Flexible and Waterproof 2D/1D/0D Construction of MXene-Based Nanocomposites for Electromagnetic Wave Absorption, EMI Shielding, and Photothermal Conversion

Zhen Xiang, Yuyang Shi, Xiaojie Zhu, Lei Cai, Wei Lu✉

ABSTRACT High-performance electromagnetic wave absorption and electromagnetic interference (EMI) shielding materials with multifunctional characters have attracted extensive scientific and technological interest, but they remain a huge challenge. Here, we reported an electrostatic assembly approach for fabricating 2D/1D/0D construction of Ti3C2Tx/carbon nanotubes/Co nanoparticles (Ti3C2Tx/CNTs/Co)nanocomposites with an excellent electromagnetic wave absorption,EMI shielding efficiency, flexibility, hydrophobicity, and photothermal conversion performance. As expected, a strong reflection loss of-85.8 dB and an ultrathin thickness of 1.4 mm were achieved. Meanwhile, the high EMI shielding efficiency reached 110.1 dB. The excellent electromagnetic wave absorption and shielding performances were originated from the charge carriers, electric/magnetic dipole polarization, interfacial polarization, natural resonance, and multiple internal reflections. Moreover, a thin layer of polydimethylsiloxane rendered the hydrophilic hierarchical Ti3C2Tx/CNTs/Co hydrophobic, which can prevent the degradation/oxidation of the MXene in high humidity condition. Interestingly, the Ti3C2Tx/CNTs/Co film exhibited a remarkable photothermal conversion performance with high thermal cycle stability and tenability. Thus, the multifunctional Ti3C2Tx/CNTs/Co nanocomposites possessing a unique blend of outstanding electromagnetic wave absorption and EMI shielding, light-driven heating performance, and flexible water-resistant features were highly promising for the next-generation intelligent electromagnetic attenuation system.

KEYWORDS Ti3C2Tx; CNTs; Co; Low-dimensional materials; Electromagnetic wave absorption; EMI shielding; Multifunction

1 Introduction

The rapid advancement of communication and increasingly compact and intelligent electronic devices have caused serious electromagnetic interference (EMI), information leakage, and even affected human health and the surrounding environment [1]. Generally, an effective electromagnetic attenuation material can reduce the reflection and transmission of undesirable electromagnetic waves. In addition, it is highly desirable to integrate lightweight, thin thickness, and flexibility into one material for the next-generation electromagnetic wave absorbing and EMI shielding applications[2-4].

Low-dimensional nanomaterials owing to their distinctive properties of large surface area, flexibility, and tunable electronic structure have been receiving great attention [5-8]. MXene (multifunctional two-dimensional (2D)layered metal carbides and nitrides) has a great potential in novel electromagnetic wave absorption and EMI shielding materials due to its unique multilayer microstructure,high specific surface area, good electrical conductivity, and metal-like properties [9-12]. For the electromagnetic wave absorption material, it was characterized by the little reflection and transmission of the incident electromagnetic wave.The moderated impedance matching and electromagnetic parameters were required to achieve the more capture and efficient attenuation of electromagnetic waves. In addition,the high electric conductivity and multiple internal reflections in MXene contributed to the formation of high-efficient EMI shielding materials [10]. Thus, it is necessary to explore the development of the electromagnetic wave absorption and EMI shielding materials by making full use of the synergy of the component loss mechanism and rationally designing a novel architecture. Recently, several materials, including intrinsic conductive polymers, magnetic nanoparticles/nanowires, and carbon nanomaterials of graphene sheets and carbon nanotubes, have been strategically combined with MXene to explore the novel electromagnetic attenuation materials. Wang et al.prepared flexible and lightweight Ti3C2Tx/Fe3O4@PANI composite, which achieved a highperformance EMI shielding performance [13]. Wang et al.reported that hierarchical Ti3C2TxMXene/Ni Chain/ZnO on cotton fabric showed self-cleaning and improved microwave absorption [14]. Wang et al.prepared hierarchical carbon fiber@MXene@MoS2composites with an efficient electromagnetic wave absorption [15]. Li et al.manufactured Ti3C2TxMXene@graphene oxide aerogel microspheres by rapid freezing assisted electrostatic-spinning, which exhibited a low filler loading, a thin thickness, and a strong reflection loss [16]. Among them, one-dimensional (1D) carbon nanotubes (CNTs) with high conductivity, mechanical reliability, lightweight were integrated with MXene to assemble MXene/CNTs composites for efficiently shielding the electromagnetic waves [4, 17, 18] and absorbing electromagnetic wave [19]. Moreover, due to their high Snoek’s limit, conductivity, strong anisotropy field, and high saturation magnetization, zero-dimensional (0D) magnetic nanoparticles(MNP) were widely used in electromagnetic wave absorbing and EMI shielding [20-24]. Thus, combining magnetic nanoparticles with MXene can achieve enhanced electromagnetic dissipation materials, such as MXene/hollow Fe3O4[25], TiO2/Ti3C2Tx/Fe3O4[26], MXene/Ni chain hybrid [27],Ti3C2Tx@NiCo2O4[28]. The 0D magnetic nanoparticles decorated 1D CNTs can be derived from the metal-organic frameworks (MOFs) due to their tunable composition and microstructure [29-31]. In addition, substantial development of electromagnetic wave absorption and EMI shielding was achieved for MXene-based composites with a novel structure. Examples included hollow structures [32-35],porous structures [36, 37, 2], and layer structures [38, 39].The laminated porous structure gives the material the advantages of a lightweight, abundant interfacial polarization, and multiple scattering and reflection, which can attenuate more electromagnetic wave energy [40-42]. However, there were few reports on the laminated porous structure with the combination of 0D magnetic nanoparticles, 1D CNTs, and 2D MXene for the high-efficiency electromagnetic wave absorption and EMI shielding. Therefore, on the basis of the complementarity in composition and structure, combining 0D magnetic nanoparticles, 1D CNTs, and 2D MXene into the laminated MXene/CNTs/MNP nanocomposites is worthy of extensive research toward high-efficiency electromagnetic attenuation materials.

Furthermore, with the popularity of emerging highly integrated fifth-generation (5G) wireless technologies and wearable devices, the electromagnetic wave absorption and EMI shielding materials have been strikingly updated by integrating their inherent absorbing and shielding capabilities as well as novel functionalities, including flexible, hydrophobic, and energy conversion functions[43]. Examples such as flexible Fe3O4@Ti3C2Tx/elastomer [44], MXene foam [37], PET-PPy/MXene textiles[45], CNF@MXene films [46], and MXene/Ag nanowire-PVA films [47]. Considering abundant surface termination(O, F, and/or OH groups) introduced during etching and delamination processes and the use under a humid condition, the degradation/oxidation of the MXene was easily proceeded in a high humidity environment, which may deteriorate their stability and reliability of electromagnetic wave absorption and EMI shielding performances. Thus,the water-resistant treatment of the surface is particularly important and urgently needed. Furthermore, light-to-heat,also known as a photothermal conversion, is an energy conversion process that harvests light energy by photothermal materials and converts it into thermal energy [48].The efficient photothermal performance would broaden the practical applications range of MXene-based composites.It is of great significance to explore the photo-responsive behavior of MXene/CNTs/MNP nanocomposites. Therefore, the development of fabrication and functionalization for hydrophobic surfaces and photothermal conversion would be profitable in improving their practicability for electromagnetic wave absorption and EMI shielding in various technological applications.

Here, we demonstrated an electrostatic assembly approach for fabricating 2D/1D/0D construction of Ti3C2Tx/CNTs/Co nanocomposites with highly integrated functions, including excellent electromagnetic wave absorption, EMI shielding efficiency, photothermal conversion performance, flexible and hydrophobic characterizations. The sea urchin-like CNTs/Co nanocomposites were introduced on 2D Ti3C2TxMXene sheets to form laminated Ti3C2Tx/CNTs/Co nanocomposites to improve the electromagnetic wave absorption and enhance the EMI shielding efficiency. As expected, a strong reflection loss of −85.8 dB, an ultrathin thickness of 1.4 mm, an ultralow filler loading of 5 wt%, and a broad EAB of 6.1 GHz were obtained. At the same time, the EMI shielding efficiency was as high as 110.1 dB. The corresponding mechanisms were discussed in detail. Moreover, a thin layer of polydimethylsiloxane rendered the hydrophilic hierarchical Ti3C2Tx/CNTs/Co hydrophobic with a water contact angle of ～ 110.3°, which can prevent the degradation/oxidation of the MXene-based composites in high humidity conditions. The photothermal conversion performances of the multifunctional film and its thermal cycle stability and adjustability were also investigated.

2 Experimental Section

2.1 Materials

Cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O), adenine[C5H5N5], ethanol (C2H6O), N, N-dimethylformamide[HCON(CH3)2, DMF], concentrated hydrochloric acid (HCl,12 M), lithium fluoride (LiF), hexadecyl trimethyl ammonium bromide (CTAB), and polydimethylsiloxane (PDMS)were purchased from Shanghai Aladdin Co., Ltd and used without further purification. Ti3AlC2powder was purchased from Foshan XinXi Technology Co., Ltd.

2.2 Preparation of Ti3C2Tx/CNTs/Co Nanocomposites

The Ti3C2Tx/CNTs/Co nanocomposites were synthesized as shown in Scheme 1. First, Ti3C2Txsheets were prepared by the revised method [49, 50]. 1 g of LiF was added to 30 mL HCl solution (9 M) and stirred for 10 min with a magnetic Teflon stir bar to completely dissolve. 1 g of Ti3AlC2powders were carefully added within 5 min and keep the reaction mixture at 40 °C for 24 h. The mixture was washed by deionized water and centrifuged(3,500 rpm, 5 min for each cycle) until the pH of the supernatant reached 5.5. The obtained sample was dispersed in 50 mL deionized water, followed by sonication at the temperature of 4 °C for 60 min in Ar atmosphere. The dark green Ti3C2Txsheets solution was collected supernatant by centrifuging mixture at 3,500 rpm for 75 min. Secondly, 0.5 mM Co(NO3)2·6H2O and 1 mM adenine were dissolved in 50 mL DMF and magnetic stirring for 60 min.The resulting solution was transferred into a three-necked flask (100 mL) and then heated up to 140 °C for 30 min with a heating rate of 20 °C min−1, and then washed with DMF and ethanol, and dried at 60 °C overnight. The sea urchin-like CNTs/Co nanocomposites were prepared by pyrolysis of Co-MOFs template in a tube furnace at 600 °C for 240 min at a heating rate of 5 °C min−1under H2:Ar (5:95 in volume %) flow. Thirdly, 10 mg CNTs/Co nanocomposites were dispersed in 50 mL CTAB solution(2 mg mL−1) by mechanical stirring for 30 min. Subsequently, 10 mL Ti3C2Txsheets solution (1 mg mL−1) was added dropwise into the above mixture, and the mechanical stirring was continued for 60 min. Finally, the Ti3C2Tx/CNTs/Co nanocomposites were collected by washing with deionized water by centrifugation, and then vacuum freeze-dried at −80 °C for 48 h. A PDMS solution was prepared by mixing elastomer and curing agent in a mass ratio of 10:1 at room temperature. The PDMS solution was evenly coated on the surface of Ti3C2Tx/CNTs/Co nanocomposites with a coating rod and then cured in a vacuum oven at 80 °C for 4 h.

2.3 Characterization

The morphology and microstructure of the synthesized samples were observed by scanning electron microscopy(SEM) and transmission electron microscopy (TEM).X-ray powder diffraction (XRD, Cu-Kα radiation) was used to analyze the crystallographic structure and phase composition of the samples. Raman spectroscopic system(633 nm laser excitation) was used to measure the Raman spectra. The room-temperature magnetic properties were tested by vibrating sample magnetometer (VSM). Nitrogen adsorption and desorption isotherms were tested by the Quad-resorb-SI instrument. The electrical conductivity of the samples was measured by using an advanced fourprobe. The optical contact angle (CA) of the composite fabric was measured by the optical CA measurement system. An 808 nm high-power multimode pump laser was used as the NIR light source with a spot radius of about 6 mm. The thermal image and temperature of the samples were recorded by an infrared thermal imaging instrument.

2.4 Electromagnetic Wave Absorption Measurements

Scheme 1 Schematic illustration of the Ti3C2Tx/CNTs/Co nanocomposites formation process

Electromagnetic parameters were measured on a vector network analyzer (VNA, 3672B-S, Ceyear, China) in the frequency range of 2-18 GHz by a transmission-reflection mode. The obtained products with a loading of 5 wt% were uniformly mixed with paraffin, and pressed into coaxial rings(Φout: 7.00 mm,Φin: 3.04 mm). The reflection loss (RL) of Ti3C2Tx, CNTs/Co, and Ti3C2Tx/CNTs/Co nanocomposites was calculated by transmission line theory [51, 52]:

whereZ0,Zin,f,c,anddwere the free space impedance,input impedance, frequency, light speed, and matching thickness of the absorber. The input impedance (|Zin/Z0|) was described as follows [53]:

The attenuation constantαwas introduced [54, 55]

2.5 EMI Shielding Measurements

EMI shielding effectiveness (SE) of all the samples was measured by a rectangular waveguide (32,117) using a 2-port network analyzer (3672B-S, Ceyear, China) in the frequency range of 8.2-12.4 GHz. A film with a diameter of 40 mm was obtained by a vacuum filtration method. Specifically, the measured volume dispersion of the MXene and CNTs/Co with a mass ratio (90:10, 80:20, 70:30, and 60:40)was filtered through a PES membrane with a pore size of 0.22 μm. The EMISEof all the samples was calculated usingS-parameters. The total EMISE(SEtotal), consisting of reflection efficiency (SER), absorption efficiency (SEA),and multiple reflection efficiency (SEMR), can be written as:

where SEMRis often negligible when SEtotal> 15 dB. SERand SEAwere expressed as reflection and absorption coefficient considering the power of the incident electromagnetic wave inside the shielding material as:

According to Simon’s formula, theSETcan be written as [9]:

whereσ,f, anddwere the electrical conductivity, frequency,and thickness of the shielding materials, respectively.

2.6 Photothermal Performance Measurements

The photothermal performance of the samples was studied under an 808 nm NIR laser with different power densities(0.2, 0.3, 0.4, 0.5, and 0.6 W cm−2). The photothermal conversion efficiency (η) of the sample is determined according to the following equation [56]:

whereh,A,ΔTmax, andIwere the heat transfer coefficient,surface area of the system, the temperature difference between the maximum temperature of the sample and ambient temperature, the light power, respectively. In order to gethA,tandθdefined as the ratio of ΔTto ΔTmaxwere introduced. The value ofhAwas derived according to the equation:

wheremandcpwas the weight of coating and the specific heat of sample, which was determined using a method of sapphire with DSC instrument. Hence,hAcan be acquired by calculating the aforementioned linear equation from the cooling period.

3 Results and Discussion

The Ti3C2Tx/CNTs/Co nanocomposites were synthesized as shown in Scheme 1. The Ti3C2TxMXene sheets were prepared by ultrasonic delamination of Ti3C2Tx-deionized water solution under the protection of Ar atmosphere, which was obtained by etching Ti3AlC2particles with LiF and HCl solution. The sphere Co-MOF precursor was successfully fabricated by a facile microwave-assisted method. The sea urchin-like CNTs/Co nanocomposites were prepared by pyrolysis of Co-MOFs template under a reducing atmosphere, in which Co2+ions were in situ reduced to 0D Co nanoparticles, and the organic components tended to be carbonized into 1D CNTs. Using an electrostatic assembly mechanism, the sea urchin-like CNTs/Co nanocomposites were adhered on the 2D Ti3C2TxMXene sheets to form laminated Ti3C2Tx/CNTs/Co nanocomposites.

The crystallographic structure and phase composition of the resultant nanocomposites were explored, as shown in Fig. 1a. The XRD curve of Ti3AlC2(MAX) powder is illustrated in Figure S1, which was identified to (002), (104), and(105) planes of Ti3AlC2with a hexagonal structure (JCPDS No. 52-0875). Figure 1a demonstrates the XRD pattern of Ti3AlC2powder after the etching, delamination, and freeze-drying treatment. A distinguished diffraction peak at 2θ= 6.8° with an interlayer spacing of 1.30 nm corresponded to the (002) plane of the Ti3C2Txsheets. The broadened diffraction peak of the Ti3C2Txsample was attributed to the water and/or cationic intercalation in the hydrophilic and negatively charged Ti3C2Txsheets [49]. In order to explore the thermal decomposition of Co-MOFs precursor, the thermal gravimetric behavior was analyzed, as shown in Fig. S2.It was seen that the weight loss reached up to 70.3 wt% and kept constant as the temperature increased above 575 ℃,which was corresponded to the thermal decomposition of Co-MOFs precursor. Thus, the pyrolysis temperature of 600℃ was selected in this work. After pyrolysis, for the formation of CNTs/Co nanocomposites, three diffraction peaks(44.2°, 51.5°, and 75.9°) were corresponded to ((111), (200),and (220)) planes of Co with cubic structure (JCPDS No.89-7093). In addition, another diffraction peak at 2θ= 26.5°was assigned to the (002) plane of the hexagonal carbon(JCPDS No. 89-7213). Moreover, the XRD patterns of Ti3C2Tx/CNTs/Co nanocomposites with different CNTs/Co content ratios (0, 25, 50, 75, and 100 wt%) are demonstrated in Fig. S3. For Ti3C2Tx/CNTs/Co nanocomposites, the characteristic diffraction peaks were also well indexed with the Ti3C2Txphase (6.8°), Co phase (44.2°, 51.5°, and 75.9°),and C phase (26.5°).

Raman spectra of Ti3C2Tx, Ti3C2Tx/CNTs/Co, and CNTs/Co nanocomposites are shown in Fig. 1b. For the Raman shift range from 100 to 800 cm−1, the Ti3C2Tx, Ti3C2Tx/CNTs/Co showed similar Raman spectra (Fig. S4). The modes at 195 and 718 cm−1were assigned to the out-ofplane stretching vibration of Ti and C atoms, meanwhile,the modes (373 and 620 cm−1) corresponded to the in-plane modes of Ti, C, and surface functional group atoms (O, F,and OH) [57]. After the incorporation of CNTs/Co, two broad bands appeared at ～ 1330 and 1592 cm−1, which were assigned to theDband andGband, respectively [58]. Thus,it was confirmed that the carbon material was present in the hybrid. Generally, theDband indicates disorder and defects in carbon materials and theGband represents the graphitic degree withsp2bonding [59, 60]. Moreover, the integrated intensityID/IGratio of Ti3C2Tx/CNTs/Co and CNTs/Co were 1.16 and 1.23, respectively. It would be due to the interaction between 2D Ti3C2Txand CNTs/Co nanocomposites. Here,to further confirm the interaction between 2D Ti3C2Txand CNTs/Co nanocomposites, X-ray photoelectron spectroscopy (XPS) was introduced to analyze the surface electronic states and chemical composition, as shown in Figs. 1c-e and S5. The XPS survey spectra demonstrated that the Ti3C2Tx/CNTs/Co nanocomposites were composed of Ti, C, O, Co,N, and F (Fig. 1c). In special, there were two strong Co 2p and N 1 s peaks in the XPS survey spectra of Ti3C2Tx/CNTs/Co nanocomposites, which were not found in the spectra of pure Ti3C2Txsheets (Fig. S5a). The high-resolution XPS spectra of Ti 2p (Figs. 1d and S5b) was deconvolved into two peaks (Ti 2p3/2, Ti 2p1/2), in which the components centered at ～ 455.0, 455.9, and 458.1 eV were assigned to the Ti-C, Ti-O, and TiO2bonds, respectively [25, 57, 61].The high-resolution C 1 s (Fig. 1e) was fitted with six components located at ～ 281.4, 282.1, 284.8, 284.1, 286.2, and 288.0 eV, which corresponded to the C-Ti, Ti-C-O, C-C,C = C, C-O, and C-F bonds, respectively [62]. It was found that the peak intensity of the C-Ti bond in Ti3C2Tx/CNTs/Co nanocomposites was smaller than that of Ti3C2Txsheets(Fig. S5c), which was attributed to the contribution of CNTs/Co nanocomposites. Thus, it endowed Ti3C2Txand CNTs/Co with strong electrostatic charges to induce the assembly of laminated Ti3C2Tx/CNTs/Co nanocomposites. Moreover,the porous properties of Ti3C2Tx/CNTs/Co nanocomposites were analyzed, as shown in Figs. 1f and S6. It was seen a long and narrow hysteresis loop, which was a typical IVtype isotherm. Thus, the Ti3C2Tx/CNTs/Co nanocomposites exhibited mesoporous characteristics based on the IUPAC classification, which achieved a surface area of 93.57 m2g−1and a total pore volume of 0.36 cc g−1. The mesoporous structure might be derived from the rational construction of porous CNTs/Co nanoparticles and 2D Ti3C2Txsheets.Based on the Maxwell-Garnett theory, the porous structure contributed to modulate permittivity values [63-65].Additionally, porous composites showed the advantage of low density [66]. The room-temperature magnetic performance of all samples is shown in Fig. S7. Ti3C2Txdid not exhibit the magnetic hysteresis due to its lack of magnetic components. In contrast, Ti3C2Tx/CNTs/Co had a saturation magnetization (Ms) value of 32.7 emu g−1, remnant magnetization (Mr) value of 8.1 emu g−1, and coercivity (Hc) value of 368.5 Oe, which was lower than that of CNTs/Co (Msof 57.1 emu/g,Mrof 14.5 emu g−1, andHcof 401.2 Oe). The lowerMsvalue was caused by the non-magnetic composition of 2D Ti3C2Txsheets and 1D CNTs, spin disorder and superparamagnetic relaxation of Co nanoparticles [67]. In addition, the lower coercivity value might be attributed to the interaction among 0D Co nanoparticles, porous carbon,and 2D Ti3C2Txsheets and the limitation of the surface spincanting effect [68].

Fig. 1 XRD curves (a) and Raman spectra (b) of Ti3C2Tx, Ti3C2Tx/CNTs/Co, and CNTs/Co nanocomposites. XPS survey spectra (c), Ti 2p XPS spectrum (d), C 1 s XPS spectrum (e), and N2 adsorption-desorption isotherms (f) of Ti3C2Tx/CNTs/Co nanocomposites

The morphology and microstructure of nanocomposites were characterized by SEM and TEM. Figure 2a shows that the Ti3C2Txpresented a sheet-like structure with an average size of 200 μm. The more energy-dispersive X-ray spectrometry (EDS) information is provided in Fig. S8.The TEM showed the flat flaky morphology of 2D Ti3C2Txsheets (Fig. 2d) and the dark-field TEM image is illustrated in Fig. S9. Co-MOFs precursor had a smooth and uniformly spherical morphology with an average diameter of 300 nm (Fig. 2b, e). After calcination, a novel sea urchinlike structure was observed for CNTs/Co nanocomposites(Fig. 2c). Furthermore, the TEM image (Fig. 2f) illustrated that the CNTs/Co nanocomposites had a porous structure,in which 0D Co nanoparticles were well-dispersed inside the carbon spheres. Interestingly, the sphere was suspended with the extended and highly flexible 1D CNTs with 0D Co nanoparticles isolated inside. The elemental mapping further demonstrated that the Co, C, N, and O elements were evenly dispersed in the CNTs/Co nanocomposites (Fig. S10).Hence, the spherical Co-MOFs precursor can provide an ideal template for the preparation of novel sea urchin-like CNTs/Co nanocomposites. As for Ti3C2Tx/CNTs/Co nanocomposites, the CNTs/Co nanocomposites were anchored onto the surface of the 2D Ti3C2Txsheets (Fig. 2g-i), which was well consistent with the TEM results of Ti3C2Tx/CNTs/Co nanocomposites (Fig. 2j-l). The EDS mapping of the SEM image in Fig. 2g further demonstrated that the Ti, C,Co, O, N and F elements were located in the Ti3C2Tx/CNTs/Co nanocomposites (Fig. 2m). Thus, SEM, TEM, and EDS jointly confirmed the successful construction of laminated Ti3C2Tx/ CNTs/Co nanocomposites.

Fig. 2 SEM images of a Ti3C2Tx, b Co-MOFs, c CNTs/Co and g-i Ti3C2Tx/CNTs/Co. TEM images of d Ti3C2Tx, e Co-MOFs, f CNTs/Co, and j-l Ti3C2Tx/CNTs/Co. m EDS mapping of Ti3C2Tx/CNTs/Co

Fig. 3 3D representations of RL values for (a) Ti3C2Tx, (b) Ti3C2Tx/CNTs/Co, and (c) CNTs/Co. 2D contour map representation of modulus of relative input impedance (|Zin/Z0|) for (d) Ti3C2Tx, (e) Ti3C2Tx/CNTs/Co, and (f) CNTs/Co. (g) The RL curves of CNTs/Co-5.0 mm, Ti3C2Tx/CNTs/Co-1.4 mm, and Ti3C2Tx-1.0 mm. (h) The attenuation constant (α) of Ti3C2Tx, Ti3C2Tx/CNTs/Co, and CNTs/Co nanocomposites

Generally, the high electrical conductivity of the material is a prominent factor in achieving an excellent EMI shielding performance [101-103]. Here, the EMI shielding efficiency of Ti3C2Tx/CNTs/Co nanocomposites was investigated in depth, considering the remarkable conductivity of the 2D Ti3C2Txand 1D CNTs in the composites. In Fig. 4a,the layered Ti3C2Tx/CNTs/Co film with slight undulating structures was clearly identified. Figure 4b shows that the Ti3C2TxMXene layer exhibited a fluffy stacked interconnected microstructure, in which the CNTs/Co nanoparticles were distributed inside, indicating that CNTs/Co nanoparticles connected between the Ti3C2Txlayers. The high-magnification SEM images (Fig. 4c, d) revealed that the Ti3C2Txsheets were wrapped on the CNTs/Co nanoparticles. The EMI shielding properties of 40-um-thick Ti3C2Tx/CNTs/Co film with different CNTs/Co contents were explored, as shown in Figs. 4g, h and S17. With increasing the CNTs/Co content, the shielding efficiency first increased and then decreased. The Ti3C2Tx/CNTs/Co film with 10 wt% of CNTs/Co showed the highestSETvalue of 62.0 dB. Figure 4i provides theSER/SETandSEA/SETvalue versus CNTs/Co content. TheSEA/SETvalue was much larger than that ofSER/SET, indicating that theSEAmade a more contribution to theSET. Interestingly, theSEA/SETreached the largest value, which corresponding to the excellent shielding performance. In addition, the thickness-dependent EMI shielding efficiency of Ti3C2Tx/CNTs/Co film with 10 wt% of CNTs/Co was also investigated and is demonstrated in Figs. 4j, k and S18. It was found that the totalSETreached the maximum value of 110.1 dB with the thickness increasing to 100 μm. To compare the EMISEof the Ti3C2Tx/CNTs/Co film and the Ti3C2Txfilm, the EMI ΔSET, ΔSEA, and ΔSERvalues of the Ti3C2Tx/CNTs/Co(10wt%)and Ti3C2Txfilms with different thicknesses are analyzed in Figs. 4l and S18.With increasing the film thickness, there was no obvious change of ΔSERvalue between the Ti3C2Tx/CNTs/Co(10wt%)and the Ti3C2Txfilms, whereas the gradual improvement of ΔSEAvalue was observed. Most importantly, the ΔSETvalue had a similar trend with the ΔSEAvalue, which suggesting an absorption-dominated electromagnetic wave attenuation mechanism in the Ti3C2Tx/CNTs/Co system.Furthermore, the EMI shielding efficiency of MXene-based composites is summarized in Table S2. It was shown that the Ti3C2Tx/CNTs/Co films exhibited a favorable EMI shielding performance.

Fig. 4 Cross-sectional SEM image of Ti3C2Tx/CNTs/Co film (a-d). The photograph of wireless power-transfer circuit without Ti3C2Tx/CNTs/Co film (e), LED on, and with Ti3C2Tx/CNTs/Co film (f), LED off. (g) 3D histogram of average SET, SEA, and SER value, (h) average SET, and(i) SEA/SET and SER/SET ratio of 40-μm-thick Ti3C2Tx/CNTs/Co nanocomposites with different contents of CNTs/Co. (j) 3D histogram of average SET, SEA, and SER value, (k) average SET of Ti3C2Tx/CNTs/Co(10 wt%) nanocomposites with different thicknesses, and (l) comparison of EMI ΔSET, ΔSEA, and ΔSER value versus thickness of Ti3C2Tx/CNTs/Co(10 wt%) and Ti3C2Tx films

Here, we further analyzed the EMI shielding mechanisms of the Ti3C2Tx/CNTs/Co film. According to Simon’s formula [9], the shielding efficiency was closely related to the conductivity and thickness of shielding materials. Figure S19 shows the electric conductivity of the Ti3C2Tx/CNTs/Co films with different CNTs/Co contents and thicknesses.As the CNTs/Co content increased to 10 wt%, the electric conductivity increased to a maximum value of 3571 S cm−1,which was corresponded to the largest EMI shielding effi-ciency. Moreover, as the film thickness increased, the electric conductivity of the Ti3C2Tx/CNTs/Co films gradually increased to the highest value of 5108 S cm−1, which was greater than that the electrical conductivity of the Ti3C2Txfilms (3410 S cm−1). Hence, it contributed to the excellent EMI shielding performance in the Ti3C2Tx/CNTs/Co films.Generally, EMI shielding performance was originated from the mobile charge carriers, electric/magnetic dipoles, and interior interfaces. Given the 3D architecture built with 0D Co nanoparticles, 1D CNTs, and 2D MXene, a 3D conductive network can efficiently consume electromagnetic energy.The electric polar functional groups as the dipoles induced the electric dipole polarization, while the ferromagnetic resonant of Co nanoparticles led to the magnetic polarization.In addition, a large number of heterogeneous interfaces in the Ti3C2Tx/CNTs/Co films produced abundant interfacial polarization, which would further dissipate the electromagnetic energy. Moreover, the laminated porous structure caused the multiple reflections and scattering of the incident microwave, thereby enhancing energy dissipation. To verify the actual EMI shielding effect of Ti3C2Tx/CNTs/Co films,a typical wireless power transmission system was built, as shown in Fig. 4e, f. The electromotive force generated by electromagnetic induction in the receiver coil can light up the light-emitting diode (LED) (Fig. 4e). When the Ti3C2Tx/CNTs/Co film was inserted between the two coils, the LED light was turned off (Fig. 4f and Movie S1). It was due to the obstruction of electromagnetic transmission by the Ti3C2Tx/CNTs/Co film. Thus, the obtained Ti3C2Tx/CNTs/Co film was expected to achieve efficient EMI shielding in practical applications.

Fig. 5 a Bending photograph of the PDMS@Ti3C2Tx/CNTs/Co film.b Water contact angle (CA) of Ti3C2Tx/CNTs/Co film and PDMS@Ti3C2Tx/CNTs/Co film. Inset is a photograph of the measurement of water adhesion (3 μL water droplet) on the film surface. c-f Photograph of the liquid droplets of water (translucent), milk (white), and coffee (yellow) solution sitting on the surface, g optical photographs of self-cleaning test on coated filter paper. (Color figure online)

In view of the excellent electromagnetic wave absorption and EMI shielding efficiency of the Ti3C2Tx/CNTs/Co nanocomposites, the water-resistant treatment of the surface is particularly important to maintain its electromagnetic attenuation performance in practical applications. In general, PDMS due to its transparency, hydrophobicity, high elasticity, biocompatibility, and easy modeling capability has been widely applied to electronic skins [104], sensors [105], and thermal management [106].In this work, the Ti3C2Tx/CNTs/Co film was coated by PDMS, which can provide the flexibility and hydrophobicity characters. Figure 5a illustrates the bending photograph of the PDMS@Ti3C2Tx/CNTs/Co film, which confirmed that the PDMS@Ti3C2Tx/CNTs/Co film had high flexibility. Furthermore, the water contact angle (CA) of the Ti3C2Tx/CNTs/Co film and the PDMS@Ti3C2Tx/CNTs/Co film was analyzed. Figure 5b shows that the CA value of the PDMS@Ti3C2Tx/CNTs/Co film (110.3°) was larger than the CA value of the Ti3C2Tx/CNTs/Co film (73.1°).Figure 5c-f shows that the liquid droplets (water, milk,and coffee) retained a spherical shape on the surface of the PDMS@Ti3C2Tx/CNTs/Co film, exhibiting hydrophobicity. Figure 5g illustrates that the water droplets from a dropper rolled off the surface due to the hydrophobicity and negligible water adhesion to the PDMS@Ti3C2Tx/CNTs/Co film. Therefore, the PDMS@Ti3C2Tx/CNTs/Co film had remarkable flexibility and hydrophobicity characteristics, which would expand its practical application range in moist or wet environments.

In addition to the above-mentioned intriguing functions,the outstanding light-to-heat conversion performance endowed Ti3C2Tx/CNTs/Co nanocomposites with potential application in wearable heaters for self-heating garments[107-110]. Here, the PDMS@Ti3C2Tx/CNTs/Co film was exposed to an 808 nm NIR laser irradiation with different power densities (0.2, 0.3, 0.4, 0.5, and 0.6 W cm−2) to explore its photothermal performance. Figure 6a shows that the surface temperature of the PDMS@Ti3C2Tx/CNTs/Co film reached about 33.5 ℃ within 20 s even under relatively low power density (0.2 W cm−2) irradiation. Overall, as the NIR light power densities increased,the surface temperature of the PDMS@Ti3C2Tx/CNTs/Co film increased significantly and reached the maximum steady-state value. In particular, under exposure to continuous NIR light at the power density of 0.6 W cm−2, the surface temperature reaches a maximum value of 126.6℃ (Movie S2). It is very interesting that regardless of the light power density, a very fast thermal response time was observed during the heating process, which demonstrated a fast thermal response of the PDMS@Ti3C2Tx/CNTs/Co film. In contrast, the PDMS film as a control group had no temperature variation under the same irradiation process.It was indicated that the PDMS@Ti3C2Tx/CNTs/Co film exhibited an efficient photothermal conversion. Figure 6b reveals the excellent real-time temperature dependence on the light power density under the increasing power density stepwise from 0.2 to 0.6 W cm−2, and decreasing to 0.2 W cm−2. It demonstrated a controllable light-to-heat performance of the PDMS@Ti3C2Tx/CNTs/Co film. Figure 6e shows infrared thermographic photographs of the PDMS@Ti3C2Tx/CNTs/Co film with the power density changing from 0.2 to 0.6 W cm−2. Clearly, the thermal image showed a uniform temperature distribution, which had high application prospects in photothermal heaters.

Fig. 6 a Photothermal heating and cooling curves of the PDMS@Ti3C2Tx/CNTs/Co film at different NIR laser power densities. b Tailored surface temperatures of PDMS@Ti3C2Tx/CNTs/Co coating under gradually changing power density. c Heating curves of the PDMS@Ti3C2Tx/CNTs/Co coating for five laser on/off cycles at different NIR laser power densities. d A linear fitting correlation between time (t) and -lnθ obtained from the cooling period. e Infrared thermographic photographs of the PDMS@Ti3C2Tx/CNTs/Co coating under power densities rise from 0.2 to 0.6 W cm−2

In order to further explore the photothermal stability and recyclability of the PDMS@Ti3C2Tx/CNTs/Co film,the recycling temperature change was assessed under different NIR irradiation (0.2, 0.4, and 0.6 W cm−2) and then naturally cooled to the ambient temperature for five on/offlight cycles. Figure 6c shows the stable and regular ascending and descending temperature cycles corresponding to turning on and off the light during the whole cycling process, demonstrating outstanding photothermal stability and recyclability of the PDMS@Ti3C2Tx/CNTs/Co film. The photothermal conversion efficiency is one of the important factors to evaluate heating performance. Figure 6d presents a linear fitting correlation between time(t) and lnθtaking a light power density of 0.6 W cm−2as an example. The photothermal conversion efficiency of the PDMS@Ti3C2Tx/CNTs/Co film was as high as 29.5%,which was higher than that of SnS nanosheets (24%)[111] and MoS2(24.37%) [112], and slightly smaller than that of PDMS@m-Ti3C2Tx/d-Ti3C2Txcoating (30.3%)[109]. Here, the photothermal conversion mechanism was explored in depth. The material composition was of importance role in absorbing photons and converting the photon energy to heat. Hence, the synergy of the unique localized surface plasmon resonance (LSPR) effect in 0D Co nanoparticles [113], the conjugation and hyperconjugation effect in 1D CNTs [114] as well as the strong light absorption and the LSPR effect in 2D Ti3C2TxMXenes[115] endowed the PDMS@Ti3C2Tx/CNTs/Co film with the favorable photothermal conversion performance. In addition, by rationally designing the microstructure to harvest light to the greatest extent, and adopting a suitable thermal insulation layer to minimize heat loss, the photothermal performance can be further significantly improved[107]. Attributing to the multiple composition and novel structures, the PDMS@Ti3C2Tx/CNTs/Co film enabled an outstanding photothermal performance. Therefore, the excellent photothermal performance with excellent cycle stability and adjustability can further broaden the possible application range of this novel multifunctional material.

4 Conclusion

We demonstrated the 2D/1D/0D construction of Ti3C2Tx/CNTs/Co nanocomposites with excellent electromagnetic wave absorption, EMI shielding efficiency, flexibility,hydrophobicity, and photothermal functions via a facile method of microwave-assisted, in situ carbonization and electrostatic assembly process. The sea urchin-like CNTs/Co nanocomposites were introduced on 2D Ti3C2TxMXene sheets to form laminated Ti3C2Tx/CNTs/Co nanocomposites to improve the electromagnetic wave absorption and enhance the EMI shielding efficiency. As expected, a strong reflection loss of −85.8 dB, a broad EAB of 6.1 GHz, an ultrathin thickness of 1.4 mm, and an ultralow filler loading of 5 wt%were achieved for the laminated Ti3C2Tx/CNTs/Co nanocomposites by the improved attenuation capability and optimized impedance matching. The investigation of the underlying mechanism revealed that the electromagnetic wave absorbing performances were enhanced by the synergistic effects between the conduction loss originated from the electronic transport in the conductive network of 1D CNTs and 2D Ti3C2Tx, the dielectric loss stemmed from the dipole polarization and abundant interface in the laminated structure, and the magnetic loss derived from the ferromagnetic resonance of 0D Co nanoparticles. The Ti3C2Tx/CNTs/Co film exhibited a high EMISEof 110.1 dB, which originated from the excellent electrical conductivity, electric and or magnetic dipole polarization, interfacial polarization, natural resonance, and multiple internal reflections. Moreover,PDMS rendered the hydrophilic hierarchical Ti3C2Tx/CNTs/Co hydrophobic with a water contact angle of ～ 110.3°,which can prevent the degradation/oxidation of the MXene under high humidity conditions. Interestingly, the PDMS@Ti3C2Tx/CNTs/Co film exhibited an excellent photothermal conversion performance with high thermal cycle stability and tenability. Thus, our multifunctional Ti3C2Tx/CNTs/Co nanocomposites possessed a unique blend of outstanding electromagnetic wave absorption and EMI shielding effi-ciency, light-driven heating performance, flexibility, and water-resistant features, which was highly promising for the next generation of intelligent electromagnetic attenuation systems.

AcknowledgementsThis project was supported by the China Postdoctoral Science Foundation (Grant No. 2020M671208), National Key Research and Development Program of China (Grant No.2019YFE0122900), National Natural Science Foundation of China(Grant No. 51971162, U1933112, 51671146), and the Program of Shanghai Technology Research Leader (Grant No. 18XD1423800).

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Supplementary InformationThe online version contains supplementary material available at https:// doi. org/ 10. 1007/s40820- 021- 00673-9.