Electropolymerization of pyrrole in propylene carbonate on zinc electrodes modified by heteropolyanions

SVIITH|TIIC flII|TRLS Synthetic Metals 65 (1994) 2%34

ELSEVIER

Electropolymerization of pyrrole in propylene carbonate on zinc electrodes modified by heteropolyanions B. Zaid, S. Aeiyach, P.C. Lacaze* lnstitut de Topologie et de Dynamique des Syst~mes de l'Universit~ Paris 7 - Denis Diderot, Associd au CNRS-URA 34, 1 m e Guy de la Brosse, 75005 Paris, France

Received 11 October 1993; in revised form 13 February 1994; accepted 4 March 1994

Abstract

The electropolymerization of pyrrole on a zinc electrode was studied in different organic and aqueous media, such as acetonitrile (MeCN), propylene carbonate (PC), methanol (MeOH) and water in the presence of various ammonium salts with PF6-, CIO4-, Br-, BF4- or pars-toluene sulfonate (TsO-) anions. Polypyrrole (PPy) film formation was achieved only in PC with TsO- anions and when the zinc electrode was chemically or electrochemically treated with heteropolyanions (HPA), such as (H2WlsO56F6)Hs-9H20, P2W18062H6-9H20 and PWI204oH3. In other media, electropolymerization failed and the zinc electrode was oxidized to Zn 2÷. Analysis of the films by IR, Raman, XPS and SEM confirmed that standard PPy films were obtained with a conductivity of about 30 S cm-1. Adhesion of PPy to the substrate was relatively poor but could be improved drastically by thermal treatment. Keywords: Electropolymerization; Pyrrole; Zinc; Electrodes; Heteropolyanions; Surfaces

1. Introduction

Over the past decade, organic conducting polymers have attracted considerable interest because of their unique physicochemical properties and the large variety of their technological applications in electrochromic displays, chemical and electrochemical sensors, 'plastic' electronic devices, batteries, etc. [1]. Recently, a new field of application has been envisaged: the protection of oxidizable metals against corrosion [2-6]. Polypyrrole (PPy), the nuclei of which are well known for their inhibitory corrosion properties [7] and which can be deposited by electropolymerization of pyrrole on a metallic substrate, is a very promising candidate [2-6]. However, the adaptation of the electropolymerization procedure to common metals constitutes one of the major difficulties to be overcome. Indeed, the high thermodynamic oxidation potential of pyrrole is incompatible with the oxidation potential of usual metals, which normally dissolve before the monomer oxidizes and, thus, prevents the formation of the polymer film. *Corresponding author.

0379-6779/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)02092-D

In order to achieve the electropolymerization of pyrrole on oxidizable metals it is necessary to reduce the rate of dissolution of the metal. By a suitable choice of the solvent and of the salt, this aim can be achieved, making it possible to electropolymerize pyrrole on metals like Fe, Cu and AI [2-6]. In the search for new treatments capable of slowing the corrosion of the metal without preventing electropolymerization of pyrrole, we envisage in this work to test a new surface treatment of Zn based on the use of heteropolyanions, which are well known for their ability to anchor to metallic surfaces and to catalyse electrochemical reactions [8]. Adsorption of heteropolyanions and oxymetalates on metallic surfaces has been widely studied by Keito and Nadjo [8]. They investigated glassy carbon electrodes modified by these compounds and showed that such surfaces were quite stable and could catalyse oxygen and proton reduction. Moreover, it has been proved that Keggin-type and Dawson-type anions can be used as immobilized dopants in the conducting polymer matrix [9-11], and thus could a priori constitute a good binder for the metallic surface and the doped PPy.

28

B. Zaid et al. / Synthetic Metals 65 (1994) 27-34

In this work, we show that polypyrrole films can be obtained on a Zn surface without provoking the dissolution of the metal. Such a result is achieved if the surface is submitted to a preliminary chemical or electrochemical treatment by heteropolyanions (HPA) and if the electropolymerization of pyrrole is carried out in propylene carbonate (PC) in the presence of paratoluene sulfonate (TsO-).

Weight measurements on PPy samples were carried out by using a Mettler AE 163 balance ( + 10 -5 g). 3. Results and discussion

The electrochemical behaviour of Zn electrodes was studied in several organic electrolyte media either in the absence or in the presence of pyrrole.

3.1. Surface modification procedure 2. Experimental

2.1. Chemical reagents

Heteropolyanions, P2W18062H6•91-120 and (H2WlsO56F6)Ha'9H20, were prepared according to the methods previously described in the literature [12,13]. Pyrrole (Janssen, 99%) was distilled twice under N2 prior to use, and water was purified by passing distilled water through a Millipore water purification system. PW12040Ha'nH20 (Fluka, Puriss), tetrabutylammonium and tetraethylammonium para-toluene sulfonate (TBATsO and TEATsO), tetrabutylammonium perchlorate (TBAC104), tetrabutylammonium fluoroborate (TBABF4) and LiAsF6 (all from Aldrich), tetrabutylammonium bromide (TBABr) and tetrabutylammonium hexafiuorophosphate (TBAPFr, Merck), sodium and potassium oxalate (Fluka), methanol (MeOH) and propylene carbonate (PC, Janssen), and acetonitrile (MeCN, Aldrich, 99%) were used without further purification. 2.2. Cell, electrochemical equipment and analysis techniques The electrochemical cell for cyclic voltammetric and galvanostatic experiments was a standard single-compartment three-electrode cell driven by a PAR system Model 363 potentiostat/galvanostat and pilot Model 175, connected to a Sefram TGM 164 XY recorder. The working Zn electrodes were made of Zn sheets (15 × 40 mm, Weber metals, purity 99%). Prior to use they were polished mechanically with abrasive paper (1200-grade), rinsed with water and then cleaned by ultrasonic stirring in acetone. The counter electrode was a stainless-steel plate (30×40 mm) and the reference electrode an Ag wire coated with AgC1 or a saturated calomel electrode (SCE). The XPS and IR techniques have been described previously [2]. Raman spectra were obtained with a Dilor XY modular spectrometer, equipped with a Spectra-Physics Model 165 argon-ion laser (514.5 nm, 5 mW). Conductivity measurements were performed classically by the four-probe technique for conducting free-standing films.

Zn surface electrodes were polished with abrasive paper, and then modified by HPA deposition from (H2WlsO56F6)H s •9H20 , P2WlsO62H 6 •9H20 and PW1204oH3"nH20 polyacids. These deposits can be obtained either by a dipping treatment or by electrolysis [14]. In the first case, the procedure is similar to an electroless process, and consists of substituting the Zn atoms of the surface by metallic ions from the solution (M ÷) according to the scheme: 2M ÷ + Z n

, Zn 2÷ +2M

(1)

A similar reaction (2) was observed between the Zn and the heteropolyanions in their oxidized state (HPA)ox [15]: (HPA)ox + (n/2)Zn + nH÷

, Hn(HPA)red + (n/2)Zn 2÷

(2)

blue Complex

The heteropolyanions are reduced at the Zn surface and yield a blue product consisting of the reduced HPA associated with Zn 2÷. Under these conditions, the homogeneity of the layers deposited at the Zn surface is very poor and their thickness very small. Therefore, in order to improve the quality of this treatment, the HPA were deposited on the Zn surface electrolytically. By using a 10-3 M aqueous solution of HPA (colourless) and by applying 0.0 V(SCE) to the Zn electrode for several minutes a thick, homogeneous blue coating was obtained, characteristic of the reduced HPA adsorbed at the Zn surface. However, it must be noted that a significant fraction of the reduced HPA is solubilized and that a considerable amount of Zn dissolves. Weightloss measurements performed on a Zn sample show that the amount of Zn dissolved is greater than that of HPA deposited, and this weight loss is proportional to the electrical charge consumption (Fig. 1).

3.2. Electropolymerization of pyrrole Electropolymerization of pyrrole at a Zn electrode modified by HPA requires optimization of the electrolytic medium as well. Therefore, in addition to the above electrolytic surface treatment of Zn by HPA, several electrolysis media such as MeCN, MeOH, PC

B. Zaid et al. / Synthetic Metals 65 (1994) 27-34

29

I(mAcn52765 ) ~

b°''

E -4 <~ 4 0

1

2

3

///;'

4

q (O.cm-') Fig. 1. Weight loss of a Z n electrode vs. quantity of electricity consumed during electrolysis in w a t e r + 10 -3 M HPA. Applied potential: 0.0 V vs. SCE.

and H20 in the presence of different salts (TBATsO, TEATsO, TEABF4, TEAPF6, TEABr and TEAC104) were tested, under potentiodynamic and galvanostatic conditions, by using 0.5 M pyrrole in 0.1 M salt solutions.

3.2.1. Electropolymerization of pyrrole under potentiodynamic conditions With MeOH, MeCN or water and in the presence of one of the previous salts, polymerization of pyrrole was unsuccessful, whether the Zn electrode was treated or not, and dissolution of Zn occurred immediately. Thus, with an untreated Zn electrode and TEAPF6, the reaction begins at - 0 . 2 V and is due exclusively to metal oxidation. After treatment of the Zn electrode by HPA, this oxidation curve is shifted to more positive potentials (+0.3 V), but no PPy film is formed. On the backward sweep, extensive dissolution of Zn is observed at negative potential values, indicating that passivation of Zn by HPA is not efficient. In contrast, the use of para-toluene sulfonate as the anion seems to be a better choice. In the absence of pyrrole, the dissolution of a Zn electrode in PC is considerably slowed (Fig. 2(a)), and it becomes negligible after treatment by HPA (Fig. 2(b)). For comparison, voltammetric curves (Figs. 2(a') and (b')) obtained in TEAPF6 clearly show that both the presence of para-toluene sulfonate in PC and the treatment of the electrode surface strongly passivate the Zn, which remains quite stable between - 1 and + 1.5 V. These new medium and treatment conditions appear particularly suitable for the electropolymerization of pyrrole. After a series of cycling potential sweeps in this medium (Fig. 3), a black deposit is formed on the electrode surface; its analysis by IR, Raman and XPS spectroscopy confirmed the presence of standard polypyrrole.

3.2.2. Electropolymerization of pyrrole under galvanostatic conditions The electropolymerization of pyrrole was carried out under the same conditions of medium as described above, but at a constant current density of 2 mA cm-2.

E(V) -1

0

1

2

Fig. 2. Cyclic voltammograms of a Z n electrode (S = 6 cm 2) in PC + (a) and (b) 0.1 M T E A T s O , and (a') and (b') 0.1 M TEAPF6. Scan rate: 10 m V s - ' . (a) a n d (a') Z n surface polished mechanically with abrasive paper (1200-grade); (b) and (b') Zn surface treated with PWl2040H3.

I nnAcrn-2)

2.5

-0.8

2 43 I

E(V)

Fig. 3. Cyclic voltammograms of PPy film growth in P C + 0 . 1 M T E A T s O +0.5 M pyrrole on a Z n electrode treated with PW,204~I-I3. Scan rate: 10 m V s - L

As previously, if the Zn electrode was not treated with HPA, only dissolution of Zn was observed and no PPy deposit occurred whatever the salt and the solvent used. After treatment with HPA (5 min at 0.0 V(SCE)), a homogeneous PPy deposit was obtained in propylene carbonate+para-toluene sulfonate, confirmed by the fact that the electrode reaches a stable positive potential at 2 V. It is worth noting that this unusual high value includes an ohmic drop of 1.2 V and is indeed consistent with the formation of PPy between 0.8 and 1 V. (The ohmic drop between the working and reference electrodes is about half of the ohmic drop between

30

B. Zaid et aL / Synthetic Metals 65 (1994) 27-34

Table 1 Faradaic yield (It) of the electropolymerization of pyrrole (0.5 M) in PC+0.1 M TEATsO at a Zn electrode treated with HPA. mth was calculated according to Ref. [3] Q (C)

Am~,,p (mg)

~nth

3.6 7.2 10.8 14.4

1.92 4.02 6.21 8.43

1.94 3.98 5.82 7.76

Y= ~rr/exp/~/1 t h

(mg) 0.99 1.01 1.06 1.09

the working and counter electrodes, i.e. 1200 f i x 2× 10 -3 A/2= 1.2 V). The first results show clearly that a synergic effect exists between the solvent, salt and treatment of the surface. PC, through the carbonate group, has passivating properties towards A1 and Fe [2,6,16,17] and probably also towards Zn. para-Toluene sulfonate ions are also well known to provide very dense and compact PPy films [18-20], which likewise can contribute to slowing the diffusion of the Zn 2÷ ions into the solution. This latter effect is also increased by the insolubility of Zn(TsO-)2 in PC, which consequently will favour the electropolymerization of the pyrrole. Electrical yields (Y), calculated by weighing the PPy films and assuming 2.3 electrons per monomer [3] are always close to 99% (Table 1), indicating that Zn oxidation is negligible. A slight increase in Y was observed when the charge Q, used during the electropolymerization reaction increased, a result which can be attributed to the fact that the doping of PPy increases slightly when the PPy film grows.

3.3. Characterization of Zn surfaces and PPy films Pure or modified Zn surfaces were analysed by XPS, IR and Raman spectrometry. PPy films were obtained at j = 2 mA cm-2 on a Zn surface modified with HPA.

3.3.1. XPS analysis results 3.3.1:1. Zn sample Prior to XPS analysis the pure Zn sample was polished and rinsed as indicated in section 2. A symmetrical Zn(2p3a) signal was observed at 1022 eV, attributable to Zn (metal) or ZnO (oxide), which cannot be differentiated owing to their similar binding energies [21]. A main C(ls) peak was found at 285 eV accompanied by two small peaks at 287.2 and 289.5 eV (relative intensities 1:0.26:0.26), attributed to contamination compounds containing C-C, C-O and C = O groups, respectively [2]. An O(ls) signal (Fig. 4(a)) consisting of two peaks at 531.9 and 529.9 eV (relative intensities 1:0.17) was also observed; these correspond to zinc hydroxide, Zn(OH)2, and zinc oxide, ZnO [21].

3.3.1.2. Modified Zn electrode After modification of the Zn electrode by HPA, the Zn(2p3/2) signal remained symmetrical and strong, only slightly shifted to 1022.6 eV, probably indicating a higher Zn oxidation state than previously. The adsorption of HPA on Zn was disclosed by the presence of two doublets arising from W(4f) (Fig. 4(b)) and attributed to W TM (W(4f7/z)= 35.1 eV and W(4fs/2) =36.2 eV) and to W v~ (W(4fT/2)=37.1 eV and W(4fs/2)=38.2 eV) [22] corresponding to an atomic ratio of 0.24 compared to other elements. The O(ls) signal also consisted of two peaks (Fig. 4(c)) located in this case at 531.6 and 532.9 eV (relative intensities 1:0.44) and attributable to Zn(OH)2 and HPA, respectively [21]. One peak at 134.1 eV corresponding to the phosphorus P(2p) of the HPA was also observed [21]; the C(ls) signal remained practically identical with that previously found. 3.3.1.3. PPy deposited on modified Zn electrode After deposition of a PPy film (] = 2 rnA cm -2, Q = 7.2 C, e = 4 /~m) on the Zn electrode modified by P2WlsO62Hr'9H/O, XPS analysis was performed on the upper surface of the film (I) and on the two sides (II and III) of the metal/polymer interface, after the film had been peeled off the Zn electrode (Fig. 5). Surface I. XPS analysis of the outer PPy layer revealed the presence of C, O, N and S, and the absence of Zn, P and W, indicating that the PPy coating is homogeneous and that there is no diffusion of Zn 2+ and HPA through the film. The C(ls) signal consisted of three peaks at 285, 286.2 and 287.7 eV (relative intensities 1:0.3:0.1). The peak at 285 eV is attributed to the pyrrolic and aromatic C-C bonds; the peak at 286.2 eV to the carbons bound to the nitrogen of the PPy (C-N) and to the sulfur (C-S) of the doping anion; the third one might be due to a carbonyl group [2]. The O(ls) signal, consisting of two components at 531.76 and 533.26 eV (relative intensities 1:0.1), arises from the R-SO3- anion and from organic compounds with C-O or C = O moieties [23], probably corresponding to traces of PC entrapped in the polymer matrix. An S(2p3/2) and S(2pl/2) doublet located at 168.1 and 169.4 eV, respectively, is attributed to the oxidized form of the sulfur (S TM) [21] of the R-SO3- anion. The O531/Sa'o,a~ atomic ratio is about 2.95, and thus confirms the presence of the R - S O 3 - group as the compensating anion of the doped PPy. Deconvolution of the N(ls) signal led to three peaks at 400.2, 401.7 and 403.4 eV (relative intensities 1:0.26:0.07), attributed to the neutral pyrrolic NH moiety, to the oxidized = N H - group of PPy [2,23-27] and to traces of ammonium salt (> N < ) entrapped in the matrix, respectively. Evaluation of the atomic ratio,

B. Zaid et al. / Synthetic Metals 65 (1994) 27-34

31

18875

131BS

J

cpe

cp. 1 °1"

10484

78£

524!

262

Binding Energy eV

(a)

(c)

Binding Energy eV

5~15

cpl 471E

~3

23s

i~~

(b)

,,.,,..

E,.,,, .~

Fig. 4. XPS spectra of Zn surfaces: (a) polished with abrasive paper (1200-grade); (b) and (c) polished and treated with PWt204on3.

Q

\\\\\

T

Fig. 5. Scheme of a PPy film with its substrate showing the different surfaces and interfaces submitted to XPS analysis.

STotai/N-rota], indicates that the PPy film is about 30-35% doped, in agreement with the value (39%) found by Baker and Reynolds [28] for the tosylate anion. Surface H. No significant change was observed for the S(2p) signal, which appeared similar to that observed

with surface I. However, some differences arose for C, O and N, and with the appearance of Zn and W signals. Thus, the C(ls) peak situated at 287.7 eV disappeared and the second O(ls) peak at 533.3 eV was replaced by a very broad, intense peak at 532.1 eV corresponding to different oxygen functions. Similarly, a new N(ls) peak appeared at 398.9 eV, probably due to some deprotonated pyrrolic nitrogen [2,23-27]. The S/N atomic ratio was found to increase dramatically to unity, indicating that the doping salt has accumulated at the metal/polymer interface [29]; this assumption was confirmed by SEM/X-ray fluorescence analysis. Quite surprisingly the zinc Zn(2p) and tungstate W(4f) signals were found to be closely similar to those obtained on Zn after treatment by HPA. Surface IlL The XPS signals of Zn, W and O were similar to those observed on Zn after treatment with

32

B. Zaid et aL / Synthetic Metals 65 (1994) 27-34

HPA; the C(ls) region is characterized by a very broad, strong peak at 285 eV, accompanied by a weak peak centred at 291.5 eV. The component at 285 eV could be due to carbon from contamination and carbons of the aromatic T s O - group, whereas the small peak at 291.5 eV is attributed to C = O and C-S groups. It must be emphasized that no N(ls) signal was detected, indicating that rupture of adhesion between the PPy film and the metal is of the adhesive type.

3.3.2. SEM analysis PPy film obtained galvanostatically under the same conditions as previously was examined by SEM. The outer face (I) was characterized by a globular texture (Fig. 6(a)) with globules of variable size, ranging from 2 to 20 ~m in diameter. The internal face (II) appeared as a plane surface on which are deposited white crystals of variable forms (Fig. 6(b)). Micro-Xray fluorescence analysis of these crystals indicates a high sulfur concentration, and thus supports the idea that Zn(TsO-)2 has accumulated at the Zn polymer interface, confirming the XPS results.

3.3.3. Infrared analysis PPy films obtained on a modified Zn surface were analysed in their doped and undoped states. The films were peeled off the substrate and analysed in the transmission mode as KBr discs. Undoped films were obtained by treating PPy with a 3% NaOH solution for 30 rain, then rinsing successively with distilled water and ethanol, and drying at 50 °C in vacuum for 24 h. The IR spectra of doped (Fig. 7(a)) and of undoped PPy films (Fig. 7(b)) exhibited features similar to those of standard PPy films described in the literature [2,26,27,29]. In particular, two sharp bands at 1129 and 1179 cm-1, corresponding to symmetric and asymmetric stretching vibrations of the TsO- group [30,31], were observed in the doped film (Fig. 7(a)). As usual, the characteristic bands of the undoped PPy (Fig. 7(b)) at 908 cm -1 (very intense) and 767 cm -1 (medium) corresponding to out-of-plane C-H vibrations and a very intense band at 1029 cm- 1 assigned to the N - H in-plane vibration deformations of the PPy [2] were observed; they were accompanied by two bands at 1170 cm -1 (strong) and at 1550-1480 cm -~ (weak) due to pyrrole ring vibrations [2,32].

3.3.4. Raman analysis The Raman spectrum of doped PPy film on a Zn electrode modified by HPA was determined. It was not significantly different from that of a PPy film obtained by electrooxidation of pyrrole on Pt in MeCN [18,33]. The two bands located at 1565 and 1619 cm -1, characteristic of the stretching vibration of C=C, and the various bands between 1475 and 1236 cm-1 due to the vibration modes of the pyrrole ring were observed as well as the three bands situated at 1044.8, 967.6 and 921.6 cm -1 related to the in-plane and out-of-plane vibration modes of N - H and C-H.

(a) ~.'

,~, ,

1.1% /

/ J

I i

(b) Fig. 6. Scanning electron micrographs of (a) surface I and (b) surface II.

2{}00

i

i

V.90

I

i

i

9BO

i

i

i

~

470 {cm-1 ) Fig. 7. IR transmission spectra (in KBr discs) of (a) doped and (b) undoped PPy.

B. Zaid et aL / Synthetic Metals 65 (1994) 27-34

3.4. Electrochemical properties of PPy films Cyclic voltammetry of PPy films prepared galvanostatically ( j = 2 mA cm -2, Q = 7 . 2 C) in PC+0.1 M TEATsO +0.5 M pyrrole on a Zn electrode modified by HPA was carried out in different solvents (PC, MeCN and H20). In PC+0.1 M TEATsO (Fig. 8), no redox peak was detected during the first potential cycle performed at different sweep rates (20, 50 and 100 mV s-l). Only after the second cycle did broad redox peaks arise at 0.5 V (oxidation peak, Ep a) and at - 0 . 4 V(Ag-AgC1) (reduction peak, Ep c) characterized by a AE= Epa-EpC=0.9 V , and a n Ipa/Ip e peak intensity ratio of about unity (Table 2). When the number of potential cycles was increased, the Ep a and E p e peak potentials shifted to more positive and negative values, respectively, and the separation between E p a and E p e became very large, while Ipa/Ip c remained approximately constant (Table 1). Such a phenomenon is unusual and is probably due to an increase in microporosity of the film, resulting

I(rnA.cm"2 ./~-~ n(lh) /

/

1.8

33

from the swelling of the PPy film in PC which, therefore, improves the transport of T s O - anions through the film [18]. However, after a series of cyclic potential scans (about 40 cycles), the curves changed dramatically and the film lost its electroactivity. The i-E curves were then characteristic of a capacitive current, probably due to the progressive detachment of the film from the electrode surface. This swelling of the film by PC and MeCN did not occur in water in the presence of the same electrolysis salt. The curves were fiat and did not exhibit any anodic and cathodic peaks. The current intensity decreased with number of cycles and was accompanied by the diffusion of white filaments from the electrode to the solution, probably due to metal dissolution.

3.5. Conductivity and adhesion of PPy films PPy films were obtained at modified Zn electrodes by electrooxidation of pyrrole at 2 mA cm -z in P C + 0.1 M TEATsO. The conductivity of these PPy films is about 30 S cm-1. Since their adhesion to the substrate is very weak they can be detached from the substrate as free-standing films. However, when dried at 180 °C for several hours in air their adhesion improved considerably and they no longer peeled off the substrate when subjected to the Sellotape test.

4

.J //

J

1

4. Conclusions

1

y

,2

E(v~

/ / " -I.8'

! ?/

Fig. 8. Cyclic voltammograms in PC + T E A T s O of a PPy film obtained at a modified Z n electrode in P C + 0 . 5 M P y + 0 . 1 M T E A T s O . Scan rate: 10 m V s -1. Table 2 Electroactivity of a PPy film synthesized in P C + 0.1 M T E A T s O + 0.5 M pyrrole in a Z n electrode treated with H P A and submitted to successive potential cyclic sweeps in the same m e d i u m without pyrrole No. of cycles

Ipa ( m A cm -2)

Ipc ( m A cm -2)

Ip'/lp c

Ep a (V)

Ep ¢ (V)

Epa-Ep ~ (V)

References

1

2 3 4 5 6 7 8

In this work we have shown it is possible to deposit PPy films on a Zn electrode by electrooxidation of pyrrole, if the Zn surface is previously treated with heteropolyanions and if the electropolymerization reaction is carried out in PC in the presence of paratoluene sulfonate. The electrical yield is close to unity, and it was confirmed by IR, XPS and Raman spectroscopy that the deposits consist of PPy with a conductivity of about 30 S cm-1. Their electroactivity in water is very poor, indicating probably that the microstructure of the polymer matrix is very compact. Their adhesion to the substrate is not very good but can be improved by a thermal.treatment at 180 °C for several hours.

0.76 1.16 1.41 1.60 1.74 1.85 1.93

0.76 1.13 1.38 1.56 1.71 1.82 1.90

1.00 1.03 1.02 1.02 1.02 1.02 1.02

0.54 0.78 0.92 1.02 1.06 1.10 1.14

- 0.40 -0.56 -0.68 - 0.74 -0.80 -0.84 -0.90

0.94 1.34 1.60 1.74 1.86 1.94 2.04

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