Observation of Photocatalytic Dissociation of Water on Terminal Ti Sites of TiO2(110)-1 x 1 Surface

The water splitting reaction based on the promising TiO2 photocatalyst is one of the fundamental processes that bears significant implication in hydrogen energy technology and has been extensively studied. However, a long-standing puzzling question in understanding the reaction sequence of the water splitting is whether the initial reaction step is a photocatalytic process and how it happens. Here, using the low temperature scanning tunneling microscopy (STM) performed at 80 K, we observed the 1 h dissociation of individually adsorbed water molecules at the 5-fold coordinated Ti (Ti-5c) sites of the reduced TiO2 (110)-1 X 1 surface under the irradiation of UV lights with the wavelength shorter than 400 nm, or to say its energy larger than the band gap of 3.1 eV for the rutile TiO2. This finding thus clearly suggests the involvement of a photocatalytic dissociation process that produces two kinds of hydroxyl species. One is always present at the adjacent bridging oxygen sites, that is, OHbr, and the other either occurs as OHt at Ti-5c sites away from the original ones or even desorbs from the surface. In comparison, the tip-induced dissociation of the water can only produce OHt or oxygen adatoms exactly at the original Ti-5c sites, without the trace of OHbr. Such a difference clearly indicates that the photocatalytic dissociation of the water undergoes a process that differs significantly from the attachment of electrons injected by the tip. Our results imply that the initial step of the water dissociation under the UV light irradiation may not be reduced by the electrons, but most likely oxidized by the holes generated by the photons.