J. Phys. Chem. 1995,99, 12305-12311
12305
Morphology Control in Electrochemically Grown Conducting Polymer Films. 3. A Comparative Study of Polyaniline Films on Bare Gold and on Gold Pretreated with p -Aminothiophenol Eyal Sabatani,? Yael Gafni, and Israel Rubinstein* Department of Materials and Inteqaces, The Weizmann Institute of Science, Rehovot 76100, Israel Received: March 28, 1995; In Final Form: June I , 1995@
Polyaniline (PANI) films deposited galvanostatically on gold electrodes precoated with self-assembled monolayers of p-aminothiophenol (PATP) exhibit significantly higher optical densities than similar polymer films deposited on bare gold, as measured by in situ ellipsometry. At the same time there is no change in the total mass of the deposit or in the amount of polymer, as indicated by in situ quartz crystal microbalance measurements and by ex situ Rutherford backscattering results, respectively. It is therefore concluded that PANI grown on Au/PATP is considerably denser than PANI grown on bare Au. Failure of other self-assembled monolayers to produce a similar effect suggests that the chemical resemblance of P A P to the aniline monomer is a key factor in its ability to alter the morphology of electrodeposited PANI. The ac-impedance measurements indicate that the morphological change (i.e., densification) is accompanied by pronounced improvement in the electrochemical response of PANI. In particular, the insulatorkonductor switching rate is enhanced for PANI grown on A u / P A P relative to PANI grown under similar conditions on bare Au. Moreover, a substantial increase in the rate of discharge is observed for fresh PANI on AuRATP when applying a cathodic bias to the electrode. It is suggested that morphological restructuring is the limiting step during initial discharge of PANI grown on bare Au.
Introduction Polyaniline (PANI) is among the most extensively studied conducting polymers, mainly due to its ease of preparation,'-6 substantial stability under ambient conditions? and potential technological applications in bateries,' electrochromic device^,^ and sensors.8 PANI is a redox polymer which is insulating (a % 52-' cm-I) in its reduced state and conducting (a % 1.0 8-'cm-I) in its partially oxidized ~ t a t e . ~ *It ~can - ~ be prepared by chemical oxidation of aniline using oxidizing agents such as persulfate' or hydrogen peroxides2 It can also be conveniently obtained by anodic oxidation of aniline in acidic media either potentio~tatically~or galvano~tatically~or by multicycling of the electrode potential4 While the chemical composition of the electrodeposit produced by the different electrochemical methods is quite similar, different morphologies and performances have been r e p ~ r t e d . ~ . ' ~Although -'~ the origin of these differences is not entirely understood, it is generally accepted that the electronic conductivity of PANI is limited by structural disc~ntinuities'~ and that the electrochemical performance is governed by morphological factor^.'^^'^^'^ Hence, some systematic work has been devoted to the study of the electropolymerization mechanism of aniline' and to improving the morphology of electrodeposited PANI.'6,'7 It is reasonable to expect that one way to affect the polymer morphology would be by controlling the polymerization process at the very early stages of the deposition, Le., during the nucleation stage. We have previously shown that this can be achieved by precoating the electrode with a self-assembling monolayer of p-aminothiophenol (PATP).Is PATP possesses an aniline functionality and a thiol binding group; hence as a self-assembled monolayer on the electrode surface it can serve '-13315
* Author to whom correspondence should be addressed.
' Present address:
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91 125. Abstract published in Advance ACS Abstracts, July 15, 1995. @
0022-3654/95/2099-12305$09.00/0
as an initial monomeric layer which improves the "wetting" of the surface by the polymer. It was shownI8that a more ordered structure of electrodeposited PANI is obtained at such a monolayer-modified electrode, resulting in higher optical and mass density. The same concept was recently used to improve the morphology and conductivity of electrochemically deposited polypyrrole film^.'^,^^ An important question related to the morphological effect of electrode precoating is whether the electrochemical response of the film is affected and if so in what way. The most prominent electrochemicalfeature of PANI (and other conducting polymers) is the switching of the polymer between an insulator (in its fully reduced state) and a conductor (in its partially oxidized ~ t a t e ) . ~This . ~ switching is associated with a charging/discharging process whose exact nature has been the subject of considerable debate.5.2'.22 it was recently reported that the redox switching of PANI depends on the electrochemical history of the p ~ l y m e r . ~It~was . ~ ~shown that the charging kinetis of the polymer are slower when the polymer is held for longer times at negative potentials23 and that the cyclic voltammetry (CV) depends upon the potential scan limits.24 Ellipsometric studies25 showed that application of negative potentials to galvanostatically grown PANI leads to significant changes in the polymer optical properties, associated with the mobility of the primary charge carriers in PANI (bipolarons). Morphological alterations are evidently responsible for the pronounced change in the optical spectrum of the polymer upon such treatment.25 In this paper we further investigate the effectiveness of PATP preadsorption in improving the morphology of galvanostatically grown PANI on gold electrodes.Is The effect of the PATP monolayer on the morphology of PANI is compared with that of other self-assembled monolayers in an attempt to understand the role of PATP as an intermediate layer between the gold substrate and the polymer. Film growth was monitored in situ using the combined ellipsometry-quartz crystal microbalance 0 1995 American Chemical Society
Sabatani et al.
12306 J. Phys. Chem., Vol. 99, No. 32, 1995 (QCM) setup described before.26 The system allows continuous in situ measurement of film thickness and mass as independent parameters and thus the determination of film density during electrochemical growth. A marked advantage of the ellipsometry-QCM system is that it provides information pertaining to the overall film morphology, while imaging techniques reflect the morphology of the exposed surface only. In addition, the effect of PATP pretreatment on the electrical properties and the electrochemical response of PANI is described. It is shown that the optical changes observed upon application of negative potential^^^ are accompanied by pronounced changes in the electrochemical response of freshly prepared thin PANI films. In order for this response to be studied, the impedance of PANI films grown galvanostatically was analyzed as a function of the applied potential using acimpedance spectroscopy. Both complex-~apacitance~~~~~ and c ~ m p l e x - a d m i t t a n c epresentations ~~,~~ are used to analyze the data obtained in the frequency range 10-2-104 Hz.
20 80
90
100
110
120
130
140
A I degrees
Experimental Section Materials. Octadecyl mercaptan (OM) (Aldrich) was recrystallized from n-hexane. Dodecyl mercaptan (DM) (Merck), thiophenol (TP) (Merck), PATP (Aldrich), and aniline (Baker Analyzed) were vacuum distilled. p-Biphenylyl mercaptan (BPM) was synthesized from biphenyl (Aldrich) in a four-step procedure, as described elsewhere.30 Bicyclohexyl (BCH) (Aldrich) and chloroform (Bio-Lab) were passed through basic alumina (ICN Biochemicals) to remove residual water. The chloroform was stabilized with 1% ethanol. Ethanol (Bio-Lab) and ether (Baker analyzed) were used as received. Acid solutions were prepared from HC104 (Baker Analyzed or Analar). All aqueous solutions were prepared with triply distilled water (typical resistivity, 1 MQ cm). Purified argon or nitrogen was used to deaerate the solutions when needed. For the sputtering treatments, purified argon and oxygen were used. Preparation of Electrodes and Monolayers. Gold electrodes were prepared by sputter-deposition of gold (-1000 8,) onto glass or quartz substrates coated with a 50 8,Ti underlayer. TP monolayers were prepared by immersion of gold electrodes for 15 min in a 1.0 mM aqueous solution of TP. BPM monolayers were prepared by immersion for 20 min in a 1.0 mM solution of BPM in ethanol. PATP monolayers were prepared by immersion for 24 h in a 10 mM solution of PATP in 1:15 ether:BCH. DM monolayers were prepared by immersion for 30 min in a 1.0 mM solution of DM in ethanol. OM monolayers were prepared by immersion for 2 h in a 1.0 mM solution of OM in BCH. All electrodes were treated with a mild (- 1.O W/cm2) oxygen plasma to remove contaminations immediately before immersion into the adsorption solutions. Electrochemical Cell and Measurements. All electrochemical measurements were performed using a three-electrode cell comprising the gold working electrode, a platinum flag or gauze counter electrode, and a saturated calomel electrode (SCE). For CV measurements, a Solartron potentiostat (Model 1286) was used with a Houston x-y recorder. For the ac measurements the same potentiostat, connected to a Solartron frequency response analyzer (Model 1250), was used. The impedance was measured at seven discrete frequencies per decade, using an ac modulation of &5.0 mV (rms). The ac data were analyzed using a software package developed by B. A. Boukamp of the University of Twente, The nether land^.^' Polyaniline Deposition. PANI was deposited galvanostatically in a three-electrode cell from a solution of 0.1 M aniline
time / min Figure 1. Ellipsometric Y vs A curves at 600 nm (a) and the QCM response (b) during the growth of PANI on bare Au (solid line) and on AuPATP (dashed line) electrodes from a solution of 0.1 M aniline + 1.0 M HC104 at a constant current of 5.0 @cm2 for 125 min. The arrows indicate the points corresponding to 17 min deposition.
+
1.0 M HC104 at a constant current of 5.0 pA/cm2. The electrodes were equilibrated in the deposition solution for 30 min under argon deaeration prior to PANI deposition. The Electrochemical-Ellipsometry-QCM Setup. A combined electrcchemical-ellipsometry -QCM setup was employed as previously described.26 The combined system allows simultaneous monitoring of the polymer thickness and mass. A semicylindrical Teflon cell, equipped with quartz windows, served as the electrochemical cell. The working electrode was 1000 8, sputtered gold (with 50 8,Ti underlayer) on AT-cut l/2 in. disk quartz crystal. The quartz crystal resonance frequency is -5 MHz, and the conversion factor from the measured frequency change to mass is 60 Hz cm2/pg.26 The working electrode was positioned in the optical center of the ellipsometer using an appropriate holder, allowing electrical contact to the working electrode. The ellipsometer uses a photoelastic modulator (50 kHz) for modulation of the polarization of the incidence beam. The signal generated by the reflected beam was collected in three channels, corresponding to the 50 kHz, 100 kHz, and dc components. The QCM response was monitored with a frequency counter (Philips, PM6654C)whose readings, together with the electrochemical output and the ellipsometer readings, were transferred to an HP computer. The output signals were digitized, processed, and recorded using the same computer.
Results and Discussion PANI Growth at a Constant Current on Bare Au and on Au/PATP. Ellipsometric W us A plots during galvanostatic growth of PANI at 5.0 pA/cm2 on bare Au and ALIPATP electrodes are shown in Figure la. The optical properties of thin polymeric films grown on bare gold differ significantly from those of similar films grown on au/PATP.I8 In general, the ellipsometric Y vs A plots during PANI growth on bare gold curve initially with a much smaller radius than such plots for PANI growth on Au/PATP. Computer-generated ellipso-
Morphology Control in Conducting Polymer Films
J. Phys. Chem., Vol. 99, No. 32, 1995 12307
250
CHANNEL
250 300
50
solution without the monomer between -0.2 and + O S V vs SCE, which was terminated at -0.2 V.34 The electrodes were then washed thoroughly with distilled water and dried under a stream of nitrogen. Figure 2a shows the entire RBS spectrum associated with scattering from the gold film on quartz, with and without a PANI deposit. Figure 2b concentrates on the region of the spectrum from channel 800 to channel 850, where the signal is due to scattering from gold surface atoms. The shift of the backscattered a-beam to a lower energy relative to the bare gold signal is very similar at Au/PANI and Au/PATP/ PANI. This finding strongly suggests that the same overall amount of polymer was deposited on the two electrodes. Hence, it can be concluded that the different optical spectra of the polymer films grown on bare Au and on au/PATP are not due to different amounts of deposited polymer but rather are caused by different morphologies, as suggested earlier. l 8 Careful analysis of the QCM curves at the early stages of growth reveals differences in the initial mass gain. During the first 5 min of growth (Figure lb), the mass gain at the bare electrode is substantially higher than at au/PATP. A rapid increase of mass may reflect a rapid intake of solvent by a more porous polymer grown on bare Au. The rate of mass change at the two electrodes at more advanced stages of the growing process is quite similar, suggesting a decreased solvent intake by the film on bare Au, perhaps due to densification of the polymer with further growth. The QCM response suggests, therefore, that the differences in polymer morphology, evidenced by the ellipsometric results, originate at very early stages of the growing process. Hence, the PATP underlayer induces a more uniform distribution of the polymer at the early stages, leading to a polymer film which is more uniformly distributed along the substrate surface and is more compact. Growth of PANI at Other Modified Au Surfaces. In order to further elucidate the role of the PATP monolayer in improving the polymer morphology, PANI was grown at gold electrodes pretreated with other self-assembled monolayers. Adsorbates like OM and BPM form organized monolayers upon selfassembly on gold.30 TP is adsorbed to form a monolayer of a less-organized structure.30 In addition, the possible effect of random contaminations in the solvent from which PATP is adsorbed-BCH-was also considered. The growth of PANI on all pretreated gold surfaces was monitored by ellipsometry and QCM in experiments similar to those described above for PANI on AulPATP. In all the control experiments, the ellipsometry (Figure 3a) and the QCM response, (Figure 3b) closely resemble the respective responses for PANI grown on bare Au electrodes. Particularly noticeable is the very small (2-3’) total drop in Y associated with the initial branch of the ellipsometric curves, whereas with Au/PATP the drop is nearly lo’, as seen in Figure lb. It can therefore be concluded that neither random contamination of the electrode surface nor organized monolayers of different organic molecules (some of which are rather similar to P A P , e.g., TP30) affect significantly the morphology of galvanostatically electrodeposited PANI. Evidently, the aniline functionality in PATP is essential in altering the electrodeposition of PANI. This would imply that only high chemical compatibility of the underlayer with the growing polymer would induce improved morphology of the polymer. In the present case, PATP adsorption produces a monolayer comprising aniline units, which are compatible with the growing aniline polymer. Although there is still no evidence that an actual chemical bond is formed between the underlayer and the grown polymer it is a viable possibility. This would imply that PANI had been chemically grafted onto the gold surface.35
I -
0 800
I
810
820
830
850
1
850
CHANNEL
Figure 2. RBS results for bare Au (solid line), AuPANI (dotted line), and Au/PATPPANI (dashed line) measured with the detector positioned at 167”to the beam direction and with the sample tilted at an angle of
70” relative to the beam direction (to enhance the signal from the thin film). Frame b is a zoomed perspective of the 800-850 region (corresponding to energies of 1.98-2.10 MeV, respectively) in frame a. PANI films were prepared at 5 pA/cm* for 17 min. The ellipsometric and QCM readings for such films are marked by arrows in Figure 1. metric growth curves show that films with lower kf (Le., lessabsorbing films) and with nf which is close to n, (n of the solvent) exhibit smaller radii of curvature than films with a high kf and nf n,.32 Apparently, the thin PANI film grown on AulPATP is absorbing the 600 nm light (used in the experiment described in Figure 1) more than a thin PANI film grown on bare Au. The latter has a complex index of refraction which is close to that of the solvent. These findings were attributed to the more compact structure adapted by the polymer grown on AulPATP.I8 The QCM response to polymer growth under the same conditions on bare Au and on AulPATP is shown in Figure lb. The total increase in the electrode mass (expressed as a decrease in the resonance frequency) after 80 min of growth is quite similar at both electrodes. The same qualitative behavior was previously demonstratedI8 for PANI electrodeposited galvanostatically at 1.OpA/cm2. To verify that the observed difference in polymer density is not attributed to different Coulombic efficiencies, Rutherford backscattering (RBS) measurement^^^ were performed. RBS is based upon the loss of part of the kinetic energy of ions upon collision with stationary atoms and is often used for surface analysis. The energy loss of the backscattered particles depends on the atomic number, density, and depth of the “target” atoms. The existence of a deposit on the surface alters the energy loss to substrate (gold) surface atoms, and this change in the energy loss would depend upon the atomic number and the number of atoms in the deposit per unit area.33 In Figure 2 the RBS data obtained for AulPANI and A d PATPPANI are compared with RBS results for bare gold. The two polymer films were electrodeposited at 5.0 pA/cm* for 17 min (17 min of PANI growth under these conditions produces films with the Y and A values marked by arrows in Figure 1, parts a and b) and subsequently subjected to multicycling in a
Sabatani et al.
12308 J. Phys. Chem., Vol. 99, No. 32, 1995 44
l
42
-
41
-
40
-
43
39
400
2
200
1
~
l
'
l
'
I
a
'
l
~
A
-
-
>o: I
-
600
-8
~
I
I
I
I
I
I
I
I
sc;
~
+ n 0.2 v
Figure 4. Cyclic voltammograms obtained with PANI grown at 5.0 pA/cm2for 50 min on Au in 0.1 M aniline i1.O M HC104 immediately after termination of the growth (dashed line) and after 6 repetitive cycles in the same solution (solid line) (A = 1.O cm2; v = 0.1 V/s). Note that qualitatively similar voltammograms are obtained with Au/PATP/ PANI.
500
300
100
0 0
5
10
1 5 20 25
3 0 35 40
time (min) Figure 3. Y vs A curves (a) and QCM response (b) obtained during PANI growth from 0.1 M aniline 1.0 M HC104 at 5.0 pA/cm2 on bare Au (O), Au/OM (A), Au/TP (A), AulBPM (e),and AuBCH (*) electrodes (AuBCH denotes an electrode that was treated with the
+
solvent only). Lang et al.36studied the deposition of polythiophene on Pt surfaces pretreated with various thiols. An improvement in polythiophene properties was reported when short thiols, such as thiophenol, were preadsorbed. The improvement was explained as being due to prevention of initial metal oxidation. This is not the case when PANI is electrodeposited on Au, since gold oxidation occurs at significantly higher potentials. Therefore, "randomly" chosen monolayer systems are not expected to have a significant effect on the growth and properties of PANI, as indeed is shown in Figure 3. Voltammetric Behavior of Galvanostatidy Grown PAM. The CV curves shown in Figure 4 present the insulating1 conducting redox behavior of AU/PANI (grown 50 min at 5.0 pAlcm2) upon voltammetric cycling in the potential range -0.2 to +OS V at 0.1 VIS. The solid curve in Figure 4 is the wellknown steady-state ~oltammogram~-~.'* obtained after 6 cycles in this potential range. In its reduced form (negative end of the voltammogram), the polymer is insulating and exhibits very low CV currents, corresponding mainly to metal substrate double-layer charging. When the potential is scanned positively, the polymer is oxidized while gaining its conducting character. In addition to its increased conductivity, the oxidized polymer exhibits a marked capacitive b e h a v i ~ r . ~ . ' ~ - ~ ' . * ~ . ~ ~ The dashed curve in Figure 4 corresponds to the first potential excursion of the same electrode, carried out immediately after termination of the galvanostatic growth. The initial potential was + O S V, and the initial scan direction was negative. The first cycle is clearly different from. the steady-state voltammogram. The cathodic peak is much smaller in the first cycle and appears to be superimposed on a fairly large, smeared-out capacitive current. Upon reversal of the scan direction, the anodic half-cycle resembles almost completely the steady-state
voltammogram. It is clear from this simple experiment that the first half-cycle has a marked effect on the electrochemical response of the polymer. The voltammetric behavior of A d PATPPANI (not shown) in the fvst cycle is qualitatively similar to that of AdPANI (Figure 4),and the steady-state voltammetric behavior at the two electrodes is also similar. It can thus be concluded that, within the time scale of the voltammetric experiment, the two electrodes do not differ in their charge1 discharge behavior. As shown below, differences are revealed when a wide time domain is examined. AC-Impedance Measurements. Figures 5 -7 show series of complex-capacitance (C-plot) and complex-admittance (Yplot) c u ~ - v e s . ~Each ~ - ~ frame ~ in the figures presents dispersion data recorded at a certain potential, starting at +OS V, stepping down gradually to the cathodic limit of -0.2 V and back to + O S V. Unless otherwise specifield, the ac response was measured at each potential after holding the electrode at the given potential for 10 min. Note that left and right arrows in Figures 5-7 are used to specify the direction of the potential change. Hence, left arrows (+) indicate that the specified potential was reached from a more positive potential and vice versa for right arrows (+). Figure 5 presents a series of complex-capacitance plots obtained with Au/PANI immediately after completion of the galvanostatic growth. The C-plot at + O S V exhibits a single, nearly perfect semicircle, whose diameter corresponds to the overall electrode capacitance measured at that potential at very low frequencies. The effective capacitance obtained from the C-plot is equal to the constant capacitance obtained from CV at potentials 'f0.35 V with the same electrode.27 The next frame in Figure 5 was obtained at f 0 . 1 V +. The total capacitance at this potential is somewhat smaller, but the shape of the semicircle is not altered. At -0.1 V the dispersion data do not resemble a perfect semicircle anymore, but the total capacitance decreases only by about a factor of 2 compared with the capacitance at 0.5 V. Note that -0.1 V corresponds to the completion of the cathodic voltammetric PANI reduction (see solid line in Figure 4). The next three capacitance plots in Figure 5 (d-f) were recorded at -0.2 V, which is the cathodic limit used in this study, at different times after application of the potential. Starting from 10 min (Figure 5d) and continuing with 20 min (Figure 5e) and 40 min (Figure 50, a gradual decrease in the capacitance is observed. This series of C-plots demonstrates +
-
Morphology Control in Conducting Polymer Films a
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0.sov
J. Phys. Chem., Vol. 99, No. 32, I995 12309
e
-0.20v
f
-0.zov
2.0 1.0 0.0
0.IOVC
I
I
2.0
"E r&
E
0.0
c
-tO.OSV 2.4 0.2 0.0
0.4 1.0
0.2 0.0
0.0 0.0
1.0
20
3.0
4.0
0.0
1.0
2.0
3.0
4.0
5.0
0.0
0.2
0.6
0.4
c' (mF/cm')
0.0
0.2
0.4
0.6
0.8
1.0
Y' (R"cm-')
Figure 5. Experimental (circles) and simulated (solid line) complexcapacitance results obtained for AuPANI grown at 5.0 pA/cm2 for 125 min in 0.1 M aniline 1.O M HC104 immediately after completion of the growth. The electrode dc bias and the direction from which the potential has been reached (see text) are indicated in each frame. Frequencies are marked for certain experimental points; the theoretical points corresponding to the same frequencies are always closest on the simulated curve. Note the higher sensitivity in e and f.
+
3.0
0.8
Figure 7. Experimental (circles) and simulated (solid line j complexadmittance results obtained with the same electrode as in Figure 5 during the same set of experiments. Cdl
*
r
2.0 1.0
z,
0.0
=
-?7$*CLA
h
3
Figure 8. The electrical equivalent circuit used to model the electrode process and to analyze the ac response of PANI-covered electrodes. ZD, the finite transmission line, is a combination of RL and CL,as shown in the figure.
2.0
b
E
v
1.0
b 0.0 2.0
1.0
0.0 0.0
1.0
2.0
3.0
4.0
0.0
1.0
2.0
3.0
4.0
5.0
C' (mF/cm') Figure 6. Experimental (circles) and simulated (solid line) complexcapacitance results obtained for AuPATPPANI grown at 5.0 pa/cm2 1.0 M HC104 immediately after for 125 min in 0.1 M aniline completion of the growth. The electrode dc bias potential, direction, and frequencies are indicated as in Figure 5. Note the increased sensitivity in d.
+
the slow discharging process of galvanostatically grown PANI when the polymer is subjected to the f i s t cathodic bias. The discharge occurs at a potential considerably negative of the cathodic voltammetric peak of PANI, and approximately 40 min is required to reach the low capacitance shown in Figure 5f. Upon reversing the direction of the potential change, the capacitance is gradually recovered. At + O S V, however, after the completion of a full cycle, the total capacitance is larger
-
than the original capacitance measured at that potential. Also, the fact that the data points in the C-plot at this potential are not on a single semicircle suggests that the charging process in the film is not uniform anymore. The same experiment was repeated with Au/PATP/PANI prepared under similar conditions (Figure 6 ) . The ac response of AulPATPPANI at + O S V and -0.1 V immediately after termination of the galvanostatic growth closely resembles the behavior of Au/PANI at these potentials. However, unlike A d PANI, when the potential is switched to -0.2 V -, the capacitance drops rapidly (within the time required to collect the data in the entire frequency range, Le., -3 min) to a,very small value. Upon reversal of the direction of the potential change, the capacitance is almost unaltered and the C-plot is close to a semicircle throughout much of the potential range, suggesting that the polymer at the AuPATP electrode is more uniform and is not subjected to substantial changes during the initial discharginghecharging cycle. The ac data in Figures 5-7 were analyzed quantitatively using a software package designed for this purpose (see the Experimental Section). The package utilizes a nonlinear least-squares procedure to fit experimental ac-dispersion data to an equivalent circuit model. All the parameters in the model are adjusted
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Sabatani et al.
12310 J. Phys. Chem., Vol. 99,No. 32, 1995
TABLE 1: Calculated Parameters from Fitting of Alternating Current-Impedance Measurements during the First Cycle of PANI Grown at 5.0 pAlcm2for 125 Min bias pot vs SCE [VI +0.50 +o. 10 -0.10 -0.20 after 20 min at -0.20 after 40 min at -0.20 +0.05 +0.50
Au/PANI Au/P ATPPANi RI [Q cm2] CI[F ~ m - ~R2] [Q cm2] RL [Q cm2] CL[F ~ m - ~RlI [Q cm21 CI[F~ m - ~R2l [Q cm21 RL [Q cm21 CL[F ~ m - ~ l 4.6 3.0 1.6 2.3 6.3 32 1.7
3.6
2.2 2.4 4.7 2.4 7.6
x lo-) x lo-)
x x x
1.6 0.3 0.8 0.7 0
5.8 x
0
8.8 x 3.4 x
1.3 3.6
2.0 7.1 5.0 52 130 9300 24 0.7
1.7 x 10-3 1.0 x 10-3 2.4 x 2.3 x 1.3 x
2.1 2.1 3.0 743
3.7 3.5 8.2 1.8
x 10-3
4.1
x 10-3
0 0
x x
2560
5.7 18 11 5 x lo4
2.7 10-4 2.2 x 10-4 1.1 x 10-3 1.8 x
9.3 x 10-5 1.1 x 1.3 x
23 1.6
8.9 x 3.7 x 10-3
34
0
38 0.7
1.3 x 5.6 x
TABLE 2: Calculated Parameters from Fitting of Alternating Current-Impedance Measurements of PANI Grown at 5.0 uA/cmZfor 125 mln and Then Subjected to Repetitive CV bias pot. vs SCE [VI RI [Iz cm2] 10.50 +0.10 -0.05 -0.20 -0.10 +0.05 +0.10 +0.50
1.4 4.9 5.5 121 42 7 32 2
Au/PANI
Au/PATPPANI
CI[F~ m - ~R2] [Q cm21 RL [Q cm2] CL [Fcmw2] RI [Iz cm2] CI [F~ m - ~R2l [Q cm21 RL [Q cm21 CL[F ~ m - ~ l 4.2 4.4 1.2 7.1 6.1 1.5 2.1 4.7
x x 10-3 x 10-5 x x x
x 10-3 x 10-3
0.05 1.5 8.1 1490
0 16
0 3.6
5.9 2.3 58 2.3 x io4 2.1 x 10" 103 1.9 6.5
5.0 x 2.7 7.6 x 7.3 x 5.8 x 1.3 x 3.1 2.8 x
simultaneously,thereby obtaining an optimal fit to the measured dispersion data. Simulated curves produced by the fitting procedure are presented in Figure 5-7 by the solid line. The electrode reponse corresponds to a charging process with a distribution of time constants. It can be satisfactorily represented by a combination of three parallel branches as shown in the model equivalent circuit in Figure 8, which was used to fit the experimental data. The circuit comprises an ohmic resistor RQ, attributed to the combined solution resistance and the ohmic (non-faradaic) resistance of the polymer. We have not attempted to separate the two because of the uncertainty in the dispersion data at high frequencies. The ohmic resistance is connected in series to a complex element which represents the electrode-charging process. c d l is the double-layer capacitance at the metal-film interface. This capacitance is significant only when the polymer is fully reduced and the other capacitive components are smaller.38 Two slower branches can be resolved in parallel with c d l : a pure RC branch (represented by the R I C I branch in Figure 8), and a transmission line branch (represented by R2 and ZD in Figure 8). ZD is a finite transmission line which comprises a repetitive combination of capacitors CLand resistors RL, as shown in Figure 8.39 The equivalent circuit presented in Figure 8 is somewhat different from the equivalent circuit proposed previously for PANLZ7 It comprises three branches (instead of two) and an additional resistor in series with the transmission line. The differences are the result of our present attempt to use a single equivalent circuit to describe the electrode process in a broad potential range and to fit the data simultaneously in a broader frequency range. The expanded view was made possible by fitting experimental data to simulated results of both C- and Y-plots, as examplified in Figures 5 and 7, respectively. While the capacitance presentation emphasizes the frequency range below ca. 100 Hz,the admittance presentation emphasizes the range above ca. 100 Hz. This is demonstrated in Figures 5 and 7, in which the C-plots and Y-plots of the same experiment are presented. A good fit is achieved in the entire frequency range studied, and the model in Figure 8 satisfactorily describes the charging process covering the full range from the insulating, fully reduced polymer (at -0.2 V) to the conducting, oxidized polymer (+OS V). The results at positive potentials (>0.1 V)
10-4 10-3 10-4 10-5 10-3 10-3 10-4
1.5 1.9 9.6 374 170 4.7 2.0
4.1 5.2 1.2 5.4 4.9 3.1 2.7
0
x
x x 10-5 x
x x 10-5
10-3
0.1 14 1070 320 14 2.3
0.5 5.7 17 2.2 x 105 1.9 104 38 38
5.4 x 1.1 1.1 x 7.0 x 7.8 x 2.1 x 2.9 x
10-4 10-3 10-3 10-5 10-5 10-3 10-3
have to be viewed, however, with caution, since the low resistance values of the polymer in its oxidized (conducting) state make it difficult to fully resolve the three charging branches. The values obtained from fitting the experimental data to the model circuit (Figure 8) are summarized in Table 1 for the first cathodic treatment. The main difference between the Au/PANI and AdPATPPANl electrodes is the much higher values of R I ,R2, and RL obtained at much shorter times with the latter electrode following application of cathodic bias. This reflects the more effective and faster process of reduction (dedoping) in the case of the Au/PATP/PANI electrode. Table 2 summarizes the results obtained by a similar parameter-fitting procedure for Au/PANI and Au/PATP/PANI subjected to prolonged potential cycling between -0.2 and + O S V in 0.1 M HC104 (the experimental results qualitatively resemble those in Figures 5-7 and are not shown). An increase in R1, R2, and RL and a decrease in C1 and CLupon changing the potential in the negative direction are observed for the two electrodes, while the opposite trend is seen in the reverse direction. The rate of these changes is comparable at the two electrodes, and evidently the inhibiting factor observed during the initial cathodization of Au/PANI is removed upon repetitive potential cycling. However, the values derived for Au/PANI and Au/PATP/PANI are different in several respects. At + O S V, RL for Au/PANI is higher than that for Au/PATP/PANI; conversely, a larger RL is obtained for Au/PATP/PANI at -0.2 V compared with Au/PANL40 These results imply that the switching of the polymer between its insulating and conducting forms is more effective and complete at Au/PATP/PANI than at Au/PANI, even after multiple CV treatment.
Summary The galvanostatic electrodeposition of PANI on bare Au and on PATP-covered Au (AdPATP) electrodes was monitored in situ using a combined electrochemical-ellipsometery-QCM system.I8 The simultaneous measurement of the thickness and mass of as-grown polymer films enables real-time monitoring of the evolution of the polymer density. Comparison of the ellipsometry-QCM results during the polymer growth at a low
Morphology Control in Conducting Polymer Films current density (5.0 ,uA/cm2) on Au and on Au/PATP reveals substantial differences in the polymer morphology at the two electrodes, i.e., PANI grown on Au/PATP is considerably denser than PANI grown on bare Au. Further support for this conclusion is provided by RBS results, which indicate that the overall amount of polymer deposited is very similar for Au and Au/PATP, implying that the different ellipsometric observations are caused by different film morphologies. The improved polymer packing at Au/PATP is interpreted as-the result of a better chemical match between the polymer and the substrate upon coating of the substrate with a compact monolayer of the respective monomer. As shown here, pretreatment of gold electrodes with other monomolecular layers does not result in any noticeable improvement in the structure of the PANI film. The unusual voltammogram observed during the first cathodic potential scan of fresh galvanostatically grown PANI shows that the capacitive nature of the polymer“-5is maintained even when reaching potentials as low as -0.2 V. This suggest that the initial reduction of galvanostatically grown PANI is inhibited and that the dedoping of fresh PANI is not complete within the time scale of the CV experiment (at 0.1 V/s scan rate). Also, the ac response of Au/PANI during the first negative cathodic excursion differs from the response after prolonged voltammetric ~ycling,~’exhibiting slow reduction of the polymer. The voltammetric and ac response of Au/PANI during the first cathodic treatment, showing sluggish reduction of the as-grown (oxidized) PANI, may be compared with the reported sluggish reoxidation of the polymer after prolonged r e d ~ c t i o n . ~ ~ . ~ ~ Note that although the ac-impedance measurements and the cyclic voltammetry were performed at different time scales, the two techniques show qualitatively the same type of behavior, namely a marked difference between the results obtained in the first voltage excursion and those observed at the steady state. This similarity suggests that the discharge during the first cathodization of galvanostatically grown PANI is much more dependent on the applied potential than on the rate of change of the potential. The discharge is completed only when a potential of ca. -0.2 V is reached; at that potential, the discharge is time-dependent, as is evidenced from the ac response (Figure 5).
Substantially higher initial discharge rates are observed with freshly prepared Au/PATPffANI. While 40 min at -0.2 V is required to complete the discharge of Au/PANI, less than 3 min is required for discharge of Au/PATP/PANI when reaching this same bias potential. The slow discharge of Au/PANI during the first cathodic half-cycle may be due to either poor contact at the film-metal interface or, as previously suggested,25to poor packing of the polymeric segments in the bulk of the film. Based on the ellipsometric behavior of the two film^,'^.^^ it is reasonable to assume that the faster discharge is due to the superior packing of the polymer on Au/PATP. Furthermore, we suggest that the initial cathodic discharge rate is limited by morphologicalrestructuring which is less dramatic for Au/PATP/ PANI, thereby facilitating the discharge process in these films. It may also suggest that the “memory effect” 23-24 in PANI is a morphological effect activated by the cathodic bias. Analysis of the ac data reveals that even after subjecting the electrode to repetitive CV between -0.2 and + O S V, the insulator/conductor switching of PANI grown on Au/PATP is more efficient than the same process on bare Au.
Acknowledgment. The authors acknowledge support of this work by The Basic Research Foundation, Israel Academy of Sciences and Humanities, Grant No. 400/90. Special thanks to J. G. Beery (Los Alamos National Laboratory) for performing the RBS experiments and to S. Gottesfeld (Los Alamos National Laboratory) for valuable advice throughout this work.
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