Photodegradation of methylene blue in water, a standard method to determine the activity of photocatalytic coatings?

The degradation of methylene blue (MB) in aqueous solutions has been re-examined as a method to characterize the photocatalytic activity of transparent TiO(2) coatings. Increasing irradiation intensities leads to a change in the observed kinetic behavior from zero-order to pseudo-first-order regarding the concentration of MB. This is due to a diffusion inhibition of MB. In order to obtain data within a zero-order kinetic regime at an initial MB concentration of 10 mu mol/l and, thus, to avoid the diffusion control, irradiation intensities below E = 5 W/m(2) for substrates comprising higher photonic efficiencies than zeta = 0.09% have to be applied. Recommendations for a standard protocol are given.