Langmuir 1996,11, 493-497
493
Conformation of Alkanethiols on Au, Ag(lll), and Pt(ll1) Electrodes: A Vibrational Spectroscopy Study Margaret A. Hines, J. A. Todd, and P. Guyot-Sionnest" James Franck Institute, University of Chicago, Chicago, Illinois 60637 Received March 1, 1994. In Final Form: November 22, 1994@ Electrochemical effects on self-assembledmonolayers of alkanethiols, C,H2,+1SH (n = 9,10,18), on Au, Ag(lll), and Pt(ll1) are investigated using sum-frequency generation. Layers on Au and Ag(ll1) show no noticeable effects. Layers on Pt(ll1) show gauche transformations that are reversibly eliminated at negative or positive potentials.
I. Introduction Self-assembled thiol monolayers (SAM)on metal surfaces present many interesting aspects as surface modifiers, including the protective action that a S A M may provide against corrosion. In recent years, much has been learned about the self-assembly process and the monolayer structuresl-l0 using various techniques such as infrared and Raman spectroscopy, low-energy electron diffraction, X-ray diffraction, He atom scattering, ellipsometry, and scanning tunneling and atomic force microscopy. Relatively little has been done with the nonlinear optical technique of sum-frequency generation (SFG)11J2although it has the unique advantage of allowing in situ vibrational spectroscopy of the liquid-solid interface.13J4 The substrate best characterized for alkanethiol monolayers is gold. Adsorption of the layers has also been studied to a lesser extent on silver and much less on copper and platinum. It is known that the chains on gold form closely packed, ordered monolayers with excellent nonwetting characteristics and high stability. Such a closely packed layer is also observed on silver although the nature of the silver-monolayer interface is less certain. Little is known about alkanethiol monolayers on Pt except that the thiol monolayer is formed, as judged from the surface wetting.15 The structure of the thiol monolayers on gold and silver has been investigated in particular by the vibrational spectroscopic techniques of Raman, FTIR, and SFG. The Abstract published in Advance ACS Abstracts, January 15, 1995. (1)Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J . Am. Chem. SOC.1989,111, 321. (2)Steiner, U.B.; Caseri, W. R.; Suter, U.W. Langmuir 1992,8, 2771. (3)Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J . Chem. Phys. 1993, 98,678. (4)Nuzzo, R.G.; Dubois, L. H.; Allara, D. L. J . Am. Chem. SOC.1990, 112, 558. (5) Nuzzo, R. G.; Korenic, E. M.; Dubois, L.H. J . Chem. Phys. 1990, 93,767. (6) Camillone 111, N.; Chidsey, C. E. D.; Eisenberger, P.; Fenter, P.; Li. J.: Lianp.. K. S.: Liu. G.-Y.:Scoles. G. J . Chem. Phvs. 1993.99.744. '(7jCamalone II1,N.fChidsey, C. E: D.; Liu, G.-Y.;Scoles, G.>. Chem. Phys. 1993,98,4234. ( 8 ) Pan, J.; Tao, N.; Lindsay, S. M. Langmuir 1993,9,1556. (9)Sun, L.;Crooks, R. M.J . EZectrochem. SOC.1991,138,L23. (10)Dubois, L. H.; Nuzzo, R. G. Annu. Reu. Phys. Chem. 1992,43, 437. (11)Ong, T. H.; Davies, P. B.; Bain, C. D. Langmuir 1993,9,1836. (12)Harris, A.L.; Chidsey, C. E. D.; Levinos, N. J.; Loiacono, D. N. Chem. Phys. Lett. 1987,141,350. (13)Shen, Y. R.Nature 1989,337,519. (14)Guyot-Sionnest, P.; Tadjeddine, A. Chem. Phys. Lett. 1990,5, 172. (15)Lee, T.R.;Laibinis, P. E.; Folkers, J. P.; Whitesides, G. M. Pure A&. Chem. 1991,63,821. @
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monolayer spectrum is similar in peak frequency (FTIR)16 and trandgauche intensity ratio (Raman)17-19to the thiol bulk solid rather than the liquid, and these layers are often termed "solidlike", with the molecular chain in a mostly trans configuration. The SFG spectrum reveals mostly methyl resonances,ll which is also consistent with an all-trans chain conformation.11,20 It has been observed that the layer undergoes structural changes when placed in contact with an organic solvent. An early SERS investigation" of hexadecanethiol on Ag reported an increase in the gauche contribution with a decrease of the trans contribution in the presence of a CD3C1 overlayer. Similarly, a recent FTIR study16of an octadecanethiol layer (ODT)on Au with a CD3CN overlayer found small shiftsin peak frequencies and attributed these to changes in molecular conformation. Earlier FTIR work21on ODT/Au in the presence of CD30H and CC14 claimed only an increase in disorder at the chain terminus. An SFG study has also shown that overlayers of hexane and acetonitrile on ODT/Aull lead to an increase in the methylene resonances as well as small red shifts of the methyl resonances. These trends are indicative of an increase in gauche conformations along the chain.11,20 Molecular conformation changes in the presence of organic overlayers are attributed to a partial solvation of the ~ h a i n . ~ l @This J ~ also explains the absence of effect for water overlayers. As further support, an electrochemical study22has found that alkyl thiol layers on Au suppress faradaic currents of redox couples in aqueous electrolytes, but in acetonitrile the layers no longer act as a barrier against electroactivity. However, while hydrophobic repulsion may effectively prevent the incorporation of water molecules intothe densely packed layer, the added driving force from an electrical potential may overcome this hydrophobic repulsion. This should be particularly noticeable if the layer is not so densely packed. In spite of the relevance of this issue for the stability of SAMs, only a few studies have been performed with FTIR and Raman in an electrolyte environment. Interestingly, FTIR studies have provided weak evidence16that the ODT monolayer on Au in CD3CN regains some structural order and reorganizes with applied potential. (16)Anderson, M.R.;Gatin M. Langmuir 1994,10, 1638. (17)Sandroff, C. J.;Garoff, S.; Leung, K. P. Chem. Phys. Lett. 1983, 96,547. (18)Bryant, M.A,; Pemberton, J. E. J . Am. Chem. SOC.1991,113, 3629. (19)Bryant, M.A.;Pemberton, J. E. J . Am. Chem. SOC.1991,113, 8284. (20)Guyot-Sionnest, P.; Hunt, J. H.; Shen, Y. R. Phys. Rev. Lett. 1987,59,1597. (21)Stole, S. M.;Porter, M. D. Langmuir 1990,6, 1199. (22)Finklea, H. 0.; Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987, 3,409.
0 1995 American Chemical Society
Hines et al.
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the total susceptibility as the sum of a resonant and a nonresonant . nonresonant part, ~ N R ' ~ is ) , the susceptibility, x ~ ( 2 ) X N R ( ~ ) The contribution from all remote resonances, electronic and vibrational, of the molecule and the substrate. X N R ' ~ ' is varying so slowly over the spectral region of interest that it is assumed to be uniform. However, it can vary with potential due in particular to the different electronic charge of the metallic surface a t different potentials.26 The resonant part is due to specific vibrations and is taken here as a lorentzian thereby neglecting the inhomogeneous broadening. We found that in the electrolyte the line shapes are sufficiently broad that the simple lorentzian works quite well. The SFG signal can then be written as
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Figure 1. Sum-frequency spectra in air of octadecanethiol (ODT, C18H&H) adsorbed onto (a) Pt(ll1) and (b) Au.
An ex situ FTIR investigationz3 showed ordering of a dithioether ( C I ~ H Z ~ S C Z H ~ S Cmonolayer I ~ H Z ~ ) on gold cycled in an aqueous electrolyte with methylene blue. However, in aqueous electrolytes, all in situ evidence so far indicates that applied potentials have no effect on long chain alkyl thiol monolayers on Au and Ag within the ranges of chemical ~ t a b i l i t y . ~ ~ ~ ~ ~ , ~ ~ This prompted us to pursue this line of investigation by SFG since both SERS and FTIR present specific difficulties. In SERS, the rough surfaces used are not wellcharacterized, and there is no guarantee that the spectrum is representative of the average molecular orientation. For in situ FTIR spectra, the frequency shifts are difficult to detect due to broadening and increased noise level, particularly in aqueous electrolytes. On the other hand, SFG can be advantageously applied to the electrochemical interface.l4sZ6 We present our observations for Au (nominally (ill)), Ag(lll), andPt(ll1) surfaces. Consistent withworkdone by others, on Au and Ag(ll1) we have not observed any change in the monolayer vibrational spectra within the bounds of electrochemical stability of the layers. On the other hand, on Pt(ll1) we have observed clear and reversible evolution of the spectra that we interpret as molecular conformation changes that are allowed by the larger disorder already present in those layers.
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11. Experimental Section Sum-Frequency Generation Method. Sum-frequency generation (SFG)is a process in which two intense lasers interact in a nonlinear media t o generate photons at a frequency that is the sum of the two input photon frequencies, W S F = WIR + W ~ S This occurs through a polarization induced by the nonlinear response of the media. This polarization is equal to the product of the nonlinear susceptibility, ~ ( ~ 1and , the incoming electric fields: P ' 2 ' ( W S F ) = X'2':E(Wws):E(WIR)
.
(1)
In the dipole approximation, f 2 ) is zero in the bulk of a centrosymmetric substrate, and therefore the process is very surface specific.13 When the IR beam is tuned to a vibrational mode of a surface adsorbate, there will be a n enhancement to x ' ~ )as, shown for example in Figure 1. It is convenient to write (23) Barner, B. J.; Corn, R. M. Langmuir 1990, 6, 1023. (24) Pemberton. J. E.: urivate communicatinn. ~ ~ ~ ~ ~ . (25) Popenoe, D. D.; Dhnhammer, R. S.; Porter, M. D. Langmuir 1992. 8. 2521. (26) Guyot-Sionnest,P.; Tadjeddine, A,; Liebsch, A. Phys. Rev. Lett. 1990, 64, 1678. ~
~
where W L is the laser frequency, B is the effective nonresonant background, W P are the resonance frequencies, r are their line widths, and A p are their effective resonance amplitudes. A p and B are in fact products of the surface field strengths (the Fresnel factors) and tensor elements of the substrate or molecular susceptibility. In our experimental setup, where we use p polarized beams, the relevant susceptibility tensor elements x"k(2) (WSF = WIR wws) have indices z (in the plane of the surface and the plane ofincidence) andz (perpendicularto the surface plane). We also remark that the very high dielectric constant of the metal in the infrared insures that the surface IR field seen by the monolayer is essentially along z. Therefore, assuming inplane isotropy, the dominant tensor elements must be x b Z z ( 2 ) and x h J 2 ) for the molecular amplitude A p . Conversely, the IR field in the metal is mostly along x so that the amplitude of the nonresonant background B must be coming mostly from X N Q = ( ~ ) and X N b Z J 2 ) . Finally, we note that given a fixed configuration for the lasers and given the dielectric constants of the materials on either side ofthe interface, the effective resonance amplitudes A p depend only on the molecular susceptibility, the coverage density, and the average orientation. Therefore, the variations of A p as a function of the applied potential are indications of modifications at the molecular level. We have restricted ourselves to the 2800-3000 cm-l region that covers the CH stretching region. In this region the modes that are SFG active (being both IR and Raman active) are (i),for the methyl group, the symmetric stretch (r+,2874 cm-l), the Fermi resonance ofthe symmetric stretch (r'FR, 2931 cm-l), and the degenerate asymmetric stretches (r-, 2954 and 2962 cm-l) and (ii), for the methylene group, the symmetric stretch (d+, 2851 cm-l) and the Fermi resonance of the symmetric stretch (d+FR,2907 cm-'). The given frequencies are based on vibrational modes of polycrystalline alkyl thiol chains.27 In the SFG spectra of alkyl layers, the methyl resonances are usually dominant and the methylene modes absent. This is because, in straight chains, the methylenes are distributed symmetrically along the chain and cancel each other. Yet in the presence of gauche defects, the methylenes no longer canceLZ0 To analyze the spectra in terms ofthe five possible resonances, we used a least squares fitting program to fit the data to eq 2 (MicroMath Scientific Software's program, Scientist). Fitted spectra and values for IAp12 are shown in Figures 2 and 3. To reduce the number of adjustable parameters, we chose B as real and allowed for different phases of the amplitudes, A p . When comparing spectra at different potentials, the line widths were not adjusted, but the frequencies were allowed to vary slightly around literature values. The laser system has been described elsewhere,2Bas has been the use of SFG for in situ electrochemical studies.14 Briefly, the visible and the IR laser source enter the electrochemical cell in a nearly copropagating configuration with respectively 50" and 55" angles of incidence. They pass through a prismatic CaFp window. The visible (0.532 pm, 8 ps, 100 p J ) and the IR (2-11 pm, 6 ps, 50pJ) sources are overlapped and synchronized on the sample. The SFG signal from the sample is collected with a
.
(27)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. (28) Guyot-Sionnest,P. J.Electron Spectr. Rel. Phenom. 1993,1,64.
An SFG Study of Alkanethiols on Electrodes
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Langmuir, Vol. 11, No. 2, 1995 495
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Figure 2. Sum-frequency spectra of ODT on Pt(ll1) in 0.1 M HClOl a t various electrochemical potentials: (a)-0.2 V, (b)0.7 V, and (c) 1.1V. The experimental data points are shown with the calculated fit lines.
Potential (V vs SCE) Figure 3. Fitted amplitudes lAplZofthe vibrational resonances as a function of potential for the methyl group, (a)symmetric stretch, (b) Fermi resonance, and (c) degenerate asymmetric stretch; for the methylene group, (d) symmetric stretch and (e) Fermi resonance; and for the (f) nonresonant background. The lines drawn are a guide to the eye. PMT andratioed against a reference signal. The reference signal is obtained by reflecting 10% of the IR and visible beam before the electrochemical cell, passing it through a ZnS polycrystalline window (ZnS is noncentrosymmetric), and detecting the output SFG with a second PMT. Each spectrum is recorded in 2 cm-I increments with 100 laser shots per point. One spectrum takes about 6 min. FTIR Spectroscopy. Ex situ grazing angle reflection infrared spectroscopy (Nicolet Magna 550 FTIR spectrometer) has been performed to characterize the layers in air and compare the results with those found in the literature. Spectra (see Figure 4)are taken a t grazing incidence (-85") with IR lightp-polarized through a wire grid polarizer (Molectron Detector, Inc.) and detected with a liquid nitrogen cooled mercury cadmium telluride detector. We averaged 1000 scans a t 4 cm-l resolution.
2800
2850
2900
2950
3000
Wavenumber (cm")
Figure 4. FTIR spectra of ODT monolayers adsorbed onto (a) Au, (b) Ag, and (c) Pt surfaces. Sample Preparations. The Au samples are prepared by evaporation onto glass disks. The glass disks are cleaned in piranha solution (3:l HzSOdHz02), rinsed thoroughly with deionized (DI) water, and dried with Ar. A thin layer of Cr (50 8)is first evaporated onto the glass for better adhesion of the gold (2500 A). After evaporation, the gold samples are cleaned in freshly prepared chromic acid solution, rinsed, and dried. Silver samples are either evaporated as above and used for the ex situ characterization or a single crystal Ag(ll1) disk is used for en situ and in situ characterization. For adsorption on gold and silver, the thiol solution was 1mM in absolute ethanol, but 10 mM in CHC13 provided the same results. For the platinum, we used a Pt(ll1) crystal. It is prepared by annealing in a HdOz flame for 10 min, slowly cooling in an Ar environment, and finally submerging into deaerated water. The Pt(ll1)is subsequently placed into fuming sulfuric acid for 10 min. Upon removal, it is thoroughly rinsed with DI water, dried with Ar, and immediately submerged into thiol solution. Other Pt samples used for FTIR work were prepared by chemical vapor deposition (CVD) using platinum(I1) acetylacetonate in an Ar/Oz environment onto 1 x 2 in. quartz slides (Quartz Scientific, I ~ C . ) By . ~cleaning ~ in hot HzS04 we could strip the layers and reproducibly reuse the Pt film. By FTIR, we observed that on clean platinum rinsing with ethanol left a hydrocarbon residue while chloroform did not. Samples prepared in ethanolic solutions also produced poor spectral features, maybe due to the chemical interaction of alcohol and Pt. We therefore preferred to use a 10 mM thiol solution in chloroform (HPLC grade). Upon removal, the samples were rinsed in CHC13 and subsequently water. The thiols (CnHzn+lSH) nonanethiol ( n = 9),decanethiol (n = lo), and octadecanethiol (n = 18) were obtained from Aldrich Chemical Corp. They were used without purification except for a simple recrystallization of the longer chain thiol with, however, no noticeable effect on our observations. The samples were left in solution between 1 and 12 h. The Electrochemical Cell. The cell has inlets for the electrolyte solution, Ar purging gas, and a Pt counter electrode. A saturated calomel electrode (SCE) is used for the potential reference. The sample is pressed up against a prismatic CaFz window, leaving only a few microns of electrolyte in order to reduce the IR absorption. A computer controlled potentiostatl galvanostat (Intertech Systems modelPGS151)is usedto regulate the applied potential to the working electrode.
111. Results Ex Situ IR and SFG. For Au and Ag, the FTIR spectra of the layers agree well with the spectral data available in the literature (see Figure 4). (29) Rand, M. J. J.EZectrochem. SOC.1973,120, 686.
Hines et al.
496 Langmutr, Vol. 11, No. 2, 1995
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,
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Figure 5. Sum-frequency spectra of ODT on (a) AU and (b) Ag under an electrochemical potential. (a)Au, solid line, 0.4 V; dotted line, 1.1V (b) Ag, solid line, -0.8 V; dotted line 0.2 V.
The SFG spectra are also in good agreement.11J2 As shown in Figure 1, the SFG spectra have three features entirely attributable to the methyl resonances with no methylene contributions. We have to remark that the resonances appear as dips, unlike spectra reported by others in which the resonances appear as peaks. This is due to a phase difference between the nonresonant background and the resonance which is easily explained by the laser configuration. Indeed, in our case the two laser beams come from the same side (copropagating)while others use a counterpropagating geometry. The Fresnel factors corresponding to xzzzand xxsz(dominant molecular terms) have the same sign in both geometries while those for xxzxand xzxs(dominant substrate terms) have reversed signs. It follows that in the two configurations, counterpropagating or copropagating, the spectra should have reverse features ofeach other. This was checked by taking spectra in air in both geometries. We now turn to the case of platinum for which no literature data is available for either FTIR or SFG. In the FTIR spectrum (Figure 4)the methyl resonances on Pt appear to be approximately half the strength of those on Au while the methylenes are of similar magnitude. One interpretation might be that the monolayer is comprised of chains with a larger tilt angle but at a lower coverage compared to gold. The SFG spectrum of the ODT layer on Pt(ll1) shows, in addition to the three methyl resonances, the two features attributable to the methylene groups. As discussed earlier, the observation of methylene resonances for the alkyl thiols on Pt is taken as an indication of gauche defects. We therefore infer that alkyl thiols on Pt may not form close-packed, ordered layers. Contact angle measurements confirm this interpretation since the water contact angle for ODTPt (angle less than 103") is smaller than for layers of gold or silver (angle around l12").27 In Situ SFG. For gold, we used an HC104 electrolyte (0.1 M), and the potential was cycled from 0.2 to 1.4 V vs SCE. For silver, Kc104 (0.1M) was used, and the potential was cycled from -1.0 to 0.6 V vs SCE. SFG spectra of ODTon Au andAg(1ll)at therest potentials (respectively 0.2 V and 0.1 V/SCE) resemble spectra in air but broader. They possess only the three features attributable to the methyl resonances with no methylene contributions. As seen in Figure 5 , there are no detectable changes upon cycling within the previous limits. We first remark that
the absence of change of the nonresonant background readily indicates that the metal charge varies little. This is consistent with the knowledge that those layers are efficient barriers for the electrolyte. The methyl resonances are also not affected, indicating that the orientation of the methyl group, which is in contact with the electrolyte, also does not change in a detectable way at different potentials. This is also consistent with reported FTIR and SERS data. For P t ( l l l ) , the electrolyte used was HC104 (0.1 M), and the potential range studied was -0.3 V (hydrogen limit) to f 1 . 3 V. The layer was observed to be stable within the limits -0.3 V and f 1 . 0 V. The SFG spectra shown in Figure 2 and their analysis indicate a very different behavior than for Au and Ag. Indeed, we observe large changes in the nonresonant signal, indicating already that the metal charge gets significantly altered, implying that the layer is a poor barrier. The intensities of the resonances are also strongly modified in a reversible way between -0.3 and $1.0 VISCE. The changes occur on a time scale of less than 1s as checked by taking two spectra point by point at -0.2 and $0.8 VISCE and alternating the potential every second. Above 1.0 V the voltammogram indicates the beginning of an oxidation current, and the layer begins to be stripped slowly, as observed by the reduced intensities of the SFG features upon returning to less positive potentials. The time scale for removal of the layer is about 10 min at $1.3 V.
IV. Discussion We therefore focus on the spectral changes observed for F't. At very negative potentials, only CH3 resonances are observed (Figures 2 and 3). As the potential is made positive, the amplitudes of the CH3 resonances increase little before a fast decrease around 0.5 V. The CH2 resonance appears around 0 V, peaks around 0.7 V, and decreases at more positive potentials. For positive potentials, the magnitude of the CH3 resonance reaches a deep minimum around 1V but begins to increase slightly again at higher potentials. As stated earlier, within the limits -0.3 and + L O V this is reversible with no loss of signal. Such spectral changes with no reaction current are purely due to molecular conformational changes. On a metallic substrate, one has access to only one set of polarizations, and therefore, the average molecular orientation cannot be determined with certainty. We may, however, extract a few general features. First, at negative potentials -0.310.0 V, the absence of CH2 contributions indicates a straight chain. Since the CH3 contribution is rather large, the methyl group is probably pointing upward although not vertical since this would prevent an rcontribution. (If we could rely on a bond additivity model for the CH3, its orientation could be determined by comparing the r+ and r- contributions. This will be discussed in subsequent work.) For more positive potentials, the maximum in the CH2 contribution around 0.7-0.9 V indicates a kinked chain. At the same time, the CH3 contributions (both rf and r-) decrease nearly to zero, indicating a tilting of the methyl group in the plane of the surface in a more isotropic arrangement. Around 1V, the spectral features become inverted, and the fits indicate a phase change of approximately 110" between the methyl resonance and the nonresonant background. This could be a flipping down of the methyl group, but it could also be an effect from the nonresonant background of the platinum. For potentials more positive than 1.0 V, the layer begins to be slowly oxidized, but while the CH3 contribution recovers partially, the CH2 contribution
An SFG Study of Alkanethiols on Electrodes
Langmuir, Vol. 11, No. 2, 1995 497
Figure 6. Proposed molecular illustration of the thiol monolayer on platinum with applied potential.
becomes quite small, and there might be a straightening within the interior of the layer disappear to improve the of the chain. packing while the number of defects associated with the chain’s end increases. This restructuring of the layer due This behavior is not very specific to the ODT. We to an applied force could be an explanation for the effects investigated the effects of oddeven chain length on the that we observe. However, by estimating the magnitude three substrates by comparing nonanethiol (NT, C9H19of the electrostatic pressure, using reported capacitance SH) and decanethiol (DT, C1OHZlSH). In all cases, the value for the chain molecules of 5O/npF/cm2 ( n = number NT, DT, and ODT spectra appeared rather similar, except of carbon atoms in the effective pressure is only with Au, where the terminal methyl group orientation is of the order of 60 atm for a potential drop of 1V. Even known to have an odd-even a l t e r n a t i ~ n .This ~ ~ appears with such a rough estimate, it appears that this pressure also in the SFG spectra, where the amplitude of the is extremely small compared to the pressures necessary asymmetric CH3 stretch is stronger by about a factor of in the simulation to produce any effect. The absence of 1.5 in the odd chain than in the even chain. Otherwise, effects on the layers on gold and silver is also consistent monolayers of both NT and DT on Pt(ll1)exhibit similar with the numbers, and we may rule out the simple general behavior in comparison with ODT under applied electrostatic pressure mechanism. potentials, except that their oxidation and reduction occur earlier thereby reducing the extent of the observable (iii)As discussed earlier, it is believed that ordered layers changes. can be solvated by organic molecules16J7but not by aqueous electrolytes. Our observations on gold and silver confirm Given the absence of change of conformations on the this statement. On the other hand, the disorder already close-packed layers on gold and silver, but the large and reversible modifications seen on Pt, it is natural to present for the layer on Pt may reduce the energy barrier associate the initial disorder of the layer as a necessary for the introduction of the electrolyte (Figure 6). The condition for allowing molecular reorganization. The introduction of ions is in fact needed to explain the disorder of the layer on Pt is probably due to the details modification of the metal nonresonant background. Thereof the interaction between the Pt and the thiols. This fore, there must be an interaction between the ions and interaction could be complicated due to the chemical the monolayer which can then affect the molecular activity of Pt with alkyl compounds. We think that Pt is conformation. One possible explanation for the enhanced not affecting the chains in a major way since we find order at the extreme biases is based on hydrophobicity. consistent trends in contact angles, potential ranges, and Simply, as the electrolyte is driven through the layer, the FTIR and SFG spectra for long and short chain thiols. alkyl chains adopt a more close-packed configuration to However, reaction sites such as steps or kinks, as well as reduce the overall energy. This suggests that different contamination, could be a source of disorder. The smaller ions or electrolytes would produce different effects. We Pt lattice spacing, which is 3.92 compared to 4.08 A for are investigating this possibility. We also plan to use Au and 4.09 A for Ag,30may also prevent the chains from specifically disordered layers (with side-chain substitupacking in an ordered fashion on Pt. ents) to see if we can generalize these observations to the more common gold substrate. The cause for the observed molecular reorganization that we see under applied potential is not yet firmly established. V. Conclusion (i) We cannot rule out a possible restructuring at the Using sum-frequency generation, we have shown that sulfur level due to a potential dependent reconstruction long chain alkyl thiol monolayers on platinum are of the surface occurring faster than 1s. For example, it disordered with gauche defects. In situ SFG spectroscopy is known that in ultrahigh vacuum, sulfur adsorbs on that they can be straightened by applying a ~ clean R(ll1)in a coverage dependent r e c o n s t r u c t i ~ n . ~ ~ , ~indicates negative or positive electrochemical potential while the We would have to investigate the interface structure by more ordered layers on gold and silver are unaffected. We an array of techniques such as LEED, XPS (X-ray propose that the initial disorder is what allows those photoelectron spectroscopy), and EELS (electfon energy conformation changes and that the conformation changes loss spectroscopy)before a structural picture could emerge. may be due to the introduction of the electrolyte within (ii) In relation to atomic force microscopy studies, the membranelike layer. simulations have shown that pressure on a close-packed monolayer could induce transformation^.^^,^^ Upon increasing pressure (of the order of lo5atm), gauche defects Acknowledgment. We gratefully acknowledge financial support from the National Science Foundation through (30)Kittel, C. Introduction to Solid State Physics; Wiley: New York, grants DMR-8819860 and CHE-9204416. 1953. (31)Heegemann, W.; Meister, K. H.; Bechtold, E.; Hayek, K. Surf. Sci. 1975,49, 161. (32)Berthier, Y.;Perdereau, M.; Oudar, J. Surf. Sci. 1973,36,225. (33)Siepman, J. I.; McDonald, I. R. Phys. Rev. Lett. 1993,70, 453. (34)Siepman, J. I. Tenside Surf. Det. 1993,30,247.
LA940190P (35)Widrig, C . A,; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991,310,335.
