Photoluminescence of nitrogen-doped anatase

Photoluminescence depending on nitrogen concentration was investigated using anatase-type TiO₂ prepared by the calcination of a mixture of titanyl sulfate hydrate and urea. The substitutional ratio (x) of nitrogen in TiO₂ was successfully varied from 0.004 to 0.022 by changing the molar ratio of the mixture. The absorbance at 380–560 nm due to the formation of mid-gap states was proportional to the substitutional ratio of nitrogen controlled by the preparation conditions. In contrast, the fluorescent intensity at 382 nm originating from the band-to-band transition monotonically decreased with an increase in the substitutional ratio with an expansion of the anatase lattice. On the other hand, the maximum intensity of photoluminescence at 560 nm excited at 350 nm, which could be associated with the transition from the conduction band to the mid-gap states, was observed at x = 0.01. The optimal substitutional ratio for the emission was almost agreed with that for the photocatalytic decomposition of methylene blue and acetaldehyde under visible-light illumination. The photoluminescence was fundamentally determined by the balance between photoexcitation originating from a sufficient number of mid-gap states and deactivation of excited electrons and holes due to lattice distortion or defective states induced with the nitrogen doping.