a Laboratory for Manufacturing Low Carbon and Functionalized Textiles in the Universities of Shandong Province, College of Textiles & Clothing, Qingdao University, Qingdao 266071, China
b Research Centre of Textiles for Future Fashion, JC STEM Lab of Sustainable Fibers and Textiles, School of Fashion and Textiles, The Hong Kong Polytechnic University, Hong Kong 999077, China
c Collaborative Innovation Center for Eco-Textiles of Shandong Province and the Ministry of Education Collaborative, Qingdao University, Qingdao 266071, China
Microwave absorption (MA) materials are essential for protecting against harmful electromagnetic radiation. In this study, highly efficient and ultrawide-band microwave-absorbing fabrics with superhydrophobic surface features were developed using a facile dip-coating method involving in situ graphene oxide (GO) reduction, deposition of TiO2 nanoparticles, and subsequent coating of a mixture of polydimethylsiloxane (PDMS) and octadecylamine (ODA) on polyester fabrics. Owing to the presence of hierarchically structured surfaces and low-surface-energy materials, the resultant reduced GO (rGO)/TiO2-ODA/PDMS-coated fabrics demonstrate superhydrophobicity with a water contact angle of 159° and sliding angle of 5°. Under the synergistic effects of conduction loss, interface polarization loss, and surface roughness topography, the optimized fabrics show excellent microwave absorbing performances with a minimum reflection loss (RLmin) of −47.4 dB and a maximum effective absorption bandwidth (EABmax) of 7.7 GHz at a small rGO loading of 6.9 wt%. In addition, the rGO/TiO2-ODA/PDMS coating was robust, and the coated fabrics could withstand repeated washing, soiling, long-term ultraviolet irradiation, and chemical attacks without losing their superhydrophobicity and MA properties. Moreover, the coating imparts self-healing properties to the fabrics. This study provides a promising and effective route for the development of robust and flexible materials with microwave-absorbing properties.
Human living environments and health are increasingly threatened by electronic radiation from wireless communication equipment, high-power signal transmitters, and household network electronic equipment [1], [2], [3], [4], [5], [6], [7]. Thus, the development of high-performance microwave-absorbing materials that protect people from electromagnetic pollution has gained significant attention. Various microwave-absorbing materials, such as magnetic loss materials [8], dielectric loss materials [5], [9], [10], metamaterials, and chiral materials [11], [12], [13], [14], have been developed based on different working mechanisms. Carbon materials are one of the most commonly used microwave absorption (MA) materials because of their corrosion resistance, light weight, and diverse morphologies and structures [15], [16]. Wang et al. [17] prepared porous cocoon-like reduced graphene oxide (rGO) through a non-annealed freeze-drying process using ascorbic acid as the reducing agent. The developed material exhibited a minimum reflection loss (RLmin) of −29.05 dB at 15.96 GHz and an effective absorption bandwidth (EAB) of 5.27 GHz at a mass filling ratio of only 7.0%. However, pure carbon materials still face challenges in MA applications, such as small MA bandwidth and relatively low MA capacity.
Nonmagnetic carbon-based materials normally have limited attenuation ability toward electromagnetic waves. Therefore, various magnetic nanoparticles (NPs) have been introduced to improve their impedance matching and magnetic loss abilities [11], [18], [19], [20]. Zhou et al. [21] prepared CoNi@SiO2@C composites using a multistep method. When the loading of CoNi@SiO2@C-3 was 50 wt%, the RLmin reached −46 dB while the EABmax was approximately 5.6 GHz. Zhang et al. [22] prepared C/Fe3C nanocomposites by annealing the product from the chemical blowing of polyvinylpyrrolidone (PVP) and Fe(NO3)3·9H2O. When mixed with 90 wt% paraffin, the obtained C/Fe3C nanocomposites demonstrated an RLmin of −37.4 dB and an EABmax of 5.6 GHz. However, the application of nonconductive magnetic NPs as heterogeneous loadings in MA carbon materials also has negative effects, such as damage to the conductive network continuity and magnetic loss occurring mainly in the relatively low-frequency region.
Recently, flexible wearable MA materials demonstrating sufficiently high mechanical stability and flexibility have attracted considerable attention because of their high application potential in electromagnetic wave protection, aerospace, radar stealth, and many other fields [23]. Most existing MA materials cannot satisfy these requirements owing to their high mechanical brittleness, large loading, and narrow absorption bands [20], [21]. Fabric-based MA materials with high flexibility, an integrated molding structure, and mechanical and chemical resistance are ideal candidates for the fabrication of flexible wearable MA materials. Guo et al. [24] prepared WS2/CoS2@carbonized cotton fibers (CCF) via tungsten etching, vulcanization, and carbonization of Zeolitic imidazolate framework 67 (ZIF-67)/metal-organic frameworks (MOFs) nanosheet-anchored cotton fibers. The resultant WS2/CoS2@CCF/paraffin wax composite, which had a thickness of 2 mm, exhibited an RLmin and an EABmax of −51.26 dB and 6.72 GHz, respectively, at 17.36 GHz. Song et al. [25] constructed a three-dimensional (3D) conductive frame based on silica textile substrates through in situ reduction of GO and then immersed it in phenolic resin (PF) to obtain rGO/silica/PF textile composites. The obtained products, at a low rGO loading of 4.1 wt%, had an RLmin of −36 dB and effective absorption of the full X band.
The degradation of MA materials in harsh conditions (such as ultraviolet (UV) irradiation, washing, abrasion, and chemical corrosion) should also be considered. For instance, MA materials used in real environments are prone to contamination by dust or deterioration under wet conditions, resulting in reduced MA capability and a shortened service life span. To alleviate this problem, it is essential to impart durable liquid-repellent properties to MA fabric materials, which, to the best of our knowledge, have not yet been reported.
In this study, we developed a robust superhydrophobic and superior MA composite coating on polyethylene terephthalate (PET) fabrics using a two-step coating technique involving in situ graphene oxide (GO) reduction with the aid of a reducing agent (fatty acid methyl ester ethoxylate sodium sulfonate (FMES)). TiO2 NPs were deposited on the fabrics, followed by polydimethylsiloxane (PDMS)/octadecylamine (ODA) coating treatment. The coated fabric was superhydrophobic, with a water contact angle (WCA) of 159° and a small sliding angle (SA) of 5°. Owing to the combined effect of conduction loss, interface polarization loss, and their hierarchical surface structure, the optimized fabrics exhibit excellent microwave-absorbing performance with an RLmin of -47.4 dB and an EABmax of 7.7 GHz. These composite coatings are durable and can withstand harsh conditions without losing their superhydrophobicity and MA capabilities. Moreover, the coated fabrics self-heal, which further enhances the stability of the MA coating against various types of physical and chemical damage. This study provides a promising design inspiration for the development of flexible multifunctional fabrics with durable MA performance.
2. Experimental
2.1. Materials
FMES (C18H36CHSO3Na (OCH2CH2)7) was purchased from Shanghai Xihe Fine Chemical Co., Ltd. (China). TiO2 NPs (20 nm) were supplied by Aladdin (USA). A GO (solid content = 6 mg·mL−1) aqueous suspension was kindly provided by Changsheng Textile Technology Development Co., Ltd. (China). ODA was purchased from Macklin Biochemical Co., Ltd. (China). The polydimethylsiloxane resin (Sylgard 184) was purchased from Dow Corning (USA). Cyclohexane (CYH, analytical reagent (AR)), tetrahydrofuran (THF, AR), N,N-dimethylformamide (DMF, AR), HCl (37%) and alkali (NaOH, AR) were purchased from Sinopharm Group Chemical Reagents Co., Ltd (China). N-methylpyrrolidone (NMP, AR) and ethanol (AR) were obtained from Aladdin. The PET fabrics, milk, and milky tea were obtained from a local market.
2.2. Preparation of rGO/TiO2 coated fabric
Briefly, 10 g of FMES and 0.5 g of TiO2 NPs were added to a 100 mL GO dispersed aqueous solution (3 mg·mL−1) and ultrasonicated for 10 min to form a uniform GO/FMES/TiO2 nanoparticulate coating suspension. The PET fabric was dip-coated in the prepared suspension and then cured at 150 °C for 40 min to form an rGO/TiO2 coating and achieve the in situ reduction of GO on the fabric surface. Subsequently, an rGO/TiO2-coated fabric was obtained.
2.3. Preparation of rGO/TiO2-ODA/PDMS coated fabric
ODA (8 g) and PDMS (4 g, mass ratio was 10:1) were dissolved in a 40 mL ethanol solution by 30 min of magnetic stirring to form a homogeneous PDMS/ODA coating solution, which was then applied onto the as-prepared rGO/TiO2-coated fabric through dip-coating treatment. Finally, the rGO/TiO2-ODA/PDMS-coated fabric was obtained after curing at 150 °C for 30 min.
2.4. Characterizations
The surface morphology of the coated fabric materials was analyzed using scanning electron microscopy (SEM; Phenom Pure; Phenomenon World, the Netherlands) at an accelerated voltage of 10 kV. The crystal structure of the samples was characterized by X-ray diffraction (XRD; Ultima IV, Japon; Cu Kα radiation with a step size of 10 (°)·min−1). The chemical structure of the coating was analyzed by X-ray photoelectron spectroscopy (XPS; 60 kV, Cu target, −110°-+168°; Axis Supra+, Japan) and a laser microraman spectrometer (Raman; resolution < 2 cm−1, spectral repeatability over ±0.2 cm−1; inVia, UK). The WCA and SA were measured using a contact angle goniometer (Dataphysics OCA 25, Germany) and deionized water droplets of 5 and 30 μL, respectively; all the WCA and SA values represented the mean value of five measurements. The ultraviolet protection factor (UPF) was measured using a UV measuring system (YG(B)912E; Wenzhou Darong Textile Instrument Co., Ltd., China) to evaluate the UV protection performance of the coated fabrics. The air permeabilities of the fabrics before and after the coating treatment were tested using an air permeability meter (YG461E-II, China) following the SIST EN ISO 9237-1999 standard.
2.5. MA test
The relative permeability and permittivity of the coated fabrics were acquired using an E5080B-ENA vector network analyzer (VNA; Agilent, USA) in the frequency range of 2-18 GHz to calculate the reflection loss. The coated fabric samples (thickness of 0.5 mm) were cut into standard toroidal shapes with an outer diameter of 7 mm and an inner diameter of 3.04 mm using direct laser writing equipment (Spirit SI-60TI; GCC Laser Pro, USA). In addition, the reflection loss of the coated fabrics was simulated according to the transmission line theory.
2.6. Coating durability tests
The abrasion resistance of the coated fabrics was evaluated using the Martindale wear test machine according to the American Society for Testing Materials (ASTM) D4966 standard method [26], which used a rotation speed of 47.5 r·min−1 and a loading pressure of 9 kPa. The washing durability was tested according to the American Association of Textile Chemists and Colorists (AATCC) test method (61-2006 Test No. 2A) [27]. A standard washing cycle of 45 min was used, which is equivalent to five simulated household washes. The UV resistance of the coating was measured on a camera obscura UV lamp that had a wavelength of 254 nm and an intensity of 20.8 mW·cm−2. The UV protection (UPF value) performance of the coated fabric was measured using the standard AATCC-183 method [28]. The resistance to soiling of the MA fabrics was determined as follows: 40 g of soil was dropped into 40 mL of water to form a mud-water dispersion by stirring; the coated fabric was then soaked in the dispersion and stirred thoroughly, followed by washing in 40 mL of clean water. The treatment process was repeated for at least five cycles and the change in MA was measured.
3. Results and discussion
Fig. 1(a) illustrates the fabrication of the rGO/TiO2-ODA/PDMS coating on PET fabrics. A two-step dip-coating technique was used in this study. Briefly, the fabric was first coated in a TiO2/GO suspension containing FMES as the reducing agent for 40 min to allow the in situ reduction of GO; after heating at 150 °C for 30 min, a thin layer of rGO/TiO2 hierarchical coating was formed on the fabric surface; accordingly, the color of the fabric changed from its original white to black (Fig. S1 in Appendix A). The fabric was then dip-coated in the ODA/PDMS solution for 3 min under magnetic stirring to allow ODA grafting onto TiO2 and further decrease the surface free energy. After curing at 150 °C for 30 min, the cross-linked PDMS polymeric network immobilized the above coating materials on the fabric surface, hence improving the coating durability, and the final rGO/TiO2-ODA/PDMS-coated fabric was thus obtained. SEM images of the fabrics before and after the coating treatment are shown in Figs. 1(b)-(d). After the rGO/TiO2 coating treatment, the fabrics demonstrated a hierarchical surface morphology by integrating nanoscale TiO2 NPs and microscale rGO. The resulting hierarchical surface roughness may not only improve liquid repellency but also enhance MA capability. The base weights of the uncoated and coated fabrics were 220 and 237.5 g·m−2, respectively, with a coating weight of 17.5 g·m−2. The digital photographs in Fig. 1(e) verify that the coated fabric still exhibited excellent flexibility and remained stable on the flower without bending. More importantly, the presence of rGO and micro/nano-structured coating provided the fabric with outstanding MA capability, with an RLmin and an EABmax of -47.4 dB and 7.7 GHz, respectively.
Fig. 1(f) shows the wettability results of the rGO/TiO2-ODA/PDMS-coated fabric. After the coating treatment, the fabric became superhydrophobic (WCA of 159° and SA of 5°) and showed strong repellency to various aqueous liquids such as strong acid (HCl, pH 1), alkali (NaOH, pH 14) solutions, milk, salt water, and milky tea. All the droplets had a round shape and remained stable on the fabric surface without spreading until complete evaporation. When the coated fabric was immersed in water, an air cushion was formed, which reflected light and created a bright silver mirror effect in the submerged area. When the fabric was subjected to a continuous water jet, the water immediately slid from the surface upon contact, leaving no trace of water. This indicates that the coated fabric exhibits strong water repellency.
In this study, rGO-functionalized PET fabric was fabricated by in situ chemical reduction of GO using the SO32− anion in FMES, which has GO-reducing capability in weakly acidic aqueous solutions [29], [30]. To confirm the reduction of GO, GO and GO/FMES sheets were prepared and heated at 150 °C, respectively. The SEM images showed that, without FMES, the GO sheet had a relatively smooth surface. After applying FMES to GO, a rough surface morphology was observed, which was probably due to the shrinkage of the graphene layer and release of trace gases during the GO reduction process, indicating the reduction of GO (Figs. S2(a) and (b) in Appendix A) [31]. XPS analysis was conducted to further confirm GO reduction (Figs. S2(c) and (d) in Appendix A). The peaks at 284.8, 286.9, and 287.6 eV in the high-resolution C 1s spectra corresponded to C-C/C=C, C-O and C=O, respectively. The intensity of the C=O peak in GO/FMES was much lower than that in GO, indicating that a large quantity of GO was converted into rGO in the GO/FMES system. In the XRD patterns (Fig. S2(e) in Appendix A), a characteristic peak was observed at approximately 10.8°, which represents the typical diffraction pattern of the GO crystal structure (001) [32]. However, for GO/FMES, the characteristic peak at 10.8° disappeared, and a new peak appeared at 21.3°, confirming the partial depletion of oxygen-containing groups from the basal plane of GO and the reduction in the interlayer distance.
An XPS analysis of the rGO/TiO2-ODA/PDMS-coated fabric was conducted. After the coating treatment, C, O, Ti, N, and Si were observed in the XPS survey spectra (Fig. S3 in Appendix A), which verified the presence of TiO2, ODA, and PDMS in the coating layer. In the high-resolution C 1s spectrum (Fig. 2 (a)), the characteristic peaks at 284.8, 285.4, 286.0, and 286.9 eV can be attributed to C-C, C-N, C-O, and C=O, respectively. The small intensity of the C=O peak and the strong intensities of the C-C and C-O peaks were attributed to the reduction of GO, indicating an rGO coating on the surface of the fibers. In the Si 2p spectrum (Fig. 2(b)), the peaks at 99.7 and 100.6 eV indicated the presence of Si-C and Si-O groups from PDMS. In the Ti 2p spectrum, the peaks at 455.7 and 461.1 eV were attributed to Ti 2p3/2 and Ti 2p1/2, respectively, confirming the presence of TiO2 (Fig. 2(c)) [33].
EDS analysis was also performed to study the elemental compositions of the control fabric and the rGO/TiO2-ODA/PDMS-treated fabric, as shown in Figs. 2(d)-(g). C and O were only observed in the control fabric, with atomic weight ratios of 47.93% and 52.07%, respectively (Figs. 2(d) and (e)). After coating, C, O, Si, and Ti were observed with atomic ratios of 36.31%, 57.88%, 4.23%, and 1.58%, respectively, resulting from the rGO/TiO2-ODA/PDMS coating. Elemental distribution maps confirmed that the coating was uniformly distributed on the fabric surface, as shown in Figs. 2(f) and (g).
In the coating system, both the micro-scale rGO and nano-scale TiO2 NPs provided hierarchical surface roughness. PDMS is a hydrophobic polymer that not only renders the surface hydrophobic but also acts as a bonding agent to enhance the adhesion of the coating to the substrate. The long carbon chain of ODA grafted on TiO2 further reduced the surface free energy, thus improving the hydrophobicity of the coating. Therefore, a durable superhydrophobic coating was developed under the synergistic effect of all components in the coating system.
The durability of the coating against physical and chemical damage was also evaluated. Fig. 3(a) shows the changes in the WCAs and SAs of the coated fabric after repeated washing treatments. Increasing the number of washing cycles resulted in only a slight decrease in the WCA, and after 100 laundry cycles, the fabric still exhibited superhydrophobicity with a WCA of 155° and SA of 14°. A Martindale abrasion test was also conducted on the coated fabric (load pressure of 9 kPa). As shown in Fig. 3(b), after 500 abrasion cycles, the coated fabric exhibited WCA and SA values of 151° and 30°, respectively. Upon further increasing the abrasion to 1000 cycles, the fabric remained hydrophobic with a WCA of 143.9° (Fig. 3(c)). SEM images revealed that the coating on the top fiber surface was partially damaged, resulting in reduced surface roughness (Fig. S4(a) in Appendix A). However, after a 130 °C heating treatment for 10 min, the fabric recovered its superhydrophobicity with a WCA of 154.6°. In addition to self-healing from abrasion, the coating also recovered from plasma damage (Fig. 3(d)). After 3 min of plasma treatment, the coated fabric became hydrophilic, with a WCA of 0°. After heat treatment, the WCA increased to 157.6°, almost being restored to its original value. The plasma damage/recovery process is repeatable (Fig. 3(e)). When the coating was subjected to repeated abrasion or plasma treatment, the coating surface was damaged by the partial removal of ODA molecules or the introduction of hydrophilic groups, which increased the surface free energy and reduced the surface hydrophobicity. However, heat treatment can induce small hydrophobic ODA molecules inside the coating to migrate towards the surface under the driving force provided by the tendency to minimize the surface free energy, resulting in the re-accumulation of ODA on the coating surface and restoring its superhydrophobicity. A similar self-healing mechanism has been proposed in previous studies [34], [35].
The PDMS used in this work is a type of silicone elastomer that is stable and flexible between 50 and 200 °C. To investigate the effect of heat treatment on the coating, the coated fabric was subjected to a 200 °C heat treatment for 1 h. The coating did not exhibit cracks on the surface, as shown in Fig. S4(b) and Appendix A. The tensile strength results (Fig. 3(f)) showed that the coated fabric had slightly higher tensile stress than the control PET fabric after heat treatment at 200 °C, indicating that the tensile strength of PET fabrics was not affected by the heat treatment. In contrast, the PDMS coating could slightly improve the fabric’s mechanical strength. The chemical stability of the coated fabrics was evaluated by soaking them in various solvents (ethanol, CYH, DMF, NMP, THF, acid, and alkali solutions). As shown in Fig. 3(g), after 24 h of soaking, rinsing with deionized water, and drying at ambient temperature, all the fabrics exhibited WCAs exceeding 150°, indicating superior chemical stability.
The UV resistance of the coated fabrics was evaluated using a standard method. As shown in Fig. 3(h), prolonged UV exposure resulted in only a slight decrease in the WCA, whereas the SA gradually increased. After 500 h of UV irradiation, the WCA and SA values were 152° and 20°, respectively. When the UV irradiation was prolonged to 96 h, the coated fabric still exhibited a WCA of 150° (Fig. S4(c) in Appendix A). Moreover, the coated fabric exhibited high UV protection performance because of its significantly higher UPF value compared to the pristine PET fabric. The UV transmittance behaviors of the fabrics were demonstrated in the ultraviolet region. As shown in Fig. 3(i), the control fabric showed a UPF value of only 66.7, while the rGO/TiO2-ODA/PDMS-coated fabric exhibited a UPF value as high as 447.8. The UV-blocking property of the coated fabric is attributed to the synergistic effect of the UV absorbers rGO and TiO2. rGO has strong short-wave UV absorption capability, and its unique two-dimensional planar structure enables long-wave reflection. TiO2 (with an energy bandgap of 3.2 eV) can effectively absorb UV rays in the range of 200-400 nm [36], [37], [38], [39], [40].
An ideal MA material should have excellent impedance matching and strong microwave attenuation ability. Generally, RL ≤ –10 dB is associated with effective MA. Surprisingly, the rGO/TiO2-ODA/PDMS-coated fabric demonstrated excellent MA performance. The MA performances of the coated fabrics were assessed by measuring their key electromagnetic parameters, including complex dielectric constant (εr= ε′ − jε′′, εr is complex permittivity and j is an imaginary unit) and complex permeability (μr= μ′ − jμ′), where the real parts ε′ and μ′ represent the storage capabilities for electric energy and magnetic energy, respectively. The imaginary parts ε′′ and μ′′ represent the loss of electrical energy and magnetic energy, respectively [41], [42], [43]. The dependency of the real and imaginary parts of the complex dielectric constants on frequency (2–18 GHz) is indicated by the fact that both ε′ and ε′′ increase as the rGO loading increases and decrease as the frequency increases (Figs. S5 (a) and (b) in Appendix A). This is because the rGO dipole cannot adjust quickly in the presence of an applied electric field [44]. The dielectric loss (tanδε) and magnetic loss (tanδμ) of the coated fabrics to an electromagnetic field can be expressed by the loss angle tangent according to Eqs. (1), (2) [45], [46], [47], which are used to represent the loss capacity of electromagnetic waves. In general, a larger tanδε value means that the MA materials have a stronger loss effect on electromagnetic waves and better absorbing capability. When the loading of rGO was 6.9 wt%, the rGO/TiO2-ODA/PDMS-coated fabric had the largest tanδε, indicating excellent MA performance (Fig. S5(c) in Appendix A). Since carbon materials lack magnetism, the coating’s complex permeability remained essentially unchanged with μ′ and μ′′ at approximately 1 and 0, respectively (Fig. S5(d) in Appendix A), confirming that the rGO/TiO2-ODA/PDMS coating system did not involve magnetic loss.
The attenuation constant (α) and impedance matching (Z) are the key parameters for evaluating MA performances, which can be calculated using Eqs. (3), (4), respectively [5], [48], [49], [50].where Zin in Eq. (4) represents the input impedance, which is calculated using Eq. (5). c is the speed of light in free space, f represents the frequency of the electromagnetic wave, and d is the thickness of the electromagnetic wave absorption layer.
The good impedance matching of the MA materials is attributed to the fact that their Zin and Z0 are consistent. When the impedance-matching Z value is close to 1, microwaves are likely to be absorbed instead of reflected [51]. In addition to favorable impedance matching, a strong α is also a significant indicator for MA performance, which can directly reflect microwave attenuation ability. The larger the α value, the greater the ability to dissipate electromagnetic waves. As shown in Figs. 4 (a) and (b), the impedance matching value (Zin/Z0) and α curves indicated that α gradually increased with the increasing rGO loading. When the rGO loading was 6.9 wt%, the coated fabric had a moderate α value and good impedance matching, indicating that the microwave could enter the fabric interior and be absorbed instead of being reflected.
Cole-Cole plots have also been applied to explore the possible mechanism of MA, which describes the relationship between ε′ and ε′′. Each semicircle in the plots corresponds to one Debye relaxation process. As shown in Fig. 4(c), each downward semicircle is associated with a polarization relaxation phenomenon. The elongated tails and semicircles indicate the conduction and polarization losses. Moreover, the multiple resonant peaks in the middle- and high-frequency ranges indicate the existence of multiple polarization relaxation losses in the coated fabric [9], [52], [53]. These results indicate that the treated fabrics exhibit good impedance matching and high attenuation performance, with enhanced polarization relaxation and conduction loss.
Based on the measured complex permittivity and permeability, the RL values of the fabrics with different loadings were also calculated using the transmission line theory (Eq. (6)) [54], [55].
The reflection loss versus rGO loading is shown in Figs. 4(d)-(f). When the loading was 5.5 wt% (Fig. 4(d)), the coated fabric had an RLmin of -26 dB and showed inferior MA properties. After increasing the rGO loading to 6.9 wt%, the fabric exhibited improved MA property with an RLmin of −47.4 dB at 7.4 GHz, as shown in Fig. 4(e). Further increasing the rGO loading to 8.9 wt% resulted in reduced MA properties, with an RLmin of -41.8 dB at 7.1 GHz (Fig. 4(f)). The main reason for this could be an imbalance in impedance matching inside the coated fabrics, which led to a weakened absorption capacity. The RL curve also shows that the coated fabric with increasing coating thickness tended to absorb electromagnetic waves of longer wavelengths and lower frequencies, which conforms to the principle of quarter wavelength [56]. In addition, the EAB values of different coating thicknesses were calculated. When the rGO loading was 6.9 wt%, a broad EAB of 7.7 GHz was obtained with thicknesses of 3.5, 4.0, and 4.5 mm (Fig. S6(a) in Appendix A). For comparison, the RL of the uncoated fabric was also measured, and an RLmin of only -10 GHz was obtained when the thickness was 5.5 mm (Fig. S6(b) in Appendix A). These results indicate that after the rGO/TiO2-ODA/PDMS coating treatment, the fabric exhibited excellent MA properties. Moreover, the MA properties of the coated fabrics under different loadings were compared in detail, as shown in Figs. 4(g)-(i); when the loading was 6.9 wt%, an RLmin of -47.4 dB and an EABmax of 7.7 GHz could be achieved. Therefore, a loading of 6.9 wt% led to the most advantageous MA performance.
More importantly, our rGO/TiO2-ODA/PDMS-coated fabric exhibited durable MA properties under various harsh conditions, such as repeated washing, abrasion, and soiling. As shown in Figs. 5 (a) and (d), after 20 laundry cycles, the coated fabric still showed an RLmin and an EABmax of -36.3 dB and 7.4 GHz, respectively. The MA properties of the coated fabric after 100 abrasion cycles, at a loading pressure of 9 kPa, are given in Figs. 5(b) and (e); although the RLmin value dropped to -22.2 dB after 100 abrasion cycles, the EABmax still remained at 7.0 GHz. The decrease in RLmin was probably due to the removal of rGO from the top fabric surface under the action of the abrasion force, and the damage to the conductive network resulted in the weakening of the MA (Fig. S7 in Appendix A). The dielectric constant of water is 78.36 F·m−1 at 25 °C, and the imaginary part is very small. When MA materials are wetted, their dielectric constants increase, leading to an imbalance in the internal impedance matching. As a result, the MA properties can be critically weakened or eliminated. The silicone elastomer PDMS clearly affected the hydrophobicity and MA properties of the rGO/TiO2-ODA/PDMS coating. In the absence of PDMS, the rGO/TiO2-ODA-coated fabric only demonstrated hydrophobicity with the largest WCA of 147° (ODA concentration = 2 wt%), as shown in Fig. S8(a) in Appendix A. Figs. S8(b)-(d) in Appendix A show the MA properties of the rGO/TiO2-ODA-coated fabric after wetting with water. The Zin/Z0 of the wetted fabric was far from 1, and the impedance matching was unbalanced. When the RLmin was only −14 dB, the MA properties of the wetted fabric decayed significantly. In comparison, the rGO/TiO2-ODA/PDMS coating with superhydrophobicity was non-wettable, and the wet environment had almost no influence on the MA properties. Therefore, water-repellent properties are crucial for MA. The soiling test results for the coated fabric are shown in Figs. 5(c) and (f). After five cycles of soiling, the fabric retained its excellent MA properties, with RLmin and EABmax values of -41.5 dB and 7.7 GHz, respectively. When subjected to soiling, the fabric surface remained clean without any contamination owing to the surface superhydrophobicity, which rendered the fabric self-cleaning (Video S1 in Appendix A).
The outstanding properties of the rGO/TiO2-ODA/PDMS-coated fabric were attributed to the synergistic effects of the coating materials. The underlying rGO/TiO2 coating provided high MA absorption capability and micro/nanoscale surface roughness, and the top ODA/PDMS coating endowed the fabric with durable and self-healing superhydrophobicity. Based on the above analysis, the possible mechanisms of electromagnetic wave absorption and superhydrophobicity of the coated fabric can be explained as shown in Fig. 6. The MA properties of the coated fabrics were mainly derived from the dielectric loss induced by conduction loss, dipole polarization, and interfacial polarization, which play key roles in the absorption of electromagnetic waves (Fig. 6(a)). GO contains numerous oxygen-containing groups at the plane edge of the graphene sheets and is chemically reactive. After the partial reduction of these functional groups, the resultant rGO exhibited improved electrical conductivity and simultaneously retained the defective structure. rGO can also build a conductive network between fibers, resulting in conduction loss, which is reflected by the decreasing loss of electrical energy (ε′′) with the increasing frequency of the electromagnetic wave. When electromagnetic waves collide inside a coating, electrons and holes move under the excitation of an electric field to generate a current. Owing to electrical resistance, the current generates heat such that the electromagnetic energy is dissipated in the form of heat. Defects in rGO create local dipoles, and under the action of an alternating electromagnetic field, dipole alignment is induced to produce dipole polarization. This results in dielectric loss, which causes electromagnetic wave attenuation. In addition, owing to the existence of numerous heterogeneous interfaces between rGO, GO, and TiO2, interface polarization occurs, resulting in enhanced multireflection and electromagnetic wave scattering within the coating layer and further improving the attenuation of electromagnetic waves.
The ODA and PDMS in the top coating layer played a key role in reducing the surface energy of the coated fabric. In addition, the TiO2 particles and rGO sheets in the underlayer form a rough structure, thereby imparting superhydrophobicity to the coated fabric. PDMS is a silicone elastomer that strongly adheres to fabric substrates, endowing the coating with outstanding washing and abrasion fastness. It also protects the rGO/TiO2 MA coating from various types of damage (e.g., washing, abrasion, or sewage erosion). After plasma treatment, repeated washing, and wear treatment, the creation of hydrophilic polar groups or the loss of hydrophobic ODA molecules on the fiber surface causes an increase in surface energy and deteriorates surface hydrophobicity. Heat treatment induces ODA molecules inside the PDMS matrix to migrate to the surface to minimize the surface energy, and the reaccumulation of ODA on the surface restores the superhydrophobicity of the coating (Fig. 6(b)).
The functional fabrics with MA properties reported in recent years are summarized in Table 1 [24], [25], [54], [55], [56], [57], [58], [59], [60], [61], [62]. For comparison, our developed rGO/TiO2-ODA/PDMS-coated fabrics exhibited several advantages: ➀ The coated fabric only had an rGO loading weight of 6.9 wt%, which is significantly smaller than the majority of their counterparts reported. ➁ The fabric demonstrated high-efficiency ultrawide-band MA and ➂ superhydrophobic features for durable MA performance. Although MA samples with WCA values greater than 150° have already been reported, the effect of superhydrophobicity on MA performance has not yet been discussed. In this study, the durability test results verified that the superhydrophobicity of the rGO/TiO2-ODA/PDMS coating not only improved the MA durability but also expanded its application range. ➃ The fabrication strategy developed in this study is simple and has potential for industrial applications.
4. Conclusions
Superhydrophobic fabrics showing high-efficiency MA at ultrawide absorption bandwidths were successfully developed using a facile two-step dip-coating method. Under the synergistic effect of all the coating materials, the coated fabrics exhibit excellent MA capability, with an RLmin of -47.4 dB and an EABmax of 7.7 GHz. The coating was robust and could withstand repeated washing, abrasion, long-term UV irradiation, and chemical attacks without losing its superhydrophobicity and MA capability. Moreover, the coating exhibited a self-healing ability, which further enhanced the stability of the MA coating against physical and chemical damage. This simple and effective coating system provides a promising strategy for the development of multifunctional fabrics with durable MA performance.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (22372087), the Natural Science Foundation of Shandong Province (ZR2021ME039), the Applied Basic Research Programs of National Textile Industry Federation (J202106), the Newtech Textile Technology Development (Shanghai) Co., Ltd., China, and the Jiangsu New Vison Advanced Functional Fiber Innovation Center. Xungai Wang wishes to acknowledge support from both the Research Centre of Textiles for Future Fashion at The Hong Kong Polytechnic University and The Hong Kong Jockey Club Charities Trust.
Compliance with ethics guidelines
Zhong Zhang, Yaxin Meng, Xinrui Fang, Qing Wang, Xungai Wang, Haitao Niu, and Hua Zhou declare that they have no conflict of interest or financial conflicts to disclose.
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