Spectroscopic and Spectroelectrochemical Characterization of

Smita Sarkar, and S. Sampath*. Department of Inorganic and Physical ... Susmita Behera, S. Sampath, and C. Retna Raj. The Journal of Physical Chemistr...
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Langmuir 2006, 22, 3396-3403

Spectroscopic and Spectroelectrochemical Characterization of Acceptor-Sigma Spacer-Donor Monolayers Smita Sarkar and S. Sampath* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India ReceiVed July 9, 2005. In Final Form: October 18, 2005 A novel self-assembled monolayer based on benzoquinone-sigma spacer-ferrocene assembly has been prepared on the surface of gold. The preceding paper gives the reaction conditions for the synthesis of the monolayer and a detailed electrochemical characterization of the assembly. In the present paper, the monolayer structure and the orientation of the redox moieties are followed by a detailed spectroscopic analysis. The monolayer has been characterized by reflectance absorbance IR spectroscopy, FT-Raman spectroscopy, and X-ray photoelectron spectroscopy at every step of modification. Additionally, spectroelectrochemical studies have been carried out by coupling Raman spectroscopy with cyclic voltammetry to elucidate the structural variations associated with the monolayer as a function of applied dc bias.

Introduction The development of a monolayer assembly conferring to acceptor-sigma spacer-donor architecture (A-σ spacer-D) has significant importance in the fundamental studies on electrontransfer processes1 and in the development of novel applications2 such as sensors and molecular rectifiers. However, to have a controlled tunability on the surface properties, it is necessary to understand how the designed changes in preparative surface chemistries affect the structure of the adsorbed layer. A variety of spectroscopic techniques such as Raman spectroscopy, reflectance absorbance IR spectroscopy (RAIR), X-ray photoelectron spectroscopy (XPS), etc. have proved useful to understand the chemical functionalities and the molecular orientation of the monolayer. Raman spectroscopy of adsorbed molecules on metal surfaces, particularly Au and Ag,3-7 has received widespread attention in the study of interfacial systems. It provides the ability to acquire spectroscopic information in a range of environments with submonolayer sensitivities.8 The RAIR technique has proved to be a very useful technique to study the vibrational modes of an assembled monolayer.9-11 In the RAIR technique, a polarized IR beam is used, and as a result, the transition dipoles perpendicular to the surface are enhanced. A wealth of information on the surface functionalities, the spatial location of the chemical functionality in the monolayer, and the surface concentration of * Corresponding author. E-mail: [email protected]. (1) (a) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40. (b) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (c) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem. ReV. 1996, 96, 759. (d) PaddenRow, M. N. Acc. Chem. Res. 1994, 27, 18. (2) (a) Lahav, M.; Katz, E.; Willner, I. Electroanalysis 1998, 10, 1159. (b) Metzger, R. M. J. Mater. Chem. 1999, 9, 2027. (c) Metzger, R. M. Acc. Chem. Res. 1999, 32, 950. (3) Cotton, T. M.; Kim, J.-H.; Chumanov, G. D. J. Raman Spectrosc. 1991, 22, 729. (4) Kim, Y. H.; Kim, K.; Kim, M. S. J. Phys. Chem. 1990, 94, 2552. (5) Taylor, C. E.; Schoenfisch, M. H.; Pemberton, J. E. Langmuir 2000, 16, 2902. (6) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284. (7) Sandhyarani, N.; Pradeep, T. Vacuum 1998, 49, 279. (8) (a) Jeanmaire, D. L.; Van Duyne, R. P. J. Electroanal. Chem. 1977, 84, 1. (b) Albercht, M. G.; Creighton, J. A. J. Am. Chem. Soc. 1977, 99, 5215. (9) Porter, M. D.; Bright, T. B.; Allara, D.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (10) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (11) Chailapakul, O.; Sun, L.; Xu, C.; Crooks, R. M. J. Am. Chem. Soc. 1993, 115, 12459.

the chemical species could be probed using XPS technique.12 The spectroscopic characterization of redox active as well as nonredox active monolayers on substrates such as gold and silver has been reported in the literature. The redox active monolayers comprise of electroactive molecules such as anthraquinone,13,14 hydroquionone,15-17 phenanthrenequinone,18 ferrocene,19-21 etc., and the nonredox active molecules are generally based on thiol monolayers. The preceding paper presents the synthesis and electrochemical characterization of a novel benzoquinone-σ spacer-ferrocene assembly, where the distance between the two moieties has been varied with different chain lengths of diaminoalkane. To gain insight into the structural aspect of the monolayer, a detailed spectroscopic analysis for the benzoquinone-sigma spacerferrocene SAM has been carried out. The assembly has been characterized by RAIR, FT-Raman spectroscopy and XPS techniques at every stage of modification. Additionally, spectroelectrochemical studies have been carried out by coupling Raman spectroscopy with cyclic voltammetry to elucidate the structural variations associated with the monolayer as a function of applied dc bias. Experimental Section Chemicals. The solvents and chemicals used for the preparation of the benzoquinone-σ spacer-ferrocene monolayer on gold surfaces and their purification procedures have been described in the preceding paper.22 (12) (a) Yanagida, M.; Kanai, T.; Zhang, X.-Q.; Kondo, T.; Uosaki, K. Bull. Chem. Soc. Jpn. 1998, 71, 2555. (b) Zhong, C.-J.; Brush, R. C.; Anderegg, J.; Porter, M. D. Langmuir 1999, 15, 518. (13) Han, S. W.; Ha, T. H.; Kim, C. H.; Kim, K. Langmuir 1998, 14, 6113. (14) Han, S. W.; Joo, S. W.; Ha, T. W.; Kim, Y. Kim, K. J. Phys. Chem. B 2000, 104, 11987. (15) Ye, S.; Yashiro, A.; Sato, Y.; Uosaki, K. J. Chem. Soc., Faraday Trans. 1996, 3813. (16) Bae, I. T.; Sandifer, M.; Lee, Y. W.; Tryk, D. A.; Sukenik, C. N.; Scherson, D. A. Anal. Chem. 1995, 67, 4508. (17) Sasaki, T.; Bae, I. T.; Scherson, D. A.; Bravo, B. G.; Soriaga, M. P. Langmuir 1990, 6, 1234. (18) Ishioka, T.; Uchida, T.; Teramae, N. Bull. Chem. Soc. Jpn. 1999, 72, 2713. (19) Nishiyama, K.; Uoda, A.; Tanoue, S.; Koga, T.; Taniguchi, I. Chem. Lett. 2000, 930. (20) (a) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687. (b) Popenoe, D. D.; Randall, S. D.; Porter, M. D. Langmuir 1992, 8, 2521. (21) Han, S. W.; Seo, H.; Chung, Y. K.; Kim, K. Langmuir 2000, 16, 9493.

10.1021/la051858a CCC: $33.50 © 2006 American Chemical Society Published on Web 03/02/2006

Characterization of A-σ Spacer-D Monolayers Substrate Preparation. Highly oriented single crystalline Au (111) surfaces were used for the RAIR measurements. Gold samples were prepared by vacuum evaporation of gold on glass or mica surfaces. Gold films of 1500-2000 Å thickness were deposited onto a clean glass surface that had been pre-covered with 150 Å thick chromium layer. Prior to the deposition of the metals, the glass slides were cleaned following the reported procedure.23 Glass slides were placed in hot piranha solution (1:3 by volume of 30% H2O2 and concentrated H2SO4) for 20 min, taken out and thoroughly rinsed with double distilled water and dried. (Caution: piranha reacts violently when it comes in contact with organic compounds.) The deposition of metal films was carried out at a rate of 1.0 Å/s for chromium and 4.0 Å/s for gold. During deposition, the substrate temperature was maintained at 573 K (300 °C) at a pressure of 2 × 10-5 mbar, and the substrate was rotated at a speed of 4-6 rotations/ min. The powder XRD (X-ray diffraction) data confirmed the formation of a highly oriented 〈111〉 phase. The surface profiles taken using a profilometer (Taylor-Hobson, U.K.) yielded reproducible average roughness of 12 ( 2 nm for the gold films deposited on glass and 3 nm for gold films on mica. The single crystalline gold surfaces were washed with ethanol for about 15 min to ensure the removal of any impurity that might have been deposited on the surface while transferring the gold slides from the coating unit to the desiccator. All of the FT-Raman studies have been carried out using electrochemically roughened gold substrates before modification. The surface roughening was carried out following a reported procedure, where the gold surface was subjected to 20 potential sweep oxidation reduction cycles (ORCs) in 0.1 M KCl between -0.2 and +1.2 V vs saturated calomel electrode (SCE) at 0.1 V/s.6 Roughness factors of approximately 6 were obtained for the electrochemically treated surfaces. The surface modification and subsequent electrochemical response are found to be similar for both unroughened and roughened surfaces. After the rigorous cleaning procedure, the gold surfaces were immediately used for modification with monolayers. It should be noted that the redox-active monolayers modified on both single crystal and polycrystalline gold surfaces resulted in a similar electrochemical response. Reflectance Absorbance Infrared Spectroscopy (RAIR). Reflectance absorbance infrared (RAIR) spectra were recorded on a Perkin-Elmer (Spectrum GX, Switzerland) FT-IR spectrometer equipped with a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector. The sample chamber was purged with ultrahigh pure nitrogen prior to the experiment. Reference spectra were obtained on freshly cleaned evaporated Au (111) surface. The perpendicularly polarized infrared beam (p-polarized) was reflected from the SAM modified surfaces at an angle of 82° to the surface normal. The spectra were averaged over 1024 scans with a resolution of 4 cm-1. Raman Spectroscopy. Fourier transform Raman (FT-Raman) spectra were recorded using a Bruker RFS-100/S FT-Raman spectrometer. An Nd:YAG laser with a source wavelength of 1064 nm was used. Laser power between 100 and 500 mW was used depending on the experiment, with a radiation spot of 0.1 mm diameter. The scattered light was collected at an angle of 180° to the incident light. Each spectrum was averaged over 500 scans, and the resolution was 4 cm-1. A liquid nitrogen cooled germanium detector was used. Solid samples were ground to a fine powder and then mounted on a stainless sample holder. Raman Spectroelectrochemical Studies. The FT-Raman based spectroelectrochemical studies were carried out using the monolayermodified gold foil as the working electrode, platinum mesh as the counter electrode and standard calomel electrode (SCE) as the reference electrode. The electrodes were placed in a quartz cuvette and firmly secured such that the three electrodes do not touch each other. The cuvette was then filled with either 0.1 M phosphate buffer (pH 7.2) or 0.1 M phthalate buffer (pH 4.6). The setup was placed in the sample-holder of the FT-Raman spectrometer with the working (22) Preceding paper. (23) Bharathi, S.; Joseph, J.; Lev, O. Electrochem. Solid State Lett. 1999, 2, 284.

Langmuir, Vol. 22, No. 7, 2006 3397 electrode facing the laser beam. The leads of the electrodes were then connected to the electrochemical system. Raman spectra were recorded at various applied dc bias potentials. During each measurement the working electrode was held at the desired potential until the measurement was complete. The monolayer was stable during the entire course of the experiment and this was confirmed based on the electrochemical characterization of the SAM modified surfaces before and after spectroelectrochemical measurements. All of the experiments were carried out under inert atmosphere. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an ESCA-3 Mark II spectrometer (VG Scientific Ltd., England) using Al KR radiation (1486.6 eV). The Au (4f), C (1s), O (1s), N (1s), and S (2p) regions were recorded for all of the monolayer samples using a takeoff angle (TOA) of 90°. Some experiments were also performed where the Fe (2p) region was followed. The pass energy was kept at 44 eV for recording the spectra of Au (4f), C (1s), and O (1s) and at 90 eV for recording the spectra of N (1s), S (2p), and Fe (2p). To study the spatial distribution of the elements in the film, additional experiments were carried out with a TOA value of 10°. The Au (4f) peak at 84.0 eV was used to check the binding energy scale of the instrument and also as the reference. The measurements were carried out at a vacuum level of 10-9 Torr. Prior to mounting the sample into analyzing chamber, it was kept in the preparation chamber at 10-9 Torr for 5 h in order to desorb any volatile species present on the sample. The experimental data were curve fitted using Gaussian distribution after subtracting a linear background. The concentrations of different species were estimated from the area of the respective Gaussian peaks. The surface concentration ratios of element 1 (E1) to element 2 (E2) were calculated by using the following formula: [E1] [E2]

)

IE1 σE2λ E2DE2 IE2 σE1λE1DE1

where IE, σ, λ, and DE are the intensity, photoionization crosssection, mean escape depth, and geometric factor, respectively. The subscripts 1 and 2 denote elements 1 and 2, respectively. The spectral analysis, for all of the spectroscopic techniques mentioned above, was carried out using Microcal Origin 6.0 software.

Results and Discussion Cystamine Monolayer. FT-Raman Studies. Figure 1 shows the FT Raman spectra of the cystamine monolayer adsorbed on a roughened gold surface. The absence of the S-S stretching band around 510 cm-1 indicates that chemisorption of disulfide happens with the cleavage of the S-S bond similar to the mechanism reported for the adsorption of disulfides.24,25 The band at 275 cm-1 is assigned to the Au-S stretch,26 which further confirms the chemisorption of cystamine onto the Au surface. The two bands observed at 641 and 724 cm-1 are characteristic of the C-S stretching vibrations. The first band is assigned to the gauche conformation, whereas the second one is typical of the trans conformer.27 As a comparison, the normal Raman spectrum of the solid cystamine.2HCl sample has also been recorded. The cystamine SAM on gold shows a high fraction of the gauche conformers on the surface similar to the observations on silver surfaces.28 The strong bands at 973 and 1027 cm-1 are due to the C-C stretching vibrations coupled to the C-N stretching vibrations. The band at 1290 cm-1 is attributed to the C-N stretching vibration and the band at 1584 cm-1 is due to the N-H bending mode. The band at 1444 cm-1 is assigned to the scissoring mode of the CH2 group present in the cystamine (24) Lee, H.; Kim, M. S.; Suh, S. W. J. Raman Spectrosc. 1991, 22, 91. (25) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (26) Li, Z.; Lieberman, M.; Hill, W. Langmuir 2001, 17, 4887. (27) Li, T. T.-T.; Liu, H. Y.; Weaver, M. J. J. Am. Chem. Soc. 1984, 106, 1233. (28) Kudelski, A.; Hill, W. Langmuir 1999, 15, 3162.

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Figure 2. XPS spectra obtained for the cystamine monolayer in the regions of (a) C (1s), (b) N (1s), (c) O (1s), and (d) S (2p) with a TOA of 85°. The actual data are given as point symbols. The curves connecting the points are for the sum of the individual deconvoluted curves.

Figure 1. (a) FT-Raman spectrum of cystamine monolayer adsorbed on Au surface. (b) The CH2 symmetric and antisymmetric vibrations for the cystamine monolayer are shown. Table 1. Frequencies and Assignments of Bands Observed in the SERS Spectra of BQ-II SAM and BQ-I SAM frequencies (cm-1) BQ-II SAM

BQ-I SAM

assignments

240a 275 464 540 643 732 819a 1010a 1278 1335 1382 1443 1515 1590 1665 2859 2911

238a 277 466 540 639 728 815a 1012a 1277 1347 1386 1443 1515 1586 broad shoulder 2857 2913

τ(chair), ω(CH) (BQ) ν (Au-S) δ(2, 5), b(CdO) (BQ) νa(C-C), δ(2, 5), b(CdO) (BQ) ν (C-S)G ν (C-S)T ω(CdO), τ(chair) (BQ) b(C-H) (BQ) ν (C-N) (cystamine) b (C-H) (BQ) νa (C-C), b (C-H) (BQ) δ (CH2) ν (CdC) (BQ) ν (CdO) (diamino BQ) + ν (CdC) ν (CdO) (monoamino BQ) νas (CH2) (cystamine) νs (CH2) (cystamine)

a Frequencies and assignments denote out of plane modes of benzoquinone.

monolayer. The C-H stretching region (2800-3000 cm-1) shows prominent peaks at 2855 and 2909 cm-1 corresponding to the symmetric and antisymmetric stretching frequencies of the CH2 groups (Figure 1b). This is in accordance with the values reported in the literature for short chain alkane thiol monolayers.6,7 All of the band assignments in the Raman spectrum of cystamine SAM are listed in Table 1 (Supporting Information). RAIR Characterization. The RAIR spectrum of cystamine monolayer is noisy, and no characteristic peaks are observed. The short chain-based thiol monolayers are known to form disorganized structures leading to a very poor signal-to-noise ratio.29 The presence of predominantly gauche conformers in the monolayer from Raman studies, the absence of a good IR spectrum and a poor blocking behavior toward external redox probes as

observed in the voltammetric studies, indicate that cystamine does not form a very well ordered monolayer on the gold surface. XPS Characterization. The survey spectrum reveals the presence of C, N, S, and O, and Figure 2 show the XPS spectra of the cystamine monolayer for a takeoff angle of 85° along with the deconvoluted peaks. The peak positions, peak area and line width (full width at half-maximum) of various components are given in the Supporting Information. In the S (2p) region, three different components can be identified based on the deconvoluted spectrum at 165.5, 162.5, and 161 eV.30 The peak at 162.5 eV having the maximum intensity is attributed to the thiolate species formed between cystamine and gold. The S (2p) feature that occurs at 162.5 eV is in agreement with the RS--Au+ type of coordination similar to the values reported in the literature for alkanethiols and disulfides.31-34 The peaks at 165.5 and 161 eV correspond to the unbound sulfur and the oxidized sulfur species, respectively. The formation of oxidized sulfur species, such as sulfonates and sulfinates, has previously been found for SAMs of alkanthiols on gold.32,35,36 The intensities of the two peaks are comparatively lower than that of the peak corresponding to the Au-S bond. This shows that majority of sulfur in the monolayer is present in the form of thiolate that coexists with a small percentage of unbound and oxidized form of sulfur. The O (1s) spectrum features two main peaks at approximately 532.7 and 531.3 eV, respectively, where the former component is ascribed to sulfonate and the latter is likely due to the contamination from water. The C (1s) region for the cystamine monolayer shows two peaks at 284.7 and 285.9 eV. The peak at 284.7 eV is assigned to the carbon present in alkyl chains.31,32,37 Wirde and co-workers30 have reported a C (1s) peak at 285.9 eV due to impurities associated with cystamine SAM on gold. The N (1s) region (29) Yam, C.-M.; Pradier, C.-M.; Salmain, M.; Marcus, P.; Jaouen, G. J. Colloid Interface Sci. 2002, 235, 183. (30) Wirde, M.; Gelius, U.; Nyholm, L. Langmuir 1999, 15, 6370. (31) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1996, 118, 10211. (32) Bindu, V.; Pradeep, T. Vacuum 1998, 49, 63. (33) Lukkari, J.; Kleemola, K.; Meretoja, M.; Ollonqvist, T.; Kankare, J. Langmuir 1998, 14, 1705. (34) Zubragel, C.; Deuper, C.; Schneider, F.; Neumann, M.; Grunze, M.; Schertel, A.; Woll, Ch. Chem. Phys. Lett. 1995, 238, 308. (35) Rieley, H.; Kendall, G. K.; Zemicael, F. W.; Smith, T. L.; Yang, S. Langmuir 1998, 14, 5147. (36) Lee, M.-T.; Hsueh, C.-C.; Freund, M. S.; Feguson, G. S. Langmuir 1998, 14, 6419. (37) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmiur 1992, 8, 1330.

Characterization of A-σ Spacer-D Monolayers

Figure 3. FT-Raman spectra of (a) BQ-II SAM and (b) BQ-I SAM on gold.

shows a peak at 400 eV that is assigned to the amine groups33 present in the monolayer. Based on the area of the peaks for different functionalities, the ratios for C/N, C/S, and N/S have been calculated to be 3, 2.4, and 0.8, respectively. The values are close to the ratios expected for the cystamine monolayer based on stoichiometry. As the value of TOA changes from 85° to 30°, the C/S and N/S ratios are reduced to 1.57 and 0.48, respectively. On the other hand, the area corresponding to the S (2p) peak shows an increase in the area at a TOA of 30° as compared to area observed at a TOA of 85° indicating that S lies close to the gold surface. This is an additional proof to show that adsorption of cystamine on gold surface takes place through the S and not the “N” end of the molecule. Benzoquinone Monolayer. FT-Raman Characterization. It should be noted that BQ-II SAM (preceding paper) contains both mono- and diamino derivatives of the quinone, whereas BQ-I SAM predominantly comprises the diamino derivative of benzoquinone. The bands for the quinone monolayer are assigned in Table 1.38-40 The FT-Raman spectra of the two quinone monolayers, BQ-II SAM and BQ-I SAM, are shown in Figure 3, panels a and b, respectively. Both of the SAMs show the presence of Au-S stretch, the two conformers for C-S stretching mode and the scissoring modes of the CH2 groups. The bands for symmetric and antisymmetric stretching modes for the CH2 group (not shown) are observed at 2911 cm-1 and 2859 cm-1, respectively and the intensity of the two bands are attenuated compared to the cystamine SAM. A strong band observed at 464 cm-1 is attributed to the coupling of two quinonoid modes, the ring bending mode and the carbonyl-bending mode. The band observed at 540 cm-1 could be due to a combination of a number of modes in quinone namely, antisymmetric stretching of the C-C bond, the quinonoid ring bending, and the carbonyl bending mode. The C-H bending mode of BQ is observed at about 1335 cm-1, and the band due to antisymmetric C-C stretch (coupled to C-H bending) is present at 1382 cm-1. Apart from this, several out of plane modes are also observed at 240, 819, and 1010 cm-1 (38) Dunn, T. M.; Francis, A. H. J. Mol. Spectrosc. 1974, 50, 1. (39) Yamakita, Y.; Tasumi, M. J. Phys. Chem. 1995, 99, 8524. (40) Mohandas, P.; Umapathy, S. J. Phys. Chem. A 1997, 101, 4449.

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for both of the BQ monolayers, and the corresponding assignments are given in the Supporting Information. In the cystamine monolayer, the C-N stretching frequency is shown at 1290 cm-1, but on reaction with BQ, there is a downshift in the stretching frequency value, and it occurs at about 1278 cm-1. The downshift indicates weakening of the C-N bond that could be attributed to the delocalization of electron density on nitrogen after the reaction with BQ. The BQ-I SAM shows two bands at 1586 and 1515 cm-1 that can be assigned to the carbonyl stretch and CdC stretch of quinone. In the case of BQ-II SAM, apart from the bands at 1590 and 1515 cm-1, there is an additional band present at 1665 cm-1. It should be noted that BQ-I SAM would predominantly have diamino derivative of quinone, whereas BQ-II SAM would have both mono- and diamino derivatives on the surface. This additional band at 1665 cm-1 can be assigned to the CdO stretch of the monoamino derivative of the quinone present on the surface. Thus, the three bands at 1665, 1590, and 1515 cm-1 observed for BQ-II SAM can be assigned to CdO of monoamino derivative, CdO of diamino derivative, and the CdC stretch of the quinonoid ring. This assignment of monoamino- and diamino derivatives of benzoquinone is further confirmed based on spectroelectrochemistry that will be discussed later. RAIR Characterization. The RAIR spectrum of BQ-II SAM shows characteristic bands between 1800 and 600 cm-1 (Supporting Information). The strong band observed at 1652 cm-1 is assigned to the carbonyl stretch of the quinone.41 The incorporation of quinone to the cystamine monolayer leads to ordering of the SAM and is evident by the presence of the CH2 scissoring mode that is observed at 1470 cm-1 in BQ-II SAM. The RAIR spectrum of BQ-I SAM shows a smaller number of bands than those observed in the case of BQ-II SAM (Supporting Information). A reduction in the number of bands indicates a different orientation of the quinone moiety. If the diamino derivative of quinone is present in a face-on orientation, the number of bands associated with the quinonoid moiety is expected to be small. The bands due to C-C and CdC stretch of quinone are observed in the RAIR of BQ-II SAM but are absent in the case of BQ-I SAM. This is generally taken to be a diagnostic criterion for the difference in orientation observed with the diamino derivative of quinone. In the IR spectrum of the oxidized form of hydroquinone monolayer, no bands are reported16 at 1590 cm-1, whereas Sasaki and co-workers17 and Ye and co-workers15 have reported the presence of bands at 1530 and 1600 cm-1, respectively, corresponding to the CdC stretch. In the case where SAM shows the presence of a band at 1600 cm-1, the authors have concluded that the adsorbate, both in the reduced and oxidized forms, is coordinated to the surface with the molecular plane perpendicular rather than parallel to the substrate.15-17,42 This observation is in direct correlation with the observations made in the present study. Thus, quinone molecules present in BQ-I SAM can be assigned to have the molecular plane parallel to the surface, and as a result, the RAIR does not show any band at 1590 cm-1. On the other hand, the monolayer having both mono- and diamino derivatives shows a behavior where the molecular plane of quinone is lying perpendicular to the substrate. Spectroelectrochemical Studies. Spectroelectrochemical characterization is expected to reveal information on the orientation differences of the quinonoid moiety. Figure 4a shows the in situ FT Raman spectra of BQ-II SAM in 0.1 M phosphate buffer (pH (41) Davies, M.; Prichard, F. E. Trans. Faraday Soc. 1963, 59, 1248. (42) Nishiyama, K.; Tahara, S.-I.; Uchida, Y.; Tanoue, S.; Taniguchi, I. J. Electroanal. Chem. 1999, 478, 83.

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Figure 4. (a) Raman spectra of BQ-II SAM as a function of applied dc bias (i) -0.052 V, (ii) -0.15 V, (iii) -0.23 V, (iv) -0.34 V, (v) -0.425 V, (vi) -0.54 V, and (vii) -0.632 V. (b) Cyclic voltammogram of the BQ-II SAM in 0.1 M phosphate buffer (pH 7.2) at a scan rate of 0.05 V/s. Reference electrode: SCE

7.2) as a function of applied dc bias. The potential is varied in the negative direction to convert the quinone to hydroquinone species. The reduction peak potential of the two derivatives are observed at -0.2 V for the monoamino BQ and -0.56 V for the diamino BQ (Figure 4b). The presence of electrolyte in the spectroelectrochemical cell leads to attenuation in the intensities of the bands as compared to that obtained in the dry state. As a result, appreciable changes are observed only for the in-plane modes that are quite intense in the spectra obtained for the dry state SAM as compared to the out-of-plane modes of the quinone. When the quinone is in the oxidized state, that is when a potential bias of -0.052V is applied, the spectrum matches well with that obtained in the dry state. The bands due to carbonyl stretching frequencies of both mono- and diamino derivatives of quinone are observed at 1660 and 1590 cm-1, respectively. As the potential is changed to negative values, the intensity of the band at 1660 cm-1 reduces and finally disappears at the peak potential of -0.2 V. Correspondingly, an increase in intensity is observed for the band at 1386 cm-1. The band at 1590 cm-1 attributed to the quinone of the diamino derivative is still observed. The intensities of other bands in the spectrum do not change considerably till the first redox peak potential is reached. As the potential is swept further from -0.2 V to negative values, a reduction in intensity is observed for all of the in-plane bands of quinone. The intensity of all of these bands beyond the second peak potential of -0.56 V reduces drastically and almost vanishes. This strongly indicates that both the quinone derivatives are present in two different orientations as relative variations in the band intensities observed are very large only upon the electrochemical reduction of the diamino derivative of the quinone. Apart from a decrease in intensity for the bands mentioned above, a small increase in intensity is observed for the band at 1344 cm-1. This is attributed to the C-O stretch of the hydroquinone moiety generated on reduction of the quinone.

Sarkar and Sampath

A reduction in intensity is observed for all of the bands that have contributions from the CdO modes. This is understood based on the changes in the redox state from quinone to the hydroquinone form, the CdO bond gets reduced to the corresponding enolic form that results in a change in the intensity of the bands that are associated with the vibrational modes of CdO bond. The only exception to this observation is the band at 1386 cm-1. This band has been assigned to the C-H bending mode, which is coupled to the antisymmetric stretch of the C-C bond of BQ. Thus, a change in intensity of this band indicates that the quinone immobilized to the SAM undergoes a change in orientation when the there is a change in the redox state. A similar change in intensity has also been reported for anthraquinone monolayer.42 The intensity of the bands at 272, 640, and 723 cm-1 remain unchanged and are assigned to the Au-S stretching frequency and the gauche and the trans conformers of the C-S stretching modes, respectively. These bands would be little affected when the redox state of the quinone is changed from the reduced state to the oxidized state. The following section describes the spectroscopic characterization of the monolayer when the redox state is reversed from the hydroquione to the quinone state. The BQ-II SAM shows two peaks for oxidation of the mono- and diamino derivatives of quinone at -0.46 and -0.15 V in the cyclic voltammogram. The bands due to the quinone form are not observed till the first oxidation peak potential is reached (-0.46 V). As the potential is swept to positive values, the bands corresponding to the inplane modes of BQ revive and increase in intensity (figure not shown). The weak band at 1344 cm-1 that is attributed to the C-O stretching frequency, decreases in intensity, upon oxidation of hydroquinone to the corresponding quinone. Hence, the spectroelectrochemical study of BQ-II SAM confirms the fact that the bands at 1660 and 1590 cm-1 are due to the mono- and the diamino derivatives of benzoquinone on the surface. A change in intensity observed for the quinonoid bands as a function of applied potential is attributed to a change in the redox state of the quinone and also a change in the orientation of the redox moieties as part of the monolayer. XPS Characterization. The XPS spectra for the BQ-II SAM and the corresponding peak areas, line widths, and peak positions for various functional groups are listed in the Supporting Information. The C (1s) region of the monolayer shows two peaks at 284.5 and 286.6 eV. The peak at 284.5 eV is also observed in the cystamine monolayer and has already been assigned to the carbon present in the alkyl chain of the SAM. The additional peak observed at 286.6 eV is assigned to the carbonyls present in the quinonoid moiety33,43 The N (1s) region shows a broad peak at 400.6 eV. The noticeable change in this region is the increase in line width from 2.15 eV in cystamine SAM to 3.49 eV in the BQ-II SAM. An increase in line width strongly indicates the presence of multiple functional groups, which have very close values of binding energy. The spectroscopic and electrochemical studies have indicated the presence of both mono- and diamino derivatives of benzoquinone in the BQ-II SAM. It has been reported in the literature that both mono-31 and diamino derivatives42 of benzoquinone have similar values of 400.5 eV for the N (1s) binding energy. Thus, the chemical heterogeneity that exists for the N (1s) peak in BQ-II SAM is possibly shown as an increase in the line width of the peak.33,44 The S (2p) region shows two peaks at 165.2 and 162.5 eV similar to that of the cystamine monolayer. The S/N ratio is (43) Ohta, T.; Yamada, M.; Kuroda, H. Bull. Chem. Soc. Jpn. 1974, 47, 1158. (44) Han, M.; Helms, A. B.; Hu, Y. Nikles, D. E.; Nikles, J. A. Sharma, R. Street, S. C. Warren, G. W. IEEE Trans. Magn. 1999, 35, 2763.

Characterization of A-σ Spacer-D Monolayers

Figure 5. (a) Raman spectrum of BQ-DA10 SAM. The deconvoluted Raman spectra of (b) BQ-DA10 SAM and (c) BQ-II SAM are also shown.

calculated to be 1.01, which is in good accordance with the expected value. In the O (1s) region, two peaks are observed at 532.8 and 530.8 eV and are assigned to oxygen present in sulfonate and quinonoid functionalities. Benzoquinone Diaminoalkane Spacer SAM. FT-Raman Characterization. The FT-Raman spectrum of the monolayer after the introduction of diaminodecane spacer is shown in Figure 5a. The most noticeable changes observed are the disappearance of the band at 1665 cm-1 and an increase in intensity observed for the band at 1594 cm-1. This indicates that, upon reaction with diamino decane, the monoamino derivative of the quinone present on the surface is converted to the diamino derivative. As a result, the CdO stretching mode due to monoamino derivative at 1665 cm-1 disappears, and the band at 1594 cm-1 due to diamino derivative shows an enhanced intensity. To clearly understand this conversion, the Raman spectra obtained for BQII SAM and BQ-DA10 SAM are deconvoluted and shown in Figure 5, panels b and c. There are distinct changes observed in the spectra before and after the modification. The band at 1665 cm-1 is absent in the deconvoluted spectrum of the diamine attached BQ-II SAM, whereas this band is present in the deconvoluted spectrum of the BQ-II SAM. This is in accordance with the results obtained from the spectroelectrochemical studies performed on the BQ-DA10 SAM. The band position due to CdO stretch after diamine modification matches well with that of BQ-I SAM, which primarily contains only the diamino derivative. Several other bands are observed that correspond to the diamine-modified quinone and the underlying base monolayer (Supporting Information). Upon functionalization with the diamino decane, the band due to C-N stretch shifts back to 1295 cm-1 as observed for the cystamine SAM, indicating the introduction of free NH2 groups on the surface. RAIR Characterization. In the RAIR spectrum (Supporting Information) of the BQ-DA10 monolayer, a very strong band is observed at 1242 cm-1 corresponding to the C-N stretching frequency showing the presence of free NH2 groups on surface. The bands present at 1115, 906, and 823 cm-1 are due to different modes of the immobilized quinone. The incorporation of a long alkyl chain shows the symmetric and antisymmetric stretching frequencies for the CH2 groups at 2931 and 2855 cm-1, respectively. However, several other bands in the region between 1800 and 1300 cm-1 are observed as a broad feature. Upon deconvolution (Figure 6b), four bands occurring at 1640, 1581,

Langmuir, Vol. 22, No. 7, 2006 3401

Figure 6. (a) RAIR spectrum of the BQ-σ10-Fc SAM. (b) Deconvoluted spectrum in the range between 1800 and 1300 cm-1.

1509, and 1451 cm-1 are observed. The first three bands are assigned to the CdO stretch, CdC stretch, and C-C stretch of the quinone, respectively, and the latter is due to the CH2 scissoring mode present in the SAM. Spectroelectrochemical Characterization. The Raman spectra of the BQ-DA10 SAM have been recorded as a function of applied dc potential. Upon oxidation and reduction of the quinonoid moieties, the bands observed in the spectra show similar variations in intensity as observed for the BQ-II SAM (figures not shown). XPS Characterization. The XPS spectrum of the monolayer after the incorporation of diamino decane spacer and the peak areas, line widths, and peak positions obtained for different functional groups are listed in the Supporting Information. The C (1s) region shows a peak at 284.8 eV that is assigned to the carbon present in the long alkyl chain of the monolayer. As compared to BQ-II SAM, this peak shows an increase in intensity by 3.5 times for the BQ-DA10 SAM. This increase is attributed to the incorporation of a long alkyl chain spacer into the monolayer assembly. A similar increase in area is observed for the peak observed at 400.3 eV in the N (1s) region. However, the line width is reduced from 3.5 to 3.0 eV indicating a reduction in the chemical heterogeneity of the monolayer modified with diaminoalkane. The S (2p) region shows a relatively weak band as compared to the previous stages of modification. Benzoquinone-Sigma Spacer-Ferrocene SAM. FT-Raman Characterization. The Raman spectrum of the complete D-σ spacer-A assembly shows very few bands that correspond to the ferrocene moiety attached to the monolayer (Supporting Information, Table 2). A broad shoulder is observed at 1604 cm-1 due to the carbonyl stretch of the amide bond. The Au-S stretch is observed at 277 cm-1 and a shoulder observed at 310 cm-1 is assigned to the metal ring stretching frequency of ferrocene.45-47 Upon reaction with ferrocene acetic acid, the band intensities for the stretching frequencies of benzoquinone are reduced to a large extent. RAIR Characterization. In sharp contrast, the RAIR shows characteristic bands for ferrocene as shown in Figure 6, and the band assignments are given in the Supporting Information. The (45) Bailey, R. T.; Lippincott, E. R. Spectrochim. Acta 1965, 21, 389. (46) Willis, J. N.; Ryan, M. T.; Hedberg, F. L.; Rosenberg, H. Spectrochim. Acta 1967, 24A, 1561. (47) Bodenheimer, J. S.; Low, W. Spectrochim. Acta 1973, 29A, 1733.

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Sarkar and Sampath

Table 2. Frequencies and Assignments of the Raman Spectrum of BQ-σ10-Fc SAM on Gold

a

frequencies (cm-1)

band assignment

277 310 470 535 648 735 800a 1340 1386 1450 1510 1604

ν (Au-S) νs (ring-Fe) δ(2, 5), b(CdO) (BQ) νa(C-C), δ(2, 5), b(CdO) (BQ) ν (C-S)G ν (C-S)T ω(CdO), ring deformation (BQ) intraring stretch (Fc) νa(C-C), b(C-H) (BQ) δ (CH2) ν (CdC) (BQ) ν (CdO) (amide I Fc)

Frequency and assignment denote out of plane modes of BQ.

band at 1044 cm-1 is assigned to be due to the δ⊥(C-H) mode of ferrocene having an out-of-plane character. The in-plane mode of the ferrocene is observed at 1645 cm-1 (amide I) and the band at 1096 cm-1 is attributed to the γ (C-H) mode of ferrocene. The ferrocenyl C-C and C-H stretching modes couple to give two bands at 1379 and 1350 cm-1. The region between 1800 and 1380 cm-1 is deconvoluted into five bands as shown in Figure 6b. The amide I band is shown at 1645 cm-1, and the amide II band is present at 1587 cm-1. The band at 1502 cm-1 is due to the metal-carbon stretching frequency.20 The scissoring mode of CH2 is observed at 1457 cm-1, and the symmetric and antisymmetric modes are observed at 2927 and 2854 cm-1, respectively. The quinonoid moiety in the BQ-σ10-Fc assembly shows the characteristic bands as listed in the table in Supporting Information. Spectroelectrochemical Characterization. The spectroelectrochemical study of the BQ-σ10-Fc monolayer has been carried out in 0.1 M phthalate buffer (pH 4.6). Figure 7a corresponds to the changes observed in the Raman spectra of the SAM for the reduction of the quinone, and Figure 7b shows the changes associated with the Raman spectra upon oxidation of ferrocene in the SAM. The FT-Raman spectrum of the assembly shows weak bands for BQ in the presence of an electrolyte though the changes observed as a function of potential are very similar to the changes associated with the BQ-DA10 and BQ-II SAM. The bands associated with CdO vibrational modes as well as the band at 1386 cm-1 show a decrease in intensity, whereas the band at 1335 cm-1 (attributed to the C-O stretching of hydroquinone) shows a small increase in intensity at the reduction peak potential. When the redox state is reversed from the hydroquinone to the quinone state, the in-plane modes of BQ revive and increase in intensity (figure not shown). Nishiyama and co-workers19 have reported two strong bands for the ferrocene monolayer at 1106 and 320 cm-1, respectively. The band at 1106 cm-1 is attributed to the breathing mode of the cyclopentadienyl ring of ferrocene, and the intensity of this band is strongly dependent on the orientation of the ferrocenyl moiety. This band is absent in the FT-Raman spectrum of the monolayer indicating a different orientation for ferrocene in the BQ-σ10-Fc assembly as compared to that of 8-ferrocenyloctanethiol monolayer on gold.19 The other strong band observed at 320 cm-1 corresponds to the stretching mode of the Fe(II)-cyclopentadienyl ring. This band occurs in combination with the Au-S stretch and results in a broad feature in the region 200-350 cm-1. When a potential bias is applied in the positive direction, the ferrocene molecules get oxidized to form ferricinium moieties in the monolayer. At the peak potential, the Raman spectrum shows a shift in the position of the band corresponding to the

Figure 7. Raman spectra of BQ-σ3-Fc SAM on gold as a function of applied dc bias (i) -0.11 V, (ii) -0.25 V, (iii) -0.395 V, (iv) -0.56 V, (v) 0.15 V, (vi) 0.26 V, and (vii) 0.4 V for (a) the reduction of benzoquinone and (b) the oxidation of ferrocene in the monolayer. (c) Cyclic voltammogram of the BQ-σ3-Fc SAM in 0.1 M phthalate buffer (pH 4.6) at a scan rate of 0.05 V/s. The reference electrode used is SCE.

Fe (III)-cyclopentadienyl ring vibration from 320 to 311 cm-1. The quinonoid bands show a change in intensity, particularly with the bands present at 1386 and 1590 cm-1. As mentioned earlier, a change in intensity in the band at 1386 cm-1 strongly indicates a change in the orientation of the quinonoid moiety in the monolayer. The electrochemical experiments do not show any interaction between the donor and acceptor molecules; hence, the change in intensities observed for the quinonoid bands upon oxidation of ferrocene is attributed to a change in the orientation of BQ in the assembly. XPS Characterization. The XPS spectra of the BQ-σ10-Fc monolayer and the peak areas, line widths, and peak positions obtained for different functional groups are shown in the Supporting Information. The C (1s) region shows three peaks at 284.3, 285.1, and 286.6 eV that correspond to the carbon present as part of the aromatic ring, the alkyl chain, and the quinonoid moiety, respectively. The aromatic carbon functionality is introduced into the monolayer after the covalent attachment of ferrocene to the SAM. It is interesting to note that the line width for the N (1s) peak, present at 400.3 eV, has further reduced to

Characterization of A-σ Spacer-D Monolayers

a value of 2.57 eV. This value is close to that obtained for the N (1s) peak in the cystamine monolayer. This strongly indicates a very homogeneous chemical environment for nitrogen in the BQ-σ10-Fc monolayer compared to the BQ-II SAM and BQDA10 SAM. The BQ-σ10-Fc monolayer shows a peak for Fe (2p) at 708.5 eV, which is characteristic of Fe present in the ferrocene molecule.48

Conclusions A detailed spectroscopic analysis has been carried out for the Benzoquinone-σ spacer-Ferrocene monolayer based on reflectance absorbance IR spectroscopy, FT-Raman spectroscopy and X-ray photoelectron spectroscopy, at every step of modification. The cumulative analysis based on Raman, RAIR, and spectroelectrochemical studies offers an insight into the possible orientation difference that exists between the monoamino and the diamino derivatives of benzoquinone. The diamino derivatives show a face-on orientation on the surface, whereas the monoamino (48) Fischer, A. B.; Wrighton, M. S.; Umana, M. Murray, R. W. J. Am. Chem. Soc. 1979, 101, 3442.

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derivatives are present in an end-on orientation. The presence of monoamino and diamino derivatives in the BQ-II SAM is clearly revealed in the spectroscopic and spectroelectrochemical experiments. The spectroscopic results further indicate a good monolayer integrity for the benzoquinone-σ spacer-ferrocene SAM and is in line with the electrochemical data presented in the preceding paper. Acknowledgment. This work was supported by the funding from DST, CSIR and CHT, New Delhi. The authors gratefully acknowledge the useful discussions with Prof. S. Umapathy and also thank Dr. Rama Rao, IUC, Indore for the XPS measurements. Supporting Information Available: The XPS spectra for the BQ-II SAM and the corresponding peak areas, line widths, and peak positions for various functional groups are listed in the Supporting Information; the XPS spectrum of the monolayer after the incorporation of diamino decane spacer and the peak areas, line widths, and peak positions obtained for different functional groups; the XPS spectra of the BQ-σ10-Fc monolayer and the peak areas, line widths, and peak positions obtained for different functional groups. This material is available free of charge via the Internet at http://pubs.acs.org. LA051858A