Stability of TiO₂-coated ZnO photocatalytic thin films for photodegradation of methylene blue

Investigations on the stability of titanium dioxide (⁠$TiO2$⁠)-coated zinc oxide (ZnO) thin films upon repeated uses for methylene blue (MB) degradation were conducted. Photocorrosion of ZnO, upon exposure to light in aqueous media, can affect the photocatalytic performance due to loss of material. Hence, coating with a more stable metal oxide was seen as a way to suppress the effects of photocorrosion. In this study, homogeneous wurtzite ZnO nanostructured thin films were obtained from thermal oxidation of sputter-deposited Zn films on glass substrates. $TiO2$ was subsequently deposited onto the ZnO nanostructured thin films using a reactive magnetron sputtering system in an admixture of argon and oxygen gases. After deposition, the thin films were annealed at 500 $°$C for 1 h. The photocatalytic efficiency and stability of the thin films were investigated after multiple degradation cycles. The addition of a $TiO2$ film increased the surface roughness and blueshifted the absorption edge of the ZnO thin films. The coated films obtained up to 94.3% degradation efficiency of MB after a 180-min exposure cycle using a solar light simulator. After three cycles, degradation efficiency decreased for the uncoated ZnO photocatalysts. Analysis of the MB solution after one degradation cycle revealed the presence of Zn$2+$ ions attributed to the effects of photocorrosion. Higher Zn$2+$ concentrations were observed when the ZnO surface is uncoated. This study showed that the addition of a thin, antiphotocorrosion material such as $TiO2$ layer decreased the dissolution of ZnO caused by photocorrosion without a significant reduction in the photodegradation efficiency.