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Mar 8, 2016 - mechanical force exerted due to falling water on the device has been converted to electrical current by the RGO/PMMA based nanogenerators.
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Reduced Graphene Oxide-Based Piezoelectric Nanogenerator With Water Excitation Ruma Ghosh, Manojit Pusty, and Prasanta Kumar Guha

Abstract—Generation of electrical power using mechanical energies that are readily available in the environment has become indispensible to provide source energy to all the electronic devices that are used in day-to-day lives. In this study, reduced graphene oxide (RGO)/poly methyl methacralyte (PMMA)-based hybrid film has been employed for nanogeneration. The thin films were characterized using atomic force microscopy, field emission scanning electron microscopy, and X-ray photoelectron spectroscopy. The mechanical force exerted due to falling water on the device has been converted to electrical current by the RGO/PMMA based nanogenerators. The magnitude of generated current varied with volume of water dropped on the device. The roles of amount of functional groups present in RGO, metal stripes as bottom electrode and the spacing between the fingers have been explored. It was demonstrated that the device can also operate in selfpowered mode, i.e., without applying any bias. It is believed that this study would help in development of highly efficient RGO-based nanogenerator in a cost effective manner. Index Terms—Energy harvesting, piezoelectric nanogenerator, RGO/PMMA nanogenerator, water excitation.

I. INTRODUCTION ECENT advances in technologies enable us to produce micro and nano devices and sensors which consume ultra low power (micro or even nano watt). In fact, so little is the power requirement, that many of these devices are going wireless and can be driven by batteries. However batteries need to be replaced or charged on a regular basis, thus it will incur cost and cumbersome maintenance (disposal of batteries needs to be tightly regulated). Hence a more effective alternative approach is to harvest ambient energy locally to run these devices. Energy harvesting is a process by which ambient energy (e.g., solar, thermal, mechanical) is captured and converted to electricity to drive a device. Harvesting mechanical energy is an interesting prospect because harnessing solar energy is not possible in dark and also areas with less sun light. Piezoelectric (phenomenon of converting mechanical energy to electrical energy) nanogenerators fabricated with zinc oxide (ZnO) nanostructures are particularly

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Manuscript received September 10, 2015; accepted January 11, 2016. Date of publication January 20, 2016; date of current version March 8, 2016. The work of P. K. Guha was supported by SRIC, IIT Kharagpur (ISIRD grant). The review of this paper was arranged by Associate Editor Y.-H. Cho. R. Ghosh and P. K. Guha are with the Department of Electronics & Electrical Communication Engineering, Indian Institute of Technology Kharagpur, West Bengal 721302, India (e-mail: [email protected]; pkguha@ ece.iitkgp.ernet.in). M. Pusty is with the Centre for Material Science and Engineering, Indian Institute of Technology Indore, Indore 453441, India (e-mail: phd1401281001@ iiti.ac.in.). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNANO.2016.2520019

promising for this application [1], [2]. Apart from ZnO, other piezoelectric structures which have been reported so far for converting mechanical energy into electricity are indium nitride nanowires [3], lead zirconate titanate nanofibers [4], poly(vinylidene fluoride) nanofibers [5], and cadmium sulphide nanowires [6]. In recent years, there have been reports of carbon nano material based nanogenerators (e.g., with carbon nanotube and graphene) [7]–[9]. Graphene is a 2-D material which consists of monolayer of sp2 hybridized carbon atoms arranged in a hexagonal fashion. Its high aspect ratio, outstanding electronic and mechanical properties make it a suitable candidate for piezoelectric material [10]. Graphene synthesized using chemical vapour deposition technique and epitaxial method is of high purity; such graphene layer does not contain defects and functional groups [11]. Also, these synthesis techniques require high temperature and are expensive. In this respect, chemically reduced graphene oxide (RGO) gains advantages. First, synthesis of RGO is easy and cost effective. Also, RGO contains functional groups and defect sites which support generation of electricity [12], [13]. It was reported recently that electricity can be generated by the flow of slightly acidic water over graphene sheet [14]. But acidic water is hard to find naturally. Que et al. have reported performance of GO sheets on flexible substrate for converting electrical energy from acoustic energy [15]. Valentini et al. also reported generation of electricity from acoustic energy by RGO film and they showed its performance is better than GO film [16]. Tian et al. demonstrated a nanogenerator from GO film on flexible substrate which converts pressure energy to electrical energy [17]. In spite of these recent reports on graphene/ GO based nanogenarators, there is a lot of room for improvements. The current generation from nanogenerators are still quite poor and also the performance is not up to mark to drive nano devices and sensors reliably. In this work, a RGO/PMMA nanogenerator was developed on a glass substrate. The energy generation was observed by dropping different amount of water on the RGO/PMMA film. Extensive material characterizations of the film were carried out using atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS). A systematic investigation was carried out to maximise the energy conversion. It was found that the electric current generation is directly proportional to the volume of water droplets dropped over the samples. The effect of bottom electrode spacing on energy conversion was studied. In order to gain an insight to the effect of functional groups on nanogeneration, RGO was synthesized by reducing graphene oxide (GO) thermally for three different time periods. The results of our findings have been discussed in details in later sections.

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II. EXPERIMENTAL SECTION A. Sample Preparation Fine graphite powder was purchased from Loba Chemie Pvt. Ltd. All the other chemicals were purchased from Merck, India. GO was prepared by modified Hummers and Offeman method [18]. The hence synthesized GO was thermally reduced (partially) at 160 °C for three different time durations viz. 15 min, 30 min, 60 min to obtain three different samples of RGO. 2 × 2 cm glass slides were used as substrates for RGO deposition. The glass slides were cleaned using acetone, twopropanol and DI water followed by drying in oven. RGO was then drop casted on the glass slides and were dried in open air. The thicknesses of all the RGO layers were maintained between 0.15–0.2 μm. PMMA solution was prepared by dissolving 50 mg of PMMA in 1 mL toluene. The hence prepared PMMA solution was spin coated at the rate of 1000 r/min for 60 s on the RGO films which were pre-deposited on the glass substrates. After that the RGO/PMMA film coated glass substrates were dipped in DI water at a temperature of 95 °C, until the film completely detaches from the substrate. The floating film was then transferred over aluminium electrodes pre deposited on another glass substrate of same dimensions substrate followed by top metal electrode evaporation. The device was then tested for the current generation using water as the excitation source. The steps of our device fabrication have been schematically represented in Fig. 1(a). Aluminium electrodes were fabricated by thermal evaporation technique using Hind Hivac 15F6 thermal evaporator. For the fabrication of bottom electrode arrays, shadow masks were used to deposit 1 mm wide electrodes having three different spaces (0.5, 1 and 1.5 mm) between them. The thicknesses of the electrodes were around 250 nm. Also a uniform layer of 250 nm thick aluminium was evaporated on a glass slide to have a flat and continuous bottom electrode-device without any gaps in between. For fabrication of top electrodes, a small patch of aluminium was thermally evaporated after successfully transferring the floating film over the bottom electrodes. The schematic of the finally fabricated device is shown in Fig. 1(b). The space between the bottom electrodes provided RGO/PMMA film, the required space to bend. But in order to bias all the bottom electrodes at once they were shorted using another Al metal layer as shown in Fig. 1(b). B. Material Characterizations and Nanogeneration In order to ensure the proper exfoliation of graphite powder to GO, AFM characterization was done using Veeco Nanoscope-IV. FESEM characterization was done using Gemini Supra 40 to get the morphology of the RGO/PMMA film. Also, to know the extent of reduction in amount of functional groups by reducing GO for three different times, XPS was done using PHI 5000 Versa Probe II (ULVAC-PHI Inc., Japan). The device was then probed using Cascade Microtech PLV50 probe station which is interfaced with a computer through Agilent 4155C semiconductor parameter analyzer (SPA). The water droplets were introduced to the sample using variable volume micropipette (Tarsons-T100).

Fig. 1. (a) Schematic description of the steps of fabrication of the RGO/PMMA based nanogeneration device (b) Schematic of the finally fabricated nanogenerator device (not to scale).

III. RESULTS AND DISCUSSION A. Material Characterizations In order to ensure proper exfoliation of graphite powder to GO different characterizations were carried out. The AFM image of the synthesized GO is shown in Fig. 2(a). The thickness of the GO flakes varied from 2–5 nm as shown in Fig. 2(a). Hence, it was ensured that the GO got properly exfoliated. The GO was then thermally reduced at 160 ºC for three different durations. The FESEM image of RGO/PMMA composite film is shown in Fig. 2(b). It shows the surface morphology of the composite film. The wrinkles visible in the FESEM image signify that the coated film is multilayered. The XPS results of GO and RGO obtained after reducing for 15, 30 and 60 min are shown in Fig. 3. The 15 minutes reduced GO shows no significant decrease in the intensities at 286.5 and 288.4 eV as compared to GO as shown in Fig. 3. This implies that the amount of functional groups present in RGO after reducing for 15 min was almost same as that of GO. But it was observed that the film got transferred properly after reducing it for 15 min whereas GO film could not be transferred properly owing to its hydrophilic nature. For the samples prepared after reducing GO for 30 and 60 min, the C–O and C = O peaks got significantly reduced as

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Fig. 3. XPS results of (a) GO (b–d) thermally reduced GO for (b) 15 min (c) 30 min (d) 60 min.

Fig. 4. Current generated of 15 min reduced GO/PMMA generator placed over 1.5 mm spaced bottom electrode arrays for five different volumes of water. Fig. 2.

(a) AFM image of GO (b) FESEM image of RGO/PMMA film.

can be seen at 286.5 and 288.4 eV respectively as is also evident from Fig. 3. B. Nanogeneration Results The fabricated device as shown in Fig. 1 was then probed and a bias of 1V was applied at the bottom electrodes. The stability of the initial current before introducing water droplets to the sample was observed and then five different volumes of water (25–150 μL) were dropped over the samples and then removed. The current increased from hundreds of pA to hundreds of nA when water was dropped on the sample. The nanogeneration result is shown in Fig. 4. Fig. 4 signifies that the current increased with increase in volume of water. Also, the current of the device decreased to its initial value when the water was removed/ desorbed from the device. It was observed during our experiments, that the current increased steadily when 25–75 μL of water was dropped. But the generated current tends to saturate when the volume of the water was increased beyond 75 μL. The probable reason for such

behaviour of the device is with increase in volume of water after certain value, the RGO/PMMA film does not get enough space to bend further. And the bending of the film is significantly important for the current generation as has been explained in details in later section. The performances of the device were examined by varying different parameters of the device and the material. In order to check if there is any significant role of the spacing between the metal stripes acting as bottom electrodes in current generation, the current generated by the RGO/PMMA generator were observed by preparing the samples on three different spaced bottom electrode arrays as well as on continuous metal electrodes which did not have any gaps in between. The comparative result of the same is shown in Fig. 5. Very less current generation was observed in the device prepared by transferring the RGO/PMMA film over the flat and continuous aluminium bottom electrode. Also, the generated current in the continuous bottom electrode was almost same as that generated in 0.5 mm spaced metal striped based device. But it was observed that the current increased as the space between

GHOSH et al.: REDUCED GRAPHENE OXIDE-BASED PIEZOELECTRIC NANOGENERATOR WITH WATER EXCITATION

Fig. 5. Comparative current generations as observed in 15 min reduced GO/PMMA generators transferred over bottom electrodes having three different spaces in between (0.5–1.5 mm) and on continuous aluminium electrode for three different volumes of water (50–150 μL).

Fig. 6. (a) Current generated in directly coated RGO/PMMA film over metal electrodes for three different volumes of water (b) Comparative current generation results of 15, 30 and 60 min reduced GO/PMMA generator placed over 1.5 mm bottom electrode against five different volumes of water (c) current generated only PMMA film based nanogenerator (d) current generation in PMMA/RGO/PMMA film coated over 250 nm thick metal stripes with 1.5 mm distance in between for four different volumes of water (25–100 μL).

the electrodes array increased. The reason for this mechanism has been explained in the later section. The protruded bottom electrode structure was important for nanogeneration as the space between the electrodes played a vital role. To prove this, we also prepared a device in which the RGO film was directly drop casted over 1.5 mm spaced bottom electrode. Then the PMMA was spin coated and the top electrode was evaporated. Effectively, the gaps between the bottom electrodes were absent as RGO filled in the space. The device was then subjected to water excitation and the generation results are shown in Fig. 6(a). The initial current of the device fabricated by directly coating RGO film (and not by transferring) was in range of few nA, which is quite higher than what we got in earlier cases (tens to hundreds of pA). When water was dropped over this sample, the current increased a bit from 2 to 11 nA against 50 μL water

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but this increment is very less than what we got in the devices fabricated by transferring the films. This once again prompted that current generation in our samples were majorly assisted by the air gap between the metal stripes acting as bottom electrodes. RGO contains functional groups and defect sites. The amount of functional groups can be modulated by changing the time of reduction. To gain an insight to the effect of functional groups in amount of current generated, GO was reduced thermally for three different times. Since the optimum generation was observed 1.5 mm spaced bottom electrodes, three samples were prepared by transferring the different time reduced RGO/PMMA samples over 1.5 mm spaced electrode array. The comparative current generation result is shown in Fig. 6(b) which signifies that the presence of functional groups is very important for nanogeneration. As the amount of functional groups decreased (which is also supported by our XPS results), the magnitude of the generated current also gradually got reduced. This observation led us to fabricate a GO/PMMA device instead of RGO/PMMA as more functional groups are available in GO than RGO. But the GO film didn’t get transferred properly owing to its highly hydrophilic nature. Although the nanogeneration was carried out on GO/PMMA film but the generated current was very less (results not shown) compared to that observed in RGO/PMMA films. This is because the area of GO/PMMA transferred film was very small compared to the RGO/PMMA films. Further, it is a well known that the polymers show swelling effect in presence of water/humid air [19]. Due to this the resistance of the polymer decreases in presence of water. As PMMA layer is directly exposed to the water droplet, so, to ensure that the current generation we observed was due to piezoelectric effect produced by RGO/PMMA hybrid film and not due to swelling effect of PMMA film, a device was fabricated. Here only PMMA film was transferred over 1.5 mm spaced bottom electrodes and then top electrode was thermally evaporated. The generation result of the hence fabricated device is shown in Fig. 6(c) which clearly shows that there was no generation when only PMMA layer was used as nanogenerator. So, the current generated in the aforementioned devices are due to the RGO/PMMA hybrid layer. The above observation also led us to a conclusion that the RGO-bottom electrode contact also played a vital role in the current generation demonstrated by our samples. Although the structure of the device reported by Que et al. on nanogeneration by GO based films, included a thin insulator layer (teflon film) in between the GO layer and the bottom electrode (PEDOT: PSS) [15] but in our case the observation was different. A PMMA/RGO/ PMMA multilayered film was transferred over 1.5 mm spaced metal striped bottom electrodes and then the generation was observed by evaporating the top electrode by thermally evaporating aluminium. The current generated in the device fabricated by transferring PMMA/RGO/PMMA film as shown in Fig. 6(d) was found to be lesser than what was demonstrated by 15 min reduced GO/PMMA film transferred over 1.5 mm spaced striped aluminium electrodes. This signifies that the contact between the RGO film and the bottom aluminium electrode is very crucial for better nanogeneration.

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Fig. 7. Comparison between the current generated by RGO/PMMA nanogenerator at 0 and 1 V bias for five different volumes of water.

Fig. 8.

Schematic representation of nanogeneration mechanism.

All the results discussed above were observed after applying a bias of 1 V to the bottom electrodes. But to check whether the device can generate current without any external biasing, the experiment with 15 min reduced GO and 1.5 mm spaced bottom electrodes was carried out at 0 V bias and the results is shown in Fig. 7. The above result ensures that even if there is no bias the device can generate current, i.e., the device can operate solely on self-powered mode but the magnitude of current generated in self-powered mode would be remarkably lesser than that generated with 1 V bias. C. Nanogeneration Mechanism The nanogeneration observed in our samples is due to piezoelectric effect. When water is not dropped over the nanogenerator, i.e., the sample is in rest condition, the charges are balanced inside it and so, the net dipole moment is zero. When water is dropped over the sample, it exerts a force on the RGO/PMMA film which impelled the film to bend. This bending of the film generated a stress in the film. Due to which charge imbalance gets created thus generating an effective dipole moment. This in turn results in flow of an external current in the circuit and thus, nanogeneration was observed. The mechanism has been schematically represented in Fig. 8. It was observed that the current generation increased with increase in the gap between the bottom electrodes as shown in Fig. 5. This happened because the space for RGO/PMMA film to bend increased with increase in spacing between the bottom electrodes. And charge imbalance created got increased and thus the generated external current also got enhanced. Similarly, the devices prepared by directly coating RGO over the 1.5 mm

spaced electrodes and also the device which had flat and continuous bottom electrode demonstrated very less current generation because the RGO/PMMA film did not get enough space to bend when water was dropped over it. In our devices, we observed a pulse of current generated when the water was dropped over the RGO/PMMA film. And the duration of the pulse was few tens of seconds. This raised a concern on the mechanism of current generation in our devices. But it was found in recently reported literature [16], and also in a review paper [20] that a pulse of current can get generated in piezoelectric nanogenerators. Further, to ensure that the external current measured is due to piezoelectricity and not due to change in resistance of the film due to ionization of RGO or PMMA in presence of water, the open circuit voltage (Vo c ) of the sample was measured. It was observed that when water was not dropped over the sample, Vo c across the sample was measured to be 0 V. But when 50 μL water was dropped over the sample, the Vo c was measured to be around 150 mV which further increased to ∼200 mV when 100 μL water was dropped over it. Thus the maximum generated power (I × Vo c ) in our sample was 1.1 × 10−7 Watts which was found be much more than the reported values in recent literature [10], [15], [16]. Such explanations and calculations regarding nanogeneration due to piezoelectricity were also carried out in recent reported work [21]. This enhanced generation in our sample is attributed to the space between the metal stripes acting as bottom electrode. Due to available space, the RGO/PMMA film could bend more, thereby creating a large amount of charge difference and hence, the current generated was also large. IV. CONCLUSION RGO/PMMA based hybrid film was employed as piezoelectric nanogenerator. The role of the spacing between the bottom metal layers was demonstrated. It was observed that with increase in the gap between the metal stripes of the bottom electrode of the device, the current generation increased. The optimum performance was observed in 1.5 mm spaced metal striped device. Also, it was shown that the functional groups present in RGO assisted the current generation. That’s why the maximum current was observed in 15 min reduced GO which consisted of maximum amounts of functional groups as was evident from XPS results. Also, the piezoelectric effect and current generation mechanism was studied and analysed in details. It is believed that this work would lead to development of cost effective and very environment friendly nanogenerator to power our electronic devices which would generate power by absorbing moisture and humidity from the atmosphere. ACKNOWLEDGMENT The authors would like to thank DST-FIST Lab (in Department of Physics, IIT Kharagpur), MEMS and Microelectronics Lab (in E&ECE Department, IIT Kharagpur) and Central Research Facility, IIT Kharagpur for providing us XPS facility, Probe Station and SPA, and AFM and FESEM respectively. The authors (R. Ghosh and M. Pusty) have contributed equally to this work.

GHOSH et al.: REDUCED GRAPHENE OXIDE-BASED PIEZOELECTRIC NANOGENERATOR WITH WATER EXCITATION

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Ruma Ghosh received the M.Tech. degree in nanotechnology from the Maulana Azad National Institute of Technology, Bhopal, India, in 2011. She is currently working toward the Ph.D. degree in the Department of Electronics and Electrical Communication Engineering, Indian Institute of Technology Kharagpur, Kharagpur, India. She is carrying out her research work under the supervision of Dr. P. K. Guha. Her research interests include chemo-resistive sensors and graphene-based electronic devices.

Manojit Pusty received the B.Tech. degree in electronics and instrumentation engineering under West Bengal University of Technology, Kolkata, India, in 2011. In 2014, he received the M.Tech. degree in electronics and communication engineering from the Indian Institute of Technology Kharagpur, Kharagpur, India. He is currently working toward the Ph.D. degree at the Centre for Material Science and Engineering, Indian Institute of Technology Indore, Indore, India. His research interests mainly include nano energy harvesting devices and biological applications of nanomaterials.

Prasanta Kumar Guha received the Ph.D. degree from the University of Cambridge, Cambridge, U.K., in 2008. He has been working as an Assistant Professor in the Department of Electronics and Electrical Communication Engineering, Indian Institute of Technology, Kharagpur, Kharagpur, India, since 2010. His research interest includes energy harvesting using nanomaterials, nano electronics, chemical sensors, sensor integration with CMOS and flexible platform, and nano materials.