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, θ, 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 <i>f</i>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. <i>Langmuir</i> <b>1989</b>, <i>5</i>, 288). Extending this model to include a Langmuir–Hinshelwood based kinetic analysis of <i>f</i>₂ as a function of time allowed for calculation of an expected value for θ immediately prior to illumination, that is, at illumination time <i>t</i> = 0. Such expected values of θ at <i>t</i> = 0 were calculated using two possible values of θ₁, the contact angle on a pristine unilluminated homogeneous TiO₂ surface: (i) θ₁ = 4° as suggested by, <i>inter alia</i>, 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 × 1). Dominant Role of the Photooxidation of Adsorbed Hydrocarbons Causing Wetting by Water Droplets. <i>J. Phys. Chem. B</i> <b>2005</b>, <i>109</i>, 15454); and (ii) where θ₁ = 25°, 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 θ₁ → 0° (Fujishima, A.; Zhang, X.; Tryk, D. A. TiO₂ photocatalysis and related surface phenomena <i>Surf. Sci. Rep.</i> <b>2008</b>, <i>63</i>, 515). Analysis of data from our experiments and from selected literature sources demonstrates better agreement between these calculated and experimental values of θ at <i>t</i> = 0 when θ₁ is taken to be 4°, 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