An Effective Wind Energy Harvester by Paper-ash Mediated Rapid

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 292−299

pubs.acs.org/journal/ascecg

An Effective Wind Energy Harvester of Paper Ash-Mediated Rapidly Synthesized ZnO Nanoparticle-Interfaced Electrospun PVDF Fiber Md. Mehebub Alam,† Sujoy Kumar Ghosh,† Ayesha Sultana,† and Dipankar Mandal*,†,‡ †

Organic Nano-Piezoelectric Device Laboratory, Department of Physics, Jadavpur University, Kolkata 700032, India Institute of Nano Science and Technology, Phase-10, Sector-64, Mohali-160062, India

‡

S Supporting Information *

ABSTRACT: In this paper, we report an innovative technique for the rapid and simple synthesis of ZnO-containing paper ash (ZPA) within 30 min. It is demonstrated that as-synthesized ZPA can be an alternative for conventional semiconducting filler that is used to improve the electroactive β-phase content in the electrospun PVDF nanofiber (NF). Furthermore, the PVDF nanofiberbased nanogenerator (NFNG) that enables a high output voltage of 4.8 V under an exerted wind pressure of 145 Pa is fabricated. In addition, the detection capability of minute wind pressure, even from human mouth blowing, makes the NFNG an active sensor that might be utilized for charging a mobile phone during conversation. Thus, this strategy is expected to be a promising platform for harvesting wind energy that is beneficial for the design of integrated self-powered wearable electronic devices. KEYWORDS: Paper ash-assisted ZnO nanoparticle, PVDF nanofiber, Electrospinning, β-Phase induction, Wind energy harvesting

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INTRODUCTION Mobile electronic devices and wireless sensors are gradually shrinking in size, and the power requirements are also decreasing because of tremendous technological development.1 However, powering these devices by conventional batteries becomes challenging as the number of devices is going to be huge and recharging or replacing the batteries becomes an ambitious task.2,3 In an alternative way, if the devices can be run through self-powered mode by using the energy that is available in the surrounding environments, then that would be a permanent solution for replacing the batteries. Therefore, harvesting energy from ambient mechanical vibrations becomes a possible strategy for designing self-powered electronic devices. Thus, a piezoelectric nanogenerator (NG) that can convert a mechanical vibration into electrical energy becomes the most likely power source as vibration is present everywhere forever.4−10 A NG has been employed to harvest energy from various sources of mechanical vibration such as body movement (handling, winding, pushing, stretching, bending, clapping, talking, breathing, eyelash trembling, etc.), air flow, friction, vibrations (acoustic and ultrasonic waves), and hydraulic forces (ocean waves, waterfall, or even body fluid and blood flow).11,12 Among various sources of ambient energy, wind energy harvesting has gradually become more interesting because of the abundance, ubiquity, steady supply, and large energy capacity of wind energy.13−15 Lee et al.13 reported that a piezoelectric NG can generate output power in a light wind and serve as an energy-harvesting device under the waving motion of a flag. Sun et al.14 developed a NG for harvesting energy from low-speed air flow and also demonstrated the possibility © 2017 American Chemical Society

of harvesting energy from simulated respiration. A wind-driven pyroelectric NG has been also recently reported by Xie et al.15 Alongside experimental evolution, theoretical and analytical efforts have also been undertaken to demonstrate the generation of electric potential in piezoelectric materials.16,17 Boxberg et al.18 reported that the piezoelectric property of compound semiconductor core−shell nanowires (NWs) might be utilized for the separation of photon-generated electron− hole pairs and, thereby, for photovoltaic applications. However, from the material point of view, PVDF is the best choice as an active material for NG fabrication because it is flexible, lightweight, cost-effective, and easy to process in comparison to the inorganic piezoelectric materials and shows the maximum piezoelectric coefficient among the polymers.19−21 It exhibits different crystalline phases (i.e., α, β, γ, δ, and ε), among which the nonpolar α phase is the most kinetically favored phase.22 Thus, induction of an electroactive β phase that is the most desirable phase for mechanical energy harvesting becomes a challenging task.22,23 Among different techniques (i.e., stretching, heat treatment, electrical poling, etc.), electrospining has recently become the most fascinating technique as it is a one-step method for β-phase induction in PVDF fiber via in situ mechanical stretching and electrical poling during fiber formation.24,25 Thus, a recent literature survey showed that the electrospun PVDF nanofibers are extensively being utilized in energy conversion,26,27 wearable Received: July 20, 2017 Revised: November 17, 2017 Published: November 24, 2017 292

DOI: 10.1021/acssuschemeng.7b02441 ACS Sustainable Chem. Eng. 2018, 6, 292−299

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) XRD pattern of as-synthesized ZPA in the 2θ region from 10° to 80°. The inset shows the digital image of the burned solid that was crushed into ZPA powder. (b) FE-SEM image of the as-synthesized ZPA with a histogram profile of particle size as the inset. (c) TGA curves of PA and ZPA powders. (d) UV−vis absorption spectra of as-synthesized ZPA in the wavelength region from 300 to 800 nm.

power generation,28,29 and various types of sensors.30,31 However, in most of the cases, PVDF electrospun fibers still showed the coexistence of a nonpolar α phase with the induced polar β phase, and thus, researchers focused on the doping of different types of inorganic nanofillers (i.e., MWCNTs, ZnO, MoS2, BaTiO3, and Fe3O4) to improve the β-phase content of PVDF fiber.32−39 However, the synthesis of such inorganic fillers needs a multistep fabrication procedure and thus is timeconsuming. In this context, quick fabrication of the filler by a cost-effective approach is beneficial for the overall functionality of the PVDF-based energy generation system. Zinc oxide (ZnO) is a well-known biocompatible nanomaterial, and great attention has been paid for its uses in photocatalytic devices, gas sensors, photovoltaics, piezoelectric energy harvesters, and biomedical applications.40 Various techniques have been proposed for the synthesis of ZnO nanomaterials, such as sol−gel, hydrothermal, precipitation, microemulsion, electrospinning, and spray pyrolysis.40 However, most of the methods mentioned above involve complicated procedures, expensive materials, high reaction temperatures, long reaction times, and toxic reagents.41 On the other hand, combustion synthesis (CS) is an effective, ecofriendly, simple, rapid, and low-cost method for the production of various industrially useful nanomaterials.42 Meanwhile, paper is a biodegradable material made of cellulose fibers and has various applications in our daily life. Paper is an attractive low-cost functional material that could be used in various sensing applications. Paper is used to design electronic devices, energy devices, strain sensors, gas sensors, and electroactive piezoelectric paper.43 In this paper, we report the synthesis of nanosized ZnO within 30 min by a combustion method using paper as fuel. It is also demonstrated that the ZnO-containing paper ash can effectively improve the piezoelectric β-phase content in the electrospun PVDF nanofiber. In addition, the nanofiber-based

nanofiber-based nanogenerator (NFNG) is found to be an excellent piezoelectric material, suitable for wind energy harvesting applications. The NFNG also exhibits a high output response under human mouth blowing.

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EXPERIMENTAL SECTION

Materials. Zinc acetate (ZnAc2·2H2O) (Merck), printing paper, poly(vinylidene fluoride) (PVDF) pellets (Mw ≈ 275000; SigmaAldrich), N,N-dimethylformamide (DMF), and acetone (Merck Chemical) were used. Synthesis of ZnO-Containing Paper Ash (ZPA). ZnAc2·2H2O (1 wt %) was dissolved in deionized water, and a clear solution was prepared under constant stirring at room temperature. Printing paper was cut into small pieces (dimensions of ∼6 cm × 4 cm) and immersed in the solution for only 10 min. The wet paper pieces were then collected carefully and dried with a blow dryer. Those pieces were then burned with a lighter. One edge of the paper was held with a tweezers, and the other edge was placed in the flame to burn. Then it was quickly removed from the flame to burn by spontaneous combustion with the liberation of a large amount of gaseous products and became a burned solid. Finally, to obtain the ZnO-containing paper ash (ZPA), a pestle and mortar was used to crush and grind the burned solid into a powder. Preparation of a Mat of NFs. PVDF [12 wt % (w/v)] was dissolved in a mixed DMF/acetone solvent [6:4 (v/v)] under constant stirring at room temperature. Therefore, a low content (0.5 wt %) of ZPA was placed in the solution and kept under constant stirring at room temperature. The prepared solution was added to a 10 mL hypertonic syringe with a 0.8 mm diameter needle, and ZPAcontaining PVDF nanofibers (ZPA-PNFs) were fabricated by using commercial electrospinning equipment under a high voltage of 13 kV with a needle tip−collector distance (aluminum foil-covered plate collector) of 12 cm. A syringe pump was applied to feed solutions to the needle at a rate of 1 mL/h. A mat of pure PVDF nanofibers (PNFs) was also fabricated to serve as a reference specimen. Fabrication of the NFNG. The as-prepared mat of ZPA-PNFs with a thickness of 150 μm was cut into a rectangular shape with dimensions of 9.5 cm × 8.7 cm, and then two conducting fabrics (Ni− 293


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Figure 2. FE-SEM images of mats of (a) PNFs and (b) ZPA-PNFs with corresponding fiber diameter distribution histogram profiles in the insets. (c) FT-IR spectra of mats of PNFs and ZPA-PNFs in the wavenumber region from 1600 to 600 cm−1. (d) XRD patterns of mats of PNFs and ZPAPNFs. (e) Typical P−E hysteresis loop and (f) corresponding S−E hysteresis loop of the mats under an electric field of ±8 MV/m. Cu−Ni-plated polyester fabric) with an area of 8.2 cm × 7.2 cm are attached on both sides of the mat to prepare a sandwichlike NFNG. Characterization. X-ray diffraction (XRD) patterns were determined using a Bruker, D8 Advance diffractometer with Cu Kα radiation (λ = 0.154 nm). The surface morphology was evaluated by field emission scanning electron microscopy (FE-SEM, FEI, INSPECT F50). Fourier transform infrared spectroscopy (FT-IR, Bruker, Tensor II) was executed with 16 scans and a 4 cm−1 spectral resolution. Optical properties were determined with an ultraviolet−visible (UV− vis) (Shimadzu, 3110PC) spectrometer. The ferroelectric measurements were taken with a FE test system (aixact TF Analyzer 2000). Thermogravimetric analyses (TGAs) were performed in a thermogravimetric analyzer (TGA/SDTA851e, Mettler Toledo AG) with a heating rate of 10 °C min−1 under a N2 atmosphere from room temperature to 1000 °C. Open-circuit output voltages were measured through a National Instrument (NI) data acquisition (DAQ) system (NI, USB 6000) interfaced with a computer with a stand-alone version of LabVIEW.

values of ∼29.5°, ∼39.5°, and ∼43.3° are also observed, indicating the presence of a small quantity of CaCO3 in the asprepared ZPA powder. The average crystallite size of ZnO is 26 nm, estimated by Debye−Scherrer’s equation: D = Kλ/(μ cos θ), where K is a constant (0.89), λ is the wavelength of the Xray radiation, μ is the full width (in radians) at half-maximum (fwhm) of the intense diffraction peak, and θ is the angle of diffraction.The morphology of the synthesized ZPA powder is examined by FE-SEM (Figure 1b). It revealed that the ZPA powder is mainly composed of irregularly aggregated 15−50 nm particles as shown in the histogram profile (inset of Figure 1b). To calculate the content of ZnO in ZPA, TGA measurements were taken on both specimens, viz., only paper ash (PA) and ZnO-containing paper ash (ZPA), from room temperature to 1000 °C in a N2 atmosphere. It was found that PA and ZPA showed 36.5 and 15% weight losses, respectively, at 1000 °C as shown in Figure 1c. Therefore, the difference between the weight losses of PA and ZPA gives rise to the content of ZnO, which is 21.5% (w/w). The UV−vis absorption spectrum (Figure 1d) shows a well-defined exciton band at ∼397 nm corresponding to the band gap value of 3.12 eV that matches well with the typical UV−vis absorbance spectra of ZnO NPs.44,45 However, it shows a red shift of ∼24 nm relative to the bulk exciton absorption (373 nm). The

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RESULTS AND DISCUSSION Structural Properties of ZPA. The X-ray diffraction (XRD) pattern of the collected ZPA, after the burned solid (shown in the inset of Figure 1a) had been crushed into a powder, shows the characteristic diffraction peaks of ZnO corresponding to the typical wurtzite ZnO phase (JCPDS Card No. 36-1451) as shown in Figure 1a. Some weak peaks at 2θ 294


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interaction with the macromolecular polymer chain of PVDF.46 In addition, the increase in the conductivity of the electrospining solution also leads to a decrease in fiber diameter, which also improves the β-phase content. To investigate the effect of improved β-phase content in ZPA-PNFs, the macroscopic polarization responses have been measured. Figure 2e illustrates the P−E loops of the mats of NFs, measured at 10 Hz. It is found that under an electric field of 8 MV/m the remnant polarization (Pr) of ZPA-PNFs is effectively increased in comparison to that of the PNFs. This is attributed to the fact that the β-phase content of ZPA-PNFs is larger than that of PNFs. Pr also is directly related with piezoelectric coefficient d33 as shown in eq 249

reason for the shift of the absorption band could be the presence of defect energy levels in the synthesized ZnO nanostructures. In this method, defects in ZnO NPs might originate in the surface defects in the presence of paper ash. Structural Properties of a Mat of NFs. FE-SEM images of the fabricated mats of PNFs and ZPA-PNFs along with the histogram profile are shown in panels a and b of Figure 2, respectively. It is found that both mats are made of randomly oriented fibers where ZPA-PNFs show a distinctly smaller average fiber diameter (dav ∼ 195 nm) in comparison with that of the mat of PNFs (dav ∼ 240 nm) because of the increase in conductivity of the PVDF solution after doping of ZPA. This decrease in the fiber diameter also promises an improvement in the mechanical and piezoelectric properties of the mat of ZPA-PNFs in comparison with those of the PNFs.36 The induction of the piezoelectric β phase in both mats is confirmed by vibrational peaks at 841 and 1275 cm−1 in FTIR spectra (Figure 2c).23,25,46 The improved peak intensities ensure the improvement in the β-phase content in ZPA-PNFs compared to those in PNFs. The relative proportion of the β phase [F(β)] is estimated from eq 1 Aβ F (β ) = × 100% Kβ A + A α β K

( )

d33 = −S33Pr

where S33 is the elastic compliance. Thus, ZPA-PNFs are more suitable for energy harvesting applications than mats of PNFs. Furthermore, strain (S) developed in a piezoelectric materials under an electric field (reverse piezoelectric effect) is a quadric function of P as shown in eq 350,51 S = QP 2

where Aα and Aβ are the absorbance intensities and Kα and Kβ the absorption coefficients at 766 and 841 cm−1, respectively.23 Interestingly, the mat of ZPA-PNFs shows an F(β) value (∼92%) that is higher than that of the mat of PNFs (∼86%). The F(β) content of ZPA-PNFs is similar to several previously reported results for cases in which different fillers were doped with PNFs as shown in Table 1. Thus, the as-synthesized ZPA Table 1. Comparison of F(β) Values in PVDF NFs When Different Fillers Are Used filler

F(β) (%)

ref

ZnO MoS2 BaTiO3 Fe3O4 CoFe2O4 CNTs ZPA

87 95 80 95.1 90 84 92

35 36 37 39 46 47 this work

(3)

where Q is an electrostriction constant ranging from −2.5 to −2.1 m4/C2.52 Hence, a butterfly-shaped symmetrical S−E hysteresis loop corresponding to the P−E loop was found and is shown in Figure 2d. It is found that much greater strain is developed in the mat of ZPA-PNFs at applied electric fields of 8 MV/m. This is direct evidence that the mat of ZPA-PNFs is a superior piezoelectric material than the mat of PNFs. It should also be mentioned that the strains are large compared to those of ceramics, which is consistent with the previously reported results.53−55 Wind Energy Harvesting Performance of the NFNG. The design for characterizing the wind energy harvesting capability of the ZPA-PNFs comprising NFNG is shown in Figure 3a. An air gun with an adjustable flow speed was employed as a wind source, and NFNG was attached to a rigid frame that was kept 30 cm from the air gun. It is found that when the NFNG is subjected to a vertical wind pressure with a wind velocity of 10 m/s, an output voltage of 1.8 V is generated (Figure 3b). Basically, the NFNG experienced a compressive strain when it was subjected to a vertical wind pressure. As a result, the crystal structures of the PVDF NFs as well as ZnO were deformed, which leads to a piezoelectric output response through repeated flipping of the stable state to the deformed state or vice versa.56 Thus, piezoelectric potential is developed between the top and bottom electrodes of the NFNG. A theoretical analysis has also been conducted using the finite element method (FEM) to gain a quantitative explanation of the piezopotential distribution inside the deformed ZPA-PNFs of the NFNG under wind pressure with a wind velocity of 10 m/s (material properties listed in Table S1). A network structure in which eight NFs were entangled with each other was considered for the aforementioned theoretical analysis. The resulting deformation (i.e., displacement) distribution and the piezopotential distribution are shown in panels c and d of Figure 3, respectively. It is found that, under wind pressure, maximum deformations (∼34.8 μm) of the ZPA-PNFs occur in the interaction region between the lower and upper layers of the fibers. In the non-interactive regions, the deformations gradually diminish. This indicates that the network structure of NFs enhances the resulting polarization in comparison to that

(1)

α

(2)

may be the alternative to conventional semiconducting fillers to improve the piezoelectric β-phase content of PNFs for energy harvesting applications. The XRD patterns also reveal that the β phase is induced in the mats of NFs as shown in Figure 2d. It is also noticeable that the corresponding peak intensity of the β phase at 2θ = 20.9° and 36.6° becomes intense in ZPA-PNFs compared to that in mats of PNFs.37,48 These observations are well matched with the FTIR results. The crystallinity (χc) was also calculated from XRD results (details of the calculations are shown in Text S1). It is found that after incorporation of ZPA, ZPA-PNFs exhibit a 12% improvement in crystallinity compared to that of the mat of PNFs (Figure S1). The simultaneous in situ mechanical stretching and electrical poling at a high electric field during electrospining is responsible for the induction of the β phase in the NF mats. In addition, the improvement in the β phase in ZPA-PNFs is due to the doping of ZPA filler that has made an interfacial 295


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Figure 3. (a) Digital photograph showing that strain was given by an air gun and the NFNG was attached to a rigid frame for piezoelectric voltage generation. (b) Output voltage under wind pressure. Simulation result of the (c) displacement and (d) piezopotential distribution in the network structure of NFs under wind pressure. (e) Driving of a blue light-emitting diode (LED) by the NFNG under wind pressure. The NFNG is directly connected to a blue LED to light it up. The inset shows a schematic of a circuit diagram for LED lighting. (f) Performance of capacitor charging of the NFNG under wind pressure. The inset shows a schematic of a circuit diagram for capacitor charging. The ac output voltage generated from the NFNG is converted to dc voltage via the full wave rectifier and is utilized to charge the capacitor.

seen with a single fiber because of cooperative electromechanical mutual interaction among the adjacent fibers under the applied pressure.57 Furthermore, this deformation yields a quantitative explanation of the piezopotential distribution inside the deformed NFs of the NFNG. The simulated piezopotential (maximum piezopotential of 2.3 V) is slightly higher than the experimentally measured piezopotential (∼1.8 V). This deviation is probably due to the voltage drop resulting from internal leakage paths and charge loss in the conductor− dielectric−conductor (CDC) structure.49 It is also found that the output performance of the NFNG shows a proportional relation with wind velocity (WV), which is consistent with the previously reported results.14 The increase in wind velocity from 10 to 15 ms−1 results in an improvement in the output voltage from 1.8 to 4.8 V as shown in Figure S2. It should be mentioned that the output voltage is much higher than those of several reported wind energy harvesters as shown in Table S2. This is expected because the increasing wind speed results in an increase in the strain on the NFNG. Moreover, the output voltage is enough to power a commercial blue light-emitting diode (LED) continuously without any external power source

or storage devices as shown in Figure 3e (the corresponding circuit diagram is shown in the inset). For better illustration, we also show the LED of being lit in the dark. This led us to assume that proper design of the NG can explore its potential application to power different portable electronic devices used in our everyday life. For example, relative air flow during the driving of a car usually exceeds 20 m/s, so energy can be harvested when the NFNG is attached to a car. The applicability of the NFNG is also verified by testing the capacitor charging results that ensure the reliability and feasibility of the NFNG. For the experiments, the output voltage from the NFNG is first rectified through a typical bridge circuit and then a 1 μF capacitor is connected across the rectified voltage to charge it (the corresponding circuit diagram is shown in the inset of Figure 3f). The capacitor charging results are shown in Figure 3f. It is observed that the charging voltage curves increase exponentially and finally reach steady state values of 0.6, 1.8, and 4 V, corresponding to external pressures of 65, 93, and 145 Pa, respectively, generated from wind velocities of 10, 12, and 15 ms−1, respectively. The energy (W) and power (P) stored in the capacitor are calculated from eqs 4 and 5 respectively, 296


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Figure 4. (a) Schematic demonstration of voltage generation under human mouth blowing. (b) Output voltage under human mouth blowing. Simulation results of the (c) piezopotential distribution and (d) displacement distribution in the network structure of NFs under human mouth blowing.

W=

CVS2 2

(4)

P=

CVS2 2t

(5)

technique. It indicates that ZPA may be an alternative to conventional semiconducting filler that can effectively improve the electroactive β-phase content of the electrospun PVDF nanofiber that makes it superior for piezoelectric drive power generation. The as-fabricated NFNG delivers a high throughput of 4.8 V under 145 Pa of exerted pressure of wind flow. Up to 1.0 V is obtained when the NFNG is deformed by human mouth blowing, which shows its ability to serve as an active sensor. Thus, it is expected that in the near future it might be utilized to charge a mobile phone during a conversation. As a result, this strategy may provide a unique platform for wind energy harvesting that leads us to design self-powered wearable electronic devices.

where C is the capacitance of the capacitor, VS is the saturation voltage, and t is the time required to reach the saturation voltage. The maximum energy and power stored in the capacitor were found to be 8 μJ and 0.47 μW, respectively. The storage performance of the capacitor at different wind velocities is summarized in Table S3. This indicates that the NFNG can be utilized as an alternative energy-harvesting power source for tiny portable electronic devices.58 The flexible NFNG can also be used as an active sensor capable of detecting a small amount of wind pressure from human mouth blowing. To demonstrate this possibility, the NFNG was placed close to (within 8 cm) the human mouth (Figure 4a) and driven by mouth blowing. An output voltage spike of 0.2 V was obtained when the NFNG was deformed by the pressure of feeble mouth blowing (WV ∼ 1 ms−1). In addition, the theoretical simulation of the piezopotential distribution (Figure 4c) shows that the resulting deformation (Figure 4d) distribution is confined to a small area. It is also found that the output voltage linearly increases from 0.2 to 1 V with an increase in mouth blowing wind flow from 1 ms−1 (corresponding to an exerted pressure of ∼0.65 Pa) to 5 ms−1 (∼16 Pa) as shown in Figure S3. Therefore, the NFNG is demonstrated to be an excellent candidate for day and night energy harvesting from respiration considering the fluctuating wind flow speed during normal breath (WV ∼ 2 ms−1) that generally increases 4−8-fold during exercise.14 This capability is ideal for harvesting energy from environmental wind flow or respiration, which makes the NFNG suitable for various applications, including charging mobile phones during conversations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02441. Additional information about the material properties of PVDF fiber for finite element model simulation, comparison of the data for the NFNG with other reported data, and output voltages of the NFNG under wind pressures at different wind velocities and as a function of human mouth blowing wind flow (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Telephone: +91-8336-017243. Fax: +91-33-2413-8917. ORCID

Dipankar Mandal: 0000-0003-2167-2706 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Science and Engineering Research Board (SERB/1759/2014-15), Government of India.

CONCLUSION In summary, ZnO-containing paper ash (ZPA) is synthesized within 30 min through a simple but innovative synthesis 297


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composite film: an excellent material for energy storage applications and piezoelectric throughput. Nanotechnology 2017, 28, 015503. (20) Li, J.; Claude, J.; Norena-Franco, L. E.; Seok, S. I.; Wang, Q. Electrical energy storage in ferroelectric polymer nanocomposites containing surface-functionalized BaTiO3 nanoparticles. Chem. Mater. 2008, 20, 6304−6306. (21) Kawai, H. The piezoelectricity of poly (vinylidene fluoride). Jpn. J. Appl. Phys. 1969, 8, 975−976. (22) Lovinger, A. J. Ferroelectric Polymers. Science 1983, 220, 1115− 1121. (23) Martins, P.; Lopes, A. C.; Lanceros-Mendez, S. Electroactive phases of poly(vinylidene fluoride): determination, processing and applications. Prog. Polym. Sci. 2014, 39, 683−706. (24) Fang, J.; Wang, X.; Lin, T. Electrical power generator from randomly oriented electrospun poly(vinylidene fluoride) nanofibremembranes. J. Mater. Chem. 2011, 21, 11088−11091. (25) Mandal, D.; Yoon, S.; Kim, K. J. Origin of piezoelectricity in an electrospun poly(vinylidene fluoride-trifluoroethylene) nanofiber webbased nanogenerator and nano-pressure sensor. Macromol. Rapid Commun. 2011, 32, 831−837. (26) Chang, C.; Tran, V. H.; Wang, J.; Fuh, Y.-K.; Lin, L. Direct-write piezoelectric polymeric nanogenerator with high energy conversion efficiency. Nano Lett. 2010, 10, 726−731. (27) Gheibi, A.; Bagherzadeh, R.; Merati, A. A.; Latifi, M. Electrical power generation from piezoelectric electrospun nanofibers membranes: electrospinning parameters optimization and effect of membranes thickness on output electrical voltage. J. Polym. Res. 2014, 21, 571. (28) Sorayani Bafqi, M. S.; Bagherzadeh, R.; Latifi, M. Nanofiber alignment tuning: An engineering design tool in fabricating wearable power harvesting devices. J. Ind. Text. 2017, 47, 535−550. (29) Gheibi, A.; Latifi, M.; Merati, A. A.; Bagherzadeh, R. Piezoelectric electrospun nanofibrous materials for self-powering wearable electronic textiles applications. J. Polym. Res. 2014, 21, 469. (30) Zandesh, G.; Gheibi, A.; Sorayani Bafqi, M. S.; Bagherzadeh, R.; Ghoorchian, M.; Latifi, M. Piezoelectric electrospun nanofibrous energy harvesting devices: Influence of the electrodes position and finite variation of dimensions. J. Ind. Text. 2017, 47, 348−362. (31) Wang, Y. R.; Zheng, J. M.; Ren, G. Y.; Zhang, P. H.; Xu, C. A flexible piezoelectric force sensor based on PVDF fabrics. Smart Mater. Struct. 2011, 20, 045009. (32) Yu, H.; Huang, T.; Lu, M.; Mao, M.; Zhang, Q.; Wang, H. Enhanced power output of an electrospun PVDF/MWCNTs-based nanogenerator by tuning its conductivity. Nanotechnology 2013, 24, 405401. (33) Ahn, Y.; Lim, J. Y.; Hong, S. M.; Lee, J.; Ha, J.; Choi, H. J.; Seo, Y. Enhanced piezoelectric properties of electrospun poly(vinylidene fluoride)/multiwalled carbon nanotube composites due to high βphase formation in poly(vinylidene fluoride). J. Phys. Chem. C 2013, 117, 11791−11799. (34) Liu, Z. H.; Pan, C. T.; Lin, L. W.; Lai, H. W. Piezoelectric properties of PVDF/MWCNT nanofiber using near-field electrospinning. Sens. Actuators, A 2013, 193, 13−24. (35) Soryani Bafqi, M. S.; Bagherzadeh, R.; Latifi, M. Fabrication of composite PVDF-ZnO nanofiber mats by electrospinning for energy scavenging application with enhanced efficiency. J. Polym. Res. 2015, 22, 130. (36) Maity, K.; Mahanty, B.; Sinha, T. K.; Garain, S.; Biswas, A.; Ghosh, S. K.; Manna, S.; Ray, S. K.; Mandal, D. Two-dimensional piezoelectric MoS2-modulated nanogenerator and nanosensor made of poly(vinlydine fluoride) nanofiber webs for self-powered electronics and robotics. Energy Technol. 2017, 5, 234−243. (37) Chanmal, C. V.; Jog, J. P. Electrospun PVDF/BaTiO3 nanocomposites: polymorphism and thermal emissivity studies. Int. J. Plast. Technol. 2011, 15, 1−9. (38) Bodkhe, S.; Turcot, G.; Gosselin, F. P.; Therriault, D. One-step solvent evaporation-assisted 3D printing of piezoelectric PVDF nanocomposite structures. ACS Appl. Mater. Interfaces 2017, 9, 20833−20842.

Md.M.A. is supported by a UGC-BSR fellowship (P1/RS/191/ 14). S.K.G. acknowledges an INSPIRE fellowship (IF130865). A.S. is supported by a Maulana Azad National Fellowship [F117.1/2015-16/MANF-2015-17-WES-53885/(SA-III/Website)] from UGC.

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