Photo-fixation of SO2 in nanocrystalline TiO2 films prepared by reactive DC magnetron sputtering

We report on photo-fixation of SO₂ onto nanostructured TiO₂ thin films prepared by reactive DC magnetron sputtering. The films were exposed to 50 ppm SO₂ gas mixed in synthetic air and illuminated with UV light at 298 and 473 K. The evolution of the adsorbed SO_x species was monitored by in situ Fourier transform infrared specular reflection spectroscopy. Significant photo-fixation occurred only in the presence of UV illumination. The SO₂ uptake was dramatically enhanced at elevated temperatures and then produced strongly bonded surface-coordinated SO_x complexes. The total SO_x uptake is consistent with Langmuir adsorption kinetics. The sulfur doping at saturation was estimated from X-ray photoelectron spectroscopy to be ~ 2.2 at.% at 473 K. These films were pale yellowish and had an optical absorption coefficient being ~ 3 times higher than in undoped film. The S-doped films exhibit interesting oleophobic properties, exemplified by the poor adherence of stearic acid. Our results suggest a new method for sulfur doping of TiO₂ to achieve combined anti-grease and photocatalytic properties.