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Rapid synthesis of size-controllable YVO4 nanoparticles by microwave irradiation
Solid State Communications 130 (2004) 465–468 www.elsevier.com/locate/ssc
Rapid synthesis of size-controllable YVO4 nanoparticles by microwave irradiation HaiYan Xua, Hao Wanga,*, YongQiang Mengb, Hui Yana a
The Key Laboratory of Advanced Functional Materials, Education Ministry of China, Beijing University of Technology, Beijing 100022, China b College of Materials Science and Engineering, HeBei University of Science and Technology, Shijiazhuang 050054, China Received 18 January 2004; received in revised form 23 February 2004; accepted 24 February 2004 by M. Cardona
Abstract Sized-controlled YVO4 nanoparticles have been synthesized by a simple microwave irradiation processing. The products were characterized by X-ray diffraction, transmission electron microscopy, and ultraviolet – visible spectroscopy. The results showed that the size of as-synthesized YVO4 powders was in the range of 5 – 18 nm and was extremely dependent on the solution pH value. The optical measurements displayed the obvious quantum-size effect of the products. q 2004 Elsevier Ltd. All rights reserved. PACS: 81.05.Ys; 81.20.Zx; 81.40.Tv Keywords: A. Nanostructures; B. Chemical synthesis; E. Microwave irradiation
1. Introduction Yttrium orthovanadate (YVO4) has been extensively used as a red phosphor with several rare-earth metal ions as dopant in cathode ray tubes (CRTs) and color television in powder form [1,2]. Bulk YVO4:Eu is a highly photoluminescence material with 70% photoluminescence quantum yield of the europium [3] and has a strong luminescence efficiency upon electron-beam excitation. Moreover, YVO4 is also a promising polarizer [4] and laser host material in single crystal form [5]. In recent years, much attention has been paid to the synthesis and characterization of nanomaterials due to their interesting properties, which mainly come from the quantum-size effect in nano-systems, as well as the high surface/volume ratio [6]. Some new synthetic methods including solution combustion process [7], hydrolyzed colloid reaction (HCR) technique [8], microemulsion* Corresponding author. Address: Department of Materials Science and Engineering, Beijing University of Technology, ChaoYang district, Beijing 100022, China. Tel.: þ 86-106-7392733; fax: þ 86-106-739-2412. E-mail address: [email protected] (H. Wang). 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.02.045
mediated method [9], hydrothermal processing [10], and wet chemical method [11 – 13], etc. have been developed to synthesize YVO4 nanoparticles in view of its important application in optoelectronic field. However, these methods need either long reaction time, high temperature treatment, or complicated processing. The microwave synthesis is a fast, simple and efficient method to prepare nanosized inorganic materials [14 – 19]. Compared with the conventional methods, the microwave synthesis has the advantages of rapid growth, small particle size and narrow particle size distribution due to fast homogenous nucleation [20]. In this study, a simple and rapid microwave irradiation method was adopted to synthesize YVO4 nanoparticles with controlled particle size, and their optical property dependent on particle size was investigated.
2. Experimental All reagents were of analytical grade and used without further purification. The NaVO3 stock solution was prepared by mixing V2O5 and sodium hydroxide (NaOH) and
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H.Y. Xu et al. / Solid State Communications 130 (2004) 465–468
vigorously stirring in deionized water for 12 h. A solution of Y(NO3)3 (5.0 mmol) was blended with the NaVO3 stock solution (5.0 mmol). To investigate the influence of pH value, a series experiments were designed. The pH of the final mixture was adjusted to 4, 6, 7, 8 and 11 by adding acetic acid (CH3COOH) or ammonia (NH3·H2O). The ready-adjusted mixture was stirred constantly in a beaker at room temperature for 20 min. The beaker was then placed in a household type microwave oven of 700 W power with a refluxing system and the reaction was performed under ambient air for 10 min. The microwave oven followed a working cycle of 6 s on and 10 s off (37% power). After cooling to room temperature, the precipitate was centrifuged, washed with deionized water, and dried in air. The final products were characterized by X-ray diffractometry (XRD, Bruker Advance D-8) using Cu Ka radiation and transmission electron microscopy ðl ¼ 1:5405 AÞ; (TEM, JEM-2000FX microscopy, 160.0 KV). Optical absorption studies were carried out using a UV – Vis-NIR spectrophotometer (Shimadzu UV-3101PC).
3. Results and discussion Fig. 1 shows the XRD patterns of the products obtained via microwave irradiation at different pH value, which was varied from 4 to 11. Despite the broadening caused by the small mean size of the particles, all the five patterns are found to be consistent with the tetragonal zircon-type structure of YVO4 (JCPDS card No. 17-0341). It indicates that phase-pure tetragonal structure YVO4 has been successfully synthesized at different pH value via microwave irradiation for 10 min. The mean particle size can be
Fig. 1. XRD patterns of YVO4 powders synthesized by microwave irradiation for 10 min. at different pH values: pH ¼ (a) 4, (b) 6, (c) 7, (d) 8, (e) 11.
roughly determined from the full width at half maximum (FWHM) of (200) peaks by using the Scherrer formula. The mean size of the particles obtained at different pH (4, 6, 7, 8 and 11) is 18, 12, 5, 8 and 15 nm, respectively. In Fig. 2 the particle size is plotted as a function of pH value. It shows that the smallest size appears at the neutral medium (pH ¼ 7). Apart from this point, both the increasing and decreasing of pH value lead to the increase of the particle size. The morphology and particle size of the YVO4 powders were characterized by TEM. Fig. 3 shows the typical TEM image of YVO4 particles obtained at pH ¼ 7. The average size of the particles is about 5 nm, which is consistent with the result of XRD pattern. The particles have mono-shape and are well dispersed. The YVO4 powders obtained at the other pH values display similar shape to the particles obtained at pH ¼ 7, the micrographs of which are not shown here. Moreover, the particle size of the samples obtained at the other pH conditions observed by TEM is essentially in agreement with the XRD data. From the above results we can see that the particle size of as-synthesized YVO4 samples is sensitive to the pH of solution. The dependence of particle size on pH value has also been investigated in hydrothermal synthesis of YVO4 [9,10]. In the basic media, the following reactions would occur: Y3þ þ OH2 ! YðOHÞ3
ð1Þ
2 32 þ VO2 3 þ OH ! VO4 þ H
YðOHÞ3 þ
VO32 4
ð2Þ 2
! YVO4 þ 3OH
ð3Þ
Here nanosized Y(OH)3 precipitates served as nucleation sites. VO32 were mobile species which incorporated into 4 Y(OH)3 grains, resulting in the nucleation and growth of YVO4. According to Eq. (2), high pH favors the formation of VO32 4 species, which consequently favors the formation
Fig. 2. Variation of particle size of as-synthesized YVO4 powders with different pH values.
H.Y. Xu et al. / Solid State Communications 130 (2004) 465–468
467
ize the optical property of the YVO4 particles. Fig. 4 gives the UV – Vis absorption spectra of the YVO4 particles obtained at different pH (4, 6, 7, 8 and 11). As seen from Fig. 4, the absorption peaks of the particles vary from each other. The dependence of absorption peaks on the particle size of the samples is illustrated in Fig. 5. Obviously, with increasing particles size, the position of absorption peak of the as-synthesized YVO4 samples shifts considerably to lower wavelength. This is the typical behavior of the quantum-size effect of nano-system. As the host matrix for rare-earth metal ions, YVO4 can be excited by UV radiation owing to VO32 4 absorption. The energy then transfers to the localized states of the doping ion, which results in the emission at the luminescence center. The blue shift of the absorption peak will affect the emission properties of the rare-earth metal doped YVO4. Further work is under way to apply the microwave method to synthesize nano-sized rareearth metal doped YVO4 phosphors and investigate their luminescent properties. Fig. 3. TEM image of an YVO4 sample synthesized at pH ¼ 7.
of YVO4 products in Eq. (3). Therefore, the higher the pH value, the faster the YVO4 particles form, a fact which leads to the formation of larger particles. In acidic media, the vanadium species exists as anionic oligomers [10], while Y(OH)3 would dissolve to form yttrium reactive species Y3þ based on the reaction: YðOHÞ3 þ 3Hþ ! Y3þ þ 3H2 O
ð4Þ
Then a normal base – acid reaction between yttrium cations and vanadium anions yielded YVO4 products. From Eq. (4) we can see that, low pH favors the formation of the Y3þ species, which benefits the formation of large YVO4 particles. Sun et al. [9] used a microemulsion-mediated hydrothermal process to synthesize YVO4 nanoparticles with the size of 8.9 – 47.2 nm in the pH range of 7 – 10. The microemulsion droplets afforded a confinement environment for the growth of nanoparticles. However, the surfactant used in the microemulsion process might introduce organic impurities in the final products, which is detrimental to the optical performance of YVO4-based phosphors. Wu et al. [10] found that micrometer size YVO4 crystals were obtained in the acidic media, while YVO4 nanoparticles with size of about 50 nm were obtained in the basic media. In this study, YVO4 nanoparticles with the size less than 20 nm have been obtained in the pH range of 4 – 11 by the surfactant-free microwave irradiation method for only 10 min. Microwave synthesis is considered as a fast, simple and energy efficient synthesis method, it can also avoid competing reactions in the known processes [21]. The energy transfer from microwaves to reactive species is so efficient that the nucleation and growth of YVO4 crystallites can be achieved in a very short time. In other words, the reaction time is too short to form large particles. UV – Vis spectroscopy has been employed to character-
4. Conclusion We have fabricated YVO4 nanoparticles with controlled particle size through a microwave irradiation method in a solution with a wide pH range. The particle size of the YVO4 powders ranges from 5 to 18 nm and is very sensitive to the pH value. The smallest size appears at pH ¼ 7 while both increasing and decreasing the pH results in forming larger particles. UV – Vis spectra demonstrate an obvious blue shift of absorption peak with decreasing particle size, which is due to the quantum-size effect. This microwave irradiation method, without using surfactants or templates and requiring no expensive equipments, ensures higher purity in the products and greatly reduces the production
Fig. 4. UV–Vis spectra of as-synthesized YVO4 powders at different pH values: pH ¼ (a) 4, (b) 6, (c) 7, (d) 8, (e) 11.
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H.Y. Xu et al. / Solid State Communications 130 (2004) 465–468
Fig. 5. Dependence of the wavelength of the absorption peak on particle size of the as-synthesized YVO4 powders.
cost. It thus offers a simple and rapid synthetic route for YVO4-based phosphors and other functional inorganic materials.
Acknowledgements The authors are grateful to the Project of New Star of Science and Technology of Beijing for financial support.
References [1] A.K. Levine, F.C. Palilla, Appl. Phys. Lett. 5 (1964) 118.
[2] M. Yu, J. Lin, Z. Wang, J. Fu, S. Wang, H.J. Zhang, Y.C. Han, Chem. Mater. 14 (2002) 2224. [3] W.L. Wanmaker, A. Bril, J.W. ter Vrugt, J. Broos, Philips Res. Rep. 21 (1966) 270. [4] E.A. Maunders, L.G. Deshazer, J. Opt. Soc. 61 (1971) 684. [5] J.R. O’Connor, Appl. Phys. Lett. 9 (1966) 407. [6] C.H. Yan, L.D. Sun, C.S. Liao, Appl. Phys. Lett. 82 (2003) 3511. [7] S. Ekambaram, K.C. Patil, J. Alloys Compd. 217 (1995) 104. [8] S. Erdei, J. Mater. Sci. 30 (1995) 4950. [9] L.D. Sun, Y.X. Zhang, J. Zhang, C.H. Yan, C.S. Liao, Y.Q. Lu, Solid State Commun. 124 (2002) 35. [10] H. Wu, H. Xu, Q. Su, T. Chen, M. Wu, J. Mater. Chem. 13 (2003) 1223. [11] K. Riwotzki, M. Haase, J. Phys. Chem. B 102 (1998) 10129. [12] A. Huignard, T. Gacoin, J.P. Boilot, Chem. Mater. 12 (2000) 1090. [13] A. Huignard, V. Buissette, G. Laurent, T. Gacoin, J.P. Boilot, Chem. Mater. 14 (2002) 2264. [14] I. Girnus, K. Hoffmann, F. Marlow, J. Caro, G. Doering, Microporous Mater. 2 (1994) 537. [15] X.C. Xu, W.S. Yang, J. Liu, L.W. Lin, Adv. Mater. 3 (2000) 195. [16] X.H. Liao, J.M. Zhu, J.J. Zhu, J.Z. Xu, H.Y. Chen, Chem. Commun. 10 (2001) 937. [17] O. Palchik, J.J. Zhu, A. Gedanken, J. Mater. Chem. 10 (2000) 1251. [18] J.J. Zhu, O. Palchik, S.G. Chen, A. Gedanken, J. Phys. Chem. B 104 (2000) 7344. [19] O. Palchik, I. Felner, G. Kataby, A. Gedanken, J. Mater. Res. 15 (2000) 2176. [20] J.C. Jansen, A. Arafat, A.K. Barakat, H. Van Bekkum, in: M.L. Occelli, H. Robson (Eds.), Synthesis of Microporous Materials, vol. 1, Van Nostrand Reinhold, New York, 1992, p. 507. [21] K.J. Rao, B. Vaidhyanathan, M. Ganguli, P.A. Ramakrishnan, Chem. Mater. 11 (1999) 882.
Rapid synthesis of size-controllable YVO4 nanoparticles by microwave irradiation HaiYan Xua, Hao Wanga,*, YongQiang Mengb, Hui Yana a
The Key Laboratory of Advanced Functional Materials, Education Ministry of China, Beijing University of Technology, Beijing 100022, China b College of Materials Science and Engineering, HeBei University of Science and Technology, Shijiazhuang 050054, China Received 18 January 2004; received in revised form 23 February 2004; accepted 24 February 2004 by M. Cardona
Abstract Sized-controlled YVO4 nanoparticles have been synthesized by a simple microwave irradiation processing. The products were characterized by X-ray diffraction, transmission electron microscopy, and ultraviolet – visible spectroscopy. The results showed that the size of as-synthesized YVO4 powders was in the range of 5 – 18 nm and was extremely dependent on the solution pH value. The optical measurements displayed the obvious quantum-size effect of the products. q 2004 Elsevier Ltd. All rights reserved. PACS: 81.05.Ys; 81.20.Zx; 81.40.Tv Keywords: A. Nanostructures; B. Chemical synthesis; E. Microwave irradiation
1. Introduction Yttrium orthovanadate (YVO4) has been extensively used as a red phosphor with several rare-earth metal ions as dopant in cathode ray tubes (CRTs) and color television in powder form [1,2]. Bulk YVO4:Eu is a highly photoluminescence material with 70% photoluminescence quantum yield of the europium [3] and has a strong luminescence efficiency upon electron-beam excitation. Moreover, YVO4 is also a promising polarizer [4] and laser host material in single crystal form [5]. In recent years, much attention has been paid to the synthesis and characterization of nanomaterials due to their interesting properties, which mainly come from the quantum-size effect in nano-systems, as well as the high surface/volume ratio [6]. Some new synthetic methods including solution combustion process [7], hydrolyzed colloid reaction (HCR) technique [8], microemulsion* Corresponding author. Address: Department of Materials Science and Engineering, Beijing University of Technology, ChaoYang district, Beijing 100022, China. Tel.: þ 86-106-7392733; fax: þ 86-106-739-2412. E-mail address: [email protected] (H. Wang). 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.02.045
mediated method [9], hydrothermal processing [10], and wet chemical method [11 – 13], etc. have been developed to synthesize YVO4 nanoparticles in view of its important application in optoelectronic field. However, these methods need either long reaction time, high temperature treatment, or complicated processing. The microwave synthesis is a fast, simple and efficient method to prepare nanosized inorganic materials [14 – 19]. Compared with the conventional methods, the microwave synthesis has the advantages of rapid growth, small particle size and narrow particle size distribution due to fast homogenous nucleation [20]. In this study, a simple and rapid microwave irradiation method was adopted to synthesize YVO4 nanoparticles with controlled particle size, and their optical property dependent on particle size was investigated.
2. Experimental All reagents were of analytical grade and used without further purification. The NaVO3 stock solution was prepared by mixing V2O5 and sodium hydroxide (NaOH) and
466
H.Y. Xu et al. / Solid State Communications 130 (2004) 465–468
vigorously stirring in deionized water for 12 h. A solution of Y(NO3)3 (5.0 mmol) was blended with the NaVO3 stock solution (5.0 mmol). To investigate the influence of pH value, a series experiments were designed. The pH of the final mixture was adjusted to 4, 6, 7, 8 and 11 by adding acetic acid (CH3COOH) or ammonia (NH3·H2O). The ready-adjusted mixture was stirred constantly in a beaker at room temperature for 20 min. The beaker was then placed in a household type microwave oven of 700 W power with a refluxing system and the reaction was performed under ambient air for 10 min. The microwave oven followed a working cycle of 6 s on and 10 s off (37% power). After cooling to room temperature, the precipitate was centrifuged, washed with deionized water, and dried in air. The final products were characterized by X-ray diffractometry (XRD, Bruker Advance D-8) using Cu Ka radiation and transmission electron microscopy ðl ¼ 1:5405 AÞ; (TEM, JEM-2000FX microscopy, 160.0 KV). Optical absorption studies were carried out using a UV – Vis-NIR spectrophotometer (Shimadzu UV-3101PC).
3. Results and discussion Fig. 1 shows the XRD patterns of the products obtained via microwave irradiation at different pH value, which was varied from 4 to 11. Despite the broadening caused by the small mean size of the particles, all the five patterns are found to be consistent with the tetragonal zircon-type structure of YVO4 (JCPDS card No. 17-0341). It indicates that phase-pure tetragonal structure YVO4 has been successfully synthesized at different pH value via microwave irradiation for 10 min. The mean particle size can be
Fig. 1. XRD patterns of YVO4 powders synthesized by microwave irradiation for 10 min. at different pH values: pH ¼ (a) 4, (b) 6, (c) 7, (d) 8, (e) 11.
roughly determined from the full width at half maximum (FWHM) of (200) peaks by using the Scherrer formula. The mean size of the particles obtained at different pH (4, 6, 7, 8 and 11) is 18, 12, 5, 8 and 15 nm, respectively. In Fig. 2 the particle size is plotted as a function of pH value. It shows that the smallest size appears at the neutral medium (pH ¼ 7). Apart from this point, both the increasing and decreasing of pH value lead to the increase of the particle size. The morphology and particle size of the YVO4 powders were characterized by TEM. Fig. 3 shows the typical TEM image of YVO4 particles obtained at pH ¼ 7. The average size of the particles is about 5 nm, which is consistent with the result of XRD pattern. The particles have mono-shape and are well dispersed. The YVO4 powders obtained at the other pH values display similar shape to the particles obtained at pH ¼ 7, the micrographs of which are not shown here. Moreover, the particle size of the samples obtained at the other pH conditions observed by TEM is essentially in agreement with the XRD data. From the above results we can see that the particle size of as-synthesized YVO4 samples is sensitive to the pH of solution. The dependence of particle size on pH value has also been investigated in hydrothermal synthesis of YVO4 [9,10]. In the basic media, the following reactions would occur: Y3þ þ OH2 ! YðOHÞ3
ð1Þ
2 32 þ VO2 3 þ OH ! VO4 þ H
YðOHÞ3 þ
VO32 4
ð2Þ 2
! YVO4 þ 3OH
ð3Þ
Here nanosized Y(OH)3 precipitates served as nucleation sites. VO32 were mobile species which incorporated into 4 Y(OH)3 grains, resulting in the nucleation and growth of YVO4. According to Eq. (2), high pH favors the formation of VO32 4 species, which consequently favors the formation
Fig. 2. Variation of particle size of as-synthesized YVO4 powders with different pH values.
H.Y. Xu et al. / Solid State Communications 130 (2004) 465–468
467
ize the optical property of the YVO4 particles. Fig. 4 gives the UV – Vis absorption spectra of the YVO4 particles obtained at different pH (4, 6, 7, 8 and 11). As seen from Fig. 4, the absorption peaks of the particles vary from each other. The dependence of absorption peaks on the particle size of the samples is illustrated in Fig. 5. Obviously, with increasing particles size, the position of absorption peak of the as-synthesized YVO4 samples shifts considerably to lower wavelength. This is the typical behavior of the quantum-size effect of nano-system. As the host matrix for rare-earth metal ions, YVO4 can be excited by UV radiation owing to VO32 4 absorption. The energy then transfers to the localized states of the doping ion, which results in the emission at the luminescence center. The blue shift of the absorption peak will affect the emission properties of the rare-earth metal doped YVO4. Further work is under way to apply the microwave method to synthesize nano-sized rareearth metal doped YVO4 phosphors and investigate their luminescent properties. Fig. 3. TEM image of an YVO4 sample synthesized at pH ¼ 7.
of YVO4 products in Eq. (3). Therefore, the higher the pH value, the faster the YVO4 particles form, a fact which leads to the formation of larger particles. In acidic media, the vanadium species exists as anionic oligomers [10], while Y(OH)3 would dissolve to form yttrium reactive species Y3þ based on the reaction: YðOHÞ3 þ 3Hþ ! Y3þ þ 3H2 O
ð4Þ
Then a normal base – acid reaction between yttrium cations and vanadium anions yielded YVO4 products. From Eq. (4) we can see that, low pH favors the formation of the Y3þ species, which benefits the formation of large YVO4 particles. Sun et al. [9] used a microemulsion-mediated hydrothermal process to synthesize YVO4 nanoparticles with the size of 8.9 – 47.2 nm in the pH range of 7 – 10. The microemulsion droplets afforded a confinement environment for the growth of nanoparticles. However, the surfactant used in the microemulsion process might introduce organic impurities in the final products, which is detrimental to the optical performance of YVO4-based phosphors. Wu et al. [10] found that micrometer size YVO4 crystals were obtained in the acidic media, while YVO4 nanoparticles with size of about 50 nm were obtained in the basic media. In this study, YVO4 nanoparticles with the size less than 20 nm have been obtained in the pH range of 4 – 11 by the surfactant-free microwave irradiation method for only 10 min. Microwave synthesis is considered as a fast, simple and energy efficient synthesis method, it can also avoid competing reactions in the known processes [21]. The energy transfer from microwaves to reactive species is so efficient that the nucleation and growth of YVO4 crystallites can be achieved in a very short time. In other words, the reaction time is too short to form large particles. UV – Vis spectroscopy has been employed to character-
4. Conclusion We have fabricated YVO4 nanoparticles with controlled particle size through a microwave irradiation method in a solution with a wide pH range. The particle size of the YVO4 powders ranges from 5 to 18 nm and is very sensitive to the pH value. The smallest size appears at pH ¼ 7 while both increasing and decreasing the pH results in forming larger particles. UV – Vis spectra demonstrate an obvious blue shift of absorption peak with decreasing particle size, which is due to the quantum-size effect. This microwave irradiation method, without using surfactants or templates and requiring no expensive equipments, ensures higher purity in the products and greatly reduces the production
Fig. 4. UV–Vis spectra of as-synthesized YVO4 powders at different pH values: pH ¼ (a) 4, (b) 6, (c) 7, (d) 8, (e) 11.
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H.Y. Xu et al. / Solid State Communications 130 (2004) 465–468
Fig. 5. Dependence of the wavelength of the absorption peak on particle size of the as-synthesized YVO4 powders.
cost. It thus offers a simple and rapid synthetic route for YVO4-based phosphors and other functional inorganic materials.
Acknowledgements The authors are grateful to the Project of New Star of Science and Technology of Beijing for financial support.
References [1] A.K. Levine, F.C. Palilla, Appl. Phys. Lett. 5 (1964) 118.
[2] M. Yu, J. Lin, Z. Wang, J. Fu, S. Wang, H.J. Zhang, Y.C. Han, Chem. Mater. 14 (2002) 2224. [3] W.L. Wanmaker, A. Bril, J.W. ter Vrugt, J. Broos, Philips Res. Rep. 21 (1966) 270. [4] E.A. Maunders, L.G. Deshazer, J. Opt. Soc. 61 (1971) 684. [5] J.R. O’Connor, Appl. Phys. Lett. 9 (1966) 407. [6] C.H. Yan, L.D. Sun, C.S. Liao, Appl. Phys. Lett. 82 (2003) 3511. [7] S. Ekambaram, K.C. Patil, J. Alloys Compd. 217 (1995) 104. [8] S. Erdei, J. Mater. Sci. 30 (1995) 4950. [9] L.D. Sun, Y.X. Zhang, J. Zhang, C.H. Yan, C.S. Liao, Y.Q. Lu, Solid State Commun. 124 (2002) 35. [10] H. Wu, H. Xu, Q. Su, T. Chen, M. Wu, J. Mater. Chem. 13 (2003) 1223. [11] K. Riwotzki, M. Haase, J. Phys. Chem. B 102 (1998) 10129. [12] A. Huignard, T. Gacoin, J.P. Boilot, Chem. Mater. 12 (2000) 1090. [13] A. Huignard, V. Buissette, G. Laurent, T. Gacoin, J.P. Boilot, Chem. Mater. 14 (2002) 2264. [14] I. Girnus, K. Hoffmann, F. Marlow, J. Caro, G. Doering, Microporous Mater. 2 (1994) 537. [15] X.C. Xu, W.S. Yang, J. Liu, L.W. Lin, Adv. Mater. 3 (2000) 195. [16] X.H. Liao, J.M. Zhu, J.J. Zhu, J.Z. Xu, H.Y. Chen, Chem. Commun. 10 (2001) 937. [17] O. Palchik, J.J. Zhu, A. Gedanken, J. Mater. Chem. 10 (2000) 1251. [18] J.J. Zhu, O. Palchik, S.G. Chen, A. Gedanken, J. Phys. Chem. B 104 (2000) 7344. [19] O. Palchik, I. Felner, G. Kataby, A. Gedanken, J. Mater. Res. 15 (2000) 2176. [20] J.C. Jansen, A. Arafat, A.K. Barakat, H. Van Bekkum, in: M.L. Occelli, H. Robson (Eds.), Synthesis of Microporous Materials, vol. 1, Van Nostrand Reinhold, New York, 1992, p. 507. [21] K.J. Rao, B. Vaidhyanathan, M. Ganguli, P.A. Ramakrishnan, Chem. Mater. 11 (1999) 882.