A Second Harmonic Generation Study of a Physisorbed Precursor to

Mar 22, 2001 - (F) Second harmonic generation signal recorded during a potential .... Blank experiments have shown that the contribution of the substr...
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A Second Harmonic Generation Study of a Physisorbed Precursor to the Electrodeposition of a Monolayer of Alkanethiols M. Byloos, S. Rifai, H. Al-Maznai, M. Laferrie`re, and M. Morin* Department of Chemistry and Ottawa-Carleton Chemistry Institute, 10 Marie Curie, University of Ottawa, Ottawa, Ontario, Canada, K1N 6N5 Received July 17, 2000. In Final Form: January 9, 2001 Physisorbed thiolates are found to be precursors in the oxidative chemisorption of hexadecanethiolates. In alkaline electrolyte solution, the physisorbed species undergo a slow, nonelectrochemical, reaction that shifts their oxidative adsorption to more positive potentials. This reaction, which makes the formation of a self-assembled monolayer more difficult, was monitored using second harmonic generation (SHG) spectroscopy. The SHG signal is found to be constant for the first 3 min that the thiolates are physisorbed. After this induction period, there is a slow decrease of the SHG signal that lasts 15 min. The magnitude of the SHG decrease is a function of the applied potential. A larger decrease is observed when the potential is close to the hydrogen evolution region. These results suggest that the reaction is a protonation of the physisorbed thiolates via a reaction with adsorbed hydrogen atoms. We also compare the electrodeposition of alkanethiolates with the adsorption of alkanethiols in absence of an applied electric field.

Surfaces modified by organic monolayers are being used as chemical sensors.1-3 To obtain the sensitivity and selectivity required for such applications requires very good control of the surface coverage and the packing density of the adsorbates. These sensors often are mixed monolayers in which sensing molecules are homogeneously dispersed among inert molecules. Thiols which adsorb spontaneously on gold and form organized monolayers4 are often used to make modified surfaces for chemical sensors. The current method of formation of the so-called thiols “self-assembled monolayers” consists of the adsorption of the thiols from a solution onto a gold substrate. This simple method reproducibly yields a complete monolayer of thiols. However, sub-monolayer coverages and mixed monolayers are difficult to obtain using this approach.5,6 Electrodeposition can be used to form self-assembled monolayers. For example, thiols have been electrodeposited on gold7,8 and silver9,10 substrates. The coverage and properties of the electrodeposited monolayers have been shown to be identical to those of chemically (i.e., without an applied electric field) deposited monolayers. The electric field driven adsorption of thiols gives better control of the surface coverage. This method also allows selective

adsorption/desorption of adsorbates.11 These are useful advantages in the formation of mixed monolayers. The mechanism of electrodeposition is not known in as much detail as the chemical deposition. In the case of chemical deposition,12-16 the adsorption of alkanethiols follows a Langmuir mechanism. There is a subsequent slow organization of the monolayer. The oxidative adsorption of alkanethiols can be easily followed with electrochemical methods. The electrodeposition of alkanethiols on gold follows a nucleation and growth mechanism.8 However, on silver9 there is adsorption of two-thirds of a monolayer via a nucleation and growth process and the remaining one-third of a monolayer is electrodeposited via a Langmuir mechanism. There is limited knowledge of the nonfaradaic processes occurring after the oxidative adsorption of thiols. It is clear that the electric double layer will undergo important changes upon adsorption of organic molecules. Such a interfacial reorganization could influence the properties of electrodeposited monolayers. Recently, a precursor (physisorbed) state to the oxidative chemisorption of insoluble alkanethiols has been reported.17-21 For example, hexadecanethiol, which is insoluble in aqueous solutions, remains at the surface after its reduction and can be oxidatively redeposited. The same

(1) Mandler, D.; Turyan, I. Anal. Chem. 1997, 69, 894. (2) Malem, F.; Mandler, D. Anal. Chem. 1993, 65, 37. (3) Toby, A.; Jenkins, A.; Bushby, R. J.; Boden, N.; Evans, S. D.; Knowles, P. F.; Lui, Q.; Miles, R. E.; Ogier, S. D. Langmuir 1998, 14, 4657. (4) Ulman, A. An introduction to ultrathin organic films from Langmuir-Blodgett to self-assembly; Academic Press, Inc.: New York, 1991. (5) Truong, K. D.; Rowntree, P. A. J. Phys. Chem. 1996, 100, 19917. (6) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 6560. (7) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860. (8) Calvente, J. J.; Gil, M.; Andreu, R.; Roldan, E.; Dominguez M. Langmuir 1999, 15, 1842. (9) Hatchett, D. W.; Uibel, R. H.; Stevenson, K. J.; Harris, J. M.; White, H. S. J. Am. Chem. Soc. 1998, 120, 1062. (10) Hatchett, D. W.; Stevenson, K. J.; Lacy, W. B.; Harris, J. M.; White, H. S. J. Am. Chem. Soc. 1997, 119, 6596.

(11) Imabayashi, S.; Hobara, D.; Kakiuchi, T.; Knoll, W. Langmuir 1997, 13, 4502. (12) Dannenberger, O.; Buck, M.; Grunze, M. J. Phys. Chem. B 1999, 103, 2202. (13) Buck, M.; Grunze, M.; Eisert, F.; Fischer, J.; Trager, F. J. Vac. Sci. Technol., A 1992, 10, 926. (14) Dannenberger, O.; Wolff, J. J.; Buck, M. Langmuir 1998, 14, 4679. (15) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 3315. (16) Yamada, R.; Uosaki, K. Langmuir 1998, 14, 855. (17) Zhing, C. J.; Porter, M. D. J. Electroanal. Chem. 1997, 425, 147. (18) Yang, D.; Wilde, C. P.; Morin, M. Langmuir 1997, 13, 243. (19) Koichiro, D. H.; Miyake, K.; Imabayashi, S. I.; Niki, K.; Kakiuchi, T. Langmuir 1998, 14, 3590. (20) Badia, A.; Arnold, S.; Scheumann, V.; Zizlsperger, M.; Mack, J.; Jung, G.; Knoll, W. Sens. Actuators, B 1999, 54, 145. (21) Byloos, M.; Al-Maznai, H.; Morin, M. J. Phys. Chem. B 1999, 103, 6554.

Introduction

10.1021/la001010g CCC: $20.00 © 2001 American Chemical Society Published on Web 03/22/2001

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molecules go from a chemisorbed to a physisorbed state. There are no alkanethiolates adsorbed from the solution after the initial formation of a monolayer. This simplifies the study of the electrodeposition process since no diffusion step needs to be considered. We used this model system to examine the stability of physisorbed alkanethiolates and their effects on the oxidative chemisorption of thiols. The data reported here provide information relevant to the development of electrochemical methods for the fabrication of self-organized monolayers. Experimental Section The preparation of the Au(111) electrode has been described elsewhere.18 All electrochemical measurements were done in a three-electrode cell containing a 0.1 M KOH solution (Aldrich, semiconductor grade) made with deionized distilled water. The 1-hexadecanethiol and 1-butanethiol were from Aldrich. The electrochemical measurements were done with a three-electrode potentiostat and an impedance analyzer. The potential values are all relative to a saturated calomel electrode (SCE). The IR (SNIFTIRS, subtractively normalized interfacial Fourier transform IR spectroscopy) and second harmonic generation (SHG) experimental setups have been described previously.21,22 The SHG measurements were made with a 10 Hz, picosecond, activepassive, mode-locked Nd:YAG laser. A p-polarized 1064 nm beam was used and p-polarized light at 532 nm was detected. The plane of the 1064 nm incident beam was along one of the 〈110〉 directions of the Au(111) substrate. The 1064 nm beam had a diameter of 1-2 mm at the surface and the energy per pulse was kept below 0.7 mJ/pulse to prevent laser damage. The angle of incidence was 45° from the surface normal.

Results and Discussion Figure 1 illustrates the electrodeposition of physisorbed thiolates as a function of the time they spent physisorbed. In these cyclic voltammograms, the potential was first scanned at a rate of 20 mV s-1 from -0.30 to -1.30 V, in a 0.1 M KOH solution containing no thiols, to reduce the hexadecanethiols chemisorbed on a Au(111) electrode. We have discussed the origin of the two reductive current peaks observed during the reduction in previous studies.18,21 The first reductive current peak at the most positive potential has been assigned to the reduction of a monolayer of hexadecanethiols, whereas the smaller reductive current peak at the more negative potential was associated with a capacitive current. An alternative interpretation assigns the two reductive current peaks to different domains of alkanethiols.17 At -1.30 V, a monolayer of physisorbed alkanethiolates has been formed. On the subsequent positive-going potential scan, the potential was held at -1.05 V for 1, 5, and 15 min before recording the oxidative redeposition. The voltammogram of the oxidative redeposition, measured after a holding time of 1 min, in Figure 1A is identical to the one measured in an uninterrupted cyclic voltammogram.17-20 After 5 min at -1.05 V, we see in Figure 1B a substantial decrease of the oxidative current peaks at -0.98 and -0.91 V. In addition, a broad peak appears at a more positive potential of -0.85 V. After a period of 15 min (Figure 1C), there is a further decrease of the peaks at -0.98 and -0.91 V. Most of the oxidative redeposition charge is shifted to the new peak at -0.85 V, and the deposition now occurs from -1.0 to -0.3 V. For holding times up to 30 min, the cyclic voltammograms are similar to the one in Figure 1C. The appearance of a broad peak at a more positive potential indicates that it is harder to oxidatively redeposit the hexadecanethiolates onto Au(111) when they spend (22) Revesz, E.; Keefe, C. D.; Morin, M. Can. J. Chem. 1997, 75, 449.

Figure 1. Cyclic voltammograms of a monolayer of hexadecanethiols on Au(111) in 0.1 M KOH recorded as follows: The potential was scanned at 20 mV s-1 from -0.30 to -1.30 V to reduce the chemisorbed hexadecanethiols. On the subsequent positive potential scan, the potential was held at -1.05 V for (A) 1 min, (B) 5 min, and (C) 15 min before continuing the positive potential scan back to -0.30 V.

more time in a physisorbed state. This stabilization of the physisorbed state is a slow process, taking as much as 15 min to occur. The oxidative charges of all holding experiments are almost the same ((5%) as the absolute value of the charge of the reduction of the monolayer prior to the potential holding experiment at -1.05 V.17,18,20,21 Hence, no substantial loss of thiols occurs during the potential hold experiments, and the observed changes are not caused by a variation of the surface concentration of the hexadecanethiols. We recorded cyclic voltammograms at a scan rate of 0.5 mV s-1 and the oxidative current-potential profile was almost identical to the voltammogram recorded after holding the potential at -1.05 V for 15 min (see Figure 1C). Most of the oxidative charge is shifted to more positive potentials compared to was is seen in a voltammogram recorded at 20 mV s-1. This observation suggests that the slow stabilization of the physisorbed monolayer is linked to the time spent in the physisorbed state. We measured the differential capacitance (DC) of the physisorbed thiolates while holding the potential at -1.05 V after recording a voltammogram from -0.30 to -1.30 V and back to -1.05 V. We see in Figure 2 that at -1.05 V the DC decreases during the first 2-3 min. This is followed by a slower decrease during the next 27-28 min. The DC indicates a two-step decrease of the interfacial charge. This decrease of the interfacial charge could be caused by a reorganization of the physisorbed monolayer. For example, if some hexadecanethiolates lie down on the surface, the capacitance could decrease. A neutralization of the thiolates by ion pairing or protonation would make the monolayer neutral and would also lead to a reduction of the interfacial charge due to a better screening of the electric field.

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Figure 2. Differential capacitance measured in 0.1 M KOH while holding the potential at -1.05 V after recording a voltammogram first from -0.30 to -1.30 V and then from -1.30 to -1.05 V for a monolayer of hexadecanethiols on Au(111) at a potential scan rate of 20 mV s-1. The differential capacitance measurements were done with a 20 Hz, 5 mV rms ac potential.

We previously reported that the oxidative redeposition peaks at -0.98 and -0.91 V shift to more positive potentials and merge in one broad peak when the pH of the electrolyte is decreased.18 At pH of 10.5 the oxidative redeposition voltammogram was characterized by a broad peak at a potential ca. -0.80 V. The most important changes in the voltammogram occurred at a pH close to the pKa (10-12) of alkanethiols. The positive shift and merging of the two oxidative redeposition peaks were thus assigned to a protonation of the physisorbed thiolates. This protonation adds the breaking of S-H bonds to the energetics of the oxidative redeposition and increases the value of the potential required to cause the oxidative redeposition. The positive shift of the oxidative current as a function of the time spent in a physisorbed state is similar to the effect of lowering the pH of the electrolyte solution.18 First, the potential of the new peak, formed after 15 min in the physisorbed state of a monolayer (ca. -0.85 V), is close to the potential of the oxidative peak at a pH of 10.5 (ca. -0.80 V). Hence, the magnitude of the positive potential shift that we observed is compatible with protonation. Cyclic voltammograms recorded after our potential hold experiments display the typical features of a cyclic voltammogram recorded without holding the potential at -1.05 V.17,18,20 This is an indication that the stabilization of the physisorbed state is a reversible process just as is the protonation of the thiolates. The most likely source of protons for this reaction is the reductive decomposition of water molecules (i.e., the hydrogen evolution) since at -1.05 V we are near the onset of hydrogen evolution in 0.1 M KOH. It may seem unlikely that thiolates can be protonated in a solution with a pH higher (i.e., 13) than the solution pKa of 10-12. However, it has been reported that the pKa of carboxylate-terminated thiolates adsorbed on a gold electrode is shifted to higher values.23 The pKa of a carboxylate dissolved in an aqueous solution is about 4 whereas the pKa of an adsorbed carboxylate is 8. This study lends strong support to our suggestion of protonation of the physisorbed thiolates. (23) Wang, W.; Frostman, L. M.; Ward, M. D. J. Phys. Chem. 1992, 96, 5224.

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In situ vibrational spectroscopy (SNIFTIRS) was used to detect the protonation of the physisorbed hexadecanethiolates. A SNIFTIRS spectrum was obtained by subtracting spectra recorded at -1.05 V (while thiolates are physisorbed) and -0.30 V. No S-H stretching band in the 2600-2540 cm-1 region was observed. However, the weakness of this IR band,24 our sample size (nanomole), and the surface selection rule at a metal surface25 might have prevented its detection. We also recorded a SNIFTIRS spectrum every 3 min for 30 min when the potential was at -1.05 V and then subtracted each one from a spectrum at -0.30 V. We did not see any changes in the SNIFTIRS spectra of the CH stretching bands of the physisorbed alkanethiolates for times of up to 30 min. Hence, there is no substantial reorientation of the alkanethiolates related to the changes displayed in Figure 1. This makes it unlikely that the stabilization is due to the reorganization of the physisorbed monolayer since this would change the intensity of the CH stretching bands. SHG spectroscopy is surface specific, and the intensity of the SHG signal is related to the second-order polarizability at the interface.26 Interfacial reactions in which bonds are formed or broken modify the interfacial polarizability. This is particularly the case for solution reactions since charged species are often involved. A variation of the interfacial charge (i.e., the differential capacity) is also indicative of changes of the interfacial polarizability since both are related to the electronic density at the interface. Hence, we used SHG spectroscopy to monitor the changes reported in Figure 1. In our experiments, the intensity of the SHG signal, ISHG, at a given potential, E, and a time, t, can be described by the three-term equation

ISHG(E,t) ) A|χsub(E,t) + χint(E,Γ(t)) + χphys(E,Γ′(t))|2(Iω)2 (1) where A is a constant that depends on the experimental setup (i.e., angle of incidence, polarization of the light, optical constants of materials). Iω is the intensity of the incident laser beam. The suceptibility is divided into three terms: χsub, the second-order susceptibility of the gold substrate. The contribution of the hexadecanethiols to ISHG is separated in two components. The first term, χint, is assigned to the chemisorbed thiols. It represents the adsorbate-substrate interaction and is a function of the surface concentration of the adsorbate, Γ. The contribution of the physisorbed (protonated) thiols is χphys. This term is also a function of the surface concentration Γ′. Each suceptibility has a magnitude and a phase. Hence their addition could lead to complex variations (e.g., minimum, maximum) of ISHG. In our experiment the fundamental (1064 nm) and second harmonic (532 nm) beams are not in resonance with electronic transitions of surface or solution species.13 We first measured the effect of the applied potential on the SHG signal on an uncoated Au(111) electrode. The SHG signal coming from a clean Au(111) surface in 0.1 M KOH was monitored while the potential of the substrate jumped between -0.30 and -1.20 V. The SH signal was acquired at -0.30 V for the first 2.5 min to establish a baseline. The potential was then jumped from -0.30 to -1.20 V and held there for 3 min, after which the potential (24) Silverstein, R. M.; Bassler, G. Clayton; Morrill, Terence C. Spectrometric identification of organic compounds; Wiley: New York, 1991. (25) Greenler, R. G. J. Chem. Phys. 1996, 44, 310. (26) Shen, Y. R. The principles of nonlinear optics; Wiley: New York, 1984.

Physisorbed Precursor to Electrodeposition

Figure 3. Second harmonic generation signals recorded during potential steps from -0.30 V to (A) -1.12 V, (B) -1.14 V, (C) -1.16 V, (D) -1.18 V, and (E) -1.20 V and back to -0.30 V for a monolayer of hexadecanethiols on Au(111) in 0.1 M KOH. (F) Second harmonic generation signal recorded during a potential step from -0.30 V to -1.00 V and back to -0.30 V for a monolayer of butanethiols on Au(111) in 0.1 M KOH. The first arrow, A, indicates the time of the potential jump from -0.30 V to the potentials (A-F) and the second arrow, B, indicates the time of the potential jump back to -0.30 V. Each data point is an average of 500 laser shots.

jumped back to -0.30 V. No changes in the intensity of the SHG signal were observed. Hence, χsub is not a strong function of the potential in our experiments. The substrate’s contribution to the SHG signal is assumed to be constant under these conditions. SHG signals A-E of Figure 3 show the SHG signal measured during potential steps from - 0.3 V to potentials between - 1.12 and - 1.20 V for the hexadecanethiolcovered Au(111). The experiments were carried out in the same fashion as that used for the blank experiment described in the previous paragraph. In all cases we observe an instantaneous (i.e., a few seconds) change of the SHG signal intensity when the potential is jumped between the two values. When the potential step is negative going, the monolayer gets reduced and we observe an increase in the SHG signal. A positive-going potential step oxidatively redeposits the monolayer and we then observe a decrease of the SHG signal back to its initial value. The relative change (i.e., the percentage change with respect to the baseline value acquired initially at -0.30 V) of the SHG signal intensity observed for potential steps to -1.16 V (Figure 3C), -1.18 V (Figure 3D), and -1.20 V (Figure 3E) are equal to 2.7 ( 0.2% during the 3 min at these potentials. For potential steps to -1.12 V (Figure 3A) and -1.14 V(Figure 3 B), the SHG signal intensity after the initial rapid increase continues to slowly increase during the next 3 min. Thus, the average change observed for steps to -1.12 and -1.14 V have lower values of 1.8

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( 0.6% and 2.4 ( 0.6%, respectively. However, the increase levels off, after 3 min, to the same value observed for potential steps to -1.16, -1.18, and -1.20 V. Consequently, the change in the SHG signal intensity is the same (within the uncertainties) for all potential steps after 3 min. The slower increases at -1.12 and -1.14 V are related to a slower reduction of the monolayer at these potentials.21 The constant value of the SHG signal for the physisorbed thiolates is compatible with the absence of change in the cyclic voltammograms for a holding time of 3 min or less at -1.05 V. The SHG changes are faster than the capacitance changes shown in Figure 2. This difference might be related to the fact that SHG spectroscopy is more sensitive to chemical changes than the differential capacitance measurements (see below). The validity of our model in which the physisorbed (unprotonated) thiolates do not contribute to the SHG signal was checked. This was done by comparing the SHG signals measured in parts A-E of Figure 3 for the hexadecanethiols to the SHG signals of the reductive desorption and the oxidative redeposition of 1-butanethiol on Au(111). Butanethiols are soluble in 0.1 M KOH and thus do not remain physisorbed on the Au(111) electrode after their reduction. No recovery (oxidative deposition) is observed when a monolayer of butanethiols is reductively removed in a 0.1 M KOH solution containing no butanethiolates. Also, in a solution containing 10-3 M of butanethiolates in 0.1 M KOH, we held the potential at -1.00 V while removing the Au(111) electrode from the solution. We then transferred the electrode to a 0.1 M KOH solution containing no butanethiolates and recorded a cyclic voltammogram. We observed no reduction peak corresponding to chemisorbed butanethiols. Hence, there are no physisorbed butanethiolates at -1.00 V. From these results we conclude that there are no physisorbed butanethiolates and that consequently they do not contribute to the SHG signal. The binding of butanethiols and hexadecanethiols with the Au(111) are identical.12,27 Their χint values are also identical.12 The alkane chain does not contribute significantly to the SHG signal.12 The only difference between these alkanethiols is therefore the presence (or not) of physisorbed thiolates at the electrolyte/ substrate interface. The butanethiolates were electrodeposited from a 0.1 M KOH solution containing 10-3 M butanethiolates. This was done by bringing a clean Au(111) substrate in contact with the electrolyte solution at a potential of -0.30 V for a few seconds to form a monolayer. Cyclic voltammograms were then recorded in the solution containing butanethiolates under the same conditions used for the hexadecanethiol monolayer. On the negative-going scan, the butanethiol monolayer gets reduced at -0.89 V. We observe the oxidative deposition of the butanethiol monolayer on the positive-going scan at -0.78 V. We chose the negative limit for the potential step in the SHG experiments to be about -100 mV from the cathodic (reductive) current peak similarly to the conditions used for the hexadecanethiol monolayer. In Figure 3F, the SHG signal intensity was monitored while the potential jumped between -0.30 and -1.00 V. The experiments were carried out as described above. We observe the same changes of the SHG signal intensity with time as for hexadecanethiols. There are instantaneous increases and decreases of the SHG signal intensity for negative- and positivegoing potential steps, respectively. The change of the SHG signal intensity is constant at 2.3 ( 0.4% for the 3 min (27) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335.

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at -1.00 V. This is identical (within the uncertainties) to the change observed for hexadecanethiols (2.7 ( 0.2%). The same experiment was repeated for potential steps from -0.30 to -1.20 V, and identical results were obtained. During the first 3 min after the reduction of the thiols, the physisorbed (unprotonated) hexadecanethiolates do not have a measurable effect on the SHG signal. This gives support to our model. Hence, the instantaneous increase of the SHG signal intensity accompanying negative-going potential jumps is ascribed to the breaking of the S-Au bond and the creation of physisorbed thiolates. Similarly, we associate the instantaneous decrease of the SHG signal intensity accompanying positive-going potential jumps with the oxidative adsorption of thiolates and the formation of the S-Au bond. These observations indicate that the term χint in eq 1 goes from a maximum value at -0.30 V to zero at -1.20 V and that χphys is zero (i.e., no physisorbed thiols are formed). The instantaneous changes of the SHG signal with potential jumps agree with chronoamperometric studies28 which show that the reduction and oxidation of alkanethiols on Au(111) occur in a second or less. The directions of the SHG signal intensity changes for the positive- and negative-going potential jumps agree with qualitative predictions. When the S-Au bond is broken, electrons at the interface become more delocalized, thus causing an increase in the polarizability of the interface (and vice versa). Our results are also compatible with SHG results, for the adsorption of alkanethiols onto gold substrates from ethanolic solutions,12 which show a decrease of the SHG signal upon adsorption. We monitored the long-term (i.e., more than 3 min) stability of the physisorbed hexadecanethiolates with SHG spectroscopy. The uncoated gold response to a potential step was first measured as follows. The SHG signal was acquired at -0.30 V for the first 5 min to establish a baseline. The potential was then jumped from -0.30 to -1.20 V and held there for 30 min, after which the potential was brought back to -0.30 V. We observed no change of the SHG intensity during the potential steps. Thus we assume that χsub is constant during the experiment described below. Figure 4 presents the SHG signal intensity observed when a hexadecanethiol monolayer is reductively desorbed and held in a physisorbed state at -1.20 V for 30 min. The experiment was carried out as described above. When the potential is jumped from -0.30 to -1.20 V, we observe a instantaneous increase of approximately 2.5% in the SHG signal intensity. During the first 3 min at -1.20 V, the SHG signal intensity remains constant. Beyond 3 min, the SHG signal intensity decreases exponentially by ∼10% and then levels off after 15 min. We note a small increase of 1.5-2% of the SHG signal between 15 and 20 min. The signal is constant between 20 and 30 min. The origin of the small increase between 15 and 20 min is not clear. It could be related to the fact that cyclic voltammograms show that a small amount (typically less than 10%) of the hexadecanethiols is desorbed after the 30-min holding experiment. Upon the potential jump from -1.20 to -0.30 V, the SHG signal intensity rapidly (within a minute) increases back to the initial (baseline) value. Similar results were observed for potential jumps to -1.18 and -1.22 V. For potential jumps to values between -1.14 and -1.12 V, we observe a slower decrease beyond 3 min such that after 30 min at these potentials the decrease is typically less than 4%. At these potentials only 70-90% of the monolayer is reduced during the first 3 min.21 This (28) Yang, D.; Morin, M. J. Electroanal. Chem. 1998, 441, 173.

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Figure 4. Second harmonic generation signal recorded during a potential jump from -0.30 V to -1.20 V (first arrow, A) and back to -0.30 V (second arrow, B) for a monolayer of hexadecanethiols on Au(111) in 0.1 M KOH. Each data point is an average of 100 laser shots.

Figure 5. Second harmonic generation signal recorded during a potential jump from -0.30 V to -1.20 V (first arrow, A) and back to -0.30 V (second arrow, B) for a monolayer of butanethiols on Au(111) in 0.1 M KOH. Each data point is an average of 100 laser shots.

shows that the magnitude of the SHG decrease is related to the amount of thiols that are reduced. The magnitude of the change is also related to the amount of hydrogen evolution since the closer the applied potential is to values where hydrogen evolution occurs, the larger is the decrease of the SHG signal. The SHG signal intensity observed for hexadecanethiols was compared to the one of butanethiols measured in identical experiments. One such experiment is shown in Figure 5. During the 30 min at -1.20 V, the SHG intensity remains constant at ∼2.5%. The same results were obtained when the potential was jumped to -1.00 and -1.22 V and held at these values for 30 min. This comparative study unequivocally demonstrates that the presence of physisorbed hexadecanethiolates (χphys) is the cause of the changes of the SHG signal intensity after 3 min at the most negative potential (i.e., -1.20 V). For the first 3 min after the reduction of hexadecanethiols and butanethiols chemisorbed on Au(111), the SHG signal is constant. Hence, as we have discussed above,

Physisorbed Precursor to Electrodeposition

the initial variation of the SHG signal can be assigned to the reduction of the chemisorbed thiolates. The magnitude of the SHG intensity change also shows that it is related to the amount of thiol that is reduced/oxidized since butanethiols and hexadecanethiols give the same results. This result agrees with a previous SHG study which found that the alkane chain of the thiol does not contribute to the SHG signal.12 The induction period during which the oxidative voltammogram and SHG signal of the physisorbed state are constant could be linked to a reorganization of the thiolates that would be needed to allow hydrogen evolution to occur. The decrease of the capacitance during the first 3 min in the physisorbed state supports the suggestion of a reorganization of the interfacial region. There is a clear relation between the magnitude of the decrease of the SHG intensity after 3 min in the physisorbed state and the amount of hydrogen evolution. Also the positive shift of the potential of oxidative deposition of the physisorbed hexadecanethiolates is similar to what is observed when the same experiments are done in an electrolyte of lower pH.18 These observations suggest that a slow protonation of the physisorbed hexadecanethiolates occurs. The fact that no such changes are observed for butanethiols provides more support for a surface protonation. The decrease of the SHG signal after 3 min is compatible with a protonation since the electron localized on the sulfur is now involved in the S-H bond and thus is less polarizable. The full recovery of the SHG signal on the oxidative deposition step indicates the reversibility of the process. This latter result is also compatible with the fact that electrodeposited hexadecanethiols have the same properties has chemically deposited monolayers. The absence of change in the SNIFTIRS spectrum as a function of time (for up to 30 min) indicates that there is no substantial reorientation of the physisorbed molecules as a function of time at -1.20 V. The protonation of thiolates could decrease the repulsion between adsorbates and could lead to a different packing. However, in concentrated electrolyte solutions, such as 0.1 M KOH, the interadsorbate repulsion is significantly attenuated. Furthermore, it has been reported that both neutral and charged surfactants form physisorbed micellar structures in concentrated electrolyte solutions.29,30 The absence of reorientation after the protonation of the thiolates is compatible with these results. The results described in the previous paragraph are helpful in interpreting the variation of the differential capacity shown in Figure 2. The initial decrease of the capacitance in the first 3 min can be assigned to a relaxation of the electric double layer after the formation of a monolayer of physisorbed hexadecanethiolates. Furthermore, it has been reported that physisorbed thiolates form micelles.18,19,21 This will obviously change the distribution of charge at the interface. Such a reorganization should not significantly modify the energetics of the oxidative redeposition of the thiolates. After 3 min, the gradual increase of the oxidative current peak at -0.85 V is probably related to the fraction of thiolates that are protonated. The small and slow variation of the capacity after 3 min is expected since a protonation is not expected to cause major changes in the structure of the physisorbed monolayer in a concentrated electrolyte solution (see above). (29) Bizzotto, D.; Lipkowski, J. Prog. Surf. Sci. 1995, 50, 237. (30) Burgess, I.; Jeffrey, C. A.; Cai, X.; Szymanski, G.; Galus, Z.; Lipkowski, J. Langmuir 1999, 15, 2607.

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The slow variations of electrochemical and spectroscopic properties of the physisorbed thiolates are related to the kinetics of the protonation which involves chemisorbed hydrogen atoms.31 On gold there is no hydrogen underpotential deposition and the surface concentration of hydrogen atoms is low.32,33 If the hydrogen evolution process remains the same on a gold surface covered with physisorbed thiolates, we would expect the protonation process to be slow because of the low concentration of adsorbed hydrogen. In the micellar structure of the physisorbed thiolates18,19,21 only a fraction of alkanethiolates are in direct contact with the gold surface and the hydrogen atoms. The rate of protonation would depend on the reorientation of the physisorbed thiolates which is a slow process. On the basis of these data, the protonation is not expected to be fast. A potential-dependent protonation of physisorbed organic molecules has been reported recently. The electrodeposition and electrodissolution of monolayers of physisorbed N,N′-bipyridines on gold show that the structure of the adlayer is a function of the pH of the electrolyte solution.34 In 0.05 M H2SO4 it was suggested that at negative potentials the physisorbed bipyridines become protonated whereas at positive potentials the bipyridines are deprotonated and adopt a different packing arrangement. These studies12,13,34 as well as ours show that slow reactions and reorganizations of adsorbed organic molecules with ionizable or polarizable functions occur and they depend on the applied potential. The three-term equation for the SHG signal (eq 1) provides a good description of our observations. First, in the negative-going potential step experiments, χint goes from its maximum value to zero since all chemisorbed thiols are reduced. Once the physisorbed thiolates have been created, χphys will increase proportionally (not necessarily linearly) with the rate of formation of thiols. This causes a decrease of the SHG signal. The smooth decrease of ISHG after 3 min suggests a simple relation between χphys and the surface concentration of (protonated) thiols Γ′ and a constant difference between the phases of χphys and χsub. Blank experiments have shown that the contribution of the substrate is constant in the range of potentials used in our experiments. On the basis of these observations, we assign the slow decrease of ISHG to the protonation of the thiolates. An estimate of the rate constant for the protonation can be obtained by making the assumption that χphys varies linearly with the surface concentration of thiols and that the protonation is first order in the concentration of thiolates. A fit of ln(ISHG1/2) vs time yields a pseudo-first-order rate constant (i.e., the concentration of adsorbed H is assumed to be constant) of 1.4 ((0.1) × 10-2 s-1 at -1.20 V for the protonation of the physisorbed thiolates. A comparison with the SHG studies of Buck et al.12,13 of the adsorption of thiols from solution reveals differences between chemical and electrochemical adsorption processes. These authors found that the rate-limiting step for the chemical deposition and self-organization is the displacement of solvent molecules from the surface.12 The same authors observed postadsorption changes of the SHG signal for the chemical adsorption of 12-(4-nitroanilino)(31) Bockris, J. O’M.; Reddy, A. K. N. Modern electrochemistry; Plenum/Rosetta: New York, 1977; Vol. 2. (32) Hamelin, A.; Stoicoviciu, S. C.; Chang, S. C.; Weaver, M. J. J. Electroanal. Chem. 1991, 307, 183. (33) Perez, J.; Gonzalez, E. R.; Villullas, H. M. J. Phys. Chem. B 1998, 102, 10931. (34) Dretschkow, Th.; Lampner, D.; Wandlowski, Th. J. Electroanal. Chem. 1998, 458, 121.

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dodecanethiol on polycrystalline gold from ethanol and n-hexane solutions.13 They assign the decrease of the SHG signal to the term χint. Their decrease of the SHG signal is slower (many minutes) and larger than what we observed for the oxidative adsorption for less than 3 min in the physisorbed state. Clearly, in our experiments, solvent displacement plays a less important role since the molecules are (initially) deprotonated and physisorbed. This makes a direct comparison with the work of Buck et al.12,13 difficult. Our results probe the surface steps of the adsorption process. In our SHG and voltammetric data, we identify two different rate-limiting steps for the oxidative adsorption of thiolates. The first one is seen when physisorbed hexadecanethiolates exist for less than 3 min. In this case, the rate-limiting step in the oxidative adsorption is the electrospreading of micelles of thiolates.21 This process is rapid (a few seconds) since it involves a potential-induced reorientation of the thiolates. The second type of process starts 3 min after the creation of the physisorbed thiolates and is completed after 15 min. We assign it to a protonation of the physisorbed thiolates. Once protonated, the thiols remain in the shape of micelles. The oxidative deposition after the protonation becomes slower because the ratelimiting step is now the dissociation of the S-H bonds. The oxidative charge does not vary when the physisorbed thiolates become protonated. Furthermore, the oxidative charges, once corrected for capacitive currents, are compatible with a complete monolayer.18 These observations provide insights into the mechanism of adsorption of thiols. The fact that (unprotonated) thiolates and (protonated) thiols give rise to the same amount of oxidative charge allows us to rule out that molecular hydrogen is produced during adsorption. The formation of molecular hydrogen from protons is a two-electron reductive process. Hence, no net current would be gener-

Byloos et al.

ated for the adsorption of thiols if H2 is produced. We thus propose the following one-electron mechanism for the oxidative adsorption of thiols in alkaline solutions:

OH- + RSHphys + Au f RS-Au + H2O + 1e-

(2)

In this reaction, the proton produced in the oxidative adsorption of the thiols is neutralized by the hydroxyls. The production of a proton is compatible with the fact that there is no atomic hydrogen adsorbed on gold at the potentials where the oxidative adsorption of thiols occurs.32 This contrasts with the open-circuit adsorption of thiols which is believed to generate molecular hydrogen.4 However, both electrochemical and open-circuit methods yield monolayers with identical properties. We do not know if the SH bond dissociation is a unimolecular or nucleophilic reaction. Clearly more studies are needed to elucidate the mechanism of the dissociation of the S-H bond under electrochemical and open-circuit conditions. In summary, this study has shown that insoluble alkanethiols are in a physisorbed state prior to being chemisorbed. These physisorbed thiolates undergo a nonelectrochemical reaction at potentials where the reductive decomposition of water molecules occurs. This reaction makes the oxidative chemisorption more difficult and causes a decrease the interfacial capacity. Important variations in the SHG signal which track the electrochemical results have been observed. These results are compatible with a slow protonation of the physisorbed thiolates. Acknowledgment. M. B. thanks the Government of Ontario for an OGS scholarship. S.R. and M.L. thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for scholarships. The financial support of NSERC is gratefully acknowledged. LA001010G