Photoinduced superhydrophilicity : a kinetic study of time dependent photoinduced contact angle changes on TiO 2 surfaces

Foran, Philip and Boxall, Colin and Denison, Kieth (2012) Photoinduced superhydrophilicity : a kinetic study of time dependent photoinduced contact angle changes on TiO 2 surfaces. Langmuir, 28 (51). 17647–17655. ISSN 0743-7463

Abstract

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 Gee1, these measurements were used to calculate the time dependence of f2, the fractional surface coverage of the TiO2 surface by adventitious contaminating organics. Extending this model to include a Langmuir-Hinshelwood based kinetic analysis of f2 as a function of time allowed for calculation of an expected value for θ immediately prior to illumination i.e. at illumination time t = 0. Such expected values of θ at t = 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, inter alia, Zubkov et al.2; and (ii) where θ1 = 25°, as suggested by Fujishima et al.3, representative of a more hydrophobic homogeneous TiO2 surface that reconstructs upon exposure to ultra-band gap illumination into a hydrophilic surface where θ1  0°. Analysis of data from our experiments and from selected literature sources demonstrates better agreement between these calculated and experimental values of θ at t = 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 photo-induced super-hydrophilic effect.