Computational Study of ZnO Surface Catalysis: Adsorption of H₂O or/and O₂ as a Pathway to ROS Formation

Reactive oxygen species (ROS) play a central role in photocatalytic processes relevant to environmental remediation and clean energy. This work focused on the computational investigation of ZnO surface reactivity toward H₂O and O₂ adsorption, as a preliminary step in understanding ROS generation pathways. Surface stability and adsorption energies for isolated and co-adsorbed H₂O and O₂ molecules on different ZnO surfaces (both in their pristine form and with oxygen vacancies) were evaluated using DFT calculations at the PBE-D3 level under various surface coverages. The introduction of vacancies on the pristine (001) and (100) surfaces enhanced O₂ binding, particularly in inclined configurations at the defect sites, with the adsorption energies reaching -2.63 eV and -2.04 eV, respectively. However, the (110) surface showed very strong H₂O binding, but weak O₂ adsorption, which only modestly improved with vacancies. Co-adsorption of H₂O and O₂ exhibited synergistic stabilization, especially on the (001) and (100) surfaces, where ROS were formed through proton transfers either between adsorbed H₂O and O₂ or between H₂O and surface oxygen atoms. These findings provide detailed insight into the mechanistic role of surface defects in ROS generation and support the rational design of ZnO-based photocatalysts.