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Synthesis of well-ordered mesoporous titania powder with crystallized framework
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Materials Letters 62 (2008) 1410 – 1413 www.elsevier.com/locate/matlet
Synthesis of well-ordered mesoporous titania powder with crystallized framework Hua Li a,b , Jian-lin Shi b,⁎, Jian Liang b , Xia Li b , Lei Li b , Meilin Ruan b a
b
Inorganic materials department, School of Material engineering, Soochow University, 178 Ganjiang east roat, Suzhou, Jiangsu province, PR China State Key Lab of High Performance Ceramic and Superfine Microstructure, Shanghai Institute of Ceramic, Chinese Academy of Science, 1295 DingXi Road, Shanghai 200050, PR China Received 7 April 2007; accepted 25 August 2007 Available online 4 September 2007
Abstract Pure well ordered mesoporous TiO2 powder materials with crystallized frameworks were synthesized by carefully controlling the hydrolysis process of Ti precursor during self-assembly and the aging processes. A complete morphological characterization of the sample was performed using XRD, TEM, and N2 sorption isotherms. The resulted powder indicates well crystallization with two phases: anatase with main phase and the other, rutile; the SAXRD shows a peak at 0.8°, which indicates ordered mesoporous structure's formation. The thick frameworks (7 nm) and the narrow pore size distribution with the peak size within mesoporous size (5 nm) suggest the meoporous structure as well. In addition, CuO doped mesoporous TiO2 with even better ordered mesostructure were also reported in this work. © 2007 Elsevier B.V. All rights reserved. Keyword: Mesoporous TiO2 powder; Nanomaterials; Porosity; Crystallized frameworks; Hydrolysis controlling; Aging processes controlling
1. Introduction Titania has been a researching focus for many years [1–4] especially since it was found to possess photocatalytic property to decompose water and various kinds of organics [5]. When used as a catalyst, titania with large surface area and appropriate crystalline structure is highly expected. Plenty of chemical routes have been developed for the preparation of nanosized titania. Among them, mesoporous titania have been synthesized and studied by several groups [6–9]. There are several factors [10–14] should be taken into account during the synthesis of mesoporous titania for the structural and property control: first, most titania precursors are easily hydrated; second, chemical composition and phase structure of titania are highly uncontrollable; and finally, the structure and performance of titania are highly sensitive to many environmental factors such as aging conditions, the ratio among the reactors and so on. When using
⁎ Corresponding author. Tel.: +86 21 52410802; fax: +86 21 52413122. E-mail addresses: [email protected] (H. Li), [email protected] (J. Shi). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.08.072
EISA (evaporation-induced self-assembly process) process to synthesize mesoporous titania films, environmental conditions such as relative humidity, aging temperature, etc., are especially important, as the formation of titania meso-structure is a kinetic controlled process. We have recently reported the preparation of mesoporous titania/silica composite films with a evaporationinduced self assembly (EISA) process and the factors effecting the structure of the materials [15]. However, most reported work of mesoporous titania with ordered mesostructures were focused on the film form, rare work can be found for the preparation of mesoporous titania powder materials with both ordered mesoporous structure and crystallized framework. In the case of catalytic applications, however, powder materials will be more flexible and efficient for practical uses. In this paper, we reported the successful synthesis of pure mesoporous titania powder materials using a similar but significantly modified route recently developed by us [15]. The use of hydrolysis controlling agents acetylacetone for titanium alkoxide during self-assembly and the careful control of the aging process are found to be crucial for the formation of the wellordered mesoporous titania. The prepared materials have a typical mesoporous structure and well crystallized framework.
H. Li et al. / Materials Letters 62 (2008) 1410–1413
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Fig. 1. A: SAXRD pattern and B: WAXRD pattern of mesoporous TiO2.
Copper oxide doped mesoporous titania with well-defined mesoporous structure were also prepared. 2. Experimental 2.1. Synthesis In a typical synthesis of mesoporous TiO2, 1.5 g Pluronic P123 (PEO20PPO70PEO20, BASF) was dissolved in 15 ml ethanol containing 1.2 ml concentrated HCl (38 wt.%) and 0.2 ml distilled water at 10 °C under a 70% relative humidity. Then, 0.2 ml acetylacetone was added into this transparent solution. After that, 7.0 ml tetrabutyl titanate was added and the
mixture was stirred in a sealed bottle for 21 h. The resultant solution was transferred into an open Petri dish at room temperature and then aged in air for 7 days to form gel. During this experiment process above, the temperature and relative humidity were kept constant. After the gel was dried, the assynthesized samples were heat treated at 400 °C for 6 h. With above procedure, the desired mesoporous TiO2 materials were produced. 2.2. Analysis/characterization The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-IIA powder diffractometer. This diffractometer system employed nickel-filtered Cu Kα (λ = 0.15418 nm) radiation and was operated at 40 kV and 20 mA. The N2 adsorption–desorption isotherms of the samples were obtained as adsorbate at 77 K. The outgassing temperature was slowly raised to 200 °C and maintained at this temperature for 6 h. Pore structure data were analyzed from the adsorption curve by the Barrett–Joyner Halenda (BJH) method. 3. Results and discussion In the above experimental procedures, acetylacetone was used as a chelate agent to control the hydrolyzing process of tetrabutyl titanate
Fig. 2. TEM images of mesoporous TiO2. A: ordered pore channels taken with the electron beam perpendicular to the pore channels; B: image taken with the electron beam parallel with the pore channels; C: selected-area electron diffraction pattern.
Fig. 3. N2 adsorption–desorption isotherm curves and pore size distribution (inset) of the sample after calcination.
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H. Li et al. / Materials Letters 62 (2008) 1410–1413
Fig. 4. TEM Images of the calcined sample with CuO(1 − X) contents of A and B: 5 mol%, C and D: 10 mol%.
during the self-assembly process which is the premise to form ordered meso-structure. Besides, the aging temperature of as low as 10 °C was also important to slower down the meso-structure formation process. Finally, long aging time (7 days) was also necessary for the sake of full condensation in the framework, which is an effective way to avoid the walls collapse during heat treatment. The small angle XRD (SAXRD) patterns demonstrate that ordered meso-structure has been achieved, in which a peak about 0.8° can be identified (Fig. 1A); Fig. 1B of the wide angle XRD pattern proves that the mesoporous framework is well crystallized. From the wide angle XRD (WAXRD) pattern, both anatase (JCPDF no. 21-1272) and rutile (JCPDF no. 65-0190) phases are present in the mesoporous TiO2 framework with the anatase as the main crystal phase. As we know, the anatase TiO2 is an important photocatalyst, which indicates the application perspective of the materials we synthesized here. The TEM images directly show the ordered pore channels in mesoporous TiO2. From the images, pore size could be estimated to be around 5 nm (Fig. 2A), and framework of the materials is relative thick, which is about 7 nm. The thick pore walls are helpful for keeping pore structure from collapsing during calcinations [15]. The selected-area electron diffraction pattern shows that the materials are well crystallized. In contrast to the mesoporous TiO2/SiO2 composite, the synthesis of pure mesoporous TiO2 is much more difficult to achieve due to the very fast formation process which is difficult to control. In the synthesis of the mesoporous TiO2/SiO2 composite, vitrified SiO2 plays two important roles in the formation of mesoporous structure: the first one is the high viscosity of hydrolyzed silica gel which prevented the oligomeric Ti-oxo species or titania clusters from aggregating with each other too fast; and the other one is that the vitrified SiO2 act as adhesive to bond TiO2 clusters together into the mesoporous structure. These two factors make the formation process of mesoporous TiO2/ SiO2 composite controllable during self-assembly process and heat treatment. With the significant modification for the synthesis process developed for the preparation of mesoporous TiO2/ SiO2 composite,
pure mesoporous titania powder materials can be synthesized by using a hydrolysis controlling agent combined with the careful control of the aging processes, and the material shows similar but slightly deteriorated meso-structure as compared to mesoporous TiO2/SiO2 composite [15]. The pore structure analysis demonstrates that the pure TiO2 mesoporous materials have narrow pore size distribution in mesoporous size range and high special surface area (Fig. 3) i.e., the material processes well defined mesoporous structure. From Fig. 3, a well defined step at relative pressure (p/p0) ≈ 0.5–0.7 clearly indicates the mesoporous structure of the synthesized pure titania. The material has a BET surface area of 158 m2/g, a BJH pore volume of 0.2 cm3/g and an average pore size of 5.0 nm. From the fact that SiO2 can be used as an effective additive for controlling the mesoporous structure of TiO2, we tried to use CuO as a dopant during the synthesis of mesoporous pure TiO2 in order to achieve well ordered mesostructure. There are two reasons for using CuO as dopant: firstly, Cu ions have valences of +1 or +2, which is far different from that of Ti ions (+4). The large valence difference between Cu and Ti ions will efficiently disturb the formation process of TiO2 nanocrystals, which may, as we expected, lead to the retarded hydrolysis and self-assembly of TiO2 precusors and/or clusters by Cu+/Cu2+. As a result, the well ordered mesostructure could be developed by a sufficient selfassembly process. In addition, the presence of CuO would also prevent the over-quick growing of TiO2 clusters and the fast aggregation between them. The synthesis of CuO doped TiO2 mesoporous materials are similar to pure TiO2 mesoporous materials, but part TiO2 (5–10 mol%) was replaced with corresponding mole ratio of CuCl2. TEM images (Fig. 4) show that CuO is indeed favorable for the formation of ordered mesostructure. At increased contents of CuO in the composites, the mesostructure in the materials becomes better wellordered. The SAXRD patterns supported the conclusion (not shown), and the WAXRD showed that frameworks of the composite mesoporous materials are amorphous.
4. Conclusions In conclusion, this work shows that pure ordered mesoporous TiO2 powder materials with well crystallized framework, high surface area and narrow mesopore size can be successfully prepared when the hydrolysis process for selfassembly and aging conditions were carefully controlled. When copper oxide was added, mesoprous structure could be further improved while the mesoporous framework became amorphous. Acknowledgement This work was supported by Shanghai nano-project of NO.05 nm05030. References [1] S.N. Frank, A.J. Bard, J. Phys. Chem. 81 (1977) 1484. [2] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431. [3] S. Ikezawa, H. Homyara, T. Kubota, Thin Solid Films 386 (2001) 173. [4] C.H. Ao, S.C. Lee, C.L. Mak, L.Y. Chan, App. Catal., B Environ. 42 (2003) 119.
H. Li et al. / Materials Letters 62 (2008) 1410–1413 [5] A. Fujishima, K. Honda, Nature 238 (1972) 37. [6] X.C. Wang, J.C. Yu, H.Y. Yip, L. Wu, P.K. Wong, S.Y. Lai, Chem. Eur. J. 11 (2005) 2997. [7] C.Y. Jimmy, L.Z. Zang, J.G. Yu, New. J. Chem. 26 (2002) 416. [8] Q. Dai, N. He, Y. Guo, Chem. Lett. 11 (1998) 1113. [9] S.Y. Choi, M. Mamak, N. Coombs, N. Chopra, G.A. Ozin, Adv. Funct. Mater. 14 (2004) 335. [10] J. Livage, M. Henry, C. Sanchez, Prog. Solid State Chem. 18 (1988) 259. [11] P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Nature 396 (1998) 152.
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[12] P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Chem. Mater. 11 (1999) 2813. [13] C.J. Erinker, Y. Lu, A. Sellinger, H. Fan, Adv. Mater. 11 (1999) 579. [14] E.L. Crepaldi, G.J. De, A.A. Soler-Illia, D.G.F. Cagnol, F. Ribot, C. Sanchez, J. Am. Chem. Soc. 125 (2003) 9770. [15] Hua Li, Wei-hua Shen, Jian-lin Shi, Liang-ming Xiong , Jian Liang , Meilin Ruan, J. Mater. Res. 21 (2006) 380.
Materials Letters 62 (2008) 1410 – 1413 www.elsevier.com/locate/matlet
Synthesis of well-ordered mesoporous titania powder with crystallized framework Hua Li a,b , Jian-lin Shi b,⁎, Jian Liang b , Xia Li b , Lei Li b , Meilin Ruan b a
b
Inorganic materials department, School of Material engineering, Soochow University, 178 Ganjiang east roat, Suzhou, Jiangsu province, PR China State Key Lab of High Performance Ceramic and Superfine Microstructure, Shanghai Institute of Ceramic, Chinese Academy of Science, 1295 DingXi Road, Shanghai 200050, PR China Received 7 April 2007; accepted 25 August 2007 Available online 4 September 2007
Abstract Pure well ordered mesoporous TiO2 powder materials with crystallized frameworks were synthesized by carefully controlling the hydrolysis process of Ti precursor during self-assembly and the aging processes. A complete morphological characterization of the sample was performed using XRD, TEM, and N2 sorption isotherms. The resulted powder indicates well crystallization with two phases: anatase with main phase and the other, rutile; the SAXRD shows a peak at 0.8°, which indicates ordered mesoporous structure's formation. The thick frameworks (7 nm) and the narrow pore size distribution with the peak size within mesoporous size (5 nm) suggest the meoporous structure as well. In addition, CuO doped mesoporous TiO2 with even better ordered mesostructure were also reported in this work. © 2007 Elsevier B.V. All rights reserved. Keyword: Mesoporous TiO2 powder; Nanomaterials; Porosity; Crystallized frameworks; Hydrolysis controlling; Aging processes controlling
1. Introduction Titania has been a researching focus for many years [1–4] especially since it was found to possess photocatalytic property to decompose water and various kinds of organics [5]. When used as a catalyst, titania with large surface area and appropriate crystalline structure is highly expected. Plenty of chemical routes have been developed for the preparation of nanosized titania. Among them, mesoporous titania have been synthesized and studied by several groups [6–9]. There are several factors [10–14] should be taken into account during the synthesis of mesoporous titania for the structural and property control: first, most titania precursors are easily hydrated; second, chemical composition and phase structure of titania are highly uncontrollable; and finally, the structure and performance of titania are highly sensitive to many environmental factors such as aging conditions, the ratio among the reactors and so on. When using
⁎ Corresponding author. Tel.: +86 21 52410802; fax: +86 21 52413122. E-mail addresses: [email protected] (H. Li), [email protected] (J. Shi). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.08.072
EISA (evaporation-induced self-assembly process) process to synthesize mesoporous titania films, environmental conditions such as relative humidity, aging temperature, etc., are especially important, as the formation of titania meso-structure is a kinetic controlled process. We have recently reported the preparation of mesoporous titania/silica composite films with a evaporationinduced self assembly (EISA) process and the factors effecting the structure of the materials [15]. However, most reported work of mesoporous titania with ordered mesostructures were focused on the film form, rare work can be found for the preparation of mesoporous titania powder materials with both ordered mesoporous structure and crystallized framework. In the case of catalytic applications, however, powder materials will be more flexible and efficient for practical uses. In this paper, we reported the successful synthesis of pure mesoporous titania powder materials using a similar but significantly modified route recently developed by us [15]. The use of hydrolysis controlling agents acetylacetone for titanium alkoxide during self-assembly and the careful control of the aging process are found to be crucial for the formation of the wellordered mesoporous titania. The prepared materials have a typical mesoporous structure and well crystallized framework.
H. Li et al. / Materials Letters 62 (2008) 1410–1413
1411
Fig. 1. A: SAXRD pattern and B: WAXRD pattern of mesoporous TiO2.
Copper oxide doped mesoporous titania with well-defined mesoporous structure were also prepared. 2. Experimental 2.1. Synthesis In a typical synthesis of mesoporous TiO2, 1.5 g Pluronic P123 (PEO20PPO70PEO20, BASF) was dissolved in 15 ml ethanol containing 1.2 ml concentrated HCl (38 wt.%) and 0.2 ml distilled water at 10 °C under a 70% relative humidity. Then, 0.2 ml acetylacetone was added into this transparent solution. After that, 7.0 ml tetrabutyl titanate was added and the
mixture was stirred in a sealed bottle for 21 h. The resultant solution was transferred into an open Petri dish at room temperature and then aged in air for 7 days to form gel. During this experiment process above, the temperature and relative humidity were kept constant. After the gel was dried, the assynthesized samples were heat treated at 400 °C for 6 h. With above procedure, the desired mesoporous TiO2 materials were produced. 2.2. Analysis/characterization The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-IIA powder diffractometer. This diffractometer system employed nickel-filtered Cu Kα (λ = 0.15418 nm) radiation and was operated at 40 kV and 20 mA. The N2 adsorption–desorption isotherms of the samples were obtained as adsorbate at 77 K. The outgassing temperature was slowly raised to 200 °C and maintained at this temperature for 6 h. Pore structure data were analyzed from the adsorption curve by the Barrett–Joyner Halenda (BJH) method. 3. Results and discussion In the above experimental procedures, acetylacetone was used as a chelate agent to control the hydrolyzing process of tetrabutyl titanate
Fig. 2. TEM images of mesoporous TiO2. A: ordered pore channels taken with the electron beam perpendicular to the pore channels; B: image taken with the electron beam parallel with the pore channels; C: selected-area electron diffraction pattern.
Fig. 3. N2 adsorption–desorption isotherm curves and pore size distribution (inset) of the sample after calcination.
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H. Li et al. / Materials Letters 62 (2008) 1410–1413
Fig. 4. TEM Images of the calcined sample with CuO(1 − X) contents of A and B: 5 mol%, C and D: 10 mol%.
during the self-assembly process which is the premise to form ordered meso-structure. Besides, the aging temperature of as low as 10 °C was also important to slower down the meso-structure formation process. Finally, long aging time (7 days) was also necessary for the sake of full condensation in the framework, which is an effective way to avoid the walls collapse during heat treatment. The small angle XRD (SAXRD) patterns demonstrate that ordered meso-structure has been achieved, in which a peak about 0.8° can be identified (Fig. 1A); Fig. 1B of the wide angle XRD pattern proves that the mesoporous framework is well crystallized. From the wide angle XRD (WAXRD) pattern, both anatase (JCPDF no. 21-1272) and rutile (JCPDF no. 65-0190) phases are present in the mesoporous TiO2 framework with the anatase as the main crystal phase. As we know, the anatase TiO2 is an important photocatalyst, which indicates the application perspective of the materials we synthesized here. The TEM images directly show the ordered pore channels in mesoporous TiO2. From the images, pore size could be estimated to be around 5 nm (Fig. 2A), and framework of the materials is relative thick, which is about 7 nm. The thick pore walls are helpful for keeping pore structure from collapsing during calcinations [15]. The selected-area electron diffraction pattern shows that the materials are well crystallized. In contrast to the mesoporous TiO2/SiO2 composite, the synthesis of pure mesoporous TiO2 is much more difficult to achieve due to the very fast formation process which is difficult to control. In the synthesis of the mesoporous TiO2/SiO2 composite, vitrified SiO2 plays two important roles in the formation of mesoporous structure: the first one is the high viscosity of hydrolyzed silica gel which prevented the oligomeric Ti-oxo species or titania clusters from aggregating with each other too fast; and the other one is that the vitrified SiO2 act as adhesive to bond TiO2 clusters together into the mesoporous structure. These two factors make the formation process of mesoporous TiO2/ SiO2 composite controllable during self-assembly process and heat treatment. With the significant modification for the synthesis process developed for the preparation of mesoporous TiO2/ SiO2 composite,
pure mesoporous titania powder materials can be synthesized by using a hydrolysis controlling agent combined with the careful control of the aging processes, and the material shows similar but slightly deteriorated meso-structure as compared to mesoporous TiO2/SiO2 composite [15]. The pore structure analysis demonstrates that the pure TiO2 mesoporous materials have narrow pore size distribution in mesoporous size range and high special surface area (Fig. 3) i.e., the material processes well defined mesoporous structure. From Fig. 3, a well defined step at relative pressure (p/p0) ≈ 0.5–0.7 clearly indicates the mesoporous structure of the synthesized pure titania. The material has a BET surface area of 158 m2/g, a BJH pore volume of 0.2 cm3/g and an average pore size of 5.0 nm. From the fact that SiO2 can be used as an effective additive for controlling the mesoporous structure of TiO2, we tried to use CuO as a dopant during the synthesis of mesoporous pure TiO2 in order to achieve well ordered mesostructure. There are two reasons for using CuO as dopant: firstly, Cu ions have valences of +1 or +2, which is far different from that of Ti ions (+4). The large valence difference between Cu and Ti ions will efficiently disturb the formation process of TiO2 nanocrystals, which may, as we expected, lead to the retarded hydrolysis and self-assembly of TiO2 precusors and/or clusters by Cu+/Cu2+. As a result, the well ordered mesostructure could be developed by a sufficient selfassembly process. In addition, the presence of CuO would also prevent the over-quick growing of TiO2 clusters and the fast aggregation between them. The synthesis of CuO doped TiO2 mesoporous materials are similar to pure TiO2 mesoporous materials, but part TiO2 (5–10 mol%) was replaced with corresponding mole ratio of CuCl2. TEM images (Fig. 4) show that CuO is indeed favorable for the formation of ordered mesostructure. At increased contents of CuO in the composites, the mesostructure in the materials becomes better wellordered. The SAXRD patterns supported the conclusion (not shown), and the WAXRD showed that frameworks of the composite mesoporous materials are amorphous.
4. Conclusions In conclusion, this work shows that pure ordered mesoporous TiO2 powder materials with well crystallized framework, high surface area and narrow mesopore size can be successfully prepared when the hydrolysis process for selfassembly and aging conditions were carefully controlled. When copper oxide was added, mesoprous structure could be further improved while the mesoporous framework became amorphous. Acknowledgement This work was supported by Shanghai nano-project of NO.05 nm05030. References [1] S.N. Frank, A.J. Bard, J. Phys. Chem. 81 (1977) 1484. [2] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431. [3] S. Ikezawa, H. Homyara, T. Kubota, Thin Solid Films 386 (2001) 173. [4] C.H. Ao, S.C. Lee, C.L. Mak, L.Y. Chan, App. Catal., B Environ. 42 (2003) 119.
H. Li et al. / Materials Letters 62 (2008) 1410–1413 [5] A. Fujishima, K. Honda, Nature 238 (1972) 37. [6] X.C. Wang, J.C. Yu, H.Y. Yip, L. Wu, P.K. Wong, S.Y. Lai, Chem. Eur. J. 11 (2005) 2997. [7] C.Y. Jimmy, L.Z. Zang, J.G. Yu, New. J. Chem. 26 (2002) 416. [8] Q. Dai, N. He, Y. Guo, Chem. Lett. 11 (1998) 1113. [9] S.Y. Choi, M. Mamak, N. Coombs, N. Chopra, G.A. Ozin, Adv. Funct. Mater. 14 (2004) 335. [10] J. Livage, M. Henry, C. Sanchez, Prog. Solid State Chem. 18 (1988) 259. [11] P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Nature 396 (1998) 152.
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[12] P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Chem. Mater. 11 (1999) 2813. [13] C.J. Erinker, Y. Lu, A. Sellinger, H. Fan, Adv. Mater. 11 (1999) 579. [14] E.L. Crepaldi, G.J. De, A.A. Soler-Illia, D.G.F. Cagnol, F. Ribot, C. Sanchez, J. Am. Chem. Soc. 125 (2003) 9770. [15] Hua Li, Wei-hua Shen, Jian-lin Shi, Liang-ming Xiong , Jian Liang , Meilin Ruan, J. Mater. Res. 21 (2006) 380.