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Synthesis of high surface area mesoporous carbonates in novel ionic liquid
Materials Letters 63 (2009) 1061–1064
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis of high surface area mesoporous carbonates in novel ionic liquid Yawei Hou, Aiguo Kong, Xinhua Zhao, Haiyan Zhu, Yongkui Shan ⁎ Department of chemistry, East China Normal University, Shanghai, 200062, PR China
a r t i c l e
i n f o
Article history: Received 13 December 2008 Accepted 3 February 2009 Available online 10 February 2009 Keywords: Nanomaterials Straw-like Porosity Carbonate Ionic liquid
a b s t r a c t High surface area mesoporous SrCO3, CaCO3 and MnCO3 with straw-like bundles morphology were synthesized in a novel imidazolium pyruvate ionic liquid. These carbonates were characterized by X-ray diffraction, Fourier transform infrared spectra and transmission electron microscopy. The results revealed that these carbonates have pure orthorhombic and rhombohedral structures, respectively, and the prepared products all show the unique straw-like morphology. Mesoporous properties and high surface areas of SrCO3 (118 m2/g), CaCO3(43 m2/g) and MnCO3 (122 m2/g) were also confirmed by the corresponding N2 sorption analyses. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental section
High surface area nanomaterials with special morphologies and structures have attracted growing interests because of their numerous potential applications [1]. As a special solvent, room temperature ionic liquid (RTIL) provides an opportunity to create many unique nanomaterials with different morphologies or structures. Many metals nanoparticles, nanorods and nanowires [2], hollow TiO2 microspheres [3], well-ordered mesoporous SiO2 [4] and mesoporous γ-Al2O3 [5] have been obtained in RTIL. SrCO3 and CaCO3 are important components in minerals, skeleton and many mineralizing organisms and are applied to many industrial productions [1,6]. Nano-scaled SrCO3 or CaCO3 with rodlike, whiskerlike or flowerlike morphologies [1,6] have been synthesized in aqueous system. Han et al. reported the mesoporous SrCO3 spheres and hollow CaCO3 spheres prepared in guanidinium ionic liquid, but the surface area of those products is very low, even only 17 m2/g [7]. It is still attractive to prepare high surface areas mesoporous carbonates with unique morphology and structure. In this paper, mesoporous SrCO3 and CaCO3 with unique straw-like bundles morphology have been synthesized in a novel imidazolium pyruvate ionic liquid. These prepared carbonates have high surface areas, even up to 118 m2/g, and show the typical mesoporous properties. Furthermore, this method can be applied to the synthesis of mesoporous transition metal carbonates (i.e. MnCO3) with the similar straw-like morphology and high surface area.
2.1. Synthesis of ionic liquid
⁎ Corresponding author. Tel./fax: +86 021 62233505. E-mail address: [email protected] (Y. Shan). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.02.005
The 1-methyl-3-pentylimidazolium bromide (PMIMBr) and silver pyruvate (CH3COCO2Ag) were prepared according to the literatures [8,9]. The synthesized CH3COCO2Ag was dispersed in anhydrous ethanol. Then the equal molar PMIMBr was added drop by drop and the mixture was stirred for 5 h. The resultant precipitation was removed, and the filtrate was vaporized under vacuum to remove ethanol. Finally, a clear slightly yellow liquid [PMIM][CH3COCO2] was obtained. IR (KBr pellet): 3418.82, 3146.46, 3093.36, 2962.52, 2933.95, 2874.17, 1705.84, 1622.14, 1469.37, 1386.31, 835.06, 758.67, 625.83 cm− 1. 1H NMR (500 MHz, D2O): δ = 8.60 (s, 1H), δ = 7.37 (s, 1H), δ = 7.32 (s, 1H), δ = 4.09 (t, 2H), δ = 3.79 (s, 3H), δ = 2.27(s, 3H), δ = 1.74 (m, 2H), δ = 1.23 (m, 2H), δ = 1.18 (m, 2H), δ = 0.78 (t, 3H). 2.2. Synthesis of SrCO3, CaCO3 and MnCO3 The typical synthesis of SrCO3 was carried out as follows: 10 ml of the CO2-saturated [PMIM][CH3COCO2] RTIL (bubbling with CO2 for 1 h) and 0.8 g SrCl2.6H2O powders were mixed in a flask at room temperature, and 1.2 ml of 5 M NaOH aqueous solution was added. The
Scheme 1. Chemical structure of the ionic liquid.
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Y. Hou et al. / Materials Letters 63 (2009) 1061–1064
resultant mixtures were refluxed at 100 °C under stirring for 10 h. During the reactions, SrCl2 was partly dissolved in this RTIL system containing little water, and reacted with CO2 at the alkaline conditions to produce the carbonates. The final carbonate sample (~ 86% isolated yield) was obtained by centrifugation, washing with ethanol and drying at 60 °C. CaCO3 and MnCO3 samples were prepared by the same procedure using CaCl2 and MnCl2d 4H2O as the raw materials, and the isolated yields are about 81% and 83%, respectively. 2.3. Characterization 1
Fig. 1. XRD patterns of the SrCO3 (a), CaCO3 (b) and MnCO3 (c) prepared in ionic liquid system.
H NMR of the RTIL was recorded on Brucker DRX-500 spectrometer. FT-IR spectra were recorded on a Nexus 670 Fourier transform infrared spectrophotometer (Nicolet). The differential scanning calorimetry (DSC) experiment was measured using a Pyris 1 DSC (Perkin-Elmer) with a heating rate of 10 °C min− 1. The thermal stability of the RTIL was investigated on a TGA/SDTA851 thermogravimetric analyzer with a heating rate of 10 °C min− 1. X-ray diffraction (XRD) patterns of the carbonates were recorded on a D8advance diffractometer (Cu Ka radiation; l = 0.154 nm). Transmission electron microscopy (TEM) images were taken with a JEOL JEM 2100 transmission electron microscope using an accelerating voltage of 200 kV. BET surface area and pore distribution of the carbonates were
Fig. 2. TEM images of the SrCO3 (A,B), CaCO3 (C,D) and MnCO3 (E,F) synthesized in ionic liquid system.
Y. Hou et al. / Materials Letters 63 (2009) 1061–1064
1063
MnCO3, respectively (JCPDS 47-1743 and 86-0172). In addition, the broadened XRD diffraction peaks were also observed, which suggest the small particle size of these carbonates. The particle sizes of three carbonates estimated from the Scherrer equation [10] are about 9 nm (SrCO3), 30 nm (CaCO3) and 12 nm (MnCO3), respectively. The FT-IR spectra (not shown) of these samples revealed the characteristic peaks of typical carbonates, in agreement with the XRD analyses. The strong absorption bands at about 1470, 850 and 700 cm− 1 can be attributed to the ν3, ν2 and ν4 modes of the carbonates ion, respectively [11]. The morphologies of the prepared carbonates were also observed by TEM images (see Fig. 2). Especially, all these carbonates samples show the special straw-like bundles morphology, and carbonates with such unique morphology are little reported. A few tiny particles and many inconsecutive tiny particle lines can also be observed in these images. It seems that the tiny particles stack or grow into lines and these particle lines further aggregated into bundles. As a result, the special straw-like bundles morphology is formed. TEM images clearly indicate that the particle size of CaCO3 is slightly larger than that of SrCO3 and MnCO3, which is in good agreement with the results of XRD. 3.3. Porous property The porous properties of the SrCO3, CaCO3 and MnCO3 products have been investigated by the N2 adsorption–desorption analyses (Fig. 3). The SrCO3, CaCO3 and MnCO3 samples show typical type IV isotherms with hysteresis loops in the P/Po region of about 0.4–0.8, 0.4–0.8 and 0.5–0.8, respectively (see Fig. 3A a, b and c). Their BET specific surface areas are 118, 43 and 122 m2/g, respectively. Comparatively, the SrCO3 and CaCO3 samples show higher surface areas than that of the reported SrCO3 or CaCO3 [7,12,13]. The pore size distribution analyses of SrCO3 and MnCO3 reveal their narrow pore size distribution (see Fig. 3B) center at 4 nm (SrCO3) and 5 nm (MnCO3), respectively, while CaCO3 has a narrower pore size distribution around 4 nm (see Fig. 3B b). Obviously, the porous properties of carbonates are mainly from the stack of their nanoparticles, which is in agreement with the TEM results. 3.4. Formation of nanostructure carbonates
Fig. 3. N2 sorption isotherms (A) and the corresponding pore size distributions (B) for the synthesized SrCO3 (a), CaCO3 (b) and MnCO3 (c).
During the formation of crystalline carbonates, the CO2 absorbed in the RTIL reacted with hydroxyl and metal ion to form carbonates deposit gradually. The reaction can be written as follow [7]: 2þ
M determined from N2 adsorption–desorption isotherms using a Micromeritics ASAP 2000 system. 3. Results and discussion 3.1. Property of the novel ionic liquid The structure (Scheme 1) of this RTIL has been determined by the FT-IR spectra and 1H NMR (listed above). Its thermal behavior was investigated by DSC measurement heating from −100 °C to 100 °C. There is only a glass transition temperature at −59 °C during heating. The decomposition temperature of the RTIL measured from thermogravimetric analysis is about 105 °C. The conductivity is 0.16 mS/cm at room temperature. 3.2. Microstructure of the synthesized carbonates Fig. 1 shows the XRD patterns of the SrCO3, CaCO3 and MnCO3 synthesized in this RTIL system. The diffraction peaks in Fig. 1a can be indexed to the pure orthorhombic phase of SrCO3 (JCPDS 71-2393). Fig. 1b and c indicate the rhombohedral structures of CaCO3 and
þ CO2 þ 2OH−→MCO3 þ H2 O ðM ¼ Sr; Ca and MnÞ
And then, the pyruvate anion may react with a small quantity of O2 from air in the alkaline system and slowly decarboxylate to form acetate and CO2 [14,15] during heating. As a result, this compensated for the decrease of CO2 in reaction and promoted the production of carbonates. As discussing above, the reaction formula of the decomposition could be inferred as follow: CH3 COCOO
−
+ 1 = 2 O2 →OH
−
CH3 COO
−
+ CO2
Here, imidazolium pyruvate ionic liquid acts as not only a classical media for the storage of CO2, but also the origin of CO2. In addition, the characters of RTIL, such as low interface energies, low interface tensions [16] and the ability to form hydrogen bonds [17] may also play important roles in the formation of mesoporous carbonates with special morphology and high surface areas. 4. Conclusions Mesoporous SrCO3, CaCO3 and MnCO3 with straw-like bundles morphology have been synthesized in a new [PMIM][CH3COCO2] ionic
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liquid system. Especially, these prepared carbonates exhibit high surface areas of 118, 43 and 122 m2/g, respectively. Acknowledgements We thank the Natural Science Foundation of China (No. 20273021) and key project of Shanghai Science and Technology Committee (No. 08JC1408600, 06DZ05025) for the financial support. References [1] [2] [3] [4]
Cao MH, Wu XL, He XY, Hu CW. Langmuir 2005;21:6093–6. Zhu YJ, Wang WW, Qi RJ, Hu XL. Angew Chem, Int Ed 2004;43:1410–4. Nakashima T, Kimizuka N. J Am Chem Soc 2003;125:6386–7. Zhu KK, Pozgan F, D'Souza L, Richards RM. Microporous Mesoporous Mater 2006;91:40–6. [5] Park HS, Yang SH, Jun YS, Hong WH, Kang JK. Chem Mater 2007;19:535–42.
[6] Shen Q, Wei H, Wang LC, Zhou Y, Zhao Y, Zhang ZQ, et al. J Phys Chem, B 2005;109:18342–7. [7] Du JM, Liu ZM, Li ZH, Han BX, Huang Y, Zhang JL. Microporous Mesoporous Mater 2005;83:145–9. [8] Bonhote P, Dias AP, Papageorgiou N, Kalyanasundaram K, Gra1tzel M. Inorg Chem 1996;35:1168–78. [9] Skell PS, May DD. J Am Chem Soc 1983;105:3999–4008. [10] Birks LS, Friedman H. J Appl Phys 1946;17:687–92. [11] Sivakumar M, Gedanken A, Zhong W, Du YW, Bhattacharya D, Yeshurun Y, et al. J Magn Magn Mater 2004;268:95–104. [12] Wang L, Zhu YF. J Phys Chem, B 2005;109:5118–23. [13] Tsuzuki T, Pethick K, McCormick PG. J Nanopart Res 2000;2:375–80. [14] Korff RW. In: Lowenstein J, editor. Methods in Enzymology, 13. New York: Academic Press; 1969. p. 519–23. [15] Kakkar R, Pathak M, Radhika NP. Org Biomol Chem 2006;4:886–95. [16] Antonietti M, Kuang D, Smarsly B, Zhou Y. Angew Chem, Int Ed 2004;43:4988–92. [17] Elaiwi A, Hitchcock PB, Seddon KR, Srinivasan N, Tan YM, Welton T, et al. J Chem Soc Dalton Trans 1995;21:3467–72.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis of high surface area mesoporous carbonates in novel ionic liquid Yawei Hou, Aiguo Kong, Xinhua Zhao, Haiyan Zhu, Yongkui Shan ⁎ Department of chemistry, East China Normal University, Shanghai, 200062, PR China
a r t i c l e
i n f o
Article history: Received 13 December 2008 Accepted 3 February 2009 Available online 10 February 2009 Keywords: Nanomaterials Straw-like Porosity Carbonate Ionic liquid
a b s t r a c t High surface area mesoporous SrCO3, CaCO3 and MnCO3 with straw-like bundles morphology were synthesized in a novel imidazolium pyruvate ionic liquid. These carbonates were characterized by X-ray diffraction, Fourier transform infrared spectra and transmission electron microscopy. The results revealed that these carbonates have pure orthorhombic and rhombohedral structures, respectively, and the prepared products all show the unique straw-like morphology. Mesoporous properties and high surface areas of SrCO3 (118 m2/g), CaCO3(43 m2/g) and MnCO3 (122 m2/g) were also confirmed by the corresponding N2 sorption analyses. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental section
High surface area nanomaterials with special morphologies and structures have attracted growing interests because of their numerous potential applications [1]. As a special solvent, room temperature ionic liquid (RTIL) provides an opportunity to create many unique nanomaterials with different morphologies or structures. Many metals nanoparticles, nanorods and nanowires [2], hollow TiO2 microspheres [3], well-ordered mesoporous SiO2 [4] and mesoporous γ-Al2O3 [5] have been obtained in RTIL. SrCO3 and CaCO3 are important components in minerals, skeleton and many mineralizing organisms and are applied to many industrial productions [1,6]. Nano-scaled SrCO3 or CaCO3 with rodlike, whiskerlike or flowerlike morphologies [1,6] have been synthesized in aqueous system. Han et al. reported the mesoporous SrCO3 spheres and hollow CaCO3 spheres prepared in guanidinium ionic liquid, but the surface area of those products is very low, even only 17 m2/g [7]. It is still attractive to prepare high surface areas mesoporous carbonates with unique morphology and structure. In this paper, mesoporous SrCO3 and CaCO3 with unique straw-like bundles morphology have been synthesized in a novel imidazolium pyruvate ionic liquid. These prepared carbonates have high surface areas, even up to 118 m2/g, and show the typical mesoporous properties. Furthermore, this method can be applied to the synthesis of mesoporous transition metal carbonates (i.e. MnCO3) with the similar straw-like morphology and high surface area.
2.1. Synthesis of ionic liquid
⁎ Corresponding author. Tel./fax: +86 021 62233505. E-mail address: [email protected] (Y. Shan). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.02.005
The 1-methyl-3-pentylimidazolium bromide (PMIMBr) and silver pyruvate (CH3COCO2Ag) were prepared according to the literatures [8,9]. The synthesized CH3COCO2Ag was dispersed in anhydrous ethanol. Then the equal molar PMIMBr was added drop by drop and the mixture was stirred for 5 h. The resultant precipitation was removed, and the filtrate was vaporized under vacuum to remove ethanol. Finally, a clear slightly yellow liquid [PMIM][CH3COCO2] was obtained. IR (KBr pellet): 3418.82, 3146.46, 3093.36, 2962.52, 2933.95, 2874.17, 1705.84, 1622.14, 1469.37, 1386.31, 835.06, 758.67, 625.83 cm− 1. 1H NMR (500 MHz, D2O): δ = 8.60 (s, 1H), δ = 7.37 (s, 1H), δ = 7.32 (s, 1H), δ = 4.09 (t, 2H), δ = 3.79 (s, 3H), δ = 2.27(s, 3H), δ = 1.74 (m, 2H), δ = 1.23 (m, 2H), δ = 1.18 (m, 2H), δ = 0.78 (t, 3H). 2.2. Synthesis of SrCO3, CaCO3 and MnCO3 The typical synthesis of SrCO3 was carried out as follows: 10 ml of the CO2-saturated [PMIM][CH3COCO2] RTIL (bubbling with CO2 for 1 h) and 0.8 g SrCl2.6H2O powders were mixed in a flask at room temperature, and 1.2 ml of 5 M NaOH aqueous solution was added. The
Scheme 1. Chemical structure of the ionic liquid.
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resultant mixtures were refluxed at 100 °C under stirring for 10 h. During the reactions, SrCl2 was partly dissolved in this RTIL system containing little water, and reacted with CO2 at the alkaline conditions to produce the carbonates. The final carbonate sample (~ 86% isolated yield) was obtained by centrifugation, washing with ethanol and drying at 60 °C. CaCO3 and MnCO3 samples were prepared by the same procedure using CaCl2 and MnCl2d 4H2O as the raw materials, and the isolated yields are about 81% and 83%, respectively. 2.3. Characterization 1
Fig. 1. XRD patterns of the SrCO3 (a), CaCO3 (b) and MnCO3 (c) prepared in ionic liquid system.
H NMR of the RTIL was recorded on Brucker DRX-500 spectrometer. FT-IR spectra were recorded on a Nexus 670 Fourier transform infrared spectrophotometer (Nicolet). The differential scanning calorimetry (DSC) experiment was measured using a Pyris 1 DSC (Perkin-Elmer) with a heating rate of 10 °C min− 1. The thermal stability of the RTIL was investigated on a TGA/SDTA851 thermogravimetric analyzer with a heating rate of 10 °C min− 1. X-ray diffraction (XRD) patterns of the carbonates were recorded on a D8advance diffractometer (Cu Ka radiation; l = 0.154 nm). Transmission electron microscopy (TEM) images were taken with a JEOL JEM 2100 transmission electron microscope using an accelerating voltage of 200 kV. BET surface area and pore distribution of the carbonates were
Fig. 2. TEM images of the SrCO3 (A,B), CaCO3 (C,D) and MnCO3 (E,F) synthesized in ionic liquid system.
Y. Hou et al. / Materials Letters 63 (2009) 1061–1064
1063
MnCO3, respectively (JCPDS 47-1743 and 86-0172). In addition, the broadened XRD diffraction peaks were also observed, which suggest the small particle size of these carbonates. The particle sizes of three carbonates estimated from the Scherrer equation [10] are about 9 nm (SrCO3), 30 nm (CaCO3) and 12 nm (MnCO3), respectively. The FT-IR spectra (not shown) of these samples revealed the characteristic peaks of typical carbonates, in agreement with the XRD analyses. The strong absorption bands at about 1470, 850 and 700 cm− 1 can be attributed to the ν3, ν2 and ν4 modes of the carbonates ion, respectively [11]. The morphologies of the prepared carbonates were also observed by TEM images (see Fig. 2). Especially, all these carbonates samples show the special straw-like bundles morphology, and carbonates with such unique morphology are little reported. A few tiny particles and many inconsecutive tiny particle lines can also be observed in these images. It seems that the tiny particles stack or grow into lines and these particle lines further aggregated into bundles. As a result, the special straw-like bundles morphology is formed. TEM images clearly indicate that the particle size of CaCO3 is slightly larger than that of SrCO3 and MnCO3, which is in good agreement with the results of XRD. 3.3. Porous property The porous properties of the SrCO3, CaCO3 and MnCO3 products have been investigated by the N2 adsorption–desorption analyses (Fig. 3). The SrCO3, CaCO3 and MnCO3 samples show typical type IV isotherms with hysteresis loops in the P/Po region of about 0.4–0.8, 0.4–0.8 and 0.5–0.8, respectively (see Fig. 3A a, b and c). Their BET specific surface areas are 118, 43 and 122 m2/g, respectively. Comparatively, the SrCO3 and CaCO3 samples show higher surface areas than that of the reported SrCO3 or CaCO3 [7,12,13]. The pore size distribution analyses of SrCO3 and MnCO3 reveal their narrow pore size distribution (see Fig. 3B) center at 4 nm (SrCO3) and 5 nm (MnCO3), respectively, while CaCO3 has a narrower pore size distribution around 4 nm (see Fig. 3B b). Obviously, the porous properties of carbonates are mainly from the stack of their nanoparticles, which is in agreement with the TEM results. 3.4. Formation of nanostructure carbonates
Fig. 3. N2 sorption isotherms (A) and the corresponding pore size distributions (B) for the synthesized SrCO3 (a), CaCO3 (b) and MnCO3 (c).
During the formation of crystalline carbonates, the CO2 absorbed in the RTIL reacted with hydroxyl and metal ion to form carbonates deposit gradually. The reaction can be written as follow [7]: 2þ
M determined from N2 adsorption–desorption isotherms using a Micromeritics ASAP 2000 system. 3. Results and discussion 3.1. Property of the novel ionic liquid The structure (Scheme 1) of this RTIL has been determined by the FT-IR spectra and 1H NMR (listed above). Its thermal behavior was investigated by DSC measurement heating from −100 °C to 100 °C. There is only a glass transition temperature at −59 °C during heating. The decomposition temperature of the RTIL measured from thermogravimetric analysis is about 105 °C. The conductivity is 0.16 mS/cm at room temperature. 3.2. Microstructure of the synthesized carbonates Fig. 1 shows the XRD patterns of the SrCO3, CaCO3 and MnCO3 synthesized in this RTIL system. The diffraction peaks in Fig. 1a can be indexed to the pure orthorhombic phase of SrCO3 (JCPDS 71-2393). Fig. 1b and c indicate the rhombohedral structures of CaCO3 and
þ CO2 þ 2OH−→MCO3 þ H2 O ðM ¼ Sr; Ca and MnÞ
And then, the pyruvate anion may react with a small quantity of O2 from air in the alkaline system and slowly decarboxylate to form acetate and CO2 [14,15] during heating. As a result, this compensated for the decrease of CO2 in reaction and promoted the production of carbonates. As discussing above, the reaction formula of the decomposition could be inferred as follow: CH3 COCOO
−
+ 1 = 2 O2 →OH
−
CH3 COO
−
+ CO2
Here, imidazolium pyruvate ionic liquid acts as not only a classical media for the storage of CO2, but also the origin of CO2. In addition, the characters of RTIL, such as low interface energies, low interface tensions [16] and the ability to form hydrogen bonds [17] may also play important roles in the formation of mesoporous carbonates with special morphology and high surface areas. 4. Conclusions Mesoporous SrCO3, CaCO3 and MnCO3 with straw-like bundles morphology have been synthesized in a new [PMIM][CH3COCO2] ionic
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Y. Hou et al. / Materials Letters 63 (2009) 1061–1064
liquid system. Especially, these prepared carbonates exhibit high surface areas of 118, 43 and 122 m2/g, respectively. Acknowledgements We thank the Natural Science Foundation of China (No. 20273021) and key project of Shanghai Science and Technology Committee (No. 08JC1408600, 06DZ05025) for the financial support. References [1] [2] [3] [4]
Cao MH, Wu XL, He XY, Hu CW. Langmuir 2005;21:6093–6. Zhu YJ, Wang WW, Qi RJ, Hu XL. Angew Chem, Int Ed 2004;43:1410–4. Nakashima T, Kimizuka N. J Am Chem Soc 2003;125:6386–7. Zhu KK, Pozgan F, D'Souza L, Richards RM. Microporous Mesoporous Mater 2006;91:40–6. [5] Park HS, Yang SH, Jun YS, Hong WH, Kang JK. Chem Mater 2007;19:535–42.
[6] Shen Q, Wei H, Wang LC, Zhou Y, Zhao Y, Zhang ZQ, et al. J Phys Chem, B 2005;109:18342–7. [7] Du JM, Liu ZM, Li ZH, Han BX, Huang Y, Zhang JL. Microporous Mesoporous Mater 2005;83:145–9. [8] Bonhote P, Dias AP, Papageorgiou N, Kalyanasundaram K, Gra1tzel M. Inorg Chem 1996;35:1168–78. [9] Skell PS, May DD. J Am Chem Soc 1983;105:3999–4008. [10] Birks LS, Friedman H. J Appl Phys 1946;17:687–92. [11] Sivakumar M, Gedanken A, Zhong W, Du YW, Bhattacharya D, Yeshurun Y, et al. J Magn Magn Mater 2004;268:95–104. [12] Wang L, Zhu YF. J Phys Chem, B 2005;109:5118–23. [13] Tsuzuki T, Pethick K, McCormick PG. J Nanopart Res 2000;2:375–80. [14] Korff RW. In: Lowenstein J, editor. Methods in Enzymology, 13. New York: Academic Press; 1969. p. 519–23. [15] Kakkar R, Pathak M, Radhika NP. Org Biomol Chem 2006;4:886–95. [16] Antonietti M, Kuang D, Smarsly B, Zhou Y. Angew Chem, Int Ed 2004;43:4988–92. [17] Elaiwi A, Hitchcock PB, Seddon KR, Srinivasan N, Tan YM, Welton T, et al. J Chem Soc Dalton Trans 1995;21:3467–72.