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Self-Assembled Alkanethiol Monolayers on a Zn Substrate: Structure and Organization C. Nogues† and P. Lang* ITODYS (CNRS UMR 7086), UniVersity Paris 7, 1 rue Guy de la Brosse, 75005 Paris, France ReceiVed January 22, 2007. In Final Form: March 19, 2007 This work concerns a first step toward the use of self-assembled monolayers derived from thiols as coupling layers between a zinc surface and organic coatings. The adsorption, structure, and aging of alkanelthiol monolayers on zinc substrates have been studied by contact angle measurements, infrared spectroscopy, and electrochemistry. The thiols self-assemble on the zinc surface to form a highly hydrophobic monolayer. The molecules are well organized with very few gauche defects, oriented nearly normal to the surface, and protect the zinc from oxidation in a neutral aqueous medium.
Introduction Over the past 20 years, self-assembled monolayers (SAMs) of various chain lengths and terminal groups have been extensively studied.1-3 SAMs give strongly bound and densely packed organic molecules on various surfaces. The character of the bond created between the SAM and the substrate is governed by both the nature of the surface and the reactive group of the molecules. In particular, thiol moieties4 have a high affinity toward metals (oxidation state 0) such as Au5-10 or Ag,11,12 Pt,13-18 Cu,19-23 * Corresponding author. E-mail:
[email protected]. † Present address: Department of Structural Biology, Weizmann Institute of Science, Rehovot, Israel. (1) See, for example, Ulman, A. An Introduction to Ultrathin Organic Films from LB to Self-Assembly; Academic Press: Boston, 1991; Chapter 3 and references therein. (2) Stewart, K. R.; Whitesides, G. M.; Godfried, H. P.; Silve, I. F. ReV. Sci. Instrum. 1986, 57, 1381. (3) Young, J. T.; Boerio, F. J.; Zhang, Z.; Beck, T. L. Langmuir 1996, 12, 1219. (4) Ulman, A. Self-Assembled Monolayers of Thiol Thin Films; Academic Press: San Diego, CA, 1998; Vol. 24. (5) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (6) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (7) Bain, C. D.; Troughton, E. B.; Tao, Y. D.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (8) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (9) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 1, 723. (10) Laibinis, P. E.; Whitesides, L. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 715. (11) Walzak, M. M.; Chung, C.; Stole, S. M.; Widrig, C.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370. (12) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 828. (13) Lang, P.; Mekhalif, Z.; Garnier, F. J. Chim. Phys. 1992, 89, 1063. (14) Hines, M. A.; Todd, J. A.; Guyot-Sionnest, P. Langmuir 1995, 11, 493. (15) Mekhalif, Z.; Lang, P. Garnier, F. J. Electroanal. Chem. 1995, 399, 61. (16) Lang, P.; Mekhalif, Z.; Garnier, F.; Rat, R. J. Electroanal. Chem. 1998, 441, 83. (17) Vilar, M. R.; Bouali, Y.; Kitakatsu, N.; Lang, P.; Michalitsch, R.; Garnier, F.; Dubot, P. Thin Solid Films 1998, 329, 236. (18) Petrvyckh, D. Y.; Kamura-Suda, H.; Opdahl, A.; Richter, L. J.; Tarlov, M. J.; Whitman, L. Langmuir 2006, 22, 2578-2587. (19) (a) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022-9028. (b) Jennings, G. K.; Munro, J. C.; Laibinis, P. E. AdV. Mater. 1999, 11, 1000-1003. (c) Jennings, G. K.; Yong, T. H.; Munro, J. C.; Laibinis, P. E. J. Am. Chem. Soc. 2003, 14, 2950 and references therein. (20) Zamborini, F. P.; Crooks, R. M. Langmuir 1998, 17, 3279. (21) Ron, H.; Cohen, H.; Maltis, S.; Rappaport, M.; Rubinstein, I. J. Phys. Chem. B 1998, 102, 9861-9869. (22) Laffineur, F.; Delhalle, J.; Guittard, F.; Geribaldi, S.; Mekhalif, Z. Colloids Surf., A 2002, 198-200, 817. (23) Sung, M. M.; Sung, K.; Kim, C. G.; Lee, S. S.; Kim, Y. J. Phys. Chem. B 2000, 104, 2273-2277.
Al,24,25 Fe,26,27 Ni,28-30 Ti,31 and Zn.32,33 More recently, thiols have been adsorbed onto nonmetallic substrates: GaAs,34 InP,35 ITO,36-38 and ZnSe.39 Because of their intrinsic properties, SAMs provide an original path for surface modification and open up new perspectives for applications in materials science. For instance, surface wettability can be controlled by the adsorption of a specific SAM to give an antifouling coating.40 It is important to control the surface energy and the chemical reactivity of a modified surface because these factors strongly influence the adhesion and structure of deposited organic thin films. In addition, it has been shown that SAMs provide protection against the corrosion of oxidizable metals such as Fe26,27 and Cu.19-23,41 (24) (a) Lang, P.; Mekhalif, Z.; Regis, A.; Garnier, F. In Short and Long Chains at Interfaces; Daillant, J., Ed.; XVth Moriond Workshop; Editions Frontieres: Gif-sur-Yvette, France, 1995; p 331. (b) Mekhalif Z.; Lang P.; Garnier F.; Caudano R., Delhalle J. In Polymer-Solid Interfaces: From Model to Real Surfaces; Pireaux, J. J., Bertrand, P., Bredas, J. L., Eds.; Presses Universitaires de Namur: Namur, Belgium, 1998; pp 311-325. (25) (a) Lang, P.; Mekhalif, Z.; Regis, A.; Garnier, F. J. Electrochem. Soc. 1999, 146, 2913. (b) Lang, P.; Mekhalif, Z.; Garnier, F. In Organic Coating; Lacaze, P.C., Ed.; 1995; p 176. (26) (a) Volmer, M.; Czodrowski, M.; Stratmann, M. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 1335. (b) Volmer, M.; Stratmann, M.; Viefhaus, H. Surf. Interface Anal. 1991, 16, 278. (c) Stratmann, M. AdV. Mater. 1990, 2, 191. (27) (a) Uehara, J.; Aramaki, K. J. Electroanal. Soc. 1991, 138, 3245. (b) Pirlot, C.; Delhalle, J.; Pireaux, J.-J.; Mekhalif, Z. Surf. Coat. Technol. 2001, 138, 166. (28) Mekhalif, Z.; Laffineur, F.; Couturier, N.; Delhalle, J. Langmuir 2003, 19, 637-645. (29) (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. (30) Vogt, A. D.; Han, T.; Beebe, T. P. Langmuir 1997, 13, 3397. (31) Mekhalif, Z.; Lang, P.; Garnier, F.; Caudano, R.; Delhalle, J. Synth. Met. 1998, 96, 165-175. (32) Mekhalif, Z.; Massi, L.; Guittard, F.; Geribaldi, S.; Delhalle, J. Thin Solid Films 2002, 405, 186. (33) Michalitsch, R.; Garnier, F.; Lang, P.; Nogues, C.; Mauer, D. European Patent; Bekaert & Cie, WO0023505, 2000. (34) Sheen, C. W.; Matensson, J.; Parikh, A. N.; Allara, D. L. J. Am. Chem. Soc. 1992, 114, 1514. (35) Gu, Y.; Butara, R. A.; Smentkowski, V. S.; Waldeck, D. H. Langmuir 1995, 11, 1849. (36) Yan, C.; Golzhauser, A.; Grunze, M. Langmuir 2000, 16, 6208. (37) Brewer, S. H.; Brown, D. A.; Franzen, S. Langmuir 2002, 18, 6857. (38) Karsi, N.; Lang, P.; Chehimi, M.; Delamar, M.; Horowitz, G. Langmuir 2006, 22, 3118. (39) Noble-Luginbuhl, A. R.; Nuzzo, R. G. Langmuir 2001, 17, 3937. (40) Otsuni, E.; Chapman, R. G.; Liang, M. N.; Meluleni, G.; Pier, G.; Ingber, D. E.; Whitesides, G. M. Langmuir 2001, 17, 6336. (41) Jennings, G. K.; Munro, J. C.; Laibinis, P. E. AdV. Mater. 1999, 11, 1000-1003.
10.1021/la0701879 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/27/2007
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Figure 1. Static contact angles, measured with water, versus adsorption time tads: (a) CH3(CH2)9-SH/Zn and (b) CH3(CH2)17-SH/Zn. (The solid line is just a guide for the eye.)
Zinc used as a pure metal or in an alloy is very important in the domain of surfaces and coatings. It can form a passivation layer against corrosion and is easily deposited electrochemically in the metallic state from aqueous media.42,43 However, the stabilization of such galvanized substrates under corrosive conditions and the further fixation of various coatings on these surfaces remains a serious problem. As a possible solution, we have exploited the self-assembly of alkanethiols on zinc, which give a barrier against ions, dioxygen, and water diffusion and then protection against corrosion. Furthermore, the thiols can act as a grafting agents if they possess a second suitable terminal function that is able to bind chemically or electrochemically to a subsequent layer. Bipolar molecules such as HS-(CH2) n-Y have already been tailored and used successfully to graft conducting polymers/oligomers to noble metals44 but also to Al25 and Ti31 and to improve the adhesion of polyisoprene to Zn wire.45 In this work, we study the adsorption of SAMs on a metallic zinc surface through thiol chemisorption at atmospheric pressure. We characterized the self-assembly and the structure of the alkanethiol on zinc as a function of the alkyl chain length and the adsorption time. The resulting layers are densely packed, and the molecules are increasingly organized on the surface for longer adsorption times. The sulfur-zinc bond is strong enough to resist ultrasonic treatment. In addition, the stability of the layer under corrosive conditions, as studied by electrochemistry, reveals that the value of the oxidation potential of zinc increases significantly. (42) Zinc-Based Steel Coating Systems: Metallurgy and Performance; Krauss, G., Matlock, D. K., Eds.; Minerals, Metals & Materials Society: Warrrendale, PA, 1990. (43) Notoya, T.; Poiling, G. W. Corrosion 1979, 35, 19. (44) (a) Collard, D. M.; Sayre, C. N. J. Electroanal. Chem. 1994, 375, 367. (b) Mekhalif, Z.; Lang, P.; Garnier, F. J. Electroanal. Chem. 1995, 61, 399. (c) Sabatini, E.; Gafni, Y.; Rubinstein, I. J. Phys. Chem. 1995, 9, 12305. (d) Willicut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 10823. (e) Willicut, R. J.; McCarley, R. L. Langmuir 1995, 11, 296. (f) Liedberg, B.; Yang, Y.; Engquist, I.; Wirde, M.; Gelius, U.; Gotz, G.; Bauerle, P.; Rummel, R. M.; Ziegler, C. H.; Gopel, W. J. Phys. Chem. B 1997, 101, 5951. (g) Ng, S.-C.; Miao, P.; Chen, Z.; Chan, H. S. O. AdV. Mater. 1998, 10, 782. (h) Michalitsch, R.; Nogues, C.; Najari, A.; El Kassmi, A.; Yassar, A.; Lang, P.; Garnier, F. Synth. Met. 1999, 101, 5. (i) Berlin, A.; Zotti, G.; Schiavon, G.; Zecchin, S. J. Am. Chem. Soc. 1998, 120, 13453. (j) Michalitsch, R.; Lang, P.; Yassar, A.; Nauer, G.; Garnier, F. AdV. Mater. 1997, 9, 321. (k) Michalitsch, R.; El Kassmi, A.; Yassar, A.; Lang, P.; Garnier, F. J. Electroanal. Chem. 1998, 457, 129. (l) Michalitsch, R.; Nogues, C.; Najari, A.; El Kassmi, A.; Yassar, A.; Lang, P.; Rei Vilar, M.; Garnier, F. Synth. Met. 1999, 102, 1319. (45) (a) Mauer, D.; Lang, P.; Najari, A.; Garnier, F. Proc. Rubber Bonding 2000 2000, 1. (b) Michalitsch, R.; Brabant, J. V.; Mauer, D.; Garnier, F.; Lang, P. U.S. Patent US2002/055011, 2002. (c) Najari, A. Ph.D. Thesis, University of Paris 7, 2003.
Table 1. Contact Angles Measured on a Monolayer (Long Adsorption Time) of CH3(CH2)n-SH θa (deg, H2O) Au11 Ag10 Cu11 This work Electrodeposited Zn Zn (platelet)
n)9
n ) 17
115 110 120
115 116 120
123 ( 3 111 ( 2
129 ( 3 113 ( 2
Experimental Section Preparation of Zn Substrates. Two different zinc samples were used for the adsorption of self-assembling monolayers. For contact angle measurements and electrochemistry, zinc was electrodeposited from 1 M aqueous solutions of ZnSO4 (Aldrich, 99.99%) adjusted to pH 2.5 with H2SO4. The platinum electrode was a spherical polyoriented single crystal. To remove the singlecrystal properties of the platinum surface prior to electrodeposition, it was cycled in 0.5 M H2SO4 (Suprapur, Merck) between -0.7 and 0.8 V versus Hg/HgSO4, sat. K2SO4 (sweep rate 500 mV‚s-1). Metallic zinc was then deposited on the platinum with sonication59 by scanning the potential to -1.7 V (50 mV‚s-1), which was maintained until the reduction charge reached 2.1 C‚cm-2 (ca. 1 µm thickness of zinc). The current density was ca. j ≈ 50 mA‚cm-2. For infrared spectroscopy, a Zn sheet (Goodfellows, 99.99%) (1 × 4 cm2) was polished with abrasive paper (4000 Struers). Ultrapure water for solutions and for rinsing was taken directly from an Elgstat UHQ II (18 MΩ‚cm) water purification system. Sonication was performed in a type T-460H Bioblock-Scientific ultrasonic bath at a frequency of 35 kHz and a power of 35 W. Preparation of Monolayers. All vessels were carefully cleaned by immersion in hot sulfuric acid (Ultrapure, Merck) overnight and then rinsed thoroughly with ultrapure water. n-Decanethiol (DT), n-octadodecanethiol (ODT), CH3(CH9)-SH, and CH3(CH17)-SH (Aldrich, 99%) were used as received. Just after Zn electroplating, the spherical electrode was rapidly rinsed with argon-saturated acetonitrile (distilled under argon) and transferred to argon-purged neat thiol for different adsorption times ranging from a few seconds to 48 h. Because n-octadecanethiol is solid at room temperature, it is melted at 40 °C prior to and during adsorption. For the spectroscopic study, the zinc sheets are first polished in neat thiol in a glovebox under argon. After 10 min, the substrates are immersed in thiol for a few seconds up to 48 h. All Zn samples were subsequently rinsed twice with dichloromethane (Aldrich, 99%) in the ultrasonic bath for 2 min to remove molecules that were not strongly grafted. Analytical Methods. Contact angle measurements using the spherical electrode have been described in detail.16 Static contact
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Figure 2. IR spectrum of n-decanethiol and n-octodecanethiol. (Top) transmission of the liquid. (Bottom) reflection of the monolayer/Zn: (-) experiment, (---) calculated. Table 2. Wavenumbersa and Full Width at Half-Height (fwhh) of νC-H Stretching Vibrations for Adsorbed Alkanethiol Monolayers and Corresponding Neat Liquids or Solids DT/Zn assignment νaCH3 ipb νaCH3 opc νaCH3 + 2δCH3, FR1d νaCH2 νsCH2 + 2δCH2, FR2 νsCH3 νsCH2
ν
(cm-1
)
2966 2960 2939
neat liquid DT fwhh
-1
ν (cm )
ODT/Zn -1
fwhh
ν (cm ) 2966 2958 2934
2958 7
2936
6
2919.5 2905
16.5
2925 2903
20
2879 2850
7 12
2871 2853
7 16
solid ODT fwhh
-1
ν (cm )
fwhh
2957 14
2938
13
2919 2900
17.5
2918 2900
8-12
2877 2851
10 10
2872 2851
8-12 8-12
a The νCH and νCH peak positions are determined on the fitted spectra and are accurate to within 1 cm-1. b The νCH peak is not resolved in 2 3 3 the spectra of monolayers; ip: in plane. c op: out of plane. d FR: Fermi resonance.
angles on flat zinc substrates were measured using a Digidrop GBX equipped with a CCD camera and analysis software. Grazing angle infrared spectroscopy was performed on a Nicolet 850 FTIR (thermoelectron) spectrometer with a resolution of 4 cm-1 by summing 1000 scans with an optical velocity of 0.47 cm‚s-1. The reference of thiol-free substrate (bulk Zn plate) was recorded after the oxidation-desorption of the thiol layer using a homemade ozonizer. The thiol-free substrate was kept under argon during transfer to the spectrometer. Baselines were electronically adjusted to zero absorbance. Electrochemical experiments were conducted with an EG&G PAR 273 potentiostat. Controlled cyclic voltammetry (CV) experiments were carried out in a single-compartment three-electrode cell with a saturated calomel reference electrode (SCE) and a platinum counter electrode. The supporting electrolyte was aqueous 0.1 M KCl at pH 7. The scan rate in all measurements was 50 mV‚s-1.
Results Monolayer Organization and Structure. Contact Angle Measurements. Contact angles of the monolayers as a function of the adsorption time were measured for zinc electrodeposited on a platinum single crystal, making it possible to perform the measurements in a controlled atmosphere and resulting in very reproducible data. We compared these results with contact angles measured on a flat polycrystalline-treated zinc surface. Figure 1a,b reports the contact angles measured as a function of adsorption time for n-decanethiol (DT) and n-octadecanethiol (ODT) adsorbed on electrodeposited zinc. Both thiols give rise to a highly hydrophobic surface after 1 min of incubation. For long adsorption times, the contact angle is 123 ( 3° for DT and 129° ( 3° for ODT. For both monolayers, saturation is reached after 5 min. At shorter adsorption times, lower contact angles are found for DT (60 ( 4°) than for ODT (103 ( 4°). Therefore, the time required for the monolayer to be complete is shorter for the longer alkyl chain.
We report in Table 1 the values for planar substrates and values found in the literature for the same thiols on various metals. The contact angles measured on an electrodeposited zinc surface are significantly higher than on a flat polycrystalline substrate. The electrodeposition of zinc leads to roughness that is likely to trap air during the immersion of the electrode in water. It was shown that on very hydrophobic, rough substrates air bubbles can be trapped in hollows, thereby increasing the contact angle, which is a phenomenon called superhydrophobicity.46 On the flat zinc substrate, the contact angles are still very high and are similar to those measured on gold and silver (Table 1). SAMs of alkanethiols on these two metals are dense and highly organized. We conclude that the zinc surface allows the formation of dense SAMs of CH3(CH2)9-SH and CH3(CH2)17-SH. In addition, high contact angles found after sonication indicate that the molecules are strongly bound to the zinc surface. Molecular Self-Assembly and Orientation by IR Spectroscopy. IRRAS Spectra of CH3(CH2)9-SH and CH3(CH2)17SH on Zn. To characterize the organization of the monolayer and the interaction between the alkyl chains, IRRAS spectroscopy was performed on Zn platelets. To minimize the oxidation of the zinc surface before the adsorption of the monolayer, the polycrystalline zinc substrate was polished in neat thiol under argon for 10 min and directly immersed in the thiol for 48 h. Figure 2 compares the infrared spectra of CH3(CH2)9-SH and CH3(CH2)17-SH adsorbed on zinc with those of the thiols in their liquid phase. The assignments of the different absorption bands are given in Table 2. n-Decanethiol. For a CH3(CH2)9-SH monolayer, the frequencies of methylene stretches νaCH2 (2919 cm-1) and νsCH2 (2850 cm-1) are shifted toward lower energies than those found for the (46) (a) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457. (b) Marmur, A. Langmuir 2004, 20, 3517.
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is no significant influence of the chain length on the molecular packing and organization on the zinc surface. This is in agreement with the literature on thiol SAMs adsorbed on various metals and in which a length of n ) 9 to 10 is sufficient to obtain close packing and self-assembly.4,47 QuantitatiVe Determination of the Molecular Orientation. The selection rule for metallic surfaces implies that the infraredactive modes are those having a component of the vibrational transition dipole moment normal to the surface, Miz. Therefore, the integrated intensity I of a band corresponding to vibrational mode i is proportional to Miz2
I(Mi)# (Mi‚E)2# Miz2
(1)
in which E is the electric field normal to the surface of the metal. Introducing the absorption intensity Ai for vibrational mode i in the bulk isotropic product, we get for two vibrations i and j Figure 3. Definition of angles, rotation in the positive sense for R, and orientation of vibration polarizations (δ ) 35.5°). Table 3. Geometry and Angular Functions of Selected Vibrational Modesa modes
direction of Mi
⊥ to C-CH3 bond, in the C-C-C plane νaCH3 out-of-plane both ⊥ to C-C-C plane and to the C-CH3 bond νsCH2 | H-C-H plane bisecting the HCH angle νsCH3 | to C-CH3 bond νaCH3 in-plane
⊥ to C-C-C plane
νaCH2 a
Fi (R, β) cos R sin δ + sin R cos β cos δ sin R sin β sin R cos β cos R cos δ sin R cos β sin δ sin R sin β
R is the tilt angle, β is the twist angle, and δ ) (180°-109°)/2. Table 4. Values of Twist Angle β for n-Decanethiol and n-Octodecanethiol Monolayers on Zn β
DT/Zn
ODT/Zn
50 ( 2°
51 ( 2°
liquid-phase (Table 1). Such a shift indicates a crystal-like structure in which the alkyl chains are fully extended with a nearly all-trans conformation. Strong interactions between the alkyl chains are responsible for such ordering.4,5,22 To date, the largest shift was obtained on SAMs of the 16-carbon alkanethiol. The vibrational bands are then at 2914-2917 and 2848-2850 cm-1, respectively. They are close to the frequencies characteristic of alkyl chains in the crystal phase. Therefore, the frequencies of νaCH2 (2919 cm-1) and νsCH2 (2850 cm-1) of CH3(CH2)17SH adsorbed on polycrystalline zinc demonstrate that the monolayer is densely packed and organized. Furthermore, the small peak widths at half-height (fwhh), 16.5 and 12 cm-1 of the νaCH2 and νsCH2 peaks, respectively, confirm that most of the alkyl chains are in the trans conformation, with broader peaks being associated with disordered gauche-rich chains.25,26 In addition, the frequency of the νsCH3 vibration of the monolayer is shifted toward higher energies than the corresponding vibration of the molecules in the liquid. This is also characteristic of monolayer organization. The intensities of the methyl vibrations are stronger than the methylene vibrations although there is one methyl for nine methylenes. This is an indication of a highly oriented monolayer with the molecules nearly normal to the surface. n-Octadecanethiol. The qualitative characteristics of the spectrum for an ODT monolayer adsorbed on a zinc surface are similar to those described for a DT monolayer. Therefore, there
( ) [
]( ) [
Miz 2 Fi(R, β) Iiz ) ) Ijz Mjz Fj(R, β)
2
]( )
Mi 2 Fi(R, β) ) Mj Fj(R, β)
2
Ai Aj
2
(2)
where Fi is a function depending on the angle between Mi and E.
Fi(R, β) )
Mi ) cos(Mi, E) Miz
(3)
R and β are the tilt and twist angles, respectively (Figure 3). EValuation of Orientation Angles R and β. Table 3 indicates the direction of the transition dipole moment and the value of Fi/j for the C-H bond stretching. The twist angle β and the tilt angle R are calculated from the ratio between the relative integrated intensities of methyl and methylene vibrations of the molecules adsorbed on the surface (I) and of the neat liquid (A).47,48 The relative integrated intensities of the methyl and methylene groups determined in the monolayer (I) and in the isotropic liquid thiol (A) give the monolayer orientation. The twist angle β is directly deduced from the νaCH2 and νsCH2 intensities and by using the selected angular functions
β ) arc tan
x
I(νaCH2) A(νsCH2)
, 0 e β e π/2
I(νsCH2) A(νaCH2)
(4)
The values of β (Table 4) are similar for the two thiols and are close to the values found in the literature.4-11,48 Two solution couples are found for each of the chosen vibrations (Supporting Information). The differences between the positive angles (R2+ - R1+) are very small whatever the thiol length, whereas the negative ones are quite different. As has been shown,48,49 this difference is related to the presence of gauche defects and should be zero for an all-trans chain. Introducing a number of gauche defects into the calculation should give the same value for R2 and R1. As we show now, the true solution can be found on the basis of physical considerations. (47) (a) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta, Part A 1978, 34, 395. (b) Snyder R. G. J. Chem. Phys. 1976, 47, 1316. (c) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (d) Atre, S. V.; Liedberg, B.; Allara, D. L. Langmuir 1995, 11, 3882. (e) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (f) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (g) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (48) (a) Buffeteau, T. Ph.D. Thesis, University of Bordeaux I, France, 1988. (b) Blaudez, D.; Buffeteau, T.; Desbat, B.; Orrit, M.; Turlet, J. M. Thin Solid Films 1992, 210, 648. (c) Buffeteau, T.; Desbat, B.; Devaure, J.; A Salami, A.; Turlet, J. M. J. Chim. Phys. 1993, 90, 1855-1870. Buffeteau, T.; Desbat, B.; Devaure, J.; Salami, A.; Turlet, J. M. J. Chim. Phys. 1993, 90, 1871-86. (49) Blaudez, D. Ph.D. Thesis, University of Bordeaux I, France, 1993.
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Figure 5. I ) f(V) curves for bare zinc and for CH3(CH2)9-SH adsorbed on Zn.
Figure 4. Representation of gauche defects on the outermost CH2CH2 bond for R > 0 and for the case of CH3(CH2)n-SH with odd n. Table 5. Calculated Values of Tilt Angle r for the Two Vibrational Couples vibrations used νaCH3, νsCH2 νsCH3, νsCH2
tilt angle R
DT/Zn
ODT/Zn
R1+ R1R2+ R2-
14.5° -10° 14.6° -18.9°
18.5° -11.4° 18.6° -26.9°
Table 6. Tilt Angles r+(r1, r2+) and r-(r1-, r2-) and Corresponding Percentages of Gauche Defects
Figure 6. Current intensity at -0.95 V/SCE versus adsorption time.
DT/Zn
IR spectra, which showed that the monolayers are highly organized. On the contrary, the percentage of gauche defects extracted from the negative values of the tilts is higher than 50%, which is not compatible with the same IR spectra. Therefore, the negative solution of the tilt angle has no physical meaning and can be rejected. We consider that the tilt angle of the molecules on the surface is +14.5° for CH3(CH2)9-SH and +18.5° for the CH3(CH2)17-SH monolayer with less than 1% gauche defects. Gauche defects are very rare and are independent of the alkyl chain length. The positive angle indicates that the metal-sulfurcarbon angle has increased compared to its value for a zero tilt angle of the molecule. It is interesting to compare the tilt angle R and its sign as well as the concentration of gauche defects for a monolayer adsorbed on zinc with values found in the literature for different metals. Porter et al.5,11 showed that on silver the tilt angle of the molecules increases from +11 ( 6° to +15 ( 6° when the alkyl chain increases from 12 to 18 carbons. Laibinis et al.10 have studied in great detail the structure of monolayers of different chain lengths on silver and copper. They observed that the tilt angle alternates between positive and negative for even and odd numbers of carbons in the alkyl chain, respectively. The number of gauche defect was not deduced from the IR spectra. On gold (111),4-9,52 the tilt angle with respect to our conventions is always negative whatever the parity of the number of CH2 groups (close to -30° with 45% gauche defects5,10 or ca. 2-5% in our case53). On silver and copper, the tilt angle is about +13°. Therefore, we can conclude that the self-assembled monolayer of alkyl thiol adsorbed on zinc behaves like monolayers on silver and copper (see Discussion).
R+ (good solution) R- (wrong solution)
ODT/Zn
tilt angle
gauche defects ((5%)
tilt angle
gauche defects ((5%)
14.5° -11.9°
0.9% 64%
18.5° -13.8°
1% 77%
EValuation of Gauche Defects. In the SAMs, gauche defects correspond to the rotation of the CH3 group around the outermost CH2-CH2 of the CH3 group. It is generally assumed that at room temperature the gauche defects are located only at the extremity of the alkyl chain.4,50,51 In our case, the latter assumption seems very likely in view of the highly organized nature of the monolayer. Figure 4 clearly shows the influence that a gauche defect located at the extremity of an alkanethiol adsorbed on a surface has on Miz. For a small positive value of R and for an odd number of CH2 groups, the projected transition moment Miz of the in-plane νsCH3 decreases when a trans conformation is changed to a gauche defect. The opposite behavior is observed for the in-plane νaCH3. For both conformations, the Miz component of the outof-plane νaCH3 depends only on the twist angle β. As a consequence, the calculated values of R1( and R2( using the intensities of νaCH3 and νsCH3 vibrations vary with the presence of gauche defects, and their difference expresses the number of gauche defects. Table 6 gives the results for gauche defects in both monolayers for solutions R+(R1+, R2+) and R- (R1-, R2-) The number of gauche defects for both monolayers and for positive tilt angles does not exceed 1%. This is in agreement with the position and width of the νC-H bands observed in the (50) Hautman, J.; Klein, M. L. J. Phys. Chem. 1989, 91, 4994. (51) Prathima, N.; Harini, M.; Rai, R.; Chandrashekara, R. H.; Ayappa, K. G.; Sampath, S.; Biswas, S. K Langmuir 2005, 21, 2364.
(52) Srisvastava, P.; Chapman W. G.; Laibinis P. E. Langmuir 2005, 21, 12171. (53) Our own experiments on CH3(CH2)11SH on gold (111) (annealed Arrandee substrate) exhibit a tilt angle of 22 ( 3° with 6 ( 3% gauche defects using the same procedure, in agreement with MD simulations in ref 51.
8390 Langmuir, Vol. 23, No. 16, 2007
Nogues and Lang
Figure 7. Influence of alkyl chain length on the I ) f(V) curve for adsorption times of (a) 10 min and (b) 48 h (KCl, neutral pH). Table 7. Comparison of Tilt Angles and Signs of Molecules Adsorbed on Different Metals with Our Positive Sign Convention bulk crystallized Ag10,11*
Cu10
Zn (this work)
Hg57
Au4-10
12°
12°
14.5°, 18.5°
0°
27°
n even, 30°; n odd, 0°
R>0 R > 0a; R < 0b 0.44 13°
R>0 R0
R
