Ultraviolet Light-Induced Hydrophilicity Effect on TiO₂(110)(1×1). Dominant Role of the Photooxidation of Adsorbed Hydrocarbons Causing Wetting by Water Droplets

Abstract

The UV photoproduction of a hydrophilic TiO 2 (110)(1×1) 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.

Influence of plasma and ultraviolet treatment of zirconia on initial attachment of human oral keratinocytes: Expressions of laminin γ2 and integrin β4

“…In this model, UV light irradiation creates surface O2 vacancies at bridging O2 sites. Wetting is caused by surface physicochemistry alteration by TiO 2 photocatalytic activity in association with photocatalytic removal of hydrocarbons from the surface 31,32) . Decomposition of hydrocarbon is accelerated by UV with wavelengths of less than 260 nm ( · = · 103 kcal/mol), which corresponds to UVC.…”

Superwetting of TiO 2 by light-induced water-layer growth via delocalized surface electrons

“…The phenomenon has been attributed to production of surface radical groups (1,(3)(4)(5) or photo-oxidation of hydrophobic surface contaminants (6)(7)(8), but its origin remains controversial (9,10) in part because conventional contact-angle analysis alone cannot fully account for the complex surface wettability (11). To address the fundamental mechanism responsible for superwetting, one has to go beyond the contact-angle measurement that reflects the specific surface properties and to probe the temporal growth dynamics of the water layer itself adsorbed on TiO 2 under wide-spectrum illumination of visible and near-infrared (NIR) in addition to UV light.…”

Wetting Angles on Illuminated Ta2O5 Thin Films with Controlled Nanostructure

“…Table 1). According to Yates et al, 32 contamination of the surface of oxides by carbonaceous rests may produce an increase in contact angles. It is likely that such contamination occurs in our films because of their handling in air, thus leading to contact values higher than 90°for the normal evaporated and IBAD thin films (note that the measured values of these two thin films is close to 95°; cf.…”