Nucleation and Growth of Silver at Zeolite A-Modified Electrodes - The

A general model pertaining to the nucleation and growth of silver at zeolite-modified ... The Journal of Physical Chemistry B 2003 107 (13), 3040-3050...
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J. Phys. Chem. B 1997, 101, 10390-10397

Nucleation and Growth of Silver at Zeolite A-Modified Electrodes Darren H. Brouwer and Mark D. Baker* Department of Chemistry and Biochemistry, Guelph-Waterloo Centre for Graduate Work in Chemistry, UniVersity of Guelph, Guelph, Ontario, Canada N1G 2W1 ReceiVed: June 13, 1997; In Final Form: September 16, 1997X

In this paper the nucleation and growth of silver deposits at tin oxide and tin oxide modified with silver ion-exchanged zeolite A (AgxNa12-xA) is studied. We show via chronoamperometry that, in the case of the zeolite-modified electrodes, the lateral motion of silver cations to active sites is impeded by the zeolite layer, resulting in distinct nucleation and growth at defect sites and/or at the bulk tin oxide surface. Solution-phase silver deposition at tin oxide fits the Scharifker-Mostany model extremely well, showing that on average only one active site for silver nucleation and growth is available for each 10 zeolite particles. Furthermore, the scan rate and loading dependence of the cyclic voltammetry are explained in terms of ion-exchange kinetics and supersaturation of silver atoms around the active nucleation site. A general model pertaining to the nucleation and growth of silver at zeolite-modified electrodes is presented. Cyclic voltammetry recorded in aqueous electrolytes containing cations of different sizes (i.e., ammonium, tetramethylammonium, and sodium) supports the general model involving nucleation and growth of silver at the electrode surface.

Introduction Suggestions that the zeolites and zeotypes could have an impact in advanced application areas surfaced in the early 1980s,1 and several subdisciplines have since emerged. In the electrochemical field, dispersion electrolysis and modified electrodes have been explored with a view to the development of new catalysts and sensors. Another interesting and potentially viable way of marrying electrochemistry to zeolite science would be the template-assisted growth of nanostructures at electrode surfaces.2,3 Certainly the growth in nanochemistry these days is explosive. Reviews of this field are appearing in profusion. Template-synthesized nanometals have many interesting optical, magnetic, and electronic properties,3 and the quest for even smaller nanoscopic metals has aroused considerable interest. Further advance in zeolite thin film synthesis4,5 may indeed pave the way for the use of zeolites as templates for nanode array formation in a controlled manner. However, the electrochemistry associated with the deposition of silver from ZMEs still remains controversial. The reduction of transition-metal ions at zeolite-coated electrodes can occur via two broad mechanisms. These have been termed as intra- or extrazeolitic. The latter calls for the electron transfer to occur at the electrode-solution interphase and in the case of Ag+ zeolite-modified electrodes (ZMEs) would result in deposition of silver either on the electrode surface or onto the external surfaces of the zeolite. In this paper and its companion,6 we supply further evidence for the extrazeolite mechanism in terms of silver nucleation and growth at the electrode surface. This is achieved via a combination of cyclic voltammetry and chronoamperometry. Experimental Section Aqueous Solutions. Water used in all experiments was deionized and further purified using a Barnstead water purification system containing ion-exchange and organic removal towers. Final resistivity of water was 18 MΩ cm. Zeolites. Zeolite A (Valfor 100), donated by UOP Corp. (Whistler, AL), was used in all experiments. The average zeolite particle size was approximate 1 µm in diameter. Prior to Ag+ X

Abstract published in AdVance ACS Abstracts, November 1, 1997.

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ion exchange, removal of impurity extraframework cations was performed in 0.1 M NaNO3 for 24 h (pH 6.5), followed by washing with water, filtering, and drying in air. All ion exchange was performed in the dark. Various Ag+ cationexchange levels were achieved by stirring the zeolite A in silver nitrate (Aldrich) solutions of concentrations less than 0.01 M, followed by filtration and drying. Silver-exchange levels were determined by AA analysis of filtrates to within 5% error. Three Ag+-exchange levels were used in this study: x ) 2.6, 6, and 12 (AgxA). Some hydrolysis of the zeolite may have occurred in the ion exchange, but analysis for H was not carried out. Zeolite-Modified Electrodes. The conductive substrate onto which Ag+-exchanged zeolite A films were laid was tin oxide on glass, donated by Libby Owens Ford (Toledo, OH). A suspension of 21 mg of Ag+-exchanged zeolite A in 3 mL of ethanol was prepared. As the suspension was stirred, 20 µL was carefully pipetted onto the conducting surface of a clean 1 × 3 cm tin oxide electrode, warmed slightly above room temperature by a hot plate. Upon evaporation of the ethanol, an approximately 1 cm2 film of 140 µg of Ag+-zeolite A results. Last, 10 µL of a 50 µg/mL polystyrene in tetrahydrofuran solution is pipetted onto the zeolite film to provide mechanical stability. A standard two-compartment, threeelectrode cell was used in all experiments. The working electrode was the previously described zeolite-modified electrode held in a custom-made electrode holder. All potentials were biased versus a platinum-wire quasi-reference electrode. The counter electrode was a 1 cm2 platinum flag, separated from the working and reference electrodes by a glass frit. The cell was purged with dry nitrogen prior to and during the experiments. All electrochemical measurements were performed using a Princeton Applied Research PAR 273A Potentiostat (EG&G Instruments) controlled by PC-run electrochemical software. Results and Discussion The Ag+-zeolite A-modified electrode system exhibits a complex and interesting electrochemical behavior.7 Understanding the processes at work that give rise to this electrochemical behavior is key to discerning between intra- and extrazeolite electron-transfer mechanisms and rationally further© 1997 American Chemical Society

Silver at Zeolite A

J. Phys. Chem. B, Vol. 101, No. 49, 1997 10391 surface under which the current decays and approaches the t-1/2 dependence described by the Cottrell equation. According to Scharifker and Hills, instantaneous nucleation corresponds to the formation of N nuclei per unit area at t ) 0. Consequently, all nuclei will be the same age and grow at the same rate through the experiment. For progressive nucleation, the number density of nucleation sites is function of time, N(t) ) AN∞t, where A denotes the steady-state nucleation rate constant per site and N∞ denotes the number density of active sites available for nucleation. Therefore, the nuclei will be of varying age and size through the experiment. Scharifker and Mostany9 showed that instantaneous and progressive nucleation are the limiting cases of a more general model, whereby nucleation occurs randomly on a limited number density of active sites, N0. As these active sites become converted into nuclei, the number of available active sites, N0,t, decreases and is described by the differential equation

Figure 1. Cyclic voltammetry of Ag6A at a scan rate of 20 mV/s. The insets show chronoamperometry recorded at the indicated potentials.

ing the design of advanced ZME applications. Typical results gathered for the Ag6A-modified electrode system are shown in Figure 1. The feature of primary interest is the presence of two reduction waves in the CV, a somewhat surprising feature in view of the simplicity of the probe redox system, viz., Ag+ f Ag0. It is the aim of this paper to interpret the nature of these two reduction waves, i.e., to understand the process or mechanism that gives rise to each reduction wave. The first clue to understanding the nature of the two reduction waves is given by chronoamperometry experiments. These are shown as insets in Figure 1 at several key potentials. It is clear that the shape of the current transient is very dependent on the potential. At potentials corresponding to the growth of the first reduction wave in the CV (e.g., -150 mV), the chronoamperometry current transient shows a growth-and-decay shape. At a potential between the two reduction waves in the CV (e.g., -400 mV), the current transient is a peculiarly shaped decay. The current transient of a chronoamperometry experiment taken at a potential corresponding to the region of growth of the second reduction wave in the CV (e.g., -500 mV) yields a decay-growth-decay shape. Finally, at potentials cathodic of the two reduction waves (e.g., -800 mV), the current transient decays in a non-Cottrellian fashion. It is interesting to note that the two potential ranges at which the two current growth and decay shapes are observed in chronoamperometry experiments correspond to the potentials at which the two reduction waves occur in the CV. Understanding the genesis of these transient shapes is key to understanding the nature of the two reduction waves. In the following discussion we will show how these are linked to nucleation and growth of silver at the conductive tin oxide substrate. It is first pertinent to briefly review current understanding of electrochemically assisted nucleation and growth. The growth and decay features of chronoamperometry current transients can be explained in terms of the nucleation and growth of numerous small metal deposits (nucleation centers) on the electrode surface. Scharifker and Hills8 have developed a model to describe the current transients of three-dimensional nucleation and growth under diffusion control for the limiting cases of instantaneous and progressive nucleation. In essence, the current growth is due to the increase in the electroactive area, as each independent nucleus grows in size and/or the number of nuclei also increases. As time progresses, the hemispherical diffusion zones about each nuclei grow and eventually overlap with each other. As the diffusion zones overlap, hemispherical mass transfer gives way to linear mass transfer to an effectively planar

dN/dt ) AN0,t ) AN0 exp(-At) I(t) )

zFD1/2c × π1/2t1/2

(

[

(1)

])

N0πD(8πcM/F)1/2 (t - (1 - e-At)) 1 - exp A

(2)

where A is the nucleation rate constant per active site. Using this model for the time dependence of nuclei number density, Scharifker and Mostany derived the following expression for a current transient of diffusion-controlled three-dimensional nucleation at an electrode surface: solving dI/dt ) 0 to get expressions for Im and tm yields the following dimensionless expression. 2

I2 1 (1 - exp[-xt/tm + R(1 - exp(-xt/tm))]) ) (3) 2 t/t Im (1 - exp[-x + R(1 - exp(-x/R))])2 m where x and R are dimensionless parameters defined by

x ) [N0πD(8πcM/F)1/2]tm

(4)

R ) N0πD(8πcM/F)1/2/A

(5)

and

where M and F are the atomic mass and density of silver. Scharifker and Mostany show that instantaneous and progressive nucleation, as described by these equations, are indeed the limiting cases of this model as R f 0 and R f ∞, respectively. If the constants D, c, M, and F are known, and the parameters x and R can be fit to the current transient, the number density of active sites, N0, and the nucleation rate per active site, A, can be determined for an electrode at a particular overpotential. Solution-Phase Ag+ Deposition. The deposition of silver from the solution phase onto tin oxide electrodes without the zeolite-modifying film was studied in order to illustrate the nucleation kinetics involved in silver deposition and to obtain information about the activity of the tin oxide electrode surface as a function of potential. In addition, these data clearly show the difference between the naked and modified tin oxide electrodes. The current transients of silver nucleation onto tin oxide for potentials in the range -130 to -230 mV are shown in Figure 2. Note that this range of potentials corresponds to the reduction wave in the cyclic voltammogram of solutionphase Ag+ deposition presented in Figure 3. The dimensionless plots shown in the insets of Figure 2 show the fits to the current

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Brouwer and Baker

Figure 4. Potential dependence of the number density of active sites, N0, for silver deposition onto tin oxide.

Figure 2. Current transients for the deposition of silver (1 mM AgNO3 in 0.1 M NaNO3) from solution onto tin oxide electrodes at various overpotentials in the range of the reduction wave in the cyclic voltammogram. The insets are the experimental dimensionless current transients (b), along with fits to eq 3 (s). The upper and lower curves (---) correspond instantaneous and progressive nucleation (see text).

Figure 5. Potential dependence of nucleation rate, A, for silver deposition onto tin oxide.

parameters according to eqs 4 and 5. Note that the diffusion coefficient was determined from the current transients at long times, as the current decay is described by the Cottrell equation

I(t) ) Figure 3. Cyclic voltammogram for silver deposition onto tin oxide electrode from 1 mM Ag+ in 0.1 M NaNO3 aqueous electrolyte. Scan rate ) 20 mV s-1.

TABLE 1: Experimental, Fitting, and Calculated Parameters for the Dimensionless Current Transients in Figure 1; D ) 1.3 × 10-5 cm2 s, c ) 1 × 10-6 mol cm-3, M ) 107.9 g mol-1, G ) 10.5 g cm-3 η/mV -130 -140 -150 -160 -170 -180 -190 -200 -210 -220 -230

Im/mA 0.0463 0.0463 0.0658 0.0900 0.111 0.139 0.170 0.207 0.245 0.259 0.294

tm/s 32.83 15.91 6.07 3.10 2.13 1.21 0.829 0.495 0.362 0.278 0.247

x 2.47 2.49 2.68 2.42 2.54 2.28 2.39 2.13 2.04 1.84 1.78

R

N0/cm-2

A/s-1

0.875 0.795 0.920 0.839 0.905 0.750 0.823 0.644 0.604 0.406 0.370

1.17 × 2.43 × 105 6.86 × 105 1.21 × 106 1.85 × 106 2.93 × 106 4.48 × 106 6.69 × 106 8.75 × 106 1.07 × 107 1.12 × 107

0.0860 0.197 0.480 0.930 1.32 2.52 3.50 6.68 9.33 16.9 19.4

105

transients according to Scharifker and Mostany (eq 3). The upper and lower dashed lines are the instantaneous and progressive limits of this model. The current maxima, Im, and its corresponding time, tm, and the parameters x and R used to fit each current transients are listed in Table 1, along with the values of N0 and A which were extracted from the fitting

zFScD1/2 π1/2t1/2

(6)

where S is the total electrode area (1 cm2 in this case). The diffusion coefficient for aqueous silver was found to be 1.3 × 10-5 cm2 s-1, which agrees reasonably well with the literature value of 1.648 × 10-5 cm2 s-1.10 The potential dependence of N0 and A is illustrated in Figures 4 and 5. The values of N0 and A are similar to those for lead deposition on glassy carbon.11 It is of importance to note that the number density of active sites at -220 mV is approximately 107 cm-2. This means that for the ZME there is approximately one active site per ten 1 µm diameter zeolite particles. Nucleation and Growth in ZME System. This concept of nucleation and growth can be extended to the Ag+-zeolite A-modified electrode system. As noted before, chronoamperometric experiments at potentials where the two reduction waves in cyclic voltammetry experiments occur give rise to current transients with growth and decay shapes (see Figure 1) which suggest the nucleation/growth process is operational. Figure 6 presents the results of a comprehensive set of chronoamperometry experiments on the Ag6A ZME system. For clarity, the currents have been normalized so that the shapes of the curves can be readily compared. The current transients recorded at potentials in the 0 to -300 mV range show the current growth and decay shape typical of the nucleation/growth process described earlier. Also note that

Silver at Zeolite A

J. Phys. Chem. B, Vol. 101, No. 49, 1997 10393

Figure 6. Normalized chronoamperometry current transients for Ag6A for the first 10 s at the potentials indicated. Figure 8. Current-time-potential surface for Ag6A. Note that this surface links together the cyclic voltammetry and chronoamperometry observed for this electrode. Constructed from chronoamperometry at potentials from +50 to -900 mV.

Figure 7. An illustration of sequential nucleation events followed by chronoamperometry. This qualitatively explains the current transients observed at -475 to -600 mV in Figure 6.

the current maximum shifts to shorter times at more cathodic bias potentials. This observation is similar to that observed in the solution-phase Ag+ nucleation results presented in Figure 2. This shift is a consequence of an increase in the number of active sites and the nucleation rate constant at more cathodic potentials (see Figures 4 and 5). At potentials cathodic of -300 mV, the initial current growth is not observed as the current maximum is reached before the first data point is collected. Therefore, only the decay of this nucleation process is observed at potentials cathodic of -300 mV. The current transients in the -475 to -600 mV range have a very interesting shape. There is a “bump” or what appears to be a second growth and decay curve superimposed on the decay of the first nucleation process, as illustrated in Figure 7. This second growth and decay is interpreted to be a second nucleation and growth process that also occurs at the electrode surface but requires a more cathodic potential to occur. The current maximum associated with this second nucleation/ growth process shifts to lower times with increasingly cathodic potential, an observation also made for the first nucleation process in this same ZME system and in the solution-phase Ag+ nucleation. At potentials cathodic of -600 mV, nucleation is not explicitly observed as the number of active sites is large enough that essentially the entire electrode surface can be said to be active toward silver deposition. Consequently, the current transients at these very cathodic potentials show only the current decay associated with a diffusion-controlled process. It is important to note that the electrochemical equations typically used to model chronoamperometry experiments (i.e., the Cottrell equation, Scharifker and Mostany’s nucleation model) are not quantitatively valid in many zeolite-modified electrode systems because the condition of semiinfinite diffusion is not upheld. This can occur at a ZME when the diffusion layer extends past the zeolite film since there is no bulk solution reservoir of electroactive ions. Second, intrazeolite diffusion is far more complex than diffusion in an aqueous medium.12 However, the observation of growth and decay shapes of current

transients that the nucleation model predicts strongly suggests that nucleation and growth processes are occurring. Further direct evidence showing the presence of silver deposits via AFM imaging will be presented elsewhere.6 The potentials at which these two nucleation processes are observed in chronoamperometry experiments correspond well with the potentials at which the two reduction waves occur in the CV. It is possible to extrapolate the findings of this chronoamperometry study to understanding the nature of the CV’s two reduction waves. It is important at this stage, particularly for the nonelectrochemist, to be aware of the link between chronamperometry and cyclic voltammetry. This is best visualized by considering an i-V-t surface.13 On this surface, for any given potential, i vs t curves can be constructed. A diagonal cut through this surface produces a series of curves that show current versus potential (i.e., cyclic voltammograms). The direction of the cut defines the scan rate in the CV experiment. This is a useful notion since it allows us to approach the complex cyclic voltammetry of AgA ZMEs from an in-depth understanding of the chronoamperometry. The current-time-potential of the Ag6A ZME system is presented in Figure 8. It is clear from this plot that a diagonal cut across this surface will result in more than one current maximum, in qualitative agreement with the cyclic voltammetry observed for this particular ZME (see also Figure 10). Model To Explain Two Nucleation/Growth Processes. The chronoamperometry experiments with Ag6A ZMEs suggest that the two reduction waves in the CV are two nucleation/growth processes. It is important to discuss the mechanism by which two nucleation/growth processes may be manifest. Two distinct nucleation/growth events will arise at two types of sites on the electrode surface. At these, nucleation and growth can occur, the difference between these two types of sites being the potential at which each become “active” toward nucleation. It is proposed that the first nucleation and growth process occurs on defect sites of the electrode surface and that the second nucleation occurs on the bulk electrode. It is useful to compare the cyclic voltammetry of Ag+ reduction from a zeolite-modifying film (Figure 1) and from solution (Figure 3). An important similarity to note is that the CV reduction wave for solution-phase Ag+ deposition and the first Ag+ ZME reduction wave occur over approximately the same potential range, although the ZME reduction wave is broader and shifted to a slightly more anodic potential. The

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Figure 9. Schematic representation of the electrodeposition of Ag+ (denoted by x) from a zeolite-modified electrode during a cyclic voltammetry scan: (a) ion exchange of Ag+ with the electrolyte cations; (b) nucleation of exchanged Ag+ at accessible defect sites; (c) growth of nucleation centers (note Ag+ comes only from those zeolite particles that have access to nucleating centers); (d) nucleation of Ag+ at bulk electrode once the required potential is reached; (e) growth of the second nucleation centers to which the remaining Ag+ have access.

fact that the systems share this similar reduction wave suggests that the nucleation of silver is occurring at the same type of active site on the tin oxide electrode in both systems. The most obvious and perhaps the most important difference between Ag+ deposition from solution and from a zeolite-modifying layer is that only one CV reduction wave appears in Ag+ deposition from solution. What is it about the zeolite film that allows two separate nucleation/growth electrodeposition processes to occur? To explain the differences in the electrochemical response of Ag+ deposition from solution and from the zeolite-modifying layer, an understanding of the formation of nuclei and their subsequent growth is required. The first step in this nucleation/ growth process is the formation of nuclei at the electrode surface. There are essentially two factors involved with this step. A supersaturation of metal atoms at the electrode surface is required so that the atoms will cluster together. The second condition is that there must be a site on the electrode surface at which the atoms can cluster and form the nucleussa so-called “active site.” As the process continues, there is a competition between the formation of new nuclei and the incorporation of metals atoms into already established nuclei. The growth of

Brouwer and Baker nuclei usually dominates over the formation of new nuclei. In essence, the deposition of metal ions onto nuclei of reduced metal occurs more readily than the deposition onto the bulk electrode surface, and therefore, the nucleation centers will grow. In solution-phase deposition of Ag+, only one reduction wave is observed. Nucleation at sites on the electrode surface will occur when a suitable supersaturation of Ag0 is achieved and the potential applied to the electrode is such that these sites have enough free energy to cause reduction of Ag+. In general, the sites at which nucleation occur are defect sitesssites that are different from the bulk of the surface in that they possess more free energy and thus require less energy (via the applied potential) in order bring about reduction of Ag+. Once the defect sites are nucleated, these nucleation centers will grow. Nuclei growth will dominate over the formation of any new nuclei on the bulk of the electrode as the Ag+ and Ag0 are free to move about the electrode surface and impinge upon a nucleation center to be incorporated into. It is this freedom of Ag motion that gives rise to only reduction peak in the CVsnucleation and growth at defect sites. However, the situation is much different at a zeolite-modified electrode. The presence of the film of zeolite particles on the electrode surface disrupts the free motion of Ag at the electrode surface. Each zeolite particle confines and restricts its Ag+ ions to only that area of the electrode surface at which the particle is located above. Consequently, the nature of that particular area of the electrode surface to which a zeolite particle, or, more realistically, group of zeolite particles, has access to will govern the electrochemistry of the Ag+ contained within these zeolite particles. The Ag+ in the ZME system can thus be separated into two types: Ag+ contained within zeolite particles that have access to defect sites on the electrode surface and Ag+ within zeolite particles that have access only to the bulk electrode surface. Note that for this system (vide supra and Figures 4 and 5), that at the potentials used here, there is approximately one active site per 10 zeolite particles. This model is presented schematically in Figure 9. The initial state of the ZME is depicted in Figure 9a. In general, the zeolite film will not be in perfect registry with the substrate. There will be pinholes, cracks, and possibly multiple layers of particles. The situation depicted in Figure 9a consists of two groups of zeolite particlessone group has access to a defect site on the electrode surface, and an active site is inaccessible for the other group. The arrows depict an ion exchange in which a number of Ag+ move out of the zeolite, a process that is coupled with the movement of electrolyte cations into the zeolite to balance the negative charge of the zeolite framework. Figure 9b depicts the nucleation of Ag+ at a defect site. The rate at which this initial nucleation process occurs is dependent on the concentration of Ag+ at the electrode surface, i.e., on the amount of Ag+ exchanged out of the zeolite, and on the potential applied to the electrode, since a supersaturation of Ag0 must be achieved and the defect site must have enough free energy to bring about Ag+ reduction. The number of Ag0 formed, and thus the supersaturation, will be effected by the flux of Ag+ from the zeolite. Once formed the silver atoms can move laterally across the surface (albeit impeded by the zeolite). If the silver atoms are in large enough numbers to meet supersaturation, nucleation will occur. Also, deposition at an existing nucleus can occur. Once the nucleus is established, it will grow as the Ag from those zeolite particles that access the nucleation center diffuse to the nucleation center, as pictured in Figure 9c. This first growth process will continue until these zeolite particles become depleted of Ag+. Figure 9d illustrates the second nucleation event. Once a potential is reached such that the bulk electrode

Silver at Zeolite A

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Figure 10. Cyclic voltammetry of solution-phase Ag+ (1 mM) and AgxA ZMEs: dependence on scan rate (ν) and Ag+ loading level (x). The electrolyte solution for all experiments was 0.1 M NaNO3. First scan is denoted by a solid line and the second scan by a dashed line.

surface is active toward Ag+ deposition, nuclei will form at all locations where there is a sufficient supersaturation of Ag, i.e., those regions of the ZME that did not have access to the first nucleation and growth process that occurred at the defect sites. Figure 9d shows that these nuclei will grow as the remaining Ag+ diffuses to the these nucleation centers on the bulk electrode. Note that the initial reduction of silver at highly cathodic potentials was not seen for the solution-phase experiments. At low Ag+ concentrations one would expect potential shifts due to supersaturation arguments. Indeed, similar shifts are observed for UPD of silver, in accord with the Nernst equation. Evidence in Support of the Proposed Model. Evidence for this model is provided by experiments where the cyclic voltammetry scan rate, ν, is varied and the Ag+ loading, x, of the zeolite particles is varied. The results of these experiments are presented in Figure 10 and are similar to the work of Li et al.,7 who studied the cyclic voltammetry of Ag+-zeolite A-modified glassy carbon electrodes. The cyclic voltammetry of solution Ag+ shows little variation with scan rate, and the second reduction wave is never observed at any scan rate. There are three important trends to note for the cyclic voltammetry of the various Ag+ loadings. First, the shapes of the CVs are very dependent on the scan rate. As the scan rate is increased, the magnitude of the second reduction wave relative to the first increases. Conversely, with slower scan rates, the magnitude of the first reduction wave relative to the second increases. Also, for small x, the size of the second wave relative to the first is much greater; for large x, the size of the first wave relative to the second wave is much larger. Finally, as the Ag+ loading is increased, the position of the first reduction wave is shifted to more anodic potentials. These observations are explained by the proposed model. Essentially, each trend is a consequence of the Ag+ exchange and nuclei formation steps in the model (Figure 9a,b). As more

Ag+ is exchanged out of the zeolite, there will be a higher Ag+ concentration at the electrode surface. Thus, the condition of supersaturation required for the initial formation of nuclei has an increased probability of being fulfilled. Since reduction of Ag+ can only occur once nuclei form, a greater probability of supersaturation results in increased nucleation and hence increased reduction of Ag+. Thus, for slow scan rates, a greater amount of time is allowed for the Ag+-exchange step, and more time is spent at these anodic potentials. If more Ag+ exchanges to the electrode surface, the probability of fulfilling the supersaturation requirement for the nucleation at defect sites will increase. Thus, a larger number of defect sites are nucleated, and the size of the first reduction wave increases. Since an increased fraction of Ag+ is reduced at defect sites during the first wave, the magnitude of the second wave will consequently decrease. For fast scan rates, less Ag+ exchanges to the electrode surface, the probability of reaching supersaturation decreases, the number of defect sites nucleated decreases, the amount of silver reduced at defect sites during the first reduction wave decreases, and hence the amount of silver deposited on bulk electrode during the second wave increases. Prior to discussing the Ag+ loading level dependence, it is pertinent to describe the different sites at which Ag+ cations are located within zeolite A. Four different cation positions in zeolite A have been crystallographically identified,14,15 and these sites have been shown to coordinate Ag+ with various strengths. The sites that coordinate Ag+ the strongest fill first. Therefore, for low Ag+ loading zeolite particles, the Ag+ cations are strongly coordinated, and the exchange out of the zeolite occurs slowly. For high Ag+ loading, a larger proportion of Ag+ are held at weakly coordinating sites, and the exchange out of the zeolite occurs at a faster rate. Once again we emphasize the importance of the rate of ion-exchange kinetics in ZMEs. The shift of the first reduction wave with varying Ag+ loading

10396 J. Phys. Chem. B, Vol. 101, No. 49, 1997

Figure 11. Normalized chronoamperometry of Ag2.6A. The current transients are for the first 30 s following application of the potential bias.

is thus due to this supersaturation condition. For Ag12A, the reduction wave begins at a relatively anodic potential of approximately 0 mV. Because most of the Ag+ are held weakly in the zeolite and are easily exchanged out, there will be a large concentration of Ag+ at the electrode surface. This large Ag+ surface concentration leads to a supersaturation that requires less energy for the defect sites to nucleate. Therefore, a less cathodic potential, i.e., less energy, is required to form nuclei. On the other hand, Ag2.6A requires a potential of approximately -200 mV for the first reduction process to begin. In this case, the Ag+ are held more strongly by the zeolite framework, reducing the mobility and exchange ability of Ag+. Consequently, there will be a lower Ag+ concentration at the defect sites on the electrode surface, and more energy, i.e., a more cathodic potential, is required for the formation of nuclei. The first wave for Ag6A begins at approximately -50 mV, falling between the results of Ag2.6A and Ag12A as one would expect. A similar argument can be made to explain how the relative magnitudes of first and second reduction waves depend on Ag+ loading. Increasing Ag+ loading increases the number of mobile Ag+ that can exchange out of the zeolite to the electrode surface, which in turn increases the probability of supersaturation at defect sites which increases both the rate at which nuclei are formed and the number of nuclei formed at defect sites. With more defect sites nucleated, more Ag+ reduces at the defect sites during the first reduction wave, and thus the amount of bulk deposition during the second reduction wave decreases. Evidence for the two nucleation/growth processes occurring according to this model is also found in the chronoamperometry experiments. A series of chronoamperometry experiments with Ag2.6 A ZMEs over a wide range of potentials are presented in Figure 11. The first nucleation/growth process appears at rather cathodic potentials (-200 mV) and reaches a current maximum at long times, indicating that this process is slow. The second nucleation/growth process is evident in the -475 to -900 mV range and is also occurring slowly. The current transient at -525 mV clearly shows the two nucleation/growth processes occurring simultaneously. The fact that both nucleation/growth process are occurring slowly and that cathodic potentials are required for the low loading Ag2.6A ZMEs supports the idea that the rate of nucleation and growth is regulated by the amount of Ag+ exchanged into solution and also the rate at which Ag+ exchanges. With higher Ag+ loading, the rate at which nuclei form at defect sites and number of defect sites nucleated increased dramatically, as shown by the chronoamperometry experiments of Ag12A ZMEs (Figure 12). Nucleation and growth is observed

Brouwer and Baker

Figure 12. Normalized chronoamperometry of Ag12A ZMEs. The current transients are for the first 10 s following application of the potential bias.

at a very anodic potential of +50 mV, indicating that supersaturation is achieved around the defect sites. The second nucleation is not observed in the current transients, which agrees with the cyclic voltammetry experiments. It is likely that the current response due to the second nucleation and growth process is small in comparison to the first nucleation/growth process and is hidden in the current decay of the much larger first nucleation/growth process. The chronoamperometry experiments of Ag6A ZMEs, presented earlier in Figure 6, fall between these two extremes, as one would expect. Experiments performed in different electrolytes also support the proposed model. Larger electrolyte cations such as NH4+ and tetramethyammonium (TMA+) will slow the rate at which Ag+ exchanges out of the zeolite. Consequently, the supersaturation concentration of Ag at the electrode surface decreases. As a result, it would be expected that the first nucleation occurring at defect sites will be hindered, and the magnitude of the second CV reduction wave (nucleation and growth at the bulk electrode) will increase relative to the first reduction wave. Also, it would be expected that the reduction waves will shift to cathodic potentials as more energy will be required to nucleate at the lower supersaturation of Ag. The cyclic voltammetry of Ag6A ZMEs in 0.1 M electrolyte solutions of N(CH3)4NO3, NH4NO3, and NaNO3 at scan rates of 2, 5, 10, 20, and 50 mV s-1 are presented in Figure 13. With the large TMA+ cation, only one broad reduction wave is observed. It appears that the first nucleation process is entirely suppressed or that there is a massive cathodic shift occurring. In the case of NH4+, there are two reduction waves with the second more prominent than the first and shifted to a more cathodic potential in comparison to the second wave in the Na+ electrolyte. Once again this is entirely in accord with the model proposed and described schematically in Figure 9. Note that with TMA+ the charge passed is approximately an order of magnitude smaller than with Na+ or NH4+. The diameter of the TMA+ ion is ∼6.4 Å, which may render it size excluded. However, true size exclusion may difficult to achieve. For zeolite Y (pore diameter 7.4 Å) a molecule must be 10 Å or larger to be size excluded. Nonetheless, it is likely on the time scale of the electrochemical experiment that the silver exchanged with the TMA ion originated from surface sites. We finally note that the CVs shown in this paper which are interpreted as being due to silver electrodeposition at TO resemble those of Li et al.,7 recorded on glassy carbon (GC) substrates, where intrazeolite redox is proposed. However, there are distinct differences in the relative charges passed in each

Silver at Zeolite A

J. Phys. Chem. B, Vol. 101, No. 49, 1997 10397

Figure 13. Effect of electrolyte cation size and scan rate (ν) on the cyclic voltammetry of Ag6A zeolite-modified electrodes in 0.1 M solutions of NaNO3, NH4NO3, and N(CH3)4NO3. First scan is denoted by a solid line and the second scan by a dashed line.

wave, likely signaling a substrate effect. The model proposed in this paper focuses on the active site density, the supersaturation of Ag, and how this and the lateral motion of silver are influenced by the zeolite layer. The numbers of active sites per unit area are similar for TO and GC (vide infra). Also, the solution-phase peak is at a similar potential for GC and TO. Further studies on the importance of substrate are in progress. These will involve electrochemical measurements and AFM imaging of the electrodeposited silver. Summary and Conclusions The cyclic voltammetry of Ag+-zeolite A-modified electrodes exhibits two reduction waves which are caused by two nucleation and growth processes occurring at defect sites and at the bulk electrode surface. Since these processes occur at the electrode surface, the electron transfer must occur via an extrazeolite mechanism and not by the intrazeolite mechanism described by Li et al.7 In chronoamperometry experiments, nucleation and growth processes give rise to growth and decayshaped current transients, and this current response can be modeled, under ideal conditions, by Scharifker and Mostany’s model for three-dimensional nucleation under diffusion control. The chronoamperometry of Ag+-zeolite A-modified electrodes shows two growth and decay current transients superimposed upon one another. Results of chronoamperometry that show two nucleation and growth processes can be extrapolated to explain the nature of the two reduction waves in the cyclic voltammetry via a current-time-potential surface. A model is proposed that describes how two nucleation and growth processes can be simultaneously operational whereby the zeolite film contains and restricts the motion of Ag+ to localized regions on the electrode surface. Essentially, the Ag+ in the ZME system can be broken down into two types: those Ag+ that can access defect sites on the electrode surface and those that cannot. The model consists of several steps: (1) Ag+ exchange, the exchange of Ag+ out of zeolite particles to produce a concentration of Ag+ at the electrode surface; (2) formation of nuclei at defect sites, if a suitable localized concentration of Ag+ and a suitable potential are applied to the electrode surface, supersaturation of Ag0 occurs leading to the

formation of nuclei at defect sites on the electrode surface; (3) growth of nuclei, once a nucleated defect site is established, the Ag+ that have access to this nucleation center will be reduced and incorporated into the growing nucleation center; (4) nucleation at bulk electrode, the Ag+ that does not have access to defect sites will then nucleate and deposit at the bulk electrode surface if a suitably cathodic potential is applied; (5) growth of second nuclei, once nuclei are formed on the bulk electrode surface, the nuclei will grow as Ag+ is reduced and incorporated into the nuclei. Evidence for this model is provided in cyclic voltammetry experiments by the effects of changing scan rate and the Ag+ loading level. Also, the dramatically different chronoamperometry of high- and low-Ag+ loading ZMEs agrees with this model. Acknowledgment. We gratefully acknowledge financial assistance from the Natural Sciences and Engineering Research Council of Canada. It is also a pleasure to thank Professor Jacek Lipkowski and Mr. Ian Burgess for several enlightening discussions. References and Notes (1) Ozin, G. A.; Kuperman, A.; Stein, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 359. Ozin, G. A.; Ozkar, S. AdV. Mater. 1992, 4, 11. (2) Foss, C. A., Jr.; Hornyak, G. L.; Stockart, J. A.; Martin, C. R. J. Phys. Chem. 1992, 96, 7497. (3) Martin, C. R. Science 1994, 266, 19 and references therein. (4) Koegler, J. H.; Arafat, A.; Van Bekkum, H.; Jansen, J. C. Stud. Surf. Sci. Catal. 1997, 105, 2163. (5) Klap, G. J.; Wubbenhorst, M.; van Turnhout, J.; Jansen, J. C.; van Bekkum, H. Stud. Surf. Sci. Catal. 1997, 105, 2093. (6) Baker, M. D.; McBrien, M.; Burgess, I. Manuscript in preparation. (7) Li, J.-W.; Pfanner, K.; Calzaferri, G. J. Phys. Chem. 1995, 99, 2119. (8) Scharifker, B. R.; Hills, G. Electrochim. Acta 1983, 28, 879. (9) Scharifker, B. R.; Mostany, J. J. Electroanal. Chem. 1984, 177, 13. (10) CRC Handbook; CRC Press: Boca Raton, FL, 1985. (11) Mostany, J.; Mozota, J.; Scharifker, B. R. J. Electroanal. Chem. 1984, 177, 25 (12) Baker, M. D.; McBrien, M.; Zhang, J. J. Phys. Chem. 1995, 99, 6635. (13) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley and Sons, Inc.: New York, 1980. (14) Kim, Y.; Seff, K. J. Phys. Chem. 1978, 82, 1071. (15) Thoni, W. Z. Kristallogr. 1975, 142, 142.