Reaction Pathway of Photocatalytic Water Splitting on Two-Dimensional TiO₂ Nanosheets

Using density functional theory and many-body Green’s function theory, we examine the mechanism of the water splitting reaction photocatalyzed by two-dimensional TiO₂ nanosheets, which has gained attention recently in the TiO₂ community. We reveal the barriers and the hole-transfer pathway in the reaction. Compared with its bulk isomer phases, such as the rutile TiO₂, the advantage of the TiO₂ nanosheet in this reaction is that the key reaction intermediate Ti-OH has good hole-trapping capacity, which is extremely beneficial for the oxidation half-reaction. However, the poor stability of Ti-OH relative to its precursor is a negative factor for the reaction on the TiO₂ nanosheet and might have a huge effect on the photocatalytic efficiency of the nanosheet. We find that introducing Ti vacancies into the nanosheet to convert the TiO₂ material into a p-type semiconductor can make bridging oxygen atoms become favorable active sites for water splitting and hence may be an effective way to increase the photocatalytic efficiency.