Flexible High-Performance Lead-Free Na0.47K0.47Li0.06NbO3Microcube-Structure-Based Piezoelectric Energy HarvesterManoj Kumar Gupta,*,† Sang-Woo Kim,‡ and Binay Kumar*,§

†Department of Physics, Indian Institute of Science Education and Research, Bhopal, I.T.I. (Gas Rahat) Building, Govindpura,Bhopal, Madhya Pradesh 462023, India‡School of Advanced Materials Science and Engineering, SKKU Advanced Institute of Nanotechnology (SAINT), Center for HumanInterface Nanotechnology (HINT),and IBS Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS),Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea§Department of Physics & Astrophysics, University of Delhi, Delhi 110007, India

*S Supporting Information

ABSTRACT: Lead-free piezoelectric nano- and microstructure-basedgenerators have recently attracted much attention due to thecontinuous demand of self-powered body implantable devices. Wereport the fabrication of a high-performance flexible piezoelectricmicrogenerator based on lead-free inorganic piezoelectricNa0.47K0.47Li0.06NbO3 (NKLN) microcubes for the first time. Thecomposite generator is fabricated using NKLN microcubes andpolydimethylsiloxane (PDMS) polymer on a flexible substrate. Theflexible device exhibits excellent performance with a large recordablepiezoelectric output voltage of 48 V and output current density of 0.43μA/cm2 under vertical compressive force of 2 kgf, for which an energyconversion efficiency of about 11% has been achieved. Piezoresponseand ferroelectric studies reveal that NKLN microcubes exhibited highpiezoelectric charge coefficient (d33) as high as 460 pC/N and a well-defined hysteresis loops with remnant polarization and coercive field of 13.66 μC/cm2 and 19.45 kV/cm, respectively. Thepiezoelectric charge generation mechanism from NKLN microgenerator are discussed in the light of the high d33 and alignmentof electric dipoles in polymer matrix and dielectric constant of NKLN microcubes. It has been demonstrated that the developedpower generator has the potential to generate high electric output power under mechanical vibration for powering biomedicaldevices in the near future.

KEYWORDS: lead-free piezoelectric, microstructures, ferroelectricity, piezoelectric generator, energy harvesting

1. INTRODUCTION

Piezoelectric micro/nanogenerators are emerging as promisingenergy harvesting tools for converting irregular and tinymechanical energy with variable frequency and amplitudesuch as heartbeat, contraction of blood vessels, musclestretching or eye blinking into electrical energy.1−9Recently,lead-free piezoelectric materials have been extensively inves-tigated for power generating device applications and theirvarious potential applications in biomedical self-powerednanodevices and nanosystems, especially in implantablebiomedical devices have been successfully demonstrated.10−17

Piezoelectric generator has been successfully utilized to powersmall electronic devices, such as the lighting liquid crystaldisplay (LCD) screen and light emitting diodes (LED). Self-powered nanosensors such as gas, magnetic, mercury-iondetection, tactile have been also recently demonstrated usingmicro and nanogenerators.18−23 Up to now, various types ofenergy harvesters based on vertically and randomly alignedpiezoelectric ZnO nanowires, GaN, and InN nanowires/

nanorods, etc. have been reported for scavenging mechanicalenergy from the living environment.1,24 However, due to theircoupling of piezoelectric and semiconducting dual properties,piezoelectric power output greatly reduced because of piezo-electric potential screening effect caused by the free electro-ns.25Although an extensive efforts such as doping, thermalannealing, plasma treatment have been devoted to improve thepower efficiency of nanogenerators, the output power was stillnot sufficiently high enough to operate a sustainable biomedicaldevices.25−27 In this regard, to enhance the power output, anew type of polymer assisted hybrid generators based onvarious piezoelectric nanostructures such as NaNbO3, BaTiO3,

LiNbO3, and KNbO3 have been recently designed and reportedby many researchers because of their high piezoelectric chargecoefficients compared to ZnO and GaN.14,16,28−32 In addition,

Received: October 8, 2015Accepted: January 6, 2016Published: January 6, 2016

Research Article

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© 2016 American Chemical Society 1766 DOI: 10.1021/acsami.5b09485ACS Appl. Mater. Interfaces 2016, 8, 1766−1773

various lead based PbZrxTi1−xO3 (PZT) nanowire assisted NGsand PbZrTiO3 ribbon-based nanogenerators using graphenetransparent electrodes are also reported with an output voltageof 1.6 and 2.0 V, respectivily.33,34 However, using lead and toxicmaterials in nanogenerator fabrication causes severe pollutionto the environment during the synthesis, fabrication anddisposal processes, which restricted their real application forbody implantable devices. Although, polymer composite basedhybrid power generators are attractive because of simplefabrication process, cost-effectiveness, and mechanical robust-ness, the realization of stable and large power outputperformance energy harvester is still challengeable to operatesmall scale electronics devices.35 To achieve high outputvoltage/current, alignment of the electric dipoles of ferro-electric nanostructures in polymer matrix via electric poling ofdevice is usually required in composite based energy harvesters.Therefore, enhanced ferroelectric property as well as high d33 ofa material is key factor to design the high performance powergenerators, especially in composite based piezoelectric powergenerators. Moreover, many high d33 based materials are stillnot explored for NG fabrication and, therefore huntingbiocompatible materials having large d33 values are desperatelyneeded for sustainable and stable self-powered medicaldevices.35,36

In this regard, lead-free biocompatible piezoelectric materialsalkali based perovskite titanates such as sodium potassiumbismuth titanates (KBT-NBT) and noibates such as sodiumpotassium niobate (KNN) are known for high d33.

35−38

However, pure KNN and NBT showed usually limited andsmall d33, as a result of which low electric signal is expectedfrom respective micro nanogenerators under application ofexternal pressure. Further, doping of alkali metal into A or Bsites, enhanced its various properties such as ferroelectricity,pyroelectricty, and piezoelectricty.39−48

In this article, we report the fabrication of high performanceflexible micro generator device based on single-crystalline leadfree biocompatible piezoelectric Na0.47K0.47Li0.06NbO3 (NKLN)microstructures and polydimethylsiloxane (PDMS) polymer.The NKLN microcubes were prepared by solid state reactionmethod.49,50 Indium tin oxide (ITO)-coated polyethyleneterephthalate (PET) substrates was used as flexible substrate,which also serve as the bottom electrode for device. Aluminumcoated poly(ether sulfone) (Al/PES) was mechanicallyintegrated on the NKLN:PDMS composite layer for topelectrode fabrication. The piezoelectric output electric signals ofthe fabricated composite type energy harvester were measured

under vertical mechanical strain. We achieved a very stable andhigh electric output voltage of 48 V and current density of 0.43μA/cm2 under application of 2 kgf vertical stress.

2. EXPERIMENTAL SECTION2.1. Synthesis of Na0.47K0.47Li0.06NbO3 Microcubes and

Fabrication of Energy Harvester. Single-crysta l l ineNa0.47K0.47Li0.06NbO3(NKLN) microcubes was synthesized using theconventional solid state reaction method. In this process, high purityK2CO3 (>99%) and Na2CO3 (>99%), Nb2O5 (99.95%), and Li2CO3(99.999%) of Sigma-Aldrich were used according to the stoichiometry.The powders were mixed homogeneously using ball mill for 10 h andthen calcined at 850 °C for 10 h. After calcinations, powders werereground and mixed with binder (poly(vinyl alcohol)). Calcinedpowder was pressed into pellets after adding 2% weight of poly(vinylalcohol). The pellets of the different samples were kept at 600 °C toburn the binder. Further, pellets were sintered at 1090 °C for 2 h. Thesintered pellets were then ground using agate mortar for 3 h to getpowders of NKLN microcubes. Proportion of each element in theNa0.47K0.47Li0.06NbO3 is given in Table S1. Finally, obtained NKLNmicrocubes powders were thoroughly mixed with poly-(dimethylsiloxane) (PDMS) with the volume ratio of 40:60, andthen spin coated on the ITO coated PET substrate at 500 rpm for 15 s.100 nm thick Al layer was deposited on PES substrates using a thermalevaporator as the top electrode, which was integrated with compositelayers. The 10 nm thick chromium (Cr) was deposited on PES film bythermal evaporation prior to deposition of Al layer.

2.2. Characterization and Measurements. The synthesizedNKLN microcubes underwent structural and crystallographic charac-terizations using XRD (Bruker D8 DISCOVER) with Cu Kα radiation(λ = 1.54 Å). The crystal shape of the samples was probed with ZEISSscanning electron microscope. The dielectric constant was measuredby Agilent E4980A LCR meter. The piezoelectric charge coefficientwas measured directly using a piezometer system (Piezotest PM300)by applying a tapping force of 0.25 N and frequency 110 Hz at RT.Ferroelectric hysteresis loop of the sample was recorded using aSawyer−Tower circuit at 50 Hz of an automatic P−E loop trace. Apushing tester (Labworks, Inc., model no. ET-126-4) was used toproduce a vertical compressive strain on the device. A Tektronix DPO3052 Digital Phosphor Oscilloscope with input impedance 1 MΩ anda low-noise current preamplifier (model no. SR570, Stanford ResearchSystems, Inc.) were used for electrical measurements. The device waselectrically poled by applying a direct electric field of 50 kV/cm for 24h at room temperature (RT) before the piezoelectric output signalmeasurements. The active size of the NKLN:PDMS based device wasabout 2.0 × 3.0 cm2.

3. RESULTS AND DISCUSSIONLarge-scale NKLN microcubes were synthesized through asimple solid state reaction method.42 The morphologies and

Figure 1. (a) SEM image of NKLN microcubes with edge size in the range of 700 nm−4 μm. (b) X-ray diffraction pattern of NKLN microcubes,showing perovskite structure.

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crystal structure of the as-synthesized perovskite NKLNsamples were characterized using field-emission scanningelectron microscopy (FE-SEM) and X-ray diffraction measure-ments techniques, respectively. SEM image and X-raydiffraction (XRD) results of the NKLN microcubes is shownin Figure 1. Figure 1a shows the well-defined cubic morphologywith an edge size of about in the range of 700 nm to 4 μm.Figure 1b presents the XRD pattern of NKLN showing theformation of a pure perovskite phase. No additional peak isdetected in XRD pattern, confirming that the sample is of highquality with good crystallinity. The XRD pattern of themicrocubes is identified according to the Joint Committee onPowder Diffraction Standards (JCPDS 11-0274) demonstratingthat perovskite-structure NKLN is formed. The refined latticeconstants of a = 3.8125 Å and c = 4.0659 Å for tetragonalcrystal structure and a = 4.0005 Å, b = 4.0368 Å, and c = 3.9758Å, for the orthorhombic phase were determined.42 Further,XRD pattern of NKLN microcubes indicates that the ratio ofthe peak intensities at (002) and (200) was nearly equal whichsuggests the formation of an equally mixed orthorhombic andtetragonal phase.42

The piezoelectric charge coefficient (d33) and ferroelectricproperties of the as grown microcubes were investigated withpiezometer and ferroelectric P−E loop tracer at RT. Tomeasure the piezoelectric charge coefficient of NKLN micro-cubes, electric poling with dc field of 40 kV/cm was applied atRT. A very high piezoelectric charge coefficient (d33) about 450

pC/N was obtained, which is much higher than other reportedlead-free perovskite samples.42−44 It has been reported thatuniform grains formed with mixed orthorhombic and tetragonalphase (PPT) near RT leads to such a high value of piezoelectriccoefficient of NKLN microcubes.51−53 To harvest mechanicalenergy, we subsequently had fabricated the power generatingdevice, using the piezoelectric NKLN microcubes, it is notedthat higher electric output is expected from micro ornanogenerators with high d33 piezoelectric materials.Figure 2a−d presents the schematic diagram of the

fabrication process of composite type microgenerator devicebased on the NKLN:PDMS composite structure. As shown inFigure 2, initially, the powder of NKLN microcubes and PDMSpolymer in the volume ratio of 40:60 are homogeneouslymixed, degassed, and uniformly spin-coated on the surface ofITO coated PET substrate. The composite was heated at 70 °Cfor 4 h. Then, Al/PES was mechanically integrated on thecomposite layer of NKLN:PDMS for top electrode preparation.The original image of flexible NKLN:PDMS composite wasshown in the Figure 2e. Random distribution of NKLNmicrocubes in PDMS matrix was obtained in both sides of thecomposite film, as shown in FE-SEM images in Figure 2f (topview of composite) and Figure S1(bottom view of composite).Further, a cross-sectional FE-SEM image of flexible film isshown in Figure S2, which shows that the thickness thecomposite film was about 525 μm. It is also clear thatpiezoelectric NKLN microcubes are randomly oriented inside

Figure 2. Schematic diagrams for the fabrication of flexible NKLN microcube-based power generating device: (a) microcubes are mixed with PDMSpolymer; (b) the solution was deposited by spin coating on a bare ITO/PET substrate and (c) subsequently heated at 70 °C for 4h; and (d) Al filmcoated on flexible PES substrate are used as top electrode. (e) Original image of flexible NKLN:PDMS composite film on ITO/PET (f) SEM imageof top surface of composite film, showing random distribution of NKLN microcubes.

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the PDMS matrix, which results the electric dipoles arerandomly aligned between the top and bottom electrodes inabsence of any external field or pressure.The piezoelectric output voltage and current from NKLN

power generating device were measured under vertical pushingand releasing conditions using force simulator. The observedoutput voltage and current from the device are shown in Figure3 and 4, respectively. A large output voltage of 48 V and currentdensity of 0.43 μA/cm2 were obtained under verticalcompressive of 2 kgf at a frequency of 3 Hz, as shown inFigures 3 and 4, respectively. Switching-polarity tests were alsocarried out to confirm that the output voltage originated fromthe piezoelectric phenomenon. An opposite output signal isobserved when the device is connected in reverse connection asshown in Figures 3b and 4b. Enlarged view of one cycle ofoutput voltage (Figure 3c,d) and output current (Figure 4c,d)confirmed that the electric signals were reversible. Underforward and reverse connection, the difference of peak valuesbetween compress and release conditions are detected becauseof the difference in straining rate when applying and releasingthe stress on the NKLN:PDMS device.14,54 The equivalentelectric circuit of NKLN:PDMS piezoelectric power generatoris given in Figure S3 of Supporting Information. To confirmthat the large output voltage is produced from NKLN:PDMSbased energy harvester, only PDMS based device (withoutNKLN microcubes) was also fabricated under identicalcondition and its output voltage is carefully measured. Anegligible output voltage (∼0.03 V) is obtained (Figure S4,

Supporting Information) under the same mechanical stress. It isnoted that, the device was electrically poled before the electricsignal measurement. In the present work, the obtained electricoutput from the NKLN:PDMS based device are comparativelyhigher than previously reported composite-based piezoelectricenergy harvesters (Table 1). It is demonstrated that the highoutput power of the generator can be used as a power sourcefor self-powered biomedical devices.The working mechanism and high performance of the power

generator was also investigated on the basis of piezoelectricproperties of NKLN microcubes, electric dipoles orientation,and the dielectric constant of the materials. It is expected thatthe charge-pumped electron flow is driven back and forththrough an external load similar to phenomena of capacitor,where charging and discharging process results in alternatingcurrent (AC)-type charge generation. The details of powergeneration mechanism can be easily understood with the helpof electric dipole alignment under application of mechanicalstress and electric poling effect. It is noteworthy to point outthat the electric dipoles can be oriented or switched in aparticular direction after applying suitable electric field due toferroelectric property of the NKLN microcubes.Therefore, to confirm the capabilities of alignment and

switching of electric dipoles, we have also investigated theferroelectric property of the sample at RT by tracing itsferroelectric hysteresis loops. The ferroelectric study wasperformed and a well saturated P−E loop was obtained atRT, confirming the ferroelectric properties of NKLN micro-

Figure 3. (a) Piezoelectric output voltage generated from the fabricated energy harvester under vertical compressive force (forward connection). (b)Output voltage under reverse connection (switching- polarity test). (c) Enlarged view of one cycle of output voltage under forward connection. (d)Enlarged view of one cycle of output voltage under reverse connection.

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cubes. The corresponding remnant polarization (Pr) andcoercive field (Ec) were obtained as 13.66 μC/cm2 and 19.45kV/cm, respectively, at RT.42 The details of the ferroelectricand piezoelectric studies are given in our previous publica-tion.42 The ferroelectric property of NKLN:PDMS compositefilm is further investigated using Sawyer−Tower ferroelectricloop tracer. The obtained P−E loop is shown in the Figure 5.The result indicates that although the composite film does notshow well saturated hysteresis like obtained P−E loop ofNKLN microcubes42 due to nonferroelectric characteristics ofPDMS polymer, a ferroelectric property with comparativelysmaller remnant polarization (0.2835 μC/cm2) and slightlyhigher coercive field of 15.5 kV/cm than Pr and Ec of pureNKLN microcubes is still maintained. This result confirmed

that electric dipoles alignment in the direction of applied field ispossible under application of suitable electric poling.Therefore, without any electric poling, electric dipoles inside

the NKLN microcubes are randomly aligned in the PDMSmatrix between the top and bottom electrodes (black arrows inFigure 6a). Hence, when no external force on the device wasapplied, net dipole moment remains zero, leading to theabsence of any piezoelectric electric charge generation betweenthe electrodes Figure 6a (step 1). Further, when the device waselectrically poled, the electric dipoles of NKLN in polymer

Figure 4. (a) Piezoelectric output current density generated from the fabricated energy harvester under vertical compressive force (forwardconnection). (b) Output voltage under reverse connection (switching-polarity test). (c) Enlarged view of one cycle of output current under forwardconnection. (d) Enlarged view of one cycle of output current under reverse connection.

Table 1. Comparison of Output Performance of CompositeEnergy Harvesters

composite piezoelectricgenerator

outputvoltage (V)

output current/current density ref

LiNbO3:PDMS 0.46 9.11 nA 28BaTiO3:PDMS 5.50 350 nA 48NaNbO3:PDMS 3.20 16 nA/cm2 14ZnSnO3:PDMS 9.70 0.9 μA/cm2 13PMN−PT:PDMS 7.80 2.29 μA 55Na0.47K0.47Li0.06NbO3:PDMS 48.00 0.43 μA/cm2 present

work

Figure 5. Ferroelectric P−E hysteresis curve of NKLN:PDMScomposite.

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matrix are aligned along the electric-field direction from top tobottom. However, under the absence of any external pressure,the device remains in equilibrium state and no electric signal isdetected.When a vertical compressive force is applied (step 2), a

piezoelectric potential is developed, which results in a change ofpolarization across the electrodes in such a way negativecharges generated on the top side and positive charge isgenerated on the bottom side of device, due to strongalignment of electric dipoles in a single direction which resultsin a significant potential across the electrodes is created (Figure6b). Further, in order to screen the piezoelectric electricpotential, positive and negative charges are accumulated at thetop and bottom electrodes, respectively, resulting in ageneration of voltage and current output signals from thedevice. Further, when the vertical compressive force is removed,the compressive strain is released and the piezoelectric effectdiminishes, and the accumulated charges transported back tothe reverse direction and an electric signal is detected inopposite direction as shown in Figure 6c (step 3).Therefore, AC-type voltage and current output signals from

the NKLN device was obtained during continuous applicationand releasing of the compressive strain. It is also noted that, inaddition to high d33, output voltage/current of the device arealso greatly influenced by the dielectric constant. In the presentstudy, the output voltage and current from single cell of theNKLN based device are much higher than previously reportedcomposite type piezoelectric power generators (Table 1), toconfirm the influence of dielectric constant on piezoelectricpower output, we have carefully measured the dielectricconstant of NKLN at RT. It was observed that dielectricconstant showed a decreasing trend with frequency in which isdue to the space charge contribution at lower frequencies (datanot shown here for NKLN, details).42 Dielectric constantreached up to the value of 551 at RT, which is relatively smallerthan other reported values of lead-based piezoelectric materials

such as PbZnNbO3−PbTiO3 [PZNT], PbMnNbO3−PbO3[PMNT] etc.55,56 Piezoelectric output voltage from the devicecan be expressed as V = g33εE, where g33 is the piezoelectricvoltage constant, ε is the strain, and E is the Young’s modulusof the materials.8,49 Further, piezoelectric voltage constant, g33,is directly proportional to the d33 and their mathematicalrelation can be expressed as g33 = d33/(ε0K), where ε0 is thepermittivity of free space and K is the relative dielectricconstant of the NKLN microcubes, which also suggest that,piezoelectric output signal increased with decreasing dielectricconstant. Moreover, dielectric constant of NKLN:PDMScomposite film was also measured at various appliedfrequencies (Figure 7) at RT. It is observed that the dielectric

constant of composite was significantly reduced (72.9 at 200Hz), because of lower dielectric constant of PDMS (2.5) thanNKLN microcubes.57

The average energy conversion efficiency of the flexiblepiezoelectric NKLN:PDMS-based device was estimated andfound to be ≈11.12%. The energy conversion efficiency is

Figure 6. (a) No piezoelectric output electric is generated in the absence of any external mechanical strain (step 1). (b) Once electric dipoles arealigned in a single direction after electric poling of device, the piezoelectric potential induced electrons flow from the top electrode to the bottomelectrode side through the external load under vertical compression of device (step 2). (c) As the compressive strain is removed, the piezoelectricpotential inside the device instantly disappears, the accumulated electrons from the bottom electrode move back to the top electrode, and an electricsignal is observed in opposite direction (step 3).

Figure 7. Variation of dielectric constant with frequency at ofNKLM:PDMS composite at RT.

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obtained by dividing the output electrical energy (1.63 × 10−8

J) per cycle with strain energy (1.46 × 10−7 J) developed in thedevice under mechanical pressure per cycle. Details about thecalculation are given in the Supporting Information.

4. CONCLUSIONSIn conclusion, we fabricated a high-performance lead-freeNKLN-microcube-based generator for scavenging mechanicalenergy. The flexible power generating device was fabricatedusing composite structure of NKLN:PDMS (40:60) on ITOcoated PET substrate. The output voltage and current from thegenerator reached a record value of 48 V and 0.43 μA/cm2,respectively, under vertical compressive force, which is muchlarger than other reported lead-free energy harvesters. Thehigh-output voltage and current from the device was explainedin the light of high d33(430 pC/N), low dielectric constant, andstrong alignment of electric dipoles due to its well-definedferroelectric properties. Our results indicate that the device hasa great potential for self-powered implantable device due to itshigh power output.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.5b09485.

Low-resolution FE-SEM image of NKLN:PDMS com-posite, cross-sectional FE-SEM image of NKLN:PDMScomposite, high-resolution cross-sectional FE-SEMimage, equivalent circuit of NKLN:PDMS powergenerator, output voltage from only PDMS-based deviceunder mechanical stress, and detail of calculations ofenergy conversion efficiency of flexible NKLN:PDMSpower generator. Proportion of each element in theNa0.47K0.47Li0.06NbO3 microcubes. (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*Tel: +91-755-4092310. Fax: +91-755-6692392. E-mail:mkgupta@iiserb.ac.in.*E-mail: b3kumar69@yahoo.co.in.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSM.K.G. is grateful to the Department of Science & Technology,Govt. of India, for awarding the DST-INSPIRE FacultyFellowship [IFA-13 PH-81]. B.K. is thankful for the financialsupport received in DST project (Sanction No. SR/S2/CMP-0068/2010) and DRDO project (Sanction No. ARMREB/MAA/2015/163).

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ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.5b09485ACS Appl. Mater. Interfaces 2016, 8, 1766−1773

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