Quantitative XAFS/EELS analyses of nitrogen species in titanium oxide photocatalysts

Abstract

With a view to rational designing of a highly functional visible-light TiO2 photocatalyst, nitrogen atoms were doped into TiO2 samples by an ion implantation technique which enables to control the depth and concentration of dopants. Although the absorbance in the visible-light region of the sample increased by the nitrogen doping, photocatalytic activity of the sample was not directly connected with the photo-absorbance. The N K-edge X-ray absorption near edge structure (XANES) spectrum of the photocatalytic active sample (A-cat) showed a characteristic double peak at 398 and 401 eV, and the XANES spectrum of the inactive sample (I-cat) a distinct single peak around 401 eV. These features of the XANES spectra were well reproduced by theoretical simulations based on the model where an O atom in TiO2 was replaced by N ((N)s) for A-cat, and that of quasi NO2 molecule ((NO2)s) for I-cat. Therefore, we have concluded that the nitrogen atom occupying the oxygen site of TiO2 is photocatalytic active species effective for visible light photocatalysis. In addition, the quantitative XANES/ELNES analysis has revealed that the photo-absorbance ratio of I-cat to A-cat corresponds well to the ratio of total doped nitrogen concentration rather than photocatalytic active nitrogen ((N)s) concentration. This result indicates that not only (N)s but also (NO2)s also absorb the visible light. Thus, the absorbance in the visible-light region is not necessarily an indication of the visible-light response of a photocatalyst.