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Flexible Lead-Free BiFeO3/PDMS-Based Nanogenerator as Piezoelectric Energy Harvester Xiaohu Ren, Huiqing Fan, Yuwei Zhao, and Zhiyong Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04497 • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 10, 2016

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ACS Applied Materials & Interfaces

Flexible Lead-Free BiFeO3/PDMS-Based Nanogenerator as Piezoelectric Energy Harvester Xiaohu Ren, Huiqing Fan*, Yuwei Zhao, Zhiyong Liu State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China KEYWORDS: Nanogenerator; Lead-Free; BiFeO3; Piezoelectricity; Composites ABSTRACT Perovskite ferroelectric BiFeO3 has been extensively researched in many application fields, but which have rarely been investigated for the energy conversion of tiny mechanical motion between electricity in spite of its large theoretical remnant polarization. Here we demonstrate the fabrication of a flexible piezoelectric nanogenerator based on BiFeO3 nanoparticles (NPs), which was synthesized using a sol-gel process. The BiFeO3 NPs-PDMS composite device exhibits an output open circuit voltage of ~3 V and short circuit current of ~ 250 nA under repeating hand pressing. The output generation mechanism from the PNG is discussed on the basis of the alignment of electric dipoles in composite film. It is demonstrated that the output power from the PNG can directly drive light emitting diode (LED) and charge capacitor. These results demonstrate that BiFeO3 nanomaterials have the potential for large-scale lead-free piezoelectric nanogenerator applications.

1 INTRODUCTION Power generation based on fossil energy has faced many critical problems such as shortage of resources and environmental pollution, which is expected to be aggravated in several years of the future1. In order to mitigate this tendency, making use of various renewable energy resources has attracted increasing attention2. Especially, small-scale energy harvesting approaches by converting environmental energy such as solar3,4, thermal5,6 and biomechanical energy7-9 into power have been extensively researched to realize self-powering electronic devices10-12. Among these methods, piezoelectric nanogenerator (PNG) is seen as a viable and promising approach for its relatively less influences from external conditions13. Up to now, various types of PNGs based on non-centrosymmetric oxides such as ZnO14,15, PZT16,17 and BaTiO313,18 have been reported for harvesting energy application. ZnO has received vivid attraction for NGs because of its unique non-centrosymmetric structure, but its intrinsically low piezoelectric coefficient effects further improvement of device performances19. Furthermore, ferroelectric perovskites exhibit high polarization and piezoelectric coefficient, especially the Pb-based perovskites have always investigated and applied extensively as piezoelectric materials candidates20,21. But considering toxicity of lead, lead-free materials would be more eco-friendly for extensive applications22. Among the lead-free perovskites, much attention has been paid to BiFeO3 due to its unique crystalline structure and characteristics. However, the studies of BiFeO3 are mainly 1

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focused on the multiferroic23, ion conduction24, photovoltaic effect25, electromagnetic properties26 and magnetic properties27. It is known that the energy harvesting performance of piezoelectric materials are closely related to their piezoelectric coefficient (d33∝εPr)28. Thus, the larger remnant polarization (Pr) of materials is desirable for enhancing performance of PNG devices. Ravindran et al.29 reported their theoretical studies using density functional theory, predicting a large spontaneous polarization of about 90 µC/cm2 along [111] direction, which is larger than that of other materials such as ZnO (Pr∼5 µC/cm2)30, KNbO3 (Pr∼23 µC/cm2)31, LiTaO3 (Pr∼32 µC/cm2)32, AgNbO3 (Pr∼52 µC/cm2)33, NaNbO3 (Pr∼40 µC/cm2)34 and ZnSnO3 (Pr∼59 µC/cm2)35. Although BiFeO3 with the R3c space group exhibits large spontaneous polarization, it has not been as extensively investigated as other lead-free perovskite materials in piezoelectric application. The one reason is the large difficulty of preparing single-phase BiFeO3 bulk36, another main reason is the problem of the high electrical conductivity of BiFeO3, which not only limits its applicability, but makes it difficult to investigate its basic ferroelectric and piezoelectric properties. However, the pure phase BiFeO3 nanoparticles is easier to synthesize compared to bulk due to low preparing temperature. In addition, compositing oxide nanoparticles with polymer matrix can overcome the problem of difficulty in poling process due to low electrical resistance. Therefore, it is significant to research the energy harvesting performance of composite based on BiFeO3 nanoparticles and practical application in PNG. Here, we report a flexible PNG based on the composite film obtained by dispersing BiFeO3 nanoparticles (NPs) in PDMS matrix. The BiFeO3 NPs were synthesized by a convenient sol-gel method and the composite thin film was prepared by the spin-coating process. In the PNG, BiFeO3 NPs act as power generation sources and the PDMS as flexible matrix. Under human hand impacting, the output voltage and current of the fabricated PNG were measured. It is obtained a stable output voltage of around 3 V and current density of 0.12µA/cm2. The generating power can instantly light a commercial light-emitting diode (LED). Besides, the capacitor charging capability of the PNG demonstrates that it might be an alternative candidate as a piezoelectric energy transformer. In addition, it is low-cost, simple and potential for large-scale fabrication for the approach we report here.

2 EXPERIMENTAL 2.1 Synthesis of BiFeO3 Nanoparticles. BiFeO3 nanoparticles were synthesized via previously reported sol-gel method [], using bismuth nitrate pentahydrate, iron nitrate nonahydrate, acetic acid, ethylene glycol and citric acid as starting ingredients. Initially, stoichiometric bismuth nitrate and iron nitrate were respectively dissolved in acetic acid and ethylene glycol. Then the two solutions were mixed together after stirring for 30 minutes, after which the citric acid monohydrate solution was added. The mixture was stirred constantly at 80 ºC until a yellow-brown sol was obtained by volatilization of the excess solvent. After drying the sol at 100 ºC, the obtained gel was preheated to 400 ºC to remove residual oxynitrides and hydrocarbons impurities. After further calcined at 600 ºC for 2h, the BiFeO3 nanoparticles were obtained finally. 2.2 Fabrication of PNG. The PNG fabrication process can be described as the following. At first, the polydimethylsiloxane (PDMS; Sylgard 184, Dow Corning Corp., Auburn, MI, USA) solution prepared by adding curing agents to base (where weight ratio of PDMS and curing agent=10:1), and then the synthesized BiFeO3 NPs were dispersed into the PDMS at various concentrations of 10, 20, 30 and 40 2


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wt%. Next, the mixture is spin-casted onto Al electrode with a spinning rate of 500 rpm for 30 seconds and then cured at 70 ºC for 10 min. Thereafter, another Al electrode was pasted to the top surface of BiFeO3-PDMS composite film directly and fully cured. Further, the two electrodes were both pasted onto the PET substrate. Finally, the PNG device was poled using an applied electric field of 200kV/cm for 10 h at 150 ℃. 2.3 Characterization. Structural and crystallographic characterizations of the synthesized BiFeO3 NPs were performed via using x-ray diffraction (XRD; X'Pert PRO MPD, Philips, Eindhoven, Netherlands) with Cu Kα radiation (λ=1.54 Å). The morphologies of BiFeO3 NPs and structure of composites were observed by a field emission scanning electron microscopy (FESEM; Supra 55, Zeiss, Germany). The output voltage and current from the PNGs under repeating hand impact were measured by a digital oscilloscope and a source measurement unit (2410 SMU, Keithley, Beaverton, OR, USA) respectively. Ferroelectric and piezoelectric characteristics were investigated by a piezoelectric force microscopy (PFM; Dimension Icon, Bruker, Santa Barbara, CA, USA).

3 RESULTS AND DISCUSSION The XRD Rietveld refinements for BiFeO3 were performed by using the GSAS-EXPGUI program to characterize structure of the prepared BiFeO3 nanoparticles at room temperature37. The XRD pattern for BiFeO3 is shown in Figure 1a, which was well-fit by the rhombohedral structure with R3c symmetry.

(a) Observed Calculated Difference Bragg reflections

Intensity (a.u.)

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20

40

60

80

100

120

2 Theta (°)

Figure 1. Structural and morphology of BiFeO3 NPs. (a) Rietveld refinement analysis of XRD spectra of BiFeO3. (b) FE-SEM image showing the crystalline surface morphology of BiFeO3 NPs. (c) Crystal structure of BiFeO3.

Figure 1b shows FE-SEM image of BiFeO3, exhibiting a spherical shape with homogeneous particle sizes of about 100 nm. The summary of crystal data and R factors is listed in Table S1 (Supporting Information). The final factors, Rwp, Rp, and χ2 are 6.92%, 5.05%, and 2.73. Table 1 lists 3



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the atomic occupancies, coordinates as well as isotropic thermal parameters of BiFeO3 at room temperature. To understand the crystallographic structure of BiFeO3, a rhombohedral unit cell of BiFeO3 is depicted in Figure 1c on the basis of the refined result. It is observed that the crystal possesses a perovskite structure with high distortion and rhombohedral symmetry. In the ideal cubic perovskite structure the oxygen ions would occupy the face-centered sites. However low tolerance factor of BiFeO3 indicates that Bi ion may be difficult to stabilize in the high-symmetric position of the cubic perovskite structure due to lattice mismatch between the cations 29. Therefore, as shown in Figure 1c, a lattice distortion arises by the displacement of FeO6 octahedra along the [111]rho direction of the rhombohedral unit cell to reduce the lattice mismatch and enhance stability, which leads to off-centering of Fe3+ in lattice thereby forming strong spontaneous polarization. Thus, a piezoelectric nanogenerator (PNG) fabricated based on BiFeO3 NPs is expected. Table 1 Atomic coordinates, occupancies and isotropic thermal parameters of BiFeO3. Atom

Site

x

y

z

100Uiso (Å2)

Occupancy

Bi

6a

0

0

0.00117(15)

0.0202(2)

1

Fe

6a

0

0

0.22217(18)

0.0196(8)

1

O

18b

0.4448(13)

0.0133(11)

0.95256(28)

0.0024(2)

1

Contributions to the Ps of each ion (µC/cm2) -32

-24

-16

-8

0

8

16

24

32

Total P =22.76 (µC/cm2) s O

Fe ∆x

Bi

Ps

-0.8

-0.4

0.0

0.4

0.8

Atomic displacement along c axis (Å)

Figure 2. Ion displacement and contribution to the total spontaneous polarization (Ps) of each ion in BiFeO3.

For BiFeO3 with R3C space group, atomic displacements are along c axis ([001]hex direction) from the corresponding locations in the hexagonal cause ferroelectric spontaneous polarization (Ps). In the light of the atomic displacements in unit cell, the total Ps of displace-type ferroelectrics can be calculated by Shimakawa’s mode 38 ݉௜ × ∆‫ݔ‬௜ × ܳ௜ ݁ ܲ௦ = ෍ ܸ ୧

Here, mi, V, ∆xi, and Qie represents the site multiplicity, unit cell volume, atomic displacement along the c axis direction from the corresponding location in the symmetric structure and the ionic charge of the ith corresponding ion respectively. According to the crystallographic data obtained from refinement (Tables 1 and S1), the ion displacements along the c axis direciton and the contributions of 4


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each ion to the total Ps of BiFeO3 are shown in Figure 2. The calculated Ps is 22.76µC/cm2, which is consistent with some experimental result reported previously39, 40. The schematics of the PNG fabricating process are shown in Figure 3a. The PNGs were fabricated by covering the top and bottom surfaces of BiFeO3 NPs-PDMS nanocomposite with aluminum (Al) foil as electrodes on a polyethylene terephthalate (PET) substrate. The detail information about fabrication process is described in Experimental section. The optical image in Figure 3b shows the bent by human fingers, confirming its suitability in flexible and embedded electronics. Figures 3c exhibits cross-sectional SEM image of PNG that is composed of composite film and two metal electrodes, indicating that the composite film is about 100µm thick. A magnified cross-sectional SEM image shown in Figure 3d suggests that the BiFeO3 nanoparticles were well-distributed in PDMS matrix by and large except for slight agglomeration. Random orientation of BiFeO3 nanoparticles inside PDMS will result the electric dipoles are randomly aligned between the two electrodes before electric poling.

Figure 3. (a) Schematic diagrams of the PNG fabricating process. (b) Photograph of the BiFeO3-PDMS composite film bended by fingers. (c) The cross-sectional SEM image of the PNG device. (d) The magnified cross-sectional SEM image of BiFeO3-PDMS composite film with 40wt% BiFeO3 NPs.

The generated outputs voltage and current of the poled PNG during the periodic vertical pressing and releasing process were measured respectively. The open circuit voltage and short circuit current of PNG (40 wt% of BiFeO3 NPs concentration) under repeating impacting is shown in Figure 4a and d respectively. It is clearly seen that an open circuit voltage of around 3 V and a short circuit current of around 250 nA. Table 2 shows that BiFeO3 NPs mixed with PDMS in present work exhibits output performance comparable to other piezoelectric nanogenerators. In order to verify that the output electric signal purely resourced from the piezoelectric phenomenon of PNG, switching-polarity tests were carried out. As shown in Figure 4a and b, when the PNG is forward connected to a measurement 5



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device, a positive voltage generated can be detected under compress condition. While under reverse connection, an opposite output voltage signal are measured. It is indicated that the detected outputs are the true electric signals produced from PNG strained by compressing motions41. Figure 4c exhibits an enlarged graph of the single voltage pulse. A difference of voltage peak values between compress and release conditions are observed, which can be attributed to the difference of straining rate in process of applying and releasing the stress on the PNG 42. In addition, there are two opposite responses in releasing process of one cycle. The former one generates when the surface recover to initial position after press releasing, and the latter one can be ascribed to reverse strain due to elasticity of BiFeO3-PDMS composite. Table 2. Output performance comparison of piezoelectric nanogenerators.

Output performance Piezoelectric filler

Matrix

PZT Nanowires PZT Microsphere BaTiO3 nanowires BaTiO3 nanoparticles + CNT NaNbO3 nanowires LiNbO3 nanowires BiFeO3 nanoparticles

Voltage

Current/Current density

Ref.

PDMS PDMS PVC

6V 6V 1.9 V

50 nA 0.2 µA/cm2 24 nA

17 43 44

PDMS

3V

0.3 µA

41

PDMS PDMS

3.2 V 0.46 V

16 nA/cm2 9.11 nA/4.64 nA/cm2

PDMS

3V

0.25 µA/0.12 µA/cm2

42 45 This work

To confirm the contribution of BiFeO3 NPs within PNG, the PNG with pure PDMS was measured under the same test conditions. The result is shown in Figure S1 of Supporting Information. It is observed that the output voltage of pure PDMS PNG is much smaller than the outputs of PNG with BiFeO3 NPs. It works out that the high output of PNG is mainly attributed to addition of BiFeO3 NPs. Furthermore, to research the importance of poling process, a comparative trial of the PNG without poling was carried out. From the Figure S2 in Supporting Information, it suggests that the output voltage of the unpoled PNG is about 100 mV and smaller than the poled PNG, which proves that the output is associated with ferroelectricity of BiFeO3-PDMS composite. So the poling process is an indispensable step to improve the output capability of the PNG. The PFM measurement was used to investigate the piezoelectricity of the BiFeO3-PDMS composite film containing 40 wt% BiFeO3 NPs. Figure 5a and b shows the piezoelectric/ferroelectric response phase and amplitude respectively under applying direct-current (dc) bias voltage. Before measuring, two layers of Au film were coated to top and down surfaces of the sample to eliminate the effect of electrostatic cantilever-sample interaction 46. The piezoelectric response amplitude (nm) was calculated by dividing the measured amplitude (mV) with the cantilever sensitivity (0.00934 V/nm for the tip). The difference of phase ∆Φ (Figure 5a) between the opposite signals is around 180°, meaning a 180° domain switching of the permanent polarization, and a classic butterfly-shaped amplitude loop (Figure 5b) corresponds to the 6


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strain-voltage (S-V) phenomenon of piezoelectric materials. Those results indicate nonlinear characteristic of the composite film as expected from the strong piezoelectricity of the component materials 47.

Figure 4. (a) The open circuit voltage of the PNG in the forward connection. (b) The open circuit voltage of the PNG in the reverse connection. (c) Enlarged view for one cycle of output voltage under forward connection. (d) The short circuit current of the PNG during the periodic pressing and releasing motions.

a 200

b 60 Amplitude (pm)

50 Phase (Degree)

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150 100 50 0

40 30 20 10 0

-20

-10 0 10 DC Bias (V)

20

-20

-10 0 10 DC Bias (V)

20

Figure 5. The phase (a) and amplitude (b) hysteresis loop of BiFeO3-PDMS composite film with 40 wt% BiFeO3 NPs.

In figure 6, we schematically show the work mechanism of PNG device. Initially, without any electric poling, electric dipoles within the BiFeO3 NPs randomly align in the PDMS matrix between the two electrodes (Figure 6a). When an electrical field is applied on the PNG, the dipoles within BiFeO3 NPs tend towards align along the applied electric-field direction as depicted in Figure 6b. If no external force on the device is applied, no electric signal will be detected due to that the device remains an equilibrium state. Further, when the PNG suffers a vertical compressive force, a change of 7



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total polarization of the composite is generated due to the compressive strain, which results in a piezoelectric potential between the top and bottom electrodes. In such a way external free charges will move to and accumulate at the electrodes to screen the piezoelectric potential. During this process an output signal can be detected from the PNG (Figure 6c). When the vertical applied mechanical force is released, the vertical strain and piezoelectric potential between the two electrodes of the PNG disappear. The accumulated charges transport back in a reverse direction and a negative electric signal is generated (Figure 6d). After the first returning to original state, a small tensile strain will be produced due to good elasticity of PDMS matrix, resulting in a reverse piezoelectric potential. To screen the piezoelectric potential, the motion and accumulation of free charges happen again as shown in Figure 6e. So a weak negative electric signal can be detected. As discussed above, output pulse signals from the PNG device is obtained during continuously applying and releasing of the external compressive force.

Figure 6. (a)The dipoles in BiFeO3 NPs randomly align before poling process. (b)After poling, the dipoles in BiFeO3 NPs will align in one direction along the electric-field. (c)When the compressive force is applied on the PNG, a piezoelectric potential is generated. (d)As the compressive force is released, the accumulated electrons flow back along the opposite direction. (e)After returning to original state, a reverse piezoelectric potential produced from tensile strain due to good elasticity of PDMS.

In order to study the influence of BiFeO3 NPs concentration on the output capability of the PNG, a series of PNGs were fabricated based on BiFeO3-PDMS composites with various mass fraction of BiFeO3 NPs (10, 20, 30, and 40 wt%). To select suitable electric poling process, we measured output performance of the PNGs with different BiFeO3 NPs contents polarized by various poling times, which is shown in Figure S3 of Supporting Information. Finally, we poled each PNGs applying a electric field of 2kV for 10h at 150 ℃. Typical time-dependent output voltages measured for the various PNGs during cyclic pressing and releasing process are shown in Figure 7a. It can be obtained output voltages around 0.2, 0.7, 1.5 and 3 V from the PNGs, for BiFeO3 NPs fractions of 10, 20, 30, and 40 wt% respectively. We can see that the output voltage gradually increases with increasing content of BiFeO3 NPs up to 40 wt%, and gains a maximum output voltage of around 3 V. Further, we simulated the distribution of induced piezoelectric potential in vertically stressed PNGs based on BiFeO3 NPs by using COMSOL multiphysics software for finite element calculation, by which 8


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piezoelectric potential distribution in PNGs with different amounts of piezoelectric particles dispersed in matrix was calculated respectively. Differences of the piezoelectric potential were clarified by color code, as a compressive stress of 10 kPa was applied to the BiFeO3-PDMS composite. It can be found that the piezoelectric potential difference between two electrodes of PNGs increases linearly with increasing amount of piezoelectric particles, as shown in Figure 7b. For comparison, the experimentally obtained output voltages from the PNGs with different concentration of BiFeO3 particles are also depicted in Figure 7b. It is found that the tendency of output voltages obtained experimentally is consistent with the simulated result.

(a)

4 20%

10%

30%

40%

3 Voltage (V)

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


2 1 0 -1 -2

0

10

20 Time (S)

30

40

Figure 7. (a)Time-dependent output voltage for PNGs with 0-40% BiFeO3 NPs concentration. (b)The variation of open circuit voltage and COMSOL simulation result about distribution of piezoelectric potential for PNGs with different BiFeO3 contents.

To test the feasibility of practical application of the PNG, the PNG is directly connected with two commercial LED in parallel but opposite directions without any storage units (a schematic circuit diagram is displayed in figure 8a). When the PNG is impacted by hand, the red LED is instantly lighted and the green LED is lighted by reverse output after releasing as shown in Figure 8b and supplementary video file. And then, a typical rectifier was employed in order to convert the generated AC to DC output. Figure 8c shows the output voltage signal rectified. To test the capacitor’s charging capability from the PNG, the capacitors of 1.0, 2.2 and 4.7 µF were charged by the rectified output with the PNG respectively, the concerning transient response is exhibited in Figure 8d. It is observed that the charging voltages increased gradually and eventually reached the maximum value of 1.26, 1.68, and 2.58V within 55, 170, and 527s respectively. The above result indicates that the PNG can be seen as a promising power source device for tiny energy harvesting application 48, 49 and self-powering electronic devices 50, 51. Moreover, in order to prove the stability of the PNG, a durability test was carried out over 1000 cycles. As shown in Figure 9, there is no degradation in the output performance is observed even after 1000 cycles. The BiFeO3-PDMS based nanogenerator shows exceptionally durable and reliable energy harvesting performance.

9



Figure 8. (a) The circuit schematic of LED driving and capacitor charging, the switch is connected to ‘A’ firstly to drive LED by ac output generated by the PNG, and then connected to ‘B’ to charge capacitor. (b) The red and green LED was driven successively by output generated from hand pressing and releasing. (c) Rectified output voltage of the PNG. (d) The capability of capacitor charging from the PNG by periodic pressing.

4 3 Voltage (V)

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

2 1 0 -1 0

50

100

150 200 Times (s)

250

300

Figure 9. Durability test of the PNG, the inset shows an enlarged view of the open circuit voltage signal in process of the durability test.

Conclusion In summary, BiFeO3 nanoparticles were synthesized via a sol-gel method. We have also fabricated a flexible piezoelectric nanogenerator based on BiFeO3 NPs-PDMS composite structure for scavenging mechanical energy. The output voltage and current of the PNG reached up to ∼3 V and ∼300 nA which corresponds to a current density of ∼0.12 µA/cm2 under repeating hand impacting. The single output pulse generated from the PNG can light up a commercial LED directly. It is concluded that the BiFeO3 nanoparticles is a valuable candidate for energy harvesting application.

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ASSCIATED CONTENT Supporting Information Refined crystal data for BiFeO3, The output voltage generated from the PNG without BiFeO3 NPs. The output voltage generated from the PNG before poling process. The output voltage of the PNGs with different BiFeO3 NPs contents polarized by various poling times (PDF) Video S1, power up the LED (AVI)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: +86-29-8849 2642. Tel: +86-29-8849 4463. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (51172187), the 111 Program (B08040) of MOE, the Xi'an Science and Technology Foundation (CXY1510-2), the Fundamental Research Funds for the Central Universities (3102014JGY01004) of China.

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