High Performance Piezoelectric Nanogenerators Based on

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C: Energy Conversion and Storage; Energy and Charge Transport

High Performance Piezoelectric Nanogenerators Based on Electrospun ZnO Nanorods/PVDF Composite Membranes Jingjing Li, Sheng Chen, Wentao Liu, Runfang Fu, Shijian Tu, Yinghui Zhao, Liqin Dong, Bin Yan, and Yingchun Gu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

High Performance Piezoelectric Nanogenerators Based on Electrospun ZnO Nanorods/PVDF Composite Membranes Jingjing Lia, Sheng Chena*, Wentao Liub, Runfang Fuc, Shijian Tud, Yinghui Zhaoa , Liqin Donga, Bin Yana, Yingchun Gua*

aFunctional

Polymer Materials Laboratory, College of Light Industry, Textile and Food

Engineering, Sichuan University, Chengdu, Sichuan 610065, China bKey

Laboratory of Leather Chemistry and Engineering (Sichuan University), Ministry of

Education, Chengdu 610065, China cDepartment

of Chemical Engineering, Faculty of Engineering, Monash University-

Clayton, Victoria 3800, Australia dZhonghao

Chenguang Research Institute of Chemical Industry, Fushun, Zigong 643201,

China

ABSTRACT

ACS Paragon Plus Environment

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A novel flexible zinc oxide/poly(vinylidene fluoride) (ZnO/PVDF) nanocomposite was prepared by electrospinning for fabricating piezoelectric nanogenerator (PNG). The ZnO nanoparticles (NPs) and nanorods (NRs) were used as nanofillers of piezoelectric PVDF to prepare fibrous nanocomposite membranes. It has been found that the addition of piezoelectric ZnO NPs and NRs can improve the overall performance of the PNGs fabricated with the electrospun membranes. A large electrical throughput (open circuit voltage ~85 V and short circuit current ~2.2 A) from the ZnO NRs/PVDF fiber membrane based PNG (ZR-PNG) indicates that ZnO NRs are effective functional fillers for PVDF. The high aspect ratio and flexibility characteristics of ZnO NRs were found to be highly beneficial for improving the piezoelectric properties of the nanocomposites. ZnO NRs act as nucleating agents of -phase PVDF, and ZnO NRs can also produce piezoelectric charges when they deform with the composite fibrous membrane. It has been concluded that the obvious synergistic effects between the piezoelectric nanofillers and the electroactive -crystals of PVDF in the ZnO NRs/PVDF composites are useful for the construction of the high performance flexible PNG. In addition, the fabricated ZR-PNG can light up commercial Light Emitting Diodes (LEDs) (40 white, 36 blue) and charge the


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capacitors in very short time (3 V is accomplished in 25 s), which indicates the potential of the ZR-PNG for portable, wearable, flexible, or self-powered electronic devices.

INTRODUCTION

A number of flexible piezoelectric materials have been demonstrated to be useful in energy-harvesting

(piezoelectric

nanogenerator)

and

high-precision

sensors

(piezoelectric sensors) fields because of their flexibility and unique ability to achieve mechanical-to-electrical energy conversion.1-4 As a representative flexible piezoelectric material, poly(vinylidene fluoride) (PVDF) and its copolymer have been widely studied owing to their inherent superiority in terms of convenient processing characteristics and high sensitivity to small mechanical forces.5-7

It is well known that PVDF have five distinct crystallite polymorphs (   , and ), among which the polar  phase is most desirable as it exhibits the highest piezoelectric energy harvesting properties8. However, the common polymorph of PVDF derived from traditional film-forming technology is the non-polar -phase.8, 9 So in order to obtain an enhanced piezoelectric performance of PVDF film, extra post processing treatment which


3


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can make -phase transform into  phase must be applied, such as mechanical stretching and high voltage electric field poling.10, 11 Thus enhancing the content of  phase and simplifying the preparation process are very important to achieve the practical application of PVDF piezoelectric material. For this reason, electrospinning is extensively studied to prepare high piezoelectric PVDF fibers and membranes.2,

12, 13

During the one-step

electrospinning process, the ejection of the PVDF solution, volatilization of the solvents, stretching of the electrospinning streams, and the high voltage polarization of PVDF fibers are instantaneously completed, and the resultant micro-nanofibers form a nonwoven membrane with high polar -phase.14, 15 Compared with the traditional piezoelectric PVDF film manufacturing method, the electrospinning technique is simple and quick, for which the additional post-processing procedures (e.g. post mechanical drawing and electrical poling) are not necessary.3, 16

To further improve the performance of PVDF nanofibers based nanogenerators, many kinds of fillers have been dispersed into the polymer matrix to alter the microstructure of the piezoelectric materials and to promote their piezoelectricity, such as barium titanate


4

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(BaTiO3),17, 18 carbon nanotubes (CNTs),11, 19 cellulose nanocrystals (CNCs),20 graphene nanosheets.21,

22

We have previously demonstrated the piezoelectric effects of

CNCs/PVDF20 and BaTiO3 nanoparticle/PVDF23 fibrous membranes for application as piezoelectric nanogenerators. The results suggested that CNCs with high aspect ratio can introduce more crystallites than BaTiO3 nanoparticles. In addition, Huang et al. reported that single-walled carbon nanotubes (SWCNTs) can form more content of  phase compared with multiwalled carbon nanotubes (MWCNTs) in the electrospinning of PVDF nanofibers.24 All the above studies indicate that the piezoelectric behaviors of the composites based on PVDF fibrous membranes are highly dependent on the morphologies and chemical structures of nanofillers. However, to the best of our knowledge, little information is available about the effect of nanofillers with the same chemical structures but different morphologies on the properties of organic-inorganic composite piezoelectric materials. It is quite crucial to understand such an intrinsic mechanism for the piezoelectric PVDF system.


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ZnO was reported as a piezoelectric nanogenerator by Wang et al, in the form of vertically aligned ZnO nanorod arrays.25 Meanwhile, it is of low cost and easy preparation, and can be prepared into various morphologies (nanowires, nanobelts, nanorings, and nanotubes) by using different fabrication methods.26, 27 Herein, in this work, the concentrations of ZnO nanoparticles (NPs) and nanorods (NRs) in PVDF nanofibers were optimized by using output voltages of the fabricated piezoelectric nanogenerators (PNGs), and the effect of ZnO morphology on the electroactive  phase and piezoelectric properties for energyharvesting-device applications was studied. The increases of  phase as well as the piezoelectric values of the composite PVDF fiber membranes were discussed in detail in terms of oriented polymer chain and synergistic effect of ZnO and PVDF. Thereafter, two highly superior, flexible, and lightweight PNGs (with dimensions of 3.5 cm×1.5 cm) were demonstrated using the PVDF fiber membranes conjugated with ZnO NPs and NRs, respectively. In comparison, the ZnO NRs/PVDF fiber membrane based PNG showed a superior performance than the ZnO NPs/PVDF based PNG. These findings suggest the synergistic effect between the rodlike piezoelectric nanofillers and the electroactive -


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crystals of PVDF, which plays an important role in enhancing the piezoelectric behaviors of ZnO NRs/PVDF nanocomposite.

EXPERIMENTAL SECTION

Materials: Zinc nitrate hexahydrate (ZnNO3 • 6H2O, Chengdu Kelong Chemical Plant, China), ZnO NPs (Huaxia Reagent, China), Poly(vinylidene fluoride) (PVDF) pellets (Mw ≈2,000,000-3,000,000) (Zhonghao Chenguang Research Institute of Chemical Industry, China), N,N- Dimethylformamide (DMF) and acetone (Chengdu Kelong Chemical Plant, China).

Synthesis of ZnO NRs: ZnO-NRs were prepared by solvothermal method, which is a facial, cost-effective, low temperature, and quick-synthesis process. Initially, 0.6 g of ZnNO3•6H2O, 72 ml of DMF and 8 ml of deionized water were successfully added into a glass bottle with a capacity of 120 ml. Afterward, the resulting solution was fully stirred for several minutes to form a transparent solution and then maintained at 120 ℃ for 2.5 h in an oil bath. After cooling down to room temperature, the white precipitation was collected


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by washed several times with ethanol to ensure the removal of contaminated residues. This white precipitation was dried at 80 ℃ for 24 h in an oven.

Fabrication of ZnO/PVDF nanocomposites and PNGs: A detail flow diagram of ZP-PNGs and ZR-PNGs fabrication is presented in Figure 1. At first, in order to disperse greatly ZnO NPs and ZnO NRs in the polymer matrix. The ZnO NPs and ZnO NRs were dispersed separately in 1:1 (w/w) DMF/acetone mixture solvents under magnetic stirring and sonication. Meanwhile, PVDF powders was dissolved in mixed solvent of DMF and acetone (1:1, w/w) by stirring at 60 ℃ for 6 h to form transparent solutions. Afterward the required amounts of ZnO NPs and ZnO NRs were added into the PVDF transparent solution under continuous stirring for 12 h to form the 5 wt % PVDF spinning solution under the same condition (Figure 1a, c).


8

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(c)

(a) ZnO NPs

ZnO NPs/PVDF membrane

Electrospinning with high take-up speeds

(d)

Assembly of the piezoelectric nanogenerators PET film

(b)

Copper foil

Syringe

film Nanofiber membrane

PVDF solution

(f)

(e) ZnO NRs DC power

Roller collection

PVDF solution

Photograph of the device ZnO NRs/PVDF membrane

Figure 1. Schematic illustration of PNG fabrication. Spinning solution of (a) ZnO NPs/PVDF and (e) ZnO NRs/PVDF. (b) Schematic diagram of the electrospinning system. Digital and SEM images of (c) ZnO NPs/PVDF and (f) ZnO NRs/PVDF flexible fibrous membranes. (d) Schematic diagram of the structure and digital image of PNG.

The electrospinning process is shown in Figure 1b. The prepared polymer solutions were filled into a 10 mL plastic syringe with a 0.8 mm inner diameter needle connected to a high-voltage power supply of 10 kV. The syringe was placed in an automatic pump. The distance from the needle tip to the collector was set up 20 cm. electrospinning was done with an ejection rate of 2 mL/h from the syringe. A rotating disk with 2000 rpm was used


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for the collection of the electrospun fibrous membrane. Another reference sample was also prepared where piezoelectric nanofiller was not utilized. After preparation of nanocomposite fibrous membrane (Figure 1c, e), copper foils (dimension =35 mm×15 mm) were staked on both sides for serving as top and bottom electrodes of PNG. Finally, the entire structure (45 mm in length, 22 mm breadth) including two copper wire connection was encapsulated with Polyethylene terephthalate (PET) to protect from any external mechanical damage and named P-, ZPX-, ZRX-PNG for pure PVDF, ZnO NPs/PVDF and ZnO NRs/PVDF (where X signifies the mass percent of nanofillers) (Figure 1f). It is worth mentioning that selection of amount (wt%) of ZnO NPs and ZnO NRs in PVDF was preferred as per the optimize output of PNG (optimization of output voltage is provided in Figure S1-S2 in the Supporting Information).

Characterizations: X-ray diffraction (XRD) patterns and the crystalline phases of all samples were obtained by Ultima IV diffractometer (Rigaku Corporation, Japan). Fourier transform infrared spectroscopy (FT-IR) spectra were recorded with spectrometer (Shimadzu, Tracer 100). The surface morphologies of the as-prepared composite were observed by field emission scanning electron microscopy (FE-SEM, JSM-7500F). The


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diameters of all samples were measured using Nano Measurer software. Differential scanning calorimetry (DSC) analysis was performed in differential scanning calorimetry analyzer (DSC 200 PC, NETZSCH, Germany) with a heating rate of 5 ℃ min-1 under nitrogen atmosphere from 60 ℃ to 200 ℃ . The open circuit voltage and short circuit current from the PNGs under repeating finger bending were recorded by oscilloscope (GA1102CAL, Glarun Atten) and a digital multimeter (34465A, Agilent) respectively. The capacitor charging performance was employed via a typical rectifier bridge circuit unit. The mechanical properties test of ZnO/PVDF membranes was performed on a tensile test machine (LLY-06A, Yuanmore, China). Tensile test samples were obtained from the above mentioned composite fibrous membranes with a nominal gauge length of 40 mm, and a width of 2 mm. The samples were loaded in constant deformation mode at a speed of 5 mm/min.

RESULTS AND DISCUSSION

In order to investigate the effect of morphology on piezoelectric property of PVDF fibers, rod-liked and sphere-liked ZnO nano-powders were synthesized via the solvothermal


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method and commercially obtained from Huaxia Reagent Company respectively. Figure 2a and b show the FE-SEM images of ZnO NPs and synthesized ZnO NRs powders whose dimensions and morphologies are distinctively different. The ZnO NPs show small aspect ratio with the average diameter of ~150 nm. The as prepared ZnO powder show well-defined rod structure and relatively uniform morphologies with the average diameter of about 70 nm and length of about 830 nm. Figure 2c illustrate the XRD patterns of the ZnO NPs and ZnO NRs. All characteristic diffraction peaks of ZnO NPs and ZnO NRs perfectly coincide and correspond to the planes hexagonal wurtzite structure (JCPDS Card No. 36-1451). The absence of any additional peak indicates the high purity of the synthesized ZnO NRs powder.


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(a)

(b)

50 40 30 20 10 0

0

50 100 150 200 250 300

Diameter (nm)

(d)

(g)

(e)

Frequency (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Frequency (a.u.)

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50

(c)

40 30 20 10 0

0

50 100 150 200 250 300

Diameter (nm)

(f)

(h)

(i)

Figure 2. FE-SEM images of (a) ZnO NPs and (b) ZnO NRs. In the inset, the statistical diameter distribution of the corresponding particles, (c) XRD patterns of ZnO NPs and NRs. SEM images of ZnO/PVDF electrospun fiber (d) pure PVDF, (e) ZP30/PVDF, (f) ZR5/PVDF. The diameter distributions of the electrospun fibers (g) pure PVDF, (h) ZP30/PVDF, (i) ZR5/PVDF.


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Different contents of ZnO NPs and ZnO NRs were added into PVDF to establish the optimizing contents of nanofillers which are 30 wt% and 5 wt% respectively. The optimization of output voltage is provided in Figure S1-S2 (in the Supporting Information). SEM images of pure PVDF, ZP30/PVDF and the ZR5/PVDF samples are shown in Figure 2d, e, f. It is shown in Figure 2d that the surface morphology of pure PVDF fibers is relatively smooth. However, in Figure 2e, the additive ZnO NPs mass or lump can be observed on the surface of fibers, which lead to remarkable increasing roughness. The results are similar to reported morphologies of nanofibers containing SiO2.28, 29 In Figure 2f, ZnO NRs can be observed to align well along the fiber axis. This phenomenon is similar to CNTs/ Polyacrylonitrile composite nanofibers30 and can be explained by the theoretical model built by Dror et al31 who explained the alignment behavior of rod-like CNTs in electrospinning. Initially the ZnO NRs were randomly gradually oriented, but due to the sinklike flow in the cone they were gradually oriented mainly along the streamlines of the electrospinning solution. The addition of nanofillers to the polymer not only affect the morphology of the fiber, but also affect the diameter of the fiber. As shown in Fig.2g, h, and i, the ZP30/PVDF and ZR5/PVDF composite fibers show larger mean diameters than that of


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the pure PVDF fibers because the addition of the ZnO nanoparticles or ZnO nanorods led to the increase of the viscosities of the electrospinning solutions.32, 33 Furthermore, the mechanical properties of the electrospun fibers were studied also (Figure S3 in the Supporting Information). Compared with the tensile strength of pristine PVDF membrane (21 MPa), the addition of 10% nanoparticles and 5% nanorods into polymer solution led to a significant enhancement of tensile strength (34 MPa for ZP10/PVDF and 37 MPa for ZR5/PVDF, respectively). This phenomenon is similar to a few of researches.28, 33 This should be due to the increase of the crystallinities when the ZnO nanoparticles and ZnO nanaorods were added into PVDF substrate. Interestingly, the elongation at break for the ZnO NRs/PVDF fiber membranes did not decrease, which should be attributed to the large aspect ratio of ZnO nanorods.34 Effective nucleation of highly electroactive  crystallite in the NRs doped PVDF fibrous membrane has been confirmed from FT-IR, XRD and DSC studies. Figure 3a represents the FT-IR spectra of pure PVDF, ZP30/PVDF and the ZR5/PVDF composite fibers in the range of 700 to 1500 cm-1. According to the related reports in the literature,35-37 for the pure PVDF film, the characteristics absorption bands of non-polar α are observed at 763 cm-1, 795 cm-1, 855 cm-1, 976 cm-1, and bands of the polar β-phase are observed at 840 cm-1, 1276 cm-1, 1431 cm-1 in the FT-IR spectra. Interestingly, in our study, the characteristics absorption bands of α prepared by electrospinning is not observed except


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the broad peak at about 792 cm-1- 815cm-1, while the characteristics absorption peaks of the polar β-phase, as mentioned above, are all observed. This indicates that electrospinning technology is an efficient method to obtain the electroactive β-phase.38 Evaluation of electroactive  crystal content (F()) in the samples is executed using the lambert-beer law,39

F(β) =

Aβ

((Kβ Kα)Aα + Aβ)

× 100 %

Here, A and A refer to the absorbance of α and -phases (at 763 and 840 cm-1), respectively. K and K represent the absorption coefficients with the value 6.1×104 and 7.7×104 cm2mol-1, respectively. The evaluated F() is graphically represented in Figure 3b. It is found that ZnO NPs and ZnO NRs nanofillers can improve the F() of PVDF nanofibers. What’s more, the mat of ZnO NRs/PVDF nanofiber shows higher F() value (90.7 %) than that of the mat of ZnO NPs/PVDF nanofiber (87.5 %). The nucleation of the  phase crystallite in NPs and NRs doped PVDF composite fibrous membrane can be attributed to an effective interaction between the CH2 groups of the PVDF chains, having a positive charge density, and the surface of NPs and NRs with negative charge density.39,


16

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40

Figure 3c shows the conformation of  and  phase and interaction between ZnO NPs,

ZnO NRs and the CH2 groups of the PVDF polymer chains. The F() content of ZnO NRs/PVDF composite fiber membranes is similar to several previously reported results for cases in which different fillers were doped with PVDF fibers as shown in Table S1(in the Supporting Information). Thus, the ZnO NRs may be the alternative to other nanofillers to improve the piezoelectric -phase content of PVDF nanofiber for energy harvesting applications.

(b)

(a)





(c)

 

 phase nO NPs  phase nO NRs

Carbon

Hydroge

Fluorine

n

(d)

(f)

(e)

 

 ℃


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Figure 3. (a) FT-IR spectra of ZnO/PVDF electrospun fibers, (b) -phase contents of the samples, (c) schematic representation of  phase formation mechanism in the presence of ZnO NPs and NRs, (d) XRD patterns of electrospun fiber, (e) DSC thermographs of electrospun fiber, (f) crystallinity calculated using DSC data.

The XRD patterns of the samples further confirms the nucleation of highly electroactive  phase nucleation in ZnO NRs doped PVDF sample. Figure 3d represent the XRD pattern of unblended and NPs and NRs doped PVDF fibrous membranes. The XRD spectrum of pure PVDF consists of the characteristic peak of nonpolar  crystal at 18.3°4,

35

and

electroactive  crystallite at 20.3°.35 In the diffraction patterns of the ZP30/PVDF sample, existence of NPs prominently diminish the peak associated with nonpolar  crystal. Additional diffraction peaks at 2 = 31.9° (100), 34.5° (002), 36.4° (101), 47.7° (102), 56.7° (110), 63.0° (103), 66.5° (200), 68.1° (112), 69.2°(201), 72.6°(004), 77.0° (202) correspond to the characteristic peaks of ZnO.39 When ZnO NRs are added as the nano fillers, the peak at 18.3° almost disappears, and only a single peak is observed at a 2 ≈ 20.3°. This results is consistent with the FT-IR analysis of the samples.


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The crystalline behavior of the samples are verified using DSC data which are presented in Figure 3e. The DSC thermographs demonstrate the shifting of melting temperature values to higher temperature by ~ 0.3 ℃ in the thermograph of ZR5/PVDF fibrous membrane inferring  phase crystallite in the sample.39, 41 The melting enthalpy values (∆𝐻𝑚) and degree of crystallinity (c) are also raise with ZnO NPs and ZnO NRs loading in PVDF matrix (Figure 3e, f). The c value reached maximum ~ 53.1 % for 5 wt% incorporation of NRs. The percent crystallinity (c) was calculated using the following equation:4 ∆Hm

c (%) = (1 ― ∅)∆H0 ×100 % m

Where, ∆Hm is the melting enthalpy of the composite nanofiber, ∆H0mis the melting enthalpy of 100 % crystalline PVDF (104.50 J/g) and ∅ is the weight percentage of ZnO in the composite fibrous membranes. Figure 3f shows the degree of crystallinity PVDF with different morphology of ZnO, It indicates that the highest crystallinity of PVDF is formed by adding ZnO nanorod fillers.


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(a) Crystallization

Electrospinning

phase Pure PVDF

Orientated Polymer chains

Non-orientated Polymer chains

phase

(b) Electrospinning

Crystallization

ZP30/ PVDF ZnO NPs

(c) Electrospinning

ZR5/ PVDF

Crystallization

ZnO NRs

Figure 4. Schematic showing the proposed mechanism for the electrospinning of Pure PVDF, ZP30/PVDF, ZR5/PVDF composite fibers.

Combining above results, we give a proposed mechanism about the changes of crystalline structure of the electrospun ZnO/PVDF composite fibers. In the case of pure PVDF, during electrospinning, the crystalline structure is formed due to the orientation of the polymer chains which is caused by extensional flow of the polymer solution under the electric filed and this process is the major factor leading to formation of polar  phase


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crystallite (in Figure 4). In contrast, when ZnO NRs is added into the PVDF solution, orientated polymer chains could be formed because of ZnO NRs which have well oriented along the fiber axis due to their large aspect ratio seen from SEM image. In addition, the large surface area of ZnO NRs with negative charged could also act as  phase crystallite nucleating agent due to the effective interaction between the CH2 groups of the PVDF chains and the surface of ZnO NRs with negative charge, as depicted in Figure 3c.


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(a)

(d)

(g)

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(b)

(c)

(e)

(f)

(h)

Figure 5. Measured open circuit voltage (Voc) and short circuit current (Isc) under continuous finger bending (a, d) P-PNG, (b, e) ZP30-PNG, (c, f) ZR5-PNG. The output curve features during one cycle of finger bending (g) open circuit voltage (Voc), (h) short circuit current (Isc).


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The nanocomposite fibrous membranes (ZP30/PVDF and ZR5/PVDF) with the highest electroactive -crystals are chosen for device fabrication to achieve optimum piezoelectric response. The structures of PNG are schematically illustrated in Figure 1, which is composed of a piece of fiber membrane sandwiched with copper foil electrodes and PET substrates. The piezoelectric properties of the PNGs were examined by a periodic bending-releasing. In order to make more fibers suffer a large bending, one end of the piezoelectric device was fixed with a clip (the schematic illustration of device bending is shown in Figure 7c). The output characteristics i.e. the open circuit output (Voc) and short circuit currents (Isc) from P-PNG, ZP30-PNG and ZR5-PNG under repeating bending and relaxation with frequency ~ 4 Hz are presented in Figure 5. Here, the output curve features of our fabricated PNGs during one cycle of bending can be similar to a free vibration system with damping (Figure 5g, h). The ZR5-PNG builds open circuit voltages (Voc) ~ 85 V and short circuit currents (Isc) ~ 2.2 A, which are about four times of the PPNG's outputs. The output voltages and current of ZP30-PNG are about 60 V and 1.7 A, respectively, which are also obviously lower than those of ZR5-PNG (Figure 5). The


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generated output voltage value is higher than that of the previously reported BaTiO3,23 CNCs20 composite nanofibers based flexible PNG.

The piezoelectric output of PVDF fibrous membrane is related to both proportion of phase and the crystallinity degree in PVDF fibers42. Ultimately, the polar configuration of -phase is the source of piezoelectric response, we propose a value of the product of F() and c as “content of effective piezo-phase” (CEP for short), which means the actual content of -phase in PVDF. The output voltage of devices fabricated by PVDF fibrous membranes under different fillers mentioned above as function of CEP are shown in Figure 6a and b, interestingly, the piezoelectric response shows linear dependence on the effective piezoelectric phase. The linear fitting equation is y =5.68x-204 and y=6.10x-219 with variance of 0.89 and 0.93 for the output voltage of ZnO NPs and ZnO NRs, respectively. The distinct correlation between CEP and piezoelectric response is much useful to predict piezoelectric performance of PVDF device by characterizing on phase structure. Moreover, the difference of the linear slope of ZR5/PVDF and ZP30/PVDF is


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observed. The results demonstrate that the contribution of piezoelectricity derived from ZnO NRs is higher than ZnO NPs under the same CEP.

(a)

(c)

Pure PVDF Fiber

(b)

Be bended difficultly ZP30/PVDF Fiber

Be bended easily ZR5/PVDF Fiber

Figure 6. The relationship of CEP and output voltage (a) ZP-PNGs, (b) ZR-PNGs, (c) schematics of the energy generation mechanism of the composite nanofibers.

In order to understand the different piezoelectric contribution of ZnO NPs and ZnO NRs in the fibrous nanocomposite membranes, a possible working mechanism of the PNGs was proposed (as shown in Figure 6c). The suggested working mechanism of our PNGs


25


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may be explained in terms of synergistic effect of electroactive -crystals present in PVDF and the piezoelectric nanofillers (ZnO NPs and NRs). For the pure PVDF, When P-PNG is subjected to the mechanical stress by bending motions, the piezoelectric charges are generated only by the piezoelectric -phase of PVDF which is formed by strong drawing of electric field during the electrospinning process. However, when an external mechanical force is applied on the ZP30-, ZR5-PNG, more piezoelectric charges are than the P-PNG. Firstly, ZnO NPs and ZnO NRs acting as nucleating agent that results in more polar -phase formation in PVDF. Secondly they can also generate piezoelectric charges due to the bending of themselves43. As shown in Figure 5, though the content of ZnO NRs of is lower than that of ZnO NPs in PNGs, ZR5-PNG exhibits better piezoelectric performance than ZP30-PNG. The reason is that the ZnO NRs with small diameters and large aspect ratios are easily deformed by external force. Shin et al also demonstrated that ZnO nanoneedle with small diameter have a relatively small elastic modulus the large one.44


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To test the feasibility of practical implementation of the prepared PNG, a typical bridge rectifier circuit is employed (Figure 7c), which can convert the AC output generated by the PNG to DC signals. The output piezoelectric energy created by periodic finger bending and releasing is directly utilized to successfully operate several commercial LEDs (without connecting a capacitor) with different color (white and blue) emissions (a schematic circuit diagram is demonstrated in Figure 7). As shown in panels a and b of Figure 7, the P-, ZP30- and ZR5-PNG are capable of lighting up 10, 31 and 40 white LEDs connected in spelling the letters “SCU” which are the acronym of “Sichuan University (Video S1 in the Supporting Information) and 10, 30 and 36 blue LEDs connected in a series, respectively (Video S2 in the Supporting Information). The capability of our fabricated PNG to charge a storage system has been confirmed by charging a capacitor with capacitance of 4.7 F. The capacitor is separately charged up by the three individual PNGs. The concerning transient response is presented in Figure 7d. The maximum values of charging voltage that P-, ZP30- and ZR5-PNGs can charge are 1.5 V, 2.8 V and 3 V within 78 s, 50 s and 26 s, respectively. The capacitor charging result shows that the performance of ZR5-PNG is the best among them.


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(a)

(b)

b

Pure PVDF

Page 28 of 42

ZP30/PVDF

ZP30/PVDF

(c)

Pure PVDF

ZP30/PVDF

ZP30/PVDF

(d)

LEDs

v

4.7 F

Figure 7. Snapshots of the commercial white and blue LEDs driven using P-PNG, ZP30PNG and ZR5-PNG connected in (a) 10，31 and 40 white LEDs and (b) 10, 30 and 36 blue LEDs. (c) Schematic circuit diagram of the full bridge rectifier and electrolytic capacitor. (d) Voltage-charging time relationship of 4.7 F capacitor by continuous bending of the flexible P-PNG, ZP30-PNG and ZR5-PNG.

CONCLUSIONS

The flexible nanocomposite fibrous membranes of PVDF, ZnO NPs/PVDF and NRs/PVDF were prepared and their piezoelectric properties were studied. It has been


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found that the piezoelectricity is different when the same kind of substance with different morphologies are used as nanofillers of PVDF. The output voltage of ZnO NRs/PVDF nanocomposite fibrous membrane was found to be maximum (85 V) with a short circuit current of 2.2 A. Particularly, our study demonstrates that the obviously synergistic effect between the piezoelectric ZnO NRs nanofillers and the electroactive -crystals of PVDF plays a crucial role in enhancing piezoelectricity of the nanocomposites. On one hand, ZnO NRs act as nucleating agents that result in more polar -phase (90.7 %) formation in PVDF. On the other hand, the ZnO NRs could also produce obvious deformation when the composite fibrous membrane is bent and exhibit enhanced piezoelectric effect. The ZR5-PNGs can drive commercial multicolor LEDs and charge a capacitor in a very short time (reach 3 V in 25 s), indicating that the electrospun ZnO nanorods/PVDF composite is a promising candidate for producing lightweight, large-scale, and flexible piezoelectric power harvester in the application of portable electronic devices.

SUPPORTING INFORMATION

The Supporting Information is available free of charge on the ACS Publications website.


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Content optimization of ZnO NPs and ZnO NRs; Mechanical properties of ZnO NPs/PVDF and ZnO NRs/PVDF composite fibrous membranes; The comparison of F() values in PVDF fibers (PDF); Demonstration of

lighted White LED by PNG(Video S1);

Demonstration of lighted blue LED by PNG(Video S2).

Corresponding Author

* Sheng Chen. E-mail: [email protected]. * Yingchun Gu. E-mail: [email protected].

Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

This work was supported by the Sichuan Province Science and Technology Foundation (No. 2017GZ0429), the National Natural Science Foundation of China (No. 21606156), and the open project program of Key Laboratory of Eco-textiles, Ministry of Education


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(No. KLET1510). We would like to thank the Analytical & Testing center of Sichuan University for structured illumination microscopy work and we would be grateful to Hui Wang for her help of SEM images.

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Toc Graphic


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High Performance Piezoelectric Nanogenerators Based on

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