Preparation of lamellar hybrid inorganic–organic films of layered titanate and cationic or anionic surfactants
Introduction
Since researchers at Mobil first demonstrated that the mesoporous silica with uniform pores, which is called MCM-41, could be synthesized by utilizing hexagonally packed micelle of surfactants [1], [2], mesoporous materials have attracted a great deal of attention. The film fabrication [3], [4], [5] and applications [6], [7], [8] of mesoporous silica and the syntheses of mesoporous transition metal oxides [9], [10], [11], [12], [13], [14], [15], [16], [17] have been studied extensively. In addition, the folding of the silicate layers of the clay called kanemite was applied to the fabrication of mesostructured silicates [18], [19]. When an alkylammonium surfactant with a long alkyl chain is intercalated into the interlayer of kanemite, the mesoporous silica called KSW-2 is synthesized by the folding of the silicate layers [18]. The mesoporous silica called FSM-16 is also synthesized in a similar way [19]. It is envisaged that mesostructured transition metal oxides may be synthesized from layered metallic acids or their salts by interaction with surfactants. For example, when the cation in the interlayer of a layered metallate is ion-exchanged for a cationic surfactant, a hybrid inorganic–organic material consisting of the cationic surfactant and the anionic metallate is formed. The removal of the surfactant from the hybrid material by firing or other methods may lead to synthesis of a mesostructured transition metal oxide. As there are many kinds of layered titanate salts [20], [21], they can be applied to syntheses of mesostructured hybrid inorganic–organic materials consisting of surfactants and titanates and mesostructured TiO2. As TiO2 is a photocatalyst with a superior property, the application of mesoporous TiO2 films with a large specific surface area to photocatalysts would be interesting. So far, we demonstrated that aqueous sols including the colloids of layered titanate salts could be prepared by a very easy method, that is, by diluting mixtures of titanium tetraisopropoxide (Ti{OCH(CH3)2}4; TIP) and tetramethylammonium hydroxide (N(CH3)4OH; TMAOH) with water [22], [23]. Not only the hydrolysis and condensation reactions of TIP but also the acid–base reaction of the formed Ti species with TMAOH occur in the mixtures, to form a tetramethylammonium (TMA) titanate, not titanium hydroxide, as follows: nTi(OH)4 +mTMAOH → (TMA)mTinO(m + 4n)/2 + (m/2 + 2n) H2O. At a mixing ratio of TMAOH/TIP < 0.4, precipitation occurred, whereas at TMAOH/TIP > 0.4, transparent aqueous sols including TMA titanate colloids were obtained. When the sol was coated on a glass substrate and dried under an ambient condition, the XRD pattern of the obtained sample featured a layered structure, because the reciprocals of the d-spacings of the peaks had a ratio of 1:2:3, as shown in the Results and discussion section. Thus, the colloids dissolved in the sols have a layered structure with anionic titanate layers and TMA+ cations and water molecules in the interlayer. As the layered titanate is obtained as an aqueous sol, it is easy to mix it with surfactants and to fabricate films of the aqueous mixtures by dip or spin coating method.
The purpose of this study is to fabricate hybrid inorganic–organic films by using the above-mentioned TMA titanate aqueous sol and different surfactants. The intercalation of the surfactants into the interlayer of the layered TMA titanate was examined by means of XRD measurements. It is anticipated that as the titanate layer has a negative charge, cationic surfactants are effective as the surfactant. However, as anionic surfactants are easily available because of their good detergency and low cost of manufacture, the synthesis of hybrid films using anionic surfactants is interesting. [16], [17], [24], [25] In this study, the film fabrication was also examined using an anionic surfactant as the surfactant.
Section snippets
Experimental procedure
Aqueous colloidal solutions of titanate salts were prepared according to the procedure that we reported previously [22], [23]. 20 mmol of TIP was added in 12.15 g of 15% TMAOH aqueous solution, which includes 20 mmol of TMAOH. The mixture was stirred for 1 h, and diluted with distilled water by 40 mL, resulting in a transparent sol with a Ti concentration of 0.5 M and a TMAOH/TIP molar ratio of 1. Cetyltrimethylammonium bromide (C16H33(CH3)3NBr; CTAB) and dodecyltrimethylammonium bromide (C12H25
Hybrid films of cationic surfactants and layered titanates
TIP and TMAOH were mixed, and then diluted with water, resulting in transparent sols. As described in the Introduction section, the transparent sols include TMA titanate colloids with a layered structure. For ion exchange of the TMA+ ions present in the TMA titanate colloids for cationic surfactants, CTAB or DTAB was added in the TMA titanate sols at CTAB or DTAB/Ti = 0.1. The obtained mixtures became turbid, as shown in Fig. 2a. The formed particles were so small that even when the mixtures were
Conclusions
Transparent aqueous sols including layered TMA titanate colloids were prepared by mixing TIP, TMAOH, and water. The fabrication of hybrid inorganic–organic films was examined by using the titanate aqueous sols and cationic and anionic surfactants. CTAB and DTAB were used as a cationic surfactant, while DP was used as an anionic surfactant. In all cases, either the mixing of the surfactants with the TMA titanate colloid solutions or the hydrothermal treatment of the mixture sols did not lead to
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