On the dependence of structural and sensing properties of sputtered MoO₃ thin films on argon gas flow

Highlights: • MoO₃ thin films are sputter coated and their structure are analyzed. • Effect of argon gas flow on the structural and some properties is studied. • CO sensing ability of MoO₃ increases with argon gas flow. • MoO₃ nano-strain decreases with argon gas flow. - Abstract: Nitrogen and carbon oxides (CO, NO and NO₂), released from combustion facilities and automobiles, are known to be extremely harmful to the human body and also are the main cause of air pollution. Therefore, effective methods to monitor and suppress the carbon and nitrogen oxides have been highly demanded for atmospheric environmental measurements and controls. It is known that molybdenum oxide (MoO₃) can be a good semiconductor material for use as a gas sensor in monitoring CO, NO and NO₂. In this paper we report the structural characteristics and sensing properties of the sputtered MoO₃ thin films as a function of argon gas flow. MoO₃ thin films were deposited by DC reactive magnetron sputtering technique on glass substrates at different argon gas flows in the range of 5–20 sccm. X-ray diffraction (XRD) analysis was used for studying crystallographic structure. XRD results showed that all of our films were of polycrystalline structure and of α-MoO₃ stable orthorhombic phase. Results also showed that crystallite size increases while compressive nano-strain in the structure of the films decreases with increasing the argon gas flow. Atomic force microscope and the field emission scanning electron microscope studies showed granular structures for all samples, which increased in size consistent with the XRD results, with argon gas flow, while the surface roughness of the films also increased with argon gas flow. Chemical composition study showed optimum reaction between oxygen and molybdenum atoms for films produced at 15 sccm flow of argon gas. The electrical response of samples was measured in the vacuum and the CO environments in the temperature range of 150–350 K. All samples showed Ohmic behavior and the electrical resistances of the films measured in the CO environment were lower than those measured in vacuum. This study showed that the sensing ability of MoO₃ for CO improves with increasing the argon gas flow