Photocatalysis at the Molecule/Metal Oxide Interface Is Driven by Asymmetric Photocarrier Transfer: Ab Initio Quantum Dynamics Simulation

The
photocatalytic efficiency of a molecule/metal oxide interface
critically depends on the dynamic competition between forward carrier
transfer to the molecule and reverse transfer to the substrate. Using
CH3O–/TiO2 as a prototypical
system and employing nonadiabatic molecular dynamics simulations with
hybrid density functional theory, we reveal the complete cascade of
atomistic processes following photoexcitation. Our results demonstrate
that accurately predicting carrier dynamics in photocatalysis requires
a comprehensive understanding of the full photochemical sequence involves
carrier trapping, post-trapping stabilization, and subsequent dissociation.
In the trapping process, despite an unfavorable 0.55 eV HOMO-VBM offset
under static conditions that would suggest minimal hole trapping,
thermal fluctuations dynamically modulate adsorbate–substrate
hybridization, enabling transient photogenerated hole capture. Critically,
when trapped hole stabilization occurs via CH3O· radical formation, the reverse transfer time scale dramatically
extends from sub-10 fs to sub-10 ps, a rise of 3 orders of magnitude.
This radical state elevates the trap-state energy, suppressing reverse
transfer and extending carrier lifetimes sufficiently to enable subsequent
chemistry. The metastable radical further reduces the C–H dissociation
barrier, driving spontaneous photodecomposition via proton-coupled
charge transfer. Our findings reconcile long-standing theory-experiment
discrepancies by demonstrating that a dual energy alignment framework
in the molecule/metal oxide interface: initial charge capture requires
pretrapping energy matching, while post-trapping stabilization via
chemical intermediates creates an energetic asymmetry that effectively
suppresses carrier dissipation. This mechanistic understanding of
interfacial charge dynamics provides fundamental design principles
for the rational development of high-efficiency photocatalytic systems.