Novel Hierarchical Structure of MoS2/TiO2/Ti3C2Tx Composites for Dramatically Enhanced Electromagnetic Absorbing Properties


 In order to prevent the microwave leakage and mutual interference, more and more microwave absorbing devices are added into the design of electronic products to ensure its routine operation. In this work, we have successfully prepared MoS2/TiO2/Ti3C2Tx hierarchical composites by one-pot hydrothermal method and focused on the relationship between structures and electromagnetic absorbing properties. Supported by comprehensive characterizations, MoS2 nanosheets were proved to be anchored on the surface and interlayer of Ti3C2Tx through a hydrothermal process. Additionally, TiO2 nanoparticles were obtained in situ. Due to these hierarchical structures, the MoS2/TiO2/Ti3C2Tx composites showed greatly enhanced microwave absorbing performance. The MoS2/TiO2/Ti3C2Tx composites exhibit a maximum reflection loss value of −33.5 dB at 10.24 GHz and the effective absorption bandwidth covers 3.1 GHz (13.9–17 GHz) at the thickness of 1.0 mm, implying the features of wide frequency and light weight. This work in the hierarchical structure MoS2/TiO2/Ti3C2Tx composites opens a promising door to the exploration of constructing extraordinary electromagnetic wave absorbents.


Introduction
Electronic products are developing towards the direction of high power and high frequency, but there is a major problem for these electronic products that they come with leakage and mutual interference of microwaves [1]. In order to resolve this problem, more and more electromagnetic wave absorbing devices are added into the design of electronic products to ensure routine operation. For practical microwave absorbing device applications, a new generation electromagnetic wave absorbers are expected to possess strong absorption capability and light weight, etc. To date, 2D nanomaterials such as graphene, graphene oxides (GOs)[2], MoS2 [3] and MXenes [4], have proven to be potential candidates for ideal microwave absorbers.
MXenes, an emerging group of layered materials, were synthesized by etching MAX phases by Gogotsi and co-workers in 2011 [5]. They have a general formula of Mn+1XnTx, where M represents an early transition metal (e.g., Ti, Mo, Nb, etc.), X usually is C and/or N, n = 1, 2, or 3, and Tx denotes surface terminations (=O, -OH, and/or -F). The distinguishing characteristics of MXenes, including special accordion- 5 like morphology, outstanding electrical conductivity, and rich functional groups surface, have captured enormous attention in many applications, especially for microwave absorption (MA) [6,7].
Since 2016, Ti3C2Tx MXene has been developed as a potential microwave absorber candidate due to its positive dielectric loss ability [8]. Unfortunately, pure Ti3C2Tx MXene alone is always limited to meet the requirement for practical application. Hence, incorporation of a second phase is very important. Semiconductors such as ZnO [9], SiC [10], MoS2 [11] and TiO2 [12] are usually introduced into Ti3C2Tx to enhance microwave absorbing abilities due to their moderate dielectric constant, effective heterojunction formation and better impedance matching. Recently, a sandwich-like structure of the MoS2/TiO2/Ti3C2Tx as nanocomposite was shown to possess improved microwave absorption ability when compared to the pure Ti3C2Tx [11].
However, the maximum reflection loss was just -16.0 dB, which is not satisfactory in practical applications.
The electromagnetic wave absorption properties of a material are closely related to the micro-structure 13 . Recent research in hierarchically structured materials opens a possible door into the exploration of constructing extraordinary electromagnetic wave absorbents 14 . So, in this work, we have successfully prepared MoS2/TiO2/Ti3C2Tx hierarchical composites by one-pot hydrothermal method and focused on the relationship between structures and electromagnetic wave absorbing properties.
Supported by comprehensive characterizations, MoS2 nanosheets were proved to be anchored vertically on the surface and interlayer of Ti3C2Tx through a hydrothermal process and in situ generated TiO2 nanoparticles were obtained simultaneously. Due to these hierarchical structures, the MoS2/TiO2/Ti3C2Tx composites showed greatly enhanced microwave absorbing performance.

Preparation of Ti3C2Tx
The Ti3C2Tx nanosheets were prepared by a typical etching process of MAX phase Ti3AlC2 15 . 3.0 g Ti3AlC2 powder was added into 30 mL HF solution and stirred at room temperature for 12 h. Then, the Ti3C2Tx suspension was centrifuged and washed by ethanol and deionized water several times until the pH value was above 6. The obtained precipitates were then freeze dried.

Preparation of MoS2/TiO2/Ti3C2Tx composites
To synthesize MoS2/TiO2/Ti3C2Tx composites, 0.1 g Ti3C2Tx were dispersed in 70 mL deionized water. The solution was ultrasonicated for 5 min to form a uniform Ti3C2Tx dispersion. After that, Na2MoO4·2H2O and CH4N2S were dissolved into the Ti3C2Tx suspension followed by stirring. The above solution was then transferred into a 100 ml Polyphenylene (PPL)-lined autoclave. The autoclave was kept at 200 °C for 12 h. After cooling naturally, the black precipitates were washed with deionized water and ethanol for many times. The samples were dried in a vacuum oven at 80 °C for 12 h. Based on the weight ratio of Ti3C2Tx to MoS2, i.e., 2:1, 1:1 and 1:2, the obtained 7 MoS2/TiO2/Ti3C2Tx composites were labeled as S1, S2, and S3. For comparison, pure MoS2 and TiO2/Ti3C2Tx powders were treated under the same procedure and condition.
The schematic of the synthetic route and formation mechanism for the MoS2/TiO2/Ti3C2Tx composites is depicted in Fig. 1.

Characterization
The morphology and microstructure of MoS2, TiO2/Ti3C2Tx and MoS2/TiO2/Ti3C2Tx composites were identified by field emission scanning electron microscopy (FESEM, JEOL JSM-7001F) and transmission electron microscopy (TEM, Jem-2100F). The crystal structure and phase composition were measured by X-ray diffractometer (XRD, Rigaku Ultima IV). Raman spectroscopy (HORIBA LabRAM HR Evolution, λ = 532 nm) was applied to determine the surface characteristics of the composites. X-ray Photoelectron Spectroscopy (XPS, ThermoFischer ESCALAB 250Xi) was employed to analyze the chemical states of the major elements on the sample surface.

Electromagnetic Measurements
For electromagnetic parameter measurements, S1, S2, S3 and MoS2 powders were homogeneously dispersed in the paraffin wax with 60 wt%, respectively. All the above mixtures were pressed into toroidal-shaped (Outer Diameter = 7.00 mm and Inner Diameter = 3.04 mm). The relative complex permittivities of samples were measured using a vector network analyzer (Agilent, N5244A) in the frequency range of 2.0-18.0 GHz with the coaxial-line method. 8 The typical XRD patterns of MoS2, TiO2/Ti3C2Tx and MoS2/TiO2/Ti3C2Tx composites are shown in Fig. 2 (a). The diffraction peak at 8.8 o is considered to be a characteristic peak of Ti3C2Tx [3]. Compared with the pure Ti3C2Tx, there are two main diffraction peaks in TiO2/Ti3C2Tx composites at 25.6° and 27.4°, ascribed to anatase TiO2 phase (JCPDS No. 21-1272) and rutile TiO2 phase (JCPDS No. 83-2243), respectively [3]. It also could be seen that, after coupling with MoS2, the relative peak intensity of TiO2 is significantly weakened, while the diffraction peak intensity of Ti3C2Tx is sharply enhanced, indicating that the formation of MoS2 had inhibited the transformation process from Ti3C2Tx to TiO2. The characteristic diffraction peaks at 14.3 and 28.9 assigned to the (100) and (103) planes of MoS2 (JCPDS card no. 37-1492) [16], while the peak of (002) plane of Ti3C2Tx slightly shifts to 7.1 o , demonstrating the d-spacing between Ti3C2Tx nanosheets was expanded, and implying the successful anchoring of MoS2 on the Ti3C2Tx layers. The corresponding magnification of the range from 5-10 o is shown in Fig. 2(b). It shows that the d-space of (002) plane of Ti3C2Tx decreases with increase in MoS2 content.

Results and discussion
Raman spectroscopy of TiO2/Ti3C2Tx and MoS2/TiO2/Ti3C2Tx(S2) composites are shown in Fig. 3. The characteristic bands of Ti3C2Tx at 261, 411, 606 cm -1 were observed [17]. In the spectrum of S2, the strong E2g 1 mode at ~374 cm −1 corresponds to in-plane vibration and the A1g mode at ~402 cm −1 is ascribed to the out-of-plane vibration. These are mainly due to the worse crystallinity of MoS2, and due to the interaction between MoS2 and Ti3C2Tx nanosheets [18], which indicates that MoS2 formed in the S2 samples. Additionally, the Ti-O peak appeared in all the samples.
Compared with Ti3C2Tx, the Ti-O peak of both TiO2/Ti3C2Tx and MoS2/TiO2/Ti3C2Tx(S2) composites shifts to ~154 cm −1 , suggesting that more defects were generated during the hydrothermal process [19]. Furthermore, the strong D and G peaks of the composites are at 1382 and 1576 cm -1 , respectively. This illustrates that amorphous C was formed in the hydrothermal process [20]. TEM and HRTEM were further applied to investigate the MoS2/TiO2/Ti3C2Tx composites. As displayed in Fig. 5 (a), the MoS2/TiO2/Ti3C2Tx composites are composed of Ti3C2 matrix and MoS2 blanket with the average thickness around 80 nm.
The interlayer spacing was considered to be about 0.61 nm, which matched the crystal planes of MoS2 (002), as shown in Fig. 5 (b). These images further supported the hypothesis of the heterojunction structure of MoS2/TiO2/Ti3C2Tx composites.
To confirm the surface properties of MoS2/TiO2/Ti3C2Tx composites, S2 samples were analyzed by XPS. The survey spectra (Fig. 6a) and the high resolution XPS of Ti2p, C1s, O1s, Mo3d and S2p (Fig. 6b-f) are shown in Fig. 6. As shown in Fig. 6(a 20 . In Fig. 6c, the C1s spectra can be separated into two peaks, related to C-C (284.7 eV) and C-O (285.9 eV ), the weak peak at 281.8 eV is assigned to Ti-C bond [21]. The O1s profile was fitted by five symmetrical peaks. The fitting peak located at 530.3 eV attributes to TiO2. The peaks at 530.9, 531.7, 532.5, and 533.5 eV are assigned to C-Ti-Ox, C-Ti-(OH)x, Al2O3, and H2O, respectively [22].
As for the Mo3d spectra in Fig. 6 (e), the obvious peaks located at 232.2, 229.0, and 226.1 eV are assigned to Mo3d3/2, Mo3d5/2, and S2s, respectively, confirming the Mo +4 ion exists in the nanocomposites [24]. There is an extremely small peak at 235.6 eV, which is correspond to the Mo (+6) 3d3/2 orbit, probably caused by the incompletely reduced MoO4 2during the hydrothermal procedure [23]. The S spectra of the MoS2/TiO2/Ti3C2Tx composites can be divided into two peaks, and the peak at 163.2 eV and 161.9 eV corresponds to the S 2p3/2 and S 2p1/2, respectively.
Relative complex permittivity in the frequency range of 2-18 GHz was measured to clarify the electromagnetic wave absorbing property of the MoS2 and MoS2/Ti3C2Tx composites at room temperature. The real parts (ε') and the imaginary part (ε'') represent the storage and the loss capability of electromagnetic energy 20 . As shown in Fig. 7(a) and (b), we can find that for the bare MoS2, the ε' value is approximately 7.5 and almost kept constant with frequency, meanwhile, the ε'' is within 1-2.3. After coupled with Ti3C2Tx, the complex permittivity is clearly enhanced, as the Ti3C2Tx content increased, the real part reaches to 13.8-34.5 and the imaginary parts fluctuates in the range of 2.1-

12.9.
Higher complex permittivity demonstrates that the composites can store and dissipate much more electromagnetic energy, the real part of complex permittivity is an expression of interfacial and relaxation polarization mechanism [11]. For the MoS2/Ti3C2Tx composites, higher ε' can be attributed to the improved interfacial and relaxation polarization effect arising from the multiple interfaces between the Ti3C2Tx and MoS2 nanosheets.
Based on the theory of free electric, the imaginary part (ε'') is related to the conductivity, it can be illustrated as the following equation [25]: Here σ, f and ε0 represent the electrical conductivity, electromagnetic wave frequency and permittivity of free space, respectively. Consequently, higher conductivity brings higher ε'' value. Owing to the excellent conductivity of Ti3C2Tx [26], the higher Ti3C2Tx content results higher conductivity of MoS2/TiO2/Ti3C2Tx composites.
In addition, Fig. 7(c)  According to the Debye's dipolar theory, the relationship of between ε' and ε'' can be described as the following equation [27]: composites exhibit an enhanced electromagnetic wave absorption characteristic, the maximum RL value of S1, S2 and S3 is -29.2 dB, -33.5 dB and -18.2 dB, respectively, as shown in Fig.9(b-d). Especially for S2, the effective absorption bandwidth (<10 dB) covers 3.1 GHz (13.9-17 GHz) at the thickness of 1.0 mm, implying the characteristic of wide frequency and light weight.
In general, the MoS2/TiO2/Ti3C2Tx composites (S2) exhibited excellent electromagnetic wave absorption performance, which originated from three aspects: enhanced interface polarization relaxation, scattering effects and appropriate conductivity loss. The possible electromagnetic wave absorption mechanism of MoS2/TiO2/Ti3C2Tx composites is proposed in Fig. 10. Firstly, the conductive laminate Ti3C2Tx could enhance the conductance loss, and the unique laminated structure with MoS2 as blanket could generate multiple reflections and scattering to attenuate electromagnetic waves. Furthermore, TiO2 as a semiconductor, which could adjust the impedance matching properties, and the interface between MoS2, TiO2 and Ti3C2Tx could also enhance the interface polarization effects.

Conclusion
MoS2/TiO2/Ti3C2Tx composites were successfully prepared by a simple hydrothermal method. All the MoS2/TiO2/Ti3C2Tx samples share the same lamellar structure, and the MoS2 nanosheets are grown on the Ti3C2Tx nanolayers, which could increase the specific surface area and expand the Ti3C2Tx layer spacing. The