Photocatalytic Characteristics and Structural Evaluation of TiO[sub 2] Thin Films Reactively DC Sputtered with H[sub 2]O

Abstract

TiO 2 thin films containing Ti-OH bonds have been fabricated using dc reactive sputtering of Ti in a H 2 O ambient. The structures of these films and their photocatalytic characteristics were analyzed. The structures of the TiO 2 thin films were analyzed using Fourier transform infrared spectroscopic analysis, X-ray diffraction, scanning electron microscope inspection, nuclear magnetic resonance, and photoluminescence. To evaluate the photocatalytic characteristics of the TiO 2 films, we used a wet methylene blue method and pure-water contact angle measurement. We found the following: ͑i͒ The total quantity of OH groups decreases as the substrate temperature is increased. (ii) OH groups can be either densely or sparsely distributed in the TiO 2 film. (iii) The abundance ratios of the concentrated and sparse OH groups to the total quantity of OH groups change as the substrate temperature is increased. Measurements made with nuclear magnetic resonance and photoluminescence show that the sparse OH groups (͓OH͔ n ) are effective in removing oxygen deficiency and other deficiencies, while the localized, concentrated OH groups (͓OH͔ w ) are ineffective. We concluded that the distribution of OH groups in the TiO 2 thin film is an important factor that must be considered for improvement of photocatalytic characteristics.In 1972 Fujishima and Honda reported that water breaks down into hydrogen and oxygen due to the oxidation-reduction potential of electrons and holes generated when ultraviolet rays irradiate a TiO 2 electrode: this finding is known as the Honda-Fujishima effect. 1 TiO 2 photocatalysis has subsequently drawn a great deal of attention as a means of creating clean energy. TiO 2 photocatalysis is a photocatalytic decomposition process that takes place on the surface of a TiO 2 film. When the TiO 2 photocatalytic phenomenon occurs, electrons and holes generated by photoexcitation react with water and oxygen on the surface of the TiO 2 film, producing highly oxidative hydroxyl radicals ͑•OH͒ and superoxide anions (•O 2 Ϫ ).Various products are being developed based on TiO 2 photocatalysis: soil-resistant air cleaners for automobile interiors, for home and industrial use, soil-resistant tiles and tents that become soil resistant when exposed to sunlight, antimicrobial medical appliances such as antimicrobial catheters, products with a deodorizing function, and so forth. 2 It has been found that the surface of the TiO 2 film becomes ultrahydrophilic due to light excitation. 3 Products developed based on this ultrahydrophilic property of TiO 2 , such as the defogging of wing mirrors of automobiles, are now expanding their market. The ultrahydrophilic phenomenon can be explained as follows:Some of the electron-hole pairs generated during UV irradiation behave differently from others: they do not react with surface adsorbates, and their electrons reduce Ti 4ϩ and their holes oxidize O atoms in TiO 2 , thus producing O 2 gas and neutral O deficiencies (O v ), consequently giving rise to the ultrahydrophilic phenomeno...