Hierarchical FeCo@MoS2 Nanoflowers with Strong Electromagnetic

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Hierarchical FeCo@MoS2 Nanoflowers with Strong Electromagnetic Wave Absorption and Broad Bandwidth Chenhui Zhou, Chen Wu, and Mi Yan ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01203 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 5, 2018

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ACS Applied Nano Materials

Hierarchical FeCo@MoS2 Nanoflowers with Strong Electromagnetic

Wave

Absorption and

Broad

Bandwidth Chenhui Zhou, Chen Wu* and Mi Yan* State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Keywords: (FeCo@MoS2 composites, hierarchical nanoflowers, electromagnetic wave absorption, effective bandwidth, impedance matching)

ABSTRACT: Advanced electromagnetic (EM) wave absorbing materials are technologically important with extensively increasing utilization of EM waves in the GHz range. Hierarchical structure containing FeCo nanoparticles composited with MoS2 nanoflowers has been developed as an effective EM wave absorber. Large surface area and sufficient voids provided by the MoS2 nanoflowers enable uniformly dispersed FeCo nanoparticles with single domain size for enhanced permeability and natural resonance. Existence of the large amount of the FeCo/MoS2 interfaces also improves the dielectric loss. The composite not only exhibits satisfactory impedance matching over a wide frequency range, but also provides synergistic effect between magnetic and dielectric loss for effective dissipation of the wave energy. Excellent wave absorption performance have been achieved for the composite with a maximum reflection loss of

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-64.64 dB at 14.4 GHz and a broad effective absorption bandwidth (RL < -10 dB) of 7.2 GHz at a small thickness of 2 mm. Consequently, a novel type of high-performance wave absorber has been developed and insights for the design of advanced EM wave absorbers with strong absorption capacity and broad effective bandwidth have also been provided.

1. Introduction The development of electromagnetic (EM) wave absorbing materials with strong absorption capacity and broad effective bandwidth at a small thickness is strategically important to meet the ever-rising requirements for electronic and telecommunication devices.1-7 Various materials including carbon,8 conducting polymers,9 magnetic ferrites10 and alloys11 have been exploited for effective absorption. Among these candidates, magnetic alloys such as FeCo exhibit satisfactory absorption due to their high saturation magnetization and permeability.12-14 The permeability in the GHz range however, is restricted by the Snoek’ limit. To break such limit, easy magnetization planes have been introduced by making the magnetic powders into flake shape.1517

An alternative method is to reduce the particle size to nanometer range for enhanced surface

anisotropy.18-21 It is however, difficult to achieve well-dispersed nanoparticles due to the large surface energy, and agglomeration of the nanoparticles leads to deteriorated permeability and magnetic loss.22-23 Another challenge in the application of FeCo nanoparticles as effective EM wave absorber is to achieve impedance matching over a wide frequency range.24 Many efforts have been devoted to fabricating composites containing FeCo and dielectric materials, such as FeCo/Al2O3,25 FeCo/ZnO26 and FeCo/C/BaTiO3.27 Despite of tunable impedance in these cases, the permeability and magnetic loss are sacrificed due to magnetic dilution. Consequently, investigations on simultaneous achievement for strong absorption and broad effective bandwidth remain limited.


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Two-dimensional (2D) materials including graphene, MXenes and transition metal dichalcogenides (TMDs) have been intensively investigated due to its potential applications in catalysis, sensing and energy storage.28-33 As a typical TMD, MoS2 has attracted significant attention owing to its unique electrical, optical and mechanical properties.34-36 Recently, both chemically exfoliated and hydrothermally synthesized MoS2 nanosheets have been used as EM wave absorbers due to the large dielectric loss derived from interfacial and dipole polarization.3738

Here MoS2 has been used as a suitable dielectric component to composite with FeCo

nanoparticles due to the following reasons: i) abundant defects for enhanced dipole polarization, ii) large surface area supporting uniform growth and distribution of FeCo nanoparticles and iii) heterogeneous interfaces between MoS2 and FeCo for improved interfacial polarization. Here MoS2 with a unique three-dimensional (3D) flowerlike structure have been used which provides plenty of nucleation sites and voids for the uniform growth of FeCo nanoparticles. The dispersed FeCo nanoparticles exhibit single domain effect for enhanced permeability and natural resonance in the GHz range. Synergistic effect between the magnetic and dielectric loss not only gives rise to effective attenuation of the EM energy but also is beneficial to achieve impedance matching over a wide frequency range. This study sheds light on the design of novel EM wave absorbers with strong absorption capacity and broad effective bandwidth. 2. Results and Discussion


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Figure 1. Schematic illustration showing the synthesis of the FeCo@MoS2 nanoflowers. Figure 1 schematically illustrates the synthesis of the hierarchical FeCo@MoS2 nanoflowers where ultrathin MoS2 nanosheets self-assemble into flowerlike spheres via the hydrothermal process. The MoS2 was then dispersed in a solution containing Fe3+ and Co2+ which coprecipitate to form CoFe2O4 nanoparticles covering the petals of the MoS2. The CoFe2O4@MoS2 samples were then subjected to H2 reduction in the formation of FeCo@MoS2 composites. The blank MoS2 is denoted as S1, while other samples with the FeCo: MoS2 ratio of 1:6, 1:3 and 1:1 are referred as S2, S3 and S4, respectively.


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Figure 2. a) XRD curves of FeCo@MoS2 composites compared with the blank MoS2. Typical XPS spectra taken from the S3, showing the b) survey scan and the c) Mo, d) S, e) Fe and f) Co peaks. Phase constitution of the samples has been investigated by X-ray diffraction (XRD) as shown in Figure 2a. For the S1, diffraction peaks at 2θ = 14.38°, 33.51°, 39.54°, 49.79°, and 58.33° are observed, corresponding to the (002), (100), (103), (105) and (110) reflections of the MoS2 (JCPDS Card No. 04-0831). Additional peaks with increased intensity appear at 2θ = 44.75° and 65.11° for the S2, S3 and S4 which match with the (110) and (200) planes of the Fe7Co3 (JCPDS Card No. 48-1816). The XRD curve in Figure S1 (Supporting Information) confirms the formation of the Fe7Co3 without the addition of MoS2 during the fabrication. Figure S2 shows typical XRD curve for the S3 before H2 reduction, which contains the CoFe2O4 and MoS2 peaks. The disappearance of the CoFe2O4 peaks in Figure 2a indicates complete reduction of the CoFe2O4 into the FeCo during annealing. Figure 2b presents the X-ray photoelectron


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spectroscopy (XPS) survey scan taken from the S3. Detailed Mo 3d (Figure 2c) and S 2p (Figure 2d) spectra show the Mo 3d5/2 and 3d3/2 peaks at approx. 229.4 eV and 232.5 eV, and the S 2p3/2 and 2p1/2 peaks at around 162.2 eV and 163.5 eV, indicating the formation of the 2H phase, since the binding energy of the 1T phase is lower.39-40 Figure 2e exhibits the Fe 2p spectrum where the Fe 2p3/2 and 2p1/2 peaks at approx. 707.3 eV and 720.0 eV correspond to the metallic Fe, while the peaks at 710.7 eV & 724.2 eV as well as 713.7 eV & 725.7 eV can be assigned to the Fe3+ at the octahedral and tetrahedral site, respectively.41-42 The Co spectrum in Figure 2f can also be deconvoluted into the Co0 peaks at 778.5 eV and 794.3 eV as well as the Co2+ peaks at 781.9 eV and 797.4 eV.43 The intensity of the Fe3+ and Co2+ peaks exceeds those of the metallic Fe and Co in the XPS spectra while the oxides are not observed in the XRD data in Figure 2a, indicating that the oxidation only occurs at the thin surface of the FeCo nanoparticles since the XPS is a surface-sensitive technique.


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Figure 3. SEM images taken from the a) & e) S1, b) & f) S2, c) & g) S3 and d) & h) S4. i) Typical TEM image taken from the S3 with Region 1 and Region 2 enlarged as j) and k). l) SAED pattern for S3, confirming the coexistence of the MoS2 and the FeCo. m-q) HAADF image and EDS mappings showing the element distribution of the Mo, S, Fe, and Co. Figure 3a-3h show the scanning electron microscopy (SEM) images taken from the samples with varied FeCo loadings. The blank MoS2 exhibits a flowerlike spherical feature which is maintained for all the samples with increased FeCo content (top panel in Figure 3). The SEM image at higher magnification reveals that the petals of the flower consist of nanosheets with a thickness of approx. 13.6 ± 1.2 nm (Figure 3e). The interspacing between the nanosheets exceeds


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67.1 ± 13.1 nm, serving as ideal nucleation sites for the formation of the FeCo nanoparticles. Uniform distribution of the FeCo nanoparticles (22.9 ± 4.4 nm) is achieved for the S2 with the FeCo: MoS2 ratio of 1:6 (Figure 3f). Increased FeCo: MoS2 ratio of 1:3 in S3 gives rise to saturating nanoparticle loading with almost completely filled interspaces between the nanosheets as shown in Figure 3g. When the FeCo: MoS2 ratio reaches 1:1, large FeCo content in S4 leads to complete cover of the MoS2 and the interspaces is invisible (Figure 3h). Severe aggregation of the FeCo particles is observed in Figure S3 without MoS2 addition, indicating that the abundant surfaces of the MoS2 is beneficial for homogeneous nucleation of the FeCo nanoparticles. Similar morphology to the FeCo@MoS2 composites is observed for the CoFe2O4@MoS2 before H2 reduction (Figure S4). Transmission electron microscopy (TEM) image in Figure 3i reveals detailed microstructure of the S3 where regions with different contrast, including the gray nanosheets and dark nanoparticles are observed. Region 1 and 2 indicated in Figure 3i are enlarged as Figure 3j and 3k where measured lattice spacing in the high-resolution TEM images are approx. 2.02 Å and 6.40 Å, corresponding to the (110) plane of the Fe7Co3 and the (002) plane of the MoS2, respectively. Selected area electron diffraction (SAED) pattern in Figure 3l exhibits ring feature with diffraction spots corresponding to the crystalline MoS2 and the FeCo. EDS mappings of the Mo, S, Fe and Co taken from the area as shown in Figure 3m illustrates uniform distribution of the Mo and the S (Figure 3n and 3o), while granular distribution of the Fe and the Co is observed in Figure 3p and 3q, corresponding to the nanoparticles.


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Figure 4. a) M-H curves of the FeCo@MoS2 composites. b) Detailed saturation magnetization (Ms) and coercivity (Hc) of the S2, S3 and S4. Hysteresis loops measured from the FeCo@MoS2 composites all exhibit the ferromagnetic feature as shown in Figure 4a. Detailed saturation magnetization (Ms) and coercivity (Hc) of the S2, S3 and S4 are plotted in Figure 4b. The Ms increases significantly from 6.2 emu/g to 40.6 emu/g with raised FeCo ratio from 14.3 mol% to 50.0 mol%. The Hc of the composites exhibits initial decreasing followed by increment with raised FeCo content, which can be explained by variation of the grain size of the nanoparticles as shown in Figure S5.44 The pure FeCo sample presents the highest Ms of 210.7 emu/g and the lowest Hc of 200.8 Oe due to severe aggregation of the nanoparticles (Figure S6) since the particle size also affect the Hc.45-46


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Figure 5. Reflection loss calculated for the a) S1, b) S2, c) S3 and d) S4. Figure 5 shows the reflection loss (RL) calculated based on the measured complex permittivity and permeability for given absorber thicknesses previously reported.47-48 The MoS2 alone (S1) exhibits the poorest absorption with the minimum RL of -4.77 dB at 3 mm (Figure 5a). Slightly improved absorption is observed for the S2 with the minimum RL of -9.42 dB obtained at 2.5 mm (Figure 5b). The best absorption performance is achieved for S3 with the minimum RL of 64.64 dB at 14.4 GHz together with an effective absorbing bandwidth (EAB) (RL < -10 dB) of 7.2 GHz (9.84 GHz -17.04 GHz) at a small absorber thickness (2 mm) in Figure 5c. The absorption performance deteriorates with further increased FeCo content in S4 for which the minimum RL is -36.53 dB at 4.8 GHz with significantly reduced EAB (1.6 GHz) at 3 mm


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(Figure 5d). The FeCo alone also exhibits weak absorption ability with the absorption peaks shift to the low frequency range (Figure S7). For the S3 with the optimal EM wave absorption, effects of the paraffin ratio on the RL have also been investigated as shown in Figure S8. The sample containing 58 vol% paraffin still presents a broad bandwidth of 6.4 GHz (10.88 GHz -17.28 GHz) with a reduced minimum RL of 26.08 dB (Figure S8a), and the minimum RL is only 10.16 dB when the paraffin ratio reaches 70 vol% (Figure S8b). Consequently, the best wave absorption performance can be achieved for S3 with the addition of 45 vol% paraffin.

Figure 6. Comparison of the minimum RL and EAB with related material systems. Figure 6 shows detailed minimum RL and EAB of the FeCo@MoS2 composites compared with other MoS2- and FeCo-based EM wave absorbers reported recently.49-52 It has been rather challenging to achieve RL< -60 dB and the effective bandwidth > 6 GHz simultaneously. The hierarchical FeCo@MoS2 nanoflowers with FeCo: MoS2 ratio of 1:3 obtained in this work exhibit both large RL (-64.64 dB) and EAB (7.2 GHz), which is promising to be highperformance EM wave absorbers.


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Figure 7. a) Real part (ε′) and b) imaginary part (ε″) of the permittivity measured from S1 to S4. c) Dielectric loss tangent (tan δe) calculated based on the ε′ and ε″. d) Real part (µ′) and e) imaginary part (µ″) of the permeability measured from S1 to S4. f) Magnetic loss tangent (tan δm) calculated based on the µ′ and µ″. The absorption performance is determined by the complex relative permittivity (εr = ε′ - jε″) and permeability (µr = µ′ - jµ″).53 The dependence of ε′ and ε″ on the frequency are shown in Figure 7a and 7b. The ε′ of S1 - S4 decreases with increased frequency from 2 GHz to 18 GHz and raised FeCo content gives rise to both increased ε′ and ε″. Figure 7c exhibits variation of the dielectric loss tangent (tan δe = ε″/ε′) which increases from around 0.2 to 0.5 from S1 to S4. Based on the Debye theory, change in the dielectric loss can be attributed to polarization and conductive loss. Cole-Cole curves54-55 have been used to investigate the polarization behavior, which contain semicircles and each semicircle associates with one polarization process. Plot of the ε″-ε′ for S1 (Figure S9a) exhibits two semicircles, which correspond to the MoS2/paraffin interfacial polarization and the dipole polarization originated from the intrinsic defects in MoS2.


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An additional semicircle is observed for the S2, S3 and S4 (Figure S9b-d) which may derive from the interfacial polarization between the FeCo and the MoS2. The conductive loss serves as another important factor which contributes to the overall dielectric loss. Enhanced conductive loss is obtained with raised FeCo content due to the formation of conductive network evidenced by the improved electrical conductivity (Figure S10). Hence the enhanced interfacial polarization and conductive loss simultaneously lead to increased dielectric loss with the introduction of the FeCo nanoparticles. Figure 7d and 7e show the measured complex permeability of the samples. With increased FeCo content from S1 to S4, the µ′ increases from 1 to around 1.8 due to increased Ms and the µ″ also enhances from 0 to around 0.7. The magnetic loss tangent (tan δm = µ″/µ′) follows similar trend with the µ″ (Figure 7f), which may be related to the domain-wall resonance, hysteresis loss, eddy current effect, natural and exchange resonance.56 The domain-wall resonance normally exists for multidomain materials at low frequency (< 100 MHz) and can be neglected here.57 The hysteresis loss is related to the coercivity which depends on the particle size of FeCo. The existence of the MoS2 is critical to the nucleation and distribution of the FeCo particles (22.9 ± 4.4 nm). The particle size is comparable to the single domain size of FeCo (about 10 nm),58 which results in large coercivity and hysteresis loss. Further analysis of the C0 in Figure S11 indicates that natural and/or exchange resonance dominate in the magnetic loss rather than the eddy current effect. The resonance peaks induced by the magnetic nanoparticles can be analyzed by the exchange resonance mode where the resonance frequency f is given by59 = +

(1)

where γ0 is the gyromagnetic ratio (3×107 Oe-1s-1), k is exchange constant (1.73×10-5 erg/cm), the Hc, Ms and D represent the coercivity, the saturation magnetization and the crystal size,


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respectively. The f corresponding to the natural and exchange resonance is related to the γ0Hc and (D2Ms)-1 respectively. In this work, the calculated γ0Hc (around 15 GHz) is close to the resonance frequency of the magnetic loss in Figure 7f and the variation of resonance frequency also agrees with the change of the Hc, indicating that the resonance peaks observed corresponds to the natural resonance. For the pure FeCo, the natural resonance peak is located at around 6 GHz and the peak at 17 GHz may correspond to the exchange resonance (Figure S12). The significant increment in the frequency of the natural resonance with the addition of MoS2 may be resulted from strengthened single-domain effect, due to the small size of the FeCo nanoparticles in the composites. Consequently, the hysteresis loss and natural resonance are the main factors that contribute to the magnetic loss, and the increment of the magnetic loss from S1 to S4 is mainly due to strengthened natural resonance evidenced by the enhanced resonance peak. The simultaneously enhanced dielectric and magnetic loss discussed above leads to the boosted EM wave absorption performance.


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Figure 8. a) The reflection loss of the S3 with the thickness of 2 mm, b) dielectric loss tangent (tan δe) and magnetic loss tangent (tan δm) as well as c) the normalized impedance (Z) as a function of frequency. The strong absorption and broad bandwidth obtained for the S3 (Figure 8a) can be further explained by the synergistic effect of the tan δm and tan δe. Figure 8b shows that the tan δm is close to the tan δe, and both of them exhibit satisfactory contribution for a wide frequency range


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of 9.84 GHz to 17.04 GHz. This gives rise to enhanced dissipation of the EM wave, and the excellent impedance matching as shown in Figure 8c where the real part of the impedance (Z′) is close to 1 and imaginary part (Z″) approaches 0.60-62

Figure 9. Schematic illustration showing the mechanisms involved during the EM wave absorption for the FeCo@MoS2 composite. Based on the analysis above, Figure 9 summarizes the mechanisms involved for the ultrahigh EM wave absorption performance achieved for the FeCo@MoS2 composite. Firstly, excellent impedance matching is achieved by tuning the FeCo content to allow maximum EM wave to enter into the absorber. Secondly, the entering EM wave interacts with the FeCo@MoS2 nanoflowers which induce multiple reflections for significantly enhanced dissipation of the EM energy. Last but not the least, the combination of the FeCo and MoS2 in the composite gives rise


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to multiple loss mechanisms such as the magnetic loss involving the hysteresis loss and natural resonance, as well as the dielectric loss provided by the MoS2 support. The introduction of FeCo nanoparticles also improves conductive loss, as well as generates FeCo/MoS2 and FeCo/paraffin interfaces for enhanced interfacial polarization. 3.Conclusions FeCo@MoS2 nanoflowers with unique microstructure and excellent EM wave absorption performance have been fabricated. The 3D flowerlike microstructure provides large surface area and sufficient voids for the nucleation and growth of the FeCo nanoparticles with uniform distribution and single domain size, which efficiently enhances the permeability and natural resonance. Introduction of appropriate amount of FeCo also improves the conductive loss and generates FeCo/MoS2 interfaces for enhanced dielectric loss. The magnetic and the dielectric loss synergistically function for improved impedance matching and absorption (RL = -64.64 dB at 14.4 GHz) over a wide frequency range (7.2 GHz for RL< -10 dB). This work provides insights and inspiration for the design of novel wave absorption materials with strong absorption capacity and broad effective bandwidth.

4. Experimental Section Materials: Sodium molybdate dihydrate (Na2MoO4·2H2O, 99%), thiourea (H2NCSNH2, 99%), cobalt

nitrate

hexahydrate

(Co(NO3)2·6H2O,

98.5%),

iron(III)

nitrate

nonahydrate

(Fe(NO3)3·9H2O, 98.5%), ammonium hydroxide (NH3·H2O, 25%) and ethanol absolute (C2H5OH, 99.7%) were supplied by Sinopharm Chemical Reagent, China. Synthesis of the Flowerlike MoS2: Na2MoO4·2H2O (14 mmol) and H2NCSNH2 (60 mmol) were dissolved in deionized (DI) water (72 ml) and stirred for 30 min. The mixture was


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transferred into a Teflon-lined stainless steel autoclave (100 ml) and maintained at 220 °C for 6 h. After cooling down to the room temperature, the products were collected by centrifugation, washed with DI water and ethanol for five times, followed by drying at 60 °C in vacuum. Synthesis of the FeCo@MoS2 Composites: Co(NO3)2·6H2O (1.67 mmol) and Fe(NO3)3·9H2O (3.33 mmol) were dissolved in DI water (50 ml) under constant stirring for 15 min. The asprepared MoS2 was added to the solution and stirred for another 30 min followed by ultrasonication for 15 min. The FeCo: MoS2 ratio was varied from 1:6 to 1:3 and 1:1, which was achieved by changing the addition of the MoS2 from 30 mmol to 15 mmol and 5 mmol, respectively. The mixture was then maintained in water bath at 60 °C followed by dropwise addition of the ammonium solution until the pH reached 11. After stirring for 3 h, the black products were collected using the same process described for the blank MoS2. The resultant was annealed under H2 flow at 400 °C to obtain the FeCo@MoS2 composites. Pure FeCo nanoparticles were synthesized using the same preparation procedures without the addition of the MoS2. Characterization and Performance Evaluation: Phase constitution and chemical state of the FeCo@MoS2 composites were investigated by X-ray diffraction (XRD, Rigaku D/Max-2550pc) with Cu Kα radiation (λ = 1.5406 Å) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific ESCALAB 250 Xi). TEM samples were prepared via sonicating the solution containing S3 (20 mg) and ethanol (20 ml) for 30 min, followed by dropping the solution to the carbon covered copper TEM microgrid. The morphology and detailed microstructure were examined by scanning electron microscopy (SEM, Hitachi SU-8010) and transmission electron microscopy (TEM, Titan G2 80-200, FEI). A superconducting quantum interference device (SQUID, Quantum Design MPMS-XL-5) was used to measure the static magnetic properties of


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the nanoflowers, while the electromagnetic parameters (ε′, ε″, µ′, µ″) were investigated with a vector network analyzer (VNA, Agilent PNA N5234A). The volume fraction of paraffin was calculated based on the weight and density of the paraffin, as well as the measured ring volume.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS publications website at DOI:

10.1021/an-2018-01203a

XRD and SEM data of the pure FeCo and CoFe2O4@MoS2 composites, grain size for the FeCo@MoS2 composites, hysteresis loop of the pure FeCo, reflection loss of the pure FeCo and S3 sample with varying paraffin content, ε″- ε′ curves, electrical conductivity and µ″(µ′)-2f-1- f curves of the FeCo@MoS2 composites, complex permeability of the pure FeCo. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS


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This work was supported by the National Natural Science Foundation of China (NSFC11404284, 51571176, 51590881), the Public Technology Application Research Projects of Zhejiang Province (2016C31008) and the National Key Research and Development Program (2016YFB0700902).

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Hierarchical FeCo@MoS2 Nanoflowers with Strong Electromagnetic

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