J. Phys. Chem. B 2000, 104, 3563-3569
3563
Molecular Orientation and Ordered Structure of Benzenethiol Adsorbed on Gold(111) Li-Jun Wan,†,§ Mimi Terashima,‡ Hiroyuki Noda,‡ and Masatoshi Osawa*,† Catalysis Research Center and Graduate School of EnVironmental Earth Science, Hokkaido UniVersity, Sapporo 060-0811, Japan ReceiVed: September 17, 1999; In Final Form: January 22, 2000
The adsorption of benzenethiol on Au(111) from aqueous solution has been investigated by using surfaceenhanced infrared absorption spectroscopy (SEIRAS), scanning tunneling microscopy (STM), and cyclic voltammetry (CV). This molecule is dissociatively adsorbed on the surface as a Au benzenethiolate and forms a well-ordered monolayer with a commensurate (x13×x13)R13.9° symmetry. The ordered phase is formed within a few minutes in 0.1 mM aqueous solution, and further immersion of the substrate results in the deposition of multilayers. The phenyl ring is tilted about 30° from the surface normal in the monolayer. The coverage of this molecule is calculated to be 0.31 (or 0.71 nmol cm-2) from the (x13×x13)R13.9° symmetry, but it was estimated to be 0.23 (or 0.53 nmol cm-2) from the reductive desorption of the monolayer in alkaline solution. The discrepancy between the two measurements may be ascribed to the coadsorption of sulfur atoms and the presence of vacancies.
Introduction Self-assembled monolayers (SAMs) of organothiols and disulfides on metal surfaces are promising systems for use in chemical sensing, biocompatibility, wetting, lubrication, corrosion inhibition, lithographic patterning, molecular electronics, and other technologically important applications.1-4 So far, alkanethiol SAMs have been investigated most extensively by using a range of techniques because of their ease-to-preparation, long-term stability, and characteristic closed-packed structures. In recent years, SAMs of aromatic thiols have come to receive strong attention due to their high conductivity, nonlinear optical properties, and other various functionalities.5-10 For example, the SAM of 4-mercaptopyridine on Au promotes the heterogeneous redox reaction of cytochrome c.8-10 However, much less is known about aromatic thiol SAMs compared with alkanethiol counterparts. Benzenethiol (BT) is one of the simplest aromatic thiols. In the past decades, the adsorption of BT on metal surfaces from the gas phase and solution phase was investigated. Studies on the BT adsorption from the gas phase onto transition metals such as Mo, Ni, and Rh were focused mainly on catalytic desulfurization.11-21 This molecule is dissociatively adsorbed on these metal surfaces via the sulfur atom to form benzenthiolate. The phenyl ring was found to tilt slightly from the surface normal. On Mo(110), near-edge X-ray absorption fine structure (NEXAFS) studies indicate a tilt of ∼23° away from the surface normal.12,22 The S-C bond is nearly normal to the surface for benzenethiolate adsorbed in 4-fold sites of Ni(100).23 Benzenthiolate decomposes at 300-400 K to yield benzene and hydrogen. Other products such as biphenyl, diphenyl sulfide, and dibenzenethiophene have also been observed on Au(110).17 On the other hand, the studies on the BT adsorption from the solution phase has been directed toward the self-assembly * Corresponding author. E-mail:
[email protected]. Fax: +8111-709-4748. † Catalysis Research Center. § Present address: Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan. ‡ Graduate School of Environmental Earth Science.
of compact monolayers on coinage metal surfaces (Au, Ag, and Cu). Despite a large number of studies, there remains a controversy regarding the orientation of the molecule. Surfaceenhanced Raman scattering (SERS) was employed to determine the molecular orientation.24-28 Completely different conclusions were deduced from almost identical SER spectra. Sandroff and Herschback,24 Joo et al.,25 and Szafranski et al.26 concluded that the phenyl ring lies nearly flat on Ag and Au surfaces, whereas Takahashi et al.27 and Carron and Hurley28 concluded that the phenyl ring is nearly perpendicular to the surfaces. By quantitative analyses of the spectra in terms of electromagnetic theories, Carron and Hurley28 estimated the tilting angle of the phenyl ring from the surface normal to be 14° on Au, whereas Szafranski et al.26 estimated it to be about 76°. High-resolution electron energy loss spectroscopy (HREELS) was also employed to determine the orientation of benzenthiolate on the (111) surfaces of Ag, Pt, and Au.29-31 From a comparison of intensities of in-plane and out-of-plane vibrational modes, vertical and nearly vertical orientations were concluded. The thickness of the BT monolayer on Au(111) determined by X-ray photoelectron spectroscopy (XPS) is 0.6-0.8 nm,31 which is a reasonable value for vertically oriented benzenethiolate. However, a film thickness of 0.1-0.15 nm was measured by ellipsometry.5,32 There is a controversy also regarding the two-dimensional ordering of BT molecules on single-crystal surfaces. Gui et al.30 observed by low-energy electron diffraction (LEED) that the adsorption of BT on Ag(111) yields an ordered phase with a (x7×x31, 88°)R40.9° symmetry. On the other hand, it was reported that BT does not form any ordered phases on Au(111) due to poor molecular packing.32,33 However, the surface coverage of BT on Au(111) reported in the literature34 is identical to that on Ag(111) (0.24).30 Very recently, Jung et al.35 carried out molecular dynamics calculations of the ordering of BT molecules on Au(111) and predicted that any long-range ordered phases will not be formed in the framework of (x3 × x3) and (2 × 2) structures due to repulsive interactions between phenyl rings. Nevertheless, it is noteworthy that some aromatic thiols form well-ordered phases on Au(111); (x3×x3)R30°
10.1021/jp993328r CCC: $19.00 © 2000 American Chemical Society Published on Web 03/28/2000
3564 J. Phys. Chem. B, Vol. 104, No. 15, 2000 phases for 4-aminothiophenol, benzyl mercaptans, 4-biphenylmethanethiols, and 4-hydroxythiophenol,32,36,37 (2x3×x3)R30° phases for oligo(phenylethynyl)benzenethiols,33 and (5×x3) phases for 4-mercaptopyridine.38,39 It is still unclear why BT does not form any ordered phases on Au(111). In the present study, we have characterized BT SAMs formed on Au(111) from the solution phase by using surface-enhanced infrared absorption spectroscopy (SEIRAS),40 scanning tunneling microscopy (STM), and cyclic voltammetry (CV) to make clear the controversies mentioned above. Since the orientation and ordering of BT molecules will be strongly affected by sample preparation conditions, we paid great care to selfassembling the SAM. We report herein the first STM observation of a well-ordered monolayer of BT on Au(111). On the basis of the surface selection rule in SEIRAS,40,41 we show that the phenyl ring is inclined by about 30° from the surface normal in the ordered phase. CV is used to estimate coverage of BT, which is compared with STM images. All the measurements were carried out in solution under potential control unless otherwise noted. The observation in solution has a benefit to protect the surface from contamination. SEIRAS is a new infrared technique suitable to probe solidliquid interfaces at a higher sensitivity with less interference from the bulk solution compared with the so-called infrared reflection-absorption spectroscopy.40 The absorption enhancement primarily stems from an electromagnetic mechanism.40,41 The island structure of a thin metal film used in the measurements facilitates production of a strong electric field through the excitation of plasma oscillations of the metal, resulting in absorption enhancement. The enhanced electric field is polarized perpendicular to local surfaces of each particle and thus only vibrations having dipole derivative components perpendicular to the surface are infrared active. This surface selection rule can be used to determine molecular orientation. The enhanced electric field decays very sharply within a few molecules distance from the surface. Thus, absorption enhancement is significant for the first monolayer directly attaching to the surface, which enables us to selectively monitor the Au/solution interface.
Wan et al. STM studies were carried out with a Nanoscope E (Digital Instruments, Santa Barbara, CA) in 0.1 M HClO4. The tunneling tips were 0.25 mm tungsten wires electrochemically etched in 1 M KOH and insulated with clear nail polish so as to minimize the faradaic current occurring at the tip. STM images were acquired in the constant current mode. STM scanners were calibrated laterally with the lattice constant of Au(111)-(1×1) (aAu ) 0.288 nm) and vertically using monatomic height steps (0.249 nm). SEIRAS measurements were carried out using the so-called Kretschmann attenuated-total-reflection (ATR) configuration (a prism/thin Au film/solution geometry).40 The thin Au film (mass thickness of 20 nm measured with a quartz microbalance) was prepared by vacuum evaporation of Au on top of a hemicylindrical silicon prism and then flame-annealed shortly (about 10 s) to yield a preferentially oriented (111) surface.46 The thin film consisted of metal particles with a dimension of 50-80 nm. STM observation revealed that the metal particles are covered with (111) terraces.46 Electrochemical behaviors of the film were nearly identical to those of well-defined bulk Au(111) electrodes. Electrochemical cell and details of the experimental procedures were described elsewhere.47 The Fourier transform IR spectrometer used was a Bio-Rad FTS-575C or FTS-60A/896 equipped with a liquid N2 cooled MCT detector and operated at a spectral resolution of 4 cm-1. Spectra are shown in the absorbance units defined as -log(I/I0), where I and I0 represent intensities of the infrared beam reflected from the Au/solution interface after and before modifying the electrode surface with a BT SAM, respectively. The solution background can be canceled out completely by this procedure. Electrochemical measurements were performed with a potentiostat (EG&G, Model 263A). Electrolyte solutions were prepared from HClO4 (Merck, supurapure), KOH (Wako, super special grade), and Milli-Q water and deaerated by argon gas bubbling before use. The reference electrode was a platinum wire in STM measurements, and a saturated calomel electrode (SCE) in SEIRAS and electrochemical measurements. However, all potentials in this paper are quoted with respect to SCE. Results and Discussion
Experimental Section A (111) facet on a single-crystal Au bead, prepared by melting an Au wire (99.999% in purity, 0.8 nm in diameter), was used as the substrate for the STM studies. In electrochemical measurements, a hemispherical single-crystal Au(111) electrode, prepared by cutting a single-crystal Au bead, was used as the working electrode. The electrode surface was polished with diamond pastes down to 0.05 µm and then annealed in an electric furnace at 900 °C for 24 h to remove damages on the surface. The substrate was further annealed before each measurement by a hydrogen-oxygen flame and quenched into ultrapure Milli-Q water saturated with hydrogen. Cyclic voltammograms for the Au(11) electrode in 0.1 M H2SO4 were in good agreement with literature reports.42,43 The roughness factor of the surface estimated from the reduction wave of the surface oxide layer in the voltammograms42,44 was 1.04. Benzenethiol (Kanto Chemical, special grade reagent) was used as received without further purification. BT SAMs were repaired by immersing the substrate into 0.1 mM aqueous solution for 1-2 min. A freshly prepared solution was always used to minimize the adsorption of the sulfur ion produced by the decomposition of BT in solution.45 Then the substrate was sonicated in ultrapure water to remove excess adsorbate from the surface.
Adsorption Kinetics. Gui et al.30 observed by LEED that the adsorption of BT on Ag(111) from a saturated (about 0.1 mM) aqueous solution yields an ordered monolayer with a (x7×x31, 88°)R40.9° symmetry. However, no ordered phases have been observed on Au(111). In the most previous studies using Au(111) surfaces, organic solutions (methanol, ethanol, and acetone) were commonly used in self-assembling BT SAMs. Recently, Taniguchi and co-workers45 reported that water is more suitable than organic solvents in self-assembling wellordered monolayers of 4-mercaptopyridine on Au(111). On the basis of these previous findings, we prepared a BT SAM with 0.1 mM aqueous solution and found that a well-ordered phase is formed, as described later. The ordered structure was observed only when the Au substrate had been immersed into the solution for less than 2 min. As the immersion period increases, disordered patches looking like clouds appeared on the wellordered monolayer and grew in size. The disordered patches could be removed partly by prolonged sonication in water. These observations suggest the multiplayer deposition of BT (probably as biphenyl disulfide because BT is soluble in water). The multiplayer deposition seems to prevent observation of the ordered phase in previous STM studies. To confirm the monolayer formation within a few minutes, the kinetics of the BT adsorption on an evaporated Au(111)
Benzenethiol Adsorbed on Gold(111)
J. Phys. Chem. B, Vol. 104, No. 15, 2000 3565
Figure 2. Cyclic voltammogram for an Au(111) electrode modified with a BT SAM measured in 0.5 M KOH at 10 mV s-1. The SAM was prepared by immersing the Au(111) electrode into a 0.1 mM aqueous solution of BT for 1 min.
Figure 1. Series of infrared spectra showing the adsorption of BT from 0.1 mM aqueous solution onto an Au(111) electrode at rest potential (A) and plot of the integrated intensity of the 1472 cm-1 band as a function of time (B). The spectra were acquired every 15 s since the injection of BT into the cell. The background spectrum of the electrode was measured before the injection of BT.
electrode from 0.1 mM aqueous solution was examined by realtime spectral acquisitions. The reference spectrum of the bare electrode was measured first in pure water. Then BT was injected into the electrochemical cell to be a concentration of 0.1 mM, and the spectra were accumulated sequentially at every 15 s. To enhance the diffusion of BT to the electrode surface, the solution was continuously bubbled with argon gas. A series of spectra thus obtained are shown in Figure 1A. Two positivegoing bands characteristic of benzenethiolate (vide infra) appear at 1472 and 1574 cm-1 in the spectral range between 1300 and 1800 cm-1 soon after the injection of BT and grow in intensity with time. No peak shifts were observed, suggesting moleculesubstrate and molecule-molecule interactions are constant during the adsorption process. This result is interpreted as that BT is adsorbed on the surface via nucleation-growth kinetics.31 Concomitant with the growth of these bands, the bending mode of water appears around 1640 cm-1 as a negative-going peak. The negative intensity represents that water molecules are repelled from the interface by the BT adsorption. The adsorption of BT also causes the rise of the baseline level of the spectrum. All these spectral changes are synchronized. As plotted in Figure 1B, the integrated intensity of the 1472 cm-1 band appears to saturate within about 1 min and then increases gradually. Since the absorption enhancement is significant for the first monolayer,40 the latter gradual increase in intensity may be ascribed to the multiplayer deposition. For a further support of the monolayer formation, coverage (or surface concentration, Γ) of BT on the Au(111) surface was evaluated by CV as a function of immersing period. Figure 2 shows a typical voltammogram for the Au(111) electrode on which BT was adsorbed from a 0.1 mM aqueous solution with an immersing period of 1 min. The electrolyte solution used here was 0.5 M KOH, and the scan rate was 10 mV s-1. The voltammogram exhibits a strong cathodic peak at -0.6 V
attributed to the reductive desorption of BT from the Au(111) surface34,45 and an additional weak peak at -0.95 V. The charge for the BT desorption, the area under the major peak, is measured to be 51 µC cm-2. In this calculation, the doublelayer charging current was subtracted by using a straight capacitive-baseline approximation. The charge was constant within an experimental error of (10% for immersing periods longer than 1 min. The charge measured in the present study is very close to that reported by Zhong and Porter34 for a BT SAM prepared from a 0.1 mM methanol solution with an immersing period of 24 h (52 µC cm-2), indicating that full coverage is reached within about 1 min. Assuming a one-electron reductive process (PhS-Au + e- f PhS- + Au),34 the coverage of BT is calculated to be 0.23 (Γ ) 0.53 nmol cm-2) from the charge of 51 µC cm-2. Although a larger coverage of 0.45 was measured by XPS,31 it is noted that this value is larger than the theoretically highest surface coverage of 0.34 (Γ ) 0.79 nmol cm-2) calculated using van der Waals dimensions of BT (0.64 × 0.33 nm2 for a vertically oriented phenyl ring). The origin of the minor peak at -0.95 V is not clear, but two possibilities are considered; the desorption of sulfur atoms45,48 and the desorption of BT from defect sites on the Au(111) surface.49,50 Taniguchi et al.45 have suggested that a trace amount of sulfur is included in commercially available thiols as an impurity and that sulfur is adsorbed faster than thiols. The charge for the minor peak was typically 10 µC cm-2. Considering the roughness factor of the electrode (1.04), this peak is much too large to attribute to the desorption of BT from defect sites on the surface and more likely to be the desorption of sulfur atoms. If we assume a two-electron reductive process for the sulfur desorption (Au-S + 2e- f Au + S2-), sulfur coverage is estimated to be 0.02 (Γ ) 0.05 nmol cm-2) from the charge. Molecular Orientation. In Figure 3, a spectrum extracted from Figure 1 (collected at 5 min after the BT injection) is compared with the transmission spectrum of neat BT (a and b, respectively). The band assignments were taken from ref 28, where a1, b1, and b2 represent symmetries of the phenyl ring vibrations (under an assumption of C2V symmetry). A notable difference between the two spectra is the absence of the SH stretching mode (ν(SH) at 2568 cm-1) in the surface spectrum. Ring modes are observed at almost the same frequencies as for neat BT. These results clearly indicate that BT is adsorbed on the surface as an Au benzenethiolate. Nevertheless, the surface spectrum is simpler than the bulk spectrum. All the clearly identified bands are a1 modes, and b2 modes (at 3073, 1680,
3566 J. Phys. Chem. B, Vol. 104, No. 15, 2000
Wan et al.
Figure 3. Comparison of the infrared spectra of BT adsorbed on an Au(111) surface (a) and neat BT (b). Spectrum a was extracted from Figure 1 (collected at 5 min after the injection of BT). The band assignments are taken from ref 28.
Figure 5. Transmission infrared spectra of BT adsorbed on a 6 nm thick Au film evaporated on a silicon water (a) and Au(I) benzenethiolate (b).
where I° represents the intrinsic intensity of the corresponding mode (∝|dµ b/dQ|2). From these three equations, the following two equations are introduced.
tan2 θ ) Figure 4. Representation of the model used in estimating molecular orientation and the directions of dipole moment changes of a1, b1, and b2 modes. Experimental and molecular systems are represented by XYZ and xyz axes, respectively. θ: tilt angle from the surface normal. χ: twist angle of the molecular plane around the S-phenyl bond (χ ) 0° when the y-axis is parallel to the surface). φ: rotation angle of the molecular system around the Z axis.
1442, and 1092 cm-1 in the bulk spectrum) are hardly observed. The preferential observation of a1 modes can be ascribed to a specific orientation of BT in terms of the surface selection rule. The orientation is discussed quantitatively below. The surface electric field, B E, effectively has only a normal component (Z direction in Figure 4).40,47 The intensity of a vibrational mode is proportional to the square of scalar product of the electric field and the dipole moment derivative of the mode, dµ b/dQ.51
I∝
| | | |
dµ b 2 dµ b2 2 |E B| cos2 R B E ) dQ dQ
(1)
where R is the angle between B E and dµ b/dQ. The coordinate system is defined in Figure 4, where θ is the tilt angle of the z axis of the molecular system from the surface normal (Z), χ is the twist angle of the molecular plane around the z axis (which is 0° when y is parallel to the surface), and φ is the rotation angle of the molecular system around the z axis. In-plane a1 and b2 modes have dipole moment derivatives along z and y axes, respectively. On the other hand, out-of-plane b1 modes have dipole moment derivatives perpendicular to the phenyl ring (along x axis). The molecular-fixed axis system xyz can be correlated with the experimental axis system XYZ by three Eulerian angles θ, χ, and φ.52 The intensities of a1, b1, and b2 modes are represented as follows by rewriting eq 1 using this correlation.
I(a1) ∝ cos2 θ I°(a1)
(2)
I(b1) ∝ sin2 θ cos2 χ I°(b1)
(3)
I(b2) ∝ sin2 θ sin2 χ I°(b2)
(4)
I(b1) I°(a1)
1 I(a1) I°(b1) cos2 χ
tan2 χ )
I(b2) I°(b1) I(b1) I°(b2)
(5)
(6)
The very weak intensities of b2 modes clearly indicate that BT is oriented with y axis nearly parallel to the surface (χ ≈ 0°). To determine the tilt angle θ, the intensity of a b1 mode is necessary to be compared with that of an a1 mode. Unfortunately, the spectral range below 950 cm-1 where b1 modes locate could not be measured due to the low transmittance of the silicon prism and the low intensity of the light source. To observe the low-frequency region, the SEIRA spectrum of BT adsorbed on a 6 nm thick Au film, evaporated onto a 0.5 mm thick silicon wafer, was measured in air in the transmission mode. The surface selection rule mentioned above is applicable also to the transmission SEIRA spectrum.41 Figure 5a shows a typical transmission SEIRA spectrum, where the CH out-of-plane mode is clearly seen at 736 cm-1. Note that the spectral features in the spectral range above 1000 cm-1 are identical to that measured in solution with the ATR configuration (Figure 3a). Using eq 5, θ is estimated to be about 30° from a comparison with the transmission spectrum of Au(I) benzenethiolate synthesized through the reduction of HAu(III)Cl4 in the presence of BT53 (Figure 5b). The molecular orientation of BT on Au(111) deduced by SEIRAS is close to a slightly tilted orientation (θ ∼ 23°) on Mo(110) determined by NEXAFS.12,22 The result is also close to a tilted orientation determined using SERS by Carron and Hurley (θ ) 14° and χ ) 0°)28 but largely different from a flat orientation (θ ) 76 ( 5°, χ ) 0°) reported by Szafranski et al.26 Two-Dimensionally Ordered Structure. The ordered structure of BT was investigated by STM in 0.1 M HClO4 under potential control. Prior to the STM investigation, the potential range in which the monolayer can stably exist was examined by CV and SEIRAS. Figure 6 shows voltammograms for the Au(111) electrode in 0.1 M HClO4 before and after modifying with a BT SAM (dashed and solid curves, respectively). The voltammogram of the unmodified Au(111) electrode is identical to that reported in the literature42 and exhibits a pair of peaks
Benzenethiol Adsorbed on Gold(111)
J. Phys. Chem. B, Vol. 104, No. 15, 2000 3567
Figure 6. Cyclic voltammograms for an Au(111) electrode in 0.1 M HClO4 measured before (dashed trace) and after (solid trace) modifying the surface with a BT SAM. The scan rate was 50 mV s-1.
Figure 8. High-resolution STM image (5 × 5 nm2) showing the ordered structure of a BT SAM on Au(111) observed in 0.1 M HClO4 at 0.2 V (A) and cross sectional profiles along a-a′, b-b′, and c-c′ lines shown on the image (B). An oblique unit cell is overlaid on the image. Figure 7. STM image of a BT SAM on Au(111) observed in 0.1 M HClO4 at 0.2 V.
at around 0.4 V, which are assigned to the adsorption and desorption of perchlorate.42,47 The weak feature completely disappears by the adsorption of BT. The noticeable decrease of the double-layer charging current is characteristic of organic adsorption. The cathodic current observed at potentials less positive than 0 V is due to hydrogen evolution. SEIRAS measurements revealed that BT is desorbed from the surface associated with hydrogen evolution. The desorption of BT was observed also at potentials more positive than 1 V where the underlying Au surface is oxidized. In the potential range between 0 and 1 V, no changes were observed in the SEIRA spectrum and STM image. Therefore, we report hereafter only the results obtained at a constant potential of 0.2 V. A typical STM image (scan area of 20 × 20 nm2) of a BT SAM on Au(111) is shown in Figure 7, which shows a wellordered structure. This ordered phase was consistently observed over terraces, and disordered domains were hardly observed. However, a close inspection revealed that there exist many molecular scale (∼0.5 nm) defects, which are seen as black spots on the image (especially, at the center of the image). The origin of the defects will be described later. Figure 8A shows a typical high-resolution STM image (5 × 5 nm2) of the ordered BT phase. Bright and less bright spots are arranged in a hexagonal structure. Since sulfur atoms are generally imaged brighter than other atoms (or atom groups) due to a stronger interaction with the surface,36,54 we tentatively assign the spots to the sulfur headgroups of BT molecules. The cross-sectional profiles in Figure 8B show that the nearest neighbor inter-spot spacing is 0.52 ( 0.2 nm and the periodicity
along lines a-a′ and b-b′ is 1.04 ( 0.2 nm. The angle between the two lattice vectors, γ, is about 60°. An oblique unit cell is overlaid on the image. The observed structure is apparently different from the (x7×x31, 88°)R40.9° structure found on Ag(111).30 So far, ordered phases with (mx3×nx3)R30° (m and n are integers) symmetries have been observed for many aromatic thiols32,33,36,37 and alkanethiols55-57 on Au(111). The observed structure is close to a (2x3×2x3)R30° symmetry (a ) b ) 2x3aAu ) 1.0 nm and γ ) 60°). However, a careful inspection of the molecular layer and the underlying Au(111) surface revealed that the unit vectors of the ordered BT layer are deviated 14 ( 3° from the close-packed lattice directions (〈110〉 directions) of the underlying Au surface. The 〈110〉 directions were determined by observing an atomic resolution image of the Au surface at the same potential (0.2 V) after BT molecules had been desorbed from the surface by holding the electrode potential at -0.4 V for a few minutes. On the basis of these observations, we propose a commensurate (x13×x13)R13.9° structure shown in Figure 9. Different contrast of the spots is assumed to arise from the difference in adsorption sites: bright spots at the four corners of the unit cell locate in on-top sites and less bright spots locate in bridge sites (this can be reversed). The unit cell parameters of this model (a ) b ) x13aAu ) 1.04 nm, γ ) 60°, and δ ) 13.9°) are in good agreement with the observations. Ab initio calculations predict that 3-fold hollow and bridge sites on Au(111) are energetically more stable than on-top sites for the adsorption of HS and CH3SH.58 However, no consistent structures that include 3-fold hollow sites were found. The (x13×x13)R13.9° unit cell contains four spots. Assuming that all the spots correspond to BT molecules, coverage of BT is calculated to be 0.306 (Γ ) 0.709 nmol/cm2). This
3568 J. Phys. Chem. B, Vol. 104, No. 15, 2000
Wan et al. sulfur atoms and the presence of vacancies at which BT molecules are not adsorbing. Acknowledgment. This work was supported by the Ministry of Education, Culture, Science, and Sports, Japan, through a Grant-in-Aid for Scientific Research on Priority Area of “Electrochemistry of Ordered Interfaces” (No. 09237101) and Grant-in-Aid for COE Research. References and Notes
Figure 9. Commensurate (x13×x13)R13.9° model structure for the BT SAM on Au(111). Open circles represent the surface Au atoms, and sulfur headgroups (shaded circles) of the molecules are assumed to locate in on-top and bridge sites.
Figure 10. High-resolution STM image (height-shaded 3D plot) of a BT SAM observed around the center part in Figure 7. Arrows denote weak and missing spots locating in the centers of unit meshes.
value is larger than that evaluated by CV (0.23) even if the (10% error in the CV measurements is taken into account. The discrepancy will be ascribed to the molecular scale defects observed in Figure 7. The defects are observed as very weak or missing spots on the mildly filtered high-resolution STM image shown in Figure 10 (indicated by arrows). These spots are speculated to be coadsorbing sulfur atoms and vacancies. These spots always locate in the centers of unit meshes, suggesting that this site is less favorable than other sites for the BT adsorption, probably due to steric hindrance and repulsive interactions between phenyl rings.35 Conclusions We investigated the orientation and structure of BT adsorbed onto Au(111) from 0.1 mM aqueous solution using SEIRAS, STM, and CV. In situ real-time SEIRAS and CV measurements revealed that a compact monolayer is formed within a few minutes. From SEIRA spectra, the phenyl ring plane was deduced to be tilted about 30° from the surface normal. We observed for the first time that the adsorption of BT on a Au(111) surface yields a well-ordered phase. The ordered phase was observed only when the substrate was immersed into the solution for less than 2 min, probably due to a multilayer deposition of BT at longer immersion periods. High-resolution STM imaging allowed us to propose a commensurate (x13×x13)R13.9° structure for the ordered phase. The surface coverage of BT estimated by STM was 0.306, whereas it was estimated to be 0.23 from the reductive desorption in alkaline solution. The discrepancy was attributed to the coadsorption of
(1) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991. (2) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312. (3) Dubois, L. H.; Nuzzo, R. Annu. ReV. Phys. Chem. 1992, 43, 437. (4) Ulman, A. Chem. ReV. 1996, 96, 1533. (5) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (6) Dhirani, A.; Lin, P.-H.; Guyot-Sinnost; Zehner, R. W.; Shita, L. R. J. Chem. Phys. 1997, 106, 5249. (7) Zehner, R. W.; Sita, L. R. Langmuir 1997, 13, 2973. (8) Armstrong, F. A.; Hill, H. A. O.; Walton, N. J. Acc. Chem. Res. 1988, 21, 407. (9) Lamp, B. D.; Hobara, D.; Porter, M.; Niki, K.; Cotton, T. M. Langmuir 1997, 13, 736. (10) Taniguchi, I.; Yoshimoto, S.; Nishiyama, K. Chem Lett. 1997, 353. (11) Agron, P. A.; Carison, T. A. J. Vac. Sci. Technol. 1982, 20, 815. (12) Robert, J. T.; Friend, C. M. J. Chem. Phys. 1988, 88, 7172. (13) Friend, C. M.; Roberts, J. T. Acc. Chem. Res. 1988, 21, 394. (14) Huntley, D. R. J. Phys. Chem. 1992, 96, 4550. (15) Shen, W.; Nyberg, G. L.; Kiesengang, J. Surf. Sc. 1993, 298, 143. (16) Rafael, T. S.; Huntley, D. R.; Mullins, D. R.; Gland, J. L. J. Phys. Chem. 1994, 98, 13022. (17) Jaffey, D. M.; Madix, R. J. J. Am. Chem. Soc. 1994, 116, 3020. (18) Weldon, M. K.; Napier, M. E.; Wiegand, B. C.; Friend, C. M.; Uvdal, P. J. Am. Chem. Soc. 1994, 116, 8328. (19) Bol, C. W.; Friend, C. M.; Xu, X. Langmuir 1996, 12, 6083. (20) Kane, S. M.; Rufael, T. S.; Gland, J. L.; Huntley, D. T.; Fisher, D. A. J. Phys. Chem. B 1997, 101, 8486. (21) Chen, D. A.; Friend, C. M.; Xu, H. Surf. Sci. 1998, 395, L221. (22) Sto¨hr, J.; Outka, D. A. Phys. ReV. B 1987, 36, 7891. (23) Takata, Y.; Yokoyama, T.; Yagi, S.; Happo, N.; Sato, H.; Seki, K.; Ohta, T. Surf. Sci. 1991, 259, 266. (24) Sandroff, C. J.; Herschbach, D. R. J. Phys. Chem. 1982, 86, 3277. (25) Joo, T. H.; Kim, M. S.; Kim, K. J. Raman Spectrosc. 1987, 18, 57. (26) Szafranski, C. A.; Tannaer, W.; Laibinis, P. E.; Garrell, R. Langmuir 1998, 14, 3570. (27) Takahashi, M.; Fujita, M.; Ito, M. Surf. Sci. 1985, 158, 307. (28) Carron, K. T.; Hurley, G. J. Phys. Chem. 1991, 95, 9979. (29) Stern, D. A.; Wellner, E.; Salaita, G. N.; Languren-Davidson, L.; Lu, F.; Batina, N.; Frank, D. G.; Zapien, D. C.; Walton, N.; Hubbard, A. T. J. Am. Chem. Soc. 1988, 110, 4885. (30) Gui, J. Y.; Stern, D. A.; Frank, D. G.; Lu, F.; Zapien, D. C.; Hubbard, A. T. Langmuir 1991, 7, 955. (31) Whelan, C. M.; Smyth, M. R.; Barnes, C. J. Langmuir 1999, 15, 116. (32) Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L. Langmuir 1997, 13, 4018. (33) Dhirani, A.-A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319. (34) Zhong, C.-J.; Porter, M. D. J. Am. Chem. Soc. 1994, 116, 11616. (35) Jung, H. H.; Won, Y. D.; Shin, S.; Kim, K. Langmuir 1999, 15, 1147. (36) Kim, Y.-T.; McCarley, R. L.; Bard, A. J. J. Phys. Chem. 1992, 96, 7416. (37) Jin, Q.; Rodriguez, J. A.; Li, C. Z.; Darici, Y.; Tao, N. J. Surface Sci. 1999, 425, 101. (38) Sawaguchi, T.; Mizutani, F.; Taniguchi, I. Langmuir 1998, 14, 3565. (39) Wan, L.-J.; Hara, Y.; Noda, H.; Osawa, M. J. Phys. Chem. B 1998, 102, 5943. (40) Osawa, M. Bull. Chem. Soc. Jpn. 1997, 70, 2861. (41) Osawa, M.; Ataka, K.; Yoshii, K.; Nishikawa, Y. Appl. Spectrosc. 1993, 47, 1497. (42) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. Electrochim. Acta 1986, 31, 1051; J. Electroanal. Chem. 1987, 228, 429. (43) Dretschkow, Th.; Wandlowski, Th. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 749.
Benzenethiol Adsorbed on Gold(111) (44) Uchida, H.; Ikeda, N.; Watanabe, M. J. Electroanal. Chem. 1997, 424, 5. (45) Yoshimoto, S.; Yoshida, M.; Kobayashi, S.; Nozute, S.; Miyawaki, T.; Hashimoto, Y.; Taniguchi, I. J. Electroanal. Chem. 1999, 473, 85. (46) Sun, S.-G.; W, -. B. C.; Wan, L.-J.; Osawa, M. J. Phys. Chem. B 1999, 103, 2460. (47) Ataka, K.; Yotsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996, 100, 10664. (48) Weisshaar, D. E.; Waleczk, M. M.; Porter, M. D. Langmuir 1993, 9, 323. (49) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (50) Zhong, C.-J.; Zak, J.; Porter, M. J. Electroanal. Chem. 1997, 421, 9.
J. Phys. Chem. B, Vol. 104, No. 15, 2000 3569 (51) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (52) E. B. Wilson, J.; Decius, J. C.; Cross, P. C. Molecular Vibrations; MaGraw-Hill: New York, 1955. (53) Puddenphatt, R. The Chemistry of Gold; Elsevier Scientific Publishing: Oxford, U.K., 1978; p 61. (54) Venkataraman, B.; Flynn, G. W.; Wilbur, J. L.; Folkers, J. P.; Whitesides, G. J. Phys. Chem. 1995, 99, 8684. (55) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805. (56) Dubios, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (57) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (58) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389.
