GGA plus U Study on the Mechanism of Photodecomposition of Water Adsorbed on Rutile TiO2(110) Surface: Free vs Trapped Hole

The initial step of O-2 evolution reaction on a TiO2 surface is a long-standing puzzle. A recent scanning tunneling microscopy experiment showed that the H2O molecule adsorbed on rutile TiO2(110) surface could decompose under ultraviolet illumination (Tan, S. J.; et al. J. Am. Chem. Soc., 2012, 134, 9978). The underlying reaction mechanism is now examined by our GGA+U study, in which the oxidation of the H2O molecule by both free and trapped holes has been carefully investigated. It is found that the transfer of the hole trapped at the bridge oxygen to the molecule is hindered by the mismatch between the energy and spatial symmetry of the trapped hole orbital and the highest occupied molecule orbital of H2O. The entire oxidation reaction has a high energy barrier and is barely exothermic. In contrast, the oxidation of the molecule by the free hole is energetically more favorable. The free hole is transferred to the H2O molecule via the in-plane oxygen atom when the molecule stays in the transient dissociation state. This mechanism may also be applicable to the photooxidation of other R OH type molecules adsorbed on the rutile TiO2(110) surface.