An active, robust and transparent nanocrystalline anatase TiO2 thin film -: preparation, characterisation and the kinetics of photodegradation of model pollutants

The preparation, characterisation, and photocatalytic activity of robust and transparent nanocrystalline thin films of anatase TiO2 are described. The films, which are prepared from a sol-gel can be either dip-coated or spin-coated, however, the latter have better optical and mechanical properties, since dip coating results in the retention of a significant amount of the organic stabilisers. used in the sol-gel preparation. Quantum yields for the photooxidation of 4-chlorophenol ((Phi (4CP)) on these spin-coated films are similar to those reported previously for TiO2 films and dispersions, with Phi (4CP) = 0.5-1%. For films of different thicknesses, and therefore different absorption efficiencies, Phi (4CP) depends only upon the total number of photons absorbed, i.e. Phi is independent of both the absorbed photon flux and the distribution of charge carriers in the film. By way of contrast, the rate of photoreduction of methylene blue is independent of film. thickness and appears to be limited by the surface area of the film. Hence, quantum yields for methylene blue reduction (Phi (MBR)) fall significantly from 1.7 to 0.5% as film thickness, and hence photoabsorption efficiency, is increased. These pure anatase films do not show any deleterious sodium ion effect for either 4-chlorophenol oxidation or methylene blue reduction. Quantum yields for photooxidation of stearic acid (Phi (SA)) deposited directly onto the films are relatively high with Phi (SA) = 1-4%. This, combined with the excellent optical and mechanical properties of these films, suggests that they may be of particular interest for the development of 'self-cleaning windows'. (C) 2001 Elsevier Science B.V. All rights reserved.