Anatase nanocrystals were formed throughout the whole of poly(ethylene glycol) (PEG)-containing silica–titania films after a hot water treatment of the films, whereas in the films without PEG, anatase nanocrystals were formed only on the surface of the films. The photocatalytic activities of the anatase nanocrystal-dispersed silica films obtained with and without PEG were examined from the decomposition of methylene blue, MB. Both anatase nanocrystal-dispersed silica films with and without PEG showed slightly higher photocatalytic activities than pure anatase films in spite of their lower titania contents. The amounts of MB photocatalytically decomposed by the films with PEG increased with an increase in the film thickness. On the other hand, the amounts of MB decomposed by the films without PEG were almost independent of the film thickness. These results indicate that the effective photocatalytic decomposition of MB occurs not only at the surface but also in the inside of the films obtained with PEG, and that it occurs mainly at the surface of the films obtained without PEG.
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Kotani et al. (2001) studied this question.