Modification of a Au (111) Electrode with Ethanethiol. 2. Copper

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Modification of a Au(111) Electrode with Ethanethiol. 2. Copper Electrodeposition H. Hagenstro¨m, M. A. Schneeweiss, and D. M. Kolb* Department of Electrochemistry, University of Ulm, 89069 Ulm, Germany Received April 12, 1999. In Final Form: June 14, 1999 Copper electrodeposition from sulfuric acid solutions onto ethanethiol-modified Au(111) electrodes was studied by in-situ scanning tunneling microscopy and cyclic voltammetry. The ethanethiol adlayer undergoes an order-disorder transition before Cu underpotential deposition starts around +0.20 V versus SCE. Five percent of a monolayer is deposited positive of the Nernst potential at a sweep rate of 10 mV s-1. At low overpotentials the Cu deposit exhibits a ramified monatomic high morphology, if the ethanethiol adlayer is dense. In all cases three-dimensional growth nucleating at large substrate defects is found in addition. Cyclic voltammetry revealed two characteristic deposition features: firstly, a sharp cathodic peak at -0.18 V which is ascribed to the insertion of a Cu monolayer between Au and the organic adlayer and, secondly, a current loop due to Cu bulk deposition. The corresponding stripping peaks are found at 0.08 and 0.35 V, respectively.

1. Introduction Modification and functionalization of metal electrodes with self-assembled thiol adlayers1,2 opens manifold possibilities for electrochemical investigations. This very active field currently concentrates on the study of the electrochemical properties of self-assembled monolayers (SAMs)3,4 as well as electron-transfer processes through the alkane chain.5-8 In an electroanalytical context, functionalized thiol adlayers are used as ion selective electrodes9,10 and defects in the adlayer can function as microelectrodes.11 Furthermore, well-known electrochemical reactions such as the corrosion/oxidation or electrodeposition of metals are expected to be inhibited or blocked in the presence of such organic monolayers. A dense and structurally well-defined SAM can function as a model system for metal deposition onto nonconductors and in a wider sense is hoped to enhance our understanding of organic/inorganic interfaces. Detailed UHV studies concerning metal deposition onto surfaces modified with self-assembled monolayers of alkanethiols [CH3(CH2)n-1SH, abbreviated Cn] have been published.12-14 The central question for the system substrate/SAM/deposit still deals with the exact location of the deposited metal, which is closely related to the interaction between SAM and deposit. * Corresponding author. Fax: +49-731-502 5409. E-mail: [email protected]. (1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (2) Ulman, A. Chem. Rev. 1996, 96, 1533. (3) Hayes, W. A.; Shannon, C. Langmuir 1996, 12, 3688. (4) Bryant, M. A.; Crooks, R. M. Langmuir 1993, 9, 385. (5) Cooper, J. M.; Greenough, K. R.; McNeil, C. J. J. Electroanal. Chem. 1993, 347, 267. (6) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (7) Chidsey, C. E. D.; Bertozzi, C.-R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (8) Song, S.; Clark, R. A.; Bowden, E. F.; Tarlov, M. J. J. Phys. Chem. 1993, 97, 6564. (9) Steinberg, S.; Rubinstein, I. Langmuir 1993, 8, 1183. (10) Turyan, I.; Mandler, D. Anal. Chem. 1997, 69, 894. (11) Sabatani, E.; Rubinstein, I. J. Phys. Chem. 1987, 91, 6663. (12) Jung, D. R.; Czanderna, A. W. Crit. Rev. Solid State Mater. Sci. 1994, 19, 1. (13) Herdt, G. C.; Jung, D. R.; Czanderna, A. W. Prog. Surf. Sci. 1995, 50, 103. (14) Herdt, G. C.; King, D. E.; Czanderna, A. W. Z. Phys. Chem. 1997, 202, 163.

Jung and Czanderna have proposed three basic possibilities: (1) metal bonded to or complexed with the SAM, thus spreading out on top of the SAM; (2) metal bonded weakly to the organic layer and hence clustering on top of the SAM, and (3) metal penetrating the SAM and connecting directly to the metal substrate underneath the SAM.12,13 There have been several reports on the “metallization” of organic layers (case 1) by creating functionalized surfaces that allow selective extraction of metal ions from solution. Grummt et al. succeeded in producing silver clusters via electroless deposition on a mixed SAM with functionalized anchoring molecules embedded in a C12 matrix.15 Kern and co-workers were able to create Pd and Co islands of controllable size on an aminothiolate SAM with the same approach.16 Reifenberger and co-workers were successful in binding gold clusters deposited from a cluster beam onto a dithiol-covered substrate.17,18 An example of the third case is a report by Tarlov on the evaporation of silver onto octadecanethiol SAMs. They come to the conclusion that Ag nucleates as clusters underneath the organic monolayer.19 A number of studies have recently been carried out with organic monolayers that were self-assembled on top of UPD layers on Au(111), thus representing a configuration of a metal monolayer between Au and S, which is one of the possibilities mentioned above. Jennings and Laibinis found that these SAMs present a far better protection against corrosion of the substrate than alkanethiol SAMs directly assembled on Au(111).10,21 Yoneyama and coworkers found the same for Cu and Ag, but they inserted the metal monolayer after the modification of the bare substrate by means of an underpotential deposition (15) Grumnmt, U.-W.; Geissler, M.; Schmitz-Huebsch, Th. Chem. Phys. Lett. 1996, 263, 581. (16) Kind, H.; Bittner, A. M.; Cavalleri, O.; Kern, K. J. Phys. Chem. 1998, 102, 7582. (17) Dorogi, M.; Gomez, J.; Osifchin, R.; Andres, R. P.; Reifenberger, R. Phys. Rev. B 1995, 52, 9071. (18) Andres, R. P.; Bein, Th.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R.G.; Reifenberger, R. Science 1996, 272, 1323. (19) Tarlov, M. J. Langmuir 1992, 8, 80. (20) Jennings, G. K.; Laibinis, P. E. Langmuir 1996, 12, 6173. (21) Jennings, G. K.; Munro, J. C.; Yong, T.-H.; Laibinis, P. E. Langmuir 1998, 14, 6130.

10.1021/la9904307 CCC: $18.00 © 1999 American Chemical Society Published on Web 08/19/1999

Copper Electrodeposition on Au(111) Electrodes

through the SAM.22,23 Crooks and co-workers studied the corrosion of a Cu UPD layer on Au(111) covered by alkanethiols of different chain length and found that the protection depends on chain length but also on the functional group and whether the chain contained aromatic units.24 Several studies about Cu electrodeposition onto alkanethiol-modified gold electrodes with varying chain length have been published. The very fact that the initial stages of Cu deposition on the bare Au surfaces have been characterized in detail makes it a popular model system.25 Sun et al. used STM images of Cu UPD islands to detect individual defect structures contained within the organic monolayer.26 Sondag-Huethorst et al. reported that the overpotential needed to deposit copper on thiol-covered gold increased with increasing chain length, and they found deposition of hemispherical nuclei in contrast to homogeneous flat copper films on bare gold.27 Whelan et al., using heterocyclic thiols, conducted a thiol coveragedependent study of Cu deposition and also found for complete coverage a suppression of the Cu UPD and a significant hindering of bulk deposition.28 A short thiol (butanethiol) was found to not completely block the UPD process.22 Kern and co-workers monitored the two- and three-dimensional growth of copper clusters and nodules on alkanethiols of various chain lengths by in situ STM.29-32 Alkire and co-workers studied the copper deposition on gold surfaces covered with alkanethiols of different chain length with AFM.33 In the first part of this paper we have described in detail the structure and electrochemical response of the C2 adlayer in sulfuric acid solution.34 The influence of the ethanethiol SAM on Cu electrodeposition is the topic of this part. We studied the electrochemistry by cyclic voltammetry and characterized the interface structure with in-situ STM. 2. Experimental Section The experimental setup has been described in Part 1 of our work.34 The copper-containing solutions were prepared from H2SO4 (Merck, suprapure), CuSO4 (Fluka, puriss. p.a.), or CuO (Merck, p.a. > 99.9%) and Milli-Q water. For STM measurements, a copper wire (Aldrich, > 99.9%) served as a convenient, lownoise reference electrode, whereas a saturated calomel electrode (SCE) was employed for the voltammetric experiments. The Nernst potential of the Cu/Cu2+ redox couple in the solutions we used for the STM experiments, which all contained 0.5 mM CuSO4, is close to 0.0 V versus SCE. All potentials are quoted (22) Nishizawa, M.; Sunagawa, T.; Yoneyama, H. Langmuir 1997, 13, 5215. (23) Oyamatsu, D.; Nishizawa, M.; Kuwabata, S.; Yoneyama, H. Langmuir 1998, 14, 3298. (24) Zamborini, F. P.; Campbell, J. K.; Crooks, R. M. Langmuir 1998, 14, 640. (25) 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. (26) Sun, L.; Crooks, R. M. J. Electrochem. Soc. 1991, 138, L23. (27) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1995, 11, 4823. (28) Whelan, C. M.; Smyth, M. R.; Barnes, C. J. J. Electroanal. Chem. 1998, 441, 109. (29) Gilbert, S. E.; Cavalleri, O.; Kern, K. J. Phys. Chem. 1996, 100, 12123. (30) Cavalleri, O.; Gilbert, S. E.; Kern, K. Chem. Phys. Lett. 1997, 269, 479. (31) Cavalleri, O. Ph.D. thesis, E Ä cole Polytechnique Fe´de´rale de Lausanne, Switzerland, 1997. (32) Cavalleri, O.; Bittner, A. M.; Kind, H.; Kern, K.; Greber, T. Z. Phys. Chem. 1999, 208, 107. (33) Eliadis, E. D.; Nuzzo, R. G.; Gewirth, A. A.; Alkire, R. C. J. Electrochem. Soc. 1997, 144, 96. (34) Hagenstro¨m, H.; Schneeweiss, M. A.; Kolb, D. M. Langmuir 1999, 15, 2435.

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Figure 1. Cyclic voltammograms of ethanethiol-modified Au(111) electrodes in Cu2+-free (a) and Cu2+-containing (b-d) sulphuric acid solution (sweep rate: 10 mV s-1): (a) 0.1 M H2SO4; (b) 1 mM CuSO4 + 0.1 M H2SO4, first cycle; (c) after bulk deposition; (d) after an oxidation-reduction cycle. against SCE. The Au(111) electrodes were modified in a 1 mM solution of ethanethiol (Fluka; purum, >97%) in absolute ethanol (Merck, extra pure) for 16-20 h. Flame-annealed Au(111) films on glass were employed for the STM measurements, and a Au(111) single crystal was employed for cyclic voltammetry. 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. The tips were coated with electrodeposition paint to minimize faradaic currents at the tip-electrolyte interface.35

3. Results and Discussion 3.1. Cyclic Voltammetry. In Figure 1 the cyclic voltammogram (CV) of an ethanethiol-covered Au(111) electrode in the underpotential deposition (UPD) range of Cu is shown. The curve in Figure la was recorded in 0.1 M H2SO4 to check the quality of the C2 adlayer. The low, purely capacitive currents indeed indicate that the thiol layer is dense and well defined. Figure lb shows the current response of a C2-covered Au(111) electrode in 0.1 M H2SO4 + 1 mM CuSO4, which is markedly different from that for the bare gold electrode (cf. Figure 1d). The C2 film reduces the double-layer capacity as compared to a bare electrode, but the adlayer is not entirely blocking charge transfer. Since no peaks are found in 0.1 M H2SO4 (cf. Figure la), the transferred charge has to be attributed to Cu deposition. Positive of the Nernst potential for bulk Cu deposition (i.e., positive of 0 V versus SCE) the transferred charge amounts to only approximately 20 µC cm-2 at a sweep rate of 10 mV s-1, which corresponds to about 5% of a Cu monolayer on bare Au(111). Other samples modified under the same conditions showed moderately higher charges ( 8), where the UPD is completely blocked. Furthermore, charge transfer on freshly-prepared alkanethiol-modified Au(111) can be largely reduced by temperature treatment of the electrodes. An almost completely passivated electrode can be obtained by annealing the modified surface at temperatures around 325 K.30 Electrodes modified with hexanethiol treated in this way showed practically no charge transfer in the whole UPD range. Finally, we and other authors observed that a certain irreproducibility has to be reckoned with, as the same (or at least very similar) modification processes result in varying densities of the SAMs.27 Yoneyama and co-workers22 and Alkire and coworkers33 show CVs of propane- and pentanethiol-modified Au(111) with higher charge transfer than that for the shorter (!) ethanethiol, as is displayed in Figure 1 parts b and c. In their CVs a separate cathodic peak around 0.12 V is visible in the UPD range that is absent in our case. We also observed such a separate maximum for samples that showed a higher current at all potentials, indicating a higher defect density of the film. The voltammograms of Cu bulk deposition on the C2 covered electrode are shown in Figure 2. Several interesting features can be distinguished. We find a very sharp cathodic peak (C1) at -0.18 V, the charge under which corresponds to roughly one monolayer of Cu, before the deposition current increases again below -0.30 V. Obviously, nucleation sites are blocked and have to be activated at an overpotential, since the curve forms a loop after reversing the scan direction. The same loop was found by Sondag-Huethorst and Fokkink27 and Alkire and coworkers33. This behavior is readily explained by the inhibitive properties of the adlayer toward charge transfer. The regular Cu bulk stripping peak A2 is found around 0.07 V, covering about 1000 µC cm-2. Finally, the current maximum A1 at 0.35 V, due to Cu dissolution in the UPD range, is found again, but the corresponding charge is considerably larger (by a factor of 20) than that for the UPD cycles in Figure 1 parts b and c. Here, it amounts to roughly 450 µC cm-2 which is close to a full monolayer of Cu. The current maximum C1 was not previously reported. The corresponding charge is transferred in a narrow potential range, and the peak shape resembles a twodimensional phase transition. The shape is markedly different from diffusion-limited bulk growth (cf. Figure (36) Kolb, D. M.; Al Jaaf-Golze, K.; Zei, M. S. DECHEMA-Monographien; VCH: Weinheim, 1986; Vol. 102, pp 53-64.

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Figure 2. Cyclic voltammograms for Cu bulk deposition on ethanethiol-modified Au(111) electrodes in 1 mM CuSO4 + 0.1 M H2SO4 (sweep rate: 10 mV s-1): (a) first cycle; (b) third cycle; (c) after oxidation-reduction cycle.

2c) or the reductive desorption peak of C2 described in the first part of our work.34 The peak C1 remains almost unaffected in the following deposition cycles (cf. Figure 2b); only the total cathodic charge increases. Reductive desorption of the C2 adlayer in 0.1 M H2SO4 takes place at more negative potentials. If C1 were connected with reductive desorption, creating a considerably less dense C2 adlayer, the subsequent Cu dissolution should be similar to the one shown in Figure 2c, which is not observed. Thus, the current maximum C1 can be safely assigned to Cu deposition. It is important to note that we do not see a large effect of bulk Cu deposition on the C2 monolayer integrity (cf. Figure lc), which is in agreement with the findings of Alkire and co-workers.33 Interestingly, the C2 film is not desorbed during bulk Cu deposition, although the applied potential (-0.4 V) would be sufficient to do so. In 0.1 M H2SO4 a cathodic maximum at -0.31 V was found to correspond to the reductive desorption of the SAM.34 Two possible configurations might explain this unexpected stability of the C2 adlayer. Either the Cu deposit is found on top of the C2 adlayer, completely shielding the SAM against reductive desorption, or the Cu is in a buried-monolayer configuration, enhancing the C2 adlayer stability toward negative potential excursions (cf. Figure 3). The first case is rather unlikely, because integrating the cathodic charge under the curves of Figure 2 parts a and b, positive of the desorption potential (-0.31 V; in 0.1 M H2SO4) yields only

Copper Electrodeposition on Au(111) Electrodes

Figure 3. Model for the two possible locations of a twodimensional Cu deposit.

450 µC cm-2 (∼1.0 ML) and 480 µC cm-2 (∼1.1 ML). This would require a very smooth Cu adlayer on top of the alkanethiol SAM in order to protect it from reductive desorption, which is not compatible with the STM results (this point will be further discussed below). The buriedmonolayer configuration was previously suggested by Yoneyama and co-workers in a slightly different context.22 They worked with a propanethiol adlayer studying Cu deposition at underpotentials and showed that their buried Cu monolayer prevented reductive desorption of the C3 SAM in 0.5 M KOH down to -1.2 V. Likewise, Jennings and Laibinis found that such a configuration with a Ag UPD layer was found to be very stable in a corrosive environment.20 Consequently, there are some indications that a Cu ML inserted between the Au substrate and an alkanethiol SAM presents a stable configuration. A more detailed voltammetric study concerning this point will be presented in a forthcoming publication.37 3.2. In-Situ STM. In Figure 4 a sequence of images of C2-covered Au(111) in 0.05 M H2SO4 + 0.5 mM CuSO4 is presented. The image quality for the C2 adlayer is affected by tip-sample interactions that were found to be markedly stronger for Cu2+-containing electrolytes than for the pure base electrolytes (cf. part 1 of this work). However, (p × x3) domains of the adsorbed C2 are recognized on the atomically flat terrace at 0.35 V (Figure 4a). Lowering the electrode potential leads to a sudden surface transformation (Figure 4b), which proceeds in a narrow potential range, but the transition potential varies from sample to sample between 0.38 and 0.28 V. The new phase spreads out in a two-dimensional island-growth mechanism (1.5 ( 0.2 Å high), forming some additional islands on top (2.9 ( 0.5 Å high; white spots in image 4b). In Figure 4b some of the remaining darker patches of the not yet transformed and ordered C2 adlayer can still be seen (arrow). We stress the point that no charge transfer was seen in cyclic voltammetry in this region between 0.28 and 0.38 V (cf. Figure 1a and b), so that the underpotential deposition of Cu is strongly disfavored as a possible explanation for this process.30 We do not rule out that at potentials below +0.20 V some Cu is deposited at the small islands of the transformed C2 adlayer, but positive of 0.2 V, where the islands evolve, no substantial deposition current was recorded by cyclic voltammetry. The deposition current only increases below 0.2 V, where almost no island growth (see Figure 4, parts c and d) was observed by STM. Furthermore, in the base electrolyte (0.1 M H2SO4) we also observed a surface transformation preceding the reductive desorption around -0.31 V versus SCE.34 The onset potential for this transformation was likewise varying, but at lower potentials, between +0.1 and -0.1 V. Additionally, we observed an unstable intermediate network phase that was not seen with the copper-containing electrolyte. (37) Hagenstro¨m, H.; Schneeweiss, M. A.; Kolb, D. M. Electrochim. Acta, in print.

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However, the final state of the transformation is the same in both electrolytes: The surface is covered by a smooth but disordered phase with additional small islands on top. We therefore conclude that the same C2 adlayer transformation proceeds in Cu2+-free and Cu2+-containing electrolytes. The retransformation of the film into its ordered state (e.g. into the (p × x3) structure) occurs upon scanning the potential positive above 0.25 V (Figure 5; the potential was scanned from 0.25 to 0.40 V at 5 mV s-1, as indicated by arrows), which is very similar to the retransformation potential of the Cu2+-free electrolyte.34 Here, no hysteresis was observed for this process. The islands disappear and the molecular order is reestablished within about 1 min (not shown here). More defects appear in the SAM with every transformation, as was observed earlier in Cu2+free 0.1 M H2SO4. We will now continue to discuss Cu growth in the low overpotential range. The morphology of the Cu deposit in this potential region was found to depend on the varying density of islands that emerged during the film transformation. Since the structural transformation of the C2 layer precedes reductive desorption in 0.1 M H2SO4, we concluded that the emerging islands may consist of material that is expelled from the adlayer, resulting in a disordered C2 adphase of lower density.34 Following this interpretation, the amount of expelled material (islands) would be inversely proportional to the film density after the transition. In Figure 4d an example is given for a high density of C2 islands and, as will be shown below, with in-situ STM we observed only three-dimensional growth of Cu clusters at step-bunching sites on such island-rich samples when further lowering the potential. This will be discussed later on together with the images presented in Figure 8. At low island density a two-dimensional ramified deposit was found at low overpotentials (Figure 6). The ramified adlayer is 2.6 ( 0.2 Å high and tends to have threefold symmetry. The growth proceeds very slowly; the images were recorded after 13 min (Figure 6b), 21 min (Figure 6c), and 33 min (Figure 6d) at practically the same electrode potential. Since ramified 2D growth has often been observed during metal epitaxy in UHV,38 we are led to conclude that the ramified deposit found here also consists of a metal (Cu). The same growth morphology has been found for Cu on hexanethiol-covered Au(111) by Kern and coworkers, but they reported continued layer-by-layer growth in the OPD region at room temperature.30,31 We were not able to follow this growth mode beyond a coverage of about half a monolayer, since 3D growth originating at neighboring step bunching regions quickly buried the slowly growing ramified structures. We only observed monatomic high 2D structures just positive of the Nernst potential with a coverage of about half a monolayer that remained after the dissolution of Cu bulk deposit (not shown here). We did not find evidence for a layer-by-layer growth of Cu on C2-modified Au(111). The dissolution of the ramified Cu structure is displayed in Figure 7. At +0.02 V the structure is stable (Figure 7a), but dissolution starts around 0.08 V. Remainders of the ramified structure are seen up to 0.30 V. The dissolution process does not start at distinct sites; the process rather resembles an arbitrary “evaporation” of the adsorbed material. Interestingly, some islands completely resist the dissolution below 0.30 V (see arrows in Figure 7). This presents another strong indication that these small islands (38) Brune, H. Surf. Sci. Rep. 1998, 31, 121.

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Figure 4. STM images of ethanethiol-modified Au(111) electrodes in 0.5 mM CuSO4 + 0.05 M H2SO4 in the underpotential and low overpotential range: (a) 100 × 100 nm2, E ) 0.35 V versus SCE, showing the ordered C2 adlayer ((p × x3) domains) on an atomically flat terrace; (b) 96 × 96 nm2, E ) 0.29 V, order-disorder transition of the C2 adlayer (different sample); (c) 120 × 120 nm2, E ) 0.15 V, transformed adlayer with islands; (d) 120 × 120 nm2, E ) -0.07 V, the same terrace as in (c).

that emerge during the order-disorder transition are chemically different from the ramified Cu deposit. It is difficult to draw a straightforward conclusion from the voltammetric data in respect to the above-mentioned two-dimensional growth process, as was observed by insitu STM. The general behavior of the cathodic current response in the OPD range (cf. Figure 2) is a continuously increasing charge transfer onto which a single sharp peak at -0.18 V is superimposed. These two current features may correspond to the two different growth modes which we observed by STM. The peak C1 covers a charge equivalent to roughly 1 ML of Cu. Additionally, the peak shape resembles UPD peaks on bare metal electrodes, which points toward a comparable process. Of course, with STM the ramified structures were observed 0.1 V positive of the C1 peak potential, but this could explain the very slow growth. There was no possibility to observe twodimensional growth by STM at potentials as low as -0.18 V, because the overall roughness of the employed Auon-glass films was much higher than that of the Au crystal with which the electrochemical experiments were performed, strongly promoting bulk deposition, as already mentioned. Another problem is the disparity of time scales

Figure 5. STM image (128 × 128 nm2) showing the reordering of the C2 adlayer in 0.5 mM CuSO4 + 0.05 M H2SO4 during a potential sweep from 0.25 to 0.40 V versus SCE at 5 mV s-1.

Copper Electrodeposition on Au(111) Electrodes

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Figure 6. STM images (250 × 250 nm2) of the ramified growth of Cu on C2-covered Au(111) in 0.5 mM CuSO4 + 0.05 M H2SO4 at low overpotentials: (a) E ) -0.05 V versus SCE; (b) E ) -0.06 V, after 13 min; (c) E ) -0.07 V, after 21 min; (d) E ) -0.07 V, after 33 min.

between the two methods employed. The electrode was held for 40 min at low overpotentials while recording the images in Figure 6, whereas the negative polarization in the CVs of Figure 2 lasted only 80 s. It is therefore difficult to separately observe by STM ramified and 3D growth, which proceed simultaneously. At this point, the location of the Cu deposit with respect to the SAM will be discussed again. Kern and co-workers conclude from an XPS study of C18-modified Au samples that were emersed at 0.02 V from Cu2+-containing electrolyte that Cu is found between the gold surface and the alkanethiol monolayer.32 Our STM results confirm the statement that the Cu deposit penetrates the organic adlayer, as the ramified structures are perfectly stable in the low overpotential range. No deposit was displaced by the proximity of the tip, which could be expected for a Cu layer that presumably loosely adheres to the methyl end groups of ethanethiol. It is furthermore very unlikely that a ramified but smooth structure can evolve on a mobile substrate such as an alkanethiol film. Infrared spectroscopy39 and helium diffraction measurements40 of such systems that probe the chain packing and the end group

symmetry, respectively, detected only a low degree of or no order at room temperature. Dissolution experiments (Figure 7) reveal that some remainders of the deposit persist at potentials as high as 0.30 V, which is again unlikely for a deposit in direct contact with the electrolyte. The dissolution is even slightly delayed with respect to that of the Cu UPD layer on bare Au(111), which is evident also from cyclic voltammetry, since the dissolution peak A1 is found at 0.35 V. This is more positive than the most positive UPD peak for Cu on Au(111) (cf. Figure 1d). Although STM is a priori not capable of detecting unequivocally the location of the ramified structure with respect to the SAM, there are several indications that the Cu is actually penetrating the C2 adlayer and forming a Cu monolayer on the Au substrate with the SAM on top. In this respect the current peaks C1 and A1 can be understood as the corresponding counterpeaks of the formation and dissolution of the buried Cu monolayer. So far, we have described surface structural changes in the underpotential and low overpotential range. Bulk Cu deposition on a C2-modified Au(111) electrode at high overpotentials is shown in Figure 8. A large scan area

(39) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, 93, 767.

(40) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503.

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Figure 7. STM images (124 × 78 nm2) of the dissolution of the ramified Cu deposit on C2-covered Au(111) in 0.5 mM CuSO4 + 0.05 M H2SO4: (a) E ) 0.02 V versus SCE; (b) E ) 0.08 V; (c) E ) 0.15 V; (d) E ) 0.30 V.

(3 × 3 µm2) was chosen to facilitate a distinction between very different nucleation sites (terraces (1), steps (2), stepbunching sites (3)). Figure 8a shows the surface topography of a flame-annealed Au(111) film on glass at -0.08 V. The images are shaded in order to better recognize small topographical features on the rather rough substrate background. Large Au grains with atomically flat terraces on top (1) can be seen in Figure 8a. In the top, right corner the first Cu cluster has grown (arrow). A substantial increase in deposited material requires a higher overpotential such as in Figure 8b (-0.15 V). The tip was retracted between the acquisition of these two images, resulting in a small displacement of the scanned surface area (the overlap of the two images is indicated by the dashed rectangles). At such low overpotentials Cu crystallites preferentially nucleate and grow at grain boundaries, whereas the flat areas are devoid of any 3D deposit. High overpotentials are required to cover the entire substrate. At -0.25 V the whole surface is covered by a rough copper film, but the influence of the substrate topography is still obvious. Figure 8d was recorded on a different sample at -0.02 V after the deposition potential had been set around -0.15 V for 45 min. A small number of very large Cu crystallites has evolved. On top of the crystallites the typical Moire´ pattern of sulfate on Cu(111) was found (cf. inset in Figure 8d).41,42 No traces of adsorbed C2 were found on the Cu deposit by inspection (41) Wilms, M.; Broekmann, P.; Kruft, M.; Park, Z.; Stuhlmann, C.; Wandelt, K. Surf. Sci. 1998, 402-404, 83. (42) Li, W.-H.; Nichols, R. J. J. Electroanal. Chem. 1998, 456, 153.

Figure 8. STM images (3000 × 3000 nm2) of Cu bulk deposits on terraces (1), at steps (2), and at step-bunching sites (3) on ethanethiol-modified Au(111) in 0.5 mM CuSO4 + 0.05 M H2SO4: (a) E ) -0.08 V versus SCE, arrow indicates first nucleation site; dashed rectangle in (a) and (b) marks same area on surface; (b) E ) -0.15 V, progressing nucleation at grain boundaries; (c) E ) -0.25 V, surface is almost entirely covered with Cu bulk deposit; (d) different sample, E ) -0.02 V, after deposition at E ) -0.15 V for 45 min; inset, 10 × 10 nm2.

Copper Electrodeposition on Au(111) Electrodes

of the STM images. Therefore, the C2 adlayer, which we believe is found on a buried Cu monolayer, must be incorporated into the bulk deposit at high overpotentials. This is in agreement with our voltammetric results and earlier findings of Alkire and co-workers.33 In their galvanostatic experiments they found a low density of Cu clusters on a C5-covered electrode at low current densities and smooth deposits at higher current densities (which corresponds to higher overvoltages). The current loop in the CVs in Figure 2 is evidence for a blocking of nucleation sites that is overcome below -0.2 V. Deposition at potentials positive of this barrier for longer times (3/4 h, cf. Figure 8d) leads to a low density of large, flat-topped Cu(111) crystallites. 4. Conclusions We have studied Cu deposition from sulfuric acid solutions onto ethanethiol (C2)-modified Au(111) electrodes by cyclic voltammetry and in-situ STM. In the potential range of Cu deposition, the C2 adlayer is in a disordered state. The structure of this state cannot be resolved by STM on a molecular level. One characteristic feature of the disordered C2 adlayer, however, is small round islands of about 2.9 Å in height, which are formed during an order-disorder transition within the adlayer that occurs at more positive potentials. The islands are believed to consist of C2 molecules expelled from the ordered state as it undergoes the transition to the lessdensely-packed disordered adlayer. Copper underpotential deposition onto the C2-covered Au(111) surface starts around +0.20 V versus SCE, but at a sweep rate of 10 mV s-1 a total charge corresponding to only 5% of a full Cu monolayer is transferred. Since the exact amount of Cu deposited at underpotentials markedly depends on the defect density of the C2 adlayer, these defects are believed to act as nucleation centers for Cu

Langmuir, Vol. 15, No. 22, 1999 7809

deposition in this potential range. The potential of the corresponding anodic stripping peak is at +0.35 V. At -0.18 V versus SCE a pronounced current peak is seen on the negative scan in the cyclic voltammogram. The respective charge roughly corresponds to a full Cu monolayer. STM images reveal the formation of a ramified, monatomic high layer, which we assign to a Cu monolayer that grows in between the Au(111) surface and the C2 adlayer. This buried Cu monolayer is desorbed around +0.35 V, that is, at a potential more positive than the stripping potentials for a Cu monolayer on bare Au(111). Obviously, the C2 layer on top of the Cu monolayer markedly influences the energetics as well as the kinetics of the Cu UPD process. The latter point will be addressed in more detail in a forthcoming publication. Scanning the electrode potential even more negative leads to nucleation and growth of 3D Cu clusters at large surface imperfections such as grain boundaries or stepbunching regions. These Cu clusters rapidly cover the whole surface and often mask the much slower Cu monolayer formation. Interestingly enough, the C2 adlayer withstands being buried by a thick Cu overlayer and remains essentially intact during deposition and dissolution of bulk Cu. No indications were found for C2 molecules desorbing from the Au(111) surface during bulk Cu deposition and readsorbing on top of the Cu clusters. In the presence of Cu2+ ions in solution, reductive desorption of C2 was not observed up to a negative potential of -0.4 V, whereas it occurred in pure 0.1 M H2SO4 at -0.31 V. This indicates that the buried Cu monolayer considerably stabilizes the C2 SAM. Acknowledgment. One of us (H.H.) gratefully acknowledges a grant from the Deutsche Forschungsgemeinschaft through Graduiertenkolleg 328. LA9904307