Electrochemical Properties of Thin Films of Polythiophene

Jan 27, 2009 - 45-03, Edificio 451, Bogotá, Colombia ... It was found that the electrochemical properties, ion transport ... Universidad Nacional de ...
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J. Phys. Chem. B 2009, 113, 1899–1905

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Electrochemical Properties of Thin Films of Polythiophene Polymerized on Basal Plane Platinum Electrodes in Nonaqueous Media Marco F. Suarez-Herrera*,†,‡ and Juan M. Feliu† Instituto de Electroquı´mica, UniVersidad de Alicante, Apartado Postal 99, E-03080 Alicante, Spain, and Departamento de Quı´mica, Facultad de Ciencias, UniVersidad Nacional de Colombia, Cra 30 No. 45-03, Edificio 451, Bogota´, Colombia ReceiVed: October 10, 2008; ReVised Manuscript ReceiVed: NoVember 13, 2008

In this paper the electrochemical properties of polythiophene thin films synthesized on single-crystal platinum electrodes are studied. It was found that the electrochemical properties, ion transport kinetics, and morphology of the polythiophene films depend on the surface orientation of the single-crystal platinum electrode used for their electropolymerization. Different oxidation levels, regarded as neutral, polaron, bipolaron, and metallic states, are usually found in conjugated heterocyclic polymers. However, the transitions between the different oxidation levels were never clearly observed in cyclic voltammetry. Instead the voltammograms usually show broad oxidation and reduction peaks with some shoulders. With the use of single-crystal platinum electrodes, it was found that polythiophene has a well-defined redox process at low potential, not observed before, possibly related to the conversion from the neutral state to polarons. On the other hand, two well-defined consecutive steps were found during the ion exchange reaction of thin films of polymer, both characterized by nucleation kinetics. This is the first report of two consecutive nucleation processes during the ion exchange process of a conducting polymer. The results presented here could further illuminate the mechanism in which the electron is transported in organic semiconductor materials. Introduction Electrodes coated with conducting polymers show intriguing electrochemical features such as electronic conduction, ionic conduction, ion exchange ability, ion selectivity, accumulation of charge, and voltage-controlled chromatic.1-3 These features, however, have not been fully applied industrially to produce electrochemically controlled materials, partly because of its fragility and poor processability. A systematic study of the effect of parameters such as solvent, potential, electrolyte, and monomer concentration have on the nucleation and growth mechanism of polythiophene has been reported by Del Valle et al.4 The authors reported that in acetonitrile and with the use of a polycrystalline platinum disk as working electrode the j vs t transient presents three contributions corresponding to the following mechanisms: twodimensional instantaneous nucleation, three-dimensional progressive nucleation under charge-transfer control, and threedimensional progressive nucleation under diffusion control. The authors claimed as well that the main nucleation and growth process at the first stage of polythiophene growth corresponds to a charge transfer controlled reaction where adsorbed thiophene molecules are likely involved. Li and Alberyt5 reported that the electrochemical deposition of poly(thiophene-3-acetic acid) proceeds via a two-dimensional layer-by-layer nucleation and growth mechanism, following the first monolayer deposition through oxidative adsorption of the monomer on a bare Pt surface. On the other hand, it has been reported that when thiophene molecules adsorb on a Ge(100) surface, they preferentially form one-dimensional molecular * To whom correspondence should be addressed. Telephone: +571 3165000, ext. 14459. Fax: +571 3165220. E-mail: [email protected]. † Universidad de Alicante. ‡ Universidad Nacional de Colombia.

chains with Ge-S dative bonding configurations via a Lewis acid-base reaction.6 All the above facts point out that the surface state of the electrode must be determinant for the kinetics of polymer growth at the very beginning. The basic structural information on polythiophenes is usually obtained from electron diffraction, solid-state NMR, X-ray analysis, IR, and Raman spectroscopy. However, the method used to prepare the conducting polymer strongly affects the structure, and this fact, coupled with the difficulties in producing highly crystalline products, makes very difficult the interpretation of the experimental data.7-9 Many strategies have been used to synthesize highly crystalline polythiophene films.10,11 Particularly, Jin and Xue12 reported that low-potential electrochemical deposition of polythiophene can be carried out in the presence of a Lewis acid in a solvent with a suitable donor number and in dry condition. They obtained a high-quality conducting polythiophene where the conductivity parallel to the film surface was more than 104 times that across the film thickness. In a previous report13 we have proved that the electropolymerization of polypyrrole in aqueous media is a structuresensitive process on platinum single-crystal electrodes with basal orientations. The platinum surface structure determines adhesion, coverage level, charge-transfer properties, and morphology. The aim of this report is to give a step forward in this research project and study the electrochemical properties of polythiophene films synthesized on single-crystal platinum electrodes from nonaqueous solution. It is expected that working with well-defined surfaces, in the sense of cleanliness and definition of the energy states, and in the absence of nucleophilic solvents, such as water that can stop the polymerization reaction, polymers of much better quality and with particular properties can be synthesized. In this work, by using single-crystal platinum electrodes and different electroanalytical techniques such as chronoamperom-

10.1021/jp8089837 CCC: $40.75  2009 American Chemical Society Published on Web 01/27/2009

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Suarez-Herrera and Feliu

etry at constant potential, cyclic voltammetry, and electrochemical impedance expectroscopy (EIS), we demonstrate that it is possible to obtain polythiophene films in well-defined redox states, which have electrochemical and ion exchange properties not reported before. Experimental Section The electropolymerization of thiophene on platinum singlecrystal surfaces was carried out in sodium perchlorate solution in acetonitrile using a classical two-compartment electrochemical cell with a platinum counter electrode and Ag/AgCl reference electrode. The main experimental procedures related to the preparation of the clean single-crystal electrode surfaces and the design of the electrochemical cell were the same as those reported in a previous paper.13 The electrochemical measurements were performed using a computer-controlled potentiostat/ galvanostat µAutolab III (EcoChemie). Polymer films were grown under galvanostatic conditions, applying a current density of 2 mA cm-2 to the working electrode during a specific period of time. For ion exchange experiments or cyclic voltammetry, after electropolymerization, the polythiophene films were rinsed with dry solvent to get rid of the monomer and then immersed in dry electrolyte solution (containing only the solvent and supporting electrolyte). Tapping mode atomic force microscopy (AFM) images were obtained under ambient conditions using a Multimode atomic force microscope (Veeco Metrology) equipped with a Nanoscope III controller (Veeco) and using silicon scanning tips purchased from Nanosensors (model PPP-NCHR), which have a tip radius smaller than 10 nm (force constant, 42 N/m; resonance frequency in the range of 300 kHz). The cantilever drive frequency was chosen in such a way as to be 5% smaller than the resonance frequency. The free amplitudes of the TAFM tips used were adjusted by trial and error until a good-quality image was achieved. The films used for the AFM experiments were galvanostatically grown for 5 s at a constant current of 2 × 10-3 A cm2. For the electrochemical impedance spectroscopy (EIS) experiments and the study of the ion exchange kinetics about 101 ( 17 nm thick films (galvanostatically grown at 2 mA cm-2 and during 20 s) were used. This film thickness was measured by imaging the border of the film produced by scratching the film on the electrode gently with a filter paper. EIS measurements of the polythiophene films were carried out in the potential range between +0.65 and -0.4 V vs Ag/AgCl by superimposing 5 mV alternating current (ac) to the applied direct current (dc) potential at frequencies ranging from 100 kHz to 0.01 Hz. The impedance data were fitted utilizing equivalent-electrical circuits and using the FRA fitting program (EcoChemie). Before each EIS measurement, the electrode was held at the desired dc potential during an equilibrium time of 100 s. The EIS measurements were made in 0.1 M NaClO4 solution in acetonitrile. The electrolyte was 0.1 M sodium perchlorate (Merck) solution in acetonitrile, to which 0.1 M thiophene (Alfa Aesar) was added. The thiophene used was always purified a half-hour before each experiment by distillation. In all cases, the solutions were prepared with HPLC acetonitrile (Merck), and they were dried using molecular sieve 5 Å beads 8-12 mesh (CAS 6991279-4) and deareated with argon (ALPHAGAZ N50) before any electrochemical experiments were carried out. Measurements were made under ambient temperature conditions of 20 ( 2 °C. The stability and electrochemical properties of the polymer are affected by parasitic electrochemical processes from the

Figure 1. Voltammogram of the Pt(111) electrode in 0.1 M NaClO4 in acetonitrile (ACN) as received (thick line) and voltammogram obtained when the electrolyte has been treated with molecular sieve (thin line). Scan rate: 0.25 V s-1.

supporting electrolyte solutions. It has been reported that the electropolymerization of thiophene in the presence of water corresponds to the formation of a nonconducting or passive film.4 This behavior can be attributed to adsorbed water molecules that can react with the cation radicals inhibiting the formation of a conducting film.14 To overcome these difficulties, we have elaborated a new strategy where well-defined singlecrystal platinum surfaces and dry electrolytes were used to polymerize thiophene. The thick-line voltammogram of Figure 1 shows the characteristic voltammetric features of a Pt(111) electrode in 0.1 M NaClO4 in acetonitrile, when the electrolyte is not dried, related possibly to the hydrogen evolution (below -1.5 V), to hydrogen (between -1.5 and -0.9 V) and hydroxide (anodic peak between -0.6 and -0.15 V) adsorption, platinum bulk oxidation (between -0.15 and +1.6 V), oxygen evolution (over +1.6 V), and reduction of platinum oxides (reduction peak between -1 and -0.5 V in the negative-going sweep). The lack of masstransport and H+ concentration changes upon cycling break the symmetry, which is observed in acidic or alkaline aqueous media, between cathodic and anodic peaks. The latter voltammetric features almost disappear when the electrolyte is dried during 2 h using a molecular sieve (40 g/(100 mL of electrolyte)), as it is shown in Figure 1, thin line. The small voltammetric features that still can be seen in Figure 1 (thin line) are possibly due to the presence of a small amount of water, which cannot be removed by the molecular sieve, and the subsequent formation/removal of surface oxides at potentials above 0.4 V. To avoid the oxidation of the electrode, to preserve as much as possible its well-defined surface structure until upper potential limits up to +1.8 V and avoid side reactions between water (and its oxidation products) with the thiophene radicals, the polymerization of thiophene was done always in the presence of this molecular sieve. Results and Discussion Voltammetry of Polythiphene Films on Basal Plane Platinum Electrodes. The polythiophene films usually exhibit broad redox signals containing two or three shoulders or peaks between about -1 and +1 V vs Ag/AgCl.15,16 Chen and Ingana¨s17 reported that at low temperatures three reversible couples of redox peaks could be distinguished for thin films of poly(3,4-ethylenedioxythiophene) using an ultramicroelectrode. The authors attribute the three peaks to three redox steps: conversion from the neutral state to polarons, from polarons to

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Figure 3. Evolution of the voltammetric profile of the Pt(111) electrode coated with a polythiophene film synthesized using a current density of 2 × 10-3 A cm-2 during 20 s. The other conditions are the same as those in Figure 2.

Figure 2. Evolution of the voltammetric profile of the platinum singlecrystal electrodes coated with a polythiophene film synthesized using a current density of 2 × 10-3 A cm-2 during 30 s: (a) Pt(100), (b) Pt(110), and (c) Pt(111). The electrolyte was 0.1 M NaClO4 in dry ACN, and the scan rate was 0.25 V s-1.

bipolarons, and finally from bipolarons to the metallic state, each step involving monoelectronic electron transfers. Figure 2 shows that the polythiophene films synthesized on basal plane platinum electrodes and in dry conditions exhibit voltammetric features very different from those which have been reported for polythiophene film synthesized on polycrystalline platinum electrodes.15-18 Furthermore, our data show a welldefined redox process not observed before between 0 and -0.8 V, likely associated with the polaron redox reaction. The observation of this signal at such low potentials suggests the presence of longer polymer chains and/or an ordered and stacked structure of polymer chains that can enhance an interchain hopping of polarons, leading to higher polaron mobilities or polymer condutivity. It is important to notice that the polymer films show very stable response when the potential is cycled between -1 and +0.15 V and the well-defined redox process observed becomes more reversible upon cycling (Figure 3). The later property is very important for the potential application of these conducting polymers in different fields such as rechargeable batteries, capacitors, and electrooptic devices, etc.1-3 The small decrease

of the peak height upon cycling, during the first cycles, is likely due to the fact that deeper sites in the conducting polymer can trap the inserted charge for long times and/or because compact zones of the polymer formed during the potential cycling cannot exchange anions at high rates.19 The shoulder observed in the left side of the anodic peak can be related to a discharge of trapped charges. At potentials higher than +0.15 V three signals can be seen from the polymer films grown on Pt(110) and Pt(111) electrodes (Figure 2b,c) jointly with small changes in the definition of the first couple of peaks reported below: one oxidation peak that grows upon cycling between +0.15 and +0.35 V at Pt(110) (Figure 2b) and a smaller similar process at Pt(111) accompanied by two quite reversible signals (Figure 2b,c) in both electrodes (the first one between +0.35 and +0.65 V, and the last one between +0.65 and +1.1 V). Latter we will analyze the origin of these signals, but it is already evident that cycling up to +1.1 V diminishes the definition of the quasireversible signals at low potentials, suggesting a degradation of the polymer film. Figure 2 shows that higher currents and better defined redox processes are observed for the thin polymer films on the Pt(111) and Pt(110) surfaces in comparison to the films synthesized on the Pt(100) surface. It is evident that the electropolymerization of thiophene on platinum electrodes is a surface-sensitive reaction. In a previous communication13 it was demonstrated that the Pt(110) and Pt(111) surfaces are more suitable for obtaining polypyrrole films with higher conductivity and charge storage capacity than the Pt(100) surface in aqueous media. In this report again it is found that the Pt(100) is the worst basal plane platinum electrode for synthesizing conducting polymers in nonaqueous media. If the polaron/bipolaron concept,20 which comes from semiconductors theory, is going to be used to rationalize the electric properties of conducting polymers, it is necessary to work with polymers that can exhibit a well-defined polaron transition from the reduced state, such as the polymers synthesized in our experimental conditions. This would enable comparison of their properties with those derived from theoretical models16 without interference of processes leading to higher oxidation states of the polymer. Ion Exchange Kinetics of Crystalline Platinum Electrodes Coated with Polythiophene Films. The oxidation and reduction kinetics of polytihophene have been studied extensively by Otero et al.21 They have reported that the kinetic characteristics of the conducting polymer oxidation are influenced by the initial

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Figure 4. Chronoamperometry of the basal plane platinum electrodes coated with a polythiophene film synthesized using a current density of 2 × 10-3 A cm-2 during 15 s. The electrolyte was 0.1 M NaClO4 in dry ACN. The sequence of potential steps was as follows: standby potential, +1.1 V; then -1 V; and finally +1.1 V vs Ag/AgCl.

packed state of the polymer if the reaction occurs under chemical kinetic control. Their experimental results have been rationalized using the electrochemical relaxation model.21 This model states that after polarization at low potentials the film is partially reduced, shrunk, and closed. More packed conformations are obtained for rising reduced states. This model predicts only one nucleation peak for the current vs time trace obtained during the potentiostatic oxidation of the conducting polymers, as it has been observed many times.21 According to the conformational relaxation model, the origin of this chronoamperometric maximum is attributed to the initial nucleation/relaxation control when the oxidation starts from a conformational packed film.21 Another explanation for the origin of the current maximum is that the nucleation processes are due to the nonconducting/ conducting transitions.22 However, Figure 4 shows that the kinetics of nucleation of polythiophene thin films on single-crystal platinum electrodes is much more complex. Particularly, two well-defined nucleation peaks are observed for the polymer film grown on Pt (110) and Pt(111) substrate electrodes during the oxidation process and two associated shoulders during the reduction process are also observed. As far as the authors’ knowledge is concerned, this complex ion exchange kinetics, with two consecutive nucleation processes, has not been reported before. This observation is unexpected if we take into account that the actual substrates are more uniform and better defined from the topographic point of view than the polycrystalline samples previously used. The oxidation trace for Pt(100)-modified electrode shows only a single oxidation peak (Figure 4). This behavior must be linked to the different quality of the thiophene film on this electrode in comparison to the other two electrode substrates, also evidenced in its voltammetric features (Figure 2). To demonstrate that the two nucleation peaks are associated with different oxidation states, intermediate potential step experiments were done (Figure 5) in the potential range of -1 and +0.15 V, where a reversible oxidation process is observed (Figure 2), and between +0.15 and +1.1 V, where two broad signals are observed (Figure 2). Figure 5 (gray thin line) clearly shows that the reversible redox process observed at low potential (Figures 2 and 3) is associated with a transition that involves nucleation kinetics features during both reduction and oxidation processes. It is remarkable the definition of the nucleation peak during the reduction step and the shoulder during the oxidation one.

Suarez-Herrera and Feliu

Figure 5. Chronoamperometry of the Pt(111) electrode coated with a 101 ( 17 nm polythiophene film. Three different experiments are shown (three different traces) with the following sequence of potential steps: (black thick line) standby potential of +0.15 V and then +1.1, +0.15, +1.1, and +0.15 V; (medium gray line) standby potential of 0.15 V and then -1, +0.15, -1, and +0.15 V; (black thin line) standby potential of +1.1 V and then -1, +1.1, -1, and +1.1 V. The film was synthesized using a current density of 2 × 10-3 A cm-2 during 20 s. The electrolyte was 0.1 M NaClO4 in dry ACN.

Figure 6. Chronoamperometry of the Pt(111) electrode coated with a 101 ( 17 nm polythiophene film. Potential steps from +0.15 V to (thickest line) +1.8 V, (medium thick line) +1.5 V, and (thinnest line) +1.1V vs Ag/AgCl. The other conditions are as in Figure 5. The second step is shown.

Figure 5 (black thick line) also shows that the redox transitions that take place between +0.15 and +1.1 V are associated with another nucleation process. Particularly, the nucleation peak during the oxidation steps is observed much better at higher positive potentials (Figure 6), but the higher limiting electric current densities observed at those potentials suggest the presence of a faradaic process, possibly linked to the polymer overoxidation. The reduction steps for this case do not show peaks, but the linear relationship between current density and time at the beginning of the potential steps is possibly due to a kinetically controlled process. Figure 5 clearly shows that the trace obtained between -1 and +1.1 V (black thin line) is the “sum” of the other two traces. In summary, the polythiophene films grown on Pt(110) and Pt(111) have two independent and well-defined chemical and/or conformational transitions in the potential range between -1 and +1.1 V. The increasing limiting current at higher potentials observed in Figure 6 has been reported earlier.23 Applying a potential greater than the polymerization potential degrades the electroactivity of the conducting films. This is known as overoxidative degradation. The degradation of polythiophenes also occurs at potentials lower than the overoxidation potential. This phenomenon is associated with the breakage of the backbone.23 On the

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Figure 8. Equivalent circuit for the Pt(111)/polythiophene electrode used to simulate the EIS data between +0.65 and -0.4 V vs Ag/AgCl.

Figure 7. Effect of the potential cycling on the ion exchange kinetics of the polythiophene films synthesized on Pt(111). The potential was changed between -1.2 and +1.1 V during (a) 0, (b) 10, (c) 30, and (d) 80 cycles using a scan rate of 0.25 V s-1. The potential was stepped from -1 to +1.1 V. The film was synthesized as in Figure 5.

TABLE 1: Magnitude of the Different Elements of the Equivalent Circuit Shown in Figure 8 for the Pt(111) Electrode Covered with a 101 ( 17 nm Polythiophene Film CPE E C1 Y0 (V vs Ag/ R1 (Ω cm 2) (mF cm-2) (S cm-2 sR) AgCl) 0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0 -0.05 -0.10 -0.20 -0.30 -0.40 max error (%)

11.58 11.73 11.86 12.07 12.34 12.75 13.35 14.4 16.13 17.55 17.29 16.38 18.11 18.93 18.77 17.89 17.97 17.18 17.86 15

1.197 1.089 1.032 0.975 0.875 0.752 0.611 0.470 0.354 0.282 0.266 0.321 0.346 0.357 0.371 0.392 0.510 0.596 0.575 3

3.00 × 10-4 2.79 × 10-4 2.59 × 10-4 2.23 × 10-4 1.77 × 10-4 1.25 × 10-4 7.84 × 10-5 3.89 × 10-5 1.14 × 10-5 1.16 × 10-6 1.98 × 10-7 4.56 × 10-7 3.19 × 10-7 1.55 × 10-7 3.96 × 10-8 7.58 × 10-9 4.27 × 10-9 2.10 × 10-8 5.70 × 10-10 60

R 0.710 0.692 0.669 0.653 0.641 0.632 0.622 0.598 0.540 0.468 0.449 0.523 0.506 0.462 0.399 0.342 0.320 0.315 0.290 7

C2 (mF R2 (Ω cm2) cm-2) 5.4 5.8 6.6 8.1 10.8 16.2 25.7 42.0 65.6 80.0 136.2 310.7 631 996 1358 1465 419 260 618 30

1.727 1.770 2.000 2.000 1.877 1.424 0.995 0.616 0.371 0.260 0.218 0.195 0.205 0.217 0.180 0.132 0.330 0.533 0.313 30

other hand it has been reported that poly-3-methylthiophene films are dissolved at high potentials up to disappearance in terms of the electrochemical oxidation, and the main dissolved species are derivatives with a thiophene ring.23 In Figure 7 we address the important issue of the stability of the polymeric films upon consecutive cycling in the potential range typical for the p-doping process. The polythiophenemodified electrodes were cycled up to 80 times between -1.2 and +1.1 V. It can be seen that during cycling the faradaic efficiency of the p-doping process gradually decreases. This fact can be assigned to the progressive overoxidation of the polymer. Beyond the potential of +1.1 V, a rapid irreversible degradation and dissolution of the polymer film was observed. Notice that the trace “a”, for the fresh polymer film, has two well-defined peaks and one shoulder at short times. Upon cycling, the traces are modified until the limiting case in which only a single peak is observed. Figure 7 shows that upon continuous cycling we can obtain the reported behavior for the oxidation of polymer films synthesized on polycrystalline platinum electrodes.21 In other words, if we overoxidized the polymer films grown on basal plane platinum electrodes, their behavior becomes similar to those polymer films synthesized. The latter facts support one

Figure 9. (a) Variation of R2 and (b) (() C1 and (b) C2 with the applied potential for the Pt(111) electrode coated with a 101 ( 17 nm polythiophene film. Other conditions as in Figure 5.

of the main conclusions of this paper: thin polythiophene films synthesized on basal plane platinum electrodes (especially on Pt (111) and Pt(110)) have different electrochemical properties in comparison to those synthesized on polycrystalline platinum electrodes, and the former films seem to have better electrochemical properties. Electrochemical Impedance Spectroscopy Studies of Polythiophene Film. The electrochemistry of the polythiophene films on Pt(111) was studied using EIS. This study was done using 101 nm film galvanostatically grown as it was explained before. The electrochemical impedance spectra were similar to those reported for conducting polymers,24 and the equivalent circuit used to fit the impedance data, obtained in the potential window where the main reversible redox processes are present, is shown Figure 8. Good fitting between the experimental data and the equivalent circuit depicted in Figure 8 was found for frequencies ranging from 100000 to 0.01 Hz. The maximum error estimated for the different elements in the whole potential window is shown in Table 1. It is important to notice that the maximum error of the electric elements is a function of the applied potential. The fit is much better at higher potential, and the estimated error for all electric elements was below 20%

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Figure 10. AC voltammogram at 5 Hz for the Pt(111) electrode coated with a 101 ( 17 nm polythiophene film. Other conditions as in Figure 5.

between +0.65 and 0 V. At lower potentials the estimated error increased particularly for the value of Yo related to the CPE. The circuit consists of an electrolyte resistance (R1) in series with the double-layer capacitance (constant phase element). Then, in series with the last circuit, the dielectric properties of the polymer film were simulated using an equivalent circuit that is a slight variation of the Debye equivalent circuit. This circuit is often used in the interpretation of ac impedance data for solid electrolyte systems.25 The film resistance (R2) was used to simulate the electric properties of the conducting polymer film. The resistance R2 can be ascribed to a charge-transfer resistance related to the doping/undoping process of the polymer. The CPE and capacitance C2 account for the charge storage capacity of the film, and the capacitance C1 is related to the charge storage capacity of the electric double layer at the film/electrolyte interface. Table 1 shows that the values of the parameter R increase with the applied potential. It indicates that the CPE describes a capacitor with a wide range of relaxation times and/or the influence of a diffusion capacitance, especially at lower potentials. The influence of the applied potential on R2 is presented in Table 1 and in Figure 9a. It is clear that at potential higher than -0.1 V R2 decreases when a more positive potential is applied. This behavior deserves a few words of comment. The lower R2 value at higher potential suggests that there must be some potential influence on the polythiophene structure which consequently facilitates the counterion flux and/or increases the number of charge carriers (polarons and bipolarons), leading to higher polymer conductivity. The decrease of R2 at potentials lower than -0.1 V might be related to the film shrinking, that can induce a well-ordered and stacked structure of polymer chains that would enhance the interchain hopping of polarons. Figure 9b shows that a significant increase of C2 and C1 occurs from +0.2 V to higher potentials. This behavior can be

Suarez-Herrera and Feliu the consequence of an increase in the polythiophene/electrolyte real surface. In addition, the volume of the polythiophene layer can increase when more positive potentials are applied.26 Therefore, it can be concluded that a change in the applied potential is capable of providing a significant influence on the counterion flux, i.e., on R2 and film capacitance values (Figure 9b). Figure 10 shows the ac voltammogram at 5 Hz of the Pt(111) electrode coated with a 101 nm polythiophene film. A welldefined Gaussian peak between +0.1 and -0.6 V is observed. It might be related to the polaron redox reaction as it was discussed before. Interestingly, at potentials higher than +0.1 V the shape of the voltammogram is very similar to the behavior of capacitors C1 and C2 shown in Figure 9b. Taking into account the results shown in Figures 2, 9b, and 10, it is safe to conclude that the main contribution to the current density observed at potentials between +0.1 and +0.65 V comes from capacitive processes. Capacitance can explain such broad voltammetric signals that are observed in this potential range. At potentials higher than +0.65 V the overoxidation reaction of the polymer on the current density begins. Finally, the calculated R1 values increase with the film’s reduction (Table 1) as it has been reported in a previous communication.13 This result indicates a minor contribution of the film to this element. This behavior can be due to the noncompact porous structure of the film.27 AFM Images. To see the topographic differences of the polythiophene films synthesized on the basal plane platinum electrodes during the first stages of growth, much thinner films than the ones described before were synthesized and imaged using tapping mode AFM. Tapping AFM images are not purely topographic but depend on the material properties of the sample and the interaction forces between the tip and the sample. Measurements with tapping AFM, at constant amplitude, show a surface of constant damping of the cantilever oscillation. Figure 11 shows some tapping AFM images obtained for the three basal plane platinum electrodes covered by a thin film of polythiophene galvanostatically electrodeposited during 5 s at 2 × 10-3 A cm-2. All surfaces show a globular structure. It can be clearly appreciated in Figure 11 that the morphology of the polythiophene deposits on the three basal plane platinum electrodes are very different. On Pt(100) the polymer film shows circular globules with about 50 nm diameters. On Pt(110) a homogeneous thin film of polymer ellipsoidal globules is seen. The film formed on the Pt(111) surface is much flatter and the size of the globules is much smaller than those observed on Pt(100) and Pt(110). From Figure 11 it can be concluded that the growth of polythiophene during the first stages of polymerization is affected by the structure of the platinum electrode surface. Taken into account that thicker films (about 100 nm thickness)

Figure 11. Tapping mode AFM images of thin films of polythiophene on (a) (100), (b) (110), and (c) (111) platinum single-crystal electrode surfaces synthesized galvanostatically at 2.0 × 10-3 A cm-2 for 5 s. Electrolyte 0.1 M NaClO4 plus 0.1 M thiophene in acetonitrile.

Polythiophene Polymerized on Pt Electrodes synthesized on the three basal plane platinum electrodes still have significant differences on their electrochemical properties (Figure 2 and 4), it can be suggested that the structure of the first layer of polymer affects the way in which the subsequent layers of polymer are deposited. In other words, the final structure (morphology and possibly polymer chain length) of the polymer film is affected by the structure of the first layer of polymer formed on the electrode. The structure and electrochemical properties of this first layer of polymer are affected by the state of the electrode as well. This was also the case for the electropolymerization of pyrrole on basal plane platinum electrodes in aqueous media.13 In this respect we can conclude that the formation of polythiophene is a structure-sensitive process on platinum electrodes. Conclusions Polythiophene films synthesized on basal plane platinum electrodes and in dry conditions exhibit voltammetric features very different from those which have been reported for polythiophene film on polycrystalline platinum electrodes. Particularly, our data show a well-defined redox process, not observed before, between 0 and -0.8 V, likely associated with the polaron redox reaction. The kinetics of the ion exchange reaction of polythiophene films synthesized on basal plane platinum electrodes is more complex than it was believed. For polymer films grown on Pt(110) and Pt(111) substrate electrodes, this process is characterized by at least two different consecutive redox processes, with kinetics characteristic of nucleation reactions. On the other hand, the analysis of the experimental data suggested that the main contribution to the current density observed at potentials between +0.1 and +0.65 V in the cyclic voltammogram for the thiophene films comes from capacitive processes, and beyond +1.1 V a rapid irreversible degradation and dissolution of the polymer film occur. Finally the voltammetric experiments and AFM images showed that the electropolymerization of polythiophene is a structure-sensitive process on platinum single-crystal electrodes with basal orientations. Acknowledgment. This work has been done in the framework of the Project CTQ 2006-04071/BQU. M.F.S.-H. acknowledges the scholarship SAB2006-0029 from the Spanish

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