Mechanism of the Photoactivity under Visible Light of N-Doped Titanium Dioxide. Charge Carriers Migration in Irradiated N-TiO2 Investigated by Electron Paramagnetic Resonance.

The generation of surface charge carriers in N-doped TiO2 under various types of irradiation has been investigated by electron paramagnetic resonance using an approach consisting in scavenging surface migrated electrons and holes using oxygen and hydrogen, respectively. N-doped TiO2, which is moderately active in photocatalytic processes under visible light, forms surface electrons and, at lower extent, surface holes due to the synergistic effect of visible components (around 400 nm) and near-infrared ones. The visible radiation excites electrons from intra band gap NOx- states to the conduction band, while NIR frequencies excite electrons from the valence band to the NOx- centers. The limited concentration of such centers explains the moderate efficiency of the whole process and, consequently, of the photocatalytic activity of N-TiO2 in visible light with respect to the case of UV light irradiation. Despite the mentioned limits this material remains a fundamental starting point for a new generation of photocatalytic systems exploiting solar light.