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In-situ ESR spectroelectrochemical studies of overoxidation behaviour of poly(3,4-butylenedioxythiophene)
Electrochimica Acta 51 (2006) 2135–2144
In-situ ESR spectroelectrochemical studies of overoxidation behaviour of poly(3,4-butylenedioxythiophene) A. Zykwinska a , W. Domagala a , A. Czardybon b , B. Pilawa b , M. Lapkowski a,b,∗ a b
Faculty of Chemistry, Silesian University of Technology, 9 M. Strzody street, 44-101 Gliwice, Poland Institute of Coal Chemistry, Polish Academy of Sciences, 5 Sowinskiego street, 44-121 Gliwice, Poland Received 30 November 2004; received in revised form 20 March 2005; accepted 30 March 2005 Available online 6 September 2005
Abstract The behaviour of poly(3,4-butylenedioxythiophene) (PBuDOT), a relative of poly(3,4-ethylenedioxythipohnene) PEDOT within the poly(3,4-alkylenedioxythiophene) family, has been investigated at potentials above its electrochemical stability threshold using in situ ESR spectroelectrochemistry. The aim was to investigate the effect of electrochemical overoxidation on the charge carrying species, namely polarons, normally generated and annihilated during reversible redox doping and dedoping reactions, by determining the potential dependencies of spectroscopic parameters of the ESR spectra of the polymer over a selected potential range. Specific features of the trends of these dependencies allowed also for an evaluation of presence of the second type of charge carrying species—diamagnetic bipolarons and the effects of their interactions with polarons at different potentials. Around 1.5 V, where the boundary of electrochemical stability of the polymer lies, sharp drop of the concentration of paramagnetic centres has been observed together with a transitory narrowing of the ESR line. These changes were found to be irreversible as evidenced by the course of subsequent reduction half-cycle, which differed from the one for a not overoxidised polymer, observed in previous studies. Aided by the results of electrochemical studies it was concluded that the overoxidation process leads to a degradation of the polymer most probably due decrease of the conjugation length of the main chain -bond through cross-linking or addition reactions. While the electrochemical results pointed to a non-complete degradation of the polymer, the specific parameters of the ESR line in the reduction half-cycle indicate that the remaining spins are confined to isolated segments of a partially degraded polymer where their behaviour resembles oligomer-like radicals. © 2005 Elsevier Ltd. All rights reserved. Keywords: PBuDOT; Charge carriers; ESR spectroscopy; Polarons; Overoxidation; Degradation
1. Introduction The discovery and polymerisation of 3,4-ethylenedioxythiophene in the laboratories of Bayer A.G. in the late 1980-ties [1] marked the birth of one of the most successful conducting polymer in terms of both academic interest and practical applications, known to date [2,3]. Following numerous studies conducted over the last 10 years, poly(3,4ethylenedioxythipohnene) (PEDOT) is nowadays well known for its high conductivity, interesting electrical and spectrochemical properties associated with its low band gap, ∗
Corresponding author. Tel.: +48 32 237 1743; fax: +48 32 237 1509. E-mail address: [email protected] (M. Lapkowski).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.03.081
electrochromic and antistatic properties and good stability [4,5]. The success of PEDOT gave birth to a family of 3,4alkylenedioxythiophene polymers, research into which has been undertaken to tailor and fine-tune selected properties of this polymer into specific applications [6], as well as to try to overcome PEDOT’s few limitations and drawbacks like insolubility and infusibility. It was this idea which led to first syntheses of PEDOT analogues with alkylene chains of different size, namely poly(3,4-propylenedioxythiophene) (PProDOT) by Jonas and co-workers [4,7] later followed by poly(3,4-butylenedioxythiophene) (PBuDOT) by the group of Reynolds and co-workers who soon reported the first systematic study of electro- and spectrochemical properties of a number of different 3,4-alkylenedioxythiophenes [8,9].
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The purpose behind enlarging the alkylene ring was to space the polymer chains apart and by increasing the flexibility of the alkylene substituent, introduce some degree of disorder in the relative arrangements of the polymer chains, all of which could grant the respective polymer solubility in common organic solvents. To date however this approach alone proved unsuccessful in preparing a soluble 3,4alkylenedioxythiophene polymer. Nevertheless, members of this polymer family displayed some interesting properties, sometimes markedly superior to their parent relative PEDOT. The spectroscopic studies indicated that PProDOT displays a high degree of regularity along the polymer backbone while PBuDOT displays one of the fastest switching times together with one of the highest optical contrasts (%T amounting to 63%) in the poly(3,4-alkylenedioxythiophene) family [4]. Although the conductivity of PBuDOT is markedly impaired relative to PEDOT, it has been identified as one of the candidates for further studies on fast electrochromic materials. Despite these promising initial results, to date and to the best of our knowledge no reports of further studies concerning this polymer have appeared. The electrochemical and spectroscopic properties of conducting polymers depend on their doping state. One of the most accurate and reproducible methods of such doping is the electrochemical oxidation or reduction of the polymer. During doping of the polymer two types of charge carriers appear in the polymer – polarons and bipolarons [10–12] each dominating at different doping levels. When operating within the boundaries of stability of the polymer these two types of charge carriers coexist together and can be reversibly generated and annihilated. The question arises as to how stable these redox properties are and also what happens beyond the stability threshold. Right form the first studies, PEDOT was known as a very stable polymer [7], resistant to different ageing factors of both physical and chemical nature. Studies of PEDOT’s stability commenced almost concurrently with its first studies. Heinze and co-workers reported that overoxidation of PEDOT up to 3.0 V leads to evolution of a strong irreversible oxidation peak [7]. Venture to such high potentials led to complete loss of electrochemical properties of the material however. Kvarnstr¨om et al. have evaluated the onset of degradation potential at 1.9 V [13], which compared with 1.4 V for other polythiophenes [14] indicates that PEDOT is indeed an electrochemically stable material. Lapkowski and Pron have demonstrated a finite potential window of conductivity of PEDOT [15] while Du and Wang have reported in a recent study, the effect of polymerisation potential on PEDOT’s conductivity and ESR response [16]. When PBuDOT is concerned however, to date we did not come across any report concerning its stability studies. Since the polymer displays interesting spectroscopic and electrochemical properties it would be valuable to investigate its behaviour beyond normal application regimes in which it could be used. Recent detailed study of electrochemical dissolution of polythiophene presented by Aoki and co-workers [17] can provide some clues as to such behaviour. Their work gives
some insight into the degradation mechanisms involved in the degradation of an electroactive polymer—polythiophene, upon its overoxidation. The course of the degradation of polythiophene main chain studied through identification of its fragmentation products may be an indication as to what happens to the main polymer chain of PBuDOT during overoxidation. Refaey et al. have also employed electrochemical impedance to study the electrochemical degradation of polythiophene [18]. Our group has already presented the results of basic electrochemical studies of PBuDOT [19] together with ESR spectroelectrochemical studies in the stable potential range [20]. Following our previous studies of the effect of overoxidation on the properties of charge carriers in PEDOT [21], we decided to undertake a similar study for PBuDOT in order to investigate the intrinsic behaviour of charge carriers at high oxidation potentials for this close analogue of PEDOT. Of the two charge carriers present in organic conducting materials, polarons carry a magnetic moment and can therefore be observed and distinguished from diamagnetic bipolarons by means of ESR spectroscopy. Their spectroscopic response being sensitive to the chemical environment they reside in, enables however to draw some conclusions regarding the presence of bipolarons since these, alongside paramagnetic species, constitute part of that environment. This influence will be evident not only at high doping levels where polarons become oxidised to bipolarons but also at intermediate levels where both species coexist togehter [22]. The results of such studies have to be taken with great caution however. Degradation process of the polymer is never simple but rather a result of several parallel and/or consecutive reactions all taking place at different speeds, triggered by different factors. It is also difficult to unambiguously elucidate all the variables governing this process, as some may be hard to be expressed numerically making an observed degradation path difficult to reproduce. Nonetheless these information may help to determine and possibly explain the effects of extreme potential conditions on the redox properties of the material, supplementing the necessary information when potential applications of PBuDOT may be considered.
2. Experimental 2.1. Electrochemistry Cyclic voltammetry measurements were done on an AUTOLAB potentiostat–galvanostat model PGSTAT20 (EcoChemie, Netherlands) under the control of a PC class computer. The following electrodes were used: Platinum wire (φ = 1 mm) sealed using TeflonTM tape, with a geometric area of ca. 0.10 cm2 was used as working electrode. Ag wire was used as a quasireference electrode. Platinum coil of 8 mm diameter inside of which the two electrodes were positioned, served as an auxiliary electrode. All potentials are
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given versus the Ag quasireference electrode used. The E0 potential of ferrocene couple versus this electrode was equal to +0.40 V. The monomer 2,3,4,5-tetrahydrothieno[3,4-b][1,4]dioxocine (BuDOT) was synthesised in our group in a multistep synthesis, described by Kumar et al. [8]. Poly(3,4butylenedioxythiophene) was obtained via cyclic electropolymerisation on platinum electrode from 0.01 M solution of the monomer in 0.1 M (Bu)4 NPF6 in CH3 CN with maximum oxidation current control. For a given electrode area, the current value at which the oxidation half-cycle was reversed, did not exceed 3.5 × 10−4 A. Thanks to such optimised procedure we were able to obtain polymer films having reproducible CV responses each time.
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Fig. 1. Cyclic voltammogram of PBuDOT film showing the overoxidation peak of the polymer together with the background CV of the electrolyte −0.1 M Bu4 NPF6 in CH3 CN, both recorded at 0.5 mV s−1 . Inset shows the lower potential part of the CV in detail with an overlaid CV of PBuDOT in the potential range up to 1.0 V recorded at 0.3 mV s−1 .
field (Br ), were evaluated. g-Factor was calculated from the resonance condition: hν g= βBr 2.2. ESR measurements In situ ESR measurements were performed using ELPAN SE/X-2543 series X-band (9.3 GHz) reflective type ESR spectrometer (Radiopan, Poznan, Poland) with magnetic modulation of 100 kHz and with recording of the microwave resonance frequency. For spectroelectrochemical measurements, analogue ELPAN EP-21 type potentiostat driven by ELPAN EG-20 potential generator was used in conjunction with the ESR spectrometer. The spectroelectrochemical cell was a thin cylindrical glass tube narrowed at the bottom (3 mm in diameter). PBuDOT films were studied directly on the platinum electrode they were synthesised on utilising the same set of electrodes as described above except for an auxiliary electrode as a smaller diameter Pt spiral was used in this case. The electrolyte was 0.1 M (Bu)4 NPF6 in CH3 CN—the same as for polymerisation. The ESR spectra, recorded as the first derivative of resonance absorption curves, were computer collected. The spectra were measured with an attenuation of microwave power of 3 dB (∼15 mW), which came out to be still enough to avoid signal saturation. For all the spectra recorded in the potential range studied, the same modulation amplitude was used = 0.08 mT. Lineshape of the ESR spectra was numerically analysed using an Origin 6.0 program package form Microcal. Decomposition of the complex ESR spectra has been performed. The experimental spectra were fitted with superpositions of Gauss and Lorenz lines. The parameters of the lines of best fit (having the lowest χ2 value): linewidth (Bpp ) and resonance
where h is Planck constant, ν is microwave frequency, β is Bohr magnethon, and Br is resonance magnetic field.
Fig. 2. ESR spectra of PBuDOT film at different applied potentials during (a) oxidation half-cycle, carried out from −0.7 to 2.0 V and (b) reduction one from 2.0 to −0.7 V.
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The dependence of relative concentrations of paramagnetic centres in the studied polymer upon the potential applied to the polymer film was determined. Concentration of paramagnetic centres in the sample is proportional to the area under the absorption curve, thus double integration of the first-derivative ESR spectra was performed.
3. Results Studies of basic electrochemical properties of PBuDOT following its synthesis and first electropolymerisation attempts [19] led us to identify the oxidation potential boundary of PBuDOT around 1.5 V versus the same reference electrode as used in this study. To investigate the behaviour of the polymer in an extended potential window, a −0.7 to 2.0 V range was designated for this study. Cyclic voltammogram of PBuDOT in this potential range is presented in Fig. 1. The salient feature is the large and virtually irreversible oxidation peak at 1.65 V. Similar peak has been observed in PEDOT by Heinze and co-workers [7] and Zotti et al. [23], as well as in our previous studies of PEDOT’s overoxidation [21]. In this case however the peak is not as tall as in PEDOT, compared to the current magnitude of the rest of the CV response. In order to confirm the assignment of this peak, a CV of the electrolyte, shown in Fig. 1, has been recorded. Comparison of the
two indicates that the oxidation peak can be ascribed solely to the polymer, representing some sort of large-scale process, taking place at these potentials. This process leads to permanent changes in the polymer as evidenced by the course of the reduction half-cycle of the polymer, shown in more detail on the inset CV in Fig. 1. Comparing the reduction of the overoxidised film with the response of the film in the potential range of its electrochemical stability (Fig. 1 insert), one notes that the two clear reduction peaks are replaced with a broad and diffused one for the overoxidised film. These changes may suggest that a degradation process of some sort accompany the overoxidation of the polymer. ESR spectra of PBuDOT film highly depend upon the potential applied to the polymer film. These are shown in Fig. 2, arranged one after another as a function of progressively changed potential. Two maxima are observed in the oxidation half-cycle, at 0.1 and 1.4 V. At these potentials the onsets of the oxidation and overoxidation peaks of the polymer are located. In the reduction half-cycle the ESR signal is no longer so potential sensitive. The general trend is an increase of the ESR signal intensity interrupted only by a shallow dip at 0.2 V. Analysing the ESR spectra in Fig. 2 one can only compare their relative amplitudes. To allow for their complete comparison, selected spectra from both half-cycles were plotted, one above another, in Fig. 3. It can be seen that changes
Fig. 3. Selected ESR spectra of PBuDOT at various potentials in: (a) oxidation and (b) reduction half-cycles.
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Fig. 4. Concentration of paramagnetic centres in PBuDOT film as a function of the potential during oxidation and reduction half-cycles. Above a CV response of PBuDOT in monomer-free electrolyte −0.1 M Bu4 NPF6 in CH3 CN, recorded at 0.5 mV s−1 —a speed equivalent to the pace of the ESR spectroelectrochemical experiment. Inset shows the voltammograms in the same electrolyte as above, of PBuDOT before and after completion of the voltammetric cycle up to 2.0 V, shown below.
in the intensity of the ESR line are mutually coupled with changes in peak-to-peak (Bpp ) linewidth. In general, as the intensity of the signal decreases, the line broadens and viceversa. Detailed discussion of the potential dependency of the linewidth parameter—Bpp will be given further on. By doubly integrating the ESR signals, the relative concentration of paramagnetic centres in the polymer film was evaluated. These results are presented in Fig. 4. At the start of the experiment in the oxidation half-cycle a notable concentration of spins prevails decreasing slightly before a major rise commences around −0.1 V. At this point on the voltammogram (Fig. 4, inset CV) an oxidation pre-peak forms thought to represent the transition of the insulating film into a conducting form at light doping levels [20]. This pre-peak was observed in our previous studies of PBuDOT, and there too its presence was not reflected on the potential dependency of concentration of spins’ curve. As recently reported by Randriamahazaka and co-workers for PEDOT [24] this pre-peak becomes increasingly well defined and standalone, the lower the reduction potential applied to the polymer film is and the longer the duration of such treatment lasts. In the potential range of 0–0.3 V the concentration of spins increases sharply reaching a maximum at roughly the same potential as the oxidation peak of the polymer. Following that, a decrease of
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the concentration of spins is observed, up to a minimum at around 1.6 V. At this point, the total concentration of spins is roughly the same as in the potential range in which the polymer is undoped. At ca. 1.5 V the overoxidation peak of the polymer forms with its apex appearing at potentials when the concentration of spins is already at its minimum. In the reduction half-cycle the potential dependency is no longer similar to the oxidation half-cycle one as in the case of studies in the potential window up to 1.0 V [20]. A point to note is that at the end of the experiment, the concentration of paramagnetic centres is greater than at the beginning of the experiment. Studies of PEDOT and PBuDOT in the potential range of electrochemical stability revealed that the spectra of both polymers in the doped state are complex, being made up of two components one Gaussian and one Lorenzian [25]. Extending the potential rage in these studies, a question arose whether the same structure of the spectra prevails at these potentials. Fig. 5(a–d) shows four different spectra at different potentials modelled by the Gauss + Lorenz superposition. It can be seen that in all four cases the experimental spectra are well modelled. Though at different potential ranges the mutual relationship between the two components changes, in general, high and narrow Gauss line is observed accompanied by a small and wide Lorenz one carrying the major fraction of the concentration of spins. Decomposition of the experimental spectra according to this model yielded potential dependencies of three parameters—concentration of spins, Bpp linewidths, and g-factors of both components. Changes of concentrations of paramagnetic centres of each component with potential in the oxidation and reduction half-cycles are presented in Fig. 6(a and b), respectively. The potential dependence of the Lorenz component maps the total concentration curve very closely whilst the Gauss component takes an independent shape. The sudden concentration decrease between 1.4 and 1.5 V, associated with the overoxidation of the polymer, practically applies only to the Lorenz component. In the reverse reduction half-cycle the concentration of the Lorenz component, again follows the trend of the total concentration of spins (Fig. 6b). The concentration of the Gauss component keeps low, yet higher than in the oxidation half-cycle, again being more or less independent of potential in its whole range. Potential dependencies of linewidths (Bpp ) of different groups of paramagnetic centres are presented in Fig. 7. At first in the oxidation half-cycle, linewidths decrease very slowly, starting to increase above 0.1 V (Fig. 7a) as the polymer becomes doped. In the potential range in which the polymer is doped and both component paramagnetic centres can be distinguished, we find the linewidth of the Lorenz line decreasing monotonously until 1.5 V and that of the Gauss line oscillating with no clear trend. In the reduction half-cycle the dependence of linewidth of Lorenz component takes a shape similar to the dependence of concentration of spins with changing potential with a maximum at 0.2 V. The Gauss component has a linewidth virtually independent of potential down to 0.2 V.
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Fig. 5. Selected complex ESR spectra of PBuDOT showing the decomposition of the experimental line into two components – Gauss and Lorenz lines (a) and (b) oxidation; (c) and (d) reduction half-cycles.
The g-factor potential dependencies are presented in Fig. 8. In the potential range of −0.7 to 0.5 V the g-factor of the experimental line holds up above the free electron’s value of 2.0023. In general, the g-factor of the Lorenz component is lower than that of Gauss one. In both the oxidation and reduction half-cycles, little correlation with potential dependencies of other spectroscopic parameters is observed. In Fig. 4 we presented two voltammograms of a PBuDOT film—before and after oxidation up to 2.0 V. The polymer after oxidation up to 2.0 V does not appear to be fully degraded yet, however it is not known how far it is from complete degradation. In order to gain some insight into the stages of degradation of PBuDOT, we have recorded step by step CVs of the polymer upon its oxidation up to 2.0 V at 100 mV s−1 , results of which are shown in Fig. 9. It is observed that a much broader peak whose apex falls at 1.9 V replaces the large oxidation peak at 1.65 V registered at 0.5 mV s−1 . This shift is a further indication of the irreversibility of the processes taking place in the film at these high potentials. The biggest difference between two consecutive cycles is observed between the 1st and the 2nd one. In the following cycles the overoxidation peak diminishes quickly together with characteristic redox peaks of the polymer being replaced by numerous broad peaks from which in the end two dominant ones remain. In the final stages of degradation, the
remaining peaks decrease evenly until a stable CV is attained being similar to the background one. In total it takes nearly 100 continuous cycles (1.5 h) to fully degrade the polymer. In spite of that, at the end of the experiment the electrolyte solution remained clear and colourless and the film remained intact on the electrode and continued to adhere well to it even during cleaning of the electrode, suggesting that mechanical disintegration of the film has not taken place.
4. Discussion In our previous ESR studies of PBuDOT, we have investigated the spectroscopic properties of this polymer in the potential range of −0.7 to 1.0 V in which it is observed to be well stable [20]. The first eminent observation at potentials above 1.0 V is the sudden drop in concentration of spins observed in the narrow potential range between 1.4 and 1.6 V (Fig. 4) signalling sudden changes taking place in the population of polarons at these potentials. Possible explanations include their oxidation and/or recombination to spinless bipolarons [18] or some reaction with species generated in the overoxidation process represented by the large oxidation peak at 1.65 V on the polymer’s CV (Fig. 1). The magnitude of this CV peak indicates that profound changes in the polymer
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Fig. 6. Concentrations of spins of different groups of paramagnetic centres at different potentials in the (a) oxidation and (b) reduction half-cycles.
may be taking place and its irreversibility suggests that these changes are permanent. Bearing in mind that the potential boundary of electrochemical stability of PBuDOT is located around 1.5 V [19], this suggests that the overoxidation peak represent a process leading to the degradation of the polymer film. In this light the second interpretation of the drop of concentration of spins appears more justified and there are also other reasons to support it. Firstly, the polarons and bipolarons coexist in practically whole potential range of the doped polymer and it is only the relative position of equilibrium between the two, which shifts with changing potential [26]. A shift in this equilibrium, favouring the formation of bipolarons is probably responsible for the steady decrease in the concentration of spins at higher potentials in the range of 0.3–1.0 V. Secondly, the drop in concentration of spins around 1.5 V is virtually irreversible, as only a small rise around this potential is observed in the back reduction half-cycle. Under normal conditions, the polaron–bipolaron transition is reversible upon dedoping of the polymer. If this had been the case then a reduction peak reflecting this process should have been observed on the polymer’s CV. The absence of this peak indicates that the overoxidation process has led to irreversible reactions within the polymer film. Going further into the reduction half-cycle we also observe that the characteristic two redox peaks observed on PBuDOT’s CV up
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Fig. 7. ESR signal linewidths (Bpp ) of different groups of paramagnetic centres at different potentials in the (a) oxidation and (b) reduction halfcycles.
to 1.0 V (Fig. 1, inset) are now missing, replaced by a very broad peak whose shallow apex falls at around 0.1 V, matching the potential at which a maximum concentration of spins is observed. Also a voltammogram of the film recorded at 100 mV s−1 after the slow scan rate experiment compared with CV recorded before it (Fig. 4, inset), reveals a substantial change in the electrochemical response of the film. Apart from the coalescing of the oxidation peaks a decrease in the current magnitude is observed which in the absence of dissolution of the film implies that the electroactive area of the polymer film has decreased. These observations suggest that degradation of PBuDOT takes place during its overoxidation above 1.5 V, leading to partial or complete loss of its electroactive properties. Step by step degradation voltammograms of PBuDOT presented in Fig. 9 give us some information regarding the path of the degradation process of the polymer. As already noted the biggest changes in the CV of the polymer are observed in the first cycles. Here, an interesting feature is observed—an “undercut” of the CV in the range between 1.8 and 1.3 V in the reduction half-cycle. We have observed, in overoxidation studies up to potentials close to the polymer’s degradation threshold, that it is the first sign of commencing degradation of the polymer before any other changes appear in the shape
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Fig. 8. g-Factor of different groups of paramagnetic centres at different potentials in the (a) oxidation and (b) reduction half-cycles.
of the CV. The large current drop upon reversing of the potential run at the end of the oxidation half-cycle of the polymer film (Fig. 4 insert) is an indication of its high double layer capacity and its sharpness is an effect of high conductivity (low resistance) of the film. Changes observed in this part of the CV upon degradation in the form of this “undercut”, may signify that structural changes leading to alteration of these two properties of the film take place before changes in the redox properties of the polymer do. In the first cycles around 0 V a sharp pre-peak of the main oxidation peak of the polymer is present. This peak can be ascribed to the process of transition of the film from the insulating to the conductive form comprising conformational change in the film and the sudden increase in the double layer capacity of the electrodefilm system [20,24]. Its continued presence in the first stages of degradation may indicate that the film still retains its electroactive properties though the shift of this peak into positive potentials means that it becomes ever more difficult to oxidise the film into its conductive state. This may be an effect of a decrease in the conjugation length of the polymer’s bond occurring as a result of the degradation reactions taking place in the film. Another indication of this process may be the complex shape of the CVs at intermediate stages where they appear to be made up of multiple peaks. These may signal the emergence of oligomer-behaving conjugated segments of different length along the polymer backbone. If we
Fig. 9. Electrochemical degradation of PBuDOT at Pt electrode induced by its cyclic oxidation up to a potential of 2.0 V in 0.1 M Bu4 NPF6 in CH3 CN at 100 mV s−1 . Progressive CVs showing the changes of the electrochemical response of the polymer film at (a) initial, (b) final stages until a stable CV is attained. For clarity, only selected cycles are shown. Arrows indicate the direction of changes of different parts of the CV. The cycle in (a) shown in dashed line, corresponds to the CV of PBuDOT (inset of Fig. 4, solid line CV) after the electrochemical degradation experiment, and may be considered to represent the electrochemical response of the polymer after the ESR spectroelectrochemical experiment (Fig. 2).
compare the shape of the CV of PBuDOT after slow scan rate degradation experiment (Fig. 4 insert, solid line CV) with these consecutive degradation CVs, we find closest match being the voltamogram around 40th cycle (Fig. 9a, dashed line CV). Considering that the CV peaks at these stages of degradation come from oligomer-like segments on the polymer main chain, it may be these segments where the Lorenz type spins reside and undergo electrochemical reactions in the reduction half-cycle. This may thus explain PBuDOT’s residual electroactivity after the ESR spectroelectrochemical experiment, as it is not fully degraded yet. Decomposition of the experimental spectra according to the Gauss–Lorenz model gave us information regarding the contribution of each group of spins to the overall ESR response (Fig. 6). These results may help to shed some light on what happens in the polymer upon its degradation. In our previous studies we have identified the Gauss centres as heterogeneously distributed pinned defects of the polymer backbone, conjugation defects and radical trapped at chain ends during electropolymerisation. Lorenz centres were
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attributed to homogeneously distributed polaronic spins appearing on the polymer’s conjugated -bond upon doping, thus thought to represent the electrons of conductivity in the polymer. This assignment seems valid in this study as the concentration of Lorenz type centres maps the overall concentration curve while the Gauss type centres are practically potential insensitive. The fact that it is only the Lorenz type centres that diminish during the sudden drop at 1.5 V further justifies their assignment to polaronic spins. This decrease can be caused by sudden oxidation to diamagnetic bipolarons as discussed above or by recombination with free radicals that may be generated upon overoxidation of the polymer, which seems more probable in light of the irreversibility of this process. One possible source of these free radicals may be the cleaved electron-rich C O bonds between the thiophene meric unit and the 3,4-butylenedioxythiophene ring. Radicals from the electrolyte or the solvent can rather be excluded since voltammetric studies of the background response revealed no decomposition peaks up to 2.0 V (Fig. 1). One cannot exclude however that the solvent molecules or the PF6 − ions may react with the heavily oxidised polymer chain leading to introduction of functional groups which may be responsible for interruption of conjugation in the main chain. Abstraction of hydrogen cation leading to interchain crosslinking reaction may also take place especially considering that no loss of cohesiveness of the film has been observed. Interesting is the absence of changes in the concentration of Gauss type centres around 1.5 V—a distinct behaviour compared to the parent relative PEDOT where these centres also diminish at this point [21]. This insensibility indicates that these spins are indeed very stable or that they do not interact with the Lorenz type ones. Puzzling is the rise of concentration of Gauss spins in the back reduction half-cycle around 0.7 V as no special feature on either the voltammogram (Fig. 1) or potential dependencies of other spectroscopic parameters of Gauss spins is observed. Nevertheless compared to the oxidation half-cycle, a slightly higher concentration of this type of spins may be an indication that their population has been augmented by defects created during overoxidation (degradation). The Lorenz type spins on the other hand are more potential sensitive. At first a small rise in their concentration around 1.5 V indicates that some process, reverse to the overoxidation one could be taking place. It is followed by a rise up to 0.2 V when their concentration reaches around 3/4 of their maximum concentration in the oxidation half-cycle. This together with their follow-up decrease suggests that the film has retained some electroactivity. The potential sensitiveness of population of paramagnetic centres is much weaker however compared to PBuDOT film studied in the potential range up to 1.0 V [25]. Comparing the concentration dependencies in the reduction half-cycle in this study with the ones for a film studied up to 1.0 V substantial differences are observed. Not only is the peak concentration smaller but also it appears at a lower potential and the final concentration at the end of the experiment is relatively higher. Such behaviour may be due
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to shortening of the conjugation length of the polymer’s bond either by saturation of individual carbon-carbon bonds, cross-linking, or scission of the polymer’s main chain which could have taken place during overoxidation. The higher concentration of spins at the end of the experiment compared to its start, also observed in PEDOT [21], cannot be explained by increased number of spin type defects as the concentration of Gauss type spins is to low for this (Fig. 6b). Rather it may be the difficulties in charge carriers’ transport through a polymer whose electroactive properties have been impaired due to partial degradation, that hinder prompt reduction of Lorenz type spins at the end of the reduction half-cycle. Linewidth of ESR line reflects the type of magnetic interactions between the paramagnetic centres and their surroundings and the centres themselves [27]. From the start of the experiment we observe practically constant linewidth of the experimental ESR line until 0 V when broadening takes place. This broadening is caused by increasing dipolar and exchange interactions between polarons in their growing population at the onset of doping. Onward from 0.5 V the two types of paramagnetic centres behave differently. The linewidth of Gauss centres is again practically potential insensitive however a shallow decrease may be picked out around 1.5 V. On the contrary the linewidth of Lorenz type centres decreases constantly until 1.5 V after which it stabilises. This line narrowing may be explained in terms of a polarons’ pairing process, in which generated bipolarons begin to separate the polaronic spins. Contrary to PEDOT no sudden narrowing of the linewidths is observed at the potential of overoxidation of the polymer, which implies that in PBuDOT overoxidation reactions are less sudden and spread over a wider potential range. Nevertheless, the final narrowing of Lorenz line may indicate that at high oxidation potentials, mutual interactions of these homogeneously distributed spins weaken considerably up to a point that they may end up separated and confined in the so-called “spin packets”. Small changes in the linewidth of heterogeneously distributed Gauss centres indicate that these spins are indeed stable and not interacting with the polaronic spins. The linewidth decrease around 1.5 V may be caused by changes in the paramagnetic environment caused by changing Lorenz type centres, or by chemical changes inflicted on the polymer upon its overoxidation. In the reduction half-cycle the potential dependence of linewidth of the Lorenz line is very similar to the concentration dependence of this type of centres (Fig. 6b), suggesting that changes in their population are closely linked, and determine the strength of dipolar interactions between them. The fact that the linewidth of Gauss type centres decreases below 0.2 V suggests that upon dedoping of the polymer exchange interactions even between these centres weaken as a result of disappearance of polarons from the still electroactive polymer. Overall the observed linewidth dependencies in the reduction half-cycle provide evidence that specific interactions still exist between spins and the polymer matrix even in a partially degraded film. Analysis of the g-factor potential dependencies (Fig. 8) can give some insight into the environment of spins in the
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film. A first observation is that this parameter is rather insensitive to the sharp changes taking place in the film around 1.5 V during oxidation. Changes in g-factor value indicate, among other things, changes in the spin-orbit coupling effect. Increase in the spin-orbit coupling may be the reason behind the observed rise in g-factor values of both Lorenz and Gauss component lines above 0.5 V. In the reverse, reduction halfcycle, an opposite mechanism can be recalled to explain the observed gradual decrease in the g-factor of the Gauss line, most rapid between 0.8 and 0.2 V when the concentration and the ESR linewidth of the second—Lorenz component increase. Interestingly however the Lorenz g-factor remains practically constant until −0.2 V, which would indicate that the environment of these spins change little. The magnitude of the g-factor of the Gauss line, being higher than a value of 2.0023, indicates that for this group of spins a spin-orbit coupling takes place presumably with C or O atoms. For the Lorenz group its g-factors are peculiarly low and as such we are unable to convincingly explain this phenomenon. 5. Conclusions Using ESR spectroelectrochemistry the effects of overoxidation on the properties of charge carriers in PBuDOT have been investigated. Upon traversing of the potential boundary of electrochemical stability a sharp drop in the number of free spins in the polymer has been observed together with changes in their spectroscopic properties. This behaviour is irreversible as evidenced by the course of changes of subsequent spectroscopic parameters in the reduction half-cycle, being different from that observed for PBuDOT when studied in the potential range in which it is stable. These phenomena were attributed to degradation reactions taking place in the polymer at these potentials. Irreversible oxidation of polarons to bipolarons which are thought to undergo followup reactions and shortening of the conjugation length of the polymer’s -bond through spin pinning and crosslinking are thought to be one of these. Decomposition of the ESR spectra was possible in the overoxidation potential range, according the Gauss + Lorenz model originally found to apply for PBuDOT in the potential range below the overoxidation boundary. This decomposition allowed assessing of individual contribution of different groups of spins to the overall spin concentration after overoxidation. The appearance of Lorenz type spins together with changes in their spectra linewidth in the reduction half-cycle suggest that PBuDOT retains some of its electroactivity after overoxidation. Step by step voltammetric degradation of PBuDOT enabled following of the degradation process of the polymer in more detail. Subsequent changes in the voltammetric response of the polymer were explained in terms of shortening of the conjugation length leading to appearance of oligomer-like segments in the polymer’s main chain. Their consecutive shortening in the course of degradation was pro-
posed to be the prime reason for the loss of electroactive properties of the film, in the end leading to its complete electroinactiveness with the film being mechanically intact. Comparison of the CV of PBuDOT after the spectroelectrochemical experiment with these degradation CVs gave information about the relative extent of degradation of the polymer and corroborated the observed residual electroactivity of the overoxidised film.
Acknowledgement The authors gratefully acknowledge partial financing of this research by The Foundation for Polish Science.
References [1] Bayer A.G., Eur. Patent 339340 (1988). [2] L. “Bert” Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J.R. Reynolds, Adv. Mater. 12 (2000) 481. [3] A. Pron, P. Rannou, Prog. Polym. Sci. 27 (2002) 135. [4] F. Jonas, L. Schrader, Synth. Met. 41–43 (1991) 831. [5] G. Heywang, F. Jonas, Adv. Mat. 4 (1992) 116. [6] L. “Bert” Groenendaal, G. Zotti, P.-H. Aubert, S.M. Waybright, J.R. Reynolds, Adv. Mater. 15 (2003) 855. [7] M. Dietrich, J. Heinze, G. Heywang, F. Jonas, J. Electroanal. Chem. 369 (1994) 87. [8] A. Kumar, D.M. Welsh, M.C. Morvant, F. Piroux, K.A. Abboud, J.R. Reynolds, Chem. Mater. 10 (1998) 896. [9] D.M. Welsh, A. Kumar, M.C. Morvant, J.R. Reynolds, Synth. Met. 102 (1999) 967. [10] J. Chen, A.J. Heeger, F. Wudl, Solid State Commun. 58 (1986) 251. [11] J.A. Van Haare, E.E. Havinga, J.L.J. Van Dongen, R.A.J. Janssen, J. Cornil, J.L. Bredas, Chem. Eur. J. 4 (1998) 1509. [12] Y. Harima, T. Eguchi, K. Yamashita, K. Kojama, M. Shiotani, Synth. Met. 105 (1999) 121. [13] C. Kvarnstr¨om, H. Neugebauer, S. Blomquist, H.J. Ahonen, J. Kankare, A. Ivaska, Electrochim. Acta 44 (1999) 2739. [14] J. Bobacka, M. Grzeszczuk, A. Ivaska, Synth. Met. 44 (1991) 9. [15] M. Lapkowski, A. Pron, Synth. Met. 110 (2000) 79. [16] X. Du, Z. Wang, Electrochim. Acta 48 (2003) 1713. [17] I. Mukoyama, K. Aoki, J. Chen, J. Electroanal. Chem. 531 (2002) 133. [18] S.A.M. Refaey, G. Schwitzgebel, O. Schneider, Synth. Met. 98 (1999) 183. [19] A. Czardybon, M. Lapkowski, Synth. Met. 135–136 (2003) 27. [20] A. Zykwinska, A. Czardybon, B. Pilawa, M. Lapkowski, J. Solid State Electrochem. 8 (2004) 369. [21] A. Zykwinska, W. Domagala, B. Pilawa, M. Lapkowski, Electrochim. Acta 50 (2005) 1625. [22] Y. Harima, Y. Kunugi, H. Tang, K. Yamashita, M. Shiotani, J. Ohshita, A. Kunai, Synth. Met. 113 (2000) 173. [23] G. Zotti, S. Zecchin, G. Schiavon, Chem. Mater. 12 (2000) 2996. [24] V. No¨el, H. Randriamahazaka, C. Chevrot, J. Electroanal. Chem. 542 (2003) 33. [25] A. Zykwinska, A. Czardybon, B. Pilawa, M. Lapkowski, Chem. Phys. 292 (2003) 31. [26] Y. Harima, T. Eguchi, K. Yamashita, K. Kojama, M. Shiotani, Synth. Met. 105 (1999) 121. [27] J.E. Wertz, J.R. Bolton, Electron Spin Resonance. Elementary Theory and Practical Applications., Academic Press, London, New York, 1986.
In-situ ESR spectroelectrochemical studies of overoxidation behaviour of poly(3,4-butylenedioxythiophene) A. Zykwinska a , W. Domagala a , A. Czardybon b , B. Pilawa b , M. Lapkowski a,b,∗ a b
Faculty of Chemistry, Silesian University of Technology, 9 M. Strzody street, 44-101 Gliwice, Poland Institute of Coal Chemistry, Polish Academy of Sciences, 5 Sowinskiego street, 44-121 Gliwice, Poland Received 30 November 2004; received in revised form 20 March 2005; accepted 30 March 2005 Available online 6 September 2005
Abstract The behaviour of poly(3,4-butylenedioxythiophene) (PBuDOT), a relative of poly(3,4-ethylenedioxythipohnene) PEDOT within the poly(3,4-alkylenedioxythiophene) family, has been investigated at potentials above its electrochemical stability threshold using in situ ESR spectroelectrochemistry. The aim was to investigate the effect of electrochemical overoxidation on the charge carrying species, namely polarons, normally generated and annihilated during reversible redox doping and dedoping reactions, by determining the potential dependencies of spectroscopic parameters of the ESR spectra of the polymer over a selected potential range. Specific features of the trends of these dependencies allowed also for an evaluation of presence of the second type of charge carrying species—diamagnetic bipolarons and the effects of their interactions with polarons at different potentials. Around 1.5 V, where the boundary of electrochemical stability of the polymer lies, sharp drop of the concentration of paramagnetic centres has been observed together with a transitory narrowing of the ESR line. These changes were found to be irreversible as evidenced by the course of subsequent reduction half-cycle, which differed from the one for a not overoxidised polymer, observed in previous studies. Aided by the results of electrochemical studies it was concluded that the overoxidation process leads to a degradation of the polymer most probably due decrease of the conjugation length of the main chain -bond through cross-linking or addition reactions. While the electrochemical results pointed to a non-complete degradation of the polymer, the specific parameters of the ESR line in the reduction half-cycle indicate that the remaining spins are confined to isolated segments of a partially degraded polymer where their behaviour resembles oligomer-like radicals. © 2005 Elsevier Ltd. All rights reserved. Keywords: PBuDOT; Charge carriers; ESR spectroscopy; Polarons; Overoxidation; Degradation
1. Introduction The discovery and polymerisation of 3,4-ethylenedioxythiophene in the laboratories of Bayer A.G. in the late 1980-ties [1] marked the birth of one of the most successful conducting polymer in terms of both academic interest and practical applications, known to date [2,3]. Following numerous studies conducted over the last 10 years, poly(3,4ethylenedioxythipohnene) (PEDOT) is nowadays well known for its high conductivity, interesting electrical and spectrochemical properties associated with its low band gap, ∗
Corresponding author. Tel.: +48 32 237 1743; fax: +48 32 237 1509. E-mail address: [email protected] (M. Lapkowski).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.03.081
electrochromic and antistatic properties and good stability [4,5]. The success of PEDOT gave birth to a family of 3,4alkylenedioxythiophene polymers, research into which has been undertaken to tailor and fine-tune selected properties of this polymer into specific applications [6], as well as to try to overcome PEDOT’s few limitations and drawbacks like insolubility and infusibility. It was this idea which led to first syntheses of PEDOT analogues with alkylene chains of different size, namely poly(3,4-propylenedioxythiophene) (PProDOT) by Jonas and co-workers [4,7] later followed by poly(3,4-butylenedioxythiophene) (PBuDOT) by the group of Reynolds and co-workers who soon reported the first systematic study of electro- and spectrochemical properties of a number of different 3,4-alkylenedioxythiophenes [8,9].
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The purpose behind enlarging the alkylene ring was to space the polymer chains apart and by increasing the flexibility of the alkylene substituent, introduce some degree of disorder in the relative arrangements of the polymer chains, all of which could grant the respective polymer solubility in common organic solvents. To date however this approach alone proved unsuccessful in preparing a soluble 3,4alkylenedioxythiophene polymer. Nevertheless, members of this polymer family displayed some interesting properties, sometimes markedly superior to their parent relative PEDOT. The spectroscopic studies indicated that PProDOT displays a high degree of regularity along the polymer backbone while PBuDOT displays one of the fastest switching times together with one of the highest optical contrasts (%T amounting to 63%) in the poly(3,4-alkylenedioxythiophene) family [4]. Although the conductivity of PBuDOT is markedly impaired relative to PEDOT, it has been identified as one of the candidates for further studies on fast electrochromic materials. Despite these promising initial results, to date and to the best of our knowledge no reports of further studies concerning this polymer have appeared. The electrochemical and spectroscopic properties of conducting polymers depend on their doping state. One of the most accurate and reproducible methods of such doping is the electrochemical oxidation or reduction of the polymer. During doping of the polymer two types of charge carriers appear in the polymer – polarons and bipolarons [10–12] each dominating at different doping levels. When operating within the boundaries of stability of the polymer these two types of charge carriers coexist together and can be reversibly generated and annihilated. The question arises as to how stable these redox properties are and also what happens beyond the stability threshold. Right form the first studies, PEDOT was known as a very stable polymer [7], resistant to different ageing factors of both physical and chemical nature. Studies of PEDOT’s stability commenced almost concurrently with its first studies. Heinze and co-workers reported that overoxidation of PEDOT up to 3.0 V leads to evolution of a strong irreversible oxidation peak [7]. Venture to such high potentials led to complete loss of electrochemical properties of the material however. Kvarnstr¨om et al. have evaluated the onset of degradation potential at 1.9 V [13], which compared with 1.4 V for other polythiophenes [14] indicates that PEDOT is indeed an electrochemically stable material. Lapkowski and Pron have demonstrated a finite potential window of conductivity of PEDOT [15] while Du and Wang have reported in a recent study, the effect of polymerisation potential on PEDOT’s conductivity and ESR response [16]. When PBuDOT is concerned however, to date we did not come across any report concerning its stability studies. Since the polymer displays interesting spectroscopic and electrochemical properties it would be valuable to investigate its behaviour beyond normal application regimes in which it could be used. Recent detailed study of electrochemical dissolution of polythiophene presented by Aoki and co-workers [17] can provide some clues as to such behaviour. Their work gives
some insight into the degradation mechanisms involved in the degradation of an electroactive polymer—polythiophene, upon its overoxidation. The course of the degradation of polythiophene main chain studied through identification of its fragmentation products may be an indication as to what happens to the main polymer chain of PBuDOT during overoxidation. Refaey et al. have also employed electrochemical impedance to study the electrochemical degradation of polythiophene [18]. Our group has already presented the results of basic electrochemical studies of PBuDOT [19] together with ESR spectroelectrochemical studies in the stable potential range [20]. Following our previous studies of the effect of overoxidation on the properties of charge carriers in PEDOT [21], we decided to undertake a similar study for PBuDOT in order to investigate the intrinsic behaviour of charge carriers at high oxidation potentials for this close analogue of PEDOT. Of the two charge carriers present in organic conducting materials, polarons carry a magnetic moment and can therefore be observed and distinguished from diamagnetic bipolarons by means of ESR spectroscopy. Their spectroscopic response being sensitive to the chemical environment they reside in, enables however to draw some conclusions regarding the presence of bipolarons since these, alongside paramagnetic species, constitute part of that environment. This influence will be evident not only at high doping levels where polarons become oxidised to bipolarons but also at intermediate levels where both species coexist togehter [22]. The results of such studies have to be taken with great caution however. Degradation process of the polymer is never simple but rather a result of several parallel and/or consecutive reactions all taking place at different speeds, triggered by different factors. It is also difficult to unambiguously elucidate all the variables governing this process, as some may be hard to be expressed numerically making an observed degradation path difficult to reproduce. Nonetheless these information may help to determine and possibly explain the effects of extreme potential conditions on the redox properties of the material, supplementing the necessary information when potential applications of PBuDOT may be considered.
2. Experimental 2.1. Electrochemistry Cyclic voltammetry measurements were done on an AUTOLAB potentiostat–galvanostat model PGSTAT20 (EcoChemie, Netherlands) under the control of a PC class computer. The following electrodes were used: Platinum wire (φ = 1 mm) sealed using TeflonTM tape, with a geometric area of ca. 0.10 cm2 was used as working electrode. Ag wire was used as a quasireference electrode. Platinum coil of 8 mm diameter inside of which the two electrodes were positioned, served as an auxiliary electrode. All potentials are
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given versus the Ag quasireference electrode used. The E0 potential of ferrocene couple versus this electrode was equal to +0.40 V. The monomer 2,3,4,5-tetrahydrothieno[3,4-b][1,4]dioxocine (BuDOT) was synthesised in our group in a multistep synthesis, described by Kumar et al. [8]. Poly(3,4butylenedioxythiophene) was obtained via cyclic electropolymerisation on platinum electrode from 0.01 M solution of the monomer in 0.1 M (Bu)4 NPF6 in CH3 CN with maximum oxidation current control. For a given electrode area, the current value at which the oxidation half-cycle was reversed, did not exceed 3.5 × 10−4 A. Thanks to such optimised procedure we were able to obtain polymer films having reproducible CV responses each time.
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Fig. 1. Cyclic voltammogram of PBuDOT film showing the overoxidation peak of the polymer together with the background CV of the electrolyte −0.1 M Bu4 NPF6 in CH3 CN, both recorded at 0.5 mV s−1 . Inset shows the lower potential part of the CV in detail with an overlaid CV of PBuDOT in the potential range up to 1.0 V recorded at 0.3 mV s−1 .
field (Br ), were evaluated. g-Factor was calculated from the resonance condition: hν g= βBr 2.2. ESR measurements In situ ESR measurements were performed using ELPAN SE/X-2543 series X-band (9.3 GHz) reflective type ESR spectrometer (Radiopan, Poznan, Poland) with magnetic modulation of 100 kHz and with recording of the microwave resonance frequency. For spectroelectrochemical measurements, analogue ELPAN EP-21 type potentiostat driven by ELPAN EG-20 potential generator was used in conjunction with the ESR spectrometer. The spectroelectrochemical cell was a thin cylindrical glass tube narrowed at the bottom (3 mm in diameter). PBuDOT films were studied directly on the platinum electrode they were synthesised on utilising the same set of electrodes as described above except for an auxiliary electrode as a smaller diameter Pt spiral was used in this case. The electrolyte was 0.1 M (Bu)4 NPF6 in CH3 CN—the same as for polymerisation. The ESR spectra, recorded as the first derivative of resonance absorption curves, were computer collected. The spectra were measured with an attenuation of microwave power of 3 dB (∼15 mW), which came out to be still enough to avoid signal saturation. For all the spectra recorded in the potential range studied, the same modulation amplitude was used = 0.08 mT. Lineshape of the ESR spectra was numerically analysed using an Origin 6.0 program package form Microcal. Decomposition of the complex ESR spectra has been performed. The experimental spectra were fitted with superpositions of Gauss and Lorenz lines. The parameters of the lines of best fit (having the lowest χ2 value): linewidth (Bpp ) and resonance
where h is Planck constant, ν is microwave frequency, β is Bohr magnethon, and Br is resonance magnetic field.
Fig. 2. ESR spectra of PBuDOT film at different applied potentials during (a) oxidation half-cycle, carried out from −0.7 to 2.0 V and (b) reduction one from 2.0 to −0.7 V.
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The dependence of relative concentrations of paramagnetic centres in the studied polymer upon the potential applied to the polymer film was determined. Concentration of paramagnetic centres in the sample is proportional to the area under the absorption curve, thus double integration of the first-derivative ESR spectra was performed.
3. Results Studies of basic electrochemical properties of PBuDOT following its synthesis and first electropolymerisation attempts [19] led us to identify the oxidation potential boundary of PBuDOT around 1.5 V versus the same reference electrode as used in this study. To investigate the behaviour of the polymer in an extended potential window, a −0.7 to 2.0 V range was designated for this study. Cyclic voltammogram of PBuDOT in this potential range is presented in Fig. 1. The salient feature is the large and virtually irreversible oxidation peak at 1.65 V. Similar peak has been observed in PEDOT by Heinze and co-workers [7] and Zotti et al. [23], as well as in our previous studies of PEDOT’s overoxidation [21]. In this case however the peak is not as tall as in PEDOT, compared to the current magnitude of the rest of the CV response. In order to confirm the assignment of this peak, a CV of the electrolyte, shown in Fig. 1, has been recorded. Comparison of the
two indicates that the oxidation peak can be ascribed solely to the polymer, representing some sort of large-scale process, taking place at these potentials. This process leads to permanent changes in the polymer as evidenced by the course of the reduction half-cycle of the polymer, shown in more detail on the inset CV in Fig. 1. Comparing the reduction of the overoxidised film with the response of the film in the potential range of its electrochemical stability (Fig. 1 insert), one notes that the two clear reduction peaks are replaced with a broad and diffused one for the overoxidised film. These changes may suggest that a degradation process of some sort accompany the overoxidation of the polymer. ESR spectra of PBuDOT film highly depend upon the potential applied to the polymer film. These are shown in Fig. 2, arranged one after another as a function of progressively changed potential. Two maxima are observed in the oxidation half-cycle, at 0.1 and 1.4 V. At these potentials the onsets of the oxidation and overoxidation peaks of the polymer are located. In the reduction half-cycle the ESR signal is no longer so potential sensitive. The general trend is an increase of the ESR signal intensity interrupted only by a shallow dip at 0.2 V. Analysing the ESR spectra in Fig. 2 one can only compare their relative amplitudes. To allow for their complete comparison, selected spectra from both half-cycles were plotted, one above another, in Fig. 3. It can be seen that changes
Fig. 3. Selected ESR spectra of PBuDOT at various potentials in: (a) oxidation and (b) reduction half-cycles.
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Fig. 4. Concentration of paramagnetic centres in PBuDOT film as a function of the potential during oxidation and reduction half-cycles. Above a CV response of PBuDOT in monomer-free electrolyte −0.1 M Bu4 NPF6 in CH3 CN, recorded at 0.5 mV s−1 —a speed equivalent to the pace of the ESR spectroelectrochemical experiment. Inset shows the voltammograms in the same electrolyte as above, of PBuDOT before and after completion of the voltammetric cycle up to 2.0 V, shown below.
in the intensity of the ESR line are mutually coupled with changes in peak-to-peak (Bpp ) linewidth. In general, as the intensity of the signal decreases, the line broadens and viceversa. Detailed discussion of the potential dependency of the linewidth parameter—Bpp will be given further on. By doubly integrating the ESR signals, the relative concentration of paramagnetic centres in the polymer film was evaluated. These results are presented in Fig. 4. At the start of the experiment in the oxidation half-cycle a notable concentration of spins prevails decreasing slightly before a major rise commences around −0.1 V. At this point on the voltammogram (Fig. 4, inset CV) an oxidation pre-peak forms thought to represent the transition of the insulating film into a conducting form at light doping levels [20]. This pre-peak was observed in our previous studies of PBuDOT, and there too its presence was not reflected on the potential dependency of concentration of spins’ curve. As recently reported by Randriamahazaka and co-workers for PEDOT [24] this pre-peak becomes increasingly well defined and standalone, the lower the reduction potential applied to the polymer film is and the longer the duration of such treatment lasts. In the potential range of 0–0.3 V the concentration of spins increases sharply reaching a maximum at roughly the same potential as the oxidation peak of the polymer. Following that, a decrease of
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the concentration of spins is observed, up to a minimum at around 1.6 V. At this point, the total concentration of spins is roughly the same as in the potential range in which the polymer is undoped. At ca. 1.5 V the overoxidation peak of the polymer forms with its apex appearing at potentials when the concentration of spins is already at its minimum. In the reduction half-cycle the potential dependency is no longer similar to the oxidation half-cycle one as in the case of studies in the potential window up to 1.0 V [20]. A point to note is that at the end of the experiment, the concentration of paramagnetic centres is greater than at the beginning of the experiment. Studies of PEDOT and PBuDOT in the potential range of electrochemical stability revealed that the spectra of both polymers in the doped state are complex, being made up of two components one Gaussian and one Lorenzian [25]. Extending the potential rage in these studies, a question arose whether the same structure of the spectra prevails at these potentials. Fig. 5(a–d) shows four different spectra at different potentials modelled by the Gauss + Lorenz superposition. It can be seen that in all four cases the experimental spectra are well modelled. Though at different potential ranges the mutual relationship between the two components changes, in general, high and narrow Gauss line is observed accompanied by a small and wide Lorenz one carrying the major fraction of the concentration of spins. Decomposition of the experimental spectra according to this model yielded potential dependencies of three parameters—concentration of spins, Bpp linewidths, and g-factors of both components. Changes of concentrations of paramagnetic centres of each component with potential in the oxidation and reduction half-cycles are presented in Fig. 6(a and b), respectively. The potential dependence of the Lorenz component maps the total concentration curve very closely whilst the Gauss component takes an independent shape. The sudden concentration decrease between 1.4 and 1.5 V, associated with the overoxidation of the polymer, practically applies only to the Lorenz component. In the reverse reduction half-cycle the concentration of the Lorenz component, again follows the trend of the total concentration of spins (Fig. 6b). The concentration of the Gauss component keeps low, yet higher than in the oxidation half-cycle, again being more or less independent of potential in its whole range. Potential dependencies of linewidths (Bpp ) of different groups of paramagnetic centres are presented in Fig. 7. At first in the oxidation half-cycle, linewidths decrease very slowly, starting to increase above 0.1 V (Fig. 7a) as the polymer becomes doped. In the potential range in which the polymer is doped and both component paramagnetic centres can be distinguished, we find the linewidth of the Lorenz line decreasing monotonously until 1.5 V and that of the Gauss line oscillating with no clear trend. In the reduction half-cycle the dependence of linewidth of Lorenz component takes a shape similar to the dependence of concentration of spins with changing potential with a maximum at 0.2 V. The Gauss component has a linewidth virtually independent of potential down to 0.2 V.
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Fig. 5. Selected complex ESR spectra of PBuDOT showing the decomposition of the experimental line into two components – Gauss and Lorenz lines (a) and (b) oxidation; (c) and (d) reduction half-cycles.
The g-factor potential dependencies are presented in Fig. 8. In the potential range of −0.7 to 0.5 V the g-factor of the experimental line holds up above the free electron’s value of 2.0023. In general, the g-factor of the Lorenz component is lower than that of Gauss one. In both the oxidation and reduction half-cycles, little correlation with potential dependencies of other spectroscopic parameters is observed. In Fig. 4 we presented two voltammograms of a PBuDOT film—before and after oxidation up to 2.0 V. The polymer after oxidation up to 2.0 V does not appear to be fully degraded yet, however it is not known how far it is from complete degradation. In order to gain some insight into the stages of degradation of PBuDOT, we have recorded step by step CVs of the polymer upon its oxidation up to 2.0 V at 100 mV s−1 , results of which are shown in Fig. 9. It is observed that a much broader peak whose apex falls at 1.9 V replaces the large oxidation peak at 1.65 V registered at 0.5 mV s−1 . This shift is a further indication of the irreversibility of the processes taking place in the film at these high potentials. The biggest difference between two consecutive cycles is observed between the 1st and the 2nd one. In the following cycles the overoxidation peak diminishes quickly together with characteristic redox peaks of the polymer being replaced by numerous broad peaks from which in the end two dominant ones remain. In the final stages of degradation, the
remaining peaks decrease evenly until a stable CV is attained being similar to the background one. In total it takes nearly 100 continuous cycles (1.5 h) to fully degrade the polymer. In spite of that, at the end of the experiment the electrolyte solution remained clear and colourless and the film remained intact on the electrode and continued to adhere well to it even during cleaning of the electrode, suggesting that mechanical disintegration of the film has not taken place.
4. Discussion In our previous ESR studies of PBuDOT, we have investigated the spectroscopic properties of this polymer in the potential range of −0.7 to 1.0 V in which it is observed to be well stable [20]. The first eminent observation at potentials above 1.0 V is the sudden drop in concentration of spins observed in the narrow potential range between 1.4 and 1.6 V (Fig. 4) signalling sudden changes taking place in the population of polarons at these potentials. Possible explanations include their oxidation and/or recombination to spinless bipolarons [18] or some reaction with species generated in the overoxidation process represented by the large oxidation peak at 1.65 V on the polymer’s CV (Fig. 1). The magnitude of this CV peak indicates that profound changes in the polymer
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Fig. 6. Concentrations of spins of different groups of paramagnetic centres at different potentials in the (a) oxidation and (b) reduction half-cycles.
may be taking place and its irreversibility suggests that these changes are permanent. Bearing in mind that the potential boundary of electrochemical stability of PBuDOT is located around 1.5 V [19], this suggests that the overoxidation peak represent a process leading to the degradation of the polymer film. In this light the second interpretation of the drop of concentration of spins appears more justified and there are also other reasons to support it. Firstly, the polarons and bipolarons coexist in practically whole potential range of the doped polymer and it is only the relative position of equilibrium between the two, which shifts with changing potential [26]. A shift in this equilibrium, favouring the formation of bipolarons is probably responsible for the steady decrease in the concentration of spins at higher potentials in the range of 0.3–1.0 V. Secondly, the drop in concentration of spins around 1.5 V is virtually irreversible, as only a small rise around this potential is observed in the back reduction half-cycle. Under normal conditions, the polaron–bipolaron transition is reversible upon dedoping of the polymer. If this had been the case then a reduction peak reflecting this process should have been observed on the polymer’s CV. The absence of this peak indicates that the overoxidation process has led to irreversible reactions within the polymer film. Going further into the reduction half-cycle we also observe that the characteristic two redox peaks observed on PBuDOT’s CV up
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Fig. 7. ESR signal linewidths (Bpp ) of different groups of paramagnetic centres at different potentials in the (a) oxidation and (b) reduction halfcycles.
to 1.0 V (Fig. 1, inset) are now missing, replaced by a very broad peak whose shallow apex falls at around 0.1 V, matching the potential at which a maximum concentration of spins is observed. Also a voltammogram of the film recorded at 100 mV s−1 after the slow scan rate experiment compared with CV recorded before it (Fig. 4, inset), reveals a substantial change in the electrochemical response of the film. Apart from the coalescing of the oxidation peaks a decrease in the current magnitude is observed which in the absence of dissolution of the film implies that the electroactive area of the polymer film has decreased. These observations suggest that degradation of PBuDOT takes place during its overoxidation above 1.5 V, leading to partial or complete loss of its electroactive properties. Step by step degradation voltammograms of PBuDOT presented in Fig. 9 give us some information regarding the path of the degradation process of the polymer. As already noted the biggest changes in the CV of the polymer are observed in the first cycles. Here, an interesting feature is observed—an “undercut” of the CV in the range between 1.8 and 1.3 V in the reduction half-cycle. We have observed, in overoxidation studies up to potentials close to the polymer’s degradation threshold, that it is the first sign of commencing degradation of the polymer before any other changes appear in the shape
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Fig. 8. g-Factor of different groups of paramagnetic centres at different potentials in the (a) oxidation and (b) reduction half-cycles.
of the CV. The large current drop upon reversing of the potential run at the end of the oxidation half-cycle of the polymer film (Fig. 4 insert) is an indication of its high double layer capacity and its sharpness is an effect of high conductivity (low resistance) of the film. Changes observed in this part of the CV upon degradation in the form of this “undercut”, may signify that structural changes leading to alteration of these two properties of the film take place before changes in the redox properties of the polymer do. In the first cycles around 0 V a sharp pre-peak of the main oxidation peak of the polymer is present. This peak can be ascribed to the process of transition of the film from the insulating to the conductive form comprising conformational change in the film and the sudden increase in the double layer capacity of the electrodefilm system [20,24]. Its continued presence in the first stages of degradation may indicate that the film still retains its electroactive properties though the shift of this peak into positive potentials means that it becomes ever more difficult to oxidise the film into its conductive state. This may be an effect of a decrease in the conjugation length of the polymer’s bond occurring as a result of the degradation reactions taking place in the film. Another indication of this process may be the complex shape of the CVs at intermediate stages where they appear to be made up of multiple peaks. These may signal the emergence of oligomer-behaving conjugated segments of different length along the polymer backbone. If we
Fig. 9. Electrochemical degradation of PBuDOT at Pt electrode induced by its cyclic oxidation up to a potential of 2.0 V in 0.1 M Bu4 NPF6 in CH3 CN at 100 mV s−1 . Progressive CVs showing the changes of the electrochemical response of the polymer film at (a) initial, (b) final stages until a stable CV is attained. For clarity, only selected cycles are shown. Arrows indicate the direction of changes of different parts of the CV. The cycle in (a) shown in dashed line, corresponds to the CV of PBuDOT (inset of Fig. 4, solid line CV) after the electrochemical degradation experiment, and may be considered to represent the electrochemical response of the polymer after the ESR spectroelectrochemical experiment (Fig. 2).
compare the shape of the CV of PBuDOT after slow scan rate degradation experiment (Fig. 4 insert, solid line CV) with these consecutive degradation CVs, we find closest match being the voltamogram around 40th cycle (Fig. 9a, dashed line CV). Considering that the CV peaks at these stages of degradation come from oligomer-like segments on the polymer main chain, it may be these segments where the Lorenz type spins reside and undergo electrochemical reactions in the reduction half-cycle. This may thus explain PBuDOT’s residual electroactivity after the ESR spectroelectrochemical experiment, as it is not fully degraded yet. Decomposition of the experimental spectra according to the Gauss–Lorenz model gave us information regarding the contribution of each group of spins to the overall ESR response (Fig. 6). These results may help to shed some light on what happens in the polymer upon its degradation. In our previous studies we have identified the Gauss centres as heterogeneously distributed pinned defects of the polymer backbone, conjugation defects and radical trapped at chain ends during electropolymerisation. Lorenz centres were
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attributed to homogeneously distributed polaronic spins appearing on the polymer’s conjugated -bond upon doping, thus thought to represent the electrons of conductivity in the polymer. This assignment seems valid in this study as the concentration of Lorenz type centres maps the overall concentration curve while the Gauss type centres are practically potential insensitive. The fact that it is only the Lorenz type centres that diminish during the sudden drop at 1.5 V further justifies their assignment to polaronic spins. This decrease can be caused by sudden oxidation to diamagnetic bipolarons as discussed above or by recombination with free radicals that may be generated upon overoxidation of the polymer, which seems more probable in light of the irreversibility of this process. One possible source of these free radicals may be the cleaved electron-rich C O bonds between the thiophene meric unit and the 3,4-butylenedioxythiophene ring. Radicals from the electrolyte or the solvent can rather be excluded since voltammetric studies of the background response revealed no decomposition peaks up to 2.0 V (Fig. 1). One cannot exclude however that the solvent molecules or the PF6 − ions may react with the heavily oxidised polymer chain leading to introduction of functional groups which may be responsible for interruption of conjugation in the main chain. Abstraction of hydrogen cation leading to interchain crosslinking reaction may also take place especially considering that no loss of cohesiveness of the film has been observed. Interesting is the absence of changes in the concentration of Gauss type centres around 1.5 V—a distinct behaviour compared to the parent relative PEDOT where these centres also diminish at this point [21]. This insensibility indicates that these spins are indeed very stable or that they do not interact with the Lorenz type ones. Puzzling is the rise of concentration of Gauss spins in the back reduction half-cycle around 0.7 V as no special feature on either the voltammogram (Fig. 1) or potential dependencies of other spectroscopic parameters of Gauss spins is observed. Nevertheless compared to the oxidation half-cycle, a slightly higher concentration of this type of spins may be an indication that their population has been augmented by defects created during overoxidation (degradation). The Lorenz type spins on the other hand are more potential sensitive. At first a small rise in their concentration around 1.5 V indicates that some process, reverse to the overoxidation one could be taking place. It is followed by a rise up to 0.2 V when their concentration reaches around 3/4 of their maximum concentration in the oxidation half-cycle. This together with their follow-up decrease suggests that the film has retained some electroactivity. The potential sensitiveness of population of paramagnetic centres is much weaker however compared to PBuDOT film studied in the potential range up to 1.0 V [25]. Comparing the concentration dependencies in the reduction half-cycle in this study with the ones for a film studied up to 1.0 V substantial differences are observed. Not only is the peak concentration smaller but also it appears at a lower potential and the final concentration at the end of the experiment is relatively higher. Such behaviour may be due
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to shortening of the conjugation length of the polymer’s bond either by saturation of individual carbon-carbon bonds, cross-linking, or scission of the polymer’s main chain which could have taken place during overoxidation. The higher concentration of spins at the end of the experiment compared to its start, also observed in PEDOT [21], cannot be explained by increased number of spin type defects as the concentration of Gauss type spins is to low for this (Fig. 6b). Rather it may be the difficulties in charge carriers’ transport through a polymer whose electroactive properties have been impaired due to partial degradation, that hinder prompt reduction of Lorenz type spins at the end of the reduction half-cycle. Linewidth of ESR line reflects the type of magnetic interactions between the paramagnetic centres and their surroundings and the centres themselves [27]. From the start of the experiment we observe practically constant linewidth of the experimental ESR line until 0 V when broadening takes place. This broadening is caused by increasing dipolar and exchange interactions between polarons in their growing population at the onset of doping. Onward from 0.5 V the two types of paramagnetic centres behave differently. The linewidth of Gauss centres is again practically potential insensitive however a shallow decrease may be picked out around 1.5 V. On the contrary the linewidth of Lorenz type centres decreases constantly until 1.5 V after which it stabilises. This line narrowing may be explained in terms of a polarons’ pairing process, in which generated bipolarons begin to separate the polaronic spins. Contrary to PEDOT no sudden narrowing of the linewidths is observed at the potential of overoxidation of the polymer, which implies that in PBuDOT overoxidation reactions are less sudden and spread over a wider potential range. Nevertheless, the final narrowing of Lorenz line may indicate that at high oxidation potentials, mutual interactions of these homogeneously distributed spins weaken considerably up to a point that they may end up separated and confined in the so-called “spin packets”. Small changes in the linewidth of heterogeneously distributed Gauss centres indicate that these spins are indeed stable and not interacting with the polaronic spins. The linewidth decrease around 1.5 V may be caused by changes in the paramagnetic environment caused by changing Lorenz type centres, or by chemical changes inflicted on the polymer upon its overoxidation. In the reduction half-cycle the potential dependence of linewidth of the Lorenz line is very similar to the concentration dependence of this type of centres (Fig. 6b), suggesting that changes in their population are closely linked, and determine the strength of dipolar interactions between them. The fact that the linewidth of Gauss type centres decreases below 0.2 V suggests that upon dedoping of the polymer exchange interactions even between these centres weaken as a result of disappearance of polarons from the still electroactive polymer. Overall the observed linewidth dependencies in the reduction half-cycle provide evidence that specific interactions still exist between spins and the polymer matrix even in a partially degraded film. Analysis of the g-factor potential dependencies (Fig. 8) can give some insight into the environment of spins in the
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film. A first observation is that this parameter is rather insensitive to the sharp changes taking place in the film around 1.5 V during oxidation. Changes in g-factor value indicate, among other things, changes in the spin-orbit coupling effect. Increase in the spin-orbit coupling may be the reason behind the observed rise in g-factor values of both Lorenz and Gauss component lines above 0.5 V. In the reverse, reduction halfcycle, an opposite mechanism can be recalled to explain the observed gradual decrease in the g-factor of the Gauss line, most rapid between 0.8 and 0.2 V when the concentration and the ESR linewidth of the second—Lorenz component increase. Interestingly however the Lorenz g-factor remains practically constant until −0.2 V, which would indicate that the environment of these spins change little. The magnitude of the g-factor of the Gauss line, being higher than a value of 2.0023, indicates that for this group of spins a spin-orbit coupling takes place presumably with C or O atoms. For the Lorenz group its g-factors are peculiarly low and as such we are unable to convincingly explain this phenomenon. 5. Conclusions Using ESR spectroelectrochemistry the effects of overoxidation on the properties of charge carriers in PBuDOT have been investigated. Upon traversing of the potential boundary of electrochemical stability a sharp drop in the number of free spins in the polymer has been observed together with changes in their spectroscopic properties. This behaviour is irreversible as evidenced by the course of changes of subsequent spectroscopic parameters in the reduction half-cycle, being different from that observed for PBuDOT when studied in the potential range in which it is stable. These phenomena were attributed to degradation reactions taking place in the polymer at these potentials. Irreversible oxidation of polarons to bipolarons which are thought to undergo followup reactions and shortening of the conjugation length of the polymer’s -bond through spin pinning and crosslinking are thought to be one of these. Decomposition of the ESR spectra was possible in the overoxidation potential range, according the Gauss + Lorenz model originally found to apply for PBuDOT in the potential range below the overoxidation boundary. This decomposition allowed assessing of individual contribution of different groups of spins to the overall spin concentration after overoxidation. The appearance of Lorenz type spins together with changes in their spectra linewidth in the reduction half-cycle suggest that PBuDOT retains some of its electroactivity after overoxidation. Step by step voltammetric degradation of PBuDOT enabled following of the degradation process of the polymer in more detail. Subsequent changes in the voltammetric response of the polymer were explained in terms of shortening of the conjugation length leading to appearance of oligomer-like segments in the polymer’s main chain. Their consecutive shortening in the course of degradation was pro-
posed to be the prime reason for the loss of electroactive properties of the film, in the end leading to its complete electroinactiveness with the film being mechanically intact. Comparison of the CV of PBuDOT after the spectroelectrochemical experiment with these degradation CVs gave information about the relative extent of degradation of the polymer and corroborated the observed residual electroactivity of the overoxidised film.
Acknowledgement The authors gratefully acknowledge partial financing of this research by The Foundation for Polish Science.
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