Rutile TiO2 (110) single crystals were doped with nitrogen by heating at 675C in flowing NH3 gas. This caused a red-shift in the band edge and an increase in visible region absorption. Nitrogen depth profiles obtained using dynamic secondary ion mass spectrometry could best be fitted by assigning three distinct diffusion coefficients. X-ray photo-electron spectroscopy indicated the presence of two surface nitrogen states with binding energies of 395.6eV (substitutional N) and 399.8eV (interstitial N). Angle-resolved X-ray photo-electron spectroscopy measurements permitted the linking of X-ray photo-electron spectroscopy environments to the secondary ion mass spectrometry diffusion profiles. Subsequent air annealing at 500C led to asymmetrical diffusion of the interstitial nitrogen into the bulk, removal of substitutional nitrogen into the gas phase, a decrease in the concentration of Ti3+ and a blue-shift in the band edge. These changes could be related to variation in the optical band gap, and it was found that the principle cause of band-gap narrowing was substitutional rather than interstitial nitrogen.

Nitrogen Diffusion in Doped TiO2 (110) Single Crystals: a Combined XPS and SIMS Study. Palgrave, R.G., Payne, D.J., Egdell, R.G.: Journal of Materials Chemistry, 2009, 19[44], 8418-25