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

Abstract

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 x -loaded LaTiO 2 N photocatalyst. CoO x -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 LaTiO 2 N 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 H 2 evolution because the loaded Pt could not effectively capture the trapped electrons from LaTiO 2 N. The electron transfer was slow, proceeding in 0–100 μs, and was thus ineffective. However, in the case of CoO x loading, we have clearly observed, for the first time, that the holes are captured rapidly by CoO x 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 x and LaTiO 2 N, 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 x 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 x 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.