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Synthesis of Co3O4 nanoparticles via an ionic liquid-assisted methodology at room temperature
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Materials Letters 62 (2008) 1976 – 1978 www.elsevier.com/locate/matlet
Synthesis of Co3O4 nanoparticles via an ionic liquid-assisted methodology at room temperature Dingbing Zou a , Chao Xu a , Hao Luo a , Ling Wang a , Taokai Ying a,b,⁎ a
College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, PR China b Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, PR China Received 14 September 2007; accepted 29 October 2007 Available online 1 November 2007
Abstract Cobalt oxide (Co3O4) nanoparticles have been successfully synthesized through a facile solution oxidation route at room temperature and normal pressure for 6h at first time, using an ionic liquid 1-n-butyl-3-methylimidazolium hydroxide ([BMIM]OH) as an assisted agent and electrical conductor. The structure and morphology of as-prepared Co3O4 nanoparticles were characterized by powder X-ray diffraction (XRD), infrared spectrum (IR), and scanning electron microscope (SEM). XRD studies indicated that the product was well-crystallized cubic phase of Co3O4 with a cell constant of a = 8.0722Å. The SEM images showed that the obtained Co3O4 powders consisted of dispersive quasi-spherical particles with the size ranged from 10 to 50nm. The possible formation mechanism of Co3O4 nanoparticles was simply proposed as well. © 2007 Elsevier B.V. All rights reserved. Keywords: Co3O4; Ionic liquids; Nanomaterials; Semiconductors
1. Introduction Nanometer-scale materials with the size of 1–100nm have attracted considerable interest in recent years due to the departure of properties from bulk phases arising from quantum size effect. Over the past decade, a variety of techniques have been applied to fabricate nanostructures of a broad class of materials, ranging from ceramic dielectrics [1], semiconductors [2], metals [3], and metal oxides [4,5]. Among these materials, transition metal oxides have been the subjects of scientific and technological attention owing to their interesting properties [6,7]. As an important functional material, the p-type semiconductor Co3O4 is used in heterogeneous catalysis [8], solidstate sensors [9], anode materials in Li ion rechargeable batteries [10], and energy storage [11]. Furthermore, it is an important magnetic material [12]. Therefore, the synthesis of nanocrystalline Co3O4 has been the target of material chemists [13,14]. In this field, various strategies have been developed for ⁎ Corresponding author. College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, PR China. Tel.: +86 579 82282780; fax: +86 579 82282269. E-mail address: [email protected] (T. Ying). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.10.056
the synthesis of Co3O4 such as the thermal decomposition of cobalt precursors under oxidizing condition (210–815°C) [15–18], chemical spray pyrolysis (350–400°C) [19,20], chemical vapor deposition (CVD, 550°C) [21], and the traditional sol–gel method (above 260°C) [22]. Nevertheless, all of the above mentioned methods need some special instruments, harsh conditions, and/or relatively high performance temperature. Moreover, the other limitation of the traditional synthesis method of Co3O4 nanocrystal is the low product yield [23]. As a result, the preparation of nanocrystalline Co3O4 is difficult and inconvenient to obtain. Therefore, there is a need for more effort toward the preparation of the spinel-type Co3O4 nanocrystal. The most important issue is to design a lowtemperature synthesis route which would allow the synthesis of Co3O4 at relatively low temperature. Room-temperature ionic liquids (RTILs), a set of environmentally benign solvents with a relatively wide electrochemically stable window, good electrical conductivity, high ionic mobility, a broad range of room temperature liquid compositions, negligible vapor pressure, and excellent chemical and thermal stabilities, have attracted increasing interest [24]. Research on ionic liquids has focused on the synthesis and organic chemistry in ILs. Recently, however, ILs have also received attention from the
D. Zou et al. / Materials Letters 62 (2008) 1976–1978
Fig. 1. XRD pattern of the Co3O4 nanoparticles.
inorganic materials community. Ionic liquids can act as solvents for reactants and morphology templates for the products at the same time, which enable the synthesis of inorganic materials with novel or improved properties. In principle, the ionic liquids can be retrieved after synthesis and thus it provides an ecologically friendly and economical approach to inorganic materials [25]. However, to the best of our knowledge, there has been no report on the preparation of spinel-type Co3O4 in the presence of ionic liquids at room temperature. Herein, we report a novel ionic liquid-assisted method that is more facile and effective to synthesize Co3O4 nanoparticles.
1977
homogeneous suspension. Then, 4ml of NaOH was added dropwise into the above suspension with continuous stirring. About 30min later, 5ml of H2O2 was dropped in the mixture within half an hour. After that, the mixed solution was maintained stirring at room temperature for 6h. The black–brown precipitate was separated by centrifugation, washed several times with distilled water and absolute ethanol in sequence to remove impurities, and then dried in an oven at 80°C for 10h. The ionic liquid was collected and reused for subsequent reactions. As a result, 0.25g of product was obtained. X-ray diffraction (XRD) patterns were collected on a PhilpsPW3040/60 X-ray diffractometer with Kα radiation (λ = 0.15418nm). Infrared spectra (IR) were recorded on a Nicolet Nexus 670 spectrometer in the range of 400–4000cm− 1 using KBr pellets. The morphologies of the products were observed and determined by the scanning electron microscope (SEM) (HITACHI S-4800). 3. Result and discussion
All chemicals used in this work were of analytical reagent grade, obtained from the commercial market and used without further purification. Ionic liquid (1-n-butyl-3-methylimidazolium hydroxide, [BMIM]OH) was synthesized according to the literature [26]. In a typical procedure, 0.93g of Co(NO3)2·6H2O was put into 2.0g of [BMIM]OH at room temperature to form a
The representative XRD pattern of the product synthesized by the ionic liquid-assisted method is shown in Fig. 1. All diffraction peaks of the sample can be readily indexed to the cubic spinel Co3O4 with lattice parameter α = 8.0722Å, which is consistent with the literature results (JCPDS No. 76-1802). No characteristic peaks of impurity phases such as CoO and CoOOH are present, indicating the high purity of the final products. The considerable broadening of the diffraction peaks demonstrates the nanoparticles character of the Co3O4 powders. The IR spectrum of the as-prepared Co3O4 nanoparticles synthesized in the presence of [BMIM]OH is shown in Fig. 2. The absorption peaks at 668.91 and 585.14cm− 1 are assigned to the ν(Co–O) modes [27], which confirm the formation of Co3O4 nanocrystals. The broad band centered at 3340.80cm− 1 and 1624.29cm− 1 are assigned to O–H stretching and bending modes of water respectively [28]. The imidazolium ν(C–H) stretching between 3200 and 3000cm− 1 [29] disappears completely. The shape of the Co3O4 product is further examined with scanning electron microscopy (SEM), as shown in Fig. 3. From the image, it can be seen remarkably that the sample consisted of a large quantity of
Fig. 2. IR spectrum of the as-prepared Co3O4 nanoparticles.
Fig. 3. SEM image of the Co3O4 nanoparticles.
2. Experimental
1978
D. Zou et al. / Materials Letters 62 (2008) 1976–1978
dispersive nanoparticles with the size ranged from 10 to 50 nm. These nanoparticles are relatively uniform and seemed as quasi-spheres. Jiang et al. [30] have reported the synthesis of Co3O4 by hydrothermal reaction of CoSO4·7H2O, ammonia, and H2O2 at 180 °C. According to their experimental results, the processing temperature played a critical role in the reaction. Co3O4 was not formed when the reaction temperature was lower than 100 °C in the absence of ionic liquids. Thus, all the evidences in our experiment indicate that the ionic liquid has a unique function in the reactions. The reactions involved in the process can be shown as follows: Co2þ þ 2OH− →CoðOHÞ2
½BMIMOH
3CoðOHÞ2 þH2 O2 Y Co3 O4 þ 4H2 O r:t; stirring
ð1Þ
ð2Þ
Based on the experimental results, a possible formation mechanism for the Co3O4 nanoparticles in the present ionic liquid-assisted solution oxidation condition is clarified as follows: (i) In our experiment, ionic liquid was used as solvent owning to its high surface activity, and the reactants could disperse in ionic liquid which forms thousands of tiny reactors; (ii) RTILs really had polarity, however, their low interfacial tension made the inorganic species have a high nucleation ratio, which propelled the formation of the nanocrystal; (iii) Reaction (2) was an emblematical redox reaction involving electron transfer. However, it could not react in the absence of ionic liquids at ambient temperature as low as room temperature based on the experimental result. Therefore, the ionic liquid here acted as both a reaction medium and an electrical conductor which drove the above reaction resulted from accelerating electron transfer.
4. Conclusions In summary, the ionic liquid-assisted strategy has been adopted to prepare Co3O4 nanoparticles at room temperature. The ionic liquid [BMIM]OH has a significant influence on the reaction as the assisted agent and electrical conductor. Compared with the traditional preparation methodology, this route has promoted the reaction to act at as low as room temperature and simplified the reaction procedures. According to the XRD pattern, no impurity was present. Additionally, the ionic liquid has been reused for the subsequent reactions, so the product was obtained in high yield. Furthermore, the probable formation mechanism of Co3O4 nanocrystal was proposed. The strategy presented in this work is expected to prepare nanostructures of other transition metal oxides.
References [1] C. Liu, B. Zou, A.J. Rondinone, Z.J. Zhang, J. Am. Chem. Soc. 123 (2001) 4344–4345. [2] M.H. Huang, Y.Y. Wu, H. Feick, N. Tran, E. Weber, P.D. Yang, Adv. Mater. 13 (2001) 113–116. [3] B. Kim, S.L. Tripp, A. Wei, J. Am. Chem. Soc. 123 (2001) 7955–7956. [4] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 126 (2004) 8124–8125. [5] J.J. Teo, Y. Chang, H.C. Zeng, Langmuir 22 (2006) 7369–7377. [6] W.F.S. Spear, D.S. Tamhuser, Phys. Rev., B 7 (1973) 831–833. [7] R.N. Singh, J.F. Koenig, G. Poillerat, P. Chartier, J. Electroanal. Chem. 314 (1991) 241–257. [8] H. Kim, D.W. Park, H.C. Woo, J.S. Chung, Appl. Catal., B Environ. 19 (1998) 233–243. [9] E.M. Logothesis, K. Park, A.H. Meitzler, K.R. Laud, Appl. Phys. Lett. 26 (1975) 209–211. [10] E. Antolini, Mater. Res. Bull. 32 (1997) 9–14. [11] S. Noguchi, M. Mizuhashi, Thin Solid Films 77 (1981) 99–106. [12] S.A. Makhlouf, J. Magn. Mater. 246 (2002) 184–190. [13] J.S. Yin, Z.L. Wang, J. Mater. Res. 14 (1999) 503–508. [14] B.B. Lakshmi, C.J. Patrissi, C.R. Martin, Chem. Mater. 9 (1997) 2544–2550. [15] A. Rumplecker, F. Kleitz, E.L. Salabas, F. Schüth, Chem. Mater. 19 (2007) 485–496. [16] W.W. Wang, Y.J. Zhu, Mater. Res. Bull. 40 (2005) 1929–1935. [17] L.X. Yang, Y.J. Zhu, L. Li, L. Zhang, H. Tong, W.W. Wang, et al., Eur. J. Inorg. Chem. (2006) 4787–4792. [18] Y.K. Liu, G.H. Wang, C.K. Xu, W.Z. Wang, Chem. Commun. (2002) 1486–1487. [19] R.N. Singh, J.F. Koenig, G. Poillerat, P. Chartier, J. Electrochem. Soc. 137 (1990) 1408–1413. [20] M. Hamdani, J.F. Koenig, P. Chartier, J. Appl. Electrochem. 18 (1988) 568–576. [21] Y. Xuan, R. Liu, Y.Q. Jia, Mater. Chem. Phys. 53 (1998) 256–261. [22] M.E. Baydi, G. Poillerat, J.L. Rehspringer, J.L. Gautier, J.F. Koenig, P. Chartier, J. Solid State Chem. 109 (1994) 281–288. [23] T. Sugimoto, E. Matijevic, J. Inorg. Nucl. Chem. 41 (1979) 165–172. [24] J.S. Wilkes, M.J. Zaworotko, J. Chem. Soc., Chem. Commun. 13 (1992) 965–967. [25] A. Taubert, Angew. Chem., Int. Ed. 43 (2004) 5380–5382. [26] B.C. Ranu, S. Banerjee, Org. Lett. 7 (2005) 3049–3052. [27] J.A. Gaddsden, Infrared Spectra of Minerals and Related Inorganic Compounds, 1st Ed., Butterworth, London, 1975. [28] K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compound, 4th Ed., Chemical Industry Press, Beijing, 1991. [29] Y. Zhou, M. Antonietti, J. Am. Chem. Soc. 125 (2003) 14960–14961. [30] Y. Jiang, Y. Wu, B. Xie, Y. Xie, Y.T. Qian, Mater. Chem. Phys. 74 (2002) 234–237.
Materials Letters 62 (2008) 1976 – 1978 www.elsevier.com/locate/matlet
Synthesis of Co3O4 nanoparticles via an ionic liquid-assisted methodology at room temperature Dingbing Zou a , Chao Xu a , Hao Luo a , Ling Wang a , Taokai Ying a,b,⁎ a
College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, PR China b Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, PR China Received 14 September 2007; accepted 29 October 2007 Available online 1 November 2007
Abstract Cobalt oxide (Co3O4) nanoparticles have been successfully synthesized through a facile solution oxidation route at room temperature and normal pressure for 6h at first time, using an ionic liquid 1-n-butyl-3-methylimidazolium hydroxide ([BMIM]OH) as an assisted agent and electrical conductor. The structure and morphology of as-prepared Co3O4 nanoparticles were characterized by powder X-ray diffraction (XRD), infrared spectrum (IR), and scanning electron microscope (SEM). XRD studies indicated that the product was well-crystallized cubic phase of Co3O4 with a cell constant of a = 8.0722Å. The SEM images showed that the obtained Co3O4 powders consisted of dispersive quasi-spherical particles with the size ranged from 10 to 50nm. The possible formation mechanism of Co3O4 nanoparticles was simply proposed as well. © 2007 Elsevier B.V. All rights reserved. Keywords: Co3O4; Ionic liquids; Nanomaterials; Semiconductors
1. Introduction Nanometer-scale materials with the size of 1–100nm have attracted considerable interest in recent years due to the departure of properties from bulk phases arising from quantum size effect. Over the past decade, a variety of techniques have been applied to fabricate nanostructures of a broad class of materials, ranging from ceramic dielectrics [1], semiconductors [2], metals [3], and metal oxides [4,5]. Among these materials, transition metal oxides have been the subjects of scientific and technological attention owing to their interesting properties [6,7]. As an important functional material, the p-type semiconductor Co3O4 is used in heterogeneous catalysis [8], solidstate sensors [9], anode materials in Li ion rechargeable batteries [10], and energy storage [11]. Furthermore, it is an important magnetic material [12]. Therefore, the synthesis of nanocrystalline Co3O4 has been the target of material chemists [13,14]. In this field, various strategies have been developed for ⁎ Corresponding author. College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, PR China. Tel.: +86 579 82282780; fax: +86 579 82282269. E-mail address: [email protected] (T. Ying). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.10.056
the synthesis of Co3O4 such as the thermal decomposition of cobalt precursors under oxidizing condition (210–815°C) [15–18], chemical spray pyrolysis (350–400°C) [19,20], chemical vapor deposition (CVD, 550°C) [21], and the traditional sol–gel method (above 260°C) [22]. Nevertheless, all of the above mentioned methods need some special instruments, harsh conditions, and/or relatively high performance temperature. Moreover, the other limitation of the traditional synthesis method of Co3O4 nanocrystal is the low product yield [23]. As a result, the preparation of nanocrystalline Co3O4 is difficult and inconvenient to obtain. Therefore, there is a need for more effort toward the preparation of the spinel-type Co3O4 nanocrystal. The most important issue is to design a lowtemperature synthesis route which would allow the synthesis of Co3O4 at relatively low temperature. Room-temperature ionic liquids (RTILs), a set of environmentally benign solvents with a relatively wide electrochemically stable window, good electrical conductivity, high ionic mobility, a broad range of room temperature liquid compositions, negligible vapor pressure, and excellent chemical and thermal stabilities, have attracted increasing interest [24]. Research on ionic liquids has focused on the synthesis and organic chemistry in ILs. Recently, however, ILs have also received attention from the
D. Zou et al. / Materials Letters 62 (2008) 1976–1978
Fig. 1. XRD pattern of the Co3O4 nanoparticles.
inorganic materials community. Ionic liquids can act as solvents for reactants and morphology templates for the products at the same time, which enable the synthesis of inorganic materials with novel or improved properties. In principle, the ionic liquids can be retrieved after synthesis and thus it provides an ecologically friendly and economical approach to inorganic materials [25]. However, to the best of our knowledge, there has been no report on the preparation of spinel-type Co3O4 in the presence of ionic liquids at room temperature. Herein, we report a novel ionic liquid-assisted method that is more facile and effective to synthesize Co3O4 nanoparticles.
1977
homogeneous suspension. Then, 4ml of NaOH was added dropwise into the above suspension with continuous stirring. About 30min later, 5ml of H2O2 was dropped in the mixture within half an hour. After that, the mixed solution was maintained stirring at room temperature for 6h. The black–brown precipitate was separated by centrifugation, washed several times with distilled water and absolute ethanol in sequence to remove impurities, and then dried in an oven at 80°C for 10h. The ionic liquid was collected and reused for subsequent reactions. As a result, 0.25g of product was obtained. X-ray diffraction (XRD) patterns were collected on a PhilpsPW3040/60 X-ray diffractometer with Kα radiation (λ = 0.15418nm). Infrared spectra (IR) were recorded on a Nicolet Nexus 670 spectrometer in the range of 400–4000cm− 1 using KBr pellets. The morphologies of the products were observed and determined by the scanning electron microscope (SEM) (HITACHI S-4800). 3. Result and discussion
All chemicals used in this work were of analytical reagent grade, obtained from the commercial market and used without further purification. Ionic liquid (1-n-butyl-3-methylimidazolium hydroxide, [BMIM]OH) was synthesized according to the literature [26]. In a typical procedure, 0.93g of Co(NO3)2·6H2O was put into 2.0g of [BMIM]OH at room temperature to form a
The representative XRD pattern of the product synthesized by the ionic liquid-assisted method is shown in Fig. 1. All diffraction peaks of the sample can be readily indexed to the cubic spinel Co3O4 with lattice parameter α = 8.0722Å, which is consistent with the literature results (JCPDS No. 76-1802). No characteristic peaks of impurity phases such as CoO and CoOOH are present, indicating the high purity of the final products. The considerable broadening of the diffraction peaks demonstrates the nanoparticles character of the Co3O4 powders. The IR spectrum of the as-prepared Co3O4 nanoparticles synthesized in the presence of [BMIM]OH is shown in Fig. 2. The absorption peaks at 668.91 and 585.14cm− 1 are assigned to the ν(Co–O) modes [27], which confirm the formation of Co3O4 nanocrystals. The broad band centered at 3340.80cm− 1 and 1624.29cm− 1 are assigned to O–H stretching and bending modes of water respectively [28]. The imidazolium ν(C–H) stretching between 3200 and 3000cm− 1 [29] disappears completely. The shape of the Co3O4 product is further examined with scanning electron microscopy (SEM), as shown in Fig. 3. From the image, it can be seen remarkably that the sample consisted of a large quantity of
Fig. 2. IR spectrum of the as-prepared Co3O4 nanoparticles.
Fig. 3. SEM image of the Co3O4 nanoparticles.
2. Experimental
1978
D. Zou et al. / Materials Letters 62 (2008) 1976–1978
dispersive nanoparticles with the size ranged from 10 to 50 nm. These nanoparticles are relatively uniform and seemed as quasi-spheres. Jiang et al. [30] have reported the synthesis of Co3O4 by hydrothermal reaction of CoSO4·7H2O, ammonia, and H2O2 at 180 °C. According to their experimental results, the processing temperature played a critical role in the reaction. Co3O4 was not formed when the reaction temperature was lower than 100 °C in the absence of ionic liquids. Thus, all the evidences in our experiment indicate that the ionic liquid has a unique function in the reactions. The reactions involved in the process can be shown as follows: Co2þ þ 2OH− →CoðOHÞ2
½BMIMOH
3CoðOHÞ2 þH2 O2 Y Co3 O4 þ 4H2 O r:t; stirring
ð1Þ
ð2Þ
Based on the experimental results, a possible formation mechanism for the Co3O4 nanoparticles in the present ionic liquid-assisted solution oxidation condition is clarified as follows: (i) In our experiment, ionic liquid was used as solvent owning to its high surface activity, and the reactants could disperse in ionic liquid which forms thousands of tiny reactors; (ii) RTILs really had polarity, however, their low interfacial tension made the inorganic species have a high nucleation ratio, which propelled the formation of the nanocrystal; (iii) Reaction (2) was an emblematical redox reaction involving electron transfer. However, it could not react in the absence of ionic liquids at ambient temperature as low as room temperature based on the experimental result. Therefore, the ionic liquid here acted as both a reaction medium and an electrical conductor which drove the above reaction resulted from accelerating electron transfer.
4. Conclusions In summary, the ionic liquid-assisted strategy has been adopted to prepare Co3O4 nanoparticles at room temperature. The ionic liquid [BMIM]OH has a significant influence on the reaction as the assisted agent and electrical conductor. Compared with the traditional preparation methodology, this route has promoted the reaction to act at as low as room temperature and simplified the reaction procedures. According to the XRD pattern, no impurity was present. Additionally, the ionic liquid has been reused for the subsequent reactions, so the product was obtained in high yield. Furthermore, the probable formation mechanism of Co3O4 nanocrystal was proposed. The strategy presented in this work is expected to prepare nanostructures of other transition metal oxides.
References [1] C. Liu, B. Zou, A.J. Rondinone, Z.J. Zhang, J. Am. Chem. Soc. 123 (2001) 4344–4345. [2] M.H. Huang, Y.Y. Wu, H. Feick, N. Tran, E. Weber, P.D. Yang, Adv. Mater. 13 (2001) 113–116. [3] B. Kim, S.L. Tripp, A. Wei, J. Am. Chem. Soc. 123 (2001) 7955–7956. [4] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 126 (2004) 8124–8125. [5] J.J. Teo, Y. Chang, H.C. Zeng, Langmuir 22 (2006) 7369–7377. [6] W.F.S. Spear, D.S. Tamhuser, Phys. Rev., B 7 (1973) 831–833. [7] R.N. Singh, J.F. Koenig, G. Poillerat, P. Chartier, J. Electroanal. Chem. 314 (1991) 241–257. [8] H. Kim, D.W. Park, H.C. Woo, J.S. Chung, Appl. Catal., B Environ. 19 (1998) 233–243. [9] E.M. Logothesis, K. Park, A.H. Meitzler, K.R. Laud, Appl. Phys. Lett. 26 (1975) 209–211. [10] E. Antolini, Mater. Res. Bull. 32 (1997) 9–14. [11] S. Noguchi, M. Mizuhashi, Thin Solid Films 77 (1981) 99–106. [12] S.A. Makhlouf, J. Magn. Mater. 246 (2002) 184–190. [13] J.S. Yin, Z.L. Wang, J. Mater. Res. 14 (1999) 503–508. [14] B.B. Lakshmi, C.J. Patrissi, C.R. Martin, Chem. Mater. 9 (1997) 2544–2550. [15] A. Rumplecker, F. Kleitz, E.L. Salabas, F. Schüth, Chem. Mater. 19 (2007) 485–496. [16] W.W. Wang, Y.J. Zhu, Mater. Res. Bull. 40 (2005) 1929–1935. [17] L.X. Yang, Y.J. Zhu, L. Li, L. Zhang, H. Tong, W.W. Wang, et al., Eur. J. Inorg. Chem. (2006) 4787–4792. [18] Y.K. Liu, G.H. Wang, C.K. Xu, W.Z. Wang, Chem. Commun. (2002) 1486–1487. [19] R.N. Singh, J.F. Koenig, G. Poillerat, P. Chartier, J. Electrochem. Soc. 137 (1990) 1408–1413. [20] M. Hamdani, J.F. Koenig, P. Chartier, J. Appl. Electrochem. 18 (1988) 568–576. [21] Y. Xuan, R. Liu, Y.Q. Jia, Mater. Chem. Phys. 53 (1998) 256–261. [22] M.E. Baydi, G. Poillerat, J.L. Rehspringer, J.L. Gautier, J.F. Koenig, P. Chartier, J. Solid State Chem. 109 (1994) 281–288. [23] T. Sugimoto, E. Matijevic, J. Inorg. Nucl. Chem. 41 (1979) 165–172. [24] J.S. Wilkes, M.J. Zaworotko, J. Chem. Soc., Chem. Commun. 13 (1992) 965–967. [25] A. Taubert, Angew. Chem., Int. Ed. 43 (2004) 5380–5382. [26] B.C. Ranu, S. Banerjee, Org. Lett. 7 (2005) 3049–3052. [27] J.A. Gaddsden, Infrared Spectra of Minerals and Related Inorganic Compounds, 1st Ed., Butterworth, London, 1975. [28] K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compound, 4th Ed., Chemical Industry Press, Beijing, 1991. [29] Y. Zhou, M. Antonietti, J. Am. Chem. Soc. 125 (2003) 14960–14961. [30] Y. Jiang, Y. Wu, B. Xie, Y. Xie, Y.T. Qian, Mater. Chem. Phys. 74 (2002) 234–237.