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Electrochimica Acta 53 (2008) 3779–3788

Fluorinated functionalized EDOT-based conducting films

Alessandro Benedetto a, Mirela Balog b, Houari Rayah b, Franck Le Derf b,∗,Pascal Viel a,∗, Serge Palacin a, Marc Salle b,∗∗

a Laboratoire CSI, CEA Saclay, DSM/DRECAM/SPCSI, F-91191 Gif-sur-Yvette Cedex, Franceb Universite d’Angers, Laboratoire CIMMA, CNRS UMR 6200, 2 Bd Lavoisier, F-49045 Angers Cedex, France

Received 15 June 2007; received in revised form 29 August 2007; accepted 31 August 2007Available online 8 September 2007

bstract

Three ethylenedioxithiophene (EDOT) derivatives bearing either perfluoro- or ether perfluoro-alkyl chains were synthesized with the objectiveo prepare films with dry and chemically immobilized lubrication properties. The corresponding fluorinated PEDOT films were deposited onlatinum surface by electropolymerization. Cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) analyses (bothuartz resonant frequency and resonant admittance) of the growing steps are described. The electroactive behavior of the films versus their dopingevel was followed by electrochemical impedance spectroscopy (EIS) through equivalent circuit fitting procedure and compared to their CVesponses. Hysteresis in fit parameters corresponding to capacity and film resistance between the forward and the backward scans are observed

nd discussed. Control of the chemical structures and charge effects on PEDOT chains are followed by an XPS analysis. From these analyses, itppears that the fluorinated side-arm does not alter both the growing and the electrical properties of the films in respect to the pristine PEDOTaken as reference.

2007 Elsevier Ltd. All rights reserved.

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eywords: Ethylenedioxithiophene (EDOT); Fluorinated; Dry lubricant film;pectroscopy (EIS)

. Introduction

The functionalization of metallic surfaces with organicolecules to impart “lubricating” properties becomes a subject

f major interest both from fundamental and technologi-al points of view. Extensive works have been conductedn functionalized surfaces with different organic moleculess thiols [1,2], surfactants [3] or electro-grafted polymers4,5,7,8]. In such a scope, intrinsically conducting polymers areaterials of great interest. Among these polymers, poly(3,4-

thylenedioxithiophene) (PEDOT) has led to materials amonghe most successful. At the moment, thanks to remarkableroperties in terms of electrical conduction and chemical sta-

ility, PEDOT is used in several industrial application includingntistatic coatings, electrode-material in solid state capacitors,ubstrates for electroless metal deposition in printed circuit

∗ Corresponding authors.∗∗ Corresponding author. Tel.: +33 2 41735439; fax: +33 2 41735405.

E-mail address: marc.salle@univ-angers.fr (M. Salle).

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013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2007.08.075

ochemical quartz crystal microbalance (EQCM); Electrochemical impedance

oards and hole conducting material in organic/polymer-basedight-emitting diodes (OLEDs/PLEDs) [6].

We have therefore focused on PEDOT derivatives whichhould associate the inherent conducting properties of the con-ugated polyheterocyclic backbone to the lubricating characterf fluorinated lateral substituents. Such fluorine-rich chains arexpected to bring a better compatibility with other fluorinatedolymers and to reduce surface energy contribution (limitingdhesion).

In addition, a key feature of EDOT derivatives lies on theirbility to be polymerized under controlled conditions using elec-rochemical methods. It has been shown recently the possibilityf growing a polyconjugated polymer through a pre-graftednsulating polymer film [9,10]. Finally, electrochemistry allowsor tuning PEDOT conductibility by electrochemical controlledoping [6].

We present here the synthesis and characterization of two

ew perfluoro-alkyl and ether perfluoro-alkyl EDOT monomers1) and (2) as well as the already described ester analogue3) for comparison [11]. Their synthesis is presented togetherith polymer film fabrication and characterization in terms

3780 A. Benedetto et al. / Electrochimica Acta 53 (2008) 3779–3788

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f chemical structure and behavior as electro-active polymersScheme 1).

. Experimental

.1. Synthesis of fluorinated ethylenedioxithiopheneerivatives (1–3)

Compound (3) was synthesized according to the methodescribed [11] from the hydroxymethyl EDOT [12], and wereviously depicted the synthesis of iodoalkyl EDOT (4)Scheme 2) [13]. Target derivatives (1) and (2) were synthe-ized by a Williamson reaction between the 1-iodo derivative4) and the suitable 1-hydroxy fluorinated chain, treated withodium hydride (Scheme 2).

NMR spectra were recorded with a Bruker Advance DRX500pectrometer operating at 500 and 125.7 MHz for 1H and 13CMR spectroscopy, respectively; δ values are given in ppm (rel-

tive to TMS). Mass spectra were recorded on a Bruker BIFLEXII (MALDI-TOF) spectrometer or on a JEOL JMS 700 B/ESESI) spectrometer.

.1.1. Synthesis of (1)

The fluorinated polyether alcohol (0.36 g, 1.3 mmol) was dis-

olved in dry THF (50 mL) under nitrogen. Sodium hydride60% suspension in oil, 1.9 mmol) was added to this solutionnd the mixture was stirred at room temperature for 2 h. Com-

2

i

Scheme 2

.

ound (4) (0.5 g, 1.3 mmol) in 5 mL THF was added dropwise atoom temperature and the mixture was stirred at 50 ◦C for 3 days.HF was vacuum-evaporated and the residue was extracted withichloromethane, washed with water and dried over MgSO4.he crude product was purified by silica gel chromatography

cyclohexane/AcOEt: 7/1) affording target compound (1) as aellow oil (yield: 48%).

1H NMR (500 MHz, CDCl3): δ = 1.36 (m, CH2, 4H),.60 (m, CH2, 4H), 3.48 (t, J = 6.5 Hz, CH2O, 2H), 3.58t, J = 6.5 Hz, CH2OCH2CF2, 2H), 3.59 (dd, gemJ = 10.5,= 5.0 Hz, –OCH2CH, 1He), 3.67 (dd, gemJ = 10.5, J = 5.0 Hz,CH2CH, 1Hd), 3.80 (t, J = 10 Hz, –OCH2CF2–, 2H),.05 (dd, gemJ = 11.7, transJ = 7.7 Hz, = C(C)OCH2, 1Ha), 4.23dd, gemJ = 11.7, cisJ = 2.2 Hz, = C(C)OCH2, 1Hb), 4.29 (m,= 11.7 Hz, CHCH2OCH2, 1Hc), 6.32 (s, CHthiophene, H), 6.33

s, CHthiophene, H).19F NMR (500 MHz, CDCl3): δ = −91.33, −89.02, −78.03,

55.64.13C NMR (125 MHZ, CDCl3): δ = 25.59, 25.74, 29.39, 29.41,

6.19, 69.09, 69.64, 71.86, 72.61, 72.92, 99.53, 99.65, 141.52,41.55.

Anal. calc. for C18H21F9O6S—C: 40.30, H: 3.95, found—C:9.49, H: 3.91.

.1.2. Synthesis of (2)The fluorinated alcohol (0.59 g, 1.3 mmol) was dissolved

n dry THF (50 mL) under nitrogen. Sodium hydride (60%

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A. Benedetto et al. / Electroch

uspension in oil, 1.9 mmol) was added to this solution and theixture was stirred at room temperature for 2 h. Compound (4)

0.5 g, 1.3 mmol) in 5 mL THF was added dropwise at roomemperature and the mixture was stirred at 50 ◦C for 3 days.HF was vacuum-evaporated and the residue was extracted withichloromethane, washed with water and dried over MgSO4.he crude product was purified by silica gel chromatography

cyclohexane/AcOEt: 7/1) affording target compounds (2) as aolorless oil (yield: 40%).

1H NMR (500 MHz, CDCl3): δ = 1.37 (m, CH2, 4H),.60 (m, CH2, 4H), 3.48 (t, J = 6.5 Hz, CH2O, 2H), 3.58t, J = 6.5 Hz, CH2OCH2CF2, 2H), 3.59 (dd, gemJ = 10.5,= 5.0 Hz, –OCH2CH, 1He), 3.67 (dd, gemJ = 10.5, J = 5.0 Hz,CH2CH, 1Hd), 3.91 (t, J = 13.7 Hz, –OCH2CF2–, 2H),.05 (dd, gemJ = 11.7, transJ = 7.7 Hz, = C(C)OCH2, 1Ha), 4.23dd, gemJ = 11.7, cisJ = 2.2 Hz, = C(C)OCH2, 1Hb), 4.29 (m,= 11.7 Hz, CHCH2OCH2, 1Hc), 6.32 (s, CHthiophene, H), 6.33

s, CHthiophene, H).19F NMR (500 MHz, CDCl3): δ = −126.59, −123.89,

123.20, −122.44, −120.08, −81.21.13C NMR (125 MHZ, CDCl3): δ = 25.59, 25.75, 29.38, 29.39,

0.92, 66.19, 67.57, 67.77, 67.97, 69.10, 71.87, 72.62, 73.14,9.54, 99.65, 141.53, 141.56.

Anal. calc. for C22H21F17O4S—C: 37.51, H: 3.00,ound—C: 37.87, H: 2.86.

.2. Electrochemical experiments

Cyclic voltammetry experiments have been carried outn a potentiostat–galvanostat EG&G PARK model 273 or73A, with solvents and electrolytes of electrochemicalrade.

The substrate solution was placed in an electrolysis cellncluding three electrodes: a working electrode (platinum disk)n which takes place the electropolymerization (diameter:mm,), an auxiliary electrode (platinum wire), and a referencelectrode (Ag/AgCl) (or a silver wire which potential was deter-ined versus Ag/AgCl). The solvent was acetonitrile in the

resence of 0.1 M tetrabutylammonium hexafluorophosphateTBAPF6) as the supporting electrolyte.

Electrochemical impedance spectroscopy was carried outn a Solartron 1286 potentiostat equipped with a SolartronI 1255 HF frequency response analyzer. Measurements wereade in the frequency range from 0.1 MHz to 0.02 Hz, with ac

oltage of 10 mV amplitude superimposed on different dc poten-ials (measured with respect to a Ag|AgClO4 0.01 M referencelectrode). We made sure that the electrochemical steady-stateas attained by the cell by waiting for 1 min between the

pplication of the dc potential and the data recording. Duringhis time, the current passing through the electrochemical cellecomes lower than the �A. Electrolytic medium was consti-uted by tetraethylammonium perchlorate 0.05 M in acetonitrileAldrich, electrochemical grade). Prior to EIS experiments the

ell was purged with N2 for 10 min and then experimentsere conducted under a N2 atmosphere. For clarity reasons, allotentials reported in this article are expressed versus Ag/AgCleference electrode.

ep

Acta 53 (2008) 3779–3788 3781

.3. EQCM experiments

Electrochemical quartz crystal microbalance experimentsere carried on a SEIKO EG&G Quartz Crystal AnalyzerCA917. The electrochemical was a standard three-electrode

ell, composed by a platinum counter electrode foil, a refer-nce electrode (Ag/AgClO4 0.01 M) and the working electrodeonstituted by an AT 9 MHz cut crystal quartz platinum coatedarea 0.2 cm2). For clarity reasons, all potentials reportedn this article have been expressed versus Ag/AgCl refer-nce electrode. The quartz microbalance was calibrated byu electrolytic deposition. Electrolytic medium was consti-

uted by tetraethylammonium perchlorate 0.05 M in acetonitrileAldrich, electrochemical grade). Experiments were conductednder Argon atmosphere in a dry box.

.4. XPS experiments

XPS spectra were recorded on a Vacuum GeneratorSCALAB 210, using an Al K� source monochromatized at486.6 eV. We used a hemispheric analyzer working at passnergy of 50 eV for the survey spectrum, and 20 eV when focus-ng on the sole core levels.

. Results and discussion

.1. Electrochemical preparation of the PEDOT-baseduorinated polymer films

Poly(3,4-ethylenedioxithiophene) (PEDOT) has been exten-ively studied over the last decade due to its remarkable optical,lectrochemical and electrical properties [6] with notably aroad window of electroactivity which makes it a suitable matrixor preparing modified surfaces.

The irreversible oxidation of monomers 1–3 appears at a usualalue for EDOT derivatives (ca. Epa = 1.6 V vs. Ag/AgCl). Elec-ropolymerization of all monomers 1–3 was carried out underimilar concentration conditions. Formation of the polymer filmsas realized either under potentiostatic (Eappl = 1.40 V) or poten-

iodynamic conditions (0–1.40 V) giving rise to stable films. Anllustrative example is given in Fig. 1, showing the film growinghrough potentiodynamic conditions in the representative case of

onomer 1 (Fig. 1a), as well as the corresponding electrochemi-al response of the modified surface (poly(1)) in a monomer–freelectrolytic medium (Fig. 1b). The polymerization could be car-ied out in acetonitrile (10−2 mol/L), pointing out the efficiencyf the process. The CV response (Fig. 1b) exhibits a very broadedox system characteristic of the poly(EDOT) doping/dedopingrocess in the +0.1 to +1.1 V region. Finally, it has to be pointedut that all of these electrodes exhibit a remarkable stability,ince no loss of the electrochemical signature has been observedpon repeated cycling, or after standing for days in solution.

.2. Electrochemical impedance spectroscopy

EIS spectroscopy was performed on the films obtained bylectropolymerization to study the differences in the dopingrocess and in the conduction properties of the films.

3782 A. Benedetto et al. / Electrochimica Acta 53 (2008) 3779–3788

Fig. 1. (a) Electropolymerization of 1 (10 mM), in 0.10 M Bu4NPF6, CH3CN, 25 cycles at 50 mV s−1, Pt Φ = 2 mm (left); (b) CV response of poly(1), CH3CN,Bu4NPF6 (0.1 M), 100 mV s−1 Pt Φ = 2 mm (right). Insert: plot of the peak intensity as a function of the scan rate.

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where γ is a factor (between 0 and 1) to account for the nonideality of the capacitance Cf. This non ideality is often ascribed

ig. 2. Nyquist (a) and Bode (b) EIS plot on poly(1) film taken for different desults.

For poly(1) film taken as an illustrative example, the dc poten-ial was swept from 0.85 V down to −1.2 V (forward scan) andhen backward to 0.85 V. Polarization for 1 min at the dc poten-ial to reach a steady-state was performed before starting the

easurement. Fig. 2 reports the complex plane results duringorward scan. In the frequency window that has been consid-red, and for the most positive polarization (0.85 V), the filmehave almost as a pure capacitor. A wide circle in the Nyquistlot starts to develop when sweeping toward more negative dcotentials. This evolution can be interpreted as a modification oflectroactive film by the expulsion of perchlorate counterions.he transition from a doped “conducting state” to an undopedinsulating state” can be easily observed on the Bode diagramFig. 2b) in the low frequencies |Z| values.

The modification of film properties with dc potential varia-ion is expected from the theory of electroactive polymer filmsIS behaviour as formalized by Vorotyntsev et al. [14]. This the-ry is based on a homogenous film model within an irreversiblehermodynamic framework. The ion transport is described by aiffusional process since a supporting electrolyte large excesss used that allows the migration contribution to flux to beeglected. It considers that two mobile charge carriers partic-pate in the process, and due to bulk electroneutrality of thelm, the coupled chemical diffusion appears in the form of a

inary diffusion coefficient. In our case the charges correspondo holes (positive charges delocalized on the polymer film) thatre injected at the metal/polymer interface, leading to the forma-ion of, and to the diffusion of charge compensating perchlorate

rization potentials. Symbols report experimental points while lines report fits

ons through the polymer/solution interface. Therefore, the con-entration of the charge-compensating counterions in the filmsepends on the oxidation level of the polymer film.

In the case that we considered, at least for the frequencyindows used for carrying on the experiments, ion diffusion is

ast enough for impedance to be dominated by the capacitiveehaviour.

With the aim of summing up polymer films behavior morehan a deep EIS analysis, we tried to fit the experimental datay the simple homogeneous film equivalent circuits presentedn Fig. 3. The considered parameters are sufficient for fittingur experimental data and there is good agreement between theodel and the experimental points as shown in Fig. 2. Symbols

epresent experimental data while the fit results are representedy lines. The capacitor CPEf is modeled with a constant phaselement. The impedance of this element is expressed as

1

Fig. 3. Equivalent circuit used for fitting EIS experimental data.

A. Benedetto et al. / Electrochimica

Fig. 4. Parameters values obtained by fitting EIS results poly(1) film at differentdc potentials. C is represented by filled symbols and R by empty symbols.Fb

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orward scan (from positive to negative potentials) is represented by circles andackward scan (from negative to positive potentials) by squares.

o surface roughness [15]. In our experiments γ was found inhe range 0.6–0.95. Adding circuit elements such as Warburgmpedance did not improve our fits.

Rs accounts for solution resistance. It is found to be constantn our experimental conditions at any dc potential and equalo 261 �. Cf and Rf describe film capacity and resistance. Fittatistical errors are in the order of 0.5% for Cf and 4% for Rfor our experiments.

Fit results at the different dc potentials during forward andackward scan are summed up in Fig. 4. Error bars report thetting statistical errors. The bars in the case of Cf cannot beeen because they are smaller than the symbols. One can seehat the poly(1) film shows very high capacity response at oxi-ized potentials in the order of 30 mF/cm2. When the potentials decreased below −0.5 V the capacitance starts to decreasend reaches a plateau with values in the order of 20 �F cm−2.

symmetrical behavior is found for Rf. We interpret this ashe transition from a conducting doped state down to an intrinsi-ally semiconductor state. In the doped state the polymer shows aigh capacitance corresponding to a state where positive charges

arried by polymer main chain are compensated by negativeounterions. In the intrinsic undoped state the capacity decreaseso the normal electrochemical double-layer capacity (as foundor a bare platinum surface in the same electrolyte). The resis-

aesf

Fig. 5. Nyquist (a) and Bode (b) EIS plots for PEDOT at different dc potentia

Acta 53 (2008) 3779–3788 3783

ance is presumably dominated by the ionic carrier more thanhe electronic component. However within the framework ofhe applied simple model, it is not possible to discriminate elec-ronic and ionic charge transfer resistance components, or tonow which one is dominant; but both increase when passingrom a conducting state to a more resistive one corresponding tontrinsic poly(1), resulting in Rf increase. Varying films thick-esses changed the absolute value of the impedance response butid not change the general trend that seems to be thickness inde-endent for the studied films (from 100 to 400 nm, as measuredy mechanical profilometry).

We observe a hysteresis in the value of Cf and Rf in betweenhe forward and the backward scan.

The hysteresis of redox peaks on PEDOT CV response is aell known phenomenon, and a potential shift (0.6–0.4 V) isbserved between the oxidation and reduction peaks. Hass etl. [16] have also observed a hysteresis for PEDOT in EIS andhown that it is correlated to CV hysteresis. The EIS responsehey have reported for PEDOT films (obtained on ITO substratesor a different electrolytic medium) partially differ from what weeport here, and the model used for fitting experimental data hado include a further capacitance to account for low-frequencyiffusion behavior observed.

Therefore we carried out EIS on parent PEDOT films usinghe same conditions as for poly(1), to check whether there isn effect caused by the chemical PEDOT modification with theuorinated lateral chain.

Fig. 5 reports some of the EIS Nyquist and Bode dia-rams taken during the forward scan. The results with thearent PEDOT system appear similar to that presented aboveor poly(1), showing a transition from “conducting states” inhe oxidized highly capacitive form to the “insulating states”n the intrinsic form. The inset in Fig. 5 reports a zoom at theighest frequencies. No particular features are observed for therequency range used here, and a semicircle at the high frequen-ies could be expected. Hass et al. [16] correlate this behavior atigh frequencies to the interfacial properties while the medium

nd low frequencies behavior is correlated to the charging prop-rties. However a complete study of the interfaces is out of thecope of our article and phenomena at high frequencies were noturther studied.

ls. Symbols represents experimental points, lines represents fits results.

3784 A. Benedetto et al. / Electrochimica

Fig. 6. Cf (filled symbols) and Rf (empty symbols) values resulting from fitptp

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rocedure at different applied dc potentials, during forward scan (from positiveo negative potentials, circles) and backward scan (from negative to positiveotentials, squares) for PEDOT.

Again a hysteresis between the forward and the backwardcan is present (see Fig. 6). In the PEDOT case the transition isroader, spanning from 0.2 V down to −0.8 V, while for poly(1),he transition is located between −0.1 and −0.6 V. We suggesthat hysteresis phenomenon is correlated to the energy level dif-erence between the lowest unoccupied level of the doped statehen electronic injection is performed from metal to polymernder the forward scan and the highest occupied level state ofhe undoped state when injection from polymer to metal underackward scan occurs.

Fig. 7a reports a comparison of Cf parameter at differentc potentials for PEDOT and polymer electrodeposited fromonomers 1, 2 and 3. The capacitance was normalized on the

umber of repetitive units for each film (obtained from the chargeecessary to deposit the polymer film in the approximation thathe faradic yield is unitary) and expressed in elementary charge1 F = 1 C/1 V ∼= (1.602E−19)−1 × 1 e/1 V). It can be seen thatn the oxidized states the values of the capacitance for repetitivenit is similar for the different polymers, which indicates thathe nature of the fluorinated side chain has not dramatic effectsn the film doping.

One can see that all Cf curves present a maximum. ForEDOT and poly(2), the maximum is situated at about 0.25 V

hile for poly(1) and poly(3) it is shifted at about 0.05 V.CV responses for the same potential zone are reported in

ig. 7b. One can see that the films showing a broad transitionrom “conducting states” and “insulating states” in EIS experi-

3

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ig. 7. (a) Cf comparison of different polymer films (forward scan). Curves were resoly(3), and PEDOT. v = 20 mV/s, five cycles, TEAP 0.05 M in acetonitrile.

Acta 53 (2008) 3779–3788

ents show a CV response that is also broader and characterizedy ill-defined redox peaks. On the other hand poly(1) and poly(2)hich show characteristic and well defined redox peaks tend toave narrower transition in EIS experiments.

We suggest here that this difference possibly come from aifferent degree in polymers chains organization. It is known17] that the presence of side chains on polyconjugated poly-ers, thanks to their interdigitation, leads to a better organization

esulting in an increased conjugation length. Moreover theresence of fluorinated extremities is favourable to chains self-rganizing [18].

Therefore the presence in poly(1) and poly(2) of a fluorinatederminal fragment in the lateral chain, separated from the con-ugated polyheterocyclic backbone by a flexible aliphatic chaincting as spacer, could allow for a higher degree of organization,esulting in a narrower DOS for the energetic levels in the bandap created by the doping process with respect to parent PEDOT.

In poly(3), the CV response shows broader features whichesemble those of the PEDOT again. The shorter side chainnd the absence of an aliphatic spacer should be responsibleor a lower organization degree. The fluorinated polymers elec-ronic structure will be further studied by XPS valence band andpectroelectrochemistry measurements.

The capacitance measured by EIS can be compared to thene measured from CV by the slope of the I (scan rate) curvee.g. the slope in Fig. 1b, insert). For poly(2) a capacitance of.083 e V−1 (monomer unit)−1 is found at 0.3 V. From the graphn Fig. 7a, we find a capacitance value, measured at the sameotential by EIS, of about 0.045 e V−1 (monomer unit)−1. Asxpected measurements obtained by EIS give lower values ofhe capacitance than the ones obtained by CV. Indeed, CV cor-espond to a system that is not in a steady-state, and for which theurrent measured results from two contributions: the faradic oneue to the redox processes on the polymer and the capacitive oneue to the charging/discharging currents. Therefore the value ofhe capacitance obtained by the slope of the curve I (scan rate)verestimates the intrinsic capacitance of the electroactive filmecause of the contribution of the redox processes.

.3. Electrochemical quartz crystal microbalance analysis

The doping process by perchlorate ions was further investi-ated by EQCM experiments. Fig. 8 reports the EQCM signal

caled to facilitate lecture. (b) Comparison of CV response for poly(1), poly(2),

A. Benedetto et al. / Electrochimica Acta 53 (2008) 3779–3788 3785

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ig. 8. (a) EQCM cyclic voltammogram corresponding to the electrodepositiondmittance.

orresponding to the deposition process of poly(1). Fig. 8bhows a peculiar stair profile for both resonant frequency vari-tion and resonant admittance. Each step corresponds to theotential being swept at the beginning of the oxidation peakf compound 1.

In the case of a thin rigid film the resonant frequency varia-ion is directly proportional to the variation of mass as describedy Sauerbrey equation [19]. Resonant admittance is related toesonance peak width. Moving from a rigid environment toore viscous one causes the widening of resonance peak anddiminution of the admittance. Such variation in energy dis-

ipation mechanism is associated also to a resonant frequencyariation as shown by Kanazawa and Gordon [20].

In the case presented in Fig. 8, the frequency steps alwaysorrespond to steps in the admittance. In such case we cannotpply the Sauerbrey approximation and the frequency variations not directly correlated to mass increase because the deposi-ion of the “thick” energy-dissipating polymer film causes theariation of quartz viscoelastic properties. It anyway demon-trates that for each cycle corresponding to oxidation potentialf monomer 1, a new polymer layer is deposited. The electro-hemical doping by perchlorate ion was then studied followingy EQCM the polymer response in TEAP 0.05 M in acetoni-rile. CV shows a very particular redox response with two verytrong peaks at ca. −0.15 V (oxidation) and −0.4 V (reduction),ollowed by two redox processes at about 0.2 V (oxidation) and.05 V (reduction), Fig. 9a. The presence of such intense and

arrow peaks that usually are less visible in parent PEDOTsee Fig. 7b), can be probably correlated to the transitionbserved in EIS response, much broader in the case of parentEDOT.

titc

ig. 9. (a) EQCM experiment, poly(1) CV response in TEAP 0.05 M in acetonitrile, (

ly(1).v = 20 mV/s; (b) corresponding quartz resonant frequency and resonant

Fig. 9b reports the corresponding variation in resonant fre-uency. One can see that over five cycles there is almost noariation in the resonant admittance and therefore we can applyauerbrey approximation, and consider the frequency variationirectly proportional to mass variation. A diminution in fre-uency corresponds to an increase of resonating crystal mass.or potential lower than −0.8 V �f presents a plateau, whilever this potential the decrease is almost linear. We can assignhis mass increase to the insertion of perchlorate ions into thelectroactive films, to compensate the charge generated on theolymer chain. More interestingly we can see that the begin-ing of the mass increase potential, around −0.8 V, correspondso the lowest limit of the transition observed in the EIS transi-ion. Therefore EQCM measurements show that the “insulatingtates” observed in EIS correspond to the polymer that is coun-erion free, which means undoped.

In the case of the applicability of Sauerbray equation, ourystem was previously calibrated in order to associate fre-uency variation to mass variation [21]. It is found that aecreasing in frequency of 1 Hz corresponds to an increasingn mass of 5 ng cm−2. Therefore the variation in frequencyresented in Fig. 9b of about −760 Hz (between −0.9 and.6 V on the second scan) corresponds to a variation of ca..8 �g cm−2. We can express the mass increase for each repet-tive unit (obtained from the charge necessary to deposit theolymer film in the approximation that the faradic yield is uni-ary) that is 1.13E−26 kg (monomer unit)−1 which corresponds

o 6.8 u (monomer unit)−1. If we assume that no perchlorate ions present in the film at the lowest potential, this is equivalento a mass gain of 0.068 perchlorate ion per monomer unit. Weannot exclude that some of the mass gain is also due to the

b) frequency and admittance variation corresponding five cycles, v = 20 mV/s.

3786 A. Benedetto et al. / Electrochimica Acta 53 (2008) 3779–3788

Table 1Atomic ratio calculated from XPS peaks area (Scofield factor normalized) compared with values expected from chemical formulas

Cexpected Cexperimental Fexpected Fexperimental Oexpected Oexperimental Sexpected Sexperimental

Poly(1) 0.53 0.50 0.27 0.31 0.18 0.16 0.03 0.03PP

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cipet2a CH2. The area ratio is consistent with the hypothesis.

The component at 286.2 eV is assigned to aromatic carbons(subjected to S effect), and the component at 284.6 to aliphaticcarbons.

oly(2) 0.50 0.52 0.39 0.38oly(3) 0.43 0.45 0.43 0.42

ncorporation of solvent molecules solvating the perchlorate inhe film and therefore the value given is an overestimation of theoping rate of the film.

The value is comparable with what we also find by measuringhe response of poly(1) by cyclic voltammetry. Integration of theurrent with respect to the time while cycling the polymer in theolvent in presence of the electrolyte gives the charge that istocked in the polymer during the cycle. The ratio of the chargeeasured during response and the charge used for depositing

he film gives an approximation of the doping level. For poly(1)n different films values of about 6% are found. As discussedn Section 3.2 this is also an overestimation of the doping level.n this case just the faradic component of the current shoulde considered to measure the quantity of perchlorate introducedo compensate the positives charges on the polymer, but it isardly possible to separate this component from the capacitivene.

The measurements taken by EIS are made in steady-statend the fitting procedure allowed to separate capacitive com-onent from the resistive ones. Integration of the values of Cfexpressed in the measure units used in Fig. 7a) with respecto the potential allows for the calculation of the charge that istocked in the polymer for each repetitive unit at a given poten-ial. For poly(1) integration of Fig. 7a) gives doping level of.043 for each monomer unit at 0.6 V. This value is lower thanhat estimated by CV response and EQCM, but is more likely

o correspond to the true value of the doping level because thether two techniques overestimate the doping level as discussedbove.

The ratio in between the doping level as measured by EQCMnd the one measured from EIS is about 1.6, which may bexplained by assuming that each perchlorate inserted in thelm upon oxidation is accompanied in average by 1.4 solventolecules.At 0.8 V (which is the maximum potential we used in EIS)

he doping level for repetitive unit is found at 0.053. Experi-ental optimizations were not carried out, and the doping levels

eported here are somewhat lower than usually encountered forlectrochemical doping levels reported in literature for PEDOTnd PEDOT derivatives [6].

Looking more closely to Fig. 8, it is possible to observe thatnput/output of perchlorate ions within the films was alreadyhown during the film growth. Actually the plateau steps shownn Fig. 8b changes during the growth of the film. After the firsttep, the �f signal corresponding to the area where no oxidation

f the monomer occurs is completely flat, while in the followingteps it becomes more and more “roof-shaped” correspondingo the leaving of perchlorate during film reduction, followed bye-entering of ions when the film is oxidized.

0.09 0.08 0.02 0.020.11 0.11 0.03 0.03

As one can observe from Fig. 9b the hysteresis observed inIS measurements corresponds also to a small mass variationysteresis due to the perchlorate ions film filling.

.4. XPS analysis

Polymers deposition and electrochemical doping ofonomer 1 was confirmed by XPS measurements. By integrat-

ng all elements having most intense core peaks, and normalizingreas by Scofield factor, we could estimate the chemicalomposition of the electrodeposited films. Ratio between theormalized areas and the chemical formulas are consistent, withverage differences close to the percent (the maximum differ-nce found was at 4%) showing that electropolymerization doesot induce lateral chain degradation. The results are summed upn Table 1.

The survey spectrum of a reduced-state polymer is reportedn Fig. 10. No platinum peak can be seen, which insures the films thick in respect to the XPS mean free path, homogeneousnd covering. The absence of chlorine peaks is also indica-ive of the absence of perchlorate ions in the films in intrinsictate.

Fig. 11 (circles) reports the C 1s core level spectrum. Onean distinguish clearly at least four peaks, the peak at 295.3 eVs assigned to the –CF3 components, the broad and asymmetriceak at 292.8 eV is assigned to a –CF2– component, with highernergy component at 293.0 eV assigned to the carbon bondedo oxygen and another CF2 and the lower energy component at92.2 eV assigned to carbon bonded with an oxygen atom and

Fig. 10. XPS survey spectrum of films of poly(1) in the reduced state.

A. Benedetto et al. / Electrochimica Acta 53 (2008) 3779–3788 3787

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ig. 11. (a) C 1s core level spectrum of films of poly(1) in the reduced state (oly(1) in the reduced state (circles) and in the oxidized state (squares).

Fig. 11a (squares) reports the C 1s XPS core level spectrumor the same film but now electrochemically doped by apply-ng a linear sweep at 5 mV/s in TEAP 0.05 M in acetonitrilerom the equilibrium potential (−0.4) to 0.85 V. We observehe same peaks as above, but the broad peak between 282 and89 eV shows a widening on high energy side, which needs fortting an additional component centered around 286.7 eV. Thisomponent corresponds to the formation of delocalized positiveharges on the polyconjugated chain. The presence of a posi-ively charged carbon component for the oxidized state is alsovident from peaks intensities modification, since for the unoxi-ized film the peak around 286 eV is more intense than the peakround 285 whereas for the oxidized film the trend is inversed.

This presence of charge on the main chain is also shown byhe S 2p core level spectra. For the undoped form, the S 2p corepectrum (shown in Fig. 11b circles) can be decomposed in twoomponents at 163.4 and 164.6 eV, accounting for the S 2p3/2nd S 2p1/2 levels. The same spectrum for the oxidized forms reported in Fig. 11b (squares), which shows a component atigher energy accounting for positive charge presence on theolythiophenic chain [22]. It was also possible to follow theresence on perchlorate counterions by the Cl 2p core level peaksppearing in the oxidized form spectrum (not shown here).

. Conclusions

New EDOT monomers bearing fluorinated chains were syn-hesized and characterized. The corresponding EDOT basedolymers were successfully electrochemically deposited. Theirlectrochemical response was studied showing a conductingolymer behavior, with peculiarities resulting from the film orga-ization induced by the fluorinated side chain. We suggest thathe hysteresis phenomenon observed with EIS in the capaci-ies measurements is correlated to the difference between thelectronic structures of the doped and the undoped polymers.lectronic structures will be further studied by XPS or UPSalence band and by spectroelectrochemistry in order to clarifyhis suggestion.

cknowledgements

The authors are grateful to the “Agence Nationale de laecherche” for their financial support in the framework of

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) and in the oxidized state (squares); (b) S 2p core level spectrum of films of

ANOCONNECT project. MB is indebted to the Conseil gen-ral de Maine et Loire for a grant and MS thanks the Institutniversitaire de France (IUF) for its financial support.

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