CONICET | Buscador de Institutos y Recursos Humanos

The development self-cleaning surfaces is one of the most important applications of the photocatalytic properties of TiO2. In particular, the coating of ceramic tiles with TiO2 films is a base for removing chemical and biological contaminants in buildings by irradiation either with lamps or sun light. One of the problems for the one-step production of self-cleaning tiles is the high temperature (> 800ºC) in the tile fabrication. At these temperatures rutile is the dominant phase, instead of the most photoactive anatase. A second problem derived from the high temperatures is sintering which decreases the surface area of the film also decreases when submitted to high temperatures. By the other hand, the photoactivity of TiO2 films under visible illumination is poor, being this problem worst in indoor applications. The incorporation of elements with high and variable oxidation numbers, is a way to inhibit the anatase-rutile phase transition and to extend the photoactivity towards the visible range. However, the incorporation of foreign cations is not trivial, as segregation to the surface must be controlled in order to restrain the phase transition, but controlling the fraction of photoactive TiO2 at the surface. The sol gel process overcomes this difficulty for preparing TiO2 fims doped with transition metals. In this work, we present the effect of W (VI) on the photocatalytic activity of TiO2 films. Our goal is to determine the optimal synthesis parameters, such as tungsten concentration and precursors and firing temperature. TiO2 sols were synthesized by hydrolysis of titanium isopropoxide (TIP) in excess of acidulated water, followed by peptization at 80 ºC and dialysis until pH 3.5. Sols containing W/Ti 1% or 5% (mol/mol) were prepared by incorporation of ammonium metataungstate or tungsten ethoxide before TIP hydrolysis. The films were deposited by spraying the resulting sol on the ceramic tiles. For characterization purposes the film was deposited on Si waffles by dip-coating. In all cases the samples were fired at different temperatures for 30 minutes. The self-cleaning activity was determined by monitoring the evolution of the diffuse reflectance spectra of crystal violet (CV) exposed to UVA or to visible light. CV degradation depends on W content. Under UVA illumination TiO2 films were more effective while under white illumination W-TiO2 were initially more efficient than the first ones due to the ability of W(VI) to accept electrons, improving the separation charge and the photocatalytic activity. Although the differences in the chemistry of the W precursors the material exhibits similar structure and surface composition, as determined by DRX, SEM-EDX and XPS.

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