Electrochemistry of Zeolites on Thickness Shear Mode Oscillators

13590

J. Phys. Chem. B 2005, 109, 13590-13596

Electrochemistry of Zeolites on Thickness Shear Mode Oscillators Wei Xiong and Mark D. Baker* Electrochemical Technology Centre, Department of Chemistry, UniVersity of Guelph, Guelph, Ontario, Canada N1G 2W1 ReceiVed: August 27, 2004; In Final Form: May 13, 2005

This paper describes electrochemical studies of thickness shear mode (TSM) acoustic wave oscillators coated with zeolites. The frequency response of gold on AT-cut 9 MHz quartz oscillators of silver-ion-exchanged zeolite-modified electrodes (ZMEs) under an electrochemical bias is interpreted. This is achieved using a combination of cyclic voltammetry, double-potential-step chronocoulometry (DPSC), and the frequency and resistance responses of the quartz crystal oscillators. Three ZMEs were investigated including fully exchanged Ag12A plus partially exchanged Ag6.4A and Ag3.5A. In all cases, the frequency response of the quartz crystal nanobalance (QCN) could only be interpreted when motional resistance changes were considered. This determines the importance of energy storage and energy dissipation of the shear wave produced by the oscillator in the zeolite film, which was affected by the deposition of silver at the zeolite-electrode-solution interface. The silver deposit formed via the reduction of silver ions originally within the zeolite phase mechanically couples the zeolite film to the underlying substrate. The resistance changes occurring during redox are thus linked to an inner interfacial slip between the zeolite film and the underlying oscillating surface. The data presented are consistent with an extrazeolite redox mechanism.

Introduction Electrochemical redox at zeolite-modified electrodes (ZMEs) provokes intrazeolite counterdiffusion of electroactive and charge-balancing solution phase ions.1 Electron transport at ZMEs proceeds via an extrazeolite redox process,2 whereby species within the zeolite must generally exit the zeolite prior to electron transfer, which then occurs at the electrode-solution interface or possibly the electrode-zeolite-solution interface. Redox involving charge-balancing intrazeolite cations thus relies on the passage of electrolyte ions to and from the zeolite. This may occur at the open-circuit ion exchange rate, or as we show in this paper, it can be accelerated by an electrochemical bias. Ion exchange will lead to a mass change of the zeolite provided that the redox moiety and the electrolyte cations are different. This provides an opportunity to further probe the electrochemical behavior of ZMEs by monitoring the mass of the electrode as the potential bias is applied. The thickness shear mode (TSM) oscillator is well suited to this task. In its most common format, this comprises an ATcut quartz wafer coated on both sides with electrodes. An alternating electric field across the crystal piezoelectrically induces the propagation of shear waves in the thickness direction. Sauerbrey3 showed that the resonant oscillation frequency could be related to the amount of attached mass. This treatment assumes that the attached matter has the same acoustic properties as quartz. For many inorganic and biochemical films, this is not true,4 meaning that the analysis must be refined. This is addressed later in the discussion section of this paper. In this study, the TSM oscillator is used in an electrochemical environment as the working electrode. This is the basis of the electrochemical quartz crystal nanobalance (EQCN). Gold- and platinum-coated quartz crystal oscillators are well-known for their nanogram sensitivity and have been used in many electrochemical studies.5 A potential avenue of future research is to use zeolite films as templates for electrochemical syntheses

of nanostructures, and the nanobalance could be a sensitive and convenient monitor of growth. In addition, the concept of using zeolite films on a TSM oscillator as selective chemical sensors can only be realized if the response of the crystal is interpreted correctly. However, unless the mass response of the crystal is understood, this will be impossible. This entails, as discussed later, a consideration of both frequency and resistance responses of the QCN. To expedite this, we use a relatively well-studied system, namely, silver-ion-exchanged zeolite A. The frequency of a quartz crystal oscillator varies according to the mass of a deposit on the conductive electrode. If the film has the same acoustic properties as quartz, the Sauerbrey equation3 is used to determine the additional mass.

∆f ) -2f2∆m/[A(µF)1/2]

(1)

∆f (the frequency change) is thus related to the mass change (∆m), the fundamental resonant frequency (f), the area (A), and the density (F) and shear modulus (µ) of quartz. The relationship indicates that the resonant frequency decreases linearly with mass increase. In the case of a zeolite film, however, the use of this equation is inappropriate. The shear wave generated by the quartz oscillator propagates into the zeolite and potentially (depending on the film thickness) into the solution phase. The energy storage and dissipation properties of the film must therefore be considered. This can be achieved by measuring the complex impedance of the quartz oscillator. The imaginary part is representative of energy storage and is related to the frequency change (∆f), while the real part is related to the energy dissipative Q-factor or motional resistance of the device.6 These are further considered later in this paper. These properties can be experimentally observed during redox using a correctly configured electrochemical quartz crystal nanobalance (EQCN). This is conveniently achieved by using an equivalent circuit analysis.4 An equivalent circuit of an

10.1021/jp0405862 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/22/2005

Electrochemistry of Zeolites on TSM Oscillators

J. Phys. Chem. B, Vol. 109, No. 28, 2005 13591

unloaded oscillator is constructed containing resistive, capacitive, and inductive parts of the oscillator circuit. Additional loadings on the crystal via either mass, liquid, or viscoelastic changes are modeled by adding resistive and inductive elements to the circuit. These will change the resonant oscillation frequency. The most commonly used equivalent circuit is the ButterworthVan Dyke (BVD) model,7 and for a loaded resonator, a modified BVD circuit is used.8 In this study, we consider how the total observed frequency change is actually the sum of resonant frequency changes induced by resistance effects and those due to pure mass loading of the crystal. Experimental Section Zeolite A was synthesized in colloidal form using a modification of the methods developed by Schoeman and co-workers.9 Films were then formed onto 9 MHz gold-coated AT-cut quartz crystals (Perkin-Elmer, Owensboro, KY) in the following manner. The clean gold surface was first exposed to 600 µL of 10 mM γ-mercaptopropyltrimethoxy silane (MPS, 95% purchased from Aldrich) in methanol for 10 h. The silanized substrate was then rinsed with Millipore water to remove excess MPS. The surface was then hydrolyzed with 200 µL of 0.1 M HCl for 5 h. Following this, the MPS-Au surface was exposed to 400 µL of a cationic polymer (ATC 4050, EKA Chemicals, average MW 50 000). The pH was then adjusted to 8 with NaOH over a period of 4 h and then exposed to a colloidal suspension of pre-exchanged zeolite A in water. The modified electrodes were air-dried in the dark and then placed in a “well” cell. The rear of the electrode was mounted on a Viton O-ring to ensure that the liquid phase did not impinge on the rear of the oscillator. The reference and auxiliary electrodes were both platinum wires. Silver nitrate (99.993%, Aldrich) was used as received. Three silver zeolite A (AgA) samples were used in this study; Ag12A, Ag6.4A, and Ag3.5A. These were ion exchanged ex situ, and unit cell compositions were determined by silver atomic absorption spectroscopy. Film thickness was determined at 5 µm using an interference microscope (Veeco). The electrochemical quartz crystal nanobalance is comprised of a QCA922 analyzer and a model 263A potentiostat (EG&G, Princeton Applied Research, U.S.A.). The QCA922 analyzer records simultaneously both the frequency and resistance responses of the TSM resonator. Frequency and resistance could be recorded at a resolution of 0.1 Hz and 0.1 Ω, respectively. Gold-coated (300 nm thick) AT-cut quartz crystals were used, with a resonant frequency of 9 MHz and an area of 0.196 cm2. Using the Sauerbrey equation, this gives a mass sensitivity of 1.07 ng Hz-1. In this paper, we will use the following sign convention. Each resistance and frequency change is preceded by a sign indicating either a decrease (-) or increase (+) in the measured value. Results and Discussion Prior to discussing AgA-modified electrodes, we will examine the TSM resonator response to the deposition of silver on bare Au. Figure 1 shows the cyclic voltammetry of Au in 93 ppm AgNO3/0.1 M NaNO3 together with the corresponding QCN response. In these experiments, the resistance changes (∆R) were within (1 Ω. This means that the deposited silver films were not affecting energy dissipation (see the Introduction). The voltammetric and QCN data are labeled A(a) through E(e) where upper case refers to the voltammetric experiments. The scan direction in both cases is A(a) to E(e). Each trace is labeled at critical points and these are referred to in the discussion below. Around B(b) (-110 mV), Ag+ reduction is

Figure 1. EQCN response of bare gold in 93 ppm AgNO3/0.1M NaNO3. Scan rate: 10 mV/s. (O) frequency response; (b) voltammetry.

indicated by virtue of an increase in the cathodic current and a decrease in the oscillation frequency. At the switching potential C, and to D, the current remains cathodic, and correspondingly, the resonance frequency further decreases due to silver deposition. At D(d), the stripping potential is reached and the frequency of the crystal increases as silver strips into the solution phase. Integration of the cathodic wave (A to D) gives a charge of 388 µC, corresponding to 434 ng of Ag deposition on the Au surface. Using the conversion factor 1.07 ng Hz-1 (see the Experimental Section), this would lead to an observed frequency change of -406 Hz. The maximum frequency decrease at the switching point is in fact -405 Hz, in agreement with the predicted value. Similarly, integration of the anodic stripping peak (D to E) yields a charge of 369 µC, which would lead to a ∆f value (frequency increase) of +387 Hz. The difference between the cathodic and anodic charge would actually lead to an overall frequency change of -19 Hz, which is close to that observed experimentally (-15 Hz). The discrepancy of 4 Hz probably stems from inaccuracies in integration of the anodic peak due to the unknown charging currents. We now examine the QCN response of the bare gold electrode during double-potential-step chronocoulometry (DPSC). In DPSC, the potential of the electrode is stepped from a base potential (where no redox occurs) to a potential where redox will occur. The charge is recorded as a function of time, and then, the step is reversed. For the forward step, the charge varies with the square root of time according to the integrated Cottrell equation:

Q ) 2nFACπ-1/2(Dt)1/2

(2)

During the forward potential step, at time t, Ag is deposited on the electrode. The total charge passed is given by eq 3.10

QF ) 2nFACAg+(DAg+t)1/2(π)-1/2 + Qdl

(3)

where CAg+ is the bulk concentration of reactant Ag+, DAg+ is the diffusion coefficient of Ag+, and Qdl is the double-layer charge. This deals with a redox process where the oxidized species is soluble and the reduced species is deposited on the electrode surface. This case is applicable to the Ag+/Ag system. In the reverse step, Ag is reoxidized and the charge passed (QR) is given by10

QR ) 2nFACAg(DAgθ)1/2(π)-1/2 + Qdl + Qad

(4)

13592 J. Phys. Chem. B, Vol. 109, No. 28, 2005

Xiong and Baker

Figure 2. DPSC of bare Au in 2 mM AgNO3/0.1 M NaNO3. Data for two switching times (O, τ ) 2 s) and (b, τ ) 4 s) are shown.

where θ1/2 ) τ1/2 + (t - τ)1/2 - t1/2, τ is the switching time, Qad is the charge arising from the oxidation of adsorbed Ag on the electrode surface, CAg is the concentration of Ag, that is, the solubility of Ag, and DAg is the diffusion coefficient of the dissolved Ag. The charge passed due to adsorption (Qad ) is given by

Qad ) 2nFA[CAg+(DAg+)1/2 - CAg(DAg)1/2](τ)1/2(π)-1/2 t1/2

(5)

A plot of QF versus in the forward step is linear with an intercept of Qdl, while QR versus θ1/2 in the reverse step is irregular. The linear portion, however, can be extrapolated to give an intercept of Qdl + Qad. Therefore, the difference between the two intercepts is Qad. Using this method, the mass of silver deposited on the electrode can be determined. This can then be compared to the expected mass change due to changes in the resonant frequency of the quartz crystal. DPSC for silver reduction and reoxidation at Au is shown in Figure 2 for step times of 2 and 4 s. Note that as expected from eq 3 the slopes for the forward steps are identical. For the reverse step, the slopes are identical (eq 4) but the intercepts are different. This is expected because more silver is deposited for the longer step time. Linear regression for the forward steps in Figure 2 gave Qdl values of 5 and 6 µC for τ ) 2 and 4 s, respectively. Intercepts for the reverse step, (Qdl + Qad), were 277 µC (τ ) 2 s) and 391 µC (τ ) 4 s). This gives Qad ) 272 and 385 µC for τ ) 2 and 4 s, respectively. Note that the ratio Qads(τ ) 4)/Qads(τ ) 2) is 1.42, in agreement with the predicted value 1.414 (x4/2), the ratio of the square root of the switching time for τ ) 4 and 2 s. Simultaneously recorded QCN data are shown in Figure 3. In concert with the above, the resonance frequency decreases during the forward step and then returns to its original value following the stripping of silver. The resonant resistance change was (1 Ω, which was close to the noise, and thus, the resonant resistance changes can be safely ignored. One therefore expects that the resonant frequency changes will follow the Sauerbrey equation. The charge Qad (272 µC) determined from DPSC at τ ) 2 would produce a frequency decrease of -286 Hz (using the Faraday and Sauerbrey relationships). The experimentally observed frequency change in Figure 3 was -280 Hz. The data for τ ) 2 and 4 s are assembled in Table 1. Together, the voltammetry, DPSC, and QCN data give a consistent description of silver reduction at gold. In this case, the resonant frequency

Figure 3. Frequency response of bare gold in 2 mM AgNO3/0.1 M NaNO3. The switching times (O, τ ) 2 s) and (b, τ ) 4 s) correspond to the data in Figure 2.

TABLE 1: Comparison of the Charge Passed in DPSC and the Frequency Response for Silver Deposition on Bare Gold for Switching Times of τ ) 2 and 4 s intercept for charge for calcd frequency obsd frequency reverse step adsorbent decrease due to decrease due to (QIn) (Qad)a silver adsorbentb silver adsorbent (Hz) (µC) (µC) (Hz) τ)2s τ ) 4s

277 391

272 385

-286 -405

-280 -420

a Qad ) QIn - Qdl, where Qdl is the double-layer charging from a linear regression of the forward step in Figure 2. b Frequency decrease is calculated by using 1.04 Hz/µC, which is obtained from the Faraday and Sauerbrey relationships.

changes observed in both voltammetric and coulometric experiments are consistent with a metallic deposit that possesses the same acoustic properties as the TSM oscillator and thus the data are amenable to interpretation via the Sauerbrey equation. We now discuss the behavior of silver-zeolite-modified electrodes. Cyclic voltammetry of silver-zeolite-modified gold electrodes and simultaneous EQCN data were recorded, and these are shown for Ag3.5A in Figure 4. Note that completely analogous data were observed for Ag6.4A and Ag12A. The composite figure shows current, frequency change (∆f) and resistance change (∆R) as a function of potential. As before, the plots have been labeled (A to D) at critical points for ease of description and discussion. Note that the potential scan direction was from A to D, as shown by the arrows. At potentials between +400 and -70 mV, the voltammetry shows no substantial reduction of silver. Nonetheless, ion exchange between intrazeolite Ag+ and solution phase Na+ can occur, which will change the mass of the electrode. The ion exchange rate was slow, resulting in frequency and resistance changes that were very small compared to the large changes observed when redox was occurring (vide infra). We discuss elsewhere the frequency and resistance changes that occur at an open-circuit potential, that is, under normal ion exchange conditions.11 During section B of the cathodic scan, Ag+ is more rapidly reduced, causing substantial deposition. The multiple cathodic peaks observed here have been discussed elsewhere.12 Note that this alone does not increase the mass of the electrode because Ag+ was present within the zeolite, before redox occurred.



Figure 4. EQCN response of Ag3.5A in 0.1 M NaNO3: (a) cyclic voltammetry; (b) frequency change; (c) resistance change; (d) ∆f vs ∆R. Arrows indicate the scan direction.

However, as silver reduction proceeds, charge-balancing hydrated sodium ions from the electrolyte enter the zeolite, causing a mass increase (vide infra). It can be seen from the figure that the entrance of the sodium ions into the zeolite phase is substantially accelerated upon silver reduction, causing a remarkable frequency decrease (Figure 4b (A to B)). This is due to the rapid reduction of Ag+ at the electrode-solution interface causing an increased flux of silver from the zeolite and thus an increased uptake of charge-balancing Na+ from the electrolyte by the zeolite phase. Unlike the bare gold surface, there is also a concomitant increase in the resistance contribution to the frequency change (Figure 4c). This is likely due to the deposition of silver between the zeolite particles and the underlying surface as discussed in ref 12. This will couple the zeolite film more strongly to the gold resonator. We will discuss this effect later in this paper. During the reverse scan (initiated at a vertex of -500 mV), the silver deposit strips off the electrode surface, causing a mass decrease and a simultaneous frequency increase. The stripping potential is close to -200 mV. The coupling between the film and the substrate is also affected. This can be followed in Figure 4d from C to D by a rapid decrease of resistance from C to D which implies an immediate decrease of coupling between the TSM oscillator and the zeolite film. This is reasonable considering that, after the Ag stripping, the zeolite coating on the electrode surface is now only connected by a pliant polymer (MPS). In Figure 4b, from 100 to 400 mV (D region), the EQCN

response shows no frequency or resistance changes, indicating that the electrode coating remained stable after the oxidation of Ag. We now quantify these data in terms of the absolute frequency, resistance, and voltammetric responses of the electrodes. This can be achieved by first considering the relationship between the measured frequency and resistance changes of the TSM oscillator. If the quartz crystal oscillator is treated as a lossless elastic solid and the surrounding medium is treated as a purely viscous medium with no chemical interaction with the electrode, then the oscillation frequency change is given by13

∆f ) -f1.5q(Flη1/πFqµq)1/2

(6)

where f is the fundamental resonant frequency of the quartz crystal of density Fq and elastic modulus µq and the surrounding medium density and viscosity are given by Fl and ηl, respectively. The resonant resistance of the oscillator is given by14

R ) A(2πFqF1η1)1/2/K2

(7)

where A is the surface area of the resonator and K is the electromechanical coupling constant, which is an indicator of the effectiveness with which a piezoelectric material converts electrical energy into mechanical energy.


Xiong and Baker

From eq 6, the frequency change varies with (Fη)1/2 as does the resistance (eq 7). A linear relationship between ∆f/∆R has indeed been demonstrated by several authors.15-18 Note that ∆R is an experimental quantity of the resistance change observed between two regions, as shown later. The proportionality constant between ∆f and ∆R for AgxA ZMEs will be determined later in this paper. Lucklum et al. have thoroughly discussed the proportionality between ∆f and ∆R (see, for example, ref 17). The discussion was based on the variation of ∆f/∆R with several experimental parameters for a viscoelastic film immobilized on a TSM oscillator. The value of ∆f/∆R was determined to be constant for particular combinations of G′ and G′′ (defined in ref 18 as the energy loss and storage moduli of the film) and various film thicknesses. The notion that for a particular film there will be a constant relationship between ∆f and ∆R is indeed upheld, as shown later in this paper. The experimental determination of ∆f/∆R is indeed successful in correlating electrochemical and gravimetric data, as discussed below. It is important to realize that the total frequency change (∆fT) observed during electrochemical biasing of the Ag ZME stems from two sources: (1) mass-induced frequency changes, as expected from the Sauerbrey equation (∆fM) and (2) resistanceinduced frequency changes (∆fR) as discussed above. Thus,

∆fT ) ∆fM + ∆fR

(8)

The last term in eq 8 (∆fR) is influenced by several factors including the viscosity and density of the surrounding medium and also changes in the nature of the interface between the zeolite layer and the gold substrate. This will be discussed in detail later in this paper. In Figure 4, ∆fM results from the entrance of charge-balancing sodium cations into the zeolite phase and ∆fR results from changes in the zeolite coating and how it couples to the underlying gold substrate (see later). ∆fT is measured directly by the QCA922 analyzer, and ∆fM can be determined from the charge passed for the Ag+/Ag redox reaction because the number of moles of Ag+ reduced equals the number of moles of charge-balancing sodium cations that enter the zeolite to maintain charge neutrality. This enables the separation of the two contributions to the total observed frequency change. We now demonstrate this for the Ag3.5A sample. This involves the determination of ∆f/∆R, which is termed χ in the remainder of this paper. Integration of the cathodic peak in Figure 4a gives 430 µC corresponding to 4.46 nmol of redox active silver. This must evoke the entrance of 4.46 nmol (103 ng) of charge-balancing sodium cations into the zeolite and a frequency decrease of -96 Hz (Sauerbrey equation). The frequency decrease (∆fT) from A to C (-200 mV) (Figure 4b) is -258 Hz combined with a resistance increase of 28 Ω (region A to region C). This gives χ ) (-258 + 96)/28 ) -5.79 Hz/Ω. For a complete voltammetric cycle in Figure 4b (region A to D), the total frequency increase (∆fT) is +648 Hz and the resistance decrease is 60 Ω (Figure 4c, region A to D). This is expected, since silver is lost to the solution phase. Recall the frequency decrease due to the entrance of sodium (vide supra) will be 96 Hz. The total frequency change of 648 can now be understood as follows by using the χ value from the cathodic (A to C) part of the cycle:

648 ) -96 + ∆fAg + χ(-60) since

∆fT ) ∆fM + χ(∆R) + ∆fAg (substituting χ ) -5.79 into the above). This gives the frequency increase due to stripping silver (∆fAg) as 397 Hz which would stem from a charge of 381 µC (using a conversion factor of 1.05 Hz/µC from the Faraday constant and a mass sensitivity of 1.07 ng Hz-1, see the Experimental Section and Table 3). Integration of the anodic peak gives 412 µC in reasonable agreement. The difference lies in the difficulty in integrating the voltammetric peak accurately due to unknown charging currents. Note that these data are entirely consistent with the notion that redox chemistry displayed by intrazeolite cations is via an extrazeolite redox mechanism whereby the ion initially present within the zeolite is electroactive only at the electrode solution interface and is steadily lost to the solution phase during the anodic cycles. No silver is irreversibly reduced and enters the zeolite phase. The frequency increase and resistance decrease during the CV scan agreed well with the amount of Ag+ involved in ion exchange. Analogous data were collected for Ag12A and Ag6.4A. The data are assembled in Table 2. We next discuss the DPSC/QCN response of AgA. DPSC, frequency, and resistance responses produced by the potential steps are shown in Figures 5, 6, and 7, respectively. The potential step in these experiments was from an initial potential of +500 to -500 mV. In Figure 5, the step to a reducing potential was initiated at t ) 0. In each case, the frequency gradually decreased for the 10 s duration of the measurement. At t ) 10, the step to an anodic potential caused Ag to strip off the electrode and an increase in frequency occurred. We now quantify and interpret these data for fully ion-exchanged Ag12A. Once again, the behaviors of the other two zeolite samples were analogous. In the following analysis, the mass-related response of the QCN is related to the charge passed in the electrochemical experiment and then the resistance-induced frequency change is used to calculate the expected total frequency change of the resonator. In Figure 6 for the forward step Ag12A, the total charge passed was 621 µC (corrected for a double-layer charging current of 13 µC), associated with the reduction of silver and concomitant ingress of sodium ions. The charge passed corresponds to 6.44 nmol of sodium entering the zeolite phase, which in the absence of resistance changes would give a frequency decrease of -137 Hz. This was determined by a double-step experiment using a NaA zeolite where no redox processes occur. The resistance change for the forward step (Figure 7) is +87 Ω, which corresponds to a frequency decrease of -485 Hz (using χ from Table 2). This would give rise to a total frequency decrease of -622 Hz close to the observed value (-595 Hz). The resistance response (see Figure 6) was noisy (and the source was not identified) for these electrodes producing these discrepancies. For the reverse step, the DPSC extrapolated intercept (minus charging current) gives a charge of 585 µC. The overall frequency and resistance changes observed were +1717 Hz and -200 Ω (-200 × 5.58 ) -1116 Hz), respectively. Thus, the mass-induced frequency change was (1717 - 1116)601 Hz corresponding to 643 ng of silver stripped from the electrode. This would require 572 µC in good agreement with the DPSC data (585 µC). Similar data were collected for the partially exchanged zeolites, and the results are given in Table 3. The data interpretation above necessitated a consideration of the resistance- and mass-induced frequency responses of the TSM oscillator. Mass changes are associated with chargebalancing cations entering the zeolite upon redox followed by subsequent stripping of reduced silver into the solution phase.



TABLE 2: Summary of Data Collected for AgxA during Cyclic Voltammetry whole cycle frequency change (Hz)

whole cycle resistance change (Ω)

χ (Hz/Ω)

648 1001 1475

-60 -104 -170

-5.79 -5.36 -5.58

Ag3.5A-MPS-Au Ag6.4A-MPS-Au Ag12A-MPS-Au

TABLE 3: Summary of Data Collected for of AgxA during DPSC

Ag12A

F R

Ag6.4A

F R

Ag3.5A

F R

Qa (∆fM)

∆Rb (∆fR)

∆f (calcd)c

∆f (obsd)d

(Hz)

(Hz)

621 µC (-137 Hz)e 585 µC (614 Hz) 426 µC (-94 Hz)e 364 µC (382 Hz) 302 µC (-67 Hz)e 261 µC (274 Hz)

87 Ω (-485 Hz) -200 Ω (1116 Hz) 41 Ω (-220 Hz) -121 Ω (648 Hz) 19 Ω (-110 Hz) -55 Ω (318 Hz)

-622

-595 1717

1730 -314

-325

1030

1078

-177

-175

592

615

Figure 6. DPSC plot of AgxA in 0.1 M NaNO3 where x ) (1) 3.5, (2) 6.4, and (3) 12.

a Charge observed in forward and reverse steps. The figures in parentheses (forward step only) are the corresponding frequency changes due to sodium ingress. For the reverse step, charge and frequency are for silver stripped. b Observed resistance changes for forward and reverse steps. The figures in parentheses are the corresponding frequency changes using χ from Table 2. c Total frequency change expected using data from columns 3 and 4. d Observed total frequency change of the TSM oscillator. e Mass-induced frequency change is due to the ingress of Na+. F and R refer to forward and reverse steps. Conversions were done using -1.07 ng/Hz, and for coulometric data where Ag+ was reduced, 1.05 Hz/µC was used. Resistance to frequency conversion used χ from Table 2.

Figure 7. Resistance responses during DPSC in 0.1 M NaNO3: (1) Ag3.5A; (2) Ag6.4A; (3) Ag12A. Step potential: -500 mV. The overall resistance changes (from 0 to 15 s, forward and reverse steps) are -34.2, -80.4, and -112.8 Ω for electrodes 1, 2, and 3.

Figure 5. Frequency responses during DPSC in 0.1 M NaNO3: (1) Ag3.5A; (2) Ag6.4A; (3) Ag12A. Step potential: -500 mV. The overall frequency increases are 440, 753, and 1122 Hz for electrodes 1, 2, and 3.

Charge balance is afforded by hydrated cations. Ion exchange will inevitably lead to a change in the water content of the zeolite, due to the different sizes of the hydrated ions, and an associated mass change of the electrode. It is apparent from the correlation between charge passed (voltammetry and coulometric data) from the mass response of the TSM oscillator that the change in water content is small compared to cation-

induced mass changes. Indeed, the analysis in this paper ignores any contribution from changes in the water content of the zeolites. We now examine the source of frequency changes for the zeolite-coated TSM oscillator associated with resistance (i.e., ∆fR). These are apparent resonant frequency shifts that are not the direct result of mass loadings on the quartz oscillator. The source of the resistance changes appears to be the inner interfacial coupling between the zeolite and the underlying (oscillating) gold surface and necessitates a discussion of “slip” at TSM oscillators. To facilitate this, we return to a consideration of the propagation of the acoustic wave generated by the crystal oscillation. Recall that the motional resistance term is related to how energy is dissipated in the film. The displacement of the crystal surface at a very high frequency (9 MHz) and small displacements (3-5 Å) means that the shear rates are of the order of 1000 s-1. This can result in severe slip.19,20 Slip can be interpreted in several ways. For example, it can be thought of as a result of the friction between two moving surfaces.21 In essence, the film may not be able to respond to the motion of


Xiong and Baker Summary and Conclusions In this paper, we have shown that it is possible to track the redox processes occurring at zeolite-modified electrodes using an electrochemical quartz crystal nanobalance. However, the frequency changes can only be interpreted correctly when motional resistance changes are accounted for. The nanobalance can only be used to study ZMEs if the instrument is capable of simultaneous frequency and resistance determinations. Motional resistance changes of Ag ZMEs are associated with the deposition and stripping of silver to and from the conductive gold substrate. The deposition of silver apparently couples the film strongly to the quartz oscillator causing a large increase in resistance.

Figure 8. Resistance change for Ag12A as a function of charge passed in DPSC for the forward step.

the quartz oscillator, and slippage between the substrate and film can occur. If the decay length of the acoustic shear wave is of the order of 1 µm,22 unlike thin biomolecular films, only inner slip (Au zeolite) needs to be considered. In other words, the decay length of the shear wave is solely contained within the zeolite phase. As silver deposits form between the zeolite particles and the surface,12 the slip will decrease and the zeolite film will be coupled to the oscillator motion. Ellis and Thompson have recently modeled this.6 In the case of the zeolite film, as the motional resistance due to a reduction in inner slip increases, the resonant frequency will decrease. It is interesting to contemplate this as a probe of inner slip. The silver deposit can be thought of almost as a film of “glue” between the gold surface and the zeolite. As the amount of silver deposited increases, so will the film-substrate coupling. Indeed, if the amount of silver deposited is plotted against ∆R, there is a good linear dependence. The data shown in Figure 8 were produced by plotting the charge passed in DPSC (see Figure 5) against the corresponding resistance changes for Ag12A. The plot was tested for correlation using a SAS package giving a significant linear relationship with a correlation coefficient of R ) 0.91 and a p value of

Electrochemistry of Zeolites on Thickness Shear Mode Oscillators

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