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Synthesis of Brookite-typed Titania from Titanium Chloride Solution.
Satoshi Okano1, a, Saeki Yamamuro1,b and Toshiro Tanaka1,c 1 Bunkyo-tyo 3, Matsuyama, Ehime, 790-8577, Japan.
[email protected], [email protected], [email protected]
Keywords: Brookite phase, Titanium chloride solution, Hydrothermal synthesis
Abstract. The brookite-phase TiO2 was prepared by a hydrothermal synthesis of titanium chloride solution. The thermolysis time and the pH value of the solution were controlled during the synthesis. X-ray diffraction experiments showed that TiO2 powders partially containing the brookite-phase were successfully obtained. A higher amount of OH- in the reaction solution was found to be important to obtain the brookite phase because the intermediate complex leading to the brookite phase consumes more amount of OH- than other phases like the rutile.
Introduction
It is well known that TiO2 has three types of crystal structures: anatase, rutile and brookite. The photo-catalytic activity appears strongly for the anatase phase and poorly for the rutile. The anatase-phase TiO2 has been extensively researched from the scientific and industrial points of view. On the contrary, little has been reported on the photo-catalytic properties of the brookite phase, because of its synthetic difficulty. Kiyama et al. have reported the brookite-phase TiO2 was obtained by an oxidation of TiCl3 solution in the existence of CH3COONa at 95℃ in air [1]. Ohtani et al. have reported the brookite TiO2, which were prepared by the Kiyama’s method, exhibited a marked photocatalytic activity for dehydrogenation of 2-propanol in aqueous solution [2]. Additionally, pure brookite-phase was obtained by milling the anatase-phase TiO2 powders [3].
Recently, we have succeeded in obtaining TiO2 powders partially containing the brookite phase from titanium chloride solution. In this paper, we will show their synthetic method.
Experimental Procedure
A hydrothermal synthesis was applied to obtain TiO2 powders. The aqueous solution of 20%TiCl3 was diluted by distilled water. The solution was heated in an autoclave for 3-72 hours, and then cooled to room temperature. The resulting precipitates generated in the solution were separated by a filter of 0.1mm mesh, followed by three-times washing by distilled water and subsequent drying at room temperature. The crystal structures of the precipitates were evaluated by X-ray diffraction method (XRD).
Results
Figure1 shows the XRD patterns of TiO2 powders prepared at 100 °C for various reaction time. The sample for 3h shows a broad peak at around 2θ=27 °, corresponding to the (110) reflection of the rutile phase. With increasing the thermolysis time, the peak becomes sharper, indicating that the crystallinity is improved. Multiple peaks other than the rutile are also found at around 2θ=26 °, matching with the (110) of the anatase phase and the (210) and (111) of the brookite. These three phases co-exist for 6h-48h and the rutile is dominant for 48-72h.
By integrating the peak intensity, the relative abundance of each phase is evaluated as shown in Fig. 2. For all the samples, the rutile phase amounts to 60% or more. The abundance of rutile phase becomes dominant when the thermolysis time is increased. Inversely, the abundance of brookite phase decreases. This result indicates that a shorter reaction time is favored for the synthesis of the
Materials Science Forum Vols. 610-613 (2009) pp 285-287Online available since 2009/Jan/02 at www.scientific.net© (2009) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.610-613.285
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Fig. 1 XRD patterns of TiO2 powders prepared Fig. 2 The relative abundance of each phase at 100 °C.(A:Anatase B:Brookite R:Rutile) prepared at 100 °C.
brookite phsae. It is found that an increased reaction time causes a decrease in the pH value of the solution.
The relationship between the pH value and the relative abundance of each phase is shown in Fig. 3. For higher acidity, the rutile phase becomes dominant. When the basicity is increased, on the contrary, the rutile phase decreases and the brookite increases. Under the present reaction conditions, the maximum amount of the brooktie phase is 26% at pH=2.4.
Since the formation of the brookite phase is favored at higher basicity, a various amount of 1M NaOH (0~12.8ml) was added to the reaction solvent to increase the pH value. The solution was heated at 100°C for 24h. The relationship between the pH value and the abundance of each phase is shown in Fig. 4. It should be noted that the amount of brookite phase further increases in the range of 3<pH<3.2, reaching 48% in abundance. This value is about twice as much as the one obtained without an addition of NaOH. However, too much addition of NaOH resulted in a preferred formation of amorphous TiO2.
Discussion
In the present reaction, Ti3+ in the chloride solution is transformed to Ti4+ by giving electron to oxygen because Ti3+ is highly instable. O2- is transformed to OH- by being combined with H+ in the solution. These reactions will lead to an increased pH as shown below:
2Ti3+ 1/2O2 → 2Ti4+ + O2-
O2- + 2H+ → 2OH- (1)
Pottier et al. [4] have reported that the intermediate precursors for the formation of rutile and brookite phases are Ti(OH)3Cl(OH2)2 and Ti(OH)2Cl2(OH2)2, respectively. The formation ability of these two complexes depend only upon the [Cl]/[Ti] ratio in the solution. These precursors precipitate in the range of 17<[Cl]/[Ti]<34 and [Cl]/[Ti]>34, respectively. During the reaction, aquo and hydroxo ligands in the complexes induce to bridge multiple complexes by olation, which involves an elimination of aquo ligands and removes all the chloride ions from the complexes. Drying these bridged complexes finally gives rise to the formation of titanium oxide.
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Fig. 3 Relationship of between the pH value Fig. 4 Relationship between the pH value and the and each phase abundance. abundance of each phase with the addition of NaOH.
In our experiment, the addition of NaOH to the reaction solution increases the pH value, leading to an increased amount of OH-. This will promotes the formation of brookite phase because its complex consumes the amount of OH- twice as much as the rutile’s one. However, too much NaOH results in the preferred formation of amorphous TiO2 which contains much more OH- than the brookite complex.
Conclusion
TiO2 powders partially containing the brookite phase were successfully synthesized by a thermolysis of titanium chloride (III) solution. Controlling the pH value of the reaction solution is crucial to obtain the brookite phase. An increased pH value leads to a preferred formation of brookite phase and its maximum abundance of 48 %.
References
[1] M. Kiyama, T. Akita, Y. Tsutsumi and T. Takada: Chem.Lett.1, 1, 21(1972).
[2] B. Ohtani, J. Handa, S. Nishimoto, T. Kagiya: Chem, Phys. Lett 120, 3, 292(1985).
[3] T. Wakamatsu, T. Fujiwara, N. Ishihara and H. Shingu: J. Jpn. soc. Powder Powder Metallurgy. 48, 10, 950 (2001)
[4] A. Pottier, C. Chaneac, E. Tronc, et al: J. Mater. Chem. 11, 1116 (2001).
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Materials Science Forum Vols. 610-613 287
Materials Research 10.4028/www.scientific.net/MSF.610-613 Synthesis of Brookite-Typed Titania from Titanium Chloride Solution 10.4028/www.scientific.net/MSF.610-613.285
DOI References
[4] A. Pottier, C. Chaneac, E. Tronc, et al: J. Mater. Chem. 11, 1116 (2001).
doi:10.1039/b100435m
