Spectroscopic Study of the Photofixation of SO₂ on Anatase TiO₂ Thin Films and Their Oleophobic Properties

Photoinduced SO(2) fixation on anatase TiO(2) films was studied by in situ Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). The TiO(2) films were prepared by reactive DC magnetron sputtering and were subsequently exposed to 50 ppm SO(2) gas mixed in synthetic air and irradiated with UV light at substrate temperatures between 298 and 673 K. Simultaneous UV irradiation and SO(2) exposure between 373 and 523 K resulted in significant sulfur (S) deposits on crystalline TiO(2) films as determined by XPS, whereas amorphous films contained negligible amounts of S. At substrate temperatures above 523 K, the S deposits readily desorbed from TiO(2). The oxidation state of sulfur successively changed from S(4+) for SO(2) adsorbed on crystalline TiO(2) films at room temperature without irradiation to S(6+) for films exposed to SO(2) at elevated temperatures with simultaneous irradiation. In situ FTIR was used to monitor the temporal evolution of the photoinduced surface reaction products formed on the TiO(2) surfaces. It is shown that band gap excitation of TiO(2) results in photoinduced oxidation of SO(2), which at elevated temperatures become coordinated to the TiO(2) lattice through interactions with O vacancies and form sulfite and sulfate surface species. These species makes the surface acidic, which is manifested in nondetectable adherence of stearic acid to the modified surface. The modified films show good chemical stability as evidenced by sonication and repeated recycling of the films. The results suggest a new method to functionalize wide band gap oxide surfaces by means of photoinduced reactions in reactive gases at elevated substrate temperatures. In the case of anatase TiO(2) in reactive SO(2) gas, we here show that such functionalization yields surfaces with excellent oleophobic properties, as probed by adhesion of stearic acid.