Natural Sugar-Assisted, Chemically Reinforced, Highly Durable

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 44018−44032

Natural Sugar-Assisted, Chemically Reinforced, Highly Durable Piezoorganic Nanogenerator with Superior Power Density for SelfPowered Wearable Electronics Kuntal Maity,† Samiran Garain,†,‡ Karsten Henkel,‡,§ Dieter Schmeißer,‡ and Dipankar Mandal*,†,∥ †

Organic Nano-Piezoelectric Device Laboratory (ONPDL), Department of Physics, Jadavpur University, Kolkata 700032, India Applied Physics and Sensor Technology, Brandenburg University of Technology Cottbus-Senftenberg, K.-Wachsmann-Allee 17, 03046 Cottbus, Germany § Applied Physics and Semiconductor Spectroscopy, Brandenburg University of Technology Cottbus-Senftenberg, K.-Zuse-Str. 1, 03046 Cottbus, Germany ∥ Institute of Nano Science & Technology (INST), Mohali 160062, India

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‡

S Supporting Information *

ABSTRACT: Natural piezoelectric materials are of increasing interest, particularly for applications in biocompatible, implantable, and flexible electronic devices. In this paper, we introduce a cost-effective, easily available natural piezoelectric material, that is, sugar in the field of wearable piezoelectric nanogenerators (PNGs) where low electrical output, biocompatibility, and performance durability are still critical issues. We report on a high-performance piezoorganic nanogenerator (PONG) based on the hybridization of sugar-encapsulated polyvinylidene fluoride (PVDF) nanofiber webs (SGNFW). We explore the crucial role of single-crystal sugar having a fascinating structure along with the synergistic enhancement of piezoelectricity during nanoconfinement of sugar-interfaced macromolecular PVDF chains. As a consequence, the SGNFW-based PONG exhibits outstanding electricity generation capability (e.g., ∼100 V under 10 kPa human finger impact and maximum power density of 33 mW/m2) in combination with sensitivity to abundantly available different mechanical sources (such as wind flow, vibration, personal electronics, and acoustic vibration). Consequently, it opens up suitability in multifunctional self-powered wearable sensor designs for realistic implementation. In addition, commercially available capacitors are charged up effectively by the PONG because of its rapid energy storage capability. The high performance of the PONG not only offers “battery-free” energy generation (several portable units of light-emitting diodes and a liquid crystal display screen are powered up without using external storage) but also promises its use in wireless signal transmitting systems, which widens the potential in personal health care monitoring. Furthermore, owing to the geometrical stress confinement effect, the PONG is proven to be a highly durable power-generating device validated by stability test over 10 weeks. Therefore, the organic nanogenerator would be a convenient solution for portable personal electronic devices that are expected to operate in a self-powered manner. KEYWORDS: natural piezoelectric material, sugar, PVDF, organic piezoelectric nanogenerator, high performance and durability, self-powered electronics

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INTRODUCTION

However, key challenges still exist to enable adoption in practical use cases such as (i) the lack of a thin, wearable NG, (ii) limited power supply and energy storing capacity, (iii) biocompatibility, and so forth. Earlier reports either rely on the integration of bulky batteries, which severely compromise wear ability or developing thin, stretchable batteries and supercapacitors.10−13 In addition, such systems require frequent recharging. Wang and Song first reported on a PNG in 2006 based on inorganic piezoelectric nanowire arrays.14 However,

The recent trend of autonomous, wireless, smart, and wearable electronic devices has prompted a great interest in “batteryfree” environment-friendly energy harvesting devices. The growing demand of alternative energy generation leaves a significant challenge to develop self-powered energy harvesting devices such as nanogenerators (NGs).1−5 In this aspect, piezoelectric NGs (PNGs) could play a significant role for scavenging energy from any type of abundant mechanical vibration available in our environment to realize a costeffective, sustainable, and portable energy source.6−9 In particular, a considerable attention has been paid for their suitability in wearable, portable, and smart electronic devices. © 2018 American Chemical Society

Received: September 4, 2018 Accepted: November 20, 2018 Published: November 20, 2018 44018

DOI: 10.1021/acsami.8b15320 ACS Appl. Mater. Interfaces 2018, 10, 44018−44032

Research Article

ACS Applied Materials & Interfaces

In contrast, poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE)), a copolymer of PVDF, contains a high content of the piezoelectric β-phase but has several limitations that restrict its practical application such as a lower Curie temperature (Tc ≈ 80−110 °C, depending on the molar ratio of VDF and TrFE), less dipole (−CH2−/−CF2−) density due to the defect-containing TrFE unit, and much higher cost than PVDF. Therefore, nucleation of the piezoelectric β-phase in PVDF is one of the prime challenges for electronic device fabrication. Over the years, several techniques (such as mechanical stretching, poling, high-pressure annealing, casting from solutions, and spin-coating) have been used to induce the piezoelectric β-phase in PVDF.28−31 However these costintensive and tedious processes are not so attractive because of their several disadvantages including damages of flexibility and limitation of large-scale production of the films. At the same time, piezoelectric phases cannot be achieved typically unless the application of an electric field which aligns the randomly oriented molecular −CH2−/−CF2− dipoles along the electric field direction (called electrical poling). In addition, nanostructure-based piezoelectric device designing is especially appealing because of the apparent strain confinement effect and improved strain tolerance capability, which are usually not achievable by bulk material-based designs. To overcome these issues, electrospinning, a simple, scalable, and smart technique, offers to induce the piezoelectric β-phase in PVDF nanofiber webs (NFWs) where the in situ dipole alignment and electrical poling simultaneously take place.32,33 Thus, an electrospun PVDF NFW-based PNG has become highly desirable, which is lightweight and comfortable to wear especially for wearable electronic devices.33 It is worth to mention that the content, dipole orientation, and the degree of crystallinity are the crucial factors that result in the macroscopic piezoelectric response of the PNG. However, a problem associated with the β-phase orientation is that relaxation takes place during the downstream electrospinning in spite of its dominance during the early stage of rapid jetting under a high electric field.34 To overcome this problem, the incorporation of external assisting agents into the PVDF matrix has become one of the good alternatives.35,36 In this aspect, the idea behind introduction of natural sugar into the piezoelectric PVDF could greatly assist the formation and stabilization of the oriented β-phase while forming oriented −CH2−/−CF2− dipoles. Hence, the enhancement of polar β-phase and the role of piezoelectric sugar in accordance with PVDF NFWs are both of great importance for making a composite suitable for a mechanical energy harvester. In this paper, we report on a piezoorganic NG (PONG) based on a novel, flexible, lightweight hybridized composite NFWs, that is, a natural piezoelectric material containing particularly sugar in the PVDF NFWs. Here, sugar plays a crucial role in the performance enhancement of the PNG while it can synergistically act even as a spontaneous piezoelectric component as well as a stabilizing agent. Besides, the encapsulation of the PVDF matrix prevents the degradation of the sugar from moisture and oxygen. To the best of our knowledge, no relevant study has been reported so far where piezoactive natural sugar is explored as an assisting agent in a hybridized piezoorganic NG making it superior for mechanical energy harvesting and wearable electronics. The performance durability is another crucial issue in flexible PNGs, explicitly in wearable electronics owing to the use of metal foil electrodes or metal-coated thin-film

the use of inorganic material (such as BaTiO3, PZT, ZnSnO3, ZnO, ZnS, CdS, and GaN)-based PNGs has been limited especially in embedded in vivo self-powered wearable device realizations because of the brittleness, weight, flexibility, and biocompatibility issues.15−19 On the other hand, natural piezoelectric materials such as bones, cellulose, hair, sugar cane, collagen fibrils, and peptides are increasingly interesting for self-powered biomedical systems owing to their unique crystal structure, spontaneous piezoelectricity, endless availability, cost effectiveness, and adequate biocompatibility.20,21 However, many natural piezoelectric materials in this group still remain undiscovered. For example, natural sugar, a novel piezoelectric material is always available (mainly found in the tissues of most plants and present in sugar cane and sugar beet), possesses a fascinating natural structure and also treated as a biocompatible organic product. In particular, sugar is used in preparing foods (e.g., cookies, cakes, etc.) and is also incorporated to some foods and beverages. Basically, sugar is constituted of sucrose (C12H22O11), one of the most important saccharides built of fructose and glucose. The single-crystals of sucrose belong to a monoclinic structure with P21 space group and possess only element of symmetry, that is, binary rotation around the baxis.22 The other crystal axes a and c are perpendicular to the b-axis where the angle between them is about 103.3°. Therefore, the piezopotential created in the crystal because of stress application has a strong effect on the carrier transport of the materials. Thus, it is interesting to investigate the performance of natural sugar in terms of spontaneous piezoelectricity and its practical application in energy harvesting devices. However, the stability of this material is a point of agitation for its use in the self-powered technology where moisture is being considered as the main degradation source because of its intrinsic hydrophilic property.23 In such a case, encapsulation of the sugar material may be an effective alternative solution for improvement of the lifecycle of the devices. Here, an interesting functionality could be added by introducing an encapsulation layer which is simultaneously useful for protecting the materials as well as for harvesting mechanical energies. Currently, flexible piezoelectric polymer materials have been increasingly used in energy harvesting devices especially in flexible electronics owing to their several advantages over inorganic materials.24 However, it is still desirable to further raise the output power of these PNGs because of miniaturization of electronic devices and improvements in wireless communication technology. Thus, a hybrid organic energy harvester may be designed where the piezoactive natural component is being introduced in piezoelectric polymers. It has been found that flexible, lightweight, nontoxic polyvinylidene fluoride (PVDF) is one of the best known piezoelectric polymers which has excellent potential to be used in PNGs, sensors, actuators, and biomedical applications because of its spontaneous polarization, chemical resistiveness, high thermal stability, and brilliant electroactive properties, viz., piezo-, pyro-, and ferroelectricity.25,26 PVDF mainly consists of four different crystalline phases (viz., α, β, γ, and δ) in nature depending on the stereochemical macromolecular conformation, but the main drawback is that it mainly contains stable nonelectroactive α-phase (TGTG′ conformation). The most desired piezoelectric β-phase (TTTT conformation) possesses largest spontaneous polarization which is perfectly suitable for PNG fabrication.27 44019


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Figure 1. Schematic illustration of PONG fabrication step by step. (a) Sugar particles incorporated into the PVDF−DMF solution. The enlarged view in the right side shows the schematics of interaction occurring in the PVDF solution because of H-bonding between the incorporated sugar and the CF2 dipoles of the PVDF chain. (b) Electrospinning process. (c) Optical image illustrating the prepared SGNFW in the omnidirectionally stretched condition. (d) 3D network structure of the SGNFW used for the PONG fabrication. (e) Top and bottom electrodes are realized by conducting fabrics. (f) Final structure of the PONG bearing connection of two electrical leads.

electrodes. Therefore, it is a great challenge to enhance the performance of the PNGs particularly by keeping their flexibility and wear ability unchanged. Herein, we explore a simple, cost-effective, and one-step approach to fabricate a PONG where a hybridized organic composite, that is, sugarcontaining whitish PVDF NFWs (SGNFW) acts as a stretchable and flexible piezoelectric component keeping a soft conducting textile as the top and bottom electrodes to overcome the drawbacks of the existing devices. Surprisingly, the as-fabricated mechanically robust SGNFWbased PONG offers exceptionally high power-generating performance from human finger touch (power density of 33 mW/m2 and output voltage ≈ 100 V under the application of 10 kPa stress) to drum beats of a toy. As a direct proof of evidence, it can drive several light-emitting diodes (LEDs) instantly or power up a liquid crystal display (LCD) screen in accordance with the fast energy storing capability of the PONG for different commercially available capacitors (capacitances of 1, 2.2, and 4.7 μF). The high performance can be attributed to the hybridization of two piezoelectric organic materials where the single-crystal sucrose encapsulated in the crystalline (β-phase) PVDF matrixes via electrostatic interactions (such as H bonding) may be considered as an

overall composite of giant crystal-like structures. Therefore, we hypothesize that this type of interaction and overall deformation of the composite crystal having a noncentrosymmetric structure create a huge piezopotential difference during the application of external stress between the top and bottom electrodes, exhibiting an enhanced piezoelectric response. We further demonstrate the effective electric power generation via abundantly available wind vibration and subsequently by real-life device such as “massage belt” vibration giving a meaningful energy harvesting performance. In addition, the tactile sensing performance of the PONG is also tested in front of a sound speaker driven by different musical instruments in operation such as a violin, piano, guitar, and kick drum. Furthermore, the PONG transmits wireless signals and sustains its high power under repeated mechanical impact over 10 weeks of measurements exploring its potential to be used as a sustainable energy harvester in self-powered, multifunctional devices suitable for wearable electronics.

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RESULTS AND DISCUSSION Design Strategy. The fabrication steps of the PONG architecture are schematically illustrated in Figure 1a−f. The fabrication starts with the preparation of the sugar-incorpo44020


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Figure 2. FT-IR spectra of the PNFW and SGNFW in the region of (a) 3700−2900 cm−1, the marked region (3054−2952) cm−1 is shown in the enlarged view of (b). High-resolution XPS spectra of the SGNFW and PNFW in the (c) F 1s and (d) C 1s core-level regions. (e) Deconvoluted XPS spectra of the O 1s core-level of the SGNFW. (f) EDXS spectrum giving the atom fraction distribution in the SGNFW. Elemental mapping exhibiting (g) carbon (green), (h) oxygen (red), and (i) fluorine (blue) concentrations; scale bars correspond to 4 μm. (j) FT-IR spectra of the PNFW and SGNFW in the region of 1600−400 cm−1. (k) Change in the absorbance intensity of the β-phase (1276 cm−1) vibrational band of the SGNFW as a function of temperature (30−180 °C, thermal ramp ≈ 1 °C/min for clarity) and its corresponding FT-IR spectra in 10 °C intervals to guide the eye in the inset, as recorded by in situ temperature-dependent FT-IR spectroscopy. XRD patterns and their curve deconvolution of the (l) SGNFW and (m) PNFW; points show the experimental data, and the solid lines correspond to the best fit.

attaching wires (Figure 1f). Finally, the as-prepared SGNFW with wired electrodes having an effective electrode area of (10.5 × 8) cm2 were ready for characterization (Figure 1f). It is worthy to mention that here two types of electrospun NFWs were basically prepared for comparison, one in which sugar was introduced into the PVDF matrix (SGNFW) and the other one, as a reference, from only neat PVDF (PNFW), followed by fabrication of two PNGs (i.e., PONG and Ref. PNG without sugar) keeping all conditions including device dimensions unchanged. Material Characterization/Evaluation. The encapsulation of sugar into the PVDF matrix leads to a plausible electrostatic interaction and hydrogen bonding between the hydroxyl group of sugar and PVDF dipoles. In sugar, the electropositive hydrogen is present because of the attached electronegative oxygen in the hydroxyl group, and thus, the surface active positive charge forms hydrogen bonds with adjacent PVDF chains at the interface of PVDF via its negative pole (i.e., fluorine atoms) as illustrated in the schematic on the right-hand side of Figure 1a. Therefore, these bonds may have

rated PVDF−N,N-dimethylformamide (DMF) solution. Briefly, sugars were added (Figure 1a) into stock PVDF− DMF solutions (12 wt %) at an initial concentration of 0.01 g/ L, homogenized by stirring for 24 h, and then sonicated by using an ultrasonic bath at 300 W for 1 h. The initial dispersion turned clear, which indicates the completion of hybridization, and afterward, the solution was ready for the electrospinning process. The interaction between the sugar and PVDF chain is shown by a schematic in the right side of Figure 1a. The detailed phenomenon will be discussed below. Electrospinning of SGNFW was started from the mentioned solution upon a grounded plate collector wrapped by an aluminum foil at an optimized distance (Figure 1b). Then, the NFWs was dried at 60 °C for 12 h to remove the remnant solvents. Figure 1c shows an optical image of a section of the prepared SGNFW in the omnidirectionally stretched condition. In the next step, the as-prepared SGNFW (thickness of 150 μm, area 12 × 9 cm2) was cut for the fabrication of the device (Figure 1d). Then, conducting textiles were attached as bottom and top electrodes by means of pasting adhesive tapes (Figure 1e), followed by 44021


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ACS Applied Materials & Interfaces an effect in the rearrangement of fluorine atoms up to certain extent and consequently align more dipoles throughout the polymer chain. To provide evidence for the explanation given above, Fourier transform infrared spectroscopy (FT-IR) spectra of the SGNFW and PNFW were compared in the higher wavenumber region (3700−2900 cm−1). The appearance of a broad absorption band in the SGNFW data signifies the H-bonding interaction; in contrast, there are no observable peaks in the reference data of PNFW (Figure 2a).37,38 Notably, a huge broadening of this vibrational band occurs because of the variation of the microenvironments of the ground state of the −OH stretching vibration. In other words, the strong Hbonding interaction of the hydroxyl groups present in sucrose toward the F− ions of PVDF (i.e., O−H···F−C) leads the −CH2−/−CF2− dipoles in PVDF to be more unidirectional oriented. On the other hand, single-crystal sucrose may be considered as a combination of dipole as a whole. Therefore, there will be a bipolar interaction between the opposite poles of sucrose and the PVDF matrix. This type of electrostatic interaction may enhance the polar β-phase generation in PVDF and self-orientation as well, which in turn help to reduce the threshold electric field strength required to be applied during the nanoscale device fabrication.30,34 In addition, the polar PVDF can be oriented in a much more effective way along the applied electric field during the electrospinning process which plays an effective role in directing the dipole alignment.33,39 Thus, the synergistic effect of the sugar-induced polar configuration and subsequent electric field-induced dipole alignment through electrospinning result in macromolecular chains preferentially aligned along the fiber longitudinal axis and the electric dipoles (−CH2−/−CF2−) transversely isotropic to the chains in the SGNFW.32,33,39 This interfacial interaction model is quite consistent with the FT-IR results, where the −CH2 dipoles are stretched as evident from the clear shifting of the CH2 asymmetric (νas) and symmetric (νs) stretching vibrational bands toward lower frequency for SGNFW in comparison to that of the PNFW (Figure 2b). The electrostatic interactions result in a damping of the oscillations of the −CH2− dipoles and thereby support the aforementioned shifts in the vibrational bands. To get a better understanding of the interfacial interactions between the sugar and −CH2−/−CF2− dipoles of the PVDF matrix, X-ray photoelectron spectroscopy (XPS) was performed on SGNFW and PNFW.24,40 The high-resolution F 1s core-level spectra for SGNFW and PNFW exhibit a slight asymmetry when peak intensities are normalized at 688.1 eV as evident from Figure 2c.40 The asymmetry arises because of the strong interfacial interactions between sucrose and PVDF. Subsequently, this observation is strongly confirmed by the high-resolution C 1s core-level spectra (Figure 2d). The C 1s data of the PNFW show two peaks with binding energies of 286.7 and 291.2 eV attributed to the two main carbon species, that is, CH2 and CF2, respectively. Interestingly, a significant shift toward lower binding energies can be observed in the SGNFW data, where the CH2 and CF2 peaks appeared at 285.6 and 290.2 eV, respectively, (illustrated by ΔBECF2 in Figure 2d). These changes suggest a notable variation in the electronic environment of the CH2 and CF2 species of PVDF because of electrostatic interactions between the sucrose molecules and PVDF moieties in the SGNFW.27 Furthermore, in the C 1s data, a substantial difference in the peak intensity (area) between the spectra of SGNFW and PNFW is monitored,

particularly if the data are normalized to the CH2 peak. The SGNFW spectrum clearly exhibits a reduction of the CF2 peak area confirming further the aforementioned interfacial interactions.40 In addition, the O 1s spectrum of the SGNFW (Figure 2e) confirms the presence of oxygen in the sucrose molecule, whereas no related oxygen signal could be sensed for the PNFW. The deconvolution of the O 1s spectrum clearly indicates two main components, where the peak at 532.8 eV is assigned to C−O and the one at 533.7 eV to O−C−O. This observation is consistent with the strong OH stretching vibrational band in the FT-IR spectra (Figure 2a), strongly confirming the presence of sugar in the PVDF molecular chain. Subsequently, elemental quantification was performed using the energy-dispersive X-ray spectroscopy (EDXS) spectrum of the SGNFW, which shows O along with C and F (Figure 2f). The appearance of F and C signal is mainly arising from the PVDF matrix while O contributions originate from the presence of sugar in the SGNFW. It also suggests that the solvent was evaporated completely because no nitrogen signals were detected. This is further evidenced by the elemental mapping images of oxygen (O) along with carbon (C) and fluorine (F) (Figure 2g−i). Overall, the aforedescribed interfacial interaction promotes the β-phase formation of PVDF in the SGNFW. Here, it is worthy to mention that a very small amount of sugar can hinder the α-phase nucleation and increase the β-phase content in the PVDF matrix, as evident from the comparison of the FT-IR spectra of SGNFW and PNFW (Figure 2j). The vibrational bands at 614, 763, and 975 cm−1 in PNFW are mainly attributed to the nonpolar α-phase. In contrast, these bands are diminished in the spectra of SGNFW while two other arrow-marked new peaks at 1053 and 995 cm−1 are appearing, indicating the existence of sugar in the PVDF matrix. These bands are assigned to the C−O stretching modes ν(C−O) of a sucrose molecule in SGNFW.41 At the same time, the piezoelectric β-phase can be well-identified from the vibrational bands at 1276 and 840 cm−1 (Figure 2j). Because of the electrospinning process, also in the PNFW spectrum, the β-phase is clearly present; however, the β-phase-related peaks are more intense in the SGNFW data, evidencing the higher yield of the polar β-phase.24 The relative proportions of the electroactive phases (FEA) within the NFWs are quantified from the Beer−Lambert law using the following equation FEA =

A840 × 100% (K840/K 763)A 763 + A840

(1)

where A840 and A763 are the absorbance intensities at 840 and 763 cm−1, respectively, and K840 (7.7 × 104 cm2 mol−1) and K763 (6.1 × 104 cm2 mol−1) are the absorption coefficients at the respective wavenumbers.30 An FEA value of less than 90% is achieved in PNFW, whereas this value is more than 95% for SGNFW, attributing the formation of the enhanced piezoelectric β-phase because of the hybridization of sugar and PVDF. Here, the role of sugar is very crucial; it not only induced but also stabilizes the electroactive phases by rapid crystallization of SGNFW during the electrospinning process. In addition, we have also recorded the FT-IR spectra of the SGNFW under increasing relative humidity to observe its effect upon SGNFW (Supporting Information, Figure S1a). However, the respective results clearly exhibit the presence of the same content of the polar β-phase and no significant change is observed, although the relative humidity is changed. 44022


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ACS Applied Materials & Interfaces As a result, the generation of the polar crystalline β-phase is irreversible in SGNFW and no back-relaxation to the α-phase by any thermal treatment occurs. This statement is further supported by the observation from in situ temperaturedependent FT-IR spectroscopy (Figure 2k).42 The thermal behavior of the crystalline polymorph (β-phase) is achieved when the absorbance intensity of the β-phase (β1276) is plotted versus temperature (Figure 2k). Though, the melting point (tβm) of the β-phase at 158 °C is confirmed from this plot (Figure 2k), the β-phase representing the vibrational band at 1276 cm−1 still exists even at 180 °C (inset of Figure 2k). Therefore, it is expected that the device could operate up to a wide temperature region (27−158 °C), as the polar phase is mainly responsible for the piezoelectric outcome.42 Thus, the introduction of sugar in PVDF acts as an assisting agent for the crystalline β-phase in SGNFW; the all-trans conformation of PVDF is thermodynamically stabilized with interfacial interaction phenomena. In addition, the crystallization behavior of the NFWs was investigated to identify their crystalline phases and to quantify the degree of crystallinity by curve-deconvoluted X-ray diffraction (XRD) patterns (Figure 2l,m). In the XRD spectra, the curve-deconvoluted sharp peak at 20.8° [(110)/(200)] indicates that the β-phase is predominant in SGNFW as well as in PNFW where this peak is, however, a bit less intense. Furthermore, it is obvious that PNFW carries a higher fraction of the nonpolar α-phase as evident from the more pronounced diffraction peaks of the α-phase at 2θ = 19.6° (110) and 18.3° (020).24,27,30 Therefore, it implies that the content of the αphase decreases and that of polar β-phase increases in SGNFW in contrast to the neat one (PNFW). This result is in good agreement with the FT-IR spectra supporting our observations given before. The total degree of crystallinity could be deduced from the following equation χct =

∑ Acr × 100% ∑ Acr + ∑ A amr

surfaces and no bead defects were observed for PNFW and SGNFW (Figure 3a,b). Notably, the incorporated sugar is well

Figure 3. FE-SEM images of (a) PNFW and (b) SGNFW; the diameter distributions of the NFWs are shown in the respective inset; scale bars correspond to 5 μm. (c) Radial intensity distribution vs alignment angle (0°−180°) for randomly networked SGNFW with its 2D FFT image shown in the inset. (d) TEM image, right upper inset shows the HR-TEM image and left lower inset shows the corresponding SAED pattern of the SGNFW; scale bars correspond to 200 nm, 20 nm, and 10 nm−1, respectively. (e) Mechanical response of the SGNFW and PNFW. Nature of flexibility of SGNFW is shown by (f) rolling, (g) twisting, and (h) folding, respectively.

dispersed in the PVDF matrix, making a considerably reduced diameter for SGNFW (average fiber diameter ≈ 125 nm) compared to that of PNFW (average fiber diameter ≈ 250 nm) which is evident from the corresponding histogram profiles (insets of Figure 3a,b). It should be mentioned that a fixed humidity (20−30%) was maintained during the electrospinning process within the SGNFW preparation routine because the increase of humidity results in the appearance of bead defects. Beyond a certain level of humidity, the web is full of beads which are not useful for device fabrication (Supporting Information, Figures S1b,c). The two-dimensional (2D) fast Fourier transform (FFT) of the FE-SEM image of SGNFW yields the quantitative analysis on the degree of alignment. A highly symmetric, circular distribution of the pixel intensity confirms the randomly oriented NFWs along with a featureless behavior of the radial pixel intensity versus polar angle (Figure 3c and inset). In other words, a symmetric distribution of the pixel intensity implies that the particular frequency at which pixel intensities occur in the respective 2D FFT image (inset of Figure 3c) is almost identical in any direction and this observation is also confirmed from no observable peaks when plotting the sum of the pixel intensity as a function of the polar angle in the region of 0°−180° (Figure 3c). This analysis further evidences the randomly networked structure of the SGNFW. However, the molecular orientation of the SGNFW neither depends on randomized nanofiber distribution nor on nanofiber diameter rather upon the synergistic effect of the applied high electric

(2)

where ∑Acr and ∑Aamr are the summation of the integral areas of the crystalline peaks and the amorphous halo, respectively.27,30 Therefore, the degrees of β-crystallinity (χcβ) and αcrystallinity (χcα) are calculated by the following equations χcβ = χct ×

χcα = χct ×

∑ Aβ ∑ Aβ + ∑ Aα

(3)

∑ Aα ∑ Aβ + ∑ Aα

(4)

The corresponding values were calculated and are labeled in Figure 2l,m. Obviously, the interaction of sugar in the PVDF matrix leads to the yield of the piezoelectric β-phase in SGNFW (χcβ ≈ 56%) in comparison to PNFW (χcβ ≈ 43%). Thus, the total crystallinity (χct) of the SGNFW becomes higher (≈59%) than that of PNFW (≈53%). Therefore, the hybridization of the single-crystal sugar and the PVDF matrix affects the resultant properties significantly because of the overall improvement of crystallinity.30,43 Basically, NFWs were the prime component of the NG owing to their excellent flexibility. Therefore, it is necessary to study the surface morphology and nanostructure. The field emission scanning electron microscopy (FE-SEM) images reveal that the NFWs were randomly oriented having rough 44023


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ACS Applied Materials & Interfaces field and mechanical stretching during the electrospinning process.33 This is already confirmed by our FT-IR data reported above. In addition, for the decreasing of the fiber diameter, the resultant crystallinity and overall piezoelectric performance have been greatly improved.24,34,43 The present state of the art depending on the nanofiber orientation and device performance is shown in a comparison table (Supporting Information, Table S1). The transmission electron microscopy (TEM) image (Figure 3d) and high-resolution TEM image (right upper inset of Figure 3d) of the SGNFW reveal that the sugar is encapsulated within the PVDF, which implies the nanoscale interfacing of sugar within the macromolecular chain of PVDF. The corresponding selected area electron diffraction (SAED) pattern (left lower inset of Figure 3d) of the SGNFW unveils the highly crystalline structure of the sugar, and the respective lattice planes are indexed.44 The clear diffraction spots from single-crystal sugar embedded within the diffraction rings of PVDF are well identified in the SAED pattern which further confirms the presence of sugar in the PVDF matrix. In addition, the XRD peaks of SGNFW appeared at 31.9°, 33.3°, 36.0°, 37.2°, 38.3°, 46.7°, 50.6°, and 56.6° correspond to the diffraction from the lattice planes of sugar as evident from the respective XRD peaks of sugar (Supporting Information, Figure S2).44 This result further confirms the presence of sugar within the PVDF NFWs. Subsequently, the mechanical properties of the SGNFW and PNFW were investigated under extreme stress to get an idea of their applicability in environmentally harsh conditions. In general, the output response of a piezoelectric material can be presented by the constitutive piezoelectric strain−charge equation Di = dijσj = dijYkjεk

Next, we discuss the macroscopic polarization responses of the NFWs to support our previous observations. A superior response is observed for the SGNFW (Figure 4a,b) in

Figure 4. (a) Polarization vs electric field (P−E) and (b) butterflyshaped strain vs electric field (S−E) hysteresis loops of the SGNFW. Frequency-dependent dielectric properties: (c) dielectric constant (εr) and (d) loss tangent (tan δ) of the SGNFW (red) and PNFW (blue).

comparison with the PNFW (Supporting Information, Figure S3). The SGNFW possesses a high remnant polarization (Pr) of 0.13 μC/cm2 with negligible loss of spontaneous polarization (Ps) at E = 0 (i.e., Pr/Ps ≈ 0.7), whereas the PNFW exhibits Pr/Ps ≈ 0.3 as evident from Figure 4a and Supporting Information, Figure S3a. The corresponding current density versus electric field (J−E) loops of the PNFW and SGNFW are shown in the Supporting Information, Figure S3b,c. The average coercive field (Ec) of the SGNFW above which the dipoles are switched back and forth is found to be 10 kV/cm. The switching current density at Ec is 0.2 μA/cm2. The symmetrical current curves for both increasing and decreasing electric field mainly arise because of the current induced by the dipole reversal where leakage and capacitive charging currents are comparatively negligible.45 In addition, the electric fieldinduced strain amplitude (S) response corresponding to the P−E loop due to the converse piezoelectric effect is shown by the butterfly-shaped hysteresis loop (S vs E) (Figure 4b). This observation further confirms that the polarization response arises only because of the dipole reversal process and is not based on the conduction mechanism. The piezoelectric charge coefficient (d33) can be obtained as P d33 = − r = −33 pC/N (6) Y where Y is the Young’s modulus determined above (refer to Figure 3e). Hence, the SGNFW possesses a negative longitudinal piezoelectric coefficient and contracts under the electric field. In addition, the longitudinal electrostrictive coefficient (Q) can be evaluated as S = QP2 from the S−E loop. The slope of S versus P2 yields Q ≈ 0.78 m4 C−2 (Supporting Information, Figure S3d), which is a higher value than those of many inorganic and organic piezoelectric materials. Thus, the P−E and S−E hysteresis loops of the SGNFW suggest the preferential alignment of electric dipoles along the direction of

(5)

where Di is the electric charge separation, dij is the piezoelectric coefficient, σj is the applied stress, Ykj is the Young’s modulus, and εk is the applied strain. It is evident from the equation that the changes of dij and Ykj both collectively contribute to the piezoelectric output performance and affect the energy harvesting efficiency significantly.43 Thus, the mechanical aspects of piezoelectric materials are equally important along with the contribution of the piezoelectric coefficient to enhance the overall piezoelectric response. Figure 3e presents the corresponding stress−strain curves that clearly display an enhanced stretch ability of up to 60% for the SGNFW compared to 21% for the PNFW accompanied by a significantly higher Young’s modulus (Y) of 40 MPa for the SGNFW than of 9 MPa for the PNFW. The improvement of mechanical properties in SGNFW may be ascribed to the stress concentration caused by the sucrose molecule within the PVDF matrix resulting in an enhanced degree of crystallinity. It is noteworthy that polymer chains become less mobile in accordance with the raising of the crystalline part in the composite and hence it is more difficult to stretch the material, thereby assisting improved mechanical responses. Thus, the higher toughness of the SGNFW than that of the PNFW upon the application of tensile stress makes it more promising for practical applications. In addition, the flexible nature of the SGNFW is also demonstrated in the optical images by rolling, twisting, and folding (Figure 3f−h). Therefore, the good stretch ability, enhanced elasticity, and higher toughness of the SGNFW open its potential for wearable PONG fabrication. 44024


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ACS Applied Materials & Interfaces the applied electric field within the crystalline regions. This polarization response may be attributed to the combined effect of the piezoelectric β-phase acting as nanodimensional dipoles (nano-dipole) and the piezoelectret properties of microvoids in the SGNFW behaving as microdipoles under the applied electric field.39,46 It is well known that the aligned dipoles in the NFWs can create boundaries carrying the polarity charges at the nanofiber surfaces. Subsequently, a large amount of space charges are localized to form charged voids and electret dipoles between these polarized nanofibers, resulting in inhomogeneous surface charge distributions.39 Generally, charges are artificially injected into the macroscopic voids of porous polymers which subsequently assist to create the oriented “quasi-dipoles” known as electrets.45,46 In this case, large amount of charges are primarily being localized, caused by the fibrous structure of the SGNFW. In addition, the sugar incorporation influences the charge mobilization in the SGNFW and thereby assists heteropolarization inside the NFWs.39 Therefore, a charge separation has already occurred during nanofabrication of the SGNFW. Thus, this cooperative phenomenon from the piezoelectric β-phase and piezoelectret may result in a phase-lag of the loop under the sweeping of the electric field identified by a not perfectly closed hysteresis loop in Figure 4a.46,47 Figure 4c,d exhibits the dielectric permittivity (εr) and dielectric loss (tan δ) of the SGNFW and the PNFW as a function of frequency in the range of 1 kHz to 1 MHz at room temperature. The sugar incorporation assists the enhanced dielectric constant (εr ≈ 12 at 1 kHz) of the SGNFW in comparison to the PNFW (εr ≈ 6.2 at 1 kHz), whereas the dielectric loss is higher at low frequencies for the SGNFW (e.g., tan δ ≈ 0.04 at 1 kHz) compared to PNFW (e.g., tan δ ≈ 0.004 at 1 kHz). This is a well-known phenomenon that the dielectric constant of a material is related to the degree of polarization which mainly contributes to the enhancement of the dielectric constant at lower frequency, whereas at higher frequency, the molecules cannot follow the alternation of the electric field. On the other hand, the strong hydrogen bonding between the hydroxyl group of sucrose and the PVDF matrix (e.g., O−H and C−F) (scheme of Figure 1a) results in the highest dipole moment because dipole−dipole correlation is more prominent than its dipolar aprotic counterpart. Therefore, it is conclusive that this fact mainly contributes to the enhanced value of εr of the SGNFW. Piezoelectric Energy Harvesting Performance. To check the energy harvesting performance of a PONG based on the SGNFW, the PONG was impacted by a human finger (covered in a polyethylene glove) applying different compressive normal stresses (σa) periodically through pressing and releasing. The corresponding test bench setup is shown in Supporting Information, Note S1 and Figure S4. The harvested open-circuit voltage (Voc) of the PONG increases remarkably from 35 to 100 V when σa raises from 1 to 10 kPa (Figure 5a). In contrast, the Ref. PNG exhibits a smaller Voc of 12−43 V under the application of the same σa as shown in Figure 5b. The PONG easily converts the mechanical human finger touch into electricity under press-release motions (inset of Figure 5a). The average mechanical sensitivity (Sm) of the PONG can be calculated from the following equation

Sm =

ΔVoc Δσa

Figure 5. Open-circuit output voltage responses with different applied human finger impacts for the (a) PONG and (b) Ref. PNG. The enlarged marked area in (a) is shown in the lower inset while the upper inset shows the output voltages of the PONG as a function of applied pressure. (c) Output voltage response for the impact of drum beats of a toy; the enlarged marked area is shown in the middle inset, upper inset shows the demonstration, and lower inset shows a photograph of response. (d) Measured short-circuit output current response under the human finger impact of 10 kPa. (e) Dependences of output voltage and current on variable external load resistance with a schematic circuit diagram in the inset. (f) Instantaneous output power density of the PONG as a function of variable load resistance; the upper and lower insets show the glowing array of several LEDs (blue and green) by direct finger touch without using any external power source.

It is found to be 7.2 V (kPa)−1 in the pressure range of 1−10 kPa from the slope of the curve of Figure 5a (upper inset), which is superior compared to the previously reported PNGs.24,30,48,49 It implies that the hybridization of two piezoelectric materials greatly influences the output piezoelectric performance of NG devices. The superior energy harvesting performance of the PONG is attributed to the higher piezoelectric charge coefficient (d33), and as a result, it exhibits a superior piezoelectric figure of merit (FoM ≈ d33. g33 ≈ 10.23 × 10−12 Pa−1) than the Ref. PNG (FoM ≈ 7 × 10−12 Pa−1) where g3j can be calculated as, g3j =

d3j ε0εr

. Here, the

subscript “j” denotes the direction 1 (X-axis) or 3 (Z-axis) of induced strain of a three-dimensional (3D) coordinate system and ε0 is the permittivity of free space. The corresponding switching polarity test was performed by reversing the electrode connections to verify that the output voltage is governed by the piezoelectric properties of the PONG (Supporting Information, Figure S5).48,50 In this case, the same amplitude of output voltage with opposite polarization cancels out any additional contribution of artifacts or frictions.

(7) 44025


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Table 1. An Extensive Comparison between the Output Performances of Our Fabricated PONG and Those of PNGs Based on PVDF NFWsa name of additive NA NA NiCl2·6H2O NA NaNbO3 NA ZnO NA Ce3+/graphene MoS2 graphene−BaTiO3 ZnO sugar

type of additive

inorganic inorganic inorganic inorganic inorganic inorganic inorganic organic

applied force/frequency/pressure

device dimensions

device impedance

output voltage (V)

refs

2−4 Hz 10 Hz 950 ± 30 με 10 Hz, 10 N 1 Hz, 0.2 MPa 100 Hz 75 kHz 4 Hz 4 Hz, 8 N, 6.6 kPa 7 N, 8.8 kPa 2 Hz 145 Pa 10 kPa

d = 100−600 μm t = 140 μm, A = 2 cm2 t = 0.1 mm, A = 100 mm2 t = 100 μm, A = 2 cm2 t = 162 μm, A = 6.3 cm2 t = NF, A = 775 mm2 t = 120 μm, A = 4 cm2 t = NF, A = 6.3 cm2 t = NF, A = 1200 mm2 t = 150 μm, A = 93.5 cm2 t = 18−20 μm, A = 6.3 cm2 t = 150 μm, A = 82.7 cm2 t = 150 μm, A = 108 cm2

15 GΩ NF NF NF NF NF 41 MΩ ∼MΩ NF NF NF NF 13.5 MΩ

0.005−0.03 6.3 0.76 2.6 3.2 0.78 1.1 2.5 11 14 11 4.8 100

32 52 53 54 55 56 57 58 34 24 59 47 this work

a Here, t: thickness of the piezoelectric material, A: working area of the piezoelectric material, d: separation length between two electrodes, NF: not found, and NA: not applicable.

bridge-rectified instantaneous output voltage (VL) and current (IL) of the PONG are measured as a function of an externally connected load resistance (RL).24,34 It has been observed that VL across the resistance gradually increases with the change of RL accordingly and saturated at infinitely high resistance (∼30 MΩ) comparable to the open-circuit voltage (Figure 5e). The calculated IL is gradually decreased upon increasing RL (Figure 5e). The PONG is able to deliver a maximal instantaneous power density of 33 mW/m2 at RL of 15 MΩ, converting mechanical energy to electric energy taking into account the following equation

In addition, it also confirms that the piezoelectric output voltage is coming only from the SGNFW. Therefore, this is an indirect proof of dipole alignment which is in good agreement with the FT-IR spectra (Figure 2f).32,33 Additionally, for ruling out electrostatic contributions to the output electrical response of the PONG, a control device made only with a nonpiezoelectric material polydimethylsiloxane (without NFWs) keeping device dimension (area 12 × 9, thickness ≈ 150 μm) and other conditions unchanged was fabricated and tested, where indeed no considerable output voltage is observed (Supporting Information, Figure S6).48 Furthermore, the high sensitivity of the PONG is also demonstrated by attaching the device upon a drum of a toy and the respective output voltage response was recorded during the repeated drum beats (Figure 5c). It is remarkable that a significant output voltage of ∼5 V under the gentle imparting of drum beats of a toy is detected, opening the potential use of the PONG in real-life applications. Subsequently, the current measurements of the PONG showed a well-behaved periodic alternation of positive and negative peaks of short-circuit current (Isc) of about 6.2 μA during human finger imparting (10 kPa) (Figure 5d). The magnitude of the generated charge (Q) under the applied force, F ≈ 84 N, is calculated by the integration of a current pulse, that is, using the following equation Q=

∫ Isc dt

P=

Q F

(10)

The variation of P with RL is shown in Figure 5f. Thus, with this superior power density, the PONG is able to drive several blue or green LEDs separately without the need of any external battery (upper and lower insets of Figure 5f). The corresponding LED driving circuit and typical current−voltage (I−V) characteristic curve of the blue LEDs powered by the PONG are shown in Supporting Information, Figure S8. The latter one clearly indicates the threshold voltage and current requirement to light up the LEDs. Therefore, the power delivered by the PONG is well above the threshold requirement.24 Furthermore, a superior instantaneous piezoelectric energy conversion efficiency (ηpiezo) of the PONG up to 64% is also achieved (see Supporting Information, Note S2 and Figure S9, for details). Therefore, the PONG acts as an excellent power source under mechanical impact. The internal resistance (Ri) of the PONG is evaluated as 13.5 MΩ using the linear circuit theory (see Supporting Information, Note S3 and Figure S10, for details). The output power is found to be maximal approximately at Ri ≈ RL because of impedance matching between internal and external systems. To the best of our knowledge, the output performance of the PONG is much higher than that achieved with previously reported PNGs made with PVDF NFWs as shown in Table 1. Discussion of the Improved Piezoelectric Performance of the Hybrid PONG. The superior performance of the PONG could be attributed to the hybridization of sugar and PVDF. In sugar, glucose and fructose are connected via an oxygen atom (Figure 6a) where the cell dimensions are as follows

(8)

It evaluates Q ≈ 3192 pC (Supporting Information, Figure S7). Thus, the estimated magnitude of the piezoelectric coefficient is given by d33 =

VL 2 RL

(9)

which leads to the value of 38 pC/N.51 This result is in close agreement with the value calculated by the P−E loops (d33 ≈ 33 pC/N) using eq 4. Therefore, the axial strain ε achieved in the PONG by human finger application is 2.5 × 10−4, calculated from eq 5, using Y ≈ 40 MPa. The corresponding strain rate ε̇ is developed by 0.5% s−1 assuming that the PONG undertook compressive strain for a repetitive mechanical stress, using the relation ε̇ = 4fε × 100% s−1, where the average frequency of the axial stress is f ≈ 5 Hz.46 Furthermore, the 44026


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generation of piezoactive sugar is favored. Thus, the deformation of the overall giant crystal structures in the sugar−PVDF composite causes huge enhancement in the piezoelectric performance. Interestingly, when the PONG is subjected to vertical compressive pressing/releasing (Figure 6c), the crystal structure of the SGNFW is deformed creating a positive potential at the bottom electrode and a negative potential at the top electrode surface of the PONG. Thus, a huge potential difference between the top and bottom electrodes causes the electrons in the external circuit to flow back and forth, leading to the alteration of the piezoelectric charge polarization of Voc ≈ 100 V and Isc ≈ 6.2 μA with very a fast response time (5 ms) (Supporting Information, Figure S11). To get quantitative explanation on this piezoelectric mechanism exclusively, we further made theoretical simulations where compressive forces are applied in the direction normal to the plane constructed by the networked structure of the SGNFW. The simulation was conducted by the finite element method (FEM) using COMSOL multiphysics software to describe how the electromechanical interaction among nanofibers at microscale affects the resulting longitudinal polarization under compressive stress.3,49 It is mainly presented by six SGNFWs constructing a network-like structure and the piezovoltage distribution (colored z-axial scale bar) between two electrodes inside the deformed SGNFW under stress of σa ≈ 10 kPa (Figure 6d). A 3D geometry is required for the simulation in COMSOL where a single nanofiber is considered as a cylinder with a diameter of 125 nm. In this study, we consider isotropic bulk material properties of the SGNFW. The used material properties within the FEM simulation are given in the Supporting Information, Table S2. The model is meshed with a “physics-controlled mesh” and the element size “fine” is used. The meshed model is shown in Supporting Information, Figure S12. The detailed simulation process is described in Supporting Information, Note S4, Table S2, and Figure S12.3,49 The experimentally measured maximum piezopotential (∼100 V) was slightly lower than the simulated result of 118 V. This variation occurs probably because of the voltage drop from internal leakage paths and charge losses in the electrode−insulator junctions.60 The corresponding surface charge density and displacements distributions due to the applied stress are determined consecutively and are shown in Figure 6e,f, respectively. In an obvious manner, charge density differences between the top and bottom surfaces of the SGNFW exhibit potential differences as evident from the color bar, whereas the deformations are shown in the region of applied stress, resulting in attenuated displacements from its original position among the NFWs. The interaction behavior between structural and electrical properties of the PONG can be classically described by the following equations

a = 10.89 Å, b = 8.69 Å, c = 7.77 Å and β = 103°

Figure 6. (a) Molecular structure of sugar, that is, sucrose consisting of glucose and fructose and (b) typical shape of a sucrose crystal with assigned main planes indicated by arrow marks. (c) Schematic showing the working mechanisms under original, pressing, and releasing state of the PONG. Simulation results of the (d) piezovoltage distribution, (e) surface charge density distribution, and (f) deformation distribution (the solid black line indicates the original position before deformation) of the network-structured SGNFW at a constant external stress amplitude of 10 kPa.

Each unit cell consists of two sucrose molecules and most of the dipoles share approximately the same orientation via hydrogen−oxygen bonds between the molecules. The typical shape of sucrose single-crystals and the axes orientation are shown in the schematic of Figure 6b where the main planes are indicated by arrow marks. In the crystalline structure of sucrose, the molecule is folded and two additional hydrogen bonds to oxygen atoms are built within the molecule. The binding between the molecules in the crystal structure is also formed by hydrogen bonds to oxygen atoms.22 Thus, every sucrose molecule is interconnected with the adjacent molecules via several weak bonds in this way. It is noteworthy that these bonds between them are not stiff as the molecules itself are heavy. Therefore, any type of stress application easily deforms the crystalline lattice of sucrose and the stress-induced net charges can result in the variation of carrier transport in the single crystal of sugar. The plausible explanation behind the enhanced piezoresponse of the hybrid SGNFW composite is based on a combined effect of two piezoelectric materials with increased overall polarization where the single sugar crystal is attached to PVDF via electrostatic interaction/H-bonding. In other words, PVDF chains occupy the interstitial positions in sugar (and vice versa) where the −CF2 dipoles interact with H and the −CH2 dipoles with O. As a synergistic effect of this kind of interaction and intercalation, the improvement of electroactive crystalline phases in PVDF along with the

T = c ES + e TE

(11)

D = eS + ε0 εrS E

(12)

where T and cE are the stress tensor and the elasticity tensor, respectively, (e)ijk is the piezoelectric tensor, S is the mechanical strain, εrS is the dielectric tensor, ε0 is the permittivity in vacuum, and E is the electric field.61 As the piezoelectric SGNFW is anisotropic in nature, the piezoelectric polarization field (P) under a small uniform mechanical strain is given by 44027


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ACS Applied Materials & Interfaces Pi = (e)ijk (S)jk

(13)

Then, the calculated displacement current from the media polarization is JDi =

∂Pi i ∂S y = (e)ijk jjj zzz ∂t k ∂t { jk

(14)

which implies that the changing rate of the applied strain is directly proportional to the output current density of the PONG. Thus, the measured output piezoelectric performance of the PONG upon different applied strains strongly agrees with the above theory. Energy Harvesting Performance from Abundant Sources. Now, we demonstrate the piezoelectric power generation from normally wasted mechanical energy in the surrounding systems as our environment is full of abundant vibration (e.g., wind, acoustics, human movement, etc.). As a result, we can convert it into usable energy for operating electronic devices. Besides, the ability to differentiate multiple frequencies of vibrations is also critical for an efficient device performance. Therefore, at first, we started with wind vibration; the corresponding setup is shown in the scheme (Figure 7a). For exclusive optimization, we varied different depending parameters such as the distance from the wind blower to the PONG (60, 120, 176, 234, and 300 cm), speed of the wind-blower (1, 4, 8, 12, 16, and 20 m s−1), and effective area of the PONG (45, 63, 81, and 108 cm2). The respective real-time voltage signal responses from the PONG are shown in Figure 7b−d, and the insets show the voltages as a function of corresponding parameters, that is, distance, velocity, and effective area, respectively. It was expected that the variation of the distance affects the wind vibration-to-electric conversion. We observed a decreasing output voltage response and sensitivity with increasing distance (from 60 to 300 cm) between the wind blower and the PONG (Figure 7b). The speed of the blowing wind also affected the voltage output response of the PONG. When the velocity increased from 1 to 12 m s−1, the output voltage also increased from 0.5 to 3.5 V. Further increasing the speed of wind blowing (16−20 m s−1) led to a decrease in both sensitivity and voltage (Figure 7c). Also, the effective area of the PONG was found to be an important parameter for the output response when the PONG is placed in front of the wind vibration, keeping the distance and wind blowing speed fixed. The output voltage of the PONG showed a moderate increase with the increasing effective area (45−108 cm2) (Figure 7d). Apparently, it is evident that the voltage output profile and sensitivity of the PONG are greatly affected by the wind vibration influenced by different experimental parameters. This can be explained by the effect of the wind vibration upon the main piezoactive component of the PONG, that is, SGNFW. It is well-known that polymeric NFWs are flexible and thereby easy to vibrate under external mechanical waves. It is worthy to mention that the absorbed vibration in the NFW caused by an external source is mainly localized, and in this case, the nanofiber deformations differ to those resulted from compression or bending of the NFW as the mechanical deformation in the latter case spans the whole fibrous networked structure. Thus, the decreasing output response of the SGNFW-based sensor device with increasing distance of the wind blower could be attributed to the fact that the weak vibrational waves are damped in the surrounding medium. In contrast, the vibration

Figure 7. Vibrational response from PONG. (a) Schematic illustration of the wind blowing setup from an air blower and continuous data acquisition from a computer control system. The real-time voltage signal response of the PONG to wind blowing with (b) different distances, (c) different air velocities, and (d) different effective areas of the PONG (the corresponding insets show the voltages as a function of distance, velocity, and effective area, respectively). (e) Real-time voltage signal response during massage belt vibration (“off” and “on” conditions) from the PONG. The inset shows the photograph where the massage belt is attached to the human body. (f) Voltage signal in response to various musical instruments in operation (violin, piano, guitar, and kick drum). The inset scheme shows the corresponding setup in front of a sound speaker.

effect becomes higher with increased effective active area of the PONG exposed in front of the wind blower. However, the effect of the wind blowing speed is different as the output voltage response first increases to a certain point of external stimulation (12 m s−1) and beyond it decreases. Here, different effects such as turbulences in the stimulating wind or the disability of the active dipoles to follow the exciting vibrational frequency might be responsible. Then, we proceed with a real-life tool demonstration by a massage belt attached to human body parts and the PONG is fixed with the belt (inset of Figure 7e). The open-circuit voltage response is shown in Figure 7e during massage belt vibration “off” and “on” conditions, which implies the sensitivity and promising output performance of the PONG. Furthermore, the acoustoelectric conversion sensitivity of the PONG is demonstrated (Figure 7f) by different musical instruments (violin, piano, guitar, and kick drum) in operation where the PONG device is placed in front of a sound speaker connected to the instruments under the sound pressure label of 85 dB at a frequency of 130 Hz (scheme is shown in the inset of Figure 7f). The distinguishable output voltage patterns show the potential of the PONG as an acoustic sensor and as an 44028


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ACS Applied Materials & Interfaces energy harvester. Thus, the energy harvesting performance of the PONG from different abundant sources makes it a standalone device in a self-powered application system. Energy Storing Performance and Application in a Wireless System and Stability Test. Besides instantaneous energy generation, some applications effectively require fast charging or reaching a higher voltage at a certain time. In view of this practical requirement, we investigate the unique charging ability of the PONG by choosing capacitors as an energy storage system. Thus, three capacitors (e.g., 1, 2.2, and 4.7 μF) are connected to the PONG via a full-wave bridgerectifying circuit and charged them up via repeated applied stress (Supporting Information, Figure S13). Figure 8a shows

P=

CVs 2 2t

(15)

where C is the capacitance of the capacitor, Vs is the saturation voltage, and t is the time taken for the capacitor to reach the steady state (Figure 8b). Thus, the outstanding capacitor charging performances greatly promise to power up tiny portable electronics. Subsequently, for potential application of our organic NG, we used the PONG to power up a LCD screen from “off” state to “on” state from the stored energy in the 1 μF capacitor under the repeated applied stress of about 10 kPa via the bridge-rectifying circuit (mentioned in Experimental Section) without using any external power source (Figure 8b upper inset). Moreover, the PONG is also demonstrated to be useable as a power source for the wireless signal transfer of the pressure impact in terms of the applied voltage. The setup for this application can be arranged by integrating a transmitter [here an infrared (IR) LED is used] which actually transmits the signal generated by the PONG under pressure impact (∼2 kPa) and a receiver (here a photodiode is used) captures the transmitted signal. Finally, we measure the output signal by a digital oscilloscope (the circuit diagram is shown in Figure 8c). In particular, the PONGgenerated signal acted as a source that is possible to be detected wirelessly via photodetectors (Figure 8d). Thus, the PONG is expected to be suitable for a wireless signal transfer system in short scale, which is particularly useful in personal health care monitoring. Finally, the robustness of the PONG device has been tested by the stability of the electrical output to demonstrate its advantage and reliability. An average stable amplitude of the output voltage is observed during the continuous operation of 24 000 cycles (impacting the device at 3 Hz for ∼8000 s by a stepping motor under the pressure of 0.9 kPa) (Figure 8e). In addition, the long-term period (∼10 weeks) mechanical durability test shows less than ±6% of electrical output fluctuation without remarkable degradation, which strongly affirms that the environmental moisture does not affect the performance of the PONG any more (Figure 8f). Meanwhile, the nanofiber morphology of the SGNFW surface exhibits no significant distortion or serious damage after continuous impact for long-term period (10 weeks), as shown in the insets of Figure 8f. This validates the fatigue test of the PONG. Furthermore, we study the effect of different humidity conditions upon the piezoelectric performance of the PONG, keeping other conditions unchanged (Supporting Information, Figure S14). The corresponding results evidence no significant changes in the piezoelectric responses from the PONG under finger imparting (σa ≈ 10 kPa) in spite of increasing relative humidity. Thus, the outstanding robustness of the PONG enables long-term stability under harsh environment, which is considered to be a significant factor of organic NGs in mechanical energy harvesting applications. The superior performance and robustness of the PONG are primarily attributed to its all-fiber structure, which enables the enhanced collection of charges by the conducting fabric electrodes and thereby improved energy conversion efficiency of the device.

Figure 8. (a) Capacitor (1, 2.2, and 4.7 μF) charging response driven by the PONG and (b) the power stored in the respective capacitors. The upper inset shows that an LCD screen is powered up (“off” to “on” state) directly from the charged capacitor. (c) Schematic circuit diagram of wireless detection of the transmitter−receiver system driven by the PONG via an IR-LED and a photodiode. (d) Enlarged form of the transmitted signal from the receiver when the PONG drives the transmitter. (e) Fatigue test: open-circuit voltage recorded over time in response to continuous impacting over 24 000 cycles with frequency of 3 Hz under the pressure of 0.9 kPa. (f) Stability and durability test results of the PONG during 10 weeks. The inset FESEM images (initial and after 10 weeks) show that no obvious distortion or serious damage was observed on the SGNFW structure after continuous mechanical impact; scale bar corresponds to 1 μm (insets).

that the generated voltage of the PONG is rectified to charge the capacitors exponentially to reach the steady state. All of the three capacitors are charged under identical conditions, and the stored voltages increase exponentially and reached steady states in a very short time. The fast charging ensures the rapid energy supply ability of the PONG. The average power stored inside the capacitors can be calculated from the following equation

■

CONCLUSIONS In summary, we have demonstrated a fully packaged highperformance PONG based on natural sugar-introduced PVDF NFWs that can be used for enhanced electrical energy generation with superior power density for self-powered 44029


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component. Finally, the edges are encapsulated by adhesive tape to make the structure more compact. Then, electrical leads were connected to the top and bottom electrodes. Performance Characterizations. The piezoelectric performances of the PONG were recorded in terms of open-circuit output voltage using a digital storage oscilloscope (Tektronix TDS 2024C). A calibrated three-axial force pressure sensor (Flexi-Force A201) was placed underneath the PONG to directly record the applied pressure. The output voltages from the PONG under wind and continuous mechanical impact measurements were obtained using a National Instruments (NI) DAQ board (USB 6000) via LabVIEW interfacing with 1000 samples/s sampling rate. Short-circuit currents (Isc) were measured with a picoammeter (Keithley 6485). In the wireless data transfer system, a photo diode (BPW34) was used as a receiver and an IR LED (TSHG8400) was used as an IR transmitter. All of the measurements were performed in 20−30% humidity and 300 K temperature. To investigate the influence of the ambient humidity on the output performance of the PONG, the relative humidity was increased up to 60% in selected experiments.

multifunctional wearable electronics. Furthermore, the energy generation capability and sensitivity from abundant mechanical sources (such as wind, personal electronics, and acoustic vibration) are exclusively demonstrated (optimizing different depending parameters) besides its rapid energy storage capability (commercial available capacitors are charged up). The high performance is sufficient enough to power up several LEDs and an LCD screen without using any external battery besides its suitability in a wireless signal transmitting system. Finally, the mechanical durability and robustness of the PONG has been tested by observing the stable amplitude of the electrical output during continuous mechanical impact for a long-term period (∼10 weeks). Therefore, this organic NG provides an effective prospect for harvesting energy in selfpowered wearable electronics.

■

EXPERIMENTAL SECTION

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Materials. PVDF pellets (M̅ w ≈ 275 000, Sigma-Aldrich, USA), DMF, acetone (Merck Chemical, India), and sugar (Commercially available). Preparation of NFWs. The schematic of the preparation of NFWs is shown in Figure 1a,b. At first the PVDF solution was prepared by dissolving PVDF pellets in DMF/acetone (6:4) at a polymer/solvent concentration of 12% w/v under continuous stirring until a clear solution was obtained. Then, sugar was incorporated into the stock PVDF solution. The NFWs was prepared by using a typical electrospinning setup.24 Electrospinning was performed by placing 10 mL of the solution within a 15 mL commercially available plastic syringe (DISPOVAN) tipped with a 22 G stainless steel needle fed by using a syringe pump with a constant rate of 0.6 mL h−1. A positive high voltage (11 kV) was applied between the metal needle and the Al foil wrapped collector, where the needle-to-collector distance of 12 cm was adjusted. A relative humidity of 20−30% is maintained throughout the electrospinning process at room temperature. For the investigation of the influence of the humidity during the electrospinning, some SGNFWs were also prepared under elevated humidity conditions (R.H. up to 50/60%). Characterization. The crystalline phases of NFWs were identified by an FT-IR spectrometer in attenuated total reflection mode (A529P/QMIRacle-ATR-unit) (Pike), (TENSOR II, Bruker). XPS was performed by an instrument attached with a Mg Kα X-ray source and a semispherical electron analyzer (Leybold-Heraeus). The crystallographic properties of the NFWs were studied using XRD patterns measured by an X-ray diffractometer (Bruker, D8 ADVANCE). The surface morphologies and diameters of the NFWs were studied with a field emission scanning electron microscope; INSPECT F50 and HRTEM; JEOL, JEM-2100. The chemical composition and elemental mapping were obtained by using EDXS spectra recorded by using a Bruker Nano X-flash detector (410M) equipped with a FE-SEM chamber. The stress−strain measurements were performed by a universal testing machine (Tinius Olsen H50KS) to study the mechanical properties of the NFWs where four specimens were tested for each NFW. The polarization (P)−electric field (E) hysteresis loops were acquired from the ferroelectric testing system (P−E, PLC100V, Radiant Technology Precision) at room temperature. The dielectric properties were studied using a precision impedance analyzer (Wayne Kerr, 6500B) at room temperature. Fabrication of the PONG. The PONG was basically fabricated via a simple sandwich-like structure. The fabrication begins with the SGNFW placed in the middle as a piezoelectric component between two electrodes. Copper (Cu)−nickel (Ni)-plated fine knit polyester fabric (Coatex Industries, India) with surface resistance of 0.05−0.1 Ω/cm2 was used for top-bottom electrodes as shown in Figure 1e,f. The conducting fabric was cleaned several times in ethanol, acetone, and deionized water to avoid any unwanted dust. After placing the textiles, the whole structure (top electrode/SGNFW/bottom electrode) was tightly sandwiched using a roller machine ensuring that there was no air gap between the electrode and the piezoactive

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b15320.

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FT-IR spectra of SGNFW recorded under different humidity conditions; FE-SEM images of SGNFW prepared at different humidity conditions; comparison table of present state-of-the-art device performance of the PNGs in terms of nanofiber orientation; XRD patterns of the SGNFW in the range of 2θ ≈ 30°−60°; plot of polarization versus electric field (P−E) and current density versus electric field (J−E) of the PNFW; current density versus electric field (J−E) of the SGNFW; strain (S) versus P2 plot of the SGNFW; custom-built test bench for the PONG output characterizations; open-circuit output voltage responses of the PONG by reversing the electrode connections; reverse connection condition of the PONG; output voltage from the control device prepared in analogy to the PONG setup and preparation and its enlarged view of the marked region; short-circuit current of the PONG and its enlarged view of the selected positive peak; several LEDs driving circuit (connected in series connection) and its respective I−V characteristic of powering up by the PONG; recorded output voltage response at an external load of 15 MΩ and square of the output voltage; plot of the 1/VL versus 1/RL and its linear fit to calculate the internal resistance of the PONG; enlarged view of the open-circuit voltage in one cycle and response time (τR) of the PONG; table providing material properties of the SGNFW for FEM simulation; 3D FEM model for simulation after meshing; capacitor charging circuit of the PONG; and open circuit output voltage responses of the PONG under different humidity conditions (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. ORCID

Dipankar Mandal: 0000-0003-2167-2706 Notes

The authors declare no competing financial interest. 44030


Research Article


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Efficient, Flexible Piezoelectric PZT Thin Film Nanogenerator on Plastic Substrates. Adv. Mater. 2014, 26, 2514−2520. (18) Alam, M. M.; Ghosh, S. K.; Sultana, A.; Mandal, D. Lead-free ZnSnO3/MWCNTs-Based Self-poled Flexible Hybrid Nanogenerator for Piezoelectric Power Generation. Nanotechnology 2015, 26, 165403−165408. (19) Huang, C.-T.; Song, J.; Lee, W.-F.; Ding, Y.; Gao, Z.; Hao, Y.; Chen, L.-J.; Wang, Z. L. GaN Nanowire Arrays for High-Output Nanogenerators. J. Am. Chem. Soc. 2010, 132, 4766−4771. (20) Kholkin, A. L.; Pertsev, N. A.; Goltsev, A. V. Piezoelectricity and Crystal Symmetry. In Piezoelectric and Acoustic Materials for Transducer Applications; Safari, A., Akdogan, E. K., Eds.; Springer: New York, 2008. (21) Alam, M. M.; Mandal, D. Native Cellulose Microfiber-based Hybrid Piezoelectric Generator for Mechanical Energy Harvesting Utility. ACS Appl. Mater. Interfaces 2016, 8, 1555−1558. (22) Beevers, C. A.; McDonald, T. R. R.; Robertson, J. H.; Stern, F. The Crystal Structure of Sucrose. Acta Cryst. 1952, 5, 689−690. (23) Dittmar, J. H. Hygroscopicity of Sugars and Sugar Mixtures. Ind. Eng. Chem. 1935, 27, 333−335. (24) Maity, K.; Mahanty, B.; Sinha, T. K.; Garain, S.; Biswas, A.; Ghosh, S. K.; Manna, S.; Ray, S. K.; Mandal, D. Two-Dimensional Piezoelectric MoS2-Modulated Nanogenerator and Nanosensor Made of Poly(vinlydine Fluoride) Nanofiber Webs for Self-Powered Electronics and Robotics. Energy Technol. 2017, 5, 234−243. (25) Lovinger, A. J. Ferroelectric Polymers. Science 1983, 220, 1115−1121. (26) Ince-Gunduz, B. S.; Alpern, R.; Amare, D.; Crawford, J.; Dolan, B.; Jones, S.; Kobylarz, R.; Reveley, M.; Cebe, P. Impact of Nanosilicates on Poly(vinylidene fluoride) Crystal Polymorphism: Part 1. Melt-Crystallization at High Supercooling. Polymer 2010, 51, 1485−1493. (27) Martins, P.; Lopes, A. C.; Lanceros-Mendez, S. Electroactive Phases of Poly(vinylidene fluoride): Determination, Processing and Applications. Prog. Polym. Sci. 2014, 39, 683−706. (28) Sencadas, V.; Gregorio, R.; Lanceros-Mendez, S. α to β Phase Transformation and Microestructural Changes of PVDF Films Induced by Uniaxial Stretch. Macromol. Sci., Part B: Phys. 2009, 48, 514−525. (29) Davis, G. T.; McKinney, J. E.; Broadhurst, M. G.; Roth, S. C. Electric-field-induced Phase Changes in Poly(vinylidene fluoride). J. Appl. Phys. 1978, 49, 4998. (30) Garain, S.; Sinha, T. K.; Adhikary, P.; Henkel, K.; Sen, S.; Ram, S.; Sinha, C.; Schmeißer, D.; Mandal, D. Self-Poled Transparent and Flexible UV Light-Emitting Cerium Complex−PVDF Composite: A High-Performance Nanogenerator. ACS Appl. Mater. Interfaces 2015, 7, 1298−1307. (31) Chen, S.; Yao, K.; Tay, F.; Liow, C. L. Ferroelectric Poly(vinylidene fluoride) Thin Films on Si Substrate with the Phase Promoted by Hydrated Magnesium Nitrate. J. Appl. Phys. 2007, 102, 104108. (32) Chang, C.; Tran, V. H.; Wang, J.; Fuh, Y.-K.; Lin, L. DirectWrite Piezoelectric Polymeric Nanogenerator with High Energy Conversion Efficiency. Nano Lett. 2010, 10, 726−731. (33) Mandal, D.; Yoon, S.; Kim, K. J. Origin of Piezoelectricity in an Electrospun Poly(vinylidene fluoride-trifluoroethylene) Nanofiber Web-Based Nanogenerator and Nano-Pressure Sensor. Macromol. Rapid Commun. 2011, 32, 831−837. (34) Garain, S.; Jana, S.; Sinha, T. K.; Mandal, D. Design of In Situ Poled Ce3+-Doped Electrospun PVDF/Graphene Composite Nanofibers for Fabrication of Nanopressure Sensor and Ultrasensitive Acoustic Nanogenerator. ACS Appl. Mater. Interfaces 2016, 8, 4532− 4540. (35) Liu, X.; Ma, J.; Wu, X.; Lin, L.; Wang, X. Polymeric Nanofibers with Ultrahigh Piezoelectricity via Self-Orientation of Nanocrystals. ACS Nano 2017, 11, 1901−1910. (36) Huang, S.; Yee, W. A.; Tjiu, W. C.; Liu, Y.; Kotaki, M.; Boey, Y. C. F.; Ma, J.; Liu, T.; Lu, X. Electrospinning of Polyvinylidene Difluoride with Carbon Nanotubes: Synergistic Effects of Extensional

ACKNOWLEDGMENTS This work was financially supported by a grant from the Science and Engineering Research Board (SERB/1759/201415), Government of India. K.M. is thankful to the University Grants Commission (UGC), Government of India for providing his fellowship (no. P-1/RS/319/14). S.G. was partially supported by German Federal Ministry for Economic Affairs and Energy (BMWi, ZIM, 16KN033522).

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Natural Sugar-Assisted, Chemically Reinforced, Highly Durable

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