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MoS2 spheres decorated on hollow porous ZnO microspheres with strong wideband microwave absorption
Chemical Engineering Journal 380 (2020) 122625
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MoS2 spheres decorated on hollow porous ZnO microspheres with strong wideband microwave absorption
T
Juhua Luo , Kang Zhang, Mingliang Cheng, Mingmin Gu, Xinkai Sun ⁎
School of Materials Science and Engineering, Yancheng Institute of Technology, Yancheng, Jiangsu 224051, China
HIGHLIGHTS
GRAPHICAL ABSTRACT
composite is synthesized • ZnO/MoS by a facile hydrothermal method. composite exhibits out• ZnO/MoS standing microwave absorption per2 2
formance.
properties and ab• Electromagnetic sorption mechanism are investigated in detail.
ARTICLE INFO
ABSTRACT
Keywords: Hollow porous ZnO MoS2 sphere Microwave absorption property Dielectric loss
With the aim to obtain microwave (MW) absorber possess simultaneously light weight and broad absorption bandwidth, a newfangled composites of ZnO/MoS2, in which MoS2 spheres decorated on hollow porous ZnO microspheres, have been successfully prepared by a facile hydrothermal method. The phase composition and morphology of the composites are characterized by XRD, BET, SEM, TEM and XPS techniques. Because of the high specific surface area originated from the hollow porous structure, the multi-interfacial polarizations between the different components, as well as the improved impedance matching, the composites exhibit outstanding MW absorption performance, especially strong wideband MW absorption. When the filler loading ratio is 30 wt%, the minimum reflection loss reaches −35.8 dB at 11.84 GHz and the absorption bandwidth exceeding −10 dB reaches as wide as 10.24 GHz (from 7.76 GHz to 18 GHz) with a thickness of only 2.5 mm. The results indicate that the synthesized ZnO/MoS2 composites with strong absorption bandwidth can be considered as a promising candidate for high-efficiency MW absorption materials.
1. Introduction With the development of electronic products and digital systems, the emergence of electromagnetic waves (EMW) will generate severe electromagnetic radiation on the environment and human health, including breaking DNA, weakening biological immune systems as well as
⁎
threatening human health [1–3]. In addition, the rapid development of radar tracking systems poses a great threat to traditional combat weapons. Researching stealth combat weapons can enhance the offensive and defensive capabilities of weapons [4,5]. Therefore, searching a kind of material with strong wideband, highly efficient, high specific surface area and lightweight EMW absorption performance to attenuate
Corresponding author. E-mail address: [email protected] (J. Luo).
https://doi.org/10.1016/j.cej.2019.122625 Received 17 June 2019; Received in revised form 10 August 2019; Accepted 23 August 2019 Available online 24 August 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic representation of the preparation of the ZnO/MoS2 composite.
and even eliminate adverse EMW effectively in a wide range become the universal focus attentions [6–10]. In recent years, many traditional microwave (MW) absorbing materials, such as ferrite [11,12], magnetic metal powders [13–15], silicon carbide [16–18] and conductive fiber [19,20] have been investigated. Excellent MW absorption performance appears was proved in some cases, but their large density or narrow bandwidth have severely limited their practical applications. Recently, ZnO and its hybrids are used as MW absorbing materials motivated by their lightweight, semiconductive properties and outstanding dielectric performance, and also by the fact that their large-scale synthesis can be easily realized [21]. Besides, ZnO can avoid the reconnaissance of radar and play an important role in infrared stealth for its wide band gap. The MW absorption performances of various structured ZnO, such as lamellar ZnO [22], tetra-needle-like ZnO whiskers [21], crossed ZnO netlike micronanostructures [23], flower-like ZnO architectures [24], and hierarchical tree-like ZnO [25] have been previously investigated. From these results, it can be concluded that the ZnO nanostructures with complex and special morphologies manifest enhanced MW absorption properties and wider absorption bandwidth. However, hollow porous ZnO microspheres with the advantages of light weight, easy preparation, cheaper, and wide-band is rarely reported. Previous studies have proved that MoS2 possessed the superior MW absorption properties [4,5]. Here we demonstrate simple fabrication of an ultra-lightweight and highly conductive ZnO/MoS2 composite for broadband and tunable high-performance MW absorption application. The ZnO/MoS2 composite was synthesized by a facial hydrothermal method, in which MoS2 spheres decorated on hollow porous ZnO microspheres. The results indicate that the ZnO/MoS2 composite exhibits enhanced MW absorption intensity and strong wideband MW absorption. Furthermore, the EMW loss mechanism and the effect of the MoS2 content on MW absorption properties of the composite also have been explored.
Table 1 The formulas of different components of ZnO/MoS2 composites. Samples
ZnO (g)
MoS2 (g)
m (MoS2): m (ZnO)
S-1 S-2 S-3 S-4
1 1 1 1
0.1 0.2 0.3 0.4
10% 20% 30% 40%
glucose monohydrate (C6H12O6·6H2O), 25 mmol of zinc acetate dihydrate (Zn(CH3COO)2·2H2O) and 50 mmol of urea (CO(NH2)2) were dissolved in 120 mL of ethylene and distilled water with 2:1 vol ratio under vigorous stirring for 30 min until the solution become transparent. Then the above solutions were transferred into 200 mL Teflonlined stainless steel autoclave and heated in a chamber oven at 160 °C for 16 h. After the reaction for cooling down to room temperature, the black precipitates were filtered and washed with distilled water and absolute alcohol for several times. Thereafter, the washed precipitates were dried in a vacuum oven at 70 °C for 12 h. Finally, the products were calcined in air at 600 °C and 900 °C for 4 h and 3 h, respectively, with a heating rate of 2 °C/min to obtain white porous hollow ZnO spheres. 2.3. Synthesis of ZnO/MoS2 composite The ZnO/MoS2 hybrids were prepared via a facial hydrothermal method. The schematic process for the fabrication of ZnO-MoS2 is presented in Fig. 1. Initially, 0.3 g of pre-synthesized ZnO powder was dispersed in 120 mL of distilled water. Then, 0.45 g of Sodium molybdate (Na2MoO4·2H2O) and 0.28 g of thioacetamide (C2H5NS) were added into the above solution and keep stirring for 30 min. Following by that, the mixture was placed into Teflon-lined stainless steel autoclave with 200 mL of capacity and heated at 160 °C for 12 h. Finally, the black precipitates were filtered, washed by deionized water and absolute alcohol for three times and dried in vacuum oven at 60 ℃ for several hours. For comparation, we fabricated ZnO/MoS2 composite with different mass ratios and the results are showed in Table 1.
2. Experimental section 2.1. Materials Sodium molybdate (Na2MoO4·2H2O) and thioacetamide (C2H5NS) were purchased from China pharmaceutical group chemical reagent Co. Ltd. Zinc acetate dehydrate (Zn(CH3COO)2·2H2O) was purchased from Tianjin Kermel Co. Ltd. Glucose (C6H12O6·2H2O) was purchased from Tianjin bodi chemical Co. Ltd. All chemicals were analytical grade and used without further purification. Deionized water was used for all experiments.
2.4. Characterization The resulting powder was characterized by X-ray powder diffraction (XRD) through a diffractometer (RIGAKU, model D/max) with Cu Kα radiation source. The specific surface area and pore volume were collected on a BET analyzer equipped with micromeritics ASAP 2020 system. The morphology and structure of the sample were researched by field emission scanning electron microscopy (FESEM; JEOL, model JSM-7001F) and transmission electron microscopy (TEM; JEOL, model JEM2001). X-ray photoelectron spectroscopy (XPS; Thermo Scientific, model ESCALAB 250Xi) was recorded to study the surface states. The EM performance of the sample was determined by a vector network
2.2. Fabrication of hollow porous ZnO microspheres Hollow porous ZnO microspheres were fabricated by a facile solvothermal method, using zinc acetate dihydrate and glucose as zinc source and carbonaceous source, respectively. Typically, 50 mmol of D2
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and strong intensity are matched well with the phase of ZnO (JCPDS 75-0516), and no additional characteristic peaks can be observed, indicating the high purity and the successful formaiont of highly crystallized ZnO [26]. For MoS2 in Fig. 2b, the three diffraction peaks at 14°, 34°, and 59° are attributed to the (0 0 2), (1 0 1) and (1 1 0) crystal face of MoS2, respectively (JCPDS 04-0831) [27]. As displayed in Fig. 2c, only one diffraction peak for MoS2 is observed, which is mainly attributed to the low content. Besides, because of the decorated MoS2 on the microspheres, the diffraction peaks of ZnO in ZnO/MoS2 composite become weaker compared with pure ZnO. Thus, the XRD results confirm that the ZnO/MoS2 composite has been successfully fabricated. The sizes and morphology of the as-synthesized ZnO and ZnO/MoS2 composites were characterized by SEM. The SEM image of ZnO particles with different magnification are exhibited in Fig. 3a and b. It is obviously observed that these ZnO spheres, with uniform sizes (about 3.0 μm), display a hollow porous structure. As shown in Fig. 3c and d, it can be seen that the hollow porous ZnO is covered by MoS2 and all the synthesized ZnO/MoS2 products are spherical with a diameter about 3.5 μm. In order further to clarify the evolution of microstructure, nitrogen sorption measurements of the samples were carried out at 77 K. Nitrogen adsorption–desorption isothermal and the corresponding Barrett–Joyner–Halenda (BJH) pore size distribution desorption of all samples are presented in Fig. 4. All isotherm profiles of the samples can be categorized to type IV based on classification of IUPAC, indicating the existence of amounts of mesopores. Table 2 shows the results of the measured BET specific surface areas and BJH pore size distribution desorption of the samples. As can be seen, all samples have the large specific surface area and narrower pore size distribution, which will be ideal microwave absorption materials due to the high specific surface
Fig. 2. XRD patterns of (a) ZnO, (b) MoS2, (c) ZnO/MoS2 composite.
analyzer (VNA; Agilent, model N5244A) with a transmission-reflection method in the 2–18 GHz based on the coaxial-line approach. 3. Results and discussion The X-ray diffraction patterns of all the samples are presented in Fig. 2. As can be seen from Fig. 2a, these diffraction peaks with sharp
Fig. 3. FESEM images of (a-b) ZnO, (c-d) ZnO/MoS2 composite. 3
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The surface chemical composition of the ZnO/MoS2 composite is identified by XPS measurement and the results are shown in Fig. 6. As observed in Fig. 6a, the survey spectrum demonstrates the presence of Mo, S, Zn and O elements in the sample. Fig. 6b shows the XPS spectra of Mo 3d. The peaks located at 228.5 eV and 233 eV are attributed to the binding energy of Mo 3d5/2 and Mo 3d3/2, respectively [30,31]. From Fig. 6c, it can be seen that the peak at 162.5 eV corresponds to the S 2p3/2 [32]. Fig. 6d represents the XPS spectra of Zn 2p, the binding energy peaks at 1021.4 eV and 1044.5 eV, are consistent with the binding energy of Zn 2p3/2 and 2p1/2 in ZnO, respectively [26]. From the above comprehensive analysis, the as-obtained products can be identified as ZnO/MoS2 composite. The EM properties of the samples were measured by VNA through the coaxial method. As is well known, the values of ε′ and ε″ refers to storage capability and loss capability of electric energy, respectively. The complex permeability (μr) denotes the magnetic loss of the composites. In our work, the ZnO/MoS2 composites are nonmagnetic material, thus it is unnecessary to take magnetic loss into account. To investigate the MW absorption performances, the ε′ and ε″ values are showed in Fig. 7. Fig. 7a and b show the frequency dependence of ε′ and ε″ of the ZnO/MoS2 dispersed in paraffin with different mass fraction of MoS2 in 2–18 GHz. As exhibited in Fig. 7a, the ε′ values of all samples decrease as the frequency increases. It is clear the values of ε′ increases significantly with increasing the content of MoS2. With decreasing the frequency, the ε′ values decrease (S-1, 6.3–3.24; S-2, 9.37–5.72; S-3, 12.28–6.1; S-4, 15.28–9.1). According to Fig. 7b, the ε″ values of all samples tend to decline with increasing the frequency, in which higher ε″ values can be obtained by increasing the content of MoS2. In general, the dielectric loss can be expressed by the Debye theory as follows [33]: '
=
=
Table 2 The specific surface area and pore size of ZnO/MoS2 composites. Surface area (m2·g−1)
Pore size (nm)
S-1 S-2 S-3 S-4
26.22 24.67 22.78 21.43
9.86 9.72 9.52 9.34
s
1+
s 2 2
1+
(1)
2 2
+ 0
(2)
where ω, τ, εs and ε∞ are the angular frequency, the polarization relaxation time, the static permittivity, and the relative dielectric permittivity at the high frequency limit, respectively. The decrease in ε' with increasing frequency can be explained by the Eq. (1) and the dipoles and interfaces in the composite increase with enhancing the mass percentage cause the augment of ε′ and ε″ [21]. Besides, according to the free electron theory [34]:
Fig. 4. (a) N2 adsorption-desorption isotherm and (b) pore size distribution of ZnO/MoS2 composite.
Samples
+
=
1 2
f
0
(3)
where ρ is the resistivity (inversely proportional to the conductivity), f is the frequency, ε0 is the dielectric constant of vacuum. From the above equation, we can conclude that with the increase of conductivity, ρ appears a decline trend, resulting in the growth of ε″. The tangent of dielectric loss value (tanδε) of the composite can be expressed as [35]:
area and narrow pore size distribution with many channels for multireflection EMW [7]. To further illuminate the structure of ZnO/MoS2 nanospheres, HRTEM images and the corresponding element mappings are displayed in Fig. 5. From Fig. 5a and b, it is clearly seen that the center of the sphere ZnO is brighter, indicating the formation of hollow structure, and these MoS2 spheres are attached on the surface of ZnO. As shown in Fig. 5c, the ordered lattice fringes are clearly observed from a highresolution TEM image. The spacing between two neighboring lattice fringes is approximately 0.62 nm, corresponding to the (0 0 2) plane of MoS2 [28]. For ZnO, two parallel fringes with spacing values of 0.26 nm can be observed, which is ascribed to the (0 0 2) crystalline planes of ZnO [29]. In addition, the element mappings detected in the selected area (Fig. 5d and e) illustrate the existence of Zn, O, Mo and S with a homogeneous elemental distribution. It declares that the molar ration of Zn, O, Mo and S elements are 46.41%, 34.92%, 6.93%, 11.74%, respectively. The atomic ratio of Zn:O is about (1:1) and Mo:S is about (2:1), which is consistent with the stoichiometry of ZnO and MoS2.
tan
=
(4)
Fig. 8 displays the tanδε versus frequency at different loading levels of ZnO/MoS2. As can be found that the tanδε value of S-3 is greater than that of other three samples in 2–18 GHz. Meanwhile, the tanδε curves of all the samples loaded with 10–40 wt% MoS2 have several relaxation peaks in the tested frequency range, manifesting that there is a relaxation loss in the MW absorption which enhanced dielectric loss. On the basis of the above discussion about complex permittivity, the MW absorption properties of ZnO/MoS2 nanospheres with various MoS2 content are investigated in terms of RL value at a given frequency and thickness layer. In a general way, when the RL values are below −10 dB and −20 dB, it respectively means that 90% and 99% EMW can be effectively attenuated and absorbed. According to the transmission line theory, the value of RL can be calculated by the following 4
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Fig. 5. HRTEM images of (a-c) ZnO/MoS2 composite and mapping images of (d-e) ZnO/MoS2 composite.
equations [36]:
RL(dB) = 20log
Z in =
µr r
tanh j
Z in 1 Z in + 1
2 fd c
permeability of the composite respectively, f is the frequency, d is the thickness of the absorber, and c is the velocity of light in vacuum. Because ZnO and MoS2 are dielectric material, theµ' and µ of permeability are 1 and 0, respectively. Fig. 9 shows the optimized RL values of ZnO/MoS2 with different loadings. When the filler loading is 30 wt%, the minimum RL value is far more than that of 10 wt%, 20 wt% and 40 wt%, indicating that the optimum loading is 30 wt%. It is widely accepted that the higher permittivity will result in the poor MW absorption performance because of the impedance mismatch [37]. It is found that the minimum RL value of
(5)
µr
r
(6)
where Zin is the normalized input characteristic impedance, εr (r= j ) and μr (µr = µ jµ ) are the complex permittivity and
5
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(a)
228.5
(b) Intensity (a.u.)
Intensity (a.u.)
Zn2p
O1s
Mo3d
233
Mo3d S2p
1200
1000
800
600
400
200
234
0
232
230
Binding Energy (eV)
S2p
(c)
168
166
164
162
160
158
1021.4
1044.5
Intensity (a.u.) 170
226
Zn2p
(d)
162.5
Intensity (a.u.) 172
228
Binding Energy (eV)
1050
1040
1030
1020
Binding Energy (eV)
Binding Energy (eV)
Fig. 6. The high resolution XPS spectra of (a) ZnO/MoS2 composite, (b) Mo 3d, (c) S 2p and (d) Zn 2p.
S-3 is −35.8 dB at 11.84 GHz with a thickness of 2.5 mm and the effective bandwidth (RL < -10 dB) reaches as strong as 10.24 GHz, ranging from 7.76 to 18 GHz. Both the MW absorption intensity and the absorption bandwidth are much higher than other samples. Fig. 10a–d show three dimensional plots of RL of ZnO/MoS2 composite versus the frequency and thickness at different loadings. As can be observed from Fig. 10, the RL value of ZnO/MoS2 firstly increases and then tends to decrease with the increasing loading. Generally, the minimum RL value of S-1 is −14.2 dB which is demonstrated in Fig. 10a, it presents relatively inferior performance by comparing with other samples. Fig. 10b and d display the RL of S-2 and S-4 we can see the minimum RL value of which are −23.3 dB and −27.2 dB, respectively. The RL of S-3 is showed in Fig. 10c, it is observed that the minimum RL value can up to −35.8 dB with a thickness of 2.5 mm and a broad effective absorption bandwidth is obtained from 3.36 GHz to 18 GHz at a thickness of 1.5–5.0 mm. Observe all samples exhibited in Fig. 10, it can be found that the MW absorption peaks shift toward the lower frequency when the thickness changes from 1.5 to 5 mm. It can be explained by quarterwavelength theory, the relationship between the matching frequency (fm) and the theoretical matching thickness (dm) is given as follows [38]:
dm =
n nc = 4 4fm |µr || r |
( n= 1, 3, 5…)
in the above formula, dm is the absorber thickness at the minimum EMW absorption, λ is the wavelength of the EMW, c is the velocity of light in free space, fm is the corresponding peak frequency, |μr| and |εr| are the modulus of μr and εr, n is an integer. Obviously, the fm is inversely proportional to the dm which suggests that the frequency corresponding to minimum RL value can be modulated by altering the thickness of absorber. Besides, the MW absorption properties of asprepared ZnO/MoS2 composite are strongly dependent on the content of MoS2, which means that MW absorption can be easily modulated not only by varying the sample thickness, but also by changing the ratio of MoS2 in the composite [39]. Attenuation constant (α) is a key factor for EMW absorption that should be considered for an excellent absorber, which determines the attenuation properties of materials, it can be calculated by the following formula [40]:
=
2 f × c
µ
µ
+
(µ
µ ) 2 + (µ
+µ
)2
(8)
in which f is the frequency of EMW and c is the velocity of light. Generally speaking, the larger attenuation constant will generate more dielectric loss, which is beneficial to EMW absorption performance. Fig. 11 gives the frequency dependence of attenuation constant for ZnO/MoS2 composite. It is observed that S-4 possess the largest attenuation constant among the four samples in the frequency of 2–18 GHz while its MW absorption property is not the best. The
(7)
6
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0
RL (dB)
-10 -20 -30
S-1-2.5 mm S-2-5.0 mm S-3-2.5 mm S-4-2.0 mm
-40 -50
2
4
6
8
10
12
14
16
18
Frequency (GHz) Fig. 9. The reflection loss (RL) of as-prepared samples with matching thickness as a function of frequency over 2–18 GHz.
phenomenon proves that there are many factors affect the MW absorption performance. To illustrate ZnO/MoS2 composites possess an preeminent MW absorption performance, ZnO matrix composites with their corresponding MW absorption properties reported in recent literatures were summarized in Table 3. By comparing with most of them, the ZnO/MoS2 composites have more remarkable MW absorption performance and broader effective absorption bandwidth. Meanwhile, it also meet the demand of lightweight EMW absorber for its porous hollow sphere. All the above results have demonstrated that the ZnO/MoS2 hybrids synthesized in the current work can be a promising candidate for MW absorption. The excellent MW absorption properties of the ZnO/MoS2 composites could be expounded as the follows and the MW absorbing mechanism schematic diagram was demonstrated in Fig. 12. Firstly, ZnO/MoS2 with pore structure may have more chances to satisfy the Eq. (7), which makes the incident and reflected MW in the absorber completely cancel out each other. This implies that minimal reflection of the MW power or good impendence matching occurs [6,41]. Also, the pores, which can induce dipole moment as point defects, provides enhanced interfacial dipole polarizations to increase dielectric relaxations [42,43]. Secondly, the hollow ZnO spherical will furnish massive active sites to induce multiple reflection and scattering of MW [44,45]. Thirdly, due to the unique structure of ZnO/MoS2, the existence of numerous interfaces can lead to the accretion of bound charges on the interfaces, resulting in the interfacial polarization effect [27]. Additionly, MW can be attenuated by the resistance of MoS2, because the induced electrons into MoS2 may be transferred to form dissipated current [14,46].
Fig. 7. Behavior of (a) real and (b) imaginary parts of the permittivity of ZnO/ MoS2 composite with different adding of MoS2 as a function of frequency over 2–18 GHz.
4. Conclusion The ZnO/MoS2 composites with different mass ratio were successfully fabricated by a facial hydrothermal method. The composites exhibit excellent MW absorbing performance in the frequency range of 2–18 GHz. The result shows that ZnO/MoS2 composites own the best absorption performance with 30 wt% loading of MoS2. The minimum RL value is −35.8 dB at 11.84 GHz with a thickness of 2.5 mm and the effective absorption bandwidth is 10.24 GHz (from 7.76 to 18 GHz). In addition, both the thickness and attenuation constant of the absorbing material are key factors to MW attenuation, which provides an efficient
Fig. 8. Tangent loss of ZnO/MoS2 composite with different adding of MoS2.
7
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0 -38.00
-5
-33.25
-10
-28.50
RL (dB)
-15
-23.75
-20
-19.00
-25
(c)
-30
-14.25 -9.500
-35 1.5
2.0
2.5
Th
3.0 3.5 nes 4.0 s (m 4.5 m)
ick
4
2
5.0
8
6
Fr
12
10
cy
n ue
eq
14
16
18 -4.750
) Hz (G
0
0 -29.00
-5
-25.38
RL (dB)
-10
-21.75
-15
-18.13
-20
-14.50
(d)
-25
-10.88 -7.250
1.5 2.0
2.5 Th 3.0 ick 3.5 ne ss 4.0 (m 4.5 m )
5.0
2
4
8
6
y enc
qu
Fre
12
10
14
16
18
) Hz (G
-3.625 0
Fig. 10. 3D plots of the calculated reflection loss of samples loaded with different concentrations of ZnO/MoS2 composite (a-d) versus frequency and thickness.
Table 3 Comparison of the MW absorption properties about zinc oxide matrix composite. Absorber
Filler loading (wt%)
Minimum RL (dB)
Optimum thickness (mm)
RL < −10 dB band width (GHz)
References
ZnO 3D-RGO/ZnO ZnO@PANI CB/T-ZnO ZnO/CF ZnO/CNT ZnO/Zn/CF This work
– 10 – 7 60 40 – 30
−34.5 −25.95 −41 −19.3 −33 −35 −39.42 −35.8
1.5 2.5 3.5 3.0 4.35 5.0 3.5 2.5
2.0 6.4 1.0 5.76 5.1 3.7 0.6 10.24
[31] [36] [19] [21] [37] [38] [32] –
approach to adjust MW absorbing properties. And the ZnO/MoS2 composite absorber may meet the requirements for an ideal MW absorbing material with thinness, low density, wide absorption frequency, and strong absorption properties.
Fig. 11. MW attenuation constants (α) of ZnO/MoS2 composite.
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Fig. 12. The MW absorption mechanism schematic diagram of ZnO/MoS2 composite.
Acknowledgements This work is supported by Jiangsu Provincial Department of Education (Grant No. 18KJA430016) and Joint Open Fund of Jiangsu Collaborative Innovation Center for Ecological Building Material and Environmental Protection Equipments and Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province and Key Laboratory for Ecological-Environment Materials of Jiangsu Province (Grant No. JH201826).
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MoS2 spheres decorated on hollow porous ZnO microspheres with strong wideband microwave absorption
T
Juhua Luo , Kang Zhang, Mingliang Cheng, Mingmin Gu, Xinkai Sun ⁎
School of Materials Science and Engineering, Yancheng Institute of Technology, Yancheng, Jiangsu 224051, China
HIGHLIGHTS
GRAPHICAL ABSTRACT
composite is synthesized • ZnO/MoS by a facile hydrothermal method. composite exhibits out• ZnO/MoS standing microwave absorption per2 2
formance.
properties and ab• Electromagnetic sorption mechanism are investigated in detail.
ARTICLE INFO
ABSTRACT
Keywords: Hollow porous ZnO MoS2 sphere Microwave absorption property Dielectric loss
With the aim to obtain microwave (MW) absorber possess simultaneously light weight and broad absorption bandwidth, a newfangled composites of ZnO/MoS2, in which MoS2 spheres decorated on hollow porous ZnO microspheres, have been successfully prepared by a facile hydrothermal method. The phase composition and morphology of the composites are characterized by XRD, BET, SEM, TEM and XPS techniques. Because of the high specific surface area originated from the hollow porous structure, the multi-interfacial polarizations between the different components, as well as the improved impedance matching, the composites exhibit outstanding MW absorption performance, especially strong wideband MW absorption. When the filler loading ratio is 30 wt%, the minimum reflection loss reaches −35.8 dB at 11.84 GHz and the absorption bandwidth exceeding −10 dB reaches as wide as 10.24 GHz (from 7.76 GHz to 18 GHz) with a thickness of only 2.5 mm. The results indicate that the synthesized ZnO/MoS2 composites with strong absorption bandwidth can be considered as a promising candidate for high-efficiency MW absorption materials.
1. Introduction With the development of electronic products and digital systems, the emergence of electromagnetic waves (EMW) will generate severe electromagnetic radiation on the environment and human health, including breaking DNA, weakening biological immune systems as well as
⁎
threatening human health [1–3]. In addition, the rapid development of radar tracking systems poses a great threat to traditional combat weapons. Researching stealth combat weapons can enhance the offensive and defensive capabilities of weapons [4,5]. Therefore, searching a kind of material with strong wideband, highly efficient, high specific surface area and lightweight EMW absorption performance to attenuate
Corresponding author. E-mail address: [email protected] (J. Luo).
https://doi.org/10.1016/j.cej.2019.122625 Received 17 June 2019; Received in revised form 10 August 2019; Accepted 23 August 2019 Available online 24 August 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic representation of the preparation of the ZnO/MoS2 composite.
and even eliminate adverse EMW effectively in a wide range become the universal focus attentions [6–10]. In recent years, many traditional microwave (MW) absorbing materials, such as ferrite [11,12], magnetic metal powders [13–15], silicon carbide [16–18] and conductive fiber [19,20] have been investigated. Excellent MW absorption performance appears was proved in some cases, but their large density or narrow bandwidth have severely limited their practical applications. Recently, ZnO and its hybrids are used as MW absorbing materials motivated by their lightweight, semiconductive properties and outstanding dielectric performance, and also by the fact that their large-scale synthesis can be easily realized [21]. Besides, ZnO can avoid the reconnaissance of radar and play an important role in infrared stealth for its wide band gap. The MW absorption performances of various structured ZnO, such as lamellar ZnO [22], tetra-needle-like ZnO whiskers [21], crossed ZnO netlike micronanostructures [23], flower-like ZnO architectures [24], and hierarchical tree-like ZnO [25] have been previously investigated. From these results, it can be concluded that the ZnO nanostructures with complex and special morphologies manifest enhanced MW absorption properties and wider absorption bandwidth. However, hollow porous ZnO microspheres with the advantages of light weight, easy preparation, cheaper, and wide-band is rarely reported. Previous studies have proved that MoS2 possessed the superior MW absorption properties [4,5]. Here we demonstrate simple fabrication of an ultra-lightweight and highly conductive ZnO/MoS2 composite for broadband and tunable high-performance MW absorption application. The ZnO/MoS2 composite was synthesized by a facial hydrothermal method, in which MoS2 spheres decorated on hollow porous ZnO microspheres. The results indicate that the ZnO/MoS2 composite exhibits enhanced MW absorption intensity and strong wideband MW absorption. Furthermore, the EMW loss mechanism and the effect of the MoS2 content on MW absorption properties of the composite also have been explored.
Table 1 The formulas of different components of ZnO/MoS2 composites. Samples
ZnO (g)
MoS2 (g)
m (MoS2): m (ZnO)
S-1 S-2 S-3 S-4
1 1 1 1
0.1 0.2 0.3 0.4
10% 20% 30% 40%
glucose monohydrate (C6H12O6·6H2O), 25 mmol of zinc acetate dihydrate (Zn(CH3COO)2·2H2O) and 50 mmol of urea (CO(NH2)2) were dissolved in 120 mL of ethylene and distilled water with 2:1 vol ratio under vigorous stirring for 30 min until the solution become transparent. Then the above solutions were transferred into 200 mL Teflonlined stainless steel autoclave and heated in a chamber oven at 160 °C for 16 h. After the reaction for cooling down to room temperature, the black precipitates were filtered and washed with distilled water and absolute alcohol for several times. Thereafter, the washed precipitates were dried in a vacuum oven at 70 °C for 12 h. Finally, the products were calcined in air at 600 °C and 900 °C for 4 h and 3 h, respectively, with a heating rate of 2 °C/min to obtain white porous hollow ZnO spheres. 2.3. Synthesis of ZnO/MoS2 composite The ZnO/MoS2 hybrids were prepared via a facial hydrothermal method. The schematic process for the fabrication of ZnO-MoS2 is presented in Fig. 1. Initially, 0.3 g of pre-synthesized ZnO powder was dispersed in 120 mL of distilled water. Then, 0.45 g of Sodium molybdate (Na2MoO4·2H2O) and 0.28 g of thioacetamide (C2H5NS) were added into the above solution and keep stirring for 30 min. Following by that, the mixture was placed into Teflon-lined stainless steel autoclave with 200 mL of capacity and heated at 160 °C for 12 h. Finally, the black precipitates were filtered, washed by deionized water and absolute alcohol for three times and dried in vacuum oven at 60 ℃ for several hours. For comparation, we fabricated ZnO/MoS2 composite with different mass ratios and the results are showed in Table 1.
2. Experimental section 2.1. Materials Sodium molybdate (Na2MoO4·2H2O) and thioacetamide (C2H5NS) were purchased from China pharmaceutical group chemical reagent Co. Ltd. Zinc acetate dehydrate (Zn(CH3COO)2·2H2O) was purchased from Tianjin Kermel Co. Ltd. Glucose (C6H12O6·2H2O) was purchased from Tianjin bodi chemical Co. Ltd. All chemicals were analytical grade and used without further purification. Deionized water was used for all experiments.
2.4. Characterization The resulting powder was characterized by X-ray powder diffraction (XRD) through a diffractometer (RIGAKU, model D/max) with Cu Kα radiation source. The specific surface area and pore volume were collected on a BET analyzer equipped with micromeritics ASAP 2020 system. The morphology and structure of the sample were researched by field emission scanning electron microscopy (FESEM; JEOL, model JSM-7001F) and transmission electron microscopy (TEM; JEOL, model JEM2001). X-ray photoelectron spectroscopy (XPS; Thermo Scientific, model ESCALAB 250Xi) was recorded to study the surface states. The EM performance of the sample was determined by a vector network
2.2. Fabrication of hollow porous ZnO microspheres Hollow porous ZnO microspheres were fabricated by a facile solvothermal method, using zinc acetate dihydrate and glucose as zinc source and carbonaceous source, respectively. Typically, 50 mmol of D2
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and strong intensity are matched well with the phase of ZnO (JCPDS 75-0516), and no additional characteristic peaks can be observed, indicating the high purity and the successful formaiont of highly crystallized ZnO [26]. For MoS2 in Fig. 2b, the three diffraction peaks at 14°, 34°, and 59° are attributed to the (0 0 2), (1 0 1) and (1 1 0) crystal face of MoS2, respectively (JCPDS 04-0831) [27]. As displayed in Fig. 2c, only one diffraction peak for MoS2 is observed, which is mainly attributed to the low content. Besides, because of the decorated MoS2 on the microspheres, the diffraction peaks of ZnO in ZnO/MoS2 composite become weaker compared with pure ZnO. Thus, the XRD results confirm that the ZnO/MoS2 composite has been successfully fabricated. The sizes and morphology of the as-synthesized ZnO and ZnO/MoS2 composites were characterized by SEM. The SEM image of ZnO particles with different magnification are exhibited in Fig. 3a and b. It is obviously observed that these ZnO spheres, with uniform sizes (about 3.0 μm), display a hollow porous structure. As shown in Fig. 3c and d, it can be seen that the hollow porous ZnO is covered by MoS2 and all the synthesized ZnO/MoS2 products are spherical with a diameter about 3.5 μm. In order further to clarify the evolution of microstructure, nitrogen sorption measurements of the samples were carried out at 77 K. Nitrogen adsorption–desorption isothermal and the corresponding Barrett–Joyner–Halenda (BJH) pore size distribution desorption of all samples are presented in Fig. 4. All isotherm profiles of the samples can be categorized to type IV based on classification of IUPAC, indicating the existence of amounts of mesopores. Table 2 shows the results of the measured BET specific surface areas and BJH pore size distribution desorption of the samples. As can be seen, all samples have the large specific surface area and narrower pore size distribution, which will be ideal microwave absorption materials due to the high specific surface
Fig. 2. XRD patterns of (a) ZnO, (b) MoS2, (c) ZnO/MoS2 composite.
analyzer (VNA; Agilent, model N5244A) with a transmission-reflection method in the 2–18 GHz based on the coaxial-line approach. 3. Results and discussion The X-ray diffraction patterns of all the samples are presented in Fig. 2. As can be seen from Fig. 2a, these diffraction peaks with sharp
Fig. 3. FESEM images of (a-b) ZnO, (c-d) ZnO/MoS2 composite. 3
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The surface chemical composition of the ZnO/MoS2 composite is identified by XPS measurement and the results are shown in Fig. 6. As observed in Fig. 6a, the survey spectrum demonstrates the presence of Mo, S, Zn and O elements in the sample. Fig. 6b shows the XPS spectra of Mo 3d. The peaks located at 228.5 eV and 233 eV are attributed to the binding energy of Mo 3d5/2 and Mo 3d3/2, respectively [30,31]. From Fig. 6c, it can be seen that the peak at 162.5 eV corresponds to the S 2p3/2 [32]. Fig. 6d represents the XPS spectra of Zn 2p, the binding energy peaks at 1021.4 eV and 1044.5 eV, are consistent with the binding energy of Zn 2p3/2 and 2p1/2 in ZnO, respectively [26]. From the above comprehensive analysis, the as-obtained products can be identified as ZnO/MoS2 composite. The EM properties of the samples were measured by VNA through the coaxial method. As is well known, the values of ε′ and ε″ refers to storage capability and loss capability of electric energy, respectively. The complex permeability (μr) denotes the magnetic loss of the composites. In our work, the ZnO/MoS2 composites are nonmagnetic material, thus it is unnecessary to take magnetic loss into account. To investigate the MW absorption performances, the ε′ and ε″ values are showed in Fig. 7. Fig. 7a and b show the frequency dependence of ε′ and ε″ of the ZnO/MoS2 dispersed in paraffin with different mass fraction of MoS2 in 2–18 GHz. As exhibited in Fig. 7a, the ε′ values of all samples decrease as the frequency increases. It is clear the values of ε′ increases significantly with increasing the content of MoS2. With decreasing the frequency, the ε′ values decrease (S-1, 6.3–3.24; S-2, 9.37–5.72; S-3, 12.28–6.1; S-4, 15.28–9.1). According to Fig. 7b, the ε″ values of all samples tend to decline with increasing the frequency, in which higher ε″ values can be obtained by increasing the content of MoS2. In general, the dielectric loss can be expressed by the Debye theory as follows [33]: '
=
=
Table 2 The specific surface area and pore size of ZnO/MoS2 composites. Surface area (m2·g−1)
Pore size (nm)
S-1 S-2 S-3 S-4
26.22 24.67 22.78 21.43
9.86 9.72 9.52 9.34
s
1+
s 2 2
1+
(1)
2 2
+ 0
(2)
where ω, τ, εs and ε∞ are the angular frequency, the polarization relaxation time, the static permittivity, and the relative dielectric permittivity at the high frequency limit, respectively. The decrease in ε' with increasing frequency can be explained by the Eq. (1) and the dipoles and interfaces in the composite increase with enhancing the mass percentage cause the augment of ε′ and ε″ [21]. Besides, according to the free electron theory [34]:
Fig. 4. (a) N2 adsorption-desorption isotherm and (b) pore size distribution of ZnO/MoS2 composite.
Samples
+
=
1 2
f
0
(3)
where ρ is the resistivity (inversely proportional to the conductivity), f is the frequency, ε0 is the dielectric constant of vacuum. From the above equation, we can conclude that with the increase of conductivity, ρ appears a decline trend, resulting in the growth of ε″. The tangent of dielectric loss value (tanδε) of the composite can be expressed as [35]:
area and narrow pore size distribution with many channels for multireflection EMW [7]. To further illuminate the structure of ZnO/MoS2 nanospheres, HRTEM images and the corresponding element mappings are displayed in Fig. 5. From Fig. 5a and b, it is clearly seen that the center of the sphere ZnO is brighter, indicating the formation of hollow structure, and these MoS2 spheres are attached on the surface of ZnO. As shown in Fig. 5c, the ordered lattice fringes are clearly observed from a highresolution TEM image. The spacing between two neighboring lattice fringes is approximately 0.62 nm, corresponding to the (0 0 2) plane of MoS2 [28]. For ZnO, two parallel fringes with spacing values of 0.26 nm can be observed, which is ascribed to the (0 0 2) crystalline planes of ZnO [29]. In addition, the element mappings detected in the selected area (Fig. 5d and e) illustrate the existence of Zn, O, Mo and S with a homogeneous elemental distribution. It declares that the molar ration of Zn, O, Mo and S elements are 46.41%, 34.92%, 6.93%, 11.74%, respectively. The atomic ratio of Zn:O is about (1:1) and Mo:S is about (2:1), which is consistent with the stoichiometry of ZnO and MoS2.
tan
=
(4)
Fig. 8 displays the tanδε versus frequency at different loading levels of ZnO/MoS2. As can be found that the tanδε value of S-3 is greater than that of other three samples in 2–18 GHz. Meanwhile, the tanδε curves of all the samples loaded with 10–40 wt% MoS2 have several relaxation peaks in the tested frequency range, manifesting that there is a relaxation loss in the MW absorption which enhanced dielectric loss. On the basis of the above discussion about complex permittivity, the MW absorption properties of ZnO/MoS2 nanospheres with various MoS2 content are investigated in terms of RL value at a given frequency and thickness layer. In a general way, when the RL values are below −10 dB and −20 dB, it respectively means that 90% and 99% EMW can be effectively attenuated and absorbed. According to the transmission line theory, the value of RL can be calculated by the following 4
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Fig. 5. HRTEM images of (a-c) ZnO/MoS2 composite and mapping images of (d-e) ZnO/MoS2 composite.
equations [36]:
RL(dB) = 20log
Z in =
µr r
tanh j
Z in 1 Z in + 1
2 fd c
permeability of the composite respectively, f is the frequency, d is the thickness of the absorber, and c is the velocity of light in vacuum. Because ZnO and MoS2 are dielectric material, theµ' and µ of permeability are 1 and 0, respectively. Fig. 9 shows the optimized RL values of ZnO/MoS2 with different loadings. When the filler loading is 30 wt%, the minimum RL value is far more than that of 10 wt%, 20 wt% and 40 wt%, indicating that the optimum loading is 30 wt%. It is widely accepted that the higher permittivity will result in the poor MW absorption performance because of the impedance mismatch [37]. It is found that the minimum RL value of
(5)
µr
r
(6)
where Zin is the normalized input characteristic impedance, εr (r= j ) and μr (µr = µ jµ ) are the complex permittivity and
5
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(a)
228.5
(b) Intensity (a.u.)
Intensity (a.u.)
Zn2p
O1s
Mo3d
233
Mo3d S2p
1200
1000
800
600
400
200
234
0
232
230
Binding Energy (eV)
S2p
(c)
168
166
164
162
160
158
1021.4
1044.5
Intensity (a.u.) 170
226
Zn2p
(d)
162.5
Intensity (a.u.) 172
228
Binding Energy (eV)
1050
1040
1030
1020
Binding Energy (eV)
Binding Energy (eV)
Fig. 6. The high resolution XPS spectra of (a) ZnO/MoS2 composite, (b) Mo 3d, (c) S 2p and (d) Zn 2p.
S-3 is −35.8 dB at 11.84 GHz with a thickness of 2.5 mm and the effective bandwidth (RL < -10 dB) reaches as strong as 10.24 GHz, ranging from 7.76 to 18 GHz. Both the MW absorption intensity and the absorption bandwidth are much higher than other samples. Fig. 10a–d show three dimensional plots of RL of ZnO/MoS2 composite versus the frequency and thickness at different loadings. As can be observed from Fig. 10, the RL value of ZnO/MoS2 firstly increases and then tends to decrease with the increasing loading. Generally, the minimum RL value of S-1 is −14.2 dB which is demonstrated in Fig. 10a, it presents relatively inferior performance by comparing with other samples. Fig. 10b and d display the RL of S-2 and S-4 we can see the minimum RL value of which are −23.3 dB and −27.2 dB, respectively. The RL of S-3 is showed in Fig. 10c, it is observed that the minimum RL value can up to −35.8 dB with a thickness of 2.5 mm and a broad effective absorption bandwidth is obtained from 3.36 GHz to 18 GHz at a thickness of 1.5–5.0 mm. Observe all samples exhibited in Fig. 10, it can be found that the MW absorption peaks shift toward the lower frequency when the thickness changes from 1.5 to 5 mm. It can be explained by quarterwavelength theory, the relationship between the matching frequency (fm) and the theoretical matching thickness (dm) is given as follows [38]:
dm =
n nc = 4 4fm |µr || r |
( n= 1, 3, 5…)
in the above formula, dm is the absorber thickness at the minimum EMW absorption, λ is the wavelength of the EMW, c is the velocity of light in free space, fm is the corresponding peak frequency, |μr| and |εr| are the modulus of μr and εr, n is an integer. Obviously, the fm is inversely proportional to the dm which suggests that the frequency corresponding to minimum RL value can be modulated by altering the thickness of absorber. Besides, the MW absorption properties of asprepared ZnO/MoS2 composite are strongly dependent on the content of MoS2, which means that MW absorption can be easily modulated not only by varying the sample thickness, but also by changing the ratio of MoS2 in the composite [39]. Attenuation constant (α) is a key factor for EMW absorption that should be considered for an excellent absorber, which determines the attenuation properties of materials, it can be calculated by the following formula [40]:
=
2 f × c
µ
µ
+
(µ
µ ) 2 + (µ
+µ
)2
(8)
in which f is the frequency of EMW and c is the velocity of light. Generally speaking, the larger attenuation constant will generate more dielectric loss, which is beneficial to EMW absorption performance. Fig. 11 gives the frequency dependence of attenuation constant for ZnO/MoS2 composite. It is observed that S-4 possess the largest attenuation constant among the four samples in the frequency of 2–18 GHz while its MW absorption property is not the best. The
(7)
6
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0
RL (dB)
-10 -20 -30
S-1-2.5 mm S-2-5.0 mm S-3-2.5 mm S-4-2.0 mm
-40 -50
2
4
6
8
10
12
14
16
18
Frequency (GHz) Fig. 9. The reflection loss (RL) of as-prepared samples with matching thickness as a function of frequency over 2–18 GHz.
phenomenon proves that there are many factors affect the MW absorption performance. To illustrate ZnO/MoS2 composites possess an preeminent MW absorption performance, ZnO matrix composites with their corresponding MW absorption properties reported in recent literatures were summarized in Table 3. By comparing with most of them, the ZnO/MoS2 composites have more remarkable MW absorption performance and broader effective absorption bandwidth. Meanwhile, it also meet the demand of lightweight EMW absorber for its porous hollow sphere. All the above results have demonstrated that the ZnO/MoS2 hybrids synthesized in the current work can be a promising candidate for MW absorption. The excellent MW absorption properties of the ZnO/MoS2 composites could be expounded as the follows and the MW absorbing mechanism schematic diagram was demonstrated in Fig. 12. Firstly, ZnO/MoS2 with pore structure may have more chances to satisfy the Eq. (7), which makes the incident and reflected MW in the absorber completely cancel out each other. This implies that minimal reflection of the MW power or good impendence matching occurs [6,41]. Also, the pores, which can induce dipole moment as point defects, provides enhanced interfacial dipole polarizations to increase dielectric relaxations [42,43]. Secondly, the hollow ZnO spherical will furnish massive active sites to induce multiple reflection and scattering of MW [44,45]. Thirdly, due to the unique structure of ZnO/MoS2, the existence of numerous interfaces can lead to the accretion of bound charges on the interfaces, resulting in the interfacial polarization effect [27]. Additionly, MW can be attenuated by the resistance of MoS2, because the induced electrons into MoS2 may be transferred to form dissipated current [14,46].
Fig. 7. Behavior of (a) real and (b) imaginary parts of the permittivity of ZnO/ MoS2 composite with different adding of MoS2 as a function of frequency over 2–18 GHz.
4. Conclusion The ZnO/MoS2 composites with different mass ratio were successfully fabricated by a facial hydrothermal method. The composites exhibit excellent MW absorbing performance in the frequency range of 2–18 GHz. The result shows that ZnO/MoS2 composites own the best absorption performance with 30 wt% loading of MoS2. The minimum RL value is −35.8 dB at 11.84 GHz with a thickness of 2.5 mm and the effective absorption bandwidth is 10.24 GHz (from 7.76 to 18 GHz). In addition, both the thickness and attenuation constant of the absorbing material are key factors to MW attenuation, which provides an efficient
Fig. 8. Tangent loss of ZnO/MoS2 composite with different adding of MoS2.
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0 -38.00
-5
-33.25
-10
-28.50
RL (dB)
-15
-23.75
-20
-19.00
-25
(c)
-30
-14.25 -9.500
-35 1.5
2.0
2.5
Th
3.0 3.5 nes 4.0 s (m 4.5 m)
ick
4
2
5.0
8
6
Fr
12
10
cy
n ue
eq
14
16
18 -4.750
) Hz (G
0
0 -29.00
-5
-25.38
RL (dB)
-10
-21.75
-15
-18.13
-20
-14.50
(d)
-25
-10.88 -7.250
1.5 2.0
2.5 Th 3.0 ick 3.5 ne ss 4.0 (m 4.5 m )
5.0
2
4
8
6
y enc
qu
Fre
12
10
14
16
18
) Hz (G
-3.625 0
Fig. 10. 3D plots of the calculated reflection loss of samples loaded with different concentrations of ZnO/MoS2 composite (a-d) versus frequency and thickness.
Table 3 Comparison of the MW absorption properties about zinc oxide matrix composite. Absorber
Filler loading (wt%)
Minimum RL (dB)
Optimum thickness (mm)
RL < −10 dB band width (GHz)
References
ZnO 3D-RGO/ZnO ZnO@PANI CB/T-ZnO ZnO/CF ZnO/CNT ZnO/Zn/CF This work
– 10 – 7 60 40 – 30
−34.5 −25.95 −41 −19.3 −33 −35 −39.42 −35.8
1.5 2.5 3.5 3.0 4.35 5.0 3.5 2.5
2.0 6.4 1.0 5.76 5.1 3.7 0.6 10.24
[31] [36] [19] [21] [37] [38] [32] –
approach to adjust MW absorbing properties. And the ZnO/MoS2 composite absorber may meet the requirements for an ideal MW absorbing material with thinness, low density, wide absorption frequency, and strong absorption properties.
Fig. 11. MW attenuation constants (α) of ZnO/MoS2 composite.
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Fig. 12. The MW absorption mechanism schematic diagram of ZnO/MoS2 composite.
Acknowledgements This work is supported by Jiangsu Provincial Department of Education (Grant No. 18KJA430016) and Joint Open Fund of Jiangsu Collaborative Innovation Center for Ecological Building Material and Environmental Protection Equipments and Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province and Key Laboratory for Ecological-Environment Materials of Jiangsu Province (Grant No. JH201826).
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