Effect of Residual Stress on the Photochemical Properties of TiO2 Thin Films

Sputtered TiO2 films with residual stresses ranging from compressive to tensile were fabricated by use of Substrates with different thermal expansion coefficients. With these stress-controlled films, changes in the photocatalytic oxidation and photoinduced hydrophilic conversion activities were investigated. The photocatalytic oxidative decomposition of cis-9-octadecenoic acid showed little stress dependence. In contrast, the photoinduced hydrophilic conversion showed remarkable stress dependence. The rate constant for the hydrophilic conversion of a film with large tensile stress (sigma = -1120 +/- 200 GPa) was 5 times greater than that of a film with negligibly small compressive stress (sigma = +270 +/- 200 GPa). In contrast, introducing large compressive stress (sigma = +890 +/- 250 GPa) reduced it to 1/16 of that for the film with negligibly small compressive stress. On the basis of these results, we have proposed an effective pressure effect on photoinduced hydrophilic conversion and have experimentally estimated the apparent positive activation volume (ca. +5 x 10(-6) m(3)/mol) for that reaction, in accordance with transition state theory.