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

Abstract

Transparent TiO(2) thin films were prepared on quartz substrates via a reverse micelle, sol-gel, spin-coating technique. The time dependence of the TiO(2) 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 TiO(2) 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 TiO(2) 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 θ immediately prior to illumination, that is, at illumination time t = 0. Such expected values of θ at t = 0 were calculated using two possible values of θ(1), the contact angle on a pristine unilluminated homogeneous TiO(2) surface: (i) θ(1) = 4° 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 TiO(2)(110)(1 × 1). Dominant Role of the Photooxidation of Adsorbed Hydrocarbons Causing Wetting by Water Droplets. J. Phys. Chem. B2005, 109, 15454); and (ii) where θ(1) = 25°, as suggested by Fujishima et al., representative of a more hydrophobic homogeneous TiO(2) surface that reconstructs upon exposure to ultraband gap illumination into a hydrophilic surface where θ(1) → 0° (Fujishima, A.; Zhang, X.; Tryk, D. A. TiO(2) 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 θ at t = 0 when θ(1) is taken to be 4°, implying that an uncontaminated TiO(2) 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.

Surface-induced assembly of sophorolipids

“…Within the framework of our experimental conditions, we determined a contact angle of 691 AE 41 on bare gold in accordance with a previous study, 85 521 AE 61 on titanium dioxide anatase layer, and 431 AE 41 on silicon wafer. If titanium dioxide was known to be highly hydrophilic, the value we have determined was actually coherent to the values reported in the literature 86 for a non-model surface that contains environmental impurities. These data have indicated that the gold surfaces, as prepared in these experiments, were the most hydrophobic samples and silicon wafers were the most hydrophilic.…”

In Situ Characterization of Interfaces Relevant for Efficient Photoinduced Reactions

“…The self‐cleaning characteristics of semiconductors result not only from their strong oxidation power under irradiation, but also from the discovered phenomenon of the photoinduced superhydrophilicity inherent in the surface . Thus, one may consider the contribution of a self‐cleaning function combined with stability of the organized soft matter on a semiconductor for novel “intelligent” materials and nanoscale machineries.…”

Grand challenges in inorganic chemistry: toward better life quality and a more sustainable world

“…Photocatalysis is another facet of the problem (Fujishima et al, 2008) with higher ambition: in addition to producing solar fuels (typically hydrogen), one gains storage capability, a real Holy Grain since sunlight is not always available. Main efforts are focusing on water splitting (Walter et al, 2010; Valdes et al, 2012), an ideal process since energy production from hydrogen and oxygen only generates innocuous water as waste.…”