Revisiting the mechanism of photocatalytic activities in N-doped TiO$_{2}$

Photocatalysis possesses a great potential for environmental remediation and fuel production [1]. Nitrogen doped TiO$_{2}$ is a well-known visible-light sensitive photocatalyst where deep impurity states associated with substitutional nitrogen at oxygen sites (N$_{O})$ are believed to be the source of the red shift in photo-absorption edge. However, such a deep level should trap hole carriers, degrading oxidation process. The contradiction between the deep N$_{O}$ level and rather a high oxidation power of N-doped TiO$_{2}$ has been an unsolved puzzle. Here, we propose a convincing mechanism which successfully solves the riddle. N$_{O}$ strongly binds with a titanium atom at an interstitial site, forming a defect-impurity band, which consists of bonding and anti-bonding states of nitrogen $p$ and titanium $d$ and narrows the band gap. Such a newly formed band, which is connected to the valence band maximum of the host TiO$_{2}$, becomes the migration path of photo-induced hole carriers, assisting carrier transfer to the surface. This clearly explains the photocatalytic activity of N-doped TiO$_{2}$ both for the visible-light absorption and the oxidation reaction. [1] Hua Tong, Shuxin Ouyang, Yingpu Bi, Naoto Umezawa, Mitsutake Oshikiri, and Jinhua Ye, Advanced Materials DOI: 10.1002/adma.20110275