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J. Phys. Chem. C 2008, 112, 17336–17339
Potential-Induced Reorientation of Physisorbed n-Hexatriacontane on a Au(111) Electrode Studied by in-Situ Infrared Reflection Absorption Spectroscopy Osamu Endo,* Keita Tsuji, and Hiroyuki Ozaki Department of Organic and Polymer Materials Chemistry, Faculty of Technology, Tokyo UniVersity of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan ReceiVed: June 20, 2008; ReVised Manuscript ReceiVed: September 3, 2008
The potential-induced structural changes of a monolayer of n-hexatriacontane (HTC) molecules physisorbed on a Au(111) electrode have been studied in the sulfate solution by cyclic voltammetry and in situ infrared reflection absorption spectroscopy. The CCC planes of the adsorbed HTC molecules are parallel to the surface around the potential of zero charge and inclined in both the positive and negative potential regions. At positive potentials, the sulfate anions are adsorbed on the Au(111) surface of the modified electrode in the same manner as on a clean Au(111) surface. 1. Introduction Self-assembly of long alkyl chains on solid surfaces is an intriguing phenomenon, and it has gained importance in both bioscience and nanoscience. In order to form a thin layer with well-defined structures and electronic properties, it is essential to acquire a proper understanding of the behavior of alkyl chains on solid surfaces. In recent decades, the structures of n-alkane monolayers on cleaved surfaces of layered materials and single crystal surfaces of transition metals have been intensively studied by infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM), under ultrahigh vacuum (UHV) conditions1-6 as well as at liquid-solid interfaces.7-14 Such monolayers consist of a columnar assembly of molecules, whose chain axes and CCC planes are parallel to the surface (“flat-on” orientation). The columnar structure is a common characteristic of monolayers of n-alkanes and their derivatives.7-16 Furthermore, it is understood that van der Waals interactions between the substrate and the molecules play a crucial role in their assembly on solid surfaces. On the other hand, hydrophobic interactions are known to affect the aggregation of alkyl chains in an aqueous environment.17 Because of these interactions, chain molecules form a droplet or micellar structure; consequently, the area of contact between alkyl groups and water decreases. A monolayer of n-alkane molecules that have the same orientation on a solid surface in contact with an aqueous phase is appropriate for studying molecular structures and the competition between van der Waals and hydrophobic interactions at the liquid-solid interface. By using an electrolyte solution as an aqueous phase and an n-alkane monolayer as a modifier of a metal electrode, we can study the response of n-alkane molecules to electrochemical potentials and the stability of an n-alkane monolayer under electrochemical conditions, which are necessary for the development of new biosystems and nanodevices. Recently, He et al. reported results of in situ STM of a n-hexadecane monolayer formed on a Au(111) surface, in contact with an electrolyte solution under potential control.18 They found that columnar structures were maintained at 0.05-0.65 V (vs Ag/AgCl) around the potential of zero charge * To whom correspondence should be addressed. E-mail: oendo@ cc.tuat.ac.jp.
(pzc, 0.23 V vs Ag/AgCl) and were reversibly modified at both positive and negative potentials. Alkanols exhibit a similar response to the potentials. 1-Octadecanol is physisorbed on a Au(111) electrode at the pzc; however, it is desorbed from the electrode surface at negative potentials, forming aggregates near it.19,20 These results are interesting because they show that the monolayer of alkyl chains is stable under aqueous electrochemical conditions at least at potentials around the pzc. They also show that an assembly of neutral molecules can be reversibly controlled by applying suitable electrochemical potentials. To further investigate the electrochemical control of chain assemblies, it is necessary to elucidate changes in molecular structure induced by the application of a potential and effect of the electrochemical environment on van der Waals interactions between the chain molecules and the substrate. Since n-alkanes do not contain heteroatoms and functional groups, we can determine the properties of only the alkyl chains by investigating the behavior of n-alkanes on the electrode surface. It is speculated that the response of n-alkane assemblies to potentials is different from the redox-induced change in orientation, which occurs in modified electrodes including the self-assembled monolayer of ferrocenylalkanethiols21-23 and aromatic carboxylic acids.24-26 In the case of these modified electrode, charge transfer occurs through an electroactive functional group of adsorbed organic molecules. In contrast, it is suggested that some polar or ionic species should displace n-alkane molecules on the electrode surface at both positive and negative potentials.18 In this sense, it is quite appropriate to consider that the response of n-alkane is similar to that occurring in wetting equilibria of hydrocarbon droplets at a dropping mercury electrode.27 It has been reported that wetting of hydrocarbons occurs at potentials around the pzc, and specific adsorption of anions decreases the wetting range;27 this indicates that anions displace hydrocarbons at sufficiently positive potentials. However, adsorption of the corresponding species on the surface has not been observed directly. Furthermore, no information is available on the orientation and conformation of n-alkane molecules removed from the electrode surface and brought into an aqueous solution. In this paper, we report the potential-dependent structures of a Au(111) electrode modified with a n-hexatriacontane (HTC; C36H74) monolayer in a 0.5 M H2SO4 solution determined by cyclic voltammetry (CV) and in situ IRAS. It is known that
10.1021/jp805437q CCC: $40.75 2008 American Chemical Society Published on Web 10/10/2008
Reorientation of n-HTC on a Au(111) Electrode
J. Phys. Chem. C, Vol. 112, No. 44, 2008 17337
Figure 1. Cyclic voltammograms of the HTC modified (solid line) and unmodified Au(111) electrode (dashed line) in 0.5 M H2SO4 with a scan rate of 50 mV/s.
HTC molecules form stable columnar structures on the Au(111) surface at room temperature.11 Changes in the orientation and conformation of the HTC molecules and the adsorption of (bi)sulfate ions were detected via CH and SO stretching vibrational modes in the IRA spectra, respectively. The relationship between the wavenumbers of these modes as a function of the electrode potential (electrochemical frequency shift) and the characteristics of interactions between the adsorbate and the substrate were also examined in this study. 2. Experimental Details A mechanically polished Au(111) single crystal (MaTecK, Inc.) was annealed in a H2 flame for 2 h. The Au(111) surface was cleaned with a brief process of flame-annealing and then quenching in pure acetone or water for each experiment. The clean surface was immersed into the 0.2 mM acetone solution of HTC and rinsed in the pure solvent to obtain a well-defined monolayer. Electrolyte solutions were prepared with Milli-Q water (>18 MΩ) and H2SO4 (super special grade, Wako Pure Chemical Industries, Ltd.). Cyclic voltammograms (CVs) were obtained with a scan rate of 50 mV/s. Potentials were referred to a saturated Hg|Hg2SO4 electrode and subsequently converted to values against SHE. In-situ IRA spectra were obtained with a Fourier transform infrared spectrometer (FTS-45RD, Varian, Inc.) and an MCT detector with a resolution of 2 cm-1. Details of the cell and setup have been described elsewhere.28,29 3. Results and Discussion 3.1. Cyclic Voltammetry of the HTC Modified Au(111) Electrode. Figure 1 shows the CVs of the Au(111) electrode (dashed line) and the HTC modified one (solid line) in the sulfate solution. In the anodic sweep of Au(111), a sharp peak related to the reconstruction lifting, i.e., the structural transition of the top layer Au atoms from (3 × 23) to (1 × 1), appears at 0.53 V.30 Beyond this peak, bisulfate/sulfate anions start to be adsorbed randomly on the surface and the coverage increases as the potential becomes more positive. Finally at the 1.03 V peak, the ordered structures of the (3 × 7) phase are formed. This phase is ascribable to a mixed structure of water and (bi)sulfate.31 All these processes are reversible. The peak of the reconstruction lifting shifted to a more positive potential for the modified electrode. This CV is stably
Figure 2. (a) CH stretching vibration region and (b) SO stretching vibration region of the in situ IRA spectra of the HTC modified Au(111) electrode in 0.5 M H2SO4. All the spectra are referred to the spectrum at 0.3 V.
obtained after several potential cycles in the same region. In the first scan of the modified electrode immersed into the solution at 0.3 V, reconstruction lifting appears at a more positive potential. In contrast, the peak representing the formation of the ordered (bi)sulfate layer is almost the same as that of the Au(111). These two features indicate that the HTC overlayer induces a delay in the reconstruction lifting, while the (bi)sulfate anions are adsorbed in the same manner as those on the Au(111). In other words, a bare Au(111) surface is exposed to the solution at the positive potentials. A delay of the reconstruction lifting by an organic adlayer has been reported for an in situ surface X-ray scattering study of pyridine, 2,2′bipyridine, and uracil on Au(111).32 This is also consistent with the results of in situ STM wherein a n-hexadecane monolayer stabilizes the reconstructed Au(111) surface.18 3.2. In-situ IRA Spectra of the HTC Modified Au(111) Electrode. Figure 2a displays the CH stretching vibration region of the in situ IRA spectra for the HTC modified Au(111) electrode (see Table 1 for band assigments). Since all the spectra are referred to the spectrum at 0.3 V, the upward (downward) bands indicate an increase (decrease) in the intensities of the corresponding modes as compared to those at 0.3 V. The upward bands near 2909 and 2810 cm-1 are assigned to the decoupled CH stretching modes. The former is ascribed to the stretching of the CHdistal bond getting away from the surface (ν(CHdistal)) and the latter to that of the CHproximal bond of which the H atom is in contact with the surface (ν(CHproximal), the so-called “soft mode”).5,6 These modes are interpreted as proof of the flat-on orientation of the alkyl chains on metal surfaces.5,6 It should be
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TABLE 1: Band Assignments for the IRA Spectra of n-Hexatriacontane on a Au(111) Electrodea wavenumber/cm-1
direction
assignment
2810 2850 2909 2920 2957
up down up down down
CHproximal CH2 symmetric CHdistal CH2 antisymmetric CH3 antisymmetric
a
The directions “up” and “down” represent a decrease or increase in the intensity of the corresponding bands, respectively, as compared to those at 0.3 V.
pointed out that the frequencies of these decoupled modes seem to be almost unperturbed by the existence of water and ions existing above. On the other hand, the downward bands around 2920 and 2850 cm-1, observed for all but the 0.2-0.5 V spectra, are assigned to the antisymmetric (νas) and symmetric CH2 stretching vibrational modes (νs), respectively.33 The fact that both νas and νs are detected in the p-polarized IRA spectra suggests tilted CCC planes (see below), and hence, it is concluded that the orientation of the HTC molecules adsorbed on the surface is flat-on at 0.2-0.5 V and is altered to “tiltedon” outside this region. Note that the potential of zero charge for an unreconstructed Au(111) is about 0.43 V (vs SHE).34 In other words, the potential region for the flat-on orientation is distributed near the pzc (pzc region). This is in good agreement with the columnar structures maintained within this region, which was observed by in situ STM for a n-hexadecane monolayer on a Au(111) electrode in the perchlorate solution.18 It is intriguing to note the featureless spectra observed at 0.2-0.5 V. When a certain species exists at both the potential of interest and that of the reference, the frequency shift should be detected in the IRA spectra as a differential peak. Therefore, no features in this potential region indicate no frequency shift of ν(CHdistal) and ν(CHproximal). For most adsorbates on electrode surfaces, including anions and neutral molecules such as carbon monoxide, the frequency shift of the stretching vibrational mode has been reported as a function of the applied potential.35-37 There are several explanations for the frequency shift. First, since the adsorbate is under an interfacial electric field, the Stark effect might be operational on the adsorbate.38 Second, if the adsorbates exchange electrons with the substrate (donation and backdonation), the orbital populations and bond strengths vary with the potential, causing the frequency shift as well (electronic effect).35 Third, if the coverage of the adsorbate is changed with the potentials, the extent of the dipole coupling among the adsorbates is affected.39 Of these factors, the electronic effect does not contradict the lack of the frequency shift for the present HTC monolayer. Since the HTC molecules are physisorbed on the Au(111) surface, the electron donation and backdonation cannot be effective. In other words, the amount of the electron exchange is quite small compared with that involved in chemisorption. Moreover, the metal electronic states interacting with / the σCH and σCH orbitals of the alkyl chains are the d-bands located at higher binding energies,40 which are unlikely affected by the position and population of the Fermi level or the electrode potential. Thus, the change in the electrode potential does not / significantly alter the population of the σCH orbitals and the frequency of the relevant vibrational mode. If the Stark effect is the main origin of the frequency shift, it should be observed because there is an interfacial electric field over the HTC monolayer even though the molecules are physisorbed. Figure 3a shows that the frequencies of νas and νs slightly depend on the potential. Since the symmetric stretching of
Figure 3. (a) Wavenumber of the antisymmetric (νas) and symmetric stretching vibrational mode of CH2 (νs) as a function of the electrode potential. (b) Wavenumber of the symmetric stretching vibrational mode of SO3 as a function of the electrode potential. (c) Peak height ratio of the CH stretching vibrational modes (I(νs)/I(νas)) as a function of the electrode potential.
(bi)sulfate exhibits much higher frequency shift than νas and νs (Figure 3b; see below), however, the electrochemical effects discussed above are not responsible for the slight shifts in Figure 3a. The frequencies of the CH2 stretching vibrational modes are sensitive to the conformation of the alkyl chain.33 The higher frequencies are characteristic of the methylene sequences with gauche defects and the lower ones are attributed to the trans conformations. In Figure 3a, it is shown that the frequencies of νas and νs are almost maximal at potentials just above and below the pzc region and converge at 2918 and 2850 cm-1, respectively, as the potential leaves the pzc. It is deduced, therefore, that at the initial stages of the structural changes, the chain orientation is altered at a certain portion of the alkyl chain (probably near a terminal methyl group at both ends of the molecule), causing conformational disorder. As the potentials move outside the pzc region, the orientation change spreads into the middle of the molecule and finally the trans conformations are recovered. The peak height of νs(I(νs)) relative to that of νas(I(νas)) is plotted as a function of the potential in Figure 3c. The I(νs)/ I(νas) value becomes larger at the potentials outside the pzc region, which indicates that the CCC plane is tilted to the surface and inclination grows with increasing (or decreasing) potential because the transition dipole moment of νs and that of νas are parallel and perpendicular to the CCC plane, respectively. The inclination of the alkyl chain direction is, however, not very large in all the observed potentials, since both νs and νas with the transition dipole moment perpendicular to the chain axis are observed; if the chain axis were perpendicular to the surface
Reorientation of n-HTC on a Au(111) Electrode (upright or “end-on” orientation), the transition dipole moments would be orthogonalized to the electric vector and the corresponding bands would be vanished in the p-polarized IRA spectra. It is to be noted that this result does not seem to be in agreement with that of the in situ STM for n-hexadecane on Au(111).18 In the perchlorate solution, a “ring” structure was observed because of the hexadecane molecules standing upright and surrounding the Au particles. The Au particles were formed as a result of the reconstruction lifting at the positive potentials, and almost no structures attributable to the molecules were observed in the negative potentials. The difference in the behavior of the alkanes can be ascribed to the difference in the chain length (C36 vs C16). Since the mobility of n-alkane becomes lower with increasing carbon number, it is probable that most HTC molecules remain near the surface at all the potentials observed in the present experiment. The change in the orientation of the HTC molecules forms a vacancy on the surface for water molecules and ions. Traces of the OH stretching vibrational mode are observed in the spectra outside the pzc region (not shown) and the (bi)sulfate ions are clearly detected via the SO stretching vibrational mode in the in situ IRA spectra, as shown in Figure 2b. The band centered at 1210 cm-1 in the spectra at -0.1-0.2 V is assigned to the SO3 asymmetric stretching vibrational mode of the (bi)sulfate in the bulk solution.41 This mode vanishes upon adsorption due to the surface selection rule. On the other hand, bands with lower wavenumbers ∼1160-1200 cm-1 observed above 0.7 V are attributed to the SO3 symmetric stretching vibrational mode of the adsorbed (bi)sulfate with a 3-fold geometry on Au(111),36,37 which is observed at 1050 cm-1 in the bulk solution. Figure 3b displays the wavenumbers of the SO3 symmetric stretching vibrational mode of the adsorbed (bi)sulfate species both on the modified (filled squares) and unmodified Au(111) (open circles) as a function of the potentials. The peak position is almost the same for these two electrodes at all the potentials. This means that the adsorbed state of the (bi)sulfates is unperturbed by the existence of the HTC molecules at the interface, which is in agreement with the results of the CV in the previous section that the peak at 1.03 V represents the formation of the ordered (3 × 7) phase of (bi)sulfate. All these structural changes are reversible, i.e., the HTC monolayer is recovered when the potential is moved to the pzc region again. This indicates that the HTC/Au(111) interface is more stable than the water/Au(111) interface in the pzc region, while the (water + ion)/Au(111) interface is more stable in both the positive and negative potentials. 4. Conclusion The potential-induced orientation changes of the CCC planes of HTC molecules assembled on a Au(111) electrode was studied by CV and in situ IRAS. The flat-on orientation is maintained at 0.2-0.5 V, and outside this potential region, the CCC planes are inclined to make a vacancy for water and (bi)sulfate directly adsorbed on the Au(111) surface. Acknowledgment. The authors greatly appreciate the experimental support for in situ IRAS by Prof. Masatoki Ito (Keio University) and Dr. Masashi Nakamura (Chiba University).
J. Phys. Chem. C, Vol. 112, No. 44, 2008 17339 References and Notes (1) Hostetler, M. J.; Manner, W. L.; Nuzzo, R. G.; Girolami, G. S. J. Phys. Chem. 1995, 99, 15269. (2) Manner, W. L.; Bishop, A. R.; Girolami, G. S.; Nuzzo, R. G. J. Phys. Chem. B 1998, 102, 8816. (3) Bishop, A. R.; Hostetler, M. J.; Girolami, G. S.; Nuzzo, R. G. J. Am. Chem. Soc. 1998, 120, 3305. (4) Bishop, A. R.; Girolami, G. S.; Nuzzo, R. G. J. Phys. Chem. B 2000, 104, 754. (5) Yamamoto, M.; Sakurai, Y.; Hosoi, Y.; Ishii, H.; Kajikawa, K.; Ouchi, Y.; Seki, K. J. Phys. Chem. B 2000, 104, 7363. (6) Fosser, K. A.; Nuzzo, R. G.; Bagus, P. S.; Wo¨ll, C. J. Chem. Phys. 2003, 118, 5115. (7) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424. (8) Frommer, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 1298. (9) Claypool, C. L.; Faglioni, F.; Goddard, W. A., III; Gray, H. B.; Lewis, N. S.; Marcus, R. A. J. Phys. Chem. B 1997, 101, 5978. (10) Uosaki, K.; Yamada, R. J. Am. Chem. Soc. 1999, 121, 4090. (11) Marchenko, O.; Cousty, J. Phys. ReV. Lett. 2000, 84, 5363. (12) Yabion, D. G.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B 2000, 104, 7627. (13) Xie, Z.-X.; Xu, X.; Mao, B.-W.; Tanaka, K. Langmuir 2002, 18, 3113. (14) Zhang, H.-M.; Xie, Z.-X.; Mao, B.-W.; Xu, X. Chem.sEur. J. 2004, 10, 1415. (15) Endo, O.; Ootsubo, H.; Toda, N.; Suhara, M.; Ozaki, H.; Mazaki, Y. J. Am. Chem. Soc. 2004, 126, 9894. (16) Endo, O.; Furuta, T.; Ozaki, H.; Sonoyama, M.; Mazaki, Y. J. Phys. Chem. B 2006, 110, 13100. (17) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992. (18) He, Y.; Ye, T.; Borguet, E. J. Phys. Chem. B 2002, 106, 11264. (19) Lipkowski, D. B. J. J. Electroanal. Chem. 1996, 409, 33. (20) Shepherd, J. L.; Bizzotto, D. Langmuir 2006, 22, 4869. (21) Ye, S.; Sato, Y.; Uosaki, K. Langmuir 1997, 13, 3157. (22) Viana, A. S.; Jones, A. H.; Abrantes, L. M.; Kalaji, M. J. Electroanal. Chem. 2001, 500, 290. (23) Yao, X.; Yang, M.; Wang, Y.; Hu, Z. Sens. Actuators, B 2007, 122, 351. (24) Li, H. Q.; Roscoe, S. G.; Lipkowski, J. J. Electroanal. Chem. 1999, 478, 67. (25) Ikezawa, Y.; Sekiguchi, R.; Kitazume, T. Electrochim. Acta 2000, 46, 731. (26) Han, B.; Li, Z.; Wandlowski, T. Anal. Bioanal. Chem. 2007, 388, 121. (27) Ivosˇevic´, N.; Zˇutic´, V.; Tomaic´, J. Langmuir 1999, 15, 7063. (28) Matsumoto, H.; Oda, I.; Inukai, J.; Ito, M. J. Electroanal. Chem. 1993, 356, 275. (29) Shingaya, Y.; Ito, M. J. Electroanal. Chem. 1993, 372, 283. (30) Wang, J.; Ocko, B. M.; Davenport, A. J.; Isaacs, H. S. Phys. ReV. B 1992, 46, 10321. (31) Magnussen, O. M.; Hangebock, J.; Hotlos, J.; Behm, R. J. J. Faraday Discuss. 1992, 94, 329. (32) Wandlowski, T.; Ocko, B. M.; Magnussen, O. M.; Wu, S.; Lipkowski, J. J. Electroanal. Chem. 1996, 409, 155. (33) Snyder, R. G. J. Chem. Phys. 1967, 47, 1316. (34) Kolb, D. M. Prog. Surf. Sci. 1996, 51, 109. (35) Faguy, P. W.; Markovic, N.; Adzic, R. R.; Fierro, C. A.; Yeager, E. B. J. Electroanal. Chem. 1990, 289, 245. (36) Nakamura, M.; Shingaya, Y.; Ito, M. Surf. Sci. 2002, 502, 474. (37) Hoshi, N.; Kuroda, M.; Koga, O.; Hori, Y. J. Phys. Chem. B 2002, 106, 9107. (38) Lambert, D. K. Phys. ReV. Lett. 1983, 50, 2106. (39) Persson, B. N. J.; Ryberg, R. Phys. ReV. B 1981, 24, 6954. (40) Morikawa, Y.; Ishii, H.; Seki, K. Phys. ReV. B 2004, 69, 041403(R). (41) Dawson, B. S.; Irish, D. E.; Toogood, G. E. J. Phys. Chem. 1986, 90, 334.
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