Multifunctional MXene/Aramid Nanofiber Composite Films for Efficient Electromagnetic Interference Shielding and Repeatable Early Fire Detection.

Rapid development of highly integrated electronic and telecommunication devices has led to urgent demands for electromagnetic interference (EMI) shielding materials that incorporate flame retardancy, and more desirably the early fire detection ability, due to the potential fire hazards caused by heat propagation and thermal failure of the devices during operation. Here, multifunctional flexible films having the main dual functions of high EMI shielding performance and repeatable fire detection ability are fabricated by vacuum filtration of the mixture of MXene and aramid nanofiber (ANF) suspensions. ANFs serve to reinforce MXene films via the formation of hydrogen bonding between the carbonyl groups of ANFs and the hydroxyl groups of MXene. When the ANF content is 20 wt %, the tensile strength of the film is increased from 24.6 MPa for a pure MXene film to 79.5 MPa, and such a composite film (9 μm thickness) exhibits a high EMI shielding effectiveness (SE) value of ∼40 dB and a specific SE (SSE) value of 4361.1 dB/mm. Upon fire exposure, the composite films can trigger the fire detection system within 10 s owing to the thermoelectric property of MXene. The self-extinguishing feature of ANFs ensures the structural integrity of the films during burning, thus allowing for continuous alarm signals. Moreover, the films also exhibit excellent Joule heating and photothermal conversion performances with rapid response and sufficient heating reliability.

Introduction

With the booming development of electronic technologies, the telecommunication devices tend to be smaller but more powerful. A large number of strong electromagnetic signals generated by these devices are causing electromagnetic interference (EMI) that may result in disturbances to the function of nearby sensitive electronic equipment and even pose threat to human health.[1−3] Especially, the rapid development of fifth-generation (5G) communication technology and wearable electronic devices has raised the problem of electromagnetic pollution to a level never attained before.[4,5] Besides, another problem associated with device miniaturization is the potential fire hazards caused by heat propagation and thermal failure of integrated electronic components during operation, which present a fire risk to both property and life.[6,7] Given these concerns, it is highly desirable to incorporate the flame retardancy, and more desirably the early fire-warning ability, to EMI shielding materials to ensure fire safety.

In recent years, electrically conductive polymer composites have been attracting great attention for EMI shielding due to their advantages of lightweight, flexibility, tunable conductivity, and good processability.[8−19] Highly electroconductive nanofillers used for preparing such polymeric composites include carbon nanotubes (CNTs),[20,21] graphene,[22] and MXene.[23] Among them, MXene is a relatively new two-dimensional (2D) material with the formula MXT, where M is an early transition metal and X is carbon or nitrogen. The 2D layered structure of MXene leads to the formation of multiple reflections of electromagnetic waves inside the composites, and the multiple reflections are absorbed or released in the form of converted thermal energy. Unlike 2D graphene, MXene flakes are hydrophilic and electronegative because of their surface-terminated moieties, including −OH, =O, and −F,[24,25] which makes it easy to disperse MXene flakes in water and common polar solvents. In addition, the intrinsic thermal conductivity of MXene (Ti3C2T) is reported to be around 55 W/mK,[26] which is far below that of carbon nanomaterials such as graphene (5,300 W/mK)[27] and CNTs (6,600 W/mK[28]). These unique features make MXene (Ti3C2T) promising for EMI and fire thermal radiation shielding.

Although flame retardancy has been well imparted to the polymeric EMI shielding composite materials,[29−31] intrinsic fire-warning ability-integrated EMI shielding materials are still rare.[32] Upon exposure to fire, graphene oxide (GO) is thermally reduced, and thus, a sharp decrease in resistance is exhibited. This unique feature of GO has been utilized to fabricate early fire-warning sensors.[33−39] However, the inherent non-conductivity of GO before burning hinders its use as an EMI shielding material. Very recently, highly conductive MXene emerges as a promising fire-warning material owing to its good thermoelectric property, which allows the generation of electrical signals under the action of a temperature gradient without the use of an external power.[32,40−42] However, as far as we know, there is only one work that reports the MXene-enabled dual functions of EMI shielding and early fire warning, which is based on a composite aerogel containing phosphated lignocellulose nanofibrils, gelatin, and MXene.[32] Aerogels are usually rigid and do not have flexibility, which somehow inhibit their applications. Therefore, flexible composite films that possess MXene-enabled dual functions of EMI shielding and early fire warning are desirable.

Free-standing MXene films made by vacuum filtration are known to have unbeatable electromagnetic interference shielding effectiveness (EMI SE) (e.g., >50 dB for a 2.5 μm pure Ti3C2T film), originating from the excellent electrical conductivity of MXene and multiple internal reflections from MXene flakes stacked in films.[23] However, the pure MXene films are brittle, and they are usually reinforced with polymers (e.g., sodium alginate (SA),[43] chitosan,[44,45] and poly(vinyl alcohol) (PVA)[46,47]) or one-dimensional polymer nanofibers (e.g., cellulose nanofibers,[48−50] and aramid nanofibers (ANFs)[51−56]), though the EMI SE is sacrificed to some extent (e.g., 19.7 dB for a 13 μm Ti3C2T/chitosan (50/50) film[44]). Among these reinforcing components, ANFs are advantageous in the exceptional mechanical property owing to their high aspect ratio and strong interactions between the poly(paraphenylene terephthalamide) (PPTA) chains. More importantly, ANFs are non-flammable and self-extinguishing, which is an essential characteristic of materials useful for early fire-warning applications, due to the requirement of keeping the structural integrity during burning so as to continuously give alarm signals. However, previous reports on MXene/ANF composite films[51−56] have not comprehensively revealed the reinforcement role of ANFs, as well as the MXene-enabled dual functions of EMI shielding and early fire warning.

Herein, we choose ANFs to reinforce MXene films and meanwhile to realize the MXene-enabled dual functions of EMI shielding and early fire warning, with the features of good mechanical property, high EMI shielding performance, and repeatable fire-warning capability. The MXene/ANF films are prepared by vacuum filtration of the mixture of MXene and ANF suspensions. The microstructure, mechanical strength, electrical conductivity, and EMI shielding effectiveness as a function of the ration between MXene and ANFs are investigated in detail. The as-obtained composite films show outstanding mechanical properties owing to the hydrogen bonding between carbonyl groups of ANFs and hydroxyl groups of MXene, superior EMI shielding performances, as well as reliable and repeatable fire detection ability under the assistance of self-extinguishing ANFs that ensures the structural integrity during burning. Moreover, the films also exhibit excellent Joule heating and photothermal conversion performances with rapid response and sufficient heating reliability. These flexible multifunctional films have promising potentials for emerging electronic devices and wearable applications.

Experimental Section

Materials

Kevlar 29 fabrics were purchased from DuPont, USA. Dimethyl sulfoxide (DMSO, 99.95%) and potassium hydroxide (KOH, 95%) were obtained from Shanghai Macklin Biochemical Co., Ltd. Hydrochloric acid (HCl, 36%) was purchased from Chinasun Specialty Products Co., Ltd. Lithium fluoride (LiF, 99.99%) was purchased from Aladdin Inc., China. Ti3AlC2 powder (200 mesh, >98%) was purchased from Forsman Scientific (Beijing) Co., Ltd. Mixed cellulose membrane (0.22 μm pore size) and PTFE membrane (0.22 μm pore size) were purchased from Haining Chuangwei Filter Material Co., Ltd.

Preparation of Ti3C2T MXene

Delaminated Ti3C2T was prepared via a moderate etching method using LiF/HCl solutions based on a previous report.[57] First, 2 g of LiF was dissolved in 40 mL of 9 M HCl in a Teflon beaker and stirred for 30 min. Then, 2 g of Ti3AlC2 powder was slowly added into the LiF/HCl acid solution and stirred at 35 °C for 24 h. The resultant mixture was washed with deionized water continuously through centrifuging at 3500 rpm until the pH became ∼6. The slurry was then dispersed in deionized water, followed by sonication in an ice bath for 60 min. The obtained dispersion was centrifuged at 3500 rpm again for 1 h, and then by adding the required amount of water, a colloidal suspension of MXene (∼0.5 mg/mL) was prepared.

Preparation of ANFs

The ANF solution was obtained by dissolving 1 g of Kevlar fabric in 500 mL of DMSO containing 1.5 g of KOH and magnetically stirring at room temperature for 1 week.[58] Deionized water was then added into the obtained solution to form a colloidal suspension of ANFs (∼0.5 mg/mL).

Preparation of MXene, ANF, and MXene/ANF Films

A MXene film was prepared by vacuum-assisted filtration of the obtained MXene using a mixed cellulose membrane, followed by vacuum-drying. ANF and MXene/ANF films were prepared in the same way using PTFE membranes. Mixtures of ANF suspension and MXene suspension were used for filtration to obtain composite films with the weight ratio of MXene to ANF being 20/80, 40/60, 50/50, 60/40, and 80/20, respectively. The composite films are named MA-X, with X representing the weight percentage of MXene.

Characterization

A Hitachi S8100 scanning electron microscope and an FEI Tecnai G-20 transmission electron microscope were used for observing the morphology of the samples. The X-ray diffraction (XRD) patterns of MXene, ANF, and MA-50 films were measured with a Bruker D8 Advance X-ray diffractometer with a Cu Kα radiation of 1.54 Å at a generator voltage of 40 kV. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet 5700 infrared spectroscope. The surface chemistry of the samples was analyzed with an Axis Ultra HAS high-resolution X-ray photoelectron spectrometer. The tensile test was carried out on an Instron 5967 universal tensile testing machine equipped with a 30 kN load cell at a crosshead speed of 1 mm/min(25 °C). The cyclic loading–unloading test was performed for up to 10,000 cycles at a constant deformation rate of 10 mm/s. An ST-2258C multifunctional digital four-probe tester was used to measure the square resistance of MA-X films.

The EMI SE of the films was measured with a vector network analyzer (Agilent N5247A). Scattering parameters (S11 and S22) were measured to calculate the total EMI SE (SE), shielding effectiveness of absorption (SE) and reflection (SE), and the power coefficients of absorptivity (A), reflectivity (R), and transmissivity (T) in the X band (8.2–12.4 GHz) using eqs –6.where SE is the microwave multiple internal reflection effectiveness, which is negligible when SE ≥ 10 dB.[59] The shielding efficiency (%) can be calculated as

The electrothermal and photothermal conversion performance was evaluated by monitoring the temperature change of film surface under a voltage supplied by a DC power supplier (A-BF HXF300) and the irradiation of a xenon lamp (CEAULIGHT HXF300), respectively. The temperature of the film surface was recorded with a multimeter (FLUKE F289FVF). Infrared thermography (FLIR T620) was used to study the degree of uniformity of heat distribution on the film surface. Fire-warning test was conducted by connecting the film and an alarm device (HB414, Beijing Huibang Technology Co., Ltd) in series with wire.

Results and Discussion

Figure illustrates the fabrication process of the multifunctional MXene/ANF composite films. First, Ti3C2T MXene flakes were prepared by selectively etching off the Al component from Ti3AlC2 using a LiF/HCl solution, followed by ultrasonic treatment to get delaminated flakes (Figure S1). An ANF suspension was prepared by splitting pristine aramid microfibers with a DMSO solution of KOH through reducing the hydrogen bond interaction between polymer chains (Figure S2).[58] Then, free-standing MXene/ANF composite films were simply fabricated via vacuum-assisted filtration using the mixed suspensions of MXene and ANF.

Figure 1

Schematic illustration of the preparation process of MXene/ANF composite films.

Microstructure and Chemical Compositions

Figure shows the photo and scanning electron microscopy (SEM) images of the as-obtained MXene/ANF film (MA-50), as well as that of pure MXene and ANF films for comparison. The pure ANF film is highly transparent due to the smaller diameter of ANFs than the wavelength of visible light (Figure a). The surface of a pure MXene film exhibits a metallic sheen, while the metallic sheen of an MA-50 film becomes slightly weak due to the existence of ANFs. From the surface SEM images (Figure b), it can be seen that the ANF film has a relatively smooth surface, while obvious MXene flakes can be seen on the surface of the MXene film. The surface of an MA-50 film shows the existence of ANFs, but the MXene flakes are not obvious anymore. The cross-sectional SEM images show that the free-standing ANF, MXene, and MA-50 films all have a laminar structure (Figure c).

Figure 2

(a) Photos, (b) surface, and (c) cross-sectional SEM images of free-standing ANF, MXene, and MA-50 films.

The XRD patterns shown in Figure a indicate that the characteristic (002) peak of the MXene film shifts from 2θ ∼6.06 to ∼5.32° for the MA-50 film, implying that the d-spacing of MXene layers is increased due to the intercalation of ANFs. The FT-IR spectra of ANF, MXene, and MA-50 films are shown in Figure b. In the spectrum of the MXene film, the peaks at 3440, 1644, and 1029 cm–1 can be attributed to the absorption of hydroxyl groups, C=O bonding, and C–F vibrations, respectively.[60] The peaks in the spectrum of the ANF film at 3322, 1648, 1542, 1312, and 826 cm–1 correspond to the N–H stretching, C=O bonding, C–N/N–H stretching couple modes, Ph-N vibrations, and C–H (out of plane) vibrations, respectively.[51] Compared with the spectrum of the ANF film, in the spectrum of the MA-50 film, a slight shift toward the lower wavenumber side is observed in the position of C=O peak, which can be ascribed to the hydrogen bonding between the C=O bonds of ANFs and the hydroxyl groups of MXene.[51,61] Furthermore, the X-ray photoelectron spectrometry (XPS) survey spectra shown in Figure d indicate the existence of F and N elements in MXene and ANF films, respectively, and the existence of both elements in the MA-50 film. Comparing the high-resolution C 1s spectrum of the ANF film (Figure e) with that of the MA-50 film (Figure f), it can be found that the characteristic C=O peak at 288.1 eV for the ANF film shifts to a higher binding energy of 288.4 eV for the MA-50 film, further confirming the existence of hydrogen bonding between ANFs and MXene.

Figure 3

(a) XRD patterns, (b,c) FT-IR spectra, and (d) XPS survey spectra of free-standing ANF, MXene, and MA-50 films. High-resolution C 1s XPS spectra of (e) ANF film and (f) MA-50 film, respectively.

Mechanical Property

Figure a shows the tensile stress–strain curves of the free-standing ANF, MXene, and MA-X films. The pure MXene film exhibits a relatively low tensile strength of 24.6 MPa and a fracture strain of 1.33%, while the pure ANF film shows a high tensile strength of 202.3 MPa and a fracture strain of 7.5% (Figure b). For the MA-X films, the tensile strength and toughness increase by increasing the content of ANFs. When the ANF content is 20% (MA-80), the tensile strength of the film is 79.5 MPa, which is more than 3 times that of a pure MXene film. For the MA-20 film that has an ANF content of 80%, the tensile strength reaches nearly 200 MPa and the elongation at break exceeds that of a pure ANF film, which can be explained by the formation of hydrogen bonding between the C=O bonds of ANFs and the hydroxyl groups of MXene, as evidenced by the above FT-IR and XPS characterizations. The excellent mechanical properties of MA-X films enable them to be able to lift a very heavy load as compared to their own weight. For instance, a piece of the MA-80 film with a weight of only 6.3 mg is able to lift a load of 200 g (Figure c), which is more than 30,000 times its own weight.

Figure 4

(a) Typical stress–strain curves and (b) values of tensile strength and fracture strain of the free-standing ANF, MXene, and MA-X films. (c) Photo showing a piece of the MA-80 film which is able to withstand a load of 200 g.

Electrical Conductivity and EMI Shielding Performance

Electrical conductivity and film thickness are two important factors affecting the EMI shielding performance. As shown in Figure a, the pure MXene film shows a high conductivity of 2752.5 S/cm. When the MXene film is blended with ANFs, the conductivity of the film decreases significantly. When the MXene content is 50% (MA-50), the conductivity is about 75.7 S/cm, which still provides the basis for an excellent electromagnetic shielding performance as discussed below. Here, it is noted that the flexibility of the MA-50 film allows the resistance of conductivity against bending. As shown in Figure b, the conductivity of the MA-50 film just decreases by less than 10% after 10,000 bending-release cycles at a constant bending strain of 100%. Such good flexibility and reliability are critical for the practical use of the films in flexible electronic devices.

Figure 5

(a) Electrical conductivities of the MA-X films. (b) Change in the electrical conductivity of the MA-50 film with the number of bending-release cycles. (c) EMI SE of MA-X films at a thickness of 8–10 μm. (d) Total EMI shielding effectiveness (SE), microwave absorption (SE), and microwave reflection (SE) of MA-X films. (e) Shielding efficiencies of the MA-X films. (f) Comparison of SSE and stress values of MA-X films with that of some previously reported vacuum filtration films (see Table S1 in the Supporting Information for details).

The EMI shielding performance of the MA-X films was tested at frequencies ranging from 8.2 to 12.4 GHz (X-band). The results are given in Figure c. The MA-X films with the MXene content ≥40% exhibit EMI SE higher than 20 dB, which meets the demands of commercial EMI shielding applications. Such an EMI shielding performance is also relatively stable against repeated bending, with a decrease of <10% after 10,000 bending-release cycles at a constant bending strain of 100% (Figure S3). Figure d shows the average values of total EMI shielding effectiveness (SE), microwave absorption (SE), and microwave reflection (SE) in the range of 8.2–12.4 GHz. Although SE is always greater than SE, this does not mean that films are absorption-dominated EMI shielding materials.[62] Usually, the maximum value of SE can reach 20 dB for highly conductive materials, whereas for absorption-dominated EMI shielding materials, their SE should be less than 3 dB.[62,63] Herein, the SE of MA-X films ranges from 5 dB (MA-20) to 15 dB (MA-80). The R values of the MA-X films are calculated to range from 0.690 to 0.973 (Figure S4), indicating that the MA-X films attenuate microwaves mainly through reflection. The values of the shielding efficiency of MA-X films are given in Figure e. The MA-20, MA-50, and MA-80 films can block 96.12, 99.89, and 99.99% of the incident radiation, respectively.

Specific SE (SSE), the EMI SE divided by film thickness, is used to evaluate the comprehensive EMI shielding performance of MA-X films. As shown in Figure f, the MA-20, MA-50, and MA-80 films present high SSE values of 1763.8, 3280.0, and 4361.1 dB/mm, respectively. To highlight the superior EMI shielding performance and mechanical property of the as-obtained MA-X films, their SSE value and tensile strength are compared with that of other previously reported free-standing composite films prepared via vacuum-assisted filtration (Figure f and Table S1 in the Supporting Information). It is found that the simultaneous achievement of high-efficiency EMI shielding performance and good mechanical property is still challenging. The superior EMI shielding performance and mechanical property to previous counterparts make the MA-X films more suitable for various real applications.

Joule Heating Performance

The flexibility and high conductivity of MA-X films also make them suitable for use as a Joule heater for emerging wearable applications. Figure a shows the change in the surface temperature of MA-X films with a supplied voltage of 3 V. The maximum values of the surface temperatures of the MA-20 film, MA-50 film, and MA-80 film reached 32.6, 46.5, and 128 °C, respectively, within tens of seconds. Figure b shows the surface temperature change of the MA-80 film with different supplied voltages. The steady-state temperature of the MA-80 film is 32.3, 40.0, 56.5, 82.5, and 127.0 °C for 1, 1.5, 2, 2.5, and 3 V, respectively. As shown in Figure c, the steady-state temperature of the MA-80 film is approximately proportional to the square of the supplied voltage (U2), which is consistent with the theoretical prediction for electrical heaters reported previously.[64,65]Figure d shows that the steady-state temperature of the MA-80 film is higher than that of some previously reported vacuum filtration films (Figure d and Table S1 in the Supporting Information), indicating its superior electrothermal property.

Figure 6

(a) Temperature changes of MA-X films with a supplied voltage of 3 V. (b) Temperature changes of the MA-80 film with different supplied voltages. (c) Steady-state temperature of the MA-80 film as a function of U2. (d) Comparison of electrothermal performance of the MA-80 film with that of some previously reported vacuum filtration films (see Table S1 in the Supporting Information for details). Temperature changes of the MA-80 film: (e) 3 V voltage is repeatedly supplied and removed, (f) 2 and 3 V voltages are alternately supplied and removed, (g) voltage is increased in equal increment (0.5 V) and then decreased in equal decrement (−0.5 V), and (h) 3 V voltage is continuously supplied for 1 h. (i) Digital and infrared images of the de-icing process using the MA-80 film at 3 V.

In addition, the electrothermal property of the MA-X films is stable and reliable, as indicated by the almost identical temperature change curves obtained when a voltage of 3 V is repeatedly supplied and removed (Figure e), or when voltages of 2 and 3 V are alternately supplied and removed (Figure f). Figure g shows the temperature change curve obtained when the supplied voltage is increased in equal increment (0.5 V) from 0 to 3 V and then decreased in equal decrement (−0.5 V) from 3 to 0 V. The results indicate the rapid response of surface temperature to the supplied voltage, implying that the surface temperature can be easily adjusted by changing the voltage during real use. The electrothermal conversion stability was studied by continuously supplying a voltage of 3 V for 1 h. As shown in Figure h, it can be seen that the MA-80 film exhibits a very stable surface temperature of ∼128 °C over the test period of 1 h, confirming the outstanding stability and reliability of the film for Joule heating. Furthermore, the electrical heating performance was evaluated by heating a vial containing ice water using the MA-80 film with a supplied voltage of 3 V (Figure i). After 300 s, the temperature of the ice water increases from 3.8 to 10.3 °C. The ice melts completely after 600 s, and the temperature reaches 26.4 °C after 1200 s. These results indicate that the MA-X films have good electrothermal conversion ability, which makes them applicable for wearable thermal management and a variety of situations in both our daily life and industrial applications. To investigate the effect of Joule heating on the EMI SE performance, the change in EMI SE of the MA-80 film with the number of Joule heating cycles under 3 V (20 min for each cycle) was tested (Figure S5). The results indicate that after 100 cycles, the decrease in EMI SE is about 10%, which is possibly caused by the oxidation of MXene.

Photothermal Conversion Performance

The photothermal conversion performance of MA-X films was studied with a simulated solar xenon lamp light source. As shown in Figure a, when the light intensity is 200 mW cm–2, the steady-state temperature of MA-20, MA-50, and MA-80 films is 75.2, 87.3, and 97 °C, respectively, while the ANF film only shows a surface temperature of 38.2 °C, indicating a good photothermal performance of MXene. Figure b shows that the steady-state temperature increases by increasing the light intensity, and a proportional linear relationship between them can be seen in Figure c, suggesting that the surface temperature of MA-X films is controllable via adjusting the light intensity. Similarly, the reliability of the photothermal conversion was tested by repeatedly switching on and off the light. The almost identical temperature change curve in each cycle reveals the good reliability of the light-to-heat process (Figure d). The stable surface temperature during the test of 1 h irradiation (Figure e) also indicates the good photothermal conversion stability and reliability. As shown in Figure f, the MA-80 film exhibits higher photothermal conversion performance than some previously reported vacuum filtration films (see also Table S1 in the Supporting Information).

Figure 7

(a) Temperature changes of MA-X films at 200 mW cm–2. (b) Temperature changes of the MA-80 film at different light intensities. (c) Steady-state temperature of the MA-80 film as a function of light intensity. Temperature changes of the MA-80 film at 200 mW cm–2: (d) light is repeatedly switched on and off and (e) continuously irradiated for 1 h. (f) Comparison of the photothermal performance of the MA-80 film with that of some previously reported vacuum filtration films (see Table S1 in the Supporting Information for details).

Early Fire-Warning Ability

A piece of the MA-50 film (1.5 cm × 2 cm) was connected with an alarm device to construct an early fire warning system without using an external power source. Upon exposure to the flame of an alcohol lamp, the MA-50 film can generate a voltage signal under the action of the temperature difference between the flame exposure area and the end of the film owing to the thermoelectric property of MXene,[40,66] and as a result, the alarm was triggered within 10 s (Figure a; see also Video S1, Supporting Information). Since both ANF and MXene films are flame retardant (Figure S6), the MA-50 film kept its structural integrity even after 60 s burning (Figure b; see also Video S2, Supporting Information), which enables the fire-warning system to continuously alarm until the flame was removed at 30 s, and as a result, the alarm went off at 43 s (Figure a). In addition, the alarm can be triggered repeatedly for up to 10 cycles of alternate flame exposure and flame removal. As shown in Figure c, a number of particles appeared on the surface of the MA-50 film after burning, which can be attributed to the formation of TiO2 caused by the oxidation of MXene.[67] The XRD pattern of char residue from the MA-50 film shows the presence of typical diffraction peaks at 25.1 and 27.8° coming from anatase and rutile TiO2, respectively (Figure S7), and the formation of TiO2 is further proved by the significant increase in the Ti–O peak intensity in the high-resolution Ti 2p XPS spectrum of the MA-50 film after burning (Figure d).

Figure 8

(a) Snapshots showing the repeated fire detection using a piece of MA-50 film (1.5 cm × 2 cm). (b) Burning process of the MA-50 film. (c) SEM images of the MA-50 film after burning. (d) High-resolution Ti 2p XPS spectra of the MA-50 film before and after burning.

Conclusions

In summary, flexible and mechanically strong films having the main dual functions of EMI shielding and early fire warning have been prepared by utilizing not only the high electrical conductivity and thermoelectric property of MXene but also the reinforcing role and the self-extinguishing property of ANFs. Benefiting from the hydrogen bonding between the carbonyl groups of ANFs and the hydroxyl groups of MXene, the tensile strength of the film (MA-80) is increased by more than 3 times compared to that of a pure MXene film. Besides the superior EMI shielding performance, the composite films are fireproof and can maintain a good structural integrity in the fire, thus allowing the films to continuously provide alarm signals. In addition, the composite films also have excellent Joule heating and photothermal conversion performances with a rapid response and sufficient heating reliability. To realize the practical applications of such multifunctional films in various fields, more study on the development of efficient methods of protecting MXene from oxidation is deserved.