(Invited, Digital Presentation) Properties of Defects That Determine the Charge Carrier Dynamics in Photocatalysts

Abstract

Defect engineering of photocatalysts has been intensively studied as a way to improve their activity. It is reported that introducing oxygen vacancies into TiO2 and SrTiO3 improve the activity of photocatalytic water splitting. On the other hand, the effects of oxygen vacancies on WO3 photocatalyst have not yet been elucidated sufficiently, though WO3 is one of the most appealing visible-light-responsive photocatalysts. Herein, we have investigated photocarrier dynamics in WO3 powder by using a transient absorption spectroscopy from mid-infrared to visible wavelength range. We found that electrons deeply trapped by defect states decayed within 1 ns, which is in contrast to the case of TiO2 and SrTiO3. The decay of the deeply trapped electrons is further accelerated by increasing oxygen vacancies through the reduction process. This result suggests that the deep trapping states associated with oxygen vacancies in WO3 work as a recombination center. On the other hand, long-life free/shallowly trapped electrons were increased when the WO3 powder was mildly reduced, whereas they decreased when it was reduced too heavily. These results indicate that shallow trapping sites in WO3 elongate photocarrier lifetime, but the effect of recombination acceleration by deep trapping sites becomes dominant when too much oxygen vacancies are introduced. Our study revealed the positive and negative aspects of defects in WO3, and highlighted the importance of appropriate defect control on improving the photocatalytic activity.