Electric-Field-Dependent Surface Potentials and Vibrational Energy

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Electric-Field-Dependent Surface Potentials and Vibrational Energy Harvesting Characteristics of Bi(Na0.5Ti0.5)O3-Based Pb-Free Piezoelectric Thin Films Ahra Cho, Da Bin Kim, and Yong Soo Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00367 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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ACS Applied Materials & Interfaces

Electric-Field-Dependent Surface Potentials and Vibrational Energy Harvesting Characteristics of Bi(Na0.5Ti0.5)O3-Based PbFree Piezoelectric Thin Films Ahra Cho,†,# Da Bin Kim,†,# Yong Soo Cho*,† †Department

of Materials Science and Engineering, Yonsei University, Seoul 03722, Korea

ABSTRACT. The successful utilization of Pb-free piezoelectric materials is considered as critical since the piezoelectric material-based thin-film cantilever is still the preferred choice for commercial vibrational energy harvesters. Herein, we introduce a highly efficient piezoelectric energy harvester based on a Pb-free representative compound, Bi0.5Na0.5TiO3, which has not been explored so far. Applying a strong electric field for poling purposes brought unexpectedly huge changes in the dielectric constant and piezoelectric coefficient, which were responsible for the promising power density of 21.2 Wcm-2g-2Hz-1 with 537.7 mV output voltage and 2.22 µW output power for a 2 m-thick 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 thin-film cantilever. The power density value is the best so far compared to any reported values for thinfilm-based harvesters. As the origin of the effects of poling, the surface potentials across the grain structure are discussed in conjunction with the defect-dipole alignment, as evidenced by the increased oxygen vacancies on the film surface under an external bias field.

KEYWORDS: Bi0.5Na0.5TiO3, piezoelectric energy harvesting, thin films, cantilevers, poling 1 ACS Paragon Plus Environment

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INTRODUCTION The demand for new power sources beyond conventional batteries has recently increased because of their diverse usage in portable and wireless electronic devices in nonconventional environments. Piezoelectric energy harvesting is considered as one of the most attractive renewable power devices because this harvesting technique uses a variety of vibrational energy sources that are easily available without any limitations on time and space.1-4 Depending on the target applications of the energy harvesters, it is essential to design a suitable harvester structure operating under a specific mechanical source at a certain vibrational frequency.5,6 There are different types of piezoelectric energy harvesters, including thin-film cantilevers, for which various high-performance piezoelectric materials based on perovskite Pb(Zr,Ti)O3 (PZT) and (Na,K)NbO3 have been utilized. The figure of merit (FOM) defined as FOM = (-e31,f)2/(εrεo), where -e31,f is the effective transverse piezoelectric coefficient, εr is the relative permittivity, and εo is the dielectric permittivity of free space, is considered as one of the most critical parameters for characterizing harvesting performance.7-9 According to the equation, a piezoelectric film with a large e31,f and a low εr is preferred for a better harvesting performance. In this regard, numerous modifications of the basic perovskite structure by doping,10-12 the optimal design of the cantilever structure,13,14 and the adoption of competitive processing15,16 have been carried out for better performing thin-film cantilever harvesters with the focus on the possible utilization of Pb-free compositions. For example, poling with a strong electric field may be an ideal option since the orientation of the domains along the field direction provides enhanced piezoelectric properties whereas the dielectric constant is reduced due to the extension of the long-range structure orders.17,18 In this study, we focused on thin-film cantilevers based on Pb-free (Bi,Na)TiO3 (BNT) 2 ACS Paragon Plus Environment

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materials, which have been recognized as one of the most competitive Pb-free piezoelectric compositions. As general, (Na,K)NbO3 (NKN)-based materials have been more preferred as the Pb-free substitutes in most piezoelectric applications due to their higher piezoelectric coefficients and dielectric constant (r~290) than those of the BNT-based materials.8,16,19 The higher Curie temperature of ~410 oC in NKN (compared to ~325 oC in BNT) is another merit for the material selection depending on application and processing. However, a lower dielectric constant of ~230 for BNT is likely to be an advantage for the energy harvesting applications as anticipated from the FOM equation.20 There have been no reports so far on using BNT-based materials for piezoelectric thin-film energy harvesters. We initially examined the (Bi,Na)TiO3BaTiO3 material system to find an optimal composition with regard to the crystal structure and energy harvesting characteristics. As an optimal composition, 0.94BNT-0.06BT was selected for the thickness- and electric field-dependent harvesting performances. The resulting cantilever-type energy harvester showed an output power of ~2.22 µW from a vibration source at a resonance frequency of 42 Hz, which is very competitive compared to other previously reported thin-film-based cantilever harvesters.

EXPERIMENTAL SECTION (1-x)(Bi0.5Na0.5TiO3)-xBaTiO3 ((1-x)BNT-xBT; x = 0, 0.02, 0.04, 0.06, 0.08, 0.10)) thin films were prepared on Pt(111)/Ti/SiO2/Si substrates using a solution deposition technique. Bismuth acetate (Bi(CH3COO)3, >99.99%, Sigma-Aldrich), barium acetate ((CH3COO)2Ba, 99%, Sigma-Aldrich), anhydrous sodium acetate (CH3COONa, 98%, Daejung Chemicals, Korea), and titanium isopropoxide (Ti(CH3)2CHO)4, 97.0%, Sigma-Aldrich) were used. Barium acetate was first dissolved in a solvent containing 2-methoxyethanol (C3H8O2, 99.8%, Sigma-Aldrich) and acetic acid (CH3COOH, > 99%, Duksan Chemicals, Korea) at 60 oC with stirring for 30 min; the volume ratio of 2-methoxyethanol and acetic acid was 1:1. Bismuth acetate and 3 ACS Paragon Plus Environment


sodium acetate were then added to the Ba solution at 60 oC for 30 min. 10 mol% excess Bi and Na were used to compensate for their potential loss during firing. Titanium isopropoxide was finally dissolved in the Ba/Bi/Na solution with the addition of acetylacetone (CH3COCH2COCH3, >99%, Sigma-Aldrich) as a chelating agent. The final concentration of the complete precursor solution was 0.4 M. The solution was spin coated onto the Pt/Ti/SiO2/Si substrate at 3,000 rpm for 1 min. Each spin-coated layer was dried at 200 oC for 5 min and then pyrolyzed at 400 oC for 10 min. After multiple coatings for variable thicknesses (e.g., 24 coatings for ~2 m thickness), the films were annealed at 600, 700, 750, and 800 oC for 10 min in an ambient atmosphere. The poling of the annealed films was carried out in a silicon oil bath. Polydimethylsiloxane (PDMS) encapsulation was used to prevent the permeation of the silicone oil. The detailed procedure of the poling process can be referred to Figure S1 of the Supporting Information. Various poling parameters, such as the electric field, bath temperature, and poling time, were optimized for better harvesting performance, as demonstrated with an example in Figure S2. The crystal structure of the thin films was analyzed with an X-ray diffractometer (XRD; Ultima IV, Rigaku, Japan) using Cu Kα radiation (λ = 0.154056 nm) at a scan rate of 4°/min. The surface and cross-sectional microstructures of the films were obtained using field-emission scanning electron microscopy (FE-SEM; JSM 7001F, JEOL, Japan). To measure the dielectric properties, a 200 nm-thick Pt top electrode was deposited using DC-magnetron sputtering. The dielectric constant r was measured as a function of frequency using an impedance analyzer (HP 4194A, Yokogawa-Hewlett-Packard Ltd., Tokyo, Japan) in the frequency range of 100 Hz to 1 MHz. The piezoelectric coefficient of -e31 was measured with a four-point bender measurement unit (aix4PB, aixACCT, Aachen, Germany) interfaced with a thin film analyzer (TF Analyzer 2000; aixACCT, Aachen, Germany) which utilized a laser vibrometer for detecting deflections. Surface potentials across the grain boundary region of the selected films 4 ACS Paragon Plus Environment

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were measured using Kelvin probe force microscopy (KPFM; Nanoscope V Multimode, Bruker, USA) utilizing a Pt/Ir-coated Si cantilever. The contact potential difference was detected to build up a topographical image by applying an AC bias of 1 V at a second resonance frequency of 270 kHz in the cantilever.

RESULTS AND DISCUSSION Figure 1 shows the effects of the film thickness on the fundamental physical, dielectric, and piezoelectric characteristics of the 0.94BNT-0.06BT thin films annealed at the optimal temperature of 700 oC. The temperature was selected on the basis of the phase purity as seen in the XRD patterns of the BNT thin films with an annealing temperature range of 600 to 800 oC,

in which a phase-pure perovskite phase was only observed at 700 oC (See Figure S3).

Unreacted Bi2O3 and Na-deficient Bi2Ti2O7 phases were found at temperatures lower and higher than 700 oC, respectively. Preliminary experiments on the effect of the BT addition to the BNT thin films were also conducted to define the optimal composition of 0.94BNT-0.06BT, although there have been several reports on the compositional dependence in the BNT-BT system.21-23 The results on the XRD patterns, microstructures, -e31, r, and FOM of the BTvariable compositions are presented in Figure S4-S6. Conclusively, the 0.94BNT-0.06BT composition was selected as the best one as it exhibited a slightly better -e31 of 1.89 C/m2 with the best FOM value of ~1.23 GPa for the 2 m-thick films while belonging to the morphotropic phase boundary (MPB) region between the tetragonal and rhombohedral phases. Figure 1a shows the cross-sectional and surface images of the 0.94BNT-0.06BT films with different thicknesses of ~0.5, ~1.0, ~1.5, and ~2.0 µm, in which all of the films showed welldensified microstructures. Figure 1b exhibits a plot of average grain size as a function of film thickness, which was measured from the surface microstructures. The results indicate that the grain size depended on the film thickness as they had a larger average size with the thicker 5 ACS Paragon Plus Environment


film.24 Figure 1c shows X-ray diffraction patterns of the 0.94BNT-0.06BT thin films with different thickness. All the films presented a pure perovskite phase with random orientation and the peak intensities of which tended to increase with film thickness, thereby suggesting the enhanced crystallinity with a thicker film. The morphotropic phase boundary of all the samples was confirmed from the coexistence of tetragonal (002)/(200) and rhombohedral (200) peaks. Figure 1d shows the thickness dependence of the r, -e31, and FOM values, all of which demonstrated a gradual increase with increasing film thickness. The dielectric constant generally depends on the crystallinity, microstructure, film orientation, defects, and stress levels.25,26 The increase in grain size might have reduced the density of the grain boundary so that the movement of the domain walls was less restricted by the grain boundaries.27 Another possible explanation is that the clamping effect by the substrate acting against the domain wall movement was minimized by the thicker film,28 for which the formation of the electric dipoles was less confined by the substrate and thus the surface charges of the film were effectively collected, thereby leading to the higher r and -e31 values.29 For the contribution of these dielectric and piezoelectric coefficients, the FOM values were maximum for the 2.0 m BNTBT films. Accordingly, we chose an optimal film thickness of 2 µm for our subsequent studies. The influence of a strong electric field applied to the optimized BNT-BT thin films was investigated by focusing on the changes in the energy harvesting characteristics. Figure 2a-c represents the effect of the applied electric field for the poling procedure of the 2.0 µm-thick 0.94BNT-0.06BT films on the r, -e31, and FOM values. It was found that the dielectric constant decreased with increasing poling field up to ~500 kV/cm, while the piezoelectric coefficient showed the reverse trend, i.e., increased until ~500 kV/cm and then became saturated above this value. The calculated FOM followed the increasing tendency of the -e31 value. It is reasonable that the piezoelectric coefficient was saturated at a critical field strength with the gradual evolution of domain orientation toward the direction of the applied electric field. The 6 ACS Paragon Plus Environment

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saturation field of ~500 kV/cm is comparable to the reported value of ~600 kV/cm for BNTbased thin films.30 The varying tendencies with electric fields suggest that the increase of poling field improves the piezoelectric responses by aligning the domain orientation in electrical field direction.31 It was reported that the relative permittivity decreased as piezoelectric materials were poled due to the reduced domain wall density.32 Figure 2d shows the frequency dependence of the dielectric constant of the 0.94BNT0.06BT thin films in the range of 102-106 Hz after poling at a given electric field from 0-900 kV/cm. The dielectric constant of all of the samples decreased linearly with increasing frequency in the frequency range, which is typical in this film, as has been previously reported.33 It is postulated that the dipoles are more susceptible to electric displacement at lower frequencies. The decreasing tendency of the dielectric constant was reduced with increasing applied field, as can be seen in the slope change with the electric field in the inset plot of Figure 2d. An identical decreasing tendency of the dielectric constant with the applied electric field strength has been reported.15,19 As the poling field increased, the dielectric dispersion decreased, which is a reflection of the increased long-range polar ordering with a reduced degree of disorder.19,34 Figure 2e shows the XRD patterns of the unpoled and 600 kV/cm-poled 0.94BNT-0.06BT films. The inset shows the change in the peak intensities, thereby clearly indicating a structural transformation under the electric field. The increased intensity of the (002) peak is attributed to the increase in the volume fraction of the c-domain of the BNT-BT films by the applied electric field at the top and bottom electrodes.35 Figure 2f compares the effects of film thickness on the r, -e31, and FOM values of the films with and without the poling process. As the film thickness increased, the difference in the dielectric and piezoelectric constants between the unpoled and poled films increased, leading to the same trend in the variation of FOM values. This indicates that the film thickness and applied electric field were the critical factors in

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determining the dielectric and piezoelectric properties. The variations in dielectric constant with different film thicknesses, which depended on the applied electric field, can be seen in Figure S7. Figure 3a shows the schematic illustration of the experimental setup for the measurement of vibrational energy harvesting performance of BNT-BT films. The energy harvester was subject to a sinusoidal acceleration, and the clamped boundary condition of the energy harvester allowed the periodic vibration of the cantilever under an external vibration source with variable frequency.36 This vibration induced the mechanical stress in the BNT-BT films and thus generated charges by the direct piezoelectric effect. Figure 3b shows the frequency dependence of the cantilever tip displacement D and open circuit voltage Voc of the 2 µm-thick BNT-BT film-based energy harvesting device. When the frequency between two vibrations matches, the amplitude of the cantilever tip displacement shows the maximum value, leading to the improved harvesting properties. In this experiment, the resonance frequency was measured with the change in vibrational frequency by the function generator. The cantilever tip displacement showed the largest amplitude of ~2 mm at 42 Hz and then dropped as the input frequency moved away from the resonance frequency. The frequency dependence of the open circuit voltage also indicates that its resonance frequency was likely around 42 Hz with a peak Voc, max of ~751 mV after poling at 600 kV/cm. Figure 3c,d shows the measured values of output voltage and output power with load resistance for the 1 and 2 µm-thick 0.94BNT-0.06BT-based harvesting devices at the resonance frequency of 42 Hz. According to the maximum power transfer theorem, the maximum power at the load occurs when the internal resistance of the energy harvesting device is equal to the resistance of the load.13 To find the internal resistance of the harvester, a voltmeter was connected at the load resistance of the circuit and then the equipment was run to measure the


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harvesting properties while increasing the load resistance. As the vibrating piezoelectric element generates an AC voltage, the maximum output power is obtained by the equation: Pmax=VmaxImax=Imax2RL=Vmax2/RL.5,36 Therefore, the output voltage and output power in Figure 3c,d were maximum values at each load resistance. In the case of the 2 µm thick 0.94BNT0.06BT-based harvester, the maximum power, corresponding peak voltage, and load resistance of the unpoled BNT-BT film-based harvester were 0.293 µW, 153.2 mV, and 80 kΩ, respectively, while those of the poled BNT-BT film harvester under optimal conditions were 2.224 µW, 537.7 mV, and 130 kΩ, respectively, which indicates a significant improvement. It can be clearly seen in Figure 3c,d that the thicker 2 m films induced a better energy harvesting performance than the 1 m-thick cantilevers. The origin of the enhanced piezoelectricity with an electric field was further explored in terms of the electrostatic properties of the film surface for both the unpoled and poled BNTBT films with the KPFM measurements. Figure 4a shows the topographic atomic force microscopy (AFM) images of the unpoled and poled thin film samples, which were mapped with a potential field. The darker grain regions relative to the brighter grain boundary indicates higher surface potentials. The profiles of the surface potentials across the designated line in each image are included in Figure 4a, to demonstrate the clear potential difference between the grain and grain boundary regions for the unpoled and poled samples, and it is clear that the potential difference became smaller after the poling process. The average potential difference of ~125 mV decreased to ~55 mV after the poling at 600 kV/cm. The reduced difference in surface potential after poling is attributed to the domain alignments along the film thickness, as illustrated in Figure 4b. Applying a strong electric field incurred the domain orientation and thus minimized the differences in the surface potentials between the grain and grain boundary.37 The lowered potential across the grain boundary likely helped the charge transfer when the harvested charges were externally extracted with the continuous vibrating motion. 9 ACS Paragon Plus Environment


The enhanced harvesting mechanism with the poling procedure is illustrated in Figure S8, where more accumulated surface charges were induced with the higher dipole alignments in the poling case.1 We carried out an additional surface analysis using high-resolution XPS to ascertain the chemical states after the poling process. Figure 4c presents the changes in the chemical states of oxygen in a specific binding energy region after the poling treatment at 600 kV/cm for the 2 m-thick sample. XPS spectra of the cations: Ba 3d, Na 1s, Ti 2p, and Bi 4f, which had not been significantly changed by the poling, can be seen in Figure S9. It is noticeable that oxygen vacancies were created after the poling since the ratio of the concentration of oxygen vacancies OV to that of lattice oxygen OL increased from ~0.12 to ~0.28 after the poling, as is evident in Figure 4d. It has been reported that the poling process directs the oxygen vacancies toward the surface since the strong electric field reorients the defect dipoles, typically the pairs of cation and oxygen vacancies.38 With the stabilization of the dipoles under the external bias, more oxygen vacancies were likely positioned near the film surface.39 We compared our energy harvesting performance with those of other reported piezoelectric thin film-based cantilever harvesters, as shown in Figure 5.13,40-52 All relevant values, including voltage, power, and power density, are listed in the accompanying table. Extended plots of the comparative energy-harvesting performance are demonstrated in Figure S10 with more information in Table S1. Figure 5 represents the power density expressed as Wcm-2g-2Hz-1, where g is the acceleration,46 thereby suggesting the contribution of the power generation per resonance frequency. Since the target frequency and device volume are not identical in the literature, the power density in terms of dimension and resonance frequency is often directly compared.9,40 Our achieved value of 21.2 Wcm-2g-2Hz-1 for the 2 m-thick 0.94BNT-0.06BT cantilever corresponds to the best value so far among the piezoelectric thin-film-based harvesters, even though harvesting performance can be variable depending on the target usage 10 ACS Paragon Plus Environment

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and device design.42,51 This achievement is assumed to come mainly from the significant drop in the dielectric constant and the simultaneous enhancement of the piezoelectricity through the optimized poling process, which depends on the choice of piezoelectric materials.

CONCLUSIONS We demonstrated the Bi(Na,Ti)O3-based thin film cantilever harvester with the best harvesting performance, compared to any other reported thin film-based piezoelectric energy harvester, in terms of power density per resonance frequency. The reason for the best performance was believed mainly due to the optimized poling condition with the optimized composition in the Bi(Na,Ti)O3-BaTiO3 system for maintaining morphotropic phase boundary. The electric field applied for the poling purpose was found to dramatically reduce dielectric constant but to largely increase piezoelectric coefficient, depending on the level of electric field. As the optimal harvesting performance, an excellent power density of 21.2 Wcm-2g-2Hz-1 with 537.7 mV output voltage and 2.22 µW output power was obtained for the 2 m-thick 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 thin-film cantilever. The maximum output power is 7.6 times higher than the value for the unpoled case. The significant enhancement was correlated to the relative decrease of surface potential across the grain boundary and the increased oxygen vacancy as a result of the optimized poling process.



Figure 1. (a) Cross-sectional and surface images, (b) the variation in the average grain size, (c) XRD patterns, and (d) the variation in the dielectric constant εr at 1 kHz, transverse piezoelectric coefficient -e31,f, and FOM of the unpoled annealed 0.94BNT-0.06BT thin films of different thicknesses.


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Figure 2. Electric-field dependent variations of (a) dielectric constant εr at 1 kHz, (b) transverse piezoelectric coefficient -e31,f and (c) FOM of the 2 µm-thick 0.94BNT-0.06BT thin film. (d) Frequency dependence of the dielectric constant according to different electric field strength with an inset showing the slopes of εr versus frequency plots as a function of electric field, (e) XRD patterns with an inset of zooming in on the (002)/(200) peaks, and (f) variations of εr, e31,f, and FOM of the unpoled and poled films of different film thicknesses.



Figure 3. (a) A schematic illustration of the experimental setup for the measurement of vibrational energy harvesting performance with a cantilever structure, (b) the frequency dependence of cantilever tip displacement D and maximum open circuit voltage Voc,max for the 2 µm-thick 0.94BNT-0.06BT film harvester before and after poling, (c) output voltage, and (d) output power of the thin-film-cantilever harvesters at the resonance frequency of 42 Hz as a function of load resistance in the unpoled and poled cases.


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Figure 4. (a) Topographic AFM images of electronic-potential mapping with plots of the surface potentials across the white line of each sample, (b) schematic illustrations of the poling effect in the domain orientation over the film grain structure, (c) high-resolution XPS spectra of the O 1s peak, and (d) the ratios of oxygen vacancy OV to lattice oxygen OL of the unpoled and poled 0.94BNT-0.06BT film samples.



Figure 5. A comparison of the vibrational energy harvesting performances of reported piezoelectric thin film-based cantilevers with the results of the present work in terms of power density/fr (expressed in Wcm-2g-2Hz-1, where g is the acceleration and fr is the resonance frequency) in terms of resonance frequency. Note that BFO stands for BiFeO3.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: 10.1021/acsami.xxxxxx. XRD patterns of BNT thin films as a function of annealing temperature; characteristics of BNT-BT thin films as a function of BT content; frequency dependence of dielectric constant; schematic poling mechanism, additional XPS spectra; comparison of energy harvesting performance with literature; detailed poling procedure and the effects of poling conditions (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Yong Soo Cho: 0000-0002-1601-6395 Author Contributions #The

authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by grants from the National Research Foundation of Korea (NRF-2016M3A7B4910151), the Industrial Strategic Technology Development Program (#10079981), the Korea Institute of Energy Technology Evaluation and Planning (No. 20173010013340) funded by the Ministry of Trade, Industry, & Energy (MOTIE) of Korea, and the Creative Materials Discovery Program by the Ministry of Science and ICT (2018M3D1A1058536).

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TOC


Electric-Field-Dependent Surface Potentials and Vibrational Energy

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