Photocatalytic and Photovoltaic Properties of TiO2 Nanoparticles Investigated by Ab Initio Simulations

Titanium dioxide and TiO2-based materials are widely used in environmental- and energy-related applications like photocatalysis and photovoltaics, where they are usually employed as nanocrystals or nanostructures. The present contribution is aimed at filling the gap between the vast literature devoted to the simulation of electronic and photochemical properties of TiO2 crystals and surfaces, and the few theoretical studies of photoactivated processes involving instead TiO2 nanostructures. More specifically, photocatalytic and photovoltaic processes promoted by model TiO2 nanoparticles (NPs) have been investigated by using ab initio simulations based on the U-corrected density functional theory, and on the time-dependent density functional perturbation theory. We focus on well-investigated processes like the photogeneration of charge carriers in UV-irradiated NPs, the photoreduction of dioxygen and photooxidation of methanol catalyzed by NPs, and the splitting of photogenerated charge carriers occurring at a model NP-dye interface. Our results provide indications on some crucial points of such processes, showing that (i) excited charge carriers photogenerated within bare NPs are preferentially trapped as small polarons at surface undercoordinated Ti3+ and O(-)sites; (ii) dioxygen and methanol are efficient scavengers of such electrons and holes, respectively, and trigger surface redox processes likely involving proton coupled electron transfer (PCET) steps; (iii) dye-sensitized NPs are instead characterized by low-energy excited states in which electrons and holes photogenerated within the dye are efficiently split by the TiO2/dye junction, thus confirming the expected spontaneous formation of charge-separated states in TiO2-based photovoltaic devices; (iv) cost-effective theoretical tools can be fruitfully employed to obtain reliable predictions of the photocatalytic properties of nanostructured metal oxides and of the photovoltaic properties of hybrid organic photovoltaic devices.