Photocatalitic reactions pathways of oxygen and water molecules at the (101) TiO2 (anatase) surface

In recent years, applications of metal-oxides, in particular of the rutile and anatase forms of TiO2, to the photooxidation of organic and small inorganic molecules in polluted air and water have been the subject of intense study. Basically, a photocatalytic process is characterized by the following steps: adsorption of a molecule on a semiconductor surface, photogeneration of electron-hole (e-h) pairs, trapping of electrons (holes) at surface sites, reduction (oxidation) of the adsorbed molecule through e (h) transfer from the surface to the molecule, molecule desorption. A recent investigation of the photoreduction of O2 molecules in aqueous solutions performed by using an highly sensitive IR spectroscopy has assigned the measured frequencies to possible intermediates of the photocatalytic reactions and proposed some reaction schemes for various processes of the photocatalytic O2 reduction. In the present study, the same photocatalytic processes have been investigated by using first principles Density-Functional Theory methods. In particular, as in a recently proposed approach, we treat molecules adsorbed on the TiO2 surface like surface defects and investigate the electronic structure of the surface/molecule system in the theoretical framework successfully used for defects in semiconductors. Such an approach permits to investigate states induced by an adsorbed molecule in the energy gap of the semiconductor, which are the states governing the charge-transfer processes. Moreover, adsorption-desorption processes can be investigated for both neutral and charged species. Thus, this approach permits to achieve an integrated description of the adsorption/charge-transfer/desorption key-steps of the photocatalytic process cited above. The achieved results explain and complement the above IR results. In particular, they permit to identify the primary intermediates of the photocatalytic reactions and their charge states upon irradiation. Moreover, the achieved results suggest a different assignment of the IR frequencies and clarify the mechanisms underlying the photocatalytic reactions.