Ultrathin SiOx film coating effect on the wettability change of TiO2 surfaces in the presence and absence of UV light illumination

Wettability of sol-gel TiO2 film surfaces has been studied by following the H2O contact angle (theta) as functions of illumination time and subsequent dark storage time. Upon illumination of the TiO2 surface (lambda (ex) > 300 nm), the value of theta rapidly decreased to reach approximately zero (photoprocess). When the resultant superhydrophilic sample was stored in the dark, the theta value increased slowly with increasing storage time (dark process). Ultrathin SiOx films were formed on the surface of TiO2 by repeating chemisorption of 1,3,5,7-tetramethylcyclotetrasiloxane and its photooxidation. With increasing thickness of the SiOx monolayer coating, the rate for the dark process significantly decreased, while that for the photoprocess also decreased. Both X-ray photoelectron spectroscopic studies and product analyses during the photoprocess clarified that photocatalytic oxidation of organic adsorbates in air causes the superhydrophilicity of the TiO2 surfaces. (C) 2000 Academic Press.