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 an d 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 o rganic stabilisers. used in the sol-gel preparation. Quantum yields for the photooxidation of 4-chlorophenol ((Phi (4CP)) on the se spin-coated films are similar to those reported previously for TiO2 film s and dispersions, with Phi (4CP) = 0.5-1%. For films of different thicknes ses, and therefore different absorption efficiencies, Phi (4CP) depends onl y upon the total number of photons absorbed, i.e. Phi is independent of bot h the absorbed photon flux and the distribution of charge carriers in the f ilm. By way of contrast, the rate of photoreduction of methylene blue is in dependent 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 photoabso rption efficiency, is increased. These pure anatase films do not show any d eleterious sodium ion effect for either 4-chlorophenol oxidation or methyle ne 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 o f these films, suggests that they may be of particular interest for the dev elopment of 'self-cleaning windows'. (C) 2001 Elsevier Science B.V. All rig hts reserved.