Photocatalyzed oxidation of water on oxygen pretreated rutile TiO2(110)

Abstract

The oxygen evolution reaction (OER) is the bottleneck in the overall photocatalytic splitting of water. The active sites (terminal titanium or bridging oxygen) and active species (molecular or dissociative water) of the initial step of the photocatalyzed OER on the prototypical photocatalyst TiO₂, remain debatable. Herein, the photocatalytic chemistry of monolayer water on oxygen-pretreated TiO₂(110) (o-TiO₂(110)) and reduced TiO₂(110) (r-TiO₂(110)) surfaces initiated by 400 nm light illumination was investigated by time-dependent two-photon photoemission spectroscopy (TD-2PPE). The photoinduced reduction of the H₂O/o-TiO₂(110) interface rather than the H₂O/r-TiO₂(110) interface was detected by TD-2PPE. The difference in 2PPE originated from the presence of the terminal hydroxyl anions (OH_t¯) on H₂O/o-TiO₂(110), as identified by X-ray photoelectron spectroscopy and temperature-programmed desorption. Therefore, the evolution of the electronic structure of H₂O/o-TiO₂(110) was attributed to the photocatalyzed oxidation of the terminal hydroxyl anions, which most likely formed gaseous ^•OH radicals, reducing the interface. This work suggested that the oxidation of hydroxyl anions on top of the terminal titanium ions on TiO₂, which were excluded previously in solution, need to be considered in the mechanistic studies of the photocatalyzed OER.