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Solvent-assisted optimal BaTiO3 nanoparticles-polymer composite cluster formation for high performance piezoelectric nanogenerators

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 485401 (http://iopscience.iop.org/0957-4484/25/48/485401) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 132.239.1.231 This content was downloaded on 29/04/2017 at 20:12 Please note that terms and conditions apply.

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Nanotechnology Nanotechnology 25 (2014) 485401 (7pp)

doi:10.1088/0957-4484/25/48/485401

Solvent-assisted optimal BaTiO3 nanoparticles-polymer composite cluster formation for high performance piezoelectric nanogenerators Sung-Ho Shin1, Young-Hwan Kim1, Joo-Yun Jung2, Min Hyung Lee3 and Junghyo Nah1 1

Department of Electrical Engineering, Chungnam National University, Daejeon, 305-764, Korea Department of Nano Manufacturing Technology, Korea Institute of Machinery and Materials, Yuseong-Gu, Daejeon, 305-343, Korea 3 Department of Applied Chemistry, Kyung Hee University, Yongin, Gyeonggi, 446-701, Korea 2

E-mail: [email protected] Received 22 July 2014, revised 1 October 2014 Accepted for publication 13 October 2014 Published 13 November 2014 Abstract

We report on an optimal BaTiO3-P(VDF-HFP) composite thin-film formation process for high performance piezoelectric nanogenerators (NGs). By examining different solvent ratios in a solvent-assisted composite thin film formation process, the BTO nanoparticle (NPs) clustering and related performance enhancements were carefully investigated. Using the optimal process, the fabricated BTO NGs exhibited an excelling output power performance. Under a compressive force of ∼0.23 MPa normal to the surface, the measured open-circuit output voltage and shortcircuit current were over 110 V and 22 μA, respectively, with a corresponding peak output power density of 0.48 Wcm−3. Our results clearly demonstrate the effectiveness of a solvent-assisted BTO cluster formation process for fabricating high performance piezoelectric energy harvesting devices. S Online supplementary data available from stacks.iop.org/NANO/25/485401/mmedia Keywords: nanogenerator, piezoelectric, barium titanate oxide (Some figures may appear in colour only in the online journal) 1. Introduction

demonstrated as a promising candidate for piezoelectric NGs due to its inherently high piezoelectric coefficient [11–13] and its environmentally safe nature [14, 15]. Recently, high performance flexible BTO NGs were demonstrated by an approach that employed a composite structure that consists of BTO nanoparticles (NPs), filling materials that aid the even dispersion of the NPs and the encapsulating polymer [16, 17]. This approach greatly simplifies the overall fabrication process and is also suited for large-scale flexible NGs. However, it is essential to add filling materials, such as carbon nanotubes [16] and DNA [17], in the composite solution for the even distribution of BTO NPs, which determine the piezoelectric potential enhancement in the NGs. Thus, the addition

The rapidly growing number of personal electronic devices and increasing overall energy consumption related with this trend have necessitated the development of efficient, sustainable and environmentally safe energy harvesting systems for self-powered electronic devices [1]. Piezoelectric nanogenerators (NGs) [2–5], which harvest energy from various physical movements that exist in nature and in our living environment, have also gained great attention in this aspect. Since its introduction [6], piezoelectric NGs have been intensively studied to develop high performance piezoelectric NGs [7–10]. Barium titanate oxide (BTO) has been 0957-4484/14/485401+07$33.00

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© 2014 IOP Publishing Ltd Printed in the UK

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of these filling materials result in an overall fabrication cost increase and complicates the fabrication process. As an alternative approach, a simple and facile way to fabricate high performance NGs without filling nanomaterials was recently introduced [18]. In the work, the active power generation layer is separately fabricated by aggregating BTO NPs into large spherical-shaped clusters using poly(vinylidene fluoride-hexafluoropropylene) P(VDF-HFP) as a cementing polymer, followed by subsequent curing, poling and a polymer (PDMS) packing process. Using this method, NP clustering was rather promoted for the NG’s performance enhancement in contrast to the previous works [16, 17]. The cluster formation is advantageous for harvesting externally applied forces and for maximizing the total dipole moment by aligning the piezoelectric domain of NPs inside of the clusters. Nevertheless, the performance of NGs can still be greatly affected by both the density and related morphology of the composite clusters. In particular, due to high permittivity [19] and Young’s modulus of P(VDF-HFP) [20], it is desirable to limit and maximize the NGs’ role as links for NPs. Therefore, it is necessary to further investigate different composite cluster formations using a solvent-assisted process and to find an optimal condition for high performance NGs fabrication. In this work, by varying acetone ratios in the composite solution, the related effects on the BTO NP cluster formation and the NG’s output power were investigated. Our results show that a relative increase of acetone volume in the composite solution effectively promotes BTO NP cluster formation, increasing the cluster density and reducing P(VDFHFP)-covered regions on the cluster surface by residual P (VDF-HFP). By establishing an optimal BTO NP cluster formation condition, the output voltage and current, up to 110 V and 22 μA, were obtained at the applied force of ∼0.23 MPa in our NGs, with a corresponding peak output power density of ∼0.48 Wcm−3, which is one of the highest values reported to date (Supplementary table S1). The method presented here provides a very simple, reliable and costeffective way to fabricate high performance piezoelectric NGs.

acetone-to-DMF mixture ratios of (0:1), (1:1), (2:1), (3:1) and (5:1) are prepared while keeping a constant DMF volume rate. To completely dissolve the P(VDF-HFP), the solution is then heated on a hotplate at 60 °C for 40 min and cooled for 30 min in ambient conditions. Next, the 30 wt % of BTO NPs (US Research Nanomaterials, Inc.) are mixed with the prepared solution and stirred for 2 h using a magnetic stirring bar. The prepared solution is spin-coated on a Si wafer piece at 2000 rpm for 30 s, as illustrated in figure 1(c). Subsequently, the coated films are cured in an oven at 80 °C for 1 h, resulting in clustered BTO NPs. After the curing process, the films can be readily peeled off of the Si substrate. 2.2. Fabrication process of the piezoelectric NGs based on the composite films

Using the prepared film as an active power generation layer, the NGs are fabricated, as shown in figure 1(d). On two pieces of 100 nm thick Al deposited flexible substrates (Polyimide (PI) 50 μm and 200 μm thick), the PDMS (Sylgard, 184 SILICONE ELASTOMER) is spin-coated at 5000 rpm for 40 s, resulting in a ∼10 μm thick PDMS layer. The BTO NP-P (VDF-HFP) composite thin film is then sandwiched between the PDMS-coated PI layers and cured in an oven at 80 °C for 3 h, which completes the NG fabrication. The thickness of the PDMS-covered BTO NP-P(VDF-HFP) layer between the electrodes is ∼50 μm (Supplementary information figure S1). Here, the role of PDMS is twofold. First, PDMS packing can seamlessly fill the gaps between the BTO NP clusters and link them together (Figures. S9 and S10). Thus, the applied force or stress can be more effectively transmitted from the PDMS capping layer to all of the BTO clusters, greatly enhancing the NG’s efficiency. Second, the PDMS layer is an insulating layer that provides a potential barrier of infinite height. Thus, the internal leaking of the induced electrons on the electrode can be prevented even if there exist pores on the composite thin film. Lastly, the fabricated device is poled at 100 °C under a direct electric field of 100 kV cm−1 for 20 h. The high voltage poling process aligns the piezoelectric domain of the BTO NPs in the cluster parallel to the external field. 2.3. Device characterization

2. Experimental section

The output voltage and current of the NGs were measured using a voltage meter (Agilent, 34401A) and a current preamplifier (Stanford Research Systems, SR570), respectively. Using the bending stage, the NGs were measured during cyclic bending and the releasing motions. The compressive force normal to the NG’s surface was measured using a load cell (Bongshin, Inc.).

2.1. Preparation of the BTO-P(VDF-HFP) composite films

The BTO NP-P(VDF-HFP) thin film is prepared by a solventassisted composite solution coating method in which the solution is composed of BTO NPs, P(VDF-HFP), dimethylformamide (DMF), and acetone mixed at the specific ratios. The BTO NPs used in this work have an average diameter of 200 nm, as displayed in figure 1(a). The Raman spectrum peaks at 256 cm−1 [A1(TO)], 306 cm−1 [E, B1(TO + LO)], 513 cm−1 [E, A1(TO)] and 715 cm−1 [E, A1(LO)] clearly indicate the non-centrosymmetric tetragonal phase [21, 22], which has a high piezoelectric coefficient (figure 1(b)). The detailed solution preparation process is described as follows: First, P(VDFP-HFP) (Sigma-Aldrich) is melted with DMF, followed by mixing with acetone at specific ratios. The

3. Result and discussion The scanning electron micrographs (SEMs) in figure 2 show the composite thin films with BTO NP clusters, prepared by varying the acetone ratios in the composite solution. When acetone is not included in the solution, the density of the NP 2

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Figure 1. (a) Scanning electron micrograph (SEM) of BTO NPs. (b) Raman spectrum of BTO NPs retaining the tetragonal phase. (c)

Schematic flow of solvent-assisted BTO-P(VDF-HFP) composite film formation by spin-coating of the composite solution. (d) Schematic representation of the piezoelectric NG fabrication process.

Figure 2. (a–e) Tilted SEMs of BTO NP-P(VDF-HFP) composite films obtained by spin-coating the composite solution with different solvent ratios. (a) The composite thin film prepared with the solution that has an acetone-to-DMF ratio of (0:1). The inset shows that the BTO NP clusters are completely covered with P(VDF-HFP). (b) The composite film obtained by the solution with an acetone-to-DMF ratio of (1:1). (c) The BTO clusters are slightly exposed as the acetone-to-DMF ratio increases to (2:1). (d) The film prepared by an acetone-to-DMF ratio of (3:1) with clearly exposed BTO NPs on the hemisphere surface of the cluster. (e) At the acetone-to-DMF ratio of (5:1), spherical BTO clusters are created, and the cluster density is obviously increased. (Insets) Magnified SEMs of the BTO NP clusters. The NP exposed region in the cluster is marked with red-dashed lines.

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Figure 3. Effects of P(VDF-HFP) coverage on the piezoelectric output potential (a–d). (a) The BTO cluster fully packed with P(VDF-HFP) in

the PDMS. (b), (c) The piezoelectric potential gradually increases as the P(VDF-HFP) coverage in the BTO cluster shrinks. (d) The BTO cluster with minimal P(VDF-HFP) incorporation shows the highest piezoelectric output, representing the BTO clusters in figure 2(e).

cluster is lower in comparison to other cases (figure 2(a)). In addition, the clusters are covered under a thick P(VDF-HFP) layer such that the BTO NPs on the surface of the cluster are barely exposed. As a relative acetone ratio increases in the solution, it can be noticed that the BTO NP cluster density gradually increases, and the region uncovered with P(VDFHFP) on the cluster surface is enlarged (figure 2 (insets) and S2 (Supplementary Information)). At an acetone-to-DMF mixture ratio of (2:1), the NPs on the top surface start to become more exposed (figure 2(c) (inset)). As the acetone-toDMF ratio reaches (5:1), nearly spherical-shaped BTO NP clusters are formed, exhibiting both the high cluster density and large exposed NP region in the cluster (figure 2(e) (inset)). Thus, it can be clearly noticed that the shape of the BTO NP clusters and the incorporation of P(VDF-HFP) in the cluster are significantly changed, depending on the solvent mixture ratio. We also note that if the P(VDF-HFP) is not included in the solution, the BTO NP clusters are not formed, indicating the role of P(VDF-HFP) as a cementing polymer (Supplementary figure S3). The role of acetone in the composite solution is twofold. First, it enhances the solubility of P (VDF-HFP) in the composite solution and reduces the viscosity of the P(VDF-HFP) melted in DMF. Thus, NPs can be more evenly distributed in the solution. More importantly, due to the low boiling temperature of acetone (65 °C) in comparison to DMF (150 °C), P(VDF-HFP), a rapid solvent evaporation can be more readily obtained in the oven (80 °C) while maintaining the solution being agitated, preventing the

NPs from subsiding in the layer. In addition, the aggregation time between BTO NPs and P(VDF-HFP) can also be tuned to form more clusters by increasing the acetone volume in the solution. Using COMSOL simulation software, piezoelectric potentials generated in the BTO NP clusters with different P (VDF-HFP) coverages were qualitatively investigated. In the simulation, the dimensions of the PDMS region were 140 μm by 125 μm and the diameter of the BTO cluster was 12.5 μm. The Young’s modulus of PDMS and BTO were 850 kPa and 50 GPa, respectively. Also, the longitudinal piezoelectric coefficient (D33) used in the simulation was 250 pC/N. We simulated four models to reflect different P (VDF-HFP) regions covering BTO NP clusters, as shown in figure 2. For fair comparison, the compressive force of ∼0.23 MPa, the simulation parameters and the number of BTO NPs were kept the same; we only varied the P(VDFHFP)-covered regions in the BTO clusters. In figure 3(a), the BTO NP cluster is completely covered with the P(VDFHFP) layer, which is similar to the case of figure 2(a). The calculated piezoelectric potential difference between the top and bottom electrodes was only ∼3.7 V. As the P(VDFHFP)-merged area in the BTO NP cluster gradually shrank, the piezoelectric potential was proportionally increased to ∼5.8 V, as in figure 3(c). As it shrank further, the generated piezoelectric potential reached ∼7.9 V figure 3(d). The results here explicitly show that it is important to minimize the incorporation of P(VDF-HFP) in the BTO NP cluster to 4

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Figure 4. (a) Open-circuit voltages of five different NGs fabricated with the different composite thin films, prepared by varying solvent ratios. The measured voltages were proportionally increased as the acetone ratio in the solution increased. (b) The short-circuit currents of the same NGs measured in (a), exhibiting similar trend as in (a). (c) The output voltage and current of the NG under a reverse connection for the switching polarity test. The NG is fabricated with the thin film obtained by an acetone-to-DMF mixing ratio of (5:1).

Figure 5. (a, b) Mechanical stability and reliability test of the NG based on the composite film by an acetone-to-DMF mixing ratio of (5:1). (a) Measured open-circuit voltage during bending and releasing motions for 2 h over 5400 cycles. (b) Measured short-circuit current during bending and releasing motions for 2 h over 5400 cycles. The output performance was maintained for 2 h without performance degradation.

inside. Thus, it results in a smaller piezoelectric potential in comparison to the BTO cluster with a smaller P(VDF-HFP)covered region. In addition, a relatively high dielectric constant of P(VDF-HFP) compared with that of PDMS is also detrimental to the piezoelectric output voltage [24–26]. Hence, the performance of NGs can be improved by minimizing P(VDF-HFP) incorporation for optimized BTO cluster formation.

enhance the piezoelectric output performance. In our work, although P(VDF-HFP) plays an essential role in BTO NP cluster formation, it is, however, desirable to minimize its coverage in the clusters as much as possible due to its high Young’s modulus (∼GPa) [23]. When a compressive force normal to the top PDMS surface is applied to the BTO cluster covered completely with P(VDF-HFP), the force cannot be effectively transferred to the individual BTO NPs 5

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4. Conclusion

For experimental verification, we fabricated piezoelectric NGs using the composite thin films shown in figure 2. The piezoelectric power generation mechanism in our NG can be explained as follows: When the force is applied to NG, the piezoelectric potential is induced between the top and bottom electrodes due to a dipole moment generated in the cluster. We note here that even if the BTO clusters are electrically isolated from the electrodes by PDMS, it can still induce charges in the electrodes. The formed built-in potential results in an electron flow to neutralize the electric field produced by the dipoles, where the current flows only through the external circuit since the BTO cluster layer and PDMS packing layer are insulators. When the strain or stress is removed, the piezoelectric potential is faded, and an opposite built-in potential is formed by the charges accumulated at both ends of the external circuit. Thus, electrons flow back in the opposite direction until zero current is reached. Figure 4(a) displays the open-circuit voltage measurement of the five different NGs under a cyclic compressive force of ∼0.23 MPa. The active area of the measured devices was ∼2.2 cm2. We note that P (VDF-HFP) has an α-phase even after the poling process since the presence of HFP does not mediate structural transitions toward the ferroelectric PVDF. Thus, it demonstrates a negligible piezoelectric property, and we can limit the role of P(VDF-HFP) as a cementing polymer. The output characteristics of only the P(VDF-HFP)-based NG and FTIR spectrum of P(VDF-HFP) after the poling process can be found in figure S4. It can be clearly confirmed that the smallest output voltage, ∼12 V, was measured in the device fabricated with the film prepared by an acetone-to DMF ratio of (0:1). On the other hand, the output voltage exceeded ∼110 V in the device fabricated with the film produced by an acetone-to-DMF ratio of (5:1), which is approximately one order of magnitude higher in value in comparison to the smallest one. Similarly, the measured short-circuit currents are also increased from ∼2 μA to ∼22 μA as the composite thin film changes, generating a maximum power density of ∼0.48 Wcm−3, which is the one of the highest values reported to date. The results here also agree well with the tendency of the simulation calculations. To demonstrate the validity of output signals by switching the polarity test [27, 28], the device that showed the highest output performance was measured again under the reverse connection mode. (figure 4(c)) The magnitudes of the generated output voltage and current are approximately the same in the opposite direction. We note that the estimated piezoelectric coefficient (D33) of the NG is ∼180 (pC/N) (Supplementary Information Method S1). Lastly, the durability and reproducibility of the NG were tested during the cyclic bending and releasing motions by using a bending stage (figure 5). The NG fabricated using the solution with an acetone-to-DMF ratio of (5:1) was used for these measurements. The amplitudes of the generated output voltage and current were ∼20 V and ∼3 μA, respectively, and maintained over ∼5400 bending cycles without any performance degradation.

In conclusion, we systematically investigated the optimal solvent-assisted BTO NP cluster formation and the related effects on performance of the NG. By adjusting the relative acetone ratio in the composite solution, different BTO cluster formations were induced and adopted for the NG fabrication. Using the optimal BTO cluster formation process, the density of the BTO clusters on the composite thin film was greatly increased while reducing the P(VDF-HFP) incorporation in the clusters. These improvements contributed to the significantly enhanced output performance of the NGs, exhibiting a maximum open-circuit voltage and a short-circuit current of 110 V and 22 μA, respectively, which corresponds to the peak output power density of ∼0.48 Wcm−3.

Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF-2012R1A1A1041869) and the CNU research fund of Chungnam National University in 2014.

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Solvent-assisted optimal BaTiO3 nanoparticles-polymer composite cluster formation for high performance piezoelectric nanogenerators.

We report on an optimal BaTiO3-P(VDF-HFP) composite thin-film formation process for high performance piezoelectric nanogenerators (NGs). By examining ...
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