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Ionic conductivity of silica gels and dynamic properties of their pore liquids studied by impedance spectroscopy and polarized-light spectrofluorimetry
Solid State Ionics 136–137 (2000) 453–456 www.elsevier.com / locate / ssi
Ionic conductivity of silica gels and dynamic properties of their pore liquids studied by impedance spectroscopy and polarized-light spectrofluorimetry a, b M. Wasiucionek *, M.W. Breiter a
Institute of Physics, Warsaw University of Technology, Koszykowa 75, 00 -662 Warsaw, Poland b ¨ Technische Elektrochemie, TU Wien, Getreidemarkt 9, 1060 Wien, Austria Institut f ur
Abstract Long-range transport and local molecular motions in the sol / gel system have been studied during an acid-catalyzed sol–gel process in the silicate system. Tetramethoxy orthosilicate (TMOS, Si(OCH 3 ) 4 ) was used as an alkoxide precursor. The long-range ionic transport was studied by impedance spectroscopy, and local dynamics of molecules were monitored by polarized-light spectrofluorimetry. A slow, but steady decrease of the ionic conductivity and a similar time-dependence of the reciprocal microviscosity were observed during the sol–gel process. The changes of local and long-range transport properties are ascribed to interactions of molecules of the pore liquid with highly ramified surfaces of pore walls and to geometrical constraints on the freedom of their local motion. 2000 Elsevier Science B.V. All rights reserved. Keywords: Sol–gel process; SiO 2 gels; Impedance spectroscopy; Ionic conductivity; Spectrofluorimetry
1. Introduction Hybrid solid–liquid ionic conductors based on silica gels exhibit ionic conductivity as high as 10 21 S cm 21 (proton conduction) [1] and 10 22 S cm 21 (Li 1 conduction) at room temperature [2,3]. These values are usually slightly lower than the conductivity of the respective pore liquids. This discrepancy can partly be caused by structural and transport modifications of the liquids placed inside narrow pores of the gel (or other porous media). The reduced local mobility of molecules of a variety of liquids in porous media, intensively *Corresponding author. Fax: 148-22-628-2171. E-mail address: [email protected] (M. Wasiucionek).
studied by a variety of spectroscopic techniques [4–7], has been ascribed to interactions of molecules of the liquids with solid gel framework or to purely topological constraints on local molecular rotations and translations [4]. Intensive investigations of local dynamics of liquids confined inside gels have not been adequately accompanied by studies of long-range transport phenomena in such systems. In the present work, we have attempted to find a possible correlation between short- and long-range molecular motions in silica sol / gels. The major structural and dynamic changes in the sol / gels occur in the initial stage of the sol–gel process, ending soon after gelation. Therefore, we have concentrated on observing the time-dependence
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00562-2
M. Wasiucionek, M.W. Breiter / Solid State Ionics 136 – 137 (2000) 453 – 456
454
of the ionic conductivity of silicate sols / gels and the local mobility of molecules of the sol / gel liquid in this early period. In this work the long-range ion transport was studied by impedance spectroscopy and the local dynamics of pore liquid was investigated by polarized-light spectrofluorimetry [8].
2. Experimental The sols / gels were prepared from tetramethoxy orthosilicate (TMOS) using a single-step acid-catalyzed procedure described in Ref. [2]. Initial sol compositions corresponded to the formula TMOS:H 2 O:MeOH:HCl 5 1:4:3:5 3 10 24 . Additionally 10 25 M of pyranine (HPS) was added to each batch as a photophysical probe. Impedance spectra were measured at room temperature in a two-probe configuration within 5 Hz–10 MHz frequency range. The spectra were automatically acquired every 3–4 h throughout the sol–gel process. Vertically polarized excitation light at wavelength lexc 5 390 nm was used. In the spectrofluorimetric experiments emission spectra in the 400–600 nm range were recorded in both vertical and horizontal polarization.
3. Results and discussion The time-dependence of the ionic conductivity during the sol–gel process is characterized by a slow but steady decrease (Fig. 1) without a drastic change at the gel point. The conductivity of the fresh gel (at t ¯ t gel ) is ca. 3 4 5 times lower than the conductivity of the initial sol. Two important factors affecting the time-dependence of the ionic conductivity of the system under study during the sol–gel process are: (a) an increase of the volume fraction of the solid, non-conducting phase (gel framework) at the expense of the conducting one (pore liquid), (b) modifications of the local dynamic properties of conducting liquids, due to entrapment of the gel liquid in narrow, nanometer-sized pores. The volume fraction of the solid phase can be
Fig. 1. Time-dependence of the room temperature conductivity of a sol / gel of the initial composition TMOS:H 2 O:MeOH:HCl5 1:4:3:5310 24 .
qualitatively deduced from the progress of the waterproducing condensation reaction [9,10]: Si–OH 1 Si–OH → Si–O–Si 1 H 2 O
(1)
According to reaction (1), the formation of a single Si–O–Si bond (an elementary unit of the gel solid phase) is accompanied by the release of a water molecule. Therefore, in principle, it is possible to monitor the build-up of the solid phase by observing water content changes. The latter dependence is shown in the Fig. 2. Our quantitative analysis of the data from Fig. 2 gives the time-dependence of the
Fig. 2. Time-dependence of the relative water content of a sol / gel of the initial composition TMOS:H 2 O:MeOH:HCl51:4:3:53 10 24 .
M. Wasiucionek, M.W. Breiter / Solid State Ionics 136 – 137 (2000) 453 – 456
content of Si–O–Si groups during the sol–gel process similar to that obtained by 29 Si NMR [11]. Unfortunately the values of Si–O–Si contents given by these two methods differ considerably. Therefore we were not able to determine reliably the volume fraction of the solid phase from spectrofluorimetric experiments alone. Despite these problems, a slow growth of the water content (and of the content of the solid phase) during the sol–gel process is evident. There are published estimates of 0.240.35 of the solid phase at the gel point [12,13]. A preliminary estimate of the effect of increasing volume fraction of the non-conducting solid phase on the conductivity of sols / gels can be obtained from the basic version of the effective medium percolation theory (EMPT) [14]. According to it, the increase of the volume fraction of the isolating phase from 0 in the initial sol to 0.240.35 at the gel point [12,13] should lead to a decrease of the conductivity of the system by a factor of ca. 1.642 from its initial value. The sol–gel process also leads to modifications of the local properties of the liquid phase [5,13]. The quantity measured in the polarized-light spectrofluorimetric experiments to monitor molecular motions of pore liquid is the polarization anisotropy r [8]: IVV 2 IHV r 5 ]]] IVV 1 2IHV
(2)
where IVV , IHV -intensities of vertically or horizontally polarized emission light of the photoprobe excited by vertically polarized light, respectively. The parameter r is a monotone function of the microviscosity h [15], therefore its changes reflect the evolution of the local motional dynamics in the liquid. Fig. 3 presents the time-dependence of the parameter r during the sol–gel process. Similar dependencies of parameter r (or microviscosity h ) determined in spectrofluorimetric experiments, were reported for other photoprobes in silicate sol / gels (e.g. Refs. [5,13]). Also other techniques, e.g. 1 H NMR [5], show a slow gradual increase of microviscosity h during the sol–gel process. All these observations indicate that during the sol-process the mobile species in the liquid phase gradually lose the freedom of motion. This tendency is to be ascribed to the fact that during the sol–gel process increases
455
Fig. 3. Time-dependence of the polarization anisotropy r of a sol / gel of the initial composition TMOS:H 2 O:MeOH:HCl5 1:4:3:5310 24 .
the fraction of the liquid entrapped in narrow pores or cages formed by a ramified solid phase. This part of the liquid phase is affected by the entrapment phenomena [16]. It is noteworthy that microviscosity, measured in systems similar to ours, started to increase considerably after a very long plateau (until t¯600 t gel ) [13]. This observation can be ascribed to the fact that at that stage of the sol–gel process most of pore-core liquid evaporated. The remaining thin liquid layers at the pore walls determine the overall microviscosity. Since these interfacial layers are the most affected by the confinement effects the measured microviscosity should be much higher than that of the bulk liquid. The apparent correlation between the time-dependence of the conductivity and reciprocal microviscosity (Figs. 1 and 3) can be justified by applying well-known relations between the quantities characterizing transport properties. In many liquid systems, one experimentally observes proportionality between the self-diffusion coefficient D and the fluidity (reciprocal viscosity h 21 ). Also for many systems the ionic conductivity s is proportional to D. These relations are observed despite of the fact that the rigorous assumptions of the Stokes–Einstein or Nernst–Einstein equations which give their theoretical justification, are often not fulfilled. If the two proportionalities hold, then the electrical conductivity s is proportional to fluidity h 21 . Such a correlation can be deduced from Figs. 1 and 3. An additional argument for the dependence between
456
M. Wasiucionek, M.W. Breiter / Solid State Ionics 136 – 137 (2000) 453 – 456
local and long-range transport properties comes from the observation that high microviscosity of silica xerogels [13] is accompanied by their low conductivity [17].
4. Conclusions The monotone decrease of ionic conductivity during the sol–gel process has been related to two factors: an increase of the volume fraction of the solid phase and a decrease of local molecular motions due to confinement effects in the narrowpore system under study. Both of these factors lead to a steady slow decrease of the ionic conductivity.
References [1] M. Tatsumisago, H. Honjo, Y. Sakai, T. Minami, Solid State Ionics 74 (1994) 105. [2] M. Wasiucionek, M.W. Breiter, J. Appl. Electrochem. 27 (1997) 1106.
[3] P.-W. Wu, S.R. Holm, A.T. Duong, B. Dunn, R.B. Kaner, Chem. Mater. 9 (1997) 1004. [4] J.-P. Korb, X. Shu, L. Malier, J. Jonas, J. Chem. Phys. 107 (1997) 4044. [5] R. Winter, D.W. Hua, X. Song, W. Manitulin, J. Jonas, J. Phys. Chem. 94 (1990) 1392. [6] B. Dunn, J.I. Zink, Chem. Mater. 9 (1997) 2280. [7] X. Yan, C. Streck, R. Richert, Ber. Bunsenges. Phys. Chem. 100 (1996) 1392. [8] B. Valeur, in: S.G. Schulman (Ed.), Molecular Luminescence Spectroscopy, Part 3. Chemical Analysis Series, Wiley, New York, 1993, p. 5. [9] C.J. Brinker, G.W. Scherer, Sol–Gel Science. The Physics and Chemistry of Sol–Gel Processing, Academic Press, San Diego, 1990, Chapter 3. [10] V.R. Kaufman, D. Avnir, D. Pines-Rojanski, D. Huppert, J. Non-Cryst. Solids 99 (1987) 379. [11] J. Sanchez, A. McCormick, J. Phys. Chem. 96 (1992) 8973. [12] R. Zallen, The Physics of Amorphous Solids, Wiley, New York, 1983. [13] P. Audebert, P. Griesmar, Ph. Hapiot, C. Sanchez, J. Mater. Chem. 2 (1992) 1293. [14] R. Landauer, J. Appl. Phys. 23 (1952) 779. [15] G. Weber, Biochem. J. 51 (1952) 145. [16] J.M. Drake, J. Klafter, Phys. Today 43 (May) (1990) 46. [17] H. Sodolski, M. Kozl«owski, J. Non-Cryst. Solids 194 (1996) 241.
Ionic conductivity of silica gels and dynamic properties of their pore liquids studied by impedance spectroscopy and polarized-light spectrofluorimetry a, b M. Wasiucionek *, M.W. Breiter a
Institute of Physics, Warsaw University of Technology, Koszykowa 75, 00 -662 Warsaw, Poland b ¨ Technische Elektrochemie, TU Wien, Getreidemarkt 9, 1060 Wien, Austria Institut f ur
Abstract Long-range transport and local molecular motions in the sol / gel system have been studied during an acid-catalyzed sol–gel process in the silicate system. Tetramethoxy orthosilicate (TMOS, Si(OCH 3 ) 4 ) was used as an alkoxide precursor. The long-range ionic transport was studied by impedance spectroscopy, and local dynamics of molecules were monitored by polarized-light spectrofluorimetry. A slow, but steady decrease of the ionic conductivity and a similar time-dependence of the reciprocal microviscosity were observed during the sol–gel process. The changes of local and long-range transport properties are ascribed to interactions of molecules of the pore liquid with highly ramified surfaces of pore walls and to geometrical constraints on the freedom of their local motion. 2000 Elsevier Science B.V. All rights reserved. Keywords: Sol–gel process; SiO 2 gels; Impedance spectroscopy; Ionic conductivity; Spectrofluorimetry
1. Introduction Hybrid solid–liquid ionic conductors based on silica gels exhibit ionic conductivity as high as 10 21 S cm 21 (proton conduction) [1] and 10 22 S cm 21 (Li 1 conduction) at room temperature [2,3]. These values are usually slightly lower than the conductivity of the respective pore liquids. This discrepancy can partly be caused by structural and transport modifications of the liquids placed inside narrow pores of the gel (or other porous media). The reduced local mobility of molecules of a variety of liquids in porous media, intensively *Corresponding author. Fax: 148-22-628-2171. E-mail address: [email protected] (M. Wasiucionek).
studied by a variety of spectroscopic techniques [4–7], has been ascribed to interactions of molecules of the liquids with solid gel framework or to purely topological constraints on local molecular rotations and translations [4]. Intensive investigations of local dynamics of liquids confined inside gels have not been adequately accompanied by studies of long-range transport phenomena in such systems. In the present work, we have attempted to find a possible correlation between short- and long-range molecular motions in silica sol / gels. The major structural and dynamic changes in the sol / gels occur in the initial stage of the sol–gel process, ending soon after gelation. Therefore, we have concentrated on observing the time-dependence
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00562-2
M. Wasiucionek, M.W. Breiter / Solid State Ionics 136 – 137 (2000) 453 – 456
454
of the ionic conductivity of silicate sols / gels and the local mobility of molecules of the sol / gel liquid in this early period. In this work the long-range ion transport was studied by impedance spectroscopy and the local dynamics of pore liquid was investigated by polarized-light spectrofluorimetry [8].
2. Experimental The sols / gels were prepared from tetramethoxy orthosilicate (TMOS) using a single-step acid-catalyzed procedure described in Ref. [2]. Initial sol compositions corresponded to the formula TMOS:H 2 O:MeOH:HCl 5 1:4:3:5 3 10 24 . Additionally 10 25 M of pyranine (HPS) was added to each batch as a photophysical probe. Impedance spectra were measured at room temperature in a two-probe configuration within 5 Hz–10 MHz frequency range. The spectra were automatically acquired every 3–4 h throughout the sol–gel process. Vertically polarized excitation light at wavelength lexc 5 390 nm was used. In the spectrofluorimetric experiments emission spectra in the 400–600 nm range were recorded in both vertical and horizontal polarization.
3. Results and discussion The time-dependence of the ionic conductivity during the sol–gel process is characterized by a slow but steady decrease (Fig. 1) without a drastic change at the gel point. The conductivity of the fresh gel (at t ¯ t gel ) is ca. 3 4 5 times lower than the conductivity of the initial sol. Two important factors affecting the time-dependence of the ionic conductivity of the system under study during the sol–gel process are: (a) an increase of the volume fraction of the solid, non-conducting phase (gel framework) at the expense of the conducting one (pore liquid), (b) modifications of the local dynamic properties of conducting liquids, due to entrapment of the gel liquid in narrow, nanometer-sized pores. The volume fraction of the solid phase can be
Fig. 1. Time-dependence of the room temperature conductivity of a sol / gel of the initial composition TMOS:H 2 O:MeOH:HCl5 1:4:3:5310 24 .
qualitatively deduced from the progress of the waterproducing condensation reaction [9,10]: Si–OH 1 Si–OH → Si–O–Si 1 H 2 O
(1)
According to reaction (1), the formation of a single Si–O–Si bond (an elementary unit of the gel solid phase) is accompanied by the release of a water molecule. Therefore, in principle, it is possible to monitor the build-up of the solid phase by observing water content changes. The latter dependence is shown in the Fig. 2. Our quantitative analysis of the data from Fig. 2 gives the time-dependence of the
Fig. 2. Time-dependence of the relative water content of a sol / gel of the initial composition TMOS:H 2 O:MeOH:HCl51:4:3:53 10 24 .
M. Wasiucionek, M.W. Breiter / Solid State Ionics 136 – 137 (2000) 453 – 456
content of Si–O–Si groups during the sol–gel process similar to that obtained by 29 Si NMR [11]. Unfortunately the values of Si–O–Si contents given by these two methods differ considerably. Therefore we were not able to determine reliably the volume fraction of the solid phase from spectrofluorimetric experiments alone. Despite these problems, a slow growth of the water content (and of the content of the solid phase) during the sol–gel process is evident. There are published estimates of 0.240.35 of the solid phase at the gel point [12,13]. A preliminary estimate of the effect of increasing volume fraction of the non-conducting solid phase on the conductivity of sols / gels can be obtained from the basic version of the effective medium percolation theory (EMPT) [14]. According to it, the increase of the volume fraction of the isolating phase from 0 in the initial sol to 0.240.35 at the gel point [12,13] should lead to a decrease of the conductivity of the system by a factor of ca. 1.642 from its initial value. The sol–gel process also leads to modifications of the local properties of the liquid phase [5,13]. The quantity measured in the polarized-light spectrofluorimetric experiments to monitor molecular motions of pore liquid is the polarization anisotropy r [8]: IVV 2 IHV r 5 ]]] IVV 1 2IHV
(2)
where IVV , IHV -intensities of vertically or horizontally polarized emission light of the photoprobe excited by vertically polarized light, respectively. The parameter r is a monotone function of the microviscosity h [15], therefore its changes reflect the evolution of the local motional dynamics in the liquid. Fig. 3 presents the time-dependence of the parameter r during the sol–gel process. Similar dependencies of parameter r (or microviscosity h ) determined in spectrofluorimetric experiments, were reported for other photoprobes in silicate sol / gels (e.g. Refs. [5,13]). Also other techniques, e.g. 1 H NMR [5], show a slow gradual increase of microviscosity h during the sol–gel process. All these observations indicate that during the sol-process the mobile species in the liquid phase gradually lose the freedom of motion. This tendency is to be ascribed to the fact that during the sol–gel process increases
455
Fig. 3. Time-dependence of the polarization anisotropy r of a sol / gel of the initial composition TMOS:H 2 O:MeOH:HCl5 1:4:3:5310 24 .
the fraction of the liquid entrapped in narrow pores or cages formed by a ramified solid phase. This part of the liquid phase is affected by the entrapment phenomena [16]. It is noteworthy that microviscosity, measured in systems similar to ours, started to increase considerably after a very long plateau (until t¯600 t gel ) [13]. This observation can be ascribed to the fact that at that stage of the sol–gel process most of pore-core liquid evaporated. The remaining thin liquid layers at the pore walls determine the overall microviscosity. Since these interfacial layers are the most affected by the confinement effects the measured microviscosity should be much higher than that of the bulk liquid. The apparent correlation between the time-dependence of the conductivity and reciprocal microviscosity (Figs. 1 and 3) can be justified by applying well-known relations between the quantities characterizing transport properties. In many liquid systems, one experimentally observes proportionality between the self-diffusion coefficient D and the fluidity (reciprocal viscosity h 21 ). Also for many systems the ionic conductivity s is proportional to D. These relations are observed despite of the fact that the rigorous assumptions of the Stokes–Einstein or Nernst–Einstein equations which give their theoretical justification, are often not fulfilled. If the two proportionalities hold, then the electrical conductivity s is proportional to fluidity h 21 . Such a correlation can be deduced from Figs. 1 and 3. An additional argument for the dependence between
456
M. Wasiucionek, M.W. Breiter / Solid State Ionics 136 – 137 (2000) 453 – 456
local and long-range transport properties comes from the observation that high microviscosity of silica xerogels [13] is accompanied by their low conductivity [17].
4. Conclusions The monotone decrease of ionic conductivity during the sol–gel process has been related to two factors: an increase of the volume fraction of the solid phase and a decrease of local molecular motions due to confinement effects in the narrowpore system under study. Both of these factors lead to a steady slow decrease of the ionic conductivity.
References [1] M. Tatsumisago, H. Honjo, Y. Sakai, T. Minami, Solid State Ionics 74 (1994) 105. [2] M. Wasiucionek, M.W. Breiter, J. Appl. Electrochem. 27 (1997) 1106.
[3] P.-W. Wu, S.R. Holm, A.T. Duong, B. Dunn, R.B. Kaner, Chem. Mater. 9 (1997) 1004. [4] J.-P. Korb, X. Shu, L. Malier, J. Jonas, J. Chem. Phys. 107 (1997) 4044. [5] R. Winter, D.W. Hua, X. Song, W. Manitulin, J. Jonas, J. Phys. Chem. 94 (1990) 1392. [6] B. Dunn, J.I. Zink, Chem. Mater. 9 (1997) 2280. [7] X. Yan, C. Streck, R. Richert, Ber. Bunsenges. Phys. Chem. 100 (1996) 1392. [8] B. Valeur, in: S.G. Schulman (Ed.), Molecular Luminescence Spectroscopy, Part 3. Chemical Analysis Series, Wiley, New York, 1993, p. 5. [9] C.J. Brinker, G.W. Scherer, Sol–Gel Science. The Physics and Chemistry of Sol–Gel Processing, Academic Press, San Diego, 1990, Chapter 3. [10] V.R. Kaufman, D. Avnir, D. Pines-Rojanski, D. Huppert, J. Non-Cryst. Solids 99 (1987) 379. [11] J. Sanchez, A. McCormick, J. Phys. Chem. 96 (1992) 8973. [12] R. Zallen, The Physics of Amorphous Solids, Wiley, New York, 1983. [13] P. Audebert, P. Griesmar, Ph. Hapiot, C. Sanchez, J. Mater. Chem. 2 (1992) 1293. [14] R. Landauer, J. Appl. Phys. 23 (1952) 779. [15] G. Weber, Biochem. J. 51 (1952) 145. [16] J.M. Drake, J. Klafter, Phys. Today 43 (May) (1990) 46. [17] H. Sodolski, M. Kozl«owski, J. Non-Cryst. Solids 194 (1996) 241.