Photo-induced super-hydrophilicity of TiO₂ surfaces [1], providing self-cleaning and anti-fogging properties, is generally attributed to some chemical surface modifications such as an increased concentration of Ti OH groups under illumination. TiO₂ is also known to undergo light induced hydrogen insertion [2], especially in nanostructured material, but up to now no clear correlation has been evidenced between super-hydrophilicity and hydrogen evolution or intercalation.

In the present work, light-induced superhydrophilicity on nanostructured TiO₂ layers deposited by PVD on stainless steel was characterized by contact angle measurement and impedance spectroscopy, as a function of the thickness of the layer (50 to 300 nm), light intensity and duration of exposure. The dielectric constant of TiO₂ is determined by Mott-Schottky analysis from the change in the M-S slope when the depletion layer reaches the TiO₂/substrate interface, showing a significant increase under illumination, while the water contact angle decreases. Further storage in the dark, shows the reversibility of light –induced modifications. Such behaviour could be the consequence of hydrogen insertion during illumination [3] followed by extraction in the dark and which will be discussed as a function of the structure and the thickness of the titanium oxide layer.

References
[1] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigosh, T. Watanabe, Nature, 1997, 388, 431-432
[2] B.I. Lemon, J. T. Hupp, J. Phys. Chem., 1996, 100, 14578-14580.
[3] G. K. Boschloo, A. Goossens, J. Schoonman, J. Electrochem. Soc., 1997, 144, 1311-1317