ZNRs on the ZnO seed layer coated double-sided flexible polyethylene tere- phthalate (PET) at different molar concentrations (0.01, 0.05 and 0.1 M) were synthesized by controlling the asp[r]
Trang 1Original 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
Article history:
Received 2 March 2019
Received in revised form
29 July 2019
Accepted 7 August 2019
Available online xxx
Keywords:
ZnO nanorods
Spin coating method
Hydrothermal method
Energy harvesting
Piezoelectric nanogenerators
a b s t r a c t
This paper presents an investigation on the performance of ZnO nanorod (ZNR)-based piezoelectric nanogenerators (PENGs) ZNRs on the ZnO seed layer coated double-sidedflexible polyethylene tere-phthalate (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 andfield 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 piezo-electric 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/)
1 Introduction
In recent years, scientists have strived to convert wasted
envi-ronmental energy into electricity to solve the global energy
de-mands by exploiting cost-effective, simple and environmentally
friendly power sources (and energy conversion technologies)[1]
Therefore, nanogenerator technology has been developed for
har-vesting energy from the environment, and this strategy is based on
three main effects: piezoelectric and triboelectric effects for
har-vesting mechanical energy, and pyroelectric effect for harhar-vesting
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
envi-ronmental adaptability and high robustness, numerous vibrations,
such as human motions, including walking, tiny clicks of the
fin-gers, and rotating tires, are available from the surrounding
envi-ronment but are wasted in our daily life[4,5]
PENG is a nanoscale energy harvesting device that converts ki-netic energy from mechanical vibrations in the ambient environ-ment 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 wasfirst 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 piezo-electricity of ZnO as well as the Schottky barrier that is formed be-tween 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 hy-drothermal method has more advantages, due to its low cost, low
* Corresponding author.
E-mail addresses: ruaa_s_k@yahoo.com (R.S Kammel), drraad_sci@
uomustansiriyah.edu (R.S Sabry).
Peer review under responsibility of Vietnam National University, Hanoi.
Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
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/ ).
Trang 2temperature, compatibility withflexible substrates, and
environ-mental 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
differentflexible 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' ef
fi-ciency Thus, in this study, we investigated the efficiency of
ZNR-based 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
[ðCH3COOÞ2$2H2O; Scharlau] in ethanol to obtain the
concentra-tion of 0.02 M The soluconcentra-tion 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
100C 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 100C 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 [ZnðNO3Þ2:6H2O; Scharlau] and hexamethylenetetra-mine [ðCH2Þ6N4; 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 95C 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 poly-dimethylsiloxane (PDMS) was deposited onto the as-grown ZNRs using the spin-coating method The PDMS prepolymer
(Sylgard-184, 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 70C 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 inFig 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 30e70at 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 2shows 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.
Trang 3concentrations 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
distrib-uted of growth orientations along the c-axis[20,21]
Fig 3presents 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 inTable 1 At a low molar concentration of 0.01 M,
ZNRs with a small diameter and a long length (i.e high aspect ratio) were formed As shown inFig 3A1 and A2, the ZNRs are homog-enous 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 inTable 1andFig 3B1, B2, C1 and C2
Fig 4shows 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 2 XRD patterns of ZNRs grown on seeded substrates at different molar
concentrations.
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.
Table 1 Average diameter, length and aspect ratio of ZNRs at various molar concentrations.
Concentration(M) Average
diameter (nm)
Average length (nm)
Aspect ratio
Trang 4electronsflowed back via the external circuit, creating an electric
pulse in the opposite direction[23]
Fig 5shows 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 6shows 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 characteris-tics 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 fabri-cated 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 4 Images of PENG: (A) under non-pressing condition and (B) under a periodic of external pressing by using a mass of 500 g.
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.
Trang 5of 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
piezo-electric 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
propor-tional 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
synthe-sized 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,
Mustansir-iayah 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
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Fig 6 The plot of output voltage of PENG as a function of aspect ratios.