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Hierarchical PbZrxTi1-xO3 nanowires for vibrational energy harvesting Xing Zhang, Jingfan Chen, and Ya Wang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00317 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018
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ACS Applied Nano Materials
Hierarchical PbZrxTi1-xO3 Nanowires for Vibrational Energy Harvesting Xing Zhang, *† Jingfan Chen, † and Ya Wang*† †
Department of Mechanical Engineering, Stony Brook University, Stony Brook, New York, 11794-2300, United States
*Corresponding authors: Tel: +1-631–632–8322; Dr. Xing Zhang, E-mail:
[email protected]; Dr. Ya Wang, E-mail:
[email protected] Notes The authors declare no competing financial interest.
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ABSTRACT
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Perovskite piezoelectric material nanostructures have gained immense interest in
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the energy harvesting community due to their ability of converting mechanical energy
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into electric power in a much more efficient manner. In this paper, we first
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successfully synthesized PbZrxTi1-xO3 (PZT) nanowires with unique hierarchical
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nanostructures of ~ 100 nm in diameter and 6 µm long, by using a facile hydrothermal
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process. Then a unimorph cantilever energy harvester was fabricated by bonding a
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mixed hierarchical PZT nanowires with polydimethylsiloxane (PDMS) polymer
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matrix to a brass substrate. The measured open-circuit voltage from this harvester was
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2.7 V at its fundamental resonance (25.2 Hz) under 100 mg root mean square (RMS)
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acceleration input, and the measured power density was 51.8 µW cm-3 with an
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optimal load resistance of 2 MΩ, which is approximately 8 times larger than that of
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the commercial BaTiO3 nanowires energy harvester on the same scale, but driven by a
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much higher acceleration level.
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KEYWORDS: PbZrxTi1-xO3 nanowires, hierarchical nanostructure, open-circuit
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voltage, power density, vibrational energy harvesting
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INTRODUCTION
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Perovskite piezoelectric materials have attracted considerable attention because they
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can generate electrical power from ambient vibrational energy more efficiently,1, 2
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such as from engine vibrations,3 direct human walking motion,4,
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air-flowing,6-9 and even tiny biomechanical movements from muscles/organs,10 which
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could lead to a range of important potential applications for powering sensors,
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actuators, implants, etc. The energy conversion efficiency of piezoelectric devices
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largely depends on the piezoelectric coupling coefficient of nanomaterials,11 which
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significantly relies on the structures formed through the synthesis processes,12 the
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crystalline orientation,13 and the doping elements into the host lattice.14 Therefore, the
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synthesis approaches should be extensive conducted for achieving high-power-density
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energy harvesting. These approaches can be systematically investigated by adjusting
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the reaction conditions for exploiting their full potential in nanotechnology
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applications.
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gentle
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Energy harvesting properties have been successfully demonstrated in the
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literature on many high-efficient piezoelectric materials, such as ZnO,15-17 MoS2,18
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NKN,19-22 lead zirconium titanate (PZT),23-29 CdTe,30 BaTiO3,31,
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BiFeO3,35, 36 and TMCMMnCl337. Among these piezoelectric materials, PZT ceramics
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is the most cost-effective candidate for its perovskite nanostructure, high
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electromechanical coupling coefficient, high Curie temperature, spontaneous
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polarization and inexpensive fabrication process.37 Thus, it has been widely used in 3 ACS Paragon Plus Environment
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PMN-PT,33,
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many commercial products for powering high performance sensors and actuators.38, 39
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However, it is well known that PZT ceramics has the deficiency of low flexibility,
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high fragility and brittleness, which makes it difficult to be applied in flexible or
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wearable devices.40 Polydimethylsiloxane (PDMS) with the virtues of elasticity,
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thermal stability, and biocompatibility has been widely employed as a supporting
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medium in various flexible devices.41, 42 Therefore, embedding the PZT nanoparticles
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into PDMS matrix to form a readily flexible and tailorable device, may overcome
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such a restriction.
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For nanoscale piezoelectric materials, several typical piezoelectric nanostructures
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have been designed for efficiently powering small electronics. For example, vertically
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aligned BaTiO3 nanowire array energy harvester can provide an average power
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density of approximately 6.27 µW cm-3 from 1 g acceleration, which is 16 times
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higher than that of vertically aligned ZnO nanowire arrays from the same acceleration
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input.17 Recently, hierarchical PMN−PT nanocomposites34 can output the maximum
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voltage of 7.8 V, which is over 2 times larger than that of the BaTiO3 nanocomposite
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harvester
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novel and more sophisticated perovskite structures piezoelectric nanocomposite
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containing unique PZT nanostructures would have great potential for high-power
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nanogenerators and large-output signal with flexibility and low cost. Such as
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hierarchical PZT nanowires, has not been reported due to the complexity of synthesis
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methods. Therefore, the development of rational synthesized routes to assemble
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nanoscale PZT nanowires into hierarchical structures in a cost-effective manner for
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(3.2 V) and that of the NaNbO3 nanoharvester44 (3.2 V). However, the
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achieving high-efficient energy harvesting is desirable and significant, which would
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greatly promote their applications in nanodevices.
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Herein, we reported a simple and facile synthesis approach to build PZT
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nanowires with unique hierarchical nanostructures under hydrothermal conditions
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using sodium titanate nanowires as the source. The unimorph cantilever harvesting
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device was fabricated by bonding mixed hierarchical PZT nanowires with PDMS
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matrix to a brass substrate. This piezoelectric energy harvester produced much higher
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output voltage and power than other reported piezoelectric nanocomposites. This is
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potentially a
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micro/nanodevices.
key step in developing sustainable energy technology for
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EXPERIMENTAL SECTION
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Reagents. All the chemicals and materials used in this work include: Titanium (IV)
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butioxide (TBOT, Sigma Aldrich, St. Louis, MO), zirconium (IV) oxychloride
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(ZrOCl2·8H2O, Sigma Aldrich, St. Louis, MO), ethanol (EtOH, Sigma Aldrich, St.
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Louis, MO), ammonia solution (NH3, 2.0 M in ethanol, Sigma Aldrich, St. Louis,
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MO), potassium hydroxide (KOH, Sigma Aldrich, St. Louis, MO), lead (Ⅱ) nitrate
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(Pb(NO3)2, Sigma Aldrich, St. Louis, MO), and acetic acid (Sigma Aldrich, St. Louis,
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MO), were all analytical-grade purity and used as received without further
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purification. Distilled water was used in the preparation of all aqueous solutions.
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Synthesis of potassium zirconium titanate nanowires as a precursor. The
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potassium zirconium titanate nanofibers were synthesized as zirconium and titanium
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sources for the hydrothermal synthesis of PbZrxTi1-xO3. Firstly, Ti(C4H9O)4 (1.354
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mL) and ZrOCl2·8H2O (2.08 mmol) were dissolved into 15 mL ethanol under
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continuous magnetic stirring condition, and the ratios of Zr: Ti approximately is 3.5:
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6.5, then 5 mL ammonia solution (2.0 M in ethanol) added into the above mixture
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solution under continuous magnetic stirring. Followed by the addition of KOH (28 g)
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pellets and H2O (50-mL), and a white suspension was formed immediately. The white
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suspension was transferred into a 100-mL autoclave with a Teflon linear, which was
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heated at 200 °C for 16 h, and then naturally cooled to ambient temperature. The
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resulting white solid powders were collected by centrifugation and washed with
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distilled water and absolute alcohol for several times to remove residual ions. The
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final products were then dried at 80 °C for 8 h for the next step using for further
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characterization.
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Synthesis of lead zirconium titanate (PZT). In this step, potassium zirconium
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titanate precursor (0.2864 g) was dispersed in 15 mL distilled water and sonicated for
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30 min, followed by adding Pb(NO3)2 (4.0 mmol), then KOH pellets (15 g) were
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added directly and stirred for 15 min, and finally 50 mL H2O was poured into the
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above mixture solution. The final mixture was transferred into a 100-mL Teflon lined
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stainless steel autoclave. The reactor was heated in an oven at 200 °C for 24 h. After
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the hydrothermal reaction was completed, the reaction products were sequentially
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washed with a diluted acetic acid aqueous solution, water and ethanol for several 6 ACS Paragon Plus Environment
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times to remove residual ions and centrifugation collected. The final product was then
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dried at room temperature for 24 h.
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Physicochemical characterization of the samples. The X-Ray diffraction
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(XRD, Rigaku Ultima III) was conducted to confirm the crystallinity of hierarchical
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PZT nanowires. The scanning electron microscopy (SEM) was characterized with a
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Crossbeam 340 microscope (Zeiss, Oberkochen, Germany). In addition, energy
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dispersive X-ray spectroscopy (EDS) was performed with an INCA Energy 350
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AzTec Advanced system using a silicon drift detector (SDD) (Oxford Instruments,
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Abingdon, UK) for the determination of element stoichiometry.
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Fabrication of PZT-PDMS nanocomposite unimorph. The nanocomposite was
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made by mixing PZT nano-powder into the PDMS (SYLGARD, 184 silicone
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elastomer kit) matrix via sonication for 30 min in order to expel the dissolved air
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formation bubbles. The weight percentage of PZT in PDMS was approximately
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40%.45 The mixture was spin-coated on an acrylic plate and then transferred into a Cu
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foil substrate. The final geometry of the unimorph cantilever beam is 4 cm (L) × 0.7
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cm (W) × 0.004 cm (H), (the photo representation of the unimorph harvester can be
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found in the supplementary information (SI) Figure S1). After curing in the oven at
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120 °C for 6 hours, silver paste was covered on the top of the nanocomposite film to
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function as the top electrode. Then, the resulting nanocomposite was poled at 1 kV for
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6 hours (TREK model 2220) at room temperature.
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Electrical measurement. An electromagnetic shaker (APS-113, APS Dynamics
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Inc.) was used to generate mechanical vibration by accelerating the clamped end of 7 ACS Paragon Plus Environment
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the cantilever beam. The mechanical input energy was measured by mounting a shear
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accelerometer (PCB 333B32) on the root of the energy harvester. Power density
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measurement was performed across a series of load resistances from 0 to 10 MΩ
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provided by a resistance box. The electrical output signals were recorded by using an
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oscilloscope (RIGOL MSO1104). The photo representation of experimental setup,
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data acquisition, and testing processes has been provided in the SI (Figure S2).
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RESULTS AND DISCUSSION
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Fabrication and characterization of the PZT nanostructures. Figure 1 shows the
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XRD pattern of the sample hydrothermally synthesized with the potassium zirconium
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titanate nanowires (see ESI Figure S3) as Zr and Ti sources. All diffraction peaks can
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be easily indexed to the tetragonal PZT, corresponding well with the reported data of
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JCPDS No. 06-0452. The strong and sharp reflection peaks indicate that the
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as-prepared PZT product is well crystallized.
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Figure 2 shows typical SEM images of the obtained PZT sample by the
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hydrothermal method. Figure 2a shows that the product consisting of a large quantity
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of hierarchical structures. Figure 2b is a higher magnification SEM image of a
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randomly selected area. The sample images show a three-dimensional hierarchical
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structure with individual nanowires extending from cube faces directions. The length
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of some nanowires is up to 6 µm, and the length of each individual nanowire varies
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with the similar diameter of approximately 100 nm. The composition of hierarchical 8 ACS Paragon Plus Environment
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PZT nanowires was determined by scanning electron microscopy-energy dispersive
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X-ray spectroscopy. As shown in Figure S4, it clearly indicates that the single
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hierarchical PZT nanowire is composited of Pb, Zr and Ti elements.
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Investigation of the influence factors for the formation of hierarchical PZT
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nanowires. The amount of Pb(NO3)2 is critical to the configuration of the synthesized
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assembly, and thus its ratio influence on the nanostructure of the final product is
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investigated in detail in this experiment. Figure 3 shows the SEM images of the
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products synthesized with different amounts of Pb(NO3)2. When 3 mmol of Pb(NO3)2
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is added as a Pb2+ source, PZT assembled with long nanorods appeared in the product,
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as shown in Figure 3a, b, and no particles were produced. The change of morphology
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is observed by further increasing the amount of Pb(NO3)2 in the system, and Figure 3c,
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d give the SEM image of the product obtained by adding 5 mmol of Pb(NO3)2. The
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cube PZT can be obtained with the size of 2 µm.
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To get insight into the Zr and Ti ions ratio effect on the evolution of PZT
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nanostructures, a series of Zr and Ti ratios-dependent experiments were conducted.
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Figure 4 shows representative SEM images for the samples obtained with different Zr:
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Ti ratios. With no added Zr elements, the product of PbTiO3 consists of well-defined
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plate nanostructure with a rectangular outline and a side length of 500–3000 nm
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(Figure 4a), and the thickness of the nanoplates is approximately 200 nm (Figure 4b).
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When increasing the Zr: Ti ratio up to 1:9, the PZT sample shows the same nanoplate
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assembly but with much larger size of up to 10 µm (Figure 4c, d). It is very interesting
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to notice that when the Zr: Ti ratio increases up to 2:8, the obtained PZT product 9 ACS Paragon Plus Environment
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displays cube-like nanostructures with the size of 2-5 µm (Figure 4e). Obviously, the
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surface of the cube-like structure is rugged, which is assembled by small nanoplates
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(Figure 4f). These results, indicate that the higher Zr concentration could be
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favourable for the formation of three-dimensional PZT nanocube structures.
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In addition, we also investigated the effect of reaction temperature on the sample
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morphology under the same Pb2+. As shown in Figure 5a, b, cube-like assemble with
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square rods of 400 nm diameter and 5 µm long was obtained at 170 °C. While
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increasing the reaction temperature to 185 °C, the surface square rods grow bigger
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with the size of ~6 µm long (Figure 5c, d). Figure 5d shows the broken square rods
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consisting of many nanorods, and some of the rods fuse together. When the
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temperature increases to 210 °C, we could find the cube-like morphology of our PZT
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product with many nanowires (Figure 5e, f). The results indicate that the surface of
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cube-like PZT nanostructure could be tuned by manipulating the synthetic reaction
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temperature.
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The energy harvesting performances of the PZT-PDMS nanocomposites.
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Figure 6 shows the schematic representation of the energy harvester device
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fabrication processes. First, an acrylic sheet is cleaned with deionized water and
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alcohol (Figure 6a). Next, the preparing the mixture solution of PZT nano-powder and
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the PDMS matrix with weight ratio 40: 60, then spin coat the hierarchical PZT
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nanowires and the PDMS mixture (Figure 6b). The detailed mixture preparation
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process is described in the Experimental section. Next, the composite thin film with
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hierarchical PZT nanowires is peeled off from the acrylic substrate (Figure 6c). It 10 ACS Paragon Plus Environment
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should be noted that the composite thin film can be easily detached from the acrylic
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substrate after curing enough time. Next, attach the composite thin film onto the brass
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(Cu) beam with PDMS before curing (Figure 6d). The composite film can be bonded
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tightly by curing the PDMS. Finally, the conductive silver epoxy is covered on the top
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side of the PZT-PDMS film (Figure 6e).
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To demonstrate sustainable power generation from synthesized hierarchical PZT
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nanowires, the energy harvester is subjected to a sinusoidal base acceleration
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generated by a permanent magnet shaker. The clamped boundary condition of the
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energy harvester device allows simple periodic vibration of the cantilever at its
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resonant frequencies. This vibration induces compressive and tensile stress on the
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hierarchical PZT nanowires thus generating charge by the direct piezoelectric effect,
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resulting in a potential difference between the two electrodes of the beam. The
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frequency-dependent open-circuit voltage output of the energy harvester device has
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been investigated, as shown in Figure 7a. Obviously, the open-circuit voltage
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increased with the exciting frequencies from 24.5 to 25.2 Hz based on 100 mg root
209
mean square (RMS) acceleration and then decreased with the continuant increasing
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from 25.2 to 26.1 Hz. The results indicate that sinusoidal excitation at resonant
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frequency of 25.2 Hz yielded the highest peak to peak voltage ~2.7 V from 100 mg
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RMS acceleration input as shown in Figure 7b, which indicates the compressive and
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tensile stress is maximum under its resonant frequencies (25.2 Hz) when the
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amplitude up to maximum. The open-circuit voltage output of hierarchical PZT
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nanowires energy harvester at a range of RMS sinusoidal acceleration from 20 to 94 11 ACS Paragon Plus Environment
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mg is shown in Figure 7c. The open-circuit voltage output from the hierarchical PZT
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nanowires energy harvester increased with the increasing RMS acceleration. The
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higher voltage output is produced because the high dynamic input induces a higher
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alternating piezoelectric charge accumulation at the two electrodes. At this resonant
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frequency, the PZT-PDMS device was subjected to various accelerations thus
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producing the different bending strain states result in producing different output
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voltage. Therefore, the cantilever vibrating with maximum strain can generates
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maximum output voltage by increasing the amplitude of the sinusoidal acceleration at
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the resonant frequency. The hierarchical PZT nanowires also exhibits better
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performance than two-dimensional PbTiO3 nanoplates (Figure S5). The power output
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from the hierarchical PZT nanowires energy harvester is calculated by measuring the
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voltage, across several load resistors (RL), ranging from 0 to 10 MΩ. The power
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output from the hierarchical PZT-PDMS nanocomposite energy harvester increased
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rapidly as RL was increased up to 2 MΩ when a peak value of 0.58 µW is reached,
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and then reduced as RL was increased up to 10 MΩ. The peak power density across
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the optimal RL is calculated to be 51.8 µW cm-3 from 100 mg RMS base acceleration
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(Figure 7d). This power density is 8 times higher than that recorded from the BaTiO3
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based energy harvester (6.27 µW cm-3) driven by a higher base acceleration of 1.0 g
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RMS.32 It is noticed that hierarchical PZT nanowires have a higher power density than
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BaTiO3, because of the following advantages. Firstly, the PZT owns higher
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piezoelectric constants than that of the BaTiO3 and ZnO, which can effectively
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improve the voltage output, and the special hierarchical nanostructure always makes 12 ACS Paragon Plus Environment
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some nanowires of the bulk parallel to the two electrodes, which makes hierarchical
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PZT nanowires potentially have a higher effective length strain.34 Therefore, the
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significant performance improvement in the energy harvester device based on
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hierarchical PZT nanowires are largely attributed to both the higher piezoelectric
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constant and the unique hierarchical structure.
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CONCLUSIONS
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In summary, we have synthesized PZT nanowires with novel hierarchical
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nanostructures of ~100 nm in diameter and ~6 µm long, by using a facile
248
hydrothermal process. We have fabricated a unimorph energy harvester by bonding
249
hierarchical PZT nanowires with PDMS polymer mixture to a brass substrate. The
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peak output voltage from this energy harvester was 2.7 V at its fundamental resonance
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of 25.2 Hz, and the power density was 51.8 µW cm-3 with an optimal load resistance
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of 2 MΩ. This power density is 8 times higher than that recorded from the BaTiO3
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based energy harvester (6.27 µW cm-3) driven by a higher base acceleration RMS.
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This is a key step in developing sustainable energy harvesting technical for
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micro/nanodevices.
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ACKNOWLEDGEMENTS
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The authors thank the support by the US Office of Navy Research 13 ACS Paragon Plus Environment
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(N000141410230), and the Department of Energy ARPA-E (DOE-AR0000531) to
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this work. This research used resources from the Center for Functional Nanomaterials,
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which is a U.S. DOE Office of Science Facility, at the Brookhaven National
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Laboratory under Contract No. DE-SC0012704.
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Appendix A. Supplementary Data
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Supplementary data associated with this article can be found in the online version.
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Figure captions Figure 1. The XRD pattern of the hierarchical PZT nanowires sample. Figure 2. The SEM images of product synthesized hierarchical PZT nanowires: (a) Low magnification, and (b) high magnification images. Figure 3. The SEM images of products obtained with different amounts of Pb(NO3)2: (a, b) 3 mmol, and (c, d) 5 mmol. Figure 4. The SEM images of products synthesized by using precursors with different Zr: Ti ratio: (a, b) 0: 1; (c, d) 1: 9, and (e, f) 2: 8. Figure 5. The SEM images of products synthesized using different temperature: (a, b) 170 °C; (c, d) 185 °C; and (e, f) 210 °C. Figure 6. Schematic diagram of the energy harvester device fabrication process: (a) acrylic sheet substrate, (b) PZT-PDMS composite thin film cured by spin-coating, (c) peeled off the composite thin film from the acrylic sheet substrate, (d) PZT-PDMS film attachment on the brass beam top, and (e) conductor silver epoxy coated on the PZT-PDMS film. Figure 7. Sinusoidal excitation on hierarchical PZT nanowires energy harvester device: (a) open-circuit voltage generation under different vibration frequencies, (b) open-circuit peak to peak voltage versus vibration frequencies, (c) open-circuit voltage under different RMS sinusoidal acceleration input at resonant frequency (25.2 Hz), and (d) the power delivered to the load resistors versus the load resistance.
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