*3.2. Non-Metal Doping*

Gopinath et al. [94] reported that the band gap of semiconductor oxide was reduced by doping with anions because of the broadening or the upward shifting of VB. Asahi et al. [95] studied the effects of C, N, F, P, or S doping on TiO2-based photocatalysts. Their results showed that N doping can decrease the band gap of the photocatalyst due to the energy state overlapping between the N 2p states and O 2p states. Such band gap narrowing was also found for S doped photocatalysts. Up to the present, only a few studies have demonstrated the use of anion doping to improve the visible light-induced hydrogen generation performance. As non-metal doping is more difficult to prepare by conventional chemical methods, it has received little attention. Tsuji et al. demonstrated the effectiveness of halogen codoped Pb-ZnS photocatalyst in photocatalytic activity. Although Pb-doped ZnS already has a high hydrogen evolution rate, codoping of halogen is still useful, for it facilitates the relaxation of the distortion produced by doping with large Pb ions. The photocatalytic activity of halogen and Pb codoped ZnS is three times higher than that of the Pb-doped ZnS photocatalyst [96].
