For the application of ZnO nanoparticles to water disinfection, immobilization of the ZnO nanoparticles on reactor surfaces is required; otherwise, the nanoparticles aggregate and leave the reactor over time. To overcome these limitations, we formed thin ZnO films on a glass substrate using atomic layer deposition (ALD) (n = 400 at 140 degrees C). An X-ray diffraction pattern and scanning electron microscope (SEM) images demonstrated that the thin ZnO film consisted of closely packed nano-sized grains. The grains had hexagonal wurtzite crystalline structures, which were mostly oriented vertically from the substrate. The thin ZnO film did not show a change in the optical properties of ZnO: absorbance at <380 nm and band gap of 3.25 eV. Furthermore, the thin ZnO films produced reactive oxygen species (ROS) such as superoxide anion, hydroxyl radical, and singlet oxygen, by irradiation with UV-A light (350-400 nm), and the production of ROS was not diminished even after repeated use. Antibacterial activity against Staphylococcus aureus was observed when the ZnO films were placed under UV-A light. However, this was not evident when the ZnO films were in the dark or when the glass substrate without the ZnO film was exposed to UV-A light, suggesting that the antibacterial activity was due to ROS production by the ZnO film under UV-A light. Moreover, the bacterial cells exposed to the ZnO film under UV-A light showed a crushed morphology, indicating that the ROS facilitated the disruption of the bacterial cell membranes. Taken together, this study demonstrates that ALD firmly anchored the ZnO nanocrystalline structures on the substrate without hampering the ROS production or antibacterial activity for the purpose of water disinfection. (C) 2017 Elsevier B.V. All rights reserved.