Investigating the properties of nickel oxide thin films prepared via DC reactive magnetron sputtering for potential application in gas sensing

Abstract

Nanocrystalline nickel oxide (NiO) thin films were developed on Si substrate using DC reactive magnetron sputtering. The goal of our study was to investigate the impact of O2/Ar gas ratios (varied from 10% to 20%) on the properties of sputtered NiO thin films. Further, after the deposition, all the samples were annealed at 400 ℃ in air. The prepared thin films were characterized by GIXRD, UV–visible spectroscopy, and FESEM to investigate the structural, optical, and morphological properties, respectively. The GIXRD results demonstrated the formation of the polycrystalline nature of sputtered NiO films with the FCC phase. As the O2 ratio was increased, the preferred orientation of NiO thin film was found to change from (111) to (200) plane with an average crystallite size of ∼ 14 nm. The UV–visible spectroscopy measurements show the semi-transparent nature of the deposited NiO films with a decreasing trend in the transmittance value from ∼ 83% to ∼ 77% with an increase in the O2 to Ar gas ratio. This behavior could be due to the poor crystallization of the film. However, the change in the calculated band gap of NiO films with respect to O2 partial pressure was found to be negligible and lay between 3.82 eV and 3.84 eV. The surface morphology was studied using FESEM, which depicted the uniform and homogenous distribution of grains with cracks on the film that improve the gas permeability; hence it may prove beneficial for gas sensing applications. Besides, the observed contact angle, as measured by the wettability test for the NiO films at different oxygen partial pressure, was greater than 90°, which shows that all the deposited NiO thin films are hydrophobic. Additionally, we have tested the CO gas sensor responses of the NiO sample deposited at a 15% O2/Ar ratio due to the high hydrophobic nature among all three ratios.