Ultrafast Dynamics of Photongenerated Holes at a CH₃OH/TiO₂ Rutile Interface

Photogenerated charge carrier dynamics near molecule/TiO₂ interfaces are important for the photocatalytic and photovoltaic processes. To understand this fundamental aspect, we performed a time-domain ab initio nonadiabatic molecular dynamics study of the photogenerated hole dynamics at the CH₃OH/rutile TiO₂(110) interface. We studied the forward and reverse hole transfer between TiO₂ and CH₃OH as well as the hole energy relaxation to the valence band maximum. First, we show that the hole-trapping ability of CH₃OH depends strongly on the adsorption structure. Only when the CH₃OH is deprotonated to form chemisorbed CH₃O will ∼15% of the hole be trapped by the molecule. Second, we find that strong fluctuations of the HOMO energies of the adsorbed molecules induced by electron-phonon coupling provide additional channels, which accelerate the hole energy relaxation. Third, we demonstrate that the charge transfer and energy relaxation processes depend significantly on temperature. When the temperature decreases from 100 to 30 K, the forward hole transfer and energy relaxation processes are strongly suppressed because of the reduction of phonon occupation. These results indicate that the molecule/TiO₂ energy level alignment, thermal excitation of a phonon, and electron-phonon coupling are the key factors that determine the photogenerated hole dynamics. Our studies provide valuable insights into the photogenerated charge and energy transfer dynamics at molecule/semiconductor interfaces.