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Base-treated polydimethylsiloxane surfaces as enhanced triboelectric nanogenerators
Nano Energy (2015) 15, 523–529
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RAPID COMMUNICATION
Base-treated polydimethylsiloxane surfaces as enhanced triboelectric nanogenerators Byung Kil Yuna, Jae Woong Kima, Hyun Soo Kima, Kwan Wook Jungb, Yeonjin Yib, Min-Seok Jeongc, Jae-Hyeon Koc, Jong Hoon Junga,n a
Department of Physics, Inha University, Incheon 402-751, Republic of Korea Department of Physics, Yonsei University, Seoul 120-749, Republic of Korea c Department of Physics, Hallym University, Gangwondo 200-702, Republic of Korea b
Received 24 March 2015; received in revised form 7 May 2015; accepted 14 May 2015 Available online 22 May 2015
KEYWORDS
Abstract
High-efficiency triboelectric nanogenerator; NaOH treatment; Polydimethylsiloxane; Si–O bonds
Due to its flexibility, transparency, easy fabrication, and high negative polarity, polydimethylsiloxane (PDMS) has been considered as one of the most appropriate materials for the use in triboelectric nanogenerator (TENG) applications. Here, we report the significantly enhanced triboelectric surface charge of PDMS simply by sprinkling of NaOH solution. Fresh PDMS-based TENGs generated an open-circuit voltage of 3.8 V and a closed-circuit current of 65 nA after the contact/separation from an indium tin oxide (ITO) electrode. After sprinkling the PDMS surface with 1 M NaOH, in contrast, the resulting TENG generated voltage of 10.4 V and current of 179 nA. Exposing the PDMS to ultraviolet-ozone prior to sprinkling with NaOH solution resulted in a triboelectric voltage and current of 49.3 V and 1.16 μA, respectively, which are almost 15fold larger than those of fresh PDMS. The origin of the enhanced triboelectric charge is related with an increase of polar Si–O bonds at the expense of non-polar Si–CH3 bonds in PDMS. This work demonstrates a cost-effective method for producing large-area and high-efficiency PDMSbased TENGs and helps clarify the triboelectric mechanism of PDMS. & 2015 Elsevier Ltd. All rights reserved.
Introduction
n
Corresponding author. E-mail address: [email protected] (J.H. Jung).
http://dx.doi.org/10.1016/j.nanoen.2015.05.018 2211-2855/& 2015 Elsevier Ltd. All rights reserved.
Among the various alternative environmental energy sources, motion-based mechanical energy is abundant, ubiquitous, and more accessible than solar and thermal energy [1,2]. While the amount of energy harvested from
524 mechanical vibrations is relatively small, the greatly reduced power consumption of modern devices allows it to replace or at least supplement traditional batteries [3]. There have been numerous works to increase the conversion efficiency through prudent choice of appropriate onedimensional nano-materials [4–7] and by optimizing existing device structures [8–11]. In 2012, Dr. Wang group at Georgia Institute of Technology invented a new means of converting mechanical energy into electricity based on triboelectrification and electrostatic induction [12]. Compared with piezoelectric nanogenerators, triboelectric nanogenerators (TENGs) are more costeffective, generate more power, and are easier to fabricate [13–16]. Recently, Fan et al. reported a highly transparent and flexible TENG based on a polydimethylsiloxane (PDMS) polymer and an indium tin oxide (ITO) electrode [17]. This polymer-electrode TENG exhibited a significantly enhanced triboelectric charge than polymer–polymer TENGs. Especially, they reported that ultraviolet-ozone (UVO) treated PDMS films yielded full power output without a charging step, which suggests the modification of triboelectric surfaces. To enhance the triboelectric surface charge of PDMS for application, it is quite important to develop low-cost and large-area compatible techniques and understand the triboelectric mechanism of PDMS [18]. In this communication, we report a facile method to increase the triboelectric surface charge of PDMS. Using fresh PDMS film with a prolonged curing time, we obtained a low but stable triboelectric voltage and current from the beginning of contact/separation from ITO electrode. The triboelectric charge of fresh PDMS significantly increased upon irradiation of UVO. With the extended irradiation of UVO, however, the triboelectric charge started to decrease. We found that the triboelectric charge can be further enhanced by simply sprinkling the PDMS surface with a strong base solution like NaOH after the irradiation of UVO. The NaOH and UVO treated PDMS resulted in a 15-fold increase in triboelectric charge compared with fresh PDMS. Attenuated total internal reflection and X-ray photoemission spectroscopic measurements indicated significant changes in Si–OH vibrations, the atomic ratio of O/Si, and the oxidation state of Si. These changes can be explained by a modification of Si–CH3 bonds to Si–O bonds after UVO and NaOH treatments. The simple process of sprinkling NaOH solution onto PDMS after UVO irradiation to enhance its triboelectric charge, without losing transparency as well as flexibility, should be quite suitable for large-area and lowcost PDMS-based TENG applications.
Experimental section Fabrication of surface-treated PDMS-based TENGs PDMS films were fabricated using conventional spin-coating and curing methods. The PDMS elastomer and cross-linker were thoroughly mixed at a weight ratio of 10:1 and then degassed under vacuum for 30 min. A small amount of the resulting PDMS mixture was spin-coated at 500 rpm for 1 min onto commercially available ITO-coated polyethylene terephthalate (PET) substrates and cured in an oven at 100 1C for 5 h. Fresh PDMS films were exposed to UVO at a
B.K. Yun et al. power density of 28 mW/cm2 (AH1700, AHTECH) and then sprinkled with a 1 M solution of NaOH. To fabricate the TENGs, another ITO-coated PET substrate was used as a top electrode. Surface-treated PDMS and ITO-coated PET were attached to acryl plates and assembled with springs in such a way as to enable contact and separation with a gap size of approximately 2 mm. The effective surface area of the PDMS was 2.5 cm 2.5 cm and 300 μm in thickness.
Characterization of the TENG The chemical bonds of PDMS were examined using attenuated total internal reflection (ATR) Fourier transform infrared (FT-IR) spectroscopy (VERTEX 80 V, Bruker) and microRaman scattering at a laser excitation of 532 nm (LabRAM HR Evolution, Jobin Yvon). For the ATR measurements, small pieces of the PDMS film were cut and attached to a ZnSe crystal supporting multiple reflections of infrared light. Chemical bonds were additionally characterized by X-ray photoemission spectroscopy (XPS) using Al Kα radiation (E = 1486.5 eV) (PHI 5700, Physical Electronics). The optical transmittance and surface morphology of PDMS were characterized by grating spectrophotometry (Cary5, Varian) and field-emission scanning electron microscopy (SEM) (SU 8010, Hitachi), respectively. TENG performance was characterized using a custom mechanical system in which a linear motor was used to periodically apply and release compressive forces to the device. The pushing amplitude and frequency were fixed at 10 N and 0.16 Hz, respectively, over the course of the measurement. The output voltage and current of the TENGs were recorded by a Keithley 6517A electrometer and an SR570 low noise current amplifier from Stanford Research Systems, respectively. All electrical measurements were conducted in a Faraday cage to minimize noise.
Results and discussion Figure 1a-(i)–(iii) illustrates the surface treatment procedure used on the PDMS films. A small amount of PDMS solution was spin-coated onto ITO-coated PET substrates and then cured for a prolonged time. The fresh PDMS polymer was exposed to UVO in air and then sprinkled with a 1 M solution of NaOH. After drying in air, the surface-treated PDMS and ITO-coated PET were attached to acryl plates and assembled into a TENG (Figure 1a-(iv)). Four springs were used to restore initial positions after contact. The surface morphology and optical transmittance of the resulting films are shown in Figure 1b and c, respectively. Discernable differences were not observed in the surface morphology and roughness of the fresh and surface-treated PDMS films. In the visible light region, the transmittance of all three PDMS films exceeded more than 90% with a small variation of within 3%. The flexibility of fresh PDMS film was also unchanged after the surface treatments. As evidenced in the inset of Figure 1c, all three PDMS films were easily foldable. Figure 2 compares the triboelectric power generation of fresh, UVO irradiated, and NaOH-treated PDMS-based TENGs. To directly compare, we fixed pushing amplitude and frequency and used same TENGs except PDMS. During
Base-treated polydimethylsiloxane surfaces
525
Figure 1 (a) Schematic illustration of surface treatment of polydimethylsiloxane (PDMS) polymer film and a triboelectric nanogenerator (TENG) device: (i) spin-coating of PDMS onto indium tin oxide (ITO)-coated polyethylene terephthalate (PET) and curing; (ii) irradiation of ultraviolet-ozone (UVO); (iii) sprinkling of NaOH solution; and (iv) assembling of a surface treated PDMS, ITO-coated PET, acryl, and springs. (b) Scanning electron microscope (SEM) images and (c) optical transmission spectra of fresh, UVO-irradiated, and NaOH-sprinkled PDMS. In the inset of (c), we show a photograph of folded PDMS polymer films on black and white paper.
Figure 2 Comparison of the triboelectric power generation of fresh and surface-treated PDMS-based TENGs. (a) Open-circuit voltage, (b) closed-circuit current, and (c) surface charge of fresh PDMS, UVO-irradiated PDMS for 1 h (UVO (1 h)) and 2 h (UVO (2 h)), and NaOH-sprinkled PDMS for 2 h after UVO irradiation for 1 h (NaOH (2 h) +UVO (1 h)).
periodic contact and separation, the fresh PDMS-based TENG generated an open-circuit voltage of 3.8 V with a closed-circuit current of 65 nA, corresponding to a current density of 10.4 nA/cm2 (Figure 2a and b). The triboelectric voltage and current of our fresh PDMS-based TENGs were
lower than those of similar PDMS-based TENGs [17]. However, our fresh PDMS-based TENG generated a stable voltage and current without a charging step. Given the extended curing time of our PDMS films, we think that material transfer may not occur from the PDMS film to the ITO surface during the contact/separation operations [19,20]. Thus, the resulting triboelectric signal could be weak without requiring a charging step. After UVO irradiation for 1 h (UVO (1 h)), the PDMS-based TENG exhibited a significantly enhanced voltage of 22.5 V and a current of 0.72 μA, corresponding to a current density of 114.4 nA/cm2. Further increases in irradiation time, however, resulted in a decrease in both triboelectric voltage and current (Figure S1 in Supporting information), reaching 9 V and 0.39 μA (current density of 62.4 nA/cm2) after an irradiation time of 2 h (UVO (2 h)). After surface treatment with NaOH for 2 h and UVO irradiation for 1 h (NaOH (2 h)+UVO (1 h)), on the other hand, the PDMS-based TENG exhibited a significantly enhanced triboelectric voltage of 49.3 V and a current of 1.2 μA (current density of 185.6 nA/cm2), i.e., almost 15-fold larger than those of fresh PDMS-based TENGs. We notice that the enhanced triboelectric voltage and current are quite stable, at least up to 20,000 cycles of contact/ separation (Figure S2 in Supporting information). In addition, the enhancement of triboelectric voltage and current for surface treated PDMS was also observed in Al- and Au-top electrodes (Figure S3 in Supporting information). In Figure 2c, we show the induced triboelectric surface charge obtained from the integration of current–time curves
526 of TENGs. The triboelectric charges of UVO (1 h) and NaOH (2 h) + UVO (1 h) treated PDMS were approximately 33.9 and 53.5 nC, respectively, which are significantly larger than that of fresh PDMS (3.17 nC). To clarify the effects of NaOH on TENG performance, we performed two control experiments. First, we measured device performance after sprinkling the PDMS with NaOH without UVO irradiation. Figure S4 in Supporting information shows that both the triboelectric voltage and current increased with the increasing NaOH sprinkling time. Second, we measured device performance after sprinkling the PDMS surface with HCl without UVO irradiation. Figure S5 in Supporting information shows that both the triboelectric voltage and current decreased with increasing HCl sprinkling time. These control experiments suggest the importance of pH in generating the triboelectric surface charge of PDMS. The strong UVO and pH dependencies of TENG performance should be related to changes in the chemical elements and/or bonds at the PDMS surface. Two surface sensitive techniques, ATR FT-IR and XPS, were used to investigate the surface characteristics of PDMS. Figure 3a shows ATR FT-IR spectra of fresh, UVO (1 h), UVO (2 h), and NaOH (2 h) +UVO (1 h)-treated PDMS surfaces. One of the most noticeable changes is the Si–OH vibrational bands located around 850 and 3300 cm 1 [21,22]. The band near 3300 cm 1 is almost absent in fresh PDMS and is easily observable in UVO-treated PDMS. With subsequent NaOH
B.K. Yun et al. treatment, the Si–OH band almost disappears again. These drastic changes occurred only at the PDMS surface. Hence, Raman scattering measurements with a large penetration depth could not detect such changes in the Si–OH band (Figure S6 in Supporting information). The other noticeable changes is the Si–CH3 vibrational bands near 1250 and 2960 cm 1 [21,22]. These bands are reduced following UVO and NaOH treatments. Figure S7 in Supporting information presents XPS spectra of fresh, UVO, and NaOH + UVO-treated PDMS surfaces. By examining the relative heights and areas of the Si 2s, C 1s, and O 1s peaks with considering atomic sensitivity factors, we determined the atomic contents of the PDMS and showed the resulting O/Si and C/Si atomic ratios in Figure 3b. The O/Si ratio changed from 0.75 to 1.37 and to 0.92 for fresh, UVO (1 h), and NaOH (2 h) + UVO (1 h)treated PDMS, respectively. Conversely, the C/Si ratio changed from 1.37 to 0.63 and to 1.08 for fresh, UVO (1 h), and NaOH (2 h) + UVO (1 h)-treated PDMS, respectively. The changes of O/Si and C/Si atomic ratio after UVO irradiation are quite consistent with the changes of Si– OH and Si–CH3 vibrations in ATR FT-IR measurement. The drastic changes of O/Si and C/Si atomic ratio after further NaOH sprinkling are rather intriguing. Note also that these atomic ratio changes occurred at the PDMS surface. Hence, energy dispersive X-ray (EDX) spectra with a large penetration depth could not detect such changes of chemical elements (Figure S8 in Supporting information).
Figure 3 Characterization of the chemical elements and bonds at the PDMS surface. (a) Attenuated total internal reflection (ATR) Fourier transform infrared (FT-IR) spectra. (b) Atomic ratios of O/Si and C/Si. (c) High-resolution Si 2p X-ray photoemission spectroscopy (XPS) peaks of fresh, UVO (1 h)-, and NaOH (2 h) +UVO (1 h)-treated PDMS. In (c), open circles represent experimental data that were deconvoluted into three peaks (red, blue, and green lines).
Base-treated polydimethylsiloxane surfaces To clarify the intriguing atomic ratio changes, we show a high-resolution XPS spectrum of the Si 2p peak of fresh, UVO, and NaOH +UVO-treated PDMS in Figure 3c. Clearly, the spectral shape of Si 2p for fresh PDMS drastically changes after the UVO and NaOH treatment. Based on the previous literatures [23,24], we deconvoluted the peak into three components: a peak at 101.5 eV corresponding to Si bonded to one O atom; a peak at 102.2 eV corresponding to Si bonded to two O atoms; and a peak at 103.3 eV corresponding to Si bonded to three or four O atoms. The third peak is associated with a highly oxidized surface with a silica-like structure (SiOx, x = 3–4). The first peak of fresh PDMS is sharply reduced in UVO and NaOH + UVO treated PDMS, whereas the second and third peaks of fresh PDMS are increased in UVO and NaOH + UVO treated PDMS. These evolutions of spectral weight should suggest the increase of oxidation with surface treatments. Notably, the second and third peaks of UVO treated PDMS sharply increases and decreases in NaOH + UVO treated PDMS, respectively. These evolutions of spectral weight should be related with the partial replacement of O and/or OH with CH3. Such replacement should not be strongly related with the NaOH treatment but with the long time exposition of UVO treated PDMS during NaOH sprinkling [25,26]. In fact, Ren et al. reported the increase of CH3 and the decrease of OH vibrations with the increase of exposed time in air [25]. Based on the above results, we show a proposed arrangement of chemical elements and bonds in the PDMS surface (Figure 4). The fresh PDMS surface consists of mainly Si–CH3 bonds with some Si–O and Si–OH bonds [27]. When the fresh PDMS is exposed to UVO, the Si–CH3 bonds are broken and converted to Si–O, Si–OH, and Si–COOH bonds to form a mildly base and polar surface [27]. When the UVO-treated PDMS further sprinkled with NaOH solution, the Si–OH bonds are changed to Si–O bonds. When the UVO-treated PDMS further sprinkled with HCl solution, on the other hand, the Si–O bonds are changed to Si–OH bonds [21]. Remained Na +
527 ions in NaOH solution and Cl ions in HCl solution should not participate in bonding and should be removed during washing and drying process. This proposed structure and the changes thereto induced by the various surface treatments explain the device performance of our PDMS-based TENGs. During UVO treatment, the amount of Si–OH bonds increases at the expense of Si–CH3 bonds. Further NaOH treatment results in the increase of Si–O bonds at the expense of Si–OH bonds. Conversely, HCl treatment results in the increase of Si–OH bonds at the expense of Si–O bonds. Noticing the polar nature of Si–O, therefore, UVO- and NaOH-treated PDMS were able to generate a high triboelectric charge, while HCl-treated PDMS generated relatively little triboelectric charge. An increase in the silica-like bonds of PDMS with extended UVO irradiation resulted in a decrease in triboelectric charge due to the difficulty in bond breaking during contact and separation operations [28]. There are two important implications in our work for surface treated PDMS based TENG. First, enhancements to device performance can be realized by simply sprinkling NaOH solution onto PDMS immediately after UVO treatment. This is a cost-effective method that is amenable to large-area TENG applications. Optimization of UVO irradiation time and power and the pH of the NaOH solution may further increase the triboelectric charge at the PDMS surface, thereby increasing the power generation efficiency of the resulting TENG. In particular, the increased power generation of TENGS that had been sprinkled with NaOH without UVO irradiation should imply that a weak UV in our usual environment is sufficient to partially modify the chemical elements and bonds at the PDMS surface. Second, the enhancement of triboelectric charge after UVO and NaOH treatments provides important information regarding the mechanism of triboelectricity in PDMS. For tightly-cured fresh PDMS, weak but stable triboelectric signals were obtained immediately without a charging step. This finding suggests that the modification of chemical elements
Figure 4 The proposed chemical elements and bonds of (a) fresh, (b) UVO-, (c) NaOH-, and (d) HCl-treated PDMS surfaces.
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and bonds is the primary origin of triboelectricity in PDMS. Consistent with this hypothesis, no material transfer between the PDMS surface and the ITO electrode was evidenced in XPS spectra (see Figure S7 in Supporting information).
Conclusion In summary, we have demonstrated a facile method for enhancing the triboelectric charge of PDMS surfaces. Sprinkling a PDMS surface with a 1 M NaOH solution after UVO irradiation resulted in an open-circuit voltage of 49.3 V and a closed-circuit current of 1.16 μA (corresponding to a current density of 185.7 nA/cm2), when the PDMS surface was brought into contact with an ITO electrode. This is nearly 15-fold greater than the corresponding current density obtained with fresh, untreated PDMS. ATR FT-IR and XPS measurements revealed that the increase in triboelectric charge is strongly related to increases in the amount of polar Si–O bonds at the expense of non-polar Si–CH3 bonds. Additionally, these surface treatments had no effect on the roughness, transmission, and flexibility of the resulting TENGs, and are suitable for large-area, costeffective, and high-power TENG applications.
Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A2002536, 2013R1A2A2A01015734, and 2013R1A1A2006582). This research was a part of the project titled ‘Gyeonggi Sea Grant Program’, funded by the Ministry of Oceans and Fisheries, Korea.
Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2015.05.018.
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[11] J.H. Jung, M. Lee, J.-I. Hong, Y. Ding, C.-Y. Chen, L.-J. Chou, Z.L. Wang, ACS Nano 5 (2011) 10041–10046. [12] F.R. Fan, Z.Q. Tian, Z.L. Wang, Nano Energy 1 (2012) 328–334. [13] Z.L. Wang, ACS Nano 7 (2013) 9533–9557. [14] Z.L. Wang, Faraday Discuss 176 (2014) 447–458. [15] Q. Liang, Z. Zhanga, X. Yan, Y. Gu, Y. Zhao, G. Zhang, S. Lu, Q. Liao, Y. Zhang, Nano Energy (2014). http://dx.doi.org/ 10.1016/j.nanoen.2014.07.010. [16] Q. Liang, X. Yan, Y. Gu, K. Zhang, M. Liang, S. Lu, X. Zheng, Y. Zhang, Sci. Rep. 5 (2015) 9080. [17] F.R. Fan, J. Luo, W. Tang, C. Li, C. Zhang, Z. Tian, Z.L. Wang, J. Mater. Chem. A 2 (2014) 13219–13225. [18] S. Matsusaka, H. Maruyama, T. Matsuyama, M. Ghadiri, Chem. Eng. Sci. 65 (2010) 5781–5807. [19] H.T. Baytekin, A.Z. Patashinski, M. Branicki, B. Baytekin, S. Soh, B.A. Grzybowski, Science 333 (2011) 308–312. [20] H.T. Baytekin, B. Baytekin, J.T. Incorvati, B.A. Grzybowski, Angew. Chem. Int. Ed. 51 (2012) 4843–4847. [21] C.-L. Lai, Y.-J. Fu, J.-T. Chen, Q.-F. An, K.-S. Liao, S.-C. Fang, C.-C. Hu, K.-R. Lee, Sep. Purif. Technol. 100 (2012) 15–21. [22] Y.-J. Fu, H.-Z. Qui, K.-S. Liao, S.J. Lue, C.-C. Hu, K.-R. Lee, J.-Y. Lai, Langmuir 26 (2010) 4392–4399. [23] L.-A. O’Hare, B. Parbhoo, S.R. Leadley, Surf. Interface Anal. 36 (2004) 1427–1434. [24] S. Yang, J.-J. Lee, H.-J. Lee, Y.-G. Kang, H.-J. Kim, H.-Y. Jung, D.-S. Seo, Liq. Cryst. 39 (2012) 71–75. [25] X. Ren, M. Bachman, C. Sims, G.P. Li, N. Allbritton J. Chromatogr. B 762 (2001) 117–125. [26] D.C. Duffy, J.C. Cooper McDonald, O.J.A. Schueller, G.M. Whitesides, Anal. Chem. 70 (1998) 4974–4984. [27] J.J. Cole, C.R. Barry, R.J. Knuesel, X. Wang, H.O. Jacobs, Langmuir 27 (2011) 7321–7329. [28] J. Song, D. Tranchida, G.J. Vancso, Macromolecules 41 (2008) 6757–6762. Byung Kil Yun is a Ph.D. student at Department of Physics, Inha University. He received his BS degree (2009) and MS degree (2011) from Inha University. His current research interests are fabrication and characterization of piezoelectric and triboelectric nanomaterials for mechanical energy harvesting.
Jae Woong Kim is a BS student at Department of Physics, Inha University. His current research interests are fabrication of piezo/ triboelectric nanogenerators and their application to emerging electronic devices.
Hyun Soo Kim is a BS student at Department of Physics, Inha University. His current research interests are fabrication and characterization of triboelectric nanomaterials for mechanical energy harvesting.
Base-treated polydimethylsiloxane surfaces Kwan Wook Jung is a Ph.D. student in Department of Physics at Yonsei University. He received his BS degree from Yonsei University in 2013. His research focuses on organic/polymer light emitting diodes and photovoltaics.
Yeonjin Yi is a Professor in Department of Physics at Yonsei University. He received his Ph.D. from Yonsei University in Department of Physics in 2005. After working as a postdoctoral researcher at University of South Florida and as a senior researcher at Korea Research Institute of Standard and Science, he joined the Department of Physics, Yonsei University in 2011. His recent research interest is focused on the interfacial electronic structures of molecular solids and low dimensional materials and related devices. Min-Seok Jeong is a BS student at Department of Physics, Hallym University. His current research interests are spectroscopic study of piezoelectric materials, such as PZT single crystals and lead-free piezomaterials.
529 Jae-Hyeon Ko is a Professor in Department of Physics at Hallym University. He received his Ph.D. from Korea Advanced Institute of Science and Technology in Department of Physics in 2000. After working as a research associate in University of Tsukuba and a principal researcher in Samsung Corning Co. Ltd., he joined the Department of Physics, Hallym University in 2004. His recent research interest is focused on phase transition behaviors of ferroelectrics and disordered systems such as relaxors and dipolar glasses. Jong Hoon Jung is a Professor in Department of Physics at Inha University. He received his Ph.D. from Seoul National University in Department of Physics in 2000. After working as a postdoctoral researcher at Seoul National University and Spin superstructure ERATO project in Japan Science and Technology, he joined the Department of Physics, Inha University in 2004. His recent research interest is focused on piezoelectric/pyroelectric/triboelectric nanogenerators in ferroelectric nanomaterials, and electrolyte control of interfacial spins in magnetic thin films.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanoenergy
RAPID COMMUNICATION
Base-treated polydimethylsiloxane surfaces as enhanced triboelectric nanogenerators Byung Kil Yuna, Jae Woong Kima, Hyun Soo Kima, Kwan Wook Jungb, Yeonjin Yib, Min-Seok Jeongc, Jae-Hyeon Koc, Jong Hoon Junga,n a
Department of Physics, Inha University, Incheon 402-751, Republic of Korea Department of Physics, Yonsei University, Seoul 120-749, Republic of Korea c Department of Physics, Hallym University, Gangwondo 200-702, Republic of Korea b
Received 24 March 2015; received in revised form 7 May 2015; accepted 14 May 2015 Available online 22 May 2015
KEYWORDS
Abstract
High-efficiency triboelectric nanogenerator; NaOH treatment; Polydimethylsiloxane; Si–O bonds
Due to its flexibility, transparency, easy fabrication, and high negative polarity, polydimethylsiloxane (PDMS) has been considered as one of the most appropriate materials for the use in triboelectric nanogenerator (TENG) applications. Here, we report the significantly enhanced triboelectric surface charge of PDMS simply by sprinkling of NaOH solution. Fresh PDMS-based TENGs generated an open-circuit voltage of 3.8 V and a closed-circuit current of 65 nA after the contact/separation from an indium tin oxide (ITO) electrode. After sprinkling the PDMS surface with 1 M NaOH, in contrast, the resulting TENG generated voltage of 10.4 V and current of 179 nA. Exposing the PDMS to ultraviolet-ozone prior to sprinkling with NaOH solution resulted in a triboelectric voltage and current of 49.3 V and 1.16 μA, respectively, which are almost 15fold larger than those of fresh PDMS. The origin of the enhanced triboelectric charge is related with an increase of polar Si–O bonds at the expense of non-polar Si–CH3 bonds in PDMS. This work demonstrates a cost-effective method for producing large-area and high-efficiency PDMSbased TENGs and helps clarify the triboelectric mechanism of PDMS. & 2015 Elsevier Ltd. All rights reserved.
Introduction
n
Corresponding author. E-mail address: [email protected] (J.H. Jung).
http://dx.doi.org/10.1016/j.nanoen.2015.05.018 2211-2855/& 2015 Elsevier Ltd. All rights reserved.
Among the various alternative environmental energy sources, motion-based mechanical energy is abundant, ubiquitous, and more accessible than solar and thermal energy [1,2]. While the amount of energy harvested from
524 mechanical vibrations is relatively small, the greatly reduced power consumption of modern devices allows it to replace or at least supplement traditional batteries [3]. There have been numerous works to increase the conversion efficiency through prudent choice of appropriate onedimensional nano-materials [4–7] and by optimizing existing device structures [8–11]. In 2012, Dr. Wang group at Georgia Institute of Technology invented a new means of converting mechanical energy into electricity based on triboelectrification and electrostatic induction [12]. Compared with piezoelectric nanogenerators, triboelectric nanogenerators (TENGs) are more costeffective, generate more power, and are easier to fabricate [13–16]. Recently, Fan et al. reported a highly transparent and flexible TENG based on a polydimethylsiloxane (PDMS) polymer and an indium tin oxide (ITO) electrode [17]. This polymer-electrode TENG exhibited a significantly enhanced triboelectric charge than polymer–polymer TENGs. Especially, they reported that ultraviolet-ozone (UVO) treated PDMS films yielded full power output without a charging step, which suggests the modification of triboelectric surfaces. To enhance the triboelectric surface charge of PDMS for application, it is quite important to develop low-cost and large-area compatible techniques and understand the triboelectric mechanism of PDMS [18]. In this communication, we report a facile method to increase the triboelectric surface charge of PDMS. Using fresh PDMS film with a prolonged curing time, we obtained a low but stable triboelectric voltage and current from the beginning of contact/separation from ITO electrode. The triboelectric charge of fresh PDMS significantly increased upon irradiation of UVO. With the extended irradiation of UVO, however, the triboelectric charge started to decrease. We found that the triboelectric charge can be further enhanced by simply sprinkling the PDMS surface with a strong base solution like NaOH after the irradiation of UVO. The NaOH and UVO treated PDMS resulted in a 15-fold increase in triboelectric charge compared with fresh PDMS. Attenuated total internal reflection and X-ray photoemission spectroscopic measurements indicated significant changes in Si–OH vibrations, the atomic ratio of O/Si, and the oxidation state of Si. These changes can be explained by a modification of Si–CH3 bonds to Si–O bonds after UVO and NaOH treatments. The simple process of sprinkling NaOH solution onto PDMS after UVO irradiation to enhance its triboelectric charge, without losing transparency as well as flexibility, should be quite suitable for large-area and lowcost PDMS-based TENG applications.
Experimental section Fabrication of surface-treated PDMS-based TENGs PDMS films were fabricated using conventional spin-coating and curing methods. The PDMS elastomer and cross-linker were thoroughly mixed at a weight ratio of 10:1 and then degassed under vacuum for 30 min. A small amount of the resulting PDMS mixture was spin-coated at 500 rpm for 1 min onto commercially available ITO-coated polyethylene terephthalate (PET) substrates and cured in an oven at 100 1C for 5 h. Fresh PDMS films were exposed to UVO at a
B.K. Yun et al. power density of 28 mW/cm2 (AH1700, AHTECH) and then sprinkled with a 1 M solution of NaOH. To fabricate the TENGs, another ITO-coated PET substrate was used as a top electrode. Surface-treated PDMS and ITO-coated PET were attached to acryl plates and assembled with springs in such a way as to enable contact and separation with a gap size of approximately 2 mm. The effective surface area of the PDMS was 2.5 cm 2.5 cm and 300 μm in thickness.
Characterization of the TENG The chemical bonds of PDMS were examined using attenuated total internal reflection (ATR) Fourier transform infrared (FT-IR) spectroscopy (VERTEX 80 V, Bruker) and microRaman scattering at a laser excitation of 532 nm (LabRAM HR Evolution, Jobin Yvon). For the ATR measurements, small pieces of the PDMS film were cut and attached to a ZnSe crystal supporting multiple reflections of infrared light. Chemical bonds were additionally characterized by X-ray photoemission spectroscopy (XPS) using Al Kα radiation (E = 1486.5 eV) (PHI 5700, Physical Electronics). The optical transmittance and surface morphology of PDMS were characterized by grating spectrophotometry (Cary5, Varian) and field-emission scanning electron microscopy (SEM) (SU 8010, Hitachi), respectively. TENG performance was characterized using a custom mechanical system in which a linear motor was used to periodically apply and release compressive forces to the device. The pushing amplitude and frequency were fixed at 10 N and 0.16 Hz, respectively, over the course of the measurement. The output voltage and current of the TENGs were recorded by a Keithley 6517A electrometer and an SR570 low noise current amplifier from Stanford Research Systems, respectively. All electrical measurements were conducted in a Faraday cage to minimize noise.
Results and discussion Figure 1a-(i)–(iii) illustrates the surface treatment procedure used on the PDMS films. A small amount of PDMS solution was spin-coated onto ITO-coated PET substrates and then cured for a prolonged time. The fresh PDMS polymer was exposed to UVO in air and then sprinkled with a 1 M solution of NaOH. After drying in air, the surface-treated PDMS and ITO-coated PET were attached to acryl plates and assembled into a TENG (Figure 1a-(iv)). Four springs were used to restore initial positions after contact. The surface morphology and optical transmittance of the resulting films are shown in Figure 1b and c, respectively. Discernable differences were not observed in the surface morphology and roughness of the fresh and surface-treated PDMS films. In the visible light region, the transmittance of all three PDMS films exceeded more than 90% with a small variation of within 3%. The flexibility of fresh PDMS film was also unchanged after the surface treatments. As evidenced in the inset of Figure 1c, all three PDMS films were easily foldable. Figure 2 compares the triboelectric power generation of fresh, UVO irradiated, and NaOH-treated PDMS-based TENGs. To directly compare, we fixed pushing amplitude and frequency and used same TENGs except PDMS. During
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Figure 1 (a) Schematic illustration of surface treatment of polydimethylsiloxane (PDMS) polymer film and a triboelectric nanogenerator (TENG) device: (i) spin-coating of PDMS onto indium tin oxide (ITO)-coated polyethylene terephthalate (PET) and curing; (ii) irradiation of ultraviolet-ozone (UVO); (iii) sprinkling of NaOH solution; and (iv) assembling of a surface treated PDMS, ITO-coated PET, acryl, and springs. (b) Scanning electron microscope (SEM) images and (c) optical transmission spectra of fresh, UVO-irradiated, and NaOH-sprinkled PDMS. In the inset of (c), we show a photograph of folded PDMS polymer films on black and white paper.
Figure 2 Comparison of the triboelectric power generation of fresh and surface-treated PDMS-based TENGs. (a) Open-circuit voltage, (b) closed-circuit current, and (c) surface charge of fresh PDMS, UVO-irradiated PDMS for 1 h (UVO (1 h)) and 2 h (UVO (2 h)), and NaOH-sprinkled PDMS for 2 h after UVO irradiation for 1 h (NaOH (2 h) +UVO (1 h)).
periodic contact and separation, the fresh PDMS-based TENG generated an open-circuit voltage of 3.8 V with a closed-circuit current of 65 nA, corresponding to a current density of 10.4 nA/cm2 (Figure 2a and b). The triboelectric voltage and current of our fresh PDMS-based TENGs were
lower than those of similar PDMS-based TENGs [17]. However, our fresh PDMS-based TENG generated a stable voltage and current without a charging step. Given the extended curing time of our PDMS films, we think that material transfer may not occur from the PDMS film to the ITO surface during the contact/separation operations [19,20]. Thus, the resulting triboelectric signal could be weak without requiring a charging step. After UVO irradiation for 1 h (UVO (1 h)), the PDMS-based TENG exhibited a significantly enhanced voltage of 22.5 V and a current of 0.72 μA, corresponding to a current density of 114.4 nA/cm2. Further increases in irradiation time, however, resulted in a decrease in both triboelectric voltage and current (Figure S1 in Supporting information), reaching 9 V and 0.39 μA (current density of 62.4 nA/cm2) after an irradiation time of 2 h (UVO (2 h)). After surface treatment with NaOH for 2 h and UVO irradiation for 1 h (NaOH (2 h)+UVO (1 h)), on the other hand, the PDMS-based TENG exhibited a significantly enhanced triboelectric voltage of 49.3 V and a current of 1.2 μA (current density of 185.6 nA/cm2), i.e., almost 15-fold larger than those of fresh PDMS-based TENGs. We notice that the enhanced triboelectric voltage and current are quite stable, at least up to 20,000 cycles of contact/ separation (Figure S2 in Supporting information). In addition, the enhancement of triboelectric voltage and current for surface treated PDMS was also observed in Al- and Au-top electrodes (Figure S3 in Supporting information). In Figure 2c, we show the induced triboelectric surface charge obtained from the integration of current–time curves
526 of TENGs. The triboelectric charges of UVO (1 h) and NaOH (2 h) + UVO (1 h) treated PDMS were approximately 33.9 and 53.5 nC, respectively, which are significantly larger than that of fresh PDMS (3.17 nC). To clarify the effects of NaOH on TENG performance, we performed two control experiments. First, we measured device performance after sprinkling the PDMS with NaOH without UVO irradiation. Figure S4 in Supporting information shows that both the triboelectric voltage and current increased with the increasing NaOH sprinkling time. Second, we measured device performance after sprinkling the PDMS surface with HCl without UVO irradiation. Figure S5 in Supporting information shows that both the triboelectric voltage and current decreased with increasing HCl sprinkling time. These control experiments suggest the importance of pH in generating the triboelectric surface charge of PDMS. The strong UVO and pH dependencies of TENG performance should be related to changes in the chemical elements and/or bonds at the PDMS surface. Two surface sensitive techniques, ATR FT-IR and XPS, were used to investigate the surface characteristics of PDMS. Figure 3a shows ATR FT-IR spectra of fresh, UVO (1 h), UVO (2 h), and NaOH (2 h) +UVO (1 h)-treated PDMS surfaces. One of the most noticeable changes is the Si–OH vibrational bands located around 850 and 3300 cm 1 [21,22]. The band near 3300 cm 1 is almost absent in fresh PDMS and is easily observable in UVO-treated PDMS. With subsequent NaOH
B.K. Yun et al. treatment, the Si–OH band almost disappears again. These drastic changes occurred only at the PDMS surface. Hence, Raman scattering measurements with a large penetration depth could not detect such changes in the Si–OH band (Figure S6 in Supporting information). The other noticeable changes is the Si–CH3 vibrational bands near 1250 and 2960 cm 1 [21,22]. These bands are reduced following UVO and NaOH treatments. Figure S7 in Supporting information presents XPS spectra of fresh, UVO, and NaOH + UVO-treated PDMS surfaces. By examining the relative heights and areas of the Si 2s, C 1s, and O 1s peaks with considering atomic sensitivity factors, we determined the atomic contents of the PDMS and showed the resulting O/Si and C/Si atomic ratios in Figure 3b. The O/Si ratio changed from 0.75 to 1.37 and to 0.92 for fresh, UVO (1 h), and NaOH (2 h) + UVO (1 h)treated PDMS, respectively. Conversely, the C/Si ratio changed from 1.37 to 0.63 and to 1.08 for fresh, UVO (1 h), and NaOH (2 h) + UVO (1 h)-treated PDMS, respectively. The changes of O/Si and C/Si atomic ratio after UVO irradiation are quite consistent with the changes of Si– OH and Si–CH3 vibrations in ATR FT-IR measurement. The drastic changes of O/Si and C/Si atomic ratio after further NaOH sprinkling are rather intriguing. Note also that these atomic ratio changes occurred at the PDMS surface. Hence, energy dispersive X-ray (EDX) spectra with a large penetration depth could not detect such changes of chemical elements (Figure S8 in Supporting information).
Figure 3 Characterization of the chemical elements and bonds at the PDMS surface. (a) Attenuated total internal reflection (ATR) Fourier transform infrared (FT-IR) spectra. (b) Atomic ratios of O/Si and C/Si. (c) High-resolution Si 2p X-ray photoemission spectroscopy (XPS) peaks of fresh, UVO (1 h)-, and NaOH (2 h) +UVO (1 h)-treated PDMS. In (c), open circles represent experimental data that were deconvoluted into three peaks (red, blue, and green lines).
Base-treated polydimethylsiloxane surfaces To clarify the intriguing atomic ratio changes, we show a high-resolution XPS spectrum of the Si 2p peak of fresh, UVO, and NaOH +UVO-treated PDMS in Figure 3c. Clearly, the spectral shape of Si 2p for fresh PDMS drastically changes after the UVO and NaOH treatment. Based on the previous literatures [23,24], we deconvoluted the peak into three components: a peak at 101.5 eV corresponding to Si bonded to one O atom; a peak at 102.2 eV corresponding to Si bonded to two O atoms; and a peak at 103.3 eV corresponding to Si bonded to three or four O atoms. The third peak is associated with a highly oxidized surface with a silica-like structure (SiOx, x = 3–4). The first peak of fresh PDMS is sharply reduced in UVO and NaOH + UVO treated PDMS, whereas the second and third peaks of fresh PDMS are increased in UVO and NaOH + UVO treated PDMS. These evolutions of spectral weight should suggest the increase of oxidation with surface treatments. Notably, the second and third peaks of UVO treated PDMS sharply increases and decreases in NaOH + UVO treated PDMS, respectively. These evolutions of spectral weight should be related with the partial replacement of O and/or OH with CH3. Such replacement should not be strongly related with the NaOH treatment but with the long time exposition of UVO treated PDMS during NaOH sprinkling [25,26]. In fact, Ren et al. reported the increase of CH3 and the decrease of OH vibrations with the increase of exposed time in air [25]. Based on the above results, we show a proposed arrangement of chemical elements and bonds in the PDMS surface (Figure 4). The fresh PDMS surface consists of mainly Si–CH3 bonds with some Si–O and Si–OH bonds [27]. When the fresh PDMS is exposed to UVO, the Si–CH3 bonds are broken and converted to Si–O, Si–OH, and Si–COOH bonds to form a mildly base and polar surface [27]. When the UVO-treated PDMS further sprinkled with NaOH solution, the Si–OH bonds are changed to Si–O bonds. When the UVO-treated PDMS further sprinkled with HCl solution, on the other hand, the Si–O bonds are changed to Si–OH bonds [21]. Remained Na +
527 ions in NaOH solution and Cl ions in HCl solution should not participate in bonding and should be removed during washing and drying process. This proposed structure and the changes thereto induced by the various surface treatments explain the device performance of our PDMS-based TENGs. During UVO treatment, the amount of Si–OH bonds increases at the expense of Si–CH3 bonds. Further NaOH treatment results in the increase of Si–O bonds at the expense of Si–OH bonds. Conversely, HCl treatment results in the increase of Si–OH bonds at the expense of Si–O bonds. Noticing the polar nature of Si–O, therefore, UVO- and NaOH-treated PDMS were able to generate a high triboelectric charge, while HCl-treated PDMS generated relatively little triboelectric charge. An increase in the silica-like bonds of PDMS with extended UVO irradiation resulted in a decrease in triboelectric charge due to the difficulty in bond breaking during contact and separation operations [28]. There are two important implications in our work for surface treated PDMS based TENG. First, enhancements to device performance can be realized by simply sprinkling NaOH solution onto PDMS immediately after UVO treatment. This is a cost-effective method that is amenable to large-area TENG applications. Optimization of UVO irradiation time and power and the pH of the NaOH solution may further increase the triboelectric charge at the PDMS surface, thereby increasing the power generation efficiency of the resulting TENG. In particular, the increased power generation of TENGS that had been sprinkled with NaOH without UVO irradiation should imply that a weak UV in our usual environment is sufficient to partially modify the chemical elements and bonds at the PDMS surface. Second, the enhancement of triboelectric charge after UVO and NaOH treatments provides important information regarding the mechanism of triboelectricity in PDMS. For tightly-cured fresh PDMS, weak but stable triboelectric signals were obtained immediately without a charging step. This finding suggests that the modification of chemical elements
Figure 4 The proposed chemical elements and bonds of (a) fresh, (b) UVO-, (c) NaOH-, and (d) HCl-treated PDMS surfaces.
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and bonds is the primary origin of triboelectricity in PDMS. Consistent with this hypothesis, no material transfer between the PDMS surface and the ITO electrode was evidenced in XPS spectra (see Figure S7 in Supporting information).
Conclusion In summary, we have demonstrated a facile method for enhancing the triboelectric charge of PDMS surfaces. Sprinkling a PDMS surface with a 1 M NaOH solution after UVO irradiation resulted in an open-circuit voltage of 49.3 V and a closed-circuit current of 1.16 μA (corresponding to a current density of 185.7 nA/cm2), when the PDMS surface was brought into contact with an ITO electrode. This is nearly 15-fold greater than the corresponding current density obtained with fresh, untreated PDMS. ATR FT-IR and XPS measurements revealed that the increase in triboelectric charge is strongly related to increases in the amount of polar Si–O bonds at the expense of non-polar Si–CH3 bonds. Additionally, these surface treatments had no effect on the roughness, transmission, and flexibility of the resulting TENGs, and are suitable for large-area, costeffective, and high-power TENG applications.
Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A2002536, 2013R1A2A2A01015734, and 2013R1A1A2006582). This research was a part of the project titled ‘Gyeonggi Sea Grant Program’, funded by the Ministry of Oceans and Fisheries, Korea.
Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2015.05.018.
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Jae Woong Kim is a BS student at Department of Physics, Inha University. His current research interests are fabrication of piezo/ triboelectric nanogenerators and their application to emerging electronic devices.
Hyun Soo Kim is a BS student at Department of Physics, Inha University. His current research interests are fabrication and characterization of triboelectric nanomaterials for mechanical energy harvesting.
Base-treated polydimethylsiloxane surfaces Kwan Wook Jung is a Ph.D. student in Department of Physics at Yonsei University. He received his BS degree from Yonsei University in 2013. His research focuses on organic/polymer light emitting diodes and photovoltaics.
Yeonjin Yi is a Professor in Department of Physics at Yonsei University. He received his Ph.D. from Yonsei University in Department of Physics in 2005. After working as a postdoctoral researcher at University of South Florida and as a senior researcher at Korea Research Institute of Standard and Science, he joined the Department of Physics, Yonsei University in 2011. His recent research interest is focused on the interfacial electronic structures of molecular solids and low dimensional materials and related devices. Min-Seok Jeong is a BS student at Department of Physics, Hallym University. His current research interests are spectroscopic study of piezoelectric materials, such as PZT single crystals and lead-free piezomaterials.
529 Jae-Hyeon Ko is a Professor in Department of Physics at Hallym University. He received his Ph.D. from Korea Advanced Institute of Science and Technology in Department of Physics in 2000. After working as a research associate in University of Tsukuba and a principal researcher in Samsung Corning Co. Ltd., he joined the Department of Physics, Hallym University in 2004. His recent research interest is focused on phase transition behaviors of ferroelectrics and disordered systems such as relaxors and dipolar glasses. Jong Hoon Jung is a Professor in Department of Physics at Inha University. He received his Ph.D. from Seoul National University in Department of Physics in 2000. After working as a postdoctoral researcher at Seoul National University and Spin superstructure ERATO project in Japan Science and Technology, he joined the Department of Physics, Inha University in 2004. His recent research interest is focused on piezoelectric/pyroelectric/triboelectric nanogenerators in ferroelectric nanomaterials, and electrolyte control of interfacial spins in magnetic thin films.