Triboelectrification-Induced Large Electric Power Generation from a

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Triboelectrification-Induced Large Electric Power Generation from a Single Moving Droplet on Graphene/Polytetrafluoroethylene Sung Soo Kwak,†,⊥ Shisheng Lin,*,‡,⊥ Jeong Hwan Lee,†,⊥ Hanjun Ryu,† Tae Yun Kim,§ Huikai Zhong,‡ Hongsheng Chen,‡ and Sang-Woo Kim*,†,§ †

School of Advanced Materials Science and Engineering and §SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea ‡ Department of Information Science and Electronic Engineering and State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Recently, several reports have demonstrated that a moving droplet of seawater or ionic solution over monolayer graphene produces an electric power of about 19 nW, and this has been suggested to be a result of the pseudocapacitive effect between graphene and the liquid droplet. Here, we show that the change in the triboelectrification-induced pseudocapacitance between the water droplet and monolayer graphene on polytetrafluoroethylene (PTFE) results in a large power output of about 1.9 μW, which is about 100 times larger than that presented in previous research. During the graphene transfer process, a very strong negative triboelectric potential is generated on the surface of the PTFE. Positive and negative charge accumulation, respectively, occurs on the bottom and the top surfaces of graphene due to the triboelectric potential, and the negative charges that accumulate on the top surface of graphene are driven forward by the moving droplet, charging and discharging at the front and rear of the droplet. KEYWORDS: triboelectric effect, graphene, polytetrafluoroethylene, energy harvesting, water droplet of absorbed ions inside the nanotube,14 fluctuating asymmetric potential,15 and streaming potential models.16 On the other hand, graphene is a one atomic layer thin two-dimensional material, indicating strong charge interaction between graphene and the template beneath graphene. Thus, it can be proposed that the expected strong charge interaction between graphene and the template critically influences the charge interaction between graphene and a liquid droplet in the generation of electric power output. Here, we demonstrate large electric power generation from a single moving water droplet on a monolayer graphene, producing an output power of about 1.9 μW, which is about 100 times larger than the power output achieved in previous reports.11,12 This result is explained to be a result of the change in triboelectrification-induced pseudocapacitance between a water droplet and the monolayer graphene on polytetrafluoroethylene (PTFE). Positive and negative charges were found to,

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raphene is considered to have strong potential for various device applications due to its unique and multifunctional properties such as ultrahigh electron mobility, high transparency, high mechanical elasticity, high thermal stability, chemical inertness, etc.1,2 These fascinating properties make graphene a promising material for energy harvesting, storage, and saving device applications such as solar cells,3 nanogenerators,4 supercapacitors,5 batteries,6 photodetectors,7 and light-emitting diodes.8 Recent studies reported an interesting phenomenon that graphene can produce electricity when droplets of ion-containing water moves over it.9−12 A single moving droplet of seawater or ionic solution results in a power output of about 19 nW, which was explained to be a result of the pseudocapacitive effect between graphene and the single droplet of liquid.11,12 Similarly, it has been observed that electrical signals are detected when carbon nanotubes (CNTs) are dipped into flowing water or polar liquids. Various physical mechanisms have been proposed to explain the electricity-generating process between water and CNTs, such as the dragging phonons induced by the friction of the moving liquid,13 drifting © 2016 American Chemical Society

Received: May 7, 2016 Accepted: July 14, 2016 Published: July 14, 2016 7297

DOI: 10.1021/acsnano.6b03032 ACS Nano 2016, 10, 7297−7302

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Figure 1. Water-droplet-based electric power generation from a graphene/PTFE structure. (a) Voltage output from a graphene/PTFE structure by a single moving droplet of 0.6 M NaCl on a graphene surface with a reversed voltage signal output of a graphene/PTFE structure obtained by reversing the moving direction of the droplet. (b) Current output from the graphene/PTFE structure according to the forward and reverse motion of the water. (c,d) Dependence of the output voltage and current as a function of the external load resistance and corresponding maximum power output by a single moving droplet of 0.6 M NaCl, respectively.

Figure 2. Electric power generation performance of a graphene/PTFE structure under different situations. (a) Voltage output change from the graphene/PTFE structures with a different number of graphene layers; the voltage output decreases as the number of graphene layers increases with a 0.6 mL droplet. (b) Dependence of the voltage output on the size of the water droplet; the voltage output increases as the water droplet increases, and a value of greater than 0.4 V can be obtained with a water droplet size of 0.6 mL with 0.6 M NaCl. (c) Dependence of the voltage output on the concentration of NaCl. (d) Electric power generated from the monolayer graphene/PTFE structure by continuous water drops.

accumulate onto the top surface of the graphene and are driven forward by the moving droplet, charging and discharging at the front and rear of the droplet.

respectively, accumulate at the bottom and top surfaces of graphene on PTFE by the triboelectric potential generated during the graphene transfer process. The negative charges 7298

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Figure 3. Electric power generation mechanism. (a) Charge distribution of the water droplet and graphene/PTFE in the initial state and (b) charge redistribution during movement of the water droplet over the surface of the graphene/PTFE.

RESULTS AND DISCUSSION Our monolayer graphene samples were synthesized using a chemical vapor deposition (CVD) process with copper foil as a substrate as well as growth catalyst. Graphene strips with dimensions of ∼3.5 cm × 7 cm were transferred onto a PTFE substrate using poly(methyl methacrylate) (PMMA) as a sacrificial layer. The quality of the transferred graphene was identified via Raman characterization (Figure S1). An Ohmic contact with graphene was achieved using conductive wires embedded in a silver epoxy, and silicone glue was used to prevent any possible electric short due to contact with the water droplet. The experimental setup is presented in Figure S2. A droplet of 0.6 M NaCl solution was placed on the graphene/PTFE surface, and the single droplet was moved by artificially tilting the PTFE substrate. The signal for the electric power output was obtained in real time using an oscilloscope (Tektronix, DPO 3052), with its positive terminal connected to the right end of the strip (Video S1). The typical voltage output is shown in Figure 1a, and the peak of the signal can reach ∼0.40 V (left) when the droplet (0.6 mL) moves from the left-hand side to the right-hand side. By reversing the moving direction of the droplet, we can observe the voltage of the signal output to be reversed to ∼0.45 V (right). Figure 1b shows that the typical current output is ∼4.5 μA (left) and ∼4.8 μA (right) according to the direction in which the droplet is moving. These electric output signals that pass through the external circuit are generated by cation redistribution based on the change in the shape of the droplet when the droplet moves downward. The load resistance dependence of both voltage output and the current output with the resistance ranging from 10 KΩ to 1 GΩ using a single water droplet is shown in Figure 1c. With increasing resistance, output voltage was increased and output current was decreased, so the maximum output power was evaluated to be about 1.9 μW at 50 MΩ (Figure 1d). Here, it should be noted that the output power that is achieved from this single droplet moving on monolayer graphene on PTFE is about 100 times larger than the power output from a single droplet moving onto monolayer graphene on SiO2.11 To assess the change in the electric power output from a single moving droplet on graphene as a function of the number of the graphene layers, we prepared bilayer and trilayer graphene on PTFE using a multiple transfer process for monolayer graphene. The output voltage for bilayer and trilayer graphene (Figure 2a) corresponds to 1.3 and 1.2 mV, respectively, and such a significant decrease in the voltage outputs corresponding to the increases in the number of the graphene layers can be attributed to multilayer graphene screening the substrate effect by increasing the conductivity and

thickness of the graphene layers. This is further confirmed by the fact that the Au thin film deposited on PTFE with a thickness of tens of nanometers shows no voltage output when the water droplet moves (Figure S3). We also investigated the dependence of the size of the water droplet on the voltage output. The voltage output increases linearly with an increase in the size of the water droplet, as shown Figure 2b. The increase in the size of the water droplet leads to an increase in the contact area with graphene, resulting in a large difference in the cation distribution between the front and rear end of the droplet. In Figure 2c, the voltage output depending on the NaCl mole concentration shows an increasing trend under 0.6 M and a decreasing trend over 0.6 M with a droplet of 0.6 mL. Over the critical mole concentration, an increase in the amount of anions screens the electron double layer (EDL) between the graphene and water droplet because the Debye length is reciprocally proportional to the square root of the ion concentration.11,12,17 In this regard, we suggest that our proposed structure with PTFE beneath monolayer graphene is very promising in suppressing the screening effect caused by the anion in the diffuse layer of the EDL, resulting in the significant reduction in output power because the graphene/PTFE structure effectively attracts more cations on the interface between graphene and the droplet. We further demonstrate the use of a graphene/ PTFE-based energy-harvesting device that uses rain drops. Rain droplets produce a continuous voltage output (∼0.25 V) due to the continuous contact between the inclined graphene/PTFE and the sliding water droplets, as shown in Figure 2d. When graphene is transferred on the PTFE template using a wet transfer method, triboelectrification occurs at the interface of the PTFE and deionized (DI) water.18−22 Thus, DI water becomes positively charged while the surface of PTFE is negatively charged.23−29 After the DI water completely dries, the negatively charged surface of PTFE attracts holes from inside the graphene, resulting in the formation of EDL between the top surface of PTFE and the bottom surface of the graphene. Thus, the excluded electrons are localized along the surface on the top side of the graphene layer. Then, when we drop an ionic water droplet on the graphene transferred onto the PTFE template, the cations (Na+) in the water droplet are dragged and accumulate at the bottom contact surface of the droplet, close to the top side of the graphene layer. The cations accumulate into the droplet, and electrons are excluded at the top side of the graphene, forming another EDL and inducing partial capacitive behavior. For the water droplet on graphene in a steady state, there is no potential difference at both ends of water droplet, as shown in Figure 3a. However, after the water droplet achieves constant movement in one direction, the cations inside the water droplet 7299

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Figure 4. Surface potential of each sample obtained via KPFM measurements. (a) Surface potential of the PTFE at an initial state and after immersion into DI water. (b) Surface potential of transferred graphene on negatively charged PTFE substrate at an initial state and after immersion into 0.6 M NaCl solution and after 7 days.

Figure 5. Series and parallel connection of three individual graphene/PTFE structures: (a) schematic image, (b) output voltage, (c) output current for connection in series; and (d) schematic image, (e) output voltage, (f) output current for a connection in parallel. The voltage output is greater than 1.1 V in series, and the current output is greater than 8 μA in parallel.

accumulate at one side ahead of the water droplet. Due to the alteration of cations, the uniform balance of surface charges into the graphene is broken, and this begins and generates the relative potential difference at both ends of the water droplet. Therefore, this potential difference generated at the water droplet triggers the drawing of electrons in the graphene. This potential difference of the water droplet is maintained until the water droplet arrives at the end of the graphene, and electrons are constantly drawn when the water droplet has a fixed moving speed, as shown in Figure 3b. Also, as the velocity increases, the voltage output increases due to the increase in the potential difference into the water droplet by greater deformation in the shape of the droplet (Figure S4 and Video S2). The acceleration of the water droplet leads to an increase in the potential difference at both ends of the water droplet in unit time. In addition, no significant electric power output from a single moving droplet on PTFE without graphene was observed (Video S3), which further confirms that the graphene/PTFE structure is a very promising platform to produce large power output using water droplets. Due to the strong, negatively charged PTFE surface due to triboelectrification with DI water, these charges attract more cations in the water droplet to the top side of the graphene (Figure S5). To verify that the graphene/PTFE structure has a

much better electric power output performance than other structures consisting of graphene on either Kapton or SiO2, we experimentally measured the surface potential of the PTFE substrate for both the pristine state and after immersion in DI water via Kelvin probe force microscopy (KPFM), as shown in Figure 4a. The PTFE substrate immersed into DI water exhibits a surface potential more negative than that in the pristine state. Although other Kapton and SiO2 substrates also show similar behavior because Kapton and SiO2 also have a negatively charged surface potential after immersion in DI water, the surface potential of PTFE is much more negative than that for both Kapton and SiO2 (Figure S6). Futhermore, the surface potential of transferred graphene on negatively charged substrates was measured (Figure 4b and Figure S7). The surface potential of graphene/PTFE is also much more negative than that for both graphene/Kapton and graphene/SiO2, which indicates that the surface potential of graphene/triboelectrified substrates is mainly governed by the surface potential of each triboelectrified substrate. In addition, it was found that there is no significant change on the surface potential of graphene/ PTFE at an initial state and after immersion into 0.6 M NaCl solution and after 7 days (Figure 4b).30,31 Several graphene/PTFE structures are connected in both series and parallel to further increase both output voltage and 7300

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ACS Nano output current for possible device applications. As shown in Figure 5, each 0.6 mL size droplet of 0.6 M NaCl is put on the graphene layer with three individual graphene/PTFE structures connected in serial (Figure 5a) and parallel (Figure 5b). When the droplets move by tilting the integrated graphene/PTFE, very broad voltage and current peaks are observed, indicating high-performance electric power generation from the graphene/PTFE when we consider the totally integrated broad peak areas in both voltage output and current output shown in Figure 5b−f. In series connection, voltage output is added with each output voltage of structures, over 1.1 V, and the current shows an output current almost similar to that of the single graphene/PTFE structure. In contrast, the parallel connection reveals that the voltage is similar to the single device’s output voltage, and the current is added with each output current of devices, over 8 μA. This result suggests that we can further create a huge electric power output by using proper series and parallel connections of many graphene/PTFE devices for realizing graphene-based large-area electric power generation.

after immersion in DI water; surface potential of transferred graphene on a triboelectrified substrate before and after immersion of 0.6 M NaCl solution; power generation of graphene/PTFE with a single water droplet at the tilting mode and linear motion in forward and reverse electrode connection (PDF) Video S1 (AVI) Video S2 (AVI) Video S3 (AVI)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ⊥

S.S.K., S.L., and J.H.L. contributed equally.

Notes

The authors declare no competing financial interest.

CONCLUSIONS In summary, we have achieved large electric power generation from a single moving water droplet on the basis of the change in triboelectrification-induced pseudocapacitance between graphene and the droplet by introducing PTFE under the graphene layer. An output power of about 1.9 μW was achieved from a single droplet moving onto monolayer graphene/PTFE. A strong negative potential on the surface of the PTFE substrate was generated due to triboelectrification between PTFE and DI water during the graphene transfer process. The triboelectric potential induces the accumulation of positive and negative charges on the bottom and top surfaces of graphene, respectively. The negative charges that accumulate onto the top surface of the graphene are driven forward by the moving droplet, charging and discharging at the front and rear of the droplet. This graphene/PTFE structure expands the range of utilization of graphene in practical energy-harvesting devices.

ACKNOWLEDGMENTS This work was financially supported by the Framework of International Cooperation Program managed by National Research Foundation of Korea grant (NRF2015K2A2A7056357) funded by the Ministry of Science, ICT & Future Planning and “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20154030200870). REFERENCES (1) Du, X.; Skachko, I.; Barker, A.; Andrei, E. Y. Nat. Approaching Ballistic Transport in Suspended Graphene. Nat. Nanotechnol. 2008, 3, 491−495. (2) Novoselov, K. S.; Fal’ko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, K. A Roadmap for Graphene. Nature 2012, 490, 192−200. (3) Shin, K.-S.; Jo, H.; Shin, H.-J.; Choi, W. M.; Choi, J.-Y.; Kim, S.W. High Quality Graphene-Semiconducting Oxide Heterostructure for Inverted Organic Photovoltaics. J. Mater. Chem. 2012, 22, 13032− 13038. (4) Kim, S.; Gupta, M. K.; Lee, K. Y.; Sohn, A.; Kim, T. Y.; Shin, K.S.; Kim, D.; Kim, S. K.; Lee, K. H.; Shin, H.-J.; Kim, D.-W.; Kim, S.-W. Transparent Flexible Graphene Triboelectric Nanogenerators. Adv. Mater. 2014, 26, 3918−3925. (5) Liu, C.; Yu, Z.; Neff, D.; Zhamu, A.; Jang, B. Z. Graphene-Based Supercapacitor with an Ultrahigh Energy Density. Nano Lett. 2010, 10, 4863−4868. (6) Yoo, E.; Kim, J.; Hosono, E.; Zhou, H.; Kudo, T.; Honma, I. Storage of Graphene Nanosheet Families for Use in Rechargeable Lithium Ion Batteries. Nano Lett. 2008, 8, 2277−2282. (7) Xia, F.; Mueller, T.; Lin, Y.; Valdes-Garcia, A.; Avouris, P. Ultrafast Graphene Photodetector. Nat. Nanotechnol. 2009, 4, 839− 843. (8) Chung, K.; Lee, C.-H.; Yi, G.-C. Transferable GaN Layers Grown on ZnO-Coated Graphene Layers for Optoelectronic Devices. Science 2010, 330, 655−657. (9) Dhiman, P.; Yavari, F.; Mi, X.; Gullapalli, H.; Shi, Y.; Ajayan, P. M.; Koratkar, N. Harvesting Energy from Water Flow over Graphene. Nano Lett. 2011, 11, 3123−3127. (10) Yin, J.; Zhang, Z.; Li, X.; Zhou, J.; Guo, W. Harvesting Energy from Water Flow over Graphene? Nano Lett. 2012, 12, 1736−1741. (11) Yin, J.; Li, X.; Yu, J.; Zhang, Z.; Zhou, J.; Guo, W. Generating Electricity by Moving a Droplet of Ionic Liquid along Graphene. Nat. Nanotechnol. 2014, 9, 378−383.

METHODS Fabrication of the Graphene/PTFE Structure. Monolayer graphene was grown on a copper foil via CVD, using CH4 and H2 as the reaction source. Growth was carried out at 1000 °C for 60 min, with a reaction source flux ratio of 5:1 CH4/H2. To transfer graphene on the target substrate, PMMA was spin-coated on the graphene/ copper foil, and the PMMA/graphene/copper foil was dipped into an etchant (HNO3) solution for copper etching. After complete etching of the copper foil, the PMMA/graphene was cleaned using DI water several times; the PMMA/graphene was then transferred onto a PTFE substrate, and the sample was dried in an oven at 80 °C. To remove the PMMA, the samples were immersed in acetone for 2 h. Then, two electrodes were placed on both ends of the graphene/PTFE to measure the output performance, and the electrodes were sealed with silicon glue to prevent contact with the ionic solution.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b03032. Raman spectroscopy of monolayer graphene; detailed device structure and experimental setup for movement of the water droplet; dependence on moving speed of the water droplet using a linear motor; variation in the voltage output with different substrate materials; surface potential of each substrate material at an initial state and 7301

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