Preparation of Polyaniline@MoS2@Fe3O4 Nanowires with a Wide

Article Cite This: ACS Appl. Nano Mater. 2018, 1, 5865−5875

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Preparation of Polyaniline@MoS2@Fe3O4 Nanowires with a Wide Band and Small Thickness toward Enhancement in Microwave Absorption Weidong Zhang,† Xue Zhang,† Yuan Zheng,† Chao Guo,† Meiyu Yang,† Zhao Li,†,‡ Hongjing Wu,§ Hua Qiu,† Hongxia Yan,† and Shuhua Qi*,† †

Department of Applied Chemistry, School of Natural and Applied Sciences, Northwestern Polytechnical University, Xi’an 710072, China Department of Materials Science and Engineering, Frederick Seitz Materials Research Laboratory, and Beckman Insitututed for Advanced Science and Technology, University of Illinois at UrbanaChampaign, Urbana, Illinois 61801, United States § Department of Applied Physics, School of Natural and Applied Sciences, Northwestern Polytechnical University, Xi’an 710072, China

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‡

S Supporting Information *

ABSTRACT: It is well-known that the absorbing property of materials is related to the microstructure and can be enhanced by magnetic nanoparticles coating. However, the mechanism is still unclear that the magnetic coating structure plays an important role in the microwave absorption behavior. Hence, a novel kind of absorber with efficient performance and relatively clear enhancement mechanism is very necessary. To solve these problems, polyaniline@MoS2 nanowires (PANI@MoS2 NWS) and a series of polyaniline@MoS2@Fe3O4 nanowires (PANI@MoS2@Fe3O4 NWS) with hierarchical structure were successfully prepared by in situ polymerization and versatile hydrothermal reactions and investigated in microwave absorption. The results reveal that compared with the articles related to MoS2-based composites, the as-prepared PANI@MoS2@Fe3O4 NWS (e2) displays the widest absorption frequency range (6.48 GHz) and strong absorption (−49.7 dB at a thickness of 1.3 mm), and the excellent MAP can be interpreted by optimizing impedance matching and simultaneously inhibiting skin effect rather than only focusing on magnetic loss. KEYWORDS: microwave absorption performance, MoS2 nanosheets, polyaniline nanowires, magnetic materials, hierarchical structure

1. INTRODUCTION Radiated electromagnetic (EM) waves have become a severe environmental problem, not merely affecting humans’ health and life but also influencing the functioning of military devices.1−3 It is still a difficult problem to solve the requirements of practical applications.4 To date, various absorbents, such as metal powders, magnetic ferrites, carbon materials, conductive polymers composites (CPs), and chiral materials, have been investigated.5−10 Among these absorbents, PANI is prominent among the other CPs due to its no redox doping and good environmental stability.11−15 The microwave absorption performance (MAP) of PANI can be explained by the electrical conductivity and polarization as well as relaxation effects.16,17 Recent advances have shown that materials with hierarchical structure are also beneficial for the fabrication of excellent microwave absorbers. Two-dimensional (2D) compounds having hierarchical structure, such as polymorphism transition metal dichalcogenides (TMDs), MXenes, and graphene, have been prepared and used for microwave absorbents owing to their sheet-like morphology and good dielectric loss. 2D MoS2 nanosheets display excellent MAP, which can be explained by its larger interfacial polarization and high dielectric loss. Sun et al. synthesized MoS2-based © 2018 American Chemical Society

composites with heterostructures and the optimal reflection loss (RL) as well as effective absorbing bandwidth (RL < −10 dB), which are as follows: MoS2/Ni nanoparticles, −19.7 dB and 2.92 GHz; MoS2/carbon nanotubes, −47.9 dB and 5.60 GHz; MoS2/carbon layers, −69.2 dB and 4.88 GHz.18 Zhang et al. prepared NiS2@MoS2 core−shell nanospheres, and the results showed that the minimum RL can reach −41.05 dB at 12.08 GHz and the effective attenuation bandwidth can be up to 4.4 GHz.19 Mu et al. synthesized MoS2/CNT nanohybrids with a hierarchical structure, and the results show that MoS2 nanoflowers were decorated uniformly on the surface of carbon nanotubes and the optimal result of −46 dB can be reached at a thickness of 2.9 mm.20 In general, impedance matching is a key factor that should be considered while constructing an excellent absorbent. Metallic oxides possess relatively better EM parameter that can be controlled more easily, which can improve the impedance matching characteristics of materials and broad absorbing bandwidth.21 For example, Li et al. enhanced the MAP of CNTs by optimizing Received: September 3, 2018 Accepted: September 25, 2018 Published: October 16, 2018 5865

DOI: 10.1021/acsanm.8b01452 ACS Appl. Nano Mater. 2018, 1, 5865−5875

Article

ACS Applied Nano Materials

30 min and marked as solution A. Then, ferrous chloride and ferric chloride (n(ferric chloride):n(ferrous chloride) = 2:1) were dissolved in 10 mL of distilled water under nitrogen protection and continuous magnetic stirring to obtain a suspension liquid marked as solution B. Then, solution B was added to the solution A and the mixture was stirred at 80 °C for 30 min. Then, the mixed black solution was placed into a TLSS of 100 mL capacity and 2.5 mL of ammonia solution was quickly injected. Finally, after the system was heated at 110 °C for 3 h, the prepared black PANI@MoS2@Fe3O4 NWS were collected and repeatedly washed with ethanol and water. Several kinds of the PANI@MoS2@ Fe3O4 NWS with different amounts of added ferric chloride (1.08 mmol, 2.16 mmol, 3.24 mmol, and 4.32 mmol) were obtained. For convenience, the four kinds of PANI@MoS2@Fe3O4 NWS are denoted as e1, e2, e3, and e4. 2.6. Characterization. The scanning electron microscopy (SEM) images were achieved by a FE-SEM Hitachi S4800 microscope. Transmission electron microscopy (TEM) was carried out using a JEOL JSM-2010 microscope. XRD analyses were carried out on Bruker D8 Advanced X-ray diffractometer and recorded in the range of 2θ = 10−80°. The X-ray photoelectron spectroscopy (XPS) spectra were recorded using a PHI 5000 Versa probe. The magnetic property was measured by Cryogenic CFMS-14T physical property measurement system (PPMS). The EM parameters measurements were prepared by an Agilent PNA N5224A vector network analyzer and coaxial wire method in 2.0−18 GHz, the samples of which were prepared by mixing the wax with 30 wt % products and then pressing into cylindrical-shaped samples (Φout = 7.0 mm, Φin = 3.04 mm).

the Fe3O4 nanocoating structure, owing to the closely spaced Fe3O4 nanoparticles decorated on CNTs increasing the impedance matching and magnetic loss ability.22 In addition, as shown in our previous investigation, while MoS2 nanosheets show poor MAP and the RL is only −25.44 dB, when Fe3O4 nanoparticles were coated on the surface of MoS2 nanosheets, the RL increased to −26.8 dB and the effective attenuation bandwidth (RL < −15 dB) covered the whole X-band.2 However, first, few studies have been performed to systematically explore the MAP of the PANI@MoS2@Fe3O4 NWS with peculiar hierarchical and nanowires structure. Second, the origin of the effect of the magnetic particle coating on MAP is unclear. Herein, PANI@MoS2 NWS and a series of PANI@MoS2@ Fe3O4 NWS were successfully prepared. The results reveal that the PANI@MoS2@Fe3O4 NWS at the optimized [Fe3O4 nanoparticles]/[PANI@MoS2 NWS] ratio show a wide absorption frequency range (6.48 GHz) and strong absorption (−49.7 dB at a thickness of 1.3 mm). Finally, comparison with the reports in the literature related to MoS2-based composites shows that the as-prepared e2 displays the widest effective bandwidth, indicating the as prepared e2 possesses an excellent MAP. The PANI@ MoS2@Fe3O4 NWS enhanced the microwave absorption by magnetic Fe3O4 nanoparticles coating that possesses optimized [Fe3O4 nanoparticles]/[PANI@MoS2 NWS] ratio, which is due to optimizing impedance matching and simultaneously suppressing the skin effect rather than only focusing on magnetic loss.

3. RESULTS AND DISCUSSION The preparation mechanism of PANI@MoS2@Fe3O4 NWS is shown in Scheme 1. In this paper, in terms of the common sense

2. EXPERIMENTAL SECTION 2.1. Materials. Sodium molybdate (Na2MoO4·2H2O, AR), ammonium molybdate ((NH4)6Mo7O24·4H2O, AR), and thioacetamide (C2H5NS, AR) were purchased from Janus New-Materials Co., Ltd. Ammonia (NH3·H2O, 28%) and hydrochloric acid (HCl, AR) were supplied by Tianli Chemical Reagent Co., Ltd. Aniline (C6H7N, AR), ammonium persulfate (APS, AR), ferric chloride (FeCl3, AR), and ferrous chloride (FeCl2·4H2O, AR) were provided by Sinopharm Chemical Reagent Co., Ltd. All reagents were used without further purification. 2.2. Synthesis of Mo 3 O 10 (C 6 H 8 N) 2 ·2H 2 O Nanowires (Mo3O10(C6H8N)2·2H2O NWs). The Mo3O10(C6H8N)2·2H2O NWs were prepared according to the methods described in the literature.23 Briefly, 2.48 g of ammonium molybdate and 3.34 g of aniline were dissolved in 40 mL of deionized water, then dilute HCl (1 M) was dropwise added until white precipitate appeared and the pH is around 4−5. After continuous stirring at 50 °C for 2 h, the Mo3O10(C6H8N)2·2H2O NWs were obtained. The products were collected and thoroughly washed with absolute ethanol. 2.3. Synthesis of PANI@MoOx Nanowires (PANI@MoOx NWs). The PANI@MoOx NWs were prepared as reported previously.24 Typically, 0.2689 g of ammonium molybdate and 0.3400 g of Mo3O10(C6H8N)2·2H2O NWs were dispersed into 40 mL of deionized water. The polyreaction was initiated by 0.5700 g of (NH4)2S2O8 for 6 h, with the color of the above system changing from white to light red and to dark blue. Finally, dilute HCl (1 M) was dropwise added until the pH is approximately 2. The dark blue PANI@MoOx NWs were collected and washed with deionized water and ethanol. 2.4. Synthesis of PANI@MoS2 NWS. The PANI@MoS2 NWS with hierarchical structure were synthesized using the above-described PANI@MoOx NWs as precursors. Briefly, 0.8000 g of PANI@MoOx NWs and 0.9000 g of thiourea were dispersed in 20 mL of water with constant ultrasonication for 30 min, and the resulting samples were then transferred to a Teflon-lined stainless-steel (TLSS) autoclave (100 mL) and treated at 200 °C for 48 h. Then, the PANI@MoOx NWs were collected by centrifugation and washed three times with deionized water and ethanol. 2.5. Preparation of PANI@MoS2@Fe3O4 NWS. Briefly, PANI@ MoS2 NWS (200 mg) were dispersed into 60 mL of ethanol/water solution (V(ethanol):V(water) = 1:1) with constant ultrasonication for

Scheme 1. Illustration of Growth Mechanism of PANI@ MoS2@Fe3O4 NWS

that molybdenum−amine complexes are easy to prepare, we used aniline and ammonium molybdate as the raw materials and fortunately prepared a kind of molybdenum−amine complex (Mo3O10(C6H8N)2·2H2O) with nanowire structure in the room temperature. According to the oxidative polymerization properties of aniline, we obtained PANI nanowires that are covered by a small amount of molybdenum ammonium complex, which can be regarded as seeds and the positions that are fixed for the next step to in situ grow molybdenum disulfide. Therefore, it is most important to prepare PANI@MoOx NWs by in situ polymerization, which was decided by the stirring speed, reaction temperature, concentration, and ratio of molybdenum ammonium. MoS2 nanosheets was self-assembled coated on the PANI in H2S and molybdenum source presence by the hydrothermal reaction at 200 °C for 48 h. Finally, we decorated the magnetic Fe3O4 on 5866


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region of 20−25 nm, and the length is approximately 10 μm. It is observed from Figure 3b that the surface of PANI@MoOx NWs is very rough, which is the molybdenum ammonium complex. It can be seen from Figure 3c that PANI@MoS2 NWS with hierarchical structure show orderly scatter with a length of approximately 10 μm and a diameter of approximately 200 nm. Moreover, the magnified SEM image (Figure S2a in Supporting Information) confirms that the PANI surface is completely covered by MoS2 nanosheets, and they are interconnected with each other and grown perpendicularly on PANI with different directions. This implies that the PANI@MoS2 NWS with hierarchical structure possess higher porosity that can be verified by the SEM image (Figure S2b in the Supporting Information), which is suitable for promoting the EM wave repeated absorption. To obtain an excellent MAP material, we have optimized the impedance matching performance by coating magnetic Fe3O4 nanoparticles on the surface of the PANI@MoS2 NWS. As shown in Figure 3e1, e2, e3, and e4, we can see very clearly that different numbers of magnetic Fe3O4 were distributed uniformly on the PANI@MoS2 NWS. To illustrate the distribution of the individual components of PANI@MoS2@Fe3O4 NWS, the FESEM images and the elemental maps of PANI@MoS2@Fe3O4 NWS are displayed in Figure S1 in the Supporting Information, and it can be seen that the maps of Mo, S, Fe, O, and N are homogeneously distributed and surrounded the PANI@MoS2@Fe3O4 NWS frame. The morphologies of PANI@MoS2 NWS are also characterized by TEM. It can be seen from Figure S3 in the Supporting Information that the results are in accordance with the FESEM images. In Figure S4 in the Supporting Information, the isotherm of the products displays a typical type IV curve, suggesting the presence of a porous structure. The results were calculated by the BET method, the BET surface areas of PANI@MoS2@Fe3O4 NWs (e1, e2, e3, and e4), and the pore volumes are summarized in Table S1 in the Supporting Information. As has been discussed in the Figure S2b, the larger special surface area and higher pore volume of the PANI@MoS2@Fe3O4 NWs can be attributed to the interconnected MoS2 nanosheets causing the existence of many nanopores. But too much of the Fe3O4 nanoparticles coating can lead to less special surface area and small pore volume. Furthermore, the considerably large special surface area and high pore volume make PANI@MoS2@Fe3O4 NWs become a kind of promising candidate material for EM wave absorption. The magnetic properties of PANI@MoS2@Fe3O4 NWs (e1, e2, e3, and e4) were characterized by PPMS measurements at room temperature and are due to the magnetic Fe3O4 nanoparticles in the composites. In Figure 4, the values of saturation magnetization (Ms), remnant magnetization (Mr), and coercivity (Hc) are summarized in Table S2 in the Supporting Information. In general, high initial permeability (μi) means strong ability of magnetic loss, as can be expressed by

the surface of PANI@MoS2 NWs, then obtained PANI@MoS2@ Fe3O4 NWS in ferric chloride and ferrous chloride presence by the hydrothermal reaction at 110 °C for 3 h. To understand the crystal structure of PANI@MoS2@Fe3O4 NWS, XRD patterns were presented in Figure 1. The PANI@

Figure 1. XRD patterns of PANI@MoS2 NWS and PANI@MoS2@ Fe3O4 NWS.

MoS2 composites show broad diffraction peaks at approximately 13.9°, 33°, and 58.6°, which are attributed to the (002), (100), and (110) planes of MoS2 (JCPDS card no. 37-1492). For PANI@ MoS2@Fe3O4 NWS, the diffraction peaks at approximately 2θ = 18.22°, 30.0°, 35.34°, 43.0°, 53.3°, 56.8°, and 62.4° can be indexed to the (111), (220), (311), (400), (422), (511), and (440) planes of face-centered Fe3O4 (JCPDS no. 65-3107). Additionally, the broad diffraction peaks that appeared at 13.9°, 33°, and 58.6° are attributed to the diffraction pattern of the MoS2 sheets.25 Nevertheless, the peaks of PANI from PANI@MoS2 NWS and PANI@MoS2@Fe3O4 NWS cannot be obviously detected by XRD. As shown in Figure 2, the elemental components of the PANI@MoS2@Fe3O4 NWS were further confirmed by XPS. The wide scan XPS spectrum presented in Figure 2a confirms that the PANI@MoS2@Fe3O4 NWS are composed of Mo, S, Fe, O, C, and N elements, which is consistent with the results of the EDS analysis (Figure S1 in the Supporting Information). The peaks at 711.6 and 725.1 eV presented in Figure 2b are attributed to the Fe 2p3/2 and Fe 2p1/2, respectively.26 Figure 2c shows the C 1s spectra of the PANI@MoS2@Fe3O4 NWS divided into four different peaks. The peaks at 284.77, 285.67, 286.67, and 288.77 eV can be attributed to the C−C/CC, C−H, C−O, and CONH groups, respectively. The two peaks at 229.2 and 232.4 eV in the Mo 3d spectrum (Figure 2d) are in agreement with the Mo 3d5/2 and Mo 3d3/2 of MoS2.27,28 The peak at approximately 399.5 eV is attributed to the N−, and the peak at 401.9 and 402.9 eV are assigned to the N+− on the polymer backbone compensated with the Cl− and the protonated amine units, respectively.4,29 The S 2s peak is located at approximately 226.3 eV. The peaks at 162.1 and 163.2 eV (Figure 2f) correspond to the S 2p3/2 and S 2p1/2 of S2−.27,28 All of the above analyses confirm that the PANI@MoS2@Fe3O4 NWS were successfully fabricated. To further ensure the microstructure, SEM images of Mo3O10(C6H8N)2·2H2O NWs, PANI@MoOx NWs, PANI@ MoS2 NWS, and PANI@MoS2@Fe3O4 NWS are shown in Figure 3a−d. A high number of Mo3O10(C6H8N)2·2H2O NWs (Figure 3a) are smooth, the nanowire diameter is in a narrow

μi =

Ms2 ak Hc Ms + bλξ

(1)

where a and b are constants, λ is the magnetostriction constant, and ξ is an elastic strain parameter.30 According to the eq 1, the μi value can be improved by higher Ms and lower Hc.31,32 On the one hand, compared with other samples, e4 shows the strongest magnetic loss ability, suggesting that the ability of magnetic loss can be enhanced by coating magnetic nanoparticles. On the other hand, all PANI@MoS2@Fe3O4 NWs samples show stronger magnetic loss ability than the PANI@MoS2 NWs. 5867


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Figure 2. XPS spectra of (a) PANI@MoS2@Fe3O4 NWs (e2), (b) Fe 2p, (c) C 1s, (d) Mo 3d, (e) N 1s, and (f) S 2p.

Microwave Absorption Properties. To assess the MAP of absorbents, the RL value is employed. On the basis of the transmission line theory, the RL values at the given frequency could be calculated according to the EM parameters (complex permittivity (εr = ε′ − jε″) and relative complex permeability (μr = μ′ − jμ″)) by the following equations.36−38

In Figure S5 in the Supporting Information, the weight loss at the lower temperatures (under 100 °C) is attributed to the surface adsorbed water in PANI@MoS2@Fe3O4 NWs. The decomposition in 100−320 °C range is assigned to the water evaporation from the interior of PANI@MoS2@Fe3O4 NWs as well as due to the evaporation of residual aniline and the degradation of low polymerized PANI.33 In the range of 320−480 °C, the obvious loss events observed can be ascribed to the decomposition of PANI and parts of MoS2 from the PANI@MoS2@ Fe3O4 NWs.34,35 At the second stage (480−588 °C), the weight loss of approximately 1.40 wt % between 72% and 70.6% is mainly caused by the collapse of the skeleton structure from the PANI@MoS2@Fe3O4 NWs. The loss at the higher temperature (T > 840 °C) is mainly caused by the oxidation of MoS2. According to the Figure S5 and Figure S6 in the Supporting Information, we can calculate that the weight of the PANI, MoS2, and Fe3O4 is approximately 18, 40, and 42 wt %, respectively, without consideration the water.

ÉÑ ÄÅ ÑÑ ÅÅ i 2πfd y jij μr zyz j z Å zz( με zz tanhÅÅjjj Zin = Z0jjj )ÑÑÑ r r Ñ z Å ÑÑÖ ÅÅÇ k c { k εr {

(2)

Zin − Z0 Zin + Z0

(3)

RL = 20 lg|Γ|

(4)

Γ=

where Z0 is the impedance of air, Zin is the input impedance of the absorbent, c is the light velocity, f is the frequency of 5868


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it is clear that PANI@MoS2@Fe3O4 NWs (e1, e2, e3, and e4) enhanced the MAP. Figure 5b shows that the minimum RL can reach −47.3 dB at a thickness of 2.3 mm, and the bandwidth with effective attenuation of only 2.04 GHz. However, for e2 in Figure 5c, the minimum RL can reach −49.7 dB at a thickness of 1.3 mm. Simultaneously, the bandwidth with effective attenuation increased to 4.30 GHz (13.7−18.0 GHz). Moreover, the minimum RL values is −32.1 dB at a thickness of 1.7 mm, and the bandwidth with effective attenuation reaches 6.48 GHz (10.28−16.76 GHz). For e3, the conspicuous minimum RL values of −50.6 dB, −40.0 dB, −37.5 dB, and −42.7 dB are obtained for the matching thicknesses of 3.8 mm, 3.4 mm, 2.8 mm, and 2.0 mm, respectively (Figure 5d). Figure 5e shows the minimum RL value of −35.7 dB at a thickness of 2.6 mm, and RL < −10 dB can be obtained in the 8.8−12.04 GHz range. A comparison of the above-mentioned results suggests that e2 is a promising EM wave absorbent. The relative complex permittivity and the complex permeability of PANI@MoS2@Fe3O4 NWs and PANI@MoS2 NWs were measured and shown in Figure 6. The real part (ε′) of complex permittivity is associated with temporary storage of EM wave, and the imaginary part (ε″) represents the conversion and the loss capability of electric, which is related to dielectric loss. As shown in Figure 6a, the ε′ of e2 is higher than those of the other samples, implying that a higher polarization occurs in e2 under microwave radiation.39 The reasons for the higher permittivity of e2 can be interpreted as following. First, unlike PANI@MoS2 NWs, the PANI@MoS2@Fe3O4 NWs were decorated with Fe3O4 nanoparticles, which provided the optimal impedance matching performance of these materials. Second, the Fe3O4 nanoparticles decorating the surface of MoS2 nanosheets result in relatively high porosities and provide more reflection and scattering surfaces of EM waves. However, lower porosities are obtained if the number of the Fe3O4 nanoparticles on the surface of PANI@MoS2 NWs is too much.40 Thus, the ε′ values of e3 and e4 are smaller than those of e2. It is well-known that the εr originates mainly from the electronic, ionic, and electric dipolar polarization contributions.41 At the EM wave frequencies, the electronic and ionic polarizations are negligible.42 Therefore, the electric dipolar polarization may make the dominant contribution, which can be performed by the following expression:43,44 ε − ε∞ εr = ε∞ + s = ε ′ − jε ″ 1 + jωτ (5)

Figure 3. SEM images of Mo3O10(C6H8N)2·2H2O NWs (a), PANI@ MoOx NWs (b), PANI@MoS2 NWS (c, d), and PANI@MoS2@Fe3O4 NWS (e1, e2, e3, e4).

ε′ = ε∞ + ε″ =

εs − ε∞ 1 + ω 2τ 2

εs − ε∞ 2 2

1+ωτ

ωτ +

(6)

σ = εp″ + εc″ ωεo

(7)

Thus, a drastic change will appear for ε′ and ε″. As mentioned above, this explains the rapid changes of ε′ and ε″ (in Figure 6a and Figure 6b). In order to further analyze the mechanisms of dielectric loss, electrical conductivities of the as-prepared samples were evaluated and summarized in Table S3 in the Supporting Information. As shown in Figure 6b and Table S3, the higher ε″ values of e2 resulted in the higher εp″ rather than εc″. This may be ascribed to the introduction of Fe3O4 nanoparticles and the interconnected MoS2 nanosheets with hierarchical structure which may introduce more interfaces. Figure 6c and Figure 6d present the complex permeability of PANI@MoS2@Fe3O4 NWs (e1, e2, e3, and e4) and PANI@MoS2

Figure 4. Magnetization hysteresis loops of e1, e2, e3, e4, and pure Fe3O4 nanoparticles.

EM wave, d is the thickness of the absorbent, Γ is the reflection coefficient of the material, respectively. The calculated minimum RL curves of PANI@MoS2 NWs and PANI@MoS2@Fe3O4 NWs (e1, e2, e3, and e4) are shown in Figure 4. It is noted that PANI@MoS2 NWs (Figure 5a) exhibit a minimum RL of −18.9 dB at the optimal sample thickness of 1.7 mm, and the maximum bandwidth with effective attenuation increases to 5.16 GHz (from 11.4 to 16.60 GHz). When Fe3O4 nanoparticles are coated on the surface of PANI@MoS2 NWs, 5869


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Figure 5. 3D surface plots of RL for PANI@MoS2 ((a, a′), e1 (b, b′), e2 (c, c′), e3 (d, d′), and e4 (e, e′) at different thicknesses.

NWs. Compared to PANI@MoS2 NWs the values of the μ′ were higher. The μ″ values of e2 were higher than those for e1, e3, and

e4, indicating the magnetic loss of e2 is stronger than the others samples. 5870


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Figure 6. Electromagnetic parameters of PANI@MoS2 NWs and PANI@MoS2@Fe3O4 NWs (e1, e2, e3, and e4).

As demonstrated in Figure 6e, compared to PANI@MoS2 NWs, the values of the tan δe increase with increased Fe3O4 nanoparticles concentration. According to common sense, once the permittivity satisfies the impedance matching requirement, the loss tangent can be used to measure the MAP. Therefore, for e4, the values of the tan δe were smaller than those for e1, e2, and e3, indicating an excessive amount of Fe3O4 nanoparticles were coated on the PANI@MoS2 NWs, which is not conducive to EM mave absorption. Therefore, from only the perspective of dielectric loss, e3 is the best microwave absorbing material. However, e1, e2, e3, and e4 possess two unique absorption characteristics that facilitate the transformation of microwave energy into other energy: namely, the dielectric loss and the magnetic loss. As seen in Figure 6f, e2 possesses higher values of the tan δμ than other samples, which means that e2 has the strongest magnetic loss ability in MAP. The magnetic loss mainly originates from hysteresis loss (it is negligible in the megahertz range), domain wall resonance (it is occurred only in multidomain magnetic materials), natural resonance, and eddy current effects. Therefore, the last two

factors are generally considered to be the main loss mechanisms for magnetic Fe3O4 nanoparticles absorbent. For eddy current loss, it can be expressed by μ″ ≈

2πμ0 (μ′)2 σd 2f (8)

3 −2 −1

C0 = μ″(μ′) f

=

2πμ0 σd 2 3

(9)

where μ0 is the permeability of vacuum of the material. Obviously, the C0 values should be constant if magnetic loss arises solely from the eddy current loss. As shown in Figure S7 in the Supporting Information, the values of C0 change dramatically in the range of 2.0−18 GHz. This indicated that magnetic loss originates from natural resonance rather than the eddy current loss. For natural resonance, it is related to the anisotropy energy. However, with the increase of the Fe3O4 nanoparticles number, the C0 values of e4 tend to become constant in the 14−18 GHz range, which should be attributed to the eddy 5871


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ACS Applied Nano Materials current induced by the EM field. The eddy current will produce additional magnetic field, which cancels the external magnetic field and is not conducive to attenuating the EM wave. While designing an absorbent, the impedance matching (Z) and EM wave attenuation (α) are the two key factors that should be considered. The value of Z = |Zin/Z0| and α used to evaluate impedance matching and the EM wave attenuation in the interior of materials51,52 is given by

material attenuation capacity for the EM wave, the importance of impedance matching should also be considered. According to the above-mentioned discussion, the excellent MAP of PANI@MoS2@Fe3O4 NWs (e2) can be attributed to the following: (1) high porosities and multi-interfaces, which can trap and inhibit direct reflections incident EM waves. When the EM wave permeates into the absorbers, multiple reflection and scattering will occur. (2) For dielectric loss, on the one hand, lots of interfaces have been introduced, such as MoS2−MoS2, MoS2−PANI, PANI−Fe3O4, Fe3O4−Fe3O4, and MoS2−Fe3O4, leading the accumulation of bound charges and causing the interfacial polarization. On the other hand, there are many defects on MoS2 nanosheets and PANI nanowires that favor dipole polarization.53,54 (3) For magnetic loss, the PANI@MoS2 NWs were modified by Fe3O4 nanoparticles, and the materials with optimal [Fe3O4 nanoparticles]/[PANI@MoS2 NWs] ratio showed that MAP was improved only due to the effect of natural resonance. Thus, we can conclude that a coating of an excessive amount of Fe3O4 nanoparticles will lead to the generation of eddy currents by the induced current which is not conducive to attenuating microwave. However, the optimal Fe3O4 nanoparticles’ coating is beneficial for improving the impedance matching. A detailed comparison of the MAP values of the investigated materials and values reported recently in the related literature is summarized and shown in Table 1. e2 shows wider bandwidth and thinner thicknesses than those of the previously reported materials. Unfortunately, in the process of experiment, we cannot find an effective way to determine the ratio between PANI and MoS2 and investigate the effect of the content of PANI and MoS2 on the MAP of the PANI@MoS2 NWs. However, Zhang et al. prepared a type of MoS2/PANI composite by growing the PANI nanoneedles on the surface of MoS2 nanosheets and investigated the MAP. With the increase of PANI, the σ was enhanced. According to eq 7, dielectric loss is the sum of εp″ and εc″. It is obvious that εc″ is related to σ. Thus, we can conclude that the more PANI, the more energy loss but the less porosity and interface for EM wave polarization, which can be contribute to the synergistic effect of PANI and MoS2.49 The MoS2@Fe3O4@PANI composites with the lamellar structure were discussed in our previous work, and according to the analysis of Figure S8 in the Supporting Information, the special surface area and the average pore size of MoS2@Fe3O4@ PANI composites are 4.384 m2 g−1 and 372 nm, respectively. Compared to MoS2@Fe3O4@PANI composites, the PANI@ MoS2@Fe3O4 NWs possess larger specific surfaces areas and higher porosities, resulting in enhanced MAP by more electric dipolar polarization among the interface. In addition, we pay more attention to the eddy current loss and hysteresis loss as well as impedance matching rather than the influence of the skin effect. However, in this paper, we discussed how the magnetic coating structure plays an important role in the microwave absorption behavior. According to the analysis of Figure S7 and Figure S9 in the Supporting Information, the eddy loss of MoS2@Fe3O4@PANI composites and PANI@MoS2@Fe3O4 NWs can be negligible. Therefore, optimizing impedance matching and simultaneously inhibiting skin effect is an important way to enhance MAP.

2 πf c

α= ×

(μ″ε″ − μ′ε′) +

(μ″ε″ − μ′ε′)2 + (μ′ε″ + μ″ε′)2 (10)

To understand the excellent MAP of the PANI@MoS2@ Fe3O4 NWs, the importance of Z and α should be described in more detail. The Z and the α of the as-prepared PANI@MoS2 NWs, e1, e2, e3, and e4 are shown in Figure 7. It can be clearly seen from Figure 7b that e1, e2, e3, and e4 show a better Z, while

Figure 7. Minimum RL (a), modulus of Z (b) and α (c) for PANI@ MoS2 NWs, e1, e2, e3, and e4.

PANI@MoS2 NWs show a mismatch in the impedance matching characteristics. Moreover, compared with e1, e3, and e4, e2 shows a much larger α and moderate Z. As a result, e2 shows better MAP than e1, e3, and e4. Here, α denotes the attenuation ability of absorbent. Although the α value can originate from dielectric loss and magnetic loss, the α value cannot display the capability of dielectric loss or magnetic loss. In Figure 7c, e2 has a much superior α value than the other samples. The α value of e2 increased from 46.4 to 282.2, whereas the α value of PANI@ MoS2 NWs only increased from 11.89 to 105.31. As shown in Figure 7a, for PANI@MoS2 NWs, the minimum RL can be obtained at 16.7 GHz, where the value of α reaches a maximum and Z is 2.52. For e1, the minimum RL can be obtained at 5.36 GHz, where α only reaches 74 and Z is equal to 0.3478 at 5.36 GHz. For e2, Z is close to 0.7 at 16.8 GHz. The minimum RL value of −49.7 dB can be achieved at 16.8 GHz and 1.3 mm coating, and the α values can be up to maximum (282.2). For e3, Z ≈ 1 at 7.2 GHz and the minimum RL value of −50.65 dB can be achieved at 7.2 GHz and 3.8 mm coating. However, the minimum RL of e4 cannot be obtained at 16.8 GHz, while α values reach a maximum, and Z is close 0.2. Therefore, for the

4. CONCLUSION In summary, according to the analysis of the SEM, XRD, and XPS, the PANI@MoS2 NWs and PANI@MoS2@Fe3O4 NWs 5872


Article


Table 1. Comparisons of Microwave Absorption Performance between Our Work and Related Reports in the Literature material

content (wt %)

RLmin (dB)

thickness (mm)

bandwidth (RL < −10 dB)

erequency range (GHz)

ref

MoS2 NS MoS2/RGO MoS2-Ni-CNTs MoS2-Ni NPs MoS2-CNTs MoS2-CLs Fe3O4/MoS2 MoS2 nanosheets MoS2/PANI MoS2/graphene NiS2@MoS2 MoS2/CNT MoS2@Fe3O4@ PANI MoS2@Fe3O4 NS PANI@MoS2 NWs e1 e2 e3 e4

40 10 30 60 30 60 60 60 60 30 20 50 60 60 30 30 30 30 30

−54.7 −50.9 −50.0 −19.7 −47.9 −69.2 −38.4 −47.8 −44.8 −41.9 −41.0 −46 −40.9 −26.8 −18.9 −47.3 −49.7 −50.6 −35.7

3.5 2.3 2.4 5.0 3.8 1.5 2.4 2.2 1.6 2.4 2.2 2.9 2.3 2.3 1.7 2.3 1.3 3.8 2.6

4.2 5.2 6.0 2.9 5.6 4.8 4.1 5.2 2.4 5.8 4.4 2.0 4.2 4.2 5.1 2.0 6.4 2.0 3.2

8.2−12.4 12.2−18.0 11.9−18.0 4.3−5.9 16.4−17.7 11.4−16.3 9.6−13.7 12.8−18.0 13.5−15.9 12.2−18.0 10.0−14.4 5.6−7.6 8.2−12.4 8.2−12.4 11.4−16.6 3.9−6.0 10.2−16.7 6.3−8.3 8.8−12.0

45 46 47 18 18 18 48 39 49 50 19 20 2 2 this work this work this work this work this work

Cultivating Foundation of Northwestern Polytechnical University. We thank the Analytical & Testing Center of Northwestern Polytechnical University for the testing of TEM.

with hierarchical structures were successfully prepared. By tuning the [Fe3O4 nanoparticles]/[PANI@MoS2 NWs] ratio, PANI@MoS2@Fe3O4 NWs (e2) enhanced the MAP in terms of the minimum RL value (−49.7 dB at a thickness of 1.3 mm) and coating thickness (1.3−1.7 mm) as well as bandwidth with effective attenuation (6.48 GHz at a thickness of 1.7 mm). Thus, e2 is a kind of promising material for use as high-performance EM wave absorbent. More importantly, the absorption mechanisms of the as-prepared samples were investigated and the results revealed that only an optimal Fe3O4 nanoparticles coating is beneficial for optimizing impedance matching and simultaneously inhibiting eddy currents. This should also be considered while designing an excellent absorbent.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b01452. EDS mappings of PANI@MoS2@Fe3O4 NWS (e2), magnified SEM and TEM images of PANI@MoS2 NWs as well as the thickness of MoS2, absorption−desorption isotherm and TGA of PANI@MoS2@Fe3O4 NWS, electrical conductivity and CO curves of a series of PANI@MoS2@Fe3O4 NWS (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: + 86-13186052872. ORCID

Weidong Zhang: 0000-0003-3793-7289 Hongjing Wu: 0000-0002-5575-3224 Shuhua Qi: 0000-0002-6545-4682 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was from the Foundation of Aeronautics Science Fund (Grant 2017ZF53071) and from Excellent Doctorate 5873


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Preparation of Polyaniline@MoS2@Fe3O4 Nanowires with a Wide

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