Interfacing MXene Flakes on a Magnetic Fiber Network as a Stretchable, Flexible, Electromagnetic Shielding Fabric

A unique self-standing membrane composed of hierarchical thermoplastic polyurethane (TPU)/polyacrylonitrile (PAN) fibers is prepared by the electrospinning technique, followed by a simple dip-coating process. Fe3O4 nanoparticles are uniformly anchored on TPU/PAN fibers during the electrospinning process, enabling the membrane to achieve effective electromagnetic interference shielding (EMI SE) performance. Such a hybrid membrane has a high magnetization of 18.9 emu/g. When MXene (Ti3C2Tx) layers are further loaded on the TPU/PAN/Fe3O4NPs hybrid membrane, its EMI SE performance in the X band can exceed 30 dB due to the hydrogen bonds generated between the macromolecular chain of PAN and the functional group (Tx) on the surface of MXene. Simultaneously, the interfacial attraction between MXene and the TPU/PAN/Fe3O4NPs substrate is enhanced. The EMI SE mechanism of the hybrid membrane indicates that this film has great potential in the fields of wearable devices and flexible materials


Introduction
The continuous development of the intelligence level of wireless communications technology prompts a heavy use of flexible electronic equipment. Enormous quantities of such electronic equipment will produce a large amount of electromagnetic pollution, which can disturb high-precision electronic devices, and poses potential human health hazards. To address this problem, the development of high-performance shielding materials is urgently required in order to prevent or minimize harmful radiation [1][2][3][4][5]. Currently, many studies in this field are focusing on the preparation of electromagnetic interference (EMI) shielding fabrics via the integration of metal particles and polymeric or inorganic fabrics [6][7][8]. Such EMI shielding fabrics have excellent conductivity and effective protection performance. Nevertheless, the cost to fabricate such EMI shielding fabrics is very high, and their life span is short due to their susceptibility to migration under humid and high-temperature environments. In contrast to traditional metal materials, Fe 3 O 4 nanoparticles have recently been introduced into EMI shielding fabrics as mixed fillers to enhance electromagnetic wave absorption by making use of their high magnetic properties and excellent compatibility with polymeric materials. However, it is still a huge challenge to overcome the agglomeration and uncontrollable loading of Fe 3 O 4 nanoparticles on the fabrics [9,10]. Carbon-based EMI shielding materials usually have a large surface-to-volume ratio, high chemical and thermal stability, good electrical conductivity, and high EMI SE; such materials include reduced graphene [11], carbon foam [12], multiwalled carbon nanotubes [13], carbon nanofibers [14], etc. Nevertheless, to the best of our knowledge, no EMI SE materials have been reported with a combination of extremely small thickness, high flexibility, and excellent EMI SE properties. MXenes are two-dimensional metal carbide/nitride layered nanomaterials. vessel for 30 min. Next, 2 g of Ti3AlC2 was slowly added to the beaker under continuous stirring at 35 °C for 24 h. The mixture was then centrifuged until the pH value was greater than 6. The final precipitate was dispersed in deionized water and sonicated in an ice bath for 1 h. Then, it was centrifuged at 3500 rpm for 1 h, and the black/brown-colored supernatant was collected as a small-layer dispersion liquid. The uniformly dispersed supernatant exhibited the Tyndall effect ( Figure S1c).

Preparation of TPU/PAN/Fe3O4/MXene Films
The manufacturing process for obtaining the TPU/PAN/Fe3O4/MXene (TPFM) films is shown in Figure 1a. Colloidal solutions were prepared by dissolving 14 wt% TPU/PAN (TPU:PAN = 5:2) in a mixed solution containing DMF and acetone at a mass ratio of 3:1. The increased volatility of this solvent was necessary in order to reduce the formation of a large number of beads during the electrospinning process. Next, different weight ratios of Fe3O4NPs (0, 3, 6, 12, and 20 wt%) were dispersed in this solution. The electrospinning solution was then mechanically stirred at room temperature for 12 h. The prepared electrospinning solution was loaded into a plastic syringe with an 18G metal needle. A positive voltage (15 kV) was applied to the needle tip, and the collector plate was grounded by covering it with aluminum foil. The distance between the needle tip and the collector was 15 cm, and the infusion rate was 1-1.5 mL/h. All samples were electrospun under the conditions of 30-35% relative humidity and 25 ± 2 °C. The fiber membranes were then separated from the aluminum foil and dried for 12 h. These fiber The prepared electrospinning solution was loaded into a plastic syringe with an 18G metal needle. A positive voltage (15 kV) was applied to the needle tip, and the collector plate was grounded by covering it with aluminum foil. The distance between the needle tip and the collector was 15 cm, and the infusion rate was 1-1.5 mL/h. All samples were electrospun under the conditions of 30-35% relative humidity and 25 ± 2 • C. The fiber  were then separated from the aluminum foil and dried for 12 h. These fiber  membranes were denoted TPF 0 , TPF 3 , TPF 6 , TPF 12 , and TPF 20 based on their Fe 3 O 4 NP contents. Next, the TPU/PAN/Fe 3 O 4 (TPF) films with different Fe 3 O 4 NP contents were soaked in a deionized water solution containing MXene (2 mg/mL or 5 mg/mL). After drying at 50 • C for 1 h, the MXene sheets formed a uniform coating structure on the TPF films; this structure was the TPFM mat. Figure 1b shows digital photographs of TPF films with different nanomagnetic particle contents and TPF films coated with MXene.

Characterization
The morphology of the nanomaterials was observed via FE-SEM (FEI, Quanta FEG 450, Hillsboro, OR, USA), and their crystal structure was characterized by XRD (Bruker, D8 Advance, Berlin, Germany); their chemical states were identified by X-ray photoelectron spectroscopy (XPS, Thermo Scientific, ESCALAB 250, Waltham, MA, USA). FTIR spectra were taken for a uniformly dispersed sample in KBr in the compressed pellet form, using a PerkinElmer RXI FTIR spectrometer (Waltham, MA, USA) in the frequency range of 500-4000 cm −1 .
The electrical conductivity values of the TPU/PAN/Fe 3 O 4 /MXene films obtained under different preparation conditions were tested using the four-point probe method at room temperature. Each reported measurement is the average of four samples. The details are reported in our previous work.
A vibrating sample magnetometer (VSM, MicroSense, EV9, Lowell, MA, USA) was used to measure magnetic properties at 300 K. The EMI shielding performance of the sample was measured with an Agilent N5232A vector network analyzer in the 8-12 GHz (X band) microwave range, using waveguide calibration (HD-100VNAWKS, Santa Clara, CA, USA). The measured scattering parameters (S 11 and S 12 ) were used to calculate the EMI SE. The specific calculation formulae for determining total EMI SE (SE T ), absorption shielding effectiveness (SE A ), reflection (SE R ), and multiple internal reflection (SE M ) are as follows [34,35]: where T, R, and A represent the transmission coefficient, reflection, and absorption coefficient, respectively. When SE T ≥ 15 dB, the SE M can be negligible [11,28]. Figure 2 shows FE-SEM images displaying the microscopic morphology of the fabricated nanocomposite fabrics with different Fe 3 O 4 NP contents. These nanocomposite fiber membranes consist of a three-dimensional network structure formed by interspersed superfine fibers, and the magnetic nanoparticles are randomly distributed throughout the fibers. As shown in Figure 2b-e, due to the stable formation of Taylor cones and the high volatility of the precursor liquid, the TPF nanofiber membranes have no defects (such as spindles or beads). The viscosity of the precursor fluid decreases as the magnetic nanoparticle content increases; this helps the electrostatic force on the surface of the Taylor cone to overcome the viscous resistance, making the electrospinning jet thinner, and reducing the diameter of the produced fibers. In addition, as the Fe 3 O 4 NP content increases, Fe 3 O 4 NPs partially agglomerate in the solution, resulting in varying degrees of bulging on the fibers.

Results and Discussion
Under this condition, the jet is not stable under the action of the electric field, resulting in an uneven fiber surface; this is conducive to microwave energy loss. As shown in Figure 2f, the MXene sheet is continuously and uniformly dispersed on the surface of TPU/PAN/Fe 3 O 4 with a "scale" appearance, demonstrating that the fibers have thickened and filled into a fiber network. At the same time, the nanofiber network structure can be observed. The element mapping images of TPFM displayed in Figure S2 also show that the MXene layer is evenly dispersed on the fiber surface.
no defects (such as spindles or beads). The viscosity of the precursor fluid decreases as the magnetic nanoparticle content increases; this helps the electrostatic force on the surface of the Taylor cone to overcome the viscous resistance, making the electrospinning jet thinner, and reducing the diameter of the produced fibers. In addition, as the Fe3O4NP content increases, Fe3O4NPs partially agglomerate in the solution, resulting in varying degrees of bulging on the fibers. Under this condition, the jet is not stable under the action of the electric field, resulting in an uneven fiber surface; this is conducive to microwave energy loss. As shown in Figure 2f, the MXene sheet is continuously and uniformly dispersed on the surface of TPU/PAN/Fe3O4 with a "scale" appearance, demonstrating that the fibers have thickened and filled into a fiber network. At the same time, the nanofiber network structure can be observed. The element mapping images of TPFM displayed in Figure S2 also show that the MXene layer is evenly dispersed on the fiber surface. The X-ray diffraction (XRD) patterns of the specimens are shown in Figure 3. As shown in Figure 3a, the diffraction peak of MXene obtained after selective etching is at 2θ = 39°, and the most substantial diffraction peak of the MAX phase disappears; this indicates that the etching was successful. The peaks at 2θ = 8.90°, 18.24°, and 27.65° are attributed to the (002), (006), and (008) crystal planes, respectively. In addition, the (002) peak shifts to a lower angle due to the lattice expansion caused by the attachment of groups such as -F or -OH groups to the surface of the MXene. Moreover, this peak is lower in intensity, indicating electrospinning; this indicates the lower re-agglomeration of the Ti3C2 nanosheets. The XRD pattern of the Fe3O4NPs displays characteristic face-centered cubic Fe3O4 peaks at 30.2°, 35.4°, 43.7°, 56.9°, and 63.1°, which are attributed to its (220), (311), (400), (511), and (440) planes, respectively. Because the phase structure of the composite nanofiber textile material does not change, the XRD diffraction patterns of all of the TPFM composite nanofibers with different Fe3O4NP contents show similar XRD diffraction patterns. Due to the encapsulation of TPU/PAN, the relative intensities The X-ray diffraction (XRD) patterns of the specimens are shown in Figure 3. As shown in Figure 3a, the diffraction peak of MXene obtained after selective etching is at 2θ = 39 • , and the most substantial diffraction peak of the MAX phase disappears; this indicates that the etching was successful. The peaks at 2θ = 8.90 • , 18.24 • , and 27.65 • are attributed to the (002), (006), and (008) crystal planes, respectively. In addition, the (002) peak shifts to a lower angle due to the lattice expansion caused by the attachment of groups such as -F or -OH groups to the surface of the MXene. Moreover, this peak is lower in intensity, indicating electrospinning; this indicates the lower re-agglomeration of the Ti 3 C 2 nanosheets. The XRD pattern of the of their diffraction peaks are reduced compared with the diffraction pattern of pure Fe3O4NPs [36]. In addition, no characteristic diffraction peaks of any other impurities are visible. To determine the elemental composition of TPFM and investigate the nature of its surface functional groups, X-ray photoelectron spectroscopy (XPS) was performed, and the results were analyzed with Gaussian-Lorentzian curve peak fitting calculations. The Ti 2p XPS spectrum of TPFM is shown in Figure 3c. The area ratio of the three fixed dual-level peaks (Ti 2p1/2, Ti 2p3/2) at 452.9, 454.3, and 457.1 eV is 2:1; these peaks correspond to Ti-C, Ti-O-Fe, and TiO2, respectively, and they demonstrate the strong interaction between them [37]. The Fe 2p XPS spectrum of TPFM is shown in Figure 3b. The peaks at 718.73 eV and 735.58 eV correspond to Fe 2p3/2 and Fe 2p1/2, respectively [38]. The C1s XPS spectrum shown in Figure 3d proves the presence of C-Ti (280.8 eV), C-C (283.9 eV), -CH2and -CH3 (284.83 eV), CO (285.93 eV), and -COO (288.43 eV) [39].
The FTIR spectra of the samples are shown in Figure 4. The peaks at 3336 cm −1 and 2958 cm −1 in the FTIR spectrum of the original TPU fiber membrane are attributed to N-H stretching vibration and carbon-hydrogen polar covalent bond C-H stretching vibration, respectively [40]. The peak at 1726 cm −1 is the urethane bond. The peaks at 1528 cm −1 and 1171 cm −1 are caused by N-H in-plane bending vibration and C-O stretching vibration, respectively [41]. There is no significant shift in the TPU/MXene spectrum compared to the TPU spectral peak, reflecting that TPU and MXene only weakly interact. An analysis of the sudden disappearance of the PAN/Fe3O4 film signal in the 3431 cm −1 band To determine the elemental composition of TPFM and investigate the nature of its surface functional groups, X-ray photoelectron spectroscopy (XPS) was performed, and the results were analyzed with Gaussian-Lorentzian curve peak fitting calculations. The Ti 2p XPS spectrum of TPFM is shown in Figure 3c. The area ratio of the three fixed dual-level peaks (Ti 2p 1/2 , Ti 2p 3/2 ) at 452.9, 454.3, and 457.1 eV is 2:1; these peaks correspond to Ti-C, Ti-O-Fe, and TiO 2 , respectively, and they demonstrate the strong interaction between them [37]. The Fe 2p XPS spectrum of TPFM is shown in Figure 3b. The peaks at 718.73 eV and 735.58 eV correspond to Fe 2p 3/2 and Fe 2p 1/2 , respectively [38]. The C1s XPS spectrum shown in Figure 3d proves the presence of C-Ti (280.8 eV), C-C (283.9 eV), -CH 2 -and -CH 3 (284.83 eV), CO (285.93 eV), and -COO (288.43 eV) [39].
The FTIR spectra of the samples are shown in Figure 4. The peaks at 3336 cm −1 and 2958 cm −1 in the FTIR spectrum of the original TPU fiber membrane are attributed to N-H stretching vibration and carbon-hydrogen polar covalent bond C-H stretching vibration, respectively [40]. The peak at 1726 cm −1 is the urethane bond. The peaks at 1528 cm −1 and 1171 cm −1 are caused by N-H in-plane bending vibration and C-O stretching vibration, respectively [41]. There is no significant shift in the TPU/MXene spectrum compared to the TPU spectral peak, reflecting that TPU and MXene only weakly interact. An analysis of the sudden disappearance of the PAN/Fe 3 O 4 film signal in the 3431 cm −1 band clearly shows that the nanoparticles strongly interact with nitrogen, hydrogen, carbon, and oxygen atoms in the PAN chain [42]. Moreover, the PAN/Fe 3 O 4 film peak at 2247 cm −1 is attributed to -CN stretching vibration. However, the -CN peak in the PAN/Fe 3 O 4 NPs/MXene film spectrum is shifted to 2209 cm −1 , and also demonstrates a broader peak shape. The -OH stretching vibration peak of MXene shifts to 3335 cm −1 , indicating the formation of a hydrogen bond between -CN and -OH [43].  [44]. Overall, the addition of PAN and nanomagnetic particles improves interfacial interaction and coating dispersion, due to the hydrogen bonding and electrostatic interaction between the MXene and nanofibers.
clearly shows that the nanoparticles strongly interact with nitrogen, hydrogen, carbon, and oxygen atoms in the PAN chain [42]. Moreover, the PAN/Fe3O4 film peak at 2247 cm −1 is attributed to -CN stretching vibration. However, the -CN peak in the PAN/Fe3O4NPs/MXene film spectrum is shifted to 2209 cm −1 , and also demonstrates a broader peak shape. The -OH stretching vibration peak of MXene shifts to 3335 cm −1 , indicating the formation of a hydrogen bond between -CN and -OH [43]. The changes in the adhesive strength of PAN/Fe3O4NPs/MXene, indicated by the peaks at 1457 cm −1 , 1359 cm −1 , 828 cm −1 , and 767 cm −1 , also imply the existence of a force. Some peaks are observed at a low wavenumber of 574 cm −1 , indicating the presence of Fe3O4NPs in the PAN/Fe3O4 and PAN/Fe3O4/MXene films [44]. Overall, the addition of PAN and nanomagnetic particles improves interfacial interaction and coating dispersion, due to the hydrogen bonding and electrostatic interaction between the MXene and nanofibers. The magnetic properties of TPF were analyzed using a vibrating sample magnetometer at room temperature. TPF shows superparamagnetism due to the existence of the nanosized Fe3O4NPs. As shown in Figure 5b, with increasing Fe3O4NP content, the saturation magnetization increases linearly from 2.3 emu/g to 18.9 emu/g, and the coercive field strength increases linearly from 51 Oe to 125 Oe. As displayed in Figure 5a, the strong coercivity of TPF nanofilms at ~300 Oe is attributed to the shape anisotropy of the nanofiber and the ferromagnetic Fe3O4NPs acting as one domain. The magnetocrystalline anisotropy in the nanofiber films can generate more substantial magnetic cleaning and lead to higher coercivity [45]. The magnetic properties of TPF were analyzed using a vibrating sample magnetometer at room temperature. TPF shows superparamagnetism due to the existence of the nanosized Fe 3 O 4 NPs. As shown in Figure 5b, with increasing Fe 3 O 4 NP content, the saturation magnetization increases linearly from 2.3 emu/g to 18.9 emu/g, and the coercive field strength increases linearly from 51 Oe to 125 Oe. As displayed in Figure 5a, the strong coercivity of TPF nanofilms at~300 Oe is attributed to the shape anisotropy of the nanofiber and the ferromagnetic Fe 3 O 4 NPs acting as one domain. The magnetocrystalline anisotropy in the nanofiber films can generate more substantial magnetic cleaning and lead to higher coercivity [45]. Relatively high electrical properties are an important condition for efficient EMI shielding performance. Therefore, electrical conductivity testing of composite nanofibers with different Fe3O4NP loadings was carried out with a 4-point probe device. As shown in Figure 6a, the electrical conductivity reached a maximum of 18.9 S/m. Changes in the electrical conductivity of these nanofibers can be attributed to the different Fe3O4 loadings. With increasing Fe3O4NP contents, the dispersion of MXene on the fiber surface is enhanced, the physical connections between the fibers are improved, and the MXene sheet layer forms a conductive path. Figure 6b shows the tensile properties of the composite fibers produced under different processes. The tensile stress of the pure TPU film is 5.78 MPa. When a quantitative PAN and 20 wt% Fe3O4NPs are added, the tensile stress increases to 8.65 MPa; the tensile curves of the composite films fabricated with different Fe3O4NP loadings are shown in Figure S3. As the magnetic nanoparticle loading increases, the tensile stress of TPF first increases and then decreases. Tensile strains lower than that of the initial TPU film are also obtained, which is due to the dispersion-strengthening effect of the Fe3O4NPs. This strengthening mechanism explains that a large number of Fe3O4NPs are attached to the TPU/PAN fiber membrane, which increases the roughness of the fiber membrane. Consequently, friction is generated between the fibers during stretching, increasing the stretching resistance. The applied load therefore increases the strength through the transfer of magnetic particles. The TPFM stretching curve clearly shows that MXene transmits some of the tensile force on the surface layer during stretching, which increases the tensile breaking strength but slightly reduces the tensile strain compared with the original fiber membrane. The effect of Fe3O4NPs on microwave attenuation has been repeatedly proven [46,47], Figure 6c,d show the EMI of the pure TPU film and the TPFM fiber films with different MXene concentrations and Fe3O4NP contents at 8-12 GHz (the X band). The pure TPU film has no shielding ability against electromagnetic waves, while TPU/PAN/Fe3O4/MXene shows satisfactory EMI shielding ability. With an Fe3O4NP loading of 20 wt%, the EMI SE values of films fabricated with 2 mg/mL and 5 mg/mL MXene increase to 24.9 dB (TPF5-2 mg/mL) and 32.5 dB (TPF5-5 mg/mL), respectively. To more reasonably evaluate the contribution of Fe3O4NPs to electromagnetic wave attenuation, the power coefficients R, A, and T were studied (Figure 6e). With increasing magnetic nanoparticle loading, R increases and A slightly decreases, proving that the addition of magnetic particles has a positive effect on EMI SE. To measure the stability of the material's shielding performance, TPFM was tested for 500 cycles at room temperature. As shown in Figure 6f, the EMI SE of this TPFM was slightly reduced by 1.02% after 500 repeated bending tests; Relatively high electrical properties are an important condition for efficient EMI shielding performance. Therefore, electrical conductivity testing of composite nanofibers with different Fe 3 O 4 NP loadings was carried out with a 4-point probe device. As shown in Figure 6a, the electrical conductivity reached a maximum of 18.9 S/m. Changes in the electrical conductivity of these nanofibers can be attributed to the different Fe 3 O 4 loadings. With increasing Fe 3 O 4 NP contents, the dispersion of MXene on the fiber surface is enhanced, the physical connections between the fibers are improved, and the MXene sheet layer forms a conductive path. Figure 6b shows the tensile properties of the composite fibers produced under different processes. The tensile stress of the pure TPU film is 5.78 MPa. When a quantitative PAN and 20 wt% Fe 3 O 4 NPs are added, the tensile stress increases to 8.65 MPa; the tensile curves of the composite films fabricated with different Fe 3 O 4 NP loadings are shown in Figure S3. As the magnetic nanoparticle loading increases, the tensile stress of TPF first increases and then decreases. Tensile strains lower than that of the initial TPU film are also obtained, which is due to the dispersion-strengthening effect of the Fe 3 O 4 NPs. This strengthening mechanism explains that a large number of Fe 3 O 4 NPs are attached to the TPU/PAN fiber membrane, which increases the roughness of the fiber membrane. Consequently, friction is generated between the fibers during stretching, increasing the stretching resistance. The applied load therefore increases the strength through the transfer of magnetic particles. The TPFM stretching curve clearly shows that MXene transmits some of the tensile force on the surface layer during stretching, which increases the tensile breaking strength but slightly reduces the tensile strain compared with the original fiber membrane. The effect of Fe 3 O 4 NPs on microwave attenuation has been repeatedly proven [46,47], Figure 6c,d show the EMI of the pure TPU film and the TPFM fiber films with different MXene concentrations and Fe 3 O 4 NP contents at 8-12 GHz (the X band). The pure TPU film has no shielding ability against electromagnetic waves, while TPU/PAN/Fe 3 O 4 /MXene shows satisfactory EMI shielding ability. With an Fe 3 O 4 NP loading of 20 wt%, the EMI SE values of films fabricated with 2 mg/mL and 5 mg/mL MXene increase to 24.9 dB (TPF 5 -2 mg/mL) and 32.5 dB (TPF 5 -5 mg/mL), respectively. To more reasonably evaluate the contribution of Fe 3 O 4 NPs to electromagnetic wave attenuation, the power coefficients R, A, and T were studied (Figure 6e). With increasing magnetic nanoparticle loading, R increases and A slightly decreases, proving that the addition of magnetic particles has a positive effect on EMI SE. To measure the stability of the material's shielding performance, TPFM was tested for 500 cycles at room temperature. As shown in Figure 6f, the EMI SE of this TPFM was slightly reduced by 1.02% after 500 repeated bending tests; this was due to a small amount of Fe 3 O 4 NPs detaching from the fibers, as well as the destruction of the MXene conductive network. this was due to a small amount of Fe3O4NPs detaching from the fibers, as well as the destruction of the MXene conductive network.   It can be seen that the value of SE A is much greater than SE R , which is due to the coordination effect of the entire shielding system. The adhesion of the MXene sheet significantly improves the EMI SE of the TPFM film. In Figure 7b, the surface reflection of TPF 20 -5 mg MXene is enhanced. This phenomenon can be attributed to the agglomeration of Fe 3 O 4 NPs due to the large Fe 3 O 4 NPs/MXene content. The internal electron spins of Fe 3 O 4 NPs can be arranged spontaneously in a small range to form a spontaneous magnetization zone. Besides, the double-layer non-magnetic coating on the surface of the Fe 3 O 4 nanoparticles enhances the spin disorder in the TPFM composite material, and affects the transmission of microcurrents. Therefore, spin disturbance will affect the generation of microwave currents and affect the surface reflection. Figure 7c shows the SE/t comparison between the TPFM film and some EMI shielding materials reported in a previous work, where SE/t represents the shielding effect of the conductive material to remove the thickness factor. According to observations, the SE/t of the TPFM film can be as high as 72.5 (dB/mm), which achieves a high shielding effect at a very thin thickness.
Nanomaterials 2022, 12, x FOR PEER REVIEW 10 of 14 Figure 7a,b show the average SEA and SER values of films with Fe3O4NP loadings at different MXene concentrations. It can be seen that the value of SEA is much greater than SER, which is due to the coordination effect of the entire shielding system. The adhesion of the MXene sheet significantly improves the EMI SE of the TPFM film. In Figure 7b, the surface reflection of TPF20-5 mg MXene is enhanced. This phenomenon can be attributed to the agglomeration of Fe3O4NPs due to the large Fe3O4NPs/MXene content. The internal electron spins of Fe3O4NPs can be arranged spontaneously in a small range to form a spontaneous magnetization zone. Besides, the double-layer non-magnetic coating on the surface of the Fe3O4 nanoparticles enhances the spin disorder in the TPFM composite material, and affects the transmission of microcurrents. Therefore, spin disturbance will affect the generation of microwave currents and affect the surface reflection. Figure 7c shows the SE/t comparison between the TPFM film and some EMI shielding materials reported in a previous work, where SE/t represents the shielding effect of the conductive material to remove the thickness factor. According to observations, the SE/t of the TPFM film can be as high as 72.5 (dB/mm), which achieves a high shielding effect at a very thin thickness.    Figure 7d (mechanism diagram) shows the coordination effect of the entire shielding system. In the absence of Fe 3 O 4 NPs, the absorption and attenuation of MXene to electromagnetic waves is relatively weak, because spin disorder plays a vital role in the absorption phenomenon of EMI shielding applications. Similarly, without the MXene interface, it is difficult to transmit the microcurrent generated by Fe 3 O 4 NPs. The surface functional groups and local defects of MXene after etching will produce dipolar polarization, which plays a key role in the dielectric loss [48]. At the same time, MXene's multilayer structure provides a large number of interfaces, which is conducive to interface polarization. Secondly, the bridging effect of TPFM with high conductivity can generate more conductive paths for the movement of charges, which is conducive to the loss of conductivity [48]. When electromagnetic waves fall on the TPFM material, under the influence of an external electric field, the mobile charge carriers are distorted near the multiple interfaces and junctions present in the composite material. These interfaces increase the Debye relaxation process, which also helps to improve the EM absorption characteristics. Increasing the content of Fe 3 O 4 /MXene will increase the electron annulus from Fe +2 and Fe +3 , thereby improving the conductivity of the composite material. The best balance between dielectric constant and permeability can improve the impedance-matching characteristics of the absorber [49]. In addition, the hierarchical structure of TPFM composite materials can cause multiple reflections of electromagnetic waves. The SE A -to-SE R ratio of the 5 mg/mL MXene increased from 1.82 to 5.13 ( Figure S4), indicating that the primary mechanism of the TPFM composites is absorption attenuation.

Conclusions
To prepare EMI shielding materials with ultrathin thickness, high durability, and excellent EMI SE characteristics, this work proposes a combination of improved electrospinning and coating to fabricate a conductive TPFM fiber film with superparamagnetism. The average thickness of the TPFM film is~0.45 mm. The doping of PAN increases the bonding force between the conductive layer and the substrate. Moreover, this film is highly durable, with electromagnetic shielding performance maintained after repeated bending for 500 cycles. The electrospinning method allows a high loading of Fe 3 O 4 NPs to be easily and efficiently dispersed on the fibers, and the synergy of these magnetic nanoparticles with MXene provides an excellent electromagnetic shielding effect of up to 32.5 dB. The lightweight and stable mechanical properties of this film mean that it is suitable for use in flexible wearable devices and wireless human-computer interaction applications.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/nano12010020/s1, Figure S1. (a,b) SEM image of the multi-layer MXene and MXene after exfoliation, (c) The Tyndall effect of few layer MXene solution, Figure S2. (a-c) SEM image and element mapping images of TPFM, respectively, Figure S3. Stress-strain curves of the TPF-5 mg MXene with different Fe3O4NPs content, 0 wt%, 3 wt%, 6 wt% and 12 wt% respectively, Figure S4. The ratio of average SEA and SER of TPFM films as a function of Fe3O4NPs/MXene loading.