Microwave Absorption Properties of CoS2 ... - ACS Publications

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Microwave Absorption Properties of CoS2 Nanocrystals Embedded into Reduced Graphene Oxide Can Zhang,† Bochong Wang,*,†,‡ Jianyong Xiang,† Can Su,† Congpu Mu,†,‡ Fusheng Wen,*,†,‡ and Zhongyuan Liu*,† †

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, People’s Republic of China ‡ Hebei Key Laboratory of Microstructure Material Physics, Yanshan University, Qinhuangdao 066004, People’s Republic of China S Supporting Information *

ABSTRACT: CoS2 nanoparticles and CoS2/reduced graphene oxide (CoS2/ rGO) nanohybrids were fabricated by a unique single-mode microwave-assisted hydrothermal method. The microwave absorption properties of CoS2/rGO composites with different rGO proportions were investigated. Morphology analysis indicated that the CoS2 nanoparticles were uniformly embedded into rGO without aggregation. The complex permittivity of CoS2/rGO nanohybrids could be artificially tuned with the rGO proportions. For the CoS2/rGO 1:2 composite, the minimum reflection loss (RL) of −56.9 dB was achieved at 10.9 GHz for the thickness of 2.2 mm; meanwhile, the RL exceeding −10 dB were obtained in the frequency range of 9.1−13.2 GHz. Compared with other rGObased materials, CoS2/rGO composite exhibited superior microwave absorption ability at a rather thin thickness and it has great potential to be used as a highefficiency and tunable microwave absorber. KEYWORDS: CoS2, reduced graphene oxide, permittivity, microwave absorption, impedance matching pyrite structure is a ferromagnetic state (Tc ≈ 120 K) with nearly half-metallic nature and the saturation moment is around 0.86 μB per Co site at 0 K.10,11 Such behavior of CoS2 leads to potential applications in spin-electronic devices.12−15 Besides, CoS2 shows low solubility in molten electrolyte, high thermal stability and large electronic conductivity, which allow the longterm applications in lithium ion batteries and supercapacitors.16,17 These exceptional magnetic and electrical properties indicate that the CoS2 can be used as the magnetic component of the microwave absorption material. However, to the best of our knowledge, there are no related reports. Carbon-based materials with various morphologies, such as single/multiwalled carbon nanotubes,18,19 graphite,20 graphene sheets,6 fullerene,21 etc., have been extensive investigated in recent years and been widely used in the physics, chemistry, biology, and energy fields.22 As the thinnest member in the carbon family, graphene of a monolayer sp2 carbon atoms with honeycomb structure has exhibited outstanding capability as the dielectric loss type microwave absorption material because of its advantage properties including superior electrical conductivity, large interface, high thermal conductivity, light weight, easy processability, and low cost.11,18,20,23−25 However, the high electrical conductivity of graphene is also a fatal

1. INTRODUCTION With the development of communication technology, electromagnetic wave (especially the microwave in GHz band) pollution is becoming a serious problem that is a threat to human health.1−3 Searching for a high-efficiency and tunable microwave absorption material is quite urgent and necessary, not only for military purposes but also for the improvment of the human living environment. As we know, electromagnetic waves contain electrical and magnetic components, and the attenuation of any component will lead to a reduction in total energy. Theoretically, the dielectric loss corresponding to the complex permittivity and the magnetic loss corresponding to the complex permeability in the microwave absorption materials.4,5 However, only one single loss factor cannot result in an excellent absorption performance because it is difficult to satisfy the impedance matching condition.6 Mutual cooperation of the electromagnetic parameters (complex permittivity and complex permeability) is the key to improve the microwave absorption ability. Normally, the magnetic materials have large magnetic loss and the dielectric materials possess high dielectric loss in microwave frequency range.7,8 Therefore, the exploration of magnetic/dielectric composites whose electromagnetic parameters can be artificially adjusted is one of the solutions to obtain the high-efficiency microwave absorption material. Cobalt sulfides, one of transition metal dichalcogenide compounds, are of particular interest for their unique magnetic, electrical, and catalytic properties.9 Among them, CoS2 with © 2017 American Chemical Society

Received: May 17, 2017 Accepted: August 3, 2017 Published: August 3, 2017 28868

DOI: 10.1021/acsami.7b06982 ACS Appl. Mater. Interfaces 2017, 9, 28868−28875

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration for the fabrication of CoS2 embed into rGO.

Figure 2. (a) XRD and (b) Raman spectra of CoS2/rGO nanohybrids with different rGO proportions.

2. EXPERIMENTAL SECTION

volume), and ultrasonically crushed for 4 h. Then, Co(NO3)2·6H2O (0.2 mmol) and Na2S2O3·5H2O (0.4 mmol) were magnetically stirred in this mixed solution for 20 min. Next, the solution was moved in a single-mode microwave reactor and heated at 180 °C for 9 h under the microwave irradiation. This single-mode microwave-assisted hydrothermal method is benefit for the chemical reactions and preventing agglomeration of the nanoparticles. When the chemical reactions were finished, the CoS2/rGO nanohybrids were collected by centrifugation and washed several times using the deionized water. Finally, the CoS2/ rGO nanohybrids were obtained by drying at 120 °C in a vacuum oven for 12 h. The mass ratio between CoS2 and rGO were 2:1, 1:1, 1:2, and 1:3, respectively. The pure CoS2 nanoparticles were synthesized by the same procedure without graphene oxide. 2.2. Characterization. The crystal structures of CoS2/rGO nanohybrids were measured by X-ray diffraction (XRD) under Cu− Kα radiation on Rigaku SmartLab diffractometer. The structural deformation was characterized by Raman spectra at room temperature (Horiba Jobin Yvon LabRAM). The morphologies were investigated by the sacnning electron microscope (SEM, Hitachi S-4800) and the high-resolution transmission electron microscopy (HRTEM, FEI Tecnal G2 F20). The electromagnetic parameters measurements were performed by the vector network analyzer (VNA). The CoS2/ rGO nanohybrids were mixed with 50 wt % paraffin and were pressed into toroidal shape of φout 7.00 mm and φin 3.04 mm. The complex permittivity and permeability were computed from the electromagnetic parameters by VNA, and then were used to calculate the reflection loss (RL) of the samples.

2.1. Fabrication of the CoS2 Nanoparticles and the CoS2/rGO Nanohybrids. In this experiment, all reagents were of analytical grade without further purification. The cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O), sodium thiosulfate pentahydrate (Na2S2O3· 5H2O) and ethylene glycol (C2H6O2) were purchased from Alfa Aesar Chemicals Co., Ltd., China. The graphene oxide (GO) was produced by Hummers method.35 The deionized water was obtained from a Lab Pure Water System (18.25 MΩ cm). The CoS2/rGO nanohybrids were synthesized by a facile synthetic route, as shown in Figure 1. First, the graphene oxides with different mass were put in 10 mL of mixed solution of deionized water and ethylene glycol (2:1 in

3. RESULTS AND DISCUSSION The X-ray power diffraction spectra of CoS2/rGO nanohybrids with different rGO proportions are shown in Figure 2a. All CoS2/rGO nanohybrids have the main diffraction peaks located around 28.0, 32.4, 36.3, 39.8, 46.4, and 55.1°, corresponding to the (111), (200), (210), (211), (220) and (311) crystal planes of polycrystal pyrite structure CoS2, respectively.36 The component of Co oxides and S oxides are not detected, and the diffraction peaks confirm the existence of pure polycrystal

weakness to further enhance the microwave absorption ability because the electromagnetic parameters are out of balance, leading to a bad impedance matching condition and weak absorption.26 One possible way to tailor the conductive property of graphene is a controlled oxidation/reduction process.27 Experimental results revealed that the reduced graphene oxide (rGO) composites have been found much enhanced microwave absorption ability, for example the rGO/ MnFe2O4,28 rGO/CoFe2O4,29 rGO/MnO2,30 rGO/Fe3O4,31 rGO/Co 3O 4 ,32 rGO/ZnO,33 rGO/NiO.34 Most of the researches are fouced on the rGO/ferrite or metal oxides composites that mainly belong to the dielectric loss type microwave absorption material. The rGO/magnetic composites with tunable electromagnetic parameters are not fully explored. In this work, CoS2 nanoparticles and CoS2/rGO nanohybrids were obtained by the single-mode microwave-assisted hydrothermal method and the CoS2 nanoparticles were uniformly dispersed on the rGO monolayer honeycomb structure. Then, the CoS2/rGO nanohybrids with different rGO proportions were fabricated as the microwave absorber. The microwave absorption abilities of the CoS2/rGO composites were systematically investigated.

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Figure 3. SEM images of CoS2/rGO nanohybrids with different rGO proportions, (a) CoS2/rGO 2:1, (b) CoS2/rGO 1:1, (c) CoS2/rGO 1:2, (d) CoS2/rGO 1:3. (e) Representative TEM image of CoS2/rGO nanohybrids. (f) HRTEM image of CoS2/rGO nanohybrids.

IG value of rGO (1.13) is relatively larger than that of GO (0.93), suggesting the formation of new and smaller sp2 domains during the microwave irradiation process.28 The magnetic hysteresis loops of CoS2 nanoparticles were measured at 300 and 80 K (below the Curie temperature). The results are shown in Figure S2. Below Curie temperature, CoS2 nanoparticles show ferromagnetic behavior: the saturation magnetization is around 17.3 emu/g and the coercivity is 395 Oe. At 300 K, CoS2 nanoparticles change to paramagnetic property. In Figure 3a−d, the morphologies of CoS2/rGO nanohybrids with different rGO proportions were analyzed though the SEM images. The CoS2 nanocrystals show spherical shape and the distribution of particle size is mainly concentrated in the diameters of 100−300 nm. The flexible sheetlike twodimensional structure is supposed to be rGO. The CoS2 nanoparticles are dispersed without agglomeration, even in the high CoS2 concentration nanohybrids, shown in Figure 3a. With increasing the rGO proportions, CoS2 nanocrystals are uniformly anchored on the rGO, and finally coated by rGO absolutely, as seen in Figure 3c, d. From these figures, we can

CoS2 in the nanohybrids. To confirm the component of the CoS2 nanoparticles, the EDS elemental analysis was performed and the results are shown in Figure S1 and Table S1. Because of the low crystallinity and scattering power, the rGO signal can barely be detected. Raman measurement is an efficiency tool to detect the structural properties of carbon-based materials. Two typical peaks of rGO can be detected in the Raman spectra, as can be seen in Figure 2b. The peak around 1347 cm−1 is named D band, corresponding to the vibration of sp3 defects and the disorder sites; and the peak around 1588 cm−1 is called G band, corresponding to the vibration of in-plane sp2 hybridization. The defects and disorders in rGO can be revealed by the intensity ratio of D band and G band (ID/IG).6 On the basis of the experimental results, the intensity ratios ID/IG are around 1.13 regardless of the rGO proportions, which indicate that disorders and defects are keep constant in different CoS2/rGO nanohybrids. In ref 28, the D band at 1359 cm−1 and the G band at 1604 cm−1 of the graphene oxides (GO) have been reported. Clearly, the D band and G band of rGO in CoS2/ rGO nanohybrid have red-shifted which indicate a recovery of the hexagonal structure of carbon atoms. Furthermore, the ID/ 28870


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Figure 4. Frequency dependence of the (a) real part and (b) imaginary part of permittivity, (c) real part and (d) imaginary part of permeability, (e) dielectric loss tangent and (f) magnetic loss tangent of CoS2/rGO composites with different rGO proportions.

Figures 4a−d show the frequency dependence of real part and imaginary part of permittivity and permeability for CoS2/ rGO composites with differnet rGO proportions, respectively. The real part of permittivity and permeability represent the energy storage ability, whereas the imaginary part of permittivity and permeability reveal the energy dissipation and magnetic loss, respectively. In Figure 4a, b, with increasing the rGO proportions from CoS2/rGO 2:1 to 1:3, complex permittivity enhance, for example, the values of ε′ increase from 6.3 to 12.8 and the values of ε″ increase from 1.4 to 6.3 at 9 GHz. The enhancement of permittivity is due to the high electrical conductivity property of rGO, and also consistent with our prediction from the SEM images in Figure 3. And it may also due to the relaxation and polarization of the residual defects and groups of rGO surface, which can be generated by the reduction process of GO. The residual defects can act as polarization centers and generate polarization relaxation under the microwave field, resulting in the absorption of microwave. Because of the local symmetry breaking of the CoS2 surface atoms, dipole polarization can be introduced and the dipolar relaxation process is an important mechanism to enhance the microwave absorption property that is expressed by the Cole− Cole semicircle. Figure S3 shows the Cole−Cole semicircle of CoS2/rGO 1:2 nanohybrids. There is more than one semicircle suggesting the multiple Debye relaxation process in the nanohybrids. The values of permeability are determined by the content of CoS2 and not sensitive to the rGO proportions, because the rGO is nonmagnetic material. Therefore, the values

Figure 5. Attenuation constant of CoS2/rGO composites with different rGO.

predict that the dielectric properties of CoS2/rGO nanohybrids can be tuned by the rGO proportions. The HR-TEM images of CoS2/rGO nanohybrids are shown in Figure 3e, f. The interplanar spacing of CoS2 nanocrystal is measured as 0.32 nm, which corresponds to the (111) crystal planes of CoS2 and also consistents with the XRD result in Figure 2a and the EDS analysis in Table S1. To investigate the microwave absorption properties of CoS2/ rGO nanohybrids, one common method is prepare the CoS2/ rGO composite mixed with wax, and then measure the frequency dependence of the electromagnetic parameters (complex permittivity and complex permeability) using vector network analyze. 28871


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Figure 6. Color map of the reflection loss for the CoS2/rGO composites with different rGO proportions, (a) CoS2/rGO 2:1, (b) CoS2/rGO 1:1, (c) CoS2/rGO 1:2, (d) CoS2/rGO 1:3; (e) reflection loss for CoS2/rGO composites under a constant thickness.

Table 1. Microwave Absorption Properties of rGO-Based Composites material rGO MnFe2O4/rGO NiO/rGO ZnO/rGO CNTs/rGO Fe2O3/rGO MoS2/rGO CoS2/rGO a

matrix PVDF wax wax wax wax wax wax wax

b

ratio (wt %)

thickness (mm)

frequency rangea (GHz)

RLmin (dB)

ref

3 5 8 50 5 50 10 50

4.0 3.0 3.0 2.2 3.0 2.5 2.3 2.2

8.5−12.8 8.0−12.9 10.2−16.9 8.9−11.4 7.1−10.4 8.2−10.2 10.4−13.4 9.1−13.2

−25.6 −29.0 −38.0 −45.1 −44.6 −18.2 −50.9 −56.9

39 28 34 40 41 42 43 This work

Reflection loss below −10 dB. bPolyvinylidene fluoride.

4e. The enhancement of tan δe can be explained by the polarization effects of the nanohybrids, and the defects and disorders in rGO. On the other hand, tan δm is much smaller than tan δe regardless of rGO proportion and frequency. The results suggest that the microwave absorption property of CoS2/rGO composites can be easily controlled by the rGO proportion. Based on the transmission line theory, the attenuation constant α is used to evaluate the damping

are almost the same with different rGO proportions, seen in Figure 4c, d. In addition, the permeability of CoS2/rGO is independent from the frequency. The real part is around 1.3 and the imaginary part is about 0.1 in whole frequency range. Dielectric loss tangent (tan δe = ε″/ε′) and magnetic loss tangent (tan δm = μ″/μ′) are always used to represent the dielectric loss and magnetic loss capacity of the microwave absorber, respectively. It can be seen that the tan δe is enhanced from 0.2 to 0.5 with increasing the rGO proportions in Figure 28872


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ACS Applied Materials & Interfaces property of one material. The attenuation constant α can be expressed as α=

2 πf c

(μr ″εr″ − μr ′εr′) +

XRD results showed a polycrystal pyrite structure of CoS2 nanoparticles without oxidation. Raman spectra suggested that the rGO had new and smaller sp2 domains compared with graphene oxide. Morphology analysis indicated that CoS2 nanoparticles were uniformly embeded into rGO without aggregation. The microwave absorption properties of CoS2/ rGO composites with different rGO proportions were investigated. The complex permittivity of CoS2/rGO nanohybrids could be artificially tuned with the rGO proportions because of the electrical property of rGO. For the CoS2/rGO 1:2 composite, the minimum reflection loss (RL) of −56.9 dB was achieved at 10.9 GHz for the thickness of 2.2 mm; meanwhile, the RL exceeding −10 dB were obtained in the frequency range of 9.1−13.2 GHz. Compared with other rGObased materials, CoS2/rGO composite exhibited superior microwave absorption ability at a rather thin thickness and it has great potential to be used as a high-efficiency and tunable microwave absorber.

(μr ″εr″ − μr ′εr′)2 + (μr′εr″ + μr ″εr′)2

(1)

where f is the frequency, c is the velocity of light, εr′ and εr″ are the real and imaginary part of complex permittivity, μr′ and μr″ are the real and imaginary part of complex permeability, respectively. Figure 5 shows the attenuation constant of CoS2/ rGO composites with differnet rGO proportions. In whole frequency range, CoS2/rGO 1:2 and CoS2/rGO 1:3 composites have much larger values than other composites. Therefore, it can be expected that the CoS2/rGO 1:2 and CoS2/rGO 1:3 composites exhibit better microwave absorption performance than others. According to the transmission line theory, the reflection loss (RL) of CoS2/rGO composites with different rGO proportions can be estimated by the following equations37

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Z in = Z0(μr /εr)1/2 tanh[j(2πfd /c)(με )1/2 ], r r RL = 20log|(Z in − Z0)/(Z in + Z0)|

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06982. SEM image of CoS2 nanoparticles, and selected areas for the EDS elemental analysis; magnetic hysteresis loops of CoS2 nanoparticles measured at 300 and 80 K; Cole− Cole semicircle of CoS2/rGO 1:2 nanohybrids; dependence of RL on frequency at various thicknesses for the CoS2/rGO 1:2 composite; dependence of λ/4 thickness on frequency for the CoS2/rGO 1:2 composite (PDF)

(2)

where Z0 is the impedance of air, Zin is the impedance of CoS2/ rGO composite, εr is the complex permittivity, μr is the complex permeability, f is the microwave frequency, d is the thickness, and c is the velocity of light. As observed in Figure 6a−d, the RL values show similar behavior and the minimum value is around −12 dB. With increasing the rGO proportion to CoS2/rGO 1:2, the microwave absorption performances of CoS2/rGO composite are extremely enhanced. Further increasing the rGO proportion, the RL becomes worse. The reason is that the complex permittivity excessive increase while the complex permeability keeps stable (Figure 4); such unilateral change leading to the impedance mismatch of the composite. There are bright belts in the color maps, indicating the minimum RL under different thickness and frequency. It always shows an inverse proportional relationship between the thickness and frequency which can be explained by the quarterwavelength (λ/4) matching microwave absorption mechanism.6,38 The frequency dependence of RL at various thicknesses and the frequency dependence of calculated λ/4 thickness for CoS2/rGO 1:2 composite are shown in Figure S4. Clearly, the minimum RL frequencies are well consistent with the quarter-wavelength matching model. The RL values calculated at a thickness of 2.2 mm is shown in Figure 6e. The microwave absorption performances can be easily tuned by the rGO proportions. For the CoS2/rGO 1:2 composite, the RL exceeding −10 dB in the frequency range of 9.1−13.2 GHz are obtained, while an optimal RL of −56.9 dB is achieved at 10.9 GHz. A typical RL value of −10 dB means 90% microwave energy is absorbed, which is suitable for the practical application. The microwave absorption properties of rGObased composites are summarized in Table 1. Compared with the recently reports, CoS2/rGO composite exhibits superior microwave absorption ability at a rather thin thickness. Therefore, the CoS2/rGO composite has great potential to be used as a high-efficiency and tunable microwave absorber.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Tel.: +86-335-8074631. Fax: +86-335-8074545. ORCID

Congpu Mu: 0000-0003-1656-716X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant 51571172, 51672240, 51571171, 51272225, 51271214, and 51421091), Program for New Century Excellent Talents in University (NCET-13-0993), Natural Science Foundation for Distinguished Young Scholars of Hebei Province (Grant E2017203095), Science Foundation for the Excellent Youth Scholars from Universities and Colleges of Hebei Province (YQ2014009), Research Program of the College Science & Technology of Hebei Province (QN2014047).

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REFERENCES

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CONCLUSIONS In the present research, CoS2 nanoparticles and CoS2/reduced graphene oxide (CoS2/rGO) nanohybrids were prepared by a unique single-mode microwave-assisted hydrothermal method. 28873


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