Elaboration and characterization of polyaniline films electrodeposited on tin oxides

Synthetic Metals 161 (2011) 2162–2169

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Elaboration and characterization of polyaniline films electrodeposited on tin oxides C.C. Buron, B. Lakard ∗ , A.F. Monnin, V. Moutarlier, S. Lakard Institut UTINAM, UMR CNRS 6123, University of Franche-Comté, 16 route de Gray, 25030 Besanc¸on Cedex, France

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Article history: Received 11 May 2011 Received in revised form 9 August 2011 Accepted 9 August 2011 Available online 26 August 2011 Keywords: Polyaniline Tin oxides Electrochemistry Surface analysis Atomic force microscopy

a b s t r a c t This paper presents the electrochemical synthesis of polyaniline films on fluorine tin oxide (FTO) and indium tin oxide (ITO) in acidic medium by both potentiodynamic and potentiostatic methods. The use of potentiodynamic deposition showed that conductive polyaniline films were synthesized by the same process than on noble metals and allowed the determination of the electrodeposition potential used for further potentiostatic electrodepositions. The electropolymerization was then performed by chronoamperometry and appeared easier on ITO than on FTO. The electrodeposited polyaniline films were extensively characterized in terms of chemical nature, electrochemical growth, thickness, roughness and morphology. The chemical nature of the films was identified through infrared spectroscopy and X-ray diffraction analyses. The roughness and the morphology of the polymer coatings, determined by profilometry, scanning electron microscopy and atomic force microscopy, were correlated to the calculated thickness of each polyaniline film and to the nature of the substrate. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Conducting polymers have been studied extensively during the last two decades because of their possible applications as sensors [1–3], corrosion inhibitors [4,5], electrochromic devices [6,7], organic light emitting diodes [8], organic solar cells [9] and so forth. Among the conducting polymers, polyaniline (PANI) has been widely studied in the last decades due to its simple synthesis, stability in ambient environment, unique electrochemical properties and possibility of deposition on various substrates like platinum or carbon [10]. In particular, the emeraldine form of polyaniline, has gained interest among the scientific community for its easy synthesis, high stability, and interesting combination of redox [11] and proton doping [12,13] properties. PANI films can be synthesized by chemical oxidative reaction of aniline [14,15], by electropolymerization of aniline [12,13,15] or by electroless autocatalytic oxidation of aniline [15–18]. However, if the electrochemical synthesis of polyaniline on platinum substrates has been widely studied, this is not the case of the electrodeposition on metal oxides such as fluorine tin oxide (FTO) or indium tin oxide (ITO) despite the fact that these latter materials could be interesting substrates. Indeed, indium tin oxide is a conductive material that can be used as substrate for the deposition of PANI films. However, very few works are reported in the liter-

∗ Corresponding author. Tel.: +33 3 81 66 20 46. E-mail address: [email protected] (B. Lakard). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.08.021

ature that used PANI-coated ITO. In these works, PANI films were deposited using three different ways. A first way used was the electrochemical deposition of PANI on ITO, mainly for electrochromic applications [19]. A second method to modify ITO substrates by PANI films consisted in the chemical oxidation of PANI films for incorporation in sensors dedicated to mercury detection [20]. A third method used to modify ITO substrates by PANI films was the electrophoretic deposition of PANI films used as sensitive layers for cholesterol biosensing [21]. Fluorine tin oxide (FTO) is another interesting substrate since the FTO is more stable than the ITO in acidic conditions and since the FTO is of excellent transparency. At the moment, electrodeposited PANI-coated FTO has only been used to elaborate a hydrogen peroxide sensor [22], to study the fluorescence emission of PANI films [23] and to develop counter electrode for dye-sensitized solar cell [24]. Since we would like to develop applications in biology, such as enzymatic biosensors or biological cell culture substrates, using both the properties of polyaniline (conductivity, amino groups useful for sensing or adhesion of enzymes and biological cells) and the properties of tin oxides (electrical conductivity, optical transparency allowing to follow easily the immobilization of enzymes or the adhesion and growth of biological cells on the substrates), it was necessary to perform the electropolymerization of PANI on tin oxides to determine the surface properties of the electrodeposited PANI films. Consequently, the first aim of this work was to perform the electrodeposition of polyaniline on FTO and ITO in acidic medium by both potentiodynamic and potentiostatic methods. Then, after

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examination of the nature of the polymer films using InfraRed Reflection Absorption Spectroscopy (IRRAS) and X-ray diffraction (XRD), the polyaniline films were characterized using electrochemistry, profilometry, scanning electron microscopy (SEM) and atomic force microscopy (AFM). These techniques allowed us to access the surface features (thickness and roughness) and morphologies of the films, but also to evaluate the influence of the substrate on these parameters. PANI films of different thickness were also studied to determine the influence of the thickness on the film features and to follow the nucleation and growth mechanism of PANI on the tin oxides.

2. Experimental

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2.2. Surface characterization Polyaniline samples were characterized by InfraRed Reflection Absorption Spectroscopy (IRRAS) and X-ray diffraction (XRD). Polymer samples were first investigated in reflection geometry under a grazing-incidence angle of 71◦ using a Vertex 70 FT-IR spectrometer equipped with a DTGS detector. Then, the structure of the polyaniline films was investigated by X-ray diffraction (XRD). XRD analyses were performed using a D8 Advance Bruker system with Cu K␣ radiation ( = 0.154 nm) at 40 kV and 40 mA. The angular 2 range was 15–30◦ in steps of 0.01◦ and the scan speed was 0.1 s/degree. The d-spacings were deduced from the Bragg equation:  = 2 (dspacing) sin , where  is the wavelength of the X-ray, 2 is the X-ray scattering angle and d-spacing is the inter-planar spacing. The roughness of the polymer films was determined by stylusbased mechanical probe profiler (Alpha-Step IQ, KLA Tencor). Both average roughness (Ra ) and peak to peak roughness (Rq ) were estimated on a scan length of 1000 ␮m at a scan speed of 20 ␮m s−1 . Polymer surface morphology examinations were performed using a high-resolution scanning electron microscope. Once synthesized and dried, polymer samples were examined in a LEO microscope (Scanning Electron Microscopy LEO stereoscan 440, manufactured by Zeiss–Leica, Köln, Germany) with an electron beam energy of 15 keV.

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Analytical grade aniline was freshly distilled under reduced pressure and stored in dark at low temperature (4 ◦ C). Double deionized water (Milli-Q, resistivity 18 M cm) and analytical grade hydrochloric acid were used to prepare the electrolyte solutions. Aniline was used at the concentration of 0.4 mol L−1 in an aqueous solution of 1.2 mol L−1 HCl. The substrates used for polymer deposition were either FTO substrates (from Balzers, R = 120 , thickness = 2 mm) or ITO substrates (from Solems, R = 40 , thickness = 110 nm) deposited on Si (1 0 0) wafers. The substrate’s area coated by PANI films was 1.2 cm × 1.5 cm for FTO substrates, and 1.0 cm × 0.7 cm for ITO substrates. The electrochemical experiments were carried out with a Voltalab potentiostat/galvanostat PGZ301 (Radiometer, France) coupled to a computer running the VoltaMaster software. All electrochemical measurements were performed using a single-compartment cell with three electrodes, at room temperature. The electrodes used were either a FTO or a ITO substrate as the working electrode, a Saturated Calomel Electrode (SCE) as the reference electrode (Metrohm), and a platinum wire (Metrohm) as the auxiliary electrode. The electropolymerization of polyaniline was performed either by cyclic voltammetry, with a sweep rate of 100 mV s−1 between −0.2 and +1.2 V/SCE, or by chronoamperometry by applying an electrodeposition potential of +900 mV/SCE.

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The imaging of the surface topographies was performed, with a commercial atomic force microscope (AFM PicoSPM from Molecular Imaging, USA), in contact mode with aluminium coated silicon tip. The Si rectangular AFM cantilever was 450 ␮m-long and its stiffness was about 0.27 N m−1 . The experiments were done at the air and at room temperature. 3. Results and discussion 3.1. Electrochemical polymerization of polyaniline on tin oxides 3.1.1. Potentiodynamic deposition PANI films were grown on a FTO surface by sweeping the potential between −0.2 and +1.2 V/SCE at a scan rate of 100 mV s−1 in a solution containing 0.4 mol L−1 aniline and 1.2 mol L−1 HCl. Fig. 1a shows the cyclic voltammogram (CV) obtained during the potentiodynamic electropolymerization of PANI. The first scan (inset of Fig. 1a) exhibited an anodic peak corresponding to the oxidation of the aniline monomers at +0.9 V/SCE that initiated the electropolymerization of PANI. The following scans showed the peaks corresponding to the oxidation and reduction of the polyaniline films (Fig. 1a). A very similar cyclic voltammogram was obtained on an ITO substrate (Fig. 1b). Indeed, the same anodic and cathodic peaks were observed at similar potentials. Only, the value of the current density, J, was really different since the measured current density was four times more important during the electropolymerization of PANI on ITO than during the deposition on FTO

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(Jmax = 0.45 A cm−2 for FTO surfaces and Jmax = 1.80 A cm−2 for ITO surfaces). These voltammograms were different from the CVs usually obtained by electropolymerization of PANI on platinum electrodes that commonly exhibited well-defined redox peaks corresponding to a series of redox transitions: oxidation of the fully reduced insulating form to its radical cation (polaron), followed by oxidation of degradation products and/or intermediate species, and finally the transition from the delocalized polaronic state to a localized bipolaron or quinoid form. However, the voltammogram of Fig. 1a is in very good agreement with those already reported in the literature for the electropolymerization of PANI on FTO [22–24] (no CV corresponding to the electropolymerization of PANI on ITO was found in the literature). The difference of behaviour on tin oxides and Pt surfaces can be attributed to the difference of conductivity of the substrates that renders the electropolymerization easier on Pt that on tin oxides substrates [25–27]. It can also be noted that the PANI films electrodeposited here were conductive since the anodic and cathodic current density increased with the number of scans. 3.1.2. Potentiostatic deposition Polyaniline films were electrosynthesized, at a fixed potential of +900 mV/SCE, from aqueous solutions containing 0.4 mol L−1 aniline and 1.2 mol L−1 HCl on FTO or ITO substrates. Chronocoulometry (Fig. 2) was used to synthesize the polymer films since this technique allowed the determination of the charge quantity brought to the electrode in order to control the amount of deposited polymer and the thickness of the deposited films. To study the influence of the substrate on the electrochemical behaviour of PANI but also the influence of the polymer thickness, different films were grown, on ITO and FTO, during various times of electrodeposition to elaborate PANI films with various thicknesses. The eight PANI films presented here (4 being grown on FTO and 4 on ITO substrates) were chosen since they led to the most significant interpretations. Moreover, it can be deduced from the cyclic voltammograms of Fig. 1 that a potential of +900 mV/SCE allowed the oxidation of both aniline and PANI. That is why this value was taken as electrodeposition potential for the potentiostatic electropolymerization. The chronoamperometric curves (Fig. 2) obtained on FTO and ITO had the same general shape, typical of a polymerization mechanism. Indeed, Fig. 2 exhibited first a sudden increase of the current density, corresponding to the formation of radical cations by oxidation of aniline, then a steady-state was reached corresponding to the growth of PANI films by oxidation of the oligomers of PANI. On the contrary, the current density was very different depending on the nature of the substrate since it reached 3.7 mA cm−2 on ITO substrates but only 1 mA cm−2 on FTO surfaces, probably because ITO

Fig. 3. IRRAS spectra of PANI films electrodeposited on ITO (a) and FTO (b) substrates.

substrates had a better conductivity than FTO ones (the electrical resistance of naked ITO and FTO were measured and estimated to be 40 and 130 , respectively). 3.2. Characterization of polyaniline films 3.2.1. Infrared spectroscopy InfraRed Reflection Absorption Spectroscopy (IRRAS) was used to perform spectra of electrodeposited polyaniline films in the region of 4000–400 cm−1 at ambient temperature. Fig. 3 was obtained by subtracting the IR spectrum of PANI-modified FTO or ITO and the IR spectrum of the corresponding naked substrate. Thus Fig. 3 exhibits only the characteristic vibration bands of PANI films. The different vibrational bonds obtained in the IRRAS spectra were assigned by comparing their values with theoretical values and with already published spectra of PANI [28,29]. Fig. 3a describes the IR spectrum of PANI-modified ITO and contains the characteristic peaks of N–H stretching mode at 3238 cm−1 , C–H aromatic stretching mode at 3109 cm−1 and C–H aliphatic stretching mode at 2925 cm−1 and 2856 cm−1 , C C stretching mode of the quinoid rings at 1556 cm−1 , C C stretching mode of the benzenoid rings at 1476 cm−1 , C–N stretching mode at 1397 cm−1 and 1268 cm−1 , C–H deformation at 1194 cm−1 , C–H aliphatic wagging mode at 743 cm−1 and 613 cm−1 . Nearly all these vibration bands were also present in the spectrum of PANI films electrosynthesized on FTO substrates (Fig. 3b). Moreover, the vibrations bands were sharp and well-defined in the IRRAS spectrum of PANI-modified ITO when they were large in the spectrum of PANImodified FTO. This is because ITO-coated Si is a good mirror leading to a good reflection of the laser’s signal and so to an IRRAS spectrum having a good resolution. On the contrary, FTO is totally transparent and led to a dissipation of the signal and so to an IRRAS spectrum having a worst resolution. These spectra correspond very well to

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Fig. 4. XRD patterns of the PANI films electrodeposited on FTO (a) and ITO (b) substrates.

the theoretical spectrum of polyaniline and with the spectra of PANI already published [28,29]. It can thus be concluded that the polymer coatings electrodeposited on FTO and ITO substrates are polyaniline films. Moreover, these PANI films have a very similar chemical structure whatever the substrate used. 3.2.2. XRD analysis Fig. 4 shows the XRD patterns of polyaniline films electrodeposited on FTO (a) and ITO (b) substrates from an aqueous solution containing aniline and HCl. The PANI–ITO sample showed two main peaks that were not present on the XRD pattern of the naked ITO. The first peak is located at 20.1◦ 2 value and can be attributed to polyaniline. However, polyaniline must be considered as an amorphous material and the presence of peaks only indicates the partial crystalline nature of PANI doped by HCl. Consequently, the peaks observed in the XRD patterns imply the presence of a rigid chain and localized ordered structure, resulting in partial crystallinity. However, the rigid benzene ring on the molecular chain prevents formation of a completely crystalline structure. Compared with ordered crystals, the crystallinity of PANI is low and its crystalline regions are only small. Taking this into account, the peak at 20.1◦ 2 value can be linked with an orientation (1 1 0), with a d-spacing of 0.415 nm. The second peak, located at 23.8◦ 2 value, is due to the Cl− counter-ions that were incorporated in the polyaniline film during its electropolymerization. The PANI–FTO sample exhibited a sharp and intense peak at 22.8◦ 2 value. Another peak is located at 26.7◦ 2 value but this broaden peak was already present in the XRD pattern of naked FTO, so this peak must be attributed to the FTO substrate. The peak at 22.8◦ 2 value is characteristic of polyaniline with an orientation (2 0 −1), with a d-spacing of 0.389 nm, in the crystalline regions of the polymer film. Surprisingly, this XRD pattern did not reveal the presence of the Cl− ions, probably because Cl− ions were more entrapped in the PANI film deposited on FTO than in the PANI film deposited on ITO, because of the higher porosity of the PANImodified FTO coatings, as proved by the SEM images (see Section 3.4.). The attribution and location of these peaks are concordant with the literature since it has already been shown that polyaniline films deposited on FTO substrates lead to a peak at 20–25◦ 2 that represents the characteristic distance between the ring planes of benzene rings in adjacent chains or the close-contact interchain distance due to pi–pi stacking interactions [30,31], and since it was

shown that the presence of dopants, such as Cl− , SO4 2− or ClO4 − , in the polyaniline films polymerized from acidic solutions of aniline leads to peaks at around 24◦ 2 [31]. 3.3. Thickness and roughness of polyaniline films 3.3.1. Thickness The time of electropolymerization was varied to synthesize polyaniline films of different thickness either on FTO or on ITO substrates. Then, for each sample, the thickness of the PANI film was estimated assuming a current efficiency for the electropolymerization process of 100%, and using Faraday’s law of electrolysis: t = QM/AzF, where t was the film thickness (cm), Q was the electrical charge associated with polyaniline formation (C), M was the molar mass of the aniline monomer (93.13 g mol−1 ), F was the Faraday constant (96,500 C mol−1 ), A was the area of the working surface (cm2 ),  was the density of the polymer (1.4 g cm−3 ) and z was the number of electrons involved which is in this case of 2.5 [26,32]. Using Faraday’s law, it appeared that the required charge density to grow a film with an average thickness of 0.1 ␮m was 36.3 mC cm−2 . Thus, Table 1 gathered the film thickness of each polyaniline film estimated from electrical charge densities associated with aniline oxidation by application of Faraday’s law. The thicknesses calculated for PANI-modified ITO were comprised between 0.89 ␮m and 8.29 ␮m, when the thicknesses obtained for FTO-modified were comprised between 0.39 ␮m and 8.01 ␮m. Since the current density observed in the cyclic voltammograms and chronoamperometric curves during the electropolymerization of PANI was superior on ITO than on FTO, the calculated thickness corroborated the electrochemical data. Moreover, the regular increase of the polymer thickness with the increase of the time of Table 1 Calculated thickness of PANI films electrodeposited on FTO or ITO substrates. Sample

Calculated thickness (Faraday’s law) (␮m)

ITO 2 ITO 5 ITO 10 ITO 15 FTO 2 FTO 10 FTO 15 FTO 30

0.89 3.19 6.80 8.29 0.39 1.02 2.32 4.01

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electropolymerization confirmed the conductive properties of the PANI films. 3.3.2. Roughness To obtain additional information about the surface characteristics of the PANI films, the polymer films roughness was investigated using a stylus-based mechanical probe profiler (Table 2) enabling surface mechanical scanning without damaging it. Considering PANI-modified ITO substrates, the average roughness increased with the time of electropolymerization from 0.264 ␮m for the thinnest film to 1.152 ␮m for the thickest one (the same trend was observed for peak to peak roughness). Moreover, it was possible to calculate the ratio between the average thickness and the roughness of the films to determine if this roughness was impor-

Table 2 Measured roughness of PANI films electrodeposited on FTO or ITO substrates. Sample

Average roughness (Ra) (␮m)

Peak to peak roughness (Rq) (␮m)

ITO 2 ITO 5 ITO 10 ITO 15 FTO 2 FTO 10 FTO 15 FTO 30

0.264 0.388 0.619 1.152 0.219 0.454 0.654 0.762

0.334 0.499 0.742 1.383 0.269 0.575 0.829 0.948

Fig. 5. SEM images of PANI films grown on ITO and FTO substrates (magnification: 3000×).

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Fig. 6. 2D deflection images and 3D topography images obtained for PANI films grown on ITO and FTO substrates (size of the scanned surface: 20 ␮m × 20 ␮m).

tant compared to the thickness of the sample. These calculations led to a ratio of 25% for ITO 2 and to a ratio comprised between 9 and 11% for other PANI-modified ITO films. Consequently, the roughness was proportionally the most important for the thinnest film, probably because the nucleation process took place. On the contrary, for the three thickest films, the nucleation process was finished and the growth mechanism occurred, covering progressively and homogeneously the whole surface of the substrate. That is why the roughness was less important for these latter PANI films.

Now considering PANI-modified FTO substrates, the average roughness increased too with the time of electropolymerization from 0.219 ␮m for the thinnest film to 0.762 ␮m for the thickest one. But once again the ratio between the average thickness and the roughness decreased progressively, from 56% for FTO 2 to 19% for FTO 30 films, when the thickness increased. Consequently, once again, if the roughness increased with the time of electropolymerization, the ratio roughness/thickness decreased, probably for the same reasons than on ITO (nucleation and growth processes). This

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will be confirmed in the next part by the AFM images. It can finally be observed that the roughness values of PANI-modified ITO and PANI-modified FTO were not very different from each other. 3.4. Morphology of polyaniline films 3.4.1. Scanning electron microscopy (SEM) Fig. 5 shows the SEM micrographs of PANI films grown, from a strongly acidic aqueous solution of 1.2 M HCl, on ITO and FTO substrates and for different times of electropolymerization. The PANI coatings exhibited a sponge-like, branched, porous structured, high surface area polymer film, this structure being typical of an electrodeposited PANI film [33,34]. More precisely, PANI films grown on ITO surfaces consisted of nanowire networks in which the individual wire structure varied depending on the time of electropolymerization. The diameter of the individual wires was estimated to be less than 50 nm for ITO 2 and to be around 100 nm for ITO 15 . Moreover, the length of the nanowire networks was up to several micron meters. It was also observed that the nanowires appeared to be formed individually to cover the electrode surface first, then successive layers were formed from new grown nanowires. Hence, the surface of the film appeared to be relatively smooth with high porosity due to the intertwined nanowire network. The structure of the PANI films grown on FTO substrates was very similar and also consisted in a porous structure made of intertwined nanowires. The main difference came from the less important thickness of the PANI-modified FTO compared to PANI-modified ITO that led to a more dense structure and less porous structure on ITO than on FTO surfaces. Moreover, the diameter of the PANI nanowires deposited on FTO surfaces was estimated to be 200 nm for the thickest films. This was superior to the diameter of the PANI nanowires deposited on ITO surfaces. To explain the morphology of PANI films, the electropolymerization mechanism must be taken into account. Indeed, during the electropolymerization of aniline in strong acidic solution (pH ≤ 0), all aniline molecules existed in protonated form which allowed fast reaction kinetics to produce high concentration of cation radicals by lose of an electron. These cation radicals could then quickly delocalize the lone electron to the para-position and react with other cation radicals to form head to tail addition product. A rapid build-up of materials locally rendered spontaneous homogeneous nucleation to occur which did not require preferential nucleation sites but needed a local concentration gradient to build up prior to the onset of nucleation. This process occurred randomly and the result was the formation of long elongated PANI nanowires. 3.4.2. Atomic force microscopy (AFM) In addition to these SEM pictures, AFM images of the different PANI films have been done. Fig. 6 gathers the 2D deflection images and 3D topography images obtained by contact AFM. Deflection images of PANI-modified ITO showed a regular structure whatever the time of electrodeposition is. For example, black zones, corresponding to the pores of the PANI films, were regularly distributed all over the coatings confirming the porous structure observed by SEM. It can also be noticed from both 2D deflection and 3D topography images that the PANI films were more and more homogeneous when the time of electropolymerization increased. Considering the AFM images of PANI-modified FTO images, it can be deduced that FTO 2 corresponded to the nucleation step of the PANI electropolymerization since the AFM image exhibited some islands on the surface corresponding to the nucleation sites but no regular coating, thus corroborating the data obtained through profilometry. On the contrary, the other films of PANI-modified FTO were more regular and corresponded to the growth step of

the reaction. It can be noticed that PANI films were less homogeneous on FTO substrates than on ITO substrates since more valleys and mountains were observed on the AFM images. Once again, the homogeneity of the PANI film increased with the time of electropolymerization.

4. Conclusion The present work was dedicated to the study of polyaniline electrochemical polymerization in acidic environment on fluorine tin oxide and indium tin oxide. It was demonstrated that the electrosynthesis of PANI films was possible on these oxides both by potentiodynamic and potentiostatic methods. Resulting PANI films were conductive and the current density was greater on ITO substrates. This was confirmed by the determination of the thickness of PANI films electrodeposited during different times of electropolymerization at a fixed potential. The roughness of the films was also measured and correlated with AFM images of the coatings. It appeared that, after a nucleation step, the PANI films grew homogeneously on both ITO and FTO substrates. Moreover, the structure of PANI exhibited a sponge-like, branched, porous structured, high surface area polymer film. Hence, the surface of the film appeared to be relatively smooth with high porosity due to the intertwined nanowire network. This study demonstrated the possibility to electropolymerize PANI films on tin oxides using similar electrochemical parameters than on platinum. The surface properties obtained were also comparable to those obtained on platinum. These properties, associated with the electrical conductivity and optical transparency of tin oxides, are interesting to develop new applications as biosensors, using the reactivity of PANI and its porous structure than can enhance the immobilization of biomolecules in the pores of the polymer, or as cell culture substrates, using the affinity of the amino groups of PANI for proteins and cells and using the transparency of tin oxides to follow their adhesion and proliferation.

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