Elaboration of Self-Assembled Monolayers of n-Alkanethiols on Nickel

Jan 7, 2003 - of the native oxide, the thiol concentration (10-3 versus 10-1 M), and the nature of the solvent (ethanol, ... Telephone: +32-(0)81-7252...
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Langmuir 2003, 19, 637-645

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Elaboration of Self-Assembled Monolayers of n-Alkanethiols on Nickel Polycrystalline Substrates: Time, Concentration, and Solvent Effects Z. Mekhalif,* F. Laffineur, N. Couturier, and J. Delhalle Faculte´ s Universitaires Notre-Dame de la Paix, De´ partement de Chimie, Laboratoire LISE 61, rue de Bruxelles, B-5000 Namur, Belgium Received April 8, 2002. In Final Form: July 25, 2002 Self-assembled monolayers (SAMs) of n-alkanethiols on mechanically polished polycrystalline nickel substrates are presented. The effects of several preparation conditions of the surface upon the metal/sulfur bonds and monolayer properties are evidenced. Of particular importance are the electrochemical reduction of the native oxide, the thiol concentration (10-3 versus 10-1 M), and the nature of the solvent (ethanol, acetonitrile, n-heptane, toluene, and carbon tetrachloride).

Introduction Monolayers of alkanethiols and their derivatives, Y-(CH2)n-SH, self-assembled on metal substrates offer a convenient and effective approach for fabricating interfaces of well-designed composition, structure, and thickness.1-5 Via the deposition of molecules with specific end functionalities, Y, surfaces can be tailored for particular applications: modified electrodes, chemical sensors, lubrication, interfacial reactivity, and so forth.1-5 To date, gold has been the most frequently studied substrate because of its good inertness to most potential contaminants as well as the high affinity of sulfur for Au and the relative ease to obtain high-quality monolayers from a large variety of solutions of organothiols.1-5 Growing research interest also exists for the modification of oxidizable metal substrates with organothiols. Modifications of Cu,6 Fe,7 Ni,8-9 Zn,10 and some of the * Corresponding author. Telephone: +32-(0)81-725230. Fax: +32-(0)81-724530. E-mail address: [email protected]. (1) See for example: (a) An introduction to Ultrathin Organic Films; Ulman, A., Ed.; Academic Press: Boston, 1991. (b) Ulman, A. Chem. Rev. 1996, 96, 1533. (c) Whitesides, G. M.; Ferguson, G. S.; Allara, D. Crit. Rev. Surf. Chem. 1993, 3, 49. (d) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 11, 7155. (2) (a) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437, (b) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 321. (3) (a) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1992; Vol. 19, p 109. (b) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (4) Self-Assembled Monolayers of Thiols; Ulman, A., Ed.; Academic Press: 1998; p 24. (5) Schriber, F. Prog. Surf. Sci. 2000, 65, 151. (6) (a) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022. (b) Ron, H.; Cohen, H.; Maltis, S.; Rappaport, M.; Rubinstein, I. J. Phys. Chem. 1998, B102, 9861-9869. (c) Nozawa, K.; Aramaki, K. Corros. Sci. 1999, 41, 57 and references therein. (d) Mekhalif, Z.; Sinapi, F.; Laffineur, F.; Delhalle, J. J. Electron Spectrosc. Relat. Phenom. 2001, 121, 149. (e) Laffineur, F.; Delhalle, J.; Guittard, F.; Geribaldi, S.; Mekhalif, Z. Colloids Surf., A 2002, 198-200, 817. (7) (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. Bunsen-Ges. 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. (e) Pirlot, C.; Delhalle, J.; Pireaux, J.-J.; Mekhalif, Z. Surf. Coating Technol. 2001, 138, 166. (8) (a) Mekhalif, Z.; Riga, J.; Pireaux, J.-J.; Delhalle, J. Langmuir 1997, 13, 2285. (b) Mekhalif, Z.; Lazarescu, A.; Hevesi, L.; Pireaux, J.-J.; Delhalle, J. J. Mater. Chem. 1997, 13, 437. Vogt, A. D.; Han, T.; Beebe, T. P.; Langmuir 1997, 13, 3397.

corresponding alloys11,12 have been reported in the literature. In this class of substrates, copper has received the largest amount of attention and quite interesting results have been obtained, in particular in fields related to protection against alteration. Monolayer films sufficiently free from pinhole defects have been shown to decrease considerably the diffusion of oxidizing species and effectively block the dissolution of the metal. These inhibition properties are strongly dependent on the thickness, on the molecular structure of the monolayers, and on the chemical bonds at the molecule/metal interface. The requirements on the film quality and the potential reactivity of the substrate with contaminants, solvent molecules, and the terminal function, Y, of the bifunctional organothiols are such that the formation of organothiol monolayers on oxidizable metals by wet chemical treatments is quite involved. While the modification of copper7-12 is already under good control for a variety of molecule/solvent combinations, a similar command is still largely missing for the other metals, and nickel in particular. Yet, there are a number of situations where surface modifications of nickel substrates are important to ensure quality performances in the respective applications. For example, used as contact material,13 nickel requires protective treatments to prevent corrosion without scarifying contact performance. An analogous situation is found in the formation of hydrophobic coatings on Cu(1-x)Nix alloys to promote dropwise condensation of steam on horizontal tubes, while keeping heat transfer resistance to acceptable levels.14 Here questions arise as to the kinetics of thiol binding on copper compared to nickel. The same question is potentially relevant in the selective (9) (a) Mekhalif, Z.; Delhalle, J.; Pireaux, J.-J.; Noe¨l, S.; Houze´, F.; Boyer, L. J. Coating Surf. Technol. 1998, 100-101, 463. (b) Noe¨l, S.; Houze´, F.; Boyer, L.; Mekhalif, Z.; Caudano, R.; Delhalle, J. IEEE Trans. Compon. Packag. Technol. 1999, 22, 79. (10) Mekhalif, Z.; Massi, L.; Guittard, F.; Geribaldi, S.; Delhalle, J. Thin Solid Films 2002, 405, 186. (11) Das, A. K.; Kilty, H. P.; Marto, P. J.; Andeen, G. B.; Kumar, A. Trans. ASME 2000, 122, 278. (12) Laffineur, F.; Delhalle, J.; Mekhalif, Z. Mater. Sci. Eng. C 2002, 22, 331. (13) (a) Holden, C. A.; Law, H. H.; Mattoe, C. A.; Spjeta, J. J. Plating Surf. Fin. 1989, 76, 58. (b) Law, H. H.; Sapjeta, J.; Chidsey, C. E. D.; Putvinski, T. M. J. Electrochem. Soc. 1994, 141, 1977. (14) Das, A. K.; Kilty, H. P.; Marto, P. J.; Andeen, G. B.; Kumar, A. Trans. ASME 2000, 122, 278 and references therein.

10.1021/la020332c CCC: $25.00 © 2003 American Chemical Society Published on Web 01/07/2003

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adsorption of alkanethiols on patterned gold spots formed by local electrochemical oxidation of nickel coated on gold surfaces.15 From a literature search and to the best of our knowledge, studies on the modification of nickel substrates and alloys have generally overlooked the problems of surface activation, reaction time, and solvent nature, which, from our experience,8 are important parameters to consider in order to achieve control and command on the monolayers formation. In particular, we already pointed out that removing the surface nickel oxide species, NiOx, by an electrochemical treatment greatly enhanced the quality of the organothiol films and improved their resistance to atmospheric oxygen permeation. Monolayers of n-dodecanethiol on polycrystalline nickel, mechanically polished and electrochemically reduced, were shown to constitute interesting electrochemical barriers with acceptable contact resistance, an improved friction coefficient, as well as wear resistance.8-9 In our previous study8 pure n-dodecanethiol was used as modifying medium. Preliminary attempts to form monolayers from 10-3 M n-dodecanethiol in absolute ethanol (the standard concentration and solvent in the field of SAMs of organothiols) led to films of much poorer quality. These results were tentatively explained in terms of a competitive adsorption on the nickel substrate between ethanol and n-dodecanethiol molecules. A proper control of the SAMs’ formation on Ni calls for more studies on the effects of parameters such as substrate activation, solvent nature, concentration, reaction time, temperature, and so forth. The study of solvent nature and concentration with chains of increasing length and no reaction of the CH3 forms the subject of the present contribution. This paper describes the preparation and the characterization of selfassembled monolayers (SAMs) of n-alkanethiols formed on nickel substrates. We will focus on the effect of the solvent, the concentration, and the immersion time importance for the preparation of SAMs of n-alkanethiols on such metal. This metal is known to be highly active for the chemisorption of organosulfur compounds but, at the same time, also reactive with the oxygen. Experimental Section Chemicals. The normal alkanethiols considered in this work are H-(CH2)n-SH, denoted briefly as Cn, where n ) 2, 4, 6, 8, 10, and 12. They are all liquid at room temperature and were used as received: n-dodecanethiol (C12; Aldrich, 98%, D22, 1406), n-decanethiol (C10; Aldrich, 98%, D120, 3), n-octanethiol (C8; Aldrich, 98.5%, 47-183-6), n-hexanethiol (C6; Fluka, 52870), n-butanethiol (C4; Aldrich, 99%, 11,292-5), n-ethanethiol (C2; Aldrich, 97%, E370-8). Perchloric acid (HClO4 p.a.; Acros Organics, 22.331.21), sodium hydroxide monohydrate (NaOH‚ H2O; Merck, 017-C762966), acetone (Aldrich, 99.9%, HPLC grade 27, 072-5), H2O ultrapure water (18 MΩ‚cm), toluene (Romil Ltd., 99.9%, 108-88-3), acetonitrile (Aldrich, 99.9%, HPLC grade 27,072-5), absolute ethanol p.a. (Merck, 1.000983.2500), and carbon tetrachloride (Acros Organics, 14817-0010) were used without further purification. Substrate Preparation: Polishing and Electrochemical Reduction. The substrates used in this study are rectangular coupons (10 mm × 20 mm) cut from commercially polycrystalline nickel foils (thickness 1 mm, 99.99%, Advent, NI 1889) and mechanically polished using various grit diamond pastes down to 1 µm and rinsed with acetone and ethanol. Two sets of nickel substrates differing by the chemical state of their surface were considered. The first set, referred to as Niox, corresponds to Ni substrates mirror polished, cleaned as indicated (15) Ufheil, J.; Boldt, F. M.; Bo¨rsch, M.; Borgwarth, K.; Heinze, J. Bioelectrochemistry 2000, 52, 103.

Mekhalif et al. above, and directly used for the monolayer adsorption. The second set, referred to as Nired, is obtained from Niox substrates to which an electrochemical pretreatment (reduction at -0.7 V vs SCE for 20 min in 1 M HClO4 in aqueous solution) was applied prior to adsorption. After the electrochemical treatment, the Nired substrates were immediately used for the preparation of the monolayer to minimize oxidation of the surfaces due to their exposition to the laboratory atmosphere. The exposure time to the laboratory atmosphere of the Nired substrates during this transfer operation is estimated to last less than 3-5 s. The Niox abbreviation stresses the fact that the mechanical polishing does not prevent the formation of a layer of nickel oxides on the surface, while the electrochemical pretreatment leads to samples (Nired) with limited amounts of residual nickel oxides on the surface. Monolayer Preparation. The monolayers were formed by immersion of the nickel substrates in either the neat n-alkanethiol liquids or solutions of these alkanethiols in different solvents: ethanol (C2H5OH), acetonitrile (CH3CN), carbon tetrachloride (CCl4), n-heptane (C7H16), and toluene (CH3-φ). Two immersion times were considered, 15 min and 18 h, as well as, in addition to the neat liquids, two concentrations: a dilute solution (10-3 M), which has become a standard solution for preparation of SAMs onto gold, and a more concentrated solution (10-1 M). All n-alkanethiol solutions were degassed with argon and kept under argon atmosphere during modification. After immersion, all modified samples were quenched with the corresponding pure solvent and ultrasonically cleaned for 15 min in the same solvent to remove adsorbed molecules. When modified in the pure organothiols, the samples have been quenched and ultrasonically cleaned for 15 min in ethanol. Finally, the samples were blown dry under argon gas and immediately used for characterization or left exposed to the laboratory atmosphere, without any specific protection for the stability test. Film Characterizations. The monolayers were characterized by X-ray photoelectron spectroscopy (XPS) and contact angle measurements. An XPS technique is used to determine the elemental composition of the monolayer and to differentiate between the oxidation states of the nickel and sulfur atoms residing at the surface. The photoelectron spectra of the monolayers have been obtained with a SSX-100 spectrometer using monochromatized X-ray Al KR radiation (1486.6 eV), with 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 energies of the core levels were calibrated against the C 1s binding energy set at 285.0 eV, an energy characteristic of alkyl moieties. The peaks were analyzed using mixed Gausian-Lorentzian curves (80% Gaussian character). The S 2p line, which is a doublet structure where the S 2p3/2 and S 2p1/2 components are spaced by 1.18 eV and have an intensity ratio S 2p3/2/S 2p1/2 of 2, has been analyzed accordingly. With the C 1s core level peak set at 285.0 eV, the S 2p3/2 signal corresponding to free thiols arises at 163.3 eV. According to literature data,17 binding energies of 162, 164, 167, and 169 eV at the maximum of the S 2p3/2 component are generally assigned to thiolates (Ni-S-), unbound thiols (-SH), sulfinates (-SO2), and sulfonates (-SO3-), respectively. The Ni 2p level is quite sensitive to the oxidation state of nickel; hence, the Ni 2p3/2 position shall be used as a marker to discriminate between Ni, NiO, and Ni(OH)2, which arise at 852.7, 853.8, and 855.6 eV,8,16 respectively. The spacing, ∆, between the Ni 2p3/2 and Ni 2p1/2components provides additional clues for the interpretation. In the case of metallic nickel, it is equal to 17.3 eV, and it is equal to 17.6 eV for NiO and Ni(OH)2. In all our measured spectra, the characteristic peaks were found essentially constant to within (0.3 eV and typical of values reported in the literature for corresponding environments.18 Contact angles are sensitive to the surface state of the monolayer and, by comparison with literature data on similar films, turn out to be rather straightforward probes of the (16) Moulder, J. F.; Sticle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Paris, 1992. (17) Mekhalif, Z.; Riga, J.; Pireaux, J.-J.; Delhalle, J. Langmuir 1997, 13, 2285 and references cited therein. (18) Beamson, D.; Briggs, D. High-Resolution XPS of Organic Polymers, The Scienta ESCA 300 Database; Wiley: Chichester, 1992.

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molecular structure and organization. Furthermore, the evolution with time of these angles provides information on the stability of the films. The reported contact angles θeq were measured with a VCA 2000 contact angle meter on water drops (about 15 mm3) that have been allowed to equilibrate in air and at room temperature.

Results and Discussion This section is organized in two parts. The first one is devoted to a discussion of the quality of the modification of the substrates, Niox and Nired, in the pure n-alkanethiols for two immersion times, 15 min and 18 h. The second part addresses the influence of solvent nature as well as concentration of the thiol solution on grafting linear alkanethiols on nickel surfaces. Monolayer Self-Assembly from Pure n-Alkanethiols. Self-Assembly from Pure n-Dodecanethiol on Niox and Nired: 15 min and 18 h Reactions. In our previous work,8 modification of nickel in pure n-dodecanethiol has been studied as a function of the chemical state of the surface, evidencing the beneficial effect of an electrochemical reduction of the substrates. Already after 15 min, chemisorbed n-dodecanethiol monolayers were obtained exhibiting good stability toward exposition to the atmosphere. Kinetic studies of organothiol adsorption on gold indicate that longer immersion times lead to better film structure.2 Thus, it was interesting to investigate the immersion time effect on the resulting monolayer on the nickel surface. A short immersion time (15 min) and a longer immersion time (18 h) have been chosen for this purpose. Parts a and b of Figure 1 show the XPS spectra of the Ni 2p structure of C12 monolayers adsorbed on Niox and Nired substrates after immersion in neat n-dodecanethiol for 15 min and 18 h, respectively. As easily observed in Figure 1a (Niox), the Ni 2p structure is typical of NiO. The Ni 2p3/2 line is found at 854.1 eV with ∆ equal to 17.88 eV. This is corroborated by the presence of the plasmon feature at 859.7 eV. By contrast, the Nired surface similarly modified with pure n-dodecanethiol exhibits the features of a metallic nickel. The Ni 2p3/2 line is found at 852.4 eV, with a spacing ∆ of 17.3 eV, and the wide plasmon feature has disappeared, while a structure of lower intensity arises at 856.5 eV. These data show that a Nired substrate modified by n-dodecanethiol directly after the electrochemical reduction is essentially in the metallic state. The presence of the structure located at 856.5 eV with lower intensity for Nired than for Niox means that the oxide has been largely reduced but not totally eliminated from the surface. The persistence of this structure could be due to a small amount of residual oxides formed on the nickel surface during the transfer from the electrochemical cell to the n-dodecanethiol or could also be a result of a posteriori oxidation of nickel at some defects present in the monolayer. Compared to the case of Figure 1a, significant differences are noted in Figure 1b when Niox and Nired are modified with n-dodecanethiol for 18 h. In the case of Niox, features characteristic of metallic nickel (852.7 and 870.4 eV for Ni 2p3/2 and Ni 2p1/2, Figure 1b) now arise alongside those of nickel oxide (856.4 and 881.0 eV for Ni 2p3/2 and Ni 2p1/2), which, nevertheless, remain substantially intense. The presence of metallic nickel in this case could be a consequence of nickel oxide reduction by the ndodecanethiol molecules, which is more efficient for a longer time. This behavior have also been evidenced for the modification of copper oxide surfaces. Modification of Nired, already very effective after 15 min of immersion, has further improved after 18 h; the Ni 2p level is essentially characteristic of the metallic state, and nickel

Figure 1. Ni 2p core level spectrum of a CH3-(CH2)11-SH monolayer adsorbed from solvent-free product on nickel oxide Niox (top) and reduced-nickel Nired (bottom): (a) immersion time ) 15 min; (b) immersion time ) 18 h.

oxidized species are not detected. The Ni 2p spectrum is comparable to that of a nickel etched under vacuum. The features observed in the O 1s level (Figure 2) confirm the difference observed on the Ni 2p levels for Niox and Nired modified during 18 h. For modified Niox, three components of oxygen are necessary to obtain a good fit: one at 529 eV assigned to NiO, a second one at 531.6 eV which could be related to oxidized carbonaceous contaminants adsorbed on NiO and on Ni(OH)2,19 and a third one at a higher energy (533.3 eV) assigned to adsorbed water. (19) Mansour, A. N. Surf. Sci. Spectra 1994-1995, 3/3, 211.

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Figure 3. Ni 2p core level spectra of a CH3-(CH2)n-SH monolayer adsorbed on Nired from solvent-free product.

Figure 2. O 1s core level spectrum of a CH3-(CH2)11-SH monolayer adsorbed from solvent-free product on nickel oxide Niox (top) and reduced-nickel Nired (bottom): immersion time ) 18 h.

In the case of Nired, a very small amount of oxygen is detected at 532 eV which a priori could be associated to Ni2O3 and/or residual oxidized carbon contaminants. Combining results from Figures 1b, where the Ni 2p level does not show the presence of Ni(OH)2, and those of Figure 2 allows us to conclude that the small amount of oxygen detected on the surface is most likely of contamination origin. The contact angles, θeq, for modified Niox and Nired substrates are equal to 110° ( 2°, irrespective of the immersion time. They compare to values measured for the corresponding monolayers formed on evaporated gold and, accordingly, point to a good molecular organization of the films. On the basis of the above results, film formation with n-alkanethiols of increasing chain length will all be carried out with Nired substrates and an 18 h reaction time. Self-Assembly from Pure n-Alkanethiols on Nired: 18 h Reaction. As seen from the Ni 2p levels in Figure 3, the Nired substrates are successfully modified with all the n-alkanethiols under consideration (Cn, n ) 2, 4, 6, 8, and 10) when reacted 18 h in the pure mercaptans. However, films formed from short alkane chains such as in ethanethiol, even if the initial modification is remarkably good (Figure 4), show rapid degradation due to oxidation by atmospheric oxygen (Figure 5). Figure 4 shows the survey spectrum, the S 2p and Ni 2p levels of ethanethiol obtained immediately after modification of Nired, while Figure 5 provides the corresponding information on a similar sample exposed for 1 week to the laboratory atmosphere. The difference between the two surfaces is very significant. For the fresh monolayer, the intensity of the O 1s peak is very low, while it has drastically increased after 1 week of exposure to the atmosphere, indicating that oxygen has

Figure 4. XPS spectra of a CH3-CH2-SH monolayer adsorbed on Nired from solvent-free product.

penetrated through the film and reacted with both sulfur and nickel at the interface. This is evidenced by the change in spectral characteristics of both the S 2p and Ni 2p levels, which exhibit features pertaining to oxidized species; see also Table 1. The contact angle measured on a freshly modified surface is about 90°, which is characteristic of neither CH2 nor CH3. This is not surprising, as the molecule is short and the water drop reacts with the surface and changes then the wetting properties. This part of the work evidences the fact that an electrochemical reduction and a sufficient reaction time are necessary to get Ni substrates properly modified with n-alkanethiols and keep the interfacial nickel in the

SAMs of n-Alkanethiols on Ni Polycrystalline Substrates

Langmuir, Vol. 19, No. 3, 2003 641 Table 2. Contact Angle and Relative Composition of Elements from XPS Data of a CH3-(CH2)11-SH Monolayer Adsorbed on NiRed: Immersion Time ) 18 h; Different Solvents; Concentrations ) 10-3 and 10-1 M

Figure 5. XPS spectra of a CH3-CH2-SH monolayer adsorbed on Nired from solvent-free product, after a 1-week exposure to the atmosphere. Table 1. Peak Energies for Ni 2p Core-Level of n-Dodecanethiol SAMs on Nickel Oxide and Reduced-Nickel, Solvent-Free n-Dodecanethiol: (a) Immersion Time ) 15 min; (b) Immersion Time ) 18 h peak energy (eV) element/transition

Niox

Nired

peak assignment

Ni 2p3/2 Ni 2p3/2 Ni 2p3/2 Ni 2p1/2 Ni 2p1/2 Ni 2p1/2

(a) Immersion Time ) 15 min 852.4 Ni metallic 854 856 NiO 859.6 870 satellite 870.3 Ni metallic 871.8 satellite 878.2 NiO

Ni 2p3/2 Ni 2p3/2 Ni 2p3/2 Ni 2p1/2 Ni 2p1/2 Ni 2p1/2

(b) Immersion Time ) 18 h 852.7 852 Ni metallic 856.4 NiO 861.6 859.4 satellite 870.4 869 Ni metallic 874.4 875 satellite 881.0 NiO

metallic state. Contacting an oxidized nickel substrate such as Niox with the pure n-alkanethiols leads to the oxidation of the thiol function and a parallel reduction of the metal oxide(s). During this process the oxidized n-alkanethiols are probably desorbed and replaced by fresh species. As noted in Figure 1a and b, a longer reaction time corresponds to a larger decrease in the intensity of the spectral features characteristic of oxidized nickel species and, hence, in the thickness of the nickel oxide layer. This is in line with similar findings noted for other substrates such as Ag,20,21 Ti,22 Al,23 and Cu.24 In the case of silver, which has been thoroughly studied from a (20) (a) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parilkh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 71. (b) Allara, D. L.; Tao, Y. T.; Parilkh, A. N. J. Am. Chem. Soc. 1992, 114, 1990. (21) Himmelhaus, M.; Guass, I.; Buck, M.; Eisert, F.; Wo¨ll, Ch.; Grunze, M. J. Electron Spectrosc. Relat. Phenom. 1998, 92, 139. (22) Mekhalif, Z.; Delhalle, J.; Lang, P.; Garnier, F.; Pireaux, J.-J. Synth. Met. 1998, 96, 165.

conc (M)

C 1s

O 1s

S 2p bonded

S 2p free or oxidized

C/S

angle (deg)

10-1 10-3

70.7 56.8

27.7 41.6

Toluene 4.5 0.6

0.5 1.2

14.4 31.5

90 82

10-1 10-3

54.2 58.7

44.8 40.3

n-Heptane 0.4 0.6 0.4 0.6

54.2 58.7

72 54

10-1 10-3

47.7 37.7

49.9 61.1

0.7 0.5

19.9 31.4

58 36

CCl4 1.7 0.7

mechanistic viewpoint, Laibinis et al.20 interpret the adsorption effect of alkanethiols as a combination of a reduction of Ag(I) to Ag(0), a direct conversion of Ag(I) surface oxides to Ag(I) thiolates, and formation of an anionic surface phase in which silver atoms have been removed from their equilibrium lattice positions. In the case of Cu, Rubinstein et al.25 point out also that good quality SAMs can be elaborated directly on copper oxide (Cu2O). Though we have not studied this possibility in sufficient detail, it seems that nickel is required to be in the metallic state to lead to stable monolayers, hence the importance of appropriate treatments, such as an electrochemical reduction, to remove the surface oxides. As for monolayers elaborated on substrates such as gold or silver, the adsorption of n-alkanethiol on reduced Ni is quite fast, thiolates are evidenced for a short immersion time, however, stability is not achieved, the thiolates transform after a few days to sulfinates and sulfonates, and nickel moves from the metallic state to an oxidized one. Prolonged immersion times in the neat n-alkanethiols are necessary to improve the organization of the chains. This is of particular relevance for oxidizable metals if SAMs are to be used as a protective barrier against the diffusion of O2 at the monolayer/metal interface. Good stability was already noted for nickel Nired reacted 15 min in pure n-dodecanethiol and exposed 5 weeks to the laboratory atmosphere.8 However, as just pointed out, this is valid only for organothiols with sufficiently long alkyl moieties, -(CH2)n-, typically n > 8. This stresses the limitations of modifying nickel substrates with pure n-alkanethiols because, beyond a certain length (n > 12), they are either highly viscous or become solid at room temperature. Accordingly, it is important to find conditions for effective modification of nickel substrates with n-alkanethiols dissolved in appropriate solvents. Solvent and Concentration Effects on Monolayer Self-Assembly. Acetonitrile, ethanol, toluene, n-heptane, and carbone tetrachloride have been selected as potential solvents to modify Nired substrates using n-dodecanethiol as test molecule. For each solvent, two concentrations, 10-1 and 10-3 M, and 18 h of reaction have been considered. First, we consider in detail the influence of the concentration of n-dodecanethiol in ethanol, because it is the most commonly used solvent in the field of SAMs of organothiols and as such will constitute our reference to be compared with the other solvents. (23) (a) Mekhalif, Z.; Delhalle, J.; Lang, P.; Garnier, F.; Caudano, R. J. Electrochem. Soc. 1999, 146, 2913. (b) Mekhalif, Z.; Delhalle, J.; Lang, P.; Garnier, F.; Caudano, R. In Polymer-Metal interface: from Model to Real systems; Proceedings of the second International Conference, Namur, Belgium, 12-16 August 1996; Pireaux, J. J., Delhalle, J., Rudolf, P., Eds.; p 311. (24) Mekhalif, Z.; Laffineur, F.; Delhalle, J. Submitted. (25) Ron, H.; Cohen, H.; Maltis, S.; Rappaport, M.; Rubinstein, I. J. Phys. Chem. 1998, B102, 9861-9869.

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Figure 6. XPS spectrum of a CH3-(CH2)11-SH monolayer adsorbed on Nired: immersion time ) 18 h; solvent ) ethanol; concentration ) 10-3 M (bottom) and 10-1 M (top); (a) survey spectrum; (b) Ni 2p core level; (c) O 1s core level; (d) S 2p core level.

Self-Assembly of n-Dodecanethiol on Nired in Ethanol. Figure 6 provides the XPS spectra of modified Nired for both concentrations. The survey spectra (Figure 6a) reveal the presence of nickel (Ni 2p), carbon (C 1s), sulfur (S 2p), and oxygen (O 1s) on the substrates for both concentrations, but the relative intensity of the peaks changes significantly with concentration. Compared to that of the Ni 2p peak, the intensities of the C 1s and S 2p peaks are relatively low for the 10-3 solution, which suggests a small amount of n-dodecanethiol grafted on the surface. Moreover, the O 1s peak is rather intense and can be due to

oxidation of the Nired surface not enough protected against atmospheric oxygen. Figure 6b and d shows the Ni 2p and S 2p lines, respectively. For the 10-1 M solution, the Ni 2p, with binding energies 852.8 and 870.0 eV for Ni 2p3/2 and Ni 2p1/2, respectively, is typically nickel in the metallic state. A small feature is also detectable at 854 eV, which indicates the presence of a small amount of residual nickel oxide. This result is quite comparable to the one obtained in Figure 1a, where Nired was modified in pure n-dodecanethiol for 15 min. In the case of the monolayer prepared

SAMs of n-Alkanethiols on Ni Polycrystalline Substrates Table 3 bond strength (kJ/mol) for the following metals (fcc) M-Sa M-Oa

Au

Ag

Cu

Ni

418 222

217 220

276 269

344 382

a Handbook of Chemistry and Physics, 70th ed.; CRC Press: Boca Raton, FL, 1989; pp F197-F200.

in the 10-3 M solution, the main peaks also correspond to metallic nickel, but intense satellite lines typical of oxides, Ni(OH)2 and NiO, are also visible at 857.3 and 853.8 eV. In the case of the 10-1 M solution, the S 2p peak is composed of only one, well-defined, doublet peak where S 2p3/2 appears at 161.8 eV, an energy typical of thiolate species. The ratio between the S 2p3/2 and S 2p1/2 peaks is equal to 1/2, which is in accordance with the theoretical value. This points to the absence of both free thiols (nonbonded) and disulfides in the organic layer. Quite the opposite, the sulfur signal corresponding to the 10-3 M case displays two S 2p components; the one around 162 eV corresponds to thiolates, while the other at 168.1 eV is typical of oxidized sulfur.17 This result is consistent with the nickel chemical state (Figure 6b), which shows more oxides at the surface for the 10-3 M solution than in the 10-1 M case. The quantitative analysis of the XPS data yields a carbon/sulfur ratio close to 12 for the layer formed from the 10-1 M solution, that is, close to the theoretical value for n-dodecanethiol, but 19 for the other layer (10-3 M). In the latter case, more carbon than expected from the n-dodecanethiol composition is detected at the surface. This might be the result of contamination or of ethanol molecules physi- and/or chemisorbed on the surface. Figure 6c displays the O 1s level for both surfaces. It is worth noticing that the concentration of oxygen at the surface modified with the 10-1 M solution is very low and most likely due to organic contamination and water. On the contrary, in the case of the monolayer formed from the 10-3 M solution, more oxygen is found. It is related to a small amount of NiO at 529.6 eV and to a larger quantity of organic contaminants adsorbed on Ni(OH)2 at 531.4 eV as well as water and/or ethanol at 532.7 eV. Despite the large spectroscopic differences noted for layers obtained from 10-3 and 10-1 M solutions, the equilibrium contact angles measured with water are close to 110°, irrespective of the concentration. Such values are typical of compact layers rich in CH3 groups on their outer surface. Contact angle (80°) as well as XPS results on Nired modified by 10-3 M n-dodecanethiol in ethanol for 3 h8 pointed to monolayers of much lesser quality, which once again stresses the importance of the immersion time. The XPS observations indicate that chemical grafting of the n-dodecanethiol on Nired and good quality monolayers can be achieved in concentrated solutions, such as 10-1 M, with sufficient immersion time. Self-Assembly of n-Dodecanethiol on Nired in Acetonitrile, Carbon Tetrachloride, n-Heptane, and Toluene. In view of the fact that ethanol is not necessarily the solvent of choice for all situations (effective solubility of the bifunctional coupling agents, cost, operational difficulties, as well as other constraints), it is important to consider the virtues of other solvents. Figure 7 shows the XPS results obtained on C12 monolayers elaborated on Nired in acetonitrile solutions. Ni 2p, S 2p, O 1s, and N 1s core levels are shown for the low and the high thiol concentrations (part a to d of Figure 7, respectively). Nickel is mainly in the metallic state, whatever the concentration of the solution of n-dode-

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canethiol. However, significant differences are noted in the sulfur S 2p level. For the low concentration, the sulfur peak can be decomposed into three components found at the binding energies 162.0, 165.5, and 168.1 eV; they signify the presence of thiolate, sulfinate, and sulfonate functions, respectively. Higher concentration of organothiol favors the formation of thiolate functions over the sulfinates and sulfonates, since no oxidized sulfur is detected (Figure 7b, top). Thus, similarly to the case for ethanol, low concentrations of C12 (10-3 M) in acetonitrile are not adequate to obtain good quality n-dodecanethiol monolayers on Nired. It is interesting to point out that acetonitrile is prone to interact with nickel and accordingly can affect the kinetics of film formation. For the 10-3 M solution the nitrogen signal is present (Figure 7d, bottom). In comparison with the ethanol case, this leads to significant differences in both the O 1s and S 2p levels. The components at 532.7 and 168 eV of the O 1s and S 2p levels, respectively, are much higher for the monolayers obtained with 10-3 M C12 in acetonitrile rather than in ethanol with the same concentration. This situation corroborated with the results on contact angles which are equal to 109° and 102° for the monolayers obtained from 10-1 and 10-3 M C12 in acetonitrile, respectively. Similar experiments conducted with solutions of C12 in toluene, n-heptane, and carbon tetrachloride have been very deceptive, irrespective of the concentrations used (Table 2). Only small amounts of sulfur, mainly oxidized, are detected by XPS, and interfacial nickel is also found mainly oxidized. In the case of CCl4, XPS results (not shown here) reveal the presence of chlorine (Cl 2p level at 198 eV), which points to the possibility of reactive interactions of this solvent with the substrate in its metallic state. More specific studies should be carried out with these solvents to better understand the reasons for their behavior. The results shown above highlight the critical importance of the conditions for n-alkanethiol SAMs’ elaboration on nickel. For gold as substrate the effect of some of those conditions is already well-known while others are without importance because of the inertness of gold surfaces. In the case of oxidizable metals, such as nickel, the process is more complicated. The control of the surface oxidation state is the most important step because the surface has to be oxide-free for the self-assembly to be effective. The second consideration of importance is the possible interaction of the solvent with the surface substrate, which may compete with thiols’ chemisorption. One has to notice that most of the inhibitors are compounds with at least one polar function, including nitrogen, sulfur, oxygen, and in some cases selenium and phosphorus atoms. When dealing with active metals such as nickel, polar functions are potential reaction centers for competing chemisorption but also for inhibiting access of the thiol to the surface due to physisorption. In such a case, the adsorption bond strength is fixed by the electron density of the atom acting as the reaction center and by the polarizability of the function. The effectiveness of the function atoms with respect to the adsorption process, when the stabilities of the compounds are equal, can be taken as being in the following sequence: selenium > sulfur > nitrogen > oxygen.26 In our case the n-alkanethiol molecules act as corrosion inhibitors (Ni-S bond strength ) 344 kJ/mol, Table 3), because their chemisorption on the nickel associated with (26) Advances in Corrosion Science and Technology; Fontana, M. G., Staehle, R. W., Eds.; Plenum Press: New York-London, 1970; Vol. 1, p 149.

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Mekhalif et al.

Figure 7. XPS spectrum of a CH3-(CH2)11-SH monolayer adsorbed on Nired: immersion time ) 18 h; solvent ) acetonitrile; concentration ) 10-3 M (bottom) and 10-1 M (top); (a) Ni 2p core level; (b) S 2p core level; (c) O 1s core level; (d) N 1s core level.

the formation of dense thin films effectively protects against atmospheric oxidation. The solvent containing nitrogen or oxygen could react as well with nickel but not as much as the sulfur function does, and an increase in the thiol concentration favors the interaction of the nickel with the thiol. Furthermore, most of the reported studies concerning the formation of n-alkanethiols on gold, silver, and copper suggest the use of ethanol as the preferred solvent. Others studies have been performed with other solvents, and no significant effect has been detected.1 However, Bain et al. have studied the effect of the solvent on monolayer properties by wetting measurements and showed that n-hexadecanethiol monolayers adsorbed from

hexadecane lead to lower contact angles; a possible incorporation of the solvents into the monolayers was, according to them, a possible explanation.27 Uosaki results28 based on the STM observations of C10 SAMs on a well-defined surface of Au(111) show that domain size and pits are strongly dependent on the solvents used, particularly so for heptane. In-situ STM studies29 showed the formation of an island. When the n-heptane islands are removed and destroyed, a disordered phase is formed. (27) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (28) Yamada, R.; Uosaki, K. Langmuir 1998, 14, 855. (29) Yamada, R.; Uosaki, K. Langmuir 1997, 13, 5218.

SAMs of n-Alkanethiols on Ni Polycrystalline Substrates

This interpretation is in agreement with the result obtained in our case. The possible formation of such islands on C12 adsorbed on Nired and their removal during the rinsing process could privilege the formation of defects in the monolayer which are obviously more problematic in the case of nickel than that gold. The large pits created allow the oxidation of nickel and sulfur at the interface. From these studies it can be inferred that alkane solvents are not the most suitable ones for SAMs elaboration despite their inertness toward the substrate. In conclusion, the choice of the solvent should be first made according to its capability to dissolve the bifunctional thiols, then lower interaction of this solvent with thiol molecules, and, in the case of oxidizable metals, its inertness toward the substrate. Concluding Remarks Good quality n-alkanethiol monolayers can be elaborated by wet chemistry on mechanically polished polycrystalline Ni substrates provided these have been freed from their surface oxides, for example, by electrochemical reduction. Such monolayers can be obtained from the pure n-alkanethiol liquids or dissolved in polar solvents, for

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example, ethanol and acetonitrile. In the latter cases, sufficiently high concentrations and immersion times are needed, however. The apolar solvents n-heptane and toluene, which are nonreactive with the metallic substrate, do not lead to the formation on SAMs on Ni. This is not fully understood and calls for further studies. Solvents reactive with the substrate, for example, CCl4, are definitely not suitable. The results of this study are promising in many respects. For example, they open the prospect of differential modification of patterned bi- or polymetallic active metal substrates on the basis of their relative chemical affinities toward the thiol functions. However, the control and command of such modifications will require further work along the lines described in this paper. This forms the subjects of ongoing works. Acknowledgment. This study was supported by the Belgian National Interuniversity Research Program on “quantum size effects in nanostructure materials” (IUAP P5/01). LA020332C