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Sulfur-Substrate Interactions in Spontaneously Formed Sulfur Adlayers on Au(111) C. Vericat, M. E. Vela, G. Andreasen, and R. C. Salvarezza Instituto de Investigaciones Fisicoquı´micas Teo´ ricas y Aplicadas (INIFTA), (CIC-CONICET-UNLP), Sucursal 4, Casilla de Correo 16, 1900 La Plata, Argentina
L. Va´zquez and J. A. Martı´n-Gago* Instituto de Ciencia de Materiales de Madrid (CSIC), 28049-Madrid, Spain Received December 30, 2000. In Final Form: April 3, 2001 The electroadsorption of S on Au(111) from 0.1 M NaOH + 3 × 10-3 M Na2S solutions has been studied by in situ scanning tunneling microscopy (STM), electrochemical methods, and ex situ X-ray photoemission spectrocopy (XPS). By analyzing STM images, we have observed that S adsorbs on Au(111) forming a x3×x3R30° superstructure. Under potential control this lattice slowly and continuously transforms into S octomers (S8) in the range -0.7/-0.5 V (i.e., at typical potentials observed under open circuit conditions). In this potential range, mixtures of both structures are present on the Au(111) surface. An XPS study of the S 2p peak from the adlayers reveals the presence of three components that can be assigned to S forming a x3×x3R30° structure, S8, and bulk S at surface defects. The most important component is that corresponding to S8, in good agreement with the STM images. Furthermore, XPS spectra recorded for x3×x3R30° thiol adlayers on Au(111), characterized by STM and atomic force microscopy, lead to similar S 2p XPS spectra. A comparison between these cases allows us to conclude that S in spontaneously formed S8 on Au(111) exhibits the same binding energy of the core electronic levels (i.e., same chemical state) as S in x3×x3R30° spontaneously formed thiol lattices, although the adsorption sites are different.
Introduction Self-assembled monolayers (SAMs) of alkanethiols on metals have attracted considerable scientific interest.1 These fascinating two-dimensional structures have potential applications to modify wetting and wear properties of solid surfaces and to anchor different functional groups to be used in chemical and biochemical sensors. Also, they can be used to protect metal surfaces against corrosion and to be used as masks for the fabrication of nanodevices for electronics and magnetic storage media.2 One of the main problems in understanding self-assembly of SAMs on metals arises from the fact that alkanethiol-metal and alkanethiol-alkanethiol interactions are not fully understood. These interactions determine the stability of SAMs, a crucial point for their use in many technological applications. In aqueous solutions, the most important environment for technological applications, reductive electrodesorption has been used to explore SAM stability.3 In fact, SAMs on Ag(111)4 and Au(111)5 are desorbed in sharp voltammetric peaks whose peak potentials (Ep) shift in the negative direction as the length (n) of the alkanethiol hydrocarbon chains, given in C units, increases. Based on the shift in Ep, stabilizing forces acting in x3×x3R30° and related superlattices of alkanethiols adsorbed on Au(111) and Ag(111) in contact with aqueous solutions have been estimated in ≈3-4 kJ/mol C units.4,5 This energy involves van der Waals and hydrophobic forces, both stabilizing * Corresponding author. (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Haag, R.; Rampi, A. M.; Holmlin, R. E.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 7895. (3) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (4) Hatchett D. W.; Uibel, R. H.; Stevenson, K.J.; Harris, J.M.; White, H. S. J. Am. Chem. Soc. 1998, 120, 1062. (5) Zhong, C-J.; Porter, M. D. J. Electroanal. Chem. 1997, 425, 147.
SAMs. Conversely to this progress in the understanding of the lateral interactions prevailing in SAMs, the nature of the S-Au bond is not fully understood. In fact, although the chemical state of S atoms at the alkanethiol/Au interface has been extensively studied by XPS, the interpretation of experimental data still remains controversial.6-10 Recently, the behavior of electroreductive desorption curves for x3×x3R30° alkanethiol adlayers (and its c(4×2) superlattice), recorded in aqueous 0.1 M NaOH, has been compared to that found for similar S adlayers.11 It has been found that the Ep vs n plot leads to a value for n ) 0, i.e., alkanethiols adsorbed on Au in the absence of chain-chain interactions, 0.2 V positively shifted with respect to the Ep value for S electroreductive desorption. This is a strong indication that the S-Au bond in x3×x3R30° alkanethiol adlayers differs from the S-Au bond in x3×x3R30° S lattices. In this paper we investigate the nature of the S-Au bond for both spontaneously formed S and alkanethiol adlayers using in situ scanning tunneling microscopy (STM), ex situ atomic force microscopy (AFM), and photoemission experiments. We have found that the spontaneously formed S adlayer on Au consists mainly of S8 coexisting with x3×x3R30° domains and bulk S. We (6) Heiser, K.; Allara, D. L.; Bahnck, K.; Frey, S.; Zharnikov, M.; Grunze, M. Langmuir 1999, 15, 5440. Whelan, C. M.; Barnes, C. J.; Walker, C. G. H.; Brown, N. M. D. Surf. Sci. 1999, 425, 195. (7) Castner, D.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (8) Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nishida, N.; Sasabe, H.; Knoll, W. Langmuir 1998, 14, 2092. (9) Zhong, C.-J.; Brush, R. C.; Anderegg, J.; Porter, M. D. Langmuir 1999, 15, 518. (10) Ishida, T.; Choi, N.; Mizutani, W.; Tokumoto, H.; Kojima, I.; Azehara, H.; Hokari, H.; Akiba, U.; Fujihira, M. Langmuir 1999, 15, 6799. (11) Vela, M. E.; Martin, H.; Vericat, C.; Andreasen, G.; Herna´ndez Creus, A.; Salvarezza, R. C. J. Phys. Chem. B 2000, 104, 11878.
10.1021/la0018179 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/10/2001
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have found that the chemical state of S in the octomers is the same as that found for S in x3×x3R30° and related alkanethiol adlayers, although different adsorption sites are involved. Our XPS results also show that the S-Au bond in the x3×x3R30° and c(4×2) alkanethiol lattices is less ionic than that for S in the x3×x3R30° S lattice. This conclusion explains the differences observed in behavior of the electroreductive desorption curves for these adlayers. Experimental Section Two types of substrates were used for S and thiol adsorption: Au films evaporated on glass (Robax glass, AF Berliner Glas KG, Germany) annealed in a hydrogen atmosphere at 650 °C, and a Au single crystal (Mateck, Germany) cleaned in ultra-highvacuum (UHV) conditions by sputtering at 500 eV plus annealing at 700 °C. S adsorption on Au films was followed by in situ STM imaging under potential control in nitrogen-saturated 3 × 10-3 M Na2S + 0.1 M NaOH solutions using a Nanoscope III STM (Digital Instruments Inc.). A large-area Au counter electrode and a Pd/H2 reference electrode were used in the STM electrochemical cell. Potentials in the text are referred to the saturated calomel electrode (SCE). STM images were taken in the constantcurrent mode using Pt-Ir tips covered with Apiezon and etched W tips for in situ and UHV experiments, respectively. Spontaneously adsorbed alkanethiol adlayers were prepared by immersion of the substrate for 24 h in 0.05 mM X ethanolic solutions, (X ) hexanethiol and dodecanethiol). Spontaneously formed S adlayers were prepared by immersion in nitrogensaturated 3 × 10-3 M Na2S + 0.1 M NaOH for different times. Solutions were prepared with high-purity chemicals. Ex situ STM and AFM (Nanoscope III, Digital Instruments Inc.) were used to characterize alkanethiol films. The adsorption procedure leads to the formation of well-ordered S adlayers, as revealed by the STM and AFM images. The ex situ photoemission experiments were carried out in a UHV chamber. The sample was transported in air for a few seconds. The chamber is equipped with a double-pass cylinder mirror analyzer. A Mg anode in the X-ray source was used for experiments (1253.6 eV photons).
Figure 1. Typical STM images of Au substrates used for S and alkanethiol adsorption: (a) 200 × 200 nm2, vapor deposited Au on glass with Au(111) terraces, taken in 0.1 M HClO4. The 62 × 62 nm2 inset shows the 23×x3 reconstruction of the Au(111) terraces. (b) 200 × 200 nm2, Au(111) single crystal taken in UHV.
Results and Discussion Substrate Characterization. Figure 1 shows the topography of the substrates used for S and alkanethiol adsorption: Au films deposited from vapor on glass (Figure 1a) and a Au single crystal (Figure 1b). The surface of the Au film deposited from the vapor consists of atomically smooth (111) terraces 40-60 nm in size separated by monatomic height steps (Figure 1a). In a clean aqueous environment we have observed the well-known herringbone reconstruction of the Au(111) surface12 as shown in Figure 1a, inset. On the other hand, the surface of the single-crystal substrate after the cleaning procedure in UHV shows larger terraces with characteristic triangular vacancy islands monatomic in height. These terraces also exhibit the herringbone reconstruction of the Au(111) surface (image similar to the Figure 1a, inset). Comparing both images, we can conclude that although the general morphology is very similar, the average size of the terraces is larger in the single crystal (Figure 1b) than in evaporated Au surfaces (Figure 1a). Therefore, the evaporated substrate has a higher number of step edges and kinks than the single-crystal one. Electrochemical and in Situ STM Images for S Adsorption. Figure 2 shows a typical current density (j)/E profile recorded for a Au film deposited from the vapor on glass and inmersed in a 0.1 M NaOH + 3 × 10-3 M Na2S electrolyte and recorded at 0.1 V s-1.13-15 Several (12) Behm, R. J. In Scanning Tunneling Microscopy and Related Methods; Behm, R. J., Garcia, N., Rohrer, H., Eds.; NATO ASI Series; Kluwer Academic Publishers: Dordrecht, 1989; p 173.
Figure 2. Typical j vs E profile for vapor deposited Au on glass recorded at 0.1 V s-1 in 0.1 M NaOH + 3 × 10-3 M Na2S. The inset shows the ocp vs t plot.
features are observed in the figure: a net cathodic current for E < -1.15 V related to the hydrogen evolution reaction (HER), two humps at -1.13 V (AI) and -1.17 V (CI), and finally two well-defined peaks at -0.90 V (AII) and -0.96 V (CII). Similar results are obtained using the Au(111) single crystal. To correlate these features with the adlayer’s different surface structures, we have made in situ STM measurements under potential control in nitrogen-saturated 0.1 M NaOH + 3 × 10-3 M Na2S solutions. Some of the sequential in situ STM images are shown in Figure 3. Thus, we have observed that the humps in the j/E profiles are related to the adsorption/desorption (13) Vericat, C.; Andreasen, G.; Vela, M. E.; Salvarezza, R. C. J. Phys. Chem. B 2000, 104, 302. (14) McCarley, R. L.; Kim, Y.-T.; Bard, A. J. J. Phys. Chem. 1993, 97, 211. (15) Gao, X.; Zhang, Y.; Weaver, M. J. J. Phys. Chem. 1992, 96, 4156.
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Figure 3. Sequential STM images (7.3 × 7.3 nm2, raw data) of the Au(111) surface in 0.1 M NaOH + 3 × 10-3 M Na2S at different E values and times. (a) E ) -0.90 V, t ) 0 s; (b) E ) -0.7 V, t ) 30 s; (c) E ) -0.7 V, t ) 120 s; (d) E ) -0.6 V, t ) 120 s. The dynamics of the x3×x3R30° w S8 lattice transformation is shown.
of S at/from step edges.13 Peak AII corresponds to the adsorption of sulfur species at hollow sites forming a x3×x3R30° lattice with a nearest neighbor distance d ) 0.5 nm, as can be inferred from the analysis of Figure 3a, and in good agreement with previously published results.13,15 It has been already reported13 that desorption of this lattice from the Au(111) surface takes place at peak CII. The charge density (q) estimated by the integration of current peaks AII/CII is 150 ( 10 µC cm-2. This value of q for the number of atoms in the x3×x3R30° lattice implies a charge transfer of two electrons per S atom to the Au(111).15 In the range -0.90 V < E < -0.60 V (AIII) the value of j remains nearly constant during the anodic potential scans. In this potential range the formation of S octomers (S8),10,15 which are reduced at hump CIII (-0.80 V), takes place. In fact, when the potential is changed from -0.9 V (peak AII) to -0.7 V, in situ STM images (Figure 3b,c) show that the x3×x3 R30° regions transform into quasirectangular S8 species, 0.60 × 0.58 nm2 in average size. This transformation is rather slow, and it is only completed when E is changed to -0.6 V (Figure 3d). At E values more positive than -0.6 V a net anodic current related to bulk S formation is observed.15 The x3×x3R30° w S8 transformation takes place in the potential range -0.7/-0.5 V, i.e., at typical potentials observed under open circuit potential (ocp) conditions for Au electrodes in the nitrogen-saturated working solution (Figure 2, inset). In fact, the ocp changes rapidly from -0.9 to -0.7 V and then increases slowly with time to reach values close to -0.5 V. Therefore, the ocp values cover the potential range where x3×x3R30° S lattice, S8, and some bulk S is formed. This means that S8 species are spontaneously formed on the Au(111) surface, although they can coexist with domains of the x3×x3R30° S lattice and bulk S. The S8 octomers have been also observed by ex situ STM imaging of the substrate after immersion for 5-60 min in the 0.1 M NaOH + 3 × 10-3 M Na2S solution. XPS Data of Spontaneously Formed S Adlayers. We have investigated the nature of the bonding of the spontaneously formed S adlayers by ex situ XPS. Several Au(111) samples, previously annealed, were prepared by immersion in the electrolyte (0.1 M NaOH + 3 × 10-3 M Na2S) for time intervals ranging from 10 to 60 min. Immediately after rinsing, the samples were introduced in the UHV system for analysis.
Figure 4. XPS spectra (S 2p) taken from S adlayers on different Au substrates and deposition times. Peaks have been fitted to three Voight curves. Experimental points are represented by dots and the best fit is represented by a continuous line.
Figure 4a shows XPS spectra of the S 2p core level peak from evaporated Au samples with 10-min (central and lower curves) and 20-min (upper curve) immersions, respectively. In the range 20 min < t < 60 min XPS spectra are similar to that shown for t ) 20 min. In the figure, experimental data are represented by dots and the continuous line overlapping them corresponds to the best fit. Three different components were necessary for a good fitting of the spectra. Thus, the peaks have been fitted to three Voight curves with a Gaussian and Lorentzian width of approximately 1.3 and 0.1 eV, respectively. Each of the curves has two components split by 1.1 eV and with a branching ratio of 0.5 that account for the spin-orbit splitting of the S 2p core level. The binding energies of these components are 161.0, 162.0, and 163.3 eV (for the S 2p3/2). From now on we will refer to these peaks as components c1, c2, and c3, respectively. In Figure 4 it can be observed that while the intensity of c2 increases, the intensity of c1 decreases; i.e., the c2/c1 intensity ratio increases with the immersion (adsorption) time. Comparing the binding energies of the components with previously published S 2p reference peaks, an assignation to different adsorbed S species can be made. Table 1 gives a summary of some of the published binding energies (BE) for adsorbed S on Au and for some S compounds.6-10,16-19 In this table it can be seen that S forming multilayers or elemental S appears at BE between 163 and 164 eV. In these compounds S has a “neutral” valence state. Thus, we can assign the peak at 163.3 eV (c3) to either some S in multilayer or to a certain amount of elemental sulfur19 that could be formed either in defects of the substrate or in domain boundaries. In some STM images we have observed areas where S multilayers are present (data not shown). These species are weakly bound to the gold surface, and consequently, the tip moves them from one (16) Rodriguez, J. A. Private communication. (17) Handbook of X-ray Photoelectron Spectroscopy; King, Ch., Jr., Ed.; Perkin-Elmer Corporation: Eden Prairie: Minnesota, 1995. (18) Rodriguez, J. A.; Chaturvedi, S.; Jirsak, T., Chem. Phys. Lett. 1998, 296, 421. (19) Buckley, A. N.; Hamilton, I. C.; Woods R. J. Electroanal. Chem. 1987, 216, 213.
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thiols
species
S adsorbed on Au, θ e 1/3
Sn on Au
elemental S: multilayers, Sn
thiol adsorbed in x3×x3R30° and/or c(4×2)
unbounded thiol
binding energy/eV refs
161.2 9, 16, 19
162.3 19
163.5-164.0 16-18
162.0 6-9
163.5 7
STM image to the next. Furthermore, by normalizing the area of the c3 component to the sum of the other two, we have found that the normalized c3 intensity is the same for both polycrystalline samples. However, the normalized c3 intensity for the single crystal is half that found for the Au films. This observation can be correlated to the morphological analysis of the substrate: the single-crystal surface presents larger atomically ordered terraces and fewer steps and grain boundaries (Figure 1b) than the polycrystalline one. The latter presents regions where multilayers of elemental sulfur could be found in a greater extent. This component is the lowest of the spectra, suggesting that these species are present on the surface in smaller quantities. Besides, it should be noted that c3 is the widest component, indicating a disordered nature of the associated species. The species labeled as c1 and c2 have lower BE values than c3, suggesting that S is chemically bound to the Au surface atoms. The assignation of component c1 at 161.0 eV is straight from Table 1. This component presents the lowest binding energy, and its absolute value agrees, within the experimental resolution, with the BE value found for diluted S (surface coverage up to one-third) adsorbed from the gas phase in UHV on Au.16 On the other hand, the dominant component, c2, does not have a direct interpretation. To assign this component we have to correlate XPS information with STM data. In Figure 3, we have shown that two main adsorbed species coexist at the surface at potentials in the range of open circuit potential values: x3×x3R30° and S8. We have also shown that the area covered by a x3×x3R30° lattice decreases with time to favor the formation of S8 species, which finally cover most of the surface (see Figure 3). Therefore, it is reasonable to think that the intensity of the signal corresponding to S8 should increase as the immersion (adsorption) time is increased, and this is indeed what occurs with c2. This interpretation is supported by a previous study19 for S adsorption on Au that assigns the peak at 162.0 eV to Sn species (Table 1). Thus, we can conclude that components at 161.0 eV (c1) and 162.0 eV (c2) correspond to chemisorbed S in a x3×x3R30° lattice and S8 octomers, respectively. It is interesting to remark that the ratio c1/c2, which is proportional to the area covered by the x3×x3R30° structure, decreases with deposition time. However, for a similar deposition time and different surface termination (low and central spectra in Figure 4), the ratio is not constant, being lower for the single-crystal surface. This would mean that the kinetics of the global x3×x3R30° w S8 transformation is faster on the single-crystal surface; i.e., it does not depend exclusively on the time but also on morphological factors. In fact, once the transformation process is started at a site in a given terrace, the atomic rearrangement is rapidly propagated to the whole terrace (see ref 13). Therefore, this transformation is faster on the single-crystal surface, which presents larger terraces than the polycrystalline one. STM and AFM Imaging of Thiol Adlayers. We have also prepared dodecanethiol and hexanethiol adlayers on Au(111) by immersion of the Au substrates in ethanolic solutions. It is well-known that alkanethiols adsorb on
Figure 5. (a-d) STM images of alkanethiol lattices (4.5 × 4.5 nm2, raw data): (a, c) x3×x3R30°; (b) rectangular c(4×2); (d) zigzag c(4×2). (a, b) Hexanethiol, (c, d) dodecanethiol. (e, f) AFM images of alkanethiol lattices (4.5 × 4.5 nm2, raw data) showing the x3×x3R30° alkanethiol lattice: (e) hexanethiol; (f) dodecanethiol.
Au(111) forming ordered structures where molecules are bound to the Au surface through their S heads and tilted from the surface normal.1 In general, STM and AFM images of well-ordered alkanethiol covered Au(111) surfaces exhibit domains with two related surface structures, the x3×x3R30° lattice (Figure 5a,c) and its c(4×2) superlattice (Figure 5b,d).1,20 The x3×x3R30° and c(4×2) lattices are also observed, although with less resolution, by AFM imaging (Figure 5e,f). In the x3×x3R30° lattice, alkanethiol molecules are placed on the Au(111) surface with a nearest neighbor distance d ) 0.5 nm.1 Theoretical calculations have shown that the adsorption energy of thiols is lower at hcp and fcc hollow sites and depends on the Au-S-C angle.21 Recent experimental data for alkanethiol adsorption on Cu(111) reveal an equivalent occupation of fcc and hcp hollow sites.22 On the other hand, the c(4×2) superlattice exhibits a similar surface arrangement as that found in the x3×x3R30° lattice. The different contrast of the alkanethiol molecules in the STM image (Figure 5c,d) of this superlattice has been assigned to either a different tilt of the hydrocarbon chains23 or to a small displacement of a row of alkanethiol molecules from a hollow to a nearest (20) Touzov, I.; Gorman, C. B. J. Phys. Chem. B 1997, 101, 5263. Tera´n, F.; Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. Langmuir 1998, 14, 7203. (21) Beardmore, K. M.; Kress, J. D.; Bishop, A. R.; Grombech-Jensen, N. Synth. Met. 1997, 84, 317. (22) Jackson, G. J.; Woodruff, D. P.; Jones, R. G.; Singh, N. K.; Chan, A. S. Y.; Cowie, B. C. C.; Formoso, V. Phys. Rev. Lett. 2000, 84, 119.
Sulfur-Substrate Interactions in S Adlayers
Figure 6. XPS spectra (S 2p) of different alkanethiol chains and S adlayers on Au(111).
bridge site. This leads to alternating rows of alkanethiol molecules at different substrate sites.24 Note that these alkanethiol structures are closely related to the x3×x3R30° S lattice. XPS Data of Thiol Adlayers. The comparison of XPS results for spontaneously formed alkanethiol and S adlayers leads to interesting conclusions. Figure 6 shows XPS data of the spontaneously formed dodecanethiol, hexanethiol, and S adlayers. In this figure it can be seen that all three spectra resemble each other, and that the maxima appear at ≈162 eV. This is the BE assigned to the thiolate-Au bond (see Table 1) and also the BE that we have assigned to S8 surface structures on Au (c2 component, Figure 4). Actually, the same three components previously discussed for S adlayers can also be fitted in the experimentally wide alkanethiol peaks; the higher being that corresponding to the c2 component. Other authors9,10 have already reported the presence of those components. Briefly, for alkanethiols on Au, the c3 component has been related to the thiol-thiol bond,7 i.e., some amount of unbounded thiols, while the origin of the c1 component is still controversial.6-10 Recently it has been suggested that c1 reflects the adsorption of small amounts of S present in alkanethiols as a contaminant.9 The presence of a main peak c2 at 162 eV in the spectra of hexanethiol and dodecanethiol should be related to S forming either the x3×x3R30° or the c(4×2) alkanethiol lattices, as evidenced from STM and AFM images (see Figure 5). Therefore, the chemical state of the S-Au bond x3×x3R30° or the c(4×2) alkanethiol lattices in these alkanethiol lattices seems to be similar to that found in S8, as the latter species are characterized by the same c2 component. In principle, this result could support the presence of disulfide bonds in adsorbed alkanethiols on Au(111). In fact, it has been proposed that the S heads of alkanethiol molecules could form disulfide bonds (d ) 0.20.22 nm, S atoms at hollow and bridge sites) by introducing gauche defects in the molecules distorting the C-S angle.25 Therefore, to explain the STM images of x3×x3R30° or c(4×2) alkanethiol lattices with this model it is necessary to assume that it is the hydrocarbon chains, which remain separated by d ) 0.5 nm, that are sensed by the STM. However, with few exceptions, it is accepted that this (23) Anselmetti, D.; Baratoff, A.; Guntherodt, H. J.; Delamarche, E.; Michel, B.; Gerber, C.; Kang, H.; Wolf, H.; Ringsdorf, H. Europhys. Lett. 1994, 27, 365. (24) Tera´n, F.; Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. J. Chem. Phys. 1998, 109, 5703. (25) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216.
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Figure 7. Ep(vf0) vs n plot for the electrodesorption of different alkanethiols. The Ep(vf0) value was calculated by plotting Ep vs v and extrapolating for v ) 0. The slope is 0.03 V/C unit. The experimental point at n ) 0 is the Ep(vf0) value for S desorption. (Data taken from ref 11.)
technique senses the positions of the S atoms.24 There is other evidence against disulfide formation. In fact, isolated S atoms on Au(111) rearrange spontaneously to form S8 structures with d ) 0.3 nm rather than 0.20-0.22 nm; i.e., a disulfide bond is not formed.13 Therefore, disulfide formation should be more difficult in alkanethiol adlayers because, in addition, a gauche defect should be introduced in the molecule. Recent experimental results have shown the absence of disulfide bonds in alkanethiol SAMs at room temperature.26 The fact that the c2 component is characteristic of the S-Au bond in the x3×x3R30° lattice of alkanethiol (S heads at hollow sites) and of the S-Au bond in S8 (S atoms at hollow and top sites) reveals a minor role of substrate sites. Therefore, it is the nature of the S-Au bond that dominates the XPS spectra. More important, we can see that sulfur atoms of these completely different adlayers spontaneously reach a similar common chemical state. This state is less ionic than that found in the x3×x3R30° S lattice. In fact, the latter evolves spontaneously to S8 when S atoms are present in the environment.27 The chemical state of spontaneously formed alkanethiol and S adlayers explains the behavior of the x3×x3R30° alkanethiol lattice electrodesorption in 0.1 M NaOH.11 In fact, a plot of the peak potential for adlayer electrodesorption measured at a low sweep rate, Ep(vf 0), vs the hydrocarbon chain length (n) for different alkanethiols (Figure 7) leads to a straight line with a slope of ≈0.03 V mol-1/C atom unit, which accounts for chain-chain interactions and hydrophobic forces. For n ) 0 the result is Ep,n)0 ) -0.69 V, a value that reflects the S-Au bond energy in alkanethiol adlayers without any influence of the hydrocarbon chains. This refers to a hypothetical situation where we have an isolated thiol molecule adsorbed on Au(111). Surprisingly, it has been observed11 that desorption of the x3×x3R30° S lattice takes place at a potential ≈0.2 V more negative than Ep,n)0) -0.69 V. For a one-electron charge-transfer process this potential difference represents a bond energy ≈19 kJ mol-1 greater for S adsorbed in the x3×x3R30° lattice.11 Our XPS data clarify this point. In fact, the maximum of the thiol peak, corresponding to the thiolate-Au bond in the x3×x3R30° alkanethiol lattice, is ≈1 eV higher than the c1 component corresponding to the S-Au bond in the x3×x3R30° S lattice. Therefore, a less ionic and weaker S-Au bond is present in this alkanethiol surface structure. (26) Kluth, G. J.; Carraro, C.; Maboudian, R. Phys. Rev. B. 1999, 59, R10449. (27) Wan, L.-J.; Shundo, S.; Inukai, J.; Itaya, K. Langmuir 2000, 16, 2164.
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Conclusions We have demonstrated that a spontaneously formed S adlayer on Au consists of a mixture of x3×x3R30° lattice, S8, and some elemental S. It has also been found that S8 has the same chemical state as the S heads of alkanethiol molecules in x3×x3R30° and c(4×2) alkanethiol lattices. This implies that the substrate sites do not markedly affect the state of S adsorbed on Au. Our XPS results also show that the S-Au bond in the x3×x3R30° and c(4×2) alkanethiol lattices is less ionic than that found for S in the x3×x3R30° S lattice. This result explains the differences observed in the behavior of the electroreductive desorption curves for x3×x3R30° S and x3×x3R30° and c(4×2) alkanethiol lattices.
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Acknowledgment. Financial support from Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica (PICT 99 6-5030), CONICET (PIP-0897), PI1999/128 (Argentina), and Spanish Agency DGCyT (PB98/0524) is acknowledged. This work has been performed within the CONICET/CSIC and Programa de Cooperacio´n con Iberoame´rica (MEC) research programs. The authors thank Prof. J. A. Rodriguez for providing the binding energy value of S adsorbed on Au in UHV. We also thank E. Roman and O. Bo¨hme for their assistance during the XPS measurements. LA0018179
