Methylene Blue (MB) has been chosen as a model molecule to evaluate the impact of inorganic salts, present in textile waste waters, on the adsorption properties and on the photocatalytic efficiency ofTiO2. NoOH∘radical scavenging by anions such asNO3−,Cl−,SO42−,PO43−, andCO32−was observed at neutral and basic pH, while this phenomenon can be suggested at acidic pH for some anions except carbonate anions which are totally neutralized and/or eliminated asCO2in these conditions. The decrease in the rate MB photocatalytic degradation in the presence of inorganic salts was shown to be due to the formation of an inorganic salt layer at the surface ofTiO2, which inhibits the approach of MB molecules. The correlation between the amount of MB adsorbed and the rate of its photocatalytic degradation, whatever the nature of the salt, its concentration and the pH of the solution, indicates (i) that photocatalysis occurs at the surface and not in the solution and (ii) thatOH−ions added at basic pH do not participate to the increase in the photocatalytic efficiency by inducing an increase inOH∘formation. They increase the surface density in adsorption sitesTiO−. The effect of various salts is similar on various titania samples of industrial origin (MillenniumTiO2PC 500, PC 50, and Degussa P 25). It is however more important on Millennium PC 10 probably because of its smaller surface area.
ZnO-SnO 2 nanocomposites were synthesized by a facile sol-gel synthesis route and characterized through X-ray diffraction, BET specific surface area analysis, UV-vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy. The photocatalytic activity of the samples was tested using the degradation of 4-nitrophenol under UV light as model reaction. The ZnO/SnO 2 molar ratio was varied in order to study its influence on the photoefficiency of the samples. The ZnO-SnO 2 nanocomposites showed higher photoactivity than the pure oxides and in particular the sample with ZnO/SnO 2 molar ratio equal to 1/0.05 resulted the best one among the tested powders. The high activity of the mixed samples was attributed to the presence of heterojunctions between the two oxides, which allows an improved charge separation of the photogenerated electron-hole pairs, due to the differences between the energy levels of the conduction and valence bands of ZnO and SnO 2. Photovoltage measurements were performed to determine the energy band structure of the ZnO-SnO 2 heterojunction.
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