Effects of intrinsic defects and extrinsic doping on the electronic and photocatalytic properties of Ta3N5

Ta3N5 is a good candidate for an oxygen evolution photocatalyst or a photoanode for a Z-scheme device due to its n-type feature. In the present work, the formation energies and electronic structures of defects contained in Ta3N5 are studied by first principles density functional theory in detail. Our results show that the substitution of O for three-coordinated N in Ta3N5 possesses a low formation energy and introduces a shallow donor under both N-rich and N-poor conditions, making a major contribution to the n-type conductivity. By investigation of the optical transition levels, we show that the four-coordinated N vacancy in Ta3N5 is responsible for the observed 720 nm sub-band gap optical absorption. In addition, for alkali metal doped Ta3N5, our results reveal that the interstitial doping can lead to enhanced conductivity and reduced band gap, and the doping of Na and K in Ta3N5 are expected to produce higher photocatalytic activity compared to Rb and Cs. These results are useful to understand the recent experimental observations and provide guidance to engineer Ta3N5 with improved photocatalytic efficiency.