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Langmuir 1998, 14, 3590-3596
In-Situ Scanning Tunneling Microscopy Imaging of the Reductive Desorption Process of Alkanethiols on Au(111) Daisuke Hobara, Koichiro Miyake, Shin-ichiro Imabayashi, Katsumi Niki,† and Takashi Kakiuchi* Department of Physical Chemistry, Yokohama National University, Yokohama 240, Japan Received September 2, 1997. In Final Form: April 16, 1998 The reductive desorption process of self-assembled monolayers of 1-hexadecanethiol, 1-propanethiol, and 3-mercaptopropionic acid on Au(111) has been studied in 0.5 M KOH solution by in-situ scanning tunneling microscopy (STM) and cyclic voltammetry. In-situ STM images of the monolayers at the potentials between -0.2 V and the reduction potentials of each thiols show the pits that are commonly seen in STM images of thiol self-assembled monolayers. A drastic morphological change takes place in the STM image around the peak potential in a cyclic voltammogram for the reductive desorption of adsorbed thiols. The images indicate that 3-mercaptopropionic acid molecules diffuse away from the surface after the reduction because of its higher solubility, while 1-propanethiol and 1-hexadecanethiol molecules stay in the vicinity of the surface forming aggregates. The partial recovery of the 1-hexadecanethiol monolayer after the anodic scan, suggested by cyclic voltammograms, is confirmed by STM, whereas 1-propanethiol aggregates remain at the surface without being reoxidized. The difference in the reoxidation behavior reflects the different amphiphilic properties of the desorbed molecules and the resultant molecular organizations formed on the surface.
Introduction This paper describes the morphological changes in the self-assembled monolayers (SAMs) of thiols caused by the reduction and reoxidation of the monolayers studied with in-situ scanning tunneling microscopy (STM) and cyclic voltammetry. The potential-dependent structural change of the SAMs is important for the dynamical control of the surface properties of metals, such as the deposition of thiolate,1 wetting,2,3 shape of liquid lenses,4 and the structure of binary SAMs.5 It has been shown that the reductive desorption of the SAMs in an alkaline solution is useful not only for its characterization but also for controlling and designing its surface properties.5-20 Understanding the states of the thiol molecules after the * To whom correspondence should be addressed. Present address: Department of Energy and Hydrocarbon Chemistry, Kyoto University, Kyoto 606-8501, Japan. E-mail: kakiuchi@ scl.kyoto-u.ac.jp. Telephone, fax: +81-75-753-3360. † Present address: Department of Chemistry, Iowa State University, Ames, IA 50011. (1) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860-5862. (2) Abbott, N. L.; Whitesides, G. M. Langmuir 1994, 10, 1493-1497. (3) Abbott, N. L.; Gorman, C. B.; Whitesides, G. M. Langmuir 1995, 11, 16-18. (4) Gorman, C. B.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1995, 11, 2242-2246. (5) Imabayashi, S.; Hobara, D.; Kakiuchi, T.; Knoll, W. Langmuir 1997, 13, 4502-4504. (6) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335-359. (7) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687-2693. (8) Weisshaar, D. E.; Walczak, M. M.; Porter, M. D. Langmuir 1993, 9, 323-329. (9) Zhong, C.-J.; Porter, M. D. J. Am. Chem. Soc. 1994, 116, 1161611617. (10) Everett, W. R.; Welch, T. L.; Reed, L.; Fritsch-Faules, I. Anal. Chem. 1995, 67, 292-298. (11) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103-114. (12) Rojas, M. T.; Ko¨niger, R.; Stoddart, J. F.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 336-343. (13) Calvente, J. J.; Kova´cˇova´, Z.; Sanchez, M. D.; Andreu, R.; Fawcett, W. R. Langmuir 1996, 12, 5696-5703.
reduction and reoxidation is crucial for the precise control of the surface properties. The knowledge of the structural changes of SAMs by the reductive desorption and reoxidation processes is also important for the theoretical interpretation of cyclic voltammograms of the reductive desorption. Cyclic voltammetry is a powerful technique to study the reductive desorption of SAMs,6 because the peak potential and the shape of voltammograms are sensitive to the type and state of the thiol molecules in the monolayers.6,9,11,13,14,16-18,20 A typical cyclic voltammogram of a SAM shows a sharp peak corresponding to the reductive desorption of the thiols in the cathodic scan and a partial reoxidation peak in the anodic scan. The area and the shape of the reoxidation peak depend on the chain length of thiols and the pH of the electrolyte solution.6,16,18 It has been shown that a larger reoxidation peak appears for a longer chain n-alkanethiol6 and at a lower pH of the electrolyte solution.14 Such behavior has been explained by the difference in the solubilities and diffusion rates of the reduced molecules.6,14,18 The voltammetric technique, however, gives little information about the structural changes of the SAMs at a molecular level caused by the electrochemical reactions. Various in-situ techniques15,21-23 have been used to elucidate the structural changes of the SAMs accompanied (14) Yang, D.-F.; Wilde, C. P.; Morin, M. Langmuir 1996, 12, 65706577. (15) Yang, D.-F.; Al-Maznai, H.; Morin, M. J. Phys. Chem. B 1997, 101, 1158-1166. (16) Yang, D.-F.; Wilde, C. P.; Morin, M. Langmuir 1997, 13, 243249. (17) Zhong, C.-J.; Zak, J.; Porter, M. D. J. Electroanal. Chem. 1997, 421, 9-13. (18) Zhong, C.-J.; Porter, M. D. J. Electroanal. Chem. 1997, 425, 147-153. (19) Lamp, B. D.; Hobara, D.; Porter, M. D.; Niki, K.; Cotton, T. M. Langmuir 1997, 13, 736-741. (20) Imabayashi, S.; Iida, M.; Hobara, D.; Feng, Z. Q.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1997, 428, 33-38. (21) Schneider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391-12397. (22) Pan, J.; Tao, N.; Lindsay, S. M. Langmuir 1993, 9, 1556-1560.
S0743-7463(97)00985-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/02/1998
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by the reduction. An in-situ atomic force microscopy study of an octadecyl mercaptan monolayer in 100 mM NaClO4 showed no sign of a periodic molecular image at the positive potential after destroying the monolayer by applying the negative potential, indicating that the disintegration of the SAM is irreversible.22 An electrochemical quartz crystal microbalance study indicated that the readsorption occurred during the reverse scan of the applied potential in dimethylformamide.21 In an aqueous alkaline solution, ac-modulated reflectivity of a hexanethiol-adsorbed Au electrode also showed evidence of irreversible desorption after the reduction peak,20 while electrodeposited dodecanethiol monolayers retain a certain structure and exhibit the properties similar to those of their SAMs.1 The formation of micelles of nonanethiolates by the reductive desorption has been suggested from the in-situ vibrational study, which showed an intense CH stretching modes spectrum.15 In the present study, we examined the potential dependent morphological change of the SAMs using insitu STM for three different thiols, 1-hexadecanethiol (HDT), 1-propanethiol (PT), and 3-mercaptopropionic acid (MPA), having different solubility in the aqueous phase. Experimental Section Materials. HDT (Aldrich), PT (Tokyo Kasei Chemical Co.), and MPA (Dojin Chemical Laboratory) were used without further purification. All other chemicals used were of reagent grade. Water was distilled and purified with a Milli-Q system (Milli Pore Co.). Methods. The gold electrodes used in the present study were prepared by vacuum deposition of gold on mica sheets at less than 5 × 10-6 Torr. The mica was baked at 580 °C for more than 5 h prior to the deposition of gold and maintained at 580 °C during the deposition. The Au substrate was then annealed at 550 °C in a furnace for 6 h in ambient atmosphere before use. STM measurements of the gold film prepared in this way confirmed that the substrate mainly consisted of a (111) surface. The SAM was formed by immersing the gold film into a 1 mM ethanol solution of each thiol for at least 24 h. A gold substrate was then rinsed with ethanol and dried in air. In-situ STM and electrochemical measurements were carried out in a 0.5 M KOH aqueous solution with a silver/silver chloride electrode in a saturated potassium chloride solution and a platinum wire as a reference and a counter electrodes, respectively. Pt-Ir tips coated with Apiezon wax were used in STM measurements. All STM images were obtained in the constant current mode. The potential was changed from -0.2 to -1.2 V in a stepwise manner after taking two images at each potential. The time held at each potential was typically ∼6 min. The step width was 50 or 100 mV in the double layer region of the cyclic voltammograms, while the potential was changed by 10 mV in the region where the peak for the reductive desorption of the thiols was observed. The tip potential was held at +50 mV against the reference electrode throughout a measurement. All cyclic voltammograms were taken at the scan rate being 5 mV s-1 with the initial potential of -0.2 V using a cone-shaped cell.6
Results and Discussion Figure 1 shows cyclic voltammograms for the reductive desorption of MPA (curve A), PT (curve B), and HDT (curve C) at a scan rate of 5 mV s-1. The reduction peaks of the adsorbed thiols appeared at -0.63, -0.73, and -1.05 V for MPA, PT, and HDT, respectively. In parts A and B of Figure 1, a small broad peak also appeared at ∼ -0.9 V. A similar peak, aside from the main reduction peak, has been reported for HS(CH2)6CH3 SAM by Porter et al.18 They suggested that the peaks are associated with either the presence of gold-bound atomic and/or oligomeric (23) Kabasawa, A.; Matsuda, N.; Sawaguchi, T.; Matsue, T.; Uchida, I. Denki Kagaku 1992, 60, 986-991.
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Figure 1. Cyclic voltammograms for the reductive desorption of MPA (A), PT (B), and HDT (C) measured at 5 mV s-1. Labels (a-d or a-f) indicate the potentials where STM images were taken and shown in Figures 2-4. The dashed curve represent the second scan.
sulfur species,8 desorption from step sites,11 or processes following the reductive cleavage of the Au-S bond. In the reverse scan, the peak corresponding to the reoxidation of the thiol appeared at -0.95 V in the case of HDT. On the other hand, no corresponding peak was observed for MPA, while a very small peak appeared for PT at ∼ -0.6 V. In the second cycle of the voltammogram (Figure 1A, dashed line), a large reduction peak at -0.63 V was not observed. A small peak at -0.53 V is probably ascribed to the reduction of a small amount of reoxidized MPA. In the present study, we observed no splitting of the reductive and oxidative peaks for HDT, which have been recently reported for alkanethiols that have more than 11 carbons in the alkyl chain.16,18 To investigate the morphological changes in the SAMs, in-situ STM measurements were carried out at different potentials along a cyclic voltammogram. Figure 2 shows the in-situ STM images (150 nm × 150 nm) of the SAM of MPA measured when the potential of the Au substrate was held at -0.2 (Figure 2a), -0.5 (Figure 2b), -0.7 (Figure 2c), and -0.2 V (Figure 2d). All images were taken for the same area, as can be seen from the shape of the Au terraces in Figure 2. The images shown in parts a and b of Figure 2 at -0.2 and -0.5 V, which are more positive than the peak potential in the voltammogram (curve A), show Au terraces with many pits, which are characteristic of thiol SAMs.24-28 These images are similar to those observed for the STM images of alkanethiol SAMs taken in air. On the other hand, the pits disappeared in the STM image (Figure 2c) when the potential was brought to -0.7 V, which is more negative than the reduction potential. The image shows flat gold terraces with a few small islands. In addition, the shape of the step edge of the gold terrace showed indentations that were not (24) Sun, L.; Crooks, R. M. J. Electrochem. Soc. 1991, 138, L23-L25. (25) Kim, Y.-T.; Bard, A. J. Langmuir 1992, 8, 1096-1102. (26) Sondag-Huethorst, J. A. M.; Scho¨nenberger, C.; Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6826-6834. (27) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383-3386. (28) McDermott, C. A.; McDermott, M. T.; Green, J.-B.; Porter, M. D. J. Phys. Chem. 1995, 99, 13257-13267.
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Figure 2. In-situ STM images of the SAM of MPA measured at -0.2 (a), -0.5 (b), -0.7 (c), and -0.2 V (d). Scan size was 150 nm × 150 nm, and setpoint was 380 pA.
observed in parts a and b of Figure 2. The image remained unchanged when the potential was brought back to -0.2 V (Figure 2d). The result in Figure 1A and the disappearance of the pits in the STM image suggest the escape of MPA molecules from the surface. The indentations of the step edges of the gold terraces imply that the pits migrated to edges and merged into descending step edges after the desorption of MPA molecules. A reduced MPA molecule probably exists as a divalent anion and is highly soluble in the aqueous phase, because pKa of the thiol group is ∼10.29 MPA is therefore unlikely to form micelles. Apparently, MPA molecules dissolve in the aqueous phase after the reduction and diffuse away during the STM measurement. This is in agreement with the absence of the reoxidation peak in the reverse scan and a disappearance of the large reduction peak in the second cycle (Figure 1A). The in-situ STM images of a 150 nm × 150 nm area of the SAM of PT are shown in parts a-f of Figure 3. Crosssectional profiles along the line drawn on the STM images are also shown in parts g-i of Figure 3. In the images in parts a-e of Figure 3, two defects of the Au substrate (bottom left and top left) can be taken as markers for showing that the scanned area remained unchanged during the recording of these five images (Figure 3a-e). The imaged area shifted due to the drift of the tip in Figure 3f. Figure 3a shows a STM image at -0.2 V. It can be seen that the pits are distributed on the terrace. As shown in Figure 3b-d, the image gradually varied as the potential was changed from -0.66 to -0.68 V at a 0.01 V interval in the region of the rising foot of the reduction peak in the voltammogram (curve A, Figure 1). At -0.66 V, some (29) Crampton, M. R. The Chemistry of the Thiol Group: Part I; Wiley: New York, 1974; p 398.
aggregates whose size ranged from 10 to 20 nm started to appear especially around the gold defects indicated by arrows in Figure 3b. This suggests that the potential on the Au surface is not uniform around the defects, or the irregularity in the monolayer structure exists around the defects; the ionic permeability may be higher than the ordered regions of the SAM and the thiol can be reduced more easily. When the potential was brought to -0.67 V, the size of the aggregates that already appeared in Figure 3b increased (Figure 3c), while the height of the aggregates almost remained constant. In addition, new aggregates appeared in the vicinity of the existent aggregates. They continued to grow as the applied potential became more negative (-0.68 V, Figure 3d). The image in Figure 3d consists of two types of regions, the aggregated region and the relatively flat region where the original structure of the SAM is apparently maintained. New aggregates thus tend to form at the peripheries of the existent aggregates where the ionic permeability would be higher. The cross-sectional profiles (Figure 3g-i) along the line drawn on the STM images (Figure 3b-d) also show the appearance and the growth of the aggregates. The maximum apparent height of the aggregates is ∼0.5 nm. The surface was fully covered by the aggregates at ∼-0.8 V and the image was almost unchanged at a more negative potential than -0.8 V. The image taken at -1.0 V (Figure 3e) showed the aggregates whose size is ∼15 nm across. The aggregate formation observed in Figure 3b-e agrees with chronoamperometric results,13,30 which suggested that the reduction of thiols follows a nucleation and growth process.31 Unlike the case of MPA, these aggregates stayed at the surface after the potential was brought back to -0.2 V. The size of the aggregates further grew, but no (30) Yang, D.-F.; Morin, M. J. Electroanal. Chem. 1997, 429, 1-5.
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Figure 3. In-situ STM images of the SAM of PT measured at -0.2 (a), -0.66 (b), -0.67 (c), -0.68 (d), -1.0 (e), and -0.2 V (f). Scan size was 150 nm × 150 nm, and setpoint was 350 pA. Cross-sectional profiles (g through i) along the line drawn on the STM images (b through d).
pits characteristic of thiol SAMs were regenerated (Figure 3f). It is unlikely that the PT anions having a short alkyl chain form micelles,32 although the image shows ag(31) Fleischman, M.; Thirsk, M. In Advances in Electrochemistry and Electrochemical Engineering; Delahay, P., Ed.; Wiley-Interscience: New York, 1963; Vol. 3. (32) Tanford, C. The Hydrophobic Effect; Wiley: New York, 1973.
gregates on the surface. The nature of the aggregates is not clear at this moment. The in-situ STM images of the area of 100 nm × 100 nm of the SAM of HDT and the cross-sectional profiles of the images are shown in Figure 4. The images were taken at the same area except for Figure 4d, which was drifted by ∼ 40 nm toward the right direction. The observed pits
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Figure 4. In-situ STM images of the SAM of HDT measured at -0.2 (a), -1.0 (b), -1.2 (c), and -0.2 V (d). Scan size was 100 nm × 100 nm, and setpoint was 350 pA. Cross-sectional profiles (e and f) are along the line drawn on the STM images (b and c).
in the image at -0.2 V (Figure 4a) were distorted. This distortion is probably ascribed to the interaction between the STM tip and the SAM. For a SAM composed of longchain alkanethiols, the imaging at a nanoampere-level tunneling current causes the disruption of the SAM.25,33 When the potential was set to -1.0 V, the aggregates whose size ranged from 2 to 5 nm appeared and the pits disappeared (Figure 4b). The cross-sectional profile in Figure 4e shows that the apparent height of the aggregates ranges from 0.3 to 0.8 nm, showing greater size of aggregates than those for the PT desorption. The peak potential in the cyclic voltammogram depends on the scan rate. The peak potential shifts to the positive direction (33) Ross, C. B.; Sun, L.; Crooks, R. M. Langmuir 1993, 9, 632-636.
with decreasing scan rate. It seems therefore that the reduction of HDT already occurred at -1.0 V (Figure 4b), which is ∼30 mV positive of the beginning of the reduction peak in Figure 1C. The change in the image was instantaneous and the aggregates appeared all over the surface, unlike the case of PT desorption where the aggregates start to appear at around the Au defects and gradually grew with the applied potential. This suggests that the phase transition of the HDT SAM caused by the reduction of the thiols is sharper than that of PT. Indeed, Figure 1 shows that the peak for the reduction of HDT is significantly sharper (pwhm ) 20 mV), while that of PT is broader (pwhm ) 60 mV). The size of the aggregates is smaller than that of PT, and its number is greater than that of PT, indicating a more abrupt nucleation and growth
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Figure 5. Cyclic voltammograms for the reductive desorption of HDT: the second (a), the third (b), and the fourth (c) cycles of the voltammogram. Scan rate was 5 mV s-1.
in the HDT desorption. Both the size and the height of the aggregates increased at -1.2 V as shown, in parts c and f of Figure 4. The apparent height of the aggregates ranges from 0.8 to 2.0 nm, which is about twice greater than that of HDT at -1.0 V. This is different from the case of PT, where the height of the aggregates remained constant but the size increased as the potential became more negative. In contrast to the case of PT and MPA, the pits were regenerated when the potential was brought back to -0.2 V (Figure 4d), indicating the partial recovery of the SAM. The reoxidation peak area in the voltammogram represents the extent of the re-adsorbed molecules, which were desorbed in the cathodic scan, as reported previously.6,14,16,18 The image in Figure 4d is consistent with the reoxidation peak that appeared in the cyclic voltammogram in Figure 1C. Although the aggregates were observed for PT and HDT, the regeneration of SAM took place only for HDT, which suggests that the states of the molecules after the potential was bought
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back to -0.2 V depend on the molecular organizations formed by the reduction. It is unlikely that anionic micelles composed of HDT have more facile associationdissociation kinetics than micelles of PT. One possible explanation of the partial recovery for the HDT SAM is that on the Au surface certain desorbed fragments of the SAM retain its monolayer structure through the strong lateral interaction and the original structure of the SAM is more easily restored. To confirm the partial recovery and to investigate the reversibility of the re-formation of the HDT monolayer with potential cycling, a multiple cycling experiment was carried out. Figure 5 shows the second (Figure 5a), third (Figure 5b), and fourth (Figure 5c) cycles, following the first cycle shown in Figure 1C. The gradual decreases in both the reduction and reoxidation peak areas in the voltammograms to about 70% of the first scan (Figure 5) indicate the limited reversibility in the monolayer recovery. It has been reported that about 90% of the HDT monolayer is still present after several voltammetric cycles at 20 mV s-1 using a Au single-crystal electrode.16 The origin of the difference in the way of decreasing the peak is not clear. But the preparation of the Au substrate16 and the scan rate of the voltammetry would influence the desorption-readsorption characteristics.7,13 For example, rough surfaces would be less favorable for the lateral interaction of adsorbed HDT molecules, giving rise to the greater irreversibility in the monolayer re-formation. Figure 6 shows ex-situ STM images taken after one cycle (Figure 6a) and four cycles (Figure 6b) of the applied potential between -0.2 and -1.15 V. After the first cycle, the aggregates whose size ranged from ∼ 4 to ∼10 nm newly appeared in the STM image, together with the pits that existed before the cycling as shown in Figure 6a. On the other hand, the pits almost disappeared and many islands, whose size ranged from ∼3 nm to ∼12 nm, appeared with lower contrast than the aggregates after four cycles (Figure 6b). These STM images indicate the gradual disintegration of the SAM by repeating the
Figure 6. STM images of the SAM of HDT taken in air after the first cycle (a) and after the fourth cycle (b) of the voltammogram. Scan size was 150 nm × 150 nm, bias voltage 1.5 V, and setpoint 12 pA.
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potential cycling, which is consistent with the cyclic voltammogram in Figure 5.
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difference in the different thiol molecules at the surfaces after the desorption should be taken into account for better control of the state of SAMs.
Conclusion Cyclic voltammetry and electrochemical in-situ STM imaging of the thiol SAMs have revealed that MPA dissolves in the electrolyte solution after the reduction, giving no readsorption peak in an anodic scan. In contrast, HDT remains on the Au surfaces after the reduction and the monolayer partially recovered upon the reoxidation. PT remained as aggregates even after the potential was brought back to a positive value. The morphological
Acknowledgment. This work was partially supported by New Energy and Industrial Technology Development Organization (NEDO), Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists (D.H.), the Saneyoshi Scholarship Foundation (S.I.), and Grant-in-Aid for Scientific Research (09875208) from the Ministry of Education, Science and Culture, Japan (T.K.). LA9709857
