Surface fluorination of TiO2 for visible light photocatalysis
Résumé
Since many years, transition metal oxides with semiconductor behaviour have been studied for their wide range of photocatalytic applications. Among them, the TiO2 P25 presents the best properties. In this work, we described a new and efficient approach for the preparation of fluorinated TiO2 powders by thermal shock (TS) method and investigated the influence of surface fluorination on the bulk structure, surface composition, optical properties and the enhancement of UV and visible-light-driven photocatalytic activity of TiO2 using the methylene blue (MB) as the target organic pollutant. The surface fluorination was performed by thermal shock of TiO2 P25 and KF aqueous suspensions at several temperatures (400 to 950°C). X-ray diffraction, Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy have been carried out to determine the crystal structure, particle morphology and local chemical environment of atoms of the prepared samples. The optical properties of all samples were characterized by UV-Vis diffuse reflectance spectra (Figure 1) allowing the determination of the band gap. Furthermore, the UV (350 nm) and visible-light (420 nm) induced photocatalytic activities were evaluated through MB degradation to follow the effect of surface fluorination of TiO2 samples. Structure refinements reveal that the fluorinated samples with a TS temperature up to 600°C exhibit the same crystal structure as the TiO2 pristine material with a constant particle size distribution (~diameter of 20-50 nm). At higher TS temperature, the K2Ti6O13 phase appears leading to stick-like particles. From XPS analyses, the chemical environment of fluor atom has been identified: chemisorbed F- ions on the TiO2 surface for TS temperature up to 800°C observed simultaneously with F- ions in an oxygen environment above 800°C. The photocatalytic tests (Figure 2) show a net decomposition of MB in the aqueous solution with the highest performance for the sample fluorinated at 500°C, under both UV and visible light irradiation with initial rate decomposition of k = 2.09 h-1 and 0.32 h-1, respectively. Our work highlights the influence of the fluorination by TS method on the surface properties and photocatalytic activity of TiO2. This method appears useful to adsorb a large amount of F- ions leading to an enhancement of the photocatalytic activity until 600°C. Thus, the origin of enhanced photocatalytic activity, especially for the visible light response, is attributed to many factors included the presence of surface fluoride, the increase of surface hydroxyl group amount and the formation of oxygen vacancies whereas the decrease of activity above 600°C could be assigned to the formation of K2Ti6O13 and the apparition of fluoride ion in an oxygen environment.
Domaines
| Origine | Fichiers produits par l'(les) auteur(s) |
|---|---|
| Licence |
