Research on two-dimensional FeSiCr/mesoporous MXene composites and their absorption properties

Abstract

This study constructed FeSiCr/mesoporous MXene composites via electrostatic adsorption, focusing on investigating the regulatory mechanism of MXene pore structure on the material's wave absorption properties. Adjusting the oxidant concentration led to abundant mesoporous voids on the MXene surface, with pores concentrated in the small pore size range, significantly increasing the heterogeneous interfaces and electromagnetic wave transmission pathways within the material. By increasing the mesopore density, the composite's microstructure was optimized, enhancing the interfacial compatibility and the synergistic interaction between the two materials. Combined with FeSiCr, this effectively balanced impedance matching and electromagnetic wave attenuation capabilities. The mesoporous MXene structure obtained by etching with 0.2 mmol L⁻¹ of oxidizing agent exhibited outstanding S-band performance, achieving an RL_min value of −66.41 dB. At a matched thickness of 1.85 mm, the effective absorption bandwidth (EAB) reached 10.46 GHz. Results indicate that the outstanding absorptive performance is primarily attributed to the impedance matching characteristics, providing a key technological strategy for developing high-performance, wideband microwave absorbers that are thin, lightweight, and robust.

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1. Introduction

With the rapid advancement of electronic information technology, the widespread use of electronic devices has led to increasingly severe electromagnetic radiation interference issues.^1–3 This interference can disrupt the normal operation of electronic devices, pose security threats to electronic information, and have adverse effects on human health.^4,5 To address these issues, microwave absorptive materials have emerged. Currently, metal-based microwave absorbers have attracted significant attention,^6,7 besides carbon-based,^8–10 ceramic-based^11,12 and polymeric absorbers.^13,14 Based on their operating frequency, they can also be categorized as low-frequency absorbers designed to absorb frequencies below 0.3 GHz¹⁵ and high-frequency absorptive materials for electromagnetic wave absorption in the GHz band.¹⁶ The rapid advancement of electronic information technology has brought significant attention to the research of high-frequency absorptive materials. Simultaneously, the distinct absorption mechanisms of these materials result in substantial variations in their effective absorption bands (EAB).^17,18 Dielectric loss materials generate high dielectric loss through conductive particles or polar molecules, exhibiting a broad absorption band and primarily operating in the high-frequency range.¹⁹ Magnetic-loss-type materials rely on hysteresis loss, natural resonance, or eddy current loss, exhibiting significant absorptive effects near specific low-frequency resonance regions.²⁰ Therefore, among various electromagnetic absorptive materials, composite materials have become a research hotspot due to their dual characteristics of magnetic loss and dielectric loss, coupled with a broad absorption bandwidth.^21,22

Among composite materials, FeSiCr alloys possess unique advantages, such as high magnetic permeability, high saturation magnetization, high Curie temperature, and low cost, making them highly promising electromagnetic absorbers.²³ Lu et al.²⁴ synthesized the composite material FeSiCr@MoS₂ by the in situ growth of two-dimensional MoS₂ on oxidized flake-like FeSiCr surfaces via a one-step hydrothermal process. The minimum reflection loss, RL_min, reached −67.76 dB at 13.92 GHz, with a thickness of 1.71 mm and an edge-of-band frequency (EAB) of 4.25 GHz. Wu et al.²⁵ prepared cobalt ferrite and flake-shaped FeSiCr alloy powders via mechanical ball milling and hydrothermal methods, achieving an RL_min of −41.04 dB at 4.8 GHz and an EAB_max of 5.4 GHz. Wan et al.²⁶ employed ball milling to fabricate NaOH-passivated multilayer FeSiCr@Fe₃O₄ composites, achieving an EAB_max spanning 9.36–18 GHz at 8.64 GHz and delivering an optimal reflection loss of −37.00 dB at 2.32 GHz and 4.3 mm thickness. However, FeSiCr alloys exhibit limitations, such as high density, narrow EAB, and large matching thickness, which contradict the thinness, light weight, and wide bandwidth characteristics of absorptive materials.^27,28 To compensate for these shortcomings, efforts are being made to develop composite systems by combining FeSiCr alloys with other materials to enhance and optimize their properties.

As emerging two-dimensional materials, MXenes exhibit high electrical conductivity, large specific surface area, and abundant surface functional groups.^29–32 Simultaneously etching two-dimensional MXenes into mesoporous structures generates abundant interfacial polarization at the mesoporous interfaces.^33,34 Mesopores of different sizes scatter and absorb electromagnetic waves at different frequencies, broadening the absorption frequency band.³⁵ Zhu et al. obtained porous MXene materials through H₂O₂ etching, achieving an RL_min of −63.31 dB and an EAB of 6.88 GHz at a matched thickness of 2.39 mm. Liu et al.²⁹ successfully synthesized MXene/MoS₂ folded microspheres via ultrasonic spray self-assembly, achieving an RL_min of −51.2 dB at 10.4 GHz. Le et al.³⁰ employed ultrasonic atomization technology to fabricate surface-wrinkled porous MXene@Fe₃O₄ magnetic microspheres, achieving an RL_min of −63.3 dB and an EAB_max of 5.2 GHz at a matching thickness of merely 1.8 mm. Previous studies indicate that mesoporous structures significantly influence microwave absorption properties due to multiple reflections and scattering of the incident microwaves.³⁶ Simultaneously, the high magnetic properties of FeSiCr and the high dielectric properties of MXenes stimulate impedance matching in the material, refining the absorber with distinct loss mechanism components.

This study employs a simple and versatile electrostatic adsorption method to fabricate an FeSiCr/mesoporous MXene composite. By adjusting the H₂O₂ concentration used to prepare the mesoporous MXene, the microstructure of the composite was optimized by increasing the defect density, thus enhancing interfacial compatibility and the synergistic interaction between the two materials. The uniquely structured FeSiCr/mesoporous MXene exhibited a reflection loss of −68.67 dB at a low thickness of 1.65 mm. The electromagnetic absorption mechanism of the composite was thoroughly investigated, including the influence of dielectric loss, magnetic loss, and interfacial polarization on its absorption performance.

2. Experimental methods

2.1 Materials

The FeSiCr in this work was prepared via the water atomization method we previously reported,³⁷ lithium fluoride (MacLean, analytical grade), hydrochloric acid (Xilong Science, analytical grade), hydrogen peroxide (Xilong Science, analytical grade), titanium aluminum carbide (Bohua, analytical grade), solid paraffin wax (Shanghai Sinopharm Group Co., Ltd) were obtained from commercial sources. All materials were utilized as received, without any purification treatment.

2.2 Preparation of FeSiCr/mesoporous Ti₃C₂T_x MXene composites

1. Preparation of few-layered mesoporous Ti₃C₂T_x MXene. All etching procedures in this study were conducted within a fume hood. Porous MXene was synthesized using a hydrogen peroxide (H₂O₂) chemical etching method. First, the prepared Ti₃C₂T_x MXene solid particles were added to a beaker containing 20 mL of deionized water. Mechanical stirring was performed for 20 min to completely dissolve the Ti₃C₂T_x MXene in deionized water, yielding a 5 mg mL⁻¹ Ti₃C₂T_x MXene colloidal solution. A 30% H₂O₂ solution was then added dropwise to the colloidal solution using a pipette, followed by mechanical stirring at room temperature for 1 h. Subsequently, 2 M hydrochloric acid was added to the mixture and stirred for 10 minutes to remove the byproduct TiO₂. Finally, the supernatant was washed several times with deionized water until the pH approaches neutrality, yielding an etched few-layered Ti₃C₂T_x MXene colloidal solution.

2. Ball milling, surface microoxidation of FeSiCr powder. FeSiCr alloy powder, with an average particle size of 100 µm and purity of 99% (5.3%-Si, 9.9%-Cr), was purchased from Wachovia. After 6 hours of ball milling (LSM-20B disc mill, Dongguan Langling Machinery Co., Ltd), the flaky FeSiCr alloy powder was obtained and then heated to 600 °C under an oxygen atmosphere.

3. Preparation of FeSiCr/mesoporous Ti₃C₂T_x MXene composite. Mechanically stir the colloidal solution of few-layer Ti₃C₂T_x MXene for 10 min, then add ATPES surface-modified micro-oxidized flaky FeSiCr magnetic microparticles, micro-oxidized flake-shaped FeSiCr magnetic micropowder. Mechanical stirring was continued for 30 min to coat the Ti₃C₂T_x MXene etched flakes onto the FeSiCr surface. After coating, the resulting product was freeze-dried at −60 °C for 48 h to obtain the FeSiCr/porous Ti₃C₂T_x MXene composite material. The mass of the MXene solid particles constituted 15% of the FeSiCr powder. The Ti₃C₂T_x MXene colloidal solution was supplemented with 30% H₂O₂ at concentrations of 0, 0.1, 0.2, and 0.3 mmol L⁻¹, designated as samples C₀, C₁, C₂, and C₃, respectively.

4. Characterization. The phase composition and crystal structure of the materials were analyzed using X-ray diffraction (XRD, Cu Kα radiation, ultra-high-resolution XRD, X’Pert PRO MPD) over the range of 5–90°. The microstructures were observed via scanning electron microscopy and energy-dispersive spectroscopy (SEM and EDS, FEI MLA650F, USA), as well as transmission electron microscopy (TEM, FEI Tecnai G2F 20). The chemical composition and valence states of the samples were analyzed using XPS (Thermo Scientific ESCALAB 250Xi). The electromagnetic parameters, complex permittivity (ε′, ε″), and complex magnetic permeability (μ′, μ″) were measured using a 1–18 GHz network analyzer (Agilent PNA-L5230C). The prepared powder was mixed with molten paraffin wax at a 1 : 1 mass ratio and compacted into sample rings with a thickness of 1–3 mm, an outer diameter of 7.0 mm, and an inner diameter of 3.0 mm.

3. Results and discussion

The XRD patterns of flake-like FeSiCr and the mesoporous MXene-FeSiCr composite are shown in Fig. 1(a), with the right panel displaying an enlarged view of the 2θ range from 5° to 12°. The diffraction peaks of FeSiCr appeared at 2θ = 44.84°, 65.36°, and 82.57°, corresponding to the (110), (200), and (211) crystal planes (JCPDS card no. 65-5584), confirming the successful preparation of single-phase CrFe₈Si. Samples C₀, C₁, C₂, and C₃ exhibited no characteristic peak at 2θ = 38.9°, corresponding to the (104) plane of Al in Ti₃AlC₂, indicating successful etching of the MAX interlayer. The enlarged view on the right shows the (002) plane peak at 2θ = 9.73° in Ti₃AlC₂ shifting to 2θ = 5.87° after etching. This shift results from increased interlayer spacing due to the breaking of Ti–Al metallic bonds between layers. Post-etching diffraction peaks exhibited reduced intensity and broadening, attributed to increased internal defects in the MXene following strong acid etching, which partially disrupts the crystal structure and crystallinity. Samples C₁, C₂, and C₃ showed lower peak intensities than sample C₀ because H₂O₂ oxidation partially converts the Ti atoms in MXene into TiO₂ particles. The TiO₂ particles grow larger with increasing H₂O₂ concentration, and the resulting stress disrupts the sheet integrity of MXene. Simultaneously, as dilute hydrochloric acid washes away the TiO₂ particles, the stress generated by the numerous vacancies causes mesopore wrinkling in the MXene sheet structure. The increased surface defects further disrupt the crystal structure, reducing crystallinity and diminishing the intensity of the diffraction peaks.³⁸ The primary diffraction peaks of samples C₁, C₂, and C₃ show no significant difference from sample C₀, indicating that the formation of mesoporous MXene does not alter the constituent elements. Fig. 1(b) shows the Raman spectra of samples C₀, C₁, C₂, and C₃. The A₁g mode is one of the most prominent Raman features of MXenes, primarily associated with the in-plane vibrations of carbon atoms within the MXene layers. Its frequency and intensity are influenced by interlayer interactions, surface functional groups, and the number of layers in the material. Specifically, the C–Ti vibrations occurred at 202 cm⁻¹, while the C atoms exhibited vibrations at 530–680 cm⁻¹ and 727 cm⁻¹. The E_g mode is primarily associated with lattice vibrations between transition metals and carbon/nitrogen atoms within the MXene. This mode is sensitive to factors such as interlayer spacing, surface functional groups, and chemical environment. The Ti atoms exhibited in-plane vibrations at 320–470 cm⁻¹ (E_1g).³⁹ The characteristic peaks of the D band and G band were approximately 1350 cm⁻¹ and 1600 cm⁻¹.⁴⁰ D bands are typically associated with sp³-hybridized carbon sites, indicating defects or structural imperfections, while G bands correlate with sp²-hybridized carbon atoms exhibiting ordered planar vibrations.⁴¹ The intensity of the G peak reflects the strength of interlayer interactions in the MXene. Stronger interlayer interactions typically result in higher G peak intensity, indicating tighter interlayer bonding. Conversely, weaker interlayer interactions lead to reduced G peak intensity. The intensity ratio of the D peak to the G peak (I_D/I_G) is commonly used to assess the degree of carbon disorder or defects. The I_D/I_G values of samples C₀, C₁, C₂, and C₃ were 1.1232, 1.2527, 1.2152, and 1.3486, respectively. This indicates an increase in disordered structural regions during the mesopore formation process.⁴²Fig. 1(c) shows the XPS survey profiles of samples C₀ and C₂. The presence of C, Ti, and Fe elements is clearly demonstrated across different orbital energy levels. Due to the retention of the minor byproduct TiO₂ during mesopore formation, the Ti 2p_3/2 peak, with a binding energy of approximately 457.6 eV, is characteristic of the Ti⁴⁺ oxidation state in TiO₂. Fig. 1(d) presents the high-resolution XPS spectrum of Ti 2p. The five peaks of samples C₀ and C₂, located at 464.29 eV, 461.47 eV, 458.83 eV, 546.76 eV, and 455.3 eV, and 464.13 eV, 461.11 eV, 458.33 eV, 546.66 eV, and 455.1 eV, correspond to Ti–O 2p_1/2, C–Ti–T_x 2p_1/2, Ti–O 2p_3/2, Ti³⁺, and C–Ti–T_x 2p_3/2, respectively. The formation of Ti–C–T_x bonds confirms the successful preparation of the MXene.⁴³ No Ti–Al bonds were detected in the Ti 2p spectrum, consistent with characterization by XRD and Raman spectroscopy. The absence of Ti–Al–C bonds indicates that all Al atoms in Ti₃AlC₂ were etched by HF. The Ti–O peak results from TiO₂ due to slight oxidation during the experiment.⁴⁴ The preparation process of sample C₂ is more complex, resulting in a slightly higher peak for Ti–O. Fig. 2(e) presents the Fe 2p spectra, which are deconvoluted into multiple peaks based on energy level splitting. For samples C₀ and C₂, these correspond to Fe²⁺ (711.75, 725.2 and 711.08, 724.62 eV), Fe³⁺ (714.56, 727.67 and 714.12, 727.62 eV), and the 2p_1/2 and 2p_3/2 orbitals (720.07, 720.25 and 733.53, 733.53 eV), respectively. Fe³⁺ originates from Fe crystal structures in complex chemical environments, indicating that the microoxidation process successfully formed an oxide layer on the sample surface. Fig. 1(f) shows the C 1s spectrum, with peaks at 284.8, 283.5, 281.8 eV, 288.8 eV, and 286.2 eV for C₀, and 284.8 eV, 283.1 eV, 281.6 eV, 288.6 eV, and 286.6 eV for C₂, corresponding to C–C, C–O–C, O–C=O, Ti–C, and Ti–C=O bonds, respectively.^45,46

Fig. 1 (a) XRD patterns, (b) Raman spectra, and XPS spectra of (c), Ti 2p (d), Fe 2p (e), C 1s (f) of samples C₀ and C₂.

Fig. 2 SEM images of FeSiCr, MXene, mesoporous MXene, and FeSiCr/mesoporous MXene (a) to (d); SEM images and magnified views of samples C₀, C₁, C₂, and C₃ (e) to (h); EDS elemental mapping images of C₂ (i); TEM and selected area high-resolution electron micrographs (j); and selected area electron diffraction pattern (k) of C₂.

Fig. 2(a)–(c) show the SEM images of FeSiCr, MXene, and mesoporous MXene, respectively. It can be observed that FeSiCr exhibits a smooth, plate-like solid surface; the MXene displays a distinctly flat surface with minute rough textures and no noticeable agglomeration; mesoporous MXene exhibits a distinct wrinkled morphology. This arises because a portion of Ti atoms in the MXene are oxidized by H₂O₂ into TiO₂ particles. The formation and occupation of these TiO₂ particles disrupt the sheet integrity of the MXene. Upon washing with dilute hydrochloric acid, the TiO₂ particles are removed, resulting in the formation of numerous mesoporous structures on the MXene surface. Fig. 2(d) reveals mesoporous MXene coating the surface of flaky FeSiCr, with significantly increased thickness relative to FeSiCr. Multi-layered mesoporous MXene deposited on the substrate forms a distinct porous structure. Fig. 2(e)–(h) present the SEM images and corresponding local magnifications of the selected areas of samples C₀, C₁, C₂, and C₃. Sample C₀ exhibits an irregularly rough texture due to flat, flake-like MXene multilayers stacked on the FeSiCr surface. Samples C₁, C₂, and C₃ display distinct spatial structures formed by stacked mesoporous MXene layers. Transmission electron microscopy (TEM) was employed for in-depth compositional and structural analysis of sample C₂. Fig. 2(i) displays the EDS mapping of sample C₂, showing the distribution of O, Fe, C, and Ti elements. The distribution regions of Fe and O elements are similar, while Ti exhibits a more comprehensive coverage. Enhanced particle brightness at the edges indicates uniform MXene encapsulation across the sample surface. Fig. 2(j) presents the TEM image under the field of view shown in Fig. 2(g). Calibration using the Digital Micrograph software yielded interplanar spacings of 0.3626 nm and 0.117 nm, corresponding to the (211) and (1010) crystal planes of the CrFe₈Si phase, respectively. This confirms that the primary sample composition consists of two phases: CrFe₈Si and MXene. Fig. 2(k) shows an electron diffraction ring with a lattice spacing of 0.3784 nm, which also corresponds to the MXene phase.

Reflection loss (RL) is a key metric for evaluating a material's ability to absorb electromagnetic waves and is typically expressed by the following formula:⁴⁷

RL can evaluate the degree of reflection of the incident electromagnetic waves by absorptive materials. A lower RL value indicates stronger electromagnetic wave absorption performance. The electromagnetic frequency at which 90% of incident electromagnetic waves are absorbed by the absorptive material (RL ≤ −10 dB) is termed the effective absorption bandwidth (EAB). Fig. 3(a)–(e) presents the 3D RL plots of samples FSC, C₀, C₁, C₂, and C₃, illustrating the relationship between reflection loss, thickness, and frequency. All samples exhibited favorable performance in the 12–18 GHz Ku-band. Samples C₀, C₁, and C₂ achieved an RL_min of −60 dB in the Ku-band, with respective values of −61.28 dB, −68.67 dB, and −63.87 dB. Sample C₃ exhibited an RL_min of −49.67 dB. The maximum frequency range for EAB was also within the Ku band, with samples C₀, C₁, and C₃ exhibiting EAB exceeding 10 GHz at 11.13, 10.46, and 10.96 GHz, respectively. Sample C₂ demonstrated an outstanding performance in the 2–4 GHz S band, achieving an RL_min of −66.41 dB, demonstrating that mesoporous MXene can simultaneously enhance both RL and EAB properties of FeSiCr materials in the low-frequency range. Fig. 3(f) shows the EAB plots of the samples at 1.5–3.5 mm thickness. Samples FSC, C₀, C₁, C₂, and C₃ exhibited relatively broad EABs at 1–2 mm thickness. Notably, Fig. 3(g) reveals that the EABs of samples FSC, C₀, C₂, and C₃ spanned 9–10 GHz, with the RL_min increasing as the MXene etching time is prolonged and the mesopore density rises. RL_min exhibits a trend of an initial increase and then a decrease with the extension of MXene etching time and increasing mesopore density, with sample C₁ achieving an RL_min of −68.67 dB.

Fig. 3 3D RL diagrams (a) to (e) and 2D RL diagrams (a1) to (e1) for samples FSC, C₀, C₁, C₂, and C₃. EAB diagrams (f) for different thicknesses. RL_min and EAB_max diagrams (g) for each sample.

To investigate the absorptive properties of the FeSiCr/mesoporous MXene composite in depth, the complex dielectric constant (ε_r = ε′ − ε″) and complex magnetic permeability (μ_r = μ′ – μ″) were calculated and analyzed. The real components (ε′ and μ′) and imaginary components (ε″ and μ″), respectively, govern the capacity of the material to store and dissipate incident electromagnetic wave energy.^48,49 The influence of hydrogen peroxide concentration on the electromagnetic properties of the FeSiCr/mesoporous MXene composite was investigated. The electromagnetic wave absorption properties were characterized by testing the composite material in the 2–18 GHz frequency range using the coaxial method. Fig. 4(a) and (b) show that ε′ decreased with increasing frequency, exhibiting the typical dispersion behavior caused by polarization hysteresis.⁵⁰ε′ and ε″ exhibited identical trends. According to the free electron theory and effective medium theory, high electrical conductivity leads to a larger complex dielectric constant.⁵¹ Sample C₀ exhibited the highest ε′ value because its intact MXene sheets remained unetched, providing extensive contact area with the FeSiCr substrate and resulting in high overall conductivity. However, samples C₁, C₂, and C₃ exhibited lower ε′ values than C₀ due to reduced high-dielectric MXene content after etching. Increased etching depth enhances the MXene mesoporous structure, creating more contact points with FeSiCr and establishing numerous heterogeneous interfaces that amplify interfacial polarization. Multiple resonance peaks in the ε″ curve across the 6–14 GHz frequency range indicate polarization processes in all samples. tan δ_E is primarily influenced by polarization losses (dominated by interfacial and dipole polarization) and conduction losses during electron transport. Fig. 4(c) shows that FSC exhibited the highest tan δ_E, which decreased after coating with mesoporous MXene. Fig. 4(d)–(f) investigate the magnetic properties of the materials. Since MXene is non-magnetic, the amount of MXene coating has a negligible effect on the magnetic properties of the materials. Consequently, the μ″ and tan δ_M values of samples C₁, C₂, and C₃ showed little change. Sample C₀ exhibited the maximum μ′ in the Ku band, indicating the occurrence of exchange resonance in the high-frequency region.⁵² Within the 2–18 GHz range, most tan δ_M values exceeded the tan δ_E values, indicating that magnetic loss is the dominant factor.

Fig. 4 Electromagnetic parameters: real part of permittivity (a), imaginary part of permittivity (b), and tangent of permittivity (c) of the dielectric constant. Real part of permeability (d), imaginary part of permeability (e), and tangent of permeability (f) of the magnetic permeability. Cole–Cole curve (g), C0 curve (h), and attenuation constant α (i) of the samples.

Dielectric relaxation polarization is reflected by the Cole–Cole semicircle.⁵³ The relationship between the ε′–ε″ curves can be described by the Debye theory:

The Cole–Cole semicircle corresponds to polarization loss, as shown in Fig. 4(g). All samples exhibited several semicircles, indicating the presence of multiple polarization losses. Moreover, the gradual decrease in the radius of the semicircles indicates the weakening of medium polarization.^54–56 Partial semicircles exhibited distortion with irregular variations, indicating that, in addition to Debye relaxation, other loss mechanisms such as dipole polarization and interfacial polarization were present.^57,58 The diminution of the Cole–Cole semicircle with increasing mesoporous MXene content may be attributed to defects and dipole polarization effects within the composite material.

As shown in Fig. 4(h), eddy current losses exerted a minor influence on the magnetic losses of the samples and remained nearly identical across all specimens within the 6–18 GHz range. Furthermore, the C₀ values of the samples decreased with increasing frequency across the 2–18 GHz spectrum.

Absorption coefficient (α) is a key parameter for evaluating the attenuation capability of absorptive materials against electromagnetic waves and describes the degree of attenuation as electromagnetic waves propagate through the material. It can be derived using the following formula:⁵⁹

Fig. 4(i) shows that the trend of the attenuation constant was similar to that of the complex dielectric constant. Sample FSC exhibited the highest attenuation constant, while sample C₁ had the lowest. The attenuation constant gradually increased from C₁ to C₃. As indicated by the formula, the attenuation constant is primarily influenced by the complex dielectric constant and complex magnetic permeability. Since the complex magnetic permeability varies little among the samples, the attenuation constant is mainly affected by the complex dielectric constant. Although sample FSC exhibited the highest attenuation constant, its reflection loss performance was suboptimal. The microwave absorption capability of a material depends not only on its attenuation constant but also on impedance matching. Only when the input impedance on the material surface equals the output impedance,⁶⁰ the material can absorb electromagnetic waves to its maximum capacity. Therefore, analysis of this critical factor is also required.

The matching performance of absorptive materials can be evaluated using the |Δ| function.^61,62 The smaller the |Δ| value, the better the impedance matching effect. The most effective impedance matching occurs when |Δ| ≤ 0.4.⁶⁰

|Δ| = sinh²(Kfd) − M

Fig. 5(a)–(e) show the impedance matching plots for samples FSC, C₀, C₁, C₂, and C₃, respectively. Light green indicates regions with good impedance matching (|Δ| ≤ 0.4); sample C₃ exhibited the best impedance matching with an effective area of 30.84%. The impedance matching of samples FSC, C₀, and C₁ was similar, ranging between 27% and 28%. Sample C₂ exhibited the poorest impedance matching at 21.34%, yet its absorptive performance was not the worst. This indicates that to maximize the absorptive performance of absorptive materials, a balance must be achieved between their electromagnetic wave attenuation capability and impedance matching. Fig. 5(f) shows the relationship between the cumulative pore volume and pore size in the samples. Compared to C₀, C₁, and C₂, C₃ exhibited a greater concentration of pores within the smaller pore size range. Overall, their pore sizes were smaller than those of C₀, with a more pronounced proportion of small pores.

Fig. 5 Impedance matching diagrams for samples FSC, C₀, C₁, C₂, and C₃ (a)–(e). Pore size distribution curve (f) of the samples.

An octahedral model was constructed using COMSOL finite element simulation to investigate the relationship between sample impedance matching, attenuation constant, and microwave reflection loss. Sample C₂ exhibited optimal absorption performance at 3.38 GHz and the maximum attenuation constant at 17.49 GHz. For these two frequencies, the COMSOL simulation analyzed the surface current density (SCD) and volume loss density (VLD). SCD correlates with the impedance matching of an absorptive material, where a lower SCD indicates better impedance matching.⁶³ As shown in Fig. 6(a), (a1), (c) and (c₁), the SCD values of sample C₂ at 3.38 GHz and 17.49 GHz were lower than those of sample C₀, indicating that C₂ exhibits superior impedance matching compared to C₀. This result is consistent with the findings presented in Fig. 5(b) and (d). The VLD correlates with the attenuation capacity of the material per unit volume, corresponding to the attenuation constant α. A higher VLD indicates greater electromagnetic wave energy loss. Sample C₂ exhibited a higher VLD than C₀ at 3.38 GHz, indicating stronger attenuation capability; however, its VLD was lower at 17.49 GHz, reflecting weaker attenuation capability. This pattern aligns with the variation in attenuation constant α shown in Fig. 4(i). As shown in Fig. 6(e), the EAB of the FeSiCr/mesoporous MXene composite exhibits a distinct advantage over various MXene-based materials or FeSiCr-based absorbers.^{24,25,29,30,64–70} Therefore, we conclude that the comprehensive absorption performance of the FeSiCr/mesoporous MXene composite is satisfactory, demonstrating outstanding microwave absorption capabilities. This provides valuable insights for the future design of absorber materials.

Fig. 6 SCD (a) and VLD (b) of sample C₀ at 3.38 GHz. SCD (c) and VLD (d) of sample C₂ at 3.38 GHz. (a1)–(d1) correspond to the values of samples C₀ and C₂ at 17.49 GHz in (a)–(d). Performance comparison chart in recent years (e). Schematic of the electromagnetic loss mechanism in FeSiCr/mesoporous Ti₃C₂T_x MXene composites (f).

Electromagnetic wave simulation can directly reveal the differences in dominant factors affecting the microwave absorption performance of a material at various frequencies. Samples must exhibit strong electromagnetic wave attenuation and excellent impedance matching characteristics; however, achieving outstanding electromagnetic wave absorption performance hinges on balancing electromagnetic wave attenuation and impedance matching. In Fig. 6(f), an in-depth analysis of the electromagnetic loss mechanism in the FeSiCr/mesoporous Ti₃C₂T_x MXene composite is presented. First, the composite employs flake-like FeSiCr as the substrate and integrates mesoporous MXene as its surface layer. The extensive voids between these layers facilitate the reflection and scattering of electromagnetic waves within the material, enhancing its attenuation capability. Simultaneously, the highly dielectric mesoporous MXene forms extensive conductive networks on the FeSiCr substrate, enhancing free electron migration and hopping, while intensifying dipole polarization and dielectric loss. Moreover, the flake-like FeSiCr substrate generates magnetic permeability losses, such as natural resonance and eddy current dissipation. The synergistic interaction between multiple dielectric loss mechanisms and magnetic loss mechanisms achieves an optimal balance, thereby producing a magnetoelectric coupling effect. This equilibrium significantly improves impedance matching and enhances the electromagnetic wave absorption performance of the material.

4. Conclusions

In this study, we constructed FeSiCr/mesoporous MXene composites via electrostatic adsorption and systematically investigated the regulatory mechanisms of the mesoporous MXene structure and electromagnetic wave absorption properties of the composite. The mesoporous MXene structure was controlled by adjusting the H₂O₂ concentration and optimizing the composite's microstructure by increasing the mesopore density. Concurrently, using flake-like FeSiCr as the substrate, mesoporous MXene was composited onto its surface layer. The resulting structure featured abundant voids between layers, creating conditions for electromagnetic wave reflection and scattering within the material, thereby enhancing its electromagnetic wave attenuation capability. Furthermore, the flake-like FeSiCr substrate induced magnetic loss mechanisms, such as natural resonance and eddy current dissipation. At an H₂O₂ concentration of 0.2 mmol L⁻¹, the material exhibited outstanding performance in the S-band, with the RL_min reaching −66.41 dB. When the material is matched to a thickness of 1.85 mm, the EAB improved to 10.46 GHz. This work demonstrates that the formation of mesoporous MXenes and their composite with magnetic metals on the material surface represents a highly promising strategy for developing high-performance electromagnetic wave absorbers.

Conflicts of interest

All the authors listed declare that the paper is original research that has not been submitted for publication to other journals, and all the authors listed have read the paper and agree with its submission to Physical Chemistry Chemical Physics.

Data availability

The data that support the findings of this study are available from the corresponding author upon request.

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