Potential Dependent Orientation and Oxidative Decomposition of

Masayuki Okamura, Toshihiro Kondo, and Kohei Uosaki .... Toshihiro Kondo , Takuya Masuda , Motoko Harada , Osami Sakata , Yoshio Katsuya , Kohei Uosak...
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Potential Dependent Orientation and Oxidative Decomposition of Mercaptoalkanenitrile Monolayers on Gold. An in Situ Fourier Transform Infrared Spectroscopy Study Yukari Sato,† Shen Ye, Toshio Haba, and Kohei Uosaki* Physical Chemistry Laboratory, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060, Japan Received August 9, 1995. In Final Form: February 20, 1996X The potential dependent structural change and irreversible anodic decomposition of the self-assembled monolayers of 3-mercaptopropanenitrile (HSC2CN) and of 8-mercaptooctanenitrile (HSC7CN) on gold electrode were investigated by two modes of electrochemically modulated infrared spectroscopy, namely, the subtractively normalized interfacial Fourier transform infrared reflection-absorption spectroscopy (SNIFTIRS) and the difference spectra with only one potential alteration. In addition to the bands due to water molecules around 3100-3500 cm-1 and 1600-1700 cm-1, bands were observed around 2930, 2850, 2342, and 2250 cm-1 at both the HSC7CN and HSC2CN modified gold electrodes. Except for the band at 2342 cm-1, all the bands were observed only by using p-polarized light, suggesting these bands are due to adsorbed species. The 2930 and 2850 cm-1 bands are of the asymmetric and symmetric C-H stretching modes of methylene groups, respectively. The intensities of these two bands decreased a little as potential became more positive. These results suggest that the alkyl chain stands closer to surface normal when potential became more positive. The peak position of the 2250 cm-1 band shifted slightly to lower frequency than that of the same band in neat liquid of corresponding mercaptoalkanenitrile and was not affected by potential so much. On the basis of these results the 2250 cm-1 band was assigned as the CN stretching mode of the terminal CN group, which has no direct interaction with gold. The fact that the absorbance decreased as potential became more positive suggests that the orientation of the terminal nitrile group became closer to parallel to the surface. The 2342 cm-1 band is due to CO2 in solution generated by an irreversible oxidative decomposition of monolayer as this band was observed even by using s-polarized light and grew in the potential region where anodic current flowed. The irreversible anodic decomposition of the monolayer in positive potential region was confirmed by XPS measurements. Effects of alkyl chain length on both the potential dependence of peak position and intensity and the nature of anodic oxidation were also discussed.

Introduction Studies of chemical modification of metal and semiconductor surfaces by molecular layers have been carried out extensively.1,2 Although the most popular technique to form molecular layers has been the Langmuir-Blodgett (LB) method, recently a self-assembly method is often employed to obtain molecular layers with better stability.1,3-9 In this technique, a solid substrate is dipped into a solution containing adsorbing molecules which have a long alkyl chain, and the molecules chemisorb onto the solid surface by forming covalent bonds with surface atoms. The molecules then self-assemble through chain-chain * To whom correspondence should be addressed: tel, +81-11706-3812; fax, +81-11-706-3440; e-mail, [email protected]. hokudai.ac.jp. † Present address: National Institute of Bioscience and HumanTechnology, 1-1 Higashi, Tsukuba, Ibaraki 305, Japan. X Abstract published in Advance ACS Abstracts, May 1, 1996. (1) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembling; Academic: San Diego, CA, 1991. (2) Murray, R. W.; Weissberger, A. Molecular Design of Electrode Surfaces; Techniques of Chemistry; Sannders, W. H., Jr., Ed.; John Wiley & Sons: New York, 1992; Vol. 22. (3) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (4) Simon, R. A.; Ricco, A. J.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104, 2031. (5) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (6) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988. 4, 365. (7) Bain, C.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (8) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C.E.D. J. Am. Chem. Soc. 1987, 109, 3559. (9) Finklea, H. O.; Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987, 3, 409.

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interactions. Self-assembled monolayers of n-alkanethiol derivatives have been investigated through many approaches and the effects of the chain length and terminal group on the structure and properties of the monolayers have been studied thoroughly.10-18 Most of the papers, however, are on the structure of the monolayers exposed to air, and only limited information is available for the structure of the monolayers on electrodes in solution, although many research groups are interested in the electrochemical characteristics of self-assembled monolayers. The structure of the monolayers in electrolyte solutions should be quite different from the one in air because of the interaction between the adsorbed molecules and species in solution, i.e., solvent molecules and ions. Furthermore, the structure may be affected by applied potential. One of the most useful techniques to obtain information on the structure of adsorbed species on electrodes is in situ IR spectroscopy, which has been progressed signifi(10) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 31. (11) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (12) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510. (13) Sato, Y.; Itoigawa, H.; Uosaki, K. Bull. Chem. Soc. Jpn. 1993, 66, 1032. (14) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. Langmuir 1992, 8, 1385. (15) Hickman, J. J.; Ofer, D.; Zou, C.; Wrighton, M. S.; Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 1128. (16) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307. (17) Sato, Y.; Frey, B. L.; Corn, R. M.; Uosaki, K. Bull. Chem. Soc. Jpn. 1994, 67, 21. (18) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558.

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cantly in the last 15 years.19,20 The IR spectroelectrochemistry has been applied to many systems including the adsorption of neutral molecules such as CO21,22 and ions such as N3-,19,20 SCN-,19,23-25 OCN-,25 and CN- 19,24,26-30 from solution and the detection of reaction intermediates such as CO from many C1 compounds.19,31 Surprisingly, only few reports of in situ IR studies on the self-assembled monolayers are available. Popenoe et al. studied the structural change of ferrocenylalkanethiol upon redox of the ferrocene moiety.32 We constructed an FTIRRAS (Fourier transform infrared reflection adsorption spectroscopy)/EQCM (electrochemical quartz crystal microbalance) combined system and investigated the interfacial mass and structural changes during the redox reaction of ferrocene moiety in the self-assembled monolayer of ferrocenylundecanethiol on a gold electrode.33 In this paper, we investigate the potential dependent structural change and irreversible anodic decomposition of the self-assembled monolayers of mercaptoalkanenitrile on gold by electrochemically modulated IR spectroscopy. Mercaptoalkanenitrile molecules have an alkyl chain, a thiol group which can attach to a gold surface, and a nitrile group. Electrochemical and spectroscopic properties of mercaptoalkanenitrile on gold were reported very briefly by Chidsey and Loiacono by employing 11-mercaptodecanenitrile (HSC10CN).34 We chose these molecules as a typical example to study the effect of the potential on the structure and orientation of self-assembled monolayers because (1) they form self-assembled monolayer easily, (2) they are expected to have a wide potential window where no redox reaction takes place, and (3) the terminal nitrile group has a strong band in the frequency region where strong absorption by water layer is avoided. The IR study of the self-assembled monolayers of these compounds should also provide useful information for the understanding of adsorption characteristics of neutral nitrile molecules such as acetonitrile19,35 and benzonitrile24,36 from solution. Experimental Section 3-Mercaptopropanenitrile (HSC2CN) and 8-mercaptooctanenitrile (HSC7CN) were synthesized from 3-bromopropanenitrile (Tokyo Kasei Kogyo) and 8-bromooctanenitrile (Aldrich), respectively, by adopting the method reported before.37 These (19) Beden, B.; Lamy, C. Spectroelectrochemistry; Gale, R. J., Ed.; Plenum: New York, 1988; Chapter 5. (20) Ashley, K.; Pons, S. Chem. Rev. 1988, 88, 673. (21) Kunimatsu, K.; J. Phys. Chem. 1984, 88, 2195. (22) Kunimatsu, K.; Seki, H.; Golden, W. G.; Gordon, J. G., II Surf. Sci. 1985, 158, 596. (23) Ashley, K.; Samant, M. G.; Seki, H.; Philpott, M. R. J. Electroanal. Chem. 1989, 270, 349. (24) Corrigan, D. S.; Gao, P. L.; Leung, W. H.; Weaver, M. J. Langmuir 1986, 2, 744. (25) Bron, M.; Holze, R. J. Electroanal. Chem. 1995, 385, 105. (26) Ashley, K.; Lazaga, M.; Samant, M. G.; Seki, H.; Philpott, M. R. Surf. Sci. 1989, 219, L590. (27) Ashley, K.; Weinert, F.; Samant, M. G.; Seki, H.; Philpott, M. R. J. Phys. Chem. 1991, 95, 7409. (28) Paulisse, V. B.; Korzeniewski, C. K. J. Phys. Chem. 1992, 96, 4563. (29) Kim, C. S.; Korzeniewski, C. K. J. Phys. Chem. 1993, 97, 9784. (30) Stuhlmann, C.; Villegas, I.; Weaver, M. J. Chem. Phys. Lett. 1994, 219, 319. (31) Christensen, P. A.; Hamnett, A.; Trevellik, P. R. J. Electroanal. Chem. 1988, 242, 23. (32) Popenoe, D. D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521. (33) Shimazu, K.; Ye, S.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1994, 375, 409. (34) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (35) Faguy, P. E.; Fawcett, W. R.; Lin, G.; Motheo, A. J. J. Electroanal. Chem. 1992, 339, 339. (36) Gao, P.; Weaver, M. J. Phys. Chem. 1985, 89, 5040. (37) Urquhart, G. G.; Gates, J. W.; Connor, R. Org. Synth. 1955, III, 363.

Langmuir, Vol. 12, No. 11, 1996 2727 molecules were purified by column chromatography and identified by NMR, IR, and mass spectrometry. The electrolyte solution was 0.5 M NaClO4 aqueous solution prepared from Milli-Q purified water and sodium perchlorate monohydrate (Reagent grade, Wako Pure Chemicals). Two polycrystalline gold disks (10 mm diameter × 1 mm thick, 99.98% purity) were used as working electrode in the present study, and no difference in results was observed. The surface of the electrode was polished successively with 1.0, 0.3, and 0.05 µm alumina on a polishing cloth. The electrode was cleaned first in an ultrasonic bath and then electrochemically by cycling the potential between -0.2 V and +1.5 V in 0.05 M H2SO4 solution for 10 min before the surface modification. The roughness factor of the electrode determined by using the charge for the reduction of gold oxide was 2.0. The gold electrode was modified with HSC2CN or HSC7CN by dipping into 10 mM hexane solution of the thiol for 2 h. The electrode was rinsed with pure hexane for several times after the modification. FTIRRAS measuremenst were carried out by using a BioRad, FTS-30 spectrometer with a HgCdTe detector cooled by liquid nitrogen. Nitrogen gas was used to purge atmospheric water and CO2 in the spectrometer. A home-made IR spectroelectrochemical glass cell with CaF2 window was used for the measurements.33 The gold electrode was pressed against the window so that the thickness of water layer between the window and the gold electrode was minimized. The incidence angle at the electrode surface was approximately 65° with respect to the surface normal. In situ spectra of monolayers were obtained as difference spectra between two potentials by employing two different methods. One was the subtractively normalized interfacial Fourier transform infrared reflection-absorption spectroscopy (SNIFTIRS).19,20 A total of 1024 scans (2 cm-1 resolution) was recorded at each potential with the voltage being switched between the reference potential and the sample potential every 128 scans. Time kept at each potential was 2 min 15 s, although collection of 128 scans takes only 1 min 40 s. Collection of the spectra was started 10 s after the potential was varied. The SNIFTIRS spectra were obtained by using p-polarized light and the reference potential was 0 V. The spectra in positive and negative potential regions were obtained separately by using a freshly modified sample for each measurement to avoid the effect of the possible loss of the monolayer at very negative or positive potentials. In the negative potential region, the first sample potential was typically -0.3 V and the sample potential was changed negatively. After a set of spectra was obtained in negative potential region, the electrode was cleaned and modified by the methods described above. Then the measurement in the positive potential region was carried out. The sample potential was first set to typically +0.5 V and was changed positively. This method (SNIFTIRS) is known to be very sensitive for detecting reversible structural changes but does not provide quantitative spectra when irreversible structural change, desorption and decomposition of the monolayer, takes place. Thus, difference spectra between a reference potential and sample potentials with only one potential alteration were also obtained. The feasibility of this method to study the irreversible system was demonstrated by Corrigan et al. who referred to this approach as single potential alteration infrared spectroscopy (SPAIRS).38 The difference spectra were obtained by using both p- and s-polarized light. Usually 256 scans (4 cm-1 resolution) were collected first at 0 V (reference potential) and then at a sample potential. Potential was then set back to 0 V, and 256 scans were collected. Another 256 scans were collected at a different sample potential. This procedure was repeated so that the irreversible loss can be easily detected by comparing the spectra at 0 V before and after a sample potential was applied. As was the case in the SNIFTIRS measurement, the spectra in positive and negative potential regions were obtained separately by using a freshly modified sample for each measurement to avoid the effect of the possible loss of the monolayer at very negative or positive potentials. Electrochemical measurements were carried out in a threecompartment, three-electrode cell. For both electrochemical and spectroelectrochemical studies, a sodium chloride saturated calomel electrode (SSCE) and a Pt wire were used as a reference (38) Corrigan, D. S.; Leung, L.-W. H.; Weaver, M. J. Anal. Chem. 1987, 59, 2252.

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Figure 2. Transmission IR spectra of (a) HSC7CN and (b) HSC2CN (4 cm-1 resolution, 256 scans).

Figure 1. Cyclic voltammograms of gold electrodes, which were dip-treated in (a) 10 mM HSC7CN-hexane solution and (b) 10 mM HSC2CN-hexane solution for 2 h, measured in 0.5 M NaClO4 solution with sweep rate of 100 mV/s. and a counter electrode, respectively, and a potentiostat/ galvanostat (Toho Technical Research, 2001) was used to control the potential of the working electrode. A function generator (Hokuto Denko, HB-III) was used to provide an external potential to obtain cyclic voltammograms (CVs) which were recorded with an X-Y recorder (Rika Denki, RW-11T). For in situ FTIR measurements, potential pulse was provided by a personal computer (NEC, PC-8801 MH) via a 12 bit D/A converter and the link between the personal computer and an FTIR controller (SPC 3200) was made via an RS-232C interface. All the measurements were carried out at room temperature after the solution was deaerated by flowing N2 gas. All the FTIR measurements have been repeated more than 3 times. X-ray photoelectron spectroscopy (XPS) measurements were carried out by using a VG Scientific model ESCALAB MKII spectrometer. The XPS spectra were recorded using Al KR X-rays of energy of 1486.6 eV. The acceleration voltage and emission current used for generating the X-ray were 15 kV and 20 mA, respectively.

Results Electrochemical Characteristics of the Modified Electrodes. Figure 1 shows the CVs of gold electrodes modified with HSC7CN (a) and HSC2CN (b) in 0.5 M NaClO4 solution which were obtained by varing the potential scan limits. In the potential region between -0.4 and +0.7 V (vs SSCE), redox waves were not observed at both the HSC7CN and HSC2CN modified electrodes. When the potential became more positive or negative than this region, oxidative or reductive current was observed. In the case of the electrode modified with HSC2CN,

oxidative current started to increase at +0.8 V, and increased significantly as potential became more positive. In the negative potential region, reductive current started to flow at -0.5 V and a cathodic peak was observed at -0.75 V. In the case of the gold electrode modified with HSC7CN, the oxidative current started to increase around +1.0 V and grew more at the more positive potential region. Cathodic current started to flow around -0.7 V, and a cathodic peak was observed at -0.95 V. The onset potentials for both cathodic and anodic currents at the HSC7CN-modified electrode were more positive and more negative, respectively, than those at the HSC2CN-modified electrode. FTIR Spectra. Figure 2 shows transmission IR spectra of HSC7CN (a) and HSC2CN (b) which were obtained by placing a drop of the neat liquid of the molecule under investigation between two NaCl plates. The bands of 2930 and 2850 cm-1 are assigned as the asymmetric C-H stretching mode of a methylene group (νas(CH2)) and the symmetric C-H stretching mode of a methylene group (νs(CH2)), respectively.39 The 2568 cm-1 band of HSC2CN and the 2570 cm-1 band of HSC7CN are of S-H stretching.39 The band corresponding to the stretching of nitrile group was observed at 2260 cm-1 for HSC2CN and 2252 cm-1 for HSC7CN.39 It is known that the electron conjugation in the alkyl chain lowers the frequency of CN stretch.40,41 Figure 3 shows a typical in situ SNIFTIRS spectrum of gold electrodes modified with HSC7CN which was obtained at -1.2 V with p-polarized light. The spectrum showed various absorption bands in the regions of 3500-3100 cm-1, 3000-2800 cm-1, 2400-2000 cm-1, and 1700-1600 cm-1. The band around 2570 cm-1 due to the S-H stretching vibration, which was observed for the bulk molecules, was not observed at the monolayers. This is (39) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds, 4th ed.; John Wiley & Sons: New York, 1981. (40) Jesson, J. P.; Thompson, H. W. Spectrochim. Acta 1958, 13, 217. (41) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy; Academic Press: New York, 1964; p 203.

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Figure 5. Potential dependence of the intensity of the bands around 2930 (O) and 2850 cm-1 (b) in the SNIFTIR spectra of the HSC7CN-modified electrode (Figure 4). Note the vertical axis is -∆R/R which represents the absorbance. Errors in intensity are (0.005%. Figure 3. A SNIFTIRS spectrum of HSC7CN-modified electrode observed at -1.2 V in the region of 3500-1200 cm-1 measured by using p-polarized light.

Figure 4. SNIFTIRS spectra of the HSC7CN-modified electrode in the region of 3000-2500 cm-1 measured by using p-polarized light.

reasonable as it has been already confirmed that the adsorption of thiols on gold involves cleavage of the S-H bond.42 The bands observed in the region of 3500-3100 cm-1 and 1700-1600 cm-1 should be due to potential dependent structural change of the layer of water molecules near the electrode.43 Although the investigation of these bands is interesting, we focus on the bands due to adsorbed monolayers, namely, in the regions of 30002800 cm-1 and 2400-2000 cm-1. Spectra in the Region between 3000 and 2800 cm-1. (a) HSC7CN-Modified Electrode. Figure 4 shows SNIFTIRS spectra of the gold electrode which was (42) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1191, 113, 8283. (43) Kunimatsu, K.; Bewick, A. Indian J. Technol. 1986, 24, 407.

modified with HSC7CN by immersing the gold electrode into a 10 mM HSC7CN hexane solution for 2 h. In this frequency region, two vibrational bands were observed around 2930 and 2850 cm-1, both in positive and negative sample potential regions, although the signal to noise (S/ N) ratio was low because of very low IR intensity due to strong absorption by water. When the sample potential was more negative than the reference potential, peaks were downward. Upward peaks were observed when the sample potential was more positive than the reference potential and were larger at more positive potentials. The upward and downward peaks mean that the absorbance of the band at the sample potential is smaller and larger, respectively, than that at the reference potential. Thus, the absorbance of these bands decreased as potential became more positive in whole potential region. The positions of these bands seemed to be slightly affected by potential, and the bands shifted to higher frequency as potential became more positive, although it is rather difficult to determine the real position of the bands with accuracy because of low S/N ratio and interference by water band. Figure 5 shows the potential dependence of the intensities of these two bands. These two bands around 2930 and 2850 cm-1 were observed even in the difference spectra between the reference potential (0 V) and sample potential of the HSC7CN-modified gold electrode measured by SPAIRS using p-polarized light, although the bands were not as clear as those of SNIFTIRS spectra because of lower S/N ratio. Figure 6 shows difference spectra of the HSC7CNmodified gold electrode between the spectrum obtained at 0 V at the beginning of the measurement and the spectra obtained again at 0 V but after sample potentials shown in the figure were applied. If the change is reversible, no band except for background drift should be observed. Actually no clear bands were observed until the applied potential became more positive than +1.3 V. Upward peaks were observed around 2930 and 2850 cm-1 in the spectra obtained after potentials more positive than +1.3 V were applied. This means irreversible loss of the two bands. The peaks of these bands were not observed when negative potentials were applied, suggesting no irreversible loss of the bands in this potential region despite the cathodic current flow. No bands were observed within this frequency region if the spectra were obtained by using s-polarized light. (b) HSC2CN-Modified Electrode. The SNIFTIRS spectra and the difference spectra between the reference

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Figure 7. SNIFTIRS spectra of the HSC7CN-modified electrode in the region of 2500-2000 cm-1 measured by using p-polarized light. Figure 6. Difference spectra of the HSC7CN-modified electrode in the region of 3100-2700 cm-1 between the spectrum obtained at 0 V at the beginning of the measurement and the spectra obtained again at 0 V but after the spectra by SPAIRS at given potentials shown in the figure were taken. Spectra were obtained by using p-polarized light.

potential (0 V) and sample potentials measured by SPAIRS of the HSC2CN-modified gold electrode were essentially the same as those observed at the HSC7CN-modified electrode as the two bands around 2930 and 2850 cm-1 were observed, although the intensity of these peaks at the HSC2CN modified surface was much smaller. The difference spectra of the HSC2CN-modified electrode obtained in the same way as the spectra presented in Figure 6 showed two positive going peaks around 2930 and 2850 cm-1 when potential became more positive than +1.0 V, showing the irreversible loss of these bands in this potential region. The critical potential at which the bands started to appear was more negative than that at the HSC7CN-modified electrode. As was the case at the HSC7CN-modified electrode, no peaks of these bands were observed when negative potentials were applied, suggesting no irreversible loss of the bands despite the cathodic current flow in this potential region. No bands were observed within this frequency region if the spectra were obtained by using s-polarized light as was the case at the HSC7CN-modified electrode. Spectra in the region between 2400 and 2000 cm-1. (a) HSC7CN-Modified Electrode. SNIFTIRS spectra of the HSC7CN-modified gold electrode in this frequency region are shown in Figure 7. Two bands were observed in this frequency region. One appeared around 2250 cm-1 and the second was at 2342 cm-1. When the sample potential was more negative than the reference potential, a downward peak around 2250 cm-1 was observed. The 2250 cm-1 band was not clearly seen when the potential was more positive than the reference potential. The band at 2342 cm-1 appeared as a downward peak only when sample potential became more positive than +1.0 V. The peak position was not affected by applied potential and was always 2342 cm-1. The intensity of this band was very strong compared with other bands. In the potential region where the 2342 cm-1 band became strong, various new bands were observed between 2300 and 2100 cm-1. Figure 8 shows the relationship between potential and peak positions (top panel) and peak intensities (bottom

Figure 8. Potential dependence of the peak position (top panel) and intensity (bottom panel) of the bands around 2250 cm-1 in the SNIFTIR spectra of the HSC7CN modified electrode (Figure 6). Note the vertical axis is -∆R/R, which represents the absorbance. Errors in intensity are (0.005%.

panel) of the band around 2250 cm-1 at the HSC7CNmodified gold electrode observed in the SNIFTIRS spectra (Figure 7). The potential dependence of the peak position of the 2250 cm-1 band was very small. The absorbance of this band changed uniformly with potential in negative potential region between -1.2 and -0.2 V. Since almost no absorbance change was observed in the positive potential region, the results are not presented in the figure. Figure 9 shows difference spectra of the HSC7CN-modified electrode measured by SPAIRS using p-polarized light. Although all the bands observed in SNIFTIRS spectra (Figure 7) are also seen in Figure 9, the peaks are less clear in Figure 9 than in Figure 7 because of a lower S/N ratio in the spectra obtained by the SPAIRS method. One must, however, note that the band at 2342 cm-1 is clearly seen and dominated in Figure 9 when positive potentials were applied. Furthermore, a various new downward band appeared between 2300 and 2100 cm-1, the most dominant being 2280 cm-1 band, at potentials more positive than +1.3 V as shown in the enlarged spectra presented in the insets of Figure 9. Figure 10 shows difference spectra of the HSC7CNmodified electrode measured by SPAIRS using s-polarized light. While no band was observed in the negative

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Figure 9. Difference spectra of the HSC7CN-modified electrode measured by SPAIRS using p-polarized light in the region of 2400-2000 cm-1 between the reference and the sample potentials. Inset: Enlarged spectra in positive potential region.

potential region, one clear band was observed at 2342 cm-1 in the positive potential region. In difference spectra (p-polarized light, not shown) of the HSC7CN-modified gold electrode between the spectrum obtained at 0 V at the beginning of the measurement and the spectra obtained again at 0 V but after various sample potentials were applied, a downward peak at 2342 cm-1 was observed when applied potential was more positive than +0.9 V and an upward peak around 2250 cm-1 was observed when the applied potential became more positive than +1.4 V. These results indicate the irreversible gain of the 2342 cm-1 band and the loss of the 2250 cm-1 band in the positive potential region. No irreversible change was detected in the negative potential region. Figure 11 shows difference spectra of HSC7CN-modified gold electrode obtained by using s-polarized light between the spectrum at 0 V at the beginning of the measurement and the spectra obtained again at 0 V but after sample potentials shown in the figure were applied. As was the case in Figure 10 no peak but the 2342 cm-1 band was observed in the positive potential region. No clear peaks were observed in the negative potential region, showing no irreversible change of the bands in this frequency region when negative potential was applied despite the cathodic current flow. The facts that the band around 2250 cm-1 was detected only by p-polarized light and that the band at 2342 cm-1 was observed by using both p- and s-polarized light suggest that the former band is of adsorbed species and the band at 2342 cm-1 is of solution species. (b) HSC2CN-Modified Electrode. SNIFTIRS spectra of the HSC2CN-modified gold electrode showed all the bands observed at the HSC7CN-modified electrode and essential features of these bands were the same in both cases except for the fact that the 2250 cm-1 band was observed as an upward band when the potential was more positive than the reference potential. Figure 12 shows

Figure 10. Difference spectra of the HSC7CN-modified electrode measured by SPAIRS using s-polarized light in the region of 2400-2000 cm-1 between the reference and the sample potentials.

the potential dependence of peak positions (top panel) and peak intensities (bottom panel) of the band around 2250 cm-1 of the HSC2CN-modified gold electrode obesrved in the SNIFTIRS spectra. The peak frequency increased uniformly but only slightly by 10 cm-1/V as potential became more positive and the absorbance decreased uniformly as potential was made positive between -1.0 and +0.7 V. Complicated potential dependence of the peak intensity was observed in a more positive potential region. The intensity of the upward peak increased more at +0.8 V but started to decrease at +0.9 and +1.0 V. All the bands observed in the SNIFTIRS spectra are also seen in the difference spectra of the HSC2CN-modified electrode measured by SPAIRS using p-polarized light, although the bands around 2250 cm-1 were less clear because of lower S/N ratio in the spectra obtained by the SPAIRS method. The downward band at 2342 cm-1 was, however, more clearly observed in the difference spectra than in the SNIFTIRS spectra. This band began to be observed at potentials more positive than +0.8 V and became very strong (up to several percent) as potential became more positive (Figure 13) as was the case at the HSC7CN-modified electrode. The upward 2250 cm-1 band was observed at potentials more positive than +0.9 V but disappeared at potentials more positive than +1.1 V. In addition to these bands various downward peaks were observed between 2300 and 2100 cm-1, the most dominant being the 2280 cm-1 band, at potentials more positive than +1.1 V as shown in the enlarged spectra presented in the insets of Figure 13.

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Figure 11. Difference spectra of the HSC7CN-modified electrode in the region of 2500-2000 cm-1 between the spectrum obtained at 0 V at the beginning of the measurement and the spectra obtained again at 0 V but after the spectra by SPAIRS at given potentials shown in the figure were taken. Spectra were obtained by using s-polarized light.

Figure 13. Difference spectra of the HSC2CN-modified electrode measured by SPAIRS using p-polarized light in the region of 2400-2000 cm-1 between the reference and the sample potentials. Inset: Enlarged spectra in positive potential region.

Figure 12. Potential dependence of the peak position (top panel) and intensity (bottom panel) of the bands around 2250 cm-1 in the SNIFTIR spectra of the HSC2CN-modified electrode. Note the vertical axis is -∆R/R, which represents the absorbance. Errors in intensity are (0.005%.

In difference spectra (p-polarized light) of the HSC2CNmodified gold electrode between the spectrum obtained at 0 V at the beginning of the measurement and the spectra obtained again at 0 V but after various sample potentials were applied, a downward peak at 2342 cm-1 was observed when the applied potential was more positive than +0.7 V and an upward peak around 2250 cm-1 was observed when the applied potentials became more positive than +1.1 V. These results indicate that irreversible increase of the 2342 cm-1 band and the irreversible loss of the 2250 cm-1 band in the positive potential region. As was the

case at the HSC7CN-modified gold electrode, no clear peaks were observed in the negative potential region, showing no irreversible change of the bands in this frequency region. XPS Spectra. To clarify the effect of the electrode potential on the stability of the monolayer, XPS measurements were carried out for the HSC7CN-modified gold electrodes to which various potentials were applied. Figures 14-16 show the XPS spectra in the S(2p), C(1s), and N(1s) regions, respectively, of the HSC7CN monolayer modified gold electrode after various treatments. Except for the electrochemically untreated sample (curve a), 0 V was applied before the samples were removed from the electrochemical cell after the various potentials were applied. Figure 14 shows a peak around 162.8 eV corresponding to S(2p3/2) with a shoulder around 164 eV corresponding to S(2p1/2). These peak positions confirm that sulfur exists as thiolate (Au-S). Intensities of the band were almost the same for all the modified samples except for the one to which +1.3 V was applied (curve c). The shape of the band seemed to be affected by applied potential. The shoulder around 164 eV grew and became a peak when cathodic potential was applied. This band position agrees with that of S(2p3/2) of the -SH group (163.6 eV).44 In Figure 15, a broad band was observed around 285 eV corresponding to C(1s) in all cases even at bare gold (curve g), although peak positions were slightly different from sample to sample. A shoulder was noticed around 287 eV except at bare gold and at samples to which 1.3 V was applied (curve c). This shoulder should be due to (44) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. T.; Nazzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152.

Potential Dependent Orientation of Monolayers

Figure 14. XPS spectra in the S(2p) region of the HSC7CNmodified electrode after the application of various potentials; (a) as modified; (b) 0 to +0.9 V to 0 V; (c) 0 V to +1.3 V to 0 V; (d) 0 V to -0.8 V to 0 V; (e) V to -1.2 V to 0 V. (f) -0.4 V to 0 V to +0.4 V to -0.4 V. All samples were washed with water after removal from the cell.

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Figure 16. XPS spectra in the N(1s) region of the HSC7CNmodified electrode after the application of various potentials. Samples a-f are from Figure 14.

These results show that the monolayer stayed on the surface unless very positive potential was applied (curve c). The fact that the peaks due to C of the CN group, S, and N were missing at the sample to which +1.3 V was applied shows that the monolayer was totally decomposed at this potential. It should be stressed that although a cathodic peak current was observed at -0.95 V and significant cathodic current flowed at -1.2 V, intensities of XPS bands of the monolayer-modified electrode to which -1.2 V was applied were almost the same as those of the electrochemically untreated sample. Thus, it is concluded that no irreversible decomposition of the monolayer occurred in this potential region. Although the desorption of alkanethiolate monolayers from the electrodes at cathodic potential region were reported, those results were obtained in alkaline solution45,46 or nonaqeous solutions.47 Neutral solution was used in the present study. It seems that the pH of the solution affects the stability of the monolayer greatly. Discussion

Figure 15. XPS spectra in the C(1s) region of the HSC7CNmodified electrode after the application of various potentials. Samples a-f are of Figure 14 and sample g is untreated gold.

Assignment and Potential Dependence of the Bands around 2930 and 2850 cm-1. The peak positions of these bands are essentially the same as those of the asymmetric C-H stretching mode of the methylene group (νas(CH2)) and the symmetric C-H stretching mode of the methylene group (νs(CH2)) observed in free molecules. The peak positions also agree with the values reported for various LB and self-assembled monolayers formed by molecules which have a methylene group.1,17,18 The fact that these bands were not observed if the spectra were obtained by using s-polarized light confirms that they are of adsorbed species. Thus, the bands around 2930 and

the CN group.7 Thus, the peaks observed around 285 eV at samples c and g should be due to contamination. A peak corresponding to N(1s) was observed at all the monolayer-modified samples except for the one to which +1.3 V was applied (curve c) as shown in Figure 16.

(45) Schneider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391. (46) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687. (47) Everett, W. R.; Fritsch-Faules, Anal. Chim. Acta 1995, 307, 253.

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2850 cm-1 observed at the modified electrodes are assigned as the asymmetric C-H stretching mode of methylene group (νas(CH2)) and the symmetric C-H stretching mode of methylene group (νs(CH2)), respectively, of the adsorbed molecules. Although the position of these bands seemed to be dependent of potential as mentioned before, low S/N ratio makes it very difficult to discuss the potential dependence quantitatively. The absorbance of these two bands became smaller as potential became more positive (Figure 5). Potential dependent absorbance change in in situ IR spectra is usually considered to be due to the change of coverage, i.e., the amount of adsorbed species. In this case, however, no significant coverage change should be expected within the potential region between -1.2 and +0.9 V as already discussed, and therefore, the intensity change cannot be due to the coverage change. Since the p-polarized light interacts only with dipole moment perpendicular to the surface, the intensities of the C-H stretching bands of the methylene group at a given coverage should be strongly affected by the orientation of the adsorbed molecule, i.e., zero at perpendicular orientation and maximum at flat orientation.1 Thus, the lower absorbance at more positive potential suggests that the orientation of adsorbed molecules became closer to perpendicular orientation at more positive potential.48 Although the relative intensities between these two bands seem to be affected by the potential, the discussion on the origin of this change is avoided as the S/N ratios of these bands, particularly of the 2850 cm-1 band, are very low. The irreversible decrease of these bands (upward peaks in Figure 6) when potentials more positive than +1.3 V for the HSC7CN-modified electrode and +1.2 V for the HSC2CN-modified electrode were applied suggests anodic decomposition of the monolayer in this potential region. Assignment and Potential Dependence of the Band around 2250 cm-1. As mentioned in the results section, this peak was observed only by using p-polarized light and, therefore, should be due to an adsorbed species. Peak positions of this band are close to that of the CN stretching band of neat liquid of HSC2CN (2252 cm-1) and HSC7CN (2260 cm-1), suggesting this peak is of the CN stretching band. The peak position of the 2250 cm-1 band of the HSC7CNmodified electrode showed very small potential dependence (Figure 8, top panel) and the absorbance of this band decreased with potential as potential became more positive (Figure 8, bottom panel). In the case of the 2250 cm-1 band of the HSC2CN-modified electrode, the peak position shifted slightly to higher frequency by 10 cm-1/V (Figure 12, top panel) and the absorbance decreased as potential was made more positive (Figure 12, bottom panel). The peaks of the 2250 cm-1 band of these adsorbed species appeared always at lower frequency than those of the neat liquid of corresponding molecules. The frequencies of the CN stretching band of the nitrile molecules adsorbed on electrodes have been reported to be close to 2250 cm-1 by various groups. The adsorption behaviors of benzonitrile and acrylonitrile on gold elec(48) The effects of the surface roughness should be considered in the discussion of the molecular orientation change of the monolayer in a microscopic level. It should be emphasized that the information about the molecule orientation obtained by FT-IRRAS measurement in the present study represents only averaged properties of the self-assembled monolayer. A detailed study using a relatively flat (111) ordered gold electrode is in progress. A detailed procedure for the preparation of the flat (111) ordered surface by vacuum evaporation of gold onto a gold polycrystalline disk is given elsewhere: Uosaki, K.; Ye, S.; Kondo, T. J. Phys. Chem. 1995, 99, 4117.

Sato et al.

trodes were investigated by Weaver and colleagues by employing SERS and IR.24,36 They found that the CN stretching mode of benzonitrile adsorbed on electrochemically roughened gold appeared around 2239 cm-1 at -200 mV, which is higher compared with that of bulk benzonitrile (2230 cm-1) and shifted to higher frequency by ca. 23 cm-1/V as potential became more positive. They also reported that the frequency of the CN stretching mode of adsorbed acrylonitrile was ca. 2245 cm-1 at 0 V and increased as potential became more positive.36 The CN stretching band of adsorbed acrylonitrile also shifted to higher frequency compared to the bulk value (2232 cm-1). On the basis of these results they suggested that these molecules adsorb on gold via a σ nitrile nitrogen-surface atom bond. Bewick and Pons reported that the CN stretching band of adsorbed benzonitrile on gold appeared between 2248 and 2264 cm-1, depending on potential.49 These values are higher compared with the frequency of free benzonitrile in solution (2229 cm-1). They reached the same conclusion of Weaver et al. that this band came from the CN group of N-bonded benzonitrile. As mentioned before, Faguy et al. investigated adsorption of acetonitrile on gold by SNIFTIRS35 and observed a band at 2342 cm-1. In addition to this band, they found a band at 2261 cm-1, which is higher than that of bulk acetonitrile (2220 cm-1). They proposed that this band is due to the CN stretching of an acetonitrile molecule bonded through a nitrogen atom to a water molecule, which itself adsorbs on the electrode. In all the cases mentioned above, the CN stretching band shifted to higher frequency upon adsorption and showed relatively large potential dependent frequency shift. On the other hand, the CN stretching band around 2250 cm-1 of the adsorbed HSC2CN and HSC7CN observed in the present study shifted to lower frequencies than those of the free molecules, and the potential dependence of the frequency of this band was very small compared to those of the molecules mentioned above. These results suggest that the 2250 cm-1 band is not due to the CN stretching of N-coordinated species. Since both HSC2CN and HSC7CN molecules are expected to adsorb on gold surface through the Au-S bond, a terminal CN group is unlikely to be used for adsorption. Thus, it is reasonable to conclude that the 2250 cm-1 band is of the CN group which has no direct interaction with gold. The frequency of this band should be affected by potential only as a result of the Stark effect and should be smaller than the potential dependence of the CN stretching band of N-coordinated species. In the latter case, potential affects the frequency mainly through the change in the degree of back donation of electron from the gold d band to the CN antibonding π orbital.24,36 The fact that there was a small potential dependence in the CN stretching mode of the HSC2CNmodified electrode but no potential dependence in that of the HSC7CN-modified electrode shows that while the CN group of the adsorbed HSC2CN is close enough to the electrode surface to feel the field, that of the HSC7CN species is far from the electrode and does not feel the field. The decrease of the absorbance of the 2250 cm-1 band as potential became more positive should not be due to the decrease of the number of adsorbed species but as a result of the potential dependent orientation change of the CN group. As p-polarized light interacts only with a dipole moment perpendicular to the surface,19 the stronger absorption of the CN band at more negative potential means that the orientation of the CN group is closer to the surface normal.48 This is reasonable because the elec(49) Bewick, A.; Pons, S. Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley Heyden: London, 1985, Vol. 12, Chapter 1.

Potential Dependent Orientation of Monolayers

tronegativity of a N atom is larger than that of a C atom. Negative charge of the electrode attracts the carbon atom and repels the nitrogen atom of the terminal CN group. Although the value of -∆R/R at the HSC2CN-modified electrode decreased uniformly as potentials became more positive up to +0.8 V, the -∆R/R value started to increase again and became a less negative value when potentials were made more positive than this potential (Figure 12). This complicated behavior should be related with the anodic decomposition of the monolayer, which will be discussed later. Since the anodic decomposition of the monolayer leads to the decrease of the adsorbed species, the difference between the reflectivity at the reference and sample potentials should be decreased as experimentally observed. Note if there is no adsorbed species on the surface, -∆R/R should be zero. The downward band around 2280 cm-1, which is higher than the frequency of bulk molecules, observed at potentials more positive than +1.2 V at the HSC2CN-modified electrode and at potentials more positive than +1.4 V in the case of the HSC7CN-modified electrode should be due to a N-coordinated CN stretching band of alkanenitrile which is generated by anodic decomposition of adsorbed mercaptoalkanenitrile. Assignment of the Band at 2342 cm-1 and Irreversible Decomposition of the Monolayer. The band at 2342 cm-1 behaved quite differently from other bands around 2930, 2850, and 2250 cm-1. This band appeared only at a positive potential region, showed a potential independent peak position, and was observed even by s-polarized light (Figure 10). The peak intensity of this band became very large up to several percent. These features suggest that this band is due to solution species generated in the positive potential region. It is also interesting to note that this band in difference spectra with SPAIRS was more intense than that in SNIFTIRS spectra at a given potential, suggesting that this band was generated as a result of irreversible change. This was confirmed also by the difference spectra between the spectrum obtained at 0 V at the beginning of the measurement and the spectra obtained again at 0 V but after the spectra by SPAIRS at given potentials were taken. Faguy et al. observed a band at 2342 cm-1 when they investigated adsorption of acetonitrile on gold by SNIFTIRS and proposed that this is due to the CN stretching of an adsorbed acetonitrile molecule, despite the fact that the band position did not change at all with potential between +1.3 and -0.65 V (vs SCE).35 Although the band appeared at the same frequency of the one observed by Faguy et al., the 2342 cm-1 band in this study is certainly not due to the CN stretching of adsorbed mercaptoalkanenitrile because of the reasons mentioned above. This band position actually agrees with that of the CO2 band which was observed in in situ IR spectra obtained during anodic oxidation of various C1 compounds.19,31 Figure 17 shows potential dependence of the peak intensity of the band at 2342 cm-1 in the difference spectra of the HSC7CN- (Figure 9) and HSC2CHmodified (Figure 13) electrodes between the reference and sample potentials measured by SPAIRS using p-polarized light. The onset potential for the band to be observed was +0.9 and +1.05 V for the HSC2CN- and HSC7CN-modified electrode, respectively. These values are in agreement with the onset potentials for the anodic current at these electrodes (Figure 1a,b). Thus, we can conclude that this band is due to CO2 in solution generated by anodic oxidation of the adsorbed molecular layers of mercaptoalkanenitriles. Since the CV (Figure 1) showed no cathodic peak due to the reduction of gold oxide was observed even after significant anodic current flowed, it

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Figure 17. Potential dependence of the peak intensity of the band at 2342 cm-1 in the difference spectra of the HSC7CN- (O) and HSC2CN-modified (b) electrodes between the reference and sample potentials measured by SPAIRS using p-polarized light. Note the vertical axis is -∆R/R, which represents the absorbance. Errors in intensity are (0.05%.

Figure 18. A schematic model for the potential dependent orientation of the Au-SC7CN monolayer.

seems that the decomposition of the monolayer starts before the formation of surface gold oxide. The irreversible anodic decomposition of the monolayer was also supported by XPS measurement as mentioned before. When the HSC7CN-modified electrode was anodically oxidized at +1.3 V for 1 min, the peaks of S(2p) and N(1s) disappeared as shown in Figures 14 and 16. The peak of C(1s), which was still present even after this treatment, should be due to contamination as a similar peak was observed at the bare gold electrode (Figure 15). The shoulder of the C(1s) peak at 287 eV due to the nitrile carbon was not observed at the modified electrode after the anodic oxidation at +1.3 V and at the bare gold electrode. These results show that Au-S bond cleavage and CO2 generation took place at the same time at this potential, although the detailed reaction mechanism is not clear. The appearance of the new downward bands between 2300 and 2100 cm-1 and the 2342 cm-1 band became strong when potential became relatively positive clearly suggests that various types of nitrile molecules were formed as intermediates of the anodic decomposition of mercaptoalkanenitrile monolayers on gold. The details of the anodic decomposition process of self-assembled monolayers are now under investigation by using an FTIRQCM combined system.33 Conclusion The above discussions show that the alkyl chain became more perpendicular to the surface and the orientation of the CN group became closer to parallel to the surface as potential became more positive. One of the driving forces for this change may be the interaction between the dipole moment of the CN group and the charge of gold electrode. When the potential becomes more positive and the gold electrode is positively charged, the nitrogen atom of the terminal nitrile group, which is more negative than the carbon atom, is attracted by the positive charge of the

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electrode, and the orientation of the CN group becomes closer to parallel to the surface. This makes the space required to accommodate the CN group bigger and the orientation of the alkyl chain becomes more perpendicular to the surface in the positive potential region as shown in Figure 18. The irreversible anodic decomposition of the monolayers took place at a positive potential region, ca. >+0.8 V for the adsorbed HSC2CN and >+1.0 V for the adsorbed HSC7CN. It was found that CO2 was generated as soon as anodic current started to flow, although the detailed reaction mechanism is not clear. Furthermore, some decomposition products (intermediates) of anodic oxidation were also observed. Although significant current flowed, the monolayer seemed to stay on the electrode surface in cathodic potential region. XPS spectra indicate the formation of

Sato et al.

-SH groups after electrochemical reduction of the monolayer. Detailed study to clarify this reaction is now under way. Acknowledgment. Thanks are due to Mr. I. Saeki for XPS measurements, Messers, T. Kiya and N. Sakai for construction of the IR cell, and Dr. K. Kunimatsu and Professor T. Sasaki for discussions. This work was partially supported by Grants in Aids for Scientific Research (04555191), for Priority Area Research (05235201, 06235201, 07235201), and for International Research (University-to-University Cooperation: 03045011) from the Ministry of Education, Science, Culture and Sports, Japan. Y.S. acknowledges the Japan Society for the Promotion of Science for the fellowship and Dr. F. Mizutani for encouragement. LA950675T