Morphology-controlled synthesis and excellent microwave absorption performance of ZnCo₂O₄ nanostructures via a self-assembly process of flake units

Abstract

Pure dielectric microwave absorbers with strong attenuation capability and wide-band response become a challenge for efficient electromagnetic wave energy absorption. Herein, a series of ZnCo₂O₄ hierarchical structures with superior absorption performance have been achieved by tuning their surface architectures from ball-, hydrangea- to cabbage-, and pineapple-like morphologies. A facile one-step synthesis strategy using a self-assembly process with ZnCo₂O₄ crystalline flakes as structural units was proposed. The deionized water solution and urea addition were found to critically determine the formation of our unique cabbage-like ZnCo₂O₄ self-assembled morphology. The wide band and distinct absorption was dominantly contributed from dielectric ZnCo₂O₄ flakes, which could be furthermore adjusted by the above-mentioned morphologies. Due to its abundant void volume stacked by flakes, the cabbage-like ZnCo₂O₄ demonstrated the best absorption performance where the RL_max reached −36.33 dB at 9.5 GHz with an efficient bandwidth of 5.11 GHz (RL < −10 dB, 11.17–16.28 GHz). Adjusting the simulating thickness from 1 to 5 mm, the bandwidths range from 5.8 to 18 GHz. This unique structure has the polarization, conduction loss and strong dissipation capability resulting from the high density of accumulated charges trapped by the flake gap, confirmed by the analysis of electromagnetic parameters and electronic holography. It is expected that the self-assembled ZnCo₂O₄ microsphere might shed new light on the design of novel microwave absorption materials.

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Introduction

With the rapid development of the modern electronics industry, the issue of electromagnetic wave absorption has become more serious.^1–5 Therefore, it is a necessary goal to fabricate microwave absorbers with the following advantages: easy fabrication, low density, strong absorption capability and wide bandwidth. Both magnetic and dielectric absorption materials have been reported, such as ferrites,⁶ nickel,⁷ alloys,⁸ carbon derivatives⁹ and conducting polymers.¹⁰

Notably, dielectric materials have attracted a great deal of attention since they avoid the limitation of Curie temperature and the high temperature oxidation of magnetic materials. There are a number of reports concerning dielectric substances used as electromagnetic wave absorbing materials with great absorption properties, especially for low-dimensional materials.^11–13 Cao et al. have successfully prepared ZnO nanocrystals on multi-walled carbon nanotubes to form a conductive absorber.¹⁴ Yin et al. incorporated SiC nanowires into rGO foams to achieve improved thermostability and electromagnetic absorption.¹⁵ Du et al. introduced the core@shell composites of polypyrrole and polyaniline, which displayed evident interfacial polarization.¹⁶ Briefly, the nonmagnetic system with enhanced polarization and dielectric loss plays an important role in the process of dissipating microwave energy. However, the introduction of multiple components in the above structures has resulted in complex processing, low efficiency and agglomeration phenomenon. Unfortunately, hierarchical self-assembly structures composed of only single dielectric components are rare. Thus, monocomponent absorbers might become a simplified model for the study of absorption mechanisms.

Generally, the methods for adjusting the electromagnetic parameters for a single component absorber are doping,^17,18 changing size,^19,20 and tuning the morphology.^21,22 Noticeably, the electromagnetic wave absorption property is affected by the morphologies of an absorber due to the changed electron transportation path, penetration route and induced scattering effects. These above-mentioned studies suggest that absorption performance depends strongly on the geometrical morphology, which becomes an effective path to improve microwave absorption capability.^23,24 Therefore, it is valuable to explore a type of pure dielectric microwave absorber that shows strong absorption and wide response band and to further investigate the relationship between morphology and electromagnetic properties. Moreover, a facile and repeatable synthetic process is still lacking.

Zinc cobaltate (ZnCo₂O₄) has been used as an energy storage material due to its ion conductivity.^25–27 ZnCo₂O₄ has the potential to become a promising microwave absorber due to the intrinsic charge conductivity similar to other AB₂O₄ spinel structure compounds. Furthermore, Co₃O₄,^28,29 CoFe₂O₄,³⁰ NiCo₂O₄,³¹ ZnFe₂O₄,³² and MnFe₂O₄ ³³ all have been successfully reported to possess related microwave absorption properties. Therefore, the development of ZnCo₂O₄ as a novel microwave absorbing material can expand the opportunities in this field. So far, the effect of structure and morphology of ZnCo₂O₄ on its dielectric and microwave absorption performance have not been reported, although its composites have already been studied in other fields.

Herein, hierarchical ZnCo₂O₄ microspheres made up of ZnCo₂O₄ crystalline nanosheets were synthesized by using a facile hydrothermal method, which exhibits excellent microwave absorption performance. A novel strategy was proposed, wherein ZnCo₂O₄ flake units were effectively self-assembled into microspheres simply by adjusting the reaction solvent (H₂O) and additive (urea). A 5.11 GHz bandwidth with reflection loss stronger than −10 dB was achieved from our products. The dielectric absorption mechanism involves charge accumulation and multiple reflections around the nanosheets surface region and interfacial gaps resulting from their unique morphology. ZnCo₂O₄ have a significant potential to offer suitable relative complex permittivity, thus, they can be good candidates for fabricating distinct performance and wide broadband microwave absorbers. Our findings might provide new strategies for producing microwave absorption materials using single component dielectric components as a self-assembly unit for achieving morphology-dependent electromagnetic absorption properties.

Experimental

Materials

Zinc chloride (ZnCl₂), cobaltous chloride hexahydrate (CoCl₂·6H₂O), zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O), cobalt(II) acetate tetrahydrate (Co(CH₃COO)₂·4H₂O), zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), cobaltous nitrate hexahydrate (Co(NO₃)₂·6H₂O), ammonium bicarbonate (NH₄HCO₃), urea and ethylene glycol (EG) solution were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were of analytical grade and used without further purification. Deionized water obtained from a Milli-Q system (Millipore, Bedford, MA) was used in all experiments.

Synthesis of ball-, hydrangea-, cabbage-, and pineapple-like ZnCo₂O₄ hierarchical nanostructures

In a typical process for the synthesis of the cabbage-like microspheres, Zn(NO₃)₂·6H₂O (1 mmol) and Co(NO₃)₂·6H₂O (2 mmol) were dissolved in 40 mL deionized water under magnetic stirring at room temperature. Then, 15 mmol of urea was added to the above solution. After being stirred for 3 h, the resultant solution was transferred into a Teflon lined stainless-steel autoclave with a capacity of 50 mL, and heated at 120 °C for 16 h in an oven. The cabbage-like ZnCo₂O₄ precursors were collected by centrifuging, washed with water three times and with ethanol four times, and dried in a vacuum oven at 60 °C for 12 h. Finally, the black cabbage-like ZnCo₂O₄ microspheres were obtained after calcination at 350 °C for 2 h in air. By simply changing the reaction solvent (EG) and additives (NH₄HCO₃), ball-, hydrangea-, and pineapple-like ZnCo₂O₄ microspheres can be synthesized using the same method (see details in the ESI†).

Characterization

The crystallographic structure and phase purity of the as-synthesized products were studied by a Bruker, D8 ADVANCE X-ray diffractometer with Ni-filtered Cu Kα radiation (40 kV, 40 mA). The morphologies of the products were characterized using a field-emission scanning electron microscope (FESEM, Hitachi S-4800, Japan) and a field-emission transmission electron microscope (TEM, JEOL, JEM-2100F, 200 kV). For electron holography measurements, the samples were fixed in an embedding medium and cut into ultrathin sections by using an ultra-microtome. An electrostatic bi-prism (a thin conducting wire) system was installed in the microscope column perpendicular to the electron beam. The X-ray photoelectron spectroscopy (XPS) measurements were recorded on KRATOS Axis Ultra DLD equipped with a monochromatic X-ray source (Al Kα, hv = 1486.6 eV). The electromagnetic parameters were measured in the 2–18 GHz range with a N5230C vector network analyser. The measured samples were prepared by uniformly mixing with a paraffin matrix in a mass fraction of 50 wt% and compacted into a ring shape with a 7.00 mm outer diameter and 3.04 mm inner diameter.

Results and discussion

By adjusting the reaction solvents and additives, a series of ZnCo₂O₄ structures with four different morphologies can be obtained, as shown in Scheme 1. For example, the formation of cabbage-like ZnCo₂O₄ can be described as follows. First, a cobalt zinc precursor formed by Zn and Co ions coordinated with deionized water acts as the nuclei for ZnCo₂O₄ nanocrystals. To reduce the surface energy, the primary particles aggregated spontaneously into flake units during the process of Ostwald ripening. With the assistance of urea, these flake units were self-assembled into cabbage-like hierarchical structures based on their inner crystallographic orientation. Finally, the continuous growth of hierarchical branches of microspheres results in the formation of the compact assembled microspheres after the post-annealing process.

Scheme 1 Schematic illustration of the synthetic process for the ball-, hydrangea- cabbage-, and pineapple-like ZnCo₂O₄ samples.

The phase structures of four samples were determined by XRD over the range of 10° ≤ 2θ ≤ 70° (Fig. 1a). It can be seen that all the diffraction peaks are well assigned to the spinel structure of ZnCo₂O₄ (JCPDS No. 23-1390). Eight typical diffraction peaks are observed with increasing degree, related to the (111), (220), (311), (222), (400), (422), (511), and (440) crystallographic planes, respectively. At the same time, no impurity peaks are present, evidently proving that all products are highly purified and do not have any second phase. Hence, all four ZnCo₂O₄ samples with different morphology were successfully fabricated. As shown in Fig. 1b, the ZnCo₂O₄ is “normal” spinel with the +3 ions mostly on octahedral sites and the +2 ions mostly on tetrahedral sites (space group Fd3m). With the replacement of Co²⁺ by Zn²⁺ ions, spinel ZnCo₂O₄ is isomorphic to the Co₃O₄ crystal structure.³⁴ This cubic spinel structure offers better electron transmission behaviour between the metal–oxygen bond, resulting in the increased dielectric property especially for the imaginary part of the dielectric constant, that is, the electromagnetic loss capability.

Fig. 1 XRD patterns of four different ZnCo₂O₄ samples (a) and the structural illustration of the dielectric ZnCo₂O₄ (b).

The surface electronic states and more detailed information about the chemical composition of the prepared ZnCo₂O₄ can be further investigated by XPS, and the results are displayed in Fig. 2. The full wide-scan spectrum (Fig. 2a) shows the presence of Co, Zn, O and C elements. The two major peaks centred at 1020.3 eV and 1043.4 eV (Fig. 2b) can be assigned to Zn 2p_3/2 and Zn 2p_1/2, respectively. As shown in Fig. 2c, the high-resolution Co 2p XPS spectrum exhibits two strong peaks at about 779 eV and 794 eV, which are assigned to 2p_3/2 and 2p_1/2, respectively.³⁵ Furthermore, the binding energy at 779.4 and 794.7 eV correspond to Co³⁺, while the binding energy at 780.5 and 795.9 eV are assigned to Co²⁺. In the O 1s high-resolution spectrum (Fig. 2d), the peaks at 529.6 eV and 531.0 eV are usually associated with the oxygen from the metal–O bonds and oxygen vacancy in the ZnCo₂O₄, respectively. Overall, these results indicate that the chemical composition of four different ZnCo₂O₄ materials contains Zn²⁺, Co²⁺, Co³⁺, Co–O and Zn–O bonds, which are in good agreement with the reported literatures.^36,37 Moreover, this XPS data in conjunction with XRD results all confirm that the as-synthesized ZnCo₂O₄ have a pure spinel structure, which may lead to the dielectric polarization for the incident microwave.

Fig. 2 XPS spectra: (a) survey spectrum; (b) Zn 2p; (c) Co 2p; (d) O 1s for four different ZnCo₂O₄ samples.

It is confirmed that a series of ZnCo₂O₄ with different morphologies have been successfully synthesized, such as ball-, hydrangea-, cabbage- and pineapple-like (Fig. 3). The self-assembly process of flake units can also be clearly observed only by adjusting the reaction solvent and additives. In Fig. 3a and b, each monodisperse ball-like ZnCo₂O₄ exhibits a relatively narrow size distribution of 3–7 μm and is composed of many small nanospheres. A number of nanosheets were wrapped around a solid microsphere to form hydrangea-like ZnCo₂O₄ structures with an average size of about 3 μm (Fig. 3d and e). By changing the reaction solvent from EG to deionized water, it seems that the nanosheets were wrapped around an invisible core to form the cabbage-like ZnCo₂O₄ architectures that consist of many densely interconnected flake units with diameters of about 5 μm (Fig. 3g and h). Thus, the multilayered porous architecture is formed by the extremely thin flake units with similar morphologies that are densely packed. The low-magnification SEM image (Fig. 3j) shows that the pineapple-like samples are composed of many linked nanocubes with a diameter of ∼5 μm. The high-magnification image (Fig. 3k) further reveals that the surface of these nanocubes has many pores due to the loss of inorganic substances during the reaction process. Remarkably uniform distribution of elements in the four different ZnCo₂O₄ samples is further verified by EDX spectra (Fig. 3c, f, i and l). Compared to other samples, the cabbage-like ZnCo₂O₄ structures have much more flake units, pores and voids, indicating that these structures are advantageous for keeping the large surface area in the internal space, which may largely improve multiple reflection capabilities. Energy dispersive spectrometry quantitative analysis (Fig. S1†) also demonstrates that the atomic ratio of Zn, Co and O is 1 : 1.96 : 3.68, which is in good agreement with XPS O 1s data (Fig. 2d), implying a certain degree of oxygen vacancies in the ZnCo₂O₄ spinel crystal.

Fig. 3 SEM images of (a, b) ball-like ZnCo₂O₄, (d, e) hydrangea-like ZnCo₂O₄, (g, h) cabbage-like ZnCo₂O₄ and (j, k) pineapple-like ZnCo₂O₄ structures; elemental mapping of (c) ball-like ZnCo₂O₄, (f) hydrangea-like ZnCo₂O₄, (i) cabbage-like ZnCo₂O₄ and (l) pineapple-like ZnCo₂O₄ structures.

Fig. 4a and b show the typical TEM images of the ball-like ZnCo₂O₄. It can be clearly seen that the average size of microspheres is consistent with the SEM observation. As shown in Fig. 4d and e, there are numerous nanosheets closely packed on the surface of the solid microspheres in hydrangea-like ZnCo₂O₄ products. Fig. 4g and h show the cabbage-like structure, composed of a number of ultrathin nanosheets, which is in good accordance with the SEM results. The inset in Fig. 4i gives the selected-area electron diffraction (SAED) pattern with well-defined rings, indicating the polycrystalline nature of the as-prepared ZnCo₂O₄. From the image shown in Fig. 4j, the irregular edges of the microspheres are in accordance with the nanocubes. A high resolution TEM image (Fig. 4c, f, i and l) is taken from the parts of Fig. 4b, e, h and k, which clearly indicates two interplanar spacings of 0.24 and 0.28 nm, corresponding to the (311) and (220) planes of spinel ZnCo₂O₄ structures, respectively. As described above, it can be further proved that all the four different ZnCo₂O₄ samples are produced with high purity and good crystallinity.

Fig. 4 TEM images of (a, b) ball-like ZnCo₂O₄, (d, e) hydrangea-like ZnCo₂O₄, (g, h) cabbage-like ZnCo₂O₄ and (j, k) pineapple-like ZnCo₂O₄ structures; HRTEM images of (c) ball-like ZnCo₂O₄, (f) hydrangea-like ZnCo₂O₄, (i) cabbage-like ZnCo₂O₄ and (l) pineapple-like ZnCo₂O₄ samples (the inset in i are SAED patterns).

Generally, the complex permittivity (ε_r = ε′ − jε′′), complex permeability (μ_r = μ′ − jμ′′), and the impedance matching of absorbers play important roles in determining the microwave absorption properties. The real parts of complex permittivity (ε′) and complex permeability (μ′) show the storage capability of electromagnetic energy, while the imaginary parts (ε′′, μ′′) indicate the loss capability of electromagnetic energy.^38,39 According to the electromagnetic loss model, a high ε′′ and μ′′ are significantly beneficial to absorb the electromagnetic wave energy.⁴⁰Fig. 5 shows the electromagnetic parameters of the four different ZnCo₂O₄ samples in the frequency range of 2–18 GHz. The cabbage-like ZnCo₂O₄ materials with massive flake units exhibit the highest real permittivity (ε′) ranging from 7.5 to 5.4 (Fig. 5c), and may possess strong capability of both energy storage and dielectric polarization. Moreover, a noticeable valley peak appeared at about 8 GHz. With the increase in frequency, this decreasing tendency of (ε′) follows the general rule of a typical dielectric relaxation. When the number of nanosheets are reduced to form a hydrangea-like shape, ε′ values fluctuate within the range of 6.11–4.59. When the number of nanosheets further decreases to zero, ε′ rapidly declines to 4.11–2.36 without any relaxation peak. For other unique pineapple-like ZnCo₂O₄ with many linked nanocubes, the ε′ value decreases from 5.51 to 3.18. Hence, the massive nanosheets might provide the absorber superior capability to store electromagnetic energy although they have the same chemical composition. As for the imaginary permittivity (ε′′), compared to other samples, cabbage-like materials display the highest ε′′ of 2.96 to 2.74 with a relatively smooth trend. This relates to their enhanced interfacial polarization resulting from the increased specific surface area between the linked flake units. Cabbage-like ZnCo₂O₄ has a stronger ability to generate the induced charges, thereby making the ε′′ value rise and dissipating the electromagnetic wave energy. According to the Debye theory of dielectrics, ε′′ can be calculated by

where σ is the electrical conductivity.⁴¹ Thus, ε′′ is proportional to conductivity and represents the dielectric loss ability that resulted from the remarkable polarization and conductivity in the samples. Although the polarization has an effect on ε′′, the directional movement of conducting electrons leads to the conductance loss. Electromagnetic waves are further converted into thermal energy under an external electromagnetic field. The values of μ′ and μ′′ are nearly constant at 0 and 1, respectively, revealing that the microwave absorption ability of all ZnCo₂O₄ samples resulted primarily from dielectric loss rather than magnetic loss.^42,43 The dielectric loss tangent δ_ε (tan δ_ε = ε′′/ε′) was calculated to further evaluate the dielectric loss property, as shown in Fig. 5e. Generally, higher the δ_ε value, higher would be dissipation of electric energy of incident microwaves. The order of tan δ_ε values of four samples are cabbage-like > hydrangea-like ≈ pineapple-like > ball-like ZnCo₂O₄ samples, which provides the distinct evidence that the flake units are powerful aids to reinforce the microwave attenuation capacity. Moreover, microwave attenuation ability can also be evaluated by the attenuation constant (α), which can be calculated aswhere f is the frequency and c is the velocity of light. Notably, α represents integral attenuation ability.⁴⁴ Thus, a strong dielectric loss may lead to a high α, but a high α does not necessarily result in a strong dielectric loss. Fig. 5f shows the curves of attenuation constant versus frequency for the obtained four ZnCo₂O₄ samples. The cabbage-like material has large α values in the measured frequency range, indicating its distinct dissipation properties. Furthermore, because the calculated tan δ_ε values are much higher than that of tan δ_m (Fig. S2†), all the as-prepared ZnCo₂O₄ samples can be considered as a dielectric loss-type microwave absorber.

Fig. 5 The real (ε′) and imaginary (ε′′) components of relative complex permittivity (a, b), the real (μ′) and imaginary (μ′) components of relative complex permeability (c, d), tan δ_ε (e) and attenuation constant versus frequency (f) for four different paraffin composites containing 50 wt% ZnCo₂O₄.

In order to study the microwave absorption capabilities of ZnCo₂O₄ samples with four different morphologies, the reflection loss (RL) was calculated. According to the transmit line theory, RL values usually are given by the following equations:^45,46

RL(dB) = 20 log|(Z_in − Z₀)/(Z_in + Z₀)| (4)

where Z_in is the normalized input impedance of the material; Z₀ is the impedance of the free space; μ_r and ε_r are the relative complex permeability and permittivity of the composite medium, respectively; c is the velocity of light; d is the thickness of the absorber; f is the frequency of the microwave. Generally, an effective microwave absorber should have the values of RL less than −10 dB (90% electromagnetic wave attenuation). Fig. 6 shows the 3D plots of reflection loss of the four ZnCo₂O₄ samples with different thickness. Significantly, the cabbage-like ZnCo₂O₄ material exhibits the strongest microwave absorption capability compared to those of the others. The ball-like ZnCo₂O₄ sample has a very poor microwave absorption ability with the maximum RL value of only −4.63 dB (Fig. 6a), which could be ascribed to the lowest microwave loss capacity inherited from its structure. Comparatively, hydrangea-like and pineapple-like ZnCo₂O₄ have the maximum RL value of −14.78 dB and −11.62 dB (Fig. 6b and d), respectively. Particularly, as shown in Fig. 6c, cabbage-like ZnCo₂O₄ samples possess the highest maximum RL value of −36.33 dB at 9.5 GHz and the broad effective bandwidth of 5.1 GHz (from 11.1 to 16.2 GHz) at the thickness of 2.5 mm. Moreover, changing the thickness from 2.0 to 5.0 mm, cabbage-like ZnCo₂O₄ samples still have distinct microwave absorption performance with maximum RL values all less than −20 dB, suggesting its tuneable microwave absorption performance.

Fig. 6 3D plots of reflection loss for (a) ball-, (b) hydrangea-, (c) cabbage-, (d) pineapple-like ZnCo₂O₄ composites containing 50 wt% paraffin.

The best electromagnetic wave absorbing property is exhibited by cabbage-like ZnCo₂O₄ samples (Fig. 7). It should be pointed out that the optimal RL peaks are shifted toward lower frequencies with increasing sample thickness (Fig. 7a). Furthermore, the RL peak intensity first increases and then decreases. Fig. 7c indicates the dependence of matching thickness (t_m) on matching frequency (f_m) at wavelengths of λ/4, which should be expressed by the 1/4 wavelength cancellation equation.^47,48

where t_m is the frequency of complex permeability and permittivity, f_m is the matching thickness, c is the velocity of light. When the matching thickness of sample satisfies the above-mentioned equation, electromagnetic waves will be reflected from various interfaces with opposite phases by 180°, resulting in the microwave energy attenuation. In Fig. 7a, each vertical dot line from the RL peak at different absorber thickness crosses with its corresponding thickness line in Fig. 7c. All crossover points can be found in the λ/4 curves, which demonstrate that the microwave absorption for cabbage-like ZnCo₂O₄ mainly follows the quarter-wavelength cancellation model. The effective bandwidth (RL ≤ −10 dB) reaches the maximum of 5.11 GHz at a sample thickness of 2.5 mm (Fig. 7d). A comparison of cabbage-like ZnCo₂O₄ with other samples reveal that cabbage-like ZnCo₂O₄ samples are extremely suitable for microwave absorption materials with good performance and broad bandwidth. It is important to highlight that the electromagnetic response and microwave absorption of the ZnCo₂O₄ in the investigated range of frequency has never been published before. Compared with other AB₂O₄-based absorbers (Table 1), the ZnCo₂O₄ synthesized in this work showed superior advantages owing to lower thickness, strong absorption properties, and tunable absorption frequency, which distinctly cater to the requirement of wide response band of a highly efficient microwave absorber. Besides, our single component materials can achieve the performance of multi-component materials and further avoid the problem of complex processing, low efficiency and agglomeration phenomenon.

Fig. 7 Calculated reflection loss curves (a) and 3D plots (b) for cabbage-like ZnCo₂O₄ sample, (c) dependence of matching thickness (t_m) on matching frequency (f_m) at wavelengths of λ/4, (d) absorption bandwidth under various sample thickness at the level of −10 dB.

Table 1 Comparison of microwave absorption performance between the reported AB₂O₄-based composites and as-prepared ZnCo₂O₄

Absorber	Maximum RL (dB)	Matching frequency (GHz)	Matching thickness (mm)	Absorption band (GHz) RL ≤ −10 dB	Ref.
NiCo2O4	−25.5	4.5	4	—	31
Co3O4	−38	11.7	2.5	4.5	28
ZnFe2O4/RGO	−29.3	16.7	1.6	2.6	32
CoFe2O4/GO	−42	12.9	2	4.59	30
Co3O4/RGO	−32.3	12.4	2.5	—	29
MnFe2O4/RGO	−32.8	8.2	3.5	4.8	33
ZnCo 2 O 4	−36.33	9.5	3.4	5.11	this work

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The microwave absorption mechanism of the cabbage-like ZnCo₂O₄ samples loaded with an abundance of nanosheets is described from the viewpoint of the microstructure (Scheme 2). First, because of the numerous flake units embedded inside the cabbage-like ZnCo₂O₄ structures, a large number of ZnCo₂O₄–ZnCo₂O₄ cross plane around interfacial regions are formed in microspheres (Fig. 8a, e and j), leading to an enhanced interfacial polarization relaxation. Second, the massive flake units of the cabbage-like ZnCo₂O₄ material are prone to produce a dielectric polarization field at nanoscale, which further induces a redistribution of surface charge and enhances the dielectric polarization, thereby dissipating the microwave energy. Electron holography analysis was carried out to determine the charge density distribution (Fig. 8b, f and j). Both negative and positive distributions coexist at interfaces and junctions of ZnCo₂O₄–ZnCo₂O₄ (Fig. 8d, h and l). Interestingly, the defects on microsphere surfaces formed by the existence of oxygen vacancies can break the balance of the charge distribution and cause the dipole polarization and Debye relaxations, which are favourable for an enhanced microwave absorption. Third, the cabbage-like ZnCo₂O₄ samples have the most flake units and voids compared to that of others, leading to the fact that an enhanced surface area is generated. When the electromagnetic waves permeate into these unique microspheres, large amounts of specific surface areas will provide a large number of active sites to induce multiple reflection and scattering. Such processes could effectively attenuate microwave irradiations. Fourth, the collective migration of polarization electrons near the interfacial planes could generate residual holes and gather more electrons driven by the applied high frequency electromagnetic field. The imaginary part of permittivity could be enhanced by the effect of the electron migration. Because ε′′ is proportional to conductivity, the higher value of σ means better electron movement, by which electromagnetic energy can be quickly converted into thermal energy and further improve the microwave absorption ability. Fifth, according to porous architectures, microwaves may be neutralized by the opposite electromagnetic waves with similar vibrational frequency, further improving the microwave absorption capability of cabbage-like ZnCo₂O₄ microspheres.

Scheme 2 Schematic illustration of microwave absorption mechanisms for cabbage-like ZnCo₂O₄ samples.

Fig. 8 Off-axis electron holograms of cutting sections (a, e, i), charge density images (b, f, j), enlarged charge density maps (c, g, k), and the profile of charge density (d, h, l) in the region of the white arrow for the cabbage-like ZnCo₂O₄ samples.

Conclusions

In conclusion, a self-assembly process of flake units can be successfully achieved through a facile solvothermal method and thermal annealing treatment by simply changing the reaction solvent and additives. Only when the reaction solvent is deionized water and the additive is urea, cabbage-like ZnCo₂O₄ can be successfully prepared. Benefitting from the interconnected nanosheet structure with numerous pores, voids and interfaces, the cabbage-like ZnCo₂O₄ absorber shows excellent microwave absorption performance. The structural distinctions among these four samples lead to a total difference in dielectric and microwave absorption abilities. Thus, cabbage-like ZnCo₂O₄ samples show the maximum RL value of −36.33 dB at 9.5 GHz and effective bandwidth of 5.11 GHz (from 11.17 to 16.28 GHz). The enhanced microwave attenuation ability was mainly ascribed to unique architectures, induction of more interfacial polarization, dipole polarization, multiple scattering and energy dissipation. Furthermore, all the above-mentioned discussions demonstrate that this self-assembly process of flake units related to their microwave absorption performance provides an avenue for future material design.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Ministry of Science and Technology of China (973 Project No. 2018YFA0209102) and the National Natural Science Foundation of China (11727807, 51725101, 51672050, 61790581), Science and Technology Commission of Shanghai Municipality (16DZ2260600).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/C8NR08601J

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