Phase boundaries in layer-by-layer electrodeposited polypyrrole resulted from 2D–3D growths of polymer sublayers

Journal of Electroanalytical Chemistry 626 (2009) 47–58

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Phase boundaries in layer-by-layer electrodeposited polypyrrole resulted from 2D–3D growths of polymer sublayers Maria Grzeszczuk *, Jerzy Kalenik, Anna Ke˛pas-Suwara Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wroclaw, Poland

a r t i c l e

i n f o

Article history: Received 4 July 2008 Received in revised form 23 October 2008 Accepted 5 November 2008 Available online 14 November 2008 Keywords: Thin multilayer polymer structures Potentiostatic nucleation and growth Double layer relaxation EQCM SEM

a b s t r a c t Potentiostatic deposition of polypyrrole on gold and polypyrrole surfaces has been analyzed using theoretical models of metal electrodeposition. The so-called induction period of electrodeposition was not excluded from the analysis. Such global analysis of chronoamperograms of potentiostatic polymerization was presented for the first time. It was done under a general assumption that two different nucleation and growth mechanisms can proceed in parallel. At +0.8 V vs. SSCE, the mixed 2DI–3DI growth mechanism of the polymer was found and characterized by rates increasing with the electrode potential. The 2D structure of the polymer predominates at the initial stage of electrodeposition. It was proved by SEM images of the polymer layers electrodeposited at conditions derived from the mathematical analysis. A contribution of the 2D structure in the deposited polymer depends on the chemical nature of the substrate surface and it increases with the potential. The formation of the 2D polymer phase at the onset of each electrodeposition introduces new local phase boundaries in the multilayer polypyrrole electrodes prepared via layer-by-layer electrodeposition of polypyrrole on polypyrrole from solution of the same composition. The relaxation time of the decaying double layer current was found to increase with time of deposition from a usual 103 s range to a 100 s range, mainly due to an increase in the double layer capacitance. EQCM data support results of chronoamperometry on contribution of the double layer current to the initial current of potentiostatic deposition monitored on a time scale up to few seconds. The double layer process accompanying 2D–3D deposition of polypyrrole was shown to depend on the nature of the substrate electrode (gold, polypyrrole). EQCM results indicate substantial differences in compositional changes of the interphase regions between gold/aqueous and polypyrrole/aqueous systems. The Sauerbrey’s equation is shown valid only when the resonant frequency change due to a thin rigid polymer deposit exceeds that due to reconstructions of the double layer and the diffusion layer of the electrodes. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Recently we had shown that interrupted, so-called layer-bylayer, electrodeposition of polypyrrole from a solution of the same composition results in the multilayer polymer electrode that slows down transport of ions [1]. Impedance analysis indicated that rates of ion diffusion in the multilayer polymer electrodes were strongly affected due predominantly to hindrance of interlayer ion transfer. Subsequently, chronoamperograms recorded during potentiostatic deposition of single layers and multilayers of polypyrrole have been subjected to a quantitative analysis. Results of this analysis supported by EQCM data will be presented in this communication. Early studies on mechanisms of electrodeposition of polypyrrole have indicated some similarities with electrodeposition of metals [2–4]. Later on, nucleation and growth mechanisms of polythiophenes and closely related polymers were investigated [5–14]. * Corresponding author. Tel.: +48 71 375 7336; fax: +48 71 328 2348. E-mail address: [email protected] (M. Grzeszczuk). 0022-0728/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2008.11.002

There, theoretical considerations and resulting diagnostic equations for electrodeposition of metals were adapted for the polymer electrodeposition [11–18]. We will follow this approach to the multilayer polypyrrole electrodes to find out characteristic features of electrodeposition of polypyrrole on polypyrrole as compared to electrodeposition of polypyrrole on gold. The multilayer polypyrrole electrode was obtained in the procedure that should result in the same chemical composition of the sublayers. Exactly the same conditioning and redox switching measurements in the monomer free supporting electrolyte were performed after deposition of each sublayer of the polymer electrode [1]. The later treatment was shown to influence composition and/or structure of the virgin polypyrrole deposit [19]. Electrochemical preparation and resulting properties of thin multilayer polymer structures are of considerable interest in many applications including analytical sensors and electronic devices. A main purpose of this study was to find out whether the mechanism of the layer-by-layer growth might cause new phase boundaries in the multilayer structure of the same polymer material. An

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increase of the interfacial contribution to the electrical resistance of the multilayer electrode was found by us before from the EIS analysis [1]. 2. Experimental 2.1. Conditions of electrodeposition Polypyrrole was electrodeposited at a potential of +0.80 V vs. SSCE on three different substrate electrodes, namely: two polycrystalline gold electrodes Au1 and Au2 (Metrohm, Ageom = 0.07 cm2), thin film Au/quartz crystal electrodes (International Crystal Mfg. Co. Inc., Ageom = 0.25 cm2), and a number of the single layer or multilayer polypyrrole electrodes obtained by using the gold substrates described above. Thicknesses of the polymer layers, l, were in the submicrometer range. Estimations of thickness of the deposited polymer layer were based on the electric charge consumed during electrodeposition, assuming after Diaz and Castillo that 240 mC/cm2 results in 1 lm layer of polypyrrole [20]. The solution used in electropolymerization procedures was 0.6 M pyrrole in 0.04 M aqueous sodium hexafluoroaluminate. Thereafter polypyrrole electrodes were washed in water, conditioned at OCP for a few minutes in the corresponding supporting electrolyte [0.04 M(Na3AlF6)aq.] and submitted for further studies (CV, EIS, EQCM) as reported earlier [1]. Afterwards, some polypyrrole electrodes were used as substrate electrodes for electrodeposition of polypyrrole. A sequential repetition of the preparation and examination procedures resulted in the multilayer polymer electrodes. The total thickness of polypyrrole was from 0.1 lm to 1.0 lm. Additional measurements, to monitor the initial current in the millisecond time range, were performed using a polycrystalline gold electrode Au3 (Ageom = 0.03 cm2). Effects of the electrode potential, +0.6 V, +0.7 V, +0.8 V, was studied for electrodeposition of polypyrrole hexafluorosilicate on the conventional Au electrode (Ageom = 0.07 cm2) using 0.6 M pyrrole in 0.04 M (Na2SiF6)aq. solution. SEM images were made with accelerating voltage 20 kV using JSM-5800LV scanning microscope, Joel.

with a pyrrole–electrolyte solution. The initial current is predicted to bear a predominant contribution from the charging current of the substrate electrode. In addition, when electrodeposition of polypyrrole continues, a change in the charging current of the polymer modified electrode as compared to the substrate electrode has to be considered. Furthermore, diffusion of solution soluble species, i.e. the monomer and low molecular mass oligomers to/ from the electrode may influence the electrodeposition current. However, mechanism and kinetics of nucleation and growth of the new phase impose the strongest impact on chronoamperometric characteristics of electrodeposition [15–16]. In general, three successive regions in the current–time curves recorded during potentiostatic deposition of our thin polypyrrole layers were observed: a decrease in dc current followed by an increase in dc current, and a final asymptotic trend in dc current. The later might be predicted to change with increase in time of electrodeposition due

2.2. Apparatus GPES of Autolab Electrochemical Instruments, Ecochemie, and an M-106 nanobalance, UELKO, were the instrumentation used in conjunction with the one compartment conventional and EQCM electrode cells. The reference and auxiliary electrodes were a sodium saturated calomel electrode (SSCE) and a glassy carbon rod, respectively. All measurements were performed at room temperatures, i.e. mostly at (25 ± 1) °C. The nonlinear least squares, NLLS, data fitting procedures were done with MATHEMATICATM and/or Microsoft Excel. 2.3. Chemicals Pyrrole (Merck, Sigma, Fluka, Aldrich) was distilled under reduced pressure immediately before use in the electrode cell. Sodium hexafluoroaluminate (p.a. Aldrich) and sodium hexafluorosilicate (Aldrich) was used as received. Twice distilled water for the preparation and washing procedures and nitrogen for deoxygenation of the cell solution were used. 3. Results and discussion There are various processes occurring in parallel and/or in series to each other when a constant dc potential allowing polymer electrodeposition is applied to a substrate gold electrode in contact

Fig. 1. Comparison of the experimental and theoretical best fit currents of potentiostatic deposition of polypyrrole on gold and polypyrrole: (A) AuEQCM/PPYs system, (B) AuEQCM/PPYm3 system; black – experimental, red – simulated total, violet – simulated 2DI, green – simulated 3DI, orange – simulated DL. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)

M. Grzeszczuk et al. / Journal of Electroanalytical Chemistry 626 (2009) 47–58

to many factors. Therefore, the time scale of electrodeposition can be of importance for mechanistic considerations. Judging from the relative values of the minimum currents observed during potentiostatic deposition of polypyrrole (see Fig. 1 in [1]), one has to take into account the nucleation and growth of the polymer phase at times preceding the minimum, i.e. a contribution of the nucleation and growth current to the current dominated by the double layer phenomena. In our opinion to avoid misinterpretation, the quantitative analysis of experimental data should rather concern the whole current–time curve recorded under potentiostatic conditions. Usually, the separate parts of chronoamperograms had been taken for the analysis of electrodeposition of conducting polymers, and always, the so-called induction period of electrodeposition was omitted there [5–14]. To the best of our knowledge, this communication delivers the first example of the overall quantitative analysis of current–time dependencies for potentiostatic deposition of conducting polymers. 3.1. Theoretical models Any contribution of diffusion of pyrrole and/or its oligomers to deposition current of polypyrrole was excluded. A decrease in current with time, at longer times of deposition, was not observed for our polymer, even at deposition times approaching 100 s [21]. On the other hand, slow diffusion of counterions in the polymer phase can contribute to time dependent characteristics of the double layer and/or ‘‘redox” capacitor of the growing polypyrrole electrode [22]. In fact, the overall series capacitance of the polymer electrode, can be in the order of mF for our system [1]. The solution resistance is expected to decrease with time of electrodeposition due to an increase in an effective area of the electrode. Note, that the hexafluoroaluminate medium is characterized by values of the solution resistance of the order of kX [1]. Because of these factors one cannot rule out a significant contribution of charging current even at times as long as a few seconds. Also, a presence of water in the system can be of great importance for kinetics of electrodeposition of conducting polymers, as shown by Downard and Pletcher for polythiophene [23]. The functions written below were considered by us as possible contributions to the observed electrodeposition current–time dependencies [15–18,24]. They are marked as 2DI or 2DP, 3DI or 3DP, and DL to indicate instantaneous or progressive nucleation and two-dimensional growth, instantaneous or progressive nucleation and three-dimensional growth, and double layer, respectively.

  t I2DI ðtÞ ¼ a2DI  t  expðb2DI  t 2 Þ ¼ Im;2DI  t m;2DI " !# 2 2 t  t 1 m;2DI  exp   2 t 2m;2DI  2 t I2DP ðtÞ ¼ a2DP  t 2  expðb2DP  t 3 Þ ¼ Im;2DP  t m;2DP " !# 3 3 t  t 2 m;2DP  exp   3 t 3m;2DP    I3DI ðtÞ ¼ c3DI  1  exp d3DI  t2    I3DP ðtÞ ¼ c3DP  1  exp d3DP  t 3   t IDL ffi I0  exp 

sDL

ð1Þ

ð2Þ ð3Þ ð4Þ ð5Þ

The parameters of the current–time Eqs. (1)–(5) are related to the physicochemical parameters of electrodeposition (a, b, c, d, I0, sDL) and the maxima of current–time functions for the 2D nucleation and growth (Im, tm) [15–18]. Details on their meanings will be provided later when needed for a purpose of a kinetic analysis. The Eqs. (1)–(4) used in our analysis are limiting cases of the more general current–time functions shown by Abyaneh and

49

Fleischmann in [17] for 2D and 3D electrocrystallisation. The two limiting cases in question concern the number N of nuclei which would be formed in time t. They would depend on the nucleation rate constant which could be either very high or very low as compared to the time scale of growth then leading to instantaneous or progressive nucleation, respectively. The Eqs. (3 and 4) were derived for nucleation and subsequent growth of metal nuclei having the shape of a right-circular cone [15]. Another shape of the nucleus considered in the theoretical derivation is a hemisphere [15,25]. The two cases are theoretically predicted to be distinguished by different current–time dependencies. The topographic features of the polymer ‘‘nucleus” are expected to be more sophisticated as compared to the metal case. Obviously, a polymer nucleus would not have a circular base. For a polymer, as polypyrrole, one should rather expect a strong impact of the molecular mechanism of the polymer chain formation as well as interaction of monomer and/or oligomers not only with the substrate electrode material but also electrolyte on the topographic features of the new phase [19,26]. Nevertheless, we have assumed that the geometry of the ‘‘polymer nucleus” is of negligible importance in comparison with the kinetics of its nucleation and growth in different directions of space, especially when electrodeposition of the same polymer is under a quantitative analysis. Additionally, this assumption is supported by experimental data as no descending currents were observed outside the double layer current decay region, which is the behavior predicted for the 3D hemisphere case [25]. Therefore the simplest, right-circular cone model of the nucleus is adopted for the analysis of potentiostatic deposition of polypyrrole in presence of the aqueous hexafluoroanion electrolytes. 3.2. Analysis procedure Our goal has been to find the best fit between a whole chronoamperometric curve and theoretical models of phase growth. It was done for the entire time scale of experiments using the following three-stage procedure: (1) the NLLS fit with a preliminary simplified theoretical model using the experimental chronoamperogram without data points corresponding to the current minimum and its vicinity – it allows to calculate preliminary simulated Ipresim–t data encompassing entire time scale of the experimental data; (2) an inspection of a Idiff–t curve obtained as a result of subtraction of the Ipresim–t curve from the experimental I–t curve; (3) the NLLS fit of the whole experimental I–t curve with the concurrent theoretical models – the best fit corresponds to the model leading to a minimum value of variance of the fit. As the number of data points in each experimental current–time curves has been in the range of hundreds and a number of parameters to find was six, so the degree of freedom for our fitting procedure was in a range of hundreds as well. The preliminary models corresponded to the 3D growth as no peak currents characteristic for the 2D growth were observed on the recorded chronoamperograms (see Eqs. (3 and 4)). However, the peak shaped Idiff–t results indicated clearly an involvement of 2D growth of polypyrrole masked in the experimental chronoamperograms by the double layer current at the initial stage of electrodeposition. Therefore, the final step of our analysis was to find the best NLLS fit between experimental data and the following theoretical equations of currents corresponding to the parallel 2D–3D growth mechanisms [11,18,27]:

I2DI3DI ¼ IDL þ I2DI þ I3DI

ð6Þ

I2DI3DP ¼ IDL þ I2DI þ I3DP

ð7Þ

I2DP3DI ¼ IDL þ I2DP þ I3DI

ð8Þ

I2DP3DP ¼ IDL þ I2DP þ I3DP

ð9Þ

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Table 1 Goodness of approximations of experimental I–t data by different nucleation and growth mechanisms in terms of variance r2I [10+12A2]. The electrodes are described by substrate gold sample (Au1, Au2), polypyrrole (PPY) layer structure (s – single stage deposition, mn – multistage deposition with indication of a number of sublayers by n-value).

sDL ðtÞ ¼ ðR þ DR  tÞðC þ DC  tÞ  R  C þ ðR  DC þ C  DR Þ  t ¼ sDL þ DDL  t

Electrode

2DI–3DI

2DI–3DP

2DP–3DI

2DP–3DP

ln IDL ¼ ln I0 

Au2/PPYs Au1/PPYs Au1/PPYs Au1/PPYm2 Au1/PPYm3 AuEQCM/PPYs AuEQCM/PPYm3 AuEQCM/PPYm5

0.144 0.304 0.279 0.430 0.516 3.37 5.40 7.51

0.205 0.450 0.383 0.483 0.564 4.68 6.51 8.68

0.549 1.231 1.414 0.710 0.785 8.28 11.41 13.75

0.936 2.485 2.375 0.906 0.967 8.57 17.13 18.41

Results are shown in Table 1 where goodness of the each NLLS fit is represented by values of its variance. A graphical comparison of the quality of fitting with the best theoretical model is illustrated in Fig. 1 for electrodeposition of polypyrrole on EQCM gold and EQCM polypyrrole electrodes. The best fit criterion indicates the 2DI–3DI growth mechanism for polypyrrole irrespective of the substrate electrode being either bulk polycrystalline Au electrode, either a polycrystalline Au film on quartz, or the thin film polypyrrole electrode. The instantaneous nucleation of polypyrrole at +0.8 V vs. SSCE was found. This potential is very close to the highest potential of electropolymerization that allows deposition of polypyrrole in water environment without irreversible overoxidation of the polymer [19,26]. Further inspection of the variance data indicates that the 2DI–3DI mechanism is slightly better for electrodeposition of polypyrrole on gold than electrodeposition of polypyrrole on polypyrrole. However, one should consider a difference in an impact of double layer phenomena on the observed characteristics. Experimental I–t curves and corresponding best fit, i.e. I2DI–3DI–t, curves are compared in Fig. 1 for electrodeposition of polypyrrole on gold and polypyrrole on polypyrrole using the EQCM electrodes. Also, decomposition of the total current function into partial currents due to contributing processes of 2DI growth (I2DI) and 3DI growth (I3DI) of the polymer as well as double layer of the electrode (IDL) are shown there.

ð10Þ

Then Eq. (5) must be substituted by Eq. (11) in the fitting functions, i.e. in Eqs. (6)–(9).

t

ð11Þ

sDL þ DDL  t

In the above description of the double layer current, we have assumed, for simplicity that the relaxation time increases linearly with time of electrodeposition. Therefore, a linear dependence between ln I and t could not be observed. The additional measurements in the millisecond range, which was not accessible in the deposition experiments, support the assumption on the change of the parameters of the double layer with time of electropolymerization. A rough comparison of sDL-values obtained for the deposition time window experiments (see Table 2) and the additional millisecond time window experiments gives

sDL; 100  3 ¼ 103 sDL; 10

ð12Þ

where and indicate the former and the latter time window, respectively. An another probe for the double layer behavior due to electrodeposition is the absolute value of the initial current, which is expected to be higher for a larger surface area of the electrode as R is determined predominantly by the solution resistivity and the electrode surface area (assuming geometry of the electrochemical cell is fixed). As the same potential and solution conditions were kept in our measurements, values of I0, determined from the ln I  t linearization analysis should increase in the following order: AuMetrohm, AuMethrom/PPY, AuEQCM, i.e. with increasing surface area of the substrate electrode. The predicted behavior is in fact observed in our experimental data. Furthermore, it has been observed that I0-parameter obtained from the analysis in the millisecond range is always higher than I0-parameter obtained from the analysis in the second range (see Table 2) and leads to the following relation between the corresponding resistances:

I0; R ffi 6 101 I0; R

ð13Þ

Thus the ratio of the corresponding capacitances is

3.3. Charging current

s

The equivalent series capacitance and resistance of the studied electrodes are expected to change with the deposition time of polypyrrole. The former might increase significantly due to the so-called redox capacitor formed by the oxidized polymer chains and the doping counterions. If one assumes, for simplicity, that the double layer relaxation time, sDL, changes with a time unit of electrodeposition by DDL due to changes DR and DC in the solution resistance, R, and the double layer capacitance, C, respectively, so

C 103 s ¼ R  P 102 1 C R 6 10

ð14Þ

That supports the hypothesis of a predominant contribution of the growing capacitance (from lF-range values at gold to nearly mF-range values at polypyrrole [1]) to the double layer current behavior. By applying Eq. (11) instead of Eq. (5) in the fitting procedures using Eqs. (6)–(9) for the approximation functions, the goodness of

Table 2 Growth of polypyrrole on different surfaces. Values of model parameters and their standard deviations (in parentheses) obtained from the NLLS fits between the experimental data and the 2DI–3DI nucleation and growth model (see Eq. (6) for I2DI–3DI). Thicknesses estimated from polymerization charge: li – corresponding to monitored stage of deposition, lP – total. Meanings of other symbols have been explained in the text. Electrode

li [lm]

lP [lm]

I0 [lA]

sDL [s]

Im [lA]

tm [s]

c [lA]

Au2/PPYs Au1/PPYs Au1/PPYs Au1/PPYm2 Au1/PPYm3 AuEQCM/PPYs AuEQCM/PPYm3 AuEQCM/PPYm5

0.71 0.44 0.18 0.26 0.25 0.14 0.13 0.13

0.71 0.44 0.18 0.44 0.71 0.14 0.41 0.68

39.370 (0.183) 59.288 (0.294) 35.396 (0.283) 99.069 (0.538) 104.031 (0.628) 172.877 (1.084) 252.875(1.530) 271.343 (1.843)

3.761 3.830 5.196 1.128 0.889 3.870 2.596 2.154

29.152 (0.168) 38.078 (0.833) 21.815 (0.449) 27.424 (0.536) 29.809 (0.675) 78.083 (1.712) 75.021(1.966) 76.721 (1.802)

13.614(0.077) 8.665 (0.133) 8.032 (0.129) 2.641 (0.049) 2.029 (0.042) 6.293 (0.090) 4.898 (0.102) 4.829 (0.102)

68.292 106.283 55.350 54.079 54.959 170.628 161.971 151.086

(0.038) (0.064) (0.260) (0.023) (0.022) (0.145) (0.076) (0.056)

d [s2] (0.015) (0.039) (0.047) (0.035) (0.038) (0.169) (0.191) (0.226)

0.0017394 (0.0000192) 0.0050070 (0.0001559) 0.0047824 (0.0001377) 0.0399914 (0.0013440) 0.0664275 (0.0025885) 0.0071686 (0.0001889) 0.0121401 (0.0004550) 0.0118124 (0.0004559)

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plified double layer current. Therefore Eq. (5) was used for the analysis in the entire time window of the electrodeposition experiments in order to reduce a number of parameters of the fitting functions. So, the derived sDL-values should be considered as the mean values of the time dependent parameter corresponding to the time window used to monitor deposition of the polymer layer. The latter was in the range from 101 s to 102 s. This fact does not allow derivation of the double layer contribution at the very initial stages of deposition. Therefore, the determined sDL-values characterize the double layer of the growing oxidized polypyrrole/electrolyte interfaces. The time dependent relaxation time is expected to reach values of a range of seconds for the polypyrrole hexafluoroaluminate electrodes [1]. 3.4. Characteristics of 2DI–3DI growth Values of parameters of the I2DI–3DI fit function (Eq. (6)) together with values of their standard deviations are collected in Table 2. Values of standard deviation of the model parameters never exceed 5% of the parameter value, thus fulfilling requirements of a reasonably good quality model. Noticeably, the highest deviations concern sDL-values. Subsequently, the model parameters can be discussed in terms of yield and structure of deposited polymer, kinetics of electrodeposition, double layer phenomena, optimal conditions for electrodeposition. Also, eventual correlations between a mechanism of electrodeposition and electrical properties of multilayer and single layer polypyrrole electrodes can be proved. Results of decomposition of integrated chronoamperograms into electrical charges involved in double layer charging and mixed 2DI–3DI growth of polypyrrole are illustrated in Fig. 2 for the conventional gold and polypyrrole electrodes. There, the fraction of the charge calculated for the each contributing process is shown vs. the total charge density used for electropolymerization of pyrR t

Q

ðtÞ

Ipol ðtÞdt

¼ 0 Ageom . Fraction of the 2D structure in the role, qpol ðtÞ ¼ Apol geom 2DI ðtÞ , reaches a maxipolymer phase, given by f2DI ðtÞ ¼ Q DL ðtÞþQQ2DI ðtÞþQ 3DI ðtÞ mum value at the initial stage of electrodeposition where the contribution of the double layer phenomena, given by fDL ðtÞ ¼ Q DL ðtÞ , is either lower or similar. The former behavior Q DL ðtÞþQ 2DI ðtÞþQ 3DI ðtÞ characterizes electrodeposition of polypyrrole on gold. The later behavior characterizes electrodeposition of polypyrrole on polypyrrole. The Qi(t) variables (where i = DL, 2DI, 3DI) were calculated by integration of corresponding Ii (t) functions obtained from the NLLS analysis of the experimental chronoamperograms, i.e. Rt Q i ðtÞ ¼ 0 Ii ðtÞdt. The parameters of the I2DI–3DI function (see Eq. (6) with Eq. (1), Eq. (3) and Eq. (5)) are related to physicochemical parameters as follows [15–18,24]: 2

a2DI ¼

2  p  n  F  M  h  A  N 0  k1

q

;

b2DI ¼

p  M2  N 0  k21 q2 ð15a—15bÞ

c3DI ¼ n  F  A  k3 ;

Fig. 2. Distribution of electric charges due to processes contributing to electrodeposition of polypyrrole on gold, (A), and polypyrrole on polypyrrole, (B) and (C): (A) Au1/PPYs system (li = 0.18 lm), (B) Au1/PPYm2 system, (C) Au1/PPYm3 system. Meanings of colours and symbols as in Fig. 1. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)

our global approximation increases slightly. However, those changes of IDL do not influence results on nucleation and growth to an extent to change the conclusions of the analysis with the sim-

I0 ¼

DE ; R

d3DI ¼

p  M2  N0  k22 ; q2

sDL ¼ R  C

ð16a—16bÞ

ð17a—17bÞ

Symbols used above have the following meanings: N0 – number density of 2D (N0,2D) or 3D (N0,3D) active sites formed instantaneously, in [cm2]; k1 – rate constant of the 2D growth of the nucleus, in [mol cm2 s1]; k2 – rate constant of the 3D parallel (lateral) growth, in [mol cm2 s1];

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k3 – rate constant of the 3D perpendicular (outward) growth, in [mol cm2 s1]; A – electrode surface area active for 2D (A2D) or 3D (A3D) nucleation and growth; h – ‘‘height” of the 2D layer; q – density of the deposited 2D layer (q2D) or 3D layer (q3D); M – molar mass of pyrrole; n – number of electrons transferred in the deposition reaction; DE – dc potential step applied to the electrode; R – solution resistance (equivalent series resistance); C – double layer capacitance (equivalent series capacitance). The rate parameters obtained from the analysis together with electric charge densities used for charging of the double layer capacitor, DqDL, formation of the 2DI polymer deposit, q2DI,Rand fortdep

mation of the 3DI polymer deposit, q3DI (calculated as

0

Ii ðtÞdt

Ageom

,

where i = DL, 2DI, 3DI) are shown in Table 3. One notices that contribution of 3D growth increases with time of deposition. Ratios of the 3D to 2D charges are always higher for polypyrrole than for gold substrates. However, an increase in contribution of 2D growth of polypyrrole on polypyrrole is observed for the EQCM electrode as compared with the conventional electrodes. One observes a significant decrease in mass/thickness of the 2DI polymer phase, measured as q2DI, for polypyrrole electrodeposited on polypyrrole in comparison with polypyrrole electrodeposited on gold, when a similar amount of electricity per unit area of the electrode is used. For electrodeposition of polypyrrole on polypyrrole, the double layer relaxation time, sDL, follows similar dependencies as the Dq2DI-parameter. The above characteristics concern both types of substrate electrodes, i.e. conventional and quartz crystal ones. These results might indicate a correlation between 2DI growth and double layer structure and dynamics. Furthermore, the chemical nature of the substrate electrode surface affects time distribution of the processes contributing to electrodeposition. The gold surfaces have induced longer involvement and higher relative contribution of 2DI growth in electrodeposition than a polypyrrole surface does (see Fig. 2). It might result in a less ordered structure of the polymer deposited on gold than on polypyrrole, at the same electrodeposition charge per unit area in the both cases. N 0;2D k1 , and The ratio of two lateral growth rates, estimated as N0;3D k2 the outward growth rate constant, k3, were derived from the 2DI–3DI model parameters b, c and d, as b/d and c/(nFAgeom), respectively. The kinetic data shown in Table 3 again would indicate a similar growth mechanism of the polymer irrespective of the chemical nature and structure of the substrate electrode (two types of gold, thin polypyrrole layers, stacks of thin polymer layers). However, the yield of the 2DI polypyrrole phase, measured as q2DI, depends strongly on the chemical nature of the substrate surface.

3.5. Electric current/charge functions calculated from EQCM data Data on the change of the resonant frequency of the electrochemical quartz crystal microbalance with time of electrodeposition, Df(t), were used to calculate the time derivative of Df(t). If one assumes that Df(t) can be related to the electric charge consumed in the electrochemical process, Q(t), and the corresponding electrode current, I(t), as follows:

jDf ðtÞj ¼ C eqcm  DmðtÞ

ð18Þ

DmðtÞ ¼ kelchem  QðtÞ jDf ðtÞj ¼ C eqcm  kelchem  Q ðtÞ djDf ðtÞj ¼ C eqcm  kelchem  IðtÞ dt

ð19Þ ð20Þ ð21Þ

where Eq. (18) is know as the Sauerbrey’s equation, Ceqcm and kelare the EQCM and electrochemical constants, respectively [24,28]. The two functions that are predicted to be proportional to each other are shown in Fig. 3. A comparison of EQCM and chronoamperometry in view of time dependence of electrodeposition current is now possible. There is observed a clear difference between the EQCM and chronoamperometry data concerning the current functions at the initial stage of deposition. Electrogravimetry emphasizes the significant difference between the initial stages of electrodeposition on gold and polypyrrole. The contribution of double layer current to the electrode current measured at short times seems unquestionable in view of the differences in behavior observed in this region from the two different experimental observables: Df(t) and I(t). The current function derived from the EQCM frequency, djDf ðtÞj =ðC eqcm  kelchem Þ, is expected to discriminate double layer dt structure related phenomena as compared to nucleation and growth of a solid deposit on the electrode [28]. At longer times of electrolysis, the two current functions under comparison change with time in an approximately similar fashion. During the final stages of electrodeposition they differ by a factor of about 4  10+5 Hz/C which is in fair agreement with a theoretically predicted value of (Ceqcm kelchem) product for our system equal 3.9  10+5 Hz/C (calculated assuming: Ceqcm = 1.1  10+9 Hz/g [19,26,28], the oxidation level of electrodeposited polypyr3 role = 0.25 and AlF6 as the counterion, giving kelchem = 3.53  4 10 g/C). An alternative method to study correlations between the current and mass data is by integrating, instead of differentiating, the chronoamperometric curves The electric charge consumed in the monitored electrochemical process, Q(t), bears the double layer charging contribution, QDL(t), and the mass deposition contribution, Q2DI(t) + Q3DI(t). If one assumes that the double layer phenomena have negligible influence on the resonance frequency as compared to deposition of the polymer, then the following relation holds chem

Table 3 Parameters of electrodeposition of thin films of polypyrrole on gold and on polypyrrole derived from the parameters of the I2DI–3DI function. Symbols were explained earlier in the text except tdep which is the deposition time. Bold letter numbers correspond to the data for the multilayer structures. Electrode

tdep [s]

(k1’/k2’)a

109k3 [mol/(s cm2)]

q3DI [mC/cm2]

q2DI [mC/cm2]

DqDL [mC/cm2]

Au2/PPYs Au1/PPYs

185.0 75.2

1.25 1.15

5.06 7.87

159.76 95.16

9.35 7.77

2.12 3.24

Au1/PPYs Au1/PPYm2 Au1/PPYm3

58.4 83.0 81.2

1.27 1.34 1.35

4.10 4.00 4.07

36.04 60.70 61.05

4.13 1.71 1.42

2.63 1.60 1.32

AuEQCM/PPYs AuEQCM/PPYm3 AuEQCM/PPYm5

50.0 50.0 50.0

1.33 1.31 1.35

3.54 3.36 3.13

26.98 27.18 25.29

3.24 2.42 2.44

2.68 2.63 2.34

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M. Grzeszczuk et al. / Journal of Electroanalytical Chemistry 626 (2009) 47–58

ratio value with time must origin from the double layer total charge growing with time. Furthermore, before reaching the constant value, the ratio will decrease with time due to increasing contribution of the deposition charge. Plots of the ratio defined by Eq. (22), for experimental data of Fig. 3, is shown in Fig. 4. The observed characteristics differ substantially for electrodeposition of polypyrrole on gold and polypyrrole. Noteworthily, the prolonged deposition on gold resulting in estimated film thickness equal 0.75 lm follows, see Fig. 4B, characteristics of the ratio–time dependence predicted by Eq. (22). The ratio Q(t) characteristics for the long time electrodeposition of polypyrrole on gold was found reproducible. It can indicate a much slower deposition of polypyrrole on gold as compared to its deposition on polypyrrole and/or a higher contribution of the double layer in the former case. The same assumption, as used above, on the higher sensitivity of EQCM for the mass deposition than the double layer phenomena can be used for a comparison of the measured current and the resonant frequency derivative. Relevant plots corresponding to the experimental data shown in Fig. 4 are presented in Fig. 8 provided in the Appendix A because of a high noise level resulting from the differentiation of the chronoamperometric data. Eq. (22) is based on the assumption that Df originating from deposition of the polymer is much higher than that resulting from the double layer processes preceding and/or accompanying deposition of the polymer on the electrode. However, the polymer deposition must be preceded by dehydration of the gold surface, adsorption processes of pyrrole and/or its oligomers, as well as accompanied by electrolyte ions. The later in a form of counterions are becoming components of the polymer phase. When an EQCM electrode contact with liquid electrolyte, the Sauerbrey’s equation for the resonant frequency change, Df, is valid only when a contribution of an elastic mass deposit Dfm exceeds significantly a contribution of a viscous liquid Dfg. The two contributions, respectively, one finds in Eq. (23) [28,29].

"

Df ¼ C eqcm

Fig. 3. A comparison of EQCM and chronoamperometry: (A) current functions derived from EQCM data according to Eq. (21), (B) corresponding electrode currents. Both, Df(t) and I(t) recorded during potentiostatic deposition of polypyrrole. Working electrode systems: AuEQCM/PPYs (orange, s), AuEQCM/PPYm3 (pink, m3), AuEQCM/PPYm5 (green, m5). Meanings of symbols as in Table 3. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)

ratio Q ðtÞ ¼

QðtÞ 1 1 Q DL ðtÞ þ  ¼ jDf ðtÞj C eqcm  kelchem C eqcm  kelchem Q 2DI ðtÞ þ Q 3DI ðtÞ ð22Þ

where Qi(t) are the total charges involved in a given process ‘‘i”, as defined earlier in the text. It predicts a constant value of the ratio of the experimental variables at longer times of deposition of the polymer. An increase of a ratio value is predicted for a prevailing contribution of the double layer charging over the deposition charge contribution. It is important to emphasize that, under this assumption, the increase in the



qL  g L Dm þ 4p  f 0

1=2 #

¼ Dfm þ Dfg

ð23Þ

For conditions of the studied system (i.e. solution density qL, solution viscosity gL, fundamental resonant frequency f0), the deposited mass will dominate the EQCM frequency only when Dm  100 lg/cm2. In addition, a deposited polymer phase itself can be viscoelastic as well and, as a result, a plot of Df vs. Q will tend to show an apparently lower slope as compared with the purely elastic deposit case. Moreover, at early stages of the electrode polarization, the EQCM electrode experiences mainly the double layer charging. Then, the EQCM monitors (i) Dfm’ due to adsorption/desorption processes involving solvent, ions and reactants, (ii) Dfg due to changes in density and viscosity of a nearest environment of the solid surface, i.e. reorganization of a solution part of the double layer structure and possibly the diffusion layer, (iii) Dfr and DfP due to changes in roughness of a solid and hydrostatic pressure of a liquid, respectively [30]. The corresponding change in the resonant frequency will be denoted Dfdl to point out its predominantly nonfaradaic origin so to discern it from Df originating from electrochemical deposition of the polymer.

Dfdl ¼ Dfm0 þ Dfg þ Dfr þ DfP

ð24Þ

The observed differences in the EQCM frequency–time characteristics between gold and polypyrrole point out substantial differences in the surface processes between the two electrodes. One can expect differences in a structure and properties of the surface water as well as substantial differences in preferences for adsorption of hexafluoroanions and pyrrole species on gold and polypyrrole. The ratio values observed for deposition of the thin layer of polypyrrole are much lower for a gold substrate than for

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A

6.0E-06

ratioQ / C/Hz

5.0E-06 4.0E-06 3.0E-06 2.0E-06 1.0E-06 0.0E+00 0

5

10

15

20 m1

B

25 30 time /s m3

35

40

45

50

m5

3.5E-06

ratioQ / C/Hz

3.0E-06 2.5E-06 2.0E-06 1.5E-06 1.0E-06 5.0E-07 0.0E+00

0

50

100

150 time /s

200

250

300

Fig. 4. The charge ratio, calculated according to Eq. (22), for electrodeposition of polypyrrole hexafluoroaluminate. Meanings of symbols as in Table 3. (A): AuEQCM/PPYs (dark), AuEQCM/PPYm3 (dark grey), AuEQCM/PPYm5 (light grey); (B): AuEQCM/PPYs (dark, 0.75 lm).

a polypyrrole substrate. It points out higher values of Dfdl in the former case. The observed increase of the ratio Q-values with time might indicate a decrease in other than electrochemical contributions to Df, namely: Dfm due to dehydration of the surface, Dfg due to changes in structure and composition of the double layer, Dfr due to growing nuclei. An increase in a density viscosity product value is expected during the induction period of the polymer film formation due to an increase in concentration of precursors of a polypyrrole film in a vicinity of an electrode surface (pyrrole oligomers, counterions). Further in time one observes a decrease in the ratio Q-values with time that would indicate a decrease of the double layer charging contribution. Finally, the ratio Q-values should approach a (kelchem Ceqcm)1 – value that indicates a region of validity of the Sauerbrey’s equation. 3.6. Currents measured during very early stages of the potential step Representative experimental data obtained at the highest sampling rate of our instrument (50 ls) are shown in Fig. 5. The initial 3 ms range currents recorded for the eight consecutive electrodepositions lasting from tdep = 0.5 s to tdep = 2 s each, and resulting in the total deposition time equal 5 s, are shown there (see a legend of Fig. 5 for details of the experiment). Of course, the data points recorded at t 6 0.1 ms should be rejected as artifacts produced by the potentiostat circuity. In all the cases, one observes a decrease in a slope of ln I vs. t dependence with an increase in the total time of the potential step and/or the time window for the linear approximation of the initial

currents. A similar tendency in the characteristics of the initial currents were observed for longer times of the consecutive deposition, with tdep = 5 s, and the total deposition time equal 25 s. Moreover, because the corresponding mean sDL-values determined from the linear analysis are always lower for the gold substrate electrodes than for the polypyrrole substrate electrodes and increase with time, an impact of nucleation and growth of polypyrrole on the relaxation time of the double layer current is evident. The short time deposition data shown in Fig. 5 when approximated using a function given by the following equation:

  t I ¼ I0  exp  þ a2DI  t

sdl

ð25Þ

result in values of the parameters collected in Table 4. An increase in the double layer relaxation time with the deposition time is found again. However, the millisecond range a2DI-values obtained are three orders of magnitude higher than obtained from the analysis of the electrodeposition currents monitored at the time window 101 s to 10+1 s. An advanced quantitative analysis of the charging current transients and early deposition currents/EQCM responses of the polymer needs additional studies, including experimental works specially tailored for the purpose of the mechanistic–kinetic analysis. The problem is of a great importance due to many applications of conducting polymers [31,32]. EQCM measurements of very early stages of electrodeposition would require much sophisticated analytical approach due to effects of a no uniform distribution of deposited species and a contacting liquid. In addition to

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-8.0

c3

c4

c5

c6

c7

c9

ln( I [A] )

-8.4

-8.8

-9.2

-9.6 0.0000

0.0005

0.0010

0.0015 t [s]

0.0020

0.0025

0.0030

Fig. 5. The initial 3 ms parts of chronoamperograms recorded during the consecutive (c3–c10) applications of +0.8 V to the Au3 electrode immersed in the polymerization solution (pyrrole in the hexafluoroaluminate electrolyte). c3–c7: tdep = 0.5 s; c8–c9: tdep = 0.25 s; c10: tdep = 2.0 s. The total time of the polarization equal 5.0 s does not result in the polypyrrole deposit visible by naked eyes.

Table 4 Values of parameters of Eq. (25) determined from the analysis of the initial 3 ms parts of chronoamperograms recorded during the eight consecutive applications of +0.8 V to the Au3 electrode immersed in the polymerization solution. The c3-data correspond to the Au3 electrode without the former electrodeposition history. Data

I0 [mA]

sdl [ms]

a2DI [mA s1]

tdep [s]

tanal [ms]

c3 c4 c5 c6 c7 c8 c9 c10

0.256 0.266 0.266 0.266 0.266 0.262 0.264 0.262

1.3 3.0 4.0 4.6 5.0 5.0 5.3 5.1

21.0 12.0 12.0 12.0 12.0 10.5 10.6 11.8

0.5 0.5 0.5 0.5 0.5 0.25 0.25 2.0

3 3 3 3 3 3 3 3

As regards the main subject of the present work, the 2D deposition was found to precede the 3D deposition at the three studied potentials. The calculated charge values for depositions of the 2DI structure of polypyrrole hexafluorosilicate are presented in Fig. 6. An increase of the 2D deposition charge with an increase of the electrode potential was found for electrodepositions on gold and polypyrrole, irrespective of the thickness pattern of the sublayers. The analysis of the deposition currents was done under conditions of the masking double layer currents that were characterized by the relaxation times of the order of seconds. The sdl-value were found lower than for the case of polypyrrole hexafluoroaluminate and always lower for the depositions of polypyrrole on polypyrrole than for the depositions of polypyrrole on gold.

requirements of a low thickness and a rigid attachment to the surface, a mass being deposited must be uniformly distributed over the surface of the EQCM electrode to result in the frequency change predicted by the Sauerbrey’s equation.

0.0008

0.0006

Polypyrrole electrodeposited in aqueous sodium hexafluorosilicate was found to follow predominantly the 2DI–3DI electrodeposition model at electrode potentials +0.8 V, +0.7 V and +0.6 V. The results are presented in a separate communication to describe all details important for the kinetic analysis [33]. For a reason of very importance of the potential variable in the electrochemical processes, a summary of results obtained in that study must be invoked here. The depositions were performed on polycrystalline gold and on the single layer and bilayer polypyrrole hexafluorosilicate structures. Only in the case of the third layer deposition of polypyrrole at +0.6 V, the experimental current was found to fit better with the 2DP–3DI model. Contributions of the 2D structure increase with potential. Rate constants of the growth increase with potentials and the anodic transfer coefficients were found lower than 0.5. Values of the outward growth rate constants are of the order of 108 to 109 mole/(s cm2); at +0.8 V being one order of magnitude higher than for the case of polypyrrole hexafluoroaluminate.

Q/C

3.7. Effects of the deposition potential

0.6 V 0.7 V 0.8 V

0.0004

0.0002

0.0000 1

2 E/V

3

Fig. 6. Values of the electric charge used to deposit the 2D polypyrrole hexafluorosilicate calculated as a2DI/b2DI. Labels: 1, 2, 3 indicate the first (on gold), the second (on polypyrrole deposited as ‘‘1”) and the third (on polypyrrole deposited as ‘‘2”) electrodeposition of polypyrrole, respectively, except for electrodeposition at +0.8 V where label ‘‘3” indicates the single layer deposition (on gold). Thicknesses l of the sublayers in [lm] are: l(1, 0.6 V) = 0.18; l(2, 0.6 V) = 0.27; l(3, 0.6 V) = 0.27; l(1, 0.7 V) = 0.19; l(2, 0.7 V) = 0.27; l(3, 0.7 V) = 0.29; l(1, 0.8 V) = 0.45; l(2, 0.8 V) = 0.27; l(3, 0.8 V) = 0.75.

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3.8. Morphology of polypyrrole layers Surfaces of the electrodes at various stages of electrodeposition were monitored ex situ by scanning electron microscopy. Fig. 7 shows representative examples of the images for the electrode surfaces with 2D rich (A), mixed 2D–3D (B), and 3D rich (C) polymer structures. The corresponding polymer layers were obtained at conditions derived from the mathematical analysis of the electrodeposition process (see Fig. 2 to find details). The model predicts the 2D-rich polypyrrole structure at early stages of electrodeposition (thin films obtained at polymerization charge of the order of 101 mC/cm2, see Fig. 7A) and the 3D-rich polypyrrole structure for the thicker films (polymerization charge of the order of 101 mC/cm2, see Fig. 7C). At the intermediate stage of electrodeposition (polymerization charge of the order 100 mC/ cm2, see Fig. 7B) one expects similar amounts of 2D and 3D polypyrrole structures on the electrode surface. The SEM images of the polypyrrole layers confirm the predictions derived from the mathematical analysis of the experimental current–time curves. The images shown in Fig. 7, although corresponding to the independent polypyrrole samples, should be considered as representative pictures of the film morphology evolution with time and/or

electric charge of electrodeposition. Because the same resolution and surface area of the films are shown there, one can characterize the 3D deposit on the surface simply by a number and an average diameter of the sphere-like 3D features on the surface. The number of the 3D spots increases approximately by the one order of magnitude with an increase in the polymerization charge by the one order of magnitude, i.e. one observes 8  100, 8  101 and 102 spheroids on the surfaces shown in Fig. 7 for 6  101 mC/cm2 (A), 5  100 mC/cm2 (B), and 3.5  101 mC/cm2 (C), respectively. At the same time, the mean size of the 3D spheroids increases with the charge of electrodeposition up to 0.5 lm. Note that the large 3D polypyrrole structures can be seen even at the early stages of electrodeposition where the 2D polypyrrole structure dominates on the surface (see Fig. 7A). This is again in agreement with the analytical model assuming simultaneous nucleation and growth of the 2D and 3D structures of the polymer deposit. 4. Conclusions The mixed 2D–3D growth mechanism of polypyrrole hexafluoroaluminate was found on gold and polypyrrole substrate electrodes. Values of rate constants of the outward growth of the polymer phase were of the order of 109 mol/(scm2). Rates for the 2D lateral growth were 20–30% higher than for the 3D lateral growth. The 2D structure of the polymer is produced at the initial stage of electrodeposition. Electrodeposition influences the double layer relaxation time of the growing polymer electrode. The relaxation time of the decaying double layer current was found to increase with time of deposition from a usual 103 s range to a 100 s range, mainly due to an increase in the double layer capacitance. The yield and/or thickness of the 2D phase, measured as the charge density (q2DI) used for deposition of the 2D structure, were lower for electrodeposition of polypyrrole on polypyrrole than for electrodeposition of polypyrrole on gold. In general, short time regimes of electrodeposition should increase contribution of the 2D structure in the polymer deposit. Imaging of the surfaces by in situ microscopy might prove this prediction. We are not able to do this at the moment. The results of this study provide a strong indication for phase segregation in the multilayer polypyrrole electrodes prepared using so-called layer-by-layer electrodeposition of the same polymer. Thus, the conclusions of our EIS and EQCM studies on the ion transport hindrance, reported previously [1], are strongly supported by present findings on dominance of 2D growth of the polymer phase at the initial stage of electrodeposition. This is featured schematically in Scheme 1. It should be noted that the q2DI-data correlate with the ion transport rate data reported in [1]. Therefore, ion transport in the multilayer polypyrrole structure slows down due, at least in part, to increase in the interlayer ion transfer resistance between the final 3D-rich structure and the initial 2D-rich structure of the consecutive layers. The ‘‘global” quantitative analysis of chronoamperograms of electrodeposition has been proposed. Such approach was not used before in studies of conducting polymers. Furthermore, our results

Au

Fig. 7. SEM images of layers of polypyrrole on a conventional polycrystalline gold electrodes obtained for different electric charges used for electrodeposition: 0.6 mC/cm2 (A), 5 mC/cm2 (B), 35 mC/cm2 (C). The white resolution label marks 2 lm.

PPY layer1

PPY layer2

PPY layer3

Scheme 1. A schematic idealistic view of multilayer polypyrrole electrodeposited on gold. Shadows indicate changes in distribution of 2D structure in the (2D–3D) deposit. The darkest areas represent 2D-rich polymer structures.

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A 1.2E-05

ratioI / C/Hz

1.0E-05 8.0E-06 6.0E-06 4.0E-06 2.0E-06 0.0E+00

0

10

20

30

40

50

time /s m1

m3

100

150 time /s

m5

B 1.2E-05

ratioI / C/Hz

1.0E-05 8.0E-06 6.0E-06 4.0E-06 2.0E-06 0.0E+00 0

50

200

250

300

Fig. 8. The current ratio, calculated according to Eq. (26) for electrodeposition of polypyrrole hexafluoroaluminate. Meanings of symbols as in Table 3. (A): AuEQCM/PPYs (dark), AuEQCM/PPYm3 (dark grey), AuEQCM/PPYm5 (light grey); (B): AuEQCM/PPYs (dark, 0.75 lm) (see Appendix). (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)

seem to show, that the quantitative analysis of electrodeposition might be improved by refinement of the theoretical model of the double layer contribution to the mechanism of electrodeposition. Detailed analysis of EQCM data could possibly help to resolve the double layer and deposition currents. Further explorations of current–time data and corresponding EQCM frequency–time data, obtained for electrodeposition of polypyrrole at different electrode potentials, chemical environments and thicknesses of the polymer layer are required for the purpose. Results of the quantitative mechanistic–kinetic analysis of electrodeposition of polypyrrole from aqueous sodium hexafluorosilicate at +0.6 V, +0.7 V and +0.8 V indicate a decrease in the rates of nucleation and growth of the polymer with a decrease in the potential. Very recently electrochemical nucleation and growth of polycarbazole on tin oxide was presented as the mixed 2D–3D mechanism (without any considerations concerning the double layer contribution) but clearly evidenced by monitoring the electrode surface by SEM images [34]. Many fundamental aspects of electrodeposition of metals have been recalled and discussed in recent review papers [35,36]. Although EQCM method offers a new valuable insight into that process, it was used rather seldom until very recently. Our paper shows its application to study mechanisms of electrodeposition of a conducting polymer. The Sauerbry’s equation is shown valid only when the resonant frequency change due to the polymer deposit exceeds that due to the double layer reconstruction and due

to significant changes in the diffusion layer composition at the onset of electrodeposition. The proposed model of electrodeposition was finally proved by the SEM images of the polypyrrole layers of different thicknesses. Acknowledgements A financial support to J.K. through the BW grant of University of Wrocław is gratefully acknowledged. M.G. thanks Robert A. Hillman for a helpful discussion. Appendix A Applying the same assumption as for Eq. (22) to a comparison of the measured current and the resonant frequency time derivative one obtains:

IðtÞ ratio IðtÞ ¼ d½Df ðtÞ ¼ dt

1 1 IDL ðtÞ þ  C eqcm  kelchem C eqcm  kelchem I2DI ðtÞ þ I3DI ðtÞ ð26Þ

Relevant plots corresponding to the same experimental data as shown in Fig. 4 are presented in Fig. 8. Numerical calculations of derivative of the EQCM Df vs. t data produce a high noise level in the ratio I values. Nevertheless, the ratio I(t) function follows similar time characteristics as the ratio

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Q(t) function shown in Fig. 4. Also, similar values for the Ceqcmkelchem product are observed at longer times of electrodeposition where the double layer contribution to electrodeposition process can be neglected. References [1] A. Ke˛pas, M. Grzeszczuk, J. Electroanal. Chem. 582 (2005) 209. [2] E.M. Genies, G. Bidan, A.F. Diaz, J. Electroanal. Chem. 149 (1983) 101. [3] S. Asavapiriyanont, G.K. Chandler, G.A. Gunawardena, D.J. Pletcher, J. Electroanal. Chem. 177 (1984) 229. [4] A.J. Downard, D.J. Pletcher, Electroanal. Chem. 206 (1986) 139. [5] A.R. Hillman, E.F. Mallen, J. Electroanal. Chem. 220 (1987) 351. [6] F. Li, W.J. Albery, Electrochim. Acta 37 (1992) 393. [7] K. Kontturi, M. Pohjakallio, G. Sundholm, E. Vieil, Eur. J. Electroanal. Chem. 384 (1995) 67. [8] V. Peulon, G. Barbey, J.-M. Valleton, S. Alexandre, Synth. Met. 74 (1995) 15. [9] R. Schrebler, P. Grez, P. Cury, C. Veas, M. Merino, H. Gomez, R. Cordova, M.A. del Valle, Electroanal. Chem. 430 (1997) 430. [10] L.M. Abrantes, J.P. Correira, Electrochim. Acta 44 (1999) 1901. [11] H. Randriamahazaka, V. Noel, C.J. Chevrot, Electroanal. Chem. 472 (1999) 103. [12] M.J. Gonzalez-Tejera, I. Carrillo Ramiro, I. Hernandez-Fuentes, Electrochim. Acta 45 (2000) 1973. [13] I. Villareal, E. Morales, J.L. Acosta, Polymer 42 (2001) 3779. [14] M. Vignali, R.A.H. Edwards, M. Serantoni, V.J. Cunnane, J. Electroanal. Chem. 591 (2006) 59. [15] R.D. Armstrong, M. Fleischmann, H.R. Thirsk, J. Electroanal. Chem. 11 (1966) 208.

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