Excitonic effects in the optical properties of hydrogenated anatase TiO2

Optical absorption spectra and excitonic properties of hydrogenated anatase titania have been calculated by ab initio calculations based on density functional theory, GW approximation, and the Bethe-Salpeter equation. Neutral hydrogen impurities lead to enhanced absorption in the infrared and visible regions, giving spectra compatible with experimental observations. Beside a broad signal in the infrared part, a sharp peak appears in the visible region that is due to excitonic effects. The wave functions of excitons in the infrared and in the visible are very localized, while those in the UV region are more delocalized than their counterparts in pure titania. It is argued that this could lead to a decreased electron-hole recombination of excitons in the UV, possibly leading to an increase in the photocatalytic activity of the material. On the contrary, positively charged hydrogen impurities barely modify the absorption spectrum, are probably difficult to be detected by optical absorption, and are unlikely to have a big effect on the photocatalytic properties of titania. These results contribute to understand the experimentally observed variability in the optical properties of hydrogenated titania and to obtain insight into their relation with the photocatalytic activity.