Effect of Na-Doping on Electron Decay Kinetics in SrTiO3 Photocatalyst

Photocatalytic water splitting by solar light is an environment-friendly means for generating hydrogen as energy resources. For practical use, photocatalysts with higher activity are desired. Recently it was found that the photocatalytic activity of SrTiO3 is remarkably improved by Na-doping. However, why Na-doping enhances the activity has not been well understood. In this work, we found that Na-doping in SrTiO3 introduces new mid-gap states. Transient absorption measurements revealed that photoexcited electrons were trapped into these states within similar to 20 ps after excitation. These trapped electrons have longer lifetime than those in undoped SrTiO3, and number of surviving electrons in microseconds increased similar to 15 times. These electrons are trapped at the mid-gap states, but still keep reactivity with reactant molecules. Furthermore, they were effectively captured by H-2-evolution cocatalyst, indicating that they can participate in steady-state reactions. This work emphasizes the important role of electron-trapping states introduced by doping on photocatalytic activity.