TY - JOUR

T1 - Photoinduced superhydrophilicity

T2 - a kinetic study of time dependent photoinduced contact angle changes on TiO 2 surfaces

AU - Foran, Philip

AU - Boxall, Colin

AU - Denison, Kieth

PY - 2012

Y1 - 2012

N2 - 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.

AB - 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.

KW - superhydrophilicity

KW - contact angle measurement

KW - Photocatalysis

KW - Titanium dioxide

U2 - 10.1021/la3026649

DO - 10.1021/la3026649

M3 - Journal article

VL - 28

SP - 17647

EP - 17655

JO - Langmuir

JF - Langmuir

SN - 0743-7463

IS - 51

ER -