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Hierarchically Surface-Textured Ultrastable Hybrid Film for Large-Scale Triboelectric Nanogenerators

Hyunhwan Lee, Han Eol Lee, Hee Seung Wang, Seung-Mo Kang, Daewon Lee, Yun Hyeok Kim, Jung Ho Shin, Young-Woo Lim, Keon Jae Lee,* and Byeong-Soo Bae*

The supply of electrical power is indispensable in the internet of things (IoT) area. Triboelectric nanogenerators (TENGs) has been largely used as a power supply due to simple device structure and various possibilities of material selection. Although various surface modifications of TENG are utilized for improving the output performance of TENG, these methods have significant drawbacks of high-cost fabrication process, limited device size, and wearing out. Here, a mechanically robust TENG using a newly developed surface-tex-tured glass fabric reinforced siloxane hybrid film (SGH film) is presented. The large-scale SGH film is cost-effectively fabricated via a simple roll-to-plate and UV curing process. The film is repeatedly duplicated over 100 times using one pre-treated substrate mold. SGH film exhibits superior thermal and mechan-ical stability, compared to other polymer films. Finally, surface textured TENG (THTENG) is achieved using extremely stable SGH film, resulting in enhanced output power compared to flat TENG and high durability during the 100 000 cycles of TENG operation.

DOI: 10.1002/adfm.202005610

H. Lee, Dr. H. S. Wang, S.-M. Kang, Y. H. Kim, J. H. Shin, Dr. Y.-W. Lim, Prof. K. J. Lee, Prof. B.-S. BaeWearable Platform Materials Technology Center Department of Materials Science and EngineeringKorea Advanced Institute of Science and Technology (KAIST)291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of KoreaE-mail: keonlee@kaist.ac.kr; bsbae@kaist.ac.krDr. H. E. LeeDepartment of Mechanical EngineeringMassachusetts Institute of TechnologyCambridge, MA 02139, USAD. LeeDepartment of Materials Science and EngineeringUniversity of California at BerkeleyBerkeley, CA 94720, USAD. LeeMaterials Sciences DivisionLawrence Berkeley National LaboratoryBerkeley, CA 94720, USA

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202005610.

of large-scale integrated devices such as smartphones, health monitoring devices, and wearable computing systems.[11–22] A triboelectric nanogenerator (TENG), which transforms mechanical work to electrical energy by using triboelectrification and static electricity, has been suggested as a potential power source due to its low-cost and diverse possibilities of materials selec-tion, outstanding power generation, and simple device structure for integrating a system level.[23–30]

The harvested energy from a TENG is strongly dependent on its electrified surface morphology, which is related to the effective charged area of the device.[25–27,31–35] Several researchers have investigated surface-modification methods such as chemical/ion treatment, block copoly mer patterning, and micro/

nanograting molding.[25,26,36–41] However, these methods have several limitations including restriction in processable size, complex/high-cost fabrication method, and low durability of nano/microstructures. For example, a surface-modified TENG was gradually worn out after energy harvesting processes with periodic frictions, which degraded the performance degrada-tion and decreased the lifetime.[42,43] Despite previous studies about durability enhancement, the intrinsic material issue of the TENG has not been solved due to the mechanical limits of conventional polymers such as polyethylene terephthalate (PET), polyimide (PI), polydimethylsiloxane (PDMS), and poly-vinylidene fluoride (PVDF).

Recently, our group developed an organic–inorganic siloxane hybrid material (hybrimer) using a sol-gel reaction and a sub-sequent photocuring process.[44–46] The hybrimer exhibited the mechanical/chemical/thermal stabilities of inorganic mate-rial and flexible properties of organic material. In particular, the mechanical properties of the hybrimer could be further enhanced by monolithically embedding woven glass fabrics (with the elastic modulus of ≈10 GPa and mechanical stability during fatigue tests for 100000 cycles).[47–51] Although the glass fabric-reinforced hybrimer (GFRHybrimer) has been realized in a flexible substrate and chemical/thermal encapsulation, the appli-cation to energy harvesting devices has not been exploited yet.

Herein, we developed a hierarchically surface-textured GFRHybrimer (SGH) film with superior mechanical stability via a simple sol-gel reaction and rapid hydrolytic photoreac-tion. The hierarchical surface patterning for the large-scale

1. Introduction

With the advent of the internet of things (IoT) era, portable electronic devices have been spotlighted for their various appli-cations spanning from personal health monitoring to hypercon-nected social interactions.[1–10] The supply of electrical power is considered a key technology for the continuous operation

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GFRHybrimer film (370 × 470 mm2) was enabled by micro-dome molding and nanopore formation on the film surface during the synthesizing process. The overall surface area of the SGH film increased up to 210% due to its hierarchical surface nano/micro structure. We were able to repeatedly duplicate the SGH film over 100 times using one microdome mold, thus enabling a cost-effective, recyclable, and simple roll-to-plate (R2P) fabrication process. To confirm the superior mechanical properties of the proposed SGH film, we examined various mechanical analyses, including nanoindentation, tensile test, thermomechanical analysis (TMA), and dynamic mechanical analysis (DMA) with conventional polymer films. Finally, we successfully applied the SGH film to a TENG application. Prior to device fabrication, the output enhancing effect of the hier-archical structure was theoretically calculated by finite element method (FEM) simulation. The textured hybrimer-assisted TENG (THTENG) showed open-circuit voltage and short-circuit current enhancement by ≈4 times and ≈2.7 times higher than a flat TENG. The THTENG charged a 22 µF capacitor, and oper-ated 120 commercial red, green, and blue (RGB) light-emitting diodes (LEDs) without any external power supply.

2. Results

Figure 1a schematically illustrates the fabrication procedure of the proposed SGH film. The following is the process in detail: i) Prior to fabrication of the flexible SGH film, epoxy-functionalized siloxane (EFS) resin was synthesized by a non- hydrolytic sol-gel condensation reaction between methoxy groups of 3-(2-Trimethoxysilylethyl)cyclohexene oxide (ECTS) and silanol groups of dihydroxydiphenylsilane (DPSD). ii) To prepare the surface-patterned mold for the SGH film, micro-dome patterns were inversely engraved on a glass substrate by HF-based etching and surface smoothing process. The size of the engraved microdome on the glass surface was con-trolled in the range of 0.5–2.5  µm by adjusting the etching and smoothing process time. iii) EFS resin was poured on the inverse microdome-textured glass (IMT glass) mold, entirely embedding the glass fabric. iv) An R2P pressing process was carried out for the polymerization of EFS with bis[1-ethyl(3-oxe-tanyl)]methyl ether (DOX). In this step, nano-bubbles, which were initially contained in the EFS resin, were diffused and congregated at the EFS/IMT glass interface by roll compression (Figure S1, Supporting Information). The pressurized EFS was photo-cured by UV light (wavelength of 340 nm and power of 85 mW cm−2), forming a surface-textured hybrimer film (SGH film). v) The SGH film was completely fabricated by exfoli-ating the cured film from the IMT glass. As shown in the inset image of Figure  1a–v, the detached SGH film had numerous 100–500 nm-sized nanopores (brown-colored region) on its sur-face (green-colored region), which were made by nano-bubble bursts in the detachment stage. Figure 1b shows a photograph of the 370 × 470 mm2-sized SGH film (red dotted line). The developed film was fabricated by a scalable R2P pressing pro-cess as shown in the inset of Figure  1b, which enabled us to freely modulate the film size from millimeter-scale to tens of centimeter-scale. The SGH film structure was analyzed by scanning electron microscopy (SEM) and energy-dispersive

spectrometer (EDS). Figure  1c and its upper inset exhibit a tilted-view and an enlarged top-view SEM image of the SGH film. SGH film had microscale dome patterns with nanopores (diameter of 100–500 nm) on the top surface. The lower inset of Figure 1c presents a cross-sectional SEM image of the SGH film. The glass fabric (yellow-colored region) was stably weaved, embedded in cured EFS (blue-colored region). Figure  1d pre-sents EDS elemental mapping results for the SGH film, which indicate the location of the hybrimer and glass fabric. The hybrimer, which was composed of organic–inorganic hybrid material, was detected as a carbon-dominant region, while the glass fabric revealed only silicon and oxygen atoms.

To increase the effective surface area, the surface structure of the SGH film was sophisticatedly modulated by forming micro/nanopatterns. Figure 2a and its inset image exhibits the SEM image of the SGH film, consisting of micro-domes with nanopores. As shown in the inset of Figure 2a, the SGH film had several random nanopores on each microdome, which cre-ated a hierarchical structure. According to numerical calcula-tion, the surface area of the SGH film increased up to 210% compared to flat film (see Supporting Information for detailed calculation procedure of the SGH film surface area). Nano-pores at the film surface were decided by movement and con-centration of fine bubbles at the interface between the uncured hybrimer (EFS) resin and the glass mold during the pressing/curing process. Those behaviors of fine bubbles were closely related to the resin viscosity, which is decided by the molar ratio of ECTS and DPSD. To realize the elaborate hierarchical structures, a new hybrimer with low viscosity was developed by adjusting the ratio of ECTS and DPSD. The hybrimer resin was scrutinized at a molecular level by analyses with nuclear mag-netic resonance spectroscopy (NMR) and Fourier-transform infrared spectroscopy (FT-IR). Figure  2b shows the 29Si-NMR result of the EFS resin, revealing the condensation degree of siloxane bonds (SiOSi) in the organic–inorganic hybrid material. The degree of condensation was calculated by the fol-lowing equation,[52]

The degree of condensation2 2 3

2 3

1 2 1 2 3

0 1 2 0 1 2 3) )( (=+ + + +

+ + + + + +D D T T T

D D D T T T T (1)

where Dn and Tn are the peak intensity values (integrated inten-sity) of specific siloxane bonds, designated on a constitution formula in Figure  2b. According to Equation  1, the degree of condensation of the EFS resin was calculated to 82%, indicating a DPSD to ECTS molar ratio of 1:1. This molecular ratio of the developed resin indicates sparse siloxane bonds in the complex network, and lower viscosity than normal hybrimer with a 2:3 ratio (DPSD:ECTS) (Figure S2, Supporting Informa-tion).[46] Figure  2c is the result of FT-IR analysis, examining the molecular structure of the EFS resin based on the struc-tures of its building blocks (i.e., DPSD and ECTS). As depicted in Figure  2c-i, the sharp siloxane peak (wavenumber of 1000–1200 cm−1) of the EFS resin originated from the sol-gel condensation reaction with the silanol group of DPSD (peak at 3450 cm−1) and methoxy group of ECTS (peak at 2840 cm−1).[53] Figure 2c-ii displays FT-IR spectra of EFS resin and SGH film, which show the effect of the UV curing pro-cess on the chemical structure. During the UV curing process in the SGH film fabrication, epoxy rings in an EFS resin were

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destroyed by triarylsulfonium hexafluoroantimonate as a photo acid generator, which made the epoxy peak of 883 cm−1 in SGH film vanish. Figure 2d displays optical images of a flat film and a SGH film, which were made by newly synthesized hybrimer material. The textured SGH film was opaque, while the flat film was not, due to the SGH film‘s random light reflectance at the surface. Quantification of SGH film surfaces was conducted by 3D optical microscopy images, as shown in the right images of Figure 2d. The SGH film exhibited a root-mean-square surface

roughness (Rrms) of 462 nm, which is higher than the flat film (Rrms  = 60  nm). Figure  2e shows the parallel transmittance spectra of a flat film and different-sized SGH films, which is a method of optically analyzing the surface structure in a trans-parent material. Although the flat film has an intrinsic trans-parency of 90%, the transmittance of our 2.5 SGH film (SGH film with 2.5 µm-size dome patterns) was about 60% due to the diffused light reflection of its periodic hierarchical structures. According to these results, the hierarchical structure of the

Figure 1. a) Synthetic process of EFS resin and the fabrication process of surface textured glass-fabric reinforced hybrimer film (SGH film). b) Photo-graph of large-scaled (370 × 470 mm2) SGH film. The inset image exhibits the large-scalable R2P equipment. c) Tilted SEM image of SGH film. The upper inset image shows a magnified image of the nanopore surface. The lower inset image displays the cross-sectional SEM image of SGH film. d) EDS mapping of SGH film (red: carbon, green: silicon, cyan: oxygen).

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Figure 2. a) Surface SEM image of SGH film. The inset image exhibits a magnified SEM image of several microdomes with nanopores. b) 29Si NMR analysis of EFS resin. c) FT-IR analysis. i) Silane precursors and EFS resin. ii) EFS resin and SGH film. The inset image of (c-ii) represents the specific region in wavenumber including epoxy peak (900–850 cm−1). d) Photograph images (left) and 3D optical microscopy images (right) of flat film (top) and SGH film (bottom), respectively. e) Parallel transmittance graph of flat film, and SGH films. The inset displays the path of incident light passing through the film.

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SGH film surface was successfully realized by development/synthesis of optimized viscous hybrimer resin.

To make the fabrication process cost effective, reusability of the micro-patterned mold is essential to repeatedly dupli-cate the SGH film. Figure 3a depicts the micro-patterned glass mold before (Figure  3a-i) and after the film was reproduced 100 times (Figure 3a-ii). No degradation was visually observed in the mold surface despite 100 times of the repeated R2P process. In addition, gloss of glass remained constant and the 3D structure of 2.5  µm IMT mold was unchanged after 100 times of film reproduction, implying IMT mold could be used infinitely (Figure S3, Supporting Information). To verify the reproducibility of inverse microdome patterns, Alphastep analysis was conducted to the IMT glass mold before and after film duplication. Regardless of the microdome size from 1 to 2.5 µm, the IMT glass molds maintained their surface rough-ness (Figure 3b). In the inset of Figure 3b, the statistical histo-gram of microdome roughness displays the best-fit Gaussian distribution of a 2.5  µm-sized IMT glass mold, showing negligible changes of surface roughness after several film duplications.

The mechanical stability of the SGH film is a crucial factor for an enduring usage of TENG-based energy harvesting appli-cations. Since conventional flexible plastic films are easily damaged and deformed during continuative TENG operation, mechanical properties such as high elastic modulus, effective modulus, and hardness are crucial to withstand its original structures under the continuative contact/separating pro-cess.[54–57] In addition, the films have to resist from the gener-ated heat during TENG operation. Therefore, the mechanical properties of the SGH film were investigated by various anal-yses such as a nanoindentation test, tensile test, TMA, and DMA. Figure  3c presents the result of nanoindentation test, comparing with PET, polyurethane (PU), and polymethylmeth-acrylate (PMMA) film. Our SGH film outperformed other polymer films, showing hardness (H) of 0.56 GPa and an effec-tive modulus (E*) of 11.9 GPa. Figure 3d displays the excellent elastic modulus of the SGH film in the tensile test. The SGH film (10.1 GPa) had a 2–3 times higher elastic modulus than the PET (5.6 GPa) and PI (3.9 GPa) which have been the most com-monly used films for the TENG.[58] These outstanding mechan-ical properties could be attributed to the inorganic bonds and complex network in the SGH film that strongly maintained its material structure under mechanical stress.[59] Figure 3e depicts the TMA results of the SGH film and conventional polymer films, namely, PET, PI, PU, and PMMA. Based on the TMA curves, the coefficient of thermal expansion (CTE) of the SGH film (13  ppm K−1) was 4–13 times lower than that of conven-tional polymer film, such as PET (55 ppm K−1), PMMA (80 ppm K−1), and PU (170 ppm K−1). Figure 3f presents the DMA pro-file of the SGH film compared to conventional films, which evaluate the viscoelastic behavior of the samples in heating con-ditions. Our SGH film retained its storage modulus of 3 GPa in the temperature range of 20–120 °C, since glass transition of the SGH film did not occur owing to the thermal/mechanical stability of the glass fabric-based framework.[50] These results confirm that, our stable SGH film is a suitable material for a TENG, as it can endure mechanical/thermal shocks from con-tinuous energy harvesting frictions.

Figure 4a-i depicts a 3D schematic illustration of the THTENG, which is composed of SGH film with Au electrode as a base material, and PET with an indium tin oxide (ITO) elec-trode as a counter material. The PET/ITO film with advantages of convenient usage, electrode element, large area application was selected as a counter triboelectric surface without any sur-face morphological modification as shown in the Figure S4, Supporting Information. The combined effects of contact elec-trification and electrostatic induction can explain energy har-vesting mechanism of THTENG.[60] After applying mechanical pressure (Figure  4a-ii), SGH film and PET are completely in contact, equalizing the Fermi levels between two materials. The surfaces of the SGH film and PET are accumulated by the same amount of negative and positive charges, because of their dif-ferences in work function (Φ). It is expected that the surface of SGH film possesses negative charges during triboelectrifica-tion because of the epoxy-crosslinked network.[61] A large dif-ference in electrical potentials is generated by separating the two materials (Figure 4a-iii). To balance the distance-dependent potential between two friction surfaces, electrons of the Au electrode flowed to the ITO electrode through an intercon-necting wire (Figure 4a-iii). When the surface distance between the two materials was decreased by mechanical force, the elec-trons returned to their original status, generating electrical power with an alternative current (Figure 4a-v).[62] The surface-engineering effect in the THTENG device was theoretically calculated by an FEM simulation in terms of the triboelectric potential. Figure  4b-i,ii displays the potential distribution of a hybrimer-based flat TENG and THTENG (2.5 µm-sized micro-domes with 100–500  nm-sized nanopores), respectively. The flowing charges (qf) between the two electrodes are explained by Equation 2:

1 2

1 2 1 2 2 1

σε εε ε ε ε

=+ +

qd

t d tf

(2)

where d is the distance between the base and the counter material, σ is the charge density on the friction surface, εn is the relative permittivity of the dielectric material, and tn is the film thickness. The triboelectric potential (Vt) is expressed by Equation 3.

Vq

S

t td

dt

f

ε ε εσ

ε= − + +

+

0

1

1

2

2 0

(3)

where S is the film area and ε0 is the permittivity of a vacuum condition. According to FEM calculations, while the electrical potentials of the THTENG and flat TENG were respectively 201.7  and 53.2  V on the material surface, showing a triboe-lectric potential increment of ≈4 times. In pressing measure-ments of TENGs, results similar to the FEM calculation were confirmed in a different pattern-sized THTENG, as displayed in Figure 4c. The samples were classified by their microdome size on the SGH film surface, including a flat TENG (no micro-dome), a 1.5 THTENG (1.5 µm-sized microdome), a 2 THTENG (2  µm-sized microdome), and a 2.5 THTENG (2.5  µm-sized microdome). The electrical properties of the TENGs were measured under a pressing force of 20 N using a periodic vibrating/pressing system. As the surface morphology of the THTENG was roughened, the generated voltage and current

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Figure 3. a) Photographs of micro-patterned glass mold mold i) before and ii) after 100 times of film reproduction. b) Surface roughness before and after 100 times of film reproduction. The right upper inset image shows the principle of measuring the surface roughness. The right lower inset image is the statistical distribution of the surface root-mean-square based on the 3D optical microscopy image. c) Hardness and effective modulus chart of conventional polymer films and SGH film. The inset image shows the configuration of the nanoindentation test. d) Stress versus strain curve of polymer films and SGH film. The inset image shows the configuration of the tensile test. e) TMA curves of polymer films and SGH film. f) DMA curves of polymer films and SGH film. The left inset image shows the configuration of DMA measurement. The right inset graph represents the storage modulus at 25 °C.

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increased from 40  V and 4.3 µA to 80  V and 12.5 µA, respec-tively (Figure  4c,d). These results could be interpreted by the increment of the charge accumulation due to the modified sur-face area in the THTENG.

The 2.5 THTENG, which had the greatest output power compared to flat TENG and other THTENGs, was utilized for characterizing the detailed electrical/mechanical proper-ties. As described in Figure 5a, the normalized output voltage

Figure 4. a) 3D schematic illustration of THTENG. i) Initial state before tapping. ii) Contact status by external force with the triboelectrification. iii) Electron flow due to electrical potential difference with elimination of external force. iv) Returning to the initial position with complete screening of the triboelectric charges. v) Repeating the contact to triboelectrification by external force. b) FEM simulation results of i) flat TENG and ii) THTENG. c) Normalized open-circuit voltage of flat TENG and 1.5, 2, and 2.5 THTENG at tapping force of 20 N. d) Short-circuit current of flat TENG and 1.5, 2, and 2.5 THTENG at tapping force of 20 N.

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and current of our THTENG were proportional to the applied external force. The open-circuit voltage and short-circuit current of the 2.5 THTENG increased up to 210 V and 24 µA, respec-tively, at an 80 N tapping force. This output increasing effect of

the THTENG by applied force could be explained by a superior intrinsic elasticity of our organic–inorganic hybrid material, which was enabled to easily deform and return to the original shape during the pressing test. Figure  5b presents electrical

Figure 5. a) Open-circuit voltage and short-circuit current of 2.5 THTENG at an increase in the tapping force from 20 to 80 N. The inset image is a schematic of the working mechanism to increase the tapping force. b) Output voltage and current of the 2.5 THTENG according to the external load resistance. The inset graph shows the power density calculated based on the output voltage and current. c) Charging a capacitor by flat TENG and 2.5 THTENG with a rectifying circuit (inset). d) Stability of THTENG by monitoring the output voltage during 100 000 cycles of 2.5 THTENG opera-tion. The inset image shows the output current during 100,000 cycles of 2.5 THTENG operation. e) Set up for large-scale (30 × 30 cm2) THTENG. The bottom image is an image of THTENG composed of ITO/PET film and SGH film with metal electrode. f) Lighting the serially connected 750 RGB LEDs by THTENG operation without any external power sources.

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performances of the THTENG as a function of load resistance change. As the load resistance increased, the THTENG voltage improved, while the current declined due to the ohmic loss at the increased load resistance. The maximum output power of the THTENG was 824 mW m−2 at a 106 Ω load, which was the intersection region of the voltage and current graphs, as shown in the inset of Figure  5b. For comparison with other works, power density of related works was summarized in Figure S5, Supporting Information.[14,22,34,35,63–68] To verify the suitability of our THTENG for energy storage application, the charging test was conducted by connecting the THTENG to a 22 µF capacitor with a full-wave bridge rectifying circuit (Figure 5c). The peri-odically pressed THTENG (with 10 Hz frequency) successfully charged the capacitor within ≈700  s up to 880 µC, which was about 4 times faster than the hybrimer-based flat TENG. The mechanical stability of the THTENG was verified by a periodic pressing test at a force of 80 N. As plotted in Figure  5d, our THTENG exhibited an excellent durability in the harsh condi-tions of 100 000 cycles of contacting/separating movements. As shown in Figures S6 and S7, Supporting Information, the THTENG surface showed negligible abrasion after 100 000 tap-ping cycles. The inset of Figure  5d is a current versus time graph of the THTENG during the pressing test. THTENG displayed an imperceptible output degradation owing to the unchanged surface. This outstanding reliability of the THTENG could be achieved by the superior mechanical properties of the SGH film (e.g., CTE ≈13 ppm K−1, E ≈11 GPa, and H ≈0.5 GPa) despite its micro/nano-scale surface structure.

Finally, large-scale THTENG (30 × 30 cm2) was fabricated by the developed photoreaction-based R2P process, as shown in Figure 5e. The output voltage during the operation of large-scale THTENG was represented in Figure S8, Supporting Infor-mation. The output power of the THTENG was directly utilized to operate commercial light-emitting diodes (LEDs). The gen-erated electrical power repeatedly turned on 750 RGB LEDs as shown in Figure 5f. These results demonstrated that our large-scale THTENG has a potential for a practical power source in the future IoT era.

In summary, we realized a hierarchically surface-patterned hybrimer film, enabled by the photoreaction-based R2P pro-cess. The development of the hybrid material and the optimiza-tion of the fabrication process enabled the hierarchical surface structure on the SGH film with microdomes and nanopores, which increased the surface area up to 210% compared to flat film. Large-scale SGH films were repeatedly duplicated over 100 times by a microdome-patterned glass mold. The mechan-ical/thermal stability of the SGH film was experimentally analyzed by TMA, DMA, nanoindentation and tensile tests, showing 13  ppm K−1 of CTE, 3  GPa of storage modulus, and 0.56  GPa of hardness. The surface-engineering effect of the TENG was theoretically calculated by the FEM simulations, and experimentally confirmed by vertical pressing tests, generating a maximum output voltage of 210  V and a current of 24 µA. To elucidate the practical usages of SGH films, the THTENG was tested by recharging the commercial capacitor, and directly turning on the 120 RGB LEDs without additional power supply. Accordingly, we believe that the developed ultrastable and practical material suggests a new approach to generalize cost-effective triboelectric energy harvesters for future self-powered

systems. We currently have a future plan to develop a perfor-mance-enhanced THTENG by optimizing the device structure (e.g., macroscopic device shape, weave density of glass fabric) and the power generation method.

3. Experimental SectionMaterials: ECTS (ShinEtsu, Japan), DPSD (Gelest, U.S.A), barium

hydroxide monohydrate, triarylsulfonium hexafluoroantimonate (Sigma Aldrich, USA) were purchased and used without purification. The 25  µm  thick-glass fabric (Nittobo, Japan) was purchased and used without any pretreatment. The glass fabric is used in a woven form, mainly composed of SiO2 (52–56 wt%), CaO (20–25 wt%), and Al2O3 (12–16 wt%), which were used in a woven form. The density of glass fabric is 2.6 g cm−3. Even the weave density of glass fabric could affect the mechanical properties, the weave density of glass fabric was not considered in this research to reduce the variables in the experiment.[69–71]

Synthesis and Characterization of EFS Resin: ECTS, DPSD and barium hydroxide monohydrate were mixed in a two-neck flask with a molar ratio of 100:100:0.1. Then the solution was stirred vigorously and a sol-gel reaction proceeded at 80 °C for 4 h. After the reaction, clear and viscous EFS resin was obtained. The sol-gel condensation reaction was illustrated in the Supporting Information. The degree of condensation was confirmed by 29Si-NMR analysis (400  MHz NMR, Bruker). FT-IR analysis was conducted using a FT-IR spectroscopy (FT-IR 4600, JASCO).

Fabrication Procedure of SGH Film: The patterned glass was placed on an 80 °C-heated hot plate. Two layers of the 25 µm-thick woven glass fabrics were stacked on the patterned glass. The total thickness of SGH film was proportional to the number of glass fabric and in this research two layers of 25 µm-thick glass fabric were used to fabricate the SGH film for comparison with conventional polymer films with 50  µm  thickness (Figure S9, Supporting Information). The tensile property of the SGH film according to the number of glass fabric was displayed in Figure S10, Supporting Information. The EFS resin was poured on the sample (glass fabric/patterned glass). After PET film covered the resin, the roller of R2P machine pressed the sample on the patterned glass to make a thin hybrimer film with microdome-patterned surface. Numerous microdomes were confirmed at the surface of the fabricated SGH film as shown in low resolution surficial SEM image of Figure S11, Supporting Information. Since the number and the size of nanopores are closely related with the resin viscosity and the process pressure, the resin with viscosity of 105 cP was used for SGH film by 0.2 MPa-pressing process with 5000 mm min−1 of roller pressing speed, as shown in Figure S12, Supporting Information. Finally, the UV curing process was conducted at a wavelength of 340 nm and a power of 85 mW cm−2 for 30 s. The curing mechanisms are illustrated in the Supporting Information.

Characterization of SGH Film: Surface and cross-sectional SEM images and EDS elemental mapping of cross-sectional SGH film was characterized by field-emission SEM (SU5000, Hitachi). Transmittance of the SGH film was measured by UV–vis spectroscopy (Solidspec-3700, Shimadzu). Surface roughness was measured by a surface profiler (a-step IQ, KLA-Tencor). The hardness and effective modulus were obtained by nanoindentation equipment (iMicro, KLA). Stress–strain behavior was confirmed by a universal testing machine (AGS-X STD, Shimadzu). Elastic moduli were determined within the initial region of engineering stress–stress curves. TMA and DMA were carried out by a thermomechanical analyzer (SS6100, Seiko Instruments Inc.).

Fabrication Process of the THTENG: For fabrication of the THTENG, SGH film was coated by fluorosilane (Optool UD509, Daikin) to figure out the output performance. An acryl-based tapping frame was prepared for energy harvesting, which was composed of an acrylic body and 4 metallic springs of each corner column. Cr/Au layers (10/100  nm thickness) were deposited on the SGH film (2 × 2.5 cm2) by radio frequency (RF) sputtering. The ITO/PET film was sliced to the same size

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as the SGH film. The films were attached to the acrylic boards, and were electrically interconnected with Cu wires by Ag solder.

Electrical Measurement: The electrical properties of the TENGs were observed by a vibrating/pressing system, which was composed of a customized tapping load cell, a z-axis moving stage (LW 140.141-110, Labworks Inc.) and an electrometer (Keithley 6514). The tapping load cell freely adjusted the tapping force from 20 to 80 N. The distance between the base and counter materials in the TENG was regulated by the z-axis moving stage.

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

AcknowledgementsH.L. and H.E.L. contributed equally to this work. This work was supported by Wearable Platform Materials Technology Center (WMC) (NRF-2016R1A5A1009926). H.E.L. was supported by Basic Science Research Program through the NRF funded by the Ministry of Education (2019R1A6A3A12031643).

Conflict of InterestThe authors declare no conflict of interest.

Keywordsglass-fabric reinforced film, hierarchically textured surface, large-scale demonstration, siloxane hybrid material, triboelectric nanogenerator

Received: July 4, 2020Revised: August 10, 2020

Published online: September 13, 2020

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