Photocatalytic behavior of WO 3-loaded TiO 2 systems in the oxidation of salicylic acid

Abstract

The role of WO 3 in enhancing the photocatalytic activity of TiO 2 deposits has been investigated through the oxidation of salicylic acid using UV and vis-light irradiation. Different procedures of semiconductor loading and deposition were undertaken: firstly, TiO 2-Degussa P25 was impregnated with tungstic acid solution and spread on the glass substrate. Secondly, precursor of WO 3 was ammonium paratungstate, mixed with a monomer and spincoated on the glass. Finally, a gel of WO 3–TiO 2 was synthesized by sol–gel method and spincoated on the glass, TiCl 4 was used as the titanium dioxide precursor and ammonium paratungstate was introduced in the sol before precipitation of TiO 2. The films obtained were characterized by UV–vis spectrophotometry, X-ray powder diffraction and scanning electron microscopy in order to better understanding the behavior and the effective role of WO 3. A red-shift in the absorption edge wavelength was observed for coupled catalysts prepared by the first procedure, the bang gap energy decreased to respectively 2.7 and 2.6 eV for 0.5% and 2.5 wt.% of WO 3. A tendency to agglomeration and a higher percentage of rutile in the catalysts were observed after WO 3 loading. The photocatalytic activity under visible light increased consequently for catalysts with 2.5 wt.% of WO 3 and variable inhibition was observed for lower loadings, but only a positive effect of WO 3 was observed under UV light. The inhibition of photocatalysis was also observed under visible light for some WO 3 loadings for coupled catalysts prepared by the procedure 2 and a great enhancement was observed at the 0.5 wt % of WO 3 under UV light. Concerning the catalysts prepared by the sol–gel method, a positive effect of the WO 3 introduction was noted: the coverage on the glass was improved, the band gap energy decreased to 2.3 eV with 4 wt.% of WO 3 and the corresponding photocatalytic activity was remarkably enhanced under visible light. A higher photocatalytic activity and a better response to WO 3 introduction was also observed under UV irradiation. The introduction of tungsten precursor before the crystallization of TiO 2 seems to be an appropriate method to ensure good contact and better charge transfer between the two semiconductors. As expected, the photocatalytic performances were generally higher under UV light than under visible light for all the catalysts and 0.5 wt.% WO 3–TiO 2 was the common optimal loading for the 3 procedures exhibiting the best activity under UV light.