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High-Performance Triboelectric Nanogenerators Based on Electrospun Polyvinylidene Fluoride–Silver Nanowire Composite Nanofibers

Siuk Cheon, Hyungseok Kang, Han Kim, Youngin Son, Jun Young Lee, Hyeon-Jin Shin,* Sang-Woo Kim,* and Jeong Ho Cho*

The preparation of ferroelectric polymer–metallic nanowire composite nanofiber triboelectric layers is described for use in high-performance tribo-electric nanogenerators (TENGs). The electrospun polyvinylidene fluoride (PVDF)–silver nanowire (AgNW) composite and nylon nanofibers are utilized in the TENGs as the top and bottom triboelectric layers, respectively. The electrospinning process facilitates uniaxial stretching of the polymer chains, which enhances the formation of the highly oriented crystalline β-phase that forms the most polar crystalline phase of PVDF. The addition of AgNWs fur-ther promotes the β-phase crystal formation by introducing electrostatic inter-actions between the surface charges of the nanowires and the dipoles of the PVDF chains. The extent of β-phase formation and the resulting variations in the surface charge potential upon the addition of nanowires are systemati-cally analyzed using X-ray diffraction (XRD) and Kelvin probe force micros-copy techniques. The ability of trapping the induced tribocharges increases upon the addition of nanowires to the PVDF matrix. The enhanced surface charge potential and the charge trapping capabilities of the PVDF–AgNW composite nanofibers significantly enhance the TENG output performances. Finally, the mechanical stability of the electrospun nanofibers is dramatically enhanced while maintaining the TENG performances by applying thermal welding near the melting temperature of PVDF.

DOI: 10.1002/adfm.201703778

S. Cheon, H. Kang, Prof. S.-W. Kim, Prof. J. H. ChoSKKU Advanced Institute of Nanotechnology (SAINT)Sungkyunkwan University (SKKU)Suwon 440-746, Republic of KoreaE-mail: [email protected]; [email protected]. Kim, Y. Son, Prof. S.-W. KimSchool of Advanced Materials Science and EngineeringSungkyunkwan University (SKKU)Suwon 440-746, Republic of KoreaProf. J. Y. Lee, Prof. J. H. ChoSchool of Chemical EngineeringSungkyunkwan University (SKKU)Suwon 440-746, Republic of KoreaDr. H.-J. ShinDevice Lab., D&S Research CenterSamsung Advanced Institute of TechnologySuwon 443-803, Republic of KoreaE-mail: [email protected]

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201703778.

1. Introduction

Triboelectric nanogenerators (TENGs) have attracted significant attention for use in self-powering wearable electronic devices because they provide a high-output power density, are low in cost, their device fabrication is simple, and they are mechanically robust. TENGs can success-fully convert mechanical energy, such as the energy in vibrations, waves, wind, and routine motions or friction produced during human motion, into electrical energy by coupling triboelectrification and electrostatic induction.[1–5] In principle, contact between two layers can generate triboelectric charges with opposite polari-ties on the two surfaces. The induced tri-bocharges can flow back and forth through an external circuit during repeated contact and release movements.[6–11] TENG per-formance is determined by a variety of factors, including the surface morphology, dielectric constant, and triboelectric poten-tial difference between the two triboelec-tric layers.[12–15] Experimental efforts have mainly focused on the formation of pat-

terned reliefs on single component surfaces using complex fab-rication techniques.[16–19] The use of TENGs as efficient energy harvesters in practical applications would require the rational design of triboelectric materials that can induce a high surface charge density but that require simple fabrication procedures to produce triboelectric layers with a high surface-to-volume ratio.

Electrospinning is a versatile and promising technique for fabricating continuous polymeric nanofibrous structures with a high surface roughness and a high surface-to-volume ratio.[20–23] Electrospun nanofibers are produced under an elec-tric field applied between the metal tip of a syringe filled with a polymer solution and a grounded collector. Recently, several fer-roelectric polymers such as polyvinylidene fluoride (PVDF) and its copolymers of poly(vinylidenefluoride-co-trifluoroethylene) [P(VDF-TrFE)] and poly(vinylidene fluoride-co-hexafluoropro-pylene) [P(VDF-HFP)] have been electrospun into nanofibers and these nanofibrous mats were utilized as the triboelec-tric layers for the TENGs.[24–28] Electrospinning combines the uniaxial stretching and electric field poling in a single step to

Nanogenerators

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promote the formation of a crystalline β-phase of ferroelectric polymers.[29–32] Among various crystalline phases, the β-phase provides the greatest degree of spontaneous polarization per unit cell and, thus, exhibits the highest ferroelectric activities, suggesting that ferroelectric polymers are useful as triboelec-tric materials with excellent surface charge densities in high-performance TENGs.[33,34] Recently, a few attempts have been undertaken to improve further the TENG output performances by adding carbon nanotube, metal nanoparticle, and graphene oxide in the ferroelectric polymers.[35–37] Furthermore, electro-spun polymeric nanofibers display excellent mechanical flexi-bility and stretchability, rendering them suitable for use in wearable device applications.

In this manuscript, we report the fabrication of high-per-formance TENGs using electrospun PVDF–silver nanowire (AgNW) composite nanofibers and nylon nanofibers as tribo-electric layers. The uniaxial stretching of the polymer induced during the electrospinning process enhanced the formation of the highly polar crystalline β-phase of PVDF. The addition of AgNWs further promoted β-phase crystal formation because of the electrostatic interactions between the nanowires and the dipoles of the PVDF chains. The crystalline phase and the sur-face charge potential of the PVDF–AgNW composite nanofibers were systematically analyzed by X-ray diffraction (XRD) and Kelvin probe force microscopy (KPFM). The addition of metallic nanowires enhanced charge trapping of the induced tribo-charges. The enhancement in both the surface charge potential and the charge trapping capability of the nanofibrous tribo-electric layer dramatically enhanced the TENG performances.

Finally, the mechanical durability of the electrospun nanofibers was dramatically enhanced by applying thermal welding near the melting temperature of the PVDF. Electrospinning of the ferroelectric polymer−metallic nanowire composite material provides a facile approach to fabricate triboelectric layers with a large surface area and a high surface charge density.

2. Results and Discussion

Figure 1a presents a schematic illustration of the TENGs pre-pared using two electrospun triboelectric layers: (i) AgNW-embedded ferroelectric PVDF composite nanofibers with a highly negative triboelectric potential, and (ii) nylon nanofibers with a highly positive triboelectric potential. Scanning elec-tron microscopy (SEM) images of the as-spun PVDF–AgNW composite (3 wt% AgNWs) and nylon nanofibers triboelec-tric layers are shown in Figure 1b. Both layers exhibited com-pletely bead-free fibrous structures with uniform diameters of 605 (±127) nm for the PVDF–AgNWs and 219 (±55) nm for the nylon. Transmission electron microscopy (TEM) images of the PVDF–AgNW nanofiber mats (inset in Figure 1b) indicated that the AgNWs were embedded inside the PVDF nanofiber and aligned along the fiber direction. The elongation of the fluid during jet travel facilitated AgNW alignment along the fiber axis. This alignment was also observed among the carbon nanotubes embedded in the electrospun PVDF nanofibers.[36,38] Note that the nanofibrous structures of the triboelectric layers increased the effective surface area[11] and, thus, enhanced the


Figure 1. a) Schematic diagrams of the TENGs based on the PVDF–AgNW composite and nylon nanofibers prepared by electrospinning methods. b) SEM images of the electrospun PVDF–AgNW composite and nylon nanofibers. The inset shows a TEM image of the PVDF–AgNW composite nanofibers. c) KPFM images of the surfaces of the pristine PVDF and PVDF–AgNW composite nanofibers. The right panel shows the surface potentials of the nanofibers. d) Schematic illustration of the electrospinning process applied to a PVDF solution. e) Schematic band diagrams explaining the TENG operation mechanism.



mechanical contact between the two triboelectric layers, thereby generating a higher number of induced triboelectric charges and enhancing the output voltage of the TENGs (Figure S1, Supporting Information). Figure 1c shows the noncontact mode KPFM images of the electrospun pristine PVDF and PVDF–AgNW composite nanofibers obtained using a Pt tip. It is interesting to note that the addition of small amounts of AgNWs to the PVDF nanofibers shifted the surface charge potential toward negative values compared with those obtained from the pristine PVDF, even though the samples were pre-pared under the same spinning conditions. The negative shift in the surface charge potential of the PVDF–AgNW nanofibers was expected to increase the triboelectric potential difference between the top and bottom layers,[34,37,39] thereby enhancing the TENG performance.

As described in the schematic diagram of the electrospin-ning process (Figure 1d), a viscous polymer solution was emitted from a Taylor cone at the needle tip and collected at the Al foil collector upon application of a high electrostatic field between the Taylor cone and the Al foil. Uniaxial stretching of the polymer chains facilitated the formation of highly oriented crystalline β-phases (rather than randomly oriented α-phases) along the fiber axis.[29,30,36] At the same time, electrical poling by the external electric field applied between the needle and the collector substrate aligned the dipoles of the PVDF chains along the direction of the external electric field. The field-induced dipole orientation facilitated the formation of β-phase crys-tals during the electrospinning process.[29,31,37,40] The crystal-line β-phase is characterized by a larger degree of polarization per unit cell compared with the α-phase, and a higher surface

charge density may be obtained on the β-phase surface. Coope-rative mutual alignment of the PVDF chains with the AgNWs in the electrospun PVDF–AgNW composite nanofibers resulted from the interfacial interactions between the PVDF and nano-wires, which synergistically promoted β-phase formation. The crystalline phase transformation of the PVDF nanofibers will be discussed in detail below.

Figure 1e illustrates the energy band diagrams of the two nanofibrous triboelectric layers (PVDF–AgNW and nylon) pre-pared by the electrospinning processes. The surface charge potential on the PVDF–AgNW composite nanofibers was affected by the electrically induced ferroelectric polarization of the PVDF (which depended on the crystalline phase). The dipole polarization induced by the poling of pristine PVDF nanofibers shifted the Fermi level toward negative values, resulting in a negative potential on the PVDF surface (left panel of Figure 1e). The downward shift in the Fermi level increased the charge transfer to the nylon surface. Note that the addition of AgNWs to the PVDF nanofibers further shifted the Fermi level downward (right panel of Figure 1e), which enhanced the triboelectric potential difference between the two triboelectric layers. As a result, greater quantities of surface charges could be transferred to the bottom nylon nanofibers.

Figure 2a shows a schematic diagram of the operation mechanism of the contact-mode TENGs based on the top PVDF–AgNW and bottom nylon nanofiber mats. Forced con-tact between two triboelectric layers induced the collection of opposite triboelectric charges (negative charges on PVDF and positive charges on nylon) with equal densities on each poly mer surface.[7,9] Release of the contact drove electrons through the


Figure 2. a) Schematic diagram illustrating the operation of the contact-mode TENGs based on PVDF–AgNW composite and nylon nanofibers. b) Output voltage and current of the TENGs. c) Maximum output voltage and current extracted from panel (b). d) SEM and TEM images of the PVDF–AgNW composite nanofibers prepared with various AgNW concentrations. e) Surface potentials of the PVDF–AgNW composite nanofibers with various AgNW concentrations.



external circuits, resulting in a negative voltage and current. Subsequent contact induced a positive voltage and current because the transferred electrons were driven back to make elec-trostatic equilibrium. Figure 2b plots the output voltage/current produced by the TENGs based on bottom nylon nanofiber mats and top PVDF–AgNW nanofiber mats in five AgNW concentra-tions (0, 1, 3, 7, and 10 wt%). The 5 N force was applied during contact–separation cycle. As the AgNW weight ratio increased from 0 to 3 wt%, the output voltage increased sharply from 156 to 240 V. The output voltage decreased to 150 V, however, upon the addition of more nanowires (up to 7 wt%). The output cur-rent followed a trend similar to that obtained from the output voltage. The maximum voltage and current outputs of the TENGs are summarized in Figure 2c.

The surface morphologies were investigated because TENG performance is closely related to the mechanical contact area between two triboelectric layers. Figure 2d presents SEM images of the electrospun PVDF–AgNW nanofiber mats. Although the AgNW concentration increased to 3 wt%, uni-form bead-free fibrous structures without nanowire aggrega-tion were observed over a large area, and the average diameter of the nanofibers remained fairly constant. Further additions of AgNWs, however, generated film-like regions without fibrous structures on the surfaces because nanowire aggregation in the solution state disrupted the uniform jetting from the needle.[41,42] Nanowire aggregation was confirmed by collecting optical microscopy images of the spin-coated PVDF–AgNW films (Figure S2, Supporting Information). AgNW additions of 1–3 wt% resulted in nanowires that were uniformly distributed throughout the PVDF matrix. By contrast, nanowire agglom-erates were observed in both the 7 and 10 wt% films. The suppressed TENG performances of the 7 and 10 wt% PVDF–AgNW nanofibers could be explained in terms of the reduced mechanical contact area of the films.

The surface charge potential on the PVDF–AgNW nanofiber mats was monitored as a function of the AgNW content by col-lecting KPFM images using a Pt tip, as shown in Figure 2e. The surface potentials of the PVDF–AgNW nanofibers decreased from −225 to −441 mV as the AgNW concentration increased from 0 to 3 wt%. The PVDF–AgNW nanofibers correspond-ingly exhibited larger triboelectric potential differences with the bottom nylon nanofibers. A larger triboelectric potential difference resulted in a better TENG output performance. The enhanced output performances observed upon the addi-tion of up to 3 wt% AgNWs were understood as arising mainly from the enhanced triboelectric potential difference between the contact surfaces and not from the surface morphologies of the PVDF nanofibers. The addition of AgNWs in amounts exceeding 3 wt% increased (toward positive values) the surface potential, which reduced the triboelectric potential difference with the bottom nylon nanofibers. The reduction of the tribo-electric potential difference as well as the mechanical contact area (from the film-like area) was reflected in the output perfor-mances of the TENGs (see Figure 2c).

The surface charge potential, measured as a function of the AgNW content in the PVDF nanofibers, was correlated with the crystalline phase of the ferroelectric PVDF. The PVDF crystals exhibited three different chain conformations: (i) an all-trans (TTTT) conformation planar zigzag β-phase, (ii) alternating

trans-gauche (TGTG′) conformations for α- and δ-phases, and (iii) T3GT3G′ conformation for the γ- and ε-phases.[43,44] The dipoles of PVDF chains packed into crystal lattices are either additive (β-, γ-, and δ-phases) or canceled (α- and ε-phases). Because the crystalline β-phase provided the most polar crystal-line phase with the highest degree of polarization per unit cell, the highest surface charge density among the five phases could be obtained from the β-phase PVDF triboelectric surface.[45] The crystalline phases in the PVDF–AgNW composite nanofibers were investigated using XRD measurements. Figure 3a shows the XRD patterns of the PVDF–AgNW composite nanofibers prepared with various AgNW concentrations. The intensity of the Ag diffraction peak at 37.9° was directly proportional to the amount of AgNWs added to the PVDF solution. The pat-terns exhibited diffraction peaks at 2θ = 17.6°, 18.3°, and 19.9°, which corresponded to the (100), (020), and (110) α-phase reflections, respectively. The strong diffraction peak positioned at 2θ = 20.7° corresponded to the (110)(200) reflection of the crystalline β-phase. These results indicated that the electrospun PVDF–AgNW nanofibers were composed of a blend of α-phase and β-phase crystals.

The relative quantities of the β-phase and α-phase in the PVDF–AgNW nanofibers were determined as a function of the AgNW concentration (Figure 3b). The β-phase/α-phase ratio was calculated from the ratio of the area corresponding to β-phase part (red arrow in the right panel of Figure 3a) to the total area (red, orange, green, and blue arrows in the right panel of Figure 3a). The β-phase/α-phase ratio of the pristine PVDF nanofibers was 0.81, and this ratio increased sharply as the nanowire concentration increased up to 3 wt%. These results indicated that the AgNWs played a significant role in enhancing the β-phase content of the crystals. AgNWs were negatively charged due to the presence of the oxo- and hydroxo-groups on the nanowire surfaces as a result of silver oxidation.[46] The positive CH2 dipoles (δ+) in the PVDF chains could form an ion-dipole interaction with the negatively charged AgNW surface, which promoted the formation of the β-phase crystals.[47,48] The addition of AgNWs enhanced the β-phase crystal formation, which negatively shifted the surface charge potential of the PVDF–AgNW nanofibers (consistent with the KPFM results).[36,38,44] A high negative surface charge potential at the top PVDF–AgNW nanofibers yielded a large potential difference with the bottom nylon nanofibers and improved the TENG performance. Further additions of AgNWs beyond 3 wt% decreased the β-phase/α-phase ratio. The ran-domly oriented nanowire agglomerates in the film-like region (not aligned parallel to the fiber direction) in both the 7 and 10 wt% samples appeared to prohibit the unidirectional fer-roelectric dipole alignment of the PVDF chains. The reduced β-phase formation increased the surface charge potential (toward positive values), consistent with the KPFM results.

The charge trapping properties of the AgNWs embedded in PVDF were explored by measuring the specific capacitance of the AgNW–PVDF films prepared with various AgNW concen-trations as a function of frequency (Figure 3c). The dielectric constants (k) calculated from the specific capacitance at 100 Hz are summarized in Figure 3d. The k value of the AgNW–PVDF films increased gradually from 0 to 10 wt%, indicating that charge trapping at the PVDF–AgNW triboelectric layer was




enhanced by the addition of nanowires. This enhanced charge trapping, and the low (large negative) surface charge potential (confirmed by the KPFM results), enhanced the output voltage/current properties of the TENGs. The addition of more AgNWs beyond 3 wt%, however, significantly increased the leakage current through the nanowire agglomerates, as shown in Figure 3e. The nanowire agglomerates in the film-like regions of both the 7 and 10 wt% samples induced the local induction charges on the nanowire surfaces to flow along the film thick-ness direction and become neutralized in the nanofiber.[36] Both the leakage current and the film-like surface morphologies low-ered the TENG performances.

The mechanical stabilities of TENGs based on a top PVDF–AgNW and bottom nylon nanofiber mat were measured after repeated operations over 60 min (Figure 4). The TENG opera-tion was unstable (Figure 4a) because the nanofibers delami-nated from the top surface, and some fibers were attached to the bottom nylon surface during operation. Deformation of the nanofibers dramatically reduced the output performances and resulted in unstable TENG operation. The mechanical sta-bility of the as-spun PVDF nanofiber mats improved dramati-cally upon application of a welding process to the nanofibers comprising thermal treatment near the melting temperature of PVDF (≈160 °C). Thermal treatment for a short time of 30 s cre-ated an extensive fused network among the nanofibers without

destroying the fibrous structure. The thermal welding process dramatically enhanced the TENG operational stability, even after 60 min of operation, without reducing the TENG perfor-mance, as shown in Figure 4b. Further increases in the thermal treatment time to 180 s destroyed the fibrous morphology of the nanofiber mats, which reduced the voltage output, even though the durability was enhanced compared with the pristine case (Figure S3, Supporting Information). Finally, we successfully demonstrated the self-powering of a commercial liquid crystal display using our optimized TENGs as shown in Figure S4 in the Supporting Information.

3. Conclusion

In conclusion, we report the development of high-performance TENGs using an electrospun PVDF–AgNW composite and nylon nanofibers as the triboelectric layers. The electrospin-ning process and the addition of metallic nanowires to PVDF promoted the formation of the polar crystalline β-phase. The addition of AgNWs enhanced the induced tribocharge trapping capabilities. The enhanced surface charge potential and charge trapping properties of the PVDF–AgNW composite nanofibers significantly enhanced the TENG output performances. Finally, thermal annealing dramatically enhanced the mechanical


Figure 3. a) X-ray diffraction patterns obtained from the electrospun PVDF–AgNW nanofibers prepared with various AgNW concentrations. b) Relative amounts of the β-phase and α-phase in the PVDF–AgNW nanofibers. c) Specific capacitance of the PVDF–AgNW composite films prepared with various AgNW concentrations as a function of frequency. d) Dielectric constant of the PVDF–AgNW composite films as a function of the AgNW concentrations. e) Leakage current measured across the PVDF–AgNW composite films prepared with various AgNW concentrations as a function of the applied voltage.


1703778 (6 of 7) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Funct. Mater. 2017, 1703778

stability of the nanofibrous triboelectric layers while main-taining the TENG performance.

4. Experimental SectionFabrication of TENG: The AgNWs were purchased form Nanopyxis

Co (diameter = 25–35 nm and length = 20–30 nm). PVDF and nylon pellets were purchased form Sigma-Aldrich. The 28 wt% PVDF–AgNWs solutions with various AgNWs concentration of 0, 1, 3, 7, and 10 wt% [co-solvent of N,N-dimethyl acetamide and acetone (7:3 volume ratio)] were sonicated for 5 min and then stirred for 3 h to prepare uniform dispersion. The 8 wt% nylon was dissolved in formic acid. The electrospinning (eS-robot, NanoNC) was performed at a high voltage of 18 kV and the feed rate of flow-metering pump of 3 mL h−1. The distance between the syringe and collector was fixed to be 15 cm. The temperature was kept at 25 °C and relative humidity was around 30%. Nanofibers were spun over 2 h on the Al foil. The nanofibers mat collected to Al foil was cut at 2 × 2 cm2 size by scissor and then attached to the acryl support. Figure S5 in the Supporting Information shows the photographic image of the TENG device. The metallic Cu wires were used to connect Al electrodes to the measurement system. For welding of nanofibers, the thermal treatment was performed at 160 °C on hot plate and then naturally cooled down to room temperature in order to enhance the mechanical stability for TENG.

Measurements: The surface morphologies of electrospun nanofibers were measured by field-emission scanning electron microscopy (JSM-7600F, JEOL Ltd.). The AgNWs embedded in the PVDF nanofibers were observed by transmission electron microscopy (JEM-2100F, JEOL Ltd.). The crystalline structure of the nanofibers was measured by X-ray diffraction (D8 Discover) and Fourier transform infrared (IFS66v-S). The noncontact mode Kelvin probe force microscopy (Park systems XE-100) equipped with a Pt/Cr-coated silicon tip (radius < 25 nm) was employed to measure the surface charge potentials. The applied frequency and phase were 17 kHz and −90°, applying 2V AC, respectively. All electrical properties of TENG were measured by using a Tektronix DPO 3052 Digital Phosphor Oscilloscope and a low noise current preamplifier (Model No. SR570, Stanford Research Systems, Inc.). The capacitance measurements of the samples were carried out at 0.05 V (AC) using a Hewlett Packard 4284A Precision LCR Meter.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsS.C. and H.K. contributed equally to this work. This work was financially supported by a grant from the Korea Institute of Industrial Technology as“Development of smart textronic products based on electronic fibers and textiles” (kitech JA-17-0045) and the Center for Advanced Soft Electronics (CASE) under the Global Frontier Research Program (NRF-2013M3A6A5073177), Korea. J. Y. Lee was financially supported by Basic Research Science Program through the National Research Foundation of Korea (NRF) grants funded by Korean Government (MSIP) (2014M3A7B4052200), Korea.

Conflict of InterestThe authors declare no conflict of interest.

Keywordselectrospinning, ferroelectric polymers, nanofibers, silver nanowires, triboelectric nanogenerators

Received: July 9, 2017Revised: September 13, 2017

Published online:

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Figure 4. a) SEM image of as-spun PVDF–AgNW composite nanofibers. The right panel shows the output voltage performances of the TENGs after repeated operation over 60 min. b) SEM image of the PVDF–AgNW composite nanofibers annealed at 160 °C for 30 s. The right panel shows the output voltage performances of the annealed TENGs after repeated operation for 60 min.


1703778 (7 of 7) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Funct. Mater. 2017, 1703778

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