Scalable synthesis of 2D Ti2CTx MXene and molybdenum disulfide composites with excellent microwave absorbing performance

The signal crosstalk and electromagnetic interference (EMI) problems direly need to be resolved in the rapid development of modern microwave communication technology for a better working frequency and transmission power of electronic systems. Where the new absorbing materials such as molybdenum disulfide (MoS2)/titania (TiO2)/Ti2CTx and MoS2/Ti2CTx composites could meet the requirement of “thin, strong, light weight, and wide band” for excellent absorbing performance. In this work, a lighter Ti2CTx material was selected as the matrix, and MoS2 was in-situ grown on Ti2CTx matrix by traditional hydrothermal method and microwave solvothermal method. The fabricated composite exhibited synergic effect of two-dimensional heterostructural interface and double dielectric elements, where a small amount of TiO2 and a certain proportion of MoS2 jointly improve the impedance matching of the composite material. In here, the extreme reflection loss (RLmin) can reach − 54.70 dB (with a frequency of 7.59 GHz, 3.39 mm thickness), and the maximum effective absorption bandwidth (EABmax) can reach 4 GHz. Polyethylene glycol 200 was used as the solvent instead of water to make Ti2CTx less oxidized during the composite process, where the microwave heating would attain fast speed, short time, high efficiency, and uniform product. Since, the MoS2/Ti2CTx composite without oxidizing possessed a wider effective absorption bandwidth (EAB) at a thinner thickness, thus resulting in the excellent microwave absorption performance and confirming the validity and rationality of new microwave absorption materials.


Introduction
With the rapid development of modern microwave communication technology, the working frequency and transmission power of electronic systems are getting higher while continuously raising the issues regarding signal crosstalk and EMI [1,2]. In order to effectively avoid the damage caused by the electromagnetic wave pollution and interference, the development of microwave absorbing (MA) materials with stronger microwave absorption capacity could be the emerging way of resolving the concerned matters. Among these, the traditional absorbing materials (TAM) mainly including ferrite, barium titanate, carbon black, metal powder, and silicon carbide are widely used in military and industrial fields [3][4][5][6][7][8]. Although traditional Baoji Miao and Yange Cao contributed equally to this work.
* Qingsong Zhu qingsong_zhu@haut.edu.cn * Muhammad Asif Nawaz Mnawaz@us.es absorbing materials have excellent absorbing performance and low price; however, it contain high density that make it inappropriate to be widely adopted in a renewable zone. Since, in order to realize the goal of "thin, wide, light, and strong" absorbing material, (NAM) new absorbing materials (such as plasma materials, electromagnetic metamaterials, and multi-spectrum composites) can be a better approach. NAM are usually categorized into zeronanometer particles, one-nanometer wires/tubes, and twodimensional nanosheets according to their dimensions. In which, the two-dimensional materials with high surface area (such as MXene, MoS 2 and graphene) are the most ideal candidates for microwave absorption. The surface of MXene (Ti 3 C 2 T x , Ti 2 CT x ) with accordion-like layered structure and various active sites presents excellent performance in battery electrode, catalysis, electromagnetic absorption (EA), and other energy sectors [9][10][11][12]. Since 2011, Naguib et al. [9] prepared Ti 3 C 2 MXene phase by etching Ti 3 AlC 2 for the first time, where only Ti 3 C 2 T x had been widely studied in the field of microwave absorption among the 30-35 synthesized MXene materials afterward. Where, Ti 3 C 2 T x MXene was first investigated in 2016 for its absorbing performance, while comparing it with that of Ti 3 AlC 2 . The experimental results revealed that the thickness of the material was 1.4 mm at the filling ratio of 50 wt%, and the extreme reflection loss (RL min ) of Ti 3 C 2 T x could reach − 17 dB, which was much lower than the RL min of Ti 3 AlC 2 [13]. Similarly, Han et al. [14] annealed Ti 3 C 2 T x at high temperature under argon atmosphere to obtain a layered structure with local TiO 2 /C interlayer. They found that with 1.7 mm thickness at 50 wt% TiO 2 /Ti 3 C 2 T x , the RL min value was − 48.7 dB and the effective absorption bandwidth (EAB) was 2 GHz. Asia et al. [15] made a translucent Ti 3 C 2 T x antenna with a thickness of about 100 nm and a reflection loss (RL) of less than − 10 dB, where they achieved the RL min value of − 65 dB by decreasing the antenna thickness to 8 mm. Guo et al. [16], constructing the Ti 3 C 2 T x @SiO 2 core-shell structure with thick edges and thin middle through the Stöber method, revealed that the abundant functional groups at the edge of Ti 3 C 2 T x enable more SiO 2 to be formed at the edges, significantly enhancing the interface polarization loss. At the same time, the SiO 2 layer with adjustable thickness can balance the surface impedance and allow more microwaves to penetrate the absorber, thus reducing the reflection of the incident wave.
Therefore, owing to these qualifications, Ti 2 CT x with a similar structure to Ti 3 C 2 T x in MXene family, and excellent microwave absorption performance, is a spectacular contestant that can be obtained by selective etching of Al atomic layer in terpolymer Ti 2 AlC ceramic using HF or HCl + LiF [17,18]. Due to the abundant -OH, -F, and = O functional groups (represented by T x ) formed on the surface of the material during the wet chemical etching process, Ti 2 CT x possess excellent hydrophilicity and dispersibility. Different density functional theory (DFT) simulation results interpreted that the formation of carbon vacancy improves the conductivity of the material, where Ti 2 CT x inherits Ti 2 AlC's accordion structure for the conducive electromagnetic attenuation [19,20]. Since, the stable layered structure, high conductivity, and abundant surface functional groups place a great prospect of Ti 2 CT x in the field of electromagnetic shielding and microwave absorption. However, the high conductivity of Ti 2 CT x gives it a large dielectric constant, resulting in the impedance mismatches. By combining MoS 2 with MXene material with the large-scale two-dimensional heterostructures, the absorption performance of MXene material can be improved by adjusting the impedance matching [21]. It has been demonstrated in a recent work that the dielectric loss and impedance matching can be tuned with core layer structure in CF@MXene@MoS 2 nanocomposite by electrostatically self-assembling Ti 3 C 2 nanosheets on the surface of carbon fiber (CF) and further anchoring MoS 2 nanosheets on the surface of Ti 3 C 2 by hydrothermal method [22]. Similarly, engineering of three-dimensional MXene/MoS 2 folded microspheres with MXene and molybdenum disulfide nanosheets [23], and MoS 2 /Ti 3 C 2 T x composite [24] has also been spotted in deviating the impedance mismatches and energy barriers via regulating the relative RL min , thickness, packing, and the frequency values.
In this work, MoS 2 /TiO 2 /Ti 2 CT x and MoS 2 /Ti 2 CT x composites with excellent wave-absorbing properties were synthesized by traditional hydrothermal method and microwave solvothermal method (Fig. 1). The fabricated composite exhibited synergic effect of two-dimensional hetero structural interface and double dielectric elements, where the small amount of TiO 2 and a certain proportion of MoS 2 jointly improve the impedance matching of the composite material. The RL min can reach − 54.70 dB (with a frequency of 7.59 GHz, 3.39 mm thickness), and EAB max can reach 4 GHz. While the deployment of polyethylene glycol 200 as the solvent hindered the oxidation of Ti 2 CT x during the composite process which facilitated the MoS 2 /Ti 2 CT x composite in possessing a wider EAB at a thinner thickness.

Preparation of Ti 2 CT x nanosheets
The first step includes the preparation of Ti 2 AlC MAX phase. A certain proportion of titanium carbide, titanium powder, and aluminum powder were mixed evenly and pressed into blocks, which were sintered in a tubular furnace at 1300 ℃ and held for 180 min, and finally grounded into Ti 2 AlC powder.
Secondly, the prepared Ti 2 AlC was etched to obtain Ti 2 CT x material. 20 mL of concentrated hydrochloric acid (to make an 8 M hydrochloric acid solution) was added into a 10 mL deionized water in a 50 mL sized beaker. Gradually, 2.5 g lithium fluoride was added to the above solution and stirred until completely dissolved, and then 2 g of homemade Ti 2 AlC was slowly mixed. The reaction was carried out at 40 °C for 48 h with continuous magnetic agitation. At the end of the reaction, the mixture was washed with deionized water and centrifuged (to make the supreme clear liquid), followed by washing with dilute hydrochloric acid and centrifuged twice (to remove the unfinished LiF), and finally washed with ethanol and centrifuged twice (to make the dried fraction disperse well). The obtained centrifugal products were put into a vacuum drying oven and dried at 60 °C for 12 h to get the Ti 2 CT x powder.

Preparation of MoS 2 /TiO 2 /Ti 2 CT x nanocomposites
Ti 2 CT x nano-powder measuring 0.2 g was added into a beaker containing 60 mL deionized water. The raw material of molybdenum sulfide synthesis was weighed by the theoretical mass ratio of MoS 2 : Ti 2 CT x (weighed by the ratio of molybdenum to sulfur of 5:1). The raw materials of ammonium molybdate tetrahydrate and thiourea were respectively weighed by calculation and added into the beaker with Ti 2 CT x nano powder, under continuous magnetic stirring for 30 min. After stirring, the mixed solution was transferred to a stainless-steel high-pressure reactor and heated in an oven at 200 °C for 18 h. The resulting reaction product was washed several times with deionized water and ethanol. Finally, the reaction products were put into a vacuum drying oven and dried at 60 °C for 12 h to get MoS 2 /TiO 2 / Ti 2 CT x nanocomposites. In this experiment, three samples in different proportions of MoS 2 and Ti 2 CT x were prepared and labeled as MTT-1 (MoS 2 : Ti 2 CT x = 1:1), MTT-2 (MoS 2 : Ti 2 CT x = 1:2), and MTT-3 (MoS 2 : Ti 2 CT x = 1:3).

Preparation of MoS 2 / Ti 2 CT x nanocomposites
Ti 2 CT x powder measuring 0.1 g was added to a beaker containing 150 mL polyethylene glycol 200. The raw material for the synthesis of molybdenum sulfide was weighed by the theoretical mass ratio of MoS 2 : Ti 2 CT x = 8:1 (The ratio of molybdenum to sulfur was 5:1). Where 0.8824 g ammonium molybdate tetrahydrate and 1.9020 g thiourea were added to the beaker with Ti 2 CT x powder and stirred for 90 min with magnetic force. After stirring, the mixed solution was poured into 250 mL three-way flask placed in the microwave reaction device, by setting the target temperature of 200 °C, holding time of 30 min, and heating rate of 10 °C/min. The resulting reaction product was washed several times with deionized water and ethanol. Finally, the reaction products were placed in a vacuum drying oven and dried at 60 °C for 12 h to obtain MoS 2 / Ti 2 CT x nanocomposites, which were labeled as MT-4.

Characterization
X-ray diffraction (XRD; Bruker D8 ADVANCE) was used for phase analysis in the 2θ range of 5-90°. The surface Ti (C, Mo, S, or O) states of the samples were obtained by X-ray photoelectron spectrometry (XPS, Thermo SCI-ENTIFIC ESCALAB 250Xi). The surface morphology and composition of the samples were characterized by field emission scanning electron microscope (SEM; FEI INSPECT F50) and field emission transmission electron microscopy (TEM; FEI Tecnai G2 F30). In order to test the microwave absorption characteristics of the prepared material, the test material was mixed with paraffin wax in a certain proportion (Sample: Paraffin = 7:3) and compressed into a coaxial ring with an inner ring diameter of 3 mm and an outer ring diameter of 7 mm. The electromagnetic parameters of the material were measured by a vector network analyzer (VAN; Agilent, N5234A), and the test frequency range was 2 ~ 18 GHz.  [26], may indicate that Ti 2 CT x is easy to oxidize during the hydrothermal reaction, thus introducing TiO 2 phase. Meanwhile, as compared to the spectral line of MTT-2, it can be found that MT-4 has the diffraction peak of Ti 2 CT x at 7.02° (002) and 18.1° (111), with the obvious deviation the of peak (002) position to a smaller angle, indicating that the introduction of MoS 2 increases the layer spacing of Ti 2 CT x . In addition, no characteristic peak of TiO 2 appeared in the spectral line, interpreting that the use of microwave solvothermal method effectively prevented the oxidation of Ti 2 CT x into TiO 2 during the reaction process. Where, the spectral line of MTT-1 and MT-4 at 9.3° as compared to that of MTT-2 and MTT-3 for partially intercalated crystal plane of MoS 2 (002) may illustrate the insertion of more thiourea one molecules between MoS 2 layers with the increase of sulfur source concentration, thus increasing the layer spacing and shifting the diffraction peak to a smaller angle [27]. Further evaluating the surface chemical composition and bonding nature of the as-prepared samples, Ti 2 CT x , MTT-2, and MT-4 were representatively characterized using X-ray photoelectron spectroscopy (XPS). The overall elemental analysis can be respectively found in the overall survey of The comparative analysis demonstrates that the relative intensity of O1s photoelectron peak at 530 eV of MTT-2 is higher than that of Ti 2 CT x and MT-4, due to the fact that Ti atoms are easily oxidized in the hydrothermal reaction, and Ti 2 CT x combines with a large number of O atoms from the external environment, thus resulting in the increase of O1s photoelectron peak. In addition, the photoelectron peak intensity of F1s and Cl1s of MTT-2 and MT-4 decreased significantly, indicating that the content of -F and -Cl functional groups decreased greatly after the recombination reaction. Taking MTT-2 as an example, the narrow region fine scanning patterns of Ti2p, C1s, O1s, Mo3d, and S2p were analyzed. The two Ti2p-related peaks can be classified as TiO 2 at the binding energy of 458.9 eV and 464.7 eV [28,29], respectively (Fig. 2d). Due to the oxidation of Ti element after hydrothermal synthesis, the peak of TiO 2 become dominant, where the detected Ti content is small and the photoelectron peak intensity of Ti element is low, that can be attributed to the covering of Ti 2 CT x by MoS 2 for less than 10 nm detection depth of XPS. On the other hand, the prominent characteristic peak of TiO 2 reveal the easier detection of TiO 2 being distributed on the surface and edges of Ti 2 CT x after oxidation. The three C1s peaks (Fig. 2e) at the binding energies of 284.6 eV, 286.3 eV, and 287.7 eV can be assigned to C-C, C-O, and O-C = O, respectively [30,31]. Where, the presence of C-C main peaks may come from external polluted carbon or amorphous carbon in the structure of Ti 2 CT x , while C-O and C = O are related to the adsorption of oxygen in the air on the surface of the material. Figure 2f displays three O1s associated peaks at 529.7 eV, 530.1 eV, and 531.6 eV being indexed to TiO 2 , C-Ti-O x , and C-Ti-(OH) x , respectively [32,33]. In here, TiO 2 reflects the oxidation state of Ti, and C-Ti-O x , while C-Ti-(OH) x reflects the binding state of O in the surface functional groups. Similarly, the seven Mo3d peaks can be attributed to Mo 4+ 3d5/2 (228.6 eV, 229.4 eV, and 232.2 eV), Mo 4+ 3d3/2 (232.8 eV and 234.7 eV), and Mo 6+ 3d3/2. In here, Mo 4+ 3d5/2 and Mo 4+ 3d3/2 correspond to Mo in MoS 2 , and the appearance of Mo 6+ (MoO 3 ) characteristic peak is related to the insufficient reaction of molybdenum source [34]. While, the characteristic peak at 225.7 eV corresponds to the binding energy of S2s orbital in MoS 2 (S 2− ) (Fig. 2g). Moreover, it can be seen from Fig. 2h that there are two forms of S in the sample, namely S 2− and S 2 2− /S n 2− . The two characteristic peaks at 161.57 eV and 162.79 eV reflect the orbital binding energy of MoS 2 (S 2− ) 2p1 and 2p3, where the characteristic peak of 163.91 eV corresponds to the binding energy of double (multiple) (S 2 2− /S n 2− ) 2p1/2 orbital [35].   Fig. 3a, the layered structures of Ti 2 AlC are closely packed together, while the SEM images of Ti 2 CT x (Fig. 3b), respectively indicate that the lamellar structure of Ti 2 CT x is relatively complete, being conducive to the growth of MoS 2 . Figure 3c-e shows MoS 2 /TiO 2 /Ti 2 CT x materials of different proportions synthesized by hydrothermal method. It can be seen from the figure that, due to the high content of MoS 2 in MTT-1, MoS 2 grows on the matrix of Ti 2 CT x , and part of MoS 2 accumulates between layers. However, MoS 2 load in MTT-2 and MTT-3 decreased, while it grew evenly on the surface of Ti 2 CT x matrix and between layers. The layers that had been thickened due to the attachment of a large number of MoS 2 gradually became thinner and clearer. Since, the comparative analysis of Fig. 3f indicates that the MT-4 material synthesized by microwave solvothermal synthesis has more uniform MoS 2 growth on the surface of Ti 2 CT x . TEM images of Ti 2 CT x , MoS 2 /TiO 2 /Ti 2 CT x (MTT-2), and MoS 2 /Ti 2 CT x (MT-4) have been depicted in Fig. 4. Where, Fig. 4a-c shows that Ti 2 CT x flakes have a distinct multilayer feature with the characteristic spacing between the lattice fringes of d = 0.26 nm in the corresponding highpower transmission electron microscopy (HR-TEM) images [36]. Similarly, the surface topography of MTT-2 and local magnification in Fig. 4d-f shows that a large number of MoS 2 is loaded on Ti 2 CT x layer, where the measured spacing between lattice fringes of 0.61 nm could be corresponded to (002) crystal plane of MoS 2 . Interestingly, MoS 2 grows on the surface of Ti 2 CT x in the case of MT-4 as can be seen in Fig. 4g and h, where the diffraction rings in the selected area electron diffraction (SAED) pattern of Fig. 4i corresponding to the Ti 2 CT x and MoS 2 could be the portray of excellent crystalline nature of MoS 2 /Ti 2 CT x .

Microwave absorption properties
Generally, the electromagnetic absorbing materials can simultaneously exhibit polarization and magnetization responses to external electromagnetic fields, which are described by complex dielectric constant (ε = ε′ − jε′′) and complex permeability (μ = μ′ − jμ′′). Where ε′′ is the real part of the dielectric constant and represents the polarization intensity of the material under the action of the electromagnetic field; ε′′ is the imaginary part of the dielectric constant and represents the dielectric loss of the material under the action of the electromagnetic field; μ′ is the real part of the magnetic permeability which represents the magnetization under the action of the electromagnetic field; and μ′′ is the imaginary part for the permeability which represents the magnetic loss of the material under the action of the electromagnetic field. As there are no magnetic components in these samples, since μ value is almost constant. This kind of absorbing material which attenuates electromagnetic wave through relaxation process and polarization effect of medium in alternating electromagnetic field is also known as dielectric loss absorbing material [37][38][39]. Figure 5a    According to the free electron theory ε′′≈ σ/(2πfε 0 ) (σ for electrical conductivity) [40], the imaginary part of the dielectric constant is positively correlated with the conductivity. The MT-4 material with the most MoS 2 load has the smallest imaginary part of dielectric constant and therefore has a small conductivity, which means that the conductivity of the material becomes worse. Moreover, Ti 2 CT x , MTT-3, and MT-4 have obvious fluctuations in the imaginary part of the dielectric constant curves, where these fluctuations are the dielectric response peaks, thus indicate that the material has obvious polarization behavior. Alternatively, the dielectric loss tangent (tenδ ε = ε′/ε′′) is typically used to characterize the dielectric loss [41]. As shown in Fig. 5c, within the frequency range of 2 ~ 11 GHz, the tenδ ε of MTT-2 and MTT-3 is relatively large, while that of MT-4 is relatively small. However, in the frequency range of 11 ~ 18 GHz, the tenδ ε of MT-4 and Ti 2 CT x samples increased, and the peak value was higher than other curves. MTT-3 samples decreased in the frequency range of 11 ~ 18 GHz, and the lowest value was lower than other curves. By comprehensive comparison, it can be demonstrated that MTT-2 has a stronger microwave loss ability in the range of 2 ~ 18 GHz, while MT-4 has a stronger microwave loss ability at a specific frequency. By calculating the attenuation constant (α) with the following formula, it can be analyzed that how fast the microwave attenuates inside the material, such as the larger the attenuation constant the faster the microwave attenuates inside the material [42].
where f is the frequency, c is the speed of light. Figure 5d shows the curve of the attenuation constants for Ti 2 CT x , MTT-1, MTT-2, MTT-3, and MT-4 samples as a function of frequency. It can be observed that the attenuation constants of MTT-1 and MTT-2 materials increase with the increase of frequency. MTT-3 increases first and then decreases rapidly with the increase of frequency, where Ti 2 CT x and MT-4 materials fluctuate greatly with the frequency, and the attenuation constants are lower than those of MTT-1 and MTT-2 materials at most frequencies. However, MT-4 has a higher peak value at 12.5 GHz than other materials, with a peak value of 170.
According to Debye theory [43], dielectric loss of materials mainly depends upon the conduction loss and polarization loss [44]. Ti 2 CT x is a highly conductive material, so the layered structure of Ti 2 CT x provides a conduction pathway for carrying and attenuating microwaves, which makes Ti 2 CT x , MoS 2 /TiO 2 /Ti 2 CT x , and MoS 2 /Ti 2 CT x materials for conduction losses. Polarization loss is caused by the electron polarization, dipole polarization, ion polarization, and interface polarization, since the electron polarization and ion polarization in the GHz range are negligible [45]. Cole-Cole semicircle equation is used to describe ε′ and ε″, and the relationship between each semicircle represents a Debye relaxation process. Figure SI 2 shows the Cole-Cole diagram of Ti 2 CT x , MTT-2, and MT-4 materials, exhibiting that there are several Cole-Cole semi-circles in Ti 2 CT x , MTT-2, and MT-4 materials as the portray of multiple relaxation polarization processes in the materials. Where, the possible relaxation polarization processes of Ti 2 CT x , MTT-2, and MT-4 materials can be categorically described as follows: interface polarization caused by the interface of Ti 2 CT x and MoS 2 ; Ti 2 CT x surface defects caused by functional groups on Ti 2 CT x and etching treatment, in-situ growth of MoS 2 nanosheets, surface lattice defects of TiO 2 produced by oxidation, and dipole polarization caused by unsaturated suspension bonds. In summary, Ti 2 CT x and its composites with MoS 2 possesses the high dielectric loss due to their conductivity loss and high polarization loss.
Microwave absorption needs the material to reflect less and absorb more electromagnetic waves, which requires the material to have a good impedance (Z) matching, and conducive to the electromagnetic wave entering the absorption material. When the impedance of the material is equal or close to the impedance of free space (Z = 1), it can lead to the better the impedance matching. The high conductivity of Ti 2 CT x material results in partial reflection of electromagnetic wave after it reaches the surface of the material.
However, the synergistic effect between Ti 2 CT x and MoS 2 can improve the impedance matching of the composite and make it easier for electromagnetic wave to enter the material [46]. Figure 6 shows the impedance matching contour maps of Ti 2 CT x , MTT-1, MTT-2, MTT-3, and MT-4 samples. As shown in Fig. 6a, there are fewer yellow areas and poor impedance matching. While the impedance matching diagram of Ti 2 CT x and MoS 2 composites in Fig. 6b-e obviously indicates that the yellow area of the composite material is significantly more than that of Ti 2 CT x material. Where it could be related to the low dielectric phase of MoS 2 , as the greater the content, the better impedance matching of the composite material. In addition, in the reaction process of MTT-1, MTT-2, and MTT-3 materials synthesized by hydrothermal method, part of Ti 2 CT x was oxidized to TiO 2 , and the appropriate amount of TiO 2 was also conducive to impedance matching, but excessive TiO 2 will destroy the structure of Ti 2 CT x , which is not conducive to microwave absorption performance. Therefore, the control of TiO 2 production has a certain effect on MoS 2 and Ti 2 CT x composites.
The microwave absorption capacity of any absorbing material is greatly related to the minimum reflection loss, thickness of the material, and the effective bandwidth. If the RL value of the absorbing material is less than -10 dB, it indicates that the microwave consumption is up to 90%, and the material can be predicted as an ideal absorbing material [47]. Figure 7 shows the 3D reflection loss (RL) profiles and contour maps from 2 to 18 GHz for the absorbing materials of Ti 2 CT x , MTT-1, MTT-2, MTT-3, and MT-4. The three-dimensional reflection loss curves shown in Fig. 7a 1 -e 1 intuitively illustrate that the combination of a certain proportion of MoS 2 and Ti 2 CT x can be effectively reduced. The minimum reflection loss values of the five materials and their corresponding thickness, frequency, and effective bandwidth lists are compared, and presented in Table 1. It can be seen from the comparison that the RL min of the composites prepared by the traditional hydrothermal method decreases first and then increases with the decrease of MoS 2 loading. The RL min of MTT-2 and MTT-3 materials is lower than that of pure phase Ti 2 CT x , which are − 54.70 dB (thickness is 3.39 mm) and − 46.78 dB (thickness is 1.57 mm) respectively, and the RL min of MT-4   While, the concentrated yellow area of MT-4 material at high frequency illustrates that the MT-4 material shows excellent absorbing ability at high frequency. As shown in Fig. 8, the EAB max of pure phase Ti 2 CT x , MTT-1, MTT-2, and MTT-3 materials is 2.1 GHz, 3.5 GHz, 4.0 GHz, and 4.2 GHz respectively, which continues to decrease with the increase of thickness. Since all of the four samples show that the minimum reflective loss value of the material will move to the low frequency direction with the increase of thickness, being consistent with the law of quarter-wavelength attenuation. Therefore, the overall absorption bandwidth of the material is extended, which is conducive to the material's function in a wider frequency range. However, it can be noted that the EAB max of MT-4 material is 4.11 GHz, where the increase in thickness does not affect the movement of the reflection loss value to the low frequency. Thus, it can be reflected that the effective absorption of MT-4 material is mainly concentrated in the high frequency range with the widest effective bandwidth. The microwave absorption properties of similar materials reported in the literature has been compared with that of current work ( Table 2) in view of enhancing the key features of the composites in this work. It can be seen that the microwave absorption performance of MoS 2 /TiO 2 /Ti 2 CT x (1:2) and MoS 2 /Ti 2 CT x (8:1) samples prepared in this study exceeds that of the most reported MXene-based and MoS 2 -based materials by comprehensive analysis of RL min , EAB max , and thickness. Figure 9 shows the absorbing mechanism of MTT-2 material as an example, where the excellent absorbing ability of the composite material can be mainly attributed to the following four aspects: By optimizing impedance matching, more electromagnetic waves enter the interior of the composite material, and the incident microwave is dissipated mainly through conductivity loss, multiple reflection loss, polarization loss [55], which can be explained by following aspects: (1) The high conductivity of pure Ti 2 CT x leads to its impedance mismatch, while the introduction of dielectric MoS 2 optimizes the comprehensive conductivity and finally achieves the matching effect; (2) The existence of functional groups or defects on the surface of Ti 2 CT x can form a large number of polarization sites, which enhances dipole polarization and thus enhances polarization loss; (3) The formation of heterogeneous interfaces such as Ti 2 CT x -MoS 2 , MoS 2 -TiO 2 , and Ti 2 CT x -MoS 2 is conducive to improving the interface polarization, thus increasing the polarization loss; (4) Ti 2 CT x layer is equipped with conductive carbon core, which provides a fast channel for transferring electrons, resulting in conductive loss; (5) The layered structure of Ti 2 CT x is conducive to the multiple reflection and scattering process of incident waves, which is equivalent to increasing the transmission distance of microwave in the material, maximizing the conversion of incident microwave into heat energy, so that its energy is attenuated.

Conclusions
In summary, MoS 2 /TiO 2 /Ti 2 CT x and MoS 2 /Ti 2 CT x composites prepared in this experiment meet the requirements of new wave absorbing materials, such as "thin thickness, strong performance, light weight, and wide band." In this experiment, lighter Ti 2 CT x was selected as the matrix, and MoS 2 was in-situ grown on Ti 2 CT x matrix by traditional hydrothermal method and microwave solvothermal method. The composite showed the synergic effect of twodimensional heterotructural interface and double dielectric elements, which enhanced the wave absorption effect. The MoS 2 /TiO 2 /Ti 2 CT x composite material obtained by traditional hydrothermal method contains a certain amount of TiO 2 (produced by oxidation of Ti 2 CT x ), where a small amount of TiO 2 and a certain proportion of MoS 2 jointly improve the impedance matching of the composite material. The RL min can reach − 54.70 dB (with a frequency of 7.59 GHz, and 3.39 mm thickness), and EAB max can reach 4 GHz. The microwave solvothermal synthesis of MoS 2 / Ti 2 CT x composite also has excellent wave absorption performance, having RL min up to − 53.26 dB (with of frequency 14.50 GHz, and 3.14 mm thickness) and EAB max up to 4.11 GHz. In addition, microwave heating has the advantages of fast speed, short time, high efficiency, and uniform product, and Ti 2 CT x is not easy to oxidize in the composite process by using polyethylene glycol 200 with higher boiling point instead of water. While the MoS 2 /Ti 2 CT x composite without oxide has wider EAB at thinner thickness. The excellent wave absorption performance of MoS 2 /TiO 2 / Ti 2 CT x and MoS 2 /Ti 2 CT x materials without oxidizing and possessing a wider EAB at a thinner thickness, confirms the validity of our scheme for rationally designed "thin, light, wide, and strong" new microwave absorption material composite that may find a new era of utilization in the defense, medical, and other energy sectors.