Dynamics of Photogenerated Charge Carriers on Ni- and Ta-Doped SrTiO₃ Photocatalysts Studied by Time-Resolved Absorption and Emission Spectroscopy

The behavior of photogenerated charge carriers on SrTiO₃ photocatalysts doped with transition metals (such as Ni and Ta) was examined by time-resolved visible to mid-IR absorption and emission spectroscopy. When SrTiO₃ was co-doped with Ni and Ta, the catalyst absorbed visible light and exhibited photocatalytic activity under visible light irradiation. However, activity under UV light was decreased significantly compared to that before doping. The results of time-resolved measurements showed that monodoping of Ni or Ta accelerated the recombination but co-doping Ni with Ta increased the lifetime of charge carriers compared to those without doping. Furthermore, electrons excited by a visible laser pulse had longer lifetimes compared to those excited by a UV laser pulse. Time-resolved photoluminescence measurements suggested that doped Ni cations act as recombination centers, giving a luminescence peak at ∼8000 cm^-1 due to the downward d-d transition at Ni²⁺. However, the lifetime of the emission was much shorter than that of free or shallowly trapped electrons. These results suggest that recombination at the Ni cations is not the dominant process. In addition, the reactivity of photogenerated electrons was decreased dramatically by doping; electrons did not react with exposed O₂, although holes maintained reactivity with MeOH. These results confirm that the decrease in the steady-state activity of doped SrTiO₃ under UV light irradiation is responsible for the decrease in reactivity of photogenerated electrons.