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Er3+/ Fe3+ Stimulated Electroactive, Visible Light Emitting and High Dielectric Flexible PVDF Films Based Piezoelectric Nanogenerators: A Simple and Superior Self-Powered Energy Harvester with Remarkable Power Density Nur Amin Hoque, PRADIP THAKUR, Swagata Roy, Arpan Kool, Biswajoy Bagchi, Prosenjit Biswas, Md. Minarul Saikh, Farha Khatun, Sukhen Das, and Partha Pratim Ray ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 14 Jun 2017 Downloaded from http://pubs.acs.org on June 14, 2017

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ACS Applied Materials & Interfaces

Er3+/ Fe3+ Stimulated Electroactive, Visible Light Emitting and High Dielectric Flexible PVDF Films Based Piezoelectric Nanogenerators: A Simple and Superior Self-Powered Energy Harvester with Remarkable Power Density Nur Amin Hoque1, Pradip Thakur2*, Swagata Roy1, Arpan Kool1, Biswajoy Bagchi1+, Prosenjit Biswas1,Md. Minarul Saikh1#, Farha Khatun1, Sukhen Das1*, Partha Pratim Ray1* 1

Department of Physics, Jadavpur University, Kolkata- 700032, India

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Department of Physics, Netaji Nagar College for Women, Kolkata- 700092, India.

+

Present address: Fuel Cell and Battery Division, Central Glass and Ceramic Research Institute, Kolkata- 700032, India. #Government General Degree College at Pedong, Pedong, Kalimpong, Pin- 734311, India *Corresponding author. E-mail address: [email protected] Mob: +919433091337 : [email protected] Ph. No: 033-2457-2844 : [email protected] Mob: +919830366215

ABSTRACT Design of energy harvesting unit with superior output characteristics i.e. high power density is of great technological challenge in the present time. Here, a simple, light weight, flexible and cost effective piezoelectric nanogenerators (PENGs) have been fabricated integrating the aluminum electrodes on to Er3+/ Fe3+ stimulated electroactive, visible light emitting and large dielectric PVDF films where ErCl3. 6H2O and Fe(NO3)3 · 9H2O act as the catalytic agents for electroactive β polymorph nucleation and enhancement of dielectric properties. The developed PENGs exhibit excellent energy harvesting performance with very high power density and very fast charging ability compared to the previously reported PVDF assisted prototype nanogenerators. The PENGs lead to very large power density ~ 160 mWcm-3 and ~ 55.34 mWcm-3 under periodic finger imparting for Er3+ and Fe3+ stimulated PVDF film based energy harvester unit respectively. The fabricated self-powered PENG is also able to glow 54 commercially available light emitting diodes.

Keywords: PVDF, Electroactive β-phase, Dielectric constant, Self-powered, Piezoelectric nanogenerator.

1. INTRODUCTION

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Superior self-powered portable devices such as mobile phones, wearable electronic products, implantable medical goods, roll-up displays, actuators and sensors are essential tools of our modern daily life, health care and environmental monitoring.1-5 Increasing power demands in such devices stimulate the researchers or engineers to think about the development of environment friendly, biocompatible, light weight, flexible, thin self-powered nanogenerators capable of harvesting electrical energy from the abundant resources in nature and our living system. Such resources include solar energy, rain fall, sea water wave, geothermal, and biomass energies, nuclear energy, thermal energy and mechanical resources in terms of human movement, touch, walking and talking.7-14 Harvesting of energy from the living systems of nature is more important to reduce the use of conventional power resources such as fossil fuels and thereby alleviate the environmental degradations caused by the latter. Among these, the mechanical energies are most environment friendly and available in nature as well as in our daily life style.15-17 Recently, piezoelectric nanogenerators (PENG), have come to light as a promising green technique for harvesting electrical energy from mechanical energy resources via piezoelectric effect. Fabrication and performance of several prototype flexible piezoelectric nanogenerators (FPENGs) using traditional piezoelectric materials or ceramics (Like ZnO1, PMN-PT,18,19 (Na,K)NbO3,5 BaTiO3,20 PZT21) have been studied. Most recently, poly(vinylidene fluoride) (PVDF) and its copolymers have emerged as most favourable prospect for designing sophisticated FPENGs because of their light weight, flexibility and environmental compatibility.22 PVDF is an electroactive semi-crystalline thermoplastic polymer having five main crystalline polymorphs α, β, γ, δ and ε. α, β and γ are the most common crystallite forms of PVDF, though the melt processing directly results in the dominating non-polar α crystal. β polymorph with cell all-trans (TTTT) conformation is more important than other crystallite forms of PVDF as it leads to optimum piezoelectric, pyroelectric, ferroelectric and dielectric properties as well as good elastic strength.23-25 The γ-phase has TTTGTTG' conformation which shows average piezoelectricity.26,27 Thus, a technique to develop the electroactive β-phase nucleation in simple and low cost method to optimize their application possibilities in the field of sensors, actuators, storage devices, nanogenerators as well as in biomedical and electronic industries is required.25-28 The most widely used procedure for β phase nucleation is ‘poling’ which is attained by mechanical stretching or application of electrical field on PVDF. However, self-poled PVDF (without any physical stretching) can be easily obtained by incorporating fillers like metal nanoparticles(NPs), metal oxide NPs, solution growth, addition of metal salts, blending with polymers consisting of carbonyl groups, carbon nanotubes, graphene, quaternary phosphorus salt functionalized graphene, ceramics, carbon nanofibers, carbon black, and electrospinning etc.23-28 can be used for the purpose. However, literatures show some studies on the effect of rare earth salts (R3+ cations) on the dielectric properties and crystalline structure of PVDF29, 30 but a comparative study with effect of transition metal cations are very rare. Rare earth cations (R3+) are relatively large in size and low in capacity that form covalent bond. The ability of formation the covalent bond affects the chemical properties of the R3+ cations differently than transition metal cations like Cr3+, Al3+, Fe3+ etc. In our previous study by Thakur et al., electroactive β-phase nucleation and high dielectric constant by the incorporation of cerium (III)/yttrium (III) nitrate 2


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hexahydrate in PVDF was reported.23 Hassen et al. and El-Sayed et al. also studied the dielectric properties of lanthanum (III) chloride (LaCl3)/PVDF29 and ErCl3 or GdCl3/PVDF30 thin films respectively. The differences in chemical properties, non-equality in size and capacity of R3+ cations and transition metal cations may have some dissimilar effect on electroactive β phase crystallization and the physio-chemical properties (i.e. optical properties, dielectric properties etc.) of PVDF. In this circumstances, our present work demonstrates the fabrication of selfpoled piezoelectric PVDF films by incorporation of Er3+ and Fe3+ ions and studies their effect on the electroactive β phase nucleation and dielectric properties for energy harvesting device applications. The enrichment of β phase nucleation as well as the dielectric value of the composite self-poled PVDF films has been elaborately discussed in terms of strong electrostatic interaction via formation of hydrogen bonds and interfacial polarization in between the salt molecules and the PVDF chains. Thereafter, two highly superior, costeffective, flexible and light weight FPENGs (with dimension = 2.4 cm x 2 cm x 48 µm) are demonstrated using two PVDF thin films conjugated with Er3+ and Fe3+ ions respectively showing maximum electroactive β phase nucleation as well as good dielectric properties. The Er3+/PVDF thin film based FPENG, named as EPENG shows better performance than the Fe3+/PVDF thin film based FPENG, named as IPENG.

2. EXPERIMENTS AND METHOD 2.1. Synthesis of Er3+ and Fe3+ ion doped PVDF thin films Erbium (III) chloride hexahydrate/PVDF and iron nitrate (III) nonahydrate/PVDF thin films were prepared by simple solution casting method. Initially, 250 mg PVDF (Sigma Aldrich, Germany. Mw: 180 000 GPC, Mn: 71 000) was dissolved in 5 ml dimethyl sulfoxide (DMSO) (Merck, India) at 60O C under magnetic stirring for clear solution and then required amount of Erbium (III) chloride hexahydrate (Sigma Aldrich, USA) and Iron nitrate (III) nonahydrate (Merck, India) (1-20 mass %) were added to the clear solution under continuous stirring for 14 hours under the same conditions. Afterwards the whole solution were cast in clean petri dishes and dried at 80 oC in a dust free oven to obtain composite thin films. The same process was followed to prepare pure PVDF thin film (supporting information figure S1). The samples thus prepared are designated as ErX and IX (where X= 1, 5, 10, 15 and 20) for Erbium (III) chloride hexahydrate/PVDF and Iron nitrate (III) nonahydrate thin films with varying loading concentration of salts (1-20 mass%) respectively. 2.2. Fabrication of the FPENGs To fabricate our FPENGs, we take the two flexible Er5 and I10 thin films with dimension 2.4 cm x 2 cm x 48 µm. First, 40 µm thick aluminium electrodes were attached on both side of the films and two wires were extended out from both side of each film for measuring the performance of the nanogenerators. Then both films containing the electrodes and connecting wires were sealed with the polydimethylsiloxane (PDMS) (Sylgard 184, Dow Corning, ratio, 1:10) by immersing it in PDMS gel and dried for 15 minutes in vacuum followed by drying at 60 oC for 1 hour to remove the bubbles from the mixture and to obtain our FPENGs. The dimension of the as fabricated devices (FPENG and IPENG) is 5 cm x 3 cm x 0.2 cm. 3



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3. RESULTS AND DISCUSSION 3.1. Characterization of Er3+ and Fe3+ ion incorporated PVDF thin films 3.1.1. Investigation of electroactive β phase nucleation The morphology, microstructure, different phase behaviour and thermal properties of the samples are investigated using field emission scanning microscopy (FESEM) (INSPECT F50, Netherland), X-ray diffraction (XRD) (Model-D8, Bruker AXS Inc., Madison, WI), Fourier transform infrared (FTIR) spectroscopy (FTIR-8400S, Shimadzu), thermal gravimetric analysis (TGA) (TGA/SDTA851e, Mettler Toledo AG) and differential scanning calorimetry (DSC-60, Shimadzu, Singapore). Figure 1 represents the FESEM images of pure PVDF and the salt loaded PVDF samples. Surface images of the Er3+ and Fe3+ ion doped PVDF thin films show the formation of spherulites with diameter ~ 5 µm confirming the nucleation of electroactive β crystals in the composite samples. Whereas the spherulites with diameter ~ 40 µm in pure PVDF film specify the presence of nonpolar α crystals mostly23.

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Figure 1. FESEM images of (a) pure PVDF and the ErCl3. 6H2O / PVDF thin films (Er1, Er5, Er10, Er15, Er20) and (b) pure PVDF and the Fe (NO3)3 · 9H2O / PVDF thin films.

Nucleation of β polymorphs in composite samples have been further verified by XRD spectra. Figure 2a and c illustrate the XRD patterns of pure PDVF and erbium (III) chloride hexahydrate / iron nitrate (III) nonahydrate doped PVDF thin films. The peaks at 2θ=17.6O (100), 18.3O (020), 19.9 O (021) and 26.6 O ((201), (310)) directly indicate the maximum presence of non-polar α crystals. A tiny peak at 2θ = 38.3 O is also observed in the diffraction 5



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spectrum of pure PVDF due to reflection from either (002) plane which corresponds to α crystals or (211) plane which assigns as γ crystals.31,32 This issue is concluded by FTIR study of the pure PVDF film. The addition of the salts leads to vanishing of all XRD peaks correspond to α and γ crystals. Only one characteristic peak at 2θ = 20.5 ((110), (200)) raises strongly confirming the nucleation of electroactive β crystals in all salt loaded PVDF thin films.26 Closer examination of the XRD patterns stipulate the maximum intensity of main β crystal diffraction peak at 2θ = 20.5 for 5 mass% loading of ErCl3. 6H2O and 10 mass% inclusion of Fe (NO3)3 · 9H2O confirming more electroactive β phase crystallization in these samples. The tentative amount of α phase and β phase content sometimes are measured by the ratio of I20.5 and I18.3 (shown in Figure 2b and d). The maximum value of these ratios have reached 18 for Er5 and 7 for I10 indicating more β crystal acceleration ability of the rare earth Er3+ ion than the transition metal ion Fe3+. Figure 2e and 2g represent the FTIR spectra of pure PVDF and Er3+/ Fe3+ assisted PVDF thin films in the range of 400 to 1350 cm-1. The spectrum of pure PVDF are well packed with the characteristic absorbance bands of nonpolar α- crystals at 488 cm-1 (CF2 waging) 532 cm-1 (CF2 bending), 615 and 764 cm-1 (CF2 bending and skeletal bending), 796 and 976 cm-1 (CH2 rocking). A small peak at 840 cm-1 (CH2 rocking, CF2 stretching and skeletal C-C stretching) is also observed in the spectrum of pure PVDF which may be due to slight presence of β or β+γ crystals (Figure 2 e, g) as reported previously by different research groups. The absence of main characteristic absorbance band of γ crystals ~ 1234 cm-1 directly concludes that the small band at 840 cm-1 is due to the presence of β polymorph i.e. some alignment of the polymer chains in TTTT conformation in pure PVDF film. Whereas in the composite samples all characteristic peaks of nonpolar α-crystals are completely diminished. Only the absorbance bands at 445 cm-1 (CF2 rocking and CH2 rocking), 479 cm-1 (CF2 deformation) 510 cm-1 (CF2 stretching), 600 cm-1 (CF2 wag), 840 cm-1 and 1274 cm-1 are raised prominently. The appearance of 510 cm-1 and 840 cm-1 bands along with 445 cm-1 and 1274 cm-1 bands and the absence of the characteristic absorbance band of γ crystals at 1234 cm-1 confirm the nucleation of electroactive β crystals in the salts doped PVDF films.33 Thus, β crystals are mainly nucleated in our samples due to the catalytic effect of the Er3+ / Fe3+ in PVDF matrix. Closer observation of the FTIR spectra of the composite samples revels that the intensity of the characteristic bands of β crystals are seemed to be maximized for Er5 and I10. These results are well consistent with the XRD data. Further we have qualitatively quantified the fraction of electroactive β-phase content (F(β)) using Lambert-Beer law, F (β)

…………………………………………………………….. (1)

Where, Aα = the absorbance at 764 cm-1, Aβ = the absorbance at 840 cm-1, Kβ =7.7 x 104 cm2 mol-1 and Kα =6.1 x 104 cm2 mol-1 are the absorption coefficients at 840 cm-1 and 764 cm-1 respectively.34,35 This provides an idea about the amount of β polymorph present in composite samples as this phase leads to highest piezoelectric, ferroelectric and dielectric properties. The fraction of β-phase content increases with the loading concentration of each salt and maximizes for 5 mass% ErCl3. 6H2O (F(β) ~ 83.65%) and 10 mass% of Fe (NO3)3 · 9H2O (F(β) ~ 82.15%) loading (Figure 2f and h). Further doping of each salt reduces the β crystal formation by quenching the free chain movements of PVDF matrix.36 6


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Figure 2. (a, c) XRD spectra of pure PVDF and the ErCl3. 6H2O / PVDF (Er1, Er5, Er10, Er15, Er20), Fe (NO3)3 · 9H2O / PVDF (I1, I5, I10, I15, I20) thin films respectively, (b, d) the ratio of I20.5 and I18.3, (e, g) FTIR spectra of ErCl3. 6H2O / PVDF and Fe (NO3)3 · 9H2O / PVDF thin films respectively and (f, h) β phase content of the samples.

The phase crystallization and melting behaviour of pure PVDF and salt doped PVDF thin films are also analysed by differential scanning calorimetry (DSC), the complimentary technique of XRD and FTIR spectroscopy. Figure 3a and b presents the DSC thermographs of the thin films. The melting peak at 164.5 oC in the thermograph of pure PVDF film directly indicates the presence of nonpolar α polymorph in it,25 whereas, for the salt doped PVDF films the melting peak is shifted to higher temperature suggesting the nucleation of electroactive β polymorph in composite samples. The degree of crystallinity (Xc) of the films have been evaluated by the formula Xc= ∆Hm / ∆H100%, (where, ∆Hm = the enthalpy of fusion of the samples and ∆H100% = the enthalpy of fusion of 100% crystallite PVDF (104.6 J/g)).27,38 The enthalpy of fusion and crystallinity of the samples have been presented in Figure 3c and 3d. The enthalpy of fusion and crystallinity seem to increase up to 5 mass% (Xc ~54.68 %) and 10 mass% (Xc ~ 53.82 %) loading of ErCl3. 6H2O and Fe (NO3)3 · 9H2O respectively in PVDF matrix due to the catalytic effect of the salt molecules and formation of β crystals in composite samples. Further, loading of the salts reduce the crystallinity values which may be due to the restriction in free chain movements of PVDF matrix.26

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Figure 3. (a, b) DSC thermograph of pure and salts modified PVDF thin films, (c, d) Variation of enthalpy of fusion and (e, f) crystallinity with salt content and (g, h) TGA graph of pure and salts loaded PVDF thin films.

Thermal stability of the thin films has also been investigated by TGA thermographs illustrated in Figure 3e and 3f. The study is carried out placing the sample under N2 atmosphere in temperature range of 30 oC to 600 oC with a heating rate 10 oC/min. Thermograph of pure PVDF film shows only one degradation peak at 416 oC whereas the thermographs of salt doped PVDF thin films show a shifting of this degradation temperature to higher temperatures by 26 oC to 40 oC indicating better thermal stability of the salt assisted PVDF thin films than the pure one. The thermal stability increase may be attributed to strong interaction between the polymer chains closer to the salt interface and crystallization of more β crystals in the composite samples. 36 An extra mass loss has also been observed in the TGA thermographs around 250 oC and 150 oC for Erbium and Iron salt doped composite films. First weight loss is observed up to temperatures 300 oC and 220 oC for ErCl3. 6H2O) and Fe (NO3)3 · 9H2O doping in PVDF respectively due to thermal evolution of water molecule in the salts.23 Thus, the formation of β phase crystallization has been thoroughly investigated with the above mention techniques and the thermal stability has been observed to increase in composite samples. The possible interaction mechanism is very similar to the previous study done by Thakur et al. using two hydrated rare earth salts Ce(NO3)3. 6H2O or Y(NO3)3. 6H2O. Strong electrostatic interaction between the water molecules of salts and the polar –CF2 via formation of hydrogen bonds may be the possible driving factor for nucleation of polar β crystals in ErCl3. 6H2O) and Fe (NO3)3 · 9H2O assisted PVDF thin films. Finally, two best piezoelectric thin films Er5 and I10 having highest electroactive β polymorphs have been achieved with potential application probabilities in the field of sensors, actuators and piezoelectric energy harvesting system. Furthermore, the photoluminescence property and dielectric properties have also been studied for finding multi-functional application possibilities of our composite thin films.

3.1.2. Photoluminescence property The photoluminescence (PL) emission properties have been studied at room temperature in the range of 450-600 nm by Agilent Technologies Cary Eclipse fluorescence spectrophotometer. ErCl3. 6H2O and Fe (NO3)3 · 9H2O doped PVDFs show peaks in the emission spectra at λex = 335 nm and 260 nm respectively. The emission spectra are graphically represented in Figure 4. Two prominent emission peaks at 495 nm and 524 nm have been observed in ErCl3. 6H2O doped PVDF films due to 4G11/2 4 I15/2 and 2 H11/2 4 I15/2 electronic transition respectively.37 Whereas, the PL emission spectra of Fe (NO3)3 · 9H2O doped PVDF represent only one broader blue emission peak ~ 432 nm. The intensities of all emissions bands have been observed to increase with doping concentration of salts implying good and uniform dispersion of the salt molecules in PVDF matrix.

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Figure 4. Photoluminescence spectra of pure PVDF and (a) ErCl3. 6H2O/ PVDF, (b) Fe (NO3)3 · 9H2O/ PVDF thin films.

3.1.3. Dielectric properties The capacitance (C) and tangent loss (tan δ) data were collected by digital LCR meter (Agilent, E4980A) in the frequency range 20 Hz to 2 MHz applying 1 V ac voltage across the two opposite surfaces of the samples under ambient conditions. The dielectric constant (ε) and the ac conductivity (σac) of the samples have been evaluated using equations (2) and (3) respectively, ε = C.d / ε0A

(2)

σac= 2πfε0 ε tanδ

(3)

where, d, A and tan δ are the thickness, area and tangent loss of the samples respectively, f is the applied frequency in Hz and ε0 is free space permittivity (8.854 x 10-12 F.m-1)26. The dielectric constant increases with the increase of salt doping concentration for 5 mass % of ErCl3. 6H2O and 10 mass% of Fe (NO3)3 · 9H2O salt loading in PVDF matrix respectively and then reduces for higher doping (Figure 5). The dielectric constant also decreases with increasing frequency (Figure 5a and b). The increase in value of the dielectric constant with doping concentration and its reduction with increasing frequency may be explained by Maxwell–Wagner–Sillars (MWS) interfacial polarization. At low frequency, dipoles may move and follow the applied ac electric field but for higher frequencies the dipoles are unable to follow the ac electric field frequency and lag behind the electric field resulting in decrease of dielectric constant.

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Figure 5. (a, b) Frequency dependent dielectric constant of pure PVDF and salts modified PVDF films, (c, d) Salt content dependent dielectric constant of the samples at 20 Hz, (e, f) and (g, h) Frequency dependent ac conductivity and tangent loss of pure PVDF and ErCl3. 6H2O / Fe (NO3)3 · 9H2O doped PVDF films respectively.

This enrichment of dielectric constant up to 5 mass% of ErCl3. 6H2O and 10 mass % of Fe (NO3)3 · 9H2O loading in PVDF matrix may be due to the formation of maximum electroactive β crystals (Figure 2g and h) and maximization of the average localised polarization complemented with the salt molecules and coupling with the polymer matrix for these certain percentages and large accumulation of surface charge between salts surface and PVDF matrix via MWS interfacial polarization.26 Highest dielectric constant was measured to be ε~795 for 5% ErCl3. 6H2O (Er5) and ε~790.7 for 10% Fe (NO3)3 · 9H2O (I10) doping concentration (Figure 5c and 5d) at 20 Hz. Figure 5e and 5f presents the ac conductivity of pure PVDF and salt modified PVDF samples with increasing frequency. Ac conductivity increases with increasing frequency for both salts (ErCl3. 6H2O and Fe (NO3)3 · 9H2O) due to MWS effect between the interface of salt and the polymer chain.23 The maximum ac conductivity has been observed for Er15 and I10 samples at 20 Hz. Tangent loss also seems to reduce with frequency for pure PVDF and the salt doped PVDF thin films at lower frequencies due to MWS effect (Figure 5g and h).26 The excellent dielectric properties of the samples (Er5 and I10) may be a potential candidate for self-charging energy storage devices associated with nanogenerators and as separator for lithium ion battery in modern electronic field.27 3.1.2. Polarization (P) – electric field (E) loop Room temperature ferroelectric and piezoelectric P-E hysteresis loop for Er5 and I10 thin films are shown in figure 6a. For comparison both samples are polarised at 200 kV/cm range at 50 Hz and measured by Hysteresis Version: 4.9.0 - Radiant Technologies. PE hysteresis loop of Er5 and I10, clearly show the strength of polarization and is a reflection of the amount of β-phase. Whereas α-phase dominated pure PVDF film shows a weak P-E curve.38 The area within the loop is caused by the heterogeneous charge and gives the charge storage ability of the material. This definable area under the curves increases upon introduction of dielectric materials as reflected in the P-E curve shown in figure 6.38 Remanent polarization (Pr) values for Er5 and I10 samples are 1.88 and 1.73 µC/cm2 (Pr = 0.038 µC cm−2 at 100 Hz for pure PVDF). This high Pr value attributed the ferroelectric behaviour and hint of polarisation reversal of ferroelectric nature also accelerated the heterogeneous polarization of the films.38 The ratio between Pr and spontaneous polarization (Ps) is ~1 for both Er5 and I10 which are significantly high values at the same high coercive electric field Ec=114 kV/cm. This indicates that the material is inherently polar and piezoelectric in nature.39 The improved piezoelectric coefficient (d33) is the desired aptitude for electrochemical applications. The piezoelectric coefficient (d33) is defined as the change in polarization with applied uniaxial stress. At zero applied potential: d33 = - Pr/Y, where Y is the Young’s modulus.40 The Young’s modulus of Er5 and I10 were measured to be 151.8 N/mm2 and 241 N/mm2 respectively (Supplementary information figure S2). Using the above equation and Pr values, the d33 for Er5 and I10 are calculated to be -124 pC/N and -72.4 pC/N respectively. In contrast, undoped PVDF have d33 in the range of -24 to -34 pC/N.41 The piezoelectric 13



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coefficient d33=2Q33εrε0Pr, where Q33 is electrostriction coefficient and εr dielectric constant, from this equation we can calculate the value of Q33= 0.997 m4/C2 and 1.053 m4/C2 for Er5 and I10 samples respectively (Supplementary table 1).40 Again the longitudinal strain (S3) is equal to Q33 P2 where P3 the polarization.41 Figure 6b shows the strain –field plot which gives a butterfly curve due to the converse piezoelectric effect and polarization reversal.

Figure 6. (a) The measured polarization versus electric field for Er5 and I10 samples. (b) Strain-Field butterfly loop for Er5 and I10 samples. 3.2. Performance of the FPENGs: The detailed device fabrication procedure is described in the experimental section and schematic device configuration has been shown in Figure 7a. The films (Er5 and I10) with highest electroactive β crystals and dielectric value are chosen for device fabrication for achieving maximum piezoelectric response and spontaneous polarization. The excellent flexibility of the films as well as the fabricated devices has been depicted in Figure 7b-e. Figure 7f and 7g show the open circuit output voltage of EPENG (fabricated using Er5 as energy harvester) and IPENG (fabricated using I10 as energy harvester) respectively by continuous finger imparting directly detected using a digital storage oscilloscope (Keysight, Oscilloscope DSO-X 3012A). During periodic human finger imparting, the EPENG produces positive open circuit voltage (Voc) ~ 115 V and short circuit current (Isc) 32 µA which correspond to the power density ~160 mWcm-3 as illustrated in Figure 6f and 6g, whereas, the IPENG shows Voc ~ 75 V and Isc ~ 17 µA with power density ~55.34 mWcm-3 under same condition (7h and 7i). In releasing condition the EPENG and IPENG show negative open circuit voltage ~ 58 V and 17 V. The maximum power density of both our PENGs is ~5700 times (for EPENG) and ~2000 times (for IPENG) larger than the more recent reported PENG based on self-poled PVDF/AlO-rGO composite by Karan et al.45 The output characteristics of our developed devices are many times larger than the previously reported prototype devices fabricated using similar type cations till date.39 The novelty of the study revolves around the superior performances of our developed materials i.e. devices.

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Figure 7. (a) Schematic diagram of the device configuration, (b-e) Digital photograph of flexibility test of devices and Er5 and I10 films, Open circuit voltage (Voc) and short circuit current (Isc) under continuous finger imparting (f, g) EPENG and (h, i) IPENG and (j) Schematic working mechanism of the devices due to piezoelectric response under applied force. 15



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Figure 7j represents the possible working mechanism of electrical signal generation by the PENGs from mechanical energy under periodic finger imparting. The suggested working mechanism of our PENGs may be explained in terms of synergistic effect of the dipoles present in PVDF i.e. electroactive β crystals and the salt molecules (ErCl3. 6H2O and Fe (NO3)3 · 9H2O). The presence of salt molecules trigger the piezoelectric β phase formation by strong electrostatic interaction between the water molecules of salts and the negative –CF2 dipoles of PVDF via formation of hydrogen bonds. Moreover, when an external mechanical force is applied on the PENGs via periodic finger imparting a secondary potential is produced in the salt molecules which additionally arrange the PVDF dipoles in same direction of the applied mechanical force via stress associated polarization. Thus, due to dual effect of mechanical stress and surface charge assisted polarization, the PVDF has been self-polarized in an approved direction without any external bias field. It is quite difficult to understand the self-polarization technique for ferroelectric and piezoelectric materials due to their complex behaviour but one may effectively use this method by eliminating this complexity coming from the conventional electrical poling method for ferroelectric and piezoelectric materials based energy harvester systems. When a vertical mechanical compression is driven on the PENG (EPENG and IPENG) a positive piezoelectric potential at the top electrode and a negative potential at the bottom one is produced due to generation of self-polarization via the deformation of the crystalline structures of the salt doped PVDF thin films (Er5 and I10) (Figure 7j). This self-polarization i.e. potential difference between the two electrodes drives the flow of electrons from one electrode to another electrode through an external load. The piezoelectric potential sharply diminishes after immediate release of the compressive force and the electrons accumulated at the bottom electrode flow back to other electrode through the external circuit resulting in an opposite electrical output.43-45 This procedure is repeated in both PENGs under periodic compression and relaxation process and to obtain the periodic output electric signals from the PENGs. For realistic application of our PENGs, we investigate the energy creation capability of the developed PENGs to glow the commercially available light emitting diodes (LEDs). The EPENG and IPENG are capable to light up 54 and 42 blue LEDs connected in parallel (without connecting capacitor) and 37 and 15 same LEDs connected in series as shown in Figure 8(a, b) and (c, d) respectively under gentle finger imparting (supporting video S1, S2, S3 and S4). A full wave bridge rectifier has been used to rectify the PENG generated output voltage for driving the LEDs.

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Figure 8. Snapshots of the commercial blue LEDs driven using EPENG and IPENG connected in series (a, b) and parallel (c, d), (e) Capacitor charging circuit and time voltage charging graph using (f) EPENG and (g) IPENG.

The capability of our fabricated PENGs for charging a storage system has also been confirmed by charging a capacitor (1 µF) connected through full wave four probe bridge rectifier circuit (Figure 7e) under periodic finger imparting and releasing. The typical exponential charging of the capacitor implies the high energy storage capability, as illustrated 17



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in Figure 8f and g. The capacitor has been charged up to ~ 3.8 V in 18 seconds and 2.6 V in 22 seconds under cyclic finger imparting and releasing circumstances using EPENG and IPENG respectively which is significantly higher than the previous prototype PENG investigated in same manner.45,46

CONCLUSIONS: To summarise, we first investigate the effects of ErCl3. 6H2O and Fe (NO3)3 · 9H2O salt molecules on the nucleation of electroactive β-phase in salt modified samples. The maximum β -phase crystallization achieved are ~83.65% and ~82.15 % by adding 5% ErCl3. 6H2O and 10% Fe (NO3)3 · 9H2O in PVDF respectively due to effective electrostatic or ion-dipole interaction via hydrogen bond formation between the water molecules of the salts and the negatively charged –CF2 dipoles of the polymer matrix. Thermal stability, visible light emitting properties has also been improved. The ErCl3. 6H2O doped composites show bluishgreen and green light emissions whereas Fe (NO3)3 · 9H2O doped PVDF thin films lead to light emission in blue regions. The dielectric properties (ε~795 for Er5 and I10) are also improved in composite samples due to large interfacial polarization and nucleation of electroactive β crystals in them. Thus, the ability of rare earth cation (Er3+) to nucleate β polymorph and the dielectric properties is more effectual than the transition metal cation (Fe3+) because of their large size and talent to form covalent bonds. Then, two simple and highly efficient flexible PENGs consisting Er5 in one and I10 in other, have been fabricated. The EPENG nanogenerator exhibits a remarkable improvement in open circuit output voltage (Voc) to very large value ~ 115 V and short circuit current (Isc) ~ 32 µA associated with the power density ~ 160 mWcm-3 under periodic finger imparting. The IPENG also leads to very high Voc ~ 75 V and Isc ~ 17 µA, corresponding to power density ~ 55.34 mWcm-3 in same finger imparting. These output characteristics of our PENGs are better than any previous PVDF based prototype PENGs reported till date. Our PENGs are highly efficient to charge a capacitor in very short span of time (~ 3.8 V in 18 seconds for EPENG and 2.6 V in 22 seconds for IPENG). The fabricated PENGs are capable to glow instantly many number of LEDs. Thus, the fabricated flexible Er3+/ Fe3+-PVDF based PENGs are very promising candidates for next generation portable electronics devices and they may lead to a new manifesto for energy harvesting materials which are able to covert small mechanical energy from human activities into electrical energy.

SUPPORTING INFORMATION: Preparation process of PVDF composite films. Stress-Strain mechanical characteristics, tabulated form of the calculation of piezoelectric coefficients and electrostriction coefficient and video demonstration of glowing of LEDs.

ACKNOWLEDGEMENTS We are thankful to University Grants Commission (UGC) (BSR fellowship to N. A. Hoque) and Department of Science and Technology, Government of India for the financial assistance. 18


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Graphical abstract

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Fe3+ Stimulated Electroactive, Visible Light ... - ACS Publications

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