Electrodesorption Kinetics and Molecular Interactions at Negatively

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Electrodesorption Kinetics and Molecular Interactions at Negatively Charged Self-Assembled Thiol Monolayers in Electrolyte Solutions O. Azzaroni,† M. E. Vela,† H. Martin,‡ A. Hernańdez Creus,‡ G. Andreasen,† and R. C. Salvarezza*,† Instituto de Investigaciones Fisicoquı´micas Teo´ ricas y Aplicadas (INIFTA), Universidad Nacional de La Plata - CIC - CONICET, Sucursal 4, Casilla de Correo 16, (1900) La Plata, Argentina, and Departamento de Quı´mica-Fı´sica, Facultad de Ciencias, Universidad de La Laguna, Spain Received January 5, 2001. In Final Form: May 11, 2001 The electrodesorption of self-assembled monolayers of methyl-terminated alkanethiols, 11-mercaptoundecanonic acid, and 3-mercaptopropionic acid adsorbed on Au(111) has been investigated by scanning tunneling microscopy and electrochemical methods. Results show that 11-mercaptoundecanonic acid and 3-mercaptopropionic acid form x3×x3 R30° and c(4×2) lattices similar to those found for methyl-terminated thiols. For both methyl- and COO--terminated alkanethiols, the electrodesorption process follows a combined adsorption-desorption nucleation and growth-of-holes model under charge-transfer control. The nucleation rate of holes at COO--terminated alkanethiol adlayers is smaller than at methyl-terminated alkanethiol adlayers, which explains the slower electrodesorption kinetics. The activation energy to remove a COO-terminated alkanethiol molecule from the edge of a growing hole is smaller than that of a methyl-terminated molecule because of the repulsive forces introduced by the negatively charged groups. The magnitude of these repulsive forces, which is partially compensated for by ion binding between the COO- groups and the counterions in the electrolyte, depends on both the nature of the cation and the pH.

Introduction Self-assembled monolayers (SAMs) of thiols on metals have attracted considerable scientific interest because they provide a method for creating well-defined surfaces with controllable chemical functionality.1,2 The possible application of SAMs ranges from nanotechnology to fundamental surface science. SAMs can be used to prevent corrosion,3-5 to modify wetting and wear properties6-8 of solid surfaces, to develop nanodevices for electronics,9 and for pattern formation.10-12 In particular, by anchoring specific chemical groups to these molecular self-assemblies, well-ordered structures with controlled chemical features can be achieved to be used in molecular recognition,13 protein adsorption,14 and templates for crystallization of inorganic salts.15 SAM stability becomes a crucial point in all these potential applications. SAM * Corresponding author. e-mail: [email protected]. † Universidad Nacional de La Plata. ‡ Universidad de La Laguna. (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, pp 109335. (3) Jennings, G. K.; Munro, J. C.; Yong, T.-H.; Laibinis, P. E. Langmuir 1998, 14, 6130. (4) Zamborini, F. P.; Crooks, R. M. Langmuir 1998, 14, 3779. (5) Boubour, E.; Lennox, R. B. Langmuir 2000, 16, 4222. (6) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (7) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990. (8) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (9) Haag, R.; Rampi, M. A.; Holmlin R. E.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 7895. (10) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (11) Moffat, T. P.; Yang, H. J. Electrochem. Soc. 1995, 142, L220. (12) Xia, Y.; Kim, E.; Mrksich, M.; Whitesides, G. M. Chem. Mater. 1996, 8, 601. (13) Chailapakul, O.; Crooks, R. M. Langmuir 1993, 9, 884. (14) Kidoaki, S.; Matsuda, T. Langmuir 1999, 15, 7639.

stability depends on the S-Au bond, lateral interactions among the hydrocarbon chains, and hydrocarbon chainenvironment interactions. In aqueous solutions, an environment in which these systems have potential applications,2 reductive electrodesorption has been used to explore SAM stability.16,17 In fact, SAMs on Ag(111)18 and Au(111)19 are desorbed in sharp voltammetric peaks whose peak potentials (Ep) shift in the negative direction as the length (n) of the hydrocarbon chains,17 given in C units, of alkanethiol molecules increases. On the basis of the shift in Ep, stabilizing forces acting in x3 ×x3 R30° and related superlattices of alkanethiols adsorbed on Ag(111) and Au(111) in contact with aqueous solutions have been estimated in ≈2.5-4 kJ/mol of C units.20 This energy difference involves van der Waals and hydrophobic forces, both stabilizing SAMs. It is well-known that, by functionalizing the terminal group of alkanethiol molecules with carboxylic (COOH) and amine (NH2) groups, charged templates similar to a Langmuir film can be synthesized. In electrolyte solutions interactions between these groups and counterions in the electrolyte should affect the packing, ordering, and stability of SAMs.21 Carboxylate-terminated alkanethiol monolayers assembled onto Au electrodes have shown considerable sensitivity and permselectivity that can be (15) Ku¨ther, J.; Seshadri, R.; Nelles, G.; Assenmacher, W.; Butt, H.J.; Mader, W.; Tremel, W. Chem. Mater. 1999, 11, 1317. (16) Walczak, M. M.; Alves C. A.; Lamp B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (17) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (18) Hatchett, D. W.; Stevenson, K. J.; Lacy, W. B.; Harris, J. M.; White, H. S. J. Am. Chem. Soc. 1997, 119, 6596. (19) Hobara, D.; Sasaki, T.; Imabayashi, S.; Kakiuchi, T. Langmuir 1999, 15, 5073. (20) Hatchett D. W.; Uibel, R. H.; Stevenson, K. J.; Harris, J. M.; White, H. S. J. Am. Chem. Soc. 1998, 120, 1062. (21) Lin, J.; Liang, K. S.; Scoles, G.; Ulman, A. Langmuir 1995, 11, 4418.

10.1021/la010019v CCC: $20.00 © 2001 American Chemical Society Published on Web 09/13/2001

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modulated by changing pH and electrolyte composition.22,23 Recently, it has been reported that electrodesorption curves of COO--terminated SAMs are displaced to more positive potentials than those obtained for the corresponding methyl-terminated alkanethiols.24 These results suggest that repulsive interactions between the negatively charged groups make these SAMs less stable. However, this effect should be partially compensated for by counterions in the electrolyte. In fact, a recent X-ray reflectivity study revealed the formation of dense counterion overlayers of Ca2+, Cd2+, and Pb2+ on COOH-terminated SAMs at pH > 8.21 The role of counterions at charged interfaces is determinant in several processes of considerable interest such as colloidal suspension, biomineralization, and ionic transport through biomembranes. The importance of monovalent25 and divalent cations26 as fusogenic agents in the aggregation of unilamellar phospholipid vesicles is well-known.27 The relevance of the ionic environment in SAM properties is also supported by recent work in which different techniques, such as laser-induced temperature-jump experiments28 and quartz crystal microbalance measurements,29 were used. However, to compare kinetic and energetic data obtained from electrodesorption curves of different thiols, the same twodimensional thiol structure should be present on the Au(111) surface before the reductive electrodesorption takes place. In fact, in many cases it is assumed that COO-terminated alkanethiols form x3 × x3 R30° lattices on Au(111) similar to those found for methyl-terminated thiols. SAMs of alkanethiols consist of domains with different surface structures and surface coverage that slowly evolve to form the x3 × x3 R30° and its c(4 × 2) superlattice.30,31 In this article we compare the surface structure and electrodesorption data of ordered methyl-terminated alkanethiol SAMs with those corresponding to COO-terminated alkanethiol SAMs on Au(111) by using different electrolytes. Results show that 11-mercaptoundecanoic acid (MUA) and 3-mercaptopropionic acid (MPA) form x3 × x3 R30° and c(4 × 2) lattices on Au(111) similar to those found for methyl-terminated alkanethiols. For both methyl- and COO--terminated alkanethiols, the electrodesorption process follows a combined adsorptiondesorption nucleation and growth-of-holes model under charge-transfer control. The nucleation rate of holes on COO--terminated alkanethiol layers is smaller than that found on the corresponding methyl-terminated alkanethiol layers, which explains its slower electrodesorption kinetics. We have found that the activation energy to remove a COO--terminated alkanethiol molecule from the edge of a growing hole is smaller than that of a methylterminated molecule because of the repulsive forces introduced by the negatively charged groups. The magnitude of these repulsive forces, which are partially compensated for by ion binding between the COO- groups (22) Cheng, Q.; Brajter-Toth, A. Anal. Chem. 1995, 67, 2767. (23) Cheng, Q.; Brajter-Toth, A. Anal. Chem. 1996, 68, 4180. (24) Imabayashi, S.; Iida, M.; Hobara, D.; Feng, Z. Q.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1997, 428, 33. (25) Helm, C. A.; Laxhuber, L.; Lo¨sche, M.; Mo¨hwald, H. Colloid Polymer Sci. 1986, 264, 46. (26) Lo¨sche, M.; Mo¨hwald, H. J. Colloid Interface Sci. 1989, 131, 56. (27) Ohki, S.; Du¨zgu¨nes, N.; Leonards, K. Biochemistry 1982, 21, 2127. (28) Smalley, J. F.; Chalfant, K.; Feldberg, S. W.; Nahir, T. M.; Bowden, E. F. J. Phys. Chem. B 1999, 103, 1676. (29) Sastry, M.; Patil, V.; Mayya, K. S. J. Phys. Chem. B 1997, 101, 1167. (30) Terań Arce, F.; Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. J. Chem. Phys. 1998, 109, 5703. (31) Terań Arce, F.; Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. Langmuir 1998, 14, 7203.

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Figure 1. (a) STM image (220 × 220 nm2) of a hydrogen flameannealed gold electrode used on the electrochemical runs; (b) cross-sectional analysis showing smooth terraces and monoatomic steps. Step height corresponds to 0.24 nm.

and the counterions in the electrolyte, depends on both the nature of the cation and the pH. Experimental Section Au films evaporated on glass (250 nm thick onto Robax glass, AF Berliner Glass KG, Germany) with a 2-nm-thick chromium undercoating were used as substrate for electrochemical runs and scanning tunneling microscopy (STM) imaging. These films exhibit atomically smooth (111) terraces 60-150 nm in size (Figure 1a) separated by monatomic steps in height (Figure 1b). Adsorbed layers on Au(111) were prepared by immersion for 24 h in 0.05 mM propanethiol, hexanethiol, dodecanethiol, hexadecanethiol, MUA in ethanolic solutions, and in 0.05 mM MPA. All solutions were prepared with high-purity chemicals. STM imaging was performed with a Nanoscope III STM (Digital Instruments Inc.). STM images were taken in the constant current mode with Pt-Ir tips. Potentiodynamic electrodesorption curves were made in a conventional three-electrode glass cell with a large-area Au counter electrode and a saturated calomel electrode (SCE) as reference electrode. The electrodesorption curves were recorded at different scan rates (v) from -0.3 to -1.4 V in nitrogen saturated either in 0.1 M NaOH or 0.1 M KOH. Some runs were also made in 0.1 M NaCl, 0.1 M KCl, and 0.1 M CaCl2.

Results and Discussion Typical STM images of methyl-terminated alkanethiolcovered Au(111) are shown in Figure 2a. Well-ordered domains consisting of rows separated by ≈1.0 nm, corresponding to the rectangular c(4 × 2) lattice and dense

Electrodesorption of Negatively Charged SAMs

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Figure 2. STM images showing well-ordered domains of alkanethiols on Au(111). (a) hexanethiol (20 × 20 nm2)- and (b) MUA (18 × 18 nm2)-covered Au(111) surfaces. The rows and the dense regions correspond to c(4 × 2) and x3 × x3 R30° domains, as shown in the high-resolution (4 × 4 nm2) STM images.

Figure 3. STM images (200 × 200 nm2) of Au(111) surfaces covered by (a) dodecanethiol and (c) MUA. The black regions correspond to monatomic deep holes as shown in the corresponding cross-sections (b and d).

regions corresponding to the x3 × x3 R30° lattice are clearly observed. Similar images were obtained for COO-terminated alkanethiols, MPA, and MUA. In Figure 2b the rows corresponding to the zigzag c(4 × 2) lattice are clearly visible. In addition, the x3 × x3 R30° lattice is observed at the bottom (right-hand side) of the STM image. In situ STM imaging for MPA adsorbed on Au(111) has

revealed the presence of ordered p × x3 lattices.32 It is possible that these lattices slowly evolve to the more stable x3 × x3 R30° and c(4 × 2) lattices as proposed in ref 30. MUA adsorbed on Au(111) has also been studied by in (32) Giz, M. J.; Duong, B.; Tao, N. J. J. Electroanal. Chem. 1999, 465, 72.

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Figure 5. Peak potential (Ep) vs chain length (n) for the reductive electrodesorption of spontaneously assembled nalkanethiol monolayers from Au(111) electrodes. The data were taken from the electrodesorption curves recorded at 20 mV s-1 in 0.1 M NaOH.

For a propanethiol-covered Au(111) surface desorption of the x3 × x3 R30° and c(4 × 2) lattices takes place at current peak located at -0.82 V (v ) 20 mV s-1) (Figure 4a, dashed line). On the other hand, desorption of the x3 × x3 R30° and c(4 × 2) lattices for butanethiol (Figure 4b, dashed line), hexanethiol (Figure 4c, dashed line), dodecanethiol (Figure 4d, dashed line), and hexadecanethiol (Figure 4e, dashed line) takes place at Ep ) -0.86 V, -0.97 V, -1.15 V, and -1.22 V, respectively. For these alkanethiols Ep values change only 0.01 V when v is changed from 5 mV s-1 to 50 mV s-1. The Ep,v ) 20 mV s-1 vs n plot (Figure 5) leads to a straight line with a slope ≈0.03 V mol-1 of C units.17,20 The charge density (q) involved in the desorption peak is ≈90 µC cm-2, indicating a one-electron transfer for desorption of a thiol molecule from the x3 × x3 R30° and c(4 × 2) lattices. Therefore, the reductive desorption of alkanethiols from the x3 × x3 R30° lattice can be written as follows17:

1e + Au(111)-SR a R-S- + Au(111)

(1)

situ STM,33 although no molecular resolution has been reported. Our STM images demonstrate that COO-terminated alkanethiols also organize in x3 × x3 R30° and c(4 × 2) lattices as methyl-terminated alkanethiols. As previously reported,32,33 both Au(111) covered by methyl (Figure 3a, b) and Au(111) covered by COO--terminated alkanethiols exhibit typical pits, monatomic in depth (black regions), induced by thiol adsorption (Figure 3c, d).

For this one-electron charge-transfer process the value 0.03 eV mol-1 of C unit involves both van der Waals and hydrophobic forces stabilizing SAMs. This result is close to that reported for alkanethiol adsorption on Ag(111) and Au(111) electrodes.16,20 The contribution of hydrophobic forces to stabilize SAMs is demonstrated by performing reductive electrodesorption in methanolic solutions.20 In this case the slope of the Ep vs n plot for the n > 6 decreases, whereas it becomes 0 for n < 6, because the methanol/Ag(111) interaction becomes dominant for desorption of the shorter thiols.20 Similar electrodesorption curves were recorded for MUA and MPA adsorbed on Au(111) that exhibit the same x3 × x3 R30° and c(4 × 2) lattices. The electrodesorption curves recorded at v ) 20 mV s-1 in 0.1 M NaOH (Figure 4f, g) show q values close to those expected for the corresponding alkanethiols, undecanethiol, and propanethiol. However, as previously reported,24 Ep is shifted in the positive direction ∆Ep ≈ 0.05 V, which suggests a decrease in the SAM stability. The presence of negatively charged groups at the end of the hydrocarbon chains should produce a destabilizing effect because of repulsive forces acting in the adlayer.34 We have also made the electrodesorption curves of these alkanethiols at a higher sweep rate v ) 50 mV s-1 (Figure

(33) Hobara, D.; Ota, M.; Imabayashi, S.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1998, 444, 113.

(34) Doblhofer, K.; Figura, J.; Fuhrhop, J. H. Langmuir 1992, 8, 1811.

Figure 4. Electrodesorption curves (j vs E profiles) recorded for different SAMs of alkanethiols in 0.1 M NaOH: (a) propanethiol, (b) butanethiol, (c) hexanethiol, (d) dodecanethiol, (e) hexadecanethiol, (f) MPA, (g) MUA. (dashed line) v ) 20 mV s-1; (continuous line) v ) 50 mV s-1. The insets (8 × 8 nm2) show c(4 × 2) and/or x3 × x3 R30° domains.


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4a-g, solid line). Although Ep values are slightly shifted in the negative direction with respect to those values observed at v ) 20 mV s-1 for the methyl-terminated thiols (Figure 4, dashed line), the Ep values for COO--terminated thiols are shifted more than 0.05 V in the negative direction (Figure 4, solid line). This difference in the dependence of Ep on log v (Figure 6a) should be related to a slow kinetics for desorption.35 In fact, we have recorded current-time transients for the desorption of SAMs of propanethiol and MPA from the Au(111) surface (Figure 6b). These transients were recorded by stepping the potential from -0.3 V to -0.95 V, the corresponding peak potentials being -0.80 and -0.83 V (v ) 20 mV s-1) for MPA and propanethiol, respectively. In both cases the currenttime transients exhibit a maximum in current consistent with a electrodesorption process involving nucleation and growth of holes.36-38 Note that complete electrodesorption takes place faster for propanethiol than for MPA despite the fact that for propanethiol the potential step implies a smaller potential excursion in the cathodic direction with respect to the corresponding peak potential. In principle, we tried to reproduce the experimental current-time transients for the electrodesorption of the x3 × x3 R30° and c(4 × 2) thiol lattices by using nucleation and growth models (instantaneous or progressive)39 and the Frumkin adsorption model.40 However, large deviations between the experimental and calculated data were observed. A remarkable feature of the experimental transients is that the final decay in current from nucleation and growth models is slower than expected. This feature has also been observed for the electrodesorption of SAMs of methyl-terminated alkanethiols from Au surfaces.36 Recently, we found similar current-time transients for the x3 × x3 R30° S lattice electrodesorption from the Au(111) surface in 0.1 M NaOH.41 These current-time transients can be reproduced by using the combined adsorption-desorption nucleation and growth model under charge-transfer control proposed in ref 35. Therefore, we use this model to fit the experimental currenttime transients for both methyl- and COO--terminated thiols. In the combined adsorption-desorption nucleation and growth model two different phases compete for the electrode surface. The first one is a diluted phase obeying a Langmuirian adsorption-desorption isotherm, whereas the second one is a condensed phase that follows a nucleation and two-dimensional growth kinetics under charge-transfer control. The condensed “ordered” phase consists of growing holes that are nucleated at defects of the thiol lattice, whereas the other consists of vacancies (the diluted “disordered” phase) produced by direct thiol desorption. Let us consider the stripping/formation of a thiol monolayer according to eq 1. The rate of change of surface coverage by a diluted adsorbed phase of vacancies at a given E is35

dX ) ka(1 - S - X) - kdX dt Figure 6. (a) Peak potential (Ep) vs potential scan rate (v) logarithm representation for the reductive desorption of dodecanethiol (solid line) and MUA (dashed line) SAMs on Au(111). Electrolyte: 0.1M NaOH. (b) Current-time transients at constant potential of propanethiol (b) and MPA (n) on Au(111) electrodes in 0.1 M NaOH. Potential step from -0.3 V to -0.95 V (vs SCE), (c) (n) experimental (see b) and (ss) calculated current-time transient (eqs 2 and 7) for propanethiol electrodesorption, (d) (n) experimental (see b) and (ss) calculated current-time transient (eqs 2 and 7) for MPA electrodesorption.

(2)

(35) Noel, M.; Chandrasekaran, S.; Ahmed, B. C. J. Electroanal. Chem. 1987, 225, 93. (36) Vinokurov, I. A.; Morin, M.; Kankare, J. J. Phys. Chem. B 2000, 104, 5790. (37) Calvente, J. J.; Kovaćova´, Z.; Sańchez, M. D.; Andreu, R.; Fawcett, W. R. Langmuir 1996, 12, 5696. (38) Vela, M. E.; Martıń, H.; Vericat, C.; Andreasen, G.; HernańdezCreus, A.; Salvarezza, R. C. J. Phys. Chem. B 2000, 104, 11878. (39) Thirsk, H. R.; Harrison, J. A. A Guide to the Study of Electrode Kinetics; Academic Press: London, 1972.

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where X is the fraction of the surface covered by the diluted adsorbed phase (vacancies), S is the fraction of the surface covered by the condensed phase (holes) involved in nucleation-growth, ka and kd are the vacancy adsorption and desorption potential-dependent rate constants, respectively. The fraction of the electrode surface covered by holes can be estimated by the Avrami equation35

S ) (1 - X)(1 - exp(-Sex))

(3)

where Sex is the extended area covered by nucleation and growth of holes t dr dz dy ∫0t (dN dt )y[∫y ( dt )z ] 2

Sex ) π

(4)

N is the number and r the radius of the nuclei. For an instantaneous nucleation,

dN ) Noδ(t) dt

(5a)

where No is the total number of sites available for nucleation and δ(t) the Dirac delta function. For a progressive nucleation

dN ) NoA dt

(5b)

where A is the conversion frequency of a site into a hole. For two-dimensional circular growth under chargetransfer control

dr M ) k dt F

(6)

In eq 6 k is a potential-dependent constant, M is the molecular weight, and F is the density. The current density j(t) related to this process is given by

dX dS + σmL j ) σmA dt dt

(7)

where σmL and σmA are the maximum charges related to the condensed phase of holes and to the diluted adsorbed phase of vacancies, respectively. By introducing eqs 3-6 into eq 2 we have 2 dX ) ka(1 - X)e-βt - kdX dt

(8)

where β ) πM2Nok2/F2 for an instantaneous nucleation and β ) πM2ANok2/3F2 for a progressive nucleation.39 Equation 8 can be solved to give finally a differential equation,41 which cannot be easily managed by conventional fitting procedures. Therefore, to reproduce the current-time transients we proceeded in the following way. First, we numerically solved eq 8 by using the RungeKutta fourth-order method, obtaining X for different t values. For this purpose ka and kd were estimated from the tail, whereas β was estimated from the rising part of the experimental current-time transients. Then, using the obtained X data and eq 3, we calculated the S values. Finally, j(t) was obtained by introducing dS/dt and dX/dt into eq 7. (40) Gileadi, E. Electroadsorption; Plenum Press: New York, 1967. (41) Martıń, H.; Vericat, C.; Andreasen, G.; Hernańdez-Creus, A.; Vela, M. E.; Salvarezza, R. C. Langmuir 2001, 17, 2334.

For methyl-terminated alkanethiols the analysis of the rising part and the maximum indicate that an instantaneous nucleation is involved (eq 5a, β ) πM2Nok2/F2). The experimental current transients for the electrodesorption of methyl-terminated alkanethiols can be reproduced by using this procedure (Figure 6c). On the other hand, to reproduce the current-time transients for the electrodesorption of the COO--terminated alkanethiols progressive (eq 5b, β ) πM2ANok2/3F2) rather than instantaneous nucleation should be used (Figure 6d). The fact that the progressive nucleation is involved in the electrodesorption of COO--terminated thiols is consistent with their sharper electrodesorption peaks and their greater dependence of Ep on v.35 The analysis of the parameters derived from the model shows that under the present experimental conditions σmL . σmA, that is, nucleation and growth dominates the electrodesorption process. In principle, the small value of σmA could be related to the small current peaks observed at the negative potential side of the main electrodesorption peaks (Figure 4). These current peaks have been assigned to the electrodesorption of thiol molecules from step edges or other surface defects.42 In fact, we have found by in situ STM that thiol molecules are only removed from step edges when the applied potential lies the potential region of these small cathodic current peaks.43 However, the electrodesorption potential for the current-time transients shown in Figure 6b-d (E ) -0.95 V) is more positive than those corresponding to the small current peaks. Thus, the contribution of thiol electrodesorption from surface defects can be discarded in the discussion of our experimental data. Despite the small contribution of vacancy desorption to the overall electrodesorption charge, it must be included to obtain a good fit of the experimental current-time transients, that is, simple nucleation and growth models cannot reproduce the current-time transients. The ability of Na+, K+, and Ca2+ to compensate for the repulsive interactions produced in SAMs by the presence of COO- groups was investigated in different electrolytes. We have recorded electrodesorption curves for propanethiol in 0.1 M KOH. As reported in ref 17, the difference in the Ep value (∆Ep) for short-chain methylterminated alkanethiols measured in KOH with respect to those obtained in 0.1 M NaOH is negligible (Figure 7a). Conversely, for MPA this difference is ∆Ep ≈ 0.13 V in the positive direction (Figure 7b), i.e., the stability of this SAM is smaller than that produced in 0.1 M NaOH. Note that, as observed in 0.1 M NaOH, the increase in v to 50 mV s-1 cancels ∆Ep (Figure 7c, d) because of the slow desorption kinetics of the COO--terminated thiols. We also compared the electrodesorption curves of propanethiol and MPA SAMs in 0.1 M NaCl, KCl, and CaCl2 at pH 7 to increase the solubility of Ca2+ salts. For propanethiol the Ep values in these electrolytes (Figure 8, dashed line) show no significant dependence of Ep on the nature of the cation, as discussed above, and pH, i.e., ∆Ep w 0. Recent data for methyl-terminated thiols have shown that as the pH value is decreased from 13 to 5.9, the position of the electroadsorption peak moves in the positive direction, although the Ep values related to the electrodesorption peaks remain practically constant.44 However, for MPA the Ep values depend markedly on the nature of the cation and pH. In fact, SAMs of negatively charged thiols are more stable (42) Wong, S.-W.; Porter, M. D. J. Electroanal. Chem. 2000, 485, 135. (43) Vericat, C.; Andreasen, G.; Vela, M. E.; Martin, H.; Salvarezza, R. C. J. Chem. Phys., in press. (44) Yang, D.-F.; Wilde, C. P.; Morin, M. Langmuir 1996, 12, 6570.


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Figure 9. Reductive electrodesorption curves for MPA (solid line) and propanethiol (dashed line) SAMs from Au(111) electrodes in electrolyte solutions containing different pH values: (a) KOH, pH ) 13; (b) KCl, pH ) 7. Scan rate: 20 mV s-1.

Figure 8. Reductive electrodesorption curves for MPA (solid line) and propanethiol (dashed line) SAMs from Au(111) electrodes in different electrolyte solutions: (a) 0.1 M CaCl2; (b) 0.1 M NaCl; (c) 0.1 M KCl. Scan rate: 50 mV s-1.

moves in the positive direction, i.e., the stability of SAMs decreases. In principle this is a surprising result because the number of COO- groups, and accordingly the repulsive forces, decreases as the pH is decreased from 13 to 7. In fact, this change in pH implies a change in the COO-/ COOH ratio from 2.5 × 107 to 25, considering pKa ≈ 5.6 for MPA.45,46 However, the decrease in the COO-/COOH ratio results in a nonhomogeneous, negatively charged layer on which only a disordered counterion overlayer can be formed. These results are in close agreement with recently reported atomic force microscopy measurements where a considerable increase in interfacial metal ion concentration at higher pH values was found for alkaline ions.47 Although the net repulsive force decreases, the ability of the counterions to compensate for the repulsion is markedly reduced. Therefore, the net effect of a decrease in pH is a reduction of the SAM stability and a shift of Ep in the positive direction. On the other hand, at pH < 5.9 Ep values for short methyl-terminated thiols on Au(111) move markedly in the positive direction as the pH value is decreased.48 We found Ep ) -0.39 V for propanethiol and MPA from electrodesorption curves recorded at a low sweep rate (20 mV s-1) in 0.01 M H2SO4 + 0.1 M Na2SO4. The fact that both thiols exhibit the same Ep at this pH value can be explained considering the presence of a COOH-terminated thiol layer, that is, the repulsion among COO- groups is canceled. The extra stabilizing factor arising from hydrogen bonding among the COOH groups49 does not seem to be important in determining the Ep value, because both MPA and propanethiol exhibit the same electrodesorption potential. Therefore, we propose the following scenario for stripping the x3 × x3 R30° and c(4 × 2) thiol lattices from Au(111) surfaces. Two different phases compete for the

in Na+ and Ca2+ than in K+-containing electrolytes (Figure 8, solid line). We propose that Na+ and Ca2+ are small enough to accommodate in the x3 × x3 R30° lattice of COO-,21 whereas the larger K+ cannot do it leading to a more defective (less stable) overlayer. The pH value also affects the Ep value for the COO--terminated thiols. The analysis of Figure 9 shows that, as the pH decreases, Ep

(45) Zhao, J.; Luo, L.; Yang, X.; Wang, E.; Dong, S. Electroanalysis 1999, 11, 1108. (46) Shimazu, K.; Teranishi, T.; Sugihara, K.; Uosaki, K. Chem. Lett. 1998, 669. (47) Klane, V.; Mulvaney, P. Langmuir 1998, 14, 3303. (48) Hagenstro¨m, H.; Schneeweiss, M. A.; Kolb, D. M. Langmuir 1999, 15, 2435. (49) (a) Cooper, E.; Leggett, G. J. Langmuir 1999, 15, 1024 (b) Imae, T.; Torii, H. J. Phys. Chem B 2000, 104, 9218.

Figure 7. Electrodesorption curves from Au(111) electrodes performed in 0.1 M KOH for (a) propanethiol at 20 mV s-1, (b) MPA at 20 mV s-1, (c) propanethiol at 50 mV s-1, (d) MPA at 50 mV s-1.

6654


electrode surface: one phase consists of growing holes (the condensed “ordered” phase) that are nucleated at defects of the thiol lattice, and the other consists of vacancies (the diluted “disorder” phase) produced by direct desorption from the lattice. For methyl-terminated alkanethiols adsorbate-sustrate interactions (EA-S), van der Waals (EvW), and hydrophobic forces (EH) stabilize SAMs, whereas the hydrophilicity of the Au surface (EW) destabilizes molecules in the adlayer. To grow a hole, an alkanethiol molecule should be removed from the edge of a growing hole. The energy barrier for this process should be E ) EA-S + EvW + EH - EW. For the COO--terminated alkanethiols the repulsion arising from the partially screened COO- groups (ER) should be considered. Our results show that the magnitude of ER depends on the nature of the counterion and pH. Therefore, in this case, E ) EA-S + EvW + EH - EW - ER. This explains the shift in Ep toward more positive potential values for COO-terminated thiols when v w 0. However, the nucleation rate of holes (ANo) is smaller in adlayers of COO-terminated thiols (progressive nucleation) than in adlayers of methyl-terminated thiols (instantaneous nucleation). The presence of the charged layer of COO- and cation overlayer would result into a more rigid lattice21,50 making the nucleation process more difficult. In fact, it has been reported that high concentrations of Na+ ions, on the order of 0.1 M, at pH > 10 increase the degree of order in SAMs by their binding with the negatively charged carboxylate groups.50 Next, we discuss the possible role of cation transport17 in the context of the nucleation and growth process that dominates the thiol electrodesorption. In the growth stage a hole expands by removing thiol molecules from the hole edge. The transport of cations, to compensate for thiolate anion formation, takes place more easily through the electrolyte in the hole, which is in contact with the metal surface and thiol molecules at the hole edge, rather than (50) Kakiuchi, T.; Iida, M.; Imabayashi, S.; Niki, K. Langmuir 2000, 16, 5397.

Azzaroni et al.

through the thiol layer. The rate of the hole growth depends on the energy barrier E (as discussed above). Conversely, to nucleate a hole (nucleation stage) in the thiol layer the transport of cations through the thiol layer should be important. For COO--terminated thiols the dissociation of the cation-carboxylate complex and their subsequent migration to the substrate surface should occur to nucleate a hole. Interadsorbate interactions between two carboxylates via a bridging cation would slow the nucleation rate in relation to the nucleation rate in methyl-terminated thiols. Conclusions The electrodesorption of SAMs of methyl-terminated alkanethiols, MUA, and MPA adsorbed on Au(111) has been investigated by STM and electrochemical methods. 1. MUA and MPA form x3 × x3 R30° and c(4 × 2) lattices on Au(111), similar to those found for methylterminated alkanethiols. 2. The electrodesorption process for methyl- and COO-terminated alkanethiols follows a combined adsorptiondesorption nucleation and growth-of-holes model. 3. The nucleation rate of holes on COO--terminated alkanethiols is smaller than that on methyl-terminated alkanethiols because of the presence of the COO- and cation overlayer. 4. The activation energy for the growth rate of holes in COO--terminated alkanethiol layers, that is, the energy for removing a molecule from a hole edge is smaller than that in a methyl-terminated layers because of the repulsive forces introduced by the negatively charged groups. 5. The magnitude of the repulsive forces depends on both the nature of the cation and the pH value. Acknowledgment. The authors thank Agencia Nacional de Promocioń Cientı´fica y Tecnolo´gica (PICT 995030) and CONICET (PIP-0897) (Argentina) for the financial support of this work. LA010019V

Electrodesorption Kinetics and Molecular Interactions at Negatively

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