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Electrochemical and Spectroscopic Study of the Self-Assembling Mechanism of Normal and Chelating Alkanethiols on Copper V. S. Dilimon, J. Denayer, J. Delhalle, and Z. Mekhalif* Laboratory of Chemistry and Electrochemistry of Surfaces, FUNDP-University of Namur, Rue de Bruxelles 61, B-5000 Namur, Belgium S Supporting Information *

ABSTRACT: The self-assembly of aliphatic thiol (RSH), dithiol (R(SH)2), and dithiocarboxylic acid (RS2H) onto mildly oxidized and highly oxidized copper was studied in real time by in situ electrochemical impedance spectroscopy (EIS). Ex situ characterization of the films was carried out using linear sweep voltammetry (LSV), polarization modulation infrared reflection absorption spectroscopy (PMIRRAS), and X-ray photoelectron spectroscopy (XPS). In situ EIS studies found a very fast adsorption of RSH, R(SH)2, and RS2H (within 10−15 s). This fast adsorption step is followed by the long-term additional adsorption and consolidation of SAM. However, the self-assembly of RS2H passes through an intermediate step of molecule rearrangement for around 10 to 30 min after around 2 to 7 min of self-assembly. The binding of both sulfur moieties of R(SH)2 with Cu happens simultaneous. The oxide reduction capacity of RSH, R(SH)2, and RS2H was good. However, the XPS studies showed the decomposition of RS2H-based SAMs to Cu2S. Monolayers prepared on both mildly oxidized and heavily oxidized Cu with R(SH)2 had the highest stability. Monolayers of RS2H showed the least stability on both mildly oxidized and heavily oxidized Cu. Although RSH-based SAMs had good organization on both mildly oxidized and highly oxidized Cu, R(SH)2-based SAMs did not show good organization in either case. The RS2H monolayer had good organization only on mildly oxidized Cu.

1. INTRODUCTION Noble metals such as gold and silver are the best choice as substrates for surface modification with organothiol selfassembled monolayers (SAMs).1,2 However, the high electromigration resistance and high thermal as well as electrical conductivity of Cu make it an attractive alternative to Al and expensive Au in various applications, for example, in small integration circuit assemblies.2 Therefore, the surface modification of Cu with SAMs has attracted a significant amount of attention.3−9 The stability of SAMs is an important criterion in determining their applications.10 Easily removable protective overlayers are attractive for certain technologies (e.g., soft lithographic patterning10−12). However, stable layers are highly desirable for applications such as corrosion prevention and thin-film lubrication.3,10,13 Lee et al. reported chelating SAMs generated by the adsorption of aliphatic dithiocarboxylic acids onto the Au surface.10,14 Compared to the normal alkanethiolbased SAMs, these SAMs are relatively easy to remove and can find use in applications where temporary nanoscale coatings are required.10 However, compared to normal alkanethiol-based SAMs, the chelating SAMs derived from the chelating dithiols are very stable on the Au substrate because of their chelating effect and the energetically disfavored formation of highly strained cyclic disulfides.15−24 However, nothing had been reported on the formation of chelating dithiol or dithiocarboxylic acid-based SAMs on Cu until our previous report on the © 2012 American Chemical Society

preparation of aliphatic thiol, dithiol, and dithiocarboxylic acid SAMs on electroreduced polycrystalline Cu.25 We showed that aliphatic thiol, dithiol, and dithiocarboxylic acid SAMs could be prepared on Cu and that their stability followed a trend similar to that on Au.25 Furthermore, the electroassisted self-assembly of these molecules on Cu has been proven to be very efficient and profitable for obtaining SAMs with improved adherence, organization, and protection features.26 Therefore, selfassembled monolayers of these molecules on Cu are attractive. However, no attempt has been made so far to study the selfassembly process of these molecules, especially with selfassembling time, on Cu. The use of electrochemical impedance spectroscopy (EIS) as a powerful and reliable technique for the real time in situ study of SAM formation on Cu has been described in our recent paper.27 The interfacial capacitance variation of the substrate with time was used to study the self-assembly process.27 In the case of SAM formation in ethanol solution, unlike with Au substrate, the interaction of Cu with solvent is very significant.4 However, the interfacial capacitance variation of Cu caused by organothiol self-assembly is very significant in making reasonable conclusions from the EIS study.27 Therefore, in the present work, the self-assembly mechanism of nReceived: January 3, 2012 Revised: March 8, 2012 Published: April 11, 2012 6857

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a deaerated 0.5 M aqueous solution of HClO4 (Acros Organics, 70%). The electrochemically reduced substrates were immediately rinsed with absolute ethanol and blown dry with high-purity N2. The substrates removed from the HClO4 electrolyte were in contact with atmospheric air for 1 min before being transferred to the selfassembling solution. These substrates will be referred to as mildly oxidized Cu in this article. Strong oxidation for the electrochemically reduced substrates was carried out by UV O3 activation for 10 min. These substrates will be referred to as highly oxidized Cu in this article. Self-assembly was carried out in deaerated absolute ethanol (VWR) solution, and the solution was blanketed with N2 throughout the selfassembly. The SAMs were rinsed thoroughly with absolute ethanol, sonicated in absolute ethanol for 5 min, and then blown dry with highpurity N2. The samples were then immediately (within 10 min for PMIRRAS and LSV analyses and within 30 min for XPS analyses) subjected to ex situ analyses. Electrochemical Analyses. Electrochemical studies were carried out using a potentiostat/galvanostat (EG&G model 273) and a Solartron impedance gain-phase analyzer (model SI 1260). In Situ EIS Analysis of SAM Formation. In situ EIS studies were carried out with a control potential versus time (control E vs t) setup in which the alternating current (ac) frequency, ac amplitude, and direct current (dc) potential are constant and the impedance behavior of the system is monitored for changes over time. A platinum foil auxiliary electrode and an SCE reference were used. Mildly oxidized or highly oxidized Cu was the working electrode. The EIS studies were carried out in deaerated ethanol solutions of the respective SAMforming molecules with a 0.1 M LiClO4 (Aldrich, 95%) supporting electrolyte. The dc potential was 0 V/SCE, and the applied ac amplitude was 5 mV peak to peak. The applied frequency ( f) was 600 Hz, which is in the stable capacitance plateau of the capacitance versus log f plot for both bare and organothiol self-assembled Cu.27 The duration of the experiments was 7 h. The electrolyte was blanketed with N2 throughout the experiment. Even though the experiment was started immediately after the Cu electrode was immersed in the electrolyte, an instrumental delay of a maximum of 10 s was there when receiving the first values of the real (Z′) and imaginary (Z″) components of the impedance. It is possible to write Z′ and Z″ for an ideal Randles-type circuit as29

tetradecanethiol (RSH), 2-dodecylpropane-1,3-dithiol (R(SH)2), and n-tetradecanedithiocarboxylic acid (RS2H) onto Cu was studied in real time by in situ electrochemical impedance spectroscopy (EIS). Characterization of the SAMs was carried out by using ex situ techniques such as linear sweep voltammetry (LSV), polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). A few studies based on the ex situ analyses of SAMs are available on the chelating SAM formation process on Au.15,16 Two kinetic regions consisting of fast bulk adsorption followed by a slow crystallization process have been recognized on Au.15,16 The aim of the present work is to study the selfassembly mechanism, stability, and molecular organization of SAMs formed with normal alkanethiol (n-tetradecanethiol (RSH)), aliphatic chelating dithiol (2-dodecylpropane-1,3dithiol (R(SH)2), and dithiocarboxylic acid (n-tetradecanedithiocarboxylic acid (RS2H)) on Cu. The structures of these molecules are given in Figure 1. Copper is very susceptible to

Figure 1. Molecular structure of n-tetradecanethiol (RSH) (a), 2dodecylpropane-1,3-dithiol (R(SH)2) (b), and n-tetradecanedithiocarboxylic acid (RS2H)) (c).

Z′ = R u +

Z″ =

oxidation in air, hence the process of organized molecular assembly on it remains incompletely understood.2 The reduction of copper oxide during oraganothiol adsorption has been claimed by various researchers based on ex situ spectroscopic analyses.4,6,8 Recently, Calderón et al studied the interaction of oxidized Cu surfaces with alkanethiols in organic and aqueous solvents.28 They studied the mechanism of Cu2O reduction by means of density functional theory calculations. Given this background, mildly oxidized and highly oxidized Cu substrates are used in the present work.

R ct 1 + ω2R ct 2Cdl 2

(1)

CdlR ct 2ω 1 + ω2R ct 2Cdl 2

(2)

where Rct and Ru are the charge-transfer resistance and uncompensated for solution resistance, respectively, Cdl is the interfacial double-layer capacitance, and ω = 2πf (where f is the frequency of the applied ac potential). In a system where ω2Rct2Cdl2 ≫ 1, (e.g., a thiol film), it can be assumed that Z″ = 1/Cdlω.29 The calculation of Cdl is thus possible from Z″ at a high enough frequency. LSV Analysis. The LSV analyses of the SAMs were carried out in 0.1 M NaOH solution from −0.9 to 0.3 V or from 0.1 to −1.0 V at a scan rate of 20 mV s−1. A spot cell (spot Ø = 5.2 mm) was used for LSV analyses. A platinum foil auxiliary electrode and an SCE reference were used. Spectroscopic Analyses. PM-IRRAS Analysis. PM-IRRAS analyses of the monolayers were carried out on a Bruker Equinox55 PMA37 spectrometer equipped with a liquidnitrogen-cooled mercury−cadmium−telluride (MCT) detector and a zinc selenide photoelastic modulator. The measurements were carried out with the photoelastic modulator set at a halfwave retardation of 2600 cm−1. The infrared light, reaching the sample surface at an angle of 80°, was modulated between s and p polarization at a frequency of 50 kHz. Signals generated from each polarization (Rs and Rp) were detected simultaneously by a lock-in amplifier and were used to calculate the differential surface reflectivity (ΔR/R) = (Rp − Rs)/(Rp + Rs). The number of scans was 512, and the resolution was 2 cm−1.

2. EXPERIMENTAL METHODS Materials. 2-Dodecylpropane-1,3-dithiol (R(SH)2) and n-tetradecanedithiocarboxylic acid (RS2H) were synthesized in the laboratory as described in our previous report.25 n-Tetradecanethiol (RSH) (Aldrich, 99%) was used. Rectangular polycrystalline Cu substrates (Goodfellow, 99.99+%) were mechanically polished to a mirror finish (1200 grit silicon carbide paper and 9, 3, and 1 μm diamond paste suspensions). The substrates were washed with ultrapure water (18.2 MΩ cm), sonicated in absolute ethanol (VWR) for 15 min, and then immediately dried in a stream of high-purity N2. SAM Preparation for Ex Situ Studies. The Cu substrates were subjected to UV O3 activation for 15 min and then electrochemically reduced (−800 mV/SCE (saturated calomel electrode)) for 10 min in 6858

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XPS Analysis. XPS analyses of the SAMs were recorded at a 35° takeoff angle (relative to the surface normal) with an SSX-100 spectrometer using monochromatized Al Kα radiation (1486.6 eV). The spot size of the XPS source on the sample was 600 μm, and the analyzer was operated with a pass energy of 20 eV. The pressure was kept below 1 × 10−9 Torr during data acquisition. The peaks were calibrated with respect to the binding energy of the C 1s line set at 285 eV. Curve fitting of the XPS spectra was conducted using WinSpec 2.09 software. Spectra were curve fitted to the minimum number of peaks required for an optimum fit. The XPS-derived S/Cu abundance ratio for the SAMs was calculated from the peak area of S 2p and Cu 2p XPS peaks. The peak area of the elements was divided by the product of the corresponding Scofield sensitivity factor and the number of scans to get their abundance.

capacitance value measured at any time t. The initial surface coverage calculated for RSH-, R(SH)2-, and RS2H-based SAMs is around 75−90% (Figure S1, Supporting Information). The fast adsorption step of RSH and R(SH)2 is followed by a long-term additional adsorption and consolidation of their SAMs (Figures 2 and 3). A similar two-step process with a fast

3. RESULTS AND DISCUSSION Study of the Adsorption Process. The capacitance versus time curves recorded during the self-assembly of RSH, R(SH)2, and RS2H on mildly oxidized Cu are compared in Figures 2−4, Figure 3. Capacitance vs time curves for the mildly oxidized Cu in different concentrations of R(SH)2: 0.5 (a), 1 (b), 5 (c), and 10 mM (d). Inset a compares the capacitance vs time curves for the mildly oxidized Cu (a) and highly oxidized Cu (b) in 5 mM R(SH)2 solution. Inset b shows the stability test for the R(SH)2-based SAMs prepared for 30 min on mildly oxidized Cu (a) and on highly oxidized Cu (b). The stability test was carried out by measuring the capacitance vs time curves for the mildly oxidized and highly oxidized Cu during SAM formation in 5 mM R(SH)2 solution for 30 min and then by measuring the capacitance vs time curves for the corresponding self-assembled samples in the blank electrolyte for 1 h. Solvent, ethanol; supporting electrolyte, 0.1 M LiClO4. Figure 2. Capacitance vs time curves for the mildly oxidized Cu in different concentrations of RSH: 0.5 (a), 1 (b), 5 (c), and 20 mM (d). Inset a compares the capacitance vs time curves for the mildly oxidized Cu (a) and highly oxidized Cu (b) in 5 mM RSH solution. Inset b shows the stability test for the RSH-based SAMs prepared for 30 min on mildly oxidized Cu (a) and on highly oxidized Cu (b). The stability test was carried out by measuring the capacitance vs time curves for the mildly oxidized and highly oxidized Cu during SAM formation in 5 mM RSH solution for 30 min and then by measuring the capacitance vs time curves for the corresponding self-assembled samples in the blank electrolyte for 1 h. Solvent, ethanol; supporting electrolyte, 0.1 M LiClO4.

first step followed by a slow second step has been reported for the adsorption of thiol and dithiol on Au.15,16,29,30 The capacitance of RSH-based SAMs becomes almost steady after around 2 or 3 h depending on the RSH concentration (Figure 2). However, R(SH)2-based SAMs attain steady capacitance values in a shorter time showing their fast consolidation due to the cooperative effect of the two sulfur atoms in the headgroup of R(SH)2 (Figure 3). Moreover, compared to the RSH-based SAMs, the steady capacitance value of R(SH)2-based SAMs is lower by around 1 μF. The capacitance is related, on the basis of the Helmholz model, to the dielectric constant (ε), the length of the molecule (or the thickness of the SAM) (d), and the permittivity of free space (ε0) by the relation Cdl = εε0/d.29 Better orientation of the molecules with smaller tilt angles can give a high thickness value.17,30 Moreover, monolayers with a higher packing density can exhibit a smaller ε value because of the exclusion of polar solvent molecules from the hydrophobic surface.17 The XPS-derived S/Cu ratios for the RSH- and R(SH)2-based SAMs on mildly oxidized Cu are 0.18 and 0.51, respectively. Therefore, a higher packing density and/or a better orientation of the molecules may be responsible for the lower capacitance values of the R(SH)2-based SAMs. Ex situ EIS characterization of the SAMs prepared with a structurally tailored series of monodentate and bidentate alkanethiols on Au have also shown lower capacitance values for the later.17 The in situ EIS study shows a different self-assembly mechanism for RS2H (Figure 4). A fluctuation in capacitance is observed after around 2 to 7 min (varies from experiment to experiment) of self-assembly and continues for around 10 to 30 min (varies from experiment to experiment). The fluctuation in capacitance is very clear when it remains for a longer time

respectively. The capacitance at any time shows a decrease, indicating an increase in the surface coverage (θ), with an increase in the concentration of SAM -forming molecules up to around 5 mM. However, the observed increase in surface coverage with the increase in RSH concentration (Figure 2) is not as significant as that observed during the adsorption of ndodecanethiol.27 The capacitance of mildly oxidized bare Cu in the blank electrolyte is around 12 μF cm−2.27 The initial capacitance values in RSH, R(SH)2, and RS2H solutions are considerably lower than 12 μF cm−2 (Figures 2−4), indicating a fast adsorption of molecules within 10 s (i.e., within the experimental delay). The surface coverage was calculated as a function of time for the optimum concentration (i.e., 5 mM) using eq 3.29

⎛ C − Ct ⎞ θ=⎜ 0 ⎟ ⎝ C0 − Cf ⎠

(3)

In eq 3, C0 and Cf are the capacitances of the bare electrode and fully covered monolayer, respectively, and Ct is the 6859

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RS2H molecules is very weak, as revealed by the assessment of their stability. The S/Cu ratios for the SAM on highly oxidized Cu have the following values: RSH, 0.18; R(SH)2, 0.41; and RS2H, 0.45. However, XPS analysis shows that RS2H-based SAMs on highly oxidized Cu also have a significant quantity of disulfides or unbound thiols and Cu2S (discussed later). The stability of the monolayers of RSH, R(SH)2, and RS2H prepared for 30 min on mildly oxidized Cu and highly oxidized Cu was immediately investigated for 1 h, after washing thoroughly with absolute ethanol, by recording their capacitance versus time curves in the blank electrolyte. The results for RSH-, R(SH)2-, and RS2H-based SAMs are given in Figure 2 (inset b), Figure 3 (inset b), and Figure 5 (inset),

Figure 4. Capacitance vs time curves for the mildly oxidized Cu in different concentrations of RS2H: 0.5 (a), 1 (b), 5 (c), 10 (d), and 20 mM (e). Inset a compares the capacitance vs time curves for the mildly oxidized Cu showing a capacitance fluctuation for a longer time in different concentrations of RS2H: 5 (a), 10 (b), 20 (c), and 50 mM (d). Inset b compares the capacitance vs time curves for the mildly oxidized Cu (a) and highly oxidized Cu (b) in 5 mM RS2H solution. Solvent, ethanol; supporting electrolyte, 0.1 M LiClO4.

(Figure 4 (inset a)). In this period, the capacitance values are almost comparable to those recorded during the self-assembly of RSH (Figure S2, Supporting Information). The XPS analysis of the SAM formed up to the time corresponding to the end of the capacitance fluctuation was carried out. The XPS study confirmed the absence of any dissociation products (discussed later). According to the literature reports, such fluctuations in capacitance signify the rearrangement of molecules on the surface during monolayer formation.31 After this transition period, the capacitance again shows a decrease, and after around 5 h, it reaches the values observed during the selfassembly of R(SH)2. In the case of RS2H-based SAMs, the higher packing density and/or better orientation of the molecules may be responsible for the lower capacitance values compared to those of RSH-based SAMs. The XPS-derived S/ Cu ratio for the RS2H-based SAMs on mildly oxidized Cu is 0.29 (0.18 for the RSH-based SAM). However, XPS analysis showed that RS2H-based SAMs prepared for 7 h have a significant quantity of disulfides or unbound thiols and Cu2S (discussed later). The presence of these dissociation products on the Cu surface can also have a great influence on the capacitance values. On the basis of these observations, we propose a three-step adsorption process for RS2H. The initial fast adsorption step is followed by a rearrangement step. The dissociation of SAM also happens during the third long-term adsorption step. The capacitance versus time curves during the self-assembly of RSH, R(SH)2, and RS2H on highly oxidized Cu are compared with those on mildly oxidized Cu in Figure 2 (inset a), Figure 3 (inset a), and Figure 4 (inset b), respectively. The behavior of curves for mildly oxidized and highly oxidized Cu are reasonably identical in RSH as well as in R(SH)2 solutions. However, in the first hour, surprisingly, the capacitance values are noticeably lower for the highly oxidized Cu. Therefore, the combined effect of the oxide layer and the thiol layer contributes to the observed capacitance values. Moreover, the chemical interaction of the thiol and copper oxide layer during copper oxide reduction can also result in a sudden interaction of thiol with the highly oxidized surface. Compared to the adsorption of RSH and R(SH)2 on highly oxidized Cu, the adsorption of RS2H on highly oxidized Cu results in a rapid decrease in capacitance. However, this rapid adsorption of

Figure 5. Stability test for RS2H-based SAMs prepared for 5 h on mildly oxidized Cu (a) and on highly oxidized Cu (b). The stability test was carried out by measuring the capacitance vs time curves for the mildly oxidized and highly oxidized Cu during SAM formation in a 5 mM RS2H solution for 5 h and then by measuring the capacitance vs time curves for the corresponding self-assembled samples in the blank electrolyte for 5 h. The inset shows the stability test for the RS2Hbased SAMs prepared for 30 min on mildly oxidized Cu (a) and on highly oxidized Cu (b). Solvent, ethanol; supporting electrolyte, 0.1 M LiClO4.

respectively. No desorption of molecules is observed in the blank electrolyte for both RSH- and R(SH)2-based monolayers prepared on mildly oxidized as well as highly oxidized Cu. However, a gradual decrease in capacitance is observed in the blank electrolyte for the RSH-based SAMs on both mildly oxidized and highly oxidized Cu (Figure 2 (inset b)), showing their further consolidation with time. No additional consolidation of R(SH)2-based SAM is observed in the blank electrolyte (Figure 3 (inset b)), probably because of the cooperative interaction of two sulfur atoms in the R(SH)2 headgroup. Monolayer of RS2H prepared for 30 min on mildly oxidized Cu shows constant capacitance values in the blank electrolyte. However, the RS2H-based SAM on highly oxidized Cu shows a gradual increase in capacitance in the blank electrolyte because of desorption (Figure 5 (inset)). Therefore, the binding force responsible for the sudden adsorption of RS2H on highly oxidized Cu is not strong. The XPS analyses (discussed later) show that RS2H-based SAMs formed for a longer time (i.e., up to the last long-term adsorption step) on both mildly oxidized and highly oxidized Cu have significant quantities of dissociation products (disulfides or unbound thiols and Cu2S). Therefore, the stability of SAMs formed on both mildly oxidized and highly oxidized Cu for 5 h was also studied in a blank electrolyte (Figure 5). Figure 5 shows that the SAMs formed on both mildly oxidized and highly oxidized Cu for 5 h 6860

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identical capacitance versus time curves for both mildly oxidized and highly oxidized Cu in RSH or R(SH)2 solutions after around 1 h. Moreover, consistent with the EIS results showing a higher interaction of R(SH)2 with Cu, the monolayers of R(SH)2 prepared for any particular self-assembly time on mildly oxidized or highly oxidized Cu show higher cathodic decomposition potentials and lower decomposition current densities than do the corresponding RSH-based SAMs. Monolayers of RS2H on mildly oxidized Cu show lower blocking and poorer stability than the monolayers of RSH and R(SH)2 (Figure 6c). As the self-assembly time increases, RS2Hbased SAMs also show cathodic shifts in the dissociation potential as well as decreases in the dissociation current density, proving their consolidation with time. Because LSV is used as an ex situ technique in the present study, the decomposition of the RS2H-based SAMs when they were kept outside of the SAM-forming solution can also be the reason for their low stability.10 In agreement with the EIS results, films of RS2H on highly oxidized Cu are very unstable (Figure 6f). An evaluation of the oxide reduction capability of the molecules would be possible from the oxide reduction current by performing LSV in the cathodic direction for the freshly prepared monolayers on highly oxidized Cu. However, it should be noted that the characteristics of the voltammograms of Cu in alkaline solutions are strongly dependent on various experimental conditions.34 Moreover, Cu(I) or Cu(II) species other than Cu2O or CuO can also take part in the reaction.35 The LSV of bare Cu shows two major reduction peaks (Figure 7). The peak at low cathodic potential (C1) is due to Cu(II) to

are unstable. These results prove that even though the SAM formed for 30 min on mildly oxidized Cu is stable, SAM decomposition is happening during the long-term adsorption step. However, the RS2H-based SAM formed on highly oxidized Cu for any time is unstable. Monolayer Stability. Linear sweep voltammograms in 0.1 M NaOH solution for the SAMs prepared for different durations of time on mildly oxidized and highly oxidized Cu are compared in Figure 6. The highly oxidized Cu and mildly

Figure 6. Linear sweep voltammograms recorded in the anodic direction for RSH-modified, mildly oxidized Cu (a), R(SH)2-modified, mildly oxidized Cu (b), RS2H-modified, mildly oxidized Cu (c), RSHmodified, highly oxidized Cu (d), R(SH)2-modified, highly oxidized Cu (e), and RS2H-modified, highly oxidized Cu (f). Voltammograms for the unmodified copper (a) and the SAMs prepared for 15 s (b), 15 min (c), 1 h (d), and 7 h (e) are given in each part. Electrolyte: 0.1 M NaOH.

Figure 7. Linear sweep voltammograms recorded in the cathodic direction for highly oxidized Cu (a), RSH-modified, highly oxidized Cu (b), R(SH)2-modified, highly oxidized Cu (c), and RS2H-modified, highly oxidized Cu (d). Electrolyte: 0.1 M NaOH.

oxidized bare Cu show three anodic peaks. The first anodic peak is related to the formation of a porous layer of Cu2O.32,33 The second anodic peak is due to the dissolution of Cu within the pores of Cu2O and the formation of Cu(OH)2.32,33 This results in the establishment of a partial passive region for a short potential range. Following this, the third anodic peak due to the conversion of Cu2O to CuO appears.32,33 Monolayers of RSH or R(SH)2, prepared for even 15 s, on mildly oxidized Cu show good blocking (Figure 6a,b). As the self-assembly time increases, their cathodic decomposition potential shifts to higher negative values and the dissociation current density decreases. These observations justify the EIS results showing an initial fast adsorption of RSH and R(SH)2 (within around 10 s), followed by the long-term additional adsorption and consolidation of SAM. The SAMs of RSH or R(SH)2 on highly oxidized Cu show good blocking and high stability, comparable to those on mildly oxidized Cu, after only 15 min or 1 h (Figure 6d,e). This observation supports the EIS results showing

Cu(I) reduction, and that at high cathodic potential (C2) corresponds to Cu(I) to Cu(0) reduction.34,36 Modification with SAM results in a considerable decrease in the oxide reduction current (Figure 7). The decrease in the oxide reduction current is the least for the RS2H-based SAM, and the R(SH)2-based SAM shows the highest decrease. However, the stability of the protective SAM on Cu can also have an influence on the measured oxide reduction current. PM-IRRAS Analysis. Figure 8 shows the PM-IRRAS spectra of monolayers prepared on mildly oxidized and highly oxidized Cu for different self-assembly times. The −C−H stretching mode region between 3000 and 2800 cm−1 shows four bands due to asymmetric −CH3 stretching (υa (−CH3)) at ∼2965 cm−1, asymmetric −CH2 stretching (υa (−CH2)) at ∼2918 cm−1, symmetric −CH3 stretching (υs (−CH3)) at ∼2877 cm−1, and symmetric −CH2 stretching (υs (−CH2)) at ∼2850 6861

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before and after self-assembly. An analysis of both of these spectral regions are required when more than one species (e.g., Cu, Cu2O, and CuO) is present.3 Even though the different oxidation states of Cu can be distinguished by using the most intense component of the LMM Auger transition, this region of the spectrum also contains other Auger transitions. In such a case, the determination of the identity and the relative abundance of various species are often complicated by the inability to resolve individual peaks.3 The LMM Auger transition intensities of any of the Cu species (e.g., Cu(0), Cu2O, CuO, etc.) can provide qualitative information about the changes in the composition of Cu. However, their relative intensities cannot be used as measures of the relative composition of the Cu surface,3 but the Cu 2p region can be used quantitatively to distinguish CuO from Cu(0) and Cu2O.3 The Cu 2p spectral regions of both mildly oxidized and highly oxidized bare Cu are compared in Figure 9. The binding

Figure 8. PM-IRRAS spectra recorded for RSH-modified, mildly oxidized Cu (a), R(SH)2-modified, mildly oxidized Cu (b), RS2Hmodified, mildly oxidized Cu (c), RSH-modified, highly oxidized Cu (d), R(SH)2-modified, highly oxidized Cu (e), and RS2H-modified, highly oxidized Cu (f). The spectra were recorded for the SAMs prepared for 15 s (a), 15 min (b), 1 h (c), and 7 h (d).

cm−1. The symmetric −CH3 stretching band splits because of Fermi resonance interactions with the lower-frequency asymmetric −CH3 deformation bands. Because of these interactions, a shoulder at ∼2934 appears in the υa (−CH2) band. The wavenumbers corresponding to υa (−CH2) and υs (−CH2) are particularly interesting in determining the monolayer organization. The shift of these bands to higher frequencies indicates a molecular organization of lesser quality with randomly oriented methylene groups with dominant gauche defects rather than a trans zigzag extended conformation.4,9,27,37,38 The positions of υa (−CH2) at 2918 cm−1 and υs (−CH2) at 2850 cm−1 for the RSH-based SAMs prepared for any selfassembly time on both mildly oxidized and highly oxidized Cu indicate their high degree of organization (Figure 8a,d). However, the positions of these bands for R(SH)2-based SAMs on both mildly oxidized and highly oxidized Cu are at higher wavenumbers (Figure 8b,e). The positions of υa (−CH2) and υs (−CH2) are at 2925 and 2854 cm−1, respectively, indicating their poor organization. Figure 8c shows that RS2Hbased SAMs prepared for 15 s on mildly oxidized Cu have poor organization (υa (−CH2) at 2924 cm−1 and υs (−CH2) at 2852 cm−1). However, the SAMs prepared for longer times on mildly oxidized Cu have good organization (Figure 8c). In situ EIS studies have shown that the initially adsorbed RS2H molecules go through a rearrangement of molecules. The PM-IRRAS results therefore show that the organization of RS2H-based SAM on mildly oxidized Cu before the rearrangement step is poor. The positions of υa (−CH2) at 2923 cm−1 and υs (−CH2) at 2852 cm−1 for the RS2H-based SAMs prepared for any selfassembly time on highly oxidized Cu show their poor organization. XPS Analysis. The Cu 2p spectral region and the CuLMM Auger region were used to study the oxidation states of Cu

Figure 9. XPS spectra of the Cu 2p spectral region of mildly oxidized Cu (a) and highly oxidized Cu (b). The solid lines show the position of the peaks due to the Cu(0) and Cu(I) species. The dashed lines show the position of peaks due to the Cu(II) species.

energy corresponding to the Cu 2p3/2 XPS peak for metallic Cu and Cu2O has the standard value of 932.6 eV.3 The binding energies corresponding to the Cu 2p3/2 XPS peaks for CuO have standard values of 933.8 (line widths of Cu(II) species are approximately twice those of Cu and Cu2O because of multiplet splitting), 940.5 (shake-up peak), and 493.5 eV (shake-up peak).3 Figure 9b shows the presence of a considerable amount of CuO on the highly oxidized Cu. The satellite peak at 962.4 eV again confirms the presence of CuO on the highly oxidized Cu. However, the mildly oxidized Cu surface contains predominantly Cu(0) or Cu2O (Figure 9a). The Cu 2p spectral region for the RSH, R(SH)2, and RS2H-based SAMs prepared for 7 h on both mildly oxidized and highly oxidized Cu shows the presence of only Cu(0) or Cu2O (Figure S3, Supporting Information). However, the determination of the identity of Cu(0) or Cu2O is difficult from the Cu 2p spectral region because both have almost the same binding energy of 932.6 eV. Therefore, the ability of RSH, R(SH)2, and RS2H to reduce Cu(II) to Cu(I) and/or Cu(0) is clear from the Cu 2p spectral region. The CuLMM Auger spectrum of the SAMs prepared for 7 h on both mildly oxidized and highly oxidized Cu is compared in Figure 10. Five binding-energy values shown in Figure 10 6862

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Cu 2O + 2R(SH)2 → 2Cu + R(SH)S−S(SH)R + H 2O (9)

The ability of RS2H to reduce copper oxide is clear from the increased Cu peak intensity after SAM formation (Figure 10). Even though the Cu 2p spectral region proved the reduction of CuO to Cu2O and/or Cu(0) because of RS2H-based SAM formation, the shift of the CuO peak (569.8 eV) to the binding energy of Cu2O (570.6 eV) is not observed for the RS2H-based SAMs in the CuLMM Auger spectrum. Moreover, the CuO peak for the RS2H-based SAMs is at 569.5 eV. It is difficult to distinguish if the peak observed at 569.5 eV is due to CuO or Cu2S. Copper sulfide can be formed by the decomposition of RS2H-based SAMs.10 The S 2p signal was therefore analyzed to understand more about the nature of the chemical bond between the adsorbate anchoring groups and the surface (Figure 11). The S 2p peak

Figure 10. XPS CuLMM Auger spectra recorded for part a: unmodified, mildly oxidized Cu (a), mildly oxidized Cu self-assembled with RSH (b), mildly oxidized Cu self-assembled with R(SH)2 (c), and mildly oxidized Cu self-assembled with RS2H (d). XPS CuLMM Auger spectra recorded for part b: unmodified, highly oxidized Cu (a), highly oxidized Cu self-assembled with RSH (b), highly oxidized Cu selfassembled with R(SH)2 (c), and highly oxidized Cu self-assembled with RS2H (d). The SAMs were prepared for 7 h.

correspond to metallic Cu (567.9 eV),39,40 Cu2S (569.5 eV),39 CuO (569.8 eV),40 and Cu2O (570.6 eV).40 Unmodified Cu, both mildly oxidized and highly oxidized, shows a broad peak at around 570 eV due to copper oxide. The analysis of the Cu 2p spectral region has shown that the highly oxidized Cu surface has CuO as the major copper oxide component and Cu2O is the major component for mildly oxidized Cu. Moreover, in the CuLMM Auger spectrum, the metallic Cu peak is low intensity for mildly oxidized Cu and is not visible for highly oxidized Cu. The intensity of the copper oxide peak is diminished, and the peak position is shifted to 570.6 eV (Cu2O) by the selfassembly of RSH on mildly oxidized Cu. A corresponding increase in the intensity of the metallic Cu peak is also observed (Figure 10). A similar shift in the copper oxide peak position with an increase in the Cu peak intensity is also observed by the self-assembly of RSH on highly oxidized Cu. Therefore, the ability of RSH to reduce copper oxide to Cu(0)4,6,8,28 is clear from the XPS results. It is clear that initially CuO is reduced to Cu2O by RSH (eq 4) and then RSH reacts with Cu2O per eq 5 or 6. 2CuO + 2RSH → Cu 2O + RS−SR + H 2O

(4)

Cu 2O + 2RSH → 2RSCu + H 2O

(5)

Cu 2O + 2RSH → 2Cu + RS−SR + H 2O

(6)

Figure 11. XPS S 2p spectral region recorded for mildly oxidized Cu self-assembled with R−SH (a), mildly oxidized Cu self-assembled with R(SH)2 (b), mildly oxidized Cu self-assembled with RS2H (c), highly oxidized Cu self-assembled with R−SH (d), highly oxidized Cu selfassembled with R(SH)2 (e), and highly oxidized Cu self-assembled with RS2H (f). The SAMs were prepared for 7 h.

positions are copper thiolate (162.7 eV),3,42 Cu2S (161.9 eV),39 disulfides/unbound thiols (163.9 eV),39,26 copper sulfinates (167 eV),26 and copper sulfonates (168.8 eV).39,26 The sulfur doublets were fitted using an intensity ratio and energy separation of 2:1 and 1.18 eV, respectively. Only copper thiolate binding is observed for the RSH-based SAM on mildly oxidized Cu. However, RSH-based SAMs on highly oxidized Cu show oxidized species such as copper sulfonates and sulfinates (broad peak at 168 eV). Monolayers of R(SH)2 on mildly oxidized and highly oxidized Cu show a copper thiolate peak at 162.6 eV. However, R(SH)2-based SAMs on highly oxidized Cu also show a very small or negligible quantity of disulfides or unbound thiols (163.8 eV). This result indicates that the thiol functions of R(SH)2 essentially bind to the surfaces of both mildly oxidized and highly oxidized Cu as thiolates without leaving unbound thiols. Monolayers of RS2H on both mildly oxidized and highly oxidized Cu show a copper thiolate peak at 162.5 eV. The peak at around 164.1 eV shows the presence of disulfides or unbound thiols. The decomposition of RS2H-based SAMs on both the mildly oxidized and

The reduction of CuO to Cu2O during the self-assembly of R(SH)2 is evident from the CuLMM Auger spectrum of SAMs prepared on both mildly oxidized and highly oxidized Cu (Figure 10). However, its efficiency for the Cu2O reduction to Cu seems to be lower than that of RSH. The reduction of copper oxide by R(SH)2 via its oxidation to cyclic disulfide is energetically disfavored because of ring strain generated in the product.41 Hence, pathways involving intermolecular disulfide formation are tenable (eqs 7−9). 2CuO + 2R(SH)2 → Cu 2O + R(SH)S−S(SH)R + H 2O (7)

Cu 2O + R(SH)2 → RS2Cu 2 + H 2O

(8) 6863

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R(SH)2-, and RS2H-based SAMs on mildly oxidized and highly oxidized Cu. XPS spectra of the S 2p spectral region of RS2Hbased SAMs prepared for 10 min on mildly oxidized Cu. This material is available free of charge via the Internet at http:// pubs.acs.org.

highly oxidized Cu surfaces to Cu2S is very clear from the intense peak at 161.8 eV. This observation shows that the CuLMM peak observed at 569.5 eV includes a significant contribution from Cu2S. Therefore, it is clear that RS2H reduces Cu(II) to Cu(I) and Cu(0) and additionally that Cu2S is a major component of the Cu(I) state. The S 2p signal for the RS2H-based SAM prepared for 10 min on a mildly oxidized surface was studied to understand if the capacitance fluctuation observed during the adsorption of RS2H (Figure 4) was due to the decomposition of the SAM. Interestingly, only a copper thiolate peak at 162.6 eV was observed (Figure S4, Supporting Information). We propose a mechanism for oxide reduction and RS2Hbased SAM formation as follows:

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AUTHOR INFORMATION

Corresponding Author

*Tel: +32 (0) 81 72 52 30. Fax: +32 (0) 81 72 46 00. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS

V.S.D. acknowledges CERUNA for financial support.

4. CONCLUSIONS The self-assembly of aliphatic thiol (RSH), dithiol (R(SH)2), and dithiocarboxylic acid (RS2H) onto mildly oxidized and highly oxidized Cu was studied. The molecules bind to Cu within 10−15 s. This fast adsorption step is followed by the long-term additional adsorption and consolidation of SAMs. However, the self-assembly of RS2H passes through an intermediate reorganization step for around 10−30 min after around 2−7 min of self-assembly. The binding of both sulfur moieties of R(SH)2 with Cu happens simultaneous. The stability of RS2H-based SAMs was low, especially on highly oxidized Cu. Monolayers of R(SH)2 had the highest stability on both mildly oxidized and highly oxidized Cu. Although RSHbased SAMs have very good organization on both mildly oxidized and highly oxidized Cu, the organization was very poor for the R(SH)2-based SAMs. The organization of RS2H-based SAMs on mildly oxidized Cu was poor before the rearrangement step. However, the organization of RS2H-based SAMs on highly oxidized Cu was poor at any time. Among the three molecules, RSH showed the highest copper oxide reduction efficiency. The present study shows that the in situ EIS technique can be used with the conventional ex situ electrochemical and spectroscopic techniques to get a better understanding of the self-assembly mechanism on oxidizable metals.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

Surface coverage versus time curves for mildly oxidized Cu in RSH, R(SH)2, and RS2H solutions. Capacitance versus time curves recorded for a short time in RSH, R(SH)2, and RS2H solutions. XPS spectra of the Cu 2p spectral region of RSH-, 6864

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