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Preparation and characterization of anatase TiO2 microspheres with porous frameworks via controlled hydrolysis of titanium alkoxide followed by hydrothermal treatment
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Materials Letters 62 (2008) 2970 – 2972 www.elsevier.com/locate/matlet
Preparation and characterization of anatase TiO2 microspheres with porous frameworks via controlled hydrolysis of titanium alkoxide followed by hydrothermal treatment Tianzhong Tong a , Jinlong Zhang a,⁎, Baozhu Tian a , Feng Chen a , Dannong He b a
Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China b Shanghai National Engineering Research Center for Nanotechnology, 245 Jiachuan Road, Shanghai 200237, PR China Received 6 September 2007; accepted 29 January 2008 Available online 5 February 2008
Abstract TiO2 microspheres with porous frameworks have been prepared by controlled hydrolysis of Ti(OC4H9-n)4 with water generated “in situ” via an esterification reaction between acetic acid and ethanol, followed by hydrothermal treatment. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, and nitrogen adsorption–desorption methods. The TiO2 microspheres, with diameters ranging from 100 to 500 nm, were constructed by pure anatase particles with average crystalline size of 16.4 nm. The formation of porous structure might be related to slow hydrolysis under ultrasound irradiation. The formation of unique anatase should be attributed to the presence of SO2− 4 ions. © 2008 Elsevier B.V. All rights reserved. Keywords: TiO2 microspheres; Crystal structure; Porosity; Catalysts; Esterification reaction
1. Introduction Since the discovery of photoelectrochemical splitting of water on n-TiO2 electrodes [1], TiO2 has been extensively used as an environmentally harmonious and clean photocatalyst due to its various merits, such as its optical and electronic properties, low cost, high photocatalytic activity, non-toxicity and chemical stability [2,3]. TiO2 has three crystalline polymorphs: anatase, rutile, and brookite. It has been reported that the photocatalytic activity of TiO2 is related to its crystal structure and anatase TiO2 often exhibits the highest photocatalytic activity in the photodegradation of most pollutants in water and air [4]. As for the preparation method of anatase TiO2, sol–gel process based on the hydrolysis and polycondensation reactions of titanium alkoxides has been extensively employed [5,6]. However, the high hydrolysis rate of titanium alkoxides may cause uncontrolled local precipitation, resulting in photocatalytic losses in ⁎ Corresponding author. Tel./fax: +86 21 64252062. E-mail address: [email protected] (J. Zhang). 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.01.085
TiO2-based material [7]. Since the work of Larbot et al. [8], excellent control of the hydrolysis rate of titanium alkoxides has been realized with “in situ” water generated in esterification reactions [9,10]. In addition, the photocatalytic activity of TiO2 is also dictated by its morphology, because the photocatalytic reactions often take place on the surface of catalysts. Therefore, it is practically and scientifically significant to explore new synthetic methods in which the crystalline phase and morphology of TiO2 can be controlled. In this study, TiO2 microspheres with porous frameworks were prepared by controlled hydrolysis of Ti(OC4H9-n)4 (Eq. (1)), followed by hydrothermal treatment. The prepared samples were characterized by XRD, SEM, TEM, FT-IR, and nitrogen adsorption–desorption methods. (1).
T. Tong et al. / Materials Letters 62 (2008) 2970–2972
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2. Experimental section The synthetic procedure was as follows: Under magnetic stirring, 20 ml HAc was added dropwise to a flask containing 10 ml of Ti(OC4H9-n)4 diluted in 30 ml C2H5OH, followed by addition of 1 ml of H2SO4. Then, the obtained clear liquid was sonicated in an ultrasonic cleaning bath (Elma, T660/H, 35 kHZ, 360 W) at 313 K for 1 h and 333 K for 3 h, resulting in the formation of a milk-like sol, which was further transferred into a 100 ml Telflon-inner-liner stainless autoclave and kept at 393 K for 13 h. The resulting precipitates were separated from the mother liquor by centrifugation, washed thoroughly with deionized water and ethanol several times, and then dried at 373 K in air for 12 h. Finally, the obtained powders were further calcined at certain temperature for 2 h. The obtained samples were labeled as TiO2-T, in which “T” refers heat-treating temperature in centigrade degree. The phase presence and crystallite sizes of TiO2 samples were characterized by XRD performed on a Rigaku D/max 2550 VB/PC X-ray diffractometer. The morphologies of the TiO2 samples were observed with TEM (JEOL JEM-100 CX II) operated at 100 kV and SEM (JEOL JSM-6360 LV) operated at 15 kV. FT-IR spectroscopy was performed on a Nicolet Magna 550 spectrometer. The porous texture of the powders was analyzed from nitrogen adsorption–desorption isotherms at 77 K by using a Micromeritics ASAP 2000 system.
Fig. 2. SEM (a) and TEM (b) images of sample TiO2-733.
3. Results and discussion Fig. 1 shows the wide-angle XRD pattern of samples TiO2-373 and TiO2-733. Both samples consist of anatase as a unique phase (JCPDS, No. 21-1272). The peaks at scattering angles of 25.24, 36.98, 48.02 and 62.74° correspond to the reflections from the (101), (004), (200) and (204) crystal planes of anatase TiO2, respectively. No crystalline phase ascribed to rutile or brookite can be found. The intensities of the diffraction peaks for samples TiO2-373 and TiO2-733 have no evident differences, indicating that sample TiO2-373 has high crystallinity and almost no amorphous TiO2 was found in this sample. In our experiment, both ultrasound irradiation and long-time hydrothermal treatment perhaps favor the formation of crystalline structure and decrease the
Fig. 1. Wide-angle XRD pattern of samples TiO2-373 and TiO2-733. The inset is low-angle XRD patterns of sample TiO2-733.
content of amorphous fraction. Here, the aim of heat treatment is to remove the residual organics adsorbed on the surface of TiO2. The residuals usually have undesirable influences on the photocatalytic activity of TiO2. The average crystallite size of anatase in the samples can be calculated by applying the Debye–Scherrer formula [11] on the anatase (101) diffraction peaks. The calculated average crystallite sizes of sample TiO2-373 and TiO2-733 were 15.1 nm and 16.4 nm, respectively. The SEM and TEM images of sample TiO2-733 are shown in Fig. 2a and b, respectively. As shown in Fig. 2a, TiO2 sample consists of
Fig. 3. N2 adsorption–desorption isotherms and BJH pore size distribution (inset) of the sample TiO2-733.
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on the surface of TiO2, suggested that the bonding SO2− 4 cannot be entirely removed by washing and calcination. The formation of unique anatase for TiO2 sample might be relative to the presence of SO2− 4 ions.
4. Conclusion
Fig. 4. FT-IR spectra of samples (a) TiO2-373, (b) TiO2-573 and (c) TiO2-733.
spherical particles in sizes varying from 100 to 500 nm in diameter. The size of spheroids observed with SEM is extremely different from the XRD calculated size (16.4 nm), indicating that each spherical particles observed from SEM is not a single crystallite of TiO2 but the agglomerates of many single crystallites. From Fig. 2b, it can be clearly seen that the spheroids observed in Fig. 2a have disordered wormholelike pores. The presence of porous structure might favor the improvement of the photocatalytic activity of TiO2, because it not only increases the specific surface area of TiO2 but also facilitates the pass of reactants and degradation products. Although sol–gel combined calcination is usually used to prepare TiO2 particles with a certain crystalline structure, the morphologies of obtained TiO2 particles are often irregular. Above-mentioned program provides a new methodology for the preparation of porous TiO2 microspheres. Fig. 3 shows the N2 adsorption–desorption isotherm and pore size distribution (BJH) of the sample TiO2-733. The isotherm is type IV with H2 hysteresis-loop, characteristic of mesoporous materials [12]. The BET surface areas (SBET), average pore diameter, and pore volumes for TiO2-733 are 97 m2/g, 4.3 nm, 0.18 cm3/g, respectively. The mesoporous structure of TiO2-733 probably comes from the agglomeration of TiO2 particles. It has been reported that slow hydrolysis under ultrasound irradiation promotes the formation of monodispersed TiO2 particles. Thus, the mesoporous TiO2 was further produced by the ultrasound-induced agglomeration of monodispersed TiO2 particles [13]. The low-angle XRD patterns of TiO2-733 (the inset of Fig. 1) indicate that no long-range order in the pore arrangement exists in the sample TiO2-733, which is consistent with the TEM result. The FT-IR spectra of the obtained samples TiO2-373, TiO2-573 and TiO2-733 are shown in Fig. 4a, b, and c, respectively. All the spectra show one broad band around 3400 cm− 1 and another around 1650 cm− 1, which can been attributed to the surface-adsorbed H2O and –OH of TiO2 [14]. The intensities of the above-mentioned peaks decrease with the increase of calcination temperature, ascribed to the loss of the adsorbed H2O. The broad band at 400–900 cm− 1 can be ascribed to the vibration of Ti–O bond in TiO2. According to the previous reference [15], the band around 1135 cm− 1 in Fig. 4 should be ascribed to the characteristic band of bidentately bonding SO2− 4 species
TiO2 microspheres with porous frameworks have been successfully prepared via controlled hydrolysis of Ti(OC4H9-n)4 with water generated “in situ” by an esterification reaction between acetic acid and ethanol, followed by hydrothermal treatment. It was found that the TiO2 microspheres, containing disordered pores, were constructed by pure anatase particles with average crystalline size of 16.4 nm. The formation of mesoporous microsphere might be related to slow hydrolysis and polymerization rates, as well as ultrasound irradiation. The formation of unique anatase should be attributed to the presence of SO42− ions. In addition, both ultrasound irradiation and long-time hydrothermal might favor the improvement of crystallinity of TiO2. Acknowledgements This work has been supported by Shanghai Nanotechnology Promotion Centre (0752nm001, 0652nm045), Shanghai Science & Technology Committee (07JC14015), National Nature Science Foundation of China (20577009, 20773039), National Basic Research Program of China (973 Program, 2007CB613301) and the Ministry of Science and Technology of China (2006AA06Z379, 2006DFA52710). A part of research work was finished in Shanghai Nanotechnology Joint Lab. References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37–38. [2] A. Fujishima, T.N. Rao, D.A. Truk, J. Photochem. Photobiol. C 1 (2000) 1–21. [3] G.Q. Li, C.Y. Liu, Y. Liu, Appl. Surf. Sci. 253 (2006) 2481–2486. [4] J.F. Zhu, W. Zheng, B. He, J.L. Zhang, M. Anpo, J. Mol. Catal., A 216 (2004) 35–43. [5] I. Moriguchi, H. Maeda, Y. Teraoka, S. Kagawa, Chem. Mater. 9 (1997) 1050–1057. [6] C.C. Wang, J.Y. Ying, Chem. Mater. 11 (1999) 3113–3120. [7] J.F. Zhu, J.L. Zhang, F. Chen, Mater. Lett. 59 (2005) 3378–3381. [8] A. Larbot, I. Laaziz, J. Marignan, J.F. Quinson, J. Non-Cryst. Solids 147–148 (1992) 157–161. [9] C. Wang, Z.X. Deng, Y. Li, Inorg. Chem. 40 (2001) 5210–5214. [10] I.H. Tseng, J.C.S. Wu, H.Y. Chou, J. Catal. 221 (2004) 432–440. [11] J. Lin, P. Liu, M.J. Meziani, L.F. Allard, Y.P. Sun, J. Am. Chem. Soc. 124 (2002) 11514–11518. [12] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, et al., Pure Appl. Chem. 57 (1985) 603–619. [13] J.C. Yu, L.Z. Zhang, J.G. Yu, New J. Chem. 26 (2002) 416–420. [14] Z. Ding, G.Q. Lu, P.F. Greenfield, J. Phys. Chem., B 104 (2000) 4815–4820. [15] B.Z. Tian, F. Chen, J.L. Zhang, M. Anpo, J. Colloid Interface Sci. 303 (2006) 142–148.
Materials Letters 62 (2008) 2970 – 2972 www.elsevier.com/locate/matlet
Preparation and characterization of anatase TiO2 microspheres with porous frameworks via controlled hydrolysis of titanium alkoxide followed by hydrothermal treatment Tianzhong Tong a , Jinlong Zhang a,⁎, Baozhu Tian a , Feng Chen a , Dannong He b a
Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China b Shanghai National Engineering Research Center for Nanotechnology, 245 Jiachuan Road, Shanghai 200237, PR China Received 6 September 2007; accepted 29 January 2008 Available online 5 February 2008
Abstract TiO2 microspheres with porous frameworks have been prepared by controlled hydrolysis of Ti(OC4H9-n)4 with water generated “in situ” via an esterification reaction between acetic acid and ethanol, followed by hydrothermal treatment. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, and nitrogen adsorption–desorption methods. The TiO2 microspheres, with diameters ranging from 100 to 500 nm, were constructed by pure anatase particles with average crystalline size of 16.4 nm. The formation of porous structure might be related to slow hydrolysis under ultrasound irradiation. The formation of unique anatase should be attributed to the presence of SO2− 4 ions. © 2008 Elsevier B.V. All rights reserved. Keywords: TiO2 microspheres; Crystal structure; Porosity; Catalysts; Esterification reaction
1. Introduction Since the discovery of photoelectrochemical splitting of water on n-TiO2 electrodes [1], TiO2 has been extensively used as an environmentally harmonious and clean photocatalyst due to its various merits, such as its optical and electronic properties, low cost, high photocatalytic activity, non-toxicity and chemical stability [2,3]. TiO2 has three crystalline polymorphs: anatase, rutile, and brookite. It has been reported that the photocatalytic activity of TiO2 is related to its crystal structure and anatase TiO2 often exhibits the highest photocatalytic activity in the photodegradation of most pollutants in water and air [4]. As for the preparation method of anatase TiO2, sol–gel process based on the hydrolysis and polycondensation reactions of titanium alkoxides has been extensively employed [5,6]. However, the high hydrolysis rate of titanium alkoxides may cause uncontrolled local precipitation, resulting in photocatalytic losses in ⁎ Corresponding author. Tel./fax: +86 21 64252062. E-mail address: [email protected] (J. Zhang). 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.01.085
TiO2-based material [7]. Since the work of Larbot et al. [8], excellent control of the hydrolysis rate of titanium alkoxides has been realized with “in situ” water generated in esterification reactions [9,10]. In addition, the photocatalytic activity of TiO2 is also dictated by its morphology, because the photocatalytic reactions often take place on the surface of catalysts. Therefore, it is practically and scientifically significant to explore new synthetic methods in which the crystalline phase and morphology of TiO2 can be controlled. In this study, TiO2 microspheres with porous frameworks were prepared by controlled hydrolysis of Ti(OC4H9-n)4 (Eq. (1)), followed by hydrothermal treatment. The prepared samples were characterized by XRD, SEM, TEM, FT-IR, and nitrogen adsorption–desorption methods. (1).
T. Tong et al. / Materials Letters 62 (2008) 2970–2972
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2. Experimental section The synthetic procedure was as follows: Under magnetic stirring, 20 ml HAc was added dropwise to a flask containing 10 ml of Ti(OC4H9-n)4 diluted in 30 ml C2H5OH, followed by addition of 1 ml of H2SO4. Then, the obtained clear liquid was sonicated in an ultrasonic cleaning bath (Elma, T660/H, 35 kHZ, 360 W) at 313 K for 1 h and 333 K for 3 h, resulting in the formation of a milk-like sol, which was further transferred into a 100 ml Telflon-inner-liner stainless autoclave and kept at 393 K for 13 h. The resulting precipitates were separated from the mother liquor by centrifugation, washed thoroughly with deionized water and ethanol several times, and then dried at 373 K in air for 12 h. Finally, the obtained powders were further calcined at certain temperature for 2 h. The obtained samples were labeled as TiO2-T, in which “T” refers heat-treating temperature in centigrade degree. The phase presence and crystallite sizes of TiO2 samples were characterized by XRD performed on a Rigaku D/max 2550 VB/PC X-ray diffractometer. The morphologies of the TiO2 samples were observed with TEM (JEOL JEM-100 CX II) operated at 100 kV and SEM (JEOL JSM-6360 LV) operated at 15 kV. FT-IR spectroscopy was performed on a Nicolet Magna 550 spectrometer. The porous texture of the powders was analyzed from nitrogen adsorption–desorption isotherms at 77 K by using a Micromeritics ASAP 2000 system.
Fig. 2. SEM (a) and TEM (b) images of sample TiO2-733.
3. Results and discussion Fig. 1 shows the wide-angle XRD pattern of samples TiO2-373 and TiO2-733. Both samples consist of anatase as a unique phase (JCPDS, No. 21-1272). The peaks at scattering angles of 25.24, 36.98, 48.02 and 62.74° correspond to the reflections from the (101), (004), (200) and (204) crystal planes of anatase TiO2, respectively. No crystalline phase ascribed to rutile or brookite can be found. The intensities of the diffraction peaks for samples TiO2-373 and TiO2-733 have no evident differences, indicating that sample TiO2-373 has high crystallinity and almost no amorphous TiO2 was found in this sample. In our experiment, both ultrasound irradiation and long-time hydrothermal treatment perhaps favor the formation of crystalline structure and decrease the
Fig. 1. Wide-angle XRD pattern of samples TiO2-373 and TiO2-733. The inset is low-angle XRD patterns of sample TiO2-733.
content of amorphous fraction. Here, the aim of heat treatment is to remove the residual organics adsorbed on the surface of TiO2. The residuals usually have undesirable influences on the photocatalytic activity of TiO2. The average crystallite size of anatase in the samples can be calculated by applying the Debye–Scherrer formula [11] on the anatase (101) diffraction peaks. The calculated average crystallite sizes of sample TiO2-373 and TiO2-733 were 15.1 nm and 16.4 nm, respectively. The SEM and TEM images of sample TiO2-733 are shown in Fig. 2a and b, respectively. As shown in Fig. 2a, TiO2 sample consists of
Fig. 3. N2 adsorption–desorption isotherms and BJH pore size distribution (inset) of the sample TiO2-733.
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on the surface of TiO2, suggested that the bonding SO2− 4 cannot be entirely removed by washing and calcination. The formation of unique anatase for TiO2 sample might be relative to the presence of SO2− 4 ions.
4. Conclusion
Fig. 4. FT-IR spectra of samples (a) TiO2-373, (b) TiO2-573 and (c) TiO2-733.
spherical particles in sizes varying from 100 to 500 nm in diameter. The size of spheroids observed with SEM is extremely different from the XRD calculated size (16.4 nm), indicating that each spherical particles observed from SEM is not a single crystallite of TiO2 but the agglomerates of many single crystallites. From Fig. 2b, it can be clearly seen that the spheroids observed in Fig. 2a have disordered wormholelike pores. The presence of porous structure might favor the improvement of the photocatalytic activity of TiO2, because it not only increases the specific surface area of TiO2 but also facilitates the pass of reactants and degradation products. Although sol–gel combined calcination is usually used to prepare TiO2 particles with a certain crystalline structure, the morphologies of obtained TiO2 particles are often irregular. Above-mentioned program provides a new methodology for the preparation of porous TiO2 microspheres. Fig. 3 shows the N2 adsorption–desorption isotherm and pore size distribution (BJH) of the sample TiO2-733. The isotherm is type IV with H2 hysteresis-loop, characteristic of mesoporous materials [12]. The BET surface areas (SBET), average pore diameter, and pore volumes for TiO2-733 are 97 m2/g, 4.3 nm, 0.18 cm3/g, respectively. The mesoporous structure of TiO2-733 probably comes from the agglomeration of TiO2 particles. It has been reported that slow hydrolysis under ultrasound irradiation promotes the formation of monodispersed TiO2 particles. Thus, the mesoporous TiO2 was further produced by the ultrasound-induced agglomeration of monodispersed TiO2 particles [13]. The low-angle XRD patterns of TiO2-733 (the inset of Fig. 1) indicate that no long-range order in the pore arrangement exists in the sample TiO2-733, which is consistent with the TEM result. The FT-IR spectra of the obtained samples TiO2-373, TiO2-573 and TiO2-733 are shown in Fig. 4a, b, and c, respectively. All the spectra show one broad band around 3400 cm− 1 and another around 1650 cm− 1, which can been attributed to the surface-adsorbed H2O and –OH of TiO2 [14]. The intensities of the above-mentioned peaks decrease with the increase of calcination temperature, ascribed to the loss of the adsorbed H2O. The broad band at 400–900 cm− 1 can be ascribed to the vibration of Ti–O bond in TiO2. According to the previous reference [15], the band around 1135 cm− 1 in Fig. 4 should be ascribed to the characteristic band of bidentately bonding SO2− 4 species
TiO2 microspheres with porous frameworks have been successfully prepared via controlled hydrolysis of Ti(OC4H9-n)4 with water generated “in situ” by an esterification reaction between acetic acid and ethanol, followed by hydrothermal treatment. It was found that the TiO2 microspheres, containing disordered pores, were constructed by pure anatase particles with average crystalline size of 16.4 nm. The formation of mesoporous microsphere might be related to slow hydrolysis and polymerization rates, as well as ultrasound irradiation. The formation of unique anatase should be attributed to the presence of SO42− ions. In addition, both ultrasound irradiation and long-time hydrothermal might favor the improvement of crystallinity of TiO2. Acknowledgements This work has been supported by Shanghai Nanotechnology Promotion Centre (0752nm001, 0652nm045), Shanghai Science & Technology Committee (07JC14015), National Nature Science Foundation of China (20577009, 20773039), National Basic Research Program of China (973 Program, 2007CB613301) and the Ministry of Science and Technology of China (2006AA06Z379, 2006DFA52710). A part of research work was finished in Shanghai Nanotechnology Joint Lab. References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37–38. [2] A. Fujishima, T.N. Rao, D.A. Truk, J. Photochem. Photobiol. C 1 (2000) 1–21. [3] G.Q. Li, C.Y. Liu, Y. Liu, Appl. Surf. Sci. 253 (2006) 2481–2486. [4] J.F. Zhu, W. Zheng, B. He, J.L. Zhang, M. Anpo, J. Mol. Catal., A 216 (2004) 35–43. [5] I. Moriguchi, H. Maeda, Y. Teraoka, S. Kagawa, Chem. Mater. 9 (1997) 1050–1057. [6] C.C. Wang, J.Y. Ying, Chem. Mater. 11 (1999) 3113–3120. [7] J.F. Zhu, J.L. Zhang, F. Chen, Mater. Lett. 59 (2005) 3378–3381. [8] A. Larbot, I. Laaziz, J. Marignan, J.F. Quinson, J. Non-Cryst. Solids 147–148 (1992) 157–161. [9] C. Wang, Z.X. Deng, Y. Li, Inorg. Chem. 40 (2001) 5210–5214. [10] I.H. Tseng, J.C.S. Wu, H.Y. Chou, J. Catal. 221 (2004) 432–440. [11] J. Lin, P. Liu, M.J. Meziani, L.F. Allard, Y.P. Sun, J. Am. Chem. Soc. 124 (2002) 11514–11518. [12] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, et al., Pure Appl. Chem. 57 (1985) 603–619. [13] J.C. Yu, L.Z. Zhang, J.G. Yu, New J. Chem. 26 (2002) 416–420. [14] Z. Ding, G.Q. Lu, P.F. Greenfield, J. Phys. Chem., B 104 (2000) 4815–4820. [15] B.Z. Tian, F. Chen, J.L. Zhang, M. Anpo, J. Colloid Interface Sci. 303 (2006) 142–148.