Computational studies of Ag₅ atomic quantum clusters deposited on anatase and rutile TiO₂ surfaces

Aiming at boosting the photocatalytic activities of rutile and anatase TiO₂ surfaces, an in-depth investigation of stoichiometric and reduced rutile TiO₂ (1 1 0) and anatase TiO₂ (1 0 1) decorated with bipyramidal and trapezoidal Ag₅ atomic quantum clusters (AQCs) is carried out. In this study, density functional theory (DFT) plus a Hubbard correction (U) is implemented to explore the geometric and electronic properties of such systems. It is found that the silver AQCs donate electrons to both stoichiometric and reduced TiO₂ surfaces resulting in the formation of a single polaron at either a fivefold coordinated (Ti_5c) atom or a sixfold coordinated (Ti_6c) atom, indicating improved surface activity. Depositing Ag₅ AQCs on both TiO₂ surfaces can produce mid-gap states within the band gap of the bulk, thereby improving the optical response of the composite in the visible and infrared. As expected, the number of mid-gap energy states increases further by introducing a single oxygen vacancy into the studied surfaces, which means that Ag₅ AQCs and oxygen vacancies can reinforce each other, leading to higher efficient photocatalytic activity. We also find that upon adsorption of Ag₅ AQCs on an anatase TiO₂ (1 0 1) surface, the energy required to form an oxygen vacancy is lower than that of rutile TiO₂ (1 1 0). Moreover, the adsorption of both bipyramidal and trapezoidal Ag₅ AQCs on both TiO₂ surfaces generally leads to significant distortion of the clusters, which accounts for the significant reduction in the total energy as compared to the pristine TiO₂. This detailed investigation provides insight into new mechanisms for enhancing photocatalytic efficiency of both rutile TiO₂ (1 1 0) and anatase TiO₂ (1 0 1) surfaces.