Evaluation of the self-cleaning ability of MoO3 thin films prepared by radio frequency magnetron sputtering using a quartz crystal microbalance technique

Abstract

This study investigated the self-cleaning ability of MoO${}_3$ thin films fabricated by radio frequency magnetron sputtering. With an increase in annealing temperature, the crystal structure of MoO${}_3$ transitioned from an amorphous phase to a $\beta$ phase and then to an $\alpha$ phase. XPS measurements clarified that the atomic ratio exceeded 3.0, indicating that the sample contained excess oxygen. Their self-cleaning abilities were evaluated by measuring the mass reduction of citric acid under UV irradiation using a quartz crystal microbalance. Samples annealed at 400 °C exhibited the highest self-cleaning ability, which was approximately six times greater than that of a commonly used photocatalyst, TiO${}_2$. After a decomposition experiment, reflectance spectroscopy revealed that MoO${}_3$ changed to a darker color, indicating a photochromic reaction. These results suggest that the combination of the photochromic reaction and high oxidation power of MoO${}_3$ contributes to its higher self-cleaning ability compared to that of TiO${}_2$.