High quality barium titanate nanofibers for flexible piezoelectric device applications

Sensors and Actuators A 233 (2015) 195–201

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

High quality barium titanate nanofibers for flexible piezoelectric device applications Feifei Wang a,b,∗ , Yiu-Wing Mai a,∗ , Danyang Wang c , Rui Ding c , Wangzhou Shi b a Centre for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia b Key Laboratory of Optoelectronic Material and Device, Department of Physics, Shanghai Normal University, Shanghai, 200234, China c School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia

a r t i c l e

i n f o

Article history: Received 15 February 2015 Received in revised form 29 April 2015 Accepted 5 July 2015 Available online 13 July 2015 Keywords: Nanofiber Ferroelectrics Piezoelectricity

a b s t r a c t Piezoelectric and ferroelectric nanostructures and devices have attracted extensive attention because they can realize the conversion between mechanical and electrical energies for sensors and energy harvesting applications. In the present work high quality lead-free BaTiO3 nanofibers were obtained by a sol-gel based electrospinning technique. X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and piezoresponse force microscopy (PFM) were utilized to characterize the morphologies, phase and domain structures along with the nanoscale electromechanical response. Well-crystallized BaTiO3 fibers were obtained with good local piezoelectric response (d33,eff of ∼40 pm/V using PFM). Further, a flexible piezoelectric device was fabricated by combining aligned BaTiO3 nanofibers and a PDMS polymer matrix. An inter-digital electrode on a flexible substrate was incorporated to enhance the output signal. The proposed device displayed an output peak–peak voltage of ∼0.45 V at a load resistance of 1 M under a periodic bending excitation of ∼45 Hz. It has the advantages of small-size, ease of processing, high flexibility and strain tolerance, and high-sensitivity to external vibration at low-frequency which may open up a range of new applications. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Harvesting clean and renewable energy from the environment has been attracting immense interest in the scientific community which provides an effective way to power nanodevices and wireless sensors [1–4]. Among the various renewable energy sources such as light, vibration, heat, and microwave radiation, vibration appears to be the most ubiquitous power source. Several transduction mechanisms have been suggested to scavenge mechanical and vibrational energy into electricity, including electro-magnetic, electrostatic, and piezoelectric mechanisms. Of these mechanisms, harvesters based on the piezoelectric effect have the advantages of high energy density and potential for miniaturization and thus attracted much attention in recent years [2–5]. A variety of piezoelectric nanogenerators based on ZnO nanowires have been successfully demonstrated for potential applications in converting

low-frequency vibration and biomechanical energy into electrical energy [2–4,6–9]. Considering the relatively low piezoelectric constant and electromechanical coefficient of ZnO, which potentially hinder the supply of sufficient power for certain applications, piezoelectric harvesters based on perovskite piezoelectric materials with high charge constant have been suggested, utilizing Pb(Zr1−x Tix )O3 (PZT) and (1−x)Pb(Mg1/3 Nb2/3 )O3 –xPbTiO3 (PMN-PT) nanofibers [10,11]. However, lead is toxic; and environmental legislation in the European Union, parts of Asia, and the US demands elimination of toxic lead for these materials systems [12]. Therefore, developing other environmental-friendly piezoelectric device with efficient conversion of mechanical energy into electrical energy has attracted continuous attention [13–16]. Based on the piezoelectric effect, the voltage generated during mechanical deformation is given by:

 Vout =

∗ Corresponding authors at: Centre for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia. Fax: +86 21 64328894. E-mail addresses: f f [email protected] (F. Wang), [email protected] (Y.-W. Mai). http://dx.doi.org/10.1016/j.sna.2015.07.002 0924-4247/© 2015 Elsevier B.V. All rights reserved.

gE (x)dx

(1)

where Vout , g, E, and (x) represent output voltage, piezoelectric constant, Young’s modulus and strain, respectively. Take the d longitudinal extensional mode for example, wherein, g33 = T 33 . 33 0

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Higher piezoelectric constant d33 and lower dielectric constant T33 thus favor improving the voltage output. For the output power, higher dielectric constant leads to high intrinsic capacitance C and 1 hence small external load resistance (Z = ωC ) is needed to extract the power. This yields higher output power and better efficiency. Thus, searching for suitable lead-free piezoelectric materials is crucial to enhance the voltage and power output simultaneously. A recent theoretical analysis by Sun et al. suggested that BaTiO3 fiber exhibits substantially enhanced power and efficiency relative to ZnO nanowire and the output level is also comparable to highperformance lead-based PMN-PT system, making it favorable for mechanical energy harvesting [17]. A piezoelectric nanogenerator based on BaTiO3 system in thin-film form was proposed by Park et al. who fabricated the nanogenerator by micro-fabrication and soft lithographic printing techniques [14]. By bending the flexible substrate, they obtained an excellent output voltage of 1.0 V [14]. Thus, in the present work, environmental-friendly BaTiO3 is selected as the functional component in order to convert the mechanical energy into electricity which will lead to high sensitivity caused by small and random ambient mechanical movements. Compared to the bulk counterpart, piezoelectric nanofibers are utilized, since they have the advantages of lightweight, small-size, high elastic compliance, and high strain tolerance (e.g., maximum break strain for ZnO nanowire is ∼7.7%, while that for bulk ZnO is ∼0.2% [18,19]). Also, the fiber form is easily accessible and the fabrication process is cost-effective and much simpler compared to the device based on thin-film materials. The high flexibility and high strain tolerance of the nanofibers can also effectively reduce the risk of potential fracture or damage of the piezoelectric materials (which often occurs in bulk counterpart) under high-frequency vibration conditions and widen their safe vibration frequency and amplitude range. As mentioned, eletrospinning has been utilized to fabricate BaTiO3 nanofibers due to the advantages of low-cost, highefficiency, and long fiber production [20,21]. Here, X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and piezoresponse force microscopy (PFM) are utilized to characterize the morphologies, phase and domain structures along with the nanoscale electromechanical responses. The BaTiO3 nanofibers were then aligned using a parallel electrode set-up. Based on the aligned BaTiO3 fibers, a flexible piezoelectric device is proposed and an inter-digital electrode on a flexible substrate is incorporated to enhance the output signal. Subjected to an applied periodic bending excitation, an output peak–peak voltage of ∼0.45 V can be extracted under a load resistance of 1 M at ∼45 Hz for a fiber area of 15 × 8 mm2 .

2. Experimental details BaTiO3 nanofibers were prepared using the electrospinning technique combined with a sol–gel process [20,21]. First, barium acetate was dissolved in acetic acid and stirred at 60 ◦ C for 0.33 h. Titanium isopropoxide was dissolved in 2-methoxyethanol and acetylacetone was chosen as ligands. Then, the two solutions were mixed under continuous stirring to form a BaTiO3 precursor solution referred to as A. 1:1 molar ratio of Ba and Ti was maintained to achieve stoichiometric BaTiO3 . A second solution, referred to as B, was obtained by dissolving polyvinyl pyrrolidone (PVP, Mw = 1300,000) in ethanol. The solution was mechanically stirred for ∼1 h. Solution A was then added drop-wise to solution B under constant stirring. The concentration of the final BaTiO3 was controlled at ∼0.3 M. After being stirred at room temperature for 2 h, the mixture was loaded into a plastic syringe. A nanofiber electrospinning (Kato Tech Co. Ltd., Japan) facility was utilized to fabricate the BaTiO3 fibers. Electrospinning was con-

ducted at 21 kV, 18 cm spacing between needle tip and collector, and a feed rate of 0.1 mm/min. The as-spun fibers were dried at 90 ◦ C for 2 h followed by thermal annealing at 750 ◦ C for 2 h. The obtained fibers were pure BaTiO3 fibers. Additionally, nanofibers were also collected directly onto the Pt-coated silicon substrate for later piezoelectric characterization. A field emission scanning electron microscope (FESEM, Zeiss ultra) was used to characterize the fiber samples and to determine the diameters of the fibers. Both the as-spun nanofibers and annealed samples were coated with gold and examined at an accelerating voltage of 5–10 kV. Annealed samples were also studied using high-resolution transmission electron microscopy (HRTEM, Jeol 3000F) at an accelerating voltage of 300 kV. The crystal structures of the BaTiO3 fibers were determined from X-ray diffraction (XRD) patterns recorded with a Siemens D6000 diffractometer. The local piezoelectric response was characterized using piezoresponse force microscopy (PFM, MFP-3D-SA, Asylum Research). For the piezoelectric device fabrication, the BaTiO3 nanofibers were first aligned using a parallel electrode set-up. Then, the annealed fibers with good alignment were transferred to the flexible plastic substrate coated with an inter-digital electrode (the distance between each pair of electrode is 1 mm). Polydimethylsiloxane (PDMS), as a matrix material, was used to cover the entire BaTiO3 fiber/electrode structure. After curing at 60 ◦ C for 2 h, the device was poled in silicone oil under an electric field of 4 kV/mm for 0.5 h at 80 ◦ C with half of the field applied in the cooling run. Under periodic bending excitation, the device electric output was recorded with a digital oscilloscope (Agilent 54,622A).

3. Results and discussion The typical surface morphologies for the as-spun and calcined BaTiO3 fiber are shown in Fig. 1 (a) and (b). The as-spun fibers were quite smooth, continuous, and long with a diameter ∼400 nm. After thermal annealing at 750 ◦ C for 2 h, the diameter of BaTiO3 fiber decreased to ∼100–200 nm owing to the volatilization of the polymer and continuous grained structure formed, indicating a crystallization process. During the electrospinning process, it was found that the diameter of the fiber depended strongly on the applied voltage and the concentration of the BaTiO3 precursor solution. Higher voltage and lower concentration both decreased the fiber diameter. (See Figs. S1 and S2 in Supplementary Data) Corresponding phase structure for the annealed BaTiO3 fiber was also examined using XRD in Fig. 1 (c). Pure perovskite structure and sharp diffraction peaks could be well observed and no second phase was detected in the fiber. The XRD pattern also indicates that the obtained fibers are polycrystalline. HRTEM was further utilized to give an insight into the microstructure shown in Fig. 1(d–f). The BaTiO3 fiber exhibited a dense structure and no pores were observed. Polycrystalline structure with grain size ∼40 nm can be seen in Fig. 1(d). The regular atomic arrangement indicates that annealed BaTiO3 fiber also has good crystallinity. The ferroelectric tetragonal phase could be confirmed both by selected area electron diffraction pattern and high resolution lattice space as shown in Fig. 1 (e) and (f). The high-resolution image (Fig. 1(f)) revealed that the individual grain was single crystalline. Furthermore, the energy disperse spectrum (EDS) was utilized to study the elemental distribution in the nanofibers. Fig. 2 is a typical EDS result of the annealed BaTiO3 fiber obtained from a selected area. All elements of BaTiO3 can be identified and distributed continuously in the fiber. To evaluate the nanoscale electromechanical response of the BaTiO3 fiber, PFM, as one of the most powerful nanoscale imaging techniques, was used to explore the local piezoelectric response [22,23]. A schematic of the measurement system is shown in Fig. 3(a). A silicon scanning probe (Budget-sensors) and a gold-

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Fig. 1. Morphology and structure of the electrospun random BaTiO3 nanofibers. (a) and (b) SEM images of as-spun and annealed fibers, respectively. (c) XRD of the annealed fibers. (d)-(f) TEM images of a BaTiO3 fiber, SAED pattern of a BaTiO3 fiber, and HRTEM image, respectively.

coating lever with a frequency of 75 kHz and force constant of 3 N/m were used during the measurements. The PFM probe was applied to the BaTiO3 fiber and an alternate voltage of 0.6 V was applied between the tip and the bottom Pt electrode under different dc bias field. The deformation of the fiber was detected by the laser interferometer and processed with a lock-in amplifier. The topology, vertical PFM amplitude and phase of the fibers were then determined from the recorded response. Fig. 3(b–d) show the corresponding surface topologies, vertical piezoresponse amplitude, and phase images of a 4 × 4 ␮m2 area, respectively. Fiber form can be clearly observed from the topology along with clear contrasts in the amplitude and phase. In order to give an insight into the local domain structure and piezoresponse, a 0.5 × 0.5 ␮m2 scanning area was further performed and clear grain and grain boundary could be observed. The distinct contrast in the magnitude (Fig. 3(e)) and phase images (Fig. 3(f)) indicated different amplitude and domain orientation inside the grain which also verified the polycrystalline

and multi-domain structure. A local piezoresponse hysteresis loop was recorded and shown in Fig. 3(g). The phase angle exhibited a 180◦ change under the reversal of the dc bias field, confirming a polarization switching process. The effective piezoelectric constant d33,eff can be determined from: A = d33,eff V Qeff

(2)

where A, V and Qeff are displacement, applied voltage and quality factor of cantilever, respectively. From the PFM results, the values of A, V, and Qeff were ∼780 pm, 0.6 V, and ∼33, respectively. Hence, d33,eff can be calculated as ∼40 pm/V. This value is superior to leadfree ZnO (d33 ∼ 26.7 pm/V) [24], NaNO3 nanowires (d33 ∼ 4 pm/V) [25], and BaTiO3 particles (d33 ∼ 28 pm/V) [26]. It is also close to the BaTiO3 single-crystal nanowire (d33 ∼ 45 pC/N) [27] prepared by chemical method and PMN-PT polycrystalline fiber (d33 ∼50 pm/V) [11]. Here, it is noted that the fiber selected for measuring the piezoresponse had a large diameter of ∼1 ␮m compared to the aver-

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Fig. 2. EDS analysis of the annealed BaTiO3 fiber.

age fiber diameter of ∼100–200 nm. This fiber was chosen as it had good contact with the Pt-coated silicon substrate, which guaranteed reliable PFM measurements. Based on the obtained BaTiO3 fibers which exhibited good global piezoelectric response, a flexible piezoelectric device was further developed. Prior to fabricating the device, BaTiO3 fibers were first aligned and the experimental set-up for obtaining the aligned BaTiO3 fibers is shown in Fig. 4(a). The corresponding mechanism for the fiber alignment was discussed in previous work [28]. Parallel electrodes were utilized during the electrospinning process whereby the repulsion Coulomb force between deposited adjacent fibers promoted the final alignment. Fig. 4(b and c) show the as-spun BaTiO3 fibers with different magnification and wellaligned long fibers were obtained. After annealing, the alignment was maintained as shown in Fig. 4(d and e). Fig. 5 shows the configuration, prototype device, and electrical output of the proposed flexible piezoelectric device. In the present device, the BaTiO3 fibers worked under a bending mode with an alternating bending force applied at the two ends of the device. In general, the bending mode has a relatively low resonance frequency compared to other piezoelectric working modes, e.g., longitudinal-, transverse-, and thickness-extensional modes, and thickness shear mode. The low resonance frequency is close to the environmental vibration frequency which is favorable for enhancing the power output. The applied force was transferred to the BaTiO3 fibers through the PDMS matrix and resulted in the charge generation. A voltage difference between the two adjacent electrodes was thus induced due to this separation of charge. The inter-digital electrodes were utilized to enhance the power output of the device. The BaTiO3 nanofibers between each pair of adjacent electrodes served as unit cells, and each cell was connected in parallel. Under a period bending at ∼45 Hz, the open voltage of the device was characterized

in Fig. 5(h). A maximum output peak–peak voltage of ∼0.45 V was obtained under a load resistance R of 1 M. The corresponding output energy U and power P are shown in Fig. 5(i) and calculated from:



T2

U= T1

P=

V2 dt R

1 T2 − T1



(3) T2

T1

V2 dt R

(4)

where R, V, T1 and T2 represent the load resistance, output voltage, starting and finishing time, respectively. V, T1 and T2 were obtained from Fig. 5(h). The calculated peak power output is ∼60 nW and the average value during the measurement period is 6 nW. These values do not seem large compared to some recent lead-based nanogenerators [29,30], but they are entirely comparable with other lead-free nanogenerators based on ZnSnO3 microbelt [13], PVDF fibers [31], and Mn-doped (Na, K)NbO3 fibers [16]. Optimization of the BaTiO3 nanofiber-based device through better structure design and processing is still underway. 4. Conclusion In summary, high quality lead-free perovskite ferroelectric BaTiO3 fibers were fabricated by a sol–gel based electrospinning method. Dense, uniform, long nanofibers were obtained with diameters ∼100–200 nm and grain size ∼40 nm. Good electromechanical response was obtained based on PFM giving the fibers an effective piezoelectric constant of ∼40 pm/V. Aligned BaTiO3 fibers were fabricated using two parallel electrodes. A flexible piezoelectric device was further designed and fabricated based on the aligned BaTiO3 fibers. Under a bending excitation at ∼45 Hz,

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Fig. 3. (a) Schematic of the PFM set-up. (b)–(d) The topology, amplitude, and phase of the BaTiO3 fiber (4 × 4 ␮m2 ), respectively. (e)–(f) The amplitude and phase of the BaTiO3 fiber (0.5×0.5 ␮m2 ), respectively. (g) The piezoelectric hysteresis loop (displacement and phase versus dc voltage) of the BaTiO3 fiber.

Fig. 4. (a) Parallel-electrode collection set-up. (b)–(c) Aligned as-spun BaTiO3 fibers with different magnification. (d)–(e) Aligned annealed BaTiO3 fibers with different magnification.

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Fig. 5. (a) Schematic of the proposed piezoelectric device. (b) Cross-sectional view of the device configuration. (c)–(e) Photos of the aligned BaTiO3 fiber, plastic substrate coated with inter-digital electrode, and PDMS. (f)–(g) Photos of the fabricated flexible device. (h)–(i) The output voltage and energy subjected to a periodic bending excitation of ∼45 Hz.

an output peak–peak voltage of ∼0.45 V under a load resistance of 1 M was obtained. This device has the advantages of smallsize, good reliability, high flexibility and sensitivity to external movement at low-frequency which may open up a range of new applications.

Acknowledgements This work was supported by the “Chenguang” Program of Shanghai Educational Development Foundation of China (Grant No. 11CG49), and the National Natural Science Foundation of China (Grant Nos. 11204179 and 61376010). One of us (FFW) was also supported by the CAMT as a Visiting Scholar at the University of Sydney.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.sna.2015.07.002

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Biographies Feifei Wang is an Associate Professor in the Department of Physics in Shanghai Normal University. He obtained his Ph.D. degree from Shanghai Institute of Ceramics,

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Chinese Academy of Sciences in 2009. From March 2008 to March 2009, he worked as a Research Assistant at the Hong Kong Polytechnic University. From May 2013 to May 2014, he worked in the University of Sydney as a Visiting Scholar. His current research focuses on smart materials (high-performance ferroelectric, dielectric, piezoelectric, and novel lead-free materials) and related devices (sensors, actuators, transducers and transformers). He has authored and co-authored 70 papers. Professor Yiu-Wing Mai is a mechanical engineering alumnus of the University of Hong Kong and obtained the degrees of BSc (Eng) and Williamson Prize in 1969, PhD in 1972, DSc in 1999 and DSc (honoris causa) in 2013. He also obtained a DEng in 1999 from the University of Sydney. He currently holds a University Chair in Mechanical Engineering at the University of Sydney. His current research interests are in the fields of advanced composite materials and fracture mechanics. Danyang Wang is currently a lecturer of functional materials at the School of Materials Science and Engineering of the University of New South Wales (UNSW), Sydney, Australia. He received his B. Eng. in 2000 from Tianjin University and his MPhil and PhD in 2002 and 2006, respectively, for the Hong Kong Polytechnic University. His research team is working on the development, fabrication and characterization of perovskite-based functional thin films and nanostructures for electronic, photonic and energy applications. Rui Ding received his Bachelor of Engineering (Chemical) and Bachelor of Commerce degrees at The University of Melbourne, Melbourne, Australia (2004–2008), followed by Master of Engineering Management degree at The University of Melbourne, Melbourne, Australia (2009–2010). He is currently a Ph.D. student studying at The University of New South Wales, Sydney, Australia (2011–Present). His Ph.D. research is on the development of high performance lead-free piezoelectric thin films. Wangzhou Shi is Professor in the Department of Physics in Shanghai Normal University. He obtained his Ph.D. degree from the Institute of Solid State Physics, Chinese Academy of Sciences in 1994. His current research interest focuses on the structure, processing, and performances of the functional oxide films and nanostructures.