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Corrosion of Alkanethiol-Covered Cu(100) Surfaces in Hydrochloric Acid Solution Studied by in-Situ Scanning Tunneling Microscopy J. Scherer, M. R. Vogt, O. M. Magnussen,* and R. J. Behm Abteilung Oberfla¨ chenchemie und Katalyse, Universita¨ t Ulm, D-89069 Ulm, Germany Received April 10, 1997. In Final Form: August 12, 1997X The surface structure and corrosion of alkanethiol-covered Cu(100) surfaces in 1 mM HCl was studied by in-situ scanning tunneling microscopy (STM) and complementary electrochemical measurements. The octanethiol (OT) and hexadecanethiol (HDT) monolayers were prepared by spontaneous adsorption from ethanolic solution onto an electropolished single crystal substrate and then immersed into 1 mM HCl solution at potentials between -0.16 and -0.26 V vs Ag/AgCl (KCl sat.). Samples prepared in this way exhibit a well-defined surface morphology, where atomically smooth Cu terraces, which are found also on thiol-free Cu(100), are covered by monoatomically high Cu islands and by pits. Keeping the potential in this regime causes slow Cu roughening via the formation of additional Cu monolayer islands and pits. This surface restructuring is probably caused by exchange of Cu or Cu-thiolates with the electrolyte. The onset of Cu corrosion is shifted anodically to potentials in the range -0.12 to -0.10 V, reflecting the inhibition by the thiol layer. Copper corrosion proceeds via the formation of many small etch pits and pronounced surface roughening. At higher potentials and/or after prolonged etch times only few larger pits are observed. Local Cu corrosion within these pits proceeds via a similar mechanism as on bare Cu(100), suggesting that the inhibiting thiol layer is completely removed at these places.
Introduction The strongly bound self-assembled monolayers (SAMs) formed by alkanethiols on various metal surfaces offer interesting new possibilities for a fundamental understanding of corrosion inhibition by thin organic layers as well as for technological applications in corrosion protection. In contrast to the extensively studied thiol SAMs on (chemically inert) Au surfaces, only a few studies of alkanethiol SAMs on more reactive metals, such as Ag,1-5 Cu,1,6-11 and Fe12,13 have been reported. These studies indicate a strong influence of the metal substrate on structure, stability, and properties of the SAMs. In-situ structural sensitive techniques, such as scanning tunneling microscopy (STM) or atomic force microscopy (AFM) gain direct structural information on these surfaces and in particular allow observation of the influence of SAMs on the corrosion behavior. However, up to now in-situ STM or AFM studies were performed only on thiol-covered Au electrodes in aqueous electrolytes.14-18 Here we report the first STM observations of alkanethiol SAMs on a more * Corresponding author: phone, XX49-731-502-5457; fax, XX49731-502-5452; e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, October 1, 1997. (1) 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. (2) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370. (3) Fenter, P.; Eisenberger, P.; Li, J.; Camillone, N.; Bernasek, S.; Scoles, G.; et al. Langmuir 1991, 7, 2013. (4) Bucher, J.-P.; Santesson, L.; Kern, K. Langmuir 1994, 10, 979. (5) Li, W.; Virtanen, J. A.; Penner, R. M. Langmuir 1995, 11, 4361. (6) Blackman, L. C. F.; Dewar, M. J. S.; Hampson, H. J. Appl. Chem. 1957, 7, 160. (7) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990. (8) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022. (9) Yamamoto, Y.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1993, 140, 436. (10) Feng, Y.; Teo, W.-K.; Siow, K.-S.; Gao, Z.; Tan, K.-L.; Hsieh, A.-K. J. Electrochem. Soc. 1997, 144, 55. (11) Jennings, G. K.; Laibinis, P. E. Colloids Surf. A 1996, 116, 105. (12) Stratmann, M. Adv. Mater. 1990, 29, 191. (13) Stratmann, M. Stahl Eisen 1993, 113, 101. (14) Pan, J.; Tao, N.; Lindsay, S. M. Langmuir 1993, 9, 1556. (15) Rohwerder, M.; de Weldige, K.; Vago, E.; Viefhaus, H.; Stratmann, M. Thin Solid Films 1995, 264, 240.
S0743-7463(97)00372-7 CCC: $14.00
reactive metal, on (100)-oriented Cu, which give new insight into the surface structure and the corrosion behavior of thiol-covered Cu(100) in HCl solution. According to previous studies densely packed SAMs of long-chain n-alkanethiols and ω-substituted alkanethiols can be formed on polycrystalline copper surfaces by chemisorption from solution.1,6-11 These monolayers have been characterized by IR-reflectance-spectroscopy,1 X-ray photoelectron spectroscopy (XPS),1,7,9-11 wetting measurements,1,7,9,11 surface-enhanced Raman scattering (SERS),9 and electrochemical methods.9,10 It was found, that alkanethiol SAMs can provide significant protection against the oxidation of copper surfaces by air.8,10,11 Investigations of the corrosion resistance of copper surfaces covered by long-chain alkanethiols revealed that the SAMs provide a high protection ability and result in an effective inhibition of oxide growth in water11 and in aerated 0.51 M NaCl solutions.10 Good protection by alkanethiol SAMs was also found in more technical experiments, where patterned SAMs were used as masks in the formation of micrometer-scale etch structures on copper.19 In contrast, in aerated 0.5 M Na2SO4 only moderate protection abilities of various alkanethiols were observed.9 Finally the effect of chemical modification of the adlayer or the substrate was investigated. Increased protection was obtained by cross-linked SAMs, in which the thiol molecules were polymerized.9,11,20-23 It was also suggested that the sample treatment before thiol adsorption can strongly influence structure and properties of the monolayers,1,10 which might explain the observed differences in the protection abilities. (16) Li, Y.-Q.; Chailapakul, O.; Crooks, R. M. J. Vac. Sci. Technol., B 1995, 13, 1300. (17) Gilbert, S. E.; Cavalleri, O.; Kern, K. J. Phys. Chem. 1996, 100, 12123. (18) Zamborini, F. P.; Crooks, R. M. Langmuir 1997, 13, 122. (19) Moffat, T. P.; Yang, H. J. Electrochem. Soc. 1995, 142, L220. (20) Itoh, M.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1994, 141, 2018. (21) Itoh, M.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1995, 142, 1839. (22) Itoh, M.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1995, 142, 3696. (23) Haneda, R.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1997, 144, 1215.
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The potential-dependent morphology of Cu(100) in the double layer range and the initial stages of Cu(100) dissolution were recently studied by in-situ STM in our group24,25 as well as by others.26-28 As a part of an ongoing study on copper corrosion we very recently reported the first in-situ STM results on corrosion inhibition by benzotriazole,29 where the organic inhibitor was directly added to the corrosive medium. Here we present in-situ STM observations of Cu(100) surfaces, which were covered with alkanethiol SAMs prior to immersion into the corrosive electrolyte and hence may serve as a simple model system for protective coatings. The monolayers were formed by adsorption of octanethiol (OT) and hexadecanethiol (HDT) from ethanolic solution onto an atomically flat Cu(100) single crystal. The results show that well-defined thiol-terminated Cu(100) surfaces can be prepared, which exhibit a drastically different corrosion behavior as compared to bare Cu(100). These findings agree well with parallel electrochemical measurements. Experimental Section Preparation of Copper Substrate. For every experiment the Cu(100) single crystal was electrochemically polished in 66% orthophosphoric acid (Riedel deHae¨n, extra pure) at an anodic value of about 2.4 V versus a Pt counter electrode as described elsewhere.24,25 After being polished, the crystal was rinsed with ultrapure water (Millipore). Then the crystal was immersed for 5-10 min in 1 mM HCl (Merck, Suprapur) to ensure an oxidefree Cu surface with a well-defined topography (see below). Preparation of Self-Assembled Monolayers. 1-Octanethiol (Merck, p.a.) and 1-hexadecanethiol (Fluka, pract.) were used as received to prepare 1 mM solutions in absolute ethanol (Merck, p.a.). The freshly prepared Cu(100) single crystal was directly transferred from the 1 mM HCl into the thiol solution. Before transfer and during immersion, the thiol solution was purged with N2 (Linde, 99.999%). After 1-2.5 h the crystal was emersed and then thoroughly rinsed in absolute ethanol and n-hexane (Merck, p.a.) to remove excess thiol. Electrochemistry. The cyclic current-voltage curves (cyclic voltammograms) and the current transients of bare and thiolcovered Cu(100) were recorded in a separate electrochemical cell by the dipping technique. A Pt-wire counter electrode and a Ag/AgCl (KCl sat.) reference electrode were used. Prior to and during the measurements the electrolyte was purged with N2. In-Situ STM. The STM experiments were carried out in a home-built STM, based on a design by Besocke.30 The tunneling tips were electrochemically etched from a polycrystalline W-wire in 2 M NaOH and subsequently coated with Apiezon wax, leaving only the very end of the tip exposed to the electrolyte. An electrochemical cell made from Kel-f with a Pt-wire counter electrode was used in the STM measurements. The potentials of sample and tip were controlled potentiostatically against a Ag/AgCl (KCl sat.) reference electrode, which was connected to the cell by a 0.01 M Na2SO4 (Merck, Suprapur) salt bridge. After being rinsed in ethanol and n-hexane, the thiol-covered crystal was quickly mounted in the cell and covered with the electrolyte (1 mM HCl). Then the cell was mounted in the STM and a potential between -0.26 and -0.16 V was applied. This potential was held constant while waiting for thermal equilibration (ca. 1-3 h). The electrolyte in the STM cell was not deaerated. All STM images were recorded in the constant current mode, with the tip potential typically between -0.30 and -0.10 V vs Ag/ AgCl (KCl sat.).
Results and Discussion Electrochemical Characterization. Typical current-voltage curves of bare and OT-covered Cu(100) (24) Vogt, M. R.; Mo¨ller, F. A.; Schilz, C. M.; Magnussen, O. M.; Behm, R. J. Surf. Sci. 1996, 367, L33. (25) Vogt, M. R.; Lachenwitzer, A.; Magnussen, O. M.; Behm, R. J. Surf. Sci., in press. (26) Suggs, D. W.; Bard, A. J. J. Phys. Chem. 1995, 99, 8349. (27) Moffat, T. P. Mater. Res. Soc. Symp. Proc. 1996, 404, 3. (28) Moffat, T. P. Mater. Res. Soc. Symp. Proc., in press. (29) Vogt, M. R.; Polewska, W.; Magnussen, O. M.; Behm, R. J. J. Electrochem. Soc. 1997, 144, L113. (30) Besocke, K. H. Surf. Sci. 1987, 181, 145.
Figure 1. Cyclic current-voltage curves of bare (dotted lines) and OT-covered (solid lines) Cu(100) surfaces in 1 mM HCl, recorded at a sweep rate of (a) 10 mV/s and (b) 0.1 mV/s. (c) Current transients of bare (dotted line) and OT-covered (solid line) Cu(100) after a potential step from -0.16 to 0.05 V.
surfaces at a potential sweep rate of 10 mV/s are presented in Figure 1a. At potentials positive of -0.10 V the cyclic voltammogram of the bare surface in 1 mM HCl (dotted line) shows an increasing anodic current density due to copper dissolution. When the potential scan is reversed, a cathodic current peak corresponding to Cu redeposition is observed. The relatively low and almost constant current density in the potential range from -0.10 to -0.40 V can be attributed to double layer charging. Between -0.40 and -0.65 V an increased current density and small peaks are visible, which can be associated with Cl adsorption/desorption and phase transitions in the adlayer.24,25,28 At potentials lower than -0.65 V cathodic H2-evolution takes place. In the cyclic voltammogram of the OT-covered Cu(100) surface (solid line) the current density for Cu dissolution is far lower, although the anodic limit of the cycle is 0.03 V higher, and the onset of strong Cu dissolution is shifted by about 0.08 V to more anodic values. This shows clearly that the alkanethiol layer hinders Cu dissolution. In addition, the current density in the double layer region is much smaller and the Cl adsorption/desorption peaks as well as the cathodic H2-evolution are suppressed. No features indicative of adsorption/desorption, reduction/ oxidation, or phase transitions of the alkanethiols are visible. For anodic potential limits below about +0.08 V no significant changes are observed in the voltammogram, even after several hours of potential cycling. Cyclic voltammograms of HDT-covered surfaces exhibit similar
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features. The data suggest that alkanethiols are adsorbed throughout the entire double layer potential range, without any changes relevant for the electrochemical behavior. From the cyclovoltammetric data the (pseudo)capacitance of the double layer was estimated. The values were about 60 µF/cm2 for bare, 9 µF/cm2 for OT-covered, and 4 µF/cm2 for HDT-covered Cu(100); i.e., the capacitance is about an order of magnitude smaller in the presence of a thiol adlayer. A similar behavior was observed for bare and thiol-covered polycrystalline Cu.9 The decrease in capacitance can be rationalized by a dense, impermeable alkanethiol layer, which increases the separation of electronic and ionic charge in the double layer and decreases the polarizability of the intermediate medium.31 In addition to these relatively fast potentiodynamic measurements also quasi-stationary potential scans at a sweep rate of 0.1 mV/s, i.e., at a time scale comparable to the STM experiments, were recorded (Figure 1b). For bare Cu(100) (dotted line) the Cu dissolution current is lower than that shown in Figure 1a due to the development of a higher metal cation concentration in the near-surface region at the slower scan rate, resulting in a correspondingly higher rate of Cu redeposition. In contrast, the quasistationary dissolution current of the OT-covered Cu(100) (solid line) is increased with respect to that in the fast scan and exhibits substantial hysteresis upon reversing the scan direction. As will be shown by the STM data (see below) this behavior can be attributed to the gradual, slow removal of the protective thiol-layer at potentials in the Cu dissolution regime. This is further supported by the different long-time current response of bare and thiolcovered Cu surfaces after potential steps from the double layer into the dissolution range. An example is shown in Figure 1c for a potential step from -0.16 to 0.05 V. For bare Cu (dotted line) the current sharply rises and then gradually decays due to the increase in the near-surface concentration of Cu cations. In contrast, for OT-covered Cu (solid line) the current slowly rises over a period of about 2 h, reflecting the slow disruption of the protective SAM. Even when a steady-state current has been reached, the dissolution current is about 50% lower on the OTcovered Cu, suggesting that the thiol layer is not completely removed. Topography of Thiol-Covered Cu(100) after Immersion in 1 mM HCl. The morphology of bare, OT-, and HDT-covered Cu(100) surfaces after relative short exposure to the electrolyte solution at potentials between -0.16 and -0.25 V is illustrated in the STM images in Figure 2. In all three experiments the potential was applied immediately after mounting the cell into the STM and held constant until the STM image was recorded about 90 min later. In agreement with previous results24-28 the bare Cu(100) surface in HCl solution (Figure 2a) exhibits a well-ordered, characteristic topography, where several hundred angstroms wide, atomically smooth terraces are separated by almost perfectly straight steps, which are oriented along the [010] and [001] directions of the substrate. The step height is 1.8 ( 0.3 Å or multiples thereof, corresponding to mono- and multisteps of the Cu(100) substrate. On the atomic scale a c(2×2) superstructure is observed, which has been attributed to an ordered Cl adlayer.24-28 A similar surface topography with extended rectangular terraces separated by mono- and multiatomic Cu steps is found on the thiol-covered surfaces (Figure 2b,c). Since the crystal was always mounted in the same way in the cell, the images can be directly compared with the image of the bare Cu(100) surface. (31) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.
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This comparison reveals that also here the steps are preferentially oriented along [010] and [001], which is related to the sample preparation procedure (see below). However, the terraces of the thiol-covered Cu(100) exhibit a significantly higher surface roughness than those of bare Cu(100) surfaces. As can be seen in Figure 2, Cu monolayer islands and pits with a depth of about 10-15 Å (arrows) are found on the terraces. The density of pits seemed to be smaller on HDT-covered surfaces than on OT-covered surfaces. In addition, also the roughness of the steps is significantly higher on the thiol-covered surfaces. Additional experiments show that the long-range surface topography of the Cu surfaces is determined by the pretreatment of the sample prior to self-assembly of the thiol adlayer. First, in complementary STM measurements of thiol-covered surfaces in air (not shown here) topographies with oriented rectangular terraces resembling those shown in Figure 2b,c were obtained. Hence, these features are formed during sample preparation and do not result from the exposure of the thiol-covered surface to the electrolyte solution. Second, in experiments where the electropolished Cu sample was immersed in 0.01 M H2SO4 (Merck, Suprapur) instead of 1 mM HCl prior to the thiol self-assembly, no oriented rectangular terraces were found. Instead, a morphology with randomly oriented Cu monosteps was observed, which resembles that observed in thiol-free sulfuric acid solution.25 Similar to the HCl pretreated samples the surface roughness was found to be increased due to additional monolayer islands and pits. On the basis of these findings we assume that the rectangular terraces are formed during the HCl pretreatment due to Cl-induced surface restructuring of the (bare) electropolished Cu surface, as described in refs 24-28. Since this pretreatment resulted in a well-defined and easily recognizable surface topography, it was adopted as the standard preparation procedure. In addition, the surface topography of thiol-covered Cu(100) also strongly depends on the immersion time in the thiol-containing solution. The most reproducible and stable STM images were obtained with immersion times up to 2.5 h. In experiments with longer immersion (12-60 h) rough Cu surfaces, which could not be imaged stable, were obtained. It is noteworthy that stable, high-resolution images of HDT-covered surfaces, especially for longer observation periods, were more difficult to obtain than images of OTcovered surfaces. This may be related to the different thickness and hence differing conductivity of the alkanethiol SAMs. The STM experiments in electrochemical environment required tunneling currents >0.2 nA and tunneling voltages of a few hundred millivolts. Under these conditions an irreversible disorganization of thiol SAMs on Au(111) substrates was observed in air, in particular for longer chain length, and attributed to penetration of the tip into the thiol layer. The experimental difficulties in imaging HDT-covered Cu(100) are probably caused by such penetration as well as by the resulting disorganization or destruction of the thiol layer.32,33 Tipinduced changes in the substrate might also occur due to subsequent chemical reactions (oxidation, Cu dissolution). In order to minimize these problems, most experiments were carried out with OT-covered surfaces. After longer immersion of the sample in the electrolyte solution (∼2-4 h) at potentials in the range -0.16 to -0.26 V significantly rougher morphologies were observed, although no electrochemical Cu dissolution in this potential regime is expected from STM experiments on bare Cu substrates and from the cyclic voltammograms of thiol(32) Kim, Y.-T.; Bard, A. J. Langmuir 1992, 8, 1096. (33) Scho¨nenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611.
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Figure 3. In-situ STM image of an OT-covered Cu(100) surface in 1 mM HCl at -0.26 V about 180 min after immersion (It ) 3.5 nA, 2400 × 2400 Å2), showing the increase in surface roughness after longer exposure to the electrolyte solution.
Figure 2. In-situ STM images of (a) a bare Cu(100) surface in 1 mM HCl at a potential of -0.20 V (It ) 10 nA, 2400 × 2400 Å2), (b) an OT-covered Cu(100) surface at a potential of -0.16 V (It ) 3.5 nA, 2400 × 2400 Å2), and (c) a HDT-covered surface at a potential of -0.25 V (It ) 1.6 nA, 2400 × 2400 Å2). All images were recorded after about 90 min of exposure to the electrolyte solution. The given potential was applied after mounting the cell and then held constant.
covered surfaces. An example of these rougher surfaces obtained with OT-covered Cu after about 3 h immersion is shown in the STM image in Figure 3. Under these conditions the terraces are covered by a large number of anisotropic islands, which are typically between 30 and 200 Å long, 20 to 50 Å wide, and 1.8 ( 0.3 Å high. Since the latter value agrees well with the monoatomic step height of the Cu substrate, these islands are attributed to Cu monolayer islands, i.e., to changes in the substrate topography. Comparable Cu islands were not observed in the STM measurements of thiol-covered surfaces in air even after several hours, indicating that the process causing the formation of these islands occurs only in solution. However, in some surface areas, in particular at pits and at Cu steps, irregular-shaped islands with a height of about 5-20 Å and a diameter of about 20-60 Å were observed in the STM experiments in air. These islands, which are not present on bare and thiol-covered surfaces in electrolyte solution, are most likely caused by oxidation of the copper substrate and/or the monolayer by air.1,8 In contrast to the observed pits and rugged step edges, which can be associated with etching processes, the origin of the Cu monolayer islands remains uncertain. Possible explanations are that (i) dissolved Cu atoms or Cu thiolates are redeposited on the surface (see below) or that (ii) the islands are remainders of a previous terrace, after partial dissolution starting from step edges, pits, or defects in the thiol layer. A tip-induced process as the only explanation for changes in the surface seems unlikely, since the same rough morphology was also observed in experiments directly after changing the scan range to different locations on the surface. Nevertheless, in some cases it was found that repeated scanning in the same area accelerated roughening and finally led to highly instable STM images. Since this phenomenon seemed to depend strongly on the tunneling conditions and the state of the tip, it is probably caused by (partial) penetration of the tip into the thiol layer (see above). The Cu dissolution and redeposition processes mentioned above (see (i) and (ii)) may occur by interaction of the thiol molecules with the Cu surface atoms or by interaction between Cu and the HCl electrolyte. It has
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Figure 4. Series of in-situ STM images showing the corrosion of an OT-covered Cu(100) surface in 1 mM HCl with increasing potential. The images were recorded at (a) -0.13 V, (b) -0.11 V, (c) -0.10 V, (d) -0.09 V, (e) -0.08 V, and (f) -0.07 V, (a) 14 min, (b) 20 min, (c) 24 min, (d) 28 min, (e) 32 min, and (f) 36 min after the potential was slowly increased from -0.16 V (It ) 3.5 nA, 2400 × 2400 Å2).
been suggested previously that gold and silver substrates are etched by thiols during adsorption in ethanolic solution via formation of metal-thiolate complexes.4,15,33-36 Due to the high chemical reactivity of Cu against sulfurcontaining compounds, comparable etching can be expected for Cu surfaces in the ethanolic solution as well as in the HCl electrolyte. In the latter case the lower solubility of Cu thiolates should result in a lower dissolution rate and an increased probability for redeposition in the electrolyte solution. On the other hand the HCl electrolyte may interact directly with the Cu surface. On bare Cu(100) in HCl solution local dissolution/redeposition processes are observed in this potential range,24,25 resulting in pronounced temporal fluctuations of the surface topography. These are apparently strongly suppressed by the thiol layer. However, a small amount of Cu dissolution/redeposition may still occur, either at already existing defects in the thiol layer or directly through the SAM, if the thiol layer is slightly permeable for water and ions and thus allows the transport of Cu species through the SAM. The permeation of water and ions into the monolayer might also attenuate van der Waals interactions between the alkyl chains, and this may additionally promote the desorption of Cu thiolate complexes. In a recent study it was found by comparing ex- and in-situ SERS results that the Cu-S bond in alkanethiol SAMs is weakened in aqueous electrolytes,9 suggesting that the stability and structural integrity of the SAMs decrease in solution. Other studies came to the conclusion that (34) Edinger, K.; Go¨lzha¨user, A.; Demota, K.; Wo¨ll, C.; Grunze, M. Langmuir 1993, 9, 4. (35) McCarley, R. L.; Dunaway, D. J.; Willicut, R. J. Langmuir 1993, 9, 2775. (36) Sondag-Huethorst, J. A. M.; Scho¨nenberger, C.; Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6826.
with decreasing alkyl chain length of the alkanethiols the degree of structural order and the stability of the SAM decrease, the number of defects increases, and the reactivity toward copper increases.1,11,31 Hence one can expect a more pronounced roughening for alkanethiols with shorter chains. In contrast to these mechanisms, where roughening proceeds via exchange with the HCl solution, the roughness might also increase due to mass transport on the Cu surface. However, the high mobility of the bare Cu(100) surface24,25 should be drastically lowered by interaction with the strongly bound SAM. In addition, a similar roughening was not observed in the STM measurements of thiol-covered surfaces in air, where no exchange via the electrolyte can occur. Therefore a significant contribution of surface diffusion processes to the roughening seems unlikely. Hence, dissolution and redeposition of Cu and/or Cu thiolates after the transfer of the thiol-covered sample to the electrochemical environment is proposed to account for the slow roughening at potentials
