Facile synthesis of highly conductive MoS₂/graphene nanohybrids with hetero-structures as excellent microwave absorbers

Abstract

Two-dimensional (2D) MoS₂/graphene nanosheet (MoS₂/GN) hybrids have been demonstrated to be promising microwave absorption (MA) materials due to their unique chemical and physical properties as well as rich impedance matching. However, the reported strategies for preparing MoS₂/GN hybrids have limited their application potential due to the complex, high-cost and inefficient preparation processes. On the other hand, it is of note that the main source of graphene is based on converting insulating graphene oxides (GO) back to conductive reduced graphene oxides (RGO). Thus, the MA performance of obtained MoS₂/RGO nanohybrids is greatly affected by the conversion process of GO. In this work, we prepared the MoS₂/GN hybrids by a facile hydrothermal method with directly introducing highly pure and electroconductive GNs. It is found that the highest reflection loss value of the sample-wax containing 40% MoS₂/GN is −57.31 dB at a thickness of 2.58 mm, and the bandwidth of RL values less than −10 dB can reach up to 12.28 GHz (from 5.72 to 18 GHz) when an appropriate absorber thickness between 1.5 and 4 mm is chosen. The excellent MA performances emanate from effective conjugation of MoS₂ with GN (Mo–C bond between the interfaces), which provides the dielectric loss caused by multi-relaxation, conductance, and polarization. Taking into account the facile synthesis route and their excellent MA performance, the MoS₂/GNs hybrid nanosheets and those composite materials with similar isomorphic hetero-structures are very promising for a wide range of MA applications.

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1. Introduction

With more and more electronic communication devices around us, the resulting electromagnetic pollution is becoming a threat to human beings. Therefore, designing and preparing efficient microwave absorbing (MA) materials is crucial to governing electromagnetic pollution.^1–6 Two-dimensional (2D) materials, specially molybdenum disulfide (MoS₂), have attracted attention as MA materials due to their size effect, high specific surface areas and remarkable mechanical properties.^7–11 However, the further application of pure MoS₂ is limited by narrow bandwidth; low RL value and the poor impedance matching of single component materials.^12–15

Therefore, one of the strategies is introducing a second phase, such as magnetic ferrite (Fe₃O₄),^16,17 carbon materials (CNT, GN),^18,19 and conductive polymers (PEDOT),^20,21 into/onto the matrix to improve the MA performance and enrich impedance matching.^22–24 Zhang et al.²⁵ synthesized MoS₂/Fe₃O₄ hybrids through a hydrothermal and coprecipitation method and they exhibited outstanding MA performance. Pan et al.²⁶ reported a porous coin-like Fe@MoS₂ composite which optimized impedance matching and showed efficient microwave absorption. Zhang et al.²⁷ fabricated MoS₂/PANI-NDs composites via an in situ oxidation polymerization which exhibited tunable electromagnetic wave attenuation performance. Liu et al.²⁸ prepared 3D hierarchical MoS₂ nanosheets/ultralong N-doped carbon nanotubes as a high-performance electromagnetic wave absorbing material.

Among these second phase materials, graphene shows great potential due to its exclusive structures and electrical properties. Wang et al.²⁹ first synthesized MoS₂/RGO hybrids by chemical vapor deposition and the highest reflection loss value is −50.9 dB. Ran et al.³⁰ reported the synthesis of MoS₂/RGO hybrids by a three-step ultrasonic method and that the highest reflection loss value was −49.7 dB. It should be noted that the preparation of MoS₂/graphene hybrids with hetero-structures involves the conversion of insulating GO into conductive RGO using reducing agents or high-temperature thermal annealing treatments.^31,32 The conversion process is usually tedious and with some troubles, such as insufficient conductivity recovery, morphological changes, phase transitions, even samples polluted with reducing agents. And the reported preparation of MoS₂/graphene hybrids with pure GNs is inefficient and high-cost, which is not viable from the viewpoint of practical application.

In this work, we develop a hydrothermal method to prepare dielectric MoS₂-nanosheets (MoS₂-NS) and MoS₂/GN hybrids with hetero-structures, using molybdic acid; thiourea to grow on pure GN (Scheme 1). This synthesis strategy for MoS₂ and MoS₂/GN hybrids could be applied in various research areas including catalysis, environmental remediation and hydrogen evolution. The as-synthesized MoS₂/GN hybrids show efficient MA performance, and we also investigate the MA performance mechanisms of MoS₂/GN hybrids.

Scheme 1 Synthetic route for the MoS₂/GN hybrids.

2. Experimental

All chemicals were in analytical grade and used without further purification. Molybdic acid (H₂MoO₄ ≥ 85%) was purchased from Shanghai Aladdin Biochemical Technology Co. Ltd. Thiourea (CH₄N₂S) was purchased from Tianjin Fuchen Chemical Reagent Factory. Natural graphite powders were purchased from Qingdao YUBO graphite Technology Co, LTD. Deionized water was purchased from Qiqihar Tianyuan Water Supply Company.

Graphene was prepared from graphite power by a liquid-phase ultrasound method.¹⁵ MoS₂/GN hybrids were prepared by an improved one-step hydrothermal method. Firstly, 40 mg graphene-nanosheets (GN) was dispersed in 40 mL of ethanol/deionized water mixture (volume ratio: 1/3). Secondly, 0.4 g H₂MoO₄ and 0.75 g CH₄N₂S were added into the above dispersions. After magnetic stirring for 30 min, the mixture was transferred to a 100 ml Teflon-lined autoclave with a magnetic stirring device. After heated and stirred for 10 hours at 200 °C, the mixtures were subsequently cooled down to room temperature at natural circumstances. Finally MoS₂/GN hybrids were washed with deionized water and ethanol for several times and dried at 60 °C under vacuum oven.

The morphology of samples was characterized by transmission electron microscope (TEM: Japan Hitachi H7650). The crystallization properties were investigated by an X-ray diffractometer (Bruker Company, D8) using the Cu-Kα radiation (λ = 0.15418 nm). The crystal structure of samples was further observed with high-resolution electron microscopy (HRTEM: Tecnai F30). X-ray photoelectron spectra (XPS) were recorded using a Thermo Scientific ESCALAB MK II with an Mg Kα excitation source. Raman spectroscopy measurements were performed via a Lab RAM HA Evolution. The complex permittivity and permeability were measured in a frequency range of 2–18 GHz with a coaxial wire method using an Agilent N5244A network analyzer.

3. Results and discussion

The XRD patterns of GN; MoS₂ and MoS₂/GN are shown in Fig. S1† and 1a. The MoS₂-nanosheets show diffraction peaks at 2θ = 14.6°, 32.8°, 39.8° and 58.5° corresponding to the (002), (100), (103), and (110) planes (PDF37-1492). It is worth noting that there are no diffraction peaks for GN at the patterns of MoS₂/GN hybrids, suggesting the weak scattering power and low crystallinity of very small amount of GNs added. However, the existence of GNs in the hybrid could be identified by Raman spectroscopy (Fig. 1b). There are two strong peaks located at 1345 and 1587 cm⁻¹, which correspond to the D- and G-bands for GNs. It is also observed that the weak E_2g and A_1g peaks for MoS₂ are located at 376 and 405 cm⁻¹.²⁹ In general, the number of layers of graphene is usually determined by the 2D/G ratios. In this work, the 2D/G ratio of MoS₂/GN hybrids is 0.21 (0.07 < 0.21 < 0.3), corresponding to triple-layer graphene sheets,³³ which consistent with pure GN (as shown in Fig. S2†).

Fig. 1 (a) X-ray powder diffraction (XRD) patterns for MoS₂ and MoS₂/GN; (b) Raman spectrum for MoS₂/GN.

Typical morphology information of the MoS₂/GN hybrids is obtained by TEM. Fig. 2a shows that 2D MoS₂ nanosheets look like a flexible flower. The corresponding selected area electron diffraction (SAED) patterns of the MoS₂-nanosheets are also presented in the inset of Fig. 2a, two diffraction rings in the SAED patterns agree well with the (110) and (100) planes of MoS₂. Fig. 2b shows that GN looks thin and light. The overall morphology image of MoS₂/GN hybrids is shown in Fig. 2c. As presented, it can be clearly seen that the flower-like MoS₂-NS are evenly attached on the GNs. The further micromorphology information of MoS₂/GN composites is displayed by HRTEM, and the interplanar spacing of 0.62 and 0.27 nm observed in Fig. 2d are corresponding to the (002) and (100) planes of hexagonal MoS₂ crystalline structure.^22,34

Fig. 2 TEM images of MoS₂ (a); GN (b); MoS₂/GN (c); HRTEM image of MoS₂/GN (d).

The MoS₂/GN hybrids were also characterized by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 3a, there are only the Mo, S, C and O elements in the survey spectrum, and the atomic contents of Mo and S are 15.52% and 31.76%, respectively. The ratio of Mo/S is very close to 1 : 2. The Mo spectrum for MoS₂/GN as shown in Fig. 3b, the bands located at binding energies of 232.05 and 228.75 eV were assigned to the Mo (3d_3/2) and Mo (3d_5/2) in the normal state of Mo⁴⁺ chemical state, respectively. Moreover, the peak deconvolution of the Mo (3d) spectrum of the MoS₂/GN hybrids indicated two other weak peaks located at 233.00 and 229.20 eV which were attributed to formation of a Mo–C bond on the hybrids, that is, formation of MoS₂/GN hetero-structures. That shows during the prepared process some carbon diffused into the MoS₂ to substitute in the lattice of MoS₂ at the interfaces.^35–37 The S spectrum of MoS₂/GN can be observed to have two peaks located at 162.82 and 161.71 eV, as shown in Fig. 3c, in consistent with the existence of MoS₂. The C 1s spectrum can be deconvoluted into two peaks located at 285.30 and 284.47 eV (Fig. 3d), which correspond to the C–N and C–C functionalities, respectively. Moreover, the band located at 283.90 eV was assigned to the presence of the Mo–C bond, which also proved the formation of hetero-structures.

Fig. 3 XPS spectra of the MoS₂/GN samples: overall spectrum as marked (a); Mo 3d (b); S 2p (c); C 1s (d).

The MA performance can be evaluated by the values of reflection loss (RL) versus frequency which can be determined according to transmission line theory.^38–41 The RL is calculated with the following formulas:

where the normalized input impedance (Z_in) is given bywhere Z₀ is the impedance of free space, c is the velocity of light, f is the frequency, and d is the thickness of the absorber. ε_r is the complex permittivity, ε = ε′ − jε′′, μ_r is the complex permeability, μ = μ′ − jμ′′.

Fig. 4 shows the RL curves of MoS₂–wax composites and MoS₂/GN–wax composites with different loading contents (from 30 to 50 wt%) in the frequency range of 2–18 GHz at the thickness of 2.5 mm. It can be found that under the same loading of 40 wt%, the value of RL for MoS₂/GN hybrid (−43.68 dB) is much larger than that of pure MoS₂ (−19.83 dB). For the MoS₂/GN-wax composites, it can be found that the MA performance of MoS₂/GN-wax composites improves with the increasing loading of MoS₂/GN from 30 to 40 wt%. Nevertheless, degraded MA performance is also observed for the sample with the MoS₂/GN loading of 50 wt%. In summary, the MoS₂/GN–wax composites containing 40 wt% loading exhibited the outstanding microwave absorption properties.

Fig. 4 RL curves of MoS₂ and MoS₂/GN composites mixed with paraffin with different loading (thickness: 2.5 mm, frequency: 2–18 GHz).

Fig. 5 shows the RL value and 3D plots for MoS₂–wax composites, GN–wax composites and MoS₂/GN–wax composites with different thickness (40 wt% loading). The minimum RL is observed to be −57.31 dB at 11.03 GHz for MoS₂/GN with a thickness of 2.58 mm; the bandwidth of RL values less than −10 dB can reach up to 12.28 GHz (from 5.72 to 18 GHz) when an appropriate absorber thickness between 1.0 and 4 mm is chosen (as shown Fig. 5a). Compared with the MoS₂/GN hybrids, the MoS₂-NS and GN-NS exhibit poor MA performances (Fig. 5b and c), indicating that the combination of MoS₂ with GN is an effective method in improving the MA performance.

Fig. 5 RL curves and 3D plots for MoS₂/GN–wax (a); GN–wax (b); and MoS₂–wax (c) composites.

The permittivity and permeability of MoS₂ and MoS₂/GN with 40 wt% loadings were investigated to better understand the probable mechanism of dielectric loss or magnetic loss. Fig. 6 shows the complex permittivity; complex permeability and dielectric loss tangents (tan δ_e = ε′′/ε′) of MoS₂ and MoS₂/GN hybrid in the range of 2–18 GHz. Obviously, as shown in Fig. 6a, both real (ε′) and imaginary (ε′′) permittivity of MoS₂/GN are found higher than that of pure MoS₂, and both real ε′ and ε′′ decrease with increasing frequency. As shown in Fig. 6b, the values of μ′ and μ′′ for both MoS₂ and MoS₂/GN are low, and are close to 1 and 0, respectively. As shown in Fig. 6c, the dielectric loss tangent of MoS₂/GN is higher than that of pure MoS₂.

Fig. 6 The complex permittivity (a); and complex permeability (b); the dielectric loss (c) and the values of C₀ = μ′′(μ′)⁻²f⁻¹ (d) in the range of 2–18 GHz for MoS₂ and MoS₂/GN composites.

In general, the change of complex permittivity can be explained according to Debye theory:^42–45

where ε_s, ε_∞, ω, τ, σ, and ε₀ are the static permittivity, the relative dielectric permittivity at the high frequency limit, the angular frequency, the polarization relaxation time, the electrical conductivity and the dielectric constant in vacuum, respectively. According to eqn (3) and (4), the decrease in ε′ and ε′′ is attributed to the increase in ω. The ε′ of MoS₂/GN is larger than that of MoS₂, which is due to the abundant defect polarization of GNs and interfacial polarization caused by interface coupling of MoS₂ and GNs. The electrical conductivities of the samples were measured by four probe conductivity meter. The conductivity of pure GNs and MoS₂/GN hybrids (the addition amount of GN is 10 wt%) are 27 800 S m⁻¹ and 109.5 S m⁻¹, respectively. Compared to reported MoS₂/RGO composites, the conductivity of MoS₂/GN hybrids in this work is close to that of 30 wt% RGO addition amount of MoS₂/RGO hybrids (113.3 S m⁻¹) in the literature.¹³ The highly conductive MoS₂/GN hybrids stem from the high electrical conductive GNs. According to the free electron theory,¹⁴ ε′′ = 1/2ε₀πρf, where ε₀, ρ and f are the permittivity of vacuum, the resistivity and the frequency of the electromagnetic wave, respectively. Therefore, the high electrically conductive GNs result in the enhanced ε′′ values of MoS₂/GN. The μ′ and μ′′ for both MoS₂ and MoS₂/GN are low and no obvious changes appear due to lack of magnetic components in the composite-wax of MoS₂ and MoS₂/GNs. However, possible magnetic loss for MoS₂ and MoS₂/GN were also investigated. As shown in Fig. 6d, the value of μ′′(μ′)⁻²f⁻¹ is almost a constant within the range of 4–18 GHz, indicating that the eddy current loss is the main contribution of the magnetic loss in MoS₂ and MoS₂/GN at 4.0–18.0 GHz.

As presented, the MA performance of MoS₂ and MoS₂/GN are mainly determined by dielectric loss. In order to investigate and better understand the dielectric loss, the relationship between the relative complex permittivity is expressed by the equation:

which corresponds to a circle centered at ((ε_s + ε_∞)/2, 0) in the spectrum, representing the Debye relaxation process. The Cole–Cole plots further demonstrate the existing of multi-relaxation in MoS₂ and MoS₂/GN samples, as shown in Fig. 7a and b. Two peaks of ε′′ diverge from the ideal Cole–Cole semicircles, indicating the leakage conductance in the MoS₂ and MoS₂/GN samples. It well illustrates that both relaxation loss and conduction loss play important roles in dielectric loss.

Fig. 7 The cole–cole plot of MoS₂ (a) and MoS₂/GN (b).

In this work, the excellent MA performance of MoS₂/GN hybrid is ascribed to the following factors. First, as shown in Fig. 8a, the addition of GN adjusts the complex permittivity of MoS₂ nanosheets, which facilitates impedance matching. Second, the additional GN also carries abundant defects (such as imperfect carbon for GN), providing a large number of dipoles, which are very helpful for dielectric relaxation. Meanwhile, the interfacial polarization in MoS₂/GN, which is considered as the capacitor-like structure, could be effective in the adsorption of electromagnetic waves (Fig. 8b). It could be supported by the mechanism of multi-polarization reported by Cao et al.⁴⁶ Third, electrons can absorb energy and leap on the composite nanosheets, resulting in eddy current losses (Fig. 8c). It also could be supported by Cao's Electron-Hopping model and Conductive–Network equation.⁴⁷ Fourth, the multiple scattering also plays a significant role in electromagnetic wave attenuation as shown in Fig. 8d, the sandwiched layers could lead to the inter-layer reflection and continuous loss of electromagnetic waves. Fifth, the materials with good electrical conductivity may produce skin effect and additional reflection at the surface between materials and air,⁴⁸ which offers effective electromagnetic attenuation.

Fig. 8 (a) Impedance matching of MoS₂/GN; (b) defects and defects polarization of GN; interface polarization of MoS₂/GN; (c) electron hopping of MoS₂/GN; (d) multiple scattering of MoS₂/GN.

As listed in Table 1, compared with the previously reported MoS₂/graphene MA materials, the minimum RL value of MoS₂/GN hybrids in this work is higher than those of MoS₂/RGO hybrids in previous reports. Meanwhile, it is also shown that the minimum RL value of MoS₂/RGO prepared by chemical vapor deposition is higher than that of MoS₂/RGO prepared by hydrothermal method. This is because the chemical vapor deposition can help convert more insulating GO back to conductive RGO.

Table 1 Electromagnetic wave absorption performance of MA materials

	Method	Thickness (mm)	Minimum RL value (dB)	Loading ratio (wt%)	Effective bandwidth (GHz)	Ref.
MoS2/GN	Liquid phase stripping, hydrothermal	2.58	−57.31	40	12.28	This work
MoS2	Hydrothermal	2.0	−22.85	40	4.5	This work
MoS2	Liquid phase stripping	2.4	−38.42	60	4.16	12
MoS2/RGO	Hummers, chemical vapor deposition	1.9	−50.9	10	5.72	29
MoS2/RGO	Hummers, hydrothermal	2.5	−41.53	10	5.92	13
MoS2/RGO	Hummers, hydrothermal	2.4	−41.9	30	5.8	14
M/MoS2/RGO	Hummers, liquid phase ultrasound	2.5	−49.7	18	5.81	30
MoS2/GN	Liquid phase stripping, hydrothermal	2.2	−55.3	20	5.6	15

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4. Conclusion

In summary, MoS₂ and MoS₂/GN hybrids were prepared by a facile one-step hydrothermal method, and the MoS₂-nanosheets were conjugated on the highly conductive GNs. The mechanisms of MA performance for MoS₂ and MoS₂/GN were also investigated. The excellent MA performance indicates that the MoS₂/GN hybrids with hetero-structures hold some advantages in both high conductance and high absorption performance devices. It is also believed that the simple hydrothermal method can promote the synthesis of hetero-structures nanohybrids at an industrial scale. Furthermore, the hetero-structure nanohybrids could also be readily applied in other fields.

Conflicts of interest

There is no conflict in the statement.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51272110, 51772160 and 51771123) and the Shenzhen Peacock Innovation Project (Grant No. KQJSCX20170327151307811).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra08086k

‡ These authors contributed equally to this work.

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