We present in this work a study of the electrochemical behaviour of terthiophene and its corresponding polymer, which is obtained electrochemically as a film by cyclic voltammetry (CV) on platinum electrode. The analysis focuses essentially on the effect of two solvents acetonitrile and dichloromethane on the electrochemical behaviour of the obtained polymer. The electrochemical behavior of this material was investigated by cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The voltammograms show that the film of polyterthiophene can oxide and reduce in two solutions; in acetonitrile, the oxidation current intensity is more important than in dichloromethane. The impedance plots show the semicircle which is characteristic of charge-transfer resistance at the electrode/polymer interface at high frequency and the diffusion process at low frequency.
As other conjugated conducting polymers, polythiophene and its oligomers can be polymerized from their monomers in solutions by electrochemical methods. The electrochemical synthesis is advantageous method: polymers are formed in the doped state; films generally possess interesting electrochemical and good semiconductor properties [
The mechanism of the electropolymerisation of conducting polymers and polyheterocycles occurred by the coupling via
Conducting polymer-modified electrodes have been widely investigated because of their potential application in areas such as electrocatalysis [
In our previous work [
The solvents acetonitrile and dichloromethane, Aldrich pure products for analysis, were used without further purification. Terthiophene (3T) was purchased from Aldrich (98%), and it was used as received; the supporting electrolyte salt tetrabutylammonium perchlorate (TBAP) was purchased from Fluka; this salt was first dried at a temperature of 80°C for 4 h before its use for the preparation of solution.
Electrochemical measurements were carried out on potentiostat/galvanostat Voltalab 40 interfaced with a PC under Voltamaster 4 software, in a three-electrode cell consisting of: platinum working electrode (diameter 2 mm), which was served as a substrate for the deposition of polyterthiophene (P3T), wire platinum as auxiliary electrode, and a saturated calomel electrode (SCE) as reference.
Electrochemical impedance spectroscopy measurements were performed using an impedancemeter Z-computer TACUSSEL controlled by microcomputer HEWLETT-PACKARD. The assembly is coupled to a plotter and a printer. The impedance spectra were recorded in the frequency range 105 Hz–10−3 Hz with an amplitude of 10 mV, in three-electrode electrochemical cell. All experiments were carried out at room temperature.
Before the electrochemical deposition of polyterthiophene for each experiment, the working electrode was polished by 0.03
Figure
Electrochemical polymerization of 3T (a) 1rst cycle, repetitive cycling (b) in a solution of TBAP 0.1 M in acetonitrile, scan rate 50 mV/s.
After polymerization, the working electrode was extracted from the cell, rinsed with acetone, and dried with a gentle nitrogen flux, then analysed by cyclic voltammetry and impedance spectroscopy in the monomer-free solution of CH3CN/TBAP and CH2Cl2/TBAP, respectively.
Figure
Cyclic voltammograms corresponding to P3T film in a solution of CH3CN/TBAP 0.1 M, recording for different scan rates: (a) 10, (b) 20, (c) 50, and (d) 100 mV/s.
The voltammogram recorded at scan rate of 10 mV/s (inset of Figure
The film formed on the platinum electrode is electrochemically active and shows a reversible change of colour, a red in the oxidation state and blue in the reduction one. This phenomenon of P3T based on reversibly coloured electrochromic materials has become the subject of many interest applications. The electrochemical stability of the film was shown by repetitive voltammograms which remain essentially stable with cycling, and no evolution of the oxidation or the reduction current is observed. The stability of the oligothiophenes in air is already discussed in the literature [
The film of P3T was furthermore studied in CH2Cl2/TBAP in absence of monomer. The corresponding voltamperograms are presented in Figure
Cyclic voltammograms corresponding to P3T film in a 0.1 M solution of TBAP in dichloromethane recording for different scan rates; (a) 10, (b) 20, (c) 50, and (d) 100 mV/s.
The voltammogram obtained here do not show clearly the anodic peaks except in the return scan; we observe a peak at 0.51 V attributable to the reduction of the film deposited on the electrode. So, contrary to the analysis realized in acetonitrile medium, in dichloromethane, the oxidation peak is poorly defined and is concealed, probably because of strong participation of capacitive current. The current intensity of the anodic and cathodic peak increases with scan rate. Also, the waves and the oxidation peaks do not appear when curves are recorded with high scan rates (
These results suggest that the thickness of the film is smaller than the diffusion layer thickness of counteranions on the cyclic voltammetric time scale used here, which must diffuse in and out during the doping and dedoping processes. The oxidation peaks shifted to more positive potential at scan rates increase; this suggests that as explained in reference [
Figure
Nyquist plots of Pt modified electrode with P3T, recorded in: (a) CH3CN/TBAP, (b) CH2Cl2/TBAP. Plots were recorded in the frequency range from 105 Hz to 10−3 Hz at free potential.
Towards low or middle frequency region, the impedance diagrams show a straight line with slope of 45° which indicates a diffusion-controlled Warburg behavior, attributable to the semiinfinite diffusion of protons at the polymer-electrolyte interface [
From the Nyquist plots, the kinetic parameters can be easily deduced. The value of transfer resistance allows us to determine the corresponding capacity
Electrical parameters corresponding to P3T in CH3CN/TBAP and in CH2Cl2/TBAP.
CH3CN/TBAP | CH2Cl2/TBAP | |
---|---|---|
100 | 500 | |
500 | 2600 | |
0.019 | 0.036 |
As shown from Table
Generally, the electrochemically results obtained from the impedance diagrams of conducting organic polymers are modelled by an equivalent electrical circuit. Many of them have been proposed, in the literature, and in general, for the most part of cases, the equivalent circuit can be assimilated to a circuit of the Randles model, more or less modified according to the experimental conditions. The electrochemical parameters of P3T film are modelled by Randles circuit (Figure
Electrical equivalent circuit corresponding to P3T/electrode interface (Randles circuit).
Electrochemical characterisation of P3T film was carried out using CV and EIS techniques. CV measurements showed that the electroactivity of polyterthiophene films intensity increases with increase of scan rates and it is more important in CH3CN than in CH2Cl2. However, the reduction peak currents are broader and displaced to more negative potential. EIS revealed that, at high frequencies, charge-transfer process dominates with a semicircle, and at low frequencies, diffusion process dominates with a slope line of 45°. The electrical parameters were found to be more significant in CH3CN.