Formation and Electrochemical Property of Pyrrole-Terminated SAMs

Langmuir 2002, 18, 7473-7481

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Formation and Electrochemical Property of Pyrrole-Terminated SAMs and the Effect of the SAMs on the Physicochemical Properties of Polypyrrole Films Electrochemically Deposited over Them Chun-Guey Wu,* Sheng-Chang Chiang, and Chiung-Hui Wu Department of Chemistry, National Central University, Chung-Li, Taiwan 32054, Republic of China Received December 12, 2001. In Final Form: April 1, 2002 ω-(N-Pyrrolyl) alkanethiols with various alkyl chain lengths were prepared to study the formation of self-assembled monolayers (SAMs) bearing a bulky headgroup and the electrochemical deposition of polypyrrole over SAM-modified gold electrodes. Polarized IR reflection-absorption, ellipsometry, and quartz crystal microbalance data showed that an alkanethiol with a longer alkyl chain is better with regard to coverage and ordering of the SAMs. The monolayer coverages are 62%, 74%, 78%, and 78% for ω-(Npyrrolyl) propanethiol, ω-(N-pyrrolyl) hexanethiol, ω-(N-pyrrolyl) nonanethiol, and ω-(N-pyrrolyl) dodecanethiol, respectively. Due to oxidative desorption of the monolayer, the cyclic voltammograms (CVs) of the SAMs of ω-(N-pyrrolyl) alkanethiols on Au in 0.1 M KCl/H2O display an irreversible oxidation wave at ca. 1.2 eV versus a Ag/AgCl electrode. No redox reaction of the SAM was observed when the CV measurements were carried out in acetonitrile, using Et4NBF4 as an electrolyte. There is also no voltammetric evidence for polymerization of the surface-confined pyrrole units at the potential cyclically scanned from 0 to +1.4 V. The electrochemical deposition of polypyrrole potentiodynamically over monolayers of ω-(Npyrrolyl) alkanethiols shows that the polymerization potential of pyrrole over long alkyl chain ω-(Npyrrolyl) alkanethiol modified gold electrodes was higher than that over short alkyl chain modified electrodes in KCl/H2O. The polymerization of pyrrole apparently occurred without monolayer oxidation. In Et4NBF4/CH3CN, SAMs of ω-(N-pyrrolyl) alkanethiols were stable under a potential scan. During the growth of polypyrrole, the pyrrole molecules in ω-(N-pyrrolyl) alkanethiols may serve as nucleation sites for the growth of polypyrrole. Therefore, polypyrrole films electrochemically deposited on ω-(N-pyrrolyl) alkanethiol monolayers have a denser morphological packing and good adhesion. The conductivity of these films is enhanced as a consequence of the change in morphology.

Introduction Organized molecular systems have attracted growing attention since the early 1980s, due to their technological potential in both optical and molecular electronics.1 The molecular self-assembly technique is one of the important methods for constructing such systems. In nature, the self-assembly phenomenon has been known for a long time, such as the folding of proteins, the bilayer structure of the cell membrane, the formation of the DNA double helix, and so forth. Self-assembled monolayers (SAMs) are the most extensively studied and well-developed nonbiological self-assembling systems. Among a number of systems known to produce SAMs, the alkanethiolates on gold2 are best characterized. The phenomenon of autoadsorption of long-chain thiolates on gold has been known for two decades3 and was extensively studied by Whitesides4 et al. The monolayers of thiolates on gold surfaces are ordered and densely packed. These discoveries provide a unique opportunity to engineer surfaces at the molecular level. SAMs are excellent model systems for studying wetting,5 (1) (a) Roberts, G. G. Adv. Phys. 1985, 34, 475. (b) Ulman, A. J. Mater. Educ. 1989, 11, 205. (c) Tour, J. M. Acc. Chem. Res. 2000, 33, 791. (2) (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (b) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266. (3) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4482. (4) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (5) Adamson, A. W. Physical Chemistry of Surfaces; Wiley: New York, 1982; Chapter 10.

adhesion,6 lubrication,7 corrosion,8 nucleation, and protein adsorption.9 They are also well suited for ultrathin resisting or passivating layers in fabricating patterns and structures with lateral dimensions at the nanometer to micrometer scale.10 In these prospects, SAMs can be used to control the surface properties and to elucidate the chemistry of the interfaces.11 The electrochemically synthesized conducting polymers have been extensively investigated because of their wide range of useful applications such as microelectronic devices.12 However, the mechanism for the deposition of these polymer films on electrode surfaces has been widely studied13 but not well understood. Marcos14 et al. reported the formation of a monolayer of monomer, which nucleates further polymer growth. Other evidence showed that an (6) Zisman, W. A. Handbook of Adhesives; Skeist, I., Ed.; Van Nostrand: New York, 1977; Chapter 3. (7) Bowden, F. P.; Taber, D. The friction and lubrication of solids; Oxford University Press: London, 1968; Part II, Chapter 19. (8) Bregman, J. I. Corrosion Inhibitors; MacMillan: New York, 1963; Chapter 5. (9) Delamarche, E.; Michel, B.; Biebuyck, H. A.; Gerber, C. Adv. Mater. 1996, 8, 719. (10) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550. (11) Whitesides, G. M. Chimia 1990, 44, 310. (12) Bond, S. F.; Howie, A.; Friend, R. H. Surf. Sci. 1995, 333, 196. (13) (a) Kim, Y. T.; Allara, D. L.; Collins, R. W.; Vedam, K. Thin Solid Films 1990, 193/4, 350. (b) Raymond, D. E.; Harrison, D. J. J. Electroanal. Chem. 1993, 361, 65. (c) Raymond, D. E.; Harrison, D. J. J. Electroanal. Chem. 1993, 355, 115. (d) Guyard, L.; Hapiot, P.; Neta, P. J. Phys. Chem. B 1997, 101, 5698. (e) Lukkari, J.; Alanko, M.; Heikklia, L.; Laiho, R.; Kankare, J. Chem. Mater. 1993, 5, 289. (f) Higgins, S. J.; Hamnett, A. Electrochim. Acta 1991, 36, 2123. (14) Marcos, M. L.; Rodriguez, I.; Gonzalez, V. J. Electrochim. Acta 1987, 32, 1453.

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electrochemically initiated chain polymerization mechanism is better than radical-radical coupling.15 Therefore, the rate-determining step for polypyrrole growth from aqueous solutions is the oxidative incorporation of monomers or oligomers into the growing polymer chains.16 Nevertheless, a generally accepted mechanism for the anodic polymerization of heteroarenes is the formation of soluble oligomers that eventually precipitate on the electrode surface and the formation of nucleation sites for the polymer film to grow. In the latter case, regulating the number and locations of the nucleation sites on the surface would allow us to control the deposition of the polymer films. On the other hand, polypyrrole (Ppy) risks delamination from an Au electrode when it undergoes volume change during electrochemical cycling, which results in large shear stresses at the Ppy/Au interface.17 Researchers have tried to use adhesion-promoting molecules to modify the electrode to increase the adhesion between the electrode and the polymer film.18 Wrighton and co-workers used N-(3-(trimethoxysilyl)propyl)pyrrole as a surface modification agent to improve adhesion of Ppy films and n-type Si.19 Sayre and Collard20 have reported the electrooxidative polymerization of 3-ethylpyrrole on gold electrodes modified with spontaneously adsorbed monolayers of alkanethiols bearing 3-substituted pyrroles. Wurm21 et al. used bis-(ω-(N-pyrrolyl)-n-undecyl)disulfide to modify the gold surface and then electrochemically deposited polyalkylpyrrole on it to get the epitaxial growth of the polypyrrole thin films. Kupila22 pretreated the electrode with various thiols and found that the thiols enhanced the early stages of polymerization. However, the effects were small and varied with both the anion and the thiols. Willicut23 et al. reported the synthesis of various ω-(Npyrrolyl) alkanethiols and the electrochemical characterizations of the resulting monolayers on Au. They found that the self-assembled monolayers of alkyl mercaptans containing pyrrole enhance the adhesion between polymer film and electrode. The short-chain ω-(N-pyrrolyl) alkanethiols they used have a liquidlike monolayer structure, which should enhance the probability of oligomer or polymer formation within the monolayer. E. Smela24 et al. have reported a series of studies on the synthesis, characterization, and electrochemical studies of the thioethylpyrrole (TEP) self-assembled monolayer as well as the electrochemical deposition of polypyrrole over the monolayer of thioethylpyrrole.25 They found that the monolayer TEPs are not stable in air and they are oxidatively polymerized during electrochemical cycling; therefore, the surface-bound pyrrole is unavailable for reacting with the pyrrole molecules in solution. Recently, Wurm and Kim26 have used an electrochemical quartz (15) Qiu, Y. J.; Reynold, J. R. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 1315. (16) Scharifker, B. R.; Garcia, P. E.; Marino, W. J. J. Electroanal. Chem. 1991, 300, 85. (17) Smela, E.; Inganas, E.; Lundstrom, I. Science 1995, 268, 1735. (18) Mekhalif, Z.; Lang, P.; Garnier, F. J. Electroanal. Chem. 1995, 399, 61. (19) Wrighton, M. S. Science 1986, 231, 32. (20) Sayre, C. N.; Collard, D. M. Langmuir 1995, 11, 302. (21) Wurm, D. B.; Brittain, S. T.; Kim, Y. T. Langmuir 1996, 12, 3756. (22) Kupila, E. L.; Kankare, J. Synth. Met. 1995, 74, 241. (23) (a) Willicut, R. J.; Mccarley, R. L. Langmuir 1995, 11, 296. (b) Willicut, R. J.; Mccarley, R. L. Anal. Chim. Acta 1995, 307, 269. (c) Willicut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 10823. (24) (a) Smela, E.; Zuccarello, G.; Kariis, H.; Liedberg, B. Langmuir 1998, 14, 2970. (b) Smela, E.; Kariis, H.; Yang, Z. P.; Uvdal, K.; Zuccarello, G.; Liedberg, B. Langmuir 1998, 14, 2976. (25) (a) Smela, E.; Kariis, H. K.; Yang, Z. P.; Mecklenburg, M.; Liedberg, B. Langmuir 1998, 14, 2984. (b) Smela, E. Langmuir 1998, 14, 2996.

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crystal microbalance (QCM) to study the growth of N-alkylpyrrole on a bis-(ω-(N-pyrrole)-n-undecyl) sulfide modified electrode and demonstrated that the surfaceconfined pyrrole units on the modified electrode serve as specific nucleation sites to induce long-range order in the polymer film. These researchers reported materials closely related to our subject. Nevertheless, none of these have investigated systematically the effect of the alkyl chain length on the properties of the monolayer of the pyrrolylalkanethiols and the process of the electrochemical deposition of polypyrrole on SAMs of these pyrrole-thiols. Furthermore, no detailed studies were focused on the physicochemical and charge transport properties of polypyrrole deposited over the monolayers of pyrrolyl-alkanethiols. In this article, we report the preparation and self-assembly phenomena of a series of ω-(N-pyrrolyl) alkanethiols with various alkyl chain lengths and study the effects of SAM’s structure on the physicochemical and charge transport properties of the polypyrrole films deposited over them. Experimental Section Chemicals. Dibromoalkanes, KOH, tetraethylammonium tetrafloroborate, KCl, MgSO4, Et4NBF4, thiourea, HCl(aq), and silica gel were obtained from commercial sources and used as received. Pyrrole and all the solvents were purified with the literature methods27 and distilled before use. The gold (99.99%) and titanium wires used for vacuum evaporation were purchased from Alpha Ventron. Silicon wafers ((100) oriented, p-type) were obtained from Topsil Co. Preparation of ω-(N-Pyrrolyl) Alkanethiols. A number of homologues of ω-(N-pyrrolyl) alkanethiols with n ) 3, 6, 9, and 12 were synthesized. In a typical reaction, 20 mmol of pyrrole was mixed with 25 mmol of KOH powder and stirred at 0 °C for 3 h. To the mixture, 60 mmol of dibromoalkane and 60 mL of dimethylformamide (DMF) were added and stirred at 0 °C for 12 h, and then the reaction was quenched with 60 mL of H2O. The product was extracted with ether and dried, and flash chromatography (1:100 v/v ethyl acetate/petroleum ether on silica gel (240-400 mesh)) gave the pale yellow liquid N-bromoalkyl pyrrole. N-Bromoalkyl pyrrole (2.0 mmol) was mixed with 11 mmol of thiourea in 50 mL of ethanol and heated at 80 °C for 16 h. The ethanol was removed, and then 50 mL of 0.3 M KOH aqueous solution was added and the mixture was heated at 85 °C for 3 h. The reaction was quenched with 100 mL of 1.0 M HCl(aq). The product was extracted with 100 mL of dichloromethane. Flash chromatography (2:100 v/v ethyl acetate/ petroleum ether on silica gel (240-400 mesh)) gave ω-(N-pyrrolyl) alkanethiols as a pale yellow liquid. 1H NMR (CDCl3, 200 MHz): ω-(N-pyrrolyl) propanethiol, δ 1.29 (t, 1H, SH), 2.11 (m, 2H, C2), 2.51 (m, 2H, C3), 4.03 (t, 2H, C1), 6.14 (t, 2H, ArH), 6.65 (t, 2H, ArH); ω-(N-pyrrolyl) hexanethiol, δ 1.31 (m, 5H, C3-4, SH), 1.60 (m, 2H, C5), 1.76 (m, 2H, C2), 2.52 (q, 2H, C6), 3.88 (t, 2H, C1), 6.14 (t, 2H, ArH), 6.63 (t, 2H, ArH); ω-(N-pyrrolyl) nonanethiol, δ 1.28 (m, 11H, C3-7, SH), 1.54 (m, 2H, C8), 1.82 (m, 2H, C2), 2.55 (q, 2H, C9), 3.82 (t, 2H, C1), 6.13 (t, 2H, ArH), 6.64 (t, 2H, ArH); ω-(N-pyrrolyl) dodecanethiol, δ 1.24 (m, 17H, C3-10, SH), 1.62 (m, 2H, C11), 1.82 (m, 2H, C2), 2.56 (q, 2H, C12), 3.78 (t, 2H, C1), 6.11 (t, 2H, ArH), 6.62 (t, 2H, ArH). Anal. Calcd for C7H11NS: C, 59.57%; H, 7.80%; N, 9.93%; S, 22.70%. Found: C, 60.02%; H, 7.55%, N, 9.85%; S, 22.58%. Anal. Calcd for C10H17NS: C, 65.57%; H, 9.29%; N, 7.65%; S, 17.49%. Found: C, 65.42%; H, 9.49%; N, 7.55%; S, 17.54%. Anal. Calcd for C13H23NS: C, 69.33%; H, 10.22%; N, 6.22%; S, 14.23%. Found: C, 69.47%; H, 10.15%; N, 6.28%; S, 14.10%. Anal. Calcd for C16H29NS: C, 71.91%; H, 10.86%; N, 5.24%; S, 11.99%. Found: C, 71.72%; H, 11.06%; N, 5,32%; S, 11.90%. Preparation of the Gold Substrate. The silicon wafer substrates were cleaned with a piranha solution (30% H2O2/ concentrated H2SO4, v/v ) 3/7) (caution: piranha solution is a (26) Wurm, D. B.; Kim, Y. T. Langmuir 2000, 16, 4533. (27) Gordon, A. J.; Ford, R. A. The Chemist’s Companion; John Wiley & Sons: New York, 1972; p 429.

Properties of Pyrrole-Terminated SAMs very dangerous solution with high oxidation power) and then sent to the vacuum chamber. Ti (500 Å) was deposited on the Si substrate followed by 3300 Å of gold. The gold-covered substrates were used right after removing from the vacuum chamber to avoid contamination. Assembly of ω-(N-Pyrrolyl) Alkanethiols on the Gold Surface. Under a dark environment (to avoid the decomposition of ω-(N-pyrrolyl) alkanethiols), the freshly prepared gold substrate was dipped in a 1.0 mM ω-(N-pyrrolyl) alkanethiol/ethanol solution for 12 h. The ω-(N-pyrrolyl) alkanethiol coated gold substrate was washed with MeOH thoroughly and then dried with nitrogen gas at room temperature. Assembly of ω-(N-Pyrrolyl) Alkanethiols on the Gold Electrode of a Quartz Crystal Microbalance. The quartz crystal microbalance was cleaned and then fixed in a homemade sample cell. The solvent (ethanol) was added in the cell, a waiting period was allowed until the vibration of the microbalance was stable, and then 10 µL of a 10 vol % ω-(N-pyrrolyl) alkanethiol/ ethanol solution was added into the sample cell. The changes of the vibration frequency of the quartz crystal microbalance versus deposition time were recorded with a computer. Electrochemical Studies of the SAMs of ω-(N-Pyrrolyl) Alkanethiols and Electrochemical Deposition of Polypyrrole Films on the ω-(N-Pyrrolyl) Alkanethiol Modified Gold Electrode. Electrochemical studies of the ω-(N-pyrrolyl) alkanethiol monolayers were performed in a single-compartment, three-electrode cell. The ω-(N-pyrrolyl) alkanethiol modified gold was used as a working electrode, the counter electrode is a Pt coil, and a Ag/AgCl electrode is used as a reference. The electrolyte solution was 0.1 M Et4NBF4 in acetonitrile or 0.1 M KCl aqueous solution (pH ) 3). Cyclic voltammograms were recorded using a model 263 EG&G PAR potentiostat/galvanostat electrochemical instrument with a scan rate of 100 mV/s. Electrochemical deposition of polypyrrole on the ω-(N-pyrrolyl) alkanethiol modified electrode was performed at the same experimental conditions except for the presence of 1.0 mM pyrrole molecules in the electrolyte solution. Physicochemical Measurements. Reflection-absorption infrared (RAIR) spectra were recorded of a ω-(N-pyrrolyl) alkanethiol monolayer on a gold substrate using a Bio-Rad 185 FTIR spectrometer equipped with a MCT detector and a Seagull reflection accessory. The infrared radiation was polarized parallel or perpendicular to the plane of incidence. UV/Vis/NIR spectra were obtained using a Varian Cary 5E spectrophotometer in the laboratory atmosphere at room temperature. Static contact angles were measured with a homemade goniometer at room temperature and ambient humidity with water as the probe liquid. A 2 µL water droplet was placed on the substrate with a syringe. The angle was obtained by estimating the tangent to the drop at its intersection with the surface, and the average of three measurements was taken for the reported contact angles. Ellipsometric measurements for the ω-(N-pyrrolyl) alkanethiol monolayer were made on a computer-interfaced Rudolph Research AitoE1 II automatic ellipsometer equipped with a HeNe laser (λ ) 632.8 nm) at an incident angle of 70°. The thickness and surface roughness of polypyrrole films were measured with a Dektak ST surface profile measuring system. The scan length is 10 mm, and the thickness and surface roughness were calculated from the average values of the scanned length. X-ray photoelectron spectroscopy (XPS) studies were carried out on a Perkin-Elmer PHI-590AM electron spectroscopy for chemical analysis (ESCA)/XPS spectrometer system with a cylindrical mirror electron energy analyzer (CMA). The X-ray sources were Al KR at 600 W and Mg KR at 400 W. An EG&G QCA917-01 system was used for measuring the frequency change of the quartz crystal during the adsorption of ω-(N-pyrrolyl) alkanethiols. The instrument was interfaced with a computer for data collection and processing. Direct current room-temperature electrical conductivity measurements of the films on substrates (2.0 cm × 1.0 cm rectangle plate) were performed in the usual four-point geometry.28 The four points on the sample surface were in line at an equal spacing of 3 mm. Each point was adhered with a gold wire as an electrode. An appropriate current (ranging from 1 nA (28) Street, G. B. In Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986; Vol. 1, p 224.

Langmuir, Vol. 18, No. 20, 2002 7475 Table 1. Static Contact Angle and Thickness of SAMs of ω-(N-Pyrrolyl) Alkanethiols SAM

contact angle (deg)

thickness (Å)

coverage (%)a

ω-(N-pyrrolyl) propanethiol ω-(N-pyrrolyl) hexanethiol ω-(N-pyrrolyl) nonanethiol ω-(N-pyrrolyl) dodecanethiol bare gold

78 74 72 65 47

6.8 10.3 14.0 17.1

62 74 78 78

a The coverage was calculated as the measured thickness divided by the theoretical length of ω-(N-pyrrolyl) alkanethiols.

to 1 mA) was maintained on two outer electrodes. The floating potential across two inner electrodes was measured to determine the condutivity.29

Results and Discussion Scientists are interested in ω-(N-pyrrolyl) alkanethiols to be used as agents promoting adhesion and/or alignment for the electrochemical deposition of polypyrrole films. Therefore, in this article, the structure and electrochemical properties of SAMs of ω-(N-pyrrolyl) alkanethiols with various alkyl chain lengths were studied in detail. The effects of the structure and properties of the SAMs on the properties of polypyrrole electrochemically deposited over them were emphasized. 1. Self-Assembly of ω-(N-Pyrrolyl) Alkanethiols on the Gold Substrate. The assembly of pyrrole-thiol on the gold surface was done by immersion of the gold substrate in 0.01 M ω-(N-pyrrolyl) alkanethiol/methanol solution for 12 h. After the sample was rinsed copiously with methanol to remove any weakly absorbed molecules, it was blown dry with a nitrogen stream and characterized immediately. The static contact angle and thickness of ω-(N-pyrrolyl) alkanethiols on gold measured from the ellipsometer are listed in Table 1. With help of the refractive index (1.5-1.8) of ω-(N-pyrrolyl) alkanethiol, the optical thickness of the monolayer was calculated based on an Au/thiol/air parallel slab model. It was found that the thickness of ω-(N-pyrrolyl) alkanethiols on gold is not proportional to the chain length of the alkyl group and all are smaller than the size of ω-(N-pyrrolyl) alkanethiols calculated from molecular models. These results indicated that the ω-(N-pyrrolyl) alkanethiol molecules are not closely packed on the gold surface, maybe due to the bulky terminal pyrrole group. Nevertheless, the surface coverage of SAMs is better for ω-(N-pyrrolyl) alkanethiols with longer alkyl chains and the magnitude of the thickness is reasonable for the formation of a monolayer of ω-(Npyrrolyl) alkanethiol. These results are consistent with those observed by other authors.30 The contact angle data (see Table 1) showed that the longer alkyl chain of ω-(Npyrrolyl) alkanethiols leads to high hydrophobicity of the corresponding SAM. Willicut31 et al. reported that SAMs of ω-(N-pyrrolyl) alkanethiols with chain lengths of 3, 5, and 6 had a contact angle of ca. 58°, which is lower than what we had observed. The higher contact angle of the SAMs prepared in this study suggests that the gold substrate has a higher surface coverage. Furthermore, we also found that the longer the alkyl chain of the ω-(Npyrrolyl) alkanethiol, the higher the contact angle of the corresponding SAM. This result indicated that ω-(Npyrrolyl) alkanethiols with longer alkyl chains formed (29) Smiths, F. M. Bell System Technical J. 1958, 710. (30) (a) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192. (b) Smela, E.; Kariis, H.; Yang, Z. P.; Uvdal, K.; Zuccarello, G.; Liedberg, B. Langmuir 1998, 14, 2976. (31) Willicut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 6, 10823.

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Figure 1. The polarized infrared spectra of the SAM of ω-(N-pyrrolyl) nonanethiol: (a) parallel and (b) perpendicular polarization.

more compact monolayers on the gold surface, consistent with the ellipsometry data. Polarized IR external reflection-absorption spectra of ω-(N-pyrrolyl) nonanethiol are shown in Figure 1. The aromatic C-H stretching (at 30003100 cm-1) signal was observed, although very weak, only when the infrared light was perpendicular polarized (polarized perpendicular to the plane of incidence). IR data suggested that the terminal pyrrole ring of the SAM oriented to the substrate preferentially. The orientation of the pyrrole ring on the SAM is perpendicular to the plane of the incident light. This means that the pyrrole ring may orient rather parallel to the substrate. 2. Self-Assembly of ω-(N-Pyrrolyl) Alkanethiols on the QCM with a Gold Electrode. The QCM was placed in a homemade cell with only one face contacting the methanol solution of ω-(N-pyrrolyl) alkanethiol. A typical figure of the change of the frequency (mass) of QCM versus deposition time is shown in Figure 2. The deposition is very fast in the first 10 min and then slowly reaches equilibrium. The mass deposited (or molecules absorbed) on the QCM calculated from the change of the frequency is listed in Table 2. Compared to the data in Table 1, the

Figure 2. The change of the frequency vs deposition time of ω-(N-pyrrolyl) dodecanethiol.

surface coverage of the ω-(N-pyrrolyl) alkanethiol calculated from the ellipsometry data is higher than that calculated from QCM data (if the surface roughness of the Au electrode of QCM was taken into account, the coverage

Properties of Pyrrole-Terminated SAMs

Langmuir, Vol. 18, No. 20, 2002 7477

Figure 3. Cyclic voltammogram of the SAM of ω-(N-pyrrolyl) dodecanethiol in (a) Et4NBF4/CH3CN and (b) KCl(aq). Table 2. Deposition of ω-(N-Pyrrolyl) Alkanethiols on the Electrode of the QCM

SAM ω-(N-pyrrolyl) propanethiol ω-(N-pyrrolyl) hexanethiol ω-(N-pyrrolyl) nonanethiol ω-(N-pyrrolyl) dodecanethiol

molecules absorbed per weight unit area deposited (molecules/cm2) (ng)a 10 15 20 27

2.16 × 1014 2.49 × 1014 2.70 × 1014 3.07 × 1014

coverage (%)b 54 62 67 77

a The deposition time is 1 h; the area of the gold electrode of the QCM is 0.198 cm2. b The coverage was calculated by assuming that the area of the ω-(N-pyrrolyl) alkanethiol is 5 Å × 5 Å.

will be even smaller than reported in Table 2). This is because the methods used to calculate the surface coverage in these two experiments are different. We assumed that the molecular area of ω-(N-pyrrolyl) alkanethiol is 5 Å × 5 Å, but this assumption may not be correct. In addition, because the QCM crystal is not stable for a longtime experiment, the deposition time in QCM experiments is only 1 h. The deposition of the ω-(N-pyrrolyl) alkanethiol on the Au electrode of the QCM may not reach equilibrium within 1 h; therefore, the surface coverage of the ω-(Npyrrolyl) alkanethiol calculated from the QCM data is lower than that calculated from ellipsometry data. Nevertheless, the monolayer coverages obtained from these two experiments have a similar tendency: the surface

coverage of the SAMs is higher for ω-(N-pyrrolyl) alkanethiols with longer alkyl chains. In other words, ω-(Npyrrolyl) alkanethiols with longer alkyl chains formed denser SAMs on the gold surface. Similar behavior was also observed in simple alkylthiols. As expected, the SAMs of ω-(N-pyrrolyl) alkanethiols are not as well packed as those of simple alkylthiols, because the former has a bulky pyrrole head. 3. Electrochemical Properties of SAMs of ω-(NPyrrolyl) Alkanethiols. To understand the function of pyrrole molecules on the gold surface during the electrochemical deposition of polypyrrole, we should know the electrochemical properties of the SAMs. Cyclic voltammetry experiments were performed using two different electrolyte systems: 0.1 M Et4NBF4 in acetonitrile and 0.1 M KCl aqueous solution. No redox peak in the cyclic voltammograms (CVs) of ω-(N-pyrrolyl) alkanethiols was observed when the SAMs were electrochemically scanned from 0 to 1.4 V (vs Ag/AgCl) in 0.1 M Et4NBF4/CH3CN solution. Nevertheless, in KCl aqueous solution, SAMs of ω-(N-pyrrolyl) alkanethiols showed an irreversible oxidation peak at a potential of ca. 1.2 V (vs Ag/AgCl, see Figure 3) and the oxidation potential is independent of the alkyl chain length of the ω-(N-pyrrolyl) alkanethiols. This oxidation peak is due to Cl--assisted oxidation of the gold

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Figure 4. XPS spectra of the SAM of ω-(N-pyrrolyl) dodecanethiol on Au (a) before and (b) after electrochemical oxidation in Et4NBF4/CH3CN solution and (c) after electrochemical oxidation in KCl/H2O solution.

metal.32 We do not know whether the oxidation of the gold substrate caused the desorption of the SAMs from the electrode or the desorption of the SAMs allowed the oxidation of the gold electrode. Nevertheless, no N (or S) signal was observed in the XPS spectrum of the SAMs on Au after the electrochemical scan in KCl aqueous solution (as shown in Figure 4c), and for the third scan cycle, the CV of the SAM is similar to that of bare gold. On the other hand, the XPS spectra of SAMs on Au after electrochemical treatment in Et4NBF4/CH3CN solution showed a strong N (and S) peak (see Figure 4b), indicating that the monolayer of ω-(N-pyrrolyl) alkanethiol was still adsorbed on the surface of the gold electrode. The effect of electrolyte solution on the electrochemical stability of alkylthiols on Au has been widely studied.33,34 In some of those articles,33 the role of the solvents in the electrochemical stability of (32) Ye, S.; Ishibashi, C.; Uosaki, K. Langmuir 1999, 15, 807. (33) (a) Everett, W. R.; Fritsch-Faules, I. Anal. Chim. Acta 1995, 307, 253. (b) Schneider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391. (c) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (34) (a) Garrell, R. L.; Chadwick, J. E.; Severance, D. L.; McDonald, N. A.; Myles, D. C. J. Am. Chem. Soc. 1995, 117, 11563. (b) Schneider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391. (c) Weisshaar, D. E.; Walczak, M. M.; Porter, M. D. Langmuir 1993, 9, 323. (d) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335.

SAMs was emphasized. In comparison to Everett’s paper,33a we found that the SAMs of ω-(N-pyrrolyl) alkanethiols on Au are more stable than SAMs of alkylthiols when the electrochemical oxidation was carried out in Et4NBF4/CH3CN. Unfortunately, from the XPS spectra we are not able to tell whether the pyrrole molecules of ω-(N-pyrrolyl) alkanethiols are polymerized. The behaviors of the SAMs in different electrolyte solutions also affect the electrochemical deposition of polypyrrole over them as discussed in the following paragraphs. 4. Electrochemical Deposition of Polypyrrole on ω-(N-Pyrrolyl) Alkanethiol Modified Gold Electrodes. One of the important properties of SAMs on the surface of the electrode is their defects. Cyclic voltammetric study, in the presence of a redox probe in the solution, is a convenient technique used to probe the integrity of blocking layers on the electrode. Electrochemical polymerization of pyrrole on ω-(N-pyrrolyl) alkanethiol modified gold electrodes was carried out (either in Et4NBF4/CH3CN or KCl aqueous solution) to study the defects of the monolayers of ω-(N-pyrrolyl) alkanethiols and the effects of the surface modification of the electrode on the growth of conducting polymers. The CVs of the electrodeposition of polypyrrole using thiol-treated gold as an electrode in


Langmuir, Vol. 18, No. 20, 2002 7479

Figure 5. Cyclic voltammograms of electrodeposition of polypyrrole over ω-(N-pyrrolyl) nonanethiol modified Au in (a) Et4NBF4/ CH3CN and (b) KCl aqueous solution (the scan rate is 100 mV/s). Table 3. Oxidative Polymerization Potentials (vs Ag/ AgCl) of Pyrrole on ω-(N-Pyrrolyl) Alkanethiol Modified Gold Electrodes electrode modification agent ω-(N-pyrrolyl) propanethiol ω-(N-pyrrolyl) hexanethiol ω-(N-pyrrolyl) nonanethiol ω-(N-pyrrolyl) dodecanethiol none a

oxidation oxidation potential in potential in Et4NBF4/CH3CN (V)a KCl/H2O (V) 0.58 ((0.01) 0.60 ((0.01) 0.60 ((0.01) 0.60 ((0.01) 0.58 ((0.01)

0.80 ((0.01) 0.81 ((0.01) 0.86 ((0.01) 0.86 ((0.01) 0.60 ((0.01)

The oxidation potential is the initial oxidation potential.

both Et4NBF4/CH3CN and KCl aqueous solution are shown in Figure 5. The oxidative polymerization potentials of pyrrole over ω-(N-pyrrolyl) alkanethiol modified gold electrodes are listed in Table 3. When Et4NBF4/CH3CN was used as an electrolyte solution, the modification of the electrode did not affect extensively the oxidation potentials of pyrrole. It seems that studying the integrity of SAMs using the organic solvent gives no conclusive result. Nevertheless, in aqueous electrolyte, the oxidation potentials of pyrrole over ω-(N-pyrrolyl) alkanethiol modified gold electrodes are higher than that over bare

gold. And the longer the alkyl chain of ω-(N-pyrrolyl) alkanethiols, the higher the oxidation potential of polypyrrole electrochemically deposited over them. This result is consistent with with the ellipsometry data, which showed that ω-(N-pyrrolyl) alkanethiols with long alkyl chains formed more compact monolayers on the gold surface and therefore exhibited higher blocking properties. On the other hand, the modification of the electrode did not affect the oxidation potential of pyrrole in Et4NBF4/ CH3CN. This phenomenon may be because SAMs of ω-(Npyrrolyl) alkanethiols on the gold surface have some defect sites. Pyrrole molecules are soluble in acetonitrile. They can penetrate through the SAM to the electrode surface; therefore, the blocking property of the SAM is not revealed. In Et4NBF4/CH3CN, although the monolayer of the ω-(N-pyrrolyl) alkanethiol on the gold electrode does not change the oxidation potential of pyrrole molecules, it does affect the surface morphology of the polymer films deposited over it. Under scanning electron microscopy (SEM) (Figure 6), polypyrrole films deposited over ω-(Npyrrolyl) alkanethiol modified gold electrodes have a smoother morphology compared to that of the film deposited over bare gold. Furthermore, polypyrrole films deposited over ω-(N-pyrrolyl) alkanethiols with longer

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Figure 6. SEM micrographs of polypyrrole films deposited over bare gold and ω-(N-pyrrolyl) alkanethiol modified gold electrodes in organic solvent.

Figure 7. SEM micrographs of polypyrrole films deposited over bare gold and ω-(N-pyrrolyl) alkanethiol modified gold electrodes in aqueous solution.

alkyl chains have a smoother morphology than that of films deposited over ω-(N-pyrrolyl) alkanethiols with shorter alkyl chains. Contact angle and ellipsometry data showed that ω-(N-pyrrolyl) alkanethiols with longer alkyl chains formed more compact and ordered SAMs on the gold surface. The consistent behavior of the SAMs and polymer films deposited over them suggested that the SAMs on the surface provide nucleation sites for polymer chains to grow. Although we do not have any evidence to show that the polypyrrole chains are tethered on the pyrrole molecules of the monolayer, the existence of the pyrrole molecules on the surface affects the deposition and growth of the polypyrrole chains and the resulting surface morphology of the polymer films. This phenomenon could be rationalized as the SAM creates an organic environment for organic polymer to deposit and grow. On the other hand, compared to the electro-oxidation in Et4NBF4/CH3CN, the polypyrrole film obtained from the KCl aqueous solution has a relatively smooth surface even when bare gold was used as an electrode. The flatter morphology of the polypyrrole film grown in KCl/H2O solution may be because the growing rate is slower as evidenced from the small oxidation current (see Figure 5). Since the polypyrrole film obtained from electrochemical oxidation of pyrrole in KCl/H2O solution is very smooth, the effect of the SAMs on the surface morphology of the polypyrrole films deposited over them is insignificant (see Figure 7).

5. Effects of the Alkyl Chain Length in ω-(NPyrrolyl) Alkanethiols on the SAM Formation and the Properties of the Polypyrrole Deposited over the SAM. The reason for doing this study was to investigate the feasibility of using surface pyrrole molecules as adhesion and polymerization promoters by electrochemical deposition of polypyrrole films over them. Several physicochemical studies, such as polarized infrared absorption, XPS, conductivity measurement, and adhesion tests, were used to probe the properties of the polypyrrole films deposited over SAMs of ω-(N-pyrrolyl) alkanethiols. (a) Polarized Infrared Absorption Spectroscopy. The polarized IR reflectance was used to probe the ordering of the polypyrrole films electrochemically deposited over SAMs of ω-(N-pyrrolyl) alkanethiols. The absorption intensities at 1553 cm-1 (the vibration of the polypyrrole backbone35) were used to calculate the dichoric ratio36 of the polymer films. It was found that all polymer films deposited over ω-(N-pyrrolyl) alkanethiol modified gold electrodes have a dichoric ratio slightly bigger than 1 and (35) Furukawa, Y.; Tazawa, S.; Fujii, Y.; Harada, I. Synth. Met. 1988, 24, 329. (36) The dichoric ratio is the ratio of the absorption intensities when the perpendicular and parallel (to the plane of incident light) polarized light passed respectively through the samples.When the dichoric ratio is equal to 1, the sample is randomly oriented. A dichoric ratio bigger (or smaller) than 1 implies that the sample is preferentially oriented parallel (or perpendicular) to the substrate.


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Table 4. Film Thickness, Surface Roughness, Conductivity, and Adhesion of Polypyrrole Films Deposited over ω-(N-Pyrrolyl) Alkanethiol Modified and Bare Gold Electrodes in Et4NBF4/CH3CN working electrode

film thickness (Å)a

roughness of the polymer film (Å)b

conductivity (S/cm)b

adhesionc

ω-(N-pyrrolyl) propanethiol/Au ω-(N-pyrrolyl) hexanethiol/Au ω-(N-pyrrolyl) nonanethiol/Au ω-(N-pyrrolyl) dodecanethiol/Au bare gold

2.0 × 104 2.0 × 104 2.9 × 104 2.6 × 104 2.3 × 104

1.6 × 103 1.1 × 103 5.9 × 102 4.5 × 102 2.0 × 103

4.8 × 10-2 1.2 × 10-1 2.3 × 10-1 2.7 × 10-1 5.1 × 10-3

good good good good poor

a The polymer films were obtained from two potential scan cycles. The scan range is 0-1.4 V, the scan rate is 100 mV/s, and the deviation of the film thickness is within 10%. b The surface roughness and conductivity were calculated by the average of three polymer films measured. c The adhesion tests were performed by pressing a Scotch tape on the film and then peeling it off or blowing the films with a stream of nitrogen gas.

Figure 8. The dichoric ratios of polypyrrole films (electrochemically deposited from Et4NBF4/CH3CN solution) vs alkyl chain length of ω-(N-pyrrolyl) alkanethiols.

as the alkyl chain length of ω-(N-pyrrolyl) alkanethiols increased, the dichoric ratio also increased, as shown in Figure 8. The dichoric ratios indicated that the order of polypyrrole films deposited over ω-(N-pyrrolyl) alkanethiols is better than that of the film deposited over bare gold. The orientation of the pyrrole rings on polypyrrole chains is similar to (but not as ordered as) that of pyrrole rings on the SAMs of ω-(N-pyrrolyl) alkanethiols (vide ante). (b) XPS. The ESCA spectra of polypyrrole films deposited over ω-(N-pyrrolyl) alkanethiol modified gold electrodes were taken to investigate the degree of doping in those polymer films.37 Regardless of the alkyl chain length, polypyrrole films electrochemically deposited over ω-(Npyrrolyl) alkanethiol modified gold electrodes in Et4NBF4/ CH3CN have a degree of doping of ca. 0.21 ( 0.1. On the other hand, the polypyrrole film obtained from KCl aqueous solution has a degree of doping of 0.18 ( 0.1, which is lower than that of films prepared from Et4NBF4/ CH3CN solution. Despite that, the modification of the electrode did not affect the degree of doping of the polypyrrole films deposited over it. Polarized infrared data showed that polymer films deposited on SAM-modified gold electrodes have a better ordering compared to that of films deposited over bare gold. These results suggested that the dopant, not the ordering of polymer chains, determined the degree of doping of the polymer films. (c) Other Physicochemical Properties of Polypyrrole Films. The film thickness, surface roughness, conductivity, and adhesion of polypyrrole films deposited over bare gold (37) The degree of doping of polypyrrole film was calculated by the ratio of the peak area of the corresponding ESCA spectrum. For the polypyrrole film grown in Et4NBF4/CH3CN, the intensity ratio of F to N is used. For the polypyrrole film grown in KCl/H2O, the intensity ratio of Cl to N is used.

and ω-(N-pyrrolyl) alkanethiol modified gold electrodes in Et4NBF4/CH3CN are listed in Table 4. The thicknesses of the polymer films deposited on both bare and SAMmodified Au electrodes, obtained from two potential scan cycles, are of the same order of magnitude. However, the polymer film deposited over ω-(N-pyrrolyl) alkanethiols is much smoother than the film deposited over bare gold. Furthermore, polymer films deposited over SAMs with long alkyl chains are also smoother than those deposited over SAMs with short alkyl chains. The conductivity studies reveal that room-temperature conductivity is increasing with respect to surface smoothness. The conductivity reported here is smaller than the literature data;38 perhaps the high deposition potential degraded the polymer film. It is also interesting to address the adhesion of polymer films. In Et4NBF4/CH3CN electrolyte solution, polymer films deposited on the bare gold electrode can be blown away with nitrogen gas. On the other hand, polymer films deposited over ω-(N-pyrrolyl) alkanethiol modified gold adhere nicely when they are blown with nitrogen gas. The result suggested that the adhesion of polymer films is improved when they are electrochemically polymerized/deposited over a monomer-primed electrode in acetonitrile. The monolayer of ω-(N-pyrrolyl) alkanethiol provides covalently bound organic nucleation sites on the surface for organic polypyrrole chains to deposit and grow. The resulting polypyrrole films are smoother, more densely packed, and more robust compared to those deposited on the bare gold electrode. It seems that the molecular self-assembly method can be used to create nucleation sites for preparing compact polymer films by using surface-anchored monolayers to promote the growth of electrically conductive polymer chains around (maybe from) the monolayer. Conclusions The formation of ω-(N-pyrrolyl) alkanethiol self-assembled monolayers on gold substrates and the physicochemical properties of the polypyrrole electrochemically deposited over them were reported. These data are critical for evaluating the potential application of these thiol molecules as adhesion- and alignment-promoting agents for the deposition of conducting polypyrrole films. The differences in the blocking and aligning properties of ω-(Npyrrolyl) alkanethiols in Et4NBF4/CH3CN and KCl/H2O solution demonstrate that the general approach to adhesion promotion through the use of a thiol-monomermodified surface has conditions and limits that must be recognized. Acknowledgment. Financial support for this work by the National Science Council of the Republic of China (NSC-89-2113-M-008-007) is gratefully acknowledged. LA011792T (38) Kanatzidis, M. G. Chem. Eng. News 1990, 68, 36.

Formation and Electrochemical Property of Pyrrole-Terminated SAMs

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