity of f-Ti2CTX/PVA composite indicates the strong interaction of f-Ti2CTX and PVA molecular. All above results are consistent with TEM and AFM results. The chemical composition of the samples were monitored by XPS shown as Figure 5c, XPS spectra of Ti2CTX reveals the main constituents of Ti, C, O and F elements, the disappearance of the peak corresponding to Al 2p (72.8 eV) and the appearance of the peak corresponding to F 1s (684.6 eV) indicate the etching of Al atoms and the introduction of fluorine terminations.37 Also, the f-Ti2CTX shows an O/Ti ratio of 1.54 indicating the introduction of oxygen functionalities after aqueous etching,38 and the f-



Ti2CTX/PVA composite has a larger O/Ti ratio (7.34) and C/Ti (14.16) than these of fTi2CTX due to the introduction of PVA molecular.


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Figure 6. The real part ɛ' (a), and imaginary part ɛ" (b) of f-Ti2CTX/PVA foams, 2D contours and reflection coefficient curves versus frequency and thickness of fTi2CTX/PVA foams, (c, d) foam-1, (e, f) foam-2, (g, h) foam-3. 3.2 Dielectric and electromagnetic wave absorption properties of f-Ti2CTX/PVA foam. The EM wave absorption performance of a dielectric material is highly associated with its complex permittivity (ε=ε′+iε″), where ε′ stands for the storage capability, and ε″ represents the loss capability of electric energy.39-42 The complex permittivity parameters of the samples were recorded using vector network analyzer in the X band. 19, 43As

shown in Figure 6a, b, the values of ε′ and ε″ aggrandize with the content of f-

Ti2CTX increasing from 10 mg to 20 mg, and 30 mg, respectively. The values of ε′ of the three samples show a similar decrease trend along with the frequency increasing from 8.2 GHz to 12.4 GHz which can be explained with Debye theory. And the obvious enhanced ε″ indicates the enhanced attenuation capability.41, 44 According to the measured permittivity of the samples, the reflection coefficient (RC) can be calculated using the following equation 2, 45

RC(dB)  20 log10

Z in  1 Z in  1

(1)

Where Zin is the input impedance of a metal-backed electromagnetic absorbing layer:

Z in 

  2 tanh j   c



  fd  

(2)

Where ε and µ are the relative permittivity and permeability, f is the frequency, d is the thickness of the samples, and c is the light velocity in vacuum.46-48 The effective absorption bandwidth (EAB) is defined as the frequency range where RC

Composite Foams for Electromagnetic Wave ... - ACS Publications

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