Interaction of Oxidized Copper Surfaces with Alkanethiols in Organic

J. Phys. Chem. C 2010, 114, 3945–3957

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Interaction of Oxidized Copper Surfaces with Alkanethiols in Organic and Aqueous Solvents. The Mechanism of Cu2O Reduction C. A. Calderoń,† C. Ojeda,† 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, Ciudad UniVersitaria, 5000 Co´rdoba, Argentina ReceiVed: May 14, 2009; ReVised Manuscript ReceiVed: January 12, 2010

The interaction of 1-octanethiol, 1,8-octanedithiol, 1-hexadecanethiol, and 16-mercaptohexadecanoic acid with polycrystalline copper surfaces was investigated comparatively using forming solutions with polar (0.05 M NaOH solution) and apolar (n-hexane) solvents. The thiol layers were formed on the freshly chemically polished copper surface as well as on the anodically oxidized surface. The effects of the alkanethiol chain length and terminal group on the blocking properties of the surface were investigated. We show for the first time that compact monolayers and multilayers can be obtained from an alkaline forming solution. Copper oxides are completely reduced in the alkaline forming solution for all of the thiols investigated after an immersion time of 45 min. On the contrary, the presence of a surface oxide was always detected after the formation of the thiol layer in the n-hexane solution. The mechanism of Cu2O reduction by thiols was investigated by means of density functional theory calculations. The surface reactions involve the protonation of the surface oxygen atoms of the oxide which act as Lewis base sites. In the alkaline electrolyte, the proton transfer involves the water molecules of the solvent, whereas in the n-hexane solution the proton transfer involves the -SH group of the alkanethiol. The surface reactions are not the rate limiting step because they have very low activation energy barriers. The higher reduction rate observed in the alkaline thiol solutions is due to the high concentration of the reacting water molecules, whereas the lower reaction rate in the n-hexane solutions correlates with the lower concentration of the reactant alkanethiol molecules. Introduction The formation of compact self-assembled monolayers on copper surfaces is a challenging task due to the oxide film that spontaneously forms on the metal upon exposure to air or water. Several strategies have been used to ensure the best conditions for the formation of a compact monolayer. They basically involve the surface pretreatment protocol and the nature of the solvent of the forming solution. The solvent is known to affect the composition and barrier properties of self-assembled monolayers on gold.1,2 On copper surfaces, it has been reported that ethanol as a solvent causes the formation of low-quality monolayers probably because of chemical interactions with Cu. Better results were obtained when toluene was used.3 In most studies, SAMs have been formed from solutions using organic solvents because of their ability to solvate long-chain alkanethiols. As water is the solvent of choice for many applications, water-soluble corrosion inhibitors are required. The use of water as the solvent was investigated for SAMs formed from sodium S-alkyl thiosulfates, as these ionic compounds are considerably more soluble in water than alkanethiols, and they adsorb to the copper surface primarily as thiolates.4 However, it was found that water lowers the quality of these SAMs.4 Enhanced copper surface protection was reported recently in aqueous solutions containing short-chain alkanoic acid potassium salts.5 The organic layer binds to the thin oxide film that spontaneously grows on the copper surface after the mechanical polishing. * Corresponding author. E-mail: [email protected]. Phone: 54-3514334169. † Departamento de Fisicoquı´mica. ‡ Departamento de Matema´tica y Fı´sica.

It is well established that, to achieve good quality SAMs on oxidizable metals, these must be oxide-free when brought into contact with the organothiols.6-13 The surface pretreatment often involves a number of etching steps to remove the native oxide.14 However, copper oxide is an exception. Several studies have shown the possibility to modify the oxidized copper surface without any treatments.15,16 Self-assembled monolayers of alkanethiols were formed on oxidized polycrystalline copper surfaces with an oxide thickness of 50 nm.17 The same monolayer structure was observed on the oxidized and unoxidized copper surfaces.17 The presence of a thin oxide film on the Cu surface does not prevent the formation of a high-quality octadecanethiol monolayer, provided that a high thiol concentration is used.3 The critical influence of the thiol concentration was also recognized in a study of dodecanethiol monolayers.18 The effect of thiol concentration was attributed to a better efficiency in reducing the oxide when the concentration increases.18 Thus, thiol molecules may act as a cleaner which reduces the copper oxide. This has found an important application in the cleaning of oxidized surfaces of copper interconnects used in semiconductor integrated circuits.19 Cupric oxide dispersed in nonpolar media is capable of oxidizing thiols to disulfides under mild conditions.20 This oxidation reaction involves the corresponding reduction of the metal oxide.19 In a study of the interaction of copper sheets and copper powder with alkanethiols, it was found that the thin oxide layer covering the metal is reduced, whereas the alkanethiols are oxidized to disulfides.21 In this paper, we show that compact monolayers and multilayers can be obtained on copper from alkaline aqueous solutions of thiolates. The copper oxide film is completely

10.1021/jp9045148  2010 American Chemical Society Published on Web 02/12/2010

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reduced in alkaline forming solutions irrespective of the initial oxide film thickness. On the contrary, the oxide film is not completely removed when the SAM is formed from hexane solutions of alkanethiols. The mechanism of Cu2O reduction is investigated by means of density functional theory calculations. The energy profiles along the reaction coordinate of the different elementary steps were calculated in order to obtain the corresponding activation energy barriers. An overall picture of the reduction mechanism of Cu2O and CuO oxides by alkanethiols and alkanethiolate anions is presented. Experimental Section The working electrode was a polycrystalline 99.99% copper rod with a 6 mm diameter. A platinum sheet was used as the counter-electrode, and an Ag/AgCl (3M) electrode was employed as the reference. The surface was first mechanically polished and then chemically polished. This procedure left a mirror-like surface. After abrasion with emery paper, the surface was polished on a metallurgical cloth with 3 µm diamond paste for 15 min. The polishing was completed with 0.05 µm Al2O3 for 15 min. The chemical polish was performed by immersing the electrode for 30 s in the following bath: 60 mL of 85% H3PO4 + 10 mL of 98% H2SO4 + 40 mL of Milli-Q water. After rinsing the electrode with abundant Milli-Q water, it was transferred to the corresponding 2 mM thiol forming solution. Two solvents were employed: n-hexane (Anedra) and 0.05 M NaOH (pH 12.75). The following thiols (Sigma-Aldrich) were employed: 1-octanethiol, 1,8-octanedithiol, 1-hexadecanethiol, and 16-mercaptohexadecanoic acid. The cyclic voltammograms (CV) were recoded with an EG&G PAR 173 potentiostat. The electrochemical impedance spectroscopy (EIS) measurements were performed with a Solartron 1260 electrochemical interface. The amplitude of the ac perturbation was 0.01 V. The STM measurements were performed with a Molecular Imaging microscope (Phoenix, Arizona), using tungsten tips electrochemically etched from a 0.25 mm diameter wire in aqueous 2.5 M NaOH. To minimize faradaic currents at the tip-electrolyte interface, the tips were coated with nail polish. A platinum wire was used in the STM cell as a counter electrode. Theoretical Methods and Surface Modeling The first-principle atomistic calculations were performed using periodic density functional theory (DFT) as implemented in the PWSCF code.22 Gradient corrections were included in the exchange correlation functional in the PBE formulation.23 Ultrasoft24 pseudopotentials were used for the atomic species. We first validated our methodology by calculating the lattice constant and bulk modulus of Cu2O by fitting energy vs volume data to the Murnaghan equation of state. The bulk calculations were performed with an 8 × 8 × 8 Monkhorst-Pack k-point grid. We obtained a bulk modulus of 109.5 GPa and a lattice constant of 4.303 Å in very good agreement with the experimental values: 112 GPa25 and 4.267 Å,26 respectively. For copper, we used the bulk constants determined in our previous work.27 For the slabs, brillouin zone integration was performed using a (4 × 4 × 1) Monkhorst-Pack mesh.28 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). Figure 1 shows top and side views of the Cu2O(111) surface. The surface was modeled with a slab of nine atomic layers. Each copper layer is sandwiched between two layers of oxygen anions, giving rise to an apolar surface (Figure 1b). The bulk Cu2O structure consists of two interpenetrating sublattices

Figure 1. (a) Top view of the 1 × 1 structure of Cu2O(111) showing the two interpenetrating lattices labeled 1 and 2. (b) Side view of Cu2O(111) showing the slab with nine atomic layers used to model the surface. A methanethiol molecule adsorbs nearly on top of a Cucus atom. (c) Four-layer metal slab used to model the Cu(111) surface. A methanethiol molecule is adsorbed in a 3 × 3 unit cell.

labeled 1 and 2 in the figure. The copper and oxygen atoms of each sublattice lie on the same plane. Each oxygen atom is coordinated to four copper atoms and each copper atom is coordinated to two oxygen atoms in a linear configuration. For the 111 surface, sublattice 1 exposes oxygen atoms which are coordinatively unsaturated (Ocus, they are 3-fold coordinated to copper atoms) and coordinatively saturated copper atoms (Cucsa). Sublattice 2 exposes coordinatively unsaturated copper atoms, Cucus, as they are only coordinated to one oxygen atom. Cucus atoms act as Lewis acid sites, whereas Ocus atoms act as Lewis base sites. The top view in Figure 1a shows that the surface unit cell of Cu2O(111) has hexagonal symmetry. The Cucus atoms are surrounded by a hexagon of copper and oxygen atoms corresponding to sublattice 1. Figure 1b shows the Cu2O(111) slab with an adsorbed methanethiol molecule on top of a Cucus atom. The Cu(111) surface was represented by a slab with four layers of metal

The Mechanism of Cu2O Reduction

Figure 2. Blocking properties of a 1-octanethiol layer on copper formed in (a) 2 mM thiol solution in n-hexane and (b) 2 mM thiol + 0.05 M NaOH solution. The black line corresponds to the first potential cycle, and the gray line corresponds to the sixth potential scan. The dashed line corresponds to the CV profile of a bare copper electrode. All CVs were performed at a sweep rate of 20 mV/s in 0.05 M NaOH.

atoms. Figure 1c shows the equilibrium geometry of a methanethiol molecule adsorbed on the Cu(111) surface. 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 thiolates. For the Cu2O(111) surface, the positions of all of the atoms in the unit cell (except the oxygen and copper atoms of the last three atomic layers) were relaxed in the potential energy determined by the full quantum mechanical electronic structure. For the Cu(111) surface, the fourth layer of copper atoms was held fixed. The convergence criterion for geometry optimizations was a rms force of 0.01 eV/Å. Results Figure 2 shows cyclic voltammograms obtained for the bare copper surface as well as for the 1-octanethiol covered surface. Three anodic peaks are observed for the bare copper electrode in 0.05 M NaOH. The anodic peak AI corresponds to the formation of a first layer of Cu(I) oxide, Cu2O, while the broader anodic peaks AII and AIII correspond to the formation of a second and mixed layer of Cu(II) oxide and hydroxide, Cu2O/CuO,Cu(OH)2.29,30 The structure of these anodic films has been examined more recently by in situ electrochemical scanning tunneling microscopy.31-35 The in situ STM measurements of the anodic oxidation of Cu(111) in 0.1 M NaOH evidence the crystalline structure of the Cu(I)/Cu(II) duplex passive films.35 The epitaxial relationships between the Cu(II) outer layers, the Cu(I) inner layers, and the Cu(111) substrate correspond to a parallel alignment of the closed packed directions of the CuO and Cu2O lattices: Cu(111) | Cu2O(111) | CuO(001).35 On the reverse sweep, the cathodic CII and CI reduction peaks correspond to the reduction of Cu(II) to Cu(I) and of Cu(I) to Cu0, respectively. The blocking effect of the 1-octanethiol SAM formed in hexane is shown in Figure 2a. The CV of the modified electrode was started at a potential of -0.9 V, and the potential was

J. Phys. Chem. C, Vol. 114, No. 9, 2010 3947 scanned in the positive going direction. A small oxidation current starts at a potential of 0.10 V, and a broad peak is observed at a potential of 0.33 V. When the potential scan is reversed, the two cathodic peaks are observed; however, they are shifted toward more negative potentials. In order to check the stability of the monolayer, the electrode was cycled between -0.9 V and the potential corresponding to the onset of the oxygen evolution reaction. The anodic current steadily increased, and the anodic peaks of the copper electrode were recovered. The gray curve in Figure 2a corresponds to the sixth potential cycle. The shape of the sixth CV profile is similar to that of the bare copper electrode. The main difference is the magnitude of the oxidation current which is lower for the modified electrode. The oxidation charge in the potential range investigated is 10.7 mC/cm2 for the bare copper electrode, whereas after the sixth cycle the corresponding charge is 5.4 mC/cm2. Figure 2b shows the same set of experiments when the selfassembly process occurs in a millimolar thiol solution of 0.05 M NaOH. The first potential scan of the freshly modified electrode shows better blocking properties than the modified electrode formed in the hexane solution. The anodic current starts at 0.25 V, and the current peak is observed at a potential of 0.40 V, which is 0.07 V more positive than that for the monolayer formed in hexane. The anodic charge after the sixth cycle is 5.03 mC/cm2 which is slightly lower than the corresponding charge in the hexane forming solution (Figure 2a). The fact that the CV profiles during the successive potential cycles are very similar to that of the bare copper electrode with the anodic current steadily increasing during the potential cycling indicates that only a fraction of the electrode area is available for oxidation. This implies that the oxidation occurs in patches and that the area of these patches increases with the number of cycles. The presence of oxide patches was confirmed by STM measurements. A freshly prepared 1-octanethiol layer formed in the alkaline thiol solution was imaged at a potential of -0.9 V in 0.05 M NaOH solution, and then, the potential was stepped to +0.0 V at which the onset of copper oxidation is observed in the CV profiles of Figure 2. Figure 3a shows a 1 µ × 1 µ scan recorded at -0.9 V where several metallic grains can be observed. The surface of the metal grains is very smooth, and we have observed that a surface smoothing process is produced during the formation of the thiol layer. The surface dynamics during the formation of alkanethiol layers on copper will be presented elsewhere.36 When the potential was stepped to 0.0 V, amorphous oxide patches appeared on the surface, as indicated by the arrows in Figure 3b. The cross section in Figure 3b shows the contrast between the smooth regions on the unoxidized surface and the protruded regions corresponding to the oxide patches. After the formation of the 1-octanethiol layer in the hexane and NaOH forming solutions, a voltammogram was recorded toward negative potentials starting from the open circuit potential (Figure 4). No reduction peak was observed for the layer formed in NaOH, whereas for the layer formed in hexane a reduction peak was observed at -0.63 V. This behavior was very reproducible for all of the experiments carried out. These results indicate that, after the 45 min dipping in the hexane forming solution, oxide patches are left on the surface and they are reduced in the negative potential sweep. On the contrary, the oxide film is completely reduced during the formation of the 1-octanethiol layer in the NaOH solution. 1-Octanethiol Layers on Anodically Oxidized Copper. The assembly of the 1-octanethiol layer from hexane and NaOH

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Figure 5. CV profiles recorded after the formation of a 1-octanethiol layer on an anodically oxidized copper surface in (a) NaOH and (b) n-hexane forming solutions. The potential was scanned in the negative going direction starting from the open circuit potential.

Figure 3. (a) In situ STM image (1 µ × 1 µ area) recorded at an electrode potential of -0.9 V of copper modified with 1-octanethiol showing smooth metallic grains. (b) The same area after stepping the potential to 0.0 V. The protruded regions on the smooth metallic grains correspond to the oxide patches (see arrows). The cross section shows a very smooth surface (within a few angstroms) for a scan length of 1 µ. Tunneling current: 1 nA. Images recorded in 0.05 M NaOH solution.

Figure 4. CV profiles of copper modified with 1-octanethiol recorded in the negative going direction from the open circuit potential. The gray curve corresponds to the layer formed in the 2 mM thiol + 0.05 M NaOH solution, and the black curve corresponds to the layer formed in the n-hexane solution.

forming solutions was also performed on electrochemically oxidized copper surfaces in order to evaluate the ability of the thiol to reduce the surface oxide layer. The electrode was first oxidized up to the potential corresponding to the onset of the

oxygen evolution reaction at +0.7 V. Under these conditions, the oxide thickness is around 5.6 nm.35 The oxidized surface was transferred to the corresponding forming solution. When the 1-octanethiol layer was formed in the NaOH solution, the first cathodic sweep showed no reduction peaks (Figure 5a), indicating that the oxide layer was completely reduced during the formation of the thiol layer in this medium. In the first sweep toward positive potentials, an anodic current peak was observed at +0.36 V corresponding to the formation of oxide patches, as in the case of Figure 2b. Figure 5b shows the CV profile for the first negative potential scan after the formation of the 1-octanethiol layer on the oxidized surface in the hexane forming solution. It becomes clear that the oxide has not been completely reduced, as two prominent cathodic peaks were observed at -0.56 and -0.84 V. In the next potential cycle toward positive potentials, the anodic current showed the peaks corresponding to the different stages of copper oxidation. It can be observed that the first oxidation peak occurs at -0.39 V, whereas on the bare copper surface this peak is observed at -0.40 V, virtually the same potential. However, the oxidation current is lower than that on the bare copper surface. Effect of Chain Length. The influence of the chain length of the alkanethiol on the surface blocking properties can be observed in Figure 6 for 1-hexadecanethiol layers formed in hexane and NaOH. The CV in the hexane forming solution (Figure 6a) is very similar to that for 1-octanethiol (Figure 2a). During the first cycle toward positive potentials, the oxidation peak is observed at nearly the same potential as for the 1-octanethiol layer, and in the first cycle toward negative potentials, the two cathodic peaks are clearly observed. In the sixth cycle toward positive potentials, the oxidation current is observed in the same potential range as for the bare copper surface. However, the oxidation current for the 1-hexadecanethiol (Figure 6a) is lower than that for the 1-octanethiol layer (Figure 2a), indicating that the density of oxide patches decreases as the chain length increases. The CV profile for the 1-hexadecanethiol formed in the NaOH solution (Figure 6b) is very different from that obtained from


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Figure 6. Blocking properties of a 1-hexadecanethiol layer formed in (a) 2 mM thiol solution in n-hexane and (b) 2 mM thiol + 0.05 M NaOH solution. The black line corresponds to the first potential cycle, and the gray line corresponds to the sixth potential scan. The dashed line corresponds to the CV profile of a bare copper electrode. (c) CV profiles of a freshly prepared 1-hexadecanethiol in NaOH (gray line) and n-hexane (black line) forming solutions. The CV was started at the open circuit potential in the negative going direction. All CVs were performed at a sweep rate of 20 mV/s in 0.05 M NaOH.

Figure 7. Blocking properties of 1,8-octanedithiol layers formed in (a) 2 mM thiol solution in n-hexane and (b) 2 mM thiol + 0.05 M NaOH solution. The black line corresponds to the first potential cycle, and the gray line corresponds to the sixth potential scan. The dashed line corresponds to the CV profile of a bare copper electrode. (c) CV profiles recorded in the negative going direction starting from the open circuit potential for the 1,8-octanedithiol layer formed in n-hexane (black line) and NaOH (gray line) forming solutions.

the hexane forming solution. During the first potential cycle toward positive potentials, a small anodic current started at a potential of 0.33 V and an anodic peak was observed at 0.56 V (see inset of Figure 6b). When the potential scan was reversed, the anodic current did not decrease and a broad oxidation current peak was observed at a potential of 0.32 V. This indicates that the surface oxidation continues in the negative sweep. It is remarkable that, in the subsequent potential cycles, no oxidation peaks were observed. During the sixth sweep toward positive potentials, for example, the electrode still remained passivated and no oxidation current peak was observed (see inset, gray curve). The onset of oxygen evolution also remained shifted toward potentials more positive than on the bare copper surface. This indicates that some sort of healing process has occurred after the reduction of the oxide at the negative potentials. In the first scan toward negative potentials, two small cathodic peaks were observed. The reduction current was considerably lower than that observed for the same thiol in the hexane forming solution (Figure 6a) and was also lower than the current observed for the 1-octanethiol layer in the same forming solution (Figure 2b). During the sixth potential scan toward negative potentials, the current of the reduction peaks also remained low.

The presence of unreduced copper oxide after the formation of the 1-hexadecanethiol layer in hexane and NaOH forming solutions was checked by performing a CV toward negative potentials on the freshly modified surfaces (Figure 6c). For the layer formed in hexane, a reduction current peak was observed at -0.62 V as in the case of the 1-octanethiol monolayer prepared in the same forming electrolyte (Figure 4). For the layer formed in the NaOH solution, no reduction peak was observed as in the case of the 1-octanethiol layer formed in this media. This indicates that no oxide patches are left on the surface after the layer formation in the NaOH solution. Effect of Terminal Group. The effect of the terminal group on the blocking properties of the layers was investigated with 1,8-octanedithiol and 16-mercaptohexadecanoic acid. The CV profiles of the 1,8-octanedithiol layers formed in hexane and NaOH are shown in Figure 7. The CV of the first cycle toward positive potentials is similar to that obtained for the monothiol (Figure 2). However, the blocking effect is more pronounced for the dithiol layer formed in NaOH (Figure 7b). The oxidation peak is observed at a potential of 0.43 V which is more positive than that obtained for the monothiol in the same forming electrolyte (0.40 V) and is also more positive that that of the dithiol formed in the hexane solution (0.31 V, Figure 7a).

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Figure 8. Blocking properties of 16-mercaptohexadecanoic acid (gray line) and 1-hexadecanethiol layers prepared (black line) in (a) n-hexane and (b) NaOH forming solutions. For comparison, the CV profile of the clean copper surface is included (dashed line).

When the potential of the freshly prepared surfaces was scanned toward negative values, no oxide reduction peak was observed for the 1,8-octanedithiol layer prepared in NaOH solution, whereas a small reduction peak was observed when the layer was formed in hexane (Figure 7c). This shows that the dithiol is more efficient in reducing the oxide layer even in the hexane forming solution. Figure 8 compares the blocking properties of 1-hexadecanethiol and 16-mercaptohexadecanoic acid layers formed from hexane and NaOH solutions. For both forming solutions, the mercaptoalcanoic acid layer has a lower blocking efficiency than the monothiol layer because higher oxidation currents are observed for the mercaptoalcanoic modified surfaces. Figure 8a shows that, before the oxidation peak at 0.33 V, a small and steadily increasing oxidation current is observed from a potential of -0.32 V. This indicates that the surface is not covered by a compact monolayer of 16-mercaptohexadecanoic acid. Copper Electrochemistry in Alkaline Solutions Containing 1-Octanethiol and 1,8-Octanedithiol. The electrochemistry of copper surfaces was also investigated in 0.05 M NaOH with a millimolar concentration of the corresponding thiol. After the surface polishing, the copper electrode was immersed in the alkaline thiol solution and the potential was cycled between -1.2 V and the potential corresponding to the onset of the oxygen evolution reaction. Figure 9a shows the CV profiles obtained in the 1-octanethiol solution. The electrode was held at a potential of -0.9 V for 2 min, and then, the scan toward positive potentials was initiated at a sweep rate of 20 mV/s. No current oxidation peaks were observed during the positive going sweep. Only an exponential current increase was observed at the most positive potentials associated with the oxygen evolution reaction and copper oxidation which we attribute to the breakdown of the thiol layer. The potential at which this current increase begins is shifted toward more positive values during the successive potential cycles. When the potential scan was reversed, the current profile showed a large hysteresis. The current in the reverse scan was larger than that in the forward scan, and it decreases more slowly. An important difference between the

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Figure 9. Succesive potential cycles toward increasingly positive potentials in a 0.05 M NaOH solution containing a 2 mM concentration of (a) 1-octanethiol and (b) 1,8-octanedithiol. The inset in part b shows the variation of the breakdown potential as a function of the number of potential cycles.

CV profiles in Figure 9 with those obtained in the absence of thiols in the working solution (Figure 2a) is that no reduction peaks were observed. This implies that no copper oxide is present on the modified surface even though an anodic current is observed at the most positive potentials after the potential scan is reversed. This point will be discussed later. The CV profiles obtained in the 1,8-octanedithiol solution are shown in Figure 9b. They are qualitatively similar to those of the monothiol with only one important difference: as the number of potential cycles increased, the potential at which the film breakdown occurs was shifted to considerably more positive potentials. After the ninth cycle, for example, the current increase was observed at around 4 V. The inset in Figure 9b shows the shift of the breakdown potential as a function of the number of potential cycles. It shows a sudden increase after the first few cycles. These results indicate that the film formed on the electrode surface has excellent blocking properties. Impedance Measurements. Impedance spectra were taken at a potential of -0.9 V, which is more negative than the potential of the oxide reduction peaks, in order to ensure that during these measurements the surfaces do not oxidize. Figure 10 shows Bode plots of log(|Z|) vs log(f) for the bare and modified copper surfaces. The bare copper surface shows a linear relationship with a slope of -1 (for frequencies lower than 1 kHz) corresponding to the double layer capacity of the copper surface. In the case of the copper surfaces modified with 1-octanethiol (Figure 10a) and 1-hexadecanethiol, (Figure 10b), the log(|Z|) vs log(f) plots show a break at intermediate frequencies. The high frequency part has a slope of close to -1 and corresponds to the capacity of the layer. The low frequency part has a slope of around -0.5 and corresponds to mass transport occurring through the thiol layer. During the impedance measurements at -0.9 V, the steady state current was around -0.1 µA/cm2. This small cathodic current corresponds to the hydrogen evolution reaction and is responsible for the mass transport behavior observed at low frequencies.



Figure 10. Impedance spectra of (a) 1-octanethiol and (b) 1-hexadecanethiol layers formed in hexane (black line) and NaOH (gray line) solutions. (c) Comparison of the impedance spectra of a 1-octanethiol monolayer (black line) formed in hexane with that of a 1,8-octanedithiol multilayer (gray line) formed after seven potencial cycles in a 0.05 M NaOH solution containing a 2 mM concentration of the dithiol. The dotted line in all figures corresponds to the spectra of the bare Cu electrode. The spectra were measured at a potential of -0.9 V.

TABLE 1: Effect of Dipping Solution on the Capacity of Different Alkanethiol Layersa dipping solution

a

alkanethiol

n-hexane

0.05 M NaOH

1-octanethiol 1,8-octanedithiol 1-hexadecanethiol

2.2 4.1 1.0

1.4 1.0 0.16

Capacities in µF/cm2.

The impedance spectra were fitted with the circuit shown in the inset. It has two time constants: one is associated with the capacitance and resistance of the film, and the other corresponds to mass transport through the film. This circuit has been used for alkanethiol SAMs with defects.37,38 Constant phase elements were employed to describe the dielectric properties of the film (exponent close to 1) and the mass transport through the film (exponent close to 0.5). Table 1 contains the capacity of the different layers. Figure 10a shows that the impedance of the 1-octanethiol layer formed from the alkaline solution is higher than that obtained from the dipping in hexane. The influence of the forming solution is more pronounced in the impedance spectra of 1-hexadecanethiol layers (Figure 10b).

The capacity of the 1-octanethiol layer formed in hexane is 2.2 µF/cm2. This value is in the range of capacities that we have obtained previously39 on Au(111): 2.9 µF/cm2 in 0.1 KOH and 1.8 µF/cm2 in 0.001 M H2SO4. The capacity of the 1-hexadecanethiol layer formed in hexane is 1.0 µF/cm2. This value is half of that for 1-octanethiol and correlates with the fact that the chain length is twice as large. Therefore, we conclude that monolayers of 1-octadecanethiol and 1-hexadecanethiol are formed on copper after 45 min of immersion in hexane. Dipping in 0.05 NaOH produced layers with lower capacitances than in hexane, as is evident from the impedance spectra of Figure 10a and b. Table 1 shows that the capacity of the 1-octanethiol layer is 1.4 µF/cm2, whereas the capacity of the 1-hexadecanethiol layer is 0.16 µF/cm2. Comparison of these values with those obtained in the hexane forming solution (2.2 and 1.0 µF/cm2, respectively) indicates that an incomplete bilayer is formed from 1-octanethiol, whereas a multilayer is formed from 1-hexadecanethiol. Taking as a reference the value of 1.0 µF/cm2 as the capacity of a monolayer of 1-hexadecanethiol on copper, the value of 0.16 µF/cm2 obtained in the NaOH forming solution corresponds to a film which is seven monolayers thick. This explains the high blocking properties of this film toward the oxidation of the copper surface shown in the CV profiles of Figure 6b. The factors leading to a facile formation of multilayers in the NaOH forming solution are considered in the Discussion section. The 1,8-octanedithol layer formed from hexane has a higher capacity (4.1 µF/cm2) than that corresponding to the monothiol. This indicates that the dithiol layer is not as compact. Probably, not all of the dithiol molecules are in an upright configuration. Howerver, when the dipping is performed in the NaOH solution, the capacity of the dithiol layer (1.0 µF/cm2) is half of that of the monothiol in hexane (2.2 µF/cm2). A lower capacitance value proves the formation of a multilayer. If we assume that the multilayer has the same dielectric constant as the monolayer (implying the same degree of compactness), the ratio of capacitance values indicates that a bilayer has been formed. Figure 10c compares the impedance spectra of the 1-octanethiol monolayer obtained from hexane with the spectra of the 1,8-octanedithiol film obtained after seven potential cycles in the dithiol alkaline solution (Figure 9b). It can be observed that the impedance of the film is much higher than that of the monolayer. A capacity of 0.1 µF/cm2 was obtained for this film. Taking as a reference the capacity of 2.2 µF/cm2 for a monolayer of 1-octanethiol and assuming that both layers have the same dielectric constant, the value of 0.1 µF/cm2 implies that the film has a thickness of around 20 dithiol monolayers. DFT Calculations. The experimental results show that spontaneously grown oxide films as well as anodic oxide films are completely reduced in alkaline solutions containing alkanethiols. This implies that the oxide reduction mechanism in alkaline forming solutions is different from that in the hexane forming solution. In order to explain these facts, we investigated the mechanism of Cu2O reduction using quantum mechanical calculations. The modeling was performed on the Cu(I) oxide because the aqueous and native oxides on the Cu(111) surface are crystalline Cu2O, as has been reported in an X-ray diffraction study on the aqueous and air oxidation of Cu(111).40 The oxide reduction requires the protonation of the Ocus atoms of Cu2O which act as Lewis base sites.41 The -SH group of the thiols or the water molecules are the source of protons, and the product may be a water molecule or a hydroxide anion, depending on the forming solution, as we shall see below.

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Calderoń et al. triangle in Figure 11b). The (3 × 3)R30° structure of Figure 11b has been observed recently in vacuum annealed Cu2O(111) surfaces.43 Our calculations confirm that the protrusions observed in the STM images43 are due to a cluster of copper atoms arising from an oxygen vacancy. The removal of an oxygen atom from Cu2O with formation of O2 in the gas phase according to

Cu2O f Cu2O (oxygen vacancy) + 1/2O2

Figure 11. (a) A (3 × 3)R30° unit cell of the perfect Cu2O(111) surface. (b) A (3 × 3)R30° unit cell with a missing oxygen atom. Note the formation of a cluster of six copper atoms around the oxygen vacancy. The atoms involved in the formation of the cluster are marked by a yellow dot.

The global reaction for Cu2O reduction by alkanethiols (RSH) can be written as

Cu2O + 2RSH f 2CuSR + H2O

(1)

In alkaline solutions, the -SH group is deprotonated and the interaction of a thiolate anion with the copper oxide has to be considered:

Cu2O + 2RS- + H2O f 2CuSR + 2OH-

(2)

In this reaction, water molecules from the solution are involved in the protonation of the oxygen atom of Cu2O. The surface chemistry of metal oxides depends on the pH and the polarity of the solvent in which they are immersed. In alkaline solutions, metal oxides are negatively charged due to the adsorption of hydroxide anions.34,42 The surface hydroxylation of Cu2O has been confirmed by STM measurements.34,42 Therefore, this negative surface charge has to be taken into account in the surface modeling. Before discussing the details of the mechanism of Cu2O reduction by thiols, we want to calculate the energetics involved in the elimination of a surface unsaturated oxygen atom and show the extent of surface relaxation that occurs upon removal of the oxygen atom. Figure 11 compares the structure of the perfect surface with that of a surface with a (3× 3)R30° structure in which one-third of the oxygen atoms are missing. After elimination of the Ocus atom, a cluster of six copper atoms forms from the three Cucsa and the three Cucus atoms (see yellow

(3)

is a very endothermic process with ∆E ) +115.4 kcal/mol. This explains the fact that annealing or ion bombardment is required to produce oxygen vacancies.43 On the contrary, the removal of an oxygen atom from Cu2O by thiol molecules is an exothermic process, as we shall see in the next section. As a model alkanethiol, we used the methanethiol molecule because it is less demanding from a computational point of view. Reaction pathways and energy barriers were calculated using the “climbing image nudged elastic band” (CI-NEB) method,44 which has proven to be a very efficient technique to determine minimum energy paths in complex chemical reactions. Reaction Path of CH3SH with Cu2O(111). Figure 12 shows the equilibrium structures for reactants, transition state, and products for the successive transfer of two protons from two methanethiol molecules to an Ocus surface atom. Figure 12a shows that the reactant methanethiol molecules are monocoordinated to Cucus atoms which act as Lewis acid sites attracting the lone pair electrons of the sulfur atoms of methanethiol. The interaction of a methanethiol molecule with the Cu2O(111) surface is much higher than that on the Cu(111) surface. The nondissociative adsorption energies are 27.0 and 6.6 kcal/mol, respectively. The strong interaction of methanethiol with the Cucus atom of Cu2O(111) is evidenced in the short S-Cu bond length 2.17 Å, whereas the corresponding bond length on the Cu(111) surface is 2.39 Å (Figure 1c). In the transition state for the transfer of the first proton (Figure 12b), the methanethiol molecule still remains monocoordinated to the Cucus atom and the hydrogen atom is approximately halfway between the sulfur and oxygen atom. This reaction has a very low activation barrier of 4.2 kcal/mol and is exothermic by 5.8 kcal/mol. Figure 12c shows the final state after the transfer of the first proton. It can be appreciated that the methanethiolate molecule is bicoordinated to two copper atoms. The structure in Figure 12c is the starting point for the transfer of the second proton. Figure 12c-e shows the transfer of the second proton from the methanethiol molecule to the surface OH group produced in the previous step. The products are two methanethiolate molecules and a water molecule (Figure 12e). The conversion of the OH group to a water molecule leads to the clustering of copper atoms due to the elimination of the oxygen atom from the surface, as shown in Figure 11. This step has an activation energy barrier of 7.4 kcal/mol, and it is exothermic by 1.0 kcal/ mol. Comparing Figure 12a and e, it can be appreciated that there is an important surface relaxation as the reaction proceeds due to the clustering of copper atoms in the final state and the bicoordination of methanethiolate molecules. The energy profile for both elementary reaction steps along the reaction path is shown in Figure 13. The low activation barriers prove the reducing nature of alkanethiols on Cu2O. The total energy change of both steps is -6.8 kcal/mol. This value contrasts with the high endothermic value (115.4 kcal/mol) required to remove an oxygen atom and produce molecular oxygen in the gas phase, as described in the previous section. Reaction Path of CH3S- with Cu2O(111). In the alkaline electrolyte, the reacting species is the thiolate anion which is a



Figure 13. Energy profile along the reaction pathway for the successive transfer of two protons from adsorbed methanethiol molecules to the Ocus surface atom. The labels indicate the corresponding structure in Figure 12.

Figure 12. (a) Reactants, (b) transition state, and (c) products for the transfer of a proton from an adsorbed methanethiol molecule toward an Ocus surface atom. (d) Transition state and (e) products for the transfer of a second proton from an adsorbed methanethiol molecule toward a surface OH group which leads to the detachment of the oxygen atom from the surface as a water molecule. Calculations performed in a 2 × 1 unit cell.

strong nucleophile. Both the thiolate and the hydroxide anions bind to the Cucus site which acts as a Lewis acid. However,

thiolate anions may displace hydroxide anions. This is due to the higher binding energy of the thiolate (which favors the interaction with the surface) and the higher hydration energy of the hydroxide which favors its desorption to the aqueous solution. The CH3S- and OH- binding energies are -39.7 and -22.6 kcal/mol, respectively, whereas the corresponding hydration energies are -89.9 45 and -104.5 kcal/mol.46 In the NaOH electrolyte, the water molecules are the source of protons for the protonation of the Ocus atom to yield a surface OH group. The reaction may involve adsorbed water molecules or molecules in solution. In this work, we only consider adsorbed water molecules as the solvent is not taken into account. The fact that water may act as a reducing agent has been suggested in several literature reports. Water-soluble silver nanoparticles were obtained recently in a single step by simple decomposition of a commercial silver complex at room temperature without the need of external reducing agents. The role of water as the reducing agent was proved in this study by the presence of hydrogen peroxide.47 The reduction potential of water molecules in the first coordination sphere may be lower than that of free water.48 Radiotracer experiments showed that coordinated water may lead to reduction of the metal center oxidizing to hydrogen peroxide.49 The calculations were performed with a 2 × 1 unit cell containing a thiolate anion and a water molecule. The net charge of -1 in this unit cell produces an average surface charge density of 8.9 µC/cm2. Figure 14a shows that the methanethiolate and the water molecule adsorb nearly on top of the Cucus atoms. The hydrogen atom of the water molecule points toward the Ocus and the hydrogen bond length of 1.89 Å is very short, indicating a strong hydrogen bond interaction. When the surface is not charged, the hydrogen bond length enlarges to 2.10 Å. This proves that part of the negative surface charge locates around the Ocus atoms. The transition state and the final products are shown in Figures 14b,c. In the final state, one OH group is adsorbed on top of the Cucus atom, whereas the newly formed OH group remains tricoordinated to copper atoms (Figure 14c). The activation barrier is 6.4 kcal/mol, and the reaction is slightly endothermic by 6.2 kcal/mol. The tricoordinated OH may then diffuse outward in a very exothermic process. Using a big (3 × 3)R30° unit cell, we next calculated the energy change in the reconstruction process in which a tricoordinated OH becomes adsorbed on top of the copper cluster which is formed after the OH groups diffuse outward. In order to reduce the number of degrees of freedoms during this optimization, we did not consider the presence of the thiolate and the other OH group

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Figure 14. (a) Reactants, (b) transition state, and (c) products for the proton transfer from an adsorbed water molecule toward an Ocus surface atom on a surface with an adsorbed thiolate anion. Calculations performed in a 2 × 1 unit cell.

(adsorbed on the Cucus atom in Figure 14c). The calculations were performed with a net charge of -1 to simulate the presence of adsorbed thiolate anions. Figure 15 shows the equilibrium structures before and after the copper clustering. Figure 15a shows the equilibrium structure of a protonated Ocus atom. The onset of the formation of a copper cluster can be appreciated as the Cu-Cu distances around the hydroxide decrease. This structure reconstructs to the pyramidal copper cluster shown in Figure 15b. This process is very exothermic with a ∆E value of -26.8 kcal/mol. In an aqueous environment, it is expected that the adsorbed hydroxide anion may then be transferred to the solution because its hydration energy (-104.5 kcal/mol46) is much higher than its binding energy to the surface (-22.6 kcal/mol). Surface Reconstruction of Thiolated CuO2(111) with Oxygen Vacancies. After the elimination of an Ocus atom from the surface, we investigated the surface reconstruction process in which a CH3S species drags a copper atom to form Cu4 pyramids (Figure 16). In order to simulate the negatively charged oxide surface in the alkaline solution, the computations were performed by adding one electron to the unit cell. The diffusion of the CH3SCu moiety on the neutral surface has an activation barrier of 22.6 kcal/mol and is endothermic by 17.1 kcal/mol. However, the activation barrier is much lower on the negatively charged surface, 14.1 kcal/mol, and the reaction is exothermic by 2.54 kcal/mol. Figure 16 shows the reactants, transition state, and products together with the energy profile along the reaction coordinate. These results show that on the negatively charged surface (alkaline electrolyte) the surface reconstruction processes will be faster than on the neutral surface (hexane forming solution). Discussion A major finding of this work is that spontaneously grown oxide films as well as anodic oxide films are completely reduced in alkaline solutions containing alkanethiols. In this forming

Figure 15. Side and top views of a surface hydroxide in a (3 × 3)R30° unit cell negatively charged (a) before and (b) after the formation of a metal cluster.

electrolyte, the growth of multilayers rapidly occurs, leaving the copper surface completely passivated. On the other hand, the reduction of copper oxides in hexane forming solutions is less effective and no multilayers were detected in the hexane forming solution for the immersion time of 45 min employed in this work. The theoretical calculations shed light on these experimental findings. The Cu2O(111) surface is very reactive toward the breakage of the S-H bond because this process has a low activation barrier of 4.2 kcal/mol. On the Cu(111) surface, a similar activation barrier has been calculated for the S-H cleavage of methanethiol: 4.4 kcal/mol.50 The breakage of the second S-H to produce a water molecule has an activation barrier of 7.4 kcal/mol. In the alkaline media, the protonation of the Ocus atoms of the surface involves the breakage of the OH bond of a water molecule and this process (Figure 14) also has a low activation barrier: 6.4 kcal/mol. These results show that the energy barriers for the elementary steps involved in the reduction of Cu2O are very small in alkaline as well as in hexane solutions. Therefore, the high efficiency of thiolate anions to reduce Cu2O as compared to that of molecular alkanethiols in hexane solutions cannot be attributed to differences in the rate constants of the elementary steps. However, the kinetics of the reactions involved in the reduction of Cu2O has important differences in both media: (a) In the alkaline media, the reactants for the protonation of surface Ocus atoms are the water molecules from the

The Mechanism of Cu2O Reduction solvent. Therefore, the high rate of oxide reduction observed in this medium is due to the high concentration of the reactant water molecules. In turn, the reaction product is a surface hydroxide anion for which it is energetically very favorable to be transferred to the solution. (b) Exactly the reverse trend is observed when alkanethiols are dissolved in an apolar solvent such as n-hexane. First, the reactants are thiol molecules which normally have millimolar concentrations as compared to the high concentration of the reactant water molecules in the alkaline solution. Second, the protonation of Ocus surface atoms to produce water molecules requires two successive elementary steps with alkanethiol molecules (Figure 12), whereas, in the alkaline media, the protonation of the Ocus atom to produce a surface hydroxide anion requires only one step (Figure 14). Third, it is not energetically favorable for the water molecules obtained as the product to dissolve into the apolar solvent. These points show that the rate of the reaction will be much slower in the hexane forming solution than in the alkaline forming solution (as observed experimentally) mainly due to the difference in the concentration of the reactants. This explains recent results by Mekhalif and co-workers18 who showed that the efficiency in reducing the oxide layer improves by increasing the thiol concentration. For the 45 min immersion time in the forming solution, we observed the formation of multilayers only in the alkaline solution. We attribute the facile formation of multilayers to the dissolution of copper favored by the solubility of copper complexes in this medium. Further reactions with thiolates in the electrolyte will give rise to the formation of polymeric thiolato complexes and finally to multilayer structures. In the NaOH forming solution, the oxide surface is negatively charged and we showed that surface restructuring easily leads (with low activation barriers) to copper clusters on which hydroxide and thiolate anions adsorb (Figures 15b and 16c, respectively). In the exchange reactions with the electrolyte, the anions may detach copper atoms from the surface. Species like CuOH and CuSRS- (originating from the dithiol), for example, are soluble in the aqueous electrolyte. In the case of dithiols, the formation of the multilayer is easily understood as the copper ions may sandwich between the thiol groups of the bifunctional molecules. The detailed structure of such a multilayer is not known yet on the copper surface, but it has been elucidated on the gold surface. Bard and co-workers reported the self-assembly of multilayers on gold using 1,6hexanedithiol and Cu(II) ions.51 They found a sandwiched structure with alternating layers of copper ions and dithiolates. X-ray photoelectron spectroscopic (XPS) data confirmed a Cu(I) oxidation state and the formation of a multilayer with intralayer disulfide bonds.51 In the case of monothiols, the formation of multilayers on copper has been reported by several authors using different techniques such as ellipsometry,21 STM,52 and radiochemistry.53 Although the structure of these multilayers is not known, it is well known that copper thiolates form layered materials with high thermal stability.54 In the layered structure, copper atoms are arranged in a plane and the sulfur atoms are attached to it from both above and below the plane. The XPS spectra of the layered material shows that the chemical state of copper corresponds to the +1 oxidation state.54 In the layered structure, the three-dimensional stacking of these two-dimensional CH3(CH2)nS-Cu-S(CH2)nCH3 layers is due to van der Waals


Figure 16. (a) Initial, (b) transition, and (c) final state for the thiol induced reconstruction of a copper atom on a negatively charged Cu2O(111) surface (1 × 1 unit cell) with missing Ocus atoms. (d) Energy along the reaction pathway.

interactions between the alkyl chains.54 Therefore, we think that, after the formation of the alkylthiolate monolayer on the copper surface, the growth of the multilayer is due to the stacking of CH3(CH2)nS-Cu-S(CH2)nCH3 layers.

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On the contrary, we did not observe the formation of multilayers in the hexane forming solution during a dipping time of 45 min. This is attributed to a much lower concentration of copper species in this medium. CuOH species are insoluble, and CuSR complexes are probably the only soluble species. The formation of mutilayers in apolar solvents has been reported.52 However, the process is much slower than in the alkaline electrolyte, as it requires from tens to hundreds of hours of immersion time in the forming solution.52 The current plateau observed at the most positive potentials on the bare copper surface (Figure 2a) corresponds to copper oxide growth together with generalized copper dissolution through the complex oxide film.55 We took advantage of this copper dissolution process to form multilayers in the NaOH electrolyte containing 1,8-alkanedithiol. In the successive cycles toward more positive potentials (Figure 9b), oxide growth occurs after the layer breakdown and thus the dissolution process accompanying the oxide growth is the source of copper for further growth of the multilayers in the dithiol containing medium. It is remarkable that, after reversing the potential scan toward negative potentials, no oxide reduction peak is observed. This indicates a fast healing process in which the oxide dissolves and the thiolate multilayer grows on the previously formed oxide patches. We did not investigate theoretically the adsorption of thiols on the CuO oxide in this work because the experimental evidence shows that this interaction is weak. It has been proposed that thiols do not adsorb on CuO and this surface is not reactive toward dipropyldisulfide,17 whereas disulfides can form monolayers on the clean copper surface via rupture of the S-S bond. A weaker thiol-CuO surface interaction is expected considering its atomic structure. The CuO oxide has a high density of oxygen atoms on the surface, which are expected to “screen” the interaction of the Cu2+ cations with the sulfur atom of the thiol. The CuO(001) surface observed for the electrochemically grown oxide in NaOH has an interatomic distance of 2.88 Å between adjacent oxygen35 atoms, whereas the interatomic distance between the Ocus atoms of Cu2O is much larger: 6.086 Å (Figure 1a). Another surface which has been reported under electrochemical conditions is the CuO(111) surface which is terminated by two layers of oxygen atoms,56 thus leaving the Cu2+ cations in the third layer. Therefore, the oxygen-rich CuO surface is expected to interact with thiols in solution which are reduced to disulfides which also remain in the solution.17,21 We think that, as oxygen atoms are removed from the CuO surface, it will be gradually converted to Cu2O. We therefore propose the following reactions in the apolar and alkaline media, respectively:

2CuO + 2RSHsol f Cu2O + H2O + RS-SRsol

(4)

2CuO + 2RSsol- + H2O f Cu2O + 2OHsol- + RS-SRsol

(5) As the oxide surface becomes copper-rich, the oxide reduction mechanism will mainly involve adsorbed thiols or thiolates (in alkaline media) due to the strong interaction with Cucus acid sites. Therefore, the reduction of the Cu2O oxide involves the following surface reactions:

Cu2O + 2RSHads f 2CuSR + H2Oads

(6)

Cu2O + 2RSads- + H2Oads f 2CuSR + 2OHads-

(7) Conclusions Compact self-assembled monolayers and multilayers can be formed on copper surfaces with low immersion times in aqueous alkaline solutions of alkanethiols. Spontaneously grown as well as anodically grown oxide films are completely reduced in alkaline solutions of alkanethiols after an immersion time of 45 min. On the contrary, the reduction of copper oxides in hexane forming solutions is less effective and produces layers with inferior blocking properties due to the presence of oxide patches. In the alkaline forming electrolyte, the formation of multilayers readily occurs and this effect is favored by the increase of the chain length or by the presence of an -SH terminal group. 1,8-Octanedithiol forms a bilayer, whereas 1-hexadecanethiol forms a multilayer which is seven monolayers thick. The formation of multilayers was attributed to the dissolution of copper moieties in the alkaline medium and to the subsequent assembly of polymeric thiolato complexes into a layered structure. The electrochemistry of copper in alkaline solutions containing 1,8-octanedithiol shows that multilayers with excellent blocking properties can be readily obtained after a few potential cycles in the potential window between the onset of the hydrogen evolution reaction and the onset of the film breakdown. The multilayer produced after nine potential cycles is 20 monolayers thick and has a breakdown potential of 4.0 V. DFT calculations were performed to elucidate the mechanisms of Cu2O reduction by alkanethiols and alkanethiolate anions. The oxide reduction requires the protonation of the surface Ocus atoms which act as Lewis base sites. In the apolar solvent, the proton is provided by the -SH group of the alkanethiol, whereas, in the aqueous alkaline medium, the proton comes from the water molecules of the solvent. The proton transfer reactions which originate a water molecule (reaction 6) or a hydroxide anion (reaction 7) have very low activation barriers (a few kcal/ mol), indicating that the surface reaction is not the limiting step. The high oxide reduction rate by alkanethiols dissolved in the alkaline aqueous solution is explained by the high concentration of the solvent water molecules which are the reactants (reaction 7). In turn, the hydroxide anions which are the reaction products are soluble in the aqueous solution. In the apolar solvent, the low reduction rate is a consequence of the low thiol concentration. The products in reaction 6 are water molecules which are insoluble in the apolar solvent and are therefore expected to condensate on the electrode surface. The fact that copper oxide was always detected for monolayers formed in hexane is attributed to the presence of surface water microdrops which block the reduction of the oxide surface by the alkanethiols in the forming solution. Acknowledgment. Financial support from FONCyT (Grant PICT 2005-32893), CONICET (Grant PIP 5903), and SECYTUNC is gratefully acknowledged. C.A.C. thanks CONICET for the fellowships granted. References and Notes (1) Kang, J. F.; Liao, S.; Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 9662. (2) Sur, U. K.; Lakshminarayanan, V. J. Electroanal. Chem. 2001, 516, 31.

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JP9045148

Interaction of Oxidized Copper Surfaces with Alkanethiols in Organic

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