Enhancement of photoinduced highly hydrophilic conversion on TiO2 thin films by introducing tensile stress

Utilizing differences in the thermal-expansion from TiO2, we fabricated sputtered anatase TiO2 thin films with tensile stress and without stress on NEX-C and quartz substrates, respectively. The differences in the photocatalytic oxidation activity and photoinduced hydrophilicity on these anatase TiO2 thin films are reported. The photocatalytic oxidation activities, which were identical for the two films, were evaluated by the initial photodegradation rate constants of cis-9-octadecenoic acid (k = 1.7 +/- 0.1 mug.cm(-2).h(-1) for NEX-C and 1.6 +/- 0.1 mug.cm(-2).h(-1) for quartz.). The photoinduced hydrophilic conversion rate, which was evaluated by monitoring the changes in water contact angles, however, was 5 times greater for the film with tensile stress than for the film without residual stress. Because the surface morphologies and the charge-separation efficiencies of these two thin films are nearly identical, it is concluded that the hydrophilic conversion on the anatase thin film can be enhanced by introducing tensile stress.