ZnSnO3

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A Facile Approach to Develop Highly Stretchable PVC/ ZnSnO3 Piezoelectric Nanogenerator with High Output Power Generation for Powering Portable Electronic Devices Sarbaranjan Paria, Sumanta Kumar Karan, Ranadip Bera, Amit Kumar Das, Anirban Maitra, and Bhanu Bhusan Khatua Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02172 • Publication Date (Web): 13 Sep 2016 Downloaded from http://pubs.acs.org on September 18, 2016

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Industrial & Engineering Chemistry Research

A Facile Approach to Develop Highly Stretchable PVC/ZnSnO3 Piezoelectric Nanogenerator with High Output Power Generation for Powering Portable Electronic Devices Sarbaranjan Paria, Sumanta Kumar Karan, Ranadip Bera, Amit Kumar Das, Anirban Maitra and Bhanu Bhusan Khatua* Materials Science Centre Indian Institute of Technology Kharagpur, Kharagpur 721302, India

*Correspondence to Dr. B.B. Khatua (email: [email protected]) Materials Science Centre, Indian Institute of Technology, Kharagpur-721302, India Tel.:91-3222-283978 1

ACS Paragon Plus Environment


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Abstract Harvesting mechanical energy from ambient environment with a piezoelectric nanogenerators (PENG) consisting of piezoelectric nanoparticles (NPs) and flexible polymer has drawn a considerable attention for developing self-powered electronic devices. Here, a flexible, lead free, solution processable PENG, composing piezoelectric ZnSnO3 NPs and plasticized PVC was fabricated by simple solution casting method. The nanogenerator shows a VOC of ~40 V, a ISC of ~ 1.4 µA and an overall power density more than ~3.7 µW cm-2 at 35 wt% loading of ZnSnO3, and these values are the highest than the values reported so far in the literature on the cubic ZnSnO3 based nanogenerator. We utilized the generated power for powering 7 different color LEDs without any external energy storage unit. Also, the nanogenerator could charge a commercial capacitor (2.2 µF) to ~ 6.7 V in ~ 129 s which can be used for powering wrist watch, mobile LCD screen and calculator.

Keywords: Mechanical energy, Piezoelectric nanogenerators, PVC, Cubic ZnSnO3, Power density

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1. Introduction Over the past few decades, waste energy harvesting from ambient environment and biological movements has been received a considerable attention as an attractive alternative for the problems from the environmental disorder like global warming, depletion of ozone layer, harmful emission of gases and as well as energy crisis.1,2 Among the various waste energy sources, mechanical vibrations and movements are one of the most promising candidates for energy production because of their high power density (0.01 to 10 mW cm-2), easy of power generation, and over the broad frequency range for power generation.3,4 For the sake of mechanical energy harvesting, piezoelectric nanogenerator (PENGs)5 and triboelectric nanogenerators (TENGs)6 have been well established. Between these two types of nanogenerators, TENGs have exhibited high energy conversion efficiency up to ≈55% 7 and high output voltage upon regular contact of the two electrification surfaces.8 However, their large size and the use of polymers as electrification materials make them unfavorable for long term uses as they suffer from wear and tear at the two contact surfaces.9,10 On the other hand, PENGs compose stacked layers without friction between the layers as they involve the piezoelectric material or piezoelectric composite in between the two electrodes. Hence, piezoelectric nanogenerators are suitable for powering micro/nanosystems, wireless transmitter, biomedical devices and wearable electronic devices. Composite based hybrid piezoelectric nanogenerators (HP-NGs) where several nanostructured piezoelectric materials like BaTiO3, ZnO, KNbO3, ZnSnO3, NaNbO3 and LiTaO3 have been used as active piezoelectric material and polymer acts as flexible matrix

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important role to solve the fragile problem of the nanogenerator. These polymer supporting nanogenerators are very much attractive because of their easy processing, large scale production, cost effectiveness and mechanical robustness. However, advanced technologies are very much 3



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affectionate to get a goal of having lead free groups with varied applications because of the toxic nature of the lead. So, it is very much important to develop a piezoelectric composite that can be used for self-powered lead free nanodevice or system.17,18 A continuous effort has been made in establishing the noncentrosymmetric (NCS) oxide groups for the development of piezotronic devices, as noncentrosymmetric oxides shows the spontaneous polarization effect.19 Among such oxides, ZnSnO3 has been received significant focal point of research interest because of its high piezoelectric and ferroelectric properties as well as high thermal stability up to 700 ºC.20 It displays exceptional polarization of ~59 C cm-2 along c-axis which makes it most promising agent for piezoelectric applications than other lead free piezoelectric materials like ZnO (~5 C cm-2), BaTiO3 (~6C cm-2), and KNbO3 (~23C cm-2). Its noncentrosymmetric property is highly responsible for high ferroelectricity, piezoelectricity, pyroelectricity, and second order nonlinear optical behavior.20 For the polymer supporting nanogenerators, several research groups have used polymers like polyvinyledene fluoride (PVDF), 11,23

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polycarbonate (PC),24 PVC,25 and Silk fibroin,26 etc.

polydimethyl siloxane (PDMS), It is noteworthy, flexible and

mechanically strong piezoelectric polymers like PVDF and its copolymers having high piezoelectric coefficients are the good candidate for PENGs.27,28 In such cases, both polymeric phase and nanostructured filler help to get high output voltage and short circuit current. But their negative piezoelectric coefficient (d33) limits their use in piezoelectric composite fabrications. However, in most of the cases PDMS is the best one because of its some attractive properties, e.g., flexibility, transparency, cost effectiveness, biocompatibility, low Young’s modulus (≈300800 kPa).29 But it has a drawback; it is so viscous that the piezoelectric filler could not be uniformly dispersed. So, for the homogeneous dispersion of the filler in the dense polymeric matrix, filler need to be surface modified or third component need to be added. For example,

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Alam et al. used multi walled carbon nanotube dispersant for the zinc stannate as well as conducting filler.30 For some cases mixed solvent also plays an important for the dispersion of the nanofiller in the polymer matrix. For example, Peng Han et al. used ethyl alcohol for dispersing the PLZT in the PVDF matrix.31 This process of using low boiling liquid is very much acceptable for the dispersion of nanostructured materials because on evaporation the solvent can completely be removed from the casted film. But, there may be possibility for small solvent molecules remain trapped inside the polymer, which in turn deteriorate certain properties of the composite. So, it is urgent need for the replacement of the PDMS, as piezoelectric properties largely depends on the dispersion of the filler in the polymeric matrix. For example, Stefano Stassi et al. have used porous PC matrix to align the ZnO nanorod to get high output performance from the nanogenerator.24 Nevertheless, porous PC are not extensively used because of its high cost. Consequently, extensively used polymer would be the better option for cost effective and large scale production of the nanogenerators. For instance, Min Zhang et al. used PVC micro fiber as the flexible matrix for dispersion of BaTiO3 nanowire.25 However, no such research reports have been found in the literature where people have used plasticized PVC as polymeric matrix for uniform nanoparticle dispersion to get high output performance from the piezoelectric nanogenerator. Here, in this article, we have synthesized ZnSnO3 nanocubes by a simple aqueous solution method at low temperature and built a lead-free nanogenerator to develop an environmentally friendly flexible energy harvester without any toxic dispersant. The fabricated ZnSnO3 nanocube based PENG exhibits recordable open circuit voltage of ~40 V and short circuit current of ~1.4 µA at 35 wt% loading on periodic fingertip compression, without any electrical poling treatment. Again, the harvested energy sources can be applicable for driving the

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liquid crystal display (LCD), light emitting diode (LED), commercial capacitor charging etc. This hybrid nanogenerator also exhibits high mechanical robustness, excellent power generation with outstanding reversibility. 2. Experimental 2.1. Materials Analytical reagent (AR) grade zinc acetate (ZnAc2, 2H2O) and sodium stannate (Na2SnO3. 3H2O) and tetrahydrofuran (THF) were purchased from Merck Chemicals, India. Commercially available plasticized PVC sheet and Al foil were procured from India. PDMS (Sylgard 184) was purchased from Dow Corning Corp., USA. All chemicals were used as received without any additional treatment. DI water was obtained from the Merck Millipore. 2.2. Synthesis of cube-like ZnSnO3 nanoparticles A simple aqueous solution method14 was followed for the preparation of single crystalline zinc stannate (ZnSnO3) nanocubes. By lowering the synthesis temperature and increasing reaction time, ZnSnO3 nanoparticles were obtained, consistent with the literature.32 In a typical procedure for the preparation of ZnSnO3 nanoparticles, 5 mM of zinc acetate dihydrate (ZnAc2, 2H2O) was dissolved in 100 ml DI water with effective stirring by a teflon coated stir bar at room temperature. In another beaker same amount of sodium stannate (Na2SnO3. 3H2O, 5 mM) was added in 100 ml DI water with stirring till the salt dissolve completely. Then, sodium stannate solution was mixed drop wise with zinc acetate solution under constant stirring for eight hours at 60 ºC. After eight hours, the precipitate was centrifuged and, washed with DI water and ethanol for several times to eliminate any impurities in the final product. Then, the precipitate was collected and dried in hot air oven at 100 ºC for overnight. Finally, the obtained dried white product was used for the fabrication of nanogenerator and subsequent characterizations. 6


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2.3. Preparation of nanocomposite film First, PVC sheet was cut into small pieces then 3 g of the small PVC sheets were dissolved in 40 ml of THF with the magnetic stirrer for 1 h at 60 ºC. In another beaker, required amount of zinc stannate powder was dispersed in 10 ml THF by means of ultrasonication for 30 min. Then, the nanoparticles dispersed solution was mixed with the polymer solution and kept for mixing by magnetic stirrer for another 2 h to get uniform mixing. After that, the solution was poured onto a petri dish and kept overnight for solvent evaporation. Composite films with different filler loadings were also prepared following the same method. 2.4. Fabrication of the nanogenerator The PVC-ZnSnO3 composite nanogenerator was fabricated by sandwiching the composite film with the Al foil as top and bottom electrode. Commercially available Al foil was used to reduce the cost of fabrication and to facilitate the scale up of the production. Finally, the composite nanogenerator was encapsulated by PDMS (elastomer to curing agent ratio was 10:1 by weight) layer to make the device mechanically robust. 3. Characterizations For structural investigation of the nanostructured ZnSnO3, X-ray diffraction (XRD) has been used with X’Pert PRO diffractometer (PANalytical, Netherlands) with nickel-filtered CuKα (λ = 0.15404 nm) at a scanning rate of 0.25° min−1. Field emission scanning electron microscopy (FESEM, Carl Zeiss-SUPRA40) and high resolution transmission electron microscopy (HR-TEM, JEM-2100, JEOL, Japan) were used for the morphological analysis. Energy dispersive X-ray spectroscopy (EDX) assembled with FE-SEM was carried out for the elemental analysis of the nanomaterials. The measurement of output voltage and short circuit current during finger imparting were performed by oscilloscope (ROHDE & SCHWARZ, RTM 2022, 200MHz, 5 7



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GSa/s) and Keithley (model 4200-SCS), respectively. Dielectric constant measurement was carried out with a computer controlled precision impedance analyzer (HIOKI 3532-50 LCR HiTESTER) under the application of an alternating electric field along the sample in the frequency range of 42 Hz-0.5MHz. The capacitor (power of 2.2 µF) charging and discharging performance was carried out with a standard two electrode system CHI 760 D work station. A FLUKE 115 true rms multimeter was used during the practical implementation experiment. Tensile behaviors of the nanocomposite films were measured by universal testing machine (UTM, Tinius Olsen H 50 KS). 4. Results and Discussion 4.1. Structural analysis of synthesized ZnSnO3 Figure 1(a) represents the X-ray diffraction (XRD) pattern of the as prepared ZnSnO3 nanocubes. As observed, all the diffraction peaks in figure 1(a) were well matched with the standard Joint Committee on Powder Diffraction Samples (JCPDS card no. 11-0274).32 It indicates a face centered perovskite structure corresponding to cubic ZnSnO3 with space group R3c. No obvious peaks from other phases can be detected; suggesting the formation of pure phase ZnSnO3 structure in the as prepared sample. The crystallographic structure of ZnSnO3 nanoparticle is shown in the inset of figure 1(a). The crystallographic structure, shown in inset of figure 1(a), consists of two octahedral frameworks of ZnO6 and SnO6. The lower side of the crystallographic structure is responsible for ZnO6 octahedron framework while the upper one is for SnO6. The two octahedral frameworks share both edges and faces with the adjacent octahedra. Each octahedron has three short bonds and three long bonds. In ZnO6 cluster, three long bonds are of ~0.2308 nm at upper side and three short bonds of ~0.2040 nm at lower side. In contrast, SnO6 has three long bonds at lower 8


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side of ~0.2093 nm and three upper short bonds of ~0.2008 nm. Therefore, it is obvious that there will be displacement of the metals ions along c-axis from their respective octahedron center. Jyh Ming Wu calculated the displacement of Zn ion in ZnO6 is 0.5 Å, whereas that of Sn ion is 0.2 Å in SnO6 cluster.33 Thus, a spontaneous polarization is developed along the c-axis which may give rise piezoelectricity in this material.

Figure 1: (a) XRD pattern of the ZnSnO3 nanocubes synthesized at 60 ºC. (b) FE-SEM image of the as synthesized ZnSnO3 nanocubes. (c) HR-TEM image of a single nanocube (inset) and

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selected area diffraction pattern of the same crystal. (d) HR-TEM image of the cryofactured surface of the nanocomposite. The surface morphology of ZnSnO3 nanoparticles, synthesized by aqueous solution method, is characterized by FE-SEM and shown in figure 1(b). The FE-SEM image of ZnSnO3 clearly reveals the formation of cubic shaped ZnSnO3 nanostructure. FE-SEM image of ZnSnO3 powder indicates the size length variation of the nanocubes in the range of 50-275 nm (shown in Supporting Information, figure S1). The energy dispersive X-ray spectroscopy (EDS) (shown in Supporting Information, figure S2) study indicates that the composed elements in ZnSnO3 are zinc, tin and oxygen. An individual magnified ZnSnO3 nanocube image of smooth surface and ruled edge, as shown in figure 1(c), reveals that the well-defined ZnSnO3 nanostructures are of side length ~200 nm. The corresponding selected area diffraction pattern of a single nanocube, as shown in figure 1(c), exhibits the periodic diffraction spots supporting single crystalline nature of the nanocube. The size of the nanoparticles plays an important role for exhibiting high power output from the nanogenerator. As reported elsewhere,34 the effective piezoelectric response from the nanoparticles increases with decrease in size of the nanoparticles. As the size of the nanoparticles decreases, the surface to volume ratio increases which contributes to the piezoelectric coefficient of the piezoelectric material. Experimental studies and atomistic simulations on piezoelectric materials have shown that the piezoelectric coefficient experiences dramatic increase when size decreases to a nanometer scale.35,36 Thus, we assume that the size of the ZnSnO3 nanocubes may be one of the most effective parameters for the high output performance from the nanogenerator. The microtomed surface HR-TEM and FE-SEM images of the nanocomposite are shown in figure 1(d) and figure S3 (Supporting Information), respectively. From figure 1(d) figure S3 it can be clearly seen that the ZnSnO3 nanocubes were

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well and uniformly distributed in the soft PVC matrix. As reported earlier, the improvement of piezoelectric output from the nanogenerator depends on the extent of dispersion of nanoparticles in the polymer matrix.30,37 When the polymer matrix is of high viscous then the nanoparticles tend to be localized at the bottom surface of the polymer matrix owing to the agglomeration of the nanoparticles that lowers the piezoelectric performance.

Figure 2: (a) Schematic diagram of fabricated PENG. (b) Photograph of fabricated PENG device. (c) Optical images of the PVC-ZnSnO3 composite film at different flexible modes. Therefore, a low viscous polymeric solution is favorable for the homogeneous dispersion of the nanoparticles. In our study, PVC solution of low viscosity has been used as polymer matrix to distribute the nanoparticles uniformly, and to obtain greater piezoelectric output from the PENG. A schematic for the fabrication of piezoelectric nanogenerator in this study is shown in figure

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2(a). The middle layer composed of PVC matrix and ZnSnO3 nanocubes acts as active layer in the device. A photograph of a fabricated PENG device with the active nanocomposite area of 15 cm2 (length 5 cm and breadth 3 cm) is shown in figure 2(b). The flexibility of the nanocomposites can be well understood from figure 2(c). It shows the optical images of the nanocomposite at different states, rolling, bending, stretching, twisting by human fingers, demonstrating the flexibility of the nanocomposite. 5. Piezoelectric Property For the measurement of piezoelectric power generating performance from the PENG, a human finger imparting process has been used. When the lead free ZnSnO3-PVC PENG is repeatedly deformed (compressing and releasing) by the human finger with a frequency of ~2 Hz, the nonuniform open circuit voltage and short circuit current are developed. To get the high output performance, we have fabricated the PENGs with various concentrations of ZnSnO3 with the same thickness and measured the output performance under the same conditions. Pure PVC film did not exhibit noticeable voltage and current outputs under finger imparting. A large output voltage of ~40 V and short circuit current of ~1.4 µA were obtained from the nanocomposite with 35 wt% ZnSnO3 loading, which is ~40 times higher than that of pure PVC film. This high power generating ability is quite stable, even after ~3000 cycles of human finger imparting. We also carried out the experiment with the nanocomposites of higher ZnSnO3 concentrations. We found that beyond 35wt% ZnSnO3 loading, the output voltage decreases with increase in the amount of ZnSnO3. This is due to the decrease in insulation of the nanocomposite on increasing the amount of ZnSnO3. On increasing the amount of ZnSnO3, the distances among the nanocubes decrease and resulting in the thinner polymeric layer on the nanocubes which leads to the electrical breakdown of the nanocomposite .14 On the other hand, the high amount (above 35

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wt%) of piezoelectric ZnSnO3 nanocubes inside the nanocomposites can lead to the deterioration of electromechanical coupling effect due to agglomeration of the filler, which results in low output voltage production.38 As a result of this, the nanocomposite with higher loading than 35 wt% generated low output voltage than that for composite with 35wt% ZnSnO3 loading. Hence, the optimized concentration of the ZnSnO3 in the nanocomposite is 35 wt%.

Figure 3: The harvested (a) open circuit voltage (forward connection) (b) short circuit current signals from the PENG device at 35wt% loading. (c) The polarity switching tests under finger imparting (reverse connection). (d) The variation of output voltage and current generated from the PENG with the ZnSnO3 nanoparticles concentration.

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A polarity switching tests were also performed to check the measured output voltage originated from piezoelectric effect of the device. The schematic diagram for the device connection with the measurement system is shown in figure 3(a) (forward bias) and figure 3(c) (reverse bias). An opposite signal was observed under reverse connection as in figure 3(c). The device delivered an average peak-to-peak output voltage of ~40 V, both in the forward and reverse connections. From the figure 3(a) and 3(c), it is clear that there is no noticeable variation of the output voltage production, as the cubic nanofillers in the nanocomposite experience the almost same amount of strain rate under forward and reverse connections. A snapshot has been provided in the figure S4 (Supporting Information) to prove the regularity in applying pressure by human finger. Again, to check the capability of the PENG of power generating performance a sewing machine has been used as a pressure source. The generated voltage, current, and the variation of voltage and current with the filler loadings have been given in figure S5 (Supporting Information, the sensitivity of PENG under sewing machine is shown in the video file 2 in the Supporting Information). We have found that the PENG can generate a VOC and ISC of ~4.4 V and ~283 nA, respectively under the periodic applying pressure with the sewing probe. As the diameter of the probe is very small (~5.45 mm), so the pressure exerted by the sample is very less i.e. effective area is very less compared to human finger, resulting in low output performance. There are two types of asymmetries: one is the unequal heights among positive voltage peaks or among negative voltage peaks; another is in the height difference in positive and negative peaks. The first case is observed when there is a change in applied pressure for each tapping, as in case of the present study where the pressure was developed on the system through finger imparting. So, the system feels unequal stretching and compressing for each tapping, resulting in unequal heights either for positive peaks or negative peaks. In the second case,

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unequal heights in positive peaks and negative peaks entirely depend on the compression and decompression phenomenon. Thus, while the system undergoes fast compression under tapping, it fails to revert back completely into its original shape upon releasing the pressure (in a short time frame) followed by further tapping, as the ability of a material to revert back to its original shape strongly depends on the material’s property (elasticity). As a result, the negative piezoelectric pulse is much less in height than the positive piezoelectric pulse.39-40 For investigation of the effect of ZnSnO3 nanocube concentrations on the output voltage and current generation performance of the PENG, we fabricated a series of nanocomposites with different nanocube concentrations (1, 5, 15, 25, and 35 wt% of ZnSnO3), as shown in figure 3(d). Piezoelectric output voltages of ~1, 2, 8, 17, 32 and 40 V were achieved from the nanogenerator device for the loading of 0, 1, 5, 15, 25, and 35 wt% of ZnSnO3, respectively. The short circuit current also increases with increase in nanocube concentrations and reaches a maximum value of ~1.4 µA at 35 wt% of ZnSnO3 loading, giving a power density of ~3.7 µW cm-2. This enhancement can be explained by the fact that with increase in nanocube concentration, the piezopotential of the nanocomposite increases linearly. With increase in ZnSnO3 concentrations in the soft PVC matrix, the gaps among the ZnSnO3 nanocubes decrease. It is well established that the interfaces in heterogeneous system gives rise interfacial or Maxwell-Wagner-Sillars polarization, which is responsible for the dielectric constant of the nanocomposite.41-42Thus, with the increase in concentration of the nanocubes, the interfacial polarization between the nanocubes and the PVC matrix becomes more prominent which attributed to the high dielectric constant of the nanocomposite (shown in figure 4(a)).

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Figure 4: (a) The variation of dielectric constants of the nanocomposites with frequency. (b) Performance of the PENG under different operating frequencies. As can be seen (figure 4(a)), the dielectric constant (formulae are provided in Supporting Information) of the nanocomposite remains almost similar to that of pure PVC at 1 wt% ZnSnO3 loading, and then increases remarkably with gradual increase in nanocubes loading up to 25 wt% of ZnSnO3. This increase in dielectric constant with the filler loading is associated with the decrease in filler-filler distances that promotes more dipole-dipole polarization in the nanocomposites. However, above 25 wt% loading of nanocubes, a marginal increase in dielectric constant was observed when the nanocomposite was formulated with 35 wt% of ZnSnO3. We assume that, the average distance among the fillers does not decrease significantly above 25 wt% ZnSnO3 loading, and thus the extent of further increasing dipole-dipole polarization become less significant at 35 wt% of ZnSnO3 loading. The output voltage, which is related to the dielectric constant, also reveals similar trend of increasing with the filler loading in the nanocomposites (figure 3(d)). When the concentration of the ZnSnO3 nanocubes reached 35wt%, the nanocubesnanocube distances become reasonably narrow. As a result, dipole polarization between the nanocubes is maximized, resulting in the high dielectric constant, which in turns gives high 16


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polarization. Thus, this increase in polarization favors more and more piezoelectric potential development across the electrode which is responsible for the high output performance of the PENG.14,38 Again, we have measured the Young modulus of the nanocomposites from the stressstrain curves provided in the figure S6 (Supporting Information). It can be seen from the figure S6 (Supporting Information) that the Young modulus of the nanocomposites increases with increase in the amount of ZnSnO3 in the nanocomposite. This is due to the increase in contact surface areas between the nanocubes and polymer matrix. When the concentration of nanocube increases up to 35 wt%, the Young modulus of the nanocomposite not as increases as for 5 to 25 wt% and the dependence of Young modulus on output performance reported elsewhere.43,44 In this situation we could expect that the surface piezopotential is the driving factor for determining the output voltage. Thus, with increase in concentration of the nanocubes, the tendency of migration of the nanocubes from the bulk to the surface increases.45 As a result, those nanoparticles, near to the surface, experience a high amount of force under applying force and hence the surface potential increases with increase in the amount of nanoparticles.45 So, with increase in concentration of the ZnSnO3, more and more number of ZnSnO3 would be the near to the surface and under applying pressure the resultant potential will be increased i.e. piezopotential increased, which in turn give rise high output performance. 5.1. Frequency Variation As the mechanical energy from the ambient environment largely varies and is irregular, it is required to study the relationship between the output performance of the PENG and different frequencies of applying force. For this investigation of the output performance of the nanogenerator, a human finger imparting test with imparting area of ~15 cm2 was conducted using several frequencies of tapping. The output signal of the PENG was measured through

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repeatedly compressing and releasing at a same frequency range for human motion (1-10 Hz).46 The time dependent generation of voltage from the nanogenerator with 35 wt% nanofiller loading under the constant tapping force of ~18.5 N (see Supporting Information) over the full frequency range of human motion is shown in figure 4(b). As shown in figure 4 (b), the output voltage slightly increases with the increase in frequency of human finger tapping. This increase in output voltage with the imparting frequency can be explained considering the change of impedance which decreases with increase in strain rate.47 This can also be explained by the fact that under the high frequency of deformation the developed positive and negative charges inside the nanogenerator are less naturally neutralized, resulting in more accumulation of charges on the electrodes.48 Again, when we increase the frequency, the electrons in the external circuit experience less time to balance the piezoelectric potential, and thus give rise to higher current. As the output voltage is the product of current and external resistance, the voltage will be higher correspondingly.49This experiment shows the possibility of using the PENG at lower frequency range of motions that could be harvested in human and ambient environmental applications. 6. Mechanism of the Power Generation The schematic diagram for the mechanism of power generation from the PENG is shown in figure 5. It has also been proved that under a longitudinal compressive stress the electric dipoles in the crystal get oriented effectively along a single direction due to an effect, called stress induced poling effect.50 This effect develops piezoelectric potential (across the surface), which enables to run the free electrons from top and bottom electrode through the external circuit. To balance the developed piezoelectric potential, these electrons then accumulate at the interfaces of the nanocubes and electrodes. And, the piezoelectric potential is eliminated when the nanocubes decompress after removing the imparting load (object) with the result of

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accumulated electrons stream going back through the external load. Hence, both positive and negative current and voltage peaks can be observed under imparting. Higher and smaller peaks were also observed when compression occurred more rapidly than decompression and the magnitude of the both output voltage and current strongly depend on the compression frequency and applied pressure.

Figure 5: The working mechanism of generating voltage from the PENG device. To eliminate the possible triboelectric effect from direct finger tapping, the measurement of output performance of the nanogenerator was performed using PVC gloves, assuming exchange of small amount of charges between the PVC and PDMS, resulting in not inevitable output voltage.51 Thus, the developed output performance from the nanogenerator is a result of piezoelectric effect of the cubic ZnSnO3 nanoparticles embedded in the polymer matrix. This has also been reported previously under similar condition (human finger imparting), where researchers have used PDMS as an encapsulating layer of the nanocomposite films.30,52 7. Capacitor Charging Discharging Performance: Furthermore, one additional process is required for the PENG to be used as an actual power source. In order to support the charge storage capability using human finger imparting,

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rectified electrical output from 35 wt% nanocubes loaded PENG was stored into a capacitor of 2.2 µF power.

Figure 6: (a) Rectified voltage signal under the basic condition of pulse signal input. (b) The accumulated voltage across a single capacitor (power of 2.2 µF) charged by PENG with a same frequency (inset shows the schematic of circuit diagram). (c) The enlarge view of the drawn circle in figure 6 (b). (d) Performance characteristics of the PENG: Dependence of output voltage and power on external load resistance (schematic of the circuit diagram in inset). Figure 6(a) shows the rectification of the generated voltage from the PENG through a four probe rectified bridge. The typical charging and discharging behavior of the capacitor is shown in figure 6(b). The corresponding circuit diagram for the rectification is shown in inset of figure

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6(b). The enlarge view of a marked (circle) portion in figure 6(b) explains the expected stepwise charging behavior on repeatedly compressing and releasing the PENG. On stepwise charging the voltage of the capacitor reaches a saturated value of ~6.7 V in ~129 s. To check the reproducibility of the fabricated device, stepwise cyclic charging and discharging were carried out. The data shows that the capacitor can be charged and discharged repeatedly. As the normal discharging (without any load) of the capacitor requires more time than the charging, we have carried out the experiment for discharging to ~5 V and repeated for twice. This integration of PENG and energy storage device dictates a potential solution to the repeated replacement of the discharged batteries. This also implies that, the integration can be of great potential for efficient power generation and energy storage from human motions for powering small electronics. 8. Output Voltage vs. Load Resistor The output voltage generated from the PENG along a load resistor constantly increases with increase in load resistance and achieved a saturated value as open circuit voltage at high resistance. The desired circuit diagram for the measurement is given in inset of figure 6(d). The instantaneous power density, obtained by using the formula, P 

V2 , has reached to a RL A

maximum value of ~4.08 µW cm-2 at resistance of 20 MΩ, well consistent with the previous works reported elsewhere.52,53 The theoretical current (~1.73 µA) can be obtained from the maximum power value with the respective load resistance using the formula ( I 

P ). This RL

theoretical value is comparatively higher than the experimental value (~1.4 µA), which could be due to the power consumption by the internal resistance present in the measurement assembly.

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9. Practical Implementation To explore the practical utility of the nanogenerator, we made an effort to power a number of commercial LEDs, mobile LCD screen, calculator, and wrist watch which are shown in figure 7.

Figure 7: Practical implementation of the generated power from the PENG (clockwise LCD screen, Wristwatch, Digital calculator, and LEDs). Under periodic imparting induced by human finger, the electric pulses and the open circuit peak voltage developed from the nanogenerator could instantaneously turn on all 7 LEDs of different colors without any use of rectification unit. Again, the stored charge in the capacitor can be utilized for powering the LCD screen, wrist watch and calculator. The corresponding videos for the demonstrations are provided in Supporting Information.

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10. Durability Study To check the durability, the PENG was repeatedly pressed and released at a driving frequency of ~2 Hz for three weeks, ~1000 cycles (500 s) each week. The PENG exhibited excellent stability of the nanogenerator, without significant drop in output voltage, as shown in figure 8(a). Figure 8(b) shows the enlarged view of the time dependent open circuit voltage generation, which demonstrates the high stability and regularity in performance of the nanocomposite nanogenerator. These results indicated that the PENG could perform well under elevated stress and prolonged use.

Figure 8: (a) The stability and durability test of the PENG. (b) The magnified view of the selected region in the figure 8 (a). 11. Different Types of Sensitivity The output piezoelectric performances of the nanogenerator device were investigated under various conditions, e.g., bending, walking, and heel pressing conditions which are shown in figure 9 (the corresponding videos for all types of sensitivities are given in the video file 2 in the Supporting Information). A comparatively weak signal was observed while bending than that 23



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obtained from walking and heel pressing mode. It is well known that the output voltage of a nanogenerator is closely related to the alignment of the dipoles in the nanofilers. Again, without any electrical poling treatment, the alignment of the dipoles depends on the stress applied on the nanocomposite, i.e., strain applied on the nanocubes embedded in the polymer matrix. Under bending mode maximum strain is intensified in the PVC matrix and as a result, the nanocubes experience the low strain as compared to that in walking or heel pressing mode. Thus, the bending mode shows low voltage output than the other two modes.

Figure 9: Output voltage of PENG under (a) bending, (b) walking, and (c) heel pressing. 12. Conclusion In summary, we have established a high output PENG using piezoelectric and ferroelectric nanoparticles embedded in plasticized PVC matrix. The nanogenerator shows an open circuit voltage of ~40 V and short circuit current ~1.4 µA under human finger imparting. The calculated output power density value (~3.7µW cm-2) from the circuit voltage and current is the highest one reported till date with ZnSnO3 based piezoelectric nanocomposite. Also, the nanogenerator is able to charge a capacitor (power of 2.2 µF) to ~6.7 V within very short time period (~129 s). The output power from the PENG is enough to light up several different colors LED bulbs directly, and to turn on calculator, mobile LCD screen, and wrist watch through an 24


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instantaneously charged capacitor. Thus, various portable devices like solid state lighting and display could be run simply through human movements in such a situation where any other mechanical forces are not available. The PENG was developed through a simple and costeffective method, which could be used for large scale production of the energy harvesting system. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Size distribution of the as synthesized ZnSnO3 nanocubes, EDS of ZnSnO3, FE-SEM images of the cryofractured and tensile fractured surfaces of the nanocomposite film, one snapshot of generated output from the oscilloscope under human finger imparting condition, output performance of the PENG under sewing machine, nature of the stress strain curve of the nanocomposite, calculation of operating pressure under human finger imparting condition, calculation of dielectric constant of the nanocomposites, and video files for the representation of the practical utilizations of the power generated from the PENG. Acknowledgements The authors acknowledge the financial support provide by University Grant Commission (UGC), India and also acknowledge Santimoy Khilari, Krishnedu Sarkar and Abhishek Ghosh for their supports in measuring the output performance of the nanocomposites.

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