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All Organic High Performance Piezoelectric Nanogenerator with Multilayer Assembled Electrospun Nanofibers Mats for Self-powered Multifunctional Sensor Kuntal Maity, and Dipankar Mandal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01862 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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ACS Applied Materials & Interfaces

All Organic High Performance Piezoelectric Nanogenerator with Multilayer Assembled Electrospun Nanofibers Mats for Self-powered Multifunctional Sensor Kuntal Maitya and Dipankar Mandala,b * a

Organic Nano-Piezoelectric Device Laboratory (ONPDL), Department of Physics, Jadavpur

University, Kolkata 700032, India b

Institute of Nano Science & Technology (INST), Habitat Centre, Phase X, Sector - 64, Mohali

Punjab-160062, India * Corresponding author. E-mail addresses: [email protected], [email protected]

Keywords: Organic piezoelectric nanogenerator, Multifunctional sensor, Electrospun nanofibers mat, PVDF, Vapor phase polymerization, PEDOT, Weight-measurement mapping.

ABSTRACT Rapid development of wearable electronics, piezoelectric nanogenerator (PNG) has been paid a special attention due to its sustainable and accessible energy generation. In this context, we present a simple yet highly efficient design strategy to enhance the output performance of an all organic PNG (OPNG) based on multilayer assembled electrospun poly(vinyledine fluoride) (PVDF)

nanofibers

(NFs)

mats

where

vapor

phase

polymerized

poly(3,4-

ethylenedioxythiphene) (PEDOT) coated PVDF NFs are assembled as electrodes and neat PVDF NFs is utilized as an active component.

In addition to the multilayer assembly, electrode

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compatibility and durability remains a challenging task to mitigate the primary requirements of wearable electronics. Multilayer networked 3D structure integrated with compatible electrode thereby provides enhanced output voltage and current (e.g. open circuit voltage, Voc ~ 48 V and short circuit current, Isc ~ 6 µA upon 8.3 kPa of applied stress amplitude) with superior piezoelectric energy conversion efficiency of 66% compared to the single-mat device. Besides, OPNG also shows ultra-sensitivity towards human movements such as foot strikes and walking. The weight-measurement mapping is critically explored by Principal Component Analysis (PCA) that may have enormous application in medical diagnosis to smart packaging industries. More importantly, fatigue test under continuous mechanical impact (over 6 months) shows great promise as a robust wearable mechanical energy harvester.

INTRODUCTION Next generation wearable electronics have placed a great demand in accessible and environmentally sustainable energy generation from ambient environment and regular human activities.1─3 Particularly this is an effective alternative approach to build “battery-free” selfpowered devices, such as NGs are shown their suitability in low power portable electronics for example smart watch, small LCD, digital thermometer, calculator, smart phone, and wireless body sensors to name a few.4,5 The monitoring and assessment of human health problems are currently limited to medical clinic and hospitals, as available most of the testing devices are not portable or wearable due to heavily dependency on traditional bulky power sources.6 In particular, there has been enormous research interest in harvesting alternative energy to reduce the fossil fuel based economy.7,8


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Recently PNGs offer an option to scavenge “waste energy” which is environmentally abundant such as mechanical energy from wind flow, water waves, acoustics sources, human movement, transportations, machine vibrations, etc. to realize a cost-effective, sustainable and portable energy source.9─11 In addition, autonomic body movement (e.g., heart beats and respiration) and human activities (e.g., walking, running, jogging, talking, cooking, etc.) also provides to design the self-powered biomedical devices and wearable electronics. The PNGs are mainly governed by inorganic materials.12─14 However, their wider practical implementation are limited because of the brittleness of the materials, complicated and cost-intensive processing, challenging scalable large area coverage and toxic constituents.13,14 In contrast, organic piezoelectric materials are promising to develop PNGs due to their machinability, mechanical flexibility, lightweight, easy processing, nontoxicity, compatibilities with biological tissues and most importantly availability of cost effective raw materials. For instance, flexible and biocompatible PVDF is the mostly studied piezoelectric material due to its spontaneous polarization, high piezoelectric coefficient, acoustic impedance matching in water and air, chemical resistiveness and high thermal stability.15,16 The main bottleneck of PVDF is that it comprising of stable non-electroactive crystalline α- phase (TGTG conformation). In contrast, β-, γ- and δ- crystalline phases are considered as electroactive in nature, among them β- phase possess largest spontaneous polarization that give rise to superior ferro-, piezo- and pyroelectric properties which is perfectly suitable for NG fabrication. Several strategies (such as mechanical stretching, electrical poling, casting from solutions, incorporation of fillers and spin coating) have been paid to induce electroactive β- phase.17─20 Nevertheless, most of these processes are cost-intensive, unscalable and obtained as films where flexibility is often compromised. To overcome these issues, electrospinning is an attractive, scalable and facile technique to induce β-phase in flexible and air



permeable PVDF nanofibers (NFs) where the in situ alignment of molecular −CH2/−CF2 dipoles occurs simultaneously and most importantly post electrical poling step is not required.21,22 Noteworthy to mention that in the films the dipoles are usually align in particular direction by employing electric field in order to get the effective piezoelectric response, called electrical poling. Thus electrospun PVDF NFs are highly desirable to design wearable sensors, actuators and energy generators due to their light weight, air-permeability, wear ability and integrate ability with textiles.23 However the choice of electrodes and their compatibility with organic and hydrophobic surface of PVDF remained challenging tasks, particularly when device stability, implant ability and final packaging are considered. In earlier attempt, mostly PNGs are designed either by physically placing electrospun PVDF mats across a pair of top and bottom planar electrodes or electrospun on two parallel electrodes.21,22 Nevertheless, both cases, the resulting devices are not compact due to physical sandwich structures, air gapes often exist in between surface of NFs mats and electrodes, on the other hand in the case of parallel electrodes the device configuration is not favorable since dipole alignment occurred in thickness direction. For example, Chang et al. first reported PVDF NFs based PNG using two parallel metallic electrodes.21 Fang et al. prepared simply by sandwiching the as-spun PVDF nonwoven membrane between two aluminum foils.24 Subsequently, Dhakras et al. physically attached copper electrodes upon the two surfaces of PVDF electrospun fiber mat and Lee et al. fabricated by the deposition of electrospun PVDF NFs across the two aluminum electrodes on a paper substrate.25,26 To take a step further, Zeng et al. reported PNG fabrication by simply sandwiching an as-spun PVDF NFs nonwoven fabric between two fabric electrodes.27 Recently our group has also reported the PNG fabrication by using Ni-Cu plated polyester fabric electrodes.23,28 However it has been widely accepted that electrodes durability and compatibility are crucial


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issues in PNG fabrication due to limited utility life under repeated mechanical deformation. So, it would be a great challenge to ensure good connectivity of PVDF NFs surface and the electrodes particularly by keeping their flexibility, integratability and stability intact. In addition, the poor adhesion and brittleness of metal electrodes on the hydrophobic PVDF surface have limited their application in some cases causing mechanical deformation of devices. To overcome this problem, metal free organic conductive polymers would be an alternative option.29 Recently, PEDOT is one of the most promising conducting polymer having current technological and commercial potential due to its facile synthesis, high conductivity, adequate flexibility and biocompatibility.30 It has attracted considerable interest to use as flexible electrode material in order to overcome the drawbacks of the existing devices.30─32 However due to hydrophobic nature of PVDF surface, surface treatment is necessary to deposit PEDOT layer.30 In addition, effective electrode deposition is not feasible in the case of surface of PVDF NFs due to air permeable and spongy type of nature.27 So the main challenge greatly lies in the coating of organic conducting polymer PEDOT upon PVDF NFs which can be synergistically acted as a piezoelectric layer as well as metal free electrode. In this work, we introduce a simple, innovative and one-step approach to fabricate an OPNG using continuous electrospinning strategy and facile vapor phase polymerization (VPP) technique. Among several polymerization methods, such as wet chemical oxidation, electrochemical polymerization, VPP is considered as a simple, attractive and facile technique for the preparation of PEDOT, in particularly electrode coating preparation.33 The other techniques suffer from drawbacks such as, poor conductivity, limited pot life, poor adherence on the substrates and solvent dependency.34,35 Noteworthy that VPP is shown to be an effective technique that circumvents the above mentioned shortcomings of PEDOT conversion from EDOT monomer.30,33,36 In VPP process a substrate (importantly



there is no restriction on the type of substrate) is coated with an oxidant followed by exposing the oxidant-coated substrate to monomer vapor inside an enclosed chamber and subsequently polymerization takes place at the surface of substrate. Thus conducting PEDOT is formed by a chemical reaction between an oxidant and EDOT obtained from vapor phase. The technique was first described by Mohammadi et al. in a chemical vapor deposition (CVD) process, using FeCl3 or H2O2 as the oxidants.37 Later, Kim et al. synthesized PEDOT thin films by pretreating substrates with ferric chloride solutions followed by exposing the dried substrates to EDOT vapor.36 Subsequently Winther-Jensen et al. also showed that the base-inhibited VPP that form a highly conductive PEDOT films.33 Thus the combination of continuous electrospinning of PVDF NFs and VPP leads to PEDOT coated PVDF NFs mat electrodes with neat PVDF NFs mat in between as a resultant multilayer 3D structure (i.e., PEDOT coated PVDF/neat PVDF/PEDOT coated PVDF) made stand along OPNG. Nevertheless, to date, there is limited progress on the study of piezoelectric performance of metal free organic electrode based electrospun PVDF NFs, particularly for designing of the highly durable wearable PNGs. To the best of our knowledge, no relevant study has been reported so far where multilayer assembled PEDOT coated PVDF NFs are synergistically utilized as a piezoelectric energy harvesting material and electrodes. The OPNG exhibits an effective conversion of mechanical energy of human finger movements (8.3 kPa of stress amplitude) into electrical energy (open circuit voltage, Voc ~ 48 V and short circuit current, Isc ~ 6 µA) with piezoelectric energy conversion efficiency of 66%. As a direct proof of evidence, it is also capable to drive several light emitting diodes (LEDs) instantly; Subsequently OPNG shows excellent ultra-sensitivity towards human movements such as walking, foot strikes and house hold machine vibrations, such as sewing, kitchen blender, mobile calling vibration, and even toys. The distinct output responses of walking of different people


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with different weight are demonstrated and corresponding weight measurement mapping is critically analyzed by PCA, which implies its significance as a weight measurement sensor in industrial scale application. Finally, fatigue test of OPNG under continuous mechanical impact (over 21000 cycles) for 6 months shows highly durable piezoelectric output exploring the potential applicability in the field of wearable as well as personal electronic devices. RESULTS AND DISCUSSION Fabrication Strategy of OPNG. A schematic illustration is presented in Figure 1 (a~g) which describes the fabrication strategy procedure of OPNG architecture. It begins with layer by layer stacking of electrospun PVDF NFs mats consecutively from two different solutions, viz., oxidant containing PVDF-DMAc solution I and neat PVDF-DMAc solution II as illustrated in Figure 1a. At first, the oxidant (FeCl3) containing yellowish PVDF NFs mat (i.e., Fe-PVDF: layer 1) was prepared from solution I upon the grounded plate collector at an optimized distance (the detail description of electrospining procedure and associated parameters are mentioned in the experimental section). After this successful layer formation, a white layer of electrospun neat PVDF NFs mat (i.e., neat PVDF: layer 2) was collected on the top surface of first layer feeding from solution II. Subsequently, layer 3 was electrospun by repeating the first step. Thus the three-layer structure of NFs mats (PVDF-Fe/neat PVDF/PVDF-Fe) were obtained by the continuous electrospinning method (Figure 1b). Then the resulting NFs mat stacking was dried at 60 oC for 12 h to remove the residual solvents. In the next step, multilayer

3D structure NFs mat (PVDF-Fe/neat

PVDF/PVDF-Fe) was placed into a VPP chamber and exposed to EDOT monomer vapor (chemical structure of EDOT is depicted in the enlarge view of upper inset of Figure 3c) at 60 oC



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for 30 minutes (Figure 1c). Here FeCl3 is acting as an oxidant for the polymerization of EDOT monomer. When EDOT vapour is passed through oxidant containing NFs mat, an addition reaction takes place via free radical mechanism pathway. Fe3+ of FeCl3 converts itself into Fe2+, taking a single electron from thiophene unit of EDOT and making EDOT a radical entity. Subsequently, the polymerisation takes place by combining the free radical units. The reaction mechanism behind the VPP is shown by a schematic (Supporting Information, Figure S1). Following the polymerization, the NFs mat is washed with methanol several times to remove unreacted oxidant, EDOT monomer and other byproducts (e.g. FeCl2), and subsequently dried in air at 60 oC for 5 min. After completion of polymerization process (chemical structure of PEDOT is shown in the enlarge view of side inset of Figure 3c) the stacked NFs mat was finally became deep bluish color that indicating that the middle neat PVDF NFs mat layer is well covered with PEDOT coated PVDF NFs mats (i.e., PEDOT coated PVDF/PVDF/PEDOT coated PVDF) (thickness ~ 260 ± 20 µm). Then, conducting wires were attached on both electrodes by means of pasting conducting carbon tapes (Figure 1d) followed by lamination with polypropylene (PP) film (thickness ~ 125 µm) (Figure 1e). Finally, ‘PEDOT coated PVDF/PVDF/PEDOT

coated

PVDF’

whole

structure

was

encapsulated

by

poly(dimethylsiloxane) (PDMS) (thickness ~ 42 ± 5 µm) to protect from any external damage (Figue 1f) and the final structure of OPNG is shown in Figure 1g. Surface morphology. The surface morphological features of the NFs mat was analyzed by field-emission scanning electron microscopy (FE-SEM) images, before and after coating of PEDOT. Figue 2 (a,b) show the respective digital photographs of the both NFs mats. The colour change of NFs mat from yellowish to deep bluish clearly suggests the successful completion of PEDOT formation from


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EDOT monomer by VPP technique.38 The corresponding FE-SEM images (Figure 2 c,d) are also in good agreement with these photographs. Figure 2c shows fibers are randomly oriented and no bead defects are observed with average diameters ranging from 90–110 nm (histogram profile of Figure 2c upper inset). After VPP, PEDOT networks are notably coated and tied onto the surface of nanofibers whereas some portions are viewed inserted into the interspace of PVDF nanofibers (Figure 2d). The successful PEDOT coating of outer two layers built up conductive network those serves as electrodes. In the middle, neat PVDF layer composes of fine nanofibers with diameters ranging from 100 to 120 nm (histogram profile of Figure 2e upper inset) remains unchanged (not coated by PEDOT) despite of VPP (Figure 2e). Thus the as-prepared OPNG has three-layer structures, viz., ‘PEDOT coated PVDF/neat PVDF/PEDOT coated PVDF’ as illustrated by the cross-sectional FE-SEM image (Figure 2f). Noteworthy to mention that PVDF NFs mat acts as an active layer that reconcile between two PEDOT coated PVDF NFs mat. The thickness of the resulting three layers structure is about 260 ± 20 µm as estimated from FE-SEM image. The cross-section of three layers demonstrates a uniform structure that indicates a well connection among them. Thus the novelty of this strategy lies in the construct of such multilayer structure using successive continuous electrospinning method followed by VPP allowing assemble of different organic NFs mats altogether.27 It gives rise a compact device structure due to all organic configurations in comparison to physical electrode stacking that often adopted in NFs made NG fabrication (a comprehensive list of electrode materials and methodology is provided in Table 1). Characterization and properties of piezoelectric PVDF NFs mat. Basically OPNG is based on multilayer structure of piezoelectric electrospun PVDF NFs mat followed by PEDOT coating for top and bottom sides, keeping PVDF as an active piezoelectric



layer in the middle. However, the orientation of the PVDF NFs mat is affected neither by the randomized distribution nor by the fiber diameter.39 Rather, randomly oriented -CH2/CF2molecular dipoles of PVDF NFs mat preferentially oriented during electrospinning process subjected to a mechanical stretching/poling due to polymer jet elongation and whipping (Figure 3a).22,39 In addition, the PEDOT coated resulting three layer-structure of PVDF NFs mats can enhance the resulting polarization due to cooperative effect of electromechanical interaction among the adjacent fibers.40,41 Thus the 3D structure is expected to show the high piezoelectric performance from OPNG and hence this is another particular merit of this study. To identify the crystalline phases present in the electrospun NFs mat before and after PEDOT coating, the Fourier transform infrared (FT-IR) spectrum is recorded. It exhibit strong vibration bands at 840 and 1277 cm─1 in both NFs mats which are typical characteristic of the piezoelectric β- crystalline phase of PVDF (Figure 3b) particularly when 1235 cm-1 band (a characteristic band of semi-polar γ-phase) is absent.23,28 Besides this, the vibration bands observed at 1484 cm─1 in VPP NFs mat is due to stretching modes of C=C originating from the thiophene ring, while the bands at 1038 cm─1 correspond to C–O–C bond stretching in the ethylene oxide group and 935 cm─1 band indicative of the C–S bond in the thiophene ring due to successful PEDOT coating of PVDF NFs by VPP.42,43 It should be noted that these vibrational bands are absent in the case of uncoated PVDF NFs mat since the measurement was recorded before the coating process. The corresponding colour change of NFs mat via VPP is also mentioned in surface morphological discussion section. We also check the FT-IR spectra of the middle layer that comprised of neat PVDF NFs mat only. It (Supporting Information, Figure S2) suggests that the middle layer also consisting of typical β crystalline phase (greater than 90%) even before and after the PEDOT coating as evidenced from typical 1277 cm─1 band.19,23


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Noteworthy to mention that PEDOT characteristic vibrational bands are not attributed in this layer as expected it is simple because of the fact that neat PVDF NFs are not affected by VPP due to absence of oxidant component. To further confirmation the chemical structure of the resulted PEDOT coated NFs mat, Raman spectra was analyzed as shown in Figure 3c. The vibrational bands observed at 434 cm−1 is due to the C-O-C deformation, while the bands at 565 and 988 cm−1 are assigned to the oxyethylene ring deformation and a band at 694 cm−1 indicative of symmetric C-S-C deformation of PEDOT.44─46 Besides, the bands at 1256 and 1362 cm−1 are due to Cα-Cα inter-ring stretching and Cβ-Cβ stretching respectively.44,46 Importantly, the characteristic band at 1437 cm−1 due to symmetric Cα=Cβ (−O) stretching is indicative of a high level of conjugation in the structure of PEDOT which is the responsible of electrical conductivity that assisted as an organic electrode.45,46 In accordance with PEDOT coating, the mechanical properties of PVDF NFs mat before and after coating since it is one of the important parameter to determine the device performance as well as stability and longevity. Figure 3d presents the corresponding stress-strain curve which shows nearly linear elasticity for before coated PVDF NFs mats, while the PEDOT coated PVDF NF mats exhibits a fast raise of stress within the strain of 5% then a moderate increase of stress. It may be ascribed to stress concentration caused by PEDOT networks coating upon PVDF NFs. When the strain is over 5%, the rigid PEDOT networks are gradually interrupted. The higher tensile stress of PEDOT coated PVDF NFs than that of before coated PVDF NFs is due to the uniform coating of PEDOT by VPP tied on the surface of PVDF NFs and remnant flexible PEDOT networks. The evaluated Young’s modulus (Y) for PEDOT coated PVDF NFs is 67 MPa from stress-strain curve (Figure 3d). Therefore, the good mechanical property of PEDOT



coated PVDF NFs discloses its potential application in OPNG. Furthermore, the coating of this conducting polymer, viz., PEDOT, expected to add significant conductivity among NFs mats those utilized as electrodes. The typical current-voltage (I−V) characteristics of OPNG are demonstrated in Figure 3e with enhanced current as a function of applied pressure indicating good conducting behavior of device. The flexible NFs mat coated with conductive PEDOT layer enables a 3D interconnecting conductive network with more contact points that can capture differing strains applied by pressure. The increase in the contact area would decrease the total resistance, and subsequently more PEDOT conductive paths can be established when the external pressure is applied, resulting to improvement in the current.47─49 Thus, these characteristics make NFs mat promising as building blocks for ultrasensitive organic piezoelectric sensors, actuators and energy generators. Performance of OPNG. The environment is full of large amount of irregular vibrations, i.e., mechanical energies, so it is necessary to evaluate the energy harvesting performances of OPNG under similar conditions. First, the output voltages (Figure 4 a,b) and current were generated by pressing and releasing under different normal stress (e.g., 1.2 kPa and 8.3 kPa of peak amplitudes) through repeatedly bare human finger covered in a polyethylene (PE) glove. The increase in imparting pressure showed remarkable enhancement of output voltage. As a promising outcome, OPNG exhibited output voltage (Voc) of ~ 48 V under 8.3 kPa of applied stress. It indicates that OPNG easily converts the human finger movements into electricity under repeating press-release motions (inset of Figure 4a). In order to check whether the output voltage signal is arising from the piezoelectric properties of the OPNG, widely accepted switching polarity test was performed by reversing the electrode connections (Figure 4b).50,51 In this case, the identical amplitude of output


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voltage with reversed polarization cancels out any additional contribution of artefacts or friction and confirms the genuineness of piezoelectric output signals coming from NFs mat. In other words, this is one way proof of dipole alignment as also evident from the FT-IR spectra in our earlier study.22 Furthermore, under forward and reverse connection, the magnitude of peak output voltage between press and release states (i.e., positive and negative peaks) are observed to be asymmetric due to the difference in straining rate when applying and releasing the stress upon the OPNG. The large positive peak is produced from the direct application of the stress. Since the fabricated PVDF NFs mat is polarized in one direction (in situ alignment of molecular −CH2/−CF2 dipoles occurs during electrospinning), the positive voltage peak is always greater than the negative one. The following negative voltage peak corresponds to the damping effect occurring when the device changes from a pressed state to a release state after lifting the initial stress. Thus the damping behavior during stress release state also proves the inherent piezoelectric properties of the OPNG.51 Additionally, to confirm that the output voltage response is arising from NFs mat, only PDMS made control device (without NFs mat) was also fabricated under identical condition where no reliable output voltage is obtained (Supporting Information, Figure S3). These observations rule out any electrostatic contribution to the output electrical signals from OPNG. On the other hand, the output response indicates that output voltage is sensitive to magnitude of applied pressure. So this piezoelectric NFs mat based device can work under wide range of frequency. The average sensitivity of the device is 6.5 VkPa-1 in the pressure range of 1─8.3 kPa. It is significant to note that OPNG shows remarkably high output performance due to the multilayer structure of PVDF NFs mat with PEDOT coated electrode. To the best of our knowledge, the output performance of OPNG is much higher than that achieved with the previous reported devices made with PVDF NFs as shown in Table 1.



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Interestingly, the measurement showed a well–behaved periodic alternation of positive and negative peaks of short-circuit current (Isc) ~ 6 µA during repeating human finger impact of 8.3 kPa of stress amplitude (Figure 4c). The continuous pressing and releasing altered the piezoelectric charge polarization of the multilayer structure NFs mat that drives the electrons in the external circuit to flow back and forth, resulting in an alternating Voc~ 48 V and Isc~ 6 µA with very fast response time (10 ms) (Supporting Information, Figure S4). Importantly it is seen that difference between the positive and the negative voltage peaks of OPNG is much greater than the difference between the current peaks. This is mainly attributed to the fact that the piezoelectric PVDF NFs mat based device is also a capacitor. Thus when a piezoelectric voltage is generated from OPNG under the application of external stress, induced charges are accumulated on the surface of the electrodes. This is due to the fact that resilience does not occur with the same rate to return to its undistorted state under stress and release motion. Then these stored surface charges will be consumed if an opposite voltage is applied resulting negative voltage always much smaller than the positive one. It should be mentioned that the induced current only depends on the piezoelectric potential strength, thus the piezoelectric potential of OPNG produced by the stress impart and release state from the damping effect have no significant impact. As a result, difference of the positive and negative current peak is not so obvious.59 The magnitude of generated charge (Q) under the applied force, F ~ 5 N, is calculated by the integration of a current pulse, i.e., using the following equation: =

(1)

It evaluates Q ~ 164 pC (Supporting Information, Figure S5). Thus, the estimated magnitude of piezoelectric co-efficient is given by,


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=

(2)

which leads to the value of 33 pC/N.60 This magnitude is in close agreement with determining the fiber dimension-dependent d33 of electrospun PVDF NFs taking into account the following equation:

log

= 1.96 + 0.19 log

(3)

Where, d is the fiber diameter.61 We achieved the empirically determined d33 value of 37 pm/V. So the axial strain, ε developed in the OPNG by finger imparting is 1.23 × 10-4, calculated from following the equation, =

where applied axial stress is σa and Y is Young’s Modulus (Y~

67 MPa). The corresponding strain rate

ε

is achieved by 0.246 % S-1 assuming OPNG

undertook compressive strain for a periodic mechanical impact, using the relation = 4 × 100 % " # , where average frequency of the axial stress is ~ 5 Hz.62 To provide more insights on the working principle, a theoretical analysis was conducted using the Finite Element Method (FEM) in order to predict a quantitative explanation on the piezovoltage distribution (colour z-axial scale bar) interaction behavior between structural and electrical domains of NFs mat under uniaxial stress σa ~ 8.3 kPa. It is basically presented by six nanofibers constructing a network structure (Figure 4d). The experimentally measured maximum piezovoltage (~48V) was relatively lower than the simulated result of 60 V that might be treated as good agreement between both. The slight deviation arises probably due to the voltage drop resulting from internal leakage paths and charge loss in the electrode insulator junction.63 The ultra-high sensitivity of the OPNG is also demonstrated from the relative changes in output voltage synchronize with the fast and slow bending of OPNG by human fingers (Figure 5a). Output voltage responses of the OPNG are seen to be influenced upon subjecting it to bending



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and releasing repeatedly. The tensile strain developed in the thickness direction (εy, parallel to the dipole orientation) during bending of the device into arc shape can be written as εy= t /2r = 0.875%, where, r = 20 mm is the bending radius and t is the thickness (~ 350 µm). The strain developed in the thickness direction is εx ~ 0.385%, using the relation of Poisson’s ratio, γ = |εx|/|εy| = 0.44 of PVDF. Therefore, at constant strain, OPNG results in remarkably higher output voltage ~ 3 V at faster bending rates (Figure 5a). Under bending, development of piezoelectric charge polarization is less compared to axial stress due to less number of charge formations on the electrodes in bending zone. Conclusively, OPNG shows capability to harvest human physiological signals during bending and releasing of human organ. For instances, it is necessary to explore the suitability of OPNG in real life application. The feasibility was demonstrated by attaching the OPNG upon a drum of a toy and the corresponding output voltage response was recorded during the repeated drum beats (Figure 5b). Enlarged response of the marked area (Figure 5b inset bottom) shows significant output voltage generation ~ 1 V under the gentle imparting of drum beats of toy opening the possibility to use in practical applications to make self-powered similar derives. The instantaneous voltage drop (VL) of OPNG was studied by connecting the output wires with the variable external resistances (RL).23,28 It has been observed that VL across the resistances gradually increases with the change of RL accordingly and saturated at infinitely high resistance (~ 40 MΩ) like open circuit voltage (Figure 5c). The output power is also examined to evaluate the performance of converting mechanical energy to electric energy. The instantaneous output power (P) was calculated as

$ =

%&' (&


(4)

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where, VL is the voltage drop across load resistance RL. The variation of P with RL is shown in Figure 5c and a high-output power density of 51 µW at the RL of 30 MΩ has been achieved which is sufficient to turn on several LEDs without using any external storage system (inset of Figure 5c). Furthermore, superior instantaneous piezoelectric energy conversion efficiency (ηpiezo) of the OPNG up to 66% is also attained (see Supporting Information, Note 1 and Figure S6 for details). Overall, OPNG is shown to be an energy harvesting power source that has potential capability across variable resistors and the result fits well with linear circuit theory (see Supporting Information, Note 2 and Figure S7 for details) which evaluates the internal resistance (Ri) of the OPNG to be 2.86 MΩ. The output power is found to be maximum approximately at Ri≈RL due to impedance matching between internal and external systems. Weight measurement mapping. The static tactile sensing performance of the OPNG was explored by monitoring human activities such as walking and foot strikes. The demonstration is shown by a schematic (Figure 6a) that exhibits the walking of a man upon OPNG by fixing on the floor and respective output voltage responses were recorded. Evidently, it signifies the human walking motions (stamping and releasing) leading to remarkable output signal (i.e., voltage) (Figure 6b). In addition, the foot strikes were also detected by the OPNG (Figure 6c) and generated voltage is quite higher than the walking steps. This could be due to the availability of more effective area deformation under foot strikes. Thus, the distinct patterns of output signals could easily differentiate the various human movements e.g., walking, foot strikes etc. The promising output responses motivate us to further use OPNG for the mapping of weight measurement. For this purpose of study, we choose 6 people with different weight (47.8, 56, 60.6, 62.3, 71.4 and 80 Kg) shown in the Figure 6d (upper inset). where W stands for the corresponding weight in Kg. For accurate optimization of



proposed study, each people simply walked upon the OPNG for at least 10 times and we recorded the corresponding output voltage response. After completion of demonstartion, the obtained data are analyzed by PCA since it has been widely used in modeling the statistics of a set of multi- dimensional data. Briefly, the linear feature extraction method reduces dimensionality of data with a minimum loss of information.64,65 In the vector space, PCA identifies the major directions, and the corresponding strengths, of variation in the data. PCA achieves this by computing the eigenvectors and eigenvalues of the covariance matrix of the dataset. The 2D PCA plot (Figure 6d) clearly point out the classification of distinct weight of different person with respect to the recorded measurement scores. Each region is distinguished from each other. The region of W 80 Kg is very far from W 47.8 Kg, similar like W 56 Kg and W 71.4 Kg. It should be noted that nearer weight like W 60.6 and W 62.3 Kg exhibits clear distinguished region. So OPNG easily mapped the weight measurement profile of different people (for example, here 47.8, 56, 60.6, 62.3, 71.4 and 80 Kg). Thus, the results pave the way of intelligent designing of the weight measurement sensor where this ultrasensitive OPNG may be an alternative approach. Vibration sensor. Our environment is full of abundant mechanical energy (energy from wind, acoustics, human movement etc.), among them vibrational energy is the most urgent valuable energy due to its greater accessibility anywhere and anytime. As demonstrator, our approaches built OPNG capable of measuring vibration to harvest energy. First, the NFs mat based OPNG acts as a vibration sensor is clarified by keeping OPNG upon a mobile as shown in Figure 7a (upper


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inset). It is interesting to note that the response includes alternating positive and negative voltage peaks (~ 0.6 V), during mobile calling vibration (Figure 7a). However no response is observed when vibration is in off condition. So it has been confirmed that OPNG is easily enable to detect mobile vibration and generate electrical response. In a second example, OPNG captures machine vibrations, where it was subjected to the periodic impact from the cylindrical probe of a sewing machine (Figure 7b upper inset) and generates 12 V (Figure 7b). So this outstanding output response paves the way of scavenging energy not only by the regular mechanical energy but also vibrational energy. At last, we demonstrated by mounting OPNG upon a kitchen blender machine as shown in Figure 7c. OPNG shows promising output response ~ 10 V during blender vibration, but no response observed in vibration-off condition. This excellent response opens the idea to use our fabricated OPNG in similar house hold devices particularly in real life applications purposes. Fatigue testing. Finally, cyclic fatigue testing was carried out by continuous periodical impacting of the device at 3 Hz of frequency over 21000 cycles (~ 7000 s) for several months by a stepping motor. The idea is to check the stability and durability of OPNG under such robust environment for long term period. Data were recorded after 1 months (Figure 8a), 3 months (Figure 8b) and 6 months (Figure 8c). The output response exhibited negligible change in over the entire period of continuous testing. The fatigue tests for several months (for 6 months) show stable amplitude of output voltage without any notable degradation. It also strongly affirms that an environmental condition (such as temperature, moisture, etc.) does not affect the performance of the OPNG. The excellent stability of the OPNG without significant drop in output voltage paves way its



applicability as an effective and robust organic nanogenerator that might be very much useful even in harsh environment for energy harvesting application. CONCLUSION Results presented here indicate that an all organic piezoelectric nanogenerator (OPNG) can be designed based on multilayer structure of PVDF NFs mats and followed by PEDOT coating. The continuous electrospinning method enables such layer structure. In addition, vapor phase polymerization (VPP) successively ensures the coating of PVDF NFs. The OPNG exhibits a high performance such as open-circuit voltage of 48 V under the normal stress of 8.3 kPa. It also shows excellent ultra-sensitivity towards human movements such as walking and foot strikes. In addition, weight measurement mapping of a group of people with different weight is performed by combining experimental and theoretical analysis. Furthermore OPNG has shown the immense potential as a vibration sensor to implement in our daily life and finally highly durable output performance (over 6 months) broadens its applicability. The collective results suggests in utility of designing OPNG as a weight measurement sensor as well as its applications in the field of self-powered wearable and portable electronics. EXPERIMENTAL SECTION Materials. PVDF pellets (M̅w ≈ 275 000, Sigma-Aldrich, USA), N,N-dimethylacetamide (DMAc), acetone (Merck Chemical, India), 3,4-Ethylenedioxythiophene (EDOT) (TCI), Iron Chrolride (Emplura), PDMS (Sylgard, 184 Silicone Elastomer) Preparation of multilayer nanofibers mats. Multilayer structure was prepared by continuous electrospinning technique. The stock viscous solution for electrospinning was prepared by dissolving 12 wt% (w/v) of PVDF pellets in the mixed solvents of DMAc and acetone (6:4),


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under continuous stirring until a clear solution was obtained. Then 3 wt% FeCl3, 6 H2O was added into the PVDF solution and continuing stirring for 12 h. The resulting solution was placed into an ultrasonic bath for 30 min to make the homogeneous dispersions in prior to electrospinning. Notably two types of solutions were prepared, one in which FeCl3, 6H2O was incorporated in PVDF solution (i.e., solution I)and another is only neat PVDF solution (i.e., solution II). The electrospun solutions (25 ml in each case) were placed into a commercially available hypodermic syringe with a diameter of 0.8 mm needle and feed by a syringe pump with a constant rate of 0.9 mL/h. An aluminum foil wrapped grounded plate was used as the collector. A positive high voltage (12 kV) was applied between the metallic needle and collector where needle-to-collector distance was fixed at 12 cm. Electrospinning was carried out at room temperature with the relative humidity maintaining at 20-30 %. Noteworthy that solution I was electrospun initially, then solution II and finally solution I, thus three layer containing electrospun nanofibers mat was achieved. PEDOT coating of PVDF NFs mats. The EDOT monomer to PEDOT conversion process was taken place inside a VPP chamber. At first a chamber was made with fixing a three stand container inside. A small amount of EDOT monomer (10-15 µl) was drop casted inside the chamber in prior to polymerization. The chamber was kept upon a hot oven at 60 0C. The as prepared three layer structure NF mats of area 2×3 cm2 (PVDF-Fe/neat PVDF/PVDF-Fe) obtained by the continuous electrospinning method was then placed upon the container inside the VPP chamber. After placing the mats the whole chamber is sealed tightly. The NF mats was exposed to EDOT monomer vapor about 30 mins. After that, the NF mats was taken out from the chamber finishing its PEDOT coating.



Fabrication of OPNG. The fabrication begins with PEDOT coated multi-layer assembled PVDF NF mats after completing VPP. Then, the thin conducting wires were attached on top and bottom electrodes (PEDOT-PVDF) by pasting carbon tapes. After that, this multi-layer structure (thickness ~ 260 ± 20 µm) was laminated with polypropylene (PP) film (thickness ~ 125 µm) for compactness. Finally, the whole laminated structure was encapsulated with PDMS layer (area ~ 28 cm2) for mechanical robustness and environmental moisture protection, prepared by 10:1 curing agent and cured in an oven at 60 °C for 30 m. The final cured thickness of PDMS layer is about ~ 42 ± 5 µm on either side of OPNG. A reference PDMS based control device (area~ 28 cm2, thickness ~ 42 ± 5 µm) without NFs mat was also fabricated for comparison. Characterization. The crystalline phases of NF mats were identified by Fourier transform infrared spectroscopy (FT-IR) in Attenuated Total Reflection (A529-P/QMIRacle-ATR-unit (Pike), TENSOR II, Bruker) mode. The surface morphologies and diameters of the NF mats were studied with a field emission scanning electron microscope (FE-SEM; INSPECT F50). The stress-strain measurement was performed by universal testing machine (Tinius Olsen H50KS) in order to study the mechanical properties of the NF mats where three specimens were tested for each NF mats. The I-V (current vs voltage) characeristics was performed by Keithley source meter (2400). A calibrated 3-axial force pressure sensor (Flexi-Force A201) was placed underneath the OPNG to record the applied pressure. Piezoelectric throughputs were recorded in terms of open-circuit output voltage using a digital storage oscilloscope (Tektronix TDS 2024C). Blue LEDs (NSPB500S) in series connection was used to lit up. The output voltages from the OPNG under continuous mechanical impact were recorded using National Instruments (NI) DAQ board (USB 6000) via online interface with PC.


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Supporting Information: Supporting Information is available from ACS Publications website. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by a grant from the Science and Engineering Research Board (SERB/1759/2014-15), Government of India. Kuntal Maity is supported by UGC fellowship (No P-1/RS/319/14). The authors would like to thank Dr. Achintya Singha, Bose Institute, Kolkata for providing facilities of Raman measurement. REFERENCES (1) Wang, Z. L.; Zhu, G.; Yang, Y.; Wang, S.; Pan, C. Progress in Nanogenerators for Portable Electronics. Mater. Today 2012, 15, 532 –543. (2) Lee, S. H.; Jeong, C. K.; Hwang, G. T.; Lee, K. J. Self-powered Flexible Inorganic Electronic System. Nano Energy 2015, 14, 111 –125. (3) Li, S.; Wang, J.; Peng, W.; Lin, L.; Zi, Y.; Wang, S.; Zhang, G.; Wang, Z. L. Sustainable Energy Source for Wearable Electronics Based on Multilayer Elastomeric Triboelectric Nanogenerators. Adv. Energy Mater. 2017, 7, 1602832. (4) Ghosh, S. K.; Mandal, D. Efficient Natural Piezoelectric Nanogenerator: Electricity Generation from Fish Swim Bladder. Nano Energy 2016, 28, 356–365.



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Figure Captions Figur 1. Schematic illustrations of OPNG design. (a) Continuous electrospinning process. (b) Multilayer structure of PVDF NFs mat. (c) VPP method. (Enlarge views of upper inset EDOT monomer and side inset PEDOT chemical structures respectively) (d) External wires connections. (e) Lamination process. (f) PDMS encapsulation. (g) Digital photograph of OPNG.


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. Figure 2. Surface morphology. Photographs of the as-prepared electrospun NFs mat (a) before and (b) after PEDOT coating. Scale bar, 1 cm. FE-SEM images of multilayer structure PVDF NFs mat (c) before VPP (PEDOT coating), upper inset shows the histogram profile of diameter distribution of FeCl3 containing NFs (d) after PEDOT coating (arrows highlight the coating of fiber and inset shows the higher magnification image. scale bar, 500 nm) and (e) middle layer (not coated). Scale bar, 3 µm. Upper inset shows the histogram profile of NFs diameter distribution. (f) Cross-sectional FE-SEM image of multilayer structure PVDF NFs mat (after PEDOT coating). Scale bar, 200 µm.

Figure 3. Characterization of PVDF NFs mat. (a) Schematic representation of the preferential molecular orientation from randomly oriented -CH2/CF2- dipoles due to continuous electrospinning. (b) FT-IR spectra of before and after PEDOT coated PVDF NFs mat. (c) Raman spectra of PEDOT coated PVDF NFs mat. (d) Mechanical properties (Stress vs Strain curve) before and after PEDOT coated PVDF NFs mat. (e) Current-voltage (I–V) characteristics of the PEDOT coated NFs mat.

Figure 4. Performance of OPNG. Open circuit output voltage responses for different applied human finger impact (a) Forward connection (inset middle shows the enlarged view of marked area). The top and bottom insets show photographs of the device during press and release, respectively. Scale bar, 1 cm. (b) Reverse connections (the schematics of electrical connections are given in the corresponding upper inset). The middle inset shows the enlarged view of a



selected reverse output voltage region (marked by lines in main panels). (c) Measured shortcircuit output current response under the impact of human finger touch. (d) Simulation result of the piezopotential distribution in the network structure of NFs mat.

Figure 5. Sensitivity of OPNG. (a) Relative changes in output voltage signal versus time for monitoring different types of bending. The top and bottom insets show photographs of the device during bending and releasing, respectively. Scale bar, 1 cm. (b) Photograph demonstrates the voltage response for the impact of drum beats of a toy. Scale bar, 3 cm. Enlarged marked area is shown below. (c) Dependences of output voltage and instantaneous power on variable external load resistance with schematic circuit diagram in the inset. The inset shows the glowing array of LEDs by direct finger touch without external power source.

Figure 6. Weight measurement mapping. (a) Schematic of experimental set up shows the walking of a man upon device for the purpose of studying the voltage response. Output voltage generation for different deformations, such as (b) Walking and (c) foot strikes. (d) PCA 2D plot for the data of output voltage responses taken during walking of 5 different people (different weight) for 10 times individually. Photographs of this respective people inserted in the upper inset with measured weight.

Figure 7. Vibration sensor. (a) Output signal responses during off and on conditions of a mobile vibration. Photographs shows OPNG kept upon the mobile and corresponding output


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voltage from DSO in the respective upper inset. Scale bar, 3 cm. (b) The machine vibration response of the OPNG using a simple portable sewing machine (upper inset).Scale bar, 2 cm. (c) Photograph shows of flexible OPNG is mounted on the kitchen blender machine and respective signal collected from DSO. Scale bar, 3 cm. Corresponding output voltage response is shown during off and on condition in the respective right inset.

Figure 8. Fatigue testing of OPNG. Open-circuit output voltage recorded over time in response to continuous impacting at a frequency of 3 Hz over 21000 cycles under the pressure of 0.9 kPa for several months. Data were recorded (a) after 1 month, (b) 3 months and (c) 6 month



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(a)

(b) Layer 3 Layer 2 Layer 1

(g) (c)

(f)

(d) (e) Laminated sheets Figure 1. Schematic illustrations of OPNG design.


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(b)

(a)

(c)

(e)

(d)

(a)

(f)

Figure 2. Surface morphology.



(a)

(b)

(d)

(c)

(e)

Figure 3. Characterization of PVDF NFs mat.


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(a)

(b)

(c)

(d)

Figure 4. Performance of OPNG.



(a)

(b)

(c)

Figure 5. Sensitivity of OPNG.


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(a)

(b)

(c)

(d)

Figure 6. Weight measurement mapping ACS Paragon Plus Environment


(a)

(b)

(c)

Figure 7. Vibration sensor.


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(a)


(b)

Figure 8. Fatigue testing of OPNG.


(c)


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Table 1: An extensive comparison between the electrode type, method and output performances of our fabricated OPNG and those of PNGs based on PVDF NFs. (Here‚ t: thickness of the piezoelectric material, A: working area of the piezoelectric material, l: separation length between two electrodes, NF: not found)

Electrode

Fabrication

Applied

Device

Output

Type

Method

Force/ Frequency Dimensions

Voltage (V)

Metallic

Physical

2-4 Hz

l= 100-600 µm

0.005-0.03

10 Hz

t = 140 µm,

References

[21]

contact

Aluminium foil

Physical

Physical

950 ±30 µε

contact

Conducting

Physical

fabric

contact

t = 0.1 mm,

0.76

[25]

A= 100 mm 2

Contact

Aluminium foil Physical

[24]

A= 2 cm 2

Contact

Copper foil

6.3

10 Hz ,

t = 100 µm ,

10 N

A= 2 cm 2

1 Hz, 0.2 MPa

t = 162 µm , A= 6.3 cm 2


2.6

[52]

3.2

[27]

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Aluminium foil Electrospun

100 Hz

Physical

75 kHz

Electrospun

4 Hz

Physical

fabric

contact

Conducting

Physical

fabric

contact

Gold

Sputter and

t= 120 µm

1.1

[53]

t= NF

2.5

[54]

11

[28]

A= 6.3 cm 2

deposition

Conducting

[26]

A= 4 cm 2

Contact

Graphene

0.78

A= 775 mm 2

deposition

Aluminium foil

t= NF

4 Hz,

t= NF

8N, 6.6 kPa

A=1200 mm2

7 N,

t= 150 µm

8.8 kPa

A= 93.5 cm2

35 Hz

NF

14

0.4

[23]

[55]

Lift off

Metallic

Aluminium foil

Physical

1 Hz

t= 50 µm

contact

0.2 MPa

A= 4 cm 2

Physical

2 Hz,

t=NF


7.9

[56]

0.1

[57]


contact

Aluminium

PEDOT

0.49 N

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A= 35 cm 2

Physical

1 Hz

t= 100 µm

Contact

10 N

A= 4 cm 2

Coated by

5N

t = 260µm

VPP

8.3 kPa

A= 6 cm 2


3.1

48

[58]

In this work

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All Organic High Performance Piezoelectric ... - ACS Publications

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