Wearable Electricity Generators Fabricated Utilizing Transparent

Jun 10, 2016 - A simple and scalable method that takes into account the excellent advantages of the unique meshy structure of a textile, blade-coating...
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Wearable Electricity Generators Fabricated Utilizing Transparent Electronic Textiles Based on Polyester/Ag Nanowires/Graphene Core− Shell Nanocomposites Chaoxing Wu,†,‡ Tae Whan Kim,*,† Fushan Li,*,‡ and Tailiang Guo‡ †

Department of Electronic and Computer Engineering, Hanyang University, Seoul 133-791, Korea Institute of Optoelectronic Display, Fuzhou University, Fuzhou 350002, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: The technological realization of wearable triboelectric generators is attractive because of their promising applications in wearable self-powered intelligent systems. However, the low electrical conductivity, the low electrical stability, and the low compatibility of current electronic textiles (e-textiles) and clothing restrict the comfortable and aesthetic integration of wearable generators into human clothing. Here, we present high-performance, transparent, smart e-textiles that employ commercial textiles coated with silver nanowire/graphene sheets fabricated by using a scalable, environmentally friendly, full-solution process. The smart e-textiles show superb and stable conduction of below 20 Ω/square as well as excellent flexibility, stretchability, foldability, and washability. In addition, wearable electricity-generating textiles, in which the e-textiles act as electrodes as well as wearable substrates, are presented. Because of the high compatibility of smart e-textiles and clothing, the electricity-generating textiles can be easily integrated into a glove to harvest the mechanical energy induced by the motion of the fingers. The effective output power generated by a single generator due to that motion reached as high as 7 nW/cm2. The successful demonstration of the electricity-generating glove suggests a promising future for polyester/Ag nanowire/graphene core−shell nanocomposite-based smart e-textiles for real wearable electronic systems and self-powered clothing. KEYWORDS: smart electronic textile, electricity generator, wearable electronics, silver nanowire, graphene

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wearable triboelectric generators harvesting the mechanical energy induced by the activity of the human body is attractive because of the promising applications in wearable multifunctional intelligent systems.15−17 A high-performance wearable electrode is one of the key components in wearable triboelectric generators, and the development of a conductor for wearable triboelectric generators with high compatibility with human clothing is crucial. Unlike commercial wearable electronic devices, ideal wearable devices should be part of human clothing and should be foldable, stretchable, and even washable. Thus, textiles, not flexible panels such as plastic or glass, should be used as unique support materials and should play a significant role in the

lexible and wearable electronic devices have inspired both scientific and commercial interests due to their excellent combination of related base functions with stretchability and foldability, leading to promising applications in multifunctional intelligent systems.1−7 Especially, when electronic devices are integrated into wearable systems, the portable green-energy supply should play an important role with respect to global energy problems. Research in the area of wearable self-powered systems that harvest energy from the environment should be a key issue in solving problems related to energy conservation and pollution control.8 Of note is that enormous amounts of ambient mechanical energy, such as mechanical friction and vibration, surround the human body and are, in most cases, going to waste. Recently, the inventions of piezoelectric and triboelectric generators have provided an effective approach to generating electricity by harvesting the body’s energy.9−14 Thus, the technological realization of © 2016 American Chemical Society

Received: December 24, 2015 Accepted: June 10, 2016 Published: June 10, 2016 6449

DOI: 10.1021/acsnano.5b08137 ACS Nano 2016, 10, 6449−6457

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Figure 1. (a) Schematic diagram of the experimental setup for e-textile formation. (b) Optical micrographs of the textile/Ag NW structure taken immediately after the textile had been coated with a Ag NW solution. (c) SEM image of the textile/Ag NW sample. The bottom is a three-dimensional optical image. (d) The sheet resistance of the Ag-NW-coated textile as a function of the number of Ag NW coating cycles.

and Ag-textiles in mechanical deformation states have not been reported yet. Thus, the possibility that the conductivities of Nitextiles and Ag-textiles might decrease dramatically when undergoing repeated mechanical deformations must be considered.37 A monolayer of graphene can be transferred to fibers, which results in a sheet resistance of about 1 kΩ/ square.29 The sheet resistance of a graphene-wrapped textile after nitric-acid treatment is approximately 700 Ω/square.33 Such a relatively low conductivity prevents applications of the graphene/textile structure in wearable electronics. Therefore, etextiles with high conductivity, high electrical stability, and high compatibility with clothing that can be fabricated by using a simple and environmentally friendly fabrication technique are urgently needed. This paper presents an all solution-fabricated and highperformance smart e-textile for wearable triboelectric generator applications. A commercial polyester precision textile serves as the template, which makes it highly compatible with clothing. The textiles were blade-coated with conductive silver nanowires (Ag NWs) to form conducting textiles. A reduced grapheneoxide (GO) coating, namely graphene, was further introduced as the principal innovation and was used to coat the Ag NW film, forming a polyester/Ag NW/graphene core−shell structure. The Ag NW coating makes the textile a highly conductive film with a sheet resistance of less than 20 Ω/ square. The protective graphene coating produces a smart etextile with outstanding electrical, chemical, and mechanical stability. By utilizing these excellent smart e-textiles, we fabricated triboelectric generators that are able to provide stable power by harvesting the mechanical energy induced by low-frequency friction. Because of the high compatibility of the smart e-textiles and clothing, an electricity-generating glove that can generate energy from the action of the fingers was demonstrated as a proof of concept for wearable, self-powered devices, which will support the development of wearable electronics. The simple and scalable fabrication process, high conductivity, the high stability, and the compatibility with human clothing of the smart e-textiles are characteristics that

development of wearable electronics due to their large mechanical strength and flexibility, lightweight, and modifiable surfaces. A smart textile should be highly conductive for practical applications, as should the substrate, so that it can act as an electrode. Research on electronic textiles (e-textiles) has been conducted to fabricate wearable generators with good performance. Typically, two routes can be used to fabricate e-textiles. The first route is the bottom-up route, in which conducting fibers or yarns are woven into textiles.18,19 Especially, conducting nanocarbon materials, such as carbon nanotubes (CNTs) and graphene, are candidates for use in e-textiles.20−23 Metal wires have also been reportedly woven into a polymer mesh, which resulted in the formation of a flexible electrode.24,25 For example, conducting cotton threads coated with CNTs can be woven into a fabric to form an electricity-generating textile.26 However, its electrical conductivity is limited due to the relatively low conductivity of CNTs. Aluminum wires can be used as electrodes, and aluminum wire-based composites can be woven into highly stretchable two-dimensional fabrics.17 Nevertheless, realizing large-scale e-textiles and mass production of those e-textiles by using this technique is difficult due to some technological issues such as its potential incompatibility with the processes and equipment currently used in the textile industry. Furthermore, the comfort of such e-textiles might be decreased due to the existence of the aluminum wires. In consideration the compatibility of e-textiles and human clothing, the second route, the up-bottom route, is more appropriate. In this approach, the commercial textile is subsequently covered with a conducting film coating to form the e-textile. Metal films, carbon nanotubes, and graphene have been extensively studied for use as such a conducting coating.27−36 Conductive Ni-textiles prepared by using electroless plating and commercially available Ag-coated textiles have been applied as electrodes in electricity-generating textiles.15,16 However, the electroless plating method is environmentally unfriendly, and the Ni and Ag coating makes the textile almost opaque, which results in the loss of its original color and luster. Furthermore, studies on the conduction stability of Ni-textiles 6450

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Figure 2. (a) Schematic diagram of the e-textile with a polyester/Ag NW/graphene core−shell structure. (b−d) SEM images of polyester/Ag NW/graphene samples with different numbers of graphene-coating cycles: (b) one cycle, (c) two cycles, and (d) three cycles. SEM images of the fiber cross-linked regions (e) before graphene coating and (f) after graphene coating. (g) Visible-light transmittance of the textile, the textile/Ag NW, and the textile/Ag NW/graphene samples; the insets are photographs.

of the mesh holes. The liquid film inside the holes is thin because of the low viscosity of the Ag NW water solution, and the film is broken after the evaporation of the water, which results in the attachment of the Ag NWs to the surface of the yarn, particularly in the cross-linked region. Figure 1, panel c shows that the Ag NWs cover the surface of the polyester yarn, even being deposited in the cross-linked region, and form networks. The random meshy structure of the Ag NWs allows the Ag NW film to conform to the shape of the polymer textile, as shown at the bottom of Figure 1, panel c. Thus, the Ag NW film can retain its high conductivity after mechanical bending, stretching, and folding. The Ag NW networks provide conducting pathways, and those conducting pathways can conform to the shape of the textile. The sheet resistance of the textile decreases with increasing number of cycles of the coating process, as shown in Figure 1, panel d. While the sheet resistance of the textiles with one layer of Ag NWs is above 20 kΩ/square, that of the textiles with a six-layered Ag NW stacking film is 20 Ω/square. Even though the Ag-NW-coated textile shows high electric conductivity, poor electrical and mechanical stability make practical applications difficult due to the easy oxidation of the Ag NWs and their weak adhesion to the polyester yarn, which will be discussed later. As for the Ag NWs coating, three important issues must be solved before the practical application of Ag NWs in smart etextiles. The first is how to make the Ag material, which is very unstable due to its high reactivity with sulfide in the air. The second is how to increase the strength of adhesion of the Ag NWs to the polyester fibers. The last is how to reduce the roughness of the Ag NW film, which increases with increasing number of Ag NWs in the film. The drawbacks of Ag-NWcoated textiles can be easily overcome by introducing a thin graphene film.42 When the as-prepared Ag-NW-coated textile was further coated with a GO film by using the same bladecoating method, the GO film was reduced to graphene in

make them important for the development of smart wearable electronics.

RESULTS AND DISCUSSION Random meshes of Ag NWs show high optical transmittances and low sheet resistances.38,39 Furthermore, a Ag NW film with a random mesh structure has good performance, thus allowing the Ag NW film to stabilize to the shape of the polymer textile. Hence, the Ag NW film can retain its high conductivity even when it undergoes mechanical bending, stretching, and folding. However, achieving an efficient method for fabricating highperformance, size-scalable Ag NW films is still challenging. Vacuum filtration from a solution and subsequent transfer enables the preparation of films from precise amounts of a nanowire solution.40 However, that is not a practical method for fabricating electrodes over large areas because the area of the electrode depends directly on the sizes of the filter membrane and the filter. Spray coating is a simple and clean method for the preparation of electrodes from carbon nanotubes. However, the spraying of long metal nanowires is technologically difficult because the high-speed flow and the tiny pipes of the spray currents may cause the metal nanowires to break. Even though the Meyer rod coating method is scalable, the use of an additional thick polymer, which has to be removed by using a subsequent heating step, is necessary.41 As a result, the flexible substrate and the metal nanowires might be damaged during the heating process. A simple and scalable method that takes into account the excellent advantages of the unique meshy structure of a textile, blade-coating, was employed in this research to deposit Ag NWs, as shown in Figure 1, panel a. A Ag NW liquid film can be formed in the mesh of the textile during the blade-coating process due to the surface tension of the Ag NW solution and the meshy structure of the textile, even though the viscosity of the Ag NW solution is low, as shown in Figure 1, panel b. After the coating, the Ag NW solution film provides instantaneous and complete filling 6451

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Figure 3. (a) Sheet resistance variations with time of the smart e-textiles with different numbers of graphene coating cycles in air at room temperature. (b) XPS spectra of the Ag NW films with and without a graphene coating film, which were placed in air for 336 h. (c) SEM images of the Ag NWs without a graphene coating film, which were placed in air for 240 h. (d) Sheet resistance variations for the textile/Ag NW and the textile/Ag NW/graphene samples with different numbers of washings. The insets show photographs of water drops on the surfaces of the textile/Ag NW and the textile/Ag NW/graphene samples. (e) Resistance variations of the e-textile when the e-textile is being curled, stretched, or folded. The resistance changes are normalized to the initial resistance. (f) Sheet resistance variations of smart e-textiles after different mechanical deformations.

sulfide in the air, which is considered to be the critical material leading to Ag oxidation, will be separated from the Ag NWs by the graphene film. X-ray photoelectron spectroscopy (XPS) and SEM measurements were carried out to identify further the important role of the graphene coating in stabilizing the conductivity of the e-textile. Figure 3, panel b shows the XPS spectra of Ag NW films with and without a graphene coating film, which were placed in air for 336 h. The intensity of the S 2p peak for the sample without a graphene-coating film is much higher than that for the sample with a graphene-coating film, indicating that silver sulfide was formed for the Ag NW sample without a graphene coating. Thus, the graphene-coated film can separate the sulfide in the air from the Ag NWs and allow the original high electric conductivity to be maintained. Figure 3, panel c presents a SEM image of Ag NWs without a graphene coating film, which was placed in air for 240 h. Randomly distributed silver-sulfide nanoparticles are attached to the Ag NWs, as indicated by the white arrow in Figure 3, panel c. The cross-linked regions between the Ag NWs produce a Ag NW network with high conductivity. However, in an oxidation process, the cross-linked Ag NWs might be separated when the nanoparticles are formed. As a result, the well-connected Ag NW networks can be destroyed. The graphene coating film can also enhance the attachment of the Ag NWs to the polyester yarn and improve the electrical stability of the smart e-textiles. The excellent flexibility and the large superficial area of the graphene nanosheets might induce a strong van der Waals force between the graphene and the substrate,45 which can further anchor the Ag NWs to the polyester surface tightly. Furthermore, the graphene coating film transforms the Ag NW/polyester textile from one with a hydrophilic behavior to one with a hydrophobic behavior due to the hydrophobicity of graphene, as shown in the inset of Figure 3, panel d.46 The as-fabricated smart e-textiles must have strong electrical stability in diverse mechanical deformation states, such as curled, stretched, or folded states, when they are

hydrazine hydrate. A schematic of the polyester yarn/Ag NW/ graphene structure is presented in Figure 2, panel a. Figure 2, panels b−f show scanning electron microscopy (SEM) images of the graphene films for Ag NW/textile surfaces fabricated with different numbers of graphene-coating cycles. The Ag NW/textile surface is partly coated with GO sheets after one coating cycle, as shown in Figure 2, panel b. Because the GO nanosheets have a negative static charge,43 during the second blade-coating process, the GO nanosheets in the solution are deposited in the non-GO region due to the electrostatic repulsive interactions between the GO sheets, as shown in Figure 2, panel c. The self-surface-limited process produces uniform GO coatings on the Ag NW/textile surfaces, as shown in Figure 2, panel d, and on the cross-linked regions, as shown in Figure 2, panels e and f. Photographs of the textile, the textile/Ag NW structure, and the textile/Ag NW/graphene structure are shown in the insets of Figure 2, panel g. While the as-fabricated e-textile is transparent, the deposition of a Ag NW/graphene layer slightly decreases the visible-light transmittance of the textile, as shown in Figure 2, panel g. Because the stability of the electric conduction is very important for practical applications, the sheet resistances of smart e-textiles with different graphene coating films in air at room temperature were measured. Figure 3, panel a shows that the sheet resistance of the sample without a graphene coating film increases with increasing air exposure time during a measurement time of 144 h and that it becomes about six-times larger than its initial state after a measuring time of 312 h. However, the sheet resistance of the sample with one layer of the graphene coating film increases only slightly. The high electric conductivity for the samples with two or three layers of the graphene coating film is maintained even after a measurement time of 336 h. Graphene membranes have been demonstrated to be vacuum-tight due to the narrow distribution of pore sizes, particularly in the Å range.44 Even though water can permeate the graphene coating film,45 the 6452

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substrate. An insulating polymer layer is partly coated over the e-textile by using the blade coating method, as shown in Figure 4, panels a and b. A schematic of the structure of the e-textile based on the triboelectric generator is shown in Figure 4, panel c.

applied to wearable electronics. The performance of the electrical stability during repeated washings is dominantly related to the chemical resistance against water and the resistance against mechanical stress. The as-fabricated e-textile was washed in deionized (DI) water by using a magnetic stirring bar for a rotation time of 20 min. No change in the sheet resistance was observed after six washing tests, as shown in Figure 3, panel d, which is a much better washing resistance than that of the sample without the graphene coating film. The e-textile also shows outstanding mechanical properties including flexibility, foldability, and stretchability (Video S1 in the Supporting Information). The normalized e-textile resistance variations when the e-textile is being curled, stretched, and folded are presented in Figure 3, panel e. The samples for the resistance measurements underwent curling with a radius of 5 mm, stretching to 1.5-times their original length, and folding in half twice. While the resistances of the etextiles in the curling and the folding deformations are smaller than its initial resistance due to the parallel connection, that of the e-textile in the stretching deformation is larger. Figure 3, panel f shows the electric conductivity endurance of the etextile against mechanical deformation; the initial sheet resistance of the e-textile was approximately 20 Ω/square, which increased to average values of ∼25, 35, and 40 Ω/square under various mechanical tests. The strong resistances against washing and mechanical deformation might be attributed to the strong adhesion of the Ag NWs to the yarn’s surface. (1) The flexibility of the Ag NW random meshes and the graphene coating film allows the Ag NWs to adhere conformally to the surface of the polyester yarn. (2) The graphene coating films help bind the Ag NWs due to the existence of large van der Waals forces between the graphene sheets and the polyester. (3) The hydrophobic properties of the graphene coating film protect the coating from the wash water. Besides polyester textiles, cotton textiles are extensively used in human clothing. Thus, the fabrication of smart e-textiles based on cotton textiles is significant for wearable electronics. Note that a cotton fiber is composed of multiple cotton fibrils, which are composed of multiple microfibrils bundled together.27 These microfibrils made of poly-D glucose chains usually arrange in crystalline, or partially crystalline, domains.27 This unique structure allows the fibers to absorb solvents, which cause the fibers to swell when placed in such solutions. The Ag NWs and graphene can also be deposited on the surfaces of cotton textiles to fabricate highly conductive textiles by using the blade-coating method presented in this work. However, the resistance stabilities of the e-textiles based on cotton against washing and mechanical deformation are still being researched due to polyester and cotton having different surface properties. After optimization of the surface treatment, the e-textile based on the cotton fabricated by using a bladecoating method may also present high conductivity with high stability. The recent invention of the triboelectric generator has allowed the triboelectric effect to be used to generate electricity by scavenging mechanical energy, and that generator has proven to be extremely efficient, reliable, and costeffective.9−11,47,48 A triboelectric generator is made of two conducting substrates on coated insulating layers with distinctly different triboelectric characteristics. The key to the triboelectric generator is that one of the insulating layers easily gains electrons, while the other easily loses electrons. The highly conducting e-textile used in this research acts as a conducting

Figure 4. (a) Optical micrograph and (b) enlarged optical micrograph of an e-textile with a PI coating film. (c) Schematic diagram of a generator based on smart e-textiles.

The e-textile based on the triboelectric generator demonstrated in this work has improved performance compared with those of previously reported triboelectric generators fabricated on plastic substrates in three ways. (1) The compatibility of the e-textile-based device with a normal textile holds promise for applications in wearable self-powered devices. (2) The choice of the e-textile acting as a conducting substrate, due to its high flexibility and stretchability, may allow two smart e-textiles to touch closely. (3) The knitted structure of the textile can increase the roughness of the surfaces of the insulating layers and improve the effect of friction during the rubbing process, all of which increase the generation of triboelectric charges. The following measurements were carried out to illustrate the basic principle of the triboelectric generator. First, the two textiles were allowed to touch and were rubbed for a few seconds, as shown in region I in Figure 5. Then, the two textiles were separated from each other, as shown in region II. Finally, the two textiles were allowed to touch again, and the same cyclic operation was conducted again, as shown in region III. The voltage output of the device under the above-mentioned operation is shown in Figure 5, panel a. Here, the triboelectric generator was assembled with poly(methyl methacrylate) (PMMA) and polyimide (PI) layers, which were acting as insulating layers. Triboelectric charges with opposite signs were generated during a continuous rubbing process, as shown in Figure 5, panel b(I), distributed on the surfaces of the two insulating layers, and formed a dipole moment. The polarity of the materials determines whether electrons are gained or lost. Some materials, namely triboelectric series, have a tendency to lose/gain electrons.49 As for our experiments, the electrons transfer from the positive side of the triboelectric series 6453

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Figure 5. (a) Output voltage of the generator as a function of time for different rubbing times from 6−13 s. (b) Schematic diagram illustrating the electricity-generation process in a full cycle. (c) The setup for electrical measurements. Output currents as functions of time for (d) the PMMA−PI configuration and (e) the PMMA−PDMS configuration for different rubbing frequencies.

Figure 6. (a) Photograph and (b) enlarged photograph of the electricity-generating glove, showing its excellent flexibility and wearability. (c) Output current of a glove that generates electricity from the motion of the fingers as a function of time. (d) Output voltage of a glove that generates electricity from the motion of the fingers as a function of time.

transferred charge is fully offset, leading to the accumulation of electrostatically induced charges on the electrodes, which is denoted as the “charging” process. Thus, the output voltage gradually decreases. When the two insulating layers are in contact, as shown in Figure 5, panel b(III), the surface triboelectric charges might form dipole moments again, and the electrostatically induced and redistributed charges might inversely build a positive potential, which would cause all of

(PMMA) to the negative side (PI). As soon as the two layers are separated from each other, as shown in Figure 5, panel b(II), opposite triboelectric charges become separated, and the dipole moment disappears. As a result, an electrical potential difference is rapidly established between the two neighboring sides. Electrons flow from the side with a lower potential to that with a higher potential through an external load to achieve equilibrium until the potential difference resulting from the 6454

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ACS Nano the transferred charges to flow in the opposite direction. Thus, the accumulated induced charges vanish, which is denoted as the “discharge process”. When a cycle is completed, the device might go back to an equilibrium state. The electrical outputs are related to the amount of transferred charge, which is determined by the amount of triboelectric charge created during the rubbing process, as shown in region I of Figure 5, and by the triboelectric properties of the insulating polymer layers. As the duration of the rubbing process was decreased from 13 to 6 s, the maximum output voltage decreased from 14.5 to 8 V, as shown in Figure 5, panel a. This results shows that the frequency of applying friction has a noticeable influence on the output of the generators. Because the mechanical energy generated from the body’s motions is always irregular and varies at low frequencies, the dependence of the output of the generator on the frequency must be investigated. Thus, the characteristics of the generator for a series of frequencies from 1.5−6 Hz were tested. The schematic setup for the electrical measurements is shown in Figure 5, panel c. The effects of insulating polymers with different triboelectric properties were addressed, as shown in Figure 5, panels d and e. The amplitude of the current output for the PMMA−PI generator decreased with increasing frequency from about 100 μA at 1.5 Hz to about 30 μA at 5 Hz. The amplitude of the current for the PMMA− polydimethylsiloxane (PDMS) generator showed a similar trend, decreasing with increasing frequency from about 400 μA at 1.5 Hz to about 50 μA at 5 Hz, as shown in Figure 5, panel e. The generator with a PMMA−PDMS configuration was more efficient than that with a PMMA−PI configuration because the PDMS has a stronger ability to gain electrons than the PI.47 The electrical outputs are related to the amount of transferred charge. At high frequencies, the charging and the discharging processes are restrained to a certain extent. However, one should note that the frequency limitation is in favor of lowfrequency friction, which will make possible the supply of power by using the irregular, low-frequency variations in the body’s motions. The generators used in this work, unlike the previously reported triboelectric generators fabricated on plastic or metal substrates, were fabricated on smart e-textiles. Thus, the fact that the generators could be naturally integrated into common clothing and form “electricity-generating clothing” is promising. Low-frequency friction phenomena occur in many areas of the body, such as the arms, legs, and fingers, during daily activities. Thus, the very large amount of mechanical energy that has been wasted until now might be harvested by wearing “electricitygenerating clothing” and might be used to provide power for other wearable electronics. As a proof of concept, the PMMA/ e-textile and the PDMS/e-textile were assembled on a glove to form an “electricity-generating glove”, and both exhibited excellent flexibility and wearability, as shown in Figure 6, panels a and b. Figure 6, panels c and d show the output current and the output voltage of the “electricity-generating glove”, in which the load resistance was 2000 Ω. When the fingers were moved, an output current larger than 2 μA and an output voltage larger than 4 mV were almost instantaneously generated. The electricity-generating process can be observed in Video S2 of the Supporting Information. The effective output power generated by a single generator with finger motion could reach about 7 nW/cm2. Even though the output power of a single generator is not enough to drive an electronic device, the simple structure of the e-textile-based generator fabricated in

this work has much room to be improved in the future by connecting a series of the generators in parallel by optimizing the insulator materials with high-performance polymers and by controlling the surface microstructure of the generator. These exciting results demonstrate that the “electricity-generating glove” can harvest energy from the body’s motions and convert it into electrical power, which holds promise for applications to wearable, self-powered electronics.

CONCLUSIONS Smart e-textiles with a textile/Ag NW/graphene core−shell structure were demonstrated to increase the e-textile’s conductivity and improve the compatibility of electricitygenerating e-textiles and human clothing. The full-solution process makes the technique for smart e-textile fabrication scalable and environmentally friendly. The obtained smart etextiles, which had a sheet resistance of 20 Ω/square, showed superb conductivity, transparency, foldability, flexibility, and stretchability as well as high compatibility with commercial textiles. Triboelectric generators based on the smart e-textiles were able to supply an electrical output, which was induced by low-frequency friction. Because of the high compatibility of the smart e-textile and clothing, smart electricity-generating etextiles were easily integrated into a glove to form a wearable electricity generator. The effective output power generated by a single generator due to finger motion reached as high as about 7 nW/cm2. Such high-performance, low-cost, and highly clothing-compatible smart e-textiles hold promise for applications in, and are at the frontiers of science for, wearable electronic devices. EXPERIMENTAL SECTION Fabrication of Smart E-Textiles. The silver nanowires were purchased from XFNANO (item NO. Agnws-200) as suspensions in water and were further diluted to a concentration of 10 mg/mL with DI water. A GO aqueous suspension with a concentration of 10 mg/ mL was prepared by using a modified Hummer method. The polyester textiles were sequentially cleaned by sonication in acetone, ethanol, and DI water. Then the textiles were placed in a plasma cleaner under an oxygen atmosphere for the surface treatment. After the plasma surface treatment of the textiles, they were vertically strained with a tension of 20 N. First, the silver solution was pipetted into a container and was then blade-coated onto the textile’s surface from the bottom up. After the textile had been heated at 80 °C for 1 min, the same coating process was carried out again. The GO suspension was pipetted into the container for the GO coating and was then bladecoated on the surface of the textile/Ag NW structure from the bottom up. After the textile had been heated at 80 °C for 1 min, the same coating process was carried for a third time. After the coatings of the Ag NWs and the GO film, the e-textile was placed into a hydrazinehydrate vapor for 24 h to reduce the GO coating. Fabrication of Triboelectric Generators. As for the e-textile/ PMMA structure, the PMMA powder was dissolved in chloroform with a concentration of 30 mg/mL and was then blade-coated onto the e-textile’s surface. As for the e-textile/PI structure, the PI precursor, namely polyamide acid, was prepared by dissolving p-phenylene biphenyltetracarboximide (BPDA-PDA)-type polyamic acid in Nmethyl-2-pyrrolidone; then it was blade-coated onto the e-textile’s surface. The polyamide-acid-coated textile was heated at 200 °C for 1 h in an Ar atmosphere to polymerize the polyamic acid into the PI layer. As for the e-textile/PDMS structure, the liquid PDMS elastomer and cross-linker were mixed, degassed, and uniformly blade-coated onto the e-textile’s surface. Then the textile was cured at 100 °C for 1 h to solidify the PDMS film. Characteristics and Electrical Output Measurements. SEM images were obtained by using Hitachi S-3000N and Nova NanoSEM 6455

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ACS Nano 230 systems, optical micrographs were obtained by using an optical microscope (Olympus), and optical 3D images were obtained by using an Olympus 3D measuring laser microscope (OLS4100). The surface elemental analyses of the Ag NWs were carried out by using XPS (Thermo Scientific ESCALAB 250). The sheet resistances of the samples were obtained by using a four-probe resistance analyzer (DMR-1C), and the electrical output performances of the triboelectric generators were obtained by using a Keithley 4200 SCS unit. For the measurements on the triboelectric generators, one of the e-textile/ polymer was fixed, and the other one was pasted on the surface of an insulating cylinder. During one rotation cycle, the two e-textile/ polymer substrates realized touch, rubbing, and separation, in sequence. The rotation speed of the cylinder was well controlled by using a stepping motor. The measurements on the generators were carried out at 25 °C under atmospheric conditions.

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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/acsnano.5b08137. Resistance variations of an e-textile in various mechanical deformation states (ZIP) Operation of an electricity-generating glove (ZIP)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

T.W.K. and C.W. conceived the project, designed and performed the experiments, and collected the data. C.W., T.W.K., F.L., and T.G. analyzed and discussed the data. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS T.W.K. acknowledges Prof. Z. L. Wang at the Georgia Institute of Technology for valuable discussion about and helpful comments on this manuscript. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2016R1A2A1A05005502). This work was also supported by the Korea Research Fellowship Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning (2015H1D3A1062276). This work was also supported by the National Natural Science Foundation of China (61377027). REFERENCES (1) Li, F.; Son, D. I.; Cho, S. H.; Kim, W. T.; Kim, T. W. Flexible Photovoltaic Cells Fabricated Utilizing ZnO Quantum Dot/Carbon Nanotube Heterojunctions. Nanotechnology 2009, 20, 155202. (2) Son, D. I.; Kim, T. W.; Shim, J. H.; Jung, J. H.; Lee, D. U.; Lee, J. M.; Park, W. I.; Choi, W. K. Flexible Organic Bistable Devices Based on Graphene Embedded in an Insulating Poly(methyl methacrylate) Polymer Layer. Nano Lett. 2010, 10, 2441−2447. (3) Bae, J.; Song, M. K.; Park, Y. J.; Kim, J. M.; Liu, M.; Wang, Z. L. Fiber Supercapacitors Made of Nanowire-Fiber Hybrid Structures for Wearable/Flexible Energy Storage. Angew. Chem., Int. Ed. 2011, 50, 1683−1687. (4) Kim, T. W.; Yang, Y.; Li, F.; Kwan, W. L. Electrical Memory Devices Based on Inorganic/Organic Nanocomposites. NPG Asia Mater. 2012, 4, e18. 6456

DOI: 10.1021/acsnano.5b08137 ACS Nano 2016, 10, 6449−6457

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DOI: 10.1021/acsnano.5b08137 ACS Nano 2016, 10, 6449−6457