Photoredox reactions of methylene blue on zinc oxide

Abstract

Photoreduction of methylene blue (MB⁺) at 366 nm (3.4 eV) in a heterogeneous medium containing ZnO (band separation = 3.2 eV) has been carried out. In oxygen-free media the product has been found to be leuco-MB which is fully converted into MB⁺ on air oxidation. An analysis of the results shows that Zno is the absorber and negligible light absorbed by MB+ cannot explain the observed conversions. Concentration of ZnO in the medium plays a more important role at higher concentrations of MB⁺. Also reaction extent decreases with increase in dye concentration but at low concentrations it is proportional to dye concentration. A unique feature of the reaction is that it is quenched by the product. Photoreduction is more efficient in solvents of high dielectric and in solvents with easily available hydrogens. Different samples of ZnO prepared from different salts behave differently in this photoreduction due to different reflectance and absorbance characteristics of these samples. A mechanism explaining the observed data and consistant with the known features of such systems have been proposed.