1-Dodecanethiol Self-Assembled Monolayers on Cobalt - Langmuir

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1-Dodecanethiol Self-Assembled Monolayers on Cobalt S
ebastien Devillers, Alexandre Hennart, Joseph Delhalle, and Zineb Mekhalif * Laboratory of Chemistry and Electrochemistry of Surfaces (CES), University of Namur (FUNDP), 61 rue de Bruxelles, B-5000 Namur, Belgium ABSTRACT:

Cobalt and its alloys are used in a broad range of application fields. However, the use of this metal is especially limited by its strongly oxidizable nature. The use of alkanethiol self-assembled monolayers (SAMs) is a very efficient way to protect against such oxidation and/or to inhibit corrosion. This surface modification method has been particularly applied to oxidizable metals such as copper or nickel, yet the modification of cobalt surfaces by alkanethiol SAMs received limited attention up to now. In this work, we study the influence of parameters by which to control the self-assembly process of 1-dodecanethiol monolayers on cobalt: nature of the surface pretreatment, solvent, immersion time, and concentration. Each of these parameters has been optimized to obtain a densely packed and stable monolayer able to efficiently prevent the reoxidation of the modified cobalt substrates. The obtained monolayers were characterized by X-ray photoelectron spectroscopy (XPS), polarization modulation infrared reflection
absorption spectroscopy, and contact angle measurements. The stability of the optimized 1-dodecanethiol monolayer upon air exposure for 28 days has been confirmed by XPS.

’ INTRODUCTION Cobalt and its alloys have a broad range of applications as catalysts, magnetic recording media, pigments, 1 dental and orthopedic implants, etc.2 The cobalt surface is a relatively complex system due to its strongly oxidizable nature and the many possible forms of oxides with the same oxidation state.3 When a cobalt substrate is immersed into a solution, many parameters such as the composition, the pH, or the temperature of this solution can influence the formation and the stability of these oxides on the cobalt surface.4
6 Many studies have addressed the formation7
9 and the dissolution10,11 of this native oxide film. Badawy et al. reported for the first time a general mechanism of the formation of cobalt oxides in aqueous solutions at different pH values.5 In neutral conditions, the native oxide film present at the cobalt surface is composed of cobalt oxide (CoO) and/or cobalt hydroxide (Co(OH)2). In acidic conditions, this oxide film is unstable and a chemical dissolution takes place, whereas a new oxide film is formed over the previous one in alkaline conditions. This new oxide film is composed of CoOOH and Co3O4. Other studies report on the stability of these cobalt oxides in the presence of some specific chemical species.12
14 Particularly, Ismail and Badawy showed the intimate relation existing between the chloride ion concentration and the resistance of the passive film.7 r 2011 American Chemical Society

Recently, the interest for cobalt alloys has grown. Several studies showed the importance of surface property control.2,15
18 In this context, the interest of using self-assembled monolayers (SAMs) is well-known, in particular for the corrosion inhibition of oxidizable metals. Nowadays, numerous works have been carried out on copper19
24 and nickel25,26 because of the economic interest of these metals. However, to the best of our knowledge, only a few studies report on the formation of SAMs on the cobalt surface. Hoertz et al. compared SAM formation on a cobalt film with that on iron and nickel films by using two different surfacebinding chemistries (thiol and isocyanide) and pointed out the importance of the surface preparation conditions.27 Cichomski et al. reported on the modification of an oxidized cobalt film with a (perfluorodecyl)trichlorosilane (FDTS) monolayer.28 By mainly theoretical approaches, Caruso and Wang studied the properties of 1,10 -biphenyl-4,40 -dimethanethiol self-assembled monolayers on cobalt substrates for their potential application in the field of spintronics.29,30 Also in the field of spintronic applications, Burtman et al. investigated the formation of a mixed self-assembled monolayer comprising molecular wires Received: July 14, 2011 Revised: October 14, 2011 Published: October 31, 2011 14849

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Langmuir (benzene-1,4-dimethanethiol) and molecular insulators (pentane1-thiol) sandwiched between two metallic electrodes.31 Similar structures are also used for the formation of magnetic tunnel junctions (i.e., ferromagnetic/insulator/ferromagnetic layers) used in magnetic random access memory (MRAM) devices. Nowadays, the insulator layer used for MRAMs is commonly composed of Al2O3 or MgO but exposed to the problem of a degradation of the ferromagnetic/insulator interface due to oxygen diffusion.32,33 Thanks to their blocking property, SAMs appear to provide an interesting alternative to these insulating materials.34 The present work aims to study the impact of different parameters on the formation of 1-dodecanethiol SAMs on cobalt substrates. Among the parameters that can strongly influence the formation of alkanethiol SAMs by immersion are the pretreatment of the surface,25,35,36 solvent of the alkanethiol solution,26,37,38 immersion time,19,26,39 and alkanethiol solution concentration.20,25,40 These parameters were systematically investigated and optimized in the frame of cobalt surface modification with 1-dodecanethiol with X-ray photoelectron spectroscopy (XPS), polarization modulation infrared reflection
absorption spectroscopy (PM-IRRAS), and contact angle measurements. Finally, we assessed the longterm (28 days) stability of the optimized monolayer in atmospheric conditions.

’ EXPERIMENTAL SECTION Chemicals. Absolute ethanol (AnalaR NORMAPUR, analytical reagent), perchloric acid (70%, Acros Organics), sodium hydroxide (98.5%, Acros Organics), sodium chloride (99.5%, Acros Organics), and 1-dodecanethiol (Aldrich, 98%) were used without further purification. Toluene (Acros Organics) and tetrahydrofuran (Acros Organics) were distillated before use. All aqueous solutions were prepared with ultrapure Milli-Q water (18.2 mΩ). Substrate Preparation. The cobalt substrates (polycrystalline cobalt, 99.9%, 10  10  0.1 cm3 plates, Goodfellow, CO000290/13) were cut into 2  1 cm2 coupons and mechanically polished down to 1 μm on a Buehler Phoenix 4000 instrument using various grit silicon carbide papers and diamond pastes. After the polishing step, the substrates were copiously rinsed with Milli-Q water, cleaned by sonication for 15 min in ethanol, flushed dry under a nitrogen flow, and stored until their modification. We call this surface state “native” as no other pretreatment is applied before the grafting of the 1-dodecanethiol monolayer (except for another cleaning step by sonication for 15 min in ethanol). Two alternative pretreatments of the surface were studied in this work: electrochemical reductions in alkaline and acidic media. Cobalt electrochemical reductions in alkaline medium were carried out in a sodium hydroxide aqueous solution (pH adjusted to 13.7), while electrochemical reductions in acidic medium were carried out in a perchloric acid aqueous solution (pH adjusted to 2.0). All the electroreduction pretreatments were carried out in a classical three-electrode electrochemical setup using a saturated calomel electrode (SCE) as the reference, a platinum foil as the counter electrode, and the cobalt substrate as the working electrode by applying a constant potential of
1 V vs SCE for 10 min to the cobalt substrate. Monolayer Preparation and Characterization. 1-Dodecanethiol monolayers were prepared by immersion of the cobalt substrate into a modification solution directly after the pretreatment (cleaning for the native substrates or electrochemical reduction for the electroreduced ones). This solution was previously deaerated with a nitrogen flow for 10 min. At the end of the immersion time, cobalt substrates were copiously rinsed with ethanol, cleaned by

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sonication for 15 min in ethanol, flushed dry under a nitrogen flow, and characterized directly. The monolayers were characterized by XPS, PM-IRRAS, and water static contact angle measurements. XPS was used to investigate the elemental composition of the monolayers. The photoelectron spectra were recorded on an SSX-100 spectrometer using monochromatized X-ray Al Kα radiation (1486.6 eV), the photoemitted electrons being collected at a 35° takeoff angle. Nominal resolution was measured as a full width at half-maximum of 1.0
1.5 eV for core levels and survey spectra, respectively. The binding energy of the core levels was calibrated against the C1s binding energy set at 285.0 eV, an energy characteristic of alkyl moieties. The peaks were analyzed using mixed Gaussian
Lorentzian curves (80% Gaussian character). The S2p peak, which is a doublet structure where the S2p3/2 and S2p1/2 components are spaced by 1.18 eV and have an intensity ratio of 2, was analyzed accordingly. Quantitative XPS analyses were carried out by calculation of relevant abundance ratios, i.e., C1s/S2p, S2p/Co2p, and O1s/Co2p. These ratios were calculated on the basis of the XPS peak experimental intensities (peak areas) taking into account the corresponding Scofield sensitivity factors (SFs) and electron effective attenuation lengths (EALs). The SFs of C1s, S2p, O1s, and Co2p are 1, 1.79, 2.49, and 13.16, respectively (values referenced to the SSX-100 setup). The EALs used to correct the peak intensities were calculated using the NIST Electron EffectiveAttenuation-Length Database. These EAL values are 3.93, 5.03, and 6.66 nm for the Co2p, O1s, and S2p photoelectrons, respectively. PM-IRRAS data were collected to assess the monolayer organization. They were registered on a Bruker Equinox 55-PMA37 equipped with a liquid nitrogen cooled mercury cadmium telluride (MCT) detector and a zinc selenide photoelastic modulator. The infrared light was modulated between s- and p-polarization at a frequency of 50 kHz and an incident angle upon the sample surface of around 85°. Signals generated from each polarization (Rs and Rp) were detected simultaneously by a lock-in amplifier and used to calculate the differential surface reflectivity ΔR/R = Rp
Rs)/(Rp + Rs). All spectra are the average of 512 scans at a spectral resolution of 2 cm
1. Static contact angle measurements were carried out using a DIGIDROP (GBX Surface Science Technology) contact angle goniometer at room temperature. A syringe was used to dispense 2 μL of probe droplets of Milli-Q water on the sample surface. The mean roughness of the different pretreated cobalt surfaces was measured using a DEKTAK 8 Stylus profiler (Veeco Metrology Group) with a 5 μm Ø diamond stylus, a 5 mg forcem and a 2 mm scan length. Each surface modification was repeated at least three times for each characterization method to ensure the reproducibility of the presented results.

’ RESULTS AND DISCUSSION Influence of the Pretreatment. The first step of this study was to compare the quality of the 1-dodecanethiol monolayers obtained with the three pretreatments described in the Experimental Section, i.e., native (no further pretreatment after the mechanical polishing), electroreduced in alkaline medium (NaOH, pH 13.7), and electroreduced in acidic medium (HClO4, pH 2.0). Note that the cobalt surface mean roughness values vary significantly with the preatreatment: the native has a mean roughness of 42 nm, while the mean roughness values measured for cobalt electroreduced in acidic and alkaline media are 68 and 127 nm, respectively. In this section, all the monolayers have been prepared by immersion of the cobalt substrates directly after the pretreatment in a 1-dodecanethiol 10
2 M ethanolic solution for 16 h (arbitrarily 14850

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Table 1. C1s/S2p, S2p/Co2p, and O1s/Co2p Ratios Calculated from XPS Spectra of Cobalt Substrates Modified with a 1-Dodecanethiol Monolayer without any Pretreatment (Native), after an Electrochemical Reduction in a NaOH Solution, pH 13.7 (Alkaline), and after an Electrochemical Reduction in HClO4, pH 2.0 (Acidic), and the Corresponding Water Contact Angle Values C1s/S2p

S2p/Co2p

O1s/Co2p

contact angle (deg)

native

31.5

0.6

4.9

64 ( 1

alkaline

21.6

0.1

1.3

79 ( 6

acidic

12.8

1.1

0.8

115 ( 1

56 ( 8

bare cobalt

Figure 1. Representative XPS survey spectra of a bare cobalt substrate (a) and cobalt substrates modified by immersion in a 1-dodecanethiol 10
2 M ethanolic solution for 16 h without any pretreatment (b), after an electrochemical reduction in a NaOH solution, pH 13.7 (c), and after an electrochemical reduction in HClO4, pH 2.0 (d).

chosen conditions on the basis of previous similar studies26,37). The obtained monolayers have been systematically characterized by XPS, PM-IRRAS, and contact angle measurements. The XPS survey spectra of these modified substrates and a reference unmodified cobalt substrate are presented in Figure 1. On the reference spectrum, the presence of the peaks characteristic of the cobalt substrate can be pointed out, i.e., Co3p, Co3s, Co2p, and Co2s centered around 60, 101, 780, and 925 eV, respectively, as well as the Auger line CoLMM around 718 eV. Besides these cobalt peaks, this spectrum exhibits an important O1s peak around 530 eV, attributed to the oxygen atoms of the oxide layer present at the surface of the substrate as well as a C1s peak around 285 eV, attributed to the presence of some physisorbed atmospheric contaminations. On the modified cobalt substrates, the appearance of sulfur photoelectron peaks can be pointed out, i.e., S2s and S2p centered around 228 and 164 eV, respectively, confirming the presence of a certain amount of 1-dodecanethiol molecules on the cobalt surface in each studied case. Moreover, the increase of the C1s peak intensity and the decrease of the O1s peak intensity coming with the modification further confirm the grafting of 1-dodecanethiol molecules in each studied case. Note also the presence of a Na1s peak centered at 1071.6 eV on the survey spectra of the cobalt substrate modified after an electrochemical reduction treatment in alkaline medium, attributed to the presence of some residual sodium hydroxide on the surface after the modification. The calculation of relevant abundance ratios has been carried out to further characterize these modified surfaces. These abundance ratios were calculated on the basis of the XPS peak experimental intensities (peak areas) taking into account the corresponding Scofield sensitivity factors and electron effective attenuation lengths. In particular, the C1s/S2p ratio (which should be theoretically close to 12 according to the grafted

molecule stoichiometry) is indicative of the amount of physisorbed carbon contaminations, the S2p/Co2p ratio the amount of grafted 1-dodecanethiol molecules, and the O1s/Co2p ratio the oxidation of the cobalt surface. All these ratios are presented in Table 1. Note that these measurements should not be treated quantitatively using a homogeneous surface model (i.e., assuming that the elemental concentrations are uniformly distributed across the sampling depth of XPS) but considered as a layered structure (i.e., the stacking of cobalt/cobalt oxide/sulfur/carbon layers corresponding to the substrate, the anchoring groups, and the alkyl chain layers, respectively), and therefore, electron effective attenuation length values have also been taken into account for the abundance ratio calculations. Note that in this layer structure the “sulfur” as well as the “cobalt oxide” layers are directly adjacent to the “cobalt” layer. These layers have thus been considered as a homogeneous system. Despite this approximation, these data are useful to draw some general conclusions and semiquantitative comparisons of the different modified surfaces. The lowest C1s/S2p ratio (12.8) is obtained for a cobalt substrate modified after an electrochemical reduction in acidic medium. Note that the surface roughness has been shown to have a significant influence on the calculated abundance ratios as the effective local photoelectron takeoff angle is quite different from the macroscopic takeoff angle (i.e., 35° in the frame of this study). For example, Tosatti et al. showed that an increase in the surface roughness induces an important increase of the carbon/metal ratio resulting from a decrease of the effective local takeoff angle.41 Therefore, as native substrates have the lowest roughness and the highest C1s/S2p, it is reasonable to think that this pretreatment leads to the formation of monolayers with the highest amount of atmospheric contaminations. Regarding the substrates electroreduced in alkaline medium, the situation is less clear as the high C1s/S2p ratio could be due to the significant roughness of the surface rather than to an important amount of physisorbed atmospheric contaminations. However, the lowest C1s/S2p ratio is obtained for monolayers formed on substrates electroreduced in acidic medium, while these substrates have a roughness slightly higher than the native ones. It thus appears that the acidic pretreatment leads to a cobalt surface state allowing the formation of a SAM with a minimum of these atmospheric contaminations. This observation can be confirmed by the analysis of the XPS high-resolution C1s photoelectron spectra (Figure 2). The C1s spectra of the monolayers formed on cobalt surfaces electroreduced in alkaline medium can be analyzed with four peaks (Figure 2b). The first peak, centered at 285.0 eV, is mainly 14851

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Figure 2. C1s (a
c) and S2p (d
f) core level XPS spectra of 1-dodecanethiol SAMs formed on cobalt substrates by immersion in a 1-dodecanethiol 10
2 M ethanolic solution for 16 h without any pretreatment (a, d), after an electrochemical reduction in a NaOH solution, pH 13.7 (b, e), and after an electrochemical reduction in HClO4, pH 2.0 (c, f).

attributed to the aliphatic carbon atoms of the alkyl chain of the 1-dodecanethiol molecule (a certain proportion of this peak also being attributable to atmospheric contaminations). The second one, centered around 286 eV, can be attributed to the contribution of the carbon atom directly bound to the sulfur in the 1-dodecanethiol molecule or to the presence of some slightly oxidized carbonaceous atmospheric contaminations. The other two peaks, centered at 287.5 and 289 eV, can be attributed to more oxidized carbonaceous atmospheric contaminations, i.e.,
C(O)
and
C(O)
O
, respectively. The C1s spectra of the monolayers formed on native cobalt surfaces and on cobalt surfaces electroreduced in acidic medium can be analyzed with only three peaks, i.e., the first two centered at 285 and 286 eV and a third one centered around 289 eV for native substrates (Figure 2a) or 287.5 eV for substrates electroreduced in acidic medium (Figure 2c). For the monolayers formed on cobalt surfaces electroreduced in alkaline medium and on native cobalt surfaces, the peak centered at 285 eV represents only 68% and 70% of the total C1s signal, respectively. On the other hand, this peak represents about 91% of the total C1s signal for monolayers formed on the cobalt surface electroreduced in acidic medium. This is another indication of the lower contamination of these monolayers. Regarding the S2p/Co2p ratio, it clearly appears that the electrochemical reduction in alkaline medium leads to the lowest amount of grafted 1-dodecanethiol molecules while an electrochemical reduction in acidic medium leads to the highest amount of grafted molecules. High-resolution XPS spectra of the S2p regions are also presented in Figure 2. For the monolayers formed on cobalt substrates electroreduced in alkaline medium and on native cobalt surfaces, the S2p spectrum exhibits only one doublet with the S2p3/2 component centered at 162.2 eV corresponding to thiolate species. The presence of these thiolate

species proves the chemical binding of the molecule to the surface. The signal being very weak, it is not possible to determine with certainty the presence of other components. On the other hand, the S2p spectrum of the monolayers formed on cobalt substrates submitted to an electroreduction in acidic medium exhibits three doublets, S2p3/2 components centered at 162.2, 163.8, and 166.2 eV, corresponding to thiolates (about 80% of the total S2p signal), unbound thiols or disulfur (about 5% of the total S2p signal), and sulfinates (about 15% of the total S2p signal), respectively. The presence of these oxidized sulfur species is here visible thanks to the much higher signal intensity as reported in Table 1. As oxidized sulfur groups are usually desorbed and replaced by fresh thiol molecules in the modification bath due to their weaker binding energy to the surface,42 the most reasonable explanation for the presence of these oxidized thiol species is a partial reoxidation of the electroreduced (and therefore extremely reactive) surface, indicating the weak ability of the formed monolayer to prevent this reoxidation. Regarding the O1s/Co2p ratio, it appears that both electroreduction preatreatments induce a significant decrease of the amount of oxides present at the surface of the substrates. However, electroreduction in acidic medium clearly appears to be the most suitable as the resulting O1s/Co2p ratio is almost 2 times lower than that obtained with an electroreduction in alkaline medium. According to the Pourbaix diagram of cobalt,3 an electroreduction at a pH of 13.7 should not lead to a stabilization of the oxide layer after the potential removal but rather to the stabilization of soluble HCoO2
species (and therefore to a dissolution of the oxide layer). Nevertheless, the examination of the corresponding O1s core level photoelectron peak spectrum (see Figure 3b) reveals the presence of an important amount of cobalt hydroxide (peak centered around 531 eV)43,44 and cobalt oxide (peak centered around 14852

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Figure 3. O1s core level XPS spectra (a
c) and PM-IRRAS spectra (d
f) of cobalt substrates modified with a 1-dodecanethiol SAM formed by immersion in a 10
2 M ethanolic solution for 16 h without any pretreatment (a, d), after an electrochemical reduction in a NaOH solution, pH 13.7 (b, e), and after an electrochemical reduction in HClO4, pH 2.0 (c, f).

529 eV)43,44 remaining on the surface. This could be explained by the fact that the cobalt substrate is immersed into the more acidic thiol solution directly after the electroreduction process, leading to a stabilization of Co(OH)2 rather than HCoO2
according to the Pourbaix diagram. The analysis of the high-resolution XPS spectra of the O1s regions has been systematically carried out. The resulting spectra are presented in Figure 3. In each studied situation, the intensity of the peak attributed to cobalt hydroxide (centered around 531 eV)43,44 is more important than that attributed to CoO and/or Co3O4 (centered around 529 eV).43,44 These spectra also exhibit two other components centered around 532 and 533.5 eV attributed to oxidized atmospheric contaminations and adsorbed water, respectively. Note that the intensity of these last two components is much more important for modified native cobalt substrates (Figure 3a). Nevertheless, it clearly appears that the electroreduction in acidic medium leads to the lowest amount of cobalt oxide at the surface and therefore appears to be the most suitable pretreatment for the grafting of a 1-dodecanethiol monolayer on cobalt substrates. All three of these monolayers were also systematically analyzed with PM-IRRAS. The corresponding spectra are presented in Figure 3. The organization of the monolayers has been assessed on the basis of the shift of the C
H vibration mode absorption bands. In each studied case, the absorption bands of the symmetric and asymmetric CH2 stretching modes are centered at 2854 and 2925 cm
1, respectively. These values are characteristic of a weakly organized monolayer as they are much higher than those referenced to an ideally organized organothiol monolayer on a monocrystalline gold surface, i.e., 2850 and 2918 cm
1.45,46 This weak organization could be explained by the fact that the modified surfaces have a far too important roughness to allow any distinction between the different monolayers in terms of organization. This important roughness resulting from electroreduction has already been

reported in the literature.27 Presently, this is something to be improved in the perspective of obtaining a better organization. Water contact angle values were also systematically measured for all these monolayers. The measured values are reported in Table 1. These measurements confirm the hypothesis of a weak organization of the monolayers obtained on native cobalt surfaces and on cobalt surfaces electroreduced in alkaline medium, which leads to water contact angles of 64° and 79°, respectively. Even if these values are higher than the contact angle of a bare polished cobalt substrate (56°), they are far below the referenced value for an ideally organized organothiol monolayer on a monocrystalline gold surface, i.e., 112°.47 On the contrary, the grafting of a 1-dodecanethiol monolayer on a cobalt substrate electroreduced in acidic medium leads to an increase of the water contact angle to 115°. All these results confirm that the electroreduction in acidic medium appears to be the most suitable of the three tested pretreatments for the elaboration of a 1-dodecanthiol monolayer of cobalt. This treatment will thus be systematically applied in the subsequent study. Influence of the Solvent. The choice of the solvent of the immersion solution is critical, especially for oxidizable metals as some interactions between the substrate and the solvent can interfere with the self-assembly process.26,37,38 Absolute ethanol is one of the most common solvents used for the elaboration of self-assembled monolayers from solution. However, it is wellknown that ethanol can react with some substrates or even chemisorb on their surface.37 In the frame of this study, three different solvents have been tested for the grafting of a 1-dodecanethiol monolayer on cobalt substrates electroreduced in acidic medium: a polar aprotic one (tetrahydrofuran or THF), a nonpolar aprotic one (toluene), and a polar protic one (absolute ethanol). The cobalt substrates were modified by immersion for 16 h in a 10
2 M 1-dodecanethiol solution in these different solvents. 14853

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Table 2. C1s/S2p, S2p/Co2p, and O1s/Co2p Ratios Calculated from XPS Spectra of Cobalt Substrates Modified by Immersion for 16 h in a 1-Dodecanethiol 10
2 M Solution in THF, Toluene, or Ethanol after an Electrochemical Reduction in HClO4, pH 2.0, and the Corresponding Water Contact Angle Values C1s/S2p

S2p/Co2p

O1s/Co2p

contact angle (deg)

4.5

0.2

2.1

72 ( 5

toluene

17.1

0.1

2.2

100 ( 6

ethanol

11.8

1.2

0.5

114 ( 2

56 ( 8

bare cobalt THF

Figure 4. Representative XPS survey spectra of a bare cobalt substrate (a) and cobalt substrates modified by immersion for 16 h in a 1-dodecanethiol 10
2 M solution in THF (b), toluene (c), and ethanol (d) after an electrochemical reduction of the substrate in HClO4, pH 2.0.

The XPS survey spectra of these modified substrates and of a reference unmodified cobalt substrate are presented in Figure 4. No matter the solvent used for the modification, the appearance of sulfur photoelectron peaks can be pointed out, confirming the presence of a certain amount of 1-dodecanethiol molecules on the cobalt surface in each studied case. However, two different trends can be observed regarding the C1s and O1s photoelectron peaks: when the modification is carried out in THF and in toluene, the O1s peak relative intensity is similar to that of the bare cobalt substrate, while when the modification is carried out in ethanol, the O1s peak has almost completely disappeared. The C1s peak intensity progressively increases from the bare cobalt substrate case to the cobalt substrate modified in an ethanolic solution. The calculation of different abundance ratios has been carried out as previously described (Table 2). From these ratios, it clearly appears that the preparation of the monolayer in ethanol leads to the lowest amount of physisorbed carbon contaminations while working in toluene leads to the highest amount of contaminations. The case of a modification in THF seems peculiar in the sense that the C1s/S2p ratio is even lower than the theoretical one (4.5 vs. 12). According to the literature,27,48 it appears that THF can form complexes with cobalt. It is thus reasonable to expect that the interaction of THF with the cobalt surface competes with the grafting of 1-dodecanethiol. Given the very low C1s/S2p ratio obtained after a modification in a 1-dodecanethiol/THF solution, it is assumed that a nonnegligible amount of THF complexes with the cobalt surface, preventing the correct grafting of the organothiol molecules. These observations are also confirmed by the examination of the XPS high-resolution C1s photoelectron spectra (Figure 5). These spectra can be analyzed with the four typical components described in the first section of this paper, i.e., one centered at 285 eV (mainly attributed to the 1-dodecanethiol alkyl chain) and three others at higher binding energies (mainly attributed to physisorbed carbonaceous contaminations), except for the C1s

spectra of the SAMs formed in ethanol requiring only three components. For the monolayers formed in THF and toluene, the peak centered at 285 eV represents only 77% and 81% of the total C1s signal, respectively, while this peak represents about 91% of the total C1s signal for monolayers formed in ethanol. This is another indication of the lower contamination of these monolayers. The analysis of the S2p high-resolution spectra of these monolayers also shows that the solvent significantly influences the nature of the cobalt
sulfur bond (Figure 5). When the modification is carried out in THF, the sulfur is mainly oxidized in sulfonate species (with the S2p3/2 component centered at 168.8 eV) with a small thiolate component centered at 162.6 eV. The same observation can be made for monolayers formed in a toluene solution. Note however that the proportion of thiolate is slightly higher with toluene than with THF. For monolayers formed in ethanolic solution, the S2p region can be analyzed with four doublet structures, the main component being attributed to strongly bound thiolate species and the other three to unbound thiols or disulfur, sulfinates, and sulfonates. The comparison of the corresponding S2p/Co2p ratios (Table 2) reveals that the modification in an ethanolic solution leads to an amount of grafted organothiols about 10 times higher than that with THF or toluene. Thus, these two solvents do not seem suitable for the elaboration of 1-dodecanethiol SAMs on cobalt surfaces. This could be due to the more important steric hindrance of the THF and toluene molecules compared to ethanol as well as to the interaction of THF with the cobalt surface during the modification. The poor quality of the monolayers obtained in THF and in toluene also explains the much more important oxidation of the cobalt substrates modified in these solvents (see O1s/Co2p ratios in Table 2). Indeed, at the end of the immersion step, the modified cobalt surfaces are exposed to air. Therefore, a quick reoxidation of the system occurs if the monolayer does not protect the surface correctly, leading to the re-formation of cobalt oxides as well as the oxidation of grafted thiolate species. PM-IRRAS analysis of these surfaces does not reveal any significant difference in terms of organization of the obtained monolayers (not shown here). Water contact angle measurements confirm the results obtained by XPS (Table 2). The lowest hydrophobicity is obtained for a cobalt substrate modified in a 1-dodecanthiol/THF solution (72°). The contact angle obtained for a cobalt substrate modified in a 1-dodecanthiol/toluene solution is slightly higher (100°), but the best hydrophobicity (114°) is obtained after a modification of the cobalt substrate in an ethanolic solution. As expected, the solvent of the immersion solution has a critical influence on the quality of the obtained organothiol SAM. 14854

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Figure 5. C1s (a
c) and S2p (d
f) core level XPS spectra of 1-dodecanethiol SAMs formed on cobalt substrates by immersion for 16 h in a 1-dodecanethiol 10
2 M solution in THF (a, d), toluene (b, e), and ethanol (c, f) after an electrochemical reduction in HClO4, pH 2.0.

Both THF and toluene lead to the formation of incomplete and poorly organized monolayers, while ethanol appears to be the most suitable solvent among the three tested ones. Therefore, ethanol has been used as the solvent for the rest of this study. Influence of the Immersion Time. The self-assembly process is well-known for substrates such as gold,39 copper,19 or nickel.26 This process can be viewed in two distinct steps: a fast one consisting in the chemisorptions of most of the molecules constitutive of the formed SAM and a much slower one consisting in the organization of the SAM by optimization of van der Waals interactions between the alkyl chains and the replacement of oxidized grafted organothiol molecules by fresh ones from the solution.47,49 To assess the impact of the immersion time on the quality of the obtained 1-dodecanethiol monolayers on electroreduced cobalt substrates, we tested four modification conditions: two short immersion times (30 min and 2 h) and two longer immersion times (16 and 24 h). Note that all these monolayers were formed by immersion of cobalt substrates in a 10
2 M 1-dodecanethiol ethanolic solution after an electrochemical reduction in acidic medium. The corresponding XPS survey spectra (presented in Figure 6) show the evolution of the monolayer with the immersion time: the sulfur S2p and S2s and carbon C1s peak intensities increase with increasing immersion time, while the oxygen O1s peak intensity decreases. Again, the calculation of different abundance ratios has been carried out to confirm these trends (Table 3). Regarding the C1s/S2p ratio, it appears that the amount of physisorbed atmospheric contaminations remains quite important after 2 h of immersion but decreases for longer immersion time. The analysis of the C1s high-resolution spectra (Figure 7) reveals the same trend: the proportion of the peak centered at 285 eV increases from 77% to 91% of the total C1s signal for monolayers prepared by immersion from 30 min to 24 h, respectively.

Figure 6. Representative XPS survey spectra of a bare cobalt substrate (a) and cobalt substrates modified by immersion in a 1-dodecanethiol 10
2 M ethanolic solution for 30 min (b), 2 h (c), 16 h (d), and 24 h (e) after an electrochemical reduction of the substrate in HClO4, pH 2.0.

Examination of the S2p high-resolution spectra (presented in Figure 7) reveals an important difference between short- and long-time immersion modification in terms of the sulfur
cobalt bond. The S2p region spectrum of the monolayer obtained after 30 min of immersion exhibits two doublet structures: a main one attributed to sulfonate species (63% of the total S2p signal) and a second one attributed to thiolate species (37% of the total S2p 14855

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signal). An increase of the immersion time leads to an inversion of this proportion. Note also that the amount of grafted 1-dedecanethiol molecules remains very low even after a 2 h immersion. A longer immersion time allows a considerable increase of the amount of grafted molecules as well as of the proportion of thiolate species (i.e., 72% and 83% of the total S2p signal after 16 and 24 h immersion, respectively). An immersion time of 30 min is thus insufficient to allow the replacement of oxidized thiol species by fresh thiol molecules from the solution. While a 2 h immersion time seems enough to obtain a significant proportion of thiolate species at the monolayer/cobalt interface, an even longer immersion time appears to be necessary to increase the monolayer density. The evolution of the O1s/ Co2p ratio with immersion time clearly indicates that immersion times of 2 h or less are not long enough to obtain SAMs with a sufficient density to avoid a reoxidation of the cobalt substrate when exposed to air. PM-IRRAS analysis of these surfaces does not reveal any significant difference in terms of organization of the obtained monolayers (not shown here). This observation is directly related to the significant roughness of the underlying cobalt Table 3. C1s/S2p, S2p/Co2p, and O1s/Co2p Ratios Calculated from XPS Spectra of Cobalt Substrates Electrochemically Reduced in HClO4, pH 2.0, and Modified in a 1-Dodecanethiol 10
2 M Solution in Ethanol by Immersion for 30 min, 2 h, 16 h, and 24 h and the Corresponding Water Contact Angle Values contact C1s/S2p

S2p/Co2p

O1s/Co2p

angle (deg)

14.4

0.1

1.3

88 ( 4

56 ( 8

bare cobalt 30 min 2h

14.2

0.2

1.3

100 ( 4

16 h

11.8

1.2

0.5

114 ( 2

24 h

12.7

1.4

0.6

111 ( 3

substrate resulting from the electrochemical reduction pretreatment in acidic medium. Water contact angle measurements confirm the results obtained by XPS (Table 3). The hydrophobicity of the obtained monolayer increases with the immersion time from 56° for a bare polished cobalt substrate to 88° and 100° for 30 min and 2 h immersions, respectively. After an immersion for 16 h, the water contact angle value further increases to a value of 114°. However, a further increase of the immersion time to 24 h leads to a similar water contact angle (i.e., 111°). It is thus clear that the hydrophobicity of the obtained monolayers reaches a plateau for immersion times of 16 h or longer. From this part of the study, it can be concluded that the immersion time definitely has a critical influence on the quality of the obtained organothiol monolayer. Short immersion times lead to incomplete monolayers unable to prevent the reoxidation of the substrate during air exposition. Long immersion times are thus necessary to obtain a complete and dense monolayer with hydrophobic properties and are efficient in preventing the reoxidation of the system after air exposure. However, as the improvement of the obtained monolayer is not significant between the two studied long immersion times (i.e., 16 and 24 h), we selected 16 h as suitable for the rest of this study. Influence of the Concentration. Several studies reporting the grafting of organothiols on copper have shown that these molecules are able to partially reduce the oxide layer during the self-assembly process.19,50 On the other hand, it has been shown that this reduction is not observed for nickel substrates25 because of the stability of the native oxide layer, and it is thus necessary to electrochemically reduce nickel surfaces prior to the grafting to obtain a correct organothiol SAM. Therefore, it can be interesting to have a sufficient amount of organothiol molecules in the modification solution to allow an easy replacement of the grafted and oxidized molecules by fresh ones and thus an easier reduction of the remaining oxides by increasing the concentration of the immersion solution. The influence of the concentration on the quantity of oxides present and the quality of the formed

Figure 7. C1s (a
d) and S2p (e
h) core level XPS spectra of 1-dodecanethiol SAMs formed on cobalt substrates by immersion in a 10
2 M ethanolic solution for 30 min (a, e), 2 h (b, f), 16 h (c, g), and 24 h (d, h) after an electrochemical reduction of the substrate in HClO4, pH 2.0. 14856

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Table 4. C1s/S2p, S2p/Co2p, and O1s/Co2p Ratios Calculated from XPS Spectra of Cobalt Substrates Electrochemically Reduced in HClO4, pH 2.0, and Modified by Immersion for 16 h in an Ethanolic 1-Dodecanethiol Solution, 10
3, 10
2, and 10
1 M, and the Corresponding Water Contact Angle Values C1s/S2p

S2p/Co2p

O1s/Co2p

contact angle (deg)

10
3 M

13.3

0.1

1.3

102 ( 6

10
2 M

11.8

1.2

0.5

114 ( 2

10
1 M

13.2

1.3

0.4

113 ( 3

56 ( 8

bare cobalt

Figure 8. Representative XPS survey spectra of a bare cobalt substrate (a) and cobalt substrates modified by immersion for 16 h in a 1-dodecanethiol ethanolic solution, 10
3 M (b), 10
2 M (c), and 10
1 M (d), after an electrochemical reduction of the substrate in HClO4, pH 2.0.

monolayers has been assessed by studying the SAMs obtained from three different concentrations: 10
3, 10
2, and 10
1 M. All these monolayers were formed by immersion of cobalt substrates for 16 h in an ethanolic 1-dodecanethiol solution after an electrochemical reduction in acidic medium. On the XPS survey spectra of these monolayers (presented in Figure 8), the appearance of sulfur characteristic peaks can be pointed out for each studied condition. Moreover, it clearly appears that an increase of the immersion solution concentration comes along with an increase of the sulfur and carbon peak intensities and a decrease of the oxygen peak intensity. To confirm this observation, the relevant abundance ratios have been calculated (Table 4). Regarding the C1s/S2p ratio, it appears that the amount of physisorbed atmospheric contaminations does not significantly change with the concentration of the immersion solution. In each case, this ratio is quite close to the theoretical one (i.e., close to 12). However, examination of the C1s high-resolution spectra (Figure 9) reveals a slightly higher proportion of contaminations for monolayers prepared in 10
3 M solution (the proportion of the peak centered at 285 eV increasing from 83% to 91% of the total C1s signal when the concentration is increased from 10
3 to 10
1 M). The nature of the cobalt
sulfur bond appears to strongly depend on the 1-dodecanethiol concentration regarding the S2p core level region high-resolution spectra of the formed monolayers (Figure 9). In the case of the modification in a 10
3 M solution, chemisorbed thiolate species are predominantly oxidized in sulfinate species representing about 74% of the total sulfur present at the surface. It thus clearly appears that grafted thiolate species are quickly oxidized while a too weak 1-dodecanethiol concentration does not allow a correct replacement of these oxidized sulfur species by fresh thiols from solution. Furthermore, the S2p/Co2p ratio (Table 4) indicates a very weak coverage of the surface, which can also

explain the important oxidation of the system (O1s/Co2p ratio of 1.3). Regarding the S2p/Co2p ratio evolution, it clearly appears that the amount of grafted thiols strongly improves when the concentration of the immersion solution is increased to 10
2 M. Note that the S2p/Co2p ratio obtained after a modification in a 10
1 M solution is very similar to that obtained after a modification in a 10
2 M solution. Thus, an increase of the immersion solution concentration allows an increase in the amount of grafted molecules but also an increase in the amount of free thiol molecules available for the replacement of oxidized sulfur species. As a consequence, a decrease of the oxidized sulfur species proportion can be pointed out when the immersion solution concentration is increased (Figure 9). For a concentration of 10
1 M, about 95% of the chemisorbed sulfur species are in the thiolate form. Therefore, even if a concentration of 10
2 M allows the formation of a dense monolayer (as indicated by the high S2p/Co2p ratio), a concentration of 10
1 M appears to be necessary to replace the oxidized chemisorbed sulfur species and to obtain a SAM able to prevent the reoxidation of the system when exposed to air as shown by the lowest O1s/Co2p ratio obtained for this concentration (Table 4). As in all previous cases, the PM-IRRAS analysis of these monolayers was carried out systematically but did not show any significant difference between the different studied monolayers. Again, this observation is directly related to the significant roughness of the underlying cobalt substrate resulting from the electrochemical reduction pretreatment in acidic medium. Water contact angle measurements confirm the trend observed by XPS (Table 4). The hydrophobic property of the obtained monolayer increases from 56° for a bare polished cobalt substrate to 102° for a monolayer formed in a 10
3 M 1-dodecanethiol solution and further increases with the concentration of the immersion solution. From a concentration of 10
2 M, the contact angle value reaches a plateau around 114°. Thus, an increase of the immersion solution concentration to 10
1 M does not induce any significant change of the hydrophobicity of the resulting monolayer. As expected, the alkanethiol concentration of the immersion solution has an important impact on the properties of the obtained monolayer as well as on the efficiency of organothiols to reduce the oxides present at the surface of cobalt substrates. A concentration of 10
3 M is clearly insufficient to allow a correct reduction of these oxides and leads to a low coverage of the surface with mainly oxidized sulfur species as anchoring groups. An increase of the concentration to 10
2 M induces an increase in the surface coverage and the hydrophobic properties of the obtained monolayer. However, this concentration appeared to be 14857

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Figure 9. C1s (a
c) and S2p (d
f) core level XPS spectra of 1-dodecanethiol SAMs formed on cobalt substrates by immersion for 16 h in a 1-dodecanethiol ethanolic solution, 10
3 M (a, d), 10
2 M (b, e), and 10
1 M (c, f), after an electrochemical reduction of the substrate in HClO4, pH 2.0.

Table 5. C1s/S2p, S2p/Co2p, and O1s/Co2p Ratios Calculated from XPS Spectra of Cobalt Substrates Electrochemically Reduced in HClO4, pH 2.0, and Modified by Immersion for 16 h in an Ethanolic 1-Dodecanethiol 10
1 M Solution Directly after the Modification and after Aging for 28 days in Atmospheric Conditions C1s/S2p

S2p/Co2p

O1s/Co2p

fresh monolayer

13.2

1.3

0.4

aged monolayer

13.2

1.4

0.3

insufficient to allow a correct reduction of the oxides present at the cobalt surface and the replacement of oxidized sulfur species by fresh thiol molecules during the modification. Therefore, the resulting monolayer still contains a high proportion of oxidized anchoring groups. Using a concentration of 10
1 M does not lead to a further significant increase of the surface coverage and of the hydrophobic properties of the resulting monolayer but clearly improves the reduction of the cobalt oxides present on the surface and the replacement of the oxidized sulfur species by fresh thiol molecules. This leads to a monolayer with a high coverage, good hydrophobic properties, and a very low proportion of oxidized sulfur species (about 5%) as well as a very low amount of oxygen (O1s/Co2p of 0.4). From this parameter optimization, it appears that cobalt can be successfully modified with a 1-dodecanethiol monolayer by immersion in a 10
1 M ethanolic solution for 16 h after an electrochemical reduction of 10 min in a perchloric acid aqueous solution (pH 2) at a constant potential of
1 V vs. SCE. Stability at Air Exposure. The stability over time of the monolayers obtained with optimized parameters has been assessed. After their modification, cobalt substrates were directly analyzed with XPS and then stored in atmospheric conditions

without any particular precaution. After an aging of 28 days, these substrates were analyzed again with XPS, and the obtained results were compared with those obtained for freshly formed monolayers. On the basis of these XPS analysis, the calculation of different abundance ratios has been carried out (Table 5). It clearly appears that an aging of the monolayer for 28 day in atmospheric conditions does not lead to any significant change of these ratios: the amount of physisorbed carbonated contaminations remains stable (see the C1s/S2p ratio), the amount of 1-dodecanethiol molecules present on the surface also remains unchanged (see the S2p/Co2p ratio), and more interestingly, the oxidation of the system does not increase (see O1s/Co2p ratio). The high-resolution XPS spectra of the S2p region also show no difference in the proportion of oxidized sulfur species (not shown here). Thus, the formed 1-dodecanethiol monolayers are revealed to be very stable in normal atmospheric conditions and to efficiently prevent the oxidation of the cobalt surface, very reactive to oxygen present in the atmosphere, and nevertheless keep it in a metallic state.

’ CONCLUSIONS The effects of electroreduction of the surface, the solvent of the alkanethiol solution, the immersion time, and the alkanethiol solution concentration on the formation of 1-dodecanethiol selfassembled monolayers on cobalt have been assessed to optimize the formed SAMs. First, it has been demonstrated that the pretreatment influences significantly the quality of these selfassembled monolayers. Indeed, the control of the surface state just before the modification step proved to be essential to form an efficient self-assembled monolayer. The presence of an oxide film on the surface was found to be unfavorable for the formation of a good monolayer. We also showed the negative influence of the 14858

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Langmuir presence of some remaining sodium hydroxide on the surface after an electrochemical reduction treatment in alkaline medium as well as a stabilization of the newly formed oxide layer in these conditions. The electroreduction in acidic medium appears to be the most suitable pretreatment for the elaboration of a 1-dodecanethiol monolayer on cobalt. Then we studied the influence of the possible interactions between the substrate and the solvent on the self-assembly process. Both THF and toluene lead to the formation of incomplete and poorly organized monolayers. In the case of modification in THF, a competition appeared between the complexation of cobalt by the solvent and the grafting of 1-dodecanethiol. The SAMs obtained using ethanol as the solvent were systematically better than those obtained in THF or toluene. The self-assembly process is well-known to intimately depend on the immersion time. In the frame of this work, we tested four immersion times: two short ones (30 min and 2 h) and two longer ones (16 and 24 h). Short immersion times lead to incomplete monolayers with many residual carbonated contaminations and unable to efficiently prevent the reoxidation of the system. Long immersion times are thus necessary to obtain a complete and dense monolayer. As an increase of the immersion time from 16 to 24 h does not lead to a significant improvement of the resulting SAM, we selected 16 h as a suitable immersion time for the grafting of 1-dodecanethiol on cobalt. The impact of the 1-dodecanethiol concentration has also been studied. A low concentration (10
3 M) leads to poor grafting density and therefore to the formation of a monolayer unable to prevent the reoxidation of the system and thus the oxidation of the anchoring groups. Higher concentrations (10
2 and 10
1 M) lead to an increase of the SAM density and seem to be suitable for an efficient reduction of the native oxide film. Finally, we assessed the stability of the optimized monolayers (formed by immersion for 16 h in a 1-dodecanethiol 10
1 M ethanolic solution after an electroreduction of the cobalt substrate in acidic medium) at air exposure. It appears that the formed monolayers are very stable and efficient to prevent the reoxidation of the substrate in normal atmospheric conditions. Compared to up to now reported procedures,27 an efficient method of modifying mechanically polished cobalt substrates has been developed which is easy to apply without specifically controlling the working ambient atmosphere (argon atmosphere vs normal atmospheric conditions). However, one of the main drawbacks of this method is the significant roughness of the surface induced by the electroreduction step impeding a better organization of the formed monolayers as shown by the IR spectroscopic results. Electroreduction methods minimizing the cobalt surface roughness are currently under investigations. In view of the results of this study, we can consider the application of such SAMs on cobalt alloys used for the design of magnetic tunnel junctions, for example.

’ AUTHOR INFORMATION Corresponding Author

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

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