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Lead-free BaTiO3 nanowires-based flexible nanocomposite generator† Cite this: DOI: 10.1039/c4nr02246g
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 Received 25th April 2014 Accepted 30th May 2014
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
DOI: 10.1039/c4nr02246g
instantaneous power (1.2 mW) of the NCG device by calculating the load voltage and current through
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the connected external resistance.
Introduction Over the past few decades, energy harvesting technologies have received signicant attention as an alternative to solve the threats associated with environmental problems (e.g., depletion of ozone layer, global warming, and emission of harmful gas) as well as energy crises.1–3 In particular, the exible energy harvesters, which can convert the electricity from more accessible mechanical energy sources than other renewable energy sources, are the most promising candidates to realize the energy generation without restraints.4,5 In 2006, Wang and co-workers used the piezoelectric ZnO nanowire (NW) arrays to harvest electrical energy from mechanical energy sources and proposed a sustainable/exible energy harvesting device called a nanogenerator.6,7 They also demonstrated energy harvesting that converts electrical signals from not only mechanical bending motions but also small movements such as movement of the human nger,8 animal heartbeat, and diaphragm activities.9 Since then, there have been attempts to fabricate the thin-lm nanogenerators utilizing the highly efficient perovskite-structured BaTiO3 and PZT thin lms.10–12 In these attempts, highoutput performance was achieved by adopting inherently high piezoelectric materials. Recently, Park et al.13,14 developed exible nanocomposite generators (NCGs) based on piezoelectric BaTiO3 13 and
The 4th Research and Development Institute-3, Agency for Defense Development (ADD), Yuseong P.O. Box 35, Daejeon 305-600, Republic of Korea. E-mail: [email protected]; Fax: +82-42-823-3400-16250; Tel: +82-42-821-4336 † Electronic supplementary information (ESI) available: PDF materials involve the linear superposition test results (Fig. S1) and the durability test results (Fig. S2) of BaTiO3 NWs-based NCG device. A video le (Video S1) shows the power up of an LCD screen by the NCG device without any external energy source. See DOI: 10.1039/c4nr02246g
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0.942(K0.480Na0.535)NbO3–0.058LiNbO3 (KNLN)14 particles for scalable, exible, and lead-free energy harvesting devices by employing simple and low-cost spin-casting or bar-coating techniques. They dispersed perovskite piezoelectric particles and additives (such as graphitic carbons and copper nanorods) as dispersant, stress reinforcement, and conduction path to produce the piezoelectric nanocomposite (p-NC). To fabricate bio-eco-friendly NCG devices without toxic dispersion enhancers, lead-free and non-toxic piezoelectric one-dimensional nanostructures, such as KNbO3 nanorods15 and BaTiO3 NWs16/nanotubes,17 have also been used. Although the fabricated exible harvesters have shown the ability to realize costeffective and exible self-powered energy systems, the proposed NCG devices made of KNbO3 nanorods and BaTiO3 nanotubes have still shown the drawbacks and limitations such as insufcient output performance15 and energy generation by mechanically compressive stress,17 respectively. Moreover, BaTiO3 NWs synthesis employing an M13 virus as a template involves complicated procedures and a high temperature annealing process for the crystallization of perovskite ceramics before dispersing the piezoelectric nanostructures in polydimethylsiloxane (PDMS) matrix.16 In this paper, we have synthesized BaTiO3 NWs via a simple hydrothermal method at low temperature and fabricated the lead-free NCG device to achieve an environmentally friendly exible energy harvester without toxic dispersion enhancers. The hydrothermally grown BaTiO3 NWs show high aspect ratio and crystallinity with a tetragonal phase. The p-NC made of mixing piezoelectric NWs into an elastomeric polymer matrix is spin-casted onto a PDMS-coated silicon (Si) wafer and subsequently cured in an oven. The completely cured PDMS/p-NC/ PDMS layers are transferred onto indium tin oxide (ITO)-coated polyethylene terephthalate (PET) plastic substrates. The
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fabricated BaTiO3 NWs-based exible NCG device generated output voltages of up to 7.0 V and current signals of up to 360 nA during periodic and regular bending and unbending motions by a bending machine. The harvested energy sources are utilized to drive a liquid crystal display (LCD). We have also characterized the recorded output signals as a function of the connected external resistance and calculated the instantaneous power (1.2 mW) of an energy harvester.
Experimental section Synthesis of BaTiO3 NWs
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substrate and fully hardened at room temperature for 1 day. Next, PDMS/p-NC/PDMS layers sliced into a size of 3 cm 3 cm were detached from the Si wafer and sandwiched between a transparent 100 nm ITO-coated thin PET plastic substrate (50 mm in thickness, SKC) and a thick PET (175 mm in thickness, Sigma-Aldrich) for fabricating the NCG device. To measure the output voltage and current signals, Cu wires were attached to the top/bottom electrodes on exible substrates by conductive epoxy (silver paste, Chemtronics). Finally, the exible NCG device was poled at 140 C while applying an electric eld ranging from 0.5 to 1.5 kV for 12 h to enhance output performance.
BaTiO3 NWs were fabricated by a two-step hydrothermal reaction. The rst step was to synthesize Na2Ti3O7 NWs as an intermediate product. 2 g of anatase titanium dioxide powder (TiO2, 97%, Sigma-Aldrich Co.) was homogeneously dispersed in 40 ml of 10 M NaOH aqueous solution. This mixture was poured into a Teon-autoclave and maintained in the oven at 200 C for 3 days. Then, the precipitates collected from the autoclave were washed with deionized water and alcohol four times 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 with stirring for 1 hour (h). Next, this solution was poured into 40 ml of 0.12 M barium hydroxide [Ba(OH)2, 98%, Sigma-Aldrich Co.] aqueous solution, and then mechanically agitated for 1 h. For the ion exchange reaction, this reactant was poured into a Teon bottle and maintained in an oven at 100 C for 24 h. Aer the freeze-drying process, well-distributed BaTiO3 NWs were obtained for an energy generation source within NCG devices.
To periodically and regularly stress the NCG devices, a customdesigned bending machine was used with a maximum horizontal displacement of 5 mm at a bending strain rate of 0.2 m s1. During the repeated bending and unbending deformation, the open-circuit voltage and short-circuit current signals generated from NCG devices were measured and realtime recorded by a measurement unit (Keithley 2612A) and a computer, respectively. Moreover, an NCG device was connected with external load resistors ranging from 200 kU to 700 MU, and then the load voltage and current to resistor were also measured to calculate instantaneous power. For removal of artifact signals induced from external charges, all the measuring performances were carried out in a Faraday cage on an optical table.
Material characterizations
Results and discussion
Scanning electron microscope (SEM, XL 30, Philips, Japan) and eld-emission transmission electron microscope (FE-TEM 300 kV, Tecnai G2 F30, FEI Co., USA) were utilized to observe the morphologies and the crystal structure of BaTiO3 nanostructures, respectively. The phases present of the piezoelectric BaTiO3 NWs were characterized by X-ray diffraction (XRD, Rigaku, D/MAX-2500 X-ray diffractometer, Tokyo, Japan) using CuKa radiation (l ¼ 0.15406 nm at 30 kV and 60 mA). Raman analysis (ARAMIS, Horiba Jobin Yvon, France) was employed to obtain 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 in the ratio of 10 : 1 and placed in a desiccator to eliminate air bubbles. To form a dielectric layer between a piezoelectric material and plastic substrates, an approximately 50 mm PDMS layer was spin-casted onto a Si wafer and then hardened at 85 C for 10 min in an oven. By dispersing the hydrothermally synthesized BaTiO3 NWs into the PDMS elastomeric at various ratios of 5, 10, 20, 30, and 40 wt%, the p-NC was obtained and then deposited onto a PDMS-coated Si wafer using a spincasting process at a spinning rate of 1500 rpm for 30 s. The top PDMS dielectric layer was also spin-casted onto a p-NC/PDMS/Si
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Measurement of output signals generated from the NCG devices
For energy harvesting using the piezoelectric BaTiO3 NWs, we adopted a widely used NCG fabrication technique, as illustrated by the schematics of overall fabrication in Fig. 1a and detailed in the Experimental section. Since the nanocomposite-based nanogenerator technique developed by Park et al. is simple, lowcost, and scalable, many researchers have attempted to demonstrate NCG devices by various piezoelectric nanomaterials.13,14,16,18–20 An NCG device consists of polymer and two plastic substrates, in which the PDMS/p-NC/PDMS layers are sandwiched between two ITO-coated PET substrates. The spincoated thin PDMS layers act as a dielectric layer that can maintain not only an electric stability during poling process but also mechanical durability. Fig. 1b presents the piezoelectric potential inside p-NC calculated by multiphysics simulation soware. For characterization of piezoelectric potential difference, we used a simplied two-dimensional model with six BaTiO3 NWs, PDMS, and two electrodes whose parameters were taken from a COMSOL package. According to the calculation of previously reported studies, the mechanically neutral plane is located inside a bottom plastic substrate; as a result, the p-NC layer is placed above the mechanically neutral plane and entirely deformed by tensile stress during the bending process.13,18 By adopting the above assumption, the simulation model 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 a elastomeric 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 the magnified SEM image of BaTiO3 NWs in the elastomeric matrix. (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.
corresponds to tensile stress of 0.211 GPa from relationships among strain, stress, and Young's modulus (67 GPa) of BaTiO3.10,13 From the piezoelectric potential illustrated by color code, the BaTiO3 NWs can be effectively used to realize the energy harvesting of NCG devices compared to nanoparticles (NPs), which are inevitably aggregated in an elastomeric matrix.13,16,18 Fig. 1c shows the cross-sectional SEM images of the NCG device (le panel) and p-NC layer (right panel), in which PDMS (50 mm)/p-NC (150 mm)/PDMS (50 mm) layers are sandwiched between two plastic substrates. From the SEM images, the BaTiO3 NWs can be well-distributed in a so PDMS matrix with no dispersing agents, whereas only the NPs-PDMS composite 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-NC layer (3 cm 3 cm) stretched by ngers without any damage. Moreover, due to the inherently sticky PDMS property, there is no separation that can be incurred at interface between p-NC
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and plastic substrates during extremely mechanical bending/ unbending deformations. Fig. 2a and b shows the SEM and FE-TEM images of the piezoelectric BaTiO3 NWs synthesized by the hydrothermal method, respectively. As shown in the inset of Fig. 2a, Na2Ti3O7 NWs as the intermediate product are obtained by the rst hydrothermal reaction. The BaTiO3 NWs synthesized by ion exchange reaction with a barium source shows a high aspect ratio (the average length of 4 mm and the average diameter of 156 nm) (Fig. 2b) and the well-distributed morphologies without agglomeration by employing the freeze-drying process (Fig. 2a). BaTiO3, however, contains a very few of agglomerated clusters, and these unintended resultants seems to be caused by an inevitably competitive reaction during the two-step hydrothermal synthesis.21 Crystallization of piezoelectric ceramic materials is essential to harvest the electric energy and enhance the energy conversion efficiency; thus, we also characterized the hydrothermally 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 the intermediate product obtained by the first hydrothermal synthesis. (b) A magnified TEM image obtained from a piezoelectric BaTiO3 NW on a TEM grid (the inset). (c) and (d) XRD pattern (c) and Raman spectrum (d) observed from hydrothermally grown BaTiO3 NWs.
Raman spectroscopy for a more comprehensive phase characterization. The distinguishable XRD patterns show the perfect crystallinity and general results of perovskite-structured materials without crystalline of by-products such as BaCO3 or TiO2 (Fig. 2c). The active modes of Raman spectra in the range of 250 to 720 cm1 are in good agreement with the tetragonal perovskite BaTiO3 ceramics (Fig. 2d).22 Furthermore, the tetragonal Raman bands such as E/B1 and E/A1 modes at 307 and 715 cm1, respectively, indicate that BaTiO3 NWs involve the cubicto-tetragonal phase transformation.23 When the BaTiO3 NWs-based NCG device is regularly deformed by a linear motor with periodical bending and unbending motions (Fig. 3a), the generated electrical output voltage and current signals of the harvester are shown in Fig. 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 and short-circuit current of 360 nA from mechanical deformation. In the switching-polarity test, to verify the measured output signals, the inversion of voltage and current signals is observed. As shown in Fig. 3b, the positive and negative output signals were measured by bending and unbending motions, respectively, in forward connection (the inset of Fig. 3b-i). On the contrary, in reverse connection, the signal polarities are inverted, as shown in Fig. 3c. We have also conducted a linear superposition test to further conrm the energy generation of the NCG device (see ESI, Fig. S1a and b†). The output voltage
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and current pulse are enhanced when the two NCG device showing different output performance are connected in series and parallel, respectively. From these conrmation results, the measured signals are introduced from the NCG device by piezoelectric effect. To investigate the mechanical stability of NCG device, the durability test is carried out by repeatedly bending and unbending motions (see ESI, Fig. S2†). During the 5000 bending and unbending cycles, there is slight deviation of output signals and the stable voltage signals are observed. This result indicates that our NCG device can be applied to harsh mechanical conditions. Optimizing the amount of BaTiO3 NWs in an elastomeric matrix is an important issue to fabricate the NCG devices. Fig. 4a shows the measurement results of an NCG device with various ratios of BaTiO3 NWs and PDMS. When the content of piezoelectric NWs is varied from 5 to 20 wt%, the generated output voltage and current of the harvesters are increased by introducing the increment of piezoelectric NWs density. These enhancements can be introduced by the high polarization due to extensive change in the dielectric constant within NWs– polymer composites.24,25 On the other hand, the inordinately high quantity (above 20 wt%) of piezoelectric NWs inside the same region of p-NC can lead to the degradation of electromechanical coupling effect owing to an overly high dielectric constant of the p-NC layer, and these behaviors will yield low output 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 from BaTiO3 NWs-based NCG device. Upon the repeatedly bending/unbending motions, the open-circuit voltage and short-circuit current generated from the NCG device in the forward (b) and reverse (c) connections.
We also compared the NCG performance before and aer an electrical poling process with external voltage from 0.5 to 1.5 kV to further verify the measured output signals obtained from piezoelectricity of BaTiO3 NWs (Fig. 4b). The non-poled NCG device without alignment of piezoelectric dipoles produces negligible signals, while the generated voltage and current of NCG device aer poling process increase with external load voltage. The strain-dependent property of a BaTiO3 NWs-based NCG device is evaluated by deforming the harvester in different displacement. As shown in Fig. 4c-i, the amplitude of the output voltage increases with the bending curvature at a constant strain rate because the internal piezoelectric potential can be enhanced by the introduced strain. Similarly, we also found that the output performance depends on the angular bending strain rate at xed strain (Fig. 4c-ii). When the NCG device is quickly bent at the xed strain, a higher output voltage signals is observed than that of slowly deformed NCG devices. This behavior seems to be caused by the reduction of accumulated or released charges due to the quite slow electron ows during slow bending and unbending motions.26 We have demonstrated the energy utilization using only the generated energy sources
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from NCG devices. As shown in Fig. 4d-i, an LCD screen taken from a table clock is directly connected with a BaTiO3 NWsbased NCG device with no external circuits. By deforming the NCG device by a bending machine, an LCD screen is driven by the positive electrical signals (Fig. 4d-ii). Furthermore, as an LCD device shows the non-polar property, we also observed the 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 be turned on without external energy sources (see ESI, Video S1†). These results show that the BaTiO3 NWs-based energy harvester can generate sufficient energy sources to operate a commercial electric device. Fig. 5a shows the load voltage and current recorded as a function of the connected external resistance from 200 kU to 700 MU. By increasing the load resistance, the load voltage through the resistor shows rising tendency and saturation at a high external load. The load current across the resistors, in contrast, is steadily decreased as a function of resistors. Finally, we characterized the instantaneous power outputs of an NCG device by calculating the load voltage and current measured
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Fig. 4 (a) The harvested electrical signals of NCG device with various ratios of BaTiO3 NWs inside PDMS matrix. (b) The output voltage and current signals generated from NCG device before and after electrical poling process. (c) Dependence of the output voltage on angular bending curvature (i) and strain rate (ii) subjected to the NCG device. (d) The captured photographs of an LCD device operated by generated energy sources when an NCG device is deformed by bending (ii) and unbending (iii) motions.
(a) The measured load voltage and current under different external resistance varying from 200 kU to 700 MU. (b) The relationship between the instantaneous power outputs and external resistance. The effective power of the BaTiO3 NWs-based harvester calculated by the load voltage and current is 1.2 mW at an external load of 20 MU. Fig. 5
with the resistors. As shown in Fig. 5b, the effective power on the load resistor by the BaTiO3 NWs-based harvester is calculated as up to 1.2 mW at an external load of 20 MU.
Conclusions In summary, we have synthesized piezoelectric BaTiO3 NWs by a simple hydrothermal method at low temperature and developed the NCG device without toxic dispersion enhancers. The
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hydrothermally synthesized BaTiO3 NWs via two-step process show an average length of 4 mm with a high aspect ratio and well-distributed morphologies without agglomeration aer the freeze-drying process. The lead-free exible energy harvester fabricated by the spin-casting of BaTiO3 NWs–PDMS composites generates a high output voltage of up to 7.0 V and current signals of up to 360 nA from the periodically bending/ unbending motions. This high energy conversion efficiency is introduced by adopting the well-dispersed nanostructure with a
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high aspect ratio. The energy sources generated from NCG device are veried by the widely used tests and a nite element method, and then are used to operate an LCD without external circuits. This simple, economical, and practical NCG technology provides a breakthrough for bio-compatible and exible energy harvesters. Furthermore, this technique can be expanded to military applications (such as movement sensors and emergency power sources) and self-powered road systems by adopting a simple pavement process.
Acknowledgements This work was supported by DAPA and ADD. The authors would like to thank Prof. K. J. Lee, Prof. D. K. Kim, Mr. C. K. Jeong, and Mr. C. Y. Baek in KAIST for their experimental and theoretical supports.
Notes and references 1 C. Beggs, Energy: Management, Supply and Conservation, Elsevier, Oxford, 2nd edn, 2002, ch. 1, pp. 14–30, ch. 5, pp.81–100. 2 G. J. Aubrecht, Energy: Physical, Environmental, and Social Impact, Pearson Education, London, 3rd edn, 2006, ch. 1, pp. 2–15. 3 S. Priya and D. J. Inman, Energy Harvesting Technologies, Springer Science, New York, 2009, ch. 1, pp. 3–39. 4 Z. L. Wang and W. Z. Wu, Angew. Chem., Int. Ed., 2012, 51, 11700–11721. 5 Z. L. Wang, Nanogenerators for Self-powered Devices and Systems, Georgia Institute of Technology, Atlanta, 1st edn, 2011, ch. 1, pp. 1–5. 6 Z. L. Wang and J. H. Song, Science, 2006, 312, 242–246. 7 S. Xu, B. J. Hansen and Z. L. Wang, Nat. Commun., 2010, 1, 93. 8 R. Yang, Y. Qin, C. Li, G. Zhu and Z. L. Wang, Nano Lett., 2009, 9, 1201–1205. 9 Z. Li, G. A. Zhu, R. S. Yang, A. C. Wang and Z. L. Wang, Adv. Mater., 2010, 22, 2534–2537.
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10 K.-I. Park, S. Xu, Y. Liu, G.-T. Hwang, S. J. L. Kang, Z. L. Wang and K. J. Lee, Nano Lett., 2010, 10, 4939–4943. 11 Y. Qi, J. Kim, T. D. Nguyen, B. Lisko, P. K. Purohit and M. C. McAlpine, Nano Lett., 2011, 11, 1331–1336. 12 K.-I. Park, J. H. Son, G.-T. Hwang, C. K. Jeong, J. Ryu, M. Koo, I. Choi, S. H. Lee, M. Byun, Z. L. Wang and K. J. Lee, Adv. Mater., 2014, 26, 2514–2520. 13 K.-I. Park, M. Lee, Y. Liu, S. Moon, G.-T. Hwang, G. Zhu, J. E. Kim, S. O. Kim, D. K. Kim, Z. L. Wang and K. J. Lee, Adv. Mater., 2012, 24, 2999–3004. 14 C. K. Jeong, K.-I. Park, J. Ryu, G.-T. Hwang and K. J. Lee, Adv. Funct. Mater., 2014, 24, 2620–2629. 15 J. H. Jung, C. Y. Chen, B. K. Yun, N. Lee, Y. S. Zhou, W. Jo, L. J. Chou and Z. L. Wang, Nanotechnology, 2012, 23, 375401. 16 C. K. Jeong, I. Kim, K.-I. Park, M. H. Oh, H. Paik, G. T. Hwang, K. No, Y. S. Nam and K. J. Lee, ACS Nano, 2013, 7, 11016–11025. 17 Z. H. Lin, Y. Yang, J. M. Wu, Y. Liu, F. Zhang and Z. L. Wang, J. Phys. Chem. Lett., 2012, 3, 3599–3604. 18 K.-I. Park, C. K. Jeong, J. Ryu, G. T. Hwang and K. J. Lee, Adv. Energy Mater., 2013, 3, 1539–1544. 19 J. H. Jung, M. Lee, J. I. Hong, Y. Ding, C. Y. Chen, L. J. Chou and Z. L. Wang, ACS Nano, 2011, 5, 10041–10046. 20 K. Y. Lee, D. Kim, J. H. Lee, T. Y. Kim, M. K. Gupta and S. W. Kim, Adv. Funct. Mater., 2014, 24, 37–43. 21 Q. Feng, M. Hirasawa and K. Yanagisawa, Chem. Mater., 2001, 13, 290–296. 22 M. Rossel, H. R. Hohe, H. S. Leipner, D. Voltzke, H. P. Abicht, O. Hollricher, J. Muller and S. Gablenz, Anal. Bioanal. Chem., 2004, 380, 157–162. 23 A. D. Li, C. Z. Ge, P. Lu, D. Wu, S. B. Xiong and N. B. Ming, Appl. Phys. Lett., 1997, 70, 1616–1618. 24 M. Arous, H. Hammami, M. Lagache and A. Kallel, J. NonCryst. Solids, 2007, 353, 4428–4431. 25 H. Hammami, M. Arous, M. Lagache and A. Kallel, J. Alloys Compd., 2007, 430, 1–8. 26 R. Yang, Y. Qin, L. Dai and Z. L. Wang, Nat. Nanotechnol., 2009, 4, 34–39.
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Lead-free BaTiO3 nanowires-based flexible nanocomposite generator† Cite this: DOI: 10.1039/c4nr02246g
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 Received 25th April 2014 Accepted 30th May 2014
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
DOI: 10.1039/c4nr02246g
instantaneous power (1.2 mW) of the NCG device by calculating the load voltage and current through
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the connected external resistance.
Introduction Over the past few decades, energy harvesting technologies have received signicant attention as an alternative to solve the threats associated with environmental problems (e.g., depletion of ozone layer, global warming, and emission of harmful gas) as well as energy crises.1–3 In particular, the exible energy harvesters, which can convert the electricity from more accessible mechanical energy sources than other renewable energy sources, are the most promising candidates to realize the energy generation without restraints.4,5 In 2006, Wang and co-workers used the piezoelectric ZnO nanowire (NW) arrays to harvest electrical energy from mechanical energy sources and proposed a sustainable/exible energy harvesting device called a nanogenerator.6,7 They also demonstrated energy harvesting that converts electrical signals from not only mechanical bending motions but also small movements such as movement of the human nger,8 animal heartbeat, and diaphragm activities.9 Since then, there have been attempts to fabricate the thin-lm nanogenerators utilizing the highly efficient perovskite-structured BaTiO3 and PZT thin lms.10–12 In these attempts, highoutput performance was achieved by adopting inherently high piezoelectric materials. Recently, Park et al.13,14 developed exible nanocomposite generators (NCGs) based on piezoelectric BaTiO3 13 and
The 4th Research and Development Institute-3, Agency for Defense Development (ADD), Yuseong P.O. Box 35, Daejeon 305-600, Republic of Korea. E-mail: [email protected]; Fax: +82-42-823-3400-16250; Tel: +82-42-821-4336 † Electronic supplementary information (ESI) available: PDF materials involve the linear superposition test results (Fig. S1) and the durability test results (Fig. S2) of BaTiO3 NWs-based NCG device. A video le (Video S1) shows the power up of an LCD screen by the NCG device without any external energy source. See DOI: 10.1039/c4nr02246g
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0.942(K0.480Na0.535)NbO3–0.058LiNbO3 (KNLN)14 particles for scalable, exible, and lead-free energy harvesting devices by employing simple and low-cost spin-casting or bar-coating techniques. They dispersed perovskite piezoelectric particles and additives (such as graphitic carbons and copper nanorods) as dispersant, stress reinforcement, and conduction path to produce the piezoelectric nanocomposite (p-NC). To fabricate bio-eco-friendly NCG devices without toxic dispersion enhancers, lead-free and non-toxic piezoelectric one-dimensional nanostructures, such as KNbO3 nanorods15 and BaTiO3 NWs16/nanotubes,17 have also been used. Although the fabricated exible harvesters have shown the ability to realize costeffective and exible self-powered energy systems, the proposed NCG devices made of KNbO3 nanorods and BaTiO3 nanotubes have still shown the drawbacks and limitations such as insufcient output performance15 and energy generation by mechanically compressive stress,17 respectively. Moreover, BaTiO3 NWs synthesis employing an M13 virus as a template involves complicated procedures and a high temperature annealing process for the crystallization of perovskite ceramics before dispersing the piezoelectric nanostructures in polydimethylsiloxane (PDMS) matrix.16 In this paper, we have synthesized BaTiO3 NWs via a simple hydrothermal method at low temperature and fabricated the lead-free NCG device to achieve an environmentally friendly exible energy harvester without toxic dispersion enhancers. The hydrothermally grown BaTiO3 NWs show high aspect ratio and crystallinity with a tetragonal phase. The p-NC made of mixing piezoelectric NWs into an elastomeric polymer matrix is spin-casted onto a PDMS-coated silicon (Si) wafer and subsequently cured in an oven. The completely cured PDMS/p-NC/ PDMS layers are transferred onto indium tin oxide (ITO)-coated polyethylene terephthalate (PET) plastic substrates. The
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fabricated BaTiO3 NWs-based exible NCG device generated output voltages of up to 7.0 V and current signals of up to 360 nA during periodic and regular bending and unbending motions by a bending machine. The harvested energy sources are utilized to drive a liquid crystal display (LCD). We have also characterized the recorded output signals as a function of the connected external resistance and calculated the instantaneous power (1.2 mW) of an energy harvester.
Experimental section Synthesis of BaTiO3 NWs
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substrate and fully hardened at room temperature for 1 day. Next, PDMS/p-NC/PDMS layers sliced into a size of 3 cm 3 cm were detached from the Si wafer and sandwiched between a transparent 100 nm ITO-coated thin PET plastic substrate (50 mm in thickness, SKC) and a thick PET (175 mm in thickness, Sigma-Aldrich) for fabricating the NCG device. To measure the output voltage and current signals, Cu wires were attached to the top/bottom electrodes on exible substrates by conductive epoxy (silver paste, Chemtronics). Finally, the exible NCG device was poled at 140 C while applying an electric eld ranging from 0.5 to 1.5 kV for 12 h to enhance output performance.
BaTiO3 NWs were fabricated by a two-step hydrothermal reaction. The rst step was to synthesize Na2Ti3O7 NWs as an intermediate product. 2 g of anatase titanium dioxide powder (TiO2, 97%, Sigma-Aldrich Co.) was homogeneously dispersed in 40 ml of 10 M NaOH aqueous solution. This mixture was poured into a Teon-autoclave and maintained in the oven at 200 C for 3 days. Then, the precipitates collected from the autoclave were washed with deionized water and alcohol four times 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 with stirring for 1 hour (h). Next, this solution was poured into 40 ml of 0.12 M barium hydroxide [Ba(OH)2, 98%, Sigma-Aldrich Co.] aqueous solution, and then mechanically agitated for 1 h. For the ion exchange reaction, this reactant was poured into a Teon bottle and maintained in an oven at 100 C for 24 h. Aer the freeze-drying process, well-distributed BaTiO3 NWs were obtained for an energy generation source within NCG devices.
To periodically and regularly stress the NCG devices, a customdesigned bending machine was used with a maximum horizontal displacement of 5 mm at a bending strain rate of 0.2 m s1. During the repeated bending and unbending deformation, the open-circuit voltage and short-circuit current signals generated from NCG devices were measured and realtime recorded by a measurement unit (Keithley 2612A) and a computer, respectively. Moreover, an NCG device was connected with external load resistors ranging from 200 kU to 700 MU, and then the load voltage and current to resistor were also measured to calculate instantaneous power. For removal of artifact signals induced from external charges, all the measuring performances were carried out in a Faraday cage on an optical table.
Material characterizations
Results and discussion
Scanning electron microscope (SEM, XL 30, Philips, Japan) and eld-emission transmission electron microscope (FE-TEM 300 kV, Tecnai G2 F30, FEI Co., USA) were utilized to observe the morphologies and the crystal structure of BaTiO3 nanostructures, respectively. The phases present of the piezoelectric BaTiO3 NWs were characterized by X-ray diffraction (XRD, Rigaku, D/MAX-2500 X-ray diffractometer, Tokyo, Japan) using CuKa radiation (l ¼ 0.15406 nm at 30 kV and 60 mA). Raman analysis (ARAMIS, Horiba Jobin Yvon, France) was employed to obtain 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 in the ratio of 10 : 1 and placed in a desiccator to eliminate air bubbles. To form a dielectric layer between a piezoelectric material and plastic substrates, an approximately 50 mm PDMS layer was spin-casted onto a Si wafer and then hardened at 85 C for 10 min in an oven. By dispersing the hydrothermally synthesized BaTiO3 NWs into the PDMS elastomeric at various ratios of 5, 10, 20, 30, and 40 wt%, the p-NC was obtained and then deposited onto a PDMS-coated Si wafer using a spincasting process at a spinning rate of 1500 rpm for 30 s. The top PDMS dielectric layer was also spin-casted onto a p-NC/PDMS/Si
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Measurement of output signals generated from the NCG devices
For energy harvesting using the piezoelectric BaTiO3 NWs, we adopted a widely used NCG fabrication technique, as illustrated by the schematics of overall fabrication in Fig. 1a and detailed in the Experimental section. Since the nanocomposite-based nanogenerator technique developed by Park et al. is simple, lowcost, and scalable, many researchers have attempted to demonstrate NCG devices by various piezoelectric nanomaterials.13,14,16,18–20 An NCG device consists of polymer and two plastic substrates, in which the PDMS/p-NC/PDMS layers are sandwiched between two ITO-coated PET substrates. The spincoated thin PDMS layers act as a dielectric layer that can maintain not only an electric stability during poling process but also mechanical durability. Fig. 1b presents the piezoelectric potential inside p-NC calculated by multiphysics simulation soware. For characterization of piezoelectric potential difference, we used a simplied two-dimensional model with six BaTiO3 NWs, PDMS, and two electrodes whose parameters were taken from a COMSOL package. According to the calculation of previously reported studies, the mechanically neutral plane is located inside a bottom plastic substrate; as a result, the p-NC layer is placed above the mechanically neutral plane and entirely deformed by tensile stress during the bending process.13,18 By adopting the above assumption, the simulation model 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 a elastomeric 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 the magnified SEM image of BaTiO3 NWs in the elastomeric matrix. (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.
corresponds to tensile stress of 0.211 GPa from relationships among strain, stress, and Young's modulus (67 GPa) of BaTiO3.10,13 From the piezoelectric potential illustrated by color code, the BaTiO3 NWs can be effectively used to realize the energy harvesting of NCG devices compared to nanoparticles (NPs), which are inevitably aggregated in an elastomeric matrix.13,16,18 Fig. 1c shows the cross-sectional SEM images of the NCG device (le panel) and p-NC layer (right panel), in which PDMS (50 mm)/p-NC (150 mm)/PDMS (50 mm) layers are sandwiched between two plastic substrates. From the SEM images, the BaTiO3 NWs can be well-distributed in a so PDMS matrix with no dispersing agents, whereas only the NPs-PDMS composite 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-NC layer (3 cm 3 cm) stretched by ngers without any damage. Moreover, due to the inherently sticky PDMS property, there is no separation that can be incurred at interface between p-NC
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and plastic substrates during extremely mechanical bending/ unbending deformations. Fig. 2a and b shows the SEM and FE-TEM images of the piezoelectric BaTiO3 NWs synthesized by the hydrothermal method, respectively. As shown in the inset of Fig. 2a, Na2Ti3O7 NWs as the intermediate product are obtained by the rst hydrothermal reaction. The BaTiO3 NWs synthesized by ion exchange reaction with a barium source shows a high aspect ratio (the average length of 4 mm and the average diameter of 156 nm) (Fig. 2b) and the well-distributed morphologies without agglomeration by employing the freeze-drying process (Fig. 2a). BaTiO3, however, contains a very few of agglomerated clusters, and these unintended resultants seems to be caused by an inevitably competitive reaction during the two-step hydrothermal synthesis.21 Crystallization of piezoelectric ceramic materials is essential to harvest the electric energy and enhance the energy conversion efficiency; thus, we also characterized the hydrothermally 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 the intermediate product obtained by the first hydrothermal synthesis. (b) A magnified TEM image obtained from a piezoelectric BaTiO3 NW on a TEM grid (the inset). (c) and (d) XRD pattern (c) and Raman spectrum (d) observed from hydrothermally grown BaTiO3 NWs.
Raman spectroscopy for a more comprehensive phase characterization. The distinguishable XRD patterns show the perfect crystallinity and general results of perovskite-structured materials without crystalline of by-products such as BaCO3 or TiO2 (Fig. 2c). The active modes of Raman spectra in the range of 250 to 720 cm1 are in good agreement with the tetragonal perovskite BaTiO3 ceramics (Fig. 2d).22 Furthermore, the tetragonal Raman bands such as E/B1 and E/A1 modes at 307 and 715 cm1, respectively, indicate that BaTiO3 NWs involve the cubicto-tetragonal phase transformation.23 When the BaTiO3 NWs-based NCG device is regularly deformed by a linear motor with periodical bending and unbending motions (Fig. 3a), the generated electrical output voltage and current signals of the harvester are shown in Fig. 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 and short-circuit current of 360 nA from mechanical deformation. In the switching-polarity test, to verify the measured output signals, the inversion of voltage and current signals is observed. As shown in Fig. 3b, the positive and negative output signals were measured by bending and unbending motions, respectively, in forward connection (the inset of Fig. 3b-i). On the contrary, in reverse connection, the signal polarities are inverted, as shown in Fig. 3c. We have also conducted a linear superposition test to further conrm the energy generation of the NCG device (see ESI, Fig. S1a and b†). The output voltage
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and current pulse are enhanced when the two NCG device showing different output performance are connected in series and parallel, respectively. From these conrmation results, the measured signals are introduced from the NCG device by piezoelectric effect. To investigate the mechanical stability of NCG device, the durability test is carried out by repeatedly bending and unbending motions (see ESI, Fig. S2†). During the 5000 bending and unbending cycles, there is slight deviation of output signals and the stable voltage signals are observed. This result indicates that our NCG device can be applied to harsh mechanical conditions. Optimizing the amount of BaTiO3 NWs in an elastomeric matrix is an important issue to fabricate the NCG devices. Fig. 4a shows the measurement results of an NCG device with various ratios of BaTiO3 NWs and PDMS. When the content of piezoelectric NWs is varied from 5 to 20 wt%, the generated output voltage and current of the harvesters are increased by introducing the increment of piezoelectric NWs density. These enhancements can be introduced by the high polarization due to extensive change in the dielectric constant within NWs– polymer composites.24,25 On the other hand, the inordinately high quantity (above 20 wt%) of piezoelectric NWs inside the same region of p-NC can lead to the degradation of electromechanical coupling effect owing to an overly high dielectric constant of the p-NC layer, and these behaviors will yield low output 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 from BaTiO3 NWs-based NCG device. Upon the repeatedly bending/unbending motions, the open-circuit voltage and short-circuit current generated from the NCG device in the forward (b) and reverse (c) connections.
We also compared the NCG performance before and aer an electrical poling process with external voltage from 0.5 to 1.5 kV to further verify the measured output signals obtained from piezoelectricity of BaTiO3 NWs (Fig. 4b). The non-poled NCG device without alignment of piezoelectric dipoles produces negligible signals, while the generated voltage and current of NCG device aer poling process increase with external load voltage. The strain-dependent property of a BaTiO3 NWs-based NCG device is evaluated by deforming the harvester in different displacement. As shown in Fig. 4c-i, the amplitude of the output voltage increases with the bending curvature at a constant strain rate because the internal piezoelectric potential can be enhanced by the introduced strain. Similarly, we also found that the output performance depends on the angular bending strain rate at xed strain (Fig. 4c-ii). When the NCG device is quickly bent at the xed strain, a higher output voltage signals is observed than that of slowly deformed NCG devices. This behavior seems to be caused by the reduction of accumulated or released charges due to the quite slow electron ows during slow bending and unbending motions.26 We have demonstrated the energy utilization using only the generated energy sources
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from NCG devices. As shown in Fig. 4d-i, an LCD screen taken from a table clock is directly connected with a BaTiO3 NWsbased NCG device with no external circuits. By deforming the NCG device by a bending machine, an LCD screen is driven by the positive electrical signals (Fig. 4d-ii). Furthermore, as an LCD device shows the non-polar property, we also observed the 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 be turned on without external energy sources (see ESI, Video S1†). These results show that the BaTiO3 NWs-based energy harvester can generate sufficient energy sources to operate a commercial electric device. Fig. 5a shows the load voltage and current recorded as a function of the connected external resistance from 200 kU to 700 MU. By increasing the load resistance, the load voltage through the resistor shows rising tendency and saturation at a high external load. The load current across the resistors, in contrast, is steadily decreased as a function of resistors. Finally, we characterized the instantaneous power outputs of an NCG device by calculating the load voltage and current measured
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Fig. 4 (a) The harvested electrical signals of NCG device with various ratios of BaTiO3 NWs inside PDMS matrix. (b) The output voltage and current signals generated from NCG device before and after electrical poling process. (c) Dependence of the output voltage on angular bending curvature (i) and strain rate (ii) subjected to the NCG device. (d) The captured photographs of an LCD device operated by generated energy sources when an NCG device is deformed by bending (ii) and unbending (iii) motions.
(a) The measured load voltage and current under different external resistance varying from 200 kU to 700 MU. (b) The relationship between the instantaneous power outputs and external resistance. The effective power of the BaTiO3 NWs-based harvester calculated by the load voltage and current is 1.2 mW at an external load of 20 MU. Fig. 5
with the resistors. As shown in Fig. 5b, the effective power on the load resistor by the BaTiO3 NWs-based harvester is calculated as up to 1.2 mW at an external load of 20 MU.
Conclusions In summary, we have synthesized piezoelectric BaTiO3 NWs by a simple hydrothermal method at low temperature and developed the NCG device without toxic dispersion enhancers. The
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hydrothermally synthesized BaTiO3 NWs via two-step process show an average length of 4 mm with a high aspect ratio and well-distributed morphologies without agglomeration aer the freeze-drying process. The lead-free exible energy harvester fabricated by the spin-casting of BaTiO3 NWs–PDMS composites generates a high output voltage of up to 7.0 V and current signals of up to 360 nA from the periodically bending/ unbending motions. This high energy conversion efficiency is introduced by adopting the well-dispersed nanostructure with a
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high aspect ratio. The energy sources generated from NCG device are veried by the widely used tests and a nite element method, and then are used to operate an LCD without external circuits. This simple, economical, and practical NCG technology provides a breakthrough for bio-compatible and exible energy harvesters. Furthermore, this technique can be expanded to military applications (such as movement sensors and emergency power sources) and self-powered road systems by adopting a simple pavement process.
Acknowledgements This work was supported by DAPA and ADD. The authors would like to thank Prof. K. J. Lee, Prof. D. K. Kim, Mr. C. K. Jeong, and Mr. C. Y. Baek in KAIST for their experimental and theoretical supports.
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