Towards a Highly Efficient ZnO Based Nanogenerator

Abstract

A nanogenerator (NG) is an energy harvester device that converts mechanical energy into electrical energy on a small scale by relying on physical changes. Piezoelectric semiconductor materials play a key role in producing high output power in piezoelectric nanogenerator. Low cost, reliability, deformation, and electrical and thermal properties are the main criteria for an excellent device. Typically, there are several main types of piezoelectric materials, zinc oxide (ZnO) nanorods, barium titanate (BaTiO₃) and lead zirconate titanate (PZT). Among those candidate, ZnO nanorods have shown high performance features due to their unique characteristics, such as having a wide-bandgap semiconductor energy of 3.3 eV and the ability to produce more ordered and uniform structures. In addition, ZnO nanorods have generated considerable output power, mainly due to their elastic nanostructure, mechanical stability and appropriate bandgap. Apart from that, doping the ZnO nanorods and adding doping impurities into the bulk ZnO nanorods are shown to have an influence on device performance. Based on findings, Ni-doped ZnO nanorods are found to have higher output power and surface area compared to other doped. This paper discusses several techniques for the synthesis growth of ZnO nanorods. Findings show that the hydrothermal method is the most commonly used technique due to its low cost and straightforward process. This paper reveals that the growth of ZnO nanorods using the hydrothermal method has achieved a high power density of 9 µWcm^-2.

Keywords: higher output power; hydrothermal method; nanogenerator; piezoelectric effect; zinc oxide (ZnO) nanorods.

Figure 1

Direct piezoelectric action schematic: (A) electrical charge creation in a piezoelectric material in the absence of an external force, (B) tension and (C) compression. The two most often used operating modes for (D) 33 (stock configuration) and (E) 31 (bending configuration); the pole (producing the net polarization ion) direction is in the order of “3” for both configurations.

Figure 2

(a) Schematic diagram of (A) PENG nanostructure under (B) longitudinal strain, (C) corresponding longitudinal electric field and (D) potential distribution and (b) the typical structure of PENGs. Reproduced with permission from reference [41].

Figure 3

Morphology of ZnO NRs. Reproduced with permission from reference [43].

Figure 4

Morphology images of (a) surface and (b) cross-sectional view of the growth of ZnO NRs. Reproduced with permission from reference [45].

Figure 5

(a) BaTiO₃ NWs arrays in top view. (b) BaTiO₃ constitutes the majority of the peaks in the X-ray diffraction spectra of BaTiO₃ NW arrays. Reproduced with permission from reference [48].

Figure 6

Morphology of PZT NWs (a,b). Reproduced with permission from reference [49].

Figure 7

Schematic diagram of a piezoelectric nanogenerator based on Ag-doped ZnO NRs. Reproduced with permission from reference [64].

Figure 8

(a) Observation of a cotton fiber without ZnO development, (b) picture demonstrating the presence of Ag-doped ZnO NRs on a cotton fiber, (c) better magnification image of Ag-doped ZnO NRs on a cotton fiber. Reproduced with permission from reference [64].

Figure 9

Mechanism energy harvesting based on V-zigzag electrodes: (a) AZO and V-zigzag structure, (b) SEM pictures of AZO NRs. Reproduced with permission from reference [65].

Figure 10

Shows the top view of pure ZnO NRs and Ni-doped ZnO NRs structure (a,b) using FESEM and the IV characteristic of Pure ZnO and Ni-doped ZnO NRs (c). Reproduced with permission from reference [68].

Figure 11

Schematic diagram of CVD process for ZnO NR growth in a horizontal tube furnace.

Figure 12

Schematic diagram of hydrothermal process for ZnO NRs.

Figure 13

Schematic illustration of ECD.

Figure 14

Schematic of Cl-doped ZnO PENGs and its output voltage and the current of the undoped and Cl doped ZnO. Reproduced with permission from reference [63].

Figure 15

Power density vs. aspect ratio based on several techniques [34,43,44,45,48,58].