Reaction intermediates and pathways in the photoreduction of oxygen molecules at the (101) TiO2 (anatase) surface

Ab initio density functional calculations in the local spin density and Hubbard U approximations have been performed to investigate the formation of some important intermediates of the O2 photoreduction at the (1 0 1) TiO2 anatase surface as well as to compare with previous local density approximation results. A recently proposed approach has been used where molecules on the surface are treated like surface defects. Thus, by applying theoretical methods generally used in the physics of semiconductors, we successfully estimate the location and donor–acceptor character of the electronic levels induced by an adsorbed molecule in the TiO2 energy gap, both crucial for the surface-molecule charge-transfer processes, and investigate the formation and the vibrational properties of charged intermediates. Present results confirm the key role played by charged intermediates in the O2 photoreduction as well as a strong, deep acceptor character of an adsorbed O2 molecule which gives a clear theoretical explanation of the O2 electron scavenger behavior suggested by the experiment. Moreover, an investigation of the vibrational properties of some surface intermediates performed by ab initio molecular dynamics methods confirms an important role of H bonds forming between surface species and their water solvation shells.