Enhanced Performance of Microarchitectured PTFE-Based

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Enhanced Performance of Microarchitectured PTFEbased Triboelectric Nanogenerator via a Simple Thermal Imprinting Lithography for Self-Powered Electronics Bhaskar Dudem, Dong Hyun Kim, Anki Reddy Mule, and Jae Su Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06295 • Publication Date (Web): 27 Jun 2018 Downloaded from http://pubs.acs.org on July 1, 2018

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ACS Applied Materials & Interfaces

Enhanced Performance of Microarchitectured PTFEBased Triboelectric Nanogenerator via a Simple Thermal Imprinting Lithography for Self-Powered Electronics Bhaskar Dudem, Dong Hyun Kim, Anki Reddy Mule, and Jae Su Yua* Department of Electronic Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-Si, Gyeonggi-do 446-701, South Korea. *Corresponding author. Email address: [email protected] (J. S. Yu)

ABSTRACT Triboelectric nanogenerator (TENG) technology is an emerging field to harvest various kinds of mechanical energies available in our living environment. Nowadays, employing the TENG for industrial applications or for large-scale areas with low-device processing cost, and high electrical output is a major issue to resolve. Herein, we designed a TENG with low-cost by employing the micro-grooved architectured (MGA)-PTFE (polytetrafluoroethylene; Teflon) and aluminum as triboelectric materials with opposite tendencies. Moreover, the MGA-PTFE was fabricated by a single-step, facile, and cost-effective thermal imprinting lithography technique via micro pyramidal textured silicon as a master mold, is developed by a wet-chemical etching method. Therefore, designing the TENG device by following these techniques can definitely reduce its manufacturing cost. Additionally, the electrical output of TENG was enhanced by adjusting the imprinting parameters of MGA-PTFE. Consequently, the MGA-PTFE was optimized at an imprinting pressure and temperature of 5 MPa and 280 °C, respectively. Thus, the TENG with an optimal MGA-PTFE polymer exhibited a highest electrical output. A robustness test of TENG was also executed and employed its output power to drive LEDs and

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portable electronic devices. Finally, the real application of TENG was also examined by employing it as the smart floor and object-falling detector.

Keywords: Triboelectric nanogenerator; thermal imprinting lithography; polytetrafluoroethylene; micro-grooved architectures; wet-chemical etching

1. Introduction Harvesting the energy from renewable resources like solar, thermal, wind, ocean wave, and mechanical energies is one of the great research interest to fulfill the global energy requirements, which is the best alternative for conventional power supplies.1,2 Among these several energy resources, researchers are showing a keen interest to harvest the mechanical energies available in our daily lives such as human motion, mechanical triggering, rotation, etc.35

Particularly, the triboelectric nanogenerators (TENGs) are widely utilized to convert these

mechanical energies into electricity, owing to their simple and cost-effective fabrication process, high output power, eco-friendly nature, and good stability.3-7 In addition, the TENG has been also utilized as a self-power device and sensor to sense motion, pressure, acceleration, etc., because of its highly sensitive ability for mechanical agitations.8-10 However, the TENG functions on the basis of contact electrification, i.e., while two triboelectric materials with an opposite triboelectric tendency are brought into contact and separated with each other.10,11 Hence, the electrical output performance of TENG mainly depends upon the choice of the triboelectric materials and their surface morphology.10-14 Several triboelectric materials were already listed and separated based on their tendency to gain or lose charges named it as triboelectric series.4,15 Thus, to enhance the electrical output performance of TENG, there is a vast choice of materials


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in the triboelectric series. Polytetrafluoroethylene (PTFE or Teflon) is also one of the triboelectric material listed at the end of triboelectric series and it is vastly applied to attain a stable and high-performance TENGs.4,15,16 In addition, the electrical output performance of TENG can be further improved by enhancing the surface roughness of the triboelectric materials, which can lead to an enlargement of the contact or friction area.4,10-18 Previously, both the nanoparticle-assisted polymer surface etching and surface replication processes were mainly investigated to enhance the surface roughness of triboelectric materials.8,12,13,19-28 For example, the surface etching process was utilized in several reports to roughen the surface of the triboelectric material and further utilized it to efficiently harvest mechanical, water wave, and wind energies.13,19-23 Although this process seems to be effective, it has certain limitations, such as it requires an expensive and high-vacuum inductive coupled plasma (ICP) etching technique. Besides, for the surface replication processes, the liquid phase polymers such as Teflon and polydimethylsiloxane are poured into the nano- or micro-architectured molds, followed by the solidification.8,12,24-28 The surface replication is a facile process to enhance the surface roughness, but the development of molds involves an expensive lithography and dry etching techniques,8,12,27,28 which are time-consuming too. Moreover, the solidification of liquid phase polymer can slow down (or time-consuming) the fabrication process of TENG.24-26 Therefore, along with the enhancement of electrical output performance of TENGs, we should ease the device fabrication process for its low cost and mass production with rapid cycle time, which helps for the commercialization of the TENG. To meet these challenges, we adopted a facile, cost-effective, and single-step thermal imprinting lithography (TIL) technique to replicate (i.e., surface replication process) the microarchitectures on the surface of PTFE via a corresponding silicon (Si) mold. However, employing



the TIL technique for the fabrication of high-performance TENGs is rarely reported.29 Especially, there are no reports on the use of TIL technique to produce the nano/micro-architectures on PTFE for TENG applications, which is widely utilized owing to its efficient triboelectric tendency. So, it is very meaningful to employ the TIL technique for the fabrication of a low-cost and high-performance TENGs. Since, this surface replication process (i.e. TIL) doesn’t involve any expensive lithography and dry etching techniques or time-consuming solidification procedures, as mentioned above. Moreover, for the TIL, we can utilize any type of nano/microstructured molds, which can sustain at thermal imprinting temperature and pressure. Herein, we utilized the micro-architectured Si as a mold, which is developed by a relatively simple and fast wet-chemical etching method.30 Consequently, designing the TENG device by following the TIL and wet-chemical etching techniques can definitely reduce its manufacturing cost. The proposed TENG device can also be extended for the large-scale production and commercialization since the Si master molds are available to large-scale or roll-to-roll fabrication.30-32 In TIL technique, once a Si mold is made, it can be repeatedly used for microarchitecture transfer. Therefore, investigating the electrical output performance of the TENG using a micro-grooved architectured PTFE (MGA-PTFE) as a triboelectric material is very beneficial in the research area of TENG. Such TENG devices with low cost and high output performance can be employed for various applications to attain great outcomes, for example, smart floors, pressure sensors, fall-detectors, etc.8-10 In this work, we firstly investigate the effect of thermal imprinting parameters including temperature, pressure, and geometric dimension of MGAs on the surface morphology of PTFE as well as the output performance of TENG. Furthermore, the reliability test of the TENG with an optimized MGA-PTFE was performed to affirm its stability and durability. Therefore, the optimum TENG with an active area of 25 cm2


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exhibited the highest open-circuit voltage (VOC) and short-circuit current (ISC) values of ∼625 V and ∼50.5 µA, respectively, under the external pushing force and frequency of 5 N and 5Hz, respectively. Moreover, an energy conversion efficiency of 62.5% was demonstrated by this TENG, and it also exhibited a high output power density of ∼252 W·m-2. Therefore, it can be expected that, as compared to the previous works or fabrication techniques,8,12,13,19-28 using the TIL to produce the patterned PTFE is efficient to reduce the processing cost of TENG and may be also utilized to enhance its performance. In addition, for practical applications, the stable electric output of TENG is directly employed to drive 68 (or 120) blue and green commercial light-emitting diodes (LEDs) connected in series and also utilized to charge the capacitor followed by powering the portable electronic devices. At last, the real application of TENG device was also examined by employing it as the smart floor and object-falling detector.

2. Experimental procedure Fabrication of micro pyramidal textured silicon (MPT-Si) mold A single-side polished silicon (Si) substrates with an area of 2 × 2 cm2 were ultrasonically cleaned with acetone, methanol, and de-ionized (DI) water, and subsequently dried with nitrogen (N2) gas flow. And then, to remove the oxide and organic contaminants on these Si substrates, they were rinsed away using a buffer oxide etchant (BOE), 5wt% hydrofluoric (HF) acid solutions, and DI water followed by the N2 gas blow. Subsequently, the cleaned Si substrates were immersed into a wet-chemical etchant solution consisting of the mixture of potassium hydroxide (KOH), isopropyl alcohol (IPA), and DI water, at a temperature of 80 °C for 30 min. Consequently, the micro pyramidal textures (MPTs) on the surface of Si substrates were realized. Afterward, the MPT-Si was further dipped into a mixture of hydrogen chloride,



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hydrogen peroxide, and DI water (HCl:H2O2:H2O = 1:1:5) solution at 80 °C for 10 min, to get rid of the remaining potassium impurities on their surface. Fabrication of MGA-PTFE In

order

to

produce

the

micro-grooved

architectures

(MGAs)-

on

PTFE

(polytetrafluoroethylene; Teflon) by the thermal imprinting lithography (TIL) technique, the MPT-Si mold, pristine-PTFE polymer, and aluminum (Al) foil are piled and sandwiched in between the two hot press platens and the imprinting pressure of 3-10 MPa was applied at an imprinting temperature of 250-310 °C for 4 min. Herein, the Al foil was utilized as a supporting layer to prevent the damage of PTFE. After the imprinting process for 4 min, the applied imprinting pressure was slowly released, and the mold, PTFE film, and Al cooled down to 30 °C. As a consequence of TIL technique, the MPTs on Si were successfully replicated on the surface of PTFE, leading to the formation of MGAs on it. Fabrication of TENG device The TENG device was simply fabricated by assembling the two components. The top component consists of the Al foil/ polyethylene (PE) tape/acrylic substrate and the bottom component is composed of the MGA-PTFE/Al/PE tape/acrylic substrate. Finally, they were combined together to design a contact-type TENG, by locating the four springs at the corners of acrylic substrates. Characterization Instrumentation: The surface morphologies of as-fabricated MPT-Si molds and MGA-PTFE polymers were observed by using a field-emission SEM (LEO SUPRA 55, Carl Zeiss). Further, the surface roughness of pristine- and MGA-PTFE polymers, and depth of MGAs were determined by the


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3D laser scanning confocal microscope (VK-X260K, KEYENCE). The pushing frequency and force applied to the top plate of TENG were controlled or monitored by using a pushing tester (JIPT-100, Junil Tech). A digital phosphor oscilloscope (DPO4104, Tektronix) and a low-noise current preamplifier (SR570, Stanford research systems) were used to measure the output voltage and current of TENG. In addition, the charge density generated by the TENG device and voltage stored in capacitor were measured by using an electrometer (Keithley 6514).

3. Results and discussion Figure 1a depicts the schematic diagram of the fabrication process of the MGA-PTFE by a single-step, facile, and cost-effective TIL via an MPT-Si mold. Initially, the MPTs on the surface of Si was fabricated by a simple anisotropic wet-chemical etching technique. The chemical etching treatment was performed by dipping the Si substrates into the etchant solution consisting of the mixture of KOH, IPA, and DI water at 80 °C for 30 min. Consequently, the MPTs were realized on the Si surface, and they are further replicated on the surface of PTFE by the TIL technique. Prior to the TIL, such micro-pattern replication process was performed by arranging the MPT-Si, pristine-PTFE polymer, and Al foil, in the sequence, and further they sandwiched between the two hot press plates and the imprinting pressure of 3-10 MPa at the imprinting temperature of 250-310 °C was applied for 4 min (Figure 1a). As a consequence, the MPTs on Si can be pressed against the surface of the pristine-PTFE polymer, resulting in the MGAs on it. Eventually, the MGA-PTFE with the thickness of ∼100 µm was produced by peeling off, from the MPT-Si mold and Al foil. Next, the as-fabricated MGA-PTFE was employed as a triboelectric material with the negative tendency to fabricate a TENG. In order to design the TENG, the MGA-PTFE was attached to the commercially available Al foil with the



area and thickness of ∼ 4 cm2 and 80 µm, respectively. Here, an Al foil can be utilized as a bottom electrode of TENG. Subsequently, the MGA-PTFE/Al was affixed on one acrylic substrate by the double-sided PE soft foam tape with 1 mm thickness and it (i.e., MGAPTFE/Al/PE tape/acrylic) was used as a bottom component of TENG (as shown in Figure 1b). Besides, another Al foil with the same area was fixed on the other acrylic substrate using a PE soft foam tape, to prepare the top component of TENG. This Al foil can play dual roles as a top electrode and a triboelectric surface with the positive tendency. At last, both the top and bottom components are combined together to design a contact-type TENG, by locating the four springs at the corners of acrylic substrates (Figure 1b). Here, the springs are employed to restore the top components of TENG into their original positions by removing the applied pressure. However, the separation distance between the top Al electrode and MGA-PTFE was noticed as ∼4 mm. Figure 1c shows the top-view and cross-sectional scanning electron microscope (SEM) images of the MGA-PTFE. The average size and depth of randomly distributed MGAs (i.e., inverse four-sided pyramids) on the surface of MGA-PTFE were noticed in the ranges of ∼7 and 6 µm, respectively. These MGAs on PTFE can enhance its surface roughness, which is more favorable to increase the contact area between the triboelectric materials (such as PTFE and top Al), thus improving the output performance of TENG.33 In addition, the high-magnified SEM images of the MGA-PTFE shown in Figure 1c, also indicating that the formation of nano wrinkles-like structures on the MGAs (i.e., hierarchical architectures) of PTFE. Along with the MGAs, the existence of nano wrinkles may further help to enhance the surface roughness of PTFE, which improves the output performance of TENG.12-14 The working principle of the TENG device to convert the mechanical energy into the electricity is schematically explained in Figure 2. As well known, the TENG operates based on


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two fundamental phenomena, i.e., contact electrification and electrostatic induction.4,10 Under an external pushing force applied to the top component of TENG, the top Al electrode of the TENG and the MGA-PTFE polymer with two different triboelectric polarities were brought into contact with each other. Resultantly, owing to the contact electrification, the opposite electrostatic charges are generated on the surface of Al and MGA-PTFE, respectively (Figure 2a). Next, by releasing the external pushing force from the top component of TENG, the top Al electrode and MGA-PTFE start to move apart from each other, which induces a potential difference across these electrodes. Thus, the resultant potential difference can drive the electrons to flow from the bottom Al electrode to the top Al electrode (Figure 2b), thus resulting in the current in opposite direction (i.e., a negative current). By completely removing the pushing force, the top and bottom components of TENG can revert back to their original positions and the triboelectric charge distribution reaches electrical equilibrium (Figure 2c). Afterward, as the external pushing force is applied once again on the top component of TENG, the top Al electrode and MGAPTFE close enough, thus inducing a reverse electrostatic potential difference across them. Consequently, the electron flow can be observed toward the bottom Al electrode from the top Al electrode (Figure 2d), which results in a positive current. Therefore, the continuous pushing force applied on to the top component of TENG can lead to a repetitive operation and generate electrical output continuously. As discussed above, the effective surface roughness of triboelectric materials is more crucial to attaining a high output performance from TENG.12-14 The triboelectric materials with high surface roughness can cause a large contact area between them (herein, PTFE and top Al act as triboelectric materials), ensuring a large charge accumulation (or generation) across their surfaces. Hence, the electrical output performance of TENG is also enhanced. To further affirm,



the effect of MGAs on the triboelectric potential of TENG device was investigated through finite elemental simulation tool (COMSOL multiphysics software), as shown in Figure 2e-f. To design the simulation model, the PTFE with the width of 500 µm and thickness of 100 µm was placed above the Al with the width of 500 µm and thickness of 100 µm and considered as the bottom component of TENG. Besides, another Al with the same dimensions was considered as a top component. The distance between these two components was fixed at 600 µm, and additionally, the force was not applied in our simulation. Moreover, the top size and depth of the MGAs on PTFE were assumed as 7 µm and 6 µm, respectively (Figure 1c). The triboelectric surface charge densities of the top and bottom electrodes of TENG were assumed as ±30 µC/m2. Figure 2e-f are clarifying that the triboelectric potential of the MGA-PTFE significantly increased than that of the pristine-PTFE. This mainly attributed to the increased contact area between the electrodes, implies a greater charge density between them. Consequently, an increase of triboelectric charge density is directly related to an increase in transferred charges and to a greater triboelectric potential difference between the electrodes. Herein, a TIL technique was employed to pattern the triboelectric material (i.e., PTFE) and to modify its surface roughness. In TIL technique, the imprinting parameters (i.e., temperature, pressure, and geometric dimension of MGAs) can play a key role in realizing the MGA-PTFE with good quality, i.e., well micro patterns replication. Thus, the effect of the imprinting parameters on the surface morphology of PTFE as well as the output performance of TENG was investigated. Out of these three parameters, imprinting temperature was first investigated, which is the most important characteristic to realize the micro-grooved architectures on PTFE. As we know, the melting temperature of PTFE is 327 °C, where it can be mechanically shaped (or transformed) owing to its highly viscous nature at that temperature.


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Hence, three different imprinting temperatures (i.e., 250, 280, and 310°C) nearby the melting temperature were applied to fabricate MGAs on the surface of PTFE and an optimum imprinting temperature was determined for realizing the MGA-PTFE. The imprinting pressure was maintained at 8 MPa for 4 min, during these thermal imprinting processes. At the imprinting temperature of ∼250 °C, the MGAs were nucleated on the surface of PTFE (Figure S1b of the Supporting Information), but not uniformly and deeply imprinted on the surface of PTFE. This may be observed due to the low imprinting temperature, where the PTFE cannot be melt and transformed into another shape. For comparison, the SEM image of pristine-PTFE without any imprinting process is also shown in Figure S1a, which is flat without any patterns. Besides, at a very high temperature of ∼310 °C, even though the micro-grooved architectures are realized on the surface of PTFE, the resulting MGA-PTFE polymer strongly sticks to the hotplate and corners of the MPT-Si mold. Thus, it is very hard to separate the MGA-PTFE from the hotplate and mold. Due to the very high temperature (i.e., very near to melting temperature), the PTFE tends to melt easily and spread throughout the hotplate. However, in between these two temperatures (i.e., at ∼280 °C), the micropattern replication process was successfully observed (Figure S2, of the Supporting Information). Since, at this temperature, the PTFE could melt sufficiently to imprint or replicate the microarchitectures from the MPT-Si mold. Therefore, out of these three imprinting temperatures, 280 °C was determined as an optimum imprinting temperature to realize the MGAs on PTFE and this temperature was used for further experiments. In order to investigate the effect of the geometric dimension of MGAs on the surface morphology of PTFE and the output performance of TENG, three different MGA-PTFE polymers were fabricated utilizing three different MPT-Si molds. The top-view and 30°-tilted view SEM images of the MPT-Si molds formed at various concentrations of KOH and IPA



solutions (i.e., alkali etchant solution) i.e., 10:0 vol%, 10:5 vol%, and 10:10 vol%, (termed as MPT-Si mold-I, -II, and -III) are shown in Figure S2a, S2c, and S2e, respectively. As shown in the SEM images, the randomly distributed micro pyramidal textures (MPTs) were developed on the surface of (100)-oriented monocrystalline Si substrate, by the well-known alkaline anisotropic wet-etching treatment.26,27 But, by varying the concentration of alkali etchant solutions, the bottom size and height of the MPTs (four-sided pyramids) were also altered. The average size/height of MPT-Si mold-I, -II, and -III were roughly estimated to be ∼8.5/6, 6.5/5.8, and 5.7 µm/4.5 µm, respectively. The structural variations of MPTs on Si can be mainly dependent upon the chemical etching rate of Si. As mentioned above, the alkali etchant solution consisting of the mixture of KOH and IPA was used as a chemical etchant and surfactant, respectively. Thus, the addition of surfactant to the etchant solution can lead to the reduction of the etching rate.31,32,34-37 Consequently, the bottom size and height of MPTs were reduced by increasing the concentration of IPA into the etchant solution. However, these three molds with various size and height of MPTs were further used to fabricate the MGA-PTFE polymers. And, the effect of the geometric dimension (i.e., size and depth) of the MGAs was further investigated on the output performance of TENG (Figure 3). The top-view and 30°-tilted view SEM images of the MGA-PTFE polymers replicated from the MPT-Si mold-I, -II, and -III, (termed as MGAPTFE-I, -II, and -III) are shown in Figure S2b, S2d, and S2f, respectively. To fabricate these MGA-PTFE films, the thermal imprinting process was carried out at the imprinting pressure, temperature, and time of 8 MPa, 280 °C, and 4 min, respectively. As shown in the SEM images, by the TIL technique, the micro-pyramidal textures of the Si molds were successfully and negatively replicated on the surface of PTFE without any large deformation, creating the MGAPTFE. Moreover, the top size and depth of the MGAs on the surface of PTFE polymers were


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observed almost similar to those of the corresponding MPT-Si molds. The average top size and depth of the MGAs on the surface of MGA-PTFE-I, MGA-PTFE-II, and MGA-PTFE-III polymers were noticed in the range of ∼8.5/5.9, 6.8/5.8, and 5.5 µm/4.6 µm, respectively. In addition, the high-magnified SEM images of the MGA-PTFE-I, -II, and -III polymers are shown in Figure S2g, S2h, and S2i, respectively, exhibiting the nano-wrinkles on MGAs. These nanowrinkles may be observed due to the sudden quenching of PTFE, while it is cooling down from 280 °C to 30 °C. Therefore, along with the MGAs on PTFE, the nano-wrinkles formed on MGAs can also play an important role in further enhancing the surface roughness of MGA-PTFE or the contact area of TENG, which results in a high output performance of TENG. Furthermore, the surface roughness of the various MGA-PTFE polymers was analyzed by 3D laser profiler. Figure 3a-d depicts the 3D-surface scan images of pristine-PTFE, MGAPTFE-I, -II, and -III, respectively. As shown in Figure 3a, the pristine-PTFE consists the flat surface without any MGAs and exhibiting the lowest root mean square roughness (Sq) value of 0.278 µm. On the other hand, the 3D-surface scan images of MGA-PTFE-I, -II, and -III polymers (Figure 3b-d) are clearly exhibiting the MGAs on its surfaces and their dimensions are clearly distinct. The average size and depth of the MGAs on PTFE-I polymer are higher than that of the MGA-PTFE-II and -III, respectively. Moreover, the density of MGAs on PTFE-III is low as compared to that of remain two PTFE polymers. The optical microscopic images and profile graphs of pristine-PTFE, MGA-PTFE-I, -II, and -III, shown in Figure S3, also supporting the above discussion. All these MGAs variations were mainly ascribed due to the various MPTmolds produced at various alkaline etchant conditions, as mentioned above. However, the MGAPTFE-II exhibited a Sq value of 2.032 µm, which is higher than that of the MGA-PTFE-I, and III (Sq = 1.819, and 0.864 µm, respectively). As a consequence, the TENG with MGA-PTFE-II



can be expects to exhibit a high electrical output as compared to the TENG with MGA-PTFE-I, III, respectively. To further affirm, the electrical output performance of TENG with these PTFE polymers were examined. Figure 3e and 3f show the open-circuit voltage (VOC) and short-circuit current (ISC) curves of the TENG with pristine-PTFE and various MGA-PTFE polymers replicated from the three different MPT-Si molds. During these measurements, the external pushing force and frequency were maintained constantly at 5 N and 5 Hz, respectively. While an external pushing force is applied/released on the top component of pristine-PTFE-based TENG, the top Al electrode and pristine-PTFE were brought into contact and separated with each other, exhibiting the VOC and ISC values of ∼85 V and ∼3 µA, respectively. Furthermore, by introducing the MGA-PTFE-I, -II, and -III films instead of the pristine-PTFE, the electrical output performance (i.e., VOC and ISC values) of TENG was considerably enhanced. Such an enhancement is mainly attributed to the increased surface contact area between the PTFE and top Al electrode, which was introduced by the hierarchical architectures (i.e., nano-wrinkles/MGAs) on the surface of PTFE polymers. Although, out of these three PTFE polymers, the TENG with MGA-PTFE-II exhibited the higher VOC and ISC values of ∼220 V and ∼10.8 µA, respectively than the TENG with MGA-PTFE-I, and -III (∼148 V/6.9 µA and ∼107 V/4 µA, respectively). This mainly attributed to the high surface roughness of MGA-PTFE-II (i.e., Sq = 2.032 µm), than that of other PTFE polymers (Figure 3a-d). The influence of thermal imprinting pressure on the electrical output performance of TENG as well as the surface morphology of MGA-PTFE was also evaluated. The MGA-PTFE polymers were formed at various imprinting pressures of 3, 5, and 8 MPa using the MPT-Si as a mold, which is fabricated at 10:5 vol% of KOH and IPA solutions. During the thermal imprinting process, the remained imprinting parameters such as temperature and time were kept


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constant at 280 °C and 4 min, respectively. Figure 4a and 4b show low- and high-magnified SEM images of the MGA-PTFE polymers at the imprinting pressures of 3 and 5 MPa, respectively. As shown in Figure 4a, the MGAs on the surface of PTFE are rarely observed owing to a low imprinting pressure, where the MPT-Si mold cannot be pressed sufficiently towards the PTFE, which results in an improper imprinting. On the contrary, at the imprinting pressures of 5 MPa (Figure 4b) and 8 MPa (Figure S2d), the MGAs on the surface of PTFE are observed very well. The electrical output performance of the TENGs with these PTFE polymers at various imprinting pressures was investigated. Figure 4c-e show the measured VOC, ISC, and charge density curves of the TENG with various MGA-PTFE polymers at different imprinting pressures of 3, 5, and 8 MPa. As shown in Figure 4c-e, the TENG device with the MGA-PTFE polymer at 5 MPa exhibited the higher VOC, ISC, and charge density values of ∼320 V, 15 µA, and 44 µC.m-2, respectively than the TENG with the other MGA-PTFE polymers (i.e., ∼180 V/8.2 µA/25 µC.m-2 and ∼250 V/11.2 µA/30 µC.m-2 for the MGA-PTFE, produced at 3 and 8 MPa, respectively). As discussed above, the MGA-PTFE at 3 MPa does not consist of any proper microstructures, so it cannot provide a high surface roughness. Therefore, this MGA-PTFE can fail to create an efficient contact area between the PTFE and Al of TENG, resulting in a low electrical output performance of TENG. In contrast, at the imprinting pressure of 8 MPa, even though the MGAs were well replicated on the surface of PTFE, the TENG with this PTFE polymer exhibited a relatively low output performance due to the high imprinting pressure. At this condition, the surface of PTFE can undergo a large deformation and may offer a lower surface roughness than that of the MGA-PTFE at 5 MPa. Therefore, the MGA-PTFE at the imprinting pressure of 5 MPa under the temperature of 280 °C for 4 min was experimentally optimized. This optimal MGA-PTFE provides a sufficient surface roughness to enhance the



contact/friction area as well as the surface charge density in between the PTFE and Al electrodes for high electrical output performance of TENG. Furthermore, to investigate the effective electric power of TENG with an optimal MGAPTFE polymer, the output performance of TENG was systematically investigated at various external load resistances. The effective electric power of TENG is essential for practical device application which strongly depends upon short-circuit and open-circuit conditions. Figure 5a shows the output current and power density values of TENG were examined at various external load resistance (RL) values from 10 to 3 × 109 Ω. These measurements were performed under the pushing force and frequency of 5 N and 5 Hz, respectively. As the RL was increased from 10 to 106 Ω, the output current of TENG was not predominantly changed. At very low RL, the triboelectric charges generated across the electrodes of TENG can easily pass through the external load resistance, which results in a high output current. In contrast, the output current of TENG was dramatically decreased from 14 to 0.5 µA as the RL was further increased from 106 to 109 Ω, and then it reached saturation at very high RL (i.e., > 109 Ω). This could be mainly due to the high RL, which can prevent the triboelectric charges from passing through it, yielding a low output current. The average power density (Wavg) of TENG was also computed by the following equation: 2 Wavg = I RL

A

.

Here, I denote the output current of TENG at various load resistances (i.e., RL) and A denotes the active area (i.e., 4 cm2) of TENG. As shown in Figure 5a, at very low (≤ 5 × 106 Ω) and high (i.e., ≥ 109 Ω) load resistances, the TENG exhibited the lowest Wavg values because of short- and open-circuit conditions. However, at moderate load resistances, the TENG exhibited high Wavg values. Particularly, the output power density of TENG was maximized at a load


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resistance of 500 MΩ, and the corresponding average power density and effective electric power values were found to be 8 mW.Cm-2 (Figure 5a) and 32 mW (Figure S4), respectively. For commercial applications, the proposed TENG should possess a stable output performance over a long period of activity. Hence, we performed the reliability test of the TENG device for 10,000 cycles. Figure 5b and 5c show the measured output voltage and current curves of TENG for 10,000 cycles, under an external pushing force and frequency of 5 N and 5 Hz, respectively. As shown in Figure 5b and 5c, even after the long period of activity (i.e., up to 10,000 cycles), the TENG exhibited a stable output performance without any significant deformation. Such TENG device with good stability over a long period of activity (i.e., long lifetime) is more suitable for commercial and real industrial applications. The stable and high electric power generated from the TENG device was further utilized to drive the low-power portable electronic devices. But, this electric output power of TENG cannot be supplied directly to the electronic devices. Firstly, the alternating current (AC) signal generated from the TENG should be converted into the direct current (DC) signal by employing a full-wave bridge rectifier (Figure 6a). The rectified output voltage pulse of the MGA-PTFEbased TENG device was noted as about ∼320 V, as shown in Figure. S5. Moreover, a commercial capacitor is required to store the rectified voltage of TENG. Finally, the energy stored (i.e., DC power) in the capacitor can be directly employed to drive portable electronic devices. Figure 6a shows the equivalent circuit diagram of the TENG connected to the capacitor and portable electronic devices (i.e., liquid-crystal displays; LCDs) through a bridge rectifier. Figure 6b shows the charging process of various capacitors (i.e., 0.1, 0.5, 2.2, and 4.7 µF) by the rectified voltage generated from the TENG device under 5 Hz of external pushing frequency and 5 N of external pushing force. As shown in Figure 6b, the voltages of all the capacitors



gradually increased and finally reached a saturation. But, the capacitor with small capacitance (i.e., 100 nF) takes very less time to reach the saturation than the other capacitors. In addition, the inset of Figure 6b also shows an enlarged view of the details. As shown in the inset, both the pushing and releasing operations can charge the capacitor due to the rectification of the output signals. These output voltage measurements clarify that the rectified current generated by the TENG can charge the capacitor continuously. Additionally, the power stored in the capacitor can be further utilized to switch on (or lit up) various LCDs such as the speedometer and digital wrist watch, for an instant, as shown in Figure 6c and 6d, respectively. Here, we used a capacitor with the capacitance of 47 µF to store the power generated from the TENG and further supplied to the LCDs. The corresponding charge and discharge curves of 47 µF capacitor also shown in Figure S6. Therefore, these results are clarifying that the MGA-PTFE-based TENG devices are effective to drive the portable electronic devices without any external power sources. Furthermore, to demonstrate the performance of our proposed device as a self-powering energy source, we directly supplied the electric power generated from the TENG to drive 68 commercial green and blue LEDs connected in series, spelled out as “KHU” (Figure 6e and Video S1 in the Supporting Information). Along with the stability, the large-area TENG device is also important for real industrial applications. Therefore, we increased the active area of TENG from 4 to 25 cm2 (Figure S7) and analyzed its electric output performance, as shown in Figure S8a and S8b. As shown in Figure S8a, under the pushing force and frequency of 5 N and 5Hz, the VOC/ISC values of TENG were enhanced from ∼320 V/15.4 µA to ∼625 V/50.5 µA by increasing its active area from 4 to 25 cm2, respectively. Furthermore, the TENG with an active area of 25 cm2 also exhibited a relatively high power density value of ∼25.2 mW·cm-2 (Figure S9) compared to the TENG with 4


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cm2 area (i.e., ∼8 mW.cm-2). Such an enhancement is mainly attributed to the increased contact area of TENG, affirming that the proposed TENG can be employed for real and large-scale industrial applications. Moreover, both the TENG devices also generated an electrical output by gently pressing (or touching) the top component of the corresponding TENGs with a finger (Figure S8b). This process can also confirm the working function of TENG in real situations, to harvest the various human activities. On the other hand, calculating the energy conversion efficiency of the above TENG devices is also important in the figure of merit. The energy conversion efficiency is defined as the ratio between the maximum electrical output energy generated by the TENG and the mechanical energy applied to the TENG38,39 (see the Discussion S1 in Supporting Information). From the calculations, it could conclude that the energy conversion efficiency of TENG is largely improved from 19.96 to 62.5%, as its active area is enhanced from 4 to 25 cm2, respectively. Therefore, along with the 4 cm2 area device, the 25 cm2 area TENG device with high output power density ∼25.2 mW·cm-2 and an energy conversion efficiency of 62.5% is also more adequate to be utilized as a self-powered system for driving the various low-power electronic devices. In addition, the effect of external pushing frequency and force on the output performance of the optimized TENG device (i.e., with an active area of 25 cm2) was systematically investigated. Figure 7a and 7b show the VOC and ISC curves of the TENG measured under various pushing frequencies ranging from 1 to 5 Hz and at a constant pushing force of 5 N, respectively. Obviously, the VOC (i.e., ∼620 V) of TENG is barely affected by the pushing frequency, while the ISC increases from ∼22 µA to ∼50.5 µA with raising the frequency from 1 to 5 Hz. This is mainly attributed to the faster-pushing cycles (i.e., increased contact or deformation rate) under the increased pushing frequency, which can lead to increase the charge flow rate



across the external load, resulting in a high output current of TENG. Meanwhile, the total number of the charges transferred in between the electrodes of TENG is constant, which can result in a constant potential difference between them. Consequently, the output voltage of TENG is invariant under various pushing frequencies.40,41 Figure 7c and 7d show the VOC and ISC curves of the TENG measured under various pushing forces ranging from 2 to 10 N, and during the measurements the pushing force was maintained constantly at 5 N. As shown in Figure 7c and 7d, the VOC/ISC values of TENG are increased from 295 V/22 µA to 754 V/70 µA by increasing the pushing forces from 2 N to 10 N, respectively. Such an increment is mainly observed due to the enhanced contact area as well as the charge density between the triboelectric materials (i.e., MGA-PTFE and Al) of TENG under the strong external pushing forces. At last, the TENG with an active area of 25 cm2 was also utilized as a smart floor and object-falling detector to harvest the human foot-fall and to detect the tennis ball-fall, respectively, as shown in Figure 7e and 7f, respectively. The electric output signals generated from the TENG, under a human foot-fall and tennis ball-fall are clearly demonstrated in Video S2 and S3 (Supporting Information), respectively. Nowadays, people are expressing a growing interest to generate the electricity from human foot-fall by locating the TENGs as a smart floor in a highly crowded area such as railway stations, markets, airports, etc.42 In similar manner, our MGA-PTFE-based TENG with a high electrical output (i.e., VOC and ISC ∼ 750 V and ~ 64 µA, respectively) can be also employed as a smart floor to harvest the human foot-fall (Figure 7g). The TENG device with special features of simple fabrication, low cost, high electrical output performance, and excellent robustness is more desirable to develop the smart floor over the large area for continuously harvesting the human activities. Furthermore, The TENG also generated high VOC and ISC values of ∼ 600 V and 58 µA, respectively, by a tennis ball-fall on it (Figure


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7h). From these results, it is clear that our TENG device can efficiently harvest the energy from the object-fall on it and also serves as a detector to provide a warning from the potential accidents or the fall of various objects. Consequently, the electric power generating from the TENG under a human foot-fall or tennis ball-fall was also utilized to light up 120 blue and green LEDs connected in series, as shown in the inset of Figure 7g and 7h.

4. Conclusions In summary, we proposed a TENG with simple fabrication process, low cost, high electrical output performance, and excellent robustness. Such TENG was designed by employing the MGA-PTFE and Al foil as a triboelectric material with negative and positive tendencies, respectively. Moreover, the fabrication process of MGA-PTFE such as TIL technique plays a key role in significantly reducing the processing cost of TENG. Furthermore, the highest surface roughness introduced by the MGA-PTFE polymer enhanced the effective contact area and surface charge density in between the PTFE and Al electrodes of TENG. Consequently, the electrical output performance of TENG was also enhanced. However, the TENG with an optimal MGA-PTFE polymer exhibited the highest VOC, ISC, and charge density values of ∼ 320 V, 15 µA, and 44 µC.m-2, respectively, under the external pushing force and frequency of 5N and 5 Hz, respectively. Such TENG also exhibited a maximum power density value of ∼ 8 mW.Cm-2 at a load resistance of 500 MΩ. Also, the robustness test of TENG device was conducted and a stable output performance was observed over a long period of activity (i.e., up to 10,000 cycles). This stable and high electric power generated from the TENG device was successful to drive several LEDs connected in series and was also utilized to charge the capacitor followed by powering the portable electronic devices. In addition, the performance of TENG was further enhanced by



increasing the active area to 25 cm2 and exhibited the VOC and ISC values of ∼625 V and 50.5 µA, respectively. Moreover, an energy conversion efficiency of 62.5% was demonstrated, and a high output power density of ∼252 W⋅m-2 was also realized. Ultimately, this optimum TENG device was also employed as a smart floor and object-falling detector to harvest the human foot-fall and to detect the tennis ball-fall. The results are clearly concluding that the proposed TENG device with a high electrical output (i.e, VOC and ISC ∼ 750 V and 64 µA) is also more effective to harvest the human foot-fall by locating it as a smart floor in a highly crowded area such as railway stations, markets, airports, etc.

Supporting Information. SEM images of MPT-Si molds produced at various etching conditions, SEM and optical microscopic images of MGA-PTFE polymers replicated/produced from various molds/at different thermal imprinting temperatures, output power and rectified voltage curves of TENG, Charge and discharge voltage curves of the 47 µF capacitor, Photographic images of TENGs with two different working areas, compression of the electrical output of TENGs with an active area of 2 × 2 and 5 × 5 cm2, output power density of TENG with an area of 5 × 5 cm2. Video S1, Electric power generated from TENG to drive LEDs; Video S2, Electrical output of TENG under a human foot-fall; Video S3, Electrical output of TENG under an object-fall. This material is available free of charge via the Internet at http://pubs.acs.org.


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Corresponding Author *Email: [email protected] (Prof. J. S. Yu)

Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017R1A2B4011998 and No. 2017H1D8A2031138).

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Figure 1. Schematic diagram of the fabrication process of MGA-PTFE via a TIL technique and the fabrication process of triboelectric nanogenerators. (a) Fabrication process of MGA-PTFE by TIL technique via an MPT-Si mold by an anisotropic chemical etching process. (b) Assembly of several components such as MGA-PTFE, Al foil, and foam tape, to design the TENG device. Inset of (b) also depicts the photographic image of TENG device. (c) Top- and side-view SEM images of the MGA-PTFE.



Figure 2. (a-d) Schematic illustration of the working principle of MGA-PTFE-based TENG. Analytical simulation results showing the triboelectric potential differences of (e) pristine-, and (f) MGA-PTFE based TENG devices, as simulated using the COMSOL Multiphysics software. In this case, MGA-PTFE exhibited the largest potential differences.


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Figure 3. 3D-surface scan images of (a) pristine-PTFE, (b-d) MGA-PTFE-I, -II, and -III, respectively, through the 72 × 96 µm2 area. (e) VOC and (f) ISC curves of the TENG with corresponding PTFE polymers.



Figure 4. Low- and high-magnified SEM images of the MGA-PTFE polymers produced at the thermal imprinting pressures of (a) 3 MPa and (b) 5 MPa, respectively. (c) VOC, (d) ISC, and (e) charge density curves of the TENG with various MGA-PTFE polymers produced at different imprinting pressures (i.e., 3, 5, and 8 MPa).


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Figure 5. Output power density and reliability test of the TENG. (a) Effect of external load resistance on the output current and average power density values of the TENG. (b,c) Reliability test analysis of the TENG device for 10,000 cycles. During these measurements, the applied pushing force and pushing frequency are maintained at 5 N and 5 Hz, respectively.



Figure 6. Practical applications of the TENG to charge capacitors and to power up portable electronic devices and LEDs. (a) Schematic diagram illustrates the TENG to charge capacitor, followed by powering the LCDs. (b) Voltage stored in various capacitors with the capacitances of 0.1, 0.5, 2.2, and 4.7 µF, by the rectified power generated from the TENG device under 5 Hz of external pushing frequency and 5 N of external pushing force. The power stored in the 47 µF capacitor to switch on (or lit up) various LCDs such as (c) speedometer and (d) digital wrist watch, for an instant. (e) Photographic images of 68 blue and green LEDs connected in series, before and after applying the pushing force on the TENG.


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Figure 7. Real applications of the TENG to detect and generate the electric power from the human foot-fall or tennis ball-fall (i.e., object-fall), respectively. Effect of external pushing (a,b) frequency and (c,d) force on the electrical output performance of the TENG with an active area of 25 cm2. Schematic and photographic image of TENGs (e) to harvest the energy from the human foot-fall, and (f) to detect the tennis ball-fall (or object-fall). The corresponding electric output (VOC and ISC) of TENGs under the (g) human foot- and (h) tennis ball-fall, respectively. Inset of (g) and (h) also depicts the photographic images of 120 blue and green LEDs connected in series, lit up by the electric power generating from the TENG under a human foot-fall or tennis ball-fall, respectively.



Table Of Contents (TOC) High-performance MGA-PTFE based TENG via TIL technique was successfully utilized as a self-powered source to drive various portable electronic devices and also employed as a smart floor and object-falling detector to harvest the human foot-fall and to detect the tennis ball-fall.


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Enhanced Performance of Microarchitectured PTFE-Based

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