Effect of aggregation of dyes adsorbed on nanocrystalline titania films on the efficiency of photodegradation

The conditions of adsorption of some dyes from aqueous solutions on nanocrystalline titania film have been studied. Adsorption follows Langmuir kinetics. Ionic dyes have the tendency to aggregate in the adsorbed state. Aggregates are easily distinguished by absorption spectrophotometry, since absorption of light is observed at a different wavelength than monomer absorption in a dilute solution. In all cases, aggregation demonstrated a hypsochromic shift, indicating repulsive interactions, which are justified in view of the fact that titania surface is charged and that adsorbed molecules are aligned with dipole moments in parallel. For a given titania film, the extent of dye aggregation and shift of absorption spectrum varies from one dye to the other. Some blue dyes demonstrate very large shifts. Photodegradation rates were slow in the case of aggregated dyes. It is concluded that the main cause for this deficiency is dye aggregation, which apparently inhibits interaction with photocatalyst. These results may be useful in the photocatalytic treatment of waste waters, in particular, from textile industry.