Visible-light-induced superhydrophilicity of crystallized WO thin films fabricated by using a newly isolated W complex salt of citric acid.

Transparent tungsten trioxide thin films, which demonstrated visible-light (Vis-light)-induced superhydrophilicity, with thicknesses of 100-120 nm, adhesion strengths greater than 49 MPa, bandgap energies of 2.8-2.9 eV, and haze values of 0.4-0.5%, were fabricated using a solution-based process on quartz glass substrates. The precursor solution was prepared by dissolving a W complex salt isolated from a reacted solution of tungstic acid, citric acid, and dibutylamine in HO, in ethanol. By heating the spin-coated films in air for 30 min at temperatures higher than 500 °C, crystallized WO thin films were obtained. The O/W atomic ratio was evaluated to be 2.90, based on the peak area analysis of X-ray photoelectron spectroscopy spectra of the thin-film surfaces, indicating the co-presence of W ions. The water contact angle on film surfaces, which was approximately 25° prior to light irradiation, decreased to less than 10° upon irradiation with 0.06 mW cm Vis-light for only 20 min at 20-25 °C and a relative humidity (RH) of 40-50%. By comparing the contact angle changes at RH values of 20-25%, it was revealed that the interaction between ambient water molecules and the partially O-deficient WO thin films plays an important role in achieving photoinduced superhydrophilicity.