Optical, Structural and Ethanol Sensing Properties of Spin-Coated ZnO Film

Abstract

Among the various nanomaterials, Zinc Oxide (ZnO) has recently attracted the attention of researchers due to its potential application in various fields such as solar cells, bio-sensors, optoelectronic devices, gas sensors, water purification, piezoelectric devices, and liquid crystal displays. The accurate knowledge of the optical and structural properties of ZnO film is important for the fabrication of high-quality devices. In this work, 0.2M ZnO thin film was prepared by the economic spin coating technique. The Swanepoel method was employed to determine the average thickness and refractive index of the film with high accuracy in the spectral region of 200-1000 nm. The transmittance spectra were utilized to determine the absorption coefficient and extinction coefficients. The bandgap (Eg) was determined using Tauc’s formula and was found to be 3.22 eV. The real and imaginary parts of the dielectric decrease sharply with the wavelength. The single oscillator model was employed to discuss the dispersion parameters. The dispersion energy (Ed) and single-oscillator energy (Eo) were found to be 7.862 eV and 6.863 eV respectively with Eo≈ 2Eg proving the validity of the Swanepoel method for ZnO film. Structural analysis revealed that the film was polycrystalline in nature with a hexagonal wurtzite structure and an average crystallite size of ~31 nm with a Zn–O bond length of 1.9435 Å. The gas sensing properties in terms of the response of the ZnO sensor towards ethanol vapour were measured in the temperature range of 100–330 °C using DC electrical resistance. The ZnO film showed the maximum response of ~7 at temperature 260 °C for 800 ppm ethanol vapour exposure which may be due to the higher reaction rate at that temperature. The response of the sensor was increased on the exposure to a higher concentration of ethanol vapour. The sample showed a faster response on exposure to higher concentrations (400-800 ppm) of ethanol with a response time of ~13 s and a good response of 3.75 for 40 ppm of ethanol vapour exposure at 260 oC.