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

The detailed structure-activity relationship of surface hydroxyl groups (Ti-OH) and adsorbed water (HO) on the TiO 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 H solid-state NMR techniques were employed to identify surface hydroxyl groups and adsorbed water molecules as well as their spatial proximity/interaction in TiO photocatalysts. It was found that although the two different types of Ti-OH (bridging hydroxyl (OH) and terminal hydroxyl (OH) groups were present on the TiO surface, only the former is in close spatial proximity to adsorbed HO, forming hydrated OH. In situ H and C NMR studies of the photocatalytic reaction on TiO with different Ti-OH groups and different HO loadings illustrated that the enhanced activity was closely correlated to the amount of hydrated OH groups. To gain insight into the role of hydrated OH groups in the h transfer process, in situ ESR experiments were performed on TiO with variable HO loading, which revealed that the hydrated OH groups offer a channel for the transfer of photogenerated holes in the photocatalytic reaction, and the adsorbed HO could have a synergistic effect with the neighboring OH 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 TiO was proposed.