Heterostructure Composites of CoS Nanoparticles Decorated on Ti3C2Tx Nanosheets and Their Enhanced Electromagnetic Wave Absorption Performance

As a typical two-dimensional material, MXene possesses excellent conductivity and tunable interlayer space, which makes it have an impressive development potential in the field of electromagnetic (EM) waves absorbing materials. In this work, we fabricated a sandwich structure CoS@Ti3C2Tx composite using a simple solvothermal process. The CoS nanoparticles are anchored on the Ti3C2Tx MXene sheets, forming a heterolayered structure. The results demonstrate that the CoS@Ti3C2Tx composites with the sandwich-like architecture showed excellent EM absorbing performance due to the synergistic effects of the conductivity loss, interface polarization, and dipole polarization. When the doping ratio was 40 wt %, the maximum reflection loss value of CoS@Ti3C2Tx was up to –59.2 dB at 14.6 GHz, and the corresponding effective absorption bandwidth (below –10 dB) reached 5.0 GHz when the thickness was only 2.0 mm. This work endows a new candidate for the design of MXene-based absorption materials with optimal performance.


Introduction
Electromagnetic (EM) pollution has emerged from the explosive development of military equipment, and communication technology has produced serious damage for human beings [1,2]. Thus, microwave absorption materials have attracted increasing attention, which effectively converts the EM energies into thermal energies [3,4]. Strong EM wave attenuation intensity and wide effective bandwidth are the pursuits for the preparation of superior absorbing materials [5]. Meanwhile, light weight and high efficiency are also two key factors affecting their wide application in daily life [6,7]. To date, a great number of absorbing materials have been reported such as carbon materials (nanoporous carbons, graphene, CNTs, carbon fibers), conductive polymers (PPy, PANI), and semiconductor transition-metal sulfides (CuS, MoS 2 ), etc. Mohd Najim et al. used nickel-phosphorus coating on the tetrapod-shaped ZnO by the electroless coating process. The Ni-P coating increased the magnetic loss of the material, and the maximum reflection loss (RL) of the Ni-P coated T-ZnO reached -36.41 dB with an effective absorption bandwidth of 10.0 GHz [8]. Wang et al. designed a CoFe 2 O 4 /N-RGO aerogel wherein CoFe 2 O 4 was embedded in the N-doped reduced graphene oxide (RGO) aerogels and the strongest RL was -60.4 dB at 14.4 GHz [9].
Notably, MXene, a typical 2D structure composed of transition-metal carbides, nitrides, and/or carbonitrides, has gained interest in the field of EM wave absorbing [10]. Due to their special laminated

Synthesis of Ti 3 C 2 T x MXene
Ti 3 C 2 T x MXene was synthesized based on the previous work [33]. First, 1 g LiF was completely dissolved in 20 mL of 9 M HCl solution with stirring for 5 min at room temperature. Then, 1 g Ti 3 AlC 2 powder was slowly added into the above solution and kept stirring at 45 • C for 24 h. Subsequently, the homogeneous mixture was washed three times using deionized water by centrifuge (3500 rpm) until the pH reached 6. Finally, the obtained black samples were dried in the desiccator under vacuum at 50 • C.

Synthesis of CoS@Ti 3 C 2 T x Hybrids
The synthesis of CoS@Ti 3 C 2 T x hybrids was carried out in a solvothermal reaction. In detail, 200 mg Ti 3 C 2 T x MXene was dissolved in 20 mL EG by ultrasound for 1 h. Then, 2.5 mmol CoCl 2 ·6H 2 O was dispersed in 30 mL EG and mixed with the former solution, stirring for 30 min. Subsequently, the 20 mL EG solution dissolved in 6.25 mmol thiourea was slowly added into the above solution with magnetic stirring for 30 min. The homogeneous suspension was moved into the 100 mL Teflon-lined autoclave (Shanghai Yuezhongyq Co.Ltd, Shanghai, China) and reacted at 180 • C for 12 h. Meanwhile, CoS nanoparticles were prepared for comparison. The schematic illustration of the synthesis of CoS@Ti 3 C 2 T x hybrids is shown in Figure 1.

Synthesis of Ti3C2Tx MXene
Ti3C2Tx MXene was synthesized based on the previous work [33]. First, 1 g LiF was completely dissolved in 20 mL of 9 M HCl solution with stirring for 5 min at room temperature. Then, 1 g Ti3AlC2 powder was slowly added into the above solution and kept stirring at 45 °C for 24 h. Subsequently, the homogeneous mixture was washed three times using deionized water by centrifuge (3500 rpm) until the pH reached 6. Finally, the obtained black samples were dried in the desiccator under vacuum at 50 °C.

Synthesis of CoS@Ti3C2Tx Hybrids
The synthesis of CoS@Ti3C2Tx hybrids was carried out in a solvothermal reaction. In detail, 200 mg Ti3C2Tx MXene was dissolved in 20 mL EG by ultrasound for 1 h. Then, 2.5 mmol CoCl2·6H2O was dispersed in 30 mL EG and mixed with the former solution, stirring for 30 min. Subsequently, the 20 mL EG solution dissolved in 6.25 mmol thiourea was slowly added into the above solution with magnetic stirring for 30 min. The homogeneous suspension was moved into the 100 mL Teflon-lined autoclave (Shanghai Yuezhongyq Co.Ltd, Shanghai, China) and reacted at 180 °C for 12 h. Meanwhile, CoS nanoparticles were prepared for comparison. The schematic illustration of the synthesis of CoS@Ti3C2Tx hybrids is shown in Figure 1.

Characterization
The microscopic morphology of Ti 3 C 2 T x MXene, CoS nanoparticles, and CoS@Ti 3 C 2 T x composite were detected by scanning electron microscopy (SEM, JSM-7800F, JEOL, Tokyo, Japan). The microstructure and elemental mapping were characterized by an energy dispersive x-ray spectroscopy (EDS, XFlash 5030T, BRUKER, Leipzig, Germany) and a transmission electron microscope (TEM, JEM-2100F, JEOL, Tokyo, Japan) with a scanning transmission electron microscope (STEM) resolution of 0.20 nm. X-ray diffraction (XRD, D8A Advance, BRUKER, Leipzig, Germany) was used to analyze the crystallite structure of the composites. The surface of the CoS@Ti 3 C 2 T x composite was performed by x-ray photoelectron spectroscopy (XPS, ESCALAB 250 xi, Shanghai, China). A vector network analyzer (VNA, N5242A PNA-X, Agilent, Palo Alto, CA, USA) was used to collect the basic EM parameters in the frequency range of 2.0-18.0 GHz at room temperature. The composite was mixed with paraffin in different filler ratios (35 wt %, 40 wt %, 45 wt %) and then pressed into a coaxial cylinder (Φ in = 3.04 mm, Φ out = 7.0 mm, d = 3.5 mm) under a pressure of 5 MPa.

Characterization of Samples
The x-ray diffraction (XRD) patterns of CoS, Ti 3 C 2 T x , and CoS@Ti 3 C 2 T x are shown in Figure 2. The typical peaks at 7.2 • , 17.6 • , 42.0 • , and 60.8 • corresponded to the (002), (004), (010), and (110) crystal planes of Ti 3 C 2 T x , respectively [34]. Obviously, the (002) peak of CoS@Ti 3 C 2 T x shifted to 6.1 • after the solvothermal reaction. According to the Bragg equation, the layer space of Ti 3 C 2 T x increased from 12.2 Å to 14.4 Å, indicating the cobalt sulfide nanoparticles had anchored on the Ti 3 C 2 T x MXene layers to form a multilayered structure. In addition, the peaks at 30.

Characterization of Samples
The x-ray diffraction (XRD) patterns of CoS, Ti3C2Tx, and CoS@Ti3C2Tx are shown in Figure 2. The typical peaks at 7.2°, 17.6°, 42.0°, and 60.8° corresponded to the (002), (004), (010), and (110) crystal planes of Ti3C2Tx, respectively [34]. Obviously, the (002) peak of CoS@Ti3C2Tx shifted to 6.1° after the solvothermal reaction. According to the Bragg equation, the layer space of Ti3C2Tx increased from 12.  To further research the surface chemical elements of CoS@Ti3C2Tx composites, the XPS spectra are shown in Figure 3. As shown in Figure 3a, the total spectrum of the CoS/Ti3C2Tx composite demonstrates the existence of Co, Ti, O, S, and C elements. Figure 3b illustrates the Ti 2p XPS spectrum of the sample and the peaks corresponding to Ti-C (454.8 eV), TixCy (457.7 eV), Ti 2p3/2 (458.3 eV), Ti-F (461.4 eV), and Ti 2p1/2 (464.1 eV) [35]. The C 1s spectra exhibited in Figure 3c contains four fitted peaks including Ti-C (281.4 eV), C-C (284.7 eV), C-O (286.3 eV), and C-F (288.7 eV) [36]. The O 1s peaks at 530.3 eV and 531.8 eV can be indexed to C-Ti-O and Ti-OH in Figure 3d [37]. Moreover, the peak at 532.8 eV confirms that there are a small number of water molecules in the Ti3C2Tx MXene layers [38]. In Figure 3e, the peak at 793.8 eV is indexed to Co 2p1/2 and the peak located at 778.8 eV belongs to Co 2p3/2 [39]. In addition, the presence of C-S-C, C-SOx-C, S 2p1/2, and S 2p3/2 can be observed in Figure 3f. Thus, the XPS analysis indicates that the heterogeneous structural CoS@Ti3C2Tx composites were prepared, which also corresponded to the XRD analysis. To further research the surface chemical elements of CoS@Ti 3 C 2 T x composites, the XPS spectra are shown in Figure 3. As shown in Figure 3a, the total spectrum of the CoS/Ti 3 C 2 T x composite demonstrates the existence of Co, Ti, O, S, and C elements. Figure 3b illustrates the Ti 2p XPS spectrum of the sample and the peaks corresponding to Ti-C (454.8 eV), Ti x C y (457.7 eV), Ti 2p 3/2 (458.3 eV), Ti-F (461.4 eV), and Ti 2p 1/2 (464.1 eV) [35]. The C 1s spectra exhibited in Figure 3c contains four fitted peaks including Ti-C (281.4 eV), C-C (284.7 eV), C-O (286.3 eV), and C-F (288.7 eV) [36]. The O 1s peaks at 530.3 eV and 531.8 eV can be indexed to C-Ti-O and Ti-OH in Figure 3d [37]. Moreover, the peak at 532.8 eV confirms that there are a small number of water molecules in the Ti 3 C 2 T x MXene layers [38]. In Figure 3e, the peak at 793.8 eV is indexed to Co 2p 1/2 and the peak located at 778.8 eV belongs to Co 2p 3/2 [39]. In addition, the presence of C-S-C, C-SO x -C, S 2p 1/2 , and S 2p 3/2 can be observed in Figure 3f. Thus, the XPS analysis indicates that the heterogeneous structural CoS@Ti 3 C 2 T x composites were prepared, which also corresponded to the XRD analysis.  The morphology of Ti3C2Tx MXene is given in Figure 4a. It can be observed that Ti3C2Tx MXene showed a similar accordion-like structure after etching. A large number of agglomerated CoS nanoparticles can be observed in Figure 4b. As shown in Figure 4c,d, CoS nanoparticles are anchored on the surface and inside the Ti3C2Tx MXene, forming a sandwich structure. It is worth noting that the interlayer spacing of the composites is significantly larger than pure MXene. From Figure 4b, it can be found that there was obvious agglomeration of pure CoS nanoparticles. As shown in Figure  4c, when CoS nanoparticles were combined with the two-dimensional material Ti3C2Tx MXene, Co 2+ ions can be absorbed and dispersed by the functional groups on the surface of MXene, which effectively solved the agglomeration of CoS nanoparticles. These CoS nanoparticles will connect with Ti3C2Tx nanosheets to form a conductive network, which may be beneficial to increase the dielectric loss of the material. Figure 5 shows the TEM of the CoS@Ti3C2Tx composites, from which a typical laminated structure of the Ti3C2Tx MXene can be observed. It can be seen from the elemental mapping images that the Ti, C, and O elements were uniformly distributed in the diagram. Due to the oxidation on the surface of the Ti3C2Tx MXene, the oxygen element was detected. The distribution of Co and S further confirms the successful composition of the CoS nanoparticles and Ti3C2Tx MXene. Moreover, the TEM image of the Ti3C2Tx MXene in Figure 6a shows that Ti3C2Tx MXene presents an ultrathin transparent laminated structure and the interlayer space of the Ti3C2Tx MXene was approximately 0.99 nm, as shown in the high resolution transmission electron microscope (HRTEM) image in Figure  6b. The average diameter of CoS nanoparticles was about 15-17 nm and it can be observed that these nanoparticles were embedded in the interlayer or surface of the Ti3C2Tx MXene nanosheets. As described in Figure 6e, the interlayer spacing of 0.25 nm and 1.03 nm corresponded to the (101) facets of the CoS nanoparticles and the (002) planes of the laminated Ti3C2Tx MXene, respectively. In order to further demonstrate the existence of CoS nanoparticles on the surface of Ti3C2Tx MXene, the corresponding EDS image is shown in Figure 6f. It can be seen that the five elements of Ti, Co, S, O, C were detected, and the atomic ratio of Co and S was around 1:1, which corresponds to the stoichiometry of CoS. Moreover, no other elements were found, which further confirms the successful preparation of CoS@Ti3C2Tx composites. The morphology of Ti 3 C 2 T x MXene is given in Figure 4a. It can be observed that Ti 3 C 2 T x MXene showed a similar accordion-like structure after etching. A large number of agglomerated CoS nanoparticles can be observed in Figure 4b. As shown in Figure 4c,d, CoS nanoparticles are anchored on the surface and inside the Ti 3 C 2 T x MXene, forming a sandwich structure. It is worth noting that the interlayer spacing of the composites is significantly larger than pure MXene. From Figure 4b, it can be found that there was obvious agglomeration of pure CoS nanoparticles. As shown in Figure 4c, when CoS nanoparticles were combined with the two-dimensional material Ti 3 C 2 T x MXene, Co 2+ ions can be absorbed and dispersed by the functional groups on the surface of MXene, which effectively solved the agglomeration of CoS nanoparticles. These CoS nanoparticles will connect with Ti 3 C 2 T x nanosheets to form a conductive network, which may be beneficial to increase the dielectric loss of the material. Figure 5 shows the TEM of the CoS@Ti 3 C 2 T x composites, from which a typical laminated structure of the Ti 3 C 2 T x MXene can be observed. It can be seen from the elemental mapping images that the Ti, C, and O elements were uniformly distributed in the diagram. Due to the oxidation on the surface of the Ti 3 C 2 T x MXene, the oxygen element was detected. The distribution of Co and S further confirms the successful composition of the CoS nanoparticles and Ti 3 C 2 T x MXene. Moreover, the TEM image of the Ti 3 C 2 T x MXene in Figure 6a shows that Ti 3 C 2 T x MXene presents an ultrathin transparent laminated structure and the interlayer space of the Ti 3 C 2 T x MXene was approximately 0.99 nm, as shown in the high resolution transmission electron microscope (HRTEM) image in Figure 6b. The average diameter of CoS nanoparticles was about 15-17 nm and it can be observed that these nanoparticles were embedded in the interlayer or surface of the Ti 3 C 2 T x MXene nanosheets. As described in Figure 6e, the interlayer spacing of 0.25 nm and 1.03 nm corresponded to the (101) facets of the CoS nanoparticles and the (002) planes of the laminated Ti 3 C 2 T x MXene, respectively. In order to further demonstrate the existence of CoS nanoparticles on the surface of Ti 3 C 2 T x MXene, the corresponding EDS image is shown in Figure 6f. It can be seen that the five elements of Ti, Co, S, O, C were detected, and the atomic ratio of Co and S was around 1:1, which corresponds to the stoichiometry of CoS. Moreover, no other elements were found, which further confirms the successful preparation of CoS@Ti 3 C 2 T x composites.

Electromagnetic Parameters and Absorption Property
To evaluate the EM wave absorption characteristics, the relative complex permittivity (εr = ε′-jε″) and relative complex permeability (μr = μ′-jμ″) of the Ti3C2Tx, CoS, and CoS@Ti3C2Tx MXene composites with different filler loading were measured using a vector network analyzer ground on the coaxial-line method in the frequency range of 2-18 GHz. The samples were mixed with paraffin and pressed into a ring model (Φ = 3.04 mm, Φ = 7.0 mm), which were then placed in a coaxial clamp. After multiple reflection and transmission between the air interface of the transmission line and the sample, the EM wave energy would attenuate and the phase would shift. Then, the scattering parameter S is measured by a vector network analyzer and the EM parameters can be calculated according to the standard Nicolson-Ross-Weir theory [40,41]. In general, the real part of the relative complex permittivity (ε′) represents the polarization capability of the composite in the electric field, the real part of the relative complex permeability (μ′) shows magnetization capability under the influence of a magnetic field. The imaginary part of the relative complex permittivity (ε″) and the relative complex permeability (μ″) represent dielectric loss and magnetic loss capacity, respectively [42]. As shown in Figure 7a, the average ε′ and ε″ value of Ti3C2Tx (35 wt %) were maintained at 6.9 and 0.8, respectively. In comparison, the average ε′ and ε″ values of CoS (35 wt %) reached 10.0 and 2.0, respectively, as shown in Figure 7b. As the filler loading in the paraffin matrix increased from 35 wt % to 45 wt %, the ε′ and ε″ values of CoS@Ti3C2Tx added up to 13.5 and 5.1, respectively, which may be illustrated by the effective medium theory [43]. The dipole polarization, interfacial polarization, and electrical conductivity may be enhanced by the increase in CoS@Ti3C2Tx weight [19]. As demonstrated in Figure 7d,e, it is worth noting that the ε′ and ε″ curves fluctuated significantly within the 8-18 GHz, and it may be related to the relaxation polarization and interfacial polarization of dielectric materials at high frequencies. In particular, the ε′ and ε″ achieved the highest values, indicating that CoS@Ti3C2Tx (45 wt %) possibly has a favorable dielectric dissipation capability to EM waves, as shown in Figure 7e. Moreover, as shown in Figure 7, due to the absence of magnetism in these composites, the μ′ and μ″ values reached 1.0 and 0, respectively. The above analysis showed that dielectric loss is the major mechanism of EM wave absorption in the CoS@Ti3C2Tx composites, while the magnetic loss can be ignored.

Electromagnetic Parameters and Absorption Property
To evaluate the EM wave absorption characteristics, the relative complex permittivity (ε r = ε -jε") and relative complex permeability (µ r = µ -jµ") of the Ti 3 C 2 T x , CoS, and CoS@Ti 3 C 2 T x MXene composites with different filler loading were measured using a vector network analyzer ground on the coaxial-line method in the frequency range of 2-18 GHz. The samples were mixed with paraffin and pressed into a ring model (Φ in = 3.04 mm, Φ out = 7.0 mm), which were then placed in a coaxial clamp. After multiple reflection and transmission between the air interface of the transmission line and the sample, the EM wave energy would attenuate and the phase would shift. Then, the scattering parameter S is measured by a vector network analyzer and the EM parameters can be calculated according to the standard Nicolson-Ross-Weir theory [40,41]. In general, the real part of the relative complex permittivity (ε ) represents the polarization capability of the composite in the electric field, the real part of the relative complex permeability (µ ) shows magnetization capability under the influence of a magnetic field. The imaginary part of the relative complex permittivity (ε") and the relative complex permeability (µ") represent dielectric loss and magnetic loss capacity, respectively [42]. As shown in Figure 7a, the average ε and ε" value of Ti 3 C 2 T x (35 wt %) were maintained at 6.9 and 0.8, respectively. In comparison, the average ε and ε" values of CoS (35 wt %) reached 10.0 and 2.0, respectively, as shown in Figure 7b. As the filler loading in the paraffin matrix increased from 35 wt % to 45 wt %, the ε and ε" values of CoS@Ti 3 C 2 T x added up to 13.5 and 5.1, respectively, which may be illustrated by the effective medium theory [43]. The dipole polarization, interfacial polarization, and electrical conductivity may be enhanced by the increase in CoS@Ti 3 C 2 T x weight [19]. As demonstrated in Figure 7d,e, it is worth noting that the ε and ε" curves fluctuated significantly within the 8-18 GHz, and it may be related to the relaxation polarization and interfacial polarization of dielectric materials at high frequencies. In particular, the ε and ε" achieved the highest values, indicating that CoS@Ti 3 C 2 T x (45 wt %) possibly has a favorable dielectric dissipation capability to EM waves, as shown in Figure 7e. Moreover, as shown in Figure 7, due to the absence of magnetism in these composites, the µ and µ" values reached 1.0 and 0, respectively. The above analysis showed that dielectric loss is the major mechanism of EM wave absorption in the CoS@Ti 3 C 2 T x composites, while the magnetic loss can be ignored. Dielectric loss is related to two important factors: polarization relaxation and conductivity loss, and the dielectric loss tangents ( = / ′ ) of CoS@Ti3C2Tx composites with different filler loadings are calculated in Figure 8a. With the increase in frequency, the tan curves showed an upward trend and some vibration peaks corresponded well to the permittivity curves. The average values of curves at 40 wt % and 45 wt % increased from 0.25 to 0.35 at 2-10 GHz, respectively. However, as the frequency continued to increase, the value of the 45 wt % curve achieved 0.65, indicating better dielectric loss capability. A similar phenomenon can be seen in the attenuation constant (α) curves in Figure 8b, which can be calculated as follows [44]: where is frequency and represents the velocity of light. The larger α value means a stronger EM wave dissipation ability. As shown in Figure 8b, CoS@Ti3C2Tx (35 wt %) had a significantly lower absorption of EM waves than 40 wt % and 45 wt %. To further investigate the polarization relaxation phenomenon of CoS@Ti3C2Tx composites, the Cole-Cole semicircle model is necessary. The Debye equation is as follows [45]: According to Equation (2), the ε′ and ε″ can be expressed as: where stands for the static permittivity; stands for the relative dielectric constant; is the polarization relaxation time; and stands for the electric field oscillation frequency. According to Equations (3) and (4), the relationship between ε′ and ε″ may be described by [46]: Dielectric loss is related to two important factors: polarization relaxation and conductivity loss, and the dielectric loss tangents (tan δ ε = ε /ε ) of CoS@Ti 3 C 2 T x composites with different filler loadings are calculated in Figure 8a. With the increase in frequency, the tan δ ε curves showed an upward trend and some vibration peaks corresponded well to the permittivity curves. The average values of curves at 40 wt % and 45 wt % increased from 0.25 to 0.35 at 2-10 GHz, respectively. However, as the frequency continued to increase, the value of the 45 wt % curve achieved 0.65, indicating better dielectric loss capability. A similar phenomenon can be seen in the attenuation constant (α) curves in Figure 8b, which can be calculated as follows [44]: where f is frequency and c represents the velocity of light. The larger α value means a stronger EM wave dissipation ability. As shown in Figure 8b, CoS@Ti 3 C 2 T x (35 wt %) had a significantly lower absorption of EM waves than 40 wt % and 45 wt %. To further investigate the polarization relaxation phenomenon of CoS@Ti 3 C 2 T x composites, the Cole-Cole semicircle model is necessary. The Debye equation is as follows [45]: According to Equation (2), the ε and ε" can be expressed as: where ε s stands for the static permittivity; ε ∞ stands for the relative dielectric constant; τ is the polarization relaxation time; and ω stands for the electric field oscillation frequency. According to Equations (3) and (4), the relationship between ε and ε" may be described by [46]:  The plot of ε′ versus ε″ can be expressed as a Cole-Cole semicircle and each Cole-Cole semicircle corresponds to a polarization relaxation process [47]. In Figure 9a, because of the effect of the multirelaxation dielectric properties, the Cole-Cole semicircle of Ti3C2Tx (35 wt %) showed a complex interlacing shape. In Figure 9b, there are four small distorted semicircles in CoS (35 wt %). The reason of semicircle distortion is that the Debye equation is an ideal model built under special conditions, and there are lattice distortion and point defects in the material [48]. In Figure 9c-e, three or four distinct semicircles can be observed in the CoS@Ti3C2Tx composites, which may result from the synergistic effects of dipole polarization and interfacial polarization [49]. More dipoles can be induced by the localized defects, oxygen functional groups, and multilayered structure of Ti3C2Tx [17,50,51]. Furthermore, there are many CoS nanoparticles embedded on the surface of the Ti3C2Tx. According to the nanometer size effect, the number of dangling bonds, dipoles, and defects in the CoS@Ti3C2Tx composites would increase significantly, which may obviously enhance the electronic polarization and dipole polarization [52,53]. In addition, based on the Maxwell-Wanger-Sillars (MWS) effect, the special multicomponent heterostructure might generate more interfacial polarization process and thus enhance the absorption capability of EM waves [54].  The plot of ε versus ε" can be expressed as a Cole-Cole semicircle and each Cole-Cole semicircle corresponds to a polarization relaxation process [47]. In Figure 9a, because of the effect of the multi-relaxation dielectric properties, the Cole-Cole semicircle of Ti 3 C 2 T x (35 wt %) showed a complex interlacing shape. In Figure 9b, there are four small distorted semicircles in CoS (35 wt %). The reason of semicircle distortion is that the Debye equation is an ideal model built under special conditions, and there are lattice distortion and point defects in the material [48]. In Figure 9c-e, three or four distinct semicircles can be observed in the CoS@Ti 3 C 2 T x composites, which may result from the synergistic effects of dipole polarization and interfacial polarization [49]. More dipoles can be induced by the localized defects, oxygen functional groups, and multilayered structure of Ti 3 C 2 T x [17,50,51]. Furthermore, there are many CoS nanoparticles embedded on the surface of the Ti 3 C 2 T x . According to the nanometer size effect, the number of dangling bonds, dipoles, and defects in the CoS@Ti 3 C 2 T x composites would increase significantly, which may obviously enhance the electronic polarization and dipole polarization [52,53]. In addition, based on the Maxwell-Wanger-Sillars (MWS) effect, the special multicomponent heterostructure might generate more interfacial polarization process and thus enhance the absorption capability of EM waves [54].
In order to further research the EM wave absorption characteristics of the CoS@Ti 3 C 2 T x composites, the reflection loss (RL) values versus frequency of the materials with different filler loadings at specific thickness are shown in Figure 10. As described by transmission line theory, the RL values can be calculated by [55]: where d denotes the thickness of the absorbers; and Z in and Z 0 stand for the normalized input characteristic impendence and the impendence of air, respectively. Figure 10a illustrates that the maximum RL values of CoS@Ti 3 C 2 T x (35 wt %) at different thickness were above −10 dB, which means that the CoS@Ti 3 C 2 T x (35 wt %) cannot absorb EM waves effectively. Comparatively, the maximum RL value of CoS@Ti 3 C 2 T x (40 wt %) was −59.2 dB at 14.6 GHz and the corresponding optimal thickness was 2.0 mm, and the effective absorbing bandwidth was 5.0 GHz (12.24-17.24 GHz), as given in Figure 10b. When the filler ratio was 45 wt %, the maximum RL value of −28.83 dB at 12.32 GHz was found at a thickness of 2.0 mm, while the absorbing bandwidth below −10 dB was 4.16 GHz (11.04-15.2 GHz). After analysis, it can be found that the CoS@Ti 3 C 2 T x (40 wt %) composite had superior EM absorption properties. Furthermore, it is worth noting that with the increase in the absorber thickness, the maximum RL locations shifted toward lower frequencies, which is consistent with the quarter-wavelength cancellation. The simulation curve of the absorption thickness (t m ) can be calculated by the 1/4 wavelength cancellation equation (t m = nλ/4 = nc/(4 f m µ r |ε r |)) [23]. In Figure 10, the pink dots represent the matching thickness. It is interesting to observe that the pink dots were accurately distributed on the quarter-wavelength simulation curve, suggesting that the absorbing mechanism of the composite conforms to the 1/4 wavelength theory. In addition, good impedance matching is also a necessary condition for the material to have excellent absorption capability. The normalized characteristic impedance (Z = |Z in /Z 0 |) versus frequency is shown in Figure 10. Combined with the RL curves, the corresponding Z of the CoS@Ti 3 C 2 T x (40 wt %) was close to 1 with a thickness of 2.0 mm, indicating that the material has optimal impedance matching and good EM wave absorbing potential. The plot of ε′ versus ε″ can be expressed as a Cole-Cole semicircle and each Cole-Cole semicircle corresponds to a polarization relaxation process [47]. In Figure 9a, because of the effect of the multirelaxation dielectric properties, the Cole-Cole semicircle of Ti3C2Tx (35 wt %) showed a complex interlacing shape. In Figure 9b, there are four small distorted semicircles in CoS (35 wt %). The reason of semicircle distortion is that the Debye equation is an ideal model built under special conditions, and there are lattice distortion and point defects in the material [48]. In Figure 9c-e, three or four distinct semicircles can be observed in the CoS@Ti3C2Tx composites, which may result from the synergistic effects of dipole polarization and interfacial polarization [49]. More dipoles can be induced by the localized defects, oxygen functional groups, and multilayered structure of Ti3C2Tx [17,50,51]. Furthermore, there are many CoS nanoparticles embedded on the surface of the Ti3C2Tx. According to the nanometer size effect, the number of dangling bonds, dipoles, and defects in the CoS@Ti3C2Tx composites would increase significantly, which may obviously enhance the electronic polarization and dipole polarization [52,53]. In addition, based on the Maxwell-Wanger-Sillars (MWS) effect, the special multicomponent heterostructure might generate more interfacial polarization process and thus enhance the absorption capability of EM waves [54].    Figure 11 shows three dimensional profiles of RL values of Ti 3 C 2 T x , CoS and CoS@Ti 3 C 2 T x at a 40 wt % doping ratio in the paraffin. From Figure 11a, the maximum RL value of Ti 3 C 2 T x was −8.24dB and cannot be used as the EM wave absorber. In Figure 11b, the CoS nanoparticles were endowed with the maximum RL value of −39.44 dB and the responding thickness was 4.0 mm at 5.2 GHz. Although the absorption strength is acceptable, the absorption thickness cannot meet the requirement of an excellent EM wave absorbing material. In Figure 11c, the CoS@Ti 3 C 2 T x composite exhibited the maximum RL value of −61.84 dB obtained at 14.3 GHz, and the absorbing thickness was only 1.84 mm. Moreover, when the thickness was 2.0 mm, the corresponding absorption bandwidth (RL < −10 dB) of the CoS@Ti 3 C 2 T x composite reached 5.0 GHz (12.24-17.24 GHz), as shown in Figure 11d. Obviously, the CoS@Ti 3 C 2 T x composite exhibited strong absorption intensity and broad effective bandwidth than that of the CoS and Ti 3 C 2 T x composite due to their limited impedance matching.
To further explain the attenuation process of EM waves in the CoS@Ti 3 C 2 T x composite, a schematic diagram of the proposed absorption mechanism is given in Figure 12. First, due to good impedance matching, more incident EM waves could enter the material and be absorbed rather than reflected. Second, the unique sandwich structure of the CoS@Ti 3 C 2 T x composite will expand the propagation path of EM waves inside the material, which may be conductive to the conversion of the EM waves into heat energy [56]. Meanwhile, based on the space-charge polarization effect, the interlayer space of the Ti 3 C 2 T x MXene increased because of the existence of these nanoparticles, which perhaps benefits the enhancement of the absorption capacity [57,58]. Third, the introduction of CoS nanoparticles will significantly increase the conductive paths in the Ti 3 C 2 T x MXene, carriers will migrate and hop between the Ti 3 C 2 T x layers more actively. The formed field induced microcurrent may contribute to the conduction loss [59]. Moreover, abundant surface defects, dangling bonds, and functional groups (-F, -O, -OH) in Ti 3 C 2 T x layers will form many polarized centers and generate a large number of dipoles, enhancing the dipolar polarization loss [60,61]. Finally, the interfacial polarization between CoS nanoparticles and Ti 3 C 2 T x MXene sheets also favors the attenuation of EM waves. Thus, under the comprehensive influence of these factors, the CoS@Ti 3 C 2 T x composite illustrates impressive absorption potential.  To further explain the attenuation process of EM waves in the CoS@Ti3C2Tx composite, a schematic diagram of the proposed absorption mechanism is given in Figure 12. First, due to good impedance matching, more incident EM waves could enter the material and be absorbed rather than reflected. Second, the unique sandwich structure of the CoS@Ti3C2Tx composite will expand the may contribute to the conduction loss [59]. Moreover, abundant surface defects, dangling bonds, and functional groups (-F, -O, -OH) in Ti3C2Tx layers will form many polarized centers and generate a large number of dipoles, enhancing the dipolar polarization loss [60,61]. Finally, the interfacial polarization between CoS nanoparticles and Ti3C2Tx MXene sheets also favors the attenuation of EM waves. Thus, under the comprehensive influence of these factors, the CoS@Ti3C2Tx composite illustrates impressive absorption potential.

Conclusions
In this work, a CoS@Ti3C2Tx composite was successfully fabricated through a solvothermal reaction. After combining with Ti3C2Tx MXene, the impedance matching of the CoS@Ti3C2Tx composite had been significantly optimized. Enhanced dielectric loss, interfacial polarization, and unique sandwich structure also contributed to the EM wave absorption. As a result, the as prepared CoS@Ti3C2Tx composite showed excellent EM wave absorbing properties with the maximum RL value reaching −59.2 dB at an optimal thickness of only 2.0 mm. The effective absorbing bandwidth was up to 5.0 GHz (from 12.24 to 17.24 GHz). Therefore, our work offers an effective way to broaden the application fields for the development of other MXene-based absorbers.

Conflicts of Interest:
The authors declare no conflict of interest.

Conclusions
In this work, a CoS@Ti 3 C 2 T x composite was successfully fabricated through a solvothermal reaction. After combining with Ti 3 C 2 T x MXene, the impedance matching of the CoS@Ti 3 C 2 T x composite had been significantly optimized. Enhanced dielectric loss, interfacial polarization, and unique sandwich structure also contributed to the EM wave absorption. As a result, the as prepared CoS@Ti 3 C 2 T x composite showed excellent EM wave absorbing properties with the maximum RL value reaching −59.2 dB at an optimal thickness of only 2.0 mm. The effective absorbing bandwidth was up to 5.0 GHz (from 12.24 to 17.24 GHz). Therefore, our work offers an effective way to broaden the application fields for the development of other MXene-based absorbers.

Conflicts of Interest:
The authors declare no conflict of interest.