- Email: [email protected]
Triboelectric nanogenerator with nanostructured metal surface using water-assisted oxidation
Nano Energy (2016) 21, 258–264
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanoenergy
FULL PAPER
Triboelectric nanogenerator with nanostructured metal surface using water-assisted oxidation Sang-Jae Park, Myeong-Lok Seol, Daewon Kim, Seung-Bae Jeon, Yang-Kyu Choin School of Electrical Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea Received 12 November 2015; received in revised form 24 December 2015; accepted 26 January 2016 Available online 3 February 2016
KEYWORDS
Abstract
Energy harvester; Triboelectric nanogenerator; Water-assist oxidation; Oxidized metal; Contact electrification
The performance of a triboelectric nanogenerator (TENG) was effectively enhanced by forming nanostructures at the contacting interface. The contacting interface usually consists of metal and polymer surface, but the formation of nanostructures have only been actively studied for the polymer so far. In this work, a simple and effective route to forming nanostructures on the metal surface is proposed, using a water-assisted oxidation (WAO) process. The one-step WAO process requires only hot water without any complicated equipment and treatment. Using the WAO process, densely packed micro and nanostructures were successfully formed on three target metal surfaces: aluminum, copper, and zinc. The output power of the TENG was enhanced after the nanostructure formation because of the increased contact area. The influence of the process conditions on the nanostructure morphology was additionally analyzed to maximize the output power. The simple and low-cost WAO process is advantageous in terms of practicality. & 2016 Elsevier Ltd. All rights reserved.
Introduction Electrical energy is essential to the operation of various devices. This electrical energy can be generated by fossil fuels, nuclear fuel, and renewable sources such as hydropower, etc. n
Corresponding author. E-mail address: [email protected] (Y.-K. Choi).
http://dx.doi.org/10.1016/j.nanoen.2016.01.021 2211-2855/& 2016 Elsevier Ltd. All rights reserved.
However, many of the technologies needed to produce energy from these sources, or their subsequent consumption, can create environmental issues. As one approach for addressing deficiencies in energy sources and environmental issues, energy harvesting has recently attracted dramatically increased interest. Energy harvesting is a renewable technology which converts waste energy sources into useful electrical energy. This renewable electrical energy can then be used for the expanding
Nanogenerator with nanostructured metal surface applications of wireless systems and portable electronics in the era of the internet of things (IoT). Among various waste energy sources can be converted to electricity, waste mechanical energy is the most spread existing energy source and it can be easily collected from wind, water motion and daily life activities such as human motion. Various methods to harvest mechanical energy have been introduced including devices based on piezoelectric [1–4], electrostatic [5–8] and electromagnetic [9–12] properties. Recently, a simple, low-cost, eco-friend triboelectric nanogenerator (TENG) has been actively studied. [13] The energy harvesting mechanism of the TENG device is based on contact electrification between two different materials. In typical triboelectric devices, one material (usually a polymer) gains negative charges while the other material (usually metal) gains positive charges after an interactive contact and separation process. The performance of the TENG is determined by the surface charge density on the two materials. There are two major approaches to increasing the triboelectric charge density. The first method is selecting a proper combination of materials, specifically, two materials which have largely different triboelectric charging polarity. The other method is increasing effective contact area to induce a large amount of surface charge density, since the generated charge density is enhanced with increment of surface contact area. In practice, it has been found that micro or nano scale structural features on the contact interface can easily increase the effective surface area [14–20]. While the contact interface is usually composed of metal and polymer surfaces, there have been less studies which have focused on forming nanostructures on the metal surface compared with investigations for the polymer surface modifications because of fabrication difficulties [21–23]. In this study, a simple, rapid, and low-cost route to create nanostructures on a metal surface by use of a water-assisted oxidation (WAO) process was applied to the TENG. Using the WAO process, various nanostructures were formed on aluminum, copper, and zinc, triboelectric metals widely adopted for the TENG, and subsequently enhanced the output power of the TENG. Each optimized metal nanostructure-embedded TENG produced 1.5 to 2 times higher
259 open-circuit voltage and short-circuit current compared with the pristine TENG without the nanostructures.
Experimental methods WAO-based TENGs consist of two parts; one is a bottom part which contains the WAO metal, and the other is a top part which contains a polytetrafluoroethylene (PTFE) layer with a thickness of 50 μm. To fabricate the WAO metal, commercially available aluminum, copper, zinc metal plates having length, width, and thicknesses of 2 cm, 2 cm, and 0.3 mm respectively were prepared. Each metal was oxidized in 95 1C deionized (DI) water for various times (Figure 1). For aluminum, the WAO process resulted in a nanograss-like structure. In the copper and zinc cases, the WAO produced cubic-like microstructures, and needle-like nanostructures, respectively. The detailed mechanisms behind the creation of such morphologies are not yet known. It is believed that the nanograss formation occurs when the intrinsic oxide layer on the surface reacts with hot water, becomes hydrolyzed, dissolved and is precipitated on the aluminum surface [24,25]. In other studies, 1-, 2-, and 3-dimensional copper oxides [26,27] and zinc oxides [28,29] have been fabricated, and their dimensionality controlled by the oxidation conditions. Each oxidation method led to different shapes and chemical characteristics. In this work, DI water was used as the solution environment to form the nano or microstructures. While the temperature was fixed at 95 1C, time was chosen as a process variable. As a consequence, the WAO metals were processed to form various nano and microstructures for utilizing as a bottom electrode. To fabricate the top electrode, chrome (100 nm) and gold (150 nm) layers were sequentially deposited on a silicon wafer by the sputtering process. Finally, a 50 μm thick layer of PTFE was attached to the chrome/gold deposited silicon wafer. The total size of the WAO-based TENG was 2 cm by 2 cm. For the quantitative characterization of the WAO-based TENG, iterative contact and separation process between electrode and PTFE surface was enabled by an electrodynamic shaker (LW140.141-110, Labworks Inc.). Major
Figure 1 Schematic of the WAO metals. Commercially available (a) aluminum, copper, and zinc plate. After 95 1C deionized water oxidation, each metal plate has unique micro and nanostructures with oxidized (b) aluminum, (c) copper, and (d) zinc surface. (e) Each WAO metal used for each WAO-based TENG.
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Figure 2 Cross section view of the WAO (a) aluminum, (e) copper, and (i) zinc. EDS mapping data of the WAO (b and c) aluminum, (f and g) copper, and (j and k) zinc. XPS data of the (d) aluminum, (h) copper, and (l) zinc TENGs before and after WAO.
Figure 3 Scanning electron microscope (SEM) image of the WAO metal surface. The WAO aluminum surface with different WAO times of (b) 15 min, (c) 30 min, (d) 45 min, and (e) 1 h. The WAO copper surface with different oxidation times of (g) 1 h, (h) 2 h, (i) 3 h, (j) 4 h. The WAO zinc surface with different oxidation times of (l) 2 h, (m) 4 h, (n) 6 h, and (o) 12 h.
parameters to influence on the performance of the TENG such as force and frequency are precisely controlled with the aid of the shaker. The contact and separation frequency
was set to 3 Hz and applied force was set to 100 N. Opencircuit voltage (Voc) and short-circuit current (Isc) were measured by an electrometer (Keithley 6514).
Nanogenerator with nanostructured metal surface
Results and discussion To analyze the electrode material properties, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) analyzes were used. In the TEM images, the WAO aluminum shows grass-like structures (Figure 2-a), the WAO copper shows cubic-like structures (Figure 2-e), and the WAO zinc shows high dense needle-like structures (Figure 2-i). From the EDS and XPS data, the micro and nanostructures of the WAO metal compounds were all found to be metal oxides. The composition of the structured materials is metal, and the metal oxide nanostructures cover the metal surfaces. The morphology of the metal nanostructures was dependent on the metal element and WAO process time (Figure 3). To investigate their formation, the morphologies of the metal nanostructures were monitored for various WAO process times. For the aluminum, the WAO process time was split into 15 min, 30 min, 45 min, 1 h, 2 h, 3 h, and 4 h. Longer WAO process times up to 1 h resulted in denser grass structures. Beyond 1 h WAO process time, the nanostructure growth was saturated. In the copper case, the WAO process time was split into 1 h, 2 h, 3 h, and 4 h. The longer WAO process times resulted in larger sized cubic structures. In the zinc case, the WAO process time was split into 1 h, 2 h, 3 h, 4 h, 6 h, 12 h, and 24 h. The longer WAO process time gave rise to bigger and longer needle structures.
261 The output performances of the WAO-based TENGs were characterized using the contact and separation method. At first, there is no charge on the WAO metal and the pristine PTFE surface. When the WAO metal and PTFE are brought into contact, negative charges are induced on the PTFE surface and positive charges are induced on the metal surface according to their different triboelectric polarity. After contact, the two parts are separated from each other. This causes current to flow from the WAO metal electrode to the PTFE electrode until an equilibrium state of charge is reached. When the WAO metal and PTFE surface are brought into contact again, current flows from the PTFE electrode to the WAO metal electrode until another charge equilibrium state is reached. Repeated contact and separation thus produces alternating current (AC). The overall performances of the WAO-based TENGs are shown in Figure S1. The WAO aluminum TENG clearly produces higher open-circuit voltage compared with the smooth surface aluminum TENG (Figure 4-a). Moreover, the enhancement of voltage was saturated when the aluminum was prepared with a WAO process time of over 1 h. This can be understood in relation to the surface area of the metal electrode. Up until 1 h of oxidation, the nanograss structures continued growing, but that growth was saturated after a WAO process time of 1 h. This means that the surface area was being enlarged up until the WAO process time of 1 h. Accordingly, the effect of the enlarged surface area on
Figure 4 Open-circuit voltage of the WAO-based TENGs. Open-circuit voltage of the WAO (a) aluminum, (b) copper, and (c) zinc with various oxidation times. (d) Voltage comparison with initial and optimized WAO-based TENGs.
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Figure 5 (a) Camera image of the WAO-based wind TENGs. (b) Working mechanism of the WAO-based wind TENGs. Short-circuit current of the optimized WAO (c) aluminum, (d) copper, and (e) zinc wind TENG.
power enhancement was saturated beyond a WAO process time of 1 h. The WAO copper TENG showed random and unstable open-circuit voltage (Figure 4-b). After measurement of the WAO copper TENG, brown-colored residues remained in the PTFE surface. It can be inferred that the microcubic structures were detached, transferred, and adhered to the PTFE surface by the interactive contact and separation process. So, the random and unstable performances were produced by weak adhesion between the copper plate and the cubic-like structures. The WAO zinc TENG also showed higher open-circuit voltage than the smooth surface zinc TENG (Figure 4-c). The optimal open-circuit voltage was observed for the zinc sample with a WAO process time of 12 h. Prior to the optimal point, the open-circuit voltage increased, however beyond the optimal point, the open-circuit voltage decreased. This tendency resulted in a bell-shape relationship. Before reaching the optimal point, the power performance is affected by an enlarged effective contact area. However, beyond the optimal point, the excessively sharp structures caused incomplete (tip-to-tip) contact between the PTFE and oxidized zinc surface, which results in degradation of the output power. Engineering the optimal metal surface enhanced the power of the TENG by 79.7% for the aluminum case, 42.6% for the copper case, and 80.9% for the zinc case (Figure 4-d). The WAO copper electrode is unlikely to be utilized in a commercial TENG device because it produced random and unstable performances. On the other hand, the optimized WAO aluminum and zinc electrodes can be useful for a practical TENG device. However, the WAO zinc sample needed a long processing time (12 h) to be optimized. So, the WAO aluminum is the most appropriate choice for enhancing the TENG performance,
because surface modification of the aluminum by WAO was the easiest, compared to the other metals. The practicality of the WAO process was confirmed by applying the WAO based electrode to a vibrating membrane type of the TENG, which can widely be utilized as a versatile platform to harvest wind energy [30–32]. Two WAO electrodes are used as a fixed top and bottom electrode, and a thin suspended vibrating PTFE membrane is positioned in between them (Figure 5-a). The mechanism of the abovementioned wind TENG is also based on contact and separation, activated by a wind vortex [30]. Based on operating measurements, each WAO-based wind TENG showed higher performance compared with a pristine flat surface wind TENG. Because the micro and nanostructures of the oxidized copper have weak adhesion to the copper surface, the WAO copper could not contact the PTFE effectively. As a result, the WAO copper showed a relatively low enhancement of performance. On the other hand, the WAO aluminum and zinc contacted the PTFE with enlarged surface areas, resulting in notable increases in performance.
Conclusions A water-assisted oxidation (WAO) process was applied to simply and eco-friendly form nanostructures on the metal electrode of a triboelectric nanogenerator (TENG). The WAO-processed TENGs were made with three different metals: aluminum, copper, and zinc. Each metal showed different surface morphologies and different output characteristics. The WAO aluminum formed grass-like nanostructures, the WAO copper formed cubic-like microstructures, and the WAO zinc formed needle-like nanostructures. Each of the nanograss, micro cubic, and nano needle
Nanogenerator with nanostructured metal surface structures led to enhanced power due to the enlarged electrode surface areas. The optimal WAO-based TENGs showed 1.5 to 2 times better performance than the pristine flat TENG. Based on the above work, the WAO process is advantageous for TENG engineering in terms of fabrication simplicity and practicality.
Acknowledgments This work was partially supported by Open Innovation Lab Project from the National Nanofab Center (NNFC) and the EndRun Project funded by the Ministry of Science, ICT and Future Planning.
Appendix A.
Supplementary material
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2016.01.021.
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263 [20] G. Zhu, Z.H. Lin, Q. Jing, P. Bai, C. Pan, Y. Yang, Y. Zhou, Z.L. Wang, Nano Lett. 13 (2013) 847–853. [21] S. Wang, L. Lin, Z.L. Wang, Nano Lett. 12 (2012) 6339–6346. [22] M. Han, X.-S. Zhang, B. Meng, W. Liu, W. Tang, X. Sun, W. Wang, H. Zhang, ACS Nano 7 (2013) 8554–8560. [23] S.-J. Park, M.-L. Seol, S.-B. Jeon, D. Kim, D. Lee, Y.-K. Choi, Sci. Rep. 5 (2015) 13866. [24] W. Vedder, D.A. Vermilyea, Trans. Faraday Soc. 65 (1969) 561. [25] Z.S. Saifaldeen, K.R. Khedir, M.F. Cansizoglu, T. Demirkan, T. Karabacak, J. Mater. Sci. 49 (2013) 1839–1853. [26] J. Liu, X. Huang, Y. Li, K.M. Sulieman, X. He, Cryst. Growth Des. (2006). [27] J. Liu, X. Huang, Y. Li, K.M. Sulieman, X. He, F. Sun, J. Mater. Chem. 16 (2006) 4427. [28] C. Lu, L. Qi, J. Yang, L. Tang, D. Zhang, J. Ma, Chem. Commun. (Camb.) (2006) 3551–3553. [29] S. Baruah, J. Dutta, Sci. Technol. Adv. Mater. 10 (2009) 013001. [30] M.-L. Seol, J.-H. Woo, S.-B. Jeon, D. Kim, S.-J. Park, J. Hur, Y.-K. Choi, Nano Energy (2014) 1–8. [31] X.S. Meng, G. Zhu, Z.L. Wang, ACS Appl. Mater. Interfaces (2014) 10–15. [32] G. Zhu, J. Chen, T. Zhang, Q. Jing, Z.L. Wang, Nat. Commun. 5 (2014) 1–9. Sang-Jae Park received his B.S. and M.S. degrees from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2013 and 2015, respectively, where he is currently working toward his Ph.D. degree. His current research interests include bio-signal detection and energy harvesting.
Myeong-Lok Seol received his B.S. and M.S. degrees from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2010 and 2012, respectively, where he is currently working toward the Ph.D. degree. His current research interests include piezoelectric and triboelectric energy harvesting.
Daewon Kim received his B.S. degree from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2009, where he is currently working toward the Ph.D. degree. His current research interests include triboelectric energy harvesting and bio-electronics.
Seung-Bae Jeon received his B.S. and M.S. degrees from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2013 and 2015, respectively, where he is currently working toward his Ph.D. degree. His current research interests include energy harvesting.
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S.-J. Park et al. Yang-Kyu Choi received his B.S. and M.S. degrees from the Seoul National University, Seoul, Korea, in 1989 and 1991, respectively, and the Ph.D. degree from the University of California, Berkeley, in 2001. He is currently a Professor with the Department of Electrical Engineering, KAIST. He has authored or coauthored over 320 papers and is a holder of more than 20 U.S. patents
and 100 Korea patents. Dr. Choi received the Sakrison Award for the best dissertation from the Department of Electrical Engineering and Computer Sciences, University of California, in 2002. He was also the recipient of “The Scientist of the Month for July 2006” from the Ministry of Science and Technology in Korea.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanoenergy
FULL PAPER
Triboelectric nanogenerator with nanostructured metal surface using water-assisted oxidation Sang-Jae Park, Myeong-Lok Seol, Daewon Kim, Seung-Bae Jeon, Yang-Kyu Choin School of Electrical Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea Received 12 November 2015; received in revised form 24 December 2015; accepted 26 January 2016 Available online 3 February 2016
KEYWORDS
Abstract
Energy harvester; Triboelectric nanogenerator; Water-assist oxidation; Oxidized metal; Contact electrification
The performance of a triboelectric nanogenerator (TENG) was effectively enhanced by forming nanostructures at the contacting interface. The contacting interface usually consists of metal and polymer surface, but the formation of nanostructures have only been actively studied for the polymer so far. In this work, a simple and effective route to forming nanostructures on the metal surface is proposed, using a water-assisted oxidation (WAO) process. The one-step WAO process requires only hot water without any complicated equipment and treatment. Using the WAO process, densely packed micro and nanostructures were successfully formed on three target metal surfaces: aluminum, copper, and zinc. The output power of the TENG was enhanced after the nanostructure formation because of the increased contact area. The influence of the process conditions on the nanostructure morphology was additionally analyzed to maximize the output power. The simple and low-cost WAO process is advantageous in terms of practicality. & 2016 Elsevier Ltd. All rights reserved.
Introduction Electrical energy is essential to the operation of various devices. This electrical energy can be generated by fossil fuels, nuclear fuel, and renewable sources such as hydropower, etc. n
Corresponding author. E-mail address: [email protected] (Y.-K. Choi).
http://dx.doi.org/10.1016/j.nanoen.2016.01.021 2211-2855/& 2016 Elsevier Ltd. All rights reserved.
However, many of the technologies needed to produce energy from these sources, or their subsequent consumption, can create environmental issues. As one approach for addressing deficiencies in energy sources and environmental issues, energy harvesting has recently attracted dramatically increased interest. Energy harvesting is a renewable technology which converts waste energy sources into useful electrical energy. This renewable electrical energy can then be used for the expanding
Nanogenerator with nanostructured metal surface applications of wireless systems and portable electronics in the era of the internet of things (IoT). Among various waste energy sources can be converted to electricity, waste mechanical energy is the most spread existing energy source and it can be easily collected from wind, water motion and daily life activities such as human motion. Various methods to harvest mechanical energy have been introduced including devices based on piezoelectric [1–4], electrostatic [5–8] and electromagnetic [9–12] properties. Recently, a simple, low-cost, eco-friend triboelectric nanogenerator (TENG) has been actively studied. [13] The energy harvesting mechanism of the TENG device is based on contact electrification between two different materials. In typical triboelectric devices, one material (usually a polymer) gains negative charges while the other material (usually metal) gains positive charges after an interactive contact and separation process. The performance of the TENG is determined by the surface charge density on the two materials. There are two major approaches to increasing the triboelectric charge density. The first method is selecting a proper combination of materials, specifically, two materials which have largely different triboelectric charging polarity. The other method is increasing effective contact area to induce a large amount of surface charge density, since the generated charge density is enhanced with increment of surface contact area. In practice, it has been found that micro or nano scale structural features on the contact interface can easily increase the effective surface area [14–20]. While the contact interface is usually composed of metal and polymer surfaces, there have been less studies which have focused on forming nanostructures on the metal surface compared with investigations for the polymer surface modifications because of fabrication difficulties [21–23]. In this study, a simple, rapid, and low-cost route to create nanostructures on a metal surface by use of a water-assisted oxidation (WAO) process was applied to the TENG. Using the WAO process, various nanostructures were formed on aluminum, copper, and zinc, triboelectric metals widely adopted for the TENG, and subsequently enhanced the output power of the TENG. Each optimized metal nanostructure-embedded TENG produced 1.5 to 2 times higher
259 open-circuit voltage and short-circuit current compared with the pristine TENG without the nanostructures.
Experimental methods WAO-based TENGs consist of two parts; one is a bottom part which contains the WAO metal, and the other is a top part which contains a polytetrafluoroethylene (PTFE) layer with a thickness of 50 μm. To fabricate the WAO metal, commercially available aluminum, copper, zinc metal plates having length, width, and thicknesses of 2 cm, 2 cm, and 0.3 mm respectively were prepared. Each metal was oxidized in 95 1C deionized (DI) water for various times (Figure 1). For aluminum, the WAO process resulted in a nanograss-like structure. In the copper and zinc cases, the WAO produced cubic-like microstructures, and needle-like nanostructures, respectively. The detailed mechanisms behind the creation of such morphologies are not yet known. It is believed that the nanograss formation occurs when the intrinsic oxide layer on the surface reacts with hot water, becomes hydrolyzed, dissolved and is precipitated on the aluminum surface [24,25]. In other studies, 1-, 2-, and 3-dimensional copper oxides [26,27] and zinc oxides [28,29] have been fabricated, and their dimensionality controlled by the oxidation conditions. Each oxidation method led to different shapes and chemical characteristics. In this work, DI water was used as the solution environment to form the nano or microstructures. While the temperature was fixed at 95 1C, time was chosen as a process variable. As a consequence, the WAO metals were processed to form various nano and microstructures for utilizing as a bottom electrode. To fabricate the top electrode, chrome (100 nm) and gold (150 nm) layers were sequentially deposited on a silicon wafer by the sputtering process. Finally, a 50 μm thick layer of PTFE was attached to the chrome/gold deposited silicon wafer. The total size of the WAO-based TENG was 2 cm by 2 cm. For the quantitative characterization of the WAO-based TENG, iterative contact and separation process between electrode and PTFE surface was enabled by an electrodynamic shaker (LW140.141-110, Labworks Inc.). Major
Figure 1 Schematic of the WAO metals. Commercially available (a) aluminum, copper, and zinc plate. After 95 1C deionized water oxidation, each metal plate has unique micro and nanostructures with oxidized (b) aluminum, (c) copper, and (d) zinc surface. (e) Each WAO metal used for each WAO-based TENG.
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Figure 2 Cross section view of the WAO (a) aluminum, (e) copper, and (i) zinc. EDS mapping data of the WAO (b and c) aluminum, (f and g) copper, and (j and k) zinc. XPS data of the (d) aluminum, (h) copper, and (l) zinc TENGs before and after WAO.
Figure 3 Scanning electron microscope (SEM) image of the WAO metal surface. The WAO aluminum surface with different WAO times of (b) 15 min, (c) 30 min, (d) 45 min, and (e) 1 h. The WAO copper surface with different oxidation times of (g) 1 h, (h) 2 h, (i) 3 h, (j) 4 h. The WAO zinc surface with different oxidation times of (l) 2 h, (m) 4 h, (n) 6 h, and (o) 12 h.
parameters to influence on the performance of the TENG such as force and frequency are precisely controlled with the aid of the shaker. The contact and separation frequency
was set to 3 Hz and applied force was set to 100 N. Opencircuit voltage (Voc) and short-circuit current (Isc) were measured by an electrometer (Keithley 6514).
Nanogenerator with nanostructured metal surface
Results and discussion To analyze the electrode material properties, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) analyzes were used. In the TEM images, the WAO aluminum shows grass-like structures (Figure 2-a), the WAO copper shows cubic-like structures (Figure 2-e), and the WAO zinc shows high dense needle-like structures (Figure 2-i). From the EDS and XPS data, the micro and nanostructures of the WAO metal compounds were all found to be metal oxides. The composition of the structured materials is metal, and the metal oxide nanostructures cover the metal surfaces. The morphology of the metal nanostructures was dependent on the metal element and WAO process time (Figure 3). To investigate their formation, the morphologies of the metal nanostructures were monitored for various WAO process times. For the aluminum, the WAO process time was split into 15 min, 30 min, 45 min, 1 h, 2 h, 3 h, and 4 h. Longer WAO process times up to 1 h resulted in denser grass structures. Beyond 1 h WAO process time, the nanostructure growth was saturated. In the copper case, the WAO process time was split into 1 h, 2 h, 3 h, and 4 h. The longer WAO process times resulted in larger sized cubic structures. In the zinc case, the WAO process time was split into 1 h, 2 h, 3 h, 4 h, 6 h, 12 h, and 24 h. The longer WAO process time gave rise to bigger and longer needle structures.
261 The output performances of the WAO-based TENGs were characterized using the contact and separation method. At first, there is no charge on the WAO metal and the pristine PTFE surface. When the WAO metal and PTFE are brought into contact, negative charges are induced on the PTFE surface and positive charges are induced on the metal surface according to their different triboelectric polarity. After contact, the two parts are separated from each other. This causes current to flow from the WAO metal electrode to the PTFE electrode until an equilibrium state of charge is reached. When the WAO metal and PTFE surface are brought into contact again, current flows from the PTFE electrode to the WAO metal electrode until another charge equilibrium state is reached. Repeated contact and separation thus produces alternating current (AC). The overall performances of the WAO-based TENGs are shown in Figure S1. The WAO aluminum TENG clearly produces higher open-circuit voltage compared with the smooth surface aluminum TENG (Figure 4-a). Moreover, the enhancement of voltage was saturated when the aluminum was prepared with a WAO process time of over 1 h. This can be understood in relation to the surface area of the metal electrode. Up until 1 h of oxidation, the nanograss structures continued growing, but that growth was saturated after a WAO process time of 1 h. This means that the surface area was being enlarged up until the WAO process time of 1 h. Accordingly, the effect of the enlarged surface area on
Figure 4 Open-circuit voltage of the WAO-based TENGs. Open-circuit voltage of the WAO (a) aluminum, (b) copper, and (c) zinc with various oxidation times. (d) Voltage comparison with initial and optimized WAO-based TENGs.
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Figure 5 (a) Camera image of the WAO-based wind TENGs. (b) Working mechanism of the WAO-based wind TENGs. Short-circuit current of the optimized WAO (c) aluminum, (d) copper, and (e) zinc wind TENG.
power enhancement was saturated beyond a WAO process time of 1 h. The WAO copper TENG showed random and unstable open-circuit voltage (Figure 4-b). After measurement of the WAO copper TENG, brown-colored residues remained in the PTFE surface. It can be inferred that the microcubic structures were detached, transferred, and adhered to the PTFE surface by the interactive contact and separation process. So, the random and unstable performances were produced by weak adhesion between the copper plate and the cubic-like structures. The WAO zinc TENG also showed higher open-circuit voltage than the smooth surface zinc TENG (Figure 4-c). The optimal open-circuit voltage was observed for the zinc sample with a WAO process time of 12 h. Prior to the optimal point, the open-circuit voltage increased, however beyond the optimal point, the open-circuit voltage decreased. This tendency resulted in a bell-shape relationship. Before reaching the optimal point, the power performance is affected by an enlarged effective contact area. However, beyond the optimal point, the excessively sharp structures caused incomplete (tip-to-tip) contact between the PTFE and oxidized zinc surface, which results in degradation of the output power. Engineering the optimal metal surface enhanced the power of the TENG by 79.7% for the aluminum case, 42.6% for the copper case, and 80.9% for the zinc case (Figure 4-d). The WAO copper electrode is unlikely to be utilized in a commercial TENG device because it produced random and unstable performances. On the other hand, the optimized WAO aluminum and zinc electrodes can be useful for a practical TENG device. However, the WAO zinc sample needed a long processing time (12 h) to be optimized. So, the WAO aluminum is the most appropriate choice for enhancing the TENG performance,
because surface modification of the aluminum by WAO was the easiest, compared to the other metals. The practicality of the WAO process was confirmed by applying the WAO based electrode to a vibrating membrane type of the TENG, which can widely be utilized as a versatile platform to harvest wind energy [30–32]. Two WAO electrodes are used as a fixed top and bottom electrode, and a thin suspended vibrating PTFE membrane is positioned in between them (Figure 5-a). The mechanism of the abovementioned wind TENG is also based on contact and separation, activated by a wind vortex [30]. Based on operating measurements, each WAO-based wind TENG showed higher performance compared with a pristine flat surface wind TENG. Because the micro and nanostructures of the oxidized copper have weak adhesion to the copper surface, the WAO copper could not contact the PTFE effectively. As a result, the WAO copper showed a relatively low enhancement of performance. On the other hand, the WAO aluminum and zinc contacted the PTFE with enlarged surface areas, resulting in notable increases in performance.
Conclusions A water-assisted oxidation (WAO) process was applied to simply and eco-friendly form nanostructures on the metal electrode of a triboelectric nanogenerator (TENG). The WAO-processed TENGs were made with three different metals: aluminum, copper, and zinc. Each metal showed different surface morphologies and different output characteristics. The WAO aluminum formed grass-like nanostructures, the WAO copper formed cubic-like microstructures, and the WAO zinc formed needle-like nanostructures. Each of the nanograss, micro cubic, and nano needle
Nanogenerator with nanostructured metal surface structures led to enhanced power due to the enlarged electrode surface areas. The optimal WAO-based TENGs showed 1.5 to 2 times better performance than the pristine flat TENG. Based on the above work, the WAO process is advantageous for TENG engineering in terms of fabrication simplicity and practicality.
Acknowledgments This work was partially supported by Open Innovation Lab Project from the National Nanofab Center (NNFC) and the EndRun Project funded by the Ministry of Science, ICT and Future Planning.
Appendix A.
Supplementary material
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2016.01.021.
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Myeong-Lok Seol received his B.S. and M.S. degrees from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2010 and 2012, respectively, where he is currently working toward the Ph.D. degree. His current research interests include piezoelectric and triboelectric energy harvesting.
Daewon Kim received his B.S. degree from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2009, where he is currently working toward the Ph.D. degree. His current research interests include triboelectric energy harvesting and bio-electronics.
Seung-Bae Jeon received his B.S. and M.S. degrees from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2013 and 2015, respectively, where he is currently working toward his Ph.D. degree. His current research interests include energy harvesting.
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S.-J. Park et al. Yang-Kyu Choi received his B.S. and M.S. degrees from the Seoul National University, Seoul, Korea, in 1989 and 1991, respectively, and the Ph.D. degree from the University of California, Berkeley, in 2001. He is currently a Professor with the Department of Electrical Engineering, KAIST. He has authored or coauthored over 320 papers and is a holder of more than 20 U.S. patents
and 100 Korea patents. Dr. Choi received the Sakrison Award for the best dissertation from the Department of Electrical Engineering and Computer Sciences, University of California, in 2002. He was also the recipient of “The Scientist of the Month for July 2006” from the Ministry of Science and Technology in Korea.