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Preparation of stable anatase-type TiO2 and its photocatalytic performance
International Journal of Inorganic Materials 3 (2001) 809–811
Preparation of stable anatase-type TiO 2 and its photocatalytic performance a, a a a b M. Inagaki *, Y. Nakazawa , M. Hirano , Y. Kobayashi , M. Toyoda a
Faculty of Engineering, Aichi Institute of Technology, Yakusa, Toyota 470 -0392, Japan b Fukui National College of Technology, Geshi, Sabae, Fukui 916 -8507, Japan Received 12 October 2000; accepted 13 August 2001
Abstract Anatase-type TiO 2 fine powders were synthesized under hydrothermal condition at 1808C. The anatase-type structure in these powders was maintained even after the calcination at 8008C. Calcined samples with higher crystallinity showed better photocatalytic performance for methylene blue decomposition. 2001 Elsevier Science Ltd. All rights reserved. Keywords: TiO 2 fine powders; Methylene blue decomposition; Calcination
Titania TiO 2 possesses various interesting properties, optical, dielectric and catalytic, which led to its industrial applications. Particularly anatase-type TiO 2 attracted great attention in relation to various environmental problems [1], of which photocatalytic activity gives promising possibilities for the decomposition of various organic pollutants in water [2–5]. It was also applied to photocatalytic purification of soil contaminated with spilled heavy oils [6]. It was known that anatase-type structure changes easily to a rutile-type one, with heat treatment at 5508C for 2 h in a kinetic study on transformation from anatase to rutile [7] and even with mechanical grinding at room temperature [8]. However, it was also pointed out that polymorphic transformation depends strongly on the grain size, which was governed by the precursor and its formation procedure. Therefore, various investigations on the preparation of anatase-type TiO 2 were carried out; from titanium tetraethoxide under hydrothermal condition above 2508C as particles with 20–30 nm size [9], vapor hydrolysis of titanium tetraisopropoxide at 2608C as nano-sized particles [10], destabilization of aqueous titanium lactate below 1008C as thin films on various substrates [11], decomposition of titania-hydrate coated on hollow glass spheres [12] etc. *Corresponding author. Tel.: 181-565-488-121; fax: 181-565-480076. E-mail address: [email protected] (M. Inagaki).
In the present work, anatase-type TiO 2 , which was stable up to 8008C, was successfully synthesized from TiOSO 4 under hydrothermal condition at 1808C and studied its photocatalytic activity through the decomposition of methylene blue in aqueous solution as a function of calcination conditions. Reagent grade TiOSO 4 was dissolved into distilled water in a concentration of 0.1 mol / l. This solution of 18 ml was heated at 1808C for 5 h in a Teflon container with a volume of 25 ml, which was backed up by a stainless steel vessel, with a constant rotation of 1.5 rev. / min. The precipitates obtained by this hydrothermal treatment were washed by distilled water until the pH value of the rinsed water became 7, separated from the solution by centrifuging and then dried in air at 908C. In order to study the high temperature stability of anatase synthesized, calcination was carried out at a temperature from 500 to 11008C for a period of 1–24 h in air. The changes in crystallinity of anatase and crystalline phases to rutile were examined by X-ray diffraction (XRD) using Cu Ka radiation. Photocatalytic activity was estimated from the decomposition of methylene blue (guaranteed reagent grade, C 16 H 18 N 3 S) in its aqueous solution with 2.94310 25 mol / l concentration. Into aqueous methylene blue solution of 20 ml, anatase powders of 0.021 g which were calcined at different conditions were added and then irradiated by ultraviolet rays with an intensity of 1.6 mW/ cm 2 under stirring for different periods. The decomposition rate was
1466-6049 / 01 / $ – see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S1466-6049( 01 )00176-3
810
M. Inagaki et al. / International Journal of Inorganic Materials 3 (2001) 809 – 811
calculated from the decrease in the concentration of methylene blue in solution, which was measured from the absorbance change at the wavelength of 650 nm with a photospectrometer using a calibration curve. In Fig. 1, X-ray diffraction patterns are shown for the samples calcined at different temperatures for different periods. As-prepared samples showed very broad peaks, of which diffraction angles corresponded to those of anatase crystal. Increasing calcination temperature, the diffraction peaks became sharp and after calcination at 7008C for 1 h three diffraction peaks, i.e. 103, 004 and 112, were clearly separated, revealing pronounced improvement of crystallinity in anatase-type structure. Increasing calcination period to 24 h, all diffraction peaks for anatase were sharpened, but any trace of diffraction peaks for rutile-type structure was not detected. The formation of a trace of rutile structure was observed after calcination at 8008C for 24 h. The calcination at 9008C showed a partial transformation of anatase to rutile clearly even after 1 h and a single phase of rutile was obtained after 11008C. Therefore, the anatase-type TiO 2 samples synthesized in the present work, from TiOSO 4 under hydrothermal condition at 1808C, is concluded to be stable up to 8008C. This stability of the present sample gave a possibility to have different crystallinity in anatase-type structures. For comparison, one grade of commercially available anatase powders was calcined at high temperatures. Rutile-
type structure was found to appear even after annealing at 7008C for 1 h. In the present work, the crystallinity of anatase was evaluated from the half widths of the 004 diffraction line, which was broad due to small crystallite size and also to the overlapping of the diffraction of 103 and 112, and became sharp after calcination because of the separation from neighboring peaks (103 and 112) and growth of crystallites. As-prepared sample gave a half width value of 1.08 in 2u (crystallite size of 32 nm calculated by Scherrer equation), but the sample calcined at 7008C for 24 h gave 0.28 in 2u (crystallite size of 78 nm). By the addition of calcined sample with high crystallinity, the absorption peak around 650 nm decreased in intensity much faster than the case of the as-prepared (low crystallinity). In Fig. 2, the decomposition rate of methylene blue in water is compared with as-prepared and calcined (at 7008C for 1 and 24 h) samples. The decomposition rate of methylene blue using the sample calcined at 7008C for 24 h decreased much faster with irradiation time than that using as-prepared; on the former almost all methylene blue was decomposed after 2 h, but on the latter still remained even after 4 h. The improvement in the decomposition performance was not so marked after calcination at 7008C for 1 h. The present result shows clearly the importance of the crystallinity of anatase for photocatalytic activity, which is estimated through the decomposition of methylene blue. It has to be mentioned that anatase fine particles with high stability can be synthesized under the present conditions, a precursor salt of TiOSO 4 and the hydrolysis under mild hydrothermal conditions. Detailed quantitative investiga-
Fig. 1. X-ray diffraction patterns of TiO 2 as-prepared under hydrothermal conditions and calcined at different temperatures.
Fig. 2. Decomposition rate of methylene blue in aqueous solution with a starting concentration of 2.94310 25 mol / l with three TiO 2 samples.
M. Inagaki et al. / International Journal of Inorganic Materials 3 (2001) 809 – 811
tions to understand the effect of crystallinity of anatase on its photocatalytic activity are being undertaken.
References [1] Ollis DF, Al-Ekabi H, editors, Photocatalytic purification and treatment of water and air, Amsterdam: Elsevier, 1993. [2] Kato K, Torii Y, Tada H, Kato T, Butsugan Y, Nihara K. Mater Sci Lett 1996;15:913–5. [3] Herrmann J-M, Tahiri H, Ait-Icho Y, Lassaletta G, Gonzalez-Elipe AR, Fernandez A. Appl Catal B 1997;13:219–28. [4] Basco RR, Kiwi J. Appl Catal B 1998;16:19–29.
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[5] Grechulska J, Hamerski M, Morawski AW. Water Res 2000;34(5):1638–44. [6] Hamerski M, Grechulska J, Morawski AW. Solar Energy 1999;66(6):395–9. [7] Ding X, Liu X. J Mater Res 1998;13(9):2556–9. [8] Suwa Y, Inagaki M, Naka S. J Mater Sci 1984;19:1397–405. [9] Yanagisawa K, Yamamoto Y, Feng Q, Yamasaki N. J Mater Res 1998;13(4):825–9. [10] Chan CK, Porter JF, Li Y-G, Guo W, Chan C-M. J Am Ceram Soc 1999;83(3):566–72. [11] Baskaran S, Song L, Liu J, Chen YL, Graff GL. J Am Ceram Soc 1998;81(2):401–8. [12] Shin D-Y, Kimura K. J Ceram Soc Japan 1999;107(9):775–9.
Preparation of stable anatase-type TiO 2 and its photocatalytic performance a, a a a b M. Inagaki *, Y. Nakazawa , M. Hirano , Y. Kobayashi , M. Toyoda a
Faculty of Engineering, Aichi Institute of Technology, Yakusa, Toyota 470 -0392, Japan b Fukui National College of Technology, Geshi, Sabae, Fukui 916 -8507, Japan Received 12 October 2000; accepted 13 August 2001
Abstract Anatase-type TiO 2 fine powders were synthesized under hydrothermal condition at 1808C. The anatase-type structure in these powders was maintained even after the calcination at 8008C. Calcined samples with higher crystallinity showed better photocatalytic performance for methylene blue decomposition. 2001 Elsevier Science Ltd. All rights reserved. Keywords: TiO 2 fine powders; Methylene blue decomposition; Calcination
Titania TiO 2 possesses various interesting properties, optical, dielectric and catalytic, which led to its industrial applications. Particularly anatase-type TiO 2 attracted great attention in relation to various environmental problems [1], of which photocatalytic activity gives promising possibilities for the decomposition of various organic pollutants in water [2–5]. It was also applied to photocatalytic purification of soil contaminated with spilled heavy oils [6]. It was known that anatase-type structure changes easily to a rutile-type one, with heat treatment at 5508C for 2 h in a kinetic study on transformation from anatase to rutile [7] and even with mechanical grinding at room temperature [8]. However, it was also pointed out that polymorphic transformation depends strongly on the grain size, which was governed by the precursor and its formation procedure. Therefore, various investigations on the preparation of anatase-type TiO 2 were carried out; from titanium tetraethoxide under hydrothermal condition above 2508C as particles with 20–30 nm size [9], vapor hydrolysis of titanium tetraisopropoxide at 2608C as nano-sized particles [10], destabilization of aqueous titanium lactate below 1008C as thin films on various substrates [11], decomposition of titania-hydrate coated on hollow glass spheres [12] etc. *Corresponding author. Tel.: 181-565-488-121; fax: 181-565-480076. E-mail address: [email protected] (M. Inagaki).
In the present work, anatase-type TiO 2 , which was stable up to 8008C, was successfully synthesized from TiOSO 4 under hydrothermal condition at 1808C and studied its photocatalytic activity through the decomposition of methylene blue in aqueous solution as a function of calcination conditions. Reagent grade TiOSO 4 was dissolved into distilled water in a concentration of 0.1 mol / l. This solution of 18 ml was heated at 1808C for 5 h in a Teflon container with a volume of 25 ml, which was backed up by a stainless steel vessel, with a constant rotation of 1.5 rev. / min. The precipitates obtained by this hydrothermal treatment were washed by distilled water until the pH value of the rinsed water became 7, separated from the solution by centrifuging and then dried in air at 908C. In order to study the high temperature stability of anatase synthesized, calcination was carried out at a temperature from 500 to 11008C for a period of 1–24 h in air. The changes in crystallinity of anatase and crystalline phases to rutile were examined by X-ray diffraction (XRD) using Cu Ka radiation. Photocatalytic activity was estimated from the decomposition of methylene blue (guaranteed reagent grade, C 16 H 18 N 3 S) in its aqueous solution with 2.94310 25 mol / l concentration. Into aqueous methylene blue solution of 20 ml, anatase powders of 0.021 g which were calcined at different conditions were added and then irradiated by ultraviolet rays with an intensity of 1.6 mW/ cm 2 under stirring for different periods. The decomposition rate was
1466-6049 / 01 / $ – see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S1466-6049( 01 )00176-3
810
M. Inagaki et al. / International Journal of Inorganic Materials 3 (2001) 809 – 811
calculated from the decrease in the concentration of methylene blue in solution, which was measured from the absorbance change at the wavelength of 650 nm with a photospectrometer using a calibration curve. In Fig. 1, X-ray diffraction patterns are shown for the samples calcined at different temperatures for different periods. As-prepared samples showed very broad peaks, of which diffraction angles corresponded to those of anatase crystal. Increasing calcination temperature, the diffraction peaks became sharp and after calcination at 7008C for 1 h three diffraction peaks, i.e. 103, 004 and 112, were clearly separated, revealing pronounced improvement of crystallinity in anatase-type structure. Increasing calcination period to 24 h, all diffraction peaks for anatase were sharpened, but any trace of diffraction peaks for rutile-type structure was not detected. The formation of a trace of rutile structure was observed after calcination at 8008C for 24 h. The calcination at 9008C showed a partial transformation of anatase to rutile clearly even after 1 h and a single phase of rutile was obtained after 11008C. Therefore, the anatase-type TiO 2 samples synthesized in the present work, from TiOSO 4 under hydrothermal condition at 1808C, is concluded to be stable up to 8008C. This stability of the present sample gave a possibility to have different crystallinity in anatase-type structures. For comparison, one grade of commercially available anatase powders was calcined at high temperatures. Rutile-
type structure was found to appear even after annealing at 7008C for 1 h. In the present work, the crystallinity of anatase was evaluated from the half widths of the 004 diffraction line, which was broad due to small crystallite size and also to the overlapping of the diffraction of 103 and 112, and became sharp after calcination because of the separation from neighboring peaks (103 and 112) and growth of crystallites. As-prepared sample gave a half width value of 1.08 in 2u (crystallite size of 32 nm calculated by Scherrer equation), but the sample calcined at 7008C for 24 h gave 0.28 in 2u (crystallite size of 78 nm). By the addition of calcined sample with high crystallinity, the absorption peak around 650 nm decreased in intensity much faster than the case of the as-prepared (low crystallinity). In Fig. 2, the decomposition rate of methylene blue in water is compared with as-prepared and calcined (at 7008C for 1 and 24 h) samples. The decomposition rate of methylene blue using the sample calcined at 7008C for 24 h decreased much faster with irradiation time than that using as-prepared; on the former almost all methylene blue was decomposed after 2 h, but on the latter still remained even after 4 h. The improvement in the decomposition performance was not so marked after calcination at 7008C for 1 h. The present result shows clearly the importance of the crystallinity of anatase for photocatalytic activity, which is estimated through the decomposition of methylene blue. It has to be mentioned that anatase fine particles with high stability can be synthesized under the present conditions, a precursor salt of TiOSO 4 and the hydrolysis under mild hydrothermal conditions. Detailed quantitative investiga-
Fig. 1. X-ray diffraction patterns of TiO 2 as-prepared under hydrothermal conditions and calcined at different temperatures.
Fig. 2. Decomposition rate of methylene blue in aqueous solution with a starting concentration of 2.94310 25 mol / l with three TiO 2 samples.
M. Inagaki et al. / International Journal of Inorganic Materials 3 (2001) 809 – 811
tions to understand the effect of crystallinity of anatase on its photocatalytic activity are being undertaken.
References [1] Ollis DF, Al-Ekabi H, editors, Photocatalytic purification and treatment of water and air, Amsterdam: Elsevier, 1993. [2] Kato K, Torii Y, Tada H, Kato T, Butsugan Y, Nihara K. Mater Sci Lett 1996;15:913–5. [3] Herrmann J-M, Tahiri H, Ait-Icho Y, Lassaletta G, Gonzalez-Elipe AR, Fernandez A. Appl Catal B 1997;13:219–28. [4] Basco RR, Kiwi J. Appl Catal B 1998;16:19–29.
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[5] Grechulska J, Hamerski M, Morawski AW. Water Res 2000;34(5):1638–44. [6] Hamerski M, Grechulska J, Morawski AW. Solar Energy 1999;66(6):395–9. [7] Ding X, Liu X. J Mater Res 1998;13(9):2556–9. [8] Suwa Y, Inagaki M, Naka S. J Mater Sci 1984;19:1397–405. [9] Yanagisawa K, Yamamoto Y, Feng Q, Yamasaki N. J Mater Res 1998;13(4):825–9. [10] Chan CK, Porter JF, Li Y-G, Guo W, Chan C-M. J Am Ceram Soc 1999;83(3):566–72. [11] Baskaran S, Song L, Liu J, Chen YL, Graff GL. J Am Ceram Soc 1998;81(2):401–8. [12] Shin D-Y, Kimura K. J Ceram Soc Japan 1999;107(9):775–9.