Synthesis of new titanium chelated complexes stabilized in aqueous solution and their stability on pH and temperature

Abstract

Titanium oxides (TiO2) as raw materials for the synthesis of barium titanate (BaTiO3), strontium titanate (SrTiO3) and lead titanate (PbTiO3) have been very important for the electrical industry. However, recently, with miniaturization of electrical devices, fine ferroelectric particles are required, and preparation methods for ferroelectric particles such as BaTiO3 are changing from dry processes using solid state reaction between TiO2 and BaCO3 to wet processes using a liquid-solid reaction or liquid state reaction. As to the titanium source, at present, titanium tetrahydroxide (Ti(OH)4) gel in aqueous solution is taking the place of the conventional TiO2 to obtain fine ferroelectric particles. Titanium ions (Ti4+) in aqueous solution are unstable, but can exist as stable entities under only strong acid solution with a pH below 2. On the other hand, in order to synthesize ferroelectric particles, it is necessary to make the pH in solution over 10, i.e., strong basic solution. However, Ti ions can react easily with hydroxide ions (OH−), and are precipitated as Ti(OH)4 gel. Ti(OH)4 gel is an aggregate composed of Ti(OH)4 clusters with sizes from 10 to 50 nm. As a result, ferroelectric particles prepared using Ti(OH)4 gel must have minimum sizes of over 10 nm, e.g., in the hydrothermal synthesis of BaTiO3, a minimum particle size was reported as 20 nm when Ti(OH)4 gel was used [1]. Therefore, it is difficult to prepare nm-sized fine ferroelectric particles if a Ti(OH)4 gel is used. On the other hand, in the sol-gel method using Tialkoxides, the synthesis of BaTiO3 fine particles with sizes of around 10 nm has been reported [2–4]. However, in these particles, there are a lot of hydroxides and hydrocarbons that originate from Ti-alkoxides. Thus, it is necessary to remove these impurities by annealing at over 500 ◦C, but, as a result, this thermal treatment makes the particle sizes larger than 50 nm. Therefore, if Ti ions can exist as stable entities in strong basic solutions over a pH of 10, it may be possible to synthesize nm-scale sized BaTiO3 and SrTiO3 fine particles. To date, there have been only a few reports of ways to stabilize Ti4+ ions in aqueous solution. Pecsok and Maverick reported that Ti-ethylenediaminetetraacetate (EDTA) chelated complex was stable below a pH of 2.5 while above a pH of 3, this complex was hydrolyzed to colloidal titanium dioxide [5]. Gastinger found that Ti ions can prepare chelated complex with H2O2 [6]. Sweetser and Bricker reported that Ti ions can form a chelated complex with both H2O2 and EDTA and this complex was stable from a pH of 0.4 to 4 in aqueous solution [7]. Musha and Ogawa also confirmed their report [8]. On the basis of these investigations, it had been established that the concentration of Ti ions, which were stabilized using chelating agents such as H2O2 and EDTA, can be determined using titration [9]. However, the titration of Ti ions was done only under a weakly acidic aqueous solution, and thus there was no investigation of the stability of a Ti chelated complex in a basic aqueous solution. Recently, Wada et al. reported that Ti ions chelated using both H2O2 and acetylacetone (AcAc), resulted in stabilized Ti ions in a weak acid solution with a pH of 3–5 [10]. After this report, there has been no report on the stability of Ti chelated complexes in aqueous solutions. In this study, the synthesis of various Ti chelated complexes was done in aqueous solution using H2O2, AcAc and various amine acetate chelating agents such as EDTA. Moreover, their stability as a function of pH and temperature was also investigated. As a result, we succeeded in synthesizing very stable a Ti chelated complex under conditions above a pH of 10 and 100 ◦C for the first time. Titanium tetrachloride, TiCl4, (Kishida Chem., >99.9%) was used as the Ti source. On the other hand, as chelating agents, the following agents were used, i.e., H2O2, AcAc (CH2(COCH3)2), EDTA ([CH2N (CH2COOH)2]2), trans-1,2-cyclohexanediamine tetraacetic acid (CyDTA, C6H10[N(CH2COOH)2]2·H2O), glycoletherdiamine tetraacetic acid (GEDTA, [CH2 OCH2CH2N(CH2COOH)2]2), diethylenetriamine pentaacetic acid (DTPA, HOOCH2N[CH2CH2N(CH2 COOH)2]2) and nitrilo triacetic acid (NTA, N(CH2 COOH)3). The Ti chelated complex must be synthesized under an acid solution because of the stability of Ti ions only in strong acid. In this study, as acids, hydrochloric acid (HCl), nitric acid (HNO3) and acetic acid (CH3COOH) were used. As to sulfuric acid (H2SO4), we stopped its use because H2SO4 can react easily with Ba ions and form precipitates such as BaSO4 in the preparation of BaTiO3 fine particles. Moreover, the effect of concentration in various acid solutions was also investigated. For this