Photocatalytic reactivities of nafion-coated TiO₂ for the degradation of charged organic compounds under UV or visible light

Nafion (perfluorinated polymer with sulfonate groups)-coated TiO ₂ particles (Nf/TiO₂) were prepared and their reactivities for the photocatalytic degradation (PCD) of charged organic substrates were investigated. The presence of Nafion adlayers drastically changed the positive TiO₂ surface charge to a negative one over the entire pH range and significantly influenced the PCD kinetics and mechanisms. The UV-induced PCD of tetramethylammonium (TMA; cationic substrate) was greatly enhanced in the presence of Nafion adlayers on TiO₂ because the ion-exchange sites within the Nafion can hold cationic substrates. On the other hand, despite the unfavorable electrostatic interaction between the Nf/TiO ₂ and anionic substrates, the PCD of dichloroacetate (DCA) and acid orange 7 (A07) with Nf/TiO₂ was not significantly inhibited. The visible-light-sensitized degradation of dyes was enhanced with Nf/TiO ₂ not only for cationic dyes (methylene blue (MB) and rhodamine B (RhB)) whose uptake on Nf/TiO₂ is enhanced, but also for an anionic dye (AO7) that is less adsorbed on Nf/TiO₂. The unexpected behavior in AO7 degradation seems to be related to the role of the Nafion layer in retarding the charge recombination. These observations indicate that Nf/TiO ₂ can enhance the PCD reactivity for cationic substrates without sacrificing the PCD reactivity for anionic substrates. In addition, it was found that the sensitized degradation of RhB followed a different path when the surface of TiO₂ was coated with Nafion. The N-de-ethylation of RhB that leads to the generation of rhodamine-110 was a prevailing path with Nf/TiO₂, whereas the cleavage of the chromophoric ring structure was dominant with pure TiO₂. The effects of Nafion adlayers on the photoinduced electron transfer and PCD kinetics and mechanisms are discussed.