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

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.