In Situ Dynamic Monitoring of Electrochemical Oxidative Adsorption

was higher than ca. 1 μM. When the VIs disappeared after the reductive desorption of the SAMs, the herringbone structure corresponding to the (3 × 2...
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In Situ Dynamic Monitoring of Electrochemical Oxidative Adsorption and Reductive Desorption Processes of a Self-Assembled Monolayer of Hexanethiol on a Au(111) Surface in KOH Ethanol Solution by Scanning Tunneling Microscopy Hiromi Wano and Kohei Uosaki* Physical Chemistry Laboratory, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Received January 25, 2005 Electrochemical oxidative formation and reductive desorption processes of a self-assembled monolayer (SAM) of hexanethiol on a Au(111) surface in KOH ethanol solutions containing various concentrations of hexanethiol were investigated by in situ scanning tunneling microscopy in real time. The generation and disappearance of vacancy islands (VIs), corresponding to the formation and desorption of the SAM, respectively, were observed as anodic and cathodic current, respectively, flowed when the thiol concentration was higher than ca. 1 µM. When the VIs disappeared after the reductive desorption of the SAMs, the herringbone structure corresponding to the (x3 × 23) structure of Au(111), was observed on the surface, indicating that a clean reconstructed surface was exposed even in the hexanethiol ethanol solution. During both oxidative adsorption and reductive desorption of the SAMs, the shape of the steps of the gold substrate changed drastically and the step lines became parallel to the 〈121〉 direction of the Au(111) surface, suggesting that gold atoms on the surface were extremely mobile during these processes. The coalescence of adjacent vacancy islands and growth of larger islands triangular in shape accompanied with the disappearance of nearby smaller islands were observed, confirming that the VIs grew according to the Ostward ripening model.

Introduction Self-assembled monolayers (SAMs) of alkanethiols on metal, especially gold, surfaces have attracted many research groups because of their wide variety of potential applications.1-9 Stable monolayers can be very easily formed by soaking a substrate in a solution containing alkanethiols. The self-assembly is achieved by a chemical bond formation between the substrate atoms and sulfur atoms of the thiols and hydrophobic interaction between alkyl chains. The formation process and the structure of thiol SAMs on Au(111) have been studied in detail by using various techniques, including IR,10-14 quartz crystal * To whom correspondence should be addressed. Phone: +81-11-706-3812. Fax: +81-11-706-3440. E-mail: uosaki@ pcl.sci.hokudai.ac.jp. (1) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Sci. 1990, 112, 4301. (2) Ulman, A. An Introduction To Ultrathin Organic Films: From Langmuir-Blodgett To Self-Assembly; Academic Press: San Diego, CA, 1991. (3) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510. (4) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (5) Fawcett, W. R.; Opallo, M. Angew. Chem., Int. Ed. Engl. 1994, 33, 2131. (6) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, p 109. (7) Ulman, A. Chem. Rev. 1996, 96, 1533. (8) Uosaki, K.; Kondo, T.; Zhang, X.-Q.; Yanagida, M. J. Am. Chem. Soc. 1997, 119, 8367. (9) Ulman, A. Thin films: Self-assembled monolayers of thiols; Academic Press: San Diego, 1998. (10) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284. (11) Sato, Y.; Frey, B. L.; Corn, R. M.; Uosaki, K. Bull. Chem. Soc. Jpn. 1994, 67, 21. (12) Bensebaa, F.; Voicu, R.; Huron, L.; Ellis, T. H.; Kruus, E. Langmuir 1997, 13, 5335. (13) Ye, S.; Haba, T.; Sato, Y.; Shimazu, K.; Uosaki, K. Phys. Chem. Chem. Phys. 1999, 1, 3653.

balance (QCM)13,15-17, scanning tunneling microscopy (STM)18-27 atomic force microscopy,24,25 X-ray diffraction,29 X-ray photoemission spectroscopy,30,31 second harmonic generation spectroscopy,32 sum frequency generation spectroscopy,33,34 and impedance spectroscopy.35,36 It is now well-known that alkanethiols on Au(111) surface are arranged in striped structures at low coverage and in a (14) Roy, D.; Fendler, J. Adv. Mater. 2004, 16, 479. (15) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. Langmuir 1992, 8, 1385. (16) Buttry, D. A.; Ward, M. D. Chem. Rev. 1992, 92, 1355. (17) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 3315. (18) Yamada, R.; Uosaki, K. Langmuir 1997, 13, 5218. (19) Poirier, G. E. Chem. Rev. 1997, 97, 1117. (20) Yamada, R.; Uosaki, K. Langmuir 1998, 14, 855. (21) Yamada, R.; Uosaki, K. Langmuir 2000, 16, 4413. (22) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (23) Yang, G.; Liu, G.-Y. J. Phys. Chem. B 2003, 107, 8746. (24) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222. (25) Xu, S.; Cruchon-Dupeyrat, S. J. N.; Garno, J. C.; Liu, G.-Y.; Jennings, G. K.; Yong, T.-H.; Laibinis, P. E. J. Chem. Phys. 1998, 108, 5002. (26) Noh, J.; Hara, M. Langmuir 2002, 18, 1953. (27) Noh, J.; Hara, M. Langmuir 2002, 18, 9111. (28) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W Langmuir 1997, 13, 1558. (29) Camillone, N., III.; Chidsey, C. D. E.; Eisenberger, P.; Fenter, P.; Li, J.; Liang, K. S.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 99, 744. (30) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (31) Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nishida, N.; Sasabe, H.; Knoll W. Langmuir 1998, 14, 2092. (32) Dannenberger, O.; Buck, M.; Grunze, M. J. Phys. Chem. B 1999, 103, 2202. (33) Petralli-Mallow, T. P.; Briggman, K. A.; Richter, L. J.; Stephenson, J. C.; Plant, A. L. Proc. SPIEsInt. Soc. Opt. Eng. 1999, 3858, 25. (34) Himmelhaus, M.; Eisert, F.; Buck, M.; Grunze, M. J. Phys. Chem. B 2000, 104, 576. (35) Boubour, E.; Lennox, R. B. J. Phys. Chem. B 2000, 104, 9004. (36) Protsailo, L.; Fawcett, W. R. Langmuir 2002, 18, 8933.

10.1021/la050209w CCC: $30.25 © 2005 American Chemical Society Published on Web 03/23/2005

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(x3 × x3)R30° or c(4 × 2) structure with a tilt angle of 30° from the surface normal at high coverage.4,18-20 STM has also revealed that there are many defects in the SAMs such as missing rows and pits.19,37-41 The missing row defects are attributed to the orientational and translational domain boundaries, and the pits were revealed to be vacancy islands (VIs) of gold atoms. We have found that the density of the pits is strongly dependent on the solvent42 and temperature43 employed for the self-assembly, although the total area of the VIs is almost constant. VIs are known to be formed in the very early stage of the self-assembly process,18,20 and it is, therefore, very important to know how the VIs are formed to fully understand the self-assembly mechanism. At least three possible mechanisms for VI formation have been proposed: chemical etching of the gold surface by alkanethiols,37-39 compression of the surface lattice,41 and ejection of more gold atoms than expected for the reconstruction lifting of the gold surface.19 Although detailed STM studies on the initial stage of the SA process of thiol in UHV have been carried out, the reason for the VI formation has not been clarified yet and it is particularly important to clarify the SA process of alkanethiols in solution since most of the SAMs are formed in the liquid phase.18,20 Since Widrig et al. reported that alkanethiolates are desorbed from a gold surface by the following one-electron reduction process in an alkaline aqueous solution44

Au-SR + e- f Au + RS-

(1)

many electrochemical studies on this reaction have been carried out because the position, area, and shape of the cathodic, i.e., reductive desorption, peak provide useful information of the SAM such as coverage, stability, and adsorption energy. For example, it was found that the longer the alkyl chain was, the more negative the reductive peak potential was, reflecting stronger van der Waals attractive interaction among alkyl chains.44 The reductive desorption process has been investigated by using various techniques, including electrochemistry,44-54 FTIR,55,56 electrochemical QCM,57,58 and in situ STM.59-62 A recent (37) Edinger, K.; Go¨lzha¨user, A.; Demota, K.; Wo¨ll, Ch.; Grunze, M. Langmuir 1993, 9, 4. (38) Scho¨nenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611. (39) Sondag-Huethorst, J. A. M.; Scho¨nenberger, C.; Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6826. (40) Scho¨nenberger, C.; Jorritsma, J.; Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. J. Phys. Chem. 1995, 99, 3259. (41) McDermott, C. A.; McDermott, M. T.; Green, J.-B.; Porter, M. D. J. Phys. Chem. 1995, 99, 13257. (42) Yamada, R.; Sakai, H.; Uosaki, K. Chem. Lett. 1999, 667. (43) Yamada, R.; Wano, H.; Uosaki, K. Langmuir 2000, 16, 5523. (44) Widrig, C. A.; Chung. C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (45) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (46) Azehara, H.; Yoshimoto, S.; Hokari, H.; Akiba. U.; Taniguchi, I.; Fujihira, M. J. Electroanal. Chem. 1996, 473, 68. (47) Yang, D.-F.; Wilde, C. P.; Morin, M. Langmuir 1996, 12, 6570. (48) Calvente, J. J.; Kova´_ova´, Z.; Sanchez, D.; Andreu, R.; Fawcett. W. R. Langmuir 1996, 12, 5696. (49) Yang, D.-F.; Wilde, C. P.; Morin, M. Langmuir 1997, 13, 243. (50) Zhong, C.-J.; Zak, J.; Porter, M. D. J. Electroanal. Chem. 1997, 421, 9. (51) Zhong, C.-J.; Porter, M. D. J. Electroanal. Chem. 1997, 425, 147. (52) Yang, D.-F.; Morin, M. J. Electroanal. Chem. 1998, 441, 173. (53) Yoshimoto, S.; Yoshida, M.; Kobayashi, S.; Nozute, S.; Miyawaki, T.; Hashimoto, Y.; Taniguchi, I. J. Electroanal. Chem. 1999, 473, 85. (54) Kondo, T.; Sumi, T.; Uosaki, K. J. Electroanal. Chem. 2002, 538-539, 59. (55) Yang, D.-F.; Al-Maznai, H.; Mario, M. J. Phys. Chem. B 1997, 101, 1158. (56) Byloos, M.; Al-Maznai, H.; Morin, M. J. Phys. Chem. B 1999, 103, 6554.

EQCM study on reductive desorption of a SAM of alkanethiols in an alkaline ethanolic solution by Kawaguchi et al. showed that the mass change determined from the observed frequency change did not agree with that calculated from the integrated charge of desorption because adsorption of cations took place upon the desorption of the SAM.58 Several groups have suggested the formation of an “aggregate” or “micelle” near or on the electrode surface after the reductive desorption of the SAM.54,56,59,61 We have found that the desorption of a SAM was initiated from the defects of the SAM, i.e., missing rows and edges of VIs, and that desorbed thiolate molecules formed aggregates in H2SO4 but a reconstructed, i.e., clean, surface of Au(111) was observed after the desorption of the SAM in KOH solution by in situ STM measurements.62 When the electrode potential is scanned back in the positive direction after an alkanethiol monolayer has been desorbed cathodically, an anodic peak corresponding to the oxidative readsorption of a fraction of the desorbed alkanethiolates remaining on and/or near the electrode surface, i.e., the reverse reaction of the reductive desorption given by eq 1, is observed.44,47,49 Thus, thiolate SAMs can be constructed by electrochemical oxidative reaction, i.e., the reverse reaction of the reductive desorption, in a solution containing corresponding thiol or thiolate.63-75 Weisshaar et al. demonstrated that a thiolate SAM, the structure and interfacial properties of which are similar to those of a SAM formed in ethanol solution without potential control, could be constructed anodically in an alkaline ethanol solution containing alkanethiol.63 Although anodic oxidative formation of a thiolate SAM seems to be an interesting alternative for SAM formation, only a few investigations have been carried out and details are not known.64-75 Recently, we reported that quantitative analysis of the position and the charge of the reductive desorption peak of an electrodeposited thiolate layer provided us with useful information on the formation process of a SAM such as the time dependencies of the adsorbed amount and the order of the SAM.72,75 Furthermore, since the oxidative formation and reductive desorption of a SAM in a solution containing a thiol seem to occur reversibly and can be controlled by applied potential, various important issues concerning the SAM formation not only in an electrochemical environment but (57) Schneider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391. (58) Kawaguchi, T.; Yasuda, Y.; Shimazu, K.; Porter, M. D. Langmuir 2000, 16, 9830. (59) Hobara, D.; Miyake, K.; Imabayashi, S.; Niki, K.; Kakiuchi, T. Langmuir 1998, 14, 3590. (60) Azzaroni, O.; Vela, M. E.; Martin, H.; Herna´ndez-Crues, A.; Andreasen, G.; Salvarezza, R. C. Langmuir 2001, 17, 6647. (61) Vericat, C.; Andreasen, G.; Vela, M. E.; Martin, H.; Salvarezza, R. C. J. Chem. Phys. 2001, 115, 6672. (62) Wano, H.; Uosaki, K. Langmuir 2001, 17, 8224. (63) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860. (64) Frubo¨se, C.; Doblhofer, K. J. Chem. Soc., Faraday Trans. 1995, 91, 1949. (65) Ron, H.; Rubinstein, I. J. Am. Chem. Soc. 1998, 120, 13444. (66) Eu, S.; Paik, W. Chem. Lett. 1998, 405. (67) Ma, F.; Lennox, R. B. Langmuir 2000, 16, 6188. (68) Qu, D.; Morin, M. J. Electroanal. Chem. 2000, 517, 45. (69) Subramanian, R.; Lakshminarayanan, V. Electrochim. Acta 2000, 45, 4501. (70) Paik, W.; Eu, S.; Lee, K.; Chon, S.; Kim, M. Langmuir 2000, 16, 10198. (71) Chon, S.; Paik, W. Phys. Chem. Chem. Phys. 2001, 3, 3405. (72) Sumi, T.; Wano, H.; Uosaki, K. J. Electroanal. Chem. 2003, 550551, 321. (73) Lee, M.-T.; Hsueh, C.-C.; Freund, M. S.; Ferguson, G. S. Langmuir 2003, 19, 5246. (74) Brett, C. M. A.; Kresak, S.; Hianik, T.; Brett, A. M. O. Electroanalysis 2003, 15, 557. (75) Sumi, T.; Uosaki, K. J. Phys. Chem. B, 2004, 108, 6422.

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also in general should be clarified by studying these processes by STM. In this study, we carried out in situ, real time STM monitoring of the oxidative adsorption and reductive desorption of hexanethiol (CH3(CH2)5SH: C6SH) SAMs on a Au(111) surface in ethanol solutions containing various concentrations of C6SH. Generation and disappearance of VIs, lifting and reappearance of the herringbone structure, i.e., (x3 × 23) structure of Au(111), dynamic change in the shapes of the steps of the gold substrate, and coalescence of adjacent VIs and growth of larger islands were observed under potential control. Experimental Section Hexanethiol (CH3(CH2)5SH, C6SH), ethanol, and KOH were reagent-grade chemicals and were used as received from Wako Pure Chemicals. Ultrapure Ar gas (99.9995%) was purchased from Air-Water. Gold single crystals were prepared by Clavilier’s method from a gold wire (>99.95%, φ ) 0.8 mm, Tanaka Precious Metal).76 A Au(111) facet was used as a substrate for electrochemical STM (EC-STM) measurements. A Au(111) face used as a substrate for the electrochemical measurements was prepared as follows. A gold single crystal was cut in parallel to the (111) facet, mechanically polished using 0.03 µm diamond slurry, and then annealed at 800 °C for ca. 8 h. Prior to each measurement, the Au(111) surface was flame-annealed in a H2 flame for about 20 s, cooled in air for several seconds, and then immersed in an ethanol solution containing 20 mM KOH. Electrochemical measurements were carried out in a threecompartment electrochemical cell with a hanging meniscus configuration using a potentiostat (HA-151, Hokuto Denko) and a function generator (HB-111, Hokuto Denko). Cyclic voltammograms (CVs) were recorded with an X-Y recorder (WX1100, Graphtec). The electrode potential was referred to an Ag/AgCl (saturated NaCl) reference electrode, and a Pt wire was used as a counter electrode. The hanging meniscus method was employed for the measurements. The electrolyte solution was deaerated by bubbling Ar for 30 min before each experiment. High quality and cleanliness of the Au(111) single crystal were ensured by recording CVs in 0.1 M H2SO4 solution. The obtained CVs were in agreement with those for clean Au(111) electrodes reported previously.77,78 In situ EC-STM measurements were carried out using a Pico SPM (Molecular Imaging) controlled by a Nanoscope E (Digital Instruments). STM tips were mechanically cut Pt/Ir (80:20) wire (φ ) 0.3 mm, Tanaka Precious Metal) insulated with Apiezon wax. STM observations were carried out in a homemade PCTFE cell, which was placed in an Ar-filled STM chamber, at room temperature with a bias of 300-850 mV and a tunneling current of 15 pA to 5 nA in a constant-current mode. Potentials of the Au(111) electrode and the STM tip were independently controlled by a bipotentiostat (PicoSTAT, Molecular Imaging), and a Ag/ AgCl (saturated NaCl) electrode and a Pt wire were used as reference and counter electrodes, respectively. An ethanol solution containing 20 mM KOH, which was deaerated by bubbling Ar for 30 min, was injected into the cell while the Au(111) electrode was held at -450 mV. After STM observation had been carried out at this potential to ensure that the gold surface was atomically flat and featureless, droplets of C6SH ethanolic solution of 10 µM, 100 µM, 1 mM, and 10 mM were injected into the cell in order to adjust the thiol concentration to 0.07, 0.3, 5, and 60 µM, respectively. All the STM images presented in this paper are typical results of many experiments, and only reproducible results are discussed.

Results and Discussion Electrochemical Characteristics. Figure 1 shows CVs of a Au(111) electrode measured in a 20 mM KOH ethanol solution containing C6SH of various concentra(76) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (77) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. J. Electroanal. Chem. 1987, 228, 429. (78) Hamelin, A. J. Electroanal. Chem. 1996, 407, 1.

Figure 1. Cyclic voltammograms of an Au(111) electrode measured in 20 mM KOH ethanol solutions (a) without thiol (dashed line) and containing 0.1 µM C6SH (solid line), (b) containing 1 µM C6SH (dashed line) and 10 µM C6SH (solid line), and (c) containing 100 µM C6SH (dashed line) and 1 mM C6SH (solid line). Scan rate was 100 mV/s.

tions. The potential of the electrode was held at -20 mV when the electrode surface was made in contact with the electrolyte solution and was then continuously cycled between 0 and -1000 mV at a scan rate of 100 mV/s. All the CVs shown in Figure 1 were recorded after the shape of the CVs became unchanged after several potential cycles, representing steady states of the adsorption and desorption of C6SH molecules. The dashed line in Figure 1a shows a CV measured in a 20 mM KOH ethanol solution without C6SH. An anodic current flowed between 0 and -300 mV and a small cathodic current flowed at potentials more negative than -350 mV, but no current peak was observed within this potential region. A CV measured in a 20 mM KOH ethanol solution containing 0.1 µM C6SH (solid line in Figure 1a) is almost the same as that measured in the ethanol solution without C6SH. Neither an anodic peak nor a cathodic peak associated with oxidative adsorption and reductive desorption, respectively, was observed, although the solution contained thiol molecules. The dashed line in Figure 1b shows a CV measured in a 20 mM KOH ethanol solution containing 1 µM C6SH. Although an anodic peak corresponding to the adsorption of thiolate was not clearly observed, a small cathodic peak corresponding to the reductive desorption of C6SH SAM was observed at -620 mV, indicating that some C6SH molecules were chemically adsorbed on the electrode surface. No anodic peak corresponding to oxidative

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Figure 2. STM images of a Au(111) electrode in a 20 mM KOH ethanol solution of 150 nm square obtained the potential was kept (a) at -450 mV and (b) at -950 mV.

adsorption suggests that the adsorption process was so slow that the anodic peak was too broad to be observed. The solid line in Figure 1b shows a CV measured in a 20 mM KOH ethanol solution containing 10 µM C6SH. A very small anodic peak corresponding to the adsorption of the thiol molecules was observed at -630 mV. The cathodic peak corresponding to the reductive desorption the SAM was larger and appeared at a more negative potential than those in a 1 µM C6SH solution. A further increase in peak size and negative shift of both anodic and cathodic peaks were observed in solutions of higher thiol concentrations as shown in Figure 1c (dashed line, 100 µM C6SH solution; solid line, 1 mM C6SH solution), but full coverage saturation was not reached, suggesting that the scan rate of 100 mV/s was too fast for a monolayer of full coverage to be formed. The concentration dependencies of the amount of adsorbed thiol and positions of the anodic and cathodic peaks are essentially the same as those reported for decanethiol in our previous papers,72,75 in which detailed analyses can be found. It is interesting to note that while the SAM of alkanethiols of high coverage can be formed even in a very dilute thiol (∼0.3 µM) heptane solution within a relatively short period,18,20 a cathodic peak corresponding to the desorption of the SAM became visible only when the concentration of C6SH was more than 1 µM, and both cathodic and anodic peaks corresponding to the reductive desorption and the oxidative adsorption, respectively, were clearly observed only when the C6SH concentration was more than 100 µM. Furthermore, the SAM formation rate in ethanol without electrolyte was found to be faster than the fastest rate in ethanol with KOH, which was observed at rather positive potential, 0.1 V, of a given thiol concentration. These results suggest the importance of the electrolyte for the formation and the desorption of the SAM. Kawaguchi et al. demonstrated by EQCM measurements that the desorption of the SAM is accompanied by the simultaneous adsorption of the cation.58 This means that the gold surface is covered with adsorbed cations in a negative potential region. Thus, there is a possibibility that the adsorbed cations remain on the surface before thiol is adsorbed when the potential is scanned to positive and inhibit the oxidative adsorption of the thiol. Another possible cause for the suppression of SAM formation is the competitive adsorption of anion. In Situ STM Observation in an Ethanol Solution without Thiol Molecules. Figure 2 shows typical STM images of a Au(111) surface obtained in a 20 mM KOH

ethanol solution at (a) -450 and (b) -950 mV. The herringbone structure was observed at both potentials, indicating that a clean surface with the (x3 × 23) reconstructed structure was exposed in a KOH ethanol solution. This is the first time that a bare Au(111) surface with the (x3 × 23) reconstructed structure was observed in a KOH ethanol solution under potential control. This observation also indicates that the adsorption of alkaline metal cations shown by EQCM and electrochemical measurement58 was not strong enough to lift the surface reconstruction. The adsorbed ions were not imaged because they either were swept away by the tip during the STM observation or did not form an ordered structure. It should be noted here that the reconstructed (x3 × 23) structure was easily observed at -950 mV but was more difficult to be observed at -450 mV. Actually, the image shown in Figure 2a was obtained only after several attempts. This was probably due to the unavoidable Faradaic current at the tip and the substrate at this potential as shown in the CV (dashed line, Figure 1a). It should be noted that the tip potential was more positive than the substrate potential. In Situ STM Observation in a Very Dilute Thiol Solution. Figure 3 shows STM images of the Au(111) surface sequentially obtained in a 20 mM KOH ethanol solution containing 0.3 µM C6SH. The gold surface was first imaged in an ethanol solution without thiol. An atomically flat surface was observed as shown in Figure 3a, confirming that the clean Au(111) surface was exposed. The herringbone structure cannot be seen in this particular image because the observation of the reconstructed structure was rather difficult at -450 mV as mentioned above. Droplets of 100 µM C6SH ethanol solution were added into the cell to prepare 0.3 µM C6SH solution while the potential of the electrode was kept at -450 mV, at which the adsorption of thiol on the surface was expected in a C6SH ethanol solution of higher concentration as shown in Figure 1. Figure 3b shows an STM image captured immediately after injection of the thiol solution. In contrast to the completely flat surface observed in the thiol free solution (Figure 3a), the surface seemed to be covered with some featureless phase. The electrode potential was then swept negatively by 5 mV/s from -450 to -1000 mV, where the thiol was expected to desorb from the surface. Figure 3c shows an STM image captured immediately after the potential became -1000 mV. The scanned area of the image in Figure 3c is slightly different from that of the image in

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Figure 3. STM images (150 nm square) of a Au(111) electrode (a) in 20 mM KOH ethanol solution at -450 mV and (b-e) in 20 mM KOH ethanol solution containing 0.3 µM C6SH at (b) -450 mV, (c) -1000 mV, and (d, e) -150 mV.

Figure 3b. The herringbone structure was clearly observed, indicating that the clean reconstructed surface was exposed even in the ethanol solution containing thiol molecules at -1000 mV. The white arrow in Figure 3c indicates a monatomic step of the gold substrate. The electrode potential was swept positively from -1000 to -150 mV by 0.5 mV/s and an STM image (Figure 3d) was captured 5 min after the potential had reached -150 mV. The monatomic step of the gold substrate indicated by a white arrow in Figure 3d is the same step indicated by the white arrow in Figure 3c. VIs, which should appear if an alkanethiol SAM is formed on the Au(111) surface, were not observed. As indicated by four black arrows, corrugated structures, which resembled a part of the herringbone structure, were still observed. Immediately after the image shown in Figure 3d had been obtained, an STM image (Figure 3e) was obtained in a region different from that of Figure 3d but on the same facet. The corrugated structure, which resembled to the herringbone structure, was also observed all over the image. A similar result was obtained in a more dilute C6SH (0.07 µM) ethanol solution. These results and the CVs in the dilute thiol solutions indicate that the SAM with an Au-S bond and an ordered structure was not formed in dilute thiol solutions even at relatively positive potentials. However, some phase, which seemed to maintain the reconstructed structure, seemed to exist on the Au(111) surface. The formation of a similar phase was also observed by Poirier and Pylant at the initial stage of the SAM formation process in ultrahigh vacuum (UHV) when a very small amount of thiol molecules was adsorbed.79 They suggested that the observed phase is a gas phase without apparent molecular order. It is also (79) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145.

possible that the phase observed in the image shown in Figure 3 in the positive potential region is a physisorbed gas phase. This is in contrast to the results obtained in very dilute (about 0.3 µM) heptane solutions without an electrolyte on a Au(111) surface, in which rapid creation of VIs at the initial stage, i.e., within 3 min after the thiol injection, followed by the formation of stripe and mesh structures, i.e., (p × x3) structures, then the formation of a (x3 × x3)R30° structure were observed.18,20 This difference confirms that the adsorption of thiol molecules was suppressed by the presence of electrolyte in the solution. SAM Growth at a Constant Potential in a 5 µM C6SH Ethanol Solution. Immediately after the STM images shown in Figure 3e had been captured, droplets of 1 mM C6SH ethanol solution were further added into the cell so that the thiol concentration became 5 µM while keeping the potential at -450 mV. Figure 4 shows STM images captured (a) 2, (b) 8, (c) 10, and (d) 12 min after the thiol concentration was adjusted to 5 µM. A bright region indicated by a white arrow was higher than the surrounding areas by a monatomic height and was used as a marker. A monatomic step of the gold substrate was also observed in the lower left corner of the image. The corrugated structure observed in Figures 3d and 3e disappeared but VIs were not yet visible after 2 min as shown in Figure 4a. After 8 min, a number of VIs became visible (Figure 4b), indicating the formation of an alkanethiol SAM. This result is in good agreement with the fact that a cathodic peak corresponding to the reductive desorption of the SAM was observed only in solutions containing thiol more than 1 µM.72,75 The sizes of the VIs increased with time as typical examples are indicated by three small white arrows in

Surface Dynamics of Electrochemical SAM Formation

Figure 4. Sequentially obtained STM images of a Au(111) electrode (150 nm square) in 20 mM KOH ethanol solution containing 5 µM C6SH captured (a) 2 min, (b) 8 min, (c) 10 min, and (d) 12 min after the thiol concentration was adjusted from 0.3 to 5 µM while the potential was kept at (-450 mV).

Figure 5. An STM image of a Au(111) electrode (15 nm square) in 20 mM KOH ethanol solution containing 5 µM C6SH about 100 min after the injection of thiol. The potential was kept at -450 mV.

Figures 4b-4d. A detailed comparison of Figures 4b-4d shows that the some of the minute VIs such as those observed within a white circle in Figures 4b and 4c disappeared in Figure 4d and that the adjacent VIs became larger at the same time, implying that VIs migrated and coalesced with each other. Two VIs indicated by a black arrow in Figure 4c also coalesced as shown by a black arrow in Figure 4d. The VIs did not grow further during a period of about 10 min after the image shown in Figure 4d had been captured. These results show that the growth of VIs follows the ripening process.80,81 That is some nuclei of VIs, which were invisible, were formed initially (Figure 4a), and then the nuclei coalesced and grew in size. Finally, the ripening process was completed and an equilibrium state, in which the size of VIs was energetically stable and did not change any more, was reached. An STM image of molecular resolution was very difficult to obtain, but the saturated (x3 × x3)R30° structure became visible about 90 min after the potential was reached at -450 mV as shown in Figure 5. This is in agreement with the previous result that it took more than (80) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966. (81) Cavalleri, O.; Hirstein, A.; Kern, K. Surf. Sci. 1995, 340, L960.

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60 min for a highly ordered C10SH SAM to be formed under potentiostatic conditions.75 Reductive Desorption of the SAM in 5 µM C6SH Solution. Disappearance of VIs and Restoration of the Herringbone Structure. After the STM images shown in Figure 4 had been captured, the potential was swept negatively from -450 to -950 mV at the scan rate of 2.5 mV/s. Figure 6 shows STM images captured while the potential was swept (a) from -620 to -850 mV and (b) from -850 to -950 mV. The potential variation within each image was different because the scan rate of the STM tip was varied in the range of 0.5-8 Hz in order to obtain the clearest image. The monatomic step observed in the lower left corner of the image in Figure 6a is the one observed in the STM images shown in Figure 4. VIs were still observed while the potential was scanned from -620 to -720 mV, indicating that the SAM still existed on the surface within this potential region. When the potential reached -720 mV, a large tip current suddenly flowed and the STM observation was impossible for ca. 10 s. Since the reductive desorption of the SAM is expected to start around this potential, the large tip current is thought to be caused by the contact of the STM tip with a large number of desorbed thiolate molecules. When the electrode potential became more negative (after 10 s), a clear STM image was able to be obtained again. The surface morphology was, however, totally different from that observed at more positive potential. Almost all the VIs, which were observed clearly in the lower half of the image when the potential was more positive than -720 mV, disappeared, and a number of very small pits were observed all over the surface, some of which are indicated by white arrows. This observation clearly indicates that the large VIs vanished as soon as the reductive desorption of the SAM took place. Figure 6b shows an STM image captured immediately after the image shown in of Figure 6a had been obtained. A large white spot indicated by a pointing finger existed at almost the same place in both images, showing the same area was imaged in Figure 6b and the upper half of Figure 6a. The imaged areas of Figure 6b and the upper half of Figure 6a seem to be different from those of Figure 4 and the lower half of Figure 6a because this spot was not observed in Figure 4 and the monatomic step observed at the lower left corner of Figures 4 and 6a was no longer visible in Figure 6b. Thus, the STM tip seemed to move when the large tip current flowed and the feedback control was lost around -720 mV. Figure 7 shows STM images captured (a) 8, (b) 12, and (c) 14 min after the potential reached -950 mV after the STM images shown in Figure 6 had been captured. The spot indicated by the pointing finger is the same spot observed as that shown in Figure 6. The herringbone structure began to be observed in the image shown in Figure 7a, indicating that the clean (x3 × 23) reconstructed surface started to be exposed after the reductive desorption of the SAM. The number of small pits decreased significantly, but a few pits were still observed on the surface as indicated by a white arrow. They existed between rows of the herringbone near the herringbone elbow. The herringbone structure grew and became clearer and the number and sizes of pits decreased further with time as shown in Figures 7b and 7c. The pits, indicated by the white arrow, became less clear with time but maintained their positions between the rows near the herringbone elbow as the herringbone structure grew and the distance between the rows of the herringbone became narrower. Figure 7c shows that the VIs indicated by the white arrow almost disappeared between the herringbone rows and the (x3 × 23) reconstructed structure was

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Figure 6. STM images (150 nm square) of a Au(111) electrode in 20 mM KOH ethanol solution containing 5 µM C6SH while the potential was swept negatively (a) from -620 to -850 mV and (b) from -850 to -950 mV with the scan rate of 2.5 mV/s.

Figure 7. Sequentially obtained STM images (150 nm square) of a Au(111) electrode in 20 mM KOH ethanol solution containing 5 µM C6SH captured (a) 8 min, (b) 12 min, and (c) 14 min after the potential was kept at -950 mV.

restored. Poirier followed the self-assembly process of mercaptohexanol in UHV and found that the preferred site for SAM nucleation is the herringbone elbow, i.e., the site of highest surface energy.82 They assumed that the ejection of excess surface Au atoms induced by the adsorption of thiol molecules to relax the compressed herringbone reconstruction is the cause of the formation of VIs. The surface holes migrate on the surface and eventually nucleate to become islands. The present results indicate that the VIs tend to remain between the rows near the herringbone elbow, i.e., the region with the highest surface energy, when the herringbone structure grew after the desorption of the SAM. Thus, the herringbone elbow is the first site for the generation of VIs during the SAM formation process and the last site for the disappearance of the VIs during the desorption process. Reductive Desorption and Oxidative Adsorption Processes in a 60 µM C6SH Solution. Change in the Structures of Step Edges and VIs. After the electrode had been immersed in 20 mM KOH ethanol solution containing no C6SH at -450 mV and an STM image had been captured to confirm that an atomically flat and featureless surface similar to that observed in the image shown in Figure 2a was exposed, droplets of 10 mM C6SH ethanol solution were added into the cell to prepare a 60 µM C6SH solution. The potential was kept at -450 mV for about 15 min. Generation and growth of VIs similar to the results presented in Figure 4 were observed. The potential was then swept negatively to -1000 mV by 1 mV/s. The VIs disappeared, and the (x3 × 23) reconstructed structure was exposed as was observed in the 5 µM C6SH (82) Poirier, G. E. Langmuir 1997, 13, 2019.

solution (Figures 6 and 7). The potential was then positively swept at a scan rate of 1 mV/s from -1000 to -450 mV. Figure 8 shows STM images captured while the potential was being positively swept (a) from -950 to -860 mV, (b) from -860 to -770 mV, (c) from -770 to -690 mV, and (d) from -690 to -565 mV. The potential variation within each image was different because the scan rate of the STM tip was varied in order to obtain the clearest image. The white arrows in all the images indicate the same position. Initially, several rows of herringbone were observed as shown in Figure 8a by black arrows. The herringbone structure was not observed clearly in the image shown in Figure 8b, suggesting that the thiol molecules started to absorb on the Au(111) surface in this potential region. It is clear that the shape of the step line of the substrate gold changed drastically in images shown in Figures 8a and 8b, indicated by a pointing finger as a typical example. This suggests that the surface gold atoms became mobile when the thiol molecules started to adsorb on the gold surface. The STM image shown in Figure 8c shows the generation of VIs and their growth to visible sizes, indicating the formation of the SAM, when the potential reached around -730 mV. The potential range of Figure 8c, i.e., from -770 to -690 mV, is in good agreement with that where the anodic peak corresponding to oxidative adsorption of the thiol was observed in the CV measured in 100 µM C6SH solution (Figure 1c). The continuous transition of the step shape implied that the first surface layer of the gold substrate was still mobile even when the thiol molecules were chemically adsorbed on the gold. Potential-induced mobilization of gold atoms of the surface layer was suggested by Schweizer et al.83 They monitored the potential-induced structure transition

Surface Dynamics of Electrochemical SAM Formation

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Figure 8. Sequentially obtained STM images (150 nm square) of a Au(111) electrode in 20 mM KOH ethanol solution containing 60 µM C6SH captured while the potential was positively swept (a) from -950 to -860 mV, (b) from -860 to -770 mV, (c) from -770 to -690 mV, and (d) from -690 to -565 mV with the scan rate of 1 mV/s and (e) while the potential was kept at -450 mV, (f) while the potential was swept negatively from -950 to -450 mV with the scan rate of 5 mV/s, and (g) 2 min and (h) 4 min after the potential had reached -950 mV.

of step edges of a Au(100) surface modified by C2SH SAM in H2SO4 aqueous solution and suggested that the goldthiolate species move on the surface, rather than only the thiol molecules sliding across a structurally unaltered surface. Figure 8e shows an STM image captured 10 min after the potential had reached -450 mV. VIs of various sizes existed all over the surface. Although it is not possible to determine the atomic arrangement of the Au(111) surface covered with the SAM by this STM image, surface X-ray diffraction study showed that the reconstruction is lifted and that the Au(111)-(1 × 1) structure is formed upon the adsorption of the SAM84 as suggested before. (83) Schweizer, M.; Hagenstro¨m, H.; Kolb, D. M. Surf. Sci. 2001, 490, L627. (84) Uosaki, K.; Toshihiro, K.; Morita, J.; Takakusagi, S.; Sumi, T. In preparation. 2005.

The image shown in Figure 8f was captured while the potential was swept negatively from -450 to -950 mV with the scan rate of 5 mV/s after the image shown in Figure 8e had been captured. VIs started to disappear at ca. -700 mV when the reductive desorption was expected to start. The shape of the step line was further changed, proving the high degree of mobility of gold atoms of the first surface layer. Pits within a white circle had monatomic depth. Figure 8g shows an STM image captured 2 min after the potential had reached -950 mV. The herringbone structure became visible again and the changes in the shape of step edges still continued. Figure 8h shows an STM image captured 4 min after the potential had reached -950 mV. The herringbone structure became more visible all over the surface, indicating that the thiol molecules had been completely desorbed.

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Figure 9. STM images of the Au(111) surface (150 nm square) obtained in 20 mM KOH ethanol solution containing 60 µM C6SH captured (a) while the potential was kept at -950 mV after the potential had been cycled twice between -450 and -950 mV with a scan rate of 10 mV/s, (b) immediately after the observation of the image shown in (a) while the potential was positively swept from -950 to -450 mV with a scan rate 5 mV/s, (c) at -450 mV after the potential had been cycled two more times between -450 and -950 mV with a scan rate 10 mV/s, and (d) at -450 mV after the potential had been cycled between -450 and -950 mV with a scan rate of 10 mV/s once more.

A comparison of the images shown in Figures 8f-8h revealed that the step edges tended to be aligned parallel to 〈121〉 directions of the substrate gold. In addition, the terrace grew as indicated by the black arrow and the pits within a white circle also became larger simultaneously. These results indicate that gold atoms are highly mobile during the adsorption/desorption processes of thiol molecules and some of the excess gold atoms are absorbed at step edges, resulting in more aligned step lines. Ripening of VIs. While the STM images presented in this study showed almost no pits on the surface once the reductive desorption of the SAM was completed, a number of relatively large triangle-shaped pits were clearly observed when a SAM prepared in ethanol solution containing only thiol without potential control was reductively desorbed in aqueous alkaline solution.43 To clarify the cause of this discrepancy and the growth mechanism of triangle-shaped pits, further study was carried out. After the atomically flat surface of the gold substrate similar to that shown in Figure 2a was imaged to confirm the exposure of a clean Au(111) surface in the 20 mM KOH ethanol solution, droplets of 10 mM C6SH ethanol solution were added into the cell to prepare 60 µM C6SH solution while keeping the electrode potential at -450 mV. Figure 9a shows the STM image captured at -950 mV after the potential had been cycled twice between -450 and -950 mV with a scan rate of 10 mV/s. The pointing finger indicates a step of the gold substrate. While several rows of the herringbone structure, indicated by white

arrows, were observed, no clear VIs were visible, indicating that thiol molecules had not adsorbed on the surface. Figure 9b shows an STM image captured while the potential was positively swept from -950 to -450 mV with a scan rate of 5 mV/s immediately after the image shown in Figure 9a had been obtained. The pointing finger indicates the same step as that observed in the image shown in Figure 9a. The thiol molecules started to adsorb, and a SAM was formed on the surface when the potential became more positive than -780 mV as indicated by the formation of VIs. It was clear that larger VIs were of equilateral triangle shape as shown by white arrows. It must be noted that each side of the triangle was aligned parallel to the main crystallographic direction, i.e., 〈121〉 direction, of Au (111). The potential was then cycled between -450 and -950 mV with a scan rate of 10 mV/s two more times and then kept at -450 mV, at which the image shown in Figure 9c was captured in a close vicinity of the image shown in Figure 9b. The sizes of VIs of equilateral triangle shape became larger than those observed in the image shown Figure 9b as indicated by white arrows. Moreover, minute VIs were generated simultaneously; typical examples are indicated by black arrows. After the STM image shown in Figure 9c had been captured, the potential was cycled between -450 and -950 mV with a scan rate of 10 mV/s once more and then kept at -450 mV, at which the STM image shown in Figure 9d was captured in a close vicinity of the image shown in Figure 9c. Triangle-shaped VIs further grew in size as

Surface Dynamics of Electrochemical SAM Formation

indicated by white arrows, but the sizes of the minute VIs did not change as indicated by black arrows. In addition, it is clear that the minute VIs did not exist near the large triangle-shaped VIs. This heterogeneous distribution of the sizes and locations of VIs strongly suggests that the large VIs were formed due to the Ostwald ripening of minute VIs. The experimental conditions used to obtain the image shown in Figure 9 were different from those used in other experiments. While the potential was kept at a negative potential, at which the surface reconstruction of Au(111) was restored, for a sufficiently long period in other experiments, the potential scan was not stopped at negative potential and, furthermore, the scan rate in this experiment was relatively fast, i.e., 10 or 5 mV/s, compared with that used for other experiments. Thus, VIs did not disappear completely in the negative potential region. Then some of the remaining VIs acted as nucleation centers and minute VIs were absorbed by larger VIs during the rapid oxidative formation of the SAM. This process was repeated during the potential cycle with a relatively fast scan rate, resulting in the formation of remarkably large triangle-shaped VIs as shown by white arrows in the images shown in Figure 9b-d. Actually, several large VIs and many minute nuclei coexisted in the images shown in Figures 9c and 9d. It must be noted, however, that there were no minute VIs in the vicinity of the large VIs as shown clearly in the image shown in Figure 9d. Once VIs had grown more than a critical size, they were energetically so stable that they remained on the surface even if the potential was kept at a negative potential for a long time, at which the surface reconstruction of Au(111) was restored. Thus, they had become stable equilateral triangle-shaped pits of the gold surface separated by step lines of monatomic height, which were parallel to the 〈121〉 direction of the Au(111) surface. We have already reported that while the fractions of the pits were constant, the size of each VI depended on solvent42 and temperature43 and the higher the formation temperature the larger the size of each VI.43 On the basis of these results, we have pro-

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posed that VIs grow due to the Ostwald ripening mechanism. The present results suggest that this mechanism is universal for thiol SAM formation on a Au(111) surface. Conclusions Electrochemical oxidative formation and reductive desorption processes of a SAM of hexanethiol on a Au(111) surface in KOH ethanol solutions containing various concentrations of hexanethiol were investigated by in situ STM in real time. The generation and disappearance of VIs, corresponding to the formation and desorption of the SAM, respectively, were observed as anodic and cathodic current, respectively, flowed when the thiol concentration was higher than ca. 1 µM. When the VIs disappeared after the reductive desorption of the SAMs, the herringbone structure, i.e., the (x3 × 23) structure of Au(111), was observed on the surface, indicating that a clean reconstructed surface was exposed even in the hexanethiol ethanol solution. During both the oxidative adsorption and the reductive desorption of the SAMs, the shapes of the steps of the gold substrate changed drastically and the step lines became parallel to the 〈121〉 direction of the Au(111) surface, suggesting that the gold atoms on the surface were extremely mobile during these processes. The coalescence of adjacent vacancy islands and growth of larger triangle-shaped islands accompanying the disappearance of nearby smaller islands were observed, confirming that the VIs grew according to the Ostward ripening model. Acknowledgment. The work was partially supported by Grant in Aid for Scientific Research in Priority Area of “Molecular Nano Dynamics” (No. 16072202) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. Professors W. K. Paik and K. Shimazu and Dr. T. Sumi are acknowledged for their useful discussions. LA050209W