EPR features of second generation photocatalysts

The N-doped TiO2 seems to be the most promising among all the so-called second generation photocatalysts. Asahi et al.1 first suggested that N doping reduces the band gap due to the mixing of N 2p with the O 2p states. Since then, there has been a great number of publications describing enhanced visible light photochemistry in N-doped TiO2 although current debate on the fundamental understanding of the electronic and structural aspects of these materials is wide open.2-4 In this work N-doped TiO2 nanocrystals, obtained by different procedures and adopting different N sources, are analyzed. Structural (by X-ray synchrotron radiation), morphological (HRTEM, BET, granulometry) and optical (DRS) characterizations are compared with data obtained by EPR spectra. The last can be attributed to Nb · (bulk nitrogen) paramagnetic defects, i.e. to different Ns (substitutional) and/or Ni (interstitial) doping nitrogen atoms5,6. A second paramagnetic contribution appears in the EPR spectrum of samples obtained by doping TiO2 with triethylamine and showing N/Ti³0.20; this component is attributed to the formation of paramagnetic F+ centers, i.e. to electron trapping by oxygen vacancies6. The role played by the synthetic route and by the redox conditions of the calcinations on the paramagnetic features is discussed. Specifically the nature of the paramagnetic species, the intensity and the stability in time of the signal are analyzed and crosscompared also with the structural and optical features of the samples.