Photogenerated Charge Carriers Dynamics on La- and/or Cr-Doped SrTiO₃ Nanoparticles Studied by Transient Absorption Spectroscopy

Elemental doping into semiconductor-based photocatalysts to narrow the bandgap, thus extending the light absorption to visible light region, is a conventional and widely adopted strategy for water splitting (or artificial photosynthesis) application. However, one critical problem but rarely addressed, is the drastically decreased photocatalytic activity under UV light despite of the increased activity in visible light region, resulting in a decreased total performance under full solar spectrum. In order to elucidate the origin of the commonly observed phenomena, herein we choose La,Cr-codoped SrTiO 3 as a proof-of-concept model to probe the effect of dopants on the behavior of photogenerated charge carriers by transient absorption spectroscopy. By observing the excitons, it is found that under UV irradiation, the excitation occurring from the Cr impurity bands limits the excitation of SrTiO 3 from valence band, causing the drastic decrease of the photocatalytic activity over doped material. The widely proposed recombination at dopant sites, however, is not dominant for declining the UV light-irradiated performance, especially in the presence of reactant gas (methanol vapor).