Electrochemical Cleaning of Surface-Confined Carbon Contamination

A protocol for electrochemical cleaning of carbon-contaminated alkanethiol SAMs at mechanically polished. (MP) Ag surfaces is characterized by surface...
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Langmuir 2000, 16, 2907-2914

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Electrochemical Cleaning of Surface-Confined Carbon Contamination in Self-Assembled Monolayers on Polycrystalline Ag and Au Mark H. Schoenfisch, Azalia M. Ross, and Jeanne E. Pemberton* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received January 21, 1999. In Final Form: October 25, 1999 A protocol for electrochemical cleaning of carbon-contaminated alkanethiol SAMs at mechanically polished (MP) Ag surfaces is characterized by surface Raman spectroscopy and electrochemistry. Vibrational information in the ν(C-S), ν(C-C), ν(C-H), and δ(C-H) regions is particularly useful in elucidating the degree of order and amount of contamination in propanethiol, dodecanethiol, and octadecanethiol monolayers before and after negative potential exposure in several aqueous electrolytes. Specifically, Raman spectra indicate that electrochemical cleaning of alkanethiol SAMs at potentials negative of the thiolate reduction removes carbonaceous species and greatly increases the film order near the sulfur headgroup.

Introduction The preparation, characterization, and application of self-assembled monolayers (SAMs) have generated much interest in the past decade.1 In most instances, the supporting metal substrates to these monolayers are exposed to the ambient atmosphere prior to film formation, and thus, these surfaces unavoidably experience contamination by carbon impurities.2 Recent literature reports claim displacement of carbon contamination upon alkanethiol SAM chemisorption.3-5 To the contrary, Raman spectral results indicate this displacement to be incomplete from alkanethiol SAMs formed at mechanically polished and electrochemically roughened polycrystalline Ag surfaces.6,7 In fact, the Raman spectral signatures of these carbon contaminants interfere significantly with the vibrational response of adsorbed alkanethiols, especially the δ(C-H) bands in the 1200 to 1500 cm-1 region. Carbonaceous contamination within self-assembled monolayers may alter the SAM structure. For example, a fraction of film pinholes indicated by cyclic voltammetry8-11 might actually be sites of carbonaceous impurity inclusion; electron transfer may preferentially occur through such sites. Thus, any method for removing carbonaceous impurities might result in SAMs containing fewer defects and improved order. Several surface cleaning protocols have been investigated to minimize carbon contamination at Ag prior to SAM formation including chemical polishing,12 Ar+ sput* To whom correspondence should be addressed. E-mail: [email protected]. (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Taylor, C. E.; Garvey, S. D.; Pemberton, J. E. Anal. Chem. 1996, 68, 2401. (3) Taylor, C. E.; Schoenfisch, M. H.; Pemberton, J. E. Langmuir 2000, 16, 2902. (4) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (5) Buck, M.; Eisert, F.; Grunze, M.; Trager, F. Appl. Phys. A 1991, 53, 552. (6) Buck, M.; Eisert, F.; Grunze, M.; Trager, F. J. Vac. Sci. Tehnol. A. 1992, 10, 926. (7) Schoenfisch, M. H. Ph.D. Thesis, The University of Arizona, 1997. (8) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (9) Finklea, H. O.; Robinson, L. R.; Blackburn, A.; Richter, B.; Allara, D. L.; Bright, T. Langmuir 1986, 2, 239. (10) Sabatini, E.; Rubenstein, I.; Mauz, R.; Sagiv, J. J. Electroanal. Chem. 1987, 219, 365. (11) Sabatini, E.; Rubenstein, I. J. Phys. Chem. 1987, 91, 6663.

tering followed by annealing,13,14 and electrochemical polishing.15,16 Not surprisingly, Ar+ sputtering is the most effective method for preparing clean (carbon-free) surfaces.2 However, surfaces roughened by sputtering (without annealing) experience significant contamination upon exposure to the ambient environment,2 where much of the SAM preparation and analysis is typically performed. Chemically polished (CP) surfaces are the cleanest surfaces prepared in the laboratory ambient.6 However, CP surfaces work to the disadvantage of Raman spectroscopy on weakly scattering systems such as alkanethiol SAMs, because the roughness features at these surfaces provide minimal Raman scattering enhancement.17 Finally, electrochemical polishing analogous to that commonly employed for Au and Pt is not possible at Ag. Electrochemical cleaning of graphitic carbon contamination at Ag has been studied previously with Raman spectroscopy.18 Mahoney and co-workers concluded that graphitic carbon impurities are reduced upon application of negative potentials (e.g., -1.2 V versus SCE), based on the disappearance of the 1360 and 1580 cm-1 bands characteristic of graphitic carbon and the appearance of intense hydrocarbon ν(C-H) bands at ca. 2900 cm-1. Interestingly, the graphitic carbon bands reappear when the potential is returned to -0.2 V (SCE), indicating that the form of carbon contamination can be changed reversibly by potential control. Based on Mahoney’s results, we sought to develop an “electrochemical cleaning” protocol for the removal of carbonaceous contamination from alkanethiol SAMs formed at mechanically polished polycrystalline Ag surfaces. The conditions of the protocol necessary for the preparation of well-ordered alkanethiol monolayers, as determined by Raman spectroscopy and cyclic voltammetry, are reported here. (12) Thomas, B.; Doubova, L.; Stoicoviciu, L.; Trasatti, S.; J. Electroanal. Chem. 1988, 244, 133. (13) Campion, A.; Hallmark, V. Chem. Phys. Lett. 1984, 110, 561. (14) Hubbard, A.; Salaita, G.; Lu, F.; Laguren-Davidson, L. Langmuir 1988, 4, 224. (15) Hamelin, A.; Valette, G. J. Electroanal. Chem. 1973, 45, 301. (16) Weaver, M.; Larkin, D.; Guyer, K.; Hupp, J. J. Electroanal. Chem. 1982, 138, 401. (17) Taylor, C. E.; Pemberton, J. E.; Goodman, G. G.; Schoenfisch, M. H. Appl. Spectrosc. 1999 53, 1212. (18) Mahoney, M. R.; Howard, M. W.; Cooney, R. P. Chem. Phys. Lett. 1980, 71, 59.

10.1021/la9900627 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/04/2000

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Experimental Section Chemicals. Propanethiol (99%), dodecanethiol (98%), and octadecanethiol (98%) were purchased from Aldrich and used as received. Ethanol (absolute) was purchased from Quantum Chemical Corp. Bulk polycrystalline Ag and Au (99.999%) were purchased from Johnson Matthey. Distilled, deionized water was purified with a reverse osmosis (RO 10 Plus) system from Millipore Corp. (Bedford, MA) and then purified further with a Milli-Q UV Plus system (Millipore). N2 (99.995% purity) was obtained from U.S. Airweld (Tucson, AZ). NaF (99%, Fluka), NaCl (99.9%, J. T. Baker Inc.), NaOH (99.99%, Aldrich), hexammineruthenium(III) chloride (32% Ru, Alfa Aesar), and KCl (99.7%, Mallinckrodt) were all used as received. Surface Preparation. Polycrystalline Ag and Au surfaces (geometric surface area ca. 0.80 cm2 for Raman and XPS, ca. (7-8) × 10-3 cm2 for cyclic voltammetry) were cleaned either by sanding with 1500 grit silicon carbide paper or by immersion in Piranha solution, respectively. (Caution! Piranha solution is a very strong oxidant and can spontaneously detonate upon contact with organic material!) The surfaces were then mechanically polished on a padded lapping wheel (Ecomet I, Buehler Ltd.) with successively finer grades of agglomerated alumina down to 0.05 µm. Subsequently, these surfaces were rinsed with water and ethanol. A Bransonic model 220 ultrasonic cleaner was employed for removal of alumina loosely bound at the surface. After mechanical polishing, the Ag and Au surfaces exhibit rms roughness of ∼4.3 and ∼2.6 nm, respectively, as ascertained by AFM. Au surfaces were either used in this state or cleaned further by cycling them (g10 times) from -0.2 V to +1.2 V at 100 mV/s in 1.0 M H2SO4. AFM measurements indicated a rms roughness of ∼9.6 nm after electrochemical cycling of Au. In addition, for surface Raman measurements, Au surfaces were slightly roughened by applying three linear potential sweep oxidation reduction cycles (ORCs) in 0.1 M KCl between -0.2 V and +1.2 V at 100 mV/s. This protocol results in ca. 10 mC/cm2 of anodic charge passed per sweep. SAM Formation. Surfaces were first rinsed with anhydrous ethanol prior to immersion in a 10 mM thiol solution. Film formation times for Au and Ag surfaces were 3 h in propanethiol and 24 h in dodecanethiol and octadecanethiol. Cleaning Protocol. To remove carbonaceous impurities, SAM-modified Ag surfaces were subjected to the appropriate negative potential (-1.1 V for propanethiol, -1.5 V for dodecanethiol, -1.8 V for octadecanethiol) for 120 s in 0.1 M NaF and immediately reimmersed in alkanethiol solution without rinsing. The ethanolic thiol solutions and reimmersion times were identical to the initial immersion conditions above. Prior to Raman analysis, these films were rinsed with ethanol and allowed to briefly air-dry. Raman Spectroscopy. Raman spectroscopy of SAM-modified Ag and Au surfaces was performed on a system described previously.2 Spectra at Ag and Au surfaces were acquired with 514.5 nm excitation from a Coherent Innova 90-5 Ar+ laser and 720 nm excitation from an Ar+ laser-pumped Lexel model 479 Ti-sapphire, respectively. Laser powers of 150 mW at the sample were used for both excitation wavelengths. Integration times were 60 s coadded 5 times for a total of 5 min for SAMs on Ag and 10 s co-added 90 times for a total of 15 min for SAMs on Au. Detection of Raman scattered radiation was accomplished using either a 512 × 512 thinned, back-illuminated TK512 CCD-based system from Princeton Instruments (Ag) or a 512 × 512 frontsideilluminated Photometrics PM512 CCD system (Au), both cooled to -110 °C. An emersion approach19-23 was utilized for acquiring surface Raman spectra of SAMs before and after exposure to electrolyte solutions and potential.24 This approach is new and offers certain advantages over conventional spectroelectrochemical cells in (19) Woelfel, K. J.; Pemberton, J. E. J. Electroanal. Chem. 1998, 456, 161. (20) Sobocinski, R. L.; Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 7152. (21) Pemberton, J. E.; Sobocinski, R. L. J. Electroanal. Chem. 1991, 318, 157. (22) Joa, S. L.; Pemberton, J. E. J. Phys. Chem. 1993, 97, 9420. (23) Shen, A.; Pemberton, J. E. J. Electroanal. Chem. 1999, 479, 32. (24) Schoenfisch, M. H.; Pemberton, J. E. Langmuir 1999, 15, 509.

Schoenfisch et al. which the SAM is separated from the window by a thin layer of solution. One important advantage is the minimization of bulk solution spectral interference as a result of rotation of the electrode surface through and out of a solvent drop. This emersion procedure leaves a very thin layer of solution on the cross-section of surface that is spectroscopically sampled,19 allowing small spectral changes in the SAM to be more easily observed. Previous emersion studies on bare metal electrodes from aqueous19 and nonaqueous environments20-23 indicate that, upon emersion of the surface, at most a very thin layer of solution is maintained on the surface in which the molecules remain organized in the same manner as in situ. Electrochemical control of the surface potential is preserved through the contacting solution drop. The reference and counter electrodes are placed in the solution reservoir from which the drop originates. There are additional advantages which make using the spectroelectrochemical emersion cell desirable.24 The cell can be purged with N2 so that a constant solvent vapor pressure is maintained and/or SAM exposure to the ambient environment is prevented. In addition, long laser exposure times (e.g., 5 min) and relatively high laser power (150 mW) can be used without inducing surface chemistry changes, since the spot sampled on the electrode is continuously changed. Cyclic Voltammetry. Cyclic voltammetry of SAM-modified surfaces was performed with a Bioanalytical Systems 100 W electrochemical workstation in a three-electrode configuration. Potentials applied during electrode cleaning were controlled with an IBM Model EC/225 Voltammetric analyzer. A Pt wire was used as the counter electrode. Potentials were measured and are reported versus a Ag/AgCl (saturated KCl) reference electrode. Interfacial capacitance values were determined from the capacitive current in the cyclic voltammetry and normalized for surface area.

Results and Discussion Contamination of SAMs on Mechanically Polished Ag and Au Surfaces. Raman spectroscopy is useful for detecting the presence of carbonaceous contamination on unmodified mechanically polished polycrystalline surfaces2 and trapped within SAM films.3 Interestingly, considerable variability in both the quantity and chemical nature of carbonaceous contamination occurs for a single mechanical polishing preparation protocol.3,7 Furthermore, the details of the polishing technique (e.g., polishing time, alumina concentration, surface pressure) influence the amount and chemical nature of contaminant carbon present at SAM-modified Ag surfaces. Raman spectra in the ν(C-S) and ν(C-C) frequency region (ca. 600-1700 cm-1) of contaminated propanethiol, dodecanethiol, and octadecanethiol SAMs on mechanically polished polycrystalline Ag are shown in Figure 1a, e, and i, respectively. In addition to the anticipated alkanethiol bands are bands observed at ca. 630, 707, 890, 990, 1026, 1063, 1085, 1128 (dodecanethiol and octadecanethiol only), and 1181 cm-1,25,26 and a broad spectral envelope in the ca. 1200 to 1500 cm-1 region indicates the presence of carbonaceous contamination.3 Raman spectra suggest the contamination is graphitic-like, as evidenced by the bandwidths, relative intensities, and frequencies of additional peaks centered at ca. 790, 912, 1130 (propanethiol only), 1295, 1330, 1350, 1460, and 1590 cm-1, as reported by Taylor.3 Notably, δ(C-H) bands attributable to the alkanethiol SAMs are not observed in the 1200-1500 cm-1 region, because of the significant spectral interference from the carbonaceous contamination bands. Carbonaceous contamination is also present in alkanethiol SAMs on Au, but it is of a different structural form. Raman spectra in the 1300-1700 cm-1 region (720 (25) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 3629. (26) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284.

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Figure 1. Raman spectra of propanethiol (a-d), dodecanethiol (e-h), and octadecanethiol (i-l) SAMs on Ag in the following environments: (a), (e), (i) ex situ, and in 0.1 M NaCl (b-d) or NaF (f-h, j-l) at the following potentials: (b) 0.0 V, (c) -1.0 V, and (d) -0.3 V after (c); (f) 0.0 V, (g) -0.7 V, and (h) -2.0 V; (j) -0.1 V, (k) -1.5 V, and (l) -0.1 V after (k). Numerical values are the frequency (cm-1) of carbon contamination modes.

Figure 2. Raman spectra of propanethiol (a, b), dodecanethiol (c, d), and octadecanethiol (e, f) SAMs on Au without (a, c, e) and with (b, d, f) electrochemical cycling of the Au surface in 1.0 M H2SO4 prior to SAM formation. Integration times: 10 s × 90 collections.

nm excitation) for propanethiol, dodecanethiol, and octadecanethiol SAMs on mechanically polished Au are shown in Figure 2a, c, and e, respectively. The bands at 1430 and 1440 cm-1 are assignable to SAM CH2 scissor modes,27 while those features at 1370 and 1590 cm-1 are attributed to graphitic carbon originating from the mechanical polishing procedure. However, the relatively weak intensity of the 1370 cm-1 band and the narrowness of the 1590 cm-1 peak suggest the presence of a more ordered graphitic carbon at Au than at Ag. A common surface cleaning protocol used for bare Au substrates involves electrochemical polishing in 1 M H2SO4. In this procedure, the electrode potential is cycled between -0.2 V and +1.2 V for formation and reduction of Au oxide. When this protocol is applied to the unmodified mechanically polished Au surface prior to film formation, spectra in Figure 2b, d, and f are observed for propanethiol, dodecanethiol, and octadecanethiol SAMs, respectively. Significantly, the absence of a 1590 cm-1 band in these (27) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.

spectra indicates the formation of carbon-free SAMs. Thus, these oxidation reduction cycles (ORCs) on Au effectively clean the surface. A simple protocol analogous to this process is not possible at Ag given its greater electrochemical reactivity; therefore, alternate electrochemical methods for the preparation of carbon-free SAMs on Ag were investigated. Research efforts with this goal are the focus of the remaining discussion. Contamination Removal from SAMs on Ag through Negative Potential Application. Given previous work reporting the removal of carbonaceous contamination from bare Ag surfaces with negative potential application,14 the extension of this approach to SAM-modified Ag surfaces was investigated. This situation is complicated by the reductive desorption of alkanethiol SAMs on metal electrodes.15,28,29 Porter and co-workers were the first to characterize the chemistry of the bound thiol headgroup; they report the reductive desorption of n-alkanethiol SAMs (28) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860. (29) Bryant, M. A. Ph.D. Thesis, The University of Arizona, 1991.

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on evaporated polycrystalline Ag and Au surfaces in 0.5 M KOH, LiOH, and NaOH.15,28 The products of this process are proposed to be Ag0 or Au0 and thiolate species, RS-. The reductive desorption behavior of propanethiol, dodecanethiol, and octadecanethiol SAMs at mechanically polished polycrystalline Ag surfaces in other aqueous electrolytes (NaF, KCl, NaCl, KOH, NaOH, NaSCN) has also been investigated and is reported elsewhere.24,29 Reductive desorption of propanethiol SAMs at Ag occurs at potentials negative of ca. -0.9 V. This process is observed as a well-defined wave in the cyclic voltammetry of these films. For longer chain alkanethiol (dodecanethiol and octadecanethiol) monolayers on Ag, a reductive desorption peak is not observed due to the overlap of this process with solvent reduction. For both short and long chain alkanethiol SAMs, however, Raman spectra suggest that reductive desorption does not completely remove the SAM from the interface.24 These observations led to the hypothesis that the carbonaceous contamination might be selectively removed by negative potentials, while leaving most of the alkanethiol layer intact. The proof of this hypothesis can be seen by further consideration of the spectra in Figure 1. Little change in the Raman spectrum of a propanethiol SAM is observed upon exposure at 0.0 V to aqueous 0.1 M NaCl, as shown in Figure 1b; however, significant spectral changes are observed at potentials negative of propanethiol reductive desorption (-1.0 V, Figure 1c). Most notably, the bands at 806, 915, 1130, 1201, 1258, 1291, 1324, 1350, 1388, 1462, and 1598 cm-1 attributed to a small PAH contaminant3 are markedly reduced in intensity, suggesting their removal. This phenomenon is irreversible, because these bands do not reappear upon returning the potential to more positive values, as shown in Figure 1d. Significantly, associated with these band intensity losses is a decrease in I [ν(C-S)G]/I [ν(C-S)T] (i.e., band intensities at ca. 630 and 705 cm-1, respectively) from 0.22 to 0.12. Thus, once these carbonaceous impurities are removed from the film, an increase in order near the headgroup results. Since a fraction of the monolayer is also removed at the applied negative potential, such behavior suggests the formation of well-ordered islands by the remaining alkanethiol molecules (or the removal of disordered alkanethiols containing the impurities around well-ordered islands.) Identical observations are made in other aqueous electrolytes including 0.1 M NaF, NaOH, NaSCN, and H2SO4. Similar behavior is observed for dodecanethiol and octadecanethiol SAMs. Little change in the Raman spectrum of dodecanethiol SAMs upon exposure to 0.1 M NaF at 0.0 V for up to 30 min (i.e., the longest time evaluated) is indicated in Figure 1f. Furthermore, application of a negative potential, but one positive of reductive desorption for dodecanethiol (e.g., -1.5 V), does not remove the carbonaceous impurities as shown in Figure 1g. Although the monolayer order increases slightly as evidenced by a small decrease in I [ν(C-S)G]/I [ν(CS)T] from 0.39 to 0.35, bands due to the presence of carbonaceous impurities are still clearly evident at 781, 828, 912, 1295, 1344, 1406, 1460, 1510, and 1588 cm-1. Only after the potential is held negative of dodecanethiol reductive desorption (e.g., -2.0 V, Figure 1h) does the contaminant band intensity diminish. As a result of this carbonaceous impurity removal, I [ν(C-S)G]/I [ν(C-S)T] decreases from 0.31 to 0.17 and both of these peaks sharpen, indicating increased order within the remaining surface-bound film. Notably, the overall intensities of the dodecanethiol bands increase markedly upon application of potentials

Schoenfisch et al. Table 1. Capacitance Values for SAM-Modified Ag Surfaces capacitance (µF/cm2)a alkanethiol SAM on Ag

as-prepared SAMb

clean SAMc

propanethiol dodecanethiol octadecanethiol

43.6 ( 21.6 27.2 ( 6.8 16.9 ( 4.7

9.1 ( 6.5 5.1 ( 2.6 4.2 ( 1.9

a Standard deviations result from the analysis of a minimum of three samples. b Following initial film formation on mechanically polished surfaces. c Following electrochemical cleaning treatment on as-prepared SAM: negative potential application beyond reductive desorption for 120 s with subsequent reimmersion in ethanolic alkanethiol solution.

negative of -1.5 V. Such behavior is attributed to slight surface roughening (undetectable by AFM measures of surface roughness) by water reduction through defect sites. To date, no reports on surface roughening through monolayer defect sights have appeared; however, hydrogen evolution at negative potentials (used to remove surface contaminants) has been reported to roughen Ag surfaces.30-32 Nevertheless, Raman spectral results suggest the presence of well-ordered films. Thus, we speculate that the alkanethiol molecules remaining at the surface at negative potentials congregate to form islands. These observations point toward one final step in the cleaning protocol, the reimmersion of the film into thiol solution to fill in spaces created by the reductive desorption cleaning process. The effect of this step on the resulting SAMs is described in more detail below. Finally, carbonaceous contamination is also present in octadecanethiol SAMs on Ag. Exposure of octadecanethiol SAMs to 0.1 M NaCl at -0.1 V for up to 30 min does not affect the monolayer structure, as shown in Figure 1j. However, at potentials negative of octadecanethiol reductive desorption (ca. -1.5 V), the contaminant bands at 786, 912, 1297, 1330, 1352, 1448, and 1583 cm-1 vanish (Figure 1k). As a result of contaminant removal, I [ν(CS)G]/I [ν(C-S)T] decreases from 0.43 to 0.16 and the modes attributed to the alkanethiol narrow significantly indicating increased order in the remaining film. These effects are irreversible when the potential is returned to -0.1 V (Figure 1l), signifying that contaminant removal by negative potential application is permanent (in contrast to the results reported by Mahoney et al.30). Electrochemical Characterization after Negative Potential Application. Evidence for the integrity of SAMs at various stages of this protocol can be obtained from electrochemical characterization.27,33,34 Toward this purpose, capacitance values of these films are determined from linear potential sweep experiments in regions in which no Faradaic processes occur in 0.1 M NaF. Surfacearea normalized capacitance values for representative asprepared (no negative potential treatment) propanethiol, dodecanethiol, and octadecanethiol SAMs on mechanically polished polycrystalline Ag surfaces are given in Table 1. These values are approximately an order of magnitude higher than those reported for well-ordered SAMs on freshly evaporated Au surfaces,27,32,34 indicating the presence of a significant number of conductive defects within our as-prepared SAMs. (Capacitance values for SAMs on (30) Mahoney, M. R.; Howard, M. W.; Cooney, R. P. Chem. Phys. Lett. 1980, 71, 59. (31) Cooney, R. P.; Mahoney, M. R.; Howard, M. W. Chem. Phys. Lett. 1980, 76, 448. (32) Billman, J.; Kovacs, G.; Otto, A. Surf. Sci. 1980, 92, 153. (33) Finklea, H. O.; Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987, 3, 409 (34) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370.

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Langmuir, Vol. 16, No. 6, 2000 2911 Table 2. % Electrochemically Active Surface for SAM-Modified Ag Surfaces % electrochemically active surfacea

Figure 3. Cyclic voltammetry of 1.0 mM Ru(NH3)63+/0.1 M KCl for (a) mechanically polished, bare Ag, and (b) propanethiol, (c) dodecanethiol, (d) octadecanethiol SAMs on Ag. Sweep rate ) 100 mV/s.

Ag have not been reported in the literature.) Thus, we propose that a significant number of these defects result from sites of carbonaceous contamination. Cyclic voltammetry of Ru(NH3)63+ at alkanethiol SAMmodified Ag electrodes was used to quantitatively estimate the fraction of the film remaining on the surface after negative potential application to remove contamination. These results are interpreted assuming that the observed current is inversely proportional to the alkanethiol surface coverage. Ru(NH3)63+ was selected as an electrochemical probe, since it represents a convenient and electrochemically reversible, one-electron redox couple. More importantly, alkanethiol SAMs are stable throughout the electrochemical window in which Ru(NH3)63+ redox chemistry occurs. Potential scans were performed between 0.0 and -0.25 V at a sweep rate of 100 mV/s. Prior to electrochemical cleaning of the SAMs, the cyclic voltammetry of Ru(NH3)63+ at bare, mechanically polished and as-prepared SAMmodified Ag surfaces were recorded; these voltammograms in 1.0 mM Ru(NH3)63+/0.1 M KCl are shown in Figure 3. Short alkanethiol SAMs (e.g., propanethiol, Figure 3b) on Ag only slightly impede the redox activity of Ru(NH3)63+ relative to that at bare Ag (Figure 3a), due to relatively efficient electron tunneling across the film and electron transfer through defects. In contrast, electron-transfer activity to Ru(NH3)63+ is almost completely blocked at long chain alkanethiol SAMs, as shown in Figure 3c and d. The shapes of the voltammograms for dodecanethiol and octadecanethiol-modified electrodes imply radial diffusion suggesting that the modified electrode functions as microelectrode arrays.35 These data were further used as a measure of exposed Ag surface area after the negative potential cleaning process described above, since the spectral evidence clearly indicates the retention of alkanethiol at the surface. The indicator of SAM surface coverage used here is the ratio of the peak reduction current for Ru(NH3)63+ at a SAMmodified electrode (iSAM) to that measured at a bare Ag surface (iAg). The quantity (iSAM/iAg) × 100% is defined as the normalized percent electrochemically active surface area (%EAS). A bare Ag surface is 100% electrochemically active and able to reduce Ru(NH3)63+ at the maximum rate. As shown in Table 2, long chain alkanethiol SAMs block electron transfer quite effectively with less than 1% of the initial bare Ag surface area available for electron transfer when the surface supports the as-prepared films. The absolute magnitudes of these values must be interpreted with care, however, since the bulk of electron transfer (35) Finklea, H. O.; Snider, D. A.; Fedjk, J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660.

alkanethiol SAM on Ag

as-prepared SAM

after negative potential applicationb

dodecanethiol octadecanethiol

0.22 ( 0.16 0.11 ( 0.05

54.0 ( 37.3 93.0 ( 24.4

after reimmersion into alkanethiol solution 0.35 ( 0.19 0.26 ( 0.14

a All values normalized to bare (unmodified) Ag surfaces. Standard deviations are reported for a minimum of 3 trials on each SAM type. b Negative potentials used: -1.5 V for dodecanethiol; -1.8 V for octadecanethiol.

Figure 4. Cyclic voltammetry of 1.0 mM Ru(NH3)63+/0.1 M KCl for (a) propanethiol, (b) dodecanethiol, (c) octadecanethiol SAMs on Ag before and after negative potential application. Sweep rate ) 100 mV/s.

occurs at defects. Therefore, the microelectrode array behavior (observed in Figure 3b-d) results in inflated iSAM values due to radial diffusion contributions at low sweep rates;36 true %EAS values are actually slightly smaller than those given in Table 2. As an aside, propanethiol SAMs on Ag surfaces support significant electron transfer as evidenced by %EAS values of 78.7 ( 13.6%. The poor blocking ability of short chain alkanethiol SAMs is well-known and arises from relatively efficient electron tunneling through the film.37 Due to this anomalous behavior, propanethiol SAMs are only qualitatively characterized using cyclic voltammetry. SAM-modified Ag surfaces containing carbon impurities were then electrochemically cleaned in an unstirred 0.1 M NaF solution for 120 s at a potential negative of reductive desorption for the alkanethiol SAM. As shown (36) Gueshi, T.; Tokuda, K.; Matsuda, H. J. Electroanal. Chem. 1979, 101, 29. (37) Miller, C.; Luendet, P.; Gratzel, M. J. Phys. Chem. 1991, 95, 877.

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Figure 5. Raman spectra of (a), (b) propanethiol, (c), (d) dodecanethiol, and (e), (f) octadecanethiol SAMs on Ag after initial immersion in thiol/ethanol solution, electrochemical cleaning in 0.1 M NaF, and reimmersion in thiol/ethanol solution.

in Figure 4 for the three SAMs studied here, a large increase in cathodic current is observed after this step, indicating the removal of carbonaceous contamination and alkanethiol with a concomitant increase in %EAS. In fact, for propanethiol SAMs, the %EAS after cleaning is greater than 100%, suggesting slight surface roughening as a result of this process. A similar increase due to surface roughening is assumed for the longer chain alkanethiol SAMs as well. For example, an increase from 0.11 to 93%EAS for octadecanethiol SAMS after negative potential application does not imply removal of the majority of the alkanethiol, but rather a combination of surface roughening and alkanethiol desorption. This explanation is consistent with the increase in Raman intensities of these SAMs at negative potentials, as discussed above. In summary, although negative potentials are effective for removing carbonaceous contamination from SAMs on Ag, these potentials remove alkanethiol molecules as well. Therefore, the result of this negative potential treatment is an incomplete SAM film. Electrode Reimmersion. One additional step was added to this cleaning protocol to address the problem of alkanethiol monolayer depletion at negative potentials. To fill in and reanneal depleted regions, the cleaned surface is reimmersed in thiol solution. A reimmersion time equivalent to that used in the original film formation procedure was used to create highly ordered monolayers as determined by Raman spectroscopy. When done properly, electrochemical cleaning and reimmersion reproducibly results in SAM-modified interfaces which are highly ordered and possess little or no carbonaceous contamination on mechanically polished Ag surfaces. Raman spectra of propanethiol, dodecanethiol, and octadecanethiol SAMs on Ag before and after this electrochemical cleaning/reimmersion protocol are shown in Figure 5. Several interesting and important spectral changes are observed after cleaning and reimmersion. First, for all SAMs, removal of carbonaceous contamination (indicated by the disappearance of the 1600 cm-1 band) and significant decrease of the background intensity in the 1200-1600 cm-1 region allows resolution of several δ(C-H) bands not previously observed or reported for SAMs on mechanically polished Ag substrates. These bands38 are assigned in Table 3. Notably, the spectral background intensity decrease after the electrochemical cleaning/ reimmersion protocol indicates significant intensity con(38) Snyder, R. G. J. Chem. Phys. 1967, 47, 1316.

Table 3. Raman Peak Frequencies (cm-1) and Assignments in the 1200-1600 cm-1 Region for Propanethiol, Dodecanethiol, and Octadecanethiol SAMs on Polycrystalline Ag Raman spectral bands (cm-1) propanethiol

1285 1325 1373 1424 1449 a

dodecanethiol

octadecanethiol

assignmenta

1212 1238

1215 1236 1259 1273 1294 1352 1377 1435 1453

CH2 twist CH2 twist CH2 twist CH2 twist T,G CH2 wag CH3 bend CH2 scissor CH2 scissor

1272 1295 1340 1378 1433 1454

Assignments taken from ref 39.

tributions to this background from the carbon contamination. The magnitude of these contributions is consistent with the assignment of this contamination to a small PAH, because these molecules are expected to have large Raman scattering and fluorescence efficiencies. Second, information about the order of clean SAMs can be obtained from the spacing of the well-resolved peaks observed in the 1150 to 1300 cm-1 region.39 The pattern of bands in this region is attributed to the progression of coupled CH2 wagging modes expected for alkyl chains in an all-trans conformation.38,40 The spacing of these peaks (∆ν) is related to the number of methylene units (n) in an all-trans conformation in an alkyl chain according to the following equation:39

∆ν ) 326/(n+1)

(1)

Equation 1 identifies an average peak spacing of 18 cm-1 for an all-trans octadecyl chain (n ) 17). For octadecanethiol on Ag, the observed average spacing of these peaks in Figure 5f is 19.1 ( 3.9 cm-1, resulting in a value of n ) 16 according to eq 1. Thus, one out of 17 methylene units exhibits a gauche conformation, most likely the methylene adjacent to the methyl group. This value corresponds to a ca. 6% average gauche population in the octadecanethiol monolayer, consistent with a high degree of order. A similar analysis for propanethiol and dodecanethiol SAMs on Ag is difficult, due to the lack of (39) Snyder, R. G.; Schachtsneider, J. H. Spectrochim. Acta 1963, 19, 85. (40) Snyder, R. G. J. Mol. Spectrosc. 1960, 4, 411.

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Table 4. Ex situ I [ν(C-S)G]/I [ν(C-S)T] Values in the Absence and Presence of Carbon Contamination alkanethiol SAM

as-prepared SAM

clean SAM

propanethiol dodecanethiol octadecanethiol

0.44 ( 0.12a 0.32 ( 0.17 0.45 ( 0.12

0.24 ( 0.06 0.17 ( 0.03 0.27 ( 0.06

a Standard deviations result from the analysis of a minimum of three samples.

Figure 7. Raman spectra of (a) mechanically polished, bare Ag surface, (b) following electrochemical pretreatment in 0.1 M NaF, (c) propanethiol SAM on Ag formed after pretreatment, (d) propanethiol SAM from (c) after standard electrochemical cleaning in 0.1 M NaF. *denotes hydrocarbon bands.

Figure 6. Cyclic voltammetry of 1.0 mM Ru(NH3)63+/0.1 M KCl for (a) propanethiol, (b) dodecanethiol, (c) octadecanethiol SAMs on Ag before (after electrochemical cleaning) and after reimmersion. Sweep rate ) 100 mV/s.

progression bands observed in the spectra of these monolayers. An increase in monolayer order near the headgroup is indicated by both the narrower bandwidths and smaller I [ν(C-S)G]/I [ν(C-S)T] values (see Table 4) in the Raman spectra of clean SAMs relative to contaminated SAMs. In other words, carbonaceous contamination adversely affects the monolayer structure closest to the surface, where this contamination is believed to reside. The full width at halfmaximum (fwhm) value for the ν(C-S)T band at 710 cm-1 changes from ca. 34 to 27 cm-1 for propanethiol, from ca. 43 to 37 cm-1 for dodecanethiol, and from ca. 48 to 41 cm-1 for octadecanethiol. Similar trends are observed when comparing the fwhm values of neat liquid and solid alkanethiol Raman bands, because narrower bands are consistent with a more crystalline monolayer. Evidence for the formation of highly ordered SAMs after the reimmersion step is also evident in the electrochemistry of these systems. Cyclic voltammetry of alkanethiol SAMs after electrochemical cleaning (before reimmersion) and after reimmersion are shown in Figure 6. As quantified in Table 2, the %EAS after reimmersion returns to values similar to those measured for the as-prepared SAMs indicating reformation of close-packed, blocking layers.

In fact, the blocking behavior for propanethiol films after this protocol is substantially better than the as-prepared films as can be ascertained by comparison of Figure 3a and b with Figure 6a. The small standard deviations in both the I [ν(C-S)G]/I [ν(C-S)T] and %EAS values indicate the extraordinary reproducibility of this electrochemical cleaning protocol. In addition, within standard deviation, clean films possess similar %EAS values compared to contaminated films suggesting that alkanethiol molecules replace sites previously occupied by carbonaceous contamination. This observation implies, quite significantly, that the carbon contamination present initially must be largely nonconductive when entrapped within the SAM. Thus, a small PAH contaminant, a conceivable precursor to graphitic carbon, is consistent with this picture. These results further suggest that this carbon contamination, although detrimental for achieving exceptional monolayer order, does not substantially alter the dielectric properties of the film. Therefore, electrochemical estimates of pinhole density within SAMs remain generally reliable, despite the presence of such contamination stemming from mechanical polishing. Capacitance measurements provide additional evidence for the formation of highly ordered SAMs after electrochemical cleaning. As shown in Table 1, the capacitance values of clean SAMs in 0.1 M NaF are considerably smaller (ca. 5 times) than those for contaminated SAMs, suggesting a larger number of defects for contaminated SAMs. Collectively, therefore, surface Raman spectroscopy, %EAS, and capacitance measurements clearly indicate that contaminated SAMs are less ordered than their uncontaminated counterparts. Electrochemical Treatment Without SAM. Koglin and co-workers have reported the use of cetylpyridinium chloride as a surfactant for removing melamine (a stable heterocyclic structure) preadsorbed on Ag surfaces.41 Both the surfactant and the melamine were shown to desorb from the surface at negative potentials. Without the surfactant present, melamine was not removed from the surface at comparable potentials. This report led us to question the role and effectiveness of the alkanethiol SAM in removal of carbon contamination. Thus, an alternate electrochemical cleaning strategy involving negative (41) Koglin, E.; Kip, B. J.; Meier, R. J. J. Phys. Chem. 1996, 100, 5079.

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potential application prior to initial SAM formation was investigated. For these studies, mechanically polished, unmodified Ag surfaces were electrochemically pretreated in 0.1 M NaF at -2.0 V for 5 min. Raman spectra before and after this treatment and after propanethiol SAM formation are shown in Figure 7. Clearly, the strong graphitic carbon bands observed in the 1200-1600 cm-1 region on bare Ag (Figure 7a) indicate that the Ag surface is the source of the carbonaceous contamination. After exposure of this surface to -2.0 V for 2 min in 0.1 M NaF, Raman peaks are observed at 801, 920, and 1047 cm-1 (Figure 7b), suggesting partial reduction of this contamination to hydrocarbons; however, most of the contamination remains unchanged and is trapped within the propanethiol film once it is formed on this surface (Figure 7c). The I [ν(C-S)G]/I [ν(C-S)T] value of 0.59 suggests that the propanethiol SAM formed after this pretreatment is poorly ordered. Long chain alkanethiol SAMs formed after exposure of bare Ag to negative potentials possess similar disorder and contamination. Thus, these studies indicate that the alkanethiol SAMs are essential components in carbon contamination removal at negative potentials. The SAM is proposed to serve as a surfactant, facilitating removal of impurities after reductive desorption from the Ag surface at sufficiently negative potentials. Notably, pyrene entrapped within a SAM film as discussed in ref 3, is completely removed after only a few seconds at -1.5 V in 0.1 M NaF, demonstrating the necessity of alkanethiol molecules and negative potential application for removal of carbonaceous contamination. Conclusions An electrochemical cleaning protocol summarized in Figure 8 has been developed for reproducible formation of highly ordered SAMs at mechanically polished Ag surfaces, free of carbonaceous contamination. Raman spectra suggest that both the application of a negative potential and the presence of an alkanethiol SAM are necessary for removal of the surface-bound carbonaceous impurities. Our electrochemical data supports simulta-

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Figure 8. Schematic of the electrochemical cleaning protocol.

neous alkanethiol desorption and contamination removal at negative potentials, and thus, we speculate the SAM functions as a surfactant for the impurities. Cyclic voltammetry indicates that the carbon contamination in self-assembled monolayers is nonconductive. Thus, although Raman spectra and capacitance measurements proved the carbon contaminated films to be more disordered, the blocking properties of impure films are not drastically compromised. Acknowledgment. The authors gratefully acknowledge support of this research by the National Science Foundation (CHE-9504345). LA9900627