Long-Life Electrons in Metal-Doped Alkali-Metal Tantalate Photocatalysts Excited under Water

Conversion of materials for artificial photosynthesis is completed in milliseconds or seconds by assembling atoms over semiconductor photocatalysts. Band-gap-excited electrons and holes reactive on this time scale are key for efficient atom assembly to yield the desired products. In this study, attenuated total reflection of infrared (IR) light was applied to characterize the electronic absorption of long-life charge carriers excited under water. Under excitation, NaTaO₃ and KTaO₃ photocatalyst particles doped with Sr or La cations absorbed IR light. A broad absorption band appeared with a maximum at 1400 cm^–1, which was enhanced by the addition of hole scavengers (e.g., methanol and Na₂SO₃) and disappeared in the presence of electron scavengers (e.g., FeCl₃, NaIO₃, and H₂O₂). This absorption corresponded to the electronic transition of band-gap-excited electrons accommodated in mid-gap states. In anaerobic n-decane, the electron absorption was enhanced by the excitation light power, P, with absorbance being proportional to P^1/2. The observed 1/2-order power law suggested deexcitation via recombination of electrons and holes. When the excitation light was stopped, the absorbance decreased as a function of time with a second-order rate law, as expected in the case of recombinative deexcitation. In addition, the 1/2-order power law and second-order decay rate law were observed in anaerobic water, with an accelerated decay rate, which was possibly due to a water-related electron-consuming reaction. This study demonstrated that long-life electrons contribute to surface redox reactions over semiconductor photocatalysts for artificial photosynthesis.