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Water droplet-driven triboelectric nanogenerator with superhydrophobicsurfacesJeong Hwan Lee1, SeongMin Kim1, Tae Yun Kim, Usman Khan, Sang-Woo Kim⁎

School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea

A R T I C L E I N F O

Keywords:Water dropletContact electrificationTriboelectricNanogeneratorSuperhydrophobic surfaces

A B S T R A C T

A bouncing water droplet not only contains mechanical energy, but also electrostatic energy due to the tribo-electric charges on its surface that are generated by its contacts with the surrounding environment. Here, wereport development of contact electrification-based water droplet-driven triboelectric nanogenerator (Wd-TENG) for harvesting energy from the water-droplet bouncing between two superhydrophobic surfaces. The Wd-TENG consists of polytetrafluoroethylene balls on the bottom and zinc oxide nanosheets on the top; two surfacesof the device obtain electric energy during the contact electrification from the bouncing water, and additionally,maximize the bouncing motions of the droplets. The device produced a short-circuit current and an open-circuitvoltage of 1.3 μA and 1.4 V, respectively. Besides, power-generating performances of the Wd-TENG at variousangles of inclination were also investigated, and it showed voltage outputs of 5.5 V, 16 V, 9.8 V, and 6.8 V at theinclination angles of 52°, 58°, 64°, and 70°, respectively. These results demonstrate that the Wd-TENG is po-tentially a strong candidate for scavenging energy from raindrops.

1. Introduction

An abundance of mechanical-energy sources are related to watersuch as ocean waves, rainwater, tides, and waterfalls. These sources arean inexhaustible supply of energy and can potentially be an alternativeto solar energy. The electrostatic induction has recently emerged as aneffective technology for mechanical-energy harvesting. Therefore, thedevelopment of novel generator structures that can harvest the me-chanical energies that are related to Earth's water resources is crucial.In the literature, water itself has been identified as one of the tribo-electric materials that can generate electricity via contact electrification[1–14]. In fact, the contact electrification between water and solid-stateinsulating polymers has already been reported in terms of the scaven-ging of the mechanical energy from water [15–20].

In conventional contact electrification devices, the triboelectriccharges are generated on the solid interface where two different tri-boelectric materials come into contact with each other [21,22]. Like-wise, the contact electrification creates triboelectric charges on theinterface between the water and a solid, or the water and air, and ittherefore presents an opportunity for the development of triboelectricnanogenerators (TENGs) driven by water. Such water-driven TENGscould be extremely beneficial, as they can scavenge energy fromflowing and dropping water, tides, rivers, and ocean waves. For

instance, the dropping water carries two kinds of energies [23,24]. Oneis the mechanical energy in the form of the motion of dropping water.The second energy is the electrostatic potential energy that results fromthe triboelectric charges in the outer surface of the water that is gen-erated via contact electrification during its motion in air or from abouncing surface.

Here a novel contact electrification-based water droplet-drivenTENG (Wd-TENG) for scavenging energy from the water-dropletbouncing between two superhydrophobic surfaces has been developedto effectively harvest both the electrostatic and mechanical energiesfrom dropping water. Polytetrafluoroethylene (PTFE) balls form thebottom superhydrophobic surface of the Wd-TENG to bounce the waterdroplet and zinc oxide (ZnO) nanosheets form the top super-hydrophobic side; the Wd-TENG with two such surfaces scavenges theelectrostatic potential energy from the bouncing water droplets.Besides, these surfaces are bidirectional to effectively enhance thedroplet bouncing motion for a higher electricity generation.

In this work the electrical characterization has shown that the Wd-TENG produces a short-circuit current (Isc) and an open-circuit voltage(Voc) of 1.3 μA and 1.4 V, respectively, from a bouncing water droplet.To increase the bouncing-water impact on the surface, the Wd-TENG'sangles of inclination were also investigated. The Voc/Isc values of5.5 V/4 μA, 16 V/10 μA, 9.8 V/8 μA, and 6.8 V/6 μA were obtained for

https://doi.org/10.1016/j.nanoen.2019.01.078Received 22 September 2018; Received in revised form 13 January 2019; Accepted 30 January 2019

⁎ Corresponding author.E-mail address: kimsw1@skku.edu (S.-W. Kim).

1 These authors contributed equally to this work.

Nano Energy 58 (2019) 579–584

Available online 31 January 20192211-2855/ © 2019 Published by Elsevier Ltd.

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the inclination angles of 52°, 58°, 64°, and 70°, respectively.Furthermore, finite element method (FEM) simulations were carried outto study the behavior of a water droplet on the surfaces at variouscontact angles (10, 80, and 150°) with time.

2. Methods

2.1. Device fabrication

An aluminum (Al) substrate (3×7 cm2) was first prepared to growthe ZnO nanosheets. Thereafter, 15-mM Zn nitrate was added to 500mlof deionized water together with 1.04 g of hexamethylenetetramine(HMT), the formula of which is C6H12N4. The mixture was then heatedat a hot-plate temperature of 250 °C for 5min. After confirming themelting of the powder, the ZnO nanosheets were grown at a hot-platetemperature of 190 °C for 3 h. Finally, 500ml of ethanol was added to 8-mM stearic acid, and the ZnO nanosheet was treated with the solutionfor 24 h to achieve the superhydrophobicity on the nanosheet surface.Alternatively, to realize the bottom plate of the Wd-TENG, 100 nmPTFE balls were dispersed in toluene for 5 h to achieve the super-hydrophobic property on the bottom surface. Thereafter, a 10-um-thickPTFE layer was coated onto the Al film using a bar coater. The film wasthen left to dry for 24 h. The Wd-TENG's top and bottom plates wereproduced on an acrylic substrate.

2.2. Characterization

The bar coater was used for the coating of the PTFE film. Thecontact angle was measured using a contact-angle meter. The fieldemission-scanning electron microscopy (FE-SEM) study was carried outto confirm the nanostructure on the Wd-TENG surfaces. The electricaloutput was measured using a two-channel oscilloscope and a currentmeter. For more accurate measurements, vibrations were generatedusing a vibration machine (ET-126–4, Labworks Inc.). The falling waterdroplets were formed using a burette. The bouncing motions of thewater droplets were confirmed using the high-speed camera (PhantomV.9.1, Vision Research).

3. Results and discussion

Superhydrophobicity of a surface is a crucial requirement in orderto achieve a bouncing motion of a water droplet from it. In order toinvestigate the influence of superhydrophobicity on the water-dropletbouncing motion, the impact of a falling water droplet on surfaces withcontact angles of 10° (hydrophilic state), 80° (normal and hydrophobicstate), and 150° (superhydrophobic state) were studied using FEM si-mulations carried out using the computational fluid dynamics (CFD)module of COMSOL Multiphysics; the simulation parameters are de-scribed in detail in the Fig. S1. Fig. 1 shows the results of the FEMsimulations and, in each case, a water droplet falls downward andimpacts the surface at 0.25ms. The post impact droplet behavior,however, is different in each case depending on the surface contactangle. Indeed, in case of the contact angle of 10° (Fig. 1a), the surfaceenergy is too low to maintain the droplet shape, and so the droplet isdispersed and it disappears. In case of the 80° contact angle (Fig. 1b),though the sufficient surface energy maintains the droplet shape, thebouncing is not achieved. However, in case of the superhydrophobicsurface with a contact angle of 150° (Fig. 1c), the surface energy isstrong enough to cause the droplet to bounce off the surface.

We have designed a Wd-TENG with two superhydrophobic surfacesfor harvesting the mechanical energies from the bouncing water dro-plets. Fig. 2a schematically describes the Wd-TENG design that consistsof the superhydrophobic-PTFE ball and the ZnO-nanosheet layers as itsbottom and top surfaces, respectively. The backs of the two super-hydrophobic layers are composed of Al electrodes that collect theelectricity and supply it to any external circuit. The Wd-TENG design ismeant to scavenge the electrostatic energy from a water dropletbouncing between its superhydrophobic surfaces, while the bouncingwater-droplet motion results from the transformation of the dropletpotential energy into kinetic energy [25–27]. Regarding the synthesis ofthe two superhydrophobic layers, the PTFE balls were coated onto an80 μm-thick Al film, and the ZnO nanostructured nanosheets were di-rectly grown on the Al film using the hydrothermal-growth method; theZnO nanosheets were treated with a stearic-acid-forming ester(-RCOOR) to achieve the superhydrophobicity of the layer, and thefabrication process is described in detail in the “Methods” section [28].

Fig. 1. Analysis of the collision of a water droplet with a surface using a COMSOL Multiphysics CFD module at three different contact angles. (a) Contact angle of 10°(i.e., hydrophilic state), (b) contact angle of 80° (i.e., hydrophobic state), (c) contact angle of 150° (i.e., superhydrophobic state).

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Fig. 2b shows the FE-SEM image and the contact angle of the as-treatedZnO nanosheets, wherein a contact angle of more than 150° shows thatthe as-treated ZnO nanosheets demonstrate superhydrophobicity. Al-ternatively, the surface of the PTFE layer is covered by high-densityPTFE balls, as shown by the SEM image of Fig. 2c, and the diameter ofthe PTFE ball is approximately 100 nm. The high-density microballsreduced the actual contact area between the ball and the water dropletand resulted in a contact angle of 150°, thereby demonstrating that thePTFE surface is also superhydrophobic. The superhydrophobicity ofboth surfaces is crucial for the bouncing motion of the water dropletthat is a key for the functionality of the Wd-TENG. Fig. 1 the FEM re-sults demonstrated that the PTFE balls with a contact angle of 150°,which are coated on the bottom surface of the Wd-TENG, are verysuitable to ensure the bouncing motion. Besides, the PTFE balls, in theform of PTFE-surface nanostructures, also increase the effective surfacecontact area that can potentially enhance the electrical power outputfrom Wd-TENG.

To confirm the bouncing motion of a water droplet from the twosuperhydrophobic surfaces of the Wd-TENG, a high-speed camera witha frame rate of 0.15ms was utilized to record the motion (SupportingVideo 1); Fig. 3 shows the selected snapshot images of the motion of a0.02-ml water droplet between the two surfaces. Initially, the droplet is

positioned on the bottom surface and a sound vibration is applied to theWd-TENG to initiate the bouncing motion, as shown in Fig. S2, whilethe input signal to the sound source with a peak-to-peak amplitude of500mV at 3 Hz is from a function generator. Upon its collision with theWd-TENG's upper surface, the droplet adopts a compressed shape at1.00ms. Then, the droplet bounces off the top surface at 1.15ms andmoves downward due to gravity, colliding with the lower surface at1.45ms. Upon its impact with the lower surface, it spreads into apancake-like shape at 1.70ms and thereafter, it starts to bounce fromthe lower surface at 1.85ms. The droplet motion continues under thesound-vibration influence, thereby confirming its bouncing motionbetween the two contact surfaces.

Supplementary material related to this article can be found online atdoi:10.1016/j.nanoen.2019.01.078.

The working principle of the Wd-TENG is schematically shown inFig. 4. The triboelectric charges of the water droplet sitting on thebottom surface are initially positive due to the contact electrificationbetween the droplet and its environment, as shown in Fig. 4a. As thedroplet moved to the upper side, the positively charged droplet inducednegative charges on the top electrode via an electrostatic inductionthrough the ZnO nanosheets (i.e., a dielectric material), as shown inFig. 4b. In fact, when the droplet approaches the upper surface, a

Fig. 2. (a) Schematic description of the Wd-TENG showinga water droplet bouncing between two superhydrophobicsurfaces of ZnO nanosheets and PTFE with Al electrodes.(b) FE-SEM image and contact angle of the ZnO na-nosheets treated with a stearic-acid surface to promotesuperhydrophobicity. (c) FE-SEM image and the PTFEcontact angle.

Fig. 3. Bouncing motion of a water droplet between the top and bottom surfaces that were observed using a high-speed camera; the snapshots are shown at intervalsof 0.15ms.

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negative voltage was produced between the top and bottom electrodesthat drove the transfer of the electrons from the bottom electrode to thetop electrode, thereby generating a negative current; however, thecurrent-flow amount is very small, i.e., it cannot be clearly seen in theinset of Fig. 4e, due to a smaller contact between the droplet and theupper surface, and this is because of gravity and the super-hydrophobicity. Alternatively, when the water droplet moved back tothe lower surface, negative charges were induced at the bottom elec-trode via the electrostatic induction though the PTFE. However, duringthe approach toward the bottom surface, a positive-potential differencewas generated between the top and bottom electrodes that drove thetransfer of the electrons from the top electrode to the bottom electrode,thereby producing the positive current that is shown in Fig. 4c, e. Acontinuous bouncing motion of the water droplet between the two

layers therefore produced an alternating current between the twoelectrodes through the repetitive processes that are shown in Fig. 4b, c.

To characterize performance of the Wd-TENG, the water dropletswere made to bounce between its two superhydrophobic surfaces at avelocity of 0.58m/s; the bouncing motion was driven by a vibrationmachine and the gap distance between the surfaces is 0.5mm. Theelectrical output of the device was measured as a function of the fol-lowing measures: (i) number of droplets, (ii) volume of a droplet (ml),and (iii) amplitude of the input signal to the vibration machine thatdetermines the amplitude vibration (mV). The electrical output fromthe device as the number of droplets is shown in Fig. 5a; the inset ofFig. 5a is a schematic depiction of the number of droplets. The Vocvalues for one, two, and three droplets are 1.12, 2.28, and 3.66 V, re-spectively. The positive Voc values of the Wd-TENG confirm the contact

Fig. 4. (a) Working mechanism of the Wd-TENG. (a) The triboelectric charges of the water droplet sitting on the bottom surface were initially positive because of thecontact electrification with the environment. (b) As the droplet approached the upper surface of the Wd-TENG, the positively charged droplet induced negativecharges on the top electrode via electrostatic induction. (c) As the droplet bounced back and approached the lower surface, it induced negative charges in the bottomelectrodes via electrostatic induction. (d) The water droplet with its positive charges rebound off the PTFE-ball film due to the superhydrophobic property of thesurface. (e) Electrical output voltage of the Wd-TENG with the corresponding position of the water droplet.

Fig. 5. (a) Output voltage of the Wd-TENG due to the different number of droplets. (b) Output voltage with an increasing volume of the water droplets. (c) Outputvoltage at the increasing peak–peak voltage of the input signal to the vibration machine. In (a)–(c), the voltage was measured across a 40-MΩ load. (d) The electrical-output voltage, current, and power of the Wd-TENG as a function of the external load resistance.

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that was made with the bottom surface. Besides, the synchronismamong the bouncing motions of the multiple water droplets is also animportant factor. For instance, if each water droplet bounces randomly,the electrical output from the device comprises multiple peaks, asshown in the Fig. S4a, b.

The volume of the water droplet can potentially influence theelectrical output. Therefore, the electric power output of the Wd-TENGas a function of the different water-droplet amounts is shown in Fig. 5b,wherein the inset is a schematic depiction of the varying dropletamount. The Voc values of 0.72, 2.51, 3.66, 4.15, and 8.1 V wereachieved for 0.02-, 0.04-, 0.06-, 0.08-, and 0.1-ml droplets, respectively.Though the output increased with the amount of the water droplet, itsshape deviated from the normal pattern, because when a driven by avibration machine started the movement of a large droplet (for ex-ample, 0.1 ml), it did not bounce off the superhydrophobic surface atonce. It actually moved around slightly before the bouncing, resultingin additional small output-voltage peaks, as shown in Fig. 5b.

Since the bouncing motion of the water droplet was induced by avibration machine, the amplitude of the electrical signal that was inputto the vibration machine strongly influenced the electrical output. Inprinciple, higher amplitude results in a higher mechanical force that istransferred to the droplet through the vibration. Fig. 5c shows theelectrical output for the different amplitudes (peak-to-peak) of the inputsignal that drive the vibration. As the amplitudes of the input signalincrease, the number of Voc peaks and their intensities also increase,but only until 600mV. Afterward, the droplet splashed around due tothe high vibration power that broke apart the water-droplet shape.Fig. 5d shows the output voltage, current, and power of the Wd-TENGas a function of the load resistance; the Wd-TENG produced its peakelectric power output at an external load resistance of 1MΩ. And thedurability for 10min of the Wd-TENG is show in Fig. S5. There wassome difference due to the evaporation of the water droplet size andbounce unbalance. But it was driven somewhat uniformly. And thestability of the output is not a potential issue as the proposed TENG hassuperhydrohobic friction layers [29], such surfaces have a self-cleaningability.

The potential of the proposed Wd-TENG means it can harvest theenergy from raindrops. To achieve the bouncing motion of the rain-drops between the two surfaces, however, the Wd-TENG must be op-erated in an inclined manner, as shown in the Fig. S6. The electricpower output from the Wd-TENG was therefore measured at variousangles of inclination (52°, 58°, 64°, and 70°), as shown in Fig. 6; theinsets of this figure show the snapshots of the droplet motion. However,the output increased with the increasing of the angle of inclination until58°; thereafter, both of these values start to decrease. The Voc and theIsc of the Wd-TENG as a function of the tilt angle from 52° to 70° areshown in Fig. 5e; the peak output was obtained at the optimum in-clination/tilt angle of 58°. At 52°, the water droplets moved rapidly dueto the low angle to cease the bouncing motion, thereby resulting in alow output. In contrast, for the optimal case of the 58° inclination, thebouncing-droplet motion is shown in the Fig. S7. In the case of 64°, theoutput was decreased with the dominance of the sliding motion of thewater droplet. At a higher inclination angle of 70°, the bouncing motionis barely discernible, and therefore the output was further decreased.The measured Voc values are 7, 16, 10, and 5 V at the inclination anglesof 52°, 58°, 64°, and 70°, respectively. Furthermore, the Wd-TENGproduced a continuous electric power output when the water dropletscontinuously fell on it, as shown in the Supporting Video 2. As seen inthe Supporting Video 3, however, the device did not produce anyoutput when a stream of water was applied to it, and this is because, inthe case of a water stream, the bouncing motion is absent. In brief, theresults of Fig. 5 and the Supporting Video 2 demonstrate that, in an

inclined manner, the Wd-TENG is well suited to harvest the energy fromraindrops.

Supplementary material related to this article can be found online atdoi:10.1016/j.nanoen.2019.01.078.

4. Conclusions

In summary, the functionality of Wd-TENG that can harvest theenergy from water droplets by enabling the bouncing motion of thedroplets between its two superhydrophobic surfaces has been demon-strated. The dependence of electric power output on the number andvolume of the droplets was investigated; as a result, it was observedthat the output increased with the number and the volume of thedroplets. To harvest the energy from falling droplets such as raindrops,an inclined arrangement of the device is proposed, and the perfor-mances at various angles of inclination were investigated to find theoptimal angle; accordingly, the highest output was obtained at an in-clination angle of 58°. It was also demonstrated that continuouslyfalling water droplets produce a continuous output; however, a water-stream output was not obtained due to the corresponding absence of thebouncing motion between the surfaces. Due to all of the unique fea-tures, it can be suggested that the potential of the Wd-TENG demon-strated in this study for harvesting raindrop energy is promising.

Fig. 6. Output voltage of the Wd-TENG due to the falling water droplet at theangles of inclination of (a) 54°, (b) 58°, (c) 64°, and (d) 70°; inset shows a high-speed camera image of the Wd-TENG with the water-droplet movement. In(a)–(d), the voltage was measured across a 40-MΩ load. (e) The output voltageand current of the Wd-TENG as a function of the angle of inclination.

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Acknowledgements

J. H. Lee and S.M. Kim contributed equally to this work. This workwas supported by the “Human Resources Program in EnergyTechnology (No. 20174030201800)” of the Korea Institute of EnergyTechnology Evaluation and Planning (KETEP) funded by the Ministry ofTrade, Industry & Energy (MOTIE, Korea), the GRRC Program ofGyeonggi Province (GRRC Sungkyunkwan 2017-B05), and KoreaElectric Power Corporation (Grant number: R18XA02) S. M. K ac-knowledges a financial support from National Research Foundation ofKorea (No. 2017R1A2B4010642)

Appendix A. Supporting information

Supplementary data associated with this article can be found in theonline version at doi:10.1016/j.nanoen.2019.01.078.

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Jeong Hwan Lee is a Postdoctoral Fellow with Prof. Sang-Woo Kim at School of Advanced Materials Science &Engineering, Sungkyunkwan University (SKKU). His re-search interests are fabrications and characterizations oftriboelectric nanogenerator energy harvesting and theirapplications in self-powered devices.

Seong Min Kim received his Ph.D. degree from Universityof Cambridge, UK, in 2009. His research interests include amathematical multi-physics modeling/atomic and mole-cular level simulation, particularly for nanogenerators and2D materials. He is currently Research Professor with Prof.Sang-Woo Kim at School of Advanced Materials Science &Engineering, Sungkyunkwan University (SKKU).

Tae Yun Kim is a Postdoctoral Fellow with Prof. Sang-WooKim at School of Advanced Materials Science &Engineering, Sungkyunkwan University (SKKU). His re-search interests include Piezoelectric FEM Simulation &Analyzation of AFM.

Usman Khan is a Research Professor with Prof. Sang-WooKim at School of Advanced Materials Science &Engineering, Sungkyunkwan University (SKKU). He re-ceived his Ph.D. degree from University of Rome “TorVergata”, Italy, in May 2014 and, thereafter, he served as apost-doc in the university for one year. His research inter-ests include the design, simulation, fabrication and char-acterization of 2D materials based MEMS and NEMS de-vices for energy harvesting, sensing and actuation.

Sang-Woo Kim is a Professor in the Department ofAdvanced Materials Science and Engineering atSungkyunkwan University (SKKU). His recent research in-terest is focused on piezoelectric/triboelectric nanogenera-tors, sensors, and photovoltaics using nanomaterials. Hehas published over 200 peer-reviewed papers and holdsover 80 domestic/international patents. Now he is a di-rector of SAMSUNG-SKKU Graphene/2D Research Centerand is leading National Research Laboratory for NextGeneration Hybrid Energy Harvester. He is currently ser-ving as an Associate Editor of Nano Energy and an ExecutiveBoard Member of Advanced Electronic Materials.

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