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Langmuir 1999, 15, 662-668
Diffusion Coefficients of Redox Probes Encapsulated within Sol-Gel Derived Silica Monoliths Measured with Ultramicroelectrodes Maryanne M. Collinson,* Pedro J. Zambrano, Hanming Wang, and Jacob S. Taussig Kansas State University, Department of Chemistry, 111 Willard Hall, Manhattan, Kansas 66506-3701 Received June 25, 1998. In Final Form: September 11, 1998
The diffusional mobilities of potassium ferricyanide (Fe(CN)63-) and ferrocene methanol (FcCH2OH) encapsulated within silica monoliths have been investigated using cyclic voltammetry and chronoamperometry at an ultramicroelectrode. In this work, a 13-µm-radius Pt microdisk working electrode and a silver chloride coated silver wire (r ) 0.5 mm) reference electrode were inserted into doped silica sols prepared by the sol-gel process. The resultant gels were aged and slowly dried under a relative humidity atmosphere of 60-70% to minimize gel cracking. Fast-scan voltammetry (up to 100 V/s) confirmed that the gelelectrode interface remained intact throughout the duration of the drying period (typically 20-50 days). During this time frame, an ca. 30% loss in mass and ca. 50% reduction in volume of the gels were observed. The diffusion coefficients of gel-encapsulated Fe(CN)63- and FcCH2OH were measured without prior knowledge of dopant concentration via normalization of the chronoamperometric response with the steadystate limiting current obtained from a 2 mV/s potential sweep. For gel-encapsulated FcCH2OH, the diffusion coefficient dropped from ca. 4 × 10-6 cm2/s to less than 0.6 × 10-6 cm2/s as the gel dried, whereas for gel-encapsulated Fe(CN)63-, a near constant value of 2 × 10-6 cm2/s was obtained. These results suggest that transport in these solids is influenced by both gel structure and by the nature of the entrapped reagent.
Introduction The sol-gel process provides a valuable way to prepare stable inorganic host structures via the hydrolysis and condensation of an appropriate metal or semimetal alkoxide.1,2 The silicate glasses, for example, can be readily prepared by the acid- or base-catalyzed hydrolysis and condensation of tetramethoxysilane.1,2 During sol-gel formation, the viscosity of the solution gradually increases as the sol becomes interconnected to form a rigid threedimensional structuresthe gel. Upon drying, a xerogel is formed. In 1984 Avnir and co-workers showed that it is possible to introduce specific reagents into the inorganic host structure by simply adding the dopant to the sol prior to gelation.3 Since that time, the utilization of these materials as stable hosts for the entrapment of organicinorganic-organometallic reagents has blossomed.4-6 Specific applications of these host structures as solid-state electrolytes, photochromic materials, nonlinear optical materials, and chemical sensors has been reported.4-12 To effectively utilize these materials in advanced applications, it is necessary to first understand the * To whom correspondence should be addressed. Tel.: (785)532-1468. Fax: (785)532-6665. E-mail:
[email protected]. (1) Brinker, J.; Scherer, G.; Sol-Gel Science; Academic Press: New York, 1989. (2) Hench, L. L.; West, J. K. Chem. Rev. 1990, 90, 33-72. (3) Avnir, D.; Levy, D.; Reisfeld, R. J. Phys. Chem. 1984, 88, 59565959. (4) Avnir, D. Acc. Chem. Res. 1995, 28, 328-334. (5) Lev, O.; Tsionsky, M.; Rabinovich, L.; Glezer, V.; Sampath, S.; Pankratov, I.; Gun, J. Anal. Chem. 1995, 67, 22A-30A. (6) Dave, B. C.; Dunn, B.; Valentine, J. S.; Zink, J. I. Anal. Chem. 1994, 66, 1120A-1127A. (7) Dunn, B.; Farrington, G. C.; Katz, B. Solid State Ionics 1994, 70/71, 3-10. (8) Lev, O.; Wu, Z.; Bharathi, S.; Glezer, V.; Modestov, A.; Gun, J.; Rabinovich, L.; Sampath, S. Chem. Mater. 1997, 9, 2354-2375.
structure and properties of them, particularly from a molecular point of view. During the past few years, considerable attention has been given to the study of the sol-gel-xerogel formation and the silica host structure via 29Si NMR, 1H NMR, fluorescence anisotropy, luminescence, Raman, and X-ray scattering.1,2,13-23 These spectroscopic methods, in particular, have provided valu(9) (a) Jeng, R. J.; Chen, Y. M.; Jain, A. K.; Kumar, J.; Tripathy, S. K. Chem. Mater. 1992, 4, 1141-1144. (b) Marturunkakul, S.; Chen, J. I.; Jeng, R. J.; Sengupta, S.; Kumar, J.; Tripathy, S. K. Chem. Mater. 1993, 5, 743-745. (10) (a) Chaput, F.; Riehl, D.; Levy, Y.; Boilot, J.-P. Chem. Mater. 1993, 5, 589-591. (b) Riehl, D.; Chaput, F.; Levy, Y.; Boilot, J.-P.; Kajzar, F.; Chollet, P. A. Chem. Phys. Lett. 1995, 245, 36-40. (11) (a) Oviatt, H. W.; Shea, K. J.; Kalluri, S.; Shi, Y.; Steier, W. H.; Dalton, L. R. Chem. Mater. 1995, 7, 493-498. (b) Yang, Z.; Xu, C.; Wu, B.; Dalton, L. R.; Kalluri, S.; Steier, W. H.; Shi, Y.; Bechtel, J. H. Chem. Mater. 1994, 6, 1899-1901. (12) Avnir, D.; Braun, S.; Lev, O.; Levy, D.; Ottolenghi, M. In SolGel Optics. Processing and Applications; Klein, L. C., Ed.; Kluwer Academic Publishers: Norwell, Massachusetts, 1994. (13) Dunn, B.; Zink, J. I. Chem. Mater. 1997, 9, 2280-2291. (14) Narang, U.; Wang, R.; Prasad, P. N.; Bright, F. V. J. Phys. Chem. 1994, 98, 17-22. (15) (a) Kaufman, V. R.; Avnir, D. Langmuir 1996, 2, 717-722. (b) Levy, D.; Avnir, D. J. Phys. Chem. 1988, 92, 4734-4738. (16) (a) Matsui, K.; Momose, F. Chem. Mater. 1997, 9, 2588-2591. (b) Matsui, K.; Matsuzuka, T.; Fujita, H. J. Phys. Chem. 1989, 93, 49914994. (17) Diaz, A. N.; Lovillo, J.; Peinado, M. C. R. Chem. Mater. 1997, 9, 2647-2651. (18) Zheng, L.; Reid, W. R.; Brennan, J. D. Anal. Chem. 1997, 69, 3940-3949. (19) Puccetti, G.; Leblanc, R. M. J. Phys. Chem. 1996, 100, 17311737. (20) Boukari, H.; Lin, J. S.; Harris, M. T. Chem. Mater. 1997, 9, 2376-2384. (21) Rodrigues, D. E.; Brennan, A. B.; Betrabet, C.; Wang, B.; Wilkes, G. L. Chem. Mater. 1992, 4, 1437-1446. (22) Fyfe, C. A.; Aroca, P. P. J. Phys. Chem. B 1997, 101, 9504-9509. (23) Glaser, R. H.; Wilkes, G. L.; Bronnimann, C. E. J. Non-Cryst. Solids 1989, 113, 73-87.
10.1021/la980764g CCC: $18.00 © 1999 American Chemical Society Published on Web 12/05/1998
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able insight into the chemical changes that occur during sol-gel formation and the physicochemical properties of the dried gel. However, of equal importance is the need to understand molecular diffusion through the porous networks present within sol-gel glasses. Explicit information about the gel microstructure and the resultant transport of entrapped reagents is essential to the use of these materials as hosts for solid-state electrochemical devices and chemical sensors. Previous work has shown that electrochemical methodologies can be utilized to evaluate changes in the diffusion coefficient of trapped redox molecules through the sol-gel evolution.24,25 Since the diffusion coefficient is directly related to the microviscosity through the Stokes-Einstein equation, these investigations provided a convenient and sensitive approach for monitoring local changes in microviscosity during sol-gel formation.25 These methods have usually employed a large electrode and consequently have suffered to some extent from gel cracking and separation.24,25 In addition, accurate measurements of the diffusion coefficient of an entrapped redox probe were also difficult due to inaccuracies associated with the determination of concentration as the volume of the gel decreases during the drying stage. In the work described herein, ultramicroelectrodes have been embedded into the silicate matrix and used to evaluate the mobility of gel-entrapped reagents during the sol-gel-xerogel transformation. Due to their small size and unique electrochemical features, ultramicroelectrodes minimize many of the problems associated with the use of large electrodes.26 Gel cracking is lessened which allows transport to be probed in the xerogel state. In addition, accurate values of diffusion coefficients can be obtained without knowledge of solution concentration using slow-scan cyclic voltammetry in conjunction with potential step chronoamperometry.27,28 With the use of both fast-scan and slow-scan cyclic voltammetry (CV), the diffusion-layer distance can be controlled, and thus, the integrity of the gel-electrode interface can be probed. In these measurements in particular, it is critical that the gel-electrode interface remain intact in order to ensure that the measured diffusion coefficient reflects the value in the solid host structure. In this paper, we show that ultramicroelectrodes can be effectively used to probe molecular transport in high-surface-area silicate solids and provide valuable information about chemical diffusivity, entrapment, and gel microstructure. Experimental Section Reagents. Tetramethyl orthosilicate (TMOS, 99%) and ferrocene methanol (FcCH2OH, 97%) were purchased from Aldrich. Hydrochloric acid, potassium ferricyanide (Fe(CN)63-), potassium chloride (KCl), and methanol were purchased from Fisher Scientific. All reagents were used as received. Water was purified to Type I using a Labconco Water Pro PS four-cartridge system. Procedures. Platinum microdisk electrodes were prepared by sealing the microscopic wire (Alfa) with a nominal radii of 12.5 µm in a 2-mm o.d. glass tube tapered to 0.4-1.5-mm o.d.26 (24) Zhang, Y. Ph.D. Dissertation, University of North Carolina, Chapel Hill, 1991. (25) (a) Audebert, P.; Griesmar, P.; Sanchez, C. J. Mater. Chem. 1991, 1, 699-700. (b) Audebert, P.; Griesmar, P.; Hapiot, P.; Sanchez, C. J. Mater. Chem. 1992, 2, 1293-1300. (c) Cattey, H.; Audebert, P.; Sanchez, C.; Hapiot, P. J. Phys. Chem. B 1998, 102, 1193-1202. (26) Wightman, R. M.; Wipf, D. O. Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15. (27) Denuault, G.; Mirkin, M. V.; Bard, A. J. J. Electroanal. Chem. 1991, 308, 27-38. (28) Amatore, C.; Azzabi, M.; Calas, P.; Jutand, A.; Lefrou, C.; Rollin, Y. J. Electroanal. Chem. 1990, 288, 45-63.
Langmuir, Vol. 15, No. 3, 1999 663 As described below, the most satisfactory results were obtained with the smaller diameter tubing. Electrical connection was established with silver conducting epoxy (Epo-Tek). The electrodes were carefully polished successfully with 600- and 1500grit silicon carbide sandpaper for several minutes and thoroughly rinsed with deionized water. The electrode radius was determined from the steady-state limiting current obtained during the performance of slow scan (CV) (2 mV/s) in either 1 mM Fe(CN)63in 0.2 M potassium chloride or 1 mM ferrocene in 0.2 M tetrabutylammonium hexafluorophosphate.26 A diffusion coefficient of 7.2 × 10-6 or 2.17 × 10-5 cm2/s was used in the calculations for the first and for the second set of conditions, respectively.29 The microelectrode was secured with epoxy in a nylon cap along with an AgCl coated Ag wire (r ) 0.5 mm). Prior to use, the microelectrode was polished with 1500-grit sandpaper and thoroughly rinsed with water and ethanol. The silica sol was prepared by combining tetramethoxysilane (TMOS) with water, methanol, and 0.01 M hydrochloric acid in a mole ratio of (A) 1:10:3:2 × 10-4 or (B) 1:6:7:2 × 10-4 Si/H2O/ MeOH/HCl. The pH of the resultant solution was approximately 4 as measured with pH paper. The dopant (Fe(CN)63- or FcCH2OH) and supporting electrolyte (KCl) were added to the sol so that the final concentration was ca. 4-5 mM and 0.15 M, respectively. The doped sol was stirred for ca. 30 min and then carefully poured into a 5-dram polystyrene vial. The microelectrode assembly was inserted into the sol and the vial secured in a rack mounted in a Faraday cage. The sol typically gelled within 12-24 h. Separate samples were prepared for mass and volume measurements during aging/drying. In another experiment, TMOS was combined with water, methanol, and hydrochloric acid in a mole ratio of 1:10:3:2 × 10-4 Si/H2O/MeOH/HCl (i.e., silica sol A). The sol was allowed to sit for ca. 45 min. Phosphate buffer (0.1 M, pH 6) containing Fe(CN)63- was then added to the sol to bring the pH to 6 and the dopant concentration to 5 mM. The doped sol was quickly poured into the polystyrene vial, the microelectrode assembly introduced, and the vial secured in the Faraday cage. This sol typically gelled within a few minutes. CV and potential step chronoamperometry were performed with a BAS 100 W potentiostat equipped with a model PA-1 preamplifier. The relative humidity in the enclosed Faraday cage was maintained between 60 and 75% to ensure that the gel dried slowly.
Results and Discussion Voltammetric Measurements within the Silica Monolith. Figure 1 shows a simplified view of the electrochemical experiment. The cell consists of a 13.3µm-radius Pt working electrode and a silver/silver chloride reference electrode imbedded in ca. 10 mL of sol in a polystyrene container. To ensure that the gel/electrode interface remains intact for as long as possible, the electrodes were fixed in a nylon cap with epoxy and this assembly secured in a rack mounted in a Faraday cage. After 3 days, a small Parafilm plug was gently removed from the nylon cap to allow the solvent to evaporate through a ca. 3-mm-diameter hole. As the silica monolith dried, the gel pulled away from the polystyrene container and shrunk in both size and mass. For a typical 50-dayold gel, an ca. 50% loss in volume and ca. 25-30% loss in mass was observed relative to that measured for an initially formed gel. An important requirement in this investigation is the need to establish that the gel-electrode interface remains intact for the duration of the experiment (typically 20-50 days). Because the inorganic gel is macroscopically rigid,1,2 the gel can crack and/or pull away from the electrode surface during drying, thus creating a solution gap or an air-solution gap at the electrode-gel interface. This hydrated or partially hydrated interface can support (29) Baur, J. E.; Wightman, R, M. J. Electroanal. Chem. 1991, 305, 73-81.
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Figure 1. Representation of the silica monolith. The electrochemical cell consists of (A) a Pt ultramicroelectrode (r ) 13.3 µm) sealed in a small glass tube (o.d. < 1 mm) and (B) an Ag/ AgCl reference/auxiliary electrode (r ) 0.05 mm). Both electrodes are sealed in a nylon cap (C) that is secured to a polystyrene vial (D) containing the silica sol (E). After 3 days, a small plug is removed, and the gel is allowed to airdry at a relative humidity of ca. 60-75% for 20-50 days.
voltammetry. The size of the gap, the extent of solvation within the gap, and the length of the diffusion layer in the electrochemical experiment are important parameters to consider. In view of the small size of the electrode, even small changes in the boundary region could potentially influence the observed voltammetry and hence the calculated value of the diffusion coefficient. High-speed voltammetry provides a means to characterize the integrity of the gel-electrode interface and ensure that the calculated diffusion coefficient reflects transport within the host structure.26 Because of the reduced double-layer capacitance, the potential applied to the microelectrode can be scanned at rates considerably larger than is possible at a conventional-sized electrode.26 In this work, rates up to 100 V/s have been utilized. At sweep rates greater than 50 V/s, the diffusion layer extends ca. 3 µm for a redox molecule with a diffusion coefficient of 2 × 10-6 cm2/s. At 2 mV/s, the diffusion layer is ca. 450 µm from the electrode surface. Thus, if the silicate host starts to pull away from the electrode as the gel dries, it should be immediately evident in the voltammetry acquired at high sweep rates whereas it may not be obvious at low scan rates. An example of this is shown in Figure 2 for Fe(CN)63- doped into a silica sol. The voltammetry at 2 mV/s, 100 mV/s, and 20 V/s is nearly ideal for data collected when the gel was 13 days old. At slow sweep rates, a sigmoidal-shaped cyclic voltammogram was obtained, whereas at high sweep rates planar diffusion prevails and a diffusional-shaped cyclic voltammogram was obtained. On day 19, however, the voltammetry was clearly distorted at high sweep rates. The peak separation increased, and the Faradaic waves became less pronounced and lost their characteristic diffusional shapes. In general, the voltammetry becomes more resistive-like, consistent with the formation of an air, or partially filled, solution gap at the electrode-gel interface. As can be seen in Figure 2, this is most apparent for data collected at high sweep rates. At slow sweep rates, the diffusion layer extends a considerable distance into the gel (more so than at high sweep rates) and subtle changes at the gel-electrode interface are not as evident in the voltammetry. The rate at which the gel pulls away from the electrode surface appears to depend on the size of the electrode assembly and the rate at which the gel dries. Under low humidity conditions with a larger electrode (i.e., Pt sealed
Figure 2. Cyclic voltammograms of gel-encapsulated Fe(CN)63prepared from silica sol A and acquired 13 and 19 days after gelation. The cross-hairs correspond to the point of zero current and zero potential.
in a 1-2 mm o.d. tapered glass tube), most of the samples showed evidence of gel-electrode separation within the first 15-25 days. In addition, significant cracks could also be visually observed in the silica monolith. However, under conditions of relatively high humidity, the gel-electrode interface, particularly with a small working and reference electrode, remained intact for periods as long as 50 days with no significant visual cracking observed. Most of the data reported in this work were obtained with a smallmicroelectrode assembly under conditions of ca. 60-70% relative humidity. Representative cyclic voltammograms of gel-encapsulated Fe(CN)63- and FcCH2OH at 2 mV/s and 20 or 30 V/s are shown in Figures 3 and 4, respectively. The nearideal voltammetry at both low and high sweep rates (ν) reflects the fact that these electrodes have not separated from the gel during drying. At low sweep rates, the steadystate limiting current was essentially invariant with scan rate whereas at high scan rates a direct proportionality with ν1/2 was observed. This voltammetry is consistent with that found in solution for a microelectrode, indicative of the rather facile mobility of the electroactive probe in the silicate matrix (vide infra).26 Although macroscopically rigid, the gel is microscopically fluid, at least during the initial stages of drying.24,25ab The magnitude of the Faradaic current depends on either the product DC (i.e., at low sweep rates) or the product D1/2C (i.e., at high sweep rates) where D and C are the diffusion coefficient and the concentration of the gel-encapsulated redox probe, respectively. Since C increases and D either stays the same (i.e., Fe(CN)63+) or decreases (FcCH2OH) (see below) with drying, distinct changes in the peak or limiting current are observed (i.e., Figures 3 and 4). The relative variations observed upon
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Figure 3. Cyclic voltammograms of gel-encapsulated Fe(CN)63prepared from silica sol A, pH 6, and acquired 1, 9, and 24 days after gelation.
Figure 4. Cyclic voltammograms of gel-encapsulated FcCH2OH prepared from silica sol A and acquired 1, 14, and 29 days after gelation.
drying depend on the scan rate and the respective values of D and C. It is also apparent in these figures that the half-wave potential (E1/2) for the gel-encapsulated probe shifts as the gel ages and dries. For gel-encapsulated FcCH2OH, a negative shift of ca. 70-100 mV in the value of E1/2 was observed over a 30-45-day period. For gel-encapsulated Fe(CN)63-, a somewhat smaller positive shift of ca. 30-40 mV in the value of E1/2 was observed. These shifts in E1/2 could be due, in part, to changes in the redox potential of the Ag/AgCl reference electrode as the chloride concentration in the gel increases during drying. For example, the Ag/AgCl reference electrode potential will shift 30 mV in the positive direction for a ca. 3-fold increase in [Cl-]. Alternatively, they could be due, in part, to specific electrostatic interactions with the silicate host as the pores/ cavities collapse and change in chemical functionality during gel drying.1,2 Future experiments will be directed toward evaluating the stability of the Ag/AgCl reference electrode during gel aging and drying. Measurement of D of Gel-Encapsulated Fe(CN)63and FcCH2OH. To characterize the transport properties of redox probes encapsulated within this microporous solid, it is necessary to establish that diffusion-controlled conditions are valid within the gel and that D (or napp) is constant with time.28 This has been verified in this work by performing chronoamperometry at a large electrode and by examining the scan-rate dependence of the peak current at a microelectrode acquired at fast scan rates. Under these conditions, planar diffusion is necessarily obeyed. Figure 5(top) shows the results from chronoamperometry of gel-encapsulated Fe(CN)63- at a Pt disk
(r ) 0.3 mm) acquired 2 and 5 days after gelation. As can be seen, the normalized current (it1/2) was nearly constant over time, indicative of a diffusion-controlled reaction. Figure 5 (bottom) shows the scan-rate dependence of peak current for gel-encapsulated Fe(CN)63- acquired 5 and 26 days after gelation using the microelectrode assembly. As predicted for a diffusion-controlled process, ip is directly proportional to the square root of the scan rate. Thus, it can be concluded in this work that D is constant during the experimental time scale, which is consistent with that noted in earlier work.25b One of the distinct advantages of using a microelectrode in this investigation is the ability it gives to measure the diffusion coefficient (D) without prior knowledge of solution concentration (C). This is necessary because both D and C change simultaneously as the gel ages and dries, as a result of a reduction in volume and an increase in the degree of crosslinking. The research groups of Amatore28 and Bard27 have previously shown, for a microelectrode, that the appropriate equations for the chronoamperometric response (id(t)) and the steady-state cyclic voltammetric response (id,ss) can be combined to yield an expression for D which is independent of C or n, the number of electrons transferred. A linear expression for the transient at long times for a microdisk electrode was reported by Bard and co-workers27 to be
id(t)/id,ss ) 1 + 2 r/(π3/2D1/2t1/2)
(1)
where r is the electrode radius. A plot of id(t)/id,ss vs t-1/2 should yield a straight line with a slope of 2r/(π3/2D1/2) and an intercept of 1. Thus, if r is known, D can be readily
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Figure 6. Chronoamperometric response divided by the steadystate limiting current (acquired at 2 mV/s) for gel-encapsulated FcCH2OH prepared from silica sol A acquired 1 and 40 days after gelation. The electrode potential was stepped from 0 to 600 mV and held for 10 s. The solid line represents the linearregression fit to the experimental data (solid points) over the range of ti of 2-10 and 5-10 s for the D1 and D40 gels, respectively.
Figure 5. (Top) normalized chronoamperometric response of gel-encapsulated Fe(CN)63- prepared from silica sol A, pH 6, and acquired 2 and 5 days after gelation. Electrode: r ) 0.3 mm, Pt. Electrode potential was stepped from 0.5 to -0.2 V and held for 10 s. (Bottom) scan-rate dependence of the anodic peak current (ipa) measured for gel-encapsulated FcCH2OH both 5 and 26 days after gelation. Electrode: r ) 13.3 µm, Pt. Solid lines represent linear-regression fit to the experimental data (solid points).
obtained from the slope. It is important to note that this particular expression is valid in the long-time limit of the more general description of the current at a microdisk electrode given by Shoup and Szabo.30 The error associated with eq 1, however, is less than 1%, provided the sampling time (ti) is chosen to be greater than r2/D.27 Given that r ) 13.3 µm and D is ca. 2 × 10-6 cm2/s, ti should be greater than ca. 1 s. Figure 6 shows the chronoamperometric (CA) response divided by the steady-state limiting current for gelencapsulated FcCH2OH acquired 1 and 40 days after gelation. The CA response was obtained by stepping the electrode potential from 0 to 0.6 V, which was at least 200 mV more positive than the redox potential of gelencapsulated FcCH2OH. The steady-state current was obtained from the CV acquired at 2 mV/s. As can be seen in Figure 6, a linear plot was obtained with an intercept near the expected value of 1. If the gel separates significantly from the electrode surface as previously described, this will also be evident in the normalized CA response as a significantly larger y intercept (i.e., typically >1.08). The solid lines in Figure 6 represent the linearregression fit of the experimental data over the sampling time ti g 2 s. The slope of this line thus provides a direct (30) Shoup, D.; Szabo, A. J. Electroanal. Chem. 1982, 140, 237.
measure of the diffusion coefficient of gel-encapsulated FcCH2OH. For the data depicted in Figure 6, diffusion coefficients were calculated to be 4 × 10-6 and 1 × 10-6 cm2/s for the 1-day-old and 40-day-old gels, respectively. As described below, these differences reflect differences in the local microstructure of the two gels at two different stages of drying. Variation in D During the Sol-Gel-Xerogel Transformation. Figure 7 depicts the variation in D (obtained in the manner described above), for FcCH2OH encapsulated in two different gels, as a function of drying time. Single measurements on three different monoliths are shown. As can be seen, D dropped steadily in both the high-methanol and low-methanol preparations as the gels aged and dried. The initial value of D obtained in the just-solidified gel is very similar to the value obtained for FcCH2OH in the sol and is consistent with the values reported for other small molecules trapped within similar silicate-host structures.25,31,32 While the gel is macroscopically rigid, microscopically it contains an open framework of solvent-filled voids, channels, and cages. Molecular transport proceeds relatively unhindered when the gel is in its “wet” state. As the gel ages and dries, solvent evaporates, the cavities and channels collapse, neighboring silanols react, and the host microscopically becomes more rigid and crosslinked.1-2,25 Hence, diffusional transport becomes increasingly hindered. The observed decrease in D for gel-encapsulated FcCH2OH is consistent with the relative variation in D reported in previous work.25a,b The diffusion coefficient of FcCH2OH appears to drop faster in the gel that initially contained a greater mole fraction of methanol (Figure 7B). This is perhaps a result of the greater volatility of methanol and hence the more accelerated evaporative losses and increased drying rate. As expected, the concentration of FcCH2OH in the gel was also observed to increase as the gel dried as depicted (31) Sieminska, L.; Zerda, T. W. J. Phys. Chem. 1996, 100, 45914597. (32) Leon, L.; Lefaucheux, F.; Robert, M. C. J. Cryst. Growth 1987, 84, 155-162.
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Figure 7. Variation in the diffusion coefficient of gelencapsulated FcCH2OH with gel drying time. (A) Silica sol A. (B) Silica sol B. Single measurements on three different monoliths are shown. The solid or dashed lines have been drawn to guide the eye. Inset: Variation in the concentration of gelencapsulated FcCH2OH with drying time.
Figure 8. Variation in the diffusion coefficient of gelencapsulated Fe(CN)63- with gel drying time. (A) Silica sol A, pH 6. (B) Silica sol A. Single measurements on three different monoliths are shown. The solid or dashed lines have been drawn to guide the eye. Inset: Variation in the concentration of gelencapsulated Fe(CN)63- with drying time.
in the inset of Figure 7. There, concentration was determined from the steady-state limiting current expression26
by base condensation), which suggests that this observation is not a function of a particular gel preparation but rather of the molecule itself. The major difference between the redox probes, Fe(CN)63- and FcCH2OH, is charge. Perhaps the presence of the negatively charged redox molecule influences the microscopic reactivity of the silicon precursors such that the framework maintains a similar pore structure in terms of diameter and/or microtortuosity as the original wet gel. More detailed experiments with other redox probes, different concentrations, and different gel preparations will be required to completely understand these results.
iss ) 4nFrCD
(2)
using the values of D obtained from the combined electrochemical transients given n ) 1 for the oxidation of FcCH2OH. For both gels, the FcCH2OH concentration in the gel increased ca. 3- to 4-fold during the 30-40-day drying period. In direct contrast to the results observed for gelencapsulated FcCH2OH, the diffusion coefficient for gelencapsulated Fe(CN)63- stayed essentially constant as the gel aged and dried, as shown in Figure 8. The value of D obtained is consistent with the values found for FcCH2OH and other small molecules in the silicate matrix.25,31,32 The fact that D did not significantly change as the gel dried is surprising. The Fe(CN)63- doped gel exhibited nearly the same loss in volume and mass as that observed for the ferrocene-doped gel. Furthermore, the concentration of Fe(CN)63- in the gel was observed to increase as the gel dried (Figure 8, inset), as is consistent with the measured reduction in volume. Since it was ascertained that the gel-electrode interface was intact, it is not likely that the gel interface had separated from the electrode surface. The nearly constant value of D was observed for Fe(CN)63- encapsulated in both the acid-catalyzed solgel and the two-step preparation (acid hydrolysis followed
Conclusion Ultramicroelectrodes can be successfully used to probe the diffusional mobility of redox probes entrapped within a silica monolith prepared by the sol-gel process. The transport of reagents trapped within this solid host matrix is important to the development of solid-state electrochemical devices and to the development of chemical sensors and catalytic materials. The use of these microscopic electrodes is advantageous in the respect that the diffusion coefficient (D) can be measured independent of concentration, gel cracking can be minimized to increase the measurement time frame, and theoretically, the need for the addition of supporting electrolyte can be eliminated. Furthermore, the utilization of relatively high sweep rates
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in a cyclic voltammetric experiment can provide a useful means to probe the gel-electrode interface and ensure that it remains intact during gelation and drying. In this work, different diffusional behavior was observed for the two different redox probes trapped within the silicate host. For gel-encapsulated FcCH2OH, D dropped as the gel dried, consistent with a reduction in the microviscosity and an increased cross-linking density. In contrast, D for gel-encapsulated Fe(CN)63- remained nearly constant during the same drying period, suggesting that the entrapped probe could influence the gel polymerization and structure at the microscopic level. Future work will
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be directed toward understanding these differences at the molecular level. Acknowledgment. We gratefully acknowledge support of this work by the Office of Naval Research. We also wish to thank Christian Amatore and Henry White for useful discussions about microelectrodes, Richard Bachamp for the nylon caps, and Jim Hodgson for the “micro” glass tubes and for helpful advice about polishing procedures. LA980764G
