High Performance Flexible Piezoelectric Nanogenerators based on

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High Performance Flexible Piezoelectric Nanogenerators based on BaTiO3 Nanofibers in Different Alignment Modes Jing Yan, and Young Gyu Jeong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02177 • Publication Date (Web): 30 May 2016 Downloaded from http://pubs.acs.org on May 30, 2016

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ACS Applied Materials & Interfaces

High Performance Flexible Piezoelectric Nanogenerators based on BaTiO3 Nanofibers in Different Alignment Modes

Jing Yan and Young Gyu Jeong*

Department of Advanced Organic Materials and Textile System Engineering, Chungnam National University, Daejeon 34134, Republic of Korea

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*Correspondence to Young Gyu Jeong ([email protected], +82-42-821-6617)

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Abstract Piezoelectric nanogenerators, harvesting energy from mechanical stimuli in our living environments, hold great promise to power sustainable self-sufficient micro/nano-systems and mobile/portable electronics. BaTiO3 as a lead-free material with high piezoelectric coefficient and dielectric constant has been widely examined to realize nanogenerators, capacitors, sensors, etc. In this study, polydimethylsiloxane (PDMS)-based flexible composites including BaTiO3 nanofibers with different alignment modes were manufactured and their piezoelectric performance was examined. For the purpose, BaTiO3 nanofibers were prepared by an electrospinning technique utilizing a sol-gel precursor and following calcination process, and they were then aligned vertically or horizontally or randomly in PDMS matrix-based nanogenerators. The morphological structures of BaTiO3 nanofibers and their nanogenerators were analyzed by using SEM images. The crystal structures of the nanogenerators before and after poling were characterized by X-ray diffraction method. The dielectric and piezoelectric properties of the nanogenerators were investigated as a function of the nanofiber alignment mode. The nanogenerator with BaTiO3 nanofibers aligned vertically in the PDMS matrix sheet achieved high piezoelectric performance of an output power of 0.1841 µW with maximum voltage of 2.67 V and current of 261.40 nA under a low mechanical stress of 0.002 MPa, in addition to a high dielectric constant of 40.23 at 100 Hz. The harvested energy could thus power a commercial LED directly or be stored into capacitors after rectification.

Keywords:

Nanogenerator,

Piezoelectricity,

BaTiO3

Polydimethylsiloxane.

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nanofiber,

Electrospinning,

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Introduction

Considering the possible depletion of fossil energy in the future, the development of alternative sustainable energy is extremely urgent.1 Even though some nature energy such as solar,2 wind3 and hydro4 powers can alleviate a lot, they are not available at any time and affected by environmental factors.5 Piezoelectric nanogenerators that accumulate electrical charges in certain solid materials in response to mechanical stress has become one of the promising energy suppliers for microsystems because the source of mechanical energy is ubiquitous and abundant in our living surrounding environments,6 such as human walking, vehicle transportation,7 fluid flow,8 etc. as well as tiny movement of finger bending,9-10 breathing, heartbeat,11 skin movement,12 etc. The generated energy is possible to power commercial light-emitting diodes (LEDs),9 micro-electromechanical systems (MEMS), implantable biosensors,13 ultra-low power wireless electronics, remote and mobile sensors and even portable/wearable personal electronics,14 by which the power consumed typically in the range of micro- to milli-watts.15 The piezoelectric nanogenerator can also be hybridized with batteries or capacitors, in which the mechanical energy is directly converted and simultaneously stored as chemical energy.16 The mechanism of nanogenerators can be described by that a shifting of the positive and negative charge centers takes place when a sufficient force is applied to piezoelectric materials, which then result in an external electrical field.17 Non-centrosymmetric structure is a prerequisite for the presence of piezoelectricity to produce electric dipole under stress, and the materials for piezoelectric nanogenerator mainly focused on perovskite materials.18 Barium titanate (BaTiO3) discovered in 1940s is an exceptionally efficient material researched extensively for piezoelectric property due to its high piezoelectric coefficient and dielectric constant.19 The “Ti” atom moves off-center according to the direction of the external field during poling because of the relatively small atom size and high charge of Ti4+, which can



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storage a large amount of potential energy.20 On the other hand, a variety of geometric structures including nanoparticles,5 nanotubes,6 nanosheets,21 nanocubes,7 nanowires,22 nanorods,23 and triangular-belts24 have been widely researched for piezoelectric energy harvesting ceramics. Among them, nanowires or nanofibers can maintain high mechanical robustness and compliance to small loads for efficiently converting mechanical energy from low intensity strain.22,25 Yun et al. investigated the power generation from NaNbO3 nanocubeand nanowire-based nanogenerators fabricated using the same procedure.26 The output voltage and current from a nanocube-based nanogenerator are almost half of those from a nanowire-based nanogenerator under the same experimental condition. Zhu et al. manufactured a nanogenerator based on vertically aligned piezoelectric ZnO nanowires, which generates a high peak open-circuit voltage of 58 V, short-circuit current of 134 µA and maximum power density of 0.78 W/cm3.27 Xu et al. reported that well aligned PbZr0.52Ti0.48O3 (PZT) nanowire arrays prepared by a chemical epitaxial growth method produce an output peak voltage of ~0.7 V, peak current density of 4µA/cm, and average power density of 2.8mW/cm.28 Sodano et al. fabricated vertically aligned BaTiO3 nanowire arrays via a two-step hydrothermal reaction and found that higher aspect ratio nanowires can generate superior voltage response under the same mechanical input due to the reduced stiffness.22,29 Persano et al. reported well aligned piezoelectric electrospun nanofibers with ultra-high sensitive to pressure by achieving 0.1 mV output voltage even at 0.1 Pa.25 Since one-dimensional nanowires or nanofibers are superior to zero-dimensional nanoparticles in percolation, nanowires- or nanofiber-based nanogenerators could deliver effectively generated piezoelectric potentials to outside electrodes. Although the geometric structural superiority of one-dimensional nanomaterials on the piezoelectric performance has been realized, the orientation effect of nanowires or nanofibers on piezoelectric property has not been investigated systematically. In addition, it is challengeable to synthesize uniformly aligned nanowires or nanofibers of piezoelectric ceramics. Typically, the nanowire arrays are


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vertically grown on conductive substrates by hydrothermal reactions.30 However, the growth process of nanowires requires high cost for the equipment, stringent reaction condition and unobservability during the process.31 Electrospinning has been extensively studied as a promising technique for preparing controlled and aligned nanofibers, since the processing procedure is relatively simple, cost-effective, and scalable.15 Nanofibers are also longer and more flexible than nanowires, although they are both one-dimensional in shape. In this research, BaTiO3 nanofibers were prepared by electrospinning technique utilizing sol-gel precursor and following calcination process. Three different types of flexible piezoelectric nanogenerators, in which BaTiO3 nanofibers were aligned vertically or horizontally or randomly in polydimethylsiloxane (PDMS) elastomer as matrix, were fabricated. The morphological feature of neat BaTiO3 nanofibers and BaTiO3/PDMS nanogenerators were characterized, and the crystal structures of BaTiO3/PDMS nanogenerator before and after poling were also examined. The dielectric and piezoelectric performance of the flexible nanogenerators with different alignment modes of BaTiO3 nanofibers was compared by taking account their output voltage, current, and power to an external mechanical compression. The research implies the feasibility and efficiency of BaTiO3 nanofiber-based flexible nanogenerators for wearable self-powered systems.

Experimental

Materials The barium acetate, titanium butoxide, poly(vinyl pyrrolidone) (PVP, Mw =1,300,000 g/mol), and ethyl acetate were purchased from Sigma-Aldrich Com. Acetic acid, 2,4-pentanedione and n-hexane were obtained from Samchun Pure Chemical Com. PDMS (Sylgard 184 Silicone Elastomer Kit, density of 1.102 g/cm3) was purchased from Dow Corning. All the materials and chemicals were used as received without further purification.



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Fabrication of BaTiO3/PDMS nanogenerators BaTiO3 nanofibers were prepared by an electrospinning technique utilizing a sol-gel precursor (Figure S1, Supporting Information), as described in literature.32 Barium acetate (2.55 g), titanium butoxide (3.4 g), and 2,4-pentanedione (2.0 g) were dissolved in acetic acid (6 ml) with magnetic stirring for 24 h to form a homogeneous sol-gel precursor. PVP (1.0 g) was then dissolved into the solution at room temperature by stirring for 12 hrs to control the viscosity. The sol-gel precursor was loaded into a plastic syringe and spun into nanofibers by using an electrospinning setup at a voltage of 15 kV, a tip-to-collector distance of 18 cm, and an extrusion rate of 1 ml/hr. The collector was rotated at a high speed of ~226 m/min to align uniaxially precursor nanofibers along the rotating direction. The fabricated precursor nanofiber sheets were stacked and calcined under pressure by increasing temperature stepwise to 1000 °C and finally holding at 1000 °C for 6 hrs to form BaTiO3 nanofibers. Flexible nanogenerators with two different alignments of BaTiO3 nanofibers in the PDMS matrix were manufactured as depicted in Scheme 1. First, a PDMS mixture with curing agent (10:1 weight ratio) diluted using n-hexane was poured to uniaxially aligned calcined BaTiO3 nanofibers. Second, after penetration and curing of PDMS at 60 °C for 2 hrs, the BaTiO3 nanofiber/PDMS composite (BaTiO3 content of 31 wt% or 7.6 vol%, BaTiO3 density of 6.02 g/cm3) was cut into pieces along the aligned nanofiber direction or transverse direction to obtain horizontally-aligned BaTiO3 nanofiber-based nanogenerators (BTNF-H) (length 1.7 mm, width 0.7 mm, and thickness 0.65 mm) or vertically-aligned BaTiO3 nanofiber-based nanogenerators (BTNF-V) (length 1.7 mm, width 0.9 mm, and thickness 0.67 mm), respectively. Third, indium tin oxide (ITO)-coated polyethylene terephthalate (PET) film (surface resistivity of 100 Ω/sq, Aldrich Com.) was attached as electrodes. Finally, a conductive silver paste was applied to connect copper wires with electrodes. For comparison, randomly-aligned BaTiO3 nanofiber-based nanogenerator (BTNF-R) was also fabricated as


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the same BaTiO3 content of 31 wt% as above. BaTiO3 nanofiber sheets were dispersed in ethyl acetate by applying ultrasonication for 1 hr using a bath-type ultrasonicator (50~60 Hz). After adding predetermined amounts of PDMS into the ethyl acetate solution including randomly dispersed short BaTiO3 nanofibers, another ultrasonication was applied for 1 hr. Subsequently, the curing agent was added into the solution, which was casted into glass petri-dishes and cured at room temperature for 24 hrs and at 60 ºC for 2 hrs. During the curing process, the ethyl acetate solvent was evaporated. The schematic drawing of BTNF-R (length 1.7 mm, width 1.3 mm, thickness 0.43 mm) was also represented in Scheme 1. Three different BaTiO3/PDMS nanogenerators were finally poled under an electric field of 5 kV/mm at 120 °C for 12 hrs.

Characterization The morphological features and dimensions of BaTiO3 nanofibers and their composites with PDMS matrix were observed by using a cold type field emission scanning electron microscope (SEM, S-4800, Hitachi) and an optical microscope (OM, S38, Bimience) equipped with a digital camera (Eyecam 2.0). The crystal structures of BaTiO3/PDMS nanogenerators were determined by a high performance X-ray diffractometer using Cu-Kα radiation (D/MAX-2200 Ultima/PC, Rigaku, Japan). The dielectric and piezoelectric properties of the nanogenerators were characterized with aids of 4980A Precision LCR Meter (Agilent) and 7½-digit graphical sampling multimeter (Keithley), respectively.

Results and discussion

Morphological characterization BaTiO3 nanofibers were fabricated by electrospinning a precursor solution and following calcination process of the precursor nanofibers. The morphologies and dimensions of the



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nanofibers and their nanogenerators was identified by SEM and OM image analyses. Figure 1(a)-(b) show the SEM images and diameter distribution (average diameter of ~493.6 nm) of as-prepared precursor nanofibers. It is seen that the precursor nanofibers with smooth surface are free of voids or beads and they are largely well aligned along the specified direction owing to the high rotating speed of the collector during the electrospinning process. The calcination was a critical step to form BaTiO3 crystals. Wei et al. characterized the morphology and phase structure of BaTiO3 nanofibers within the annealing temperature of 750-1050 ºC and found that higher annealing temperature led to better crystallization of the BaTiO3 nanofibers.32 Therefore, the calcination temperature in this study was set to be 1000 ºC to ensure a fully developed crystal structure of BaTiO3 nanofibers. On the other hand, the shrinkage of the precursor nanofibers during the calcining process was a nuisance causing cracking and curving of final BaTiO3 nanofiber sheet. To solve this problem, the calcining process was carried out with the temperature increase interval of 100 ºC and the calcining time of 1 hr per step under proper pressure, in which final temperature of 1000 ºC was applied for 6 hrs. As the result, a flat and intact BaTiO3 nanofiber sheet could be obtained after going through a slow shrinkage. Figure 1(c) gives the SEM images of finally synthesized BaTiO3 nanofibers with well-aligned integrated structure and uneven fiber surface morphology. The average diameter of the nanofibers decreases to ~354.1 nm owing to the degradation of PVP as well as the crystallization of BaTiO3 during the calcination, as can be seen in Figure 1(d). For a comprehensive analysis, randomly-aligned short BaTiO3 nanofibers were prepared by ultrasonicating the nanofibers in ethyl acetate. During the ultrasonication, the long nanofibers are shortened and dispersed in the solvent homogeneously as confirmed by the SEM and OM images of Figure 1(e). The length distribution of short BaTiO3 nanofibers is represented in Figure 1(f), which displays that due to the random breakage of the nanofibers, the length distribution is in range of several to almost one hundred micrometer, but largely located in 20-50 µm with an average length of ~33.7 µm. Figure 1(g) and (h) display the SEM images of


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cross-sections of the BaTiO3/PDMS composites (BTNF-V and BTNF-R) with vertically- and randomly-aligned BaTiO3 nanofibers. The cross-section SEM image of Figure 1(g) confirms that BaTiO3 nanofibers are well aligned vertically in the PDMS matrix without any pores or voids. Figure 1(h) exhibits that short BaTiO3 nanofibers are dispersed homogeneously in PDMS matrix. Although calcined BaTiO3 nanofibers are fragile and difficult to handle, their composites based on PDMS matrix exhibit good flexibility due to the flexible nature of PDMS,33 as shown in a photographic image of Figure 1(g), which confirms the feasibility of the composites as piezoelectric nanogenerators to external mechanical pressures.

Crystal structure characterization In order to get high output powers, piezoelectric materials should be poled under high electric fields to orient their crystal domains so that the electric dipoles can be aligned.34 The coercive electric field (a critical electric field required to initiate the switch in the domain direction) of BaTiO3 nanowires was reported to be ~3 kV/mm.30 Accordingly, to ensure full poling in our experiments, the poling electric field of 5 kV/mm was applied to BaTiO3/PDMS nanogenerators at high temperature of 120 °C for 12 hrs. Figure 2 illustrates the X-ray diffraction patterns of BTNF-V and BTNF-H before and after poling. The diffraction peak positions are in good agreement with the results in the literature.35 The crystal structure of the nanogenerators is evident to be a tetragonal phase, which is characterized by the peak splitting at 2θ ~ 45° for (002) and (200) planes at room temperature, and the lattice parameters and c/a–axis length ratio remain constant independent of poling. For BTNF-V, it is expected that the intensity of the (h00) peak is reflected from the domains whose a-axis is parallel to the nanogenerator plane surface, while the intensity of (00l) peak is reflected from the domains with pole axis (c-axis), which is vertical to the plane surface.34 In Figure 2(a), the (001) diffraction peak of the unpoled BTNF-V is inconspicuous, while it becomes apparent after poling. It means that by applying an electric field along the direction during the poling



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process, a large number of crystal domains are aligned along the same direction.36 In addition, the intensity ratio of (002) and (200) peaks, I002/I200, is enhanced noticeably from 0.62 to 0.87 after poling, which also supports the reorientation of crystal domains of BaTiO3 nanofibers aligned vertically in BTNF-V during the poling process. On the other hand, for BTNF-H, the I001/I100 and I002/I200 values change insignificantly with the poling, as can be seen in Figure 2(b), which demonstrates that BTNF-V is more strongly affected by the poling, compared to BTNF-H. This result can be caused by the difference in alignment directions of BaTiO3 nanofibers in the PDMS matrix. In case of BTNF-H, horizontally-aligned BaTiO3 nanofibers are relatively difficult to be poled because only part of the poling electric field is imposed upon the piezoelectric materials and the rest is exerted on the polymer matrix along the poling direction, as illustrated by the insert schematic drawings in Figure 2.37 On the other hand, to figure out the effect of poling direction on X-ray diffraction patterns, both top and bottom sides of BTNF-H and BTNF-V before and after poling were characterized (Figures S2 and S3, Supporting Information). As a result, it was found that X-ray diffraction patterns of both sides of BTNF-H and BTNF-V have no noticeable differences, which indicates that the up and down poling direction has no influence on the symmetric crystal structure of BaTiO3 nanofibers.

Dielectric property Dielectric constant can reflect the ability of dielectric materials to be poled under high voltage.38 Figure 3 displays frequency-dependent dielectric constant and loss tangent of neat PDMS and BaTiO3/PDMS nanogenerators. In Figure 3(a), the dielectric constants of neat PDMS and BTNF-R remain constant in the full range of the applied electric field frequency with values of 3.85 and 4.14 at 100 Hz, respectively. On the other hand, the dielectric constants of BTNF-V and BTNF-H are far higher than that of BTNF-R, although they present a little decrease with increasing the frequency, i.e., 40.23 at 100 Hz to 35.88 at 2 MHz for


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BTNF-V and 23.99 at 100 Hz to 20.72 at 2 MHz for BTNF-H. This result is believed to stem from the higher aspect ratio of BaTiO3 nanofibers in BTNF-V and BTNF-H owing to their lower percolation thresholds, compared to BTNF-R with short BaTiO3 nanofibers.37 It should be mentioned that the dielectric constants of BTNF-V over the applied frequency are almost twice as high as those of BTNF-H. This can be understood from the facts that BaTiO3 nanofiber has far higher dielectric constant of ~82032 than pure PDMS of 3.85 and that BTNF-V possesses higher BaTiO3 density than BTNF-H along the test direction. Similar result was also reported for epoxy/BaTiO3 composite systems,39 in which the perpendicular epoxy/BaTiO3 fiber composite possesses higher dielectric constant than the parallel epoxy/BaTiO3 fiber composite. This result is attributed to the different orientations of BaTiO3 nanofibers in the polymer matrix, because the electric field is more effective along the BaTiO3 fiber direction and subsequently a higher dielectric permittivity is attained, which is consistent with the result of BaTiO3/PDMS systems in this study. It is concluded that BTNF-V with higher dielectric constant possesses the strong ability to be polarized by an electric field, which is coincident to the above X-ray diffraction analysis. Unlike the high dielectric constant values of BaTiO3/PDMS nanogenerators, low loss tangent values of 0.006, 0.062 and 0.042 at 100 Hz are attained for BTNF-R, BTNF-H and BTNF-V, respectively, which are comparable with the low loss tangent of 0.005 for PDMS, as shown in Figure 3(b). It is generally accepted that the loss tangent is the proportion of the charge transferred by conduction to that stored by polarization. Therefore, it is believed that the high dielectric constants but low loss tangents of BaTiO3/PDMS nanogenerators in this study are preferable in piezoelectric and dielectric applications. It was also found that the material with higher dielectric constant possesses a higher piezoelectric coefficient.40

Piezoelectric property

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The piezoelectric properties of BaTiO3/PDMS nanogenerators were examined by applying a periodic mechanical compression with a pressure of 0.002 MPa. When the pressure is applied to the BaTiO3/PDMS nanogenerators, a piezoelectric potential difference is produced between two electrodes, which leads to a free electron flow occurring around the external circuit and is detected as an output signal. When the pressure is released, the piezoelectric potential vanishes and an opposite potential is formed. The free electrons in the external circuit flow back and forth, resulting in an alternating output.41 Figure 4(a)-(b) demonstrates the output voltage and current of the nanogenerators with three different alignments of BaTiO3 nanofibers. The average output voltage reaches to ~0.56 V, ~1.48 V, and ~2.67 V for BTNF-R, BTNF-H, and BTNF-V, respectively, which is associated with the voltage difference due to the influences of the threshold percolation and the alignment direction between electrodes.42 In particular, BTNF-V achieves a maximum average output voltage of ~2.67 V, which can be explained in two possible aspects. One is the efficient poling for BTNF-V. If no poling is applied, the output voltage of BTNF-V is only ~0.08V. The other one is that more electric charges are accumulated on the electrodes from the vertically-aligned BaTiO3 nanofibers due to the ceramic density and that the nanofibers connected vertically between electrodes are more compliant to mechanical stress. The average output current values of BTNF-R, BTNF-H, and BTNF-V are ~57.78 nA, ~103.33 nA, and ~261.40 nA, respectively, as can be seen in Figure 4(b), which exhibits the same variation pattern to the output voltage. It should be also mentioned that the output power generated from BTNF-V is high enough to light directly a commercial blue LED, as can be seen in a photographic image inserted in Figure 4(a). Figure 5 displays the piezoelectric results of BTNF-V in the forward and reserved connection directions with the schematic drawing. The output voltage and current of the nanogenerator show the corresponding opposite values, indicating the measured voltage and current signals are generated by the piezoelectric behavior of nanogenerators, not by environment effect or noise.


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The output power (P) of the nanogenerators subjected to the mechanical compression can be estimated from the following equation:43

1 U (t ) dt T∫ R 2

P=

(1)

2 where U ( t ) is the square of the real-time voltage on the external load, R is the impedance

of the external load (10 MΩ), T is the period of pressing and releasing. From the equation, the average output powers of BTNF-R, BTNF-H, and BTNF-V are calculated to be ~0.0086 µW, ~0.0905 µW, and ~0.1840 µW, respectively, as listed in Table 1. As expected, BTNF-V can generate a highest output power value of 0.1841 µW. For comparison, energy harvesting performance, structural characteristics, and working modes of BaTiO3-based piezoelectric nanogenerators reported in literatures are summarized in Table 1. It is well known that the generated output energy of nanogenerators is strongly dependent on the applied strain by mechanical stimuli such as compression, bending, tensile, etc.5-6,42,44-45 In addition, the output voltage was found to increase with the strain for a given nanogenerator, as can be seen in Table 1. After an in-depth study, it is found that our energy harvesting systems achieve an excellent average output voltage of 2.67 V for BTNF-V even under a relatively low mechanical pressure of 0.002 MPa, contrast to 0.057-340 MPa for other nanogenerators in literatures. It is also believed that the electrospinning technique adopted for fabricating high-quality BaTiO3 nanofibers is a quite efficient process in aspects of easy operation, commercial availability, and mass production, compared to other manufacturing processes of BaTiO3 nanomaterials.25 To demonstrate the practical implementation of our energy harvesting technology, the stability of piezoelectric performance of BTNF-V was evaluated under cyclic pressing and releasing of ~1 Hz, as shown in Figure 6(a). It was found that the generated output voltage remains constant over a long time cyclic operation, which supports excellent stability of the structure and the piezoelectric performance of BTNF-V nanogenerator. Since DC output is



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needed to power electrical products, the AC output generated by nanogenerators is required to convert to DC signals using a full-wave bridge rectification circuit depicted in Figure 6(b). When switch 1 in Figure 6(b) is connected to point 2, rectified voltage and current are obtained, as shown in Figure 6(c) and (d). The scavenged piezoelectric energy after rectification is also possible to be stored into capacitors by applying consecutive mechanical pressure on BTNF-V, when switch 1 in the circuit of Figure 6(b) is connected to point 3. The entire charging processes were recorded by monitoring the voltages across the capacitors (with a rated voltage of 50 V), and the charging voltages increased exponentially to reach a steady state of 0.46 V, as represented in Figure 6(e), which showed that it took 34 sec to charge a 1.0 µF capacitor, while 58 sec and 85 sec were taken for charging 2.2 and 3.3 µF capacitors to the same voltage. The stored voltage was found to be lower than the output voltage, which might be due to a voltage drop consumed at the rectifying diodes and/or a leakage of the capacitors.28 A stepwise increase of initial part of voltage-time curves can be clearly seen with each cycle of the energy conversion process, as can be seen in Figure 6(f).

Conclusion

In summary, well-aligned BaTiO3 nanofibers were fabricated by an efficient electrospinning technique and calcination process. By combined with PDMS matrix, three different flexible BaTiO3/PDMS nanogenerators, in which BaTiO3 nanofibers were aligned vertically or horizontally or randomly in the PDMS matrix, were manufactured. The dielectric and piezoelectric properties of the nanogenerators were found to be dependent on the alignment direction of the BaTiO3 nanofibers. The dielectric constants of BTNF-V and BTNF-H were far higher than that of BTNF-R, although they decreased slightly with the increase of the frequency, i.e., 40.23 at 100 Hz to 35.88 at 2 MHz for BTNF-V and 23.99 at 100 Hz to 20.72 at 2 MHz for BTNF-H. Even with the high dielectric constant values of


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BaTiO3/PDMS nanogenerators, the loss tangent remained quite low values of 0.042, 0.062, and 0.006 at 100 Hz for BTNF-V, BTNF-H, and BTNF-R, respectively. Accordingly, BTNF-V exhibited a highest output voltage of 2.67 V as well as high output current of 261.40 nA under a low mechanical pressure of 0.002 MPa, which could harvest an average electric power of ~0.1841 µW, compared to BTNF-H and BTNF-R. The generated electric power was implemented to light up commercial LED and to charge a 1 µF capacitor to 0.46 V in 34 s. The dielectric and piezoelectric results confirmed potential applications of BaTiO3 nanofiber-based flexible nanogenerators as mechanical energy harvesting and storage elements for wireless or self-powered devices and sensors.

Supporting Information. Schematic electrospinning process for fabricating BaTiO3 precursor nanofibers and X-ray diffraction patterns of both top and bottom sides of BTNF-H and BTNF-V before and after poling. (PDF)

Acknowledgments This work was supported by research fund of Chungnam National University.

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48. Park, K. I.; Xu, S.; Liu, Y.; Hwang, G. T.; Kang, S. J.; Wang, Z. L.; Lee, K. J., Piezoelectric BaTiO3 Thin Film Nanogenerator on Plastic Substrates. Nano Lett. 2010, 10 (12), 4939-4943. 49. 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 (3), 2766-2773.



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Table 1. Structural characteristics and energy harvesting performance of BaTiO3-based piezoelectric nanogenerators Sample

Shape of BaTiO3

Synthesis method of BaTiO3

Area (cm2)

Thickness (µm)

Output voltage (V)

Output current (nA)

Output power (µW)

BTNF-V (this study)

Nanofiber

Electrospinning

1.53

670

Stress of 0.002 MPa

2.67±0.11

261.40±10.88

0.1840±0.0233

BTNF-H (this study)

Nanofiber

Electrospinning

1.19

650

Stress of 0.002 MPa

1.48±0.04

103.33±4.38

0.0905±0.0223

BTNF-R (this study)

Nanofiber

Electrospinning

2.21

430

Stress of 0.002 MPa

0.56±0.04

57.78±1.45

0.0086±0.0023

Single BaTiO3 nanowire46

Nanowire

Electrospinning

-

-

Bending amplitude of 15 mm

0.21

1.3

-

Vertically aligned BaTiO3 nanowire arrays22

Nanowire

Hydrothermal process

0.2

1

Vibration excitation of 1 g acceleration

0.312

0.9

0.126×10

Single BaTiO3 nanowire/PVC fiber47

Nanowire

Topochemical synthesis

-

60 (diameter)

Bending on figure

0.9

10.5

-

BaTiO3 thin film48

-

-

0.13

0.3

Stress of 340 MPa

1

26

0.0283

Highly oriented BaTiO3 film43

Micro-platelet

Topochemical microcrystal conversion

2

1

Tapped by a paddle Tapped by ceramic block Bending

2.3 5 6.5

25 96 140

0.021 -

BaTiO3/MWCNT/PDMS generator5

Nanoparticle

Hydrothermal reaction

12

250

Stress of 0.057 MPa, Strain of 0.33 %

1.5 3.2

150 250～350

-

BaTiO3/PDMS nanogenerator6

Nanotube

Hydrothermal method

1

300

Stress of 0.2 MPa Stress of 0.4 MPa Stress of 1.0 MPa

1.0 3.1 5.5

350

-

Virus-templated BaTiO3 nanogenerator45

-

Genetically programmed self-assembly

6.25

200

Curvature radius: 10.5 cm Curvature radius: 7.5 cm Curvature radius: 6 cm Curvature radius: 5 cm

2 3 5 6

300

-

BaTiO3/P(VDF-HFP) composite thin film49

Nanoparticle

-

2.2

50

Stress of 0.23 MPa Bending

75 5

1.5×104 600

-

Working mode

19


-3

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Scheme 1. Schematic fabrication procedure of the nanogenerators based on BaTiO3 nanofibers in three kinds of alignment modes with piezoelectric test circuits.

20



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Figure 1. (a) SEM image and (b) diameter distribution of as-prepared precursor nanofibers; (c) SEM image and (d) diameter distribution of calcined BaTiO3 nanofibers; (e) SEM image with inserted optical image and (f) length distribution of BaTiO3 nanofibers after ultrasonication process; cross-section SEM images of nanogenerators with (g) vertically-aligned BaTiO3 nanofibers and (h) randomly-aligned nanofibers. The inset image in (g) is a picture of a flexible nanogenerator with vertically-aligned BaTiO3 nanofibers. ACS Paragon Plus 21 Environment

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Figure 2. X-ray diffraction patterns of (a) BTNF-V and (b) BTNF-H before and after poling. Inserts are schematic drawings of X-ray diffraction characterization where red arrows indicate the direction of electric fields by poling.

22



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Figure 3. (a) Dielectric constant and (b) loss tangent of PDMS and BaTiO3/PDMS nanogenerators as a function of frequency.

23


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Figure 4. (a) Output voltage and (b) current changes of BaTiO3/PDMS nanogenerators under a periodic mechanical compression. Inset of (a) is a photograph of a commercial blue LED lit up by the electric energy generated from BTNF-V.

24



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Figure 5. Output voltage and current changes of BaTiO3/PDMS nanogenerators in (a) forward direction and (b) reversed direction under a periodic mechanical compression.

25


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Figure 6. (a) A long term piezoelectric test of BTNF-V under cyclic pressing and releasing of ~1 Hz; (b) a schematic circuit diagram of the rectifying (when switch 1 is connected to point 2) and capacitor charging (when switch 1 is connected to point 3); (c-d) time-dependent rectified voltage and current signals; (e) voltage-time charging curve of capacitors with different capacitances; (f) an enlarged plot of voltage-time charging curves in (e). The green and yellow arrowheads in (c-d) indicate a pressing and releasing process of external load applied on BTNF-V, respectively.

26



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A Graphic for the Table of Content (TOC)

27


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High Performance Flexible Piezoelectric Nanogenerators based on

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