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Self-Assembled Monolayers of Thioalkanoate on Ag and Au Surfaces: Hydrolysis and Rearrangement at the Interface Yu-Tai Tao,*,† Subramanian Pandiaraju,† Wen-Ling Lin,† and Li-Jen Chen*,‡ Institute of Chemistry, Academia Sinica, Taipei, Taiwan, Republic of China, and Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, Republic of China Received August 12, 1997. In Final Form: October 27, 1997X Thioalkanoic acids form highly ordered monolayer assemblies upon adsorption on surfaces of evaporated silver and gold. The thiocarboxyl group coordinates through a similar polar covalent bond to the surface as in the case of an alkanethiolate monolayer but exhibits short term stability. Hydrolysis of the thiocarboxyl head group proceeds at the interface to give the corresponding oxy acid and metal sulfide. On a silver surface, the resulting carboxylic acids react with surface oxide and transform into a closely packed alkanoate monolayer, while on a gold surface, the acid molecules rearrange to form discrete crystallites of H-bonded dimer, together with some anhydride formation. The alkyl chains of the crystallites aligned nearly parallel to the surface. Atomic force microscopy shows that the clusters are of nanometer scale dimension.
Introduction The self-assembled monolayers (SAMs) of organic amphiphiles at solid surfaces have attracted much attention in recent years for interests in both fundamental studies as well as potential applications.1 Systems that are known to form well-ordered monolayer thin films include, among others, alkanethiols on transition metal surfaces such as Au, Ag, Cu, Pt, and so forth,2-6 alkanoic acids on native oxide surfaces7-10 of metals such as Ag, Al, and Cu, and chlorosilanes on hydroxylated surfaces11-14 such as SiO2 and Al2O3. Among these systems, alkanethiol on gold is one of the most well studied systems for the sake of its stability and orderliness. Various techniques, including IR spectroscopy,2,5,15 Raman spectroscopy,16 ellipsometry,5 X-ray photoelectron spectroscopy,5 electrochemistry,3,17 X-ray and helium diffraction,18 and scanning probe microscopy,19 have been used to elucidate the †
Academia Sinica. National Taiwan University. X Abstract published in Advance ACS Abstracts, December 15, 1997. ‡
(1) Ulman, A. Chem. Rev. 1996, 96, 1533-1554 and references cited therein. (2) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (3) Finklea, H. O.; Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987, 3, 409-413. (4) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733-740. (5) Laibinis, P.; Whitesides, G. M.; Parikh, A. N.; Tao, Y. T.; Allara, D. L.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (6) Bryant, M. A.; Joa, S. L.; Pemberton, J. E. Langmuir 1992, 8, 753-756. (7) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45-52 and 52-65. (8) Schlotter, N. E.; Porter, M. D.; Bright, T. B.; Allara, D. L. Chem. Phys. Lett. 1986, 132, 93-98. (9) Tao, Y. T. J. Am. Chem. Soc. 1993, 115, 4350-4358. Tao, Y. T.; Lee, M. T.; Chang, S. C. J. Am. Chem. Soc. 1993, 115, 9547-9555. (10) Smith, E. L.; Porter, M. D. J. Phys. Chem. 1993, 97, 8032-8038. (11) Cohen, S. R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986, 90, 3054-3056. (12) Wasserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (13) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 110, 6136-6144. (14) Gun, J.; Sagiv, J. J. Colloid Interface Sci. 1986, 112, 457-472. (15) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370-2378. (16) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284-8293.
detailed structural information of the monolayer system with respect to the bonding scheme, packing arrangement, conformation, stability, and so on. The rather robust film and well-defined structure lead to various applications including sensors,20 corrosion inhibition,21 lithographic resists,22,23 and so forth. In contrast, carboxylic acid monolayers on metal oxide surfaces, being the first SAM reported24 and often used in lubrication technology, are less stable because the interaction is ionic (acid-base) in nature. It nevertheless provides important information about the assembling mechanism and the interplay of various forces in determining the structure.9 It also offers a link between the self-assembling method and the Langmuir-Blodgett technique, where fatty acid salts are frequently used as building blocks. Recently, we reported a unique process of assembling-disruption-reassembling at the surface when the alkanoate monolayer was exposed to HCl or H2S vapor.25,26 A series of protonationdeprotonation reactions induced by these reagents lead to various packing states of the carboxylic acid molecules, as a result of the interplay of various intermolecular interactions and the adsorbate-substrate interaction. No counterpart of such a process was observed on a gold surface, as no appropriate binding chemistry exists between gold and carboxylic acids or alkanethiols. An order-disorder transition in chain packing was observed as a result of heating.27 (17) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335-359. (18) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216-1218. Carnillone, N., III; Chidsey, C. E. D.; Li, J.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (19) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805-2810. Alves, C. A.; Smith, E. L. Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222-1227. (20) Kepley, L. J.; Crooks, R. M.; Ricco, A. J. Anal. Chem. 1992, 64, 3191-3193. (21) Li, Y.; Chailapakul, O.; Crooks, R. M. J. Vac. Sci. Technol. B 1995, 13, 1300-1306. (22) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626-628. (23) Kumar, A.; Whitesides, G. M. Science 1994, 263, 60-62. (24) Bigelow, W. C.; Pickett, D. L.; Zisman, W. A. J. Colloid Interface Sci. 1946, 1, 513. (25) Tao, Y. T.; Hietpas, G. D.; Allara, D. L. J. Am. Chem. Soc. 1996, 118, 6724-6735. (26) Tao, Y. T.; Lin, W. L.; Hietpas, G. D.; Allara, D. L. J. Phys. Chem., in press.
S0743-7463(97)00912-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/06/1998
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Thiocarboxylic acid is more acidic than the corresponding carboxylic acid by one and a half orders of magnitude.28 It is expected to adsorb on basic oxide surfaces like a carboxylic acid does. On the other hand, it contains a thiol group and is expected to adsorb on thiophilic metals such as gold and silver. Here we report our study of selfassembling behavior of long-chain thiocarboxylic acids on gold and silver surfaces and the observation of a facile hydrolysis reaction by ambient moisture occurring at the interface and the structural reorganization within monolayer film following the hydrolysis. Experimental Section Materials: Gold (99.99%), silver (99.99%), and chromium (99.99%) were obtained from Johnson Matthey Company (Ward Hill, MA). Silicon wafers were purchased from Silicon Sense (Nashua, MA). The hexadecane, obtained from the Aldrich Company, was dried by percolating it through an alumina column twice. The thiocarboxylic acids CnH2n+1COSH (n ) 7, 15, 19, 21) were prepared according to a literature procedure from the corresponding oxy acids via the thioanhydrides.29 The products were purified by either recrystallization or column chromatography and were characterized by NMR and Mass analysis. Monolayer Preparation and Characterization. The metal substrates were prepared by vacuum deposition onto 2-in. silicon wafers using established procedures.5 The substrates were exposed to laboratory ambient as briefly as possible (less than 10 min in taking ellipsometry reading) before being immersed into the solution. Monolayers were prepared by selfassembly from 1 mM hexadecane solutions. An oleophobic surface was obtained instantaneously. Extended immersion up to several hours to overnight resulted in a wet surface. Rinsing the wet surface with organic solvent usually gave spectra characteristic of a disordered film, presumably due to the hydrolysis of the head group. Typically, an immersion time of 5-10 min was used and the surface was characterized without rinsing with organic solvent. Film thicknesses were determined by single-wavelength ellipsometry (632.8 nm and 70° angle of incidence) using a real refractive index of 1.5. Contact angles were measured on a Rame-Hart NRL Model 100 goniometer using hexadecane and purified water as probe liquids. Infrared reflection spectra (IRS) were obtained using published methods5 and are reported as -log(R/R0), where R and R0 are the reflectivities of the film-covered sample and a clean gold-coated reference wafer, respectively. Atomic Force Microscopy. All measurements were carried out with a NanoScope IIIa commercial scanning probe microscope (Digital Instruments, Inc., Santa Barbara, CA). The measurements were performed in air at room temperature by use of an etched silicon cantilever with resonant frequency 250-300 kHz. Tapping mode atomic force microscopy was applied to image the rearrangements of the C19H39COSH SAM on the gold surface. All images were recorded at the scanning rate 1.5-2 Hz and were processed using a “planefit” program in NanoScope IIIa.
Results and Discussion 1. Silver Surface. The self-assembling process was carried out using hexadecane as solvent at a temperature of 20 °C. An Evaporated silver surface was dipped into a 1 mM solution of the acid, upon which an oleophobic surface was obtained almost instantaneously for all the chain lengths of acid examined (C8, C16, C20, C22, C24). After an incubation time of 5-10 min, the substrate surface was retrieved from the solution for characterization. The characteristics of the long-chain acids (C16) were very similar. The result is exemplified by nthioeicosanoic acid, C19H39COSH. Contact angles of 51 ( 1° for hexadecane and 110 ( 2° for H2O were observed, (27) Ulman, A. Adv. Mater. 1991, 3, 298-303. (28) Patai, S., Ed. The Chemistry of Carboxylic Acids and Esters; Interscience: New York, 1969. (29) Shin, H. C.; Quinn, D. M. Lipids 1993, 28, 73-74.
Figure 1. External reflection absorption IR of a C19H39COSH monolayer on Ag: (a) immediately after preparation; (b) 4 h after preparation; (c) 1 day after preparation; (d) 3 days after preparation.
agreeing with a highly ordered and methyl-terminated monolayer. Ellipsometry measurement gave a thickness of ∼28 Å, indicating a monolayer coverage. The reflection absorption IR spectra for the monolayers were shown in Figure 1a. The νs(CH2) appears at 2850 cm-1, and the νa(CH2) band, at 2917 cm-1, respectively, suggesting a crystalline packing of chains within the film.2 The intensity pattern observed in this region resembles that of an n-eicosanethiolate monolayer on Au.5 In the lowfrequency range, a weak broad band around 1700 cm-1 is present and assigned as the carbonyl stretch for the thiocarboxylate. Besides the scissoring band for CH2 at 1460 cm-1 and the deformation band δs for CH3 at 1380 cm-1, no other significant peak was observed. It is expected that sulfur is coordinated to the metal surface to form an ester-like linkage.28 The very weak presence of a carbonyl peak can be the result of an orientation effect such that the carbonyl group aligned nearly parallel to the surface. The bonding between the head group and the surface is more than just an ionic interaction, as shown by the following: (1) Competitive adsorption experiments between the thiocarboxylic acid and an alkanethiol from solutions containing various amounts of C19H39COSH and C20D41SH showed similar surface composition and solution composition, with a slight preference for alkanethiol (Figure 3),30 while a competition between thiocarboxylic acid and oxycarboxylic acid from a solution containing equimolar amounts of C15H31COSH and C15D31COOH yields a surface containing only the thioacid. (2) Exposure of the thioacid monolayer to HCl or H2S vapor did not cause protonation and displacement of the head group from the surface as that of an oxy acid monolayer would.25,26 However, the IR spectrum of the monolayer is apparently time-dependent. A series of spectra taken after the monolayer formation are also shown in Figure 1. The (30) The surface composition was calculated on the basis of the intensities of the methylene peak at 2918 cm-1 from the thioacid and the CD2 peak at 2221 cm-1 from the deuterated alkanethiol species.
SAMs of Thioalkanoate on Ag and Au Surfaces
intensities for the methylene stretches notably decreased after 3 days, and the frequency shifted slightly to an even more ordered state of 2916 and 2850 cm-1, respectively. In the low-frequency region, a band at 1404 cm-1 grew in its intensity with time and remained nearly constant after 3 days. A small band at around 1512 cm-1 became visible as well. Also present was a series of progressional bands between 1350 and 1200 cm-1, implying highly trans zigzag conformation order in the new film state.31 These spectral features are reminiscent of the n-alkanoate monolayer on silver.9 Exposure of the surface to HCl vapor indeed leads to the formation of free carboxylic acid and concomitant rearrangement, as occurred with the alkanoate monolayer reported before.25,26 Thus the thiocarboxylate monolayer has transformed into a carboxylate one with time. It is noted that the structure of the resulting film is somewhat different from that obtained from direct adsorption of the oxy acid to the silver surface, which has a symmetrical binding and a molecular chain tilt of about 20° from the surface normal.25 The transformed carboxylate monolayer has a tilted binding group, as suggested by the presence of both νs(CO2-) at 1404 cm-1 and νa(CO2-) at 1512 cm-1, and a less tilted molecular chain, as suggested by lower methylene stretch intensities. Apparently the sulfur atom has changed the surface property such that a different binding interaction and thus packing are obtained. It is noted that the same spectral feature was observed for a monolayer restored from an alkanoate monolayer that had been exposed to H2S and undergone a reorganization.26 A less tilted alkanecarboxylate monolayer (∼14°) was obtained after the reorganization. An analogy can be drawn between the two systems: in both systems, an alkanecarboxylate monolayer on a sulfurized silver surface was obtained eventually, with sulfurization resulting from H2S exposure of the metal previously reported, while, in the present case, the sulfurization resulted from hydrolysis of the thiocarboxyl head group at the surface. In a very few cases, a transient species was observed, which was identified as carboxylic anhydride from the characteristic peaks for carbonyl at 1742 and 1802 cm-1, respectively. This is shown in Figure 2. From the analogy in the spectral pattern with an earlier report,25,26 that is, a dramatic increase in the methylene stretch intensity, the anhydride molecules should be lying nearly parallel to the surface. Eventual conversion to an upright alkanoate monolayer was also observed with ambient storage. This would suggest the anhydride molecules further hydrolyze and self-organize to the oriented assembly again at the surface. The condition under which the anhydride formed first is not clear. The formation of anhydride was, nevertheless, constantly observed as a minor competing reaction for the same monolayer on the surface of gold and will be discussed later. The shorter chain acid (C8) underwent similar transformation but exhibited typical characteristics of disordered monolayers in that the methylene stretching frequency was at 2929 and 2855 cm-1 for νa and νs, respectively. 2. Gold Surface. For a short-chain derivative, the C8 thioacid, the gold surface emerged wet from the solution. A characteristic spectral pattern of the disordered film was observed for the surface after rinsing with organic solvent. However, well-defined monolayer formation was observed for long-chain acids (gC16). An oleophobic as well as hydrophobic surface was obtained instantaneously. Results for C19H39COSH were presented here as representative. A hexadecane contact angle of 50° and a water (31) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1963, 19, 85-117.
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Figure 2. Formation of transient anhydride species during hydrolysis of C15H31COSH on Ag: (a) immediately after preparation; (b) 1.5 h after preparation; (c) 12 h after preparation; (d) 2 days after preparation.
Figure 3. Surface composition as a function of solution composition for C19H39COSH for adsorption from a mixture of C19H39COSH/C20D41SH: (b) on a silver surface; (4) on a gold surface.
contact angle of 108° were obtained. The ellipsometric thickness was ∼24 Å. As shown in Figure 4a, a crystalline monolayer film was also suggested from the peak frequencies for the methylene stretches at 2918 and 2850 cm-1. The surface linkage is similar to that formed on the surface of silver, that is with sulfur coordinated to the surface to give a monodentate, ester-like linkage, as indicated by the carbonyl stretch at 1700 cm-1. The adsorption mechanism is assumed to take place through a similar process as that of alkanethiol adsorption on gold, that is oxidative addition of the S-H bond followed by reductive elimination of the hydrogen.17 The bonding between thiocarboxylate and gold is apparently of a covalent nature. In a competitive experiment where the adsorption was carried out from a solution containing a
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Figure 4. External reflection absorption IR of a C19H39COSH monolayer on Au: (a) immediately after preparation; (b) 90 min after preparation; (c) 4 h after preparation; (d) 20 h after preparation.
mixture of C20D41SH and C19H39COSH, the surface concentration of the monolayer is similar to the solution concentration, as shown in Figure 3. This suggests that the thioacid moiety has a similar binding strength to that of alkanethiol molecules. A unique time-dependence of the spectra for the monolayer on gold was observed, as shown in Figure 4. The intensities of the methylene stretching modes νs(CH2) and νa(CH2) increase dramatically 1 day after the sample was stored in ambient conditions. The peak frequencies are still indicative of the crystalline state (2849 and 2918
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cm-1) all through the change. In the lower frequency region, many new bands appeared, particularly the growing of peaks at 1805, 1741, 1701, 1473, 1300, and 942 cm-1, respectively. A comparison with the spectral pattern observed for crystallites of H-bonded carboxylic acid dimers obtained from an alkanoate monolayer exposed to HCl or H2S leads to the conclusion that similar crystallites of the H-bonded carboxylic acid dimer formed in the present system, as a result of hydrolysis of the thiocarboxyl group at the interface. In addition to the bands associated to the acid dimer (1701, 1300, 942 cm-1), there are two bands at 1805 and 1741 cm-1, which, after comparison with the authentic KBr sample, are assigned to the carbonyl stretch of the carboxylic anhydride. This is supported by the observation that exposure of the surface to ammonia vapor leads to an amide peak at 1640 cm-1 at the expense of these anhydride peaks. Rinsing of the surface with THF totally removes the materials on the surface, which suggests that film materials are only weakly adsorbed on the surface. The results suggest that closely packed monolayers can be formed from thiocarboxylic acids on both Au and Ag surfaces, although different mechanisms may be involved. The SAMs, nevertheless, exhibit only temporal stability. The metal salts of thioacids are known to be unstable and slowly decompose to give metal sulfides.28 A similar reaction can be expected to occur at the interface, which can be described by the equation
RCOSM(surf) + H2O f RCOOH + MSH(surf) M ) Au and Ag It can be expected that exclusion of moisture will retard the reaction. It is indeed the case. Thus the monolayer sample can be stored under nitrogen for several days without change. Exposure to hot steam results in fast hydrolysis. The stability of the thiocarboxylate monolayer (time required to result in complete hydrolysis) depends also on temperature.32 Due to the change of chemical species at the interface, a change of intermolecular interaction and adsorbate-
Figure 5. Schematic representation of the structure of a thioacid monolayer on Ag and Au before and after hydrolysis.
SAMs of Thioalkanoate on Ag and Au Surfaces
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substrate interaction results, followed by a structural change. It is further complicated by the substrate chemistry itself. On a silver surface, where an oxide layer is available or forming continuously at the interface, the free acids react directly with the oxide surface to form alkanecarboxylates, which organize themselves into a monolayer, probably with minimum perturbation of the structure. Thus the net process is the formation of a carboxylate monolayer and a metal sulfide monolayer, together with a transformation of the ordered thiocarboxylate monolayer into an oxycarboxylate monolayer. On the gold surface, where no basic oxide layer is present, the interaction between the head group (carboxyl) and the substrate (gold sulfide) is much weaker. The free acids undergo a dewetting process by forming discrete crystallites of the cyclic H-bonded dimers due to the stronger driving force of the H-bond interaction. The difference between this process and the process reported earlier25,26 is that the reorganization is gradual, limited by the kinetics of hydrolysis. No clear trend can be observed for the rate of hydrolysis among the long-chain acids. This is related to the permeability of the film to water vapor. If diffusion through the hydrocarbon matrix is the major pathway for water to reach the interface and react, a strong dependence on the chain length is expected. If the moisture reaches the interface from defects and domain boundaries, a strong dependence on the density of defects and the domain boundaries is expected. The defect density and chain length can be interrelated, and thus the situation can be complicated. It is nevertheless noted that, in LangmuirBlodgett films, permeation was suggested to be through defects.33 From the similar spectral pattern in earlier reports, it is also suggested that the crystallites exhibit an orientational preference such that the crystal axis lies nearly parallel to the surface. This is favored by a greater van der Waals interaction between the chains and the surface. The formation of carboxylic anhydride appears as a competing side reaction during the hydrolysis, a suggested mechanism is shown in the following scheme. A summary of the structures proposed for the thioacid monolayers at the two metal surfaces is presented in Figure 5.
Direct imaging of the crystallites by AFM was performed. Due to the roughness of the normally deposited gold substrate, direct observation of crystallite formation was difficult. However, a template-stripped gold substrate provides a smooth enough surface for direct imaging.34 Figure 6a shows an AFM image of a gold substrate prepared against a mica template and then delaminated. The grain size of gold is as large as 0.3 µm with a quite flat surface. A cross-section analysis of this image reveals
Figure 6. AFM images of (a) the gold reference, (b) the freshly prepared monolayer of C19H39COSH on gold, and (c) the film surface after 3 days under ambient conditions.
(32) The hydrolysis reaction rate is apparently temperature-dependent, as at 30 °C, the preparation of an oleophobic surface was difficult, presumably because hydrolysis already proceeded. (33) Gaines, G. L., Jr.; Ward, W. J., III. J. Colloid Interface Sci. 1977, 60, 210-213. (34) Hegner, H.; Wagner, P.; Semenza, G. Surf. Sci. 1993, 291, 3946.
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that the surface roughness in these gold plateaus is less than 1 nm, which gives an ideal surface for observation of the aggregation of C19H39COSH. Figure 6b shows a freshly prepared sample of the C19H39COSH SAM on gold. Essentially, there is no difference between the images of the gold surface and the freshly prepared C19H39COSH SAM on the gold surface. It is believed that C19H39COSH forms a complete monolayer on the gold surface. Figure 6c shows the image after the sample was left under ambient conditions for 3 days. It can be seen that clusters of various sizes appear on the gold surface. The major portion of the clusters has a size as large as 2 nm in height and 80 nm in horizontal distance. In conclusion, the thiocarboxylic acid can self-assemble on the surface of silver and gold to form a highly ordered and strongly coordinated monolayer. But the film is of limited temporal stability under ambient conditions due
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to a spontaneous hydrolysis reaction. The head group transformed into a carboxyl group by ambient moisture. The resulting acid molecules either react directly with an available basic surface oxide (such as on the silver surface) and form a well-ordered and crystalline carboxylate monolayer or, on the gold surface, reorganize to form hydrogen-bonded dimers, which aggregate into nanometer-sized crystals. The adsorption behavior on other metals and utilization of the temporal stability (such as a temporary template in surface patterning22,23) are vigorously pursued in our laboratory. Acknowledgment. We thank National Science Council of Republic of China for financial support of the work. LA970912N
