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Langmuir 2002, 18, 9342-9349
Electrochemical Characterization of the Alkaneselenol-Based SAMs on Au(111) Single Crystal Electrode Lesia V. Protsailo and W. Ronald Fawcett* Chemistry Department, University of California, Davis, California 95616
Dale Russell and Ryan L. Meyer Chemistry Department, Boise State University, Boise, Idaho 83725 Received April 12, 2002. In Final Form: July 9, 2002 The formation and behavior of self-assembled monolayers made of n-dodecaneselenol (DDSe) on Au(111) as a blocking dielectric medium toward heterogeneous electron-transfer were assessed by electrochemical impedance spectroscopy. Microelectrode array theory was applied to study the growth kinetics of the self-assembled monolayer. The results show that adsorption from a n-dodecaneselenol solution in ethanol can be described using two different time constants. The monolayer is formed within the first minute after the electrode has been brought into contact with the deposition solution. The second step involves film reorganization and self-ordering, which can last for several hours.
Introduction Recent studies have generated a detailed picture of the structure and properties of alkanethiol monolayers on gold substrates.1 A variety of published papers on monolayers composed of alkanethiols containing different functional groups at the soluton end of the self-assembled monolayer (SAM) is available. In contrast to this, there has been a little work done to study ordered monolayers on metals, which use an anchoring group other than sulfur. Despite the strong affinity of alkanethiols for gold, they are subject to oxidation on Au.2,3 Raman spectroscopy and X-ray photoelectron spectroscopy show oxidized sulfur modes after only a few hours of air exposure, especially when a minor amount of ozone is present.4 Once oxidized, the resulting aromatic sulfonates have been claimed to subsequently desorb upon aqueous rinsing.5 In addition, the stability of alkanethiol monolayers is strongly dependent on the temperature.6 The proper choice of the functional headgroup can affect the organization of molecules with the SAM. Well-organized films using alternatives to thiol-based monolayers would be of fundamental interest. The importance of the headgroup-substrate bond in monolayers has been discussed recently.7 Studies conducted using contact angle measurements, optical ellipsometry, X-ray photoelectron spectrometry, reflection absorption infrared spectroscopy, and near-edge X-ray absorption fine structure spectroscopy have shown the surface-thiol interaction potential and the bending (1) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, p 109. (2) Rieley, H.; Price, N. J.; White, R. G., Blyth, R. I.; Robinson, A. B. Surf. Sci. 1995, 331-333, 189-195. (3) Dishner, M. H.; Feher, F. J.; Hemminger, J. C. Langmuir 1994, 10, 626-628. (4) Schoenfisch, M. H., Pemberton, J. E. J. Am. Chem. Soc. 1998, 120, 4502-4513. (5) Garrell, R. L.; Chadwick, J. E.; Severance, D. L.; McDonald, N. A.; Myles, D. C. J. Am. Chem. Soc. 1995, 117, 11563-11571. (6) Dishner, M. H.; Hemminger, J. C.; Feher, F. J. Langmuir 1996, 12, 6176-6178. (7) Rong, H. T.; Frey, S.; Yang, Y. J.; Zharnikov, M.; Buck, M.; Wu¨nk, M.; Wo¨ll, C.; Helmchen, G. Langmuir 2001, 17, 1582-1593.
potential of the substrate-headgroup-carbon angle contribute significantly to the energy balance of the SAM. In this paper the formation, structure, and stability of n-dodecaneselenol-based SAMs are examined and compared to these for SAMs based on alkanethiols. Seleniumbased SAMs are potential systems for various applications, including photocatalysts, preparation of semiconductor quantum dots of various selenides, and molecular scale electronics. The substitution of sulfur with selenium in the latter can be significant due to the more strongly pronounced metallic properties of selenium.8 Relatively little work has been reported on organoselenol monolayers. Samant et al.9 were able to form a compact layer of docosaneselenol on gold (111) and proposed a structure for the adlayer on the basis of X-ray scattering experiments. They found that the Se-based SAM is distorted ∼3% from the typical (x3 + x3)R30° structure of a thiol-based SAM. The molecular tilt angle was 15° ( 1° off-normal to the surface. STM experiments10 have shown that benzeneselenol chemisorbs on Au(111), producing initially a surface with many small islands of SAM. Then the islands undergo Ostwald ripening, eventually producing large, hexagonalshaped facets. Competitive adsorption studies performed by means of surface-enhanced Raman spectroscopy suggest that adsorption of diphenyl diselenide is even more favorable than that of diphenyl disulfide.11 Finally, some simple electrochemical experiments have shown that small aromatic diselenides such as diphenyl diselenide can form a self-assembled monolayer with 99% surface coverage on polycrystalline gold.12 (8) Reinerth, W. A.; Burgin, T. P.; Dunbar, T. D.; Bumm, L. A.; Arnold, J. J.; Jackiw, J. J.; Zhou, Ch.; Deshpande, M. R.; Allara, D. L.; Weiss, P. S.; Reed, M. A.; Tour, J. M. Polym. Mater. Sci. Eng. 1998, 78, 178179. (9) Samant, M. G.; Brown, C. A.; Gordon, J. G., II Langmuir 1992, 8, 1615-1618. (10) Dishner, M. H.; Hemminger, J. C.; Feher, F. J. Langmuir 1997, 13, 4788-4790. (11) Huang, F. K.; Horton, R. C.; Myles, D. C., Jr.; Garrell, R. L. Langmuir 1998, 14, 4802-4808. (12) Bandyopadhyay, K.; Vijayamohanan, K. Langmuir 1998, 14, 625-629.
10.1021/la0203483 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/25/2002
Alkaneselenol-Based SAMs on Au(111)
In the present paper, electrochemical studies of a selenium-based SAM on Au(111) using n-dodecaneselenol are presented. The experiments and results include studies of SAM formation as well as a study of the blocking properties of the SAM after its formation. Extensive work has been published dealing with the formation and structure of alkanethiol-based SAMs. Detailed reviews on this subject have been made by Finklea1 and Schreiber.13 Finklea1 gives a broad and detailed discussion of SAM studies including SAM formation with consideration of the different factors that influence this process, including purity of the thiol, deposition solvent, quality of the substrate, deposition concentration, time and temperature. Schreiber’s13 review is specifically focused on the alkanethiol SAM growth and discusses several more complex issues of the film-formation process. Both growth from the solution and from the gas phase are discussed for in situ and ex situ experiments. The results from the alkanethiol film formation studies are compared to those for SAMs formed from alkaneselenoles on Au single crystals in the present work. Experimental Section Chemicals. N-dodecanethiol (DDT) (98%, Aldrich) was used as received, and n-dodecaneselenol (DDSe) was synthesized via a Grignard reaction with 100 mesh, 99.5+% selenium powder (Aldrich), and 1.0 M dodecylmagnesium bromide in diethyl ether (Aldrich). Sodium perchlorate (Fluka) was dried under the vacuum at 100 °C for 48 h. K3[Fe(CN)6] and K4[Fe(CN)6] (Aldrich) were used as received. C12H26Se Synthesis. The compound was synthesized via a Grignard reaction: (1) dry Et2O
CH3(CH2)11MgBr + Seo 9 8 CH3(CH2)11SeH + (2) H3O
All glassware was thoroughly washed with solvent and acid and subsequently stored for several hours in a drying oven at 110 °C. The synthesis apparatus was set up in a fume hood and flamed with a Bunsen burner, to remove trace water from atmospheric sources. “Prepure” grade nitrogen gas (Norco) was passed though the system to remove oxygen from the apparatus. The nitrogen was left at a constant flow rate throughout the experiment, to ensure an inert atmosphere. A mineral oil bubbler was positioned at the beginning and end of the nitrogen flow to ensure a constant flow rate and further avoidance of water and oxygen contamination. Approximately 8.5 g of selenium powder was weighed under subdued light, to avoid possible photodegradation. Then 100 mL of 1.0 M dodecylmagnesium bromide was added to the threenecked round-bottom flask through a transfer needle by pressure dislocation with nitrogen. The selenium was added in small aliquots, using a powder addition tube, over a period of approximately 30 min. Heat was applied during this process, to initiate a gentle reflux and aid in the commencement of the reaction. Heat was only necessary if the addition of selenium alone did not cause the reaction to begin. Commencement of the reaction is noticeable from a variable color and consistency change, along with reflux of the solvent from the solution. The left over solution is allowed to reflux for an additional 30 min to allow for further reaction of the starting materials. The resulting pot mixture was a tan-white to dark gray solid. The pot material was then poured over ice and mixed with HCl. The mixture was filtered to remove any solid, while rinsing liberally with diethyl ether. The filtered solid was discarded, and the liquid organic and aqueous layers were then separated. The ether was allowed to evaporate from the organic layer, resulting in a viscous yellow liquid as a final product. Preparation of the Self-Assembled Monolayer. The Au(111) electrode of cylindrical shape was prepared by flame annealing and electropolishing in 0.01 M HClO4 using cyclic (13) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151-256.
Langmuir, Vol. 18, No. 24, 2002 9343 voltammetry to monitor its quality. Prior to DDSe adsorption, the Au(111) was thoroughly rinsed with Nanopure water and ethyl alcohol. The electrode was then immersed in ∼3 mM solution of the DDSe in ethyl alcohol for a time period specific for each experiment. After deposition the electrode was rinsed with ethanol and then with the working solution that was used for the electrochemical measurements. After that it was transferred into the electrochemical cell and left in contact with the electrolyte solution for approximately 3 min to equilibrate. The meniscus contact technique was used to mount the working electrode in the electrolyte solution. Experimental Procedures. All experiments were carried out in a conventional three-electrode cell made of Pyrex glass with a water jacket for temperature control. All the experiments were carried out at 23.0 ( 0.1 °C. A saturated calomel electrode (SCE), connected to the cell through a Luggin capillary, was used as the reference electrode. The auxiliary electrode was a large area gold leaf. Working solutions were prepared with Nanopure water with a resistivity of 17.9-18.1 MΩ cm. All experiments were carried out in solutions free from oxygen and were kept under an argon blanket during the course of the experiment. The working electrode was an Au(111) single-crystal (Metal Crystals and Oxides, Cambridge, UK) with a cylindrical shape. All potentials are given with respect to a saturated calomel electrode. Cyclic voltammetry (CV) was performed using an EG&G 283 potentiostat/galvanostat. The electrochemical impedance measurements were carried out with a 1255 frequency response analyzer (SOLATRON) interfaced to the potentiostat via a GPIB IEEE-488.2 interface. For the ac impedance measurements, a 5 mV rms amplitude sine wave with a frequency range from 0.1 Hz to 10 kHz was applied to the electrode. Impedance data were analyzed using ZView2 software (SOLATRON).
Results and Discussion Cyclic Voltammetry. Close-packed aliphatic SAMs on electrodes block faradaic processes that would otherwise occur in the double layer close to the electrode surface. The current observed at the perfectly monolayer-modified electrode is believed to be due to electron tunneling through the long alkyl chain of the SAM. In the case that defects and pinholes are present in the monolayer, the electrode surface areas exposed to the electrolyte may behave as an array of microelectrodes. Figure 1 shows the response obtained in a cyclic voltammetry experiment for an electrolyte solution containing 0.5 mM [Fe(CN)6]4- and 0.5 mM [Fe(CN)6]3-. Response of the system for the uncovered Au(111) electrode is shown in Figure 1A. It is a typical cyclic voltammogram for a diffusion-limited reaction. Figure 1B,C compares the behavior of the same redox couple on a n-dodecanethiol and a n-dodecaneselenol-covered Au(111) electrode. The current-potential response observed in these cases is typical for an electrode consisting of an array of individual microelectrodes. The cyclic voltammetric curve for Au (111)/n-dodecanethol monolayer (Figure 1B) shows only a slow rise in current at low overpotentials with no visible peak or plateau currents. The absence of peak or plateau currents indicates that the pinhole area fraction is extremely small.14 On the current-voltage curve for the redox couple at the n-dodecaneselenol-covered Au(111) electrode (Figure 1C), the current is larger, suggesting larger pinholes in this SAM compared to the n-dodecanethiol film. Electrochemical Impedance Spectroscopy. Figure 2 shows the impedance spectrum in a Nyquist plot representation obtained at bare Au(111) (Figure 2A) and at a n-dodecaneselenol-modified Au(111) electrode (Figure 2B) in the presence of 0.5 mM each [Fe(CN)6]3-/4- with 0.1 (14) Finklea, H. O.; Snyder, D.; Fedyk, A. J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660.
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Figure 1. Cyclic voltammograms for solutions containing 0.5 mM each of [Fe(CN)6]3-/4- recorded at bare Au(111) (A), Au(111)/DDT (B), and Au(111)/DDSe (C). 0.1 M NaClO4 was used as supporting electrolyte. The scan rate was 50 mV/s.
M NaClO4 as the supporting electrolyte. The data were collected at +0.17V versus SCE, that is, at the formal potential of the redox couple. The impedance data presented in Figure 2B were obtained after the electrode was in contact with deposition solution for 5 s. The Nyquist plot obtained for the unmodified electrode represents a combination of a slightly depressed semicircle at high frequencies and a straight line oriented at 45° with respect to each axis at low frequencies. The semicircle contains information regarding the kinetics of the studied faradaic process, and the low-frequency data contains information about the diffusion of the species to the electrode surface. This picture is typical for a system that can be described by the so-called Randles circuit (see Figure 3). The impedance of the faradaic process consists of uncompensated electrolyte resistance RΩ in series with a parallel combination of the faradaic impedance Zf and the constant phase element, which is related to the double layer capacitance Cdl:
Z ) RΩ +
[
]
1 1 + Zf ZCPE
-1
(1)
A constant phase element (CPE) was used in the present work instead of an ideal capacitor in order to account for this frequency dispersion observed at solid electrodes caused by their microscopic roughness. The CPE impedance is given by
ZCPE )
1 Q(jω)φ
(2)
where Q is a constant in F cm-2 sφ-1 and φ is related to
Figure 2. Impedance spectra for bare Au(111) (A) and Au(111)/DDSe (B) for solutions containing 0.5 mM each of [Fe(CN)6]3-/4- with 0.1 M NaClO4 at an overpotential η ) 0.0 V. The impedance spectrum for the Au(111)/DDSe system was collected after the electrode was left in the deposition solution for 5 s. Frequency range was between 0.1 Hz and 10 kHz.
Figure 3. Equivalent circuit used to describe the response of the faradaic process in the systems with SAM-covered electrodes. RΩ is the solution resistance, CPE (constant phase element) represents the electrical double layer capacitance, and Zf is the faradaic impedance that consists of a charge-transfer resistance (Rct) in series with the Warburg impedance (Zw).
the angle of rotation of a purely capacitive line on the complex plane plots. Only when φ ) 1 is purely capacitive behavior obtained (Q ) Cdl). In the present case of a single step redox reaction, Zf is represented by a charge-transfer resistance (Rct) in series with the Warburg impedance (Zw), which describes the diffusion of the species to the reaction site (see Figure 3):
Zf ) Rct + Zw
(3)
It can be seen from Figure 2B that the semicircle corresponding to the electron transfer kinetics has a larger radius, indicating that the redox reaction is much slower after only 5 s of the SAM formation process. The semicircle on the Nyquist plot becomes a dominating feature in the impedance spectrum, and its radius is 4 orders in magnitude larger compared to the impedance spectrum of the bare Au(111) electrode. The contribution of the
Alkaneselenol-Based SAMs on Au(111)
Figure 4. Impedance spectra of Au(111)/DDT and Au(111)/ DDSe obtained in solutions containing 0.5 mM each of [Fe(CN)6]3-/4- with 0.1 M NaClO4 at the overpotential η ) 0.0V. The SAM was deposited on electrode for 24 h in both cases.
Warburg impedance decreases at the same time in the frequency range of the experiment, suggesting that the direct approach of the redox species to the electrode surface is being blocked. Figure 4 shows the results of impedance spectroscopy at a Au(111) electrode modified with n-dodecanethiol and n-dodecaneselenol. Both SAMs were formed for 24 h. The electron transfer rate in both cases is strongly suppressed. Features that are due to mass transfer from the bulk of the solution to the reaction site are absent. The semicircle that describes the redox process at the Au(111)/ DDSe interface is slightly smaller compared to the same process at Au(111)/DDT. This can be due to the presence of a larger number of defects in the n-dodecaneselenol SAM or to the fact that selenium is a more metallic in character than sulfur. As a result, the barrier height for electron transfer at a Au-Se bond maybe lower than at a Au-S bond. A more detailed picture of the formation and the structure of the n-dodecaneselenol-based SAM on a Au(111) electrode is generated through time-dependent studies of SAM formation on Au(111) and its performance as a microelectrode array system. Kinetics of Adsorption and Pinhole Analysis of the SAM. Figure 5 shows how the impedance response of the studied system changes depending on the deposition time of n-dodecaneselenol on the Au(111) electrode. The radius of the characteristic semicircle on the Nyquist plot is related to the rate constant for the redox reaction. An increase in the semicircle radius in observed as the exposure time for the electrode in the n-dodecaneselenol solution increases. The results of the CNLS analysis of the data using a Randles circuit are summarized in the Table 1. As expected, Rct increases as the deposition time increases. Double layer capacitance changes dramatically in the first 10 s and then varies only slightly. As mentioned above, to extract information on how the SAM structure changes depending on formation time, the SAM-covered electrode was treated as an array of microelectrodes. The theory for the impedance response of the monolayer-covered electrode that behaves as a microarray electrode has been developed by Finklea et al.8 We used this theory to map pinholes in the system studied here and to compare it to similar information for systems with a n-alkanethiol SAM on a Au electrode. To simplify
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Figure 5. Impedance spectra for the Au/DDSe system obtained in solutions containing 0.5 mM each of Fe(CN)63-/4- with 0.1 M NaClO4. All the spectra were recorded at overpotential η ) 0.0V. Deposition times were (O) 5 s, (+) 10 s, (3) 60 s, ([) 5 min, (0) 15 h, (b) 24 h. Table 1. Results of the CNLS Fitting of the Impedance Response for Different Deposition Times of the n-Dodecyl Selenide on a Au(111) Single Crystal Electrodea deposition time, s
R s, Ω
Rct, Ω × 10-6
CPE (Cdl) × 107,b F cm2
φ
5 10 60 300 54 000 86 400
179.3 (1.10) 177.1 (1.03) 179.0 (1.28) 147.6 (1.02) 148.2 (2.37) 171.2 (0.74)
0.11 (0.50) 1.17 (0.44) 2.34 (0.69) 4.41 (0.56) 7.52 (1.91) 11.7 (0.72)
2.00 (1.50) 1.11 (0.57) 1.17 (0.64) 1.23 (0.39) 1.34 (0.95) 1.15 (0.27)
0.9795 0.9929 0.9902 0.9850 0.9567 0.9928
a The percent errors from the fit for each element are given in parentheses. b The area of the electrode was equal to 0.0856 cm-2
the calculations, the pinholes and the electrode areas covered with SAM are assumed to have a disk shape. The area fraction of pinholes is related to the pinhole and SAM island radii by
1 - θ ) re2/rc2
(4)
where θ is the SAM coverage, re, is the average radius of a pinhole, and rc is the average radius of the SAM-covered patches. It has been previously shown that the common assumption that the total pinhole area fraction is equal to the ratio of the charge-transfer resistance of the SAMcovered electrode to the corresponding term for a bare electrode cannot be used in the case of films with a high area coverage (θ g 0.999).14 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 are obtained:
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,
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which corresponds to overlapping diffusion profiles for all the pinholes, are
Zf′ ) RCT/(1 - θ) + σ/xω + [σre(0.72/D)1/2/(1 - θ)] (7) Zf′′ ) σ/xω
(8)
σ ) [x2(RT/F)]/[FAcxD]
(9)
where
In these equations, A is geometrical area of the electrode in cm2; c, the concentration of the redox species in mol cm-3; D, the diffusion coefficient of the redox species in cm s-1; and ω ) 2πf, where f is the frequency of the ac signal in hertz. It should be noted that this kind of analysis was performed on 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.15 After the effects of solution resistance and double-layer capacitance were subtracted from experimental data, the pinhole parameters were extracted from the remaining faradaic impedance. 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 high frequencies. This error in the present case was not sufficiently large to impact the parameters determined in the corresponding analysis. In Figures 6 and 7 the real and imaginary parts of the faradaic impedance are plotted as a function of ω-1/2. It can be seen from these plots that the faradaic impedance possesses features similar to the faradaic impedance for an ideal microarray electrode.14 The real component of Zf′ exhibits two almost linear domains at high and low frequencies when plotted agains ω-1/2. Some deviations from the ideal behavior that are more pronounced at low deposition times are observed. The slope of the line for Zf′ versus ω-1/2 is not constant at low frequencies. In the case of the imaginary component of the faradic impedance, the slopes of the Zf′′ lines are different from the slopes of the Zf′ lines at high frequencies. These deviations can be due to some unrealistic assumptions in the model applied. For example, the conducting sites are assumed to be welldefined disks of equal dimensions that are evenly spaced on the conducting metal. This kind of situation cannot be achieved in a real system. The plot of the imaginary part of faradaic impedance versus ω-1/2 has a peak at the so-called transitional frequency q. According to the model, this frequency separates two time-dependent diffusion profiles for the microelectrodes. Diffusion layers are isolated at short times (high frequencies at ω . q) and overlap at long times (low frequencies at ω , q). Careful analysis of the transitional frequency changes can give some insights into the changes in the SAM structure as the electrode exposure time in the deposition solution increases. Table 2 shows that the transitional frequency decreases as the deposition time for the SAM is increased. This leads to the conclusion that the diffusion profiles for the redox species that are approaching the exposed electrode areas are more sepa(15) Macdonald, J. R. Impedance Spectroscopy: Emphasizing Solid Materials and Systems: John Wiley & Sons: New York, 1987.
Figure 6. Real part of the faradaic impedance Zf′ of the redox couple at the reversible potential versus ω-1/2 at Au(111)/DDSe electrodes. The working solutions contained 0.5 mM each of [Fe(CN)6]3-/4- with 0.1 M NaClO4. Deposition times are shown next to each plot.
Figure 7. Imaginary part of the faradaic impedance Zf′′ of the redox couple at the reversible potential versus ω-1/2 at Au(111)/DDSe electrodes. Working solutions contained 0.5 mM each [Fe(CN)6]3/4- with 0.1 M NaClO4. Deposition times are shown next to each plot.
rated with time, that is, the average distance between the areas of uncovered electrode increases.
Alkaneselenol-Based SAMs on Au(111)
Langmuir, Vol. 18, No. 24, 2002 9347
Table 2. Dependence of the Transitional Frequency on the Deposition Time for n-Dodecyl Selenide SAM on a Au(111) Electrode deposition time, s
transitional frequency, Hz
deposition time, s
transitional frequency, Hz
5 10 60
63.10 39.80 12.60
300 54 000 86 400
6.31 5.00 3.98
From the graphical representations of data shown in Figure 6, the characteristic parameters for the description of pinholes and the separation between them were obtained using following equations:
θ ) 1 - σ/R rc )
re
x(1 - θ)
(7) (8)
and
re )
γ(1 - θ) σx0.72/D
(9)
where R is the slope of the Zf′ vs ω-1/2 graph in the highfrequency region, and γ is the intercept of the Zf′ vs ω-1/2 graph in the low-frequency region. A plot of surface coverage θ versus time during adsorption of n-dodecaneselenol from the ethanol solution is shown in Figure 8. To access the alkaneselenol adsorption behavior, the Langmuir rate law was used to fit the curve of θ versus deposition time. Few kinetic models have been proposed in the literature for SAM formation. Simple first-order Langmuir kinetics16 gives the following equation:
dθ ) ka(1 - θ)c - kdθ dt
(10)
After integration the time-dependent coverage has the form
θ(t) ) [kac/(kac + kd)][1 - exp(- (kac + kd))]
(11)
Here c is the bulk concentration of adsorbing molecules, and ka and kb are the rate constants for adsorption and desorption, respectively. Equation 11 can be simplified by assuming irreversible adsorption and by substitution of k ) kac, giving
θ(t) ) 1 - e-kt
(12)
where k is the observed rate constant in s-1. In addition to first-order Langmuir adsorption (LA), a diffusion-limited Langmuir (DL) model has been suggested:17
θ(t) ) 1 - e-kxt
(13)
Both models were compared with the present experimental data. Nevertheless, one should realize that application of Langmuir rate law to the SAM formation process maybe a simplification, since the deposition solutions were of high concentration. Additional errors can come from the fact that a freshly cleaned electrode was used for each deposition time, that is, for each (16) Hu, K. and Bard, A. J. Langmuir 1998, 14, 4790-4794. (17) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731-4740.
Figure 8. The surface coverage of DDSe monolayer on Au(111) as a function of the deposition time.
experiment datum, and that the data were not acquired in situ during the adsorption process. Analysis of the curve in Figure 8 shows that there are two main time constants, suggesting a two-step process of film formation. These results were compared to the information in the literature on SAM formation kinetics for the case of straight chain aliphatic thiol molecules. The results of the previous studies on SAM formation are not entirely consistent. Taking into consideration that the films were formed under different experimental conditions, there are substantial differences among the reported time scales for the adsorption process, ranging from seconds to hours. There are several factors that can be responsible for the observed differences in the reported kinetics for thiol adsorption on gold. These include the presence of contamination on the gold surface, the concentration of the thiol, and solvent effects on the deposition process.18 However, some common observations described by different research groups can be gleaned. One of the first works in this area, published by Bain et al.,19 was carried out using ex situ ellipsometry and wettability measurements. The kinetics of octadecanethiol adsorption from ethanol solutions with the bulk concentration ranging from 0.1 to 1000 µmol L-1 suggested a two-step adsorption process. During the first 2 min the film thickness reaches ∼80-90% of the final value. The second step extends over several hours and is almost independent of the concentration. Few groups have used infrared spectroscopy as a means to study adsorption kinetics of alkanethiols.20,21 Ex situ FT-IRRAS kinetic studies concluded that immersion times of 45 s are sufficient to form an ordered monolayer of alkanethiols on polycrystalline gold substrates from micromolar solutions. Troung and Rowntree21 found that the initial deposition of an imperfect monolayer is rapid at high concentrations and can take many hours in very dilute solutions. This rapid adsorption is followed by a much slower process of additional adsorption and selforganization. For example, a 10-9 M solution of butanethiol in methanol showed changes in SAM coverage and organization over 4 days. Quite interesting observations were made by Ha¨hner et al.22 They used near-edge X-ray (18) Pan, W.; Durning, C. J.; Turro, N. J. Langmuir 1996, 12, 44694473. (19) Bain, C. D.; Throughton, E. B.; Tao, Y.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (20) Bensebaa, F.; Voicu, R.; Huron, L.; Ellis, T. H.; Kruus, E. Langmuir 1997, 13, 5335-5340. (21) Truong, K. D.; Rowntree, P. A. Prog. Surf. Sci. 1995, V.50, No.14, 207-216.
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fine structure (NEXAFS) spectroscopy to investigate the intermediate steps in self-assembly of long chain thiols on gold. After the initial rapid adsorption process, the alkyl chains were shown to be strongly entangled. The slow, second step observed previously was identified as an ordering process, where the initially entangled alkyl chains are gradually straightened. There are several groups that have used a quartz crystal microbalance to study the adsorption process,23-25 but there is no agreement on the rates of monolayer formation. For example, while Pan et al.18 observed a two-step process with an initial time constant of ∼300 min-1, Karpovich and Blanchard23 report that the adsorption of 0.3 mM octadecanethiol in n-hexane on gold is quite rapid, typically forming a monolayer within 3-4 s. Unfortunately, the QCM method cannot detect technically any changes in time within the formed monolayer. In surface plasmon resonance spectroscopy studies on the length and concentration dependence of the kinetics of film formation, at least three distinct kinetic steps were found.17 One of most recent studies by second harmonic generation (SHG)26 suggests that modified Langmuir kinetics can explain SAM formation from solutions in monomer concentrations in the range between 0.5 and 20.0 µM. No published work was found on the formation kinetics of alkaneselenol-based SAMs. The results presented here are similar to those reported by Peterlinz and Georgiadis17 on alkanethiols that suggest that the formation mechanism involves adsorption of both chemisorbed and physisorbed molecules during film formation. It is also concluded that the second step probably involves molecular motion within the film. The most likely mechanism for the observed results is addition of the thiol from the physisorbed layer to the domain boundaries of the chemisorbed layer. This is consistent with the mechanism proposed by Chidsey et al.25 that alkanethiols exchange and addition occurs at domain boundaries. Our observations are also in line with the STM experiments to study the structure of benzeneselenol films on Au(111).10 These experiments showed that a surface with many small islands is initially produced. Later the islands coalesce into large hexagonal-shaped domains. Most recent attempts to describe the adsorption kinetics of alkanethiols were made by following capacitance changes with time during film formation using electrochemical impedance spectroscopy.27 These data reported values for the rate constant of the first-step in SAM formation. The first-order Langmuir isotherm (LA) and the diffusion-limited Langmuir (DL) model were used to analyze the data. A reported rate constant for dodecanethiol formation was 0.1307 s-1/2 from 5 µM solution over a period of 500 s or 0.0551 s-1 from 20 µM solution over a period of 100 s. Our data for the surface coverage dependence on time in the range of 5-300 s were fitted with both the LA and DL models. Better correlation was observed for the DL model, most likely due to the high concentration of the (22) Ha¨hner, G.; Wo¨ll, Ch.; Buck, M.; Grunze, M. Langmuir 1993, 9, 1955-1958. (23) Karpovich, D. S.; Blanchard, G. J. Langmuir 1995, 10, 33153322. (24) Frubo¨se, C., Doblhofer, K. J. Chem. Soc., Faraday Trans. 1995, 91 (13), 1949-1953. (25) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301-4306. (26) Dannenberger, O.; Buck, M.; Grunze, M. J. Phys. Chem. B 1999, 103, 2202-2213. (27) Subramanian, R.; Lakshminarayanan, V. Electrochim. Acta 2000, 45, 4501-4509.
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Figure 9. The pinhole radius (1) and separation (b) as a function of the deposition time for the DDSe SAM on a Au(111) electrode.
deposition solution. The diffusion-limited Langmuir model gives a slightly better fit with an adsorption rate constant of k of 3.28 ( 0.03 s-1. These results agree very well with the adsorption kinetics studies of alkanethiol on gold substrates by Subramanian and Lakshminarayanan27 that were also carried out using electrochemical impedance measurements. Their studies of the adsorption support the application of the Langmuir kinetics and suggest a strong dependence of the adsorption on the deposition solution concentration. Their reported value of the rate constant for SAM formation from 1 mM deposition solution is ∼2.22 s-1. Unfortunately, their application of electrochemical impedance was limited to following only the electrochemical double layer capacitance. Although it provides a simple and effective method of following the adsorption kinetics, it does not provide any information on the SAM changes with time after the film has been almost fully formed. Additional information concerning alkanethiol SAM formation can be found in the review by Schreiber.13 When comparing results from different laboratories, careful attention should be kept in mind that different experimental conditions were often used. Thiol concentration, the purity of the deposition solution, and temperature are some of the important factors that influence the kinetics of SAM formation. In most cases evaporated Au films and not Au single crystals were employed as substrates. Film substrates are known to have a high density of defects. In solution growth, the quality of the substrate is a crucial factor that can have a strong impact on the growth behavior.13 Despite extensive work in this area, there are practically no publications of SAM healing after the film has been formed. Application of microarray electrode theory to the treatment of pinholes in the analysis of electrochemical impedance data can be very useful for this purpose. Figure 9 shows plots of pinhole radius and separation between defects versus time. Despite the fact that a high concentration of the deposition solution was used in the experiment, both plots suggest a two-step process, the first with a large time constant and the second with a much smaller time constant. The radius of the pinhole changes from ∼0.25 to ∼0.75 µm as the deposition time increases from 5 s to 24 h. The separation between the pinholes increases from 15 to 80 µm. A similar tendency in changes of re and rc with time leads to conclusions about how changes occur within the SAM with time. It is suggested that as deposition time increases, the alkaneselenol molecules migrate and join larger domains in the SAM at the boundaries. This observation is in line with the results
Alkaneselenol-Based SAMs on Au(111)
published previously by Peterlinz and Georgialis,17 who proposed a similar model of SAM healing for alkanethiols. They suggested that the molecular motion within the film takes place as addition of physisorbed thiol molecules occurs to the domain boundaries of the chemisorbed layer. It should be noticed that taking into account the numerous assumptions of the pinhole model and the reliability of its application to self-assembled monolayers, the numerical values of calculated characteristic parameters such as re and rc should not be taken as absolute values. Deviations from the microelectrode array behavior are noticeable. Following the pinhole analysis of the DDSe-based SAM that develops with time, we should be able to see if the monolayer forms as continuous film from the beginning or goes through the formation of clusters. The transition between the high- and low-frequency regimes is very sensitive to the uniformity of the microarray parameters.28 In case of the uniform spacing of the pinholes and the headgroups of the film-forming amphiphile, the diffusion layers from all the pinholes over the entire surface of the electrode overlap in the same frequency range. This is the case for the ideal microarray electrode. The transition between the high- and the low-frequency regimes is expected to be quite sharp and is to be observed over approximately a 1 order of magnitude change in frequency.14 If the DDSe molecules are most likely arranged in nonuniform clusters, the diffusion profile from the uncovered electrode (pinhole-microelectrodes) falls under a different frequency domain. This leads to more a smooth transition between the low- and high-frequency parts on the plots of Zf versus ω-1/2 (Figure 6). Nevertheless, analysis of the changes in these plots with deposition time can be used to get a clearer picture of the SAM reorganization mechanism when an ideal film is being formed. Even though the numerical values for the areas of defects/pinholes and separation between them may be approximate and vary within some limits from monolayer to monolayer, the general trend is very distinct. There is a first fast step in the adsorption kinetics that depends on the concentration of adsorbing species, and (28) Scharifker, B. R. J. Electroanal. Chem. 1988, 240, 61-76.
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the following step is much slower and is more likely due to the rearrangement of the adsorbed molecules on the electrode. Conclusions The pinhole parameters in the n-dodecylselenol-based monolayer, such as fractional coverage, pinhole size, and separation, have been studied as a function of time by electrochemical impedance spectroscopy. The relation between the fractional coverage and adsorption time indicates that the formation of the n-dodecaneselenolbased monolayer is composed of an initial rapid step and a slow follow-up step. The film formation process can be characterized in terms of pinhole size and separation between pinholes. The pinhole separation and pinhole radii increase with increasing adsorption time at high electrode coverages. Numerical values for re and rc are similar to the ones obtained by Finklea et al.14 for alkanethiols. The trends in the changes of pinhole radius and separation radius with time suggest formation of a n-dodecaneselenol-based SAM similar to the Ostwald ripening mechanism that is consistent with previously reported studies for benzeneselenol film formation.10 At small deposition times the SAM forms as many small islands of dodecaneselenol film on a Au substrate. As the time of deposition increases, the islands coalesce into larger ones, so the parameter rc increases. As a result of coalescence, the number of pinholes is smaller, but their sizes are larger compared to short deposition times. Selenium-based SAMs show themselves as a good alternative to thiol-based SAMs. Additional experiments using mixed monolayers of alkanethiols and alkaneselenol by means of scanning microscopy techniques can be used to study the competitive adsorption of these two kinds of monolayers. This kind of experiment can be used to compare the relative strength of Au-S and Au-Se bonds. Acknowledgment. This research was supported by a grant from the National Science Foundation, Washington, DC (CHE-9729314). LA0203483
