Characterization of N-doped TiO2 films prepared by reactive sputtering using air/Ar mixtures

Nitrogen-doped titanium dioxide (TiO2 − xNx) thin films desirable for visible light photocatalysts were prepared by reactive sputtering using air/Ar mixtures. Using air as the reactive gas allows the process to conduct at high base pressures (low vacuum), which reduces substantially the processing time. The obtained films transformed from mixed phases to anatase phase as the air/Ar flow ratio increased. Substitutional doping of nitrogen verified by X-ray photoelectron spectroscopy accounts for the red-shift of absorption edge in the absorption spectra. Anatase TiO2 − xNx films could incorporate up to about 7.5 at.% substitutional nitrogen and a maximum of 23 at.% nitrogen was determined in the films with mixed phases. The optical band gaps of the TiO2 − xNx films calculated from Tauc plots varied from 3.05 to 3.11 eV and those of the mixed phase ranged from 2.77 to 3.00 eV, which are all lower than that for pure anatase TiO2 and fall into the visible light regime.