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Effects of the aspect ratio of ZnO nanorods on the performance of piezoelectric nanogenerators
Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd
Original Article
Effects of the aspect ratio of ZnO nanorods on the performance of piezoelectric nanogenerators Ruaa S. Kammel*, Raad S. Sabry Mustansiriyah University, Department of Physics, Baghdad, Iraq
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 March 2019 Received in revised form 29 July 2019 Accepted 7 August 2019 Available online xxx
This paper presents an investigation on the performance of ZnO nanorod (ZNR)-based piezoelectric nanogenerators (PENGs). ZNRs on the ZnO seed layer coated double-sided flexible polyethylene terephthalate (PET) at different molar concentrations (0.01, 0.05 and 0.1 M) were synthesized by controlling the aspect ratio (length/diameter) of ZNRs, that are closely related to the piezoelectric output potential voltage using a simple hydrothermal method. ZNR PENGs were fabricated with an opposite electrode of gold-coated PET (Au/PET), which was placed on both the top and bottom of the ZNR-coated double-sided PET. X-ray diffraction and field emission scanning electron microscopy images revealed that as the molar concentration increased, the orientation of the as-grown ZNRs became non-uniformly distributed along the c-axis and also along with the decreased aspect ratio. At a low molar concentration (0.01 M), the ZNR PENGs exhibited a relatively high output potential voltage (~4.48 V) under an external pressing mass (500 g). The ZNR samples grown at 0.05 and 0.1 M exhibited a lower piezoelectric voltage 2.48 and 1.84 V, respectively. These results confirmed that ZNR PENGs with a small diameter, long length (i.e. high aspect ratio) and good alignment tend to be bent more easily for the efficient generation of the piezoelectric potential. © 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Keywords: ZnO nanorods Spin coating method Hydrothermal method Energy harvesting Piezoelectric nanogenerators
1. Introduction In recent years, scientists have strived to convert wasted environmental energy into electricity to solve the global energy demands by exploiting cost-effective, simple and environmentally friendly power sources (and energy conversion technologies) [1]. Therefore, nanogenerator technology has been developed for harvesting energy from the environment, and this strategy is based on three main effects: piezoelectric and triboelectric effects for harvesting mechanical energy, and pyroelectric effect for harvesting thermal energy [2,3]. Piezoelectric nanogenerators (PENGs) are widely used because the vibration has attracted considerable attention as a renewable power source, given its excellent environmental adaptability and high robustness, numerous vibrations, such as human motions, including walking, tiny clicks of the fingers, and rotating tires, are available from the surrounding environment but are wasted in our daily life [4,5].
* Corresponding author. E-mail addresses: [email protected] (R.S. Kammel), drraad_sci@ uomustansiriyah.edu (R.S. Sabry). Peer review under responsibility of Vietnam National University, Hanoi.
PENG is a nanoscale energy harvesting device that converts kinetic energy from mechanical vibrations in the ambient environment into a usable form of electrical energy by exploiting the excellent mechanical and electrical properties of nanostructured piezoelectric materials [4,6]. The idea of PENG was first presented in 2006 as an atomic force microscope (AFM) tip sweeps across a vertically grown ZnO nanowire, an electrical voltage/current was generated [7]. Among the various piezoelectric materials, the ZnO-based PENG is important in mechanical energy harvesting because it is a biocompatible, non-toxic and direct piezoelectric material and can be easily synthesized in the required shape and size of various substrates [8e10]. The fundamental mechanism of PENGs depends on the piezoelectricity of ZnO as well as the Schottky barrier that is formed between the metal - ZnO interface [11,12]. One-dimensional ZnO nanostructures in the form of nanowires (NWs) and nanorods (NRs) are widely used to fabricate PENG due to their high mechanical flexibility and sensitivity to small mechanical stress, which allows the conversion of small mechanical energy to electricity [13,14]. Among numerous methods used to synthesize ZNRs, the hydrothermal method has more advantages, due to its low cost, low
https://doi.org/10.1016/j.jsamd.2019.08.002 2468-2179/© 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Please cite this article as: R.S. Kammel, R.S. Sabry, Effects of the aspect ratio of ZnO nanorods on the performance of piezoelectric nanogenerators, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2019.08.002
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R.S. Kammel, R.S. Sabry / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx
temperature, compatibility with flexible substrates, and environmental friendliness. Several parameters influencing the growth of ZNRs include the growth duration, growth temperature, PH of the solution, the growth concentration, and seeding of the substrate that increases the density and alignment of the NRs [15e19]. Many studies have researched the yield voltage generated from the ZnO-based PENG by designing new devices, for example, using different flexible substrates (plastic, paper, cotton fabrics), and the growth of ZnO NWs/NRs on single or double side of the conductive substrate. However, to the best of our knowledge, not many studies have assessed the effect of the aspect ratio of ZNRs on the PENGs' efficiency. Thus, in this study, we investigated the efficiency of ZNRbased PENGs by controlling the aspect ratio (length/diameter) of ZNRs grown through the hydrothermal method at various molar concentrations of the growth solution. 2. Experimental 2.1. Preparation of ZnO seed layer A flexible polyethylene terephthalate (PET) substrate was cleaned with ethanol and distilled water in an ultrasonic bath and then heated in an oven to remove the moisture. The ZnO seed layer was coated on the double-sided PET (seeded substrate) through the solegel spin-coating method to fabricate the ZnO PENG. The seed solution was prepared by dissolving zinc acetate dehydrate [ðCH3 COOÞ2 $2H2 O; Scharlau] in ethanol to obtain the concentration of 0.02 M. The solution was stirred to obtain a homogeneous and transparent sol. Afterward, by using a syringe, the seed solution was dropped onto the substrates and rotated at 1000 rpm for 30 s to attain a uniform distributed seed layer across the substrates. This process was repeated thrice. Then, the substrates were heated to 100 C in an oven for 10 min to remove the solvent and achieve good adhesion of the seed layer. This procedure (from spin coating to pre-heat treatment) was repeated thrice to ensure the complete coverage of the plastic substrates with the ZnO seed layer. Finally, the ZnO seed layer was heated to 100 C for 25 min to improve the crystalline quality. 2.2. Growth of ZNRs Various molar concentrations of the growth solution (0.01, 0.05 and 0.1 M) were used to grow ZNRs with different aspect ratios. The growth solution was prepared by dissolving zinc nitrate
hexahydrate [ZnðNO3 Þ2 :6H2 O; Scharlau] and hexamethylenetetramine [ðCH2 Þ6 N4 ; HiMedia] in distilled water with a molar ratio of 1:1. The solution was fully and evenly stirred and then transferred into a 50 ml Teflon-lined autoclave. Seeded substrate was placed vertically in the solution. The autoclave was sealed in an oven and kept at 95 C for 3 h. Finally, the substrate was removed from the solution, washed with distilled water to remove the residues on the surface and left to dry in air. 2.3. Fabrication of devices To fabricate the ZnO PENG, an insulating layer made of polydimethylsiloxane (PDMS) was deposited onto the as-grown ZNRs using the spin-coating method. The PDMS prepolymer (Sylgard184, Dow Corning, Midland, MI) was prepared by thoroughly mixing the PDMS curing agent with the PDMS base monomer at a weight ratio of 10:1. The PDMS prepolymer was then spin-coated onto the as-grown ZnO and fully cured at 70 C. The double-sided ZNRs with PDMS were sandwiched between the Au electrodes (gold-coated PET substrate by DC sputtering). The electrical contact was placed on both the top and bottom parts of the Au electrodes by using Cu wires with silver paste. The device was wrapped with Kapton tape to avoid peeling off problems. The schematic diagram of the fabrication process of the PENGs device is shown in Fig. 1. 2.4. Characterization The structural quality and orientation of the grown ZNRs were analyzed by X-ray diffraction (XRD) at 40 KV and 30 mA with Cu-Ka radiation in the range of 30 e70 at a step of 0.02 . The surface morphology of the structure of all samples was characterized by field emission scanning electron microscopy (Tescan Mira3 FESEM, Czechia). The output voltage of the fabricated ZnO PENG was measured with an oscilloscope (Twintex, TSO1102, digital storage oscilloscope). A mass of 500 g was used for typical pressing. 3. Results and discussion Fig. 2 shows the XRD patterns of ZNR samples grown on seeded substrates at different molar concentrations. In accordance with the JSPDS card no. 00-036-1451, all XRD patterns were dominated by the ZnO hexagonal wurtzite structure. The XRD patterns revealed that the (002) peak and intensities were sharper and higher than those of other peaks, implying that the growth orientation preferred the c-axis for all samples prepared with different molar
Fig. 1. The schematic diagram of the fabrication process of the PENGs device.
Please cite this article as: R.S. Kammel, R.S. Sabry, Effects of the aspect ratio of ZnO nanorods on the performance of piezoelectric nanogenerators, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2019.08.002
R.S. Kammel, R.S. Sabry / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx
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Table 1 Average diameter, length and aspect ratio of ZNRs at various molar concentrations.
Fig. 2. XRD patterns of ZNRs grown on seeded substrates at different molar concentrations.
concentrations. Furthermore, the relative intensity of the (002) diffraction peak decreased as the molar concentrations increased, it is believed that few atoms arrange and moving away from the (002) orientation for the ZnO crystal, leading to non-uniformly distributed of growth orientations along the c-axis [20,21]. Fig. 3 presents the top (upside) and cross-section (down side) views of FESEM images of ZNRs at different molar concentrations. The FESEM images revealed that at various molar concentrations (0.01, 0.05 and 0.1 M), the ZNRs were formed with hexagonal shapes, whereas their size distributions and directions of growth orientation slightly differed which matched with the XRD results. The average values of diameters, lengths and aspect ratios of NRs are summarized in Table 1. At a low molar concentration of 0.01 M,
Concentration(M)
Average diameter (nm)
Average length (nm)
Aspect ratio
0.01 0.05 0.1
66 192 317
2400 1300 1000
36 7 3
ZNRs with a small diameter and a long length (i.e. high aspect ratio) were formed. As shown in Fig. 3A1 and A2, the ZNRs are homogenous and vertically aligned on the seeded substrate. As the molar concentration increased, the amount of zinc hydroxide produced also increased. These endothermic growth processes prevented the ZnO growth along the c-axis, resulting in shorter and thicker NRs [22]. Thus, the coverage of ZnO on the seeded substrates was increased (i.e. the space between NRs was decreased as the molar concentration increased). As the molar concentration was increased to 0.05e0.1 M, the average length decreased, the average diameter increased (the aspect ratio decreased), and the spaces between NRs decreased, as shown in Table 1 and Fig. 3B1, B2, C1 and C2. Fig. 4 shows the images of the PENG: (A) under non-pressing condition and (B) under a periodic of external pressing by using a mass of 500 g. When the PENG device was subjected to external pressing, the Au top electrode applied stress to the NRs. At the same time, the bottom NRs were exposed to the stress from the bottom Au electrode. The applied stress induced a tensile strain along the growth direction of the NRs. Consequently, a piezoelectric potential was generated because of the relative displacement of the positive and negative charges at the stretched and compressed sides of the NR growth direction and the piezoelectric-generated gradient from the root to the top of the NRs at the top and bottom parts of the substrate. Thus, an electrical charge flowed through the external circuit. As stress was released by removing the external pressing load. The piezoelectric potential in the ZNRs disappeared, so the
Fig. 3. Top (upside) and cross-section views (downside) of FESEM images for ZnO grown on the seeded substrates at different molar concentrations with scale bar of 500 nm: (A1 and A2) ZnO grown at 0.01 M, (B1 and B2) 0.05 M and (C1 and C2) 0.1 M.
Please cite this article as: R.S. Kammel, R.S. Sabry, Effects of the aspect ratio of ZnO nanorods on the performance of piezoelectric nanogenerators, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2019.08.002
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Fig. 4. Images of PENG: (A) under non-pressing condition and (B) under a periodic of external pressing by using a mass of 500 g.
electrons flowed back via the external circuit, creating an electric pulse in the opposite direction [23]. Fig. 5 shows the measured output potential voltage of the PENG devices with different aspect ratios of the ZNRs grown at (A) 0.01 M, (B) 0.05 M and (C) 0.1 M under the same external pressing mass of 500 g. The maximum output voltage of the PENG with ZNRs at 0.01 M was approximately 4.48 V (Fig. 5A). The output potential voltage decreased to 2.48 V for the PENG with ZNRs grown at 0.05 M (Fig. 5B). As the molar concentration increased to 0.1 M, the output potential voltage of the PENG with ZNRs decreased to 1.48 V (Fig. 5C). Fig. 6 shows the plot of the output voltage of PENG as a function of the aspect ratio of ZNRs. As shown, the output voltage decreased as the aspect ratio decreased. The creation of a piezoelectric
potential requires enough spaces between the NRs to be bent and the preferred c-axis orientation of the ZNRs [24]. This characteristics explains the decrease in the output potential voltage of the PENG device with the increasing diameter and the decreasing length (i.e. the decreasing aspect ratio) due to the increased molar concentration of the growth solution. For the PENG device fabricated at 0.01 M, the ZNRs had a higher aspect ratio (thinner diameter and longer length) and a perfect c-axis orientation than those of the PENG devices fabricated at other molar concentrations (0.05 and 0.1 M). Therefore, the spacing between the NRs was large enough for them to be bent generating a piezoelectric potential, and moreover, the longer NRs were easily deflected under external pressing. An increase in the molar concentration led to a decrease in the spacing between the NRs because of the increased diameter
Fig. 5. Measured output potential voltage of the PENGs under the same external pressing mass of 500 g with different aspect ratios of ZNRs grown at (A) 0.01 M, (B) 0.05 M and (C) 0.1 M.
Please cite this article as: R.S. Kammel, R.S. Sabry, Effects of the aspect ratio of ZnO nanorods on the performance of piezoelectric nanogenerators, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2019.08.002
R.S. Kammel, R.S. Sabry / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx
Fig. 6. The plot of output voltage of PENG as a function of aspect ratios.
of the NRs (the aspect ratio decreased). As a result, some NRs that were bent under pressing conditions came in contact with adjacent NRs that were also bent, leading to the cancellation of the piezoelectric potential generated from each other. Also the NRs with a thicker diameter and shorter length (low aspect ratio) were difficult to be bent under pressing. Finally, the results of this study proved that the generated piezoelectric potential was strongly proportional to the aspect ratio of the as-grown ZNRs. 4. Conclusion The effects of the controlled diameter and length (the aspect ratio) of the ZNRs on the performance of PENG devices have been investigated in the present study. Hexagonal ZNRs were synthesized on seed layers coated with double-sided PET through a simple hydrothermal method. The aspect ratio was controlled by varying the molar concentration of the growth solution. It was observed at a higher aspect ratio of ZNRs, the fabricated PENG device exhibited a relatively higher output voltage (~4.48 V) because the ZNRs with thinner diameter, longer length, and good alignment tend to be bent more easily under external pressing resulting in the efficient generation of the piezoelectric voltage. By contrast, a low output potential voltage was achieved for the PENG device with thicker diameter and shorter length (lower aspect ratio) of the ZNRs because the shorter and thicker ZNRs were difficult to be bent under the same external pressing which negatively affects the PENG performance. Acknowledgements The authors are grateful to the College of Science, Mustansiriayah University to support the completion of the project. Also, the thank is extended to the Assistant Professor Dr. Osama Abdul Azeez and Khaldoon Naji Abbas for their assistance. References [1] G.H. Kim, D.-M. Shin, H.-K. Kim, Y.-H. Hwang, S. Lee, Effect of the dielectric layer on the electrical output of a ZnO nanosheet-based nanogenerator, J. Korean Phys. Soc. 67 (2015) 1920e1924. https://doi.org/10.3938/jkps.67. 1920.
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Please cite this article as: R.S. Kammel, R.S. Sabry, Effects of the aspect ratio of ZnO nanorods on the performance of piezoelectric nanogenerators, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2019.08.002
Contents lists available at ScienceDirect
Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd
Original Article
Effects of the aspect ratio of ZnO nanorods on the performance of piezoelectric nanogenerators Ruaa S. Kammel*, Raad S. Sabry Mustansiriyah University, Department of Physics, Baghdad, Iraq
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 March 2019 Received in revised form 29 July 2019 Accepted 7 August 2019 Available online xxx
This paper presents an investigation on the performance of ZnO nanorod (ZNR)-based piezoelectric nanogenerators (PENGs). ZNRs on the ZnO seed layer coated double-sided flexible polyethylene terephthalate (PET) at different molar concentrations (0.01, 0.05 and 0.1 M) were synthesized by controlling the aspect ratio (length/diameter) of ZNRs, that are closely related to the piezoelectric output potential voltage using a simple hydrothermal method. ZNR PENGs were fabricated with an opposite electrode of gold-coated PET (Au/PET), which was placed on both the top and bottom of the ZNR-coated double-sided PET. X-ray diffraction and field emission scanning electron microscopy images revealed that as the molar concentration increased, the orientation of the as-grown ZNRs became non-uniformly distributed along the c-axis and also along with the decreased aspect ratio. At a low molar concentration (0.01 M), the ZNR PENGs exhibited a relatively high output potential voltage (~4.48 V) under an external pressing mass (500 g). The ZNR samples grown at 0.05 and 0.1 M exhibited a lower piezoelectric voltage 2.48 and 1.84 V, respectively. These results confirmed that ZNR PENGs with a small diameter, long length (i.e. high aspect ratio) and good alignment tend to be bent more easily for the efficient generation of the piezoelectric potential. © 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Keywords: ZnO nanorods Spin coating method Hydrothermal method Energy harvesting Piezoelectric nanogenerators
1. Introduction In recent years, scientists have strived to convert wasted environmental energy into electricity to solve the global energy demands by exploiting cost-effective, simple and environmentally friendly power sources (and energy conversion technologies) [1]. Therefore, nanogenerator technology has been developed for harvesting energy from the environment, and this strategy is based on three main effects: piezoelectric and triboelectric effects for harvesting mechanical energy, and pyroelectric effect for harvesting thermal energy [2,3]. Piezoelectric nanogenerators (PENGs) are widely used because the vibration has attracted considerable attention as a renewable power source, given its excellent environmental adaptability and high robustness, numerous vibrations, such as human motions, including walking, tiny clicks of the fingers, and rotating tires, are available from the surrounding environment but are wasted in our daily life [4,5].
* Corresponding author. E-mail addresses: [email protected] (R.S. Kammel), drraad_sci@ uomustansiriyah.edu (R.S. Sabry). Peer review under responsibility of Vietnam National University, Hanoi.
PENG is a nanoscale energy harvesting device that converts kinetic energy from mechanical vibrations in the ambient environment into a usable form of electrical energy by exploiting the excellent mechanical and electrical properties of nanostructured piezoelectric materials [4,6]. The idea of PENG was first presented in 2006 as an atomic force microscope (AFM) tip sweeps across a vertically grown ZnO nanowire, an electrical voltage/current was generated [7]. Among the various piezoelectric materials, the ZnO-based PENG is important in mechanical energy harvesting because it is a biocompatible, non-toxic and direct piezoelectric material and can be easily synthesized in the required shape and size of various substrates [8e10]. The fundamental mechanism of PENGs depends on the piezoelectricity of ZnO as well as the Schottky barrier that is formed between the metal - ZnO interface [11,12]. One-dimensional ZnO nanostructures in the form of nanowires (NWs) and nanorods (NRs) are widely used to fabricate PENG due to their high mechanical flexibility and sensitivity to small mechanical stress, which allows the conversion of small mechanical energy to electricity [13,14]. Among numerous methods used to synthesize ZNRs, the hydrothermal method has more advantages, due to its low cost, low
https://doi.org/10.1016/j.jsamd.2019.08.002 2468-2179/© 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Please cite this article as: R.S. Kammel, R.S. Sabry, Effects of the aspect ratio of ZnO nanorods on the performance of piezoelectric nanogenerators, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2019.08.002
2
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temperature, compatibility with flexible substrates, and environmental friendliness. Several parameters influencing the growth of ZNRs include the growth duration, growth temperature, PH of the solution, the growth concentration, and seeding of the substrate that increases the density and alignment of the NRs [15e19]. Many studies have researched the yield voltage generated from the ZnO-based PENG by designing new devices, for example, using different flexible substrates (plastic, paper, cotton fabrics), and the growth of ZnO NWs/NRs on single or double side of the conductive substrate. However, to the best of our knowledge, not many studies have assessed the effect of the aspect ratio of ZNRs on the PENGs' efficiency. Thus, in this study, we investigated the efficiency of ZNRbased PENGs by controlling the aspect ratio (length/diameter) of ZNRs grown through the hydrothermal method at various molar concentrations of the growth solution. 2. Experimental 2.1. Preparation of ZnO seed layer A flexible polyethylene terephthalate (PET) substrate was cleaned with ethanol and distilled water in an ultrasonic bath and then heated in an oven to remove the moisture. The ZnO seed layer was coated on the double-sided PET (seeded substrate) through the solegel spin-coating method to fabricate the ZnO PENG. The seed solution was prepared by dissolving zinc acetate dehydrate [ðCH3 COOÞ2 $2H2 O; Scharlau] in ethanol to obtain the concentration of 0.02 M. The solution was stirred to obtain a homogeneous and transparent sol. Afterward, by using a syringe, the seed solution was dropped onto the substrates and rotated at 1000 rpm for 30 s to attain a uniform distributed seed layer across the substrates. This process was repeated thrice. Then, the substrates were heated to 100 C in an oven for 10 min to remove the solvent and achieve good adhesion of the seed layer. This procedure (from spin coating to pre-heat treatment) was repeated thrice to ensure the complete coverage of the plastic substrates with the ZnO seed layer. Finally, the ZnO seed layer was heated to 100 C for 25 min to improve the crystalline quality. 2.2. Growth of ZNRs Various molar concentrations of the growth solution (0.01, 0.05 and 0.1 M) were used to grow ZNRs with different aspect ratios. The growth solution was prepared by dissolving zinc nitrate
hexahydrate [ZnðNO3 Þ2 :6H2 O; Scharlau] and hexamethylenetetramine [ðCH2 Þ6 N4 ; HiMedia] in distilled water with a molar ratio of 1:1. The solution was fully and evenly stirred and then transferred into a 50 ml Teflon-lined autoclave. Seeded substrate was placed vertically in the solution. The autoclave was sealed in an oven and kept at 95 C for 3 h. Finally, the substrate was removed from the solution, washed with distilled water to remove the residues on the surface and left to dry in air. 2.3. Fabrication of devices To fabricate the ZnO PENG, an insulating layer made of polydimethylsiloxane (PDMS) was deposited onto the as-grown ZNRs using the spin-coating method. The PDMS prepolymer (Sylgard184, Dow Corning, Midland, MI) was prepared by thoroughly mixing the PDMS curing agent with the PDMS base monomer at a weight ratio of 10:1. The PDMS prepolymer was then spin-coated onto the as-grown ZnO and fully cured at 70 C. The double-sided ZNRs with PDMS were sandwiched between the Au electrodes (gold-coated PET substrate by DC sputtering). The electrical contact was placed on both the top and bottom parts of the Au electrodes by using Cu wires with silver paste. The device was wrapped with Kapton tape to avoid peeling off problems. The schematic diagram of the fabrication process of the PENGs device is shown in Fig. 1. 2.4. Characterization The structural quality and orientation of the grown ZNRs were analyzed by X-ray diffraction (XRD) at 40 KV and 30 mA with Cu-Ka radiation in the range of 30 e70 at a step of 0.02 . The surface morphology of the structure of all samples was characterized by field emission scanning electron microscopy (Tescan Mira3 FESEM, Czechia). The output voltage of the fabricated ZnO PENG was measured with an oscilloscope (Twintex, TSO1102, digital storage oscilloscope). A mass of 500 g was used for typical pressing. 3. Results and discussion Fig. 2 shows the XRD patterns of ZNR samples grown on seeded substrates at different molar concentrations. In accordance with the JSPDS card no. 00-036-1451, all XRD patterns were dominated by the ZnO hexagonal wurtzite structure. The XRD patterns revealed that the (002) peak and intensities were sharper and higher than those of other peaks, implying that the growth orientation preferred the c-axis for all samples prepared with different molar
Fig. 1. The schematic diagram of the fabrication process of the PENGs device.
Please cite this article as: R.S. Kammel, R.S. Sabry, Effects of the aspect ratio of ZnO nanorods on the performance of piezoelectric nanogenerators, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2019.08.002
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Table 1 Average diameter, length and aspect ratio of ZNRs at various molar concentrations.
Fig. 2. XRD patterns of ZNRs grown on seeded substrates at different molar concentrations.
concentrations. Furthermore, the relative intensity of the (002) diffraction peak decreased as the molar concentrations increased, it is believed that few atoms arrange and moving away from the (002) orientation for the ZnO crystal, leading to non-uniformly distributed of growth orientations along the c-axis [20,21]. Fig. 3 presents the top (upside) and cross-section (down side) views of FESEM images of ZNRs at different molar concentrations. The FESEM images revealed that at various molar concentrations (0.01, 0.05 and 0.1 M), the ZNRs were formed with hexagonal shapes, whereas their size distributions and directions of growth orientation slightly differed which matched with the XRD results. The average values of diameters, lengths and aspect ratios of NRs are summarized in Table 1. At a low molar concentration of 0.01 M,
Concentration(M)
Average diameter (nm)
Average length (nm)
Aspect ratio
0.01 0.05 0.1
66 192 317
2400 1300 1000
36 7 3
ZNRs with a small diameter and a long length (i.e. high aspect ratio) were formed. As shown in Fig. 3A1 and A2, the ZNRs are homogenous and vertically aligned on the seeded substrate. As the molar concentration increased, the amount of zinc hydroxide produced also increased. These endothermic growth processes prevented the ZnO growth along the c-axis, resulting in shorter and thicker NRs [22]. Thus, the coverage of ZnO on the seeded substrates was increased (i.e. the space between NRs was decreased as the molar concentration increased). As the molar concentration was increased to 0.05e0.1 M, the average length decreased, the average diameter increased (the aspect ratio decreased), and the spaces between NRs decreased, as shown in Table 1 and Fig. 3B1, B2, C1 and C2. Fig. 4 shows the images of the PENG: (A) under non-pressing condition and (B) under a periodic of external pressing by using a mass of 500 g. When the PENG device was subjected to external pressing, the Au top electrode applied stress to the NRs. At the same time, the bottom NRs were exposed to the stress from the bottom Au electrode. The applied stress induced a tensile strain along the growth direction of the NRs. Consequently, a piezoelectric potential was generated because of the relative displacement of the positive and negative charges at the stretched and compressed sides of the NR growth direction and the piezoelectric-generated gradient from the root to the top of the NRs at the top and bottom parts of the substrate. Thus, an electrical charge flowed through the external circuit. As stress was released by removing the external pressing load. The piezoelectric potential in the ZNRs disappeared, so the
Fig. 3. Top (upside) and cross-section views (downside) of FESEM images for ZnO grown on the seeded substrates at different molar concentrations with scale bar of 500 nm: (A1 and A2) ZnO grown at 0.01 M, (B1 and B2) 0.05 M and (C1 and C2) 0.1 M.
Please cite this article as: R.S. Kammel, R.S. Sabry, Effects of the aspect ratio of ZnO nanorods on the performance of piezoelectric nanogenerators, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2019.08.002
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Fig. 4. Images of PENG: (A) under non-pressing condition and (B) under a periodic of external pressing by using a mass of 500 g.
electrons flowed back via the external circuit, creating an electric pulse in the opposite direction [23]. Fig. 5 shows the measured output potential voltage of the PENG devices with different aspect ratios of the ZNRs grown at (A) 0.01 M, (B) 0.05 M and (C) 0.1 M under the same external pressing mass of 500 g. The maximum output voltage of the PENG with ZNRs at 0.01 M was approximately 4.48 V (Fig. 5A). The output potential voltage decreased to 2.48 V for the PENG with ZNRs grown at 0.05 M (Fig. 5B). As the molar concentration increased to 0.1 M, the output potential voltage of the PENG with ZNRs decreased to 1.48 V (Fig. 5C). Fig. 6 shows the plot of the output voltage of PENG as a function of the aspect ratio of ZNRs. As shown, the output voltage decreased as the aspect ratio decreased. The creation of a piezoelectric
potential requires enough spaces between the NRs to be bent and the preferred c-axis orientation of the ZNRs [24]. This characteristics explains the decrease in the output potential voltage of the PENG device with the increasing diameter and the decreasing length (i.e. the decreasing aspect ratio) due to the increased molar concentration of the growth solution. For the PENG device fabricated at 0.01 M, the ZNRs had a higher aspect ratio (thinner diameter and longer length) and a perfect c-axis orientation than those of the PENG devices fabricated at other molar concentrations (0.05 and 0.1 M). Therefore, the spacing between the NRs was large enough for them to be bent generating a piezoelectric potential, and moreover, the longer NRs were easily deflected under external pressing. An increase in the molar concentration led to a decrease in the spacing between the NRs because of the increased diameter
Fig. 5. Measured output potential voltage of the PENGs under the same external pressing mass of 500 g with different aspect ratios of ZNRs grown at (A) 0.01 M, (B) 0.05 M and (C) 0.1 M.
Please cite this article as: R.S. Kammel, R.S. Sabry, Effects of the aspect ratio of ZnO nanorods on the performance of piezoelectric nanogenerators, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2019.08.002
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Fig. 6. The plot of output voltage of PENG as a function of aspect ratios.
of the NRs (the aspect ratio decreased). As a result, some NRs that were bent under pressing conditions came in contact with adjacent NRs that were also bent, leading to the cancellation of the piezoelectric potential generated from each other. Also the NRs with a thicker diameter and shorter length (low aspect ratio) were difficult to be bent under pressing. Finally, the results of this study proved that the generated piezoelectric potential was strongly proportional to the aspect ratio of the as-grown ZNRs. 4. Conclusion The effects of the controlled diameter and length (the aspect ratio) of the ZNRs on the performance of PENG devices have been investigated in the present study. Hexagonal ZNRs were synthesized on seed layers coated with double-sided PET through a simple hydrothermal method. The aspect ratio was controlled by varying the molar concentration of the growth solution. It was observed at a higher aspect ratio of ZNRs, the fabricated PENG device exhibited a relatively higher output voltage (~4.48 V) because the ZNRs with thinner diameter, longer length, and good alignment tend to be bent more easily under external pressing resulting in the efficient generation of the piezoelectric voltage. By contrast, a low output potential voltage was achieved for the PENG device with thicker diameter and shorter length (lower aspect ratio) of the ZNRs because the shorter and thicker ZNRs were difficult to be bent under the same external pressing which negatively affects the PENG performance. Acknowledgements The authors are grateful to the College of Science, Mustansiriayah University to support the completion of the project. Also, the thank is extended to the Assistant Professor Dr. Osama Abdul Azeez and Khaldoon Naji Abbas for their assistance. References [1] G.H. Kim, D.-M. Shin, H.-K. Kim, Y.-H. Hwang, S. Lee, Effect of the dielectric layer on the electrical output of a ZnO nanosheet-based nanogenerator, J. Korean Phys. Soc. 67 (2015) 1920e1924. https://doi.org/10.3938/jkps.67. 1920.
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Please cite this article as: R.S. Kammel, R.S. Sabry, Effects of the aspect ratio of ZnO nanorods on the performance of piezoelectric nanogenerators, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/j.jsamd.2019.08.002