Epitaxial growth and characteristics of N-doped anatase TiO2 films grown using a free-radical nitrogen oxide source

This paper reports on a systematic investigation of nitrogen (N) doping in single-crystalline TiO2 films possessing an anatase phase. TiO2 films heavily doped with N were grown with high crystallinity by employing a pulsed laser ablation technique using a close lattice-matched LaAlO3 (100) substrate and NO as the source gases for N doping. N15 and O18 isotope tracing by secondary-ion-mass spectroscopy confirmed that N and O atoms were simultaneously incorporated into the films as a consequence of the gas phase reactions between Ti-related chemical species ablated by the laser and NO free radicals. The films yielded an absorption band at around 3.0 eV, as expected from the yellowish color, with band tailing becoming stronger with increasing N concentration. It is suggested that N-related complex defects in TiO2 acted as carrier compensation centers. Micro-Raman spectroscopy with a spatial resolution of 1μm2 revealed host-lattice defects and generation of secondary phases at the microscopic scale by N doping. This work showed that the visible absorption band in N-doped TiO2 films can be attributed to small structural modifications consisting of N-related complex defects and secondary phases formed at the microscopic scale. These structural modifications were associated with a concomitant enhancement in photocatalytic activity.