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Synthesis of nanostructured TiO2 particles in room temperature ionic liquid and its photocatalytic performance
Materials Letters 61 (2007) 5056 – 5058 www.elsevier.com/locate/matlet
Synthesis of nanostructured TiO2 particles in room temperature ionic liquid and its photocatalytic performance Yongai Zhai a , Yu Gao b , Fengqi Liu a , Qing Zhang a , Ge Gao a,⁎ a
College of Chemistry and MacDiarmid Laboratory, Jilin University, Changchun 130023, PR China b Peking University Health Science Center, Beijing 100083, PR China Received 15 November 2006; accepted 1 April 2007 Available online 5 April 2007
Abstract Nanostructured TiO2 particles were synthesized by sol–gel method with room temperature ionic liquid (RTIL) 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) as a reaction medium. The structure and morphology of TiO2 nanoparticles were characterized with X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The as-prepared TiO2 nanoparticles present anatase crystal phase even without being calcined at high temperature, and show better photocatalytic performance in the degradation of methyl orange. The photocatalytic efficiency increases evidently along with increasing the concentration of nanostructure TiO2, and the degradation percent can reach 100% at the optimal catalyst concentration (2.0 g/L). © 2007 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; sol–gel preparation; Ionic liquid; Photocatalytic degradation; Methyl orange
1. Introduction In recent years, room temperature ionic liquids have aroused the increasing interest of chemists and engineers as a new and novel generation of solvent. Compared to extensively use volatile organic compounds (VOCs) [1], ionic liquids have practically no vapour pressure and possess tunable solvent properties. In addition, ionic liquids have other properties, such as high thermal stability, wide electrochemical window, no combustion, nontoxicity, and unusual dissolve capability, so they have obtained more and more applications as a kind of green solvent in the field of organic synthesis, catalysis and separation etc [2–5]. [BMIM][PF6], as a kind of alkyl imidazolium ionic liquid, is water immiscible solvent and meets the green solvent demands. Certainly, the selection of [BMIM][PF6] was mainly based on the fact that it can be obtained at a relatively low price and its synthesis method is simple. Nanocrystalline TiO2 has become an ideal photolytic degradation catalyst due to its chemical stability, high catalytic ⁎ Corresponding author. Tel./fax: +86 431 8499187. E-mail address: [email protected] (G. Gao). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.04.002
efficiency, nontoxicity, low cost etc [6]. Usually, sol–gel prepared TiO2 nanoparticles are amorphous in nature, requiring further heat treatment to obtain crystallization materials. This calcination step involves rapid thermal transformation, resulting in particle agglomeration and grain growth. Therefore, it is highly desirable to synthesize crystalline TiO2 nanoparticles at low temperature without any further heat treatment. This will save energy in the preparation steps and also allow for a wider selection of support materials to immobilize TiO2 materials. Recently, there are some reports about the combination of nanomaterials with ionic liquids. Dupont et al. [7] reported that they prepared nanometal Ir particles by deoxidizing [IrCl(cod)]2 (cod = 1,5-cyclooctadiene) in the 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) IL. Nakashima et al. [8] synthesized hollow TiO2 microspheres in [BMIM][PF6]. Zhou et al. [9] synthesized very small TiO2 nanocrystals in 1butyl-3-methylimidazolium tetrafluoroborate at ambient temperature. In this work, the nanostructure TiO2 particles were synthesized using [BMIM][PF6] as a reaction medium. The structure and morphology of TiO2 nanoparticles were characterized with X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The as-prepared
Y. Zhai et al. / Materials Letters 61 (2007) 5056–5058
Fig. 1. XRD pattern of TiO2 nanoparticles.
TiO2 nanoparticles present anatase crystal phase even without being calcined at high temperature, and show a better photocatalytic performance in the degradation of methyl orange.
5057
where A0 and At are absorbencies at t = 0 and time t respectively. The photocatalytic experiments were processed at room temperature (20 ± 2 °C). The structure of [BMIM][PF6] was measured with a mercury-300BB nuclear magnetic resonance (1H-NMR). The crystallization structure of TiO2 was characterized with a Rigaku wide angle X-ray diffractometer (D/max γA, using Cu Kα radiation at wavelength λ = 1.541 Å). The detector was calibrated by using KCl powders as a standard with 2θ being 28.345° (200) and 40.507° (220) under Cu Kα radiation. The morphology of TiO2 was characterized with a JEM-2000FX transmission electron microscopy (TEM). The absorbencies in the photocatalytic experiments were measured with a 722 mode grating spectrophotometer. 3. Results and discussion The chemical structure of [BMIM][PF6] is shown below.
2. Experimental [BMIM][PF6] was synthesized according to the literature [10]. All other chemicals used were of analytical grade and were used as received without any further purification. The synthesis of nanostructure TiO2 particles was carried out referencing [9,11]: 1 mL of tetra-n-butyl titanate was mixed with 5 mL of [BMIM][PF6]. After homogenizing the mixture, 2 mL of distilled water was added slowly under stirring at room temperature followed stirring at 80 °C for another 6 h. Afterwards the produced TiO2 nanoparticles were gathered by centrifugation and a yellowish solid was obtained. The residue of ionic liquid in the product was extracted with acetonitrile in a closed vessel at 50 °C for 8 h. The extraction process was repeated until completely removing the ionic liquid from the TiO2. The obtained product was recovered by filtration, washed thoroughly with distilled water and then dried at 70 °C for 2 h in a vacuum oven to remove residual water. The final product was a white powder by grinding. Photocatalytic experiments were performed as follows: a definite mass of nanostructure TiO2 powder was mixed with the desired concentration of methyl orange solution (20 mg/L) in a quartz tube. After ultrasonic dispersion for 15 min, the mixed solution was put in a home-built photocatalytic equipment, in which a ventilate tube was inserted to provide air and the light sources were two 8 W ultraviolet lamps paralleled to the quartz tube. 4 mL of sample was taken periodically and further separated with centrifugation at 4000 rpm for 15 min. The obtained upper clear liquid was analyzed at wavelength 460 nm by a spectrophotometer. The degradation percent η was calculated according to the following expression: g¼
A0 At 100k A0
The 1H-NMR data are δ: 0.928 (t, 1H), δ: 1.355 (m, 2H), δ: 1.910 (m, 3H), δ: 4.033 (s, 8H), δ: 4.340 (t, 4H), δ: 7.685 (t, 5H or 6H), δ: 7.742 (t, 6H or 5H), δ: 8.996 (s, 7H). The XRD pattern and TEM image of nanostructure TiO2 particles are shown in Figs. 1 and 2 respectively. By using the routine sol–gel method, only amorphous TiO2 particles can be obtained, which do not present photocatalytic activity and need to go through high temperature calcination for crystallization. However, as shown in Fig. 1, the prepared TiO2 nanocrystals exhibit anatase crystal structure (JCPDS 83-2243) even if they are not being calcined at high temperature in our work. Though the mechanism is not completely clear, we prefer
Fig. 2. TEM image of TiO2 particles.
5058
Y. Zhai et al. / Materials Letters 61 (2007) 5056–5058
Fig. 3. The effect of loading TiO2 on photocatalytic degradation of methyl orange: (a) 0.2 g/L; (b) 0.5 g/L; (c) 1.0 g/L; (d) 1.5 g/L; (e) 2.0 g/L; (f) 2.5 g/L.
to support the mechanism that the ionic liquid is used as a selfassembling template like long chain surfactants [11]. The property of [BMIM][PF6], which is immiscible with water, may be due to its special chemical structure—large asymmetry cation and small anion. It acted as a template in the process of sol aging and gel drying, so it reduced the change of surface area, which was caused by gel shrinkage in routine sol–gel method. The crystallite size was calculated with a value of around 5 nm based on the following Scherrer's equation: Dhkl ¼
Kk bhkl coshhkl
K βhkl θhkl λ
4. Conclusions In conclusion, the nanocrystalline TiO2 particles were synthesized by sol–gel method with [BMIM][PF6] as a reaction medium. The as-prepared TiO2 particles present anatase crystal morphology and have the photocatalytic activity even without being calcined at high temperature. The photocatalytic efficiency increases evidently along with increasing the concentration of nanostructure TiO2, and the optimal catalyst concentration is 2.0 g/L, by which the degradation percent can reach 100% at 5 h. References
where Dhkl
reached 89% at 7 h when 0.2 g/L TiO2 catalyst was used. The photocatalytic efficiency increased evidently along with increasing the concentration of nanostructure TiO2. Nevertheless, further increasing the loading from 2.0 to 2.5 g/L decreased the degradation percent. It is evident that the probability to absorb photons will also increase along with increasing the amount of TiO2 [12], which increase degradation efficiency accordingly. However, when the photons absorbed by TiO2 reach the saturation, the degradation efficiency can't increase again. If the loading of titanium dioxide is higher than a critical concentration, the penetration depth of light in the reactor is small, scattering of light and the collision of titanium dioxide increased, and a negative effect on the reaction rate develops. Therefore there is a fitting TiO2 concentration to achieve optimal degradation efficiency. In our study, the optimal TiO2 concentration was 2.0 g/L, by which degradation percent reached 96% at 3 h and reached 100% at 5 h.
The particle size perpendicular to the normal line of (hkl) plane A constant (K = 0.9) The full width at half maximum of the (hkl) diffraction peak The Bragg angle of (hkl) peak The wavelength of X-ray
The formed nanoparticles are constructing into disordered pore structure as shown in Fig. 2, which is in favor of photocatalytic degradation, and the nanocrystalline size is basically consistent with the XRD results. The relationship of photocatalytic degradation percent with the irradiation time is shown in Fig. 3. The degradation percent
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
L.F. Yan, Q.S. Zhu, Chemistry 11 (2001) 673. T. Welton, Chem. Rev. 99 (1999) 2701. P. Wasserscheid, W. Keim, Angew. Chem., Int. Ed. Engl. 39 (2000) 3772. R. Hagiwara, Y. Ito, J. Fluorine Chem. 105 (2000) 221. K. Przemyslaw, Prog. Polym. Sci. 29 (2004) 3. D.F. Ouis, E. Pelizzetti, N. Serpone, Environ. Sci. Technol. 25 (1991) 1523. J. Dupont, G.S. Fonseca, A.P. Umpierre, P.F.P. Fichtner, S.R. Teixeira, J. Am. Chem. Soc. 124 (2002) 4228. T. Nakashima, N. Kimizuka, J. Am. Chem. Soc. 125 (2003) 6386. Y. Zhou, M. Antonietti, J. Am. Chem. Soc. 125 (2003) 14960. J.G. Huddleston, H.D. Willauer, R.P. Swatloski, A.E. Visser, R.D. Rogers, Chem. Commun. (1998) 1765. K. Yoo, H. Choi, Dionysios D. Dionysiou, Chem. Commun. (2004) 2000. S.P. Ruan, F.Q. Wu, T. Zhang, W. Gao, B.K. Xu, M.Y. Zhao, Mater. Chem. Phys. 69 (2001) 7.
Synthesis of nanostructured TiO2 particles in room temperature ionic liquid and its photocatalytic performance Yongai Zhai a , Yu Gao b , Fengqi Liu a , Qing Zhang a , Ge Gao a,⁎ a
College of Chemistry and MacDiarmid Laboratory, Jilin University, Changchun 130023, PR China b Peking University Health Science Center, Beijing 100083, PR China Received 15 November 2006; accepted 1 April 2007 Available online 5 April 2007
Abstract Nanostructured TiO2 particles were synthesized by sol–gel method with room temperature ionic liquid (RTIL) 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) as a reaction medium. The structure and morphology of TiO2 nanoparticles were characterized with X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The as-prepared TiO2 nanoparticles present anatase crystal phase even without being calcined at high temperature, and show better photocatalytic performance in the degradation of methyl orange. The photocatalytic efficiency increases evidently along with increasing the concentration of nanostructure TiO2, and the degradation percent can reach 100% at the optimal catalyst concentration (2.0 g/L). © 2007 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; sol–gel preparation; Ionic liquid; Photocatalytic degradation; Methyl orange
1. Introduction In recent years, room temperature ionic liquids have aroused the increasing interest of chemists and engineers as a new and novel generation of solvent. Compared to extensively use volatile organic compounds (VOCs) [1], ionic liquids have practically no vapour pressure and possess tunable solvent properties. In addition, ionic liquids have other properties, such as high thermal stability, wide electrochemical window, no combustion, nontoxicity, and unusual dissolve capability, so they have obtained more and more applications as a kind of green solvent in the field of organic synthesis, catalysis and separation etc [2–5]. [BMIM][PF6], as a kind of alkyl imidazolium ionic liquid, is water immiscible solvent and meets the green solvent demands. Certainly, the selection of [BMIM][PF6] was mainly based on the fact that it can be obtained at a relatively low price and its synthesis method is simple. Nanocrystalline TiO2 has become an ideal photolytic degradation catalyst due to its chemical stability, high catalytic ⁎ Corresponding author. Tel./fax: +86 431 8499187. E-mail address: [email protected] (G. Gao). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.04.002
efficiency, nontoxicity, low cost etc [6]. Usually, sol–gel prepared TiO2 nanoparticles are amorphous in nature, requiring further heat treatment to obtain crystallization materials. This calcination step involves rapid thermal transformation, resulting in particle agglomeration and grain growth. Therefore, it is highly desirable to synthesize crystalline TiO2 nanoparticles at low temperature without any further heat treatment. This will save energy in the preparation steps and also allow for a wider selection of support materials to immobilize TiO2 materials. Recently, there are some reports about the combination of nanomaterials with ionic liquids. Dupont et al. [7] reported that they prepared nanometal Ir particles by deoxidizing [IrCl(cod)]2 (cod = 1,5-cyclooctadiene) in the 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) IL. Nakashima et al. [8] synthesized hollow TiO2 microspheres in [BMIM][PF6]. Zhou et al. [9] synthesized very small TiO2 nanocrystals in 1butyl-3-methylimidazolium tetrafluoroborate at ambient temperature. In this work, the nanostructure TiO2 particles were synthesized using [BMIM][PF6] as a reaction medium. The structure and morphology of TiO2 nanoparticles were characterized with X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The as-prepared
Y. Zhai et al. / Materials Letters 61 (2007) 5056–5058
Fig. 1. XRD pattern of TiO2 nanoparticles.
TiO2 nanoparticles present anatase crystal phase even without being calcined at high temperature, and show a better photocatalytic performance in the degradation of methyl orange.
5057
where A0 and At are absorbencies at t = 0 and time t respectively. The photocatalytic experiments were processed at room temperature (20 ± 2 °C). The structure of [BMIM][PF6] was measured with a mercury-300BB nuclear magnetic resonance (1H-NMR). The crystallization structure of TiO2 was characterized with a Rigaku wide angle X-ray diffractometer (D/max γA, using Cu Kα radiation at wavelength λ = 1.541 Å). The detector was calibrated by using KCl powders as a standard with 2θ being 28.345° (200) and 40.507° (220) under Cu Kα radiation. The morphology of TiO2 was characterized with a JEM-2000FX transmission electron microscopy (TEM). The absorbencies in the photocatalytic experiments were measured with a 722 mode grating spectrophotometer. 3. Results and discussion The chemical structure of [BMIM][PF6] is shown below.
2. Experimental [BMIM][PF6] was synthesized according to the literature [10]. All other chemicals used were of analytical grade and were used as received without any further purification. The synthesis of nanostructure TiO2 particles was carried out referencing [9,11]: 1 mL of tetra-n-butyl titanate was mixed with 5 mL of [BMIM][PF6]. After homogenizing the mixture, 2 mL of distilled water was added slowly under stirring at room temperature followed stirring at 80 °C for another 6 h. Afterwards the produced TiO2 nanoparticles were gathered by centrifugation and a yellowish solid was obtained. The residue of ionic liquid in the product was extracted with acetonitrile in a closed vessel at 50 °C for 8 h. The extraction process was repeated until completely removing the ionic liquid from the TiO2. The obtained product was recovered by filtration, washed thoroughly with distilled water and then dried at 70 °C for 2 h in a vacuum oven to remove residual water. The final product was a white powder by grinding. Photocatalytic experiments were performed as follows: a definite mass of nanostructure TiO2 powder was mixed with the desired concentration of methyl orange solution (20 mg/L) in a quartz tube. After ultrasonic dispersion for 15 min, the mixed solution was put in a home-built photocatalytic equipment, in which a ventilate tube was inserted to provide air and the light sources were two 8 W ultraviolet lamps paralleled to the quartz tube. 4 mL of sample was taken periodically and further separated with centrifugation at 4000 rpm for 15 min. The obtained upper clear liquid was analyzed at wavelength 460 nm by a spectrophotometer. The degradation percent η was calculated according to the following expression: g¼
A0 At 100k A0
The 1H-NMR data are δ: 0.928 (t, 1H), δ: 1.355 (m, 2H), δ: 1.910 (m, 3H), δ: 4.033 (s, 8H), δ: 4.340 (t, 4H), δ: 7.685 (t, 5H or 6H), δ: 7.742 (t, 6H or 5H), δ: 8.996 (s, 7H). The XRD pattern and TEM image of nanostructure TiO2 particles are shown in Figs. 1 and 2 respectively. By using the routine sol–gel method, only amorphous TiO2 particles can be obtained, which do not present photocatalytic activity and need to go through high temperature calcination for crystallization. However, as shown in Fig. 1, the prepared TiO2 nanocrystals exhibit anatase crystal structure (JCPDS 83-2243) even if they are not being calcined at high temperature in our work. Though the mechanism is not completely clear, we prefer
Fig. 2. TEM image of TiO2 particles.
5058
Y. Zhai et al. / Materials Letters 61 (2007) 5056–5058
Fig. 3. The effect of loading TiO2 on photocatalytic degradation of methyl orange: (a) 0.2 g/L; (b) 0.5 g/L; (c) 1.0 g/L; (d) 1.5 g/L; (e) 2.0 g/L; (f) 2.5 g/L.
to support the mechanism that the ionic liquid is used as a selfassembling template like long chain surfactants [11]. The property of [BMIM][PF6], which is immiscible with water, may be due to its special chemical structure—large asymmetry cation and small anion. It acted as a template in the process of sol aging and gel drying, so it reduced the change of surface area, which was caused by gel shrinkage in routine sol–gel method. The crystallite size was calculated with a value of around 5 nm based on the following Scherrer's equation: Dhkl ¼
Kk bhkl coshhkl
K βhkl θhkl λ
4. Conclusions In conclusion, the nanocrystalline TiO2 particles were synthesized by sol–gel method with [BMIM][PF6] as a reaction medium. The as-prepared TiO2 particles present anatase crystal morphology and have the photocatalytic activity even without being calcined at high temperature. The photocatalytic efficiency increases evidently along with increasing the concentration of nanostructure TiO2, and the optimal catalyst concentration is 2.0 g/L, by which the degradation percent can reach 100% at 5 h. References
where Dhkl
reached 89% at 7 h when 0.2 g/L TiO2 catalyst was used. The photocatalytic efficiency increased evidently along with increasing the concentration of nanostructure TiO2. Nevertheless, further increasing the loading from 2.0 to 2.5 g/L decreased the degradation percent. It is evident that the probability to absorb photons will also increase along with increasing the amount of TiO2 [12], which increase degradation efficiency accordingly. However, when the photons absorbed by TiO2 reach the saturation, the degradation efficiency can't increase again. If the loading of titanium dioxide is higher than a critical concentration, the penetration depth of light in the reactor is small, scattering of light and the collision of titanium dioxide increased, and a negative effect on the reaction rate develops. Therefore there is a fitting TiO2 concentration to achieve optimal degradation efficiency. In our study, the optimal TiO2 concentration was 2.0 g/L, by which degradation percent reached 96% at 3 h and reached 100% at 5 h.
The particle size perpendicular to the normal line of (hkl) plane A constant (K = 0.9) The full width at half maximum of the (hkl) diffraction peak The Bragg angle of (hkl) peak The wavelength of X-ray
The formed nanoparticles are constructing into disordered pore structure as shown in Fig. 2, which is in favor of photocatalytic degradation, and the nanocrystalline size is basically consistent with the XRD results. The relationship of photocatalytic degradation percent with the irradiation time is shown in Fig. 3. The degradation percent
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
L.F. Yan, Q.S. Zhu, Chemistry 11 (2001) 673. T. Welton, Chem. Rev. 99 (1999) 2701. P. Wasserscheid, W. Keim, Angew. Chem., Int. Ed. Engl. 39 (2000) 3772. R. Hagiwara, Y. Ito, J. Fluorine Chem. 105 (2000) 221. K. Przemyslaw, Prog. Polym. Sci. 29 (2004) 3. D.F. Ouis, E. Pelizzetti, N. Serpone, Environ. Sci. Technol. 25 (1991) 1523. J. Dupont, G.S. Fonseca, A.P. Umpierre, P.F.P. Fichtner, S.R. Teixeira, J. Am. Chem. Soc. 124 (2002) 4228. T. Nakashima, N. Kimizuka, J. Am. Chem. Soc. 125 (2003) 6386. Y. Zhou, M. Antonietti, J. Am. Chem. Soc. 125 (2003) 14960. J.G. Huddleston, H.D. Willauer, R.P. Swatloski, A.E. Visser, R.D. Rogers, Chem. Commun. (1998) 1765. K. Yoo, H. Choi, Dionysios D. Dionysiou, Chem. Commun. (2004) 2000. S.P. Ruan, F.Q. Wu, T. Zhang, W. Gao, B.K. Xu, M.Y. Zhao, Mater. Chem. Phys. 69 (2001) 7.