Sequentially bridged MXene platelets for strong high‐temperature EM‐IR bi‐stealth sheets

Combination of flexible multifunctional stealth technology properties such as electromagnetic (EM) and infrared (IR) stealth is crucial to the development of aerospace, military, and electronic fields, but the synthesis technology still has a significant challenge. Herein, we have successfully designed and synthesized highly flexible MXene@cellulose lamellae/borate ion (MXCB) sheets with strong high‐temperature EM‐IR bi‐stealth through sequential bridging of hydrogen and covalent bonds. The resultant MXCB sheets display high conductivity and good mechanical features such as flexibility, stretchability, fatigue resistance, and ultrasonic damage. MXCB sheets have a high tensile strength of 795 MPa. Furthermore, MXCB sheets with different thicknesses indicate exceptional high‐temperature thermal‐camouflage characteristics. This reduces the radiation temperature of the target object (>300 °C) to 100 °C. The conductivity of MXCB sheet with 3 μm thickness is 6108 S/cm and the EM interference (EMI) shielding value is 39.74 dB. The normalized surface‐specific EMI SE absolute shielding effectiveness (SSE/t) is as high as 39312.78 dB·cm2/g, which remained 99.39% even after 10,000 times repeated folding. These multifunctional ultrathin MXCB sheets can be arranged by vacuum‐assisted induction to develop EM‐IR bi‐stealth sheet.


| INTRODUCTION
With the rapid development of science and technology, military stealth technology has been developed rapidly, especially in the field of military aircraft is widely used.2][3][4][5] The internal transmission, distribution, or use of electrical energy EM radiation equipment is also essential for military aircraft.The elimination or reduction of EM-IR radiation intensity will reduce the operational effectiveness of military aircraft.
Simultaneously, EM-IR radiation sources on the airborne radar, radio, a variety of electronic countermeasures equipment performance, and the surrounding environment, and may affect sensitive components or cause damage, thus triggering irreversible safety hazards and property security.In addition, some military equipment (e.g., stealth fighter engines and funnels of naval ships) also requires consideration of stealth technology for security protection. 6This can add more security risks if no camouflage is provided.Therefore, the development of technologies for both EM-IR stealthy aircraft is very important.
Effective EM-IR bi-stealth materials not only reduce undesired sources of radiation but also protect components from external spurious signals as well as local hotspots.8][9] In contrast, thermal camouflage is mainly achieved using the reduction of thermal radiation between the object and its surroundings. 10,118][19] However, metal shrouds increase the additional weight of smaller device components, 9 phase change materials are limited by operating temperatures, 20 insulation aerogels and foams increase the volumetric footprint, 15,16 and metal films/coatings such as stainless steel are highly susceptible to corrosion. 21Consequently, EMI shielding and thermal camouflage materials are urgently needed for portable devices, wearables, military, and even aerospace equipment.][28] Since the first synthesis of MXenes in 2011, over 30 discrete stoichiometric forms have been reported and hundreds more are expected to be synthesized, [29][30][31][32] for example, M 2 XT x , M 3 X 2 T x , M 4 X 3 T x , and M 5 X 4 T x . 24,29,338][49] Thus far, diverse polymers have been used to reinforce titanium carbides (Ti 3 C 2 T x ) MXene nanosheets, such as polyethylene, polypyrrole, polyvinyl alcohol, epoxy, 50 or polyvinylidene fluoride. 51MXene-polymer composites exhibit higher mechanical strength and stability in use, but the conductivity decreases with the embedding of the polymer. 52The reinforcement of MXene films with sodium alginate (SA) and aramid nanofibers exhibit good EMI SE values. 1,53,54owever, water-soluble polymers do not protect MXene from water and moisture absorption, resulting in unsatisfactory EMI shielding and mechanical failure of MXene.Furthermore, embedding a large polymer between the MXene nanosheets reduces the conductivity of their composites.
Cellulose nanofibers 55,56 are the most abundant renewable biopolymers with tunable and biocompatibility.Cellulose nanofibers several tens of microns thick can enhance the mechanical strength of MXene sheets but hinder their electrical conductivity and EMI shielding properties.This may be due to the relatively large diameter of cellulose nanofibers (over 10 nm) 57 resulting in a large insulating gap between the Ti 3 C 2 T x MXene nanosheets.Hereby, we have successfully designed and synthesized highly flexible MXene@cellulose lamellae/borate ion (MXCB) sheets with strong high-temperature EM-IR bi-stealth through sequential bridging of hydrogen and covalent bonds.Specifically, Ti 3 C 2 T x MXene nanosheets were exfoliated by selectively etching of Ti 3 AlC 2 MAX phase, while ultrafine cellulose lamellae (CL) were dissociated from the raw material of poplar fibers by nanodefibrillation technology.Eight MXCB sheets containing various CL were then manufactured by vacuum-assisted filtration, impregnation, rinsing, and annealing processes.MXCB sheets with 3 μm-thick exhibit ultrahigh SSE/t values of 39481.47 dB•cm 2 /g.Meanwhile, they decrease the radiation temperature of hot objects (>300 °C) by about 200 °C, which is better than reported thermal camouflage materials.These sheets also offer other benefits such as high strength, toughness, resistance to ultrasonic damage, and fatigue resistance.

| Synthesis of ultrafine CL
CL was generated by oxidation of delignified poplar wood chips and ultrasonic dissociation.The solids content of the resulting CL suspension was ~1 wt% and was stored at 4 °C for further nanodefibrillation processing.Specifically, wood chips cut along the longitudinal direction were immersed into a 5 wt% NaClO 2 solution and the pH was adjusted with acetic acid to 4.6.The chips were subsequently boiled until they became completely white (~2 h).Wash the delignification wood chips with deionized water to remove residual chemicals.The CL material was then obtained by exfoliating the delignified wood chips backbone.Finally, these CL consisting of many highly oriented fibrils in close alignment was further dissociated into ultrathin 2D CL by an ultrasonic cell disruptor (JY98-IIIDN, Ningbo Xinzhi Biotechnology Co. Ltd).

| Synthesis of Ti 3 C 2 T x platelets
Ti 3 C 2 T x nanosheets were exfoliated from multilayered Ti 3 C 2 T x particles through the minimally intensive layer delamination method. 24,45Specifically, 1.6 g of LiF was slowly added to 20 mL of HCl at a concentration of 9 mol/ L and stirred for 5 min.Then, 1 g of Ti 3 A1C 2 particle was gradually added (for 10 min to avoid exothermic and boiling sputtering) with continually stirring at 40 °C for 48 h.With deionized water as the solvent to 3500 r/min repeated centrifugal washing 6 ~8 times, so that the pH value is up to 6. Subsequently, the precipitate was collected and added to 100 mL deionized water, and sonicated for 3 h under an Ar flow-protected atmosphere.Finally, the centrifugation at 3500 r/min for 1 h to remove nonexfoliated particles and collected the supernatant for use.

| Fabrication of sequentially bridged MXene platelets
Solutions of ultrafine CL of different concentrations (0%-100%) were mixed with freshly synthesized Ti 3 C 2 T x MXene dispersions.This was followed by 20 min magnetically stirring and sonicated (60 W) for 3 min.The MXene/CL aqueous dispersion sheets were then obtained by vacuum filtration.Subsequently, the MXene/CL sheets were immersed in sodium tetraborate for 12 h to crosslink with the hydroxyl groups on the CL and MXene.Finally, the rinsed MXCB sheets were vacuum annealed at 90 °C for 4 h.The sequentially bridged MXCB sheets were obtained by annealing under a vacuum for 4 h.

| Characterization
The micromorphology was recorded by field emission scanning electron microscope (SU8010) at an acceleration voltage of 10 kV.The sectional energy dispersive X-ray spectroscopy (EDS) elemental mapping of MXCB sheet were obtained by an EDX detector of scanning electron microscopy (SEM).The MXCB sheets were cut using a focused ion beam (FIB) and the cross-sectional morphology was observed by FIB-SEM Helios NanoLab 600i.Transmission electron microscopy (TEM) and highresolution TEM images were observed by a Philips CM200 microscope on a 200 kV JEM-2200FS fieldemission electron microscope.Atomic force microscopy (AFM) of MXene platelets were obtained by a Leica TCS SP5.Force-indentation profile was measured at a z-piezo displacement speed of 100 nm/s on an Asylum Research MFP-3D system.The crystalline structure of MXCB sheet was carried out via X-ray diffraction (XRD, BRUKER D8 ADVANCE) with Cu-Kα radiation on a Shimadzu XRD-6000 at a scanning speed of 10°/min to obtain a scanning range of 5°-90°.Fourier transform IR (FTIR) spectra (400-4000 cm −1 ) were performed on a Thermo Nicolet NEXUS-470 FTIR spectrometer with a Thermo Scientific Nicolet iS20 instrument in the wave range of 400-4000 cm −1 .X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250 XI, Thermo Fisher Scientific K-Alpha) of MXCB sheet is operated at 15 kV, 15 Ma, with the spot size of 400 μm and energy step size of 1.000 eV.Electrical conductivities of MXCS sheet are tested using four-point probes resistivity measurement system (Keithley 6220/2182 A, Tektronix Co.; Resistivity range, 0.0001 ~2000 Ω•cm).The IR reflectivity was recorded using IR thermometer GM1651 (-30 °C to 1650 °C) and FOTRIC348+ (-20 °C to 1550 °C) IR thermal camera.Tensile strength was tested at a rate of 1 mm/min (using an Electromechanical Universal Testing Machine-CMT 6104 with 200 N load cell).The size of all MXCB sheets is 10 mm × 5 mm × 3 mm strips.Tensile fatigue was tested by an Instron ElectroPulsE1000 at 1 Hz.The size of all samples is 25 mm × 5 mm strips.To assess the impact of bending on features of MXCB sheets, they were repetitively folded 100 times in reverse directions.

| EMI shielding measurements
EMI SE is the ability of MXCS to attenuate IEMW energy.When the IEMW radiations interact with the material under test, the EMI SE is governed by reflection (R), absorption (A), and transmission (T).Meanwhile, R, T, and A must add up to 1, that is, R and T were measured fusing Vector network analyzer to obtain the four scattering parameters (S 11 , S 12 , S 21 , S 22 ) and caculated by Equations (2-3): (2) (3) The total EMI SE (EMI SE T ) is usually divided into SE R , SE A , and multiple internal reflections (SE MR ).SE T can be written as; When SE T is higher than 15 dB, SE MR is usually negligible.SE R and SE A can be expressed in terms of R and A coefficient; (5) 3 | RESULTS AND DISCUSSION

| Design and fabrication of MXCB sheets
The preparation diagram of MXCB sheets is shown in Figure 1.Ti 3 C 2 T x flakes were obtained using selective etching of Ti 3 AlC 2 particles by LiF/HCl.The compact hydroxyl groups on its surface can interact with the active sites (both carboxyl and hydroxyl groups) on the cellulose molecular chain (Supporting Information: Figure S1-S2).All MXene sheets can be made from pure Ti 3 C 2 T x flakes suspension by vacuum-assisted filtration (Figure 1A).The MXCB sheet is a hydrogen bonding bridge formed by vacuum filtration of MXene/CL aqueous dispersion uniform mixed solution.Then, it was immersed in sodium tetraborate solution and covalently crosslinked with cellulose molecular chain and MXene flakes.Finally, the cleaned, hydrated sheets were vacuum annealed to produce MXCB sheets (Figure 1B and Supporting Information: Figure S3).The laminated architecture of MXCB sheets is displayed schematically in Figure 1C,D and Supporting Information: Figure S4.By this method, eight MXCB sheets with CL contents from 0% to 100% were manufactured.

| Structural characterization of MXCB sheets
The Ti 3 C 2 T x (002) peaks in all diffraction (XRD) patterns of MXene/CL (Figure 2A and Supporting Information: Figure S7A) confirms the monolayer Ti 3 C 2 T x MXene platelets in MXCB sheets (Figure 2B).The proportion of boron and CL in the MXCB sheets can be determined by XPS (Supporting Information: Figure S7B).Simultaneously, we demonstrate an efficient densification strategy using a sequential bridging process of hydrogen and covalent bonding (Figure 1 and Supporting Information: Figures S6 and S7).AFM and profiling of the building blocks of MXCB sheet showed their thickness to be about 2 nm, demonstrating that the CL molecular chains were bound to Ti 3 C 2 T x nanosheets (Figure 2C).The MXCB sheets (Figure 2D) have excellent flexibility (Figure 2E and Supporting Information: Figure S5), like the previously reported MXene-polymer composites. 58,59ross-sectional SEM images of MXCB sheets (Figure 2F,G) show a denser laminated configuration.MXCB sheet has fewer voids, because many hydroxyl groups on the CL surface can more tightly link adjacent Ti 3 C 2 T x nanosheets together to form hydrogen bonds.Nevertheless, the MXCB sheet still has some voids due to the varying concentration or dispersion of some CL leading to the inability to remove the gaps between Ti 3 C 2 T x flakes.Thus, the introduced CL and the borate ions together synergistically densify by hydrogen bonding and covalent bridging.The EDS results of the sections of the MXCB sheet (Figure 2H) indicate the uniform penetration of B + into this sheet.The cross-sectional SEM images of MXCB sheets after FIB cutting (Figure 2I) show the tightness of the end-junction between Ti 3 C 2 T x nanosheets.Therefore, its porosity derived from the corresponding calculations is lower than that of the MXene sheet (Figure 2J).Simultaneously, MXene and MXCB sheets were immersed in deionized water and sonicated for 2 h, whereas the MXene sheet was dispersed completely, and the MXCB sheet was only partially exfoliated on its surface (Figure 2K).

| Mechanical characteristics of MXCB sheets
To further evaluate the relationship between CL content and mechanical strength of MXCB sheets, eight MXCB sheets (CL content: 0 to 100 wt%) were fabricated and tested, as shown in Supporting Information: Figure S8A.3B,C and Supporting Information: Figure S11).Meanwhile, the force-displacement curves of MXCB sheets were tested using nanoindentation (Figure 3D,E), where the sharp AFM tip poked only one hole without catastrophic damage (inset of Figure 3D), displaying a high fracture toughness.Furthermore, Young's modulus of MXCB sheets reached 7.7 ± 1 GPa (Figure 3E).Although better than the MXene sheets, it is lower than the theoretical prediction for Ti 3 C 2 T x nanosheets.Meanwhile, a 5 mm-wide MXCB sheet can withstand a weight of 1000 g without fracture (Figure 3F).

| Mechanical damage resistance and fracture mechanism of MXCB sheets
In addition to its excellent mechanical properties, sequential bridging MXCB sheets also has fatigue resistance to repeated cyclic stretching.The folding test is not easy to meet the standard folding operation and is stable for multiple repeated cycles.Therefore, we designed and installed a unique device for this purpose (Figure 4A).The maximum stress corresponding to the repeated cycles of the MXCB sheet is shown in Figure 4B.MXCB sheet has a cycle life of 115,000 cycles during tensile fatigue resistance, which is much higher than that of MXene and CL sheet.This may be because hydrogen bonds can fracture and reconstitute during cyclic stretching.The morphology of the MXCB sheet was intact throughout the entire repeated folding cycle from 1 to 115,000 cycles with only micro-grooves and no significant change in conductivity, which could be due to the CL sliding (Figure 4C).The stress-strain curves of MXCB sheets after 100 repeated folding cycles showed that the tensile modulus remained above 13.59GPa and the conductivity retention was 98.83%, much higher than that of MXene sheets (33.91%), as shown in Figure 4D-F.
To investigate the synergy between the constituent units of MXCB sheets, their fracture mechanisms are shown in Figure 4G.During the continuous stretching process, the hydrogen bonds fracture first and then slide with each other as the tension increases.Concurrently, CL between MXene nanosheets can provide additional frictional energy dissipation to withstand the sliding effect. 47Then, the stretched CL molecular chains dissipate more energy to impede microcrack expansion.During fracture, the gaps occur mainly at the point of stress concentration (defects, holes, and so on) and then expand in a sawtooth-shaped crack path with the interface between monolayer Ti 3 C 2 T x MXene platelets and ultrafine CL in the homogeneous system.The fracture morphology and mechanism suggest that the MXCB sheet achieves optimal strength and toughness through the laminated architecture formed by the reinforcement of CL and B − .The 2D Ti 3 C 2 T x nanosheets provide the framework for MXCB sheets, whereas the CL and B − act as fillers and adhesives.The CL also facilitate the enhancement of stress transfer as well as frictional energy dissipation by connecting the MXene flakes in the process.Because of this synergistic toughening effect, the MXCB sheets exhibit good strength, toughness, and folding resistance.

| High-temperature thermal protection and IR stealth of MXCB sheets
To investigate the thermal camouflage ability of MXCB sheets, an electric heating stage was employed to simulate the thermal radiation temperature of the object.The radiation temperature of MXCB sheets with various thicknesses (3-15 μm) placed on the heating stage (inset of Figure 5A) was recorded by an IR thermometer and IR pseudo-color map imager.Then, the surface temperatures and thermographic pseudo color image of MXCB sheets with a thickness of 3, 6, 9, 12, and 15 μm were recorded at 300 °C radiation temperature (Figure 5A).The radiation temperature of the MXCB sheet surface versus the time curve shows a decrease with increasing thickness.However, its surface radiation temperature remained constant at 238.5, 200.93, 175.69, 155.19, and 139.43 °C, respectively, indicating that MXCB sheets of different thicknesses, even 3 μm have thermal camouflage capacity.Remarkably, these ultrathin MXCB sheets maintained radiant temperature after heating the surface of an electric heat stage at 300 °C for 600 s (Figure 5B-F and Supporting Information: Figure S12).MXCB sheets have excellent thermal insulation in a high-temperature environment, which makes them promising for EM wave absorption.
A large number of molecules or atoms inside any object are in never-ending irregular thermal motion.This is mainly because they release IR EM waves continuously from the excited state to the energy state.The surface temperature of an object determines the wavelength of its outward radiation.According to Wien's displacement law, the curve of the radiation wavelength of a blackbody with the temperature is as follows 60 : where b is the Wien displacement constant (2.8978 × 10 −3 m•K), λ max is the radiation wavelength, and T is the absolute temperature.Unfortunately, IR radiation can penetrate the atmospheric windows in the 3-5 and 8-14 mm microwave bands owing to the absorption of atmospheric molecules. 61In contrast, thermal camouflage is achieved by reducing the thermal radiation between the object and its surroundings. 10,11Of course, thermal radiation is proportional to the object's IR emissivity (ε) and thermodynamic temperature (T) to the fourth power based on the Stefan-Boltzmann law. 6,17,62S3 for detailed information.(F) Compare the relationship between shielding effectiveness (SSE) to thickness (SSE/t) and thickness of MXCB sheets (red five-pointed star), and previously reported MXene-based material, carbon-based materials, and metal-based material.Please refer to Supporting Information: Table S3 for detailed information.(G) Schematic illustration of EMI shielding mechanism of MXCB sheets.
where σ is the Stefan-Boltzmann constant, 5.67037 × 10 −8 W/(m 2 •K 4 )).According to this law, IR stealth is achieved by attenuating the range and intensity of the target IR radiation (IR emissivity) and the temperature of the IR radiation source.The most typical solution is to reduce emissivity.Unfortunately, IR stealth is achieved at low emissive radiation only at specific temperatures.Also, reducing thermal radiation by reducing emissivity is less efficient.Therefore, when using thermal insulation materials to reduce the heat source temperature is an effective measure to reduce the thermal radiation intensity.This extraordinary high-temperature thermal camouflage capability proves that MXCB sheets can be employed as thermal camouflage materials.Furthermore, we measured the emissivity of 3, 6, 9, 12, and 15 μm-thick MXCB sheets at different temperatures by radiative compensation methods using a thermal IR camera (Figure 5G).In addition, MXCB sheets with different thicknesses have the indoor thermal camouflage ability to reduce the radiation temperature at the same high-temperature (Figure 5G).In comparison, the ultrathin thick MXCB sheets, both 3 and 15 μm thickness exhibit excellent thermal camouflage performance over a wide range of temperatures.Furthermore, the thermal camouflage performance of these MXCB sheets (Figure 5H) is superior to other film-based camouflage materials reported (Figure 5I).The low IR emissivity of the MXene flakes gives MXCB sheets excellent thermal camouflage ability (Figure 5J).This is also due to the compact and continuous thermal conductivity path of MXCB sheet producing high thermal conductivity values.

| EM performance
Usually, large conductivity materials have high EMI SE values.The conductivity of eight MXCB sheets with different CL contents is shown in Figure 6A.The conductivity of MXCB sheets containing 5 wt% CL decreased by only 5%.This may be due to the large aspect ratio of Ti 3 C 2 T x nanosheets, which has a more negligible effect on the conductivity of the samples at low filler loading.The conductivity of an MXCB sheet containing 80% amount of CL is only 1969 S/cm, which is only 19.96% of the conductivity of MXene membranes.This is one of the reasons why MXCB sheets with high CL content have low EMI SE (Figure 6B).This is also confirmed by the EMI SE at 8.2-12.4GHz of eight MXCB sheets with different contents of CL, as shown in Figure 6B.As a result, the EMI SE of MXCB sheets decreases with increasing CL content (Figure 6C).Meanwhile, after 100 bending and folding cycles, MXCB sheet still maintained a 99.39% EMI SE value, indicating its stable EMI shielding performance (Supporting Information: Figure S13).Figure 6D displays the average EMI SE in the X-band for different MXene membranes with thicknesses of 5 ± 0.3 μm.Meanwhile, these MXCB sheets have high tensile strength and are superior to most MXene-based materials reported (Figure 6E and Supporting Information: Table S2).Of course, the use of large and high-quality MXene nanosheets may further optimize the performance of MXCB sheets.To reliably compare the weight-sensitive shielding capability of MXCB sheets, they are usually evaluated using the ratio of density-normalized shielding effectiveness (SSE) to thickness, SSE/t.The resulting SSE/t values of MXCB sheets were as high as 39481.47dB•cm 2 /g, outperforming most reported EMI shielding materials, such as metallic (Cu, Ag, and Al), graphene, multiwalled carbon nanotubes, and MXene materials (Figure 6F).Furthermore, lightweight aerogels (graphene aerogels, MXenenanocellulose aerogels) and foams (MXene foams) can provide highly SSE/t; they have inferior mechanical strength (Supporting Information: Figure S14).The interfacial solid polarization and multiple reflections mechanism of MXCB are shown in Figure 6G.The many charge carriers on the surface of the MXCB sheet before IEMW impact cause partial IEMW reflection from the surface.In addition, the termination group induces the production of localized dipoles that contribute to the absorption of IEMW.The transmitted waves entering the MXCB sheet produce the same reflection and absorption when they encountered the next MXene nanosheet.Therefore, multiple reflections and absorptions are generated inside the MXCB sheet, thus attenuating or eliminating the IEMW.

| CONCLUSION
In summary, defects such as voids in MXene membranes can reduce their mechanical strengths and electrical conductivity.We demonstrate an effective densification strategy that closes gaps by a sequential bridging process introducing numerous hydrogen and covalent bonds to manufacture highly flexible compact MXCB sheets successfully.This sequentially bridged approach can optimize the mechanical strength, fatigue resistance, oxidation resistance, and ultrasonic decomposition resistance of MXCS sheets.The tensile strength of MXCB sheets reaches up to 795 MPa (Young's modulus 13,755.52MPa).The MXCS sheets exhibited a stress retention rate of 91% after immersion for 24 h.Simultaneously, the MXCB sheet was sonicated in deionized water for 2 h with only partially exfoliated on its surface.Furthermore, the high-temperature IR stealth of MXCB sheets with different thicknesses enables them to reduce the IR radiation temperature by 200 °C (the temperature of objects > 300 °C).The flexible MXCB sheet can reach EMI SE values of up to 39481.47 dB•cm 2 /g, which remained 99.39% even after 10,000 times repeated folding.A flexible MXCB sheet was prepared to demonstrate the potential applications for multifunctional stealth structures in flexible electronic devices and aerospace fields.

F I G U R E 1
Schematic representation of the fabrication of Ti 3 C 2 T x , Ti 3 C 2 T x /cellulose lamellae (CL), and MXene@cellulose lamellae/ borate ion (MXCB) sheets.(A) Schematic representation of Ti 3 C 2 T x sheets.Ti 3 AlC 2 MAX was used to selectively etch Al atoms and replace them with hydroxyl, oxygen, or fluorine surface terminal (T x ) by in situ forming HF with hydrochloric acid (HCl), and lithium fluoride (LiF) to obtain delaminated Ti 3 C 2 T x MXene.In addition, Ti 3 C 2 T x MXene sheet was fabricated through vacuum-assisted filtration.(B) The MXene/ CL dispersions were first processed into MXene/CL sheets using vacuum-assisted filtration.The borate ions were then infiltrated into the Mxene/CL sheets, resulting in MXCB sheets.(C) Schematic representation of Ti 3 C 2 T x /CL sheets.(D) Schematic representation of MXCB sheets.

F I G U R E 2
Structural characterization of MXene@cellulose lamellae/borate ion (MXCB) sheets.(A) X-ray diffraction (XRD) curves of Ti 3 AlC 2 MAX, Ti 3 C 2 T x MXene, and MXCB sheet using Cu-Kα radiation.(B) Scanning electron microscopy (SEM) of delaminated Ti 3 C 2 T x particles.(C) Atomic force microscopy (AFM) images of MXene/cellulose lamellae (CL) hybrid building blocks.Height profiles of MXene/ CL hybrid building blocks in (C) illustrate its thickness of 1.5 nm.(D) Photograph of an MXCB sheet.(E) Photograph of MXCB sheet exhibiting flexibility.(F) Low-resolution SEM image of MXCB sheet.(G) High-resolution SEM image of MXCB sheet.(H) Sectional energy dispersive X-ray spectroscop (EDS) elemental distribution of element B in the red area outlined in (G).(I) Cross-sectional SEM image of MXCB sheet cut by focused ion beam (FIB).(J) Porosities of pure MXene sheet and MXCB sheet derived from density measurements.(K) Photographs of MXene and MXCB sheet before and after sonication in water for 2 h, demonstrating waterproof performance.

F I G U R E 3
Mechanical characteristics of MXene@cellulose lamellae/borate ion (MXCB) sheets.(A) Representative tensile-strain curves for the MXCB sheets after being submerged in water for 24 h.(B) Young's modulus of MXCB sheets after being immersed in water for 24 h.(C) Percentages of the tensile strength of MXCB sheets after being immersed for 24 h relative to those sheets before being immersed.(D) Force-deflection curves of MXCB sheets.The inset displays an atomic force microscopy (AFM) image of a punctured MXCB sheet without catastrophic rupture.(E) Young's modulus of pure MXene sheet (0 wt% cellulose content) and MXCB sheets.(F) Optical image of MXCB sheets that can withstand the weight of 1000 g, demonstrating excellent mechanical performance.

F I G U R E 4
Mechanical damage resistance and fracture mechanism of MXene@cellulose lamellae/borate ion (MXCB) sheets.(A) The schematic of a complete true-folding process and optical images during one folding cycle on the machine.(B) Number of failure cycles of MXCB sheets and maximum stress level.(C) Conductivities with repeated folding cycles.(D) Representative stress-strain curves of MXCB sheets containing various cellulose lamellae (CL) after 500 repeated folding.(E) Young's modulus of MXCB sheets with various CL contents.(F) Percentages of the tensile strength of MXCB sheets after repeated folding for 500 cycles.(G) Model snapshots for MXCB sheets during simulative tensile stretching.

F I G U R E 5
High-temperature thermal protection and infrared (IR) stealth of MXene@cellulose lamellae/borate ion (MXCB) sheets.(A) Far-IR digital display of temperature-time profiles for closed electric furnace and center point on the backside of MXCB sheets with different thickness.The inset shows a homemade platform for temperature measurement and IR image measurement, including a far-IR digital enclosed furnace, an IR thermometer, and an IR imager.IR thermal images of the MXCB sheets with a thickness of 3, 6, 9, 12, and 15 µm on and a radiation temperature about 300 °C, respectively.(B-F) Thermal camouflage ability of MXCB sheets with the thickness of (B) 3, (C) 6, (D) 9, (E) 12, and (F) 15 µm.(G) Indoor radiation temperature measurement of MXCB sheet with different thicknesses.(H) Thermal camouflage performance relative to object radiation temperature, reduction value, and thickness.Please refer Supporting Information: Table S2 for details.(I) The relationship between radiation temperature and thickness into consideration.Please refer Supporting Information: Table S2 for details.(J) High thermal camouflage mechanism of MXCB sheets.
The introduction of CL improved the mechanical properties of MXCB sheets compared with pure MXene sheets (0 wt% CL content).The tensile stress of MXCB sheets(40 wt% CL) was 795 MPa and Young's modulus was 13755.52MPa(SupportingInformation:FiguresS8B,S9, and S10).This indicates that appropriate CL can enhance the mechanical strength of MXene sheets, but excessive CL can hinder the synergistic enhancement reinforcement effect and densification level.Moreover, ambient humidity and oxidation are among the key parameters affecting the performance of MXene sheets in the face of hazardous and complex realworld environments.Still, MXCB sheets form stable materials due to hydrogen and covalent bonding interactions of CL and boron ions (B − ).The stress-strain curves of MXCB sheets (0, 40 wt%, and 100 wt%) after 24 h of immersing in deionized water are shown in Figure3A.As a result, the mechanical strength of MXene and CL sheets were poor (stress retention of only 22% and 19%,