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Simultaneous quartz microbalance and mirage effect studies of poly(3-methoxythiophene) electrosynthesis and electrochemical characterisations
Electrochimica Acta 105 (2013) 347–352
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Simultaneous quartz microbalance and mirage effect studies of poly(3-methoxythiophene) electrosynthesis and electrochemical characterisations Marcos Roberto de Abreu Alves ∗ , Raphael Nasser Capistrano Reis, Jean Gomes de Oliveira, Hállen Daniel Rezende Calado, Claudio Luis Donnici, Tulio Matencio Departamento 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
Article history: Received 21 December 2012 Received in revised form 26 April 2013 Accepted 30 April 2013 Available online 14 May 2013 Keywords: Mirage effect Poly(3-methoxythiophene) Electropolymerisation Quartz microbalance
a b s t r a c t This work characterises the electropolymerisation of 3-methoxythiophene and presents an electrochemical study of poly(3-methoxythiophene) by cyclic voltammetry using different electrolytes in solvent mixture of water-acetonitrile (3:1, v/v) at 25 ◦ C. Simultaneous measurements of the mirage effect and the quartz microbalance were also performed. This study evaluated the influence of cations on the electrochemical properties of monomers and polymers and allowed us to observe an anomalous behaviour of 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 was observed the influence of the polymer film thickness (deposited mass) on its electrochemical properties.
1. Introduction Organic electronics is currently one of the most promising technological fields [1,2]. The possibility of obtaining low-cost, flexible materials and conjugated polymers (CPs) is promising in several areas, such as the development of flexible solar cells [3], polymeric light-emitting diodes [4], transistors, and other applications. The electrochemical properties of these materials must be determined for their application in the electronics. The factors affecting these properties should be determined, along with the influence of the external environment on the properties of the final material. It is also necessary to assess the properties and redox behaviours of CPs in electrochemical cells during electropolymerisation. In this study, the electropolymerisation of the monomer 3methoxythiophene (MOT) was evaluated using cyclic voltammetry. The same anion was used as the electrolyte (perchlorate ion, ClO4 − ), and but different alkaline metal cations (Li+ , Na+ , K+ , Rb+ , and Cs+ ) were employed to evaluate the influence of the cation size. Studies of the mirage effect and the quartz microbalance were conducted simultaneously during the electropolymerisation and electrochemical characterisation of the film. The flow of the electroactive species/electrolytes during the electropolymerisation of 3-methoxythiophene was monitored, and the electrolytes were
∗ Corresponding author. Tel.: +55 031 34095742; fax: +55 031 34095700. E-mail address: [email protected] (M.R.d.A. Alves). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.04.173
© 2013 Elsevier Ltd. All rights reserved.
characterised while analysing the redox behaviour of PMOT to obtain important information on the influences of the electrolyte on the growth of the film and the electrochemical profile of the materials. One can find in the literature studies about the influence of ionic exchanges on properties of inorganic films deposited on the surface of electrodes, such as Prussian blue derivatives [5–8]. However these works, that show the input and output cations of the film due to 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 were purchased from Aldrich. The acetonitrile used to prepare the electrolyte solution was obtained from Vetec. All solutions were prepared using distilled water. 2.2. Utilised apparatus The system used for this study was adapted from the literature [9,10]. The electrochemical data were obtained using a digital Omnimetra PG-39 potentiostat. The electrochemical cell consisted of a glass cuvette with an optical pathlength of 30 mm
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obtained from Biocell and three electrodes. A quartz crystal ATcut 5 MHz QCM (Stanford Research Systems) with a circular film of platinum/titanium (Pt/Ti) was used as the working electrode. A platinum wire was used as the auxiliary electrode, and a silver wire was used as the quasi-reference electrode. A QCM200 from Stanford Research Systems was used as the quartz microbalance. The mirage effect apparatus consisted of a 633 nm, 2 mW HeNe laser. The laser beam diameter at the lens focal point was approximately 32 m, and the beam was parallel to the working electrode. The positive deflections indicated that the ions diffused within the
solution to the electrode surface, whereas the negative deflections indicated that the ions diffused from the electrode surface to the bulk solution. 2.3. Poly(3-methoxythiophene) electrodeposition and characterisation by quartz microbalance, cyclic voltammetry and mirage effect The poly(3-methoxythiophene) (PMOT) electrodeposition was performed as described in a previous work [11]. A 35 mmol L−1
Fig. 1. PMOT electrodeposition voltammograms with different electrolytes: (a) LiClO4 , (b) NaClO4 , (c) KClO4 , (d) RbClO4 , and (e) CsClO4 .
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solution of the monomer in water:acetonitrile (3:1) was prepared. Aqueous solutions (0.1 mol L−1 ) of LiClO4 , NaClO4 , KClO4 , RbClO4 and CsClO4 electrolytes were used. The synthesis was performed by subjecting the monomer solution to 20 cycles of cyclic voltammetry in the potential range of 0 V to 1.4 V vs Ag/Ag+ . The deposition was conducted using a conventional three-electrode configuration, as specified in item 2.2. The PMOT film obtained in each synthesis with different electrolytes was studied using cyclic voltammetry (CV) in the range -0.6 V to 0.6 V vs Ag/Ag+ at different scanning speeds (5, 10, 20, 50, 75 and 100 mV s−1 ) in the corresponding free monomer electrolyte solution. All the studies were performed under an inert N2 atmosphere. 3. Results and discussion 3.1. Electropolymerisation The PMOT was electropolymerised in the presence of different electrolytes using a 1:1 stoichiometric ratio between the cations and anions. The CV electrochemical technique was employed, which allows the growth of the film to be monitored at each cycle due to increased current below the oxidation potential of the monomer (∼1.4 V) during the oxidation and reduction of the deposited polymeric film on the electrode surface between 0 V and 0.6 V vs Ag/Ag+ . The PMOT electrodeposition voltammograms are shown in Fig. 1, in which the arrow indicates the scanning direction (small arrows) and growth (higher arrow) of the polymeric film during the electropolymerisation. The polymer growth during each synthesis was monitored using a quartz microbalance (MBQ) and the mirage effect (ME). In Fig. 2, the mass versus time of the polymer deposited is shown. The end mass deposited at the electrode increased with as the size (weight) of the cation increased, except for potassium, which exhibited a different behaviour. UV-vis spectroscopic studies of the interaction of alkaline metals [12] with the alkoxy group of conjugated polymers showed a different behaviour compared with the results of previous studies. In the previous studies, which were performed with potassium and other ions, the metal exhibited a strong interaction with the alkoxy group. Interactions with the monomer (complexing) before polymerisation appeared to discourage growth. The behaviour observed
Fig. 2. 3-Methoxythiophene mass evolution during electrodeposition in the presence of various electrolytes. Region A: Beginning of film deposition in which mass gain is observed due to polymerisation. Region B: Film deposition and mass variation due to polymer reduction-oxidation.
Fig. 3. Laser deflection and current deposition versus time during PMOT electropolymerisation 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 repeated under the same conditions with solutions of equal concentration. Fig. 2 also shows that jumps in the starting mass resulting from the polymer electrodeposition appear to be constant; the deposited mass appears to be constant in each cycle (region A, Fig. 2). However, after approximately 100 seconds, the mass jumps begin to vary. The mass deposited during monomer oxidation was higher compared with the mass deposited at the end of the cycle. The mass variation behaviour in the two bounded regions (A and B) can be understood as follows: In region A, the onset of polymeric film deposition occurs simultaneously with the polymeric film oxidation. To maintain electroneutrality in the polymer structure, ions move into and out of the polymer matrix. However, these ions do not initially cause observable variations, as observed in region B, because the film thickness is too small. After 100 s, the mass gain relative to the input of ions for charge neutralisation in the polymer matrix is pronounced. Therefore, in this region, the mass gain due to the polymerisation of the monomer is accompanied by a mass gain due to the penetration of ions into the polymer matrix that counteract the charge generated by the polymer oxidation. When the polymer is reduced, the ions are displaced into solution. The polymer reaches a maximum mass at each cycle corresponding to the sum of the mass of the film and the ions that penetrate into the film. The minimum mass corresponds to the mass of the polymer minus the mass of the excess ions of the final reduction [13]. Another phenomenon observed in this study is the deflection of the laser caused by the movement of ions (electrolyte) and electroactive species (monomer) in solution. Fig. 3 presents the laser deflection and current curves versus time for the Li+ and Cs+
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Fig. 4. Laser angle deflection as a function of time for PMOT electrodeposition in the presence of LiClO4 .
electrolytes. When the polymer electrodeposition starts, the laser deflection is more pronounced, and a reduction in deflection occurs when the consequently deposited charge and the mass increase. At the start of the electrodeposition, the working electrode exhibits its maximum capacity to promote the exchange of electrons with the monomer and creates a concentration gradient that involves the diffusion of electroactive species and ions from the electrolyte. However, over time, the electrode surface becomes less conductive due to the presence of the polymeric film, which results in decreased monomer diffusion and decreased electrolyte diffusion. As a result, the deflection angle decreases with time. During the deposition cycles, two deflection peaks appear, which are likely due to a difference in the movement between electroactive species (monomer) and ions from the electrolyte, as shown in Fig. 4. In the first cycle, one deflection peak from the monomer diffuses to the electrode surface (region A) with a minor contribution from the electrolyte ions. That is, during the deposition of the polymer in the anodic sweep, the concentrations from the electrolyte ions in the electrode surface decrease as the ions penetrate into the polymer structure (electrolyte diffusion process). However, as the number of cycles increases, a second peak arises at the beginning of the anodic sweep (region B). This peak appears because of a separation diffusion process of the species in solution (monomer and electrolyte). The first peak (area B) refers to the contribution of the electrolytes that diffuse from the bulk to the electrode surface due to the electrolyte penetration into the polymer film (deposited during the first cycle) when the polymer undergoes oxidation (anodic sweep – polaron and bipolaron formation in the polymer structure). The decrease in deflection arises from the limitations of the diffusion process in the electrolyte solution. The intensity of the deflection peak increased over the cycles because the film thickness per cycle increased and because a greater number of electrolyte ions moved to the surface of the electrode. The increase in deflection in region C corresponds to the diffusion of the monomer solution (electroactive species) to the electrode surface due to the formation of electrodeposited polymer.
Fig. 5. Electrochemical PMOT behaviours at different scan rates in the presence of LiClO4 .
with the square root of the scan rate (Fig. 6a), which indicates that the electrode process is limited by diffusion. Furthermore, a quasireversible electrochemical behaviour is indicated because the peak oxidation potential varied with the rate (Fig. 6b).
3.2. Electrochemistry study The electrical properties of cyclic voltammetry at different scan rates () were studied simultaneously with measurements of the mirage effect and quartz microbalance. The electrochemical profile of PMOT in the presence of LiClO4 , an electrolyte, is shown in Fig. 5. For all of the electrolytes, an anodic peak current evolves linearly
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 can occur via two mechanisms: (a) by input anions (oxidation process - injection of positive charges in the structure of the CPs) or (b) by the output of cations. The latter mechanism is caused by the electrodeposition of the polymer in the presence of an electrolyte (i.e., the polymer is obtained with the electrolyte as an impurity). In all cases, we observed positive deflection of the laser (mirage effect) and mass gain during oxidation and negative deflection of the laser and mass loss during the reduction process. These results
Fig. 7. (a) Current, (b) mass variation and (c) laser deflection as a function of time at 20 mV s−1 .
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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 electrolyte at 20 mV s−1 . The anodic peak current obtained in the presence of the potassium ion is the smallest (Fig. 7a), which is in agreement with the smaller mass of PMOT deposited on the electrode surface (Fig. 2). Additionally, the largest PMOT mass gain was observed in the presence of K+ (Fig. 7b). This observation may be directly related to the film thickness. By observing the mass of PMOT/electrolyte deposited on the electrode surface during electrodeposition (Fig. 1), it is concluded that the thinness PMOT film was generated in the presence of K+ . In this case, the polymer film exhibits a higher ability to exchange electrons, and as a result, a greater number of counter ions tend to penetrate the PMOT film. Another possibility to consider is the greater ion-storage capacity of the thicker films. Fewer counter ions are required to enter the polymer structure to neutralise the charge during oxidation. This interpretation is based on the analysis of the ratio between the anodic 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+ . This result indicates irreversible behaviour and shows that, during the reduction process, the positive charges injected into the polymer during the oxidation process were not completely expelled during the cathodic sweep. Therefore, the diffusion of ions (ClO4 − ) from the polymeric structure to the solution is slower than the diffusion of ions from the solution into the polymeric structure. This incomplete expulsion may be the result of the reduced mobility of the counter ions inside the polymer structure, which may not be able to enter the bulk solution. The laser deflection results confirmed this hypothesis because the positive deflection of the laser is greater than the negative deflection, which indicates that more ions enter (anodic sweep) than leave the polymer structure (cathodic sweep). Thus, the difficulty the perchlorate counter ions in leaving the polymer backbone caused by the film thickness inhibits the polymer reduction process, which results in decreases in the cathodic current relative to the anodic current. Fig. 8 shows that thicker films decrease the mobility of the 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 thickness may be used to control the redox process reversibility of the polymer films.
Fig. 8. 20 mV s−1 PMOT voltammograms in the presence of different electrolytes.
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4. Conclusion The simultaneous studies evaluating quartz microbalance and the mirage effect of the electropolymerisation and electrochemical behaviour of PMOT were successful. In this work, the deposition of PMOT on the electrode surface was strongly influenced by the K+ ion, most likely because of the interaction between the alkoxy group of the monomer and the K+ ion. Furthermore, increasing the film thickness decreases the monomer’s contribution and increases the electrolyte’s contribution to the deflection of the laser. This result was expected and was clearly observed and confirmed by employing the techniques presented in this paper. Acknowledgements We acknowledge financial support from Fapemig (PRONEX EDT479/07 and PPM V 0356/11), CNPq and PRPq-UFMG. References [1] T.W. Kelley, P. Baude, C. Gerlach, D.E. Ender, D. Muyres, M.A. Haase, D.E. Vogel, S.D. Theiss, Recent progress in organic electronics: materials, devices, and processes, Chemistry of Materials 16 (2004) 4413. [2] S. Logothetidis, Flexible organic electronic devices: materials, process and applications, Materials Science and Engineering B 152 (2008) 96. [3] S. Gunes, H. Neugebauer, N.S. Sariciftci, Conjugated polymer-based organic solar cells, Chemical Reviews 107 (2007) 1324.
[4] J.F. Deus, G.C. Faria, E.T. Iamazaki, R.M. Faria, T.D.Z. Atvars, L. Akcelrud, Polyfluorene based blends for white light emission, Organic Electronics 12 (2011) 1493. [5] E. Csahok, E. Vieil, G. Inzelt, Optical beam deflection study of indiumhexacyanoferrate films, Synthetic Metals 103 (1999) 2687. [6] V. Plichon, S. Besbes, Mirage detection of counter-ion flux between Prussian blue films and electrolyte solutions, Journal of Electroanalytical Chemistry 284 (1990) 141. [7] M.A. Malik, G. Horanyi, P.J. Kulesza, G. Inzelt, V. Kertesz, R. Schmidt, E. Czirok, Microgravimetric monitoring of transport of cations during redox reactions of indium (III) hexacyanoferrate (III,II) radiotracer evidence for the flux of anions in the film, Journal of Electroanalytical Chemistry 452 (1998) 57. [8] P.J. Kulesza, M.A. Malik, M. Berrettoni, M. Giorgetti, S. Zamponi, R. Schmidt, R. Marassi, Electrochemical charging, countercation accommodation, and spectrochemical identity of microcrystalline solid cobalt hexacyanoferrate, Journal of Physical Chemistry B 102 (1998) 1870. [9] T. Matencio, M.-A. De Paoli, R.C.D. Peres, R.M. Torresi, S.I.C. Torresi, Ionic exchanges in dodecylbenzenesulfonate doped polypyrrole Part 1. Optical beam deflection studies, Synthetic Metals 72 (1995) 59. [10] H.M. French, M.J. Henderson, A.R. Hillman, E. Vieil, Ion and solvent transfer discrimination at a nickel hydroxide film exposed to LiOH by combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) techniques, Journal of Electroanalytical Chemistry 500 (2001) 192. [11] M.R.A. Alves, H.D.R. Calado, C.L. Donnici, T. Matencio, Electrochemical polymerization and characterization of new copolymers of 3-substituted thiophenes, Synthetic Metals 160 (2010) 22. [12] I. Levesque, M. Leclerc, Ionochromic and thermochromic phenomena in a regioregular polythiophene derivative bearing oligo(oxyethylene) side chains, Chemistry of Materials 8 (1996) 2843. [13] S. Servagent, E. Vieil, In-situ quartz microbalance study of the electrosynthesis of poly(3-methylthiophene), Journal of Electroanalytical Chemistry 280 (1990) 227.
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Simultaneous quartz microbalance and mirage effect studies of poly(3-methoxythiophene) electrosynthesis and electrochemical characterisations Marcos Roberto de Abreu Alves ∗ , Raphael Nasser Capistrano Reis, Jean Gomes de Oliveira, Hállen Daniel Rezende Calado, Claudio Luis Donnici, Tulio Matencio Departamento 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
Article history: Received 21 December 2012 Received in revised form 26 April 2013 Accepted 30 April 2013 Available online 14 May 2013 Keywords: Mirage effect Poly(3-methoxythiophene) Electropolymerisation Quartz microbalance
a b s t r a c t This work characterises the electropolymerisation of 3-methoxythiophene and presents an electrochemical study of poly(3-methoxythiophene) by cyclic voltammetry using different electrolytes in solvent mixture of water-acetonitrile (3:1, v/v) at 25 ◦ C. Simultaneous measurements of the mirage effect and the quartz microbalance were also performed. This study evaluated the influence of cations on the electrochemical properties of monomers and polymers and allowed us to observe an anomalous behaviour of 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 was observed the influence of the polymer film thickness (deposited mass) on its electrochemical properties.
1. Introduction Organic electronics is currently one of the most promising technological fields [1,2]. The possibility of obtaining low-cost, flexible materials and conjugated polymers (CPs) is promising in several areas, such as the development of flexible solar cells [3], polymeric light-emitting diodes [4], transistors, and other applications. The electrochemical properties of these materials must be determined for their application in the electronics. The factors affecting these properties should be determined, along with the influence of the external environment on the properties of the final material. It is also necessary to assess the properties and redox behaviours of CPs in electrochemical cells during electropolymerisation. In this study, the electropolymerisation of the monomer 3methoxythiophene (MOT) was evaluated using cyclic voltammetry. The same anion was used as the electrolyte (perchlorate ion, ClO4 − ), and but different alkaline metal cations (Li+ , Na+ , K+ , Rb+ , and Cs+ ) were employed to evaluate the influence of the cation size. Studies of the mirage effect and the quartz microbalance were conducted simultaneously during the electropolymerisation and electrochemical characterisation of the film. The flow of the electroactive species/electrolytes during the electropolymerisation of 3-methoxythiophene was monitored, and the electrolytes were
∗ Corresponding author. Tel.: +55 031 34095742; fax: +55 031 34095700. E-mail address: [email protected] (M.R.d.A. Alves). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.04.173
© 2013 Elsevier Ltd. All rights reserved.
characterised while analysing the redox behaviour of PMOT to obtain important information on the influences of the electrolyte on the growth of the film and the electrochemical profile of the materials. One can find in the literature studies about the influence of ionic exchanges on properties of inorganic films deposited on the surface of electrodes, such as Prussian blue derivatives [5–8]. However these works, that show the input and output cations of the film due to 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 were purchased from Aldrich. The acetonitrile used to prepare the electrolyte solution was obtained from Vetec. All solutions were prepared using distilled water. 2.2. Utilised apparatus The system used for this study was adapted from the literature [9,10]. The electrochemical data were obtained using a digital Omnimetra PG-39 potentiostat. The electrochemical cell consisted of a glass cuvette with an optical pathlength of 30 mm
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obtained from Biocell and three electrodes. A quartz crystal ATcut 5 MHz QCM (Stanford Research Systems) with a circular film of platinum/titanium (Pt/Ti) was used as the working electrode. A platinum wire was used as the auxiliary electrode, and a silver wire was used as the quasi-reference electrode. A QCM200 from Stanford Research Systems was used as the quartz microbalance. The mirage effect apparatus consisted of a 633 nm, 2 mW HeNe laser. The laser beam diameter at the lens focal point was approximately 32 m, and the beam was parallel to the working electrode. The positive deflections indicated that the ions diffused within the
solution to the electrode surface, whereas the negative deflections indicated that the ions diffused from the electrode surface to the bulk solution. 2.3. Poly(3-methoxythiophene) electrodeposition and characterisation by quartz microbalance, cyclic voltammetry and mirage effect The poly(3-methoxythiophene) (PMOT) electrodeposition was performed as described in a previous work [11]. A 35 mmol L−1
Fig. 1. PMOT electrodeposition voltammograms with different electrolytes: (a) LiClO4 , (b) NaClO4 , (c) KClO4 , (d) RbClO4 , and (e) CsClO4 .
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solution of the monomer in water:acetonitrile (3:1) was prepared. Aqueous solutions (0.1 mol L−1 ) of LiClO4 , NaClO4 , KClO4 , RbClO4 and CsClO4 electrolytes were used. The synthesis was performed by subjecting the monomer solution to 20 cycles of cyclic voltammetry in the potential range of 0 V to 1.4 V vs Ag/Ag+ . The deposition was conducted using a conventional three-electrode configuration, as specified in item 2.2. The PMOT film obtained in each synthesis with different electrolytes was studied using cyclic voltammetry (CV) in the range -0.6 V to 0.6 V vs Ag/Ag+ at different scanning speeds (5, 10, 20, 50, 75 and 100 mV s−1 ) in the corresponding free monomer electrolyte solution. All the studies were performed under an inert N2 atmosphere. 3. Results and discussion 3.1. Electropolymerisation The PMOT was electropolymerised in the presence of different electrolytes using a 1:1 stoichiometric ratio between the cations and anions. The CV electrochemical technique was employed, which allows the growth of the film to be monitored at each cycle due to increased current below the oxidation potential of the monomer (∼1.4 V) during the oxidation and reduction of the deposited polymeric film on the electrode surface between 0 V and 0.6 V vs Ag/Ag+ . The PMOT electrodeposition voltammograms are shown in Fig. 1, in which the arrow indicates the scanning direction (small arrows) and growth (higher arrow) of the polymeric film during the electropolymerisation. The polymer growth during each synthesis was monitored using a quartz microbalance (MBQ) and the mirage effect (ME). In Fig. 2, the mass versus time of the polymer deposited is shown. The end mass deposited at the electrode increased with as the size (weight) of the cation increased, except for potassium, which exhibited a different behaviour. UV-vis spectroscopic studies of the interaction of alkaline metals [12] with the alkoxy group of conjugated polymers showed a different behaviour compared with the results of previous studies. In the previous studies, which were performed with potassium and other ions, the metal exhibited a strong interaction with the alkoxy group. Interactions with the monomer (complexing) before polymerisation appeared to discourage growth. The behaviour observed
Fig. 2. 3-Methoxythiophene mass evolution during electrodeposition in the presence of various electrolytes. Region A: Beginning of film deposition in which mass gain is observed due to polymerisation. Region B: Film deposition and mass variation due to polymer reduction-oxidation.
Fig. 3. Laser deflection and current deposition versus time during PMOT electropolymerisation 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 repeated under the same conditions with solutions of equal concentration. Fig. 2 also shows that jumps in the starting mass resulting from the polymer electrodeposition appear to be constant; the deposited mass appears to be constant in each cycle (region A, Fig. 2). However, after approximately 100 seconds, the mass jumps begin to vary. The mass deposited during monomer oxidation was higher compared with the mass deposited at the end of the cycle. The mass variation behaviour in the two bounded regions (A and B) can be understood as follows: In region A, the onset of polymeric film deposition occurs simultaneously with the polymeric film oxidation. To maintain electroneutrality in the polymer structure, ions move into and out of the polymer matrix. However, these ions do not initially cause observable variations, as observed in region B, because the film thickness is too small. After 100 s, the mass gain relative to the input of ions for charge neutralisation in the polymer matrix is pronounced. Therefore, in this region, the mass gain due to the polymerisation of the monomer is accompanied by a mass gain due to the penetration of ions into the polymer matrix that counteract the charge generated by the polymer oxidation. When the polymer is reduced, the ions are displaced into solution. The polymer reaches a maximum mass at each cycle corresponding to the sum of the mass of the film and the ions that penetrate into the film. The minimum mass corresponds to the mass of the polymer minus the mass of the excess ions of the final reduction [13]. Another phenomenon observed in this study is the deflection of the laser caused by the movement of ions (electrolyte) and electroactive species (monomer) in solution. Fig. 3 presents the laser deflection and current curves versus time for the Li+ and Cs+
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Fig. 4. Laser angle deflection as a function of time for PMOT electrodeposition in the presence of LiClO4 .
electrolytes. When the polymer electrodeposition starts, the laser deflection is more pronounced, and a reduction in deflection occurs when the consequently deposited charge and the mass increase. At the start of the electrodeposition, the working electrode exhibits its maximum capacity to promote the exchange of electrons with the monomer and creates a concentration gradient that involves the diffusion of electroactive species and ions from the electrolyte. However, over time, the electrode surface becomes less conductive due to the presence of the polymeric film, which results in decreased monomer diffusion and decreased electrolyte diffusion. As a result, the deflection angle decreases with time. During the deposition cycles, two deflection peaks appear, which are likely due to a difference in the movement between electroactive species (monomer) and ions from the electrolyte, as shown in Fig. 4. In the first cycle, one deflection peak from the monomer diffuses to the electrode surface (region A) with a minor contribution from the electrolyte ions. That is, during the deposition of the polymer in the anodic sweep, the concentrations from the electrolyte ions in the electrode surface decrease as the ions penetrate into the polymer structure (electrolyte diffusion process). However, as the number of cycles increases, a second peak arises at the beginning of the anodic sweep (region B). This peak appears because of a separation diffusion process of the species in solution (monomer and electrolyte). The first peak (area B) refers to the contribution of the electrolytes that diffuse from the bulk to the electrode surface due to the electrolyte penetration into the polymer film (deposited during the first cycle) when the polymer undergoes oxidation (anodic sweep – polaron and bipolaron formation in the polymer structure). The decrease in deflection arises from the limitations of the diffusion process in the electrolyte solution. The intensity of the deflection peak increased over the cycles because the film thickness per cycle increased and because a greater number of electrolyte ions moved to the surface of the electrode. The increase in deflection in region C corresponds to the diffusion of the monomer solution (electroactive species) to the electrode surface due to the formation of electrodeposited polymer.
Fig. 5. Electrochemical PMOT behaviours at different scan rates in the presence of LiClO4 .
with the square root of the scan rate (Fig. 6a), which indicates that the electrode process is limited by diffusion. Furthermore, a quasireversible electrochemical behaviour is indicated because the peak oxidation potential varied with the rate (Fig. 6b).
3.2. Electrochemistry study The electrical properties of cyclic voltammetry at different scan rates () were studied simultaneously with measurements of the mirage effect and quartz microbalance. The electrochemical profile of PMOT in the presence of LiClO4 , an electrolyte, is shown in Fig. 5. For all of the electrolytes, an anodic peak current evolves linearly
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 can occur via two mechanisms: (a) by input anions (oxidation process - injection of positive charges in the structure of the CPs) or (b) by the output of cations. The latter mechanism is caused by the electrodeposition of the polymer in the presence of an electrolyte (i.e., the polymer is obtained with the electrolyte as an impurity). In all cases, we observed positive deflection of the laser (mirage effect) and mass gain during oxidation and negative deflection of the laser and mass loss during the reduction process. These results
Fig. 7. (a) Current, (b) mass variation and (c) laser deflection as a function of time at 20 mV s−1 .
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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 electrolyte at 20 mV s−1 . The anodic peak current obtained in the presence of the potassium ion is the smallest (Fig. 7a), which is in agreement with the smaller mass of PMOT deposited on the electrode surface (Fig. 2). Additionally, the largest PMOT mass gain was observed in the presence of K+ (Fig. 7b). This observation may be directly related to the film thickness. By observing the mass of PMOT/electrolyte deposited on the electrode surface during electrodeposition (Fig. 1), it is concluded that the thinness PMOT film was generated in the presence of K+ . In this case, the polymer film exhibits a higher ability to exchange electrons, and as a result, a greater number of counter ions tend to penetrate the PMOT film. Another possibility to consider is the greater ion-storage capacity of the thicker films. Fewer counter ions are required to enter the polymer structure to neutralise the charge during oxidation. This interpretation is based on the analysis of the ratio between the anodic 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+ . This result indicates irreversible behaviour and shows that, during the reduction process, the positive charges injected into the polymer during the oxidation process were not completely expelled during the cathodic sweep. Therefore, the diffusion of ions (ClO4 − ) from the polymeric structure to the solution is slower than the diffusion of ions from the solution into the polymeric structure. This incomplete expulsion may be the result of the reduced mobility of the counter ions inside the polymer structure, which may not be able to enter the bulk solution. The laser deflection results confirmed this hypothesis because the positive deflection of the laser is greater than the negative deflection, which indicates that more ions enter (anodic sweep) than leave the polymer structure (cathodic sweep). Thus, the difficulty the perchlorate counter ions in leaving the polymer backbone caused by the film thickness inhibits the polymer reduction process, which results in decreases in the cathodic current relative to the anodic current. Fig. 8 shows that thicker films decrease the mobility of the 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 thickness may be used to control the redox process reversibility of the polymer films.
Fig. 8. 20 mV s−1 PMOT voltammograms in the presence of different electrolytes.
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4. Conclusion The simultaneous studies evaluating quartz microbalance and the mirage effect of the electropolymerisation and electrochemical behaviour of PMOT were successful. In this work, the deposition of PMOT on the electrode surface was strongly influenced by the K+ ion, most likely because of the interaction between the alkoxy group of the monomer and the K+ ion. Furthermore, increasing the film thickness decreases the monomer’s contribution and increases the electrolyte’s contribution to the deflection of the laser. This result was expected and was clearly observed and confirmed by employing the techniques presented in this paper. Acknowledgements We acknowledge financial support from Fapemig (PRONEX EDT479/07 and PPM V 0356/11), CNPq and PRPq-UFMG. References [1] T.W. Kelley, P. Baude, C. Gerlach, D.E. Ender, D. Muyres, M.A. Haase, D.E. Vogel, S.D. Theiss, Recent progress in organic electronics: materials, devices, and processes, Chemistry of Materials 16 (2004) 4413. [2] S. Logothetidis, Flexible organic electronic devices: materials, process and applications, Materials Science and Engineering B 152 (2008) 96. [3] S. Gunes, H. Neugebauer, N.S. Sariciftci, Conjugated polymer-based organic solar cells, Chemical Reviews 107 (2007) 1324.
[4] J.F. Deus, G.C. Faria, E.T. Iamazaki, R.M. Faria, T.D.Z. Atvars, L. Akcelrud, Polyfluorene based blends for white light emission, Organic Electronics 12 (2011) 1493. [5] E. Csahok, E. Vieil, G. Inzelt, Optical beam deflection study of indiumhexacyanoferrate films, Synthetic Metals 103 (1999) 2687. [6] V. Plichon, S. Besbes, Mirage detection of counter-ion flux between Prussian blue films and electrolyte solutions, Journal of Electroanalytical Chemistry 284 (1990) 141. [7] M.A. Malik, G. Horanyi, P.J. Kulesza, G. Inzelt, V. Kertesz, R. Schmidt, E. Czirok, Microgravimetric monitoring of transport of cations during redox reactions of indium (III) hexacyanoferrate (III,II) radiotracer evidence for the flux of anions in the film, Journal of Electroanalytical Chemistry 452 (1998) 57. [8] P.J. Kulesza, M.A. Malik, M. Berrettoni, M. Giorgetti, S. Zamponi, R. Schmidt, R. Marassi, Electrochemical charging, countercation accommodation, and spectrochemical identity of microcrystalline solid cobalt hexacyanoferrate, Journal of Physical Chemistry B 102 (1998) 1870. [9] T. Matencio, M.-A. De Paoli, R.C.D. Peres, R.M. Torresi, S.I.C. Torresi, Ionic exchanges in dodecylbenzenesulfonate doped polypyrrole Part 1. Optical beam deflection studies, Synthetic Metals 72 (1995) 59. [10] H.M. French, M.J. Henderson, A.R. Hillman, E. Vieil, Ion and solvent transfer discrimination at a nickel hydroxide film exposed to LiOH by combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) techniques, Journal of Electroanalytical Chemistry 500 (2001) 192. [11] M.R.A. Alves, H.D.R. Calado, C.L. Donnici, T. Matencio, Electrochemical polymerization and characterization of new copolymers of 3-substituted thiophenes, Synthetic Metals 160 (2010) 22. [12] I. Levesque, M. Leclerc, Ionochromic and thermochromic phenomena in a regioregular polythiophene derivative bearing oligo(oxyethylene) side chains, Chemistry of Materials 8 (1996) 2843. [13] S. Servagent, E. Vieil, In-situ quartz microbalance study of the electrosynthesis of poly(3-methylthiophene), Journal of Electroanalytical Chemistry 280 (1990) 227.