Change of Adsorption Modes of Dyes on Fluorinated TiO₂ and Its Effect on Photocatalytic Degradation of Dyes under Visible Irradiation

Abstract

Surface-fluorinated TiO2 (F-TiO2) particles were prepared via the HF etching method. The surface characteristics of fluorinated TiO2, the adsorption modes of dyes, and the reaction pathways for the photocatalytic degradation of dye pollutants under visible light irradiation were investigated. It was found that, in the treatment of TiO2 by HF etching, F(-) not only displaces surface HO(-) but also substitutes some surface lattice oxygen. Using zwitterionic Rhodamine B (RhB) dye as a model, the change of the adsorption mode of RhB on F-TiO2 relative to that on pure TiO2 was validated by adsorption isotherms, X-ray photoelectron spectroscopy (XPS), and IR techniques for the first time. RhB preferentially anchors on pure TiO2 through the carboxylic (-COOH) group, while its adsorption group is switched to the cationic moiety (-NEt 2 group) on F-TiO2. Both the photocatalytic degradation kinetics and mechanisms were drastically changed after surface fluorination. Dyes with positively charged nitrogen-alkyl groups such as methylene blue (MB), malachite green (MG), Rhodamine 6G (Rh6G), and RhB all underwent a rapid N-dealkylation process on F-TiO2, while on pure TiO2 direct cleavage of dye chromophore ring structures predominated. The relationship between surface fluorination and the degradation rate/pathway of dyes under visible irradiation was also discussed in terms of the effect of fluorination on the surface adsorption of dyes and on the energy band structure of TiO2.

The application of chromium substituted magnetite as heterogeneous Fenton catalyst for the degradation of aqueous cationic and anionic dyes

“…As comfirmed by previous research [15,16], the increase of dye adsorption accelerates its degradation. Therefore, the variation of adsorption should not be ignored while investigating the effect of metal substitution on catalytic activity of magnetites.…”

Photocatalytic oxidation of gaseous ammonia over fluorinated TiO2 with exposed (001) facets

“…No peak for F − ions in the lattice at 688.5 eV was observed in the spectra, because the hydrothermal process could prevent the substitution of F − for O 2− in the lattice of TiO 2 [21,39,40,45]. The fluoride peak in the D-TiO 2 almost disappeared after defluorination, indicating that the surface fluorine atoms were removed by NaOH washing for 10 h [46].…”

Specific Facets-Dominated Anatase TiO2: Fluorine-Mediated Synthesis and Photoactivity

“…As elegantly demonstrated by Zhao and coworkers [104], Rhodamine B (RhB) preferentially anchors on pure TiO 2 through the carboxylic group, while its attachment switches to the cationic moiety (-NEt 2 group) on the fluorinated TiO 2 surface. Consequently, RhB de-alkylation precedes the destruction of the chromophore structure on fluorinated TiO 2 under visible light irradiation, whereas the direct cleavage of the RhB chromophore structure occurs on naked TiO 2 .…”