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.