Self-Assembled Monolayers of n-Dodecanethiol on Electrochemically

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Langmuir 1997, 13, 2285-2290

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Self-Assembled Monolayers of n-Dodecanethiol on Electrochemically Modified Polycrystalline Nickel Surfaces Z. Mekhalif,† J. Riga,† J.-J. Pireaux,† and J. Delhalle*,‡ Laboratoire Interdisciplinaire de Spectroscopie Electronique and Laboratoire de Chimie The´ orique des Surfaces et des Interfaces, Faculte´ s Universitaires Notre-Dame de la Paix, Rue de Bruxelles, 61, B-5000 Namur, Belgium Received May 30, 1996. In Final Form: October 25, 1996X Monolayers of n-dodecanethiol molecules on surfaces of polycrystalline nickel electrochemically pretreated are obtained by immersion either in pure n-dodecanethiol or in a solution of the mercaptan in absolute ethanol. The influence of the electrochemical pretreatment on the quality of the organic layer has been studied by contact angle measurements, electrochemistry, and X-ray photoelectron spectroscopy. It is found that n-dodecanethiol monolayers directly adsorbed on nickel surfaces without electrochemical pretreatment are of poor quality and exhibit a significant proportion of the thiols oxidized into sulfonates and sulfinates. On the contrary, the electrochemical pretreatment leads to monolayers with significantly improved physicochemical properties such as better chemical stability, resistance to electrochemical oxidation, and molecular order.

Introduction Self-assembled monolayers (SAMs) chemisorbed on solid surfaces have attracted much attention in the last 15 years because of their fundamental and technological promises.1 The interest for SAMs has primarily been focused on systems including chlorosilanes on silicon oxide,2 carboxylic acids on metal oxides,3 and organosulfur compounds on gold.4 The most studied SAMs are probably the alkanethiols, HS-(CH2)n-X, chemisorbed on gold surfaces with different X groups, e.g., -X ) -CH3, -COOH, -CN, -OH, -COOCH3, and ferrocenyl.5 These compounds very often lead to structurally well defined organic monolayers and allow a detailed investigation of a variety of interfacial phenomena which would otherwise be difficult to address. Systematic studies have been possible on properties such as wetting,6 electron transfer,5 molecular recognition,7 protein adsorption,8 adhesion,9 etc. * Corresponding author. † Laboratoire Interdisciplinaire de Spectroscopie Electronique. ‡ Laboratoire de Chimie Theorique des Surfaces et des Interface. X Abstract published in Advance ACS Abstracts, March 15, 1997. (1) An introduction to Ultrathin Organic Films; Ulman, A., Ed.; Academic Press: Boston, MA, 1991. (2) (a) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 192. (b) Netzer, L.; Sagiv. J. J. Am. Chem. Soc. 1983, 105, 674. (3) (a) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (b) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (c) Ogawa, H.; Chiera, T.; Taya, K. J. Am. Chem. Soc. 1985, 107, 1365. (4) (a) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (b) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (5) (a) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (b) Chidsey, C. E. D. Science 1991, 251, 919. (c) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192. (d) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688. (e) Atre, S. V.; Liedberg, B.; Allara, D. L. Langmuir 1995, 11, 3882. (6) (a) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87 and references cited therein. (b) Bain, C. D.; Whitesides, G. M. J. Am.Chem.Soc. 1988, 110, 5897. (c) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370. (d) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990. (d) Ulman, A.; Evans, S. D.; Shnidman, Y.; Sharma, R.; Eilers, J. E.; Chang, J. C. J. Am. Chem. Soc. 1991, 113, 1499. (7) Haussling, L.; Michel, B.; Ringsdorf, H.; Rohrer, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 569. (8) (a) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 12. (b) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164.

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Gold is certainly the most widely used substrate for the preparation of alkanethiol-based SAMs because of its good inertness, resistance to atmospheric contamination, and its strong chemical interaction with sulfur.10 Other metals like silver, copper, and iron have also received a great deal of attention. For instance, Whitesides et al.11 have reported that n-alkanethiols adsorb from solution onto evaporated copper films to form densely packed and oriented SAMs. As in the case of gold, it was observed that the chemisorption involves the thiol group which transforms in thiolate. The alkyl chains are primarily trans extended and oriented close to the surface normal. Oxidation of copper surfaces initially modified with alkanethiols has been studied as a function of exposure to air and chain length. Results indicate a rather effective protection by the organic monolayer of the metal surface against oxidation. The kinetics of the oxidation is compatible with a model in which the SAM acts as a barrier to oxygen transport from the ambient atmosphere to the copper surface, the rate of oxidation decreasing with increasing chain length. When oxygen reaches the metal surface, the adsorbed thiols tend to oxidize into sulfinates and sulfonates.12 Aramaki et al. confirmed these results with X-ray photoelectron spectroscopy (XPS), surface-enhanced Raman spectroscopy (SERS), and contact angle measurements on octadecanethiol monolayers chemisorbed onto polycrystalline copper surfaces.13 In this case the SAMs still exhibit order but are permeable to selected redox species in aprotic solvents. The authors have shown that it is possible to improve the protection efficiency of copper (9) (a) Allara, D. L.; Heburd, A. F.; Padden, F. J.; Nuzzo, R. G.; Falcon, D. R. J. Vac. Sci. Technol. A 1983, 376. (b) Stewart, K. R.; Whitesides, G. M., Godfried, H. P.; Silvera, I. F. Rev. Sci. Instrum. 1986, 57, 1381. (c) Czanderna, A. W.; King, D. E.; Spaulding, D. J. Vac. Sci. Technol., A 1991, 9, 2607. (10) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (11) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh. A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (12) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022. (13) (a) Itoh, M.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1994, 141, 2018. (b) Itoh, M.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1995, 142, 1839.

© 1997 American Chemical Society

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against corrosion by reacting alkyltrichlorosilanes with a SAM of 11-mercapto-1-undacenol.13 Stratmann et al.14 have studied the formation of n-decanethiol monolayers on iron surfaces, oxidized and oxide free. The oxide-free iron surfaces are obtained by electrochemical reduction. By varying the polarization of a working iron electrode, they could regulate the amount of surface oxides before transfer of the substrate under potential control to a n-decylmercaptan phase. As revealed by XPS, the modified surfaces still exhibit large proportions of oxidized sulfurs. However, using cyclic voltammetry, these authors were able to further reduce the amount of sulfonate species and thereby improve the quality of the interface. The electrochemical behavior of the modified surfaces is found to depend on the chemical state of the initial iron surface (oxidized vs oxide free). Ultimately, it appears that the chemical stability and suppression of metallic redissolution can be attained only for clean metallic iron surfaces. The above examples, by no means exhaustive, point out that the preparation of stable SAMs on oxidizable metals cannot be as straightforward as for gold and platinum, but requires suitable treatments to reduce the amount of metal oxides at the surface. Metal oxides hinder the formation of a stable chemical bond between sulfur and metallic atoms and oxidize thiolates into sulfinates and sulfonates which do not bind strongly to the surface. Our general objective is to form good quality selfassembled monolayers on surfaces of oxidizable metals. In spite of the numerous applications where specific coatings with organic ultrathin films would be valuable, little has been attempted on the formation of selfassembled monolayers of alkanethiols on nickel surfaces.15 The aim of this work is to evaluate the merits of an electrochemical pretreatment prior to adsorption of ndodecanethiol molecules on nickel surfaces and test the improvements imparted by the treatment to the properties of the monolayer. For comparison, parallel experiments were carried out on untreated nickel surfaces. The amount of oxidized sulfur species relative to the nonoxidized ones were estimated by XPS, while the quality and stability of the monolayer were assessed by contact angle and cyclic voltammetry measurements. Experimental Section Chemicals. n-Dodecanethiol (Aldrich, 98% D22, 140-6), liquid at room temperature, HClO4 p.a. (Acros, 22. 331. 21), acetone (Aldrich, 99.9%, HPLC grade 27, 072-5), H2O ultrapure water (18 MΩ‚cm), and absolute ethanol p.a. (Merck, 1.000983.2500) were used without additional purification. Substrate and Monolayer Preparation. Disk-shaped metal substrates (6 mm thickness) were cut from polycrystalline Ni rods (6.35 mm diameter, Aldrich, 99.99%, 26,707-4) and mechanically polished using various grit diamond pastes down to 1 µm. Before use for monolayer adsorption, these substrates were rinsed with copious amounts of acetone and ethanol. The monolayers were formed by immersion of the nickel substrates either in pure n-dodecanethiol liquid (15 min) or in a 1 × 10-3 M n-dodecanethiol solution in absolute ethanol (3 h). The samples were then quenched with acetone and ethanol and ultrasonically cleaned for 15 min in ethanol to remove physisorbed (14) (a) Stratmann, M. Adv. Mater. 1990, 2, 191. (b) Volmer, M.; Stratmann, M.; Viefhaus, H. Surf. Interface Anal. 1990, 16, 278. (c) Volmer, M.; Czodrowski, B.; Stratmann, M. Ber. Bunsenges. Phys. Chem. 1988, 92, 1335. (d) Stratmann, M.; Fu¨rbeth, W.; Grundmeier, G.; Lo¨sch, R.; Reinartz, C. R. In Corrosion Mechanisms in Theory and Practice; Marcus, P., Oudar, J., Eds.; Marcel Dekker: New York, 1995. (15) (a) Shustorovich, E.; Bell, A. T Surf. Sci. 1991, 248, 359. (b) Yang, H.; Caves, T. C.; Whitten, J.; Huntley, D. R. J. Am. Chem. Soc. 1994, 116, 8200. (c) Rodriguez, J. Surf. Sci. 1992, 278, 326. (d) Vogt, A. D. Private communication.

Mekhalif et al. molecules. The samples were finally dried under argon flow and used immediately for characterization. Two types of metallic substrates differing by their surface chemical state were compared. The first type consists of nickel surfaces mirror polished, cleaned as indicated above, and used directly for the monolayer adsorption. The second type concerns nickel surfaces, polished, cleaned and electrochemically reduced prior to thiol adsorption. For sake of conciseness these two types of substrates will be referred to as NiO and NiO-elec, respectively. The electrochemical pretreatment consists of a 20 min reduction in an aqueous solution of HClO4 (1 M) of the nickel substrates at 0.7 V vs the saturated calomel electrode (SCE). The immersion of the substrates in pure thiol or in the ethanol solution is carried out immediately at the end of the electrochemical treatment. The contact time of the substrates with atmosphere during this operation was estimated to be less than 3 s; hence reaction of the substrate with atmospheric oxygen should be very limited. Monolayer Characterization. The monolayers were characterized spectroscopically (XPS), by contact angle measurements, and electrochemically. XPS technique is used here to control the elemental composition of the monolayer and identify the oxidation states of the sulfur atoms residing at the surface. The photoelectron spectra of the monolayers have been recorded with a SSX-100 spectrometer using the monochromatized X-ray Al KR radiation (1486.6 eV). The sample are analyzed at a 35° takeoff angle. The Ni(2p) and S(2p) core levels were calibrated by reference to the recorded C(1s) peak conventionally set at 284.5 eV, which is a commonly used procedure for organic films.14b-d,16 In the case of alkanethiols, the C(1s) line mainly consists of carbon atoms linked to hydrogen atoms and comes as a simple and well-defined structure in the spectra. With this calibration, the energy at which the S(2p3/2) signal of free thiols would arise is 163.3 eV.14b-d Contact angles are sensitive to the surface state of the monolayer and, by comparison with literature data on similar films, turn out to be useful probes of the molecular structure and organization. Furthermore, the evolution with time of these angles provides direct information on the stability of the films. We have measured, in air and at room temperature, equilibrium contact angles, θeq, for water drops ((1µL) that have been allowed to equilibrate. The measurements were carried out with a VCA 2000 contact angle meter. Electrochemical characterizations of nickel with and without monolayer were carried out in a conventional three-electrode cell. A calomel electrode saturated with KCl was used as reference electrode, a platinum grid as counterelectrode, and the nickel disks as working electrodes. The cell is connected to a potentiostat/galvanostat TACUSSEL PJT 24 linked to an IMT1 interface. Characterizations were performed in an alkaline aqueous solution (0.1 M NaOH) deoxygenated by argon bubbling for 30 min prior to measurements. Voltammograms were digitally recorded using the IMT1VOlT/TACUSSEL program.

Results and Discussion This section is organized in two main parts. In the first part we focus on the monolayer formation, while in the second part we discuss stability aspects for the best quality monolayers obtained in this work. Monolayer Formation. The quality of the films obtained by dipping NiO and NiO-elec substrates in pure n-dodecanethiol is first discussed. Then we compare the quality of monolayers obtained by dipping NiO-elec substrates in pure n-dodecanethiol or in a solution of the mercaptan in absolute ethanol. Pure n-dodecanethiol. Figure 1 compares the survey XPS spectra of n-dodecanethiol monolayers adsorbed on a mechanically polished nickel substrate, NiO, and on a mechanically polished nickel substrate to which an electrochemical reduction has been applied in the end, NiO-elec, respectively. The monolayers have been obtained by dipping for 15 min the substrates in a bath of pure n-dodecanethiol. (16) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers. The Scienta ESCA300 Database; Wiley: Chichester, 1992.

Dodecanethiol Monolayers on Nickel

Figure 1. Survey XPS spectrum of a polycrystalline nickel modified with pure n-dodecanethiol adsorbed on NiO and NiOelec.

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Figure 2. XPS spectrum of the Ni(2p) core level of a polycrystalline nickel modified with pure n-dodecanethiol adsorbed on NiO and NiO-elec.

Significant differences are observed between the two types of samples (Figure 1). In the case of the monolayer adsorbed on NiO, the O(1s) peak is rather intense relative to the C(1s) line. By contrast, the corresponding line for the NiO-elec substrate is substantially smaller, which points to an effective reduction of the surface nickel oxides due to the electrochemical treatment. The residual amount of oxygen is likely to result from a partial reoxidation of the NiO-elec surface during the short contact time with air needed to transfer the substrate from the electrochemical cell to the thiol bath. A detailed view at the Ni(2p) structure in Figure 2 shows also a striking difference between NiO and NiO-elec modified with the mercaptan; it corroborates the observations on the intensity changes of the O(1s) line. In the absence of electrochemical treatment (NiO) the Ni(2p) structure is typical of a nickel in NiO. The Ni(2p3/2) line is found at 853.8 eV, the spacing ∆ between the Ni(2p3/2) and Ni(2p1/2) is equal to 17.5 eV, and the unmistakable wide plasmon feature occurs at 860 eV. These three structures and their location on the energy scale are in complete agreement with data reported in ref 17. By contrast, the NiO-elec surface similarly modified with pure n-dodecanethiol exhibits the features of a metallic nickel.17 The Ni(2p3/2) line is found at 852.7 eV, the spacing ∆ between the Ni(2p3/2) and Ni(2p1/2) is equal to 17.28 eV, and the wide plasmon feature has disappeared and been replaced by a structure of lesser intensity located at 856.5 eV. These data show that a NiO-elec substrate modified by n-dodecanethiol directly after the electrochemical reduction is essentially metallic. A similar inspection of the S(2p) levels for the two types of substrates (Figure 3) also reveals interesting features.

Figure 3. XPS spectrum of the S(2p) core level of a polycrystalline nickel modified with pure n-dodecanethiol adsorbed on NiO and NiO-elec.

(17) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: Eden Prairie, MN, 1992; pp 84-85 and references therein.

In the case of NiO, two main structures are found: a doublet (S(2p3/2) at 161.8 and S(2p1/2) at 163.0 eV) and a clump centered around 167 eV. The structure of the

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doublet is similar to the one obtained on gold substrates where the shift toward lower binding energies (S(2p3/2) at 161.8 eV) relative to the free thiol (S(2p3/2) at 163.3 eV) is indicative of the formation of thiolate bonds with a metallic substrate. The intensity ratio between the low(S(2p3/2)) and the high-energy (S(2p1/2)) components of the sulfur doublet is normally 2/1. However, the intensity ratio in Figure 3 is close to 1/1, which could be attributable to small amounts of disulfides, R-S-S-R, on the surface. The S(2p3/2) level of a disulfide bond, generally found at 163.3 eV, overlaps with the S(2p1/2) contribution of the thiolate and could explain the difference between the observed and theoretical intensity ratios. This uncertainty explains why in Figure 3 the word thiolates is put between quotes. Deconvolution of the wide clump at 167 eV leads to two structures which fit well with oxidized sulfur species: sulfinates at 165.5 eV and sulfonates at 168 eV. Their presence suggests that a non-negligible proportion of the adsorbed mercaptans at the surface of NiO samples oxidize, as was already reported for copper13 and iron surfaces.15 A different situation prevails for the monolayers adsorbed on NiO-elec surfaces (Figure 3). Here the clump at 167 eV is practically nonexistent, which correlates with the low-intensity O(1s) line already noted in Figure 1. Thus, the XPS data point to the fact that the electrochemical treatment has significantly reduced the amount of surface nickel oxides to the point that oxidized sulfur species are not detected. For NiO-elec, the form of the S(2p) doublet (Figure 3) is closer to the theoretical shape than NiO. Finally, the XPS results in Figure 3, where peaks characteristic of oxidized sulfur species are not detected, suggest that the electrochemical pretreatment of nickel seems to be more effective than for iron.14 Contact angles measured for water drops on the n-dodecanethiol layer adsorbed on NiO vary from 80 to 85°, while those obtained for the NiO-elec substrates vary from 108 to 110°. The values for the monolayers adsorbed on NiO-elec substrates suggest good packing and organization of the alkyl moieties in the alkanethiol molecules. In summary, from the above XPS and contact angle data, one can conjecture that in the case of NiO-elec good quality monolayers, similar to those obtained on evaporated gold, are obtained. These data fit well with the assumption that n-dodecanethiol molecules on NiO-elec bind to the metallic surface and their alkyl chains are mainly in trans conformation with the methyl groups pointing outward.1 On the contrary, the oxidized sulfur species detected in the XPS spectra and the low contact angle values recorded for the films on NiO indicate that the organic layer is of poor quality. n-Dodecanethiol 1 × 10-3 M in Absolute Ethanol. At this stage, it is interesting to compare briefly the effect of the composition of mercaptan bath on the quality of the films adsorbed on NiO-elec substrates. NiO-elec substrates were dipped for 3 h in a 1 × 10-3 M n-dodecanethiol solution in absolute ethanol. The solution has been bubbled with argon to reduce its oxygen content. The intensity of the O(1s) line relative to that of the C(1s) peak (Figure 4) is higher than in the two previously reported cases (Figure 1). With an important S(2p) feature at 167 eV and low water contact angles (about 80°), it appears that the monolayers derived from adsorption of n-dodecanethiol on NiO-elec substrates in the ethanol solution are of lesser quality than those obtained by dipping NiO substrates in neat n-dodecanethiol. This result can be tentatively explained by a competition between ethanol and n-dodecanethiol adsorption. Both thiolates and alkoxides form strong bonds to metal surfaces.15 For ethanol on Ni, Shustorovitch and Bell15a find a chemisorption energy of 75 kJ‚mol-1 and, using the

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Figure 4. Survey XPS spectrum of the polycrystalline nickel modified with n-dodecanethiol (1 × 10-3 M in absolute ethanol) adsorbed on NiO-elec.

same method for ethanethiol, Huntley18 obtains 84 kJ‚mol-1. Ethoxide and ethylthiolate also have similar calculated adsorption energies: 268 and 280 kJ‚mol-1, respectively.18 From a comparative study of the reaction selectivities and bonding configurations in mercaptoethanol (HSCH2CH2OH) and ethanedithiol (HSCH2CH2SH) on Ni(110) carried out in ultrahigh vacuum, Huntley18 concludes that the mercaptoethanol reactivity is dominated by the thiolate formation. In the present case, however, one should bear in mind that the chemisorption reactions occur at a metal-liquid interface. Also to be taken into account is the fact that the molecules involved have a single functionality (-OH or -SH), are of different size, and n-dodecanethiol molecules are outnumbered by the solvent molecules. It is thus likely that a correspondingly larger number of the short carbon chain ethanol molecules are adsorbed as ethoxide, which can explain the large relative intensity of the O(1s) peak over C(1s) observed in Figure 4. The mechanism by which the oxidized species do actually form remains an open question. In analogy with what is observed with the monolayer formation on NiO from the pure n-dodecanethiol bath, we can only conjecture that there is a proportion of ethoxide species that undergo C-O bond breaking and leave oxygen-rich zones on which n-dodecanethiol molecules oxidize. Stability of the Monolayer. In this second part, we assess the stability of the monolayers on NiO and NiOelec substrates adsorbed in pure n-dodecanethiol. As shown in previous studies,12-14 the molecular structure and packing of the films are determinant for its chemical stability and its protection against oxidation of the metal substrate. If oxygen reaches easily the metal surface, the resulting oxidation is followed by transformation of (18) Huntley, D. R. J. Phys. Chem. 1989, 93, 6156.

Dodecanethiol Monolayers on Nickel

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Figure 5. Evolution of θeq (in degrees) as a function of exposure time to atmosphere for pure n-dodecanethiol adsorbed on NiO and NiO-elec.

thiolates into oxidized sulfur species and a progressive degeneration of the monolayer. Resistance to Atmospheric Oxygen Permeation. Contact angles being very sensitive to surface modifications are probes of choice to assess the monolayer capacity to act as a barrier to air permeation. This evolution is depicted in Figure 5, the first measurement being made 1 h after the monolayer formation. As already mentioned in the previous section on the formation of films, the initial contact angle values, 8085°, are rather low for NiO substrates, which is indicative of a poor organization and defects in the monolayer. The hypothesis of having defects in the monolayers adsorbed on such substrates is consistent with the fact that contact angles reported for paraffin and polyethylene films are never smaller than 100°. The evolution of θeq with exposure time to the laboratory atmosphere shows a decrease of about 10° after 1 week and a stabilization of θeq at 60° after 5 weeks. For comparison, the contact angle obtained on a bare NiO surface, i.e., prior to monolayer adsorption but rinsed with acetone and ethanol before depositing the water drops, is 78°. Contact angles for the monolayers assembled on NiOelec substrates start at values close to 110° and stabilize at 104° after 5 weeks of continuous exposure to ambient atmosphere. This corresponds to a mere 6° decrease over that period. Considering the precision of the contact angle measurements ((3°), a 6° decrease noted for monolayer adsorbed on NiO-elec is rather small and allows us to conclude that, under air exposure, such monolayers remain stable and organized with their methyl groups pointing mainly outward. Shown in Figure 6 are the survey scan XPS spectra of a monolayer recorded 1 h, 1 week, and 5 weeks after preparation. The detailed view at the S(2p) region (Figure 7) shows no significant trace of the peak at 167 eV characteristic of sulfinate and sulfonate species. It evidences the fact that self-assembled monolayers of

Figure 6. Survey scan XPS spectra of a polycrystalline nickel modified with pure n-dodecanethiol adsorbed on NiO-elec. XPS spectra have been recorded 1 h, 1 week, and 5 weeks after preparation.

n-dodecanethiol on NiO-elec substrates act as good barrier films against oxidation by air. Electrochemical Stability. The noted differences in the wetting properties of monolayers adsorbed on NiO and NiO-elec substrates are compared with preliminary tests of the electrochemical stability and blocking efficiency of the modified NiO-elec surface. To this end, cyclovoltammetry has been carried out in a 0.1 M NaOH aqueous solution with a potential scan ranging from -0.3 to +0.6 V at 100 mV‚s-1, which are typical conditions for such studies. During the anodic potential sweep, a clean polycrystalline nickel electrode (Figure 8a) shows a peak at 0.438 V assigned to the oxidation of the metal and the formation of a passive layer of nickel oxides. The high current rise starting at +0.52 V is due to the oxygen evolution, while the cathodic peak appearing at 0.34 V corresponds to the reduction of the passive oxide layer. Figure 8b shows the voltammogram corresponding to an electrode modified with a monolayer of n-dodecanethiol adsorbed on NiO-elec. By comparison with the clean nickel surface, the modified electrode shows reduced electrochemical activity. In line with the previous observations on the monolayer stability to ambient atmosphere, the NiO-elec modified electrode exhibits almost no activity. This is additional evidence that the organic layer is firmly and densely chemisorbed to the surface to a point where water and electrolytic species are prevented from reaching the metal/mercaptan interface. It also explains why electrode reactions like metal dissolution and passivation (peak 1) and the reduction of the oxide passive layer (peak 2) are nearly inhibited.

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Figure 7. S(2p) core level of a polycrystalline nickel modified with pure n-dodecanethiol adsorbed on NiO-elec. XPS spectra have been recorded 1 h, 1 week, and 5 weeks after preparation.

Concluding Remarks The results of this study show that n-dodecanethiol molecules adsorb on clean and oxide free polycrystalline nickel surfaces and form organized and chemically stable self-assembled monolayers. In such a case, only thiolates and, most likely, some amount of disulfide species are detected in the S(2p) region of the XPS spectra. The wetting properties are similar to those reported for alkanethiols chemisorbed on gold surfaces and point to a good molecular organization of the monolayer. Without electrochemical pretreatment, oxides are present at the surface. They cause the thiolate species to transform into sulfinates and sulfonates which are known to be responsible for poor organization and stability of the

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Figure 8. Cyclovoltammogram of a polycrystalline nickel (a) clean and (b) modified with pure n-dodecanethiol adsorbed on NiO-elec (0.1 M NaOH, argon bubbling).

monolayers. Adsorption of NiO-elec in a solution of n-dodecanethiol in absolute ethanol leads to monolayers of even poorer quality. These results are only exploratory and call for further studies. We are presently investigating the possibility of forming good quality monolayers of n-dodecanethiol from different solutions by varying the nature and the concentration of the solvents as well as the dipping time. Detailed electrochemical characterization of the modified NiO-elec surfaces is also underway. Acknowledgment. This work was funded by the Belgian National Program of Interuniversity Research Project on Materials Science initiated by the Belgian State Minister Office (Science Policy Programming). LA960528A