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Constructing 2-, 0- and 1-dimensional integrated nanostructures: an effective strategy for high microwave absorption performance Yuan Sun, Jianle Xu, Wen Qiao, Xiaobing Xu, Weili Zhang, Kaiyu Zhang, Xing Zhang, Xing Chen, Wei Zhong, and Youwei Du ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11443 • Publication Date (Web): 02 Nov 2016 Downloaded from http://pubs.acs.org on November 9, 2016

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ACS Applied Materials & Interfaces

Constructing 2-, 0- and 1-Dimensional Integrated Nanostructures: an Effective Strategy for High Microwave Absorption Performance Yuan Sun,a Jianle Xu,b Wen Qiao,c Xiaobing Xu,a,d Weili Zhang,a Kaiyu Zhang,a Xing Zhang,a Xing Chen,a Wei Zhong*,a and Youwei Du a

a) Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructures and Jiangsu Provincial Laboratory for NanoTechnology, Nanjing University, Nanjing, 210093, China. b) School of Physics and Technology, Wuhan University, Wuhan, 430072, China. c) College of Science, Henan University of Technology, Zhengzhou, 450001, China d) College of electronic Engineering, Nanjing Xiaozhuang University, Nanjing, 210017, China.

KEY WORDS MoS2, carbon nanotubes, Ni nanoparticles, electromagnetic properties, microwave absorbing materials

ABSTRACT A novel ‘201’ nanostructure composite consisting of 2-dimensional MoS2 nanosheets, 0-dimensional Ni nanoparticles and 1-dimensional carbon nanotubes (CNTs) was prepared successfully by a two-step method: Ni nanopaticles were deposited onto the surface of few-layer MoS2 nanosheets by a wet chemical method, followed by chemical vapor deposition growth of CNTs through the catalysis of Ni nanoparticles. The as-prepared 201-MoS2-Ni-CNTs composites exhibit remarkably enhanced microwave absorption performance compared to Ni-MoS2 or Ni-CNTs. The minimum reflection loss (RL) value of 201-MoS2-Ni-CNTs/wax composites with filler loading ratio of 30 wt% reached -50.08 dB at the thickness of 2.4 mm. The maximum effective microwave absorption bandwidth (RL< -10 dB) of 6.04 GHz was obtained at 1

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the thickness of 2.1 mm. The excellent absorption ability originates from appropriate impedance matching ratio, strong dielectric loss and large surface area, which are attributed to the ‘201’ nanostructure. In addition, this method could be extended to other low-dimensional materials, proving to be an efficient and promising strategy for high microwave absorption performance.

INTRODUCTION Many efforts have been made to search low-thickness, light-weight and high-performance microwave absorbing materials (MAMs) in the past decades owing to the wide and important applications in different regions.1-6 Traditionally, ferrites have been investigated extensively as efficient MAMs because they can make the electromagnetic waves enter availably and avoid high-frequency skin effect.7 Nevertheless, the density of ferrites is fairly high, which greatly limits their application. To solve this problem, many low-density materials have been utilized as MAMs including conducting polymers and carbon-based materials. At the same time, with the development of nanoscience, various nanoscale materials have been synthesized for enhanced microwave absorption performance. For example, nanostructured carbon-based materials, e.g., carbon nanofibers,8-9 carbon nanocoils,10-11 carbon nanotubes12-13 and graphene sheets,14-15 have attracted much attention due to their excellent physical and chemical properties. Specifically in CNTs, many strategies have been applied to improve their microwave absorption performance, including combining CNTs with polymer matrices16-17 and magnetic coating.18-19 Cao et al.20 combined these two strategies to fabricate PANI/Fe3O4/CNTs hybrids, which exhibited broad band effective absorption (RL< -10 dB) in a frequency range of 7 GHz with the thickness of 4 mm. Likewise, 2D reduced graphene oxide is another representative of carbon materials possessing optimized microwave attenuation and shielding performance which attributed to their higher specific surface area and clustered defects.21 As another 2D material, molybdenum disulfide (MoS2), has recently attracted significant attention for its unique electrical and optical properties in contrast with its bulk counterparts. Latest progresses in fields of energy,22 photoluminescence,23 transistor24 and biotechnology25 applications of MoS2 nanosheets have further strengthened the cognitions to this promising 2D material. In addition, 2D MoS2 has great potentials in field of microwave absorbtion according to recent researches. Ning et al26 reported that chemical exfoliated MoS2 nanosheet would be an efficient MAM. The minimum reflection loss value of MoS2 nanosheet/wax was -38.42 dB at the thickness of 2.4 mm and the maximum bandwidth with effective electromagnetic wave absorption was up to 4.1 GHz (9.6–13.76 GHz). Moreover, Liang and co-workers27 found that hydrothermal synthesized MoS2 nanosheets exhibited enhanced microwave absorption performance. The optimized RL was -47.8 dB with thickness of 2.2 mm and the effective absorption bandwidth reached 5.2 GHz 2


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(12.8-18.0 GHz) with thickness of 1.9 mm. However, as a novel efficient MAM, MoS2 is inevitably faced with some problems including comparatively high density and intrinsic low conductivity loss, which hinder further practical applications. Hence, it is quite necessary to incorporate MoS2 with low-density, high-conductivity-loss materials such as CNTs, to improve its microwave absorbing performance. Up to now, hybrids of MoS2 and CNTs have been investigated for their enhanced performance in many fields including catalysts,28-29 lithium ion batteries,30-32 supercapacitors33 and field-effect transistors.34 However, as far as we are concerned, the electromagnetic properties and microwave absorption performance of MoS2/CNTs hybrid has not been reported. Herein, we report a new method to integrate 2D MoS2, 0D Ni nanoparticles with 1D CNTs by Ni nanoparticles catalyzed CNTs CVD growth on the surface of MoS2 nanosheet. With the introduction of Ni, we have constructed a novel 201-MoS2-Ni-CNTs nanostructure with enhanced microwave absorbing performance. The minimum RL value of 201-MoS2-Ni-CNTs/wax composites (with filler loading ratio of 30 wt%) was calculated to be -50.08 dB at the thickness of 2.4 mm and the maximum effective electromagnetic wave absorption bandwidth of 6.04 GHz was achieved with the thickness of 2.1 mm. Possible mechanisms accounting for these results are discussed.

EXPERIMENTAL SECTION

1. Materials All reagents were of analytical grade and used without further purification. Single layer MoS2 powder was purchased from Nanjing XFNANO Materials Tech Co., Ltd. Sodium hydroxide (NaOH), nickel chloride hexahydrate (NiCl2·6H2O), hydrazine hydrate (N2H4·H2O, 80%), ethylene glycol ((CH2OH)2) were purchased from Nanjing Chemical Reagent Co., Ltd. 2. Sythesis of Ni Nanoparticles Decorated MoS2 Nanosheets (Ni-MoS2): Ni-MoS2 were prepared using a method reported by Huang et al.35 Typically, 30 mg single layer MoS2 powder was ultrasonic dispersed in 80 mL ethylene glycol. 0.4 mL of 1 M NiCl2·6H2O solution, 1.5 mL of N2H4·H2O and 4 mL of 1 M NaOH were added into MoS2-ethylene glycol solution under stirring, respectively. The mixure was heated in a water bath at 60 °C for 3h. The black precipitates were centrifuged and dried at 60 °C under vacuum overnight. 3. Sythesis of 201-MoS2-Ni-CNTs Nanocomposites: 10 mg of the as-prepared Ni-MoS2 were transferred into a quartz plate and placed in a tube furnace. The furnace was heated to 450 °C in Ar. Then a flow of acetylene (flow 3



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rate= 23 sccm) was introduced into the tube at 450°C for 0.5 h under atmosphere pressure. After heating, the tube was cooled down to room temperature under the protection of Ar. The preparation process of 201-MoS2-Ni-CNTs is illustrated by Scheme 1.

Scheme 1. Schematic illustration of the two-step fabrication of 201-MoS2-Ni-CNTs composites. 4. Sythesis of Ni-CNTs: For better understanding of the microwave absorption performance, we also fabricated CNTs catalyzed by Ni nanoparticles. Typically, 0.4 mL of 1M NiCl2·6H2O solution, 1.5 mL of N2H4·H2O and 4 mL of 1 M NaOH were added into ethylene glycol under stirring, respectively. The mixure was heated in a water bath at 60 °C for 3 h. The black precipitates were centrifuged and dried at 60 °C under vacuum overnight. 10 mg of the as-prepared Ni nanoparticles were transferred into a tube furnace for CNTs growth. The growth method is the same as above. 5. Characterization and Measurement The morphology of the samples was examined by transmission electron microscopy (TEM) (Model JEOL-2010, Japan) and field-emission scanning electron microscopy (FE-SEM) (FEI Helios600i, USA). The element mapping was carried out by an energy dispersive spectrometer (EDS) (Oxford X-MAX 50, UK). Powder diffraction data were collected from 10° to 80° in 2θ using an X-ray diffractometer (TD-3500, China) with Cu Kα (λ = 1.5418 Å) radiation. Raman spectroscopy was carried out on a HR 800 Raman Microscope (HORIBA Jobin Yvon, France) equipped with a 633 nm laser. The content of Ni was measured on an inductively coupled plasma (ICP) spectrometry (PERKINELMER Optima 5300DV, USA). The magnetic properties of the samples were measured using a superconducting quantum interference device (SQUID) magnetometer (Quantum design MPMS-XL, USA). Surface area by the Brunauer−Emmett−Teller (BET) calculation method, and pore-size distributions by the Barret−Joyner−Halenda method (BJH) were determined from nitrogen adsorption-desorption isotherms measured by an accelerated surface area and porosimetry system (Micromeritics ASAP 2460, USA). The relative complex permittivity (εr) and permeability (µr) were measured by a vector network analyzer (Agilent PNA 5242A, USA) in the frequency range 2 −18 GHz. The absorber were made by mixing the as-prepared samples with wax at specific loading ratio and 4


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pressed into a coaxial cylinder with an outer diameter of 7.0 mm and inner diameter of 3.0 mm.

RESULTS AND DISCUSSIONS 1. Morphology of Ni-MoS2 and 201-MoS2-Ni-CNTs:

Figure 1. TEM images of Ni-MoS2. (a) Low-resolution TEM, (b) SAED pattern and (c) HRTEM image.

Figure 1 shows the typical TEM images of Ni-MoS2. It can be identified in Figure 1a that Ni nanoparticles are distributed on MoS2 nanosheet. The mean diameter of Ni nanoparticles is measured to be ca. 11 nm (Figure S2). According to the selected area electron diffraction (SAED) pattern of Ni-MoS2 hybrid (Figure 1b), the hexagonal structure of MoS2 with Ni (111) diffraction ring can be clearly identified. The high-resolution TEM (HRTEM) image of Ni-MoS2 in Figure 1c illustrates measured lattice distance of the sample, where 2.0 Å refers to Ni (111) lattice distance and 2.7 Å refers to MoS2 (100) lattice distance, respectively.

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Figure 2. TEM images of 201-MoS2-Ni-CNTs. (a,b) Low-resolution TEM, (c,d) HRTEM images.

The morphology of 201-MoS2-Ni-CNTs hybrid is investigated by TEM and FE-SEM. Typical TEM images of 201-MoS2-Ni-CNTs are shown in Figures 2a-b, which display that different size of Ni nanoparticles and CNTs are deposited on the surface of MoS2 nanosheet. The HRTEM image (Figure 2c) of 201-MoS2-Ni-CNTs hybrid clearly reveals the lattice fringe of MoS2 (002) planes (lattice distance measured to be 6.2 Å) and MoS2 (100) planes (lattice distance measured to be 2.7 Å).36 Note that only 2-3 of the MoS2 (002) fringes are discovered in Figure 2c, confirming that the MoS2 nanosheet consists of few layers. In Figure 2d, the spacing of 2.1 Å confirms the formation of crystalline Ni nanoparticles. In addition, typical lattice fringes of carbon are seen in both Figures 2c and d, indicating that CNTs are deposited on the surface of Ni-MoS2. In Figures 3a-c, FE-SEM images demonstrate that Ni-MoS2 is well-coated by network-like CNTs. Besides, the CNTs in the FE-SEM images exhibit helical and 6


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worm-like morphology.37 In Figure 3d, nozzles of CNTs are clearly visible. EDS element mapping of 201-MoS2-Ni-CNTs (Figure 4) proves the existence of Mo, S, and Ni beneath CNTs, which further confirms Ni-MoS2 is coated by CNTs.

Figure 3. FE-SEM images of 201-MoS2-Ni-CNTs. (a-c) low-resolution FE-SEM image, (d) magnified FE-SEM image, the nozzles of CNTs are marked by red arrows.

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Figure 4. EDS element mapping of 201-MoS2-Ni-CNTs.

2. Crystal structure of Ni-MoS2 and 201-MoS2-Ni-CNTs:

Figure 5. (a) XRD patterns of Ni-MoS2 and 201-MoS2-Ni-CNTs. (b) Raman spectrum of Ni-MoS2 and 201-MoS2-Ni-CNTs

Figure 5a displays the X-ray diffraction (XRD) pattern of Ni-MoS2 and 201-MoS2-Ni-CNTs. Standard diffraction peaks of 2H-MoS2 (JCPDS card No. 37-1492) and Ni (JCPDS card No. 04-0850) are also given. As the red curve in Figure 5 shown, all peaks of Ni-MoS2 are matched with standard peaks. According to Xu et al.38 and our previous work,39 the relatively weak and broad (002) peak of 2H-MoS2 appeared at 14.2°indicating that most of the MoS2 nanosheets are few-layer (2-3 8


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layers). The XRD pattern of 201-MoS2-Ni-CNTs (Figure 5a black curve) manifests MoS2 (002) peak at 14.2° and Ni (111) peak at 44.6°. Besides, two broad diffraction peaks centered at 25.6° and 44.4° are attributed to (002) and (101) reflections of hexagonal graphite of CNTs. Note that the (002) peak of MoS2 in black curve remains weak and broad as the (002) peak in red curve, and the HRTEM result above proved existence of few-layer MoS2. Combining these two facts we can find that MoS2 nanosheets remained their few-layer structure throughout CNTs growth. The crystal structure is further investigated by Raman spectroscopy with 633 nm laser excitation. The Raman spectrum of Ni-MoS2 (Figure 5b red curve) exhibits E12g (centered at 375.9 cm-1) and A1g (centered at 402.5 cm-1) peaks of MoS2.40 After CNTs growth, two strong peaks centered at 1335.9 cm-1 and 1586.6 cm-1 (Figure 5b blue curve) arise which correspond to D band and G band of CNTs, respectively. In the meantime, E12g and A1g peaks of MoS2 diminish distinctly, which attributes to the screening effect of the CNTs.30,41

3. Magnetic properties:

Figure 6. Magnetization curves of (a) 201-MoS2-Ni-CNTs and (b) Ni-MoS2 measured at 300 K. Insets are the enlarged parts of magnetization curves close to the origin.

Figure 6 shows the magnetization–coercivity (M–H) curves of 201-MoS2-Ni-CNTs and Ni-MoS2 at 300 K. The saturation magnetization (MS) and the coercivity (HC) of 201-MoS2-Ni-CNTs are 0.114 emu/g and 8.8 Oe, respectively (Figure 6a). According to ICP spectrometry analysis, the Ni content in 201-MoS2-Ni-CNTs is measured to be 0.4 wt%. Based on the fact that MS of bulk Ni is ca. 54.39 emu/g at 300 K, we estimated the MS of the sample should be 0.218 emu/g, much higher than the MS measured experimentally. This result can be explained as the uneven distribution of Ni nanoparticles in the sample, which caused nonuniformity in magnetic properties.10 9



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In addition, the M–H curves of Ni-CNTs exhibit a combination of diamagnetism and ferromagnetism (Figure S3), where the diamagnetism may arise from CNTs42 and the ferromagnetism is caused by Ni nanoparticles. Therefore, the MS and HC of 201-MoS2-Ni-CNTs are much smaller than those of Ni-MoS2, which are 6.81 emu/g and 35.6 Oe, respectively (Figure 6b), possibly due to the growth of CNTs.

4. Microwave absorption The reflection loss coefficients were simulated from the measured relative complex permittivity and permeability at various thickness of the absorber. According to transmission line theory,43 the input impedance (Zin) and the RL coefficient of the sample can be calculated by following equations:

Zin = Z0 ( µr / εr ) tanh  j(2π fd / c)( µrε r )1/2 

(1)

RL = 20log ( Zin − Z0 ) / ( Zin + Z0 )

(2)

1/2

where Zin is the input impedance of the absorber, Z0 the impedance of free space, f the frequency of microwaves, c the velocity of light, µr the relative complex permeability, µ r = µ '− j µ '' , εr the complex permittivity, ε r = ε '− jε '' , and d the thickness of the

absorber. A RL value of -10 dB is comparable to 90% microwave absorption. The frequency range where RL values below -10 dB can be defined as an effective absorption bandwidth.

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Figure 7. 3D RL plots of 201-MoS2-Ni-CNTs/wax composites with filler loading ratios of (a) 20 wt%, (b) 30 wt%, (c) 40 wt%. (d) 3D RL plots of Ni-CNTs/wax composites with filler loading ratios of 30 wt%. (e) 3D RL plots of Ni-MoS2/wax composites with filler loading ratios of 30 wt%. (f) RL curves of 201-MoS2-Ni-CNTs/wax (20, 30, 40 wt%), Ni-MoS2/wax (30 wt%)and Ni-CNTs/wax (30 wt%) composites at the thickness of 2 mm.

Table 1. Microwave absorption performance of the samples presented in our work Loading ratio (wt%)

Minimum RL (dB) (corresponding thickness)

Maximum effective absorption bandwidth (frequency range)(GHz)

20

-6.68 (2.5 mm)

N/A

30

-50.08 (2.4 mm)

6.04 (11.96-18.00)

40

-14.84 (1.5 mm)

5.40 (12.56-17.96)

Ni-MoS2/wax

30

-25.19 (1.6 mm)

4.20 (13.80-18.00)

Ni-CNTs/wax

30

-7.54 (1.2 mm)

N/A

Filler/matrix

201-MoS2-Ni-CNTs/wax

Microwave absorption performance of 201-MoS2-Ni-CNTs/wax composites with filler loading ratios of 20, 30, 40 wt% and Ni-MoS2/wax, Ni-CNTs/wax composites with filler loading ratios of 30 wt% are displayed in Figure 7. The results are listed in Table 1. It is obvious that 201-MoS2-Ni-CNTs/wax with filler loading ratio of 30 wt% reaches the best microwave absorption performance (Figure 7f and Table 1). RL values of 201-MoS2-Ni-CNTs/wax (30 wt%) composites at the thickness range of 1.5-5.0 mm are illustrated in Figure 8a. The minimum RL value of -50.08 dB is reached at the thickness of 2.4 mm (Figure 8b) and the maximum effective absorption bandwidth of 6.04 GHz (11.96 - 18.00 GHz) is achieved at the thickness of 2.1 mm (Figure 8c). Compared to other related literature (Table S1), the 201-MoS2-Ni-CNTs/wax (30 wt%) composites achieve enhanced performance.

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Figure 8. (a) RL curves of 201-MoS2-Ni-CNTs/wax composites with the filler loading ratio of 30 wt% at different thicknesses. (b) Minimum RL of -50.08 dB of 201-MoS2-Ni-CNTs/wax composites (30 wt%) at thickness of 2.4 mm. (c) Maximum effective absorption bandwidth of 6.04 GHz (11.96-18.00 GHz) of 201-MoS2-Ni-CNTs/wax composites (30 wt%) at the thickness of 2.1 mm.

As far as we are concerned, the excellent microwave absorption performance of 201-MoS2-Ni-CNTs/wax composites (30 wt%) originates from 3 aspects: appropriate impedance matching ratio, strong dielectric loss and large surface area. The relative complex permittivity (ε′, ε′′), relative complex permeability (µ′, µ′′), dielectric and magnetic loss tangent (tan δE=ε′′/ε′, tan δM=µ′′/µ′) of 201-MoS2-Ni-CNTs/wax with the filler loading ratios of 20, 30, 40 wt% are shown in Figure S4. The ε′ and ε′′ exhibit a gradual enhancement with the increasing in filler loading ratio in Figures S4a and b, which can be explained rationally by the effective medium theory.44 According to transmission line theory, microwave absorption performance can be improved by achieving a good impedance matching ratio. Therefore, compared to the filler loading ratios of 20, 40 wt%, 201-MoS2-Ni-CNTs/wax with 30 wt% achieves more appropriate ε′ and ε′′ to meet a better requirement of impedance matching, resulting in a significant enhancement in microwave absorption. In Figures S4d and e , the µ′ and µ′′ values of 201-MoS2-Ni-CNTs/wax (filler loading ratios of 20, 30, 40 wt%) remain closely to 1 and 0 at low frequency (2-6 GHz), respectively. Especially, the values of µ′′ of 201-MoS2-Ni-CNTs/wax are negative, which indicates that the magnetic energy was radiated out from the composites.45,46 However, tan δM of 201-MoS2-Ni-CNTs (filler loading ratios of 20, 30, 40 wt%) (Figure S4f) are much lower than tan δE of the sample (Figure S4c). This result indicates that magnetic loss may contribute little to the microwave absorption performance.

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Figure 9. Real part (ε′, a) and imaginary part (ε″, b) of permittivity, dielectric tangent loss values (tan δE, c), Real part (µ′, d) and imaginary (µ″, e) of permeability, and magnetic tangent loss values (tan δM, f) of 201-MoS2-Ni-CNTs/wax, Ni-MoS2/wax and Ni-CNTs/wax composites with the loading ratio of 30 wt%

Dielectric loss is another factor contributing to the microwave absorption performance of 201-MoS2-Ni-CNTs/wax composites. In general, dielectric loss contains two sections: conductivity loss and relaxation loss.5,47 On the basis of Debye theory, the ε′′ can be expressed as:

ε '' =

εs − ε∞ σ ωτ + 2 2 ωε 0 1+ ω τ

(3)

where εs is the static permittivity, ε∞ is the relative dielectric permittivity at the high frequency limit, ε0 is the dielectric constant in vacuum, ω is angular frequency, τ is polarization relaxation time and σ is conductivity. As is illustrated in Figure 9b, the ε′′ of Ni-CNTs/wax are much higher than Ni-MoS2/wax and 201-MoS2-Ni-CNTs/wax, which can be ascribed to the better conductivity that Ni-CNTs possess. In addition, no obvious relaxation peaks appeared in the inset of Figure 9b and tan δE of Ni-CNTs/wax are much higher than the other two samples in Figure 9c. According to these two facts, the conductivity loss is believed to be the main factor in the dielectric loss of Ni-CNTs/wax composites. Therefore, the growth of CNTs on Ni-MoS2 resulting in an enhancement in ε′′ and tan δE compared to Ni-MoS2 (Figure 9c) can be understandable. However, the extremely high ε′, ε′′ and tan δE causes imbalanced impedance matching in Ni-CNTs/wax and finally leads to poor microwave absorption performance (Figure 9d). Thus, in comparison with Ni-CNTs, the 13



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201-MoS2-Ni-CNTs possess more eﬃcient complementarity between the permittivity and permeability due to the contribution of few-layer MoS2 nanosheets. On the one hand, MoS2 nanosheets act as large 2D substrate for the growth of CNTs, which would modulate the transfer of carriers6 and further decrease the overwhelming permittivity of CNTs to meet a better impedance matching ratio. On the other hand, MoS2 nanosheets could provide additional opportunity to access relaxation loss which may arise from defects and interfaces. In the case of Ni-MoS2/wax composites, two strong relaxation peaks (centered at 13.84 GHz and 17.32 GHz, respectively) and two weak relaxation peaks (centered at 7.44 GHz and 9.60 GHz, respectively) are observed clearly in Figure 9b (blue curve). Whereas in 201-MoS2-Ni-CNTs/wax, these 4 peaks remain unmoved and no other relaxation peaks appear. This fact indicates that instead of MoS2 and CNTs, the interface between Ni and MoS2 is responsible for the interfacial polarization loss in 201-MoS2-Ni-CNTs/wax composites. In addition, the defects in MoS2 nanosheet may also contribute to the interfacial polarization. Figure 9d-f demonstrates the complex permeability and magnetic tangent loss of the samples and negative µ′′ values are also observed. Compared to dielectric loss, magnetic loss is less important as discussed above. In conclusion, the strong dielectric loss in 201-MoS2-Ni-CNTs/wax composites is attributed to relaxation loss and conductivity loss. The relaxation loss arises from the interface polarization of Ni and MoS2. The conductivity loss is improved by introducing CNTs in Ni-MoS2 (Scheme 2). It is proved previously that nanoporous structure with high surface area can also improve microwave absorption.48-49 The incident microwave could be scattered and reflected between numerous interfaces (Scheme 2), thus, improving the possibility of the microwave to be absorbed. In addition, more surface area causes more defects to generate defect polarization, which further enhance the absorption of incident microwave.45 In our case, the N2 absorption and desorption isotherms and size distribution based on the BJH method for the 201-MoS2-Ni-CNTs are shown in Figure S5. The BET surface area is calculated to be 97.3 m²/g. Compared to our previous work (Co/carbon nanotube– graphene, SBET= 71.4 m²/g),50 the surface area of 201-MoS2-Ni-CNTs is higher because of the formation of CNTs network. The network-like CNTs deposited on the surface of Ni-MoS2 offer an additional opportunity for multiple reflection of the incident wave, which could be partly responsible for microwave absorption and attenuation. Therefore, the relatively high surface area also contributes to the excellent microwave absorption performance of 201-MoS2-Ni-CNTs.

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Scheme 2. Schematic illustration of possible microwave absorption mechanisms of 201-MoS2-Ni-CNTs composites

CONCLUSION In summary, we developed a new method to synthesize 201-MoS2-Ni-CNTs nanostructured composites. The core of the hybrid consists of Ni nanoparticles deposited few-layer (2-3 layers) MoS2 nanosheets. The CNTs have been grown in situ onto the surface of Ni-MoS2 nanosheets catalyzed by Ni nanoparticles to form the ‘201’ nanostructure. By integrating 2D MoS2, 0D Ni with 1D CNTs, the microwave absorption performance has dramatically improved. The minimum RL of the as-prepared 201-MoS2-Ni-CNTs/wax composites is calculated to be -50.08 dB and the maximum effective absorption bandwidth (RL

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