crystals

Article Piezoelectric Nanogenerator Based on Lead-Free Flexible PVDF- Composite Films for Driving Low Power Electronics

Manisha Sahu 1, Sugato Hajra 1, Kyungtaek Lee 1 , PL Deepti 2, Krystian Mistewicz 3 and Hoe Joon Kim 1,*

1 Department of Robotics Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Korea; [email protected] (M.S.); sugatofl@outlook.com (S.H.); [email protected] (K.L.) 2 Department of Physics, Veer Surendra Sai University of Technology, Odisha 768018, India; [email protected] 3 Institute of Physics-Centre for Science and Education, Silesian University of Technology, 44100 Katowice, Poland; [email protected] * Correspondence: [email protected]

Abstract: Self-powered sensor development is moving towards miniaturization and requires a suitable power source for its operation. The piezoelectric nanogenerator (PENG) is a potential candidate to act as a partial solution to suppress the burgeoning energy demand. The present work is focused on the development of the PENG based on flexible polymer-ceramic composite films. The X-ray spectra suggest that the BTO particles have tetragonal symmetry and the PVDF-BTO composite films (CF) have a mixed phase. The dielectric constant increases with the introduction of the particles in the PVDF polymer and the loss of the CF is much less for all compositions. The BTO particles have a wide structural diversity and are lead-free, which can be further employed to make a CF. An attempt  was made to design a robust, scalable, and cost-effective piezoelectric nanogenerator based on the  PVDF-BTO CFs. The solvent casting route was a facile approach, with respect to spin coating, Citation: Sahu, M.; Hajra, S.; Lee, K.; electrospinning, or sonication routes. The introduction of the BTO particles into PVDF enhanced Deepti, P.; Mistewicz, K.; Kim, H.J. the dielectric constant and polarization of the composite film. Furthermore, the single-layered Piezoelectric Nanogenerator Based on device output could be increased by strategies such as internal polarization amplification, which Lead-Free Flexible PVDF-Barium was confirmed with the help of the polarization-electric field loop of the PVDF-BTO composite film. Titanate Composite Films for Driving The piezoelectric nanogenerator with 10 wt% BTO-PVDF CF gives a high electrical output of voltage Low Power Electronics. Crystals 2021, 7.2 V, current 38 nA, and power density of 0.8 µW/cm2 at 100 MΩ. Finally, the 11, 85. https://doi.org/10.3390/ using the fabricated PENG is done by various actives like coin dropping, under air blowing, and cryst11020085 finger tapping. Finally, low-power electronics such as calculator is successfully powered by charging µ Received: 30 December 2020 a 10 F capacitor using the PENG device. Accepted: 20 January 2021 Published: 21 January 2021 Keywords: dielectric; lead-free; piezoelectric nanogenerator; energy harvesting

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- 1. Introduction iations. In the present era, the development of a sustainable and flexible energy harvester that harvests energy from listed mechanical sources to power electronics and nanosystems is attaining huge attention [1,2]. Nanogenerators provide a pathway towards sustainable power sources due to their superior characteristics of easy, less cost fabrication, mass Copyright: © 2021 by the authors. production, improved energy conversion efficiency [3,4]. Currently, the quest is to develop Licensee MDPI, Basel, Switzerland. flexible nanogenerators working on principles of [5] and triboelectricity [6]. This article is an open access article Nanogenerators based on the piezoelectric principle have gained immense attention due to distributed under the terms and many advantages such as wide piezoelectric materials selection, easy fabrication, and quick conditions of the Creative Commons response [7,8]. The various structure and shapes of the flexible piezoelectric nanogenerator Attribution (CC BY) license (https:// can be achieved by using several polymers like PVDF, PVDF-copolymer [9], PDMS [10], creativecommons.org/licenses/by/ Nylon [11], and polyamide [12]. Among these, PVDF and its copolymers are chosen for 4.0/).

Crystals 2021, 11, 85. https://doi.org/10.3390/cryst11020085 https://www.mdpi.com/journal/crystals Crystals 2021, 11, 85 2 of 10

several studies owing to its good electromechanical coupling, chemical stability, and high piezoelectric coefficient [13,14]. The PVDF has five crystalline polymorphs such as α, β, γ, δ, and ε phases [15,16]. The presentation of the phases in the PVDF solely depends on the processing methodology [17]. The β phase bears the highest electroactive properties [18]. There are several strategies followed to achieve the electroactive β phase, such as applying stretching mechanically with heating, applying high voltage, and doping of piezoelectric materials or metallic nanoparticles [19,20]. Mokhtari et al. explored the textile architecture and achieved an energy conversion efficiency of 22.5% for the coiled hybrid piezo fiber energy generator [21]. Mokhtari and co-workers tested pizeofibers that were capable of producing a maximum voltage output of 4 V and a power density of 87 µW·cm−3, which were 45 times higher than earlier reported for piezoelectric textiles [22]. Pereria et al. fabricated the BTO-PVDF-TRFE composites and achieved a power output of 0.28 µW[23]. Zhao et al. suggested that the BaTiO3 NPs increase the piezoelectric potential and reinforce the local stress of PVDF [24]. Ponnamma et al. fabricated hybrid composite (3 wt% BaTiO3, PVDF-HFP, 1 wt% h-BN) generated a piezoelectric output voltage (2.4 V) with high dielectric constant 45 and low dielectric loss 7.8 [25]. PVDF-ZnO-BTO composite film (CF) was fabricated by Sabry et al. that was capable of generating 11 V at 1.328 N force [26]. The authors added a comparison table of the various PVDF-ceramic composites, their preparation approach, and electrical output response in Table S1. ferroelectric oxides are one of the widely investigated materials with a general chemical formula of ABO3 [27]. BTO ceramics are categorized under the oxygen octahedron group. BaTiO3 undergoes the ferroelectric-paraelectric transition (Tc) about ◦ 120 C[28]. It exhibits a cubic structure (space group Pm3m) above the Tc, and below this temperature, it bears a tetragonal structure (space group P4mm). In the cubic symmetry, the Ti4+ ion present in the oxygen octahedra center causing the non-ferroelectric behavior of BTO. During the tetragonal symmetry, BTO bears ferroelectric properties as Ti4+ ion shifts from the center towards one of the two oxygen ions along the axis. BTO bears more structural changes such as the temperature declines from 120 ◦C to below, it has orthorhombic crystal symmetry (space group Amm2) at 5 ◦C, at −90 ◦C it transforms to rhombohedral crystal symmetry, and at 1460 ◦C it transforms to hexagonal crystal symme- try. BTO is a lead-free ceramic with reported piezoelectric coefficient values ranging from 350 to 460 pC/N by several synthesis routes, and it can act as an alternative towards the toxic lead-based compositions [29,30]. The polymer ceramic composites have several merits such as mechanical flexibility and rather simple process to control the film thicknesses and sizes [31,32]. The limitations of the high brittle of the BTO ceramics can be resolved by integrating in particle forms into the PVDF, further improving the electroactive phase of PVDF. The present work focuses on the fabrication of the thin polymer-ceramic CFs via solvent casting route. Mixed phases of the PVDF and BTO are seen from the XRD spec- tra confirming the crystallinity and the distribution of the particles within the matrix is depicted from the SEM micrographs. The improved dielectric constant and low loss of the PVDF-BTO CF is also confirmed. The flexible piezoelectric nanogenerator is designed and experiment with in terms of the different wt%, and with the optimized PENG device, other experiments and several testing of charging capacitors and the powering calculator is carried out. The solvent casting route is a cheap fabrication technique that has been adapted for this work as a facile approach, as compared to complicated spin coating, elec- trospinning, or sonication routes. The introduction of the BTO particles into PVDF leads to the improvement of the dielectric constant and polarization. The Polarization-electric field loop confirms that the BTO particles could enhance the internal polarization of the PVDF. Furthermore, the single-layered device output can be increased by strategies such as internal polarization amplification. This work paves the way to fabricate new piezoelectric material for energy harvesting. Crystals 2021, 11, 85 3 of 10

2. Processing and Experimental Techniques 2.1. Synthesis of BaTiO3 Particles The BTO particles were synthesized by mixing the various precursors and heat treat- ment at high temperature. The high purity barium carbonate and titanium oxide were procured from Loba Chemie, Mumbai, India. The stoichiometric proportion of the powders were carefully measured using a digital weighing balance. The powders were transferred to an agate mortar and mixed for 2 h in a methanol medium. The homogeneously mixed powders were put into an alumina boat and calcined inside a box furnace at 1200 ◦C for 4 h. Further, the lump was ground into fine particles utilizing an agate mortar and pestle.

2.2. Synthesis of PVDF-BaTiO3 Composite Film and PENG Device The composite film (CF) of the PVDF-BTO was prepared by a cost-effective solvent casting method. For this, a solvent of N-Methyl-2-Pyrrolidone (NMP) was procured from Sisco Research Ltd., Mumbai, India. The PVDF pellets were procured from Sigma Aldrich, St. Louis, MO, USA. The PVDF pellets were measured in proper ratio and put into the NMP solvent at 60 ◦C upon a magnetic stirrer with a magnetic bead inside it. The clear transparent solution was obtained after 1 h, and the various wt% of the BTO particles were put into the solution. Dispersions with different concentrations of BTO (1 wt%, 5 wt%, 7 wt%, 10 wt%) were prepared. Further, it was allowed to mix for 1 h at room temperature until a homogenous mixture was achieved. Then the PVDF-BTO solution was poured onto a glass Petri plate and left inside an electric oven at 70 ◦C for two hours. After curing, a free-standing flexible PVDF-BTO CF was obtained. For the PENG device fabrication, the film was cut into dimensions of 1.5 cm × 1.5 cm, and the gold coating was done to act as an electrode. The electrode area was kept as 1 cm × 1 cm. Further, to measure the system’s energy output, thin copper tape was attached to the opposite side of the device. It was then transferred to antistatic tape to neglect any of the electrostatic charges present in the environment. Then the entire device was encapsulated using a PDMS layer. The PENG devices were poled at 1 kV for 30 min at RT to align dipoles.

2.3. Experimental Techniques The structural analysis of the BTO particles and PVDF-BTO CFs were performed using a powder X-ray diffraction (PANalytical Empyrean, Almelo, The Netherlands, Cu-Kα radiation) under a scanning step size of 3◦/min. The surface morphology was taken using a Hitachi S-4800 scanning electron microscope fitted with an EDS detector. The CF was cut into small pieces, and the electrode was done with gold sputtering onto opposite sides for electrical characterization at room temperature. The dielectric measurement was taken using an N4L PSM LCR meter. The periodic contact-separation of the device was performed by a linear motor (LinMot Inc., Lake Geneva, WI, USA). An electrometer 6514 (Keithley, Solon, OH, USA) was used to measured voltage and current from the piezoelectric device.

3. Results and Discussion The systematic synthesis route is the possible way to properly obtain a high crystalline material and with superior properties. In Figure1a, the mixing of the high purity barium carbonate and titanium oxide is mixed using a mortar and pestle in the presence of the methanol for 4 h and then transferred into an alumina crucible. The mixture is calcined at 1200 ◦C for 4 h. The morphological properties, such as particle size and shape of prepared BTO particles were analyzed from the captured SEM image, as shown in Figure1a. The SEM image reveals the polyhedron shape of particles with a very smooth surface and edges. The particle size histogram with a Gaussian peak fitted curve is shown in Figure S1a. It depicts the presence of particles having a size distribution in the range of 200 to 1300 nm. The average particle size was about 460 nm, which was calibrated from the Gaussian peak fitting of the particle size distribution histogram. Figure1b shows the formation of the PVDF-BTO CF. In this, the PVDF pellets are dissolved in the NMP solvent and heated at Crystals 2021, 11, x FOR PEER REVIEW 4 of 10

Crystals 2021, 11, x FOR PEER REVIEW 4 of 10 1a. The SEM image reveals the polyhedron shape of particles with a very smooth surface and edges. The particle size histogram with a Gaussian peak fitted curve is shown in Fig- ure S1a. It depicts the presence of particles having a size distribution in the range of 200 Crystals 2021,surface11, 85 and edges. The particle size histogram with a Gaussian peak fitted curve is shown 4 of 10 in Figure S1a. It depictsto the 1300 presence nm. The of particles average having particle a size size distribution was about in 460the rangenm, which of was calibrated from the 200 to 1300 nm. The averageGaussian particle peak size fitting was ofabout the particle460 nm, whichsize distribution was calibrated histogram. from Figure 1b shows the for- the Gaussian peak fittingmation of the of particle the PVDF size -distributionBTO CF. In histogram. this, the PVDF Figure pellets1b shows are the dissolved in the NMP solvent formation of the PVDF-BTO60and◦C heated CF. for 1In h this,at using 60 the °C aPVDF for magnetic 1 pelletsh using stirring. are a dissolvedmagnetic Then in thestirring. the BTO NMP particlesThen solvent the are BTO added particles to the are transparent added to and heated at 60 °C forsolutionthe 1 hour transparent using and a the magnetic solution temperature stirring. and the Then is reducedtemperature the BTO to particles room is reduced temperature are added to room with temperature continuous with mixing con- to the transparent solution and the temperature is reduced to room temperature with con- givingtinuous rise mixing to a PVDF-BTOgiving rise to homogenous a PVDF-BTO solution. homogenous It is then solution. poured It onto is then a glass poured Petri onto dish a tinuous mixing giving rise to a PVDF-BTO homogenous solution. It is then poured onto a andglass left Petri for dish curing. and left The for contact curing. angle The contact measurement angle measurement of the PVDF-BTO of the PVDF films-BTO shows films an glass Petri dish and left for curing. The contact angle measurement of the PVDF-BTO films shows an angle greater than 100 degrees, which suggests the higher hydrophobic nature shows an angle greaterangle than greater100 degrees, than wh 100ich degrees, suggests which the higher suggests hydrophobic the higher nature hydrophobic nature of the films. of the films. Figure 1cFigureof shows the films.1thec shows XRD Figure pattern the 1c XRD ofshows the pattern BTO the particlesXRD of the pattern BTOat room particles of temperature.the BTO at room particles temperature. at room temperature. The peaks The peaks are well-matchedareThe well-matchedpeaks with arreportede well with- JCPDSmatched reported data: with 01-075-0583. JCPDS reported data: TheJCPDS01-075-0583. POWD data: MULT 01-075 The -0583. POWD The MULT POWD software MULT software was used towassoftware derive used the wa tolattices derive used para thetometers derive lattice following the parameters lattice a least-square parameters following refinement following a least-square a least refinement-square refinement rule. The rule. The values of a:values rule.3.988 The( ofǺ );values a: c: 3.9883.979 of (a:Ǻ );3.988 volume: c: 3.979 (Ǻ); 64.2 c: ( 3.979); (Ǻ volume:3). (Ǻ);The volume:XRD 64.2 peaks ( 64.23). of The (Ǻthe3 ). XRD The peaksXRD peaks of the of polymer the pol- polymer ceramic CFs ceramicymershowed ceram the CFs presenceic showed CFs show of thebothed presence PVDF the presence and of BTO both of phases.PVDF both PVDFVarious and BTO and sym-phases. BTO phases. Various Various symbols symbols were bols were used to representusedwere to usedthe represent PVDF to represent phase the and PVDF the the PVDF phaseperovskite phase and phase the and perovskite ofthe BTO. perovskite It phasewas de- phase of BTO. of ItBTO. was It depicted was depicted that a picted that a higher concentrationhigherthat a higher concentration of 10concentration wt% into of 10the wt% ofPVDF 10 into wt% leads the into to PVDF an the increase PVDF leads in toleads the an BTO increase to an increase in the BTOin the phase BTO phase and a phase and a reductionreductionand in the a r PVDFeduction in thephase PVDFin intensthe phasePVDFity. It intensity. canphase be alsointensity. It noticed can be It alsothat can the noticedbe phasealso thatnoticed the phasethat the of phase the BTO of hasthe of the BTO has not been altered as it gets dissolved in PVDF, which can be confirmed as notBTO been has not altered been as altered it gets as dissolved it gets dissolved in PVDF, in which PVDF can, which be confirmed can be confirmed as there isas no there peak is there is no peak shifting occurred in the polymer-ceramic CF. shiftingno peak occurredshifting occurred in the polymer-ceramic in the polymer- CF.ceramic CF.

Figure 1. Synthesis of particle and piezoelectric nanogenerator; (a) BaTiO3 particles by solid-state reaction method and surface morphology of the polycrystalline particles; (b) the solvent casting route and systematic steps for formation of the PVDF-BTO composite film, the contact angle of the

composite film showing hydrophobic nature of their surfaces; (c) XRD pattern of the BaTiO3 parti- cles and (d) shows the XRDFigure pattern 1. ofSynthesis the PVDF-BaTiO of particle3 composite and piezoelectric films at different nanogenerator wt% of particles; (a) BaTiO3 particles by solid-state Figure 1. Synthesis of particle and piezoelectric nanogenerator; (a) BaTiO particles by solid-state (1, 5, 7, 10 wt%) into the reactionPVDF matrix. method and surface morphology of the polycrystalline particles;3 (b) the solvent casting reaction method and surface morphology of the polycrystalline particles; (b) the solvent casting route and systematic steps for formation of the PVDF-BTO composite film, the contact angle of the route and systematic steps for formation of the PVDF-BTO composite film, the contact angle of the Figure 2a shows thecomposite surface filmmorphology showing of hydrophobic the CF. It shows nature that of thetheir BTO surfaces; concentra- (c) XRD pattern of the BaTiO3 parti- tion increases in the PVDFcompositecles and and (d filmthere) shows showing is theno clusteringXRD hydrophobic pattern of ofthe naturethe particles. PVDF of their-BaTiO The surfaces; 103 composite wt% ( cBTO-) XRD films pattern at different of the BaTiOwt% of3 particles and(1, 5, ( d7,) 10 shows wt%) the into XRD the patternPVDF matrix. of the PVDF-BaTiO3 composite films at different wt% of particles (1, 5, 7, 10 wt%) into the PVDF matrix.

Figure2a shows the surface morphology of the CF. It shows that the BTO concentration increases in the PVDF and there is no clustering of the particles. The 10 wt% BTO-PVDF CF show more compactness as compared to other CF. The EDS spectra of the 10 wt% BTO-PVDF shows that all the elements are present without any trace of impurity. Figure2b shows the frequency-dependent dielectric constant at room temperature. It shows that the dielectric contact rises as the BTO particles in the PVDF increases. The pure PVDF shows Crystals 2021, 11, x FOR PEER REVIEW 5 of 10

Figure 2a shows the surface morphology of the CF. It shows that the BTO concentra- tion increases in the PVDF and there is no clustering of the particles. The 10 wt% BTO- PVDF CF show more compactness as compared to other CF. The EDS spectra of the 10 wt% BTO-PVDF shows that all the elements are present without any trace of impurity. Figure 2b shows the frequency-dependent dielectric constant at room temperature. It shows that the dielectric contact rises as the BTO particles in the PVDF increases. The pure PVDF shows 23 dielectric constant values at 1 kHz while the 10 wt% BTO-PVDF CF shows Crystals 2021, 11, 85 5 of 10 47 at 1 kHz, suggesting that there is a rise in connectivity between the particles as the BTO content rises to improve the dipole-dipole interaction [33]. The reason behind the increase in the dielectric constant in low frequency is mainly due to the effect of various polariza- 23 dielectric constant values at 1 kHz while the 10 wt% BTO-PVDF CF shows 47 at 1 kHz, tions, and with a risesuggesting in frequency, that there the is apolarization rise in connectivity activity between fades, theleading particles to constant as the BTO or content reduced dielectricrises constant to improve magnitude. the dipole-dipole Figure interaction 2c shows [33 the]. The dielectric reason behind loss versus the increase fre- in the quency at room temperaturedielectric constant for various in low frequencyCF. The isinterfacial mainly due polarization to the effect ofcould various lead polarizations, to a rise in the loss factorand in with the a low rise- infrequency frequency, region. the polarization The heterogeneity activity fades, in leading the CF to, constantwith a ris ore reduced in the BTO contentdielectric in the constantPVDF, magnitude.is also responsible Figure2c shows for increasing the dielectric the loss loss versus factor frequency as the at room BTO. Another possibletemperature cause formay various be from CF. the The air interfacial voids ionization polarization, which could leadcreates to a plasma rise in the loss factor in the low-frequency region. The heterogeneity in the CF, with a rise in the BTO as an contentis applied in the to PVDF, the CF. is alsoFigure responsible 2d shows for increasingthe digital the image loss factor of the as fabricated the BTO. Another piezoelectric devicepossible, which cause is flexible may be fromand thebendable. air voids The ionization, ferroelectric which createspolarization plasma of as the an electric pure PVDF and PVDFfield- isBTO applied (10 towt%) the CF.is compared Figure2d shows in Figure the digital S1b. imageThe remnant of the fabricated polarization piezoelectric of the PVDF is 0.52device, μC/cm which2, whereas is flexible for and the bendable. PVDF The-BTO ferroelectric CF it is 0.66 polarization μC/cm of2. theIt demon- pure PVDF and strates that as the PVDF-BTOBTO content (10 wt%)increases is compared in the inPVDF, Figure the S1b. dielectric The remnant constant polarization and ferroe- of the PVDF is 0.52 µC/cm2, whereas for the PVDF-BTO CF it is 0.66 µC/cm2. It demonstrates that as the lectric polarization are both improved in the material. It is also noted that the relation BTO content increases in the PVDF, the dielectric constant and ferroelectric polarization are below links the dielectricboth improved property in the to material. the piezoelectric It is also noted voltage that the co relation-efficient: below gij links= dij/(ε the0ε dielectricr). Therefore, as the propertydielectric to constant the piezoelectric increases, voltage the co-efficient: piezoelectricgij = doutputij/(ε0εr). Therefore,response asshould the dielectric also rise. constant increases, the piezoelectric output response should also rise.

FigureFigure 2. Properties 2. Properties of the PVDF-BTO of the PVDF composite-BTO films; composite (a) the surfacefilms; morphology(a) the surface of pure morphology PVDF and PVDF-BTO of pure PVDF composite filmsand (1, 5, PVDF 7, 10 wt%);-BTO the composite EDS spectra films of the (1, PVDF-BTO 5, 7, 10 wt%); of 10wt% the BTOEDS into spectra PVDF of is the examined, PVDF and-BTO all of the 10wt% elements BTO match well withinto itsPVDF chemical is examined composition;, and (b, call) dielectric the elements constant match and dielectric well with loss its of thechemical pure PVDF composition; and composite (b, films;c) die- (d) the imagelectric of the constant flexible PNG10 and dielectric piezoelectric loss nanogenerator of the pure device.PVDF and composite films; (d) the image of the flex- ible PNG10 piezoelectric nanogenerator device. Figure3a shows the working mechanism of the piezoelectric nanogenerator device. In general, if a mechanical force is applied to a PENG device, there is the generation of positive and negative piezoelectric potential in the materials leading to an electrical output at both ends of electrodes. It is seen as the pressure is acted upon by the piezoelectric nanogenerator, the electrons flow from one end to the other, leading to the flow of current

Crystals 2021, 11, x FOR PEER REVIEW 6 of 10

Figure 3a shows the working mechanism of the piezoelectric nanogenerator device. In general, if a mechanical force is applied to a PENG device, there is the generation of Crystals 2021, 11, 85 positive and negative piezoelectric potential in the materials leading to an electrical6 out- of 10 put at both ends of electrodes. It is seen as the pressure is acted upon by the piezoelectric nanogenerator, the electrons flow from one end to the other, leading to the flow of current through an external load. As the for forcece is removed removed,, the stored electrons move through thethe external circuit in a reversereverse mannermanner creatingcreating aa negativenegative voltage.voltage. This repeated process leads to the generation of the output of PENG. Figure 33bb showsshows thethe representationrepresentation ofof thethe dipolesdipoles;; as we apply a vol voltagetage of 1 kV across the piezoelectric CF the dipoles are aligned aligned,, which isis requiredrequired to to achieve achieve higher higher piezoelectric piezoelectric output. output. Figure Figure3c shows 3c shows the layer the bylayer layer by layerarrangement arrangement of the of PENG the PENG device; device it has; several it has several layerssandwiched layers sandwiched into one into structure. one struc- The ture.antistatic The tapeantistat is usedic tape to is remove used to any remove stray chargesany stray in charges the air atmosphere in the air atmosphere while the PDMS while theencapsulation PDMS encapsulation act as a packing act as layer.a packing Figure layer.3d,e Figure shows 3d,e the voltageshows the and voltage current and output current of outputthe various of the PENG various devices. PENG The devices. 1 wt% BTOThe in1 wt% PVDF BTO (PNG1), in PVDF 5 wt% (PNG1), BTO in 5 PVDF wt% (PNG5),BTO in PVDF7 wt% (PNG5), BTO in PVDF7 wt% (PNG7),BTO in PVDF and 10 (PNG7), wt% BTO and in10 PVDFwt% BTO (PNG10) in PVDF are (PNG10) the abbreviated are the abbreviatednames of the names fabricated of the PENG fabricated devices. PENG It is seen devices. that theIt is PNG20 seen that shows the a PNG20 higher outputshows ofa voltagehigher output 7.2 V/current of voltage 38 7.2 nA. V/current Figure3f presents38 nA. Figure the switching 3f presents polarity the switching test confirming polarity that test confirmingthe generated that outputs the generated are due outputs solely to are the due piezoelectric solely to the effect. piezoelectric It is to note effect. that It no is changeto note thatin phase no change in the peakin phase pattern, in the indicative peak pattern, of a trueindicative piezoelectric of a true response. piezoelectric response.

Figure 3. Experimental for the piezoelectricpiezoelectric nanogenerator.nanogenerator. ( a(a)) Working Working mechanism mechanism of of the the fabricated fabri- cated piezoelectric generator, (b) representation of the poling and dipole arrangement on applying piezoelectric generator, (b) representation of the poling and dipole arrangement on applying different different voltage, (c) layer by layer schematic of the piezoelectric nanogenerator device, (d,e) volt- voltage, (c) layer by layer schematic of the piezoelectric nanogenerator device, (d,e) voltage and age and current of the piezoelectric nanogenerator of various composition, (f) the reverse and for- wardcurrent voltage/switching of the piezoelectric polarity nanogenerator test of the PNG10 of various device. composition, (f) the reverse and forward voltage/switching polarity test of the PNG10 device.

Figure4a,b shows the current and voltage of the PNG10 device at various acceler- ations/frequencies. It is seen that at higher acceleration/frequencies, the output of the device is not uniform as the device may be depolarized with higher acceleration. Figure4c shows the loading matching analysis of the PNG10 device. The power density (P = V2/R) of the PNG10 device under optimized conditions is determined by varying the load match- ing R from kΩ to GΩ and measuring the corresponding voltage. It was observed that a Crystals 2021, 11, x FOR PEER REVIEW 7 of 10

Figure 4 a,b shows the current and voltage of the PNG10 device at various accelera- tions/frequencies. It is seen that at higher acceleration/frequencies, the output of the de- vice is not uniform as the device may be depolarized with higher acceleration. Figure 4c 2 Crystals 2021, 11, 85 shows the loading matching analysis of the PNG10 device. The power density7 of 10(P = V /R) of the PNG10 device under optimized conditions is determined by varying the load matching R from kΩ to GΩ and measuring the corresponding voltage. It was observed that a power density of 0.8 μW/cm2 at 100 MΩ was achieved by the PNG10 device. Figure power density of 0.8 µW/cm2 at 100 MΩ was achieved by the PNG10 device. Figure4d 4d shows the charging of the various capacitor with different capacitance. Figure 4e shows shows the charging of the various capacitor with different capacitance. Figure4e shows the chargingcharging and and discharging discharging curve curve of the of 4.7theµ 4.7F capacitor μF capacitor several several times, whichtimes, shed which light shed light upon thethe fact fact that that the the output output generated generated by theby PNG10the PNG10 device device is stable is stable for a long for time.a long time. Figure4 f4f shows shows the the stored stored charge charge in each in ea capacitorch capacitor that was that calculated was calculated using the using formula the formula Q == CV.CV.

Figure 4. 4.Electrical Electrical response response of theof the piezoelectric piezoelectric nanogenerator. nanogenerator (a,b). Voltage (a,b) Voltage and current and measure-current meas- menturement of the of PNG10 the PNG10 device device at various at various accelerations/frequencies, accelerations/frequencies (c) the instantaneous, (c) the instantaneous output power output densitypower density as a function as a offunction different of external different load external resistance, load (d resistance) charging, curve (d) charging of two capacitors curve of with two capaci- differenttors with capacitances, different capacitances (e) charging, ( ande) charging discharging and curve discharging of the 4.7 curveµF capacitors of the 4.7 showing µF capacitors stable show- ing stable output from the device, (f) the amount of stored charge in the different capacitors. output from the device, (f) the amount of stored charge in the different capacitors.

TheThe real-time real-time energy energy harvesting harvesting from from the PNG10 the PNG10 was carried was carried out using out several using activi-several activ- ties.ities. Figure Figure5 shows 5 shows the the output output voltage voltage when when a coin a was coin dropped was dropped upon the upon device the PNG10 device PNG10 it couldcould sense sense the the impact impact from from the the dropping dropping of the of coin. the Incoin. Figure In Figure5b, the PNG105b, the devicePNG10 device waswas connectedconnected to to the the fan, fan and, and when when the fanthe wasfan turnedwas turned on at itsonhighest at its highest speed, thespeed device, the device could produce 6.9 V. Figure5c,d demonstrates the biomechanical energy harvesting from could produce 6.9 V. Figure 5c,d demonstrates the biomechanical energy harvesting from the PNG10 device was tested by simple finger tapping and the voltage of 1.4 V and current ofthe 24 PNG10 nA. To demonstrate,device was tested the powering by simple of the fin calculatorger tapping in Figure and the5e,f, voltage two PNG10 of 1.4 devices V and current wereof 24 connected nA. To demonstrate, in parallel to increasethe powering the current of the produced calculator from in the Figure device, 5e and,f, thistwo wasPNG10 de- utilizedvices we directlyre connected to charge in a parallel 10 µF capacitor. to increase The ACthe signalcurrent produced produced by the from two the PNG10 device, and devicesthis was is utilized converted directly to DC to via charg a bridgee a 10 rectifier μF capacitor. circuit. WhenThe AC the signal switch produced was ON, theby the two calculatorPNG10 devices began tois converted work and the to DC capacitor via a wasbridge discharged. circuit. Figure5 fWhen shows the the switch digital was ON, imagethe calculator of the demonstration began to work of and the powering the capacitor of the was calculator. discharged. Hence Figure the fabrication 5f shows ofthe digital theimage PVDF-BTO of the demonstration CFs based flexible of the nanogenerator powering of paves the calculator. the way towards Hence the a sustainable fabrication of the power source and a promising entrant for designing self-power applications. Figure S2a PVDF-BTO CFs based flexible nanogenerator paves the way towards a sustainable power shows the result of the manual bending test: as the device is flexible, it could easily bend andsource produce and a an promising electrical output. entrant Figure for desig S2b,cning show self the-power electrical applications. response of theFigure device S2a shows whilethe result performing of thean manual exercise bending like a basic test squat: as exercise.the device The is device flexible, generated it could an electricaleasily bend and voltageproduce of an 6 V electrical and a current output. of 19 Figure nA, demonstrating S2b,c show the that electrical the presented response PENG of device the device can while beperforming used for counting an exercise the number like a basic of times squat the exercise. exercise was The performed. device generated an electrical volt- age of 6 V and a current of 19 nA, demonstrating that the presented PENG device can be used for counting the number of times the exercise was performed.

Crystals 2021, 11, 85 8 of 10 Crystals 2021, 11, x FOR PEER REVIEW 8 of 10

FigureFigure 5. Applications 5. Applications of harvestin of harvestingg energy energy and andpowering powering electronics electronics.. (a) Electrical (a) Electrical response response of of PNG10PNG10 when when a coin a coindrop drop upon upon it, (b it,) (Electricalb) Electrical response response of ofthe the PNG PNG-10-10 device device when when connected connected to to fan; (fan;c,d) (Voltagec,d) Voltage and andcurrent current response response of PNG10 of PNG10 device device upon upon finger finger tapping tapping,, (e) (connectede) connected two two PNG10PNG10 devices devices in parallel in parallel and andcharge charge a 10 a µF 10 µcapacitorF capacitor,, and and (f) ( fPower) Power supply supply to to low low power power rating rating devicesdevices like calculator like calculator (switch (switch OFF OFF and and switch switch ON ON condition). condition).

4. Conclusion4. Conclusions The ThePVDF PVDF-BTO-BTO composite composite films films are are highly highly flexible flexible and and synthesized synthesized via via cost cost-effective-effective processingprocessing known known as the as thesolvent solvent casting casting route. route. The The XRD XRD pattern pattern confirms confirms the the crystallinity crystallinity featuresfeatures,, and andthe surface the surface morphology morphology suggests suggests that that the the particles particles are are uniformly uniformly distributed distributed upon the PVDF. The BTO particles crystallize with tetragonal symmetry. The dielectric upon the PVDF. The BTO particles crystallize with tetragonal symmetry. The dielectric constant increases with the doping of the BTO particles into the PVDF, showing that there constant increases with the doping of the BTO particles into the PVDF, showing that there are dipole-dipole interactions. Individual PENG devices are tested after the polling condi- are dipoletion. The-dipole PNG10 interactions. device delivers Individual an output PENG of voltage devices 7.2 are V tested and 38 after nA. An the instantaneous polling con- dition.power The densityPNG10 ofdevice 0.8 µ deliversW/cm2 atan a output matching of voltage resistance 7.2 ofV and 100 M38Ω nA.is achievedAn instantaneous from the powerPNG10 density device. of 0.8 The μW/cm stable output2 at a matching of the device resistance can be seenof 100 from MΩ the is continuous achieved from charging the PNG10and device. discharging The curvestable ofoutput the commercial of the device capacitor. can be The seen PENG from device the continuous can power upcharging the cal- and culatordischarging via a bridgecurve rectifier,of the commercial switch, and capacitor. charging capacitor The PENG of 10deµviceF. The can proposed power up device the calculatorfabrication via a is bridge adaptable rectifier, and straightforward switch, and charging to harness capacitor energy of from 10 differentμF. The vibrationsproposed deviceor motionsfabrication in theis adaptable environment. and Itstraightforward is many benefits to like harness eco-friendly, energy cost-effective,from different and vi- brationseasily or scalable motions methods in the environment. to meet the power It is many requirements benefits oflike micro/nano eco-friendly, electronic cost-effective, devices and ineas theily future.scalable methods to meet the power requirements of micro/nano electronic de- vices in the future. Supplementary Materials: The following are available online at https://www.mdpi.com/2073-435 2/11/2/85/s1, Figure S1: (a) A particle size distribution histogram determined from the SEM images Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: of BTO particles; (b) P-E hysteresis loop of PVDF and PVDF-BTO (10 wt%), Figure S2: (a) Electrical (a) A particle size distribution histogram determined from the SEM images of BTO particles ; (b) P- response of the PENG device upon the hand bending; (b,c) voltage and current of the PENG device E hysteresis loop of PVDF and PVDF-BTO (10 wt%), Figure S2: (a) Electrical response of the PENG upon the repetitive Squat exercise Table S1: Comparison of the piezoelectric nanogenerator based on device upon the hand bending; (b,c) voltage and current of the PENG device upon the repetitive polymer ceramic composites; device output and composite film preparation. Squat exercise Table S1: Comparison of the piezoelectric nanogenerator based on polymer ceramic composites;Author Contributions:device output andConceptualization, composite filmS.H.; preparation. methodology, M.S.; formal analysis, K.L. and M.S.; data curation, P.D.; writing—original draft preparation, M.S. and S.H.; writing—review and editing, Author Contributions: Conceptualization, S.H..; methodology, M.S.; formal analysis, K.L. and K.M., S.H. and H.J.K.; supervision, H.J.K.; funding acquisition, H.J.K. All authors have read and M.S.; data curation, P.D.; writing—original draft preparation, M.S. and S.H.; writing—review and agreed to the published version of the manuscript. editing, K.M., S.H. and H.J.K.; supervision, H.J.K.; funding acquisition, H.J.K. All authors have read and agreed to the published version of the manuscript. Funding: This study is funded by the Basic Science Research Program through the National Re- search Foundation of Korea (NRF) and funded by the Ministry of Science and ICT of Korea (2018R1C1B6008041) and the DGIST R&D Program of the Ministry of Science and ICT of Korea (19-RT-01).

Crystals 2021, 11, 85 9 of 10

Funding: This study is funded by the Basic Science Research Program through the National Research Foundation of Korea (NRF) and funded by the Ministry of Science and ICT of Korea (2018R1C1B6008041) and the DGIST R&D Program of the Ministry of Science and ICT of Korea (19-RT-01). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest.

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