Photocatalysis with TiO2 and SnO2/TiO2 films examined by a kinetic model

Here we prepare TiO2 films and coupled SnO2/TiO2 films on glass substrates by using dip coating and spin coating and test their photocatalytic action with respect to the degradation of methylene blue in a water solution. The SnO2 layer of the mixed SnO2/TiO2 films is deposited on the glass by different methods to vary its properties: thickness, grain size and surface morphology. The TiO2 films are deposited on the SnO2 film by dip coating from a suspension of Degusssa P25 powder. Of better performance are the TiO2 films prepared by spin coating due to the improved film morphology. The coupled films SnO2/TiO2 (10-cycles SnO2 and 1-cycle TiO2) are of much better photocatalytic activity than the respective counterparts of pure SnO2 and TiO2 films. The SnO2 film prepared from sol-gel increases in a similar way the photocatalytic activity of the TiO2-film overcoat. In general, the kinetics of photocatalysis in a quiescent liquid is found to obey a two-stage mechanism, which is described quantitatively by a theoretical model assuming fast initial adsorption of the dye on the film followed by slow dye degradation. This model allows distinguishing fine details in the experimental data attributed to the effects of various factors on the photocatalytic action of the films.