Solvothermal synthesis of flower-like CoS hollow microspheres with excellent microwave absorption properties

Abstract

In this study, flower-like CoS hollow spheres (CHSs), synthesized via a facile solvothermal method in the presence of CTAB, were initially investigated as microwave absorbers. It was found that such microstructure was significantly different from CoS nanoparticles (CoS NPs) prepared without introducing CTAB under similar preparation conditions. Moreover, with a low filler loading of 20 wt%, CHSs based composite could reach an optimal reflection loss of −43.6 dB at 15.6 GHz, and exhibit an effective absorption bandwidth (below −10 dB) of 4.6 GHz (13.2–17.8 GHz) at the thin thickness of 2.0 mm. Furthermore, through adjusting the thickness from 2.0 to 5.0 mm, an effective absorption bandwidth of 13.2 GHz could be monitored in the frequency range of 4.8–18.0 GHz. The results indicate that the novel CHSs are intriguing microwave absorbers with the advantages of strong absorption, broad bandwidth, light weight and thin thickness, which is attributed to the unique nanoarchitecture, extra void space, and high dielectric loss.

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1. Introduction

Nowadays, the wide application of electronic devices and communication facilities makes our lives more convenient than ever before, however, serious electromagnetic interference (EMI) problems have also emerged, which not only influence the functionality of wireless communications but also harm the fitness of human beings.^1–3 Exploiting microwave absorption materials to attenuate those unwanted electromagnetic energies with superb absorption qualities at specific frequency is a desirable way to address these issues induced by EMI.^4,5 It is well known that the following factors determine the microwave absorption properties of an absorber: dielectric loss, magnetic loss, impedance matching, and the specific micro/nanostructures.⁶ Therefore, extensive efforts have been made accordingly to develop effective absorbing materials, supposed to have the features of strong, lightweight, and broadband absorbing ability.⁷ Among all the contenders, the materials with micro/nanostructures have potential as microwave absorbers due to their unique properties, including carbon-based materials,^8–12 magnetic particles,^13,14 conducting polymer^15,16 or their composites,^17,18 etc. However, most of them cannot reach the standards of ideal electromagnetic wave absorption materials for their poor dispensability, high density, corrosion susceptibility, relatively low absorbing abilities or narrow bandwidth of absorption frequency. Thus, exploring and developing new type of electromagnetic wave absorption materials are still in high demand.

Among the novel materials, hollow micro/nanostructures have characteristics such as low density, high surface area, and low coefficients of thermal expansion and refractive index, thus making them attractive in solar cells, lithium ion batteries, especially in microwave absorption field.^19,20 In detail, MnO₂ hollow microspheres were synthesized by Wang et al. via a facile hydrothermal method using SiO₂ sphere as templates and showed the optimal reflection loss of −40 dB at 14.2 GHz and effective absorption bandwidth of 3.5 GHz with a layer thickness of only 4 mm.²¹ An et al. reported the fabrication of silica–nickel–carbon composite microspheres with shelly hollow structures, and the minimum reflection loss value reached −37.6 dB at 15.3 GHz with a thin thickness of 2.4 mm, and the frequency bandwidth of effective absorption was up to 6.3 GHz.²² Unique porous hollow Ni/SnO₂ hybrids were prepared using an easy two-step method by Zhao et al., and the optimal reflection loss was up to −36.7 dB at 12.3 GHz, and the effective electromagnetic absorption band was 3.4 GHz (10.6–14.0) with a thin coating thickness of 1.7 mm.²³ Lv et al. described the synthesis of hierarchically hollow carbon@Fe@Fe₃O₄ nanospheres and the effective absorption bandwidth was up to 5.2 GHz with an optimal reflection loss value of −40 dB while the layer thickness was only 1.5 mm.²⁴ Zhao and his coworkers successfully prepared flower-like CuS hollow microspheres with self-assembly nanoflakes and the minimum reflection loss of −31.5 dB could be observed at 16.7 GHz and the effective absorption bandwidth could be tuned between 6.2 GHz and 18.0 GHz for the hollow microwave absorber by adjusting the layer thickness from 1.5 to 4.0 mm.²⁵ Based on the above-mentioned results, the materials with hollow structures have advantages of being lightweight and high energy absorption over other absorbing materials. Thus, it is promising and meaningful to select a novel hollow structure with the above features.

Semiconductor transition-metal sulfides have received considerable attention due to their remarkable properties and impressive applications in a variety of devices.^26,27 Along with many other metal sulfides, cobalt sulfide (CoS) is a promising material with potential applications in solar cells, energy storage, optoelectronic devices, and lithium ion batteries.^28–30 So far, considerable effort has been devoted to synthesizing CoS with various morphologies, including spheres,³¹ nanosheets,³² nanowires,³³ flower-like architectures,³⁴ and so on. More recently, the microwave absorption properties of transition-metal sulfides have been of much interest. For example, flower-like CuS hollow microspheres were synthesized and their CuS/paraffin composites exhibited superb microwave absorption properties.²⁵ The graphene–CdS nanocomposites with remarkable microwave absorption were directly synthesized with graphene oxide through a facile hydrothermal approach.³⁵ Also, novel CuS microspheres embedded in graphene had been successfully synthesized via an in situ growth method and excellent microwave absorption properties were observed.³⁶ Meanwhile, Wei and his coworkers selectively prepared CuS hexagonal nanoplates by a facile wet chemical approach and the CuS/PVDF composites possessed promising microwave absorption performance.³⁷ Unique MnS hollow spheres-reduced graphene oxide hybrid composites were fabricated successfully and their microwave absorption properties were extensively investigated.³⁸ However, to the best of our knowledge, reports on exploration of cobalt sulfide with unique nanostructures for microwave absorption have not been reported.

Herein, for the first time, we demonstrate a facile route for preparing flower-like CoS hollow microspheres without using any templates which are initially used for attenuating the microwave. In addition, their microwave absorption properties were also investigated in the frequency range of 2–18 GHz. Compared to the previously reported transition-metal sulfides, the as-fabricated flower-like CHSs with well-defined 3D hierarchical flower-like hollow nanostructures harness stronger absorption and wider absorption bandwidths at a lower filler loading, which will meet the current requirement of advanced microwave absorbing materials.

2. Experimental procedures

2.1 Raw materials

Cobalt chloride hexahydrate (CoCl₂·6H₂O, 99 wt%), thiourea (CN₂H₄S, 99 wt%), ethylene glycol (EG, 98 wt%), cetyltrimethyl ammonium bromide (CTAB, 98 wt%). All other chemicals applied in this experiment were of an analytically pure grade and used without further purification.

2.2 Preparation of hollow CoS hierarchical structures

The obtained CHSs were carried out by a facile solvothermal method. In a typical synthesis, 2 mmol CoCl₂·6H₂O and 2 mmol CTAB were firstly dispersed in 65 ml EG and sonicated for 0.5 h under ambient conditions. Then, 5 mmol thiourea was introduced to this system with magnetic stirring to create a steady solution. In the present case, an excess amount of thiourea was employed to compensate possible sulphur loss in the thermal process.³⁹ After 1 h, this mixture was transferred into a Teflon-lined stainless steel autoclave and then maintained at 180 °C for 16 h. After the reaction, the solution was naturally cooled to room temperature and then washed with distilled water and absolute ethanol several times to get rid of impurities attached on the surface of CoS and finally dried at 60 °C for 24 h.

For properties comparison, CoS nanoparticles (denoted as CoS NPs) were also prepared in the absence of CTAB under similar procedures.

2.3 Characterization and measurement

X-ray diffraction (XRD) measurements were performed on Smart Lab XRD spectrometer (Rigaku) with Cu Kα radiation in the range of 20–80° (2θ). Raman spectroscopy was obtained by a Thermo Fisher Raman microscopy system (laser wavelength, 514 nm; spot size, 150 μm) in the wavenumber of 200–2000 cm⁻¹. X-ray photoelectron spectroscopy (XPS) was conducted by Thermo Scientific Escalab 250Xi photoelectron spectroscopy using a monochromated Al Kα X-ray gun and a pass energy of 30.0 eV. The morphology of the as-prepared samples was observed by scanning electron microscopy (FEI Inspect F50, USA). All of the samples were coated with gold under a vacuum before observation. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were taken with a JEM-2100 high-resolution transmission electron microscope. To analyze the composition of samples, the element composition of the samples was characterized by an energy dispersive spectrometer (EDS, Oxford Instruments), associated with TEM. The electromagnetic parameters of the obtained products in the frequency range of 2 GHz to 18 GHz were investigated by a vector network analyzer (Agilent PNA N5224A) using the coaxial-line method. The testing samples were prepared by blending the as-synthesized products in a paraffin matrix and then pressed into toroidal-shaped samples (Φ_out: 7.00 mm, Φ_in: 3.04 mm). The values of reflection loss (RL) were accordingly calculated.

3. Results and discussion

3.1 Characterization of the as-prepared samples

A typical powder XRD pattern of the as-synthesized CHSs is shown in Fig. 1a. The four characteristic diffraction peaks (2θ) at 31.0°, 35.5°, 47.2° and 55.0° represent the Bragg reflection from (100), (101), (102) and (110) planes of the hexagonal phase of CoS (JCPDS card no. 65-3418), respectively. No peaks corresponding to impurities are detected, thus indicating the purity and crystallinity of the products. However, it should be pointed that the relative intensity of these peaks has slightly changed. This might be attributed to that the CHSs have a preferred orientation hinting at the unique morphology, which is in agreement with the previous report.³⁰ Fig. 1b presents the typical Raman spectra of samples corresponding to CHSs materials. As shown, three characteristic Raman peaks at 470, 515 and 678 cm⁻¹ can be observed, which are indexed to E_g, F_2g and A_1g modes of CoS materials, respectively.⁴⁰ Moreover, the composition purity is verified by EDX spectroscopy (Fig. 1c). The EDS results indicate the presence of Co, S and a small number of O elements, and this phenomenon might be due to the residual oxygen or oxides in the obtained product. In addition, the peaks of Cu element existing in the EDS images are contributed to the copper mesh. Furthermore, the inset attached to Fig. 1c shows that the atomic ratio of Co : S is 52.10 : 47.90, which is close to the chemical stoichiometry of CoS materials and correspond well with the XRD pattern. XPS analysis in Fig. 2a and b was further performed to verify chemical states of cobalt and sulfur for the CHSs. It can be observed that the detected elements Co and S accord well with the result from the EDS. The Co 2p spectrum peaks centered at 778.6 and 793.8 eV, and the S 2p peak at 162.2 eV are the characteristic signals of cobalt sulfide.⁴¹

Fig. 1 (a) XRD pattern, (b) Raman pattern and (c) EDS profile of the CHSs.

Fig. 2 XPS spectra of (a) Co 2p and (b) S 2p for the CHSs.

To provide full insights into the structure and morphology of the obtained products, SEM and TEM are carried out, respectively. Fig. 3a and b are typical low-magnification SEM images of the CHSs, which clearly reveal that the CHSs are comprised of hierarchical flower-like microspheres with a diameter ranging from 1 to 5 μm. From the high-magnification SEM image (Fig. 3c) of CHSs, we can clearly see that the flower-like microspheres consist of many crossed thin nanoflakes with the thickness of about 40 nm. By contrast, CoS nanoparticles (shown in Fig. 3d) prepared in the absence of CTAB exhibit sphere-like morphologies of uneven size, and the diameter of the spheres is just around 100 nm, which demonstrates that CTAB plays a significant role in both the hierarchical structures and microscopic dimensions for this framework.

Fig. 3 (a–c) Different magnification SEM views of CHSs; (d) SEM image of CoS nanoparticles prepared in the absence of CTAB.

The morphology and formation of hierarchical flower-like CoS microspheres are further characterized by TEM and HRTEM. Fig. 4a and b are the different magnification TEM views of CHSs, further confirming their special nanostructure. It can be found that the CHSs present hierarchically flower-like microspheres in Fig. 4a. As shown in the Fig. 4b, the contrast between the dark edge and the pale center provides evidence of the hollow nature of the microspheres, demonstrating that these hierarchical microspheres are not completely solid. This structure endows the CHSs with the specific surface area, thus making the incident microwave contact fully with CoS materials. The HRTEM image of nanoplates (Fig. 4c) detached from the hierarchitectures supplies an indicative of the crystalline nature of the CoS nanoflakes. It can be observed that the lattice pacing is 0.254 nm, in accordance with that of (101) planes on the PDF card. As shown in Fig. 4d, CoS nanoparticles prepared in the absence of CTAB still possess a hollow structure while the size is much smaller compared with the flower-like CHSs. The difference of the morphologies would be associated with different nanocrystal formation. All the results indicate that flower-like CoS hollow microspheres have been successfully synthesized through this method.

Fig. 4 (a and b) Different magnification TEM images and (c) high-resolution TEM image of CHSs; (d) TEM image of CoS nanoparticles prepared in the absence of CTAB.

It was generally acknowledged that exact mechanism of the nucleation and growth of CoS was relatively complex, where different hydrothermal treatment technique would lead to different crystal structures, morphologies and sizes, unique morphology and different hierarchical structures/crystal phase. A three-step formation mechanism possible for the cobalt source and sulfur source has been proposed by researchers,^42,43 described as follows: first was the formation of a large amount of CoS primary particles when cobalt–thiourea complexes were solvothermally treated. Second, a large amount of CoS primary particles were produced, followed by the aggregation of primary particles (verified by S1a and 1c†). Finally, larger diverse structures were obtained for these particles assembling preferentially. It was ascertained that upon heating, ligand complex would finally decompose to give free Co²⁺ ions and active S²⁻ ions in the solution, thus laying foundation for reaction between Co²⁺ and S²⁻ to form CoS, which could be explained by the following reaction equations:

Co²⁺ + nTu + mEG → [Co(Tu)_n(EG)_m]²⁺ (1)

[Co(Tu)_n(EG)_m]²⁺ → CoS (2)

In the present case, CTAB acted as an effective surfactant, which would separate the continuous system into massive small regions and adsorb on the primary particles, thus benefitting the particles to grow along one-dimension or multi-dimension to form petals or flowers in the further procedures.^44,45 It was found that the obtained products would display totally different asymmetrical sphere-like structure in the absence of CTAB (shown in Fig. 3d), indicating CTAB played a crucial role in the formation of the flake-like structure. As illustrated in Scheme 1, the reaction proceeding, outer nanoflakes crystallites grew larger at the expense of dissolving the inner crystallites proceeds for reducing the total surface energy, thus producing shell–shell–void hollow microspheres (further verified by S1b and 1d†).²⁵ After further growth and Ostwald ripening process, these microspheres finally became the final hierarchical hollow flower-like microspheres.

Scheme 1 Schematic description of formation process for CHSs.

3.2 Electromagnetic absorption performance of the as-prepared samples

In order to understand the possible electromagnetic wave absorption mechanism of the CoS materials, the complex permittivity real part (ε′), permittivity imaginary part (ε′′), permeability real part (μ′), and permeability imaginary part (μ′′) of the CHSs paraffin composites which are formed by mixing various amounts of hollow CoS flowers with paraffin are measured in the range of 2–18 GHz. As for the relative complex permittivity, the real part (ε′) and imaginary part (ε′′) are associated with the energy storage and dielectric loss, respectively.⁴⁶ The frequency-dependent real (ε′) and imaginary (ε′′) parts of the complex permittivity of the CHSs paraffin composites with various CHSs loadings are displayed in Fig. 5a and b. It can be seen that the values of ε′ increase with growing CHSs loadings, where ε′′ values experience a similar trend. Especially when the loading of CHSs is 20 wt%, the ε′ value maintains around 8, which is double of that of 10 wt% CHSs loading. Furthermore, the imaginary part is about 10 times higher than that when the loading is 10 wt%, indicative of high conductivity and high dielectric loss. Whereas further increasing the loading of CHSs to 40 wt%, the complex permittivity value of the CHSs paraffin composites exhibit such relatively high values that they go against the impedance match, which requires the complex permittivity to be close to complex permeability.^47,48 As a consequence, the complex permittivity should be controlled in a rational range for this framework. In addition, we calculated the dielectric loss tangents (tan δ_ε = ε′′/ε′) of the samples containing CHSs with different loadings, respectively. As shown in Fig. 5c, the values of the dielectric losses increase with the growing CHSs loading, which is in good agreement with the permittivity curve. It is well known that both dielectric loss and magnetic loss make possible contributions to microwave absorption, in which the permittivity and permeability mainly originate from ion polarization, electronic polarization, intrinsic electric dipole polarization, and magnetic properties.⁶ Therefore, in the appropriate range of filler concentration, the increased loading of CHS contributes to enhancement of the dielectric loss, thus benefiting the microwave absorption. Finally, due to the absence of magnetic constituents in the CHSs paraffin composites, the real and imaginary part of complex permeability are around 1.0 and 0.0, respectively (shown in Fig. 5d).

Fig. 5 Frequency dependent (a) real parts, (b) imaginary parts of the complex permittivity, (c) dielectric loss tangents of the paraffin-composites containing different CHSs loadings and the (d) real and imaginary parts of complex permeability for CHSs of 20 wt% loading.

To better evaluate the microwave absorption, based on a generalized transmission line theory, the reflection loss (RL) values of the obtained products at a given frequency and thickness layer were calculated, which is summarized as the following equations:

Z_in = (μ_r/ε_r)^1/2tanh[j(2πfd/c)(μ_r/ε_r)^1/2] (3)

in which Z_in is the input impedance of the absorber, ε_r and μ_r are the relative complex permittivity and the relative complex permeability, respectively, f is the frequency, d is the thickness and c is the velocity of electromagnetic waves in free space.

Fig. 6a shows the theoretical RL of the CHSs/paraffin composites with different loadings in the range of 2–18 GHz at a thickness of 2.0 mm obtained via eqn (3) and (4). With a loading of 10 wt%, the minimum RL value of CHSs is just −2.1 dB at 17.1 GHz. By increasing the filler loading to 20 wt%, the minimum RL value is up to −43.6 dB at 15.6 GHz, and the bandwidth of RL less than −10 dB is 4.6 GHz (13.2–17.8 GHz). With a loading of 40 wt%, the minimum RL value decreases sharply to −3.8 dB at 6.0 GHz instead. It might originate from the fact that the relatively low filler concentrations make the CHSs molecules unable to fully contact each other, where conductive CHSs networks cannot be formed, whereas the relatively high concentration of CHSs with high complex permittivity leads to impedance mismatch. It is well known that they are both unfavorable for the microwave absorption. As shown in Fig. 6b, the electromagnetic parameters (ε′, ε′′, μ′ and μ′′) of the CoS NPs paraffin-composites with 20 wt% filler loading are measured according to the above results. It can be found that the real part of complex permittivity is around 6.5 similar to that of corresponding CHSs while the imaginary part is much smaller (0.7 only) compared with that of CHSs (1.5), which suggests that the flower-like structure is helpful for the enhancement of dielectric properties of the CoS/paraffin. Fig. 7a depicts RL curve of CoS NPs obtained in the absence of CTAB according to the above electromagnetic parameters. It can be clearly seen that the CoS NPs exhibit microwave absorption capabilities with a minimal reflection loss of −23.9 dB and an effective absorption bandwidth of 2.3 GHz (15.4–17.7 GHz). As for the 20 wt% CoS NPs/paraffin, the typical attenuation peaks would shift to lower frequency and two main RL peaks appeared as the thickness increased. It is worthy to notice that 20 wt% CoS NPs/paraffin presented an obvious peak at the frequency of about 17.0 GHz on the ε′′ values (Fig. 6b), indicative of a resonance behavior, which resulted from the high conduction and significant skin effect of CoS NPs/paraffin at a certain frequency.⁴⁹ By contrast, the peak of ε′′ values for 20 wt% CHSs/paraffin (Fig. 5b) at the fixed frequency was not distinct. According to the previous report,⁵⁰ the positions of RL peaks accorded with the natural and exchanged resonances, where the corresponding position of RL peaks almost remained unchanged without shifting to lower frequencies. As shown in Fig. 7a, calculated RL curves of 20 wt% CoS NPs/paraffin composite with various thicknesses, as well as 2.0 mm, exhibited one of the dual peaks at around 17.0 GHz. Therefore, dual peaks of RL curve for CoS NPs at the thickness of 2.0 mm might be attributed to the resonance behavior occurred between CoS NPs and paraffin. For comparison, the calculated theoretical RL curve of the flower-like CHSs/paraffin composites with 20 wt% loading at different thicknesses (2.0–5.0 mm) in the range of 2–18 GHz is shown in Fig. 7b. It can be observed that the optimal reflection loss is −43.6 dB at 15.6 GHz and RL below −10 dB is 4.6 GHz (13.2–17.8 GHz) with a thickness of 2.0 mm. Moreover, the effective absorption bandwidth can be adjusted between 4.8 GHz and 18.0 GHz for the absorber with a thin thickness in the range of 2.0–5.0 mm. It is interesting to find that the attenuation peaks shift toward the lower frequency and enjoy different microwave attenuation value with the increase of thickness, which is in accordance with the previous reports⁵¹ and indicates that the thickness of microwave absorber plays a significant role in determining the microwave absorption properties. Based on the above mentioned analyses, we can conclude that the filler contents, unique morphology and layer thickness play crucial roles in determining the microwave absorption performance of CHSs/paraffin composites.

Fig. 6 (a) Calculated RL curves of CHSs-paraffin composite at the thickness of 2.0 mm with various filler loadings; (b) electromagnetic parameters of CoS NPs–paraffin composite with a 20 wt% loading.

Fig. 7 Calculated RL curves of (a) CoS NPs–paraffin composite, (b) CHSs–paraffin composite with a 20 wt% filler loading at various thicknesses in the frequency range of 2–18 GHz.

Table 1 shows the typical transition-metal sulfides based composites and their corresponding EM wave absorbing performance. It is found that both the transition-metal materials and their hybrids with RGO require much lower filler loadings (around 20 wt% loading) than previously reported studies^52–54 and exhibit outstanding microwave absorption performance. Furthermore, the optimal reflection loss and effective bandwidth of CHSs in this work are very competitive with the previously reported metal sulfurs composites, such as CuS/paraffin²⁵ and CuS–ZnS/paraffin²⁷ at low loading (<30 wt%). Moreover, the performance of transition metal composites after hybridizing with RGO (RGO/CdS³⁵ and RGO/MnS³⁸) enjoy a better microwave absorption ability and broader effective bandwidth. Thus, it is well believed that CHSs and their further hybrids (future work) are promising materials for application as microwave absorbing agents.

Table 1 Typical transition-metal sulfides based composites for electromagnetic wave absorption recently reported in literatures (RGO: reduced graphene oxide)

Filler	Matrix	Filler loading	Minimum RL (dB)	Layer thickness (mm)	Effective bandwidth^a (GHz)	Ref.
a RL below −10 dB by adjusting the thicknesses.
CoS NPs	Paraffin	20 wt%	−23.9	2.0	2.3	This work
CHSs	Paraffin	20 wt%	−43.6	2.0	13.6	This work
CuS	Paraffin	30 wt%	−31.5	1.8	6.2	25
CuS/ZnS	Paraffin	20 wt%	−22.6	3.0	—	27
RGO/CdS	Paraffin	10 wt%	−48.4	3.3	12.8	35
RGO/MnS	Paraffin	40 wt%	−52.2	2.5	11.4	38

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The superior microwave absorption performance of CHSs might be attributed to the unique 3D (three-dimensional) nanostructures of CoS and multi-interactions between the CHSs, paraffin matrix, and the air.^25,55 The former brings about multiple reflections, thus when the incident microwaves transfer through our CHSs/paraffin composites (illustrated in Scheme 2), they are hard to escape from the inner space as transmission or reflection, on the contrary, most of them diffuse scattering until being attenuated and dissipated as heat.^49,56–59 The latter provides outstanding electric polarization, dipole polarization, and interfacial polarization, which finally results in the superior absorption of electromagnetic energy.^50,60,61

Scheme 2 Schematic illustration of the absorption mechanism of CHSs/paraffin composites.

4. Conclusions

In summary, the 3D hierarchical flower-like CoS hollow spheres were successfully fabricated to initially serve as a microwave absorber through a facile and effective solvothermal method in the presence of CTAB. The formation of the hierarchitectures is ascribed to the multi-stage growth mechanism including an Ostwald ripening process. The hierarchical nanostructures exhibit the excellent electromagnetic wave absorption performance. The optimal reflection loss is −43.6 dB at 15.6 GHz and RL below −10 dB is 4.6 GHz (13.2–17.8 GHz) with the thin thickness of 2.0 mm. Furthermore, the effective absorption bandwidth can be monitored in the frequency regime of 4.8–18.0 GHz through adjusting the absorber thickness from 2.0 to 5.0 mm. Thus, it is well believed that the flower-like CHSs with strong absorption, broad bandwidth, and light weight can be used as a new kind of candidates for advanced microwave absorbers.

Acknowledgements

The authors would like to thank Dr Jianping He for his assistance with VNA test. This work is supported by the financial supports of National Natural Science Foundation of China (Grant No. 21306023, 21376051 and 51077013), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province of China (Grant No. BA2011086). We are also grateful to Key Program for the Scientific Research Guiding Found of Basic Scientific Research Operation Expenditure of Southeast University (Grant No. 3207043101) and Instrumental Analysis Fund of Southeast University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22920d

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