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Applied Surface Science

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Full Length Article

Piezoelectric energy conversion by lead-free perovskite BaTiO3 nanotubearrays fabricated using electrochemical anodization

Chang Kyu Jeonga,b,1, Jae Hoon Leec,1, Dong Yeol Hyeonc, Yeon-gyu Kimc, Seoha Kimc,Changyeon Baekd, Gyoung-Ja Leee, Min-Ku Leee, Jin-Ju Parke, Kwi-Il Parkc,⁎

a Division of Advanced Materials Engineering, Jeonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju, Jeonbuk 54896, Republic of KoreabHydrogen and Fuel Cell Research Center, Jeonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju, Jeonbuk 54896, Republic of Koreac School of Materials Science and Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu 41566, Republic of Koread Components Solution Biz Unit, Samsung Electro-Mechanics Co. 150 Maeyeong-ro, Yeongtong-gu, Suwon, Gyeonggi 16674, Republic of Koreae Sensor System Research Team, Korea Atomic Energy Research Institute, 111 Daedeok-daero, 989 Beon-gil, Yuseong-gu, Daejeon 34057, Republic of Korea

A R T I C L E I N F O

Keywords:PiezoelectricBaTiO3

Nanotube arraysEnergy harvestingNanogeneratorFlexible electronics

A B S T R A C T

Synthesis approaches for diverse morphologies of 1D piezoelectric ceramic nanomaterials are still challenging toinvestigate nanoscale properties and applications. In this work, we demonstrated a vertically grown bariumtitanate (BaTiO3; BT) nanotube (BT NT) arrays fabricated using optimized electrochemical anodization andhydrothermal reaction. The centimeter-scale synthesized BT NT arrays with a high aspect ratio of up to 300showed the complete perovskite crystal structure and the notable piezoelectric coefficient of ~180 pm·V−1. Asingle BT NT-based piezoelectric device was fabricated on a flexible plastic substrate using a highly controlledfocused ion beam-assisted method. The single BT NT device produced a piezoelectric voltage of ~150 mV and acurrent of ~3 nA when the NT was mechanically stimulated by bending and releasing, which are higher thanpreviously reported values for 1D piezoelectric nanoscale materials. Finally, a flexible piezoelectric energyharvester (f-PEH) was achieved using vertically grown BT NT arrays with an elastomer as a vertically alignednanocomposite; its generated maximum open-circuit voltage and short-circuit current were ~1.0 V and ~20 nA,respectively. The approach in this study can propose the vision for more advanced 1D piezoceramic nanoma-terials and future piezoelectric nanodevices.

1. Introduction

Piezoelectric perovskite-structured ceramics have been used prac-tically in a wide range of applications such as actuators, sensors, ac-celerators, multilayer ceramic capacitors and energy generators be-cause they have high mechanical/dielectric properties, outstandingchemical/thermal stability, environmental safety and excellent elec-tromechanical coupling efficiency [1–5]. Among the perovskite oxides,barium titanate (BaTiO3; BT) has attracted much attention in the fieldof mechanical energy harvesting research due to its relatively highpiezoelectricity, being lead-free, its biocompatibility, as well as its low-cost production [6–9]. Recent research progress in the synthesis ofnanoscale BT, such as nanoparticles (NPs) [10,11], nanowires (NWs)[12–15], nanofibers [16,17], and nanotubes (NTs) [18–20] has ac-celerated its use in of the high-performance electronic applications andflexible piezoelectric energy harvesters (f-PEHs) that can convert

flexional/vibrational deformation to useful electrical energy. Althoughthe triboelectric device technology is also attractive for mechanicalenergy harvesting due to the high performance, there are some demeritssuch as instability by mechanical abrasion, degradation by humidity,limited package by air gap existence, and so forth [21,22]. Thus, thepiezoelectric device is still highly important to achieve future self-powered applications.

Significant efforts has been made to explore how to synthesize high-performance BT nanomaterials and to develop perovskite oxide-based f-PEHs [23–25]. Some researchers have synthesized BT NPs/NWs/NTsusing facile hydrothermal methods and fabricated f-PEH by simple spin-coating of composites consisting of piezoelectric nanomaterials andpolymeric elastomers [10,26,27]. These composite-based f-PEHs are ofgreat significance because they enable brittle and bulky perovskite-structured piezoelectric ceramics to be utilized as flexible piezoelectricdevices like the thin film configuration [28–30]. Therefore, ceramic

https://doi.org/10.1016/j.apsusc.2019.144784Received 28 September 2019; Received in revised form 8 November 2019; Accepted 18 November 2019

⁎ Corresponding author.E-mail address: kipark@knu.ac.kr (K.-I. Park).

1 These authors contributed equally to this work.

Applied Surface Science 512 (2020) 144784

Available online 28 November 20190169-4332/ © 2019 Elsevier B.V. All rights reserved.

nanomaterial-embedded f-PEHs have shown various advantages, in-cluding excellent mechanical stability, scalability, and economicalfabrication. However, they have drawbacks, such as low sensitivity,insufficient power density and poor energy conversion efficiency owingto plenty of polymeric matrices with a limited amount of piezoelectricnanomaterial and two thick top/bottom plastic substrates [10,31–34].

New strategies to resolve the aforementioned problems of piezo-electric nanocomposites can mainly be addressed as follows: (i) in-creasing the content of ceramic nanomaterials as the active piezo-electric filler, (ii) arranging the piezoceramic nanomaterials in anorderly manner and (iii) connecting the active nanomaterials directly tothe two electrodes. One-dimensional (1D) piezoelectric nanomaterialsare attractive candidates because they can have preferably orientedstructures directly established on the metal electrode substrate. 1Dnanostructures are also able to induce an effective piezoelectric cou-pling coefficient resulting from its very deformable morphology[35–38]. To confirm the advantages of 1D piezoelectric nanostructuresin terms of device performance, some researchers (including our group)have demonstrated BT NW arrays grown on conductive substrates bytwo-step hydrothermal reactions and have evaluated their piezoelectricperformance under mechanical pushing or bending deformation[39–43]. It should be noted that 1D BT nanostructures can be muchmore easily synthesized than others made of 1D perovskite nanoma-terials with complex stoichiometry. Nevertheless, previous reports havementioned the difficulty in obtaining excellent uniformity, high yieldproduction, and a high aspect ratio of the anisotropic nanostructure of1D BT nanostructures [19].

In this study, the growth of vertically aligned anisotropic 1D BT NTarrays with a high aspect ratio was achieved directly on a conductivesubstrate. TiO2 NT arrays used as the initial template was grown on thinTi foil using an electrochemical anodization process, which is con-sidered as a promising method for 1D oxide nanostructures verticallygrown on a metal substrate. Finally, the vertically grown 1D BT NTarrays were obtained by the hydrothermal conversion of the TiO2 NTtemplate. We characterized the piezoelectric property of the anodizing-synthesized BT NTs by means of piezoresponse force microscopy (PFM)analysis, thereby showing their excellent piezoelectric coefficient of180 pm·V−1. Moreover, a BT single NT-based device was fabricated viafocused ion-beam (FIB)-Pt deposition on a flexible plastic substrate andsubsequently investigated for its piezoelectric output caused by bendingdeformation. The measured voltage and current signals from the singleNT were ~150 mV and ~3 nA, respectively, which are higher than forpreviously reported single 1D nanomaterials-based piezoelectric de-vices. We also used whole 1D BT NT arrays to fabricate an f-PEH device.The anodizing-synthesized BT NT array-based energy harvester gener-ated a voltage range from 0.7 to 1.0 V and a current range from 10 to20 nA with periodic bendings. A finite element analysis (FEA) simula-tion was additionally used to support the measured piezoelectric outputof the f-PEH based on the vertically aligned BT NT arrays.

2. Experimental section

2.1. Preparation of BaTiO3 (BT) NT arrays on Ti foil

Vertically grown BT NT arrays were prepared in two steps involvinganodization and subsequent hydrothermal processing. First, well-aligned TiO2 NT arrays were obtained on flexible Ti foil (> 99.6%,50 μm in thickness; Goodfellow, UK) with an area of 3 cm × 4 cm viaan anodic oxidation process. The well-cleaned Ti foil attached to themetal electrode (Ni, 3 cm × 4 cm; Sigma-Aldrich, USA) and a bareelectrode were immersed in an electrolyte solution (ethylene glycol;Sigma-Aldrich) with a constant potential difference of 60 V for 2.5 h atroom temperature and the distance between two metal electrodes of2 cm. The volume expansion of the mother layer via the anodizationprocess caused the surface of the oxide layer to become irregular withholes, resulting in deeper holes and nanotube structures. The resulting

TiO2 NT arrays/Ti substrate was sonicated for 1 min in deionized (DI)water to remove any residual electrolyte solution. Next, the conversionprocess of TiO2 to BT NTs was conducted via a hydrothermal reaction.As-anodized Ti foil was soaked in 0.05 M barium oxide octahydrate (Ba(OH)2·8H2O) solution (≥98%; Sigma-Aldrich) in a homemade Teflon-lined autoclave. The packed reactor was placed in an air-forced oven at150 °C for 4 h. After the conversion reaction, the as-synthesized BT NTswere rinsed in a sonicator with DI water for 20 s to remove any by-products and then dried at room temperature. The resulting sampleswere cut or selected for material characterization, such as a cross-sec-tion, piezoelectric response, and output signals of a single BT NT.

2.2. Material characterization

Top-view and cross-section images of the anisotropic BT NT arrayswere observed using field-emission scanning electron microscopy (SEM;JSM-6701F, JEOL, Japan). The crystallographic structure of the per-ovskite BT NT arrays was analyzed by means of X-ray diffraction (XRD;D/Max-2500, Rigaku, Japan) with Cu Ka radiation (λ = 1.54 Å) op-erated at 40 kV and 200 mA. Atomic force microscopy (AFM; NX20,Park Systems, Korea) with a conductive tip (ElectriMulti75-G-10 M,Park Systems) was used to determine the piezoelectric charge constant(dij) of the BT NTs on a Ti substrate.

2.3. Fabrication steps of a single BaTiO3 (BT) NT onto a flexible substrate

A single BT NT was connected with the electrode lines on a plasticsubstrate based on a previously reported protocol [44,45]. 100 nm-thick Au electrode lines with a width of 30 μm and an inter gap of40 μm were deposited onto a Kapton polyimide substrate (125 μm,Dupont Co., USA) by sputtering deposition and a typical sequentialmicrofabrication process. Next, a thin Cr layer was deposited onto anelectrode lines-patterned flexible substrate to prevent electric chargingduring the FIB process. To prepare the BT NT-dispersed solution, BT NTarrays on Ti foil were immersed in ethanol and sonicated for 5 min. Adrop of the solution was spin-casted onto the flexible substrates, andsubsequently, both ends of the selected single NT were connected to theadjacent electrode lines by FIB-Pt deposition in situ. Finally, two Cuwires were affixed to electrode pads using a conductive epoxy resin(CW2400, Chemtronics, USA) and connected to a high-voltage supplyfor poling at a potential difference of 1 kV for 20 h at 120 ℃.

2.4. Fabrication of an f-PEH device based on vertically aligned BaTiO3 (BT)NT arrays

For the formation of the top electrode in the f-PEH device made of1D BT NT arrays on a conducting substrate, Al foil (a thickness of17 μm) with an area of 2 cm × 3 cm was directly placed on BT NTarrays and fixed by means of a polymeric epoxy resin (polymethylmethacrylate, PMMA). Next, top and bottom electrodes were connectedwith Cu wires using an Ag-based epoxy to measure the piezoelectricoutput signals of the f-PEH device. Piezoelectric BT NT arrays sand-wiched between two metal electrodes were placed on a hardened~1.2 mm-thick polydimethylsiloxane (PDMS; Sylgard 184, DowCorning, USA) polymeric pad and packed with PDMS polymer (thick-ness of 800 μm) made of a base and hardener in the ratio of 10:1. TheBT NT array-based f-PEH encapsulated by PDMS elastomer was fullycured at 85 ℃ for 10 min in an oven. Finally, the piezoelectric energydevice was poled with an electric field from 20 to 60 kV·cm−1 at 120 ℃for 4 h.

2.5. Measurement of harvested output signals

A programmable bending machine (Bending System, SnM, Korea)was used to input the periodically mechanical bending with variousdisplacement and strain rates. BT single NT onto a flexible substrate

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was deformed by bending conditions with a strain of 0. 283% and astrain rate of 2.32%·s−1. The f-PEH based on vertically aligned BT NTarrays was subjected to deformation with horizontal deformation dis-placement of 10 mm from an original length of 5 cm at a deformationrate of 15 cm·s−1. The piezoelectric output signals converted frommechanical bending were measured with an Electrometer 6514/E(Keithley, USA) and recorded in real-time on a computer.

2.6. Multiphysics simulation of an f-PEH device made of BT NT arrays

We conducted FEA using the multiphysics COMSOL Package soft-ware to calculate the piezoelectric potential distribution inside the BTNT arrays when subjected to mechanical deformation. We designed a3D simulation model of BT NT arrays on Ti foil packed with a PDMSmatrix. The material and piezoelectric parameters of each geometricpart were taken from the COMSOL v5.4 material library. One plane ofthe simulation model was fixed while the other was deformed by theprescribed displacement along the positive direction of the y-axis. Thedisplacement was calculated by using the width of the 3D simulationmodel and the effective strain was determined as the distance (δ) fromthe neutral plane to the middle of the piezoelectric thin film and thebending radius (Rc).

3. Results and discussion

Fig. 1a shows the fabrication of electrochemical anodization-basedsynthesis of BT NT arrays and the subsequent process for the f-PEHdevice. Pristine Ti foil was anodized to fabricate vertically grown TiO2

NT arrays which acted as a template for the subsequent addition of Bato fabricate the BT NT arrays. Note that the anodization process hasvarious advantages, such as the high aspect ratio of NTs, the con-trollable morphology of the NT diameter, good scalability, high yield

time, and so forth [19]. Next, the TiO2 NT arrays were converted intoBT NT arrays using Ba(OH)2·8H2O solution in a hydrothermal reactorfor 4 h. It should be noted that we definitely optimized the anodization-based synthesis methodology of the BT NT arrays in detail, therebymaking our approach for fabricating 1D BT nanomaterials superior topreviously reported ones. Although there has previously been an ap-proach to fabricate anodizing-based BT nanomaterials, the researcherscould only establish irregular BT nanorods rather than BT NTs [19].After the synthesis of vertically grown BT NT arrays, Al foil was at-tached to them using the PMMA solution for the top electrode and wassubsequently connected to a Cu wire for measurement. Simultaneously,Ti foil acted as the bottom electrode with another Cu wire. To confirmthe mechanical and chemical stability of the BT NT array-based f-PEHdevice, the whole structure was encapsulated in the PDMS elastomer asa flexible vertically aligned nanocomposite.

Fig. 1b exhibits a photograph of finally synthesized BT NT arraysusing electrochemical anodization on flexible Ti foil in which the ver-tically grown BT NT arrays are very densely defined throughout theentire area (3 cm × 4 cm). As shown in a cross-sectional SEM image(the inset of Fig. 1b), the total thickness of the BT NT array layer on theTi foil is around 15 μm. Fig. 1c presents a top SEM image of the ver-tically grown BT NT arrays with high uniformity. The detailed dimen-sions of the BT NTs could also be measured in the magnified SEM image(the inset of Fig. 1c): an outer diameter of ~130 nm and an innerdiameter of ~82 nm; from these, the active wall width of the BT NTswas estimated as ~50 nm. Therefore, the aspect ratio of the BT NTs was300, which is much higher than for previously reported long BT NWs[39–43].

Fig. 1d presents a photograph of the final f-PEH device fabricatedwith the vertically grown BT NT arrays. The active area is around2 cm× 3 cm, corresponding to the area of Al top electrode. The smallertop electrode area is to avoid any surface discharge breakdown between

Fig. 1. (a) A schematic illustration of the synthesis of vertically grown BT NT arrays on Ti foil through the electrochemical anodization. (b) Photograph of BT NTarrays on the Ti substrate (inset: a cross-sectional SEM image of BT NT arrays). (c) Top SEM image of the BT NT arrays (inset: a magnified SEM image of a single BTNT array to observe its detailed dimension). (d) Photograph of the f-PEH device based on BT NT arrays encapsulated in PDMS.

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the top and bottom electrodes during the poling process or measure-ments. The device is mechanically flexible, reversible, and stable, whichis attributed to the PDMS encapsulation as well as the high-quality Tisubstrate.

The XRD pattern in Fig. 2a indicates that the anodization-basedsynthesis clearly established the perovskite crystal structure in thevertically grown BT NT arrays. The piezoelectric property was simplyinspected using PFM. To measure a reliable piezoelectric coefficient(d33), the PFM-based unipolar piezoresponse was measured ten times atdifferent positions on the vertically grown BT NT arrays (Fig. 2b andTable 1). The average d33 was confirmed as ~180.3 pm·V−1, which is arelatively high piezoelectric coefficient value compared to previous BTnanomaterials [46,47]. Note that the principle of the piezoelectricity ofBT nanomaterials with a dimension of 100 nm is not yet clear [11]. Themaximum measured d33 is 213 pm·V−1 whereas the minimum one is133 pm·V−1. The comparatively broad distribution of the coefficientvalues at different positions is presumably due to other electro-mechanical coupling factors (e.g. flexoelectricity) resulting from thehighly thin wall dimension and deviation in roughness on the surface of

the sample [48,49].To investigate the mechanical energy harvesting ability of a single

BT NT, we fabricated a single BT NT-based device using the FIB processwith an electrode-prepatterned flexible plastic substrate. Each single NTin ethanol can be easily dispersed through ultrasonic treatment. Thedispersed NT was spin-casted onto an electrode of prepatterned flexibleKapton polyimide substrate; thus, each single NT should be randomlypositioned somewhere within the prepatterned electrode. Last, a se-lected single BT NT was elaborately connected to the prepatternedelectrodes using FIB-Pt deposition for bridging, as shown in the SEMimage in Fig. 3a. In contrast to a device fabricated with laterally alignedNTs or NWs, one made with randomly-dispersed single NT or NW re-quires highly controlled nanofabrication [44,50]. It should be notedthat the length of the single BT NT on the device was shorter than theoriginal vertical BT NT arrays (the inset of Fig. 3a). This was becausefirst, part of the length of single BT NT was buried in the FIB-Pt de-position bridging electrode; second, it was difficult to detach the wholelength of from the Ti substrate; and third, the BT NT was easily brokenduring the ultrasonication.

Fig. 3b presents a photograph of the single BT NT-based flexibledevice. The microscale prepatterned Au electrodes were formed byconventional photolithography on the Kapton substrate. As shown inthe top inset of Fig. 3b, optical microscopy clearly defines the goodconnection of both the microscale prepatterned Au electrodes and theFIB-Pt deposition bridging electrodes. A simplified schematic of thecross-sectional structure of the single NT to understand the bridgingstructure is illustrated in the bottom inset of Fig. 3b. When the single BTNT-based flexible device was repeatedly bent and released to deformthe single BT NT in the longitudinal direction, the device generated anopen-circuit voltage of ~150 mV and a short-circuit current of ~3 nA(Fig. 3c), which are higher than those of previously reported single NW-based piezoelectric devices [44,51,52]. As presented in Fig. 3d, thegenerated signals in the reverse connection with the measurementelectrometer corresponded to those in the forward connection, whichmeans that the produced electrical signals originated from the piezo-electric effect of the single BT NT.

The energy harvesting characteristics of the BT NT array-based f-PEH device were evaluated by periodic bending and releasing de-formation with an Rc of ~1.17 cm using the customized bending ma-chine stage (Fig. 4a). The f-PEH generated an open-circuit voltage rangefrom 0.7 to 1.0 V and a short-circuit current range from 10 nA to 20 nA(corresponding to a current density range from 1.67 nA·cm−2 to3.33 nA·cm−2), as shown in Fig. 4b and 4c, respectively. The switchingpolarities of both the voltage and current peaks in the reverse con-nection were also well defined, which clarifies the piezoelectric effect ofthe device. According to the Young’s moduli, Poisson’s ratios, andthicknesses of each layer of the f-PEH, the applied strain on the verti-cally grown BT NT array layer could be calculated as 0.181% from thebending deformation through the well-known mechanical neutral planeequation [53,54]. It should be mentioned that the PDMS encapsulationis highly suitable for bending piezoelectric devices, compared topressing piezoelectric device because pressing the piezoelectric energyharvesters might bring about some secondary interruption effects suchas surface triboelectrification, mechanoradical reaction, flexoelectriccoupling, etc. [55–57].

The FEA simulation was performed using the multiphysics COMSOLsoftware to more deeply examine the piezopotential difference of thevertically grown BT NT arrays in the PDMS matrix as an aligned na-nocomposite. As shown in Fig. 5a, a 3D simulation model was initiallydesigned with the same dimensions as the real synthesized BT NT arraysand the standard material properties in the software. One face of the BTNT array-based nanocomposite model was fixed while the other wasdisplaced by the prescribed deformation to simulate the bending strain.Fig. 5b depicts the calculated piezoelectric potential distribution insidethe vertically grown BT NT array-based structure. The maximum pie-zopotential difference between the two electrodes is ~0.8 V, which is

Fig. 2. (a) XRD pattern and (b) piezoresponse measurements of the anodizing-synthesized vertically grown BT NT arrays on Ti foil.

Table 1Piezoelectric charge constant of a BT NT arrays from PFM measurements.

Data point Piezoelectric chargeconstant (d33, pm·V−1)

StandardError

Coefficient ofDetermination (R2)

Spot 1 165 0.00129 0.98460Spot 2 213 0.00141 0.98895Spot 3 207 0.00149 0.98685Spot 4 189 0.00221 0.96638Spot 5 201 0.00177 0.98071Spot 6 174 0.00116 0.98873Spot 7 151 0.00173 0.96746Spot 8 168 0.00169 0.97481Spot 9 133 0.00146 0.97024Spot 10 202 0.00217 0.97130

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slightly smaller than the measured voltage signals of f-PEH. This dis-crepancy is because the simulation model consists of the BT NT arrayscompletely buried within the full PDMS matrix. Therefore, the me-chanical stress can be slightly dissipated by the PDMS elastomeric partlike a mechanical bumper. Note that the configuration of fully in-filtrated PDMS matrix is inevitable due to feature of COMSOL model-ling. In contrast, the real device is composed of the BT NT arrays be-neath the top attached Al electrode, blocking the full PDMS infiltration;thus the PDMS in the actual device is just for an encapsulation box. Itshould be mentioned that the well-aligned piezoelectric nanostructureis superior to the randomly distributed one, as has been reported pre-viously [43].

Fig. 6a describes the voltage and current output according to theexternal load resistance. As expected by applying Ohm’s law, the vol-tage increased with the outer circuital resistance while the currentdecreased. This indicates that the piezoelectric energy harvesting de-vice can be regarded as a standard electronic component for practicaluse. The instantaneous power was directly calculated from the load

voltage and current output, as shown in Fig. 6b; the maximum poweroutput was around 3 nW (corresponding to a power density of~0.5 nW·cm−2) at the circuital resistance of ~100 MΩ. Although thepower was not high, this is the first time that the BT NT arrays havebeen used to provide piezoelectric power. The output was also affectedby the electric field of the poling process: the higher electric field couldalign more ferroelectric domains in the perovskite-structured ceramics,resulting in a larger generated voltage output which was nearly satu-rated over the field of 40 kV·cm−1 (Fig. 6c). The durability of the BT NTarray-based f-PEH device was also tested, as shown in the plot inFig. 6d; stable energy harvesting signals were maintained during hun-dreds of bending/releasing cycles.

4. Conclusions

To summarize, we demonstrated the anodization synthesis of ver-tically grown perovskite BT NT arrays and used them in an f-PEH de-vice. The synthesized perovskite BT NT arrays were well established

Fig. 3. (a) Tilted SEM image of a single BT NT connected to microscale FIB-Pt deposition line electrodes (inset: a magnified SEM image focused on the single BT NT).(b) Photograph of the single BT NT-based piezoelectric device on the electrodes-prepatterned flexible plastic substrate (top inset: an optical microscope image of thesingle BT NT connected to microscale FIB-Pt deposition line electrodes and bottom inset: a cross-sectional schematic of the single BT NT device). (c,d) Generatedopen-circuit voltage and short-circuit current signals from the single NT-based flexible piezoelectric device with bending and releasing deformation in (c) the forwardconnection and (d) the reverse connection with the measuring equipment.

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with a very high aspect ratio of ~300 and centimeter-scale morpholo-gical uniformity. The piezoelectric coefficient of the vertically grownBT NT arrays was evaluated as ~180 pm·V−1. The piezoelectric beha-vior of a single BT NT was explored in a device using sophisticatedlycontrolled nanofabrication with FIB processing. The single BT NT de-vice produced piezoelectric signals of ~150 mV and ~3 nA by bendingand releasing deformation, which is higher than for previously reportedsingle piezoelectric NW devices. Moreover, we fabricated an f-PEHdevice using vertically grown BT NT arrays within an encapsulatedstructure. The energy harvester generated an open-circuit voltage rangefrom 0.7 to 1.0 V and a short-circuit current range from 10 nA to 20 nAby bending and releasing deformation. The piezoelectric output of theBT NT array-based f-PEH structure was additionally verified via an FEAsimulation in detail. The achievement of this study could lead to solu-tions for the facile development of perovskite-structured piezoceramicNT structures and realizing the development of future piezoelectricnanodevices such as nano-electromechanical systems, including na-noactuators, nanosensors, nanotransducers, and so on.

Author contribution

C.K. Jeong, J.H. Lee, D.Y. Hyeon, and K.-I. Park conceived the ideaof this research article. D.Y. Hyeon performed the COMSOL simulation.Y.-G. Kim performed the PFM analysis. J.H. Lee and S. Kim planned andperformed the BaTiO3 nanotube array-based energy harvester. C. Baek,G.-J. Lee, M.-K. Lee, and J.-J. Park performed the material character-ization. C.K. Jeong, J.H. Lee, and K.-I. Park prepared and wrote the dataexplanation and the manuscript.

Acknowledgements

This research was supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF) funded bythe Ministry of Education (NRF-2019R1I1A2A01057073) and theMinistry of Science and ICT (NRF-2019R1C1C1002571, NRF-2018R1A4A1022260). This study was also supported by the KoreanNuclear R&D program organized by the National Research Foundationof Korea (NRF) grant funded by the Korea Government (MSIT) (NRF-2017M2A8A4017220).

Fig. 4. (a) Photographs of the original/re-leasing and bending states of the BT NTarray-based f-PEH device during energyharvesting measurements. Energy har-vesting (b) open-circuit voltage output and(c) short-circuit current output generatedfrom the BT NT array-based f-PEH devicegenerated by bending and releasing.

Fig. 5. (a) Model of the FEA simulation for the vertically grown BT NT array-based f-PEH device. (b) Generated piezopotential calculated using the FEA simulationwith the COMSOL software when the f-PEH was stimulated by mechanical strain.

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Fig. 6. (a) Generated voltage and currentoutput, and (b) produced instantaneouspower from the BT NT array-based f-PEHdevice according to the external circuitalload resistance. (c) Generated voltageoutput in accordance with the applied elec-tric field during the poling process to thevertically grown BT NT arrays before mea-surement. (d) Mechanical durability test re-sult of the BT NT array-based f-PEH device.

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