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Combined Spectroscopic Ellipsometry and Voltammetry of Tetradecylmethyl Viologen Films on Gold§ Vytas Reipa,*,† Harold G. Monbouquette,† and Vincent L. Vilker‡ Chemical Engineering Department, University of California, Los Angeles, California 90095-1592, and Biotechnology Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received April 10, 1998. In Final Form: September 2, 1998 Dynamic measurements of changes in composition and conformation during adsorption of tetradecylmethyl viologen (C14MV) on a gold electrode were made using spectroscopic ellipsometry. Film thickness measurements indicated that a monolayer is initially formed in which the viologen species is in the upright extended position and that conversion to a tilted conformation occurs during equilibration to the final monolayer film. Subsequent reduction of the film as electrode potential was scanned from -0.2 to -0.9 V resulted in linear growth to a multilayer film in which dications (C14MV2+) were reduced to radical (C14MV•+) and neutral (C14MV0) species. A gradual increase of the film charge-to-thickness ratio and of the film index of refraction during reduction indicated densification of the deposit. The pattern in the variation of film extinction coefficient with electrode potential indicated that radical dimers were the dominant species in the potential range corresponding to the first reduction step. During the reverse potential scan, the radical film detached due to faster reoxidation of the layers adjacent to the electrode surface.
Introduction Viologens, 1,1′-disubstituted-4,4′-bipyridinium compounds, have been investigated extensively over the last twenty years due to their potential application as redox mediators, display materials, and electron relays (for review, see ref 1). They can be reduced from a dication (V2+) to a radical monocation (V•+) and further to a neutral species (V0) in two single-electron-transfer steps at redox potentials that are among the most cathodic for organic species.1 Both viologen reduction potentials are affected by the chemistry of the 1,1′ substituents,1 by oligomerization,2 and by the solution anion.1 Generally, viologens with longer 1,1′ substituents are more likely to form adsorbed films on electrodes when reduced. The redox behavior of viologens is complicated by the tendency of monocation radicals to dimerize to (V•+)2 and by disproportionation and conproportionation reactions:
2V•+ f V2+ + V0 V0 + V2+ f 2V•+ The reduced forms of viologen, V•+ and V0, are less soluble in water than V2+ and may precipitate with a counterion on a bare electrode surface. This often results in observations of electrochemical irreversibility.3 Adsorbed viologen films generally are subject to “aging”, reorgani* Corresponding author. † University of California, Los Angeles. ‡ National Institute of Standards and Technology. § Certain commercial equipment, instruments, and materials are identified in this paper to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the material or equipment is necessarily the best available for the purpose. (1) Bird, C. L.; Kuhn, A. T. Chem. Soc. Rev. 1981, 10, 49. (2) Sato, H.; Tamamura, T. J. Appl. Polym. Sci. 1979, 24, 2075. (3) Wang, H. X.; Sagara, T.; Sato, H.; Niki, K. J. Electroanal. Chem. 1992, 331, 925.
zation of the film with time such that areas cannot be completely reoxidized.3 Yet, the spontaneous adsorption of asymmetric viologens with one long alkyl chain, such as N-ethyl-N′octadecyl viologen on electrode surfaces from dilute solution, provides a simple system for the study of electron transfer between electrodes and immobilized redox moieties.4-7 Such modified electrodes could be useful for electrical communication with dissolved redox enzymes having negative reduction potentials, for example, nitrate reductases8 or ferredoxins.9-11 Improved control of electrochemical behavior may be achieved by covalent attachment of thio-derivatized viologens to metal electrodes12,13 or by incorporation of viologens in polymers14-16 or artificial membrane films.17,18 Recently, the redox activity of covalently attached asymmetric viologens 12,19,20 and their use as electron-transfer mediators to nitrate reductase were demonstrated.19 Katz et al. showed that a chemisorbed thiol derivative of viologen which was initially electroinactive on gold could reorganize into an (4) Li, J.; Kaifer, A. E. Langmuir 1993, 9, 591. (5) Diaz, A.; Kaifer, A. E. J. Electroan. Chem. 1988, 249, 333. (6) Widrig, C. A.; Majda, M. Langmuir 1989, 5, 689-695. (7) Gomez, M.; Li, J.; Kaifer, A. E. Langmuir 1991, 7, 1797-1806. (8) Mellor, R. B.; Ronnengerg, J.; Campbell, W. H.; Diekmann, S. Nature 1992, 355, 717. (9) Jones, R. W.; Ingledew, W. J.; Graham, A.; Garland, P. B. Biochem. Soc. Trans. 1978, 6, 1287-1289. (10) Cosnier, S.; Innocent, C.; Jouanneau, Y. Anal. Chem. 1994, 66, 3198-3201. (11) Landrum, H. L.; Salmon, R. T.; Hawkridge, F. M. J. Am. Chem. Soc. 1977, 99, 3154. (12) Bunding Lee, K. A. Langmuir 1990, 6, 709. (13) De Long, H. C.; Buttry, D. A. Langmuir 1992, 8, 2491. (14) Katz, E.; de Lacey, L.; Fernandez, V. M. J. Electroanal. Chem. 1993, 358, 261. (15) Laurent, D.; Schlenoff, J. B. Langmuir 1997, 13, 1552. (16) Creager, S. E.; Fox, M. A. J. Electroanal. Chem. 1989, 258, 431. (17) Lee, D. K.; Kim, Y. I.; Kwon, S. Y.; Kang, Y. S.; Kevan, L. J. Phys. Chem. B 1997, 101, 5319. (18) Herrero, R.; Moncelli, M. R.; Becucci, L.; Guidelli, R. J. Phys. Chem. B 1997, 101, 2815. (19) Katz, E.; Itzhak, N.; Willner, I. J. Electroanal. Chem. 1992, 336, 357. (20) Katz, E.; Itzhak, N.; Willner, I. Langmuir 1993, 9, 1392-1396.
10.1021/la980412+ CCC: $15.00 © 1998 American Chemical Society Published on Web 10/09/1998
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electroactive state upon coadsorption of a long chain alkanethiol.19 Reorientation of the adsorbed viologen bipyridyl residues after coadsorption of the alkyl mercaptan was hypothesized to be responsible for the improved electroactivity. Although these studies demonstrate the close relationship between the viologen molecular conformation on the electrode surface and the electroactivity, very few studies have been directed toward clarification of the structure of the adsorbed viologens prior to and during a potential scan. This study was undertaken to gain more insight into the relationship between the surface conformation and redox behavior of 1-methyl-1′-tetradecyl viologen (C14MV) on gold. This lipophilic viologen may be an especially useful electron-transfer mediator for reduction reactions catalyzed by membrane-bound redox enzymes such as nitrate reductase.9 Electroenzymatic nitrate-reducing systems have been explored both for sensing10 and for elimination of nitrate,8 an important water pollutant. We investigate the C14MV equilibrium film thickness and optical properties during the adsorption, film formation, and film transformation stages as a function of electrode surface potential. Our electrode interfaces are characterized in situ using spectroscopic ellipsometry in combination with traditional electrochemical methods.21 Previous investigations focused on the study of viologen optical properties using thin layer transmission cells with transparent electrodes16,22,23 or UV-vis reflectance.3,24,25 While the measurements provided valuable data on the prevailing viologen species close to the electrode surface, these techniques cannot distinguish between optical absorption by the surface film and absorption by the adjacent solution boundary layer. We employ spectroscopic ellipsometry,21 which has enhanced sensitivity to the optical properties and thickness of the surface films. Film thickness measurements are of crucial importance for predicting the electrochemical behavior of the electrode deposits. C14MV optical absorption in the UV-vis range is determined by the bipyridine chromophore electronic state and is sensitive to the increased interaction between viologen groups that results from dimerization. The dication species is transparent in the visible but has an absorption band due to the π-π* transition at around 265 nm, while the monocation radical has absorption bands at 395 and 604 nm.26 The latter band has been reported to shift to 520-540 nm upon dimer formation.22,26,27 The tendency of the viologen radicals to dimerize increases in parallel to the hydrophobicity of the pyridine substituent.28 A weak absorption band in the near-IR around 800 nm has also been reported for radical cation dimer solutions.29 Experimental Section Materials. 1-Methyl-1′-tetradecyl-4,4′-bipyridine dichloride (C14MV) was purchased from Fluka and was used without further purification. Most measurements were conducted in buffers (21) Reipa, V.; Gaigalas, A. K.; Vilker, V. L. Langmuir 1997, 13, 3508. (22) Lee, C.; Kim, C.; Park, J. W. J. Electroanal. Chem. 1994, 374, 115. (23) Jasinski, R. J. J. Electrochem. Soc. 1978, 125, 1619. (24) Beden, B.; Enea, O.; Hahn, F.; Lamy, C. J. Electroanal. Chem. 1984, 170, 357. (25) Lezna, R. O.; Centeno, S. A. Langmuir 1996, 12, 591. (26) Lu, T.; Cotton, T. M.; Hurst, J. K.; Thompson, D. H. P. J. Phys. Chem. 1988, 92, 6978. (27) Mizuguchi, J.; Karfunkel, H. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 1466. (28) Yasuda, A.; Mori, H.; Seto, J. J. Appl. Electrochem. 1987, 17, 567. (29) Lee, C.; Lee, M. Y.; Moon, S. M.; Park, H. S.; Park, J. W.; Kim, G. K.; Jeon, J. S. J. Electroanal. Chem. 1996, 416, 139.
Reipa et al. containing 0.1 M KCl and 0.1 M KHPO4 adjusted to pH 7.4. Solutions were prepared from analytical grade reagents in purified water (18.2 Mohm, Millipore). They were deaerated at least 30 min prior to and throughout experiments by sparging with argon. Methods. Electrodes consisted of glass slides coated with thermally evaporated gold (99.99%, Johnson-Matthey) on a Cr underlayer for better adhesion. Gold films measured about 300 nm thick, and the electrodes were stored in distilled water. Just prior to use, the evaporated gold electrodes were held in a flame for several seconds and subsequently were dipped in distilled water.30 This procedure resulted in a hydrophilic gold surface, as evidenced from uniform surface wetting. Ellipsometric measurements were conducted using a Woollam M-44 ellipsometer (J. A. Woollam Co., Lincoln, NE) in the vertical arrangement using the cell and measurement procedure described previously.21 Multichannel detection allowed the simultaneous monitoring of 44 wavelength channels every 0.04 s. However, 50 data points were averaged to reduce noise, thereby giving a spectral reading every 2 s. Quantitative evaluation of the optical constants and film thicknesses was carried out using the WVASE32 software, provided by Woollam Co. The software utilizes the Levenberg-Marquardt procedure to minimize the difference between the experimental and model-calculated ellipsometric phase, ∆, and amplitude, Ψ, values. Films were treated as homogeneous, isotropic layers; therefore, effective n and k values were calculated. Evaporated gold electrodes were mounted in a custom-built 50 mL trapezoidal cell for combined spectroellipsometry and electrochemistry which was filled with argon-sparged buffer solution.21 Prior to measurements, the potential was cycled at 50 mV/s between -1.2 and 0 V for 5 min to clean the surface. Following cleaning, ∆ and Ψ were recorded as a function of wavelength at a constant potential of -0.2 V and the “bare” gold surface optical constant spectra were solved using a two-phase (substrate/ambient) optical model. Usually, the experiments were repeated several times on the different electrodes. While initial ∆ and Ψ readings for the clean gold varied within 0.1°, the calculated substrate optical constant spectra were always consistent with the available data for polycrystalline gold within the experimental errors.31 ∆ and Ψ versus potential dependencies were recorded for bare gold in buffer solution during the potentiodynamic scan from -0.2 to -0.9V before the introduction of the adsorbate. They were similar to the previously published ellipsometry data for gold in the double-layer region.21,32 C14MV was injected close to the electrode surface at open circuit potential using 0.5 mL portions of 10 mM deaerated solution. The solution was continuously purged with argon during the experiment, thus ensuring rapid mixing of the adsorbate with buffer solution. Ellipsometric parameters and electrode current were recorded as a function of adsorption time and/or electrode potential. ∆ and Ψ variations due to the gold charging in the double-layer region were subtracted from the total ellipsometric signal at each wavelength according to Chao et al.,33 therefore compensating for the substrate optical change during the potentiodynamic scan. The resulting ∆(t) and Ψ(t) dependences were checked to be reproducible within 0.05° before further analysis. Electrode potential and current were controlled using an EG&G273 electrochemical system. Ag/AgCl in saturated KCl was used as a reference (Abtech Model Re803), and a Pt wire served as the auxiliary electrode. All potentials reported below are given versus Ag/AgCl (saturatedd).
Results and Discussion Spectroscopic Ellipsometry at Open Circuit Potential. Typical ∆ and Ψ versus time traces as recorded for a gold electrode at open circuit potential (-0.39 V) for two C14MV injections (final concentrations of 125 and 250 µM, respectively) are presented in Figure 1. A total of 44 (30) Finklea, H. O. In Electroanalytical chemistry-A series of advances; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, p 117. (31) Johnson, P. R.; Christy, R. W. Phys. Rev. 1972, B6, 4370. (32) Chao, F.; Costa, M.; Lecoeur, J. Bellier, J. P. J. Electroan. Chem. 1989, 34, 1627. (33) Chao, F.; Costa, M. Surf. Sci. 1983, 135, 497.
Tetradecylmethyl Viologen Films on Gold
Figure 1. Typical evolution of the ellipsometric parameters ∆ and Ψ after two successive injections of C14MV (125 and 250 µM final buffer concentrations). The injection times are marked B and C. The direct output of the ellipsometer 647 nm channel is presented.
Figure 2. Ψ versus ∆ growth curve at λ ) 647 nm plotted from the data presented in Figure 1. The letters mark the corresponding time points of Figure 1 and trajectory segments as discussed in the text.
such traces were recorded at different wavelengths from 419 to 761 nm in a single experiment. Although the noise is evident in the ∆(t) and Ψ(t) traces, the main features following the viologen injection were reproducible. Both ellipsometric parameters change sharply immediately after injection, indicating fast change of the electrode surface condition. We did not analyze the initial [∆(t), Ψ(t)] spike arising from the perturbation of the ellipsometer beam following the injection of the concentrated solution. About 4-5 s following the injection, an exponential relaxation toward the initial ∆ and Ψ values is observed. Such relatively fast adsorption is anticipated, since both hydrophobic and electrostatic driving forces are operative between a negatively charged gold electrode and the viologen dication. We have utilized two methods to solve for the adsorbate film thickness and the complex index of refraction. First, a trajectory plot in ∆, Ψ coordinates (Figure 2) was analyzed, as the length of the linear trace on such a plot is proportional to film thickness at small values, while the slope is determined by the value of the index of refraction.34 Such a plot is referred to as a “growth curve”.35 The instrumental noise as apparent in Figures 1 and 2 was smoothed, and the resulting (34) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: Amsterdam, 1977. (35) Barbero, C.; Kotz, R. J. Electrochem. Soc. 1994, 141, 859.
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Figure 3. Adsorbed film thickness, d, calculated from the smoothed ellipsometry data presented in Figures 1 and 2 using a three-phase model (see text).
trajectory could be broken into several linear segments. The segments from A to B and from C to D in Figure 2 indicate film growth, while the segments from B to C and from D to E indicate a net decrease in the adsorbed film thickness. Fitting the experimental trajectory for a single wavelength to a model-generated curve allows determination of the dynamics of all three film parameters: the index of refraction, n; the extinction coefficient, k; and the film thickness, d. Each segment of the growth curve was fitted separately, and solutions were sequentially included into the optical model. Alternatively, we have utilized the reported transparency of the viologen dication in the visible.22,26 From six [∆(λ), Ψ(λ)] data sets taken at specific time points in the range 419 < λ < 450 nm we have solved for the film thickness d and the index of refraction spectra n(λ), assuming k(λ) ) 0 in this range. The resulting film thickness values were then used to solve for the index of refraction n(λ) and k(λ) spectra in the whole accessible range (419 < λ < 760) through global fit using software provided by Woollam Co. within a three-phase, singlefilm (gold substrate/adsorbate film/solution) model.21 Both methods provided comparable results. As shown in Figure 3, film thickness reaches a maximum value of about 24 Å in less than 5 s after the first C14MV injection with subsequent equilibration to 11 Å. This process is nearly reproduced upon the second injection but at a slower rate and to a slightly larger final thickness d. In the extended conformation, the C14MV molecule is about 25 Å long, with the chromophore contributing about 8 Å. The initial film thickness (24 Å) is therefore very close to the value expected for the monolayer film with C14MV molecules in an upright position. This conformation is not stable at the open circuit potential, however, as indicated by the ensuing decrease in d. Since our C14MV concentration in the bulk solution far exceeded the isotherm saturation level on gold36 (10-100 µM), we believe that the decrease in thickness is not likely to be caused by desorption but rather by a conformational effect. However, partial layer desorption was observed when the solution in the electrochemical cell was exchanged to pure buffer, which confirms the dynamic equilibrium between surface and solution molecules. On the basis of the measured thickness values, we can conclude that monolayer coverage occurs under open circuit conditions with the viologens initially adsorbed on end in a predominantly extended conformation. Subsequently (t > 5 s), the conformation (36) Enea, O. Electrochim. Acta 1986, 31, 789.
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Figure 5. Cyclic voltammogram of a gold electrode in 0.1 M KCl, 0.1 M KHPO4 (pH ) 7.4), and 375 µM C14MV at 20 mV/s. Voltammetry curve deconvolution (Peakfit v.4, using Lorenzian peak shapes) revealed five anodic (A1...A5) and five cathodic (C1...C5) current peaks. A blowup of peaks C1 and C2 is shown in the inset.
Figure 4. Evolution of the extinction coefficient (k) and index of refraction (n) spectra during C14MV adsorption on gold. Film optical constants were calculated from the ellipsometry data presented in Figures 1 and 2 using a three-phase optical model and a global fit.
of the monolayer changes through, perhaps, reorientation of the bypyridine ring relative to the electrode surface and/or tilting of the molecules in the layer. Bae et al.37 detected a reorientation of N-ethyl-N′-octadecyl-4,4′bipyridinium molecules in the adsorbed film on gold using FTIR. In their case the bipyridinium moiety adopted an orientation nearly parallel to the electrode surface when in the reduced state while the alkyl chains adopted a more stretched configuration. A spectrum of the imaginary part of the complex index of refraction (i.e., the extinction coefficient), k(λ), (Figure 4) can be used to identify the adsorbed species, as it is related to the reported absorption spectra, R(λ), of viologens through the relation k(λ) ) λR(λ)/4π. The extinction coefficient k(λ) spectrum variation during C14MV monolayer formation (Figure 4) reflects the transient presence of a narrow absorption band centered at 510 nm and a weaker one around 730 nm. This result is somewhat unexpected, as viologen dication solutions are known to be totally transparent in the visible.27 Reduction of the adsorbed dication is unlikely, since the open circuit potential, -0.39 V (vs Ag/AgCl (saturated)), is more positive than the first C14MV reduction wave (about -0.46 V, Figure 5). The transient optical absorption at 510 nm could originate from strong interaction between the adsorbed dication molecules and specifically adsorbed Clions on the gold surface. Such interaction may result in stress on the bipyridine inter-ring C-C bond with an effect on the torsion angle. Mizuguchi27 hypothesized that variations in the torsion angle between pyridine rings of methyl viologen influence the optical transitions in the visible region. Lee29 has also observed a 510 nm absorption band in symmetrically substituted viologen radical solutions and has attributed it to the oblique stacking of the bipyridine moieties. Extinction coefficient spectra recorded at t > 5 s (Figure 4) are consistent with the transparent dication species, indicating the instability of the surface radical-like state under open circuit conditions. We have to note here that the single film optical model
may not be totally adequate to describe the adsorbate on gold due to possible variation of the substrate optical properties during the adsorption. To fully account for such influence, extra layers would have to be introduced into the optical model, but it is only justified when additional experimental information, such as reflectivity or independent measurement of film thickness is available.38 Accordingly, substrate features in the gold interband transition region around 500 nm could distort the presented adsorbate film k spectra. In certain cases it is possible to separate the substrate influence from the film optical constant spectra using electrode surface charge information.21 Unfortunately, the present adsorption experiment was conducted under open circuit conditions, which prevented monitoring of the differential capacitance simultaneously with the ellipsometric parameters. Combined Spectroscopic Ellipsometry and Voltammetry. After injection of C14MV into the electrochemical cell to give a viologen concentration of 375 µM, several potentiodynamic scans were performed in the 20200 mV/s range (Figure 5) together with ellipsometric measurements. Two well-separated redox processes with midpoint potentials around -0.46 and -0.72 V can be assigned to one-electron reductions of the bipyridine compounds.1 The two reactions give rise to complex sets of peaks, each with a different sensitivity to scan rate that is thought to be due to differing mixtures of diffusing and surface-confined viologen entities.1 Voltammetry curve deconvolution (using PeakFit v.4 and assuming the Lorenzian shape) revealed five current peaks both for anodic (A1...A5) and cathodic (C1...C5) scans. Analysis of both peak separation and current versus scan rate plots (data not shown) indicates the presence of surface-confined species for pairs {C1, A1}, {C3, A3}, {C 4, A4}, and {C5, A5}, while pair {C2, A2} indicates a diffusing species. The integrated charge under peak C1 is ∼30 µC/cm2 and does not vary in the scan rate range 20-200 mV/s. It has been estimated that the C14MV molecule occupies about 40-50 Å2 of electrode surface when in the upright, end-on position with alkyl chains fully extended.4 On the basis (37) Bae, I. T.; Huang, H.; Yeager, E. B.; Scherson, D. A. Langmuir 1991, 7, 1558. (38) Gottesfeld, S.; Kim, Y. T.; Redondo, A. In Physical ElectrochemistrysPrinciples, Methods, and Applications; Rubinstein, I., Ed.; Marcel Dekker: New York, 1995; p 461.
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Figure 7. ∆ versus Ψ growth curves at λ ) 647 nm for two consecutive potentiodynamic cycles recorded under the conditions given with Figure 6.
Figure 6. Time dependence of the ellipsometric parameters ∆ and Ψ and electrode surface potential during a potentiodynamic scan in 0.1 M KCl, 0.1 M KHPO4 (pH ) 7.4), and 375 µM C14MV. Labels on the potential dependence indicate the approximate location of the current peaks (see Figure 5). Ellipsometric data shown correspond to the one (λ ) 647 nm) out of 44 channels recorded simultaneously.
of these molecular dimensions, the required charge for one-electron reduction of the tightly packed monolayer is ∼40 µC/cm2. A similar calculation for adsorbed viologen molecules with rings oriented parallel to the surface gives ∼16 µC/cm2. Therefore, the charge transferred at peak C1 suggests that the adsorbed C14MV film is reduced as a relatively tightly packed monolayer with some degree of disorder or tilt relative to normal. This monolayer is reduced before the onset of bulk dication reduction (peak C2). Peak A4 is very narrow (∼9 mV FHW at 20 mV/s), reflecting rather strong attractive interaction between adsorbed viologen radicals.39 Since the radical cations are positively charged, the observed interaction most likely involves electrostatic attraction with the solution anion (Cl-) and/or hydrophobic interaction with lipophilic alkyl chains. Similar associations involving various viologen derivatives have been reported to result in dimer formation.1,29 An example of the ellipsometric trace recorded at 647 nm during potentiodynamic scanning is shown in Figure 6. Significant film growth is initiated after the first reduction peak C1 and continues until the reversal of the scan, as is evident from variation in ∆ and Ψ. The growth curves for two consecutive cycles (Figure 7) have similar shapes and return close to the initial point, indicating almost complete film dissolution at the end of each cycle. Estimates of n, k, and d were determined using a series of points along the growth/dissolution curve. Film thickness, d, as determined from the three-phase model using a global fit for all wavelength channels during the first 20 mV/s potentiodynamic scan is presented in Figure 8 for both scan directions. The film grows gradually from its open circuit value (d ) 16 Å) during the cathodic scan, reaching about 140 Å at the extreme negative potential. Film thickness remains at the maximum during the anodic scan until reaching the oxidation peak, A4 at -0.7 V, where (39) Peerce, P. J.; Bard, A. J. J. Electroanal. Chem. 1980, 114, 89.
Figure 8. Thickness, d, of the viologen film on gold as a function of electrode potential during the potentiodynamic scan of Figure 6. The film thickness was determined using the growth curve method and a three-phase model.
an abrupt decrease in d was observed. This is taken to indicate the reoxidation of the insoluble surface layer into soluble radical species. The film thickness eventually returns to ∼20 Å at the conclusion of the anodic scan, which is close to the initial value. A maximum thickness of 214 Å was recorded at -0.9 V during the second cycle. Such growth behavior with respect to repeated cycling can be rationalized on the basis of a nucleation mechanism whereby a better efficiency of film synthesis occurs on a polymer-coated surface than on bare metal.1 According to this model, the residual film which is present at the sub-monolayer level at the electrode surface after the completion of the first cycle provides “seed” for nucleation during the subsequent cycle. The influence of the earlier cycle on the maximum d at -0.9 V vanishes if the electrode is held at open circuit potential for more than 1 min, thus indicating dynamic equilibrium between the surface and solution C14MV2+ molecules. Integration of cyclic voltammetry peaks to calculate total charge transferred is often used to estimate the coverage by electroactive molecules or the thickness of films.13,16 This estimation assumes that film structure remains uniform during the reaction. To test this assumption and to gain some insight into the film structure, we define the film growth “efficiency” Θ as the ratio of the charge transferred to the ellipsometric-film thickness, Q/d. This
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Figure 9. Ratio of the integrated Coulombic charge and viologen film thickness, Θ, as a function of the electrode potential during the potentiodynamic scan shown in Figure 6.
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Figure 11. Extinction coefficient, k, spectra as a function of potential during the potentiodynamic scan shown in Figure 6.
parameter increases slightly during the cathodic scan, reflecting nearly uniform film structure with little or no discrepancy between the charges transferred during the first and the second reduction steps (Figure 9), as has been observed for some surface-confined viologens.16 A gradual increase in Θ may indicate film densification during growth. The increase in index of refraction, n, (Figure 10) during the cathodic scan supports this latter interpretation, as n increases with material density.40 On the basis of previously mentioned calculations, formation of a dense C14MV monolayer would require ∼40 µC‚cm-2, assuming 1 Faraday equivalent per mole. Division by a molecular length in the extended position (25 Å, estimated using Hyperchem) gives Θ equal to 1.6 µC‚cm-2‚Å-1. A similar estimation procedure for C14MV with bipyridyl rings oriented parallel to the surface (surface area per molecule ∼ 100 Å2) gives Θ = 1.33 µC‚cm-2‚Å-1. Therefore, both Θ values are within the range of the experimental results during the cathodic scan. A large rapid increase in Θ from 1.5 to 5 µQ‚cm-2‚Å-1 was observed after the sharp current peak at -0.7 V during the backward (anodic) scan, possibly reflecting oxidation of the C14MV0 film structure to give radical monocations (C14MV•+). This abrupt increase in Θ during the anodic scan may be rationalized on the basis of the conproportionation reac-
tion.22 Oxidation of the radical cation, which starts after peak A4, would require an additional charge to oxidize radical monocations generated by a conproportionation reaction between C14MV0 in the film and C14MV2+ in solution, thus increasing Θ well above the compact layer characteristic value. This explanation has merit if the radical molecules of the film layer, formed by conproportionation, would remain soluble and would not contribute to film thickness. An alternative interpretation could be detachment of the outer film after peak A4, with a resulting rapid decrease in overall film thickness. It is consistent with the dual film model3 that reoxidation of the neutral film, adjacent to the electrode surface (peak A4, Figure 5), precedes oxidation of the outer monocation radical film. The extinction coefficient spectra, calculated along the potentiodynamic scan trajectory (Figure 11), reflects the evolution of the film composition. Comparison with published absorption spectra of C14MV in the three oxidation states2,27,29 provides a tool for surface film chemical analysis. Of all three oxidation states, only the radical monocation (C14MV•+) has an absorption band in the visible, while the totally reduced species has a narrow absorption band centered around 400 nm.27 As recorded during the cathodic scan, ellipsometric k spectra demonstrate growth of the broad band, with the center at 540 nm for -0.54 V (Figure 11). An absorption band at 544 nm is characteristic of viologen radical dimers.29 At more negative electrode potential, the intensity of this band decreases, as expected for the totally reduced C14MV. This band also experiences a red shift when the film is reduced further during scan reversal, indicating the prevalence of the monomeric viologen radical species. Even at the extreme negative potential -0.9 V, where the cyclic voltammetry curve implies total film reduction, the film absorbance spectrum suggests the presence of residual C14MV•+ radical monomer in the film of 143 Å thickness. These molecules could originate due to conproportionation or could remain in the radical state due to their inaccessibility to the electrode surface at the indicated reducing potential. Also, the dissociation of the aggregated species into monomers before further reduction is possible. Misono41 reported the conversion of the heptyl radical dimer to a monocation monomer prior to the reduction to neutral species on the basis of Raman spectra. A slight shoulder in the blue edge of the k spectrum (Figure 11),
(40) Cruz, C. M. G. S.; Ticianelli, E. A. J. Electroanal. Chem. 1997, 428, 185.
(41) Misono, Y.; Masatoshi, N.; Itoh, K. Spectrochim. Acta 1994, 50A, 1539.
Figure 10. Refractive index, n, spectra at three potentials during the cathodic part of the potentiodynamic scan shown in Figure 6.
Tetradecylmethyl Viologen Films on Gold
recorded at -0.9 V, reflects the UV absorption of totally reduced C14MV,0 which has a narrow band below 400 nm. The limited spectral range of our ellipsometer (419 nm < λ < 761 nm) prevented full resolution of this band. Ellipsometrically determined k and d values can also be used for estimation of radical species concentration in the film, provided the molar extinction coefficient is known. Using 540 ) 6.5 × 106 cm2‚mol-1 for chemically reduced poly(p-xylyl viologen),16 we can estimate a C14MV•+ concentration of 0.0021 M‚cm-3 at -0.54 V. Surface concentrations estimated from Coulometric measurements at the same potential (Figure 5) for C14MV bypyridine rings in the upright and flat orientations are 0.0017 M‚cm-3 (0.68 × 10-12 M‚cm-2) and 0.0014 M‚cm-3 (1.75 × 10-12 M‚cm-2), respectively. Given the single homogeneous film optical model used to solve for k(λ) and d, the close agreement between the ellipsometric and Coulometric concentration values gives a posteriori support for our calculated values of the ellipsometric complex index of refraction spectra and film thickness. This agreement also attests to the total electrochemical accessibility of the viologen radical in the surface film with thickness equivalent to several monolayers.
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a predominantly upright extended conformation (t < 5 s) but converts to a tilted conformation in the equilibrated monolayer (t > 5 s). (2) Adsorbed C14MV2+ film on gold is not stable when placed in pure buffer solution. When the dication was reduced to radical and neutral species during a potential scan from -0.2 to -0.9 V, linear film growth was observed. A gradual increase in the ratio of charge to thickness and in the film index of refraction during the cathodic scan demonstrated densification of the deposit. Extinction coefficient variation during the cathodic scan supported the hypothesis that the radical dimer is the dominant species in the potential range corresponding to the first reduction step. The dimer dissociation into radical monomers was observed during the initial stage of radical reduction to the neutral film. (3) A sharp current peak at -0.7 V observed during the anodic scan was followed by an abrupt loss in total film thickness, indicating the outer film detached following inner film reoxidation or monomer solubilization. Repeated potentiodynamic scans resulted in incomplete reoxidation of the radical film, which promoted further film growth through nucleation during the succeeding scans.
Conclusions In summary, utilization of in situ spectroscopic ellipsometry during the adsorption and subsequent reduction of C14MV on gold has revealed the following: (1) Tetradecylmethyl viologen molecules are reoriented during adsorption on a gold electrode at open circuit potential. Measurements of film thickness are consistent with monolayer formation where viologen is initially in
Acknowledgment. The authors express their gratitude to Dr. Anne Plant for making time available for our work on the spectroellipsometer. This project was supported by the National Science Foundation (Award#: BES9400523) and by the National Institute of Standards and Technology (Cooperative Agreement 70NANB7H0009). LA980412+
