Photoactivity assessment of TiO2 thin films using Acid Orange 7 and 4-chlorophenol as model compounds. Part I: Key dependencies

Abstract

Azo dye Acid Orange 7 (AO7) and 4-chlorophenol (4-CP) are frequently used model compounds for photocatalytic activity assessment of TiO2 thin films. This paper explores the key factors, such as: method of analysis, initial concentration, mass transfer and reaction kinetics, in order to help identify the optimal conditions for photocatalytic activity evaluation. It has been experimentally proved that simple visible absorption spectrophotometry can be used reliably to determine the concentration changes of AO7 during its photocatalytic degradation, but is inappropriate for 4-CP and liquid chromatography should be used instead. At low concentrations (10−5–10−4 M) the first order rate constant for the photocatalytic destruction of AO7 decreases with increasing [AO7], whereas for 4-CP, it remains unchanged. This effect is not due to an approaching monolayer surface coverage of the AO7 on the titania photocatalyst but instead is due to UV screening of the titania film by the dye AO7. Thus, initial degradation rates corrected for the screening effect of AO7 solution show a direct dependence on [AO7] under the experimental conditions used in this work. At stirring speeds lower than 890 rpm (for AO7) and 220 rpm (for 4-CP) the kinetics of organic destruction are controlled by the diffusion rate of a model compound (AO7 or 4-CP) to the surface of TiO2 layer.