1. Introduction
Self-cleaning applications using TiO2 thin films have become a subject of an increasing interest especially in recent years. The self-cleaning property has been known to be a combined effect between super-hydrophilicity and photo-catalysis [1-3]. The photo-catalytic property helps decompose the organic compounds that come into contact with the surface and thus prevents them from building up. The super-hydrophilic property of the TiO2 film on the surface allows water to spread completely across the surface rather than remain as droplets, thus making the surface easy to wash [4,5]. Therefore, the photocatalytic and hydrophilic properties of the TiO2 coated surface allow the water to more easily wash away deposited particles. Because of the light absorption edge of pure Titania, which is less than 380 nm, most applications are so far limited to UV-light irradiation [6,7]. For efficient photo-reactive activity, it is necessary to extend the photo-response of TiO2 from the ultraviolet to the visible region by modification of its optical properties. Further studies have been carried out for modification of the optical properties of TiO2 absorption from the ultraviolet to the visible light region, by ion doping with transitional metals such as: Cr, Fe, Ni, V, Mn, and Cu [8-10]. In the present study, V doped TiO2 thin films were prepared by the sol-gel dip coating method on the glass substrates. Then, photo-catalytic, super-hydrophilic and selfcleaning properties of films were investigated.
2. Materials and Methods
The TiO2 sol was prepared by dissolving tetra butyl orthotitanate (1 mole, TBOT, 97%) in ethanol (20 mole, 99%) and acetyl acetone (0.2 mole, 99.99%). Then acetic acid (1.5 mole, 99.7%), ethanol (20 mole, 99%) and deionized water (3 mol) were mixed separately and added to the first mixture. The final solution was stirred for two hours [11,12]. At this stage, a solution of ammonium metavanadate (NH4VO3) with certain concentration was prepared [10]. The content of V was 0.06 atomic percent. Before coating, the glass substrates (2 × 7 × 1 mm) were ultrasonically cleaned in boiled acetone and ethanol. The thin films were obtained by a dip coating method and withdrawn at a speed of 5 mm/s. The gel films were air dried for 15 h, and then heat-treated at 550˚C for 2 h in air atmosphere [11-13]. The crystal structure, thickness and surface characteristics of the thin films were evaluated with a Bruker X-ray diffract-meter (Ni-filter, Cu Kα radiation λ = 1.5406 A) and Field Emission Scanning Electron Microscopy (FE-SEM), respectively and UVVis transmittance spectra for films were obtained using a UV-Vis spectrophotometer.
The photo induced super-hydrophilicity of the films was measured by the contact angle of water droplet on the film surfaces with an experimental error of ±1. A droplet was injected on to the surface using a 5 μL micro-injector. It should be mentioned that UV light was irradiated to the surfaces by a Hg Lamp (16 W/cm2) [14].
The photo-catalytic activities of thin films under UV-irradiation were evaluated by the decoloring rate of methylene blue (C16H18N3SCl). For this purpose, one sample of thin film (surface area 14 cm2) was horizontally placed at the bottom of the testing cell containing specific amount of methylene blue solution (10 ppm). The solution was irradiated with Hg lamp. After the irradiation time, the light absorbance of methylene blue solution was measured using a UV-Vis spectrophotometer at the absorption rate (200 - 900 nm). The decoloring rate of methylene blue was used to evaluate the photo-catalytic activities of the films, with the following equation [15].
(1)
where 
is the light absorbance of methylene Blue before the irradiatation (absorbance equilibrium in dark place for 30 min) and 
is the light absorbance of methylene blue after the irradiation [15].
3. Results
3.1. FE SEM Analysis
The average thickness of the films was measured according to a FE-SEM cross section method. A FE-SEM cross section image of a TiO2-V thin film is shown in the Figure 1. The results indicated that the film thicknesses were approximately 266 and 313 nm for pure TiO2 and V doped TiO2 film, respectively.
3.2. XRD Measurements
The XRD figure is not shown here. However, the pattern illustrated that both TiO2 and V doped TiO2 thin films contain only an anatase phase.
3.3. FTIR Spectra
Figure 2 shows the UV-Vis absorption spectrum of thin films. It can be seen that the absorption edge for V doped TiO2 films shows a red shift compared with that of the pure TiO2. The shift is consistent with the incorporation of V5+ into the titania matrix. This indicates that the band gap energy in V doped TiO2 is lower than that of undoped TiO2.
3.4. UV-Vis Spectra
Figure 3 shows the UV-Vis absorption spectrum of thin
Figure 1. FE-SEM cross section image of V doped TiO2 thin film.
Figure 2. FTIR spectra of V doped TiO2 thin film.
Figure 3. UV-Vis absorption spectra of thin films.
films. It can be seen that the absorption edge for V doped TiO2 films shows a red shift compared with that of the pure TiO2. The shift is consistent with the incorporation of V5+ into the titania matrix. This indicates that the band gap energy in V doped TiO2 is lower than that of undoped TiO2.
3.5. Water Contact Angle
Figure 4 presents the results of water contact angle measurements on the thin film surfaces under irradiation. As shown in the Figure 4, TiO2-V thin film turned superhydrophilicity after 120 min irradiation. Meanwhile,
Figure 4. Water Contact Angle on the thin film surfaces under UV irradiation.
the pure TiO2 obtained super-hydrophilic after 180 min irradiation. This difference in the appearance of superhydrophilicity will be further discussed.
3.6. Photo Catalytic Activity
The photo-catalytic activities of films were characterized by the degradation of methylene blue. The methylene blue degradation rate after irradiation in the presence of thin films is shown in the Figure 5.
4. Discussion
According to the photocatalytic results, doping TiO2 thin film with 0.06 at %·V ions decrease the photocatalytic decomposition of methylene blue.
Since both photocatalytic oxidation of organic pollutants and photo induced superhydrophilicity are initiated by electron-hole pairs, the recombination of photo-generated electron-hole pairs can decrease the photoreactive efficiency of TiO2-V [16,17].
The high rate of recombination of photo-generated electron–hole pairs, which in turn prolongs the recombination time can be limited by introducing charge traps for electrons and/or holes. The beneficial effect of V5+ in photohydrophilicity can be described by considering the efficient seperation of photo-generated electons and holes. V5+ can act as a trap for photo-generated holes [10]. In order to produce hydroxyl radicals from absorbed hydroxyl ions the traped holes can migrate to the surface [10]:
(2)
(3)
(4)
(5)
(6)
hydroxyl groups have a significant effect on the photoreactivity of TiO2. Hydroxyl groups are important factors
Figure 5. The Methylene Blue degradation rate under UV irradiation in the presence of thin films.
in the TiO2 because they can reduce the recombination of electron-hole pairs. Therefore, the increase in the hydroxyl content on the surface of V+5 doped TiO2 is beneficial to the enhancement of superhydrophilicity property. On the other side, the introduction of V5+ ions in TiO2 thin film may responsible for reducing the photo-generated hole-electron recombination rate. Thus, in comprasion with pure TiO2, photocatalytic activity of thin film decreased with 0.06 at % V doping. Thus, the V doped TiO2 thin film shows higher hydrophilicity and a slight decrease in photo-catalytic effect than pure TiO2. It is then concluded that 0.06 atomic % Vanadium doped TiO2 thin film can have a noticeable effect on selfcleaning property.
5. Conclusion
In this research, V doped TiO2 thin film was immobilized on the glass substrates using the dip coating process. Water contact angle measurements and photo-catalytic methylene blue degradation indicated that the V doping improved the photo-reactivity of TiO2 film surfaces. Although TiO2 sol-gel derived thin film has better photocatalytic activity than V doped TiO2, the super-hydrophilicity effect can show a great decrease in contact angle for TiO2-V surfaces. So, this product can be useful in exhibiting a self-cleaning effect for practical purposes such as constructional applications, especially for wherever the superhydrophilicity effect would be a significant parameter.
NOTES