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Voltammetric Detection of the Surface Diffusion of Adsorbed Thiolate Molecules in Artificially Phase-Separated Binary Self-Assembled Monolayers on a Au(111) Surface Shin-ichiro Imabayashi,† Daisuke Hobara,‡ and Takashi Kakiuchi*,‡ Department of Chemistry and Biotechnology, Faculty of Engineering, Yokohama National University, Yokohama 240-8501, Japan, and Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan Received October 27, 2000. In Final Form: January 22, 2001 Surface diffusion of adsorbed alkanethiolate molecules forming a binary self-assembled monolayer (SAM) on Au(111) has been studied by monitoring the change in the shape of voltammograms for the reductive desorption of adsorbed thiolates. Artificially phase-separated SAMs of undecanethiol (UDT) and mercaptoundecanoic acid (MUA), which would form homogeneously mixed binary SAMs at the thermodynamic adsorption equilibrium, are prepared starting from the phase-separated binary SAMs composed of mercaptopropionic acid (MPA) and UDT, followed by the electrochemical partial desorption of the MPA domains and successive adsorption of MUA from ethanol to the domains originally occupied by MPA molecules. The mutual dissolution of UDT and MUA on the gold surface is monitored as the gradual merging of the initially well-separated two peaks on the voltammogram of reductive desorption of the thiolates. The average diffusion coefficient of adsorbed UDT and MUA at 60 °C in water is estimated to be about 10-18 cm2 s-1.
1. Introduction The rate of surface diffusion of adsorbed alkanethiolates on metal surfaces has been studied from several different aspects. Experimental studies of the diffusion of adsorbed thiolates in single-component self-assembled monolayers (SAMs) has mainly utilized the pit-like defects that are formed in the course of the formation of thiol SAMs.1-5 Time-dependent scanning tunneling microscopy (STM) imaging revealed that the coalescence of pits was caused by the surface diffusion of adsorbed alkanethiolate molecules at SAMs on gold substrates.6-10 For dodecanethiol SAMs on Au(111), Scho¨nenberger et al.8 and Arvia et al.10 estimated the average surface diffusion rates being of the order of 10-17 cm2 s-1 by coalescence of pits. On the other hand, the mutual surface diffusion of two different adsorbed species is of particular interest for practical applications of SAM-modified metal surfaces in that the surface diffusion can ultimately determine the resolution of the patterning11 and also the stability of artificially * To whom correspondence should be addressed. Tel: (81)75-753-5528. Fax: (81)-75-753-3360. E-mail: kakiuchi@ scl.kyoto-u.ac.jp. † Yokohama National University. ‡ Kyoto University. (1) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, p 143. (2) Scho¨nenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611-614. (3) Edinger, K.; Golzha¨user, A.; Demota, K.; Wo¨ll, Ch.; Grunze, M. Langmuir 1993, 9, 4-8. (4) McDermott, C. A.; McDermott, M. T.; Green, J. B.; Porter, M. D. J. Phys. Chem. 1995, 99, 13257-13267. (5) Poirier, G. E. Langmuir 1997, 13, 2019-2026. (6) Delamarche, E.; Michel, B.; Kang, H.; Gerber, C. Langmuir 1994, 10, 4103-4108. (7) McCarley, R. L.; Dunaway, D. J.; Willicut, R. J. Langmuir 1993, 9, 2775-2777. (8) Scho¨nenberger, C.; Jorritsma, J.; Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. J. Phys. Chem. 1995, 99, 3259-3271. (9) Arce, F. T.; Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. Langmuir 1998, 14, 7203-7212. (10) Arce, F. T.; Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. Electrochim. Acta 1998, 44, 1053-1067.
prepared two- or multicomponent SAMs such as a molecular wire embedded in SAMs.12,13 In this respect, it seems to be useful to have a means to estimate the rate of the surface diffusion of thiolates on a metal surface. The phase-separated binary SAMs, in which there exist sizable domains predominantly composed of either thiol, exhibit two well-separated peaks in a voltammogram of the reductive desorption of the SAM. The position of each peak was close to the corresponding peak potential, Ep, for the reductive desorption of singlecomponent SAMs of the two thiols.14,19 For the homogeneously mixed SAMs, on the other hand, only one peak appeared at a potential between the two Ep’s for the corresponding single-component SAMs.15,22 In the present paper, we report a simple electrochemical method that utilizes the shape of voltammograms for the reductive desorption of adsorbed thiolates to monitor the course of very slow surface diffusion processes. We used artificially phase-separated SAMs of 1-undecanethiol (UDT) and 11mercaptoundecanoic acid (MUA) on Au(111), which would form a homogeneously mixed SAM if prepared by coadsorption from a solution. The change in the mixing state from the phase-separated SAM to the homogeneously mixed SAM while the substrate was kept in water at 60 °C was followed by monitoring the change in the shape of voltammograms for the reductive desorption of adsorbed thiolates in 0.5 mol dm-3 KOH. (11) Delamarche, E.; Schmid, H.; Bietsch, A.; Larsen, N. B.; Rothuizen, H.; Michel, B.; Biebuyck, H. J. Phys. Chem. B 1998, 102, 3324-3334. (12) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705-1707. (13) Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am. Chem. Soc. 1998, 120, 2721-2732. (14) Hobara, D.; Ota, M.; Imabayashi, S.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1998, 444, 113-119. (15) Kakiuchi, T.; Iida, M.; Mae, N.; Hobara, D.; Imabayashi, S.; Niki, K. In International Forum on New Frontiers in Functional Organic Nanomaterials; 1998; pp 120-125.
10.1021/la001516z CCC: $20.00 © 2001 American Chemical Society Published on Web 04/24/2001
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2. Experimental Section 3-Mercaptopropionic acid (MPA) and UDT were purchased from Dojindo Laboratory Co. and Aldrich, respectively, and were used as received. MUA was synthesized from 11-bromoundecanoic acid.16 Water was distilled and purified with a Milli-Q system (Millipore Co.). All other chemicals were of reagent grade and used without further purification. Gold substrates were prepared by vapor deposition of gold (99.99% purity) onto freshly cleaved mica.17 Homogeneously mixed SAMs of UDT and MUA were prepared by immersing gold substrates in an ethanol solution of UDT-MUA 1:1 mixture overnight. Artificially phaseseparated mixed SAMs of UDT and MUA were prepared through the selective replacement of MPA with MUA in the UDT-MPA binary SAMs initially formed by coadsorption of the two thiols, as described previously.18,24 The mixing ratio of the two thiols in those mixed SAMs was controlled to be 1 by adjusting the composition of a mixed-thiol solution.14,15,18 Five binary SAMadsorbed substrates thus prepared simultaneously were kept in capped wide-mouth bottles (50 mL) filled with purified water. The bottles were kept at 60 °C. The change in the phase properties of the binary SAMs in the course of the equilibration of the mixing was followed by measuring cyclic voltammograms (CVs) for the reductive desorption of the SAMs20 after taking out one of the substrates from the bottle. A SAM-adsorbed gold substrate was mounted at the bottom of a cone-shaped cell using an elastic O-ring whose diameter was 8 mm. All CVs for the reductive desorption of the binary SAM were measured at 20 mV s-1 in deaerated 0.5 mol dm-3 KOH at 20 ( 2 °C. The potential was referred to a Ag|AgCl| saturated KCl electrode.
3. Results and Discussion Figure 1 shows CVs for the reductive desorption of the phase-separated binary SAMs of UDT and MUA as a function of the annealing time at 60 °C in water. Before annealing (t ) 0), two peaks appeared at -0.91 V (peak I) and -1.03 V (peak II) that correspond to the reductive desorption of MUA-rich and UDT-rich domains in the SAM, respectively (curve a in Figure 1). From the area of the two peaks, the mixing ratio of the two thiols was about 1:1, as designed. After 186 h, peak II remained at -1.03 V but its area decreased (curve b), whereas peak I at -0.91 V shifted to -0.96 V. After 212 h in water, peak I grew further and concomitantly peak II became smaller (curve c). Finally, the two peaks merged into a single peak at -1.01 V after 355 h (curve d). The decrease in the total area of the CV peaks was not observed after annealing, indicating the absence of desorption of thiol molecules caused by annealing. We note that among several measurements repeated for different sets of samples, a few substrates gave unexpected voltammograms in that a reduction peak appeared at a more positive potential, about -0.65 V, than that corresponding to the desorption of MUA, and the gradually merging two peaks at the more negative potentials diminished their heights compared with the peaks in Figure 1. This clearly suggests that in those (16) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (17) Imabayashi, S.; Iida, M.; Hobara, D.; Feng, Z. Q.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1997, 428, 33-38. (18) Imabayashi, S.; Hobara, D.; Kakiuchi, T.; Knoll, W. Langmuir 1997, 13, 4502-4504. (19) Kakiuchi, T.; Sato, K.; Iida, M.; Hobara, D.; Imabayashi, S.; Niki, K. Langmuir 2000, 16, 7238-7244. (20) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335-359. (21) Arnold, S.; Feng, Z. Q.; Kakiuchi, T.; Knoll, W.; Niki, K. J. Electroanal. Chem. 1997, 438, 91-97. (22) Kakiuchi, T.; Iida, M.; Gon, N.; Hobara, D.; Imabayashi, S.; Niki, K. Langmuir 2001, 17, 1599-1603. (23) Somorjai, A. G. Introduction to Surface Chemistry and Catalysis; Wiley Inter-science: New York, 1994. (24) Hobara, D.; Sasaki, T.; Imabayashi, S.; Kakiuchi, T. Langmuir 1999, 15, 5073-5078.
Figure 1. Cyclic voltammograms for the reductive desorption of artificially prepared phase-separated binary SAMs of UDT and MUA measured at 20 mV s-1 in 0.5 mol dm-3 KOH solution. The SAM-adsorbed gold substrate was annealed at 60 °C in pure water for 0 (a), 186 (b), 212 (c), and 355 h (d). The surface mole fraction of UDT is ca. 0.5, the initial potential is -0.2 V, and the electrode area is 0.5 cm2.
samples a certain portion of adsorbed thiolates became less stable. This interesting, but irreproducible, problem is left for future studies. In contrast, CVs for the reductive desorption of the homogeneously mixed SAMs of UDT and MUA prepared by the coadsorption of the two thiols varied little with the annealing time, as shown in Figure 2. A single peak with a constant area stayed at -1.01 V for the annealing time over 353 h. In the case of homogeneously mixed twocomponent SAMs, there always appears a single peak at the potential between the two peak potentials for the two single-components SAMs of the corresponding thiols. The peak potential shifts with the surface ratio of the two components and hence is a rough measure of the surface composition.15,21,22 The position of the peak for the singlecomponent SAM is -0.92 ( 0.01 V for MUA and -1.07 ( 0.01 V for UDT.17 The agreement of the Ep values, -1.01 V, for curve d in Figure 1 and those in Figure 2 therefore suggests that the surface ratio of UDT and MUA is about 1:1 after 355 h of equilibration. The above results clearly indicate that the change in CVs with time reflects the change in the mixing state from the phase-separated SAM to the homogeneously mixed SAM, the latter of which is thermodynamically stable for the binary SAMs of UDT and MUA. The STM image of artificially phase-separated binary SAMs of UDT and MUA revealed that the two thiols form domains whose size was greater than 30 nm in diameter.24 The number and size of the domains that consist predominantly of
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Figure 3. Cyclic voltammograms for the reductive desorption of artificially prepared phase-separated binary SAMs of UDT and MUA measured at 20 mV s-1 in 0.5 mol dm-3 KOH solution. The SAM-adsorbed gold substrate was annealed at 40 °C in pure water for 333 h (a) or at 60 °C in air for 600 h (b). The surface mole fraction of UDT is ca. 0.5, the initial potential is -0.2 V, and the electrode area is 0.5 cm2.
Figure 2. Cyclic voltammograms for the reductive desorption of homogeneously mixed binary SAMs of UDT and MUA measured at 20 mV s-1 in 0.5 mol dm-3 KOH solution. The SAM-adsorbed gold substrate was annealed at 60 °C in pure water for 0 (a), 200 (b), and 353 h (c). The surface mole fraction of UDT is ca. 0.5, the initial potential is -0.2 V, and the electrode area is 0.5 cm2.
either of the thiols in the initial artificially phase-separated SAM are reduced by mutual dissolution of thiols with time, resulting in the increase in the homogeneously mixed region. In the case of the intrinsically phase-separated mixed SAMs composed of hexadecanethiol and MPA, the domains whose area is less than 15 nm2 are not recognized as an independent macroscopic phase, as they behave as if they are homogeneously mixed at the molecular level.14 The shift in the peak potential was accompanied by the decrease in the half-width of the desorption peak for the homogeneously mixed regions (curve d in Figure 1), indicating that both the average composition and the homogeneity of mixing in these regions simultaneously vary by the mutual dissolution of thiols. The single desorption peak at -1.01 V observed after 355 h of annealing indicates that the intermixing of thiols occurred over the entire surface of the substrate. The half-width of the peak, however, is still 1.4 times larger than that for the corresponding homogeneously mixed SAM, suggesting the contribution of several domains having different compositions to this peak. The change in the mixing state in the present work is likely to be caused by the lateral diffusion of thiol molecules on the Au surface, as the substrates were in contact with water. Figure 3a represents a CV for the reductive desorption of the phase-separated binary SAM after annealing for 333 h in water at 40 °C. The CV exhibits two peaks at -0.92 and -1.02 V and is analogous to the CV
of the binary SAM annealed for 186 h in water at 60 °C shown in curve b in Figure 1, although the peaks are relatively broader in the former. This means that a longer time is required to reach a similar mixing state at a lower annealing temperature, as expected for the surface diffusion.23 The surface diffusion of MUA and UDT also occurs in air as shown in Figure 3b. After annealing for 600 h at 60 °C in air, only one peak appeared at -1.01 V in the CV for the reductive desorption of the phaseseparated binary SAM of UDT and MUA. For surface diffusion on a plane via a site-to-site hopping process, the diffusion coefficient, D, can be evaluated from the mean-square traveling distance of diffusing molecules, 〈x2〉, and the traveling time, t, using the following equation:23
D ) 〈x2〉/4t It seems that more than 300 h as required for the appearance of curve d in Figure 1 reflects the time for the largest domains to reach the homogeneously mixed state, because the shape and size of thiol domains are not uniform. Assuming that x and t are 30 nm and 300 h, respectively, D is roughly estimated to be 10-18 cm2 s-1 at 60 °C. Weiss et al. found the rate to be lower than 1 × 10-17 cm2 s-1 for the surface diffusion of thiolate on the large Au(111) terraces from coalescence of initially isolated domains of CH3(CH2)15SH in the mixed SAM with CH3O2C(CH2)15SH.25 Scho¨nenberger et al. observed that holes in a dodecanethiol SAM coalesced at the rate of ≈0.5-1 nm/min at 90 °C, which corresponds to 1-4 × 10-17 cm2 s-1.8 Arvia and co-workers also estimated the value of (3 ( 2) × 10-17 cm2 s-1 for the average surface diffusion coefficient of a single pit in a dodecanethiol SAM on Au(111).10 Compared with these reported values obtained by the sequential STM imaging of single- or (25) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636-7646.
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mixed-alkanethiol SAMs, our D value is about an order of magnitude lower. From the phase separation of thiols in the homogeneously mixed SAMs prepared from the asymmetric alkyl fluoroalkyl disulfides, on the other hand, the D at 100 °C was estimated to be of the order of 10-14 cm2 s-1.26 The presence of a strong lateral interaction between alkyl chains of adsorbed thiols is one of the reasons for the slow surface diffusion of thiols in SAMs.10 In binary SAMs, the molecular interaction between an unlike pair is not necessarily equal to that between a like pair, explaining the difference in the diffusion rate between component thiols. As described above, the MUA-rich domains disappeared at 186 h of annealing in the binary SAM with the mixing ratio of 1, whereas the UDT-rich domains still remained at 212 h of annealing. This suggests that the intermixed region of the two thiols more easily expands into the MUA-rich domains than into the UDT-rich domains. In the phase-separated binary SAMs of UDT and MUA, the greater extent of the dissolution of UDT into the domain mainly composed of MUA was suggested from the larger shift of the Ep of MUA-rich domains against that of a single-component MUA SAM than the corresponding shift for UDT-rich domains.24 This difference in the mutual solubility between UDT and MUA is in parallel with the intermixing behavior observed in the present system. It is likely that the diffusion of adsorbed thiolates on a gold surface is accompanied with the movement of gold atoms underneath the SAM.27,28 The change in the number and size of the pits seen in annealing of the SAMs29,30 can (26) Ishida, T.; Yamamoto, S.; Mizutani, W.; Motomatsu, M.; Tokumoto, H.; Hokari, H.; Azehara, H.; Fujihira, M. Langmuir 1997, 13, 3261-3265.
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also affect the rate of the diffusion process. The D value estimated above is then to be understood as the average of the ease of movement of a UDT-gold complex and that of a MUA-gold complex and may involve the effect of the global change in the SAM-covered gold surface with time. The present study has demonstrated that the mixing state of artificially phase-separated binary SAMs is gradually equilibrated at elevated temperatures. In nanometer-scale patterns fabricated by microcontact printing31 and other methods,18,32-35 care must be exercised about the stability of two-component systems against the slow but steady lateral diffusion on metal surfaces. Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research (No. 08640769 for T.K.), a Grant-in-Aid for Exploratory Research (No. 09875208 for T.K.), and Grants-in-Aid for Scientific Research on Priority Areas of “Electrochemistry of Ordered Interfaces” (No. 11118235 for T.K. and No. 11118229 for S.I.) from the Ministry of Education, Science, Sports and Culture, Japan. LA001516Z (27) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 1096610970. (28) Stranick, S. J.; Parikh, A. N.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 11136-11142. (29) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145-1149. (30) Poirier, G. E. Langmuir 1999, 15, 1167-1175. (31) Kumar, A.; Abbott, N. L.; Kim, E.; Biebuyck, H. A.; Whitesides, G. M. Acc. Chem. Res. 1995, 28, 219-226. (32) Lopez, G. P.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1513-1516. (33) Abbott, N. L.; Folkers, J. P.; Whitesides, G. M. Science 1992, 257, 1380-1382. (34) Ross, C. B.; Sun, L.; Crooks, R. M. Langmuir 1993, 9, 632-636. (35) Xu, S.; Liu, G. Langmuir 1997, 13, 127-129.
