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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    • "The nature of a TiO2 layer hypothetically includes the architecture (ie, in nanonodular form), thickness (eg, it should be 300-nm thick or more), and surface area (eg, the larger, the better). Exposing TiO2 to UV light results in the excitement of an electron from the valence band to the conduction band.20,21 This physicochemical change – the excited electron along with the positive hole created in the superficial layer of TiO2 – is part of the explanation for why UV-mediated photocatalysis occurs. "
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    ABSTRACT: Bioactivity and osteoconductivity of titanium degrade over time after surface processing. This time-dependent degradation is substantial and defined as the biological aging of titanium. UV treatment has shown to reactivate the aged surfaces, a process known as photofunctionalization. This study determined whether there is a difference in the behavior of biological aging for titanium with micro-nano-hybrid topography and titanium with microtopography alone, following functionalization. Titanium disks were acid etched to create micropits on the surface. Micro-nano-hybrid surfaces were created by depositioning 300-nm diameter TiO(2) nodules onto the micropits using a previously established self-assembly protocol. These disks were stored for 8 weeks in the dark to allow sufficient aging, then treated with UV light for 48 hours. Rat bone marrow-derived osteoblasts were cultured on fresh disks (immediately after UV treatment), 3-day-old disks (disks stored for 3 days after UV treatment), and 7-day- old disks. The rates of cell attachment, spread, proliferation, and levels of alkaline phosphatase activity, and calcium deposition were reduced by 30%-50% on micropit surfaces, depending on the age of the titanium. In contrast, 7-day-old hybrid surfaces maintained equivalent levels of bioactivity compared with the fresh surfaces. Both micropit and micro-nano-hybrid surfaces were superhydrophilic immediately after UV treatment. However, after 7 days, the micro-nano- hybrid surfaces became hydrorepellent, while the micropit surfaces remained hydrophilic. The sustained bioactivity levels of the micro-nano-hybrid surfaces were nullified by treating these surfaces with Cl(-)anions. A thin TiO(2) coating on the micropit surface without the formation of nanonodules did not result in the prevention or alleviation of the time-dependent decrease in biological activity. In conclusion, the micro-nano-hybrid titanium surfaces may slow the rate of time-dependent degradation of titanium bioactivity after UV photofunctionalization compared with titanium surfaces with microtopography alone. This antibiological aging effect was largely regulated by its sustained electropositivity uniquely conferred in TiO(2) nanonodules, and was independent of the degree of hydrophilicity. These results demonstrate the potential usefulness of these hybrid surfaces to effectively utilize the benefits of UV photofunctionalization and provide a model to explore the mechanisms underlying antibiological aging properties.
    International Journal of Nanomedicine 06/2011; 6:1327-41. DOI:10.2147/IJN.S22099 · 4.38 Impact Factor
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    • "In following years further photochemical applications of TiO 2 have been realized, most of them motivated particularly by the advantageous combination of its low cost, nontoxicity, and excellent stability against photocorrosion. One important example is the rapidly growing field of heterogeneous photocatalysis [17] [18] [19] [20] [21] [22] [23] [24] [25], in which TiO 2 has been successfully employed in photooxidation reactions utilizing aerobic oxygen for the complete removal of pollutants from water and air [17–22, 24–39], in preparation of superhydrophilic and antifogging surfaces [40] [41] [42] [43], in photocatalytic organic syntheses [44– 49], or in antitumor medicinal applications [50] [51] [52] [53] [54]. "
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    ABSTRACT: TiO2-based nanomaterials play currently a major role in the development of novel photochemical systems and devices. One of the key parameters determining the photoactivity of TiO2-based materials is the position of the band edges. Although its knowledge is an important prerequisite for understanding and optimizing the performance of photochemical systems, it has been often rather neglected in recent research, particularly in the field of heterogeneous photocatalysis. This paper provides a concise account of main methods for the determination of the position of the band edges, particularly those suitable for measurements on nanostructured materials. In the first part, a survey of key photophysical and photochemical concepts necessary for understanding the energetics at the semiconductor/solution interface is provided. This is followed by a detailed discussion of several electrochemical, photoelectrochemical, and spectroelectrochemical methods that can be applied for the determination of band edge positions in compact and nanocrystalline thin films, as well as in nanocrystalline powders.
    Advances in Physical Chemistry 01/2011; DOI:10.1155/2011/786759(1687-7985). DOI:10.1155/2011/786759
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    • "Nearly all decane was lost from both the nonirradiated and irradiated surfaces during vacuum treatment, indicating that the decane was weakly bound, that is, physically absorbed to the surface. These experiments provide support for the theory that hydrocarbon contaminants [13], not UV-induced defects [18] or UV-induced rupture of Ti–OH bonds [19], are responsible for surface hydrophilicity. "
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    ABSTRACT: Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to study illuminated surfaces under both vacuum conditions, and in the presence of organic molecules (decane and methanol). In the presence of hole scavengers, electrons are trapped at Ti(III)–OH sites, and free electrons are generated. These free electrons are seen to decay by exposure either to oxygen or to heat; in the case of heating, reinjection of holes into the lattice by loss of sorbed hole scavenger leads to a decrease in Ti(III)–OH centers. Decane adsorption experiments lend support to the theory that removal of surficial hydrocarbon contaminants is responsible for superhydrophilic surfaces. Oxidation of decane led to a mixture of surface-bound organics, while oxidation of methanol leads to the formation of surface-bound formic acid.
    International Journal of Photoenergy 01/2008; 2008. DOI:10.1155/2008/964721 · 1.56 Impact Factor
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Tykhon Zubkov