- Email: [email protected]
The photoluminescence of Co-Al-layered double hydroxide
Available online at www.sciencedirect.com
Chinese Chemical Letters 18 (2007) 1371–1373 www.elsevier.com/locate/cclet
The photoluminescence of Co-Al-layered double hydroxide Shuai Sun, Wan Guo Hou * Key Laboratory for Colloid and Interface Chemistry of Education Ministry, Shandong University, Jinan 250100, China Received 14 May 2007
Abstract We report a new optical behaviour of pure Co-Al-layered double hydroxide (LDH). It was found that the Co-Al-LDH sample could emit fluorescence without any fluorescent substances intercalated. Its excitation spectrum shows a maximum peak near the wavelength 370 nm, the maximum emission peak appears at 430 nm and the photoluminescence colour of the Co-Al-LDH sample is blue. This new optical property will be expected to extend the potential applications of LDHs in optical materials field. # 2007 Wan Guo Hou. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Layered double hydroxides; Photoluminescence; Optical materials
Layered double hydroxides (LDHs), also called as hydrotalcite-like compounds or anionic clays, are a large family of inorganic lamellar materials which are composed of brucite-like host layers and exchangeable anions as guest molecular, having the general formula, [M1 xIIMxIII(OH)2]x+Ax/nn mH2O, where MII and MIII represent bivalent and trivalent metal cations, respectively, An is the charge compensating anions or gallery anion, m is the number of moles of co-intercalated water per formula weight of the compound, x is the number of moles of MIII per formula weight of the compound [1–4]. With many great advantages, LDHs are widely applied in various fields, such as optical materials [5–7], biomaterials [8–10], catalysis [11–13], adsorption [14,15], film materials [16–18], etc. Recently, the optical properties of LDHs are widely studied. Especially, photoluminescence, magneto-optical response [16], and photochromism phenomena [19] are most attractive due to their potential applications for photochemical and photophysical devices [5–7,19,20] and biomaterials [8–10]. The conventional methods to obtain the photoluminescent LDHs composites are to exchange the interlayer anions with some fluorescent substances by intercalating, or to adsorb the fluorescent substances on the surfaces of the LDHs layers. The fluorescent substances combined with LDHs included organic dyes or chromophores [20–25], fluorescent probes and labellings [26,27], rare earth complexes [28], polyoxotungstoeuropate anions [29,30], quantum dots [31], etc. In this paper, we report a new photoluminescence behaviour of pure Co-Al-LDH, i.e. the sample itself could also emit fluorescence without any fluorescent substances intercalated. It has been reported that Co-Al-LDH nanosheets acted as nanoscale ferromagnetic layers at room temperature, and their multilayer assemblies exhibited significant magneto-optical response in the UV–vis region [16]. Now the discovery of photoluminescence of Co-Al-LDH may extend the potential application extent of LDHs in optical materials field. The Co-Al-LDH sample was synthesized by coprecipitation [1]. Mixed solution of CoCl2/AlCl3 at a molar ratio of 2.5:1 was prepared with a total metal ion concentration of 0.5 mol L 1. A 0.286 mol L 1 solution of ammonia was * Corresponding author. E-mail address: [email protected] (W.G. Hou). 1001-8417/$ – see front matter # 2007 Wan Guo Hou. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2007.09.024
1372
S. Sun, W.G. Hou / Chinese Chemical Letters 18 (2007) 1371–1373
Fig. 1. PXRD patterns (a) and TEM image (b) of the Co-Al-LDH sample.
slowly pumped into these mixed solutions under vigorous stirring. The final pH (ca. 9.5) of the dispersion was adjusted with ammonia. The precipitate was aged for 2 h in the mother solution at room temperature and then filtered and washed with deionized water to remove NH4Cl and the excess ammonia. The filter cake was peptized at about 80 8C for about 24 h to form Co-Al-LDH sol. Part of the sol was dried at 80 8C for 24 h to obtain the solid powder The powder X-ray diffraction (PXRD) patterns of the Co-Al-LDH sample (Fig. 1a) were recorded on a Bruker D8 ˚ ), revealing the sample is a advanced X-ray diffraction-meter equipped with Ni-filtered Cu Ka radiation (l =1.5418 A pure hydrotalcite-like phase. The patterns are typical of lamellar materials, with a basal reflection and associated harmonics at low angle 2u and weaker nonbasal reflections at a higher angle. The reflections for LDHs are generally ¯ indexed in a three-layer 3R polytype with rhombohedral symmetry (space group R3m), based on the structure of layered double hydroxide [32]. The TEM image (Fig. 1b) was obtained by a JEOL JEM-2100 microscope operated at 200 kV, and it shows the typical thin hexagonal plates with the size range of 30–80 nm. According to the Scherrer formula and XRD data [33], the size of the crystalline grain is estimated to be about 10–20 nm, which is different from the plate size in TEM, meaning the Co-Al-LDH particles are polycrystals. The chemical composition of the sample was analyzed by elemental analysis to be [Co0.70Al0.30(OH)2]Cl0.26(CO3)0.020.49H2O. Fig. 2 shows the excitation and emission photoluminescence spectra of the Co-Al-LDH sample in suspension. The spectra were characterized by an Edinburgh FLS920 spectrofluorimeter (Edinburgh Instruments Ltd., England) equipped with a xenon lamp and 1.0 cm quartz cells. The excitation spectrum shows the maximum peak near the wavelength 370 nm, and the maximum emission peak appears at 430 nm, which is recorded after 370 nm excitation. The emission spectrum was obtained under a 420 nm filter. Fig. 3 shows the photoluminescence images of the Co-Al-LDH sample in suspension obtained by IX81 Motorized Inverted Fluorescent Microscope (Olympus Optical Company Co., Japan) with UV light (excitation light at wavelength 360–370 nm) and WU filter. It can be seen from Fig. 3 that the Co-Al-LDH sample can emit blue photoluminescence. The above results revealed the existence of photoluminescence phenomenon of pure Co-Al-LDH. This new optical property will be expected to extend the potential application of LDHs in optical materials field such as energy storage
Fig. 2. (a) Photoluminescence excitation spectrum (lem = 430 nm) and (b) photoluminescence emission spectrum (lex = 370 nm) of the Co-AlLDH sample.
S. Sun, W.G. Hou / Chinese Chemical Letters 18 (2007) 1371–1373
1373
Fig. 3. Photoluminescence images of Co-Al-LDH suspension.
and conversion, nonlinear optics, photochemistry reaction, tunable laser luminescent materials, fluorescence labelling, fluorescence probe, etc. The mechanism of this phenomenon is yet not well-known, ongoing work at understanding the mechanism will be pursued in our laboratory. Acknowledgments This research was supported by the National Natural Science Foundation of China (No. 20573065) and the Natural Science Foundation of Shandong Province of China (Nos. Z2005B02 and Z2006B06). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
S. Miyata, Clays Clay Miner. 23 (1975) 369. S. Miyata, Clays Clay Miner. 31 (1983) 305. X. Duan, D.G. Evans, Layered Double Hydroxides, first ed., Springer, 2006. V. Rives, Layered Double Hydroxides: Present and Future, Nova Science Pub. Inc., 2001. M. Ogawa, K. Kuroda, Chem. Rev. 95 (1995) 399. H. Tagaya, A. Ogata, Microporous Mesoporous Mater. 7 (1996) 151. R. Sasai, N. Shin’ya, Langmuir 15 (1999) 413. J.H. Choy, S.-Y. Kwak, Angew. Chem. 39 (2000) 4041. J.H. Choy, S.-Y. Kwak, J. Am. Chem. Soc. 121 (1999) 1399. F. Leroux, J.-P. Besse, Chem. Mater. 13 (2001) 3507. F. Cavani, F. Trifiro`, A. Vaccari, Catal. Today 11 (1991) 173. A.L. Mckenzie, C.T. Fishel, J. Catal. 138 (1992) 547. B. Sels, D.D. Vos, Nature 400 (1999) 855. M.C. Hermosı´n, I. Pavlovic, Water Res. 30 (1996) 171. H. Cai, A.C. Hillier, Science 266 (1994) 1551. Z. Liu, R. Ma, J. Am. Chem. Soc. 128 (2006) 4872. H. Chen, F. Zhang, Adv. Mater. 18 (2006) 3089. E. Gardner, K.M. Huntoon, Adv. Mater. 13 (2001) 1263. M. Wei, X. Xu, Eur. J. Inorg. Chem. 2006 (2006) 2831. T. Kuwahara, H. Tagaya, Microporous Mesoporous Mater. 4 (1995) 247. U. Costantino, N. Coletti, Langmuir 16 (2000) 10351. U. Costantino, N. Coletti, Langmuir 15 (1999) 4454. L. Latterini, F. Elisei, Phys. Chem. Chem. Phys. 4 (2002) 2792. Z.L. Wang, Z.H. Kang, Inorg. Chem. 45 (2006) 4364. G.G. Aloisi, U. Costantino, J. Mater. Chem. 12 (2002) 3316. L. Mohanambe, S. Vasudevan, J. Phys. Chem. B 110 (2006) 14345. K.M. Tynera, M.S. Roberson, J. Control. Release 100 (2004) 399. Z. Chang, D. Evans, J. Phys. Chem. Solids 67 (2006) 1054. L. Filipa, M. Sousa, Pillinger, Eur. J. Inorg. Chem. 2006 (2006) 726. P.L. Gentili, U. Costantino, J. Mater. Chem. 14 (2004) 1656. A.V. Lukashin, A.A. Eliseev, Dokl. Chem. 383 (2002) 93. G.A. Caravaggio, C. Detellier, Z. Wronski, J. Mater. Chem. 11 (2001) 912. A.L. Patterson, Phys. Rev. A 56 (1939) 978.
Chinese Chemical Letters 18 (2007) 1371–1373 www.elsevier.com/locate/cclet
The photoluminescence of Co-Al-layered double hydroxide Shuai Sun, Wan Guo Hou * Key Laboratory for Colloid and Interface Chemistry of Education Ministry, Shandong University, Jinan 250100, China Received 14 May 2007
Abstract We report a new optical behaviour of pure Co-Al-layered double hydroxide (LDH). It was found that the Co-Al-LDH sample could emit fluorescence without any fluorescent substances intercalated. Its excitation spectrum shows a maximum peak near the wavelength 370 nm, the maximum emission peak appears at 430 nm and the photoluminescence colour of the Co-Al-LDH sample is blue. This new optical property will be expected to extend the potential applications of LDHs in optical materials field. # 2007 Wan Guo Hou. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Layered double hydroxides; Photoluminescence; Optical materials
Layered double hydroxides (LDHs), also called as hydrotalcite-like compounds or anionic clays, are a large family of inorganic lamellar materials which are composed of brucite-like host layers and exchangeable anions as guest molecular, having the general formula, [M1 xIIMxIII(OH)2]x+Ax/nn mH2O, where MII and MIII represent bivalent and trivalent metal cations, respectively, An is the charge compensating anions or gallery anion, m is the number of moles of co-intercalated water per formula weight of the compound, x is the number of moles of MIII per formula weight of the compound [1–4]. With many great advantages, LDHs are widely applied in various fields, such as optical materials [5–7], biomaterials [8–10], catalysis [11–13], adsorption [14,15], film materials [16–18], etc. Recently, the optical properties of LDHs are widely studied. Especially, photoluminescence, magneto-optical response [16], and photochromism phenomena [19] are most attractive due to their potential applications for photochemical and photophysical devices [5–7,19,20] and biomaterials [8–10]. The conventional methods to obtain the photoluminescent LDHs composites are to exchange the interlayer anions with some fluorescent substances by intercalating, or to adsorb the fluorescent substances on the surfaces of the LDHs layers. The fluorescent substances combined with LDHs included organic dyes or chromophores [20–25], fluorescent probes and labellings [26,27], rare earth complexes [28], polyoxotungstoeuropate anions [29,30], quantum dots [31], etc. In this paper, we report a new photoluminescence behaviour of pure Co-Al-LDH, i.e. the sample itself could also emit fluorescence without any fluorescent substances intercalated. It has been reported that Co-Al-LDH nanosheets acted as nanoscale ferromagnetic layers at room temperature, and their multilayer assemblies exhibited significant magneto-optical response in the UV–vis region [16]. Now the discovery of photoluminescence of Co-Al-LDH may extend the potential application extent of LDHs in optical materials field. The Co-Al-LDH sample was synthesized by coprecipitation [1]. Mixed solution of CoCl2/AlCl3 at a molar ratio of 2.5:1 was prepared with a total metal ion concentration of 0.5 mol L 1. A 0.286 mol L 1 solution of ammonia was * Corresponding author. E-mail address: [email protected] (W.G. Hou). 1001-8417/$ – see front matter # 2007 Wan Guo Hou. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2007.09.024
1372
S. Sun, W.G. Hou / Chinese Chemical Letters 18 (2007) 1371–1373
Fig. 1. PXRD patterns (a) and TEM image (b) of the Co-Al-LDH sample.
slowly pumped into these mixed solutions under vigorous stirring. The final pH (ca. 9.5) of the dispersion was adjusted with ammonia. The precipitate was aged for 2 h in the mother solution at room temperature and then filtered and washed with deionized water to remove NH4Cl and the excess ammonia. The filter cake was peptized at about 80 8C for about 24 h to form Co-Al-LDH sol. Part of the sol was dried at 80 8C for 24 h to obtain the solid powder The powder X-ray diffraction (PXRD) patterns of the Co-Al-LDH sample (Fig. 1a) were recorded on a Bruker D8 ˚ ), revealing the sample is a advanced X-ray diffraction-meter equipped with Ni-filtered Cu Ka radiation (l =1.5418 A pure hydrotalcite-like phase. The patterns are typical of lamellar materials, with a basal reflection and associated harmonics at low angle 2u and weaker nonbasal reflections at a higher angle. The reflections for LDHs are generally ¯ indexed in a three-layer 3R polytype with rhombohedral symmetry (space group R3m), based on the structure of layered double hydroxide [32]. The TEM image (Fig. 1b) was obtained by a JEOL JEM-2100 microscope operated at 200 kV, and it shows the typical thin hexagonal plates with the size range of 30–80 nm. According to the Scherrer formula and XRD data [33], the size of the crystalline grain is estimated to be about 10–20 nm, which is different from the plate size in TEM, meaning the Co-Al-LDH particles are polycrystals. The chemical composition of the sample was analyzed by elemental analysis to be [Co0.70Al0.30(OH)2]Cl0.26(CO3)0.020.49H2O. Fig. 2 shows the excitation and emission photoluminescence spectra of the Co-Al-LDH sample in suspension. The spectra were characterized by an Edinburgh FLS920 spectrofluorimeter (Edinburgh Instruments Ltd., England) equipped with a xenon lamp and 1.0 cm quartz cells. The excitation spectrum shows the maximum peak near the wavelength 370 nm, and the maximum emission peak appears at 430 nm, which is recorded after 370 nm excitation. The emission spectrum was obtained under a 420 nm filter. Fig. 3 shows the photoluminescence images of the Co-Al-LDH sample in suspension obtained by IX81 Motorized Inverted Fluorescent Microscope (Olympus Optical Company Co., Japan) with UV light (excitation light at wavelength 360–370 nm) and WU filter. It can be seen from Fig. 3 that the Co-Al-LDH sample can emit blue photoluminescence. The above results revealed the existence of photoluminescence phenomenon of pure Co-Al-LDH. This new optical property will be expected to extend the potential application of LDHs in optical materials field such as energy storage
Fig. 2. (a) Photoluminescence excitation spectrum (lem = 430 nm) and (b) photoluminescence emission spectrum (lex = 370 nm) of the Co-AlLDH sample.
S. Sun, W.G. Hou / Chinese Chemical Letters 18 (2007) 1371–1373
1373
Fig. 3. Photoluminescence images of Co-Al-LDH suspension.
and conversion, nonlinear optics, photochemistry reaction, tunable laser luminescent materials, fluorescence labelling, fluorescence probe, etc. The mechanism of this phenomenon is yet not well-known, ongoing work at understanding the mechanism will be pursued in our laboratory. Acknowledgments This research was supported by the National Natural Science Foundation of China (No. 20573065) and the Natural Science Foundation of Shandong Province of China (Nos. Z2005B02 and Z2006B06). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
S. Miyata, Clays Clay Miner. 23 (1975) 369. S. Miyata, Clays Clay Miner. 31 (1983) 305. X. Duan, D.G. Evans, Layered Double Hydroxides, first ed., Springer, 2006. V. Rives, Layered Double Hydroxides: Present and Future, Nova Science Pub. Inc., 2001. M. Ogawa, K. Kuroda, Chem. Rev. 95 (1995) 399. H. Tagaya, A. Ogata, Microporous Mesoporous Mater. 7 (1996) 151. R. Sasai, N. Shin’ya, Langmuir 15 (1999) 413. J.H. Choy, S.-Y. Kwak, Angew. Chem. 39 (2000) 4041. J.H. Choy, S.-Y. Kwak, J. Am. Chem. Soc. 121 (1999) 1399. F. Leroux, J.-P. Besse, Chem. Mater. 13 (2001) 3507. F. Cavani, F. Trifiro`, A. Vaccari, Catal. Today 11 (1991) 173. A.L. Mckenzie, C.T. Fishel, J. Catal. 138 (1992) 547. B. Sels, D.D. Vos, Nature 400 (1999) 855. M.C. Hermosı´n, I. Pavlovic, Water Res. 30 (1996) 171. H. Cai, A.C. Hillier, Science 266 (1994) 1551. Z. Liu, R. Ma, J. Am. Chem. Soc. 128 (2006) 4872. H. Chen, F. Zhang, Adv. Mater. 18 (2006) 3089. E. Gardner, K.M. Huntoon, Adv. Mater. 13 (2001) 1263. M. Wei, X. Xu, Eur. J. Inorg. Chem. 2006 (2006) 2831. T. Kuwahara, H. Tagaya, Microporous Mesoporous Mater. 4 (1995) 247. U. Costantino, N. Coletti, Langmuir 16 (2000) 10351. U. Costantino, N. Coletti, Langmuir 15 (1999) 4454. L. Latterini, F. Elisei, Phys. Chem. Chem. Phys. 4 (2002) 2792. Z.L. Wang, Z.H. Kang, Inorg. Chem. 45 (2006) 4364. G.G. Aloisi, U. Costantino, J. Mater. Chem. 12 (2002) 3316. L. Mohanambe, S. Vasudevan, J. Phys. Chem. B 110 (2006) 14345. K.M. Tynera, M.S. Roberson, J. Control. Release 100 (2004) 399. Z. Chang, D. Evans, J. Phys. Chem. Solids 67 (2006) 1054. L. Filipa, M. Sousa, Pillinger, Eur. J. Inorg. Chem. 2006 (2006) 726. P.L. Gentili, U. Costantino, J. Mater. Chem. 14 (2004) 1656. A.V. Lukashin, A.A. Eliseev, Dokl. Chem. 383 (2002) 93. G.A. Caravaggio, C. Detellier, Z. Wronski, J. Mater. Chem. 11 (2001) 912. A.L. Patterson, Phys. Rev. A 56 (1939) 978.