If necessary, the TiO2–WO3 film was modified with PWA by casting an alkoxysilane solution (NDH-300A, Nippon Soda) diluted with ethanol (50:50 by volume) containing PWA. The mole ratios of PWA/WO3 of the modified films (PWA–TiO2–WO3) were 0.1, 0.3 and 0.5. The modified films were annealed at 100 °C for 30 min. A TiO2 film containing the same amount of TiO2 as in the composite TiO2–WO3 film was also prepared the by spin coating technique (1000 rpm, 10 s) from 3 M bis(2,4-pentanedionato) titanium oxide in ethanol. The film was annealed at 400 °C for 1 h before modification with PWA. The PWA–TiO2 films were prepared to contain the same amount of PWA and TiO2 as in the PWA–TiO2–WO3 films