Oxide semiconductor is one of the most promising candidates for photocatalysts due to its light absorption ability, electronic properties, and stability. It is used in various applications such as solar-energy conversion, CO2 reduction, and water splitting. In this research, ZnO, TiO2, and ZnO/TiO2 heterostructured thin films are fabricated via atomic layer deposition (ALD), and their photocatalytic performances are evaluated. The film thickness can be controlled using ALD, and surface reactions can easily occur in thin films owing to the short distances between the active sites and charge carriers. In addition, unlike a powder catalyst, the bilayer photocatalyst is fixed in the solution; therefore, it does not make the solution turbid or disturb the light penetration. Diethylzinc and titanium tetraisopropoxide are used as precursors for Zn and Ti, and the thin films are deposited on soda-lime glass substrates at 150 °C using H2O as the reactant gas. The photocatalytic activity and stability are evaluated through photodegradation tests using methylene blue aqueous solution. The ZnO single-substance thin film exhibits a high degradation rate, but its performance significantly decreases after three consecutive experiments. The TiO2 single-substance thin film exhibits a relatively low degradation rate, but high reusability, exhibiting characteristics opposite to that of ZnO. Therefore, a TiO2 thin film is coated on ZnO to leverage both these advantages. The thin films are heat-treated at 400 °C for 10 min after deposition in a vacuum atmosphere. The surface morphology, crystal structure, and electrical characteristics of the photocatalyst specimens are analyzed through high-resolution scanning electron microscopy, Cs-corrected scanning transmission electron microscopy, and x-ray diffraction analysis. Their photocatalytic performances under ultraviolet (UV) irradiation are measured through UV–visible spectroscopy. The heat-treated ZnO/TiO2 heterostructured thin film exhibits a photodegradation rate exceeding 80%, with little degeneration after three cycles, indicating enhanced photodegradation performance and stability.

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