High-Performance Piezoelectric Nanogenerators via Imprinted Sol

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX

High-Performance Piezoelectric Nanogenerators via Imprinted Sol− Gel BaTiO3 Nanopillar Array Sung-Ho Shin,†,‡ Seong-Young Choi,†,§ Min Hyung Lee,*,§ and Junghyo Nah*,‡ ‡

Department of Electrical Engineering, Chungnam National University, Daejeon 34134, Korea Department of Applied Chemistry, Kyung Hee University, Yongin, Gyeonggi 17104, Korea

§

S Supporting Information *

ABSTRACT: We report high-performance piezoelectric nanogenerators (PENGs) with nanoimprinted sol−gel BaTiO3 (BTO) nanopillar array polarized under high electric field and ultraviolet. The PENGs fabricated using this method demonstrate greatly enhanced output voltage of ∼10 V and current density of ∼1.2 μA cm−2, respectively, in comparison to that of flat PENG. Further, the PENG demonstrates uniform output characteristics over the entire device area thanks to uniform nanoimprint pillar array. The approach introduced here is simple, effective, reliable, and reproducible way to fabricate high-performance sol−gel-based PENGs and electronic devices. KEYWORDS: sol−gel, BaTiO3 nanopillars, uniform outputs, piezoelectric nanogenerators, UV treatment

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In this work, we report greatly enhanced performance of flexible iron (Fe)-doped barium titanate (BTO) nanopillar array PENGs. The uniformly patterned piezoelectric BTO nanopillars were formed on a sol−gel-coated PET using a PDMS (polydimethylsiloxane) stamp, contributing to achieve scalable and enhanced piezoelectric output performance. In addition, UV treatment on the patterned sol−gel layer contributed to further improve crystallinity of the sol−gel BTO layer. Consequently, the output voltage and current density of the PENG with a 400 nm pitch between the pillars (active area: 1 cm2) exceed ∼10 V and ∼1.2 μA cm−2, respectively, under the applied force of 0.3 MPa, which is two orders of magnitude higher output voltage and 6-fold higher output current density compared to those of the PENG without the patterning and subsequent treatment. Figure 1a shows the scanning electron micrographs (SEMs) of the uniform Fe-doped BTO nanopillar array with a diameter of ∼120 nm, a pitch of 400 nm, and a height of ∼400 nm and the inset shows the well-aligned BTO nanopillar array. The experimental details of BTO nanopillars formation are described in Figures S1 and S4. We note that different shapes can also be prepared in a similar way. The formed BTO nanopillar array was characterized using Fourier Transform Infrared Spectroscopy (FT-IR). As indicated in Figure 1b, two strong absorption peaks at 459 and 650 cm−1 were observed from the BTO nanopillar sample coated on an PET substrate. The former peak at 459 cm−1 is attributed to bending vibration

iezoelectric nanogenerators (PENGs) have gained much attention as they can generate electricity from any physical motion, existing in nature and our living environment1−5 To date, a number of research efforts have been made to develop high-performance flexible PENGs by employing various piezoelectric materials and nanostructures.6−10 The two most efficient approaches to maximize the output performance of flexible PENGs are to adopt crystallized ferroelectric materials with high piezoelectric coefficient such as BaTiO3 (BTO) and PbZrxTi1−xO3 (PZT) and to employ nanostructures that can effectively absorb applied physical energy.11,12 Although high output voltage can be attained by these approaches, it is difficult to achieve uniform output performance because of randomly distributed piezoelectric nanoparticles inside the composite structures. Thus, it is necessary to develop a simple, reproducible, and scalable fabrication technique to construct low-dimensional nanostructures on the flexible substrates for practical applications. One of the promising methods to realize highly uniform piezoelectric nanocrystal structure on a plastic substrate is to employ the sol−gel process, which allows us to form different piezoelectric metal-oxide films using a simple spin-coating method.13,14 Nevertheless, it is still challenging to simultaneously satisfy both uniform nanostructures formation and decent crystallinity of the sol−gel film on the plastic substrates. Although thermal treatment is required to enhance crystallinity of the sol−gel film,15 which is essential for high piezoelectric output power generation of PENGs, only limited temperature can be applied to the plastic substrates. Previously, the inorganic sol−gel precursor solutions have been widely adopted to form nano/ micro-patterns by the nanoimprint lithography.16−18 © XXXX American Chemical Society

Received: August 7, 2017 Accepted: November 13, 2017 Published: November 13, 2017 A

DOI: 10.1021/acsami.7b11773 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. (a) SEM image of the well-ordered BaTiO3 nanopillar array. (b) FT-IR spectrum from the BaTiO3 noanopillar, showing the strong absorption peaks of Ba−O and Ti−O bonds. (c−e) TEM images of Fe-doped BaTiO3 sol−gel: BaTiO3 sol−gel without any treatment (c) at low magnification and (d) at high magnification. (e) BaTiO3 seeds after UV illumination and poling process where the lattice fringes show the crystallized BaTiO3.

Figure 2. Schematic representation of generation mechanism (a) without polarization and (b) with polarization. Under the same applied force, relatively higher Vpiezo‑potential is developed from the PENG with polarized BTO nanopillar array, generating higher output current. (c) Measured output voltage and current density of the PENGs treated by different postprocess: nonpoled, poled, and UV-treated and poled. The highest output performance was achieved from the PENG that is both UV-treated and poled (blue). Significantly smaller outputs were measured from the PENG with nonpoled BaTiO3. (d) Schematic drawing of Saywer-Tower circuit to measure the polarization of ferroelectric BaTiO3. (e) Polarization switching (Ps), remanent polarization (Pr), and coercive field (Ec) were improved when the BTO film is both UV-treated and poled (blue).

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Figure 3. (a) Experimentally measured output voltage and current density of the PENGs with different pitches between the BaTiO3 nanopillars. The lowest outputs were generated from the PENG with a flat BaTiO3 film. The outputs increase in inverse proportion to the pitch. (b) Output Vpiezo‑potential, calculated using COMSOL software models of with (bottom) and without (top) pillar formation. The larger Vpiezo‑potential was observed from the device with nanopillar structure when the same force was applied to the device, thanks to effective absorption of the applied stress.

PET is presented in Figure S2. The crystallinity change of BTO sol−gel was observed using a transmission electron microscope (TEM). Without any treatment, BTO seeds surrounded by a gel matrix could be observed in low magnification image (Figure 1c), and the high magnification TEM image shows the amorphous phase (Figure 1d). After UV illumination and poling process, lattice fringes were observed, meaning UV illumination and poling process changes amorphous BTO into crystallized BTO (Figure 1e). X-ray diffraction (XRD) spectra of before and after UV treatment and poling process are also shown in Figure S3. Next, we investigated the output performance of the PENGs with the BTO nanopillar arrays processed differently. The detailed device fabrication process is described in Supporting Information S4. To demonstrate the roles of domain polarization, the patterned sol−gel layer was differently processed. The domains of nonpoled sol−gel layer is expected to be randomized as in Figure 2a. On the other hand, those of poled sol−gel layer are aligned as shown in Figure 2b. The experimental results show that the PENG with nonpoled BTO nanopillars generates the lowest ∼1 V and 0.25 μA cm−2, respectively (Figure 2c, black). In comparison, the outputs were dramatically increased to ∼6 V and ∼0.8 μAcm−2 by performing high voltage poling process (Figure 2c, red). Before complete curing of BTO pillars, the quality of the film can be improved by exposing the BTO pillars to UV because gelated organic cross-linking network can be promoted by the curing effect under UV.21−23 Indeed, after UV treatment, the highest output voltage and current density were enhanced to ∼10 V and ∼1.2 μA cm−2 (Figure 2c, blue). As shown in Figure 2d, we examined the polarization-electric field (P-E) characteristics of

Figure 4. (a) Uniform output performance of the PENG with an active size of 4.5 cm × 4.5 cm. Different colors denote different regions of the device, colored with red, green, blue, and pink dashed lines in the photographs. Identical output voltage and current density were measured from all the regions, verifying the formation of uniform nanopillar array.

of Ti−O bond19 and also the latter peak at 650 cm−1 is due to the stretching of Ba−O bond.20 The FT-IR analysis of bare C


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Figure 5. (a) Measured output voltage response by varying the applied pressure normal to the surface of device. The output voltage increases in proportion to the input pressure from 0.05 to 0.3 MPa. (b) The output voltage vs applied pressure. The sensitivity (S = Δy/Δx) of 25 was measured, which can be potentially adopted to develop flexible piezoelectric pressure sensors.

with a dimension of 4.5 cm × 4.5 cm was fabricated and constant force (0.3 MPa) was repeatedly applied to the device using the push machine with an area of 1 cm2. As shown in Figure 4a and b, consistent output Vpeak‑to‑peak and Ipeak‑to‑peak were measured from four different regions of the device. This can provide great advantages in developing different piezoelectric device applications.26,27 Moreover, the self-powerd pressure sensing performance is also demonstrated in Figure 5. Here, the sensitivity (S) (S = Δy/Δx) of 25 was obtained from this device by increasing the applied pressure from 0.05 to 0.3 MPa. In summary, we have developed a high-performance BTO nanopillar PENG. Highly uniform and well-ordered BTO nanopillars were formed by soft-lithographic imprinting of the BTO sol−gel film on a plastric substrate. The UV processing combined with high-voltage poling has contributed to further improving output performance. Consequently, output voltage and current density up to ∼10 V and ∼1.2 μAcm−2 were attained from the developed PENGs. Furthermore, thanks to the extremely uniform output performance across the entire device area, this method can potentially be used to develop large-area self-powered piezoelectric sensor applications.

the differently processed BaTiO3 films using Sawyer−Tower circuit.24 In the ferroelectric P-E hysteresis loop, the larger polarization switching (Ps) of 0.27 μC cm−2, remanent polarization (Pr) of 0.12 μC cm−2, and coercive filed (Ec) of 11 kV cm−1 were confirmed from the UV-treated and poled ITO/BaTiO3/Au sample (Figure 2e, blue) in comparison to only poled sample (Figure 2e, red). We note that capacitance of the sol−gel film was also measured under double sweep mode, exhibiting ferroelectric behavior (Figure S5). Figure 3a shows the roles of BTO nanopillars formation on output performance of the PENGs. The results show that the generated output voltage and current density from the thin film BTO PENG were ∼0.6 V and ∼0.2 μA cm−2, respectively, under the applied force of 0.3 MPa. When the device is fabricated using BTO nanopillars, apparent output performance enhancement can be observed. The device with a 800 nm pitch shows the output voltage of ∼3 V and current density of ∼0.6 μA cm−2, respectively. Further, considerable enhancement in both output voltage and current density, generating ∼10 V and ∼1.2 μA cm−2, were observed from the device with a 400 nm pitch, which was approximately two orders of magnitude higher voltage and 6-fold higher output current density, respectively, by comparison to the device with flat BTO thin film. As shown in Figure 3b, to qualitatively show the roles of nanopillar formation, the piezoelectric potentials were calulated for two different structures using finite element method (COMSOL simulation software): one with nanopillars and the other with a flat film. We note that simulation conditions (material properties and applied force) were kept the same for both BTO models except for the BTO structures under PDMS. Under the same force of 0.3 MPa, it can be noticed that relatively high output potential (∼10 V) can be obtained from the simulation model with BTO nanopillar array (Figure 3b, bottom) compared with that of the flat structure model (∼5 V) (Figure 3b, top). This result shows that more effective absorption of applied force can be achieved by forming BTO nanopillars.10,25 Here, we note that in the actual device the BTO layer exists as a composite of crystallized BTO nanoparticles and non cross-linked polymers. Thus, dielectric constant of BTO layer is lower than that of ideal uniform BTO layer, which increases the output voltage of the PENG. Therefore, even if the BTO layer is not completely crystallized, which may result in lower piezoelectric constant, the overall piezoelectric output voltage, however, can be maintained by reduced dielectric constant of BTO layer as a result of composite formation (Figure S6). In Figure 4, we demonstrate uniform output performance of the PENG with BTO nanopillar array. For this test, the device

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b11773. Flowchart and photograph of Fe-doped BaTiO3 sol−gel synthesis; FT-IR spectrum of bare PET and BaTiO3coated PET; XRD spectra of non-treated BaTiO3 film and UV-treated and poled BaTiO3; fabrication process of BaTiO3 nanopillar PENG; capacitance v.s. bias voltage plot of Ag/BaTiO3/Si/Al; measured dielectric constant v.s. frequency and calculated piezoelectric potential dependence on dielectric constant measured outputs and power density by varying load resistance; reliability test of the PENGs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.H.L). *E-mail: [email protected] (J.N). ORCID

Min Hyung Lee: 0000-0001-8313-9857 Junghyo Nah: 0000-0001-9975-239X Author Contributions †

S.-H.S. and S.-Y.C. contributed equally to this work

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ACS Applied Materials & Interfaces Notes

(15) Peroz, C.; Chauveau, V.; Barthel, E.; Søndergård, E. Nanoimprint Lithography on Silica Sol−Gels: A Simple Route to Sequential Patterning. Adv. Mater. 2009, 21, 555−558. (16) Barbé, J.; Thomson, A. F.; Wang, E.-C.; McIntosh, K.; Catchpole, K. Nanoimprinted TiO2 Sol−Gel Passivating Diffraction Gratings for Solar Cell Applications. Prog. Photovoltaics 2012, 20, 143− 148. (17) Khan, S. U.; Göbel, O. F.; Blank, D. H. A.; ten Elshof, J. E. Patterning Lead Zirconate Titanate Nanostructures at Sub-200-nm Resolution by Soft Confocal Imprint Lithography and Nanotransfer Molding. ACS Appl. Mater. Interfaces 2009, 1, 2250−2255. (18) Bursill, L. A.; Brooks, K. G. Crystallization of Sol-Gel Derived Lead-Zirconate Titanate Thin-Films in Argon and Oxygen Atmospheres. J. Appl. Phys. 1994, 75, 4501. (19) Trivedi, M. K.; Nayak, G.; Patil, S.; Tallapragada, R. M.; Latiyal, O.; Jana, S. Impact of Biofield Treatment on Atomic and Structural Characteristics of Barium Titanate Powder. Ind. Eng. Manage. 2015, 4, 166. (20) Raymundo-Pereira, P. A.; Ceccato, D. A.; Junior, A. G. B.; Teixeira, M. F. S.; Lima, S. A. M.; Pires, A. M. Study on The Structural and Electrocatalytic Properties of Ba2+- and Eu3+- Doped Silica Xerogels as Sensory Platforms. RSC Adv. 2016, 6, 104529−104536. (21) Karataş, S.; Kızılkaya, C.; Kayaman-Apohan, N.; Güngör, A. Preparation and Characterization of Sol−Gel Derived UV-Curable Organo-Silica−Titania Hybrid Coatings. Prog. Org. Coat. 2007, 60, 140−147. (22) Guo, S.; Niu, C.; Liang, L.; Chai, K.; Jia, Y.; Zhao, F.; Li, Y.; Zou, B.; Liu, R. The Polarization Modulation and Fabrication Method of Two Dimensional Silica Photonic Crystals Based on UV Nanoimprint Lithography and Hot Imprint. Sci. Rep. 2016, 6, 34495. (23) Xie, T. Q.; Yang, L.; Sun, X. X.; Jiang, J.; Zhang, X. P.; Luo, Y.; Zhang, G. Q. UV Gelation of Single-Component Polyacrylates Bearing Dinitrobenzoate Side Groups. Chem. Commun. 2016, 52, 9383−9386. (24) Wang, L.; Wang, X.; Shi, J. Measurement and Estimation of Ferroelectric Hysteresis Loops. Ferroelectrics 2010, 411, 86−92. (25) Jalalian, A.; Grishin, A. M.; Wang, X. L.; Cheng, Z. X.; Dou, S. X. Large Piezoelectric Coefficient and Ferroelectric Nanodomain Switching in Ba(Ti0.80Zr0.20)O3−0.5(Ba0.70Ca0.30)TiO3 Nanofibers and Thin Films. Appl. Phys. Lett. 2014, 104, 103112. (26) Ng, T. H.; Liao, W. H. Sensitivity Analysis and Energy Harvesting for a Self-Powered Piezoelectric Sensor. J. Intell. Mater. Syst. Struct. 2005, 16, 785−797. (27) Yu, A.; Jiang, P.; Lin Wang, Z. Nanogenerator as Self-Powered Vibration Sensor. Nano Energy 2012, 1, 418−423.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.H.L. acknowledges Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (NRF2017M3D1A1039379). J.N. acknowledges Basic Research Laboratory (NRF-2017R1A4A1015744) and Basic Science Research Program (NRF-2015R1A1A1A05027235) funded by National Research Foundation of Korea.

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REFERENCES

(1) Bao Han, C.; Zhang, C.; Li, X. H.; Zhang, L. M.; Zhou, T.; Hu, W. G.; Lin Wang, Z. Self-Powered Velocity and Trajectory Tracking Sensor Array Made of Planar Triboelectric Nanogenerator Pixels. Nano Energy 2014, 9, 325−333. (2) Lorenz, M.; Lazenka, V.; Schwinkendorf, P.; Bern, F.; Ziese, M.; Modarresi, H.; Volodin, A.; Van Bael, M. J.; Temst, K.; Vantomme, A.; Grundmann, M. Multiferroic BaTiO3−BiFeO3 Composite Thin Films and Multilayers: Strain Engineering and Magnetoelectric Coupling. J. Phys. D: Appl. Phys. 2014, 47, 135303. (3) Ko, Y. J.; Kim, D. Y.; Won, S. S.; Ahn, C. W.; Kim, I. W.; Kingon, A. I.; Kim, S. H.; Ko, J. H.; Jung, J. H. Flexible Pb(Zr0.52Ti0.48)O3 Films for A Hybrid piezoelectric-pyroelectric nanogenerator under harsh environments. ACS Appl. Mater. Interfaces 2016, 8, 6504−6511. (4) Zhao, Z.; Pu, X.; Du, C.; Li, L.; Jiang, C.; Hu, W.; Wang, Z. L. Freestanding Flag-Type Triboelectric Nanogenerator for Harvesting High-Altitude Wind Energy from Arbitrary Directions. ACS Nano 2016, 10, 1780−1787. (5) Wu, L.; Huang, G.; Hu, N.; Fu, S.; Qiu, J.; Wang, Z.; Ying, J.; Chen, Z.; Li, W.; Tang, S. Improvement of The Piezoelectric Properties of PVDF-HFP Using AgNWs. RSC Adv. 2014, 4, 35896− 35903. (6) Chen, X.; Han, M.; Chen, H.; Cheng, X.; Song, Y.; Su, Z.; Jiang, Y.; Zhang, H. A Wave-Shaped Hybrid Piezoelectric and Triboelectric Nanogenerator Based on P(VDF-TrFE) Nanofibers. Nanoscale 2017, 9, 1263−1270. (7) Gupta, M. K.; Lee, J. H.; Lee, K. Y.; Kim, S. W. Two-Dimensional Vanadium-Doped ZnO Nanosheet-Based Flexible Direct Current Nanogenerator. ACS Nano 2013, 7, 8932−8939. (8) Shin, S. H.; Kim, Y. H.; Lee, M. H.; Jung, J. Y.; Nah, J. Hemispherically Aggregated BaTiO3 Nanoparticle Composite Thin Film for High-Performance Flexible Piezoelectric Nanogenerator. ACS Nano 2014, 8, 2766−2773. (9) Shin, S. H.; Kim, Y. H.; Lee, M. H.; Jung, J. Y.; Seol, J. H.; Nah, J. Lithium-Doped Zinc Oxide Nanowires-Polymer Composite for High Performance Flexible Piezoelectric Nanogenerator. ACS Nano 2014, 8, 10844−10850. (10) Wu, W.; Bai, S.; Yuan, M.; Qin, Y.; Wang, Z. L.; Jing, T. Lead Zirconate Titanate Nanowire Textile Nanogenerator for Wearable Energy-Harvesting and Self-powered Devices. ACS Nano 2012, 6, 6231−6235. (11) Kim, S. M.; Sohn, J. I.; Kim, H. J.; Ku, J.; Park, Y. J.; Cha, S. N.; Kim, J. M. Radially Dependent Effective Piezoelectric Coefficient and Enhanced Piezoelectric Potential Due to Geometrical Stress Confinement in ZnO Nanowires/Nanotubes. Appl. Phys. Lett. 2012, 101, 013104. (12) Seol, M. L.; Im, H.; Moon, D. I.; Woo, J. H.; Kim, D.; Choi, S. J.; Choi, Y. K. Design Strategy for A Piezoelectric Nanogenerator with A Well-Ordered Nanoshell Array. ACS Nano 2013, 7, 10773−10779. (13) Attia, S. M.; Wang, J.; Wu, G. M.; Shen, J.; Ma, J. H. Review on Sol-Gel Derived Coatings: Process, Techniques and Optical Applications. J. Mater. Sci. Technol. 2002, 18, 211−217. (14) Lee, W.; Kahya, O.; Toh, C. T.; Ozyilmaz, B.; Ahn, J. H. Flexible Graphene-PZT Ferroelectric Nonvolatile Memory. Nanotechnology 2013, 24, 475202. E


High-Performance Piezoelectric Nanogenerators via Imprinted Sol

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