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The 4th Research and Development Institute-

Yuseong P.O. Box 35, Daejeon 305-600, Rep

Fax: +82-42-823-3400-16250; Tel: +82-42-82

† Electronic supplementary information (Elinear superposition test results (Fig. S1) aBaTiO3 NWs-based NCG device. A video

LCD screen by the NCG device without10.1039/c4nr02246g

Cite this: Nanoscale, 2014, 6, 8962

Received 25th April 2014Accepted 30th May 2014

DOI: 10.1039/c4nr02246g

www.rsc.org/nanoscale

8962 | Nanoscale, 2014, 6, 8962–8968

Lead-free BaTiO3 nanowires-based flexiblenanocomposite generator†

Kwi-Il Park,* Soo Bin Bae, Seong Ho Yang, Hyung Ik Lee, Kisu Lee and Seung Jun Lee

We have synthesized BaTiO3 nanowires (NWs) via a simple hydrothermal method at low temperature and

developed a lead-free, flexible nanocomposite generator (NCG) device by a simple, low-cost, and

scalable spin-coating method. The hydrothermally grown BaTiO3 NWs are mixed in a polymer matrix

without a toxic dispersion enhancer to produce a piezoelectric nanocomposite (p-NC). During periodical

and regular bending and unbending motions, the NCG device fabricated by utilizing a BaTiO3 NWs–

polydimethylsiloxane (PDMS) composite successfully harvests the output voltage of �7.0 V and current

signals of �360 nA, which are utilized to drive a liquid crystal display (LCD). We also characterized the

instantaneous power (�1.2 mW) of the NCG device by calculating the load voltage and current through

the connected external resistance.

Introduction

Over the past few decades, energy harvesting technologies havereceived signicant attention as an alternative to solve thethreats associated with environmental problems (e.g., depletionof ozone layer, global warming, and emission of harmful gas) aswell as energy crises.1–3 In particular, the exible energyharvesters, which can convert the electricity from more acces-sible mechanical energy sources than other renewable energysources, are themost promising candidates to realize the energygeneration without restraints.4,5 In 2006, Wang and co-workersused the piezoelectric ZnO nanowire (NW) arrays to harvestelectrical energy from mechanical energy sources and proposeda sustainable/exible energy harvesting device called a nano-generator.6,7 They also demonstrated energy harvesting thatconverts electrical signals from not only mechanical bendingmotions but also small movements such as movement of thehuman nger,8 animal heartbeat, and diaphragm activities.9

Since then, there have been attempts to fabricate the thin-lmnanogenerators utilizing the highly efficient perovskite-struc-tured BaTiO3 and PZT thin lms.10–12 In these attempts, high-output performance was achieved by adopting inherently highpiezoelectric materials.

Recently, Park et al.13,14 developed exible nanocompositegenerators (NCGs) based on piezoelectric BaTiO3

13 and

3, Agency for Defense Development (ADD),

ublic of Korea. E-mail: kipark@add.re.kr;

1-4336

SI) available: PDF materials involve thend the durability test results (Fig. S2) ofle (Video S1) shows the power up of anany external energy source. See DOI:

0.942(K0.480Na0.535)NbO3–0.058LiNbO3 (KNLN)14 particles forscalable, exible, and lead-free energy harvesting devices byemploying simple and low-cost spin-casting or bar-coatingtechniques. They dispersed perovskite piezoelectric particlesand additives (such as graphitic carbons and copper nanorods)as dispersant, stress reinforcement, and conduction path toproduce the piezoelectric nanocomposite (p-NC). To fabricatebio-eco-friendly NCG devices without toxic dispersionenhancers, lead-free and non-toxic piezoelectric one-dimen-sional nanostructures, such as KNbO3 nanorods15 and BaTiO3

NWs16/nanotubes,17 have also been used. Although the fabri-cated exible harvesters have shown the ability to realize cost-effective and exible self-powered energy systems, the proposedNCG devices made of KNbO3 nanorods and BaTiO3 nanotubeshave still shown the drawbacks and limitations such as insuf-cient output performance15 and energy generation bymechanically compressive stress,17 respectively. Moreover,BaTiO3 NWs synthesis employing an M13 virus as a templateinvolves complicated procedures and a high temperatureannealing process for the crystallization of perovskite ceramicsbefore dispersing the piezoelectric nanostructures in polydi-methylsiloxane (PDMS) matrix.16

In this paper, we have synthesized BaTiO3 NWs via a simplehydrothermal method at low temperature and fabricated thelead-free NCG device to achieve an environmentally friendlyexible energy harvester without toxic dispersion enhancers.The hydrothermally grown BaTiO3 NWs show high aspect ratioand crystallinity with a tetragonal phase. The p-NC made ofmixing piezoelectric NWs into an elastomeric polymer matrix isspin-casted onto a PDMS-coated silicon (Si) wafer and subse-quently cured in an oven. The completely cured PDMS/p-NC/PDMS layers are transferred onto indium tin oxide (ITO)-coatedpolyethylene terephthalate (PET) plastic substrates. The

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fabricated BaTiO3 NWs-based exible NCG device generatedoutput voltages of up to 7.0 V and current signals of up to 360 nAduring periodic and regular bending and unbending motionsby a bending machine. The harvested energy sources areutilized to drive a liquid crystal display (LCD). We have alsocharacterized the recorded output signals as a function of theconnected external resistance and calculated the instantaneouspower (�1.2 mW) of an energy harvester.

Experimental sectionSynthesis of BaTiO3 NWs

BaTiO3 NWs were fabricated by a two-step hydrothermal reac-tion. The rst step was to synthesize Na2Ti3O7 NWs as anintermediate product. 2 g of anatase titanium dioxide powder(TiO2, 97%, Sigma-Aldrich Co.) was homogeneously dispersedin 40 ml of 10 M NaOH aqueous solution. This mixture waspoured into a Teon-autoclave and maintained in the oven at200 �C for 3 days. Then, the precipitates collected from theautoclave were washed with deionized water and alcohol fourtimes and were subsequently dried in vacuum oven at 75 �C.Before the second hydrothermal reaction with a barium source,0.5 g of Na2Ti3O7 NWs was dispersed in 10 ml of water withstirring for 1 hour (h). Next, this solution was poured into 40 mlof 0.12 M barium hydroxide [Ba(OH)2, 98%, Sigma-Aldrich Co.]aqueous solution, and then mechanically agitated for 1 h. Forthe ion exchange reaction, this reactant was poured into aTeon bottle and maintained in an oven at 100 �C for 24 h. Aerthe freeze-drying process, well-distributed BaTiO3 NWs wereobtained for an energy generation source within NCG devices.

Material characterizations

Scanning electron microscope (SEM, XL 30, Philips, Japan) andeld-emission transmission electron microscope (FE-TEM 300kV, Tecnai G2 F30, FEI Co., USA) were utilized to observe themorphologies and the crystal structure of BaTiO3 nano-structures, respectively. The phases present of the piezoelectricBaTiO3 NWs were characterized by X-ray diffraction (XRD,Rigaku, D/MAX-2500 X-ray diffractometer, Tokyo, Japan) usingCuKa radiation (l ¼ 0.15406 nm at 30 kV and 60 mA). Ramananalysis (ARAMIS, Horiba Jobin Yvon, France) was employed toobtain a more comprehensive phase characterization of BaTiO3

NWs using a 514.5 nm Ar+ laser source.

Fabrication process for the NCG devices

The PDMS was prepared by mixing base and curing agents inthe ratio of 10 : 1 and placed in a desiccator to eliminate airbubbles. To form a dielectric layer between a piezoelectricmaterial and plastic substrates, an approximately 50 mm PDMSlayer was spin-casted onto a Si wafer and then hardened at 85 �Cfor 10 min in an oven. By dispersing the hydrothermallysynthesized BaTiO3 NWs into the PDMS elastomeric at variousratios of 5, 10, 20, 30, and 40 wt%, the p-NC was obtained andthen deposited onto a PDMS-coated Si wafer using a spin-casting process at a spinning rate of 1500 rpm for 30 s. The topPDMS dielectric layer was also spin-casted onto a p-NC/PDMS/Si

This journal is © The Royal Society of Chemistry 2014

substrate and fully hardened at room temperature for 1 day.Next, PDMS/p-NC/PDMS layers sliced into a size of 3 cm � 3 cmwere detached from the Si wafer and sandwiched between atransparent 100 nm ITO-coated thin PET plastic substrate (50mm in thickness, SKC) and a thick PET (175 mm in thickness,Sigma-Aldrich) for fabricating the NCG device. To measure theoutput voltage and current signals, Cu wires were attached tothe top/bottom electrodes on exible substrates by conductiveepoxy (silver paste, Chemtronics). Finally, the exible NCGdevice was poled at 140 �C while applying an electric eldranging from 0.5 to 1.5 kV for 12 h to enhance outputperformance.

Measurement of output signals generated from the NCGdevices

To periodically and regularly stress the NCG devices, a custom-designed bending machine was used with a maximumhorizontal displacement of 5 mm at a bending strain rate of0.2 m s�1. During the repeated bending and unbending defor-mation, the open-circuit voltage and short-circuit currentsignals generated from NCG devices were measured and real-time recorded by a measurement unit (Keithley 2612A) and acomputer, respectively. Moreover, an NCG device was con-nected with external load resistors ranging from 200 kU to700 MU, and then the load voltage and current to resistor werealso measured to calculate instantaneous power. For removal ofartifact signals induced from external charges, all themeasuring performances were carried out in a Faraday cage onan optical table.

Results and discussion

For energy harvesting using the piezoelectric BaTiO3 NWs, weadopted a widely used NCG fabrication technique, as illustratedby the schematics of overall fabrication in Fig. 1a and detailedin the Experimental section. Since the nanocomposite-basednanogenerator technique developed by Park et al. is simple, low-cost, and scalable, many researchers have attempted todemonstrate NCG devices by various piezoelectric nano-materials.13,14,16,18–20 An NCG device consists of polymer and twoplastic substrates, in which the PDMS/p-NC/PDMS layers aresandwiched between two ITO-coated PET substrates. The spin-coated thin PDMS layers act as a dielectric layer that canmaintain not only an electric stability during poling process butalso mechanical durability. Fig. 1b presents the piezoelectricpotential inside p-NC calculated by multiphysics simulationsoware. For characterization of piezoelectric potential differ-ence, we used a simplied two-dimensional model with sixBaTiO3 NWs, PDMS, and two electrodes whose parameters weretaken from a COMSOL package. According to the calculation ofpreviously reported studies, the mechanically neutral plane islocated inside a bottom plastic substrate; as a result, the p-NClayer is placed above the mechanically neutral plane andentirely deformed by tensile stress during the bendingprocess.13,18 By adopting the above assumption, the simulationmodel is calculated with a tensile strain of 0.33%, which

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Fig. 1 (a) Schematic illustration of overall fabrication for BaTiO3 NWs-based NCG device. (b) Simulation model (i) of dispersed BaTiO3 NWs in aelastomeric matrix and calculated piezopotential distribution (ii) inside a p-NC layer. (c) The cross-sectional SEM images of the NCG device (left)and p-NC layer (right). The inset shows themagnified SEM image of BaTiO3 NWs in the elastomericmatrix. (d) Photograph of a NCG device (3 cm� 4 cm) completely bent by human fingers. The inset shows the p-NC layer stretched by fingers without any damage.

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corresponds to tensile stress of 0.211 GPa from relationshipsamong strain, stress, and Young's modulus (67 GPa) ofBaTiO3.10,13 From the piezoelectric potential illustrated by colorcode, the BaTiO3 NWs can be effectively used to realize theenergy harvesting of NCG devices compared to nanoparticles(NPs), which are inevitably aggregated in an elastomericmatrix.13,16,18 Fig. 1c shows the cross-sectional SEM images ofthe NCG device (le panel) and p-NC layer (right panel), inwhich PDMS (�50 mm)/p-NC (�150 mm)/PDMS (�50 mm) layersare sandwiched between two plastic substrates. From the SEMimages, the BaTiO3 NWs can be well-distributed in a so PDMSmatrix with no dispersing agents, whereas only the NPs-PDMScomposite shows aggregation and poor distribution.13,16,18

Fig. 1d displays a fabricated NCG device (3 cm � 4 cm)completely bent by human ngers; the inset shows the p-NClayer (3 cm � 3 cm) stretched by ngers without any damage.Moreover, due to the inherently sticky PDMS property, there isno separation that can be incurred at interface between p-NC

8964 | Nanoscale, 2014, 6, 8962–8968

and plastic substrates during extremely mechanical bending/unbending deformations.

Fig. 2a and b shows the SEM and FE-TEM images of thepiezoelectric BaTiO3 NWs synthesized by the hydrothermalmethod, respectively. As shown in the inset of Fig. 2a, Na2Ti3O7

NWs as the intermediate product are obtained by the rsthydrothermal reaction. The BaTiO3 NWs synthesized by ionexchange reaction with a barium source shows a high aspectratio (the average length of �4 mm and the average diameter of�156 nm) (Fig. 2b) and the well-distributed morphologieswithout agglomeration by employing the freeze-drying process(Fig. 2a). BaTiO3, however, contains a very few of agglomeratedclusters, and these unintended resultants seems to be caused byan inevitably competitive reaction during the two-step hydro-thermal synthesis.21 Crystallization of piezoelectric ceramicmaterials is essential to harvest the electric energy and enhancethe energy conversion efficiency; thus, we also characterized thehydrothermally synthesized BaTiO3 NWs using the XRD and

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Fig. 2 (a) A SEM image of the piezoelectric BaTiO3 NWs synthesized by two-step hydrothermal reaction. The inset shows Na2Ti3O7 NWs as theintermediate product obtained by the first hydrothermal synthesis. (b) A magnified TEM image obtained from a piezoelectric BaTiO3 NW on aTEM grid (the inset). (c) and (d) XRD pattern (c) and Raman spectrum (d) observed from hydrothermally grown BaTiO3 NWs.

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Raman spectroscopy for a more comprehensive phase charac-terization. The distinguishable XRD patterns show the perfectcrystallinity and general results of perovskite-structured mate-rials without crystalline of by-products such as BaCO3 or TiO2

(Fig. 2c). The active modes of Raman spectra in the range of 250to 720 cm�1 are in good agreement with the tetragonal perov-skite BaTiO3 ceramics (Fig. 2d).22 Furthermore, the tetragonalRaman bands such as E/B1 and E/A1 modes at 307 and 715cm�1, respectively, indicate that BaTiO3 NWs involve the cubic-to-tetragonal phase transformation.23

When the BaTiO3 NWs-based NCG device is regularlydeformed by a linear motor with periodical bending andunbending motions (Fig. 3a), the generated electrical outputvoltage and current signals of the harvester are shown inFig. 3b and c. The NCG device with an effective area of 3 cm �3 cm harvests the maximum open-circuit voltage of�7.0 V andshort-circuit current of �360 nA from mechanical deforma-tion. In the switching-polarity test, to verify the measuredoutput signals, the inversion of voltage and current signals isobserved. As shown in Fig. 3b, the positive and negative outputsignals were measured by bending and unbending motions,respectively, in forward connection (the inset of Fig. 3b-i). Onthe contrary, in reverse connection, the signal polarities areinverted, as shown in Fig. 3c. We have also conducted a linearsuperposition test to further conrm the energy generation ofthe NCG device (see ESI, Fig. S1a and b†). The output voltage

This journal is © The Royal Society of Chemistry 2014

and current pulse are enhanced when the two NCG deviceshowing different output performance are connected in seriesand parallel, respectively. From these conrmation results, themeasured signals are introduced from the NCG device bypiezoelectric effect. To investigate the mechanical stability ofNCG device, the durability test is carried out by repeatedlybending and unbending motions (see ESI, Fig. S2†). Duringthe 5000 bending and unbending cycles, there is slight devi-ation of output signals and the stable voltage signals areobserved. This result indicates that our NCG device can beapplied to harsh mechanical conditions.

Optimizing the amount of BaTiO3 NWs in an elastomericmatrix is an important issue to fabricate the NCG devices.Fig. 4a shows the measurement results of an NCG device withvarious ratios of BaTiO3 NWs and PDMS. When the content ofpiezoelectric NWs is varied from 5 to 20 wt%, the generatedoutput voltage and current of the harvesters are increased byintroducing the increment of piezoelectric NWs density. Theseenhancements can be introduced by the high polarization dueto extensive change in the dielectric constant within NWs–polymer composites.24,25 On the other hand, the inordinatelyhigh quantity (above 20 wt%) of piezoelectric NWs inside thesame region of p-NC can lead to the degradation of electrome-chanical coupling effect owing to an overly high dielectricconstant of the p-NC layer, and these behaviors will yield lowoutput performance.14,20

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Fig. 3 (a) The captured images of an NCG device at original, bending, and unbending states. (b and c) The electrical signals measured fromBaTiO3 NWs-based NCG device. Upon the repeatedly bending/unbendingmotions, the open-circuit voltage and short-circuit current generatedfrom the NCG device in the forward (b) and reverse (c) connections.

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We also compared the NCG performance before and aer anelectrical poling process with external voltage from 0.5 to 1.5 kVto further verify the measured output signals obtained frompiezoelectricity of BaTiO3 NWs (Fig. 4b). The non-poled NCGdevice without alignment of piezoelectric dipoles producesnegligible signals, while the generated voltage and current ofNCG device aer poling process increase with external loadvoltage. The strain-dependent property of a BaTiO3 NWs-basedNCG device is evaluated by deforming the harvester in differentdisplacement. As shown in Fig. 4c-i, the amplitude of the outputvoltage increases with the bending curvature at a constantstrain rate because the internal piezoelectric potential can beenhanced by the introduced strain. Similarly, we also found thatthe output performance depends on the angular bending strainrate at xed strain (Fig. 4c-ii). When the NCG device is quicklybent at the xed strain, a higher output voltage signals isobserved than that of slowly deformed NCG devices. Thisbehavior seems to be caused by the reduction of accumulated orreleased charges due to the quite slow electron ows duringslow bending and unbending motions.26 We have demonstratedthe energy utilization using only the generated energy sources

8966 | Nanoscale, 2014, 6, 8962–8968

from NCG devices. As shown in Fig. 4d-i, an LCD screen takenfrom a table clock is directly connected with a BaTiO3 NWs-based NCG device with no external circuits. By deforming theNCG device by a bending machine, an LCD screen is driven bythe positive electrical signals (Fig. 4d-ii). Furthermore, as anLCD device shows the non-polar property, we also observedthe operation of an LCD screen by unbending deformation(Fig. 4d-iii). Consequently, by the alternately bending/unbending motions of an energy harvester, a display can beturned on without external energy sources (see ESI, Video S1†).These results show that the BaTiO3 NWs-based energy harvestercan generate sufficient energy sources to operate a commercialelectric device.

Fig. 5a shows the load voltage and current recorded as afunction of the connected external resistance from 200 kU to700 MU. By increasing the load resistance, the load voltagethrough the resistor shows rising tendency and saturation at ahigh external load. The load current across the resistors, incontrast, is steadily decreased as a function of resistors. Finally,we characterized the instantaneous power outputs of an NCGdevice by calculating the load voltage and current measured

This journal is © The Royal Society of Chemistry 2014

Fig. 4 (a) The harvested electrical signals of NCG device with various ratios of BaTiO3 NWs inside PDMS matrix. (b) The output voltage andcurrent signals generated from NCG device before and after electrical poling process. (c) Dependence of the output voltage on angular bendingcurvature (i) and strain rate (ii) subjected to the NCG device. (d) The captured photographs of an LCD device operated by generated energysources when an NCG device is deformed by bending (ii) and unbending (iii) motions.

Fig. 5 (a) The measured load voltage and current under different external resistance varying from 200 kU to 700 MU. (b) The relationshipbetween the instantaneous power outputs and external resistance. The effective power of the BaTiO3 NWs-based harvester calculated by theload voltage and current is 1.2 mW at an external load of 20 MU.

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with the resistors. As shown in Fig. 5b, the effective power onthe load resistor by the BaTiO3 NWs-based harvester is calcu-lated as up to 1.2 mW at an external load of 20 MU.

Conclusions

In summary, we have synthesized piezoelectric BaTiO3 NWs by asimple hydrothermal method at low temperature and developedthe NCG device without toxic dispersion enhancers. The

This journal is © The Royal Society of Chemistry 2014

hydrothermally synthesized BaTiO3 NWs via two-step processshow an average length of �4 mm with a high aspect ratio andwell-distributed morphologies without agglomeration aer thefreeze-drying process. The lead-free exible energy harvesterfabricated by the spin-casting of BaTiO3 NWs–PDMS compos-ites generates a high output voltage of up to 7.0 V and currentsignals of up to 360 nA from the periodically bending/unbending motions. This high energy conversion efficiency isintroduced by adopting the well-dispersed nanostructure with a

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high aspect ratio. The energy sources generated from NCGdevice are veried by the widely used tests and a nite elementmethod, and then are used to operate an LCD without externalcircuits. This simple, economical, and practical NCG tech-nology provides a breakthrough for bio-compatible and exibleenergy harvesters. Furthermore, this technique can beexpanded to military applications (such as movement sensorsand emergency power sources) and self-powered road systemsby adopting a simple pavement process.

Acknowledgements

This work was supported by DAPA and ADD. The authors wouldlike to thank Prof. K. J. Lee, Prof. D. K. Kim, Mr. C. K. Jeong, andMr. C. Y. Baek in KAIST for their experimental and theoreticalsupports.

Notes and references

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Nanoscale

Electronic Supplementary Information for:

Lead-free BaTiO3 nanowires-based flexible nanocomposite generator

Kwi-Il Park*, Soo Bin Bae, Seong Ho Yang, Hyung Ik Lee, Kisu Lee, and Seung Jun Lee

The 4th Research and Development Institute-3, Agency for Defense Development, Yuseong

P.O. Box 35, Daejeon 305-600, Republic of Korea

*e-mail: kipark@add.re.kr, (Phone) +82-42-821-4336, (Fax) +82-42-823-3400-16250

This PDF file includes:

Figures S1 and S2.

Other Electronic Supplementary Information for this manuscript

Video S1.

1

Electronic Supplementary Material (ESI) for Nanoscale.This journal is © The Royal Society of Chemistry 2014

Nanoscale1. Linear superposition test results of BaTiO3 NWs-based NCG device

Fig. S1 The open-circuit voltage (a) and short-circuit current signals (b) generated from two

difference NCG devices connected in serial and in parallel, respectively.

2

Nanoscale2. Durability test result of BaTiO3 NWs-based NCG device

Fig. S2 The mechanical stability test result of ouput voltage generated from BaTiO3 NWs-based

NCG device during periodically 5000 bending cycles.

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Nanoscale3. The real-time live view showing an LCD device operated by an NCG device

Video S1.

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