Effect of visible and UV illumination on the water contact angle of TiO2 thin films with incorporated nitrogen

Doping TiO2 with nitrogen is recognized as a procedure to get sensitization of this material with visible light. In the present work incorporation of nitrogen within the structure of TiO2 thin films has been accomplished by N2 + ion implantation in TiO2 anatase thin films (50 keV ion energy for doses of 3x1016, 6x1016 and 1.2x1017 ions cm-2) and during preparation by metalorganic CVD (MOCVD) using nitrogen as carrier gas. The analysis of the samples by X-ray photoemission spectroscopy (XPS) and for the MOCVD samples also by Secondary Ion Mass Spectroscopy (SIMS) has shown that nitrogen, in the form of nitride-like species, (N/Ti ratios of 0.03 and 0.12 for the MOCVD and the implanted samples, respectively) has become effectively incorporated within the structure of TiO2. The water contact angle on the implanted thin films varied from about 80 º to around 30 º when illuminated with visible light, depending on the ion dose. Similarly, the MOCVD samples showed a sharp decrease in wetting contact angle under visible light from about 80º to 55º. In the two cases, the thin films reach total hydrophilicity by posterior UV irradiation. To account for these results, the possible existence of specific excitation mechanisms for visible or UV photons, the former involving the incorporated nitrogen atoms, is discussed.