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Preparation and conductivity of polyvinyl alcohol (PVA) films composited with molybdotungstovanadogermanic heteropoly acid
Materials Chemistry and Physics 85 (2004) 416–419
Preparation and conductivity of polyvinyl alcohol (PVA) films composited with molybdotungstovanadogermanic heteropoly acid Yanli Cui a , Jianwei Mao b , Qingyin Wu a,∗ b
a Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China Department of Biological and Chemical Engineering, Zhejiang University of Science and Technology, Hangzhou 310012, PR China
Received 4 November 2003; received in revised form 6 January 2004; accepted 29 January 2004
Abstract In this work, the polyvinyl alcohol (PVA) films composited with molybdotungstovanadogermanic heteropoly acid were prepared. Infrared spectra (IR) revealed that the Keggin structure characteristic of the GeMo2 W9 VO40 5− anion was present in PVA films. At room temperature (20 ◦ C), the conductivity of the samples of different HPA content increased from 2.46 × 10−3 to 9.92 × 10−3 S cm−1 . The results indicated that they are new kind of excellent high-proton conductor. © 2004 Elsevier B.V. All rights reserved. Keywords: Polyvinyl alcohol (PVA); Thin films; Molybdotungstovanadogermanic heteropoly acid; Conductivity
1. Introduction Polyoxometalates (POMs) are multifunctional material and have been widely used in analytical and clinical chemistry, catalysis (including photocatalysis), biochemistry (electron transport inhibition), medicine (anti-tumonal, antiviral, and even anti-HIV activity), and solid-state devices [1] owning to their unique combination of physical and chemical properties. Heteropolyanions have some very useful and interesting properties, including the high stability of most of their redox states and the possibility of multiple electron transfer. Hybrid inorganic–organic materials have received increasing attention over the last few years as a result of their specific properties [2]. These new materials have the possibility of becoming very useful, having both the advantages of organic materials such as light weight, flexibility, and good moldability and of inorganic materials such as high strength, heat stability, and chemical resistance [3]. Ancient POMs as inorganic–organic hybrid materials have received increasing attention recently [4,5]. They have been found to be an extremely versatile inorganic building block for the construction of functional solid materials [6]. So, an aspect of research in materials science has been the preparation of hybrid materials containing POMs and or∗ Corresponding author. E-mail address: [email protected] (Q. Wu).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.01.034
ganic polymers, such as polyaniline [7,8], polypyrrole [9], polyvinylpyrrolidone [10,11], and polyvinyl alcohol [12] as thin films or as fiber mats. In this paper, the preparation and conductivity of polyvinyl alcohol (PVA) films composited with molybdotungstovanadogermanic heteropoly acid is reported.
2. Experimental 2.1. Characterization Infrared (IR) spectra were recorded on a Bruker VECTOR-22 FT/IR spectrometer in the wave number range 400–4000 cm−1 using KBr pellets. Impedance measurements of the samples were performed on a Genrad 1689 Precision RLC Digibridge with copper electrodes over the frequency range from 12 to 100 kHz. UV-Vis spectra were investigated on a Shimazdu UV-2100 UV spectrometer in the range 190–800 nm. 2.2. Synthesis The molybdotungstovanadogermanic heteropoly acid H5 GeW9 Mo2 VO40 ·22H2 O (GeW9 Mo2 V) was prepared according to the literature [13]. The purity of PVA is more than 99.0%. The synthesis of GeW9 Mo2 V/PVA is described as follows: PVA (0.25 g) was dissolved in 20 ml of boiling
Y. Cui et al. / Materials Chemistry and Physics 85 (2004) 416–419
water, then appropriate amounts of H5 GeW9 Mo2 VO40 were added to the solution, and the mixture was stirred strongly until complete dissolution of HPA powder. The solution was vaporized at 50 ◦ C until the solution looked slimy. After cooled to room temperature the viscous solution was equably spread over the plastic flat and kept in the oven at 40 ◦ C until it was transformed into film (about 2 h). Three films with different compositions were prepared: 50, 67, and 80 wt.%. Transparent films were obtained for each composition. The whole process was carried out in the dark.
3. Results and discussion 3.1. Infrared spectra Fig. 1 shows the infrared (IR) spectra of the films with different compositions. Each film doped with the molybdotungstovanadogermanic acid exhibits six characteristic
417
peaks of the Keggin anion, which are also observed in the spectrum of the pure molybdotungstovanadogermanic acid crystal. The Keggin structure of GeW9 Mo2 VO40 5− consists of one GeO4 tetrahedron surrounded by four M3 O13 (M = Mo, W, or V) sets formed by three edge-sharing octahedra. The M3 O13 sets are linked together through oxygen atoms. Thus, there are four kinds of oxygen atoms in GeW9 Mo2 VO40 5− , four Ge–Oa in which oxygen atom connects with heteroatom, 12 M–Ob –M oxygen-bridges (corner-sharing oxygen-bridge between different M3 O13 sets), 12 M–Oc –M oxygen-bridges (edge-sharing oxygenbridge within M3 O13 sets) and 12 M–Od terminal oxygen atoms [14]. In the IR spectrum of the pure molybdotungstovanadogermanic heteropoly acid crystal, there are five characteristic bands: 977 cm−1 , υas M–Od ; 883 cm−1 , υas M–Ob –M; 810 cm−1 , as Ge–Oa ; 767 cm−1 , υas M–Oc –M; 459 cm−1 , δ (O–Ge–O) [13]. The wave number of the M–Od asymmetrical stretching vibration increased from 962.3 to 970.9 cm−1 with molybdotungstovanadogermanic acid content from 50 to 80 wt.%.
Fig. 1. IR spectra of PVA films with: (a) HPA 80; (b) 67 and (c) 50 wt.%.
Y. Cui et al. / Materials Chemistry and Physics 85 (2004) 416–419
3.2. Conductivity In the studied films, there are a number of hydrogen bonds among H5 GeW9 Mo2 VO40 ·22H2 O, PVA and water. The volume of GeW9 Mo2 VO40 5− is so big that the anions cannot move as a result of the strength of hydrogen bonds, but its protons can transfer along these hydrogen bonds. So, the film is a proton conductor. Impedance results showed that the conductivity of the films increased with the increase of the molybdotungstovanadogermanic heteropoly acid content. This behaviour results from the increment of proton density with the increase of HPA weight percent. The conductivity is a function of the movement of protons. It is known that heteropoly acids are sensitive to surrounding conditions such as temperature, relative humidity and content of crystal water. For example, the research on H3 PW12 O40 ·nH2 O and H3 PMo12 O40 ·nH2 O shows that a mass of hydrogen bonds were produced when the content of crystal water had increased, and at the same time the conductivity increased. At room temperature (20 ◦ C), the conductivity of the three samples increased from 2.46 × 10−3 to 9.92 × 10−3 S cm−1 (Fig. 2), which is much higher than that
11 10 9 -1 -3
Generally, the M–O stretching can be considered as pure vibration, which characteristic wave number is an increasing function of the anion–anion interaction. The decrease of M–Od asymmetrical stretching frequency of the films with molybdotungstovanadogermanic acid weight percent from 80 to 50% is attributed to the weakening of anion–anion interactions of the electrostatic type. We assumed that the anion–anion interactions are weakened due to the influence of PVA, which leads to the lengthening of the anion–anion distances. The stretching involving Ob or Oc atoms are different from M–Od stretching and they present some bending character. This can be inferred from geometrical considerations. Because M–Ob –M and M–Oc –M vibrations are not pure and cannot be free from bending character, there is a competition of the opposite effects. The electrostatic anion–anion interactions lead to an increase in the stretching frequencies, but they lead to a decrease in the bending vibrations [15]. Moreover, perturbations due to water molecules and anion–cation interactions lead to a decrease in the frequencies of vibrations and can strengthen the decreasing effect of anion–anion interactions. In the competition of the opposite effects, the decreasing effect is stronger than the increasing one, but not for M–Ob –M. So, M–Oc –M asymmetrical stretching frequencies are decreasing functions of anion–anion interaction. With the increase of dopant from 50 to 80 wt.%, the wave number of the M–Oc –M asymmetrical stretching vibrations decreases from 778.3 to 761.2 cm−1 . Because the GeO4 tetrahedron is assumed to vibrate almost independently, the Ge–Oa stretching vibration and O–Ge–O bending vibration show no relation with the dopant weight percent.
σ (X10 S.cm )
418
8 7 6 5 4 3 2 50
55
60
65
70
75
80
HPA wt%
Fig. 2. The relation between conductivity at 20 ◦ C and HPA weight percent.
of the pure PVA films and pure molybdotungstovanadogermanic heteropoly acid. The results indicated that the PVA film composited with molybdotungstovanadogermanic acid, is a new kind of excellent high-proton conductor. 3.3. UV spectra There is a strong absorption band in UV region resulting from the p–d transition of M–O bonds in the heteropoly acids. Heteropoly anions Keggin structure exhibit two strong characteristic absorption bands. The bands near 200 and 260 nm are assigned to the Od → M (Od : terminal oxygen) and Ob /Oc → M (Ob or Oc : bridge-oxygen) charge-transfer transition. The structure of complexes, equilibrating cations and substitution elements may have influence on the UV spectra of heteropoly complexes. In the films, PVA acts as an electron donor and HPA as an electron acceptor. In the PVA solution, the two strong characteristic absorption peaks of molybdotungstovanadogermanic acid are 193 and 263 nm (Fig. 3). Fig. 4 shows the UV-Vis spectrum of molybdotungstovanadogermanic heteropoly acid in PVA solution after UV radiation. Different with the UV-Vis
Fig. 3. The UV-Vis spectrum of HPA in PVA solution before UV radiation.
Y. Cui et al. / Materials Chemistry and Physics 85 (2004) 416–419
419
cate they can be used as a promising material in the coming future.
Acknowledgements The financial support from the National Natural Science Foundation of China under Grant No. 20271045, 20006013 and the Foundation of International Cooperation Project of the Ministry of Science and Technology under Grant No. 014-08 for this work is greatly appreciated. References
Fig. 4. The UV-Vis spectrum of HPA in PVA solution after UV radiation.
spectrum of 12-tungstogermanic acid in PVA, the molybdotungstovanadogermanic acid in PVA has no phenomenon of photochromism.
4. Conclusions IR spectra and UV spectra showed that GeW9 Mo2 VO40 5− anion (Keggin structure) exists in the PVA films. In the infrared spectra of the films, each peak assigned to different vibration modes was confirmed, and the regularity of the change of characteristic peaks as a function of the HPA content was described. The conductivity of films doped with 80% molybdotungstovanadogermanic heteropoly acid is 9.92 × 10−3 S cm−1 at room temperature (20 ◦ C). The studies on the conductivity of the films indi-
[1] E. Coronado, C.J. Gómez-Garc´ıa, Chem. Rev. 98 (1998) 273. [2] C.R. Mayer, V. Cabuil, T. Lalot, R. Thouenot, Angew. Chem. Int. Ed. 38 (1999) 3672. [3] K. Nakane, T. Yamashita, K. Iwakura, F. Suzuki, J. Appl. Polym. Sci. 74 (1999) 133. [4] T.Y. Zhang, W. Feng, R. Lu, X.T. Zhang, M. Jin, T.J. Li, Y.Y. Zhao, J.N. Yao, Thin Solid Films 402 (2002) 237. [5] U.L. Stangar, N. Groseij, B. Orel, A. Schmitz, P. Colomban, Solid State Ionics 145 (2001) 109. [6] A. Muller, P. Kogerler, C.J. Kuhlmann, J. Chem. Soc., Chem. Commun. (1999) 1347. [7] J. Gong, Z.M. Dai, R.S. Wang, L.Y. Qu, Synth. Met. 101 (1999) 751. [8] J. Gong, X.J. Cui, Z.W. Xie, S.G. Wang, L.Y. Qu, Synth. Met. 129 (2002) 187. [9] T.F. Otero, S.A. Cheng, E. Coronado, E.M. Ferrero, C.J. GómezGar´ıa, Chem Phys Chem (2002) 808. [10] J. Gong, X.D. Li, B. Ding, D.R. Lee, H.Y. Kim, J. Appl. Polym. Sci. 89 (2003) 1573. [11] Q.Y. Wu, H.B. Wang, C.S. Yin, G.Y. Meng, Mater. Lett. 50 (2001) 61. [12] H.Y. Zhang, L. Xu, E.B. Wang, M. Jiang, A.G. Wu, Z. Li, Mater. Lett. 57 (2003) 1417. [13] Q.Y. Wu, H.H. Lin, G.Y. Meng, Mater. Lett. 39 (1999) 129. [14] A. Teze, G. Herve, J. Inorg. Nucl. Chem. 39 (1977) 999. [15] Q.Y. Wu, H.H. Lin, G.Y. Meng, J. Solid State Chem. 148 (1999) 419.
Preparation and conductivity of polyvinyl alcohol (PVA) films composited with molybdotungstovanadogermanic heteropoly acid Yanli Cui a , Jianwei Mao b , Qingyin Wu a,∗ b
a Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China Department of Biological and Chemical Engineering, Zhejiang University of Science and Technology, Hangzhou 310012, PR China
Received 4 November 2003; received in revised form 6 January 2004; accepted 29 January 2004
Abstract In this work, the polyvinyl alcohol (PVA) films composited with molybdotungstovanadogermanic heteropoly acid were prepared. Infrared spectra (IR) revealed that the Keggin structure characteristic of the GeMo2 W9 VO40 5− anion was present in PVA films. At room temperature (20 ◦ C), the conductivity of the samples of different HPA content increased from 2.46 × 10−3 to 9.92 × 10−3 S cm−1 . The results indicated that they are new kind of excellent high-proton conductor. © 2004 Elsevier B.V. All rights reserved. Keywords: Polyvinyl alcohol (PVA); Thin films; Molybdotungstovanadogermanic heteropoly acid; Conductivity
1. Introduction Polyoxometalates (POMs) are multifunctional material and have been widely used in analytical and clinical chemistry, catalysis (including photocatalysis), biochemistry (electron transport inhibition), medicine (anti-tumonal, antiviral, and even anti-HIV activity), and solid-state devices [1] owning to their unique combination of physical and chemical properties. Heteropolyanions have some very useful and interesting properties, including the high stability of most of their redox states and the possibility of multiple electron transfer. Hybrid inorganic–organic materials have received increasing attention over the last few years as a result of their specific properties [2]. These new materials have the possibility of becoming very useful, having both the advantages of organic materials such as light weight, flexibility, and good moldability and of inorganic materials such as high strength, heat stability, and chemical resistance [3]. Ancient POMs as inorganic–organic hybrid materials have received increasing attention recently [4,5]. They have been found to be an extremely versatile inorganic building block for the construction of functional solid materials [6]. So, an aspect of research in materials science has been the preparation of hybrid materials containing POMs and or∗ Corresponding author. E-mail address: [email protected] (Q. Wu).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.01.034
ganic polymers, such as polyaniline [7,8], polypyrrole [9], polyvinylpyrrolidone [10,11], and polyvinyl alcohol [12] as thin films or as fiber mats. In this paper, the preparation and conductivity of polyvinyl alcohol (PVA) films composited with molybdotungstovanadogermanic heteropoly acid is reported.
2. Experimental 2.1. Characterization Infrared (IR) spectra were recorded on a Bruker VECTOR-22 FT/IR spectrometer in the wave number range 400–4000 cm−1 using KBr pellets. Impedance measurements of the samples were performed on a Genrad 1689 Precision RLC Digibridge with copper electrodes over the frequency range from 12 to 100 kHz. UV-Vis spectra were investigated on a Shimazdu UV-2100 UV spectrometer in the range 190–800 nm. 2.2. Synthesis The molybdotungstovanadogermanic heteropoly acid H5 GeW9 Mo2 VO40 ·22H2 O (GeW9 Mo2 V) was prepared according to the literature [13]. The purity of PVA is more than 99.0%. The synthesis of GeW9 Mo2 V/PVA is described as follows: PVA (0.25 g) was dissolved in 20 ml of boiling
Y. Cui et al. / Materials Chemistry and Physics 85 (2004) 416–419
water, then appropriate amounts of H5 GeW9 Mo2 VO40 were added to the solution, and the mixture was stirred strongly until complete dissolution of HPA powder. The solution was vaporized at 50 ◦ C until the solution looked slimy. After cooled to room temperature the viscous solution was equably spread over the plastic flat and kept in the oven at 40 ◦ C until it was transformed into film (about 2 h). Three films with different compositions were prepared: 50, 67, and 80 wt.%. Transparent films were obtained for each composition. The whole process was carried out in the dark.
3. Results and discussion 3.1. Infrared spectra Fig. 1 shows the infrared (IR) spectra of the films with different compositions. Each film doped with the molybdotungstovanadogermanic acid exhibits six characteristic
417
peaks of the Keggin anion, which are also observed in the spectrum of the pure molybdotungstovanadogermanic acid crystal. The Keggin structure of GeW9 Mo2 VO40 5− consists of one GeO4 tetrahedron surrounded by four M3 O13 (M = Mo, W, or V) sets formed by three edge-sharing octahedra. The M3 O13 sets are linked together through oxygen atoms. Thus, there are four kinds of oxygen atoms in GeW9 Mo2 VO40 5− , four Ge–Oa in which oxygen atom connects with heteroatom, 12 M–Ob –M oxygen-bridges (corner-sharing oxygen-bridge between different M3 O13 sets), 12 M–Oc –M oxygen-bridges (edge-sharing oxygenbridge within M3 O13 sets) and 12 M–Od terminal oxygen atoms [14]. In the IR spectrum of the pure molybdotungstovanadogermanic heteropoly acid crystal, there are five characteristic bands: 977 cm−1 , υas M–Od ; 883 cm−1 , υas M–Ob –M; 810 cm−1 , as Ge–Oa ; 767 cm−1 , υas M–Oc –M; 459 cm−1 , δ (O–Ge–O) [13]. The wave number of the M–Od asymmetrical stretching vibration increased from 962.3 to 970.9 cm−1 with molybdotungstovanadogermanic acid content from 50 to 80 wt.%.
Fig. 1. IR spectra of PVA films with: (a) HPA 80; (b) 67 and (c) 50 wt.%.
Y. Cui et al. / Materials Chemistry and Physics 85 (2004) 416–419
3.2. Conductivity In the studied films, there are a number of hydrogen bonds among H5 GeW9 Mo2 VO40 ·22H2 O, PVA and water. The volume of GeW9 Mo2 VO40 5− is so big that the anions cannot move as a result of the strength of hydrogen bonds, but its protons can transfer along these hydrogen bonds. So, the film is a proton conductor. Impedance results showed that the conductivity of the films increased with the increase of the molybdotungstovanadogermanic heteropoly acid content. This behaviour results from the increment of proton density with the increase of HPA weight percent. The conductivity is a function of the movement of protons. It is known that heteropoly acids are sensitive to surrounding conditions such as temperature, relative humidity and content of crystal water. For example, the research on H3 PW12 O40 ·nH2 O and H3 PMo12 O40 ·nH2 O shows that a mass of hydrogen bonds were produced when the content of crystal water had increased, and at the same time the conductivity increased. At room temperature (20 ◦ C), the conductivity of the three samples increased from 2.46 × 10−3 to 9.92 × 10−3 S cm−1 (Fig. 2), which is much higher than that
11 10 9 -1 -3
Generally, the M–O stretching can be considered as pure vibration, which characteristic wave number is an increasing function of the anion–anion interaction. The decrease of M–Od asymmetrical stretching frequency of the films with molybdotungstovanadogermanic acid weight percent from 80 to 50% is attributed to the weakening of anion–anion interactions of the electrostatic type. We assumed that the anion–anion interactions are weakened due to the influence of PVA, which leads to the lengthening of the anion–anion distances. The stretching involving Ob or Oc atoms are different from M–Od stretching and they present some bending character. This can be inferred from geometrical considerations. Because M–Ob –M and M–Oc –M vibrations are not pure and cannot be free from bending character, there is a competition of the opposite effects. The electrostatic anion–anion interactions lead to an increase in the stretching frequencies, but they lead to a decrease in the bending vibrations [15]. Moreover, perturbations due to water molecules and anion–cation interactions lead to a decrease in the frequencies of vibrations and can strengthen the decreasing effect of anion–anion interactions. In the competition of the opposite effects, the decreasing effect is stronger than the increasing one, but not for M–Ob –M. So, M–Oc –M asymmetrical stretching frequencies are decreasing functions of anion–anion interaction. With the increase of dopant from 50 to 80 wt.%, the wave number of the M–Oc –M asymmetrical stretching vibrations decreases from 778.3 to 761.2 cm−1 . Because the GeO4 tetrahedron is assumed to vibrate almost independently, the Ge–Oa stretching vibration and O–Ge–O bending vibration show no relation with the dopant weight percent.
σ (X10 S.cm )
418
8 7 6 5 4 3 2 50
55
60
65
70
75
80
HPA wt%
Fig. 2. The relation between conductivity at 20 ◦ C and HPA weight percent.
of the pure PVA films and pure molybdotungstovanadogermanic heteropoly acid. The results indicated that the PVA film composited with molybdotungstovanadogermanic acid, is a new kind of excellent high-proton conductor. 3.3. UV spectra There is a strong absorption band in UV region resulting from the p–d transition of M–O bonds in the heteropoly acids. Heteropoly anions Keggin structure exhibit two strong characteristic absorption bands. The bands near 200 and 260 nm are assigned to the Od → M (Od : terminal oxygen) and Ob /Oc → M (Ob or Oc : bridge-oxygen) charge-transfer transition. The structure of complexes, equilibrating cations and substitution elements may have influence on the UV spectra of heteropoly complexes. In the films, PVA acts as an electron donor and HPA as an electron acceptor. In the PVA solution, the two strong characteristic absorption peaks of molybdotungstovanadogermanic acid are 193 and 263 nm (Fig. 3). Fig. 4 shows the UV-Vis spectrum of molybdotungstovanadogermanic heteropoly acid in PVA solution after UV radiation. Different with the UV-Vis
Fig. 3. The UV-Vis spectrum of HPA in PVA solution before UV radiation.
Y. Cui et al. / Materials Chemistry and Physics 85 (2004) 416–419
419
cate they can be used as a promising material in the coming future.
Acknowledgements The financial support from the National Natural Science Foundation of China under Grant No. 20271045, 20006013 and the Foundation of International Cooperation Project of the Ministry of Science and Technology under Grant No. 014-08 for this work is greatly appreciated. References
Fig. 4. The UV-Vis spectrum of HPA in PVA solution after UV radiation.
spectrum of 12-tungstogermanic acid in PVA, the molybdotungstovanadogermanic acid in PVA has no phenomenon of photochromism.
4. Conclusions IR spectra and UV spectra showed that GeW9 Mo2 VO40 5− anion (Keggin structure) exists in the PVA films. In the infrared spectra of the films, each peak assigned to different vibration modes was confirmed, and the regularity of the change of characteristic peaks as a function of the HPA content was described. The conductivity of films doped with 80% molybdotungstovanadogermanic heteropoly acid is 9.92 × 10−3 S cm−1 at room temperature (20 ◦ C). The studies on the conductivity of the films indi-
[1] E. Coronado, C.J. Gómez-Garc´ıa, Chem. Rev. 98 (1998) 273. [2] C.R. Mayer, V. Cabuil, T. Lalot, R. Thouenot, Angew. Chem. Int. Ed. 38 (1999) 3672. [3] K. Nakane, T. Yamashita, K. Iwakura, F. Suzuki, J. Appl. Polym. Sci. 74 (1999) 133. [4] T.Y. Zhang, W. Feng, R. Lu, X.T. Zhang, M. Jin, T.J. Li, Y.Y. Zhao, J.N. Yao, Thin Solid Films 402 (2002) 237. [5] U.L. Stangar, N. Groseij, B. Orel, A. Schmitz, P. Colomban, Solid State Ionics 145 (2001) 109. [6] A. Muller, P. Kogerler, C.J. Kuhlmann, J. Chem. Soc., Chem. Commun. (1999) 1347. [7] J. Gong, Z.M. Dai, R.S. Wang, L.Y. Qu, Synth. Met. 101 (1999) 751. [8] J. Gong, X.J. Cui, Z.W. Xie, S.G. Wang, L.Y. Qu, Synth. Met. 129 (2002) 187. [9] T.F. Otero, S.A. Cheng, E. Coronado, E.M. Ferrero, C.J. GómezGar´ıa, Chem Phys Chem (2002) 808. [10] J. Gong, X.D. Li, B. Ding, D.R. Lee, H.Y. Kim, J. Appl. Polym. Sci. 89 (2003) 1573. [11] Q.Y. Wu, H.B. Wang, C.S. Yin, G.Y. Meng, Mater. Lett. 50 (2001) 61. [12] H.Y. Zhang, L. Xu, E.B. Wang, M. Jiang, A.G. Wu, Z. Li, Mater. Lett. 57 (2003) 1417. [13] Q.Y. Wu, H.H. Lin, G.Y. Meng, Mater. Lett. 39 (1999) 129. [14] A. Teze, G. Herve, J. Inorg. Nucl. Chem. 39 (1977) 999. [15] Q.Y. Wu, H.H. Lin, G.Y. Meng, J. Solid State Chem. 148 (1999) 419.