Improving the Working Efficiency of a Triboelectric Nanogenerator by

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Improving the Working Efficiency of a Triboelectric Nanogenerator by the Semimetallic PEDOT:PSS Hole Transport Layer and its Application in Self-Powered Active Acetylene Gas Sensing A.S.M. Iftekhar Uddin, Usman Yaqoob, and Gwiy-Sang Chung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08002 • Publication Date (Web): 21 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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ACS Applied Materials & Interfaces

Improving the Working Efficiency of a Triboelectric Nanogenerator by the Semimetallic PEDOT:PSS Hole Transport Layer and its Application in SelfPowered Active Acetylene Gas Sensing A.S.M. Iftekhar Uddin, Usman Yaqoob, Gwiy-Sang Chung* School of Electrical Engineering, University of Ulsan, 93 Daehak-ro, Nam-gu, Ulsan 44610, Republic of Korea ABSTRACT: Herein we report an enhanced triboelectric nanogenerator (TENG) based on the contactseparation mode between a patterned film of polydimethylsiloxane (PDMS) with a semi-metallic elastomer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and a nylon fiber film. The addition of ethylene glycol to the PEDOT:PSS film improves the functionality of the TENG significantly, yielding promising applicability in both indoor and outdoor (i.e., under sunlight) environments, with the maximum instantaneous power of 0.09 mW (indoors) and 0.2 mW (outdoors) for the load resistance of 3.8 MΩ. The device can also −2 generate 11.2 V and 0.08 µA cm in response to the forearm movement of a human. Additionally, by replacing the bare nylon fiber in the TENG design with a Ag@ZnO/nylon fiber film, a self-powered active sensor (triboelectric nanogenerator–based sensor; TENS) has been realized to detect acetylene (C2H2) gas. The TENS exhibits excellent sensitivity of 70.9% (indoors) and 89% (outdoors) to C2H2 gas of 1000 ppm concentration. The proposed approach for harvesting energy and sensing can be advantageous in practical applications and may stimulate new research that will enhance nanogenerators as well as wearable, self-powered active sensors. KEYWORDS: triboelectric nanogenerator, wrinkled-PDMS, PEDOT:PSS, self-powered active sensor, acetylene 1. INTRODUCTION Recently, the use of triboelectric nanogenerators (TENGs) to scavenge energy from ambient sources and convert it into electrical energy has attracted global attention as a renewable energy source that can provide environmental pollution–free, maintenance-free, uninterrupted, and long-term energy supply, including viable options for the 1-6 sustainable powering of wearable devices. In a typical TENG device, the output power generated critically depends upon the density of the triboelectrically generated surface charges, which is the vital factor for driving the free electrons induced. A number of standard triboelectric-series materials have been used for the development of TENGs. Moreover, the configuration and design of the contact area through intentionally generated structures on the material surface, ranging in size from the microscale to the nanoscale, can significantly enhance TENG output. To date, micro/nanopatterning-based surfacemodified polymer films and chemical functionalizations with various metal nanostructures have been explored to enhance the triboelectric 7-9 effect of TENGs. Toward this end, micropatterned polydimethylsiloxane (PDMS) materials with pyramid, cube, and nanoarray shapes have shown much promise in the low-pressure regime. Additionally, wrinkling of the PDMS surface can increase the

friction area and can be advantageous in enhancing 8 the triboelectric output. Conducting and semiconducting polymers have been widely used for decades as electrode, interconnect, and active layers in various applications. Among these polymeric materials, poly (3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has proven to be versatile; its conductivity can be tuned by means of matrix dilution and/or solvent treatment to suit particular applications, and thus it is widely used in organic 10-15 electronics as a hole conductor and injector. Choong et al. reported a pressure sensor using PEDOT:PSS as a potential high-conductivity stretchable electrode on a PDMS film patterned with 16 micropyramids. Most recently, Hwang et al. and Roh et al. demonstrated patchable strain sensors using PEDOT:PSS as a conductive filler, showing promise toward the development of high-performance 17,18 mechanical sensors. Moreover, it has been utilized in piezoelectric, acoustic, and thermoelectric 19-24 energy harvester by a number of researchers. These results suggest that the use of suitably functionalized PEDOT:PSS can be a promising choice for enhancing nanogenerator output. However, the usage of this polymer has not yet been explored widely in triboelectric nanogenerator applications.

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Herein, we report an enhanced TENG composed of wrinkle-patterned PDMS, a conductive PEDOT:PSS elastomer, and nylon fiber film. We expected that the addition of functionalized PEDOT:PSS as an interconnect layer between the wrinkle-patterned PDMS and the working electrode would enhance the transport of triboelectric charge and improve the device’s practical applicability. We believed that PEDOT:PSS would act as a charge accumulator, thereby accelerating the flow of electrons in the TENG circuitry. Additionally, we considered the possible application of this TENG as a sensor. Solid-state and/or flexible chemical and gas sensors have significant uses in various industrial applications related to environmental and personal safety. In recent years, in addition to the existing interest in high mechanical flexibility and stability, nanoscale device configurations, and portability, there has been considerable interest in the fabrication of self-powered active sensors that integrate nanogenerators and functional devices, with the aim of harvesting environmental energy to power the nanosensors. To enable this functionality, we explored our as-fabricated TENG as a self-powered active sensor (triboelectric nanogenerator–based sensor; TENS) for the detection of acetylene (C2H2) by slightly changing one layer of the TENG device. This active sensor demonstrated an output signal as a power source, as well as a sensing signal in response to a change in the environment. 2. EXPERIMENTAL SECTION Materials. An indium tin oxide (ITO)-coated polyethylene terephthalate (PET) substrate having −1 sheet resistance of 14 Ω sq was obtained from Mianyang Prochema Commercial Co., Ltd., China. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) aqueous solution of 1.3 wt%, ethylene glycol (EG), silver nitrate (AgNO3), and trimethylchlorosilane (TMCS) were purchased from Sigma-Aldrich. Nylon fiber sheet material was purchased from CHMLAB; PDMS elastomer and cross-linker were purchased from HSSTS-Sylgard. All chemicals were used as received without further purification. Fabrication of EG-functionalized, PEDOT:PSScoated, Micropatterned PDMS Film and Ag/ZnO/nylon Film. A porous silicon (p-Si) mold (4 × 2 4 cm ) was prepared using a modified metal-assisted 25 electroless chemical etching process as follows: A p-Si wafer was metallized by means of RF magnetron sputtering and then etched for 15 min in an etchant solution. Detail synthesis process and the surface morphology of the as-prepared porous Si mold can be found in supplementary information (Figure S1).

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The resulting p-Si master was functionalized by means of treatment with piranha solution for 10 min, followed by vacuum-phase silanization with TMCS to facilitate peeling. Then, a mixture of PDMS elastomer and cross-linker (10:1 w/w ratio) was spin-coated onto the p-Si master and incubated at 80 °C for 2 h. EG was added to 20 mL of PEDOT:PSS aqueous solution at 6 wt% EG, and the resulting solution was spin-coated onto the upper surface of the PDMS/p-Si substrate and then annealed at 110 °C. After 30 min, the EG-PEDOT:PSS (EPP)-coated PDMS/p-Si substrate was immediately immersed in the EG solution and held there for 15 min at 60 °C. The substrate was then removed and post-annealed at 110 °C for 30 min. Finally, the EPP-coated PDMS film was peeled gently from the p-Si master and 2 sliced into 1.85 × 1.6 cm pieces. A similar process was carried out to prepare EPP-based PDMS films with EPP layers of 2, 4, 8, and 10 wt% EG concentrations. ZnO thin film (thickness ~ 130 nm) was deposited onto the nylon membrane sheet and Ag nanoparticles (diameter: 15–20 nm) were subsequently deposited onto the ZnO thin film, both by means of RF magnetron sputtering. Finally, Al was laminated onto the back side of the Ag@ZnO/nylon film as a contact electrode. Device Fabrication and Characterization. To prepare a TENG, the EPP-containing side of the patterned PDMS/EPP film was attached to the ITO/PET film; Al was laminated onto nylon film as a contact electrode and then attached to PET film. Finally, both films were sealed with an adhesive spacer to ensure adequate gap distance between the PDMS/EPP/ITO/PET and nylon/Al/PET films. Friction was induced between the patterned surface of the PDMS and the surface of the nylon film. To form charge transfer circuitry, copper conducting wires were connected to the Al and ITO electrodes using silver paste. The effective size of the final device was 2 measured to be 1.85 × 1.6 cm , and its effective thickness was about 3.4 mm. To prepare TENS devices, the nylon/Al/PET film in the TENG design was replaced with Ag@ZnO/nylon/Al/PET film. Figure S2 shows a schematic illustration and a photograph of the experimental setup, and includes a photograph of the as-fabricated TENS (or TENG). A JEOL JEM-2010F system was used to carry out FESEM characterization, and an NT-MDT Ntegra system operated in tapping mode was used for atomic force microscopy (AFM) analyses. A Kruss DSA system was used to characterize the water droplet contact angles using the sessile drop method at room temperature. Optical properties of the assynthesized materials were examined using an ultraviolet-visible (UV-vis) spectrophotometer (Cary


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5000 UV-Vis-NIR). A linear motor (FASTECH, EziServo) was used to supply constant contact force to a device under test. A functional oscilloscope (Lecroy Wave Runner 610) and a low-noise current measurement system (Keithley 6485 picoammeter) were used to acquire electrical signals. Gas sensing characterization of the TENS was carried out in a sealed custom-made gas chamber at room o temperature (25 C) including an inlet for gas supply and an outlet for passage of air. A mixture of target

gas and synthetic air was supplied to the chamber at a constant flow rate of 50 sccm, with different target gas concentrations, and the chamber was purged with synthetic air between each gas sensing pulse. The response magnitude of the TENS was calculated using the following formula: S (%) = (Va − Vg)/Va × 100, where S denotes the response of the TENS, and Va and Vg are the voltages in air and in the presence of a certain amount of gas, respectively.

Figure 1. (a) Schematic of the as-fabricated TENG device. (b–d) Representative FESEM micrographs of the (b) nylon fiber (Inset: magnified view of the nylon film), (c) W-PDMS, and (d) EPP film (Inset: cross-section of EPP). 3. RESULTS AND DISCUSSION The as-fabricated TENG has a multilayered structure based on two distinct plates: an upper layer of Al metal and highly rough nylon fiber film, and a lower layer of W-PDMS, EPP, and ITO (Figure 1a). The mean diameter of the nylon fibers as determined from a FESEM image was nearly 10 µm (Figure 1b). To increase the friction area and enhance the triboelectric output, we fabricated a wrinkle-patterned PDMS (W-PDMS) surface by means of a simple molding process. We prepared porous Si using electroless metal-assisted chemical etching, and used this as a mold to prepare W-PDMS. In comparison to traditional polymer surface

modification processes based on the use of molding 3,26 or complex ultraviolet–ozone (UVO) treatment, the proposed method instead uses the relatively economical, simple, and rapid formation of a patterned polymer surface with a high yield of surface wrinkling. More importantly, the proposed method offers the possibility of making hundreds of patterned polymer replicas from a single mold, which can be beneficial for large-scale production and practical applications. FESEM analysis of the as-prepared WPDMS showed a surface morphology of high-density wrinkle-shaped structures across the entire PDMS surface (Figure 1c). It should be noted that the shape and the density of the surface patterns was closely related to the patterns on the master surface. We



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anticipated that the ethylene glycol (EG)functionalized PEDOT:PSS (EPP) film would exhibit sufficient phase separation between the PEDOT and PSS chains, ultimately allowing the formation of more conductive PEDOT channels in the EPP layer compared to the bare one. FESEM analysis of the EPP film with 6 wt% EG functionalization, conducted on the smooth side of the W-PDMS film, showed the formation of an amorphous continuous film; however,

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no phase separation or distinct PEDOT-rich areas and PSS-rich areas were observed (Figure 1d). Several EPP samples were prepared in which the ratio of EG was varied from 0 to 10 wt%, and similar results were observed in FESEM analyses of each sample. Inset of Figure 1d shows the cross-section FESEM image of the 6 wt% EG functionalized EPP film on the PDMS film, in which the thickness of the EPP layer was estimated to be nearly 210 nm.

Figure 2. Surface topographic AFM images of (a) pristine PEDOT:PSS and EPP with (b) 4 wt%, (c) 6 wt%, (d) 8 wt%, and (e) 10 wt% EG. (f) Sheet resistance of the bare PEDOT:PSS and as-prepared EPP films; inset: enlarged view within EG concentrations ranging from 4 to 10 wt%. For further analysis, the pristine PEDOT:PSS and the as-prepared EPP samples were characterized by AFM (Figure 2). AFM topographic images clearly show the morphological changes in the EPP films compared to the pristine film. These phenomena might be caused due to the conformational changes 11 with increased interchain interactions. In the bare PEDOT:PSS film, PEDOT-rich grains (brighter regions) are densely surrounded by the PSS layers (darker region) and exhibits smooth surface with root mean square (RMS) roughness of 1.63 nm (Figure 2a). Whereas, the EPP film with 4 wt% EG exhibits higher roughness with RMS = 1.82 nm (Figure 2b). Furthermore, 6, 8, and 10 wt% EG treated EPP (Figure 2c-2e) show higher roughness with RMS = 2.04 nm, 2.23 nm, and 2.51 nm, respectively. The increased surface roughness is due to the gradual removal of the PSS content and the increased grain

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size of the PEDOT. The increasing grain size of the PEDOT observed from pristine to 10 wt% EPP film is the evidence of phase segregation, which allowed to form longer PEDOT-rich chains and consequently constructed pathways of enhanced 14-16 conduction in the film. The relationship between EPP film conductivity and EG concentration was plotted (Figure 2f), showing a trend consistent with 16,27 previous reports. The sheet resistance of the bare PEDOT:PSS film (~350 nm thick) was about 90 kΩ −1 sq . With the addition of 6 wt% EG, the sheet −1 resistance was significantly reduced to 430 Ω sq . The sheet resistances gradually decreased with increasing EG concentration. Figure 3 shows the absorption spectra of the pristine PEDOT:PSS and the as-prepared EPP films. The absorption in the visible and NIR range is related to the majority charge carriers in the PEDOT:PSS


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such as positively charged polarons and bipolarons. The pristine PEDOT:PSS shows a strong absorption band within 1300–2000 nm, which can be attributed to the bipolarons; the absorption band within 7001300 nm and 400-700 nm can be attributed to the 23 polaron and the neutral polymer chain. It is observed that bipolaron absorption reduced, whereas, polaron and neutral polymer chain absorption increased with increasing EG contents. This phenomenon suggests that EG treatment can preferentially alter the PEDOT:PSS state from bipolarons to polarons and up to neutral chains. Moreover, the increased absorption demonstrates that charge carriers become more delocalized on the PEDOT chains after the EG treatment. The reduced intensity of the absorption band of PSS (originated from the aromatic rings of PSS) with increasing EG concentration denotes the reduction of the PSS chains in the EPP. This phenomenon further confirms the obvious phase separation between the PEDOT and PSS chains in the EPP.

Figure 3. UV-visible absorption spectra PEDOT:PSS and the as-prepared EPP films.

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To understand the effect of light on the bare PEDOT:PSS and the as-prepared EPP films, I-V characteristics of the samples were examined in dark -2 and under 110 mW cm light illumination as shown in Figure S3. As seen in Figure S3a, 2 wt% EGfunctionalized EPP film showing noticeable increase in current in comparison to the bare PEDOT:PSS film with the applied voltage in dark. However, slight increases in current values are observed for 4-10 wt% EG-treated EPP films. This phenomenon can be attributed to the improved conductivity of the EPP films and is well agreed with Figure 2. Furthermore, similar trend with enhanced I-V characteristics was

observed when the samples were exposed to 110 -2 mW cm light illumination as shown in Figure S3b. A notable enhancement can relate to the preferential light absorption property of the EPP samples within the visible light region in comparison to the bare PEDOT:PSS. The outcomes demonstrate that the asprepared EPP films exhibit better photosensitivity than that of the bare PEDOT:PSS film under light illumination. Various TENGs were prepared using EPP films of various EG content ranging from 0 to 10 wt%, and these devices’ open-circuit output voltages (Vpk-pk) were characterized to determine the feasible structure of the device; the result is shown in Figure S4. As the sheet resistance of the EPP decreased dramatically with increasing EG concentration, the entire carrier population underwent significant carrier localization due to Coulomb and site disorder, which ultimately promoted the accumulation of 28 electrons/holes inside the film. As a result, at the EPP–metal (ITO) interface, the carriers were transiently depleted at the junction, due to faster 29 extraction than injection of holes. This phenomenon boosted the charge-repelling force at the EPP–ITO interface. As a consequence, the flow of electrons was facilitated and the TENG showed a sharp increase in output voltage. However, increases in the EG concentration also led to drastic increases in the EPP film’s conductivity. This trend might lead to metallic–metallic contact between the EPP and the 29 ITO, resulting in local charge neutrality and thereby reducing the charge-repelling force at the interface. A gradual decrease in the output voltage was observed in TENG devices as the EG content of the EPP was increased above 6 wt% EG (Figure S4; indoors trace). Although the charge transport in EPP is still not fully understood, these phenomena can be described as hopping transport (semi-metallic behavior of EPP) or metallic transport (metallic 30 behavior of EPP) phenomena at the Fermi level. For further investigation, TENGs including the EPP films were tested under sunlight in an outdoor environment. A similar tendency in output was observed compared to that in indoor conditions, including the observation of enhanced voltage magnitude for intermediate EG content (Figure S4, outdoors trace). The enhancement could possibly be attributed to the accelerated hole accumulation efficiency of the EPP film due to its intrinsic photoreactivity. More importantly, the extent of delocalization appears in the EPP film that can drive the formation of sufficiently high charge-carrier densities at the EPP–ITO interface. This phenomenon accelerated the flow of electrons, resulting in further increases in output voltage.



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Figure 4. (a) open-circuit output voltage and (b) short-circuit output current of TENGs with and without an EPP layer, in indoor and outdoor environments.

Figure 5. TENG output dependence on external load resistance in (a, b) indoor and (c, d) outdoor environments: (a, c) output voltage (black) and current (red) versus resistance; (b, d) power versus resistance.


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Figure 6. (a) A TENG patched on a human forearm; (b, c) outputs during arm movement, of (b) voltage and (c) current density. In order to investigate the effect of addition of EPP layer and the effect of sunlight to the proposed device, the surface charge distribution of the TENGs (with and without EPP layer) in dark and under different light intensities was evaluated using Comsol Multiphysics electrostatics modeling as shown in Figure S5. It is known that the light intensity of the -2 sun on the earth’s surface is nearly 1050-1120 W m , in which among of the total energy around 52-55% infrared (above 700 nm), 42-43% visible (40031 700 nm), and 3-5% ultraviolet (below 400 nm). For this reason, we considered the light intensity within -2 1100 W m during the simulation procedure; a fixed 2 N compressive force was considered on the TENG. Figures S5a-i and S5b-i demonstrate that due to the addition of EPP layer, surface charge density at the EPP-ITO interface favorably increased. This increment can be attributed to the intrinsic hole accumulation property of the EPP layer. Moreover, the enhanced surface charge density increased the charge-repelling force at the EPP–ITO interface, accelerated the electron transfer through the external circuitry, and thus increased the electric output of the TENG. Figures S5a-ii and S5b-ii show the surface charge distribution of the TENG without and with EPP -2 layer under 110 mW cm light intensity, respectively. The enhanced charge density on the EPP-ITO interface due to the light exposure can be attributed 32,33 to the photoreactivity property of the EPP film. The overall effect of the addition of EPP layer and the light exposure on the TENG without and with EPP layer is depicted in Figure S5c.

Figure 7. (a) Schematic of the as-fabricated TENS device. (b) Representative FESEM micrograph of the Ag@ZnO/nylon fiber film.



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Figure 8. Output voltage and current density of the TENS (a) indoors and (b) outdoors under various operating conditions: (i) output voltage in air, (ii) current density in air, (iii) output voltage under 100 ppm C2H2, (iv) current density under 100 ppm C2H2.


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To investigate the device functionality, TENG was tested under different pressing (contact) forces and external pressing frequencies (Figure S6). Its triboelectric output voltage increased gradually with increasing contact force from 0.4 to 4.6 N (Figure S6a); this trend was almost saturated at 3.2 N. Similarly, the output voltage increased gradually in both amplitude and frequency as the external pressing frequencies were increased from 0.2 to 2.1 Hz (Figure S6b). Hence, the contact force of 3.2 N and the external pressing frequency of 0.5 Hz were considered optimal, and thus were used as the optimum conditions for further investigations. In addition, output voltages of the TENG (indoors) under both forward and reverse connections are presented in Figure S7. The output voltage under reverse connection showed values opposite to those under the forward connection, proving that the measured output was generated only by the TENG, rather than any external sources. The triboelectric output characteristics of TENGs with and without an EPP layer were tested in indoor and outdoor environments. The addition of an EPP layer significantly increased the open-circuit output voltage (Vpk-pk) of the TENG device (Figure 4a). The output voltage increased from 97.2 V without an EPP layer to 166.4 V with the layer. Moreover, under sunlight (in the outdoor environment), the output voltage further increased to 191.6 V. In contrast, exposure to sunlight did not yield any output voltage variation for the TENG without an EPP layer. The addition of an EPP layer also increased the peak-topeak short-circuit output current (ISC) of the TENG; the device with an EPP layer showed ISC of 3.46 µA indoors and 3.94 µA under sunlight; the device without the EPP layer showed ISC of 1.7 µA in both conditions (Figure 4b). Enlarged views of the output voltage and current density (JSC) in both environments are given in Figure S8. Maximum −2 current densities of 1.17 µA cm (power density ~ −2 −2 0.2 mW cm ) and 1.33 µA cm (power density ~ 0.3 −2 mW cm ) were measured in indoor and outdoor conditions, respectively. These results reveal that the proposed TENG is potentially feasible for applications in both indoor and outdoor environments. For practical applications, it is very important to investigate the output power at the load. For this reason, the output power of the TENG (both indoors and outdoors) was measured across load resistors ranging from 10 kΩ to 50 MΩ. Due to ohmic loss, the voltage across the load (VL pk-pk) increased and current decreased with increasing resistance (Figures 5a, 5c). The maximum instantaneous power was found at the load resistance of 3.8 MΩ, of nearly 0.09 mW indoors (Figure 5b) and 0.2 mW outdoors (Figure 5d).

To investigate the feasibility of the device in realistic applications, we applied the TENG as a patch on the forearm of a human subject, using a transparent armband; this allowed strains to be induced and measured during arm movement (Figure 6a). During the movement, a maximum pressing force of ~0.12 N was applied to the TENG. The output voltage and current density were measured under ~0.12 N pressing force, indicating that the TENG can generate a maximum output voltage of −2 11.2 V and current density of 0.03 µA cm during its operation.

Figure 9. UV-visible absorption spectra of Ag nanoparticles, ZnO thin film, and Ag@ZnO fim. With the aim of fabricating a TENS device, the upper portion of the TENG design (i.e., the nylon/Al/PET layer) was replaced with an Ag-coated ZnO thin film on the nylon fiber film (Ag@ZnO/nylon/Al/PET). The Ag@ZnO was selected as a sensing layer for the TENS to investigate its possible use in sensing acetylene (C2H2). It has already been reported that the Ag–ZnO interface is important for C2H2 sensing due to the outstanding catalytic properties of Ag and the moderate 34 chemisorption ability of Ag–ZnO heterojunctions. The as-fabricated TENS is schematically illustrated in Figure 7a. The surface morphology of the Ag@ZnO/nylon layer was analyzed by means of FESEM, showing that a uniform and thin layer of Ag@ZnO film was deposited (Figure 7b). The thickness of the ZnO film was estimated to be nearly 130 nm (see Figure S1b) and the average diameter of the Ag nanoparticles was about 20 nm (inset of Figure 7b). A high-surface-roughness nylon fiber film was chosen intentionally as the substrate for the deposition of the Ag@ZnO film to obtain a crumpled



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sensing layer, as utilizing surface roughness is an obvious means of reducing environmental effects and

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also provides greater friction area for the triboelectric process.

Figure 10. TENS output dependence on C2H2 concentration: (a) open-circuit voltage (Vpk-pk), (b) short-circuit current density (JSC), (c) power density, and (d) response magnitude versus C2H2 concentration. (b, c insets) Magnified views of outputs within the 300–1000 ppm range of C2H2 concentration. The fabricated TENS device’s sensing properties were systematically investigated in a sealed gas chamber. The measurement process is described in detail in the Experimental section. Figure 8 shows the open-circuit voltage (Vpk-pk) and corresponding current density (JSC) measurement results of the TENS in both indoor (Figure 8a) and outdoor (Figure 8b) environments, in air and after exposure to 100 ppm C2H2. The output voltage of the TENS was observed to be about one-fifth that of the TENG. This can be ascribed to the reduced triboelectric contribution of the Ag@ZnO film on the nylon fiber film. We believe that the measured output voltage of the TENS can be attributed mainly to the dominant triboelectric property of the W-PDMS, because

triboelectric charge would be rarely generated by the Ag@ZnO layer. In addition, ZnO is not a triboelectricseries material; however, it can assist the triboelectrification process owing to its finite 35 conductivity characteristics. On the other hand, Ag is a triboelectric-series material; however, in the present device it is very small in size and deposited in a discrete manner, and as a consequence it generated a low amount of triboelectricity under friction. Moreover, the TENS could generate output voltage of up to 29.6 V (Figure 8a-i) and current −2 density of 0.38 µA cm (ISC = 1.12 µA; Figure 8a-ii) in the indoor environment. Also, it could generate output voltage of up to 31.8 V (Figure 8b-i) and current −2 density of 0.45 µA cm (ISC = 1.31 µA; Figure 8b-ii) in


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the outdoor environment. When the TENS was exposed to 100 ppm C2H2, its triboelectric output voltage decreased to 18 V (Figure 8a-iii) and its −2 current density decreased to 0.17 µA cm (ISC = 0.51 µA; Figure 8a-iv). The addition of Ag onto the ZnO thin film realigned the Fermi level (Ef) at the Ag–ZnO interface, leading to a relative increase in vacancies. These sites can act as active trap centers for collecting electrons. In an air environment, the Ag@ZnO interface reacts with environmental oxygen (O2) molecules and forms chemisorbed oxygen − anions (O n (ads)) on the Ag@ZnO surface. When the TENS was exposed to a 100 ppm C2H2 gas environment, C2H2 molecules reacted with these adsorbates. Due to the chemisorption process, Ag@ZnO acquires additional surface free electrons, which can effectively screen the triboelectric charges 36-38 generated on the TENS surface. The TENS experienced a decrease in triboelectric voltage and/or

current in the presence of C2H2. Additionally, when the TENS was exposed to 100 ppm C2H2 in the outdoor environment, a slight enhancement in sensing properties was observed. In this situation, the triboelectric output voltage was measured to be nearly 17 V (Figure 8b-iii) and the current density −2 about 0.2 µA cm (ISC = 0.61 µA; Figure 8b-iv). Outdoors under sunlight, plasmonic Ag forms ‘hot electrons’ that can enhance the separation efficiency of photoexcited electron–hole pairs and thus enhance the charge carrier transport properties of the Ag@ZnO interface to form highly reactive − photoinduced oxygen anions (O n hv). These chemisorbed and photoinduced oxygen anions facilitate redox reaction and chemical activation of the sensor surface, thereby enhancing the surface reactivity to the target gas. A schematic illustration of the entire sensing mechanism is depicted in Figure S9.

Figure 11. TENS output voltage (a) indoors and (b) outdoors, versus external RH and C2H2 concentration. Notably, irradiation within visible light region can provide sufficient energy to generate plasmon excited ‘hot electrons’ in the Ag nanoparticles and can significantly modify the electronic structure of ZnO. Figure 9 shows the absorption spectroscopy of the Ag nanoparticles, ZnO thin film, and Ag@ZnO film in the ultraviolet-visible (UV-vis) spectral region. The spectrum of the ZnO thin film shows the typical band edge absorption at nearly 348 nm. The peak centered at around 390 nm corresponds to the characteristic surface plasmon resonance (SPR) peak of the Ag nanoparticles. The surface plasmon band of Ag@ZnO film shows a red shift, which might be caused due to the strong interfacial electronic 39,40 The shift in the coupling between ZnO and Ag.

ZnO band position in the Ag@ZnO can be attributed to the higher electro-negativity of Ag nanoparticles, which attract more electrons towards themselves and influence the movement of the band position of ZnO. The observed spectrum indicates that Ag loading improved the visible spectral absorption of ZnO, as well as the separation efficiency of the photogenerated electron-hole pairs. This is due to the capture effect of Ag nanoparticles for free electrons under the irradiation of light and the more effective interface charge transfer path. The enhanced absorption property of Ag@ZnO might be attributed to the phenomenal increase in oxygen related defects in the ZnO and the presence of plasmonic Ag nanoparticles, which acted as a sink for the photo-



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induced charge carriers. Moreover, in order to realize the surface reactivity of the Ag@ZnO sensing layer with and without exposure of light, we investigated the surface charge density of the sensing layer at different light intensities (light intensity: 0-110 mW -2 cm ; wavelength: 400-700 nm) using COMSOL software (see Figure S10). It is apparent that the surface charge density increased gradually with the increasing light intensity. The overall sensor characteristics are graphed in Figure 10. With increasing C2H2 concentration, the triboelectric charges on the TENS surface became substantially screened by the surface free electrons, leading to the gradual decrease observed in surface charge (Figures 10a, 10b). As the adsorption– desorption kinetics facilitated with increasing gas concentration, the triboelectric screening effect enhanced gradually, resulting in gradually decreasing triboelectric output voltage; this is consistent with the decreasing trend observed for triboelectric output current. Upon exposure to 30 ppm C2H2 in the indoor environment, the TENS’ triboelectric voltage decreased from 29.6 to 21.2 V (Figure 10a) and its −2 current density decreased from 0.38 to 0.28 µA cm (ISC = 0.82 µA; Figure 10b). Outdoors, the triboelectric screening capability of the Ag@ZnO surface was slightly enhanced. As a result, the device’s voltage decreased from 31.8 to 28.1 V (Figure 10a) and its current density decreased from −2 0.45 to 0.35 µA cm (ISC = 1.02 µA; Figure 10b). Furthermore, when the TENS was exposed to 1000 ppm C2H2, its triboelectric voltage and current density −2 were respectively reduced to 8.6 V and 5.1 nA cm −2 indoors, and to 3.5 V and 1.3 nA cm outdoors. Moreover, it was evident that the device could effectively detect C2H2 within the concentration range of 30–1000 ppm. The calculated power density variation of the TENS at various C2H2 concentrations is shown in Figure 10c. Insets in Figures 10b and 10c show enlarged views within the concentration range of 300–1000 ppm. Figure 10d shows the calculated response magnitude of the TENS at various C2H2 concentrations; the maximum responses were calculated to be 70.9% indoors and 89% outdoors under 1000 ppm C2H2 exposure. The zero percent (0%) response at 0 ppm gas concentration represents the initial response of the TENS measured in air. Figure 10d also shows that the TENS exhibited a gradual increasing trend with the increasing C2H2 concentrations. It has already been reported that humidity negatively affects triboelectric response on device 41 surfaces. However, because the effect of environmental humidity is a dominant factor affecting the practical applicability of sensors, herein we investigated the effect of water vapors on the TENS

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to further elucidate the device’s functionality and practical applicability. Output voltages of the device versus both relative humidity (RH) and C2H2 concentrations were plotted 3-dimensionally (Figure 11), showing that both indoors and outdoors, the output signal decreased with increasing humidity concentration; the overall trend of voltage versus C2H2 concentration changed negligibly with increasing RH up to 60%. This phenomenon can be attributed to the enhanced hydrophobicity of the wrinkled PDMS surface, which acted as a protective layer minimizing the influence of water molecules on the sensing surface and promoting the stability of the 42,43 sensing surface in humid environments. Figure S11a and S11b show the water contact angle measurement of the flat PDMS and the as-prepared w-PDMS film, respectively. It is observed that due the surface wrinkling on the PDMS surface, the water contact angle increased. It is known that the contact angle is directly proportional to the surface roughness and inversely proportional to surface energy. The patterning of the PDMS increased its surface roughness and reduced its surface energy, ultimately increasing its hydrophobicity. In contrast, the degradation rate at higher RH concentration (over 60%) is slightly higher (Figure 11a). This phenomenon can be attributed to the formation of a thick layer of water molecules on the sensing surface, overwhelming the protective ability of the PDMS layer and allowing the intrinsic effect of humidity to occur, suppressing the triboelectric response. In addition, outdoors, the influence of humidity was noticeably lower over the entire tested RH concentration range (0–80%; Figure 11b). This demonstrates the enhanced surface reactivity of the TENS outdoors, arising from the plasmonic activity of the Ag nanoparticles acted as a shield, which favorably protected the TENS sensing surface from the interference of ambient humidity. Moreover, the high sensitivity and stable performance observed in humid environments suggested that the proposed TENS has potential for widespread and practical in hostile applications in detecting C 2H 2 environments. Finally, the proposed TENS offers exceptional advantages over the previously reported C2H2 sensors such as portability, self-powered operation, long-term durability, and accuracy. A comparative study on the performance of the proposed TENS with the previously reported C2H2 sensors is shown in Table S1. 4. CONCLUSION A TENG using a slightly conductive PEDOT:PSS layer with wrinkle-patterned PDMS and a nylon fiber film has been proposed. The addition of an EPP layer significantly enhanced the working efficiency of the


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TENG, leading to excellent usability in both indoor and outdoor (under sunlight) environments. The TENG can generate up to 166.4 V indoors and 191.6 V outdoors. The device also generated 11.2 V and 0.08 µA in response to human movement when patched on the forearm. Moreover, a triboelectric effect–based self-powered active sensor was demonstrated for detecting C2H2 gas. A maximum −2 power density of 11.2 µW cm (29.6 V) and 14.1 µW −2 cm (31.8 V) were achieved indoors and outdoors, respectively. When the TENS was exposed to 1000 ppm C2H2, its triboelectric voltage and current density −2 were respectively reduced to 8.6 V and 5.1 nA cm −2 indoors, and to 3.5 V and 1.3 nA cm outdoors, showing maximum sensitivities of 70.9% and 89%, respectively. We expect that the fabricated TENG, with its superior performance, can be used in a variety of applications, including as a self-powered active sensor especially for environmental and personal safety purposes. ASSOCIATED CONTENT Supporting Information As-fabricated porous Si mold and cross-section of ZnO thin film. Output voltage variation with varying EG concentrated PEDOT:PSS. Simulation model for surface charge distribution. Pressing force and frequency dependence of the device. TENG output at forward and reverse connection. Output characteristics of the TENG at optimum condition. Entire experimental setup and the optical image of the fabricated TENS. Schematic illustration for gas sensing mechanism. AUTHOR INFORMATION Corresponding Author * [email protected] Notes The authors have no competing financial interests to declare. ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded in 2014 by the Ministry of Science, ICT and Future Planning (NRF2014R1A2A2A01002668). REFERENCES (1) Wang, Z. L. Triboelectric Nanogenerators as New Energy Technology for Self-Powered Systems and as Active Mechanical and Chemical Sensors. ACS Nano 2013, 7, 9533-9557.

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TOC graphic 301x200mm (96 x 96 DPI)


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Improving the Working Efficiency of a Triboelectric Nanogenerator by

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