Photooxidative self-cleaning transparent titanium dioxide films on glass
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L. Rabenberg Department of Mechanical Engineering and Center for Materials Science, The University of Texas at Austin, Austin, Texas 78712-1062
A. Heller Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712-1062 (Received 15 May 1995; accepted 25 July 1995)
In the context of studying the feasibility of photocatalytically self-cleaning windows and windshields, clear, abrasion resistant, photocatalytic films of TiO2 were formed on soda lime glass and on fused quartz by a sol-gel process. The rate of photooxidation of contaminant deposits was estimated by measuring the rate of decrease in the integrated IR absorbance associated with the C-H stretching vibrations of a thin solution-cast film of stearic acid under 365 nm (2.4 mW/cm 2 ) or 254 nm (0.8 mW/cm 2 ) irradiation. Approximately 3 X 1 0 " * stearic acid molecules were stripped per 365 nm photon in either front- or back-illuminated soda lime glass, and 6 X 10"* molecules when the films were coated on fused quartz. For thin TiO2 films on fused quartz, the rate of photooxidation, normalized by the number of photons absorbed per unit area, was independent of the wavelength. In contrast, for films on soda lime glass, the rate of photooxidation, when similarly normalized, was higher for the less penetrating wavelength. The reduced photoactivity on glass at the deeply penetrating wavelength (365 nm), as well as the greater photoefficiency on quartz than on glass, are attributed to diffusion of sodium oxide from the glass into the inner glass-contacting zone of the T1O2 layer.
I. INTRODUCTION TiO2 is a well-known photocatalyst for the airoxidation of organic compounds. Its aqueous suspensions, li2 powders,3'4 and supported coatings5 were applied in the oxidative stripping of organic reactants from air and water. Although clear and adherent films of titania are used in optical coatings,6 their photocatalytic properties have not been characterized. Such films could form the basis for self-cleaning or photooxidatively indoor air cleaning windows and automotive windshields. Three properties are particularly relevant to these applications: the absence of scattering of visible light, requiring absence of particles larger than about 30 nm; sufficient abrasion resistance to assure that the film is not damaged when cleaned or when impacted by duct particles; and adequate photooxidation rate, i.e., efficiency. The commonly used highly photocatalytic titania particles, e.g., those of Degussa P-25, bound to the surface through a photostable binder are not particularly useful in this application, because they consist of —0.1 /xm diameter, light-scattering particles. A route toward transparent self-cleaning windows exists, however, through thin, consolidated ultra-small-grained clear films. 2842
J. Mater. Res., Vol. 10, No. 11, Nov 1995
http://journals.cambridge.org
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It has been recognized7 that the reaction of sodium oxide migrating from soda glass into TiC>2 films produces either a brookite TiO2 phase or an incompl
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