Photocatalytic degradation of C.I. Reactive Blue 19 with nitrogen-doped TiO 2 catalysts thin films under UV/visible light

Abstract

Nitrogen-doped titania semiconductor thin films, with photocatalytic properties and a high transmittance in the visible range, have been deposited by unbalanced reactive magnetron sputtering on glass substrates, using d.c. pulsed power supplies. In order to increase the photocatalytic efficiency of the titania coatings the authors optimized the sputtering process, namely by using d.c. pulsed currents for better optimization of reactive gas consumption, and doped the coatings with nitrogen. With this combined and synergistic effect it was possible to enhance the catalysts absorption of visible light, by reducing its semiconductor indirect band-gap. By slightly doping the titania films with 0.7–0.9 at.% nitrogen the photocatalytic performance is ameliorated with almost one order of magnitude. This has been achieved by using nitrogen as a co-reactive gas, together with oxygen, when sputtering from a pure titanium target. The as-deposited thin films on glass are mostly amorphous; however, upon a thermal annealing in vacuum at 500 °C, the crystalline phases of anatase and rutile are developed, being anatase the most prominent polymorph. The photocatalytic performance of the N-doped titania films was evaluated by the decomposition of an organic dye (C.I. Reactive Blue 19) with combined UV/visible light irradiation. Furthermore, a mechanism for the degradation of this dye is proposed. The hydrophilic properties of these films have also been studied by means of water contact angle measurements after varied illumination periods; a minimum contact angle of ∼10° was achieved for optimized wettability conditions.