1083

Sensors and Materials, Vol. 31, No. 3 (2019) 1083–1089
MYU Tokyo
S & M 1836

ZnO Nanogenerator Prepared from ZnO Nanorods
Grown by Hydrothermal Method
Chih-Cheng Chen,1 Tung-Lung Wu,2* Teen-Hang Meen,3**Chao-Yang Chen,1 Che-Hsiang Su,3 Jenn-Kai Tsai,3Chun-Ying Lee,4 Chun-Hsien Lee,4 and Day-Shan Liu4
12

School of Information Engineering, Jimei University, Xiamen, Fujian 361021, China
School of Mechanical and Automotive Engineering, Zhaoqing University, Zhaoqing, Guangdong 516260, China
3Department of Electronic Engineering, National Formosa University, Huwei, Yunlin 632, Taiwan
4
Institute of Electro-Optical and Materials Science, National Formosa University, Huwei, Yunlin 632, Taiwan(Received November 23, 2018; accepted December 27, 2018)

Keywords: ZnO, nanorods, nanogenerator, hydrothermal method

In this study, a zinc oxide (ZnO) film was deposited by sputtering on an indium tin oxide(ITO) glass substrate. ZnO nanorods were then grown on the film by the hydrothermal method,then assembled with a gold electrode to fabricate a nanogenerator. The ZnO nanostructure andnanogenerator were analyzed by field emission scanning electron microscopy (FE-SEM), X-raydiffraction (XRD), and the measurement of current–voltage characteristics. The results of FESEM show that the length of the ZnO nanorods increased with the growth time, and the optimal
dimensions of the ZnO nanorods were a length of 2 μm and a diameter of 130 nm at the growthtime of 6 h. In the XRD pattern, ZnO (002) and (103) peaks were observed at 2θ = 34.45 and62.51°, respectively, confirming that the ZnO nanorods were grown on the substrate. Thenanogenerator was driven by an ultrasonic wave to measure its voltage and current. The
highest average current and voltage were 3.46 × 10−6 A and 5.63 × 10−2 V, respectively. These
results indicate that the ZnO nanorods prepared by the hydrothermal method are suitable for thefabrication of a nanogenerator.

1.

Introduction

Zinc oxide (ZnO) is a II-VI semiconductor material with a direct band gap of 3.37 eV,corresponding to a wavelength in the ultraviolet region, and it also has a large excitation binding
energy (~60 meV). In addition, ZnO has low resistivity and high transparency; therefore, it isconsidered as a promising material for application in optoelectronics. Recently, ZnO materials
with the characteristics of one-dimensional (1D) nanomaterials have been realized, which canexhibit different nanostructures depending on the fabrication method of the materials. Over thepast ten years, the synthesis of ZnO nanostructures such as nanowires, nanocycles, nanobelts,
and nanocombs has been successfully achieved. The most representative structure is nanowires.
*Corresponding author: e-mail: tunglung216@gmail.com
Corresponding author: e-mail: thmeen@nfu.edu.twhttps://doi.org/10.18494/SAM.2019.2228**ISSN 0914-4935 © MYU K.K.
https://myukk.org/

1084

Sensors and Materials, Vol. 31, No. 3 (2019)

Nanotechnology involves the use of nanomaterials with various dimensions to assemble thedesired structure. Nanostructures such as quantum dots and quantum wells can be used forillumination or metering. ZnO nanomaterials not only can be used for the basic theoretical
study of physical properties, such as light, electricity, magnetism, and mechanics, but alsohave great potential as nanooptical components, such as light-emitting diodes (LEDs),(1) fieldemission elements,(2) surface coatings of conductive materials,(3) laser diodes,(4–6) solar cells,
gas sensors, photonic crystals, field-effect transistors (FETs), and photodetectors.(7) In addition,
ZnO is also a piezoelectric semiconductor material when prepared as a film. Some types ofZnO film can be applied to a surface acoustic wave (SAW) device. When ZnO is prepared in
a 1D form, it can be used as a nanogenerator.(8–10) In this study, a ZnO film was deposited by
sputtering on an indium tin oxide (ITO) glass substrate, then ZnO nanorods were on the filmgrown by the hydrothermal method to fabricate a nanogenerator. The characteristics of the ZnO
nanorods and nanogenerator were investigated by field emission scanning electron microscopy(FE-SEM), X-ray diffraction (XRD), and current–voltage measurements, and the results revealthat a ZnO nanogenerator can be fabricated successfully by this low-cost hydrothermal method.

2.

Experimental Procedure

A ZnO thin film was prepared by sputtering the ZnO target (3 inch, 99.9%) onto an ITO
glass substrate as a seed layer. The thickness of the ZnO seed layer was 500 nm. ZnOnanorods were grown on the seed layer in zinc nitrate hexahydrate [Zn(NO3)2, 0.03 M] andhexamethylenetetramine (HMTA, 0.03 M) for 3, 6, 9, or 12 h at 90 ℃ by the hydrothermal
method, and then the grown ZnO nanorods were removed from the solution, rinsed with
distilled water, and dried in air. The surface morphology of the ZnO nanorods was observed
by FE-SEM. The crystallinity of the ZnO nanorods was analyzed by XRD. To assemble thenanogenerator, a gold film was deposited on the etched ITO glass substrate as an electrode. The
microcurrent of the nanogenerator was measured in DI water by driving ultrasonic waves witha frequency of 42 kHz and exhibited a power of 100 W. The ZnO nanogenerator generated an
output current and demonstrated a Schottky-like current–voltage characteristic.

3.

Results and Discussion

Nanorods with four different lengths and widths were grown on the seed layer for 3, 6, 9, and
12 h at 90 ℃ by the hydrothermal method. Figure 1 shows top-view and cross-sectional SEM
images of the ZnO nanorods. The lengths of the ZnO nanorods grown on the seed layer for 3, 6,9, and 12 h were 800 nm, 2 μm, 2 μm, and 2 μm and the widths were 80, 130, 150, and 170 nm,respectively. These results reveal that the length and width of the ZnO nanorods increase withthe growth time, and the length of the ZnO nanorods hardly changes when the growth time islonger than 6 h. Figure 2 shows the energy-dispersive X-ray spectroscopy (EDS) spectrum ofthe ZnO nanorods grown for 6 h. The result indicates that the atomic ratio of Zn to O is nearly 1,and there are other elements in the ZnO nanorods.

Sensors and Materials, Vol. 31, No. 3 (2019)1085

Fig. 1. Top-view and cross-sectional SEM images of ZnO nanorods for growth times of (a, b) 3, (c, d) 6, (e, f) 9, and (g,h) 12 h.

1086

Sensors and Materials, Vol. 31, No. 3 (2019)

Fig. 2.

Fig. 3.
6 h.

(Color online) EDS spectrum of the ZnO nanorods grown for 6 h.

XRD pattern of the ZnO nanorods grown forFig. 4. (Color online) Diagram of deposition of gold
electrode on ITO glass substrate.

Figure 3 shows the XRD pattern of the ZnO nanorods grown for 6 h. As shown in Fig. 3,a strong ZnO (002) diffraction peak was observed at 2θ = 34.45°. It indicates that the ZnO
nanorods on the substrate are mainly oriented along the c-axis. In addition, a (103) peak also
appears since not all the ZnO nanorods have c-axis orientation and some of them are skewed.
To assemble the nanogenerator, a gold film was deposited on the etched ITO glass substrate
as an electrode. After cleaning the substrate with acetone, isopropanol, and DI water, the etched
ITO glass was placed in the oven to dry. The gold film was coated by evaporation, and its
thickness was about 100 nm. Figure 4 shows a diagram of the deposition of the gold electrodeon the ITO glass substrate.

Sensors and Materials, Vol. 31, No. 3 (2019)

1087

Figures 5–7 show the current voltage characteristics of the ZnO nanogenerators fabricatedby ZnO nanorods grown for 3, 6, and 9 h, respectively. The nanogenerators were driven by afluctuation generated with an ultrasonic oscillator. The current voltage characteristics show
Schottky contact behavior between the metal and the oxide layer, and the average currents
and voltages are 9.52 × 10−8 A and 7.04 × 10−2 V for 3 h, 3.46 × 10−6 A and 5.63 × 10−2 V
for 6 h, and 4.75 × 10−7 A and 3.38 × 10−2 V for 9 h, respectively, and the output powers are6.7 × 10−9 W for 3 h, 1.95 × 10−7 W for 6 h, and 1.6 × 10−8 W for 9 h. From the viewpointof the total power, the best nanogenerator is fabricated with the ZnO nanorods grown for 6h. Note that because thinner ZnO nanorods can be bent more easily, they can generate more
output power than the thicker ZnO nanorods. From Fig. 1, the lengths of the ZnO nanorods
grown for 3, 6, and 9 h are 800 nm, 2 μm, and 2 μm and the widths are 80, 130, and 150 nm,
respectively. Because the length of the ZnO nanorods hardly changes after 6 h, the optimal
ZnO nanogenerator is fabricated with ZnO nanorods grown for 6 h. This may also be due to theZnO nanorods grown for 6 h having a suitable length of 2 μm while having a smaller diameter
than the ZnO nanorods grown for 9 h, resulting in the highest output power when driven by thefluctuation generated by the ultrasonic oscillator.

Fig. 5.

Current–voltage characteristics of the ZnO nanogenerator fabricated from ZnO nanorods grown for 3 h.

1088

Sensors and Materials, Vol. 31, No. 3 (2019)

Fig. 6.

Current–voltage characteristics of the ZnO nanogenerator fabricated from ZnO nanorods grown for 6 h.

Fig. 7.

Current–voltage characteristics of the ZnO nanogenerator fabricated from ZnO nanorods grown for 9 h.

Sensors and Materials, Vol. 31, No. 3 (2019)

4.

1089

Conclusions

In this study, a ZnO nanogenerator was fabricated from ZnO nanorods grown by thehydrothermal method. The optimal growth time of the ZnO nanorods was 6 h. The highest
average current and voltage were 3.46 × 10−6 A and 5.63 × 10−2 V, respectively. These resultsdemonstrate that ZnO nanorods prepared by the hydrothermal method can be used to fabricate ananogenerator.

References
1 Y. Ryu, T. S. Lee, J. A. Lubguban, H. W. White, B. J. Kim, Y. S. Park, and C. J. Youn: Appl. Phys. Lett. 88 (2006)
241108.
2 M. H. Huang, S. Mao, H. Feick, H. Q. Yan, Y. Y. Wu, H. Kind, E. Weber, R. Russo, and P. D. Yang: Science
292 (2001) 1897.
3 H. Kind, H. Q. Yan, B. Messer, M. Law, and P. D. Yang: Adv. Mater. 14 (2002) 158.
4 C. Soci, A. Zhang, B. Xiang, S. A. Dayeh, D. P. R. Aplin, J. Park, X. Y. Bao, Y. H. Lo, and D. Wang: NanoLett. 7 (2007) 1003.
5 Y. Z. Jin, J. P. Wang, B. Q. Sun, J. C. Blakesley, and N. C. Greenham: Nano Lett. 8 (2008) 1649.
6 H. T. Ng, T. Yamada, P. Nguyen, Y. P. Chen, and M. Meyyappan: Nano Lett. 4 (2004) 1247.
7 K. Keem, D. Y. Jeong, S. Kim, M. S. Lee, I. S. Yeo, U. I. Chung, and J. T. Moon: Nano Lett. 6 (2006) 1454.
8 J. B. Baxter and E. S. Aydil: Appl. Phys. Lett. 86 (2005) 053114.
9 X. D. Wang, J. H. Song, J. Liu, and Z. L. Wang: Science 316 (2007) 102.
10 Q. Wan, Q. H. Li, Y. J. Chen, T. H. Wang, X. L. He, J. P. Li, and C. L. Lin: Appl. Phys. Lett. 84 (2004) 3085.