A comparative study into the photocatalytic properties of thin mesoporous layers of TiO2 with controlled mesoporosity

Mesoporous films of TiO2 were prepared by a sol–gel procedure using a Pluronic® block copolymer and the dip-coating technique. The film thickness was controlled by changing the film withdrawal rate and by repetitive coatings. The amount of dye molecules adsorbed onto the mesoporous film depends on the dye properties, with thionine and methylene blue being more strongly adsorbed than rhodamine B, methyl orange and methyl red. The photocatalytic efficiency of the degradation of these dyes strongly depends on their respective degradation mechanism. It was found that the optimum efficiency is achieved when the target molecule is adsorbed only to some extent thus enabling its decomposition directly by the attack of holes. On the other hand, secondary oxidizing species such as O2− and HO2 but most likely even OH play a crucial role in the oxidation mechanism, which is particularly evident in the case of methylene red. Higher concentrations of adsorbed species to be degraded may even be detrimental as found when thionine was employed as the model compound.