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°C; (ii) dip coating from aqueous sols after extended refluxing treatment and followed by heating at ~100°C. 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.