The TiO2–WO3 films were prepared by the spin coating technique from 3 M bis(2,4-pentanedionato) titanium oxide (Tokyo Kasei) in ethanol containing WO3 (crystalline WO3 particles; ⩾99.5%; diameter observed by SEM was from a few tens of nm to ca. 500 nm, Japan New Metals) (W/Ti mole ratio = 0.3, 0.6 or 1.0). In order to obtain almost the same thickness (film mass ∼40 mg) for different film compositions, spin coating was performed at different rates as shown in Table 1. Then, the substrates were annealed at 400 °C for 1 h. The thickness of the films measured by SEM was about 1 μm. In the study of film thickness dependence, the coating was repeated a few times