The synthesis and excellent electromagnetic radiation absorption properties of core/shell-structured Co/carbon nanotube–graphene nanocomposites

Abstract

Through the reduction process involving Co₃O₄/reduced graphene oxide and acetylene, core/shell structured Co/carbon nanotube–graphene nanocomposites were synthesized on a large scale. Because of their special structure, high attenuation constant and good complementarity between magnetic loss material and dielectric loss material, the obtained Co/carbon nanotube–graphene nanocomposites exhibited very attractive microwave absorption. An optimal reflection loss (RL) of up to −65.6 dB at 12.4 GHz was observed with a thickness of 2.19 mm, and RL values below −20 dB were obtained in almost the entire frequency range. Therefore, a simple approach was proposed to explore the high-performance microwave-absorbing materials as well as expand the fields of application of graphene-based materials.

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1. Introduction

In recent decades, with the rapidly extensive application of wireless equipment, radar systems and local area networks, electromagnetic (EM) interference and EM compatibility have become serious problems, which not only limit the utilization of electronic devices and the development of a modern military, but also are potentially harmful to humans.^1–3 In order to counteract these problems, microwave-absorbing materials (MAMs), which can dissipate an EM wave by converting it into thermal energy, have attracted more and more attention worldwide.^4–6 The microwave-absorbing ability of such a material is mainly determined by its complex permittivity and complex permeability as well as by the EM impedance match of the absorbers.^7,8 If the MAMs exhibit the optimal impedance matching conditions, the transmitted microwave can be dissipated by the dielectric loss and magnetic loss of the material, and zero reflectivity of the incident microwave can be obtained. However, it is difficult to meet these conditions using materials with only dielectric loss or magnetic loss. In order to synthesize high-performance MAMs, various structures and categories of composites have been investigated and developed,^9–13 and thin low-weight MAMs with good chemical stability, a strong absorption ability and a wide range of absorption frequencies are greatly desired.

Among these structures and materials, core/shell-structured magnetic nanoparticle/carbon-based composites, which are low in weight and highly stable to chemicals, and display good complementarity between the core substance and shell material, are particularly promising candidates.^14–16 Graphene—because of its large surface area, high chemical and thermal stability, low mass density and outstanding electronic properties—has evoked extensive interest in the past years.^17–20 The previously reported theoretical and experimental results have indicated that the interfacial electronic interaction between metal particles and graphene could cause graphene in such settings to exhibit novel physical properties.^21–24 Therefore, the investigation of the microwave-absorbing properties of magnetic metal particle/graphene-based nanocomposites is of both fundamental and technological significance.²⁵ Herein, we report the synthesis of core/shell-structured Co/carbon nanotube–graphene nanocomposites and investigate their microwave-absorption properties in detail.

2. Experimental

2.1 Material preparation

A specified amount of Co₃O₄/reduced graphene oxide (Co₃O₄/RGO) powder (purchased from XFNANO Materials Tech Co., Nanjing, China) was dispersed on a ceramic plate that was placed inside a quartz reaction tube. Subsequently, the tube furnace was heated from room temperature (RT) to 400 °C in Ar, and then a flow of acetylene (flow rate = 30 mL min⁻¹) was introduced into the reaction tube at 400 °C for 2 h under atmospheric pressure. After cooling to RT in Ar, the final product was obtained.

2.2 Characterization of products

For phase identification, the samples were examined using an X-ray powder diffractometer (XRD) with Cu Kα radiation (model D/Max-RA, Rigaku) at RT. A Raman spectroscopic investigation was performed using a Jobin-Yvon Labram HR800 instrument with a 514.5 nm-wavelength Ar⁺ laser excitation. The morphologies were examined using a transmission electron microscope (TEM) (model JEM-2000EX, operated at an accelerating voltage of 20 kV). The magnetic properties of the samples were measured at 300 K using a Quantum Design MPMS SQUID magnetometer (Quantum Design MPMS-XL) equipped with a superconducting magnet capable of producing fields of up to 50 kOe. Surface properties of the samples were studied by applying the Brunauer–Emmett–Teller (BET) methods to nitrogen adsorption and desorption measurements. For microwave measurements, 30 wt% of the as-prepared sample was mixed with paraffin and pressed into a coaxial cylinder with an outer diameter of 7.0 mm and inner diameter of 3.0 mm. The complex permittivity (ε_r = ε′_r − jε′′_r) and complex permeability (μ_r = μ′_r − jμ′′_r) of the composite were measured in a frequency range of 2–18 GHz over an Agilent E8363B vector network analyzer. The attenuation constant (α) and reflection loss (RL) were calculated by using the equations^26,27where f is the frequency of the EM wave, d is the thickness of the absorber used, c is the velocity of light and Z_in is the input impedance of the absorber.

3. Results and discussion

3.1 Microstructures of Co₃O₄/RGO and Co/carbon nanotube–graphene

The morphologies of Co₃O₄/RGO and as-synthesized Co/carbon nanotube–graphene were investigated by using TEM. The low-resolution image in Fig. 1a indicates the Co₃O₄ nanoparticles (as indicated by the arrows) were inlaid in the layer of the graphene sheet. Moreover, the high-resolution image (as shown in Fig. 1b) revealed a distance between atoms of ca. 0.24 nm, which corresponds to the distance between the (311) planes of Co₃O₄. And the ultrathin structure of graphene can be observed clearly (as seen in Fig. 1b) around the Co₃O₄ nanoparticles.

Fig. 1 (a) Low- and (b) high-resolution TEM images of Co₃O₄/RGO.

The morphology and structure of the obtained Co/carbon nanotube–graphene were also investigated by using TEM. As shown in Fig. 2a, the low-resolution TEM investigation indicated that different sizes of nanoparticles and carbon nanotubes were deposited on the surface of graphene. As indicated by symbols in Fig. 2b and c, the Co nanoparticles were found to be tightly encapsulated by carbon nanotubes and layers of graphene. The high-resolution TEM image (shown in Fig. 2d) revealed a distance between atoms of ca. 0.20 nm, indicative of the formation of crystalline Co nanoparticles. Therefore, core/shell-structured Co/carbon nanotube–graphene nanocomposites were synthesized on a large scale by the method described.

Fig. 2 (a) Low- and (b–d) high-resolution TEM images of Co/carbon nanotube–graphene.

3.2 Crystal structure and magnetic properties

Fig. 3a shows the XRD pattern of the obtained Co/carbon nanotube–graphene. The diffraction peaks located at 41.8°, 44.4°, 47.3° and 51.6° can be assigned to the phase of Co (JCPDS: 01-1254). Moreover, a diffraction peak at ca. 26.2° was also clearly observed, and can be assigned to the (002) crystal plane of hexagonal phase graphite (JCPDS: 75-1621). In order to determine the presence of graphene, Raman spectra of Co₃O₄/RGO and Co/carbon nanotube–graphene were obtained. As shown in Fig. 3b, four peaks were clearly observed for Co₃O₄/RGO and Co/carbon nanotube–graphene. As indicated by the symbols in Fig. 3b, these Raman peaks can be indexed to the D (disorder-induced), G (the tangential mode of graphite structure), 2D (intrinsic peak of graphene) and D + D′ band, respectively.^28,29 The ratio (I_D/I_G) is known to be a measure of disorder in a structure. In the current study, the obtained Co/carbon nanotube–graphene yielded a low I_D/I_G value (0.97) compared to that (1.00) of Co₃O₄/RGO, which may be related to the restoration of the sp² network as reported previously.³⁰ Moreover, compared to the previously reported graphene-based nanocomposites,^31–33 the obtained Co/carbon nanotube–graphene also shows a low I_D/I_G value, which indicates its high crystallinity. Generally, all of the obtained results indicated the as-synthesized sample to be Co/carbon nanotube–graphene. Based on the obtained results, the formation of core/shell Co/carbon nanotube–graphene in our experiment may be explained by the reactions

4C₂H₂ + 5Co₃O₄ → 8CO₂ + 15Co + 4H₂O (4)

Fig. 3 (a) XRD pattern of Co/carbon nanotube–graphene, and (b) Raman spectra of Co₃O₄/RGO and Co/carbon nanotube–graphene.

The formed Co particles could be used as the catalyst of the aforementioned reactions for the effective growth of Co/carbon nanotube–graphene.³⁴

Fig. 4 shows the magnetization curves at RT. The obtained Co/carbon nanotube–graphene was found to exhibit typical ferromagnetism. The saturation magnetization (M_s) and coercivity (H_c) of the sample were measured to be ca.19.7 emu g⁻¹ and 488 Oe, respectively. These results further confirmed that the particles encapsulated in the carbon nanotubes and graphene were ferromagnetic Co nanoparticles. Moreover, compared to the reported MnO₂@Fe–graphene and Fe/graphene,^35,36 the obtained Co/carbon nanotube–graphene displayed a small M_s value.

Fig. 4 The magnetic hysteresis loop for Co/carbon nanotube–graphene at RT (inset is the enlarged part close to the origin).

3.3 Electromagnetic and microwave absorption properties

Fig. 5 shows the complex permittivity, complex permeability, dielectric loss and magnetic loss, and attenuation constant of Co/carbon nanotube–graphene in the 2.0–18 GHz frequency range. As shown in Fig. 5a, the ε′ and ε′′ values were measured to be in the range of 12.92–6.93 and 8.74–2.18, respectively. Moreover, as shown in Fig. 5b, besides some fluctuations, the μ′ values remained close to 1.0 while the μ′′ values were equal to 0. The dielectric loss tangent (tan δ_E = ε′′/ε′) and magnetic loss tangent (tan δ_m = μ′′/μ′) are commonly used to describe the dielectric loss and magnetic loss abilities. As shown in Fig. 5c, the tan δ_E values were found to be much larger than those of tan δ_m in the entire frequency range, indicating that the dielectric loss played the main role in the absorption of EM radiation. Besides, compared to the graphene-based nanocomposites reported elsewhere,^37,38 the obtained Co/carbon nanotube–graphene exhibited good complementarity between the dielectric loss and magnetic loss, which favours a strong attenuation of EM radiation. Such excellent complementarity should be related to the core/shell structure of the sample.³⁹ In order to gain a good understanding of the microwave absorption properties of Co/carbon nanotube–graphene, the attenuation constant α values [as expressed in eqn (1)] were obtained in the entire frequency range, as shown in Fig. 5d. The obtained α values were in the range of 43–197, which is much higher than the previously reported MnO₂@Fe–graphene nanocomposites.³³ Note that the attenuation constant α represents an integral attenuation ability. In other words, the higher α values may indicate the excellent attenuation or microwave absorption.

Fig. 5 (a) Complex permittivity, (b) complex permeability, (c) loss tangent, and (d) attenuation constant versus frequency of Co/carbon nanotube–graphene.

Using eqn (2) and (3), the RL values of the as-synthesized Co/carbon nanotube–graphene nanocomposites were obtained from the measured complex permeability and permittivity at the given frequency, and the results are shown in Fig. 6. Fig. 6a shows a color map of the RL values. The minimum RL was clearly observed to move toward the low-frequency region as the thickness of absorber was increased, and a minimum RL value of −65.6 dB was observed at 12.4 GHz for a thickness of 2.19 mm, which was also confirmed by the result, as shown in Fig. 6b. Moreover, RL values below −20 dB (99% of EM wave attenuation) were obtained in almost the entire frequency range. Fig. 6c shows the typical RL values obtained with the thicknesses of 1.9 mm. The optimum RL value of −43.8 dB was observed at 13.8 GHz. Generally speaking, compared to the representative graphene nanocomposites shown in Table 1, the obtained Co/carbon nanotube–graphene nanocomposites exhibited an excellent ability to absorb microwave radiation.

Fig. 6 (a) Two-dimensional representation of the RL values, and (b and c) the typical RL values obtained with thicknesses of 2.19 and 1.9 mm as a function of frequency for Co/carbon nanotube–graphene.

Table 1 Electromagnetic radiation absorption properties of graphene-based composites reported in recent representative papers

MAMs (absorbent content)	Optimum RL (dB)	Optimum thickness (mm)	Frequency range (GHz) (RL < −20 dB)	Reference
a FeNi3@SiO2–reduced graphene oxide.b MnO2@Fe–graphene.
FeNi3@SiO2–RGO^a, (10 wt%)	−49.4	3.8	5.0–11.0	31
MnO2@Fe–G^b (50 wt%)	−17.5	1.5		35
Fe@G (40 wt%)	−45	3.0	4.0–18.0	36
G–Fe3O4 (20 wt%)	−40.3	5.0	4.0–13.0	37
G–Ni (30 wt%)	−42	2.0	12.0–18.0	38
Fe2O3@G (45 wt%)	−59.6	2.5		40
G@CoFe2O4@SiO2@TiO2	−62.8	4.9	4.5–18.0	41
NiFe2O4@RGO	−47.3	2.5	7.0–15.0	42
NiFe2O4@G (60 wt%)	−29.2	2.0	5.7–17.5	43
G/hexaferrite (20 wt%)	−58	3.0	8.0–12.5	44
Co/carbon nanotube–G (30 wt%)	−65.6	2.19	2.5–18.0	This work

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In order to determine the mechanism allowing this microwave absorption to be so good, the previously reported models such as zero reflection and geometrical effects were used to interpret the enhanced EM radiation absorption properties.^45,46 It is well known that, according to EM wave theory, μ_r = ε_r should be satisfied for zero reflection. However, as shown in Fig. 5a and b, the obtained value of ε_r is much higher those of μ_r. And the geometrical effect is strongly dependent on eqn (6):

t = nλ_m/4 (n = 1, 3, 5,…) (6)

where . Using the minimum data of RL, the corresponding frequency, μ_r and ε_r, and substituting these values into eqn (6) yields a thickness of ca. 1.98 mm, which is considerably different from the obtained thickness of 2.19 mm. Therefore, the zero reflection and geometrical effects were found not to account for the excellent EM radiation absorption properties of Co/carbon nanotube–graphene. Based on the obtained results and the recent mechanisms reported by Ren et al. and Zhang et al.,^47–50 we think that the excellent microwave absorption of the Co/carbon nanotube–graphene nanocomposites resulted from their special structure and synergetic effects. First, according to the Cole–Cole dispersion laws, the Debye relaxation process can be described by the plot of, and the enhanced Debye relaxation process induced by the interfaces can improve the EM radiation absorption properties. As shown in Fig. 7a, the Co/carbon nanotube–graphene nanocomposites yielded many more semicircles in the plot than were reported by others for graphene-based nanocomposites,⁴⁹ which indicates that the interface polarization played an important role in the excellent microwave absorption. Second, the Co nanoparticles grown on the graphene sheets can decrease the resistance (R) of the absorbers. According to, a decrease of R leads to an increase of the dielectric loss. Third, as shown in Fig. 7b, the BET surface area of the Co/carbon nanotube–graphene was determined to be 71.4 m² g⁻¹, which favours the formation of many dipoles. And dipole polarizations enhance absorption of EM radiation.⁵¹ In addition, as shown in Fig. 5c, compared to the previously reported graphene-based nanocomposites,^{33,37,38,47–50} much higher values of α, and much better complementarity between dielectric loss and magnetic loss were observed for the obtained Co/carbon nanotube–graphene nanocomposites.

Fig. 7 (a) Core–core semicircles, and (b) N₂ adsorption and desorption isotherms of Co/carbon nanotube–graphene.

4. Conclusions

In this study, through a reduction process involving Co₃O₄/RGO and acetylene, core/shell-structured Co/carbon nanotube–graphene nanocomposites were synthesized on a large scale. Because of the special structure of these nanocomposites, the high α values, and the good complementarity between magnetic loss and dielectric loss, the obtained Co/carbon nanotube–graphene nanocomposites exhibited excellent absorption of microwave radiation. A minimum RL of ca. −65.6 dB at 12.4 GHz with a thickness of 2.19 mm was observed, and RL values below −20 dB were obtained in almost the entire frequency range. The results of our experiments indicate that the obtained Co/carbon nanotube–graphene nanocomposites may be a promising low-weight material effective for microwave absorption.

Acknowledgements

This work was supported by the International Cooperation Project of Guizhou Province (2012-7002), the Excellent Talents of Guizhou Province (2014-239), the National Science Foundation of Guizhou province (2014–2059), the Postdoctoral Science Foundation of China (2015M570427), the Science and Technology Innovation Team of Guizhou province (2015-4017), the National Science Foundation of China (Grant No. 11364005 and 11174132), and the Foundation of the National Key Project for Basic Research (2012CB932304 and 2011CB922102) for financial support.

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