H2 is one of the cleanest energy carriers that could be obtained from renewable water and solar light from photocatalytic water splitting. In the past decades, scientists have developed a few semiconductor photocatalysts that exhibit high photocatalytic activities for hydrogen generation from water splitting. However, most of these semiconductors have large band gaps and are only active under UV light (<400 nm) that comprises just 3% of the solar spectrum. Doping with various impurities is one of the most widely used methods to extend their optical absorption to the visible spectral region.
Doping with impurities enable absorption of visible light, while the attendant mid-gap states may also act as charge carrier recombination centers and reduce the photocatalytic efficiency. How to tune the band structure of wide-gap semiconductors to eliminate these mid-gap states? Professor Kechen Wu and Dr. Zuju Ma from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, has resolved this problem by performing a first-principles calculation on N-doped La2Ti2O7. They found that the proper combination of N anions and O vacancies can tune the band structure of wide-gap photocatalysts in a positive way to avoid mid-gap states. In addition, the interstitial N atoms are suggested to be adverse to achieve high visible-light photocatalytic activity due to the formation of unfavorable defect states deep in the gap.
Zuju Ma, Kechen Wu, Rongjian Sa, Qiaohong Li, Chao He, Zhiguo Yi. International Journal of Hydrogen Energy, Volume 40, Issue 2, 2015, Pages 980-989.
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China.
Recently, a wide-band-gap layered perovskite compound La2Ti2O7 (LTO) has been reported to show a significant enhancement of both the visible and UV light photocatalytic activities after doping with N atoms. It is known that the doping sites often act as recombination centers of the photo-exited carriers and block the redox reactions. In this work, we investigated the origin of the enhancement in photocatalytic activity of N-doped LTO by using density functional theory (DFT) calculations. More than 40 models were constructed by considering ionic state of dopants, distance between dopants, doping concentrations, and formation of oxygen vacancy. The structure-properties relation of N-doped LTO was established. We found that the model of Sub3N–3Odv (with three dispersed substitutional N atoms at O sites and one oxygen vacancy in 87-atom LTO supercell) well explains both the shift-up of the valence bands and the narrowed band gap observed in experiment. The obtained band gap of 2.46 eV agrees well with the experimental value of 2.51 eV. For the models with interstitial N atoms, the impurity states are mainly localized at the higher-energy region of the band gap, which may trap the photo-excited carriers and decrease the photocatalytic activity. The work provides a potential implication for effective band-gap narrowing of wide-gap photocatalysts.