Printed Nanocomposite Energy Harvesters with Controlled Alignment

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Printed Nanocomposite Energy Harvesters with Controlled Alignment of Barium Titanate Nanowires Mohammad H. Malakooti, Florian Julé, and Henry A. Sodano ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13643 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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ACS Applied Materials & Interfaces

Printed Nanocomposite Energy Harvesters with Controlled Alignment of Barium Titanate Nanowires Mohammad H. Malakooti1, Florian Julé1, and Henry A. Sodano1, 2,* 1Department 2Department

of Aerospace Engineering, University of Michigan, Ann Arbor, Michigan, 48109, USA

of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, 48109, USA *corresponding author, email: [email protected]

ABSTRACT Piezoelectric nanocomposites are commonly used in the development of self-powered miniaturized electronic devices and sensors. Although the incorporation of one-dimensional piezoelectric nanomaterials (i.e. nanowires, nanorods, and nanofibers) in a polymer matrix has led to the development of devices with promising energy harvesting and sensing performance, they have not yet reached their ultimate performance due to the challenges in fabrication. Here, a direct-write additive manufacturing technique is utilized to facilitate the fabrication of spatially tailored piezoelectric nanocomposites. High aspect ratio barium titanate (BaTiO3) nanowires (NWs) are dispersed in a polylactic acid (PLA) solution to produce a printable piezoelectric solution. The BaTiO3 NWs are arranged in PLA along three different axes of alignment via shear-induced alignment during a controlled printing process. The result of electromechanical characterizations shows that the nanowire alignment significantly affects the energy harvesting performance of the nanocomposites. The optimal power output can be enhanced by as much as 8 times for printed nanocomposites with a tailored architecture of the embedded nanostructures. This power generation capacity is 273% higher compared to conventional cast nanocomposites with randomly oriented NWs. The findings of this study suggest that 3D printing of nanowirebased nanocomposites is a feasible, scalable, and rapid methodology to produce high performance piezoelectric transducers with tailored micro and nanostructures. This study offers the first demonstration of nanocomposite energy harvesters with spatially controlled filler orientation realized directly from a digital design. Keywords: nanowire alignment, printed electronics, BaTiO3 nanowires, additive manufacturing, energy harvesting, piezoelectric nanocomposites

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INTRODUCTION The capacity for piezoelectric materials to convert mechanical energy into electrical energy and vice versa has made them promising functional materials for various applications such as sensing, energy harvesting, actuation and ultrasonic transduction.1-6 However, the mechanical and electrical limitations of monolithic piezoceramics and polymers have led to the development of nanocomposites with piezoelectric inclusions.

For instance, piezoceramics such as lead

zirconate titanate have high electromechanical coupling coefficient but are brittle making them prone to fracture.

This issue can be addressed by the integration of piezoceramic as a

discontinuous phase in a compliant polymer matrix to form a piezoelectric nanocomposite.7-12 While piezoelectric polymers such as polyvinylidene fluoride (PVDF) also provide mechanical flexibility and robustness to damage, they have low piezoelectric coupling coefficient and dielectric permittivity, which limits their application.13-15 Piezoelectric nanocomposites have been developed to serve as functional materials which can be tailored through modification of the filler and matrix phases to enable properties which cannot be obtained in a homogenous material. Over the past decade, different architectures of piezoelectric nanocomposites have been examined to enhance their sensing and energy harvesting performance.

Zero- and one-

dimensional piezoelectric fillers are commonly incorporated in polymer matrices to form (0-3) and (1-3) functional nanocomposites, respectively.

Zero-dimensional fillers are suspended

piezoelectric nanoscale spherical or cubical particles in a polymeric matrix while onedimensional (1D) ones are typically piezoelectric nanowires (NWs), nanorods, or nanofibers. The high aspect ratio of the 1D nanofillers enables them to efficiently convert mechanical strain to electrical charge even at minimal strains due to the larger bending compliance and flexibility of the 1D nanofillers. Moreover, compared to nanoparticles, the 1D piezoelectric nanofillers require lower percolation thresholds to deliver the generated charge from the active phase to the electrodes.

Therefore, (1-3) nanocomposites with piezoelectric NWs and nanofibers are

preferred over the (0-3) nanocomposites with nanoparticles. For instance, Jung et al. showed that piezoelectric nanogenerator devices composed of NaNbO3 NWs outperform similar devices containing NaNbO3 nanocubes.16 The output voltage and current from the nanowire-based devices were approximately two times higher than nanocube-based devices when subjected to the same compressive strain. More recently, Zhou et al. showed that flexible energy harvesting 2


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devices fabricated from dispersed 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) NWs in a polydimethylsiloxane (PDMS) matrix also surpass similar devices composed of BZT-BCT nanoparticles.17 For the nanogenerators with 40% weight fraction of BZT-BCT fillers, the peak power density of the (1-3) nanocomposites was calculated to be 2.25 W/cc, which is ~9 times higher than that of their counterpart (0-3) nanocomposites.17 In addition to the fillers’ morphological properties, the alignment of the 1D fillers in the polymer matrix also plays an important role in defining the nanocomposites’ performance. Through systematic studies, Tang et al. showed that the alignment of lead zirconate titanate NWs in nanocomposite capacitors significantly affect their dielectric permittivity and maximum energy storage.10,11

Later, well aligned piezoelectric nanofibers were utilized for sensitive

pressure sensors and high performance nanocomposite energy harvesters.18-20

Improved

performance of nanocomposites with higher degree of filler alignment is not only limited to piezoelectric devices. The efficacy of controlled filler alignment has been shown in a wide range of applications spanning from improved structural properties of composites to enhanced cell growth in nanofibrous scaffolds for tissue engineering.21-26 Despite the superior effects of filler orientation, it is known that the alignment of high aspect ratio nanofillers is challenging due to limitations in material processing. In previous studies, bulk nanocomposites with aligned micro/nanofillers were typically fabricated by uniaxial strain assembly10,27 or in situ dielectrophoretic alignment28-31 techniques.

However, each of these fabrication methods has its own major

drawbacks. In the uniaxial strain assembly technique, randomly oriented fillers in a cured nanocomposite are aligned under mechanical strain when the applied heat to the thermoplastic matrix provides sufficient filler mobility. Thus, this method is limited to nanocomposites with a thermoplastic matrix. Moreover, the non-uniform strain distribution and limited size of the stretched nanocomposites make this method suitable only for small devices with a planar geometry. Similar limitations exist in the assembly of nanofillers using in situ field assisted methods where an external electric field is applied to control the alignment of high aspect ratio fillers.28-34 In these cases, bulk specimens cannot be fabricated because extremely high electric potentials or strong electromagnets are required to create the powerful fields. Another major limitation of this method is that the nanofillers must respond to the applied external field prior to matrix polarization. Thus, this method cannot be considered as a scalable and versatile technique for the fabrication of nanocomposites with tailored filler orientation. It should be mentioned that 3



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other self-alignment techniques such as Langmuir–Blodgett troughs,35-37 microfluidics,38,39 and evaporation‐induced alignment40,41 are restricted to the alignment of fillers only on the surface of a substrate.

Hence, these methods are not generally suitable for fabrication of bulk

nanocomposites with highly anisotropic properties. Here we introduce a scalable, rapid, and versatile fabrication technique for the alignment of functional NWs in polymer matrices. This additive manufacturing technique allows the directed assembly of nanofillers in any desirable directions independent of the type of NWs and polymer matrix. In order to demonstrate the feasibility of this approach, barium titanate nanocomposite energy harvesters with tailored nanostructures are fabricated and characterized. As shown in Figure 1a, the 3D printable functional materials are prepared in the form of a solution of polylactic acid (PLA) with dispersed BaTiO3 NWs (typically termed ink42-47) in dichloromethane. The low glass transition and suitable rheological properties of PLA enables simple preparation of both inks and filaments compatible with direct-write (DW) assembly or fused deposition modeling (FDM), respectively. However, in this study on the role of NWs alignment we utilized the DW assembly rather than FDM to lower the preparation steps and number of processing parameters.

To realize nanocomposites with different axes of NWs

alignment, first a digital model of a rectangular energy harvesting device is used to generate three different print paths with 0, 45, and 90 infill angles (Figure 1b). As the print nozzle travels along the print paths, the volatile nature of dichloromethane allows the solution of BaTiO3–PLA nanocomposite to solidify immediately after deposition from nozzle (Figure 1c). Since the BaTiO3 NWs are already self-aligned in the nozzle due to presence of shear forces 43-49 (Figure 1d), the printed nanocomposite contains aligned NWs in the print direction.

As

illustrated in Figure 1e, three types of nanocomposites with aligned NWs in the 0, 45, and 90 directions which corresponds to the infill print angles were prepared for characterization. The relationship between NW orientation and energy harvesting performance in these printed nanocomposites as well as a cast nanocomposite with randomly oriented NWs is studied here. The results indicate that spatially controlled alignment of piezoelectric NWs has a significant effect on the voltage and power output from printed nanocomposites. This study is the first demonstration of printed piezoelectric nanocomposites with tailored nanowire orientation directly from digital design.

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Figure 1: (a) preparation of printable functional materials in the form of ink and filament through dispersion of BaTiO3 nanowires (NWs) in a polylactic acid (PLA) solution, (b) generated tool path for printing rectangular energy harvesting nanocomposites with 0, 45, and 90 infill print angles, (c) schematic of nanocomposite 3D printing (d) parabolic fluid velocity and wall shear stress profiles in the nozzle causing nanowire alignment, (e) schematic of printed nanocomposite with different nanostructure orientations.

RESULTS AND DISCUSSION Materials Characterization

Free-standing BaTiO3 NWs were synthesized using a two-step hydrothermal reaction which was developed by Tang et al.50,51 In the first reaction, hydrogen titanate (H2Ti3O7) NWs were produced as the precursors (Figure 2a). Then, the diffusion of barium ions in the second hydrothermal reaction produces BaTiO3 NWs with the same morphological structure, as shown in Figure 2b. Although polydispersity in the size distribution of the hydrothermally synthesized NWs is observed, the BaTiO3 NWs have a relatively high aspect ratio with average width and length of 7 m and 250 nm, respectively. The reaction parameters such as time, temperature, and concentration were kept constant through this study to eliminate any possible changes in the intrinsic electromechanical properties of the BaTiO3 NWs and their effect on the energy 5



harvesting performance of nanocomposite devices. Furthermore, prior to the fabrication of nanocomposite energy harvesters, XRD and EDS analyses were performed to ensure the synthesis of ferroelectric BaTiO3 NWs. Figure 2c and 2d show the representative XRD patterns and EDS spectra for the NWs after the first and second hydrothermal reaction confirming the synthesis of H2Ti3O7 and BaTiO3 NWs. After confirming the structural properties of the BaTiO3 NWs, they were dispersed in the PLA dichloromethane solution and used in the fabrication of nanocomposites through the direct-write assembly method.

Figure 2: Scanning electron micrographs of the synthesized (a) hydrogen titanate (H2Ti3O7) and (b) barium titanate (BaTiO3) NWs; (c) XRD patterns and (d) EDS spectra for the H2Ti3O7 and BaTiO3 NWs.

Structural Properties of Printed Nanocomposites

Tailoring the arrangement of nanofillers in active nanocomposites, such as energy harvesters, allows engineering their electromechanical response and consequently improving their energy harvesting performance. Compared to a cast BaTiO3-PLA nanocomposite specimen with 30% weight fraction of BaTiO3 (Figure 3a), significant alignment of BaTiO3 NWs is observable in the printed BaTiO3 nanocomposite specimens (Figure 3b and 3c). A robust mathematical metric 6


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utilized to quantify the level of NWs alignment based on a Fourier analysis of scanning electron microscopy (SEM) images of the nanocomposites to determine the relative orientation of fillers in both cast and printed nanocomposites. Based on this analysis, a constant termed the Herman’s orientation factor (HOF) can be assigned to different filler orientations. 11,52-55 HOF is a measure of the orientation of a chosen axis with respect to a reference axis and varies between –0.5 and 1. By defining the NWs’ axis as the axis of interest, a HOF of 1 represents perfect alignment of NWs in the parallel direction, while a HOF of –0.5 corresponds to perfect alignment of NWs in the perpendicular direction. If a preferential axis cannot be identified, the HOF will be zero which indicates fillers are randomly oriented in the matrix. As shown in Figure 3a, twodimensional fast Fourier transform (FFT) analysis of the micrograph of cast nanocomposite results in a circular shaped intensity map without a dominant axis. In contrast, FFT analysis of the printed nanocomposite micrographs results in oval shaped FFT intensity maps with a dominant axis aligned with NWs’ orientation. Considering the longitudinal axis of the printed devices as the axis of interest, the estimated HOF value for 0, 45, and 90 infill angles are 0.455, 0.236, and –0.254, respectively (Figure 3d). Despite the obvious alignment of NWs in SEM images, the calculated HOF values are lower than the theoretical HOF values for perfectly oriented high aspect ratio fillers.

The FFT analysis of SEM images captured from

nanocomposites and vertically aligned nanostructures usually underestimates the HOF values because of the low volume fraction of the fillers in nanocomposites, partial exposure of the fillers, and the absence of high grayscale contrast in SEM images.11,54,55 The HOF values of the printed nanocomposites clearly indicate that the shear forces generated during the printing process results in the alignment of NWs in the ink.

Since

dichloromethane is highly volatile, the dissolved PLA in dichloromethane allows 3D printing of freeform nanocomposites due to the rapid solvent evaporation.56-58 Once the nozzle travels off the substrate and dispenses the BaTiO3-PLA solution, the solvent evaporates and solidification begins immediately, thus the embedded NWs preserve their alignment in the PLA matrix. Hence, the orientation of microstructures was essentially designed through defining the print path during the digital processing of the CAD model. It should be mentioned that if the orientation of NWs is considered as the reference axis, all the different fill angles (0, 45, and 90) result in a similar HOF value of ~0.45. This quantification reveals that the shear-induced alignment of NWs is independent of the print direction. 7



Figure 3: Micrographs of the cast and printed BaTiO3 nanocomposites with different filler orientations along with the corresponding fast Fourier transform intensity maps; (a) randomly distributed NWs with an Herman’s orientation factor (HOF) of 0.018, (b) axially printed nanocomposite with an HOF of 0.455, (c) printed nanocomposite at 45 degree with an HOF of 0.236, and (d) calculated HOF for the cast and printed nanocomposites.

Energy Harvesting Performance of Nanocomposites

The energy harvesting performance of the nanocomposites is characterized under controlled strain conditions using a dynamic mechanical analysis (DMA) system. Precise control over the applied strain and automated length and force measurements through the DMA system allow a fair comparison between different nanocomposite energy harvesting devices. Unlike common characterization techniques used for nanogenerators that produces voltage and current spikes, all generated signals are sinusoidal which are favorable for AC power harvesting. In the following sections, the energy harvesting performance of the four different nanocomposites is compared under the same electromechanical testing conditions. The cast nanocomposite represents a device with randomly oriented NWs while 0, 45, and 90 notations represent printed nanocomposite with 0, 45, and 90 infill angles, respectively.

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As shown in Figure 4, the generated open circuit voltage (VOC) and short circuit current (ISC) produced by energy harvesters are highly dependent on the print direction of nanocomposites as well as the amount of applied strain.

The energy harvesting devices were subjected to a

harmonic axial tensile loading with constant maximum strains ranging from 0.05% to 0.35% with 0.05% strain intervals. As expected, increasing the amount of axial strain increases the generated VOC and ISC. The root-mean-square (RMS) values of the VOC generated from BaTiO3 nanocomposite energy harvesters varies between 8 mV and 486 mV (Figure 4a) while the peakto-peak (P-P) values of the VOC ranges from 30 mV to 1.4 V (Figure 4b). When the applied strain is relatively low, there is no significant difference in the generated VOC from the nanocomposite energy harvesters. However, considerable variations in the produced VOC from nanocomposites appear by increasing the strain level. As shown in Figure 4c, all of the devices are able to produce symmetric sinusoidal voltage signals when subjected to 0.35% strain with an excitation frequency of 5Hz. The 0 printed nanocomposite energy harvester in which BaTiO3 NWs were aligned in the direction of applied load produces the highest voltage output (VRMS = 486 mV and VPP = 1.4 V). The second highest level of VOC was obtained for the nanocomposites with 45 oriented BaTiO3 NWs. The cast and 90 printed nanocomposites showed a comparable, but relatively low performance where the cast nanocomposite produced slightly higher voltage at strains above 0.25%. Similar to the voltage response, the current output from the nanocomposite energy harvesters are also highly dependent on the level of applied strain and NWs’ orientation. Under 0.35% axial strain, the 0 and 90 printed nanocomposites produce the highest (IRMS = 58.5 nA and IPP = 164.5 nA) and lowest (IRMS = 20.8 nA and IPP = 58 nA) short circuit current outputs, respectively (Figure 4d and 4e). At applied strains lower than 0.1%, the generated current signals contain higher level of noise and errors due to the small current outputs. As shown in Figure 4d, the level of noise in short circuit current signals from the nanocomposites under 0.35% strain is negligible. Also, these symmetric sinewave signals with different amplitudes further confirm the piezoelectric characteristics of the BaTiO3 NWs and their alignment impact on the current output.

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Figure 4: Open-circuit voltage (VOC) and short circuit current (ISC) responses of the cast and printed BaTiO3 nanocomposites; (a) root-mean-square (RMS) values and (b) peak-to-peak (P-P) values of the VOC as a function of the applied strain, (c) voltage signals generated from BaTiO3 nanocomposites under 0.35% strain and 5 Hz excitation frequency, (d) RMS values and (e) P-P values of the ISC as a function of the applied strain, (f) current signals generated from BaTiO3 nanocomposites under 0.35% strain and 5 Hz excitation frequency.

Based on the measured VOC and ISC values, it can be concluded that the arrangement of functional NWs in a polymeric matrix plays a crucial role in the voltage and current generation of a piezoelectric nanocomposite.

We considered the 90 printed nanocomposites as the

reference energy harvesting device in order to visualize this important role of NWs alignment. As shown in Figure 5a and 5b, the normalized RMS VOC and ISC ae plotted as a function of Herman’s orientation factor at 0.35% axial strain. It is clear that the electromechanical response of the nanocomposites enhances by increasing the HOF value. This means when the alignment axis of the NWs is closer to loading the direction (longitudinal axis of devices), the piezoelectric response is larger (Figure 5a, inset). The plausible explanation is that the print infill angles directly influences the alignment of NWs and consequently the relative orientation of embedded NWs with respect to the loading direction. If the longitudinal axis of the BaTiO3 NWs are parallel to the loading direction (0 printed nanocomposite), maximum load is applied to individual BaTiO3 NWs embedded in matrix. In contrast, when the 90 printed nanocomposites are mechanically strained, the tensile load is being applied in the lateral direction of the NWs 10


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which results in minimal longitudinal stresses on the piezoelectric NWs. Therefore, the RMS voltage and current outputs from 0 printed nanocomposites are 2.6 and 2.8 times higher, respectively, when compared to the 90 printed nanocomposites. It should be noted that the performance of 45 and 0 printed nanocomposites are comparable with respect to the reference 90 printed nanocomposites. The relatively high electromechanical coupling in the 45 printed nanocomposites can be due to the presence of local shear stresses in this specific configuration. It is plausible that the 45 oriented NWs are subjected to both normal and shear stresses which enhances the overall electromechanical coupling of these nanocomposites.

Figure 5: Power harvesting performance of the BaTiO3 nanocomposites; normalized RMS (a) open circuit voltage and (b) short circuit current outputs from the nanocomposites under 0.35% strain as a function of Herman’s orientation factors (HOF), (c) RMS voltage and (d) AC power generated from the nanocomposites versus different load resistors, (e) generated peak power for the cast and printed nanocomposites with 0, 45, and 90 fill angles, (f) normalized peak power as a function of HOF.

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In order to determine the power scavenging capabilities of the nanocomposites, the voltage output across various resistive loads was measured. The nanocomposite energy harvesters can be considered as an RC circuit to estimate the maximum power and determine the optimal load resistance.3 Each nanocomposite was subjected to an axial strain with a constant maximum strain of 0.35% at a frequency of 100 Hz. Higher exciting frequency was used for power measurements to lower the range of the optimal load resistance for all of the energy harvesters as the optimal resistance (Ropt) is inversely proportional to the capacitance (C) of the energy harvester and the frequency () of applied strain, Ropt=1/(/C).3,59 Figure 5a shows the changes in the RMS voltage output as a function of the external resistor (R) ranging from 100 k to 10 M. In a similar trend for each type of nanocomposite, the RMS voltage increases with increasing load resistance until it converges to its highest value at larger values of the external resistors (above 4 M), which replicates open circuit conditions. The measured RMS voltage (VRMS) across each resistive load in series is used to estimate the AC power (PRMS=VRMS/R) in piezoelectric power harvesters.3,17 The estimated AC power as a function of external load resistor is shown in Figure 5d. It is clear that the generated power is governed by the resistive load and more importantly the nanocomposite type. The optimal load resistance varies for each device due to differences in their capacitance. It is noticeable that the nanocomposites with aligned BaTiO3 NWs in the loading direction (0 printed nanocomposites) generate the highest AC power over the entire range of load resistors with a maximum power of 15.3 nW under only 0.35% strain (Figure 5e).

The second highest power is observed for the 45 printed

nanocomposites with 10.5 nW power generation which is only 31% lower than that of the 0 printed nanocomposites. The high power harvesting performance of these two specimens is followed by the cast and 90 printed nanocomposites with 4.1 and 1.9 nW, respectively. Figure 5f visualizes the normalized optimal power output as a function of HOF by considering the 90 printed nanocomposites as the reference power harvesting device. Under 0.35% tensile strain, the maximum AC power generated from 0 printed nanocomposites is 8.05 times of that from the 90 printed nanocomposites which means the power output is increased more than 700%. Under the same testing conditions, the power increases are 452% and 116% for the 45 printed and cast nanocomposites, respectively.

It should be mentioned that the maximum power

produced from the 0 and 45 printed nanocomposites are 3.73 and 2.56 times of that for the cast

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nanocomposites. This result indicates that the power scavenging capabilities of nanocomposites are highly dependent on the structural properties of the piezoelectric phase and utilizing advanced fabrication techniques can further improve the performance of piezoelectric nanocomposites. Furthermore, having access to a scalable fabrication method that allows precise control over the microstructural morphologies of piezoelectric transducers will facilitate the transition of nanostructured devices from the development stage to the practical applications through additive manufacturing. CONCLUSIONS In this study we established the feasibility of tailoring the nanostructure-property relationships in energy harvesting nanocomposites using an additive manufacturing technique. In particular, 3D printed PLA nanocomposites with aligned and randomly oriented BaTiO3 NWs were fabricated. Nanocomposites with randomly oriented BaTiO3 NWs were prepared using a conventional cast fabrication method while direct-write assembly was employed in order to prepare nanocomposite with aligned NWs in particular directions. Three different orientation axes (0, 45, and 90) for piezoelectric NWs were assigned in the digital design of the devices prior to the printing process. Through a systematic image analysis, it was shown that the embedded NWs in PLA were perfectly aligned in the print direction while the cast samples did not show any degree of NWs alignment. Comprehensive energy harvesting characterizations were performed by subjecting the nanocomposite devices to precisely controlled axial strains. It was revealed that the electrical response (voltage, current, and power outputs) of the nanocomposites under this mechanical loading condition are highly dependent on the arrangement of BaTiO3 NWs with respect to the loading direction. The controlled alignment of BaTiO3 NWs in the axis of tensile loading increased the generated maximum AC power more than 700%. These results demonstrate the great advantage of utilizing additive manufacturing techniques for fabrication of nanowire-based functional nanocomposites. Reduced processing time and materials preparation effort, provided additional design parameters for enhanced performance, and enabled spatially controlled filler orientation in bulk nanocomposites are the other important features of the developed fabrication technique. Moreover, this method can be applied to a wide variety of one-dimensional functional nanomaterials and polymer matrices for rapid manufacturing of high performance transducers with locally tuned electromechanical properties. 13



EXPERIMENTAL METHODS Synthesis of BaTiO3 Nanowires. In the first hydrothermal reaction, relatively high aspect ratio sodium titanate NWs were synthesized in a closed reactor with a Teflon liner. 1.5 g of anatase titanium dioxide powder was mixed with 60 ml of a 10.3 M aqueous sodium hydroxide solution. Then, the prepared solution was sealed in a steel autoclave and heated in an oven at 240 °C for 12 hours. Next, the product of this reaction which is sodium titanate NWs are soaked in a dilute hydrochloric acid (Fisher, 37%) solution with a 0.2 M concentration for 4 hours to form the final products of the first step, hydrogen titanate (H2Ti3O7) NWs. Then, the H2Ti3O7 NWs are washed multiple times with water using centrifugation. After drying the H2Ti3O7 NWs, they were used in the second step of the synthesis. In the second hydrothermal reaction, the H2Ti3O7 NWs are converted to the BaTiO3 NWs. First, the H2Ti3O7 NWs were dispersed in an aqueous solution of 0.07 M barium hydroxide octahydrate solution (Sigma-Aldrich, 98%). After bath sonication of the solution, it was transferred to another Teflon liner and sealed in stainless steel reactor and kept at 210 °C for 90 min. After completing this hydrothermal process, the precipitates were collected and washed with a dilute HCl aqueous solution. Finally, the BaTiO3 NWs were obtained after further washing and drying of the products of the second hydrothermal reaction. It should be mentioned that the reaction parameters such as time and temperature affect the morphology (length and diameter) of the synthesized NWs. However, the morphology of the NWs does not typically change during the conversion process. Ink Preparation. PLA Pellets (purchased from Filabot) were dissolved in dichloromethane. Then, a weight fraction of 30% of the synthesized BaTiO3 NWs was dispersed in the dissolved PLA using a probe sonicator (Fisher Scientific Model 500) for 3 min and a shear mixer (SpeedMixer DAC 150.1 FVZ-K, FlackTek Inc.) for 10 min. 10g batches of the BaTiO3PLAink were prepared each time prior to the printing process. Printing Process. The prepared ink is transferred to a syringe and then mounted on a 3-axis printing stage (AGS15000, Aerotech, Inc.). A high precision solution dispenser (Ultimus™ V, Nordson EFD) was used to control the applied air pressure applied to the ink. Reusable stainless steel, straight needles with luer-lock connection were attached to the syringe and used for ink deposition. 26 gauge needles (400 micron internal diameters) with a length of 10 mm was used for printing the piezoelectric nanocomposites. The print paths were generated using open source 3D slicing software, called Slic3r. The generated G-code was modified manually to adjust the 14


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applied pressure and the print speed. Next, the G-code was used as the input for the XYZ motion controller (A3200 Software-Based Machine Controller, Aerotech, Inc.). All the controllers for solution dispensing and linear displacements were synched and executed through a custom made program in LabVIEW. Device Fabrication.

After completing the print process, uniform layers of gold were

sputtered on each side of the printed nanocomposite to serve as electrodes. Next, the specimens were subjected to an electric field of 10 MV/m at an elevated temperature (90C) for 12 hours in order to electrically pole the BaTiO3 NWs. The dimensions of the printed nanocomposites were 10 mm× 45 mm ×100 μm. It should be noted that the gold sputtered and electrically poled area of the nanocomposites placed between the two clamps of DMA was limited to 8 mm × 15 mm. Nanomaterials Characterization. The crystal structures of the synthesized H2Ti3O7 and BaTiO3 NWs were studied using XRD (PANalytical X’Pert Powder). Cu Kα radiation at 1.540 Å over 2θ range of 20° to 60° was utilized to obtain XRD patterns. The elemental composition of the NWs was examined through energy-dispersive X-ray spectroscopy (EDS, JEOL-7800 FLV FE SEM). The morphologies of the NWs and their arrangement in the printed and cast nanocomposite were characterized using a field-emission scanning electron microscope (SEM, JEOL-7800 FLV FE SEM). A low accelerating voltage (1 kV) was used for imaging the nanocomposites to lower the surface charges on SEM samples which resulted in melting the top surface of PLA matrix and formation of micro-/nano cracks. Herman’s Orientation Factor (HOF) Calculation. The Herman’s orientation factor of the nanocomposites was calculated based on the 2-dimensional fast Fourier transform (2D FFT) of the grayscale SEM images. 2D FFT maps the original grayscale levels of the nanocomposite micrograph from the spatial domain into the frequency domain. The obtained pattern of the 2D FFT intensity implies the level of filler (high aspect ratio features) alignment in the original SEM image. HOF is defined based on the calculated 2D FFT intensity curves and is a common formulation (Equation 1) in polymer science to precisely evaluate orientation of semi-crystalline polymers:11,55 1 HOF  (3  cos 2  1) 2

15


(1)


Page 16 of 21

where the angular brackets indicate a spatial average to account for the spatial distribution of the orientation degree and can be expressed as:

  cos   2

π /2 0

I ( )cos 2 sin( )d



π /2 0

I ( )sin( )d

(2)

where the azimuth angle  is the angle between the arbitrary reference axis and dominant filler orientation and I() is the intensity profile of anisotropy as a function of  from zero to /2. The HOF value is determined based on average 2D FFT intensity profiles as a function of azimuth angle and integrating numerically to calculate the spatial average in Equation 2.11,55 Electromechanical Testing. To accurately control the applied strain and force to energy harvesting devices, a dynamic mechanical analysis system (TA instruments DMA Q800) was used. Nanocomposite energy harvesters with different nanowire orientations (0, 45, and 90) were subjected to sinusoidal uniaxial strains (with 5 mN preload) at different frequencies while the generated current and voltage signals were measured using an electrometer (Keysight B2985A). The data acquisition was performed using a National Instrument DAQ system (NI USB-4431) and a resistance decade box (Extech 380400) was utilized for altering the resistive loads for the power measurements. ACKNOWLEDGEMENTS The authors gratefully acknowledge financial support for this research from the Air Force Office of Scientific Research under Contract No. FA9550-16-1-0087. REFERENCES 1. Qi, Y.; McAlpine, M. C. Nanotechnology-Enabled Flexible and Biocompatible Energy Harvesting. Energy Environ. Sci. 2010, 3, 1275-1285. 2. Bowen, C.; Kim, H.; Weaver, P.; Dunn, S. Piezoelectric and Ferroelectric Materials and Structures for Energy Harvesting Applications. Energy Environ. Sci. 2014, 7, 25-44. 3. Malakooti, M. H.; Patterson, B. A.; Hwang, H.; Sodano, H. A. ZnO Nanowire Interfaces for High Strength Multifunctional Composites with Embedded Energy Harvesting. Energy Environ. Sci. 2016, 9, 634-643. 4. Dagdeviren, C.; Joe, P.; Tuzman, O. L.; Park, K.; Lee, K. J.; Shi, Y.; Huang, Y.; Rogers, J. A. Recent Progress in Flexible and Stretchable Piezoelectric Devices for Mechanical Energy Harvesting, Sensing and Actuation. Extreme Mech. Lett. 2016, 9, 269-281.

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Page 17 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5. Hwang, G.; Park, H.; Lee, J.; Oh, S.; Park, K.; Byun, M.; Park, H.; Ahn, G.; Jeong, C. K.; No, K. Self‐powered Cardiac Pacemaker Enabled by Flexible Single Crystalline PMN‐PT Piezoelectric Energy Harvester. Adv. Mater. 2014, 26, 4880-4887. 6. Hwang, G.; Byun, M.; Jeong, C. K.; Lee, K. J. Flexible Piezoelectric Thin‐Film Energy Harvesters and Nanosensors for Biomedical Applications. Adv. Healthcare Mater. 2015, 4, 646-658. 7. Qi, Y.; Jafferis, N. T.; Lyons Jr, K.; Lee, C. M.; Ahmad, H.; McAlpine, M. C. Piezoelectric Ribbons Printed Onto Rubber for Flexible Energy Conversion. Nano Lett. 2010, 10, 524528. 8. Zhou, Z.; Tang, H.; Sodano, H. A. Scalable Synthesis of Morphotropic Phase Boundary Lead Zirconium Titanate Nanowires for Energy Harvesting. Adv. Mater. 2014, 26, 7547-7554. 9. Tang, H.; Sodano, H. A. Ultra High Energy Density Nanocomposite Capacitors with Fast Discharge using Ba0.2Sr0.8TiO3 Nanowires. Nano Lett. 2013, 13, 1373-1379. 10. Tang, H.; Lin, Y.; Sodano, H. A. Enhanced Energy Storage in Nanocomposite Capacitors through Aligned PZT Nanowires by Uniaxial Strain Assembly. Adv. Energy Mater. 2012, 2, 469-476. 11. Tang, H.; Malakooti, M. H.; Sodano, H. A. Relationship between Orientation Factor of Lead Zirconate Titanate Nanowires and Dielectric Permittivity of Nanocomposites. Appl. Phys. Lett. 2013, 103, 222901. 12. Xu, S.; Yeh, Y.; Poirier, G.; McAlpine, M. C.; Register, R. A.; Yao, N. Flexible Piezoelectric PMN–PT Nanowire-Based Nanocomposite and Device. Nano Lett. 2013, 13, 2393-2398. 13. Arbatti, M.; Shan, X.; Cheng, Z. Ceramic–polymer Composites with High Dielectric Constant. Adv. Mater. 2007, 19, 1369-1372. 14. Chang, C.; Tran, V. H.; Wang, J.; Fuh, Y.; Lin, L. Direct-Write Piezoelectric Polymeric Nanogenerator with High Energy Conversion Efficiency. Nano Lett. 2010, 10, 726-731. 15. Lu, X.; Qu, H.; Skorobogatiy, M. Piezoelectric Micro-and Nanostructured Fibers Fabricated from Thermoplastic Nanocomposites using a Fiber Drawing Technique: Comparative Study and Potential Applications. ACS Nano 2017, 11, 2103-2114. 16. Jung, J. H.; Lee, M.; Hong, J.; Ding, Y.; Chen, C.; Chou, L.; Wang, Z. L. Lead-Free NaNbO3 Nanowires for a High Output Piezoelectric Nanogenerator. ACS Nano 2011, 5, 10041-10046. 17. Zhou, Z.; Bowland, C. C.; Malakooti, M. H.; Tang, H.; Sodano, H. A. Lead-Free 0.5Ba (Zr0.2Ti0.8) O3–0.5(Ba0.7Ca0.3) TiO3 Nanowires for Energy Harvesting. Nanoscale 2016, 8, 5098-5105. 18. Persano, L.; Dagdeviren, C.; Su, Y.; Zhang, Y.; Girardo, S.; Pisignano, D.; Huang, Y.; Rogers, J. A. High Performance Piezoelectric Devices Based on Aligned Arrays of Nanofibers of Poly (Vinylidenefluoride-Co-Trifluoroethylene). Nat. Commun. 2013, 4, 1633. 19. Yan, J.; Jeong, Y. G. High Performance Flexible Piezoelectric Nanogenerators Based on BaTiO3 Nanofibers in Different Alignment Modes. ACS Appl. Mater. Interfaces 2016, 8, 15700-15709.

17



20. Jeong, C. K.; Baek, C.; Kingon, A. I.; Park, K.; Kim, S. Lead‐Free Perovskite Nanowire‐Employed Piezopolymer for Highly Efficient Flexible Nanocomposite Energy Harvester. Small 2018, 14, 1704022. 21. Vigolo, B.; Penicaud, A.; Coulon, C.; Sauder, C.; Pailler, R.; Journet, C.; Bernier, P.; Poulin, P. Macroscopic Fibers and Ribbons of Oriented Carbon Nanotubes. Science 2000, 290, 1331-1334. 22. Haggenmueller, R.; Gommans, H.; Rinzler, A.; Fischer, J. E.; Winey, K. Aligned SingleWall Carbon Nanotubes in Composites by Melt Processing Methods. Chem. Phys. Lett. 2000, 330, 219-225. 23. Cheng, Q.; Bao, J.; Park, J.; Liang, Z.; Zhang, C.; Wang, B. High Mechanical Performance Composite Conductor: Multi‐walled Carbon Nanotube sheet/bismaleimide Nanocomposites. Adv. Funct. Mater. 2009, 19, 3219-3225. 24. Xu, C.; Inai, R.; Kotaki, M.; Ramakrishna, S. Aligned Biodegradable Nanofibrous Structure: A Potential Scaffold for Blood Vessel Engineering. Biomaterials 2004, 25, 877-886. 25. Murugan, R.; Ramakrishna, S. Design Strategies of Tissue Engineering Scaffolds with Controlled Fiber Orientation. Tissue Eng. 2007, 13, 1845-1866. 26. Jose, M. V.; Thomas, V.; Johnson, K. T.; Dean, D. R.; Nyairo, E. Aligned PLGA/HA Nanofibrous Nanocomposite Scaffolds for Bone Tissue Engineering. Acta Biomater. 2009, 5, 305-315. 27. Ji, J.; Sui, G.; Yu, Y.; Liu, Y.; Lin, Y.; Du, Z.; Ryu, S.; Yang, X. Significant Improvement of Mechanical Properties Observed in Highly Aligned Carbon-Nanotube-Reinforced Nanofibers. J. Phys. Chem. C 2009, 113, 4779-4785. 28. Smith, P. A.; Nordquist, C. D.; Jackson, T. N.; Mayer, T. S.; Martin, B. R.; Mbindyo, J.; Mallouk, T. E. Electric-Field Assisted Assembly and Alignment of Metallic Nanowires. Appl. Phys. Lett. 2000, 77, 1399-1401. 29. Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Indium Phosphide Nanowires as Building Blocks for Nanoscale Electronic and Optoelectronic Devices. Nature 2001, 409, 66-69. 30. Mas-Torrent, M.; Den Boer, D.; Durkut, M.; Hadley, P.; Schenning, A. P. Field Effect Transistors Based on Poly (3-Hexylthiophene) at Different Length Scales. Nanotechnology 2004, 15, S265-S269. 31. Van den Ende, D.; Van Kempen, S.; Wu, X.; Groen, W.; Randall, C.; Van der Zwaag, S. Dielectrophoretically Structured Piezoelectric Composites with High Aspect Ratio Piezoelectric Particles Inclusions. J. Appl. Phys. 2012, 111, 124107. 32. Hangarter, C. M.; Myung, N. V. Magnetic Alignment of Nanowires. Chem. Mater. 2005, 17, 1320-1324. 33. Kimura, T.; Ago, H.; Tobita, M.; Ohshima, S.; Kyotani, M.; Yumura, M. Polymer Composites of Carbon Nanotubes Aligned by a Magnetic Field. Adv. Mater. 2002, 14, 13801383. 34. Lu, T.; Wissman, J.; Majidi, C. Soft Anisotropic Conductors as Electric Vias for Ga-Based Liquid Metal Circuits. ACS Appl. Mater. Interfaces 2015, 7, 26923-26929. 18


Page 18 of 21

Page 19 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


35. Kim, F.; Kwan, S.; Akana, J.; Yang, P. Langmuir− Blodgett Nanorod Assembly. J. Am. Chem. Soc. 2001, 123, 4360-4361. 36. Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.; Xia, Y.; Yang, P. Langmuir− Blodgett Silver Nanowire Monolayers for Molecular Sensing using Surface-Enhanced Raman Spectroscopy. Nano Lett. 2003, 3, 1229-1233. 37. Acharya, S.; Panda, A. B.; Belman, N.; Efrima, S.; Golan, Y. A Semiconductor‐Nanowire Assembly of Ultrahigh Junction Density by the Langmuir–Blodgett Technique. Adv. Mater. 2006, 18, 210-213. 38. Huang, Y.; Duan, X.; Wei, Q.; Lieber, C. M. Directed Assembly of One-Dimensional Nanostructures into Functional Networks. Science 2001, 291, 630-633. 39. Chung, S. E.; Park, W.; Shin, S.; Lee, S. A.; Kwon, S. Guided and Fluidic Self-Assembly of Microstructures using Railed Microfluidic Channels. Nat. Mater. 2008, 7, 581. 40. Zhang, C.; Zhang, X.; Zhang, X.; Fan, X.; Jie, J.; Chang, J. C.; Lee, C.; Zhang, W.; Lee, S. Facile One‐Step Growth and Patterning of Aligned Squaraine Nanowires Via Evaporation‐Induced Self‐Assembly. Adv Mater 2008, 20, 1716-1720. 41. Lee, S. G.; Kim, H.; Choi, H. H.; Bong, H.; Park, Y. D.; Lee, W. H.; Cho, K. Evaporation‐Induced Self‐Alignment and Transfer of Semiconductor Nanowires by Wrinkled Elastomeric Templates. Adv. Mater. 2013, 25, 2162-2166. 42. Zhu, Z.; Guo, S.; Hirdler, T.; Eide, C.; Fan, X.; Tolar, J.; McAlpine, M. C. 3D Printed Functional and Biological Materials on Moving Freeform Surfaces. Adv. Mater. 2018, 1707495. 43. Smay, J. E.; Cesarano, J.; Lewis, J. A. Colloidal Inks for Directed Assembly of 3-D Periodic Structures. Langmuir 2002, 18, 5429-5437. 44. Lewis, J. A. Direct Ink Writing of 3D Functional Materials. Adv. Funct. Mater. 2006, 16, 2193-2204. 45. Compton, B. G.; Lewis, J. A. 3D‐printing of Lightweight Cellular Composites. Adv. Mater. 2014, 26, 5930-5935. 46. Gladman, A. S.; Matsumoto, E. A.; Nuzzo, R. G.; Mahadevan, L.; Lewis, J. A. Biomimetic 4D Printing. Nat. Mater. 2016, 15, 413-418. 47. Siqueira, G.; Kokkinis, D.; Libanori, R.; Hausmann, M. K.; Gladman, A. S.; Neels, A.; Tingaut, P.; Zimmermann, T.; Lewis, J. A.; Studart, A. R. Cellulose Nanocrystal Inks for 3D Printing of Textured Cellular Architectures. Adv. Funct. Mater. 2017, 27, 1604619. 48. Cai, L.; Zhang, S.; Zhang, Y.; Li, J.; Miao, J.; Wang, Q.; Yu, Z.; Wang, C. Direct Printing for Additive Patterning of Silver Nanowires for Stretchable Sensor and Display Applications. Adv. Mater. Technol. 2018, 3, 1700232. 49. Hausmann, M. K.; Rühs, P. A.; Siqueira, G.; Läuger, J.; Libanori, R.; Zimmermann, T.; Studart, A. R. Dynamics of Cellulose Nanocrystal Alignment during 3D Printing. ACS Nano 2018, 12, 6926-6937. 50. Tang, H.; Lin, Y.; Sodano, H. A. Synthesis of High Aspect Ratio BaTiO3 Nanowires for High Energy Density Nanocomposite Capacitors. Adv. Energy Mater. 2013, 3, 451-456.

19



51. Tang, H.; Zhou, Z.; Sodano, H. A. Relationship between BaTiO3 Nanowire Aspect Ratio and the Dielectric Permittivity of Nanocomposites. ACS Appl. Mater. Interfaces 2014, 6, 54505455. 52. Ayres, C.; Bowlin, G. L.; Henderson, S. C.; Taylor, L.; Shultz, J.; Alexander, J.; Telemeco, T. A.; Simpson, D. G. Modulation of Anisotropy in Electrospun Tissue-Engineering Scaffolds: Analysis of Fiber Alignment by the Fast Fourier Transform. Biomaterials 2006, 27, 5524-5534. 53. Ayres, C. E.; Jha, B. S.; Meredith, H.; Bowman, J. R.; Bowlin, G. L.; Henderson, S. C.; Simpson, D. G. Measuring Fiber Alignment in Electrospun Scaffolds: A User's Guide to the 2D Fast Fourier Transform Approach. J. Biomater. Sci. , Polym. Ed. 2008, 19, 603-621. 54. Hayamizu, Y.; Yamada, T.; Mizuno, K.; Davis, R. C.; Futaba, D. N.; Yumura, M.; Hata, K. Integrated Three-Dimensional Microelectromechanical Devices from Processable Carbon Nanotube Wafers. Nat. Nanotechnol. 2008, 3, 289-294. 55. Bowland, C. C.; Malakooti, M. H.; Zhou, Z.; Sodano, H. A. Highly Aligned Arrays of High Aspect Ratio Barium Titanate Nanowires Via Hydrothermal Synthesis. Appl. Phys. Lett. 2015, 106, 222903. 56. Guo, S.; Gosselin, F.; Guerin, N.; Lanouette, A.; Heuzey, M.; Therriault, D. Solvent‐cast three‐dimensional Printing of Multifunctional Microsystems. Small 2013, 9, 4118-4122. 57. Guo, S.; Heuzey, M.; Therriault, D. Properties of Polylactide Inks for Solvent-Cast Printing of Three-Dimensional Freeform Microstructures. Langmuir 2014, 30, 1142-1150. 58. Guo, S.; Yang, X.; Heuzey, M.; Therriault, D. 3D Printing of a Multifunctional Nanocomposite Helical Liquid Sensor. Nanoscale 2015, 7, 6451-6456. 59. Malakooti, M. H.; Zhou, Z.; Sodano, H. A. Enhanced Energy Harvesting through Nanowire Based Functionally Graded Interfaces. Nano Energy 2018, 52, 171-182.

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TOC GRAPHIC

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Printed Nanocomposite Energy Harvesters with Controlled Alignment

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