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EQCM and in situ conductance studies on the polymerisation and redox features of thiophene co-polymers
Electrochimica Acta 45 (2000) 3851 – 3864 www.elsevier.nl/locate/electacta
EQCM and in situ conductance studies on the polymerisation and redox features of thiophene co-polymers Csaba Visy a,*, Jouko Kankare b, Emese Kriva´n a a
Institute of Physical Chemistry, Jo´zsef Attila Uni6ersity, P.O. Box 105, H-6701 Szeged, Hungary b Department of Chemistry, Uni6ersity of Turku, Turku, Fin-20 014, Finland Received 19 July 1999; received in revised form 9 November 1999
Abstract 3-Thiophene-acetic acid and 3-methylthiophene co-polymers have been prepared with the consideration that the solvation behaviour of the film could be modified. The modification is coupled with the observed splitting of the oxidation peak on the cyclic voltammograms. The redox transformation of the films was studied by electrochemical quartz crystal microbalance (EQCM) and in situ conductance (ISC) techniques. Mass changes during the electrochemical processes showed that the cation is not involved into the charge balancing of the film. Manifestation of self-doping was excluded by analogous measurements with an aprotic co-polymer of methylthiophene. Results completed with in situ conductance and spectroelectrochemical observations confirmed the assumption of a chemical step taking place after the first oxidation. In this process desolvation of the oxidised intermediate occurs, which is a necessary step for the achievement of the quasi-metallic state. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Co-polymer; 3-Methylthiophene; Electrochemical quartz crystal microbalance; Conductance; Solvation
1. Introduction In situ methods used in the study of conducting polymers deliver indispensable information about the details of their formation and redox transformations [1]. Thus, combined methods such as UV-vis, FTIR and ESR spectroscopy, mirage effect or laser beam deflection (LBD), in situ conductance (ISC) and electrochemical quartz crystal microbalance (EQCM) are widely used to complete the results obtained by merely electrochemical techniques [2]. In the case of ISC measurements [3–8], the essential behaviour of the conducting polymers, i.e. how their macroscopic insulator/conductor transformation is related to the electrochemical process, can be resolved. With EQCM technique [9–14] the global mass flux of differently * Corresponding author. Fax: +36-62-544652. E-mail address: [email protected] (C. Visy).
charged ions and that of the solvent, coupled with the charge transfer can be followed. In recent studies [15,16], the charging process of polypyrrole was studied by EQCM technique, and the resolution of the generally observed single, broad oxidation current maximum was reported. The splitting could be seen at low bulk concentration of the anion, small enough to be able to penetrate into the film. The phenomenon was successfully simulated by distinguishing fixed and releasable anions in the film. Based on EQCM results, two electrochemical processes were related to the same electrochemical process coupled with the opposite movement of differently charged ions. Thus, the observation of the peak splitting as the manifestation of the ‘classical’ and widely accepted scheme of successive polaron and bipolaron formation was questioned. In an earlier study, the splitting of the anodic current peak in the case of poly(3-methylthiophene) was al-
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ready reported [17]. The electrochemical processes were followed by both EQCM and ISC measurements, and the necessity of the modification of the traditional mechanism was already concluded by making partly parallel electrochemical steps responsible for the insulating/conducting transformation. It was demonstrated that the macroscopic conductance starts to develop only during the second anodic peak. So there is an electrochemical analogy between thiophene and pyrrole polymers. However, EQCM curves in reference [17] did not show any sign of a mass decrease, clearly visible in the case of PPY/LiClO4 [16], showing that the cation expulsion may not apply for polythiophenes. Instead, the mass versus charge curves indicated that the anion transport is coupled with opposite solvent movement. That was concluded from the value of the virtual stoichiometry definitely lower than one anion/electron, obtained from the mass versus charge relationships. On that basis a mechanism based on the distinction of solvated and non-solvated segments of the neutral polymer was assumed. In order to get further evidence for the previous conclusions [17], we decided to perturb the solvation properties of the film, and to find some opportunity for the modification of the ratio between the different (solvated and non-solvated) parts of the layer. It is assumed that it could be achieved by the polymerisation of substituted thiophene monomers, having a polar group. One possibility could be a self-dopable polymer possessing a carboxylic group [18–22]. The main goal of the present study is to get further information, which completes the previous results. In order to connect the results with those obtained previously, studies on co-polymers of 3-methylthiophene with 3-thiophene-acetic acid were also included, and some experiments also were extended to a non-dissociating (aprotonic) co-polymer with diisopropyl 2-(3thienyl)ethylphosphonate (TEPE). During the work EQCM studies were coupled with ISC and spectroelectrochemical measurements.
2. Experimental The experimental setup was already described previously [7,17,23]. The EQCM measurements were performed using a quartz crystal analyser (EG&G Seiko, model QCA917). The crystals ( f0 =8.9 MHz) were Pt-coated and had a surface area of A=0.196 cm2. The surface area of the double band electrode for ISC measurements was 5 ×10 − 3 cm2, with a gap of 2.5 mm in between. The signal of a 130 Hz reference sine wave was measured by a lock-in amplifier (Stanford Research Instruments, model SR830). The spectra were acquired with a diode-array spectrophotometer (Hewlett-Packard, HP-8452A). The electrochemical measurements
were carried out on EG&G PAR (model 283), AUTOLAB (PGSTAT10) and/or Elektroflex (type 452) potentiostat/galvanostats and were controlled by a PC. Tetrabutylammonium hexafluorophosphate, Bu4NPF6, 3-thiophene-acetic acid, TAA and 3-methylthiophene, MT (all purchased from Aldrich) were used as received, acetonitrile, AN (Aldrich) was dried over activated alumina and kept under an argon atmosphere. The water content of the solutions were controlled by coulometric Karl Fischer titration (Metrohm 652), and it was kept below 30 ppm. The synthesis of the phosphonic acid ester (TEPE) will be described elsewhere. The polymerisation of the different monomer solutions was carried out in 0.2 M Bu4NPF6 in AN by potentiostatic method at 1.8 V potential value versus an Ag/0.01 M AgNO3 + 0.2 M Bu4NPF6 in AN reference electrode. The concentration of solution was 0.3 M for TAA and this overall monomer concentration was preserved for the different (1:3 and 1:5) TAA/MT mixtures. TEPE/MT concentration ratio was 1:5 using a solution of 0.02 M and 0.1 M for the components, respectively. The thickness of the films was controlled by measuring the charge transferred during the polymerisation. The typical charge density applied for the double band electrode was 2 – 10 C/cm2, while for EQCM and optical measurements, the charge density was 50 – 150 mC/cm2. After the electropolymerisation the monomer containing solution was changed for further studies to 0.2 M Bu4NPF6 in AN under argon blanket.
3. Results and discussion
3.1. Electropolymerisation of TAA and TAA/MT mixtures Fig. 1 shows the accumulation of the deposited polymer on the electrode obtained by EQCM in the case of TAA and its co-polymerisation with three equivalents of MT. As it could be suspected on the basis of some previous work [21], the polymerisation of the TAA alone (curve 1) is problematic, as it needs a rather positive potential and large monomer concentration. In the cases of the co-polymerisation of TAA and MT (curve 2), the amount of the film deposited also during 10 s under the same conditions is definitely larger by a factor of 2.5. The thin poly (TAA) film obtained during the polymerisation is slightly electroactive in the base solution (Fig. 2a). The figure shows the well-known first scan effect, and the shape of the two subsequent voltammetric curves is rather asymmetric, resembling those registered with polyterthiophene [24], in such case the thin film consisted mainly of dimers i.e. from six thiophenic units. This small electroactivity shows up in Fig. 2b as well, where the mass versus potential curves
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obtained parallel with cyclic voltammetry are presented. The mass change of the film during the oxidation is small, at the same time a positive mass shift during the cycling — typical for the systems included in this paper — is observable. An explanation of the relatively unsuccessful polymerisation of the TAA can be seen in Fig. 3, where the results obtained during the polymerisation onto the double band electrode of the ISC method are presented. Since the conductance data for TAA are shown on a 20 times enlarged scale, the figure illustrates very well that the TAA does not form a well-conducting film. The final conductance is only some percentages of the value obtained for the co-polymer with MT. The plot is made according to the relationship derived for double band electrodes for the determination of the conductivity of the in situ forming film [7]. However, even at large MT:TAA ratio, the curve can hardly be considered to be linear in this plot, as it should be theoretically. This may indicate that the continuous deposition is disturbed by the presence of different monomers, and perhaps the thickness/gap width ratio is not large enough although the charge consumed for the film formation is big. Otherwise, the conductivity s can be calculated by applying the linear regression method by using the following formula [7] sl p where, l is the length of the double band electrodes.
slope =
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With MT alone, the well-established linear section was obtained after an induction period with a slope of 1.34 S, from which a conductivity s= 21.0 S/cm value was calculated. For comparison, conductivity values in the range of 10 − 5 S/cm were reported for TAA and its cross-linked co-polymers [25] in a recent paper.
3.2. EQCM measurements on TAA/MT co-polymers The problems presented above led us to make the further experiments with TAA/MT co-polymers. Results with our film prepared at 1:3 (TAA:MT) monomer ratio, obtained by EQCM technique during cyclic voltammetric measurements are presented in Figs. 4 and 5. Throughout the study double cycles are presented. The first scan shows the ‘memory effect’, but the second scan could be always considered as the stationary one. First the scans were run in a restricted potential range. Fig. 4a shows that scanning between −0.4 and +0.85 V, there is only one oxidation peak, as usual, and no manifestation of the splitting of the oxidation peak is observed. Coupled EQCM results (Fig. 4b) show that the mass change of the film is positive and monotonous. It is fast and its extent is almost independent of the scan rate, only the hysteresis is smaller at slower scans. It can be rather clearly seen that during the reduction of the film the mass decrease is linearly related to the potential, and at scan rates 6B 100 mV/s the film regains its starting mass at the same potential. By extending the potential scan to a
Fig. 1. Mass accumulation of the polymeric film during the potentiostatic (E =1.8 V) polymerisation of TAA (1) and a 1:3 mixture of TAA and MT (2) in 0.2 M Bu4NPF6 in AN solution.
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Fig. 2. (a) Double voltammetric curve registered with PTAA film in 0.2 M Bu4NPF6 in AN solution at 6 = 100 mV/s sweep rate. (b) Mass change of the film during the CV obtained from EQCM measurement.
more positive region, the appearance of a prepeak could be revealed. It seemed to become detectable at the very beginning of the oxidation (at around 200 mV) and at fast scan rates. The frequency curves showed practically no mass changes coupled with the electrochemical process in this region. When a still wider potential window is applied, the prepeak becomes higher (Fig. 5a). The mass versus potential curves (Fig. 5b) have the same pattern as previously, and indicate
only a larger anion incorporation during the cycles. However, if we compare Fig. 4b and Fig. 5b, a gradual shift can be revealed, which shows that the mass of the film is increasing during or between the scans. By transforming the data, we can obtain the mass change versus charge relationship, which holds a very important information, i.e. how the mass accumulation in the film is related to the electron transfer. The curves obtained this way are almost linear to a certainly
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observable S-shape. In Fig. 6 the data measured at 6=10 mV/s scan rate are used in the calculation of the virtual stoichiometric number for the moving hexafluorophosphate ion. Applying linear regression method, the curve during the oxidation section of the scan delivered a slope of 1.17×10 − 3 mg/mC. From this value, the virtual relative molar mass of the monovalent anion is 113, that means 78% of the value for the hexafluorophosphate. This value was 85% in the case of the pure MT for the final oxidation region. The reliability of the measurements could be based on the constant admittance registered, also several times during the experiments, which exhibited a very small (within 1%) admittance change. From the value definitely smaller than one for the stoichiometric number per electron, it is assumed that the anion migration is accompanied with the sorption–desorption of some other — ionic or neutral — species, which moves in the opposite direction. A very similar behaviour of the co-polymer could be observed with films obtained at a different monomer ratio. Voltammetric curves registered with a film of 1:5 ratio are presented at an enlarged scaling in Fig. 7, where a well-expressed prepeak can be observed at 300 mV/s sweep rate. Another important feature of the curves developing with decreasing sweep rates should also be emphasised; the second oxidation process and the oppositely positioned cathodic step are not in coupled redox pair relation, since the reduction peak is at a more positive potential than the oxidation peak. If we compare
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the different voltammetric curves presented here and in previous communications [17,26], we can come to the conclusion that the more time the film is spending beyond the oxidation peak the more definitely the peak potentials are ‘oppositely’ positioned. This fact might support the assumption of some kind of phase transition in the oxidised state. The pattern of the mass change of the film during one cycle was about the same as in the previous case, and the gradual mass shift of the film was also visible. However after several cycles, the prepeak could be seen on the voltammetric curve even at 50 mV s − 1, meaning that the slow mass accumulation observed during long time cyclings to more positive anodic potentials is coupled with changes causing or manifesting themselves in the prepeak.
3.3. ISC measurements on MT/TAA co-polymers In the case of the preparation of polymeric films on the double band electrode, a larger charge density was applied, resulting in the formation of thick layers. With similar thick films, the splitting of the anodic peak observed at low sweep rates was already presented for MT [17], and there was a strong evidence that the film started to be transformed into the macroscopically conducting form only during the second oxidation peak. In the case of the thick co-polymeric films, splitting in the form of a shoulder (see later e.g. in Fig. 8a) was rather
Fig. 3. ISC changes versus the logarithm of the charge during the potentiostatic (E = 1.8 V) polymerisation of TAA (1), a 1:3 mixture (2) and a 1:5 mixture (3) of TAA and MT in 0.2 M Bu4NPF6 in AN solution. For curve (1) the conductance scale is 20 times enlarged.
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Fig. 4. (a) Double voltammetric scans registered with the 1:3 TAA/MT co-polymer film (Q =100 mC cm − 2) in the potential range between − 0.4 and + 0.85 V at different sweep rates indicated on the curves. (b) Concurrently registered mass changes of the film during the CV – s.
clear from the very beginning of the voltammetric measurements in the whole studied sweep rate region between 2.5 and 100 mV s − 1. ISC data showed that these films also started to transform into the conductive form only at potentials beyond the prepeak. At the same time the evolution of the conductance curves was deeply affected by the sweep rate; the larger conductance was achieved at slow scans. In this study, only the results with
1:3 co-polymer — connected to the mass accumulation presented in the previous section — are reported. Since the mass accumulation in the film can be coupled with the prepeak which in turn has an effect on the insulator – conductor transformation, in this section of the experiments some voltammetric results and the concurrent ISC data are presented. The appearance of similar prepeaks was coupled with
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cathodic doping in several cases [27,28]. During this series of measurement we intended to see the effect of the more and more cathodic prepolarisation on the splitted oxidation peak. For that purpose double cycles were run. In order to insure the identical state, the identical ‘history’ of the film at the beginning of each measurements, the first sweep of double scans was started always from −0.4 V (just as previously). Thus,
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the second scans were started from different cathodic startpoints, but this gradually enlarged cathodic region was reached after an identical first scan, so the ‘history’ of the second scans was otherwise the same. Fig. 8a proves that with increasing negative potentials there is no cathodic process, the cathodic ‘doping’ — if it exists with tetrabutylammonium ions at all — takes place at more negative potentials. At the same time the oxida-
Fig. 5. (a) Double voltammetric scans registered with the 1:3 TAA/MT co-polymer film (Q =100 mC cm − 2) in the potential range between −0.4 and + 1.2 V at sweep rates 10, 25, 50, 100 and 200, respectively. (b) Concurrently registered mass changes of the film during the CV – s.
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Fig. 6. Mass – charge relationship during the oxidation of 1:3 TAA/MT co-polymer film (Q = 100 mC cm − 2) at a scan rate of 6 =10 mV/s. The results of the linear regression fitting are shown as an insert.
Fig. 7. Enlarged current –potential relationships registered during double scans with the 1:5 TAA/MT co-polymer film (Q =100 mC/cm2) between − 0.4 and +1.3 V at sweep rates 10, 25, 50, 100, 200, and 300 mV s − 1, respectively.
tion is delayed by the pretreatment at more negative potentials, and the first oxidation step tends to disappear, or not to develop. ISC measurements presented in
Fig. 8b show the concurrent delay of the evolution of the conductive state. These observations show that non-electrochemical changes occurring at these negative
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potentials inhibit the realisation of the oxidation, and the lack of prepeak hinders the insulator–conductor transformation. When we compare and summarise the results obtained here by different in situ methods, we can state that the occurrence of the splitting of the oxidation process depends on some non-electrochemical process. If the
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prepeak is not seen separately, or it is delayed, the film — though oxidised — is very far from being conductive. Thus, the non-electrochemical process in the negative potential region inhibits the insulator – conductor transformation. At the same time the EQCM data show a monotonous mass increase during the oxidation, and do not reveal any mass change, which
Fig. 8. (a) The second scans of double sweeps recorded at 50 mV s − 1 sweep rate with 1:5 TAA/MT co-polymer film (Q= 2 C cm − 2) deposited for ISC measurements. The first scan of the double sweeps was started always from − 0.4 V, and the cathodic turnpoint was − 1.0, −1.3, − 1.5, − 1.8 and − 2.0 V, respectively (this order is indicated by the numbers). (b) Concurrently registered conductance changes. The numbering increases with the increase in the cathodic turnpoint potential.
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would indicate cation movement. The involvement of cations is even less probable, if we consider that negative potentials — which had to be in favour of the cation incorporation — led to the disappearance of the anodic prepeak and to the postponed transformation of the film into the conductive form. Thus, in the case of thiophene type polymers, the movement of the cation can hardly be responsible for the splitting of the oxidation wave, as it was reported in the case of the polypyrrole/ Li-perchlorate film [16]. The fact that the mass of PF− 6 ion is less than 2/3 of that of the cation makes the involvement of ion pair movement also less probable, which movement would be masked only at a very special ion participation. So we may conclude that there is no cationic doping in the potential range covered here, and the Bu4N+ ion is not incorporating with the film. The only cation, which has not been taken into consideration yet, is the proton, which might be involved in the process, since carboxylic acid groups can be in the film. Their movement would cause very little frequency change so that the small mass change from proton movement may be masked. The permanent pattern of the splitting of the oxidation process is against the assumption of proton doping/undoping, since the expelled protons would represent a very small concentration in the bulk solution, so their reversible movement is not probable. However, it is possible that the dissociating protons are not removed from the film. Therefore, in order to exclude the opportunity of the manifestation of this self-doping of the TAA component of the co-polymer, we studied another co-polymer of MT also, with TEPE, where no dissociating protons are available. In this study, only the most important and relevant results are presented.
3.4. MT/TEPE co-polymers The polythiophene film obtained by electropolymerising, a mixture of 5:1 ratio (MT:TEPE) exhibited a voltammetric behaviour basically similar to those of the co-polymers with TAA, and an EQCM pattern rather similar to pure MT. As shown in Fig. 9a, the prepeak appears even at 6=10 mV s − 1 sweep rate, and it was observable at each sweep rate in the range of 10–100 mV − 1. The splitting is obtained with thin films, too. The prepeak is larger, and there was no need for any kind of treatment to develop this peak splitting. Here again we call the attention that the position of the second oxidation/reduction peaks during the redox process excludes that they form a simple redox pair. This shift in the peak potentials, which develops with the increasing time period spent at potentials beyond the second anodic peak, might suggest again the occurrence of a kind of phase transition taking place in the very oxidised state (see Fig. 4a, Fig. 5a and Fig. 7).
The behaviour of the mass change during the voltammetric curve had two characteristics, (i) there was no sign of any mass decrease during the oxidation; and (ii) there was a gradual shift of the starting value of the curves, which indicated mass accumulation in the film. The admittance curve, not presented here, proved that the film preserved its rigidity during the measurements. Current and frequency data were transformed to mass change versus charge curves. As Fig. 9b shows, the mass of the film monotonously increases with the charge but the curve has a very expressed S-shape. From the global slope delivered by linear regression, a relative equivalent mass of 65.5 is obtained as an average, which means a virtual stoichiometric number 0.45 for the anion. This linear regression is a rather rough fitting since this value seems to be even smaller at the very beginning of the oxidation. The two section pattern was previously shown for the poly(3-methylthiophene) itself [17], where the peak splitting of the anodic current did not show up. By potential step method combined with EQCM measurement, a better linearity of the m –Q curve throughout the whole insulator – conductor conversion was obtained, which might indicate that the faster perturbation masks together the distinct processes, so that the partially consecutive processes starting at slightly different potentials occur parallelly. As Fig. 10 shows, the mass of the film follows a linearity versus the charge integrated during the potential step from − 0.4 to + 1.3 V (the values applied as the anodic and cathodic turn-potential in the voltammetry). The slope is practically the same as before, and recalculation results in a virtual stoichiometric number of 0.46. The ‘deficit of mass’ is 78 g mol − 1, which is approximately the double of the molar mass of acetonitrile solvent. On the basis of the close similarities between the behaviour of this co-polymer having no dissociating hydrogen and self-dopable thiophene co-polymers, these results can be considered as proof for that the prepeak does not originate from proton release.
3.5. In situ spectroelectrochemical measurements In order to have a complete information on the redox transformation of the polymeric film, it is especially of great importance to know what kind of process takes place during the prepeak. Owing to the separation of the two anodic peaks at low sweep rates, the TEPE/MT film gives the opportunity to record the optical spectra in the two oxidation regions separately. Thus, as a next step we studied the spectroscopic changes of the MT/TEPE film during a slow cyclic voltammetric transformation. Our special purpose was to register the optical spectra in parallel to the first peak, since the similarly appearing prepeaks have been associated with previous cathodic doping [27,28]. If it were the reoxidation of cathodically conducting form, the spectral changes in this region would be opposite to those coupled with the further
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Fig. 9. (a) Double voltammetric curves registered with the 1:5 TEPE/MT co-polymer film (Q =150 mC/cm2) at 10 mV s − 1 sweep rate. (b) Concurrent mass changes vs. the charge. The insert shows the results obtained by linear regression.
oxidation. Spectral changes of the film during the cyclic voltammetric oxidation at a sweep rate of 5 mV s − 1 are presented in Fig. 11. As it can be seen, the maximum absorbance of the neutral film is around 500 nm, which indicates a relatively good conjugation in the co-polymeric film. The spectral change is monotonous and it exhibits even during the first peak a small but clear absorbance increase in the 650–820 nm wavelength range, where the absorbance associated with the forma-
tion of the oxidised form can be detected. By that way we can exclude the manifestation of a delayed reoxidtion, a cationic ‘undoping’, and we may evidence the ‘doping’ type of the current during the prepeak. We recollect that during the main part of the anionic doping, the mass versus charge curves showed an approximately linear behaviour in case of pure and co-polymeric thiophene films, but with a slope smaller than the theoretical one. From all these facts the involvement
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of the solvent can be deduced, and the gradual mass increase registered in the series of Figs. 4 and 5 can be connected to solvent accumulation. This solvent adsorption can be referred to structural changes occurring even at negative potentials where there is no Faradaic process or at open circuit [29,30]. However, the solvent can
penetrate only into a film of favourable structure. With PMT and its co-polymers with TAA this structure develops only by extending scanning into the anodic potential region. In the case of the TEPE/MT co-polymer the structure of the film makes solvent penetration easier without any
Fig. 10. Mass vs. charge relationship obtained during a potential step from −0.4 to + 1.3 V with the 1:5 TEPE/MT film. The insert shows the results obtained by linear regression.
Fig. 11. Spectral changes during the voltammetric oxidation of the 1:5 TEPE/MT film (Q =150 mC cm − 2) registered at 5 mV s − 1 sweep rate. The arrows indicate the direction of the changes during the oxidation of the film.
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treatment. The oxidation of a more solvated film is coupled with the removal of more solvent, leading to a smaller global mass increase during the oxidation process. That gives an even smaller virtual stoichiometric number for the anion in this case. Thus, the observations give further evidence to improve the previous conclusions [17], namely that solvent removal accompanies the oxidation of the film. The deficit of mass at the m–Q curves can be interpreted by the concurrent removal of the solvent during the anion adsorption of the film, and the ‘missing mass’ can be translated to the removal of 1–2 solvent molecule per one incorporated anion. This solvent expulsion is induced by the anion penetration upon the start of the charge transfer. However, it is assumed that the solvent is partially ‘trapped’ within the films, mainly at deeper places of thicker layers. This entrapment is more expressed after a negative polarisation, and it results in a delayed solvent removal (Fig. 8). In order to describe the phenomenon, the following scheme is assumed, the oxidation of the neutral, nonconducting film starts from the metal–polymer interface. It consists of two parts: the transformation of solvated and non-solvated segments. Since the slope of the mass versus charge curves is approximately constant, the oxidation processes are overlapping and they take place parallelly. When we apply relatively slow potential changes, we can detect that the oxidation of the solvated segments starts slightly before, as the slope is smaller at the beginning of the transformation. These oxidised segments created by the charge transfer are first far away from the film–solution interface. The partially localised positive charges created in both processes generate the flux of the charge balancing anions from the surface into these places. Since the anion influx comes from the solution phase into the polymer, non-solvated, assumingly more mobile carriers move to the polymer–solution interface, but the solvent coupled with oxidised polymeric segments might be trapped in the layer. By that way the charge transfer processes create first a film which contains shielding solvent molecules. In this state the layer is not conducting because mobile, non-localised holes have moved to the polymer–solution interface, and more localised, solvated charge carriers are separated from each other. In order to get the highly conducting quasi-metallic state, the interaction between the conducting islands must be insured, and for that solvent molecules have to be removed. It is ruled by the effect of the electrical field, which is continuously increasing with the potential during the CV or with the decreasing thickness of the non-conducting layer in case of potential steps. Accompanied with the replacement of solvent molecules and the repositioning of anions, the film reaches a more and more favourable structure leading finally to the highly conducting, quasi-metallic state.
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These processes taking place during the electrochemical transformation of thiophene type polymeric films can be formulated as starting with the oxidation of solvated units coupled with the anion incorporation − − Psol + A− [ [P+ solA ] +e
(1)
followed at a slightly more positive potential by the similar oxidation of the non-solvated units. P+ A− [ P+A− + e−
(2)
The non-solvated charge carriers can freely interact, but the complex formed in Eq. (1) being not the most stable form of the oxidised polymer transforms, ruled by the electrical field: n 2+ − − (3) n[P+ solA ] [ [P2 2A ] +n sol 2 During this non-faradaic reorganisation step, the exchange of the solvent and the anion takes place, and the concurrent removal of the solvent leads to the non-integer value obtained for the virtual stoichiometric number of the anion. During this slow, elongated chemical process controlled by the actual electrical field, the intraand/or inter-chain interactions between previously separated paramagnetic segments may develop, leading to a high level of conjugation. This is indicated by the formulation of the product as a dimer. So by the removal of the interstitial solvent molecules, the film may reach a much more integrated phase, and can show a quasimetallic behaviour. The identification of the solvent removal as a chemical – desolvation — step may explain the surprising peak positions in Figs. 5, 7 and 9. If the layer is thin or its structure and character prevents it from solvent adsorption, the simple dimerisation type chemical step (3) can follow the electrochemical one (1) without any delay. In these cases, the formation of the charge carriers and their interacting combination are not separated from each other in time. Thus, the transformation into the well-conducting state is not prolonged, and the process gives a relatively sharp, single anodic peak. That might refer to the case of some polythiophenes having a more open structure presented in a recent study [31]. This limiting case might give the theoretical slope of the mass versus charge curve. Solvent and cation incorporation into the neutralised film disturbes this ‘ideal’ case, and leads to the appearance of peak splitting and/or to different m–Q curves.
4. Conclusions In this study, it was demonstrated that the solvation of the film could be modified through co-polymerisation which affected the voltammetric and the EQCM behaviour of the films. The double step pattern of the voltammetric oxidation of thiophene films has gained
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further support by studying different thiophene co-polymers. The redox transformation of the film was followed by different in situ techniques. In contrast to polypyrroles, where the two oxidation steps were coupled with cation and anion movements, respectively, the EQCM results did not show any mass decrease troughout the whole oxidation region. The ISC measurements exhibited a close relationship between the detection of the prepeak and the easy realisation of the transformation to the conducting state: if the film exhibits the well separating prepeak, the macroscopic conductance is developing easily. The negative pre-polarisation, although without current flow, delays the first step, and as a consequence, the development of the conducting state is also hindered. On the basis of the results obtained by the different methods, it is concluded that the incorporation of cations and the involvement of the ion pairs in the oxidation–reduction process is improbable. The involvement of protons originating from the dissociation of the TAA was excluded by doing the analogous measurements with MT/TEPE, an ester type co-polymer with the MT, which delivered essentially the same results, although no dissociating hydrogen was available there. On the basis of the results it is assumed that the electrochemical transformation of thiophene type polymeric film contains basically parallel processes. The oxidation of the solvated units and that of the non-solvated parts takes place at slightly different potentials. The solvated oxidised complex is not the most stable form of the charge carriers in the polymer, and it transforms to its stable form, ruled by the electrical field. During this non-Faradaic reorganisation step, the slow exchange of the solvent and the anion takes place, and the concurrent removal of the solvent leads to the non-integer value obtained for the virtual stoichiometric number of the anion. This slow chemical process involves also the intra- and/or inter-chain interactions between previously separated paramagnetic segments, leading to a high level of conjugation, and the quasi-metallic phase.
[2]
[3] [4] [5] [6] [7] [8] [9] [10] [11]
[12] [13] [14] [15] [16] [17] [18] [19] [20]
[21] [22] [23]
Acknowledgements
[24]
This work has been supported by the Hungarian National Scientific Research Fund (OTKA no. T 016017 and 030083), as well as by the Hungarian Ministry of Education (FKFP no. 0733) and by the Academy of Finland (grant no. 30579).
[25] [26] [27] [28] [29]
References [30] [1] X. Yu, in: T.A. Skotheim, R.L. Elsenbaumer, J.R. Reynolds (Eds.), Handbook of Conducting Polymers, IV. Properties
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of Conducting Polymers, second ed., Marcel Dekker, New York, 1998, pp. 531 – 822. K. Doblhofer, K. Rajeshwar, in: T.A. Skothcim, R.L. Elsenbaumer, J.R. Reynolds (Eds.), Handbook of Conducting Polymers, IV. Properties of Conducting Polymers, second ed., Marcel Dekker, New York, 1998, pp. 531 – 588. B.J. Feldman, P. Burgmayer, R.W. Murray, J. Am. Chem. Soc. 107 (1985) 872. L. Olmedo, I. Chonteloub, A. Germain, M. Petit, E.M. Genies, Synth. Met. 30 (1989) 159. R. John, A. Talaie, G.G. Wallace, S. Fletcher, J. Electroanal. Chem. 319 (1991) 365. J. Ochmanska, P.G. Pickup, J. Electroanal. Chem. 297 (1991) 197 and 211. J. Kankare, E.-L. Kupila, J. Electroanal. Chem. 322 (1992) 167. G. Zotti, A. Berlin, G. Pagani, G. Schiavon, S. Zecchin, Adv. Mater. 7 (1995) 48. J.N. Kaufman, K.K. Kanazawa, G.B. Street, Phys. Rev. Lett. 53 (1984) 2461. J.R. Reynolds, N.S. Sundaresan, M. Pomerantz, S. Basak, C.K. Baker, J. Electroanal. Chem. 250 (1988) 355. A.R. Hillman, D.C. Loveday, M.J. Swann, R.M. Eales, A. Hammett, S.J. Higgins, S. Bruckenstein, C.P. Wilde, Faraday Discuss. Chem. Soc. 88 (1989) 151. G. Inzelt, J. Electroanal. Chem. 287 (1990) 171. C.S.C. Bose, S. Basak, K. Rajeshwar, J. Phys. Chem. 96 (1992) 9899. V.M. Schmidt, J. Heitbaum, Electrochim. Acta 38 (1993) 349. M.A. Vorotyntsev, E. Vieil, J. Heinze, Electrochim. Acta 31 (1996) 1913. M.A. Vorotyntsev, E. Vieil, J. Heinze, J. Electroanal. Chem. 450 (1998) 121. C.S. Visy, J. Kankare, J. Electroanal. Chem. 442 (1998) 175. A.O. Patil, Y. Ikenoue, F. Wudl, A.J. Heeger, J. Am. Chem. Soc. 109 (1987) 1858. E.E. Havinga, L.W. Van Horsen, Macromol. Chem. Macromol. Symp. 24 (1989) 67. P. Ba¨uerle, K.W. Gaudi, F. Wurthner, S. Sariciftci, H. Neugebauer, M. Mehring, C. Zhong, K. Doblhofer, Adv. Mater. 2 (1990) 490. W.J. Albery, F. Li, A.R. Mount, J. Electroanal. Chem. 310 (1991) 239. P.N. Bartlett, D.H. Dawson, J. Mater. Chem. 4 (1994) 1805. C.S. Visy, E. Kriva´n, G. Peintler, J. Electroanal. Chem. 462 (1999) 1. C.S. Visy, J. Lukkari, J. Kankare, Macromolecules 27 (1994) 3322. D.J. Liaw, B.Y. Liaw, J.P. Gong, Y. Osada, Synth. Met. 99 (1999) 53. R.H.G. Schmitz, K. Ju¨ttner, Electrochim Acta 44 (1999) 1627. M. Onoda, H. Nakayama, S. Morita, K. Yoshino, J. Electrochem. Soc. 141 (1994) 338. R. Borjas, D.A. Buttry, Chem. Mater. 3 (1991) 872. A.R. Hillman, M.J. Swann, S. Bruckenstein, J. Phys. Chem. 95 (1991) 3271. A.R. Hillman, S. Bruckenstein, J. Chem. Soc. Faraday Trans. 89 (1993) 339. G. Zotti, Synth. Met. 97 (1998) 267.
EQCM and in situ conductance studies on the polymerisation and redox features of thiophene co-polymers Csaba Visy a,*, Jouko Kankare b, Emese Kriva´n a a
Institute of Physical Chemistry, Jo´zsef Attila Uni6ersity, P.O. Box 105, H-6701 Szeged, Hungary b Department of Chemistry, Uni6ersity of Turku, Turku, Fin-20 014, Finland Received 19 July 1999; received in revised form 9 November 1999
Abstract 3-Thiophene-acetic acid and 3-methylthiophene co-polymers have been prepared with the consideration that the solvation behaviour of the film could be modified. The modification is coupled with the observed splitting of the oxidation peak on the cyclic voltammograms. The redox transformation of the films was studied by electrochemical quartz crystal microbalance (EQCM) and in situ conductance (ISC) techniques. Mass changes during the electrochemical processes showed that the cation is not involved into the charge balancing of the film. Manifestation of self-doping was excluded by analogous measurements with an aprotic co-polymer of methylthiophene. Results completed with in situ conductance and spectroelectrochemical observations confirmed the assumption of a chemical step taking place after the first oxidation. In this process desolvation of the oxidised intermediate occurs, which is a necessary step for the achievement of the quasi-metallic state. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Co-polymer; 3-Methylthiophene; Electrochemical quartz crystal microbalance; Conductance; Solvation
1. Introduction In situ methods used in the study of conducting polymers deliver indispensable information about the details of their formation and redox transformations [1]. Thus, combined methods such as UV-vis, FTIR and ESR spectroscopy, mirage effect or laser beam deflection (LBD), in situ conductance (ISC) and electrochemical quartz crystal microbalance (EQCM) are widely used to complete the results obtained by merely electrochemical techniques [2]. In the case of ISC measurements [3–8], the essential behaviour of the conducting polymers, i.e. how their macroscopic insulator/conductor transformation is related to the electrochemical process, can be resolved. With EQCM technique [9–14] the global mass flux of differently * Corresponding author. Fax: +36-62-544652. E-mail address: [email protected] (C. Visy).
charged ions and that of the solvent, coupled with the charge transfer can be followed. In recent studies [15,16], the charging process of polypyrrole was studied by EQCM technique, and the resolution of the generally observed single, broad oxidation current maximum was reported. The splitting could be seen at low bulk concentration of the anion, small enough to be able to penetrate into the film. The phenomenon was successfully simulated by distinguishing fixed and releasable anions in the film. Based on EQCM results, two electrochemical processes were related to the same electrochemical process coupled with the opposite movement of differently charged ions. Thus, the observation of the peak splitting as the manifestation of the ‘classical’ and widely accepted scheme of successive polaron and bipolaron formation was questioned. In an earlier study, the splitting of the anodic current peak in the case of poly(3-methylthiophene) was al-
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ready reported [17]. The electrochemical processes were followed by both EQCM and ISC measurements, and the necessity of the modification of the traditional mechanism was already concluded by making partly parallel electrochemical steps responsible for the insulating/conducting transformation. It was demonstrated that the macroscopic conductance starts to develop only during the second anodic peak. So there is an electrochemical analogy between thiophene and pyrrole polymers. However, EQCM curves in reference [17] did not show any sign of a mass decrease, clearly visible in the case of PPY/LiClO4 [16], showing that the cation expulsion may not apply for polythiophenes. Instead, the mass versus charge curves indicated that the anion transport is coupled with opposite solvent movement. That was concluded from the value of the virtual stoichiometry definitely lower than one anion/electron, obtained from the mass versus charge relationships. On that basis a mechanism based on the distinction of solvated and non-solvated segments of the neutral polymer was assumed. In order to get further evidence for the previous conclusions [17], we decided to perturb the solvation properties of the film, and to find some opportunity for the modification of the ratio between the different (solvated and non-solvated) parts of the layer. It is assumed that it could be achieved by the polymerisation of substituted thiophene monomers, having a polar group. One possibility could be a self-dopable polymer possessing a carboxylic group [18–22]. The main goal of the present study is to get further information, which completes the previous results. In order to connect the results with those obtained previously, studies on co-polymers of 3-methylthiophene with 3-thiophene-acetic acid were also included, and some experiments also were extended to a non-dissociating (aprotonic) co-polymer with diisopropyl 2-(3thienyl)ethylphosphonate (TEPE). During the work EQCM studies were coupled with ISC and spectroelectrochemical measurements.
2. Experimental The experimental setup was already described previously [7,17,23]. The EQCM measurements were performed using a quartz crystal analyser (EG&G Seiko, model QCA917). The crystals ( f0 =8.9 MHz) were Pt-coated and had a surface area of A=0.196 cm2. The surface area of the double band electrode for ISC measurements was 5 ×10 − 3 cm2, with a gap of 2.5 mm in between. The signal of a 130 Hz reference sine wave was measured by a lock-in amplifier (Stanford Research Instruments, model SR830). The spectra were acquired with a diode-array spectrophotometer (Hewlett-Packard, HP-8452A). The electrochemical measurements
were carried out on EG&G PAR (model 283), AUTOLAB (PGSTAT10) and/or Elektroflex (type 452) potentiostat/galvanostats and were controlled by a PC. Tetrabutylammonium hexafluorophosphate, Bu4NPF6, 3-thiophene-acetic acid, TAA and 3-methylthiophene, MT (all purchased from Aldrich) were used as received, acetonitrile, AN (Aldrich) was dried over activated alumina and kept under an argon atmosphere. The water content of the solutions were controlled by coulometric Karl Fischer titration (Metrohm 652), and it was kept below 30 ppm. The synthesis of the phosphonic acid ester (TEPE) will be described elsewhere. The polymerisation of the different monomer solutions was carried out in 0.2 M Bu4NPF6 in AN by potentiostatic method at 1.8 V potential value versus an Ag/0.01 M AgNO3 + 0.2 M Bu4NPF6 in AN reference electrode. The concentration of solution was 0.3 M for TAA and this overall monomer concentration was preserved for the different (1:3 and 1:5) TAA/MT mixtures. TEPE/MT concentration ratio was 1:5 using a solution of 0.02 M and 0.1 M for the components, respectively. The thickness of the films was controlled by measuring the charge transferred during the polymerisation. The typical charge density applied for the double band electrode was 2 – 10 C/cm2, while for EQCM and optical measurements, the charge density was 50 – 150 mC/cm2. After the electropolymerisation the monomer containing solution was changed for further studies to 0.2 M Bu4NPF6 in AN under argon blanket.
3. Results and discussion
3.1. Electropolymerisation of TAA and TAA/MT mixtures Fig. 1 shows the accumulation of the deposited polymer on the electrode obtained by EQCM in the case of TAA and its co-polymerisation with three equivalents of MT. As it could be suspected on the basis of some previous work [21], the polymerisation of the TAA alone (curve 1) is problematic, as it needs a rather positive potential and large monomer concentration. In the cases of the co-polymerisation of TAA and MT (curve 2), the amount of the film deposited also during 10 s under the same conditions is definitely larger by a factor of 2.5. The thin poly (TAA) film obtained during the polymerisation is slightly electroactive in the base solution (Fig. 2a). The figure shows the well-known first scan effect, and the shape of the two subsequent voltammetric curves is rather asymmetric, resembling those registered with polyterthiophene [24], in such case the thin film consisted mainly of dimers i.e. from six thiophenic units. This small electroactivity shows up in Fig. 2b as well, where the mass versus potential curves
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obtained parallel with cyclic voltammetry are presented. The mass change of the film during the oxidation is small, at the same time a positive mass shift during the cycling — typical for the systems included in this paper — is observable. An explanation of the relatively unsuccessful polymerisation of the TAA can be seen in Fig. 3, where the results obtained during the polymerisation onto the double band electrode of the ISC method are presented. Since the conductance data for TAA are shown on a 20 times enlarged scale, the figure illustrates very well that the TAA does not form a well-conducting film. The final conductance is only some percentages of the value obtained for the co-polymer with MT. The plot is made according to the relationship derived for double band electrodes for the determination of the conductivity of the in situ forming film [7]. However, even at large MT:TAA ratio, the curve can hardly be considered to be linear in this plot, as it should be theoretically. This may indicate that the continuous deposition is disturbed by the presence of different monomers, and perhaps the thickness/gap width ratio is not large enough although the charge consumed for the film formation is big. Otherwise, the conductivity s can be calculated by applying the linear regression method by using the following formula [7] sl p where, l is the length of the double band electrodes.
slope =
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With MT alone, the well-established linear section was obtained after an induction period with a slope of 1.34 S, from which a conductivity s= 21.0 S/cm value was calculated. For comparison, conductivity values in the range of 10 − 5 S/cm were reported for TAA and its cross-linked co-polymers [25] in a recent paper.
3.2. EQCM measurements on TAA/MT co-polymers The problems presented above led us to make the further experiments with TAA/MT co-polymers. Results with our film prepared at 1:3 (TAA:MT) monomer ratio, obtained by EQCM technique during cyclic voltammetric measurements are presented in Figs. 4 and 5. Throughout the study double cycles are presented. The first scan shows the ‘memory effect’, but the second scan could be always considered as the stationary one. First the scans were run in a restricted potential range. Fig. 4a shows that scanning between −0.4 and +0.85 V, there is only one oxidation peak, as usual, and no manifestation of the splitting of the oxidation peak is observed. Coupled EQCM results (Fig. 4b) show that the mass change of the film is positive and monotonous. It is fast and its extent is almost independent of the scan rate, only the hysteresis is smaller at slower scans. It can be rather clearly seen that during the reduction of the film the mass decrease is linearly related to the potential, and at scan rates 6B 100 mV/s the film regains its starting mass at the same potential. By extending the potential scan to a
Fig. 1. Mass accumulation of the polymeric film during the potentiostatic (E =1.8 V) polymerisation of TAA (1) and a 1:3 mixture of TAA and MT (2) in 0.2 M Bu4NPF6 in AN solution.
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Fig. 2. (a) Double voltammetric curve registered with PTAA film in 0.2 M Bu4NPF6 in AN solution at 6 = 100 mV/s sweep rate. (b) Mass change of the film during the CV obtained from EQCM measurement.
more positive region, the appearance of a prepeak could be revealed. It seemed to become detectable at the very beginning of the oxidation (at around 200 mV) and at fast scan rates. The frequency curves showed practically no mass changes coupled with the electrochemical process in this region. When a still wider potential window is applied, the prepeak becomes higher (Fig. 5a). The mass versus potential curves (Fig. 5b) have the same pattern as previously, and indicate
only a larger anion incorporation during the cycles. However, if we compare Fig. 4b and Fig. 5b, a gradual shift can be revealed, which shows that the mass of the film is increasing during or between the scans. By transforming the data, we can obtain the mass change versus charge relationship, which holds a very important information, i.e. how the mass accumulation in the film is related to the electron transfer. The curves obtained this way are almost linear to a certainly
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observable S-shape. In Fig. 6 the data measured at 6=10 mV/s scan rate are used in the calculation of the virtual stoichiometric number for the moving hexafluorophosphate ion. Applying linear regression method, the curve during the oxidation section of the scan delivered a slope of 1.17×10 − 3 mg/mC. From this value, the virtual relative molar mass of the monovalent anion is 113, that means 78% of the value for the hexafluorophosphate. This value was 85% in the case of the pure MT for the final oxidation region. The reliability of the measurements could be based on the constant admittance registered, also several times during the experiments, which exhibited a very small (within 1%) admittance change. From the value definitely smaller than one for the stoichiometric number per electron, it is assumed that the anion migration is accompanied with the sorption–desorption of some other — ionic or neutral — species, which moves in the opposite direction. A very similar behaviour of the co-polymer could be observed with films obtained at a different monomer ratio. Voltammetric curves registered with a film of 1:5 ratio are presented at an enlarged scaling in Fig. 7, where a well-expressed prepeak can be observed at 300 mV/s sweep rate. Another important feature of the curves developing with decreasing sweep rates should also be emphasised; the second oxidation process and the oppositely positioned cathodic step are not in coupled redox pair relation, since the reduction peak is at a more positive potential than the oxidation peak. If we compare
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the different voltammetric curves presented here and in previous communications [17,26], we can come to the conclusion that the more time the film is spending beyond the oxidation peak the more definitely the peak potentials are ‘oppositely’ positioned. This fact might support the assumption of some kind of phase transition in the oxidised state. The pattern of the mass change of the film during one cycle was about the same as in the previous case, and the gradual mass shift of the film was also visible. However after several cycles, the prepeak could be seen on the voltammetric curve even at 50 mV s − 1, meaning that the slow mass accumulation observed during long time cyclings to more positive anodic potentials is coupled with changes causing or manifesting themselves in the prepeak.
3.3. ISC measurements on MT/TAA co-polymers In the case of the preparation of polymeric films on the double band electrode, a larger charge density was applied, resulting in the formation of thick layers. With similar thick films, the splitting of the anodic peak observed at low sweep rates was already presented for MT [17], and there was a strong evidence that the film started to be transformed into the macroscopically conducting form only during the second oxidation peak. In the case of the thick co-polymeric films, splitting in the form of a shoulder (see later e.g. in Fig. 8a) was rather
Fig. 3. ISC changes versus the logarithm of the charge during the potentiostatic (E = 1.8 V) polymerisation of TAA (1), a 1:3 mixture (2) and a 1:5 mixture (3) of TAA and MT in 0.2 M Bu4NPF6 in AN solution. For curve (1) the conductance scale is 20 times enlarged.
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Fig. 4. (a) Double voltammetric scans registered with the 1:3 TAA/MT co-polymer film (Q =100 mC cm − 2) in the potential range between − 0.4 and + 0.85 V at different sweep rates indicated on the curves. (b) Concurrently registered mass changes of the film during the CV – s.
clear from the very beginning of the voltammetric measurements in the whole studied sweep rate region between 2.5 and 100 mV s − 1. ISC data showed that these films also started to transform into the conductive form only at potentials beyond the prepeak. At the same time the evolution of the conductance curves was deeply affected by the sweep rate; the larger conductance was achieved at slow scans. In this study, only the results with
1:3 co-polymer — connected to the mass accumulation presented in the previous section — are reported. Since the mass accumulation in the film can be coupled with the prepeak which in turn has an effect on the insulator – conductor transformation, in this section of the experiments some voltammetric results and the concurrent ISC data are presented. The appearance of similar prepeaks was coupled with
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cathodic doping in several cases [27,28]. During this series of measurement we intended to see the effect of the more and more cathodic prepolarisation on the splitted oxidation peak. For that purpose double cycles were run. In order to insure the identical state, the identical ‘history’ of the film at the beginning of each measurements, the first sweep of double scans was started always from −0.4 V (just as previously). Thus,
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the second scans were started from different cathodic startpoints, but this gradually enlarged cathodic region was reached after an identical first scan, so the ‘history’ of the second scans was otherwise the same. Fig. 8a proves that with increasing negative potentials there is no cathodic process, the cathodic ‘doping’ — if it exists with tetrabutylammonium ions at all — takes place at more negative potentials. At the same time the oxida-
Fig. 5. (a) Double voltammetric scans registered with the 1:3 TAA/MT co-polymer film (Q =100 mC cm − 2) in the potential range between −0.4 and + 1.2 V at sweep rates 10, 25, 50, 100 and 200, respectively. (b) Concurrently registered mass changes of the film during the CV – s.
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Fig. 6. Mass – charge relationship during the oxidation of 1:3 TAA/MT co-polymer film (Q = 100 mC cm − 2) at a scan rate of 6 =10 mV/s. The results of the linear regression fitting are shown as an insert.
Fig. 7. Enlarged current –potential relationships registered during double scans with the 1:5 TAA/MT co-polymer film (Q =100 mC/cm2) between − 0.4 and +1.3 V at sweep rates 10, 25, 50, 100, 200, and 300 mV s − 1, respectively.
tion is delayed by the pretreatment at more negative potentials, and the first oxidation step tends to disappear, or not to develop. ISC measurements presented in
Fig. 8b show the concurrent delay of the evolution of the conductive state. These observations show that non-electrochemical changes occurring at these negative
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potentials inhibit the realisation of the oxidation, and the lack of prepeak hinders the insulator–conductor transformation. When we compare and summarise the results obtained here by different in situ methods, we can state that the occurrence of the splitting of the oxidation process depends on some non-electrochemical process. If the
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prepeak is not seen separately, or it is delayed, the film — though oxidised — is very far from being conductive. Thus, the non-electrochemical process in the negative potential region inhibits the insulator – conductor transformation. At the same time the EQCM data show a monotonous mass increase during the oxidation, and do not reveal any mass change, which
Fig. 8. (a) The second scans of double sweeps recorded at 50 mV s − 1 sweep rate with 1:5 TAA/MT co-polymer film (Q= 2 C cm − 2) deposited for ISC measurements. The first scan of the double sweeps was started always from − 0.4 V, and the cathodic turnpoint was − 1.0, −1.3, − 1.5, − 1.8 and − 2.0 V, respectively (this order is indicated by the numbers). (b) Concurrently registered conductance changes. The numbering increases with the increase in the cathodic turnpoint potential.
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would indicate cation movement. The involvement of cations is even less probable, if we consider that negative potentials — which had to be in favour of the cation incorporation — led to the disappearance of the anodic prepeak and to the postponed transformation of the film into the conductive form. Thus, in the case of thiophene type polymers, the movement of the cation can hardly be responsible for the splitting of the oxidation wave, as it was reported in the case of the polypyrrole/ Li-perchlorate film [16]. The fact that the mass of PF− 6 ion is less than 2/3 of that of the cation makes the involvement of ion pair movement also less probable, which movement would be masked only at a very special ion participation. So we may conclude that there is no cationic doping in the potential range covered here, and the Bu4N+ ion is not incorporating with the film. The only cation, which has not been taken into consideration yet, is the proton, which might be involved in the process, since carboxylic acid groups can be in the film. Their movement would cause very little frequency change so that the small mass change from proton movement may be masked. The permanent pattern of the splitting of the oxidation process is against the assumption of proton doping/undoping, since the expelled protons would represent a very small concentration in the bulk solution, so their reversible movement is not probable. However, it is possible that the dissociating protons are not removed from the film. Therefore, in order to exclude the opportunity of the manifestation of this self-doping of the TAA component of the co-polymer, we studied another co-polymer of MT also, with TEPE, where no dissociating protons are available. In this study, only the most important and relevant results are presented.
3.4. MT/TEPE co-polymers The polythiophene film obtained by electropolymerising, a mixture of 5:1 ratio (MT:TEPE) exhibited a voltammetric behaviour basically similar to those of the co-polymers with TAA, and an EQCM pattern rather similar to pure MT. As shown in Fig. 9a, the prepeak appears even at 6=10 mV s − 1 sweep rate, and it was observable at each sweep rate in the range of 10–100 mV − 1. The splitting is obtained with thin films, too. The prepeak is larger, and there was no need for any kind of treatment to develop this peak splitting. Here again we call the attention that the position of the second oxidation/reduction peaks during the redox process excludes that they form a simple redox pair. This shift in the peak potentials, which develops with the increasing time period spent at potentials beyond the second anodic peak, might suggest again the occurrence of a kind of phase transition taking place in the very oxidised state (see Fig. 4a, Fig. 5a and Fig. 7).
The behaviour of the mass change during the voltammetric curve had two characteristics, (i) there was no sign of any mass decrease during the oxidation; and (ii) there was a gradual shift of the starting value of the curves, which indicated mass accumulation in the film. The admittance curve, not presented here, proved that the film preserved its rigidity during the measurements. Current and frequency data were transformed to mass change versus charge curves. As Fig. 9b shows, the mass of the film monotonously increases with the charge but the curve has a very expressed S-shape. From the global slope delivered by linear regression, a relative equivalent mass of 65.5 is obtained as an average, which means a virtual stoichiometric number 0.45 for the anion. This linear regression is a rather rough fitting since this value seems to be even smaller at the very beginning of the oxidation. The two section pattern was previously shown for the poly(3-methylthiophene) itself [17], where the peak splitting of the anodic current did not show up. By potential step method combined with EQCM measurement, a better linearity of the m –Q curve throughout the whole insulator – conductor conversion was obtained, which might indicate that the faster perturbation masks together the distinct processes, so that the partially consecutive processes starting at slightly different potentials occur parallelly. As Fig. 10 shows, the mass of the film follows a linearity versus the charge integrated during the potential step from − 0.4 to + 1.3 V (the values applied as the anodic and cathodic turn-potential in the voltammetry). The slope is practically the same as before, and recalculation results in a virtual stoichiometric number of 0.46. The ‘deficit of mass’ is 78 g mol − 1, which is approximately the double of the molar mass of acetonitrile solvent. On the basis of the close similarities between the behaviour of this co-polymer having no dissociating hydrogen and self-dopable thiophene co-polymers, these results can be considered as proof for that the prepeak does not originate from proton release.
3.5. In situ spectroelectrochemical measurements In order to have a complete information on the redox transformation of the polymeric film, it is especially of great importance to know what kind of process takes place during the prepeak. Owing to the separation of the two anodic peaks at low sweep rates, the TEPE/MT film gives the opportunity to record the optical spectra in the two oxidation regions separately. Thus, as a next step we studied the spectroscopic changes of the MT/TEPE film during a slow cyclic voltammetric transformation. Our special purpose was to register the optical spectra in parallel to the first peak, since the similarly appearing prepeaks have been associated with previous cathodic doping [27,28]. If it were the reoxidation of cathodically conducting form, the spectral changes in this region would be opposite to those coupled with the further
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Fig. 9. (a) Double voltammetric curves registered with the 1:5 TEPE/MT co-polymer film (Q =150 mC/cm2) at 10 mV s − 1 sweep rate. (b) Concurrent mass changes vs. the charge. The insert shows the results obtained by linear regression.
oxidation. Spectral changes of the film during the cyclic voltammetric oxidation at a sweep rate of 5 mV s − 1 are presented in Fig. 11. As it can be seen, the maximum absorbance of the neutral film is around 500 nm, which indicates a relatively good conjugation in the co-polymeric film. The spectral change is monotonous and it exhibits even during the first peak a small but clear absorbance increase in the 650–820 nm wavelength range, where the absorbance associated with the forma-
tion of the oxidised form can be detected. By that way we can exclude the manifestation of a delayed reoxidtion, a cationic ‘undoping’, and we may evidence the ‘doping’ type of the current during the prepeak. We recollect that during the main part of the anionic doping, the mass versus charge curves showed an approximately linear behaviour in case of pure and co-polymeric thiophene films, but with a slope smaller than the theoretical one. From all these facts the involvement
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of the solvent can be deduced, and the gradual mass increase registered in the series of Figs. 4 and 5 can be connected to solvent accumulation. This solvent adsorption can be referred to structural changes occurring even at negative potentials where there is no Faradaic process or at open circuit [29,30]. However, the solvent can
penetrate only into a film of favourable structure. With PMT and its co-polymers with TAA this structure develops only by extending scanning into the anodic potential region. In the case of the TEPE/MT co-polymer the structure of the film makes solvent penetration easier without any
Fig. 10. Mass vs. charge relationship obtained during a potential step from −0.4 to + 1.3 V with the 1:5 TEPE/MT film. The insert shows the results obtained by linear regression.
Fig. 11. Spectral changes during the voltammetric oxidation of the 1:5 TEPE/MT film (Q =150 mC cm − 2) registered at 5 mV s − 1 sweep rate. The arrows indicate the direction of the changes during the oxidation of the film.
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treatment. The oxidation of a more solvated film is coupled with the removal of more solvent, leading to a smaller global mass increase during the oxidation process. That gives an even smaller virtual stoichiometric number for the anion in this case. Thus, the observations give further evidence to improve the previous conclusions [17], namely that solvent removal accompanies the oxidation of the film. The deficit of mass at the m–Q curves can be interpreted by the concurrent removal of the solvent during the anion adsorption of the film, and the ‘missing mass’ can be translated to the removal of 1–2 solvent molecule per one incorporated anion. This solvent expulsion is induced by the anion penetration upon the start of the charge transfer. However, it is assumed that the solvent is partially ‘trapped’ within the films, mainly at deeper places of thicker layers. This entrapment is more expressed after a negative polarisation, and it results in a delayed solvent removal (Fig. 8). In order to describe the phenomenon, the following scheme is assumed, the oxidation of the neutral, nonconducting film starts from the metal–polymer interface. It consists of two parts: the transformation of solvated and non-solvated segments. Since the slope of the mass versus charge curves is approximately constant, the oxidation processes are overlapping and they take place parallelly. When we apply relatively slow potential changes, we can detect that the oxidation of the solvated segments starts slightly before, as the slope is smaller at the beginning of the transformation. These oxidised segments created by the charge transfer are first far away from the film–solution interface. The partially localised positive charges created in both processes generate the flux of the charge balancing anions from the surface into these places. Since the anion influx comes from the solution phase into the polymer, non-solvated, assumingly more mobile carriers move to the polymer–solution interface, but the solvent coupled with oxidised polymeric segments might be trapped in the layer. By that way the charge transfer processes create first a film which contains shielding solvent molecules. In this state the layer is not conducting because mobile, non-localised holes have moved to the polymer–solution interface, and more localised, solvated charge carriers are separated from each other. In order to get the highly conducting quasi-metallic state, the interaction between the conducting islands must be insured, and for that solvent molecules have to be removed. It is ruled by the effect of the electrical field, which is continuously increasing with the potential during the CV or with the decreasing thickness of the non-conducting layer in case of potential steps. Accompanied with the replacement of solvent molecules and the repositioning of anions, the film reaches a more and more favourable structure leading finally to the highly conducting, quasi-metallic state.
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These processes taking place during the electrochemical transformation of thiophene type polymeric films can be formulated as starting with the oxidation of solvated units coupled with the anion incorporation − − Psol + A− [ [P+ solA ] +e
(1)
followed at a slightly more positive potential by the similar oxidation of the non-solvated units. P+ A− [ P+A− + e−
(2)
The non-solvated charge carriers can freely interact, but the complex formed in Eq. (1) being not the most stable form of the oxidised polymer transforms, ruled by the electrical field: n 2+ − − (3) n[P+ solA ] [ [P2 2A ] +n sol 2 During this non-faradaic reorganisation step, the exchange of the solvent and the anion takes place, and the concurrent removal of the solvent leads to the non-integer value obtained for the virtual stoichiometric number of the anion. During this slow, elongated chemical process controlled by the actual electrical field, the intraand/or inter-chain interactions between previously separated paramagnetic segments may develop, leading to a high level of conjugation. This is indicated by the formulation of the product as a dimer. So by the removal of the interstitial solvent molecules, the film may reach a much more integrated phase, and can show a quasimetallic behaviour. The identification of the solvent removal as a chemical – desolvation — step may explain the surprising peak positions in Figs. 5, 7 and 9. If the layer is thin or its structure and character prevents it from solvent adsorption, the simple dimerisation type chemical step (3) can follow the electrochemical one (1) without any delay. In these cases, the formation of the charge carriers and their interacting combination are not separated from each other in time. Thus, the transformation into the well-conducting state is not prolonged, and the process gives a relatively sharp, single anodic peak. That might refer to the case of some polythiophenes having a more open structure presented in a recent study [31]. This limiting case might give the theoretical slope of the mass versus charge curve. Solvent and cation incorporation into the neutralised film disturbes this ‘ideal’ case, and leads to the appearance of peak splitting and/or to different m–Q curves.
4. Conclusions In this study, it was demonstrated that the solvation of the film could be modified through co-polymerisation which affected the voltammetric and the EQCM behaviour of the films. The double step pattern of the voltammetric oxidation of thiophene films has gained
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further support by studying different thiophene co-polymers. The redox transformation of the film was followed by different in situ techniques. In contrast to polypyrroles, where the two oxidation steps were coupled with cation and anion movements, respectively, the EQCM results did not show any mass decrease troughout the whole oxidation region. The ISC measurements exhibited a close relationship between the detection of the prepeak and the easy realisation of the transformation to the conducting state: if the film exhibits the well separating prepeak, the macroscopic conductance is developing easily. The negative pre-polarisation, although without current flow, delays the first step, and as a consequence, the development of the conducting state is also hindered. On the basis of the results obtained by the different methods, it is concluded that the incorporation of cations and the involvement of the ion pairs in the oxidation–reduction process is improbable. The involvement of protons originating from the dissociation of the TAA was excluded by doing the analogous measurements with MT/TEPE, an ester type co-polymer with the MT, which delivered essentially the same results, although no dissociating hydrogen was available there. On the basis of the results it is assumed that the electrochemical transformation of thiophene type polymeric film contains basically parallel processes. The oxidation of the solvated units and that of the non-solvated parts takes place at slightly different potentials. The solvated oxidised complex is not the most stable form of the charge carriers in the polymer, and it transforms to its stable form, ruled by the electrical field. During this non-Faradaic reorganisation step, the slow exchange of the solvent and the anion takes place, and the concurrent removal of the solvent leads to the non-integer value obtained for the virtual stoichiometric number of the anion. This slow chemical process involves also the intra- and/or inter-chain interactions between previously separated paramagnetic segments, leading to a high level of conjugation, and the quasi-metallic phase.
[2]
[3] [4] [5] [6] [7] [8] [9] [10] [11]
[12] [13] [14] [15] [16] [17] [18] [19] [20]
[21] [22] [23]
Acknowledgements
[24]
This work has been supported by the Hungarian National Scientific Research Fund (OTKA no. T 016017 and 030083), as well as by the Hungarian Ministry of Education (FKFP no. 0733) and by the Academy of Finland (grant no. 30579).
[25] [26] [27] [28] [29]
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