Decomposition of Methylthiolate Monolayers on Au (111) Prepared

J. Phys. Chem. C 2010, 114, 10183–10194

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Decomposition of Methylthiolate Monolayers on Au(111) Prepared from Dimethyl Disulfide in Solution Phase F. P. Cometto,† V. A. Macagno,† P. Paredes-Olivera,‡ and E. M. Patrito*,† Departamento de Fisicoquı´mica and Departamento de Matema´tica y Fı´sica, Instituto de InVestigaciones en Fisicoquı´mica de Co´rdoba (INFIQC), Facultad de Ciencias Quı´micas, UniVersidad Nacional de Co´rdoba, 5000 Co´rdoba, Argentina

H. Ascolani§ and G. Zampieri| Centro Ato´mico Bariloche, AV. Bustillo 9500, 8400 Bariloche, Argentina, and Instituto Balseiro, UniVersidad Nacional de Cuyo, AV. Bustillo 9500, 8400 Bariloche, Argentina ReceiVed: December 22, 2009; ReVised Manuscript ReceiVed: April 26, 2010

We investigated the formation and stability of layers of methylthiolate prepared on the Au(111) surface by the method of immersion in an ethanolic solution of dimethyl disulfide (DMDS). The surface species were characterized by electrochemical reductive desorption and high-resolution photoelectron spectroscopy. Both techniques confirmed the formation of a methylthiolate monolayer at short immersion times (around 1 min). As the immersion time increased, the electrochemical experiments showed the disappearance of the methylthiolate reductive desorption current peak and the appearance of a current peak at ca. -0.9 V which was attributed to sulfur species. At long immersion times, the XPS measurements showed two main components for the S 2p signal: a component at ca. 161 eV which corresponds to atomic sulfur and a component at ca. 162 eV which we attributed to polysulfide species. We propose that the breakage of the S-C bond of methylthiolate is responsible for the appearance of sulfur species on the surface. Density functional theory (DFT) calculations were performed to identify the elementary steps that may lead to the decomposition of methylthiolate. We found that the cleavage of the S-C bond is only activated by the oxidative dehydrogenation of the methyl group of methylthiolate. Thio-oxymethylene, SCH2O, is the key intermediate leading to the breakage of the S-C bond because it decomposes into atomic sulfur and formaldehyde with an activation energy barrier of only 1.1 kcal/mol. Introduction There are lots of potential applications for self-assembled monolayers (SAMs), such as for corrosion inhibition, biosensors, lithography, organic electronics, and so forth, as already described comprehensively in several review articles.1-5 So far the most studied systems are SAMs made of alkanethiols adsorbed on gold surfaces. The reason is that they are easy to prepare and form well-ordered, closely packed monolayers on Au(111).6 There are two main routes to prepare SAMs of alkanethiols on Au(111): by deposition from gas phase and by immersion in a liquid solution. There are also two types of molecules that can be used: alkanethiols (RS-H) and dialkyl disulfides (RSSR).7 For medium and long alkyl chains (n > 6) all methods give essentially the same result: an ordered layer of chemisorbed thiolates (RS-). For short alkyl chains, however, the different methods may produce different results. With short alkanethiols the problem arises that the cleavage of the S-H bond upon adsorption becomes increasingly more difficult as the number of carbon atoms decreases;8 this renders the gas-phase adsorption method impractical for alkyl chains of less than 3-4 carbon * Corresponding author. Phone: 54-351-4334169. E-mail: martin@ fcq.unc.edu.ar. † Departamento de Fisicoquı´mica, INFIQC. ‡ Departamento de Matema´tica y Fı´sica, INFIQC. § Centro Ato´mico Bariloche. | Universidad Nacional de Cuyo.

atoms because of the extreme exposures required to attain monolayer coverages.9 Immersion of the substrate in a liquid solution can still be used to prepare SAMs of short alkanethiols, although with one important exception: the shortest alkyl chain methanethiol which is a gas at room temperature. Thus, the only possibility that is left to prepare a layer of methylthiolates is to use dimethyl disulfide (DMDS). There are, however, striking differences between layers prepared by gas-phase adsorption and by immersion in a liquid solution. A large number of investigations have shown that the adsorption of DMDS in UHV conditions produces a wellordered monolayer of methylthiolate molecules.10-17 However, the adsorption of DMDS from the liquid phase seems to be more complicated, and reports of layers grown by this method are scarce. An abnormal adsorption behavior has been reported for layers formed by immersion in ethanolic solutions of DMDS.18 On one side the STM images and the S 2p photoemission spectra were found to depend on the concentration of the forming solution, and on the other side it was found that DMDS solutions behaved different than equivalent solutions of alkanethiols and disulfides of longer alkyl chains. This raises the question if the adsorption of DMDS from the liquid phase involves something that is not present in the adsorption from the gas phase and/or in the adsorption from the liquid phase of alkanethiols and disulfides of longer chains. The above considerations have motivated us to make a systematic study of the adsorption of DMDS on Au(111) from

10.1021/jp912060e  2010 American Chemical Society Published on Web 05/17/2010

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the liquid phase. Our aim has been to understand why this process is different from the same process in gas phase as well as from the adsorption from a liquid solution of longer alkanethiols and disulfides. We have used cyclic voltammetry and high-resolution photoelectron spectroscopy to characterize the layers grown on substrates immersed for different times in an ethanolic solution of DMDS. We show that a well-ordered layer of methylthiolate molecules does form at very short immersion times, but that for long immersion times this layer decomposes via S-C bond cleavage, thus leaving a layer of adsorbed sulfur atoms. We have performed also extensive DFT calculations to elucidate the mechanism leading to the S-C bond cleavage under the dipping environment. We show that in the presence of adsorbed oxygenated species, the S-C bond is activated by an oxidative dehydrogenation mechanism of the methyl group of SCH3 which yields the thio-oxymethylene molecule (SCH2O). This is the key intermediate leading to the breakage of the S-C bond because it decomposes into atomic sulfur and formaldehyde with an activation energy barrier of only 1.1 kcal/mol. Experimental Section Gold Substrates. An Au crystal, 4 mm in diameter, oriented better than 1° toward the (111)-face and polished down to 0.03 µm (MaTeck, Ju¨lich, Germany) was used for cyclic voltammetry. Before the thiol assembly process, the electrode was annealed in a hydrogen flame for two minutes, cooled in air and put in contact with water after one minute. Au films (500 nm thick) evaporated on heat resistive glass were employed as substrates for XPS measurements. These substrates were annealed in a butane flame for two minutes and cooled down to room temperature in a stream of nitrogen. Preparation of SAMs. Dimethyl disulfide (abbreviated DMDS), 1-ethanethiol (C2T), and sodium sulfide (Na2S) were purchased from Sigma-Aldrich. All chemicals were used as received without further purification. Self-assembled monolayers were prepared by immersing the previously annealed gold substrates in 1 mM ethanolic solution of DMDS for 1 min, 15 min, 30 min, 1 h, 2 h, and 24 h. The samples were prepared by immersion in a 1 mM ethanolic solution of DMDS previously bubbled with nitrogen. After the modification, the samples were removed from the solution, rinsed copiously with ethanol and Milli Q water, and then blown dry with nitrogen. Cyclic Voltammetry and ac Impedance Spectroscopy. Cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS) measurements were acquired with a Solartron 1260 electrochemical interface and a conventional electrochemical three electrode cell with separate compartments for reference (Ag/AgCl (NaCl 3M)) and counter electrode (Pt wire). The electrolyte was thoroughly deaerated by bubbling with nitrogen prior to each experiment. Electrochemical Impedance spectra (EIS) were recorded in the frequency range 0.1 Hz to 10 kHz. The signal amplitude to perturb the system was 0.01 V. XPS Measurements. The photoemission measurements were carried out at the at the D08A-SGM beamline of the Brazilian Synchrotron Light Laboratory (Campinas, Brazil). The pressure in the analyzer chamber was in the middle 10-9 Torr range. Electron energy spectra were collected with a 150 mm hemispherical analyzer placed at 90° from the light beam in the horizontal plane; all the spectra were recorded with the sample oriented such that the surface normal made an angle of 45° with both the photon beam and the emission direction.

Cometto et al. Survey spectra, C 1s and S 2p core-level spectra were measured at photon energies of 600, 350, and 300 eV, respectively. Au 4f core level spectra were measured before and after each core-level spectrum and were used to calibrate the binding energies against that of the Au 4f7/2 core level at 83.95 eV. Theoretical Methods and Surface Modeling. The firstprinciple atomistic calculations were performed using periodic density functional theory (DFT) as implemented in the PWSCF code.19 Gradient corrections were included in the exchange correlation functional in the PBE formulation.20 Ultrasoft21 pseudopotentials were used for the atomic species. The timeconsuming energy calculations along the reaction pathway were performed on a slab with three layers of metal atoms representing the Au(111) surface. A (23 × 3)R30° unit cell was employed. Some test calculations with a slab with four layers of metal atoms showed that activation energy barriers varied by no more of 0.5 kcal/mol. Binding energies of adsorbates were calculated with slabs with four layer of gold atoms. Brillouin zone integration was performed using a (4 × 4 × 1) Monkhorst-Pack mesh.22 The electron wave functions were expanded in a plane-wave basis set up to a kinetic energy cutoff of 27 Ry (180 Ry for the density). A vacuum thickness of 10 Å was introduced between the thiolated slabs to avoid spurious interactions between neighboring replicas. Only one side of the slab was covered by adsorbates. The positions of all the atoms in the unit cell (except the last layer of gold atoms which was kept fixed) were relaxed in the potential energy determined by the full quantum mechanical electronic structure. The convergence criterion for geometry optimizations was a rms force of 0.01 eV/Å. Other details of the calculations can be found in ref 23. Reaction pathways and energy barriers were calculated using the “Climbing Image Nudged Elastic Band” (CI-NEB) method,24 which has proven to be a very efficient technique to determine minimum energy paths in complex chemical reactions. Most CI-NEB calculations were performed employing seven images and in some cases we used eleven images. Results Cyclic Voltammetry. The formation of compact alkanethiol SAMs by the immersion method normally requires the overnight dipping of the gold substrate in a thiol solution of millimolar concentration.25,26 The voltammetric properties of the reductive desorption of alkanethiols provide valuable information concerning the structure, stability and surface coverage of a SAM. All this information can be extracted from the shape, peak potential and charge under the reductive desorption current peak in an aqueous alkaline solution.27 The appearance of double peaks on the voltammograms has been ascribed to possible differences in the adlayer domains structures and in the binding modes between S-Au28 or to differences in the adsorbed state (physisorption vs chemisorption). Figure 1 presents the voltammograms of substrates immersed for different times in the 1 mM ethanolic solution of DMDS. In the upper part of the figure we have included for comparison the voltammograms of two substrates immersed for 2 and 24 h in an aqueous solution of Na2S. It is seen that the voltammograms of the samples immersed in the DMDS solution undergo drastic changes with the dipping time. At the shortest times the voltammograms are composed mainly of a single peak located at around -0.7 V. At intermediate times the desorption curves display a more complex structure, and finally, at the longest

Decomposition of Methylthiolate Monolayers

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Figure 1. Set of cyclic voltammograms recorded in 0.1 M KOH showing the reductive desorption of the adsorbed species formed after the immersion of the Au(111) substrate during 1 min, 15 min, 30 min, 1 h, 2 h, 24 h, and 1 week in 1 mM ethanolic solution of DMDS (black curves). The reductive desorption of a sulfur layer formed during immersion times of 2 and 24 h in an aqueous Na2S solution is shown in the upper part (red curve). Note the different scales of the CVs for the layers prepared in Na2S and DMDS. Scan rate: 50 mV s-1. Reference electrode: Ag/AgCl (NaCl 3M).

times there is again a single peak but shifted around 0.2 V toward more negative potentials. In Figure 2 we show a set of voltammograms corresponding to substrates prepared under similar conditions in a 1 mM ethanolic solution of 1-ethanethiol (C2T). It is seen that, contrary to what is observed with DMDS, the voltammograms remain essentially the same at all dipping times. The only visible change is a monotonic decrease of the peak width with the dipping time, what is related to a better ordering of the SAMs prepared at longer times. The peak position at -0.78 V fits perfectly with the potential obtained extrapolating the relationship between peak position and chain length reported for the electrodesorption of longer alkanethiolates,29,30 which for n ) 2 gives -0.79 V. Additionally, the peak area is ∼60 µC/cm2, which according to the reaction:

RS-Au + e f RS- + Au

(1)

implies a surface density of 6.23 × 10-10 mol/cm2, slightly smaller than the density of a perfect 3 × 3 layer (7.78 × 10-10 mol/cm2). According to the peak width and surface density, we conclude that the immersion on 1 mM C2T ethanolic solution has led to the formation of a well-ordered layer of ethanethiolates on the Au(111) surface. The reduction potential shifts in the negative direction by -0.2 V per methylene group as the alkyl chain length increases.29 Therefore, considering that the reduction potential of ethanethiolate is -0.79 V, a value of -0.77 V is extrapolated for the methylthiolate monolayer. The experimental value of -0.7 V (Figure 1) is more positive than the extrapolated value. This can be attributed to a non compact monolayer. Whereas

Figure 2. Set of cyclic voltammograms showing the reductive desorption of ethanethiolate monolayers formed after the immersion of the Au(111) electrode during 1 min, 15 min, 30 min, 1 h, 2 h, and 24 h in a dilute (1 mM) ethanolic solution of ehtanethiol. Voltammograms were acquired under the same experimental conditions as in Figure 1.

the reductive desorption charge for a compact monolayer is around 80 µC/cm2, the desorption charge of the methylthiolate monolayer is 51 µC/cm2. Therefore, we think that this is the reason why the reduction potential shifts toward more positive potentials. Ion permeation through a less compact monolayer will be facilitated and this is expected to shift the reduction potential to more positive values. To characterize the evolution of the voltammograms shown in Figure 1, the current peaks have been fitted with four Gaussian-type functions. The position, full-width at halfmaximum (fwhm), and area of each feature, labeled V1, V2, V3, and V4 in order of increasing negative potential, are listed in Table 1. The voltammogram corresponding to the shortest dipping time (1 min) exhibits a prominent peak at -0.701 V (V1), and another less intense feature at -0.941 V (V4). The peak V1 is located at a less negative potential than the desorption peak of C2T (Figure 2); since this is the expected peak position for a shorter chain, we ascribe peak V1 to the desorption of methylthiolate molecules according to reaction 1. Despite the peak V1 is broader than the desorption peak of C2T, the fwhm ) 44 mV is reasonably small and indicates a good degree of order in the layer. The peak area is 51 µC/cm2, what corresponds to about 2/3 of the coverage of a complete 3x3 monolayer (ML). Therefore, at 1 min dipping time, a layer of methylthiolates does form, although it is not yet completely ordered and the surface is not yet completely covered. As the dipping time is increased, the peak V1 looses intensity and new features, V2 and V3, appear and eventually dominate. Table 1 shows that the width of the peak V1 first decreases, reaching a minimum value of 25.5 mV at 15 min, and then increases again while the peak gradually vanishes. This indicates that, as expected, the order of the methylthiolate layer tend to increase with the dipping time. However, it is evident that another process that competes with the formation of the

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TABLE 1: Fitted Parameters Obtained from the Cyclic Voltammograms Shown in Figure 1a V1 (thiolates) sample DMDS

Na2S

1 min 15 min 30 min 1h 2h 24 h 1 week 2h 24 h

V3 (sulfur at terraces)

E

Q

fwhm

–701 –684 –691 –714 –737

51 42 25.5 11 7

44 37 26 48 63

E

Q

fwhm

–893 –876 –900 –895 –922 –917 –914 –915

9 24 23.5 45.5 91 89 101 144

77 98 58 72 61 52 45 30

V2 (sulfur at terraces) E

–861 –877 –905 –896 –902

Q

16 10 17 23.5 42

fwhm

40 26 18 16 15

V4 (sulfur at steps) E

Q

fwhm

–941 –951 –957 –957 –960 –970 –968 –967 –966

8.5 3 3.5 9 5.5 4 7.5 11 15

67 32 34 48 36 40 38 42 32

a Each reductive current peak was fitted with a gaussian-type function in order to obtain the peak potential (E/mV), the full-width at the half medium (FWHM/mV), and the charge (Q/µC cm-2).

methylthiolate layer appears and eventually dominates, leading to the complete disappearance of the methylthiolate layer. The voltammograms of the samples left 24 h and 1 week in the DMDS solution are quite similar; this shows that at 24 h the transformation of the layer has already reached the final stage. It is seen that these two voltammograms are in turn very similar to those of the substrates immersed in the Na2S solution, shown in the upper part of figure 1. These last two current profiles are in good agreement with those reported in the literature for a layer of S atoms adsorbed at fcc sites in a 3 × 3 arrangement.31,32 Since the desorption reaction in this case involves two electrons

S-Au + 2e f S2- + Au

(2)

the reductive desorption charges of the S layers formed after dipping in the Na2S (143 and 144 µC/cm2) correspond quite closely to a complete 3 × 3 layer of S atoms. Then, we conclude that in the samples prepared by immersion 24 h and 1 week in the DMDS solution the initial layer of methylthiolate has transformed completely into a pure S layer. Furthermore, according to reaction 2, the charges of the desorption peaks (V2 and V3) in the voltammograms of the samples prepared with 24 h and 1 week dipping times (108 and 112 µC/cm2, respectively) correspond to a density of S atoms equivalent to that of the methylthiolate molecules in the sample prepared with 1 min dipping time. Therefore, these results suggest that all the molecules of the methylthiolate layer formed in the first minute transformed with time during immersion in the forming solution into S atoms adsorbed at fcc sites. Finally, we consider the small feature V4 that appears at the largest negative potentials. This feature is present in all the voltammograms, including those corresponding to the desorption of S adlayers. Vericat et al,31 correlating the features in the voltammograms of a S adlayer with STM images, attributed this peak to S atoms adsorbed at step edges. If one makes this assignment it is seen that the surface coverage of this type of atoms does not evolve with the dipping time. Having assigned all the features, Figure 3 shows the evolution with the dipping time of the densities of methylthiolate molecules and of S atoms as derived from the desorption peaks according to the reactions 1 and 2, respectively. It is clearly seen that the disappearance of the methylthiolate molecules correlates with the appearance of S atoms adsorbed at fcc sites. Biener et al.33-35 found that, upon exposure of the gold surface to SO2 and after annealing to 420 K, a gold sulfide layer is formed. We rule out the formation of such a phase under our

Figure 3. Surface density of adsorbed thiolate molecules (black circles), sulfur atoms at terraces (red circles), and sulfur atoms at step edges (open circles) after different dipping times.

experimental conditions. Spectroscopic and electrochemical experiments have shown that when the sulfur adlayer is formed from an aqueous Na2S solution at room temperature only adsorbed sulfur species are observed on the surface.36 As pointed out in ref 36, the reduction of a gold sulfide layer should have a charge of 220 µC cm-2 and these authors obtained a charge of ∼170 µC cm-2. In our work, after a dipping time of 2 h in a Na2S solution, we obtained a reduction charge of 144 µC cm-2. Therefore, the electrochemical measurements are inconsistent with the presence of a 2D AuS phase. Commercially available thiols may contain a trace amount of sulfur as an impurity.37 In order to rule out any effect associated with impurities, a sample prepared by a 1 min dipping in the ethanolic DMDS solution was then immersed in absolute ethanol during 24 h. The voltammogram of this electrode was identical to the one obtained after a 24 h dipping in the ethanolic solution of DMDS (see Figure 1). This proves that the sulfur content of the surface originates from the initially formed methylthiolate monolayer. We close this section by stating the most important conclusions that have been reached. First, similar to what occurs with immersion in an ethanethiol solution, immersion during a few minutes in a DMDS solution produces a layer of methylthiolates. Second, the full coverage corresponding to 1/3 ML is not reached, very likely because the immersion time is too short. Third, if one increases the dipping time beyond a few minutes, a new process sets in that leads to the S-C bond scission; this gradually transforms the methylthiolate layer into a S adlayer. For dipping times of the order of some hours nothing of the methylthiolate layer is left, and the substrate is completely



Figure 5. S 2p photoemission spectra of samples prepared with 1 min, 24 h, and 1 week dipping times.

Figure 4. (a) Bode-type impedance spectra recorded in 0.1 M KOH at -0.3 V after the immersion of the Au(111) substrate during 1 min, 15 min and 24 h in 1 mM ethanolic solution of DMDS; and 24 h in an aqueous solution of Na2S. The spectrum of bare gold recorded in 0.1 M KOH at -0.3 V is also included as a reference. The spectra were fitted with the circuit shown in the inset. (b) Capacitance of the bare Au(111) surface and the modified surfaces as a function of the dipping time in Na2S and DMDS solutions.

covered with a layer of S atoms. Fourth, this process does not occur during immersion of the substrate in the C2T solution. Electrochemical Impedance Spectroscopy. Figure 4a shows Bode-type impedance spectra of layers formed after 1 min, 15 min, and 24 h dipping time in DMDS. As a reference, the spectrum of a monolayer of atomic sulfur formed after 24 h immersion in Na2S and bare gold one are also included (Figure 4a). Impedance spectra were recorded in the KOH solution at a potential of -0.3 V (almost open circuit potential), inside the potential window of the layer stability. The spectra are dominated by the typical capacitive behavior showing a linear log|Z| vs log(f) relationship and phase angles very close to 90°. The spectra were fitted with the circuit shown in the inset of Figure 4a. A constant phase element (CPEm) was used to describe the dielectric behavior of the layer. The variation of the layer capacitance Cm as a function of dipping time in DMDS solution is shown in Figure 4b. Surprisingly, the capacitances remain practically constant with the immersion time: 10.1 µFcm-2 < Cm < 12.4 µFcm-2. The dotted lines in Figure 4b correspond to the capacitance of the bare gold electrode (∼24 µF cm-2) and the capacitance of a sulfur monolayer (∼17 µF cm-2) formed after an immersion time of 24 h in a Na2S solution. The voltammetry results of the previous section showed that the initially formed methylthiolate monolayer is fully

converted to a sulfur layer with a surface coverage which is 2/3 of the total surface coverage. However, a gold surface with incomplete sulfur coverage should have a capacitance in between that of a bare gold surface and a surface fully covered by sulfur (dotted lines in Figure 4b). Therefore, the fact that the capacitance values remain constant and lower than those of a compact sulfur layer, indicates that the surface is not covered solely by sulfur atoms. As we shall see in the next sections, there is evidence of physisorbed DMDS molecules as well as adsorbed carbon species probably originating from the ethanol solvent. XPS Measurements. To confirm the transformation of the methylthiolate layer into a S layer with the immersion time, we measured high-resolution S 2p and C 1s photoemission spectra. This technique is very well suited for this purpose because the S 2p peaks have fairly large chemical shifts, what facilitates the identification of the different species on the surface. For example, while the S 2p spectra of alkanethiol SAMs are composed of only one S 2p3/2,1/2 doublet, with the 2p3/2 peak appearing at around 162 eV,38-41 the spectrum of S2 adsorbed on Au(111) may display up to three components, with the 2p3/2 peaks appearing at around 161, 162, and 163-164 eV.32,42 Studying the relative intensities of these components as a function of the S coverage, they have been assigned to atoms adsorbed at fcc sites, to atoms adsorbed forming Sn aggregates, and to atoms in S multilayers, respectively.42 The contributions of physisorbed molecules are expected to appear at binding energies (BEs) higher than 163 eV,38 while those of oxidized S species are expected to appear at much higher BEs, beyond 165-166 eV.43 Figure 5 shows the S 2p spectra of three samples prepared by immersion 1 min, 24 h, and 1 week in DMDS solutions. The intensities are shown normalized to that of the Au 4f7/2

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TABLE 2: Parameters Obtained from the Fittings of the Photoemission Spectra Shown in Figures 5 and 6a 1 min S1 S2 S3 C1 C2 a

24 h

1 week

BE

fwhm

%

BE

fwhm

%

BE

fwhm

%

161.15 162.04 163.15 284.34 285.80

1.08 0.77 2.38 1.20 1.20

11 79 10 94 6

161.23 162.05 163.19 284.34 286.07

0.88 0.92 1.12 1.46 1.46

49 32 19 87 13

161.26 162.08 163.26 284.30 285.9

0.89 0.79 1.55 1.49 1.49

51 27 22 78 22

The BEs and FWHMs are in eV.

Figure 6. C 1s photoemission spectra of the same samples of Figure 5.

peak, which has been assigned an intensity of 100. The spectra have been fitted with three components, named S1, S2, and S3, whose positions, FWHMs, and relative intensities are listed in Table 2.44 It is seen in Figure 5 that the spectrum of the sample prepared with 1 min is dominated by the S2 component, which appears at 162.04 eV with a fwhm of 0.77 eV. Both the position and fwhm are in very good agreement with the values obtained by Zharnikov et al for a 1-dodecanethiol SAM using a slightly better energy resolution:45 162.02 and 0.52 eV, respectively. This indicates that most of the sulfur on the surface of this sample corresponds to a well-ordered layer of methylthiolate molecules, in complete agreement with the finding of the cyclic voltammetry. Besides the S2 component, the spectrum has also a small but noticeable S1 component at 161.15 eV. This component denotes the existence of a small amount of atomic S, what is also in agreement with the occurrence of a small peak in the voltammogram at 1 min dipping time assigned to desorption of atomic S at step edges. In the spectrum of the sample prepared by immersion for 24 h, there are several changes to be noted. The most important one is the large increase of the S1 component, which is now the main component with about 50% of the total intensity. Second in intensity is the S2 component, which is slightly broader than in the sample immersed for 1 min. Another change is the growth of the S3 component. The increase of the S1 component, ascribed to atomic S adsorbed at fcc sites, implies

by itself dissociation of the S-C bond of an important fraction of the methylthiolate molecules; very interestingly, the intensity of this component is similar to that of the S2 component in the sample prepared with 1 min dipping time. Therefore, we see that the number of S atoms adsorbed at fcc sites in the sample immersed for 24 h is very nearly the same as that of methylthiolate molecules in the sample immersed for 1 min, what again confirms the findings in the cyclic voltammetry section. The relatively large intensity of the S2 component, however, seems to be in conflict with the voltammogram of the sample prepared with this dipping time, which shows no signal of desorption of methylthiolate molecules. We are then forced to change the assignment for the S2 component in this sample. To do this we first note that the S 2p spectrum of this sample is very similar to that of S atoms adsorbed on Au(111) at a S coverage slightly above 1/3 ML,42 where besides the main component at 160.8 eV, assigned to S atoms in fcc sites, there is another component at around 162 eV assigned to S atoms adsorbed forming Sn aggregates. Interestingly, Vericat et al31 have found that this last S 2p component can be associated to atoms that in the STM images form rectangular-shape S8 structures, and which desorb at around -0.8 V producing only a very shallow hump in the voltammograms. Thus, we conclude that the S2 component in the sample immersed for 24 h must correspond to this type of S atoms rather than to methylthiolate molecules. Now, if one uses the surface coverages determined with the cyclic voltammetry, the intensity of the S1 component must correspond to a coverage of 2/3 of 1/3 ML, and thus the intensity of the S2 component must correspond to half that coverage. Therefore, we can conclude that at 24 h there is a coverage of S atoms equivalent to 1/3 ML, where about 66% of the atoms are adsorbed at fcc sites in a 3 × 3 array and 33% are adsorbed forming Sn structures. The origin of the S3 component is discussed below. Finally, the spectrum of the sample immersed for 1 week is similar to that of the sample prepared with 24 h; the only detectable difference is a slight narrowing of the S2 component and the increase of the S3 component. In summary, the presence of the components S1 (atomic sulfur) and S2 (polysulfides) at long immersion times, implies the conversion of the sulfur content initially present as methylthiolate (component S2 at 1 min of immersion time) into sulfur species. Therefore, the XPS results are consistent with the CV data. S 2p spectra similar to those shown in figure 4 have been reported by Noh et al18 for Au(111) substrates immersed for 1 h in 1 mM and 50 mM DMDS solutions. The spectrum of the sample prepared in the 1 mM solution is quite similar to the spectrum of our sample immersed for 1 min; there is a small component S1 and a very intense component S2, and the authors correctly conclude that this indicates the formation of a good methylthiolate layer. In the spectrum of the sample immersed in the 50 mM solution the most important change is the strong

Decomposition of Methylthiolate Monolayers increase of the S1 component, similar to what occurs in our spectra of 24 h and 1 week dipping times. We think that the authors have wrongly assigned this component at 161 eV to methylthiolate molecules chemisorbed in other configurations. Thence, the presence of S atoms, and thereby the degradation of the layer, have passed undetected. Despite a difference in the exposition scales for which we have no explanation, we think that this work confirms our findings in the sense that a good methylthiolate layer is produced at low exposition and that at high expositions this layer is partially or totally transformed into a S layer. The C 1s spectra are shown in Figure 6. The spectrum of the sample immersed for 1 min is dominated by a single component that appears at 284.34 eV, with a fwhm of 1.20 eV. Although the position is close to that predicted for methylthiolate (284.7 eV) extrapolating from measurements made on longer alkanethiols,46 the fwhm is larger than expected. The intensity of the C1s peak is about 2-3 times larger than what would be expected for a pure methylthiolate layer. Therefore, both the fwhm and the relative intensity point to the existence of spurious C on the surface of this sample. The spectra of the samples immersed for 24 h and 1 week are still dominated by the component at 284.3 eV (C1), but a new component (C2) has emerged at around 286 eV. The intensity ratios of the C 1s and S 2p peaks of these two samples are approximately the same as at 1 min. Therefore, we conclude that also in these samples there must be an important amount of C whose origin is not related to the methyltiolate molecules. The finding of an important amount C on all the surfaces may explain the results of the capacitance measurements presented in the previous section, which indicated that the surface could not be covered solely by sulfur atoms. The possible nature of these C species will be discussed below after presenting the mechanism of methylthiolate decomposition. Finally, regarding the S3 and C2 components, we assign them to undissociated DMDS molecules loosely bound to the surface. This assignment is based on the following two observations. First, the intensities of these components grow roughly proportional, suggesting a common origin. Second, the BE positions of S3 and C2 are consistent with what is expected for physisorbed molecules; on one side the difference in BEs between of S3 and S2 is 1.2 eV, close to the 1.5 eV shift expected for undissociated DMDS,13 and on the other side the BE of C2 is close to the value reported for methanethiol (CH3SH) physisorbed on a Cu(110) surface (286.3 eV).47 The adsorption of undissociated DMDS molecules on the surface of Au nanoparticles in concentrated DMDS solutions has been clearly evidenced by Raman spectroscopy.18 We think that in our experiment the adsorption of entire DMDS molecules might occur at long dipping times because the growing S layer would inhibit the access of the DMDS molecules to the bare Au surface, and thereby their dissociation. Nevertheless, the molecules could remain attached on the overlayer stabilized by the positive charges induced on the Au atoms by the electronegative S atoms. Discussion and DFT Calculations From both the cyclic voltammetry and the photoemission spectra, one arrives at the same conclusion. Short dipping times produce methylthiolate layers of good quality, although with a coverage of only 2/3 of a complete 3 × 3 ML. However, at long immersion times, the monolayer decomposes giving rise to atomic sulfur. These results show that, contrary to what occurs in gas-phase adsorption, the formation of the monolayer by the

J. Phys. Chem. C, Vol. 114, No. 22, 2010 10189 immersion method favors the cleavage of the S-C bond of methylthiolate. The formation of a monolayer by the immersion method is different than under UHV conditions because it involves an electrochemical reaction. In the case of disulfides, the reaction is cathodic because it involves a net electron transfer from the metal toward the adsorbate:48

RSSR + Au(111) + e f Au(111)-SR + RS-

(4)

A part of the thiolate RS- produced in the above reaction may be adsorbed on Au according to the following reaction:

RS- + Au(111) f Au(111)-SR + e

(5)

whereas the remaining part of RS- may diffuse away into the bulk of solution and will be converted into thiols by abstraction of protons from the solution. As reaction 4 occurs to a greater extent than reaction 5, the net current that is observed at the outer circuit is cathodic, and produces a positive charging of the electrode surface.48 We verified that the positive charging of the electrode surface reported in ref 48 after the addition of DMDS is very reproducible. An electrochemical counter reaction is required to discharge the metal/electrolyte interface. In the case of alkanethiol adsorption, which produces a negatively charged interface, the counter reaction is the reduction of oxygen molecules always present in the dipping solutions.48 In the case of disulfide adsorption, several mechanisms may contribute to the discharging of the surface. Traces of water in the ethanol solvent may decompose on the positively charged surface according to the following reactions:

H2Oads f OHads + Hsol+ + e

(6)

OHads f Oads + Hsol+ + e

(7)

which leave a surface with adsorbed hydroxyl and oxygen atoms. Ethanol may also loose its proton giving rise to adsorbed methoxy molecules

CH3CH2OHads f CH3CH2Oads + Hsol+ + e

(8)

Although the discharging reactions 6-8 have not been proved, there is experimental evidence which supports these reactions: on the positively charged gold surface, adsorption of carboxylic acids from liquid solutions occurs by deprotonation of the acid.49 Therefore, the presence of oxygenated species must be taken into account in order to elucidate the mechanism responsible for the activation of the SC bond of methylthiolate. As we will discuss below, the role of oxygenated species is to abstract a hydrogen atom of adsorbed SCH3 thus triggering a series of reactions which end up in the facile breakage of the S-C bond. The fact that electrochemical reactions are involved in the formation of the methylthiolate layer from DMDS (reaction 4) and in the appearance of oxygenated species (reactions 6-8) which decompose methylthiolate, was tested by immersing the gold surface under potential control in the DMDS forming solution. After a formation time of one hour, the electrode was

10190


Figure 7. (a) Equilibrium structure of DMDS on Au(111). One of the S atoms of DMDS is nearly ontop of an Au atom. (b) Energy pathway for the dissociation of DMDS into two methylthiolates. The insets show the top views of the reactant DMDS molecule (R), the transition state (TS) and the products (P). In the transition state, the sulfur atoms are monocoordinated to gold atoms. Calculations performed using a (23 × 3)R30° unit cell.

transferred to the NaOH solution and the reductive desorption experiment was performed. A clear trend was observed in the CV profiles: at the most negative forming potential (-0.4 V) there is a prominent peak corresponding to the reductive desorption of the thiolate monolayer. On the contrary, at the most positive forming potential (+0.4 V), the monolayer has nearly disappeared and there is a prominent peak at -0.87 V corresponding to the reductive desorption of sulfur. Therefore, the positive charging of the electrode surface favors the decomposition of the methylthiolate layer. This is attributed to the appearance of oxygenated species which decompose the methythiolate layer as we shall see in the next section. We first consider the energetics involved in the dissociation of DMDS and then present the decomposition pathway of methylthiolate coadsorbed with oxygenated species. Adsorption and Dissociation of DMDS. Figure 7a shows the equilibrium structure of the DMDS molecule adsorbed on Au(111). It adsorbs via one of the S atoms with an adsorption energy of 6.6 kcal/mol. The S atom closer to the surface locates nearly on top of an Au atom with an S-Au distance of 2.67 Å. Figure 7b shows the energy profile along the reaction path for the dissociation of DMDS into two methylthiolate molecules. The S-S bond length of the adsorbed molecule is 2.07 Å and in the transition state the separation between the S atoms is 2.49 Å. After the dissociation, the methylthiolates are bicoordinated to a nearly perfect bridge site as it has been reported in several previous works.16,23,50-54 It can be observed that the

Cometto et al. transition state is closer to the reactants than to the products. The activation barrier for the dissociation of DMDS arises from the enlargement of the S-S bond. We performed CI-NEB calculations with different orientations of the DMDS molecule which produced methylthiolates with different relative orientations. In these calculations we obtained activation energy barriers of 12.8, 13.4, and 15.4 kcal/mol. Figure 7b shows the energy profile corresponding to the lowest activation barrier. These values are in very good agreement with the experimental value of 14.6 kcal/mol reported recently in a study of the adsorption and desorption kinetics of DMDS on Au(111).55 Yates et al. observed that under low coverage conditions, an electron injection to a DMDS molecule from an STM tip produces two methylthiolates which maintain the geometrical trans conformation of the CH3 groups in the parent CH3SSCH3 molecule.56 In our calculations, the methylthiolates produced after the cleavage of the S-S bond do not maintain the trans symmetry of the original DMDS molecule due to steric effects with neighboring molecules. The CI-NEB calculation was performed under high coverage conditions because after the dissociation of DMDS, the two methylthiolates adsorbed in the (23)R30° unit cell yield the maximum surface coverage of 0.33. The activation barrier for DMDS dissociation into methylthiolates reported experimentally is rather insensitive to the surface coverage. Roper et al.55 obtained a low coverage value of 15.5 kcal/mol and high coverage value is 14.6 kcal/mol. This is due to the fact that the location of the transition state along the reaction coordinate is very close to the reactant DMDS molecule and is therefore insensitive to the final state of the products. The sulfur atoms of the resulting methylthiolates in Figure 7b are separated by a distance of 4.49 Å and their S-C bonds are not aligned. In order to reach the compact conformation in which the S atoms are separated by a distance of 5 Å with all of the S-C bonds aligned, a 30° rotation of one of the methylthiolates is required. We calculated the energy along the reaction pathway for such a rotation and obtained an activation barrier of 3.8 kcal/mol, whereas the activation barrier for the reverse process (disordering of the monolayer) is 6.8 kcal/mol. Therefore, the rotation which leads to an ordering of the monolayer has a lower activation barrier than the rotation that disorders the monolayer. These values are in good agreement with the value of 5.2 kcal/mol reported by Yates et al for the rotation of a methylthiolate molecule under low coverage conditions.56 The physisorbed DMDS molecules may freely rotate on the surface because the activation barrier is very low.56 After cleavage of the S-S bond the resulting methylthiolates will be randomly oriented on the surface and in order to form a compact monolayer, they have to overcome the activation barrier for rotation. In the case of alkanethiols with longer alkyl chains, the driving force to form an all-trans monolayer is given by the van der Waals interactions among the alkyl chains. These interactions are obviously the lowest in the case of methylthiolate. Therefore, these observations explain the fact that the fwhm of the reductive desorption peak of a freshly formed methylthiolate monolayer is higher than for longer alkyl chains. Energetics of the Direct S-C Bond Breakage. The energy profile for the decomposition of SCH3 into S and CH3 fragments is shown in Figure 8. On the clean Au(111) surface, this reaction has a high activation barrier of 29.9 kcal/mol and is endothermic by 7.3 kcal/mol. The location of the transition state along the reaction coordinate is very close to the reactant methylthiolate and the activation barrier mainly arises from the enlargement


J. Phys. Chem. C, Vol. 114, No. 22, 2010 10191 Decomposition of Methylthiolate with Coadsorbed Oxygenated Species. In the presence of coadsobed oxygen species, the activation of the S-C bond could originate from (a) the direct rupture of the S-C bond to yield adsorbed methoxy species, (b) the oxidation of the sulfur atom, and (c) the partial oxidation of the CH3 group of SCH3. For the sake of simplicity, we mainly considered the oxygen atom as the oxygenated species. Reactions with OH and O2 are also considered in some elementary steps. Adsorbed oxygen atoms greatly change the reactivity of metal surfaces. Surface science studies have shown that coadsorbed oxygen on late transition metals acts as a Brønsted base.59 The formation of methoxy according to

SCH3 + O f S + OCH3 Figure 8. Energy profile along the reaction pathway for the decomposition of SCH3 into a sulfur atom and a methyl radical.

of the S-C bond from the equilibrium value of 1.83 Å of adsorbed methylthiolate to the value of 2.40 Å in the transition state. It is well established experimentally that adsorption of thiolate species involves Au adatoms. A low-temperature STM study showed that low- and high-coverage structures of methylthiolate on the Au(111) surface have a building block consisting of two thiolate species joined by a gold adatom.57 However, a photoemission core-level shift investigation appears to favor the Auadatom-thiolate model over the Au-adatom-dithiolate model.58 In the Au-adatom-thiolate model the thiol is monocoordinated to the gold adatom23 whereas in the Au-adatom-dithiolate model the thiols are bicoordinated (coordination to the adatom and to the metal surface).57 For both the thiolate and dithiolate adatom models we calculated the activation energy barrier for the breakage of the S-C bond of methythiolate and we found values around 30 kcal/mol. This indicates that the presence of adatoms does not alter the activation energy barrier with respect to adsorption on the perfect Au(111) surface. The activation energy for the dissociation of DMDS into two thiolates does not either seem to be affected by the presence of gold adatoms on the surface. As mentioned in the previous section, we obtained activation energy barriers of 12.8, 13.4, and 15.4 kcal/mol on the perfect Au(111) surface in good agreement with the experimental value of 14.6 kcal/mol.55 In order to elucidate other possible mechanisms leading to the activation of the S-C bond, we calculated the energy profile for the dissociation of the S-C bond in the presence of a) a water molecule, b) a coadsorbed sulfur atom and c) a positively charged surface. The aim of these calculations was to evaluate the influence of the solvent, the influence of sulfur impurities on the surface and the influence of the positive surface charge of the metal/electrolyte interface during the dissociation of DMDS into methylthiolates.48 In the presence of a water molecule the activation barrier is 30.3 kcal/mol whereas in the presence of a coadsorbed sulfur atom the activation energy is 31.8 kcal/mol. These values are very close to the value of 29.9 kcal/mol on the clean Au(111) surface indicating that there is no major effect on the stability of the S-C bond. The calculation on the positively charged surface was performed by removing one electron from the unit cell. This gives a surface charge of 17.9 µC/cm2 for the (23 × 3)R30° cell used in these calculations. Under these conditions, we obtained a slightly lower activation energy barrier of 27.0 kcal/ mol. These results show that none of these mechanisms produce an important activation of the S-C bond.

(8a)

is exothermic by -21.7 kcal/mol; however, it has a high activation energy barrier of 38.3 kcal/mol. The oxidation of the sulfur atom of methylthiolate according to

SCH3 + O f OSCH3

(9)

is also exothermic (-24.3 kcal/mol) and has an activation energy barrier of 30.2 kcal/mol. However, the breakage of the S-C bond according to

OSCH3 f SO + CH3

(10)

is endothermic (14.8 kcal/mol) and has a high activation barrier (41.5 kcal/mol). Therefore, reactions 8 and 10 do not activate the breakage of the S-C bond. The high activation of reaction 10 is consistent with the experimental results presented in previous sections which show that atomic sulfur (and not sulfur oxide) is the observed species on the surface after the decomposition of methylthiolate. Hydrogen abstraction from the methyl group leads to a series of reactions with low activation energy barriers which end up in the facile rupture of the S-C bond. The hydrogen abstraction by an oxygen atom, an OH radical and dioxygen are exothermic

SCH3 + O f SCH2 + OH

∆E ) -8.9 kcal/mol (11)

SCH3 + OH f SCH2 + H2O

∆E ) -14.0 kcal/mol (12)

SCH3 + O2 f SCH2 + OOH

∆E ) -5.3 kcal/mol (13)

However, the hydrogen abstraction by the oxygen atom has the lowest activation energy barrier (10.2 kcal/mol) whereas the hydrogen abstraction by O2 has the highest barrier (20.2 kcal/mol). Figure 9 shows the energy profile along the reaction path for the dehydrogenation of SCH3 by an oxygen atom (reaction 11). In the reactant methylthiolate molecule, the Au-S bond length is 2.43 Å, and it increases to 2.71 Å in the transition state. When the SCH2 molecule adsorbs, the Au-S an Au-C bond lengths are: 2.46 and 2.13 Å, respectively. The SCH2 intermediate has the sulfur atom bicoordinated to two gold atoms and the carbon

10192


Cometto et al.

Figure 9. Energy profile along the reaction pathway for the abstraction of a hydrogen atom of SCH3 by an oxygen atom. The insets show the reactants, transition state and products.

atom monocoordinated to a gold atom. SCH2 has been proposed to be an intermediate for dehydrogenation of SCH3 on Pt(111).60 The intermediates involved in the successive dehydrogenations of the methyl group of SCH3 have increasing activation barriers for the breakage of the S-C bond. The activation barriers for the breakage of the S-C bond of SCH3, SCH2 and SCH are: 29.9 kcal/mol, 32.0 and 36.9 kcal/mol, respectively. This implies that the partial dehydrogenation of the methyl group does not activate the S-C bond. The insertion of an oxygen atom or a hydroxyl to the SCH2 intermediate is very exothermic:

SCH2 + O f SCH2O SCH2 + OH f SCH2OH

-41.6 kcal/mol

(14)

-26.2 kcal/mol

(15) These reactions have low energy barriers: 11.3 and 14.5 kcal/ mol, respectively. The dehydrogenation of the alcohol (produced in reaction 15) on the clean gold surface

SCH2OH f SCH2O + H

the Au(111) surface in the presence of atomic oxygen, in agreement with the low activation barrier of reaction 17. The presence of a hydroxyl group bound to the carbon atom produces only a small activation of the S-C bond. The activation energy barrier for the reaction

SCH2OH f S + CH2OH

(18)

is 26.8 kcal/mol. It is only a few kcal lower than that for the breakage of the S-C bond of methylthiolate (29.9 kcal/mol, see Figure 7b). Reactions 14 and 17 produce the thio-oxymethylene intermediate, SCH2O. This species has been postulated as an intermediate in the formation of formaldehyde from the oxidation of SCH3 produced from dissociation of DMDS on the Zn(0001) surface.62 Our calculations confirm that thio-oxymethylene is the intermediate which leads to the cleavage of the S-C bond with a very low activation barrier. The energy profile along the reaction pathway for the decomposition of SCH2O into sulfur and formaldehyde

SCH2O f S + CH2O

(19)

(16)

has a high activation barrier of 30.9 kcal/mol whereas in the presence of atomic oxygen

SCH2OH + O f SCH2O + OH

Figure 10. Energy profile during the decomposition of thio-oxymethylene into atomic sulfur and formaldehyde.

(17)

the reaction has a very low activation barrier of 5.6 kcal/mol and is slightly exothermic by -2.5 kcal/mol. The large difference in the magnitude of the activation barriers of reactions 16 and 17 is in agreement with the results of Mullins and coworkers on the surface chemistry of methanol on atomic and oxygen precovered Au(111).61 On the clean Au(111) surface, they observed no evidence for dissociation of methanol which correlates with the high activation barrier of reaction 16, whereas the decomposition and oxidation of methanol was activated on

is shown in Figure 10. The reaction has an activation barrier of only 1.1 kcal/mol and is exothermic by -6.1 kcal/mol. In the reactant SCH2O molecule, the S-C bond length is enlarged (1.91 Å) with respect to the S-C bond length value of SCH3 (1.84 Å). This is therefore the reaction responsible for the facile breaking of the S-C bond. Figure 9 shows that the formaldehyde molecule produced by reaction 19 does not bind to the surface implying that during the immersion process it will desorb into the solution phase. This is consistent with the XPS results which only show traces of oxygen on the surface. Our results also shed some light on the origin of the carbon species which may contribute to the C 1s signal. As the oxidative dehydrogenation of SCH3 produces formaldehyde (reaction 19), which does not adsorb on the surface (Figure 6), we rule out the carbon atom of SCH3 as one of the sources of carbon after the decomposition of methylthiolate. Therefore, the remaining



sources of carbon may be undissociated DMDS molecules and carbon species originating from the ethanol solvent. The presence of undissociated DMDS was deduced from the simultaneous appearance of the C2 and S3 peaks. The assignement of these peaks to physisorbed DMDS was verified by annealing at 250 °C. The effect of the annealing of a sample immersed for 1 week in the DMDS was to reduce the spectral weight in the regions of the C2 and S3 components, making thereby the assignment to physisorbed DMDS plausible. We think that most of the carbon content originates from the decomposition of the ethanol molecules of the forming solution. Ethanol may decompose following equivalent reactions to those of methylthiolate. Adsorbed ethoxy radicals obtained from deprotonation of ethanol on the positively charged surface may decompose giving rise to adsorbed methyl radicals by elimination of formaldehyde

dissolved in the ethanol solvent. This assures a continuous supply of O2 to decompose all the monolayer. In summary, we have found a reaction mechanism which explains the decomposition of methylthiolates produced from the dissociation of DMDS in solution phase. The activation of the S-C bond requires the oxidative dehydrogenation of the methyl group to yield a key intermediate which is the thiooxymethylene molecule, SCH2O. Below is a list of the elementary steps with the lowest activation energy barriers and their corresponding energy differences

CH3CH2O (ads) f CH3 (ads) + CH2O (gas)

It is seen in the above list that the activation barriers of the three steps that lead to the cleavage of the S-C bond are all smaller than the barrier for the dissociation of the DMDS on the clean Au surface. Since this latter reaction occurs readily at room temperature, the cleavage of the S-C bond should be limited only by presence of O atoms in the neighborhood.

(20)

Deprotonation of the methyl group of ethoxy gives rise to the CH2CH2O species which binds to the surface by both ends producing an oxametallacycle. This intermediate is very well know in the ethylene epoxidation reaction on silver.63 This species may decompose releasing formaldehyde to the gas phase and producing an adsorbed CH2 radical

CH2CH2O (ads) f CH2 (ads) + CH2O (gas)

(21)

These reactions enrich the surface with carbon species and the adsorbed radicals may further recombine. Another possible route for the oxidation of ethanol is the conversion to ethanal and finally to ethanoic acid. However, it is well-known that hard headgroups such as the carboxylate headgroup do not exhibit chemisorption on gold.64 Therefore, if ethanol is oxidized to ethanoic acid, it will desorb into the solution. In summary, the oxidation of the ethanol solvent may leave on the surface carbon species such as CH2 and CH3 (which may further recombinate to CH2CH3), but it will not leave oxygen containing species such as formaldehyde or ethanoic acid because they desorb into the solution phase. This explains the lack of oxygen observed in the XPS spectra. In a study of the short chain 2-mercaptoethanol (HSCH2CH2OH), two reductive desorption current peaks were observed.65 The sharp peak at -0.65 V corresponded to desorption of the thiol whereas the peak observed at -0.95 V was attributed to the desorption of atomic sulfur. It is quite likely that in the alkaline solution in which the reductive desorption experiments were carried out, some hydroxyls of the alcohol become deprotonated giving rise to thio-oxyethylene, SCH2CH2O. This intermediate will probably activate the S-C bond cleavage as it is the case for the SCH2O intermediate reported in this work. Breakage of the S-C bond of SCH2CH2O gives rise to the CH2CH2O intermediate which is well-known on Pd(111)63 as a key intermediate in the epoxidation of ethylene. We showed in the experimental section that a sample prepared by a 1 min dipping in the ethanolic DMDS solution was then immersed in absolute ethanol during 24 h. The voltammogram of this electrode only showed the presence of sulfur on the surface indicating the decomposition of the monolayer. We think that the decomposition mechanism does not take place according to reactions 6-8). In this medium, the decomposition may be triggered by reaction 13 which involves molecular oxygen

CH3SSCH3 f 2SCH3 ∆E ) -8.0 SCH3 + O f SCH2 + OH ∆E ) -8.9 SCH + O f SCH2O ∆E ) -41.6 SCH2O f S + CH2O ∆E ) -6.1

Ea Ea Ea Ea

) ) ) )

12.8 10.2 11.3 1.1

Conclusions The main observation of this work is the time evolution of methythiolate monolayers prepared by the immersion method from ethanolic solutions of DMDS. The adsorption of DMDS on Au(111) was investigated by cyclic voltammetry and highresolution photoelectron spectroscopy. Both techniques confirmed the formation of a well-ordered methylthiolate monolayer at short immersion times (around one minute): a reductive desorption current peak at -0.7 V and an S 2p binding energy of 162 eV. As the immersion time in the ethanolic DMDS solution increased, the CV peak at -0.7 V gradually disappeared and a new peak at -0.9 V appeared. The charge of the latter peak doubled that of the methythiolate current peak at -0.7. This observation, together with the fact that the reductive desorption of sulfur prepared from a Na2S solution occurred at the same potential of -0.9 V, led us to identify this peak as the reductive desorption of sulfur. The XPS measurements at long immersion times showed two main components for the S 2p signal at ca. 161 and 162 eV. The first one could be unambiguously identified as atomic sulfur in agreement with the electrochemical results. The S 2p component at 162 eV can be assigned to thiolates as well as to polysulfide species. Considering the CV profiles showed no thiolates at long immersion times, we attributed this peak to sulfur species present as polysulfides. The C 1s peak showed twice more carbon than sulfur on the surface. We have no definitive explanation for this. However the XPS experiments were quite reproducible and we disregard any contamination. We think that the extra carbon could originate from the solvent molecules as well as from some physisorbed DMDS molecules. The appearance of sulfur species on the surface was attributed to the breakage of the S-C bond of methylthiolate. After a systematic examination of a large number of elementary reaction steps, we found that only the partial oxidation of the methyl of SCH3 may activate the S-C bond. The oxidative dehydrogenation of methylthiolate leads to the thio-oxymethylene intermediate (SCH2O). The cleavage of the S-C bond of SCH2O occurs with a very low activation energy barrier of 1.1 kcal/ mol giving rise to atomic S, that remains adsorbed on the

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JP912060E

Decomposition of Methylthiolate Monolayers on Au (111) Prepared

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