Effect of solution pH on the adsorption and photocatalytic reaction behaviors of dyes using TiO2 and Nafion-coated TiO2

The purposes of this research were to study the effects of solution pH on the photoelectrochemical properties of TiO2 photoelectrode and the adsorption behaviors and photocatalytic reactions of two dyestuffs (Reactive Red 22, RR22 and Basic Red 2, BR2) in aqueous solution using various photocatalyst films (TiO2 and Nafion-coated TiO2, Nf/TiO2). The open circuit voltage and the short circuit photocurrent of TiO2 photoelectrode increased as solution pH decreased and indicated more photoelectrons were generated in acidic solutions. The response times reaching half maximum photovoltage and photocurrent were determined to be faster in acidic solutions than those in alkaline solutions, indicating higher electron diffusion occurred in acidic solutions. The coating of 0.5 g of Nafion per gram of TiO2 was enough to reduce the zeta potential of Nf/TiO2 to less than −20 mV in aqueous solution and exhibited a hydrophobic surface that might decrease the adsorption and photocatalytic decomposition of dye. Experimental results on the adsorption and photocatalytic decomposition of RR22 and BR2 indicating that the charges of TiO2 surface and reactant dye markedly influence the reaction rate. The photocatalytic decompositions of RR22 using TiO2 were favored to occur in acidic conditions and exhibited a minimum decomposition rate near the isoelectric point of TiO2. Nevertheless, no obvious RR22 decompositions were found in experiments conducted using Nf/TiO2. Decompositions of BR2 using both TiO2 and Nf/TiO2 were more favorable in alkaline conditions; however, decompositions of BR2 were found to be decreased in experiments conducted using Nf/TiO2.