1 Introduction

The rapid development of electronic communication devices and intelligent appliances has brought many conveniences to people's lives [1, 2]. However, the increasingly significant electromagnetic radiation pollution caused by intelligent devices poses a threat to human health and the environment [3, 4]. The development of high-performance electromagnetic wave (EMW)-absorbing materials is an effective strategy for suppressing electromagnetic radiation. At present, the design and development of lightweight EMW-absorbing materials with strong electromagnetic energy attenuation properties has become a hot topic of research in this field. According to dielectric theory, the electromagnetic energy attenuation capability of a material is closely related to its dielectric properties [5,6,7]. The dielectric loss associated with the electromagnetic energy attenuation capability of non-magnetic materials mainly includes conduction loss and polarization relaxation loss [8]. Under the action of an external electromagnetic field, electrons in the material migrate to form a microcurrent and then dissipate electromagnetic energy in the form of Joule heat, resulting in conduction loss and attenuation of electromagnetic energy [9]. Therefore, high-performance EMW-absorbing materials should have certain conductive properties. However, excessive electrical conductivity will cause the skin effect, and the input impedance cannot match the impedance of the free space, leading to EMWs not being able to reach the inside of the material and thus not being able to dissipate more electromagnetic energy [10]. The polarization relaxation loss generally includes space-charge polarization and dipolar polarization. For composite materials, the space-charge polarization mainly comes from the heterogeneous interfaces [11]. Heterogeneous interfaces are formed when two dissimilar materials come into contact. Different materials have different energy band structures and Fermi levels. The different Fermi energies of the two materials lead to electron transfer across the interface, generating built-in electric fields and space charge regions, which affect the interfacial polarization of the materials [12]. In addition, defects, heteroatomic dopants, functional groups, and edge sites in materials can disrupt the symmetry of the local microstructure and induce the formation of electric dipoles [13, 14]. These electric dipoles oscillate under the action of an applied electromagnetic field, resulting in polarization relaxation loss, which in turn can lose electromagnetic energy [15]. Therefore, the dielectric properties of materials can be effectively regulated through the effective manipulation of factors such as conductivity, heterogeneous interfaces, defects, heteroatomic dopants, functional groups, and edge sites. And regulating the dielectric properties is the key to designing high-performance EMW-absorbing materials.

Recently, novel two-dimensional (2D) transition metal carbides and/or nitrides (MXenes) have been used as dielectric loss absorbers owing to their unique integration of metallic characteristics, high specific surface area, and remarkable conductivity [16]. On the one hand, the high electron mobility of MXenes improves the conduction loss, thereby helping increase the dielectric loss characteristics [17]. On the other hand, the defects and functional groups (− O, − OH, − F, etc.) on the surface of MXenes can be regarded as polarization centers that produce oscillations under an external electromagnetic field, thus enhancing polarization loss [18]. Thus, MXenes are expected to have considerable potential in the field of EMW-absorbing materials. Currently, many MXene-based materials, such as MXene films [19], MXene/rGO composites [20], and MXene/CNT nanocomposites [21], are widely applied as electromagnetic interference shielding materials. However, their high conductivity could induce a strong reflection of the incident EMW, which is undesirable for high-performance EMW-absorbing materials; thus, developing high-performance EMW-absorbing materials is still challenging.

Introducing dielectric loss-type materials into MXenes to modulate the dielectric properties of MXene-based materials may provide an effective strategy to solve the above issues [22]. Among the different kinds of dielectric loss-type materials, 2D transition metal dichalcogenide materials (TMDs) have attracted increasing amounts of attention as dielectric loss-type EMW-absorbing materials due to their low cost, unique layer structure, and semiconducting behavior [23, 24]. A composite of 2D TMDs and highly conductive materials not only overcomes the shortcomings of poor impedance matching of a single material but also effectively optimizes the dielectric loss properties of the material, thus effectively improving the EMW absorption properties [23]. For example, Zhang et al. developed three-dimensional (3D) heterostructure WS2/CNT materials that exhibit good EMW absorption performance with a minimum reflection loss (RLmin) of − 51.6 dB and an effective absorption bandwidth (EAB10) of 5.4 GHz [23]. Inspiringly, rhenium disulfide (ReS2) stands out owing to its special characteristics that distinguish it from conventional group VI TMDs. Most group VI TMDs assume the 2H structural phase, while ReS2 is commonly in the 1 T` structural phase [24]. The decreased symmetry endows ReS2 with unique electronic anisotropy, which may provide unique conduction loss properties for dielectric loss-type materials [24]. In addition, ReS2 exhibits a direct bandgap of 1.5–1.6 eV in both bulk and monolayer/multilayer forms, demonstrating its potential for application in the field of EMW-absorbing materials [25]. When ReS2 is introduced into MXenes materials, because different substances have different band structures and Fermi levels, the Fermi energy difference between the two materials will cause the transfer of electrons at the interface, resulting in built-in electric fields and space charge regions, thus affecting charge transport and interface polarization [26]. Similarly, the difference in dielectric properties of the ReS2 and MXenes leads to an uneven distribution of positive and negative charges, resulting in a spatial electric dipole moment, which enhances polarization relaxation [12]. In addition, the contact between ReS2 and MXenes can change the mode of electron transport. Electrons can hop across the interfacial energy barrier, thereby adjusting the conduction loss [27, 28]. Therefore, the incorporation of ReS2 into MXenes to achieve effective modulation of the dielectric loss properties of the material is expected to result in excellent EMW absorption performance. However, studies on the EMW absorption characteristics of MXenes containing ReS2 have rarely been reported. Furthermore, the dispersion of ReS2 in MXenes with lightweight EMW absorbers remains a challenge.

Significantly, the morphology and microstructure have important effects on the EMW absorption performance [29, 30]. 3D hierarchically ordered porous structures have shown great advantages in the field of EMW-absorbing materials owing to the many pores available for lightweight [31, 32] and optimizing impedance matching as well as the abundant edge sites available for enhancing polarization loss. Aerogels, as a unique 3D hierarchically porous material, have attracted widespread attention in the field of EMW-absorbing materials due to their low density, large specific surface area, and high porosity [33]. Furthermore, due to their unique structural advantages, aerogels have also been widely used in environmental remediation, biomedical applications, thermal insulation, and other fields [34,35,36,37,38], which can greatly expand the application fields of EMW-absorbing materials. Thus, developing an efficient strategy for constructing 3D hierarchically ordered porous aerogel structures is highly important for practical application. Herein, a directional freeze‒drying technology was developed to synthesize a 3D hierarchically ordered porous aerogel structure based on MXenes and ReS2 (3D OPMR). 3D OPMR has numerous interfaces, highly ordered pores, abundant edge sites, adjustable dielectric properties, and lightweight properties, resulting in unexpected EMW absorption performances with a minimum reflection loss (RLmin) and an effective absorption bandwidth (EAB10) of − 66.2 dB and 4.2 GHz, respectively. Radar cross-section (RCS) images obtained by computer simulation technology (CST) confirm the practical application of these methods in the field of electromagnetic absorption. Furthermore, the as-prepared 3D OPMRs exhibit excellent infrared stealth ability, which expands their potential application in civil defense and military applications.

2 Experimental

3D OPMR was fabricated through directional freeze-drying technology. The compared samples were fabricated by a precursor system with different 3D ReS2 feed ratios under the same conditions. The detailed experimental synthesis process, material characterization, and electromagnetic parameter measurements are shown in the Supporting Information.

3 Results and discussion

3D hierarchically ordered porous structures based on Ti3C2Tx MXene and ReS2 (3D OPMR) were fabricated through directional freeze-drying technology, as illustrated in Fig. 1. In the first step, multiple-layered Ti3C2Tx MXene was synthesized by etching Ti3AlC2 MAX with HCl and LiF solution. The relatively weak Ti–Al bond allows easy etching of the Al layer, leading to the formation of multiple layers of Ti3C2Tx MXene. After ultrasonic stripping of the multiple-layered Ti3C2Tx MXene, few-layered Ti3C2Tx MXene nanosheets were obtained. Then, in a high-temperature and high-pressure reaction environment, 3D ReS2 nanoflowers (3D NFR) were formed by a reduction reaction in which NH4ReO4 acted as the source of Re, CH4N2S provided the S source, and NH2OH·HCl functioned as a highly effective reducing agent during the reaction. Finally, 3D OPMR was prepared by mixing Ti3C2Tx MXene, ReS2, and (C6H7O6Na)n (SA) into a plastic mold and subsequently performing direct freeze-drying, in which the hydrogen bonding links between the functional group of SA and the Ti3C2Tx MXene nanosheets enhance the stability of the 3D aerogel structure and Ti3C2Tx MXene and ReS2 nanoflowers are used as functionalized units [39]. During the directional freezing process, liquid nitrogen rapidly sublimes in a bottom-up direction, causing water molecules in the solution to solidify and form ice crystals parallel to the sublimation of liquid nitrogen. Subsequently, during the vacuum drying process, the oriented ice crystal evaporation results in the formation of a 3D hierarchically ordered porous structure. To explore the effect of the modulation of dielectric properties on the EMW absorption performance of 3D OPMR, the amount of 3D ReS2 nanoflowers added to the Ti3C2Tx MXene matrix was controlled, and the compared samples were denoted 3D OPMR-1 (ReS2 addition of 50 mg), 3D OPMR-2 (ReS2 addition of 100 mg), 3D OPMR-3 (ReS2 addition of 150 mg), 3D OPM (without 3D ReS2 nanoflowers), and 3D OPM-1 (without 3D ReS2 nanoflowers and (C6H7O6Na)n (SA)).

Fig. 1
figure 1

Schematic illustration of the synthesis process of 3D OPMR

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The crystalline composition of the materials was characterized via X-ray diffraction (XRD). As shown in Fig. 2a, the XRD pattern of the as-prepared 3D NFRs presents characteristic diffraction peaks at 14.5°, 32.2°, 43.8°, and 57.1°, corresponding to the lattice planes of the (100), (002), (300) and (-122) planes of the 1 T` ReS2 phase (JCPDS 82–1379) [40]. The XRD pattern of the Ti3AlC2 MAX sample shows that the characteristic diffraction pattern matches perfectly with that of Ti3AlC2 (JCPDS 52–0875), as shown in Fig. S1 [41]. For the 3D OPM, the main characteristic peak near 6.06° can be attributed to the (002) crystal face of Ti3C2Tx, indicating the successful synthesis of Ti3C2Tx MXene after etching the Al layers of the Ti3AlC2 MAX [42]. The XRD diffraction peaks of both the ReS2 and Ti3C2Tx MXene phases can be detected for the 3D OPMR samplers, indicating that the 3D ReS2 nanoflowers are well combined with the Ti3C2Tx MXene. Furthermore, the intensity of the diffraction peaks corresponding to ReS2 gradually increased with increasing amounts of ReS2 added to the 3D OPMR samples. Fourier transform infrared (FT-IR) spectroscopy was used to analyze the chemical groups or structural compositions of the samples (Fig. 2b). The characteristic peaks of Ti − O (557 cm−1), the asymmetric vibration − COOH (1626 cm−1), the symmetric vibration − COOH (1416 cm−1), and − OH (3436 cm−1) can be detected in the 3D OPMR and 3D OPM samples. The characteristic peaks at 3448, 1626, 1416, and 1024 cm−1 are assigned to the stretching vibration of − OH, the asymmetric vibration, and the symmetric vibration of the − COOH and C − O groups, respectively, in the SA samples [43]. Compared with those of the SA samples, the characteristic peaks of − OH in the 3D OPMR and 3D OPM samples are shifted, which due to the hydrogen bonding interactions between the − COOH and − OH groups of SA and the termination groups (− OH, − O, − F) of MXene [41]. The above results reveal the presence of SA between MXene nanosheets, which can enhance interlamellar interactions through abundant hydrogen bonds, resulting in the reinforcement of the aerogel skeleton [43]. Furthermore, a large number of surface functional groups can act as polarization centers, contributing to polarization loss under an external electromagnetic field and thus increasing the EMW absorption performance [11]. X-ray photoelectron spectroscopy (XPS) was used to examine the elemental composition and bonding configurations of the samples. The full-energy XPS data indicate that 3D OPMR-2 is the main constituent of Re, S, Ti, C, O, and F; 3D OPM is the constituent of Ti, C, O, and F; and 3D NFR is the main constituent of Re and S (Fig. S2). The high-resolution C 1 s XPS spectra of 3D OPMR-2 and 3D OPM indicate the presence of C − Ti (281.6 eV), Ti − C − O (282.3 eV), C − C (284.8 eV), C − O (286.5 eV) and O − C = O (287 eV) bonds (Fig. 2c) [44]. The high-resolution Ti 2p XPS spectra of 3D OPMR-2 and 3D OPM indicate the presence of Ti − O 2p1/2 (462.3 eV), Ti (II) 2p1/2 (461 eV), Ti − C 2p1/2 (458.5 eV), Ti − O 2p3/2 (456.9 eV), Ti (II) 2p3/2 (455.7 eV), and Ti − C 2p3/2 (454.8 eV) bonds [44]. According to the C 1 s and Ti 2p XPS spectra, the presence of C-Ti bonds indicates the successful fabrication of the Ti3C2Tx MXene, where C atoms are attached to Ti atoms. As shown in Fig. 2e and Fig. S3, compared with those of 3D NFR and 3D OPM, obvious peak shifts are observed in the Re 4f and O 1 s XPS spectra of 3D OPMR-2, demonstrating the existence of strong interactions between the Ti3C2Tx MXene and ReS2 [45]. As shown in Fig. S3, an additional O 1 s peak at 530.5 eV is observed in 3D OPMR-2, which can be attributed to the formation of Re − O bonds [45]. The formation of Re − O bonds by close contact between Ti3C2Tx MXene and ReS2 is conducive to charge separation and transfer. According to the S 2p XPS spectra of 3D OPMR-2 and 3D NFR (Fig. 2f), the bonding configurations at 162.08 eV and 163.28 eV correspond to S 2p3/2 and S 2p1/2, respectively. In addition, the S − O bond can be detected in the S 2p XPS spectrum, which may be due to the partial oxidation of the ReS2 surface [46]. This peak is relatively stronger in 3D OPMR-2 than in 3D NFR, possibly due to the abundant oxygen terminations on 3D OPMR-2, which can form S − O bonds during the 3D OPMR preparation process.

Fig. 2
figure 2

a XRD patterns and b FT-IR spectra of all the samples. c C 1 s and d Ti 2p XPS spectra of 3D OPM and 3D OPMR-2. e Re 4f and f S 2p XPS spectra of 3D NFR and 3D OPMR-2

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To determine the microstructure and morphology of the samples, scanning electron microscopy (SEM) was carried out. The SEM images indicate that the 3D NFRs possess a 3D nanoflower-like structure consisting of 2D nanosheets (Fig. 3a). The magnified SEM images show that the size of the open pores between the adjacent nanosheets is ≈ 70 nm (Fig. S4a). 3D nanoflower-like structure helps to increase the specific surface area of the material compared to that of the bulk material, which contributes to the enhancement of the EMW absorption performance [40]. SEM images of the 3D OPM show that the as-prepared samples exhibit directionally open pore structures and long straight paths (Fig. 3b-c). The formation of these directionally open pore structures and long straight paths is attributed to the directional solidification of numerous ice crystals from the bottom of the aerogel in a low-temperature liquid nitrogen environment. These long paths contribute to the fast transport of carriers and construct a conductive loss network, thus enhancing the EMW absorption properties [47]. As expected, the 3D OPMR samples possess directionally open pore structures and long straight paths similar to those of the 3D OPM (Fig. 3d-f and Fig. S4b-c). ReS2 was uniformly dispersed on the surface of the aerogel skeleton. The surface of the skeleton is rougher as the proportion of the ReS2 dopant increases, as shown in Fig. 3d-f and Fig. S4b-c. The corresponding SEM energy-dispersive X-ray spectroscopy (SEM–EDX) maps further indicate the presence of C, Ti, Re, and S in 3D OPMR-2 (Fig. 3g). The digital images of 3D OPMR-2 are displayed in Fig. 3h. 3D OPMR-2 exhibits lightweight properties due to its rich pore structure and as-prepared 3D OPMR-2 with densities as low as 0.04 g cm−3, and 3D OPMR-2 can lie on top of a dandelion without destroying the fluffy heads. Transmission electron microscopy (TEM) confirmed that ReS2 possesses a 3D nanoflower-like structure consisting of 2D nanosheets (Fig. 4a-b). High-resolution TEM (HRTEM) images revealed that the interlayer spacing of the ReS2 nanosheets was 0.612 nm, assigned to the (100) plane of 1 T` ReS2 (Fig. 4c) [40]. TEM images of the 3D OPM show a flaky morphology (Fig. 4d-e), indicating that the few-layered Ti3C2Tx MXene was successfully prepared. The HRTEM images show that the layer spacing between adjacent layers of ~ 1.43 nm (Fig. 4f) corresponds to the (002) crystal spacing [42], in agreement with the XRD results. According to the TEM (Fig. 4g) and HRTEM (Fig. 4h-i) images of the 3D OPMR-2, the crystal spacings of ReS2 and Ti3C2Tx MXene in 3D OPMR-2 are the same as those of the 3D NFR and 3D OPM, suggesting that the lattice state of both materials remains unchanged after composite formation. Furthermore, 3D OPMR-1 and 3D OPMR-3 exhibited morphologies and lattice characteristics similar to those of 3D OPMR-2, as shown in Fig. S5.

Fig. 3
figure 3

SEM images of (a) 3D NFR. SEM images of 3D OPM: (b) top view and (c) side view. SEM images of 3D OPMR-2: (d) top view and (e–f) side view. (g) EDS mappings of 3D OPMR-2. (h) Digital photograph of 3D OPMR-2

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Fig. 4
figure 4

a-b TEM images of 3D NFR. c HRTEM images of 3D NFR. d-e TEM images of the 3D OPM. f HRTEM images of the 3D OPM. g TEM images of 3D OPMR-2. h-i HRTEM images of 3D OPMR-2

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Electromagnetic parameters are important indicators for evaluating the EMW absorption properties of materials [48]. The complex permittivity (εr = ε′ − jε″) and complex permeability (μr = μ′ − jμ″) were measured using the coaxial line method in this work. The real parts \(\varepsilon {\prime}\) and \(\mu {\prime}\) represent the electrical and magnetic energy storage capacities, respectively, and the imaginary parts \({\varepsilon }^{{\prime}{\prime}}\) and \(\mu {\prime}{\prime}\) denote the electrical and magnetic energy dissipation capacities, respectively. In addition, loss tangents, including dielectric loss (tan δε = ε″/ε′) and magnetic loss (tan δμ = μ″/μ′), can also be used to evaluate the attenuation of electromagnetic energy in absorbers. As shown in Fig. 5a-c, the 3D OPM sample has larger ε′, ε″ and tan δε values than the other samples, possibly because the Ti3C2Tx MXene has a higher conductivity. However, overconductivity can cause an impedance mismatch issue, blocking the entry of EMWs into the material and thus affecting the EMW absorption properties [49]. The lower ε′, ε″ and tan δε values of the 3D NFR indicate its poor storage and dissipation capacity of electrical energy, which also prevents excellent EMW absorption. Compared with 3D OPM and 3D NFR, 3D OPMR materials have adjustable electromagnetic parameters. 3D OPMR-2 has ε′, ε″ and tan δε values in the range of 9.5–14.2, 3.5–5.5, and 0.2–0.4, respectively, which are greater than those of 3D OPMR-1 (9.1–12.2, 3.3–5.1, 0.2–0.5) and 3D OPMR-3 (7.8–8.3, 1.6–2.0, 0.1–0.2), indicating that the addition of ReS2 modulates the dielectric properties of the materials and the 3D OPMR-2 have the stronger dielectric loss performances than 3D OPMR-1 and 3D OPMR-3. In addition, the complex permeability values of the samples are shown in Fig. S6. The μ′, μ″, and tan δμ values are close to 1, 0, and 0, respectively, which is due to the absence of magnetic substances in the samples. Because the EMW absorption property of an absorber is dependent on both dielectric loss and magnetic loss, the lower tan δμ values than the tan δε values demonstrate that the EMW absorption properties of our designed samples are mainly attributed to their dielectric losses.

Fig. 5
figure 5

a Real part \(\varepsilon'\) and b imaginary part \({\varepsilon }^{{\prime}{\prime}}\) of the complex permittivity of all the materials. c Dielectric loss tangent of all the materials. d Conductivity of all the materials. e εc″ − f curves and f εp″ − f curves of all the materials

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To further analyze the dielectric loss, the curves of conductive loss (εc″) and polarization relaxation (εp″) in the range of 2–18 GHz are calculated and shown in Fig. 5e-f. Based on Debye theory, \({\varepsilon }^{{\prime}{\prime}}\) can be described as [50]:

$${\upvarepsilon }^{{\prime}{\prime}}=\frac{\omega \tau \left({\varepsilon }_{s }-{\varepsilon }_{\infty }\right)}{1+{\omega }^{2}{\tau }^{2}}+\frac{\sigma }{\omega {\varepsilon }_{0}}={\varepsilon }_{p}^{{\prime}{\prime}}+{\varepsilon }_{c}^{{\prime}{\prime}}$$

where ω is the loss angle, τ is the polarization relaxation time, εs is the static dielectric constant, ε∞ is the relative permittivity and σ is the conductivity at high frequencies. εc″ is related to the conductivity of the material (εc'' = σ/ωε0). The conductivities of all the samples were measured via Hall effect measurements, and the conductivities of the 3D NFR, 3D OPM, 3D OPMR-1, 3D OPMR-2, and 3D OPMR-3 were 0.02, 1.61, 0.59, 0.28 and 0.18 S cm−1, respectively (Fig. 5d). It can be found that compared with the 3D NFR, 3D OPM has higher electrical conductivity. The electrical conductivity of the 3D OPMR decreases with the increase of ReS2 content. As shown in Fig. 5e, the εc″ − f curves of the samples further indicate the introduction of ReS2 into the Ti3C2Tx MXene can remarkably adjust the material conductivity and, correspondingly, the conduction loss. Based on the εc″ values, the polarization relaxation loss (εp״ = ε״ – εc״) of the samples can be extracted. As shown in Fig. 5f, the higher εp״ values of the 3D OPM is due to its larger ε″ value. Similarly, the lower εp״ values of 3D NFR are attributed to its smaller ε″ value. The εp″ values of the 3D OPMR samples show that the polarization relaxation losses follow the order of 3D OPMR-2 > 3D OPMR-1 > 3D OPMR-3, almost consistent with the increased order of the electrical energy dissipation capacities (ε″ − f curves in the Fig. 5b). Thus, the enhanced dielectric loss properties of 3D OPMR-2 can be attributed to its increased polarization relaxations. Generally, the dielectric relaxation loss includes space-charge polarization and dipolar polarization. For composite materials, the space-charge polarization mainly comes from the interfaces. In this work, the large number of interfaces between the ReS2 and Ti3C2Tx MXene nanosheets increases interfacial polarization. In addition, according to the XPS and FT-IR results, there are many polar groups, such as − COOH, C − F, and C − O, on the surface of the materials, which can serve as polarized centers for polarization relaxations, and thus contribute to part of dielectric loss. Due to the chemical etching treatment of titanium carbide and aluminum, Ti3C2Tx MXene nanosheets have a large number of defects on their surface, which can also serve as polarization factors for inducing dipole polarization loss. There are multiple Cole‒Cole semicircles in Fig. S7 indicating that the dielectric relaxation processes occurred upon exposure of the samples to EMW radiation. It is worth noting that 3D OPM/3D NFR has the highest/lowest εp″ value probably due to its largest/smallest ε″ value. For the Ti3C2Tx MXene/ReS2 composites, the polarization relaxation losses follow the order of 3D OPMR-2 > 3D OPMR-1 > 3D OPMR-3 may be attributed to the following factors: When the content of ReS2 increases, the Ti3C2Tx MXene/ReS2 composites have more interfaces and dipoles, thus enhancing the polarization relaxation loss ability of the materials. However, when the content of ReS2 is excessive, ReS2 will agglomerate on the surface of Ti3C2Tx MXene, which will reduce the contact interface and dipoles of the composite, resulting in a decrease in the polarization relaxation loss performance. Thus, introducing proper content of ReS2 into Ti3C2Tx MXene can optimize the conduction loss of the material and is expected to solve the problem of impedance mismatch caused by the high conductivity of Ti3C2Tx MXene. At the same time, the introduction of proper content of ReS2 can effectively adjust the polarization relaxation loss characteristics of the material, thereby optimizing the dielectric loss properties.

The reflection loss (RL) can represent the EMW absorption properties of materials and is calculated according to transmission line theory [51]. Figure 6a-e show the 2D and 3D RL − f curves of the 3D NFR, 3D OPM, 3D OPMR-1, 3D OPMR-2, and 3D OPMR-3, respectively. The minimal reflection loss (RLmin) of the 3D NFR cannot reach − 10 dB, which reflects its poor EMW absorption properties. The RLmin of the 3D OPM reaches only − 10.49 dB at 13.76 GHz; thus, overconductivity can cause an impedance mismatch issue and unsatisfactory EMW absorption performance. Compared with 3D OPMR-1 (− 21.68 dB at 17.81 GHz) and 3D OPMR-3 (− 29.93 dB at 17.46 GHz), 3D OPMR-2 shows stronger EMW absorption properties with an RLmin of − 66.20 dB at 5.62 GHz. Notably, the RL values of 3D OPMR-2 in the range of 2–18 GHz with thicknesses of 1.5–5.0 mm reach − 30 dB, indicating that up to 99.9% of the incident EMW can be effectively absorbed by 3D OPMR-2. In addition, the effective absorption bandwidth (EAB10, RL ≤  − 10 dB) is another important indicator for evaluating excellent EMW absorption performance. The EAB10 of the 3D NFR, 3D OPM, 3D OPMR-1, 3D OPMR-2, and 3D OPMR-3 can reach 0 GHz, 1.60 GHz, 2.82 GHz, 4.20 GHz, and 1.52 GHz, respectively. Among these samples, 3D OPMR-2 displays stronger RLmin values, broader EAB10, and thinner thicknesses (Fig. 6f), indicating that 3D OPMR-2 has good EMW absorption performance. In addition, the 3D OPM and 3D OPM-1 exhibit comparable electromagnetic wave absorption properties (Fig. S8a), indicating the (C6H7O6Na)n (SA) in these samples do not play a crucial role in their EMW absorption properties. Furthermore, the effects of the filler ratio of the 3D OPMR-2 on the EMW absorption properties have been investigated. As shown in Fig. S8b, the RLmin value of 3D OPMR-2 with a filler ratio of 20 wt.% is –13.87 dB at 12.2 GHz, and the EAB10 is 2.50 GHz with a thickness of 2.0 mm. When the filler ratio of the 3D OPMR-2 is increased to 40 wt.%, the RLmin value is –31.58 dB at 5.8 GHz, the EAB10 up to 3.60 GHz at the thickness of 2.0 mm (Fig. S8c). These results demonstrate that the different filler ratios of 3D OPMR-2 affect the EMW absorption performance, and the optimal filling ratio of 3D OPMR-2 is 30 wt.% in this work.

Fig. 6
figure 6

2D and 3D RL − f curves of (a) 3D NFR, (b) 3D OPM, (c) 3D OPMR-1, (d) 3D OPMR-2, and (e) 3D OPMR-3. (f) Comparison of properties across all materials

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In addition, impedance matching and attenuation capability are key factors in evaluating the properties of EMW-absorbing materials. When an EMW reaches the macroscopic interface of a material, additional waves should enter the material and attenuate, thus achieving the purpose of absorbing the EMW and controlling electromagnetic radiation pollution. Hence, the primary principle in designing EMW absorbers is to achieve better impedance matching. In this work, the electromagnetic parameters of the samples are tuned by the amount of ReS2 in the Ti3C2Tx MXene/ReS2, which may meet the requirement of impedance matching characteristics for high-performance EMW absorbers. In general, the impedance matching characteristic can be described as [52,53,54,55,56,57]: Z =| Zin / Z0 |, where Z is the normalized impedance. When Z is unlimitedly close to 1, the absorber has better impedance matching characteristics [11]. Figure 7a-e shows that the Z values at thicknesses of 1.5–5.0 mm for 3D OPMR-2 are closer to 1 than those for the other samples. Therefore, the best impedance matching characteristic endows the 3D OPMR-2 to have excellent EMW absorption property. Moreover, under the premise of satisfying good impedance matching, the excellent attenuation of EMW by the material means that it has good EMW absorption properties. In addition, the attenuation constant (α) is also an important parameter to evaluate EMW absorption property of the absorber [58]. The attenuation constant represents the attenuation ability of the energy of the EMW that entered into the absorber. Under the premise of satisfying good impedance matching, the excellent attenuation of EMW by the material means that it has good EMW absorption properties. As shown in Fig. 7f, compared with those of the 3D NFR (13–65), 3D OPM (115–714), 3D OPMR-1 (25–256), and 3D OPMR-3 (2–127), the 3D OPMR-2 has larger α values (40–270) in the range of 2–18 GHz. The best impedance matching characteristic and the largest attenuation constant of the 3D OPMR-2 indicate that it has good EMW absorption properties performance. To compare the EMW absorption property of the 3D OPMR-2 with other reported MXene-based absorbers, the radar chart based on the three parameters (minimal reflection loss, filler loading, and effective absorption bandwidth) is plotted in Fig. 8 [59,60,61,62,63,64,65,66]. The triangle drawn by the minimal reflection loss, filler loading, and effective absorption bandwidth of 3D OPMR-2 has the largest area, which means that 3D OPMR-2 has potential application value in the field of EMW absorption materials (Table S1).

Fig. 7
figure 7

Z – f curves of (a) 3D NFR, (b) 3D OPM, (c) 3D OPMR-1, (d) 3D OPMR-2, and (e) 3D OPMR-3. (f) Attenuation constant values of all the materials

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Fig. 8
figure 8

a Frequency-dependent α, RL, and Z values of 3D OPMR-2 at 4 mm. b Comparison with the EMW absorption properties of reported MXene-based absorbers

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The good EMW absorption performance of 3D OPMR-2 can be attributed to the following factors. First, the large number of functional groups and defects on the surface of the materials can be used as polarization factors and contribute to the dipole polarization of the materials [67]. Second, the difference in conductivity between MXene and ReS2 leads to interface polarization due to the uneven distribution of positive and negative charges at the contact interface caused by the Schottky barrier. Third, Ti3C2Tx MXene nanosheets with good electrical conductivity provide a large number of free electrons. The ordered porous structure provides a fast transmission channel for carriers, and the conductive network constructed from the Ti3C2Tx MXene nanosheets and an appropriate amount of nanoflower-like ReS2 help to improve the conductive loss related to the dielectric loss of the material. Furthermore, 3D hierarchically ordered porous structures can enhance the multiple scattering of the incident EMW, prolong the transmission path of the EMW, and enhance the attenuation of the incident EMW, as summarized in Fig. 9.

Fig. 9
figure 9

Schematic illustration of the EMW absorption mechanisms for 3D OPMR

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Furthermore, the EMW absorption performance of the samples under real far-field conditions was evaluated via CST simulations of a perfect conductive layer (PEC) covered by the as-prepared samples [68]. The detailed simulation parameters are presented in the Supporting Information. With an angle detection of -60°–60°, the 3D radar wave scattering signals of pristine PEC and PEC covered with the as-prepared samples are shown in Fig. 10a-f. Clearly, the pristine PEC shows the maximum 3D scattering signals (Fig. 10a). A comparison of the other samples reveals that the model covered with 3D OPMR-2 has the smallest 3D scattering signals, which indicates that 3D OPMR-2 has the smallest radar cross-section (RCS) value. These RCS simulation results are consistent with the above-measured EMW absorption properties. In other words, electromagnetic signals can be effectively suppressed when the metal plane is covered by 3D OPMR-2. The 2D RCSs of all the samples under different incident angles are shown in Fig. 10g. The incident wave perpendicular to the plane of the material has the largest RCS, and the RCS gradually decreases with increasing detection angle. 3D OPMR-2 has lower RCS values than the other samples over the incident angle range of -60°–60°, indicating its strong suppression of EMWs at any incidence angle. Therefore, the above analysis verifies that 3D OPMR-2 has good EMW attenuation capability, in which strong microwave scattering signals can be successfully suppressed. Considering the application requirements in real-world environments, as-prepared 3D OPMR-2 is practically feasible for both civil and military stealth applications.

Fig. 10
figure 10

CST simulation results of the (a) PEC, a PEC covered with (b) 3D NFR, (c) 3D OPM, (d) 3D OPMR-1, (e) 3D OPMR-2, and (f) 3D OPMR-3. (g) RCS simulation curves of the samples at different scanning angles. (h) Thermal infrared images of 3D OPMR-2 placed on the heating platform for different durations at 100 °C. (i) Sample surface temperature vs. time

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A porous structure is expected to achieve efficient thermal insulation properties and is expected to extend the application of EMW-absorbing materials in thermal environments [69]. Figure 10h shows that the 3D OPMR-2 was placed on a hot disk heated to 100 ℃, and infrared temperature surveys were conducted five times within a 3 h period. After heating for 0 min, 30 min, 60 min, 120 min, and 180 min, the center temperature of 3D OPMR-2 heated continuously reached 35.6 ℃, 36.4 ℃, 37.1 ℃, 37.4 ℃, and 37.8 ℃, respectively (Fig. 10i), indicating the excellent thermal resistance of 3D OPMR-2. In this work, the thermal conductivity of the 3D OPMR-2 is mainly related to gas-phase and radiative heat transfer, respectively [70]. There is a large amount of air in the hierarchically ordered porous structure, and its thermal conductivity was much lower than that of the Ti3C2Tx MXene/ReS2 framework. Therefore, the high porosity of the 3D OPMR-2 can significantly reduce the thermal conductivity and radiative heat transfer capacity of the solid phase, giving it good thermal insulation properties. The good thermal insulation endows 3D OPMR-2 with infrared stealth properties, which can enhance its stealth capabilities to effectively counteract enemy infrared detection.

4 Conclusion

In summary, a dielectric modulation engineering strategy is proposed for obtaining absorbing materials with excellent EMW absorption properties. A lightweight 3D hierarchically ordered porous structure based on MXene and ReS2 (3D OPMR) was fabricated through directional freeze-drying technology. The 3D nanoflower-like structure of ReS2 effectively modulates the dielectric properties of the materials, while ultrasonic stripping results in few-layered Ti3C2Tx MXene nanosheets that have good conductive loss capability. The unique nanoflower-like structure of ReS2 is uniformly distributed on the Ti3C2Tx MXene nanosheets, and the porous backbone, nanostructure, and multilayer configuration optimize the heterogeneous interface and maximally improve the material impedance matching. By adjusting the ratio of ReS2, tunable electromagnetic parameters and EMW absorption properties were achieved. As expected, 3D OPMR-2 exhibits unexpected EMW absorption when RLmin and EAB10 reach − 66.20 dB and 4.20 GHz, respectively. Therefore, the effective combination of these two elements can provide materials with excellent EMW absorption properties. The CST simulation results validate the attenuation effect of 3D OPMR-2 on electromagnetic energy under real application conditions. In addition, the unique hierarchical pore structure endows the material with good infrared stealth properties, which expands the application of EMW-absorbing materials in military and civil defense fields. Our results provide unique design perspectives and inspirations for the construction of enhanced EMW-responsive materials, as well as insights for the exploration of other multifunctional materials to cope with harsh environments.