Efficient piezoelectric ZnO nanogenerators based on Au-coated silica sphere array electrode
© Ko et al.; licensee Springer. 2013
Received: 9 October 2013
Accepted: 23 November 2013
Published: 5 December 2013
We reported ZnO nanorod-based piezoelectric nanogenerators (NGs) with Au-coated silica sphere array as an efficient top electrode. This electrode can readily bend the ZnO nanorods due to its enhanced surface roughness, thus resulting in more increased and regular piezoelectric charge output. Under a low external pushing force of 0.3 kgf, the output current and voltage were increased by approximately 2.01 and 1.51 times, respectively, in comparison with a conventional Au top electrode without silica spheres. Also, the effect of Au-coated silica spheres on the bending radius of ZnO nanorods was theoretically investigated.
Energy harvesting technology, capturing ambient waste energy from human movements or machinery vibrations, offers a promising solution for self-powered, wireless, and sustainable operation on various applications such as portable electronic devices, touch sensors, and implanted biosensors [1–3]. Since piezoelectric zinc oxide (ZnO) nanogenerators (NGs) were demonstrated for electric power conversion from mechanical energy in 2006 , they have been considered as a key technique for realizing the environment-friendly energy harvesting technology. As an external mechanical force is applied to vertically aligned ZnO nanowires or nanorods using an atomic force microscope (AFM) tip, the positive/negative potential is induced at the stretched/compressed side of ZnO, thus leading to a piezoelectric charge generation . By utilizing this principle of piezoelectric ZnO NGs, over the last decades, there have been considerable efforts to improve the performance and efficiency of piezoelectric ZnO NGs in regard to various types/properties of ZnO nanostructures and surface contact of electrodes. The first one has been achieved by growing ZnO nanowires, nanorods, and nanobelts on the flexible polyethersulfone or polyethylene terephthalate (PET) substrate via a chemical solution method [6, 7]. The other one was an alternative way in which zig-zag-shaped or network electrodes (consisting of patterned noble metals, carbon nanotubes, or graphene) were employed as a top electrode to efficiently bend the ZnO nanostructures for transmitting the external mechanical energy as well as possible [8, 9]. However, these kinds of top electrodes needed a somewhat sophisticated fabrication process for the preparation of patterned electrodes or synthesis of carbon-based nanomaterials.
On the other hand, one-dimensional (1D) ZnO nanostructures including nanowires or nanorods provide an effective deformation (i.e., stretch and compression) under external mechanical energy due to their high aspect ratio which generates the piezoelectric charges . Additionally, they have been reliably synthesized and vertically integrated on various flexible substrates with ZnO seed coating by hydrothermal or electrochemical deposition (ED) method [11–14]. Particularly, the ED method has many advantages for growing 1D ZnO nanostructures because the electric energy enables a short time process at low temperature . In this work, we prepared ZnO nanorod arrays (NRAs) on an indium tin oxide (ITO)-coated PET substrate (i.e., ITO/PET) using the ED method and fabricated ZnO NRA-based NGs with an efficient top electrode which was obtained by evaporating gold (Au) onto the surface of silica spheres. Herein, the multilayer of silica spheres was facilely deposited on the PET substrate by rolling the colloidal solution of silica spheres.
Results and discussion
We successfully fabricated the efficient top electrode for ZnO NRA-based NGs by incorporating the Au-coated silica sphere array on the PET substrate. When Au was deposited onto the multilayer of silica spheres, it formed as a highly rough surface with angulated morphology. By computational simulations for the strain distribution when bending ZnO nanorods, the rough surface of Au-coated silica sphere array could be expected to further increase the bending radius under an external pushing force. For an experimental analysis, the NGs were fabricated with ZnO NRAs on ITO/PET via the ED method and different top electrodes (i.e., Au film on PET and Au-coated silica sphere array on PET). Under an external pushing force of 0.3 kgf, the Au-coated silica sphere array contributed to the improvement in output current and voltage by about 2.01 and 1.51 times with regular curves. From these results, the Au-coated silica sphere array could be useful for an efficient top electrode in various ZnO nanostructure-based piezoelectric NG applications.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (no. 2013–010037).
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