Construction of high aspect ratio carbon nanotube networks decorated with MoS₂ nanoflowers for superior microwave absorption properties

Abstract

Unique structural design and precise compositional adjustment are universally accepted as effective methods for the enhancement of microwave absorption capacity. However, a profound mechanism is still lacking. Recently, diverse structural construction of MoS₂ sheets has attracted considerable attention. This research utilizes flower-like structural construction and composition regulation strategies to enhance the efficiency of MoS₂-based microwave absorbing materials. It aims to thoroughly reveal the correlations between structures, components, dielectric properties and the impedance matching characteristics. The high aspect ratio of carbon nanotubes is utilized to construct a complex conductive network. The MoS₂ flowers facilitate multiple scattering and reflection, increasing the transmission path of microwaves. Moreover, the heterojunction interfaces between MoS₂ and carbon nanotubes enhanced the microwave attenuation mechanism. The results demonstrate that the samples exhibited a strong microwave response at 1.95 mm, with a maximum attenuation of −41.59 dB. Simultaneously, CST simulations verify the actual microwave absorption performance of the composite under far-field conditions. The vertical incident radar cross-section values, compared to PEC, were reduced by a maximum of 36.81 dBm². These findings clearly indicate that the composite possesses excellent impedance matching and high absorption efficiency. This study provides an in-depth analysis of the microwave loss mechanism in dielectric microwave absorption materials, offering reference for enhancing the performance of similar materials.

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1. Introduction

The advancements in novel communication technologies have resulted in increased electromagnetic pollution, intensifying the conflict between technological innovation and human health.^1,2 In addition, advancements in radar technology pose significant challenges to aircraft stealth, prompting a demand for enhanced radar stealth capabilities.^3,4 Therefore, the innovation of microwave absorption materials (MAMs) with strong absorption, thin thickness, wide bandwidth, and lightweight holds the potential to mitigate the harm caused by electromagnetic interference (EMI) to the human body and ensure national defense security.^5,6

As a novel semiconductor material, MoS₂ belongs to two-dimensional transition metal dichalcogenides (2D TMDC) and features triangular S–Mo–S structures. Its unique electrical, optical, and mechanical properties attract widespread attention, including high room-temperature mobility (≈900 cm² V⁻¹ S⁻¹),⁷ an adjustable intrinsic bandgap (1.2–1.9 eV),⁸ and high specific surface area. These properties make it widely used in the fields of batteries,^9–12 photoelectric sensors, and flexible electronics. Additionally, MoS₂ has been extensively studied as a high-performance MAM. The research by Ning et al.¹³ in 2015 was the first to report the application of MoS₂ sheets as a dielectric loss MAM, obtaining good microwave attenuation performance. It is well known that MoS₂ has three phases: 1T, 2H, and 3R, with different coordination patterns that give it different electrical properties.¹⁴ The 2H phase has high stability, and the 1T phase has better conductivity. To further improve its conductivity, Ning et al.¹⁵ prepared a metal semiconductor mixed phase (2H/1T-MoS₂), and achieved tunable dielectric properties by adjusting the 1T phase content. This enhances the dipole distribution dynamics and electron transfer ability, achieving good microwave absorption (MA) performance. In 2022, Che et al.¹⁶ developed a spin-shaped MoS₂ hetero-structure with adjustable 1H and 2H phase ratios using AlOOH as a template. It contained 39% of 1T-MoS₂, expanding the range of effective absorption bandwidth (EAB) to cover up to 6.3 GHz. The above studies demonstrate the positive effect of adding 1T-MoS₂ on the conductivity. However, compared to the thermodynamically metastable phase 1T-MoS₂, 2H-MoS₂ is easier to obtain and has enhanced stability. Therefore, it remains a significant challenge to improve the conductivity and impedance matching (IM) of 2H-MoS₂ while effectively maintaining its advantages.

Numerous studies have been done to enhance the MA properties of MoS₂ composites, including the use of multiple materials, interface construction and hierarchical structure design. Carbon has gained significant attention in the MA absorption field owing to its superior electrical conductivity, lightweight, robust chemical stability, and easy availability.¹⁷ Qi et al.¹⁸ prepared a hollow carbon@MoS₂ nanosphere using a template etching and template thermal treatment method. At a thickness of 2.06 mm, it demonstrated powerful MA absorption (−42.63 dB) and exhibited a broad EAB (6.00 GHz). Wu et al.¹⁹ synthesized a CF@MoS₂ composite by growing MoS₂ nanosheets on carbon fibers. It exhibited a minimum reflection loss (RL_min) of −21.4 dB and showed an EAB of 10.85 GHz at 3.80 mm. Luo et al.²⁰ obtained a novel NDC/MoS₂ composite with a wide EAB of 6.08 GHz by designing the material's structure and composition. The interconnected porous structure enhances the material's polarization and conductivity, optimizes IM, and promotes multiple scattering of microwaves. From the above cases, it is found that the addition of carbon can compensate for the low conductivity and have a positive effect on MA absorption.²¹ High-aspect-ratio carbon nanotubes (CNTs) enable rapid charge transfer and increased interfacial polarization along the axial direction due to their 1D microstructure.^22,23 Therefore, the combination of CNTs and MoS₂ in the MAM design has great potential for achieving high MA efficiency.

The structural configuration is widely recognized as playing a key role in promoting the performance of MAMs.²⁴ To achieve higher absorption capacity, many researchers designed various types of samples with special structures, including one-dimensional structures,²⁵ honeycomb structures,²⁶ and star structures.²⁷ Among the numerous nanostructures, flower-like structures comprising a multitude of 2D nanosheets have unique advantages in optimizing IM, extending transmission paths and reducing material density.²⁸ Jia et al.²⁹ synthesized a porous flower-like Co/ZnO@CMWCNTs/Ti₃C₂T_x composite through electrostatic self-assembly, resulting in a wide EAB of 4 GHz at 1.9 mm. Qi et al.³⁰ prepared a flower-like multicomponent nanocomposite (CoSe₂/FeSe₂@MoSe₂) through a two-step hydrothermal reaction, achieving an EAB of 4.60 GHz at 1.72 mm. In addition, Lu et al.³¹ prepared a MoS₂ nanoflower/honeycomb porous composite using seaweed as a carbon source through a hydrothermal process. It has achieved strong microwave absorption capacity with a RL_min of −75.94 dB (1.68 mm). The flower-like structure facilitates the accumulation of charge at the heterojunction interface. The resulting interfacial polarization and multiple reflections/scattering will result in the consumption of the incident electromagnetic wave (EMW). The high aspect ratio of CNTs helps to form complex conductive networks and also allows them to bond with flower-like MoS₂ to enhance microwave dissipation through strong polarization effects.³²

In summary, this work aimed to explore the effects of structural design and use of multiple components on the dielectric performance of MoS₂. MoS₂/CNTs were successfully prepared through a simple process involving hydrothermal self-growth and high-temperature annealing. Their material composition, morphology, and structure were characterized, and MA properties were tested. MoS₂/CNTs exhibited broad EAB, strong MA characteristics, and good IM. The EAB spans 5.33 GHz, and the RL_min reached −41.59 dB at 1.95 mm. This study comprehensively analyzes the dielectric properties and MA mechanisms of the material. It simplifies the preparation method for MoS₂ composites, offering a reference for the design of lightweight and high performance MAMs.

2. Experimental

2.1. Chemicals

Thiourea (CS(NH₂)₂, AR) and sodium molybdenum oxide dihydrate (Na₂MoO·2H₂O, AR) were brought from Shanghai Aladdin Co., Ltd. Carbon nanotubes (CNTs) were brought from Nanjing XFNANO Co., Ltd. Argon (99.99%), nitric acid, deionized (DI) water and industrial ethanol were also utilized in the study.

2.2. Preparation of acidified CNTs

In a typical operation, 100 mg of CNTs are acidified in 200 mL of nitric acid solution (30 wt%) and dispersed by continuous sonication for 6 h. The acidified CNTs are then washed to become neutral using DI water, separated by centrifugation and dried in a vacuum at 40 °C.

2.3. Preparation of MoS₂/CNT composites

First, 2.0 g of CS(NH₂)₂ and 0.5 g of Na₂MoO·2H₂O are introduced into 50 mL of DI water and stirred for 5 min. Second, 25 mg of acidified CNTs are added to the mixture of Na₂MoO·2H₂O and thiourea and sonicated for 30 min to achieve a uniformly black solution. Third, the black solution is moved to a 50 mL reactor and subjected to a hydrothermal reaction at 190 °C. Fourthly, the solution is repeatedly washed with DI water and absolute ethanol, and then centrifuged to obtain a black MoS₂/CNT precursor and dried at 50 °C in a vacuum. In the end, the samples labeled as MoS₂/CNTs X (X = 600, 700, 800) are synthesized by annealing in an argon atmosphere at a heating rate of 5 °C min⁻¹ at X °C (X = 600, 700, 800), and are held at high temperature for 2 hours, respectively.

2.4. Characterization

The crystal structure and sample composition are examined using an X-ray diffractometer (XRD). The thermal stability, MoS₂ content and carbon content of the composites are analyzed using a thermogravimetric analyzer (TGA) of the TG 209F1 Iris type. Carbon component types are examined using a Raman spectrometer of the HORIBA/LabRAM HR Evolution type. X-ray photoelectron spectroscopy (XPS) is used to investigate the valence states of the elements within the composites. The microscopic morphology and characteristics of MoS₂/CNTs are visualized using a FEI Quanta 250 FEG scanning electron microscope (SEM), and the lattice with high resolution and element mapping are observed using a FEI/Tecnai G220 transmission electron microscope (TEM). The coaxial method is utilized to determine the complex permittivity (ε_r = ε′ − jε′′) and permeability (μ_r = μ′ − jμ′′) for MoS₂/CNT composites at a loading level of 30 wt%, using a vector network analyzer (VNA) (N5234A, USA).

3. Results and discussion

3.1. Morphology and structure of MoS₂/CNTs

Fig. 1a shows the manufacturing process of MoS₂/CNTs. In this experiment, MoS₂ nanoflowers are prepared by a hydrothermal method using CH₄N₂S as a sulfur source and Na₂MoO₄ as a molybdenum source, and subsequently decorated on CNTs. During the synthesis, anion exchange and reduction reactions are observed. CH₄N₂S acts as both a sulfur source and a reducing agent. Firstly, CH₄N₂S undergoes decomposition to produce S²⁻ anions via hydrolysis at elevated temperatures. Subsequently, it combines with Mo⁶⁺ cations to generate MoS₃ (intermediate). MoS₃ is rapidly reduced to MoS₂ (final product) and then attached to the surface of CNTs.³³ Finally, the materials undergo heat treatment in an inert atmosphere to obtain MoS₂ nanoflower-modified CNT composites (MoS₂/CNTs).

Fig. 1 (a) Schematic fabrication process, (b) XRD spectrum and (c) Raman spectrum.

XRD can characterize the composition and crystal structure of MoS₂/CNTs, as shown in Fig. 1b. Diffraction peaks for the MoS₂/CNTs are evident at 2θ = 13.9°, 33.2°, 39.5°, and 58.9°, which can be assigned to the (002), (100), (103), and (110) planes of 2H-MoS₂ (PDF#37-1492) with a hexagonal crystal structure, respectively.³⁴ Moreover, the peak detected at 2θ = 25.9° indicates the (002) plane associated with carbon (PDF#75-1621).^31,35 The peak intensity rises with increasing temperature, which means that high temperature facilitates the generation of graphitic carbon.

The electrical conduction properties of carbon are linked to the graphitization degree and are commonly analyzed using a Raman spectrometer. The D peak (∼1350 cm⁻¹) indicated the disorders and defects, and the G peak (∼1580 cm⁻¹) is associated with the ordered structure of graphitic carbon.^36,37 Generally, the graphitization degree of carbon is computed by the ratio of the D peak to the G peak (I_D/I_G). As shown in Fig. 1c, I_D/I_G values of MoS₂/CNTs 600, MoS₂/CNTs 700 and MoS₂/CNTs 800 are 0.89, 0.82 and 0.81, respectively. According to the three-stage model of the transition from amorphous carbon to graphitic carbon proposed by Ferrari and Robertson, the I_D/I_G ratio typically first increases and then decreases.³⁸ Specifically, the I_D/I_G ratio gradually increases during the transition from amorphous carbon to nanocrystalline graphite, and gradually decreases after reaching the transition point from nanocrystalline graphite to perfect graphite.³⁹ The graphite carbon diffraction peaks in the XRD results confirm the occurrence of graphitization in the samples. Based on the three-stage model, it can be concluded that the MoS₂/CNT samples undergo the transition from a nanocrystalline to a perfect graphitized state. Therefore, the graphitization degree of the MoS₂/CNTs increases with the rise in calcination temperature. It can be attributed to the fact that high temperatures enhance the graphitization degree and reduce the N content in samples, thereby increasing the graphitization degree of the composite.^40,41

TGA can analyze thermal stability and component content. Fig. S1 (ESI†) shows the TGA curve of the MoS₂/CNTs. Between 100 °C and 250 °C, the dehydration of crystallization water results in a slight weight loss as the temperature increases.⁴² The downward trend of the curves from about 290 °C to 370 °C is attributed to the MoS₂ phase transformation.⁴³ Carbon starts to oxidize to CO₂ at approximately 370 °C, while MoS₂ is oxidized to MoO₃, leading to a rapid decrease in the curves.⁴⁴ In order to verify the final product, MoS₂/CNT composites after oxidation at 700 °C are analyzed by Fourier transform infrared (FT-IR) spectroscopy. Three distinct vibrations at approximately 562, 859 and 996 cm⁻¹ are ascribed to Mo–O (3), Mo–O (2) and Mo–O (1) stretching modes, respectively.^45,46 It can be concluded that the residue above 700 °C is predominantly composed of MoO₃. The residual masses of MoS₂/CNTs 600, MoS₂/CNTs 700 and MoS₂/CNTs 800 are 78.6 wt%, 76.3 wt%, and 77.1 wt%, respectively. On the basis of formula (1), the carbon contents are calculated to be about 11.2 wt%, 13.1 wt%, and 12.5 wt%, respectively, and the MoS₂ contents are about 88.8 wt%, 86.9 wt%, and 87.5 wt%, respectively. wt% R and M represent the remaining mass percentage and the relative molecular mass of the compound, respectively. This suggests that the calcination temperature has less effect on the carbon content and MoS₂ content of MoS₂/CNTs.

SEM images show the morphology of the MoS₂/CNTs. As seen in Fig. 2a and Fig. S2a–c (ESI†), MoS₂ nanosheets attach to the CNT surface, growing interleaved and forming uniformly distributed flower-like microspheres with pores of various sizes. Fig. 2b and Fig. S2d–f (ESI†) show further magnified images where the CNTs are intertwined with each other, providing a long enough channel for the flow of electrons. The thickness of the nanosheets in Fig. 2b is measured, and the particle size analysis result reveals that the thickness is about 10 nm, as shown in Fig. S3 (ESI†). MoS₂ flowers in each composite are well preserved. However, the size of MoS₂ flowers in MoS₂/CNTs 600 is larger than that of the flowers in MoS₂/CNTs 700 and MoS₂/CNTs 800. This is typical of the shrinkage that occurs in precursors during the carbothermal reduction process.

Fig. 2 (a) and (b) SEM patterns of MoS₂/CNTs, (c) and (d) TEM images, (e) HR-TEM images, the lattice fringes of (f) graphitic carbon and (g) MoS₂, and (h) EDS mapping.

The micro-morphology, structure and lattice fringe spacing can be analyzed by TEM. Fig. 2e and Fig. S4a (ESI†) show the high-resolution TEM (HR-TEM) photographs. The lattice fringe distances of CNTs and MoS₂ are clearly displayed, confirming that 0.35 (Fig. 2f) and 0.65 nm (Fig. 2g) correspond to the (002) planes of carbon and MoS₂, respectively.^47,48 In Fig. 2c and d, the flower-like MoS₂ and CNTs are modified with blue and orange, respectively. The size of MoS₂ nanosheets (Fig. S4b and c, ESI†) is uniform, which proves that the MoS₂ hydrothermal method has good stability and feasibility. Besides, elemental mapping images (Fig. 2h and Fig. S5, ESI†) show that S, Mo, and C species are regularly distributed in the sample, and the content of C species is low. The distribution density of each species in the image and the characterization results of TG are mutually verified, and a small amount of N doping provides some defect sites in the composite.⁴⁹

The element species and surface chemical valence states are characterized by XPS.⁵⁰ The XPS spectra are shown in Fig. 3. The peaks of O, Mo, C and S species in the XPS spectrum (Fig. 3a) prove that MoS₂/CNTs contain the above elements. Fig. 3b shows the C 1s high-resolution spectrum. It could be observed that the fitted peaks are located at 284.6, 285.3, and 286.6 eV, correlating with C–C, C–S, and C–N, respectively.^21,51 The presence of C–S and C–C bonds further confirms the interaction of MoS₂ with CNTs and the N-doped C obtained from the carbonization of CNTs.⁵² The Mo 3d high-resolution spectrum (Fig. 3c) can be deconvoluted into six distinct peaks. Specifically, the peaks at 232.6 eV and 229.4 eV are attributable to Mo 3d_3/2 and Mo 3d_5/2, while the peaks at 233.1 eV and 229.7 eV are identified as two satellite features. The peak at 235.8 eV can be attributed to the existence of hexavalent molybdenum compounds. Moreover, the fitted peak at 226.5 eV can be assigned to the S 2s peak.⁵³ The S 2p spectrum of MoS₂/CNTs (Fig. 3d) consists of two fitting peaks: S 2p_1/2 (163.4 eV) and S 2p_3/2 (162.2 eV).^54,55

Fig. 3 XPS spectra of (a) MoS₂/CNTs, (b) C ls, (c) Mo 3d, and (d) S 2p.

3.2. Electromagnetic characteristics of MoS₂/CNTs

Electromagnetic characteristics are crucial for assessing the efficacy of microwave absorption. ε′ and μ′ denote the capacity to store electrical energy or magnetic energy, while ε′′ and μ′′ indicate the potential for losing electrical energy or magnetic energy.^56,57 In Fig. 4a and b, two typical phenomena are observed. One is that the value of ε′ tends to decrease with increasing frequency from 6.25, 7.86 and 10.11 to 5.00, 6.35 and 7.04, respectively. This phenomenon is characteristic of frequency dispersion effects.⁵⁸ The second is the appearance of two dielectric resonance peaks of ε′′ at about 9 and 12 GHz. This is mainly attributable to the local motion of bound charges and the variation of the dipole moment in an alternating electromagnetic field, which in turn affects both ε′ and ε′′ values within a relatively narrow frequency region.^59,60 The Debye and free electron theories are helpful for analyzing polarization and microwave loss mechanisms:^61,62

Fig. 4 (a) ε′, (b) ε′′, (c) tan δε, (d) , (e) , (f)–(h) Cole–Cole semicircle, (i) μ′, (j) μ′′ and (k) tan δμ of MoS₂/CNT composites.

Based on the method provided by Su et al.⁶³ and with some improvements, the conduction loss () and polarization loss () values of the samples were simulated in MATLAB software. As shown in Fig. 4d, the value of the samples increases with the rise in calcination temperature, further indicating that high-temperature calcination facilitates the sp² transition of carbon, which positively impacts the dielectric loss capability of the composite. This is consistent with the Raman characterization results, demonstrating that by controlling the calcination temperature, the dielectric properties can be effectively tuned. As seen in Fig. 4e, the values of the samples also increase with the rise in calcination temperature, among which MoS₂/CNTs 800 exhibits a stronger polarization loss capability.

In accordance with formula (3), these notable differences imply that temperature is a key factor influencing the ε_p′′ and ε_c′′ loss characteristics of MoS₂/CNT. To facilitate an in-depth analysis of the dielectric loss properties, the Cole–Cole model is introduced in this study as illustrated below:^64,65

The Debye relaxation formula can be described as a semicircle with a radius of (ε_s − ε_∞)/2 and a center at (0, (ε_s + ε_∞)/2). Each Cole–Cole semicircle in Fig. 4f–h signifies a relaxation process, and the semicircle number reflects the intensity of polarization. MoS₂/CNTs X (X = 600, 700, 800) show two Cole–Cole semicircles in the range of 2–18 GHz, indicating the occurrence of multiple relaxation processes, which may be due to the defects and polar groups in the carbon component.^66,67 A twisted semicircle means that it is not an ideal model for the Debye relaxation theory, and other dielectric loss mechanisms such as dipole polarization (point-to-point) and interfacial polarization (plane-to-plane) play an important role in the final performance.⁶⁸ The number of polarization processes roughly corresponds to the number of resonance peaks in the ε′′ curve.⁶⁹ This indicates that each sample exhibits similar polarization characteristics. According to reports, the interfacial polarization response relaxes at lower frequencies, and the resonance peaks observed on ε′′ curves within the 7–12 GHz range are more influenced by the effects of interfacial polarization.⁷⁰ This can be attributed to the interfaces created by the contact between CNTs and MoS₂ flowers, as well as between MoS₂ nanosheets and paraffin or air, forming solid–solid and solid–air interfaces. The differing electronegativity of the components results in a variation in their electron attraction capabilities. This leads to an imbalance in charge distribution, accumulation of electrons around heterojunctions, and migration through heterojunctions under the influence of external electromagnetic fields.⁷¹ The resonance peaks in the 12–16 GHz range can be ascribed to defect dipole polarization caused by trace nitrogen atoms, defects in CNTs, and residual groups in the composite.⁷² N doping or other intrinsic C vacancies can, on the one hand, increase the orientation force, thereby contributing to the dipole polarization under an altering EM field.⁷³ On the other hand, the nitrogen atoms acting as the donor dopants can provide additional electrons for higher conductivity, which improves the conduction loss.⁷⁴ In addition, the Cole–Cole curves have straight lines at the tail, which is attributed to the conduction loss of MoS₂/CNTs to microwaves. The curvature of the lines declines with the rise in the calcination temperature, indicating an improvement in conduction losses. Due to the similar carbon content across all samples, the improvement in conduction losses is linked to the pyrolysis temperature. It is evident that higher temperatures effectively enhance the graphitization degree of carbon, thereby improving conductivity. Usually, according to the free electron theory, the strength of the conductive losses is positively correlated with the conductivity, which explains the highest ε_c′′ values for MoS₂/CNTs 800.⁷⁵ Additionally, since active electric dipoles in materials are more inclined to interact with incident EMWs, restricted electron migration/hopping typically leads to unsatisfactory conduction losses.⁷⁶ This drawback can be completely compensated by the growth of CNTs. This is because CNTs will create a more conductive network through physical contacts, extending the transmission path of micro-currents and increasing electron transfer, thereby promoting conduction losses.⁷⁷ Hence, dipole polarization, interfacial polarization, and electronic conduction losses collectively contribute to ε′′.

The dielectric loss tangent (tan δε) demonstrates the strength of the dielectric loss capability. The tan δε curves in Fig. 4c also show multiple relaxation peaks, which are attributed to the asymmetric charge distribution caused by heterojunctions, vacancies and dipoles. In addition, staggered CNTs form a conductive network to dissipate microwaves, and multiple factors work together to enhance the MA capability. MoS₂/CNTs 800 has a higher loss capacity compared to MoS₂/CNTs 600 and MoS₂/CNTs 700. This phenomenon is analyzed in conjunction with Cole–Cole curves, which may be related to its higher conductance loss.

Fig. 4i and j illustrate the μ′ and μ′′ values of MoS₂/CNTs. They have similar magnetic storage capacities, and the μ′ and μ′′ values fluctuate around 1.01 and 0.02, respectively. The magnetic loss tangent (tan δμ) values are shown in Fig. 4k, and all of them are small.

3.3. Microwave absorption properties and mechanism

The RL_min, EAB and MAM thickness can directly reflect the MA capability. The MA capabilities of MoS/CNTs are calculated using the transmission line theory.^78–80where c and d represent the speed of light and material thickness, respectively. 2D RL plots, EAB plots, and 3D RL plots are shown in Fig. 5. MoS₂/CNTs 600 exhibits an MA performance at 2.30 mm thickness with an EAB of 4.70 GHz (Fig. 5b) and a RL_min of −15.66 dB (Fig. 5c). MoS₂/CNTs 700 has an EAB of 4.70 GHz (Fig. 5e) with a RL_min of −19.37 dB (Fig. 5f) at 2.00 mm. In addition, MoS₂/CNTs 800 achieved the best MA performance, and has a wide EAB of 5.33 GHz (Fig. 5h) and a strong RL_min of −41.59 dB (Fig. 5i) at 1.95 mm. In summary, MoS₂/CNTs 800 has the characteristics of broadband and strong absorption, and shows wide application prospects of anti-jamming, radar stealth, and human electromagnetic protection.

Fig. 5 2D reflection loss of (a) MoS₂/CNTs 600, (d) MoS₂/CNTs 700, (g) MoS₂/CNTs 800; EAB of (b) MoS₂/CNTs 600, (e) MoS₂/CNTs 700, (h) MoS₂/CNTs; 3D reflection loss of (c) MoS₂/CNTs 600, (f) MoS₂/CNTs 700 and (i) MoS₂/CNTs 800.

Outstanding impedance matching (IM) contributes positively to the improvement of MAM efficiency.⁸¹ IM charts are created to assess material compatibility across varying microwave frequencies and explore the correlation between the microwave frequency range and material thickness. As shown in Fig. 6, the IM of the composites is calculated using formula (8), where K and M are constants, f is the microwave frequency and d is the sample thickness.⁸² The effective area ratio of MoS₂/CNTs 600 is only 15.81% (Fig. 6a), that of MoS₂/CNTs 700 is 27.78% (Fig. 6b), and that of MoS₂/CNTs 800 is 42.90% (Fig. 6c). The low effective area ratio of the IM hampers the composite's compatibility with microwaves. This leads to increased scattering or reflection of microwaves at the surface, preventing their absorption in the interior of the composite. Therefore, MoS₂/CNTs 800 demonstrates superior IM characteristics, facilitating the penetration and absorption of more microwaves.

Fig. 6 IM of (a) MoS₂/CNTs 600, (b) MoS₂/CNTs 700, and (c) MoS₂/CNTs 800; (d) attenuation factors, and (e) and (f) performance of MoS₂/CNTs compared with that reported previously.

In addition, in order to comprehend the correlation between the attenuation factor and the MA performance, the attenuation factors are further calculated and analyzed (formula (9)).^83,84 As shown in Fig. 6d, MoS₂/CNTs 800 exhibits higher microwave loss capability compared with MoS₂/CNTs 600 and MoS₂/CNTs 700. A high attenuation factor value can lose more microwaves. Therefore, the superior IM and attenuation factor work together to enable MoS₂/CNTs 800 to exhibit wide EAB and strong reflection loss for microwaves compared to materials that have been reported previously, as shown in Fig. 6e and f.^85–87

|Δ| = |sinh²(Kfd) − M| (8)

The broadband and strong MA performance of MoS₂/CNTs are attributed to their special structure, good dielectric loss capability, and higher effective area ratio of IM. Fig. 7 shows the MA mechanism of MoS₂/CNTs. When the composites are exposed to an external electromagnetic environment, a large number of pores inside the composites help to improve IM and enable microwaves to smoothly enter the composites. Flower-like MoS₂ combined with CNTs constructs more heterogeneous interfaces. The difference in electronegativity between the components leads to charge accumulation under the influence of an alternating electric field, triggering interface polarization. The large number of defects in MoS₂ nanosheets and nitrogen doping can act as polarization centers leading to dipole polarization and defect polarization. Meanwhile, MoS₂ nanosheets are cross-linked to form MoS₂ flowers. The mesoporous structure on the surface induces multiple reflections and scattering of EMWs, which prolongs the transmission path and thus enhances the loss of EMWs. In addition, the highly conductive CNTs interlace to form a complex network, which provides conditions for free electron migration and hopping and thus exacerbates the conductivity loss in the composites.⁸⁸ The combined effect of various dissipation methods can effectively improve the MA intensity and width of MoS₂/CNT composites.

Fig. 7 Simplified model of the MA mechanism of MoS₂/CNTs.

3.4. Far-field simulation of MoS₂/CNTs

To discuss the MA performance under actual conditions, the radar cross-section (RCS) is simulated. The RCS value is a physical quantity equivalent to the projected area of a metal sphere with the same echo. It describes the reflection intensity under microwave irradiation, with a smaller value indicating stronger MA.^89–92 In this work, a double-layer square (20 cm × 20 cm) structure is created in CST Microwave Studio, including a perfect electrical conductor (PEC) layer and a MA layer, as shown in Fig. 8a. The MA layer is composed of 30 wt% MoS₂/CNTs and 70 wt% paraffin, with the thickness and testing frequency corresponding to the optimal performance for each sample. The thickness of the PEC model is 1 mm. The Y-axis is set to the direction of incident plane waves, phi is the incidence angle, and theta is the observation angle. Fig. 8b–i displays 2D and 3D RCS color-mapped images of MoS₂/CNTs 600, MoS₂/CNTs 700, MoS₂/CNTs 800 and the PEC layer. Covering the PEC layer with the MA material results in varying levels of suppression of microwave reflection. Consistent with actual conditions, direct exposure to incident microwaves leads to increased radiation and stronger reflection intensity, posing significant disadvantages for human health and stealth capabilities of aircraft. The RCS values of MoS₂/CNTs 600 and MoS₂/CNTs 700 are above −10 dBm² at vertical angle, suggesting weaker MA for head-on waves, and the RCS of PEC is even higher than 10 dBm². In contrast, the maximum RCS of MoS₂/CNTs 800 is close to −20 dBm², indicating superior absorption performance. Fig. 8j illustrates the RCS at Theta = 90° within the range of −90° ≤ Phi ≤ 90°, with the values for MoS₂/CNTs X (X = 600, 700, 800) being lower than 1.84, −1.75, and −19.8 dBm², respectively. The RCS is associated with the area exposed to microwave illumination, and larger areas lead to increased scattering of microwaves. At Phi near −90° and 90°, a lower RCS of PEC is attributed to its smaller side area. Fig. 8k shows the absolute difference in RCS between each sample and PEC. When Phi = 0, 36.81 dBm² of scattering intensity is reduced by MoS₂/CNTs 800, which is much higher than that in the case of MoS₂/CNTs 600 (15.17 dBm²) and MoS₂/CNTs 700 (18.75 dBm²). Overall, the MoS₂/CNTs 800 is expected to be a promising material for practical applications.

Fig. 8 (a) Far-field simulation model for MoS₂/CNTs. 3D and 2D color-mapped images of the scattering signals from (b) and (c) MoS₂/CNTs 600, (d) and (e) MoS₂/CNTs 700, (f) and (g) MoS₂/CNTs 800, and (h) and (i) PEC. (j) RCS simulated curves and (k) RCS reduction chart of MoS₂/CNTs.

4. Conclusions

In summary, MoS₂/CNTs were successfully and efficiently fabricated through a one-step hydrothermal strategy and high temperature calcination. By varying the calcination temperature of the precursor, we found that the properties of the composites can be selectively adjusted. Higher calcination temperatures contribute to improved conductivity and reduced the diameter of the flower-like MoS₂. The MoS₂ nanosheets are connected to form a 3D flower-like MoS₂, which increases internal reflection and scattering of microwaves and effectively adjusts the IM. In addition, the presence of a small amount of N, CNTs, and sufficient heterojunction interfaces endows the material with rich conductive losses, interfacial polarization, and dipolar polarization. Finally, MoS₂/CNTs 800 has excellent MA properties, achieving a broad EAB of 5.33 GHz and a RL_min of −41.59 dB at 1.95 mm. The far-field simulation results also show that MoS₂/CNTs 800 has good absorption properties. Due to its simple preparation process and good stability, this approach can be extended to the design and preparation of different types of MoS₂-based composites. It provides guidance for studying broad and strong MAMs and simulating RCS.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.†

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We are thankful for financial support from the National Natural Science Foundation of China (No. 51902359 and No. 52377026), the Taishan Scholars and Young Experts Program of Shandong Province (No. tsqn202103057), the Henan Key Technologies R & D Program (No. 242102231067), the Guangdong Basic and Applied Research Foundation (No. 2024A1515011908), the General Program (No. K2023MS009), and Youth Master's Guide Training Program, Scientific Research Foundation for Doctors of Zhongyuan University of Technology.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc03287j

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