- Email: [email protected]
Structural properties and proton conductivity of the 12-tungstophosphoric acid doped aluminosilicate gels
Solid State Ionics 125 (1999) 417–424 www.elsevier.com / locate / ssi
Structural properties and proton conductivity of the 12-tungstophosphoric acid doped aluminosilicate gels U.B. Miocˇ a , *, S.K. Milonjic´ b , V. Stamenkovic´ a , M. Radojevic´ c , Ph. Colomban d , M.M. Mitrovic´ e , R. Dimitrijevic´ f a
Faculty of Physical Chemistry, University of Belgrade, P.O. Box 137, Yu-11001 Belgrade, Yugoslavia b Institute of Nuclear Sciences, P.O.Box 522, Yu-11001 Belgrade, Yugoslavia c Institute of General and Physical Chemistry, P.O. Box 551, Yu-11001 Belgrade, Yugoslavia d Laboratoire de Dynamique Interaction et Reactivite´ , CNRS, 2 rue Henri Dunant, 94320 Thiais, France e Faculty of Physics, University of Belgrade, P.O. Box 368, Yu-11001 Belgrade, Yugoslavia f ˇ 7, 11000 Belgrade, Yugoslavia Faculty of Mining and Geology, Department of Crystallography, University of Belgrade, Djusina
Abstract Preparation of aluminosilicate gels bulk, containing 12-tungstophosphoric acid (WPA) in mesopores, as well as their structural and conduction characteristics are reported. Doped gels are prepared from aluminum–silicon alkoxide AlSi(OR)x , iso-propanol, water and WPA ? 29H 2 O mixtures. Characteristics of the obtained doped gels depend on the WPA content. Results such as WPA-filled mesopores and infra red (IR) band shifts indicate differences with respect to material obtained by mechanical alloying and interaction between the WPA and aluminosilicate host framework. According to the obtained results, especially of the higher conductivity (s | 10 23 S / cm), WPA-doped aluminosilicate gel is a promising material for solid electrolytes. 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Protonic conductivity; Sol; Gel Materials: Aluminosilicate alkoxide; 12-Tungstophosphoric acid
1. Introduction The heteropolyacids (HPA) are superionic protonic conductors at room temperature, and, therefore, they are promising materials for potential applications as solid electrolytes in sensors, fuel cells etc. These compounds are sensitive to surrounding conditions: relative humidity, partial pressure of hydrogen and temperature [1]. *Corresponding author. Fax: 1 38-11-118-7133. ˇ E-mail address: [email protected] (U.B. Mioc)
The sol–gel process is one of the most practical synthetic routes for porous matrix preparation because of the advantage of being able to make thermally and chemically stable solid, with a nanoscale pore diameter and narrow pore size distribution [3]. These matrices offer new possibilities in the field of solid state ionics by increasing the stability and conductivity of various inorganic material films on gel matrices. A 12-tungstophosphoric acid (WPA) has been incorporated in optically clear gel films to get chromogenic film displays [4]. The conductivity of a-layered zirconium phosphate (a-ZrP) has been
0167-2738 / 99 / $ – see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S0167-2738( 99 )00204-0
418
U.B. Miocˇ et al. / Solid State Ionics 125 (1999) 417 – 424
enhanced by mixing colloidal dispersions of delaminated a-ZrP with silica [7]. Our previous paper [9] reported silica gel containing different WPA content. Papers referred there deal with pure silica gel or TiO 2 gel as host framework. The aim of this contribution is to give a better understanding of the process of gelation of WPA-doped aluminosilicate gel, its surface, pore structure and its influence on the proton conductivity of these new materials.
2. Experimental
2.1. Synthesis Aluminosilicate gels with a ratio of Al / Si 5 1 (i.e., Al 2 O 3 2SiO 2 ) containing different amounts of WPA were prepared from mixtures of water, isopropanol-2, AlSi(OR) x alkoxide and WPA ? 29H 2 O. The water–solvent–alkoxide volume ratio was 1:4:2. Two volumes of solvent and one volume of water with the proper amount of WPA ? 29H 2 O, were slowly poured into a mixture of two volumes of solvent and two volumes of alkoxide. The solution was rapidly stirred. The molar water / alkoxide ratio was 10. The process of drying for all samples was performed in Petri dishes at controlled conditions (temperature 608C, relative humidity 45%) changing the WPA ? 29H 2 0 content from 10, 18, 30 to 50 mass %. Gelation time was 24 h and did not depend on WPA content.
2.2. Techniques The system stability was checked by differential thermal analysis (DTA) and simultaneous thermogravimetry (TG) between 20 and 7008C. Measurements were performed using a StantonRedcroft 1000 instrument in an atmosphere of nitrogen, with a flow-rate of 50 ml / min. The scanning rate was 108C / min. Dimensions, specific surface areas, and total volume of pores were determined by a standard method of nitrogen adsorption at 77 K, using BET isotherm on a Quantachrome instrument model Autosorb 6. IR spectra of samples in KBr pellets were recorded on a Perkin-Elmer 983G spectrophotometer. The X-ray powder diffraction (XRPD) patterns
were obtained by a Philips PW-1710 automated diffractometer, using a Cu tube operated at 40 kV and 35 mA. The instrument was equipped with a diffracted beam curved graphite monochromator and an Xe-filled proportional counter. Diffraction data were collected in the 2u angle range 4–70 counting for 2.5 s at 0.028 steps. Fixed 18 divergence as well as 0.18 receiving slits were used. The investigation of the samples surface morphology was carried out by a scanning electron microscope (SEM), Joel model JSM 840A. The samples were gold sputtered in a JFC 1100 ion sputterer. Impedance measurements of tablet samples were performed on an EG&G Princeton Applied Research Lock-in Analyzer model 5208 coupled with EG&G Princeton Applied Research Potentiostat Galvanostat 233 over the frequency range from 10 Hz to 100 kHz at room temperature. The powder was pressed to a tablet under a pressure of 2.9 ? 10 8 N / m 2 .
3. Results and discussion
3.1. Keggin’ s ion entrapping and homogeneity Our investigations were focused on the influence of the gelation process on the structure and characteristics of gels doped with different contents of WPA. This is necessary for standardization and comparison of gel characteristics with our previous report [9], where these were controlled by the aforementioned experimental techniques. The dimensions of pores mostly correspond to the mesopores (1.3–5.5 nm), valid for all gels [3]. It is evident that Keggin ion according to its size (r | 1.2 nm) could be incorporated into these pores. At a WPA concentration of 50 mass % of the total specific surface area of pores was reduced from 499 to 27.7 m 2 / g, respectively,(Table 1). According to results obtained for a WPA content of 50 mass %, total pore volume of 0.07 cm 3 and specific surface of 27.7 m 2 / g, it could be concluded that almost all mesopores are filled with WPA. The morphology of the gel surface was controlled by SEM, (Fig. 1). In Fig. 1a, pure aluminosilicate gel, repeating optically transparent amorphous blocks can be seen. The aluminosilicate gel with 10 mass % of WPA has the same morphology as pure gel, Fig.
U.B. Miocˇ et al. / Solid State Ionics 125 (1999) 417 – 424
419
Table 1 Gel characteristics Sample (Mass % WPA)
Specific surface area (m 2 / g)
Total pore volume (cm 3 )
Average pore radius (nm)
Aluminosilicate gel Gel 1 WPA 10% Gel 1 WPA 18% Gel 1 WPA 30% Gel 1 WPA 50%
499 405 319 160 27.7
0.312 0.256 0.233 0.159 0.07
1.30 1.35 1.46 1.98 5.05
1b. In the process of doping morphology transformations are evident. For the sample of aluminosilicate gel with 18 mass % of WPA the big blocks break forming smaller tiles, Fig. 1c. Further addition of WPA caused the appearance of clusters of a new phase, Fig. 1d, gel with 30 mass % of WPA. With an increase in WPA concentration the morphology of the aluminosilicate gel with 50 mass % of WPA changes and two phases are obvious: blocks of aluminosilicate and crystals of WPA. The grooves of amorphous aluminosilicate gel are evident and they are filled by crystals of WPA, Fig. 1e.
3.2. WPA–aluminosilicate surface interaction IR spectroscopy was used to define the structure of given gels. IR spectra, Fig. 2, show that in all cases Keggin’s structure of WPA ions is preserved in the aluminosilicate gel skeleton. In the IR spectrum of the pure gel, Fig. 2a, several intense peaks at 1114, 1048, 608, and 450 cm 21 can be seen. The 1114 cm 21 peak is assigned to the Si–O stretching, and the peaks at about 1048 and 608 cm 21 originate from A1 2 O 3 . The band at 450 cm 21 corresponds to Si–O bending vibrations. In the other spectra, besides the bands characteristic for aluminosilicate matrix, vibrational bands of Keggin’s anion are evident [2], (Fig. 2b,c). Systematic shift of Keggin’s anion peaks at 1079, 981, 891 and 814 cm 21 to 1069, 968, 900 and 821 cm 21 for doped samples reveal red and blue frequency shifts of P–O, inter W–O–W, intra W–O–W and terminal W–O stretching modes. These shifts do not exist in the samples prepared as a mechanical mixture of the gels and WPA. The frequency shifts of these modes could be correlated with electrostatic repulsion between Keggin’s ions in various crys-
talline heteropoly acids and salts [10]. Fig. 2 shows that the frequency of terminal W–O mode increases by the other to mode frequency decrease with the increasing concentration of WPA. The reason for that is the change of the distance between oxygen of the neighboring WPA in gels. For matrix bands no systematic shift is observed in the spectra as a function of WPA concentration. Intensities of the bands characteristic for Keggin’s anion increase with increase in WPA concentration. Opposite to our previous results with doped colloidal silica gels [9], the XRPD patterns of doped aluminosilicate gels, Fig. 3, show characteristics of amorphous substance. Figs. 3a–c represent diffractograms of pure aluminosilicate gel and samples doped with 10 and 18 mass % of WPA, respectively. Increasing of WPA content caused the appearance of a very broad peak at 88 Bragg angle, Figs. 3d,e. The origin of this peak could be explained as X-ray diffraction from nanostructured particles of WPA crystallized in the mesopores of aluminosilicate matrix. It could be well compared with corresponding peaks on Fig. 3f. This figure represents a powder pattern of mechanical mixture of 50:50 mass % ratio of crystalline WPA and amorphous aluminosilicate gels, where conditions for x-ray diffraction are much better than in situ crystallized WPA in mesopores of gel Fig. 3e. Thermal stability of gels was controlled by means of thermal analyses (DTA) and (TGA) in the temperature range from 22 to 7008C (Fig. 4 and Table 2). In this range pure aluminosilicate gel framework is stable, Fig. 4a. The process of dehydration occurs at 105.58C and 163.88C. In the temperature interval from 20 to 2008C aluminosilica gel looses water molecules, and it can be regarded that water from macro-, meso- and micropores is desorbed complete-
420
U.B. Miocˇ et al. / Solid State Ionics 125 (1999) 417 – 424
Fig. 1. SEM photomicrographs obtained in secondary electron mode: (a) pure aluminosilicate gel, (b) aluminosilicate gel with 10 mass % of WPA, (c) aluminosilicate gel with 18 mass % of WPA, (d) aluminosilicate gel with 30 mass % of WPA and (e) aluminosilicate gel with 50 mass % of WPA.
U.B. Miocˇ et al. / Solid State Ionics 125 (1999) 417 – 424
421
Fig. 3. XRPD patterns: (a) pure aluminosilicate gel, (b) aluminosilicate gel with 10 mass % of WPA, (c) aluminosilicate gel with 18 mass % of WPA (d) aluminosilicate gel with 30 mass % of WPA, (e) aluminosilicate gel with 50 mass % of WPA and (f) mechanical mixtures of aluminosilicate gel with 50 mass % of WPA.
Fig. 2. IR spectra of samples: (a) pure aluminosilicate gel, (b) doped aluminosilicate gel with 30 mass % of WPA, (c) doped aluminosilicate gel with 50 mass % of WPA and (d) pure WPA ? 29H 2 O.
ly. Regarding the obtained results, the thermally analysed molecular formula of aluminosilicate gel is Al 2 O 3 2SiO 2 ? 7H 2 O. Below 2008C, the pure aluminosilicate gel samples exhibit two ( | 1058 and | 1658C) endothermic transformations which indicates two different types of water molecules physisorbed and chemisorbed (Fig. 4a, Table 2). It is obvious that the temperature of the first endothermic transition decreases with an increase of WPA content and that the other transition is not present for the WPA contents higher than 18 mass %. It could be concluded that the number of endothermic transformations depends on the WPA concentration and only one characteristic temperature ( | 958C) is evident for the higher WPA concentration, (Fig. 4c, Table 2). According to BET measurements the increase of number of dopant mole-
cules reduces total pore volume, which prevents the presence of two types of water. Exothermic transitions on samples with 30 and 50 mass % of WPA correspond to the solid– solid re-crystallization of WPA to phosphorous bronzes [2] (Fig. 4c, Table 2). This process occurs at 5708C, below the temperature of the transformation for pure WPA at 6018C which indicates a catalytic effect of aluminosilicate matrix similar as in our previous investigations [9]. From Table 2, it is obvious that total mass loss depends on WPA concentration. Increase of the WPA content decreases total mass loss. It is expected because total mass loss of pure WPA ? 29H 2 O in this temperature interval is about 13% [2].
3.3. Conductivity Impendance conductivity measurements show that WPA is more stable when incorporated in aluminosilicate gels. Some of doped gels have conductivity higher by two orders of magnitude (a (1 ? 10 23 S / cm) than pure gel (s 5 1.3 ? 10 25 S / cm) and eight orders higher than WPA ? 6H 2 O (s 5 3.0 ? 10 211 S / cm) [11], under the same experimental conditions. The presence of higher hydrates in traces, or the saturated solution forming a film on the aluminosilicate gel surface could be the
422
U.B. Miocˇ et al. / Solid State Ionics 125 (1999) 417 – 424
Fig. 4. DTA and TGA curves of: (a) pure aluminosilicate gel, (b) doped aluminosilicate gel with 18 mass % of WPA ? 29H 2 O and (c) doped aluminosilicate gel with 50 mass % of WPA ? 29H 2 O.
U.B. Miocˇ et al. / Solid State Ionics 125 (1999) 417 – 424
423
Table 2 Thermal analysis data Sample (Mass% of WPA) Total mass loss (%)
Gel
Gel 1 WPA 10%
Gel 1 WPA 18%
Gel 1 WPA 30%
Gel 1 WPA 50%
WPA (29H 2 O)
36.6
35.8
31.2
28.9
23.8
13
Endothermic peaks (8C)
105.5 163.8
101.7 157.6
97.0 150.1
92.6 2
94.2 2
50 170
Exothermic peaks (8C)
2
2
2
578.3
570.1
601
reason of higher conductivity of WPA-doped aluminosilicate gel. The relation between conductivity at room temperature and mass % of dopant, for samples stabilized at 608C from alkoxide / isopropanol / water / WPA mixtures, is shown in Fig. 5. It is obvious that the conductivity increases from 1.0 ? 10 25 to 1.1 ? 10 23 S / cm as dopant mass increases. Straight line extrapolation to 100 mass % WPA gives the conductivity of 29 WPA (s | 0.1 S / cm). The results obtained lead to the conclusion that WPA is well protected by the aluminosilicate matrix. The same phenomenon of higher conductivity was detected previously [4]. In comparison with our results obtained on colloidal silica [9], aluminosilicate gels
have lower conductivity caused by different matrix properties. The higher conductivity in silica gels can be related to the pH of the surface: silica is relatively acidic (pH 5 5) although the presence of alumina (pure alumina surface pH 5 9) increases the surface pH which can entrap the proton. The slope on Fig. 5. is perhaps correlated to the surface characteristics of the framework. Conductivity enhancement of the composite conductors could be interpreted according to defect chemistry in heterogeneous systems [12], in which space–charge regions are responsible for pronounced conductivity increase. These polycrystalline and multiphase systems show formation of extra defect in the space charge regions as a consequence of interface processes (internal charge transfer). Internal defect chemical interactions between defects (such as proton and proton vacancy) have a significant kinetic (buffer) effect on chemical diffusion and ionic conductivity.
4. Conclusion
Fig. 5. Conductivity of doped aluminosilicate gels (\) and silica gels (h) [8] as a function of WPA ? 29H 2 O concentration. Conductivity of different WPA hydrates (.): WPA ? 29H 2 O (s 5 8.0 ? 10 22 S / cm), WPA ? 21H 2 O (s 5 1.0 ? 10 22 S / cm) and WPA ? 14H 2 O (s 5 5.0 ? 10 24 S / cm) are also reported [10].
This work contributes to the clarification of the nature of WPA incorporated in porous matrix ensuring the reproducible synthesis of these gels as good protonic conductors. The obtained results, as well as previous ones on silica gel [9], confirmed that the sol–gel process is a promising way for obtaining materials with new characteristics suitable for solid state ionics applications. The knowledge of the molecular precursors, their structure and morphology as well as the gelation process allow a better control of the characteristics of doped silica gels. Results of this work show that WPA is stabilized within the aluminosilicate skeleton resembling the stabilization
424
U.B. Miocˇ et al. / Solid State Ionics 125 (1999) 417 – 424
effect of silica gel [9]. In both cases two forms of WPA are found: the free one and the ‘hemisorbed’.
Acknowledgements This work was partially supported by the Ministry of Science and Technology, Belgrade, Serbia.
References [1] O. Nakamura, T. Kodoma, J. Ogino, Y. Miyake, Chem. Lett. (1979) 17. ˇ R. Dimitrijevic, ´ M. Davidovic, ´ Z.P. Nedic, ´ M. [2] U. Mioc, ´ Ph. Colomban, J. Mater. Sci. 29 (1994) 3705. Mitrovic, [3] V. Vendange, Ph. Colomban, J. Mater Res. 11 (1996) 518. [4] M. Tatsumisago, K. Kishida, T. Minami, Solid State Ionics 59 (1993) 171.
[5] M. Tatsumisago, H. Honjo, Y. Salsai, T. Minmi, Solid State Ionics 74 (1994) 105. ˇˇ [6] U. Lovrencic-Stangar, B. Orel, A. Regis, Ph. Colomban, J. Sol.–Gel. Sci. Tecnol. 8 (1997) 965. [7] R.C.T. Slade, H. Jinku, J.A. Knowles, Solid State Ionics 50 (1992) 287. [8] G. Alberti, M. Casciola, U. Costantino, A. Peraio, T. Rega, J. Mater. Chem. 5 (1995) 1809. ˇ S.K. Milonjic, ´ D. Malovic, ´ V. Stamenkovic, ´ Ph. [9] U.B. Mioc, Colomban, M.M. Mitrovic´ et al., Solid State Ionics 97 (1997) 239. ˇˇ [10] B. Orel, U. Lavrencic-Stangar, M.G. Hutchins, K. Kalcher, J. Non-Cryst. Solids 175 (1994) 251. ˇ M. Davidovic, ´ N. Tjapkin, Ph. Colomban, Solid [11] U. Mioc, State Ionics 46 (1991) 103. [12] J. Maier, J. Electrochem. Soc. 134 (1987) 1524. [13] Y. Saito, J. Maier, J. Electrochem. Soc. 142 (1995) 3078. [14] J. Maier, Solid State Ionics 75 (1995) 139. [15] J. Maier, Solid State Ionics 86–88 (1996) 55.
Structural properties and proton conductivity of the 12-tungstophosphoric acid doped aluminosilicate gels U.B. Miocˇ a , *, S.K. Milonjic´ b , V. Stamenkovic´ a , M. Radojevic´ c , Ph. Colomban d , M.M. Mitrovic´ e , R. Dimitrijevic´ f a
Faculty of Physical Chemistry, University of Belgrade, P.O. Box 137, Yu-11001 Belgrade, Yugoslavia b Institute of Nuclear Sciences, P.O.Box 522, Yu-11001 Belgrade, Yugoslavia c Institute of General and Physical Chemistry, P.O. Box 551, Yu-11001 Belgrade, Yugoslavia d Laboratoire de Dynamique Interaction et Reactivite´ , CNRS, 2 rue Henri Dunant, 94320 Thiais, France e Faculty of Physics, University of Belgrade, P.O. Box 368, Yu-11001 Belgrade, Yugoslavia f ˇ 7, 11000 Belgrade, Yugoslavia Faculty of Mining and Geology, Department of Crystallography, University of Belgrade, Djusina
Abstract Preparation of aluminosilicate gels bulk, containing 12-tungstophosphoric acid (WPA) in mesopores, as well as their structural and conduction characteristics are reported. Doped gels are prepared from aluminum–silicon alkoxide AlSi(OR)x , iso-propanol, water and WPA ? 29H 2 O mixtures. Characteristics of the obtained doped gels depend on the WPA content. Results such as WPA-filled mesopores and infra red (IR) band shifts indicate differences with respect to material obtained by mechanical alloying and interaction between the WPA and aluminosilicate host framework. According to the obtained results, especially of the higher conductivity (s | 10 23 S / cm), WPA-doped aluminosilicate gel is a promising material for solid electrolytes. 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Protonic conductivity; Sol; Gel Materials: Aluminosilicate alkoxide; 12-Tungstophosphoric acid
1. Introduction The heteropolyacids (HPA) are superionic protonic conductors at room temperature, and, therefore, they are promising materials for potential applications as solid electrolytes in sensors, fuel cells etc. These compounds are sensitive to surrounding conditions: relative humidity, partial pressure of hydrogen and temperature [1]. *Corresponding author. Fax: 1 38-11-118-7133. ˇ E-mail address: [email protected] (U.B. Mioc)
The sol–gel process is one of the most practical synthetic routes for porous matrix preparation because of the advantage of being able to make thermally and chemically stable solid, with a nanoscale pore diameter and narrow pore size distribution [3]. These matrices offer new possibilities in the field of solid state ionics by increasing the stability and conductivity of various inorganic material films on gel matrices. A 12-tungstophosphoric acid (WPA) has been incorporated in optically clear gel films to get chromogenic film displays [4]. The conductivity of a-layered zirconium phosphate (a-ZrP) has been
0167-2738 / 99 / $ – see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S0167-2738( 99 )00204-0
418
U.B. Miocˇ et al. / Solid State Ionics 125 (1999) 417 – 424
enhanced by mixing colloidal dispersions of delaminated a-ZrP with silica [7]. Our previous paper [9] reported silica gel containing different WPA content. Papers referred there deal with pure silica gel or TiO 2 gel as host framework. The aim of this contribution is to give a better understanding of the process of gelation of WPA-doped aluminosilicate gel, its surface, pore structure and its influence on the proton conductivity of these new materials.
2. Experimental
2.1. Synthesis Aluminosilicate gels with a ratio of Al / Si 5 1 (i.e., Al 2 O 3 2SiO 2 ) containing different amounts of WPA were prepared from mixtures of water, isopropanol-2, AlSi(OR) x alkoxide and WPA ? 29H 2 O. The water–solvent–alkoxide volume ratio was 1:4:2. Two volumes of solvent and one volume of water with the proper amount of WPA ? 29H 2 O, were slowly poured into a mixture of two volumes of solvent and two volumes of alkoxide. The solution was rapidly stirred. The molar water / alkoxide ratio was 10. The process of drying for all samples was performed in Petri dishes at controlled conditions (temperature 608C, relative humidity 45%) changing the WPA ? 29H 2 0 content from 10, 18, 30 to 50 mass %. Gelation time was 24 h and did not depend on WPA content.
2.2. Techniques The system stability was checked by differential thermal analysis (DTA) and simultaneous thermogravimetry (TG) between 20 and 7008C. Measurements were performed using a StantonRedcroft 1000 instrument in an atmosphere of nitrogen, with a flow-rate of 50 ml / min. The scanning rate was 108C / min. Dimensions, specific surface areas, and total volume of pores were determined by a standard method of nitrogen adsorption at 77 K, using BET isotherm on a Quantachrome instrument model Autosorb 6. IR spectra of samples in KBr pellets were recorded on a Perkin-Elmer 983G spectrophotometer. The X-ray powder diffraction (XRPD) patterns
were obtained by a Philips PW-1710 automated diffractometer, using a Cu tube operated at 40 kV and 35 mA. The instrument was equipped with a diffracted beam curved graphite monochromator and an Xe-filled proportional counter. Diffraction data were collected in the 2u angle range 4–70 counting for 2.5 s at 0.028 steps. Fixed 18 divergence as well as 0.18 receiving slits were used. The investigation of the samples surface morphology was carried out by a scanning electron microscope (SEM), Joel model JSM 840A. The samples were gold sputtered in a JFC 1100 ion sputterer. Impedance measurements of tablet samples were performed on an EG&G Princeton Applied Research Lock-in Analyzer model 5208 coupled with EG&G Princeton Applied Research Potentiostat Galvanostat 233 over the frequency range from 10 Hz to 100 kHz at room temperature. The powder was pressed to a tablet under a pressure of 2.9 ? 10 8 N / m 2 .
3. Results and discussion
3.1. Keggin’ s ion entrapping and homogeneity Our investigations were focused on the influence of the gelation process on the structure and characteristics of gels doped with different contents of WPA. This is necessary for standardization and comparison of gel characteristics with our previous report [9], where these were controlled by the aforementioned experimental techniques. The dimensions of pores mostly correspond to the mesopores (1.3–5.5 nm), valid for all gels [3]. It is evident that Keggin ion according to its size (r | 1.2 nm) could be incorporated into these pores. At a WPA concentration of 50 mass % of the total specific surface area of pores was reduced from 499 to 27.7 m 2 / g, respectively,(Table 1). According to results obtained for a WPA content of 50 mass %, total pore volume of 0.07 cm 3 and specific surface of 27.7 m 2 / g, it could be concluded that almost all mesopores are filled with WPA. The morphology of the gel surface was controlled by SEM, (Fig. 1). In Fig. 1a, pure aluminosilicate gel, repeating optically transparent amorphous blocks can be seen. The aluminosilicate gel with 10 mass % of WPA has the same morphology as pure gel, Fig.
U.B. Miocˇ et al. / Solid State Ionics 125 (1999) 417 – 424
419
Table 1 Gel characteristics Sample (Mass % WPA)
Specific surface area (m 2 / g)
Total pore volume (cm 3 )
Average pore radius (nm)
Aluminosilicate gel Gel 1 WPA 10% Gel 1 WPA 18% Gel 1 WPA 30% Gel 1 WPA 50%
499 405 319 160 27.7
0.312 0.256 0.233 0.159 0.07
1.30 1.35 1.46 1.98 5.05
1b. In the process of doping morphology transformations are evident. For the sample of aluminosilicate gel with 18 mass % of WPA the big blocks break forming smaller tiles, Fig. 1c. Further addition of WPA caused the appearance of clusters of a new phase, Fig. 1d, gel with 30 mass % of WPA. With an increase in WPA concentration the morphology of the aluminosilicate gel with 50 mass % of WPA changes and two phases are obvious: blocks of aluminosilicate and crystals of WPA. The grooves of amorphous aluminosilicate gel are evident and they are filled by crystals of WPA, Fig. 1e.
3.2. WPA–aluminosilicate surface interaction IR spectroscopy was used to define the structure of given gels. IR spectra, Fig. 2, show that in all cases Keggin’s structure of WPA ions is preserved in the aluminosilicate gel skeleton. In the IR spectrum of the pure gel, Fig. 2a, several intense peaks at 1114, 1048, 608, and 450 cm 21 can be seen. The 1114 cm 21 peak is assigned to the Si–O stretching, and the peaks at about 1048 and 608 cm 21 originate from A1 2 O 3 . The band at 450 cm 21 corresponds to Si–O bending vibrations. In the other spectra, besides the bands characteristic for aluminosilicate matrix, vibrational bands of Keggin’s anion are evident [2], (Fig. 2b,c). Systematic shift of Keggin’s anion peaks at 1079, 981, 891 and 814 cm 21 to 1069, 968, 900 and 821 cm 21 for doped samples reveal red and blue frequency shifts of P–O, inter W–O–W, intra W–O–W and terminal W–O stretching modes. These shifts do not exist in the samples prepared as a mechanical mixture of the gels and WPA. The frequency shifts of these modes could be correlated with electrostatic repulsion between Keggin’s ions in various crys-
talline heteropoly acids and salts [10]. Fig. 2 shows that the frequency of terminal W–O mode increases by the other to mode frequency decrease with the increasing concentration of WPA. The reason for that is the change of the distance between oxygen of the neighboring WPA in gels. For matrix bands no systematic shift is observed in the spectra as a function of WPA concentration. Intensities of the bands characteristic for Keggin’s anion increase with increase in WPA concentration. Opposite to our previous results with doped colloidal silica gels [9], the XRPD patterns of doped aluminosilicate gels, Fig. 3, show characteristics of amorphous substance. Figs. 3a–c represent diffractograms of pure aluminosilicate gel and samples doped with 10 and 18 mass % of WPA, respectively. Increasing of WPA content caused the appearance of a very broad peak at 88 Bragg angle, Figs. 3d,e. The origin of this peak could be explained as X-ray diffraction from nanostructured particles of WPA crystallized in the mesopores of aluminosilicate matrix. It could be well compared with corresponding peaks on Fig. 3f. This figure represents a powder pattern of mechanical mixture of 50:50 mass % ratio of crystalline WPA and amorphous aluminosilicate gels, where conditions for x-ray diffraction are much better than in situ crystallized WPA in mesopores of gel Fig. 3e. Thermal stability of gels was controlled by means of thermal analyses (DTA) and (TGA) in the temperature range from 22 to 7008C (Fig. 4 and Table 2). In this range pure aluminosilicate gel framework is stable, Fig. 4a. The process of dehydration occurs at 105.58C and 163.88C. In the temperature interval from 20 to 2008C aluminosilica gel looses water molecules, and it can be regarded that water from macro-, meso- and micropores is desorbed complete-
420
U.B. Miocˇ et al. / Solid State Ionics 125 (1999) 417 – 424
Fig. 1. SEM photomicrographs obtained in secondary electron mode: (a) pure aluminosilicate gel, (b) aluminosilicate gel with 10 mass % of WPA, (c) aluminosilicate gel with 18 mass % of WPA, (d) aluminosilicate gel with 30 mass % of WPA and (e) aluminosilicate gel with 50 mass % of WPA.
U.B. Miocˇ et al. / Solid State Ionics 125 (1999) 417 – 424
421
Fig. 3. XRPD patterns: (a) pure aluminosilicate gel, (b) aluminosilicate gel with 10 mass % of WPA, (c) aluminosilicate gel with 18 mass % of WPA (d) aluminosilicate gel with 30 mass % of WPA, (e) aluminosilicate gel with 50 mass % of WPA and (f) mechanical mixtures of aluminosilicate gel with 50 mass % of WPA.
Fig. 2. IR spectra of samples: (a) pure aluminosilicate gel, (b) doped aluminosilicate gel with 30 mass % of WPA, (c) doped aluminosilicate gel with 50 mass % of WPA and (d) pure WPA ? 29H 2 O.
ly. Regarding the obtained results, the thermally analysed molecular formula of aluminosilicate gel is Al 2 O 3 2SiO 2 ? 7H 2 O. Below 2008C, the pure aluminosilicate gel samples exhibit two ( | 1058 and | 1658C) endothermic transformations which indicates two different types of water molecules physisorbed and chemisorbed (Fig. 4a, Table 2). It is obvious that the temperature of the first endothermic transition decreases with an increase of WPA content and that the other transition is not present for the WPA contents higher than 18 mass %. It could be concluded that the number of endothermic transformations depends on the WPA concentration and only one characteristic temperature ( | 958C) is evident for the higher WPA concentration, (Fig. 4c, Table 2). According to BET measurements the increase of number of dopant mole-
cules reduces total pore volume, which prevents the presence of two types of water. Exothermic transitions on samples with 30 and 50 mass % of WPA correspond to the solid– solid re-crystallization of WPA to phosphorous bronzes [2] (Fig. 4c, Table 2). This process occurs at 5708C, below the temperature of the transformation for pure WPA at 6018C which indicates a catalytic effect of aluminosilicate matrix similar as in our previous investigations [9]. From Table 2, it is obvious that total mass loss depends on WPA concentration. Increase of the WPA content decreases total mass loss. It is expected because total mass loss of pure WPA ? 29H 2 O in this temperature interval is about 13% [2].
3.3. Conductivity Impendance conductivity measurements show that WPA is more stable when incorporated in aluminosilicate gels. Some of doped gels have conductivity higher by two orders of magnitude (a (1 ? 10 23 S / cm) than pure gel (s 5 1.3 ? 10 25 S / cm) and eight orders higher than WPA ? 6H 2 O (s 5 3.0 ? 10 211 S / cm) [11], under the same experimental conditions. The presence of higher hydrates in traces, or the saturated solution forming a film on the aluminosilicate gel surface could be the
422
U.B. Miocˇ et al. / Solid State Ionics 125 (1999) 417 – 424
Fig. 4. DTA and TGA curves of: (a) pure aluminosilicate gel, (b) doped aluminosilicate gel with 18 mass % of WPA ? 29H 2 O and (c) doped aluminosilicate gel with 50 mass % of WPA ? 29H 2 O.
U.B. Miocˇ et al. / Solid State Ionics 125 (1999) 417 – 424
423
Table 2 Thermal analysis data Sample (Mass% of WPA) Total mass loss (%)
Gel
Gel 1 WPA 10%
Gel 1 WPA 18%
Gel 1 WPA 30%
Gel 1 WPA 50%
WPA (29H 2 O)
36.6
35.8
31.2
28.9
23.8
13
Endothermic peaks (8C)
105.5 163.8
101.7 157.6
97.0 150.1
92.6 2
94.2 2
50 170
Exothermic peaks (8C)
2
2
2
578.3
570.1
601
reason of higher conductivity of WPA-doped aluminosilicate gel. The relation between conductivity at room temperature and mass % of dopant, for samples stabilized at 608C from alkoxide / isopropanol / water / WPA mixtures, is shown in Fig. 5. It is obvious that the conductivity increases from 1.0 ? 10 25 to 1.1 ? 10 23 S / cm as dopant mass increases. Straight line extrapolation to 100 mass % WPA gives the conductivity of 29 WPA (s | 0.1 S / cm). The results obtained lead to the conclusion that WPA is well protected by the aluminosilicate matrix. The same phenomenon of higher conductivity was detected previously [4]. In comparison with our results obtained on colloidal silica [9], aluminosilicate gels
have lower conductivity caused by different matrix properties. The higher conductivity in silica gels can be related to the pH of the surface: silica is relatively acidic (pH 5 5) although the presence of alumina (pure alumina surface pH 5 9) increases the surface pH which can entrap the proton. The slope on Fig. 5. is perhaps correlated to the surface characteristics of the framework. Conductivity enhancement of the composite conductors could be interpreted according to defect chemistry in heterogeneous systems [12], in which space–charge regions are responsible for pronounced conductivity increase. These polycrystalline and multiphase systems show formation of extra defect in the space charge regions as a consequence of interface processes (internal charge transfer). Internal defect chemical interactions between defects (such as proton and proton vacancy) have a significant kinetic (buffer) effect on chemical diffusion and ionic conductivity.
4. Conclusion
Fig. 5. Conductivity of doped aluminosilicate gels (\) and silica gels (h) [8] as a function of WPA ? 29H 2 O concentration. Conductivity of different WPA hydrates (.): WPA ? 29H 2 O (s 5 8.0 ? 10 22 S / cm), WPA ? 21H 2 O (s 5 1.0 ? 10 22 S / cm) and WPA ? 14H 2 O (s 5 5.0 ? 10 24 S / cm) are also reported [10].
This work contributes to the clarification of the nature of WPA incorporated in porous matrix ensuring the reproducible synthesis of these gels as good protonic conductors. The obtained results, as well as previous ones on silica gel [9], confirmed that the sol–gel process is a promising way for obtaining materials with new characteristics suitable for solid state ionics applications. The knowledge of the molecular precursors, their structure and morphology as well as the gelation process allow a better control of the characteristics of doped silica gels. Results of this work show that WPA is stabilized within the aluminosilicate skeleton resembling the stabilization
424
U.B. Miocˇ et al. / Solid State Ionics 125 (1999) 417 – 424
effect of silica gel [9]. In both cases two forms of WPA are found: the free one and the ‘hemisorbed’.
Acknowledgements This work was partially supported by the Ministry of Science and Technology, Belgrade, Serbia.
References [1] O. Nakamura, T. Kodoma, J. Ogino, Y. Miyake, Chem. Lett. (1979) 17. ˇ R. Dimitrijevic, ´ M. Davidovic, ´ Z.P. Nedic, ´ M. [2] U. Mioc, ´ Ph. Colomban, J. Mater. Sci. 29 (1994) 3705. Mitrovic, [3] V. Vendange, Ph. Colomban, J. Mater Res. 11 (1996) 518. [4] M. Tatsumisago, K. Kishida, T. Minami, Solid State Ionics 59 (1993) 171.
[5] M. Tatsumisago, H. Honjo, Y. Salsai, T. Minmi, Solid State Ionics 74 (1994) 105. ˇˇ [6] U. Lovrencic-Stangar, B. Orel, A. Regis, Ph. Colomban, J. Sol.–Gel. Sci. Tecnol. 8 (1997) 965. [7] R.C.T. Slade, H. Jinku, J.A. Knowles, Solid State Ionics 50 (1992) 287. [8] G. Alberti, M. Casciola, U. Costantino, A. Peraio, T. Rega, J. Mater. Chem. 5 (1995) 1809. ˇ S.K. Milonjic, ´ D. Malovic, ´ V. Stamenkovic, ´ Ph. [9] U.B. Mioc, Colomban, M.M. Mitrovic´ et al., Solid State Ionics 97 (1997) 239. ˇˇ [10] B. Orel, U. Lavrencic-Stangar, M.G. Hutchins, K. Kalcher, J. Non-Cryst. Solids 175 (1994) 251. ˇ M. Davidovic, ´ N. Tjapkin, Ph. Colomban, Solid [11] U. Mioc, State Ionics 46 (1991) 103. [12] J. Maier, J. Electrochem. Soc. 134 (1987) 1524. [13] Y. Saito, J. Maier, J. Electrochem. Soc. 142 (1995) 3078. [14] J. Maier, Solid State Ionics 75 (1995) 139. [15] J. Maier, Solid State Ionics 86–88 (1996) 55.