Ultraviolet Light-Induced Hydrophilicity Effect on TiO 2 (110)(1×1). Dominant Role of the Photooxidation of Adsorbed Hydrocarbons Causing Wetting by Water Droplets

Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA.
The Journal of Physical Chemistry B (Impact Factor: 3.3). 09/2005; 109(32):15454-62. DOI: 10.1021/jp058101c
Source: PubMed


The UV photoproduction of a hydrophilic TiO(2)(110)(1x1) surface has been investigated in a pressurized ultrahigh vacuum apparatus under controlled conditions of hydrocarbon concentration in oxygen gas at 1 atm pressure. Water droplet contact angles have been measured continuously as the droplet is exposed to UV irradiation, yielding the first observations of a sudden wetting process during irradiation. Using hexane as a model hydrocarbon, it is found that when low partial pressures of hexane are present, the sudden onset of surface wetting occurs during UV irradiation after an induction period under photooxidation conditions. The induction period to reach the critical condition for sudden wetting increases when the partial pressure (and equilibrium surface coverage) of hexane is increased. These results indicate that the removal of adsorbed hydrocarbons by photooxidation is the critical factor leading to the UV-induced hydrophilicity phenomenon on TiO(2). The phenomenon does not occur in the absence of O(2) gas. A concept concerned with kinetic screening of the TiO(2)-H(2)O interface from O(2) by water droplets is presented to explain the observation of sudden wetting in our experiments, compared to gradual wetting which is observed following UV irradiation in all other experiments reported in the literature. Complementary infrared spectroscopy measurements of the effect of UV irradiation in an O(2) atmosphere on adsorbed Ti-OH groups and on adsorbed H(2)O on the surface of a high-area TiO(2) powder show that no spectroscopic changes occur. This indicates that UV-induced changes in the -OH coverage or the nature of -OH bonding to TiO(2), as suggested by others, cannot be used to explain the photoinduced hydrophilicity effect.

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    • "Titanium dioxide (TiO 2 ) as photocatalyst has been widely used for industrial and medical applications due to its useful physical and biological properties in the last two decades, such as disposal of wastewater, decontamination of air pollutants , and sterilization of bacteria [1] [2] [3] [4]. It has been well known that photoinduced electrons and holes could be generated on the TiO 2 surface under exposure to ultraviolet (UV) light [5] [6]. These excited electrons and holes have strong reduction and oxidation activities and could further react with hydroxyl ions or water resulting in the formation of various reactive oxygen species (ROS) which have been proved to significantly damage cancer cells [7] [8] [9]. "
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