Enhancing effect of SiOx monolayer coverage of TiO2 on the photoinduced oxidation of rhodamine 6G in aqueous media

The effect of coating TiO2 with a SiOx monolayer on the photoinduced oxidation of rhodamine 6G (R-6G, counteranion X = Cl-, NO3-) has been studied. The surface treatment increased 2.6-fold the saturated adsorption amount (Gamma(s)) and 2.6-fold the adsorption strength (beta) of R-6G(+) ions. The rate of the liquid-phase oxidation was found to be dependent on X and increased remarkably with the SiOx monolayer coating, in contrast to its inhibitive effect in the gas-phase oxidation. A modified Langmuir-Hinshelwood mechanism, where the reaction rate is assumed to be proportional to the coverage of the R-6G(+) ion determined by the balance between the adsorption rate and the decomposition rate at the photostationary state, was proposed. As a result of kinetic analyses, the promoting effect in the liquid-phase oxidation was attributable to the increases in beta, Gamma(s), and/or the decrease in the adsorption amount of counteranions, which cannot be realized in the gas-phase reaction.