Piezoelectric and optoelectronic properties of electrospinning hybrid PVDF and ZnO nanofibers

Jian Ma; Qian Zhang; Kabin Lin; Lei Zhou; Zhonghua Ni

doi:10.1088/2053-1591/aab747

Introduction

Due to the limited quantity of fossil fuel energy sources, there is a pressing need to develop alternative energy sources. One mechanism that has attracted interest is the use of energy harvesting materials that are able to convert mechanical forces into electrical energy that can power small electronic devices [1]. One class of such energy-harvesting materials are piezoelectric semiconductor nanowires, made from ZnO, [2–5] InN, [6] GaN, [7, 8] CdS, [9, 10] and ZnS [11]. These piezoelectric energy harvesters have shown the ability to convert mechanical vibrations into electrical energy. However, this group of nanogenerators requires accurately well ordered producing conditions and also they are brittle and hard to integrate into a such soft material as a plastic film or fabrics [12]. An additional class of energy harvesting materials are piezoelectric polymers, among which PVDF is the most popular one due to its relatively large piezoelectric coefficients, attractive mechanical properties, ease of processing, and biocompatibility [12–24]. These features render PVDF particularly attractive for wearable and implantable energy harvesting devices. PVDF exists in at least five different crystalline forms: α, β, γ, δ, and ε depending on the chain conformation of trans (T) and gauche (G) linkages [1, 25–27]. Among them, the α-phase is considered to be most abundant naturally occurring form, while the β-phase is responsible for most of PVDF's piezoelectric response. This is due to its polar structure with oriented hydrogen and fluoride (CH₂-CF₂) unit cells along the carbon backbone. Thus, in order to obtain piezoelectric β-phase, untreated PVDF has to be stretched and poled in a strong electric field to align the dipoles.

One technique to produce β-phase PVDF is electrospinning. Electrospinning [28–30] is a versatile process that can produce polymer nanofibers from a wide range of materials. Due to the strong elongation flow in the jet, polymer molecules are usually stretched and aligned along the fiber axis. Previous work has shown that the conventional electrospinning process can transform non-polar α-phase PVDF to polar β-phase material, resulting in piezoelectric nanofibers [31–33]. The near-field electrospinning process also can fabricate piezoelectric PVDF nanofibers on working substrates with in situ mechanical stretching and electrical poling [13, 17, 34]. However, the content of β-phase in electrospun PVDF nanofiber is still low.

To improve the β-phase content of electrospun PVDF nanofibers, several methods have been reported. Damaraju et al found that increasing the electrospinning voltage resulted in an increased β-phase fraction. They report an optimized electrospinning voltage of 25 kV to achieve the highest β-phase fraction [27]. Yu et al found that adding 5% multi-walled carbon nanotubes (MWCNT) to the PVDF electrospinning solution increased both the crystallinity and the proportion of the β-phase [35]. With additional MWCNTs, the surface conductivity of the PVDF nanofiber mats increased, which is thought to further enhance the output power. Both methods can improve the β-phase fraction. In addition, ZnO is an important semiconducting material with unique electronic, optical, and piezoelectric properties. In particular, the combination of both semiconducting and piezoelectric behavior yields unique advantages and novel applications. J Lee et al have demonstrated a highly sensitive and multifunctional sensor using a PVDF/ZnO nanorod composite thin film [36]. Additionally, M Lee et al demonstrated hybrid fiber generators consisting of both ZnO NWs and PVDF infiltrating polymer [37]. Li et al also demonstrated a novel hybrid PVDF and ZnO NWs generator film. In this instance, it is suggested that the ZnO NW serve not only as a piezoelectric material, but also as an additive that increases the formation of β-phase and the stability of the PVDF film by drawing and enlarging the contact surface area [38].

In this work, we utilize electrospinning to fabricate pure PVDF and hybrid PVDF/ZnO composite nanofibers. By comparing the energy harvesting efficiency of a PVDF/ZnO hybrid nanofibers to that of a pure PVDF, we show that the added nanofillers dramatically improves the performance of this device. To investigate the microstructural origins of this increase, we use micro-Raman spectroscopy to measure the relative fraction of β-phase PVDF in individual hybrid and pure nanofibers. Finally, to further demonstrate the potential optoelectronic applications of this novel material system, we measured the electrical behavior of hybrid nanofibers exposure to UV light and observed an increase in electrical conductivity, while such effect is absent in pure PVDF nanofibers.

Experimental investigation

Electrospinning is a versatile electrostatic process capable of producing continuous micro/nanoscale fibers, with diameters ranging from micrometers to several tens of nanometers, from a wide range of materials. To electrospin hybrid PVDF/ZnO nanofibers, 10% (wt%) ZnO nanowires were first ultrasonically dispersed in DMF solution for 12 h. Then 20% (wt%) PVDF powder was dissolved in the resulting suspension. The PVDF and ZnO mixture was stirred at room temperature until the powder completely dissolved. A pure PVDF solution was also prepared at the same weight concentration. The prepared solution was then poured into a glass syringe. A 20 G metal needle was connected to the end of the syringe reservoir. The glass syringe was mounted on a syringe pump, which provided a constant flow rate of 10 μl min⁻¹. Electrospinning was performed at 10–40 kV, and at a fixed distance of 150 mm between the tip of the spinneret and the collector. The high voltage was supplied by an EMCO DX250 DC-DC converter, and measured using a BK precision high voltage digital multimeter probe. To prepare suspended nanofiber samples for easy manipulation, the nanofibers were collected on two grounded electrodes separated by a 5 cm gap (figure 1(a)), resulting in an aligned mat [39, 40]. These suspended nanofibers can be transferred onto other devices for characterization of Raman spectra and their ferroelectric and optoelectronic properties.

Figure 1. Refer to the following caption and surrounding text. — **Figure 1.** Fabrication and structural characterization of PVDF and ZnO hybrid nanogenerator. (a) Schematic of electrospinning PVDF/ZnO hybrid nanofibers onto a substrate. (b) Optical image of the laser focused on a single PVDF nanofiber during Raman spectra measurement. (c) TEM image of the PVDF/ZnO hybrid nanofiber. The ZnO nanowires almost align along with the PVDF nanofiber.
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The Raman spectra of the PVDF and hybrid PVDF/ZnO nanofibers were collected from individual free-standing ones, suspended over Si trenches, at room temperature with the use of 10 mW of radiation at 532 nm (Laser Quantum) at the sample (figure 2(a)). Spectra were accumulated for 3 min and taken with a slit width equivalent to 1.5 cm⁻¹ resolution with a Horiba LabRam HR800 system.

Figure 2. Refer to the following caption and surrounding text. — **Figure 2.** Raman Spectra of PVDF and ZnO hybrid nanogenerator. (a) α-phase of PVDF peaks at 276 cm⁻¹, 413 cm⁻¹, 609 cm⁻¹ and 796 cm⁻¹ while β-phase peaks located at 262 cm⁻¹, 510 cm⁻¹, 839 cm⁻¹ and 1278 cm⁻¹. (b) The hybrid PVDF/ZnO nanofiber show both peaks of ZnO nanowires and β-phase PVDF nanofiber. Compared with pure PVDF nanofiber, the I₈₃₉/I₇₉₄ ratio of the hybrid PVDF/ZnO nanofiber has increased from 1.7 to 2.6, indicating a higher fraction of β-phase in hybrid PVDF/ZnO nanofiber deposited at the same electrospinning voltage.
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To compare the voltage output from both pure PVDF nanofibers and hybrid PVDF/ZnO nanofibers, individual nanofibers (either PVDF or PVDF/ZnO, deposited at 10 kV) were transferred onto a device designed to measure the piezoelectric output voltage (figure 3(a)). The substrate of this device consisted of a 5 mm thick flexible PDMS membrane. The surface of the PDMS was covered by a layer of ESD tape to enhance electrostatic discharge performance [13]. Then double-sided conductive carbon tape was cut and affixed to the top of the ESD tape and two conductors were formed using copper tape. Individual nanofibers were transferred to span the two copper electrodes for piezoelectricity measurements. The contact between the fiber and the two electrodes is very long to ensure sliding is minimized. Figure 3(b) shows the actual experimental strain loading setup. The PDMS substrate was fixed on a long cantilever. The loading force on the plastic substrate was exerted by an electromagnet controlled by a function generator (Stanford Research Systems DS345). The piezoelectric output voltage was measured by a shielded rack-mountable connector (National Instruments BNC-2090) with a low-noise preamplifier (Stanford Research Systems SR560). The I–V measurement is carried out using a Keithley source-measure unit (2400 SourceMeter). For optoelectronic characterization, fibers were exposed to UV light from a 352 nm laser (LaserQuantum).

Figure 3. Refer to the following caption and surrounding text. — **Figure 3.** Piezoelectric and electric conductivity property of individual PVDF and ZnO hybrid nanofiber. (a) Optical image of a nanogenerator comprised of a single hybrid PVDF/ZnO nanofiber, two contact electrodes, and a PDMS substrate. (b) Photo of strain loading experimental setup showing two parts: a fixed cantilever and a fixed electromagnetic. (c) Output voltage of a single nanogenerator measured with respect to time under an applied strain at 10 Hz.
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Results and discussion

Structural characterization

During electrospinning, the elongational flow causes the PVDF molecules to stretch and form crystalline lamellar domains. Thus, disordered molecules transform their structure to a more regular conformation in the crystalline phase. In our previous work, we have demonstrated that the strong elongational forces experienced by the jet during the electrospinning process result in the formation of nanofibers with a high degree of molecular orientation and crystallinity [41, 42]. Raman spectroscopy has shown many advantages for the study of individual fibers, as it provides molecular level information about conformation, interactions, and crystallinity, and requires only a small sample volume. This technique is able to distinguish between the molecular orientation of the crystalline and amorphous phases. Bellan and Craighead first proposed using micro-Raman to characterize the molecular orientation within single nylon-6 electrospun nanofibers [43]. Constantino et al used micro-Raman spectroscopy to investigate the phase transition of PVDF film under controlled rates of stretching [25]. In this study, the phase transition from the non-polar α-phase to the polar β-phase of individual electrospun PVDF nanofibers was investigated using micro-Raman spectroscopy.

Figure 2(a) presents the Raman spectra recorded with the 532 nm laser line for PVDF powder and PVDF nanofiber electrospun at 20 kV and 40 kV separately. The assignment of bands in the Raman spectrum of PVDF and ZnO is given in detail in table 1 [25, 26, 44]. The Raman spectra of PVDF powder showed characteristic α-phase peaks at 276 cm⁻¹, 413 cm⁻¹, 609 cm⁻¹, and 796 cm⁻¹. In contrast, the electrospun PVDF nanofiber exhibited β-phase peaks located at 262 cm⁻¹, 510 cm⁻¹, 839 cm⁻¹, and 1278 cm⁻¹. In addition, the intensity of these β-phase peaks became larger when the electrospinning voltage was increased to 40 kV. The transition from α-phase to β-phase can be monitored by comparing the relative intensities of the bands at 794 cm⁻¹ (indicative of α-phase) and 839 cm⁻¹ (indicative of β-phase) [25, 26]. It is seen in figure 2(a) that the relative intensity of the band at 794 cm⁻¹ is much higher than 839 cm⁻¹ in the Raman spectra of PVDF powder Raman spectra (indicating a predominance of α-phase), while this relationship is reversed in the Raman spectra of PVDF electrospun nanofiber (indicating a dramatic increase in the β-phase content). Quantitatively, the ratios between the bands at 839 and 794 cm⁻¹ are 0.41, 1.7, and 3.8, for the PVDF powder, PVDF nanofiber electrospun at 10 kV and 40 kV, respectively. It is know that, in solution, some PVDF molecular chains are folded locally in the form of small crystallites, usually termed as 'lamellae' [45, 46], while other molecular chains are randomly entangled; the 'lamellae' serve as common joints for the entangled PVDF molecular chains [45, 47]. When these PVDF molecular chains are stretched by the electrospinning jet, both lamellae and entangled molecular chains will be aligned in linear orientation. As the voltage increases, the PVDF molecular chains experience stronger elongational forces at higher electrospinning voltage, giving rise to fibers with a higher degree of β-phase.

Table 1. Band assignments for Raman spectra of PVDF and ZnO nanofiber.

Band	Assignments
262	β-CF₂ twisting
276	α-CF₂ twisting/wagging
413	α-CF₂ rocking
510	β-CF₂ scissoring
609	α-CF₂ scissoring/CCC scissoring
796	α-CH₂ rocking
839	CH₂ rocking/CF₂ antisymmetric stretching
1426	CH₂ scissoring/CH₂ wagging
436	the E₂ (high frequency) optical phonon mode of ZnO

Figure 1(c) is a high resolution TEM image of an individual hybrid PVDF/ZnO nanofiber. The TEM image shows that the ZnO nanowires are generally aligned along the PVDF nanofiber axial axis and mostly continuous. To determine whether there is a difference in performance between a pure PVDF nanofiber and a hybrid PVDF/ZnO nanofiber, it is needed to ensure each hybrid PVDF/ZnO nanofiber measured actually contains ZnO NWs inside. Figure 2(b) shows the Raman spectra of ZnO nanowires, a pure PVDF nanofiber and a hybrid PVDF/ZnO nanofiber. The Raman spectrum of ZnO nanowires exhibited peaks located at 327 cm⁻¹, 377 cm⁻¹, 436 cm⁻¹, and 1147 cm⁻¹, [44] the pure PVDF nanofiber show peaks indicative of the various PVDF crystalline phases, while the hybrid PVDF/ZnO nanofiber shows peaks related to both ZnO nanowires and pure PVDF nanofiber. Compared to the pure PVDF nanofiber, the I₈₃₉/I₇₉₄ ratio of the hybrid PVDF/ZnO nanofiber has increased from 1.7 to 2.6. This indicates that, at the same electrospinning voltage, the addition of ZnO nanowires causes an increased fraction of β-phase. This phenomenon likely has a great effect on the output voltage of the hybrid PVDF/ZnO nanogenerator as discussed as follows. During the electrospinning process, PVDF molecule chains and ZnO nanowires have different electrospinning jet speed for their different flow characteristics in the solvent. Due to the different flying speed there exists a relative stretch between PVDF molecule chains and ZnO nanowires, which would introduce more β-phase formation. Another possible reason is that the additional ZnO NWs increase the local electric field during the electrospinning and in situ poling processes, leading to a greater Coulomb force, which induces the formation of a greater amount of β-phase crystalline compared with that produced in the bare PVDF nanofibers [24, 35].

Ferroelectric and optoelectronic properties characterization

A piezoelectric potential will be generated when an axial stretch is applied on the suspended nanofiber by bending the PDMS substrate. When the substrate is stretched and released repeatedly, piezovoltage outputs can be recorded. Figure 3(c) shows the piezovoltage output of a single pure PVDF nanofiber (red) and a hybrid PVDF/ZnO nanofiber (blue). Strains were applied to the nanogenerator at a frequency of 10 Hz. The peak voltage produced by an individual pure PVDF nanofiber is 14 mV, whereas the peak voltage from an individual hybrid PVDF/ZnO nanofiber is about 40 mV. Thus, the electrical output of a single hybrid PVDF/ZnO nanofiber is almost 300% that of a single pure PVDF nanofiber. According the reference [13, 24], the output voltage, V, can be expressed as

$\begin{eqnarray}&&V={{\rm{d}}}_{33}\varepsilon YL\end{eqnarray} \tag{ 1 }$

Here ${{\rm{d}}}_{33}$ is the piezoelectric voltage coefficient, $\varepsilon$ is the applied strain rate, $Y$ is the Yong's modulus of the material and $L$ is the length of the sample. As we can see the experimental results are consistent with the theory. This improvement can be explained from following reasons. First from the Raman spectra result, we have confirmed that the additional ZnO nanowires can lead more PVDF domains to transition from α-phase to β-phase. Secondly from the aligned piezoelectricity of the hybrid structure, the ZnO NWs increased the Young's modulus of the hybrid fiber which lead the output voltage increased according equation (1).

According the formula (1), the output voltage is proportional to the applied strain rate $\varepsilon .$ From the figure 3(c), the output of the PVDF/ZnO nanogenerator stretch is high to 40 mV, while when the output of the release process is about 35 mV. This might be the strain rate of the release process is smaller than the stretch process during the cantilever bending.

As expected, the nanogenerator output voltage is a function of the magnitude of bending (i.e. strain). Figure 4(a) shows the responses of the hybrid PVDF/ZnO nanogenerator under different levels of bending. The output voltage of the PVDF nanogenerator could be enhanced by serial and parallel connections [13]. figure 4(b) shows the output voltage of a bundle of pure PVDF electrospun fibers and a bundle of hybrid PVDF/ZnO electrospun fibers. The output voltage from the pure PVDF bundle is about 143 mV while the output voltage the PVDF/ZnO bundle is 480 mV (note that this is not normalized by the number of fibers, which may not be the same). The electrical outputs of serial and parallel connections should be approximately the sum of the individual nanogenerators [48].

Figure 4. Refer to the following caption and surrounding text. — **Figure 4.** (a) The amplitude of the output voltage decreased with the cantilever bending attenuation. (b) The output voltage of a bundle of pure PVDF fibers and a bundle of hybrid PVDF/ZnO fibers.
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ZnO is a well-known semiconducting, piezoelectric, and photoconducting material. The combination of these properties results in several interesting properties, including a strong dependence of electrical conductivity on amounts of ultraviolet light exposure [49, 50]. We sought to determine whether our system demonstrated this behavior. Figure 5(a) plots the electric conductivity measurement on a single nanofiber under controlled UV light exposure. A single PVDF/ZnO nanofiber was suspended on two Pt electrodes and the electric conductivity was measured under various optical conditions. Figure 5(b) show the optical image of a single nanofiber suspended on two Pt electrodes. Figure 5(c) plots of the current-voltage (I–V) curves measured on both a pure PVDF nanofiber and a hybrid PVDF/ZnO nanofiber. The I–V curve of the pure PVDF nanofiber measured under a dark condition completely overlapped with its I–V curve measured under UV light exposure, clearly indicating that UV light exposure could not impact the electrical conductivity of the pure PVDF nanofiber. On the other hand, the slope of I–V curve (i.e. conductivity) of the hybrid PVDF/ZnO nanofiber increases dramatically when under UV light exposure. The electric conductance of the hybrid PVDF/ZnO nanofiber increased from 0.007 nS to 0.03 nS, by more than a factor of 4.

Figure 5. Refer to the following caption and surrounding text. — **Figure 5.** Measurement of light-sensitive conductivity of single nanofibers. (a) Illustration of the electrical conductance measurement device. (b) Optical image of a single nanofiber suspended on two Pt electrodes. (c) Current-voltage (I–V) curves measured for a pure PVDF nanofiber and a hybrid PVDF/ZnO nanofiber in the dark and under UV light exposure.
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The energy conversion efficiency of the PVDF nanogenerator can be estimated as the ratio between generated electrical energy (W_e) and applied mechanical energy(W_M) [13]. The output electrical energy by the piezoelectric nanofiber is calculated as ${W}_{e}=\displaystyle \int VIdt,$ where V and I are the measured output voltage and current, respectively. If we ignore the contact resistance, the current I can be simply expressed as I = VG, whre G is the conductance of the nanofiber. According the figure 5(c), the conductance of the hybrid PVDF/ZnO nanofiber under the UV light exposure is more than 4 times of the pure PVDF nanofiber. At the same applied mechanical energy, the efficiency of the converting mechanical energy to electrical energy can be improved at least 4 times under the UV light exposure.

Conclusions

In summary, we have electrospun hybrid PVDF/ZnO nanogenerators and measured the piezoelectric responses of a single PVDF/ZnO nanofiber. The Raman spectra of isolated fibers shows that the addition of ZnO NWs induced a greater transition from α-phase PVDF to β-phase PVDF. Compared to the pure PVDF nanofiber, the presence of 33 wt% ZnO NWs enhances the output voltage by over a factor of 3. The ZnO NWs can serve not only as a piezoelectric material, but also as a photoconductive material. The light-sensitive behavior of the hybrid PVDF/ZnO nanofibers suggest they may be promising materials for applications in blue or ultra-violet photonic devices, photodetectors, and micromechanical devices.

Acknowledgments

The authors thank the National Natural Science Foundation of China (No.51705075) and Natural Science Foundation of Jiangsu Province (BK20150636) for financial support. Qian Zhang appreciates the financial support from the U S National Science Foundation (Grant# CMMI-1462866).

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