Behaviors and Energy States of Photogenerated Charge Carriers on Pt or CoOx Loaded LaTiO2n Photocatalysts Studied By Time-Resolved Absorption Spectroscopy

For the development of highly efficient photocatalysts, energy states as well as the behaviors of photogenerated charge carriers should be elucidated. Femtosecond to second time-resolved visible to mid-IR absorption spectroscopy is useful to investigate the matters, therefore we applied this method to study the role of cocatalysts loaded on visible-light responsible LaTiO₂N photocatalyst: this catalyst has poor activity for H₂ evolution but highest activity for O₂ evolution when CoO_x was loaded. Time-resolved absorption study revealed that most of the electrons are trapped in the mid-gap states, created below 0.74 eV (6000 cm^-1) from the conduction band. As a result, the number of free electrons giving the transient absorption below 2000 cm^-1 was much smaller than that of the trapped electrons. When Pt was loaded, the electrons were captured by Pt and the lifetime of the holes (which give the absorption band at 17000 cm^-1) became longer, however these processes proceeded at tens of microseconds and the efficiency was not high enough. On the contrary, the CoO_x-loading resulted in the capture of holes within a few ps, and drastically elongated the lifetime of electrons to second region. Additionally, the depth of the electron trap became shallower from 0.74 eV (6000 cm^-1) to 0.49 eV (4000 cm^-1), suggesting the reactivity of the trapped electrons becomes higher. These perturbations to the electrons and holes are responsible for the increase of the photocatalytic activities. We expected that co-loading of Pt and CoO_x further increased the activities but the recombination was accelerated.