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Langmuir 1998, 14, 2996-3002
Thiol-Modified Pyrrole Monomers: 4. Electrochemical Deposition of Polypyrrole over 1-(2-Thioethyl)pyrrole Elisabeth Smela* Laboratory of Applied Physics, University of Linko¨ ping, S-581 83 Linko¨ ping, Sweden Received August 4, 1997. In Final Form: February 3, 1998 The electrochemical deposition of polypyrrole (PPy) potentiodynamically and potentiostatically over monolayers of 1-(2-thioethyl)pyrrole (1-TEP) is described. The relationship between monolayer oxidation and PPy growth depended strongly on the electrolyte. In propylene carbonate/LiClO4, the PPy polymerization potential was higher than that for 1-TEP monolayer oxidation, and the monolayer oxidation peak was identical to that seen in monomer-free electrolyte. In water/LiClO4, the oxidation and polymerization potentials were almost the same, with the result that the onset of polymerization was prevented until after the monolayer oxidation process had been completed. In water/(sodium dodecylbenzenesulfonate), an anionic surfactant, the PPy polymerization potential was lower than that in water/LiClO4, and polymerization apparently occurred without monolayer oxidation. In all cases, PPy growth was found to be essentially the same on 1-TEP, oxidized 1-TEP, and clean gold as evidenced by cyclic voltammograms, chronoamperograms, and film appearance. These results show that the 1-TEP monolayers probably decomposed before PPy polymerization began or were, in the case of sodium dodecylbenzenesulfonate, most likely inaccessible because they were covered by the surfactant. This indicates that 1-TEP cannot be used to covalently bind PPy to gold. These results also demonstrate that the general approach to adhesion promotion through the use of thiol-modified pyrroles has conditions and limits that must be recognized. The electrochemistry of PPy grafting to surface-bound pyrrole depends critically on the electrolyte, which is important for application of this technique in real devices. In addition, the oxidation reactions undergone by such a monolayer cannot be assumed to be different in the absence and presence of pyrrole. Finally, the technique will probably not work with surfactant anions, which are of most practical interest.
1. Introduction In this fourth paper on 1-(2-thioethyl)pyrrole (1-TEP) and 3-(2-thioethyl)pyrrole (3-TEP), the electrochemical deposition (growth) of polypyrrole (PPy) over 1-TEP monolayers in three different electrolytes is described. This is the last paper in the series on these thiolsubstituted pyrrole compounds, and it should be read in conjunction with the previous three. The TEP monolayers were designed to be used as adhesion promoters between electrochemically deposited PPy and gold, with the pyrrole moieties incorporated into the PPy film and the thiol bound to the metal. Paper 1 describes the synthesis, characterization, and polymerization of these monomers; paper 2 examines freshly deposited monolayers adsorbed on gold; paper 3 details the electrochemistry of the monolayers. This type of monolayer, with a pyrrole group tethered to the surface by an alkyl chain, has been made previously (see the introduction of paper 1). It has been assumed that the pyrrole moieties are incorporated into the PPy because it has been observed that this type of monolayer increases the adhesion of overlying films and affects their properties. For example, Willicut et al. found increased adhesion and smoothness of PPy films deposited on thiolmodified pyrrole monolayers.1,2 Although Collard et al. failed to deposit PPy over monolayers of long-chain (n ) 11) thiol-modified pyrroles, which was not unexpected since long alkane chains are insulating, poly(3-ethylpyrrole) was successfully deposited and found to be smoother and more strongly adherent than to bare gold.3 Adhesion * Address for correspondence: Condensed Matter Chemistry and Physics Department, Risø National Laboratory, FYS-124, P.O. Box 49, DK-4000 Roskilde, Denmark. (1) Willicut, R. J.; McCarley, R. L. Adv. Mater. 1995, 7, 759. (2) Willicut, R. J.; McCarley, R. L. Langmuir 1995, 11, 296. (3) Collard, D. M.; Sayre, C. N. Synth. Met. 1995, 69, 459.
was not enhanced on alkanethiol surfaces lacking terminal pyrrole groups. Mekhalif et al. also reported that adhesion of polybithiophene to platinum was improved by thiolmodified phenyl monolayers, but not by alkylthiols without the aromatic ring.4 They also demonstrated that substituents on the bound phenyl groups affected the properties of the overlying polymer. Lukkari et al. modified indium tin oxide (ITO) with thiophene derivatives covalently bound to the surface via bridging agents; two bridging agents resulted in increased nucleation and uniform coverage, but the third did not.5 Nevertheless, although this circumstantial evidence is strong, there has been no direct verification of covalent linking of the monolayer with the overlying film; the improved adhesion might be due to other, weaker, favorable interactions with the surface, because even electrodes treated only with amorphous carbon have been shown to increase adhesion compared with bare gold.6 It is also possible that the PPy is bound to monolayer decomposition products (see paper 3); for example, allyl alcohol also enhances the nucleation process in some cases.7 Lukkari et al.8 have studied the initial phases of poly(3-methylthiophene) film formation using photocurrent spectroscopy and found only that, on surfaces modified with monomer-containing groups, the addition of monomers from solution to the surface-bound species could not (4) Mekhalif, Z.; Lang, P.; Garnier, F. J. Electroanal. Chem. 1995, 399, 61. (5) Lukkari, W.; Tuomala, R.; Ristima¨ki, S.; Kankare, J. Synth. Met. 1992, 47, 217. (6) Helms, J. H.; Everson, M. P.; Howard, K.; Plummer, J. Abstracts of Papers, 208th National Meeting of the American Chemical Society, Washington, DC, Aug 21-26, 1994; American Chemical Society: Washington, DC, 1994; COLL 214. (7) Kupila, E.-L.; Kankare, J. Synth. Met. 1995, 74, 241. (8) Lukkari, J.; Alanko, M.; Pitka¨nen, V.; Kleemola, K.; Kankare, J. J. Phys. Chem. 1994, 98, 8525.
S0743-7463(97)00863-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/08/1998
Electrochemical Deposition of Ppy over 1-TEP
be ruled out as a first step of electropolymerization. However, the major route to film formation was still the deposition of solution-formed oligomers, and strong interactions with the surface favored the deposition of short oligomers evenly on the electrode. In addition, the proportion of short oligomers in the film remained high even during later stages of polymerization, the structure of the film being dictated by the homogeneity of the initial nucleation and growth, so that on these electrodes the mechanical properties of the conducting polymer films actually deteriorated during the course of polymerization. Although this might not be a problem in some applications, in the case of others, such as actuators,9,10 a structurally weak film would be even worse than a nonadherent one. These results contradicted those of Mekhalif et al., who found that the conjugation length increased within the first 50 Å, but then decreased again so that the main effects of the monolayer were on nucleation and the formation of the first several polymer layers.4 The adherence improvement has been found to depend strongly on the anion.7 Initial adsorption of counteranions on the electrode surface may affect the nucleation process significantly.7 PPy doped with different anions thus seems to require different surface characteristics. In the previous paper of this series, we showed that during electrochemical cycling in the absence of monomers in solution, the TEP monolayers underwent oxidation and subsequent irreversible follow-up chemical reactions leading to their decomposition. Monolayer oxidation peaks have also been observed during deposition of PPy over other, longer chain silane- and thiol-modified monolayers and attributed to the formation of radical cations.1,11 The onset of polymerization of 3-ethylpyrrole on such a monolayer occurred at a higher potential than on bare gold: the modified electrode remained electrochemically inactive until the monolayer was oxidized.12 During the growth of polybithiophene on aromatic thiol monolayers on platinum, however, the phenyl groups were not oxidized.4 It must be stressed that the electrochemical behavior of those monolayers was different from that of the TEPs and that decomposition may be unique to these very short-chain TEP compounds. Since spectroscopic data on other oxidized aromatic thiol monolayers have not been presented, however, this is speculative. If the monolayers are to function as designed to improve the performance of devices based on PPy, the pyrrole moieties of the bound TEP molecules must be able to polymerize with the pyrrole monomers in solution, thereby binding the polymer film to the substrate. However, if the monolayer reacts with oxygen, polymerizes with itself, decomposes, or adopts an unfavorable conformation, all of which may be possible during its electrochemical oxidation, it cannot perform its role (although it could still function as an adhesion-promoting layer by providing, if not the intended, then still a favorable carbonaceous surface). In addition, for applications in which the PPy film is to be electrochemically cycled, if the monolayer is subject to gradual attack or desorption, then it will degrade as an adhesion layer. We needed to know how the TEP monolayers behaved in the presence of pyrrole and how the growth of PPy was affected by the monolayers. In (9) Otero, T. F. In Vol. 4. Conductive Polymers: Transport, Photophysics, and Applications; Nalwa, H. S., Ed.; John Wiley & Sons: New York, 1997; pp 517-594. (10) Smela, E.; Ingana¨s, O.; Lundstro¨m, I. Science 1995, 268, 1735. (11) Simon, R. A.; Ricco, A. J.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104, 2031. (12) Collard, D. M.; Sayre, C. N. J. Electroanal. Chem. 1994, 375, 367.
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addition, we wondered how these interactions were altered by changing the electrolyte. We examined three growth solutions, all of which contained 0.1 M pyrrole and 0.1 M of the background electrolyte: propylene carbonate (PC) with LiClO4, deionized water with LiClO4, and deionized water with dodecylbenzenesulfonate, sodium salt (DBS). This combination allowed us to compare the effect of changing from an organic solvent to water while retaining the LiClO4, which is a frequently used background electrolyte for the deposition of PPy. PC is a good organic electrolyte for electrochemical studies, which is why we used it to study the monolayers in the previous paper; we therefore continued with it for this study. Aqueous electrolytes are of particular interest for device applications, and water/ LiClO4 is often used for actuators.13 Finally, anions that are immobile, amphiphilic, and/or aromatic confer higher conductivity and advantageous actuator properties (see, for example, refs 10, 14-20), so we were also interested in the water/DBS system. In this paper we concentrate on 1-TEP because of the greater instability of the 3-TEP monomers and monolayers (see papers 1 and 2), which makes them difficult to work with. All experimental details have been given in the previous papers. Cyclic voltammograms were performed in order to compare these results with those in paper 3 and with those of other authors. These were taken at 10 mV/s. Chronoamperograms were also recorded to enable comparison with the previous monolayer results and because PPy film deposition for device applications is normally done potentiostatically or galvanostatically. 2. Results 2.1. PC/LiClO4. Polypyrrole was potentiodynamically polymerized between -0.4 and +0.8 V (vs Ag/AgCl) over clean gold and gold modified with monolayers of 1-TEP. Some monolayers were used directly after deposition and others were oxidized by electrochemical cycling two times beforehand in monomer-free PC/LiClO4 between 0 and 1 V. Electrochemical oxidation of the 1-TEP monolayer in this electrolyte was shown in paper 3 to result in the destruction of the pyrrole moieties, so the latter surfaces cannot form the desired linkages to pyrrole in solution. The first potential excursions in PC/LiClO4/pyrrole for the three surfaces are shown in Figure 1a. The oxidation peak for the previously uncycled 1-TEP monolayer appeared at the same potential and had the same shape and size as in the monomer-free electrolyte. (See paper 3.) The monolayer was oxidized completely before PPy polymerization began; in this electrolyte, the potential required to deposit PPy was higher than that required to oxidize the monolayer. The plain gold and cycled 1-TEP monolayer surfaces had, as usual in PC/ LiClO4, a small step near 0.4 V. On plain gold this was followed by a peak at the foot of the PPy polymerization peak that has previously been observed by others.21 The cycled 1-TEP monolayer was missing this feature. The (13) Otero, T. F.; Sansin˜ena, J. M. Third ICIM/ECSSM ’96, Lyon. 1996, p 365. (14) Pei, Q.; Ingana¨s, O. Synth. Met. 1993, 55-57, 3718-3723. (15) Pei, Q.; Ingana¨s, O. Sol. State Ion. 1993, 60, 161-166. (16) Gandhi, M. R.; Murray, P.; Spinks, G. M.; Wallace, G. G. Synth. Met. 1995, 73, 247-256. (17) Naoi, K.; Lien, M.; Smyrl, W. H. J. Electrochem. Soc. 1991, 138, 440-445. (18) Hepel, M. Electrochim. Acta 1996, 41, 63-76. (19) Bhattacharya, A.; De, A.; Das, S. Polymer 1996, 37, 4375-4382. (20) Ouyang, J. Y.; Li, Y. F. Polymer 1997, 38, 3997-3999. (21) Beck, F.; Oberst, M. Makromol. Chem., Macromol. Symp. 1987, 8, 97.
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Figure 2. PPy growth on a freshly deposited 1-TEP monolayer (heavy black line), 1-TEP monolayer oxidation (thin black line), and P(1-TEP) growth (dotted line) by potential cycling in PC/ LiClO4.
Figure 1. Cyclic voltammograms in PC/LiClO4/pyrrole of a 1-TEP monolayer (heavy solid line), a previously electrochemically oxidized 1-TEP monolayer (thin solid line), and a clean gold surface (dotted line): (a) first cycle; (b) seventh cycle.
onset of polymerization occurred at exactly the same potential on all three surfaces. If the first steps of polymerization were enhanced by the 1-TEP monolayer, for example, by improved nucleation, then this would not have been the case. The voltammograms of the seventh cycles on the same surfaces are shown in Figure 1b. All three were identical in shape and size. Note that the onset of polymerization occurred 50 mV lower than that shown in Figure 1a; this demonstrates that nucleation and growth are easier on an existing PPy surface. The polymerization current density was no higher for the freshly deposited 1-TEP surface than on the other two, so the growth rate was the same (assuming the same polymerization efficiency). Chronoamperograms for PPy deposition on the various electrodes were also similar (and are therefore not shown). The PPy films that were produced by that method, a few thousand angstroms thick, also looked alike, with the same thickness for a given charge density and/or deposition time and the same roughness and uniformity as seen by the eye and under a light microscope. (On gold, thin layers (less than 1 µm) of PPy show colored interference fringes that we have correlated with profilometry measurements and provide quite an accurate determination of film thickness.) Therefore, in this electrolyte the 1-TEP monolayer did not markedly affect the growth of the overlying polypyrrole, although it cannot be ruled out that scanning electron microscopy or atomic force microscopy might reveal a difference on the microscopic level. However, it should be noted that for some thiol-modified pyrroles, the effects on PPy have been reported to be large. For example, PPy films on similar monolayers were much thicker than on octadecanethiol, or very smooth and reflective compared to those grown on Au.22 A comparison of the growth of PPy on a freshly deposited 1-TEP monolayer, the oxidation of a 1-TEP monolayer (see also paper 3), and the polymerization of 1-TEP monomers to form poly(1-TEP) (see paper 1) is shown in Figure 2. The monomer concentration was 20 mM for the polymerization of both pyrrole and 1-TEP. It is clear that (22) Willicut, R. J.; McCarley, R. L. Langmuir 1995, 11, 296.
the same 1-TEP monolayer oxidation process, with its characteristic multiple peaks, occurred in all three cases. With pyrrole in solution, it was finished before polymerization began, but with 1-TEP in solution, polymerization started before the monolayer oxidation was complete. Therefore, the data indicate that the addition of pyrrole does not affect the monolayer oxidation process in PC/LiClO4 and that the PPy film is deposited over a decomposed monolayer lacking pyrrole moieties. In this electrolyte, then, the monolayer cannot form the desired pyrrole-pyrrole bonds and is thus unsuitable as an adhesion promotion layer (although, as mentioned above, it could still provide a favorable hydrocarbon surface, or the pyrrole could react with unintended groups such as acids formed during monolayer destruction). The prevailing assumption that the monolayer oxidation reactions differ in the presence of pyrrole (i.e., that the radical cation reacts with pyrrole in solution rather than leading to decomposition) is therefore questionable. On the expanded current scale of Figure 2, one can also note that for PPy deposition over 1-TEP, the current on the return scan looped over the curve from the initial scan. This again shows that nucleation/polymerization on an existing PPy film is easier than on the 1-TEP monolayer. This is as expected if the monolayer has decomposed. The current on the initial forward potential excursion was not limited by monomer diffusion, but the reactivity of the surface. If the 1-TEP monolayer had been as desirable a surface for polymerization as the PPy, then the curve would have resembled that for P(1-TEP) in Figure 1a or the curves in Figure 1b instead, with a lower oxidation potential and lower current on the return scan. 2.2. Water/LiClO4. The solvent was changed from PC to water but the same supporting electrolyte was retained; the potentiodynamic polymerization of pyrrole was performed between 0 and 0.75 V on clean gold and freshly deposited 1-TEP monolayers (see Figure 3). (It is impossible to dissolve 1-TEP in water/LiClO4 (see paper 1), so comparison with the deposition of P(1-TEP) could not be made.) Note that, again, the currents on the return scans were higher than during the initial scans. The small monolayer oxidation peak appeared at the same potential in water/LiClO4 with and without pyrrole monomers, as was the case for PC/LiClO4. In this electrolyte, however, the PPy polymerization peak was not positioned completely after the monolayer oxidation peak, but superimposed over it. Furthermore, on the clean gold surface polymerization started at a lower potential, as did polymerization during the second scan on the 1-TEP monolayer. (The current was larger on the bare gold
Electrochemical Deposition of Ppy over 1-TEP
Figure 3. Potentiodynamic polymerization of PPy in water/ LiClO4/pyrrole over clean gold (dotted line) and 1-TEP monolayers (first scan, heavy solid line; second scan, thin solid line). The cyclic voltammogram of a 1-TEP monolayer in monomerfree water/LiClO4 is shown for comparison (solid gray line). Arrows indicate the forward and return curves.
electrode at 0.75 V because the polymerization had started earlier.) The monolayer thus inhibited the growth of PPy until after the monolayer had started to oxidize. What the monolayer oxidation reactions were, and whether they differed from those in the absence of pyrrole, is unclear. (We did not perform for water/LiClO4 the surface analysis that was done in paper 3 for monolayers in PC/LiClO4, so the results of electrochemically cycling the monolayer in this electrolyte are unknown.) One might have expected that it would be easier to oxidize the pyrrole moieties in the monolayer, because they are bound to the electrode, than the pyrrole in solution.12 However, the pyrrole oxidation peak was at a lower potential. Thus, as in PC/LiClO4/pyrrole, the monolayer oxidation reactions are probably more extensive than simple cation radical formation. Perhaps, as in PC/LiClO4, the monolayer decomposes. Unfortunately, it would be difficult to ascertain the reactions using the techniques of paper 3 because of the overlying PPy layer. However, it is obvious that the 1-TEP monolayer did not enhance nucleation and deposition of PPy, but rather inhibited it in the initial stages. Since these are short-chain thiolates that do not block the electrode, this cannot be attributed to the presence of insulating alkyl chains. It has previously been found that the onset of polymerization on a modified electrode occurred at a higher potential than on bare gold, and that the SAM-modified electrode remained electrochemically inactive until the monolayer itself was subject to oxidation.12 Again in this case, the monolayer oxidation reactions under those conditions are unknown. To see whether deposition was improved over longer time scales, polypyrrole films were also deposited potentiostatically in this electrolyte at various potentials over clean gold and 1-TEP-treated surfaces. The potential was applied at time t ) 0, which can be seen in Figure 4 by the current spike that comes from double-layer charging and the decay of residual current. In pyrrole-free PC/ LiClO4, the chronoamperograms of 1-TEP monolayers had a nucleation-like peak that occurred after the initial capacitive current fell (see paper 3). The same peak can be seen in Figure 4. For low voltages, such as 0.60 V versus Ag/AgCl, the rise in current that indicates the beginning of polymerization did not occur on the clean gold electrode until approximately 35 s after the potential was applied, and the current rise was slight. On the 1-TEP-treated gold electrode, the current rose almost immediately after the potential was applied due to monolayer oxidation but reached a steady state after approximately 10 s; then, at about 40 s the current rose again, in parallel with that for the gold electrode. At 0.65 V, the polymerization current began for the gold electrode
Langmuir, Vol. 14, No. 11, 1998 2999
Figure 4. Chronoamperograms of potentiostatic growth of PPy in water/LiClO4/pyrrole. Gray lines indicate gold electrodes and black lines 1-TEP monolayers. Solid lines were taken at 0.60 V, dotted lines at 0.65 V, and dashed lines at 0.75 V; potentials are vs Ag/AgCl. Current scales are noted in the figure; the scale for the 0.75 V curves is reduced by a factor of 4 relative to that for the others. The insert has an expanded time axis to show the monolayer peak at the beginning of polymerization at 0.75 V.
after approximately 10 s, as did that for the 1-TEP electrode, but the latter also had a small peak in the current at 2 s due to the monolayer oxidation. At the even higher potential of 0.75 V, polymerization began even sooner, after about 1 s for both gold and 1-TEP, but the latter again showed the monolayer peak before that, at approximately 0.2 s (see insert). As in the cyclic voltammograms, the monolayer oxidation peak was again seen in these chronoamperograms occurring before polymerization began. Furthermore, the onset of polymerization occurred at the same time for the gold and 1-TEP electrodes at all potentials, although the current for the plain gold electrodes was consistently slightly higher than that for the 1-TEP. In water/LiClO4, then, as in PC/LiClO4, the 1-TEP monolayer is oxidized prior to the onset of PPy deposition, despite the fact that in water/LiClO4 the oxidation potential for PPy formation is lower than that for the monolayer oxidation. The surface modification does nothing to enhance nucleation or growth during potentiostatic deposition, and during potentiodynamic deposition the monolayer delays the onset of polymerization. In this electrolyte, too, the monolayer would appear to be ineffective. However, although they indicate otherwise, the data cannot actually rule out the possibility that there is covalent bonding between the monolayer and the PPy, and adhesion tests are the topic of current studies. 2.3. Water/DBS. Switching the supporting electrolyte to DBS and retaining water as a solvent, potentiodynamic and potentiostatic polymerizations were performed on gold electrodes modified with 1-TEP and 3-TEP. (For results of deposition of P(1-TEP) in water/DBS, see paper 1.) The electrolyte concentration, 0.1 M, was well above the critical micelle concentration (CMC).23 Because DBS is a surfactant, above the CMC it is expected to coat the electrode surface24 and enclose the pyrrole in micelles, radically altering the way pyrrole is delivered and polymerized.25,26 There is a question, therefore, of whether the monolayer is available to the growing PPy. The pH of 0.1 M DBS is approximately 10, so this solution is alkaline. However, the pKa for deprotonation of pyrrole is 17.5 in aqueous (23) Gershman, J. W. J. Phys. Chem. 1957, 61, 581-584. (24) Rusling, J. F. Electroanal. Chem. 1994, 18, 2. (25) Naoi, K.; Oura, Y.; Maeda, M.; Nakamura, S. J. Electrochem. Soc. 1995, 142, 417. (26) Barr, G. E.; Sayre, C. N.; Connor, D. M.; Collard, D. M. Langmuir 1996, 12, 1395.
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had not been oxidized at this low potential, it is still possible that the pyrrole moieties were intact and able to covalently bind to the polymer in this electrolyte. If this occurred, though, there was no evidence for it in the growth rate or deposition potential. Moreover, the supporting electrolyte obviously plays an important role in determining the surface condition, especially if it is a surfactant. The amphiphilic DBS would be expected to coat the surface as soon as it was immersed28 and probably screened the monolayer. The electrochemical behavior is consistent with this hypothesis.
Figure 5. First cycle of potentiodynamic growth of PPy in water/DBS on bare gold (dotted line), 1-TEP (heavy black line), and 3-TEP (thin black line). For comparison, the first cycle of a 1-TEP monolayer in water/DBS is also shown (dashed line). The insert shows subsequent cycles on 1-TEP: first cycle, heavy solid line; second and third, dotted lines; fourth, thin solid line.
Figure 6. Chronoamperograms of the first 1 s of potentiostatic growth of PPy at 0.55 V vs Ag/AgCl in water/DBS/pyrrole on two surfaces each of 1-TEP (black lines), 3-TEP (gray lines), and bare gold (dotted lines).
solutions,27 so the TEP monolayers are not expected to be deprotonated at pH 10. The results of scans between 0 and 0.5 V are shown in Figure 5. In this electrolyte, a still different result was obtained from the previous two. Polymerization began at 0.4 V, well below the polymerization potential of 0.65 V in water/LiClO4. This electrolyte clearly facilitates PPy deposition. In addition, the oxidation of the monolayer did not occur, but nevertheless PPy was deposited on the surface. (In pyrrole-free water/DBS, the monolayers did not oxidize before gold did.) Both 1-TEP and 3-TEP had very small steps (note current scales in the figure) at 0.2 V of unknown origin that were not due to monolayer oxidation. In subsequent scans on all three surfaces, polymerization began at a lower potential than during the first scans, at a lower potential indeed than that of the first return loop (see insert). The PPy(DBS) is apparently an even more favorable surface for growth than bare gold, either of the TEP monolayers, or PPy(ClO4). The potentiostatic results are shown in Figure 6 for two samples each of bare gold, 1-TEP, and 3-TEP with an applied potential of 0.55 V. The polymerization started at the same time on all of them. The 1-TEP surface had the lowest amount of current before polymerization started, but the highest acceleration in the initial stages. There were small variations from sample to sample, but after the first second the polymer grew at essentially the same rate on all of them. With DBS, there was no inhibitory effect from the 1-TEP monolayer, and polymerization occurred at the same potential and time as on bare gold. Because the monolayer (27) Gilchrist, T. L. Heterocyclic Chemistry, 2nd ed.; John Wiley & Sons: New York, 1992; pp 193-195.
3. Discussion These results in various electrolytes demonstrate that the pyrrole moieties of such monolayers cannot be assumed to be incorporated into the growing pyrrole film. We showed in the previous paper of this series that in PC/ LiClO4 the 1-TEP monolayer oxidation peak does not represent the simple formation of radical cations of pyrrole, but the decomposition of the monolayer. In the presence of pyrrole, the identical characteristic peaks were observed before the onset of PPy polymerization, so we conclude that the same chemical processes that lead to monolayer destruction must occur in this case also. The pyrrole in the PC/LiClO4 solution thus affords no protection to the monolayer and does not lead to different chemical reactions, such as the hoped-for copolymerization. Others have found that in acetonitrile/LiClO4, the oxidation potential for pyrrole was higher than the oxidation potential of thiol-modified monomers, and the monolayer was oxidized prior to, and independent of, oxidation of pyrrole in solution.12 In the absence of monomer in solution, the oxidized form of the monolayer was subject to reaction, leading to loss of electroactivity.12 The second cycle was similar to that observed for monolayers of dodecanethiol, indicating that the monolayer was present after oxidation but that the pyrrole rings had been rendered electrochemically inert.29 The reactions that occurred in the presence of monomer in solution were attributed to covalent bonding between the monolayer and the deposited film,29 but they are actually unknown. These data also highlight the sensitivity of the electrochemical oxidations to the electrolyte. Whether the monolayer oxidation potential was lower or higher than that of PPy, or whether the monolayer oxidized at all, depended on the electrolyte. Intuitively, one might expect that, because of their proximity to the electrode, it would always be easier to oxidize the pyrrole moieties in the monolayer than the pyrrole in solution to form the radical cations necessary for polymerization. Although this was true in PC/LiClO4 (the monolayer peak was at a lower potential than the onset of polymerization current), it was the opposite in water/LiClO4 (the polymerization peak started at a lower potential) and water/DBS (which showed no monolayer peak). This sensitivity could be the case for other surface-bound pyrrole monolayer treatments, too, and whether they succeed or fail at improving the properties or adhesion of PPy could depend on the electrolyte used for deposition. Initial adsorption of anions on the electrode may affect the nucleation process, and adherence improvement has been found to depend on the anion.7 Monolayer oxidation has also previously been found to be different in PC30 and acetonitrile2 using the same salt. (28) Rusling, J. F. In Electroanalytical Chemistry: A Series of Advances; Bard, A. J., Ed.; Marcel Dekker: New York, 1994; Vol. 18, pp 13-14, 46, 48, 52-53, 79. (29) Sayre, C. N.; Collard, D. M. Langmuir 1995, 11, 302. (30) Willicut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 10823.
Electrochemical Deposition of Ppy over 1-TEP
The 1-TEP monolayers are known to be disordered. How do imperfections and the possible presence of pinholes affect their performance? It should be noted that the monolayers formed from longer chain thiol-substituted aromatic rings that are usually studied are also expected to be disordered,2 monolayer coverage is 50% of that for unsubstituted alkanethiols,22 yet they had a beneficial effect on PPy deposition nevertheless. If the hypothesis were true that the bound pyrroles were readily oxidized to form radical cations and that these sites were nucleation centers for PPy, then an imperfect monolayer should also promote growth and adhesion. In fact, due to steric considerations, it may be preferable to a perfectly packed monolayer: the growing PPy needs access to those sites. Pinholes to the gold would be less attractive than the bound pyrroles to the nascent PPy chains: the surface-immobilized pyrrole should be subject to oxidation at lower potentials and give rise to more rapid deposition,12 and the monolayer should promote the growth of the polymer.12 The fact that the presence of an imperfect 1-TEP monolayer was able to shift the polymerization potential of the PPy in water/LiClO4 is another indication that pinholes are not responsible for the results presented above. Finally, it should be noted that because the alkyl chains are short, the 1-TEP molecules are not insulating, and the potential at pinholes should be essentially the same as over the monolayer. In PC/LiClO4, we observed that the electrochemistry of PPy deposition on 1-TEP-treated and clean gold electrodes was the same. Willicut et al.2 have also reported that the voltammetry and chronoamperometry of PPy deposition over short-chain 1-substituted alkanethiol pyrrole monomers (in acetonitrile) was unaffected by the presence of the monolayer and that the voltammetry of the PPy films produced was virtually the same.22 But, for that surface, adhesion was greatly increased and the morphology of the films differed.22 A beneficial result, therefore, may not be apparent in the electrochemistry. However, others have observed strong effects, such as a doubling of the peak polymerization current attributed to an increased number of nucleated sites.12 In addition, the growth rate was found to be slower on modified electrodes because the resulting PPy films were smoother than they were on clean gold.29 Because the influence of a monolayer is not necessarily apparent in the electrochemical behavior of the PPy, whether adhesion is improved must therefore be tested separately. Whereas some workers have reported that PPy did not adhere to clean gold, under our experimental conditions freshly deposited films did adhere quite well, as measured by a tape test. Thus, we could not see an improvement in adhesion without further, more thorough evaluation. The adhesion depends critically on the surface of the gold film, particularly its morphology, which is determined by the evaporation conditions. For PPy devices that undergo electrochemical cycling, adhesion must also be tested in both the oxidized and reduced states because the oxidation level of the polymer has been found to affect adhesion of thiol-substituted pyrroles.31 Most importantly, it must be examined under the experimental conditions of most interest: after prolonged oxidationreduction cycling. The properties of the PPy films produced on such surfaces, such as their mechanical strength, also need to be investigated, as suggested in ref 8. These are topics for future work. Because the 1-TEP and 3-TEP monolayers were both electrochemically silent in water/DBS, there are no clear (31) Kowalik, J.; Tolbert, L.; Ding, Y.; Bottomley, L. A. Synth. Met. 1993, 55-57, 1171.
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indications of whether the overlying PPy film copolymerized with them. Nor has the analysis that was performed in paper 3 for monolayers in PC/LiClO4 been done for water/DBS, so the results of electrochemical cycling in this electrolyte are unclear. However, there are no monolayer oxidation peaks in the 0-0.5-V potential window in pyrrole-free water/DBS. Radical cation formation in the monolayer could have occurred simultaneously with pyrrole polymerization, or the monolayer could have remained inert. In other electrolytes with longer chain thiol- and silane-modified pyrroles, an oxidation peak was observed before polymerization,1,11 but for modified phenyls it was not,4 and both resulted in increased adhesion. The important implication is that if the surfactant DBS does interfere with the coupling by coating the electrode, then it is impossible to promote adhesion of PPy(DBS) using such a method, regardless of the monolayer oxidation behavior. It may be necessary to grow an intermediate layer of PPy doped with another anion first. This is of considerable significance to those groups making PPy devices, many of whom have found this type of anion beneficial. Unlike in most of the previously published studies on monolayers with aromatic rings, in none of the electrolytes did the PPy films grown on the 1-TEP monolayers appear markedly smoother, thicker, or more uniform to the eye or under the microscope. This difference with other published results probably arise from the extreme instability of the TEP molecules (discussed in papers 1-3). The 1-TEP seems to be unlike slightly longer chain analogues because it readily decomposes upon electrochemical oxidation, and so the latter may be more promising for future work. 4. Conclusions In PC/LiClO4/pyrrole, the PPy polymerization potential was higher than that for monolayer oxidation, and the monolayer oxidation peak was identical to that seen in a monomer-free electrolyte. Judging from this, the oxidation reactions must have been the same, leaving the monolayer decomposed and inactive. From the cyclic voltammograms and visual inspection of the films, PPy growth was unaffected by the 1-TEP on the electrode. In water/LiClO4, pyrrole polymerization required a slightly lower potential than that for monomer oxidation. Rather than enhancing nucleation by providing reactive sites, however, the monolayer prevented the onset of polymerization until after the monolayer oxidation process had been completed. The surfactant anion in the third electrolyte had a significant effect on PPy deposition, lowering the polymerization potential. The interactions between the monolayer, the DBS, and the PPy are unclear, but the growth of PPy again seemed to behave the same way as on untreated gold and it is most likely that the monolayer was simply covered by a layer of DBS. The main conclusion of this paper is that because the relative oxidation potentials of a thiol-modified pyrrole monolayer and pyrrole in solution can vary from one electrolyte to the next, whether a given modified-pyrrole surface treatment functions as designed must be examined on a case-by-case basis. The use of anionic surfactant electrolytes would appear to be completely ruled out. A second conclusion is that the assumption that monolayer oxidation reactions differ in the presence of pyrrole, so that instead of oxidizing to other compounds they copolymerize with PPy, should be reexamined. Other reactive surfaces may also increase nucleation and adhesion. Finally, there is no evidence that the 1-TEP monolayers
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react with pyrrole to anchor the PPy to the gold substrate or change the properties of the overlying PPy. Given the results of the previous paper, this was not unexpected. Acknowledgment. I acknowledge the support of Dr. Olle Ingana¨s, in whose laboratory part of this work was
Smela
performed. I would also like to acknowledge the financial support of Volvos Forskningsstiftelse & Volvos Utbildningsstiftelse and the Swedish Research Council for Engineering Sciences, TFR. LA970863E
