Tailoring Ti₃C₂T_x nanosheets to tune local conductive network as an environmentally friendly material for highly efficient electromagnetic interference shielding

Abstract

Environmentally friendly materials that exhibit high-performance electromagnetic interference (EMI) shielding are extremely necessary. Herein, we fabricated ultrathin Ti₃C₂T_x (U-Ti₃C₂T_x) MXene nanosheets (NS) by atomic-layer tailoring the layer thickness of Ti₃C₂T_x MXene. The U-Ti₃C₂T_x NS composites with highly efficient EMI shielding effectiveness can reduce secondary reflection, demonstrating its environmentally friendly performance. The U-Ti₃C₂T_x NS composite with 80 wt% loading exhibits an EMI shielding effectiveness of 58.1 dB at a thickness of 1 mm. Shielding performance analysis of different layer thicknesses shows that electron transport has an important contribution to the EMI shielding performance. Furthermore, the polarization induced by defects and terminal atoms plays an important role in the EMI shielding performance. Based on the electromagnetic (EM) wave response mechanism, a novel approach to effectively tune the EMI attenuation and shielding effectiveness can be achieved by adjusting the local conductive network. These findings will offer an effective strategy for designing environmentally friendly 2D materials with high-performance EMI shielding.

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Introduction

Electromagnetic (EM) radiation pollution has been severely increased with the rapid development of modern electronic technology.^1–10 Particularly, EM radiation is a great hazard to human health, and long-term EM radiation can cause serious diseases such as leukemia and cancer. Therefore, the development of effective EM wave absorption and shielding materials has attracted more and more attention.^11–33

Two-dimensional (2D) materials are widely used as multi-functional materials due to their unusual electronic, mechanical, and optical properties. They also show excellent performances in EM wave absorption and shielding.^34–41 MXene, a new member of 2D materials, was first prepared by high-concentration HF etching of Ti₃AlC₂.⁴² Typically, these materials can be written as M_n+1X_nT_x, where M denotes the transition metal, X represents C/N, and T_x is used to represent functional groups (=O, –OH and –F).⁴³ As a new 2D material, MXenes with high metallic conductivities and hydrophilic surfaces exhibit excellent performances in energy storage,^43–47 sewage purification,⁴⁸ electrocatalysis,⁴⁹ photoelectric transmission,⁵⁰ and many other applications.⁵¹ For example, Jiang et al. reported that a pseudocapacitance-dominated MoS₂-in-Ti₃C₂ superstructure exhibits a highly reversible Li⁺ storage capacity.⁵²

Moreover, researchers are gradually becoming interested in the EM wave absorption and shielding characteristics of MXenes. Initially, Gogotsi et al. prepared a 45 μm Ti₃C₂T_x film exhibiting an electromagnetic interference (EMI) shielding effectiveness (SE) of 92 dB, which is the highest among fabricated materials prepared heretofore.⁵³ Simultaneously, a series of studies on modified Mxenes reported by Yin et al. shows that modified Mxenes exhibited high EM wave absorption and shielding properties.^54–57 Subsequently, Yu et al. fabricated lightweight, hydrophobic, and flexible MXene foams, where MXene foams exhibited excellent performance in EMI shielding.⁵⁸ Additional researches describing the EM wave absorption and shielding performance of MXene have been reported.^59–71 In conclusion, MXenes have become rising stars showing potential advantages in the field of EM wave absorption and shielding. However, the investigation of the EM wave shielding performances of MXenes is still in the exploratory stage, and the realization of strong absorption and low secondary reflection for high-performance EMI shielding effectiveness is still a challenge.

In this paper, ultrathin Ti₃C₂T_x (U-Ti₃C₂T_x) MXene nanosheets (NS) were successfully fabricated by a modified Gogotsi's method. The formation mechanism of U-Ti₃C₂T_x MXene NS is illustrated. The electromagnetic properties of Ti₃C₂T_x MXenes based on different layer thicknesses were systematically investigated. The roles of electron transport and local conductive network in the EMI shielding performance of Ti₃C₂T_x MXenes are revealed in detail. Notably, the important roles of defects and terminal atoms in EMI shielding performance were systematically analyzed. More significantly, by atomic layer tailoring (ALT) the layer thickness of Ti₃C₂T_x MXenes to induce the formation of local conductive network, a high-performance EMI shielding U-Ti₃C₂T_x MXene NS composite was obtained that could effectively reduce secondary reflection illustrating its environmentally friendly performance.

Experimental

Experimental materials

Ti₃AlC₂ powders (purity >98%, ∼300 mesh) were provided by Lianlixin Technology Ltd (Beijing, China). Lithium fluoride (LiF) was supplied by Macklin Technology Ltd (Shanghai, China). Hydrochloric acid (HCl) (35 wt%) and hydrofluoric acid (HF) (40 wt%) were supplied by Beijing Chemical Factory (Beijing, China). All the chemicals were used without further purification.

Preparation of multilayered Ti₃C₂T_x MXenes

Multilayered Ti₃C₂T_x (M-Ti₃C₂T_x) MXenes were synthesized by etching a Ti₃AlC₂ phase with HF as previously reported.⁷² 1 g Ti₃AlC₂ was added into a 20 mL HF solution (40 wt%) and was stirred at room temperature for 24 h. After this reaction, the resultant product was repeatedly washed with deionized water and centrifuged at 3500 rpm until an almost neutral pH (≥6) was reached. Then, the supernatant, which is a colloidal solution of M-Ti₃C₂T_x, was gathered. Finally, the colloidal solutions were filtered through a membrane and dried in a desiccator under vacuum at room temperature.

Preparation of ultrathin Ti₃C₂T_x MXenes

Ultrathin Ti₃C₂T_x (U-Ti₃C₂T_x) MXenes were prepared by using a modified Gogotsi's method. 1 g of LiF was added into 20 mL of 6 M HCl and the contents were allowed to mix completely for 5 minutes at room temperature. Then, 1 g of Ti₃AlC₂ was immersed in the mixture and stirred at 40 °C for 16 h. After this reaction, the resultant product was repeatedly washed with deionized water and centrifuged at 3500 rpm until an almost neutral pH (≥6) was reached. After that, the supernatant, which is the colloidal solution of U-Ti₃C₂T_x nanosheets (NS), was gathered. Finally, the colloidal solutions were filtered through a membrane and dried in a desiccator under vacuum at room temperature.

Preparation of samples

In a typical experiment, the M-Ti₃C₂T_x MXenes (20, 40, 60, 80 wt% loading) were mixed with SiO₂ nanopowders. SiO₂ is an EM wave transparent material and is used as the matrix. SiO₂ powder was mixed with M-Ti₃C₂T_x and ground into ultrafine grains. Then, the mixed powders were cold-pressed into rectangular-shaped samples with dimensions of 22.86 mm × 10.16 mm × 1 mm under a pressure of 5 MPa. Similarly, the preparation of U-Ti₃C₂T_x/SiO₂ (20, 40, 60, 80 wt% loading) follows the same procedure as described above.

Characterization

X-ray diffraction (XRD) was measured on an X'Pert PRO system (Cu Kα). X-ray photoelectron spectroscopy (XPS) measurements were performed by a PHI Quantera system. Scanning electron microscopy (SEM) images were obtained on a scanning electron microscope system (SEM Hitachi S-4800). Atomic force microscopy (AFM) images were obtained on a Veeco Dimension Fast Scan system. Transmission electron microscopy (TEM) images were collected by a TEM-2100F microscope. The S parameters of samples were measured by using a wave guide method with a vector network analyzer (VNA Anritsu 37269D) in the X-band region. Meanwhile, their DC conductivities (σ) were measured by a Keithley 4200-SCS semiconductor characterization system.

Results and discussion

Factors affecting layer thickness of MXene

Fig. 1 explains the preparation of M-Ti₃C₂T_x and U-Ti₃C₂T_x MXenes. After centrifugation, Ti₃AlC₂ that was etched by HF has a multilayered structure (referred to as “M-Ti₃C₂T_x”), while Ti₃AlC₂ that was etched by LiF/HCl can be separated into an ultrathin structure (referred to as “U-Ti₃C₂T_x”). These very different results may be due to the different surface functional groups in the etched samples.^73,74 The content of –F terminal groups in M-Ti₃C₂T_x is high while the content of =O terminal groups in U-Ti₃C₂T_x is high.⁷³ Additionally, during the LiF/HCl etching, the temperature was increased from 35 °C to 40 °C. The reaction time was shortened from 24 hours to 16 hours compared with the previous work,⁷⁵ which could effectively improve the efficiency of producing U-Ti₃C₂T_x.

Fig. 1 Synthetic illustration of (A) M-Ti₃C₂T_x and (B) U-Ti₃C₂T_x MXenes.

The different layered structures of MXenes fabricated by two distinct methods are displayed in the results of the powder XRD analysis. As shown in Fig. S1 (ESI†), there is a shift of the (002) peak of Ti₃AlC₂ at ∼9.5° to ∼6.0° in the diffraction patterns of M-Ti₃C₂T_x and U-Ti₃C₂T_x, which is ascribed to the elimination of the Al from Ti₃AlC₂. XPS is implemented to further investigate the surface environment and the oxidation states of MXenes. Fig. 2A and B show the survey spectra of the M-Ti₃C₂T_x and U-Ti₃C₂T_x. The four main elements on the surface of MXenes are F, C, O and Ti. Moreover, the illustrations of Fig. 2A and B show the Ti 2p spectra of M-Ti₃C₂T_x and U-Ti₃C₂T_x. The signal labelled C-Ti-T_x 2p_3/2 and 2p_1/2 correspond to Ti interactions with terminal atoms and carbons. A slight increase in the intensity can be seen in the Ti 2p spectra of U-Ti₃C₂T_x at ∼459 and ∼465 eV compared with that of M-Ti₃C₂T_x in the same position (Fig. S2†). This can be further proved by the comparison of the component peak-fitting of XPS spectra for M-Ti₃C₂T_x and U-Ti₃C₂T_x Ti 2p at TiO₂ 2p_3/2 (Fig. 2C) and TiO₂ 2p_1/2 (Fig. 2D). These phenomena suggest that the oxidation degree of U-Ti₃C₂T_x is higher than that of M-Ti₃C₂T_x. According to the previous theoretical work, the area of U-Ti₃C₂T_x exposed to air is more than that of M-Ti₃C₂T_x, which will lead to faster oxidation.⁷⁶

Fig. 2 The survey spectra of (A) M-Ti₃C₂T_x and (B) U-Ti₃C₂T_x, high-resolution XPS for Ti 2p of M-Ti₃C₂T_x (inset in (A)) and U-Ti₃C₂T_x (inset in (B)). Comparison of component peak-fitting of XPS spectra for M-Ti₃C₂T_x and U-Ti₃C₂T_x Ti 2p at (C) TiO₂ 2p_3/2 and (D) TiO₂ 2p_1/2, where TiO₂ 2p_3/2 and TiO₂ 2p_1/2 represent the titanium in TiO₂ that produces distinctive signals at ∼459 and ∼465 eV, respectively.

The SEM images reveal the morphology of the M-Ti₃C₂T_x and U-Ti₃C₂T_x MXenes. As shown in Fig. 3C, the as-obtained M-Ti₃C₂T_x MXene has a typical multilayered morphology. It can clearly be seen that the M-Ti₃C₂T_x MXene has an accordion-like structure (Fig. 3D–F), and the average particle size is found to be ∼5 μm for the M-Ti₃C₂T_x MXene. The thickness of the U-Ti₃C₂T_x MXene is obviously thinner compared with that of the M-Ti₃C₂T_x MXene; the majority of the U-Ti₃C₂T_x MXene is of 1–4 μm in diameter (Fig. 3J–L). Above all, the SEM results demonstrate that the multilayered Ti₃C₂T_x MXene can be effectively separated into an ultrathin structure by ALT (Fig. 3A, B, G and H).

Fig. 3 (A) Physical schematic of M-Ti₃C₂T_x. (B) The atomic schematic of M-Ti₃C₂T_x. (C) SEM image of M-Ti₃C₂T_x. (D)–(F) The high-magnification SEM image of M-Ti₃C₂T_x. (D), (E) and (F) correspond to the rectangle I, II and III in (C), respectively. (G) Physical schematic of U-Ti₃C₂T_x. (H) The atomic schematic of U-Ti₃C₂T_x. (I) SEM image of U-Ti₃C₂T_x. (J)–(L) The high-magnification SEM image of U-Ti₃C₂T_x. (J), (K) and (L) correspond to the circle I, II and III in (I), respectively.

AFM was used to investigate the thicknesses and shapes of the M-Ti₃C₂T_x and U-Ti₃C₂T_x. As shown in Fig. 4A and B, the M-Ti₃C₂T_x shows an accordion-like structure. The formation of different interlayer distances (3.7 nm, 2.3 nm, 8.1 nm, 4.3 nm, as shown in Fig. 4D) was observed in the M-Ti₃C₂T_x MXenes based on the height profile measurement along the red dashed line in Fig. 4B. Additionally, Fig. 4C shows a 3D image of the M-Ti₃C₂T_x, further revealing that the M-Ti₃C₂T_x has an accordion-like structure. Fig. 4E–G show AFM images of the U-Ti₃C₂T_x. It can be clearly seen that the U-Ti₃C₂T_x MXenes have an average height of ∼2.6 nm, which is in accordance with the previous work.⁷⁷ The results of AFM further confirm that U-Ti₃C₂T_x has an ultrathin structure.

Fig. 4 (A) and (B) AFM images of the M-Ti₃C₂T_x. (C) 3D images by AFM of the M-Ti₃C₂T_x corresponding to (B). (D) AFM height profile measured along the red dashed line in (B). (E)–(G) AFM images of the U-Ti₃C₂T_x.

TEM was performed to further characterize the Ti₃C₂T_x MXenes fabricated by different methods. The M-Ti₃C₂T_x MXene has an accordion-like structure (Fig. 5A–D and Fig. S3A–D†), and the U-Ti₃C₂T_x MXene exhibits an ultrathin structure (Fig. 5G–J and Fig. S3E–H†), which are in accordance with the results of SEM (Fig. 3) and AFM (Fig. 4). High-resolution (HR) TEM images show a hexagonal arrangement of atoms in the plane (Fig. 5E and K), indicating that the crystal structures are the same for both MXenes, which is further corroborated by the selected area electron diffraction (SAED) patterns (Fig. 5F and L). However, the red dashed circle (Fig. 5F and Fig. S4A†) in the SAED pattern shows that there is a point of separation in the M-Ti₃C₂T_x MXene, which is different from the red dashed circle marked in the SAED pattern of U-Ti₃C₂T_x MXene (Fig. 5L and Fig. S4B†). This phenomenon is mainly due to the secondary diffraction caused by the multilayered structure of the M-Ti₃C₂T_x.

Fig. 5 (A) TEM image of M-Ti₃C₂T_x. (B)–(D) The high-magnification TEM images of M-Ti₃C₂T_x. (C) and (D) correspond to the rectangle I and II in (B), respectively. (E) HRTEM image of M-Ti₃C₂T_x. (F) The SAED pattern of M-Ti₃C₂T_x. (G) TEM image of U-Ti₃C₂T_x. (H)–(J) The high-magnification TEM images of U-Ti₃C₂T_x. (I) and (J) correspond to the circle I and II in (H), respectively. (K) HRTEM image of U-Ti₃C₂T_x. (L) The SAED pattern of U-Ti₃C₂T_x.

Fig. 6A–E show the atomic phase of U-Ti₃C₂T_x. It can be clearly seen that there are four kinds of vacancy defects on the surface of the U-Ti₃C₂T_x. Fig. 6F shows the generation of vacancy defects: one single Ti vacancy (V_Ti) and two V_Ti occurring within the same sub-layer; three V_Ti and two V_Ti occurring within two different sub-layers.⁷⁸ Defects can only be formed at specific locations. These defects were formed during the etching process.^78,79

Fig. 6 (A)–(E) Atomic phase of U-Ti₃C₂T_x. (B, C, D and E) correspond to the circle I, II, III and IV in (A), respectively. (F) Schematic illustration for defects generation: one single Ti vacancy (V_Ti) and two V_Ti occurring within the same sub-layer; three V_Ti and two V_Ti occurring within two different sub-layers.

Effect of layer thickness on EMI shielding performance

For the purpose of investigating the EMI attenuation performances of the M-Ti₃C₂T_x and U-Ti₃C₂T_x composites, the scattering parameters (S₁₁ and S₂₁) were used to calculate the absorption (A), reflection (R), and transmission (T) coefficients of the M-Ti₃C₂T_x and U-Ti₃C₂T_x MXene composites.

Fig. 7A and B show the absorption coefficients of the M-Ti₃C₂T_x and U-Ti₃C₂T_x. The absorption coefficients of the M-Ti₃C₂T_x and U-Ti₃C₂T_x NS composites achieve the maximum values of 0.21 and 0.26, respectively. Moreover, the maximum values of A for M-Ti₃C₂T_x and U-Ti₃C₂T_x are achieved in the composites with 60 wt% loading, which means that the maximum reflection loss (RL) is reached when the loading content of the M-Ti₃C₂T_x and U-Ti₃C₂T_x are 60 wt%, and this performance is due to good electromagnetic matching. Moreover, since the increased effective thickness of the composites with reducing wavelength, the A values of the M-Ti₃C₂T_x and U-Ti₃C₂T_x composites increase with the increase in the frequency (Fig. S5A–D†). Actually, a material absorbs EM waves and converts them into heat, and this conversion ability is determined by the conduction loss and the polarization loss. Polarization loss has an important contribution to the absorption of EM waves. In fact, the defects formed during the etching process (Fig. 6A–E) lead to the asymmetry in the spatial distribution of electrons to form dipole moments. Moreover, terminal atoms attached on the surface of MXenes (Fig. 2A and B) lead to the asymmetric distribution of charge density, which in-turn causes the formation of dipoles. Under an alternating electromagnetic field, these dipoles will break loose and rotate directionally. When their rotation cannot keep up with the change in electromagnetic field, polarization relaxation occurs with EM energy loss.¹² Recent researches have mainly focused on the EMI shielding performance of Ti₃C₂T_x MXene films.^53,58 However, EM waves are mostly reflected by high conductivity in the Ti₃C₂T_x film and this behavior is the same as a metal shielding the EM wave. Only a small amount of EM waves can enter the body of films and be absorbed. Compared with the MXene films that mainly use high conductivity to reflect EM waves, the composites made by dispersing the U-Ti₃C₂T_x MXene NS into the SiO₂ matrix can reduce the conductivity and absorb more EM waves by polarization loss. This means that more EM waves can be attenuated so that secondary reflections will be effectively reduced, demonstrating their environmentally friendly performance.

Fig. 7 Absorption coefficients of (A) M-Ti₃C₂T_x and (B) U-Ti₃C₂T_x. Reflection coefficients of (C) M-Ti₃C₂T_x and (D) U-Ti₃C₂T_x. Transmission coefficients of (E) M-Ti₃C₂T_x and (F) U-Ti₃C₂T_x.

Fig. 7C and D show the reflection coefficients of the M-Ti₃C₂T_x and U-Ti₃C₂T_x. The R values of both the M-Ti₃C₂T_x and U-Ti₃C₂T_x composites increase with the increase in mass ratio. This can be confirmed from the Fig. S5E–H.† The higher the concentration of Ti₃C₂T_x MXene is, the larger the conductivity will be. And this can cause more EM wave to be reflected. Meanwhile, the R values of the M-Ti₃C₂T_x and U-Ti₃C₂T_x composites decrease with an increase in the frequency, which is similar to the behavior of graphene. This may be due to the similar layer-structure.^35,41

The transmission coefficients of the M-Ti₃C₂T_x and U-Ti₃C₂T_x are shown in Fig. 7E and F. An increase in the mass ratio for the M-Ti₃C₂T_x and U-Ti₃C₂T_x leads to a drastic abatement in the T values. Additionally, the T values of the U-Ti₃C₂T_x composite are less than those of the M-Ti₃C₂T_x composite (Fig. S5I–L†). The reason for this is that the high A and R values of the U-Ti₃C₂T_x composite lead to a remarkable decline in the T values, which means that more microwave radiation is shielded and absorbed by the U-Ti₃C₂T_x composite. This can be directly confirmed by the values of SE_A and SE_R (Fig. S6†).

Analysis of EMI shielding mechanism

As a matter of fact, the EMI shielding efficiency (SE) of the material is often used to investigate the efficiency of the shielding materials, which is a judgement of the ability to dissipate EM wave. Based on the measured S parameters, the EMI SE can be obtained by:^80,81

EMISE = 10 log(P_I/P_T) = 10 log(1/T), (1)

where P_I is the incident power and P_T is the transmitted power. The as-achieved EMI SE of the Ti₃C₂T_x MXene composites are shown in Fig. 8. The EMI SE of the M-Ti₃C₂T_x and U-Ti₃C₂T_x NS composites mainly depend on the mass ratio (Fig. 8A). The 80 wt% U-Ti₃C₂T_x composite shows an excellent shielding performance; the average EMI SE for the 80 wt% U-Ti₃C₂T_x composite is 58.1 dB, which is approximately two-fold higher than that of M-Ti₃C₂T_x at the same concentration. This can be clearly seen in Fig. 8C–F. A large change can be seen in the EMI SE of the M-Ti₃C₂T_x and U-Ti₃C₂T_x composites, when the concentration of the Ti₃C₂T_x varies from 60 wt% to 80 wt%. The average EMI SE for the 60 wt% M-Ti₃C₂T_x and U-Ti₃C₂T_x composites are 15.1 dB and 26.7 dB (Fig. 8E), respectively. The corresponding conductivities (σ) of the 60 wt% M-Ti₃C₂T_x and U-Ti₃C₂T_x composites are 6.3 × 10⁻³ S m⁻¹ and 0.42 S m⁻¹ (Fig. 8B), respectively. Evidently, the great differences in σ lead to a large change in the EMI SE of M-Ti₃C₂T_x and U-Ti₃C₂T_x. In addition, there is an obvious positive trend in the EMI SE of 80 wt% U-Ti₃C₂T_x/SiO₂ at 10.5 GHz (Fig. 8F), which is due to the significant changes in the value of SE_A (Fig. S6C†).

Fig. 8 (A) EMI SE of the M-Ti₃C₂T_x and U-Ti₃C₂T_x composites. (B) DC conductivity (σ) vs. mass ratio of the M-Ti₃C₂T_x/SiO₂ and U-Ti₃C₂T_x/SiO₂ composites. The inset shows log 10 DC conductivity (σ) vs. mass ratio. (C)–(F) EMI SE comparison of the M-Ti₃C₂T_x and U-Ti₃C₂T_x composites at each mass ratio.

Regulation of local conductivity network

Usually, the conductivity of the Ti₃C₂T_x MXene composites is decided by the electronic mobility and formation of local conductive network. Herein, the electronic transport model and local conductive network theory are proposed to illustrate the EMI shielding performance of the M-Ti₃C₂T_x and U-Ti₃C₂T_x composites.

As shown in Fig. 9A, the free electrons are uniformly distributed on the surfaces of U-Ti₃C₂T_x under electrostatic balance. These electrons do not collide with each other during transmission. In contrast, the electrons collide during transmission in the interlayer channels of M-Ti₃C₂T_x, which leads to the dropped electronic mobility showing reduced conductivity (Fig. 9B).^82,83

Fig. 9 Schematic diagrams of the electronic transport channel and their scattering mechanisms for (A) U-Ti₃C₂T_x and (B) M-Ti₃C₂T_x. The E_α on the right represents the electric field distribution of U-Ti₃C₂T_x and M-Ti₃C₂T_x.

Electronic transport has an important contribution to the conductivity of the pure material, whereas the formation of a local conductive network is more important for the conductivity in composites. Briefly, the local conductive network dominates the conductivity in composites. As shown in Fig. 10C, the electrons cannot jump between two layers of M-Ti₃C₂T_x MXenes when the distance between the layers of M-Ti₃C₂T_x MXenes gets larger. This leads to the failure in the formation of local conductive network in the M-Ti₃C₂T_x composites. Atomic layer tailoring the layer thickness of the Ti₃C₂T_x MXenes induces the formation of local conductive network (Fig. 10D). This leads to a much better shielding performance. As shown in Fig. 10E, the microwave has been further attenuated by multiple internal reflections in the M-Ti₃C₂T_x composite. The attenuation of microwaves through multiple scattering in the U-Ti₃C₂T_x composite is larger compared with that of the M-Ti₃C₂T_x composite (Fig. 10F). In brief, U-Ti₃C₂T_x MXene composites show a much better EMI shielding performance compared with the M-Ti₃C₂T_x MXene composites through the formation of local conductive networks (Fig. 10G and H). Moreover, different rising trends of the EMI SE with the increase in the mass ratios of M-Ti₃C₂T_x and U-Ti₃C₂T_x composites are presented (Fig. 10G and H). The reason for this phenomenon is that by atomic layer tailoring, the U-Ti₃C₂T_x composite can more easily form local conductive networks. To date, very little research has been done on the EMI shielding performance of Ti₃C₂T_x MXene composites. The U-Ti₃C₂T_x NS composites obtained by ALT show excellent EMI shielding performance, compared with M-Ti₃C₂T_x MXene composites,^54,65 which further confirms that EMI SE can be effectively improved by tailoring the layer-thickness of Ti₃C₂T_x MXene to tune the local conductive network.

Fig. 10 Microstructure of (A) M-Ti₃C₂T_x and (B) U-Ti₃C₂T_x. Local conductive network of (C) M-Ti₃C₂T_x and (D) U-Ti₃C₂T_x. Microwave propagation model in (E) M-Ti₃C₂T_x and (F) U-Ti₃C₂T_x composites. Regulation of local conductive network and shielding performance for (G) M-T₃C₂T_x and (H) U-Ti₃C₂T_x.

Conclusions

In summary, U-Ti₃C₂T_x MXene NS was successfully fabricated by a modified Gogotsi's method. By ALT the layer thicknesses of Ti₃C₂T_x NS to induce the formation of the local conductive network, high-performance EMI shielding U-Ti₃C₂T_x NS composites were obtained. The U-Ti₃C₂T_x NS composites could effectively reduce secondary reflections, demonstrating environmentally friendly performances. We have found that electron transport has an important contribution to EMI shielding performance by comparing the electromagnetic properties of M-Ti₃C₂T_x and U-Ti₃C₂T_x MXenes. Furthermore, the defects and terminal atoms can lead to dipoles, in-turn causing polarization, which has a critical contribution to EM attenuation. The EMI attenuation and shielding effectiveness can be effectively tuned by tailoring the local conductive networks. Our findings will provide a new way for designing 2D materials in the future and will help to expand the application of U-Ti₃C₂T_x MXene NS to other areas such as energy storage, microwave absorption, and metamaterial design.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

Financial support from the National Natural Science Foundation of China (No. 11774027, 51132002, 51072024 and 51372282), and Graduate Technological Innovation Project of Beijing Institute of Technology (2018CX20028) are gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nr10489a

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