Action spectrum analyses of photoinduced superhydrophilicity of titania thin films on glass plates

Samples of titania-thin-film-coated glass plates were prepared by spin-coating of commercial titania-coating liquids using a home-made spin coater (1000 rpm, 30 s) and then calcined in air at 673 K for 2 h. In a part of experiments, samples were stored in an oven at 393 K overnight. A piece of sample was set in a 10-mm rectangular quartz cell, and monochromatic light (FWHM of ca. 17 nm, intensity of ca. 108 einstein s− 1) was irradiated in ambient air using a grating multiple-wavelength photoirradiation instrument (Jasco CRM-FD). Water contact angle (WCA) of samples before and after monochromatic irradiation was measured by a contact angle analyzer. Changes in WCA, as a function of wavelength of monochromatic irradiation, a kind of action spectrum, of several titania samples showed that the sample surfaces became superhydrophilic, i.e., WCA became almost zero, only under irradiation with a wavelength shorter than the absorption edge, corresponding to the band gap, of samples. Analysis of changes in WCA showed that there are at least two stages when the samples stored at 393 K overnight before irradiation to give relatively high WCA of ca. 40° were used: first stage of WCA > 10° and second stage of WCA < 10°. The action spectra of WCA change in these two stages are discussed.