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

The effect of metal ions (Cu2+, Fe3+, Zn2+, Al3+, and Cd2+) on the photodegradation of several dyes: sulfo-rhodamine B (SRB), alizarin red (AR), and malachite green (MG) has been investigated in aqueous TiO2 dispersions under visible irradiation (λ > 420 nm). Trace quantities of transition metal ions such as Cu2+ and Fe3+ having suitable redox potentials alter the electron-transfer pathway involving the dye, O2 and TiO2 particles, and markedly depress the photodegradation of all three dyes under visible irradiation. Other metal ions, such as Zn2+, Cd2+, and Al3+, have only a slight influence on the photoreaction by altering the adsorption of dyes. Photogeneration of H2O2 and reactive radicals, and the changes in fluorescence emission of SRB in TiO2 aqueous dispersions were examined to elucidate the role of the metal ions. Addition of Cu2+ or Fe3+ decreases the reduction of O2 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 O2 is essential for the photodegradation of dyes under visible irradiation.