Study of excited states and electron transfer of semiconductor-metal-complex hybrid photocatalysts for CO2 reduction using picosecond time-resolved spectroscopies.

We have previously reported on a semiconductor-metal-complex hybrid photocatalyst for CO2 reduction composed of nitrogen-doped Ta2O5 as a semiconductor photosensitizer and Ru complex as a CO2 reduction catalyst that operates under visible light (> 400 nm) with high selectivity for HCOOH formation of more than 75 %. The electron transfer from a photoactive semiconductor to the metal-complex catalyst is a key process for photocatalytic CO2 reduction with hybrid photocatalysts. To understand the mechanism of electron transfer from a semiconductor to the metal-complex catalyst, here we describe the excited-state dynamics of several hybrid photocatalysts using time-resolved emission and infrared absorption spectroscopies. Our results show that electron transfer from the semiconductor to the metal-complex catalyst does not directly occur upon photoexcitation, but that the photoexcited electron transfers to a new excited state. Based on the present results and previous reports, we suggest that the excited state is a charge transfer state located between shallow defects of the semiconductor and the metal-complex catalyst.