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

Super-hydrophilic and photocatalytic properties of Ag-TiO2 thin films prepared by sol\u2013gel technique

A comparative study of photocatalytic degradation of methyl orange (liquid-solid interface) and methyl stearate (solid-solid interface) was performed with three different kinds of transparent pure and Ag-doped TiO2 films, prepared on glass by sol-gel dip-coating technique. The optical, morphological and hydrophilic properties of the thin films were characterized by UV-Vis spectrophotometer, atomic force microscope (AFM) and water contact angle technique, respectively; whereas the crystalline form, the specific surface area and the chemical state/composition were determined by X-ray diffraction, BET nitrogen sorption analysis and X-ray photoelectron spectroscopy (XPS), respectively. Ag doping increases the thin films surface roughness and inhibits the agglomerates structure formation of nano-size TiO2 particles during heat treatment. The XPS results showed that the silver is present in the ionic state and that its concentration is much higher at the surface of the film than in the inner layers, suggesting Ag ions migration from the interior to surface of the film during film preparation and thermal treatment. The influence of air contamination and UV irradiation on water contact angle was systematically examined in order to understand the hydrophobic/hydrophilic properties and their effect on the photocatalytic activity for methyl stearate photodegradation. It has been observed that small dopant concentration enhanced photocatalytic activity of TiO2 film remarkably towards methyl orange aqueous solution, whereas the same thin film showed slightly lower activity than pure TiO2 towards methyl stearate layer. The photodegradation results of methyl stearate and methyl orange indicate that the activity of pure TiO2 thin films is favored at the solidsolid interface, while Ag doped TiO2 is more active at the liquid-solid interface

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

Photoinduced superhydrophilicity and photocatalytic properties of ZnO nanoplatelets

The present work is devoted to the investigation of photoinduced superhydrophilicity (PSH) and photocatalytic oxidation (PCO) activity of quasi-2Dzinc oxide nanoplatelets obtained by Chemical VaporDeposition (CVD). In the case of PCO, the degradation of a fatty compound layer (methyl stearate) was chosen as a test reaction. The peculiar textural and morphological features of such systems resulted in a PSH behavior and in a PCO activity appreciably higher than those of a reference commercial Pilkington® ActivTMGlass. Relevant results are presented and discussed as a function of the structure and morphology of these systems, whose control enables a direct tailoring of the functional response

IR- and NMR time-resolved studies on the hydrolysis and condensation of methacryloxyalkylsilanes

The sol\u2013gel reactions of the two methacrylate-modified silanes methacryloxymethyltriethoxysilane (MAMTES) and methacryloxypropyltrimethoxysilane (MAPTMS) were followed by using two independent time-resolved spectroscopic methods, viz., IR ATR and NMR with the aim to optimise their pre-hydrolysis times and consequently their use as precursors for hybrid materials. Time resolved measurements were carried out on the two systems under sol\u2013gel conditions. Whereas the hydrolysis of both siloxanes is very fast, condensation proceeds gradually and is not completely finished within 5 h

Epitaxial-like growth of Co 3O 4/ZnO quasi-1D nanocomposites

The development of quasi-1D Co3O4/ZnO nanocomposites by a two-step plasma enhanced-chemical vapor deposition (PE-CVD) process is presented. Arrays of < 001 > oriented ZnO nanorods were first grown on Si(100) and subsequently used as templates for the PE-CVD of Co3O4, whose amount was tailored as a function of deposition time. The obtained composites were thoroughly characterized by means of a multitechnique approach, involving field emission-scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDXS), micro-Raman and Fourier-transform infrared (FT-IR) spectroscopies, X-ray photoelectron and X-ray excited Auger electron spectroscopies (XPS, XE-AES), glancing incidence X-ray diffraction (GIXRD), and reflection high energy electron diffraction (RHEED). The use of moderate deposition temperatures (<= 300 degrees C), together with the unique activation provided by nonequilibrium plasmas, prevented state reactions between the two oxides and promoted Co3O4 growth on the tips of vertically aligned ZnO nanostructures. In particular, the resulting quasi-1D Co3O4/ZnO composites were characterized by an interface epitaxial-like relationship, an important issue for the development of semiconductor-based functional nanosystems. Photoinduced hydrophilic (PH) and photocatalytic (PC) performances of the present nanocomposites were preliminarily investigated, showing attractive results toward the possible fabrication of advanced smart materials