A Comparative Study of N-Type ZnO and P-Type NiO Nanostructures for the Detection of Aqueous Ethanol Solution

Abstract

N-type ZnO nanorod arrays and p-type NiO thin films have been prepared on the ITO substrates by chemical bath deposition (CBD) and sol-gel spin coating method, respectively, then followed by thermal treatment at 400 °C for 1 h. The ZnO nanorod arrays have length of around 1.5 μm and diameter of around 55 nm; the thickness of the NiO film is around 340 nm with average grain size of 22 nm. The sensing properties of the n-type ZnO and the p-type NiO for the detection of aqueous ethanol solutions at room temperature are investigated using electrical impedance spectroscopy (EIS). The EIS results reveal that the series resistance (Rs) is as low as 27 Ω; the charge-transfer resistance (Rct) remarkably contributes to the sensing properties of the ethanol sensor, and decreases with a decreasing concentration (99.5%-1%) from 17.0 kΩ to 2.6 kΩ. An equivalent circuit with capacitors and resistors was developed to investigate the conduction process according to complex impedance (Nyquist) diagrams. In low concentration range, the electron conduction process is dominated by the Rct and a constant phase element (CPE); however, in high concentration ethanol solution, the conduction process is dominated by polarization and decomposition of the absorbed water with larger Rct observed. Moreover, I-V measurements were carried out to evaluate the sensitivity of n-ZnO and p-NiO nanostructures at room temperature.