A Unique Dual-Shell Structure with Highly Active Ni@SiC/CNT/CNF Microwave Catalysts

Abstract

Microwave-assisted catalytic oxidation (MACO) is a novel wastewater treatment technology for the efficient treatment degradation of organic wastewater. However, a single carbon material or SiC has limited absorption of electromagnetic waves, and the efficiency of using it as a microwave-assisted organic catalyst is not satisfactory. To improve the absorption and microwave-assisted degradation performance of carbon matrix composites, a new carbon magnetic composite Ni@SiC/CNT/CNF microwave catalyst is constructed. By controlling the introduction of nickel, different numbers of carbon nanotubes are grown on the surface of carbon nanofibers, and C and SiC double-shell structures were formed on the top of the carbon nanotubes, which catalyzed the generation of active groups by the thermal effect generated by the plasma discharge under the action of microwave field, thus realizing the highly efficient catalytic degradation of wastewater dyes. The results show that the Ni@SiC/CNT/CNF with the lowest reflection loss of RL_min = −9.26 dB exhibit excellent degradation capabilities with a degradation efficiency of 99.9% for methylene blue within 90 s under 450 W microwave irradiation.

1. Introduction

With the rapid development of human society, the economy, and industrialization, the arbitrary discharge of dye wastewater, phenol and phenol derivative wastewater, surfactant wastewater [1,2,3,4,5], antibiotic wastewater, and pesticide wastewater is becoming more and more serious [6,7,8], and the pollution caused by these toxic and hazardous wastewaters poses a serious threat to environmental, green, and sustainable development. While posing a serious threat to human health, wastewater pollution has also caused damage to agriculture, industry, livestock, farming, and other industries and other organisms [9,10]. Therefore, the development of novel catalysts capable of rapid and efficient degradation of dye wastewater with recycling and low cost has become a priority.

Recently, the use of microwave radiation to accelerate the degradation of organic dyes in wastewater has received widespread attention. Microwave radiation can significantly increase the rate of degradation reactions, which cannot be achieved with conventional degradation methods [11,12,13,14,15]. Compared with conventional heating degradation methods, microwave radiation treatment of wastewater has the advantages of high efficiency, short reaction time, low cost, and no secondary pollution. The application of microwave catalysis technology in wastewater treatment provides a feasible method for the subsequent development of efficient catalysts [16,17,18,19,20,21]. Cai et al. [22] reported that a bifunctional Fe/Fe₃C@C microwave catalyst with magnetic properties was successfully prepared using the sol–gel method, and the degradation and removal rate of methyl orange reached 100% within 30 s under microwave irradiation, which realized the high-efficiency degradation of methyl orange wastewater dye. To date, several studies have reported the use of microwave technology to treat wastewater: microwave-induced catalytic degradation (MICD), microwave-enhanced catalytic degradation (MECD), and microwave-assisted catalytic degradation (MACD) [23,24,25,26]. Although the above microwave technologies (MICD, MECD, and MACD) for wastewater treatment have high efficiency, all of these methods require the addition of additional catalysts or inducers during the treatment of wastewater; thus, the development of catalyst materials with excellent wave-absorbing properties, high catalytic efficiency, and structural stability has become a hot topic of current research. Tian et al. [27] synthesized Co₃O₄ microwave catalysts with near-spherical morphology using a two-step method of hydrothermal treatment and calcination. The degradation rate of 100 mg of Co₃O₄ on 50 mL of 100 mg·L⁻¹ tetracycline reached more than 95% under microwave irradiation of 800 W, and complete degradation could be realized within 15–20 min.

Carbon materials have excellent electrical conductivity, significant microwave absorption, and good corrosion resistance. Most carbon-based catalysts are thermally stable and do not lose catalytic activity as the reaction temperature increases, thus effectively preventing localized high temperatures due to hot spots in the microwave reaction [28]. Carbon nanotubes (CNTs), as a typical carbon nanomaterial, have excellent electron mobility and ideal thermal conductivity. Compared with other microwave-absorbing materials, the excellent dielectric properties of CNTs make them pivotal in the field of microwave absorption [29,30]. However, the severe impedance mismatch and a single loss mechanism just for CNTs alone as microwave absorbers can greatly reduce their microwave absorption performance, limiting their applications in related fields [31]. To solve this problem, there is mature research to composite CNTs with some metal magnetic materials to construct multi-system, multi-interface carbon magnetic composites. This method can not only solve the defect of the impedance mismatch of single carbon materials but also expand the overall loss mechanism of the composite material, which can significantly improve the microwave absorption as well as the catalytic degradation ability of the material [32,33,34]. Transition metals, as important wave-absorbing materials, have the advantages of high permeability, saturation magnetization, and snooker limit, and combining them with dielectric materials C or SiC can make up for their respective deficiencies, satisfy the impedance matching, and improve the absorption and utilization of microwaves by dielectric materials, which is an ideal complementary material [35,36]. Li et al. [37] investigated the effect of different Ni contents on the microwave absorption properties of Ni@C composites. The results showed that the Ni@C composites embodied the best microwave absorption performance when the Ni content was increased, with the lowest value of RL_min in the range of 5 mm being −58.7 dB for a low thickness of 1.66 mm. The excellent performance was attributed to the fact that the composites with high metallic Ni content had relatively small grain boundary areas, providing higher magnetic losses, impedance matching, and attenuation coefficient. In addition, magnetic materials under the action of microwaves can provide materials with higher magnetic loss, when microwave irradiation to metal particles produces a plasma discharge effect, resulting in local high temperatures to accelerate the formation of hot spots and improve the degradation rate of organic matter. Zhou et al. investigated and characterized the factors affecting the plasma discharge density produced by the microwave irradiation of metals, such as metal type, metal mass, and metal size. The results showed that when pure liquid paraffin oil was put into a microwave, the temperature only slightly increased. When metal was added to the liquid paraffin oil, the microwave- and metal-induced discharges led to a significant increase in the temperature of the liquid paraffin, and the discharge process was accompanied by a large amount of heat release [38,39].

In this work, a multi-system Ni@SiC/CNT/CNF microwave catalyst is constructed using an electrostatic spinning process. By controlling the Ni content in the composite, a unique double-shell structure with a double cladding layer was catalyzed at the top of CNTs. This structure generates plasma discharge under microwave irradiation for efficient degradation of wastewater dyes. The morphology, microstructure, and physical phase composition of the Ni@SiC/CNT/CNF are characterized by means of complementary analysis. The microwave catalytic performance of Ni@SiC/CNT/CNF is evaluated with methylene blue as the target containment. A synergistic mechanism for the degradation of methylene blue dye wastewater by Ni@SiC/CNT/CNF catalysts under microwave irradiation is proposed.

2. Results

2.1. Microscopic Morphology and Structural Composition of Ni@SiC/CNT/CNF Composites

The preparation methods of Ni@SiC/CNT/CNF composites mainly include the formulation of precursor liquid, electrostatic spinning, pre-oxidation, low-temperature heat treatment, and high-temperature carbonization. Among them, pre-oxidation can make the initially spun fibers obtain a certain degree of flexibility and maintain the fiber morphology, while final high-temperature carbonization promotes the further transformation of the PCS precursor into SiC.

Figure 1 shows the XRD patterns and Raman spectra of Ni@SiC/CNT/CNF (0.15, 0.30 g) composites with different Ni contents (hereafter abbreviated as NSCT15 and NSCT30). As can be seen in Figure 1a, Ni in the composites corresponds to the (121), (002), (311), and (320) crystal planes at 2θ = 45.5°, 48.9°, 49.2°, and 53.4°, and exhibits the (002) crystal plane for C and the (111) crystal plane for SiC at 26.2° and 35.6°. The crystal sizes of the corresponding C, SiC, and Ni diffraction peaks are 11.7 nm, 36.6 nm, and 24.6 nm, respectively. The intensity of the C diffraction peak becomes sharp with the increase in Ni content, which indicates that the metal Ni has a significant effect on the catalytic graphitization of the composites. In addition, the diffraction peaks of SiC changed significantly with the increase in Ni content, and the peak intensity of SiC increased significantly when the Ni content of the precursor solution was increased from 0.15 g to 0.30 g. This indicates that metal Ni has a significant effect on the catalytic graphitizing of the composites [40]. The peak intensity of SiC increased significantly with the increase in Ni content in the precursor solution. At high temperatures, C and Si in the composites are solidly dissolved into molten Ni particles, and when the heat treatment temperature reaches a certain point, C and Si react to form SiC.

Figure 1b,c show the Raman spectra of NSCT15 and NSCT30. As seen from the figure, the I_D/I_G values of NSCT30 are lower than those of NSCT15, which are 0.694 and 0.838, respectively, and the increase in Ni content leads to a significant decrease in the I_D/I_G values of the composites, which further confirms that the increase in Ni has a significant effect on the catalytic graphitization of the composites. Here, the intensity ratio (I_D/I_G) of NSCT30 is 0.694, indicating a high degree of graphitization of NSCT30, which is consistent with the XRD results. The higher degree of graphitization can make the composites have a higher dielectric loss and enhance their microwave absorption capacity [41].

X-ray photoelectron spectroscopy (XPS) results were further analyzed to reveal the chemical composition and atomic states of the NSCT15 composites, as shown in Figure 2. As shown in Figure 2a, the elemental full spectrum of NSCT15 demonstrates that the composite has the presence of C, Si, Ni, and O elements. The C element is derived from C_f, Ni is derived from C₄H₆NiO₄-4H₂O, and Si is derived from the high-temperature cracking of PCS. Figure 2b shows the high-resolution spectrum of C 1s of the composite, with three peaks at 283.5, 284.6, and 285.8 eV, which are C-Si, C-C, and C-O peaks, respectively [42,43]. Figure 2c shows the Si 2p spectrogram, which can be fitted as two peaks with typical binding energies of 101.4 and 103.4 eV, which can be indexed as Si-C and Si-O bonds, respectively [44,45]. The peaks retrieved from the Ni 2p spectrogram at 853.1 eV and 873.2 eV belong to Ni 2p3/2 and Ni 2p1/2 [46], respectively, as shown in Figure 2d. The relatively low Ni elemental strength of the composites may be because most of the Ni particles on the surface are covered by C and SiC layers, which also reinforces the cladding structure on the surface of Ni particles.

The microstructural photographs of NSCT15 and NSCT30 are shown in Figure 3. As shown in Figure 3a,b,d,e, the fiber bodies all exhibit a continuous network structure, which suggests that the micro-morphology of the fiber bodies is not greatly affected by the increase in Ni. However, it is worth noting that the increase in Ni content leads to a significant increase in the number of vertically grown CNTs on the surface of the fiber body, and the number of CNTs on the surface of NSCT30 fibers is significantly higher than that of NSCT15. In other words, Ni acts as a catalyst for the growth of CNTs, and when the Ni content increases, it can catalyze the growth of CNTs accordingly. Similarly, the Ni particles of both composites are present at the top of the CNTs. The growth of the CNTs in the C_f body enriches the composite interface of the composites, which is conducive to the enhancement of the interfacial polarization of the materials and the broadening of polarization paths of the composites. The growth of CNTs in the C_f host enriches the composite interface, enhances the interfacial polarization, and broadens the polarisation path of the composites, thus providing excellent microwave-catalytic degradation capability.

The microstructure of NSCT30 composites was further analyzed using TEM. Figure 4a shows that the number of CNTs grown on the C_f surface in NSCT30 significantly exceeds that of NSCT15, which is consistent with the SEM results. The TEM image of NSCT15 is shown in Figure S1. In the TEM image of NSCT15, the diameter of the CNTs is about 200 nm, there are black Ni particles at the top of the CNTs, and there is an obvious covering layer around the Ni particles, with a thickness of about 5 nm and a diameter of about 50 nm, which is in agreement with the previous SEM results. To further reveal the internal structure of the particulate matter at the top of the CNT, high-resolution TEM images (HR-TEM) were taken, as shown in Figure 4c. Lattice fringes of about 0.34 nm, 0.25 nm, and 0.18 nm corresponding to the C (002), SiC (111), and Ni (002) crystal faces were derived from the measurement of the crystal face spacing, and a thin layer of SiC was deposited on the surface of the Ni particles, followed by a graphitic C layer, whereas there were curved and discontinuous flakes outside the C layer [47,48], and a high number of defects were generated at these locations. Defect-induced inhomogeneous charge distribution promotes dipole production, which favors the loss of electromagnetic waves. In addition, this fused-clad structure not only effectively protects the metal particles from corrosion in harsh environments [49,50] but also increases the interfacial layer between the materials and increases the interfacial polarization of the materials.

To further confirm the specific composition of the Ni particles, we performed EDS energy spectroscopy analysis, as shown in Figure 4d. The results show that the major elements of the particulate matter are Ni and Si, and the atomic ratio of Si is 90.83%, which is less compared to NSCT15. This is because when the total Si content in the composite system is constant, the solid solution of Si into the unit Ni particles becomes less when the number of Ni particles increases. Although the Si content of individual Ni particles is relatively reduced, they are still able to saturate with Ni solid solution first, and eventually, the excess Si precipitates out and reacts with C on the surface of Ni particles to form SiC.

Through the above analysis, the formation mechanism of SiC and C coatings is proposed, as shown in Figure 5. At high temperatures, C and Si will be solidly dissolved into the molten Ni particles; the graphite C layer is preferentially precipitated at a low temperature of 700 °C, encapsulated on the surface of Ni particles. Subsequently, Si will form a SiNi alloy with molten Ni after the temperature reaches 1300 °C [47,48] and then precipitate on the surface of Ni particles after Si reaches supersaturation in Ni particles, and after further reaction with C, a SiC cladding layer forms.

2.2. Degradation of Methylene Blue Wastewater Dye

Prior to the microwave-catalyzed degradation of MB, the samples were first immersed in MB solution and placed in a dark environment for 30 min for the dark reaction to reach adsorption equilibrium.

By comparing the absorption peaks at 664 nm of both NSCT30 and NSCT15 catalysts, as shown in Figure 6a,b, we determined the UV-vis spectra of both catalysts. It can be seen that the adsorption peak at 664 nm of the NSCT30 catalyst decreases significantly with the increase in time, while the absorption peak at 664 nm of the NSCT15 catalyst decreases slowly, indicating that the degradation rate of NSCT15 is slower compared to that of NSCT30 under the same experimental conditions. In addition to this, in order to evaluate the plasma discharge phenomenon generated by the composites during microwave catalysis, the strength of the catalyst plasma discharge phenomenon was evaluated by observing the degree of continuity of the sparks and the area of the sparks generated by the MB solutions with the addition of the composites with different Ni contents under microwave, as shown in Figure 7. The experimental phenomenon showed that when the Ni content is 0.15 g, the sound of the solution is intermittent and the sparks in the solution are also intermittent, which indicates that the plasma discharge phenomenon of NSCT15 with a lower Ni content is weaker, as in Figure 7a. When the Ni content is increased to 0.30 g, the sound similar to being ionized from the solution obviously becomes continuous and the sparks in the solution have a larger area, indicating that the plasma discharge phenomenon is obviously strengthened, and the organic wastewater MB reaches boiling in a shorter time, indicating that the NSCT30 composite material has a strong thermal effect in the microwave field, which promotes a rapid increase in the temperature of the system, as shown in Figure 7b.

Figure 8a shows the degradation efficiency curves of the composites for MB under microwave radiation. The degradation efficiencies of NSCT15 and NSCT30 were determined to be 19.8% and 99.9%, respectively, after 90 s of reaction under microwave radiation. The microwave-catalyzed degradation performance of NSCT30 is higher than that of NSCT15, which can efficiently degrade methylene blue within 90 s. Based on the analysis of the morphology and physical phase of the composites, the excellent catalytic degradation performance of NSCT30 is attributed to the presence of more CNTs in the composites as well as the presence of a unique double-layered core-shell structure at the top of the composites, which brings about a rich interface to significantly enhance the interfacial polarization of the composites. Moreover, the strong plasma discharge can generate more reactive groups for the efficient degradation of methylene blue [51]. In contrast, NSCT15 has a lower number of CNTs grown due to the lower nickel content, and the number of top core-shell structures is not ideal, so its interfacial polarization and plasma discharge phenomenon are weakened, and the degradation ability of methylene blue is reduced. The presence of the SiC layer in the double-shell structure not only gives the material more interfacial polarization but also acts as a polar molecule to produce dipole polarization under the action of microwaves, and the polarization phenomenon under the action of microwaves promotes the catalyst’s absorption and transformation of electromagnetic waves and enhances the thermal effect in the reaction. NSCT30 has the strongest plasma discharge phenomenon and thermal effect, which can significantly promote the chemical reaction of the system under the action of the microwave field. NSCT30 has the strongest plasma discharge phenomenon and thermal effect, which can significantly promote the degradation ability of the system under a microwave field. The degradation kinetics of MB degradation with different catalysts in 90 s is plotted in Figure 8b, and all of the points are linear, which indicates that the degradation kinetics of MB degradation with two catalysts under microwave conditions in 90 s belongs to the first-order reaction kinetics. The microwave-induced MB degradation rate constants of different catalysts are shown in Figure 8c, and the k of NSCT15 and NSCT30 is 0.00254 mol·L⁻¹·min⁻¹ and 0.76005 mol·L⁻¹·min⁻¹, respectively. The above experimental results show that NSCT30 has a stronger MW degradation ability.

2.3. Electromagnetic Wave Absorption Properties of NSCT Composite Materials

The electromagnetic wave absorption characteristics of composites are mainly determined by electromagnetic parameters, including dielectric loss and magnetic loss. Figure 9a,b shows the ε′ and ε″ curves of the complex permittivity of NSCT15 and NSCT30, which indicate that ε′ and ε″ are inversely proportional to the frequency in the investigated frequency range of 2.0–18.0 GHz due to the dispersion effect, which is mainly related to the polarization mechanism of the materials (atomic, electronic, and dipole polarization) [52,53]. The ε′ decreases from an initial 44.01 to 25.73 for NSCT15 and from an initial 32.40 to 19.46 for NSCT30. The magnitude of the real part of the complex permittivity ε′ for both catalysts is in the order of NSCT15 > NSCT30 throughout the frequency band. It can be seen from Figure 9b that the variation in the imaginary part of the curves of the complex dielectric constants of the two catalysts is similar to that of the real part, which also shows a decreasing trend. And the ε value of NSCT30 is higher than that of NSCT15, implying that it has a better dielectric loss to electromagnetic waves.

The μ′ of magnetic permeability also shows a decreasing trend with increasing frequency and the real part values are close to each other for both catalysts, as shown in Figure 9d. The order of the size of the real part of the composites in the low-frequency band of 2–13 GHz is NSCT30 > NSCT15, which indicates that the Ni content is positively related to the real part of the magnetic permeability μ′, while in the high-frequency band of 13–18 GHz, the real part of NSCT15 is significantly larger than that of NSCT30, showing an inverse trend. Figure 9e demonstrates that there is up-and-down fluctuation in the μ″ of the magnetic permeability of the composite as the frequency increases, with a resonance peak at a frequency of 11.3 GHz and a significant increase in the intensity of the resonance peak at 11.3 GHz for NSCT30. The tanδ_ε curves of NSCT15 and NSCT30, as shown in Figure 9c. From the figure, it can be observed that the dielectric loss angle tangent tanδ_ε of NSCT30 is much larger than that of NSCT15 in the 2–18 GHz band, which indicates the excellent dielectric loss capability of NSCT30. Figure 9f exhibits the tanδ_μ curves of NSCT15 and NSCT30, from which it can be seen that the tanδ_μ curves show a similar trend to the imaginary part of the magnetic permeability, which indicates that the magnetic energy dissipation capability is the key factor of the magnetic loss, while the magnetic loss appears to peak at high frequencies, indicating that the natural resonance and exchange resonance dominate the magnetic loss process. By comparing the values of tanδ_ε and tanδ_μ, tanδε is significantly larger than tanδ_μ. This indicates that the dielectric loss is the main contribution to the excellent microwave absorption performance compared to the magnetic loss.

To further investigate the effect of dielectric and magnetic losses of the composites on the material properties, the Cole–Cole curves of the samples were analyzed as shown in Figure 10. Each semicircle represents a Debye relaxation process and the number and radius of semicircles are positively correlated with the degree of material surface polarisation. Compared to Figure 10a,b, NSCT15 and NSCT30 have four and five circles of different sizes, respectively, and the semicircles are twisted, indicating the presence of other mechanisms such as conductive loss and interfacial polarization. The NSCT30 composite has the highest number of semicircles, which indicates that it has the highest degree of polarization. In addition, there are almost no straight lines at the end of the Cole–Cole plot [54], indicating that there is almost no conductive loss throughout the process.

Figure 11 shows the reflection loss (RL) patterns of NSCT15 and NSCT30 in the frequency range of 2–18 GHz. Through comparison, it can be seen that the reflection loss of NSCT15 and NSCT30 is −6.67 dB and −9.26 dB, respectively, and the reflection loss of NSCT30 is larger than that of NSCT15, and the frequency corresponding to its maximum loss is the smallest, and the RL_min is −9.26 dB at 12.5 GHz, which shows good electromagnetic wave absorption performance. Moreover, −9.26 dB at 12.5 GHz shows good electromagnetic wave absorption performance.

Based on previous analyses, the mechanism of efficient degradation of methylene blue by Ni@SiC/CNTs/CNFs under microwave irradiation can be summarized in several forms, as shown in Figure 12. Firstly, the three-dimensional network structure formed by interwoven carbon nanofibers increases the multiple reflections after microwave incidence, and the growth of a large number of one-dimensional carbon nanotube structures effectively lengthens the electron conduction paths, resulting in the composites having good microwave absorption properties. Secondly, the bilayer core-shell structure present at the tip of carbon nanotubes has a small number of defects at the graded interfaces, and under microwave irradiation, inhomogeneous electrons accumulate at the heterogeneous interfaces to produce dipole polarization. Thirdly, the phase interface existing between CNTs and carbon nanofibers generates more interfacial polarization and multiple relaxation. The heat brought by dipole polarization and interfacial polarization will put the electrons in an unstable state, and the hole–electron pairs generated by the electrons undergoing leaps will react with O₂ and H₂O to generate reactive substances such as ·OH, O₂^−·, thus promoting the degradation of methylene blue. Finally, microwave irradiation of metals produces a unique plasma discharge, which, as the plasma is released, causes high localized temperatures in the degradation system to facilitate microwave heating [55,56,57]. However, the unique selective heating of the microwave results in a temperature gradient between the different components of the material, which creates a hotspot and accelerates the reaction of the system while generating reactive groups such as ·OH, O₂⁻ and H⁺, which enable the efficient degradation of methylene blue.

3. Materials and Methods

3.1. Preparation of Ni@SiC/CNT/CNF Composites

In this paper, Ni@SiC/CNT/CNF composites (SiC is silicon carbide, CNFs are nanofibers, and CNTs are carbon nanotubes) were prepared via electrostatic spinning combined with a heat treatment carbonization process. First, 0.16 g of PCS (polycarbon silane), 1.00 g of PAN (polyacrylonitrile), a quantitative amount of C₄H₆NiO₄·4H₂O (Nickel acetate tetrahydrate), and 0.15 g of CH₄N₂S (thiourea) were mixed and dissolved in 10 mL of DMF (dimethylformamide) to form precursor solutions. The precursor solution was magnetically stirred at room temperature for 24 h to ensure uniform and stable dispersion. Subsequently, the prepared precursor solution was loaded into a 10 mL plastic syringe with an injection flow rate of 7 μL/min, a distance of 17 cm between the needle and the receiver (the “receiver” is the aluminum foil, on which we perform all of the spinning), a rotational speed of 600 r/min, and a voltage of 17 kV, and the primed fibers were obtained at the end of spinning. To obtain a certain flexibility as well as to maintain the fiber morphology, the primary spun fibers were pre-oxidized in air at 280 °C for 2 h. The oxidized fibers were then subjected to low-temperature carbonization at 700 °C for 1 h under N₂ atmosphere. Finally, the spun fibers were subjected to high-temperature carbonization at 1300 °C in an Ar environment to obtain Ni@SiC/CNT/CNF composites. Figure 13 illustrates a flow chart of the preparation of the material.

3.2. Characterization

The crystal structure and phase composition of the samples were analyzed using a powder X-ray diffractometer (XRD, Rigaku D/max-2200PC, Tokyo, Japan) under Cu Kα (λ = 0.15418 nm) radiation. The microscopic morphology of the samples was observed using a field emission scanning electron microscope (Hitachi, Chiyoda City, Japan, FE-SEM, S-4800).

X-ray photoelectron spectroscopy (XPS) was used to qualitatively and quantitatively analyze the elements contained in the sample to determine the surface composition and chemical state of the sample.

High-resolution transmission electron microscopy (HRTEM) combined with energy dispersive spectroscopy (EDS) observation was performed on a Thermo Fisher Scientific, Waltham, MA, USA FEI Tecnai G2 F20S-TWIN system at 200 kV.

The molecular structure of the prepared samples was analyzed using a Renishaw-invia Raman instrument manufactured by Renishaw (London, UK), with lasers: 532 nm; 785 nm, wavelength range: 100~4000 cm⁻¹, optical plate size: 1~2 μm, and power: 0.005%~100% adjustable.

The UV–vis absorption spectra of the degradation were recorded on a UV/vis/NI R Spectrophotometer (LAMBDA950, PerkinElmer, Shanghai, China).

The dielectric properties and magnetic permeability of the catalysts were analyzed using a vector network analyzer (Agilent 85071 E, Santa Clara, CA, USA). The electromagnetic wave absorption properties of the samples in the 2–18 GHz band were tested using the coaxial method.

3.3. Microwave Degradation

To examine the catalytic activity of the catalysts, degradation experiments were carried out using a controllable microwave oven (WD750B, Galanz Company, Foshan, China) equipped with a self-made glass reactor and a condensing tube. The procedure was as follows: a methylene blue (MB) solution was prepared at a concentration of 20 mg/L, 20 mL of methylene blue solution and 20 mg of catalyst were added to a 50 mL reaction vessel, and degradation experiments were carried out.

The Teflon reaction vessel was followed by microwave irradiation (450 W, 2450 MHz). At the desired reaction time, the mixture was cooled to room temperature. The reaction mixture was then analyzed to evaluate degradation efficiency using an ultraviolet spectrophotometer (UV-vis, UV-2450, SHIMADZU, Kyoto City, Japan). The removal rate of MB was calculated using the following formula:

$$\mathrm{Removal} \mathrm{rate}(\%)={\displaystyle \frac{C_0-C_t}{C_0}}\times 100\%$$

C₀ and C_t are the initial concentration and the concentration after treatment time t of MB solution, respectively.

4. Conclusions

In this study, we prepared samples by varying the amount of Ni introduced into the electrospinning precursor solutions NSCT15 and NSCT30. The influence of the microstructure of the composite material on its ability to degrade MB dye and microwave absorption characteristics was studied, and the following conclusions were drawn:

(1) As the Ni content increased, the number of CNTs on the fiber surface also increased, with lengths ranging from 200 to 250 nm. In addition, composite materials exhibited a dual-core shell structure in their microstructure;

(2) Compared with NSCT15, NSCT30 showed the best microwave absorption performance, with an RL_min of −9.26 dB (12.5 GHz/1.816 mm). Under microwave radiation with a power of 450 W, the degradation efficiency of the NSCT30 composite reached 99.9% of MB within 90 s, while the degradation rate of NSCT15 was 29.8%, which was relatively low. At the same time, the reaction rates of the two materials were 0.00254 mol·L⁻¹·min⁻¹ and 0.76005 mol·L⁻¹·min⁻¹, respectively. NSCT30 composites showed efficient degradation of MB dye;

(3) The excellent catalytic degradation performance of NSCT30 composites is attributed to their unique nuclear structure and high content of SiC layers. More SiC layers increase the dipole polarization and interfacial polarization of the composite, resulting in a higher thermal effect of the material in the microwave field, promoting the overall heating of the system. Meanwhile, the plasma discharge effect of the NSCT30 catalyst is the most significant. The synergistic thermal effect and plasma discharge effect accelerate the production of ·OH, ·O₂⁻, and H⁺ active groups and promote the degradation of organic matter.

In conclusion, the excellent microwave catalytic degradation performance of Ni@SiC/CNTs/CNFs can provide a new method for the application of carbon magnetic composites in wastewater treatment.