Defect-Induced Acceleration and Deceleration of Photocarrier Recombination in SrTiO₃ Powders

Defects in powders play key roles in photocatalysis since they sometimes accelerate but other times prevent the recombination of photocarriers. However, how to differentiate the roles, as well as their actual effects, is still unclear. Here, we found that defects have dual properties: initially they accelerate recombination, but later they prevent it. These two contradictory properties are differentiated by whether electrons and holes are trapped in the vicinity or far away from each other. Just after photoirradiation, defects prevent carrier diffusion to keep them closer, resulting in accelerating recombination. However, once the carriers can escape from the initial trapping, they can spread and be trapped far away. These trapped carriers have to travel long distances by repeatedly hopping and tunneling; hence, the carrier lifetime becomes longer. The fate of carriers is determined by how they escape the initial trapping by using excess energy after photoexcitation; i.e., control of this process is the key to improving the photocatalytic activity.