Self-Supported Construction of Three-Dimensional MoS2 Hierarchical

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Self-Supported Construction of 3D MoS Hierarchical Nanospheres with Tunable High-Performance Microwave Absorption in Broadband Xiaojuan Zhang, Sheng Li, Shan-Wen Wang, Zong-Jie Yin, Jia-Qiang Zhu, Ao-Ping Guo, Guang-Sheng Wang, Penggang Yin, and Lin Guo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06661 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 7, 2016

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The Journal of Physical Chemistry

Self-supported Construction of 3D MoS2 Hierarchical Nanospheres with Tunable High-performance Microwave Absorption in Broadband

Xiao-Juan Zhang,a Sheng Li,a Shan-Wen Wang,a Zong-Jie Yin,b Jia-Qiang Zhu,a Ao-Ping Guo,a Guang-Sheng Wang*,a Peng-Gang Yin*a and Lin Guoa

a

Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of

Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing 100191, PR China b

Guangzhou Special Pressure Equipment Inspection and Research Institute, No.9,

Keyan Road, Huangpu District, Guangzhou, Guangdong 510100, P.R. China

Correspondence should be addressed to Guang-Sheng Wang, E-mail: [email protected]; [email protected] 1

ACS Paragon Plus Environment


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Abstract: The 3D MoS2 hierarchical nanospheres which assembled spontaneously by 2D lamina have been successfully designed and fabricated in large-scale

via

a

simple

hydrothermal

process.

Subsequently,

the

electromagnetic wave absorption properties of hierarchical MoS2 nanospheres compounded with polyvinylidene fluoride (PVDF) were investigated in a broad frequency range of 2-40 GHz. The results indicated that the MoS2/PVDF nanocomposites possess adjustable and enhanced wave absorption performance. Furthermore, the MA performance can be effectively tuned by absorber’s thickness and filler content. In addition, the peculiar hierarchical nanostructure of MoS2 is beneficial to microwave absorption property compared with the bulk-MoS2 and micron-sized MoS2. Moreover, the main microwave absorption mechanism including various polarization, destructive interference theory and multiple reflection have been described in detail.

2


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INTRODUCTION Electromagnetic pollution has raised serious concerns because of the wide applications in the field of military and civil devices such as information security, synthetic aperture radar, satellite communication, personal digital assistants and other communication equipment.1-3 Apart from emerging serious electromagnetic interference (EMI) and information leakage, electromagnetic wave radiation is also harmful to human body. For example, much research has verified that EM wave radiation will induce potential damage such as brain tumor, glioma, acoustic neuroma and so on.4,5 In view of this, developing microwave absorption (MA) materials have been extensively studied to overcome these problems. To date, among many EM-wave absorbing materials, much attention is being paid to nanomaterials because of their low density, distinct size effect and special nanostructure.6-9 Currently, nanostructure engineering has been demonstrated as an effective approach for improving the microwave absorption properties. Numerous nanomaterials with unique morphology have applied in EM wave absorption area, such as aligned CNT films,10 ZnO hollow spheres,11 SnO2/Ni walnut nanocomposites,12 dendrite-like NiCu alloys13 and ultrafine hollow magnetic fibers.14 Furthermore, outstanding EM wave absorption has been achieved by designing hierarchical nanostructures, such as foam,1,15 flower-like superstructures16,17 etc. Nevertheless, the electromagnetic properties of nanomaterials with frequcency extending the range of 2-18 GHz is rarely reported, mainly due to the limitations of testing methods. 18,19 As our previous research, the metal sulfides possess superior wave absorption 3



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performance on account of their excellent physical and chemical properties.20-23 In recent years, as a representative transition-metal dichalcogenides, two-dimensional (2D) few-layer molybdenum disulfide (MoS2) have become an advanced research hotpot due to their distinct physical properties and important application in energy conversion,24 saturable absorber,25 electric devices26 and supercapacitor electrodes.27 The sandwich structure that covalently bound S-Mo-S tri-layers held together by van der Waals attraction is beneficial to form 2D lamellar structure for MoS2 nanomaterials.28 It is interesting to note that the flaky-shaped nanoparticles with high aspect ratio possess an enhanced dielectric relaxation, which makes a significant contribution in exceeding the wave absorption performance.23,29,30 However, as similar to graphene, the few layer nanosheets tend to restack and agglomerate inevitably because of their high surface energy, which will limit the performance in many applications.31 Much research has proved that constructing hierarchical three-dimensional (3D) architectures based on 2D nanosheets is a valid approach to prevent its restacking and aggregation.32-35 To prevent its restacking and enhance the absorption properties in the practical application, herein, we have devised and synthesized 3D MoS2 hierarchical nanospheres which assembled by 2D lamina on a large scale via a simple hydrothermal process and firstly investigated their electromagnetic wave absorption properties in a wide frequency range of 2-40 GHz. Then we compound the hierarchical MoS2 nanospheres with polyvinylidene fluoride (PVDF) for further investigation. The MA performance of MoS2 nanospheres or MoS2/PVDF 4


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nanocomposites can be efficiently tuned via loading content and thickness. Furthermore, the peculiar hierarchical nanostructure of MoS2 is also confirmed to be helpful to enhance the microwave absorption property of the nanocomposites.

METHODS AND MATERIALS Preparation of Hierarchical MoS2 Nanospheres. In a typical synthesis, 0.1 g of sodium molybdate (Na2MoO4·2H2O) was dispersed into 50 mL of de-ionized water by stirring for 10 min. Then 0.2 g of L-cysteine was added. After stirring for 30 min, the homogeneous suspension was transferred into Teflon-lined stainless steel autoclave (100 mL). After reacting in 200 °C for 20 h, the solution was cooled to room temperature. The resultant black precipitate of MoS2 nanospheres was washed several times with distilled water and absolute ethanol and finally dried at 60 °C for 12 h for further characterization. Preparation of MoS2/PVDF Membrane. To fabricate the MoS2/PVDF membrane, PVDF was dissolved in N, N-dimethylformamide (DMF) at room temperature. After the solution became transparent, various contents of synthesized MoS2 powder were added and sonicated for several minutes, then poured them into glassy petri dishes and dried in the oven at 100 °C for 2 h. Characterization. XRD analyses were carried out on an X-ray diffractometer (D/MAX-1200, Rigaku Denki Co. Ltd., Japan). The XRD patterns with Cu Ka radiation (λ= 1.5406 Å) at 40 kV and 40 mA were recorded in the range of 2θ =5o-80o. Field emission scanning electron microscopy (FE-SEM) was on a JSM-6700F 5



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microscope. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) investigations were carried out by a JEOL TEM-2100F microscope. X-ray photon spectroscopy was studied with a Thermo Scientific ESCALAB 250 Xi XPS system. EM Absorption Measurement. The composites used for EM absorption measurement were prepared by mixing the products with wax and PVDF in different mass

percentages,

respectively.

The

mixtures

were

then

pressed

into

cylindrical-shaped samples (Φout = 7.00 mm and Φin = 3.04 mm) and cuboid-shaped samples (a1=10.60 mm, b1=4.20 mm; a2=7.10 mm, b2=3.50 mm). The complex permittivity and permeability values were measured in the 2-18 GHz range with coaxial wire method and in the 18-40 GHz with waveguide method by an Agilent N5230C PNA-L Network Analyzer.

Results and Discussion From an overview of field-emission scanning electron microscopy (FESEM) image (Figure 1a), the hierarchical MoS2 nanospheres are composed of intercrossed curved nanosheets with a thickness of several nanometers. A magnified FESEM image embedded in Figure 1a reveals that the average diameter of MoS2 nanospheres is about in the range of 150-200 nm. Furthermore, the low-resolution SEM images shown in Figure S1 indicate the polydispersity of MoS2 nanospheres. Transmission electron microscopic (TEM) observation shown in Figure 1b further confirms the results of FESEM. High-resolution TEM, selected area electron diffraction (SAED) 6


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and XRD measurement are used to study the crystal structure of MoS2 nanospheres. As shown in Figure 1c, the ordered lattice fringes are clearly observed from a high-resolution TEM image of rectangular area in Figure 1b. The spacing between two neighboring lattice fringes are approximately 0.62 nm and 0.27 nm, corresponding to the (002) and (100) plane respectively. The SAED pattern provided in Figure 1d indicates that the nanosheet is polycrystalline. These results are well accordance with that of XRD pattern (Figure S2), which is in agreement with literature data.36,37 The (002) diffraction peak represents the formation of a well-stacked hierarchical structure composed by MoS2 nanosheets during the hydrothermal process. In addition, the point-scan EDX spectrum displayed in Figure 1e shows that there are only Mo and S elements existed in the nanospheres. And their atomic ratio is close to 2:1. The elemental mapping of Mo and S elements in line-scanning mode corresponds to the EDX result (Figure S3). Furthermore, the good distribution of Mo, S and C elements can be revealed by elemental mapping for the fracture section of MoS2/PVDF membrane in area-scanning mode, indicating that the hierarchical MoS2 nanospheres disperse in PVDF matrix uniformly, as seen in Figure S4. To further verify the chemical state of Mo atom, the elemental analysis of the as-synthesized MoS2 powders is performed through the X-ray photoelectron spectroscopy (XPS). It certainly turns out that the Mo atom exists as Mo4+ ion. The two peaks at 232.5 eV (Mo4+ 3d3/2) and 229.1 eV (Mo4+ 3d5/2) in Figure 1f are from the semiconducting 2H-phase MoS2, and those at 231.8 eV (Mo4+ 3d3/2) and 228.6 eV (Mo4+ 3d5/2) are assigned to metallic 1T-phase MoS2.38 7



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Figure 1 (a) FESEM image. Inset: magnified FESEM image, (b) TEM image; (c) High-resolution TEM image, (d) SAED pattern, (e) EDX spectrum, (f) XPS spectra of hierarchical MoS2 nanospheres (a. b: scale bar = 100 nm; c: scale bar = 10 nm; d: scale bar = 10 1/nm).

As mentioned in the experimental section, the hierarchical MoS2 nanospheres are synthesized by a hydrothermal method using sodium molybdate and L-cysteine as initial precursor. With the increase of reaction temperature, L-cysteine begins to decompose and release H2S, which acts as the sulfur source and reducing agent for the formation of MoS2 nanosheets. Meanwhile, multifunctional groups (-SH, -NH2, and –COO–) of L-cysteine can also conjugate with Mo-containing ions and contribute to the growth of MoS2 nanosheets.31,39 Due to the laminar growth habit of MoS2, the 8


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agglomerated nuclei tend to self-assemble into a sphere-like microstructure to reduce the interfacial energy of nanosheets.35,40 The possible growth mechanism of hierarchical MoS2 nanospheres is depicted in Scheme 1.

Scheme 1. The growth mechanism of hierarchical MoS2 nanospheres.

To further verify the growth mechanism of hierarchical MoS2 architecture in detail, the growth processes are systematically studied by analyzing the samples at different growth stages. It is found that the reaction time and concentration of L-cysteine have important effect on the morphology of final products. Figure 2a-c show a series of FESEM images of MoS2 samples by varying the reaction time from 6 h, 8 h to 12 h at 200 °C. As shown in Figure 2a, the product is composed of irregular nanosheets aggregated together. With the increase of reaction time, the obtained nanospheres are assembled closely by nanosheets spontaneously. When the reaction time reaches to 12 h, the nanospheres can be formed completely (Figure 2b and 2c). Except for reaction time, the concentration of L-cysteine is another important factor to influence morphology of MoS2. When the concentration of L-cysteine reduces to 0.1 g, the synthesized nanospheres are larger and more hierarchical due to the increased system 9



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space (Figure 2d). On the other hand, when the concentration of L-cysteine increases to 0.3 g, it can also be clearly observed that there is no enough space for nanosheets to assemble into nanospheres (Figure 2e). Similarly, the nanospheres cannot be formed when

increasing

the concentration

of

sodium

molybdate and

L-cysteine

simultaneously (0.2 g/0.4 g), as seen in Figure 2f.

Figure 2 FESEM images of MoS2 samples under different conditions. At various reaction time: (a) 6 h, (b) 8 h and (c) 12h at 200 °C. At various concentrations of L-cysteine (200 °C, 20 h): (d) 0.1 g, (e) 0.3 g, (f) at reduplicated concentrations (Na2MoO4:0.2 g/ L-cysteine: 0.4 g, 200 °C, 20 h) (scale bar = 100 nm).

The material’s MA ability of different frequency band can be evaluated via coaxial wire method (2-18 GHz) and waveguide method (18-40 GHz) respectively. Just as shown in Figure S5a, the test samples from left to right represent for wave absorption 10


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measurement in the frequency range of 2-18 GHz, 18-26.5 GHz and 26.5-40 GHz severally. Except for high-performance MA property, the MoS2/PVDF membrane has good ductility and flexibility, as shown in Figure S5b, S5c. Moreover, in preparation stage, the repeatability and stability of hierarchical MoS2 nanospheres can make the products be put into mass production (as seen in Figure S6). To investigate the electromagnetic wave absorption properties of hierarchical MoS2 nanospheres, various contents of the products are mixed with wax or PVDF to form composites by a simple blending method. Based on the measured data of permittivity and permeability, the reﬂection loss (RL) of MoS2/wax and MoS2/PVDF composites can be calculated under the transmission line theory,41 R = 20 log

Z in − 1 Z in + 1

(1)

Where Zin is the input characteristic impedance, which can be expressed as:42 Z in =

µr  2 fπd  tanh  j ( ) µ rε r  c εr  

(2)

Where, εr and µr (for MoS2, µr is thought as 1) are the complex permittivity and permeability of the composite absorber, respectively; ƒ is the frequency; d is the thickness of the absorber, and c is the velocity of light in free space. Figure 3a shows the theoretical reflection loss (RLs) of MoS2/wax and MoS2/PVDF nanocomposites at a thickness of 2.5 mm in the frequency range of 2-18 GHz. It is observed that the MoS2/PVDF composites with filler loading of 25 wt% possess the most enhanced EM wave absorption property. The minimum reflection loss value can reach -26.11 dB at 11.36 GHz, and the frequency bandwidth less than -10 dB is from 9.92 to 13.36 GHz. Figure 3b-f show the three-dimensional presentations of 11



calculated theoretical RLs of the prepared nanocomposites with different thickness (2-5 mm) in the range of 2-18 GHz at various filler contents, respectively. This indicates the microwave absorption ability of MoS2/wax and MoS2/PVDF composites at different frequencies can be tuned effectively by controlling the thickness of the absorbers and the filler contents.

(a)

2.5 mm

0 Reflection Loss (dB)

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-5 -10 -15 10 wt% sample+wax 20 wt% sample+wax 10 wt% sample+PVDF 20 wt% sample+PVDF 25 wt% sample+PVDF 30 wt% sample+PVDF

-20 -25 -30 2

4

6

8 10 12 14 16 18 Frequency (GHz)

Figure 3 (a) Microwave RL curves of the composites with a thickness of 2.5 mm in the frequency range of 2-18 GHz. Three-dimensional representations of the RL of (b) MoS2/wax with a loading of 20 wt%, MoS2/PVDF composites with a loading of (c) 10 wt%, (d) 20 wt%, (e) 25 wt%, (f) 30 wt%. 12


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In addition, to study the material’s MA ability in higher frequency, the theoretical reflection loss (RLs) of MoS2/wax and MoS2/PVDF nanocomposites under different loading contents in the frequency range of 18-40 GHz have been shown in Figure 4. It also turns out that the MA performance of the prepared nanocomposites can be tuned very efficiently by thickness and loading content. Within the scope of 18-40 GHz, the minimum reflection loss value varies along with the change of absorber’s thickness and filler content. For example, the minimum reflection loss of -27.47 dB is observed at 18.47 GHz for MoS2/PVDF nanocomposites (30 wt%) with a thickness of 1.5 mm (Figure 4a). Moreover, as shown in Figure 4d, the minimum reflection loss value can reach -32.67 dB at 28.93 GHz for MoS2/PVDF composites with filler loading of 20 wt% at a thickness of 3.5 mm. More microwave RL curves of the composites at various thicknesses in the frequency range of 18-40 GHz are shown in Figure S7 and S8. As the increasing of the thickness or loading content, the minimum reflection loss value tends to shift to lower frequency according to the follow equation:43 fm =

nc

(3)

4tm ε r µ r

Where fm and tm represent the matching frequency and thickness of a RL peak, respectively.

13


Reflection Loss (dB)

(a)

0

1.5 mm (b)

0 -5

-5 -10 -15

10 wt% sample+wax 20 wt% sample+wax 30 wt% sample+wax 40 wt% sample+wax 5 wt% sample+PVDF 10 wt% sample+PVDF 20 wt% sample+PVDF 30 wt% sample+PVDF

-20 -25 -30 -35


-5

9.0 mm (d)

-10 -15 10 wt% sample+wax 20 wt% sample+wax 30 wt% sample+wax 40 wt% sample+wax 5 wt% sample+PVDF 10 wt% sample+PVDF 20 wt% sample+PVDF 30 wt% sample+PVDF

-20 -25 -30

18 19 20 21 22 23 24 25 26 Frequency (GHz) (e) 0 6.0 mm -5 -10 -15 -20 -25 -30


28

30 32 34 36 Frequency (GHz)

38


0

5.0 mm

-10 -15 10 wt% sample+wax 20 wt% sample+wax 30 wt% sample+wax 40 wt% sample+wax 5 wt% sample+PVDF 10 wt% sample+PVDF 20 wt% sample+PVDF 30 wt% sample+PVDF

-20 -25 -30 -35

0

3.5 mm

-5 -10 -15


-20 -25 -30 -35 28

(f) Reflection Loss (dB)

(c)

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18 19 20 21 22 23 24 25 26 Frequency (GHz)

18 19 20 21 22 23 24 25 26 Frequency (GHz)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



40


0 -5 -10 -15 -20 -25 -30 -35 -40

38

40

10.0 mm


28


38

40

Figure 4 Microwave RL curves of the composites with various thicknesses in the frequency range of (a, b, c) 18-26.5 GHz and (d, e, f) 26.5-40 GHz. In order to research the possible mechanisms for microwave absorption properties, the complex permittivity of several materials are measured in the frequency range of 2-40 GHz. Figure 5 displays the frequency dependence of the real part (ε′) and imaginary part (ε′′) of the complex permittivity, which represent the storage and inner dissipation capabilities of electromagnetic energy, respectively.44 On the basis of the Debye theory,45 ε′ and ε″ can be described as 14


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ε '= ε∞ +

ε '' =

εs − ε∞ 1 + ( 2πf ) 2τ 2

(4)

2πfτ (ε s − ε ∞ ) 1 + ( 2πf ) 2τ 2

(5)

Where f, εs, ε∞, and τ are frequency, static permittivity, relative dielectric permittivity at the high frequency limit and polarization relaxation time, respectively. As shown in Figure 5a and b, when the filler content is low (such as 10 wt%), the values of ε′ and ε″ remain nearly constant over the entire frequency range. Then with increase of MoS2 loading (from 10 wt% to 30 wt%), the ε′ begins to decrease in the 2-11 GHz frequency range which is attributed to the increase in f according to eq. (3) and remains almost constant in the range of 11-18 GHz. Furthermore, significant enhancement is also achieved in both real (ε′) and imaginary (ε″) permittivity. The increment of ε′ may be attributed to the fact that the increasing loading ratio of MoS2 will increase the dipole polarization. The more polarization results in more energy dissipation. With the further increase of frequency, the movement of dipole cannot keep up with the variation of frequency. Subsequently, ε′ and ε″ will change slowly (Figure 5c-f). The tangent of dielectric loss angle (δε) of the material can be expressed as tanδε=ε″/ε′. As displayed in Figure S9, the variation tendency of ε″ and tanδε are nearly the same. The lags of dipole polarization induce the reflection loss being fluctuant.46 Except for dipole polarization, the space charge polarization and interfacial polarization are another two important polarization mechanism for MoS2/wax and MoS2/PVDF composites. As mentioned before, the MoS2 nanospheres are composed of intercrossed curved nanosheets. It has been proved that flaky structure are more easily polarized, and the space-charge polarization is enhanced by 15



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increasing the surface area.47 In general, interface polarization arises when the neighboring phases differ from each other in a dielectric constant, conductivity, or both, at testing frequencies.48 When MoS2 nanospheres compound with PVDF, the special nanostructure of MoS2 contributes to form more interface, leading to more interface polarization. And the difference in complex permittivity between MoS2 and PVDF would generate interface scattering which is beneficial to wave absorption property.49 Moreover, as PVDF is a typical dielectric material, the synergetic effect between MoS2 and PVDF could also enhance the wave absorption abilities.

16


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9 6 3 4

6

10 wt% sample+wax 20 wt% sample+wax 10 wt% sample+PVDF 20 wt% sample+PVDF 25 wt% sample+PVDF 30 wt% sample+PVDF

8 Dielectric Constant (ε′′)

12

2

6 4 2 0 2

8 10 12 14 16 18 Frequency (GHz)

4

6

(d) 7

(c) 7 Dielectric Constant (ε′)

(b)

10 wt% sample+wax 20 wt% sample+wax 10 wt% sample+PVDF 20 wt% sample+PVDF 25 wt% sample+PVDF 30 wt% sample+PVDF

15


6 5

Dielectric Constant (ε′′)

Dielectric Constant (ε′)

(a)

4 3 18 19 20 21 22 23 24 25 26 Frequency (GHz) 10 wt% sample+wax 20 wt% sample+wax 30 wt% sample+wax 40 wt% sample+wax 5 wt% sample+PVDF 10 wt% sample+PVDF 20 wt% sample+PVDF 30 wt% sample+PVDF

6 5 4 3 2 1

0 18 19 20 21 22 23 24 25 26 Frequency (GHz)


38

40


4 3 2 1 0

28

8 10 12 14 16 18 Frequency (GHz) 10 wt% sample+wax 20 wt% sample+wax 30 wt% sample+wax 40 wt% sample+wax 5 wt% sample+PVDF 10 wt% sample+PVDF 20 wt% sample+PVDF 30 wt% sample+PVDF

(f) Dielectric Constant (ε′′)

(e) 10 9 8 7 6 5 4 3 2

Dielectric Constant (ε′)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


28


38

40

Figure 5 Frequency dependence on real and imaginary part of the complex permittivity in the frequency range of (a, b) 2-18 GHz, (c, d) 18-26.5 GHz and (e, f) 26.5-40 GHz. Another important wave absorption mechanism proposed here is destructive interference theory. When electromagnetic radiation is incident on a material, the incident microwave is divided into two parts: the reflected microwave and the absorbed microwave. The phase of the reflected microwave and the absorbed 17



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microwave is contrary. When the frequency of reflected microwave is the same as that of absorbed microwave, the interference phenomenon will be generated. If the two-wave interference occurs, the destructive amplitude phenomenon will happen after the wave path difference between them is an odd multiple of half wavelengths. Just as:50 ∆ = 2d = (2n + 1)

λ

(6)

2

That is: d = ( 2n + 1)

λ

(7)

4

As shown in Figure 4f, there are two reflection loss peaks for several samples. For example, in the 5 wt% MoS2+PVDF, after calculation, the minimum reflection loss appears in the frequency of 38.65 GHz is attributed to destructive interference theory. Besides, as mentioned before, the hierarchical 3D-MoS2 nanospheres are formed with thin nanosheets. It has been proved that the laminated structure is able to make the incident microwave generate multiple reflection between the different layers, which will extend the propagation path of electromagnetic wave and greatly enhance their EM wave absorption ability.51 To further verify the importance of hierarchical nanostructure, the MA ability of bulk-MoS2/PVDF composites and micron-sized MoS2/PVDF composites were measured in the frequency range of 2-18 GHz for comparison (Figure 6). It turns out that the MoS2 with hierarchical nanostructure possesses the best absorption performance. The minimum reflection loss value of hierarchical MoS2/PVDF composites (-26.11 dB) is much higher than that of bulk-MoS2/PVDF composites (-11.94 dB) and micron-sized MoS2/PVDF composites 18


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(-12.27 dB) at a thickness of 2.5 mm. On the other hand, to observe more clearly, the amplifying SEM images of bulk- MoS2 and micron-sized MoS2 are showed in Figure S10. Moreover, the possible wave absorption mechanism has been depicted in Scheme 2 intuitively.

Figure 6 Microwave RL curves of the (a) bulk-MoS2/PVDF composites and (b) micron-sized MoS2/PVDF composites with loading content of 25 wt% at a thickness range of 2.0-5.0 mm in 2-18 GHz frequency range. (c) Microwave RL curves of the composites with a thickness of 2.5 mm in the frequency range of 2-18 GHz.

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Scheme 2. The possible wave absorption mechanism of MoS2/PVDF composites.

Conclusions In conclusion, 3D MoS2 hierarchical nanospheres have been successfully fabricated on a large scale by a simple hydrothermal process. The results indicated that the MoS2/PVDF nanocomposites possess excellent wave absorption performance in a broad frequency range of 2-40 GHz. Furthermore, the MA performance can be effectively tuned by absorber’s thickness and filler content. It is observed that the minimum reflection loss value of MoS2/PVDF composites with filler loading of 25 wt% can reach -26.11 dB at 11.36 GHz at a thickness of 2.5 mm, and the frequency bandwidth less than -10 dB is from 9.92 to 13.36 GHz. Within the scope of 18-40 GHz, the minimum reflection loss of -27.47 dB at 18.47 GHz and -32.67 dB at 28.93 GHz for MoS2/PVDF nanocomposites (30 wt% and 20 wt%) with a thickness of 1.5 mm and 3.5 mm can be found, respectively. Moreover, the main microwave absorption mechanism including various polarization, destructive interference theory 20


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and multiple reflections have been described in detail.

Acknowledgment. This work was supported by the National Natural Science Foundation of China (51472012) and the Fundamental Research Funds for the Central Universities.

Supporting Information Available: XRD data of hierarchical MoS2 nanospheres, FESEM images and corresponding elemental mapping images of MoS2 nanospheres and MoS2/PVDF membrane, photograph of test samples and MoS2 powder, flexibility of MoS2/PVDF membrane, microwave RL curves and dielectric loss of composites were shown. This information is available free of charge via the Internet at http://pubs.acs.org/.

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TOC IMAGE

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Self-Supported Construction of Three-Dimensional MoS2 Hierarchical

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