Article pubs.acs.org/JPCC
Factors Affecting UV-Induced Superhydrophilic Conversion of a TiO2 Surface Alexei V. Emeline,*,† Aida V. Rudakova,† Munetoshi Sakai,‡ Taketoshi Murakami,‡ and Akira Fujishima*,‡ †
Faculty of Physics, Saint-Petersburg State University, Saint-Petersburg, Russia Kanagawa Academy of Science and Technology, Kawasaki-shi, Kanagawa, Japan
‡
S Supporting Information *
ABSTRACT: The present study explored the effects of several factors (wetting, light intensity, spectral variation of the actinic light, heating, and surface acidity) on the hydrophilic conversion of the surface of TiO2 nanocoatings. The experimental dependencies of the efficiencies of photoinduced hydrophilic surface conversion on the intensity and wavelength of the actinic light clearly indicate the role of electronic photoexcitation in hydrophilic surface transformation. Particularly, the maximum extrema in spectral dependence of the efficiency of photoinduced hydrophilic conversion correspond to the energies of the first indirect and first direct electronic band-to-band transitions in TiO2. At the same time, temperature dependence and the effect of the surface acidity on the hydrophilic behavior of the TiO2 surface demonstrate the importance of the multilayer hydrate structure in both the original hydrophilicity of the surface and the direction of the photoinduced hydrophilic conversion. Estimation of the surface energy alteration under photoexcitation suggests that only specific surface sites (10−3−10−4 monolayer) are responsible for the effect of photoinduced superhydrophilicity of a TiO2 surface.
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INTRODUCTION The effect of surface photoinduced superhydrophilicity represents one of those few examples in the history of scientific exploration in which the fundamental studies of the phenomenon and its practical application began practically at the same time shortly after the discovery of the phenomenon.1,2 Since then, vigorous applied studies of the effect have resulted in the development of self-cleaning and antifogging glass and self-cleaning coatings for exterior and interior surfaces of buildings, tunnels, road signs, and so forth. At the same time, these applied studies are still based mainly on the intuition of the researchers working in this area because fundamental exploration of the effect of the surface photoinduced superhydrophilicity has not yet answered questions about the reasons and mechanisms responsible for the observed effects on some metal oxide surfaces. Presently one can distinguish several hypotheses regarding the mechanism of surface photoinduced superhydrophilicity. A first hypothesis suggests that enhancement of the surface hydrophilicity occurs because of photocatalytic decomposition of surface organic contaminations existing on the surface of metal oxides.3,4 To prove this hypothesis, White, Szanyi, and Henderson irradiated the surface of a TiO2 single crystal covered with 3-methyl-acetate possessing initially highly hydrophobic properties. UV irradiation of such a surface in the presence of gaseous oxygen leads to photodecomposition of adsorbed molecules and an increase of hydrophilic behavior of the surface. A similar explanation of the effect was given by Yates and co-workers who also studied this phenomenon on a TiO2 single crystal surface.4 However, the data presented by Miyauchi and co-workers5,6 contradict the reviewed hypothesis. © 2013 American Chemical Society
These authors conducted comparative studies of photocatalytic activity of various metal oxides and their abilities to exhibit the effect of photoinduced surface superhydrophilicity. It was shown that only a few metal oxides behaving as active photocatalysts in organic decomposition also demonstrate the ability of surface transformation to a hydrophilic state under the action of light. In fact, only two of them (ZnO and TiO2) can reach the superhydrophilic state of the surface. According to the alternative hypothesis,7,8 the effect of photoinduced superhydrophilicity is caused by electronic photoexcitation of solids yielding the formation of free charge carriers. As a result of surface trapping of photoholes, the breaking of bridge bonds Ti4+−OH−−Ti4+ occurs. That leads to formation of a new surface site for water adsorption: Ti4 +−OH−−Ti4 + + h → Ti4 +−OH− Ti4 + → Ti4 +−OH− Ti4 +−OH 2
An alternative approach infers that both bridging oxygen bonds are being broken to form an oxygen vacancy as an adsorption site for a water molecule: Ti4 +−O2 −−Ti4 + + 2h → Ti4 +[]Ti4 + +
1 O2 ↑ 2
Nevertheless, the results presented in refs 3 and 4 do not confirm the possibility of UV-induced formation of surface Received: January 14, 2013 Revised: May 14, 2013 Published: May 17, 2013 12086
dx.doi.org/10.1021/jp400421v | J. Phys. Chem. C 2013, 117, 12086−12092
The Journal of Physical Chemistry C
Article
photoinduced superhydrophilic conversion of the surface can be considered a sort of “memory” effect because the superhydrophilic state of the surface can persist for a certain time after irradiation is canceled.
oxygen vacancies and their possible role in increasing surface hydrophilicity. Another hypothesis, by M. Anpo and co-workers,9 proposes that thermal action of light causes the desorption of weakly bonded water molecules from external surface hydrolayers. Consequently, the increase of the surface hydrophilicity results from restoration of the original structure of hydrated layers. However, the experimental data presented by Fujishima and coworkers10 demonstrate the significance of electronic factors connected to the variation of the sample potential, rather than thermal action, in alteration of hydrophilic behavior of the sample surface. A similar conclusion about the importance of the electronic photoexcitation can be reached on the basis of the spectral dependence of the water contact angle presented by Ohtani and co-workers,11 where the actinic light with photon energy higher than the band gap results in surface hydrophilic transition. At the same time, interesting results related to the effect of surface photoinduced superhydrophilicity were obtained in several studies.12−15 Particularly, it was shown12 that the hydrate cover of the real surface consists of up to 14 monolayers and UV irradiation results in an increase to 21 monolayers. These data correlate with NMR results obtained by Nosaka13 showing the increase of the weakly bonded water signal after UV excitation of a TiO2 surface. Obviously, the presence of a multilayer hydrate cover on the real surface causes problems for the formation of oxygen vacancies or significant bridge bond cleavage. From this point of view, it seems more reasonable that in the presence of hydrate multilayers the alteration of the structure of the hydrate cover can result in superhydrophilic conversion of the surface. This idea was supported by the results of and conclusions reached in other works14,15 demonstrating the alteration of diffuse reflectance infrared Fourier transform spectra of adsorbed water caused by UV irradiation. Particularly, the authors proposed that photogeneration of free charge carriers changes the surface potential and therefore the structure of the surface hydrate layer. This conclusion coincides with the effect of the applied potential on the surface hydrophilicity.10 Thus, the mechanism of the effect of photoinduced surface superhydrophilicity remains elusive, and the conclusions reached by different authors contradict each other. The major problem in research is poor reproducibility (particularly, the initial water contact angle may vary from 20° to 60°) and characterization of the initial state of the TiO2 surface. It is unknown what ambient conditions are responsible for lower hydrophilic properties of the initial state of a TiO2 surface that would otherwise be highly hydrophilic. Also, the data obtained for a single crystal surface and the surface of nanocoatings are rather difficult to compare because of different surface morphology. At the same time, there is currently no common parameter characterizing the efficiency of hydrophilic conversion, which makes the comparison of the data obtained by different research groups practically impossible. From basic thermodynamics, it is obvious that surface transition from a weakly hydrophilic state to a superhydrophilic state induced by UV irradiation is caused by increasing the surface energy, specifically by its polar fraction and possibly by the hydrogen bond fraction, leading to the growth of the surface affinity with water molecules. However, there are too many possible reasons for such alterations of the surface energy, and the details of the mechanism of surface transformation remain unknown. It is also important to recall that the
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EXPERIMENTAL SECTION TiO2 nanofilm was formed on SiO2-coated glass by the dipcoating method using NDH-510C (Nisso) solution and then heating in air at 500 °C. The film was formed by nanoparticles with crystallite size of ∼10 nm (see Supporting Information). Film thickness was estimated by electronic microscopy (see Supporting Information). Film surface smoothness was appraised by atomic force microscopy to be ±3 nm. X-ray diffraction and Raman spectroscopy data confirmed the anatase crystal structure of the coating. The contact angle values were measured using an optical tensiometer (Bioline Theta Lite), and total surface energy was calculated by the Owens−Wendt approach using the two-liquid method (water contact angle versus methylene iodide contact angle). Monochromatic irradiation of the film surface was carried out with a setup consisting of a 1000 W Hg−Xe lamp (Oriel) equipped with a 10 cm water filter outlet and monochromator, a UV Hg lamp, and a set of band-pass filters (Oriel) selecting the 365 nm Hg line at higher light intensities. The dependence of transformation efficiency on light intensity was measured using monochromatic light (365 nm) within the range of 0− 21.5 mW/cm2. Wavelength dependencies were measured with the spectral resolution of monochromatic light being about Δλ = ±5 nm. Reproducibility of the Surface Initial State. At the initial stage of our studies, the formation of the TiO2 nanofilm was performed by the standard dip-coating procedure described elsewhere:10,16 withdrawing from the solution at a velocity of 2.5 mm/s, heating at 500 °C for 30 min, and cooling to room temperature in an oven. The initial state of the formed nanofilm was characterized by poor reproducibility of the water contact angle, with initial values ranging from 25° to 40°. To improve the reproducibility, the heating time was increased to 10 h. In this case, the initial state of the nanofilm surface became superhydrophilic, characterized by a water contact angle of
