Optimizing Piezoelectric Coefficient in PVDF Fibers: Key Strategies for Energy Harvesting and Smart Textiles

With the advancement in smart electronic devices and self‐powered devices, the demand for piezoelectric polymers has found potential research interest. Among these, electrospun polyvinylidene fluoride (PVDF) fibers have gained attention for energy harvesting due to their flexibility and higher piezoelectric coefficient. In the current work, various methods are compared to enhance PVDF's piezoelectric properties, including different solvents (DMAc, DMF), conductive filler (rGO), and annealing as post‐treatment. The results indicate that PVDF/rGO fibers in DMAc solvent exhibit the highest β phase fraction and crystallinity. Moreover, for the first time, the piezoelectric properties of PVDF/rGO electrospun single fiber is presented using high voltage switching spectroscopy piezoelectric force microscopy (HVSS‐PFM). The highest piezoelectric coefficient (d33) is measured for PVDF/DMAc‐rGO composite fibers. Notably, PVDF/rGO in DMAc solvent significantly improves the piezoelectric coefficient, leading to a remarkable fourfold increase in power density compared to pure PVDF, making it a promising material for energy harvesting applications.


Introduction
[3] To date, mechanical energy is considered the extensively used renewable energy harvesting approach due to its vast availability everywhere. [4]ence, piezoelectric materials have found immense research attention due to their unique property of converting mechanical energy into electrical energy and vice-versa. [5]Various piezoelectric materials such as lead zirconate titanate (PZT), [6] barium titanate (BaTiO 3 ), [7] zinc oxide (ZnO), [8] gallium nitride (GaN), [9] DOI: 10.1002/aelm.202300404 and sodium potassium niobate (KNN) [10] were used to generate energy.The relatively low piezoelectric power output from GaN and ZnO limits their practical applications as flexible energy harvesters. [10]Several studies have been performed on PZT and its composites due to the highest piezoelectric coefficient of PZT (175 pC N−1 ). [6,11]Though, employing toxic Pb-based piezoceramics poses a hazard to the environment as well as human health. [12]Wu et al., [10] prepared a flexible lead-free polydimethylsiloxane (PDMS) composite film consisting of KNN piezoelectric particles with a composition of 0.91K 0 .48 Na 0 .52 NbO 3 -0.04Bi0 .5 Na 0 .5 ZrO 3 -0.05AgSbO 3 -0.2%Fe 2 O 3 (KNN-BNZ-AS-FE).However, preparing the KNN-BNZ-AS-FE particles is quite complex and involves many steps.Also, the PDMS-KNN-BNZ-AS-FE film requires poling at the high electric field.Contrary to above mentioned piezoelectric materials, piezoelectric polymers possess many advantages, such as flexibility, lightness, high deformability, and easy fabrication. [11,13]This makes them a suitable candidate for wearable, flexible, biomedical, and implantable electronic devices. [14,15]However, the polymer's relatively low piezoelectric coefficient compared to ceramic materials limits their practical applications. [16]olyvinylidene fluoride (PVDF) and its copolymers found great research interest owing to its excellent piezoelectric, pyroelectric, and ferroelectric properties. [11]Amongst the existing piezoelectric polymers, it has the highest piezoelectric coefficient and dielectric permittivity. [1,17]The low cost, high flexibility, mechanical stability, low density, and biocompatibility of PVDF polymers further strengthen its applications in numerous fields, especially in energy harvesters, smart textiles, and small electronic devices. [18,19]PVDF is a semicrystalline polymer and has different chain conformations in five crystalline phases, namely,  (TGTG'-trans-gauche-trans-gauche),  (TTT-all trans),  (TGTG'), ϒ(T3GT3G'), and Ɛ (T3GT3G'). [20]The piezoelectric property of PVDF is caused by the intense electric dipole moment that occurs in the PVDF monomer.This is due to the higher electronegativity of fluorine atoms than hydrogen and carbon atoms.From the PVDF chain conformation,  and Ɛ have zero dipole moment, though , , and ϒ (electroactive phase) possess a net dipole moment.[22] The non-polar  phase is mainly formed when PVDF is cooled from a melt form. [23]An external polarization process or mechanical stretching increases PVDF's polar  phase content. [24]Nevertheless, the polarization at a high electric field can cause electrical breakdown and structural damage. [25]In this context, electrospinning is considered a facile and efficient technique for producing self-poled PVDF nano/microfibers. [26,27]Through electrospinning, PVDF polymer with enhanced piezoelectric properties can be prepared by a single-step mechanical stretching and electrical poling process. [1,28]Moreover, using this technique, it is possible to tune the surface morphology and properties of polymer fibers by controlling specific parameters such as applied voltage, [29] polymer, [30] temperature, [31] humidity, [32] solution feed rate and many more. [1,33]The structural changes affect not only the piezoelectric properties but also the mechanical and surface properties. [34,35]n addition to the electrospinning conditions, other factors, such as solvents, polymer concentration, molecular weight, solution conductivity, surface tension, and viscosity, significantly improve the  phase content and crystallinity of the PVDF electrospun fibers. [36,37]The major impact on PVDF crystallinity and the piezo response was noticed with solvents, additions of fillers based on graphene oxide (GO) or ceramic nanoparticles, and postprocessing, such as annealing.Several studies have recently been published on the structural property's relationship between the effects of different solvents and electrospinning parameters on the morphology and crystallinity of PVDF electrospun fiber. [1,38,39]The most commonly used solvent is DMF in most of the electrospun PVDF studies. [38,40]Still, there is a literature gap on how the solvents affect PVDF's crystallinity,  phase content, and piezoelectric coefficient.Also, no comprehensive study was reported comparing the electrospinning of PVDF solution with dimethylacetamide (DMAc) and DMF solvents, which are widely used to produce PVDF fibers.Therefore, we carried out a detailed study on crystallinity,  phase content, and piezoelectric coefficient of PVDF single fibers and mats based on DMAc and DMF solvents.
[43] Additionally, certain fillers such as carbon nanotubes (CNTs), [44] nanoclays, [45] ZnO, [46] KNN, [47] BaTiO 3 , [48] graphene and its derivatives [49] induce an electroactive phase in the polymer matrix.Recently, GO and reduced graphene oxide (rGO) fillers have received significant research attention due to their high surface area and excellent mechanical and electrical properties. [50,51]Aside from increasing the -phase content, induced charge transfer directly impacts electrical power generation. [28,52]Because of their high electrical conductivity, GO and rGO are used as conductive fillers, allowing energy harvesting and conversion applications. [53,54]ang et al. [52] compared the effects of GO and in situ reduced rGO on PVDF electrospun nanofibers.The study demonstrates that the conductivity of rGO is more than four times higher than that of GO.
Post-treatment is another common practice to enhance the piezoelectric response of PVDF and its copolymers. [55,56]In addition to mechanical stretching and electric polarization, thermal annealing is one of the effective methods to alter the phase transition, orientation of chains, and crystallinity of PVDF. [57,58][60][61] However, no comprehensive study has been reported based on the effect of annealing post-treatment on electrospun PVDF and composite and comparing the piezoelectric constant of individual fibers and bulk fiber mats.
So far, it has been proved that the abovementioned affect the piezoresponses of electrospun PVDF.Still, there is no comparison of which is the most effective strategy.Therefore, in this study, we aim to verify the effect of the two best solvents: DMAc and DMF, adding rGO to fibers and annealing as the posttreatment.Here, we compare these strategies regarding  phase content, crystallinity, and piezoelectric constant of PVDF fibers.Furthermore, the first-time high voltage switching spectroscopy piezoelectric force microscopy (HVSS-PFM) results for individual fibers were compared with the piezoelectric coefficient of the fiber mat.In the current trends of smart textiles developments and their importance for energy harvesting applications, selecting the best way to produce high-piezoresponse PVDF fibers is crucial for enhancing energy generation performance and saving production costs.

Results and Discussion
We produced randomly oriented PVDF and PVDF-rGO nanocomposite fibers using two solvents, DMAc and DMF, via electrospinning.Comparing the synergistic effects of solvents, rGO filler, and the post-annealing treatment allowed us to control the piezoelectric coefficient.Therefore, it helps to choose the best method to prepare the PVDF mats on a large scale for smart textiles and energy harvesting applications.The schematic representation of the steps followed to prepare the PVDF/rGO composite is presented in Figure 1.Finally, all produced materials were analyzed based on their electroactive characteristics.
The SEM micrographs for all the PVDF fibers and composites are presented in the Supporting Information in Figure S1 (Supporting Information).We have obtained bead-free and uniform PVDF fibers by optimizing the electrospinning parameters shown in Table S1 (Supporting Information).From the fiber diameter distribution curve (Figure 2a,b), it can be observed that the average fiber diameter of the PVDF/DMAc was 1.39 ± 0.4 μm, which is less than the average fiber diameter of PVDF/DMF fibers (1.63 ± 0.4 μm).The evaporation-stretching function can cause a difference in fiber diameter between the two electrospun membranes during electrospinning.Also, solution conductivity plays an important role in stretching the charged polymer jet in electrospinning, [62] which causes structural changes in deposited fibers. [63]The conductivity measured for the DMAc solvent was 0.300 ± 0.013 μScm −1 , which is higher than the conductivity of the DMF solvent 0.241 ± 0.013 μScm −1 .The SEM and corresponding fiber diameter distribution curves of the annealed PVDF/DMAc and PVDF/DMF fibers revealed that the morphology of the annealed fiber membrane looks like the pristine PVDF fibers both in DMAc and DMF solvent.No noticeable morphological changes have occurred in the annealed fibers compared with pristine fibers.The annealing treatment could not significantly change the fiber diameter in aPVDF/DMAc or aPVDF/DMF fibers.Satthiyaraju et al. [64] found similar results by annealing the PVDF fiber at different temperatures and observed no significant change in the morphology of fibers.Gee et al. [65] performed optimization studies for PVDF nanofiber membranes with three different solvents, DMF, DMSO, and N-Methyl-2pyrrolidone (NMP), and demonstrated that DMF/Acetone produced membrane had the smallest average fiber diameter compared to DMSO and NMP.Also, DMF/Acetone showed the highest  phase fraction among DMSO and NMP.However, there is no information regarding the relative humidity of electrospinning, which can vary fiber morphology and chemical composition of surfaces. [26]in et al. [5] studied the effect of solvents (DMF, DMSO) and electrospinning parameters on the piezoelectric properties of PVDF polymer with an atmospheric environment of a temperature of 20 ± 3 • C and humidity of 45 ± 5%.Contrary to the results showed by Gee et al [65] ., Yin et al. presented that DMSO/Acetone solvent had a smooth surface and uniform shape of the nanofibers. [5,65]However, DMF/Acetone solvent results in beaded nanofiber with various diameters and consistent morphology.The authors described that this is due to the higher conductivity of the DMSO/Acetone solvent.Also, they reported that the evaporation of the solvent effect has less effect on the fiber diameter than the conductivity of the solution.These agree with our results, though the boiling point is higher for DMAc than DMF.However, DMAc has higher solution conductivity and allows for obtaining a lower fiber diameter.This confirms the more pronounced effect of conductivity on the process than the solvent evaporation during electrospinning.
Adding 1 wt% rGO to PVDF/DMAc and PVDF/DMF electrospun fibers results in a bead-free structure (see Figure S1, Supporting Information).Moreover, no volatilized solvents were found in the composite fibers.Furthermore, the fiber diameter of PVDF/DMAc-rGO and PVDF/DMF-rGO was 0.61 ± 0.3 and 0.70 ± 0.3 μm, respectively.The addition of rGO filler caused an increase in the electrical conductivity of the electrospinning solution.Hence, the fiber diameter was reduced in the PVDF-rGO composite compared to the pristine PVDF fibers.At a high voltage applied during the electrospinning, the conductive rGO can be easily charged.Hence, the electrostatic repulsion and Coulomb force increased on the Taylor cone, resulting in the subsequent decrease in fiber diameter. [52,66]hus, irrespective of the solvents, rGO improved the conductivity and reduced the diameter of the PVDF fiber.The composite PVDF/DMAc-rGO and PVDF/DMF-rGO fibers' annealing shows very similar morphology and fiber diameter, which is evident from the fiber diameter distribution graph, shown in Figure 2a,b.
For the FTIR spectrum shown in Figure 2c,d, for all the samples, the vibrational bands at 613, 763, and 976 cm −1 were the characteristic peaks for the non-polar  phase.Whereas the peaks at 840, 1276, and 1430 cm −1 were attributed to the  phase. [21,67]ompared with the PVDF fibers, the characteristic peak intensities of  phase in electrospun rGO/PVDF composite fibers decreased.The relative fraction of the  phase (F()) for each sample was calculated by analyzing the FTIR results using the following Equation (1) [67] ; where A  and A  denote absorptions at 840 cm −1 and 763 cm −1 , respectively.K  and K  represented the absorption coefficients of corresponding wave numbers, which were 6.1 × 10 4 and 7.7 × 10 4 cm 2 mol −1 , respectively. [52,67]The  phase fraction in the composite fiber incorporating rGO was significantly higher than in the pristine PVDF fiber.Furthermore, with the addition of rGO, the absence or negligible absorbance of  phase at 613, 763, and 976 cm −1 was observed.As seen, the intensity of the  phase was increased in the annealed fibers.However, the intensity of the  phase remains the same.In the case of the PVDF/DMAc-rGO composite (Figure 2c), the absorbance intensities of the  phase (840 and 1276 cm −1 ) were higher than those of the PVDF/DMAc fiber.This result reveals that adding rGO enhances the  phase fraction in the PVDF fibers.The  phase content (%) for PVDF/DMAc and aPVDF/DMAc were found to be 60% and 63%, respectively, while incorporating 1wt% of rGO into the PVDF matrix increased the  phase fraction to 69%.For aPVDF/DMAc-rGO fiber, the increase was 70% (Table S2, Supporting information).About 10% higher amounts of  phase content were obtained for the PVDF/DMAc-rGO composite than for the pristine PVDF fiber.Figure 2d represents the FTIR spectra of the PVDF/DMF-rGO composite.Compared with the PVDF fiber prepared in DMAc solvent, the PVDF/DMF fiber exhibits a decrease in  phase content (59%), whereas an increment in  phase content up to 62% was observed for the aPVDF/DMF fiber.The absorbance intensity peaks of the  phase were lower for PVDF and PVDF-rGO composite prepared in DMF solvent than for the PVDF/DMAc composite fiber.Additionally, the corresponding  phase peaks at 613, 763, and 976 cm −1 were more pronounced in PVDF/DMF and PVDF/DMF-rGO composites compared to the PVDF/DMAc and rGO composites.The addition of rGO and annealing of the composite fibers in DMF solvent led to an increase in the  phase content of 64%. Figure 3 illustrates the  phase content of all samples from the FTIR analysis.From the FTIR results, we can conclude that PVDF in DMAc solvent is more responsive toward rGO filler and annealing effects.Unlike DMAc solvent, DMF does not respond significantly to filler and the annealing effects.In literature, Yin et al. [5] studied the effect of solvents (DMF and Dimethyl sulfoxide (DMSO)) and electrospinning parameters on the piezoelectric properties of PVDF polymer.The authors reported that DMSO/Acetone (2/1) solvents showed the highest crystallinity (52.36%),  phase fraction (88.01%), and piezoelectric voltage output compared to DMF.Gee et al. [40] analyzed three solvents' effects (DMF, DMSO, NMP) on an electrospun PVDF membrane.Their study revealed that DMF/Acetone exhibited the highest  phase fraction (90.9 ± 1.81 %) among DMSO and NMP.In this study, DMAc/Acetone showed higher  phase content (60%) than DMF due to the higher electrical conductivity of DMAc.In the case of PVDF-DMAc/rGO, the increased  phase content of 69% can be explained as follows: rGO acted as a nucleating agent and creating a substrate enables the growth of PVDF crystal nucleation and promoting  phase formation of the PVDF chains by strong interactions at the interface. [68]In Further, thermal analysis was carried out using differential scanning calorimetry (DSC) to determine the solvents, annealing, and addition of rGO effects on electrospun PVDF's melting and crystallization behavior.The DSC scans for PVDF/DMAc-rGO and PVDF/DMF-rGO composites are presented in Figure 2e,f.Table S2 (Supporting Information) shows the melting temperature (T m ), melting enthalpy (∆H m ), and crystallinity content ( c ) of all the samples.The crystallinity content ( c ) of the PVDF fibers was calculated using the following equation. [28] where ∆H 100 represents the melting enthalpy of 100% crystalline PVDF (104.54J•g −1 ). [28]rom Figure 2e, annealing and addition of rGO fillers displayed higher melting enthalpy (∆H m ) and crystallinity compared to the pristine PVDF/DMAc fibers.PVDF/DMAc exhibited a melting temperature of 169.58 ˚C, a melting enthalpy of 44.09 J•g −1, and a crystallinity content of 42.17%.The annealing effect on the electrospun fiber mildly affects the melting enthalpy and, thereby, the crystallinity of the annealed fiber.The slight increment in crystallinity of the annealed fiber can be attributed to the high energy taken from the polymer chains to transform the amorphous form into a crystalline form. [64]Compared to pristine PVDF fiber, the incorporation of 1wt% of rGO led to a significant increase in T m (171.57˚C), ∆H m (50.07 J•g −1 ), and crystallinity content (47.89%) of the PVDF composite.This increment in crystallinity indicates that the rGO filler acted as a nucleating agent for the electroactive phases in the PVDF during the electrospinning.This result agrees with the FTIR spectra for PVDF/DMAc-rGO and PVDF/DMF-rGO composites.The higher content of  phase and crystallinity indicates the excellent interaction and affinity between the rGO filler and the PVDF matrix. [69,70]aur et al. [71] prepared the PVDF thick film by solution casting and investigated the influence of annealing on structural, crystalline, dielectric, and polarization behavior.The study unveiled that annealing PVDF films at 100 ˚C improves dielectric permittivity and polarization due to an increased crystalline  and phase interface.Nevertheless, a few studies were reported on the annealing effect of PVDF electrospun fibers for energy harvesting applications. [57,64]Satthiyaraju et al. [64] examined the annealing of electrospun PVDF at different temperatures.Here, from the graph in Figure 2e, it observed a twin peak through the melting response of aPVDF/DMAc-rGO.The annealing of PVDF-rGO causes further amorphization of the polymer fiber and reduces the crystallinity of the composite.Similar trends were observed for the PVDF/DMF-rGO composite (Figure 2f).Compared to PVDF/DMAc, a slight decrease in the melting enthalpy and crystallinity of PVDF/DMF can be observed.Satthiyaraju et al. [64] reported the increased crystallinity for PVDF nanofibrous film annealed at 100 ˚C.However, in our study, the annealing at 100 ˚C showed a slight enhancement in crystallinity regardless of the solvent used.Additionally, no comprehensive study has been reported on the annealing effect of PVDF-rGO composite electrospun fibers.Similar to FTIR results, Yin et al. [5] reported the higher crystallinity content from DSC for DMSO/Acetone than DMF/Acetone.It confirms that the solution with higher conductivity shows improved -phase content and crystallinity.
Figure 3 presents the relationship between crystallinity and  phase fraction of all the PVDF fibers and composites.All types of PVDF samples with DMAc show higher crystallinity and  phase.Adding rGO to the PVDF/DMF raises the crystallinity from 41.68% to 43.48%.However, compared with the PVDF/DMAc-rGO, the PVDF/DMF-rGO showed a lower crystallinity content.Corresponding to the aPVDF/DMAc-rGO composite, aPVDF/DMF-rGO also displayed a decrease in crystallinity compared to PVDF/DMF-rGO.This result further confirms the amorphization of the composite after annealing.Importantly, FTIR and DSC confirm that adding 1 wt% of rGO into PVDF/DMAc leads to the highest content of  phase and crystallinity.
Interestingly, the annealing effects on the fibers were not prominent in DMAc or DMF solvents.Also, for PVDF/rGO composites, the annealing effect reduces the crystallinity irrespective of the solvents.Therefore, we selected PVDF/DMAc, PVDF/DMAc-rGO, PVDF/DMF, and PVDF/DMF-rGO samples to further verify the piezoelectric response of the electrospun fibers by HVSS-PFM.The topographic images of the fibers from atomic force microscopy (AFM) are shown in Figure S2 (Supporting Information).The AFM images agree with the SEM results that rGO filler reduces the fiber diameter of PVDF fibers.HVSS-PFM data presented in Figure 4 for PVDF/DMAc, PVDF/DMAc-rGO, PVDF/DMF, and PVDF/DMF-rGO were measured by applying the voltage from −30 to +30 V by keeping the tip at a fixed point on the surface of the PVDF single fiber.In Figure 4, the PFM phase and amplitude loops are presented, and from them, the hysteresis behavior under the effect of an electric field was determined.The PFM phase versus voltage graph for PVDF/DMAc and PVDF/DMAc-rGO, in Figure 4a, exhibits a square-shaped hysteresis loop with a 180°phase shift between the positive and negative voltages.Figure 4b shows the amplitude versus voltage butterfly curve for PVDF/DMAc and PVDF/DMAc-rGO.The forward and reverse coercive voltages were obtained at +14 and −14 V, respectively, for PVDF/DMAc.At a DC bias of 60 V, the maximum amplitude reached 106 ± 3 pm.In the case of PVDF/DMAc-rGO, the highest amplitude value was 104 ± 2 pm.Correspondingly, the forward and reverse coercive electric fields were noted at +17 and −4 V, respectively.PVDF/DMAc-rGO fiber has a narrower butterfly loop than PVDF/DMAc fiber, indicating that a smaller electric field was required to generate a piezoelectric response to switch the polarization.
For PVDF/DMF and PVDF/DMF-rGO fibers, the butterfly loops, and hysteresis curves, presented in Figure 4c,d are qualitatively similar to PVDF/DMAc fibers with a phase change of 180˚.However, PVDF/DMF showed a lower maximum amplitude value of 69 ± 4 pm(Figure 4d) with a forward coercive field of 1 V and reverse coercive field of −26 V. Adding rGO increased the amplitude value for PVDF/DMF and obtained the peak amplitude value of 76 ± 3 pm.PVDF/DMF-rGO also showed a narrow hysteresis curve with a forward and reverse coercive voltage of 11 and −7 V, respectively.This result confirms that adding rGO reduces the electric field required to switch the polarization of PVDF and provides an enhanced piezoelectric response.Additionally, we must note that the amplitude value may change according to the different cantilevers and microscopes used to measure the fiber response.
Moreover, the piezoelectric coefficient (d 33 ) measurements were carried out to understand the relationship between the crystallization and piezoelectricity for bulk fiber mats.Table S3 (Supporting Information) lists the d 33 values of all PVDF mats and the amplitude values from HVSS-PFM.The d 33 values of PVDF/DMAc and PVDF/DMAc-rGO, PVDF/DMF and PVDF/DMF-rGO are 7.3 ± 0.2, and 11.9 ± 0.2, 5.6 ± 0.2, and 7.2 ± 0.2 pC N N−1 , respectively, see Figure 5.This result agrees well with the FTIR and DSC results, where PVDF/DMAc-rGO fiber has the highest  phase content and crystallinity.Therefore, it displayed a higher d 33 value.From the literature studies, we can find that in the case of electrospun fibers, the type of electrode coating affects the obtained value of the d 33 coefficient. [72,73]Most literature studies lack information regarding the electrode material used to measure the d 33 piezoelectric coefficient.However, considering the porosity of the electrospun fiber mat (approximately 90%) [74,75] for the d 33 measurements, the contact electrode is mostly measuring the air.Therefore, the d 33 cannot be expected to be as high as for typically spin-coated or casted PVDF films. [24,76]rom our study, using DMAc as a solvent and adding rGO as filler is the most promising strategy for electrospun PVDF mats to obtain high efficiency in energy harvesting or sensing applications.Notably, the highest conductivity of DMAc (0.300 ± 0.013 μScm −1 ) over DMF (0.241± 0.013 μScm −1 ) and just 1% addition of rGO is able to increase the  phase fraction over standard PVDF by 9%. Figure 5 shows the relation between the piezoelectric coefficient of the PVDF mats,  phase fraction of the mats, and the amplitude of the single PVDF fibers.The amplitude values between PVDF/DMAc and PVDF/DMAc-rGO were almost similar.Therefore, the difference in the performance between the individual fibers and mats for the PVDF/DMAc-rGO individual fibers measured using HVSS-PFM can be related to the enhancement of piezoresponse performance when more fibers are in contact with the mat.In addition, the random distribution of rGO for the whole mat has an intensified effect than in the individual fibers.
From the above characterization studies and discussions, it is evident that the higher electroactive  phase is induced in electrospun fiber in DMAc solvent and the presence of rGO particles.Thus, the PVDF/DMAc-rGO mat is suitable for fabricating a piezoelectric energy harvester to convert mechanical energy into useful electrical energy .We have selected PVDF/DMAc as a reference material to compare the energy harvesting performance of the PVDF/DMAc-rGO device.A detailed description of the device fabrication was given in the Experimental Section.The piezoelectric responses of the fabricated device were recorded by applying repeated human finger tapping and releasing movements, see Video S1 (Supporting Information).Figure 6a-d presents the open circuit voltage and short circuit current signals obtained from PVDF/DMAc and 1 wt% PVDF/DMAc-rGO composite der finger tapping and releasing.The device can convert the finger tapping motion (mechanical movements) into electricity by repeated pressing and releasing movements. [24]By finger tapping, we change the polarization of the PVDF, and a piezoelectric potential is developed between the two electrodes.Simply, the potential difference is causing the transitory passing of the free electrons from one electrode to another, leading to a positive voltage peak.The instant release of stress caused by the finger press, a negative output voltage appears owing to the decrease in the piezo potential and discharging of the accrued electrons. [24]From Figure 6a,b, PVDF/DMAc-rGO showed a higher output voltage (6 ± 1.4 V) than pure PVDF/DMAc (3.4 ±1 V).Similarly, the current signals exhibited more than double the output value of 320 ± 81 nA from the PVDF/DMAc-rGO (Figure 6d).
The PVDF/DMAc displayed an output current of 133 ± 45 nA (Figure 6c).The higher output voltage and current signals from PVDF/DMAc-rGO can be explained by the highest amplitude and piezoelectric coefficient values from HVSS-PFM and d 33 studies.Furthermore, rGO promoted and induced the nucleation of electroactive -phase in the PVDF matrix as in the DSC section.In addition, rGO creates a conduction network that efficiently diminishes the internal resistance of the piezo device. [77,78]Subsequently, it enhances the transfer of induced charge produced by PVDF and thus exhibits the high piezoelectric performance of the device. [52]Also, rGO causes an increase in the conductivity of the PVDF solution during electrospinning, and it helps to provide more nucleation sites and increase the PVDF's electrical properties. [52,66]Here, for PVDF/DMAc-rGO, the combined effect of DMAc solvent with higher conductivity and rGO filler's electrical conductivity improves the piezoelectric performance.Further, the power density of the piezo device was calculated.Figure 7a illustrates the power densities of PVDF and rGO/PVDF composite.PVDF/DMAc gives a power density of 0.452 ± 0.21 μW.cm −2 , and PVDF/DMAc-rGO showed an increased power density of 1.92 ± 0.71 μW.cm −2 , which is more than 4 times higher.
Previously, a few studies have been reported on PVDF/rGO electrospun composite.A comparison graph with the rGO content and the energy harvesting outputs (power density, voltage, and current) values from the literature and the results obtained in this work are illustrated in Figure 7b.Ongun et al., [66] demonstrated the finger tapping and releasing test on PVDF and rGO/PVDF composites with 0.4 and 0.8 wt% rGO loading.The solvents used to prepare the electrospun PVDF and composites are DMF and acetone (1:1 v/v).The results indicated that 0.8 wt % rGO/PVDF has the highest  phase content and generated an output voltage of 4.38 V, which is approximately 9 times higher than pure PVDF (0.50 V).However, there is no information regarding the generated current and power output values.Yang et al., [52] fabricated the piezoelectric nanogenerator (PENG) based on electrospun PVDF and PVDF/rGO nanocomposite, using DMF solvent and rGO additions 1, 1.5, and 2 wt%.The authors reported that, under finger pressing, the 2wt% rGO-PVDF had reached a short circuit current of 700nA, voltage of 16 V, and a power density of 2.8 μW.cm −2 for a film area of 6.25 cm 2 .The power density for 2wt% rGO-PVDF is 150 times higher than pristine PVDF (18.75 nW.cm −2 ).In this study, an active PVDF fiber area of 1×1 cm 2 under finger tapping reaches a power density of 0.452 μW.cm −2 , voltage, and current output of 3.4 V, and a current of 133 nA.Therefore, the results again proved that the PVDF fibers prepared in DMAc solvent have better piezoelectric performance than DMF.In addition, from our studies, 1 wt % rGO generated a power density of 1.92 ± 0.71 μW.cm −2 for an area of only 1×1 cm 2 .Hence, these results supported that the PVDF/DMAc-rGO had enhanced piezoelectric properties that can be applied in energy harvesting and sensor applications.

Conclusion
In this study, we verify different strategies, such as the effect of most used solvents (DMAc, DMF), annealing, and addition of rGO filler to electrospun PVDF membrane to control the piezoelectric response.By comparing all three effects, the addition of rGO showed the highest crystallinity,  phase fraction, and piezoelectric coefficient compared to annealing.On the contrary, for the composite fibers (PVDF/DMAc-rGO, PVDF/DMF-rGO), annealing reduces the crystallinity and  phase fraction due to the amorphization of the polymer fiber.In literature, DMF is the most used solvent for PVDF electrospinning.However, from our findings, DMAc exhibited a higher piezoelectric response compared to DMF due to the higher conductivity of DMAc.Also, adding rGO to PVDF/DMAc has a noticeable impact on the piezoelectric response.The FTIR and DSC results revealed that PVDF/DMAc-rGO presented the highest crystallinity,  phase fraction, and piezoelectric coefficient.Here, rGO acted as a nucleating agent and induced the PVDF chain's  phase formation.For the first time, we are reporting the piezoelectric properties of PVDF/rGO single electrospun fiber in DMAc and DMF solvents by using the HVSS-PFM technique.The HVSS-PFM results further confirm the FTIR and DSC results and have the highest amplitude for PVDF/DMAc-rGO fiber.
Nonetheless, in alignment with FTIR and DSC results, PVDF/DMF-rGO has not significantly improved the piezoelectric coefficient compared to PVDF/DMF fiber from the HVSS-PFM study.Therefore, to present the best combination among the methods mentioned above and make it useful at the application level, we have measured the d 33 value of the PVDF membrane.From d 33 measurements, we again confirm that PVDF/DMAc-rGO and PVDF/DMAc exhibited the highest value of 11.9 ± 0.2 pCN −1 and 7.3 ± 0.2 pCN −1 , respectively.Therefore, by comparing different strategies to control the piezoelectric properties of PVDF electrospun fiber, we found that the best method is to use DMAc as the solvent and conductive filler (rGO) for energy harvesting applications.Therefore, by comparing different strategies to control the piezoelectric properties of PVDF electrospun fiber, we found that the best method is to use DMAc as the solvent and conductive filler (rGO) for energy harvesting applications.The finger-tapping test was conducted to show the straightforward practical application of the PVDF/DMA-rGO mat.The results revealed that 1 wt% rGO content achieved a power density of 1.92 ± 0.71 μW.cm −2 , which is more than 4 times higher than pure PVDF.The rGO with high conductivity enhanced the transferring of induced charge generated by PVDF, thereby improving the piezoelectric energy harvesting and conversion ability of PVDF.
PVDF pellets were dissolved in DMAc/Acetone solvent in a 1:1 (wt/wt) ratio by constant magnetic stirring.The temperature was set at 55˚C on a hot plate (IKA RCT basic, Germany) for 4 h at a constant speed of 650 rpm to prepare a homogenous 24 wt% PVDF/DMAc solution.Similarly, the PVDF/DMF solution of 24 wt% was prepared by dissolving PVDF pellets in DMF and acetone (1:1 wt/wt).The solution was stirred on a hotplate at 55˚C for 4 h at a speed of 650 rpm.Then, the 1 wt% of rGO was added to the PVDF/DMAc and PVDF/DMF solutions in the following ways to obtain the PVDF/DMAc-rGO and PVDF/DMF-rGO solutions, respectively.First, by ultrasonication (Emag, Emmi-E20, Germany) for 4 h, rGO was dispersed in DMAc and an acetone mixture (1:1).Subsequently, it was added to the prepared PVDF/DMAc (24 wt%) solution and the reaction mixture was stirred for 2 h at 55˚C.Finally, the PVDF/DMAc-rGO solution was subjected to ultrasonication for 2 h prior to electrospinning.Correspondingly, the PVDF/DMF-rGO solution was prepared in the same way.First, the rGO was dispersed in the DMF/Acetone mixture (1:1) by ultrasonication for 4 h, added to the PVDF/DMF (24 wt%) solution, and stirred for 2 h at 55˚C.Lastly, ultrasonication was carried out for 2 h to obtain a homogenized dispersion of rGO in the PVDF/DMF solution before electrospinning.
Electrospinning of PVDF and the PVDF-rGO composites in DMF and DMAc solutions was carried out with SKE Technologies (Italy) with the climate control system.Electrospinning was initiated by loading the PVDF/DMAc solution in a syringe (5 mL) connected with a stainless needle with an outer diameter of 0.8 mm and an inner diameter of 0.5 mm.A positive voltage polarity (+18 kV) was applied to the needle to electrospun PVDF/DMAc and PVDF/DMAc-rGO solutions.The flow rate was set at 1 ml h −1 with a distance of 18 cm between the needle tip and the grounded collector plate for the PVDF/DMAc and PVDF/DMAc-rGO solutions.The electrospun PVDF/DMF fibers were produced by applying a positive voltage of 20 kV with a flow rate of 1 ml h −1 at a distance of 18 cm from the collector plate.PVDF/DMF-rGO solution electrospinning was carried out by applying +18 kV with a flow rate of 1 ml h −1 and kept at the distance of the needle tip to the collector plate at 20 cm.For all samples, ambient temperature and relative humidity (RH) during the electrospinning were kept constant at 25˚C and 30%, respectively.The electrospinning parameters for all the samples are summarized in Table S1 (Supporting Information).In addition, the physical properties of the solvents are presented in Table S4 (Supporting Information).The produced fibers were collected on Al foil for SEM, FTIR, and DSC analysis.For PFM measurements, the fibers were electrospuned directly on Indium tin oxide (ITO) glass.
Postprocessing Treatment: After fabricating electrospun PVDF fibers and PVDF/rGO composite in DMAc and DMF solvents, the samples were kept at room temperature (24˚C).It helps to remove any residual solvents that may be present in the electrospun fibers.The as-obtained PVDF/DMAc, PVDF/DMAc-rGO, PVDF/DMF, and PVDF/DMF-rGO electrospun fibers were annealed at 100˚C in an airflow oven for 3 h.The morphology,  phase content, crystallinity, and piezoelectric response of the annealed samples were compared with those of the electrospun fibers.
Preparation of Piezo Device: The piezo device consisted of a 1 × 1 cm active layer of PVDF mat sandwiched between an Al foil as electrodes on both sides of the mat.A Cu tape was used to take the connection from both electrodes to measure the generated voltage and current.The voltage and current measurements were conducted with three samples for each material, and the average values were taken.
Scanning Electron Microscopy (SEM): The morphology of the PVDF fibers was evaluated using SEM (Merlin Gemini II, Zeiss, Germany) at an accelerating voltage of 3 kV and a working distance of 6 mm.Before imaging, all samples were Au coated with a thickness of 8 nm using a rotary pump sputter coater (Q150RS, Quorum Technologies, UK).The fiber diameters (D f ) were measured from SEM micrographs utilizing the ImageJ (v.1.53c, USA) software and correlated into a histogram provided with standard deviation.
Conductometer: The conductivity of all solvents was measured with a Mettler Toledo Conductometer (Seven Compact S210, Zurich, Switzerland).Five measurements were taken for each solvent, and the average value was taken.
Fourier Transform Infrared Spectroscopy (FTIR): FTIR was performed to analyze the crystallinity and distinguish the different phases in PVDF fibers.Nicolet iS5 FT-IR spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA), with a Ge crystal, was used to conduct the spectroscopy by the ATR technique.During measurements, 64 spectra were repeated with a resolution of 1 cm −1 over the wavenumber range from 600 to 4000 cm −1 .Three measurements were carried out for each sample.
Differential Scanning Calorimetry (DSC): Thermal characterization was performed using a differential scanning calorimeter (Mettler Toledo, Columbus, OH, USA) at a heating rate of 10 K•min −1 in 50-270˚C temperature range.The samples were placed in an Al pan, and the measurements were carried out in a dynamic Ar atmosphere.For each sample, three tests were performed.
Piezoelectric Force Microscopy (PFM): The piezoelectric response of single electrospun PVDF fibers and their composites was analyzed using CoreAFM (Nanosurf, Switzerland).The samples for PFM measurements were prepared by electrospinning the PVDF fibers directly on the ITO substrate (Ossila, Germany).
For the measurements, a conductive Pt/Si coated Si cantilever (PtSi-FM, nanosensors, Switzerland) was used with a spring constant of 2.15 Nm −1 and a resonance frequency of 6 kHz.The topography images were obtained in static PFM mode by applying an AC voltage of 10 V between the cantilever tip and the bottom electrode.The amplitude and phase of the piezoresponse of the fiber were measured by applying a sequence of pulses of DC bias voltage.The spectroscopic technique helps to identify the required voltage for domain switching.This study used a highvoltage (HV) amplifier connected to the user output to achieve the coercive electric field.DC pulses between -30 V and +30 V were applied through the HV amplifier to induce the piezoelectric response.We use tip bias for electrical pulses with a voltage range of 60 V within 20 steps and a pulse time of 0.05 s.The PFM measurements were performed in "on and off states," indicating the DC bias voltage status.Here, AC voltage was continuously applied to measure the piezoelectric signal, and the DC voltage was applied to change the polarization during domain switching.This method helps to reduce the electrostatic effect. [79]S-PFM measurements were performed with three different individual fibers.
Piezoelectric d 33 Meter: The piezoelectric coefficients of the electrospun PVDF membrane were examined with a d 33 meter (YE2730A d 33 meter, Sinocera, China).This instrument was designed specifically for directly measuring the d 33 values of piezoelectric materials.A 20 nm Au layer was coated as an electrode on both sides of the PVDF fiber for the test.Five measurements were performed for each sample, and the average d 33 value was taken.The voltage and current value from the piezo device were measured by a Keithley DMM6500 6 1/2digital multimeter (China).

Figure 3 .
Figure 3. Relationship between crystallinity and  phase fraction of PVDF fiber mats.

Figure 5 .
Figure 5. Relation between the piezoelectric coefficient of the mat, amplitude response of single fiber from HVSS-PFM, and  phase fraction of the mat.

Figure 7 .
Figure 7. a) Power density of PVDF/DMAc and PVDF/DMAc-rGO.b) PVDF/rGO composites energy harvesting performance comparison from literature and this work.