Determination of the Energy Levels of Paramagnetic Centers in the Band Gap of Nanostructured Oxide Semiconductors Using EPR Spectroscopy

A detailed analysis of the nature and photo induced reactions of paramagnetic centers (PCs) in TiO2 based semiconductor nanoparticles has been performed, and energy diagrams of the investigated samples with the energy level positions in the band gap are determined using electron paramagnetic resonance (EPR) technique under illumination in situ. This study gives a new method for constructing the zone diagram of nanostructured semiconductors. N-center dot, Ti3+, and Mo5+ PCs were detected in TiO2/MoO3 samples, and Ti3+ and V4+ centers were observed in the TiO2/MoO3:V2O5 one. The determined energy position of PCs in the band gap of nanostructured semiconductors are located from the valence band on 2.9 eV for Ti3+ ions, 2.7 eV for Mo5+ ions, 2.2 eV for V4+ PCs, and 1.4 eV below the bottom of the conduction band for N-center dot radicals. The effect of illumination is reversible during a long time: approximately 24 h because of a separation of the photogenerated charge carriers just after excitation between different semiconductor oxide nanosized particles connected by nanoheterojunctions. These results can be useful for understanding the mechanism of photocatalytic reactions and further practical applications of the nanoheterojunction materials such as TiO2/MoO3 and TiO2/MoO3:V2O5 oxides.