4.1 Fabrication and Structural Characterization of Large-Area MXene/TPU Films
Multilayer-stacked MXene nanosheets were successfully synthesized via in situ etching. EDS mapping revealed a uniform distribution of Ti, C, O, and F elements on the nanosheet surfaces, confirming the characteristic surface composition of MXene (Fig. S1a, S1b). XRD patterns further verified the transformation of the Ti₃AlC₂ precursor into Ti₃C₂Tₓ (Fig. 3a), as evidenced by the distinct shift of the (002) diffraction peak from 9.68° to 6.44°, indicating enlarged interlayer spacing and effective removal of Al atoms [40]. The obtained MXene nanosheets were subsequently dispersed in DI water, and after ultrasonication and cell disruption, a stable colloidal suspension with a pronounced Tyndall effect was formed (Fig. S1c). TEM images revealed single and few-layer nanosheets with sharp edges and smooth surfaces (Fig. S2), corroborating their efficient exfoliation and excellent dispersibility, thereby providing a solid foundation for the subsequent fabrication of uniform and compact composite films.
Fig.2 a Schematic illustration of the heterogeneous crosslinking-induced blade-coating process of MT films. Inset: MT ink and a digital photograph of a large-area MT film (35 cm × 20 cm). b Cross-sectional and surface SEM images of pure MXene and MT films. c Thickness and surface morphology of MT films with different MXene/TPU mass ratios. d Electrical conductivity and thickness of MT films prepared with different MXene solution concentrations. e Droplet contraction and local cracking phenomena in MT-10 films. f AFM image of the surface of pure MXene film. g AFM images of both surfaces of MT film
MT ink was prepared by mixing MXene with a Waterborne TPU solution via magnetic stirring and vacuum degassing (Fig. 2a). Online Resource 1 also demonstrates the process of preparing films using the MT ink. To obtain MXene conductive films with outstanding performance, the mass ratio of MXene to TPU was systematically optimized. The results indicate that when the MXene dispersion concentration was 40 mg/mL or 50 mg/mL, only the MT films prepared at 90 wt.% exhibited both good electrical conductivity (Fig. S3) and dense, smooth surface morphology (Fig. 2c). This is because TPU is an insulating polymer, and excessive content weakens the conductive network among MXene nanosheets, leading to reduced conductivity. In contrast, an appropriate TPU fraction does not significantly disrupt the conductive pathways, while improving film flexibility and formability, thereby enabling synergistic optimization of conductivity and surface quality. Further tuning of the MXene concentration in the inks revealed that increasing nanosheet content enhanced electrical conductivity by establishing continuous conductive networks, while uniform film thickness was obtained only at moderate concentrations (Fig. 2d). At low MXene concentrations (10 mg/mL), excess water molecules increase the surface tension of the ink above the substrate surface energy, resulting in poor wettability, droplet shrinkage, and local film cracking (Fig. 2e). At high concentrations (70 mg/mL), the ink flow and spreading were restricted, preventing spontaneous thickness leveling and leading to local accumulation or depression of the film.
As shown in Fig. 2b, the cross-sectional SEM image of the MT film reveals a compact layered structure formed through multicomponent crosslinking-induced assembly. In contrast, the pure MXene film prepared by vacuum filtration exhibits a loosely stacked multilayer configuration, where the nanosheets are primarily connected by weak van der Waals forces. The absence of external structural constraints results in a fragile architecture, thereby limiting its feasibility for practical applications. In the MT films, the incorporation of waterborne TPU facilitates the formation of a hydrogen-bonded network, in which the hydrophilic groups of TPU interact with the abundant surface terminations of MXene (–OH, –COOH, –F), giving rise to a stable 3D interpenetrating structure (Fig. 2a). AFM scanning of a 9 μm × 9 μm surface region further confirmed the superior surface quality of MT films. Guided by the shear force during blade coating, one surface exhibited a markedly smoother morphology with a root-mean-square roughness (Sq) of only 34 nm (Fig. 2g), while the opposite side showed slightly higher roughness but still performed significantly better than pure MXene films (Sq = 119 nm). Statistical analysis of long-range height fluctuations along random surface regions indicated that the amplitude variations of the two MT film surfaces were only 0.16 and 0.37 times those of pure MXene films (Fig. S4). These results highlight the remarkable advantages of MT films over conventional MXene films in terms of structural uniformity and interfacial quality.
4.2 Heterogeneous Crosslinking and Hydrogen Bonding of MXene/TPU Films
Notably, the abundant chemical bonds in aqueous TPU can form compounds or hydrogen bonds with the surface terminations of MXene through heterogeneous crosslinking. Such interactions are the key factor facilitating the construction of a 3D IPN between MXene and TPU. To gain deeper insights into the interfacial mechanism, FTIR, Raman spectroscopy, XRD, and XPS analyses were performed. As shown in Fig. 3b, the FTIR spectrum of the MT-40 film exhibits a broadened, intensified, and red-shifted absorption peak at 3438 cm⁻¹ compared with pure MXene. This result indicates strong hydrogen bonding between the abundant surface functional groups of MXene (–OH, –COOH, –F) and the –NH and C=O groups in the urethane chains of TPU, leading to a stable hydrogen-bonded network. In addition, the C=O stretching vibration peak shifts from 1727 cm⁻¹ to 1599 cm⁻¹ with noticeable broadening, suggesting the possible presence of synergistic covalent interactions [41].
Fig. 3 a XRD patterns of Ti₃AlC₂ and Ti₃C₂Tx. b FTIR spectra of pure MXene films, TPU films, and MT-40 films. c XRD patterns of TPU and MT films at different MXene concentrations. d XPS spectra of pure MXene and MT-40 films. e Ti 2p spectra and f O 1s spectra of pure MXene and MT-40 films
The interfacial interactions were further verified by Raman spectroscopy (Fig. S5). The characteristic peaks of MXene appeared at 122 cm⁻¹ (ω₁), 201 cm⁻¹ (ω₂), 283/375 cm⁻¹ (Ti₃C₂(OH)₂, ω₅), and 594/725 cm⁻¹ (Ti₃C₂O₂, ω₄/ω₃). Notably, the peaks at 375 cm⁻¹ and 725 cm⁻¹ exhibited red shifts, indicating that the =O functional groups on the MXene surface reacted with the functional groups in TPU. In addition, the emergence of a new Raman-active peak at ~156 cm⁻¹ suggested the formation of new interfacial structures or interactions during the composite process [42, 43]. XRD analysis further corroborated these structural changes (Fig. 3c). The characteristic (002) diffraction peak of MXene gradually shifted from 6.44° to 4.78° with varying MXene concentrations, demonstrating that the introduction of TPU effectively regulated the interlayer spacing of MXene nanosheets. Such interlayer spacing variation can be attributed to intercalation of TPU molecules or interfacial interactions that induced structural rearrangements, thereby further validating the proposed hydrogen-bonding/covalent crosslinking mechanism.
XPS analysis revealed trace signals of Li 1s and F 1s at 58.67 and 684.86 eV, respectively (Fig. 3d), confirming the selective etching of Al from Ti₃AlC₂ to produce Ti₃C₂Tₓ. In the high-resolution Ti 2p spectrum of MXene (Fig. 3e), spin–orbit coupling splits the Ti 2p levels into Ti 2p₁/₂ and Ti 2p₃/₂ doublets, with an energy separation of ~5.7 eV [44]. The peaks located at 454.45, 455.38, 456.44, and 458.01 eV correspond to Ti–C (2p₃/₂), Ti(II) (2p₃/₂), Ti–O (2p₃/₂), and TiO₂ (2p₃/₂) bonds, respectively. Upon the incorporation of TPU, the main Ti 2p peaks exhibited a positive shift of ~0.1–0.5 eV with significantly increased intensity, indicating changes in the electronic environment around Ti atoms. This shift may originate from hydrogen bonding or coordination interactions between polar groups in TPU (e.g., –C=O, –NH) and MXene [45, 46]. In the O 1s spectrum (Fig. 3f), the peaks at ~529, 530.39, 532.09, and 532.89 eV were assigned to O–Ti (TiO₂), C–Ti–Oₓ (I), C–Ti–(OH)ₓ (II), and H₂O, respectively [47]. After the introduction of TPU, these O 1s peaks also exhibited pronounced shifts and intensity changes, further confirming the presence of interfacial interactions. Collectively, these XPS results demonstrate that the interface of MXene and TPU is not limited to physical blending but also involves stable interfacial chemical crosslinking through hydrogen bonding and possible covalent interactions, which contribute to enhanced structural stability and overall performance of the composite films.
4.3 Mechanical Flexibility and Environmental Adaptability of MXene/TPU Films
To assess the application potential and scalability of MT films in diverse scenarios, systematic evaluations of their mechanical properties, wettability, and thermal stability were performed. The incorporation of waterborne TPU markedly enhanced the structural integrity of the films while simultaneously improving their flexibility and environmental adaptability.
As shown in Figs 4a and 4b, the stress–strain curves reveal a remarkable enhancement in the mechanical properties of MT films. In contrast, pure MXene films exhibit weak interlayer interactions and poor structural compactness, leading to limited tensile strength and fracture toughness. Brittle fracture behavior is typically observed in pure MXene films, which severely restricts their mechanical adaptability in practical applications. With the incorporation of waterborne TPU, however, the tensile stress of MT-20 films was increased by nearly 5-fold and the strain increased by 40-fold, resulting in an ultimate elongation of 60%. Such improvements enable effective resistance to complex tensile stresses and external impacts, thereby demonstrating outstanding flexibility. This superior mechanical performance is primarily attributed to the formation of a 3D IPN, constructed through the synergistic interaction of MXene and TPU (Fig. 4c). Within this structure, the flexible soft segments of TPU penetrate into the interlayers and lamellar gaps of MXene, acting as flexible bridges to reinforce interlayer bonding. Consequently, stress transfer efficiency and strain dissipation are enhanced, effectively suppressing failure modes such as interfacial debonding and structural collapse. The MT films exhibited excellent mechanical compliance, allowing repeated twisting without structural damage and achieving tight conformability to irregular surfaces (Online Resource 2). Notably, MT-20 films maintained robust mechanical performance while simultaneously offering superior IR stealth properties. At a relatively low MXene concentration (20 mg/mL), the emissivity values in the mid- and far-IR ranges were reduced to 0.38 (3–5 μm) and 0.28 (8–14 μm), respectively (Fig. S6). When the MXene content was increased to 40 mg/mL (MT-40), the emissivity further decreased to 0.26 (3–5 μm) and 0.19 (8–14 μm), thereby significantly enhancing the IR camouflage capability. Furthermore, the film thickness can be precisely controlled via a spin-coating process, achieving a minimum thickness of 0.034 μm (Fig. S7), combining lightweight, flexible, and multifunctional integration advantages.
Surface wettability is a key parameter that determines the environmental adaptability and practical usability of materials, and it becomes particularly important for IR stealth materials that are required to withstand complex environments. In MT films, the introduction of waterborne TPU with relatively low surface energy, combined with the shear-induced alignment of MXene nanosheets during the blade-coating process, facilitates the formation of a dense and uniform surface structure. Consequently, the MT films exhibit weakly hydrophobic yet low-adhesive wetting behavior, together with a certain degree of self-cleaning capability (Fig. 4d and Online Resource 3). Moreover, when pure MXene films and MT-40 films were subjected to 5 h of ultrasonic treatment followed by 24 h storage under ambient conditions, the MT-40 films maintained good structural integrity (Fig. 4d), whereas the pure MXene films were completely destroyed. This result demonstrates the strong interfacial interactions between TPU and MXene, which markedly improve the structural stability and environmental durability of the composite films in humid environments (Fig. 4f).
Fig. 4 a Stress-strain curve of MT-20 film. b Stress-strain curves of pure MXene film and MT films at different MXene concentrations. c Mechanical stretching mechanism of MT films. d Wetting characteristics of pure MXene films and MT-40 films. e TGA and DTG curves of MT-40 composite films. f Environmental adaptation properties of MT films
The thermal stability of pure MXenes and MT-40 films was evaluated by TGA. The pure MXene film exhibited no significant mass loss when heated from room temperature to 800 °C, retaining 90% of its residual mass at the end. The observed weight loss of MXene can be attributed to the release of adsorbed water, the desorption or decomposition of surface functional groups (e.g., -OH and -F), as well as the oxidation or structural degradation of MXene itself [48] (Fig. S8a). In comparison, the thermal stability of MT films decreased as the MXene content was reduced (Fig. S8b). Nevertheless, even at relatively low MXene loading, the MT-40 film maintained 35.6% residual mass at 800 °C. The residue primarily consisted of MXene decomposition products, including TiO₂ and titanium carbide phases [49, 50], since both the soft and hard segments of waterborne TPU were completely degraded at 442 °C (Fig. S8c). Notably, the major weight loss of the MT-40 film occurred between 363–420 °C, whereas the film preserved good structural stability and resistance to thermal decomposition below 360 °C (Fig. 4e). This finding highlights the application potential of MT-40 films in medium- to high-temperature environments (Fig. 4f).
4.4 MXene Nanosheet-Driven Janus Fabrics for Combined Infrared Stealth and Electromagnetic Shielding
4.4.1 Infrared Stealth Performance
Given the excellent mechanical properties and environmental adaptability of MT films, we further expand their application scenarios by integrating them efficiently with fabrics, which are inherently soft, lightweight, and structurally supportive. This integration leads to the construction of Janus structures with distinctly different IR responses on the two sides. Such spatially differentiated functionality enables the fabrics to flexibly adapt to diverse environmental backgrounds simply by flipping, thereby achieving adaptive IR stealth responses (Fig. 5a). The resulting samples exhibit an ultrathin thickness of only 0.4 mm and an areal density of 0.03 g/cm² (Fig. 5b), highlighting their ultralight and flexible characteristics. These features lay a solid foundation for potential applications in camouflage tents, tarpaulins, equipment skins, and intelligent wearable devices.
According to Planck's law, any object with a temperature above 0 K emits thermal radiation [30]. The Stefan–Boltzmann law further states that the radiation intensity of an object is proportional to the fourth power of its surface temperature and to the IR emissivity of the material [9]. Based on these principles, the IR camouflage performance of Janus fabrics was evaluated by measuring the IR emissivity of MT-20@Fabric (MT20F), MT-30@Fabric (MT30F), MT-40@Fabric (MT40F), and their flipped surfaces (FMT) in the 3–14 μm wavelength range (Fig. 5c). The results show that MT40F exhibits an emissivity as low as 0.125, with an average value of only 0.185, surpassing most reported MXene-based IR stealth materials (Fig. 5i and Table S1). The low emissivity of MT40F is mainly attributed to the metallic conductivity and high free carrier density of MXene, which effectively reflects thermal radiation in the mid- and long-wave IR regions. In addition, its two-dimensional layered structure and smooth surface further reduce IR penetration and scattering, thereby enhancing the shielding effect. As the MXene content in the MT films decreases, the emissivity gradually increases, confirming the dominant role of MXene in achieving low-emissivity performance. Notably, the opposite surface of MT40F (FMT) exhibits a high emissivity of 0.838, and its strong IR radiation capability is more favorable for thermal dissipation and camouflage regulation in high-temperature environments.
Fig. 5 a Janus fabric integrating MT film and fabric is suitable for diverse background environments. b Digital images of the areal density and thickness of Janus fabric. c IR emissivity of MT20F, MT30F, MT40F, and FMT. d Thermal IR image of the hand in ambient temperature conditions. e IR temperature variation on the surface of MT40F under high-temperature and low-temperature environments. f Thermal IR images of MT20F, MT30F, MT40F, and fabric on a 150°C heating platform. g Thermal conductivity coefficients of the FMT side and MTF side of the Janus fabric. h Temperature variation on the FMT side of the Janus fabric under different solar radiation intensities. i Comparison of IR emissivity versus thickness for this work and other recently reported composite materials in the IR band
To comprehensively evaluate the IR thermal camouflage performance of MT40F, tests were conducted under ambient, high-temperature, and low-temperature conditions. In ambient conditions, where the temperature of the human hand is consistently higher than that of the surrounding environment, the MT40F fabric effectively blocked the thermal signal of the hand through thermal insulation and reduced surface IR emissivity, lowering the thermal radiation temperature difference between the hand and the environment to 0.5 °C. This reduction rendered the covered target invisible in the IR image (Fig. 5d). In high-temperature environments, MT20F, MT30F, MT40F, and FMT samples were placed on a heating stage at 150 °C, and their average surface temperature variations were recorded (Fig. 5f). During 60 min of heating, the MTF series exhibited only minor temperature fluctuations, with MT40F showing the smallest fluctuation (<0.5 °C) and maintaining a temperature difference with the environment below 71 °C throughout the process (Fig. 5e), further confirming its feasibility as an IR camouflage material. In low-temperature environments, when the MT40F sample was attached to the surface of an ice block (Fig. S9), its temperature remained close to the ambient room temperature with a relatively stable temperature difference (Fig. 5e), further demonstrating its excellent thermal camouflage capability under cold conditions. In summary, MT40F maintained a low IR radiation level under different thermal backgrounds, indicating outstanding environmental adaptability and thermal camouflage performance.
4.4.2 Electromagnetic Shielding Performance
In addition to the outstanding adaptive IR stealth capability, the Janus fabric also exhibits remarkable electromagnetic shielding performance owing to its unique structural design and high electrical conductivity, making it an effective barrier against EMI. Taking MT40F as an example, its average SE in the X-band reaches 45 dB, with a maximum value of 58 dB (Fig. 6a), corresponding to an electromagnetic wave shielding efficiency of over 99.99%. Furthermore, normalized analysis reveals that its specific shielding effectiveness (SSE) reaches as high as 1933 dB·cm²·g⁻¹, indicating that even under lightweight and ultrathin conditions, the fabric maintains excellent shielding capability sufficient to meet the EMI protection requirements of most electronic devices.
It is worth emphasizing that the mass fraction of MXene is a key factor in determining both the electromagnetic SE and electrical conductivity of Janus fabrics (Fig. 6b). With increasing MXene content, the nanosheets gradually stack and form continuous conductive networks, which not only facilitate efficient electron transport but also extend the propagation path of electromagnetic waves within the material and enhance multiple reflections. Moreover, the synergistic effect between the macroscopic conductive yarns in the fabric and the surface-embedded MXene nanosheets leads to the formation of a hierarchical conductive network. At the macroscopic scale, good continuity is ensured, while at the microscopic scale, high-density charge transfer channels are provided, thereby strengthening the interactions between electromagnetic waves and the material. Under this mechanism, incident electromagnetic waves are first strongly reflected at the MXene layer, while the unreflected portion undergoes further absorption and scattering at the MXene–fabric interface, gradually establishing a multi-stage shielding pathway of reflection, absorption, scattering, and attenuation, which significantly improves the overall shielding efficiency (Fig. 6c). In contrast, MT20F, with its lower MXene content, fails to form complete conductive pathways, resulting in insufficient conductivity and limited shielding performance. Therefore, rational regulation of MXene content, combined with fabric structural design, is a key strategy for improving the electromagnetic SE of Janus fabrics. Notably, the composite Janus fabrics rely more on absorption than on reflection as the dominant shielding mechanism (Fig. S10), achieving a balance between high shielding efficiency and low reflection that better meets the stringent requirements for electromagnetic compatibility in complex application scenarios.
Fig. 6 a Electromagnetic SE of MT20F, MT30F, MT40F, and fabrics. b Electrical conductivity of MT20F, MT30F, MT40F, and fabrics. c Stress-strain curves of Janus fabric and conductive fabric. d Electromagnetic shielding mechanism of Janus fabric. e Performance comparison between MT40F fabric and other MXene-integrated materials (MXene/PI/mE[51], MXene/ ANF[52], MXene/Cotton[53], MXene/polyvinyl alcohol[54])
In addition, the Janus fabric demonstrates excellent flexibility and mechanical load-bearing capacity when resisting external forces and adapting to complex stretching conditions. As shown in the stress–strain curve in Fig. 6d, the Janus fabric can withstand a maximum tensile force of 1196 N, and exhibits a certain creep tendency at medium and high load stages due to yarn slippage and local microstructural adjustments. In comparison, the Janus composite structure benefits from multi-interface reinforcement and interfacial constraint effects, which effectively enhance both ultimate strength and ductility while suppressing creep behavior to some extent, thereby ensuring superior mechanical stability and structural integrity. Furthermore, compared with previously reported MXene-based integrated materials (Fig. 6e), the Janus fabric not only maintains outstanding mechanical and electromagnetic shielding properties but also combines ultralow IR emissivity, ultrathin and lightweight characteristics, as well as good scalability and controllable large-area fabrication capability, highlighting its remarkable advantages in the field of flexible protective materials.
4.5 Multi-Scenario Adaptability and Application Prospects of Janus Fabrics
Large-area and controllable fabrication of MXene films was achieved through a blade-coating process, followed by efficient integration with fabrics via a simple and effective one-step hot-pressing method, resulting in flexible composite fabrics with Janus characteristics. Benefiting from the dual-sided functional differences and material synergistic effects, the Janus fabric exhibits excellent dynamic IR response regulation and stable electromagnetic shielding performance under complex environments, highlighting its outstanding potential for multi-scenario adaptability. In terms of mechanical performance (Online Resource 4), the fabric achieved a maximum tensile strength of 60 MPa at a stretching rate of 10 cm/min and maintained good interfacial adhesion during fracture, demonstrating superior structural stability and load-bearing capacity. Moreover, owing to its flexibility and conformability, the fabric can seamlessly adhere to surfaces with large curvatures and multiple orientations (Online Resource 5), showing great promise for applications in various critical fields.
Fig. 7 Application Prospects and Fields of Janus Fabric. a IR stealth applications of Janus fabric's FMT side and b MTF side as tarps and tents in a different background environment. c Electromagnetic shielding and IR stealth applications of Janus fabric in portable power bank tarps. d Applications of Janus fabric in human motion and health monitoring
In the field of military protection, battlefield environments are complex and variable, often involving drastic temperature fluctuations and diverse enemy detection methods, where conventional single-mode stealth materials are insufficient to meet practical demands. In contrast, the Janus fabric, with its reversible dual-sided structure, can freely switch between low-emission and high-emission modes while simultaneously providing electromagnetic shielding. This enables environmentally adaptive stealth protection for camouflage tents, tarpaulins, and equipment skins, which holds significant strategic importance (Fig. 7a, Fig. 7b). In the field of electronics and information security, Janus fabrics not only achieve efficient EMI shielding but also reduce the IR radiation signature of devices through the MTF side, thereby enhancing concealment and information confidentiality. Meanwhile, the FMT side, owing to its high emissivity and thermal conductivity, can rapidly dissipate heat, ensuring the stable operation of high-performance devices under harsh environmental conditions (Fig. 7c). In the field of intelligent wearables, the intrinsic conductive network of Janus fabrics enables them to serve as flexible sensing layers that can be integrated with sensors for physiological or environmental monitoring, thereby promoting the development of protective textiles toward multifunctionality and intelligence (Fig. 7d). In summary, Janus fabrics combine environmental adaptability with multifunctional protective features, showing broad application prospects in future military security, information protection, and civilian smart textiles.