Effect of Transition Metal Ions on the TiO_2-Assisted Photodegradation of Dyes under Visible Irradiation: A Probe for the Interfacial Electron Transfer Process and Reaction Mechanism

摘要

The effect of metal ions (Cu~(2+), Fe~(3+), Zn~(2+), Al~(3+), and Cd~(2+)) on the photodegradation of several dyes: sulforhodamine 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_2O_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.