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Cite this: Nanoscale, 2016, 8, 17632
Received 22nd July 2016,Accepted 13th September 2016
DOI: 10.1039/c6nr05784e
www.rsc.org/nanoscale
A flexible energy harvester based on a lead-freeand piezoelectric BCTZ nanoparticle–polymercomposite
Changyeon Baek,a Jong Hyuk Yun,a Ji Eun Wang,a Chang Kyu Jeong,a,b
Keon Jae Lee,a Kwi-Il Park*c and Do Kyung Kim*a
Lead-free piezoelectric 0.5(Ba0.7Ca0.3)TiO3–0.5Ba(Zr0.2Ti0.8)O3 (BCTZ) nanoparticles (NPs) composed of
earth-abundant elements were adopted for use in a flexible composite-based piezoelectric energy
harvester (PEH) that can convert mechanical deformation into electrical energy. The solid-state
synthesized BCTZ NPs and silver nanowires (Ag NWs) chosen to reduce the toxicity of the filler materials
were blended with a polydimethylsiloxane (PDMS) matrix to produce a piezoelectric nanocomposite
(p-NC). The naturally flexible polymer-based p-NC layers were sandwiched between two conductive
polyethylene terephthalate plastic substrates to achieve a flexible energy harvester. The BCTZ NP-based
PEH effectively generated an output voltage peak of ∼15 V and a current signal of ∼0.8 μA without time-
dependent degradation. This output was adequate to operate a liquid crystal display (LCD) and to turn on
six blue light emitting diodes (LEDs).
Introduction
Nanotechnology has opened new approaches to advancedtechnologies and has provided a way to realize a new energyharvesting system based upon the existing piezoelectrictechnology.1 A piezoelectric energy harvester (PEH) can effec-tively scavenge electricity without any restraints, because theforms of mechanical energy sources (such as human/machineryvibration, bending, and pressure) all around us are more access-ible than other sustainable energy sources.2–5 Moreover, PEHsbuilt on flexible substrates can harvest electricity from tiny bio-mechanical energy resources to form power sources for mobile,robotic, and implantable devices.6–8 Since the ZnO nanowire(NW)-based PEH was proposed by Wang and co-workers,9
many researchers have developed various types of PEHs usingdiverse materials [e.g. GaN,10 BaTiO3,
11,12 PbZrxTi1−xO3
(PZT),13–16 (1 − x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT)17–19]and fabrication processes including densely aligned
one-dimensional nanostructure growth,9,20,21 transfertechniques,14,17,22,23 and electrospinning.24–26
Among many kinds of energy harvesting devices, the nano-composite generator (NCG) technology has the merits of a verysimple process and scalable device fabrication with lowcost.11,13,27–32 In 2012, Park et al. firstly demonstrated an NCGdevice incorporating BaTiO3 nanoparticles (NPs),11 and thenimproved the output performance of PEHs by using lead (Pb)-based piezoelectric materials13,19 with inherently excellentelectromechanical coupling. Recently, the 0.942(K0.480Na0.535)NbO3–0.058LiNbO3 (KNLN)-based NCG device with remarkablyimproved performance was suggested as a new candidate forlead-free self-powered systems that might address the limits ofPb-based piezoelectric materials (environmental issues andgeneral toxicity to living organisms).31
In accordance with this research trend, 0.5(Ba0.7Ca0.3)TiO3–
0.5Ba(Zr0.2Ti0.8)O3 (BCTZ) composite materials, which are madeof earth-abundant elements, have attracted attention owing totheir superior piezoelectric properties (piezoelectric couplingcoefficient, ∼620 pC N−1) comparable to lead-based materialssuch as PZT or PMN-PT.33–36 Although flexible PEHs made onlyof piezoelectric BCTZ nanomaterials, which were obtained byelectrospinning or hydrothermal reactions, have been introducedto harvest electricity from biomechanical or flapping mechanicalenergies, their relatively low output performance limits their usefor operating commercial electronic devices.24,25,37
In this paper, we report our work on a bio-compatibleenergy harvesting device incorporating lead-free and non-toxic
aDepartment of Materials Science and Engineering, Korea Advanced Institute of
Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon, 34141,
Republic of Korea. E-mail: [email protected]; Fax: +82-42-350-3310;
Tel: +82-42-350-4118bKAIST Institute for the NanoCentury (KINC), 291 Daehak-ro, Yuseong-gu, Daejeon,
34141, Republic of KoreacDepartment of Energy Engineering, Gyeongnam National University of Science and
Technology (GNTECH), 33 Dongjin-ro, Jinju-si, Gyeongsangnam-do, 52725, Republic
of Korea. E-mail: [email protected]; Fax: +82-55-751-3889; Tel: +82-55-751-3884
17632 | Nanoscale, 2016, 8, 17632–17638 This journal is © The Royal Society of Chemistry 2016
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piezoelectric BCTZ NPs as an energy generation source andsilver NWs as a filler. The microstructures including themorphology, crystal structure, and vibration modes of the syn-thesized BCTZ NPs were carefully characterized via scanningelectron microscopy (SEM), X-ray diffraction (XRD), trans-mission electron microscopy (TEM), Raman spectroscopy, andpiezoelectric force microscopy (PFM). Lead-free piezoelectricBCTZ NPs showed an average size of around 500 nm withirregular morphologies and pure perovskite structures. Underthe periodically bending and releasing motions, the BCTZ NP-based NCG showed efficient high electric output performance.The generated output voltage and current pulse reached up to∼15 V and ∼0.8 μA, respectively, which correspond to instan-taneous output power around 8 μW. The output achievedmade it feasible to activate commercial electronic devices suchas a liquid crystal display (LCD) and light emitting diodes(LEDs). The fabricated BCTZ NCG was also successfullyutilized for the nimble sensing of the biomechanical move-ments such as articular motions.
Experimental procedurePreparation of BCTZ nanoparticles
The BCTZ nanopowders were prepared using a solid-statemethod. Barium acetate (Ba(CH3COO)2, 99%), calcium acetatemonohydrate (Ca(CH3COO)2H2O, ≥99%), titanium butoxide(Ti(OC4H9)4, 97%) and zirconium butoxide (Zr(OC4H9)4, 80 wt%in 1-butanol) (all reagents from Sigma-Aldrich) were used toprepare 0.5Ba(Ti0.8Zr0.2)O3–0.5(Ba0.7Ca0.3)TiO3 (BCTZ). Thiscomposition was chosen owing to its high piezoelectriccoefficient (d33) as reported in many studies. The designedstoichiometric amount of barium acetate and calcium acetatemonohydrate precursors were dissolved in deionized water andstirred at 100 °C. Titanium butoxide and zirconium butoxidewere mixed with high purity ethanol (>99.9%; Merck) andheated up to 80 °C. A mixed (Ti and Zr) solution was hydro-lyzed using ammonium hydroxide solution (28.0–30.0% NH3
basis; Sigma-Aldrich). The barium and calcium acetate solu-tion was poured into the milky white solution that containedtitanium and zirconium hydroxide precipitates. This mixedsolution was heated to 100 °C with magnetic stirring, until allsolvents were evaporated. After evaporating all solvents, awhite colored powder was obtained. The resultant powderswere collected and ground using mortar and pestle. Finally, aBCTZ powder with a particle size of ∼500 nm was synthesizedafter calcination at 1150 °C for 2 h.
Material characterization
The microstructure of the BCTZ particles and Ag NWs wasinvestigated by X-ray diffraction (XRD) (Rigaku D/Max-RB(12 kW), Japan) with Cu Kα radiation (λ = 1.5148 Å) operatingat 40 kV and 300 mA. A high-resolution dispersive Ramanmicroscope (LabRAM HR UV/Vis/NIR, Horiba Jobin Yvon,France) with the 514.5 nm line of an Ar+ laser was used tomeasure the vibration modes in the synthesized BCTZ powder.
A field-emission scanning electron microscope (FE-SEM)(XL 30, Philips, Netherlands) and a field-emission trans-mission electron microscope (FE-TEM) (Tecnai G2 F30S-Twin,Netherlands) were used to observe the morphologies andcrystal structure of the prepared powders.
Fabrication procedure of a nanocomposite generator
BCTZ nanoparticles and Ag NWs (Hongwu Nanometer Co.)were sonicated in ethanol (Merck Millipore Co.) and thenmixed at 80 °C under continuous magnetic stirring (800 rpm)until the solvent was evaporated. The mixed nanopowderswere sieved for granulation using a testing sieve with meshnumber #200. The granulated powder was blended with thefluidic PDMS (Sylgard 184, Dow Corning, base to curing agentratio: 10 : 1) to make the p-NC. The multi-layered structures ofPDMS/p-NC/PDMS were fabricated by sequential spin coatingof each fluid onto a silicon bulk wafer. To prevent the undesir-able intermixing of layer components, each as-coated layer wascured at 70 °C for 5 min in an oven before another layer wascoated on it. The semi-cured layered film was placed betweentwo ITO-coated polyethylene terephthalate (PET) plastic filmshaving different thicknesses (175 μm and 125 μm; both pur-chased from Sigma-Aldrich). The sandwiched structure wasstored for 1 day to allow complete hardening; subsequently,Cu wires were connected to the conductive ITO-coated plane ofthe PET films using conductive epoxy (Chemtronics). The finalNCG device was poled overnight at 100 °C under an externalapplied voltage of 1.5 kV to improve its piezoelectricperformance.
Measurement of output signals generated from the NCGdevice
The electric signals created from the NCG device during therepeatedly bending and releasing motions were collected andrecorded in real-time using a sourcemeter unit (Keithley2612A). A custom-designed stage was used to reproduciblycontrol for achieving the regular displacement and strain rate.The entire measurement process was performed in a Faradaycage on an optical table to prevent disturbance from any exter-nal sources.
Results and discussion
The performance of an NCG is highly dependent on the piezo-electric materials which are dispersed within the PDMSmatrix. The BCTZ nanoparticles were synthesized via a solidstate reaction following the procedures shown in Fig. 1a. Thedetailed powder preparation process is explained in theExperimental procedure section.
The morphology of the powders used for the NCG deviceswas observed via scanning electron microscopy (SEM) and theimages are shown in Fig. 1b–d. The obtained BCTZ nano-particles had an average size of ∼500 nm with an irregularshape. BCTZ powders suitably provide the benefits such ascost effectiveness (earth-abundant elements) and convenient
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processing (non-volatile elements) for mass production com-pared to other lead-free materials.38–40 Ag nanowires (NWs)with a length of 1 to 3 μm and a diameter of 50 to 200 nmwere adopted11,13,31 for multiple purposes including dis-persion of the piezoelectric nanoparticles and enhancement ofthe piezopotential in the p-NC layer. Ag NWs can enhance thestress applied to piezoelectric BCTZ NPs by reinforcing thestress of a polymer-based composite; moreover, their excellentelectric properties can effectively reduce the internal deviceresistance.11,13,31 Fig. 1d shows the well-distributed powdermixture composed of BCTZ NPs and Ag NWs.
As shown in Fig. 1e, the crystal structure of the synthesizedBCTZ NPs was investigated using X-ray diffraction (XRD).These patterns indicated a cubic perovskite structure (p4mm)without definite peak separation (or splitting), which rep-resents anisotropic distortion along the c-axis in the lattice.The composition near the morphotropic phase boundary(MPB) region often results in the coexistence of multiplephases, i.e., rhombohedral, tetragonal, and cubic systems.36,41
The materials with a composition near the MPB region arepossible to have very high piezoelectric coefficients owing tothe contractional and rotational changes of polarization.42,43
The Raman spectrum was characterized for further investi-
gation into the phases of the BCTZ powders (the inset ofFig. 1e). Among all Raman modes of the BaTiO3-based perovs-kite materials, the sharp peak of E, B1(TO + LO) modes appear-ing around 300 cm−1 means the existence of the tetragonalphase in the lattice.44–47 Therefore, the decrease in intensity ofthese Raman modes indicates that the tetragonal phase rarelyoccupies the phasic system of the BCTZ powder. Compared tothe general Raman spectrum of tetragonal BaTiO3 NPs, allbands in the BCTZ Raman spectrum are broadened, whichimplies the highly disordered state in the ceramic alloy system.It might be originated from the lattice distortion causedby the cationic substitution with different ionic radii; Zr4+
and Ca2+ partially substitute for the Ti4+ and Ba2+ sites,respectively.24,36,48
Fig. 2a presents the high resolution transmission electronmicroscopy (HRTEM) images used to analyze the crystal struc-ture of the as-synthesized BCTZ nanopowder. The directlymeasured interlayer distance (Fig. 2a, center), which wasmeasured from the lattice fringes, was 3.987 Å matching withthe d-spacing of the (100) plane calculated by the XRD pattern.The sharp selected area electron diffraction (SAED) patterns(Fig. 2a, right), when the electron beam was coherent along
Fig. 1 (a) Schematic illustration for the preparation process of the BCTZnanopowder synthesized by using a solid-state reaction. SEM images of(b) the as-synthesized BCTZ NPs, (c) the as-purchased Ag NWs with theinset of the XRD pattern, and (d) the mixed powder made of BCTZ NPsand Ag NWs. (e) XRD pattern and Raman spectrum (the inset) of theBCTZ powder.
Fig. 2 (a) HRTEM image (left), magnified scale with lattice fringes(center) and SAED patterns (right) along the [012] and [001] directions.(b) STEM and EDX mapping images showing well distributed elementsBa, Ca, Ti, Zr, and O. (c) Schematic illustration for the PFM analysis of aBCTZ single nanoparticle and the measured deformation along theZ-axis with respect to the applied voltage.
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with two different in-planes, represented all the diffractionspots from a cubic lattice symmetry. Taking into account thesize (typically a few hundreds of nanometers) of an areastudied by SAED, the faint diffraction spots, which rarelyappear in between the bright diffraction spots, would be origi-nated from the smaller particles that attached onto the largerBCTZ particles. Based on the SAED observation, the individualBCTZ NPs were well crystallized with the crystallite size of 300to 400 nm without polycrystalline domains.
From the STEM images shown in Fig. 2(b), it appears thatthe BCTZ NPs were synthesized without internal pores. Suchpores usually appear in particles synthesized via wet-chemicalroutes such as hydrothermal reactions.49 All the elements inthe BCTZ were visualized via energy dispersive X-ray spectro-scopy (EDX) mapping and they were well distributedthroughout individual particles. The piezoelectric constant(d33) of the BCTZ single nanoparticle was directly measured viaPFM, as shown in the left panel of Fig. 2c. By inducing theexternal voltage by a Pt tip mounted-cantilever, the particlewas deformed along the direction of the applied field due tothe piezoelectric response; subsequently, we drew a plot of thepiezo-response displacement versus the applied AC bias voltage(see the right panel of Fig. 2c). Table 1 indicates the d33 valuescalculated from the slope of the applied voltage to the dis-placement graphs; the unpoled BCTZ single nanoparticleshows a piezoelectric constant up to 238.2 pm V−1 with anaverage value of 220.5 ± 17.2 pm V−1.
Fig. 3a shows schematic images for the BCTZ NP-basedNCG device when subjected to deformation by mechanicalbending. The BCTZ NPs and Ag NWs were well distributed inthe PDMS matrix and the detailed fabrication process is dis-cussed in the Experimental procedure section. Compared tothe previously reported composite-based flexible energyharvesters, we employed Ag nanomaterials instead of toxic gra-phitic carbons or copper nanorods for wearable and implan-table applications (Fig. 3b). The detailed energy generationmechanism of the composite-based flexible energy harvester isdescribed in the schemes indicating that BCTZ NPs inside theNCG device produce the piezoelectric potential from an exter-nal force (Fig. 3c). The randomly oriented dipole moments canbe aligned by an induced strong electric field with temperaturebelow the Curie temperature. By tensile stress from mecha-nical bending, the BCTZ NPs inside a p-NC layer are stressed
and introduced the piezoelectric potential between two electro-des. Consequently, the generated electrons flow through theexternal circuit due to dielectric PDMS thin layers: this move-ment produces the electrical signals. The removal of piezo-potentials when the applied stress disappears allows the accu-mulated electrons to move back to the original electrode, whichcauses electric pulses in the opposite direction. Finally, thepositive and negative signals are repeatedly measured underthe periodically bending and unbending motions of the NCGdevice.
The BCTZ NP-NCG device made of naturally flexible struc-tures can harvest the electrical energies from mechanicaldeformation, as shown in Fig. 3d and e. The output open-circuit voltage and short-circuit current of an NCG device withan activation area of 3 × 3 cm2 reached up to ∼15 V and∼0.8 μA, respectively, which were obtained by bendingmotions with a displacement of 5 mm at a strain rate of0.2 m s−1. The generated electrical voltage value is higher thanthat of the previously reported composite-based flexible energyharvesters.13,27,31,32 In the well-known switching polarity test,the polarity of the output signals produced when the NCGdevice was forward-connected to a measurement kit wasswitched to reverse connection. From these results, we verifiedthat the measured electrical yields are derived from the piezo-electric potential built in the p-NC between the two electrodes.
To characterize the dependence of strain-related experi-mental parameters on the output performance, we evaluatedthe output voltage under the periodically bending motions
Fig. 3 (a) Illustration of the prepared composite generator representingthe bending and unbending states. (b) Photograph of the flexible BCTZNP-based nanocomposite generator device. (c) Schematics of theenergy generation mechanism of the BCTZ NP-NCG device. The gener-ated (d) open-circuit voltage and (e) short-circuit current when theexternal circuit is connected in the forward and reverse directions,respectively.
Table 1 Piezoelectric constant of a BCTZ single nanoparticle measuredvia PFM
Data pointPiezoelectric
constant (d33, pm V−1) Std. error R2
Particle 1 Spot 1 238.2 0.00329 0.9537Spot 2 235.1 0.00423 0.9237
Particle 2 Spot 1 205.1 0.00306 0.9463Spot 2 197.6 0.00208 0.9726
Particle 3 Spot 1 214.3 0.00296 0.9537Spot 2 232.9 0.00294 0.9609
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with various displacements from 1 to 7 mm (Fig. 4a); thesevalues indicated that the amplitude of the open-circuit voltagewas enhanced by the degree of the angular bending radius. Asshown in Fig. 4b, the measurement processes with differentbending strain rates were carried out to investigate the strainrate dependent properties of the BCTZ NP-based NCG device.The converted output signals when an NCG device was quicklydeformed at a fixed displacement were higher than those har-vested from a slowly stressed device. We also conducted amechanical durability test under 10 000 harsh bending cycles.The naturally flexible p-NC allows the NCG device to maintainits output performance as shown in Fig. 4c. The collectedoutput signals do not appear to have changed significantlyafter 1500, 5000, and 8500 bending cycles (around 500, 1500,and 2500 s, respectively).
Fig. 5a shows the load voltage and current signals recordedfrom the NCG device with diverse external resistors rangingfrom 100 kΩ to 1 GΩ to discover the instantaneous poweroutput. With increasing resistance, the load voltage values rosesteadily, whereas the load current signals gradually dimin-ished. As a result, we figure out the effective power by employ-ing these above numerical values, yielding the maximum of∼8 μW at a resistance of around 100 MΩ (Fig. 5b). To utilizethe BCTZ NP-NCG device as a power source for low-powerdevices, the peak-type electricity from piezoelectric energy har-vesters should be charged to energy storage devices after therectification process. Fig. 5c shows the schematic circuitdiagram (top) and a photograph (bottom) composed of anenergy harvester, a full-wave-bridge rectifier, and a super-capacitor (capacity of 22 mF) for achieving energy rectificationand storage. The alternating electric signals were converted
into direct current (DC)-type outputs (the inset of Fig. 5d):these energies successfully charged a supercapacitor, as shownin Fig. 5d. The charging curve of the supercapacitor from 0 to0.42 V by using a BCTZ NP-energy harvester with a displace-ment of 5 mm at a strain rate of 0.2 m s−1 and a frequency of4 Hz is shown.
Fig. 5 (a) Measured voltage and current under varied external resist-ance loadings in the range from 100 kΩ to 1 GΩ. (b) Calculated instan-taneous output power with respect to the load. (c) Schematic diagram(top) and photograph (bottom) of the circuit composed of an NCG, afull-wave-bridge rectifier, and a supercapacitor for demonstrating theenergy generation and storage system. (d) The charging curve of asupercapacitor with a capacity of 23 mF by the harvested electricity of aBCTZ NP-based energy harvester.
Fig. 4 The open-circuit voltage variation with different (a) displace-ments and (b) strain rates of the bending stage. (c) The test results forthe reliability and durability of the fabricated NCG device under harshstrain-release conditions for up to 10 000 cycles.
Fig. 6 (a), (b) Photographs of a commercial LCD device (top) and six lit-up blue LEDs (bottom) connected in series operated by bending of anNCG energy harvester. The generated output voltage (open-circuitvoltage, Voc) and current (short-circuit current, Isc) when the BCTZNP-NCG device attached onto the wrist (c) and elbow (d) are stressedby the intermittent and fast biomechanical bending motions,respectively.
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We have demonstrated the practical applications using theelectrical source generated from the BCTZ NP-based NCGdevice alone (no external power sources). As shown in Fig. 6aand b, commercial electronic devices were successfully drivenusing the alternately sufficient electricity from the flexibleNCG device deformed repeatedly by using a customizedbending machine. An LCD screen could be operated duringthe bending and releasing motions of this NCG device owingto its non-polar properties (top image of Fig. 6b). The capturedbottom image of Fig. 6b shows six lit-up blue LEDs with aforward turn-on voltage of ∼16 V. The NCG device could alsobe utilized for the agile sensing of the human articularmotions such as bending and unbending of the wrist (Fig. 6c)and elbow (Fig. 6d). The instantaneous responses (a voltageof ∼10 V and a current of ∼0.8 μA by wrist motions; a voltageof ∼4 V and a current of ∼70 nA by elbow motions) aremeasured from the BCTZ NP-NCG device on a human arm;as a result, the composite-based energy harvester can be usednot only for energy harvesting from biomechanical movementsbut also for monitoring the motions without external energysources.
Conclusions
In summary, lead-free and high piezoelectric BCTZ NPs weresynthesized via a facile solid-state route and adopted to fabri-cate a high performance and non-toxic NCG device with theaddition of conductive Ag NWs as a filler. The well-crystallizedBCTZ NPs enable the NCG to generate electric signals frommechanical stimulation. The generated output voltage andcurrent reached up to ∼15 V and 0.8 μA, respectively, whichwere enough to operate commercial electronic devices such asan LCD and LEDs. BCTZ powders suitably provide the benefitssuch as cost effectiveness and convenient processing for massproduction compared to other lead-free materials. Silver is alsoknown to be a more eco-compatible, often used for themedical and real-life applications. With these material advan-tages, the BCTZ-based NCG provides an innovative advance-ment toward a lead-free and biocompatible composite-basedenergy harvester. Furthermore, this technology could beexpanded to power sources for various consumer wearable andimplantable applications.
Acknowledgements
This work was supported by the Gyeongnam NationalUniversity of Science and Technology (GNTECH) Grant 2015.This work was also supported by the Future Planning and bythe Climate Change Research Hub of KAIST (Grant No.N11160019).
Notes and references
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Paper Nanoscale
17638 | Nanoscale, 2016, 8, 17632–17638 This journal is © The Royal Society of Chemistry 2016
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