Determination of catalytic properties of TiO2 coatings using aqueous solution of coumarin: Standardization efforts

International audienceMany types of supported photocatalytic TiO2 continue to be the subject of extensive development worldwide. Besides industrial production and practical use of the new photocatalytic materials, there is an increasing need for a simple and reliable procedure for characterization of photocatalytic activities of newly developed materials. The aim of our work was to develop a method for the determination of quantum yields of supported photocatalysts by employing an aqueous solution of a model organic compound and different thin TiO2 films. Additionally, also a newly defined parameter, the so-called mass efficiency, was introduced as an advantageous way of defining the photocatalytic activities. Coumarin (CM) was found to be an appropriate candidate for being a probe molecule when the photocatalytic degradation mainly occurs via the HO{radical dot} radical-mediated mechanism. An advantage of using CM is easy determination of highly fluorescent 7-hydroxycoumarin (7OHC). Different TiO2 films were deposited via sol-gel methods on soda-lime glass slides that were precovered with a thin SiO2 film, and via the sedimentation process from aqueous suspensions. Aqueous solutions of CM were irradiated using 365 nm radiation in the presence of titania films. Although Degussa P25 films showed approximately four times higher degradation rates compared to the sol-gel processed, its quantum yield was not more than 2.5 times higher. This could be explained by higher absorbance of Degussa P25 films per amount of the catalyst compared to sol-gel films. Since no information on the absorption characteristics of the material per unit of mass of the catalyst is included within the calculation of quantum yield, mass efficiency is suggested for the evaluation of photocatalytic properties of the coatings. It is evident from the present study that it is obligatory to evaluate the photocatalytic efficiencies of different area densities of the same photocatalyst in order to properly characterize the material's efficiency

Photocatalytically active TiO2 thin films produced by surfactant-assisted sol-gel processing

Thin TiO2 films were prepared from a titanium isopropoxide precursor by sol\u2013gel processing with or without various nonionic surfactant molecules (Brij 56, Triton X-100 or Pluronic F-127). The photocatalytic efficiency of the transparent films obtained by a dip-coating technique was found to depend strongly on the use of and type of surfactant added. Titania/Pluronic sols resulted in homogeneous and crack-free TiO2 anatase films with a thickness as much as 300 nm after one dipping and heat-treatment (500 \ub0C) cycle. Optical properties of the films were characterized by UV-Vis spectroscopy and crystalline structures by X-ray diffraction. A surfactant-assisted sol\u2013gel process retarded crystallization of the anatase titania films, which resulted in smaller grain sizes (down to 10 nm) and presumably a larger active surface area. The morphology of the film surfaces as obtained by SEM techniques could be also correlated with the results of our photodegradation studies. The photocatalytic activity of the films was enhanced by first coating the glass substrate with a SiO2 protective layer prior to the deposition of the titania film. For our in situ studies of photodegradation we chose the monoazo dye Plasmocorinth B as a model compound as it is stable under environmental conditions and its degradation products are not coloured. The highest photobleaching rate was found for films deposited from the sol with addition of the Pluronic surfactant and it was almost twice as high as that for films deposited from sols without the surfactant

TiO2-anatase films made by sol-gel processing and their photodegradation activity towards pollutants in water

Titanium dioxide has become the material of choice for hydrophilic photocatalytic surfaces and the sol-gel technique has emerged as one of the most promising techniques for growing TiO2 thin films. This chapter describes our sol-gel preparation and utilization of thin titania films on glass substrates as catalysts for photodegradation of organic pollutants, such as azo dyes and pesticides in aqueous solutions. The good photoefficiency of the as-prepared films relies on their high contact surface area and nanocrystalline structure, which facilitates efficient photoinduced electron-hole pair generation. Transparent TiO2-anatase films on soda-lime glass supports were produced by two different sol-gel processing routes: (i) dip coating from alcoholic sols containing surfactants and followed by heat treatment at 500\ub0C; (ii) dip coating from aqueous sols after extended refluxing treatment and followed by heating at ~100\ub0C. In both cases the starting precursor was titanium alkoxide and the final coating consisted of a dominant anatase crystalline phase. In case of the high-temperature processing route, the detrimental effect of interdiffused sodium ions from the glass substrate during heat treatment was prevented by depositing a thin silica barrier layer prior to titania deposition. The intermediate barrier layer was not needed in case of the lowtemperature processing route, where the crystallization of anatase has been already promoted during the formation of the sol. X-ray absorption spectroscopy and X-ray diffraction were applied to determine the structure development at different stages from the precursor solution to the solid thin film. Surface morphology, characterized by monodispersed or joint nanoparticles and variable roughness, was investigated with atomic force microscopy, while the surface and indepth composition of films were analyzed by X-ray photoelectron spectroscopy. A photocatalytic activity of the as-prepared films was studied in two different tailor-made photoreactors filled with an aqueous solution of certain pollutant. In case of an azo dye, the films were immersed in its colored solution and photobleaching was followed in-situ with the help of UV-VIS spectroscopy. The degradation of the pesticide was monitored by HPLC analysis and its mineralization by ionic chromatography. The insecticide thiacloprid was stable under irradiation (wavelength range 310-400 nm) in the absence of TiO2 films during 8 hours long period, whereas in the presence of best-performing titania films the half time of the parent molecule was typically 15 minutes. The titania catalyst can be easily removed from the solution, which is one of the principal advantages of using the immobilized films as catalysts rather than powders

Photocatalytic TiO2 Coatings: Effect of Substrate and Template

Transparent TiO2 films with a high photodegradation activity towards an azo dye in aqueous solution were prepared by sol\u2013gel processing. Films on soda\u2013lime glass supports protected with a thin silica barrier layer exhibited better crystallization and monodisperse nanoparticles, higher absorption of light below 370\u2009nm, and higher photocatalytic activity than those films deposited on bare glass supports proving the detrimental effect of interdiffused sodium ions on the development of the anatase nanostructure. The effect of substrate was more pronounced in thinner films (300\u2009nm) than in thicker ones (1200\u2009nm), which were achieved by adding a template (i.e. Pluronic F127) to the sol

Low-temperature synthesis and characterization of TiO2 and TiO2-ZrO2 photocatalytically active thin films

Transparent TiO2 and TiO2\u2013ZrO2 (molar ratio Zr/Ti = 0.1) thin films were produced by low-temperature sol\u2013gel processing from nanocrystalline aqueous based solutions. The structural features and compositions of the films treated at room temperature, 100 \ub0C and 500 \ub0C were investigated by X-ray diffraction, X-ray photoelectron spectroscopy and thermal analysis. Addition of zirconia increased specific surface area (140\u2013230 m2 g 121) and hindered the growth of anatase crystallites, exhibiting a constant size of 6\u20137 nm in the whole temperature range. These significant changes with respect to pure TiO2 in anatase crystalline form did not result in significantly and systematically different photocatalytic activity, which was evaluated in terms of aqueous pollutant degradation (azo-dye in water) and self-cleaning ability (fatty contaminant deposit). The films treated at only 100 \ub0C showed excellent photocatalytic activity towards azo-dye degradation. Contact angle measurements of aged and contaminated surfaces revealed a fast or sharp hydrophilicity gain under UVA illumination. Accordingly, the results of this study confirmed the potential application of advantageous low-temperature films in water treatment as well as for self-cleaning surfaces

Evaluation of photochemical degradation of digoxin by Na,K-ATPase assay

A simple Na,K-ATPase assay is described as a suitable method for testing of digoxin photodegradation. The exposure of Na,K-ATPase to the photodegraded samples exhibited reduced inhibition of the enzyme, compared to the unirradiated samples containing equal initial concentrations of drug. The degree of inhibition was dependent on the irradiation time. The concentrations of digoxin in irradiated samples were evaluated by HPLC analysis. Excellent agreement of the results obtained by both methods was observed. The investigation of the influence of irradiated samples on Na,K-ATPase inhibition revealed no side products acting as Na,K-ATPase inhibitors. The cytokinesis block micronucleus test (CBMN) was applied in order to investigate the cytotoxicity of the possible degradation products after exposure to UV irradiation. The results confirmed that the photochemical treatment did not induce the cytotoxic side products. Zero order kinetics, which was observed for digoxin photodegradation and the associated reaction mechanism are also discussed. (c) 2005 Elsevier B.V. All rights reserved