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

Abstract

Photogenerated charge carrier dynamics near molecule/TiO 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 CHOH/rutile TiO(110) interface. We studied the forward and reverse hole transfer between TiO and CHOH as well as the hole energy relaxation to the valence band maximum. First, we show that the hole-trapping ability of CHOH depends strongly on the adsorption structure. Only when the CHOH is deprotonated to form chemisorbed CHO 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/TiO 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.

Ab initio nonadiabatic molecular dynamics investigations on the excited carriers in condensed matter systems

“…Since both the hole transfer and relaxation dynamics at CH 3 OH/TiO 2 interface are affected by the e–p interaction, it is interesting to investigate the temperature dependence. As shown by Chu et al, decreasing the temperature from 100 to 30 K suppress the forward hole transfer and the hole relaxation process. Recently, both the forward and reverse hole transfer were measured in CH 3 OH/C 3 N 4 system.…”

“…Due to the e–p interaction, the HOMO state crosses TiO 2 valance band (VB) states frequently during a MD run, which lead to sufficient AD charge transfer. As discussed in Reference , the CO stretching mode at 900 cm −1 plays an important role in the reverse hole transfer.…”

“…Such adsorption‐induced acceleration can attribute to the e–p coupling of the CH 3 OH HOMO, which shows strong time‐dependent energy fluctuations as shown in Reference . Level crossings, which are induced by such fluctuations, favor the AD charge transfer and as a result additional energy relaxation channels are opened.…”

“…The reverse hole transfer process, which determines the lifetime of trapped holes, was also discussed by Chu et al The initial photogenerated hole was generated on HOMOs of CH 3 OH molecules. However, because of the strong hybridization between CH 3 OH and TiO 2 , the initial hole state has substantial contribution from TiO 2 surface.…”

“…(a) Averaged forward hole transfer from TiO 2 bulk states to adsorbed molecules at 100 K. (b) Averaged reverse hole transfer from adsorbed molecules to TiO 2 bulk states at 100 K. (c) Time dependence of energy relaxation of photogenerated holes with different initial energies at 100 K. (Reprinted with permission from Reference . Copyright 2016 American Chemical Society)…”