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A two-step electropolymerization of pyrrole on Zn in aqueous media
PII:
Electrochimica Acta, Vol. 43, Nos 16±17, pp. 2331±2339, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0013-4686(97)10161-X 0013±4686/98 $19.00 + 0.00
A two-step electropolymerization of pyrrole on Zn in aqueous media B. Zaid,a S. Aeiyach,a P. C. Lacazea* and H. Takenoutib a
b
Institut de Topologie et de Dynamique des SysteÁmes ITODYS, URA 34 du CNRS, Universite Paris 7±Denis Diderot, 1 rue Guy de la Brosse, 75005 Paris, France
Laboratoire de Physique des Liquides et Electrochimie, UPR 15 du CNRS, Universite Paris 6±Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France (Received 21 July 1997; accepted 24 October 1997)
AbstractÐSubstrate-adherent polypyrrole ®lms are obtained by electrochemical oxidation of pyrrole on a Zn electrode in an aqueous sodium oxalate medium provided that the zinc surface has been treated previously in a sul®de medium with the view to reduce its reactivity. The best pretreatment was achieved when the electrode was polarized anodically at 0.35 mA cmÿ2 for 3 min in 0.2 M Na2S solution. Various electrochemical techniques (cyclic voltammetry, chronopotentiometry, impedancemetry) accompanied by XPS studies were used to characterize the surface modi®cations of the zinc electrode induced by this pretreatment. It was found that the formation of surface ®lm occurs in two steps. At low anodic potentials, a porous layer is formed and at high anodic potentials the surface ®lm is modi®ed and acquires good protective properties. The surface layer consists of a mixture of ZnS (ca. 65%) and ZnOxHy (ca. 35%) with 1 R xR 2, and 0 R y R 1. Best electropolymerization conditions for obtaining homogeneous and adherent polypyrrole ®lm involved an aqueous solution of 0.9 M pyrrole + 0.1 M Na2C2O4 with galvanostatic polarization and after the substrate metal had been treated as indicated previously. # 1998 Elsevier Science Ltd. All rights reserved Key words: Zn electrode, pyrrole, aqueous media, anodic polymerization, polypyrrole ®lm, organic coatings.
INTRODUCTION Among various conducting polymers, polypyrrole (PPy) ®lms synthesized by the electrochemical oxidation of pyrrole in aqueous media are particularly promising on account of their unique physicochemical properties and the wide variety of their technological applications [1, 2]. Generally, adherent PPy ®lms are deposited on inert anodes like Pt, Au, glassy carbon (GC) or stainless steel [1, 2]. Extension of this technique to common metals such as Fe, mild steel, Al or Zn, with a view to the protection of metals against corrosion, has been recently envisaged [3±6], but was confronted with serious diculties. The main one is due to the fact that strong anodic dissolution of the metal will occur before the electropolymerization reaction *Author to whom correspondence should be addressed. E-mail: [email protected]
takes place. In the case of iron, mild steel or Al several ways of overcoming this diculty have been found [2±5]. The case of zinc, which is widely used by metallurgists, is much more complex since the metal is more electropositive than iron and is not protected by an insulating oxide layer like Al, and therefore its dissolution rate will be very high at the oxidation potential of pyrrole. Among various attempts to reduce the reactivity of Zn, we found that preliminary chemical treatment of its surface by sul®de compounds could aord a solution to this problem and allow the deposition of homogeneous and adherent PPy ®lms. After immersion of the zinc plate in aqueous 0.2 M Na2S for at least 12 h, the surface was covered by a mixed layer containing zinc sul®de and zinc oxide [6]. This layer slows down Zn dissolution dramatically without preventing pyrrole electropolymerization and markedly increases the adherence of PPy ®lms. This chemical
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B. Zaid et al. was three times the frequency at which electrochemical impedance was measured. The experiments were controlled by homemade computer software, that made possible to repeat EIS measurements automatically at a given time, every 5 min for instance. RESULTS AND DISCUSSION
Fig. 1. Potentiodynamic curves (10 mV/s) of a Zn plate in aqueous 0.2 M Na2S.
treatment is, however, too long and thus unsuitable for possible exploitation of the process on an industrial scale. Consequently, with the view to increasing the speed of the treatment, we have developed in this work, a new procedure consisting in an electrochemical conversion of zinc electrodes in aqueous Na2S solution. It considerably shortens the treatment time and also leads to better control of the modi®cations of the zinc surface, compatible with electropolymerization of pyrrole and the formation of strongly adherent PPy ®lms. EXPERIMENTAL SECTION All compounds (K2C2O4, Na2C2O4, H2C2O4, Na2S) were purchased from Fluka or Prolabo, and were used without further puri®cation. Pyrrole (Aldrich or Janssen) was distilled twice under argon. Water was puri®ed by passing through a Millipore puri®cation system. All voltammetric and galvanostatic experiments were performed using a one-compartment threeelectrode cell driven by a Princeton Applied Research (PAR) Model 363 potentiostat/galvanostat and Model 175 Universal Programmer, connected to a Sefram TGM 164 XY recorder. The working electrode was a zinc sheet (15 40 mm, Weber metals, purity 99%); the counter-electrode was a stainless-steel plate and the reference electrode a Tacussel KCl saturated calomel (SCE). XPS spectra were recorded with a VG Escalab MK1 apparatus and Infrared spectra with a Nicolet FTIR 60-SX spectrometer. Electrochemical impedance spectroscopy (EIS) measurements were carried out with Solartron equipment, comprising a transfer function analyzer (FRA 1254) coupled with a potentiostat (ECI 1286). To improve the signalto-noise ratio of ac signals an analogue ®lter (KEMO VBF 8.24 dB per octave) was inserted between the output of the potentiostat and the input of the transfer function analyzer. The cut-o frequency for the low pass mode of the ac signal
The ®rst part of the discussion is devoted to the study of the zinc electrode polarized in Na2S aqueous solutions and to the characterization of the ®lms formed on the surface. In the second part of this work we show that these optimally passivated layers allow electropolymerization of pyrrole without Zn dissolution. Electrochemical behavior of zinc electrode in aqueous Na2S solution Cyclic voltammetry. Figure 1 illustrates the oxidation curves of a Zn electrode in an aqueous solution of 0.2 M Na2S. Prior to each experiment, the electrode was polished with 1200 grade abrasive paper, initially held at its rest potential (ErZn = ÿ 1.1 V), and then polarized linearly at 0.1 V sÿ1 up to 1.4 V. The forward sweep exhibits two oxidation peaks, A and B, at about ÿ0.65 and 1.1 V, respectively (Fig. 1). These two peaks are separated by a potential zone where the current density is fairly constant and is about 0.25 mA cmÿ2 for the ®rst cycle. Unlike the previous forward polarization curve, the back-sweep does not exhibit any reduction peak from 1.4 to ÿ1.1 V, and the residual current density is practically nil. On the second potential cycle, the electrode remains passive from ÿ1.1 to 1 V, the residual current density being negligible in this potential range for both directions of potential sweep. Linear variation of the current density for peak A with the sweep rate ranging from 10 to 100 mV sÿ1 is observed; this might be associated with the formation of zinc sul®de species adsorbed at the electrode surface. It can be inferred that the sul®de adsorbs rapidly on the zinc giving an intermediate product, (Zn. . .S2ÿ x ) [7, 8], according to equation (1): ÿ4 Zn ÿ . . . S2ÿ Zn S2ÿ x ÿ x
1
(Zn. . . S2ÿ x ) is further oxidized to ZnS which leads to an insoluble ®lm, according to equations (2) and (3). ÿ ÿ4ZnS ÿ Zn . . . S2ÿ x 2e x ÿ
2
ZnSx S2ÿ ÿ ÿ4ZnS ÿ S2ÿ x
3
This ®lm is probably very porous and, therefore, zinc oxidation can continue to proceed through it. (It is worth noting that at this negative potential no oxidation of S2ÿ occurs, since the same reaction is
A two-step electropolymerization
Fig. 2. Potentiodynamic curves (10 mV/s) of a Zn plate in 0.25 M NaOH, (a) 0 mM Na2S, (b) 0.5 mM Na2S and (c) 5 mM Na2S.
observed on Pt at about 0.25 V [9].) The current increase for potentials greater than 0.4 V can be attributed to an acceleration of zinc oxidation through the porous layer resulting from the growing electric ®eld and followed immediately by new formation of the ZnS layer, leading to greater passivation. A similar phenomenon was found in the case of an iron electrode in the presence of the sul®de ions [9], and was attributed to a direct precipitation reaction between Fe3+ and S2ÿ ions. The electrochemical passivation process on Zn surface is followed by precipitation of ZnS, according to equations (4) and (5). 2 2eÿ Znÿ ÿ4Zn ÿ
4
ÿ4ZnS ÿ Zn2 S2ÿ ÿ
5
Equation (5) is in agreement with the fact that the current intensity of peak B varies linearly with n1/2 (n: sweep rate) involving in this case either diusion control of oxidation [10] or more probably ®lm formation under ohmic resistance control [11, 12]. Cyclic voltammetry carried out in 0.25 M NaOH alone (pH>12) showed (Fig. 2) that passivation of zinc is also occurring in this medium. The ®rst oxidation peak at ÿ0.85 V, probably due to the formation of Zn(OH)2, is followed by a residual oxidation current of about 2 mA cmÿ2 with a broad ¯attened peak centered at 0.35 V. Addition of small quantities of Na2S leads to a marked change in the Zn oxidation curves. Thus, when 0.5 mM Na2S is added to the 0.25 M NaOH solution, the intensity of the ®rst oxidation peak (ÿ0.93 V) decreases from
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4.3 to 2.1 mA cmÿ2, that of the residual current from 2 to about 1 mA cmÿ2, and the second oxidation peak disappears [Fig. 2(b)]. Similar behavior was observed during the backsweep. In the case of NaOH alone, a practically nil residual current is observed between 0.6 and ÿ1.2 V, and a very small reduction peak (0.25 mA cmÿ2) occurs at ÿ1.2 V, probably due to the reduction of oxidized Zn species. When 0.5 mM Na2S is added to this solution, the cathodic current decreases still more signi®cantly and remains close to 0.1 mA cmÿ2. With 5 mM Na2S, the rate of zinc dissolution is much lower [Fig. 2(c)] and the potentiodynamic curves become similar to those obtained in 0.2 M Na2S (Fig. 1). These results are in favor of preferential adsorption by sul®de ions and support the mechanisms previously proposed [10], in particular the oxidation of an adsorbed intermediate, believed to be (Zn. . . S2ÿ x ). Composition of the oxidation peaks at ÿ0.85 V for the 0.25 M NaOH [Fig. 2(a)] and 0.25 M NaOH + 5 mM Na2S [Fig. 2(c)]; reveals that the quantity of electricity consumed is about 17 to 18 mC cmÿ2 in the ®rst case, and only about 2.6 mC cmÿ2 in the second case. Assuming that the or oxygen ion is about area occupied by S2ÿ x 0.1 nm2, an approximate calculation leads to an equivalent adsorption of about 50 and 8 monolayers, respectively. If the crystal lattice of ZnS or ZnO is estimated to be 0.6 nm, then the surface layer must be about 5 nm thick in the presence of 5 mM Na2S. These results con®rm that the inhibiting eect of sul®des is more ecient than hydroxide ions. This eect is particularly striking and leads to an exponential-like decrease in the ®rst peak current located near ÿ1 V as the Na2S concentration in 0.25 M NaOH is increased. The peak current varies from 4.5 to 0.4 mA cmÿ2, when the concentration of Na2S increases from 0 to 5 mM, and then decreases slowly when the Na2S concentration is greater than 50 mM. Interestingly, the passivation of the electrode is also eective, as can be seen in Fig. 3, even when the polarization cycle is limited to between ÿ1.5 and 0 V. However, a new cycling of the electrode potential between ÿ1.5 and +1.5 V leads to a new current increase from ÿ0.2 V, which con®rms that the layer formed is porous. The zinc electrode is passivated again when potential cycling is continued (Fig. 3). Zinc electrode behavior under constant current polarization Chronopotentiometric measurements. The zinc electrode was held at its rest potential (ÿ1.2 V) in 0.2 M Na2S and then was polarized at constant cur-
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B. Zaid et al.
Fig. 3. Potentiodynamic curves (10 mV/s) of a Zn plate in 0.2 M Na2S. The potential sweep range for the ®rst two cycles was limited to between ÿ1.2 and 0 V, and then extended to between ÿ1.2 and 1.5 V.
rent between 0.1 and 4 mA cmÿ2. The subsequent potential changes are illustrated in Fig. 4. At low current densities ( j < 1 mA cmÿ2) the potential variations reveal clearly two break-points, marked A and B. The potential variation can be divided into three domains whose length and amplitude depend on the current density. The higher the current, the shorter the length of each domain, and the larger the potential variation. For instance, at j = 0.1 mA cmÿ2, and within 60 s, the potential increases linearly with time from ÿ1.2 to ca. 0.35 V (point A), then the potential continues to increase more slowly from A and for about 12 min up to about 1.6 V. After this time (break-point B) the potential remains fairly constant and the steady state is reached. At j = 1 mA cmÿ2 points A and B are reached more quickly (12 and 60 s, respectively) and at higher potentials (1 and 3.2 V, respectively). It is important to remark that the two break-points observed for the dierent applied current densities correspond roughly to the same electrical charge, 6 and 60 mC cmÿ2, respectively. When the potential at break-point A is plotted vs the current density ( j) a linear relationship is observed [Fig. 5(a)]. Calculation of its slope by
Fig. 4. Chronopotentiometric curves of a Zn plate at dierent current densities in 0.2 M Na2S.
Fig. 5. Current±potential relationships extracted from Fig. 4. (a) At break-point A and (b) at the steady state (t = 15 min) where the potential was corrected for ohmic drop due to solution resistance Rsol (65 O cm2) and ®lm resistance R®lm (150 O cm2).
means of a linear regression gives a resistance value close to 640 O cm2. This resistance may be divided into two contributions: solution resistance (Rsol) and resistance (Rf) of the ®lm formed at the electrode surface. Since the charge involved to reach break-point A is approximately constant, the amount of surface species may be also considered to be constant, provided that the current eciency remains the same. Furthermore, the extrapolation of f( j, E) to j = 0 indicates a critical potential value of 0.25 V, corresponding roughly to the end of peak A and the beginning of peak B observed in Fig. 1. The variation of the slope of the f(E, t) curve beyond point A (Fig. 4) may indicate that another kind of ®lm is formed at the electrode surface or more probably that a modi®cation of the ®lm packing density has occurred. Due to a high current density, the surface ®lm is probably doped by solution anions, leading to decrease in its resistivity. As was observed in Figs 1±3, there are two potential ranges corresponding to two dierent processes, which are likely to lead to two dierent surface ®lm entities. In the ®rst period, the formation of ZnS involves an adsorbed intermediate species (Zn. . . Sx2ÿ ) and its oxidation according to equations (1)±(3). In the second period, the surface ®lm continues to grow according to equations (4) and (5), and probably the latter surface species would be characterized by a more organized surface ®lm formed by the mixture of ZnS and ZnOxHy as will be described further. Beyond point B, dierent variations of the potential occur, depending on the applied current density. For j < 1 mA cmÿ2, the potential remains fairly
A two-step electropolymerization
Fig. 6. Nyquist plot of EIS of a Zn plate in 0.2 M Na2S polarized at j = 0.35 mA cm2 for 30 min.
constant, and since the charge necessary to reach B is also nearly constant, it is believed that further polarization does not modify the ®lm thickness. On the contrary, for a higher current density ( j r 3 mA cmÿ2) and beyond point B, a potential decrease is observed; this could be explained by a reorganization of the surface ®lm making its electrical resistance smaller, an eect which could be due to doping of the oxide ®lms by anions. Impedance measurements. In order to obtain more details on the passivation of the Zn surface and its evolution, impedance experiments were carried out under polarization at constant current. Measurements were performed in 0.2 M Na2S for 30 min every 3 min in the 0.1 and 104 Hz frequency range. Figure 6 illustrates a typical Nyquist plot obtained at 0.35 mA cmÿ2 and after 30 min polarization. It can be seen that the impedance obtained above 1 Hz can be represented by one depressed capacitive loop and therefore can be expressed as: Z Rsol
Re 1 jo Re C a
6
The high frequency limit of the impedance may be attributed to the solution resistance Rsol which remains essentially constant (~65 O cm2) and independent of the current density at which the electrode is polarized. Re is determined as the chord of the capacitive arc, and parameter a is introduced to take account of a depressed feature of the impedance diagram (constant phase element). The value of a is found to be about 0.9 whatever the experimental conditions. The value of C was determined at the frequency fm corresponding to the apex of the capacitive loop: C
1 2pfm Re
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above for chronopotentiometric curves where the potential becomes almost constant after the same time. At the beginning (t = 3 min), the capacity C is about 3 mF cmÿ2 and decreases to 2 mF cmÿ2. Though not illustrated here, the value of C is smaller when the polarization current is higher. For instance, at j = 0.1 mA cmÿ2, C decreases from 5 to 3 mF cmÿ2, whereas when the current density is equal to 4 mA cmÿ2 it decreases from 0.8 to 0.6 mF cmÿ2. These values are too small to correspond to the double layer capacitance generally encountered for a bare metal. This indicates that ®lm formation by the anodic current takes place immediately and that the thickness increases with the polarization current. If the dielectric constant e of the surface ®lm is assumed to be 10, a value often found for a metal oxide (in a dry state), then one can evaluate the ®lm thickness through the expression of the planar capacitance: C
ee0 S , d
where e0 is the permeability of vacuum (8.85 10ÿ14 F cmÿ1), S is the surface area (cm2), and d is the thickness of the surface ®lm (cm). Figure 8(a) illustrates the change in the ®lm thickness computed according to equation (8) after a polarization period of 30 min for dierent applied current densities. From this Figure and by applying the linear least squares method, one obtains the following relationship: d 2:71 10ÿ7 E ÿ 0:748
9
which shows that the ®lm thickness at the steady state increases linearly when E is greater than 0.75 V. This threshold potential may be considered as a Flade potential beyond which the surface ®lm is thermodynamically stable. The resistance Re depends also on the current density applied to the electrode. Figure 8(b) depicts its variation with respect to the reciprocal of the current density, from which one gets the following
7
The time change of Re and C determined from impedance measurements at 0.35 mA cmÿ2 are displayed in Fig. 7. Both values tend to become constant after about 5 min. This observation is in agreement with the potential change described
8
Fig. 7. Variations of Re and C determined from Fig. 6.
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B. Zaid et al. Rt j
1 RT b nbF
14
Since b is equal to 9 Vÿ1 according to equation (13), and setting n = 2 for the oxidation process of Zn, one ®nds a b value close to 0.12. The value is small but is located within the theoretical range. However, an alternative hypothesis could explain the variation of Rt with j if it is accepted that the potential drop could be due to the ionic conduction induced by a high electrical ®eld. Indeed, the current density can be expressed by the relationship: nbFs E ÿ EF 15 j j0 exp d 0 RT
Fig. 8. Results calculated from EIS data with respect to the potential E at the steady state (t = 30 min) for dierent applied current densities. (a) Film thickness d of the surface layer and (b) diameter of capacitive loop Re.
relationship: Re 149224
11625 j
10
According to equation (10), Re can be divided into two components, Rf and Rt. The former can be considered as the resistance of the ®lm and is independent of the polarization current (Rf=149). The second term, Rt, depends on the current density, and can be interpreted in two dierent ways. In the ®rst hypothesis, Rt is considered as the charge transfer resistance, and thus: Re Rf Rt
with Rt j 11625 mV
11
This charge transfer resistance (Rt) can be related to the activation energy, parameters deduced from the Tafel relationship: bnF j j0 exp bZ with b 12 RT In this relationship, j0 is the exchange current density at the equilibrium potential (mA cmÿ2), b is the Tafel coecient (Vÿ1), b the transfer coecient (a constant between 0 and 1), Z the overpotential (V), n the number of electrons involved in the charge transfer reaction; F, R and T are the usual physical constants. From equation (12), one derives readily: 1 dj j0 b exp bZ jb Rt dZ and thus:
13
where d ' is the thickness of surface ®lm, EF the Flade potential or potential at d ' = 0, and s the jumping distance of the charge carrier, typically the atomic distance between passive species [13, 14]. The potential gradient close to 106 V cmÿ1, according to equation (9), is in agreement with ionic migration through the ®lm under high electrical ®eld. However, if one considers that the ®lm thickness d ' is equal to that de®ned in equation (9), then one may conclude that the current density should be independent of the potential. This condition is in general ful®lled under potential control. That is, the ®lm thickness d ' changes, so that the current ¯ow through the ®lm becomes equal to the passive current density, corresponding to its dissolution. Under current regulation where j is constant, both E and d ' change simultaneously according to equation (15). By subtracting the ohmic drop of the solution Rsol=(65 O cm2) and also the ohmic drop of the ®lm (Rf=150 O cm2) from the overall potential obtained at t = 15 min (cf. Figure 4), the potential±current relationship at the (almost) steady state is obtained [Fig. 5(b)]. j 0:033 exp 1:05E
16
If the value of b is assumed to be 0.5, equation (15) gives d '/s = 36, equivalent to 36 monolayers or 13 mC cm2. If s is about 0.3 nm, then the overall thickness of the surface layer is 10 nm. The calculated charge is about one ®fth of that involved at break point B in Fig. 4, i.e. the current eciency to form the surface layer is about 20%, and the charge loss might correspond to the fact that a large amount of Zn2+ ions escape into the solution according to equation (4). This result is in contradiction with equation (9) which stipulates a much lower thickness. The origin of this inconsistency has not been established, but may be due to the fact that e is not constant and could be higher than 10 if we assume that the ®lm contains many more water molecules at higher current densities.
A two-step electropolymerization
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Table 1. XPS-determined atomic ratio of some elements of the surface layer formed on a Zn electrode at dierent Na2S concentrations Na2S (M)
S/O 0.05 0.1 0.2 0.4
S/Zn
O/Zn
0.70 0.60 0.64 0.67
1.14 1.15 1.19 1.16
0.54 0.52 0.53 0.53
(S + O)/Zn 1.84 1.74 1.83 1.83
j = 0.35 mA cmÿ2 for 3 min.
Concerning the impedance data, an explanation may be that the electrode impedance results from two ®lm resistances, Rf and Rm. Rf could correspond to the ®lm formed at lower polarization, associated with break-point A in Fig. 4. As stated above, the electrical charge corresponding to this point is practically independent of the applied current, and therefore its thickness may be also considered as constant over this ®lm, and up to breakpoint B another ®lm having a semiconducting nature Rm is formed. Beyond point B, further current ¯ow no longer contributes to the growth of this surface ®lm. However, it is worth noting that this behavior holds only for current densities lower than 1 mA cmÿ2, and for higher j values (4 mA cmÿ2) a decrease of the potential occurs after point B. This phenomenon has not been elucidated but could be due to strong doping of the layer which should lower its resistivity. The ®lm resistance Rf obtained experimentally is markedly higher than that given in the literature, 200 O cm [15] and may be explained by the fact that the resistivity determined in this study is due essentially to ionic conduction through the ®lm, whereas that given in the literature corresponds to electronic conduction.
XPS STUDIES OF THE ZINC SURFACES TREATED WITH AQUEOUS Na2S XPS analysis was performed on Zn plates treated electrochemically at 0.35 mA cmÿ2 for 3 min in dierent Na2S solutions (0.05, 0.1, 0.2 and 0.4 M). XPS analysis of these modi®ed Zn surfaces shows the presence of C, S, O and Zn. The signal of carbon consists of three peaks (relative intensities 1:0.24:0.14), located at 285, 286 and 288.7 eV and attributable to C±C, C.O and O.C±O groups, respectively [16, 17]. The S2p signal shows the two components, S2p1/2 and S2p3/2, at 162 and 163.3 eV, respectively [18], characteristic of a metallic sul®de. A peak of low intensity at 168.5 eV due to sulfates or sul®tes was also detected [16, 18]. The O1s signal is characterized by a very large peak centered at 532 eV, corresponding to zinc oxides and zinc hydroxides. The Zn2p signal at 1021.9 eV is very strong, and can be attributed to oxidized zinc and to zinc sul®de [16].
The atomic ratios, S/O, S/Zn, O/Zn and (S + O)/ Zn, were calculated for the various Na2S concentrations (Table 1). From these results, it appears that: (i) no signi®cant variation of the surface composition was detected with Na2S concentrations varying from 0.05 to 0.4 M; (ii) the hypothesis of a pure ZnS layer must be rejected since the O/Zn atomic ratio of about 1.1 is far higher than the S/ Zn ratio (0.65); (iii) dierent varieties of Zn oxides such as ZnO, Zn(OH)2, hydrated oxides and probably oxygen contamination must be present at the surface since the (S + O)/Zn ratio (about 1.80) is much higher than 1. It is also worth noting that a surface of Zn mechanically cleaned, then dried immediately with air depicts a very intense O1s signal, with an O/Zn atomic ratio of about 2.3, but also an important C1s signal, resulting from contamination (C/ Zn = 1.6). Deconvolution of the C1s signal leads to three main components at 285, 287.2, and 289.5 eV with relative intensities (1:0.2:0.2), the latter two probably being due to carbonyls and carbonate groups [16]. From these data, it is concluded that mixed layers of ZnS (65±70%) and ZnO or Zn(OH)2 (30±35%), are formed on the Zn surface, in agreement with a (S + O)/Zn ratio greater than 1. In conclusion, all the results obtained by potentiodynamic polarization, impedance measurements and XPS analysis, indicate that after treatment in Na2S the Zn surface is modi®ed by mixed layers of ZnS and zinc oxide, typically ZnOxHy with 1 R xR 2, and 0 R yR 1. These layers are extremely thin (thicknesses <10 nm, con®rmed by SEM analysis) and have a high resistivity (r>106 O cm). The zinc passivation which is also observed in alkaline medium, increases dramatically with the presence of sul®des which even at very low concentrations (0.1 mM Na2S in 0.25 M NaOH solution) slow down Zn dissolution. PYRROLE ELECTROPOLYMERIZATION ON PRETREATED ZINC ELECTRODE In order to synthesize homogeneous and adherent PPy ®lms on Zn electrodes several physical and chemical parameters must be optimized. The time and the current density of treatment of the zinc surface in Na2S aqueous solution, the pyrrole concen-
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B. Zaid et al. Galvanostatic polarization
Fig. 9. Chronopotentiometric curves obtained in aqueous 1 M Na2C2O4+0.9 M Py (pH = 6) at dierent current densities. j: (a) 0.1RjR 1 mA cmÿ2; (b) 5 mA cmÿ2; (c) 10 mA cmÿ2; (d) 20 mA cmÿ2.
tration, the nature of the electrolytic salt and the choice of the polarization technique are all-important. The best conditions were found when the zinc surface was polarized at 0.35 mA cmÿ2 for 3 min and then electropolymerization of pyrrole performed with aqueous 0.1 M Na2C2O4+0.9 M pyrrole solutions. PPy ®lms having poor homogeneity and moderate adherence on Zn were obtained in the case of potentiodynamic polarization. On the contrary, by galvanostatic polarization homogeneous and strong adherent PPy ®lms were obtained, as will be shown in the following.
By using the above optimal conditions very dierent chronopotentiometric curves were obtained with applied current densities ranging from 0.1 to 20 mA cmÿ2 (Fig. 9). Black, homogenous PPy ®lms can be deposited on a pre-treated zinc electrode only if the applied density is greater than 2 mA cmÿ2. For values of j equal to or smaller than 1 mA cmÿ2, the potential of the electrode remains negative and corresponds therefore to oxidation of zinc [Fig. 9(a)]. At 5 mA cmÿ2, a rapid increase of the potential up to 1.2 V is ®rst observed, followed by a fast decrease to 0.5 V. This indicates that the underlying zinc is oxidized ®rst followed by a rapid increase up to 2.2 V corresponding to pyrrole oxidation through the resistive layer. It is followed again by a slow decrease and a stabilization at 1.3 V [Fig. 9(b)]. For very high current densities ( j = 20 mA cmÿ2, Fig. 9), overoxidation of the PPy occurs and large amounts of gas are released at the anode due to the oxidation of oxalate ions: ÿ COO2ÿ ÿ42CO ÿ 2 2e 2 ÿ
17
The PPy ®lm obtained in this case has a dendritic texture. Moreover, OHÿ ions are formed at the cathode in large amounts, thus increasing the pH of the solution which must be readjusted by addition of oxalic acid.
Fig. 10. XPS spectra of N1s. PPy ®lm formed in 0.1 M Na2C2O4+0.9 M Py at 4 mA cmÿ2 for 45 s.
A two-step electropolymerization From these experiments, it appears that the optimal conditions of electrolysis require current density ranges from 3 to 10 mA cmÿ2, and a Zn electrode which has been treated in 0.2 M Na2S at 0.35 mA cmÿ2 for 3 min. The presence of oxalate is necessary to obtain PPy ®lms. Surprisingly, if the same galvanostatic conditions are used without pretreatment of the Zn electrode in sul®de medium, no PPy deposit is observed, and the electrode potential rises to very high values, indicating the formation of an insulating layer identi®ed by XPS as Zn-oxalate. Similarly, if electrolysis is performed with a sul®de-pretreated 2ÿ Zn electrode but with another salt (ClOÿ 4 , SO4 , ÿ ÿ NO3 , PF6 ), no PPy ®lm is formed. It appears, therefore, that there is a synergistic eect between oxalate and the mixed ZnS + ZnOxHy layer. XPS analysis reveals that this layer is modi®ed by inclusion of zinc oxalate. This new layer structure which now allows electropolymerization of pyrrole seems to be a compromise between a compact insulating layer (ZnC2O4) and a more porous one (ZnS + ZnOxHy). Spectroscopic characterization of PPY ®lms. With the standard Zn treatment previously de®ned (0.35 mA cmÿ2, 0.2 M Na2S, 3 min) and using an aqueous solution of 0.9 M pyrrole + 0.1 M Na2 C2O4 for the electropolymerization reaction, electrode specimens were analyzed after dierent electrolysis time at j = 4 mA cmÿ2. After 10 s of polarization, the N1s signal centered at 399 eV is extremely weak, and the S2p doublet which appears at 162.2 and 163.4 eV is characteristic of sul®de [18]. The very strong Zn2p and O1s signals indicate that PPy is not formed at this stage of the electrolysis. After 45 s, the N1s signal increases and comprises mainly three components at 398.6 (17%), 400.4 (66%) and 401.8 eV (17%) (Fig. 10) characteristic of a doped PPy (17%). A strong S2p signal at 168.3 and 169.9 eV was also found, attributable to sulfate ions [16, 17], meaning that zinc sul®de is oxidized to ZnSO4 or ZnSO3. Interestingly, in the absence of pyrrole, neither sulfate nor sul®te was detected in the surface ®lms. This can be due to the fact that partial oxidation of sul®de to sulfate leads, in the absence of pyrrole, to the direct diusion into the solution whereas, when electropolymerization is occurring, sulfate and sul®te anions are incorporated into the PPy ®lm and participate in its doping. Full doping of the PPy is thus obtained by oxalate, sulfate and sul®te ions. The Zn2p signal is also strong, indicating that the metal is substantially oxidized at this stage of the electropolymerization. After 90 s of polarization, the zinc signal has completely disappeared, and the nitrogen signal is identical to that observed after 45 s. The doping level is the same as previously, and the sulfur signal remains fairly constant (S2p/NTotal=0.05), indicating that the zinc sul®de layer is oxidized slightly,
2339
and that the sulfate or sul®te ions diuse through the PPy layer. After 3 min polarization, IR and Raman spectroscopy con®rm that the layers formed are identical to the oxidized polypyrrole obtained in the literature [19, 20]. CONCLUSION From these results we can conclude that the electrochemical treatment of Zn in 0.2 M Na2S gives mixed layers containing ZnS and ZnOxHy whose composition appears to be constant for a standard current intensity regardless of the Na2S concentration. These layers are very thin and must be optimized to allow pyrrole electropolymerization. A kinetic compromise has to be found for the electrodeposition of PPy, and the best results are obtained at constant current and for intensities ranging between 2 and 10 mA cmÿ2. REFERENCES 1. T. A. Skotheim, (ed.), Handbook of Conducting Polymers, Vol. 1. Marcel Dekker, New York, 1986, and references therein. 2. H. S. Nalwa, (ed.), Handbook of Organic Conductive Molecules and Polymers, Vol. 2. John Wiley and Sons, Chichester, New York, 1997, and references therein. 3. W. Janssen and F. Beck, Polymer 30, 353 (1989). 4. P. HuÈlser and F. Beck, J. Appl. Electrochem. 20, 596 (1990). 5. C. A. Ferreira, S. Aeiyach, J. J. Aaron and P. C. Lacaze, Electrochim. Acta 41, 1801 (1996). 6. C. A. Ferreira, B. Zaid, S. Aeiyach and P. C. Lacaze, in Proc. Organic Coatings, ed. P. C. Lacaze. AIP Press, Paris, 1995, p. 159. 7. H. Gerischer, Z. Elektrochem. 54, 540 (1950). 8. P. L. Allen and A. Hickling, Trans. Faraday Soc. 53, 1626 (1957). 9. D. W. Shoesmith, P. Taylor, M. G. Bailey and B. Ikeda, Electrochim. Acta 23, 903 (1978). 10. A. J. Bard and R. Faulkner, (ed.), Electrochimie, Principes, MeÂthodes et Applications. Masson, Paris, New York, 1983. 11. A. J. Calandra, N. R. de Tacconi, R. Pereiro and A. J. Arvia, Electrochim. Acta 19, 901 (1974). 12. P. Bernard, M. Keddam and H. Takenouti, J. Electroanal. Chem. 396, 325 (1995). 13. K. J. Vetter, Electrochim. Acta 16, 1923 (1971). 14. N. Cabrera and N. F. Mott, Rept. Progr. Phys., 1948± 1949, 12, 163. 15. C. Weast, (ed.), Handbook of Chemistry and Physics, 67th edn. CRC Press, Inc., Boca Raton, Florida, 1986±1987. 16. C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder and C. E. Muillenberg, (ed.), Handbook of X-Ray Photoelectron Spectroscopy, 1978, PerkinElmer, MN. 17. E. T. Kang, K. G. Neoh and K. L. Tan, Adv. Polym. Sci. 106, 135and references therein (1993). 18. G. Deroubaix and P. Marcus, Surf. Interf. Anal. 18, 39 (1992). 19. J. P. Marsault, K. Fraoua, S. Aeiyach, J. Aubard, G. Levi and P. C. Lacaze, J. Chim. Phys. 89, 1167 (1992). 20. K. M. Cheung, D. Bloor and G. C. Stevens, J. Mater. Sci. 25, 3814 (1990).
Electrochimica Acta, Vol. 43, Nos 16±17, pp. 2331±2339, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0013-4686(97)10161-X 0013±4686/98 $19.00 + 0.00
A two-step electropolymerization of pyrrole on Zn in aqueous media B. Zaid,a S. Aeiyach,a P. C. Lacazea* and H. Takenoutib a
b
Institut de Topologie et de Dynamique des SysteÁmes ITODYS, URA 34 du CNRS, Universite Paris 7±Denis Diderot, 1 rue Guy de la Brosse, 75005 Paris, France
Laboratoire de Physique des Liquides et Electrochimie, UPR 15 du CNRS, Universite Paris 6±Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France (Received 21 July 1997; accepted 24 October 1997)
AbstractÐSubstrate-adherent polypyrrole ®lms are obtained by electrochemical oxidation of pyrrole on a Zn electrode in an aqueous sodium oxalate medium provided that the zinc surface has been treated previously in a sul®de medium with the view to reduce its reactivity. The best pretreatment was achieved when the electrode was polarized anodically at 0.35 mA cmÿ2 for 3 min in 0.2 M Na2S solution. Various electrochemical techniques (cyclic voltammetry, chronopotentiometry, impedancemetry) accompanied by XPS studies were used to characterize the surface modi®cations of the zinc electrode induced by this pretreatment. It was found that the formation of surface ®lm occurs in two steps. At low anodic potentials, a porous layer is formed and at high anodic potentials the surface ®lm is modi®ed and acquires good protective properties. The surface layer consists of a mixture of ZnS (ca. 65%) and ZnOxHy (ca. 35%) with 1 R xR 2, and 0 R y R 1. Best electropolymerization conditions for obtaining homogeneous and adherent polypyrrole ®lm involved an aqueous solution of 0.9 M pyrrole + 0.1 M Na2C2O4 with galvanostatic polarization and after the substrate metal had been treated as indicated previously. # 1998 Elsevier Science Ltd. All rights reserved Key words: Zn electrode, pyrrole, aqueous media, anodic polymerization, polypyrrole ®lm, organic coatings.
INTRODUCTION Among various conducting polymers, polypyrrole (PPy) ®lms synthesized by the electrochemical oxidation of pyrrole in aqueous media are particularly promising on account of their unique physicochemical properties and the wide variety of their technological applications [1, 2]. Generally, adherent PPy ®lms are deposited on inert anodes like Pt, Au, glassy carbon (GC) or stainless steel [1, 2]. Extension of this technique to common metals such as Fe, mild steel, Al or Zn, with a view to the protection of metals against corrosion, has been recently envisaged [3±6], but was confronted with serious diculties. The main one is due to the fact that strong anodic dissolution of the metal will occur before the electropolymerization reaction *Author to whom correspondence should be addressed. E-mail: [email protected]
takes place. In the case of iron, mild steel or Al several ways of overcoming this diculty have been found [2±5]. The case of zinc, which is widely used by metallurgists, is much more complex since the metal is more electropositive than iron and is not protected by an insulating oxide layer like Al, and therefore its dissolution rate will be very high at the oxidation potential of pyrrole. Among various attempts to reduce the reactivity of Zn, we found that preliminary chemical treatment of its surface by sul®de compounds could aord a solution to this problem and allow the deposition of homogeneous and adherent PPy ®lms. After immersion of the zinc plate in aqueous 0.2 M Na2S for at least 12 h, the surface was covered by a mixed layer containing zinc sul®de and zinc oxide [6]. This layer slows down Zn dissolution dramatically without preventing pyrrole electropolymerization and markedly increases the adherence of PPy ®lms. This chemical
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B. Zaid et al. was three times the frequency at which electrochemical impedance was measured. The experiments were controlled by homemade computer software, that made possible to repeat EIS measurements automatically at a given time, every 5 min for instance. RESULTS AND DISCUSSION
Fig. 1. Potentiodynamic curves (10 mV/s) of a Zn plate in aqueous 0.2 M Na2S.
treatment is, however, too long and thus unsuitable for possible exploitation of the process on an industrial scale. Consequently, with the view to increasing the speed of the treatment, we have developed in this work, a new procedure consisting in an electrochemical conversion of zinc electrodes in aqueous Na2S solution. It considerably shortens the treatment time and also leads to better control of the modi®cations of the zinc surface, compatible with electropolymerization of pyrrole and the formation of strongly adherent PPy ®lms. EXPERIMENTAL SECTION All compounds (K2C2O4, Na2C2O4, H2C2O4, Na2S) were purchased from Fluka or Prolabo, and were used without further puri®cation. Pyrrole (Aldrich or Janssen) was distilled twice under argon. Water was puri®ed by passing through a Millipore puri®cation system. All voltammetric and galvanostatic experiments were performed using a one-compartment threeelectrode cell driven by a Princeton Applied Research (PAR) Model 363 potentiostat/galvanostat and Model 175 Universal Programmer, connected to a Sefram TGM 164 XY recorder. The working electrode was a zinc sheet (15 40 mm, Weber metals, purity 99%); the counter-electrode was a stainless-steel plate and the reference electrode a Tacussel KCl saturated calomel (SCE). XPS spectra were recorded with a VG Escalab MK1 apparatus and Infrared spectra with a Nicolet FTIR 60-SX spectrometer. Electrochemical impedance spectroscopy (EIS) measurements were carried out with Solartron equipment, comprising a transfer function analyzer (FRA 1254) coupled with a potentiostat (ECI 1286). To improve the signalto-noise ratio of ac signals an analogue ®lter (KEMO VBF 8.24 dB per octave) was inserted between the output of the potentiostat and the input of the transfer function analyzer. The cut-o frequency for the low pass mode of the ac signal
The ®rst part of the discussion is devoted to the study of the zinc electrode polarized in Na2S aqueous solutions and to the characterization of the ®lms formed on the surface. In the second part of this work we show that these optimally passivated layers allow electropolymerization of pyrrole without Zn dissolution. Electrochemical behavior of zinc electrode in aqueous Na2S solution Cyclic voltammetry. Figure 1 illustrates the oxidation curves of a Zn electrode in an aqueous solution of 0.2 M Na2S. Prior to each experiment, the electrode was polished with 1200 grade abrasive paper, initially held at its rest potential (ErZn = ÿ 1.1 V), and then polarized linearly at 0.1 V sÿ1 up to 1.4 V. The forward sweep exhibits two oxidation peaks, A and B, at about ÿ0.65 and 1.1 V, respectively (Fig. 1). These two peaks are separated by a potential zone where the current density is fairly constant and is about 0.25 mA cmÿ2 for the ®rst cycle. Unlike the previous forward polarization curve, the back-sweep does not exhibit any reduction peak from 1.4 to ÿ1.1 V, and the residual current density is practically nil. On the second potential cycle, the electrode remains passive from ÿ1.1 to 1 V, the residual current density being negligible in this potential range for both directions of potential sweep. Linear variation of the current density for peak A with the sweep rate ranging from 10 to 100 mV sÿ1 is observed; this might be associated with the formation of zinc sul®de species adsorbed at the electrode surface. It can be inferred that the sul®de adsorbs rapidly on the zinc giving an intermediate product, (Zn. . .S2ÿ x ) [7, 8], according to equation (1): ÿ4 Zn ÿ . . . S2ÿ Zn S2ÿ x ÿ x
1
(Zn. . . S2ÿ x ) is further oxidized to ZnS which leads to an insoluble ®lm, according to equations (2) and (3). ÿ ÿ4ZnS ÿ Zn . . . S2ÿ x 2e x ÿ
2
ZnSx S2ÿ ÿ ÿ4ZnS ÿ S2ÿ x
3
This ®lm is probably very porous and, therefore, zinc oxidation can continue to proceed through it. (It is worth noting that at this negative potential no oxidation of S2ÿ occurs, since the same reaction is
A two-step electropolymerization
Fig. 2. Potentiodynamic curves (10 mV/s) of a Zn plate in 0.25 M NaOH, (a) 0 mM Na2S, (b) 0.5 mM Na2S and (c) 5 mM Na2S.
observed on Pt at about 0.25 V [9].) The current increase for potentials greater than 0.4 V can be attributed to an acceleration of zinc oxidation through the porous layer resulting from the growing electric ®eld and followed immediately by new formation of the ZnS layer, leading to greater passivation. A similar phenomenon was found in the case of an iron electrode in the presence of the sul®de ions [9], and was attributed to a direct precipitation reaction between Fe3+ and S2ÿ ions. The electrochemical passivation process on Zn surface is followed by precipitation of ZnS, according to equations (4) and (5). 2 2eÿ Znÿ ÿ4Zn ÿ
4
ÿ4ZnS ÿ Zn2 S2ÿ ÿ
5
Equation (5) is in agreement with the fact that the current intensity of peak B varies linearly with n1/2 (n: sweep rate) involving in this case either diusion control of oxidation [10] or more probably ®lm formation under ohmic resistance control [11, 12]. Cyclic voltammetry carried out in 0.25 M NaOH alone (pH>12) showed (Fig. 2) that passivation of zinc is also occurring in this medium. The ®rst oxidation peak at ÿ0.85 V, probably due to the formation of Zn(OH)2, is followed by a residual oxidation current of about 2 mA cmÿ2 with a broad ¯attened peak centered at 0.35 V. Addition of small quantities of Na2S leads to a marked change in the Zn oxidation curves. Thus, when 0.5 mM Na2S is added to the 0.25 M NaOH solution, the intensity of the ®rst oxidation peak (ÿ0.93 V) decreases from
2333
4.3 to 2.1 mA cmÿ2, that of the residual current from 2 to about 1 mA cmÿ2, and the second oxidation peak disappears [Fig. 2(b)]. Similar behavior was observed during the backsweep. In the case of NaOH alone, a practically nil residual current is observed between 0.6 and ÿ1.2 V, and a very small reduction peak (0.25 mA cmÿ2) occurs at ÿ1.2 V, probably due to the reduction of oxidized Zn species. When 0.5 mM Na2S is added to this solution, the cathodic current decreases still more signi®cantly and remains close to 0.1 mA cmÿ2. With 5 mM Na2S, the rate of zinc dissolution is much lower [Fig. 2(c)] and the potentiodynamic curves become similar to those obtained in 0.2 M Na2S (Fig. 1). These results are in favor of preferential adsorption by sul®de ions and support the mechanisms previously proposed [10], in particular the oxidation of an adsorbed intermediate, believed to be (Zn. . . S2ÿ x ). Composition of the oxidation peaks at ÿ0.85 V for the 0.25 M NaOH [Fig. 2(a)] and 0.25 M NaOH + 5 mM Na2S [Fig. 2(c)]; reveals that the quantity of electricity consumed is about 17 to 18 mC cmÿ2 in the ®rst case, and only about 2.6 mC cmÿ2 in the second case. Assuming that the or oxygen ion is about area occupied by S2ÿ x 0.1 nm2, an approximate calculation leads to an equivalent adsorption of about 50 and 8 monolayers, respectively. If the crystal lattice of ZnS or ZnO is estimated to be 0.6 nm, then the surface layer must be about 5 nm thick in the presence of 5 mM Na2S. These results con®rm that the inhibiting eect of sul®des is more ecient than hydroxide ions. This eect is particularly striking and leads to an exponential-like decrease in the ®rst peak current located near ÿ1 V as the Na2S concentration in 0.25 M NaOH is increased. The peak current varies from 4.5 to 0.4 mA cmÿ2, when the concentration of Na2S increases from 0 to 5 mM, and then decreases slowly when the Na2S concentration is greater than 50 mM. Interestingly, the passivation of the electrode is also eective, as can be seen in Fig. 3, even when the polarization cycle is limited to between ÿ1.5 and 0 V. However, a new cycling of the electrode potential between ÿ1.5 and +1.5 V leads to a new current increase from ÿ0.2 V, which con®rms that the layer formed is porous. The zinc electrode is passivated again when potential cycling is continued (Fig. 3). Zinc electrode behavior under constant current polarization Chronopotentiometric measurements. The zinc electrode was held at its rest potential (ÿ1.2 V) in 0.2 M Na2S and then was polarized at constant cur-
2334
B. Zaid et al.
Fig. 3. Potentiodynamic curves (10 mV/s) of a Zn plate in 0.2 M Na2S. The potential sweep range for the ®rst two cycles was limited to between ÿ1.2 and 0 V, and then extended to between ÿ1.2 and 1.5 V.
rent between 0.1 and 4 mA cmÿ2. The subsequent potential changes are illustrated in Fig. 4. At low current densities ( j < 1 mA cmÿ2) the potential variations reveal clearly two break-points, marked A and B. The potential variation can be divided into three domains whose length and amplitude depend on the current density. The higher the current, the shorter the length of each domain, and the larger the potential variation. For instance, at j = 0.1 mA cmÿ2, and within 60 s, the potential increases linearly with time from ÿ1.2 to ca. 0.35 V (point A), then the potential continues to increase more slowly from A and for about 12 min up to about 1.6 V. After this time (break-point B) the potential remains fairly constant and the steady state is reached. At j = 1 mA cmÿ2 points A and B are reached more quickly (12 and 60 s, respectively) and at higher potentials (1 and 3.2 V, respectively). It is important to remark that the two break-points observed for the dierent applied current densities correspond roughly to the same electrical charge, 6 and 60 mC cmÿ2, respectively. When the potential at break-point A is plotted vs the current density ( j) a linear relationship is observed [Fig. 5(a)]. Calculation of its slope by
Fig. 4. Chronopotentiometric curves of a Zn plate at dierent current densities in 0.2 M Na2S.
Fig. 5. Current±potential relationships extracted from Fig. 4. (a) At break-point A and (b) at the steady state (t = 15 min) where the potential was corrected for ohmic drop due to solution resistance Rsol (65 O cm2) and ®lm resistance R®lm (150 O cm2).
means of a linear regression gives a resistance value close to 640 O cm2. This resistance may be divided into two contributions: solution resistance (Rsol) and resistance (Rf) of the ®lm formed at the electrode surface. Since the charge involved to reach break-point A is approximately constant, the amount of surface species may be also considered to be constant, provided that the current eciency remains the same. Furthermore, the extrapolation of f( j, E) to j = 0 indicates a critical potential value of 0.25 V, corresponding roughly to the end of peak A and the beginning of peak B observed in Fig. 1. The variation of the slope of the f(E, t) curve beyond point A (Fig. 4) may indicate that another kind of ®lm is formed at the electrode surface or more probably that a modi®cation of the ®lm packing density has occurred. Due to a high current density, the surface ®lm is probably doped by solution anions, leading to decrease in its resistivity. As was observed in Figs 1±3, there are two potential ranges corresponding to two dierent processes, which are likely to lead to two dierent surface ®lm entities. In the ®rst period, the formation of ZnS involves an adsorbed intermediate species (Zn. . . Sx2ÿ ) and its oxidation according to equations (1)±(3). In the second period, the surface ®lm continues to grow according to equations (4) and (5), and probably the latter surface species would be characterized by a more organized surface ®lm formed by the mixture of ZnS and ZnOxHy as will be described further. Beyond point B, dierent variations of the potential occur, depending on the applied current density. For j < 1 mA cmÿ2, the potential remains fairly
A two-step electropolymerization
Fig. 6. Nyquist plot of EIS of a Zn plate in 0.2 M Na2S polarized at j = 0.35 mA cm2 for 30 min.
constant, and since the charge necessary to reach B is also nearly constant, it is believed that further polarization does not modify the ®lm thickness. On the contrary, for a higher current density ( j r 3 mA cmÿ2) and beyond point B, a potential decrease is observed; this could be explained by a reorganization of the surface ®lm making its electrical resistance smaller, an eect which could be due to doping of the oxide ®lms by anions. Impedance measurements. In order to obtain more details on the passivation of the Zn surface and its evolution, impedance experiments were carried out under polarization at constant current. Measurements were performed in 0.2 M Na2S for 30 min every 3 min in the 0.1 and 104 Hz frequency range. Figure 6 illustrates a typical Nyquist plot obtained at 0.35 mA cmÿ2 and after 30 min polarization. It can be seen that the impedance obtained above 1 Hz can be represented by one depressed capacitive loop and therefore can be expressed as: Z Rsol
Re 1 jo Re C a
6
The high frequency limit of the impedance may be attributed to the solution resistance Rsol which remains essentially constant (~65 O cm2) and independent of the current density at which the electrode is polarized. Re is determined as the chord of the capacitive arc, and parameter a is introduced to take account of a depressed feature of the impedance diagram (constant phase element). The value of a is found to be about 0.9 whatever the experimental conditions. The value of C was determined at the frequency fm corresponding to the apex of the capacitive loop: C
1 2pfm Re
2335
above for chronopotentiometric curves where the potential becomes almost constant after the same time. At the beginning (t = 3 min), the capacity C is about 3 mF cmÿ2 and decreases to 2 mF cmÿ2. Though not illustrated here, the value of C is smaller when the polarization current is higher. For instance, at j = 0.1 mA cmÿ2, C decreases from 5 to 3 mF cmÿ2, whereas when the current density is equal to 4 mA cmÿ2 it decreases from 0.8 to 0.6 mF cmÿ2. These values are too small to correspond to the double layer capacitance generally encountered for a bare metal. This indicates that ®lm formation by the anodic current takes place immediately and that the thickness increases with the polarization current. If the dielectric constant e of the surface ®lm is assumed to be 10, a value often found for a metal oxide (in a dry state), then one can evaluate the ®lm thickness through the expression of the planar capacitance: C
ee0 S , d
where e0 is the permeability of vacuum (8.85 10ÿ14 F cmÿ1), S is the surface area (cm2), and d is the thickness of the surface ®lm (cm). Figure 8(a) illustrates the change in the ®lm thickness computed according to equation (8) after a polarization period of 30 min for dierent applied current densities. From this Figure and by applying the linear least squares method, one obtains the following relationship: d 2:71 10ÿ7 E ÿ 0:748
9
which shows that the ®lm thickness at the steady state increases linearly when E is greater than 0.75 V. This threshold potential may be considered as a Flade potential beyond which the surface ®lm is thermodynamically stable. The resistance Re depends also on the current density applied to the electrode. Figure 8(b) depicts its variation with respect to the reciprocal of the current density, from which one gets the following
7
The time change of Re and C determined from impedance measurements at 0.35 mA cmÿ2 are displayed in Fig. 7. Both values tend to become constant after about 5 min. This observation is in agreement with the potential change described
8
Fig. 7. Variations of Re and C determined from Fig. 6.
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B. Zaid et al. Rt j
1 RT b nbF
14
Since b is equal to 9 Vÿ1 according to equation (13), and setting n = 2 for the oxidation process of Zn, one ®nds a b value close to 0.12. The value is small but is located within the theoretical range. However, an alternative hypothesis could explain the variation of Rt with j if it is accepted that the potential drop could be due to the ionic conduction induced by a high electrical ®eld. Indeed, the current density can be expressed by the relationship: nbFs E ÿ EF 15 j j0 exp d 0 RT
Fig. 8. Results calculated from EIS data with respect to the potential E at the steady state (t = 30 min) for dierent applied current densities. (a) Film thickness d of the surface layer and (b) diameter of capacitive loop Re.
relationship: Re 149224
11625 j
10
According to equation (10), Re can be divided into two components, Rf and Rt. The former can be considered as the resistance of the ®lm and is independent of the polarization current (Rf=149). The second term, Rt, depends on the current density, and can be interpreted in two dierent ways. In the ®rst hypothesis, Rt is considered as the charge transfer resistance, and thus: Re Rf Rt
with Rt j 11625 mV
11
This charge transfer resistance (Rt) can be related to the activation energy, parameters deduced from the Tafel relationship: bnF j j0 exp bZ with b 12 RT In this relationship, j0 is the exchange current density at the equilibrium potential (mA cmÿ2), b is the Tafel coecient (Vÿ1), b the transfer coecient (a constant between 0 and 1), Z the overpotential (V), n the number of electrons involved in the charge transfer reaction; F, R and T are the usual physical constants. From equation (12), one derives readily: 1 dj j0 b exp bZ jb Rt dZ and thus:
13
where d ' is the thickness of surface ®lm, EF the Flade potential or potential at d ' = 0, and s the jumping distance of the charge carrier, typically the atomic distance between passive species [13, 14]. The potential gradient close to 106 V cmÿ1, according to equation (9), is in agreement with ionic migration through the ®lm under high electrical ®eld. However, if one considers that the ®lm thickness d ' is equal to that de®ned in equation (9), then one may conclude that the current density should be independent of the potential. This condition is in general ful®lled under potential control. That is, the ®lm thickness d ' changes, so that the current ¯ow through the ®lm becomes equal to the passive current density, corresponding to its dissolution. Under current regulation where j is constant, both E and d ' change simultaneously according to equation (15). By subtracting the ohmic drop of the solution Rsol=(65 O cm2) and also the ohmic drop of the ®lm (Rf=150 O cm2) from the overall potential obtained at t = 15 min (cf. Figure 4), the potential±current relationship at the (almost) steady state is obtained [Fig. 5(b)]. j 0:033 exp 1:05E
16
If the value of b is assumed to be 0.5, equation (15) gives d '/s = 36, equivalent to 36 monolayers or 13 mC cm2. If s is about 0.3 nm, then the overall thickness of the surface layer is 10 nm. The calculated charge is about one ®fth of that involved at break point B in Fig. 4, i.e. the current eciency to form the surface layer is about 20%, and the charge loss might correspond to the fact that a large amount of Zn2+ ions escape into the solution according to equation (4). This result is in contradiction with equation (9) which stipulates a much lower thickness. The origin of this inconsistency has not been established, but may be due to the fact that e is not constant and could be higher than 10 if we assume that the ®lm contains many more water molecules at higher current densities.
A two-step electropolymerization
2337
Table 1. XPS-determined atomic ratio of some elements of the surface layer formed on a Zn electrode at dierent Na2S concentrations Na2S (M)
S/O 0.05 0.1 0.2 0.4
S/Zn
O/Zn
0.70 0.60 0.64 0.67
1.14 1.15 1.19 1.16
0.54 0.52 0.53 0.53
(S + O)/Zn 1.84 1.74 1.83 1.83
j = 0.35 mA cmÿ2 for 3 min.
Concerning the impedance data, an explanation may be that the electrode impedance results from two ®lm resistances, Rf and Rm. Rf could correspond to the ®lm formed at lower polarization, associated with break-point A in Fig. 4. As stated above, the electrical charge corresponding to this point is practically independent of the applied current, and therefore its thickness may be also considered as constant over this ®lm, and up to breakpoint B another ®lm having a semiconducting nature Rm is formed. Beyond point B, further current ¯ow no longer contributes to the growth of this surface ®lm. However, it is worth noting that this behavior holds only for current densities lower than 1 mA cmÿ2, and for higher j values (4 mA cmÿ2) a decrease of the potential occurs after point B. This phenomenon has not been elucidated but could be due to strong doping of the layer which should lower its resistivity. The ®lm resistance Rf obtained experimentally is markedly higher than that given in the literature, 200 O cm [15] and may be explained by the fact that the resistivity determined in this study is due essentially to ionic conduction through the ®lm, whereas that given in the literature corresponds to electronic conduction.
XPS STUDIES OF THE ZINC SURFACES TREATED WITH AQUEOUS Na2S XPS analysis was performed on Zn plates treated electrochemically at 0.35 mA cmÿ2 for 3 min in dierent Na2S solutions (0.05, 0.1, 0.2 and 0.4 M). XPS analysis of these modi®ed Zn surfaces shows the presence of C, S, O and Zn. The signal of carbon consists of three peaks (relative intensities 1:0.24:0.14), located at 285, 286 and 288.7 eV and attributable to C±C, C.O and O.C±O groups, respectively [16, 17]. The S2p signal shows the two components, S2p1/2 and S2p3/2, at 162 and 163.3 eV, respectively [18], characteristic of a metallic sul®de. A peak of low intensity at 168.5 eV due to sulfates or sul®tes was also detected [16, 18]. The O1s signal is characterized by a very large peak centered at 532 eV, corresponding to zinc oxides and zinc hydroxides. The Zn2p signal at 1021.9 eV is very strong, and can be attributed to oxidized zinc and to zinc sul®de [16].
The atomic ratios, S/O, S/Zn, O/Zn and (S + O)/ Zn, were calculated for the various Na2S concentrations (Table 1). From these results, it appears that: (i) no signi®cant variation of the surface composition was detected with Na2S concentrations varying from 0.05 to 0.4 M; (ii) the hypothesis of a pure ZnS layer must be rejected since the O/Zn atomic ratio of about 1.1 is far higher than the S/ Zn ratio (0.65); (iii) dierent varieties of Zn oxides such as ZnO, Zn(OH)2, hydrated oxides and probably oxygen contamination must be present at the surface since the (S + O)/Zn ratio (about 1.80) is much higher than 1. It is also worth noting that a surface of Zn mechanically cleaned, then dried immediately with air depicts a very intense O1s signal, with an O/Zn atomic ratio of about 2.3, but also an important C1s signal, resulting from contamination (C/ Zn = 1.6). Deconvolution of the C1s signal leads to three main components at 285, 287.2, and 289.5 eV with relative intensities (1:0.2:0.2), the latter two probably being due to carbonyls and carbonate groups [16]. From these data, it is concluded that mixed layers of ZnS (65±70%) and ZnO or Zn(OH)2 (30±35%), are formed on the Zn surface, in agreement with a (S + O)/Zn ratio greater than 1. In conclusion, all the results obtained by potentiodynamic polarization, impedance measurements and XPS analysis, indicate that after treatment in Na2S the Zn surface is modi®ed by mixed layers of ZnS and zinc oxide, typically ZnOxHy with 1 R xR 2, and 0 R yR 1. These layers are extremely thin (thicknesses <10 nm, con®rmed by SEM analysis) and have a high resistivity (r>106 O cm). The zinc passivation which is also observed in alkaline medium, increases dramatically with the presence of sul®des which even at very low concentrations (0.1 mM Na2S in 0.25 M NaOH solution) slow down Zn dissolution. PYRROLE ELECTROPOLYMERIZATION ON PRETREATED ZINC ELECTRODE In order to synthesize homogeneous and adherent PPy ®lms on Zn electrodes several physical and chemical parameters must be optimized. The time and the current density of treatment of the zinc surface in Na2S aqueous solution, the pyrrole concen-
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B. Zaid et al. Galvanostatic polarization
Fig. 9. Chronopotentiometric curves obtained in aqueous 1 M Na2C2O4+0.9 M Py (pH = 6) at dierent current densities. j: (a) 0.1RjR 1 mA cmÿ2; (b) 5 mA cmÿ2; (c) 10 mA cmÿ2; (d) 20 mA cmÿ2.
tration, the nature of the electrolytic salt and the choice of the polarization technique are all-important. The best conditions were found when the zinc surface was polarized at 0.35 mA cmÿ2 for 3 min and then electropolymerization of pyrrole performed with aqueous 0.1 M Na2C2O4+0.9 M pyrrole solutions. PPy ®lms having poor homogeneity and moderate adherence on Zn were obtained in the case of potentiodynamic polarization. On the contrary, by galvanostatic polarization homogeneous and strong adherent PPy ®lms were obtained, as will be shown in the following.
By using the above optimal conditions very dierent chronopotentiometric curves were obtained with applied current densities ranging from 0.1 to 20 mA cmÿ2 (Fig. 9). Black, homogenous PPy ®lms can be deposited on a pre-treated zinc electrode only if the applied density is greater than 2 mA cmÿ2. For values of j equal to or smaller than 1 mA cmÿ2, the potential of the electrode remains negative and corresponds therefore to oxidation of zinc [Fig. 9(a)]. At 5 mA cmÿ2, a rapid increase of the potential up to 1.2 V is ®rst observed, followed by a fast decrease to 0.5 V. This indicates that the underlying zinc is oxidized ®rst followed by a rapid increase up to 2.2 V corresponding to pyrrole oxidation through the resistive layer. It is followed again by a slow decrease and a stabilization at 1.3 V [Fig. 9(b)]. For very high current densities ( j = 20 mA cmÿ2, Fig. 9), overoxidation of the PPy occurs and large amounts of gas are released at the anode due to the oxidation of oxalate ions: ÿ COO2ÿ ÿ42CO ÿ 2 2e 2 ÿ
17
The PPy ®lm obtained in this case has a dendritic texture. Moreover, OHÿ ions are formed at the cathode in large amounts, thus increasing the pH of the solution which must be readjusted by addition of oxalic acid.
Fig. 10. XPS spectra of N1s. PPy ®lm formed in 0.1 M Na2C2O4+0.9 M Py at 4 mA cmÿ2 for 45 s.
A two-step electropolymerization From these experiments, it appears that the optimal conditions of electrolysis require current density ranges from 3 to 10 mA cmÿ2, and a Zn electrode which has been treated in 0.2 M Na2S at 0.35 mA cmÿ2 for 3 min. The presence of oxalate is necessary to obtain PPy ®lms. Surprisingly, if the same galvanostatic conditions are used without pretreatment of the Zn electrode in sul®de medium, no PPy deposit is observed, and the electrode potential rises to very high values, indicating the formation of an insulating layer identi®ed by XPS as Zn-oxalate. Similarly, if electrolysis is performed with a sul®de-pretreated 2ÿ Zn electrode but with another salt (ClOÿ 4 , SO4 , ÿ ÿ NO3 , PF6 ), no PPy ®lm is formed. It appears, therefore, that there is a synergistic eect between oxalate and the mixed ZnS + ZnOxHy layer. XPS analysis reveals that this layer is modi®ed by inclusion of zinc oxalate. This new layer structure which now allows electropolymerization of pyrrole seems to be a compromise between a compact insulating layer (ZnC2O4) and a more porous one (ZnS + ZnOxHy). Spectroscopic characterization of PPY ®lms. With the standard Zn treatment previously de®ned (0.35 mA cmÿ2, 0.2 M Na2S, 3 min) and using an aqueous solution of 0.9 M pyrrole + 0.1 M Na2 C2O4 for the electropolymerization reaction, electrode specimens were analyzed after dierent electrolysis time at j = 4 mA cmÿ2. After 10 s of polarization, the N1s signal centered at 399 eV is extremely weak, and the S2p doublet which appears at 162.2 and 163.4 eV is characteristic of sul®de [18]. The very strong Zn2p and O1s signals indicate that PPy is not formed at this stage of the electrolysis. After 45 s, the N1s signal increases and comprises mainly three components at 398.6 (17%), 400.4 (66%) and 401.8 eV (17%) (Fig. 10) characteristic of a doped PPy (17%). A strong S2p signal at 168.3 and 169.9 eV was also found, attributable to sulfate ions [16, 17], meaning that zinc sul®de is oxidized to ZnSO4 or ZnSO3. Interestingly, in the absence of pyrrole, neither sulfate nor sul®te was detected in the surface ®lms. This can be due to the fact that partial oxidation of sul®de to sulfate leads, in the absence of pyrrole, to the direct diusion into the solution whereas, when electropolymerization is occurring, sulfate and sul®te anions are incorporated into the PPy ®lm and participate in its doping. Full doping of the PPy is thus obtained by oxalate, sulfate and sul®te ions. The Zn2p signal is also strong, indicating that the metal is substantially oxidized at this stage of the electropolymerization. After 90 s of polarization, the zinc signal has completely disappeared, and the nitrogen signal is identical to that observed after 45 s. The doping level is the same as previously, and the sulfur signal remains fairly constant (S2p/NTotal=0.05), indicating that the zinc sul®de layer is oxidized slightly,
2339
and that the sulfate or sul®te ions diuse through the PPy layer. After 3 min polarization, IR and Raman spectroscopy con®rm that the layers formed are identical to the oxidized polypyrrole obtained in the literature [19, 20]. CONCLUSION From these results we can conclude that the electrochemical treatment of Zn in 0.2 M Na2S gives mixed layers containing ZnS and ZnOxHy whose composition appears to be constant for a standard current intensity regardless of the Na2S concentration. These layers are very thin and must be optimized to allow pyrrole electropolymerization. A kinetic compromise has to be found for the electrodeposition of PPy, and the best results are obtained at constant current and for intensities ranging between 2 and 10 mA cmÿ2. REFERENCES 1. T. A. Skotheim, (ed.), Handbook of Conducting Polymers, Vol. 1. Marcel Dekker, New York, 1986, and references therein. 2. H. S. Nalwa, (ed.), Handbook of Organic Conductive Molecules and Polymers, Vol. 2. John Wiley and Sons, Chichester, New York, 1997, and references therein. 3. W. Janssen and F. Beck, Polymer 30, 353 (1989). 4. P. HuÈlser and F. Beck, J. Appl. Electrochem. 20, 596 (1990). 5. C. A. Ferreira, S. Aeiyach, J. J. Aaron and P. C. Lacaze, Electrochim. Acta 41, 1801 (1996). 6. C. A. Ferreira, B. Zaid, S. Aeiyach and P. C. Lacaze, in Proc. Organic Coatings, ed. P. C. Lacaze. AIP Press, Paris, 1995, p. 159. 7. H. Gerischer, Z. Elektrochem. 54, 540 (1950). 8. P. L. Allen and A. Hickling, Trans. Faraday Soc. 53, 1626 (1957). 9. D. W. Shoesmith, P. Taylor, M. G. Bailey and B. Ikeda, Electrochim. Acta 23, 903 (1978). 10. A. J. Bard and R. Faulkner, (ed.), Electrochimie, Principes, MeÂthodes et Applications. Masson, Paris, New York, 1983. 11. A. J. Calandra, N. R. de Tacconi, R. Pereiro and A. J. Arvia, Electrochim. Acta 19, 901 (1974). 12. P. Bernard, M. Keddam and H. Takenouti, J. Electroanal. Chem. 396, 325 (1995). 13. K. J. Vetter, Electrochim. Acta 16, 1923 (1971). 14. N. Cabrera and N. F. Mott, Rept. Progr. Phys., 1948± 1949, 12, 163. 15. C. Weast, (ed.), Handbook of Chemistry and Physics, 67th edn. CRC Press, Inc., Boca Raton, Florida, 1986±1987. 16. C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder and C. E. Muillenberg, (ed.), Handbook of X-Ray Photoelectron Spectroscopy, 1978, PerkinElmer, MN. 17. E. T. Kang, K. G. Neoh and K. L. Tan, Adv. Polym. Sci. 106, 135and references therein (1993). 18. G. Deroubaix and P. Marcus, Surf. Interf. Anal. 18, 39 (1992). 19. J. P. Marsault, K. Fraoua, S. Aeiyach, J. Aubard, G. Levi and P. C. Lacaze, J. Chim. Phys. 89, 1167 (1992). 20. K. M. Cheung, D. Bloor and G. C. Stevens, J. Mater. Sci. 25, 3814 (1990).