Transparent titania-zirconia-silica thin films for self-cleaning and photocatalytic applications

Highly transparent thin films of TiO₂-ZrO₂-SiO₂ composites on glass were prepared by sol-gel process, wherein different amounts of zirconia precursor were mixed with titania precursor solution (0–20 mol% Zr with regard to Ti). For better adhesion of the films on glass supports, a silica binder was added to the mixture of alkoxides (27 mol% Si vs. Ti). The prepared samples were characterized by various techniques to obtain information on their structural, optical and surface properties. The presence of anatase titania polymorph as the only crystalline phase was detected. Unexpectedly, the higher loading of Zr promoted the growth of anatase particles with progressive spherical shape and concomitant decrease of specific surface area, which adversely influenced the photocatalytic activity. Decrease of the photodegradation activity with increasing amount of Zr was evidenced by contact angle measurements of a fatty deposit, spectrofluorometrically using terephthalic acid probe and by EPR spectroscopy monitoring indirectly the non-persistent radicals generation. On the other hand, the higher content of Zr improved the mechanical stability of resulting thin films. An optimal Zr content around 10 mol% appears to be a good compromise between photocatalytic activity and mechanical robustness of the films.