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Growth of Polyaniline Nanoneedles on MoS Nanosheets, Tunable Electro-Response and Electromagnetic Wave Attenuation Analysis Wen Ling Zhang, Degang Jiang, Xiaoxia Wang, Bo Nan Hao, Ying Dan Liu, and Jingquan Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11656 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 22, 2017

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The Journal of Physical Chemistry

Growth of Polyaniline Nanoneedles on MoS2 Nanosheets, Tunable Electro-response and Electromagnetic Wave Attenuation Analysis Wen Ling Zhang, * † # Degang Jiang, †# Xiaoxia Wang, † Bo Nan Hao, ‡ Ying Dan Liu, ‡ Jingquan Liu*† †

College of Materials Science and Engineering, Institute for Graphene Applied Technology

Innovation, the Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, China ‡

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University,

Qinhuangdao 066004, China #

W. L. Zhang and D. Jiang contributed equally.

ABSTRACT: The purpose of this work was to fabricate high-performance dielectric materials for electrorheological (ER) application and electromagnetic (EM) wave attenuation. Commercial MoS2 bulks were exfoliated into nanosheets via combination of ball-milling and bath sonication procedures, which were used as template for in situ grafting of PANI nanoneedles (PANI-NDs) to afford MoS2/PANI-NDs. The length-diameter (L/D) ratio of PANI-NDs on MoS2 nanosheets was feasibly tuned via modulating the polymerization time. Therefore, the tunable electrical

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conductivity and dielectric properties of the obtained MoS2/PANI-NDs were achieved. Compared with bare MoS2 nanosheets, MoS2/PANI-NDs based ER fluids constructed more robust fibril-like structure governed by the external electric energy and exhibited higher dynamic yield stress (186.8 Pa at 3 kV/mm) with wider applied electric field strength (0-3.0 kV/mm). Furthermore, as a novel EM wave absorbing material, the maximum reflection loss (RL) values of MoS2 nanosheets reached –44.4 dB at 11.48 GHz with the thickness of 3.0 mm while the similar RL values of MoS2/PANI-NDs (-44.8 dB) reached at 14.5 GHz with the thickness of only 1.6 mm. The broad effective EM absorption bandwidth (RL values less than -10 dB) for MoS2/PANI-NDs was observed owing to the synergistic effect of PANI-NDs and MoS2 nanosheets.


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INTRODUCITON Inspired by the extensive achievements of graphene-related researches, other emerging twodimensional (2D) materials such as hexagonal boron nitride (h-BN), phosphorene, graphitic carbon nitride (g-C3N4) and transition metal dichalcogenides (TMDs) have also sparked tremendous interest because of their unique magnetic, thermoelectric, optical and electrochemical properties.1–4 The TMDs materials (e.g., MoS2, MoSe2, WS2 and WSe2) have a general expression of MX2, where a transition metal M is inserted in two layers of chalcogen X. The central transition metal M of MX2 benefits strong coordination with nitrogen atoms in conducting polymers such as polyaniline (PANI) or polypyrrole (PPy), which allows for the growth of conducting polymers onto the TMDs matrix.5,6 Therefore, reasonable combination of MX2 with conducting polymers would be an ideal way to achieve multi-functional materials. As a widely known MX2, MoS2 possesses lamellar structure analogous to graphene. It exhibits an indirect bandgap of 1.2 eV in multi-layer structure and turns into a direct bandgap of 1.8 eV after exfoliation to single layer.7,8 In addition, MoS2 can reach a high current on/off ratio above 108.8-10 Therefore, MoS2 will be an attractive next-generation 2D material like graphene. Graphene and graphene oxide (GO) based materials have been extensively explored as eletrorheological (ER) fluids owing to their superb properties.11-13 The inherent semiconducting behavior of MoS2 is favorable for ER applications to avoid electric short during the measurements. Thus, MoS2 can also be considered as a competitive material for ER applications, though there are few investigations for the ER performance of MoS2 based materials.14,15 Intelligent materials, which spontaneously respond to external stimuli such as electric or magnetic field, mechanical stress, pH or photon radiation, have received a great deal of attention in various fields.16 ER fluids are a kind of smart suspensions consisting of fine polarizable


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particles (the dispersed phase) dispersed in an insulating medium (the continuous phase).17-20 When exposed to an electric field, the dispersed particles are polarized and attract to each other to bridge fibrillar structure in the gap of the electrodes, generating reversible changes in rheological properties in milliseconds.21 The ER fluids usually display distinguishing advantages, such as low electrical power consumption, reversible behavior, fast response and have been widely utilized for numerous controllable engineering applications, including medical haptic devices, vibration dampers, engine mounts, brakes, waves and shock absorbers.22,23 Very recently, Tao et al. reported the production of much healthier and tastier chocolate using the changeable rheological behaviors of cocoa powders in liquid chocolate under an electric field.24 In spite of the widespread potential applications of ER fluids, there are still several limitations for practical commercialization, such as relatively low yield stress, a narrow work temperature and particle sedimentation. There are some effective approaches to enhance the ER performances, such as adjusting the architectures of dispersed particles and doping ions, etc.25-28 Among these advanced materials, composites with conducting polymers and inorganics are desirable owing to their high dielectric behaviors and tunable electric conductivity. Over the past years, PANI has been studied as common conducting polymers because of the ease of synthesis, chemical redox reversibility, good chemical stability and low cost.29-31 Herein, we synthesized a novel architecture by coupling PANI nanoneedles (PANI-NDs) on MoS2 nanosheets matrix via an in-situ oxidation polymerization. MoS2 bulks were previously exfoliated into nanosheets via the combination of ball milling and sonication procedure. Ball milling can effectively cleave MoS2 bulks into nanosheets from the top/bottom surfaces through shearing force and compression force. Subsequent liquid phase sonication further breaks larger sheets into smaller nanosheets. The obtained MoS2 nanosheets possess high surface area and


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active edges, which could generate new properties. In general, the freshly prepared MoS2 layers can easily re-stack together via Van der Waals interaction. The attachment of PANI-NDs on MoS2 nanosheets through electrostatic attraction afforded MoS2/PANI-NDs architectures with layered structure, which possess electrochemical active components, high surface area and tunable dielectric properties. These unique features will facilitate the absorption of electromagnetic (EM) microwaves. The effective microwave absorbers need to possess some features, including high reflection loss, broad EM wave absorption bandwidth, lightweight and thin matching thickness.32-34 Traditionally, EM wave absorbing materials are mainly based on nano-sized dielectric or magnetic materials.35-37 Significant efforts have been devoted to the development of PANI-based composites for EM wave absorbing materials, where PANI could improve the dielectric loss, reduce the particle density, and realize impedance matching. The unusual properties of MoS2 based materials impart them a new type of attractive EM wave absorbers.38,39 In this study, composites of PANI-NDs on MoS2 matrix were fabricated via an insitu oxidation polymerization, their ER behaviors and EM wave absorbing performances were investigated.

EXPERIMENTAL SECTION Materials All chemicals were used as received without further purification. MoS2 bulk powder and ammonium persulphate (APS) were purchased from Tianjin Heowns Biochemical Technology Co. Ltd. Aniline monomer and ethanol were supplied by Tianjin Guangfu Technology Development Co. Ltd. 1-Methyl-2-pyrrolidinone (NMP) was obtained from Sinopharm Chemical Reagents Co. Ltd.


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Fabrication of MoS2 nanosheets Suspension of MoS2 powder (0.5 g) and NMP (400 mL) was put into a stainless steel container. The ball milling process was carried out at a rotation speed of 350 rpm for 12 h. Then the suspension was sonicated for 24 h to exfoliate the MoS2 layers. The suspension was centrifuged at 2000 rpm for 10 min, followed by washing with ethanol to afford the final MoS2 nanosheets product. Fabrication of MoS2/PANI-NDs MoS2/PANI-NDs were synthesized via an in-situ oxidation polymerization. In brief, the assynthesized MoS2 nanosheets (50 mg) were dispersed into the mixture of 1 M HCl (10 mL) aqueous solution and ethanol (13 mL) by sonication. Aniline (40 µL) was added into the above solution. APS aqueous solution with a molar ratio of 1:1.5 (APS: aniline) was quickly added in the suspension. The final solution was kept at −15 °C in fridge for 24 h (denoted as MoS2/PANINDs-24) or 10 h (denoted as MoS2/PANI-NDs-10). The products were collected after washing by ethanol and ultrapure water. The electrical conductivity of MoS2/PANI-NDs was adjusted via a dedoping process at different pH values. The pH value was adjusted to be 9.5 by either NaOH (1 M) or HCl (1 M) aqueous solution. Characterization Field-emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F) and transmission electron microscopy (TEM, JEOL JEM-2100) were used to observe the morphology of the obtained samples. Scanning transmission electron microscopy (STEM) (Model JEM 2100F, JEOL) was carried out to characterize the distribution of the elements. X-ray photoelectron spectroscopy (XPS, Escalab 250XI) signals were acquired. Raman spectra were measured using a 488 nm laser, RENISHAW RM2000 Raman system. The Brunauer-Emmett-Teller (BET)


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specific surface area was conducted on a BET surface area analyzer (V-sorb 2800S). Fourier Transform infrared spectroscopy (FT-IR) spectra were measured on a Nicolet Nexus FT-IR spectrometer. The ball milling process was carried out by a planetary ball mill (BXQM2L, Nanjing Telunxin Instrument Co., Ltd) using stainless steel balls of 4 mm (200 grains) and 8 mm (100 grains) in diameter. As for main topic in this study, the ER fluids were fabricated by dispersing the obtained particles in silicone oil (100 cSt) using a vortex and sonicator, the particle concentration is 15 wt%. The ER performances were measured using a rotational rheometer (MCR 502, Physica, Austria). The suspension was loaded into a cup of a concentric cylinder cell (CC17, gap distance is 0.71 mm). For EM parameters measurements, the samples were prepared with 60 wt% of as-obtained composites and 40 wt% paraffin ceresin, and then the resulting mixtures were pressed into a cylindrical shaped mold with the outer and inner diameters of 7.00 mm and 3.04 mm, respectively. Based on the transmission/reflection coaxial line method the complex relative permittivity and permeability of the composites were measured from 2.0 to 18.0 GHz employing an Agilent N5244A vector network analyzer. RESULTS AND DISCUSSION

Scheme 1. Schematic illustration of the synthesis of the MoS2/PANI-NDs.


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The overall synthetic procedure of MoS2/PANI-NDs involves two steps as illustrated in Scheme 1. MoS2 powders were dispersed in NMP with ball-milling and sonication process, respectively. After washing using ethanol, the exfoliated MoS2 nanosheets were re-dispersed into the mixture of HCl and ethanol by sonication. After the addition of aniline into the above solution, the oxidizing agent, APS was added. The final suspension was kept at −15 °C in fridge for 24 h or 10 h, the resultant products were collected after washing with ethanol and ultrapure water.

Figure 1. TEM images of MoS2 bulks (a), MoS2 nanosheets (b), TEM and SEM images of MoS2/PANI-NDs10 (c, e) and MoS2/PANI-NDs-24 (d, f). STEM image of MoS2/PANI-NDs-24 (g) and the corresponding elemental mapping of C, N, Mo, and S (h).

The shape and size are crucial factors in controlling the chemical and physical performances of nanomaterials. Figure 1a, b shows the TEM images of MoS2 bulks and MoS2 nanosheets,


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respectively. Compared with the opaque MoS2 bulks (Figure 1a), the much thinner MoS2 nanosheets were observed (Figure 1b). In addition, the size of MoS2 nanosheets was significantly reduced to less than 500 nm, evidencing the successful exfoliation procedure. From the TEM and SEM images of MoS2/PANI-NDs (Figure 1c-f), PANI-NDs were grown uniformly and vertically on MoS2 nanosheets matrix, exhibiting the sea cucumber-like morphology. Moreover, the length to diameter (L/D) ratio of PANI-NDs increased obviously with the increasing polymerization time. The STEM image and elemental mapping of MoS2/PANI-NDs-24 were obtained (Figure 1h, g). As shown in Figure 1h, the distributions of Mo and S elements are consistent, as those for the N and C elements. It is a remarkable fact that the distribution areas of N and C elements are larger those of the Mo and S elements, which indicated that the PANI-NDs were grown on the surface of MoS2 matrix.


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Figure 2. FT-IR (a) and Raman (b) spectra of MoS2 nanonanosheets (i), MoS2/PANI-NDs-10 (ii) and MoS2/PANI-NDs-24 (iii).

FT-IR spectra of the resulting samples are shown in Figure 2a. For the MoS2/PANI-NDs, typical peaks of PANI were observed. The characteristic bands at 1562 and 1480 cm-1 correspond to the C=C stretching vibration mode of the quinoid and benzenoid rings, respectively.40, 41 The peak located at 1298 cm−1 attributes to the C−N stretching vibration of an aromatic conjugation. The peaks at 1130 and 802 cm−1 should be associated with the C−H inplane bending and out-of-plane bending. The peak at 1653 cm−1 is attributed to the N-H vibration.6 The Raman spectrum of MoS2 nanosheets exhibited three peaks approximately at 282, 376, and 403 cm−1 (Figure 2bi), which are similar to the peaks observed for MoS2/PANI-NDs (Figure 2bii, iii), indicating that the MoS2 nanosheets were not re-stacked after the attachment of PANI-NDs. The nitrogen adsorption−desorption isotherms of the resultant samples are shown in Figure S1. A typical hysteresis loop was observed in all of the samples, indicating their porous structure. The specific surface area of MoS2 nanosheets after mechanical exfoliation was increased from 3.49 m2/g to 27.75 m2/g. After the growth of PANI-NDs-24, the surface area was further increased to 33.30 m2/g which attributes to the unique structure of PANI-NDs.


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Figure 3. XPS survey spectra for MoS2/PANI-NDs-10 and MoS2/PANI-NDs-24 (a), Mo 3d and S 2p spectra for the commercial bulk MoS2 (b, c), Mo 3d, S 2p and N 1s spectra for MoS2/PANI-NDs-10 (d-f) and MoS2/PANI-NDs-24 (g-i).

To further clarify the chemical states of the samples, X-ray photoelectron spectroscopy (XPS) measurements were investigated and the results are shown in Figure 3. From the survey spectra of MoS2/PANI-NDs (Figure 3a), only signals of S, Mo, C, and N were detected. As shown in Mo 3d spectrum of commercial bulk MoS2 (Figure 3b), two peaks at 229.5 and 232.6 eV can be indexed as the doublet Mo 3d5/2 and Mo 3d3/2, respectively.42,43 In the S 2p spectrum (Figure 3c), the peaks at 162 and 163.5 eV should be attributed to the S 2p3/2 and S 2p1/2, respectively,44 evidencing the oxidation state of divalent sulfide ions (S2−), respectively. For the Mo 3d spectrum of MoS2/PANI-NDs (Figure 3d, g), two peaks at 229 and 232.6 eV should be related to Mo 3d5/2 and Mo 3d3/2. As shown in Figure 3e, h, the peaks at 163.6 and 162.5 eV assigned to S 2p were observed. From the N 1s spectra as shown in Figure 3f, i, the peaks at about 395.5 eV were observed, which are the characteristic peaks of PANI as reported.42, 44


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Figure 4. Shear stress of MoS2 nanosheets (a), MoS2/PANI-NDs-10 (b), and MoS2/PANI-NDs-24 (c) based ER fluids with shear rate ramped from 1 to 1000 1/s. For the mentioned ER fluids: Re-plotted τy vs. E, the solid

lines are obtained from the equation τy∝Em (d), plot of ̂ and , the solid line is obtained from Eq. 4 (e) and

plot of ̂ and , the solid line is obtained from Eq. 5 (f).

Rheological behaviors of the ER fluids were tested to verify their flow behaviors in a controlled shear rate (CSR) mode. As shown in Figure 4a-c, without the external electric field, the shear stress increased in proportion to the growth of the shear rate, similar to a Newtonianlike fluid.45 Under an escalated electric field, the shear stress increased progressively and the dynamic yield stress generated, representing the typical Bingham plastic behavior. At a constant


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electric field, the shear stress exhibited a plateau region until the critical shear rate ( ). At a low shear rate, the electrostatic interactions developed by the electric fields are dominant compared to the hydrodynamic interactions induced from the shearing flow field. With the increasing shear rate, the constructed fibrillar structures are destroyed by the shearing deformation, and the broken structures tend to re-form with the assistant of applied electric field. Above a critical value, the destruction velocity of the fibrils became faster than the re-formation velocity. The turbulent fluid destroyed the fibril-like structures completely, a fluid-like behavior appeared again.46 In addition, the plateau regions of MoS2/PANI-NDs based ER fluids (Figure 4b, c) were wider than that of the MoS2 nanosheets based ER fluid (Figure 4a). The reasonable explanation is that the unique MoS2/PANI-NDs arrays can build robust and dense chain-like structures under the electric field and generate strong electrostatic interactions. Compared with MoS2 based ER fluid (1.8 kV/mm), a higher max. applied electric field strength (3 kV/mm) for MoS2/PANI-NDs based ER fluids was observed, which should be contributed to the decreased electrical conductivity of PANI though the dedoping process. The ER efficiency I, is a critical parameter to estimate the ER activities. It can be defined as I=(τE-τ0)/τ0 (where τE and τ0 are the shear stress with and without an external electric field, respectively). As shown in Table 1, the ER efficiency of MoS2/PANI-NDs based ER fluids was higher than that of bare MoS2 based ER fluid at a constant electric field. The MoS2/PANI-NDs24 based ER fluid exhibited highest ER efficiency especially at higher electric field strength. This can be contributed to the fact that the MoS2/PANI-NDs-24 composites could densely compact and form robustly field-induced fibril-like structures. The results strongly suggested that the introduction of high dielectric PANI-NDs in MoS2 matrix can largely enhance the ER performance and the applied electric field intensity for the MoS2 nanosheets based ER fluids.


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Table 1. ER efficiency of the ER fluids at shear rate of 0.1 1/s.

In addition to the ER efficiency, we also investigate the dependence of dynamic yield stress (τy) on applied electric field strength (E) and the results are shown in Figure 4d. Generally, τy exhibits a power-law dependence on E as follows: ∝

(1)

For an electrostatic polarization model (m = 2.0), ER effect is mainly dominated by the electrostatic polarized interactions between the particles.47,48 In a conduction model (m=1.5), the flow behaviors of ER fluid arise due to the mismatch (conductivity or permittivity) between particles and medium.12 In previous studies on the relationship between τy and E, m deviates from 2 and goes to 1.5, particularly at higher electric field. To study these issues, Choi et al. have proposed a universal yield stress equation:49

⁄

τ E αE

⁄

(2)

where α is related to the dielectric properties of the fluid, particle volume fraction or other analogous parameters, Ec represents the critical electric field strength, which can be obtained at the crossover point of the slopes from 2.0 to 1.5, respectively. Eq. 2 can be simplified using E0 as shown in Eq. 3.


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τ αE ∝ E , for E ≪ E' τ αE ⁄ ∝ E(.* , for E ≫ E' (3)

As shown in Figure 4d, all of these ER fluids exhibited a crossover point from 2.0 to 1.5, the value of Ec was 1 kV/mm. In order to fit these data into a single curve, we normalized Eq. 3 using Ec and τy(Ec): / / 0/ tanh E τ, 1.313E

/ ≡ E ⁄E' and τ, ≡ τ E ⁄τ E' (4) where E

As shown in Figure 4e, there was an obviously bias from the lines and the points. To fit the experimental data accurately, one additional parameter b was employed. Then, Eq. 4 can be derived as: / 0/ tanh7E / τ, 1.313E

/ ≡ (89 and τ, ≡ τ, :9 where E

(5)

Figure 4f shows the universal fitting using Eq. 5 with b = 0.62, 0.6 and 0.54 for MoS2, MoS2/PANI-NDs-10 and MoS2/PANI-NDs-24, respectively. All of these yield stress points were located in the universal line precisely.


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Figure 5. Effect of switching the applied electric field on shear viscosity (the shear rate was 1 1/s) of MoS2/PANI-NDs based ER fluids with a square voltage pulse (t=20 s). The applied electric field strength was 0.3 kV/mm, 0.5 kV/mm, 0.7 kV/mm, 0.9 kV/mm, 1.0 kV/mm, 1.2 kV/mm, 2.5 kV/mm and 3 kV/mm, respectively.

The response of shear viscosity to a voltage pulse signal was measured to study the reproducibility of ER fluids. As shown in Figure 5, when an external electric field was switched on, the shear viscosity immediately jumped to a higher level. The shear viscosity of MoS2/PANINDs-24 reached a higher value than that of MoS2/PANI-NDs-10 based ER fluid at a constant electric field strength.


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Figure 6. Frequency sweep of MoS2/PANI-NDs-10 (a) and MoS2/PANI-NDs-24 (b) based ER fluids.

To study the viscoelastic properties of the ER fluids, oscillation measurements were conducted. Figure 6 shows the respective plots of storage modulus (G′) and loss modulus (G") vs. angular frequency (ω) in the linear viscoelastic region. The viscoelastic performances of the MoS2/PANI-NDs based ER fluids changed in an analogous pattern. The values of G′ became mostly constant at a fixed electric field as the ω increased up to 100 rad/s, indicating the stable field-responsive elastic properties. Moreover, the value of G′ was always higher than that of G", indicating that the field-induced elastic property was dominant compared with the viscosity. Focusing on the higher value of G′ for MoS2/PANI-NDs-24 than that of the MoS2/PANI-NDs-


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10, PANI-NDs with increasing L/D ratio can build robust fibril structure developed by electric field, thus higher elasticity can be obtained. As novel microwave absorption (MA) materials, the L/D ratio of PANI-NDs could influence the conductivity of MoS2/PANI-NDs, and then influence their MA properties, which could be compared and evaluated by the values of RL, based on the transmission line theory by the following equations: 50-52

RL(dB) = 20log

Z in + Z0 Zin + Z0

(6)

Where Z0 is the impedance of free space and Zin is the input characteristic impedance, which can be expressed as below:

Z in = Z 0

µr 2π fd tanh(j µr ε r ) εr c

(7)

Where d, c, and ƒ represent the absorber thickness, the velocity of light, and the microwave frequency. Based on the measured real (ɛ', µ') and imaginary (ɛ'', µ'') parts of the complex relative permittivity and permeability, and using the following equations: ɛr = ɛ'-jɛ'', µr = µ'-jµ'', the values of ɛr and µr were obtained. In general, a suitable absorber for actual application should meet the requirement of RL value less than -10 dB, which means more than 90% incident electromagnetic wave was absorbed.


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Figure 7. Real (a) and imaginary parts (b) of complex permittivity and real (c) and imaginary parts (d) of permeability of pure MoS2 nanosheets and MoS2/PANI-NDs-10 in the frequency range of 2.0-18.0 GHz.

The real and imaginary parts of relative permittivity (ε' and ε'') of pure MoS2 nanosheets and MoS2/PANI-NDs-10 were investigated in the frequency range of 2.0-18.0 GHz and the results are shown in Figure 7. Obviously, MoS2/PANI-NDs-10 showed much higher ε' and ε'' values than pure MoS2 due to the addition of conductivity of PANI-NDs. The ε' values of MoS2/PANINDs-10 were in the range of 4.19-9.12 GHz, and the ε'' values decreased gradually except a resonance peak at about 15 GHz.53 The variation tendency of complex permittivity for pure MoS2 nanosheets was similar to the change of MoS2/PANI-NDs-10 except the resonance peak appeared at about 10.5 GHz.38 This result demonstrated that the dielectric properties of MoS2/PANI-NDs composites could be tuned through the addition of PANI-NDs. From the free electron theory 54: ε'' = 1/2ε0πρƒ, where ε0 is the permittivity of vacuum, ρ is the resistivity, and ƒ is the frequency of the microwave, it can be concluded that the high electronic conductivity can lead to high ε'' value. Therefore, the conductivity of MoS2/PANI-NDs-10 is higher than pure MoS2 due to the addition of high conductivity of PANI. However, too high ε''


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means that dielectric loss and magnetic loss are easily out of balance, and this would induce poor MA performance.55 Figure 7c and 7d showed the complex variation of µ' and µ'' values of the samples. The low µ' and µ'' values should be originated from the poor magnetic properties of MoS2 and PANI. The µ' and µ'' values of samples exhibited more fluctuation, which might be due to the natural resonance. It is noteworthy that many µ'' values of the imaginary permeability of samples were negative, which indicated that the magnetic energy have been radiated out from the samples.55

Figure 8. 3D representations of RL of MoS2 nanosheets (a) and MoS2/PANI-NDs-10 (b).

To reveal the influence of the frequency and the thickness of absorber on the MA performance, three-dimensional representation RL curves of pure MoS2 nanosheets and MoS2/PANI-NDs-10 with various thicknesses (0.5-5 mm) were achieved. As shown in Figure 8, the minimum RL values of pure MoS2 nanosheets and MoS2/PANI-NDs-10 were -44.40 dB at 11.48 GHz and -44.8 dB at 14.5 GHz with thickness of 3.0 and 1.6 mm, respectively, and the corresponding RL values less than -10 dB were in the ranges of 11.09-11.51 GHz (the bandwidth was 0.42 GHz) and 13.5-15.9 GHz (the bandwidth was 2.4 GHz), respectively. These MA results demonstrated that although the minimum RL value for MoS2/PANI-NDs-10 was not improved significantly compared with MoS2 nanosheets, the absorbing bandwidth of MoS2/PANI-NDs-10 was broadened obviously due to the synergistic effect between MoS2 and PANI-NDs, and this is


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a favourable factor to absorb EM waves located at different frequency. To prove the high conductivity of PANI have important influence on the MA performance for MoS2/PANI-NDs, the Three-dimensional (3D) representation of RL values of MoS2/PANI-NDs-24 were investigated (Figure S2a and S2b). The minimum RL value was only -8.6 dB at 14.3 GHz with the thickness of 1.5 mm, which demonstrated that the higher conductivity of PANI from the increasing L/D ratio had negative impact on the MA performance. In addition, the permittivity and MA performance of MoS2 bulks were investigated, too (Figure S2c and 2d). The minimum RL value of -16.2 dB was obtained at 9.3 GHz with the thickness of 4 mm. Therefore, the morphology of PANI-NDs will produce a profound effect on the conductivity of MoS2/PANINDs, and then deeply influence the RL values and the frequency bandwidth. By adjusting the L/D ratio of MoS2/PANI-NDs array, wide bandwidth frequency MA attenuation could be obtained for MoS2/PANI-NDs-10. For MoS2 nanosheets, the defect dipole polarization arising from Mo and S vacancies and the high specific surface area will all contribute to the enhanced MA performance.38


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Figure 9. Illustrations of novel architecture of MoS2/PANI-NDs for ER polarization model (a) and EM propagation model (b).

The understanding of morphology−performance relationship can help us design desirable functional materials. The excellent ER behaviours of the MoS2/PANI-NDs are closely related to their special architecture feature as illustrated in Figure 9a. The thin MoS2 nanosheets and flexible PANI-NDs promote fast electron transportation from one MoS2/PANI-NDs unit to another. Under an external electric field, the MoS2/PANI-NDs array with high-dielectric properties triggers the increasing synergistic polarization, which is beneficial to enhance the ER performances. The unique architecture also leads to outstanding EM wave attenuation as illustrated in Figure 9b. When an incidence EM wave enters into the MoS2/PANI-NDs, the high interfacial polarization loss should be contribute to the MA performance for the MoS2/PANINDs-10 due to a number of electrons can migrate along the MoS2 interlayer to PANI-NDs (Figure 7b). In addition, the propagation paths of ER wave will increase owing to the enlarged surface area of MoS2 nanosheets and densely packed PANI-NDs array, generating multiple reflection loss and excellent EM absorption performances. CONCLUSION In summary, the commercial MoS2 bulks have been successfully exfoliated into MoS2 nanosheets via effective ball-milling and bath sonication procedures. Subsequently, the PANINDs were grown on MoS2 nanosheets matrix via the in-situ oxidative polymerization to afford the MoS2/PANI-NDs arrays. The unique architectures of MoS2/PANI-NDs provided enhanced dielectric properties, fast electron transportation and tunable electric conductivity. These advantageous properties led to excellent electro-responsive effects with widely applied electric field strength. The outstanding MA behaviours of MoS2 nanosheets and MoS2/PANI-NDs were


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also observed. Particularly, the maximum RL value of MoS2 nanosheets was –44.4 dB at 11.48 GHz with thickness of 3 mm and the maximum RL value of MoS2/PANI-NDs reached -44.8 dB at 14.5 GHz with much thinner thickness of 1.6 mm. Broad bandwidth frequency for the MoS2/PANI-NDs was observed owing to the synergistic effect of PANI-NDs and MoS2 nanosheets. The novel architecture of MoS2/PANI-NDs has opened a new strategy for designing smart materials in aerospace and Mechanical automation fields. Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.” The N2 adsorption-desorption isotherms of MoS2 bulks, MoS2 nanosheets, and MoS2/PANINDs-24. The real and imaginary parts of complex permittivity and permeability, and the threedimensional representation of RL for MoS2 bulks and MoS2/PANI-NDs-24. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (W. L. Zhang) *E-mail: [email protected] (J. Liu)

Author Contributions #

W. L. Zhang and D. Jiang contributed equally.

ACKNOWLEDGMENT This work was supported by Qingdao Basic & Applied Research project (15-9-1-100-jch), the National Natural Science Foundation of China (Grant No. 21603115), China Postdoctoral Science Foundation (2016M592136), the Open Fund of State Key Laboratory of Metastable


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Growth of Polyaniline Nanoneedles on MoS 2 ... - ACS Publications

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