Enhanced Piezoelectric Energy Harvesting Performance of Flexible

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Enhanced Piezoelectric Energy Harvesting Performance of Flexible PVDF-TrFE Bilayer Films with Graphene Oxide Venkateswarlu Bhavanasi, Vipin Kumar, Kaushik Parida, Jiangxin Wang, and Pooi See Lee* School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 S Supporting Information *

ABSTRACT: Ferroelectric materials have attracted interest in recent years due to their application in energy harvesting owing to its piezoelectric nature. Ferroelectric polymers are flexible and can sustain larger strains compared to inorganic counterparts, making them attractive for harvesting energy from mechanical vibrations. Herein, we report, for the first time, the enhanced piezoelectric energy harvesting performance of the bilayer films of poled poly(vinylidene fluoride-trifluoroethylene) [PVDFTrFE] and graphene oxide (GO). The bilayer film exhibits superior energy harvesting performance with a voltage output of 4 V and power output of 4.41 μWcm−2 compared to poled PVDF-TrFE films alone (voltage output of 1.9 V and power output of 1.77 μWcm−2). The enhanced voltage and power output in the presence of GO film is due to the combined effect of electrostatic contribution from graphene oxide, residual tensile stress, enhanced Young’s modulus of the bilayer films, and the presence of space charge at the interface of the PVDF-TrFE and GO films, arising from the uncompensated polarization of PVDF-TrFE. Higher Young’s modulus and dielectric constant of GO led to the efficient transfer of mechanical and electrical energy. KEYWORDS: nanogenerator, PVDF-TrFE, graphene oxide, electrostatic energy harvesting, enhanced Young’s modulus

1. INTRODUCTION

electronics to realize the self-powered electronic devices and sensors.8,10−12 Various inorganic and organic materials exhibit ferroelectric and piezoelectric nature. In general, the inorganic materials such as lead zirconium titanate, barium titanate, and so on have higher piezoelectric coefficients but are brittle, whereas the polymer ferroelectrics, poly(vinylidene fluoride-trifluoroethylene) [PVDF-TrFE], have lower piezoelectric coefficients but are flexible and can sustain larger strains compared to inorganic counterparts.13,14 In the present work polymeric ferroelectric PVDF-TrFE is chosen due to its high piezoelectric coefficients among polymer ferroelectrics15 and can readily form into the ferroelectric phase.16 Various approaches have been employed to improve the energy harvesting performance using piezoelectric/ferroelectric materials. One-dimensional nanostructures of the ferroelectric materials are employed, which have improved the crystalline

Piezoelectric materials have attracted interest in recent days in energy harvesting due to their ability to convert mechanical energy into electrical energy. Energy harvesting has become increasingly important as there is a need for self-powered systems particularly in miniaturization of devices. There is an abundant source of mechanical energy available in nature. In the decade before this, energy harvesting from piezoelectric materials has been largely based on bulk materials and have usually been of cantilever type, which requires special designs to operate at lower frequencies (in a frequency range < 100 Hz).1,2 Wang and Song then introduced a nanogenerator, to represent the energy harvesting devices which convert mechanical energy into electrical energy from nano-piezoelectric materials utilizing low frequency mechanical vibrations available in ambient atmosphere and human body motions.3 Thin film and nanostructure counterparts of various inorganic and organic piezoelectric materials were then studied to further improve the energy harvesting performance.3−9 Piezoelectric energy harvesters have been particularly useful in low power © XXXX American Chemical Society

Received: October 7, 2015 Accepted: December 7, 2015

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DOI: 10.1021/acsami.5b09502 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) X- ray diffraction pattern of PVDF-TrFE films; (b) voltage output of the poled PVDF-TrFE film under a compression pressure of 0.32 MPa.

component of the graphene oxide (due to the negative charges in the GO), residual tensile stress in bilayer films, and the complex interface between PVDF-TrFE and GO, which have nonzero charges, together with the higher modulus of the bilayer films led to the enhanced energy harvesting performance. We show that the high Young’s modulus and dielectric constant of graphene oxide led to the efficient transfer of electrical and mechanical energy.

nature (compared to bulk) and exhibit strain confinement. This led to improved energy harvesting performance in both inorganic17,18 and polymers ferroelectric materials5,19−21 compared to their bulk counterpart. In addition, hybrid devices are made to combine the various vibration energy harvesting mechanisms in one device such as electrostatic and piezoelectric energy harvesting22 (using ZnO nanorods and polyethylene dielectric layer), a piezoelectric/triboelectric and electromagnetic approach23 to improve the energy harvesting performance. Piezoelectric materials are combined with photovoltaic devices to harvest energy from mechanical stimuli as well as solar energy.24 Recently, graphene oxide (GO) has attracted significant interest due to its versatile functionality attributed to the different functional groups such as carboxyl, ketonic, hydroxyl, and epoxy.25,26 GO and functionalized GO have found applications in sensing,26 as electrode material for a supercapacitor, and so on.27 GO layers possess negative charges due to the presence of functional groups.25,28,29 Utilizing the charge on GO, an electrostatic nanogenerator has been fabricated to harvest the energy from mechanical vibrations.29 PVDF generally crystallizes in nonferroelectric α phase whereas its co-polymer PVDF-TrFE readily crystallizes in the ferroelectric β phase by simple spin coating and annealing. Composites of PVDF with reduced graphene oxide (RGO) or graphene oxide (GO) using spin coating and annealing were found to crystallize the PVDF in ferroelectric phase.30−35 The presence of electrostatic interaction and/or hydrogen bonding between the oxygen functionalities of RGO (or GO) and PVDF leads to the nucleation of ferroelectric γ or β phase.32 Rahman and Chung33 showed the formation of the ferroelectric β phase in composite films with 0.3 wt % loading of RGO in PVDF. Composites of PVDF with Fe doped RGO led to the nucleation of the ferroelectric γ phase that could harvest mechanical energy without further electrical poling.32 Furthermore, graphene has been used as the transparent conducting electrode on PVDF-TrFE in a transparent flexible energy harvester and actuator.9,36 In contrast to the literature, in the present work, drop casted graphene oxide onto ferroelectric PVDF-TrFE resulted in a bilayer film that improves the energy harvesting performance of ferroelectric PVDF-TrFE films. By combining the piezoelectric PVDF-TrFE with a charged material (GO), it is possible to harvest both piezoelectric and electrostatic components upon application of a force. The bilayer films of poled PVDF-TrFE and graphene oxide showed enhanced energy harvesting performance with 2 times the voltage output and 2.5 times the power output than that of the PVDF-TrFE films alone. The electrostatic

2. EXPERIMENTAL METHODS PVDF-TrFE granules (mole ratio, 65:35) were dissolved in butanone2 with a weight fraction of 70 mg/mL. The thickness of the films made from drop casting was ∼30 μm. The films were annealed at 135 °C for 2 h and cooled to room temperature naturally to improve the ferroelectric crystalline phase. The films were then poled at fields of approximately 30 MV/m at 105 °C for 60 min. ITO was used as the bottom electrode, and sputter coated gold on to the PVDF-TrFE films was used as the top electrode. Graphene oxide was synthesized according to the procedure reported in our previous work.27,37 Preoxidized graphite flakes were prepared by mixing 10 g of phosphoric acid (H3PO4) and 10 g of potassium persulfate (K2S2O8) into 30 mL of concentrated sulfuric acid (H2SO4) containing 20 g of graphite flakes. The as-prepared mixture was stirred at 80 °C for 5 h to obtain preoxidized graphite flakes. The as-produced preoxidized graphite flakes (∼3 g) and about 9 g of potassium permanganate (KMnO4) were mixed into 70 mL of concentrated sulfuric acid (H2SO4) solution, while keeping the reaction temperature well below 20 °C. The reaction temperature was increased to 40 °C after the completion of reaction. Thereafter, 350 mL of DI water was mixed into the solution. Then 30% H2O2 (7.5 mL) was added into the aforementioned solution to maximize the rate of oxidation reactions. Then the solution was kept aside for some time to allow the oxidized product to settle down. The as-obtained sediment product was washed and centrifuged with DI: HCl (10:1 by volume) and left for sonication for several hours. The as-sonicated solution was further washed with DI:HCl (10:1 by volume) and DI water, respectively, to obtain a brown solution of GO nanoflakes. The bilayer films of PVDF-TrFE and GO were obtained by drop casting the GO solution onto the PVDF-TrFE film followed by vacuum drying at room temperature. The crystalline phase of the PVDF-TrFE films is characterized by using X-ray diffractometer (Shimadzu). The thicknesses of the PVDFTrFE and GO films are measured using profilometer (KLA tencor). The Young’s moduli of the films are measured by dynamical mechanical analysis (TA Instruments, DMA Q800). Dielectric constant and capacitance versus voltage measurements are carried out using an LCR meter (Agilent E4980A). The adhesion strengths of the graphene oxide onto the PVDF-TrFE films are measured using a tensile tester (Chatillon TCD 110 series) in tensile mode using the pullout method. Energy harvesting performance of the bilayer and PVDF-TrFE films are measured by subjecting them to dynamical compression pressure as described in our earlier report.5 Instron 8516 is used to apply the mechanical compression force onto the films. The B

DOI: 10.1021/acsami.5b09502 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a) FTIR spectra of GO films; (b) capacitance versus voltage measurements on Al/p-Si/SiO2/GO/Au and Al/p-Si/SiO2/Au capacitors.

Figure 3. Micrographs and macroscopic images of bilayer energy harvesting device: (a) schematic of the bilayer films energy harvesting device; (b, c) surface morphology of PVDF-TrFE film and graphene oxide layer, respectively; (d) cross-section image of bilayer films; (e) bilayer and (f) PVDFTrFE film energy harvester devices (both are freestanding, flexible, bendable, and rollable). films are placed on the bottom platen of the mechanical tester, which is moved to hit the top platen to apply the mechanical force. The force applied is measured using the force sensor located on to the top platen. Initially the bottom platen is 4 mm away from the top platen. By increasing the movement of the bottom platen to greater than 4 mm, the amount of force applied onto the films can be increased. The force is applied at 1 Hz frequency. Oscilloscope (Yokogawa DL1620) is used to measure the voltage output from the bilayer and PVDFTrFE films. The power output of the bilayer films is measured by measuring the voltage output across the load resistors range from 43 k Ω to 6.7 MΩ (0.043, 0.1, 0.576, 1, and 6.7 MΩ). Short circuit current from the PVDF-TrFE and bilayer film devices are measured using current preamplifier SR 570 (Stanford Research).

ing to (110)/(200) planes along with (201)/(111) orientations. The PVDF-TrFE films were released from ITO glass and then sputter coated with gold to measure the energy harvesting performance. The nonpoled films did not show any voltage output, upon being subjected to the compression pressure. In contrast, the poled PVDF-TrFE films showed a maximum voltage output of approximately 2.2 V (Vav = 1.9 ± 0.2 V) as shown in Figure 1b, when subjected to a dynamic compression pressure of 0.32 MPa at 1 Hz. Similar voltage output values were reported for the PVDF-TrFE films in literature.38 Bae et al. reported a voltage output of approximately 2−3 V for PVDF-TrFE films with graphene as electrodes.9 Chen et al., reported a voltage output of 1−2 V for the mesoporous PVDFTrFE films.39 Lee et al. reported a piezoelectric voltage output of approximately 0.7 V under 30% stretching.40 FTIR spectra of the GO films (Figure 2a) show the presence of functional groups such as epoxide (970 cm cm−1), hydroxyl

3. RESULTS AND DISCUSSION Ferroelectric β phase formation of the PVDF-TrFE films is confirmed by using X-ray diffraction measurement as shown in Figure 1a. The crystals are predominantly oriented correspondC

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Figure 4. Working mechanism of the bilayer films energy harvesting device: (a) piezoelectric voltage output from the poled PVDF-TrFE film; (b) electrostatic component (voltage output from the bilayer films, with nonpoled PVDF-TrFE layer); (c) voltage output from the bilayer films device consisting of both electrostatic and piezoelectric components.

(1150 cm−1), COO- (1558 and 1575 cm−1), ketonic (1739 cm−1), and C−O groups (1030 cm−1). The presence of various functional groups makes the GO intrinsically charged. Figure 2b shows the capacitance versus voltage (C−V) measurements that are carried out with a metal dielectric semiconductor capacitor configuration (Al/p-Si/SiO2/GO/Au) in order to study the nature of the charge and amount of charge present in the GO films. For the p-type silicon, the depletion region of Al/ p-Si/SiO2/Au is observed in the negative voltage region. By incorporating GO films, for Al/p-Si/SiO2/GO/Au capacitor, the depletion region of the C−V curve is shifted to positive values; this suggests the charge in GO is negative. The amount of charge on GO is calculated from the shift in flat band voltage (δV) of C−V curves with and without GO layer. The measured charge on GO is approximately 1.7 × 10−4 C cm−3 (Q = δV/C, where C is the capacitance). Figure 3a shows the schematic of the energy harvesting device made from bilayer films of poled PVDF-TrFE and GO. As shown in field emission scanning electron micrograph in Figure 3b, PVDF-TrFE films show the typical needle shaped ferroelectric β phase. Figure 3c shows the graphene oxide morphology. The typical corrugated surface morphology is observed. From the cross-section images of bilayer films (Figure 3d), it is observed that there are no gaps between the PVDF-TrFE and GO layers. The thickness of the graphene oxide layer is approximately 19 μm and the PVDF-TrFE layer is 32 μm. The fabricated energy harvester device made from bilayers is flexible, bendable, and rollable and is free-standing as shown in Figure 3e,f. GO films are deposited on poled and nonpoled PVDF-TrFE films to obtain the bilayer films. The adhesion of the GO films with the nonpoled PVDF-TrFE films is poor because of the hydrophobicity and chemical inertness of PVDF-TrFE (the measured adhesion strength is 0.062 N/mm2, Supporting Information (SI) Figure S1). On contrast, the GO

films adhered well with the poled PVDF-TrFE films (with the hydrogen terminated surface); suggesting the electrostatic interaction between the poled surface of PVDF-TrFE and functional groups in the GO films (adhesion strength is 0.12 N/mm2, SI Figure S1). In bilayer configuration, the GO films are in contact with the hydrogen terminated surface of the PVDF-TrFE films. The bilayer films are then tested for the energy harvesting performance by subjecting them to dynamical compression pressure at 1 Hz frequency. Bilayer films, with nonpoled PVDFTrFE layer, showed a voltage output of approximately 0.3 V at compression pressure 0.32 MPa (Figure 4b). This voltage output corresponds to the electrostatic component, which is due to the change in the capacitance of the films corresponding to the thickness variation when subjected to the compression pressure.29 The bilayer films, with poled PVDF-TrFE layer, showed a maximum voltage output of approximately 4.3 V (Vav = 4 ± 0.26 V) at a compression pressure of ∼0.32 MPa applied at 1 Hz frequency as shown in Figure 4c with a response time of ∼25 ms (SI Figure S2). (From here on, bilayer films refer to the bilayer films obtained from poled PVDF-TrFE with GO layer on the hydrogen terminated surface, unless otherwise stated.) The reverse occurs in the polarity of voltage output of bilayer films when reversing the contacts of the device, suggesting that the device output is of piezoelectric nature (SI Figure S3). It is also observed that the voltage output of the bilayer films, fabricated from poled PVDF-TrFE films, is greater than the sum of the voltage output from single poled PVDFTrFE films and the electrostatic component and is approximately two times higher than that of pure PVDF-TrFE films. This clearly shows the enhanced voltage output of the PVDFTrFE films in the presence of GO. Kim et al.6 and Gupta et al.41 have reported a rectifying output when nanostructured (nanosheets) piezoelectric materials are in contact or D

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ACS Applied Materials & Interfaces terminated with a charged surface. The rectified output is due to the presence of unipolar surface, resulting from the anionic hydroxides, at one end of the piezoelectric material. In contrast, in our work, the PVDF-TrFE film is in contact with the charged material, graphene oxide, leading to the enhancement in the AC voltage output values due to the nonuniform distribution of charge in GO associated with the presence of various functional groups. Because the fabricated bilayer energy harvester device is freestanding and is flexible, bendable, and rollable (Figure 3e), the energy harvester can be easily integrated with the flexible electronic devices to obtain the self-powered system. The thickness of the GO layer in bilayer films is varied between 3 and 30 μm (3, 20, and 30 μm) to optimize the device performance. With 3 μm GO films, the peak voltage output of bilayer films is approximately 3 V. The output of the bilayer films increases to 4 V, when the GO films are of 20 μm thickness. The output voltage remains the same (4 V) when increasing the thickness of the GO films to 30 μm. The increased voltage output with increased GO thickness from 3 to 20 μm is due to the higher charge in GO. This can be further confirmed from the energy harvesting measurements on the samples consisting of solely the electrostatic components measured from the bilayer films formed from nonpoled PVDF-TrFE films (for the 3 μm thick GO film, the voltage output is approximately 0.1 V; for the 20 μm thick GO film the voltage output is approximately 0.3 V); there is not much change in the voltage output values for even higher thickness of GO films; this may be due to the poor adhesion between the GO and nonpoled PVDF-TrFE films with increasing thickness). Therefore, by further increasing the GO thickness in bilayer films, made from poled PVDF-TrFE, the output voltage remains approximately at 4 V, due to the screening of electrical output in the graphene oxide; there is not much contribution from the electrostatic component with increasing the thickness further. Furthermore, bilayer films are fabricated using 10 μm thick PVDF-TrFE films by keeping the GO layer at approximately 20 μm. Enhanced voltage output is still observed even with 10 μm thick PVDF-TrFE films. The 10 μm films showed a voltage output of 0.5 V (lower output voltage compared to the voltage output of PVDF-TrFE films of thickness 32 μm, since the piezoelectric voltage output is proportional to thickness), whereas the bilayer films showed a voltage output of approximately 1.5 V (SI Figure S4). This suggests enhanced voltage output can be observed from bilayer films, independent of the thickness of the PVDF-TrFE film. The bilayer films obtained from depositing the GO layers on fluorine terminated (negatively poled) surface showed lower voltage output compared to the PVDF-TrFE films (SI Figure S5). The lower voltage output is either due to the poor adhesion of negatively charged GO on to the fluorine terminated polymer surface or the electrostatic component may be just subtracted in this case. The power output of the bilayer and PVDF-TrFE films is extracted by measuring the voltage across a load resistor. Voltage is measured across the resistors ranging from 43 kΩ to 6.7 MΩ (0.043, 0.1, 0.576, 1, and 6.7 MΩ). Figure 5 shows the power output measured across various resistors for bilayer films and PVDF-TrFE films. A maximum in the power output is observed across a load resistor of 1 MΩ for both bilayer films and PVDF-TrFE films. A maximum power value of 4.41 μW cm−2 is observed for the bilayer films, whereas the PVDF-TrFE films have a maximum power output of 1.17 μW cm−2.

Figure 5. Power output of the bilayer and PVDF-TrFE films.

SI Figure S6 illustrates the dynamic current output of the PVDF-TrFE and bilayer film devices upon application of a dynamic compression pressure. PVDF-TrFE films exhibited a peak current output of ∼0.96 μA, whereas the bilayer films have a current output of approximately 1.88 μA, for the device area of 1 cm2. The power output (P = VI, calculated from the open circuit voltage and short circuit current) of the PVDF-TrFE film is 1.83 μW cm−2 and bilayer film is 7.52 μW cm−2 upon application of a dynamic compression pressure of approximately 0.32 MPa applied at 1 Hz. Table 1 summarizes the power density and output voltage and current of bilayer films and PVDF-TrFE films alone. The Table 1. Energy Harvesting Performance of Bilayer and PVDF-TrFE Films power density (μW cm‑2)

voltage output (V) PVDF-TrFE film bilayer films of PVDF-TrFE and graphene oxide

1.9 ± 0.2 4 ± 0.26

current (μA)

measured across a load resistor of 1 MΩ

from open circuit voltage and short circuit current

0.96

1.77

1.83

1.88

4.41

7.52

output voltage of bilayers films is 4 V, which is two times that of PVDF-TrFE films, and power density of bilayer films is 4.41 μW cm−2, which is approximately 2.5 times that of PVDF-TrFE films (power output from bilayer films is approximately four times higher than that of films considering open circuit voltage and short circuit current measurement). From Table 1, it is further observed that the power density calculated from voltage and current is overestimated compared to the power density measured across a load resistor. This is because the voltage is measured in open circuit condition (measured across a very high resistance) and the current is measured in short circuit condition (measured across a very low resistance).5,42 Since the energy harvester is required to be operated across a finite load resistance during practical applications, the power density measured across a load resistor gives the best practical values for real time applications. The output voltage of the bilayer and PVDF-TrFE films are further measured with respect to the applied pressure. PVDFTrFE films are poled with fields of 30 MV/m at 105 °C. Figure 6 shows the applied pressure versus voltage output of the bilayer and PVDF-TrFE films. The voltage output is measured in the compression pressures range from 0.024 to 0.32 MPa. It is observed that the voltage output increases with increasing applied compression pressure and is almost linear in the E

DOI: 10.1021/acsami.5b09502 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. Compression pressure versus voltage output of bilayer and PVDF-TrFE films.

Figure 8. XRD spectra of poled PVDF-TrFE and bilayer films.

measured compression pressure range. The increase in output voltage with applied pressure in both bilayer and the single PVDF-TrFE film suggests that the output voltages of the samples in this work are of piezoelectric origin. This further confirms that there is no voltage originating from the sliding of the graphene sheets or sliding at the interface between the PVDF-TrFE films and GO films. Furthermore the device is tested for cycling stability. Dynamic compression pressure of 0.32 MPa is applied at a frequency of 1 Hz, and corresponding voltage output is measured over 1000 cycles as shown in Figure 7. In the

Voltage output from the piezoelectric energy harvesting device45 V = g33(stress applied)d

(1a)

where g33 = piezoelectric voltage coefficient, stress applied = Young’s modulus of the device × strain, and d = thickness: V = g33 × Young’s modulus × strain × d

(1b)

In the bilayer form, the stress applied depends on the Young’s modulus of the bilayer films. It is measured that the Young’s modulus of the bilayer film is 2.88 GPa, whereas that of the PVDF-TrFE film is 1.5 GPa from the stress−strain curves (Figure 9). Increased Young’s modulus of the bilayer film

Figure 7. Cycling stability of bilayer films.

measured cycles range, the output voltage is almost constant; no degradation in the performance is observed even after 1000 cycles. (The slight change in the output voltage during the cycling is due to the slight fluctuation in the applied pressure, limited by the instrument). The increase in the voltage output in the bilayer films compared to PVDF-TrFE alone is due to the following reasons. The applied dynamic compression force leads to the change in the film thickness and hence the capacitance, giving rise to the voltage output. This corresponds to the electrostatic component (Figure 4b, due to the presence of charges on GO). In addition, it is observed that the films slightly curled after the formation of GO layer on to the PVDF-TrFE. This suggests the residual stress is created in the PVDF-TrFE layer with the deposition of the GO layer. Figure 8 shows the XRD spectra of PVDF-TrFE films before and after deposition of the GO layer. A shift is observed in the XRD peak positions to higher 2θ values for PVDF-TrFE (110)/(200) planes after deposition of GO. The shift in the XRD peak towards the higher 2θ values suggests the tensile stress created in PVDFTrFE due to the bonding of GO layers. The internal tensile stress created in the PVDF-TrFE films could have aligned the molecular chains and hence led to the enhanced piezoelectric ability43,44 of the films and contributed to the observed enhanced piezoelectric energy harvesting performance of the bilayer films.

Figure 9. Stress−strain curve of bilayer and PVDF-TrFE films.

furthermore leads to the higher values of electromechanical coupling factor (k = d33[E/ε]1/2, a measure of the ability to convert input mechanical energy into electrical energy, where d33 = piezoelectric coefficient, E = Young’s modulus, and ε = dielectric constant) for bilayer configuration as compared to the PVDF-TrFE films alone. In addition, since the force is applied in compression mode, the strain in the PVDF-TrFE films is not limited by the high modulus GO layer. Enhanced Young’s modulus is a contributing factor in the observed improved energy harvesting performance of the nanocomposite films20,45,46 (electrospun composite nanofibers of PVDF-TrFE and CNT and nanocomposite films of ZnO nanoparticles, reduced graphene oxide, PDMS, and composites of PVDF and BTO nanoparticles). Enhanced electromechanical coupling has been observed in unimorph actuators, when the piezoelectric material is coupled with an elastic material (metal plate) with higher Young’s modulus compared to piezoelectric material.47 In addition, high performance actuators are fabricated using high modulus flexible carbon fiber as the support in unimorph actuators.48 This explains the efficient conversion of mechanical energy into electrical energy for the bilayer films compared to the F

DOI: 10.1021/acsami.5b09502 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces PVDF-TrFE films alone. In addition, the mechanical and electrical energy efficiently transferred within the bilayer film due to the high Young’s modulus and high dielectric constant of GO compared to the PVDF-TrFE films, with the following expressions:49

4. CONCLUSION We successfully fabricated bilayer films of PVDF-TrFE and graphene oxide. Energy is harvested successfully by means of both piezoelectric and electrostatic manner. The bilayer films showed enhanced energy harvesting performance with voltage output approximately 2 times and power density 2.5 times that of the PVDF-TrFE films. The enhanced voltage and power output of the films is due to the electrostatic contribution of the graphene oxide films, internal stress created in the PVDF-TrFE films, and complex interface and increased Young’s modulus of the bilayer films of PVDF-TrFE and graphene oxide. It can be further concluded that, utilizing the charged material (characterized with high k dielectric constant and high Young’s modulus) interface with the ferroelectric material is an effective approach of improving the energy harvesting performance of ferroelectric materials.

effciency of mechanical energy transfer: ηmec = 1/(1 + dGOEp/d pEGO)

(2a)

effciency of electrical energy transfer: ηelec = 1/(1 + dGOεp/d pεGO)

(2b)

where dGO and dP are thicknesses, εGO and εP are the dielectric constants, and EGO and EP are the Young’s moduli of graphene oxide and PVDF-TrFE, respectively. From the literature, the Young’s modulus of GO monolayers is approximately 200 GPa50 and that of graphene oxide paper is 32 GPa,51 which is much higher than the Young’s modulus of PVDF-TrFE films (1.5 GPa). The dielectric constant of GO films is ∼1.75 × 103, whereas the dielectric constant of PVDF-TrFE films is 9.25 at 1 kHz (SI Figure S7). According to eq 2, it can be understood that higher Young’s modulus and dielectric constant of GO help in efficient transfer of electrical and mechanical energy within the bilayer films. In bilayer films, GO (negatively charged) is in contact with the hydrogen terminated (positively charged) surface of PVDFTrFE. At the interface of PVDF-TrFE and GO, there exists an uncompensated charge, which is the result of the aligned dipoles in the PVDF-TrFE (positive charge due to hydrogen terminated surface) and negatively charged functional groups in GO. This results in a complex interface in the bilayer films between the PVDF-TrFE and GO with uncompensated charge along with negative charges throughout the GO films. Such complex interface leads to enhanced piezoelectric properties as in the case of bilayer films of PVDF and nylon11.52 The present work provides a strategy to enhance energy harvesting performance of the piezoelectric films in conjunction with the charged layer of flexible high modulus and dielectric constant. By integrating the bilayer devices, various selfpowered systems can be developed. The enhanced voltage output can be particularly useful for a highly sensitive pressure sensor. Because the prepared bilayer films are flexible, bendable, and rollable, the energy harvesting device can be easily integrated with flexible electronics to achieve self-powered systems. In addition, the energy harvester can be used for electronic skin applications with improved sensing ability. To the best of our knowledge, this is a pioneering work showing that the augmentation of AC voltage output can be realized with piezoelectric film in contact with charged dielectric. In summary, we report an effective approach to improve the energy harvesting performance of ferroelectric material (PVDFTrFE) by the addition of a charged dielectric film (GO) with high Young’s modulus and high dielectric constant. Enhanced voltage (2 times) and power (2.5 times) output for the bilayer films of PVDF-TrFE and graphene oxide compared to only PVDF-TrFE films are presented. The enhanced performance of the bilayer films are ascribed to the electrostatic contribution of GO films, internal stress created in the PVDF-TrFE films, and increased Young’s modulus of the bilayer films. We rationalize that any charged material (with high dielectric constant and Young’s modulus) interface with the ferroelectric material can lead to enhanced energy harvesting performance.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09502. Switching polarity test of the voltage output of the bilayer films, energy harvesting performance of bilayer films in various conditions, and dielectric constant of PVDF-TrFE and GO (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (65)-67906661. Fax: (65)6790 9081. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Research Foundation Competitive Research Programme, Award No. NRF-CRP-132014-02. The work is also supported by NTU-HUJ-BGU Nanomaterials for Energy and Water Management Programme under the Campus for Research Excellence and Technological Enterprise (CREATE) that is supported by the National Research Foundation, Prime Minister's Office, Singapore. V.B. acknowledges the research scholarship provided by Nanyang Technological University, Singapore.

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REFERENCES

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DOI: 10.1021/acsami.5b09502 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX