Novel nanoporous carbon derived from metal–organic frameworks with tunable electromagnetic wave absorption capabilities

Abstract

Nanoporous carbon materials derived from metal organic frameworks (MOFs) have attracted considerable attention due to their low density for microwave absorption. Nevertheless, their poor impedance matching has reduced the absorber performance. The design and fabrication of complex nanocarbon materials with outstanding impedance matching is still a challenge. Here, we prepared a core–shell structured ZIF-8@ZIF-67 crystal through a new seed-mediated growth method. After the thermal treatment of ZIF-8@ZIF-67 crystals, we obtained selectively nanoporous carbon materials consisting of ZnO/NPC as the cores and highly graphitic Co/NPC as the shells. The shell thicknesses of ZIF-67 can be tuned simply by varying the feeding molar ratios of Co²⁺/Zn²⁺. The composites exhibited excellent impedance matching and strong absorption. The composite ZnO/NPC@Co/NPC-0.5 samples filling with 50 wt% of paraffin show a maximum reflection loss (RL) of −28.8 dB at a thickness of 1.9 mm. In addition, a broad absorption bandwidth for RL <−10 dB which covers from 13.8–18 GHz can be obtained. Our study not only bridges diverse carbon-based materials with infinite metal–organic frameworks but also opens a new avenue for artificially designed nano-architectures with target functionalities.

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Introduction

With the swift development of electromagnetic wave technology, electromagnetic interference (EMI) has become a serious problem in daily life, which may not only interrupt electronic devices, but also threaten human health. In order to solve these problems, microwave absorbing materials are indispensable. In general, the basic design principles for absorbing materials are electromagnetic wave attenuation capability and impedance matching. Clearly, weak attenuation would result in unsatisfactory behavior. In addition, poor impedance matching characteristics also lead to low reflection loss due to increased reflections on the surface. Moreover, an efficient absorbing material should not only satisfy the basic requirements of strong absorption but also the needs of practical applications, such as thin coating thickness and lightweight. In the past decade, extensive studies have been performed to develop lightweight absorbing materials including hollow structures,¹ core–shell structures² and magnetic/carbon composites.³ Among these structures, a kind of effective absorbing material, carbon-based composites, has drawn great attention due to their lower density, low cost and high electrical conductivity.⁴ Most studies have concentrated on graphene,^5–9 carbon nanotubes^10–13 and mesoporous carbon.^14–16 Although carbon based absorbing materials in recent years have achieved good development, their microwave absorbing properties also need to be further improved. In addition, carbon material is a kind of dielectric material with relatively high complex permittivity and poor magnetic permeability, which cause the mismatch of impedance and limit its use as a microwave absorbing material. Therefore, a simple yet effective method to obtain carbon-based composites with outstanding microwave absorbing performance is urgently needed.

Metal organic frameworks (MOFs)^17–20 are a class of ultraporous materials fabricated by assembling metal ions with organic ligands in appropriate solvents, which have attracted considerable attention because of their exceptionally high accessible surface area, diverse structural topologies as well as tunable functionalities.²¹ Moreover, the potential applications of MOFs can be further improved and extended by incorporating other functional species within the MOFs. Most importantly, by taking advantage of their thermal behaviour and chemical reactivity, various porous carbons and metal oxides can be achieved easily through the direct pyrolysis of MOFs.^22,23 For instance, microporous carbon polyhedra were produced through the calcination of Zn-based MOFs (ZIF-8) under an inert atmosphere.²⁴ There also exist some core–shell structures of ZIF-8 such as ZIF-L@ZIF-8,²⁵ ZIF-8@PDMS,²⁶ metal/ZIF-8 crystals²⁷ and ZIF-67@ZIF-8.²⁸ In addition, Yamauchi et al.²⁹ displayed that nanoporous carbon with magnetic Co nanoparticles (Co/NPC) can be prepared by thermal decomposition of a Co-based zeolitic imidazolate framework material (ZIF-67). The Co nanoparticles can be well protected by the carbon layer, which is attributed to the uniformity of the Co nanoparticles over the whole nanoporous carbon matrix. Moreover, our previous report also suggests that this kind of porous carbon material is a promising candidate for a lightweight microwave absorber due to its higher EM wave attenuation and lower density.³⁰ Unfortunately, in the gigahertz (GHz) band range, nanoporous carbons with magnetic Co nanoparticles (Co@NPC) derived from the thermal decomposition of ZIF-67 do not show good EM absorbing properties due to their relatively high complex permittivity compared with that of air, which leads to impedance mismatching between the materials and air. To overcome these issues, heterogeneous hybridization is an effective methodology to fuse the advantages of different materials and endows the hybrid materials with novel chemical and physical properties and interfacial functionality.^31,32 However, ZnO-containing nanomaterials can be used as high efficiency microwave-absorbing materials due to their complex permittivity and permeability.^33–35 Thus, our target in this study is to realize core–shell ZnO/NPC@Co/NPC derived from core–shell MOFs, which can bring out novel chemical and physical properties that are not attainable from a single MOF precursor.

A zeolitic imidazolate framework (ZIF) is a well-known subfamily of MOFs formed through the coordination interaction between the metal ions and the imidazole derivatives.³⁶ So we set out to prepare core–shell ZIF-8@ZIF-67 crystals, which consist of ZIF-8 crystals as the core and ZIF-67 crystals as the shell, through a seed-mediated growth method that is usually adopted to prepare core–shell structured materials.³⁷ By the thermal treatment of ZIF-8@ZIF-67 crystals, novel nanoporous ZnO/NPC@Co/NPC materials consisting of ZnO/NPC as the core and highly graphitic carbon Co/NPC as the shell are successfully prepared (Fig. 1). The permittivity and permeability of the ZnO/NPC@Co/NPC and the shell thickness of Co/NPC significantly depend on the molar ratios of Co²⁺/Zn²⁺. The as-prepared ZnO/NPC@Co/NPC-0.5 showed outstanding microwave absorption properties. A maximum reflection loss (−28.8 dB) at 1.9 mm can be achieved. In addition, the approach is quite simple and useful for large-scale preparation.

Fig. 1 Synthesis scheme of ZIF-8 crystals and ZnO/NPC; ZIF-67 crystals and Co/NPC; core–shell ZIF-8@ZIF-67 crystals and ZnO/NPC@Co/NPC. MeIm represents 2-methylimidazole.

Experimental section

Synthesis of ZIF-8 and ZIF-67 crystals

In a typical synthesis of ZIF-8, methanolic solutions of zinc nitrate hexahydrate (810 mg, 40 mL) and methanolic solutions of MeIm (526 mg, 40 mL) were mixed under stirring. Then the mixture was transferred into an autoclave and was kept for 12 h at 100 °C. A white powder was collected by centrifugation, washed several times with methanol, and dried at 60 °C. In the synthesis of ZIF-67, methanolic solutions (80 mL) of cobalt chloride (519 mg), polyvinylpyrrolidone K30 (PVP K30) (600 mg) and MeIm (2630 mg) were mixed under stirring. Then the mixture was kept at room temperature for 24 h. A bright purple powder of ZIF-67 was collected by centrifugation, washed several times with methanol, and dried at 60 °C.

Synthesis of core–shell ZIF-8@ZIF-67 crystals and ZnO/NPC@Co/NPC

In a typical synthesis of core–shell ZIF-8@ZIF-67 crystals, ZIF-8 seeds (80 mg) were firstly well-dispersed in methanol (10 mL) under sonication for 30 min. Then a methanolic solution of cobalt chloride (177 mg, 3 mL) and after stirring for 20 min, a methanolic solution of MeIm (895 mg, 3 mL) were stepwise injected into the above mixture. After stirring for another 5 min, the mixture was transferred into an autoclave and kept at 100 °C for 12 h. During this time, the core–shell ZIF-8@ZIF-67 crystals were obtained. After cooling to room temperature, the sample was collected by centrifugation, washed several times with methanol, and dried at 80 °C. The molar ratio of Co²⁺/Zn²⁺ in the obtained ZIF-8@ZIF-67 crystals was 0.5. The shell thickness of ZIF-67 can be tuned by changing the amounts of methanolic solutions of cobalt and MeIm. The obtained core–shell ZIF-8@ZIF-67 crystals are denoted as ZIF-8@ZIF-67-x, where x indicates the molar ratio of Co²⁺/Zn²⁺ in the products. All the precursors were thermally converted to ZnO/NPC@Co/NPC materials through carbonization under a N₂ flow at 700 °C for 2 h, with a heating rate of 2 °C min⁻¹. In addition, the number of the core–shell ZnO/NPC@Co/NPC-x is listed in Table 1. As a control experiment, ZnO/NPC and Co/NPC were prepared by using single precursors (ZIF-8 and ZIF-67) under the same thermal conditions, which were denoted as S1 and S5, respectively.

Table 1 Number, molar ratio and thickness of samples

Sample	Thermal conditions	Number	Molar ratio of Co^2+/Zn^2+	Thickness of samples (mm)
Co/NPC	N2, 700 °C, 2 h	S1	—	—
ZnO/NPC@Co/NPC-0.25	N2, 700 °C, 2 h	S2	0.25	71
ZnO/NPC@Co/NPC-0.5	N2, 700 °C, 2 h	S3	0.5	99
ZnO/NPC@Co/NPC-0.75	N2, 700 °C, 2 h	S4	0.75	120
ZnO/NPC	N2, 700 °C, 2 h	S5	—	—

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Sample preparation for microwave absorption performance

The relative complex permittivity (ε_r) and permeability (μ_r) were measured by using a vector network analyzer (VNA, N5242A PNA-X, Agilent) in the frequency range 2–18 GHz. The measured samples were prepared by uniformly mixing 50 wt% of a powder hybrid with a wax matrix at 90 °C. The mixture was then pressed into ring shaped samples with an outer diameter of 7.00 mm and an inner diameter of 3.04 mm. In a coaxial wire analysis, ε_r of the dielectric material has been calculated from the experimental scattering parameters S11 (or S22) and S21 (or S12) using the standard Nicolson–Ross–Weir (NRW) algorithm.³⁰

Characterization of the samples

Powder X-ray diffraction (XRD) patterns were recorded using a Bruker D8 X-ray diffractometer with Ni filtered Cu Kα radiation (40 kV, 40 mA). The morphologies and sizes of the samples were characterized by using a field-emission scanning electron microscope (FESEM Hitachi-4800) and a transmission electron microscope (JEOL JSM-2010). The Brunauer–Emmett–Teller (BET) specific surface areas of the samples were determined using a high speed automated area and pore size analyzer (ASAP 2010). A vibrating sample magnetometer (VSM, Lakeshore, Model 7400 series) was used to measure the samples’ magnetic properties under an applied magnetic field of 10 kOe.

Results and discussion

The core–shell MOFs (ZIF-8@ZIF-67-x) were prepared by a seed-mediated growth technique, as illustrated in Fig. 2c–e. ZIF-8 and ZIF-67 are possible candidates for the preparation of core–shell MOFs (ZIF-8@ZIF-67-x) due to their isoreticular structures as [M(MeIm)₂]_n (M = Zn for ZIF-8 and Co for ZIF-67) and their similar unit cell parameters between ZIF-8 (a = b = c = 16.9910 Å)³⁸ and ZIF-67 (a = b = c = 16.9589 Å),³⁶ which were determined by single crystal X-ray diffraction studies. To achieve this goal, uniformly sized ZIF-8 seeds and ZIF-67 seeds were first synthesized by the coordination reaction of Zn²⁺ ions and Co²⁺ with MeIm. SEM images confirmed that the obtained ZIF-8 and ZIF-67 consisted of uniformly dispersed crystals of rhombic dodecahedral shape, and the diameters of ZIF-8 and ZIF-67 were about 1 μm (Fig. 2a and b).

Fig. 2 (a–e) SEM images of ZIF-8, ZIF-67, core-shell ZIF-8@ZIF-67-0.25, ZIF-8@ZIF-67-0.5 and ZIF-8@ZIF-67-0.75 crystals; SEM and TEM images of core–shell ZnO/NPC@Co/NPC-0.5 (f–g). The inset in (g) is the size distribution of S3; (h–j) TEM image and elemental mappings of the core–shell ZIF-8@ZIF-67-0.5 crystals.

For the preparation of core–shell ZIF-8@ZIF-67 crystals, here the ZIF-8 seeds were sonicated to gain the dispersed crystals. The good dispersity is quite important for uniform coating with the ZIF-67 shell. After adding a methanolic solution of CoCl₂, the Co²⁺ ions were immobilized on the surface of ZIF-8 seeds through the coordinative interaction with MeIm units exposed on the surface, followed by the growth of the ZIF-67 shell via the interaction with additive MeIm linkers. The obtained core–shell MOF crystals by using ZIF-8 seeds were abbreviated as ZIF-8@ZIF-67-x (x = 0.25, 0.5 and 0.75, respectively).

The obtained core–shell ZIF-8@ZIF-67 crystals were carefully characterized. Fig. 2h and i show the TEM image and elemental mappings for the sample consisting of ZIF-8 cores and ZIF-67 shells. The shell thickness of ZIF-8@ZIF-67 crystals was easily controlled by changing the feeding ratios of Co²⁺/Zn²⁺ (Fig. 2c–e). The core–shell ZIF-8@ZIF-67-x crystals show a well-defined rhombic dodecahedral morphology, which corresponds to the original morphology of the ZIF-8 seeds (Fig. 2a). As shown in Fig. 2c–e, when the edge part with cracks was observed, it was found that the ZIF-67 shell uniformly coated the ZIF-8 core with a thickness of 71 nm, 99 nm and 120 nm, respectively (Table 1). Fig. 2g and its inset show the size of S3 (around 1.055 μm). The digital photo of S3 is presented in Fig. 3 (the inset is its corresponding SEM images). It is clearly seen that the particle of S3 remains uniform in wax. The wide-angle PXRD patterns of ZIF-8, ZIF-67 and core–shell ZIF-8@ZIF-67-x crystals provided a better understanding of the core–shell crystals (Fig. 4a). The positions of the diffraction peaks of our prepared ZIF-8 and ZIF-67 crystals corresponded to the PXRD patterns simulated from the single crystal structures of ZIF-8³⁵ and ZIF-67.³⁶ In addition, the wide-angle XRD patterns for S2, S3 and S4 (Fig. 4b) exhibited diffraction peaks at 41.2^°, 44.2^° and 47.5^° that are identical to the diffraction of carbon, Co and ZnO. The signals of graphite can hardly be detected from the powder pattern, due to the weak scattering power and low crystallinity of graphite.³⁹ Nevertheless, graphite in the hybrid can be readily identified by Raman spectroscopy (Fig. 5). Two strong peaks appearing at 1346 and 1587 cm⁻¹, respectively, are the characteristic Raman signals of graphite.³⁰ From Fig. 2f and g, one can see that the obtained S3 retained the original rhombic dodecahedral shape of the core–shell ZIF-8@ZIF-67 crystals with a slight shrinkage and distortion.

Fig. 3 Digital photos and SEM images of S3.

Fig. 4 (a) Wide-angle PXRD patterns of the as-synthesized ZIF-8, ZIF-8@ZIF-67-0.25, ZIF-8@ZIF-67-0.5, ZIF-8@ZIF-67-0.75 and ZIF-67 crystals.(b) XRD patterns of S1, S2, S3, S4 and S5 samples.

Fig. 5 Raman spectra of S2, S3 and S4.

As revealed by SEM images (Fig. S1a†), the obtained ZnO/NPC particles retained the original rhombic dodecahedral shape from the parent ZIF-8 and exhibited a smooth surface without any large pores or cracks. In contrast, the Co/NPC particles prepared from ZIF-67 shrank significantly and showed a distorted, bumpy surface (Fig. S1d†). A similar situation was observed with ZnO/NPC@Co/NPC particles prepared from ZIF-8@ZIF-67 crystals (Fig. S1b and d†). With the increase of ZIF-67 shell thickness, the ZnO/NPC@Co/NPC particles that carbonized from ZIF-8@ZIF-67 displayed a more seriously distorted surface.

The graphitic degree of the samples was investigated in more detail by Raman spectroscopy. As shown in Fig. 5 and S2,† all samples present two strong peaks at 1346 cm⁻¹ and 1587 cm⁻¹, which can be assigned to the typical D and G bands of the carbon materials, respectively. This result indicates the existence of carbon in the samples. The D band is usually related to lattice disorder in sp²-hybridized C atoms or amorphous C deposits, whereas the G band is ascribed to the highly oriented sp² hexagonal graphitic lattice.⁴⁰ For the ZnO/NPC sample (Fig. S2a†), the intensity of the G band was uniform with the D band, indicating that the carbon in ZnO/NPC has a low degree of graphitization in the atomic carbon structure. In contrast, for other samples, the intensity of the D band was lower than that of the G band, revealing that the carbon has a higher degree of graphitization in the atomic carbon structure. The obvious enhancement of the G band indicates a higher degree of graphitization compared to the carbon structure as a result of metal Co catalysis for carbon at a high calcination temperature and long calcination time.³⁰

The nitrogen adsorption isotherms obtained by means of DFT⁴¹ are shown in Fig. 6. The specific surface areas and total pore volumes were analyzed and are summarized in Table 2. The values of all the samples are extremely low in comparison to the original ZIF-67 crystals (1738 m² g⁻¹) due to the collapse of the well-defined microporous structure of ZIF-67 caused by the graphitization of amorphous carbon.^25,42 The S2 and S3 with a thin Co/NPC thickness showed a high surface area (30.40 m² g⁻¹ and 168.23 m² g⁻¹), which also indirectly proves a successful tuning of the Co/NPC shell thickness. The micropore-size distribution curves of all the ZnO/NPC@Co/NPC-x samples show a great deal of disordered nanopores and a pore size distribution from 1 to 10 nm, which may be attributed to the collapse of the well-defined microporous structure of ZIF-67. Nevertheless, ZnO/NPC@Co/NPC-x still possesses a high surface area and total pore volume, which provide more active sites for reflection and scattering of electromagnetic waves, promoting the electromagnetic wave repeated absorption process more efficiently.

Fig. 6 (a) N₂ adsorption–desorption isotherms and (b) pore size distribution plots of S2, S3 and S4.

Table 2 Surface areas, total pore volumes and saturation magnetization of S2, S3 and S4

Sample	S BET (m^2 g^−1)	S Langmuir (m^2 g^−1)	V pore (cm^3 g^−1)	M s (emu g^−1)
S2	30.40	42.30	0.1591	7
S3	168.23	214.76	0.2887	38
S4	318.80	358.40	0.2028	55

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The magnetic hysteresis of ZnO/NPC@Co/NPC-x was carried out at room temperature using a vibrating sample magnetometer (Fig. 7), and the related saturation magnetization (M_s) values are listed in Table 2. It can also be found that the M_s of ZnO/NPC@Co/NPC-x increases from 7 to 55 emu g⁻¹, which is attributed to the decreased carbon content²⁶ and the increased Co concentration in the Co/NPC shells. ZnO/NPC@Co/NPC-0.75 has the largest saturation magnetization which may induce a high permeability. As shown in Fig. S3,† the ZnO/NPC@Co/NPC-0.75 has a higher real part of permeability.

Fig. 7 Field-dependent magnetization curve of ZnO/NPC@Co/NPC-x (x = 0.25, 0.5 and 0.75) measured at room temperature.

In terms of the novel nanostructure of the modified samples, ZnO/NPC@Co/NPC-x, the dependence of samples’ complex permittivity (ε_r = ε′ − jε′′) and permeability ((μ_r = μ′ − jμ′′)) on frequency was investigated, where the real parts of complex permittivity (ε′) and complex permeability (μ′) represent the storage capability of electric and magnetic energy, and the imaginary parts (ε′′ and μ′′) stand for the loss capability of electric and magnetic energy.⁴³ The μ′ and μ′′ values of all the samples are almost constant over the whole frequency range with a slight fluctuation μ′ ≈ 1.0 and μ′′ ≈ 0.0 (Fig. S3†), respectively, which suggests that all the obtained samples are one kind of weakly magnetic but high dielectric loss material.³⁹ Here, we focus on the change in the complex permittivity of samples with Co/NPC shells of different thicknesses.

Compared with the ε′ and ε′′ of S1–S5 (Fig. 8), for the Co/NPC, the ε′ and ε′′ values were very high, indicating the very high storage capability of electric energy and dielectric loss of the Co/NPC, which can be attributed to the higher electrical conductivity of carbon. However, the ε′ and ε′′ values of ZnO/NPC were the lowest. Furthermore, the effect of the thickness of the Co/NPC shell during the synthesis process of ZnO/NPC@Co/NPC-x was studied. The results reveal that the permittivity and EM wave absorbing performance of the ZnO/NPC@Co/NPC-x samples significantly depend on the thickness of the Co/NPC shell. It can be seen that the ε′ and ε′′ values of ZnO/NPC@Co/NPC-x are augmented with an increase in the thickness of the Co/NPC shell. It is reasonable to conclude that the complex permittivity is crucially determined by the thickness of Co/NPC. In general, the real permittivity is an expression of the polarization ability of a material which mainly arises from the dipolar polarization and interfacial polarization at the microwave frequency.^44–47 In this case, the dipolar polarization is provided mainly by nanoporous carbon, and the interfacial polarization comes mainly from the interface of the cooperative consequence of the Co shell, the core/shell interfaces and the dielectric ZnO cores.⁴⁸ With the increase of Co/NPC shells, the total pore volume of ZnO/NPC@Co/NPC-0.5 was the largest according to the results in Table 2. This is because when the molar ratio of Co²⁺/Zn²⁺ is 0.75, the sample has a more serious distorted surface. Therefore, the ZnO/NPC@Co/NPC-0.75 has a lower pore volume. The interface of ZnO/NPC and Co/NPC is augmented as a result of increasing the thickness of Co/NPC shells, which brings about an increase of the interfacial polarization.

Fig. 8 Measured frequency dependence of the permittivity of sample–paraffin (50 wt%) composites. (a) Real parts of the permittivity and (b) imaginary parts of the permittivity.

The RL coefficients of the as-obtained samples were calculated from the measured relative complex permittivity and permeability values at a given frequency and layer thickness. According to transmission line theory, the RL coefficient (dB) can be defined with the following equations:^49,50

Z_in = Z₀(μ_r/ε_r)^1/2tanh[j(2πfd/c)(μ_rε_r)^1/2] (1)

RL = 20log|(Z_in − Z₀)/(Z_in + Z₀)| (2)

where Z_in is the input impedance of the absorber, Z₀ is the impedance of free space, μ_r is the relative complex permeability, ε_r is the complex permittivity, f is the frequency of microwaves, d is the thickness of the absorber, and c is the velocity of light. For common applications, it requires the microwave absorbing materials to possess a RL value lower than −10 dB which means that 90% of the incident microwaves can be absorbed. Fig. 9 shows the reflection loss curves of the as-obtained products in the frequency range of 2–18 GHz for the 50 wt% products/paraffin composites with a thickness of 1.9 mm. We can find that the reflection loss values of S1, S2, S4 and S5 were very poor and all the reflection loss was below −10 dB. It is satisfactory that S5 exhibited an outstanding microwave absorption performance. For example, the maximum reflection loss of S5 that can be achieved is −28.8 dB and a broad absorption bandwidth of 4.2 GHz (RL values exceeding −10 dB) covers the 13.8–18 GHz frequency range.

Fig. 9 RL curves of S1–S5 composites with the filler loading of 50 wt % at various thicknesses.

Furthermore, Fig. S4† shows the variation of RL values versus frequency for ZnO/NPC (Fig. S4a†) and Co/NPC (Fig. S4b†) at different thicknesses. However, the RL values for ZnO/NPC and Co/NPC cannot reach −10 dB within a thickness range of 1.0–5.0 mm. The poor RL values indicate that the ZnO/NPC and Co/NPC could not be used for practical applications. It is gratifying that the ZnO/NPC@Co/NPC-0.5 shows outstanding EM absorption performance (Fig. 10b). For ZnO/NPC@Co/NPC-0.5, a minimum RL of −45 dB can be obtained at 12.5 GHz with a thickness of 2.2 mm, and RL values less than −10 dB can be gained in the 4.3–18 GHz range with absorber thicknesses of 1–5 mm. In terms of ZnO/NPC@Co/NPC-0.25 (Fig. 10a), the minimum RL is only −13.7 dB at 15 GHz with a thickness of 2.5 mm. Unfortunately, in the frequency range of 2–18 GHz, the RL values of ZnO/NPC@Co/NPC-0.75 (Fig. 10c) cannot reach −10 dB with a thickness range of 1.0–5.0 mm. Moreover, the dependence of RL values on the sample thickness can be analyzed, as shown in Fig. 10, for all the samples, it is clear that the absorption peaks shift toward low frequency regions as the absorber thickness increases. The dependence of the RL peak on the sample thickness can be analyzed by the quarter-wavelength matching model, in which the relationship between the absorber thickness (d) and the peak frequency (f) can be described by using the following equation:^51,52

Fig. 10 RL curves and 3D plots of the paraffin-based composites (50 wt%). (a) ZnO/NPC@Co/NPC-0.25; (b) ZnO/NPC@Co/NPC-0.5; (c) ZnO/NPC@Co/NPC-0.75.

According to the quarter-wavelength matching model, the absorption peaks shift to the lower frequency range with an increase of thickness. Furthermore, based on eqn (2), when Z_in is close to Z₀, the optimal absorption performance of the absorber can be achieved. According to previous reports,⁵³ when the matching thickness of the samples satisfies eqn (3), Z = |Z_in/Z₀| is close to 1. Therefore, when the relationship between the matching thickness and the matching frequency of the samples satisfies eqn (3), the minimum RL can be obtained. Compared with the EM absorption performance of the Co/NPC obtained from pure ZIF-67 and ZnO/NPC acquired from pure ZIF-8, the results suggest that the ZnO/NPC@Co/NPC-0.5 has remarkable absorbing characteristics in wide frequency bands in the 2–18 GHz range with thicknesses of 1.0–5.0 mm, which indicates that the application of EM wave absorption over the whole band (2–18 GHz) can be realized by adjusting the matching thickness.

It is generally accepted that there are two key factors which should be considered for an excellent absorber. One is the impedance match, requiring the equality of the electromagnetic parameters; the other is the best electromagnetic wave attenuation in the interior of the absorber. The value of Z = |Z_in/Z₀| obtained using eqn (2) was used to evaluate impedance matching. The attenuation constant α, which determines the attenuation properties of materials, can be determined as:⁵⁴

Here, we take ZnO/NPC@Co/NPC-0.5 as an example to elaborate the importance of considering the two factors for an outstanding absorber. Fig. 11 shows the frequency dependence of Z = |Z_in/Z₀| and the attenuation constant α for ZnO/NPC@Co/NPC-0.5, hence, the relationship between the frequency and the RL at the matching thickness (d = 2.2 mm) is also presented. When the attenuation constant α reaches a maximum value of 16.5 GHz, the minimum RL cannot be obtained and the relevant Z is close to 1.05 at the matching thickness of 2.2 mm. However, when the matching frequency is 12.3 GHz, the minimum RL can be achieved, meanwhile, the relevant Z is close to 1 at the matching thickness of 2.2 mm. Therefore, the results provide a significant guide for the design required for the microwave absorber. We not only consider the material attenuation capacity for the EM wave, but also must take into account the importance of impedance matching.

Fig. 11 The frequency dependence of RL values for ZnO/NPC@Co/NPC-0.5 (50 wt%) with 2.2 mm thickness; attenuation constant for ZnO/NPC@Co/NPC-0.5 (50 wt%) and the modulus of normalized input impedance (|Z_in/Z₀|) for ZnO/NPC@Co/NPC-0.5 (50 wt%).

To understand the microwave absorbing performance enhancement mechanism of ZnO/NPC@Co/NPC-0.5, the dependence of the characteristic impedance and attenuation constant α on the frequency was investigated. From eqn (4), the higher values of ε′′ would result in higher α. As shown in Fig. 12, Co/NPC has the largest α in all frequency ranges, indicating excellent attenuation which is attributed to its higher ε′′. For Co/NPC, the outstanding electromagnetic wave damping capacity may come from electrical loss and polarization relaxation loss. The electrical loss is attributed to the generation of a leaking current in the conductive networks consisting of nanoporous carbon and metal Co.

Fig. 12 Attenuation constant of sample–paraffin (50 wt%) composites versus frequency of S1–S5.

The polarization relaxation mainly arises from dipolar polarization and interfacial polarization. In this case, the dipolar polarization is provided mainly by the pore wall edge of nanoporous carbon, and the interfacial polarization mainly comes from the interface of Co/carbon. However, the attenuation constant, α, of ZnO/NPC@Co/NPC-0.5 is significantly reduced due to its low ε′′. Nevertheless, ZnO/NPC@Co/NPC-0.5 also possesses a higher capacity for converting electromagnetic waves to other forms of energy. The high ε′′ of Co/NPC implies that Co/NPC has an outstanding attenuating capacity, but the higher ε′ also leads to a stronger reflection of incident EM waves on the material surface.⁵⁵ Based on the above discussion, for improved EM-wave absorption properties, the main contribution should come from impedance matching. Thus, the introduction of Co/NPC to the obtained ZnO/NPC@Co/NPC-x is helpful.

In the above description, we can realize that the ε′ and ε′′ values of ZnO/NPC@Co/NPC-x decreased with an increase in the thickness of the Co/NPC shell. For a multiphase mixture, the most commonly used equation for predicting the dielectric constant of the composite is the logarithmic law of mixing, as shown in eqn (5):⁵⁶

ln ε′ = V₁ln ε′₁ + V₂ln ε′₂ + V₃ln ε′₃ (5)

where ε₁, ε₂, and ε₃ are the real parts of complex permittivity (ε′) of ZnO/NPC, Co/NPC and air, respectively. V₁, V₂, and V₃ are the volume fractions of ZnO/NPC, Co/NPC and air, respectively. With the increasing thickness of the Co/NPC shell, V₂ increases and the other volume fractions decrease. The ε₂ is much higher than ε₁. Therefore, ε′ will decrease with the shrinking thickness of the Co/NPC shell. A lower ε′ is advantageous to the impedance matching of the material. However, a smaller ε′ is more favorable for EM wave absorption. More importantly, the suitable ε′ range of the composite is in favor of the absorption of EM waves.

ε′′ is an expression of the electrical conductivity of the material. Based on the Debye theory, this can be expressed by using the following formula:⁵⁷

where δ is the electrical conductivity and ε′ is the dielectric constant in a vacuum. According to eqn (6), an increase in conductivity leads to high ε′′. The ZnO/NPC obtained from carbonized ZIF-8 is a semiconductor with a relatively lower conductivity. Nevertheless, the carbon obtained from the carbonized ZIF-8 has a relatively low electrical conductivity.⁵⁸ Therefore, the electrical conductivity of ZnO/NPC@Co/NPC-x obtained from ZIF-8 covered ZIF-67 was larger with an increase in the thickness of the Co/NPC shell due to its high electrical conductivity. All of the above results suggest that only appropriate amounts of Co/NPC are more beneficial for enhancing the absorption efficiency of electromagnetic waves.

Conclusions

Core–shell ZIF-8@ZIF-67 crystals, which integrate the properties of single ZIF-8 and ZIF-67, are elaborately designed for the first time by applying a seed-mediated growth technique. The shell thicknesses of ZIF-67 can be tuned simply by varying the feeding molar ratios of Co²⁺/Zn²⁺. After the direct carbonization of core–shell ZIF-8@ZIF-67 crystals, a new type of selectively functionalized ZnO/NPC@Co/NPC material remains and the ZnO/NPC@Co/NPC-0.5 exhibited outstanding microwave absorption performance. The maximum reflection loss that can be achieved is −28.8 dB at an absorbent thickness of 1.9 mm and a broad absorption bandwidth of 4.2 GHz (RL values exceeding −10 dB) covers the 13.8–18 GHz frequency range. This study provides a good reference for future preparation of other carbon-based lightweight microwave absorbing materials derived from MOFs. Furthermore, the potential applications of the carbon-based materials can be developed and extended from water treatment and catalysis to the microwave absorbing domain.

Acknowledgements

The authors acknowledge the financial support from the National Nature Science Foundation of China (no.: 11575085), the Aeronautics Sciences Foundation of China (no.: 2014ZF52072), the Open Research Fund of Jiangsu Provincial Key Laboratory for Nanotechnology of Nanjing University, the Qing Lan Project, Six Talent Peaks Project in Jiangsu Province (no.: XCL-035) and the project also founded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Footnote

† Electronic supplementary information (ESI) available: Additional photographs, and permittivity and permeability of samples. See DOI: 10.1039/c6qi00359a

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