Change from a bulk to a surface coupling mechanism in the electrochemical polymerization of thiophene

ELSEVIER

Synthetic

Metals

87 (1997)

81-87

Change from a bulk to a surface coupling mechanism in the electrochemical polymerization of thiophene Csaba Visy a,*, Jukka Lukkari b, Jouko Kankare b ‘Institute of Physical Chemistql, Attila Jdzsef Universiry, H-6701 Szeged, Hungary b Department of Chemistry, Universiiy of Turku, FIN-20500 Turku. Finland Received

11 July 1996; accepted 25 November

1996

Abstract

Theelectrochemical polymerizationof thiophenehasbeenstudiedby spectrovoltammetric, spectrogalvanostatic andelectrochemicalquartzcrystalmicrobalance(EQCM) techniques. Experimentalsupportwasfoundfor achangefrom abulk to a surfacecouplingmechanism during the processat weakIy adsorbingelectrodeslike indium-tin oxide (ITO). This conclusionhasgainedfurther proof from experimentsin connectionwith the effect of a strongdeprotonatingagent(DBN) It wasobservedthat thesolutionphaseinhibitor is inefficientwhenit was addedonly duringthe process,whenthesurfacehadalreadybeencoveredby a prepolymerizedlayer. The film formationundisturbed by the late additionof the inhibitor wasillustratedby evidenceobtainedfrom spectroelectrochemical andEQCM measurements. Keywords:

Polythiophene;

Electropolymerization;

Coupling

mechanism;

Weakly

1. Introduction

Since the time of its discovery in the late 197Os,the electrochemical polymerization of heteroaromaticmoleculeshas beenintensively studied [ l-81. In spiteof the extensive work concentratedon the elucidation of the mechanismof the process, there are still different and even contradictory conclusions in this respect. Although the formation of a cation radical, asthe first electrochemical step,is generally accepted [ 91, there are differences considering the further stepsof the process.For the pyrrole the radical-radical coupling (RRC) mechanism has been evidenced [lo], but, with thiophene giving rise to a much more reactive monomer cation radical, it is not so easy to study this problem directly, and the conclusions are not so evident. In many casesthe RRC mechanism was assumedmerely in analogy with the pyrrole, but a radical-monomer coupling (RMC) mechanism has been concluded from the studieson the catalytic effect of bithiophene and terthiophene on the electropolymerization of thiophene [ 1l] and from current efficiency-concentration dependence[ 121. According to Wei et al. [ 111,the monomer cation radical reacts with the monomer, and the resulting dimer oxidizes to a dimer cation radical. This again attacks a thiophene molecule so that the polymerization proceedsin the solution phaseas the sequenceof initiation (oxidation) * Corresponding

author.

0379~6779/97/$17.00 Pr1s0379-6779(96)04902-8

0 1997 Elsevier

Science S.A. All rights reserved

adsorbing

electrode

and propagation (coupling) steps.The rate of this latter type of reaction decreaseswith the chain length, and the longer oligomeric cation radicals participate predominantly in recombination, which is in agreementwith the recent conclusion drawn by Audebert et al. [ 131 that the RRC mechanism is valid for quinquethiophenes.Longer oligomers formed in the solution are deposited on the surface [ 141 and: being easily oxidizable, they transform to their cationic (doped) form, insuring in this way that the charge transfer proceeds. However, the reality might be even more complicated. For pyrrole, Scharifker and Fermin [ I.51 found that the rate of the film growth on an electrode already covered by polymer was independentof the rotation rate of the electrode. They concluded that once the electrode is covered by the polymer, further polymerization occurs without the involvement of the intermediatesparticipating in the oligomer formation in the solution. In the caseof pyrrole the electrochemically initiated chain polymerization [ 161 assisted by the hydrogen bonding between the monomer cation radical and certain anions (BF4-, C104- and PF6-) has also been concluded. The ‘tosylate effect’, i.e. the improved quality of the PPy prepared in the presenceof this anion [ 171, has been interpreted by ion pairing through hydrogen bonding, which leadsto highly reactive radicals. The proton-catalysed cationic polymerization of pyrrole hasalso been assumed,which goesin parallel with the elec-

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Cs. Visy et al. /Synthetic

trochemical one, and gives rise to the formation of an insulating film [ 181. This process can be depressed by depleting the proton concentration in the reaction zone, e.g., by adding some percentage of water to the solution, which explains its positive effect discovered already ten years ago [ 191, although there is another interpretation taking the deprotonating ability of water into consideration [ 201. At the same time the inhibiting effect of water has been reported in the case of thiophene polymerization [ 21] . In the case of the electropolymerization of thiophene, even the early studies reported on distinct stages during the growth of the film [ 61 and it was observed that the species generated during the first current spike are in the solution phase [7]. For pyrroie it was assumed in [ 151 that, after the formation of the first thin layer, the polymerization occurs without the involvement of soluble intermediates participating in the oligomer formation, so there might be a change in the polymerization mechanism. In the case of noble metal electrodes the monomer adsorbs well on the surface [ 4,5,7] ; therefore, the control of this type of mechanism change is not easy. However, at weakly interacting electrodes like ITO, the oligomers form in the solution first [7,14] and, when the solvent becomes saturated, they are deposited on the surface. Thus, the formation of oligomers in the solution phase is better separated in time from the film growth, which gives an opportunity to control the reality of the assumed mechanism change. There is another approach which can help in solving the problem. Recently, the inhibiting effect of not only nucleophilic (Lewis-type) but protophilic (Bronsted-type) additives leading to no film at all or dominantly insulating film of poIythiophene has been evidenced [ 221. It was concluded that the deprotonation of the monomer cation radical before coupling leads to unwanted side reactions, since the neutral radical is much more reactive and less selective. Later it has been shown that the neutral (deprotonated) monomer radical can abstract a hydrogen atom from some solvents, e.g. acetonitrile, and this side reaction decreases the overall current efficiency [ 231. As the proton scavenger is assumed to react in the solution phase, the pattern of its effect can be used to distinguish between solution phase and possible heterogeneous chain-propagation steps: if the further increase of the polymeric film after the first nucieation and early growth follows the same mechanism, the proton scavenger, added at whatever stage, will be able to inhibit the process. If the film grows on the polymer electrode in a reaction between the oxidized film and the monomer, the reaction will not be inhibited by the protophilic additive in this section. As it comes below, if the deprotonating agent was added to the solution during the polymerization, when the electrode surface was already covered by an oxidized film, it could not inhibit the further development of the polymeric film, meaning that the reaction zone of the chain growth shifted from the solution phase to the polymer/solution interface.

Metals

87 (1997) 81-87

2. Experimental The spectroelectrochemical cell and equipment were the same as previously described [ 241. In a one-compartment cell the working electrode was made of IT0 (about 20 R/ 0, Planar International, Finland), and a platinum and a silver electrode served as auxiliary and reference electrode, respectively. Galvanostatic polymerization [ 25,261 was done from dried solution of 0.1 M thiophene and 0.2 M Bu,NPF, in acetonitrile (AN) or propylene carbonate (PC). Cyclic voltammograms were taken in solutions of 0.1 M for thiophene and 0.001 M for 2,2’:5’,5”-terthiophene, respectively. The water content of the solutions was controlled by coulometric Karl Fischer titration and kept below 30 ppm. Reagent grade 1$diazabicyclo [ 4.3.01 non-5-ene (DBN, Aldrich) was used as received. Its concentration was 3.75 X lo-*M, which had caused the total inhibition of the film formation when it had been added to the reaction mixture in advance [ 221. The spectra were recorded [ 271 on an optical multichannel analyser equipped with a diode array detector (PAR Ml461). The electrochemical and optical measurements were synchronized by a computer using an ASYST language programme. Electrochemical quartz crystal microbalance (EQCM) measurements were done in a flow-through cell by using a quartz crystal analyser (EG&G Seiko) connected to a potentiostat (EG&G PAR model 283). Platinum-coated crystals oscillating at about 8.9 MHz were used.

3. Results and discussion Fig. 1 shows the current-potential curves taken in the -0.4 to i-2.2 V potential region in two groups. The first scans (Fig. 1 (a) ) show that the current on the reversed section is larger, the phenomenon called the ‘nucleation loop’ character in the literature [ 191. During the subsequent scans (3-4) the oxidation starts at lower and lower potential values, going down to 1.8-1.9 V. Thus, at this weakly adsorbing electrode the process exhibits an autocatalytic effect which

0.5

1.0

1.5

2.0

Fig. 1. Cyclic voltammetric curYes in thiophene/Bu,NPF,/AN solution at 100 mV/s sweep ra&e: (a) first four cycles; (b) 5th, 10th and 15th cycies.

Cs. Visy et al. /Synthetic

increases the current at the same potential, and shifts the oxidation threshold potential to less positive values from cycle to cycle. On the second reversed sweep the small cathodic current indicates the appearance of reducible film, and from this point the reversible redox transformation of increasing amounts of deposited material shows up. Fig. 2 shows that detectable optical changes appear on the second scan. These spectra have the same pattern as those which can be registered at the beginning of the galvanostatic polymerization (Fig. 3(a) ). In order to distinguish the absorption of the deposited film from that of soluble oligomers, the film formation was artificially inhibited by DBN [ 221. Fig. 3 (b) shows the clear difference between the specu-al changes: soluble species have a small absorbance in the 350-550 nm range even at the end of the ‘polymerization’, while the deposited thin layer, becoming oxidized, has an elongated absorption tail at smaller energy excitations and this absorption becomes dominant very soon. Thus, it is con-

0.00

400

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87 (1997)

eluded that spectral changes obtained during the second voltammetric polarization (Fig. 2) are due to the formation of a thin film. The figure also proves that the deposition occurs as the potential reaches the region of the new polymerization, indicated by the voltammetric curve in Fig. 1(a). Since the time elapsed until this moment was for the formation of soluble oligomers, and their oxidation is easier than that of the monomer, the current hysteresis and the gradual negative potential shift of the oxidation potential can be connected to the formation and presence of oligomers at the electrode. Fig. 4 shows the spectraregistered during the fourth anodic scan. The first part in the less positive potential range indicates the neutral form of the film which remains unchanged until the potential reaches the region of its oxidation. At these potentials the spectral changes reflect the gradual transformation of the neutral film into its oxidized form. The next part of the spectra between 1.4 and 1.8 V clearly shows that the layer is already totally oxidized and does not change for

500

600

wavelength Fig. 2. Absorbance

spectra registered

0.00

during

a

700

/ nm

the second cycle in Fig, 1 (a) at 1.9, 2.0, 2.1, 2.2, and again 2.1 V (this order is indicated

400

500

600

wavelength Fig. 3. Evolution DBN, A.t=4 s.

83

81-87

of the spectra during the galvanostatic

700

800

0.00

500

600

wavelength

/ nm

polymerization

400

of thiophene

at i=5

mA/cm’:

700

by the arrow)

t

/ nm

(a) without

DBN,

repetition

time At=

0.2 s; (b) with

Cs. Visy et al. /Synthetic

Metals

57 (1997)

81-87

0.6

I

0.5

0.4

0.3

0.2

0.1

0.0 I

I

I 500

400

I 600

wavelength

I nm

Fig. 4. Absorbance spectra obtained during the4th cycle in Fig. 1 from -0.4 to + 2.2 V and at 2.1 V (reversed the spectrum registered at 1.9 V from where the new polymerization current can be seen on the voltammogram

The arrow indicates

well-oxidized form according to the spectra), but by the depletion of the soluble oligomers in the diffusion iayer. So, the potential shifts can be well explained by the formation and concentration change of the solution phase oligomers which are more easily oxidizable than the monomer itself. Since the threshold potential returns to the original discharge potential value of the thiophene monomer (2.1 V), and the polymeric film being already deposited is in the highly oxidized form, it is concluded that from this moment the film growth is the result of the reaction between the oxidizedchain and the monomer molecule: the monomer oxidation and the chain propagation take place on the surface, assumingly in closely related elementary steps. In order to get further support for this conclusion, the effect of the proton scavenger was studied. As mentioned in the Introduction, the protophilic additives present from the beginning inhibited totally the polymerization of thiophene, reacting with the monomer cation radical by forming a deprotonated neutral radical [22]. In the next part of the

seconds until the potential reaches 1.9 V (marked curve). From this potential the polymerization continues (see Fig. 1 (a) ) , and the modification of the spectroscopic behaviour is analogous with that in Figs. 2 and 3 (a), namely, it indicates the new deposition. The reversibility of the spectral changes during and after the scanning proved that the layer was not distorted by overoxidation processes. When continuing the cycling further (Fig. 1 (b) ), the loop disappears and the oxidation from the solution phase starts at more and more anodic potential values. This shift was interpreted previously by the increase of the film resistance with its thickness [ 281. However, when comparing the phenomenon with the case of the electropolymerization of terthiophene (Fig. 5), it is rather clear that there is no loop on the voltammograms, no change in the threshold potential, neither negative at the beginning nor positive after several scans, although polyterthiophene has a lower conductivity [ 9 1. Thus, the positive potential shift obtained with thiophene cannot be caused by the resistance of the film (as it is in the

-0.2

scan) at 100 mV intervals. in Fig. 1 (a).

0.4

0.6

0.8

1.0

1.2

1.4

I 0.4

I 0.6

I 0.8

1.0

1.2

1.4

potential I V Fig. 5. Cycfic

voltammograms

of terthiophenelBu,NPF,/AN

soIution at 100 mV/s

sweep rate: (a) first three scans; (b) 7th; 8th and 9th scans.

Cs. Visy et al. /Synthetic

Metals

experiments we studied the electropolymerization of thiophene galvanostatically in the absence, presence and with the delayed addition of the deprotonator. On the basis of our previous conclusions the similarity of the first and third cases is expected. Fig. 6 shows the chronopotentiometric curves registered without (a) and with DBN (b) , as well as in the case when the scavenger was added only at 3-4 s of the polymerization (c) . With the inhibitor (b) the potential gradually increases to a relatively positive region causing overoxidation. When the addition of DBN was made during the process (c), its appearance in the reaction zone might be connected to the slow and small potential increase at the elapsed time of around 5 s, but the polymerization went on following the pattern of the undisturbed case (a) and the potential did not rise to the overoxidative region. The small (below 30 mV) differences between the first and third curves are within the limit of the differences generally observed in the case of galvanostatic polymerization at IT0 electrodes.

87 (1997)

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85

As for the spectroscopic results they totally fulfil the expectations. In the case when the DBN was present from the beginning, no film formed on the electrode either in AN or PC solutions (see Fig. 3(b) ). However, in the two other cases (Fig. 7)) the evolution of the spectra is identical, proving that the late addition of the deprotonating agent could not affect further film formation. So the DBN could not inhibit the process through deprotonation in the solution phase any more. The effect of the addition of DBN has been studied also by EQCM with the aim of getting further experimental support for the mechanism change with another technique. Three polymerization cases are compared again: ( 1) without, (2) with DBN and (3) in the case when the polymerization was restarted in the presence of the inhibitor, but with the polymer electrode obtained in the first case. In the absence of the inhibitor the film can be obtained and the mass-charge relationship is presented in the first part of Fig. 8. In the presence of the proton scavenger, starting with a bare electrode, there

2’7-

II

2.4

1 0

2.2

, 5

I 10

I

I 15

20

in AN: (a) without;

(b) with DBN;

--I

time I s Fig, 6. Potential process.

change during

the galvanostatic

polymerization

of thiophene

1.6 ,

,

1.6,

1.0

1.0

ti e

0.8

0.8

2 m

0.6

0.6

0.4

0.4

0.2

0.2

500

0.0 600

wavelength Fig. 7. Absorbance

spectra registered

s of the

(b)

8

400

was added at 34

I

(4

0.0

(c) when DBN

during the galvanostatic

700

800

400

I nm polymerization

500

600

wavelength of thiophene

700

800

/ nm

in AN: (a) without;

(b)

with DBN added at 34

s of the process.

Cs. Viny et al. /Synthetic

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87 (1997)

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charge /mC Fig. 8. Mass vs. charge relationship during the polymerization of thiophene in PC without and in the presence of DBN. curve obtained by assuming a doping level of 25% and a solvent/anion ratio of 2 for the solvation.

was no frequency change and we could not get a film in AN and/or PC solution. But, when the polymerization was restarted with the film obtained in case (1) from a solution containing the same inhibitor at the same concentration, the film formation went on. The results can also be seen in Fig. 8, where the dotted line means the continuation of the process in the solution containing the deprotonator, as well. The slope for the first section (0.71 mg/C), which seems to continue further in the presence of the additive, is in good agreement with the theoretical value (0.78 mg/C) obtained by assuming a 25% doping level and a solvation number of 2 for the hexafluorophosphate anion by PC [ 291. The theoretical value was obtained by starting from the value of 4.6X lo-’ mol thiophene transformed by 1 mC charge with the assumption of 100% current efficiency. The accelerated mass increase during the last section of the polymerization has also been observed in the case of pyrrole [ 301 and was interpreted by the incorporation of the electrolyte through hydrogen bonding with the N-H group. As it is not possible with thiophene, this mass increase, not coupled with charge transfer and seemingly behaving as a catalytic effect of the proton scavenger initializing a chain reaction [ 161, is still an open question and subject to further investigation. However, we emphasize that the aim of the present experiments was to illustrate the inefficiency of the inhibitor for the polymerization at the polymer-modified electrode, and to monitor in this way the continued and undisturbed formation of the polymeric layer.

Dashed line represents

the theoretical

state experiments, the soluble oligomers are oxidized preferentially. Later, with the formation of a film, the concentration of the oligomers decreases, and the process gradually transforms to one in which the monomer oxidation becomes dominant. As it takes place on the oxidized ‘prefilm’, the chain growth can occur right away on the surface by coupling of the monomer cation radical and the oxidized chain in the film. As there is no intermediate in the solution phase, the deprotonating agent cannot inhibit the film formation any more. From the point of film preparation, it is interesting that the deposition of shorter oligomers of variable length may be prevented just by adding a proton scavenger. In principle, this might lead to a method of fabricating morphologically more regular polymer films by forming a regular ‘seed’ film on the electrode by surface modification [ 14,3 11, and starting the electropolymerization process in the presence of a proton scavenger. Work in this field is in progress.

Acknowledgements Financial aid from the Hungarian National Research Fund (OTKA-T016017), the Academy of Finland and OMFBCIMO (No. 5) is gratefully acknowledged.

References 4. Conclusions There is strong support for the mechanism in which the electrochemical polymerization of thiophene at a weakly adsorbing electrode like IT0 changes from a bulk to a surface process. In the very beginning the chain-growing process is exclusively homogeneous, starting with the formation of soluble oligomers. In this stage, which might be short in steady-

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[ 181 T.F. Otero and J. Rodriguez, J. Electroanal. Chem., 379 (1994) 513. [ 191 A.J. Downard and D. Pletcher, J. Electroanal. Chem., 206 (1986) 139. [20] G. Zotti, G. Schiavon, A. Berlin and G. Pagani, Electrochim. Acta, 34 (1989) 881. [21] A.J. Downard and D. Pletcher, J. Electroad. Chem., 206 (1986) 147. [22] Cs. Visy, J. Lukkari and J. Kankare, Synth. Met., 66 (1994) 61. [23] Cs. Visy, J. Lukkari and J. Kankare, J. Electroanal. Gem., $01 (1996) 119. [24] J. Kankare, J. Lukkari and Cs. Visy, Synth. Met., 41-43 (1991) 2839. [25] Cs. Visy, J. Lukkari and J. Kankare, Macromolecules, 26 (1993) 3295. [26] Cs. Visy, J. Lukkariand J. Kankare, Macromolecules, 27 (1994) 3322. [27] J. Lukkari, J. Kankare and Cs. Visy, Synth. Met., 48 (1992) 181. [28] B.R. Scharifker, E. Garcia-Pastorica and W. Marino, J. Electroanal. Chem., 300 (1991) 85. [29] S. Servagent and E. Vieil, J. Electroanal. Chem., 280 (1990) 227. [30] G. Schiavon, G. Zotti, N. Comisso, A. Berlin and G. Pagani, J. Phys. Chem., 98 (1994) 4861. [31] J. Lukkari, R. Tuomala, S. Ristimaki and J. Kankare, Synth. Metals, 47 (1992) 217.