Effect of Transition Metal Ions on the TiO2-Assisted Photodegradation of Dyes under Visible Irradiation: A Probe for the Interfacial Electron Transfer Process and Reaction Mechanism

Abstract

The effect of metal ions (Cu 2+, Fe 3+, Zn 2+, Al 3+, and Cd 2+ ) on the photodegradation of several dyes: sulfo-rhodamine B (SRB), alizarin red (AR), and malachite green (MG) has been investigated in aqueous TiO 2 dispersions under visible irradiation (λ > 420 nm). Trace quantities of transition metal ions such as Cu 2+ and Fe 3+ having suitable redox potentials alter the electron-transfer pathway involving the dye, O 2 and TiO 2 particles, and markedly depress the photodegradation of all three dyes under visible irradiation. Other metal ions, such as Zn 2+, Cd 2+, and Al 3+, have only a slight influence on the photoreaction by altering the adsorption of dyes. Photogeneration of H 2 O 2 and reactive radicals, and the changes in fluorescence emission of SRB in TiO 2 aqueous dispersions were examined to elucidate the role of the metal ions. Addition of Cu 2+ or Fe 3+ decreases the reduction of O 2 by the conduction electrons, subsequently blocks the formation of reactive oxygen species (O -•, • OH), and depresses the degradation of dyes under visible irradiation. We deduce that the reduction of O 2 is essential for the photodegradation of dyes under visible irradiation.