Computational Simulation of Trapped Charge Carriers in TiO₂ and Their Impacts on Photocatalytic Water Splitting

Titania dioxide (TiO₂) stands out as one of the most promising photocatalytic materials in the fields related to energy and environment. Despite extensive experimental studies on TiO₂-based systems in photocatalysis, knowledge at the atomic scale on the physical characteristics and catalytic impacts of trapped photo-charges, catalytic mechanisms and kinetics of photoreactions, remains largely elusive. Density functional theory calculations, either with proper on-site Coulomb interaction correction (DFT+U) or via hybrid functional methods (e.g. HSE06), have been widely conducted to elucidate the above scientific issues. In this chapter, we present calculated results regarding the geometric and electronic features of radical species, the impacts of trapped electrons or holes on surface catalytic events, the electron transfer across the metal/TiO₂ interfaces, the size-dependent activity of platinum cocatalysts, and complete pathways and kinetic trends for photocatalytic oxygen evolution (OER). We show that surface oxygens with low coordination, such as the hydroxyl group and bridge oxygen, are generally efficient hole traps, which however become less effective in trapping holes in the presence of liquid water owing to the stabilization via H-bonds. A novel proton promoted electron transfer mechanism and the optimal morphology of supported platinum nanoparticles in photocatalytic hydrogen production were evidenced from our results, rationalizing many intriguing experimental observations. The key obstacle that limits OER efficiency was identified as the low concentration of surface-reaching holes rather than the inadequate catalytic activity of TiO₂. These results may provide a new scenario for understanding the complicated photochemistry and assisting the photocatalyst design toward better water splitting efficiency.