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: 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 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.

Photosensitized Degradation of Dyes in Polyoxometalate Solutions Versus TiO2 Dispersions under Visible‐Light Irradiation: Mechanistic Implications

“…Recently, we suggested that O 2 is essential for the photodegradation of dyes under visible-light irradiation, and in fact, cannot be substituted by other electron acceptors. [52] By considering the intrinsic and nonselective character of reactive oxygen radical species such as O 2 C À , and the manner in which SOD suppresses RhB degradation in TiO 2 dispersions (Figure 7, curve d), we infer that O 2 C À plays an important role in the degradation of dyes in TiO 2 dispersions. In particular, cleavage of the RhB chromophore structure is attributed to attack of the dye (more possibly its radical cations) by O 2 C À /COOH ions.…”

The effects of copper ions on the catalytical degradation of azo dye acid chrome blue K

“…It was well known that some transition metal ions decreased the formation of reactive oxygen species by scavenging electrons on the catalysis surface. During this process, the metal ion was reduced [10]. In this experiment, during UV-light irradiation, the concentration of Cu 2+ remained unchanged.…”

“…For example, such transition or posttransition metal ions as Cu 2+ , Zn 2+ , Al 3+ , and Fe 3+ are often contained in the wastewaters of textile factories [9]. They can significantly influence the photocatalytic reactions for removal of the pollutants [10][11][12][13]. The degradation of acid red B was inhibited by the trace quantities of Mn 2+ ; Ni 2+ , and also depressed the photocatalytical degradation of cationic blue X-GRL; but Ca 2+ , Mg 2+ and Al 3+ could enhance the degradation of acid red B [14].…”

“…The decrease in the degradation rate could be caused by precipitation of dissolved metals as hydroxides onto the surface of catalyst [15]. Fe 3+ inhibited the degradation of alizarin red and malachite green by altering the electron-transfer pathway involving dyes, O 2 and TiO 2 particles [10]. On the contrary, metal ions such as Fe 2+ and Ag + could scavenge photogenerated electrons on the catalyst surface, reduce the non-desired electron/hole recombination, result in an increased rate of active oxygen species, and has positive effect on pollutants' degradation [16,17].…”

A Review on Current Status of Stability and Knowledge on Liquid Electrolyte-Based Dye-Sensitized Solar Cells

“…The production of H 2 O 2 is initiated by the reduction of O 2 from conduction band electrons in TiO 2 leading to superoxide anion O 2 −• which in turn will react with water. It should also be stressed that because of the instability in nature of the superoxide anion radical, it can also directly participate in internal chemical reactions to other organic cell components [181].…”