Redox processes by photoinduced charge carriers at TiO2 surface

Bulk excitons formed upon UV irradiation are found to become self-trapped, consistent with the observation of temperature dependent Urbach tails in the absorption spectrum and a large Stokes shift in the photoluminescence band of anatase. The electron and hole polaron trapping energy is considerably larger at the surface making energetically favorable for the polarons to travel from the bulk to the surface where the trapping sites correspond to undercoordinated Ti3+5c and O–2c surface atoms, or to isolated OH species in the case of a hydroxylated surface [1]. However, adsorbed molecular species can be better traps of photoinduced electrons and holes than surface undercoordinated ions or OH groups. In such cases a direct charge carrier transfer to the adsorbate takes place resulting in a net redox process. Here we explore the behaviour of a number of organic adsorbates as hole scavengers and of molecular oxygen as an electron one [2]. [1] C. Di Valentin and A. Selloni, J. Phys. Chem. Lett. 2, 2223 (2011). [2] C. Di Valentin and D. Fittipaldi, in preparation.