Electrochemical Reactions in Adsorption of Organosulfur Molecules

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Electrochemical Reactions in Adsorption of Organosulfur Molecules on Gold and Silver: Potential Dependent Adsorption Woon-kie Paik,* Seunghun Eu,† Kanghee Lee, Seungwhan Chon, and Minsok Kim Department of Chemistry, Sogang University, Seoul 121-742, Korea Received March 20, 2000. In Final Form: September 28, 2000 The early stage of adsorption of organosulfur molecules on gold and silver was investigated by electrochemical and quartz crystal microgravimetric methods. The potential shifts of the substrate metals, current flowing through the substrates, and the surface mass increase observed during the adsorption process were found to support newly proposed electrochemical mechanisms of chemisorption steps: Thiols adsorb on Au and Ag by an anodic oxidation, whereas dialkyl disulfides adsorb through a reaction that results in net cathodic current. The relative rates of the adsorption depended on the potential of the substrate in accordance with the electrochemical mechanisms. The increase in surface mass due to adsorption was observed to closely parallel the charge expended in the electrochemical reactions.

Introduction The past several years have seen enormously increasing interest in the self-assembled monolayers (SAM) of organosulfur molecules on gold and other metals.1-5 A large number of applications of SAMs are envisioned because the monolayers provide surfaces of well-organized organic materials, and the metal surfaces can be modified to have tailored properties.1-6 Densely packed and well-organized SAMs of alkanethiolate monolayer are known to form relatively easily on the metal surfaces when the metal substrate is brought into contact with a dilute solution of an alkane thiol or an organic disulfide. SAM formation undoubtedly starts with adsorption followed by organization of the adsorbed molecules. The time required for a well-organized SAM to be formed is reported to be, variously, from less than several minutes to tens of hours. Integrity of a SAM is usually not complete. Uosaki et al.7 observed stripe patterns or mesh patterns in the scanning tunneling microscopy (STM) images of adsorbed layer that was formed when the concentration of a thiol was near or below the micromolar range. In their in situ observation of the adsorbed layer, those stripe or mesh patterns transformed with increasing thiol concentration, or on prolonged exposure to thiol solution, into the hexagonal array of dots corresponding to the (x3 × x3)R30° surface structure of the well-organized SAM on Au(111) face. They also observed that pits in the SAM were formed in the monolayer during that period. The same type of pits had been observed in the STM images of SAMs by previous * To whom correspondence should be addressed (telephone: +82705-8440, fax: +82-701-0967, E-mail: [email protected]). † Present address: LG Chemical, Ltd., Research Park, Taejon 305-380, Korea. (1) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (2) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (3) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-assembly; Academic Press: New York, 1991. (4) Ulman, A. Chem. Rev. 1996, 96, 1533. (5) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel-Dekker: New York, 1996; Vol. 19. (6) Sawaguchi, T.; Mizutani, F.; Taniguchi, I. Langmuir 1998, 14, 3565. (7) Yamada, R.; Uosaki, K. Langmuir 1997, 13, 5218.

workers.8-10 Therefore, it is obvious that the processes leading to the SAM formation are not simple, despite the simplicity of the recipe for building SAMs. Perhaps for the very reason that the SAMs are easily prepared, not much investigation has been made into the process of their formation, in contrast to the extensive studies on the structure and properties of the monolayers. The adsorption of thiols on gold from solution phase has been tacitly assumed to involve breaking of S-H bond driven by simultaneous formation of Au-S bond and molecular hydrogen:2,4,11

RS-H + Au f RS-Au + 1/2H2

(1)

The monolayer formation from a dialkyl disulfide has been supposed to occur through breaking of S-S bond accompanied by Au-S bond formation:2,4,11

RS-SR + AuAu f RS-AuAu-SR

(2)

The above reactions, however, have not been supported by experimental evidence. Evolution of molecular hydrogen as hypothesized in reaction 1 has not been detected. These single-step reactions do not involve heterogeneous electron transfer steps, although they are basically oxidation or reduction. In previous communications we reported ellipsometric and electrochemical observations that strongly indicate involvement of electrochemical electron-transfer steps in the adsorption of alkane thiols and dialkyl disulfides on gold.12,13 For adsorption reaction of thiols on gold,

RSH + (Au) f RS-Au + H+ + e-(Au)

(3)

where e-(Au) represents an electron on gold metal. For adsorption of dialkyl disulfides, the reaction appeared to (8) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966. (9) McDermott, C. A.; McDermott, M. T.; Green, J.-B.; Porter, M. D. J. Phy. Chem. 1995, 99, 13257. (10) Edinger, K.; Grunze, M.; Wo¨ll, Ch. Ber. Busenges. Phys. Chem. 1997, 101, 1811. (11) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528. (12) Eu, S.; Paik, W. Chem. Lett. 1998, 405. (13) Eu, S.; Paik, W. Mol. Cryst. Liq. Cryst. 1999, 337, 49.

10.1021/la000421u CCC: $19.00 © 2000 American Chemical Society Published on Web 11/28/2000

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be a cathodic process:13

RSSR + e- (Au) f RS-(Au) + RS-

(4)

We have expanded the investigation on the reaction mechanism of the adsorption processes by including silver and gold as the substrate and quartz crystal microgravimetry (QCM) as the experimental tool in the present work. Effects of potential of the substrates on the adsorption of thiols and disulfides are presented in this paper. We also discuss the significance of the electrochemical adsorption mechanisms in the formation of selfassembled monolayers. Experimental Section We used the QCM techniques along with electrochemical measurements. The measurements were made on the substrate metals Au and Ag while organosulfur molecules adsorbed on the metals. The electrochemical measurements included recording of the open-circuit potential of the substrate metals while the organosulfur molecules were injected into solutions in contact with the substrate, and recording of current when the potential was fixed. Dilute LiClO4 solutions in organic solvents were used. The cells were equipped with a Pt counter electrode and a reference electrode. The reference electrode was Ag/AgCl in 3 M NaCl separated from the rest of the cell by a tube with a capillary ending. A digitally controlled potentiostat was used for the potential control or for recording the open-circuit potential. Acetonitrile used as solvent for thiols or disulfides was purified before use by distillation. Absolute ethanol was used without further purification. Reagent grade lithium perchlorate (99.99%) from Aldrich was used as the electrolyte without purification. Thiols and dialkyl disulfides of the highest purity grade from Aldrich were dispensed from a micropipet to attain the desired concentration of the resulting solution, mostly 1 mM. Gold-coated quartz crystal oscillator electrodes supplied by International Crystal Manufacturing or by Elchema were used for the electrochemical quartz crystal microbalance (EQCM) experiments. In EQCM, one of the exciting oscillator electrodes of the quartz crystal was plated with gold and served as the working electrode of the electrochemical cell as well. The increase in the mass on the Au substrate due to adsorption was monitored by the shift of the resonant oscillating frequency of the quartz crystal according to the Sauerbrey equation.14,15 The EQCM from Elchema in combination with an Au-plated 10 MHz quartz crystal oscillator electrode had 1 Hz resolution and gave 4.42 ng Hz-1 sensitivity. When only the electrochemical measurements were made without QCM involved, a short length of gold wire (99.99%) or silver wire (99.99%), or a gold flag was used as the substrate. All of the Au electrodes were precleaned by repeatedly scanning the potential between the hydrogen evolution and the oxygen evolution regions in dilute sulfuric acid. For most of the experiments the solutions were deaerated with purified (99.999%) nitrogen to avoid complication due to oxygen.

Results and Discussion Open-Circuit Potential. The potential of the gold or silver substrate, previously cleaned in dilute sulfuric acid, was cycled in the acetonitrile-LiClO4 solution by linear sweeps between the limits where significant anodic or cathodic currents begin to flow. The potential scans were ended in the middle of the scan ranges, and the potential was left to drift for several minutes under open-circuit condition before a sample of the organosulfur was introduced into the solution. The potential, under open-circuit condition, shifted abruptly to the negative direction as a sample of thiol was injected into the acetonitrile-LiClO4 (14) Buttry, D. A.; Ward, M. D. Chem. Rev. 1992, 92, 1355. (15) Bruckenstein, S.; Robert Hillman, A. In The Handbook of Surface Imaging and Visualization; Hubbard, A. T., Ed.; CRC: Boca Raton, FL, 1995.

Figure 1. Potential shifts of Au and Ag substrates under opencircuit situation in acetonitrile-0.1 M LiClO4 solution after addition of decanethiol. The concentration of the organosulfur compound in the final solution was 1 mM.

Figure 2. Potential shifts of Au and Ag substrates under opencircuit condition after addition of dipropyl disulfide. The conditions of the solution were the same as in Figure 1.

solution as shown in Figure 1. Contrary to the case of thiols, injection of a dialkyl disulfide caused the opencircuit potential of the Au and Ag substrates to shift to the positive direction (Figure 2). The potential shifts of Au substrate with the two kinds of organosulfur compounds agree with our previous results.12,13 The present results show that Au and Ag substrate behave similarly. The magnitudes of the positive shifts with the disulfides were smaller compared to the negative shifts with thiols. The magnitudes of the potential shifts were dependent on the initial open-circuit potential, which was not always reproducible, and did not show a systematic trend with the alkyl chain length of the compounds. However, the potential always changed in the negative direction when a thiol was added, and always changed in the positive direction when a disulfide was added. The initial abrupt and large drop in the potential with a thiol was usually followed by gradual further decrease except when air was admitted to the solution, in which case the potential rose back to near the original rest potential. Two-step decreases were sometimes observed as shown in the figure. Current. When the gold and silver substrates were maintained at a constant potential by means of the potentiostat, transient anodic currents were observed following the injection of a thiol (Figures 3 and 4). The size of the current peaks increased with increasingly

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Figure 3. Current densities at Au substrate at different potentials after addition of nonanethiol.

Figure 4. Currents at Ag substrate at different potentials after addition of decanethiol. The geometric surface area of the substrate was 0.27 cm2.

Figure 5. Current densities at Au substrate at different potentials after addition of dibutyl disulfide.

positive applied potential. Injection of a disulfide, on the other hand, gave rise to transient cathodic current peaks both on Au and Ag (Figures 5 and 6), opposite to the case of a thiol. In this case the size of the negative current peaks decreased with increasingly positive applied potential. Our previous results12,13 with Au were confirmed here. The current densities on Au are obtained by dividing the current by the real surface area, which is the product of the apparent surface area and the roughness factor.

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Figure 6. Current densities at Ag substrate at different potentials after addition of dipropyl disulfide. The geometric surface area of the substrate was 0.46 cm2.

The roughness factor was estimated by the area of gold oxide peak in the cyclic voltammogram in the dilute sulfuric acid solution, taking the charge for gold oxide formation to be 400 µC cm-2.16,17 When a platinum substrate was used, the transient currents were very much smaller than with Au or Ag substrate. Figure 7 shows the current behavior of Pt electrode on addition of decanethiol (A) and dipropyl disulfide (B) in comparison to Au and Ag. Microgravimetry. The quartz frequency decreased as a thiol sample was injected into the electrochemical quartz crystal microbalance cell, indicating mass increase at the Au surface. The frequency decrease was converted to increase in mass using the Sauerbrey equation. The mass increases were represented in mass per square centimeter of the real surface in Figure 8 along with the transient current peaks. The corresponding changes with a disulfide sample are shown in Figure 9. Similar results were obtained at various potentials, a few tenths of a volt above and below the usual open-circuit potentials. Adsorption Reaction of Thiols. As can be seen from Figures 1-6, the trends of the shifting substrate potential or of the transient current peaks with adsorption of organosulfur molecules on Ag substrate were the same as those on Au. The potential shifts were in the negative direction with thiol adsorption and in the positive direction with disulfide adsorption. The transient currents were anodic with thiol adsorption and cathodic with disulfide adsorption. These facts indicate that the adsorption process of thiol is associated with an electrochemical oxidation reaction, whereas the adsorption of dialkyl disulfide is associated with an electrochemical reduction reaction on both Au and Ag substrates. The reaction mechanism of adsorption of a thiol that we previously proposed for adsorption on Au substrate, reaction 3,12,13 seems to apply for both Au and Ag substrates:

RSH + M f RS-M + H+ + e-(M) (M ) Au, Ag)

(5)

where M represents surface atoms (not necessarily a single atom) of Au or Ag. The stable position of the sulfur atom on the Au surface is known to be on the 3-fold site on Au(111) surface or on the 4-fold site on Au(110) surface (16) Woods, R. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1976; Vol. 9, p 124. (17) Michri, A. A.; Pshenichnikov, A. G.; Burshtein, R. K. Electrochim. 1972, 8, 364.

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Figure 8. Current and change in mass on the Au electrode at different potentials with adsorption of decanethiol. The expended charges are presented in the bottom part.

Figure 7. Comparison of current densities at Pt, Au, and Ag substrates on addition of decanethiol (A) and dipropyl disulfide (B). The potentials of the electrodes were maintained at 0.00 V in each case. The current densities in this figure refer to the apparent surface areas of the metals.

in a well-organized SAM. On polycrystalline substrates, the adsorption sites for the S atoms will be those 3-fold and 4-fold sites in addition to other sites. The above reaction is a heterogeneous electron-transfer reaction involving oxidation of metal and M-S bond formation. When the substrate metal is fixed at a constant potential and is connected through an external circuit to a counter electrode, the flow of electronic charge appears as anodic current. The above reaction mechanism is also borne out by the EQCM result, Figure 8, which shows that the anodic current is accompanied by mass increase on the Au surface. The mass increase usually lags slightly behind the current peak, which will be dealt with in more detail later (see later discussion). When the substrate is left unconnected to an external circuit, the electrons generated by reaction 5 charges the metal/electrolyte interface. Depending on the double layer capacitance at the interface, a fraction of a monolayer adsorption according to reaction 5 could shift the potential of the electrode up to 1 V. The anodic reaction is, however, retarded by the negative potential shift. Therefore, the adsorption process can continue only when there is a cathodic reaction consuming the electrons. In the usual practice of making a SAM, one dips a substrate piece into the solution containing a thiol. The solvent invariably contains dissolved oxygen in this case. The oxygen consumes the electron on the metal by its cathodic reduction, so that reaction 5 can continue as long as the

Figure 9. Current and change in mass on the Au electrode at different potentials with adsorption of dibutyl disulfide.

surface site is available. Therefore, reaction 5 is the key step in the mechanism of adsorption of thiols. It can be seen from Figures 3 and 4 that the higher the potential of the electrode, the larger the resulting current peak, as one would expect because the reaction is anodic. It is expected that if the substrate metal is left at its own shifting potential in the absence of oxygen or any other reducible species, the adsorption cannot proceed to the monolayer coverage. The drop of open-circuit potential such as in Figure 1 is typically less than 0.3 V. The double layer capacitance of the Au-solution interface decreases as the Au surface is covered with thiols or other organosulfur molecules from about 40 µF cm-2 down to a few microfarads per square centimeter.18,19 If 20 µF cm-2 is taken as the crude average of the double layer capacitance during the adsorption, the double layer charge associated with the 0.3 V potential shift is 6 µC cm-2, which is far (18) Yang, D.-F.; Wilde, C. P.; Morin, M. Langmuir 1996, 12, 6570. (19) Fawcett, W. R.; Fedurco, M.; Kovacova, Z.; Borkowska, Z. J. Electroanal. Chem. 1994, 368, 275.

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Figure 10. Response in the crystal frequency, converted into mass change, and the current on stepping the potential of Au electrode from -905 mV to 0 V in the presence of 1 mM decanethiol.

less than the usual value of charge associated with monolayer adsorption reactions,18 in the order of 100 µC cm-2. Evidently, in the open-circuit potential experiment, extensive adsorption is hindered by the negative shift of potential, whereas at a constant potential the adsorption by anodic oxidation is favored because the electrons generated are continuously removed through the external circuit. The charge expended in the adsorption processes estimated from the current peaks such as in Figures 3-6 is typically 1 order of magnitude greater than the aboveestimated charge from the potential shifts. (See below for discussion on charge and mass.) In the presence of dissolved oxygen in the solution, the adsorption reaction is facilitated by cathodic reduction of oxygen on the metal surface, which raises the open-circuit potential. Therefore, the success of the usual recipe for making the SAM of organosulfur molecules was fortuitous because of the ubiquitous dissolved oxygen. Observations of potential shifts and current peaks at Au electrodes with injection of a thiol that are similar to the present results were also reported by Zhong et al. recently,20 albeit under different conditions, i.e., with oxygen present. Unlike in the present experiment where oxygen was excluded by deaeration, the above authors used aerated solutions and hence the data showed different features from the present results. The open-circuit potential reversed its initial shift and approached the starting open-circuit potential. Nevertheless, the direction of the initial potential shift and the sign of current were the same as the present results. They derived an explanation for the observed effects assuming partial charge transfer from the thiol S to Au, followed by discharge of the charge on Au surface by reduction of H of thiol. Partial charge transfer between the molecular layer and the Au surface is, however, an unlikely explanation of the external currents observed. The adsorption reaction can be suppressed entirely by holding the substrate potential at a sufficiently negative value because the reaction is anodic. Figure 10 is the result of a potential-jump experiment, in which a thiol sample was added into the solution in a QCM cell while the potential of Au was held at -0.905 V, and subsequently the potential was stepped up to 0.00 V. The mass increase started instantly with the potential jump. The sharp current peak appearing at the moment of the potential (20) Zhong, C.-J.; Woods, N. T.; Dawson, G. B.; Porter, M. D. Electrochem. Commun. 1999, 1, 17.

Paik et al.

jump is mostly due to the double layer charging, the current associated with the adsorption reaction being buried in the spike. Before the potential jump was applied, no current peak or mass jump was noticed because no adsorption took place. The possibility of convenient control of adsorption by potential is clearly shown. Numerous works have been reported in which adsorbed alkyl sulfur molecules on Au or Ag were desorbed cathodically at large negative potentials and the desorbed species were anodically readsorbed at higher potentials.18-25 The reversibility of the desorption and adsorption makes the anodic reaction mechanism of thiol adsorption more credible because adsorption of the desorbed species, thiolates, is a similar process as thiol adsorption. The oxidative adsorption of systeine on Hg was described by Kolthoff and Barnum in a very early report,26 and also by Stanovich and Bard.27 The systeine molecular layer on Au was also reported to cathodically desorb and anodically readsorb by Johnson et al.28 The anodic mechanism of thiol adsorption has an analogy in corrosion of metals. Although the corrosion of metals in the wet environment occurs spontaneously and without an electrochemical control or external current flow, the reactions that occur on the metal surfaces, oxidation of metallic elements and reduction of oxygen, are electrochemical in nature, analogous to the adsorption reaction of thiol molecules. Rubinstein et al.29 assembled alkanethiol monolayers in ethanol solution on Au surface held at positive potentials and monitored the formation of SAM through oxidation current of ethanol. They reported that adsorption of thiols on Au held at an anodic potential or on preanodized Au gave a good quality SAM, and the SAM formation was significantly faster than without applied potential. They also reported that anodic potential was not beneficial for SAM formation from dialkyl disulfides. Clearly, the mechanism of disulfide adsorption differs from that of thiols. Adsorption Reaction of Disulfides. The positive shift of the open-circuit potential (Figure 2) and the cathodic currents associated with adsorption of an dialkyl disulfide (Figures 5 and 6) lead us to propose the following cathodic reaction as the adsorption process of disulfides on both Au and Ag substrates, similar to our previous proposal for Au substrate:13

RSSR + M + e- (M) f RS-M + RS(M ) Au, Ag)

(6)

where M represents the metal atoms at the adsorption sites. Although reaction 6 is a reduction resulting in a 1-electron transfer, it actually involves a 2-electron reduction of the disulfide molecule and a 1-electron (21) Schneider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391. (22) Hatchett, D. W.; Stevenson, K. J.; Lacy, W. B.; Harris, J. M.; White, H. S. J. Am. Chem. Soc. 1997, 119, 6596. (23) Weisshar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860. (24) Calvente, J. J.; Kovacova, Z.; Sanches, D.; Andreu, R.; Fawcett, W. R. Langmuir 1996, 12, 5696. (25) Hagenstrom, H.; Schneeweiss, M. A.; Kolb, D. M. Langmuir 1999, 15, 2435. (26) Kolthof, I. M.; Barnum, C. J. Am. Chem. Soc. 1940, 62, 3061. (27) Stankovich, M. T.; Bard, A. J. J. Electroanal. Chem. 1977, 75, 487. (28) Tudos, A. J.; Vandeberg, P. J.; Johnson, D. C. Anal. Chem. 1995, 67, 552. (29) Ron, H.; Rubinstein, I. J. Am. Chem. Soc. 1998, 120, 13444.

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oxidation of the metal simultaneously with M-S bond formation. A part of the thiolate RS- produced in the above reaction will be adsorbed on Au according to the following reaction, which is analogous to reaction 5.

RS- + M f RS-M + e-(M)

(7)

The remaining part of RS- will diffuse away into the bulk of solution and be converted into thiols by abstraction of protons from the solution, or become oxidized to disulfide. The net current that is observed at the outer circuit should be cathodic, as was actually observed, because reaction 6 occurs to a greater extent than reaction 7. One would expect that adsorption of a disulfide would be more favorable at more negative potentials because reaction 6 is the initiating and possibly the rate-determining step. This is borne out by the trend in Figures 5 and 6. In our first published letter on this subject,12 the following cathodic reaction sequence was put forward as the possible steps in the adsorption of a disulfide.

RSSR + 2e- (Au) f 2RS-

(8)

RS- + Au f RS-Au + e-(Au)

(9)

However, it is not feasible for reduction reaction 8 to occur independently at potentials accessible in the usual conditions of adsorption. The electrode potentials for such reactions of dialkyl disulfides are mostly more negative than -1.5 V vs saturated calomel electrode (SCE).30 On the other hand, because of strong chemical affinity of sulfur to Au and Ag (and possibly to Hg and Cu), reaction 6, which is a one-step reaction involving simultaneously M-S bond formation and electron transfer, should be feasible. The mechanism of adsorption involving reaction 6 is also supported by the EQCM result, Figure 9, which shows that mass increase accompanies the cathodic current. The currents associated with adsorption of the organosulfur molecules showed oscillations with two or more current peaks in some cases on silver (Figures 4 and 6), and less frequently on gold (Figure 3). Although the reason for these oscillations is not clear at present, it could be due to the presence of different adsorption sites on the metals. Because we used polycrystalline metal electrodes, there obviously exist steps and kinks in addition to different crystal faces, which have different affinities and activation energies toward the thiol molecules. The second and third peaks might have occurred when adsorption took place on less favorable sites after the most favored sites were filled. The exact shape of the current curves was not reproducible. The attainment of full coverage by either thiols or disulfides should be faster with controlled potential than under an open-circuit situation without oxygen if the controlled potentials are in the proper range. Such comparisons of adsorption rates with and without potential control are shown in Figure 11 by the relative coverage increasing with time. The relative coverage is represented as the instantaneous increase in mass divided by the steady state value. The controlled potential was maintained at 0.00 V. With a thiol, Figure 11(A), the relative coverage without potential control showed slowing after an initial growth stage, especially in the absence of dissolved oxygen. In the presence of oxygen the adsorption without potential control was slower than under potential control, but was faster than without oxygen because (30) Chambers, J. Q. Organic Sulfur Compounds in Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Lund, H., Eds.; Marcel Dekker: New York, 1978; Vol. XII.

Figure 11. Comparison of the increasing relative coverages at Au electrode under controlled potential and under opencircuit condition, with and without dissolved oxygen. Adsorbent added: decanethiol (A); dipropyl disulfide (B).

cathodic reduction of oxygen diminishes lowering of the potential. With a disulfide, Figure 11(B), the adsorption rate without potential control was again slower than under potential control. The open-circuit potentials encountered in the usual process of preparing SAM are generally in the range where both the cathodic adsorption of disulfides and anodic adsorption of thiols are possible. Comparison with Platinum. In contrast to Au and Ag, platinum has little chemical affinity to sulfur, and hence chemisorption of sulfur compounds on platinum is expected to be less likely. Figure 7 shows currents at the platinum electrode compared to Au and Ag as a thiol or a disulfide is added in the solution. Compared to Ag and Au where large transient currents are observed, platinum shows only very small shifts in current levels. The currents are too small to be considered as resulting from adsorption reactions, and may possibly be attributed to shifts in the background currents in the microampere range and to changes in the double layer structure and the potentials of zero charge of Au. Open-circuit potential of the Pt electrode was disturbed slightly with injection of a thiol or a disulfide. However, the magnitudes of the shifts were less than 10 mV, which is much smaller than those of Au or Ag. Such a small change can be expected in the absence of an electron transfer step because the double layer changes in the presence of the organosulfur molecules. Undoubtedly, physical adsorption of the sulfur compounds on Pt substrate should be possible. Charge and Mass Increases. By integrating the current accompanying the adsorption of thiols, the charge

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expended by the anodic adsorption reaction was obtained and compared with the theoretical charge based on reaction 3. The adlayer of thiols is known to have a superlattice structure of (x3 × x3)R30° on Au(111) and less dense structures on other surfaces.31 Assuming that the average situation can be closely represented by the Au(111) surface where 7.8 × 10-10 mol cm-2 monolayer coverage is calculated, the reaction leading to a monolayer coverage would require 75 µC cm-2 of charge. In Figure 8, the curves in the bottom part show the charge accumulated as the adsorption proceeds, which was obtained by integrating the currents. The integration of the current curves is inaccurate especially for longer times or at very low potentials because of the background current that is not negligible compared to the currents in the submicroampere range. The approximate anodic charges from the current curves in Figure 3 and from Figure 8 are in the range of 20-56 µC cm-2 if the data at potentials lower than 0 mV are not included (somewhat smaller than the calculated charge for a monolayer). The mass increases due to adsorption in Figures 8 and 9 are comparable to the calculated values for Au(111) surface. At sufficiently high potentials, where the current peaks are high and the tails are short, the error in the integrated charge can be small. The effective number of electrons per molecule n calculated from data obtained at 500 mV (the highest potential we studied and not included in Figure 8) was 1.1 at a short time after the thiol injection (80 s) and decreased to about 1.0 at 100 s. This indicates that the anodic current is used almost entirely for reaction 5. It is interesting to note that Hatchett et al. obtained values close to unity (1.04 ( 0.06; 0.90 ( 0.14) as the number of electrons transferred in adsorption of each ethanethiolate ion on Ag(111), although they emphasized that the numbers include the effect of double layer change.22 For disulfides, a fraction of thiolate ions generated by the 1-electron reduction (6) can be adsorbed by oxidation reaction 7. If we let this fraction be x, net electronic charge of (1 - x)e is consumed while (1 + x) molecules are adsorbed. From the ratio of measured charge to the mass increase x can be calculated. We used one set of our data obtained at 300 mV, where the mass increase was fast, to estimate x. The x value in this particular case increased from 0.69 (at 80 s) to a steady value of 0.78 (after 300 s). This calculation is only a rough estimation because the adsorption rate is influenced by diffusion and by convection of the solution at the electrode surface, which was not controlled in the experiment. The lifting of the gold surface atoms, as suspected from the time lag between the current and mass changes as will be discussed in the following section, further complicates the calculation. Therefore, the above-estimated figure should be regarded as roughly representing the situation only in that particular experiment. One noticeable feature of Figure 8 is that although the charge increases roughly in parallel to the mass increase, the rises in the mass curves definitely lag behind the rises in the charge curves. The time lag is barely noticeable when the potential is higher than 0 V, but increases with decreasing potential. Occasionally the mass curves show fluctuations before a rapid increase starts. If the adsorption occurs by reaction 5 that produces the currents, and there are no other changes occurring before the effective electrode mass increases, there should have been no such a lag. One explanation for the above delay can be found in the detachment of Au atoms from the surface before (31) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546.

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Figure 12. Result of EQCM experiment of dodecanethiol adsorption on Au in ethanol solution (A: current, B: mass) and the current of adsorption of decanethiol on Au in diethyl ether solvent (C).

thiol molecules cover the surface. STM examinations of SAM-covered Au single crystals in many studies revealed that pits exist on SAM layers formed on Au(111).7-10 The depth of the pits was reported to be that of one Au atom layer. Edinger et al. found evidence that the pits are formed by dissolution of the surface layer of Au by thiol molecules, and that the dissolution stops as soon as the Au surface is covered by the well-organized SAM of thiols.10 The mass decrease due to the dissolution will counteract the mass increase due to adsorption. Porter et al.9 reported that STM examination reveals that the pits are also covered with an ordered layer of thiol molecules. The slight gradual decrease in mass before the adsorption potential was applied in Figure 10 is considered also to be due to the dissolution of Au in the presence of thiol before the surface was protected by adsorption of the thiol molecules. There may be other reasons for the time lags between the current and the frequency changes (translated into mass change) accompanying the adsorption process. A thorough understanding of the problem may require further experimental investigations, perhaps using singlecrystal substrates, as we are planning to do. The time lags between the current rises and the mass increases were longer when the applied potential was lower, presumably due to slower covering of the surface that provides protection against the gold atom detachments. The pit formation can presumably be suppressed by holding the substrate at relatively high potentials where the anodic reaction leading to adsorption of thiols is fast. This will provide a convenient method to obtain pit-free SAM layers. The electrochemical nature of the adsorption reactions is not expected to be influenced much by the solvent. Figure 12 shows EQCM result obtained in ethanol solution for thiol adsorption and current response in diethyl ether used as solvent for thiol. The general features of the EQCM result in ethanol solvent and the transient current obtained in diethyl ether solvent are close to those obtained with acetonitrile as solvent. The diethyl ether solvent gave a noisy anodic current, presumably because of lower conductivity of the solution. Scoles et al.32 observed that when alkane thiol gas was introduced into an ultrahigh vacuum (UHV) chamber, a (32) Lavrich, D. J.; Wetterer, S. M.; Bernasek, S. L.; Scoles, G. J. Phys. Chem. B 1998, 102, 3456.

Adsorption of Organosulfur on Gold and Silver

low coverage physisorbed thiol layer was formed on Au(111) first by lateral van der Waals interaction of the hydrocarbon chain with the substrate, and then the physisorbed molecules underwent chemisorption. Disulfides were observed to directly chemisorb without being physisorbed. These observations are all understandable in terms of the electrochemical reactions in adsorption steps. Direct chemisorption of thiols should be difficult in the gas phase, because there can be no counter reaction to the oxidative adsorption. Direct chemisorption should be easier for disulfides because the electron transfer steps of eqs 6 and 7 occurring simultaneously do not require electrochemical counter reactions. The fact that the adsorption steps of thiols and disulfides occur via electrochemical reactions has important implications. Potential control of the substrate may be used, for example, to obtain SAMs of high integrity, in shorter time. An experimental study on the SAM formation process may be made under better-controlled conditions by controlling the potential of the substrate. Rate measurement of adsorption can have an unambiguous meaning only when the measurement is made under potential control. Summary Electrochemical and QCM measurements were made on gold and silver while organosulfur compounds, thiols and disulfides, adsorbed on these metal substrates. The potential of both the Au and Ag substrates shifted to the negative direction when thiols adsorbed, and to the positive

Langmuir, Vol. 16, No. 26, 2000 10205

direction when dialkyl disulfides adsorbed. In cases where the potential of the metal was fixed, anodic currents were observed when thiols adsorbed, and cathodic currents were observed when disulfides adsorbed. Increase in the electrode mass almost paralleled the charge passed. From these observations, the following reaction mechanisms were deduced for the adsorption: Thiol molecules adsorb on Au and Ag through an anodic reaction, whereas dialkyl disulfides adsorb by a reaction that gives a net cathodic current.

RSH + M f RS-M + H+ + e-(M) (M ) Au, Ag) RSSR + M + e- (M) f RS-M + RS- (M ) Au, Ag) The relative rates of the adsorption depended on the potential of the substrate in accordance with the electrochemical adsorption mechanisms. These findings have important implications, for example, in designing the preparation methods for good quality SAMs and designing better-controlled experiments to study the adsorption processes. Acknowledgment. This research was supported by Grants from Korea Science and Engineering Foundation (996-0300-001) and from The Ministry of Education (BSRI 98-4311). Discussions with Professor K. Uosaki were informative and helpful. Professor Il Cheol Joen kindly provided one of the EQCM cells we used. LA000421U