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Electrochimica Acta 105 (2013) 347– 352

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

imultaneous quartz microbalance and mirage effect studies ofoly(3-methoxythiophene) electrosynthesis and electrochemicalharacterisations

arcos Roberto de Abreu Alves ∗, Raphael Nasser Capistrano Reis, Jean Gomes de Oliveira,állen Daniel Rezende Calado, Claudio Luis Donnici, Tulio Matencio

epartamento de Química, ICEx/Universidade Federal de Minas Gerais, 31270-901, Belo Horizonte, Minas Gerais, Brazil

a r t i c l e i n f o

rticle history:eceived 21 December 2012eceived in revised form 26 April 2013ccepted 30 April 2013

a b s t r a c t

This work characterises the electropolymerisation of 3-methoxythiophene and presents an electrochem-ical study of poly(3-methoxythiophene) by cyclic voltammetry using different electrolytes in solventmixture of water-acetonitrile (3:1, v/v) at 25 ◦C. Simultaneous measurements of the mirage effect andthe quartz microbalance were also performed. This study evaluated the influence of cations on the elec-

vailable online 14 May 2013

eywords:irage effect

oly(3-methoxythiophene)

trochemical properties of monomers and polymers and allowed us to observe an anomalous behaviourof the synthesised polymer in the presence of potassium ions (K+) when compared with other cations.The reversibility of the polymers was also evaluated with respect to the various electrolytes and wasobserved the influence of the polymer film thickness (deposited mass) on its electrochemical properties.

lectropolymerisationuartz microbalance

. Introduction

Organic electronics is currently one of the most promising tech-ological fields [1,2]. The possibility of obtaining low-cost, flexibleaterials and conjugated polymers (CPs) is promising in several

reas, such as the development of flexible solar cells [3], polymericight-emitting diodes [4], transistors, and other applications.

The electrochemical properties of these materials must be deter-ined for their application in the electronics. The factors affecting

hese properties should be determined, along with the influence ofhe external environment on the properties of the final material. Its also necessary to assess the properties and redox behaviours ofPs in electrochemical cells during electropolymerisation.

In this study, the electropolymerisation of the monomer 3-ethoxythiophene (MOT) was evaluated using cyclic voltammetry.

he same anion was used as the electrolyte (perchlorate ion, ClO4−),

nd but different alkaline metal cations (Li+, Na+, K+, Rb+, ands+) were employed to evaluate the influence of the cation size.tudies of the mirage effect and the quartz microbalance wereonducted simultaneously during the electropolymerisation and

lectrochemical characterisation of the film. The flow of the elec-roactive species/electrolytes during the electropolymerisation of-methoxythiophene was monitored, and the electrolytes were

∗ Corresponding author. Tel.: +55 031 34095742; fax: +55 031 34095700.E-mail address: marcosrabreu@yahoo.com.br (M.R.d.A. Alves).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.04.173

© 2013 Elsevier Ltd. All rights reserved.

characterised while analysing the redox behaviour of PMOT toobtain important information on the influences of the electrolyteon the growth of the film and the electrochemical profile of thematerials.

One can find in the literature studies about the influence of ionicexchanges on properties of inorganic films deposited on the sur-face of electrodes, such as Prussian blue derivatives [5–8]. Howeverthese works, that show the input and output cations of the film dueto changes in the oxidation state of iron in the material structure,not study the ion exchange during the electrodeposition.

2. Experimental

2.1. Reagents and solvents

Lithium perchlorate (LiClO4), sodium perchlorate (NaClO4),potassium perchlorate (KClO4), rubidium perchlorate (RbClO4),caesium perchlorate (CsClO4)) and 3-methoxythiophene werepurchased from Aldrich. The acetonitrile used to prepare theelectrolyte solution was obtained from Vetec. All solutions wereprepared using distilled water.

2.2. Utilised apparatus

The system used for this study was adapted from the lit-erature [9,10]. The electrochemical data were obtained using adigital Omnimetra PG-39 potentiostat. The electrochemical cellconsisted of a glass cuvette with an optical pathlength of 30 mm

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btained from Biocell and three electrodes. A quartz crystal AT-ut 5 MHz QCM (Stanford Research Systems) with a circular filmf platinum/titanium (Pt/Ti) was used as the working electrode. Alatinum wire was used as the auxiliary electrode, and a silver wireas used as the quasi-reference electrode. A QCM200 from Stanfordesearch Systems was used as the quartz microbalance.

The mirage effect apparatus consisted of a 633 nm, 2 mW HeNeaser. The laser beam diameter at the lens focal point was approxi-

ately 32 �m, and the beam was parallel to the working electrode.he positive deflections indicated that the ions diffused within the

Fig. 1. PMOT electrodeposition voltammograms with different electrol

a Acta 105 (2013) 347– 352

solution to the electrode surface, whereas the negative deflectionsindicated that the ions diffused from the electrode surface to thebulk solution.

2.3. Poly(3-methoxythiophene) electrodeposition andcharacterisation by quartz microbalance, cyclic voltammetry and

mirage effect

The poly(3-methoxythiophene) (PMOT) electrodeposition wasperformed as described in a previous work [11]. A 35 mmol L−1

ytes: (a) LiClO4, (b) NaClO4, (c) KClO4, (d) RbClO4, and (e) CsClO4.

himica Acta 105 (2013) 347– 352 349

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olution of the monomer in water:acetonitrile (3:1) was prepared.queous solutions (0.1 mol L−1) of LiClO4, NaClO4, KClO4, RbClO4nd CsClO4 electrolytes were used. The synthesis was performedy subjecting the monomer solution to 20 cycles of cyclic voltam-etry in the potential range of 0 V to 1.4 V vs Ag/Ag+. The depositionas conducted using a conventional three-electrode configuration,

s specified in item 2.2.The PMOT film obtained in each synthesis with different elec-

rolytes was studied using cyclic voltammetry (CV) in the range0.6 V to 0.6 V vs Ag/Ag+ at different scanning speeds (5, 10, 20,0, 75 and 100 mV s−1) in the corresponding free monomer elec-rolyte solution. All the studies were performed under an inert N2tmosphere.

. Results and discussion

.1. Electropolymerisation

The PMOT was electropolymerised in the presence of differentlectrolytes using a 1:1 stoichiometric ratio between the cationsnd anions. The CV electrochemical technique was employed,hich allows the growth of the film to be monitored at each

ycle due to increased current below the oxidation potential ofhe monomer (∼1.4 V) during the oxidation and reduction of theeposited polymeric film on the electrode surface between 0 V and.6 V vs Ag/Ag+. The PMOT electrodeposition voltammograms arehown in Fig. 1, in which the arrow indicates the scanning direc-ion (small arrows) and growth (higher arrow) of the polymericlm during the electropolymerisation.

The polymer growth during each synthesis was monitored using quartz microbalance (MBQ) and the mirage effect (ME). In Fig. 2,he mass versus time of the polymer deposited is shown. The end

ass deposited at the electrode increased with as the size (weight)f the cation increased, except for potassium, which exhibited aifferent behaviour.

UV-vis spectroscopic studies of the interaction of alkaline met-ls [12] with the alkoxy group of conjugated polymers showed aifferent behaviour compared with the results of previous studies.

n the previous studies, which were performed with potassium and

ther ions, the metal exhibited a strong interaction with the alkoxyroup. Interactions with the monomer (complexing) before poly-erisation appeared to discourage growth. The behaviour observed

ig. 2. 3-Methoxythiophene mass evolution during electrodeposition in the pres-nce of various electrolytes. Region A: Beginning of film deposition in which massain is observed due to polymerisation. Region B: Film deposition and mass variationue to polymer reduction-oxidation.

Fig. 3. Laser deflection and current deposition versus time during PMOT elec-tropolymerisation in 0.1 M (a) LiClO4 and (b) CsClO4 in a water:acetonitrile (3:1)mixture.

in our study was reproducible: the deposition profile was repeatedunder the same conditions with solutions of equal concentration.

Fig. 2 also shows that jumps in the starting mass resulting fromthe polymer electrodeposition appear to be constant; the depositedmass appears to be constant in each cycle (region A, Fig. 2). How-ever, after approximately 100 seconds, the mass jumps begin tovary. The mass deposited during monomer oxidation was highercompared with the mass deposited at the end of the cycle.

The mass variation behaviour in the two bounded regions (A andB) can be understood as follows: In region A, the onset of polymericfilm deposition occurs simultaneously with the polymeric film oxi-dation. To maintain electroneutrality in the polymer structure, ionsmove into and out of the polymer matrix. However, these ions donot initially cause observable variations, as observed in region B,because the film thickness is too small. After 100 s, the mass gainrelative to the input of ions for charge neutralisation in the polymermatrix is pronounced. Therefore, in this region, the mass gain dueto the polymerisation of the monomer is accompanied by a massgain due to the penetration of ions into the polymer matrix thatcounteract the charge generated by the polymer oxidation. Whenthe polymer is reduced, the ions are displaced into solution. Thepolymer reaches a maximum mass at each cycle corresponding tothe sum of the mass of the film and the ions that penetrate into thefilm. The minimum mass corresponds to the mass of the polymerminus the mass of the excess ions of the final reduction [13].

Another phenomenon observed in this study is the deflectionof the laser caused by the movement of ions (electrolyte) andelectroactive species (monomer) in solution. Fig. 3 presents thelaser deflection and current curves versus time for the Li+ and Cs+

350 M.R.d.A. Alves et al. / Electrochimica Acta 105 (2013) 347– 352

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reversible electrochemical behaviour is indicated because the peakoxidation potential varied with the rate (Fig. 6b).

ig. 4. Laser angle deflection as a function of time for PMOT electrodeposition inhe presence of LiClO4.

lectrolytes. When the polymer electrodeposition starts, the lasereflection is more pronounced, and a reduction in deflection occurshen the consequently deposited charge and the mass increase. At

he start of the electrodeposition, the working electrode exhibitsts maximum capacity to promote the exchange of electrons withhe monomer and creates a concentration gradient that involveshe diffusion of electroactive species and ions from the electrolyte.owever, over time, the electrode surface becomes less conduc-

ive due to the presence of the polymeric film, which results inecreased monomer diffusion and decreased electrolyte diffusion.s a result, the deflection angle decreases with time.

During the deposition cycles, two deflection peaks appear,hich are likely due to a difference in the movement between

lectroactive species (monomer) and ions from the electrolyte, ashown in Fig. 4. In the first cycle, one deflection peak from theonomer diffuses to the electrode surface (region A) with a minor

ontribution from the electrolyte ions. That is, during the deposi-ion of the polymer in the anodic sweep, the concentrations fromhe electrolyte ions in the electrode surface decrease as the ionsenetrate into the polymer structure (electrolyte diffusion pro-ess). However, as the number of cycles increases, a second peakrises at the beginning of the anodic sweep (region B). This peakppears because of a separation diffusion process of the species inolution (monomer and electrolyte).

The first peak (area B) refers to the contribution of the elec-rolytes that diffuse from the bulk to the electrode surface due tohe electrolyte penetration into the polymer film (deposited dur-ng the first cycle) when the polymer undergoes oxidation (anodicweep – polaron and bipolaron formation in the polymer struc-ure). The decrease in deflection arises from the limitations of theiffusion process in the electrolyte solution. The intensity of theeflection peak increased over the cycles because the film thicknesser cycle increased and because a greater number of electrolyte ionsoved to the surface of the electrode. The increase in deflection

n region C corresponds to the diffusion of the monomer solutionelectroactive species) to the electrode surface due to the formationf electrodeposited polymer.

.2. Electrochemistry study

The electrical properties of cyclic voltammetry at different scan

ates (�) were studied simultaneously with measurements of theirage effect and quartz microbalance. The electrochemical profile

f PMOT in the presence of LiClO4, an electrolyte, is shown in Fig. 5.or all of the electrolytes, an anodic peak current evolves linearly

Fig. 5. Electrochemical PMOT behaviours at different scan rates in the presence ofLiClO4.

with the square root of the scan rate (Fig. 6a), which indicates thatthe electrode process is limited by diffusion. Furthermore, a quasi-

Fig. 6. (a) Anodic peak current as a function of the square root of the scan rate and(b) anodic peak potential as a function of scan rate.

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The charge stabilisation generated in the structure of the CPs canccur via two mechanisms: (a) by input anions (oxidation process

injection of positive charges in the structure of the CPs) or (b)y the output of cations. The latter mechanism is caused by thelectrodeposition of the polymer in the presence of an electrolytei.e., the polymer is obtained with the electrolyte as an impurity).

n all cases, we observed positive deflection of the laser (mirageffect) and mass gain during oxidation and negative deflection ofhe laser and mass loss during the reduction process. These results

ig. 7. (a) Current, (b) mass variation and (c) laser deflection as a function of timet 20 mV s−1.

a Acta 105 (2013) 347– 352 351

are in agreement and indicate that the ClO4− plays a major role in

the redox process.Fig. 7 shows (a) current, (b) weight and (c) laser deflection as

a function of time for PMOT in the presence of each electrolyteat 20 mV s−1. The anodic peak current obtained in the presence ofthe potassium ion is the smallest (Fig. 7a), which is in agreementwith the smaller mass of PMOT deposited on the electrode surface(Fig. 2). Additionally, the largest PMOT mass gain was observed inthe presence of K+ (Fig. 7b). This observation may be directly relatedto the film thickness. By observing the mass of PMOT/electrolytedeposited on the electrode surface during electrodeposition (Fig. 1),it is concluded that the thinness PMOT film was generated in thepresence of K+. In this case, the polymer film exhibits a higher abilityto exchange electrons, and as a result, a greater number of counterions tend to penetrate the PMOT film.

Another possibility to consider is the greater ion-storage capac-ity of the thicker films. Fewer counter ions are required to enter thepolymer structure to neutralise the charge during oxidation. Thisinterpretation is based on the analysis of the ratio between theanodic and cathodic peak currents (Ia/Ic = 2 (LiClO4), 2.1 (NaClO4),1.4 (KClO4), 1.97 (RbClO4), and 2 (CsClO4)). The anodic current(Ia) is approximately two times greater than the cathodic current(Ic) for almost all of the cases, except in the presence of K+. Thisresult indicates irreversible behaviour and shows that, during thereduction process, the positive charges injected into the polymerduring the oxidation process were not completely expelled duringthe cathodic sweep. Therefore, the diffusion of ions (ClO4

−) fromthe polymeric structure to the solution is slower than the diffu-sion of ions from the solution into the polymeric structure. Thisincomplete expulsion may be the result of the reduced mobilityof the counter ions inside the polymer structure, which may notbe able to enter the bulk solution. The laser deflection results con-firmed this hypothesis because the positive deflection of the laser isgreater than the negative deflection, which indicates that more ionsenter (anodic sweep) than leave the polymer structure (cathodicsweep). Thus, the difficulty the perchlorate counter ions in leav-ing the polymer backbone caused by the film thickness inhibitsthe polymer reduction process, which results in decreases in thecathodic current relative to the anodic current.

Fig. 8 shows that thicker films decrease the mobility ofthe counter ions in the polymeric matrix and, consequently,increase the distance between the anodic and cathodic peaks

(Ea − Ec = 0.254 V (LiClO4), 0.421 V (NaClO4), 0.246 V (KClO4),0.394 V (RbClO4), and 0.421 V (CsClO4)). Thus, the film thicknessmay be used to control the redox process reversibility of the poly-mer films.

Fig. 8. 20 mV s−1 PMOT voltammograms in the presence of different electrolytes.

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. Conclusion

The simultaneous studies evaluating quartz microbalance andhe mirage effect of the electropolymerisation and electrochemicalehaviour of PMOT were successful. In this work, the depositionf PMOT on the electrode surface was strongly influenced by the+ ion, most likely because of the interaction between the alkoxyroup of the monomer and the K+ ion. Furthermore, increasing thelm thickness decreases the monomer’s contribution and increaseshe electrolyte’s contribution to the deflection of the laser. Thisesult was expected and was clearly observed and confirmed bymploying the techniques presented in this paper.

cknowledgements

We acknowledge financial support from Fapemig (PRONEXDT479/07 and PPM V 0356/11), CNPq and PRPq-UFMG.

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