Electropolymerized polythiophene layer extracted from the interface between two immiscible electrolyte solutions: Current–time analysis

Journal of

Electroanalytical Chemistry Journal of Electroanalytical Chemistry 591 (2006) 59–68 www.elsevier.com/locate/jelechem

Electropolymerized polythiophene layer extracted from the interface between two immiscible electrolyte solutions: Current–time analysis Marinka Vignali b

a,b,*

, Robert A.H. Edwards b, Marina Serantoni c, Vincent J. Cunnane

a,c

a Chemical and Environmental Science Department, University of Limerick, Limerick, Ireland European Commission, DG Joint Research Centre, Renewable Energies, Via Fermi 1, T.P. 450, 21020 Ispra, Va, Italy c Materials and Surface Science Institute, University of Limerick, Limerick, Ireland

Received 10 November 2005; received in revised form 10 March 2006; accepted 17 March 2006 Available online 6 May 2006

Abstract Polythiophene was formed by electropolymerization at the interface between two immiscible electrolyte solutions, using terthiophene as the starting monomer in 1,2-dichloroethane. The water phase contained a redox couple to allow removal of electrons through the interface. For the first time, a layer of polythiophene was produced which was strong enough to extract. The mechanism of electropolymerization was found to be similar to that in the electrodeposition of polythiophene on metals: progressive nucleation and 2D growth precedes 3D growth, ascribed to precipitation of oligomers from solution. The polymer extracted was found to be partially oxidized (irreversibly doped) to a conductive state, and stable in air.  2006 Elsevier B.V. All rights reserved. Keywords: 2D growth; Electropolymerization; Conducting layer; Polythiophene; ITIES

1. Introduction Polythiophene has many attractive properties for electrooptics and other applications [1]. It is easy to control its doping over a wide range from n-type to p-type semiconductor to metallic conduction. As well as high conductivity, polythiophene absorbs visible light strongly in the doped state [2]. It is also probably the most stable conductive polymer yet tested for resistance to water, air and light [3]. However, it cannot be formed into membranes by solvent evaporation, because it is insoluble. Polythiophene can be deposited onto conducting substrates by electrochemical polymerization in organic electrolytes. The present work aims to produce free-standing membranes of polythiophene by electropolymerization at the interface between two immiscible liquids and to understand the mechanism of deposition. * Corresponding author. Address: European Commission, DG Joint Research Centre, Renewable Energies, Via Fermi 1, T.P. 450, 21020 Ispra, Va, Italy. Tel.: +39 0332789190; fax: +39 0332789992. E-mail address: [email protected] (M. Vignali).

0022-0728/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2006.03.033

The mechanism of polymerization of polythiophene onto solid conductive substrates has been much investigated. Polymerization of electropolymers can proceed by reaction of monomer with radical cations formed by oxidizing the existing polymer, but in polythiophene, the growth is essentially independent of the substrate: it is necessary to oxidize the dissolved species (monomer or short oligomers) to form radical cations [4,5]. Originally, it was assumed that two radical cations combine to form an unstable dihydrogenated dication, which then released protons to form a longer thiophene chain (Scheme 1). This always seemed unlikely because the two cations would repel each other. In fact, Wei et al. [6] showed convincingly that the radical cation adds onto the end of a neutral chain (losing an electron and two protons). An important point is that radical cations can attach to monomer and short oligomers in solution, as well as onto deposited polymer chains [4]. The shorter oligomer products are soluble in organic media, can diffuse away from the interface and cannot link to the deposited polymer. Like electroplating, deposition of conducting polymers by electropolymerization takes place by nucleation and

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Formation of the radical cation (polaron) from monomer in equilibrium with dication (bipolaron) formation: (M)⇔(M)+. +e-⇔ (M)2++eDihydrodication (via α-α coupling): . (M)n+. +(M)+ ⇔(M)n+ (M)+ Deprotonation: (M)n+ (M)+ ⇔ (M)n+1+2H+

Lengthening: (M)n+. +(M)+. ⇔ (M)n+1+2H+

Disproportion (polaron bipolaron): 2(M)n+. ⇔(M)n2++(M)n

and so on, starting from first step.

Scheme 1. Early mechanism on the polymerization of heterocycles.

growth [7]. The properties of the films depend on the nucleation mechanism. For example, nucleation and 2D growth gives a compact film, 3D growth from instantaneous nuclei will tend to give columnar grains, and 3D growth from progressively-formed nuclei will give a more equiaxed grain structure. In the case of polythiophene electrodeposition, apparently contradictory results have been reported. The problem was resolved when it was realized that there are two stages in the deposition process [8]. First, instantaneous nucleation and 2D layer-by-layer growth over the surface forms a relatively compact polymer [9,10]. Then, if the current density is sufficient, and after a delay, 3D growth starts, forming a much thicker and more porous grain structure on top. So the time-frame of the experiments determines which nucleation mechanism is identified. In the 3D growth region, nucleation is ascribed to precipitation of oligomers from locally-saturated solution. The number of nuclei increases steadily with time so that the current (at constant potential) increases until the grains impinge [7], after which it remains roughly constant. The two constituent parts of the layer show reversible oxidation peaks (i.e., doping) at very different electrode potentials [11]. This suggests more than a difference in chain length and morphology: perhaps the oligomers from solution are chemically associated with other species. In the present case an aqueous solution is used as one of our immiscible liquids and, as such, the effect of water dissolved in the organic phase on the polymer and its formation is an issue. It is known that the presence of a few percentage of water (or methanol) in acetonitrile leads to oxidation of polythiophene. First, one, and then two, oxygen atoms attach to a sulphur atom in the ring, with accompanying loss of protons [12]. At this stage the conjugated carbon chain is still intact, and the polythiophene remains conductive, although it is irreversibly oxidized. This stage finishes when about one in six monomer units has two oxygen attached [12]. We prefer to reserve the pejorative term ‘‘overoxidation’’ to the following oxidation stage, in which the chain is finally broken, with release of SO2. This requires a higher water concentration, or a higher oxidizing potential. The remaining steps are successive oxidations of the broken

chain-ends, forming first carbonyl and finally carboxylic acid groups [12]. Whilst Morgan and Kwolek [13] extracted polyamide at a L/L interface by interfacial polycondensation, it was Koryta who pioneered the study of the interface between two immiscible electrolyte solutions, ‘‘ITIES’’ in 1976 [14]. The two solvents must be immiscible, chemically compatible, and have different density. The most popular solvent for electropolymerization of polythiophene is acetonitrile, but this mixes with water. Of more hydrophobic solvents, 1,2-dichloroethane (DCE) is preferred because it combines high permittivity, low viscosity and chemical stability. The high permittivity (e = 10.36) allows it to dissociate salts into ions despite its limited polarity, whilst the low viscosity makes the ions more mobile. This means that DCE can be the basis for a high-conductivity supporting electrolyte. The chemical stability allows use of a wide range of electrode potentials [7,15]. Previous work with polyterthiophene formation at a liquid/liquid interface had shown that DCE with tetraphenylarsonium-tetraphenylborate pentafluoride (TPAsTPBF20) was a suitable medium for polymer formation [16,17], but this is the first attempt to extract the polymer as a free-standing film using ITIES and to compare the mechanism with that of electropolymerization onto a solid substrate. 2. Experimental details The aqueous solution contained 0.2 M H2SO4 (BHD 98%), 0.1 M Li2SO4 (Aldrich 99%) as a supporting electrolyte, 0.1 M Ce(SO4)2 (Aldrich 99%) and 0.01 M Ce2(SO4)3 (Fluka 97%) as a redox couple. The organic solution (DCE, Fluka 99.5% G.C. grade) contained 1 · 103 M tetraphenylarsonium tetrakis (pentafluorophenyl) borate (TPAsTPBF20), as a supporting electrolyte and 1 · 103 M 2,2 0 :5 0 200 terthiophene (Fluka > 99%) as monomer. TPAsTPBF20 was prepared as in the literature [18] from lithium tetrakis(pentafluorophenyl)borate etherate (Boulder Scientific > 99%) and TPAsCl (Fluka > 95%). The cell-compartment for the reference electrode contained 0.1 M tetraphenylarsonium-chloride hydrate (TPAsCl) (Fluka > 95%) in H2O (18.2 MX cm).

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2.1. Experimental set-up A Solartron SI 1287 Electrochemical Interface for potentiostatic and galvanostatic analysis at short and long times was used. The surface and the physical structure of the polymer was investigated, using an Olympus optical microscope and an AFM surface probe microscope. (TopoMetrix– ThermoMicroscope–VEECO). Non-contact mode was used throughout. The tips used were high resonance frequency silicon tips (frequency range 354–409 kHz), with a 120 lm long cantilever and tip of 3–6 lm base, 10–20 lm long, 20 nm tip radius. The raw data collected were processed by the TopoMetrix SPMLab NT Version 5.0, using first order levelling and left shadowing. The extracted polymer was analyzed by a IR spectrometer, Bohem MB, in the form of tablets (KBr method) prepared by using a Carver Inc. Press Mod. 3392.

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The initial electrochemistry cell used was a classical four electrode cell, with interfacial area = 0.196 cm2 (SIAC, small interfacial area cell): Ag=AgCljTPAsClðH2 OÞjTPAsTPBF20 =MonomerðDCEÞjj CeðSO4 Þ2 =Ce2 ðSO4 Þ3 =Li2 SO4 =H2 SO4 ðH2 OÞjPt where i represents the organic/aqueous interface. The potentials quoted in this paper are the potential of the reference electrode in the aqueous phase measured with respect to the reference electrode in the organic phase. The aqueous phase occupied the upper half of the cell. The reference electrode in this phase was a simple platinum wire in the Ce3+/Ce4+ solution. The potential of the organic phase was sensed using a Ag/AgCl electrode in aqueous chloride solution: the cation (TPAs+) was chosen to be the same as in the organic phase,

Fig. 1. Large interface cell (LIAC), interfacial area = 19.64 cm2 ((b) scale 1:2.8).

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allowing transfer across the interface and a stable interface potential [19]. A screw-driven plunger varied the volume of the lower half-cell, to allow positioning of the interface between the Luggin capillaries. The cell was filled by feeding the organic solution first, then the reference solution, and finally the inorganic solution. A wider cell was designed to allow extraction of the film, where the area of interface was 19.64 cm2, 50 mm diameter (Fig. 1) (LIAC, large interfacial area cell). The outermost 2 mm of the cell forms a toroidal Luggin capillary. This isolates the film-forming liquid–liquid interface from the walls of the cell, eliminating edge effects caused by the meniscus, and facilitating the extraction of the film. Furthermore, there are no obstacles in the upper part of the cell to impede removal of the formed polymer layer from the interface: the top electrode consisting either of platinum coil bent around the inside of the glass or a removable platinum rod (U = 2 mm). The bottom electrode consists of a 50 mm platinum foil. The reference electrodes are immersed in the two side-‘‘towers’’; the one for the organic phase, on the right, contains aqueous TPAsCl solution. 3. Results and discussion 3.1. Potentiostatic analysis Monomer oxidation was observed to start at around 1.75 V in SIAC cell. The potential for this reaction depends on the redox potential of the aqueous phase with respect to the reference potential in the organic phase (here the aqueous phase can be thought of as a metal). In the literature the reaction is reported to occur at 1.04 V vs. SCE [20]. The first potentiostatic experiments with the SIAC were at potentials close to the monomer oxidation potential. A green soluble product, presumably oligomers, was formed in the organic phase near the interface, but there was no visible deposition of solid polymer. The potential was increased in order to obtain a stable polymer at the interface. Below 1.8 V there is only a small, transient, exponentially decaying current due to charging of the double layer. Above 1.8 V, the terthiophene oxidation potential, currents are much higher and decrease much more slowly, showing signs of hidden peaks. The same features as potentiostatic analyses of conventional electropolymerization in the literature [1,7,21–24] were observed. Fig. 2 shows two of the curves to better understand these features. After charging the double layer, the first decay of the current is generally associated with the diffusion controlling the monomer oxidation; then a peak is observed (Fig. 2a at about 3 s), generally attributed to nucleation and growth, followed by overlapping of growing nuclei [25], which reduces the active surface area, lowering the current. The behavior at longer times depends on the applied potential: at 2.3 V or below, the current continues to decay (Fig. 2a), and the polymer layer is too thin to be extracted.

Fig. 2. Potentiostatic run, SIAC, A = 0.196 cm2, 1 mM 3T, 1 mM TPAsTPBF20. No iR compensation. (a) at 1.9 V showing nucleation, growth of polymer and decay of the current. Imax = 1.22 · 105, tmax = 3.55 s. No polymer could be extracted from the interface. (b) at 2.4 V, the current increases after the first decay and a polymer layer is observed at the interface.

At 2.4 V the current starts to increase again (Fig. 2b), producing an extractable polymer layer. These potentials are again not compensated for iR drop between the ends of the Luggin capillaries and the interface. In all runs above 1.8 V, a greenish layer of solution, a few mm thick, built up in the organic phase below the liquid/liquid interface, and at high currents this layer formed fingers which flowed towards the bottom of the tube. Mukoyama et al. [26] observed a similar phenomenon when electrodes coated with poly(3-methylthiophene) were polarized anodically in an electrolyte based on nitrobenzene, in the absence of monomer. He associated the fingers (dark blue in his case) with soluble products formed by the oxidative decomposition of his pre-existing polymer layer. The potential required was similar to that for polymerizing poly(3-methylthiophene) (P3MT). He showed that the fingers in his experiment are driven away from the electrode by the electric field (rather than by gravity). It is possible that the fingers, in the present case, are caused by a similar phenomenon, even though it was found that the polymer itself, extracted at the end of the experiment, was not over-oxidized (see below). An alternative

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explanation is that the green coloration is caused by directly-oxidized oligomers. Radical cations, dications and dihydrodications of oligomers are formed at the liquid–liquid interface (or, later, at the polymer–organic interface). To form a polymer they must react with the growing polymer chains at the interface. However, oligomers are generally all soluble: they can also migrate or diffuse away from the interface before they have time to form a polymer. Subsequently, they may react with monomer or other oligomer cations to form longer oligomers in the bulk solution. Whichever explanation holds, the convection generated by the formation of fingers brings fresh solution nearer the interface, and increases the diffusion-limited current. This explains why the formation of fingers is associated with the long-term increase in current, which also produces a thick polymer layer. Mukoyama et al. [26] showed that soluble oxidative product formation only starts after several tens of seconds, and our finger-formation is similarly slow. By contrast, electopolymerization starts within seconds. Therefore, the initial part of our curves can be analysed in terms of polymer nucleation mechanisms, following the method used by Li and Albery in 1992 [7]. Since the behavior of electropolymerization at a L–L interface was rather variable, a statistical approach was used to study the I vs. tx dependence, to identify the growth mechanism of the polymer. A set of 55 potentiostatic curves was chosen from the experiments and the exponent x analyzed in the first seconds (before the second raising of the transient, generally 25 s long, including the time to charge the capacitor). Most results fell into clearly defined exponents corresponding to x = 0.5, x = 0.5, x = 1 or x = 2. Fig. 3(a–d) show the I vs. t and I vs. tx fits for the different behaviors. Only values of R P 0.999 have been accepted in assigning the behaviors to the different categories in Table 1. Fifty-five curves were analysed: Table 1 shows the % fitting in to each category of the exponent x. A dependence of I vs. t2 (27%) is normally associated with the nucleation and growth of the polymer, controlled by a charge-transfer step (Fig. 3a) [8,22,27,28]. The second relationship (indeterminate) is not understood at the moment. It is possible that this is a case of multiple dependence, as reported in study of PEDOT polymerization [1]. A t1/2 dependence (Fig. 3b) has been reported in cases where the current is limited by the diffusion either of the monomer or by the return of the oxidized monomer. If interactions of growing centres have to be considered (typical common behavior at long time), a I vs. t1/2 dependence is expected (Fig. 3c). If the nucleation of growth centres is essentially instantaneous, a dependence of I vs. t will be observed (Fig. 3d). The most common dependence is I vs. t2 and therefore this formed the basis of further investigation. Such a dependence could be the result of either progressive 2D nucleation and growth (2DP) (Eq. (1)) or instantaneous

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3D nucleation and growth (3DI) (Eq. (2)), because the initial growth equations are similar [7,23,27]: pzFhAN 0 k 2 t2 MW q 2pzFN 0 k 3 t2 MW2 I¼ q I¼

ð1Þ ð2Þ

where: MW = molecular weight, N0 = number of available nucleation sites, k = growth rate constant (mol cm2 s1), A = nucleation rate constant (s1), q = density of the material deposited, z = charge transferred from each monomer unit, h = height of the 1D disc shaped nucleus, F = Faraday constant. However, the two cases can be distinguished by looking at what happens when the nuclei start to impinge. In the case of progressive nucleation where N = AN0t [23,27], the overall current associated with 2D growth involving overlap can be described by the following equation [27]:   pzFhMAk 2 N 0 t2 pM 2 Ak 2 N 0 t3 I¼ exp ð3Þ q 3q2 If expressed in the reduced form in terms of the maximum Im and the corresponding time tm, one can write:  2  3  t 2ðtm  t3 Þ exp I=I m ¼ ð4Þ tm 3t2m where Im and tm are potential dependent through the rate constants Ak2: o log I m o logðAk 2 N 0 Þ ¼ 3oE oE o log tm o logðAk 2 N 0 Þ ¼ 3oE oE

ð5Þ ð6Þ

This is a very interesting result since the product Imtm is independent of potential for the 2D mechanism alone, even for a 2DI process where N0 nuclei are formed instantaneously at the moment the potential is applied. On the contrary, 3D processes controlled by either electrochemical step or diffusion give a product Imtm strongly influenced by potential. Therefore, by looking at how Imtm varies with different applied potentials 2D and 3D processes can be differentiated. To enable the statistical approach, experiments at each potential were repeated (using refreshed cells) many times (Table 2). This approach has wider implications than just this system. The roughly constant value of the product Imtm for the most common t2 dependence confirms a mechanism of 2D nucleation and growth for the polymerization of the terthiophene. A very interesting result, but not often reproduced, is shown in Fig. 4. It only happens with very fresh solution (solutions used in the first minutes after preparation); similar behavior was reported for deposition of poly(thiophene-3-acetic acid) in particular cases of extremely clean electrodes and fresh solutions [7].

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Fig. 3. I vs. t and I vs. tx fits for the first 20 s as example of current time dependence analysis: (a) x = 2, E = 2.1 V; (b) x = 0.5, E = 2 V; (c) x = 0.5, E = 2.1 V; (d) x = 1, E = 2.2 V.

Table 2 Nucleation peak values (Im and tm) in potentiostatic runs Table 1 Statistical approach of I vs. t dependence %

x

27.3 25.43 21.8 16.3 9.2

2 Indeterminate 1 0.5 0.5

% is the percentage of an exponential dependence x vs. the entire number of the experiments analyzed.

E applied (V)

Im (A)

tm (s)

Imtm (C)

1.8a 2 2.2 2.2 2.2 2.4 2.4 2.4

7.9E-6 2.4E-5 1.89E-5 2.04E-5 4.08E-5 3.71E-5 4.41E-5 2.99E-5

5.03 1.45 1.95 1.86 1.03 1.04 1.05 1.05

3.97E-5 3.48E-5 3.68E-5 3.79E-5 4.20E-5 3.85E-5 4.63E-5 3.14E-5

a

One of the few case in which this maximum is shown at 1.8 V.

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Fig. 4. Potentiostatic run, SIAC, at 2.2 V, showing 2D nucleation and growth, A = 0.196 cm2, 1 mM 3T, 1 mM TPAsTPBF20. No iR compensation.

The interpretation was that the peaks were formed by the nucleation and growth of successive individual layers of polymer. The peaks are distinguishable in this case because, nucleation being relatively difficult in the clean conditions, a whole polymer layer could grow over the surface before the next layer nucleated [29–31]. In the more common case where peaks are not seen, heterogeneous nucleation on impurities meant that nucleation of the next layer could occur whilst a layer was still growing. One can estimate the amount of polymer accumulated at the interface using the area of the narrow current-peaks. Taking the average charge under well-separated peaks gives an average charge of 30 ± 5 lC/cm2 (for comparison, deposition of poly(thiophene-3-acetic acid) gave peaks corresponding to 213 lC/cm2 [7]). Converting the charge to the thickness using the Faraday equation, a thickness of 0.27 ± 0.05 nm is calculated, which is also about the diameter of a thiophene ring, calculated from bond lengths [32]. The size of the current peaks is consistent with the formation of successive monolayers of polythiophene, with the polymer chains lying along the surface, and with the thiophene rings perpendicular to the surface. It is assumed that when solution is stored in air, oligomers are formed (shown by a green coloration): these make nucleation of polymer layers easier. In case of LIAC (Fig. 5), there is much less noise associated with the current–time plot, because of the larger interfacial area. The most noticeable difference with SIAC is that an extractable polymer layer can be grown at a much lower applied potential. The most likely explanation is that in SIAC only a limited volume of solution was accessible, and this easily became depleted in monomer, or acidified by build-up of protons from the polymerization reaction. In Fig. 5 first there is a small current transient due to charging of the double layer and the formation of a diffusion boundary layer at the interface. For the next 12 s the current stays high and constant. This was attributed to ion transfer across the interface (probably Li+ from water to organic phase). The high current masks the initiation of the polymer layer. However, as soon as the first monolayer of polymer is complete, this source of current is abruptly shut off and only the smaller contribution of the current due to electron

Fig. 5. Potentiostatic experiment, LIAC, at 1.7 V, showing that an organic layer is able to stop ion transfer across ITIES from the abrupt decrease of the current, A = 19.64 cm2, 1 mM 3T, 1 mM TPAsTPBF20. No iR compensation.

transfer remains (confirmed by cyclic voltammetry (CV) analysis [37] where the integrated positive current passed is much bigger than the integrated negative current). Indeed, the appearance of the polymer layer at the liquid–liquid interface separates the double-layers, which form the capacitor at the liquid–liquid interface. These results confirmed Maeda et al.’s results [33,34] that a polymer grown at a liquid–liquid interface is able to stop ion transfer across it, acting as a mechanical barrier. This result is confirmed as well by AFM images (Fig. 6) showing in the nanometric scale a compact and continuous (no pores) polymer surface. This steady-state growth rate gradually declines as the solution becomes depleted in monomer. It is thought that the ‘‘noise’’ in the later part of the growth plateau is due to electrochemistry, not an artifact. It is caused by the superposition of separated growth peaks for individual polymer layers over significant fractions of the area. This behavior was obtained with the SIAC only in the best experimental conditions (fresh solutions etc.), but SIAC enabled complete monolayers to form, so that the peaks were more noticeable and regular (Fig. 4) than in Fig. 5 after 25 s. The phenomenon develops only after some seconds. Perhaps this is because it is favoured by depletion of the monomer in the solution, which forces the polymer growth to be more uniform over the surface. 3.2. Qualitative analysis of the surface A polymer layer formed at the interface in LIAC was extracted onto a glass slide, more or less intact, although there was some damage during extraction, and a few splits formed during drying. The film was not strong enough to allow a net to be used. A first evaluation of the uniformity of the polymer was carried out with an optical microscope at 20· magnification. In transmitted-light, the layer was seen to be a green-

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nuclei, as reported by [8]. These structures are similar to those found for polyterthiophene electropolymerized on carbon fibers. A year later, after storage in ambient air and exposed to indoor light, the layer showed no measurable change in resistance (Fig. 7). 3.3. Infrared spectroscopy Infrared spectroscopy was performed using the KBr pellet method. The extracted polymer layer was washed with water and DCE, dried and ground up. 0.74 wt.% ground polymer was mixed with KBr powder, which had been dried to constant weight (the instrument specifies 0.2– 1%). The mixture was compressed into pellets. The transmittance of the polymer was then measured in the range 500–4000 cm1. A strong absorption at 1136 cm1 was found and is characteristic of the sulphoxide group: it indicates the presence of polythiophene oxide. This is what one would expect when the layer is formed in contact with an aqueous solution. However, there is no trace of the characteristic absorption peak of carboxylic groups at 1680 cm1 [12,36]. This means that the polymer is not what one would consider over-oxidized: it should still conduct due to an intact conjugated chain of single and double C bonds. This conclusion is confirmed by the presence of a cathodic ‘‘undoping’’ peak in cyclic voltammetry, which increased with each successive voltammetry cycle [37] The broad absorption at high wavenumbers correlated to the extended p-system due to the formation of new intragap states [12,38,39]. The characteristic absorption of alcohol groups also falls in this range, but it should not be present even in overoxidized polymer [36]. OH groups

Fig. 6. AFM images of the polymer layer extracted (a) at SIAC after successive potential runs at 1.8, 2, 2.2 and 2.4 V, 100 s/run. Break between runs: 1 min. (b, c, d) At LIAC at 1.7 V, 100 s. No pores are visible at the nanometric scale, confirming continuity of the layer. (c) Line scan shown as line in image (b); (d) image of the same sample at higher magnification.

ish brown; the colour associated with fully-oxidized polythiophene. Overoxidized polythiophene is black [35]. No microscopic holes or pores are observed. The AFM analysis (Fig. 6) confirms the continuity of the layer with an average roughness of 90 nm, although the surface is not uniform. In Fig. 6d, grains of polyterthiophene are visible (70 nm · 40 nm in size) which could be the tops of columns of polymer grown on the original 2D

Fig. 7. complex impedance analysis at different bias voltage, showing the polymer still conductive after 1 year at ambient conditions (h is the phase angle and jZj the impedance in X). The complex impedance graphs show no change of impedance over wide range of frequency, which implies pure ohmic conduction. The high frequency effects can be ascribed to capacitance of the contacts.

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could arise from water, which can be present as residual water of crystallization in the KBr discs. Water would give two peaks: a broad one at 3450 cm1 (covered by the p-system) and a weak band at 1615 cm1 [40]. Unfortunately, the second band is covered by the strong band due to C@C stretch of the aromatic rings at 1614 cm1 [36], so the presence of water cannot be definitively excluded. In general, peaks between 600 and 1000 cm1 are associated with bending of CH attached to C@C bonds or aromatic rings. So, the strong peak at 910 cm1 could be associated to the C–H out of plane bending in presence of C@C bond [41]. Weak peaks at 785 cm1 are characteristic of the C–H out of plane bend of the electrochemical synthesised polythiophene [42], such as at 735 and 750 cm1 for aromatic rings. These contributions are probably due to the incorporation of negative ions (TPBF 20 , 4 aromatic rings), to balance the positive charge of the polymer. Peaks at around 690 cm1 are associated to the Cc–H of the electrochemical synthesised polythiophene [42]. Following the interpretation of Schaffer and Heeger [43], the peak at 645 cm1 could be assigned to ring vibrations containing C–S stretching character. 4. Conclusions For the first time a layer of conducting polymer produced by electropolymerization at the interface between two immiscible liquids was extracted and analyzed. By improving the design of the electrochemical cell, and using terthiophene as a monomer, it was possible to achieve a potential at the interface sufficiently anodic to allow fast electropolymerization. The layer was pore-free, because it blocked ions transferring across the interface. In this potential range, IR analysis and the colour of the obtained polymer indicate a polymer that is doped, but not over-oxidized and which is conductive. The extracted polymer layer was stable in ambient air. The polyterthiophene layer was brittle after drying. One would expect a more flexible layer if one used a thiophene monomer with side-chains attached. It might also be easier to electro-polymerize, requiring a lower potential. However, it would probably not survive so well in air and light. Similar to electropolymerization on solid substrates, the initial stage of electropolymerization is progressive nucleation and 2D growth at the liquid–liquid interface, although the transition between the initial 2D growth and later 3D growth is less clearcut. References [1] H. Randriamahazaka, V. Noe¨l, C. Chevrot, J. Electroanal. Chem. 472 (1999) 103–111. [2] E.M. Conwell, in: Hari Singh Nalwa (Ed.), Conductive Polymers: Organic Conductive Molecules and Polymers, vol. 4, Wiley J. & Son, Chichester, 1997 (Chapter 1). [3] S.N. Hoier, S. Park, J. Phys. Chem. 96 (1992) 5188–5193. [4] S.N. Hoier, S. Park, J. Electrochem. Soc. 140 (1993) 2454–2463.

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[5] M. Gratzl, D.F. Hsu, A.M. Riley, J. Janata, J. Phys. Chem. 94 (1990) 5973. [6] Y. Wei, C.C. Chan, J. Tian, G.W. Jang, K.F. Hsueh, Chem. Mater. 3 (1991) 888–897. [7] F. Li, W.J. Albery, Electrochimica Acta 37 (3) (1992) 393–401. [8] F. Biscarini, M. Murgia, F. Dinelli, M. Cavallini, P. Levy, D. De Leeuw, E-MRS 2002 Spring Meeting, Strasbourg, France, June 18– 21, 2002. [9] R. Schrebler, P. Grez, P. Cury, C. Veas, M. Merino, H. Gomez, R. Cordova, M.A. del Valle, J. Electroanal. Chem. 430 (1997) 77. [10] E. Budevski, G. Staikov, W.J. Lorenz, Electrochemical Phase Formation and Growth, VCH, Weinheim, 1996. [11] M. Skompska, Electrochimica Acta 44 (1998) 357–362. [12] U. Barsh, F. Beck, Electrochimica Acta 41 (11/12) (1996) 1761–1771. [13] P.W. Morgan, S.L. Kwolek, J. Polym. Sci. 40 (137) (1959) 299– 327. [14] J. Koryta, P. Vany´sek, M. Brˇezina, J. Electroanal. Chem. 67 (1976) 263. [15] (a) S. Kihara, M. Suzuki, K. Maeda, K. Ogura, S. Umetani, M. Matsui, Z. Yoshida, Anal. Chem. 58 (1986) 2954; (b) S. Kihara, M. Suzuki, K. Maeda, K. Ogura, M. Matsui, J. Electroanal. Chem. 210 (1986) 147; (c) S. Kihara, M. Suzuki, M. Sugiyama, M. Matsui, J. Electroanal. Chem. 249 (1988) 109; (d) H. Alemu, T. Solomon, J. Electroanal. Chem. 237 (1987) 113; (e) H. Alemu, T. Solomon, J. Electroanal. Chem. 261 (1989) 259; (f) M. Senda, T. Kakiuchi, T. Osakai, Electrochimica Acta 36 (2) (1991) 253–262. [16] U. Evans-Kennedy, J. Clohessy, V.J. Cunnane, Macromolecules 37 (10) (2004) 3630–3634. [17] K. Gorgy, F. Fusalba, U. Evans, K. Kontturi, V.J. Cunnane, Synthetic Met. 125 (3) (2001) 365–373. [18] D. Fermin, H.D. Duong, Z. Ding, P.F. Brevet, H.H. Girault, Phys. Chem. Chem. Phys. 1 (1999) 1461. [19] P. Vany´sek, CRC Handbook of Chemistry and Physics, sixty-fifth ed., CRC Press, Boca Raton, FL, 1984. [20] J. Roncali, F. Garnier, M. Lemaire, R. Garreau, Synthetic Met 15 (1986) 323–331. [21] S. Asavapiriyanont, G.K. Chandler, G.A. Gunawardena, D. Pletcher, J. Electroanal. Chem. 177 (1984) 229. [22] E.B. Budevski, in: B.E. Conway, J.O’M. Bockris, E. Yeager, S.U.M. Khan, R.E. White (Eds.), Comprehensive Treatise of Electrochemistry, vol. 7, Plenum Press, New York, 1983, p. 339. [23] M. Fleischmann, H.R. Thirsk, in: P. Delahay (Ed.), Handbook of Chemistry and Physics, vol. 3, Wiley–Interscience, New York, 1963, p. 123. [24] S. Fletcher, C.S. Halliday, D. Gates, W. Westcolt, T. Lwin, G. Nelson, J. Electroanal. Chem. 159 (1983) 267. [25] E. Bosco, S.K. Rangarijian, Faraday Trans. 77 (1981) 1673. [26] I. Mukoyama, K. Aoki, J. Chen, J. Electroanal. Chem. 531 (2002) 113–129. [27] J.A. Harrison, H.R. Thirsk, in: A.J. Bard (Ed.), Electroanalytical Chemistry, vol. 5, Marcel Dekker, New York, 1971, p. 67. [28] R. De Levie, H. Gerischer, C.W. Tobias, Advances in Electrochemistry and Electrochemical Engineering, vol. 13, Wiley, New York, 1984, p. 1. [29] R.D. Armstrong, J.A. Harrison, J. Electrochem. Soc. 116 (1969) 328. [30] S.K. Rangarijian, J. Electroanal. Chem. 46 (1973) 125. [31] R.D. Armstrong, A.A. Metcalfe, J. Electroanal. Chem. 63 (1975) 19. [32] R. Lide David, in: Handbook of Chemistry and Physics, seventyninth ed., Chemical Rubber Company, Florida, 1998. [33] K. Maeda, H. Ja¨nchenova´, A. Lhotsky´, I. Stibor, J. Budka, V. Marecˇek, J. Electroanal. Chem. 516 (2001) 103–109. [34] V. Marecˇek, J. Honz, Coll. Czech. Chem. Commun. 38 (1973) 965. [35] T.A. Skotheim, Handbook of Conductive Polymers, vol. 1, Marcel Dekker, New York, 1986. [36] T.W. Graham Solomons, in: Chimica Organica, Editoriale Grasso, Bologna, 1988.

68

M. Vignali et al. / Journal of Electroanalytical Chemistry 591 (2006) 59–68

[37] M. Vignali, R. Edwards, V.J. Cunnane, J. Electroanal. Chem., in press. [38] A. Cravino, H. Neugebauer, N.S. Sariciftci, M. Catellani, S. Luzzati. In: Proceedings of the 1999 fall meeting material research society, November 29–December 3, 1999, Boston, Massachusetts, vol. 598. [39] A. Cravino, H. Neugebauer, S. Luzzati, M. Catellani, A. Petr, L. Dunsch, N.S. Sariciftci, J. Phys. Chem. B 106 (2002) 3583–3591.

[40] T. Tanaka, S. Nagao, H. Ogawa, Anal. Sci. 17 (Suppl. 2001) (2001) i1081–i1084. [41] R.J. Fessenden, J.S. Fessenden, in: Organic Laboratory Techniques, third ed., Brooks/Cole Publishing Co, Pacific Grove, 1993 (Chapter 15). [42] S. Rane, G. Beaucage, in: J.E. Mark (Ed.), Polymer Data Handbook, Oxford University Press, New York, 1999. [43] H.E. Schaffer, A.J. Heeger, Solid State Commun. 59 (1986) 415.