In Situ Characterization of Self-Assembled Butanethiol Monolayers on

STM Study on Nonionic Fluorosurfactant Zonyl FSN Self-Assembly on Au(100): Molecular Lattice, Corrugations, and Adsorbate-Enhanced Mobility. Jiawei Ya...
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Langmuir 2003, 19, 830-834

In Situ Characterization of Self-Assembled Butanethiol Monolayers on Au(100) Electrodes F. Loglio,† M. Schweizer, and D. M. Kolb* Department of Electrochemistry, University of Ulm, D-89069 Ulm, Germany Received August 29, 2002. In Final Form: November 9, 2002 We have studied butanethiol self-assembled monolayers on Au(100) using cyclic voltammetry and in situ scanning tunneling microscopy (STM). The butanethiol adlayer shows ordered domains with a striped structure, the stripes running parallel to the main crystallographic axes of the substrate. After modification the surface reveals a 50% coverage of monoatomic high gold islands, but no vacancy islands were observed. Reductive and oxidative desorption of the film, previously studied by electrochemistry, were monitored by STM.

1. Introduction Over the past 15 years the interest in chemically modified electrodes has increased tremendously. An especially attractive and widely used approach involves the formation of highly ordered, monomolecularly thin films of organic material through self-assembly. These films are called self-assembled monolayers (SAMs) and can be easily prepared by adsorbing thiols on gold and taking advantage of the strong sulfur-gold interaction.1-8 Most of the work reported so far was performed in a vacuum or in air, employing Au(111) surfaces.6-8 More recently, the electrochemical properties of SAMs, particularly of alkanethiol SAMs were investigated by several groups.9-18 Such surfaces provide good model systems for fundamental studies of wetting,19,20 corrosion,21 or adhesion phenomena,22 and they represent an important interface between a metal electrode and a biosurface.23 * Corresponding author. Fax: +49-731-502-5409. E-mail: [email protected] † Permanent address: Via della Lastruccia n. 3, Polo Scientifico Universitario, I-50019-Sesto Fiorentino (Fi), Italy. (1) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (2) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (3) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (4) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (5) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533. (6) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151, and references therein. (7) Poirier, G. E. Chem. Rev. 1997, 97, 1117. (8) Ulman, A. Chem. Rev. 1996, 96, 1533. (9) Pan, J.; Tao, N. J.; Lindsay, S. M. Langmuir 1993, 9, 1556. (10) Rohwerder, M.; deWeldige, K.; Vago, E.; Viefhaus, H.; Stratmann, M. Thin Solid Films 1995, 264, 240. (11) Dakkouri, A. S.; Kolb, D. M.; Edelstein-Shima, R.; Mandler, D. Langmuir 1996, 12, 2849. (12) Wan, L.-J.; Hara, Y.; Noda, H.; Osawa, M. J. Phys. Chem. B 1998, 102, 5943. (13) Sawaguchi, T.; Mizutani, F.; Taniguchi, I. Langmuir 1998, 14, 3565. (14) Sawaguchi, T.; Sato, Y.; Mizutani, F. Electrochemistry 1999, 12, 1178. (15) Cavallini, M.; Bracali, M.; Aloisi, G.; Guidelli, R. Langmuir 1999, 15, 3003. (16) Hsieh, M.-H.; Chen, C.-h. Langmuir 2000, 16, 1729. (17) Rampi, M. A.; Schueller, O. J. A.; Whitesides, G. M. Appl. Phys. Lett. 1998, 72, 1781. (18) Green, J.-B. D.; McDermott, C. A.; McDermott, M. T.; Porter, M. D. In Imaging of Surfaces and Interfaces; Lipkowski, J., Ross, P., Eds.; Wiley-VCH: 1999; p 249. (19) Ulman, A. Thin Solid Films 1996, 273, 48. (20) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (21) Stratmann, M. Adv. Mater. 1990, 2, 191.

In a recent publication the structure and the electrochemical behavior of ethanethiol on Au(100) has been reported.24 It was shown that the SAM on Au(100) revealed properties that were distinctly different from those on Au(111). Besides marked structure transitions within the thiol SAM induced by the electrode potential, which apparently involved also the gold surface proper, no vacancy islands were observed directly after surface modification. The purpose of this paper is to show the electrochemical behavior of another short chain alkanethiol (butanethiol) on Au(100), to determine the structure and the stability of the adlayer, and to compare it with those on Au(100), investigated with UHV-STM by Poirier.25 2. Experimental Section The STM measurements were performed with a Topometrix TMX 2010 Discoverer, using tungsten tips (0.25 mm diameter wire) electrochemically etched in 2 M NaOH. The tips were coated with an electrophoretic paint (ZQ 84-3225 0201, BASF) to reduce the faradaic current at the tip-electrolyte interface to less than 50 pA. Tip and sample potentials were controlled independently of each other by a bipotentiostat in a four-electrode arrangement. Pt wires were used as quasireference electrode and as counter electrode. However, all potentials are quoted with respect to the saturated calomel electrode (EPt vs ESCE ) +0.55 V in this system). All images were obtained in the constant-current mode with a tunneling current of 1-3 nA, and the tip potential was held at -0.05 V vs SCE. The STM images are displayed as top views with different shades of gray (dark areas indicating lower parts and light areas higher parts of the surface). For the STM studies, a Au(100) single crystal (SPL, Zaandam, NL) with a 12 mm diameter was used, which was annealed at red heat in a hydrogen flame for 5 min and cooled to room temperature in a stream of nitrogen. This led to large, atomically flat (100) terraces, and the well-known (hex)-reconstruction could be clearly imaged by STM after addition of the electrolyte under potential control.26 The electrolyte, 0.1 M H2SO4, was prepared from H2SO4 (Merck, suprapure) and ultrapure water (USF Deutschland GmbH, TOC < 2 ppb, 18.2 MΩ cm). Butanethiol SAMs were prepared by immersing the annealed gold crystal into a 1 mM solution of butanethiol (Acros, 99+%) (22) Ferguson, G. S.; Chaudhury, M. K.; Sigal, G. B.; Whitesides, G. M. Science 1991, 253, 776. (23) Murray, R. W. In Molecular Design of Electrode Surfaces; Wiley: New York, 1992. (24) Schweizer, M.; Hagenstroem, H.; Kolb, D. M. Surf. Sci. 2001, 490, L627. (25) Poirier, G. E. J. Vac. Sci. Technol. B 1996, 14(2), 1453. (26) Kolb, D. M. Prog. Surf. Sci. 1996, 51, 109.

10.1021/la026493y CCC: $25.00 © 2003 American Chemical Society Published on Web 12/31/2002

SAM of Butanethiol on Au(100) Electrodes

Figure 1. Cyclic voltammograms of bare (dotted line) and butanethiol-covered (solid line) Au(100) in 0.1 M H2SO4, starting with freshly prepared electrodes at -0.25 and -0.15 V vs SCE in negative direction, respectively. Scan rate: 10 mV/s. Inset: first and second cycle for butanethiol-covered Au(100) into the oxidation region. in ethanol (Merck, extra pure) for 12-16 h at room temperature. All chemicals were used as received without further purification. After the modification the electrode was rinsed thoroughly, first with ethanol and then with ultrapure water, to clean the surface from residues of the modification solution. Then the crystal was transferred to the STM cell and the electrolyte, 0.1 M H2SO4, was added under potential control. After each experiment, the thiol was removed from the surface by electrochemical polishing,24 which also helped to restore the surface quality.

3. Results and Discussion 3.1. Cyclic Voltammetry. Figure 1 shows the cyclic voltammograms at 10 mV/s in 0.1 M H2SO4 of a flameannealed bare Au(100) electrode (dashed line) and a butanethiol-modified Au(100) electrode (solid line). The bare electrode shows the characteristic anodic peak at +0.32 V vs SCE due to the lifting of the (hex)-reconstruction.26 Such a peak is completely absent for the alkanethiolcovered Au(100) electrode, which demonstrates that the reconstruction has been lifted during the assembly of the thiol layer, as in the case of Au(111).27 The double-layer region for the butanethiol-covered Au(100) extends from -0.2 V up to at least +0.6 V vs SCE in acidic solution (pH 1), with a nearly constant double-layer capacity of Cdl ) 3.8 ( 0.2 µC cm-2 for the whole potential range. This value is about one-fifth of that for the bare gold surface, indicating that the butanethiol forms a compact, ionblocking monolayer. The double-layer region is limited by either the reductive desorption of the butanethiol (ESCE < -0.25 V) or the oxidation of the SAM at potentials >+0.8 V vs SCE, as seen in Figures 1 (inset) and 2, respectively. To separate hydrogen evolution from thiol reduction, which overlaps in 0.1 M H2SO4 (see inset of Figure 2), measurements were performed in 0.1 M NaOH (i.e. pH 13) (Figure 2). In this solution the film was found to be stable between -0.2 and -0.8 V vs SCE. Scanning the potential in the negative direction at a scan rate of 5 mV/s revealed a sharp cathodic peak at -1.03 V vs SCE due to the reduction of the film. From the area of the reduction peak, a charge of 106 µC cm-2 was calculated. Only a small anodic peak with a charge of ∼10 µC cm-2 (i.e., about 10% of the charge of the previous reductive current peak) was observed for the reoxidation of the film during the positive scan. Cyclic (27) Hagenstro¨m, H.; Schneeweiss, M. A.; Kolb, D. M. Langmuir 1999, 15, 2435.

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Figure 2. Reductive desorption of the butanethiol SAM on Au(100) in 0.1 M NaOH. Scan rate: 5 mV/s. Inset: Reductive desorption in 0.1 M H2SO4 (scan rate: 10 mV/s)

voltammograms for butanethiol-coated Au(100) electrodes in 0.1 M H2SO4 between -0.5 V and +0.6 V vs SCE (inset of Figure 2) show a broad reductive peak, which is superimposed by the beginning of H2 evolution. Also an increased reoxidation peak is observed, in good agreement with previous measurements by Yang et al.28-30 on alkanethiol-coated Au(111) electrodes. Here the redeposited amount of alkanethiols increases with increasing chain length. This was explained by the increasing solubility of the short-chain alkanethiols in alkaline solution. To estimate the thiol coverage, we follow the calculation shown by Yang et al.28 The charge associated with the reductive desorption in 0.1 M NaOH of the alkanethiol includes a capacitive contribution, which arises from charging of the now uncoated electrode. The capacitive charge is ∆Q ) Qc - Qb, Qc and Qb being the double-layer charges of the coated and bare Au(100) electrode, respectively. To calculate this charge, we apply the following equation: ∆Q ) Cdl,c(Ec - Ec,pzc) - Cdl,b(Eb - Eb,pzc). From capacitance measurements in 0.1 M NaOH (not shown here) we obtained double-layer capacities of the coated and bare Au(100) electrode of 3.8 ( 0.2 µF cm-2 (Ec ) -0.9 V) and 17 ( 0.5 µF cm-2 (Eb ) -1.1 V), respectively. The potential of zero charge (pzc) of the bare reconstructed Au(100) electrode (Eb,pzc) is +0.30 V vs SCE,31 but unfortunately the pzc of the coated electrode (Ec,pzc) is not known. However, because of Cdl,c , Cdl,b the uncertainty in Ec,pzc, which according to Yang et al. may be estimated to lie between -0.45 and +0.30 V vs SCE, has little effect on the overall capacitive charge ∆Q, the latter being about 18-20 µC cm-2. Considering a one-electron process for the reduction of the alkanethiol adlayer on gold32,33 and taking into account a capacitive charge of ∆Q ) 18-20 µC cm-2, a total coverage of Θ ) 0.44 ( 0.04 ML (ML being normalized to the gold substrate) was estimated from the reductive desorption of butanethiol on Au(100). This value is significantly higher than that for ethanethiol-modified Au(100) electrodes.24 Also for alkanethiol SAMs on Au(111), the coverage is remarkably smaller, namely Θ ) 0.33 ML, assuming a (x3 × x3)R30° overlayer.27,28 (28) Yang, D. F.; Wilde, C. P.; Morin, M. Langmuir 1996, 12, 6570. (29) Yang, D. F.; Wilde, C. P.; Morin, M. Langmuir 1997, 13, 243. (30) Yang, D. F.; Morin, M. J. ElectroAnal. Chem. 1998, 441, 173. (31) Kolb, D. M.; Schneider, J. Electrochim. Acta 1986, 31, 929. (32) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (33) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687.

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Figure 3. Two in situ STM images of a butanethiol-modified Au(100) electrode in 0.1 M H2SO4, showing monoatomic high gold islands and the striped structure of the butanethiol SAM.

To study the oxidation of the thiol film, cyclic voltammograms were performed with 0.1 M H2SO4. The oxidation was found to start at a potential of +0.85 V vs SCE (inset of Figure 1). The first cycle shows a broad current peak at about +1.15 V vs SCE, the charge of which is 394 µC cm-2 higher than that attributed to the oxidation of the bare gold (580 µC cm-2) (second cycle, dashed line). On the negative potential scans only the reduction of the gold was observed in both cases, so a complete desorption of the film during the first cycle can be assumed. This result is also consistent with reports on alkanethiols on Au(111).27 Considering the previously estimated coverage of 0.44 ML, we assume the complete oxidation of sulfur to sulfonic acid in a five-electron process according to eq 1:

AuSC4H9 + 3H2O f Au(0) + 5e- + H9C4SO3- + 6H+ (1) This value lies within the range discussed in the literature for the oxidative desorption of alkanethiols on Au(111). For example, Widrig et al.32 suggested a three-electron process, while Yang and co-workers34 assume an 11electron process, but intermediate values were also reported.35 In the following we will show the structural behavior of the butanethiol-covered Au(100) in dependence of the applied electrode potential, including the reductive and the oxidative desorption of the thiol. 3.2. In Situ STM. The aim of this in situ STM investigation was the structure determination for butanethiol SAMs on Au(100), formed in an ethanol solution, and to search for a possible influence of the electrode potential on the adlayer structure. So far, only UHVSTM work has been published, where the SAM was formed by evaporation of the thiol (and also by assembling in ethanol) onto the reconstructed Au(100).25 In that study a network of monoatomic high islands, which covered 50% of the total surface, was reported. A (1 × 4) missing-row structure of the gold substrate was proposed to explain the high island fraction as well as to give a conclusive answer for the commensurate c(2 × 8) structure of the butanethiol SAM on Au(100) that was observed. Figure 3a illustrates a large-scale STM image of a butanethiol SAM on Au(100) in 0.1 M H2SO4 at -0.15 V vs SCE, i.e., at a potential where the film is stable. The surface is strewn with monoatomic high gold islands (0.2 nm high) with a rectangular shape, covering about 50% of the surface. The islands are often interconnected by (34) Yang, D. F.; Al-Maznai, H.; Morin, M. J. Phys. Chem. B 1997, 101, 1158. (35) Hagenstro¨m, H. Ph.D. Thesis, Universita¨t Ulm, 2000, p 70.

Figure 4. Molecularly resolved STM image (15 × 15 nm2) of butanethiol-modified Au(100) in 0.1 M H2SO4 at +0.55 V vs SCE. The distances between molecules along the indicated directions are (- - -) 0.69 nm, (s) 0.48 nm, and (‚‚‚) 0.53 nm.

narrow bars with the width of a single SAM stripe (see below). As in the case of vapor-deposited butanethiol on Au(100),25 these longish islands as seen in the left part of Figure 3a are typical features of the surface. However, this network of islands is interrupted in the vicinity of steps (see the right part of Figure 3a; the step is not shown here). All islands are aligned parallel to the main crystallographic axes of the substrate, reflecting the 2-fold symmetry of the latter. The high island fraction of about 50%, which was also reported by Poirier,25 clearly exceeds the 25% coverage expected from lifting of the Au(100)(hex)-reconstruction and may be tentatively explained by a lattice expansion of the thiol-covered Au(100) surface.24 It is also notable that vacancy islands, which are regularly observed for alkanethiol-covered Au(111),7,36-40 were never found on Au(100) surfaces. A closer view on the SAM-covered surface (Figure 3b) shows an ordered butanethiol adlayer with a striped structure. The rows run parallel to the main crystallographic ([011] and [01 h 1]) axes but change directions near the 90° corners of the gold islands in gentle curves. With respect to the gold islands, it is interesting to note that their width always appears to be multiples of the SAM rows. Figure 4 shows a molecularly resolved STM image of the butanethiol SAM on Au(100). From this image, it is clearly seen that the individual molecules undergo a height modulation that leads to rows along the main crystallographic directions of the gold substrate, with a periodicity of 1.73 ( 0.05 nm (i.e., 6aAu-Au) and a corrugation of about 0.08 nm. The molecules are approximately arranged in squares with next-neighbor distances of about 0.48 ( 0.02 nm and 0.53 ( 0.02 nm and with the diagonal axis essentially along the rows. The distance between molecules (36) Dishner, M. H.; Feher, F. J.; Hemminger, J. C. Chem. Commun. 1996, 1971. (37) Cavalleri, O.; Hirstein, A.; Kern, K. Surf. Sci. 1995, 340, L960. (38) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966. (39) Sondag-Huethorst, J. A. M.; Scho¨nenberger, C.; Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6826. (40) McDermott, C. A.; McDermott, M. T.; Green, J.-B.; Porter, M. D. J. Phys. Chem. 1995, 99, 1325.

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Figure 6. In situ STM image (50 × 50 nm2) of butanethiolmodified Au(100) in 0.1 M H2SO4, showing the reductive desorption of the SAM after a potential step to -0.4 V vs SCE. Disordered regions “d” are growing at domain bounderies and step edges. Ordered domains are indicated by “o”. Formation of 0.2 nm deep holes within the disordered region is indicated by arrows.

Figure 5. STM image (80 × 80 nm2) of butanethiol-modified Au(100), recorded in air immediately after modification. Sample bias 200 mV, It ) 2 nA.

in the direction of the rows is 0.69 ( 0.03 nm, i.e., approximately 1.2 times that of 2aAu-Au. Unfortunately, we were not able to image the bare Au(100) surface atomically resolved within one and the same experiment; hence, we lack an internal calibration for an accurate structure determination. Although one might suspect that the angles of 44° and 53° in Figure 4 are close enough to 45° to be explained by thermal drift of perfect squares, these angles were repeatedly observed and their values are an average of at least eight different experiments. Nevertheless, the present situation does not allow us to give precise LEED-terminology-type structure information of this apparently incommensurate adlayer. There are three more observations worthwhile to mention: (i) Close inspection of Figure 4 reveals a faint corrugation along the rows, which is in agreement with an incommensurable structure. (ii) The rows usually have a periodicity of five molecules, and in rare cases, a larger distance (e.g., six molecules) is noted. This means that the structure in Figure 4 is the most common one, albeit not the only one. (iii) In contrast to the pronounced potential-dependence of the ethanethiol SAM structure on Au(100),24 no influence of the electrode potential on the structure of butanethiol was found. The SAM structures of Figures 3 and 4 are independent of potential. We also recorded images of the butanethiol SAM on Au(100) in air immediately after the modification process and before adding the electrolyte. Figure 5 shows a typical ex situ STM image, 80 × 80 nm2, of the butanethiol-covered Au(100). One can already see the elevated network of gold islands and the striped structure of the SAM, but there is an increased number of disordered domains in comparison with images recorded in situ under potential control. This is also in contrast to the ethanethiol SAM on Au(100), where no long-range order of the adlayer was seen in air immediately after surface modification.24 Finally, we turn to the reduction and oxidation of the film in 0.1 M H2SO4 as studied by in situ STM. Figure 6 illustrates the behavior of the butanethiol SAM during reductive desorption at negative potentials. As a first step the thiol adlayer becomes disordered at domain bounderies and step edges (indicated by “d” in Figure 6), then the formation of about 0.2 nm deep holes within the disordered regions is observed (see arrows in Figure 6). The latter

Figure 7. (a) In situ STM image (50 × 50 nm2) of butanethiolmodified Au(100) in 0.1 M H2SO4, while oxidizing the SAM at a potential of +1.15 V vs SCE. (b) Potential-induced (hex)reconstruction of Au(100) in 0.1 M H2SO4 at -0.22 V vs SCE, monitored by STM after the oxidative removal of the butanethiol film.

corroborates our assumption of a thiol-induced gold lattice expansion as an explanation of the high gold island fraction after SAM formation. Unfortunately, it was impossible in most cases to monitor the desorption process in more detail because of hydrogen evolution interfering. Nevertheless, our observations are consistent with a model proposed by Yang and co-workers,30 who describe the reduction of the butanethiol SAM in two steps: The reduction begins with the diffusion of ions (0.1 M NaOH) across the aliphatic layer at defects, which are called etching centers. During the removal of the butanethiol from the gold surface, the etching centers grow. When the potential was stepped to values positive of +0.85 V vs SCE for SAM oxidation, disorder was created near steps and island edges. After a few minutes, the whole surface was dominated by the disordered structure, the shape of the initially rectangular islands changing drastically (Figure 7a). Stepping the potential from + 1.15 V vs SCE, where the butanethiol film is immediately oxidized, to -0.22 V vs SCE allows the potential-induced (hex)reconstruction of the now bare Au(100) to become visible (Figure 7b). It was now possible to measure the distance between the well-known reconstruction lines and thus, to calibrate the STM scanner, albeit not with atomic precision. We tried to get an atomic resolution of the gold surface, but due to the oxidized thiol (i.e., contaminations) in the electrolyte, it was not possible to obtain a surface clean enough for such a measurement. This procedure confirmed our earlier conclusions that the butanethiol rows have the same directions as the (hex)-reconstruction

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lines and that the adlayer structure is incommensurate with the Au(100) surface. 4. Summary We have investigated the electrochemical behavior of a butanethiol-modified Au(100) electrode. The reductive desorption of the thiol adlayer as studied in 0.1 M NaOH, yields a current maximum at -1.02 V vs SCE, whereas the oxidative desorption studied in 0.1 M H2SO4 was found to superimpose with the oxidation of gold and leads to a current maximum at +1.2 V vs SCE. Both the reductive and the oxidative desorption of the film were imaged by in situ STM. The desorption process started at domain boundaries and proceeded with the growth of monoatomic deep holes in the substrate. Butanethiol forms an ordered adlayer on Au(100) with stripes running in [011] and [01 h 1] directions. The adlayer structure was found to be independent of electrode potential, in contrast to ethanethiol on Au(100).24 No vacancy islands were seen after modification, but a very

Loglio et al.

high fraction of gold islands (about 50% of the total surface) was observed, which clearly exceeded the 25% expected for lifting of the (hex)-reconstruction. The high island fraction obtained by modification in an ethanolic solution as well as the formation of monoatomic deep holes in the substrate surface after adlayer desorption point toward a thiol-induced expansion of the gold lattice. The molecular structure of the SAM differs from that obtained in an UHV study. In our case, we have no indication of a commensurate adlayer formed by the butanethiol. Acknowledgment. One of the authors (F.L.) gratefully acknowledges the financial support of the European Science Foundation under the ALENET network. This work was supported by the Fonds der Chemischen Industrie and by the Deutsche Forschungsgemeinschaft through grant SFB 569. LA026493Y