Self-Cleaning, Chemically Stable, Reshapeable, Highly Conductive

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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Self-Cleaning, Chemically Stable, Reshapeable, Highly Conductive Nanocomposites for Electrical Circuits and Flexible Electronic Devices Ximing Zhong, Hengfeng Hu, and Heqing Fu* School of Chemistry and Chemical Engineering, Guangdong Provincial Key Lab of Green Chemical Product Technology, South China University of Technology, Guangzhou 510640, P.R. China

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ABSTRACT: Materials with multiple functions are highly desirable in practical applications. Developing multifunctional nanocomposites by a straightforward process is still a challenge. Here, a versatile nanocomposite has been developed by simple blending and pressing of multiwalled carbon nanotubes (MWCNTs) and modified polydimethylsiloxane (MPDMS). Because of the synergistic effect of MWCNTs and MPDMS, this nanocomposite exhibits outstanding hydrophobic property, striking self-cleaning capability, and excellent chemical stability against strong acid and strong base, which makes it possible to work under wet and even extreme chemical conditions. Besides, because of its flexibility, this nanocomposite can be reshaped, bended, twisted, and molded into on-demand patterns for special applications. Owing to the good distribution of MWCNTs, the nanocomposite shows high conductivity (with a sheet resistance of 86.33 Ω sq−1) and high healing efficiency (above 96.53%) in an electrical field, and it also exhibits outstanding performance in various electrical circuits and flexible electroluminescent devices. Furthermore, the inherent portability, recyclability, and reusability of this nanocomposite make it more convenient and environmentally friendly for practical applications. Thus, our work provides a new strategy to develop a multifunctional nanocomposite, and it shows tremendous potential in flexible electronics. KEYWORDS: multifunctional nanocomposites, reshapeable, highly conductive, electrical healing, self-cleaning, chemically stable, flexible electronics

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INTRODUCTION Flexible conductive nanocomposites, generally composed of electrically conductive materials and flexible polymer matrices, are of importance for their tremendous potential in electronic devices,1,2 sensors,3−7 actuators,8,9 and energy storage devices.10,11 A substantial amount of effort has been put on the research of conductive components.12 To obtain high electrical conductivity, metal nanoparticles/nanowires,13,14 ionic liquids,15 conductive polymers,16,17 and carbon-based fillers,18,19 such as carbon nanotubes (CNTs) and graphene, are used frequently. Although exhibiting striking conductivity, silver nanowires, silver flakes,20 and ionic liquids are costly. For organic conductive polymers, low conductivity and aging problem limit their further applications. CNTs exhibit not only high conductivity but also superlative mechanical and thermal properties, which make them an ideal candidate for high-performance and multifunctional nanocomposites,21 and the resultant nanocomposites inherently exhibit favorable characteristics, such as lightweight, costeffectiveness, facile processability, and scalability.22 Generally, in insulating polymer matrices, by increasing the contents of © XXXX American Chemical Society

conducting nanofillers, the transition from insulator to conductor occurs in nanocomposites, and the critical nanofiller content for conductive behavior is referred to as the percolation threshold. The CNT content for a conductive nanocomposite in most polymer matrices is below 5 wt % depending on their aspect ratio and dispersion state.23,24 Furthermore, the excellent chemical stability provided by CNTs endows nanocomposites with high corrosion resistance against various climatic actions and aggressive media.25 Aside from the abovementioned merits, the hydrophobicity and roughness of CNTs make them potentially applicable in superhydrophobic functional nanocomposites. To combine the superhydrophobicity and conductivity of multiwalled CNTs (MWCNTs), Zhang et al. fabricated a smart coating with superhydrophobic property against water and acid/alkali stress for wearable sensing electronics by spray-coating MWCNTs dispersed in a thermoplastic elastomer solution, followed by Received: May 9, 2018 Accepted: July 6, 2018 Published: July 6, 2018 A

DOI: 10.1021/acsami.8b07575 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces ethanol treatment.26 To the best of our knowledge, superhydrophobic surfaces, generally created by etching,27 templating,28 electrospinning,29 layer-by-layer growth,30 and lithographing,31 followed by posttreatment with low-surface-energy materials, are typically characterized by a high water static contact angle (>150°) and a low sliding angle (95 wt % purity) were purchased from Nanjing XFNANO Materials Tech Co., Ltd. Methoxy-2-propyl B

DOI: 10.1021/acsami.8b07575 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration and essential characterizations of the MWCNT/MPDMS nanocomposite. (a) Fabrication procedure of the nanocomposite. (b) Optical image of a nanocomposite film. (c) SEM image showing the morphology of a nanocomposite surface. (d) Comparison of XRD spectra of the MWCNT, MPDMS, and MWCNT/MPDMS nanocomposite.

Figure 2. Hydrophobicity, chemical stability, and self-cleaning capability performed on a nanocomposite. (a) XPS analysis of the nanocomposite. (b) Variation of the contact angle as a function of droplets (5.0 μL in volume) with different pH values. Acid solutions correspond to the aqueous H2SO4 solution with various concentrations, whereas base solutions represent the aqueous NaOH solution with various concentrations. Inset images are the corresponding droplets on the nanocomposite surface. (c) Time-lapse photographs of water (pH = 7), strong acid (aqueous H2SO4 solution, pH = 1), and strong base (aqueous NaOH solution, pH = 14) droplets (0.5 mL in volume) absorbed by a homemade tissue strip with a pH-indicator paper inserted. Droplets gradually shrank and were finally absorbed, leaving no trace on the nanocomposite surface. (d) Snapshots showing the movement of water, strong acid, and strong base droplets (40.0 μL in volume) on a nanocomposite surface at various times. When the pH-indicator paper is wetted by water, strong acid, and strong base, its color will become yellow, red, and dark violet, respectively, as seen in (c,d) (scale bar, 10 mm). pastes were used as electrodes. Except the nanocomposite with a dimension of 10 × 10 × 10 mm3, which was used for IV curve measurement to demonstrate isotropic conductivity, other nanocomposites employed for IV curve measurement had a dimension of 15 × 4 × 4 mm3. Also, the nanocomposites immersed in solvents for

5° to 80°. An RTS-9 four-probe detector from Guangzhou 4 PROBS TECH was employed to measure the sheet resistance, electrical resistivity, and conductivity of the nanocomposite. Also, the IV curves were recorded by a CHI 660E electrochemical workstation with a maximum voltage of 10.0 V and a maximum current of 0.25 A. Silver C

DOI: 10.1021/acsami.8b07575 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces 24 h were rinsed with deionized water for 5 min and then were placed at 50 °C overnight before the IV curve measurement. Mechanical properties of the nanocomposite were measured via a CMT-series microcomputer electronic universal testing machine (MST), and tensile tests were carried out at ambient temperature and a constant speed of 1 mm/min. The nanocomposites with a dimension of 55 × 10 × 4 mm3 were used for tensile tests. A two-probe digital multimeter (VICTOR 86E) linked with a laptop was used to record the resistance changes and current variations. It was also used to measure the voltage generated from the solar panel. An accurate pH value was determined with a PHS-3C pH meter, and visual color changes caused by strong acid and strong alkali were demonstrated by pH-indicator papers. The DataPhysics OCA40 Micro instrument was used to measure the contact angles and sliding angles at room temperature. The droplet used for contact angle measurement was 5.0 μL in volume, whereas it was 40.0 μL for sliding angle measurement. All of the reported contact angle and sliding angle values represented the average of five measurements.

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the nanocomposite surface not only indicated the introduction of MPDMS, but was also believed to provide a surface with low surface tension. Because the MPDMS and MWCNTs we used are water repellent, the resultant nanocomposites are supposed to be hydrophobic. To verify our speculation, the wetting properties of this nanocomposite were evaluated by determining the static contact angle and the sliding angle on the nanocomposite surface of water with a surface tension of ∼72 mN m−1.39 As we know, the maximum achievable static contact angle of water never exceeds ∼120° on a smooth surface.48 Strikingly, owing to the synergic effect of MPDMS and MWCNTs, the static contact angle and sliding angle of water on the nanocomposite surface turned out to be 112.6° ± 0.9° and 49.0 ± 1.2°, respectively, which is indicative of high hydrophobicity of the nanocomposite. Furthermore, liquids with different pH values ranging from 0 to 14 were used to evaluate the chemical stability of the nanocomposite. As seen in Figure 2b, for the droplets with pH from 1 to 13, their contact angles were stable. However, under extreme chemical conditions, pH = 0 and pH = 14 (i.e., strong acid and strong base), the contact angles on the nanocomposite surface slightly decreased, demonstrating the superb chemical stability of the nanocomposite. To further demonstrate hydrophobicity and chemical stability, a halfcoated glass plate was laid horizontally, and a home-made bibulous tissue strip with a pH-indicator paper inserted was used to absorb the test droplets (water, strong acid, and strong base, each 0.5 mL in volume) sitting on the surface of the nanocomposite film. As shown in Figure 2c, when in contact with droplets, tissue strips were quickly wetted, and the pHindicator papers inserted simultaneously exhibited a characteristic color to indicate the nature of droplets. At the same time, droplets gradually shrank into small sizes instead of being pinned on the surface and were completely absorbed within 4 s, leaving behind a clean nanocomposite surface, indicating high hydrophobicity and chemical stability of the nanocomposite. Because of excellent hydrophobicity and chemical stability, the nanocomposite was also endowed with self-cleaning property against water, strong acid, and strong base. As seen in Figure 2d, a half-coated glass plate was tilted at 52.0° to ensure the sliding of droplets, and 40.0 μL of water cleanly glided down the nanocomposite surface without leaving noticeable traces along its path. More strikingly, as for strong acid and strong base droplets, the nanocomposite also exhibited a similar self-cleaning property; no trace left along the path of the droplet movement on the nanocomposite surface. However, it was not applicable for concentrated sulfuric acid (98 wt %) because of its potent corrosive property. Apart from fabulous repellency against strong acid and strong base, the resistance of the nanocomposite against ethanol was also investigated. A nanocomposite film with a dimension of 10 × 10 × 1 mm3 was immersed into an absolute ethanol solution for 12 h, and the water contact angle was measured after the removal of the nanocomposite film from the solution and the complete evaporation of ethanol at room temperature. Instead of being thoroughly dissolved or disintegrated, the nanocomposite film maintained its original shape, but a slightly rough surface was created. Different from highly cross-linked PDMS-based coatings with an excellent ethanol-resistant performance,38 little cross-linking in this nanocomposite caused the loss of surficial MPDMS by ethanol

RESULTS AND DISCUSSION

Figure 1a illustrates the fabrication procedure of MWCNT/ MPDMS nanocomposites, and it briefly includes three sections. First, the PDMS reacted with the IPDI to achieve a transparent MPDMS (the complete consumption of isocyanate groups was confirmed by FT-IR in Figure S1). Second, the asprepared MPDMS was mixed thoroughly with MWCNTs under stirring with the aid of ethanol. At last, after the complete evaporation of ethanol, the resultant mixture was pressed to form a compact nanocomposite. Before mixing with MWCNTs, the gel-like MPDMS was sticky and readily attached to that which it contacted, which made it impossible for practical applications. Considering the enhancement effect of MWCNTs on polymers, different weight ratios of MWCNTs to MPDMS were investigated. As shown in Figure S2, before the MWCNT content reached 25.0 wt %, the resultant MWCNT/MPDMS nanocomposites remained sticky, and black traces were left after the removal of nanocomposites (Figure S2a−c). When the MWCNT content exceeded 25.0 wt %, though no mark was retained, the nanocomposite became brittle and tended to fracture (Figure S2e). Besides, the electrical resistivity of the nanocomposite, after measurements, varied slightly from 25.0 to 30.0 wt % (Figure S3a), and the nanocomposite also showed a linear Ohmic response (Figure S3b). Thus, the nanocomposite with 25.0 wt % MWCNTs was suitable for further investigations, and the exact feed ratio of MWCNTs was confirmed via thermogravimetric analysis, as shown in Figure S4. After being pressed by an external force, the nanocomposite became compact, and a smooth nanocomposite film was successfully formed (Figure 1b). Similar to previous works,38,39 according to the SEM image (Figure 1c), this PDMS-based film exhibited a fairly smooth surface. In addition, the AFM characterization revealed that the root-mean-square roughness of the surface was only 2.58 nm (Figure S5), which further confirmed the smoothness of the nanocomposite surface. In XRD spectra, a sharp and strong diffraction peak at 2θ = 25.8° and a shorter band at 2θ = 42.7° were observed, corresponding to the (002) and (100) Bragg reflection of the hexagonal graphite structure of MWCNTs.47 As for the MWCNT/ MPDMS nanocomposite, the expected MWCNT diffraction peaks, although decreased, were also found, indicating the presence of MWCNTs in the nanocomposite. Besides, according to the X-ray photoelectron spectroscopy (XPS) survey spectrum (Figure 2a), a high content of Si (21.86%) on D

DOI: 10.1021/acsami.8b07575 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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However, the fan stopped operating when the circuit was disconnected (Figure S7c). Furthermore, this nanocomposite could also serve as a single-pole double-throw switch to dominate switching and interconnecting functions. As shown in Figure 3d, by gently relocating the switch, the LED in a different pathway could be interconnected and switched to light. Thus, the nanocomposite could be employed as an emergency circuit repair material for diverse electrical circuits. Different from aligned structures constructed for anisotropic properties,45,50 in view of the preparation method we employed, MWCNTs should be well distributed, and the nanocomposite would show an electrically isotropic property. Figure 4 shows the vertical and horizontal cross-sectional SEM images of the nanocomposite containing 25.0 wt % MWCNTs at different magnifications, demonstrating the good distribution of MWCNTs in the nanocomposite. Thus, this nanocomposite is supposed to show electrical conductivity in all directions. To verify our speculation, a rectangle-shaped nanocomposite was used for investigation. Owing to the uniform structure inherent in the nanocomposite, the LED was lighted up when nanocomposite wires contacted the rectangleshaped nanocomposite positioned at different directions (A to B, C to D, E to F, and B to A; see Figure S8), offering compelling evidence of isotropic conductivity in this system. For further verification, the electrical measurement was conducted on the nanocomposite with a dimension of 10 × 10 × 10 mm3 from different directions. Also, the IV curves recorded from x-axis, y-axis, and z-axis overlapped and showed a linear Ohmic response (Figure S9), indicating the same electrical resistivity in all directions, confirming the isotropic conductivity of the nanocomposite. Furthermore, the healing process in the electrical field was investigated by separating the nanocomposite completely with a blade and subsequently bringing together the two resultant detached parts. As seen in Figure 5a−c, the LED was off when the nanocomposite was separated, while the LED worked as two fragments were reconnected. Thus, the nanocomposite showed healing capability in the electrical field, and the proposed electrical healing mechanism is represented in Figure 5d. As demonstrated in Figure 4, MWCNTs were densely distributed in a polymer matrix. Consequently, MWCNTs were also densely distributed on the fresh-cut surfaces. Thus, the separation of the nanocomposite naturally blocked the transference of electrons because of the breakdown of the conductive network, whereas the reconnection of the two detached parts resulted in the recontact of MWCNTs, leading to the recovery of the MWCNT network on the contact interface, bringing about the re-formation of continuous electrical conductive paths and finally causing the recovery of the conductive network. Owing to the highly dense distribution of MWCNTs in the nanocomposite, the electrical healing efficiency, defined as the ratio of nanocomposite conductivities after/before healing, was above 96.53% even after 50 separating−reconnecting cycles (Figure S10). However, some voids inevitably existing in the contact interface and the imperfect reconnection of MWCNTs resulted in the decrease of conductivity compared with the intact one. Besides, we found that the nanocomposite could be reshaped into various patterns. As mentioned before, because the as-prepared MPDMS was sticky, the MPDMS could be reintegrated under external forces because of the mobility of MPDMS chains. The addition of MWCNTs enhanced the

and a slightly rough surface created by the uncovered MWCNTs (Figure S6a). Owing to the compact structure and sufficient prevention of insoluble MWCNTs against further permeation of ethanol, the immersed nanocomposite film was able to retain its pristine shape. To the best of our knowledge, when the intrinsic contact angle is below 65°, an increase in roughness will bring about a decrease in the contact angle, whereas the contact angle will increase in response to greater roughness when the intrinsic contact angle is above 65°.49 Consequently, it is conceivable that the water contact angle on the ethanol-treated nanocomposite surface is supposed to be more than 112.6° because of a greater roughness. Consistent with our speculation, the water contact angle on the ethanol-treated surface turned out to be 123.2° (inset in Figure S6a), which was unachievable for a smooth surface, further demonstrating the existence of roughness. However, on this treated surface, water droplets pinned tightly instead of being slid down, which could be potentially used for liquid transportation. As for pristine MWCNTs (scattered onto a 3M double-sided tape in the case of the movement of MWCNTs by water) (Figure S6b), a rougher surface resulted in a water contact angle of 152.2°, and water droplets readily rolled off without wetting the surface, corresponding to a superhydrophobic surface. Thus, different morphologies of the surface bring about diverse wetting properties, and both of them show tremendous potential in practical applications. Besides hydrophobic property, self-cleaning performance, and chemical stability, this nanocomposite also exhibited outstanding electrical conductivity because of the sufficient incorporation of MWCNTs. After measurement, the sheet resistance of the nanocomposite reached 86.33 Ω sq−1. Because of high conductivity, this nanocomposite could be utilized as a conductive material for diverse electrical circuits. As seen in Figure 3a−c, series circuit, parallel circuit, and

Figure 3. Nanocomposite wires for different electrical circuits that performed switching and interconnecting functions. (a) Series circuit. (b) Parallel circuit. (c) Series−parallel circuit. (d) Single-pole doublethrow switch showing switching and interconnecting functions. By relocating the nanocomposite switch, the electrical circuit can be reconnected and the LED in the connected path will be switched to light. Direct voltage applied for (a−c) was 6.0 V, whereas 3.0 V was applied for (d) (scale bar, 10 mm).

series−parallel circuit were achieved by connecting three LEDs using nanocomposite wires. When direct voltage was applied, LEDs were lighted up simultaneously. Besides, it could also be employed to connect a solar-powered device. Basking in natural sunlight, the voltage of a solar panel (53.0 × 29.0 mm2) reached up to 2.252 V (Figure S7a), and a fan worked when the circuit was connected by the nanocomposite (Figure S7b). E

DOI: 10.1021/acsami.8b07575 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. Cross-sectional SEM images of the MWCNT/MPDMS nanocomposite in vertical and horizontal directions. (a1−a3) Vertical crosssectional images at different magnifications. (b1−b3) Horizontal cross-sectional images at different magnifications. Cross-sectional SEM images confirmed the good distribution of MWCNTs in the nanocomposite.

Figure 5. Healing process in the electrical field. (a) Intact nanocomposite wire with LED lighting. (b) Damaged nanocomposite wire left the LED unlit. (c) LED working once the two fractured parts were pushed together. (d) Schematic illustration for the electrical healing process (scale bar, 10 mm).

Figure 6. Reshapeable, bendable, moldable, and twistable properties of the nanocomposite. (a) Halloween pumpkin lantern shaped by the nanocomposite. (b) Reshaped methane model. (c) Nanocomposite wire bent into a heart-shaped pattern placed on a 3D spherical surface. (d) Heart-shaped pattern hanging on the columns of a sand glass. (e) LED-connected alphabetic string (SCUT patterns) molded from different Teflon molds. (f) LED-connected seagull-like patterns constructed by several bent strips molded from a same Teflon mold. (g) Ladder constructed by several nanocomposite wires. (h) DNA model twisted by the nanocomposite ladder from (g). Because of the high conductivity of the nanocomposite, LEDs connected in series were lighted up once direct voltage was applied. Direct voltage applied for (a−h) was 3.0 V, except for (f), for which it was 6.0 V (scale bar, 10 mm).

mechanical strength of MPDMS and then brought about the nonsticky nanocomposite suitable for practical application. However, although the mechanical enhancement of MWCNTs to MPDMS was effective, there was little cross-linking between MWCNTs and MPDMS. Therefore, the nanocomposite could be reshaped because the mobility and reintegration of MPDMS chains could be achieved under external forces, leading to the reshapeability of the nanocomposite. As shown in Figure 6a, a Halloween pumpkin was shaped by hand kneading using a nanocomposite piece, and then it was reshaped into a methane model (Figure 6b). The mechanical properties of the pristine nanocomposite, reshaped nanocomposite, and reconnected nanocomposite (Figure 5c) were measured, and the stress−strain curves are shown in Figure S11. As for the pristine nanocomposite, the maximum stress was 0.21 MPa, and the maximum strain was 42.71%. Obviously, when the maximum stress was achieved (30.90% strain), crack occurred on the nanocomposite, and the crack was extended under constant stretching, leading to the rapid decrease of stress. Also, the reshaped nanocomposite showed a similar stress−strain curve, indicating that the reshaped nanocomposite maintained the mechanical property of the pristine one. As for the reconnected nanocomposite, because the two cut surfaces were only directly contacted, only a part of the nanocomposite was reintegrated, leading to less stress (0.08 MPa) and strain (2.52%) compared with the

pristine one. Besides, the resistance change along with the stretching of the intact nanocomposite is shown in Figure S12. Combined with Figure S11, the result indicates that, before maximum stress, the resistance of the nanocomposite increased slightly and steadily with the increase of strain. However, when the stress exceeded the maximum value, because of the continuous extension of the crack on the nanocomposite caused by constant stretching, the resistance increased sharply because of the loss of the conductive network. Besides the reshapeability, the nanocomposite was also endowed with bendable, moldable, and twistable properties. Because of flexibility, nanocomposite wires could be bent into different patterns located on a 3D spherical surface (Figures 6c and S13) or hanging on the columns of a sand glass (Figure 6d). As seen in Figure 6e, alphabetic patterns (SCUT) were molded and peeled off from different Teflon molds and subsequently connected by three LEDs to form a conductive F

DOI: 10.1021/acsami.8b07575 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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S14b), indicating that the nanocomposite could be used under various chemical conditions. At last, we found that this nanocomposite could serve as electrodes to fabricate electroluminescent devices with a patterned display. As seen in Figure 8a, the electroluminescent

path. In addition, molded strips with 5.0 mm thickness were bent to construct a two-flying-seagulls pattern (Figure 6f). To demonstrate twistability, a 2D nanocomposite ladder laid horizontally was readily twisted into a 3D DNA model by hand (Figure 6g,h). Because of high conductivity, LEDs used to connect nanocomposite patterns were lighted when voltage was applied. It is worth noting that this nanocomposite is easy to be removed, recycled, and reused. These characteristics make it more economical and a potential candidate for practical applications. Furthermore, a comprehensive test for hydrophobicity, chemical stability, and electrical conductivity was implemented on the reshapeable nanocomposite. As demonstrated in the schematic illustration (Figure 7a), the nanocomposite was

Figure 8. Nanocomposite-based electroluminescent device. (a) Schematic illustration of a nanocomposite-based electroluminescent device. The device was composed of a bottom and a top nanocomposite layer with a sandwiched ZnS:Cu emissive layer. When a high-frequency alternating voltage was applied, the electroluminescent region was sandwiched between the top and the bottom layer. However, because of the opacity of the nanocomposite, only the profile of the top layer was exhibited. (b) Photograph of an electroluminescent device with a star-shaped top layer without applied voltage. (c) Clear profile of the top nanocomposite layer exhibited when alternating voltage was applied (scale bar, 5 mm).

Figure 7. Comprehensive test on hydrophobicity, chemical stability, and electrical conductivity of the reshapeable nanocomposite as a function of time. (a) Schematic illustration of the test. (b) At the beginning of the test (0 h). (c) After 12 h of testing. Because of darkness, the photograph was taken with the help of a spotlight. (d) After 24 h of testing. Because of excellent properties, even after 24 h of immersion in water, ethanol, strong acid, and strong base, the boxshaped nanocomposite boats with an average thickness of 0.8 mm showed no evidence of leaking, and the LED kept working. The red solution and blue solution were replenished to original volume every 8 h because of the evaporation of water and ethanol. Direct voltage applied was 4.5 V (scale bar, 10 mm).

device comprised of top and bottom nanocomposite layers with a sandwiched ZnS:Cu/Sylgard 184 (Dow Corning) emissive layer. When a high-frequency alternating voltage was applied, the emissive layer afforded most of the voltage, and blue luminescence was emitted simultaneously. Because of the opacity of the nanocomposite, the profile of the nanocomposite pattern was visualized by the emitting blue luminescence (Figure 8c). Thus, by designing and shaping the nanocomposite electrodes into on-demand patterns, pattern display can be readily achieved, and such a multifunctional nanocomposite exhibits huge potential in flexible electronic devices.

shaped into two box-shaped boats with a blue solution (solution of water and methylene blue) and a red solution (solution of ethanol and methyl red) within, floating on the strong acid and strong base solution, respectively. Also, two boats were linked by nanocomposite wires and an LED to form a series circuit. With an average thickness of 0.8 mm, if the nanocomposite shows a weak resistance against water, ethanol, strong acid, or strong base, the boats would be corroded and permeated, and the clear strong acid and strong alkali solution would be colored. However, after 24 h of immersion, the solution remained clear and colorless, and the LED kept emitting light as well, confirming the durability of the nanocomposite against water and extreme chemical conditions. Furthermore, the IV curves of the nanocomposites after immersion in water, ethanol, strong acid, and strong base for 24 h are shown in Figure S14a, and all the nanocomposites showed a linear Ohmic response. Except the fact that the resistivity of the ethanol-treated nanocomposite decreased slightly, mainly because of the loss of the insulating MPDMS on the nanocomposite surface (Figure S6a), other treated nanocomposites showed little variation in resistivity (Figure

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CONCLUSIONS In summary, we have developed a flexible multifunctional MWCNT/MPDMS nanocomposite that can be reshaped, bended, molded, and twisted into various patterns. This nanocomposite exhibits striking hydrophobicity, excellent selfcleaning performance, and outstanding chemical stability against strong acid and strong base. Thus, it can be applied under wet and extreme chemical conditions. In addition, because of the good distribution of MWCNTs, this nanocomposite is highly conductive and shows no evidence of anisotropic conductive behavior, and it also demonstrates a high electrical healing efficiency above 96.53%. As a result, it can serve as a conductive material for diverse electrical circuits and be applied as a switch to perform switching and reconnecting functions. Moreover, it can be used to construct electroluminescent devices and shows tremendous potential in flexible electronics. With the advantages of portability, G

DOI: 10.1021/acsami.8b07575 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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recyclability, and reusability, this nanocomposite has more potential in practical applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b07575. FT-IR spectrum of MPDMS; photographs of nanocomposites with different weight ratios of MWCNTs; electrical measurements of the nanocomposites with different contents of MWCNTs; thermogravimetric analysis; AFM image of the nanocomposite surface; SEM image of the ethanol-treated nanocomposite surface and pure MWCNTs; photographs of a fan driven by a solar panel with the nanocomposite connected; photographs showing the isotropic conductivity behavior of the nanocomposite in all directions; IV curves of the nanocomposite recorded from different directions; healing efficiency of the nanocomposite in an electrical field at different separating/reconnecting cycles; stress−strain curves of the pristine nanocomposite, reshaped nanocomposite, and reconnected nanocomposite; resistance change of the nanocomposite as a function of strain; photographs of on-demand patterns bent by nanocomposite wires on 3D spherical surfaces; IV curves and the corresponding resistivity of the nanocomposites before and after being immersed in various solvents for 24 h (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86 020 87114919. ORCID

Heqing Fu: 0000-0002-1298-8306 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the Science Foundation of State Key Laboratory of Structural Chemistry for financial support under grant no. 20160027.

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REFERENCES

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DOI: 10.1021/acsami.8b07575 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.8b07575 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX