Here, we report the direct writing of laser-induced graphene (LIG) on a Kevlar textile, which is further used in the fabrication of intelligent protective clothing. The transformation of Kevlar into graphene can be attributed to the photothermal effect induced by laser irradiation. The Janus graphene/Kevlar textile, which has porous graphene on the front side and Kevlar fibers on the back side, can be prepared in air with optimizing the experimental conditions (laser power of 6.5 W and writing speed of 50 mm·s –1 ). This structure enables the introduction of functions into the textile while retaining its wearing comfort. The as-prepared LIG exhibits high electrical conductivity (10.6 Ω, 2 × 0.5 cm 2 ), granting its applications in electronics. We demonstrated the applications of the graphene/Kevlar textile in flexible devices, including Zn–air batteries, electrocardiogram (ECG) electrodes, and NO 2 sensors. As a proof of concept, we fabricated self-powered protective clothing for detecting NO 2 based on the graphene/Kevlar textile.

Graphene, due to its high electrical conductivity, light weight, and outstanding stability, is a promising candidate for fabricating flexible and wearable electronics. (14−16) Typical graphene-based electronics are fabricated by transferring as-prepared graphene onto the supporting materials. (17−19) In this case, the graphene tends to exfoliate from the supporting materials due to the weak interactions in the interface, which can result in the degradation of the device performance. Researchers recently reported that graphene could be prepared through directly laser scribing on polyimide, wood, or paper. (20−23) This laser writing method for preparing graphene is simple, efficient, and design-flexible. On the basis of the fact that there are similar polymer structures in textiles with the above-reported precursors, we proposed that the laser writing technique may be applied on polymer textiles, which enables the facile fabrication of graphene-based textile electronics.

Personal safety and security are of great importance for human society. Protective clothing, as an important part of personal protective wearables, plays a vital role in the electronic industry, medical service, firefighting, and private security. (1−4) Ideal protective clothing should not only be able to protect the human body from injury but also have intelligent functions such as monitoring physiological signals and detecting potential hazard. High-performance polymer textiles, represented by Kevlar, are an indispensable component of protective clothing due to their excellent mechanical performance. (5−7) In order to introduce other functions into the textile, it is necessary to combine functional materials with fibers or textiles. The typical state-of-the-art fabrication processes to functionalize textiles are dipping, evaporation, and wax printing, through which functional components such as carbon nanotubes, graphene, Ag, or poly(3,4-ethylenedioxythiophene) can be deposited onto textiles. (7−10) Functionalized textiles could be used for monitoring sleeping, radiative outdoor cooling, and diagnosing disease. (11−13) Generally, these methods require multistepped routes and time-consuming precursor preparation processing. (7−10,14) Therefore, it is still challenging to fabricate intelligent protective clothing through a straightforward approach, especially those with arbitrary patterns or custom-designed functions.

Results and Discussion ARTICLE SECTIONS Jump To

Figure 1 Figure 1. Formation of graphene on a Kevlar textile induced by laser writing. (a) Schematic of the fabrication of the graphene/Kevlar textile from Kevlar. (b) SEM image of LIG patterned into the shape of a bird. (c) Digital image of LIG patterned into a human side face shape. (d) SEM image of the LIG circled in (b). (e) Magnified SEM image of the LIG corresponding to the square area in (d). (f) TEM image of the LIG. Average lattice space is ∼3.4 Å, which is consistent with the distance between neighboring (002) planes in few-layer graphene. Inset is the SAED pattern derived from (f). (g) Raman spectra of the LIG and Kevlar fiber. (h) High-resolution XPS C 1s spectrum of the LIG. (i) XRD curve of the LIG.

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that LIG possesses a porous morphology, nanoshaped ripples, and few-layer carbon atom features ( Figure 1 d–f and Figure S6 ). As seen in Figure 1 e, the as-obtained material showed a continuous and porous morphology, indicating the merging of neighboring Kevlar fibers during the laser-scribing process. The average lattice space was ∼0.34 nm ( Figure 1 f), which was consistent with the distance between neighboring (002) planes in few-layer graphene. Furthermore, the selected area electron diffraction (SAED) pattern of LIG shows intrinsic features of a graphitic polycrystal, which was significantly different from that of the lacey carbon film on the TEM grid (inset of Figure 1 f and Figure S7 ).

–1), the G peak (∼1580 cm–1), and the 2D peak (∼2700 cm–1) of graphene. The 2D peak centered at 2700 cm–1 is consistent with that of bilayer graphene,2-carbons. The sp2-carbons also leads to a shift of the XPS C 1s spectrum of LIG compared to that of the Kevlar textile. The X-ray diffraction (XRD) result also evidenced the formation of LIG. The XRD pattern of Kevlar showed intense peaks centered at 20.5°, 22.8°, and 28.6°, corresponding to (110), (200), and (004) diffractions The porous material exhibited three prominent peaks in the Raman spectrum ( Figure 1 g), which was different from that of pristine Kevlar. The three peaks correspond to the D peak (∼1350 cm), the G peak (∼1580 cm), and the 2D peak (∼2700 cm) of graphene. The 2D peak centered at 2700 cmis consistent with that of bilayer graphene, (24,25) indicating the formation of few-layer graphene. Our experimental results showed that there were significant differences between the spectrum of LIG and that of carbonized Kevlar textile at 1050 °C ( Figure S8 ) or glassy carbon, (26) which could be understood considering the fact that the LIG was highly graphitized carbon obtained with high temperature (∼2800 K), in contrast to the carbon materials with a low degree of graphitization or even amorphous carbon. The X-ray photoelectron spectroscopy (XPS) C 1s spectrum of LIG showed a peak of C–O and a lower C–N peak compared with that of Kevlar ( Figure 1 h and Figure S9 ), indicating that LIG was dominated by sp-carbons. The sp-carbons also leads to a shift of the XPS C 1s spectrum of LIG compared to that of the Kevlar textile. The X-ray diffraction (XRD) result also evidenced the formation of LIG. The XRD pattern of Kevlar showed intense peaks centered at 20.5°, 22.8°, and 28.6°, corresponding to (110), (200), and (004) diffractions (27) Figure S10 ), respectively. In contrast, the XRD pattern of LIG revealed intense peaks centered at 25.9° and 42.9°, which agreed with an interlayer spacing (∼0.34 nm) between (002) planes and (100) reflections in graphitic materials, respectively ( Figure 1 i). Furthermore, we found there is no significant difference in Raman spectra of the LIGs obtained from aramid textiles with different crystallinity ( Figure S11 ). Besides, it should be noted that all the experiments in this work were carried out in air conditions, and different results may be obtained under other gas atmospheres. (28)

Figure 2 Figure 2. Laser-induced transformation of Kevlar into graphene-functionalized textile. (a) Schematic illustration showing the transformation of Kevlar to LIG. (b) GC-MS chromatogram of the gas from laser irradiation of Kevlar. (c) Major compounds identified from peaks i to x in the GC-MS chromatogram in (b).

–1). The I G /I D ratio increased monotonically from 1.5 to 6.2 as the laser power rose from 5.5 W to 6.5 W, which may be attributed to increased temperatures under high laser power.I G /I D ratio showed small variation when the laser power rises from 6.5 W to 8.0 W (L a value (the crystalline size on the a axis) calculated with the equation in the R) of LIG (2 × 0.5 cm2) with the increasing of the laser power (R > 1.5 × 109 Ω) with the laser power below the threshold power of 5.5 W, while the R of LIG gradually decreased to ∼10.6 Ω as the laser power increases to 8.0 W. Furthermore, the R of LIG (2 × 0.5 cm2) increased from 10.8 to 1.2 × 106 Ω as the writing speed increased from 20 to 350 mm·s–1, at a fixed laser power of 6.5 W (R of LIG. On the one hand, severe damage of the Kevlar textile would occur under a high laser power or slow writing speed such as 8.0 W and 20 mm·s–1 due to the large energy input per unit area (–1 could result in failed LIG conversions because of the little energy input per unit area (–1, To optimize the properties of the graphene/Kevlar textile, we systematically investigated the structure-processing-property relationship of the laser writing process. Figure 3 a exhibited representative Raman spectra of the LIG obtained under different laser powers (5.5–8.0 W), with 0.5 W increments at fixed writing speed (50 mm·s). Theratio increased monotonically from 1.5 to 6.2 as the laser power rose from 5.5 W to 6.5 W, which may be attributed to increased temperatures under high laser power. (20,34) However, theratio showed small variation when the laser power rises from 6.5 W to 8.0 W ( Figure S16 ), indicating the formation of high-graphitized graphene during these processes. Thevalue (the crystalline size on theaxis) calculated with the equation in the Methods section was ∼120 nm using a laser power of 6.5 W ( Figure S16 ), further indicating that 6.5 W was the best choice to fabricate LIG among different laser powers. Notably, there was a transition of resistance () of LIG (2 × 0.5 cm) with the increasing of the laser power ( Figure 3 b). The samples were insulators (> 1.5 × 10Ω) with the laser power below the threshold power of 5.5 W, while theof LIG gradually decreased to ∼10.6 Ω as the laser power increases to 8.0 W. Furthermore, theof LIG (2 × 0.5 cm) increased from 10.8 to 1.2 × 10Ω as the writing speed increased from 20 to 350 mm·s, at a fixed laser power of 6.5 W ( Figure 3 c). The energy input under different conditions contributed to the transition ofof LIG. On the one hand, severe damage of the Kevlar textile would occur under a high laser power or slow writing speed such as 8.0 W and 20 mm·sdue to the large energy input per unit area ( Figures S17 and S18 ). On the other hand, the low laser power or fast writing speed such as 5.0 W and 350 mm·scould result in failed LIG conversions because of the little energy input per unit area ( Figures S17 and S18 ). A Janus graphene/Kevlar textile, which has porous graphene on the front side and Kevlar fibers on the back side, can be prepared under optimizing the experimental conditions (laser power of 6.5 W and writing speed of 50 mm·s Figure S19 ). Besides, the mechanical properties of the optimized Janus graphene/Kevlar textile were also investigated ( Figure S20 ), and the results indicated that the Janus graphene/Kevlar textile possessed comparable mechanical strength with the pristine Kevlar textile. We used these experimental conditions to fabricate graphene/Kevlar textile based high-performance flexible electronics.

Figure 3 Figure 3. (a) Raman spectra of the LIG prepared with different laser powers. (b) Resistance of LIG samples (2 × 0.5 cm2) produced by different laser powers under a fixed scan rate of 50 mm·s–1. (c) Resistance of graphene/Kevlar textile samples (2 × 0.5 cm2) produced by different scan rates under a fixed laser power of 6.5 W.

3 O 4 -graphene/Kevlar textile as a cathode, which was prepared through electrodeposition and annealing processes. The SEM image, XPS Co 2p spectrum, and X-ray spectroscopy (EDS) of the Co 3 O 4 -graphene/Kevlar textile all demonstrated the successful deposition of Co 3 O 4 on the graphene/Kevlar textile (–2 in 1.0 M KOH, 3 O 4 -graphene/Kevlar textile exhibited better oxygen evolution reaction (OER) catalytic activity (∼1.57 V), which was comparable to that of commercial RuO 2 (∼1.55 V, 3 O 4 nanosheets offer a large surface area and numerous active sites, which are beneficial for high OER performance. 3 O 4 -graphene/Kevlar textile also showed good oxygen reduction reaction (ORR) catalytic activity in terms of positive onset potential (∼0.94 V) and half-wave potential (E 1/2 = ∼0.79 V) in O 2 -saturated 1.0 M KOH solution ( 3 O 4 and LIG provide nice ORR activity ( As a proof of concept, we demonstrated the applications of the graphene/Kevlar textile in flexible devices, including Zn–air batteries, ECG electrodes, and gas sensors. In order to achieve Zn–air batteries with high electrochemical performances, we used a Co-graphene/Kevlar textile as a cathode, which was prepared through electrodeposition and annealing processes. The SEM image, XPS Co 2p spectrum, and X-ray spectroscopy (EDS) of the Co-graphene/Kevlar textile all demonstrated the successful deposition of Coon the graphene/Kevlar textile ( Figure 4 a,b and Figure S21 ). Compared with the pure graphene/Kevlar textile (∼2.03 V, 10 mA·cmin 1.0 M KOH, Figure 4 c), the Co-graphene/Kevlar textile exhibited better oxygen evolution reaction (OER) catalytic activity (∼1.57 V), which was comparable to that of commercial RuO(∼1.55 V, Figure S22 ). The Conanosheets offer a large surface area and numerous active sites, which are beneficial for high OER performance. (35,36) The Co-graphene/Kevlar textile also showed good oxygen reduction reaction (ORR) catalytic activity in terms of positive onset potential (∼0.94 V) and half-wave potential (= ∼0.79 V) in O-saturated 1.0 M KOH solution ( Figure S23 ). The coordination of N with Co cations and synergistic coupling between Coand LIG provide nice ORR activity ( Figures S24 and S25 ). (37,38)

Figure 4 Figure 4. Electrochemical performances of flexible all-solid-state Zn–air battery based on graphene/Kevlar textile. (a) SEM image of the Co 3 O 4 -graphene/Kevlar textile electrode. Upper right inset is the corresponding higher magnification SEM image. (b) High-resolution XPS Co 2p spectrum of the Co 3 O 4 -graphene/Kevlar textile. (c) OER polarization curves of the graphene/Kevlar textile and Co 3 O 4 -graphene/Kevlar textile electrode in 1.0 M KOH solution. (d) Schematic of the textile-based Zn–air battery. (e) Rate discharge curves of the textile-based Zn–air batteries at different current densities. (f) Galvanostatic discharge–charge cycling curves at 1.0 mA cm–2. (g, h) Photographs of a green LED (∼3.0 V) powered by three textile-based Zn–air batteries connected in series at various bending conditions. These Zn–air batteries are hid in a lotus flower (g) and heart shape (h), respectively.

3 O 4 -graphene/Kevlar textile cathode showed a high open-circuit voltage (1.37 V, 3 O 4 -graphene/Kevlar textile cathode showed higher discharge voltage plateaus at the same current densities and better rate capability than the battery with a graphene/Kevlar textile cathode (–2, A rechargeable flexible Zn–air battery was fabricated on the basis of the promising bifunctional catalytic activities presented above ( Figure 4 d and Figure S26 ). The battery with a Co-graphene/Kevlar textile cathode showed a high open-circuit voltage (1.37 V, Figure S26 ), which was 0.21 V higher than that of the battery with a pristine graphene/Kevlar textile cathode (1.16 V, Figure S27 ). The battery with a Co-graphene/Kevlar textile cathode showed higher discharge voltage plateaus at the same current densities and better rate capability than the battery with a graphene/Kevlar textile cathode ( Figure 4 e), which can be ascribed to the difference in the catalytic activities of the cathodes. This difference was pronounced with increasing current density. Thirty discharge–charge cycles were achieved between 1.0 and 2.0 V (1.0 mA·cm 4 Figure f), indicating the excellent cycling performance of the Zn–air battery. The performances of this textile-based Zn–air battery are comparable with or even superior to results reported recently. (38−41) In particular, our Zn–air batteries can be merged into various patterns ( Figure 4 g and h), benefited from the custom design of the textile. We further demonstrated that three Zn–air batteries connected in series could power a green LED at various bending conditions, indicating the excellent flexibility of our textile-based Zn–air batteries.

P-wave, QRS complex, and T-wave were shown in the magnified ECG signal ( Ideal protective clothing should not only be able to protect the human body from injury but also have intelligent functions such as monitoring physiological signals and detecting potential hazards. High-quality ECG signals were recorded by the electrodes made of graphene/Kevlar textile, which were mounted onto a volunteer’s arms ( Figure 5 a,b and Figure S28 ). Furthermore, clearwave,complex, andwave were shown in the magnified ECG signal ( Figure 5 c), indicating the high performance of the graphene/Kevlar electrode. (21) It is noted that the textile structure endows the fabricated graphene/Kevlar textile electrode with good air permeability, ensuring the comfort of the volunteer.

Figure 5 Figure 5. Graphene/Kevlar textile for an ECG electrode. (a) Schematic illustration showing the graphene/Kevlar textile electrode for ECG measurement. (b) ECG signals from the arms measured by graphene/Kevlar textile electrodes. (c) Magnified ECG signal indicating clear P-wave, QRS complex, and T-wave.

2 gas sensors. With an increasing NO 2 concentration from 10 to 200 ppm, the absolute value of the response of the sensor (|ΔR|/R 0 ) increases monotonically from 0.33% to 4.02% ( 2 is readily comprehensible. Graphene is a p-type semiconductor and NO 2 is an electron-withdrawing molecule. 2 , thereby the resistance of the sensor would decrease. The as-prepared gas sensor exhibited a stable response (ΔR/R 0 = −2.87%) with a small standard deviation (0.06%) under 100 ppm of NO 2 for three successive cycles ( 2 (|ΔR|/R 0 = 2.95%, 100 ppm) compared with some other gases including Cl 2 (|ΔR|/R 0 = 0.85%, 2500 ppm), methanoic acid (|ΔR|/R 0 = 0.67%, 2500 ppm), and acetic acid (|ΔR|/R 0 = 0.49%, 2500 ppm), even though the concentrations of the others were 25 times higher than the NO 2 . Similar to the reported mechanism, 2 is a typical strong electron-withdrawing gas, while the interferential gases possess weak electron-withdrawing or -donating groups and are, thus, unable to cause a significant change in resistance. Therefore, the graphene/Kevlar textile sensors are selectively sensitive to NO 2 . The performances of the graphene/Kevlar textile sensors are comparable with recently reported graphene-based gas sensors. The graphene/Kevlar textile electrodes could also be used as NOgas sensors. With an increasing NOconcentration from 10 to 200 ppm, the absolute value of the response of the sensor (|Δ) increases monotonically from 0.33% to 4.02% ( Figure 6 a). The response mechanism of the graphene/Kevlar textile sensor toward NOis readily comprehensible. Graphene is a p-type semiconductor and NOis an electron-withdrawing molecule. (42) The hole concentration in LIG would rise after it adsorbs NO, thereby the resistance of the sensor would decrease. The as-prepared gas sensor exhibited a stable response (Δ= −2.87%) with a small standard deviation (0.06%) under 100 ppm of NOfor three successive cycles ( Figure 6 b), indicating its excellent repeatability. As shown in Figure 6 c, the sensor showed a higher absolute value of response toward NO(|Δ= 2.95%, 100 ppm) compared with some other gases including Cl(|Δ= 0.85%, 2500 ppm), methanoic acid (|Δ= 0.67%, 2500 ppm), and acetic acid (|Δ= 0.49%, 2500 ppm), even though the concentrations of the others were 25 times higher than the NO. Similar to the reported mechanism, (42,43) NOis a typical strong electron-withdrawing gas, while the interferential gases possess weak electron-withdrawing or -donating groups and are, thus, unable to cause a significant change in resistance. Therefore, the graphene/Kevlar textile sensors are selectively sensitive to NO. The performances of the graphene/Kevlar textile sensors are comparable with recently reported graphene-based gas sensors. (43−47)

Figure 6 Figure 6. Integrated intelligent protective clothing based on graphene/Kevlar textile. (a) Response curves of the graphene/Kevlar textile gas sensor to different NO 2 concentrations (10–200 ppm). (b) Three cycle response curves of the LIG-Kevlar gas sensor upon exposure to 100 ppm of NO 2 . (c) Selective response of the graphene/Kevlar textile gas sensor toward 100 ppm of NO 2 and 2500 ppm of some other interferential gases, including Cl 2 , methanoic acid, and acetic acid. (d) Response and recovery curves of the graphene/Kevlar textile gas sensor driven by the textile-based Zn–air battery when inputting 200 ppm of NO 2 . (e) Digital image of the graphene/Kevlar textile based integrated intelligent protective clothing. (f, g) Magnified digital image of the self-powered system corresponding to the square area in (e) with primary colors (f) and false color (g), respectively.