Core/shell structured C/ZnO nanoparticles composites for effective electromagnetic wave absorption

Abstract

Core/shell structured C/ZnO nanoparticles were synthesized by a two-step process based on hydrothermal method. The experimental results show that ZnO nanoparticles attach on the surface of carbon spheres through the surfacial functional groups. The core/shell structure enhances the electromagnetic wave attenuation capability owing to defects, multiple interfaces and optimal impedance match. Different mass percentages of C/ZnO nanoparticles were mixed in paraffin wax to investigate the electromagnetic wave absorbing and shielding performance. When the filler loading is 40 wt%, the composite shows a minimum reflection coefficient of −52 dB at 11 GHz with a sample thickness of 1.75 mm. When the mass ratio is 50 wt%, the sample has an electromagnetic shielding performance of 14.85 dB dominated by absorption. Compared with pure carbon spheres and ZnO hollow spheres, the core/shell structure of C/ZnO composites exhibits a promising route to design electromagnetic wave absorbing materials with high dielectric loss and moderate impedance match.

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Introduction

Electromagnetic (EM) wave absorbing and shielding materials have attracted extensive attention since the rapid development of electromagnetic apparatuses, which have a variety of applications in telecommunication, military and other related fields.^1–4 Among different structures of non-ferromagnetic absorbing materials, multilayered design is a classical and effective way to achieve dielectric loss and impedance match, which are two key factors of EM wave attenuation. Some studies focused on multilayered coatings on the surfaces of matrices to adjust the relative complex permittivity.^5–8 To a certain extent these multilayered structures make more EM waves propagate into the absorbers, but the thicknesses of the coatings have to increase accordingly.⁹

Recently, core/shell structured absorbers with more phase interfaces along with large specific surface area have demonstrated to be a new stage to fabricate EM wave absorbing materials.^10–16 For example, Liu and co-workers have successfully prepared Fe₃O₄@TiO₂ (Fe₃O₄ cores and TiO₂ shells) microspheres, and the epoxy-based composites demonstrated the minimum reflection coefficient (RC) approaching −23 dB.¹² Similarly, Chen et al. wrapped porous Fe₃O₄ nanorods in carbon, showing enhanced EM attenuation about −27.9 dB when the sample thickness is only 2 mm.¹⁷ It is noted that most of the current researches focused on core/shell structured hybrids based on the magnetic particles, which are potential to achieve broader frequency range of effective EM wave absorption (RC < −10 dB). However, magnetic materials usually suffer from relatively high density and ease of oxidation, which limit their applications, including some non-ferromagnetic environments and lightweight devices. Therefore, non-ferromagnetic core/shell structured composites are still required to be further extended.

Compared to conventional magnetic metals, alloys and their oxides, carbon materials exhibit more advantages in low density, excellent chemical resistance and diverse microstructures. In the last decade, significant progress have been made in EM shielding applications of carbon materials, mainly due to their high conductivity and permittivity, including carbon black, carbon nanotubes, carbon fibers, porous carbon and graphene.^18–22 However, the high reflective characteristic hampers their performance for effective EM wave absorption. Core/shell structure can effectively adjust the relative complex permittivity (ε = ε′ − jε′′) to realize relative low ε′ and intermediate ε′′, which are necessary for better impedance match and higher dielectric loss. Zinc oxide (ZnO) nanoparticles with low dielectric constants are also promising for EM wave attenuation due to their small crystal size and corresponding increase in specific surface area, which arise more polarization effects.^23–25 Furthermore, the dielectric properties of composites enormously depend on the interface interaction between absorbing phases.^26,27 Therefore, compared to simple hybrid structures, core/shell structured composites based on carbon microspheres and ZnO nanoparticles could be considered as an effective strategy to adjust impedance match and enhance EM wave attenuation with low cost.

Since interface effect plays an important role in EM wave absorbing mechanisms of core/shell composites, dielectric loss phases with large surface area and smaller crystal size are expected to achieve excellent absorbing performance.^10,28–30 Based on this concept, here we report the synthesis of core/shell structured C/ZnO nanoparticles microspheres and its enhanced EM wave absorbing performance in X-band (8.2–12.4 GHz). The synthesis process of core/shell structure is simple and controllable. More importantly, the permittivity can be adjustable through the size changes of core and shell during the prepared process. The as-prepared C/ZnO microspheres exhibit uniform size with a diameter about 600 nm, whose shell are homogeneous nanoparticles coating on the smooth surface of carbon microspheres. Moreover, the effects of C/ZnO microspheres loading content in paraffin composites on dielectric and EM wave absorption properties are studied, and its optimal permittivity was compared with pure carbon spheres and ZnO hollow spheres.

Experimental

Preparation of C/ZnO microspheres

All the chemicals in the experiments were analytical grade and used without further purification. C/ZnO microspheres were synthesized using a two-step process. Firstly, carbon microspheres were prepared with hydrothermal method. 2.5 g glucose was dissolved in 30 mL of deionized water. The transparent solution was transferred into a 50 mL Teflon-sealed autoclave and maintained at 160 °C for 12 h. After hydrothermal reaction, the puce precipitates were centrifuged, and then washed with deionized water and absolute alcohol for three times. Secondly, as-prepared carbon spheres were dispersed in the mixed solution include 30 mL of absolute alcohol, 2 mL of ethanolamine and 1.5 g of ZnAc₂·2H₂O. The mixture were ultrasonicated for 30 min, and then transferred into autoclave and maintained at 160 °C for 12 h. The product was collected as the previous process of centrifugation. After that, the product was thermally treated at 700 °C for 2 h in flowing Ar and air atmosphere, respectively. For comparison, pure carbon spheres were also annealed at 700 °C in Ar atmosphere.

Characterizations

The morphology of the as-prepared C/ZnO microspheres was examined by scanning electron microscopy (SEM; S-4700, Hitachi, 15 kV, Tokyo, Japan) and transmission electron microscopy (TEM; G-20, FEI-Tecnai, 200 kV, Hillsboro, USA). X-ray diffraction (XRD) measurements were carried out on an X'Pert Pro system (Philips, Almelo, The Netherlands), using Cu Ka (λ = 1.54 Å) radiation. X-ray photoelectron spectra (XPS) were recorded using a Thermo Scientific X-ray photoelectron spectrometer (K-Alpha, Thermo Scientific, Waltham, MA, USA). Raman spectra were obtained on a Renishaw Ramoscope (Confocal Raman Microscope, inVia, Renishaw, Gloucester-shire, U.K.) equipped with a He–Ne laser (λ = 514 nm).

The samples used for EM wave absorption were prepared by mixing C/ZnO microspheres, carbon spheres and ZnO hollow spheres with paraffin wax in different mass ratio. To avoid the aggregation, the composites and paraffin were dispersed into hexane solution by ultrasonication. The relative complex permittivity and the values of scattering parameters (S-parameters) of the as-received samples with dimensions of 22.86 × 10.16 × 1.5 mm³ were measured by waveguide method using a vector network analyzer (VNA, MS4644A; Anritsu, Japan) in X-band. The conductivities of the samples were measured using the four-point probe device (ET9000, Eastchanging, China).

Results and discussion

Characterization of C/ZnO nanoparticles composites

Fig. 1 shows typical SEM and TEM images of as-prepared colloidal carbon spheres and core/shell structured C/ZnO nanoparticles spheres. As exhibited in Fig. 1a, colloidal carbon microspheres which were obtained through hydrothermal carbonization of glucose are observed. It can be seen that the spherical surface is smooth from the mono-dispersed carbon sphere with a diameter about 200–400 nm. Fig. 1b presents a view of well-dispersed C/ZnO microspheres that have a diameter in 500–700 nm range. Their spherical surfaces are coarse, owing to ZnO nanoparticles which is synthesized through the solvothermal process. The elemental mappings of C/ZnO microsphere are shown in Fig. 1c. C element can be detected in the core region, while Zn and O element exhibit larger sphere areas compared with C map. Further evidence for the core/shell structure of C/ZnO microsphere is provided by the morphology of C/ZnO microspheres which were annealed at 700 °C in air atmosphere. Fig. 1d shows the TEM image of monodispersed hollow ZnO microsphere after the colloidal carbon core was oxidized absolutely. The interface between ZnO shell and carbon core can be seen clearly.

Fig. 1 SEM images of colloidal carbon spheres (a); core/shell structured C/ZnO nanoparticles composite (b); a single C/ZnO sphere with its elemental mappings (c); a single hollow ZnO hollow sphere after annealing (with corresponding SAED pattern as inset) (d).

The schematic diagram of the synthetic procedure and morphology of core/shell structured C/ZnO composites is presented in Fig. 2. Colloidal carbon spheres were obtained by the carbonization of glucose monomers, which was attributed to the cross-linking induced by intermolecular dehydration of macromolecules.³¹ The surface of as-prepared carbon spheres has a distribution of residual hydroxyl and carbonyl groups. During the solvothermal process, zinc ions were absorbed on the surface of carbon spheres without any modifications. Finally, core/shell structured C/ZnO composite which aggregated ZnO nanoparticles formed the extrinsic layer of carbon spheres was obtained. Impedance match with free space was achieved by the micro-bilayer design of the obtained composites.

Fig. 2 A schematic illustration of the formation process of C/ZnO nanoparticles composite.

Fig. 3a presents the typical XRD patterns of carbon spheres, C/ZnO without heating treatment and C/ZnO composites. As shown in Fig. 3a, the broad diffraction peak at 2θ = 20.8° are reflection from the (002) plane of graphite, indicating that the pure colloidal carbon spheres are amorphous.³² When ZnO nanoparticles were coated on the surface of carbon spheres through solvothermal process, the peaks at 2θ = 31.46°, 34.17° and 35.95° appear, which correspond to the (100), (002) and (101) planes of hexagonal wurtzite structured ZnO (JCPDS card no. 36-1451). After annealing at 700 °C in an Ar atmosphere, the peak of (002) plane shifts to high angle, while the new weak peak at 2θ = 44.6° was observed, which can be indexed to the (101) plane of hexagonal graphite (JCPDS card no. 75-1621). They indicate higher graphitic carbon structure after heating treatment. In addition, the angle shift of ZnO peaks indicates that oxygen vacancies of ZnO nanoparticles increase when the composites are annealed in the inert atmosphere.³³ Fig. 3b shows the Raman spectra of carbon spheres, C/ZnO without heating treatment and C/ZnO nanoparticles composites. All the samples consist of two broad overlapping peaks at around 1360 and 1585 cm⁻¹. The peak at 1585 cm⁻¹, known as G band, originates from the in-plane vibration of sp² carbon atoms. The peak at around 1360 cm⁻¹, known as D band, indicates the disorder amorphous carbon due to the process of hydrothermal carbonization. It is noted that C/ZnO composite possesses higher intensity ratio of D band to G band (I_D/I_G), which shows opposite change of I_D/I_G with the general microcrystalline graphite. This is ascribed to the hysteresis of amorphous carbon caused by annealing treatment in visible Raman spectroscopy.^34,35 Therefore, the increasing value of I_D/I_G implied the increasing graphitic degree of carbon spheres, which is in good agreement with previous XRD analysis. This characteristic will make a great contribution to the complex permittivity of composites. The upper left magnified spectra exhibit typical peaks of ZnO nanocrystals.

Fig. 3 (a) XRD spectra and (b) Raman spectra of carbon spheres, C/ZnO nanoparticles without annealing, and C/ZnO nanoparticles after annealing.

Further details of structural evolution were characterized by XPS. Fig. 4 presents the XPS C1s spectra of C/ZnO composites before and after annealing. As shown in Fig. 4a, the binding energies of 284.6, 285.8 and 287.7 eV correspond to C–C, C–O and C=O band, respectively, indicating the existence of a large number of residual oxygen functional groups,^36,37 which is ascribed to the carbonization of glucose monomers. After annealing, the peak intensities of hetero-carbon components decrease obviously than unannealed C/ZnO composite (Fig. 4b). This process associated with the removal of residual functional groups is analogous to the thermal reduced process of graphene oxide.³⁸ The decrease of residual oxygen functional groups can enormously affect the dielectric and conductive properties of the composite, which will be discussed later.

Fig. 4 XPS spectra of C/ZnO nanoparticles composite before (a) and after (b) annealing in the C1s region.

Dielectric and electromagnetic properties of C/ZnO nanoparticles composites

The real part and imaginary part of the relative complex permittivity were measured for five samples composed of 10, 30, 35, 40 and 50 wt% C/ZnO nanoparticles in paraffin matrix, as shown in Fig. 5a and b. With the increase of frequency, both the real part and imaginary part decrease, indicating the dielectric relaxation characteristic of the composites. With the increase of the filler loading, the value of the real part increases obviously, owing to the fact that the large number of C/ZnO nanoparticles increase the polarization consisting of space charge and orientation polarizations, since the real part represents the polarization capacity of materials. Similarly, the value of the imaginary part also presents an increasing trend with the increasing mass ratio of C/ZnO composite, which is ascribed to the increasing electric conductivity and polarizations. Fig. 5c illustrates the tangent loss (tan δ = ε′′/ε′) of the samples. The increasing value of tangent loss implies the increasing capability of dielectric loss with the increasing filler loading. It is noted that the average value of tangent loss reaches 0.81, when the mass ratio of C/ZnO composite is 50 wt%. The excessive complex permittivity and dielectric loss indicates that the samples with 50 wt% filler loading are suitable for EMI shielding material rather than EM wave absorption.

Fig. 5 The (a) real part (b) imaginary part of permittivity and (c) the tangent dielectric loss as a function of frequency for the samples with different mass ratio.

Based on the moderate dielectric loss and impedance match, the dielectric properties of C/ZnO spheres, pure carbon spheres and ZnO hollow spheres with the same mass ratio of 40 wt% in paraffin wax were compared, as shown in Fig. 6. The real and imaginary part of ZnO spheres remains almost unchanged with the increasing frequency, and their average values were 2.96 and 0.41. Its low tangent loss make few contribution to EM wave attenuation. Meanwhile, the pure carbon spheres show high permittivity with 21.6 (ε′) and 19.4 (ε′′). The excessive tangent loss leads to impedance mismatch with free space. Therefore, adjusted dielectric properties of core/shell structured C/ZnO composites exhibit an obvious advantage in EM wave absorption.

Fig. 6 The permittivity and the tangent dielectric loss for ZnO hollow spheres, C/ZnO spheres and C spheres with the same mass ratio in paraffin matrix.

In order to further identify the contribution of conductivity on the permittivity of the composites, Fig. 7 shows the electrical conductivity (σ_dc) of the C/ZnO/paraffin samples as a function of mass fraction, compared with the average values of the real and imaginary part. It can be observed obviously that the conductivity rises with the increasing mass fraction of C/ZnO composites, ranging from 2.6 × 10⁻¹¹ to 1.1 × 10⁻³ S cm⁻¹. The electrical conductivity of the dielectric materials can be evaluated by the following equation:

where σ is the conductivity, ε₀ is the permittivity of free space, and f is the frequency. As shown in the Fig. 7, it exhibits the same tendency with the average values of the real and imaginary part. Polarizations have more effects on the imaginary permittivity of the composites with 10, 30, 35, and 40 wt% filler loading, where the values of conductivity are too low to take obvious effect on the imaginary part. However, the electronical conductivity can contribute to the dielectric loss, when the mass ratio is 50 wt%, which is in agreement with the eqn (1).

Fig. 7 Conductivity and complex permittivity of the C/ZnO/paraffin samples as a function of mass fraction.

The measured permittivity was used to evaluate the EM wave attenuation of C/ZnO composites. Base on the model of metal backplane, the reflection coefficient (RC) is calculated according to the following equations:³⁹

where f is the microwave frequency, d is the thickness of the sample, c is the light velocity, ε and μ is the relative complex permittivity and permeability of the C/ZnO composites, respectively.

Fig. 8a–c show the theoretical RC of the typical C/ZnO composites with mass ratio of 30, 40 and 50 wt% in wax matrix, which depends on the sample thickness and frequency. As shown in Fig. 8a, the composite with a loading of 30 wt% exhibits a weak RC at all thickness, which is ascribed to its low dielectric loss. Fig. 8b suggested that the composite with 40 wt% C/ZnO nanoparticles shows the highest RC up to −52 dB at 11 GHz with a thickness of 1.75 mm, and its effective absorbing bandwidth (RC < −10 dB) reaches 2.5 GHz ranging from 9.9 to 12.4 GHz. When the thickness is 2 mm, the minimum RC is −29.1 dB at 9.6 GHz. It can be observed that the minimum RC shifts to lower frequency with the increasing absorber thickness, implying that EM wave attenuation efficiency at requested frequency can be achieved by adjusting the thickness. When the filler loading increases to 50 wt%, the RC of effective absorption cannot be found at any thickness and frequency, as shown in Fig. 8c. The weak EM wave attenuation is caused by large impedance mismatch with increasing filler loading. Based on the mechanism of EM wave attenuation, the absorbing capability exceedingly depends on the impedance match conditions between the absorbers and free space.^40,41 However, the composites with high absorber loadings can be applied as EMI shielding materials, and the corresponding EMI shielding effectiveness will be discussed later. Fig. 8d illustrates the theoretical RC of the C/ZnO/wax composites with different mass ratios at a thickness of 2 mm. It can be observed that the minimum RC is enhanced and shifts to lower frequency with the increasing mass ratios of C/ZnO composites from 10 to 40 wt%, and the composite with 40 wt% filler loading exhibits the best EM wave absorption performance with −29 dB.

Fig. 8 Reflection coefficient calculated for the samples with 30 (a), 40 (b) and 50 wt% (c) mass ratio at different thicknesses, (d) the reflection coefficients calculated for the samples with a thickness of 2 mm.

The above results demonstrate core/shell structured C/ZnO nanoparticles possess effective EM wave attenuation capacity. The fundamental EM wave absorption mechanisms of the composite based on orientational polarization, space charge polarization and electrical conductivity are proposed, as shown in Fig. 9. As is commonly known, polarizations consist of electronic polarization, ironic polarization, orientation polarization and space charge polarization. For electronic and ironic polarization, the frequency and temperature effects are negligible when the frequency is lower than 10¹⁰ Hz. Therefore, the visible frequency dependent behavior may refer to orientation polarization and space charge polarization. Firstly, the multiple interfaces arise more orientational polarization associated with relaxation process, owing to the unique core/shell structure between carbon spheres and ZnO nanoparticles. Secondly, the surface functional groups of carbon spheres, dangling bonds and the defects of ZnO nanoparticles can increase space charge polarization under the alternating electromagnetic field. Thirdly, the electrical conductivity of carbon spheres also attribute to the dielectric loss. The relationship between core/shell structure and corresponding electrical properties is explained by a resistor–capacitor circuit model. As shown in Fig. 9, the basic resistors in the circuit are composed of pure ZnO resistors (R_ZnO) and C/ZnO resistors (R_C/ZnO), and capacitor-like structures form at the interfaces between carbon sphere and ZnO nanoparticles. The abundant capacitors can contribute to attenuate the incident EM waves. It is noteworthy that pure carbon spheres exhibit large impedance mismatch which lead to strong reflection, owing to their high conductivity and permittivity. The shell of ZnO nanoparticles in C/ZnO composite which has lower complex permittivity plays an important role to adjust the overall permittivity to meet the impedance match conditions.

Fig. 9 A schematic approach to enhance the EM wave absorption of C/ZnO nanoparticles composites.

As shown in Table 1, the composites with typical core/shell structured fillers and their corresponding EM wave absorbing performance have been listed. It is noted that most researches of core/shell structured absorbers focused on magnetic materials, including magnetic metals, alloys and metal oxides.^{10–17,42,43} Compared to their EM attenuation performance, C/ZnO composites exhibit a lower minimum reflection coefficient and thinner absorber thickness. In addition, the composite shows lower cost with simple process than other carbon-based absorbers, such as graphene, carbon nanotube and porous carbon. Therefore, core/shell structured C/ZnO composite could be a promising EM wave absorbing material.

Table 1 Typical core/shell structured composites for EM wave absorption

Core	Shell	Matrix	Filler loading (wt%)	RCmin (dB)	Optimal thickness (mm)	Frequency range (RC < −10 dB) (GHz)	Ref.
Barium ferrite	PEDOT	—	—	−22.5	—	—	10
CoNi	C	Wax	40	−35	2	12–18	11
Fe3O4	TiO2	Epoxy	20	−23.3	2	∼3–18	12
FeCo	Al2O3	Wax	40	−30.8	2	∼9–13	13
Polyaniline	Fe3O4	Wax	75	−15.6	2	—	14
Ni	Al2O3	Wax	40	−33.03	2	7.5–13.3	15
Fe3O4	ZrO2	Epoxy	20	∼−25	—	—	16
Fe3O4	C	Wax	55	−27.9	2	—	17
ZnO	ZnAl2O4	Wax	40	−25	2.86	8.2–12.4	41
Fe3O4	TiO2	Wax	50	−20.6	5	∼16–18	42
Fe3O4	ZnO	Wax	50	−35	2	∼9–11.5	43
Carbon spheres	ZnO	Wax	40	−52	1.75	9.9–12.4	This work

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Considering the relatively high conductivity and permittivity of the composites with high filler loadings, their electromagnetic interference (EMI) shielding performance were investigated based on S parameters (S₁₁ and S₂₁). The transmission power (T; T = |S₂₁|²), reflectivity power (R; R = |S₁₁|²) and absorption power (A) meet the equation of A + R + T = 1. EMI shielding effectiveness (SE_T), reflection effectiveness (SE_R) and absorption effectiveness (SE_A) associated with the incident wave P_I and transmitted wave P_T,^20,44 could be described as:

SE_T = −10 log₁₀(P_T/P_I) (4)

SE_R = −10 log₁₀(1 − R) (5)

SE_T = −10 log₁₀[T/(1 − R)] (6)

Fig. 10 shows the SE_T, SE_R and SE_A of the C/ZnO composites with the filler loading of 30, 40 and 50 wt% in the X-band. It can be observed that the EMI SE_T of the composites increases obviously with the increase of the filler loading. When the mass ratio of C/ZnO composite is 50 wt%, the composite exhibits a maximum SE_T of 14.85 dB, which is 2 times higher than that of the composites with 30 wt% filler loading. It is noteworthy that the SE_A shows a significant enhancement with the increasing filler loadings, while the value of SE_R increases slightly. Furthermore, the value of absorption efficiency suggests that the composite with 50 wt% C/ZnO nanoparticles presents efficiency up to 90%, indicating its high attenuation contribution. Therefore, it is believed that the composite with higher filler loading can possess a high EMI shielding performance.

Fig. 10 EMI shielding effectiveness of C/ZnO nanoparticles composites with loading of (a) 30, (b) 40, and (c) 50 wt%.

Conclusions

In this work, core/shell structured C/ZnO composites are prepared by a two-step process including hydrothermal and solvothermal methods. ZnO nanoparticles attached on the surface of carbon spheres, which optimizes the impedance matching condition and electromagnetic parameters. The EM wave attenuation capability are enhanced by the heterogeneous interface between carbon spheres and ZnO nanoparticles. The minimum reflection coefficient reaches −52 dB with a thickness of 1.75 mm, when the filler loading is 40 wt% in wax matrix. The EMI shielding effectiveness of the composite with 50 wt% loading reach 14.85 dB. Compared with the core/shell structured composites based on magnetic materials, C/ZnO nanoparticles could be a competitive and potential EM wave absorbing material with low cost.

Acknowledgements

This work was financially supported by the Nature Science Foundation of China (Grant: 51332004 and 51221001).

Notes and references

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