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Fast scan rate cyclic voltammetry for conducting polymers electropolymerized on ultramicroelectrodes
153
J. Electroanal. Chem., 305 (1991) 153-162 Elsevier Sequoia S.A., Lausanne
Preliminary note
Fast scan rate cyclic voltammetry for conducting polymers electropolymerized on ultramicroelectrodes C.P. Andrieux, P. Audebert and P. Hapiot Laboratoire d’Elecirochimie Mol&daire de I’Universitb Paris 7, Unitd associke au CNRS. No. 438, 2 Place Jussiey 75251 Paris Cedex 05 (France)
M. Nechtschein and C. Odin Laboratoire de Dynamique de Spin et de PropriPt& Electroniques, Service de Physique, Dipartement de Recherches Fondamentales, Centre d’Etudes Nuclt!aires de Grenoble, 2 Avenue des Martyrs, BP 85 X, 38041 Grenoble Cedex (France) (Received
11 February
1991; in revised form 8 March
1991)
INTRODUCTION
Since they were discovered ten years ago [1,2], conducting polymers have aroused much interest in the field of electrochemistry due to their original and attractive characteristics. The redox processes in conducting polymers such as polyaniline, polypyrrole, polythiophene, . . . are complex and the results have not fully agreed with theoretical considerations. This is due in part to the difficulty of getting all the necessary experimental information especially for short time-scales. However, with the exception of a recent article in which the authors examine the behaviour of polyaniline at scan rates up to the relatively rapid rate of 100 V/s [3], almost all the electrochemical studies performed by means of cyclic voltammetry on this class of comp’ounds were limited to low or to very low scan rates. The reason for this limitation was mainly the ohmic drop between the working and reference electrodes which alters the current response at fast scan rates and prevents kinetic information about the fast redox processes in the film being obtained [4]. One way to decrease the ohmic drop is to use ultramicroelectrodes (electrodes with a diameter of about 10 pm) to perform cyclic voltammetric or potentiostatic experiments. These techniques have permitted the use of very high scan rates in cyclic voltammetry [5-91 which allows for example to investigate closely the reactivity of some pyrrolic monomers [lO,ll]. Recently, the development of a potentiostat-current recorder equipped with positive feedback compensation permitted voltammograms at scan rates of 100,000 V/s with negligible ohmic drop distortions to be obtained 112,131. 0022-0728/91/$03.50
0 1991 - Elsevier Sequoia
S.A.
154
However, in the field of modified electrodes, the work reported with modified ultramicroelectrodes was performed at relatively low scan rates [3,4] often for the purpose of making a sensor, as for example in the case of the monitoring of electroactive biosubstances in rat brains [14]. We have found recently that the use of ultram~cr~l~trodes and a fast potentiostat made possible the observation of the electrochemical response of several polyheterocycle deposits at scan rates as large as 300,000 V/s in the case of polyaniline with a slight alteration due to the ohmic drop. With the aim of showing the possibility of obtaining fast kinetic information about the redox behaviour of conducting polymer films we have examined three classical polymers in the most widely studied families: polyaniline, poly(3,4-dimethylpyrrole) and poly(3-methylthiophene) under conditions in which they are usually studied. The dependence on the scan rate of peak potentials and currents was analysed and compared. In particular, it can be shown that the limiting scan rate for which the electrochemical signal from the polymer can be observed depends on the kinetics of the charge transfer which are generally fast and closely related to the intrinsic morphology of the polymer. EXPERIMENTAL
Preparation of polymers
The synthesis of polyaniline films was performed as previously reported by controlled potential oxidation at 0.7 V versus SCE after prepola~zation at 0.8 V versus SCE for a few seconds in 2 M hydrochloride acid containing 0.1 M of aniline on a 20 pm or a 5 pm diameter platinum ultramicroelectrode. Poly(3,4_dimethylpyrrole) [lS] and poly(3-methylthiophene) [16] were prepared by cyclic voltammetry of a solution of monomer. The 3,4-dimethylpyrrole was synthesized by a previously published procedure 1171.Film thicknesses were estimated by integration of the first oxidation peak by previously published procedures for polyaniline [4], polypyrrole [18] and poly(3-methylthiophene) [19]. These values are rough estimations since an uncert~nty remains about the exact surface of the electrode effectively coated with the polymer. Experience with ~~tram~croe~ectrodes
The ultramicroelectrodes were prepared by sealing platinum wires (Goodfellow Metals Ltd.) according to a previously described procedure [20]. The electrode was polished with 0.25 ym alumina paste. The cell was placed in a Faraday cage. Two kinds of homebuilt fast-response time potentiostats were used. When the ohmic drop was negligible (scan rate below 5,000 V/s, very conductive solvent or small current. . . ), a simple potentiostat with a three-electrode configuration was used [7,21]. The reference electrode was an aqueous SCE and the counter electrode a platinum grid. In contrast for scan rates higher than 5,000 V/s, and to limit the effect of the ohmic drop, a potentiostat with a two-electrode configuration equipped
155
with positive feedback compensation was used [7,13]. This potentiostat is able to compensate resistance up to 20 kQ for scan rates up to 200,000 V/s without any bandpass limitations. In the two-electrode configuration, the reference (counter electrode) was a silver wire of 1 mm diameter and 10 cm length dipped previously in concentrated nitric acid and cleaned in pure water. The potential of the silver wire electrode can be monitored versus a true reference electrode between each scan [13]. The procedure used for the compensation of the ohmic drop was the same as that which we described previously [13]. The residual resistance at the beginning of the experiments with the reduced polymer on the electrode is estimated to be less than 1,000 L? from the difference between the real experimental conditions and the oscillatory situation [13,22,23]. The voltammograms were recorded on a Nicolet 4094C/4080 (8 bits, 5 ns minimum sampling time) digital oscilloscope. The function generator was an Enertec 4431 triggered by another generator (PAR 175) providing the same wait-time at the starting potential during a set of experiments (one set consists of a given wait-time followed by 3 cycles). Each set of measurements was repeated at least 10 times and averaged to improve the signal/noise ratio. Electrochemical experiments were performed in a solution of 1 M H,SO, in water for polyaniline, in 0.6 M tetraethylammonium perchlorate solution in acetonitrile for the poly(3,4-dimethylpyrrole) and in 0.1 M tetraethylammonium perchlorate solution in acetonitrile for the poly(3-methylthiophene).
RESULTS AND DISCUSSION
The three heterocycles that we examined, all polymerize readily on a 20 pm diameter ultramicroelectrode in conditions analogous to those described previously on analytical size electrodes ‘[1,2]. The very good polymerisability of the polyaniline allowed us to deposit this polymer also on 5 pm diameter electrodes. In the case of the thiophene or pyrrole series, the choice of the methyl substituted derivatives is directed by the lower potential range of observation which lowers the relative importance of background currents considerably and therefore allows work with very thin films. Also in the case of poly(dimethylpyrrole) the peaks are better defined than in the case of polypyrrole. Polyaniline was deposited by constant potential electrooxidation, while it is necessary to deposit the polyheterocycles by cycling at a relatively high scan rate (we used 1,000 V/s) to avoid the formation of a too thick deposit. In fact, too thick deposits impede the ensuing use of very high scan rates because of the current limit of the apparatus. When transferred into clean electrolytes, the conducting polymer films can be cycled at scan rates between 1,000 V/s and 50,000 V/s in the case of the polyheterocycles or even 350,000 V/s in the case of polyaniline (Fig. 1). It is worthwhile noticing that in all cases the typical voltammetric pattern characteristic of the corresponding polymer is obtained. Therefore, the processes inside the coatings during the electrochemical conversion are certainly fast, with some relative differences according to the polymer that are detailed in the next parts.
156
”
“.d
I.”
”
Y.”
Fig. 1. Cyclic voltammograms of a polyaniline film in water + 1 M H,SO, on a 5 pm diameter platinum disc working electrode. Wait-time at the starting potential before the first scan: 200 ms. Scan rates: (a) 8,900, (b) 22,000, (c) 43,000, (d) 93,000, (e) 190,000, (f) 364,000 V s-‘. Thickness of the film about 5 nm. Potentials are versus SCE.
Behaviour of polyaniline In the case of polyaniline, very well-behaved voltammograms
can be obtained at very high scan rates (Fig. 1). Two peaks have been observed in the oxidation of the polyaniline film corresponding to two transfers as has been reported previously for low scan rate experiments [24]. The figure shows both the steady-state cyclic voltammogram and the relaxed one after a given wait-time (that is, the first voltamrnogram registered after holding the polymer for a defined wait-time in the reduced state) for different scan rates. As described before [25] for the first sweep, the first oxidation peak position changes to more positive values with both the
157
Ep / V(SCE)
0.7
0.5
A A
0.3
3 0.1
: :
l 0
. l
A l
l
.
’
I
I
I
I
2
3
4
5
6
Fig. 2. Cyclic voltammetry of a polyaniline film in water + 1 M H2S04 on a 5 pm diameter platinum disc working electrode. Variation of the first-scan (m, A) and steady-state (0, +) peak potentials with the logarithm of the scan rate for two films of (0, n) 20 and (A, +) 5 nm. Other conditions are the same as in Fig. 1.
relaxation time and the scan rate, while its shape becomes sharper. The wait-time for Fig. 1 was set arbitrarily at 200 ms, a value longer than the scan-times used in our experiments (however, this effect, associated with the relaxation of the polymers still continues to be observable at much shorter times [26]). By contrast, the position of the second transfer peak never varies with the scan rate, up to at least 200,000 V/s; it is similarly remarkable that the potential reduction peak associated with the first transfer behaves exactly the same way. As a consequence of this feature, the potential of the first transfer becomes so positive at high scan rates that it finally fuses with the second one to give a single wave, a unique observation up to now. Figure 2 shows the dependence of its peak potential values (I?,), for both the steady-state and the relaxed peaks. From this curve, we can see that the first oxidation step in the polymer film is rapid, since no significant variation of the peak potential with the scan rate is observed below 10,000 V/s. The effect of the ohmic drop on the peak potentials at most reaches the value of 3 mV at 8,000 V/s and 50 mV at 200,000 V/s (see below) showing that the variations observed are clearly not due to the ohmic drop but to kinetic processes. The use of the three-electrode potentiostat without compensation below 5,000 V/s gives exactly the same results as the compensated two-electrode system in the same range of sweep rates; this feature confirms the preceding estimate of the ohmic drop effect. The shape of the variation of E, with the scan rate suggests an apparent kinetic limitation by the electron transfer between the electrode and the film (passage from a slope close to 0 to a slope close to 120 mV per log(u)) [27]. Therefore, the electron transfer kinetics seem to be slower for the relaxed peak than for the steady-state peak. Sweep rates in the range of some tens of kilovolts per second are needed to observe a change in the value of the peak potential. Taking a thin layer model for the electrochemical
158 2 log
(I/PA
)
l-
log(v/v -1
3
t
I
4
5
2)
6
Fig. 3. Cyclic voltammetry of a polyaniline film in water + 1 M H,SO, on a 5 pm diameter platinum disc working electrode. Variation of the logarithm of the peak current with the logarithm of the scan rate for the first-scan (m) and the steady-state (0) voltammograms. The straight lines are for a slope of 1 (thin layer behavior) and 0.5 (diffusive behavior). Other conditions are the same as in Fig. 1.
behavior of the film in this range of sweep rates, it is possible to extract an approximate value of 10 s-l [27] for the heterogeneous rate constant k,, defined by the relation: i=nFSk,[r,e-
aF/RT(E--EO) _ q e(l-a)F/R?-(E-EO)
I
for the transfer between the film and the electrode (l?,, I’, number of electroactive species per unity of surface, n number of electron transferred, (Ycoefficient transfer, E o standard potential, E applied potential, S electrode area). The logarithmic variation of the peak currents with the scan rate (Fig. 3) confirms this analysis by displaying a + 1 slope (characteristic of thin layer behaviour) up to about 70,000 V/s and a + 0.5 slope (characteristic of semi-infinite diffusion behaviour) above this limit. It is rather striking that both the first oxidation and the second reduction waves, and, the second oxidation and the first reduction waves exhibit similar characteristics (shifting of the peak potential at very high scan rates and strong relaxation in the former case, no shift at all and relaxation noticeable in the latter). Behavior of the polyheterocycles Like polyaniline, poly(3,4_dimethylpyrrole) and poly(3-methylthiophene) can be cycled at very high scan rates on a 20 pm electrode. With this size of electrode and in acetonitrile, it is more difficult to get an accurate compensation [13], so that our studies were limited to lower sweep rates than in the case of polyaniline in water. We have not been able to deposit thin films of polyheterocycles on a 5 pm electrode, probably because of their very rapid polymerization kinetics. In an organic solvent and on a 20 pm diameter electrode, we found that the use of scan rates above 50,000 V/s brings out ill-defined peaks, a lower value compared with that obtainable with polyaniline. This result may indicate that the electron transfer processes involved in
159
I -0.5
I 0
I 0.5
E/V
I
-0.5
I
0
I
E/\(
0.5
Fig. 4. Cyclic voltammograms of a poly(dimethylpyrrole) film on a 20 gm diameter platinum disc working electrode in a 0.6 M tetraethylammonium perchlorate solution in acetonitrile. Wait-time at the starting potential before the first scan: 1 s. Scan rates: (a) 2,000, (b) 3,600, (c) 25,000, (d) 50,000 V s-l. Thickness of the film about 5 nm. Potentials are versus SCE.
the film oxidation begin to be slow at the highest sweep rates, although a consequence of the different experimental conditions cannot be completely excluded here (the effect of the uncompensated ohmic drop). The data obtained with poly(dimethylpyrrole) are depicted in Fig. 4 and show that the characteristic shape of the polypyrrole voltammogram is perfectly retained with a marked capacitive current and well-defined peaks. In Fig. 5 is depicted the dependence of the peak currents with the scan rate. It can be seen that thin layer behaviour is observed over the entire range tested, indicating the absence of diffusion associated with fast electron transfer. By contrast (Fig. 6), the difference between the cathodic and the anodic peak potential values (A&,) increases with the scan rate, probably owing to the relatively slow formation of bipolarons. The oxidation peak during the first scan is different from the next one, suggesting that a relaxation effect also exists in this polymer. However, the changes in peak position and shape are smaller than those we observed previously with polyaniline. With poly(3-methylthiophene) (Fig. 7), a large change is observed for the oxidation peak between the first cycle and the next, showing a large relaxation effect.
160
60.0
/
I/pa
4 /
40.0
20.0
v/v s-’ 0.0 0
20000
10000
30000
40000
50000
Fig. 5. Cyclic voltammograms of a poly(dimethylpyrrole) film on a 20 pm diameter platinum disc working electrode in a 0.6 M tetraethylammonium perchlorate solution in acetonitrile. Variation of the logarithm of the steady-state peak current with the scan rate. Other conditions are the same as in Fig. 4.
However, owing to the difficulty in preparing and cycling poly(3-methylthiophene), it has not been possible to record in this preliminary study the same amount of data for this polymer on a similar film. The very low values obtained sometimes for the estimates of film thicknesses also lead us to suppose that the electrode is not homogeneously covered by the polymer. However, well-defined voltammograms could be obtained on a platinum electrode for scan rates higher than 15,000 V/s (Fig. 7).
0.25 AEp/
V l
0.2
l
a 0.15
-
l 0 log(v/v
l 0.1
s-‘)
I
3
4
5
Fig. 6. Cyclic voltammetty of a poly(dimethylpyrrole) film on a 20 pm diameter electrode in a 0.6 M tetraethylammonium perchlorate solution in acetonitrile. potential difference with the logarithm of the scan rate for the steady-state conditions are the same as in Fig. 4.
platinum disc working Variation of the peak voltammogram. Other
161
I
I
I
0.9
I
3N4
I
I
I
0.5
1.3
c
J
Fig. 7. Cyclic voltammograms of poly(3-methylthiophene) films on a 20 pm diameter platinum disc working electrode in a 0.1 M tetraethylammonium perchlorate solution in acetonitrile. Wait-time at the starting potential before the first scan: (a) 2, (b, c) 0.1 s. Scan rates: (a) 260, (b) 6,700, (c) 17,000 V s-l. Formal thicknesses of films about (a) 10 nm, (b) 1 mn. Potentials are versus the silver quasi-reference.
CONCLUSION
We have shown for the first tune that it is possible to perform very high speed cyclic voltammetry on some conducting polymers with the use of ultramicroelectrodes and a fast potentiostat equipped with ohmic drop compensation, thus yielding valuable information on the kinetics of the limiting processes in the films with negligible distortions due to the ohmic drop. The shape of the voltammograms obtained is similar to that observed at low scan rates. As previously stated, in all cases the first-scan voltammogram is different from the others (after the first cycle the subsequent voltammograms are identical and are regarded as the steady-state voltammograms), owing to the relaxation effect in the polymers as previously pointed out by Odin and Nechtschein [28,29]. As detailed before, the relaxation effect appears to be larger with polyaniline and poly(3-methylthiophene) than with poly(3,4-dimethylpyrrole). Even in the case of polyaniline, which has the fastest electronic charge transfer kinetics of the three polymers studied, it is possible to obtain information about the
162
kinetics. A further, detailed study of this kind of behaviour is in preparation will be the topic of a subsequent full paper [26].
and
ACKNOWLEDGEMENTS
The authors warmly thank Dr. D. Blauch for kindly reading the manuscript and for his helpful advice. REFERENCES 1 G.B. Street, in T.A. Skotheim (Ed.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1986, pp. 265-291. 2 Reviewed by J. Heinz, in Topics in Current Chemistry, Vol. 152, Springer, Berlin-Heidelberg-New York, 1990, vol. 152. 3 M. Kalaji, L.M. Peter, L.M. Abrantes and J.C. Mesquita, J. Electroanal. Chem., 274 (1989) 289. 4 J.C. Lacroix, K.K. Kanazawa and A. Diaz, J. Electrochem. Sot, 136 (1989) 1308. 5 R.M. Wightman and D.O. Wipf, in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol 15, Marcel Dekker, New York, 1989, pp. 267-353. 6 C.P. Andrieux, P. Hapiot and J.-M. Savtant, Chem. Rev., 90 (1990) 723. 7 C.P. Andrieux, P. Hapiot and J.-M. Saveant, Electroanalysis, 2 (1990) 183. 8 A. Fitch and D.H. Evans, J. Electroanal. Chem., 202 (1986) 83. 9 C. Amatore, A. Jutand and F. Pfliiger, J. Electroanal. Chem., 218 (1987) 361. 10 C.P. Andrieux, P. Audebert, P. Hapiot and J.-M. Saveant, J. Am. Chem. Sot., 112 (1990) 2439. 11 C.P. Andrieux, P. Audebert, P. Hapiot and J.-M. Savtant, in preparation. 12 C. Amatore, C. Lefrou and F. Pfliiger, J. Electroanal. Chem., 270 (1989) 43. 13 D. Garreau, P. Hapiot and J.-M. Saveant, J. Electroanal. Chem., 281 (1990) 73. 14 E.W. Kristensen, W.G. Kuhr and R.M. Wightman, Anal. Chem., 59, (1987) 1752. 15 T.C. Clarke, J.C. Scott and G.B. Street, IBM J. Res. Dev., 27 (1983) 213. 16 R.J. Waltman, J. Bargon and F. Diaz, J. Phys. Chem., 87 (1983) 1459. 17 M. Farmer and M.P. Foumari, Bull. Sot. Chim. Fr., (1975) 2335. 18 W. Wemet and G. Wegner, Makromol. Chem, 188 (1987) 1465. 19 P. Marque, J. Ron&i and F. Gamier, J. Electroanal. Chem., 218 (1987) 107. 20 C.P. Andrieux, D. Garreau, P. Hapiot, J. Pinson and J.-M. Saveant, J. Electroanal. Chem., 243 (1988) 321. 21 D. Garreau, P. Hapiot and J.-M. Saveant, J. Electroanal. Chem., 272 (1989) 1. 22 D. Garreau and J.-M. Saveant, J. Electroanal. Chem., 35 (1972) 309. 23 D. Britz, J. Electroanal. Chem., 88 (1978) 309. 24 E.M. Genies, A. Boyle, M. Lapkowski and C. Tsintavis, Synth. Met., 36 (1990) 139. 25 B. Villeret and M. Nechtschem, Phys. Rev. Lett., 63 (1989) 1285. 26 C. Odin, M. Nechtschein and P. Hapiot, to be published. 27 E. Laviron, J. Electroanal. Chem., 101 (1979) 19. 28 C. Odin and M. Nechtschein, in H. Kuzmany (Ed.), Electronic Properties of Conjugated Polymers III, Vol. 91, Springer Series on Solid-State Sciences, 1989, p. 172. 29 C. Odin and M. Nechtschein, Synth. Met., in press.
J. Electroanal. Chem., 305 (1991) 153-162 Elsevier Sequoia S.A., Lausanne
Preliminary note
Fast scan rate cyclic voltammetry for conducting polymers electropolymerized on ultramicroelectrodes C.P. Andrieux, P. Audebert and P. Hapiot Laboratoire d’Elecirochimie Mol&daire de I’Universitb Paris 7, Unitd associke au CNRS. No. 438, 2 Place Jussiey 75251 Paris Cedex 05 (France)
M. Nechtschein and C. Odin Laboratoire de Dynamique de Spin et de PropriPt& Electroniques, Service de Physique, Dipartement de Recherches Fondamentales, Centre d’Etudes Nuclt!aires de Grenoble, 2 Avenue des Martyrs, BP 85 X, 38041 Grenoble Cedex (France) (Received
11 February
1991; in revised form 8 March
1991)
INTRODUCTION
Since they were discovered ten years ago [1,2], conducting polymers have aroused much interest in the field of electrochemistry due to their original and attractive characteristics. The redox processes in conducting polymers such as polyaniline, polypyrrole, polythiophene, . . . are complex and the results have not fully agreed with theoretical considerations. This is due in part to the difficulty of getting all the necessary experimental information especially for short time-scales. However, with the exception of a recent article in which the authors examine the behaviour of polyaniline at scan rates up to the relatively rapid rate of 100 V/s [3], almost all the electrochemical studies performed by means of cyclic voltammetry on this class of comp’ounds were limited to low or to very low scan rates. The reason for this limitation was mainly the ohmic drop between the working and reference electrodes which alters the current response at fast scan rates and prevents kinetic information about the fast redox processes in the film being obtained [4]. One way to decrease the ohmic drop is to use ultramicroelectrodes (electrodes with a diameter of about 10 pm) to perform cyclic voltammetric or potentiostatic experiments. These techniques have permitted the use of very high scan rates in cyclic voltammetry [5-91 which allows for example to investigate closely the reactivity of some pyrrolic monomers [lO,ll]. Recently, the development of a potentiostat-current recorder equipped with positive feedback compensation permitted voltammograms at scan rates of 100,000 V/s with negligible ohmic drop distortions to be obtained 112,131. 0022-0728/91/$03.50
0 1991 - Elsevier Sequoia
S.A.
154
However, in the field of modified electrodes, the work reported with modified ultramicroelectrodes was performed at relatively low scan rates [3,4] often for the purpose of making a sensor, as for example in the case of the monitoring of electroactive biosubstances in rat brains [14]. We have found recently that the use of ultram~cr~l~trodes and a fast potentiostat made possible the observation of the electrochemical response of several polyheterocycle deposits at scan rates as large as 300,000 V/s in the case of polyaniline with a slight alteration due to the ohmic drop. With the aim of showing the possibility of obtaining fast kinetic information about the redox behaviour of conducting polymer films we have examined three classical polymers in the most widely studied families: polyaniline, poly(3,4-dimethylpyrrole) and poly(3-methylthiophene) under conditions in which they are usually studied. The dependence on the scan rate of peak potentials and currents was analysed and compared. In particular, it can be shown that the limiting scan rate for which the electrochemical signal from the polymer can be observed depends on the kinetics of the charge transfer which are generally fast and closely related to the intrinsic morphology of the polymer. EXPERIMENTAL
Preparation of polymers
The synthesis of polyaniline films was performed as previously reported by controlled potential oxidation at 0.7 V versus SCE after prepola~zation at 0.8 V versus SCE for a few seconds in 2 M hydrochloride acid containing 0.1 M of aniline on a 20 pm or a 5 pm diameter platinum ultramicroelectrode. Poly(3,4_dimethylpyrrole) [lS] and poly(3-methylthiophene) [16] were prepared by cyclic voltammetry of a solution of monomer. The 3,4-dimethylpyrrole was synthesized by a previously published procedure 1171.Film thicknesses were estimated by integration of the first oxidation peak by previously published procedures for polyaniline [4], polypyrrole [18] and poly(3-methylthiophene) [19]. These values are rough estimations since an uncert~nty remains about the exact surface of the electrode effectively coated with the polymer. Experience with ~~tram~croe~ectrodes
The ultramicroelectrodes were prepared by sealing platinum wires (Goodfellow Metals Ltd.) according to a previously described procedure [20]. The electrode was polished with 0.25 ym alumina paste. The cell was placed in a Faraday cage. Two kinds of homebuilt fast-response time potentiostats were used. When the ohmic drop was negligible (scan rate below 5,000 V/s, very conductive solvent or small current. . . ), a simple potentiostat with a three-electrode configuration was used [7,21]. The reference electrode was an aqueous SCE and the counter electrode a platinum grid. In contrast for scan rates higher than 5,000 V/s, and to limit the effect of the ohmic drop, a potentiostat with a two-electrode configuration equipped
155
with positive feedback compensation was used [7,13]. This potentiostat is able to compensate resistance up to 20 kQ for scan rates up to 200,000 V/s without any bandpass limitations. In the two-electrode configuration, the reference (counter electrode) was a silver wire of 1 mm diameter and 10 cm length dipped previously in concentrated nitric acid and cleaned in pure water. The potential of the silver wire electrode can be monitored versus a true reference electrode between each scan [13]. The procedure used for the compensation of the ohmic drop was the same as that which we described previously [13]. The residual resistance at the beginning of the experiments with the reduced polymer on the electrode is estimated to be less than 1,000 L? from the difference between the real experimental conditions and the oscillatory situation [13,22,23]. The voltammograms were recorded on a Nicolet 4094C/4080 (8 bits, 5 ns minimum sampling time) digital oscilloscope. The function generator was an Enertec 4431 triggered by another generator (PAR 175) providing the same wait-time at the starting potential during a set of experiments (one set consists of a given wait-time followed by 3 cycles). Each set of measurements was repeated at least 10 times and averaged to improve the signal/noise ratio. Electrochemical experiments were performed in a solution of 1 M H,SO, in water for polyaniline, in 0.6 M tetraethylammonium perchlorate solution in acetonitrile for the poly(3,4-dimethylpyrrole) and in 0.1 M tetraethylammonium perchlorate solution in acetonitrile for the poly(3-methylthiophene).
RESULTS AND DISCUSSION
The three heterocycles that we examined, all polymerize readily on a 20 pm diameter ultramicroelectrode in conditions analogous to those described previously on analytical size electrodes ‘[1,2]. The very good polymerisability of the polyaniline allowed us to deposit this polymer also on 5 pm diameter electrodes. In the case of the thiophene or pyrrole series, the choice of the methyl substituted derivatives is directed by the lower potential range of observation which lowers the relative importance of background currents considerably and therefore allows work with very thin films. Also in the case of poly(dimethylpyrrole) the peaks are better defined than in the case of polypyrrole. Polyaniline was deposited by constant potential electrooxidation, while it is necessary to deposit the polyheterocycles by cycling at a relatively high scan rate (we used 1,000 V/s) to avoid the formation of a too thick deposit. In fact, too thick deposits impede the ensuing use of very high scan rates because of the current limit of the apparatus. When transferred into clean electrolytes, the conducting polymer films can be cycled at scan rates between 1,000 V/s and 50,000 V/s in the case of the polyheterocycles or even 350,000 V/s in the case of polyaniline (Fig. 1). It is worthwhile noticing that in all cases the typical voltammetric pattern characteristic of the corresponding polymer is obtained. Therefore, the processes inside the coatings during the electrochemical conversion are certainly fast, with some relative differences according to the polymer that are detailed in the next parts.
156
”
“.d
I.”
”
Y.”
Fig. 1. Cyclic voltammograms of a polyaniline film in water + 1 M H,SO, on a 5 pm diameter platinum disc working electrode. Wait-time at the starting potential before the first scan: 200 ms. Scan rates: (a) 8,900, (b) 22,000, (c) 43,000, (d) 93,000, (e) 190,000, (f) 364,000 V s-‘. Thickness of the film about 5 nm. Potentials are versus SCE.
Behaviour of polyaniline In the case of polyaniline, very well-behaved voltammograms
can be obtained at very high scan rates (Fig. 1). Two peaks have been observed in the oxidation of the polyaniline film corresponding to two transfers as has been reported previously for low scan rate experiments [24]. The figure shows both the steady-state cyclic voltammogram and the relaxed one after a given wait-time (that is, the first voltamrnogram registered after holding the polymer for a defined wait-time in the reduced state) for different scan rates. As described before [25] for the first sweep, the first oxidation peak position changes to more positive values with both the
157
Ep / V(SCE)
0.7
0.5
A A
0.3
3 0.1
: :
l 0
. l
A l
l
.
’
I
I
I
I
2
3
4
5
6
Fig. 2. Cyclic voltammetry of a polyaniline film in water + 1 M H2S04 on a 5 pm diameter platinum disc working electrode. Variation of the first-scan (m, A) and steady-state (0, +) peak potentials with the logarithm of the scan rate for two films of (0, n) 20 and (A, +) 5 nm. Other conditions are the same as in Fig. 1.
relaxation time and the scan rate, while its shape becomes sharper. The wait-time for Fig. 1 was set arbitrarily at 200 ms, a value longer than the scan-times used in our experiments (however, this effect, associated with the relaxation of the polymers still continues to be observable at much shorter times [26]). By contrast, the position of the second transfer peak never varies with the scan rate, up to at least 200,000 V/s; it is similarly remarkable that the potential reduction peak associated with the first transfer behaves exactly the same way. As a consequence of this feature, the potential of the first transfer becomes so positive at high scan rates that it finally fuses with the second one to give a single wave, a unique observation up to now. Figure 2 shows the dependence of its peak potential values (I?,), for both the steady-state and the relaxed peaks. From this curve, we can see that the first oxidation step in the polymer film is rapid, since no significant variation of the peak potential with the scan rate is observed below 10,000 V/s. The effect of the ohmic drop on the peak potentials at most reaches the value of 3 mV at 8,000 V/s and 50 mV at 200,000 V/s (see below) showing that the variations observed are clearly not due to the ohmic drop but to kinetic processes. The use of the three-electrode potentiostat without compensation below 5,000 V/s gives exactly the same results as the compensated two-electrode system in the same range of sweep rates; this feature confirms the preceding estimate of the ohmic drop effect. The shape of the variation of E, with the scan rate suggests an apparent kinetic limitation by the electron transfer between the electrode and the film (passage from a slope close to 0 to a slope close to 120 mV per log(u)) [27]. Therefore, the electron transfer kinetics seem to be slower for the relaxed peak than for the steady-state peak. Sweep rates in the range of some tens of kilovolts per second are needed to observe a change in the value of the peak potential. Taking a thin layer model for the electrochemical
158 2 log
(I/PA
)
l-
log(v/v -1
3
t
I
4
5
2)
6
Fig. 3. Cyclic voltammetry of a polyaniline film in water + 1 M H,SO, on a 5 pm diameter platinum disc working electrode. Variation of the logarithm of the peak current with the logarithm of the scan rate for the first-scan (m) and the steady-state (0) voltammograms. The straight lines are for a slope of 1 (thin layer behavior) and 0.5 (diffusive behavior). Other conditions are the same as in Fig. 1.
behavior of the film in this range of sweep rates, it is possible to extract an approximate value of 10 s-l [27] for the heterogeneous rate constant k,, defined by the relation: i=nFSk,[r,e-
aF/RT(E--EO) _ q e(l-a)F/R?-(E-EO)
I
for the transfer between the film and the electrode (l?,, I’, number of electroactive species per unity of surface, n number of electron transferred, (Ycoefficient transfer, E o standard potential, E applied potential, S electrode area). The logarithmic variation of the peak currents with the scan rate (Fig. 3) confirms this analysis by displaying a + 1 slope (characteristic of thin layer behaviour) up to about 70,000 V/s and a + 0.5 slope (characteristic of semi-infinite diffusion behaviour) above this limit. It is rather striking that both the first oxidation and the second reduction waves, and, the second oxidation and the first reduction waves exhibit similar characteristics (shifting of the peak potential at very high scan rates and strong relaxation in the former case, no shift at all and relaxation noticeable in the latter). Behavior of the polyheterocycles Like polyaniline, poly(3,4_dimethylpyrrole) and poly(3-methylthiophene) can be cycled at very high scan rates on a 20 pm electrode. With this size of electrode and in acetonitrile, it is more difficult to get an accurate compensation [13], so that our studies were limited to lower sweep rates than in the case of polyaniline in water. We have not been able to deposit thin films of polyheterocycles on a 5 pm electrode, probably because of their very rapid polymerization kinetics. In an organic solvent and on a 20 pm diameter electrode, we found that the use of scan rates above 50,000 V/s brings out ill-defined peaks, a lower value compared with that obtainable with polyaniline. This result may indicate that the electron transfer processes involved in
159
I -0.5
I 0
I 0.5
E/V
I
-0.5
I
0
I
E/\(
0.5
Fig. 4. Cyclic voltammograms of a poly(dimethylpyrrole) film on a 20 gm diameter platinum disc working electrode in a 0.6 M tetraethylammonium perchlorate solution in acetonitrile. Wait-time at the starting potential before the first scan: 1 s. Scan rates: (a) 2,000, (b) 3,600, (c) 25,000, (d) 50,000 V s-l. Thickness of the film about 5 nm. Potentials are versus SCE.
the film oxidation begin to be slow at the highest sweep rates, although a consequence of the different experimental conditions cannot be completely excluded here (the effect of the uncompensated ohmic drop). The data obtained with poly(dimethylpyrrole) are depicted in Fig. 4 and show that the characteristic shape of the polypyrrole voltammogram is perfectly retained with a marked capacitive current and well-defined peaks. In Fig. 5 is depicted the dependence of the peak currents with the scan rate. It can be seen that thin layer behaviour is observed over the entire range tested, indicating the absence of diffusion associated with fast electron transfer. By contrast (Fig. 6), the difference between the cathodic and the anodic peak potential values (A&,) increases with the scan rate, probably owing to the relatively slow formation of bipolarons. The oxidation peak during the first scan is different from the next one, suggesting that a relaxation effect also exists in this polymer. However, the changes in peak position and shape are smaller than those we observed previously with polyaniline. With poly(3-methylthiophene) (Fig. 7), a large change is observed for the oxidation peak between the first cycle and the next, showing a large relaxation effect.
160
60.0
/
I/pa
4 /
40.0
20.0
v/v s-’ 0.0 0
20000
10000
30000
40000
50000
Fig. 5. Cyclic voltammograms of a poly(dimethylpyrrole) film on a 20 pm diameter platinum disc working electrode in a 0.6 M tetraethylammonium perchlorate solution in acetonitrile. Variation of the logarithm of the steady-state peak current with the scan rate. Other conditions are the same as in Fig. 4.
However, owing to the difficulty in preparing and cycling poly(3-methylthiophene), it has not been possible to record in this preliminary study the same amount of data for this polymer on a similar film. The very low values obtained sometimes for the estimates of film thicknesses also lead us to suppose that the electrode is not homogeneously covered by the polymer. However, well-defined voltammograms could be obtained on a platinum electrode for scan rates higher than 15,000 V/s (Fig. 7).
0.25 AEp/
V l
0.2
l
a 0.15
-
l 0 log(v/v
l 0.1
s-‘)
I
3
4
5
Fig. 6. Cyclic voltammetty of a poly(dimethylpyrrole) film on a 20 pm diameter electrode in a 0.6 M tetraethylammonium perchlorate solution in acetonitrile. potential difference with the logarithm of the scan rate for the steady-state conditions are the same as in Fig. 4.
platinum disc working Variation of the peak voltammogram. Other
161
I
I
I
0.9
I
3N4
I
I
I
0.5
1.3
c
J
Fig. 7. Cyclic voltammograms of poly(3-methylthiophene) films on a 20 pm diameter platinum disc working electrode in a 0.1 M tetraethylammonium perchlorate solution in acetonitrile. Wait-time at the starting potential before the first scan: (a) 2, (b, c) 0.1 s. Scan rates: (a) 260, (b) 6,700, (c) 17,000 V s-l. Formal thicknesses of films about (a) 10 nm, (b) 1 mn. Potentials are versus the silver quasi-reference.
CONCLUSION
We have shown for the first tune that it is possible to perform very high speed cyclic voltammetry on some conducting polymers with the use of ultramicroelectrodes and a fast potentiostat equipped with ohmic drop compensation, thus yielding valuable information on the kinetics of the limiting processes in the films with negligible distortions due to the ohmic drop. The shape of the voltammograms obtained is similar to that observed at low scan rates. As previously stated, in all cases the first-scan voltammogram is different from the others (after the first cycle the subsequent voltammograms are identical and are regarded as the steady-state voltammograms), owing to the relaxation effect in the polymers as previously pointed out by Odin and Nechtschein [28,29]. As detailed before, the relaxation effect appears to be larger with polyaniline and poly(3-methylthiophene) than with poly(3,4-dimethylpyrrole). Even in the case of polyaniline, which has the fastest electronic charge transfer kinetics of the three polymers studied, it is possible to obtain information about the
162
kinetics. A further, detailed study of this kind of behaviour is in preparation will be the topic of a subsequent full paper [26].
and
ACKNOWLEDGEMENTS
The authors warmly thank Dr. D. Blauch for kindly reading the manuscript and for his helpful advice. REFERENCES 1 G.B. Street, in T.A. Skotheim (Ed.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1986, pp. 265-291. 2 Reviewed by J. Heinz, in Topics in Current Chemistry, Vol. 152, Springer, Berlin-Heidelberg-New York, 1990, vol. 152. 3 M. Kalaji, L.M. Peter, L.M. Abrantes and J.C. Mesquita, J. Electroanal. Chem., 274 (1989) 289. 4 J.C. Lacroix, K.K. Kanazawa and A. Diaz, J. Electrochem. Sot, 136 (1989) 1308. 5 R.M. Wightman and D.O. Wipf, in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol 15, Marcel Dekker, New York, 1989, pp. 267-353. 6 C.P. Andrieux, P. Hapiot and J.-M. Savtant, Chem. Rev., 90 (1990) 723. 7 C.P. Andrieux, P. Hapiot and J.-M. Saveant, Electroanalysis, 2 (1990) 183. 8 A. Fitch and D.H. Evans, J. Electroanal. Chem., 202 (1986) 83. 9 C. Amatore, A. Jutand and F. Pfliiger, J. Electroanal. Chem., 218 (1987) 361. 10 C.P. Andrieux, P. Audebert, P. Hapiot and J.-M. Saveant, J. Am. Chem. Sot., 112 (1990) 2439. 11 C.P. Andrieux, P. Audebert, P. Hapiot and J.-M. Savtant, in preparation. 12 C. Amatore, C. Lefrou and F. Pfliiger, J. Electroanal. Chem., 270 (1989) 43. 13 D. Garreau, P. Hapiot and J.-M. Saveant, J. Electroanal. Chem., 281 (1990) 73. 14 E.W. Kristensen, W.G. Kuhr and R.M. Wightman, Anal. Chem., 59, (1987) 1752. 15 T.C. Clarke, J.C. Scott and G.B. Street, IBM J. Res. Dev., 27 (1983) 213. 16 R.J. Waltman, J. Bargon and F. Diaz, J. Phys. Chem., 87 (1983) 1459. 17 M. Farmer and M.P. Foumari, Bull. Sot. Chim. Fr., (1975) 2335. 18 W. Wemet and G. Wegner, Makromol. Chem, 188 (1987) 1465. 19 P. Marque, J. Ron&i and F. Gamier, J. Electroanal. Chem., 218 (1987) 107. 20 C.P. Andrieux, D. Garreau, P. Hapiot, J. Pinson and J.-M. Saveant, J. Electroanal. Chem., 243 (1988) 321. 21 D. Garreau, P. Hapiot and J.-M. Saveant, J. Electroanal. Chem., 272 (1989) 1. 22 D. Garreau and J.-M. Saveant, J. Electroanal. Chem., 35 (1972) 309. 23 D. Britz, J. Electroanal. Chem., 88 (1978) 309. 24 E.M. Genies, A. Boyle, M. Lapkowski and C. Tsintavis, Synth. Met., 36 (1990) 139. 25 B. Villeret and M. Nechtschem, Phys. Rev. Lett., 63 (1989) 1285. 26 C. Odin, M. Nechtschein and P. Hapiot, to be published. 27 E. Laviron, J. Electroanal. Chem., 101 (1979) 19. 28 C. Odin and M. Nechtschein, in H. Kuzmany (Ed.), Electronic Properties of Conjugated Polymers III, Vol. 91, Springer Series on Solid-State Sciences, 1989, p. 172. 29 C. Odin and M. Nechtschein, Synth. Met., in press.