Tomokazu Ohya

Development of New Modifiers for Titanium Alkoxide-Based Sol-Gel Process

The effects of several hydroxyketones such as acetol, actoin, γ-ketobutanol themselves and their combinations with monoethanolamine (MEA) or ethylenediamine (ED) on the stabilization of titanium tetraisopropoxide (TTIP) in isopropanol solution are examined. Acetoin itself and the imine derivatives of acetol and acetoin were found to show extraordinarily strong stabilizing effect for the alkoxide. The properties including the crystal modifications and refractive index of TiO2 films that were dip-coated using each stabilized solution are examined and discussed in comparison with those of the films obtained from the diethanolamine (DEA) systems. The effect of UV-light irradiation to the gel films on the crystallization of TiO2 is also examined and discussed.

Preparation and Characterization of Titania Thin Films from Aqueous Solutions

Dip- or spin-coating and characterization of titania (TiO2) thin films from various aqueous solutions have been studied. The aqueous titanium solutions mainly used in this study were halogen- and chelate-free solutions with the concentrations up to 1.4 M derived from titanium isopropoxide (TIP) with tetramethylammonium hydroxide (TMAOH) or some alkylamines, while aqueous and alcoholic solutions containing titanium atoms stabilized chelating ligands were examined for comparison. The TiO2 films prepared from the TIP-TMAOH solution were already crystallized at 350°C to anatase form and those formed at 600°C had high transparency and refractive indices of 2.40. No carbon residue in the film prepared at 400°C was detected by XPS. The pure anatase form was sustained up to 850°C. Interestingly, it was found that the (004) preferentially oriented anatase films were obtained from TIP-lactic acid (LA) system until 700°C. The solutions containing citric acid (CA) or alkanolamines yielded anatase and rutile form fired at the temperatures equal to or higher than 600°C. Carbon residue was detected in the film fired at 400°C. The film thickness monotonically decreased from the upper to the bottom ends of the substrate. However, it was found that the thickness uniformity was drastically improved by an addition of sucrose to the aqueous solutions. The effects of the solution composition and polyhydroxy compounds on the crystal modifications of formed films and the film uniformity are discussed.

Effect of α-Hydroxyketones as Chelate Ligands on Dip-Coating of Zirconia Thin Films

The stabilization effects of α-hydroxyketones such as acetoin, acetol, and their imine derivatives of monoethanolamine (MEA) on the 2-propanol solution of metal alkoxides (Al, Ti, Zr, Nb and Ta) were investigated. The hydroxyketones, especially acetoin which has a moderate boiling point around 150°C, were found to have a stabilization effect on the alkoxides of TiIV, NbV and TaV already in their molar ratios of 2, while the ZrIV alkoxide was stabilized at the molar ratio of 4. By sol-gel processing using this acetoin-stabilized solution, crystallized ZrO2 thin films could be prepared at temperatures as low as 400°C, which was lower than that from a diethanolamine (DEA)-stabilized precursor (450°C). The films prepared in this study showed a high transparency (more than 90%, UV-VIS spectroscopy), suggesting that the use of chelate ligands with low boiling points is a good way to lower the crystallization temperature of sol-gel-derived ZrO2 thin films. The Zr complexes of the imines made by the reaction between the hydroxyketones and MEA were stable and photosensitive (<500 nm), making it possible to pattern ZrO2 films by photo-irradiation of their gel films.

Aqueous titanate sols from Ti alkoxide-α-hydroxycarboxylic acid system and preparation of titania films from the sols

Aqueous titanate sols were prepared by reactions of titanium tetraisopropoxide (TIP) with α-hydroxycarboxylic acids in water. IR and Raman spectra, and elemental analyses of the precipitates obtained from the sols revealed that the carboxylates were chelated to titanium but the Ti species were polymerized to form a cluster that had a colloidal nature. Spin-coating of titania (TiO2) thin films from these sols was also examined. Interestingly, it was found that (004) preferentially oriented anatase films with refractive index of 2.54 were obtained from TIP-lactic acid-NH3 (1:1:1, molar ratio) aqueous sol. This crystallographic orientation was characteristic of the TIP-lactic acid-NH3 (1:1:1) system, and was not affected by the kinds of substrates used and the heating rate of the film. TEM observation indicated that small anatase grains had already formed at 200°C. Therefore, the crystallographic orientation might depend strongly on the structure of the chemical species of the precursor solutions.

Photocatalysis of a transparent titanate aqueous sol prepared from titanium tetraisopropoxide and tetramethylammonium hydroxide

Abstract Photocatalysis of a transparent titanate aqueous sol, which was prepared by mixing titanium isopropoxide and tetramethylammonium hydroxide or triethylamine, followed by diluting with water, was examined via the photo-fading of rhodamine B. An irradiation of UV light to the sol containing rhodamine B caused the colour fading by the photocatalytic reactions of the titanate colloids in the sol. The combination of the UV-irradiation with the bubbling of N2 gas accelerated the fading, whereas that with the bubbling of O2 led to the decrease in the fading rate, indicating that the fading was caused by the photocatalytic reduction with the titanate colloid, which competed with the photocatalytic reduction of the dissolved oxygen in the sol. On the other hand, the aqueous suspension of a TiO2 fine powder also exhibited a similar photo-fading of rhodamine B. However, the combination of the photoirradiation with the O2 bubbling increased the fading rate, whereas that with the N2 bubbling decreased the fading rate, indicating that the colour fading was caused by the photocatalytic oxidation of rhodamine B with the TiO2 powder. Consequently, it was found that the photocatalytic reaction mechanism of the titanate sol is different from that of the TiO2 powder. The cyclic voltammogram of the titanate sols showed that the reduction of rhodamine B adsorbed onto the titanate colloid occurred at a more positive potential than that of rhodamine B itself.