The Unique Polaron Behavior at Anatase TiO ₂ (101) Surfaces under Water Modification: A Constrained Density Functional Theory Study

Polarons in anatase TiO2 are critical to its photoelectric properties, yet the influence of aqueous environments on their behavior remains inconclusive. Here, we investigate the spatial distribution and transport of polarons on both perfect and reduced anatase (101) surfaces using constrained density functional theory (CDFT) and Marcus theory. On the pristine surface, we find that while a large polaron is favored in dry conditions, water adsorption stabilizes the formation of a small polaron with a trapping energy (Etrap) between −0.03 and −0.09 eV. Hydration also enhances the subsurface to surface transfer rate and reverses the transport mode from a two-step to a one-step process when water coverage (δ) exceeds 1/3. For the Ov-reduced surface, the most stable bipolaron on the dry surface has one electron localized on the surface and the second electron on the inner-surface. Increasing water coverage promotes the migration of the inner polaron to the surface, achieving complete surface localization at δ = 1 This process is accompanied by a modulation of the transfer rate and pathway, with an optimal rate at δ = 1/3. Mechanistically, polaron stability correlates with the splitting of the Ti-3d orbitals, while transport behavior is dictated by electronic coupling and transfer distance. These findings provide a detailed understanding of polaron dynamics at the anatase-water interface.