engleski

Transparent, immobilised photocatalysts for pollutant degradation based on modified titania nanostructured thin films

Nanostructured TiO2 has taken the scientific focus because of the large specific surface area compared to bulk, increased photo-induced reactions, light absorption, photo-generated charge carrier densities, photo-reduction and contact with pollutants in photo-catalysis. TiO2 nanostructures, in general, have drawbacks in photo-catalysis, since their wide band gaps (anatase Eg = 3.2 eV, rutile Eg = 3.0 eV) limit absorbance of the complete solar spectrum and lead to fast recombination of photo-generated electron– hole pairs. Properties of nanostructured TiO2 materials can be improved by doping or decorating with other materials. In this study, the photocatalytic degradation of water and air pollutants was examined using transparent immobilized thin films of TiO2- based nanostructures altered by different modifiers (tungsten, barium and cupper-based compounds). An immobilized photocatalysts were designed for the flow-through reactor using simple, inexpensive and fast synthesis. The strategies used for the improvement of their photocatalytic activity will be presented. A series of transparent porous thin films (WxTi1- xO2) were obtained by co-sputtering tungsten-titanium in different atomic ratios on FTO glass and then anodizing them in ethylene glycol solution containing NH4F. The transparent TiO2 nanotubes arrays (TNT) modified with Cu-based compound were obtained in 2 steps: (1) anodization of the titanium layer on FTO glass prepared using radio frequency magnetron sputtering and (2) modification with different concentrations of Cu(NO3)2×3H2O by wet- impregnation method. For Ba- modified TNT, after the same first step, modification was obtained by annealing solutions of Ba(OH)2 in different concentrations spin coated on TNT. The morphology and structure of thin films were characterized by SEM, XRD and Raman spectroscopy. Different techniques were used for optical and photo-electrochemical characterization. The photocatalytic activity of WxTi1- xO2 under solar irradiation was studied using methylene blue as the target pollutant. Bamodified TNT was studied for degradation of diclofenac in water, while Cu-modified TNT had dual use ; degradation of NH3 and relative humidity sensing. The photocatalytic activity of TiO2-based nanostructures was considerably increased by doping or surface modifications. Thus, WxTi1-xO2 porouse thin film with 1% W showed the maximum photocurrent, while photoelectrochemical results revealed that the charge recombination decreased with increasing the amount of W. MB degradation using WxTi1-xO2 was performed at different pH and an enhanced photodegradation rate was obtained at higher pH. Among Ba-modified TNT the most efficient photocatalyst was the sample prepared with 25 mM Ba(OH)2 which showed 90% diclofenac degradation after 60 min. This result was in agreement with cyclic voltammetry that showed the largest increase in photo-oxidation current densities for the same sample due to the increased photogeneration of ·OH radicals and efficient charge separation. The TNT modified with 2M of Cu(NO3)2×3H2O demonstrated improved ammonia oxidation and superior relative humidity sensing properties. That is a consequence of successful charge carrier separation, which increased the ·OH radicals generation responsible for NH3 oxidation. The same sample showed the most pronounced increase in the humidity that is related to increase in the concentration of charge carriers on the surface of the sample. The mechanism behind the improvement in photocatalytic activity will be discussed in the view of morphology and structure.

photocatalytic pollutant degradation ; transparent Tio2 nanotubes arrays ; tungsten doping ; barium and copper modification

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