Enhanced interfacial polarization loss induced by hollow engineering of hollow alloyed CoFe-ZIF nanocages/carbon nanofibers for efficient microwave absorption

Abstract

Metal organic frameworks (MOFs) have been widely studied in the field of microwave absorption due to their high porosity and large specific surface area. The weak dielectric loss limits the enhancement of their absorption performance. In this study, hollow alloyed CoFe-ZIF/CNF composite fibers were successfully synthesized by electrospinning and high-temperature carbonization. The carbon fiber with dielectric loss wraps hollow alloyed CoFe-ZIF nanocages with magnetic loss to form a bamboo-shaped composite fiber to achieve magnetoelectric synergy. The core advantages of the hollow structure are optimized impedance matching, extended propagation path and enhanced multi-mechanism loss. The construction of the hollow structure of hollow alloyed CoFe-ZIF nanocages and the combination of carbon fiber not only enriches substantial heterogeneous interfaces, but also optimizes impedance matching, which is beneficial for the attenuation dissipation of EMW. The results show that hollow alloyed CoFe-ZIF/CNF composite fibers exhibit excellent electromagnetic wave absorption performance. When the filling is only 10 wt%, the minimum reflection loss is −59.61 dB, and the effective absorption bandwidth reaches 6.64 GHz. This study used a combination of MOF alloy cages and carbon fibers to regulate the absorption properties, providing new insights into the preparation and application of 1D structural composite absorbers.

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1. Introduction

Electronic devices with features such as multiple functions and high power have provided great convenience to human beings.^1–4 However, the large-scale use of electronic devices has led to serious problems such as information security and electromagnetic wave (EMW) pollution.^5–8 Basically, EMW absorbing materials are considered promising candidates to solve this serious problem, which convert EMW energy into electrical, heat or other forms of energy to eliminate EMW. Currently, ferromagnetic materials (Co,⁹ Fe₃O₄,¹⁰ and FeCo ¹¹) with excellent absorption strength and carbon materials (RGO,¹² carbon nanotubes,¹³ carbon fibers,¹⁴etc.) with stable chemical properties and superior complex dielectric properties are of great concern. Research shows that pure ferromagnetic materials or carbon materials have limitations as EMW absorbing materials. The biggest problem is the impedance mismatch caused by higher dielectric properties or magnetic properties. Therefore, the development of dielectric–magnetic composites is an effective strategy to obtain excellent impedance matching and high performance EMW absorption.

Metal organic frameworks (MOFs) are mesoporous materials with periodic network structures whose pore structures are controlled by self-assembly technology.¹⁵ In recent years, MOF materials with high porosity, large specific surface areas and controlled magnetic/dielectric compositions have been extensively studied as microwave absorbers.^16–18 Therein, higher porosity and larger specific surface area lead to strong multiple scattering and reflection, which effectively attenuate EMW. Diverse magnetic/dielectric components effectively optimize the impedance matching, which provides an excellent carrier for the design and preparation of high efficiency microwave absorbing materials with special microstructures and controllable properties.^19,20 Currently, to further optimize the performance of MOF-derived absorbers, a hybridization strategy is used to control the microstructure and chemical composition of the MOF absorbers.²¹ For instance, Tao et al. introduced nickel particles using a thermal reduction method. The obtained graphite/CoNi alloy hollow porous composites improved impedance matching by combining graphite and the CoNi alloy. The best absorption value was −63.79 dB, and the absorption bandwidth with absorption performance less than −10 dB reached 7.63 GHz.²² Peng and his colleagues synthesized a hierarchical cobalt-based MOF-derived composite material by annealing cobalt-based MOFs. The composite had a minimum reflection loss of −40.1 dB and an effective absorption bandwidth of 5.6 GHz.²³ As seen in previous reports, alloying the metal framework in the MOF hybrid material with a magnetic metal not only harvests additional magnetic losses, but also obtains hollow structures that cause the incident waves to be lost by multiple scattering and reflections inside the material.^24,25 However, the combination of these components and structures cannot enhance the conduction loss of the EMW absorbing material. Therefore, enhancing the conduction loss by improving the conductivity of the composite material becomes an important direction for improving the absorption performance of MOF.²⁶

Conduction loss benefits from the construction of local micro-current networks. Carbon materials have excellent conduction properties, especially one-dimensional carbon fibers.^27,28 1D carbon fibers can broaden the local microcurrent network and improve the conductivity in the case of small filling, which is conducive to obtaining excellent EMW absorption performance. Our research group has also done related 1D fiber work. For example, short carbon fibers prepared by using metal catalysts have shown excellent wave absorption performance.²⁹ This kind of carbon fiber greatly enhances the conduction loss of the material and improves the wave absorption performance. However, short fibers cannot construct substantial local microcurrent networks due to their small aspect ratio. Constructing continuous carbon fibers with a high aspect ratio can yield a large number of micro-conductive networks at a low filling ratio.³⁰ Therefore, we prepared continuous carbon fiber-based composite absorbers by electrospinning to enhance the conduction loss. The use of silica microspheres to improve the high dielectric properties of carbon fibers leads to the problem of material impedance mismatch. At the same time, many spherical heterogeneous interfaces and local conductive networks are constructed. The combined effect of these factors makes the EMW absorption performance of the absorber reach −59.53 dB (minimum reflection loss) and 6 GHz (effective absorption bandwidth).³¹ This shows that the construction of continuous carbon fibers is conducive to improving the conduction loss of the SiO₂@SiC@C nanofiber composites. At the same time, the combination of a MOF and carbon nanofibers introduces a large number of heterogeneous interfaces, which also enhances heterogeneous loss of the material. Therefore, constructing MOF-based composites with conduction loss and heterogeneous loss becomes an effective method to improve EMW absorption performance.

Herein, heterointerface nanostructures in Fe-ZIF-67/CNF composite nanofibers with hollow alloyed CoFe-ZIF nanocage structures are carefully designed and constructed via heterointerface engineering and conductive networks to investigate the internal mechanisms of heterointerface polarization/conduction losses and elucidate their contributions in electromagnetic wave attenuation. Due to the hollow structure and the multi-interface of Fe-ZIF-67 in Fe-ZIF-67/CNF composite nanofibers for regulation of the dielectric properties, carbon fibers with a large aspect ratio are more likely to build micro-local networks, thereby enhancing the dielectric loss of the Fe-ZIF-67/CNF absorber. In addition, hollow alloyed CoFe-ZIF nanocages form more dipoles inside and make up for the defect of the single dielectric loss of carbon fiber. Under this synergistic effect, excellent electromagnetic wave absorption performance is achieved at a lower filling level of hollow alloyed CoFe-ZIF nanocages. The composite nanofibers exhibit excellent electromagnetic wave absorption performance at a low filler content of 10 wt%, with a maximum absorption intensity of up to −59.61 dB and an effective absorption bandwidth of up to 6.64 GHz at 2.02 mm. This study provides a new idea for synergistically promoting electromagnetic wave absorption based on MOF heterogeneous engineering and conductive network construction, and provides a platform with an adjustable structure for the combination of alloyed CoFe-ZIF and carbon fibers.

2. Experimental section

2.1 Synthesis of ZIF-67

5 mmol of Co(NO₃)₂·6H₂O was dissolved in 100 mL of methanol and dispersed ultrasonically for 5 min and the resulting solution was noted as B. 40 mmol of 2-methylimidazole was weighed and dissolved in 100 mL of methanol, and the solution obtained after ultrasonic dispersion for 5 min was noted as A. Then, A was added to B and stirred magnetically at room temperature for 10 min, followed by standing for 12 h. A purple solution containing a precipitate was obtained. The precipitate was separated by centrifugation (8000 rpm, 3 min) and washed three times by centrifugation with anhydrous ethanol (8000 rpm, 3 min), and then dried in an oven at 65 °C for 12 h to obtain the blue−violet powder ZIF-67.

2.2 Synthesis of hollow alloyed CoFe-ZIF nanocages

0.40 g of ZIF-67 was dissolved in 100 mL of anhydrous ethanol, sonicated for 15 min, and then stirred with a magnetic stirrer for 15 min at room temperature, and the resulting solution was recorded as C. 0.20 mmol of FeSO₄·7H₂O was dissolved in 30 mL of deionized water, and ultrasonically dispersed for 15 min, and the resulting solution was recorded as D. D was added to C, and the black turbid solution containing a precipitate was obtained by centrifugation (8000 rpm, 5 min). The black turbid liquid containing the precipitate was obtained in 1 h. The precipitate in the turbid liquid was separated by centrifugation (8000 rpm, 5 min), and the obtained precipitate was washed by centrifugation with anhydrous ethanol (8000 rpm, 3 min) several times, and dried at 65 °C for 24 h, and then a dark purple powdery sample was obtained.

2.3 Synthesis of hollow alloyed CoFe-ZIF/PAN composite nanofibers

Hollow alloyed CoFe-ZIF/PAN nanofibers were synthesized using the electrostatic spinning method. n g (n = 0.20, 0.30, 0.40, 0.50, 0.60) of hollow alloyed CoFe-ZIF was dissolved in 10 mL of DMF and stirred magnetically at room temperature for 4 h. Then 1.00 g of PAN was added to the above solution and stirred vigorously for 4 h. The mixed solution was loaded into a 10 mL syringe and injected with a 23-gauge stainless steel needle. The electrostatic spinning device was fed at a rate of 0.03 mL min⁻¹ with a voltage of 22 kV and a collection distance of 15 cm.

2.4 Synthesis of hollow alloyed CoFe-ZIF/CNF composite nanofibers

The hollow alloyed CoFe-ZIF/PAN nanofibers were pre-oxidized under an air atmosphere at 230 °C for 2 h. Finally, CoFe-ZIF/CNF (x%) was obtained by annealing the nanofibers under an argon atmosphere at 800 °C at a rate of 2 °C min⁻¹ for 2 h. The CoFe-ZIF/CNF (x%) was obtained, where x% is the mass percentage of Fe-ZIF-67 and PAN during electrostatic spinning.

3. Results and discussion

The phase structure of hollow alloyed CoFe-ZIF/PAN composite nanofibers prepared by electrospinning and then subjected to high temperature pyrolysis was studied. Fig. 1b shows the XRD patterns of hollow alloyed CoFe-ZIF/CNF composite nanofibers. For the CoFe-MOF/CNF composites, distinct characteristic peaks are present at 44.9°, 65.3°, and 82.7°, which correspond to three different crystalline facets ((110), (200), and (211)) of hollow alloyed CoFe-ZIF (JCPDS PDF#49-1568). The presence of sharp characteristic peaks indicates better crystallinity of the prepared CoFe alloys. The presence of characteristic peaks at 26° is evident in the CoFe-ZIF/CNF composites, which was attributed to the formation of graphite carbon after PAN pyrolysis. However, with the introduction of CoFe-ZIF, the intensity of the diffraction peaks weakened, which was because hollow alloyed CoFe-ZIF destroyed the regularity of graphitic carbon. The above analytical results demonstrate the successful synthesis of CoFe-ZIF/CNF composites. Fig. 1c shows the Raman spectra of CoFe-ZIF/CNFs. There are two characteristic peaks at ∼1340 cm⁻¹ and ∼1590 cm⁻¹ in Fig. 1c, denoting the D and G peaks of carbon, respectively. The D peak indicates sp³ defects or disorder, while the G peak indicates sp² hybridization. The evaluation of the degree of carbon graphitization often depends on the intensity ratio (I_D/I_G) of the D and G peaks. Fig. 1b clearly shows that the I_D/I_G values for CoFe-ZIF/CNF (20%), CoFe-ZIF/CNF (30%), CoFe-ZIF/CNF (40%), CoFe-ZIF/CNF (50%), and CoFe-ZIF/CNF (60%) are 0.96, 0.96, 0.96, 0.96, and 0.95. The I_D/I_G values of CoFe-ZIF/CNF composites hardly changed with the increase of the CoFe-ZIF content. This indicates that the change in the ratio of hollow alloyed CoFe-ZIF nanocages to CNFs has no catalytic effect on the graphitization reconstruction of carbon in the CoFe-ZIF/CNF composites.

Fig. 1 (a) Schematic diagram for the formation process of CoFe-ZIF/CNFs, (b) XRD patterns and (c) Raman spectra of composites. (d) Full-scan spectrum and high-resolution XPS of (e) C 1s, (f) N 1s, (g) O 1s, (h) Fe 2p, and (i) Co 2p for CoFe-ZIF/CNFs.

Fig. 1d–i show the XPS full spectrum and high-resolution C 1s, N 1s, O 1s, Fe 2p and Co 2p spectra of CoFe-ZIF/CNF (30%). Five characteristic peaks at 780.44, 712.29, 531.64, 399.41, and 284.80 eV, which correspond to Co 2p, Fe 2p, O 1s, N 1s, and C 1s, respectively, can be seen in Fig. 1d. Fig. 1e shows the high-resolution spectrogram of C 1s, from which two characteristic peaks can be observed, which correspond to C–C/C=C (284.80 eV) and C–N (285.84 eV), which suggests the presence of abundant carbon-containing functional groups in the hollow alloyed CoFe-ZIF/CNF composite nanofibers. The presence of the C–C/C=C bond is due to the residual carbon skeleton after pyrolysis of the zeolite imidazole.³² The presence of C–N bonds indicates that the carbonization retains N-containing functional groups. The high-resolution N 1s spectrum (Fig. 1f) clearly exhibits three characteristic peaks located at 398.03, 398.89, and 400.85 eV, which indicate the presence of N-containing polar functional groups (such as pyridinic N, pyrrolic N, and graphitic N).³³ N induces dipole polarization chirality when stimulated by an alternating electromagnetic field. Fig. 1g shows the high-resolution spectra of O 1s, in which three characteristic peaks at 530.27 eV (–OH), 532.18 eV (C=O) and 533.52 eV (C–O) are observed. These oxygen-containing polar groups can provide dipole polarization as dipoles and are an important component of polarization loss. According to Fig. 1h, the high-resolution spectrum of Fe 2p can be observed with four sub-peaks: Fe⁰ (710.04 eV), Fe²⁺ (712.29 eV), Fe³⁺ (713.85 eV), and a satellite peak (715.99 eV). Characteristic peaks located at 779.00 eV (Co⁰), 779.81 eV (Co²⁺), 780.49 eV (Co³⁺), and 794.78 eV (Co²⁺), as well as two broad satellite peaks centered at 781.63 eV and 796.38 eV, are clearly observed in the high-resolution Co 2p spectra of Fig. 1i. The presence of Fe⁰ and Co⁰ proves that the metal ions are reduced to monomers.³⁴ Since there is a potential difference between Co and Fe metals in hollow alloyed CoFe-ZIF nanocages, there are also substantial interfaces in the CoFe alloy, which provide interface losses.

The structure of the composite has an important influence on its performance. The distribution of alloyed CoFe-MOF nanoparticles in carbon fibers was studied by FESEM. Fig. 2a shows FESEM images of alloyed CoFe-ZIF nanoparticles, revealing that the particles have a dodecahedral structure and a surface granular structure. The packing size of single alloyed CoFe-ZIF nanoparticles is about 300 nm. Fig. 2b–f show FESEM images of CoFe-ZIF/CNF composites, where all nanofibers are inlaid with different numbers of polyhedral nanoparticles. The diameter of the fiber is about 400 nm. When alloyed CoFe-ZIF nanocages are embedded inside, the diameter expands to 500 nm, so that the carbon fiber can better wrap and protect the magnetic particles. The structure of the CoFe-ZIF nanocages is mostly well preserved, which may be due to the protection of the outer carbon fibers during the carbothermal reduction process. As the amount of alloyed CoFe-ZIF nanocages embedded increases, more alloy polyhedral boundaries are formed in the CNF fibers. It can be clearly observed that the alloy polyhedral boundaries begin to aggregate in CoFe-ZIF/CNF (40%). The alloyed CoFe-ZIF nanocage is a hollow structure as shown in Fig. 2c. The hollow alloyed CoFe-ZIF nanocage can effectively adjust the impedance between the air and hollow alloyed CoFe-ZIF/CNF composite nanofibers, while reducing the weight of the absorber, which is beneficial for the impedance matching and light weight of hollow alloyed CoFe-ZIF/CNF composite nanofibers. As shown in Fig. 2g−i, the element distribution map confirms that Co and Fe elements are uniformly distributed on the alloyed CoFe-ZIF nanocages. We can also see that the alloyed CoFe-ZIF nanocage in the carbon fiber has a layered structure and the distribution of alloy particles is shown in the TEM image (Fig. 2j). At the same time, EDS element mapping was performed on the composite material near the nanocage to analyze the component distribution. It was confirmed that Co and Fe elements were distributed in the form of nanoparticles in the nanocage, while C was uniformly distributed on the particles and nanofibers at the same time, further explaining the formation of the CoFe alloy.

Fig. 2 FESEM images of (a) CoFe-ZIFs, (b) CoFe-ZIF/CNF (20%), (c) CoFe-ZIF/CNF (30%), (d) CoFe-ZIF/CNF (40%), (e) CoFe-ZIF/CNF (50%), and (f) CoFe-ZIF/CNF (60%); (g), (h) and (i) elemental distribution images of CoFe-ZIFs; (j) and (k) HRTEM images and (l) HAADF elemental mapping image of CoFe-ZIF/CNF (30%).

The average pore diameter and specific surface area of the CoFe-ZIF/CNF composites were further investigated by BET. In Fig. 3a–e, the typical type IV isotherm shows the existence of a mesoporous structure in the CoFe-ZIF/CNF composites. The specific surface areas of CoFe-ZIF/CNF (20%), CoFe-ZIF/CNF (30%), CoFe-ZIF/CNF (40%), CoFe-ZIF/CNF (50%) and CoFe-ZIF/CNF (60%) are 13.71, 214.99, 185.36, 131.79 and 53.19 m² g⁻¹, respectively. The CoFe-ZIF/CNF (30%) composites have the largest specific surface area, which is also conducive to establishing more surface interfaces in the material and providing a basis for enhancing interface loss. Fig. 3f shows the pore size distribution of the CoFe-ZIF/CNF composites. The average pore sizes of CoFe-ZIF/CNF (20%), CoFe-ZIF/CNF (30%), CoFe-ZIF/CNF (40%), CoFe-ZIF/CNF (50%) and CoFe-ZIF/CNF (60%) are 17.42, 12.75, 12.94, 13.20 and 12.22 nm, respectively, indicating the formation of a mesoporous structure in the CoFe-ZIF/CNF composites. The resulting porous structure can effectively coordinate the dielectric difference between the hollow alloyed CoFe-ZIF/CNF composite nanofibers and free air, allowing more electromagnetic waves to enter the material.³⁵ In addition, the porous structure can also reduce the mass of EMW absorbing materials, which is conducive to the realization of lightweight characteristics.

Fig. 3 N₂ adsorption desorption curves of (a) CoFe-ZIF/CNF (20%), (b) CoFe-ZIF/CNF (30%), (c) CoFe-ZIF/CNF (40%), (d) CoFe-ZIF/CNF (50%), and (e) CoFe-ZIF/CNF (60%) and (f) pore diameter distribution curves.

The dielectric constant, magnetic permeability and loss tangent of the hollow alloyed CoFe-ZIF/CNF composite nanofibers are closely related to the electromagnetic wave absorption performance of the material,³⁶ and their relationship with frequency is shown in Fig. 4. The ε′ and ε′′ values of the composite decrease with the increase of the electromagnetic wave frequency as shown in Fig. 4a and b, which may be related to the polarization of polar dipoles. The dipoles are rearranged under the influence of an external electromagnetic field. When these dipole rearrangements cannot match the changes in the electromagnetic field, the electronic polarization effect caused by the dipole will be very weak. As a result, the electronic response shows a decreasing trend in the test band. With an increase of the ratio of CoFe-ZIFs to CNF, the ε′ and ε′′ values of hollow alloyed CoFe-ZIF/CNF composite nanofibers first increase and then decrease, and the ε′ and ε′′ values of CoFe-ZIF/CNF (30%) are the highest, which indicates that CoFe-ZIF/CNF (30%) has strong power storage capacity and power loss capacity. The ε′ value of the hollow alloyed CoFe-ZIF/CNF composite nanofibers fluctuates slightly at high frequencies, which is attributed to the increase in polarization hysteresis at high frequencies. The ε′′ curve of the hollow alloyed CoFe-ZIF/CNF composite nanofibers has multiple resonance peaks at high frequencies, which is related to the obvious polarization relaxation process in the composites. The trend of tan δ_ε of the CoFe-ZIF/CNFs composites changing with frequency is shown in Fig. 4c. The tan δ_ε value of CoFe-ZIF/CNF (30%) is larger than those of other materials, which indicates that CoFe-ZIF/CNF (30%) has a stronger dielectric loss ability. Fig. 4d−f show the trends of μ′, μ′′ and tan δ_μ values of the hollow alloyed CoFe-ZIF/CNF composite nanofibers varying with frequency. The μ′, μ′′ and tan δ_μ values of hollow alloyed CoFe-ZIF/CNF composite nanofibers show a fluctuating decrease in the range of 2.00–18.00 GHz. The fluctuation is attributed to the exchange of energy between magnetic particles.³⁷ The hollow alloyed CoFe-ZIF nanocages in CoFe-ZIFs have high conductivity and produce induced currents in the process of polarization. High conductivity provides conductivity loss to convert electrical energy into thermal energy, while the other part produces magnetic coupling to fluctuate the magnetic energy.^11,38 At the same time, the μ′′ and tan δ_μ values are negative in some bands. This value is due to the eddy current effect shielding the material from EMW penetration, thus reducing the permeability of the material, resulting in μ′′ and tan δ_μ values less than 0. Obviously, the ε′ and ε′′ values of the hollow alloyed CoFe-ZIF/CNF composite nanofibers are much bigger than their μ′ and μ′′ values, indicating that the composite materials have stronger power storage capacity and power loss capacity. In addition, the hollow alloyed CoFe-ZIF/CNF composite nanofibers show a higher tan δ_ε value than tan δ_μ value, which proves that the hollow alloyed CoFe-ZIF/CNF composite nanofibers are a kind of EMW absorbing material in which dielectric loss is the main factor and magnetic loss is the auxiliary factor.

Fig. 4 CoFe-ZIF/CNFs of (a) ε′, (b) ε′′, (c) tan δ_ε, (d) μ′, (e) μ′′ and (f) tan δ_μ.

Since the dielectric loss of the hollow alloyed CoFe-ZIF/CNF composite nanofibers plays a dominant role in the attenuation of electromagnetic waves, we conducted a detailed analysis of its dielectric loss process. Generally speaking, the dielectric loss of a material is closely related to the internal polarization relaxation, and the internal polarization relaxation process can be analyzed using the Cole–Cole curve, which can be expressed by the formula:^19,39,40

The straight line and semicircle on the Cole–Cole curve represent conduction loss and polarization relaxation, respectively. The Cole–Cole curves of the CoFe-ZIF/CNF composites are shown in Fig. 5a–e. Semicircles appear on the Cole–Cole curves of the hollow alloyed CoFe-ZIF/CNF composite nanofibers, indicating that all of them have strong relaxation behavior. The hollow alloyed CoFe-ZIF/CNF composite nanofibers all have a large number of semicircles, which indicates that the combination of alloyed CoFe nanocages and carbon fibers can produce strong polarization relaxation. This may be due to the fact that there are different potential differences of each phase composition, which leads to the existence of a large number of heterogeneous interfaces (CoFe-ZIFs//CNF, CoFe-ZIFs//air, air//CNF and CoFe//C), defects and vacancies in the hollow alloyed CoFe-ZIF/CNF composite nanofibers, which leads to strong interface polarization and defect and vacancy polarization, thus forming more relaxation processes.⁴¹ In addition, it can be observed in Fig. 5a–e that the end of the Cole–Cole curve of the hollow alloyed CoFe-ZIF/CNF composite nanofibers is a straight line, which means that there is conduction loss in them to dissipate EMW, and the slope of the straight line often determines their conduction loss capacity. The slopes of the straight line part in the Cole–Cole curves of CoFe-ZIF/CNF (20%), CoFe-ZIF/CNF (30%), CoFe-ZIF/CNF (40%), CoFe-ZIF/CNF (50%) and CoFe-ZIF/CNF (60%) are 0.87, 1.78, 0.69, 1.05 and 0.48, respectively. The large linear slopes indicate their strong conduction loss. The slope of CoFe-ZIF/CNF (30%) reaches a maximum of 1.78, which indicates that it has the strongest conduction loss. From the Debye theory, ε′′ can be divided into polarization loss and conductivity loss , expressed as follows in eqn (2):^27,42,43

where corresponds to a parallel circuit of resistance (R) and capacitance (C) and corresponds to a series circuit of resistance (R) and capacitance (C).²⁴ In Fig. S3a,† the conduction loss of CoFe-ZIF/CNFs decreases with the increase of frequency in the frequency range of 2–18 GHz. The conduction loss of CoFe-ZIF/CNF (30%) is higher than those of others at the whole test frequency, which is consistent with the results of the Cole–Cole curve analysis. In Fig. S3b,† the polarization loss of CoFe-ZIF/CNF (30%) in the frequency range of 2–18 GHz is accompanied by a fluctuating decrease with increasing frequency, and the polarization losses of CoFe-ZIF/CNF (20%), CoFe-ZIF/CNF (40%), CoFe-ZIF/CNF (50%), and CoFe-ZIF/CNF (60%) are accompanied by a fluctuating increase with increasing frequency. The polarization loss of CoFe-ZIF/CNF (30%) is higher than those of others throughout the test frequency, which indicates that the result of the Cole–Cole curve analysis is correct. In addition, the polarization loss is obviously larger than the conduction loss, indicating that the EMW absorption of CoFe-ZIF/CNFs is dominated by polarization loss and supplemented by conduction loss.

Fig. 5 Cole–Cole and C₀ curves of CoFe-ZIF/CNFs.

Alloyed CoFe nanocages introduce magnetic loss to carbon fibers, enhancing the microwave absorption properties of the hollow alloyed CoFe-ZIF/CNF composite nanofibers. In the 2.00–18.00 GHz frequency band we measured, the natural/exchange resonance and eddy current loss are the main magnetic loss forms. The specific loss properties can be determined by the change of the C₀ value with frequency. Fig. 5f shows the variation of the C₀ value of the CoFe-ZIF/CNF composites with frequency. Then in this frequency band, eddy current loss is the main provider of magnetic loss, and no matter how the frequency fluctuates, the value of C₀ will be relatively stable or unchanged.⁴⁴ There is a significant change in the C₀ value in the range 2.00–8.00 GHz, which indicates that the magnetic loss of the composites in this band is natural resonance. It can be observed that the strongest fluctuation of CoFe-ZIF/CNF (60%) in the range 2.00–8.00 GHz is due to the higher content of alloyed CoFe nanocages, which have a stronger natural resonance. There is almost no change in C₀ between 8.00 and 14.00 GHz, which means that the eddy current loss is the main magnetic loss mechanism in this band. The obvious resonance peak on the C₀–f curve of the CoFe-ZIF/CNF composites in the range 14.00–18.00 GHz is attributed to the exchange resonance of magnetic loss.

The performance of absorbers in EMW absorption is evaluated by RL, matching thickness and EAB. Based on the transmission line theory, the RL value of the hollow alloyed CoFe-ZIF/CNF composite nanofibers can be calculated using the following formulas:⁴⁵

Notice that we consider those frequency bands where the RL value is less than −10 dB because these frequency bands mean that the EMW absorbing material can effectively absorb 90% of the incident EMW. Fig. 6 shows 3D and 2D RL diagrams and 2D RL curves of the hollow alloyed CoFe-ZIF/CNF composite nanofibers in the range 2.00–18.00 GHz. The RL_min value of CoFe-ZIF/CNF (20%) is −16.87 GHz at 17.00 GHz, and the widest EAB is 4.40 GHz. The RL_min value of CoFe-ZIF/CNF (30%) reaches −59.61 dB when the matching thickness is 3.46 mm. With a matching thickness of only 2.02 mm, the widest EAB achieved by CoFe-ZIF/CNF (30%) is 6.64 GHz (11.36 GHz-18.00 GHz). The strong EMW loss ability of CoFe-ZIF/CNF (30%) can be attributed to strong interfacial polarization caused by multiple heterointerfaces between CoFe-ZIFs and CNFs and the enhanced magnetic loss caused by CoFe-ZIFs, as well as the synergistic effect of dielectric–magnetic loss. The RL_min values of CoFe-ZIF/CNF (40%), CoFe-ZIF/CNF (50%) and CoFe-ZIF/CNF (60%) are 26.59 dB (1.84 mm), 27.30 dB (1.83 mm) and 22.47 dB (1.82 mm), respectively, with the widest EAB values of 5.52 GHz (12.48–18.00 GHz), 6.08 GHz (11.92–18.00 GHz) and 5.12 GHz (12.88–18.00 GHz), respectively. To sum up, CoFe-ZIF/CNF (30%) has a low RL_min value, wide EAB and low filling rate, indicating that the cooperation of the alloyed CoFe nanocages and carbon fibers can result in excellent EMW attenuation and lightweight properties at the same time.

Fig. 6 RL versus frequency and thickness for (a) CoFe-ZIF/CNF (20%), (b) CoFe-ZIF/CNF (30%), (c) CoFe-ZIF/CNF (40%), (d) CoFe-ZIF/CNF (50%), and (e) CoFe-ZIF/CNF (60%).

An ideal EMW absorbing material should meet the requirements of excellent impedance matching and strong attenuation ability. When the EMW reaches the surface of the absorbing material, appropriate impedance matching can make more EMW enter the absorbing material, and the strong attenuation ability further ensures the maximum dissipation of the incident EMW.⁴⁶ According to the equation:^47,48

If the Z value is close to 1, most of the EMWs from the air enter the hollow alloyed CoFe-ZIF/CNF composite nanofibers without being reflected.⁴⁹ As shown in Fig. 7a–e, Z values in the range of 2.00–18.00 GHz and 1.00–5.00 mm were calculated, where the Z values between 0.80 and 1.20 are indicated. It can be seen that samples CoFe-ZIF/CNF (20%), CoFe-ZIF/CNF (40%), CoFe-ZIF/CNF (50%), and CoFe-ZIF/CNF (60%) have two relatively narrow sections in the area with good impedance matching, while CoFe-ZIF/CNF (30%) has good impedance matching in the larger area in the middle, which is consistent with the strongest EMW absorption capacity of CoFe-ZIF/CNF (30%).⁵⁰ The delta function can also be used to calculate the impedance matching degree of the electromagnetic wave absorber surface, and the formulae are given in eqn (S1), (S2) and (S3).†^27,42,51,52 The CoFe-ZIF/CNFs have better impedance matching characteristics in the range 2.00–18.00 GHz, which is confirmed by the calculated delta |Δ| values (Fig. S2†).

Fig. 7 (a–e) 2D |Z_in/Z₀| plots and (f) α curves of CoFe-ZIF/CNFs.

The higher the attenuation constant (α), the stronger the attenuation ability of EMWs. The value of α is expressed by the following formula:^53,54

As shown in Fig. 7f, the α value of the composite increases with the increase of frequency. The α value of CoFe-ZIF/CNF (30%) is higher than those of other materials in the whole frequency, which means that CoFe-ZIF/CNF (30%) has strong EMW attenuation characteristics. Compared with other samples, CoFe-ZIF/CNF (30%) has the highest attenuation ability and excellent impedance matching. The synergistic effect of these two makes CoFe-ZIF/CNF (30%) have excellent electromagnetic wave absorption performance.

According to the above analysis of the absorption mechanism, Fig. 8a shows the EMW attenuation mechanism of CoFe-ZIF/CNF (30%). Firstly, the conductive network established by the CNF with a high aspect ratio in the hollow alloyed CoFe-ZIF/CNF composite nanofibers enriches the conduction path, which is beneficial for the axial migration of electrons and the electron jump between adjacent fibers, resulting in the enhancement of conduction loss of the hollow alloyed CoFe-ZIF/CNF composite nanofibers.⁵⁵ At the same time, the established micro-scale conductive network responds to the incident EMW and induces the current, thus attenuating the incident EMW and converting it into thermal energy.¹⁴ Secondly, the combination of alloy CoFe nanocages and carbon nanofibers constructs substantial heterogeneous interfaces. CoFe-ZIF/CNF (30%) has a variety of heterogeneous interfaces (such as CoFe-ZIFs//CNF, CoFe-ZIFs//air, air//CNF and Co//Fe), which significantly improves the interface polarization.^56,57 Due to the differences in electromagnetic properties such as conductivity and dielectric constant between the two phases at the heterogeneous interface, under the action of an alternating electric field, charges will accumulate at the interface and change their distribution, forming polarization.⁵⁸ Because of the time needed for the charge to move, polarization lags the electric field changes, resulting in energy loss.²⁶ Thirdly, after pyrolysis, CoFe-ZIF/CNF (30%) is obtained, which contains abundant nitrogen atoms, oxygen-containing groups, carbon structural defects, and CoFe with different electronegativity as polarization centers, forming dipole polarization.⁵⁹ When subjected to an alternating electric field, the electric dipole tries to rotate with the orientation of the electric field. However, due to the internal friction and damping of the material, the rotation lags the change of the electric field, and the consumed electromagnetic energy is converted into thermal energy.⁶⁰ Finally, the coupling effect of magnetic particles Co and Fe provides eddy current loss, exchange resonance and natural resonance for the CoFe-ZIF/CNF (30%) composite,⁶¹ which improves the impedance matching of the hollow alloyed CoFe-ZIF/CNF composite nanofibers and realizes the synergistic effect of magnetic loss and dielectric loss. Based on the synergistic effect of the above loss mechanisms, CoFe-ZIF/CNF (30%) shows excellent wave-absorption performance.

Fig. 8 (a) Mechanism of EMW attenuation of CoFe-ZIF/CNF (30%), (b) simulation curves of the RCS values (−60° < θ < 60°), and (c) RCS reduction values obtained by subtracting the sample coatings with PEC.

In addition to the RL value obtained from the transmission line equation, RCS is another important indicator to characterize the absorption characteristics of absorbing materials in practical applications.^62–64 To evaluate the effect of the ratio of CoFe-ZIFs to CNF on the RCS of composites, the CST simulation of PEC covered with the hollow alloyed CoFe-ZIF/CNF composite nanofibers was carried out, and the corresponding data are shown in Fig. 8b and c. Fig. 8b shows the RCS simulation curve of PEC and CoFe-ZIF/CNF composites. It is not difficult to see that there are obvious differences in the radar wave scattering signals between pure PEC and CoFe-ZIF/CNF composite materials. The scattering signal of the PEC board is significantly higher than that of the hollow alloyed CoFe-ZIF/CNF composite nanofibers, while the scattering signal of CoFe-ZIF/CNF (30%) is the lowest, indicating that it reduces the reflection of the incident EMW. Apparently, the CoFe-ZIF/CNF (30%) coating significantly reduces the radar signal, which results in CoFe-ZIF/CNF (30%) having a smaller RCS value while exhibiting more effective electromagnetic wave absorption. The RCS of CoFe-ZIF/CNF (30%) decreases by 25.31 dB m² as shown in Fig. 8c when the incident angle is 45°. Fig. S3† shows the 3D radar wave scattering signals of PEC and CoFe-ZIF/CNFs. Obviously, the scattering signal of the PEC model is the strongest, and the PEC scattering signal when covered with CoFe-ZIF/CNF (30%) is the weakest, indicating that the RCS value is the smallest and the EMW attenuation ability is stronger. The results of RCS analysis are in good agreement with the excellent EMW absorption properties of CoFe-ZIF/CNF (30%) observed in Fig. 6, which confirms the great potential of the composite in practical applications.

4. Conclusions

In this work, we successfully prepared alloy CoFe nanocage/CNF composite fibers with light weight and excellent EMW absorption performance through a simple and efficient strategy. The alloy CoFe nanocages with CoFe coupling magnetic loss and a hollow structure and carbon fibers with a large number of defect sites, conductive networks and polarization relaxation achieve magneto-electric synergy, which makes the composites have excellent impedance matching and EMW absorption performance. Benefiting from the synergistic advantage, the synthesized magnetic carbon fiber shows excellent EMW attenuation performance at a low filler content of 10 wt%. The maximum absorption intensity at 3.46 mm is up to −59.61 dB, and the effective bandwidth is as high as 6.64 GHz at 2.02 mm. In addition, the maximum RCS reduction of CoFe-ZIF/CNF (30%) relative to PEC is 25.31 dB m² under far field conditions, which means that the prepared composites are very promising in practical applications. This study shows that electromagnetic wave absorbing materials combined with an MOF and carbon fiber have the potential to become high-performance electromagnetic wave absorbing materials.

Data availability

Data will be made available on request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 52102036) and the Sichuan Science and Technology Program (2021JDRC0099).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5qi00118h

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