Electrochemical Impedance Spectroscopy at Alkanethiol-Coated Gold

Langmuir 2002, 18, 8933-8941

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Electrochemical Impedance Spectroscopy at Alkanethiol-Coated Gold in Propylene Carbonate Lesia V. Protsailo and W. Ronald Fawcett* Department of Chemistry, University of California, Davis, California 95616 Received February 4, 2002. In Final Form: July 9, 2002 Electrochemical impedance spectroscopy was successfully employed to investigate ionic permeability into self-assembled monolayers (SAMs) deposited on a Au(111) single crystal in propylene carbonate electrolyte solutions. The ionic permeability strongly depends on the applied dc potential, implying the existence of different potential dependent phases in monolayers exposed to propylene carbonate solutions. Increasing the temperature changes the ionic permeability of the SAM irreversibly. This is suggested to be due to changes in film structure due to Ostwald ripening. An appropriate equivalent circuit for characterization of the ionic conductivity of these self-assembled monolayers was developed.

1. Introduction Despite the tremendous interest in SAMs during the last two decades, only scattered reports regarding the electrochemistry of immobilized electroactive adsorbates and electron-transfer through the blocking films in nonaqueous electrolyte solutions have appeared.1-4 Attempts to use alkanethiol monolayers as barrier layers in nonaqueous solvents have for the most part been unsuccessful, presumably because of the low stability of the monolayer in these media. This is unfortunate, since nonaqueous solvents usually allow one to work in a wider range of potentials compared to that for aqueous media. Many interesting redox-active probe molecules that one might like to investigate at SAM-covered electrodes are incompatible with aqueous media, because they are insoluble or unstable in water. When the approach of electroactive solutes to the interface is blocked by a SAM, electron transfer must occur over a relatively long distance across the monolayer; this is of fundamental interest with respect to the mechanism of long-range electron transfer. Varying the solvent is an important strategy in this kind of research, since contemporary theory makes specific predictions regarding how such reactions should be affected by changes in solvent properties.5,6 In this paper results of studies of alkanethiol-based SAMs on gold single crystal electrodes in propylene carbonate (PC) are presented. PC is an interesting and widely used solvent in electrochemistry because it has a high relative permittivity but no acidic protons. Since the properties of water as a solvent are determined not only by its high permittivity but also by the fact that it can provide hydrogen bonding as a mechanism of solvation, a comparison of SAM properties in water and PC would be interesting. In addition, the properties of SAMs in PC have not been studied previously. In this initial study the structure and behavior of the electrode/SAM/electrolyte interface at equilibrium are (1) Ravenscroft, M. S.; Finklea, H. O. J. Phys. Chem. 1994, 98, 3843. (2) Curtin, L. S.; Peck, S. R.; Tender, L. M.; Murray, R. W.; Rowe, G. K.; Creager, S. E. Anal. Chem. 1993, 65, 386. (3) Everett, W. R.; Welch, T. L.; Reed, L.; Fritsch-Faules, I. Anal. Chem. 1995, 67, 292. (4) Groat, K. A.; Creager, S. E. Langmuir 1993, 9 (12), 3668. (5) Baranski, A.; Winkler, K.; Fawcett, W. R. J. Electroanal. Chem. 1991, 313, 367. (6) Winkler, K.; McKnight, N.; Fawcett, W. R. J. Phys. Chem. B 2000, 104, 3575.

reported. More specifically, the behavior of an alkanethiolbased SAM on a Au(111) electrode in an inert electrolyte was investigated for a variety of experimental conditions. 2. Experimental Section 2.1. Chemicals. Hexadecanethiol (HDT) (92%, Aldrich) was used as received. The impurities are alkanethiols with longer or shorter alkyl chains so that some nonuniformity in SAM thickness is anticipated. Sodium perchlorate (Fluka) was dried under vacuum at 100 °C for 48 h. Tetrapropylammonium perchlorate (TPAP) was recrystallized from acetone and dried under vacuum at 100 °C for 48 h. [Ru(NH3)6](ClO4)3 was prepared from the corresponding chloride using a synthesis based on their different solubilities in water. Propylene carbonate (HPLC grade, Aldrich, 99.9% pure) with residual water less than 0.005% was purified by vacuum distillation from potassium permanganate.6 2.2. Electrochemical Measurements. All of the electrochemical experiments were performed in a three-electrode cell made of Pyrex glass with a water jacket for temperature control and a Teflon cap. The cell was enclosed in a grounded Faraday cage. All the experiments, except when specifically stated, were carried out at 25 ( 0.1 °C. Temperature dependent studies were carried out using a circulating water bath around the cell to control cell temperature with a precision of (0.1 °C. Electrolyte solutions were prepared with purified PC, purged with nitrogen gas (99.997% pure) before electrochemical measurements, and kept under a nitrogen atmosphere during the course of the experiment. The working electrodes were single crystals (Metal Crystal and Oxides, Cambridge, England) with a cylindrical shape. The single crystals were mechanically polished on the face exposed to the solution using alumina powder of different grain sizes (from 3 to 0.05 µm). The electrodes were cleaned and brought to the same conditions before each experiment by electropolishing in 0.01 M HClO4 until the characteristic oxidation peaks on cyclic voltammograms were identical. Gold foil of large area was used as an auxiliary electrode. A silver wire immersed in 0.01 M silver perchlorate and 0.1 M NaClO4 or TPAP in propylene carbonate and separated from the analyzed solution by a Vycor tip (Bioanalytical System Inc.) served as the reference electrode. The silver perchlorate solution was replaced daily because of the instability of Ag+ due to photoreduction in PC. The stability of the reference electrode was checked using the [Fe(CN)6]3-/4- redox couple. The electrode was stable with a precision of (5 mV for up to 10 h. Cyclic voltammetry (CV) was performed using an EG&G 283 potentiostat/galvanostat. The electrochemical impedance measurements (EIS) were carried out with a 1255 frequency response analyzer (SOLATRON) connected with the potentiostat via a GPIB IEEE488.2 interface. The impedance data were automatically acquired using M398 electrochemical impedance software (EG&G). Impedance data were collected at 50 frequencies in the

10.1021/la0201218 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/19/2002

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Langmuir, Vol. 18, No. 23, 2002

range from 0.5 to 10 kHz. An ac potential amplitude of 10 mV rms was normally added to the dc potential of the working electrode. Both the in-phase (Z′) and out-of-phase impedance (Z′′) were extracted from the data at the same time and analyzed with the software ZView (Scribner Associates, Inc.) on the basis of Macdonald’s algorithm (LEVM 7) using a complex nonlinear regression technique (CNLS). The data weighting was chosen so that the weight for each data point was normalized by the magnitude of the datum. The impedance measurements were performed at various potentials using the following procedure: the dc potential was varied from +0.4 to -1.8 V vs the nonaqueous reference electrode. The modified electrode was preconditioned at each potential by holding it at that potential in the system for about 2 min before impedance data were collected; the potential was then stepped to a more negative value, and the procedure was repeated. 2.3. SAM Preparation. The SAM was prepared as follows. The n-alkanethiol was dissolved in ethanol to make a 30 mM solution. The Au single crystal electrode of cylindrical shape was cleaned in 0.01 M HClO4, flame annealed in a natural gas/air flame, and then quenched with Nanopure water. The electrode was then immersed in the n-alkanethiol solution for a period of 16 h to form a monolayer. The electrode was removed from the solution and rinsed with pure ethanol and then with propylene carbonate. After that, it was immediately transferred into the electrochemical cell containing the working solution. The residual water in the propylene carbonate used for preparation of the working solution was less than 0.005%. The electrode was mounted in the electrolyte solution using the meniscus contact technique, so that only the circular end of the gold cylinder touched the solution.

3. Model Choice and Data Fitting Considerations in EIS The impedance response of a particular kinetic phenomenon depends on the characteristic time constant of the process. In EIS, these time constants vary over a wide range. For example, the diffusion coefficient for an ionic species in an electrolyte is small. As a result, the time constant for mass transfer processes is relatively large, so that mass transfer determines the impedance response in the low-frequency range. On the other hand, electron transfer kinetics and double layer effects are more important at higher frequencies. For example, the time constant for double layer charging may correspond to a frequency of ∼10 kHz, the characteristic region for most electrochemical reactions is in the range 100-1000 Hz, and characteristic frequencies for diffusion fall into the range 1-10 Hz. For systems with several relaxation processes, it is often difficult to separate the various time constants. In this case it is often useful to look at different representations of the data from the impedance spectra. In this work Bode phase and Bode amplitude graphs are examined in addition to traditional Nyquist plots. Bode phase and amplitude graphs are particularly useful in the case of multi-time-constant responses because they give directly the response as a function of frequency. One of the problems of EIS is the ambiguity of the model chosen. The choice of a model of an equivalent circuit to fit EIS data should be based on the following criteria: (a) the model should be appropriate to the physical description of the given system; (b) additional experimental techniques should be applied to confirm the model when they are available; (c) statistical tools should be used to determine the relative importance of each element in the equivalent circuit. Previous experiments on the structure of SAMs in aqueous solutions using different techniques suggest that the SAM structure depends on a number of factors including temperature, chain length of the adsorbed molecule, coverage, substrate defects, and nature of the

Protsailo and Fawcett

Figure 1. Equivalent circuit used to describe the ionic permeability in the Au(111)/HDT (PC) system in the absence of the redox species. Rs is the solution resistance, CPE (constant phase element) represents the electrical double-layer capacitance, and RSAM is the resistance of the monolayer and accounts for the permeability of ions within the SAM.

functional groups on the adsorbate.8-15 Many studies have considered the formation of defects in the SAM. Attempts to use the equivalent circuits proposed earlier for systems involving monolayer covered electrodes12,15,16 failed. Attempts to obtain fits with these models resulted in higher errors for the individual circuit elements. Reliable simultaneous determination of every element in such models was not possible. χ2 and weighted sum of squares values were at least an order of magnitude higher compared to the ones obtained using the circuit proposed in the present work (Figure 1). To obtain reliable numerical values for all the elements in the circuits proposed earlier,12,15,16 the values of at least some elements should be previously known. Multiple answers are possible if one attempts to determine all the elements incorporated in those circuits on the basis of one set of data. As a result, it was concluded that it is sufficient to include in the model additional resistance in parallel to double layer capacitance to account for penetration of ions into the body of the SAM. Thus, the ionic conductivity of the SAM is expressed in terms of RSAM in the equivalent circuit used to analyze the data (Figure 1). There are two possible sources of ambiguity in CNLS fitting. First of all, a given fit may seem to be satisfactory but may correspond to a local minimum on the residual error surface rather than the deepest minimum available from the least-squares analysis. To avoid this problem, the model chosen to describe the system should be as simple as possible and contain as few parameters as possible to achieve a fit. The error in determining each element in the circuit rather than the overall error in the analysis should be taken into consideration. Very large error estimates for separate elements are typical when an incorrect model is chosen, for example, one that contains more elements than can be accommodated by the data. If the model contains too many elements, the addition of another element has no effect on the goodness of fit. The general fitting procedure for CNLS analysis in EIS defines the best set of parameters minimizing the error function S:

S)

∑i [wi′(Ie′ - It′)i2 + wi′′(Ie′′ - It′′)i′′]

(1)

The sum is taken over the total number of data points. I′ (7) Perrin, D. D.; Armarego, W. L. Purification of Laboratory Chemicals, 3rd ed.; Pergamon Press: Oxford, U.K., 1988. (8) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (9) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (10) Yamada, R.; Wano, H.; Uosaki, K. Langmuir 2000, 16, 5523. (11) Poirier, G. E. Langmuir 2001, 17, 1176. (12) Boubour, E.; Lennox, R. B. J. Phys. Chem. B 2000, 104, 9004. (13) Badia, A.; Lennox, R. B. Angew. Chem. 1994, 33, 2332. (14) Boubour, E.; Lennox, R. B. Langmuir 2000, 16, 4222. (15) Boubour, E.; Lennox, R. B. Langmnuir 2000, 16, 7464. (16) Nahir, T. M.; Bowden, E. F. Electrochim. Acta 1994, 39 (16), 2347.

Alkanethiol-Coated Gold in Propylene Carbonate

Langmuir, Vol. 18, No. 23, 2002 8935

is the real part of the immitance and I′′ is its corresponding imaginary part. The subscripts e and t denote the experimental values and those calculated from the model. wi′ and wi′′ are the weights related to the real and imaginary components of the i-th experimental point. Three choices for weights in the error analysis were considered. The assumption that wi′ ) wi′′ ) w, where w is any positive number (in the simplest case w ) 1), is equivalent to the assumption of unity weighting. This corresponds to an unweighted data analysis. This is usually inappropriate for models with frequency dependent elements because the data variance should be independent of frequency and the magnitude of the immitance. In the case of unity weighting, only large values contribute significantly to the regression analysis and poor parameter estimates are obtained. Other alternatives are modulus and proportional weighting. In modulus weighting the weights are normalized by the magnitude of each data point (eq 2).

S)

∑i

[

]

(Ie′ - It′)i2 + ((Ie′′ - It′′)i2) (|I|e)i2

(2)

The weights wi′ and wi′′ are set equal to the modulus of the immitance |I|i. If the errors of the real and imaginary parts of the immitance are strongly correlated, modulus weighting is appropriate. In proportional weighting, weights are estimated by normalizing the real and imaginary parts separately (eq 3). Then wi′ ) (Iei′)-2 and wi′′ ) (Iei′′)-2.

S)

∑i

[( ) ( Ie′ - It′ Ie′

2

+

i

)]

Ie′′ - It′′ Ie′′

2

(3)

i

Modulus weighting was chosen in the data analysis presented in this paper because it gave the most random uncorrelated residual structure.

Figure 2. Cyclic voltammogramns of propylene carbonate solutions containing 1 mM [Ru(NH3)6]3+ (A) and 0.2 mM ferrocene (B). Dotted lines represent voltammograms recorded at the bare Au(111), and solid lines show the voltammograms of the Au(111)/SAM systems. The scan rate is 100 mV/s. 0.1 M NaClO4 (A) and 0.1 M TPAP (B) were used as supporting electrolytes. (C) Potential dependence of the capacitance for Au(111)/NDT electrodes in propylene carbonate. Measurements were carried out at a frequency of 100 Hz, and the working solution contained 0.1 M NaClO4.

4. Results and Discussion 4.1. Blocking Properties of the SAM toward Electron Transfer. One of the goals of this study was to compare the structure and integrity of alkanethiol blocking films on gold in nonaqueous media with the corresponding information for similar aqueous systems. The solvent chosen for the present study is frequently used in practical devices such as batteries. It relatively easily dissolves a variety of electrolytes and is stable in a wide potential window, so a broad range of redox-active probes can be studied in PC solutions. The blocking properties of n-hexadecanethiol deposited at an Au(111) single crystal electrode surface were first studied using cyclic voltammetry. Figure 2 shows that successful blocking of the [Ru(NH3)6]3+ reduction at the electrode is observed. Somewhat weaker blocking properties are observed toward ferrocene reduction. Capacitance measurements carried out at an ac frequency of 100 Hz show that the alkanethiol-based SAM exhibits a good barrier layer property in a very wide range of potentials. The capacity remains very low and varies minimally in the potential range -1.70 to +0.40 V vs the Ag/Ag+(PC) reference electrode. The numerical value of the capacitance is around 20 times smaller in comparison with that at a bare gold electrode (∼20 µF cm-2). At potentials more negative than -1.7 V, the capacitance dramatically increases irreversibly, indicating an altering of the SAM blocking properties. Two factors could contribute to such a change: the isolating properties of these SAMs could be

Figure 3. Impedance spectra for bare Au(111) (A) and the SAM modified Au (B) for the [Ru(NH3)6]3+/2+ redox couple at the reversible potential in propylene carbonate with 0.1 M NaClO4. The upper part of each graph is a Nyquist plot showing the dependence of Z′′ vs Z′, and the lower part is a Bode amplitude plot in which log |Z| is plotted vs log f.

abruptly reduced by potential-induced desorption of the alkanethiol, or significant changes in the SAM structure could take place. The irreversibility of these changes indicates that the reductive dissolution of the alkanethiol takes place. Figure 3 shows how the impedance response for [Ru(NH3)6]3+ reduction on Au(111) changes after the HDT deposition. The Nyquist plots provide clear information

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on the system response for the cases of bare electrode and SAM covered Au(111) electrode. No information on the kinetics of [Ru(NH3)6]3+ reduction can be obtained using impedance spectroscopy at the bare Au(111) electrode (Figure 3A). The Nyquist plot of the data collected in the frequency range of interest shows results typical for the Warburg impedance. This suggests that diffusion of the species to the interface is the rate determining process, so that the estimation of the electron transfer kinetics for the reaction is impossible. Figure 3B represents the impedance response for [Ru(NH3)6]3+ reduction at a SAM covered Au(111) electrode in a 0.1 M NaClO4 propylene carbonate solution. The Nyquist plot is a semicircle with a large radius. The Warburg impedance is not present in the studied frequency range. This indicates that a monolayer is present on the surface of the gold electrode, forming a barrier that prevents direct approach of the redox species to the electrode surface. 4.2. Pinhole Analysis. A theory for the impedance response of a monolayer covered electrode behaving as a microarray electrode has been developed by Finklea et al.17 This theory was used here to map pinholes in the systems studied here and to compare the results to those of the equivalent system in aqueous solutions. To simplify the calculations, the pinhole and electrode areas covered with the SAM are assumed to be disk shaped. The area fraction of pinholes is related to the pinhole and SAM island radii by

1 - θ ) rc2/re2

(4)

where θ is the SAM coverage, re is the average pinhole radius, and rc is the average radius of the SAM covered patches. It has been previously shown17 that the common assumption that the total pinhole area fraction is equal to the ratio of charge transfer resistance of the SAM covered electrode to the corresponding term for a bare electrode is not applicable when coverage is high (θ > 0.999). For a total pinhole area fraction less than 0.1, both the real and imaginary components of the faradaic impedance are related to ω-1/2. At high frequencies, corresponding to almost isolated diffusion profiles for each microelectrode, the following expressions were obtained:12

Figure 4. Faradaic impedance of the redox couple at the reversible potential versus ω-1/2 at a SAM coated Au(111) electrode. The data give the real part of the faradaic impedance (b) and the imaginary part (O) obtained after correction for solution resistance (Rs ) 900 Ω) and double-layer capacity (Cdl ) 2 µF cm-2).

faradaic impedance; A is the electrode geometrical area in cm2; c is the redox species concentration in mol cm-3; and D is the diffusion coefficient of the redox species in cm s-1. In Figure 4 the real and imaginary parts of the faradaic impedance are plotted as a function of ω-1/2 from an experiment carried out with an n-hexadecanethiolcoated Au(111) electrode for a 1 mM solution of redox couple in propylene carbonate with 0.1 M NaClO4 as electrolyte. From the graphical representations of the data shown in Figure 4, the following characteristic parameters of the SAM were obtained:

θ ) 1 - σ/R rc )

re

x(1 - θ)

(10) (11)

and

re )

γ(1 - θ) σx0.72/D

(12)

Zf′ and Zf′′ are the real and imaginary components of the

where R is the slope of the Zf′ versus ω-1/2 plot in the high-frequency region, and γ is the intercept of the Zf′ versus ω-1/2 plot in the low-frequency region. It is worth mentioning that the mathematical removal of the interfacial capacitance from the total impedance may introduce some error into the faradaic impedance at the high frequencies. In the present case this error was not sufficiently large to impact the parameters determined in the corresponding analysis. Analysis of the data shown (Figure 4) yields a θ value of 0.9955, rc ) 3.57 µm, and re ) 0.02 µm and agrees with the corresponding parameters evaluated for the alkanethiol-coated gold electrodes in aqueous solutions.17 It should be noted that this kind of analysis was performed on the data collected at very low overpotentials. The dc potential was set to the formal potential of the redox couple, so essentially the amplitude of the ac voltage was the only external driving force for the reaction. In this case, tunneling currents have a minimal effect on the pinhole currents. The data were fit to the Randles circuit,18 and the pinhole parameters were extracted from

(17) Finklea, H. O.; Snyder, D. A.; Fedyk, J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660.

(18) Macdonald, J. R. Impedance Spectroscopy: Emphasizing Solid Materials and Systems; John Wiley & Sons: New York, 1987.

Zf′ ) RCT/(1 - θ) + σ/xω + σ/[(1 - θ)xω]

(5)

Zf′′ ) σ/xω + σ/[(1 - θ)xω]

(6)

The equivalent expressions for the low-frequency case, which corresponds to overlapping diffusion profiles for all the pinholes, are

Zf′ ) RCT/(1 - θ) + σ/xω + [σra(0.72/D)1/2/(1 - θ)] (7) Zf′′ ) σ/xω

(8)

σ ) [x2(RT/F)]/[FAcxD]

(9)

where


Figure 5. Impedance spectra of [Ru(NH3)6]3+ reduction at the SAM modified Au(111) electrode at E ) -750 mV (b), -800 mV (O), -850 mV (2), and -900 mV (4) vs Ag/Ag+(PC): (A) Nyquist plot; (B) Bode phase plot.

the faradaic impedance. The effects of the solution resistance and the double-layer capacitance were subtracted from the experimental data in this analysis. 4.3. Modulation of the Alkanethiol-Based SAM Structure in Propylene Carbonate under an Applied DC Voltage. The reduction rate of [Ru(NH3)6]3+ at a bare Au(111) electrode increases with increasing applied overpotential, as is expected according to heterogeneous electron transfer theory. For the gold single crystal electrode modified with alkanethiols of different length in aqueous solutions, this dependence is still observed.19 In this case, it is assumed that the electron transfer takes place exclusively via an electron tunneling mechanism. All the results follow the dependencies predicted by classical electron transfer theory. For the systems in propylene carbonate studied in this work, the observations are quite different. A characteristic semicircle is observed in the Nyquist plot for the impedance data acquired at a potential that is close to the formal potential of the [Ru(NH3)6]3+ reduction (Figure 5A). The radius of the semicircle decreases with increasing overpotential, as expected for an electron transfer process that takes place exclusively via electron tunneling. This demonstrates a decreasing charge transfer resistance with increase in overpotential and implies that the electron transfer rate constant increases correspondingly. This trend is observed until the potential E reaches a value of -850 mV. At a slightly larger overpotential (-900 mV), the radius of the semicircle increases significantly. This suggests that changes in some competitive processes in the system occur along with electron tunneling through the hydrocarbon chains of the SAM. A closer look at the Bode phase plot (Figure 5B) shows that the irregularities found for the potential dependence of the impedance response of the studied systems are strongly pronounced at low frequencies (f < 10Hz) where mass transport is the rate controlling process. Clearly, the substantial changes in the value of RCT found in this experiment reflect potential dependent (19) Protsailo, L. V.; Fawcett, W. R. Electrochem. Acta 2000, 45, 3497.

Langmuir, Vol. 18, No. 23, 2002 8937

Figure 6. Potential dependence of the impedance response for Au(111)/HDT in PC for 0.1 M TPAP: (A) Nyquist plot; (B) Bode phase plot. Edc ) -0.5 V (O), -0.7 V (b), -1.1 V (2), and -1.3 V (4) vs Ag/Ag+(PC).

changes in the SAM structure and its effectiveness as a barrier to electron transfer. Potential dependent studies in a similar system but in the absence of a redox species help to clarify the situation. The effect of applied dc potential on impedance spectra obtained with TPAP in PC is shown in Figure 6, where the Nyquist and Bode representations of the data are shown. In the Nyquist plot (Figure 6A) only impedance spectra collected at E > -0.7 V can be fitted to the model shown in Figure 1 with RSAM set equal to zero. Significant curvature of these plots is observed for more negative potentials (E < -0.7 V). An equivalent circuit that includes the resistance RSAM in parallel to the constant phase element was used to fit these data. The Bode phase plot is particularly informative in this case. It shows that the dependence of the signal response is strongly potential dependent at low frequencies. For the data acquired at E > -0.7 V the phase angle remains close to 90°. The decrease in phase angle at low frequencies occurs when a more negative potential is applied. Similar results were obtained for a 0.1 M NaClO4 solution in PC. Numerical simulations were done to show the difference between the ideal blocking monolayer and the results obtained in the present experiments. If the hexadecanethiol SAM behaved as an ideal capacitor, an equivalent circuit including only a solution resistance (Rs) in series with a capacitor (Cdl) would describe the data. It has been shown previously that this representation holds only for a mercury electrode. A constant phase element (CPE) is more appropriate than that of an ideal capacitor20 in the case of solid electrodes. The CPE accounts for the capacitive dispersion observed in the range of low frequencies. Numerical simulations for this kind of system are shown in Figure 7. All the parameters for the simulation were chosen to be close to the ones expected (20) Lasia, A. In Modern Aspects of Electrochemistry, No. 32; Conway, B. E., Bockris, J., White, R. E., Eds.; Kluwer Academic/Plenum Publishers: New York, 1999; p 143.

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Figure 8. Investigation of the reversibility of the potential induced changes of the Au/HDT SAM in propylene carbonate: E1 ) -0.7 V (O) vs Ag/Ag+(PC) as an initial potential; E2 ) -1.3 V (4); return to E3) -0.7 V (b).

Figure 7. Simulated impedance spectra for ideal blocking behavior of the SAM in the absence of redox species in the solution: (A) Nyquist plot; (B) Bode phase plot; (C) Bode amplitude plot.

for the studied systems. Attention is drawn to the Bode phase plot (Figure 7B). In the case of the ideal CPE, the phase angle is slightly smaller than 90° for frequencies less than 10 Hz and monotonically decreases. These changes are readily apparent on the Bode plot because the data are plotted versus log frequency. Solution resistance is determined from the impedance amplitude value at high frequencies (Figure 7C). The present experiments show that in a certain potential range the phase angle φ is markedly smaller than 70° (Figure 6B). At the same time, the Nyquist plots are curved. CPE dispersion cannot account for these observations. In this potential range the monolayer becomes permeable to ions, so that RSAM must be included in the circuit to account for the ionic conductivity. The SAM capacitance is not affected by the applied potential over the range of +0.4 V to -1.8 V vs Ag/Ag+(PC). On the other hand, certain potential-induced changes in the SAM structure occur as seen from the changes in RSAM. These changes are not accompanied by SAM reductive dissolution and are almost reversible, as can be seen from Figure 8. The applied potential was changed from E1 ) -0.7 V to E2 ) -1.3 V and then returned back to E3 ) -0.7 V. The phase angle observed at E2 at the frequency 1 Hz was equal to 55° at -1.3 V. After changing the applied potential back to -0.7 V, the phase angle at this frequency is 83°, that is, close to the initial value measured at this potential (φ ) 88°). This suggests that the structural changes of the SAM that lead to an increase in the ionic conductivity of the monolayer are almost reversible. The fitted results for RSAM for the studied systems in TPAP and NaClO4 are shown as a function of potential in Figure 9. There is a strong dependence of RSAM and, therefore, of the ionic permeability of the SAM on electrode potential.The dependence of RSAM on potential implies a phase transition of SAM structure, which is accompanied

Figure 9. CNLS results obtained for RSAM vs applied dc potential: (A) results for 0.1 M TPAP solution; (B) results for 0.1 M NaClO4 solution.

by high resistance toward ion penetration into the SAM at the maximum. The location of the maxima in RSAM may give an indication of the position of the potential of zero charge of the Au(111) single-crystal electrode. As a result of electrostatic interaction, cations penetrate the SAM if the charge on the electrode is negative (Edc < -1.1 V vs Ag/Ag+(PC)), and mainly anions penetrate into the monolayer at a positively charged electrode (Edc > -1.1 V vs Ag/Ag+(PC)). This idea is supported by the fact that the position and amplitude of RSAM peaks for electrolytes that have the same anion but different cations are similar. When the value of RSAM between the peaks reaches the minimum (E ) -1.1 V), the charge on the electrode is zero and the SAM resistivity toward anions and cations is equal. Similar experiments on ω-functionalized monolayers in aqueous systems also showed that the permeability of a SAM depends on applied potential.15 In contradiction to PC systems, only one critical potential at which the properties of the SAM changed was observed. This clearly suggests that PC and water influence SAM behavior in different ways. It is clear that a more detailed description of the phase transition observed in the studied systems


Langmuir, Vol. 18, No. 23, 2002 8939 Table 1. Results of the CNLS Fitting of the Temperature Dependence Experimental Data for the System Au/HDT in 0.1 M TPAP in Propylene Carbonatea T/°C

Rs/Ω

RSAM/MΩ

Cdlb/(µF cm-2)

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

1038 (0.31) 852.6 (0.49) 919.9 (0.76) 763.5 (0.86) 646.5 (0.65) 579.8 (0.74) 569.1 (0.97) 532.6 (0.94) 509.3 (1.27) 555.9 (1.16) 542.9 (1.15) 487.5 (1.40) 539.2 (0.79) 481.4 (0.84) 457.2 (1.16)

61.4 (3.36) 30.9 (3.08) 20.9 (3.21) 16.4 (3.42) 16.8 (3.34) 13.7 (3.13) 13.0 (3.03) 12.0 (3.01) 8.81 (3.16) 6.58 (2.56) 5.34 (2.66) 4.73 (2.60) 1.25 (0.82) 0.863 (0.92) 0.858 (1.15)

1.65 (0.15) 1.94 (0.23) 2.03 (0.36) 2.24 (0.42) 2.23 (0.32) 2.29 (0.35) 2.48 (0.44) 2.52 (0.43) 2.88 (0.57) 3.06 (0.56) 2.81 (0.58) 2.90 (0.68) 2.76 (0.49) 2.69 (0.55) 2.69 (0.77)

a The percent errors from the fit for each element are given in parentheses. b Cdl was evaluated from the CPE value.14

Figure 10. Temperature effect on the Au/HDT structure in 0.1 M TPAP solution in propylene carbonate at Edc ) -0.7 V vs Ag/Ag+(PC): (A) Nyquist plots; (B) Bode phase plots. The temperatures at which the experimental data were acquired are shown next to the Nyquist plots.

would require other experimental techniques, for example, scanning tunneling microscopy. 4.4. Au/SAM/Electrolyte Behavior as a Function of Temperature. It is well-known now that self-assembly relies on a balance between intermolecular and interfacial interactions. The appearance of crystalline imperfections and domain walls within the monolayer as well as adsorbate-induced defects in the substrate largely determines the formation kinetics and thermodynamics of these films. Temperature has been emphasized as playing a major role in further rearrangements within the chemisorbed layers. The mechanism by which these changes take place has been intensively studied previously. Unfortunately, the published work mainly concerns the SAM interface with vacuum or air.21-26 The short communication by Badia et al.13 is one of the few reports that describe the changes observed in SAM structure in aqueous solutions. There is no published work at present that shows the influence of SAM rearrangement on the permeability properties of the monolayer in solutions of nonaqueous electrolyte. Thus, all the published studies are only marginally helpful in understanding the present investigations which deal with the SAM/PC interface. The temperature effect on the impedance response of the Au/SAM in propylene carbonate with 0.1 M TPAP is shown in Figure 10. These studies were carried out at the potential at which the highest value of RSAM was observed (E ) -0.7 V). This allowed us to see any possible (21) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Putvinski, T. M.; Scoles, G. J. Chem. Phys. 1991, 94, 8493. (22) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, 93, 767. (23) Bensebaa, F.; Ellis, T. H.; Badia, A.; Lennox, R. B. Langmuir 1998, 14, 2361-2367. (24) Bensebaa, F.; Ellis, T. H.; Badia, A.; Lennox, R. B. J. Vac. Sci. Technol. 1995, A13 (3), 1331-1336. (25) Delamarche, E.; Michel, B.; Kang, H.; Gerber, Ch. Langmuir 1994, 10, 4103-4108. (26) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262-1269.

Figure 11. Investigation of the reversibility of the temperature induced changes of the Au/HDT SAM in propylene carbonate: (b) data acquired at 10 °C; (O) data acquired after the system was heated to 80 °C and then cooled back to 10 °C.

temperature induced changes in SAM structure more clearly. At temperatures below ∼35 °C, the SAM behaves as a dielectric barrier impermeable to the ionic species. The Nyquist plot response is a straight line slightly tilted with respect to 90°, as one expects for a CPE at low frequencies. At temperatures above 40 °C, the phase angle in the low-frequency domain is much lower than 90°. This decrease is more accentuated as the temperature is increased up to 80 °C. The results of the CNLS fitting of the experimental data to the equivalent circuit (Figure 1) are summarized in Table 1. It is noted that RSAM significantly decreases after the temperature reaches 40 °C, implying a high ionic conductivity of the SAM above this temperature. Similar trends were observed in aqueous systems13 at a temperature close to 35 °C. The changes observed in the studies here are found to be quite irreversible (see Figure 11). Initial measurements were taken at T ) 10 °C and compared to the data acquired at this temperature but after the heating of the system to 80 °C. The two curves do not overlap. The phase angle at low frequencies decreases as the temperature increases,

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Figure 12. Temperature dependent irreversible phase transitions of alkanethiol films of gold single-crystal electrodes in propylene carbonate solutions: (A) low-temperature solidlike phase; (B) mid-temperature coexistence of solid- and liquidlike phases; (C) high-temperature liquidlike phase.

and it stays low even after cooling the solution back to 10 °C. These observations demonstrate the increased ionic conductivity of the SAM at elevated temperatures. Possible explanations for the higher ionic conductivity in alkanethiol SAMs in propylene carbonate at high temperatures are the following: (a) Crystalline-to-Liquid Phase Change. Previous studies using low-energy He diffraction,21 FTIR spectroscopy,22-24 X-ray photoelectron spectroscopy, contact angle microscopy, scanning tunneling microscopy (STM) measurements,25 13C NMR spectroscopy, and differential scanning calorimetry26 suggest that the surfaces of CH3 terminated SAMs become disordered as the temperature of the monolayer increases. Molecular dynamics studies by Hautman and Klein27,28 indicate that the chain tilt angle decreases continuously from 32° at 200 K to 21° at 350 K. Gradual movement to the perpendicular orientation and conformational disorders represents a phase transition from crystalline-like to gel/liquidlike in the SAM structure. As a result, the ions penetrate into the disorganized SAM containing a low density of chains (Figure 12). This model does not fit our results which show that the temperature change produces irreversible impedance changes. (b) Ostwald Ripening of the Vacant Islands of the Substrate. Other studies using an ex-situ STM technique support a theory stating that the temperature increase to quite high values does not increase disorder but rather gives the opposite situation. The SAM undergoes Ostwald ripening and forms larger ordered domains.29,30 At low temperatures there are many small islands of the SAM with many small vacant islands of the Au substrate. The (27) Hautman, J.; Klein, M. L. J. Chem. Phys. 1990, 93, 7483. (28) Hautman, J.; Klein, M. L. J. Chem. Phys. 1989, 91, 4994. (29) Bucher, J. P.; Santensson, L.; Kern, K. Langmuir 1994, 10, 979983. (30) Yamada, R.; Wang, H.; Uosaki, K. Langmuir 2000, 16, 55235525.


Figure 13. Temperature dependent irreversible gradual phase transitions of alkanethiol films of gold single-crystal electrodes in propylene carbonate solutions caused by Ostwald ripening. T1 < T2. As the temperature increases, the pinhole areas and separation between them increase as well.

distances between them are small, and this prevents the penetration of the ions inside the film, keeping the ionic conductivity of the SAM low. At high temperatures the separate islands of the SAM tend to coalesce together, and as a result the total number of SAM islands and vacant islands decreases but they are larger in size. Thus, the supporting electrolyte ions can freely penetrate into the SAM and approach the substrate (Figure 13). This increases the ionic conductivity of the SAM, and low values of the phase angle in the impedance spectra are observed. Our studies of the temperature induced changes in the SAM structure support the idea that the self-assembled monolayer undergoes Ostwald ripening and reaches a stable state at T e 80 °C of highly ordered SAM domains and large domains of bare gold available for penetration by ions. 5. Conclusions Direct current potential induced areas of high and low ionic conductivity were detected in the studies performed on alkanethiol based SAMs on Au(111) in propylene carbonate. This ionic permeability in SAMs is reversible within a certain potential range. Similar ionic permeability caused by temperature changes was observed and was irreversible at the high temperatures studied here. Temperature induced ionic permeability is apparently caused by Ostwald ripening of SAM islands. An equivalent circuit was designed to account for the changes in the ionic permeability within the SAM on Au(111) in PC solutions. The χ2 and weighted sum of squares for the CNLS fits demonstrated that excellent fits of the model to the data were obtained. RSAM was introduced into the conventional equivalent circuit as a characteristic of ionic conductivity/permeability in the SAM. RSAM may depend on different factors such as the relationship between the film structure, temperature, applied dc potential, solvent-monolayer partition coefficient, diffusion coefficient of the redox couple in the


monolayer, nature of the supporting electrolyte ions, and nature of the redox couple. Correlation between RSAM and the point of zero charge (pzc) of the electrode was suggested. As previous work31 shows, a shift in pzc can be influenced by introduction of various tail groups to the molecules used to form the SAM. This kind of experiment should be performed in future work to support the RSAMpzc correlation. It has been shown that EIS can be successfully employed for evaluation of the SAM ionic conductivity but that some additional techniques are necessary in order to determine the conductivity origin. Scanning probe techniques could (31) Sinniah, K.; Cheng, J.; Terrettaz, S.; Reutt-Robey, J. E.; Miller, J. J. Phys. Chem. 1995, 99, 14500-14505.

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be used for this purpose, and the work in this area is currently in progress in this laboratory. It is clear that n-alkanethiol SAMs on gold in propylene carbonate should be considered as structures with rich phase properties and complex reorganization kinetics under potential and temperature control rather than as ideal films with uniform properties determined by the nature of the hydrocarbon chains. Acknowledgment. This research was supported by a grant from the National Science Foundation, Washington, DC (CHE-9729314). LA0201218

Electrochemical Impedance Spectroscopy at Alkanethiol-Coated Gold

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