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Langmuir 1996, 12, 3756-3758
Organic Monolayers as Nucleation Sites for Epitaxial Growth. 1. Electrochemical Polymerization of N-Alkylpyrrole D. Brad Wurm, Scott T. Brittain,† and Yeon-Taik Kim* Department of Chemistry, University of Alabama, Tuscaloosa, Alabama 35487 Received February 15, 1996. In Final Form: June 10, 1996X Optically flat polymer films of poly(N-hexadecylpyrrole) (PHDPy) have been successfully prepared on gold electrodes modified with self-assembled monolayers of bis(ω-(N-pyrrolyl)-n-undecyl) disulfide (BPUS). During the growth of PHDPy, the surface-confined pyrroles in BPUS serve as nucleation sites for the epitaxial growth of PHDPy instead of being laterally polymerized to form polymeric monolayers. We provide visual evidence to confirm the chemical reaction between the surface-confined pyrroles and HDPy in the solution. Polymer growth on the modified and unmodified electrodes is investigated using cyclic voltammetry, optical microscopy, and scanning probe microscopy.
Epitaxial growth of thin films on a crystal surface is important not only for understanding fundamental mechanism of thin film growth but also for producing crystalline thin films for commercial use.1 In numerous cases, however, there has been a lack of substrates for epitaxial growth. Here, we attempt to use organic monolayers on gold to mimic a substrate for homoepitaxial growth. Poly(N-alkylpyrroles) have been recognized for potential electronic materials due to their excellent stability in air. However, the electrical, optical, and morphological details of electrochemically polymerized poly(N-alkylpyrroles) have not been extensively studied owing to their poor film quality. It was found to be difficult to form uniform films from long chain poly(N-alkylpyrroles) (propyl or butyl) on noble metal electrodes. The resulting films were very rough and brown in appearance.2 Furthermore, they were easily peeled from the electrode surface because of poor adhesion. Contrary to this, we have electrochemically produced robust and mirror-like poly(N-hexadecylpyrrole) (PHDPy) films. This was achieved by electropolymerization of n-hexadecylpyrrole on self-assembly modified gold electrodes. Long-chain alkanethiols and their derivatives spontaneously form monolayers on gold surfaces upon immersion of Au substrates into thiol-containing solutions.3 The monolayers are ordered and densely packed. These findings provide a unique opportunity to engineer surfaces at the molecular level. Since the importance of the nucleation process in the electrochemical growth of polypyrrole was recognized early on,4 we took advantage of the self-assembly method to create electrochmical * To whom correspondence may be addressed: tel, 205-348-0610; Fax, 205-348-9104; e-mail,
[email protected]. † Current address: Department of Chemistry, Harvard University, Cambridge, MA 02138. X Abstract published in Advance ACS Abstracts, July 15, 1996. (1) (a) Pashley, D. W. Adv. Phys. 1965, 14, 327. (b) Bauer, E.; Hoppa, H. Thin Solid Films 1972, 12, 167. (2) (a) Diaz, A. F.; Castillo, J.; Kanazawa, K. K.; Logan, J. A.; Salmon, M.; Fajardo, O. J. Electroanal. Chem. 1982, 133, 233. (b) Diaz, A. F.; Castillo, J.; Logan, J. A.; Lee, W.-Y. J. Electroanal. Chem. 1981, 129, 115. (c) Iyoda, T.; Ando, M.; Taneko, T.; Ohtani, A.; Shimidzu, T.; Honda, K. Tetrahedron Lett. 1986, 27, 5633. (3) (a) Ulman, A. An Introduction to Ultra Thin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (b) Finklea, H, O.; Blackburn, A.; Richter, B.; Allara, D. L.; Bright, T. Langmuir 1986, 2, 239. (c) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370. (d) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidesy, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (4) Asavapiriyanont, S.; Chandler, G. K.; Gunawardena, G. A.; Pletcher, D. J. Electroanal. Chem. 1984, 177, 229.
S0743-7463(96)00139-4 CCC: $12.00
reaction sites for the epitaxial growth of PHDPy on a gold electrode. Modification of the electrode surface by self-assembly has proven useful for improving physical characteristics of polymer films such as adhesion, resistance to photocorrosion, and toughness.5-7 Recently, self-assembled monolayers of alkyl mercaptans containing pyrrole monomers were shown to enhance the adhesion of polypyrrole and poly(3-ethylpyrrole) to the modified gold electrode surface.8,9 Here, we wish to report the initial investigation of epitaxial growth of N-hexadecylpyrrole by electrochemical polymerization. The epitaxial growth was achieved by designing nucleation sites through self-assembly of bis(ω-(N-pyrrolyl)-n-undecyl)disulfide (BPUS) on gold surfaces. This work not only demonstrated the importance of a nucleation step for epitaxial growth but also produced an optically smooth film of long chain poly(N-alkylpyrrole). Experimental Section Chemicals. Pyrrole, acetonitrile, and tetrabutylammonium hexaflurophosphate (Bu4N(PF6)) were purchased from Aldrich. Pyrrole was vacuum distilled before the experiment. Bu4N(PF6) was recrystallized and stored in a glovebox to prepare for an 0.1 M electrolyte solution. Acetonitrile was purified by distillation over CaH2. N-Hexadecylpyrrole (HDPy) was synthesized according to the procedure reported in literature.10 Bis(ω-(Npyrrolyl)-n-undecyl) disulfide (BPUS) as seen in Figure 1 was synthesized and will be reported elsewhere.11 Self-Assembly. Thin film gold working electrodes were prepared by vapor deposition of 100 Å of Cr undercoat followed by 1500 Å of Au onto glass microscope slides at a pressure of 2 × 10-6 Torr. After backfilling the deposition chamber with nitrogen, the gold electrodes were immediately removed and placed in solution for self-assembly modification or promptly used for electrochemistry experiments. Self-assembly modification of the electrodes involved placing them in a 1 mM solution of BPUS in hexane for 12-24 h. Cyclic Voltammetry. A three electrode, single compartment glass electrochemical cell was used with a silver wire quasireference electrode and platinum gauze counter electrode. The Ag quasi-reference electrode was calibrated by determining the (5) Simon, R. A.; Ricco, A. J.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104, 2031. (6) Rubinstein, I.; Rishpon, J.; Sabatani, E.; Redondo, A.; Gottesfeld, S. J. Am. Chem. Soc. 1990, 112, 6135. (7) Kowalik, J.; Tolbert, L.; Ding, Y.; Bottomley, L.; Vogt, K.; Kohl, P. Synth. Met. 1993, 55, 1171. (8) (a) Willicut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 10824. (b) Willicut, R. J.; McCarley, R. L. Langmuir 1995, 11, 296. (9) Sayre, C. N.; Collard, D. M. Langmuir 1995, 11, 302. (10) Josey, A. D. Org. Synth. 1967, 47, 81. (11) Zong, K.-K.; Brittain, S. T.; Wurm, D. B.; Kim, Y.-T. Submitted for publication.
© 1996 American Chemical Society
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Langmuir, Vol. 12, No. 16, 1996 3757
Figure 1. Chemical structure of bis(ω-(N-pyrrolyl)-n-undecyl) disulfide (BPUS). oxidation potential of ferrocene in 0.1 M Bu4NPF6 in acetonitrile. The Ag quasi-reference potential was found to be approximately 150 mV lower than a saturated calomel reference electrode. In this paper, all the potentials were referred to a silver quasireference electrode. The silver wire was cleaned between experiments to ensure a consistent reference potential by soaking in nitric acid for 1 min and then rinsing with deionized water and dried by blowing with nitrogen gas. HDPy was polymerized by cycling between 0.3-1.3 V relative to a silver quasi-reference electrode. All the voltammograms were measured in air with a PAR 273 potentiostat equipped with software provided by the company. Microscopy. The morphology of the polymers formed on the modified and unmodified gold electrodes was imaged by an inverted optical microscope (Nikon, Epiphot-TME). In addition, a Park Scientific model CP Scanning Probe Microscope (SPM) was used to image the morphology. A 100 µm scanner with a Si3N4 cantilever with a spring constant of 0.05 N/m was used. A scan rate of 1.0 lines/s and constant force of 1.2 nN were used for scanning parameters.
Results and Discussion Typical cyclic voltammograms (CV’s) at a scan rate of 100 mV/s are shown in Figure 2a during the PHDPy film
growth on the unmodified electrode surface. The voltammogram is characterized by irreversible monomer oxidation followed by nucleation of PHDPy as determined by a nucleation loop in the first scan. In the following scans, monomer oxidation is decreased. In addition, an increase in the oxidation/reduction peak of PHDPy is noticed as indication of the polymer growth. The onset of monomer oxidation occurs at roughly 1.05 V vs Ag. A polymer oxidation peak is observed from the second scan around 0.8 V. At the end of the scan, a brownish, rough, and wrinkled film had formed on the electrode surface as reported in the previous work.2 It is noteworthy that the polymer oxidation potential is unstable with each additional scan. We attribute this behavior to the random orientation of hexadecyl chains in the polymer matrix resulting in the fluctuation of conductivity on the electrode surface. It is also noteworthy that HDPy starts to oxidize at lower potentials than the first monomer oxidation from the second scan, which is the opposite tendency to the regular electrochemical growth of conducting polymers.12 Figure 2b is an optical micrograph of the PHDPy formed on the unmodified electrode surface. It is clear that the nucleation is not uniform by the observation of the bare gold surface adjacent to the polymer formations. The polymer does not form a continuous film and can be easily peeled away from the electrode surface, even being removed with brisk solvent rinsing. We then tested the effect of preassembled nucleation sites on the electrochemical polymerization of PHDPy. The preassembled nucleation sites were built by the self-assembly method using BPUS, which contains the pyrrole moiety at the terminal group. The inset of Figure 2c shows a typical cyclic voltammogram for the surface confined monolayer polymerization of a BPUS-modified working electrode. This involves cycling between 0.2 and 1.1 V in a 0.1 M Bu4N(PF6)
a
c
b
d
Figure 2. (a) Cyclic voltammetry of n-hexadecylpyrrole at a bare Au film electrode scanned between 0.3 and 1.3 V vs Ag. Scan rate ) 100 mV/sec. (b) An optical micrograph (200×) of poly(n-hexadecylpyrrole) (PHDPy) formed in (a). (c) Cyclic voltammetry of n-hexadecylpyrrole at a DPUS-modified Au film electrode scanned between 0.3 and 1.3 V vs Ag. Scan Rate ) 100 mV/s. Inset: Cyclic voltammetry of monolayer polymerization scanned between 0.2 and 1.1 V vs Ag. Scan rate ) 100 mV/s. (d) An optical micrograph (100x) of PHDPy formed on the previously polymerized electrode surface (marked as 1) compared to polymer formed on the unpolymerized modified surface (marked as 2).
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acetonitrile solution. The first scan to anodic potentials yields an oxidation peak centered at 1.05 V vs Ag. Subsequent scans show no redox behavior. Similar behavior has been reported for electrochemical polymerization of pyrrole-terminated organothiol monolayers in both aqueous and organic media.8,9 Integration of the area beneath the oxidative wave on the first scan to obtain the charge passed yields 65 ( 4 µC/cm2. These pyrrole units form radical cations upon oxidation. At potential sufficiently positive to oxidize pyrrole, these surfaceconfined radical cations can then undergo a radical coupling reaction with cation radicals of the HDPy monomer in solution. In this way, they serve as nucleation sites for PHDPy epitaxial growth. The cyclic voltammetry behavior of the PHDPy during the growth on the modified electrode surface is different from that for the growth on the bare electrode surface. The onset of monomer oxidation and polymer oxidation/ reduction is at the same potential, but the polymer oxidation potential consistently shifts to more positive potential with subsequent scans. Furthermore, the monomer oxidation currents were the same from the second scan as seen in Figure 2c. The observation tells that the same amount of HDPy is oxidized from the second scan, which implies the electroactive surface area is constant. We prepared a modified electrode to prove the chemical involvement of the surface-confined pyrrole during the growth. The electrode was partially immersed to polymerize a portion of the surface-confined pyrrole in a 0.1 M Bu4N(PF6) solution. Immediately, the electrode was rinsed and a larger portion of the electrode including the already polymerized region was placed in a HDPy solution to form PHDPy by cyclic voltammetry. The resulting film indicates two different film morphologies as seen in Figure 2d. The PHDPy film on the unpolymerized BPUS monolayers is shiny and reflective, while that formed on the previously polymerized monolayers resembles the morphology formed on an unmodified bare gold electrode. This strongly suggests that a chemical reaction between the BPUS monolayer and HDPy is responsible and results in homoepitaxial growth. The PHDPy on the modified electrode is optically flat and cannot be distinguished from a gold surface by scanning electron microscopy (SEM). It was noted that SEM is not powerful enough to resolve morphology showing little contrast in the depth. It was only possible to resolve morphologies of the PHDPy formed on the modified electrode surface with SPM. Figure 3a is an SPM image of PHDPy obtained on the shiny area in Figure 2d. The film was found to be continuous and consist of small particles 70-140 Å in height with peak to peak distances of 3000-6000 Å. As shown in Figure 3b, SPM images of the PHDPy formed on the unmodified electrode showed a nonuniform and discontinuous polymer film that was peeling away from the electrode surface. Furthermore, the PHDPy film formed on the bare gold electrode was not stable. It disappeared upon continuous cycling in the potential range of 0.1-0.9 V in a blank electrolyte. On the other hand, the PHDPy film formed on the DPUS-modified electrode was very stable upon repeated cycling in the same condition. It showed a constant polymer redox peak similar to the voltammogram shown in Figure 2c in the potential range of 0.1-0.9 V. (12) Skotheim, T. A., Ed. Handbook of Conducting Polymers Marcel Dekker: New York, 1986.
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Figure 3. (a, top) An SPM image of poly(n-hexadecylpyrrole) obtained at the DPUS-modified electrode surface. A scanned area is 8 µm × 8 µm. (b, bottom) An SPM image of poly(nhexadecylpyrrole) obtained at the unmodified electrode surface. A scanned area is 20 µm × 20 µm.
In conclusion, we produced optically flat poly(n-hexadecylpyrrole) films using epitaxial growth. The substrate for the epitaxial growth was prepared by self-assembly. We also provide evidence for the chemical interaction of the surface-confined pyrrole and the HDPy monomer. Currently, we are using in situ real-time techniques such as spectroscopic ellipsometry, scanning probe microscopy, and electrochemical quartz crystal microbalance to understand and control the growth mechanism of conducting polymers. Acknowledgment. Y.T.K. greatly acknowledges financial support for this work from a start-up fund, the School of Mines and Energy Development grant, and Research Grant Committee fund at the University of Alabama. D.B.W. thanks a fellowship from the Alabama Space Grant Consortium, NASA Training Grant NGT40010. LA960139G
