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Electropolymerization of 2,2′:5′,2″ terthiophene at an electrified liquid–liquid interface
Synthetic Metals 125 (2002) 365±373
Electropolymerization of 2,20:50,200 terthiophene at an electri®ed liquid±liquid interface Karine Gorgya, Florence Fusalbaa, Una Evansa, Kyosti Kontturib, Vincent J. Cunnanea,* a
Materials and Surface Sciences Institute, University of Limerick, Lonsdale Building, Limerick, Ireland Laboratory of Physical Chemistry and Electrochemistry, Kemistintie 1, Helsinki University of Technology, FIN-02150 Espoo, Finland
b
Received 16 January 2001; accepted 11 June 2001
Abstract A new way to synthetize conducting polymers has recently been reported and is investigated further in this study. Poly(2,20 :50 ,200 terthiophene) is electrochemically prepared at an electri®ed liquid±iquid interface between two immiscible electrolyte solutions (ITIES) and initiated by a heterogeneous electron transfer (HET) between an aqueous couple (Ce(IV)/Ce(III)) and the 2,20 :50 ,200 terthiophene monomer in 1,2-dichloroethane (1,2-DCE). In the present study, we suggest the growth mechanism for the polymer formation to be different than that observed at a metal electrode. Indeed, the nature of the interface generally regarded as a region formed by several mixed solvent layers, has a signi®cant effect on the polymerization as well as on the electrochemical and physico-chemical properties of the resulting polymer. Galvanostatic polymerization, electrochemical impedance spectroscopy and FTIR spectroscopy performed on this polymer allow us to con®rm this effect. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Conducting polymers; Electri®ed liquid±liquid interface; Galvanostatic polymerization; Semi-conductors
1. Introduction Conjugated conducting polymers such as polyaniline, polypyrrole and polythiophenes (PTs) have many potential applications including anticorrosion ®lms, light-emitting diodes, lightweight battery electrodes, electrochromicdevices, electrical contacts in semi-conductors (porous silicon), selective membranes in biosensors and in catalysis [1±6]. The electrodeposition of conductive PT ®lms by electrochemical oxidation of thiophene and its derivatives has been widely described in recent years [7,8]. Usually, electropolymerization of thiophene and its derivatives has been carried out in organic media, in the presence of various supporting electrolytes, but has been limited to platinum (Pt), gold, graphite or indium±tin oxide (ITO) working electrodes [9±11]. On the other hand, studies on the electrochemical processes occurring at the interface between two immiscible electrolyte solutions (ITIES) (oil±water) have attracted much attention due to the wide range of applicability of these systems in chemistry and biology [12]. However, whilst copper ®lms [13] and gold nanoparticles [14] have been deposited using electron transfer reactions at the ITIES and ITIES theory has been applied to the Williamson transfer ether synthesis [15], * Corresponding author. E-mail address: [email protected] (V.J. Cunnane).
most research into heterogeneous electron transfer (HET) has concentrated on simple reversible systems [16±20] instead of deposition or synthesis type processes. Indeed, these two areas of research (conducting polymers liquid±liquid interfaces) combined together may be used as a means for studying the mechanism of polymerization of conducting polymers and therefore for ®nding novel synthetic applications (e.g. semi-conductors). Interfaces between two immiscible electrolyte solutions can be studied utilizing many methods of electrochemical investigation. A simple differential adapter that can convert any potentiostat to a 4-electrode potentiostat is needed for this work [21]. It was previously shown [18] from voltammetric experiments with microelectrodes that the replacement of a metal electrode by a solution of a fast redox couple leads to the same half wave potential for electron transfer to an organic redox couple in an organic phase. While an oil± water interface has been used by Uredat et al. [22] to study interfacial polymerization of 4,40 -isopropylidenediphenoldimethacrylate induced by irradiation with ultraviolet light (photopolymerization), Cunnane and Evans [23] have shown for the ®rst time that electron transfer reactions can be brought about between an aqueous-based redox system and an organic-based monomer unit (1-methylpyrrole and 1-phenylpyrrole) at an electri®ed liquid±liquid interface resulting in the formation of oligomers in the organic phase.
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 4 7 4 - X
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It was presumed that the initial electron transfer results in the formation of a radical cation in the organic phase. Later the system was expanded to look at another electroactive monomer (2,20 :50 ,200 terthiophene) and organic systems (tetraphenylarsonium tetrakis(penta¯uorophenyl)borate (TPAsTPBF20)/1,2-DCE) [24,25] producing polyterthiophene. Considering the interface between two immiscible electrolytes as a mixed solvent region, the effect of aqueous and organic mixed solutions on the electropolymerization of thiophene is an interesting point to elucidate. In a recent study [10], it was shown that the use of a suitable ratio of water to acetonitrile could lower the onset oxidation potential of bithiophene in an aqueous/organic mixed solution. The authors demonstrated that the chemical structure of polybithiophene synthetized by this technique is very similar to that of the polymer formed in organic media. However, the volume ratio of water to acetonitrile has a signi®cant effect on the electropolymerization as well as on the electrochemical properties and morphology of the formed ®lms. One of the reactions which is expected to occur is the anodic overoxidation of PTs in wet organic electrolytes. In their report, the formation of a dioxide sulphone group at one of three monomer units was revealed by FTIR spectroscopy just after galvanostatic electropolymerization. Nevertheless, it was observed that this partial overoxidation had almost no in¯uence on the electronic conductivity. On the other hand, at a higher water concentration, further oxidation was shown to proceed corresponding to an oxidative SO2 elimination and the formation of carbonyls in the 2-, 3- and 5-position. These results opened the possibility for a systematic anodic modi®cation of the conducting polymer through organic electrochemistry in the solid state. The authors also emphasized that transport processes are not limiting due to the porosity of the polymer layer. Different measurements techniques have been applied to the study of electrochemical polymerization of thiophene and its derivatives. Most investigations were performed under potentiostatic control. More recently [11], electrochemical impedance spectroscopy (EIS) was used to investigate the electrochemical polymerization of bithiophene under galvanostatic conditions. It was shown that the successful model applied in studying the impedance spectra was strongly dependent on the ®lm thickness deposited on a Pt electrode and the corresponding current density. It was suggested that the deposition of polymer ®lm onto a metal electrode follows a 3-D growth mechanism during the ®rst stages of polymerization, and pseudo-2-D growth for polymerization at charges higher than 15±20 mC cm 2. Important parameters of the polymer ®lm (e.g. apparent diffusion coef®cient, ionic conductivity, etc.) were estimated from the impedance spectra. In this study, a novel electrochemical±chemical type mechanism reaction initiated by a HET between an aqueous couple (Ce(IV)/Ce(III)) and a monomer in DCE at ITIES is reported. Galvanostatic polymerization at ITIES and at solid macroelectrodes leading to the formation of poly(2,20 :50 ,200
terthiophene) in addition to EIS under galvanostatic control are the main electrochemical techniques used to investigate this mechanism. FTIR spectroscopy is employed to characterize the resulting polymer. 2. Experimental 2.1. Chemical 1,2-Dichloroethane (1,2-DCE) from Fluka (99.5% (GC grade) was used as received as the organic solvent in all experiments. A 18.2 MO cm water used throughout was prepared using the Maxima ultra pure water system (Elga). Li2SO4 (Fluka >98%) was used as received as the aqueous supporting electrolyte. Tetraphenylarsonium tetrakis(penta¯uorophenyl)borate (TPAsTPBF20) was the supporting electrolyte in the organic phase and was prepared using tetraphenylarsonium chloride (TPAsCl) (Fluka 95%) and lithium tetrakis(penta¯uorophenyl)borate etherate (LiTPBF20) (Boulder Scienti®c Co.) according to the following procedure: 2.5 g TPAsCl were dissolved in 200 ml H2O and added to 4.09 g LiTPBF20 in 200 ml methanol 215 (super purity solvent, Romil Ltd. >99.8%) resulting in a white precipitate; the white compound was filtered under vacuum and washed with a 50/50 mix of water and methanol, recrystallized in methanol; the salt was then dried at 608C for 3 days. Cerium(IV)sulfate (Aldrich), Cerous(III) sulfate anhydrous (Fluka), sulfuric acid (H2SO4) (Merck 95±97%), 2,20 :50 ,200 terthiophene (Fluka >99%) were also used as received. 2.2. Procedure 2.2.1. The 3-electrode configuration cell Electrodeposition was performed galvanostatically at various current densities (0.05±0.5 mA cm 2) without stirring. The working electrode was Pt, GC or gold macroelectrodes (Metrohm, 0.0707 cm2). A Pt ¯ag was used as a counter electrode and either a standard calomel electrode (SCE) or an Ag/AgCl (silver/silver chloride wire dipped in 1 mM TPAsCl/H2O) or an Ag/AgTPBF20 (silver wire anodically polarized for 5000 s at 1 mA in a 1 mM TPAsTPBF20/DCE solution) were used as reference electrodes, respectively. The electrolyte compositions in DCE were 2±10 mM monomer and 1±4 mM TPAsTPBF20 (cell 1). The total polymerization charge during this experiment usually reached 10 mC cm 2 except for the EIS experiment on Pt macroelectrode (100 mC cm 2). 2.2.2. The 4-electrode configuration cell The cell was custom-made at University of Helsinki as reported before [16]. The interface between the organic and
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aqueous phase has an area of 0.126 cm2. The applied potential difference at the interface is supplied by two Pt electrodes and the potential difference at the interface is measured by means of the two reference electrodes which are connected to the interface by two Luggin capillaries. In the aqueous phase the reference electrode is a Pt wire cleaned with sulfuric acid. The organic reference electrode is a non-polarizable reference system Ag/AgCl/TPAsCl or Ag/AgTPBF20/TPBF20 . The aqueous phase is a 0.1± 0.05 M Ce(IV)(SO4)2/0.01±0.005 M Ce(III)2(SO4)3 in 0.1 M Li2SO4/0.2 M H2SO4/H2O solution. The organic phase is 2±10 mM 2,20 :50 ,200 terthiophene, 1±4 mM TPAsTPBF20 in 1,2-DCE (depending on the experiment, see cells 2 and 3). Cell 1 Ag/AgCl/TPAsCl (1 mM/H2O) or Ag/AgTPBF20 or SCE/2.5, 5 or 10 mM 2,20 :50 ,200 terthiophene (0.75, 1 or 5 mM TPAsTPBF20)(1,2-DCE)/Pt or GC or Au. Cell 2 Ag/AgCl/TPAsCl (1 mM/H2O)/5 or 10 mM 2,20 :50 ,200 terthiophene (1 mM TPAsTPBF20/1,2-DCE)/d/0.1 M Ce(IV)(SO4)2, 0.01 M Ce(III)2(SO4)3, 0.1 M Li2SO4,0.2 MH2SO4(H2O)/Pt(d electrified liquid±liquidinterface). Cell 3 Ag/AgTPBF20/2.5 mM 2,20 :50 ,200 terthiophene, 3 or 5 mM TPAsTPBF20(1,2-DCE)/d/0.05 M Ce(IV)(SO4)2, 0.005 M Ce(III)2(SO4)3, 0.1 M Li2SO4, 0.2 M H2SO4(H2O)/Pt (d electrified liquid±liquid interface). Cyclic voltammetry studies in both 3 and 4 electrode mode were performed using a 1287 Solartron Electrochemical potentiostat±galvanostat interfaced with DC Corrware for Windows software (Scribner Associates, version 2.1). EIS measurements were performed with a 1260 Solartron Frequency Response Analyzer model coupled to a 1287 Solartron Electrochemical Interface model. Data were collected and analyzed using a PC and Zplot Software for Windows (Scribner Associates, version 2.1), respectively. While most investigations are performed under potentiostatic control, in this study EIS was used to investigate the electrochemical polymerization of 2,20 :50 ,200 terthiophene under galvanostatic conditions (i.e. 0.1 mA cm 2). The current perturbation amplitude was ®xed at 0.001 and 0.0001 mA for the electri®ed liquid±liquid interface and the macroelectrode, respectively. The EIS measurements were performed at intervals of 15 s. In order to avoid polymer deposition during the impedance measurements, the cell current was switched to 0.0 mA cm 2 for the duration of the perturbation pulses and subsequent data transmission to the computer. Frequencies varied typically from 5000 to 0.5 Hz. 2.3. Sample characterization The FTIR spectra were recorded with a series FTIR spectrometer (Bomem MB). Typically, 20 scans were recorded between 4000 and 400 cm 1 (resolution 1 cm 1).
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FTIR data were collected with Win±Bomem EasyTM Software for Windows (version 3.04). Poly(2,20 :50 ,200 terthiophene) was synthetized by galvanostatic polymerization (0.1 mA cm 2) at ITIES. Several batches of 10 mC cm 2 polymer charges were prepared from an organic electrolyte of 2.5 mM 2,20 :50 ,200 terthiophene/3 mM TPAsTPBF20/1,2DCE in contact with the solution 0.05 M Ce(IV)(SO4)2/ 0.005 M Ce(III)2(SO4)3 in 0.1 M Li2SO4/0.2 M H2SO4/H2O aqueous solution (cell 3). The dark green polymer was ®ltered under vacuum, washed with ultra pure water (18.2 O cm) and acetone, dried at 608C in the oven for 3 days. The samples were prepared by mixing the polymer with KBr powder and pressing the mixture into a pellet. 3. Results and discussion 3.1. Chronopotentiograms of electropolymerization Because ITIES behaves similarly to metal electrode± solution interfaces, many techniques of electrochemistry have previously been adopted to ITIES studies [21]. Galvanostatic polymerization has the main advantage of allowing better control of the charge state of a polymer than potentiostatic methods. Fig. 1 (cell 1) shows chronopotentiograms of 2,20 :50 ,200 terthiophene obtained at a Pt electrode for different anodic current densities. When the current density is less than 0.1 mA cm 2, the potential is nearly constant in the chronopotentiograms. This indicates that the electrochemical reaction is mainly determined by electron transfer control. The electropolymerization reaction is instantaneous, the potential reaches a high value almost immediately, indicating a faradaic process. However, at higher current density, e.g. 0.5 mA cm 2, the potential increases slightly with time. This implies that the reaction is mainly controlled by monomer diffusion at higher current density. This phenomenon is not unusual, as the reaction is faster at high current densities creating a de®ciency of monomer
Fig. 1. Galvanostatic electropolymerization of 2,20 :50 ,200 terthiophene at a Pt macroelectrode (A 0:0707 cm2) for different current densities: i 0:05, 0.1, 0.5 mA cm 2. Solution: 5 mM 2,20 :50 ,200 terthiophene/ 1 mM TPAsTPBF20/1,2-DCE. Polymerization charge: q 10 mC cm 2. Inset: i 0:5 mA cm 2. Organic reference: Ag/AgCl/TPAsCl.
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species near the working electrode [10]. When the terthiophene is electropolymerized at the ITIES (cell 2, Fig. 2), the formation of a dark green polymer with an open structure is observed. It can be seen from Fig. 2 that the reaction potential also slightly increases with time and that such an increase is observed for current densities lower than those reported for the Pt electrode (e.g. 0.1 mA cm 2). This means that the reaction is controlled by monomer diffusion. At higher current densities (e.g. 0.2±0.5 mA cm 2), there is no real potential stabilization. The potential is not reaching high values immediately as observed for the solid electrode (Fig. 1) but slightly increases and then decreases. There is no stationary potential due to the rapid consumption of terthiophene in the interfacial region. This is also true if the monomer concentration is increased by a factor of two (cell 2, Fig. 3). It is important to note that, to date, the question of the ITIES structural model still remains unclear [26±31]. One model is that of a mixed solvent layer and, indeed, if a mixed
solvent layer model [28] at a polarizable water±DCE interface seems realistic, investigations of ITIES of the type described here may allow the development of an easily accessible model. Since water has a much higher viscosity than DCE and also since the solubility of terthiophene is expected to decrease when there is more water in the mixed solution (for instance, at the ITIES), the monomer diffusion rate should slow down in solution if there is a mix of water and DCE. This could explain the discrepancy in the behavior observed during galvanostatic polymerization at ITIES compared with the solid electrode (Pt or indeed GC) for the same current density. As mentioned by Hu et al. [10], electropolymerization of thiophene at metal electrodes can be carried out in aqueous/organic solution systems although the monomer is insoluble in water. They have shown that it was possible to tune the voltage of the electropolymerization at constant current density by changing the volume ratio of water to organic solvent (the voltage is lowered by increasing the water content). Unfortunately, because the stabilization potential at a liquid±liquid interface in fact includes an ohmic drop corresponding to the resistance of the ®lm and the solution resistance and because of the difference in the polymer af®nity with its electrode support, it remains dif®cult to compare directly the stabilization potentials of each technique. However, in order to approximate the polymerization behavior of 2,20 :50 ,200 terthiophene at ITIES, the same electropolymerization was carried out at a GC macroelectrode with different volumes of aqueous electrolyte added to the organic deposition solution (see cell 1, Fig. 4). The ratios of water (actually, 0.05 M Ce(IV)(SO4)2/0.005 M Ce(III)2(SO4)3 in 0.1 M Li2SO4/0.2 M H2SO4/H2O) in the organic solution system studied were 0, 1 and 2% v/v. The solution was stirred before electrochemical measurements. In Fig. 4 it can be seen that, as already observed for water addition [10], the polymerization potential decreases with increasing volume ratio of aqueous phase to DCE in the
Fig. 3. Galvanostatic electropolymerization of 2,20 :50 ,200 terthiophene at ITIES (A 0:126 cm2) for two monomer concentrations, at a current density of 0.1 mA cm 2; q 10 mC cm 2. Organic solution: 5 or 10 mM 2,20 :50 ,200 terthiophene/1 mM TPAsTPBF20/1,2-DCE. Aqueous solution: 0.1 M Ce(IV)(SO4)2/0.01 M Ce(III)(SO4)3 in 0.1 M Li2SO4/0.2 M H2SO4/ H2O. Organic reference: Ag/AgCl/TPAsCl.
Fig. 4. Galvanostatic electropolymerization of 2,20 :50 ,200 terthiophene at a GC macroelectrode (A 0:0707 cm2) for different aqueous solution content: 0, 1 and 2% v/v. Constant current density: 0.1 mA cm 2; q 10 mC cm 2. Solution: 2.5 mM 2,20 :50 ,200 terthiophene/1 mM TPAsTPBF20/1,2-DCE. Organic reference: Ag/AgTPBF20.
Fig. 2. Galvanostatic electropolymerization of 2,20 :50 ,200 terthiophene at ITIES (A 0:126 cm2) for different current densities: i 0:1, 0.2, 0.5 mA cm 2; q 10 mC cm 2. Organic solution: 5 mM 2,20 :50 ,200 terthiophene/1 mM TPAsTPBF20/1,2-DCE. Aqueous solution: 0.1 M Ce(IV)(SO4)2/0.01 M Ce(III)(SO4)3 in 0.1 M Li2SO4/0.2 M H2SO4/H2O. Organic reference: Ag/AgCl/TPAsCl.
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mixed solution (constant current density of 0.1 mA cm 2). When the addition of the aqueous solution is less than 1% (i.e. 0.5% v/v, not shown), the potential is almost constant in the chronopotentiograms. As such the electrochemical reaction is mainly determined by electron transfer control. However, at higher aqueous phase content (2% v/v), the reaction potential increases with time showing the same kind of shape as that observed at ITIES for a similar current density (e.g. 0.1 mA cm 2). The monomer diffusion rate is slowed down due to the higher viscosity of the aqueous phase compared to the organic one. There is also a decrease in the solubility of terthiophene when there is more water in the mixed solution. This behavior was further detailed earlier in this discussion and it shows that a mixed solvent layer model at a polarizable water±DCE interface seems realistic. In order to be sure that the presence of the redox couple Ce(IV)/Ce(III) in the aqueous phase did not in¯uence the reaction in solution prior to the analysis at the GC electrode, this experiment was repeated with just H2O added (not shown). The same trend was observed. 3.2. Cyclic voltammetry (CV) Fig. 5a and b show typical cyclic voltammograms for a terthiophene polymeric ®lm formed at a metal electrode
Fig. 5. Cyclic voltammograms of 2,20 :50 ,200 terthiophene (a) at a gold macroelectrode (A 0:0707 cm2), scan rate: 100 mV s 1. Solution: 10 mM 2,20 :50 ,200 terthiophene/0.75 mM TPAsTPBF20/1,2-DCE after deposition of the polymer at 1.05 V; (b) at ITIES (A 0:126 cm2), scan rate: 50 mV s 1 after a deposition of a charge of 10 mC cm 2. Organic solution: 5 mM 2,20 :50 ,200 terthiophene/1 mM TPAsTPBF20/1,2-DCE. Aqueous solution: 0.1 M Ce(IV)(SO4)2/0.01 M Ce(III)(SO4)3 in 0.1 M Li2SO4/0.2 M H2SO4/H2O, organic reference: Ag/AgTPBF20.
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(gold macroelectrode in TPAsTPBF20/1,2-DCE) and a polymeric ®lm formed at an immiscible electrolyte solution interface (water±1,2-DCE, cell 2). While the conditions of cycling are different (electrode support area, scan rate, charge deposited, etc.) and therefore does not allow direct comparison between ITIES and the gold macroelectrode, the shape of the CVs are similar with quite the same waves of oxidation±reduction (i.e. potential position) as already observed by Hu et al. [10] for ®lms of PT electropolymerized in different mixed (water/organic) solutions. One point of interest is that in the ITIES case the current does not start at the zero point as is the case for the metal electrode. It is believed this is due to the changing position of the interface with time due to the formation of the polymer. 3.3. Electrochemical impedance spectroscopy (EIS) The process of electrochemical growth of poly(2,20 :50 ,200 terthiophene) at ITIES was investigated by means of EIS and the time-resolved impedance spectra are shown here, measured during controlled-current polymerization (see Section 2). The impedance spectra were analyzed using the wellknown model of Ho et al. [32], for low current density (e.g. 0.1 mA cm 2) conditions. The impedance spectra obtained are shown in Fig. 6 (cell 3). The total polymerization charge during this experiment reached 10 mC cm 2. The main dif®culties of this type of investigation are ®rstly that the shape of the interface may change with the potential difference across the interface and with the increase of the polymer weight at the interface. Secondly, the reference electrode employed in the organic media (Ag/AgCl in TPAsCl/H2O) may not be suitable for EIS measurements. Indeed, both should generate artefacts in the impedance spectra as shown by Schiffrin et al. [33]. It is rather dif®cult
Fig. 6. Complex plane impedance spectra obtained during 2,20 :50 ,200 terthiophene galvanostatic polymerization (current density: 0.1 mA cm 2) at ITIES for two electropolymerization charges: q 4:5 and 10 mC cm 2. Frequency range: 5000±0.5 Hz. I appl 0 mA cm 2. Amplitude: 0.001 mA. Organic solution: 2.5 mM 2,20 :50 ,200 terthiophene/3 mM TPAsTPBF20/1,2DCE. Aqueous solution: 0.05 M Ce(IV)(SO4)2/0.005 M Ce(III)(SO4)3 in 0.1 M Li2SO4/0.2 M H2SO4/H2O. Organic reference: Ag/AgTPBF20.
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to control the ®rst parameter (e.g. the variation of the position of the interface), the only control being to use low polymerization charges. The second parameter was avoided by the use of a new reference electrode that of Ag/AgTPBF20, which appears to obviate the high value of the resistance of the Luggin capillary in the organic phase (as observed by Kobmehl et al. [34] with Ag/AgTPB) [35]. The use of current control instead of potential control for the EIS measurements should induce a lower modi®cation of the interface and allow at least a better control of the steady-state. 3.3.1. Equivalent circuit More comprehensive treatments of the impedance spectra of oxidized conducting polymers take into account the ®nite diffusion of counter ions through the polymer layer, assumed to be homogeneous, in analogy with the analysis given by Ho et al. [32] for the case of Li intercalation into WO3 electrodes. An equivalent circuit which describes the behavior of an electrode (GC or Pt macroelectrode) coated with a polymer layer according to the model of Ho et al., as shown in Scheme 1, is also used to ®t the impedance spectra at ITIES, assuming that the replacement of a metal electrode by a solution of a fast redox couple leads to the same behavior. In this case, Rs is the ohmic series resistance, Rct the charge transfer resistance, Cdl the double layer capacitance, and a second charge transfer resistance Rp has been introduced by Popkirov et al. [11] to account for the polymerization reaction. The restricted ®nite Warburg impedance ZRFW is given by: ! 1 dE coth l iwD1=2 ZRFW (1) nF dc iwD1=2 where n is the number of electrons transferred, F Faraday constant, E the electrode potential, D the apparent diffusion constant for anions in the polymer, l the film thickness, w the frequency and c the bulk concentration of the oxidized polymer. The total impedance of the equivalent circuit from cell 3 can thus be given by: 1 1 1 Z iwCdl (2) S Rs ZRFW Rct Rp where S is the surface area of the electrode. 3.3.2. IES data analysis: ITIES case The semi-circle observed at higher frequencies (Fig. 6) can be related to the charge transfer in the oxidation/ reduction reaction, and can be represented by the faradaic
Scheme 1.
resistance Rct and the double layer capacitance Cdl. At intermediate frequencies ion diffusion through the polymer layer becomes dominant, leading to an impedance behavior of Warburg type. A large circle is thus observed corresponding to the polymerization resistance Rp and not observed when impedance spectra are computed in the absence of monomer (not shown). At lower frequencies, however, the capacitive character due to the ®nite thickness of the layer and the limited amount of polymer able to take part in the oxidation/reduction reaction, is not observed. A complex ®tting program was used to ®t Eq. (2) to the experimental EIS data obtained on the polymer ®lm at the interface during the initial stages of the polymerization and to evaluate the parameters of the suggested equivalent circuit. It is rather dif®cult to evaluate the ®lm thickness at the interface. However, we could estimate it using the polymer layer thickness of 80 nm obtained for a bithiophene ®lm polymerized on a Pt electrode for a charge of 30 mC cm 2 (at 0.15 mA cm 2) using a value for polymer density of 1.5 g cm 3 [11]. This thickness approximation allows us to evaluate the apparent diffusion constant D. The parameters Rct, Cdl and Rp as a function of the polymerization degree (charge of polymer) from the data of Fig. 6 are presented in Fig. 7a±c. The charge transfer resistance and the double layer capacitance increases and decreases, respectively, during the polymerization, indicating that the interface available for the oxidation/reduction reaction of the polymer changes signi®cantly. This is not in agreement with the results obtained at a Pt electrode by Popkirov et al. [11] where no signi®cant change was reported. In addition, the increase in estimated polymerization charge transfer resistance Rp is probably due to an increase in the activation energy for monomer oxidation and/ or polymer precipitation at the interface already covered with polymer. Accordingly, an increase of the polymerization potential is observed during the galvanostatic experiment. The opposite behavior, e.g. a decrease of Rp and the corresponding stabilization potential, was reported at a Pt electrode [11]. This indicates that a different process is occurring at ITIES for polymer growth. 3.3.3. Macroelectrode case In order to compare the polymerization process at ITIES with solid electrodes, the same experiment was carried out at a GC and Pt electrode (Fig. 8 shows the complex plane impedance for a GC electrode). Similar trends and impedance spectra were observed at both Pt and GC macroelectrodes. The parameters Rct, Cdl and Rp (from the ®tting with Ho et al. [32] equivalent circuit for thin ®lms) as a function of the polymerization degree (charge of polymer) are presented in Fig. 9a±c (for the Pt electrode). The charge transfer resistance, Rct, and the double layer capacitance, Cdl, slightly increases during the polymerization in good agreement with literature [11], indicating that the interface available for the oxidation reaction of the conducting polymer does not change signi®cantly. The polymerization charge
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Fig. 8. Complex plane impedance spectra obtained during 2,20 :50 ,200 terthiophene galvanostatic polymerization (current density: 0.1 mA cm 2) at a GC macroelectrode for two electropolymerization charges: q 4:5 and 10 mC cm 2. Frequency range: 5000±0.5 Hz. I appl 0 mA cm 2. Amplitude: 0.001 mA. Organic solution: 2.5 mM 2,20 :50 ,200 terthiophene/5 mM TPAsTPBF20/1,2-DCE. Organic reference: Ag/AgTPBF20.
Fig. 7. (a) Charge transfer resistance Rct of the oxidation/reduction of polyterthiophene; (b) double layer capacitance Cdl; and (c) charge transfer resistance Rp of the polymerization reaction as estimated by applying the model of Ho et al. [32] to impedance spectra measured at ITIES during polymerization at 0.1 mA cm 2. Organic reference: Ag/AgTPBF20.
transfer resistance Rp decreases as reported by Popkirov et al. [11], indicating a decrease in the activation energy for monomer oxidation and/or polymer precipitation at the electrode already covered with polymer. The difference in the shape of the impedance spectra computed for ITIES and GC or Pt electrodes (which provide the same spectra) con®rmed the difference in the kinetics of the polymerization reaction at liquid±liquid interface compared to solid electrodes. At this stage the question of the polymer conductivity when electrosynthetized at ITIES compared to the electrosynthetic product at solid electrodes is of interest. Additional work is currently under way to characterize the electronic properties of the resulting polymeric material using techniques such as XPS measurements. Impedance spectra were computed at a GC (or Pt) electrode and ITIES before starting the galvanostatic polymerization (not shown). This experiment allows us to evaluate the ohmic series resistance or the solution resistance. It
Fig. 9. (a) Charge transfer resistance Rct of the oxidation/reduction of polyterthiophene; (b) double layer capacitance Cdl; and (c) charge transfer resistance Rp of the polymerization reaction as estimated by applying the model of Ho et al. [32] to impedance spectra measured at a Pt macroelectrode during polymerization at 0.1 mA cm 2. Organic reference: Ag/AgTPBF20.
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seems strange to observe that at ITIES, Rs values are lower than at a GC electrode. This could be attributed to the enhanced solution conductivity when more water is present in the mixed solutions. A ®rst approximation of the apparent diffusion constant D using the formula W s T l2 =D (W s T, Warburg ®tting parameter) indicates a higher diffusion rate for the ions in the polymer at the Pt electrode compared to ITIES. The values for D, however, are lower than expected (ca. 10 12 cm2 s 1 compared to 10 8 cm2 s 1 [32]) probably due to an under-estimation of the ®lm thickness (l 27 nm). The authors agree that four orders of magnitude for D is a large under-estimation, in addition, we believe, as it was already shown [11], that the model of Ho et al. [32] should be applied cautiously for electrodes (or interfaces) covered with small amounts of polymer. It remains clear that additional work is required to elucidate the polymerization growth mechanism at ITIES. Some caution should be taken with the EIS interpretation. Indeed, in a future work, spectra will be collected with higher electrode surface areas in order to more completely avoid artefacts due to the low currents used, as suggested by Schiffrin et al. [33] in relation to EIS measurements at ITIES. 3.4. FTIR spectroscopy of the formed polymer Fig. 10 shows the FTIR spectrum of freshly electropolymerized 2,20 :50 ,200 terthiophene at an electri®ed liquid±liquid interface which had been prepared and removed for analysis. The organic and aqueous solvents in contact were 2.5 mM 2,20 :50 ,200 terthiophene/3 mM TPAsTPBF20/1,2-DCE and 0.05 M Ce(IV)(SO4)2/0.005 M Ce(III)2(SO4)3 in 0.1 M Li2SO4/0.2 M H2SO4/H2O (cell 3). At high wavelengths,
Fig. 10. FTIR spectrum of poly(2,20 :50 ,200 terthiophene) after galvanostatic electropolymerization at ITIES. Polymerization conditions: 2.5 mM 2,20 :50 ,200 terthiophene/3 mM TPAsTPBF20/1,2-DCE in contact with the 0.05 M Ce(IV)(SO4)2/0.005 M Ce(III)(SO4)3 in 0.1 M Li2SO4/0.2 M H2SO4/H2O aqueous solution. Current density: 0.1 mA cm 2. Mixture of polymer with KBr powder pressed into a pellet.
the usual strong absorption due to the extended p-system can be seen. The two strongest bands at 1155 and 1326 cm 1 can be attributed to the symmetric and asymmetric valency vibrations of the sulphone group ±SO2, respectively, as mentioned by Barsch and Beck [36]. The band at 1638 cm 1 can be related to a carbonyl group eventually in conjugation to a C=C double band. These results conclude that PTs electropolymerised at ITIES can be oxidized as already observed for a polymer ®lm grown on Pt [36] and yield the dioxide (sulphone). On the other hand, a further oxidation (overoxidation) was not observed here. Future research investigating the nature of the polymer and the degree of its oxidation state has to be carried out. For instance, the use of techniques such as X-ray photoelectron spectroscopy (XPS) would allow us to observe the presence of terminal ±COOH groups (from the cleavage of a C±C bond). The complete mechanism for the anodic oxidation (overoxidation) of PTs in the presence of water is given in [36]. Indeed, the presence of sulphone groups in the poly(2,20 :50 ,200 terthiophene) is another clue to the ITIES structural model which seems to tend to a mixed solvent model. 4. Conclusion It is currently too early to state a growth mechanism for polymer formation at ITIES. However, we suggest this mechanism to be different than the one observed at a metal electrode. Considering galvanostatic polymerization and FTIR spectroscopy results, the ITIES structural model which seems to favor a mixed solvent model, could be at the origin of the discrepancies in the growth mechanism at solid electrodes and ITIES. It has already been shown that electrolyte impurities, oxygen and water affect the stability of conducting polymers under electrochemical conditions [37]. These aforementioned drawbacks will probably have a strong in¯uence on polymers synthetized at ITIES and therefore on their potential applications. To go further, XPS measurements should allow us to con®rm these remarks. It should also be of interest to investigate the polyterthiophene morphology when synthetized at ITIES in order to compare to literature. Differences in the surface structures of the ®lms prepared at ITIES could be studied by atomic force microscopy (AFM) since it was shown by Hu et al. [10] that polybithiophene ®lms prepared in mixed aqueous/organic solutions have a grainy structure with the grain size increasing with more water in the mixed solution. Larger size of grains suggest that there may be more voids existing in the ®lms synthetized in solutions containing more water. It means that such ®lms are less dense compared with ®lms with smaller grain size. This ®lm morphology observation should be consistent with our impedance results indicating poorer electrochemical stability and conductivity however this kind of structure should allow a good rate of charge migration.
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Acknowledgements This research was funded by the EU, Training and Mobility of Researchers Network, ODRELLI (Organization, Dynamics and Reactivity at Electri®ed Liquid±Liquid Interface), Contract No. ERBFMRXT960078. We thank Marina Serantoni, from the University of Limerick for her advice and useful discussions.
References [1] S. Li, C.W. Macosko, H.S. White, Science 259 (1993) 957. [2] Y. Sun, E. Ruckenstein, Synth. Metals 82 (1996) 5. [3] P. Novak, K. Muller, K.S.V. Santhanam, O. Hass, Chem. Rev. 97 (1997) 207. [4] J.S. Miller, Adv. Mater. 5 (1993) 587. [5] J.S. Miller, Adv. Mater. 5 (1993) 671. [6] J.D. Moreno, M.L. Marcos, F. AgulloÂ-Rueda, R. Guerrero-Lemus, R.J. MartõÂn-Palma, J.M. MartõÂnez-Duart, J. GonzaÂlez-Velasco, Thin Solid Films 348 (1999) 152. [7] H.S. Nalwa (Ed.), Handbook of Organic Conductive Molecules and Polymers, Conductive Polymers: Transport, Photophysics and Applications, Vol. 4, Wiley, New York, 1997. [8] T. Skotheim, in: M. Pomerantz, R.L. Elsenbaumer, J.R. Reynolds (Eds.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1998. [9] S. Aeiyach, E.A. Bazzaoui, P. Lacaze, J. Electroanal. Chem. 434 (1997) 153. [10] X. Hu, G. Wang, T.K.S. Wong, Synth. Metals 106 (1999) 145. [11] G.S. Popkirov, E. Barsoukov, R.N. Schindler, J. Electroanal. Chem. 425 (1997) 209. [12] M. Senda, T. Kakiuchi, T. Osaka, Electrochim. Acta 36 (1991) 253. [13] M. Guainazzi, G. Silvestri, G. Serravale, J. Chem. Soc., Chem. Commun. (1975) 201.
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Electropolymerization of 2,20:50,200 terthiophene at an electri®ed liquid±liquid interface Karine Gorgya, Florence Fusalbaa, Una Evansa, Kyosti Kontturib, Vincent J. Cunnanea,* a
Materials and Surface Sciences Institute, University of Limerick, Lonsdale Building, Limerick, Ireland Laboratory of Physical Chemistry and Electrochemistry, Kemistintie 1, Helsinki University of Technology, FIN-02150 Espoo, Finland
b
Received 16 January 2001; accepted 11 June 2001
Abstract A new way to synthetize conducting polymers has recently been reported and is investigated further in this study. Poly(2,20 :50 ,200 terthiophene) is electrochemically prepared at an electri®ed liquid±iquid interface between two immiscible electrolyte solutions (ITIES) and initiated by a heterogeneous electron transfer (HET) between an aqueous couple (Ce(IV)/Ce(III)) and the 2,20 :50 ,200 terthiophene monomer in 1,2-dichloroethane (1,2-DCE). In the present study, we suggest the growth mechanism for the polymer formation to be different than that observed at a metal electrode. Indeed, the nature of the interface generally regarded as a region formed by several mixed solvent layers, has a signi®cant effect on the polymerization as well as on the electrochemical and physico-chemical properties of the resulting polymer. Galvanostatic polymerization, electrochemical impedance spectroscopy and FTIR spectroscopy performed on this polymer allow us to con®rm this effect. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Conducting polymers; Electri®ed liquid±liquid interface; Galvanostatic polymerization; Semi-conductors
1. Introduction Conjugated conducting polymers such as polyaniline, polypyrrole and polythiophenes (PTs) have many potential applications including anticorrosion ®lms, light-emitting diodes, lightweight battery electrodes, electrochromicdevices, electrical contacts in semi-conductors (porous silicon), selective membranes in biosensors and in catalysis [1±6]. The electrodeposition of conductive PT ®lms by electrochemical oxidation of thiophene and its derivatives has been widely described in recent years [7,8]. Usually, electropolymerization of thiophene and its derivatives has been carried out in organic media, in the presence of various supporting electrolytes, but has been limited to platinum (Pt), gold, graphite or indium±tin oxide (ITO) working electrodes [9±11]. On the other hand, studies on the electrochemical processes occurring at the interface between two immiscible electrolyte solutions (ITIES) (oil±water) have attracted much attention due to the wide range of applicability of these systems in chemistry and biology [12]. However, whilst copper ®lms [13] and gold nanoparticles [14] have been deposited using electron transfer reactions at the ITIES and ITIES theory has been applied to the Williamson transfer ether synthesis [15], * Corresponding author. E-mail address: [email protected] (V.J. Cunnane).
most research into heterogeneous electron transfer (HET) has concentrated on simple reversible systems [16±20] instead of deposition or synthesis type processes. Indeed, these two areas of research (conducting polymers liquid±liquid interfaces) combined together may be used as a means for studying the mechanism of polymerization of conducting polymers and therefore for ®nding novel synthetic applications (e.g. semi-conductors). Interfaces between two immiscible electrolyte solutions can be studied utilizing many methods of electrochemical investigation. A simple differential adapter that can convert any potentiostat to a 4-electrode potentiostat is needed for this work [21]. It was previously shown [18] from voltammetric experiments with microelectrodes that the replacement of a metal electrode by a solution of a fast redox couple leads to the same half wave potential for electron transfer to an organic redox couple in an organic phase. While an oil± water interface has been used by Uredat et al. [22] to study interfacial polymerization of 4,40 -isopropylidenediphenoldimethacrylate induced by irradiation with ultraviolet light (photopolymerization), Cunnane and Evans [23] have shown for the ®rst time that electron transfer reactions can be brought about between an aqueous-based redox system and an organic-based monomer unit (1-methylpyrrole and 1-phenylpyrrole) at an electri®ed liquid±liquid interface resulting in the formation of oligomers in the organic phase.
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 4 7 4 - X
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It was presumed that the initial electron transfer results in the formation of a radical cation in the organic phase. Later the system was expanded to look at another electroactive monomer (2,20 :50 ,200 terthiophene) and organic systems (tetraphenylarsonium tetrakis(penta¯uorophenyl)borate (TPAsTPBF20)/1,2-DCE) [24,25] producing polyterthiophene. Considering the interface between two immiscible electrolytes as a mixed solvent region, the effect of aqueous and organic mixed solutions on the electropolymerization of thiophene is an interesting point to elucidate. In a recent study [10], it was shown that the use of a suitable ratio of water to acetonitrile could lower the onset oxidation potential of bithiophene in an aqueous/organic mixed solution. The authors demonstrated that the chemical structure of polybithiophene synthetized by this technique is very similar to that of the polymer formed in organic media. However, the volume ratio of water to acetonitrile has a signi®cant effect on the electropolymerization as well as on the electrochemical properties and morphology of the formed ®lms. One of the reactions which is expected to occur is the anodic overoxidation of PTs in wet organic electrolytes. In their report, the formation of a dioxide sulphone group at one of three monomer units was revealed by FTIR spectroscopy just after galvanostatic electropolymerization. Nevertheless, it was observed that this partial overoxidation had almost no in¯uence on the electronic conductivity. On the other hand, at a higher water concentration, further oxidation was shown to proceed corresponding to an oxidative SO2 elimination and the formation of carbonyls in the 2-, 3- and 5-position. These results opened the possibility for a systematic anodic modi®cation of the conducting polymer through organic electrochemistry in the solid state. The authors also emphasized that transport processes are not limiting due to the porosity of the polymer layer. Different measurements techniques have been applied to the study of electrochemical polymerization of thiophene and its derivatives. Most investigations were performed under potentiostatic control. More recently [11], electrochemical impedance spectroscopy (EIS) was used to investigate the electrochemical polymerization of bithiophene under galvanostatic conditions. It was shown that the successful model applied in studying the impedance spectra was strongly dependent on the ®lm thickness deposited on a Pt electrode and the corresponding current density. It was suggested that the deposition of polymer ®lm onto a metal electrode follows a 3-D growth mechanism during the ®rst stages of polymerization, and pseudo-2-D growth for polymerization at charges higher than 15±20 mC cm 2. Important parameters of the polymer ®lm (e.g. apparent diffusion coef®cient, ionic conductivity, etc.) were estimated from the impedance spectra. In this study, a novel electrochemical±chemical type mechanism reaction initiated by a HET between an aqueous couple (Ce(IV)/Ce(III)) and a monomer in DCE at ITIES is reported. Galvanostatic polymerization at ITIES and at solid macroelectrodes leading to the formation of poly(2,20 :50 ,200
terthiophene) in addition to EIS under galvanostatic control are the main electrochemical techniques used to investigate this mechanism. FTIR spectroscopy is employed to characterize the resulting polymer. 2. Experimental 2.1. Chemical 1,2-Dichloroethane (1,2-DCE) from Fluka (99.5% (GC grade) was used as received as the organic solvent in all experiments. A 18.2 MO cm water used throughout was prepared using the Maxima ultra pure water system (Elga). Li2SO4 (Fluka >98%) was used as received as the aqueous supporting electrolyte. Tetraphenylarsonium tetrakis(penta¯uorophenyl)borate (TPAsTPBF20) was the supporting electrolyte in the organic phase and was prepared using tetraphenylarsonium chloride (TPAsCl) (Fluka 95%) and lithium tetrakis(penta¯uorophenyl)borate etherate (LiTPBF20) (Boulder Scienti®c Co.) according to the following procedure: 2.5 g TPAsCl were dissolved in 200 ml H2O and added to 4.09 g LiTPBF20 in 200 ml methanol 215 (super purity solvent, Romil Ltd. >99.8%) resulting in a white precipitate; the white compound was filtered under vacuum and washed with a 50/50 mix of water and methanol, recrystallized in methanol; the salt was then dried at 608C for 3 days. Cerium(IV)sulfate (Aldrich), Cerous(III) sulfate anhydrous (Fluka), sulfuric acid (H2SO4) (Merck 95±97%), 2,20 :50 ,200 terthiophene (Fluka >99%) were also used as received. 2.2. Procedure 2.2.1. The 3-electrode configuration cell Electrodeposition was performed galvanostatically at various current densities (0.05±0.5 mA cm 2) without stirring. The working electrode was Pt, GC or gold macroelectrodes (Metrohm, 0.0707 cm2). A Pt ¯ag was used as a counter electrode and either a standard calomel electrode (SCE) or an Ag/AgCl (silver/silver chloride wire dipped in 1 mM TPAsCl/H2O) or an Ag/AgTPBF20 (silver wire anodically polarized for 5000 s at 1 mA in a 1 mM TPAsTPBF20/DCE solution) were used as reference electrodes, respectively. The electrolyte compositions in DCE were 2±10 mM monomer and 1±4 mM TPAsTPBF20 (cell 1). The total polymerization charge during this experiment usually reached 10 mC cm 2 except for the EIS experiment on Pt macroelectrode (100 mC cm 2). 2.2.2. The 4-electrode configuration cell The cell was custom-made at University of Helsinki as reported before [16]. The interface between the organic and
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aqueous phase has an area of 0.126 cm2. The applied potential difference at the interface is supplied by two Pt electrodes and the potential difference at the interface is measured by means of the two reference electrodes which are connected to the interface by two Luggin capillaries. In the aqueous phase the reference electrode is a Pt wire cleaned with sulfuric acid. The organic reference electrode is a non-polarizable reference system Ag/AgCl/TPAsCl or Ag/AgTPBF20/TPBF20 . The aqueous phase is a 0.1± 0.05 M Ce(IV)(SO4)2/0.01±0.005 M Ce(III)2(SO4)3 in 0.1 M Li2SO4/0.2 M H2SO4/H2O solution. The organic phase is 2±10 mM 2,20 :50 ,200 terthiophene, 1±4 mM TPAsTPBF20 in 1,2-DCE (depending on the experiment, see cells 2 and 3). Cell 1 Ag/AgCl/TPAsCl (1 mM/H2O) or Ag/AgTPBF20 or SCE/2.5, 5 or 10 mM 2,20 :50 ,200 terthiophene (0.75, 1 or 5 mM TPAsTPBF20)(1,2-DCE)/Pt or GC or Au. Cell 2 Ag/AgCl/TPAsCl (1 mM/H2O)/5 or 10 mM 2,20 :50 ,200 terthiophene (1 mM TPAsTPBF20/1,2-DCE)/d/0.1 M Ce(IV)(SO4)2, 0.01 M Ce(III)2(SO4)3, 0.1 M Li2SO4,0.2 MH2SO4(H2O)/Pt(d electrified liquid±liquidinterface). Cell 3 Ag/AgTPBF20/2.5 mM 2,20 :50 ,200 terthiophene, 3 or 5 mM TPAsTPBF20(1,2-DCE)/d/0.05 M Ce(IV)(SO4)2, 0.005 M Ce(III)2(SO4)3, 0.1 M Li2SO4, 0.2 M H2SO4(H2O)/Pt (d electrified liquid±liquid interface). Cyclic voltammetry studies in both 3 and 4 electrode mode were performed using a 1287 Solartron Electrochemical potentiostat±galvanostat interfaced with DC Corrware for Windows software (Scribner Associates, version 2.1). EIS measurements were performed with a 1260 Solartron Frequency Response Analyzer model coupled to a 1287 Solartron Electrochemical Interface model. Data were collected and analyzed using a PC and Zplot Software for Windows (Scribner Associates, version 2.1), respectively. While most investigations are performed under potentiostatic control, in this study EIS was used to investigate the electrochemical polymerization of 2,20 :50 ,200 terthiophene under galvanostatic conditions (i.e. 0.1 mA cm 2). The current perturbation amplitude was ®xed at 0.001 and 0.0001 mA for the electri®ed liquid±liquid interface and the macroelectrode, respectively. The EIS measurements were performed at intervals of 15 s. In order to avoid polymer deposition during the impedance measurements, the cell current was switched to 0.0 mA cm 2 for the duration of the perturbation pulses and subsequent data transmission to the computer. Frequencies varied typically from 5000 to 0.5 Hz. 2.3. Sample characterization The FTIR spectra were recorded with a series FTIR spectrometer (Bomem MB). Typically, 20 scans were recorded between 4000 and 400 cm 1 (resolution 1 cm 1).
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FTIR data were collected with Win±Bomem EasyTM Software for Windows (version 3.04). Poly(2,20 :50 ,200 terthiophene) was synthetized by galvanostatic polymerization (0.1 mA cm 2) at ITIES. Several batches of 10 mC cm 2 polymer charges were prepared from an organic electrolyte of 2.5 mM 2,20 :50 ,200 terthiophene/3 mM TPAsTPBF20/1,2DCE in contact with the solution 0.05 M Ce(IV)(SO4)2/ 0.005 M Ce(III)2(SO4)3 in 0.1 M Li2SO4/0.2 M H2SO4/H2O aqueous solution (cell 3). The dark green polymer was ®ltered under vacuum, washed with ultra pure water (18.2 O cm) and acetone, dried at 608C in the oven for 3 days. The samples were prepared by mixing the polymer with KBr powder and pressing the mixture into a pellet. 3. Results and discussion 3.1. Chronopotentiograms of electropolymerization Because ITIES behaves similarly to metal electrode± solution interfaces, many techniques of electrochemistry have previously been adopted to ITIES studies [21]. Galvanostatic polymerization has the main advantage of allowing better control of the charge state of a polymer than potentiostatic methods. Fig. 1 (cell 1) shows chronopotentiograms of 2,20 :50 ,200 terthiophene obtained at a Pt electrode for different anodic current densities. When the current density is less than 0.1 mA cm 2, the potential is nearly constant in the chronopotentiograms. This indicates that the electrochemical reaction is mainly determined by electron transfer control. The electropolymerization reaction is instantaneous, the potential reaches a high value almost immediately, indicating a faradaic process. However, at higher current density, e.g. 0.5 mA cm 2, the potential increases slightly with time. This implies that the reaction is mainly controlled by monomer diffusion at higher current density. This phenomenon is not unusual, as the reaction is faster at high current densities creating a de®ciency of monomer
Fig. 1. Galvanostatic electropolymerization of 2,20 :50 ,200 terthiophene at a Pt macroelectrode (A 0:0707 cm2) for different current densities: i 0:05, 0.1, 0.5 mA cm 2. Solution: 5 mM 2,20 :50 ,200 terthiophene/ 1 mM TPAsTPBF20/1,2-DCE. Polymerization charge: q 10 mC cm 2. Inset: i 0:5 mA cm 2. Organic reference: Ag/AgCl/TPAsCl.
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species near the working electrode [10]. When the terthiophene is electropolymerized at the ITIES (cell 2, Fig. 2), the formation of a dark green polymer with an open structure is observed. It can be seen from Fig. 2 that the reaction potential also slightly increases with time and that such an increase is observed for current densities lower than those reported for the Pt electrode (e.g. 0.1 mA cm 2). This means that the reaction is controlled by monomer diffusion. At higher current densities (e.g. 0.2±0.5 mA cm 2), there is no real potential stabilization. The potential is not reaching high values immediately as observed for the solid electrode (Fig. 1) but slightly increases and then decreases. There is no stationary potential due to the rapid consumption of terthiophene in the interfacial region. This is also true if the monomer concentration is increased by a factor of two (cell 2, Fig. 3). It is important to note that, to date, the question of the ITIES structural model still remains unclear [26±31]. One model is that of a mixed solvent layer and, indeed, if a mixed
solvent layer model [28] at a polarizable water±DCE interface seems realistic, investigations of ITIES of the type described here may allow the development of an easily accessible model. Since water has a much higher viscosity than DCE and also since the solubility of terthiophene is expected to decrease when there is more water in the mixed solution (for instance, at the ITIES), the monomer diffusion rate should slow down in solution if there is a mix of water and DCE. This could explain the discrepancy in the behavior observed during galvanostatic polymerization at ITIES compared with the solid electrode (Pt or indeed GC) for the same current density. As mentioned by Hu et al. [10], electropolymerization of thiophene at metal electrodes can be carried out in aqueous/organic solution systems although the monomer is insoluble in water. They have shown that it was possible to tune the voltage of the electropolymerization at constant current density by changing the volume ratio of water to organic solvent (the voltage is lowered by increasing the water content). Unfortunately, because the stabilization potential at a liquid±liquid interface in fact includes an ohmic drop corresponding to the resistance of the ®lm and the solution resistance and because of the difference in the polymer af®nity with its electrode support, it remains dif®cult to compare directly the stabilization potentials of each technique. However, in order to approximate the polymerization behavior of 2,20 :50 ,200 terthiophene at ITIES, the same electropolymerization was carried out at a GC macroelectrode with different volumes of aqueous electrolyte added to the organic deposition solution (see cell 1, Fig. 4). The ratios of water (actually, 0.05 M Ce(IV)(SO4)2/0.005 M Ce(III)2(SO4)3 in 0.1 M Li2SO4/0.2 M H2SO4/H2O) in the organic solution system studied were 0, 1 and 2% v/v. The solution was stirred before electrochemical measurements. In Fig. 4 it can be seen that, as already observed for water addition [10], the polymerization potential decreases with increasing volume ratio of aqueous phase to DCE in the
Fig. 3. Galvanostatic electropolymerization of 2,20 :50 ,200 terthiophene at ITIES (A 0:126 cm2) for two monomer concentrations, at a current density of 0.1 mA cm 2; q 10 mC cm 2. Organic solution: 5 or 10 mM 2,20 :50 ,200 terthiophene/1 mM TPAsTPBF20/1,2-DCE. Aqueous solution: 0.1 M Ce(IV)(SO4)2/0.01 M Ce(III)(SO4)3 in 0.1 M Li2SO4/0.2 M H2SO4/ H2O. Organic reference: Ag/AgCl/TPAsCl.
Fig. 4. Galvanostatic electropolymerization of 2,20 :50 ,200 terthiophene at a GC macroelectrode (A 0:0707 cm2) for different aqueous solution content: 0, 1 and 2% v/v. Constant current density: 0.1 mA cm 2; q 10 mC cm 2. Solution: 2.5 mM 2,20 :50 ,200 terthiophene/1 mM TPAsTPBF20/1,2-DCE. Organic reference: Ag/AgTPBF20.
Fig. 2. Galvanostatic electropolymerization of 2,20 :50 ,200 terthiophene at ITIES (A 0:126 cm2) for different current densities: i 0:1, 0.2, 0.5 mA cm 2; q 10 mC cm 2. Organic solution: 5 mM 2,20 :50 ,200 terthiophene/1 mM TPAsTPBF20/1,2-DCE. Aqueous solution: 0.1 M Ce(IV)(SO4)2/0.01 M Ce(III)(SO4)3 in 0.1 M Li2SO4/0.2 M H2SO4/H2O. Organic reference: Ag/AgCl/TPAsCl.
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mixed solution (constant current density of 0.1 mA cm 2). When the addition of the aqueous solution is less than 1% (i.e. 0.5% v/v, not shown), the potential is almost constant in the chronopotentiograms. As such the electrochemical reaction is mainly determined by electron transfer control. However, at higher aqueous phase content (2% v/v), the reaction potential increases with time showing the same kind of shape as that observed at ITIES for a similar current density (e.g. 0.1 mA cm 2). The monomer diffusion rate is slowed down due to the higher viscosity of the aqueous phase compared to the organic one. There is also a decrease in the solubility of terthiophene when there is more water in the mixed solution. This behavior was further detailed earlier in this discussion and it shows that a mixed solvent layer model at a polarizable water±DCE interface seems realistic. In order to be sure that the presence of the redox couple Ce(IV)/Ce(III) in the aqueous phase did not in¯uence the reaction in solution prior to the analysis at the GC electrode, this experiment was repeated with just H2O added (not shown). The same trend was observed. 3.2. Cyclic voltammetry (CV) Fig. 5a and b show typical cyclic voltammograms for a terthiophene polymeric ®lm formed at a metal electrode
Fig. 5. Cyclic voltammograms of 2,20 :50 ,200 terthiophene (a) at a gold macroelectrode (A 0:0707 cm2), scan rate: 100 mV s 1. Solution: 10 mM 2,20 :50 ,200 terthiophene/0.75 mM TPAsTPBF20/1,2-DCE after deposition of the polymer at 1.05 V; (b) at ITIES (A 0:126 cm2), scan rate: 50 mV s 1 after a deposition of a charge of 10 mC cm 2. Organic solution: 5 mM 2,20 :50 ,200 terthiophene/1 mM TPAsTPBF20/1,2-DCE. Aqueous solution: 0.1 M Ce(IV)(SO4)2/0.01 M Ce(III)(SO4)3 in 0.1 M Li2SO4/0.2 M H2SO4/H2O, organic reference: Ag/AgTPBF20.
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(gold macroelectrode in TPAsTPBF20/1,2-DCE) and a polymeric ®lm formed at an immiscible electrolyte solution interface (water±1,2-DCE, cell 2). While the conditions of cycling are different (electrode support area, scan rate, charge deposited, etc.) and therefore does not allow direct comparison between ITIES and the gold macroelectrode, the shape of the CVs are similar with quite the same waves of oxidation±reduction (i.e. potential position) as already observed by Hu et al. [10] for ®lms of PT electropolymerized in different mixed (water/organic) solutions. One point of interest is that in the ITIES case the current does not start at the zero point as is the case for the metal electrode. It is believed this is due to the changing position of the interface with time due to the formation of the polymer. 3.3. Electrochemical impedance spectroscopy (EIS) The process of electrochemical growth of poly(2,20 :50 ,200 terthiophene) at ITIES was investigated by means of EIS and the time-resolved impedance spectra are shown here, measured during controlled-current polymerization (see Section 2). The impedance spectra were analyzed using the wellknown model of Ho et al. [32], for low current density (e.g. 0.1 mA cm 2) conditions. The impedance spectra obtained are shown in Fig. 6 (cell 3). The total polymerization charge during this experiment reached 10 mC cm 2. The main dif®culties of this type of investigation are ®rstly that the shape of the interface may change with the potential difference across the interface and with the increase of the polymer weight at the interface. Secondly, the reference electrode employed in the organic media (Ag/AgCl in TPAsCl/H2O) may not be suitable for EIS measurements. Indeed, both should generate artefacts in the impedance spectra as shown by Schiffrin et al. [33]. It is rather dif®cult
Fig. 6. Complex plane impedance spectra obtained during 2,20 :50 ,200 terthiophene galvanostatic polymerization (current density: 0.1 mA cm 2) at ITIES for two electropolymerization charges: q 4:5 and 10 mC cm 2. Frequency range: 5000±0.5 Hz. I appl 0 mA cm 2. Amplitude: 0.001 mA. Organic solution: 2.5 mM 2,20 :50 ,200 terthiophene/3 mM TPAsTPBF20/1,2DCE. Aqueous solution: 0.05 M Ce(IV)(SO4)2/0.005 M Ce(III)(SO4)3 in 0.1 M Li2SO4/0.2 M H2SO4/H2O. Organic reference: Ag/AgTPBF20.
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to control the ®rst parameter (e.g. the variation of the position of the interface), the only control being to use low polymerization charges. The second parameter was avoided by the use of a new reference electrode that of Ag/AgTPBF20, which appears to obviate the high value of the resistance of the Luggin capillary in the organic phase (as observed by Kobmehl et al. [34] with Ag/AgTPB) [35]. The use of current control instead of potential control for the EIS measurements should induce a lower modi®cation of the interface and allow at least a better control of the steady-state. 3.3.1. Equivalent circuit More comprehensive treatments of the impedance spectra of oxidized conducting polymers take into account the ®nite diffusion of counter ions through the polymer layer, assumed to be homogeneous, in analogy with the analysis given by Ho et al. [32] for the case of Li intercalation into WO3 electrodes. An equivalent circuit which describes the behavior of an electrode (GC or Pt macroelectrode) coated with a polymer layer according to the model of Ho et al., as shown in Scheme 1, is also used to ®t the impedance spectra at ITIES, assuming that the replacement of a metal electrode by a solution of a fast redox couple leads to the same behavior. In this case, Rs is the ohmic series resistance, Rct the charge transfer resistance, Cdl the double layer capacitance, and a second charge transfer resistance Rp has been introduced by Popkirov et al. [11] to account for the polymerization reaction. The restricted ®nite Warburg impedance ZRFW is given by: ! 1 dE coth l iwD1=2 ZRFW (1) nF dc iwD1=2 where n is the number of electrons transferred, F Faraday constant, E the electrode potential, D the apparent diffusion constant for anions in the polymer, l the film thickness, w the frequency and c the bulk concentration of the oxidized polymer. The total impedance of the equivalent circuit from cell 3 can thus be given by: 1 1 1 Z iwCdl (2) S Rs ZRFW Rct Rp where S is the surface area of the electrode. 3.3.2. IES data analysis: ITIES case The semi-circle observed at higher frequencies (Fig. 6) can be related to the charge transfer in the oxidation/ reduction reaction, and can be represented by the faradaic
Scheme 1.
resistance Rct and the double layer capacitance Cdl. At intermediate frequencies ion diffusion through the polymer layer becomes dominant, leading to an impedance behavior of Warburg type. A large circle is thus observed corresponding to the polymerization resistance Rp and not observed when impedance spectra are computed in the absence of monomer (not shown). At lower frequencies, however, the capacitive character due to the ®nite thickness of the layer and the limited amount of polymer able to take part in the oxidation/reduction reaction, is not observed. A complex ®tting program was used to ®t Eq. (2) to the experimental EIS data obtained on the polymer ®lm at the interface during the initial stages of the polymerization and to evaluate the parameters of the suggested equivalent circuit. It is rather dif®cult to evaluate the ®lm thickness at the interface. However, we could estimate it using the polymer layer thickness of 80 nm obtained for a bithiophene ®lm polymerized on a Pt electrode for a charge of 30 mC cm 2 (at 0.15 mA cm 2) using a value for polymer density of 1.5 g cm 3 [11]. This thickness approximation allows us to evaluate the apparent diffusion constant D. The parameters Rct, Cdl and Rp as a function of the polymerization degree (charge of polymer) from the data of Fig. 6 are presented in Fig. 7a±c. The charge transfer resistance and the double layer capacitance increases and decreases, respectively, during the polymerization, indicating that the interface available for the oxidation/reduction reaction of the polymer changes signi®cantly. This is not in agreement with the results obtained at a Pt electrode by Popkirov et al. [11] where no signi®cant change was reported. In addition, the increase in estimated polymerization charge transfer resistance Rp is probably due to an increase in the activation energy for monomer oxidation and/ or polymer precipitation at the interface already covered with polymer. Accordingly, an increase of the polymerization potential is observed during the galvanostatic experiment. The opposite behavior, e.g. a decrease of Rp and the corresponding stabilization potential, was reported at a Pt electrode [11]. This indicates that a different process is occurring at ITIES for polymer growth. 3.3.3. Macroelectrode case In order to compare the polymerization process at ITIES with solid electrodes, the same experiment was carried out at a GC and Pt electrode (Fig. 8 shows the complex plane impedance for a GC electrode). Similar trends and impedance spectra were observed at both Pt and GC macroelectrodes. The parameters Rct, Cdl and Rp (from the ®tting with Ho et al. [32] equivalent circuit for thin ®lms) as a function of the polymerization degree (charge of polymer) are presented in Fig. 9a±c (for the Pt electrode). The charge transfer resistance, Rct, and the double layer capacitance, Cdl, slightly increases during the polymerization in good agreement with literature [11], indicating that the interface available for the oxidation reaction of the conducting polymer does not change signi®cantly. The polymerization charge
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Fig. 8. Complex plane impedance spectra obtained during 2,20 :50 ,200 terthiophene galvanostatic polymerization (current density: 0.1 mA cm 2) at a GC macroelectrode for two electropolymerization charges: q 4:5 and 10 mC cm 2. Frequency range: 5000±0.5 Hz. I appl 0 mA cm 2. Amplitude: 0.001 mA. Organic solution: 2.5 mM 2,20 :50 ,200 terthiophene/5 mM TPAsTPBF20/1,2-DCE. Organic reference: Ag/AgTPBF20.
Fig. 7. (a) Charge transfer resistance Rct of the oxidation/reduction of polyterthiophene; (b) double layer capacitance Cdl; and (c) charge transfer resistance Rp of the polymerization reaction as estimated by applying the model of Ho et al. [32] to impedance spectra measured at ITIES during polymerization at 0.1 mA cm 2. Organic reference: Ag/AgTPBF20.
transfer resistance Rp decreases as reported by Popkirov et al. [11], indicating a decrease in the activation energy for monomer oxidation and/or polymer precipitation at the electrode already covered with polymer. The difference in the shape of the impedance spectra computed for ITIES and GC or Pt electrodes (which provide the same spectra) con®rmed the difference in the kinetics of the polymerization reaction at liquid±liquid interface compared to solid electrodes. At this stage the question of the polymer conductivity when electrosynthetized at ITIES compared to the electrosynthetic product at solid electrodes is of interest. Additional work is currently under way to characterize the electronic properties of the resulting polymeric material using techniques such as XPS measurements. Impedance spectra were computed at a GC (or Pt) electrode and ITIES before starting the galvanostatic polymerization (not shown). This experiment allows us to evaluate the ohmic series resistance or the solution resistance. It
Fig. 9. (a) Charge transfer resistance Rct of the oxidation/reduction of polyterthiophene; (b) double layer capacitance Cdl; and (c) charge transfer resistance Rp of the polymerization reaction as estimated by applying the model of Ho et al. [32] to impedance spectra measured at a Pt macroelectrode during polymerization at 0.1 mA cm 2. Organic reference: Ag/AgTPBF20.
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seems strange to observe that at ITIES, Rs values are lower than at a GC electrode. This could be attributed to the enhanced solution conductivity when more water is present in the mixed solutions. A ®rst approximation of the apparent diffusion constant D using the formula W s T l2 =D (W s T, Warburg ®tting parameter) indicates a higher diffusion rate for the ions in the polymer at the Pt electrode compared to ITIES. The values for D, however, are lower than expected (ca. 10 12 cm2 s 1 compared to 10 8 cm2 s 1 [32]) probably due to an under-estimation of the ®lm thickness (l 27 nm). The authors agree that four orders of magnitude for D is a large under-estimation, in addition, we believe, as it was already shown [11], that the model of Ho et al. [32] should be applied cautiously for electrodes (or interfaces) covered with small amounts of polymer. It remains clear that additional work is required to elucidate the polymerization growth mechanism at ITIES. Some caution should be taken with the EIS interpretation. Indeed, in a future work, spectra will be collected with higher electrode surface areas in order to more completely avoid artefacts due to the low currents used, as suggested by Schiffrin et al. [33] in relation to EIS measurements at ITIES. 3.4. FTIR spectroscopy of the formed polymer Fig. 10 shows the FTIR spectrum of freshly electropolymerized 2,20 :50 ,200 terthiophene at an electri®ed liquid±liquid interface which had been prepared and removed for analysis. The organic and aqueous solvents in contact were 2.5 mM 2,20 :50 ,200 terthiophene/3 mM TPAsTPBF20/1,2-DCE and 0.05 M Ce(IV)(SO4)2/0.005 M Ce(III)2(SO4)3 in 0.1 M Li2SO4/0.2 M H2SO4/H2O (cell 3). At high wavelengths,
Fig. 10. FTIR spectrum of poly(2,20 :50 ,200 terthiophene) after galvanostatic electropolymerization at ITIES. Polymerization conditions: 2.5 mM 2,20 :50 ,200 terthiophene/3 mM TPAsTPBF20/1,2-DCE in contact with the 0.05 M Ce(IV)(SO4)2/0.005 M Ce(III)(SO4)3 in 0.1 M Li2SO4/0.2 M H2SO4/H2O aqueous solution. Current density: 0.1 mA cm 2. Mixture of polymer with KBr powder pressed into a pellet.
the usual strong absorption due to the extended p-system can be seen. The two strongest bands at 1155 and 1326 cm 1 can be attributed to the symmetric and asymmetric valency vibrations of the sulphone group ±SO2, respectively, as mentioned by Barsch and Beck [36]. The band at 1638 cm 1 can be related to a carbonyl group eventually in conjugation to a C=C double band. These results conclude that PTs electropolymerised at ITIES can be oxidized as already observed for a polymer ®lm grown on Pt [36] and yield the dioxide (sulphone). On the other hand, a further oxidation (overoxidation) was not observed here. Future research investigating the nature of the polymer and the degree of its oxidation state has to be carried out. For instance, the use of techniques such as X-ray photoelectron spectroscopy (XPS) would allow us to observe the presence of terminal ±COOH groups (from the cleavage of a C±C bond). The complete mechanism for the anodic oxidation (overoxidation) of PTs in the presence of water is given in [36]. Indeed, the presence of sulphone groups in the poly(2,20 :50 ,200 terthiophene) is another clue to the ITIES structural model which seems to tend to a mixed solvent model. 4. Conclusion It is currently too early to state a growth mechanism for polymer formation at ITIES. However, we suggest this mechanism to be different than the one observed at a metal electrode. Considering galvanostatic polymerization and FTIR spectroscopy results, the ITIES structural model which seems to favor a mixed solvent model, could be at the origin of the discrepancies in the growth mechanism at solid electrodes and ITIES. It has already been shown that electrolyte impurities, oxygen and water affect the stability of conducting polymers under electrochemical conditions [37]. These aforementioned drawbacks will probably have a strong in¯uence on polymers synthetized at ITIES and therefore on their potential applications. To go further, XPS measurements should allow us to con®rm these remarks. It should also be of interest to investigate the polyterthiophene morphology when synthetized at ITIES in order to compare to literature. Differences in the surface structures of the ®lms prepared at ITIES could be studied by atomic force microscopy (AFM) since it was shown by Hu et al. [10] that polybithiophene ®lms prepared in mixed aqueous/organic solutions have a grainy structure with the grain size increasing with more water in the mixed solution. Larger size of grains suggest that there may be more voids existing in the ®lms synthetized in solutions containing more water. It means that such ®lms are less dense compared with ®lms with smaller grain size. This ®lm morphology observation should be consistent with our impedance results indicating poorer electrochemical stability and conductivity however this kind of structure should allow a good rate of charge migration.
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Acknowledgements This research was funded by the EU, Training and Mobility of Researchers Network, ODRELLI (Organization, Dynamics and Reactivity at Electri®ed Liquid±Liquid Interface), Contract No. ERBFMRXT960078. We thank Marina Serantoni, from the University of Limerick for her advice and useful discussions.
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