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Controlled synthesis of ultra-long vertically aligned BaTiO3 nanowire arrays for sensing and energy harvesting applications
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 375603 (http://iopscience.iop.org/0957-4484/25/37/375603) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 160.36.178.25 This content was downloaded on 27/08/2014 at 12:04
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 375603 (10pp)
doi:10.1088/0957-4484/25/37/375603
Controlled synthesis of ultra-long vertically aligned BaTiO3 nanowire arrays for sensing and energy harvesting applications Aneesh Koka1, Zhi Zhou2, Haixiong Tang2 and Henry A Sodano1,2 1
Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL, 32611, USA 2 Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA E-mail: hsodano@ufl.edu Received 11 June 2014, revised 23 July 2014 Accepted for publication 24 July 2014 Published 22 August 2014 Abstract
A novel approach for the synthesis of ultra-long (up to ∼45 μm) vertically aligned barium titanate (BaTiO3) nanowire (NW) arrays on an oxidized Ti substrate is developed. The fabrication method uses a two-step hydrothermal reaction that firstly, involves the growth of ultra-long aligned sodium titanate NW arrays and secondly, involves the transfer of these precursor sodium titanate NW arrays to BaTiO3 NW arrays while retaining the shape of the template nanowires. The ion-exchange during the second hydrothermal reaction in barium hydroxide solution results in the structural transformation from single-crystal sodium titanate NW arrays to BaTiO3 NW arrays. This synthesis approach is low-cost, scalable, and enables control over the morphology and aspect ratio of the resulting BaTiO3 NW arrays by tuning the hydrothermal reaction parameters. In addition to the synthesis methods reported here, the energy harvesting behavior of the BaTiO3 NW arrays is evaluated as a function of their aspect ratio and demonstrated to produce significant impact on the energy produced. The newly developed hydrothermal synthesis process for controlled growth of ultra-long, vertically aligned BaTiO3 NW arrays provides a promising method for their efficient utilization in nano-electromechanical system-based sensors, energy harvesters, and nano-scale electronic devices. S Online supplementary data available from stacks.iop.org/NANO/25/375603/mmedia Keywords: hydrothermal, nanowires, barium titanate, energy harvesting, sensing (Some figures may appear in colour only in the online journal) 1. Introduction
high energy density nano-composite capacitors [15, 16]. However, increasing environmental concerns over the use of lead have motivated researchers to develop high performance lead-free nanostructures such as barium titanate (BaTiO3) that possess high electro-mechanical coupling [17–20]. In this regard, many methods have been reported on the synthesis of BaTiO3 NWs such as hydrothermal synthesis [21–23], electro-spinning [24, 25], sol–gel synthesis [26, 27], and molten salt synthesis techniques [28, 29]. Among all these methods, hydrothermal processes have received more attention because they are inexpensive, scalable and enable control over the resultant NW morphology by tuning reaction parameters for implementation in large scale
One-dimensional (1D) piezoelectric nanowires (NWs) with electro-mechanical coupling behavior have demonstrated unique transducer applications in nano-electromechanical systems (NEMS) as energy harvesters [1–4] and advanced sensors [5–8]. Thereby, extensive research has been conducted on different synthesis methods and characterization techniques to exploit their full potential in nanotechnology applications [9, 10]. Among piezoelectric NWs, ferroelectric, lead zirconate titanate (PZT) NWs [11, 12] with high piezoelectric strain coefficients and high dielectric permittivity have been utilized efficiently in nanogenerators [13, 14] and 0957-4484/14/375603+10$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
A Koka et al
Nanotechnology 25 (2014) 375603
manufacture [30, 31]. Consequently, the hydrothermal approach has been used to synthesize BaTiO3 nanostructures from different precursors such as hydrogen titanate [32], TiO2 [18–20, 33, 34], and layered alkali titanate [35, 36], nanostructures. As reported in previous research, the morphology of the resultant BaTiO3 NWs from the hydrothermal reactions is highly dependent on both the precursor’s morphology and the reaction parameters chosen to enable shape retention [30, 37]. Here, the focus is on developing a controlled hydrothermal synthesis of vertically aligned BaTiO3 NW arrays with a high aspect ratio that can be utilized as piezoelectric and ferroelectric elements in advanced nanoscale electronic devices. BaTiO3 in the form of aligned NW arrays as compared to free-standing NWs is beneficial as it allows easier integration with electrical contacts thereby enabling a simple fabrication technique for achieving highly efficient NW-based transducers. Recently, Koka et al [38] demonstrated the use of novel ultra-long aligned BaTiO3 NW arrays as a high sensitivity NEMS accelerometer. In addition, a low-frequency energy harvester composed of ultra-long BaTiO3 NW arrays was also developed recently by Koka et al [39]. However, the authors here elaborate in-detail on the synthesis approach along with the influence of tuning the hydrothermal reaction parameters for achieving control over the morphology and microstructure of the resulting ultra-long BaTiO3 NW arrays for sensing and energy harvesting applications. Based on prior hydrothermal synthesis of BaTiO3 nanostructures from various precursors [35, 40], it is evident that the development of aligned BaTiO3 NW arrays is highly dependent on the type of precursor NW arrays. Synthesis approaches that achieve arrays of polycrystalline BaTiO3 nanotubes and thin films from hydrothermal treatments using aligned arrays of single crystalline TiO2 nanotubes as precursors have been reported by Padture et al [34], and Deng et al [41]. Additionally, there has been a synthesis procedure reported for free-standing BaTiO3 NWs converted directly from sodium titanate (Na2Ti3O7) nanorods and NWs [30, 31] with enhanced shape control by tuned reaction conditions. These past experiments provided the motivation to synthesize and utilize aligned sodium titanate NW arrays as precursors for transformation to BaTiO3 NW arrays. In this regard, Guo et al [42] developed an oriented layer of sodium titanate (Na2Ti2O5 · H2O) NWs on Ti foil via an alkaline hydrothermal reaction that is similar to the synthesis process reported by Liu et al [43]. However, the resulting NW arrays from this approach had a very high aspect ratio (∼540) causing severe bending and wicking of the NWs at the top surface from the capillary forces during the drying process after synthesis [44, 45]. More recently, Chatterjee et al [46] reported the use of a higher concentration of alkaline solution in the hydrothermal reaction to synthesize an aligned array of hydrogen titanate (H2Ti3O7) NWs. These H2Ti3O7 NWs, produced from a dilute HCl wash of sodium titanate (Na2Ti3O7) NWs, on oxidized Ti foil were sufficiently rigid without wicking. Additionally, they discussed the morphology control of these NWs by varying the reaction duration. Consequently, this research work is derived from these previous findings and led to the development of ultra-long
Figure 1. Schematic illustrating the synthesis approach used for
achieving controlled growth of ultra-long, vertically aligned BaTiO3 NW arrays.
aligned BaTiO3 NW arrays using ultra-long aligned sodium titanate NW arrays as templates with improved morphology retention. Furthermore in this paper, the authors demonstrate the impact of controlling the dimensions of ultra-long BaTiO3 NW arrays, using the two-step hydrothermal approach, on the output voltage response and resonant frequency from the BaTiO3 NW array-based NEMS vibrational energy harvesters (‘nanogenerators’) and sensors (‘NEMS accelerometers’) that were fabricated using the same proof mass when driven by mechanical base acceleration.
2. Experimental section The preparation of ultra-long BaTiO3 NW arrays was approached using a two-step hydrothermal reaction as shown in figure 1. First, ultra-long sodium titanate NW arrays on the Ti substrate were prepared by a single hydrothermal reaction. Ti foil substrates (MTI Corporation, 99.9%, 100 μm thick) were first cleaned in a bath sonicator for 30 min in an acetone, 2-proponal and deionized water (1 : 1 : 1) solution. Sequentially, they were oxidized in a furnace at 750 °C for 8 h. Next, the oxidized Ti foil substrates were immersed in a Teflon lined autoclave filled with 37.5 ml of a 10–13 M NaOH solution, (Fill Factor: 50%, 97% Alfa Aesar). The sealed autoclave was placed in an oven with a temperature ranging from 200–225 °C for different durations to control the length and the diameter of the resulting aligned sodium titanate NW arrays on the substrate. The resulting substrates were washed using deionized water and ethanol and then dried at room temperature. Subsequently, the as-synthesized sodium titanate NWs arrays were converted to BaTiO3 NW arrays by a second hydrothermal reaction. To do so, the dried sodium 2
A Koka et al
Nanotechnology 25 (2014) 375603
titanate NW arrays were immersed in a Teflon lined autoclave containing aqueous Barium Hydroxide Octahydrate (Ba (OH)2 · 8H2O) solution, (Fill Factor: 33%, Sigma Aldrich) and filled with argon gas. The sealed autoclave was then placed in an oven at 210 °C for 12 h. The samples were lastly washed with dilute nitric acid, deionized water, and ethanol and then dried to yield BaTiO3 NW arrays. The morphology and crystalline structure of the NWs were characterized by scanning electron microscopy (FESEM; 6335F, JEOL) and x-ray diffraction (XRD) using a diffractometer equipped with a curved position sensitive detector (CPS120, Inel) with Cu Kα radiation, respectively. The element composition of the NWs was studied using energy dispersive x-ray spectroscopy (EDS). The crystal structure and lattice parameters of individual NWs were studied using a FEI Tecnai F30 (Philips) high resolution transmission electron microscope (HRTEM) that operates with 300 kV accelerating voltage provided by a field-emission electron gun. Phase transition behavior in the BaTiO3 NW arrays around the Curie temperature were characterized using thermal analysis from a differential scanning calorimeter (DSC; TA instruments, Q20). NEMS composed of different aspect ratio ultra-long, aligned BaTiO3 NW arrays for vibration sensing and energy harvesting applications were fabricated by releasing the BaTiO3 NW arrays’ surface carefully from the original oxidized Ti substrate following an acid wash in dilute HCl solution. The ultra-long, BaTiO3 NW arrays’ surface (∼4 mm × ∼4 mm) was then transferred and affixed to the borosilicate glass substrate (∼1 cm × ∼1 cm) using a thin uniform layer of conductive silver epoxy paste (MG Chemicals) such that the top of the NW arrays was exposed to make contact with the top electrode. The glass substrate with the NW arrays was then heated to cure the silver epoxy and improve adhesion. The silver epoxy layer also acted as the bottom electrode. For the top electrode, a Ti foil (Alfa-Aesar, 99.9%, 250 μm thick) spin coated with conductive organic Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) (3–4% in H2O, high conductivity grade, Aldrich) was used. The conductive PEDOT: PSS was spincoated on Ti foil at 2500 rpm for 30 s using WS-400-6NPP spin coater (Laurel Technologies Corporation) and heated at 60 °C for 5 min. The Ti foil/PEDOT: PSS top electrode configuration also served as the proof mass providing the BaTiO3 NWs NEMS with vibrational sensing and energy harvesting capability when subjected to mechanical base vibration [38, 39]. The dimension of the Ti foil coated with PEDOT: PSS was adjusted such that the total proof mass was kept same (∼30 mg) during the fabrication of different NEMS devices composed of BaTiO3 NW arrays of varying aspect ratio so that the influence of the NWs dimension on the output voltage response can be accurately investigated. The Ti foil/ PEDOT:PSS substrate was then placed on top of the NW arrays and then the entire NEMS assembly was heated at 120 °C for 10 min to enable adhesion between the PEDOT: PSS layer and the NWs. The two electrodes sandwich the aligned BaTiO3 NW arrays in the NEMS device configuration and the signal wires
were attached to the two electrodes using silver epoxy. The as-fabricated BaTiO3 NWs NEMS was then poled by supplying a high dc electric field of ∼75 KV cm−1 (TREK 477A Supply/Amplifier) for 12 h to ensure the electric dipoles within the BaTiO3 NWs align in the electric field direction which is normal to the plane of the two electrodes. Next, the BaTiO3 NWs NEMS was mounted on a Miniature Permanent Magnet shaker (Labworks. ET-132) that provides the mechanical vibration source and the voltage measurements from the NEMS subjected to vibration excitation was performed using a voltage buffer amplifier with unity gain from Linear Technologies (LTC6240CS8). The input base acceleration supplied by the vibration shaker to the NEMS was measured accurately by an instrumentation-grade shear accelerometer (PCB352C22). The grounded faraday cage surrounded the experimental setup to reduce the extrinsic 60 Hz power-line noise from electromagnetic interference [39]. The white noise excitation and sine wave excitation used for the frequency response function (FRF) and the output voltage (V) response characterization from NEMS was performed using NI SignalExpress software that operated a high accuracy DAQ system (NI USB 4431). All signals were reexamined during data acquisition for accuracy using an oscilloscope (Tektronix, DPO 3014 Digital Phosphor Oscilloscope).
3. Result and discussion The fabrication procedure begins with the growth of vertically aligned sodium titanate NW arrays on oxidized Ti foil. It is well known that Ti foil can be thermally oxidized at high temperatures ranging from 550–1000 °C to form the rutile TiO2 [47]. In this experiment, the rutile phase of the TiO2 layer provided the Ti source for the formation of aligned sodium titanate NW arrays during the hydrothermal reaction [46]. The as-prepared sodium titanate NW arrays have the open structure of a layered alkali titanate which facilitates their strong ion-exchange property and therefore, excel as precursors for the hydrothermal synthesis of aligned barium titanate NW arrays with enhanced morphology retention [30, 31, 48]. The sodium content in the as-synthesized sodium titanate NW arrays precursor is highly dependent on the intensity of proton (H+ ion) exchange that takes place from the washing procedure used after synthesis [49]. Temperature is a key parameter in the hydrothermal reaction because of its influence on the nucleation and the crystal growth of these sodium titanate nanostructures [50]. As reported by Lan et al [50], higher temperatures (greater than 180 °C) resulted in the morphology of the nanostructures formed from the hydrothermal synthesis using concentrated NaOH (10 M NaOH) to be isolated straight nanorods whereas at lower temperature (125–150 °C) nanotubes were formed. Here, the focus is on synthesizing precursor sodium titanate NW arrays for conversion to BaTiO3 NW arrays and hence a higher temperature (greater than 200 °C) is used for all the hydrothermal reactions in this work. Additionally in this work, three different temperatures (200, 210, and 225 °C) are 3
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Nanotechnology 25 (2014) 375603
Figure 2. The cross-sectional SEM images of sodium titanate NW arrays on oxidized Ti substrates after hydrothermal reaction in 10 M NaOH at 225 °C for different reaction time: (a) 5 h, (b) 7 h, and (c) 9 h respectively (the scale bar is 10 μm). (d) Average length of sodium titanate NW arrays as a function of reaction time.
used and their effect on the adhesion between the as-formed vertically aligned sodium titanate NW arrays’ surface and the Ti growth substrate is discussed. In the first hydrothermal reaction from this work, ultralong sodium titanate NW arrays are obtained on oxidized Ti foil at a reaction temperature of 225 °C with 10 M NaOH concentration from different reaction time as shown in figures 2(a)–(c). Reaction duration of 3 h yielded long sodium titanate NW arrays larger than 30 μm on the Ti substrates. With increasing reaction time, the alkali metal titanate NW arrays became longer with better vertical alignment since the nucleation reaction takes place more rapidly and is sustained through the entire duration of the hydrothermal reaction at 225 °C [12]. The relationship between NW length and growth duration in 10 M NaOH at 225 °C is shown in figure 2(d). The length of the NWs increases from ∼33 μm to ∼44 μm and finally saturates around ∼45 μm beyond nine hours of the hydrothermal reaction. This saturation is due to the consumption of rutile TiO2 and the exhaustion of reactant Ti ions in the solution. The results from the SEM analysis demonstrated that a reaction temperature of 225 °C can yield ultra-
long sodium titanate NW arrays on the Ti substrates; however, this high temperature affects the adherence between the sodium titanate NWs layer and the Ti foil growth substrate. During thermal oxidation, micro cracks are produced at the interfaces between the Ti foil and the outer rutile TiO2 layers because of their mismatch in the thermal expansion coefficient. These micro cracks then expand under the high temperature and pressure during the hydrothermal reaction, causing the layer of sodium titanate NW arrays to lose contact with the Ti substrate [46]. In order to establish better adhesion between the vertically aligned sodium titanate NWs and the Ti growth substrates, the alkaline hydrothermal reaction was carried out at a lower temperature (200 °C) with different reaction durations and concentrations of NaOH solution. Here, the influence of the NaOH solution concentration on the diameter and the length of the sodium titanate NW arrays are studied by using the SEM images as shown in figures 3(a)–(d). The adhesion between the resulting sodium titanate NWs layer and the substrate improved from the lower temperature reaction at 200 °C with different concentrations of NaOH solution. The diameter of the NWs increased with 4
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Nanotechnology 25 (2014) 375603
Figure 3. SEM images of the top view of sodium titanate nanowires from different concentrations of NaOH solution after a hydrothermal reaction at 200 °C for 8 h: (a)–(d) shows the top view after 10 M, 11 M, 12 M, and 13 M reactions respectively (the scale bar is 10 μm). (e) Average diameter and length of the sodium titanate nanowires as a function of the NaOH concentration after a hydrothermal reaction at 210 °C for 8 h. (f) XRD patterns of the initial Ti foil (JCPDS No. 65-3362), the oxidized rutile TiO2 after heating Ti foil at 750 °C for 8 h (JCPDS No. 65-0191) and the as-synthesized sodium titanate nanowires; H denotes sodium hexatitanate (Na2Ti6O13) peaks (JCPDS No. 731398) and T denotes sodium trititanate (Na2Ti3O7) peaks (JCPDS No. 31-1329).
an increase in the concentration of the NaOH solution from 10 M to 13 M (figure 3(e)). However, the increase in the diameter of the NWs results in a tradeoff with its length because there is a saturation limit to the amount of Ti source available from the rutile TiO2 precursor for the growth of these NW arrays. The NW arrays prepared from 10 M NaOH yielded higher aspect ratio NWs with small diameter (
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My IOPscience
Controlled synthesis of ultra-long vertically aligned BaTiO3 nanowire arrays for sensing and energy harvesting applications
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 375603 (http://iopscience.iop.org/0957-4484/25/37/375603) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 160.36.178.25 This content was downloaded on 27/08/2014 at 12:04
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 375603 (10pp)
doi:10.1088/0957-4484/25/37/375603
Controlled synthesis of ultra-long vertically aligned BaTiO3 nanowire arrays for sensing and energy harvesting applications Aneesh Koka1, Zhi Zhou2, Haixiong Tang2 and Henry A Sodano1,2 1
Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL, 32611, USA 2 Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA E-mail: hsodano@ufl.edu Received 11 June 2014, revised 23 July 2014 Accepted for publication 24 July 2014 Published 22 August 2014 Abstract
A novel approach for the synthesis of ultra-long (up to ∼45 μm) vertically aligned barium titanate (BaTiO3) nanowire (NW) arrays on an oxidized Ti substrate is developed. The fabrication method uses a two-step hydrothermal reaction that firstly, involves the growth of ultra-long aligned sodium titanate NW arrays and secondly, involves the transfer of these precursor sodium titanate NW arrays to BaTiO3 NW arrays while retaining the shape of the template nanowires. The ion-exchange during the second hydrothermal reaction in barium hydroxide solution results in the structural transformation from single-crystal sodium titanate NW arrays to BaTiO3 NW arrays. This synthesis approach is low-cost, scalable, and enables control over the morphology and aspect ratio of the resulting BaTiO3 NW arrays by tuning the hydrothermal reaction parameters. In addition to the synthesis methods reported here, the energy harvesting behavior of the BaTiO3 NW arrays is evaluated as a function of their aspect ratio and demonstrated to produce significant impact on the energy produced. The newly developed hydrothermal synthesis process for controlled growth of ultra-long, vertically aligned BaTiO3 NW arrays provides a promising method for their efficient utilization in nano-electromechanical system-based sensors, energy harvesters, and nano-scale electronic devices. S Online supplementary data available from stacks.iop.org/NANO/25/375603/mmedia Keywords: hydrothermal, nanowires, barium titanate, energy harvesting, sensing (Some figures may appear in colour only in the online journal) 1. Introduction
high energy density nano-composite capacitors [15, 16]. However, increasing environmental concerns over the use of lead have motivated researchers to develop high performance lead-free nanostructures such as barium titanate (BaTiO3) that possess high electro-mechanical coupling [17–20]. In this regard, many methods have been reported on the synthesis of BaTiO3 NWs such as hydrothermal synthesis [21–23], electro-spinning [24, 25], sol–gel synthesis [26, 27], and molten salt synthesis techniques [28, 29]. Among all these methods, hydrothermal processes have received more attention because they are inexpensive, scalable and enable control over the resultant NW morphology by tuning reaction parameters for implementation in large scale
One-dimensional (1D) piezoelectric nanowires (NWs) with electro-mechanical coupling behavior have demonstrated unique transducer applications in nano-electromechanical systems (NEMS) as energy harvesters [1–4] and advanced sensors [5–8]. Thereby, extensive research has been conducted on different synthesis methods and characterization techniques to exploit their full potential in nanotechnology applications [9, 10]. Among piezoelectric NWs, ferroelectric, lead zirconate titanate (PZT) NWs [11, 12] with high piezoelectric strain coefficients and high dielectric permittivity have been utilized efficiently in nanogenerators [13, 14] and 0957-4484/14/375603+10$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
A Koka et al
Nanotechnology 25 (2014) 375603
manufacture [30, 31]. Consequently, the hydrothermal approach has been used to synthesize BaTiO3 nanostructures from different precursors such as hydrogen titanate [32], TiO2 [18–20, 33, 34], and layered alkali titanate [35, 36], nanostructures. As reported in previous research, the morphology of the resultant BaTiO3 NWs from the hydrothermal reactions is highly dependent on both the precursor’s morphology and the reaction parameters chosen to enable shape retention [30, 37]. Here, the focus is on developing a controlled hydrothermal synthesis of vertically aligned BaTiO3 NW arrays with a high aspect ratio that can be utilized as piezoelectric and ferroelectric elements in advanced nanoscale electronic devices. BaTiO3 in the form of aligned NW arrays as compared to free-standing NWs is beneficial as it allows easier integration with electrical contacts thereby enabling a simple fabrication technique for achieving highly efficient NW-based transducers. Recently, Koka et al [38] demonstrated the use of novel ultra-long aligned BaTiO3 NW arrays as a high sensitivity NEMS accelerometer. In addition, a low-frequency energy harvester composed of ultra-long BaTiO3 NW arrays was also developed recently by Koka et al [39]. However, the authors here elaborate in-detail on the synthesis approach along with the influence of tuning the hydrothermal reaction parameters for achieving control over the morphology and microstructure of the resulting ultra-long BaTiO3 NW arrays for sensing and energy harvesting applications. Based on prior hydrothermal synthesis of BaTiO3 nanostructures from various precursors [35, 40], it is evident that the development of aligned BaTiO3 NW arrays is highly dependent on the type of precursor NW arrays. Synthesis approaches that achieve arrays of polycrystalline BaTiO3 nanotubes and thin films from hydrothermal treatments using aligned arrays of single crystalline TiO2 nanotubes as precursors have been reported by Padture et al [34], and Deng et al [41]. Additionally, there has been a synthesis procedure reported for free-standing BaTiO3 NWs converted directly from sodium titanate (Na2Ti3O7) nanorods and NWs [30, 31] with enhanced shape control by tuned reaction conditions. These past experiments provided the motivation to synthesize and utilize aligned sodium titanate NW arrays as precursors for transformation to BaTiO3 NW arrays. In this regard, Guo et al [42] developed an oriented layer of sodium titanate (Na2Ti2O5 · H2O) NWs on Ti foil via an alkaline hydrothermal reaction that is similar to the synthesis process reported by Liu et al [43]. However, the resulting NW arrays from this approach had a very high aspect ratio (∼540) causing severe bending and wicking of the NWs at the top surface from the capillary forces during the drying process after synthesis [44, 45]. More recently, Chatterjee et al [46] reported the use of a higher concentration of alkaline solution in the hydrothermal reaction to synthesize an aligned array of hydrogen titanate (H2Ti3O7) NWs. These H2Ti3O7 NWs, produced from a dilute HCl wash of sodium titanate (Na2Ti3O7) NWs, on oxidized Ti foil were sufficiently rigid without wicking. Additionally, they discussed the morphology control of these NWs by varying the reaction duration. Consequently, this research work is derived from these previous findings and led to the development of ultra-long
Figure 1. Schematic illustrating the synthesis approach used for
achieving controlled growth of ultra-long, vertically aligned BaTiO3 NW arrays.
aligned BaTiO3 NW arrays using ultra-long aligned sodium titanate NW arrays as templates with improved morphology retention. Furthermore in this paper, the authors demonstrate the impact of controlling the dimensions of ultra-long BaTiO3 NW arrays, using the two-step hydrothermal approach, on the output voltage response and resonant frequency from the BaTiO3 NW array-based NEMS vibrational energy harvesters (‘nanogenerators’) and sensors (‘NEMS accelerometers’) that were fabricated using the same proof mass when driven by mechanical base acceleration.
2. Experimental section The preparation of ultra-long BaTiO3 NW arrays was approached using a two-step hydrothermal reaction as shown in figure 1. First, ultra-long sodium titanate NW arrays on the Ti substrate were prepared by a single hydrothermal reaction. Ti foil substrates (MTI Corporation, 99.9%, 100 μm thick) were first cleaned in a bath sonicator for 30 min in an acetone, 2-proponal and deionized water (1 : 1 : 1) solution. Sequentially, they were oxidized in a furnace at 750 °C for 8 h. Next, the oxidized Ti foil substrates were immersed in a Teflon lined autoclave filled with 37.5 ml of a 10–13 M NaOH solution, (Fill Factor: 50%, 97% Alfa Aesar). The sealed autoclave was placed in an oven with a temperature ranging from 200–225 °C for different durations to control the length and the diameter of the resulting aligned sodium titanate NW arrays on the substrate. The resulting substrates were washed using deionized water and ethanol and then dried at room temperature. Subsequently, the as-synthesized sodium titanate NWs arrays were converted to BaTiO3 NW arrays by a second hydrothermal reaction. To do so, the dried sodium 2
A Koka et al
Nanotechnology 25 (2014) 375603
titanate NW arrays were immersed in a Teflon lined autoclave containing aqueous Barium Hydroxide Octahydrate (Ba (OH)2 · 8H2O) solution, (Fill Factor: 33%, Sigma Aldrich) and filled with argon gas. The sealed autoclave was then placed in an oven at 210 °C for 12 h. The samples were lastly washed with dilute nitric acid, deionized water, and ethanol and then dried to yield BaTiO3 NW arrays. The morphology and crystalline structure of the NWs were characterized by scanning electron microscopy (FESEM; 6335F, JEOL) and x-ray diffraction (XRD) using a diffractometer equipped with a curved position sensitive detector (CPS120, Inel) with Cu Kα radiation, respectively. The element composition of the NWs was studied using energy dispersive x-ray spectroscopy (EDS). The crystal structure and lattice parameters of individual NWs were studied using a FEI Tecnai F30 (Philips) high resolution transmission electron microscope (HRTEM) that operates with 300 kV accelerating voltage provided by a field-emission electron gun. Phase transition behavior in the BaTiO3 NW arrays around the Curie temperature were characterized using thermal analysis from a differential scanning calorimeter (DSC; TA instruments, Q20). NEMS composed of different aspect ratio ultra-long, aligned BaTiO3 NW arrays for vibration sensing and energy harvesting applications were fabricated by releasing the BaTiO3 NW arrays’ surface carefully from the original oxidized Ti substrate following an acid wash in dilute HCl solution. The ultra-long, BaTiO3 NW arrays’ surface (∼4 mm × ∼4 mm) was then transferred and affixed to the borosilicate glass substrate (∼1 cm × ∼1 cm) using a thin uniform layer of conductive silver epoxy paste (MG Chemicals) such that the top of the NW arrays was exposed to make contact with the top electrode. The glass substrate with the NW arrays was then heated to cure the silver epoxy and improve adhesion. The silver epoxy layer also acted as the bottom electrode. For the top electrode, a Ti foil (Alfa-Aesar, 99.9%, 250 μm thick) spin coated with conductive organic Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) (3–4% in H2O, high conductivity grade, Aldrich) was used. The conductive PEDOT: PSS was spincoated on Ti foil at 2500 rpm for 30 s using WS-400-6NPP spin coater (Laurel Technologies Corporation) and heated at 60 °C for 5 min. The Ti foil/PEDOT: PSS top electrode configuration also served as the proof mass providing the BaTiO3 NWs NEMS with vibrational sensing and energy harvesting capability when subjected to mechanical base vibration [38, 39]. The dimension of the Ti foil coated with PEDOT: PSS was adjusted such that the total proof mass was kept same (∼30 mg) during the fabrication of different NEMS devices composed of BaTiO3 NW arrays of varying aspect ratio so that the influence of the NWs dimension on the output voltage response can be accurately investigated. The Ti foil/ PEDOT:PSS substrate was then placed on top of the NW arrays and then the entire NEMS assembly was heated at 120 °C for 10 min to enable adhesion between the PEDOT: PSS layer and the NWs. The two electrodes sandwich the aligned BaTiO3 NW arrays in the NEMS device configuration and the signal wires
were attached to the two electrodes using silver epoxy. The as-fabricated BaTiO3 NWs NEMS was then poled by supplying a high dc electric field of ∼75 KV cm−1 (TREK 477A Supply/Amplifier) for 12 h to ensure the electric dipoles within the BaTiO3 NWs align in the electric field direction which is normal to the plane of the two electrodes. Next, the BaTiO3 NWs NEMS was mounted on a Miniature Permanent Magnet shaker (Labworks. ET-132) that provides the mechanical vibration source and the voltage measurements from the NEMS subjected to vibration excitation was performed using a voltage buffer amplifier with unity gain from Linear Technologies (LTC6240CS8). The input base acceleration supplied by the vibration shaker to the NEMS was measured accurately by an instrumentation-grade shear accelerometer (PCB352C22). The grounded faraday cage surrounded the experimental setup to reduce the extrinsic 60 Hz power-line noise from electromagnetic interference [39]. The white noise excitation and sine wave excitation used for the frequency response function (FRF) and the output voltage (V) response characterization from NEMS was performed using NI SignalExpress software that operated a high accuracy DAQ system (NI USB 4431). All signals were reexamined during data acquisition for accuracy using an oscilloscope (Tektronix, DPO 3014 Digital Phosphor Oscilloscope).
3. Result and discussion The fabrication procedure begins with the growth of vertically aligned sodium titanate NW arrays on oxidized Ti foil. It is well known that Ti foil can be thermally oxidized at high temperatures ranging from 550–1000 °C to form the rutile TiO2 [47]. In this experiment, the rutile phase of the TiO2 layer provided the Ti source for the formation of aligned sodium titanate NW arrays during the hydrothermal reaction [46]. The as-prepared sodium titanate NW arrays have the open structure of a layered alkali titanate which facilitates their strong ion-exchange property and therefore, excel as precursors for the hydrothermal synthesis of aligned barium titanate NW arrays with enhanced morphology retention [30, 31, 48]. The sodium content in the as-synthesized sodium titanate NW arrays precursor is highly dependent on the intensity of proton (H+ ion) exchange that takes place from the washing procedure used after synthesis [49]. Temperature is a key parameter in the hydrothermal reaction because of its influence on the nucleation and the crystal growth of these sodium titanate nanostructures [50]. As reported by Lan et al [50], higher temperatures (greater than 180 °C) resulted in the morphology of the nanostructures formed from the hydrothermal synthesis using concentrated NaOH (10 M NaOH) to be isolated straight nanorods whereas at lower temperature (125–150 °C) nanotubes were formed. Here, the focus is on synthesizing precursor sodium titanate NW arrays for conversion to BaTiO3 NW arrays and hence a higher temperature (greater than 200 °C) is used for all the hydrothermal reactions in this work. Additionally in this work, three different temperatures (200, 210, and 225 °C) are 3
A Koka et al
Nanotechnology 25 (2014) 375603
Figure 2. The cross-sectional SEM images of sodium titanate NW arrays on oxidized Ti substrates after hydrothermal reaction in 10 M NaOH at 225 °C for different reaction time: (a) 5 h, (b) 7 h, and (c) 9 h respectively (the scale bar is 10 μm). (d) Average length of sodium titanate NW arrays as a function of reaction time.
used and their effect on the adhesion between the as-formed vertically aligned sodium titanate NW arrays’ surface and the Ti growth substrate is discussed. In the first hydrothermal reaction from this work, ultralong sodium titanate NW arrays are obtained on oxidized Ti foil at a reaction temperature of 225 °C with 10 M NaOH concentration from different reaction time as shown in figures 2(a)–(c). Reaction duration of 3 h yielded long sodium titanate NW arrays larger than 30 μm on the Ti substrates. With increasing reaction time, the alkali metal titanate NW arrays became longer with better vertical alignment since the nucleation reaction takes place more rapidly and is sustained through the entire duration of the hydrothermal reaction at 225 °C [12]. The relationship between NW length and growth duration in 10 M NaOH at 225 °C is shown in figure 2(d). The length of the NWs increases from ∼33 μm to ∼44 μm and finally saturates around ∼45 μm beyond nine hours of the hydrothermal reaction. This saturation is due to the consumption of rutile TiO2 and the exhaustion of reactant Ti ions in the solution. The results from the SEM analysis demonstrated that a reaction temperature of 225 °C can yield ultra-
long sodium titanate NW arrays on the Ti substrates; however, this high temperature affects the adherence between the sodium titanate NWs layer and the Ti foil growth substrate. During thermal oxidation, micro cracks are produced at the interfaces between the Ti foil and the outer rutile TiO2 layers because of their mismatch in the thermal expansion coefficient. These micro cracks then expand under the high temperature and pressure during the hydrothermal reaction, causing the layer of sodium titanate NW arrays to lose contact with the Ti substrate [46]. In order to establish better adhesion between the vertically aligned sodium titanate NWs and the Ti growth substrates, the alkaline hydrothermal reaction was carried out at a lower temperature (200 °C) with different reaction durations and concentrations of NaOH solution. Here, the influence of the NaOH solution concentration on the diameter and the length of the sodium titanate NW arrays are studied by using the SEM images as shown in figures 3(a)–(d). The adhesion between the resulting sodium titanate NWs layer and the substrate improved from the lower temperature reaction at 200 °C with different concentrations of NaOH solution. The diameter of the NWs increased with 4
A Koka et al
Nanotechnology 25 (2014) 375603
Figure 3. SEM images of the top view of sodium titanate nanowires from different concentrations of NaOH solution after a hydrothermal reaction at 200 °C for 8 h: (a)–(d) shows the top view after 10 M, 11 M, 12 M, and 13 M reactions respectively (the scale bar is 10 μm). (e) Average diameter and length of the sodium titanate nanowires as a function of the NaOH concentration after a hydrothermal reaction at 210 °C for 8 h. (f) XRD patterns of the initial Ti foil (JCPDS No. 65-3362), the oxidized rutile TiO2 after heating Ti foil at 750 °C for 8 h (JCPDS No. 65-0191) and the as-synthesized sodium titanate nanowires; H denotes sodium hexatitanate (Na2Ti6O13) peaks (JCPDS No. 731398) and T denotes sodium trititanate (Na2Ti3O7) peaks (JCPDS No. 31-1329).
an increase in the concentration of the NaOH solution from 10 M to 13 M (figure 3(e)). However, the increase in the diameter of the NWs results in a tradeoff with its length because there is a saturation limit to the amount of Ti source available from the rutile TiO2 precursor for the growth of these NW arrays. The NW arrays prepared from 10 M NaOH yielded higher aspect ratio NWs with small diameter (
