In Situ, Real-Time Monitoring of the Reductive Desorption Process

In situ electrochemical STM measurements were carried out using a Pico SPM (Molecular .... Cathodic current corresponding to hydrogen evolution starte...
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Langmuir 2001, 17, 8224-8228

In Situ, Real-Time Monitoring of the Reductive Desorption Process of Self-Assembled Monolayers of Hexanethiol on Au(111) Surfaces in Acidic and Alkaline Aqueous Solutions 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 June 27, 2001 Reductive desorption processes of a self-assembled monolayer (SAM) of hexanethiol on an Au(111) surface both in H2SO4 and KOH aqueous solutions were investigated by in situ scanning tunneling microscopy (STM) in real time. The structure of the SAM in the electrolyte solutions before the desorption was confirmed to be same as that in the air, that is, the (x3×x3)R30° structure. The desorption of the thiolate (thiol) molecules was found to be initiated from the defects in the SAM, that is, missing rows and the edge of the vacancy islands. Although the desorbed thiolate (thiol) molecules formed aggregates both in H2SO4 and KOH solutions, the number and the size of the aggregates monitored by STM were higher in H2SO4 solution than in KOH solution, reflecting the lower solubility and diffusion rate of the desorbed molecules in H2SO4 solution than in KOH solution. The reconstructed, that is, (x3×23), structure of Au(111) was observed after the desorption of the SAM in KOH solution, showing that the clean surface was exposed, although the vacancy islands of monatomic height, which were observed on the SAM-covered gold, remained to be observed on the surface even after the desorption.

Introduction Self-assembled monolayers (SAMs) of alkanethiols on a gold surface have attracted many research groups because they have a wide variety of potential applications.1-3 Stable monolayers can be very easily formed by soaking a substrate in a solution containing the alkanethiols. The self-assembly (SA) is achieved by a chemical bond formation between the substrate atoms and sulfur atoms of the thiols and the hydrophobic interaction between alkyl chains.3 The structure of the SAMs of alkanethiols on Au(111) has been studied by various techniques in detail, and it is known that the alkanethiols are arranged in striped structures initially when the coverage was low and then in a (x3×x3)R30° or c(4x3×2x3) structure with a tilt angle of 30° from the surface normal when the coverage became high.4-6 The scanning tunneling microscopy (STM) investigations also revealed that there are many defects in the SAMs such as missing rows and pits.5 The missing-row defects are attributed to the orientational and translational domain boundaries, and the pits were revealed to be vacancy islands (VIs) of the gold atoms.7-9 We have found that the density of the pits is strongly dependent on the solvent * Corresponding author. E-mail: [email protected]. Fax: 81-11-706-3440. (1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (2) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (3) Ulmann, A. Chem. Rev. 1996, 96, 1533. (4) Ulmann, A. Self-Assembled Monolayer of Thiols; Academic Press: San Diego, CA, 1998. (5) Poirier, G. E. Chem. Rev. 1997, 97, 1117 and references therein. (6) Yamada, R.; Uosaki, K. Langmuir 1997, 13, 5218; Langmuir 1998, 14, 855. (7) Scho¨nenberger, C.; Sondsag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611. (8) McDermott, C. A.; McDermott, M. T.; Green, J.-B.; Porter, M. D. J. Phys. Chem. 1995, 99, 13257. (9) Edinger, K.; Go¨lzha¨user, A.; Demota, K.; Wo¨ll, Ch.; Grunze, M. Langmuir 1993, 9, 4. Edinger, K.; Grunze, M.; Wo¨ll, Ch. Ber. BunsenGes. Phys. Chem. 1997, 101, 1811.

and temperature employed for the self-assembly.10,11 Many reactions such as replacement reactions of the SAMs12,13 and the underpotential deposition reaction of metals on the SAM-covered metal surface14,15 seem to be initiated from these defects because ions and molecules, which induce the reactions, can penetrate from these defects into the SAMs. The reductive desorption reaction shown by eq 1 is one of the most important reactions of the SAM of alkanethiolates on gold electrodes not only for the characterization of the SAM but also for the control and design of the surface properties.16-37

X(CH2)nS-Au + e- f X(CH2)nS- + Au

(1)

(10) Yamada, R.; Wano, H.; Uosaki, K. Langmuir 2000, 16, 5523. (11) Yamada, R.; Sakai, H.; Uosaki, K. Chem. Lett. 1999, 667. (12) Kakiuchi, T.; Sato K.; Iida, M.; Hobara, D.; Imabayashi, S.; Niki, K. Langmuir 2000, 16, 7238. (13) Hobara, D.; Ota, M.; Imabayashi, S.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1998, 444, 113. (14) Oyamatsu, D.; Nishizawa, M.; Kuwabata, S.; Yoneyama, H. Langmuir 1998, 14, 3298. (15) Oyamatsu, D.; Kuwabata, S.; Yoneyama, H. J. Electroanal. Chem. 1999, 473, 59. (16) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860. (17) Imabayashi, S.; Hobara, D.; Kakiuchi, T.; Knoll, W. Langmuir 1997, 13, 4502. (18) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (19) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687. (20) Weisshaar, D. E.; Walczak, M. M.; Porter, M. D. Langmuir 1993, 9, 323. (21) Zhong, C.-J.; Porter, M. D. J. Am. Chem. Soc. 1994, 116, 11616. (22) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (23) Yang, D.-F.; Al-Maznai, H.; Morin, M. J. Phys. Chem. B 1997, 101, 1158. (24) Byloss, M.; Al-Maznai, H.; Morin, M. J. Phys. Chem. B 1999, 103, 6554. (25) Yang, D.-F.; Morin, M. J. Electroanal. Chem. 1997, 429, 1.

10.1021/la010990h CCC: $20.00 © 2001 American Chemical Society Published on Web 11/27/2001

Reductive Desorption of Hexanethiol SAMs

Several laboratories have reported a variety of interesting aspects of the reductive desorption process based on the results of electrochemistry, FT-IR, in situ STM, and so on. In situ FT-IR study showed an intense CH stretching vibration after the reductive desorption,23,24 suggesting the formation of micelles of thiolates, and chronoamperometry showed that such a micelle formation of the thiolate proceeded by a nucleation and growth process.25,26 The micelles formed on the surface of Au(111) were observed by in situ STM after the reductive desorption of the SAM of various thiols.27 Recently, the reconstructed structure of Au(111), that is, the (x3×23) structure, was observed by in situ STM after the reductive desorption of the SAM of 2-mercaptoethanesulfonic acid in KOH solution, showing that a clean surface was exposed after the reductive desorption.28 Electrochemical study showed that a larger reoxidation peak appeared in electrolyte solutions of lower pH, and such behavior has been considered to be due to the difference in the solubility and diffusion rate of the desorbed thiolate (thiol) molecules.29,30 It is, however, not clear how the desorption process is initiated and proceeds and how the pH of the solution affects the desorption process at the molecular level. Understanding the morphological change in the SAMs during the reductive desorption in situ in real time is crucial for understanding the desorption process. In this paper, we investigated the reductive desorption of the hexanethiol SAMs constructed at high temperature, which had a smaller number of defects and a larger area of individual VI and domain than those constructed at room temperature10 in H2SO4 and KOH solutions, by in situ STM. We found that (1) the desorption and the aggregation of the desorbed thiolate (thiol) were initiated from the defects in the SAMs, that is, the missing rows and the edge of the VIs, and proceeded by a nucleation and growth process, (2) the aggregates scarcely diffused into the H2SO4 solution (pH ) 2), in which the desorbed thiolate (thiol) molecules have low solubility and low diffusion rate, and remained as the grown aggregates on the surface yet diffused almost completely into the KOH solution (pH ) 12), and a clean Au(111) surface with the reconstructed (x3×23) structure was observed after the reductive desorption, and (3) VIs, which were the characteristic structure of the SAM-covered gold, remained even after the desorption of the SAM. Experimental Section Hexanethiol (CH3(CH2)5SH, C6SH), ethanol, H2SO4, and KOH were reagent grade chemicals and were used as received from Wako Pure Chemicals. High-purity N2 and Ar gas were purchased from Air Water. An Au(111) cut plane and an Au(111) facet of a single crystal prepared by Clavilier’s method were used as substrates for the electrochemical measurement and EC-STM (26) Yang, D.-F.; Morin, M. J. Electroanal. Chem. 1998, 441, 173. (27) Hobara, D.; Miyake, K.; Imabayashi, S.-I.; Niki, K.; Kakiuchi, T. Langmuir 1998, 14, 3590. (28) Hobara, D.; Yamamoto, M. K.; Kakiuchi, T. Chem. Lett. 2001, 374. (29) Yang, D.-F.; Wlide, C. P.; Morin, M. Langmuir 1996, 12, 6570. (30) Yang, D.-F.; Wlide, C. P.; Morin, M. Langmuir 1997, 13, 243. (31) Zhong, C.-J.; Zak, J.; Porter, M. D. J. Electroanal. Chem. 1997, 421, 9. (32) Zhong, C.-J.; Porter, M. D. J. Electroanal. Chem. 1997, 425, 147. (33) Lamp, B. D.; Hobara, D.; Porter, M. D.; Niki, K.; Cotton, T. M. Langmuir 1997, 13, 736. (34) Imabayashi, S.; Iida, M.; Hobara, D.; Feng, Z. Q.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1997, 428, 33. (35) Nishizawa, M.; Sunagawa, T.; Yoneyama, H. J. Electroanal. Chem. 1997, 436, 213. (36) Liu, J.; Kaifer, A. E. Isr. J. Chem. 1997, 37, 235. (37) Badia, A.; Arnold, S.; Scheumann, V.; Zizlsperger, M.; Mack, J.; Jung, G.; Knoll, W. Sens. Acuators, B 1999, B54, 145.

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Figure 1. Cyclic voltammograms of the hexanethiol SAM modified Au(111) electrode (solid line) and a bare Au(111) electrode (dashed line) in 10 mM H2SO4. Scan rate: 20 mV s-1. experiments, respectively.38 The Au surface was flame-annealed for about 20 s, cooled in air for several seconds, and then immersed in pure ethanol, which was kept at 60 °C prior to the surface modification. The modification was carried out by immersing the substrate into 1 mM C6SH ethanol solution at 60 °C. After the modification, the substrate was rinsed with pure ethanol and dried with N2 gas. Electrolyte solutions were prepared using H2SO4, KOH, and Milli-Q water (Nihon Millipore) and were deaerated by bubbling Ar for 20 min before each experiment. Electrochemical measurements were carried out using a potentiostat (HA-151, Hokuto Denko) and a function generator (HB111, Hokuto Denko), and cyclic voltammograms (CVs) were recorded with an X-Y recorder (WX1100, Graphtec). The electrode potential was referred to an Ag/AgCl electrode, and the counter electrode was a Pt wire. In situ electrochemical 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 (Tanaka Precious Metal) insulated with Apiezon Wax. The measurements were carried out in a homemade PCTFE cell sealed in an Arfilled STM chamber at room temperature. STM images were recorded in a constant-current mode. Electrochemical potentials of the Au(111) substrate and the STM tip were independently controlled by a bipotentiostat (PicoSTAT, Molecular Imaging), and an Ag/AgCl and a Pt wire were used as the reference and counter electrodes, respectively.

Results and Discussion In H2SO4 Solution. Electrochemical Characteristics. Figure 1 shows CVs of bare (dashed line) and hexanethiol SAM modified (solid line) Au(111) electrodes measured in 10 mM H2SO4 aqueous solution. At the bare Au(111) electrode, cathodic current corresponding to hydrogen evolution started to flow at ca. -520 mV and increased monotonically as the potential became more negative. The current-potential relation in the reverse (positive-going) scan was identical to that of the forward (negative-going) scan. At the hexanethiol SAM modified Au(111) electrode, cathodic current started to flow at a more negative potential and was smaller at all potentials investigated than that at the bare Au(111). Although no clear cathodic peak corresponding to the desorption of the SAM was observed, some of the thiolate (thiol) molecules were desorbed by -520 mV as an anodic peak corresponding to readsorption of the thiolate (thiol) was observed in the reverse scan at -400 mV. These results indicate that the hydrogen evolution reaction was inhibited not only by the SAM but also by the desorbed thiolate (thiol) molecules. The CV of the second scan was shifted to a positive direction compared to the first scan, showing that the number of adsorbates decreased. Morin et al. reported that the solubility and diffusion of desorbed thiolate were smaller in solutions of lower pH and that the desorbed thiolates formed physisorbed aggregates on the electrode surface.29,30 Thus, almost all of the desorbed molecules (38) Clavilier, J. J. Electroanal. Chem. 1980, 107, 211; 1980, 107, 205.

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Figure 2. Sequentially obtained STM images of the hexanethiol SAM modified Au(111) electrode in 10 mM H2SO4 (a) immediately after the potential was stepped to -340 mV, showing an area 75 nm square and (a′) 25 nm square, (b) immediately after the potential was stepped to -350 mV (75 nm square), (c) 8 min after the potential was stepped to -350 mV (75 nm square), (d) 10 min after the potential was stepped to -350 mV (75 nm square), (e) immediately after the potential was stepped to -360 mV (75 nm square), (f) 5 min after the potential was stepped to -360 mV (75 nm square), and (g) immediately after the potential was stepped to -500 mV (100 nm square). The bias was 350-600 mV, and the tunneling current was 30 pA-4 nA.

remained on the electrode surface and inhibited the hydrogen evolution in H2SO4 solution because of the low solubility and diffusion of the desorbed molecules. In Situ STM Investigation. Figure 2 shows STM images of the hexanethiol SAM on Au(111) sequentially obtained in 10 mM H2SO4 under potential control. At the beginning of the observation, the potential of the electrode was 0 mV, and the potential was stepped by 10 mV to a more negative potential after confirming that no significant morphological change in the surface took place (about 5-10 min). Although the imaged areas were slightly moved due to the thermal drift, all images are in a close vicinity. The typical structure of SAMs with VIs, missing rows, and domains was observed at -340 mV (Figure 2a). A higher resolution image of a domain (Figure 2a′) shows the molecularly ordered (x3×x3)R30° structure. Figure 2b shows an STM image captured immediately after the potential was stepped to -350 mV. A white spot as indicated by a white arrow appeared at the edge of the VIs. The white spot grew with time as shown in Figure 2c, which was captured 8 min after the potential was stepped to -350 mV. These white spots must be the

aggregates of the desorbed thiolate (thiol) molecules as reported by Kakiuchi et al.27 New aggregates were observed at the edge of the VIs as indicated by black arrows in Figure 2d, which was captured 10 min after the potential was stepped to -350 mV. Figure 2e shows an STM image captured immediately after the potential was stepped to -360 mV. The aggregate indicated by a black arrow in Figure 2e was the same as that indicated by the longest black arrow in Figure 2d. It is clearly shown that this aggregate grew. As the finger arrow in Figure 2e indicates, a new aggregate was formed at the edge of another VI. The removal of the SAM was observed at the bottom right corner of the image. The aggregate indicated by the finger arrow grew, and the area of the region where the SAM was removed increased with time as shown in Figure 2f which was captured 5 min after the potential step to -360 mV. Figure 2g shows an STM image captured immediately after the potential was stepped to -500 mV after Figure 2f was captured. Note that the scanned area was enlarged to 100 nm square. Large numbers of aggregates were formed, but only at the edges of the VIs and missing rows. These aggregates seemed to nucleate at the defects in the

Reductive Desorption of Hexanethiol SAMs

Figure 3. Cyclic voltammograms of the first scan (solid line) and the second scan (dashed line) of the hexanethiol SAM modified Au(111) electrode in 10 mM KOH. Scan rate: 20 mV s-1.

SAM when the potential was stepped to a very negative potential. These results revealed that the reaction sites of the reductive desorption were the defects in the SAM and that the desorbed thiolate (thiol) molecules formed the aggregates through a nucleation and growth process in H2SO4 solution. In KOH Solution. Electrochemical Characteristics. Figure 3 shows CVs of the first scan (solid line) and the

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second scan (dashed line) of the hexanethiol SAM modified Au(111) electrode measured in 10 mM KOH solution. In the first forward (negative-going) scan, cathodic current corresponding to the reductive desorption of the SAM started to flow at ca. -800 mV and a cathodic peak was observed at ca. -900 mV. Cathodic current corresponding to hydrogen evolution started to flow at ca. -1270 mV. In contrast to the result in H2SO4 solution, no anodic peak corresponding to readsorption of thiolate was observed in the reverse (positive-going) scan. In the second forward scan, cathodic current corresponding to the reductive desorption was not observed, and cathodic current corresponding to hydrogen evolution started to flow at a less negative potential (ca. -1220 mV) than that in the first scan. The CV of the second scan was similar to that of the bare Au(111), indicating that the thiolate molecules were desorbed from the electrode surface and were completely diffused into KOH solution during the first potential cycle. In Situ STM Investigation. Figure 4 shows STM images of the hexanethiol SAM modified Au(111) sequentially obtained in 10 mM KOH during the reductive desorption process. The potential of the electrode was 0 mV at the beginning of the observation and was stepped initially by

Figure 4. STM images of the hexanethiol SAM modified Au(111) electrode in 10 mM KOH (a) immediately after the potential was stepped to -770 mV (100 nm square), (b) immediately after the potential was stepped to -850 mV, showing an area 100 nm square and (b′) 40 nm square, (c) immediately after the potential was stepped to -865 mV, showing an area 100 nm square and (c′) 25 nm square, (d) 5 min after the potential was stepped to -870 mV (100 nm square), (e) 8 min after the potential was stepped to -870 mV (100 nm square), (f) 10 min after the potential was stepped to -870 mV, showing an area 100 nm square and (f ′) 40 nm square. The bias was 350-880 mV, and the tunneling current was 15 pA-5 nA.

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30-50 mV to a more negative potential. The potential was kept at each potential for ca. 5 min to confirm that no morphological change in the surface was observed. Figure 4a shows an STM image captured at -770 mV. The structure of the SAM was essentially the same as that observed in air. When the potential was more negative than -800 mV where the cathodic current corresponding to the reductive desorption started to flow, the potential was stepped at a smaller potential interval (10 mV) and kept at each potential for ca. 10 min. All images were of 500 nm square on the same Au(111) facet, although the scanned region was changed for each image. Figure 4b shows an STM image captured at -850 mV where the reductive desorption reaction of the SAM was expected as the CV in Figure 3 suggested. Missing rows were broadened and some of the domain became disordered, showing that the desorption of the molecules had started. A higher resolution image of the ordered region of the domain (Figure 4b′) shows that the molecularly ordered (x3×x3)R30° structure was still maintained. When the electrode potential became close to the reductive desorption peak, noise appeared in the scanning direction of the tip, that is, the lateral direction of the image. Because of this noise, it was impossible to image the initial stage of the desorption and to see the reaction sites despite several attempts. The noise was observed only in KOH solution and was considered to be caused by the contact of the STM tip with the desorbed thiolate molecules. The solubility and diffusion rate of desorbed molecules were far larger in KOH solution than in H2SO4 solution, and therefore many more desorbed molecules were in contact with the tip than in H2SO4 solution, leading to noise in the STM images. A higher resolution image (Figure 4c′) shows the coexistence of the ordered phase with the (x3×x3)R30° structure (indicated by the arrow) and the disordered phase where some of the molecules had already desorbed. The results in Figure 4b,c demonstrated that the reductive desorption took place domainwise and not in a random fashion and that the desorption of molecules started from the domain edge. When the electrode potential became more negative, the noise disappeared and a drastic morphological change was observed. Figure 4d shows an STM image captured 5 min after the potential was stepped to -870 mV. The aggregates of the desorbed thiolate molecules were found at the edge of VIs, and the bare Au(111) surface with the (x3×23) reconstructed structure began to be observed. The area with the (x3×23) reconstructed structure increased with time (Figure 4e, 8 min) and finally covered all of the surface (Figure 4e, 10 min). As shown in the higher resolution images captured before (Figure 4b′) and after (Figure 4f ′) the reductive desorption, the detailed surface structure was clearly different, proving that the molecules were actually desorbed from the surface. Although Kakiuchi et al. reported that the (x3×23) reconstructed structure was observed after the desorption of the SO3H(CH2)2SH SAM, it is interesting that a clean surface was exposed even

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after the desorption of the SAM with a long alkyl chain. These results in KOH solution were quite different from those in H2SO4 solution where the desorbed thiolate (thiol) molecules scarcely diffused into the solution bulk and formed the physisorbed aggregates which covered the whole surface. In KOH solution, the desorbed thiolate molecules diffused almost completely and scarcely remained on the surface, reflecting a large solubility and a large diffusion rate. These results in H2SO4 solution and in KOH solution were in good agreement with those obtained electrochemically by Morin et al.29,30 As clearly shown in Figure 4c′-f, the VIs, which were the characteristic features when the thiol molecules absorbed, remained on the surface after the desorption. It has been suggested that the formation of VIs is due to the lifting of the excess gold atoms on the (x3×23) reconstructed structure of the bare Au(111) caused by the adsorption of the thiol molecules.39,40 Thus, one would expect that the pits disappeared when the reconstructed structure is formed. The present results clearly showed that the VIs remained on the surface even after the reductive desorption in contradiction to this expectation. Although the detailed process is not revealed, the present results must provide an important clue for the understanding of the mechanism of the pit formation. Conclusions The detailed morphological change of a self-assembled monolayer (SAM) of hexanethiol on an Au(111) electrode surface during the reductive desorption reaction in the electrolyte solution was revealed by in situ STM. When the potential of the electrode was less negative than that at which cathodic current corresponding to the reductive desorption started to flow, the structure of the SAM in the electrolyte solution was confirmed to be the same as that in air. It was proved that the reductive desorption was initiated from the defects in the SAM, that is, the missing rows and edges of the VIs, and that the desorbed thiolate (thiol) molecules formed aggregates following a nucleation and growth process. After the reductive desorption, the electrode surface was covered with the aggregates of desorbed thiolate (thiol) molecules in H2SO4 solution, reflecting the low solubility and diffusion rate. On the other hand, the bare Au(111) with the (x3×23) reconstructed structure was observed in KOH solution, showing that a clean surface was exposed after the desorption of the SAM, although the VIs, which were observed on the SAM-modified Au(111), were found to remain on the surface even after the reductive desorption. Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture, Japan. LA010990H (39) Poirier, G. E. Langmuir 1997, 13, 2019. (40) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145.