Photoassisted Degradation of Dye Pollutants. V. Self-Photosensitized Oxidative Transformation of Rhodamine B under Visible Light Irradiation in Aqueous TiO₂ Dispersions

Abstract

Chemical oxygen demand (CODCr) and proton NMR, UV-vis, and spin trapping EPR spectroscopic evidence is presented to demonstrate the inverse photosensitized oxidative transformation of tetraethylated rhodamine (RhB) under visible illumination of aqueous titania dispersions. Both de-ethylation and oxidative degradation take place with the former proceeding in a stepwise manner to yield mono-, di-, tri-, and tetra-de-ethylated rhodamine species. Intermediates present after each de-ethylation step remain in a fast dynamic equilibrium between the titania particle surface and the bulk solution. The concentration of • OH radicals, formed from the inverse photosensitization process through the superoxide radical anion, increases upon addition of the anionic dodecylbenzene sulfonate surfactant (DBS) because a larger number of RhB excited states are able to inject an electron into the conduction band of the TiO 2 particles. Also, intermediates that can no longer absorb the visible light, (i.e., once the dye solution is competely bleached) are unable to drive the photosensitized degradation further. A mechanism for the competitive photoreactions between degradation and de-ethylation is described, in which de-ethylation {ζ ∼ 0.0035} is mostly a surface occurring process, whereas degradation {ζ ∼ 0.0015}of the RhB chromophore is predominantly a solution bulk process.

A review on plasmonic metal⿿TiO2 composite for generation, trapping, storing and dynamic vectorial transfer of photogenerated electrons across the Schottky junction in a photocatalytic system

“…The Ag nanoparticles are photoexcited due to SPR effect and charge separation is accomplished by the transfer of photoexcited electrons from the Ag nanoparticles to the TiO 2 CB and the authors further propose simultaneous transfer of compensative electrons from donor Cl − ions to the Ag nanoparticles (Fig. 14) [191][192][193]. In another study Ag/AgCl/TiO 2 nanocomposite thin films prepared by the same method exhibited enhanced photocatalytic decomposition of 4-CP under visible light irradiation [194].…”

A Comparative Study of Nanostructured TiO2, ZnO and Bilayer TiO2/ZnO Dye-Sensitized Solar Cells

“…In addition, reactive oxygen species (mainly oxygen ions) may be formed with time, which react with oxidized dye molecules under visible or ultraviolet light irradiation in the presence of TiO 2 particles. 45,46 As a result of this reaction, the number of oxidized dye molecules may reduce. Dye degradation also depends on the adsorption sites 47 and constitutes a major problem, not only for the simple organic dyes but also for the highly efficient metal complex dyes.…”

Dye-sensitized photoelectrochemical cell on plasmonic Ag/AgCl @ chiral TiO 2 nanofibers for treatment of urban wastewater effluents, with simultaneous production of hydrogen and electricity

“…For the particular case of N719 dye, since the redox potential of ROS/H 2 O (~ 2.85 V vs. NHE) [32] is higher than that of N719/sensitized N719 (~1.0 V vs. NHE) [37] it is conceivable that the leached dye could be oxidized by ROS after the total consumption of TOC from the wastewater.…”

“…The strong electronic coupling between Ag and AgCl conduction bands which injects photoinduced electrons from the Ag nanoparticles, quickly and efficiently, into the AgCl [31] and into the TiO 2 helical structure, yielding superoxide radicals and finally HO· radicals from the reductive pathway [32]. However, this effect seems negligible for the removal of the contaminants since the direct charge transfer from noble nanoparticles (Hagglund et al, 2008) or the internal photoemission from TiO 2 and AgCl [33] only play a supporting role in the photoelectrical performance.…”