Behavior and Energy State of Photogenerated Charge Carriers in Single-Crystalline and Polycrystalline Powder SrTiO₃ Studied by Time-Resolved Absorption Spectroscopy in the Visible to Mid-Infrared Region

The effects of defects on the behavior of photogenerated charge carriers in SrTiO₃ (STO) are studied by time-resolved absorption spectroscopy from the visible to mid-IR region. In the case of defect-free single-crystalline STO, free and shallowly trapped electrons are dominant, but they recombine within 50 ns. By contrast, in the case of defect-rich powder STO, the electron lifetime is much longer than 1 ms. The transient absorption spectra show that most of the charge carriers in powder STO are trapped in the defects, which elongates their lifetime. We found that these trapped carriers are nevertheless reactive toward O₂ or CH₃OH that depends on the trap depth. The steady-state photocatalytic activity is strongly correlated with the lifetime and the reactivity of the trapped charge carriers: the energy state of electrons can be deduced from the spectral shape, especially in the mid-IR region

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

Behavior and Energy States of Photogenerated Charge Carriers on Pt- or CoO_<i>x</i>‑Loaded LaTiO₂N Photocatalysts: Time-Resolved Visible to Mid-Infrared Absorption Study

Femtosecond to second time-resolved visible to mid-infrared absorption spectroscopy was applied to investigate the behavior of photogenerated electrons and holes on a Pt- or CoO_<i>x</i>-loaded LaTiO₂N photocatalyst. CoO_<i>x</i>-loaded catalyst exhibits the highest activity for water oxidation under visible light (<600 nm) excitation, and the quantum efficiency reaches up to ∼30%. Transient absorption spectra suggest that most of the photoexcited electrons in LaTiO₂N lose activity by deep trapping in the mid-gap states created at 0.74 eV (6000 cm^–1) below the conduction band. In this case, Pt loading was not so effective for H₂ evolution because the loaded Pt could not effectively capture the trapped electrons from LaTiO₂N. The electron transfer was slow, proceeding in 0–100 μs, and was thus ineffective. However, in the case of CoO_<i>x</i> loading, we have clearly observed, for the first time, that the holes are captured rapidly by CoO_<i>x</i> in a few picoseconds, and the lifetimes of electrons are dramatically prolonged to the second region. This implies that the photogenerated holes and electrons are separated effectively in CoO_<i>x</i> and LaTiO₂N, respectively. Furthermore, the electron trap becomes shallower, its depth decreasing from 0.74 eV (6000 cm^–1) to 0.49 eV (4000 cm^–1) upon CoO_<i>x</i> loading, suggesting that the reactivity of the trapped electrons increases. These perturbations of electrons and holes are what cause the dramatic increase in photocatalytic activity. We expected that coloading of Pt and CoO_<i>x</i> would further increase the activity, but it was significantly reduced. It was demonstrated that the undesirable process of recombination is accelerated under high loading and coloading