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Design of In-situ Poled Ce3+ Doped Electrospun PVDF/ Graphene Composite Nanofibers for Fabrication of Nanopressure Sensor and Ultrasensitive Acoustic Nanogenerator Samiran Garain, Santanu Jana, Tridib Kumar Sinha, and Dipankar Mandal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11356 • Publication Date (Web): 01 Feb 2016 Downloaded from http://pubs.acs.org on February 1, 2016

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ACS Applied Materials & Interfaces

Design of In-situ Poled Ce3+ Doped Electrospun PVDF/Graphene Composite Nanofibers for Fabrication of Nano-pressure Sensor and Ultrasensitive Acoustic Nanogenerator Samiran Garain,† Santanu Jana,†,‡ Tridib Kumar Sinha,§ and Dipankar Mandal*,† †

Organic Nano-Piezoelectric Device Laboratory, Department of Physics, Jadavpur University, Kolkata

700032, India ‡

Department of Electronics, Netaji Nagar Day College, 170/436 N. S. C Bose Road, Kolkata 700092,

India §

Materials Science Centre, Indian Institute of Technology (IIT), Kharagpur 721302, India

KEYWORDS: Ce3+ doped PVDF/graphene nanofiber, piezoelectric generator, ultrasensitive, acoustic nanogenerator, mechanical energy harvester ABSTRACT We report an efficient, low-cost in-situ poled fabrication strategy to construct a large area, highly sensitive, flexible pressure sensor by electrospun Ce3+ doped PVDF/graphene composite nanofibers. The entire device fabrication process is scalable and enabling to large-area integration. It can able to detect imparting pressure as low as 2 Pa with high level of sensitivity. Furthermore, Ce3+ doped PVDF/graphene nanofiber based ultrasensitive pressure sensors can also be used as an effective nanogenerator as it generating an output voltage of 11 V with a current density ∼6 nA/cm2 upon repetitive application of mechanical stress that could lit up 10 blue light emitting diodes (LEDs) instantaneously. Furthermore, to use it in environmental random vibrations (such as wind flow, water fall, transportation of vehicles, etc.), nanogenerator is integrated with musical vibration that exhibits to power up three blue LEDs instantly that promises as an ultrasensitive acoustic nanogenerator (ANG). The superior sensing properties in 1 ACS Paragon Plus Environment


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conjunction with mechanical flexibility, integrability and robustness of nanofibers enabled realtime monitoring of sound waves as well as detection of different type of musical vibrations. Thus ANG promises to use as an ultrasensitive pressure sensor, mechanical energy harvester and also effective power source for portable electronic and wearable devices. INTRODUCTION Highly sensitive, cost-effective, flexible and lightweight pressure sensors hold an essential position in the development of artificial sensing system, robotic component and lab-on-a chip healthcare monitoring. Renewable and non-exhaustible resources energy such as wave power and wind has rekindled considerable intensive interest to prevent the global warming and resources dwindling. Devices that exploit mechanical motions as natural sources of power can be particularly valuable. Therefore, human-motion-based energy harvesting has attracted tremendous interest due to growing trends of portable smart electronics that sought for a selfsufficient and sustainable power source. Thus scalable and integrated nanogenerators (NGs) for portable, embedded and wearable electronic self-powered nanosystems has become more technologically feasible due to the advancement of extremely low power consumption in tiny nano/micro-electronic based devices.1,2 NGs based on piezoelectric materials including ZnO, BaTiO3, PbZrxTi1-xO3, (K,Na)NbO3, ZnSnO3, CdS and piezoelectric polymers have been developed so far those exhibit promising energy harvesting performances.3-12 In contrast, electrospun poly(vinylidene fluoride) (PVDF) and its copolymers such as poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), Poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) nanofibers (NFs) have been widely adopted to fabricate NGs due to the high energy conversion efficiency and efficient pressure sensing properties with respect to its film counterpart.10,11,13 In addition, for the development of PVDF film based energy harvester, mechanical stretching followed by external electrical poling under high electric field (as high as 2 ACS Paragon Plus Environment

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100 kV/mm) and elevated temperature is mostly adopted techniques that needs special precautions and high degree of power consumption.2,14-16 The electrical breakdown and inhomogeneous surface morphology is another major hurdle due to leakage current under high electric field and mechanical stretching respectively. The electrospinning process produces a high degree of crystallinity and aligns the molecular dipoles during the electrospinning stage resulting the in-situ polarization unlike in processes such as film drawing and fiber melt extrusion where a separate poling stage is needed.2,11,14,17 The applied electric field in electrospinning techniques are known to induce the phase transition from non-polarized α-phase to polarized β-phase in PVDF and also induce polarization in the as-spun mats that produces a strong piezoelectric response, thus post electrical poling and mechanical drawing steps is not necessary.11,13 Regardless of power up the portable electronic devices, the piezoelectric output from NG has been demonstrated to directly drive electrochemical reactions including lithium ion intercalation, pH sensing, and electrochemical water splitting.18-20 Such piezoelectric power generation aims to capture from the normally wasted mechanical energy surrounding a system and converts it to usable energy for operating personal electronic devices. However, the mechanical energy sources used in most of the previous attempts are artificially supplied (e.g., continuous pressure imparting probe, mechanical shaker, electronic hammer, etc.) those are hard to supply continuously. In contrast, the sound (noise or even music) that always exists in our everyday life and environment that usually overlooked as a source for piezoelectric power generation despite the fact that it is a form of mechanical energy. There should be a way to turn sound energy from speech, music or noise into electrical power with NGs. To improve the performance of NFs based NGs, particularly the electro-mechanical coupling of PVDF and its copolymer, a number of carbon based materials, such as carbon nanotubes (CNTs),

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carbon black (CB) and graphene have been proposed to alter the polymer microstructure and promote piezoelectricity.15,21,22 Among them, graphene has a two-dimensional sheet of sp2-bonded carbon atoms packed in a honeycomb crystal lattice. It possesses attracted considerable attention because of its extraordinary flexibility, high aspect ratio, large specific surface area (2600 m2/g), excellent mechanical strength, and good electrical and thermal conductivities.23,24 Graphene has been widely used to prepare graphene˗polymer composites for enhancement of mechanical and thermal properties of the polymer nanocomposites, energy generation efficiency and storage properties.22,23 It has been considered as a promising material for next generation electronics. Various chemical methods have been used to modify graphene oxide (GO) for graphene-polymer composites with high dielectric constant and improvement of piezoelectric property, such as graphene-TiO2

nanorod

hybrid

nanostructures,

hyperbranched

aromatic

polyamide-

functionalized graphene sheets, graphene oxide-encapsulated carbon nanotube hybrids and PVDF-graphene nanocomposite.16,22-26 It was also found that the addition of graphene can induce nucleation of the piezoelectric β-phase in PVDF, improving the piezoelectric properties after drawing and a step-wise poling method.16 Furthermore, recent studies have shown that large scale continuous synthesis of high-quality graphene is also possible in cost effective manner and it demonstrates better biocompatibility than other carbon based materials, making it significantly more attractive for commercial and industrial applications.27,28 Several research groups have attempted to improve the piezoelectric energy harvesting performance by stabilizing the electroactive phases in PVDF with fillers such as Fe doped rGO, graphene, MWCNTs, grapheneCuS, nanoclay, graphene-ZnO, and PMMA-RGO, particularly as a free standing film.22,29-32 However, to date, there is limited progress on the study of piezoelectric properties of graphene


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based electrospun PVDF NFs, particularly for designing the pressure sensors and NGs. To our best knowledge, no relevant study has been reported so far where Ce3+ doped PVDF/graphene composite NFs are utilized as a piezoelectric energy harvesting material. In this work, we demonstrate an in-situ poled ultrasensitive pressure sensor via electrospinning process to massively fabricate composite NFs, where few monolayer graphene sheets are encapsulated in the Ce3+ doped PVDF matrix. The piezoelectric properties of these NFs, as affected by the influence of graphene are studied through a series of vibration tests on NGs. It exhibited the ultra-high pressure sensing properties, even at exceptionally small value such as 2 Pa that resembles to use as a nano-pressure sensor (NPS). Under an external pressure of 6.6 kPa, which is comparable to the peak plantar pressure from a normal human finger touch, we obtained a stable and high open-circuit peak voltage of 11 V and a current density of 6 nA/cm2. While the power output reaches the optimized output power of 6.8 µW at matched resistance of 1 MΩ. Besides this, NG is harvesting sound wave energy, which is capable of delivering a maximum output voltage of 3 V under a sound pressure level (SPL) of 88 dB makes it suitable to perform as an acoustic nanogenerator (ANG). Thus fabrication of flexible ANG from large-area Ce3+ doped PVDF-graphene composite NFs based NG may open up an important avenues to applications in acoustic energy harvesting. EXPERIMENTAL SECTION Materials Poly(vinylidene fluoride) (PVDF) pellets (Mw ≈ 275 000, Sigma-Aldrich, USA), N,Ndimethylformamide (DMF), (Merck Chemicals, India), laboratory synthesized graphene.33 (corresponding Raman spectrum analysis is given in the Supporting Information, Figure S1 and



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text S1) and ammonium cerium sulfate dihydrate (CAS) ((NH4)4Ce(SO4)4·2H2O, Alpha-Aesar, USA), Ni-Cu plated polyester fabric (DE2-280C, EMS Inc., Korea). Preparation of Ce3+ Doped Electrospun PVDF/Graphene Nanofiber (NFs) To prepare spinning solution, the solvent has been made using a mixture of DMF and acetone (6:4, v/v). PVDF and CAS were first immersed in the mixed solvent by stirring for 4 h and a complex

is

formed

with

DMF,

namely

Cerium(III)-N,N-dimethylformamide-bisulfate

[Ce(DMF)(HSO4)3] (Ce3+ complex).12 Then the graphene was added and continuing stirring for 6 h at 60 oC. The resulting (PVDF-Ce-G) solution was placed into an ultrasonic bath for 30 min to make the homogeneous dispersions in prior to electrospinning. The concentrations of PVDF, CAS and graphene were 12, 0.2 and 1 wt%, respectively, with respect to the mixed solvent. Another solution was also prepared where graphene was not added, for performing reference experiment in similar way designated as PVDF-Ce solution. The above solutions were filled into a 10 mL hypertonic syringe with a diameter of 0.8 mm needle. The PVDF-Ce and PVDF-Ce-G electrospun nanofibers (NFs) were fabricated by using commercial electrospinning equipment. All nanofibers were electrospun under a high voltage of 12 kV, and the needle tip was located at a distance of 10 cm from the aluminum foil covered plate collector. The syringe pump was applied to feed solutions to the needle at a rate of 0.8 mL/h. In order to obtain the stable NFs, fresh composite electrospun NFs mats (shown in Figure 1d,e) were allowed to dry at 60 oC for 12 h to remove the remnant solvents for the characterization and device fabrications. We fabricated a simple, low-cost, light weight and large area (~30 mm × 40 mm) hybrid composite NG, using Ce3+ doped PVDF/graphene composite NFs to use as an ultrasensitive nano-pressure sensor, mechanical energy harvester and ANG or vice-versa. A simple


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electrospinning process was used to fabricate the composite NFs, this technique is applicable to largescale NG fabrication for real-world applications. To explore the feasibility of hybridizing the composite materials, we demonstrated randomly oriented electrospun NFs of graphene and Ce3+ doped PVDF in a weight ratio of 1 : 12 to imbue of ANG with adequate flexibility by electrospinning technique. Characterization. X-ray diffraction (XRD) patterns and the crystalline phases of the composite NFs were obtained by X-ray diffractometer (Bruker, D8 Advance) with a Cu Kα (λinc.~1.54 Å) radiation source under an operating voltage and current of 40 kV and 40 mA, respectively. The crystalline phases of the composite NFs were further confirmed by Fourier transform infrared spectroscopy (FT-IR) (Shimadzu, FTIR- 8400S) results. The morphologies and diameters of the as-prepared composite NFs were observed by a field emission scanning electron microscopy (FE-SEM, INSPECT F50) operated at an acceleration voltage of 20 kV and high resolution transmission electron microscope (HRTEM, Hitachi-H600) operated at 40 kV and 40 mA. The Raman spectra of the prepared graphene was acquired using MODEL T64000 (Jobin Yvon Horiba) spectrometer with an Argon-Krypton mixed ion gas laser (~514 nm). NGs (those also use as a pressure sensor and ANG) were fabricated by simply pasting Ni-Cu plated polyester fabric as electrodes (30 mm × 40 mm) onto both sides of the NFs mat (thickness ~3.5 µm). Open-circuit output voltages were recorded using a digital storage oscilloscope (Agilent, DSO3102A). The capacitor charging performance was employed via a typical rectifier bridge circuit unit.8



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RESULTS AND DISCUSSION Crystallographic Phase Identification For the crystallinity characterization of the electrospun NFs and related polarized phases, the spectroscopic evidence for the effective transformation from paraelectric to a high fraction of induced electroactive β- and γ-phase had been evaluated from the curve deconvoluated XRD pattern (Figure 1)12,34 In the electrospun NFs, the α-characteristic diffractions (17.6o, 18.2o and 19.7o) are diminished, whereas the peaks from the electroactive β- and γ-phase (18.7°: γ(202), 20.3°: γ(110) and 20.8o: β(110/200)) are appeared (Figure 1a,b).34,35 The curve deconvoluated diffraction peaks at 18.7° (202) and 20.3° (110) are attributing the presence of lower content of γ-phase with the coexistence of the major β-phase content as revealed from its sharp diffraction peak at 20.8° (110/200).35,36 As graphene was added, the broad shoulder diffraction at 20.3o slightly decreased and only the diffraction peak area of the β crystals (20.8o) was enhanced. Thus, in the case of PVDF-Ce-G NFs, the β-phase is predominantly crystallized (Figure 1b) due to the specific interaction between PVDF and graphene layer.22 During the process of electrospinning, the PVDF molecular chains are uniaxially stretched by high electrical forces, resulting in-situ orientation of the −CH2/−CF2 dipoles of PVDF NFs.11,13,37 The degree of the total (

ct

), β- (

cβ

) and γ-crystallinity (

cγ

) are calculated and labeled in

Figure 1a,b (detail calculation is shown in the Supporting Information, S2). Soin et al. and Abolhasani et al. reported that

ct

of neat electrospun PVDF NFs is ∼ 53 %.9,38 Whereas the

incorporation of graphene visibly improves the

ct

of PVDF (Figure 1b), which directly

influences the material properties.39 The increased crystallinity is probably because graphene can efficiently restrict and order the PVDF chain arrangement (defined as a ‘‘molecule movement 8 ACS Paragon Plus Environment

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restriction’’ effect), due to its super mechanical strength (Figure S2, Supporting Information).40 Furthermore, the resulting content of the piezoelectric β-phase is also found to be higher (i.e.,

cβ

∼ 46%) than that of the non-graphene added NFs (i.e.,

cβ

∼ 42%). Therefore, it is expected

that the PVDF-Ce-G NFs should exhibit the better piezoresponse, as it is directly proportional to the β-crystallinity and degree of dipole orientations.11 To further identify the crystal structures present in the electrospun NFs, FT-IR measurement is another useful method to distinguish different crystalline forms of PVDF, as shown in Figure 1c. The piezoelectric β-phase (TTTT) can be well-characterized from the band at 1276 cm-1, whereas the lower content of semipolar γ-phase (TTTG) is evident from the peak broadening at 1236 cm1 34-36

.

Generally, in most cases, the absorption bands of the β- and γ-phases of the PVDF are

superimposed due to common TTT in the chain conformation giving rise to some common vibrational bands such as at 841 and 510 cm-1 (Figure 1c).22,36,39 Thus, FT-IR results are in agreement with those obtained by XRD analysis. The absorption intensity of 841 cm-1 band is designated to quantify the relative proportion of electroactive phases, i.e., FEA assigns both βand γ-phases (text S3, Supporting Information).41 A FEA value of 96 % is achieved at very small amount of CAS salt (0.02 wt %) and then it approaches 99 % in PVDF-Ce-G NFs due to the presence of graphene. The probable interaction between the Ce3+ complex and the –CH2–/–CF2– dipoles of PVDF leads to the formation of the electroactive phase. Furthermore, –CH2– dipoles are attracted to the delocalized π-electrons present in graphene, resulting in the formation of high fraction of the electroactive phase (Figure S2, Supporting Information).22 Usually, in order to largely enhance the β-phase in PVDF, mechanical stretching and electrical poling processes are indispensable to directionally align the dipoles in the β-crystalline PVDF structures. Thus, both



these trivial process are automatically undertaken in electrospinning process, so in situ poling of the piezoelectric β-phase is conclusively be realized.11,13

(a) ∼ 53 %

∼ 42 %

Intensity (a.u.)

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γ ∼ 11 %

(b) ∼ 56 %

(d)

∼ 46 %

(e)

γ ∼ 10 %

Figure 1. XRD patterns and their curve deconvolution of (a) PVDF-Ce, (b) PVDF-Ce-G NFs. The dotted points are experimental data and the solid lines correspond to the best curve fit. (c) FT-IR spectra of PVDF-Ce, and PVDF-Ce-G NFs in the wavenumber region from 1600 to 400 cm-1. Photographs of the large piece (d) PVDF-Ce and (e) PVDF-Ce-G NFs prepared via electrospinning (10 × 13 cm).


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Morphological Analysis PVDF plays a crucial role in this device, not only as a piezoelectric, but also by restricting aggregation between the graphene sheets. FE-SEM images of the electrospun PVDF composite NFs are shown in Figure 2a,b. The surface of the prepared NFs matrix are relatively smooth, and the additive (graphene and CAS) mass or lump are not seen in the surface. Due to the carbonbased structure of graphene and PVDF polymer, the graphene was well dispersed in the polymer matrix and agglomeration on the surface was not observed. In addition, no cracks were detected on the surface, indicating that the NFs did not become brittle by the addition of the graphenebased material and that they had proper stability (Figure 2b).42 The as-spun PVDF-Ce and PVDF-Ce-G NFs mats were made from randomly oriented NFs with diameter of ∼85 nm and ∼80 nm respectively, as shown in inset of Figure 2a,b which is controllable with the electrospinning conditions. The dispersed graphene sheet could be easily charged when a high voltage was applied to the spinning solution, leading to increases of electrostatic repulsion and Coulombic force on the Taylor cone, which caused the fiber diameters to decrease in PVDF-CeG NFs mats. Figure 2c shows TEM images of the PVDF NFs containing cerium and graphene. It should be noted that these fibers are drawn for 2 min on carbon coated copper grid keeping rest of the parameters unchanged, indicating best suitable sample preparation technique for electrospun fibers. It is evident from the TEM images that individual graphene and cerium components are well dispersed in the NFs matrix and most of the graphene sheets embedded in the NFs without aggregation. The high-resolution TEM micrograph (inset of Figure 2c) shows an inclusion of graphene sheet in the middle part of the fiber and alignment with the fiber direction. The PVDF easily absorbs on the surface of graphene, resulting the formation of the graphene embedded



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PVDF NFs and then the PVDF dendrimer generated on the surface of graphene may help to avoid the restacking of graphene sheets.16,43 Guided by the random distribution of PVDF electrospun fibers, the decorated graphene sheets construct a continuous network into the NFs. This result indicates that the graphene sheets are uniformly embedded onto the PVDF NFs. PVDF layer is finely dispersed on the surface of graphene sheet. In contrast, solution casting graphene/PVDF composites film, the conductive fillers are tend to aggregate into clusters.44 Thus, the morphology of PVDF/graphene NFs is expected to affect the piezoelectric properties and hence they are of particular interest in this study. The slightly stretched black regions (Figure 2d) are indicative of the scrolls, folds and lattice fringes of the graphene sheets into the PVDF NFs matrix.43 The interface between the graphene sheet and the PVDF confirmed the graphene sheet is closely attached to the PVDF.


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(a)

(b)

(c)

(d)

Figure 2. FE-SEM images of (a) PVDF-Ce and (b) PVDF-Ce-G NFs. In the inset, the statistical size distribution of the corresponding NFs. (c) TEM images and a higher-resolution image (inset) of PVDF-Ce-G NFs and (d) the interface between PVDF and graphene layers. SAED pattern is shown in the inset. The selected area electron diffraction (SAED) pattern from the PVDF and graphene (inset Figure 2d) reveals that the graphene lattices are present.45 The blurry rings are attributed to the polycrystalline and amorphous PVDF covered the graphene surfaces.46 The SAED pattern showed that the graphene sheets exhibited the hexagonal diffraction pattern with sharp 13 ACS Paragon Plus Environment


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diffraction spots (Figure S3, Supporting Information), indicating the crystallinity of the graphene sheet (inset in Figure 2d).28 The intensity of the inner diffraction sports are stronger than outer sports that further affirms that the few monolayer character of the graphene sheet.43,47 Performance of NPS and ANG PVDF-Ce-G NFs enable precise pressure sensing, at a level suitable for detecting the touch of an extremely lightweight object. To evaluate the sensitivity quantitatively of the piezoelectric nanopressure sensor (NPS), the soft thermocols with different size (inset of Figure 3a) are dropped from a fixed distance of 5 cm of pressure to the ANG, while the electrical response was measured. Figure 3a highlighted a well-behaved, linear variations of output voltages due to different effective pressures (e.g., 2, 3.6 and 10 Pa). It indicates the potential to use as an ultrasensitive pressure sensor. The output performance under the frequency of ∼4 Hz by simply applying a repeating pushing force and pressure of 8 N and of 6.6 kPa (text S4, Supporting Information) respectively (Figure 3b), to the top of the ANG in the vertical direction using a bare human finger covered in a polyethylene (PE) glove. As a promising outcome, the ANG containing 1 wt% of graphene in Ce3+ doped PVDF exhibited an output voltage of 11 V, whereas 4.5 V is observed in ref ANG (PVDF-Ce) made of Ce3+ doped PVDF (Figure 3b). When a strain is applied to the flexible ANG, the NFs is subjected to tensile strain and also the crystal structures of the composite NFs were deformed, resulting strong enhancement in piezo-response by interchanging the deformed structure to stable one or vice-versa. Then, piezoelectric potential difference is induced between two electrode ends of piezoelectric NG, where the “±” signs represent the polarity of the local piezoelectric potential created inside the NFs.42 If the ANG is subjected to a vertical compressive strain, a positive piezoelectric potential is developed at the bottom electrode of the ANG and an opposite negative potential is set up at the top electrode


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surface. As the strain is released, the piezoelectric potential immediately diminishes and the accumulated electrons near the electrode flow back through the external circuit and an electric signal is observed in opposite direction.48 The output performance of the ANG was seen to also influenced upon squeezing repeatedly by using human finger (Figure S4, Supporting Information). However it results in lower output voltage due to bending and releasing strain that is antiparallel with the –CH2–/–CF2– dipole orientations.49 Owing to the preferential orientations of the electroactive –CH2–/–CF2– dipoles along the thickness direction, i.e., the perpendicular to the fiber length, tensile strain developed in the length direction during the squeezing process leads to the lower change in dipole moment.11 It is apparent that the utilization of the graphene in the nanofibers resulted in better piezoelectric throughput performance (see the comparison of output voltage of ANG made with PVDF-Ce and PVDF-Ce-G in Figure 3b). This improvement attribute to the contribution of the graphene layer embedded intermittently into the Ce3+ doped PVDF NFs.16 High-quality crystal structure and presence of delocalized π-electrons remarkably enhance the electronic properties of graphene.24 The layered structure composed of a carbon network of hexagonal rings on the basal planes,43 the electrons are induced to flow throughout the graphene network under the driving force of the change in piezoelectric potential, which produces the enhanced voltage response. Thus PVDF-Ce-G ANG with this high output performance could operate various self-power generated electric devices using an integrated rectifier bridge circuit system. We could turn on 10 blue LEDs immediately (Video file V1, Supporting Information) with a series connection upon human finger imparting, that shows graphene added ANG could be suitable as a real-time power source for mobile electronic devices.



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(a)

(b)

(c)

(d)

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Figure 3. (a) The output voltage response from the PVDF-Ce-G at different applied pressures (2, 3.6 and 10 Pa). Pressures applied at the upper surface of the ANG using cubical blocks made with thermocol of different sizes, illustrated in the inset. (b) Open-circuit output voltage from the ANG made with PVDF-Ce and PVDF-Ce-G NFs during the repeating human finger impact at the pressure amplitude of 6.6 kPa. (c) The output voltage, theoretical current amplitude. (d) Power output of PVDF-Ce-G NFs made ANG as a function of the variable load resistance and associated circuit diagram displayed in the inset.


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The variation of output voltage and calculated current signal (I= ) from ANG across different

load resistance (RL) is illustrated in Figure 3c, where P is the power output. The output voltage amplitude increases with increasing the load resistance and reached a peak value (i.e., 11 V) at theoretically infinite high resistance similar to open circuit voltage. Whereas, the maximum current density is estimated to 6 nA/cm2 at the optimum power transfer condition and after that gradually decreases with increasing load resistance owing to the Ohmic loss (Figure 3c). These results are good agreement with the other piezoelectric based devices.38,50 The effective electric power output is obtained by measuring the voltage across different load resistors from 105 to 108 Ω with an applying force of 8 N at 4 Hz. Figure 3d shows the instantaneous power (P =

) has

been reached to the maximum value of ~ 6.8 µW at 1 MΩ of resistance (circuit diagram is shown at inset of Figure 3d). The output power obtained from the PVDF-Ce-G NG is sufficient to turn on 10 commercial blue LEDs instantly without any external power source and storage devices (Video file V1, Supporting Information). It should be noted that ANG exhibiting high sensitivity in response of even small force could be utilized as self-powered pressure sensors. Acoustic Energy Harvesting ANG could harvest acoustic energy verified by simply attaching the ANG to a sound speaker (Figure S5, Supporting Information), when the music is played with a sound pressure level (SPL) of 88 dB. As represented in Figure 4a, the harvested acoustic energy from a pop song showed periodic output spikes to maximum 3 V, while a solo piano, guitar and violin piece (“National Anthem of India”) generated up to 1.2 V instantaneously (Figure 4c) under the same SPL. It is interesting to note that, the wave front generation from different musical instruments are also different that indicates ANG might be also suitable to map input voice/speech signal. Thus, it is



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expected to have futuristic application in voice reorganization, speech therapy and ultrasound imaging. To validate the practical application of ANG as a sound driven energy harvester, it was connected to a few commercial blue light emitting diodes (LEDs) in series through a bridge rectifier circuit. It demonstrates that under the SPL of 88 dB, turn on at least three LEDs as illustrated in the inset of Figure 4a. To confirm the integration with energy storage component, capacitors (i.e., 1, 2.2 and 4.7 µF) are connected individually using a full-wave bridge rectifying circuit. It seems that ANG has capability to charge up the three capacitors even from acoustic energy (Figure 4b). Based on the capacitor charging performance curve (Figure 4b), the energy stored in the capacitors can be calculated as Wstored =

, where C is the capacitance of a

capacitor, V is voltage across the capacitor, and t is the time taken to reach the steady state. It shows that maximum of 3 µJ can be stored in 4.7 µF capacitor. It is interesting to note that, the ANG is sensitive to the frequency at similar SPL of acoustic energy. The voltage was increased from 62 Hz of sound source and then maximized from 110 to 130 Hz (max. output voltage ∼2.6 V). The voltage is significantly decreased at 150 Hz where compared with the 210 Hz, the ANG could harvest only a small amount of the energy from the sound source (Figure 4d). These observations are also confirmed with 3 blue LEDs. The LEDs are turned on from 90 Hz and then, the light intensity of the LEDs is maximized from 105 to 140 Hz (inset of Figure 4d). Afterwards significantly decreased from 155 Hz. It completely diminishes at 185 Hz and a result ANG could not light up 3 blue LEDs (inset of Figure 4d) (Video file V2, Supporting Information). This supports the contention that the ANG is promising as flexible, durable and very effective energy harvester, not only for mechanical energy such as that generated by direct impact (such as human motion) but also from sound, noise, air and water flow as well. 18 ACS Paragon Plus Environment

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(a)

(b)

(c)

(d)

Figure 4. (a) Output voltage and (b) capacitor charging response of different capacitors (1, 2.2 and 4.7 µF) driven from the ANG made with PVDF-Ce-G NFs with the music (pop song) under an SPL of 88 dB. Instantaneous lighting of 3 blue LEDs shown in the inset of (a). Open circuit voltages from ANG with (c) various instrumental (flute, guitar and violin) of the National anthem of India ‘Jana Gana Mana’ and (d) output voltage from increasing frequency at similar SPL. Images of the frequency driven light up 3 LEDs connected in series shown in the inset of (d).



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Wind Energy Harvesting As shown in Figure 5a, ANG could also harvest wind energy demostrating by attaching it to one edge of a table (Figure 5b and S6, Supporting Information). Under the wind blowing at open environment employed from a blower with various velosity, 0.6 to 3.2 V are generated instantaneously with variable distances (d) between the ANG and air blower (Figure 5b). The output voltage is increased from the 0.5 ms-1 (output voltage ∼0.6 V) and maximised at 8 ms-1 (output voltage ∼3.2 V). Afterwards, the voltage is significantly reduced with increasing air velocity, and it becomes negligiable at 15 ms-1 or higher velocity. It is noteworthy that the harvested waveform is sensitive to the various pressure of wind flow. Benefiting from this methodology, practical applications of ANG can be largely expanded for long-time usage and for harvesting mechanical energies with fluctuating intensities, such as environmental wind flow, transportation of vehicle, beach air flow etc.

(a)

(b)

Figure 5. (a) Open circuit voltage from PVDF-Ce-G ANG by wind blowing with different air velocity, and (b) scheme of the wind blowing setup with variable distance, d.


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Conclusion In summary, we demonstrated a novel flexible and ultrasensitive acoustic nanogenerator based on Ce3+ doped electrospun PVDF/graphene NFs. The use of electrospinning process enabling excellent response and high piezoactive β fraction without further processing (for example, post electrical poling treatment). A voltage of as high as 11 V and 6.8 µW of maximum power have been achieved by the application of a 6.6 kPa of pressure amplitude to the ANG. We also demonstrated the sound-driven power generation, for example when sound with an intensity of ∼88 dB was applied to the ANG, an AC output voltage of about 3 V is obtained. The sound wave was used to vibrate the light weight ANG that generates electric potential through the PVDF-Ce-G NFs. The nano-pressure sensor (NPS) made with PVDF/graphene NFs offers exceptional piezoelectric characteristics, to enable ultra-high sensitivity for measuring pressure, even at extremely small values (2 Pa). Thus, we developed a nano-pressure sensor that enable to detect very minute external impact/pressure deviation caused by lightweight object (i.e., thermocol) and wind flow. The ANG suggest the utility as a diverse range of sensors/actuators and energy harvesting components, where lightweight construction and large area integration is one of the prime factor such as in wearable electronic, acoustic and wind energy harvesting applications. ASSOCIATED CONTENT Supporting Information Calculation of the crystallinity and FEA of the ANGs. Input pressure (σ) calculation. Raman spectrum of the synthesized graphene and SAED pattern of the ANG. Interaction between the βphase of PVDF and graphene sheet. Output voltage response due to squeeze the ANG laterally by human finger. Open circuit output voltage response from the ANG driven by sound vibration



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and the Photograph of the experimental setup of air flow with variable distances. This information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Phone: (+91) 9433373530. E-mail: [email protected]. ACKNOWLEDGMENTS This work was financially supported by a grant from the Science and Engineering Research Board (SERB/1759/2014-15), Government of India. S.G. is supported by UGC-BSR fellowship (No. P-1/RS/79/13). 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. Mao, Y.; Zhao, P.; McConohy, G.; Yang, H.; Tong, Y.; Wang, X. Sponge-Like Piezoelectric Polymer Films for Scalable and Integratable Nanogenerators and Self-Powered Electronic Systems. Adv. Energy Mater. 2014, 4, 1301624−1301631. 3. Wang, Z. L.; Song, J. Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays. Science 2006, 312, 242−246. 4. Park, K. I.; Xu, S.; Liu, Y.; Hwang, G. T.; Kang, S. J. L.; Wang, Z. L.; Lee, K. J. Piezoelectric BaTiO3 Thin Film Nanogenerator on Plastic Substrates. Nano Lett. 2010, 10, 4939−4943. 5. Kang, H. B.; Chang, J.; Koh, K.; Lin, L.; Cho, Y. S. High Quality Mn-Doped (Na,K)NbO3 Nanofibers for Flexible Piezoelectric Nanogenerators. ACS Appl. Mater. Interfaces 2014, 6, 10576−10582.


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Absorption

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on

Graphene–CuS

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49. Ghosh, S. K.; Sinha, T. K.; Mahanty, B.; Mandal, D. Self-poled Efficient Flexible “Ferroelectretic” Nanogenerator: A New Class of Piezoelectric Energy Harvester. Energy Technol. 2015, 3, 1190–1197. 50. Jeong, C. K.; Park, K.; Ryu, J.; Hwang, G. T.; Lee, K. J. Large-Area and Flexible Lead-Free Nanocomposite Generator Using Alkaline Niobate Particles and Metal Nanorod Filler. Adv. Funct. Mater. 2014, 24, 2620−2629.



Table of Content 6

Response (V)

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4 2 0 0.0

0.5

1.0

1.5

Time (s)

ACS Paragon Plus Environment

2.0

Graphene

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