Photocatalytic degradation of organic dyes using titania modified with adsorbent nanolayers

The silica and silica-alumina nanolayers were prepared on an individual titania nanoparticle surface using photocatalytically-induced reactions. The spectroscopic and kinetic analyses for the degradation of the organic dye molecules were discussed in order to elucidate the overall mechanism. The silica and silica-alumina nanolayer surface, more acidic than the titania, enhanced the adsorption of the cationic methylene blue (MB), while the silica nanolayer surface, much more acidic than silica-alumina and titania, suppressed the adsorption of the neutral or anionic fluorescein (FC). The photocatalytic degradation of MB mainly proceeded by the titania excitation, while it was slightly triggered by the MB excitation. The dye-sensitized degradation process, i.e., the electron transfer from the dye molecules to the titania, is negligible. On the other hand, the FC degradation proceeded by both the titania excitation and the FC excitation depending on the FC species adsorbed on the titania and silica surface. The main species are the dianion interacting with the surface on the titania and the neutral or anionic one on the silica. The silica suppressed the adsorption and degradation of the FC due to its acidity. The electron transfer from the adsorbed FC molecules to the titania promoted the dye-sensitized degradation. The silica nanolayer effectively functioned as an adsorbent and enhanced the photocatalytic degradation of MB. A higher density of photons effectively decomposed a higher amount of the MB adsorbed on the photocatalyst surface. The dye adsorption process plays an important role in the dye degradation during the photocatalytic reaction.