Environmental Photochemistry on Semiconductor Surfaces:  Photosensitized Degradation of a Textile Azo Dye, Acid Orange 7, on TiO2 Particles Using Visible Light

Photosensitized degradation of a textile azo dye, Acid Orange 7, has been carried out on TiO2 particles using visible light. Mechanistic details of the dye degradation have been elucidated using diffuse reflectance absorption and FTIR techniques. Degradation does not occur on Al2O3 surface or in the absence of oxygen. The dependence of the dye degradation rate on the surface coverage shows the participation of excited dye and TiO2 semiconductor in the surface photochemical process. Diffuse reflectance laser flash photolysis confirms the charge injection from the excited dye molecule into the conduction band of the semiconductor as the primary mechanism for producing oxidized dye radical. The surface-adsorbed oxygen plays an important role in scavenging photogenerated electrons, thus preventing the recombination between the oxidized dye radical and the photoinjected electrons. Diffuse reflectance FTIR was used to make a tentative identification of reaction intermediates and end products of dye degradation. The intermediates, 1,2-naphthoquinone and phthalic acid, have been identified during the course of degradation. Though less explored in photocatalysis, the photosensitization approach could be an excellent choice for the degradation of colored pollutants using visible light.