Modification of a Au (111) Electrode with Ethanethiol. 1. Adlayer

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Langmuir 1999, 15, 2435-2443

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Modification of a Au(111) Electrode with Ethanethiol. 1. Adlayer Structure and Electrochemistry H. Hagenstro¨m, M. A. Schneeweiss, and D. M. Kolb* Department of Electrochemistry, University of Ulm, D-89069 Ulm, Germany Received October 29, 1998. In Final Form: December 21, 1998

We have used in-situ scanning tunneling microscopy and cyclic voltammetry to study self-assembled monolayers of ethanethiol on Au(111) electrodes. The adlayer was found to consist of domains of two different ordered structures, one corresponding to a (p × x3), well-established for other short-chain alkanethiols, and the other to an oblique primitive (4 × 3) superstructure, not previously reported for nonfunctionalized alkanethiols. At potentials slightly negative of 0 V vs SCE the adlayer undergoes a structural transformation that eventually leads to the formation of small pits and islands on the surface. Electrochemical studies in 0.1 M H2SO4 have revealed that around -0.31 V vs SCE the ethanethiol adlayer is reductively desorbed. Oxidative desorption of ethanethiol takes place at 1.15 V. The cathodic as well as the anodic desorption of the monolayer was monitored by scanning tunneling microscopy.

1. Introduction It is well-established that a number of compound classes spontaneously form ordered monolayers upon adsorption on certain substrates. A large amount of work has been published on these so-called self-assembled monolayers (SAMs) during the last fifteen years.1 One of the most important systems of this kind are thiol adlayers on a gold substrate (usually Au(111)), the former consisting mostly of simple alkanethiols2-8 [CH3(CH2)n-1SH, abbreviated Cn]. A recent review summarizes the contribution of scanning tunneling microscopy (STM) studies, which have created a wealth of structure information on this subject.9 Although initially most of the work was performed in a vacuum or in air, electrochemists became increasingly interested in SAM-covered electrodes for a variety of reasons: electrochemical properties of surfaces can be vastly altered by SAMs,10,11 and for electron-transfer reactions the alkane chains constitute an interesting barrier which can be varied in a systematic fashion.12-15 The electrochemical properties of the SAM itself have also been subject of several studies.16,17 Defects in the adlayer * Corresponding author. Fax: [email protected].

+49-731-502 5409. E-mail:

(1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (3) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (4) Porter, M. D.; Bright, T. B.; Allara, D. L.;. Chidsey, C. E. D J. Am. Chem. Soc. 1987, 109, 3559. (5) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.;. Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (6) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (7) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (8) Scho¨nenberger, C.; Jorritsma, J.; Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. J. Phys. Chem. 1995, 99, 3259. (9) Poirier, G. E. Chem. Rev. 1997, 97, 1117. (10) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (11) Finklea, H. O.; Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987, 3, 409. (12) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (13) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (14) Cooper, J. M.; Greenough, K. R.; McNeil, C. J. J. Electroanal. Chem. 1993, 347, 267. (15) Song, S.; Clark, R. A.; Bowden, E. F.; Tarlov, M. J. J. Phys. Chem. 1993, 97, 6564.

can function as microelectrodes,18 and in an electroanalytical context, functionalized thiol adlayers are used as ion-selective electrodes.19,20 Furthermore, well-known electrochemical reactions such as the corrosion or oxidation21,22 of a metal, or the electrodeposition23 of a metal, are expected to be significantly affected by the presence of organic monolayers. This issue will be addressed in part 2 of this work.24 Besides alkanethiols, aromatic,25,26 heterocyclic,27 and functionalized28-30 thiols have also been frequently investigated. However, self-assembled monolayers consisting of very short alkanethiols (n < 4) remain comparatively less well-documented. To the best of our knowledge, an ethanethiol monolayer has not yet been imaged under electrochemical conditions. An STM study of ethanethiol adlayers on Au(111) under ambient conditions by Porter and co-workers revealed a hexagonal arrangement of the admolecules corresponding to a (x3 × x3)R30° structure.31 Dubois et al. used low-energy electron diffraction (LEED) and reflection-absorption infrared spectroscopy (IRRAS) to determine the structure of short alkanethiol adlayers on Au(111) in UHV.32 Again, they found a (x3 × x3)R30° superstructure in the LEED pattern. However, in addition to the intense x3 superstructure spots, extra spots and streaking along high-symmetry azimuths were also (16) Bryant, M. A.; Crooks, R. M. Langmuir 1993, 9, 385. (17) Hayes, W. A.; Shannon, C. Langmuir 1996, 12, 3688. (18) Sabatani, E.; Rubinstein, I. J. Phys. Chem. 1987, 91, 6663. (19) Steinberg, S.; Rubinstein, I. Langmuir 1993, 8, 1183. (20) Turyan, I.; Mandler, D. Anal. Chem. 1997, 69, 894. (21) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (22) Zamborini, F. P.; Crooks, R. M. Langmuir 1998, 14, 3279. (23) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1995, 11, 4823. (24) Hagenstro¨m, H.; Schneeweiss, M. A.; Kolb, D. M., in preparation. (25) Carron, K. T.; Hurley, L. G. J. Phys. Chem. 1991, 95, 9979. (26) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (27) Boland, T.; Ratner, B. D. Langmuir 1994, 10, 3845. (28) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1995, 11, 2237. (29) Poirier, G. E.; Pylant, E. D.; White, J. M. J. Chem. Phys. 1996, 105, 2089. (30) Dakkouri, A. S.; Kolb, D. M.; Edelstein-Shima, R.; Mandler, D. Langmuir 1996, 12, 2849. (31) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805. (32) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678.

10.1021/la9815291 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/09/1999

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observed for C1 and C2 adlayers, thus indicating the existence of more complex structures. In the following we present results of a systematic study of C2-modified Au(111) in sulfuric acid solution, obtained by cyclic voltammetry (CV) and in-situ STM measurements. The electrochemical behavior of the C2-adlayer is shown and the structure and potential-induced surface processes are imaged by in-situ STM. 2. Experimental Section The STM measurements were performed with a Topometrix TMX 2010 Discoverer, using tungsten tips electrochemically etched from a 0.25 mm diameter wire in 2 M NaOH. To minimize faradaic currents at the tip-electrolyte interface, the tips were coated with polyethylene or electrodeposition paint.33 Tip and sample potential were controlled independently of each other by means of a bipotentiostat. The gold samples for the STM studies were 500 nm thick films evaporated onto a special glass (AF 45, Berliner Glas KG) which had a 2 nm Cr undercoating for better adhesion. The samples were annealed in a hydrogen flame for 2 min at yellow heat to yield large, atomically flat (111) terraces.34 They were cooled to room temperature in a stream of nitrogen. Ethanethiol SAMs were prepared by immersing the annealed gold sample into a 1 mM solution of ethanethiol (Fluka; purum, >97%) in absolute ethanol (Merck, extra pure) for 16-20 h (overnight). All chemicals were used as received without further purification. It has been reported that annealing at moderate temperatures (typically 80 °C) of long-chain alkanethiol SAMs leads to increased ordering.8,35,36 Such a procedure, however, proved to be damaging to the short-chain thiol investigated in this study, as could be inferred from cyclic voltammograms of samples thus treated. Temperatures higher than 120 °C have been reported to lead to decomposition of the molecules.37 For this reason none of the C2-SAMs used in the following study were subjected to the annealing treatment. They were taken from the modification solution, washed with ethanol, dried under a stream of nitrogen, and then introduced into the electrochemical STM cell which is open to air. The reconstruction of the flameannealed Au(111) surface is lifted upon modification with alkanethiols;38 therefore, the Au/C2 interface is not reconstructed in our experiments. The electrolyte solutions were prepared from H2SO4 (Merck, suprapure) and Milli-Q water (Millipore Corp., USA). In the STM cell two platinum wires served as a quasi reference electrode (Pt vs SCE ) +0.6 V in our system) and counter electrode. However, all potentials are quoted with respect to the saturated calomel electrode (SCE). All STM images were recorded in the constant current mode and are displayed as top views with different shades of gray representing different heights (dark areas indicating low parts and light areas high parts of the surface). The cyclic voltammograms were obtained with standard electrochemical equipment and a conventional electrochemical cell with separate compartments for the reference and counter electrodes. Here, the Au(111) electrode was a 4 mm thick singlecrystal disk (MaTeck, Ju¨lich) with a diameter of about 4 mm and a gold wire attached to the back for better handling. Prior to each modification the gold crystal was annealed for 2 min in a Bunsen burner flame, cooled in air, and brought into contact with the modification solution at room temperature. After 16-20 h of assembling time the electrode was dipped in Milli-Q water in order to clean the electrode surface of residues of the modification solution. Then, the crystal was transferred to the electrochemical cell and brought into contact with the electrolyte under potential (33) Bach, C. E.; Nichols, R. J.; Beckmann, W.; Meyer, H.; Schulte, A.; Besenhard J. O.; Jannakoudakis, P. D. J. Electrochem. Soc. 1993, 140, 1281. (34) Will, T.; Dietterle, M.; Kolb, D. M. In Nanoscale Probes of the Solid/Liquid Interface; Gewirth, A. A., Siegenthaler, H., Eds.; NATO ASI Series E; Kluwer: Dordrecht, 1995; Vol. 288, p 137. (35) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447. (36) Cavalleri, O.; Hirstein, A.; Bucher, J.-P.; Kern, K. Thin Solid Films 1996, 284-285, 392. (37) Jaffey, D. M.; Madix, R. J. Surf. Sci. 1994, 311, 159. (38) Poirier, G. E. Langmuir 1997, 13, 2019.

Figure 1. Cyclic voltammograms of an ethanethiol-modified Au(111) electrode in 0.1 M H2SO4 (sweep rate: 10 mV s-1). Top row: scans in the double-layer region; Bottom row: cathodic desorption scans to the onset of hydrogen evolution. control. The electrolyte was thoroughly deaerated by bubbling with nitrogen. A saturated calomel electrode (SCE) served as the reference electrode. A short remark about the quality (i.e., medium terrace width) of the employed single crystal seems appropriate. This crystal was used for a large number (∼100) of electrochemical experiments, modified with different alkanethiol adlayers. In the course of these experiments the medium terrace width of the singlecrystal surface decreased as could be concluded from cyclic voltammograms for the bare Au(111) electrode in 0.1 M H2SO4 by the decreasing height of the current spike at 0.78 V.39 Obviously, modification with alkanethiols promotes the deterioration of the single-crystal surface, since unmodified Au(111) shows no such dramatic changes with potential cycling in 0.1 M sulfuric acid. Nevertheless, all fingerprint features of a wellprepared Au(111) surface are still found for our gold sample (compare the CV of the bare Au(111) electrode in Figure 1).

3. Results and Discussion 3.1. Cyclic Voltammetry. The electrochemical desorption of n-alkanethiols (n ) 3-10, 12, 16) from polycrystalline Au in an alkaline electrolyte has been studied by Porter and co-workers.10 They found the differential capacitance (Cd) and the reductive desorption potential (Erd) to be linearly dependent on chain length. For C3 they obtained values of about Cd ) 3 µF cm-2 and a reduction charge σrd ) 90 ( 7 µC cm-2 at Erd ) -0.7 V vs SCE. Assuming a (x3 × x3)R30° packing (θ ) 1/3) within the SAM, the reductive desorption was described as a one-electron-reaction:

AuSR + 1 e- f Au (0) + RS-

(1)

For the oxidative desorption (starting positive of 1 V vs SCE in their study) a three-electron oxidation of the adsorbed sulfur was proposed:

AuSR + 2H2O f Au (0) + RSO-2 + 3e- + 4H+

(2)

In the following we will discuss our results with regard to these findings. In Figure 1 a sequence of voltammograms for a freshly modified Au(111) electrode in 0.1 M H2SO4 is shown, alternating between scans in the double-layer region and the cathodic range. All CVs were started in the cathodic direction at 0.4 V. The first cycle (a) between 0 and 0.85 V reveals a considerable reduction of the double(39) Ho¨lzle, M. H. Ph.D. Thesis, University of Ulm, Germany, 1995.

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layer charging current (∼80 nA cm-2) up to 0.5 V. From the sweep rate of 10 mV/s a double-layer capacity of ∼8 µF cm-2 is estimated, which is about one-third of the value of the bare electrode (∼25 µF cm-2). Beyond 0.5 V the current rises and there is a corresponding cathodic wave upon reversal of the sweep direction. Subsequent cycles between 0 and 0.85 V (not shown here) look almost identical. The observed charge-transfer positive of 0.5 V may be due to sulfate adsorption on Au, strongly hindered by the almost defect-free C2-adlayer. The second cycle (b) extends cathodically to the onset of hydrogen evolution at -0.45 V. There is a cathodic peak (AC) at -0.31 V, covering about 80 µC cm-2, which corresponds roughly to the reductive desorption of onethird of a monolayer (ML) for a one-electron reaction (assuming 222 µC cm-2 to correspond to one ML on Au(111)). It is difficult to exactly determine the charge associated with this process because hydrogen evolution commences at -0.45 V. At -0.3 V a broad anodic peak AA is found, covering approximately 30 µC cm-2, which is considerably less than that for AC. In the subsequent anodic sweep (c) we find a new pair of peaks (CA and CC) at 0.45 V. Since the C2-adlayer permits charge transfer more easily after cathodic polarization we think that the cathodic peak AC is caused by desorption of C2. The respective charge of the readsorption peak AA is equivalent to less than one-half a complete C2 layer. Consequently, the desorption process is partly irreversible and we attribute the current maxima at 0.45 V to sulfate adsorption on Au at defects of the SAM generated in the preceding cathodic polarization. This explanation is supported by the following potential cycles. In the fourth cycle (d) the (cathodic) desorption peak is reduced, the onset of hydrogen evolution shifts anodically, and readsorption is diminished as well. In Figure 1, parts e-g, we show the two subsequent cathodic (6th and 8th) cycles and the final double-layer cycle (9th cycle). At 0.8 V two small peaks (denoted DA and DC) evolve, indicating the formation of ordered sulfate structures on bare Au(111). As these peaks are shifted anodically and are smaller compared to the corresponding peaks for a bare Au(111) surface (Figure 1j), we assume that only small islands of ordered sulfate are formed. In a next step the electrode potential was cycled to 1.4 V and back (oxidation-reduction cycle, ORC; not shown) and the subsequent double layer and cathodic cycles were recorded (Figure 1, parts h and i). At 0.29 V a small peak (BA) indicates lifting of the Au reconstruction (1h and 1i) and no cathodic peak due to C2 desorption (i) is recorded any more. These findings are in qualitative agreement with the work of Porter et al., although we found a substantially more positive potential for reductive desorption of C2. One should bear in mind, however, that our experimental setup differs from Porter’s in two points: We used a single-crystal electrode and, more importantly, worked with an acidic electrolyte. Nevertheless, the shapes of the CVs are very similar; thus, the same reductive desorption mechanism seems to be operating. The complete oxidative removal of the C2 adlayer is demonstrated in Figure 2. In the first cycle the anodic charge is larger by about 500 µC cm-2 than that in the second cycle. The reduction peaks are equal in both cases and the second ORC is very similar to the ORC of bare Au(111),40 so that an almost complete desorption of C2 during the first ORC can be assumed. Since we measured (40) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. J. Electroanal. Chem. 1987, 228, 429.

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Figure 2. Cyclic voltammograms of the first (solid line) and second (dashed line) oxidation-reduction cycle (ORC) of an ethanethiol-modified Au(111) electrode in 0.1 M H2SO4 (sweep rate: 10 mV s-1).

markedly higher anodic charges during the oxidative desorption of ethanethiol than Porter and co-workers for propanethiol (∼280 µC cm-2),10 a different reaction mechanism seems to take place. At present we cannot provide a conclusive explanation for this difference, but we may speculate that ethanethiol is further oxidized to ethanesulfonic acid. As a first conclusion, we learned from cyclic voltammetry that (a) the C2-covered electrode has a much-reduced double-layer capacity, as would be expected for an aliphatic chain molecule, (b) C2 desorbs at cathodic potentials below 0 V (with a current maximum at -0.31 V), and (c) C2 can be oxidized and effectively removed from the electrode in the course of an ORC. In the following, we want to show that these surface changes can be imaged by in-situ STM. 3.2. In-situ STM. First, we have investigated the C2adlayer structure at potentials where it is stable according to the cyclic voltammograms (Figures 1 and 2). Figure 3a shows an 100 × 100 nm2 large image of a Au(111) terrace covered with a monolayer of ethanethiol. The image was recorded in situ in 0.1 M H2SO4 at a potential of 0.2 V vs SCE. Two distinct features are clearly recognizable: First, ethanethiol obviously forms domains of ordered structures, and second, the terrace is covered with pits, the depth of which corresponds to one monolayer of gold. Pits such as these are a well-known feature of any gold surface modified with alkanethiol and they have been shown to correspond to substrate vacancy islands.41 These monatomic deep depressions in the gold surface are also covered with the ordered thiol adlayer and thus do not represent defects in the SAM. We mention in passing that different areas of the same sample vary substantially in terms of pit density. Furthermore, no pits were observed on narrow terraces (e.g., less than about 30 nm). On wide terraces the pit density decreased in the vicinity of step edges. Vacancy island migration in alkanethiol monolayers has been studied by Poirier45 in UHV and by Kern and co-workers36 under electrochemical conditions. They found coalescence and annihilation of vacancy islands at step edges, which explains our findings, although no migration was detectable on the time scale (∼2 min/image) of our measure(41) Edinger, K.; Go¨lzha¨user, A.; Demota, K.; Wo¨ll, Ch.; Grunze, M. Langmuir 1993, 9, 4. (42) Scho¨nenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611. (43) Sondag-Huethorst, J. A. M.; Scho¨nenberger, C.; Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6826. (44) McDermott, C. A.; McDermott, M. T.; Green, J.-B.; Porter, M. D. J. Phys. Chem. 1995, 99, 13257. (45) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966.

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Figure 3. STM images of ethanethiol-modified Au(111) in 0.1 M H2SO4. (a) 100 × 100 nm2 region showing the two different superstructures and rotational domains. (b) 36 × 36 nm2, a pinstripe, and two (4 × 3) domains. (c) 16 × 16 nm2, high-resolution image of the pinstripes with p ) 7.5. (d) 17 × 17 nm2, high-resolution image of two (4 × 3) mirror domains.

ments. An influence of the modification time, of the duration of the experiment, or of other such parameters was not systematically examined in the course of our investigation. The domain boundaries seem for a large part to coincide with the etch pits (cf. Figure 3a). At this point we have no indication whether the pits (i.e., substrate defects) influence the evolution of the domains or vice versa. There are ordered domains of two different ethanethiol structures and both exhibit rotational domains and several substructures. Figure 3b (36 × 36 nm2) focuses on three such domains. We can distinguish a striped (S) and an oblique primitive (O) superstructure. No parameter was found which is responsible for the relative distribution of the two phases. For most samples only a small fraction (1020%) of the surface was covered by structure O. Occasionally, no structure O at all was observed and in some rare cases half the surface was covered by it. However, the S-phase was always present. The domain boundaries appear slightly disordered besides exhibiting a certain frizzyness (cf. Figure 3b), which would indicate a dynamic behavior of the film in these regions, but a clear phase transition from one structure to the other was never observed during the STM measurements. A reduced driving force for ordering as a consequence of the short carbon chain may imply very long and strongly varying times for this transition to occur in solution. A close inspection of Figure 3a reveals that three rotational domains are formed by both phases. Each of

these domains is aligned along distinct directions. The preferential direction of structure S is always rotated by 30° or 90° with respect to the preferential direction of structure O. Figure 3, parts c and d, presents images which show the molecular arrangements in more detail. We will first describe the S-phase (Figure 3b,c). This structure is in agreement with previously described (p × x3) structures (often referred to as pinstripe structures) of other short-chain alkanethiols.46,47 The lattice constant along the lines is 5.1 ( 0.2 Å (which is in accordance with x3 times the interatomic distance a ) 2.885 Å of Au(111)). Perpendicular to the lines the smallest periodicity p was determined to be p ) 4.5a (12.7 ( 0.3 Å), which results from every third row missing in the next-nearest-neighbor direction. However, p ) 7.5a is encountered most frequently. The corresponding unit cell is formed by every fifth row missing. A model is displayed in Figure 4a, where we assume for simplicity that the sulfur is bound in (x3 × x3)R30° 3-fold hollow sites. A recent work by Fenter et al. using X-ray standing waves (XSW) gave evidence that the buried S/Au interface is actually more complex.48 These topographic differences in the sulfur-bonding site could explain why only one of the two adjacent molecular (46) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383. (47) Camillone, N., III; Eisenberger, P.; Leung, T. Y. B.; Schwartz, P.; Scoles, G. J. Chem. Phys. 1994, 101, 11031. (48) Fenter, P.; Schreiber, F.; Berman, L.; Scoles, G.; Eisenberger, P.; Bedzyk, M. J. Surf. Sci. 1998, 412/413, 213.

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Figure 4. Models of the two superstructures of ethanethiol SAMs on Au(111). Low-index directions are indicated by arrows. Left side: most common pinstripe structures (p × x3) with p ) 4.5 (every third row missing) or p ) 7.5 (every fifth row missing). C2 molecules are depicted by shaded balls and are arranged in (x3 × x3)R30° adsorption sites. Right side: Three rotational domains of the (4 × 3) superstructure with mirror domains. Only the corner molecules of the oblique unit cell are depicted by shaded balls.

rows for p ) 4.5 and three of the four rows (between the missing rows) of a pinstripe with p ) 7.5a are clearly resolved in Figure 3c. These rows could be topographically hidden, and hence, difficult to detect. Unit cells with p > 7.5 are found less often. Figure 3c clearly shows that the surface is devoid of molecules between the pinstripes (i.e., rows of molecules are truly missing). The periodicity normal to the rows (p ) 4.5, 7.5, or others) varies at random between adjacent pinstripes (cf. Figure 3, parts b and c). Also, sudden transitions of the pinstripe periodicity p (perpendicular to the stripes) can be found along the x3 direction (see arrow in Figure 3b). The S-domains vary in size between ∼100 and 10 000 nm2, depending on the pit density and the number of domains of the other phase. Structural defects influence the domain size, as domain boundaries are always found to run between defects. The average size of an S-domain is around 1000 nm2. We now focus onto the O-phase, which at a first glance resembles a distorted hexagonal structure (see Figure 3d). However, molecular resolution of structure O never produced a hexagonal but rather an oblique (4 × 3) unit cell. Figure 3d presents a molecular resolution image of such a (4 × 3) domain. In the left part one domain of the oblique superstructure is found. Close inspection of the superstructure reveals that every corner point of the primitive oblique unit mesh contains several molecular sites. In the middle portion of the image, indicated by two arrows, there is another domain of the same superstructure, but the orientation is obviously changed. In the bottom right corner of the picture the structure is again the same as in the left part. The length of the three lattice vectors (A, B, and C, in Figure 4) of the oblique cell is the same (within an accuracy of 2%) in both domains: A ) 8.6 Å (8.66 Å ) 3a), B ) 10.3 Å (10.40 Å ) x13a), C ) 11.4 Å (11.54 Å ) 4a). Only the lattice vector direction is different. Whereas the short vector A stays the same for both domains, the other two are mirrored with A being the mirror axis. Since the nearest-neighbor direction is the same for both domains and the length of its lattice vector is 3a, this mirror phase cannot be created by a

rectangular unit cell such as the (3 × 2x3) missing row structure that was reported by Dishner et al.49 for methanethiol on Au(111) under ambient conditions. We therefore propose a (4 × 3) structure for ethanethiol as shown in Figure 4. The same superstructure was found by Poirier and coworkers for an hydroxyl-terminated hexanethiol SAM on Au(111).29 They argued that the deviation from the (x3 × x3)R30° packing was driven by end-group interactions. However, we did find the same structure for a very short methyl-terminated thiol. Whether the STM truly images the individual sulfur sites remains to be seen, but the overall arrangement into an oblique unit cell must be mediated by sulfur-sulfur interactions, since the Au lattice imposes a hexagonal bonding symmetry. Interferences of the tip with the carbon chain would not be expected to disturb the imaging. The unit cell of structure O covers 86.2 Å2 which is the same area as that for the coverage-saturated (3 × 2x3) structure determined for longer chain alkanethiols.6,31 We were not successful in determining unambiguously the unit cell’s exact internal structure. Images with even higher resolution than that in Figure 3d, not presented here, revealed four topographical maxima per unit cell, but of different height and not on equivalent (3-fold hollow) adsorption sites. The unit cell was the same everywhere; only the imaging contrast varied with scan direction. Assigning one molecule to each of the four resolved spots of a (4 × 3) corner point would result in an area of 21.5 Å2/molecule. This implies that the (4 × 3) structure is closely packed. Figure 5 illustrates the behavior of the C2-adlayer at negative potentials. The images were acquired while stepwise lowering the potential from +0.2 to -0.12 V. At 0.2 V we see the ordered domains of the ethanethiol adlayer on a large Au(111) terrace with one single curved monatomic high step. The pit density here was very low, while it was clearly higher on other parts of the same sample. (49) Dishner, M. H.; Hemminger, J. C.; Feher, F. J. Langmuir 1997, 13, 2318.

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Figure 5. 194 × 194 nm2 STM images of the ethanethiol-SAM transformation below 0 V vs SCE in 0.1 M H2SO4. (a) E ) +0.2 V, inset in the derivative mode; (b) E ) -0.08 V; (c) and (d) E ) -0.12 V; (e) E ) +0.4 V; (f) E ) +0.2 V.

Some very small monatomic high islands are seen as well, which result from an earlier potential excursion into the cathodic range. Freshly prepared samples are free of such islands (cf. Figure 3). Part of Figure 5a is shown as a shaded image, to enhance the contrast between different domains. Lowering the potential to -0.08 V initiates the development of small islands along domain boundaries (1), at steps (2) and other adlayer defects (3) indicated by arrows in Figure 5b. Setting the potential to -0.12 V results in the formation of a striped network (Figure 5c) transforming itself after some minutes into a structureless film (Figure 5d). This intermediate phase T of Figure 5c could not be stabilized. The network stripes do not show

any internal structure, run along the main crystallographic axes of the Au(111) substrate, and are separated by 17.1 ( 0.2 Å. The network lies 1.5 ( 0.2 Å above the ordered C2-film. At the same time 3.1 ( 0.2 Å high islands grow on top of the transformed film but not on the original and ordered film areas, still visible as darker patches. In Figure 5d the transformation is almost complete; the etch pits survived and only few network stripes remain. Some of the new islands are triangular in shape, reflecting the substrate symmetry. The transformation process is reversible to a large extent. Negative of +0.2 V nothing happens, but at +0.4 V the new phase dissolves (see Figure 5e). After re-

Modification of a Au(111) Electrode

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Figure 6. STM images showing the cathodic desorption and partial readsorption of an ethanethiol SAM on Au(111) in 0.1 M H2SO4. (a) 300 × 300 nm2, E ) +0.2 V, showing three terraces predominantly covered by pinstripes. (b) 300 × 300 nm2, E ) -0.32 V, same surface area, but devoid of C2, with etched terrace edges. (c) 300 × 300 nm2, E ) +0.2 V, same area at the initial potential, no ordered C2. (d) 300 × 300 nm2, E ) +0.6 V, same area, readsorbed C2 forming small pinstripe patches. (e) 86 × 86 nm2, E ) +0.6 V, higher resolution of the pinstripe islands. (f) 18 × 18 nm2, E ) +1.0 V, area between the pinstripe patches showing the ordered (x3 × x7)R19° sulfate structure.

establishing the ordered C2 film at +0.4 V, the adlayer is shown at +0.2 V, however, with numerous defects (Figure 5f). A large number of small pits and 3.1 ( 0.2 Å high islands remain. Inspection of higher-resolved images revealed more domain boundaries and even disordered areas. The islands dissolve at higher potentials. The whole process is even more fascinating since it can be induced in a very similar fashion by variation of the tip potential. The latter was done just before recording this series of pictures and it created the small islands.

At this point we cannot provide a comprehensive explanation of the transformation. The process resembles ordinary adsorption and desorption, but the electrolyte (0.1 M H2SO4) did not contain any species which would adsorb during a cathodic potential sweep. There are some indications for a transformation of the film itself. First, as we concluded from the CV, desorption of the C2-SAM starts at more negative potentials. So, the onset of desorption may be connected with the surface transformation leading to a lower-density phase. In fact, after

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reversing the potential, the film is considerably less ordered, the molecular film is pitted, and a large number of islands remains. The latter are very probably out of material that was expelled from the molecular monolayer. At positive potentials this material is again incorporated in the film, healing the defects. Second, the network stripes of the transformed film show a 3-fold symmetry only. They run along the crystallographic main axes of the Au(111) surface and do not reflect the symmetry of the pinstripe domains (S). If the transformation was an adsorption process, both symmetries should be found. As a consequence the observation can only be understood in terms of a transformation process. Looking more closely at the images in Figure 5 we see that this transformation starts at structural defects, such as domain boundaries, islands, and steps and that the (lower density) pinstripe domains are more easily transformed. The onset potential of the transformation varied between samples from +0.1 to -0.1 V, probably depending on the varying densities of the SAMs. Stepping the sample potential more negative leads to the complete reductive desorption of the adlayer as described in the first section. In Figure 6a the initial state of the C2-SAM at +0.2 V is shown. The terraces are covered with pinstripe domains and some disordered areas are visible, too. At the desorption potential (-0.32 V) (Figure 6b) the terrace contour and the surface texture suddenly change. A high surface mobility is manifested in the almost instantaneous disappearance of the etch pits of the C2adlayer, creating atomically flat terraces with etched steps. Back at the initial potential of +0.2 V the terrace is further etched (Figure 6c). We raised the potential to 0.6 V until small patches of the pinstripes reappear (Figure 6d and e). Between these S-phase islands the bare Au(111) substrate at 0 V (not shown) and the ordered sulfate (x3 × x7)R19.1°-adlayer at 1 V (Figure 6f) were resolved. This proves the surface to be devoid of C2 next to the ordered patches. As we already concluded from cyclic voltammetry, the C2-SAM desorbs around -0.3 V and is only partly readsorbed after reversing the potential. The incomplete C2-SAM consists of small ordered (low-density) pinstripe patches, leaving space for the formation of sulfate islands at the respective potential. This agrees well with the voltammetric data. In the anodic direction, the cyclic voltammogram shows the oxidative desorption of the molecule starting at 0.95 V. This process can also be monitored by in-situ STM. (Figure 7). The film does not change up to 0.75 V; only the small islands (remainders of a negative potential sweep) slowly dissolve. We see etch pits on an atomically flat terrace and the usual two structures (Figure 7a). The area is mainly covered by pinstripes, enclosing a (4 × 3) domain. At a potential of 0.95 V, the film loses its order and disordered wormlike structures emerge (Figure 7b). At even more positive potentials image resolution is completely lost. Electrochemical measurements show that the onset of film oxidation and desorption takes place at 0.95 V. As soon as the surface exhibits uncovered areas, Au oxidizes as well. Images acquired after several oxidationreduction cycles show the surface to be devoid of any ethanethiol and to be extremely pitted because of the oxidation process.50 4. Summary We have studied the electrochemical behavior of a C2modified Au(111) electrode in 0.1 M H2SO4 by cyclic (50) Trevor, D. J.; Chidsey, C. E. D.; Loiacono, D. N. Phys. Rev. Lett. 1989, 62, 929.

Hagenstro¨ m et al.

Figure 7. 56 × 56 nm2 STM images of the oxidative desorption of the C2 adlayer. (a) E ) +0.75 V; (b) E ) +0.95 V, the C2 SAM forms wormlike structures prior to complete desorption.

voltammetry. We found current maxima related to the reductive and oxidative desorption of the SAM (at -0.31 and +1.15 V vs SCE, respectively). The reductive desorption was found to be partially reversible. The structural changes of the surface related to these desorption processes were successfully imaged by in-situ STM. A minor fraction of the C2-adlayer is readsorbed in small patches showing the pinstripe structure. Prior to the oxidative desorption the C2-SAM was found to form wormlike structures. We have shown by in-situ STM that ethanethiol forms two different adlayer structures on Au(111) at potentials between 0 and 0.75 V vs SCE. The first is a pinstripe (p × x3) structure, well-established for longer alkanethiols. Ethanethiol was found to exhibit varying values of p, the smallest being 4.5 equivalent to a missing row structure in the next-nearest-neighbor direction with every third row missing. p ) 7.5 was most common, but larger unit cells were also encountered. Moreover, coexisting with the pinstripes, we found an oblique primitive (4 × 3) superstructure not previously reported for methylterminated alkanethiols. Assigning a molecule to each of the four resolved spots of every corner of the unit cell leads to the conclusion that the molecules in this structure are densely packed. The individual molecules were imaged on nonequivalent adsorption sites.

Modification of a Au(111) Electrode

Additionally, preceding the reductive desorption, we found a transformation of the molecular film into a structureless but smooth phase with some small islands while scanning the potential to -0.12 V. The transformation proceeds via an intermediate state showing domains with a network of stripes (markedly different from the pinstripes) separated by 17 Å without any substructure. This process is reversed, if the potential is raised back to 0.4 V. The film is slightly damaged in the

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course of this potential excursion, showing numerous small pits and islands. The islands slowly dissolve at positive potentials. Acknowledgment. H. H. gratefully acknowledges a grant from the Deutsche Forschungsgemeinschaft through Graduiertenkolleg 328. LA9815291