Enhancing Effect of SiOx Monolayer Coverage of TiO2 on the Photoinduced Oxidation of Rhodamine 6G in Aqueous Media

The effect of coating TiO 2 with a SiO x monolayer on the photoinduced oxidation of rhodamine 6G (R-6G, counteranion X = Cl -, NO 3 - ) has been studied. The surface treatment increased 2.6-fold the saturated adsorption amount (Γ s ) and 2.6-fold the adsorption strength (β) of R-6G + ions. The rate of the liquid-phase oxidation was found to be dependent on X and increased remarkably with the SiO x 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 β, Γ s, and/or the decrease in the adsorption amount of counteranions, which cannot be realized in the gas-phase reaction.