Electropolymerization and characterization of polyaniline films using a spectroelectrochemical flow cell

Analytica Chimica Acta 573–574 (2006) 20–25

Electropolymerization and characterization of polyaniline films using a spectroelectrochemical flow cell ´ Emma Mu˜noz, Alvaro Colina, Ar´anzazu Heras, Virginia Ruiz, Susana Palmero, Jes´us L´opez-Palacios ∗ Area de Qu´ımica Anal´ıtica. Universidad de Burgos, Pza. Misael Ba˜nuelos, s/n. 09001 Burgos, Spain Received 30 November 2005; received in revised form 9 January 2006; accepted 10 January 2006 Available online 17 February 2006

Abstract A new spectroelectrochemical flow cell is presented and its capability to provide a better understanding of reaction mechanisms is illustrated with the study of the electrosynthesis and characterization of a conducting polymer, polyaniline (PANI). A spectroelectrochemical study of electropolymerization of aniline under flow conditions has been performed for the first time. The significant influence that the flow rate of feeding monomer solution has on the electropolymerization process and, consequently, on the electrochromic properties of the resulting polymer has been demonstrated. © 2006 Elsevier B.V. All rights reserved. Keywords: Conducting polymers; Polyaniline; Spectroelectrochemistry; Flow cell; Electrochemistry

1. Introduction Conducting polymers are widely used as material for sensors. In particular, polyaniline (PANI) is among the most employed ones [1–12]. Electrosynthesis of PANI is a very complex process in which a variety of intermediate compounds and side-reaction products (dimers, oligomers, cross-linked polymer, p-aminodiphenylamine, p-benzoquinone, quinoneimines, p-aminophenol, etc.) can be generated [13–25]. The structure and properties of the resulting polymer are highly influenced by the presence of these compounds, the generation of whom depends on the polymerization potential. Hence, a rigorous control of this parameter can avoid the generation of some undesired intermediates [26]. However, some compounds affecting the properties of the synthesized polymer are always generated. When the electropolymerization is performed in a flow cell, changes in the flow rate can lead to removal of some intermediate compounds allowing to generate a specific polymer. In this work, we have used a new spectroelectrochemical flow cell which has allowed us not only to control the flow rate during the electropolymerization but also to follow the intermediate ∗

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chemical species and, eventually, to test the electrochemical and optical performance of the resulting polymer. The new spectroelectrochemical flow cell operates in near normal incidence reflection mode [27,28]. One of the main advantages of the proposed cell is that commercial electrodes can be used. Since their introduction 40 years ago, spectroelectrochemical techniques have been extensively applied to fundamental studies of electrode process and related phenomena [25,29–35]. Therefore, spectroelectrochemical flow cells of small volume offer novel possibilities for enhanced analytical selectivity of characterization reactions in flow analysis. 2. Experimental 2.1. Chemicals All chemicals were reagent grade, used without further purification. Aqueous solutions have been prepared using high-quality water (MilliQ gradient A10 system, Millipore, Bedford, MA). 2.2. Instrumentation All the experiments were performed using the spectroelectrochemical flow cell shown in Fig. 1. The body of the cell is a

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Fig. 1. Scheme of the spectroelectrochemical flow cell.

cylinder of 22 mm diameter and 15 mm height, made of Teflon. The working electrode and an optic fibre spectroscopic reflection probe are tightly inserted in holes drilled in the opposite planar faces of the cylindrical cell body. The reflection probe (Avantes, The Netherlands) consists of seven 200 ␮m fibres, six illumination fibres around a central read fibre. Thus, light was irradiated and collected perpendicularly to the working electrode surface, sampling a spot of 1 mm2 approximately. The effective cell volume can be modified for each application by varying the distance between the reflection probe and the working electrode. Solution flowed through the cell via inlet and outlet tubes fitted into holes made in the cylinder walls, with the flow direction parallel to the working electrode surface. A conventional three-electrode system was placed in the cell body. A commercial gold working electrode (BAS Inc.) was used in this work, but other electrode materials (Pt, glassy carbon, . . .) could also be employed. A home-made Ag/AgCl microreference electrode and a Pt auxiliary electrode, both with a diametrer of 1 mm aproximately, were placed in the inlet and outlet channel, respectively, near the working electrode to minimize

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the ohmic drop. It is important to place the auxiliary electrode downstream at the outlet of the flow cell to avoid contamination of the cell chamber by reaction products generated at this electrode. Potential was controlled using a PGSTAT 10 (Eco Chemie/ Autolab, Utrecht, The Netherlands). The light beam, supplied by a halogen light source (Avalight-HAL, Avantes, The Netherlands) was both taken to the cell and collected from it through the reflection probe and conducted to an S2000 Fibre Optic Spectrometer from Ocean Optics (USA), made up of a 2048element diode array. The flow system consists of a Miniplus 3 peristaltic pump (Gilson S.A., France) equipped with polyvinyl chloride tubing of appropriate diameter (0.2–4.0 mm), a Valco multiposition valve (Tecknokroma, Spain) and the flow cell made in our laboratory. Using the peristaltic pump and the valve, six different solutions can be introduced into the flow cell. 3. Results and discussion 3.1. Electropolymerization All PANI films were prepared potentiostatically at + 0.9 V versus Ag/AgCl for 100 s in a 0.5 M HNO3 solution containing 0.1 M aniline. The potential applied during the polymerization was selected to generate the most common intermediate compounds and to avoid the overoxidation of the polymer. In order to study the influence of flow rate on the electropolymerization of aniline, experiments at five different flow rates (0, 0.26, 0.52, 0.78 and 1.04 mL min−1 ) were done. Fig. 2a shows the charge recorded versus time for the five experiments. The final value of charge passed during the experiments was not very different, but it decreased slightly when the flow rate was increased, as summarized in Table 1. The amount of charge passed at the highest flow rate was clearly smaller than the other ones. From the charge data, we could think that the

Fig. 2. (a) Chronocoulograms and (b) chronoabsorptograms at 720 nm corresponding to the electropolymerization of aniline at different flow rates: (䊉) 0.00, (×) 0.26, (+) 0.52, () 0.78, (—) 1.02 mL min−1 .

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Table 1 Values of charge passed during the electropolymerization at t = 100 s (Qt=100 ) and t = 10 s (Qt=10 ); absorbance at 720 nm at t = 100 s (At=100 ) and t = 10 s (At=10 ); time at which the absorbance at 720 nm reaches a local maximum within the first 10 s (tlocal max ) and wavelength of the maximum absorbance for PANI synthesized at the different flow rates: 0.00, 0.26, 0.52, 0.78, 1.02 mL min−1 (λmax ) Flow rate (mL min−1 )

Qt=100 (mC)

Qt=10 (mC)

At=100 (au)

At=10 (au)

tlocal max (s)

λmax (nm)

0 0.26 0.52 0.78 1.02

1.92 1.86 1.72 1.79 1.39

0.53 0.56 0.58 0.60 0.45

1.34 0.71 0.42 0.29 0.12

0.28 0.16 0.08 0.04 0.01

10.00 7.83 4.56 3.48 2.76

715 650 633 600 576

Data from Fig. 2 and Fig. 3c.

final amount of polymer generated does not vary significantly between experiments at flow rates lower than 0.78 mL min−1 . Higher flow values led to a clearly minor amount of electrogenerated polymer. Hence, we selected 1.04 as limiting flow rate in our experiments. However, absorbance values at 720 nm recorded throughout the potential steps (Fig. 2b) evidence a high dependence of the amount of PANI obtained on the flow rate (values in Table 1). Therefore, incorrect conclusions can be drawn if

only the electrochemical signal is taken into account. The polymer growth rate under hydrodynamic conditions is slower than the growth rate measured under identical conditions in quiet solutions, as was previously stated from rotating disk electrode experiments [37]. Inspection of the chronoabsorptometric data provides not only quantitative information about the generated polymer but also very valuable qualitative information. Analyzing

Fig. 3. Spectra corresponding to the electropolymerization of aniline at different flow rates: (䊉) 0.00, (×) 0.26, (+) 0.52, () 0.78, (—) 1.02 mL min−1 , and at different times: (a) tlocal max (as stated in column 6 of Table 1), (b) t = 25 s and (c) t = 100 s.

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the absorbance at 720 nm registered during the first 10 s of each experiment, we observed that in all the experiments an absorbance local maximum is reached before t = 10 s when convection occurs in the solution. Conversely, such maximum was not seen in the absence of flow (Table 1). A close inspection of the charge passed during the first 10 s indicates that the current at the beginning of the polymerization increased with the flow rate, Table 1, because the thickness of the diffusion layer decreased. However, absorbance values at 720 nm measured at t = 10 s dropped considerably with increasing flow rate (see Table 1), indicating that the absorbing reaction products were being removed from the electrode/solution interface. That is, during the first seconds of the electropolymerization most of the charge was spent in the generation of oligomers and soluble side-reaction products. When the synthesis is carried out in absence of flow, all these compounds can react further to generate polymer; but if a flow is introduced, soluble species like low molecular weight oligomers and side-reaction products can leave the diffusion layer and be removed from the electrode proximity due to the convective movement of the solution. Consequently, the local maximum of absorbance (attained within the first 10 s) marks the beginning of the process in which soluble compounds leave the diffusion layer and are pushed by the flow. This time was shorter as the flow rate was increased, i.e. as the thickness of the diffusion layer diminished, Table 1. Fig. 3a shows the spectra corresponding to the time when the local absorbance maximum at 720 nm (before t = 10 s) was attained for experiments at different flow rates. Table 1 summarizes the time (tlocal max ) when the spectra displayed in Fig. 3a were recorded. In all cases, two bands around 550 and 720 nm were observed, indicating that oxidized oligomers and radical dimmer cations [19,25,36] were initially formed. These bands tend to disappear at longer times with growing flow rates, proving that species with high diffusion coefficients are removed by the flow. Fig. 3b displays the spectra recorded at t = 25 s for the five flow rates. When the flow rate is zero, the band around 720 nm is the most distinctive feature in the spectrum. At a flow rate of 0.26 mL min−1 , a broad band appears around 600 nm, although the band at 720 nm is still apparent. From 0.52 mL min−1 upwards, only the band centered at around 600 nm is present in the spectra. This broad band can be most likely assigned to a mixture of precipitated oxidized polymer and larger oligomers which are very close to the electrode and are not depleted because of the hydrodynamic conditions. If the intermediate compounds remaining in the diffusion layer vary with the flow conditions of each electropolymerization, the resulting polymer has to be also different. This conclusion could not be drawn only from the electrochemical data, but the spectra provide very valuable qualitative information about the process. Fig. 3c shows the spectra at the end of each polymerization experiment, evidencing notable differences both in the structure and amount of PANI generated at each flow rate. The spectrum of the polymer synthesized without flow exhibits an absorption maximum peaking at 715 nm. A hypsochromic shift of the PANI absorbance band with increasing flow rate was observed (Table 1), proving that a different

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Fig. 4. Voltamograms corresponding to the fifth cycle of a series of potential scans between −0.1 and +0.8 V recorded during the characterization of the polymers synthesized at the different flow rates: (䊉) 0.00, (×) 0.26, (+) 0.52, () 0.78, (—) 1.02 mL min−1 .

PANI film was obtained in each case. Usually, the explanation of this phenomenon is based on the assumption of the generation of polymer chain segments of different effective conjugation lengths [30,38]. 3.2. Characterization of electrosynthesized PANI films Voltabsorptometric characterization of the PANI films electropolymerized at different flow rates was always performed without flow, in a 0.5 M HNO3 solution, cycling the electrode potential between −0.1 and +0.8 V five times to ensure the stabilization of the electrochromic response. Fig. 4 shows the fifth cyclic voltammogram of the different PANI films. The voltammetric response further evidences the smaller amount of PANI electrodeposited as increasing flows were used during the polyTable 2 Values of the ratio of the derivative absorbance of the second peak with respect to the first one (dAp2 /dAp1 ), and of the derivative absorbance of the second peak with respect to the third one (dAp2 /dAp3 ) obtained from the anodic and the cathodic scans recorded during the characterization of the PANI films synthesized at the different flow rates: 0.00, 0.26, 0.52, 0.78, 1.02 mL min−1 Flow rate (mL min−1 )

Anodic

Cathodic

dAp2 /dAp1

dAp2 /dAp3

dAp2 /dAp1

dAp2 /dAp3

(a) 600 nm 0 0.26 0.52 0.78 1.02

3.05 2.61 2.08 2.09 1.79

1.38 1.23 0.76 0.66 0.39

2.20 2.44 2.11 1.94 1.88

1.85 1.29 0.74 0.61 0.40

(b) 720 nm 0 0.26 0.52 0.78 1.02

1.89 1.50 1.26 1.17 1.15

3.94 4.56 2.41 1.94 1.16

1.30 1.33 1.37 1.26 1.09

5.14 3.59 2.34 1.93 1.12

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Fig. 5. Voltabsorptograms corresponding to the fifth cycle recorded during the characterization of the polymers synthesized: without flow (a) at 600 nm and (b) at 720 nm; at 1.02 mL min−1 flow rate (c) at 600 nm and (d) at 720 nm.

merization. Now the electrochemical response provides quantitative information that could not be obtained from the charge during the polymerization step. Voltammograms exhibit similar shapes in all cases and do not seem to reveal significant structural differences between films formed at different flow rates. Three reversible pairs of peaks can be distinguished at potentials around +0.15, +0.47 V and +0.73 V. The first and the third pairs have been ascribed to the polaron and bipolaron forms of the PANI [34]. The origin of the second peak is much more complex and it has been attributed to many different intermediate and degradation products (cross-linked polymer, benzoquinone, etc.) [13–25]. Some authors [15,20,39] refer to this pair of peaks with the term “middle peaks” to indicate the complexity of this part of the voltammogram. The non faradaic current influences notably the shape of the voltammogram, making it difficult to perform an in-depth analysis of the electrical signal in order to ascertain the effect of the flow rate on the resulting polymer. Only quan-

titative information about the amount of synthesized polymer can be easily extracted from the voltammetric response. On the contrary, the spectroscopic signal enables us to highlight the influence of the flow rate during the polymerization on the resulting film. Fig. 5 shows the corresponding derivative voltabsorptograms in the fifth cycle at two different wavelengths, 600 and 720 nm, obtained during the characterization of PANI films synthesized without flow (a and b) and at the highest flow rate (c and d). Both the shape and the peak intensity of the voltabsorptograms corresponding to the two electrogenerated polymers are remarkably different. The figure confirms that PANI films with different electrochromic behavior have been generated. A close inspection of the dA/dt peak values, dAp , in the derivative voltabsorptograms reveals that the ratios dAp2 /dAp1 , i.e. “middle peak” to first one, and dAp2 /dAp3 , i.e. “middle peak” to third one, are completely distinct. Table 2 shows these ratios of the derivative absorbances obtained for the anodic and cathodic peaks at the wavelengths of

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(a) 600 and (b) 720 nm. In all cases, the higher the flow rate during the electropolymerization, the lower the ratio between the second and first (or third) peaks, indicating a smaller proportion of intermediate and degradation products. Therefore, not only the spectral shape but also the electro-optical behavior of the polymers is very different. From the ratios in Table 2, we can deduce that control over the flow rate of aniline solution during the polymerization enables us to remove most of the soluble intermediate compounds from the film, which can smear out the PANI typical features, provoking a substantial change in the film absorbance spectrum. In the polymer synthesized at the highest flow rate, side-reaction products are not easily retained, and the second peak could be most likely ascribed to cross-linked polymer. 4. Conclusions The spectroelectrochemical flow cell presented here allows us to perform a more controlled electropolymerization in order to obtain a polymer with desired specific electrochromic properties. Thus, incorporation of degradation and side-reaction products into the electrodeposited polymer film can be minimized by increasing the flow rate of feeding monomer solution. This was confirmed by the voltammetric responses of the resulting PANI films, in which the “middle peaks” ascribed to side-reaction and degradation products diminished as the flow rate in the polymerization process was increased. Moreover, the effect of the flow is manifest in the absorption spectra of the resulting films. The PANI absorbance band shifts hypsochromically with increasing flow, evidencing the formation of films with a distribution of polymer chain segments of different effective conjugation lengths. The main aim of these results is to illustrate the possibilities that the new spectroelectrochemical flow cell offers in order to tailor the properties of a synthetic material for a specific application. Acknowledgements Junta de Castilla y Le´on (BU011A05) and Ministerio de Educaci´on y Ciencia (MAT2003-07440) are gratefully acknowledged. V.R. also thanks the Ministerio de Educaci´on y Ciencia for a Juan de la Cierva contract. References [1] H. Xue, C. Li, Z. Shen, Biosens. Bioelectron. 20 (2005) 2330. [2] I. Jureviciute, A. Malinauskas, K. Brazdziuviene, L. Bernotaite, B. Salkus, Sens. Actuators B Chem. 107 (2005) 716.

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[3] N.G.R. Mathebe, A. Morrin, E.I. Iwuoha, Talanta 64 (2004) 115. [4] M. Snejdarkova, L. Svobodova, G. Evtugyn, H. Budnikov, A. Karyakin, D.P. Nikolelis, T. Hianik, Anal. Chim. Acta 514 (2004) 79. [5] M. Trojanowicz, Microchim. Acta 143 (2003) 75. [6] D. Nicolas-Debarnot, F. Poncin-Epaillard, Anal. Chim. Acta 475 (2003) 1. [7] X. Zhang, B. Ogorevc, J. Wang, Anal. Chim. Acta 452 (2002) 1. [8] P.J. O’Connell, C. Gormally, M. Pravda, G.G. Guilbault, Anal. Chim. Acta 431 (2001) 239. [9] A.A. Karyakin, L.V. Lukachova, E.E. Karyakina, M. Vuki, J. Wang, A.V. Orlov, G.P. Karpachova, Anal. Chem. 71 (1999) 2534. [10] U.W. Grummt, A. Pron, M. Zagorska, S. Lefrant, Anal. Chim. Acta 357 (1997) 253. [11] A.A. Karyakin, O.A. Bobrova, L.V. Lukachova, E.E. Karyakina, Sens. Actuators B Chem. 33 (1996) 34. [12] A.A. Karyakin, I.A. Maltsev, L.V. Lukachova, J. Electroanal. Chem. 402 (1996) 217. [13] E.M. Genies, C. Tsintavis, J. Electroanal. Chem. 195 (1985) 109. [14] E.M. Genies, M. Lapkowski, J. Electroanal. Chem. 236 (1987) 189. [15] E.M. Genies, M. Lapkowski, J.F. Penneau, J. Electroanal. Chem. 249 (1988) 97. [16] S.N. Hoier, S.M. Park, J. Electrochem. Soc. 140 (1993) 2454. [17] C.Q. Cui, L.H. Ong, T.C. Tan, J.Y. Lee, Electrochim. Acta 38 (1993) 1395. [18] L. Duic, Z. Mandic, S. Kovac, Electrochim. Acta 40 (1995) 1681. [19] B.J. Johnson, S.M. Park, J. Electrochem. Soc. 143 (1996) 1277. [20] L.D. Arsov, J. Solid State Electrochem. 2 (1998) 266. [21] A. Malinauskas, R. Holze, Electrochim. Acta 43 (1998) 2413. [22] A. Zimmermann, U. K¨unzelmann, L. Dunsch, Synth. Met. 93 (1998) 17. [23] M.I. Boyer, S. Quillard, M. Cochet, G. Louarn, S. Lefrant, Electrochim. Acta 44 (1999) 1981. [24] L. Jiang, Q.J. Xie, L. Yang, X.Y. Yang, S.Z. Yao, J. Colloid Interface Sci. 274 (2004) 150. [25] S.Y. Hong, Y.M. Jung, S. Bin Kim, S.M. Park, J. Phys. Chem. B 109 (2005) 3844. [26] E.M. Genies, M. Lapkowski, J. Electroanal. Chem. 236 (1987) 189. [27] C.H. Pyun, S.M. Park, Anal. Chem. 58 (1986) 251. [28] C.J. Zhang, S.M. Park, Anal. Chem. 60 (1988) 1639. [29] A. Neudeck, A. Petr, L. Dunsch, Synth. Met. 107 (1999) 143. [30] A. Malinauskas, R. Holze, J. Appl. Polym. Sci. 73 (1999) 287. [31] J. Lopez-Palacios, A. Colina, A. Heras, V. Ruiz, L. Fuente, Anal. Chem. 73 (2001) 2883. [32] V. Ruiz, A. Colina, A. Heras, J. Lopez-Palacios, R. Seeber, Helv. Chim. Acta 84 (2001) 3628. [33] V. Ruiz, A. Colina, A. Heras, J. Lopez-Palacios, R. Seeber, Electrochem. Commun. 4 (2002) 451. [34] H.S. Shim, I.H. Yeo, S.M. Park, Anal. Chem. 74 (2002) 3540. [35] L. Pigani, A. Heras, A. Colina, R. Seeber, J. Lopez-Palacios, Electrochem. Commun. 6 (2004) 1192. [36] Y.-B. Shim, M.-S. Won, S.-M. Park, J. Electrochem. Soc. 137 (1990) 538. [37] D.E. Stilwell, S.M. Park, J. Electrochem. Soc. 135 (1988) 2254. [38] V. Brandl, R. Holze, Phys. Chem. Chem. Phys. 101 (1997) 251. [39] J. Kan, R. Lv, S. Zhang, Synth. Met. 145 (2004) 37.