Langmuir 1997, 13, 3397-3403
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Adsorption of 11-Mercaptoundecanoic Acid on Ni(111) and Its Interaction with Probe Molecules Andrew D. Vogt, Taejoon Han, and Thomas P. Beebe, Jr.* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 Received October 18, 1996
The gas-phase adsorption of 11-mercaptoundecanoic acid (HOOC(CH2)10SH) on Ni(111) in ultrahigh vacuum was studied using angle-dependent X-ray photoelectron spectroscopy (ADXPS) and temperatureprogrammed desorption (TPD). We present evidence which shows that 11-mercaptoundecanoic acid is adsorbed to Ni(111) via the sulfur atom with the carboxylic acid group disposed away from the surface. The monolayer thickness of this molecule, 10.4 ( 1.7 Å, estimated by photoelectron attenuation in ADXPS, was comparable to ellipsometric measurements of related n-alkanethiols on Au(111). The interaction of polar (CH3OH) and nonpolar (n-C6H14) probe molecules with this acid-terminated surface showed that the former interact more strongly than the latter as studied by TPD. A Redhead analysis was used to analyze the TPD spectra of CH3OH and n-C6H14 desorbing from this thiol-covered, acid-terminated Ni(111) surface. The calculated low-coverage desorption energy for CH3OH desorption from HOOC(CH2)10SH/Ni(111) was 41 kJ mol-1. The calculated low-coverage desorption energy for n-C6H14 desorption from HOOC(CH2)10SH/ Ni(111) was 37 kJ mol-1.
Introduction Self-assembled monolayers (SAMs) formed from methylterminated n-alkanethiols adsorbed on Au(111) have been the most extensively characterized SAM system.1 Gold substrates have an advantage over other substrates because they have a strong preferential interaction with sulfur and are the least reactive toward oxidation in ambient conditions. SAMs of n-alkanethiols on more reactive substrates such as Ag,2,3 Cu,2 and GaAs4 have been produced and observed but not characterized extensively. These other substrates are more appealing in some respects than gold for the study of SAMs because they have some advantages for industrial applications, including higher durability and lower cost. In addition, investigations of n-alkanethiols on reactive substrates may provide further fundamental understanding of the role of the substrate in the self-assembly process. Molybdenum and nickel single crystals have been employed as model catalytic substrates to help develop an understanding of their role in the mechanism for hydrodesulfurization.5,6 These studies have focused on short-chain n-alkanethiols (i.e., HSCmH2m+1, m < 5) and, in particular, methanethiol.7-9 Not surprisingly, the interaction of these thiols with molybdenum and nickel surfaces occurs at the sulfur atoms, resulting in adsorbed thiolate and adsorbed hydrogen at surface temperatures of less than 150 K. Parker and Gellman have suggested, using temperature-programmed desorption (TPD) and sulfur coverage studies, that short-chain n-alkanethiols self-assemble in a hexagonal and upright fashion on Ni(111).10 * Tel: (801)581-5383. FAX: (801) 581-8433. E-mail: beebe@ chemistry.chem.utah.edu. X Abstract published in Advance ACS Abstracts, June 1, 1997. (1) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (2) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (3) Li, W.; Virtanen, A.; Penner, R. M. J. Phys. Chem. 1994, 98, 11751. (4) Sheen, C. W.; Shi, J.-X.; Martensson, J.; Parikh, A. N.; Allara, D. L. J. Am. Chem. Soc. 1992, 114, 1514. (5) Schuman S. C.; Shalit, H. Catal. Rev. 1970, 4, 245. (6) Roberts, J. T.; Friend, C. M. J. Am. Chem. Soc. 1987, 109, 3872. (7) Wiegand, B. C.; Uvdal, P.; Friend, C. M. Surf. Sci. 1992, 279, 105. (8) Castro, M. E.; White, J. M. Surf. Sci. 1991, 257, 22. (9) Neff, L. D.; Kitching, S. C. J. Phys. Chem. 1974, 78, 1648.
S0743-7463(96)01011-6 CCC: $14.00
The physical and chemical differences between gold and nickel suggest that the adsorption and assembly of n-alkanethiols on these two surfaces may occur by different adsorption mechanisms. Although the nickel-nickel distance (2.49 Å) is smaller than the gold-gold distance (2.88 Å) for the (111) surface, the molecular diameter of self-assembled n-alkanethiols (4.97 Å)11 is a close match with the S-S spacing for dissociated H2S on Ni(111) as determined by scanning tunneling microscopy (≈5 Å).12 If n-alkanethiols adsorbed on Ni(111) have the same overlayer structure as n-alkanethiols adsorbed on Au(111) [i.e., (x3 × x3)R30°],13,14 then interchain steric effects in the SAM would be presumed to be present and large. The S-S overlayer structure for H2S on Ni(111), (5x3 × 2),12 and the spacing suggest that n-alkanethiols adsorbed on Ni(111) initially order to minimize their repulsive interactions upon chemisorption of the sulfur head group and then perhaps proceed to order in a similar mechanism as in the formation of n-alkanethiols on Au(111). ω-Substituted n-alkanethiols bind to the gold surface with Au-S bonds, the ω-terminal groups disposed away from the gold substrate.15 The present work is motivated by the fact that this orientation allows for the production of “organic surfaces” exposing a variety of ω-terminal groups. These surfaces can then be studied in terms of their interactions with various chemical probes. Weakly interacting probe molecules and neutral metal atoms can form clusters on the monolayer, can penetrate into the monolayer, or can react with the terminal group of the monolayer.16-18 In principle, weakly interacting chemical probes that do not penetrate into the monolayer can provide information about the orientation of n-alkanethiols adsorbed on nickel using TPD. In this report, the adsorption of an eleven-carbon, acid-terminated straight(10) Parker, B.; Gellman, A. J. Surf. Sci. 1993, 292, 223. (11) Ulman, A.; Eilers, J. E.; Tillman, N. Langmuir 1989, 5, 1147. (12) Ruan, L.; Stensgaard, I.; Besenbacher, F.; Lægsgaard, E. J. Vac. Sci. Technol. B 1994, 12, 1772. (13) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805. (14) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (15) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (16) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 4739. (17) Duevel, R. V.; Corn, R. M. Anal. Chem. 1992, 64, 337. (18) Jung, D. R.; Czanderna, A. W. Crit. Rev. Solid State Mater. Sci. 1994, 19, 1.
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chain thiol on Ni(111) is discussed. The results suggest that the thiol molecules self-assemble with the -COOH terminal group extended away from the surface. X-ray photoelectron spectroscopy (XPS) was used to study the adsorption and orientation of 11-mercaptoundecanoic acid on Ni(111). Angle-dependent X-ray photoelectron spectroscopy (ADXPS) was exploited for its ability to nondestructively depth profile the overlayer of surface adsorbates to obtain an orientation and approximate overlayer thickness for 11-mercaptoundecanoic acid on Ni(111). TPD was also used to complement the ADXPS data and make an interfacial comparison of the interaction of polar and nonpolar probe molecules with 11-mercaptoundecanoic acid adsorbed on Ni(111). Experimental Procedures Experiments were completed in a custom-built ultrahigh vacuum (UHV) surface analysis chamber.19 The UHV chamber contains a hemispherical analyzer, an Al KR (1486.7 eV) X-ray source, an electron gun, a collimated capillary array doser, and a quadrupole mass spectrometer. The base pressure of the chamber is 99.9%, Fisher Scientific) were purified by three freezepump-thaw cycles in a stainless steel, turbo-pumped gas handling system. Their purity was verified in vacuo by mass spectrometry. Both methanol and n-hexane were dosed from a gas handling system, connected directly to the UHV chamber, through a microcapillary array doser to ensure uniform dosing across the substrate with a high local flux (≈2.0 × 1013 molecules s-1 cm-2) and minimal background fluence. Fluences for methanol and hexane in these experiments ranged from 5.2 × 1013 to 5.2 × 1015 molecules cm-2. One monolayer is ≈2.4 × 1015 molecules cm-2 for methanol and ≈4.5 × 1014 molecules cm-2 for hexane, based on a close-packed structure. X-ray Photoelectron Spectroscopy. Survey XPS spectra were acquired with a constant pass energy Ep of 100 eV (FWHM, ∼1 eV) while the high-resolution XPS spectra were recorded with a constant pass energy Ep of 50 eV (FWHM, ∼0.5 eV). The Al anode power for all XPS scans was 240 W. Both survey and high-resolution XPS spectra were acquired for HS(CH2)10COOH adsorbed on Ni(111) [hereafter referred to as HOOC(CH2)10SH/Ni(111)] at various takeoff angles. The binding energies of the S 2s, S 2p, C 1s, O 1s, and Ni 3p XPS peaks were referenced to the Ni 2p photoelectron peak at 852.3 eV, and compared to reference spectra. The C 1s and O 1s photoelectron peaks obtained at a takeoff angle of 90° were fitted with Gaussian curve fitting functions with a Lorentzian contribution of 10% and integrated using commercial software to obtain elemental quantitation.22 Each peak was also fitted to the minimum number of C 1s and O 1s atom contributions in 11-mercaptoundecanoic acid using an estimated FWHM ) 0.80 eV and by providing initial guesses of the peak areas and peak centers, followed by an iterative integration process. There were no constraints imposed on the iterative fitting routine. In TPD experiments, the substrate was heated resistively via a button heater (SpectraMat) and monitored by a chromel/alumel thermocouple spot welded to the side of the nickel crystal. In all cases a linear heating rate was employed through feedback control.19 The masses resulting from the thermal desorption process were detected by a computer-controlled, multiplexed quadrupole mass spectrometer with a 5-mm entrance aperture located 5 mm from the front face of the Ni(111) crystal. In each experiment, the thiol-covered substrate was cooled by liquid nitrogen to e120 K before the adsorbate of interest was dosed. TPD was carried out only to T e 300 K since the 11-mercaptoundecanoic acid begins to dissociate on the Ni(111) surface at T ≈ 360 K. This was ascertained by monitoring desorption species in control experiments on the undosed HOOC(CH2)10SH/Ni(111) surface, which exhibited H2 and other desorption products beginning at T ≈ 360 K (spectra not shown). Results and Discussion Adsorption of HOOC(CH2)10SH on Ni(111). The XPS survey spectrum of HOOC(CH2)10SH/Ni(111) (see Supporting Information) revealed a C:O:S atomic percent ratio, determined using peak areas and appropriate sensitivity factors, but uncorrected for photoelectron attenuation, of 81:12:7,23 with nickel and no other detectable species. The C:O atomic ratio obtained from the survey spectrum was ∼5.6:1 [cf. C:O ratio from C 1s and (22) Microcal Origin 3.5. (23) The stoichiometric ratio of C:O:S for 11-mercaptoundecanoic acid is 79:14:7.
Adsorption of 11-Mercaptoundecanoic Acid on Ni(111)
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Figure 1. C 1s and O 1s high-resolution XPS spectra recorded at a 90° takeoff angle for HOOC(CH2)10SH adsorbed on Ni(111) at room temperature. (a) The C 1s high-resolution XPS spectrum was fitted with three peaks (see text), revealing components at 285.0 eV for the methylene carbon, 287.1 eV for the thiol carbon and the β-carbon of the carboxyl carbon, and 289.1 eV for the carboxyl carbon.24-26,43 (b) The high-resolution XPS spectra for the O 1s region was fitted with two peaks (see text) and revealed components at 533.6 eV for the hydroxyl oxygen and 531.9 eV for the carboxyl oxygen.26
O 1s XPS high-resolution spectra (Figure 1), 5.8:1], and was calculated using the peak areas and respective sensitivity factors only. The stoichiometric C:O ratio is 5.5:1. High-resolution scans of the C 1s and O 1s photoelectron regions, shown in Figure 1, were fitted with multiple Gaussians with a Lorentzian contribution of 10% of three and two peaks, respectively. In 11-mercaptoundecanoic acid there are three carbon types that correspond to the methylene, the carboxyl, and the thiol carbons and two oxygen types that correspond to the carboxyl and hydroxyl oxygens. It can be seen that under the raw peak in the C 1s high-resolution XPS spectrum, in Figure 1a, there are at least two distinct components, one at 285.0 eV which was assigned to the methylene carbon24 and the other at 289.1 eV which was assigned to the carboxyl carbon. From the knowledge of the molecule adsorbed onto the nickel surface and to improve the overall fit of the raw peak a third peak was added. Numerous fits using identical fitting conditions yielded similar peak fits between 286.2 and 287.1 eV. In Figure 1a, we have shown the fit that resulted in a peak at 287.1 eV. The area under the peak at 287.1 eV and the peak’s position (cf. 286.0287.1 eV from the literature, ref 24) suggests that the fitted peak has a contribution from the thiol-carbon (-CS-) and perhaps from the β-carbon of the carboxyl carbon (-CH2COOH). In Figure 1b, curve fitting of the O 1s XPS spectrum resulted in two peaks at 533.6 and 531.9 eV for the hydroxyl and the carboxyl oxygens, respectively.25,26 Although the peak-area ratio between the two oxygen types is 1.2:1 and not 1:1 as expected, there was sufficient carboxyl-oxygen signal to suggest that the acid was adsorbed on the Ni(111) surface in a manner that preserved its stoichiometric values. Using the same curve-fitting routine to fit the high-resolution XPS spectrum of the S 2p region (not shown), the fit of this region resulted in two peaks at 162.5 and 163.7 eV. These were assigned to the S 2p3/2 and S 2p1/2 photoelectrons, respectively. The peak-area ratio between the S 2p3/2 and S 2p1/2 components was 2:1. Thiols bound to a gold surface as thiolates have S 2p binding energies of ∼162 eV, whereas unbound thiols (e.g., multilayer) have binding energies of 164-165 eV. The S 2p3/2 peak of HOOC(CH2)10SH/Ni(111) is close to the S 2p3/2 binding energy for alkanethiols on gold;27,28 this is supporting evidence (24) XPS binding energies are commonly assigned to chemical states, given some knowledge of the system, and when literature values have been well documented. Chemical shifts for E(COOH) ) 288.7-289.2 eV, E(C)O) ) 531.1-531.8, E(C-OH) ) 532.3-533.3 eV. See refs 15, 21, 26, and 27. (25) Desimoni, E.; Casella, G. I.; Morone, A.; Salvi, A. M. Surf. Interface Anal. 1990, 15, 627. (26) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365.
Figure 2. XPS peak area ratios versus takeoff angle for O:C, O:S, and S:C.
for the formation of a thiolate-bound species to nickel.29 Furthermore, the observed Ni 2p3/2-Ni 2p1/2 peak splitting (∼17.3 eV), measured from the spectrum acquired at a takeoff angle of 90°, suggests an oxide-free nickel surface (∼17.4 eV).21 This evidence and further observations below support the suggestion that there is negligible interaction of the carboxyl end of 11-mercaptoundecanoic acid with the nickel surface. Additionally, ADXPS was performed to obtain information regarding the structural orientation of HOOC(CH2)10SH/Ni(111). When the takeoff angle is low (e.g., 10°, defined as the angle between the outgoing electron and the surface plane), there is a lower relative sampling of electrons originating from deeper in the monolayer, making the measurement more surface-sensitive, while at higher takeoff angles (e.g., 90°) there is a higher relative sampling of electrons originating from deeper in the monolayer, making the measurement less surface-sensitive. Figure 2 describes ADXPS results for 11-mercaptoundecanoic acid adsorbed on Ni(111). The curves are meant to show the trends in the data; the error bars were calculated with the areas under the peaks of the respective high-resolution XPS spectra. When the takeoff angle was increased, the XPS peak-area ratios for O:C and O:S decreased, while XPS peak-area ratio for S:C increased, indicating that the monolayer was oriented with the (27) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (28) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (29) Rufael, T. S.; Huntley, D. R.; Mullins, D. R.; Gland, J. L. J. Phys. Chem. Soc. 1995, 99, 11472.
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carboxylic acid group extended away from the surface and that the sulfur end was oriented toward the surface. HOOC(CH2)10SH Overlayer Thickness Adsorbed on Ni(111). Thin-film thicknesses are commonly determined by ellipsometry15 and occasionally by scanning tunneling microscopy,30 while ADXPS has also been used for this purpose.15,31 The XPS signal intensity of a photoelectron originating from a substrate atom (Is) is a function of the overlayer thickness d through which it must pass to reach vacuum and can be described by the following equation32
Is ) Is° exp[-d/λ(Es) sin θ]
(1)
where Is° is the XPS signal intensity from the pure substrate, λ(Es) is the inelastic mean free path of the substrate electron with a kinetic energy Es, and θ is the takeoff angle. This signal is usually normalized to another substrate reference signal Ir, which is different from Is [i.e., λ(Er)]. The thickness is given by
d)
λs(Es)λr(Er) λs(Es) - λr(Er)
( )
sin θ ln
IsIr° Is°Ir
(2)
Peak areas were used for the analysis. Is and Ir were chosen to be two nickel peaks, Ni 2p3/2 (KE ) 634.4 eV) and Ni 3p (KE ) 1418 eV). Is°, the signal from Ni 2p3/2 in pure Ni, and Ir°, the signal from Ni 3p in pure Ni, were measured from the XPS spectrum of clean Ni(111) under the same conditions. The inelastic mean free path was estimated from the following equation:33
λ(Ei) ) kEip
(3)
where k is a constant that depends on the type of overlayer through which the photoelectron must pass (i.e., k ) 0.31 for n-alkanethiols)34 and p is a constant, which was determined experimentally for n-alkanethiols on three different substrates and is approximately 0.67 ( 0.11.34 Since the substrate signal is normalized with respect to another substrate signal, as described in eq 2, the constant k is independent of the overlayer thickness and drops out of the analysis. The calculated values for λ(Ni 2p3/2) and λ(Ni 3p) through the range of p, using eq 3, vary from 11.5 to 47.6 Å and from 18.0 to 89.0 Å, respectively. The large ranges in the mean free paths arise from the large error associated with p which appears in the exponent; these were propagated through the calculation and are reflected in the error bars on the thickness reported below. The values for λ(Ni 2p3/2) and λ(Ni 3p) at p ) 0.67, the most probable value, are 23.4 ( 2.6 and 40.1 ( 4.4 Å, respectively, which corresponded to a monolayer thickness of 10.4 ( 1.7 Å. Although no ellipsometric data have been obtained for 11-mercaptoundecanoic acid on nickel surfaces, the thicknesses obtained above by ADXPS for the Ni(111) system are reasonable for alkanethiols of this kind. The nominal length of all-trans 11-mercaptoundecanoic acid is 15.3 Å.35 The measured thickness of 10.4 ( 1.7 Å implies a methylene chain tilt angle of 43° ( 9° for 11-mercaptoundecanoic acid adsorbed on Ni(111). Values for the (30) Han, T.; Beebe, T. P., Jr. Langmuir, 1994, 10, 2705. (31) Leavitt, A. J.; Williams, J. M.; Wenzler, L. A.; Beebe, T. P., Jr. J. Phys. Chem. 1994, 98, 8742. (32) Briggs, D.; Seah, M. P. Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; John Wiley & Sons, Ltd.: New York, 1990. (33) Wagner, C. D. Anal. Chem. 1977, 49, 1282. (34) Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1991, 95, 7017.
inelastic mean free path for the Ni 2p3/2 and Ni 3p photoelectrons were calculated based on the previously reported 16-mercaptohexadecanoic acid monolayer thicknesses on Au(111) of approximately 15 Å,15 using eq 3, to be 30.3 and 53.6 Å, respectively (for p ) 0.71). These values lie close to the value for the inelastic mean free paths given by the most probable constant p, of 0.67, suggesting that 11-mercaptoundecanoic acid is densely packed on Ni(111). Adsorbate Interactions of Methanol and Hexane with Clean Ni(111) and the Acid-Terminated Ni(111) Surface. TPD can be used as a chemical means by which to probe the structure and reactivity of an adsorbate/ substrate system. In the context used here, “structure” refers to layering, if any, along the axis perpendicular to the surface, and to the integrity of the self-assembled monolayer. Adsorption of a chemical probe molecule and its subsequent desorption from that adsorbate/substrate system can yield quantitative energetic information about functional groups present at the interface. In what follows, methanol and hexane were used to study the HOOC(CH2)10SH/Ni(111) system in this manner. We chose both a polar hydrogen bonder (CH3OH), and a nonpolar, non-hydrogen bonder (n-C6H14) to interact with the ω-terminated polar carboxylic acid groups of the SAM. Although their surface chemistry is completely different on clean Ni(111) vs acid-terminated Ni(111), methanol and n-hexane were first studied on clean Ni(111) in control experiments to obtain a baseline understanding of the system in our apparatus. CH3OH/Ni(111). The desorption of CH3OH from Ni(111) has been well characterized, and for this reason it will not be analyzed in great detail here.36-39 Previous studies of the adsorption and decomposition of CH3OH on nickel surfaces have shown that methanol, which is stable up to ≈160 K, decomposes to form an adsorbed methoxy intermediate. At higher temperatures, between 200 and 300 K, some of the methoxy intermediate present on the nickel surface recombines and desorbs as methanol, while the remainder of the methoxy intermediates decompose to form adsorbed CO and H. By 385 K, H desorbs as H2 and CO desorbs as CO and CO2. Thermal desorption experiments were only carried out up to 300 K in the present experiments to avoid decomposing the thiol/Ni monolayer system. The TPD spectra of CH3OH from a clean Ni(111) single crystal, for CH3OH fluences that range from 3.5 × 1015 to 1.7 × 1015 molecules cm-2, are shown in Figure 3. For all fluences, CH3OH was adsorbed on Ni(111) at Tads < 120 K and the TPD spectra were collected for m/z 31 amu (CH3O+). Mass 31 is the predominant fragmentation species for CH3OH ionization in the mass spectrometer. Figure 3 shows that CH3OH desorbed from clean Ni(111) in the form of a single peak at 202 K for the lowest fluence of CH3OH. At higher fluences, the peak maxima in the spectra shifted to lower temperatures. The peak in each spectrum is generally broad (although we call the reader’s attention to the relatively narrow range of temperatures (35) The nominal length of 11-mercaptoundecanoic acid was calculated using the nominal CsC, SsC, CdO, and CsO bond lengths, obtained from the CRC Handbook of Chemistry and Physics, 70th ed., for an all-trans configuration. It was confirmed using HyperChem. (36) Rubloff, G. W.; Demuth, J. E. J. Vac. Sci. Technol. 1977, 14, 419. (37) Baudais, F. L.; Borschke, A. J.; Fedyk, J. D.; Dignam, M. J. Surf. Sci. 1980, 100, 210. (38) Bare, S. R.; Stroscio, J. A.; Ho, W. Surf. Sci. 1985, 155, L281. (39) Russell, J. N., Jr.; Chorkendorff, I.; Yates, J. T., Jr. Surf. Sci. 1987, 183, 316.
Adsorption of 11-Mercaptoundecanoic Acid on Ni(111)
Figure 3. TPD spectra of CH3OH desorption from clean Ni(111) at various fluences, with detection of mass 31 (CH3O+). The sample heating rate was 3.0 K s-1; Tads < 120 K.
Figure 4. TPD spectra of n-C6H14 desorption from clean Ni(111) at various fluences, with detection of mass 43 (C3H7+). The sample heating rate was 1.0 K s-1; Tads < 120 K.
shown here, in comparison to typical TPD results) and exhibits a tail on the high-temperature side of the peak. At the highest fluence the peak maximum occurred at 143 K; this was the point beyond which no further shift in temperature with an increase in coverage occurred. n-C6H14/Ni(111). The TPD spectra of n-C6H14 from Ni(111), for hexane fluences that extend to 7.7 × 1014 molecules cm-2, are shown in Figure 4. For all fluences, the TPD spectra were collected for m/z 43 amu (C3H7+), the predominant fragmentation species for n-C6H14 in the mass spectrometer’s ionizer. There was a single desorption state at 159 K for the lowest fluence. Higher fluences of n-C6H14 resulted in single desorption peaks for each fluence, except at the highest two fluences. At the two highest fluences there are two peaks present at 154 and 135 K which represent two desorption states. A monolayer of n-C6H14/Ni(111) was formed at a fluence of between 3.6 × 1014 and 5.2 × 1014 molecules cm-2. The lack of a significant shift in the peak maxima of the monolayer feature as the fluence was increased suggests a first-order desorption process for n-C6H14 desorption from Ni(111). At fluences above a monolayer of n-C6H14 a second desorption peak develops at 135 K, due to multilayer desorption. CH3OH/HOOC(CH2)10SH/Ni(111). The TPD spectra of CH3OH desorption from HOOC(CH2)10SH/Ni(111), for fluences that range from 1.2 × 1014 to 3.5 × 1015 molecules
Langmuir, Vol. 13, No. 13, 1997 3401
Figure 5. TPD spectra of CH3OH desorption from HOOC(CH2)10SH adsorbed on Ni(111), with detection of mass 31 (CH3O+). The inset shows the lowest fluences of CH3OH. The sample heating rate was 3.0 K s-1; Tads < 120 K.
cm-2, are shown in Figure 5. For all fluences, the TPD spectra were collected for m/z 31 amu. TPD spectra were not collected above 300 K to preserve the integrity of the monolayer. The desorption spectra of CH3OH from HOOC(CH2)10SH/Ni(111) illustrate a number of interesting features. First, the spectra are generally much sharper for desorption from the SAM than for desorption from clean Ni(111). Second, the peak temperature or maximum desorption rate at the lowest CH3OH fluence on the acidterminated surface (160 K) was lower than that for CH3OH desorption from clean Ni(111) (202 K), and the peak maximum temperature at saturation (156 K) shifted much less than for CH3OH desorption from clean Ni(111) (143 K). Both surfaces exhibited what appeared to be firstorder desorption processes at lower fluences. At higher fluences, the TPD spectra were much more similar. Although the mechanistic details of the desorption processes from the clean and acid-terminated Ni(111) surfaces are completely different, the observations at high fluence are consistent with a condensed phase process; the underlying substrate plays little role above a monolayer in either case. Third, for each spectrum there was a single desorption peak and essentially no tailing. Fourth, at the lowest fluence, the peak maximum was 160 K for the acid-terminated surface (see Figure 5 inset); it shifted to lower temperatures as the fluence was increased. At an intermediate fluence the peak maxima begin to shift back only very slightly toward higher temperatures, and broaden on the high-temperature side, saturating at 156 K for the highest fluence. The estimated desorption energy for CH3OH desorbing from HOOC(CH2)10SH/Ni(111), based on a first-order Redhead analysis,40 is approximately 41 kJ mol-1 at the lowest fluence, shifting only by 1 kJ mol-1 at most over the entire coverage range. Desorption from the acid-terminated monolayer ought to be dominated essentially by hydrogen-bonding interactions. These are the same kinds of interactions that should dominate in a condensed-methanol phase at higher coverages, so very little temperature shift is expected and observed. There is some evidence in the peak shapes at higher coverages that the first-order desorption process becomes a zeroorder process. n-C6H14/HOOC(CH2)10SH/Ni(111). The TPD spectra of n-C6H14 desorption from HOOC(CH2)10SH/Ni(111), for n-C6H14 fluences that range from 1.0 × 1014 to 5.2 × 1015 molecules cm-2, are shown in Figure 6. The desorption (40) Redhead, P. A. Vacuum 1962, 12, 203.
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Figure 6. TPD spectra of n-C6H14 desorption from HOOC(CH2)10SH adsorbed on Ni(111), with detection of mass 43 (C3H7+). The inset shows the lowest fluences of n-C6H14. The sample heating rate was 1.0 K s-1; Tads < 120 K. Table 1. Summary of TPD Results
adsorbate/substrate CH3OH/Ni(111) n-C6H14/Ni(111) CH3OH/HOOC(CH2)10SH/Ni(111) n-C6H14/HOOC(CH2)10SH/Ni(111)
estimated Tp ∆Tmax zero-order Eda (K) (K) onset (ML) (kJ mol-1) 202 -59 159 -5 160 -5 143 -9
∼1 ∼1
40.5 37.4
a Estimated activation energy of desorption using the Redhead analysis assuming a first-order desorption based on the temperatures of maximum desorption rate, Tp, for the lowest fluences.
of n-C6H14 from HOOC(CH2)10SH/Ni(111) exhibited similar characteristics to the desorption spectra of CH3OH from this acid-terminated surface. At the lowest fluence, the peak maximum occurred at 143 K (see Figure 6 inset) and shifted only slightly to lower temperatures as the fluence was increased and then shifted back to 143 K at intermediate fluences. At the highest fluence a shoulder at ∼134 K appeared on the low-temperature side of the peak at 143 K. The estimated desorption energy from a firstorder Redhead analysis, at the lowest exposure for the desorption of n-C6H14 from HOOC(CH2)10SH/Ni(111) was ∼37 kJ mol-1, shifting only to ∼36 kJ mol-1 at intermediate fluences. The TPD of a chemical probe (e.g., CH3OH and n-C6H14) in excess of a monolayer from either the clean Ni surface or the acid-terminated surface should, in principle, occur at the same desorption temperature. The TPD spectra of n-C6H14 desorption from Ni(111) and HOOC(CH2)10SH/ Ni(111) shown in Figures 4 and 6, respectively, reveal this expected property. Multilayer n-C6H14 desorption was observed to occur at 135 K from the clean Ni(111) surface and at 143 K with a shoulder at 134 K from the acid-terminated surface. There were no multilayer desorption peaks observed in the TPD spectra for methanol desorption from the clean Ni(111) surface at the fluences used here (Figure 3). There appears to be an onset for zero-order desorption of methanol from the acid-terminated surface (Figure 5) above ∼1 × 1015 molecules cm-2. Clean Ni(111) and HOOC(CH2)10SH/Ni(111) are very different surfaces, as suggested by the distinct TPD spectra. A summary of results for the TPD spectra and estimated desorption energies, based on a Redhead analysis for a first-order desorption process, is compiled in Table 1 for comparison of the interactions of n-C6H14 and CH3OH with the acid-terminated surface. The temperatures of maxi-
mum desorption rate, Tp, are tabulated in the second column, and the temperature shifts from the lowest fluence to the highest fluence for submonolayer desorption are tabulated in the third column. The final two columns show the approximate coverages at which the onset of zero-order desorption occurs and the estimated first-order Redhead desorption energies at the lowest exposures. The TPD spectra for methanol desorbed from the acidterminated surface are similar to the TPD spectra for n-hexane desorbed from the acid-terminated surface. Nevertheless, there are some subtle differences between these two sets of TPD spectra that point out some differences between the polar-polar and the polarnonpolar interactions with the acid-terminated surface. At the highest fluences for both methanol and n-hexane interacting with the acid-terminated surface, desorption processes characteristic of a zero-order desorption process were observed. When the leading edge of the desorption peak remains constant with respect to coverage in a series of TPD spectra starting from varying coverages, it indicates a zero-order desorption process.41 Zero-order desorption processes for each adsorbate-substrate system studied occurred at coverages of about a monolayer as suggested by their TPD spectra. This onset suggests that any additional adsorbate-substrate [e.g., CH3OHHOOC(CH2)10SH] and/or adsorbate-adsorbate (e.g., CH3OH-CH3OH) interactions become insignificant above these zero-order coverages. One explanation for the observed multilayer zero-order desorption processes is that methanol and n-hexane were desorbed from a condensed phase where the behavior of the condensed phase is generally characterized by a single desorption energy that is independent of the surface coverage at high fluences. At low fluences, the methanol and n-hexane interfacial interactions with the acid-terminated surface were stronger and underwent different desorption processes, as compared to their respective higher fluence interactions. These differences were observed in the behavior and peak shapes of their TPD spectra. The desorption spectra at low fluences for methanol and n-hexane from the acidterminated surface reflected the behavior and peak shapes characteristic of first-order desorption processes, but their narrow widths and their narrow range of temperature shifts over the range of fluences have made it difficult to identify the desorption process or processes that have occurred. In spite of that, n-hexane (a nonpolar molecule) at low coverage would not be likely to wet the polar acidterminated surface and might form a droplet-like structure on this surface. This is in contrast to the interaction that might be expected to occur between methanol (a polar molecule) and the polar acid-terminated surface, where methanol would likely wet the surface upon formation of hydrogen bonds. In general the wetting of a surface implies a stronger interaction between the adsorbate and the surface than non-wetting, and would be one explanation for the difference in the behavior of the TPD spectra at low fluences. Another explanation of the slightly stronger interaction that occurred with the methanolacid-terminated system can be attributed to hydrogen bonding between methanol the carboxylic acid end of the organic surface rather than the weaker dispersive interactions possible with n-hexane. For the acid-terminated surfaces studied in this work, the TPD spectra exhibited unusually sharp, and only slightly shifted, desorption peaks with increasing coverage, compared to what is commonly observed in TPD spectra.42 The amount by which TPD spectra shift, at varying coverages, can provide (41) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1992, 112, 570.
Adsorption of 11-Mercaptoundecanoic Acid on Ni(111)
information about adsorbate-adsorbate and adsorbatesubstrate interactions; TPD spectral shifts can also provide information about the desorption kinetics of chemical probes desorbed from their respective substrates. Nearly identical TPD experiments were performed on gold substrates by Dubois et al.41 In their experiments, the larger temperature shifts in the TPD spectra of methanol were attributed to hydrogen-bonded species, whereas smaller temperature shifts were attributed to weaker van der Waals bonded species (e.g., n-C6H14). CONCLUSION The gas-phase adsorption of 11-mercaptoundecanoic acid on Ni(111) has been characterized using ADXPS and TPD. This thiol adsorbs preserving its stoichiometric ratio on the Ni(111) surface. X-ray photoelectron spectroscopy evidence shows that the carboxylic acid group is extended away from the Ni(111) surface and that the estimated monolayer thickness is 10.4 ( 1.7 Å. The numerical value obtained for the overlayer thickness, and the carboxylic acid group orientation information suggest that the thiol film is one monolayer thick. Furthermore, the interactions of CH3OH and n-C6H14 probe molecules with this monolayer, HOOC(CH2)10SH/ Ni(111), indicate that the carboxylic acid group is extended away from the surface. The TPD results reflecting the (42) Niemantsverdriet, J. W. Spectroscopy in Catalysis: An Introduction; VCH Publishers: New York, 1995, pp 11-36. (43) Clark, D. T.; Thomas, H. R. J. Polym. Sci.: Polym. Chem. 1978, 16, 791.
Langmuir, Vol. 13, No. 13, 1997 3403
different interactions of CH3OH and n-C6H14 with clean Ni(111) and HOOC(CH2)10SH/Ni(111) suggest that TPD can be used to help characterize the orientation of the monolayer, giving information that complements the XPS data. These adsorbate interactions also illustrate the ability of TPD to characterize the bonding and/or interaction of probe molecules with functionalized SAMs. For example, the hydrogen bonding of CH3OH could be distinguished from the weaker interactions associated with n-C6H14 on the acid-terminated surface based on the temperature shifts and magnitudes of desorption energies observed or calculated from the TPD spectra. Since zeroorder desorption processes seemed to have occurred at all but submonolayer fluences, the desorption of the two probe molecules could have resulted from a condensed phase at higher fluences. Acknowledgment. The authors thank Professor Christopher E. D. Chidsey for providing the 11-mercaptoundecanoic acid used in these experiments. This work was supported by the National Science Foundation (CHE9357188), the Camille and Henry Dreyfus Foundation, and the Alfred P. Sloan Foundation. Supporting Information Available: XPS survey spectrum of 11-mercaptoundecanoic acid adsorbed on Ni(111) (2 pages). Ordering information can be found on any current masthead page. LA9610112
