Photocatalytic behavior of WO3-loaded TiO2 systems in the oxidation of salicylic acid

The role of WO3 in enhancing the photocatalytic activity of TiO2 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, TiO2-Degussa P25 was impregnated with tungstic acid solution and spread on the glass substrate. Secondly, precursor of WO3 was ammonium paratungstate, mixed with a monomer and spincoated on the glass. Finally, a gel of WO3–TiO2 was synthesized by sol–gel method and spincoated on the glass, TiCl4 was used as the titanium dioxide precursor and ammonium paratungstate was introduced in the sol before precipitation of TiO2.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 WO3.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 WO3. A tendency to agglomeration and a higher percentage of rutile in the catalysts were observed after WO3 loading. The photocatalytic activity under visible light increased consequently for catalysts with 2.5 wt.% of WO3 and variable inhibition was observed for lower loadings, but only a positive effect of WO3 was observed under UV light. The inhibition of photocatalysis was also observed under visible light for some WO3 loadings for coupled catalysts prepared by the procedure 2 and a great enhancement was observed at the 0.5 wt % of WO3 under UV light.Concerning the catalysts prepared by the sol–gel method, a positive effect of the WO3 introduction was noted: the coverage on the glass was improved, the band gap energy decreased to 2.3 eV with 4 wt.% of WO3 and the corresponding photocatalytic activity was remarkably enhanced under visible light. A higher photocatalytic activity and a better response to WO3 introduction was also observed under UV irradiation. The introduction of tungsten precursor before the crystallization of TiO2 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.% WO3–TiO2 was the common optimal loading for the 3 procedures exhibiting the best activity under UV light.

► The photocatalytic activity under visible light increased for catalysts doped with WO3.
► Catalysts prepared by spin coating have homogeneous, transparent aspect and exhibit high photocatalytic performance.
► Positive effect of the WO3 doping was noticed, for Catalysts prepared by sol–gel method, in particular the decrease in the band gap energy.
► 0.5 wt.% WO3 is the common optimal ratio for the catalysts prepared by the different procedures, which exhibits the highest photocatalytic activity.
► The positive effect of loading by WO3, would be the result of 2 ways charge transfer occurs between the two activated semiconductors.