Photo-oxidatively self-cleaning transparent titanium dioxide films on soda lime glass: The deleterious effect of sodium
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Photo-oxidatively self-cleaning transparent titanium dioxide films on soda lime glass: The deleterious effect of sodium contamination and its prevention Y. Paza) and A. Heller Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712-1062 (Received 22 May 1996; accepted 3 March 1997)
In the context of photocatalytically self-cleaning windows and windshields, clear, abrasion resistant, thin (60 6 10 nm) photocatalytic films of TiO2 were formed by a sol-gel process on (a) soda lime glass, (b) the proton-exchanged surface of soda lime glass, and (c) fused silica. The hypothesis that diffusion of sodium oxide from the soda lime glass into the titanium dioxide layer during the calcination step causes the lower photoefficiency in films on glass was tested and proven. At high concentration sodium prevented formation of the photoactive anatase phase and, at low concentration, introduced surface and bulk recombination centers. Sodium transport was efficiently blocked by a thin layer at the interface of proton-exchanged (“hydrogen”) glass and nascent TiO2 , formed at 400 ±C of a poly(titanyl acetylacetonate) TiO2 precursor. The sodium transport blocking layer did not form and the highly photocatalytic film was not obtained when the TiO2 -precursor film was applied to glass that was not proton exchanged. Furthermore, only a much less effective sodium transport blocking layer was formed on glass that was proton-exchanged, but was calcined at 400 ±C prior to application of the TiO2 precursor layer, showing that the sodium depleted glass surface, by itself, was a less effective barrier against sodium transport than the interfacial product of hydrogen glass and the TiO2 precursor.
I. INTRODUCTION
TiO2 is a photocatalyst for oxidation of organic contaminants in air and water.1 Thin, transparent, abrasion resistant non light-scattering films of titania on soda lime glass form the basis for potentially self-cleaning windows. For example, when coated on the cabin side of automotive windshields, films of plasticizer from automotive upholstery and carpeting and of cigarette smoke components that often accumulate on windshields could be photocatalytically oxidized. On the outside of the windshield organic grime could be oxidized. Recent work at TOTO in Japan also showed that photocatalytic TiO2 coated glass surfaces do not fog upon condensation of water droplets, because their spreading on the hydrophylic surface leads to a uniform, thin water film. Coatings formed of available photocatalytic titania, such as Degussa’s P25, scatter visible light. Clear films of photoactive titanium dioxide supported on glasses were formed earlier through spin coating,2,3 or spray pyrolysis of titanium alkoxides on fused quartz and borosilicate glasses.4,5 Unlike the films on fused quartz and those on borosilicate glasses, the films formed on common soda lime glass6 were not photoactive, a)
Current address: Department of Chemical Engineering, The Technion– Israel Institute
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