Stretchable Electrospun PVDF-HFP/Co-ZnO Nanofibers as Piezoelectric Nanogenerators

Herein, we investigate the morphology, structure and piezoelectric performances of neat polyvinylidene fluoride hexafluoropropylene (PVDF-HFP) and PVDF-HFP/Co-ZnO nanofibers, fabricated by electrospinning. An increase in the amount of crystalline β-phase of PVDF-HFP has been observed with the increase in Co-doped ZnO nanofiller concentration in the PVDF-HFP matrix. The dielectric constants of the neat PVDF-HFP and PVDF-HFP/2 wt.% Co-ZnO nanofibers are derived as 8 and 38 respectively. The flexible nanogenerator manipulated from the polymer nanocomposite (PVDF-HFP/Co-ZnO) exhibits an output voltage as high as 2.8 V compared with the neat PVDF-HFP sample (~120 mV). These results indicate that the investigated nanocomposite is appropriate for fabricating various flexible and wearable self-powered electrical devices and systems.

Owing to the escalating worldwide concern over energy calamity and environmental issues, attention has been diverted towards future energy. Recently, rechargeable power and renewable energy sources have been considered as an attractive alternative for environmental problems 1 . Among them, mechanical energy is one of the most abundant energy sources, and easily convertible to other useful energy forms. Piezoelectric materials can convert the mechanical energy produced by various activities into electrical energy. e mostly employed piezoelectric materials include barium titanate and lead zirconate titanate, which exhibit superior e ciency for energy conversion and high piezoelectric constants. However, these materials show disadvantages such as cost intensive nature, toxicity, brittleness and they are non-environmentally friendly. In order to overcome these problems, the research community is strongly motivated to identify new piezoelectric materials that are lightweight, exible, biocompatible and electroactive [2][3][4][5][6][7] .
Some of the semi crystalline polymers such as polyvinylidene uoride (PVDF) and its copolymers are well known materials for fabricating piezoelectric devices 8 . ese polymers are used in many elds, such as portable electronic devices, exible pressure sensors, energy harvesting systems, water purifying devices and in gas separation [8][9][10] . In the neat form, these polymers have poor electrical and mechanical performance and the enhancement of their piezoelectricity is still a challenge 11 . Reports suggest the addition of nano llers in various dimensions to PVDF and its copolymers like polyvinylidene uoride hexa uoropropylene (PVDF-HFP) to enhance the piezoelectric, pyroelectric and ferroelectric performances. e piezoelectricity of PVDF-HFP is mainly contributed by polar crystalline phases such as β-phase and γ-phase, rather than the α-phase. Among the various crystalline phases, the electrocactive β-phase imparts the highest dipole moment resulting in high piezoelectricity 12 . e β-phase content of such polymers can be increased by various techniques, such as stretching, combined stretching and poling of polymer lms or ber formation methods at given temperatures. Among the various techniques, electrospinning is the most e ective in producing self-poled piezoelectric nano bers because of the high stretching forces exerted on electri ed solution jets. However, PVDF-HFP lms or bers partially depolarize a er mechanical stretching and even electrospinning via thermal motion can bring back the polymer to the stable curled state. Recently, the addition of nano llers has emerged as an alternative chemical method to increase the β-phase content by introducing speci c polymer-ller interactions and stabilize the β-phase nanocrystals [13][14][15] .
Here we report the fabrication of PVDF-HFP/Co-doped ZnO composite nanofibers by electrospinning method, and their piezoelectric properties. To the best of our knowledge, no prior work regarding the piezoelectric properties of PVDF-HFP/Co-ZnO nanocomposites has been reported till date. To investigate their structural, morphological and dielectric behaviours, the nano bers were characterized by X-ray di raction technique (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM). e composite SCIENTIFIC REPORTS | (2018) 8:754 | DOI:10.1038/s41598-017-19082-3 containing 2 wt.% of Co-doped ZnO nanoparticles achieved a piezoelectric performance of 2.8 V. e detailed investigation on the mechanism related to the piezoelectric property has also done.
Co-doped ZnO and undoped ZnO nanoparticles were prepared by hydrothermal method. In this process, zinc acetate and cobalt chloride salts were dissolved in water and stirred for 2 h to obtain a clear and homogeneous solution. To avoid the agglomeration of nanoparticles, PEG was added to the resultant solution which was used as a surfactant and again stirred for another 1 h. e resulting solution was transferred to Te on capped autoclave and kept at 140 °C for 15 min. A er the reaction process, the autoclave was cooled down to room temperature. e obtained precipitate was washed with water and ethanol, and the whole process was repeated for the neat ZnO. Finally, the powders were dried at 80 °C in a hot air oven for 12 h and then annealed in tube furnace at 400 °C for 2 h.
PVDF-HFP solution was prepared by the addition of 2 g PVDF-HFP pellets to a 1:1 mixture of DMF and acetone (15 ml solvent) and stirred for 3 h at 70 °C. e undoped ZnO (1 wt.%) and Co-doped ZnO (0.5, 1 and 2 wt.%) nanopowders were dispersed in the same solvent mixture (5 ml) and then sonicated for 2 h. e dispersed ller solutions were mixed with each of the PVDF-HFP solution and the whole mixture was magnetically stirred overnight to obtain a homogeneous mixture. Finally, those solutions were used for electrospinning. Figure 1 shows the sample preparation procedure and the electrospinning setup to obtain the composite nanobers. e PVDF-HFP and its nanocomposite solutions were electrospun at a rate of 1 ml/h and the nano bers were collected in a rotor, rotating at a speed of 500 rpm. e tip-to collector distance was xed as 15 cm and the applied voltage was 12 kV a er optimization. e electrospun nano bers were collected on an aluminium foil substrate pasted on the collector. e morphology and structure of the nanopowders and the polymer composites were investigated by scanning electron microscope (SEM, XL-30E Philips Co., Holland), transmission electron microscope (FEI TECNAI G 2 TEM), XRD di ractometer (Mini Flex 2, Rigaku equipped with Nickel ltered CuKα radiation (λ = 0.1564 nm) operated at 30 V and 15 mA in the 2θ range of 10-30° at a scanning speed of 1.8°/min) and FTIR spectroscope (PerkinElmer Spectrum 400 spectrophotometer in the range 400-4000 cm −1 with a resolution of 2 cm −1 ). Dielectric properties were tested by broadband dielectric/impedance spectroscopy-Novocontrol at a frequency range of 10 1 to 10 7 . e piezoelectric property of the samples were analysed by means of an experimental setup consisting of a frequency generator, shaker and the output measurement system. A schematic of the PVDF-HFP composite nano bers based energy harvester is shown in Fig. 2 along with the experimental setup. e nanogenerator shown in Fig. 2a was made by silver electroding at both surfaces and wires were attached through aluminium foil electrodes. Conducting carbon tape was used to x the device to the substrate. e sample was placed on the top of a shaker and speci c weight was placed over it. An ampli ed sinusoidal voltage generated by a frequency generator was sent to the vibrating shaker, which vibrates according to the weight placed on the top of it (Fig. 2b). e

Results and Discussion
e surface morphology of undoped ZnO and Co-doped ZnO samples were characterized by SEM as shown in Fig. 3(a,b). For ZnO, the nanorods are oriented and assembled to form a ower like structure as given in Fig. 3a. Addition of the Co dopant, develops individual rods with hexagonal cross sections (Fig. 3b). e surface morphology was further con rmed by TEM studies (Fig. 3c,d), which also substantiates the ower like and hexagonal morphology for the undoped ZnO and Co-doped ZnO. e structural properties of ZnO and Co-doped ZnO nanoparticles, synthesized by the hydrothermal method were characterized by X-ray di raction technique and the obtained patterns are shown in Fig. 3e,f. All the diffraction peaks were well matched with the joint committee on powder di raction standard (JCPDS card no: 89-0510) which indicates the hexagonal wurzite structure for the ZnO and modi ed ZnO. No di raction peaks related to Co agglomeration or cobalt oxide can be observed, which implies that the crystal structure of ZnO does not change due to the incorporation of Co ions. is also suggests the uniformly substituted Co ions in the ZnO lattice sites. Figure 3f shows the e ect of Co doping in ZnO lattice, it is observed that the (100) peak is shi ed towards a lower 2θ value when cobalt is incorporated in ZnO and also the intensity of the peak is decreased and broadened for Co-doped ZnO sample, con rming the decreased crystallite size. e crystallite size is an important parameter for the enhancement of output performance of the nanogenerator 16 . According to Paria et al. 17 , the piezoelectric response was increased with a decrease in the size of the nanoparticles. We have investigated the crystallite size of undoped and Co-doped ZnO nanoparticles following the De-bye Scherrer's formula 18 .
Where λ is the X-ray wavelength (1.5406 Å), β is the full width at half maximum and θ is the Braggs angle. e calculated crystallite size for undoped ZnO and Co-doped ZnO were 23 nm and 18 nm respectively, con rming the decreased crystallite size with the addition of cobalt ions. e phase and crystallinity of the neat PVDF-HFP and PVDF-HFP/Co-ZnO nano bers were studied by analysing the X-ray di raction pattern as shown in Fig. 4. e observed di raction peaks correspond to the semicrystalline PVDF [19][20][21] . e crystalline α, β and γ phases of PVDF-HFP were well-tted with a Gaussian function for all the samples and the electroactive polarized α, β and γ phases were calculated from the de convoluted peaks.
It is also observed from Fig. 4 that the increase in Co-doped ZnO nano ller concentration enhanced the β-phase for PVDF-HFP/Co-ZnO. A similar result was reported by Kumar et al. 26 , with doped AlO-rGO lled PVDF-HFP composite system, in which the β-phase crystallinity was enhanced. In addition, the di raction peaks correspond to Co-doped ZnO nanoparticles were also observed in the range of 30°-60° for the PVDF-HFP/ Co-ZnO nanocomposites. It is concluded from the XRD results that the addition of the Co-doped ZnO nano ller enhanced the β-phase formation in the PVDF-HFP nano bers. e intensity of the β-phase peaks increases with the increasing the Co-ZnO ller concentration as well (Fig. S1 in supporting information).
The crystallinity of PVDF-HFP/Co-ZnO nanofibers were further analyzed by FTIR spectra and represented in Fig. 5. e vibrational peaks observed at 611, 760, 795, 1146, 1210 cm −1 were due to the α-phase of PVDF-HFP [27][28][29] . e peaks at 1270, 840, 878 cm −1 correspond to the β-phase 30 . It is clear from the FTIR spectra that the intensity of the peak at 840 cm −1 for the doped samples is higher than the neat PVDF-HFP nano bers. It can also be observed that the α-phase at 611, 760, 795, 971, 1146 and 1210 cm −1 disappear in Co-ZnO nanocomposites con rming the increase in β-phase with the ller concentration. Many researchers have reported that the surface charges on the Co-doped ZnO nano llers interact with the molecular dipoles (CH 2 or CF 2 ) of PVDF-HFP and improves the β-phase content of the composites 31,32 . is can also be explained on the basis of positive charges that present in the Co-ZnO nano llers that interact with -CF 2 -dipoles of the PVDF-HFP segments in the nanocomposites. e peaks at 3030-2930 cm −1 wavelength (Fig. S2, Supporting Information) are assigned to the symmetric (ν s ) and asymmetric (ν as ) stretching vibration bands of PVDF-HFP and its nanocomposites. e symmetric and asymmetric vibration bands shi towards lower frequency region in the composites when compared to the neat polymer. is indicates the interaction of surface charges of the Co-doped ZnO nanoparitlces with CH 2 and CF 2 dipoles of PVDF-HFP matrix 33 . In addition, the spectra exhibited a broad band in the 3800-3200 cm −1 region (Fig. S2, Supporting Information) due to the formation of intermolecular H-bonds between the  Table 1, from which it is clear that, compared to the neat PVDF-HFP, the β-crystalline phase content is more for the Co-doped ZnO composites. e morphology of PVDF-HFP and PVDF-HFP/Co-ZnO samples were investigated by FE-SEM and the images are depicted in Fig. 6. e defect free electrospun bers obtained for neat PVDF-HFP and the PVDF-HFP/ Co-ZnO composites suggest no bead formation. e formation of straighter, homogeneous, dense and defect free nano bers is attributed to the increase in charge density of the polymer solution, as reported by Mandal et al. 33 . It is also seen that all the nanoparticles are distributed uniformly in the PVDF-HFP matrix with no agglomeration.
Di erential scanning calorimetry (DSC) is used to study the thermal properties of the samples (Fig. S3, Supporting Information). e melting peaks were found to increase with increase in the ller loading, which can be due to the homogeneous dispersion of Co-doped ZnO nano llers in the polymer matrix and also the nano llers act as nucleating agents (that can increase the crystallinity of the composites) 36 . e degree of crystallinity (X c ) was calculated using the following Equation 37 .
Where ∆H m and ∆H m o are the melting enthalpies of the composites and neat polymer, respectively. e melting enthalpy for neat PVDF-HFP is xed as 104.5 Jg −1 . e calculated values for the crystallinity are also included in Table 1. is is in accordance with the FTIR results. Figure 7a,b show the frequency dependent dielectric constant (ϵ′) and dielectric loss (ϵ″) of the neat PVDF-HFP and its nanocomposites. It can be seen that the ϵ′ increases by the addition of Co-ZnO nano llers. e values for both ϵ′ and ϵ″ are high in the low frequency region due to space charge e ects and interfacial polarization 38 . Also, the dielectric constant decreases at the high frequency region due to the slower dipole mobility 39 . e observed increase in dielectric constants with the ller loading is related with the decrease in ller-ller distances that enhances dipole-dipole polarization in the nanocomposites 40 . e high dielectric relaxations    happening at low frequency called Maxwell-wagner relaxations are usually generated due to the interfacial e ects arising between the llers and polymer matrix 41 . For piezoelectric nanogenerator, high dipole polarization is important for the high output performance 42 . e conductive behaviour or electric heterogeneous nature of the composites or interfacial polarization between ller and polymer interfaces can be responsible for the high dielectric constant 43 . Figure 7c shows the variations in conductivity of the samples with respect to frequency. For all composites the conductivity shows similar behaviour with that of the neat polymer. is is because the lower concentrations of the ZnO and Co-ZnO semiconducting llers within the composites. e reciprocal of the frequency at which the composite shows lowest dielectric loss is de ned as the relaxation time and those calculated values are represented in Fig. 7d. e decrease in relaxation time from 191 µs for the neat polymer to 84 µs for the Co-doped ZnO sample con rms the dominating dipole polarization in the composite. Figure 8a shows the piezoelectric properties of PVDF-HFP and PVDF-HFP nanocomposites films. Piezoelectric output voltages of 2 V, 2.4 V and 2.8 V were respectively achieved from the nanogenerator containing ller loadings of 0.5, 1 and 2 wt.% of Co-ZnO. e output voltage was very low; 120 mV for neat PVDF-HFP and it is high for all the other nanocomposites. is can be attributed to the presence of piezoelectric ZnO in the nanocomposites in various concentrations 44,45 . In addition, the increase in output voltage is due to the relative proportions of polar β-phase present in the nanocomposites. It is evident from the XRD and FTIR results that the addition of Co-doped ZnO enhances the β-phase of PVDF-HFP. us, the piezopotential of the nanocomposites increases linearly with an increase in nano ller concentration. is can be the reason for the enhanced output voltage for the Co-doped ZnO nanocomposites. Also, the XRD results con rmed the enhancement of β-crystalline phase in the Co-ZnO nanocomposites.
In other words, the β-phase for the polymer nanocomposite increases with increasing ller concentration and it can be attributed to the interaction between the oppositely charged Co-ZnO surface and the -CF 2 -/ -CH 2 -dipoles of PVDF-HFP. is enhances the nanoparticles nucleation and the piezo electric polar β-phase can be stabilized by surface charge induced polarization 46 . In addition, by an application of mechanical force, the crystal structure of the PVDF-HFP/Co-ZnO bers were deformed and the external pressure creates a potential in the Co-doped ZnO nanorods, which aligns the dipoles in the PVDF-HFP matrix 47 . ese types of nanocomposites are suitable for touchable sensors such as those on the foot paths, bridges, shoes, vehicles and self-charging battery separators 48 . Fig. 8b shows the time-dependent generation of output voltage with the ller loading concentration under the constant tapping force of 2.5 N and the frequency at 50 Hz. Figure 9 shows the time-dependence of output voltage from the nanogenerator with 2 wt.% nano ller loading over the full frequency range of mechanical vibration. It is important to study the relationship between the output performance of the piezoelectric nanogenerator and the di erent frequencies of applied force, because the mechanical energy from the ambient environment largely varies and is irregular 49 . In our case, the output voltage was measured repeatedly in the frequency range 15-50 Hz and from the gure, the output voltage slightly increases with the increase in frequency. e power generation mechanism for the piezoelectric nanogenerator is shown in Fig. 10. It is established that, by an application of mechanical force, the electric dipoles in the crystal get oriented along a direction which is called the stress-induced poling e ect 50 . When the force is released then the electron stream goes back through the external load, and hence both positive and negative voltage peaks can be observed under pressing and releasing during vibration. From the gure, a positive and negative piezoelectric potential was observed which is due to the deformation of the crystal structure. It can also be explained, in terms of the dipole alignment in the PVDF-HFP matrix and surface charges on the Co-ZnO nanorods. When the force is applied to a material, it creates a potential di erence on the nanoparticles surface which aligns the dipoles uniformly in the direction of applied force. When the mechanical force is released, the electrons ow back to the electrode and produce  electric signal in opposite direction 51 . e composite bers upon stretching, twisting and bending modes are also represented in the gure, which supports the use of this material in self-powering devices of wearable electronics.

Conclusion
In summary, we have successfully prepared PVDF-HFP/Co-ZnO nano bers by electrospinning method. e structural characterization of the samples explored by XRD and FTIR studies shows higher β-phase content for PVDF-HFP/Co-ZnO (2 wt.%) nano bers than the neat PVDF-HFP sample. e incorporation of Co-doped ZnO nano llers enhances the nucleation and stabilization of the piezoelectric polar β-phase. It was inferred that the electrospinning method and Co-doped ZnO nanoparticles has a strong e ect on structural and morphological properties of the nanocomposites which reveals a signi cant e ect on piezoelectric properties. e highest piezoelectric output voltage of 2.8 V was observed for 2 wt.% Co-ZnO/PVDF-HFP nano bers. e increasing output voltage is due to the impact of Co-doped ZnO nano ller on the electroactive β-phase of PVDF-HFP in addition to the in uence of modi ed ZnO. e enhanced piezoelectric e ciency suggests the use of these nano bers in electronics and biomedical elds.