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Preparation and performance of PVA films composited with 12-tungstogermanic heteropoly acid
August 2001
Materials Letters 50 Ž2001. 61–65 www.elsevier.comrlocatermatlet
Preparation and performance of PVA films composited with 12-tungstogermanic heteropoly acid Qingyin Wu a,) , Huaibing Wang b, Chuanshi Yin b, Guangyao Meng b b
a Department of Chemistry, Zhejiang UniÕersity, Hangzhou 310027, China Department of Materials Science and Engineering, UniÕersity of Science and Technology of China, Hefei 230026, China
Received 30 June 2000; received in revised form 10 November 2000; accepted 14 November 2000
Abstract PVA films containing 12-tungstogermanic heteropoly acid have been successfully prepared for the first time. Infrared 4y ŽIR. spectra revealed that the Keggin structure characteristic of the GeW12 O40 anion was present in the PVA films. The characteristic IR peaks have been identified and their change in wave number interpreted as a function of heteropoly acid ŽHPA. content. The film exhibited considerably high proton conductivity, which increased with the increase of the 12-tungstogermanic heteropoly acid content. At room temperature Ž208C., the conductivity of the PVA film containing 80 wt.% 12-tungstogermanic heteropoly acid is 2.11 = 10y2 S cmy1. The photochromism of the PVA film was observed. Its color changed into blue. q 2001 Elsevier Science B.V. All rights reserved. Keywords: PVA; Films; 12-tungstogermanic heteropoly acid; Conductivity; Photochromism
1. Introduction Heteropoly acids ŽHPA. such as H 3 PW12 O40 P nH 2 O and H 3 PMo 12 O40 P nH 2 O ŽKeggin structure. have been attracted a lot of attention because of their high proton conductivity w1x and their potential application as solid electrolyte in hydrogen–oxygen fuel cells, electrochromic displays, desiccators, Hq sensors at low temperature, solid modified electrodes, etc. w2,3x. The structural water in heteropoly acid crystal obviously shows thermal movements. The study on 1 H NMR of heteropoly acids shows that the protons are internally transferred by hydrogen bonds quickly. Although H 3 PW12 O 40 P n H 2 O and )
Corresponding author. E-mail address: [email protected] ŽQ. Wu..
H 3 PMo 12 O40 P nH 2 O possess a high level of conductivity, a crucial problem is encountered in their application. The problem is the loss of crystal water which makes the conductivity decrease quickly. So, the preparation of novel HPA materials with good stability and high proton conductivity is important. It is also important to prepare protonic conducting films that can be applied in commercial electrochemistry apparatuses. As known, polyŽvinyl alcohol. PVA shows excellent insulation performance as a polymer materials. For pure dry materials, its conductivity can reach 10y1 0 –10y14 S cmy1 w4x. In these macromolecule materials, the doping impurity ions act as the primary electric carrier. In this work, the preparation and performance of PVA films composited with 12-tungstogermanic heteropoly acid is reported.
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 4 1 4 - 6
62
Q. Wu et al.r Materials Letters 50 (2001) 61–65
2. Experimental 2.1. Synthesis The purity of PVA is more than 99.5%. The 12-tungstogermanic heteropoly acid H 4 GeW12 O40 P 14H 2 O was prepared according to the literature w5x. PVA Ž0.5 g. was dissolved in 200 ml of boiling water, then appropriate amounts of H 4 GeW12 O40 P 14H 2 O were added to the solution, and the mixture was stirred strongly until complete dissolution of HPA powder. The solution was vaporized at 508C until the solution looked slimy. The viscous solution was equably spread over the plastic flat and kept in the oven at 408C until it was transformed into film Žabout 10 h.. Three films with different compositions were prepared: 50, 67, and 80 wt.%. Transparent films were obtained for each composition. 2.2. Characterization Infrared ŽIR. spectra were recorded on a Bruker VECTOR-22 FTrIR spectrometer in the wave number range 400–2000 cmy1 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 spectra were investigated on a Shimazdu UV-2100 UV spectrometer in the range 190–800 nm.
3. Results and discussion 3.1. Infrared spectra Fig. 1 shows the infrared spectra ŽIR. of the films with different compositions. Each film doped with the 12-tungstogermanic acid exhibits six characteristic peaks of the Keggin anion, which are also observed in the spectrum of the pure 12-tungstogermanic acid crystal. The Keggin structure of 4y GeW12 O40 consists of one GeO4 tetrahedron surrounded by four W3 O13 sets formed by three edgesharing octahedra. The W3 O 13 sets are linked together through oxygen atoms. Thus, there are four
4y , four Ge–Oa in kinds of oxygen atoms in GeW12 O40 which oxygen atom connects with heteroatom, 12 W–O b –W oxygen-bridges Žcorner-sharing oxygenbridge between different W3 O13 sets., 12 W–Oc –W oxygen-bridges Žedge-sharing oxygen-bridge within W3 O 13 sets. and 12 W–Od terminal oxygen atoms w6x. In the IR spectrum of the pure 12-tungstogermanic heteropoly acid crystal, there are six characteristic bands: 977 cmy1 , n as ŽW–Od .; 890 cmy1 , n as ŽW–O b –W.; 826 and 533 cmy1 , n as ŽGe–Oa .; 772 cmy1 , n as ŽW–Oc –W.; 462 cmy1 , d ŽO–Ge–O. w7x. The wave number of the W–Od asymmetrical stretching vibration increased from 972.6 to 976.7 cmy1 with 12-tungstogermanic acid content from 50% to 80%. Generally, the W–Od stretching can be considered as pure vibration, which characteristic wave number is an increasing function of the anion–anion interaction. The W–Od asymmetrical stretching frequency of the films with 12-tungstogermanic acid wt.% from 80% to 50% decreases from 972.6 to 976.7 cmy1 . This 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 O b or Oc atoms are different from W–Od stretching and they present some bending character. This can be inferred from geometrical considerations. Because W–O b –W and W–Oc –W 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 w8x. 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 W–O b –W. So, W–Oc –W 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 W–Oc –W asymmetrical stretching vibrations decreases from 788.0 to 775.2 cmy1 . Because the GeO4 tetrahedron is assumed to vibrate almost independently, the Ge–Oa
Q. Wu et al.r Materials Letters 50 (2001) 61–65
63
Fig. 1. IR spectra of PVA films with: Ža. 80 wt.% HPA; Žb. 67 wt.% ; Žc. 50 wt.%.
stretching vibration and O–Ge–O bending vibration show no relation with the dopant wt.%. 3.2. UV spectra There is a strong absorption band in UV region resulting from the pe–de transition of W–O bonds in the heteropoly acids. Heteropoly anions ŽKeggin structure. exhibit two strong characteristic absorption bands. The bands near 200 and 260 nm are assigned to the OdWW ŽOd : terminal oxygen. and O brOcWW ŽO b or Oc : bridge-oxygen. charge–transfer transition
w9x. The structure of complexes, equilibrating cations and substitution elements may have influence on the UV spectra of heteropoly complexes. In the PVA solution, the two strong characteristic absorption peaks of 12-tungstogermanic acid are 198.9 and 265.0 nm ŽFig. 2., respectively. 3.3. ConductiÕity In the studied films, there are a number of hydrogen bonds among H 4 GeW12 O40 , PVA and water. 4y The volume of GeW12 O40 is so big that the anions
64
Q. Wu et al.r Materials Letters 50 (2001) 61–65
Fig. 2. UV spectrum of HPA in PVA solution.
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 12tungstogermanic heteropoly acid content. This behaviour results from the increment of proton density with the increase of HPA wt.%. 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 H 3 PW12 O40 P nH 2 O and H 3 PMo 12 O40 P nH 2 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 Ž208C., the conductivity of the three samples increased from 2.84 = 10y3 to 2.11 = 10y2 S cmy1 ŽFig. 3., which is much higher than that of the pure PVA films and pure 12-tungstogermanic heteropoly acid w10x. The results indicated that the PVA film, composited with
Fig. 4. Visible spectra of a solution of PVA with 80 wt.% 12-tungstogermanic acid: Ža. before and Žb. after UV radiation.
12-tungstogermanic acid, is a new kind of excellent high-proton conductor. 3.4. Photochromic performance In the films, the effect of the structural water on protons is so weak that the hydrogen bond conjunction between heteropoly acid and PVA becomes stronger. The protons in PVA framework are distracted to HPA, and the number of the valence 4y electron of GeW12 O40 anion increases w11x. In the films, PVA acts as an electron donor and HPA as an electron acceptor. Under UV radiation, reduction occurs due to the transfer of active hydrogen protons in the hydroxyl groups from PVA to HPA, e.g. W VI is reduced into W V. Its color changes into blue. This phenomenon can also be observed in the PVA solution containing 80 wt.% 12-tungstogermanic acid. Under ultraviolet radiation, the heteropoly anion in PVA solution has been also reduced ŽFig. 4..
4. Conclusions
Fig. 3. The relation between conductivity at 208C and HPA wt.%.
4y IR spectra and UV spectra showed that GeW12 O40 anion ŽKeggin 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% 12-tungstogermanic heteropoly acid is 2.11 = 10y2 S cmy1 at room temperature Ž208C.. The studies on the conductivity and photochromism of the films indicate they
Q. Wu et al.r Materials Letters 50 (2001) 61–65
can be used as a promising material in the coming future.
Acknowledgements We greatly appreciate financial supports from the Science Foundation of Zhejiang University and China Postdoctoral Science Foundation.
References w1x O. Nakamura, T. Kodama, I. Ogino, Y. Miyake, Chem. Lett. 1 Ž1979. 17.
65
w2x M.T. Pope, A. Muller, Angew. Chem., Int. Ed. Engl. 30 ¨ Ž1991. 34. w3x O. Nakamura, I. Ogino, T. Kadama, Solid State Ionics 3–4 Ž1981. 347. w4x S.S. Zhu, L.H. Tang, B. Yue, B.F. Lin, Acta Chim. Sin. 57 Ž1999. 533. w5x Q.Y. Wu, S.W. Tao, H.H. Lin, G.Y. Meng, Mater. Sci. Eng., B 68 Ž2000. 161. w6x A. Teze, G. Herve, J. Inorg. Nucl. Chem. 39 Ž1977. 999. w7x Q.Y. Wu, S.X. Xu, Y.L. Song, Chin. Chem. Lett. 5 Ž1994. 59. w8x Q.Y. Wu, H.H. Lin, G.Y. Meng, J. Solid State Chem. 148 Ž1999. 419. w9x Q.Y. Wu, E.B. Wang, J.F. Liu, Polyhedron 12 Ž1993. 2563. w10x Q.Y. Wu, S.W. Tao, H.H. Lin, G.Y. Meng, Mater. Chem. Phys. 64 Ž2000. 25. w11x X.S. Cheng, Y.C. Su, C.X. Cheng, Acta Chim. Sin. 58 Ž2000. 277.
Materials Letters 50 Ž2001. 61–65 www.elsevier.comrlocatermatlet
Preparation and performance of PVA films composited with 12-tungstogermanic heteropoly acid Qingyin Wu a,) , Huaibing Wang b, Chuanshi Yin b, Guangyao Meng b b
a Department of Chemistry, Zhejiang UniÕersity, Hangzhou 310027, China Department of Materials Science and Engineering, UniÕersity of Science and Technology of China, Hefei 230026, China
Received 30 June 2000; received in revised form 10 November 2000; accepted 14 November 2000
Abstract PVA films containing 12-tungstogermanic heteropoly acid have been successfully prepared for the first time. Infrared 4y ŽIR. spectra revealed that the Keggin structure characteristic of the GeW12 O40 anion was present in the PVA films. The characteristic IR peaks have been identified and their change in wave number interpreted as a function of heteropoly acid ŽHPA. content. The film exhibited considerably high proton conductivity, which increased with the increase of the 12-tungstogermanic heteropoly acid content. At room temperature Ž208C., the conductivity of the PVA film containing 80 wt.% 12-tungstogermanic heteropoly acid is 2.11 = 10y2 S cmy1. The photochromism of the PVA film was observed. Its color changed into blue. q 2001 Elsevier Science B.V. All rights reserved. Keywords: PVA; Films; 12-tungstogermanic heteropoly acid; Conductivity; Photochromism
1. Introduction Heteropoly acids ŽHPA. such as H 3 PW12 O40 P nH 2 O and H 3 PMo 12 O40 P nH 2 O ŽKeggin structure. have been attracted a lot of attention because of their high proton conductivity w1x and their potential application as solid electrolyte in hydrogen–oxygen fuel cells, electrochromic displays, desiccators, Hq sensors at low temperature, solid modified electrodes, etc. w2,3x. The structural water in heteropoly acid crystal obviously shows thermal movements. The study on 1 H NMR of heteropoly acids shows that the protons are internally transferred by hydrogen bonds quickly. Although H 3 PW12 O 40 P n H 2 O and )
Corresponding author. E-mail address: [email protected] ŽQ. Wu..
H 3 PMo 12 O40 P nH 2 O possess a high level of conductivity, a crucial problem is encountered in their application. The problem is the loss of crystal water which makes the conductivity decrease quickly. So, the preparation of novel HPA materials with good stability and high proton conductivity is important. It is also important to prepare protonic conducting films that can be applied in commercial electrochemistry apparatuses. As known, polyŽvinyl alcohol. PVA shows excellent insulation performance as a polymer materials. For pure dry materials, its conductivity can reach 10y1 0 –10y14 S cmy1 w4x. In these macromolecule materials, the doping impurity ions act as the primary electric carrier. In this work, the preparation and performance of PVA films composited with 12-tungstogermanic heteropoly acid is reported.
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 4 1 4 - 6
62
Q. Wu et al.r Materials Letters 50 (2001) 61–65
2. Experimental 2.1. Synthesis The purity of PVA is more than 99.5%. The 12-tungstogermanic heteropoly acid H 4 GeW12 O40 P 14H 2 O was prepared according to the literature w5x. PVA Ž0.5 g. was dissolved in 200 ml of boiling water, then appropriate amounts of H 4 GeW12 O40 P 14H 2 O were added to the solution, and the mixture was stirred strongly until complete dissolution of HPA powder. The solution was vaporized at 508C until the solution looked slimy. The viscous solution was equably spread over the plastic flat and kept in the oven at 408C until it was transformed into film Žabout 10 h.. Three films with different compositions were prepared: 50, 67, and 80 wt.%. Transparent films were obtained for each composition. 2.2. Characterization Infrared ŽIR. spectra were recorded on a Bruker VECTOR-22 FTrIR spectrometer in the wave number range 400–2000 cmy1 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 spectra were investigated on a Shimazdu UV-2100 UV spectrometer in the range 190–800 nm.
3. Results and discussion 3.1. Infrared spectra Fig. 1 shows the infrared spectra ŽIR. of the films with different compositions. Each film doped with the 12-tungstogermanic acid exhibits six characteristic peaks of the Keggin anion, which are also observed in the spectrum of the pure 12-tungstogermanic acid crystal. The Keggin structure of 4y GeW12 O40 consists of one GeO4 tetrahedron surrounded by four W3 O13 sets formed by three edgesharing octahedra. The W3 O 13 sets are linked together through oxygen atoms. Thus, there are four
4y , four Ge–Oa in kinds of oxygen atoms in GeW12 O40 which oxygen atom connects with heteroatom, 12 W–O b –W oxygen-bridges Žcorner-sharing oxygenbridge between different W3 O13 sets., 12 W–Oc –W oxygen-bridges Žedge-sharing oxygen-bridge within W3 O 13 sets. and 12 W–Od terminal oxygen atoms w6x. In the IR spectrum of the pure 12-tungstogermanic heteropoly acid crystal, there are six characteristic bands: 977 cmy1 , n as ŽW–Od .; 890 cmy1 , n as ŽW–O b –W.; 826 and 533 cmy1 , n as ŽGe–Oa .; 772 cmy1 , n as ŽW–Oc –W.; 462 cmy1 , d ŽO–Ge–O. w7x. The wave number of the W–Od asymmetrical stretching vibration increased from 972.6 to 976.7 cmy1 with 12-tungstogermanic acid content from 50% to 80%. Generally, the W–Od stretching can be considered as pure vibration, which characteristic wave number is an increasing function of the anion–anion interaction. The W–Od asymmetrical stretching frequency of the films with 12-tungstogermanic acid wt.% from 80% to 50% decreases from 972.6 to 976.7 cmy1 . This 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 O b or Oc atoms are different from W–Od stretching and they present some bending character. This can be inferred from geometrical considerations. Because W–O b –W and W–Oc –W 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 w8x. 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 W–O b –W. So, W–Oc –W 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 W–Oc –W asymmetrical stretching vibrations decreases from 788.0 to 775.2 cmy1 . Because the GeO4 tetrahedron is assumed to vibrate almost independently, the Ge–Oa
Q. Wu et al.r Materials Letters 50 (2001) 61–65
63
Fig. 1. IR spectra of PVA films with: Ža. 80 wt.% HPA; Žb. 67 wt.% ; Žc. 50 wt.%.
stretching vibration and O–Ge–O bending vibration show no relation with the dopant wt.%. 3.2. UV spectra There is a strong absorption band in UV region resulting from the pe–de transition of W–O bonds in the heteropoly acids. Heteropoly anions ŽKeggin structure. exhibit two strong characteristic absorption bands. The bands near 200 and 260 nm are assigned to the OdWW ŽOd : terminal oxygen. and O brOcWW ŽO b or Oc : bridge-oxygen. charge–transfer transition
w9x. The structure of complexes, equilibrating cations and substitution elements may have influence on the UV spectra of heteropoly complexes. In the PVA solution, the two strong characteristic absorption peaks of 12-tungstogermanic acid are 198.9 and 265.0 nm ŽFig. 2., respectively. 3.3. ConductiÕity In the studied films, there are a number of hydrogen bonds among H 4 GeW12 O40 , PVA and water. 4y The volume of GeW12 O40 is so big that the anions
64
Q. Wu et al.r Materials Letters 50 (2001) 61–65
Fig. 2. UV spectrum of HPA in PVA solution.
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 12tungstogermanic heteropoly acid content. This behaviour results from the increment of proton density with the increase of HPA wt.%. 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 H 3 PW12 O40 P nH 2 O and H 3 PMo 12 O40 P nH 2 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 Ž208C., the conductivity of the three samples increased from 2.84 = 10y3 to 2.11 = 10y2 S cmy1 ŽFig. 3., which is much higher than that of the pure PVA films and pure 12-tungstogermanic heteropoly acid w10x. The results indicated that the PVA film, composited with
Fig. 4. Visible spectra of a solution of PVA with 80 wt.% 12-tungstogermanic acid: Ža. before and Žb. after UV radiation.
12-tungstogermanic acid, is a new kind of excellent high-proton conductor. 3.4. Photochromic performance In the films, the effect of the structural water on protons is so weak that the hydrogen bond conjunction between heteropoly acid and PVA becomes stronger. The protons in PVA framework are distracted to HPA, and the number of the valence 4y electron of GeW12 O40 anion increases w11x. In the films, PVA acts as an electron donor and HPA as an electron acceptor. Under UV radiation, reduction occurs due to the transfer of active hydrogen protons in the hydroxyl groups from PVA to HPA, e.g. W VI is reduced into W V. Its color changes into blue. This phenomenon can also be observed in the PVA solution containing 80 wt.% 12-tungstogermanic acid. Under ultraviolet radiation, the heteropoly anion in PVA solution has been also reduced ŽFig. 4..
4. Conclusions
Fig. 3. The relation between conductivity at 208C and HPA wt.%.
4y IR spectra and UV spectra showed that GeW12 O40 anion ŽKeggin 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% 12-tungstogermanic heteropoly acid is 2.11 = 10y2 S cmy1 at room temperature Ž208C.. The studies on the conductivity and photochromism of the films indicate they
Q. Wu et al.r Materials Letters 50 (2001) 61–65
can be used as a promising material in the coming future.
Acknowledgements We greatly appreciate financial supports from the Science Foundation of Zhejiang University and China Postdoctoral Science Foundation.
References w1x O. Nakamura, T. Kodama, I. Ogino, Y. Miyake, Chem. Lett. 1 Ž1979. 17.
65
w2x M.T. Pope, A. Muller, Angew. Chem., Int. Ed. Engl. 30 ¨ Ž1991. 34. w3x O. Nakamura, I. Ogino, T. Kadama, Solid State Ionics 3–4 Ž1981. 347. w4x S.S. Zhu, L.H. Tang, B. Yue, B.F. Lin, Acta Chim. Sin. 57 Ž1999. 533. w5x Q.Y. Wu, S.W. Tao, H.H. Lin, G.Y. Meng, Mater. Sci. Eng., B 68 Ž2000. 161. w6x A. Teze, G. Herve, J. Inorg. Nucl. Chem. 39 Ž1977. 999. w7x Q.Y. Wu, S.X. Xu, Y.L. Song, Chin. Chem. Lett. 5 Ž1994. 59. w8x Q.Y. Wu, H.H. Lin, G.Y. Meng, J. Solid State Chem. 148 Ž1999. 419. w9x Q.Y. Wu, E.B. Wang, J.F. Liu, Polyhedron 12 Ž1993. 2563. w10x Q.Y. Wu, S.W. Tao, H.H. Lin, G.Y. Meng, Mater. Chem. Phys. 64 Ž2000. 25. w11x X.S. Cheng, Y.C. Su, C.X. Cheng, Acta Chim. Sin. 58 Ž2000. 277.