Photoinduced Superhydrophilicity: A Kinetic Study of Time Dependent Photoinduced Contact Angle Changes on TiO2 Surfaces

Transparent TiO2 thin films were prepared on quartz substrates via a reverse micelle, sol gel, spin-coating technique. The time dependence of the TiO2 film photoinduced superhydrophilicity (PISH) was measured by goniometric observation of the contact angle, theta, of sessile water drops at the film surfaces. In these measurements, the TiO2 substrate was illuminated by 315 nm light and drops were sequentially applied at a range of illumination times. Using a model for the wetting of heterogeneous surfaces derived by Israelachvili and Gee, these measurements were used to calculate the time dependence of f(2), the fractional surface coverage of the TiO2 surface by adventitious contaminating organics (Israelachvili, J. N.; Gee, M. L. Contact angles on chemically heterogeneous surfaces. Langmuir 1989, 5, 288). Extending this model to include a Langmuir-Hinshelwood based kinetic analysis of f(2) as a function of time allowed for calculation of an expected value for theta immediately prior to illumination, that is, at illumination time t = 0. Such expected values of theta at t = 0 were calculated using two possible values of theta(1), the contact angle on a pristine unilluminated homogeneous TiO2 surface: (i) theta(1) = 4 degrees as suggested by, inter alia, Zubkov et al. (Zubkov, T.; Stahl, D.; Thompson, T. L.; Panayotov, D.; Diwald, O.; Yates, J. T. Ultraviolet Light-Induced Hydrophilicity Effect on TiO2(110)(1 x 1). Dominant Role of the Photooxidation of Adsorbed Hydrocarbons Causing Wetting by Water Droplets. J. Phys. Chem. B 2005, 109, 15454); and (ii) where theta(1) = 25 degrees, as suggested by Fujishima et al., representative of a more hydrophobic homogeneous TiO2 surface that reconstructs upon exposure to ultraband gap illumination into a hydrophilic surface where theta(1) -> 0 degrees (Fujishima, A.; Zhang, X.; Tryk, D. A. TiO2 photocatalysis and related surface phenomena Surf. Sci. Rep. 2008, 63, 515). Analysis of data from our experiments and from selected literature sources demonstrates better agreement between these calculated and experimental values of theta at t = 0 when theta(1) is taken to be 4 degrees, implying that an uncontaminated TiO2 surface is inherently hydrophilic. The results of this study are discussed in the context of the current debate over the origin of the photoinduced superhydrophilic effect.