Transfer Channel of Photoinduced Holes on a TiO₂ Surface As Revealed by Solid-State Nuclear Magnetic Resonance and Electron Spin Resonance Spectroscopy

Abstract

The detailed structure-activity relationship of surface hydroxyl groups (Ti-OH) and adsorbed water (H₂O) on the TiO₂ surface should be the key to clarifying the photogenerated hole (h⁺) transfer mechanism for photocatalytic water splitting, which however is still not well understood. Herein, one- and two-dimensional ¹H solid-state NMR techniques were employed to identify surface hydroxyl groups and adsorbed water molecules as well as their spatial proximity/interaction in TiO₂ photocatalysts. It was found that although the two different types of Ti-OH (bridging hydroxyl (OH_B) and terminal hydroxyl (OH_T) groups were present on the TiO₂ surface, only the former is in close spatial proximity to adsorbed H₂O, forming hydrated OH_B. In situ ¹H and ¹³C NMR studies of the photocatalytic reaction on TiO₂ with different Ti-OH groups and different H₂O loadings illustrated that the enhanced activity was closely correlated to the amount of hydrated OH_B groups. To gain insight into the role of hydrated OH_B groups in the h⁺ transfer process, in situ ESR experiments were performed on TiO₂ with variable H₂O loading, which revealed that the hydrated OH_B groups offer a channel for the transfer of photogenerated holes in the photocatalytic reaction, and the adsorbed H₂O could have a synergistic effect with the neighboring OH_B group to facilitate the formation and evolution of active paramagnetic intermediates. On the basis of experimental observations, the detailed photocatalytic mechanism of water splitting on the surface of TiO₂ was proposed.