Synthetic Metals 105 Ž1999. 91–98
Electrochemistry of conductive polymers XXIII: polyaniline growth
studied by electrochemical quartz crystal microbalance measurements
Sheng-Yun Cui, Su-Moon Park
)
Department of Chemistry and Polymer Research Institute, Pohang UniÕersity of Science and Technology, Pohang, 790-784, South Korea
Received 26 January 1999; received in revised form 26 March 1999; accepted 7 April 1999
Abstract
Polyaniline ŽPAn. growth has been studied in 0.50 M H 2 SO4 containing 0.030 M aniline using an electrochemical quartz crystal
microbalance in situ during potentiodynamic and potentiostatic growth experiments. A quantitative analysis of the in situ gravimetric data
and the faradaic charge suggests that the oxidative deposition of aniline is made of a few different steps with different mechanisms. The
nucleation processes were observed during the first few potential cycles leading to the formation of short chain oligomers. Then, the
number of electrons transferred for deposition approached to about 2.4 for each aniline molecule, suggesting that the number of electrons
transferred for polymer growth almost agrees with the generally accepted stoichiometry. In later stages of polymerization, the number of
electrons involved became less than 1.0, indicating that the PAn growth follows the autocatalytic mechanism. Analysis of weight data
also suggests that there must be a change in polymer morphology as well as in growth mechanisms. q 1999 Elsevier Science S.A. All
rights reserved.
Keywords: Polymers; Polyaniline; Electrochemical quartz crystal microbalance
1. Introduction
Polymers of aniline derivatives have been studied extensively due to the fundamental interest in reaction mechanisms as well as for their possible applications to practical devices including energy storage devices, electrochemicalrchemical sensors, electrochromic devices, and others
w1–10x. Most applications require repeated injection and
removal of charges from the polymer via doping and
de-doping processes. This requires the reversibility of doping and de-doping processes, and degradation reactions or
structural changes taking place during the dopingrde-doping processes greatly influence the reversibility. In order to
obtain high quality polyaniline ŽPAn. for various applications, it is necessary to better understand the growth
mechanism and the dopingrde-doping processes.
The mechanism of the aniline polymerization reaction
has been studied by a number of investigators w1,2x. The
oxidative polymerization of aniline was shown to proceed
via the formation of dimer molecules during the nucleation
process, which is followed by the oligomerization reaction
)
Corresponding author. Fax: q82-562-279-3399; e-mail:
[email protected]
in earlier stages of polymer growth w11–19x. Once short
chain oligomers are formed, they grow to polymers via
what is known as an autocatalytic mechanism w20–26x.
These processes have been studied by transient electrochemical techniques including chronoamperommetric,
cyclic voltammetric and spectroelectrochemical measurements w20–27x. However, few studies have addressed a
question on the quantitative aspects of the electron stoichiometry and growth mechanisms in different stages of
the aniline polymerization reaction. The stoichiometry,
2 xC 6 H 5 y NH 2 ™ Ž C 6 H 4 y NH . 2 x q 2 x Hqq 2 xey,
has been generally accepted for the polymerization reaction of aniline without experimental evidences w1,2x.
In our present report, PAn films were grown on platinum electrodes by potentiodynamic methods in a positive
potential region, and the PAn growth was studied by in
situ gravimetric analysis employing electrochemical quartz
crystal microbalance ŽEQCM. measurements in efforts to
address the quantitative aspect of the above stoichiometry.
By measuring changes in resonant frequencies during the
polymerization reaction, changes in mass can be monitored, which makes possible the quantitative interpretation
of the polymerization reaction. In addition to the nucle-
0379-6779r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.
PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 0 7 9 - X
92
S.-Y. Cui, S.-M. Park r Synthetic Metals 105 (1999) 91–98
ation process at the very beginning of the polymerization
reaction, more than one steps of PAn growth were recognized from these measurements. The first step was the
formation of oligomers with relatively low columbic efficiencies; then polymer growth on the electrode surface
follows Coulomb’s law with the stoichiometry summarized
above. After the polymer grows to a certain extent, the
polymer growth appears to be accelerated with a change in
the growth mechanism. During the sequence of reactions,
the structural change was also noticed through changes in
admittance.
2. Experimental
The EQCM measurements were carried out with a
Seiko EG & G model 917 quartz crystal analyser ŽQCA.
along with an EG & G model 273 potentiostatrgalvanostat.
An AT-cut, 9 MHz platinum-plated quartz crystal Žmodel
QA-AM9-PT. was used as a resonator and an electrode.
The platinum-coated quartz crystal working electrode was
mounted on a model QA-CL3 electrode holder. A threeelectrode system, which had the above working, a platinum wire counter, and an AgrAgCl Žin saturated KCl.
reference electrodes, was employed and the measurements
were carried out in a homemade electrochemical cell after
the solution had been thoroughly de-aerated. The electrode
was first pretreated by ultrasonically washing in acetone
and then in doubly distilled, de-ionized water. Chemicals
used in the experiments were of reagent grade and the
solutions were prepared with doubly distilled, de-ionized
water. The QCA, the potentiostatrgalvanostat, and the
computer were connected through a GPIB interface card,
which was controlled by EG & G PAR 270r250 electro-
Fig. 1. Ža. Potentiodynamic polymerization of aniline at a platinum-coated quartz crystal electrode for 12 consecutive potential cycles in a 0.030 M aniline
solution in 0.50 M H 2 SO4 at a scan rate of 40 mV sy1 ; Žb. Frequency shifts concurrently recorded with the cyclic voltammograms shown in Ža..
S.-Y. Cui, S.-M. Park r Synthetic Metals 105 (1999) 91–98
chemistry software. The sensitivity of the quartz crystal
electrode was calibrated by potentiostatic deposition of
silver from a silver nitrate solution using a relation,
93
3. Results and discussion
D f s yM w Cf Qr Ž nF . ,
3.1. PAn growth by potentiodynamic and potentiostatic
methods
where M w is the atomic weight of silver, Q is the cathodic
charge passed for the deposition, n is the number of
electrons involved in the electrochemical process, F is the
Faraday constant, and Cf is the sensitivity constant derived
from the Sauerbrey relationship w28x. We obtained a sensitivity factor of 8.0 " 0.6 ng cmy2 Hzy1 for a few quartz
crystal electrodes used for our measurements, which is in
reasonable agreement with the theoretically calculated sensitivity from the Sauerbrey equation using appropriate
constants Ž5.45 ng cmy2 Hzy1 .. The deviation might have
been arisen from the difference between the real and
geometric areas of the electrode due to the surface roughness, differences between different electrodes, etc.
Figs. 1 and 2 show cyclic voltammograms ŽCV. and
corresponding frequency shifts concurrently recorded during consecutive potential cycles in a 0.50 M H 2 SO4 solution containing 0.030 M aniline in a potential range between y0.10 and 1.2 ŽFig. 1. or 1.0 V ŽFig. 2.. Steady
PAn growth is observed from the decrease in frequency
with the increasing number of cycles. Deposition of oxidized aniline starts from the very first cyclic voltammetric
peak current for aniline oxidation ŽPeak F. during the first
potential cycle. The deposition is seen more clearly in the
dD frdt plot shown in Fig. 3, which actually displays the
deposition rate. As can be seen from Fig. 3a, there is a
Fig. 2. Ža. Potentiodynamic polymerization of aniline for 15 consecutive potential cycles in a 0.030 M aniline solution at a positive vertex potential of 1.0
V with a scan rate of 20 mV sy1 ; Žb. Frequency vs. potential plot concurrently recorded with CVs shown in Ža..
94
S.-Y. Cui, S.-M. Park r Synthetic Metals 105 (1999) 91–98
Fig. 3. The dD frdt plots obtained from the D f vs. E plot shown in Fig. 1b for the first potential cycle Ža., sixth cycle Žb. and 12th cycle Žc.. Here dotted
lines indicate CVs, while solid lines are for dD frdt.
significant time delay during the first cycle Ž; 300 mV in
voltage scale, ; 7 s in time scale. between the current rise
and the potential, where the deposition of aniline starts.
Also noticed is the continued deposition of aniline during
the cathodic potential scan as judged from the negative
dD frdt values until the potential reaches ; 400 mV.
This is probably because the small oligomer molecules are
not precipitated on the surface until they are supersatu-
rated, which takes some time. Thus, the delay in deposition
results from a time delay between the time for the generation of precursors for the deposition reaction and the time
for actual deposition, rather than a potential dependent
phenomenon. A large fraction of these molecules may be
dissolved in solution and diffuse away from the electrode.
This should be the reason for the very low electron efficiency observed for the oxidative deposition of aniline
S.-Y. Cui, S.-M. Park r Synthetic Metals 105 (1999) 91–98
Table 1
Anodic Ž Qa . and cathodic Ž Qc . charges, corresponding changes in frequency Ž D fa and D fc ., and net mass Ž Mnet . of polymer accumulated
during potentiodynamic growth of PAn at a scan rate of 40 mV sy1 to a
vertex potential of 1.20 V
CN a
Qa
ŽmC.
D fa
ŽHz.
Qc
ŽmC.
D fc
ŽHz.
D Mnet
Žmg.
Qnet
ŽmC.
nb
1
2
3
4
5
6
7
8
9
10
11
12
4215.2
2425.0
2328
2404.1
2545.7
2707.3
2867.1
3009.6
3149.8
3256.7
3348.0
3448.6
y290
y450
y470
y510
y570
y660
y700
y694
y596
y550
y516
y488
445.7
422.5
456.8
533.4
653.1
794.3
946.5
1095.7
1279.1
1467
1675.8
1748.7
10
40
70
70
130
120
150
166
192
232
264
280
0.448
0.656
0.64
0.704
0.704
0.864
0.880
0.845
0.646
0.509
0.403
0.333
3769.5
2002.5
1871.2
1870.8
1892.6
1913.0
1920.6
1913.9
1870.8
1789.7
1672.2
1699.9
8.14
2.95
2.83
2.57
2.60
2.14
2.11
2.19
2.80
3.40
4.01
4.94
a
b
CNs potential cycle number.
ns number of electrons transferred.
during the first few cycles Žmore on this below.. As the
film grows, the anion insertion and de-insertion becomes
increasingly important as can be seen from the relative
rates of weight increase during the 6th and 12th cycles as
shown in Fig. 3b and c. The rate of weight increase due to
the dopingrde-doping process overwhelms the growth reaction in the 12th cycles as can be seen in Fig. 3c,
although the currents Žor charges. for the two processes are
comparable. This is probably because of degradation reactions taking place when the film is thick and the counter
ion diffusion limits the PAn growth.
In general, the deposition process shows time delays
compared to the current as can be seen in Fig. 3. This is
because of the impedance of ion insertion and expulsion
processes during oxidation and reduction of the polymer,
which is readily expected. As a result, the dD frdt curves,
which should follow the current if there is no mass transport impedance, are not as well defined as the currents.
Mass transport impedance experienced during doping and
de-doping of conductive polymers has been treated recently by Yang and Kwak w29,30x.
As pointed out above, significant decreases in frequency are observed in later potential cycles when the
potential passes CV peak A, which corresponds to the
anion insertion due to the p-doping of the polymer. Also
noticed in Figs. 1Žb., 2Žb. and 3Žb. is that the weight
increase becomes more significant past peak D than peak
A except during the first cycle. In other words, the weight
increase is observed only after the oxidation of aniline
during the first cycle. However, from the second cycle on
the weight increase is also observed beyond peak D without having to oxidize aniline. This observation is consistent with the earlier contention that the polymer grows as
long as a polymer species produced at peak D is generated
w22x. From the second cycle on, peak F disappears and
95
another peak E is visible, suggesting that short chain
oligomers formed on the surface are being oxidized. As the
potential cycles increase, peak E shifts slowly to peak D
indicating that the chain length becomes longer.
Results obtained from the CV and EQCM experiments
are summarized in Tables 1 and 2, in which mass gains,
electrical charges during anodic and cathodic potential
scans, and the number of electrons required for the polymer growth are listed. Charges were calculated from the
integration of the cyclic voltammetric currents. The number of electrons transferred for polymer growth was calculated from the net anodic charge used for aniline oxidation
and the net amount of aniline units deposited. The net
anodic charge Ž Qnet . used for aniline oxidation was obtained from Qa, n y Qc, n , where subscripts a and c denote
anodic and cathodic processes during the nth cycle. Since
Q c, n contains the charge used for de-doping the doped PAn
obtained during the nth anodic scan and Qa, n measures the
charge spent for oxidation of aniline in the bulk solution
along with that used for doping the de-doped PAn grown
during the nth cycle, their difference, Qa, n y Q c, n , represents a net charge used for the growth during the nth
potential scan. The charge calculated in this way includes
the charge to oxidize a new batch of aniline without the
charge required for doping the newly grown polymer. The
number of electrons transferred for each anilinium
monomer unit deposited is then calculated from the net
anodic charge and the weight deposited. This calculation is
based on the assumption that: Ž1. the sulfate or bisulfate
ions inserted during the oxidative deposition of aniline on
the surface are completely expelled from the polymer
surface during the cathodic scan, and Ž2. there are no or
negligible amounts of solvent Žwater. molecules trapped
during the deposition of aniline. These assumptions are
Table 2
Anodic and cathodic charges, corresponding changes in frequency, and
net mass accumulated during potentiodynamic growth of PAn at a scan
rate of 40 mV sy1 to a vertex potential of 1.0 V
CN
Qa
ŽmC.
D fa
ŽHz.
Qc
ŽmC.
D fc
ŽHz.
D Mnet
Žmg.
Q net
ŽmC.
n
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1412.0
1076.6
1141.5
1302.9
1508.4
1783.4
2142.3
2668.0
3252.2
4015.9
4859.8
5748.9
6692.0
7654.9
8580.9
9923.4
y50
y100
y130
y190
y260
y390
y540
y720
y870
y1020
y1190
y1430
y1632
y3016
y3018
y3310
138.0
124.4
148.8
197.5
275.0
392.2
517.5
743.9
989.1
1308.3
1734.8
2264.0
2861.8
3723.8
4383.8
5931.9
20
30
20
40
40
70
90
120
220
290
410
452
y704
y232
y20
y140
0.048
0.112
0.176
0.240
0.352
0.512
0.720
0.960
1.040
1.168
1.248
1.565
3.738
5.197
4.861
5.520
1274.0
952.1
992.7
1105.4
1233.4
1391.2
1624.8
1924.1
2263.1
2707.6
3125.1
3484.8
3830.2
3931.1
4197.1
3991.5
28.45
8.11
5.59
4.65
3.61
2.85
2.35
2.17
2.33
2.51
2.75
2.48
1.14
0.89
0.97
0.97
S.-Y. Cui, S.-M. Park r Synthetic Metals 105 (1999) 91–98
96
justified from Orata and Buttry’s report, in which quantitative, stoichiometric insertion and de-insertion of anions
were observed w31x.
There are two points to be noticed from the data listed
in Table 1. First, the number of electrons required for the
deposition of a mole of aniline monomer units in the first
cycle is very large at about eight per each mole of aniline.
This means that the columbic efficiency is very low,
suggesting that electrolysis products obtained during the
first voltammetric cycle must have small molecular weights
and, thus, have a relatively large solubility. We therefore
believe that the product is made of mostly various dimers
in agreement with earlier reports w13,32,33x, as larger
molecules would precipitate almost quantitatively on the
surface when formed. From the second cycle on, the
number of electrons required for the deposition of PAn
quickly approaches two. When the aniline deposition is
relatively hefty as in the case shown here, the polymer
growth takes place in a relatively short time. Second, the
n-value increases back to three to five again during the last
few cycles. This is probably because the degradation reaction starts to play roles in determining the amount of
aniline deposition when the PAn growth becomes significant. The degradation reaction produces soluble products
such as p-benzoquinone andror p-aminophenol, which
would reduce the amount of aniline deposition w34–36x.
The degradation reaction is more severe when the aniline
concentration is low and the vertex potential is high w34x.
When the positive vertex potential is reduced to 1.00 V,
the nucleation process becomes slower in comparison to
that at 1.20 V Žsee Fig. 2b; Table 2.. However, the rate of
polymer growth at the vertex potential of 1.00 V in later
stages is comparable to that at 1.20 V. This observation
suggests that the polymer growth would be about the same
as long as peak D is produced on the polymer if other
experimental conditions are about the same. Here, the
number of electrons for aniline deposition in the first cycle
is much larger by about three times compared to the value
at 1.20 V ŽTable 1.. Then it decreases relatively slowly to
about two in about five to six cycles. It then decreases
further to less than one during the last few cycles. From
this point on, the autocatalytic mechanism may take over,
and just the oxidation of PAn films to produce peak D
would lead to polymerization of aniline without having to
oxidize aniline itself w22–24x. In this case, the number of
electrons would become smaller because the electrons
extracted during the polymer oxidation are cancelled out
during the cathodic scans in our charge calculation. If the
PAn growth solely follows the autocatalytic mechanism,
extraction of an electron from each aniline molecule in
solution would be required for growth as the generation of
PAn oxidation peak D would lead to polymerization of
aniline according to the stoichiometry,
sem
ox
Ž An. x ™ Ž An. x q xey
ox
ox
red
Ž An. x q An ™™™ Ž An. xy1 Ž An. 2
where An is an each anilinium unit, ŽAn. sem
is a partially
x
oxidized form of PAn with cation radicals or polarons
present in the polymer backbone, and ŽAn. ox
is a fully
x
oxidized form, i.e., dications or bipolarons present. This
mechanism w22x requires an electron for each aniline
molecule deposited, which is consistent with our observation. Thus, the fact that the extraction of approximately
one electron is required for the polymer to grow suggests
that the polymer growth takes place primarily via the
autocatalytic mechanism. Although we might have had
some contribution from the degradation reaction, the contribution must be relatively unimportant as the degradation
reaction would not be significant at this potential.
The general pattern described here was reproducibly
observed when two different vertex potentials were used,
although the specific numbers vary to some extent. Unfortunately, we were not able to vary the growth conditions
such as aniline concentrations, scan rates and other experimental variables known to control the PAn growth, to a
large extent. Even when the scan rate was changed to
slower than 20 mV sy1 , which speeded up the growth
quite a bit due to the long polymerization time, the frequency decrease became too large to make further measurements difficult with our equipment. The polymerization time during the potential scan is just the time spent
while the potential stays more positive than peak D. As a
result, we had to use a very low aniline concentration,
lower than ; 0.050 M, for the growth experiments. In this
case, however, the growth is known to compete with
degradation w22x. This was the reason why we lowered the
vertex potential to see the effects of slow polymer growth.
The change in the growth mechanism can also be seen
clearly from a plot of the amount of aniline deposited vs.
net anodic charge shown in Fig. 4, which used the data in
Table 2 from the 7th to 16th cycles. Two linear parts are
realized, suggesting that two different mechanisms of polymer growth are at work depending on the amount of the
polymer film on the electrode surface. The first linear part
Fig. 4. The amount of aniline monomeric units accumulated vs. the net
anodic charge. The data were taken from the 7th to 16th cycles in Table
2.
S.-Y. Cui, S.-M. Park r Synthetic Metals 105 (1999) 91–98
indicates the stoichiometric growth while the second part
the autocatalytic growth. The number of electrons involved
is calculated to be about 2.18 from the slope during
stoichiometric growth, which is larger than 2.0 by about
10%. As pointed out, this is due primarily to the degradation of the polymer, albeit small. From the 13th cycle, the
n-value drops to about 0.75, suggesting that the autocatalytic growth plays a more important role than the degradation reaction. When the vertex potential is high at 1.20
V, the degradation reaction overwhelms the autocatalytic
growth of the polymer film particularly when the film is
thick, leading to a large n-value as already discussed
above.
Aniline was also polymerized by a potentiostatic method
at an applied potential of 1.0 V, and the amount of
anilinium units deposited vs. the consumed charge recorded
during the potentiostatic polymerization is shown in Fig. 5.
It is more difficult to interpret the data as we have no way
of knowing the amount of counter anions inserted during
the polymerization reaction. During potentiodynamic polymerization reactions, counter anions are supposed to be
expelled at the end of each cathodic scan and the polymer
would then be completely de-doped. At least, this was the
assumption made, which some reports supported w31x.
However, the same cannot be stated during the potentiostatic polymerization as counter anions are being accumulated as long as the polymerization reaction proceeds.
Therefore, it is difficult to make a quantitative analysis on
the data shown in Fig. 5. Nonetheless, it is clear that
changes in slopes are noticed during the polymerization
experiment, suggesting that there are changes in growth
mechanisms. The larger the slope is in this plot, the
smaller the n-value is. Thus, the n-value starts out with an
intermediate number, which starts to show an increase
when the charge passed is more than about 15 mC. Then, a
large decrease is observed from about 45 mC. These
changes in slopes indicate the changes in growth mecha-
Fig. 5. The amount of PAn vs. consumed charges during the potentiostatic polymerization at 1.0V in a 0.50 M H 2 SO4 solution containing
0.030 M aniline.
97
Fig. 6. Frequency shifts recorded during potentiodynamic PAn growth at
scan rates of: Ža. 40 mV sy1 and Žb. 20 mV sy1 in a 0.030 M aniline
solution in 0.50 M H 2 SO4 .
nisms in three stages, i.e., the formation of oligomers first,
followed by stoichiometric growth and autocatalytic
growth, as was discussed more quantitatively during the
discussion of potentiodynamic growth.
3.2. Parameters affecting PAn growth
We studied the effects of various parameters on the
polymer growth reaction, although we were not able to
vary the experimental parameters to large extents. The first
parameter studied was the scan rate, which controls the
polymerization time w22x. Fig. 6 shows the frequency shifts
recorded during the potential scans at 40 ŽA. and 20 ŽB.
mV sy1 . As can be seen, the rate of polymer growth is
faster at 20 than 40 mV sy1 . This is because the reaction
time for polymerization is longer during slower potential
scans as was pointed out in our previous work w22x. When
the polymer grows to a certain extent, the growth becomes
Fig. 7. Frequency shifts recorded in: Ža. 0.030 and Žb. 0.040 M aniline at
20 mV sy1 in a 0.5 M H 2 SO4 solution.
98
S.-Y. Cui, S.-M. Park r Synthetic Metals 105 (1999) 91–98
faster. The same is true when the aniline concentration is
high, resulting in a faster growth ŽFig. 7.. The break in the
growth rate was observed in all the experiments, where the
amount of polymer deposited is greater for each electron
extracted during the polymerization than what it used to be
before this point. In other words, the n-values decrease
after a certain point. This is because the PAn growth
undergoes a change in mechanism at this stage of polymerization.
As we see here, PAn growth is related to the monomer
concentration, potential scan rate and applied potential as
previously reported w22,23,37x. When the frequency shift is
plotted along with the number of cycles in EQCM measurements, a large frequency shift is observed in a more
concentrated aniline solution, at a slower scan rate, and at
a more positive vertex potential during potentiodynamic
PAn growth. An interesting observation is that a sharp
break in frequency shifts was always observed in later
stages of PAn growth when the amount of grown polymer
is large ŽFigs. 6 and 7..
The sharp break in the frequency shifts during the
polymer growth during both the potentiostatic and potentiodynamic growth may be explained by a variety of
different mechanisms. The first is the change in mechanism as we pointed out above, resulting in a different
electron efficiency. That is to say, the intermediate species
produced on the electrode surface may act in a different
capacity depending on the amount of polymer and the
environment. The second possibility may be due to the
change in properties of the polymer film such as morphology, as the amount of polymer accumulated on the electrode surface increases.
4. Conclusion
Three different stages of aniline polymerization, in
which different numbers of electrons involved, have been
identified during the potentiodynamic and potentiostatic
PAn growth. These include: the nucleation process with an
n-value of much larger than 2, the stoichiometric growth
with n ; 2.0, and the autocatalytic process with n ; 1.0.
The large n-values during the first few cycles indicate low
electron efficiencies during the nucleation process. Gradually decreasing n-values from the second cycle on are
explained by the next series of reactions of oxidized
oligomers to form larger oligomers. When the oligomers
grow to the polymer, the polymer-coated electrode now
acts as a catalyst for growth and the growth almost follows
the columbic law with n-values smaller than 2.0. When the
aniline concentration is low and the vertex potential high,
the degradation reaction overwhelms the autocatalytic
growth, leading to larger n-values. However, autocatalytic
growth dominates in determining the n-value when the
vertex potential is low.
Acknowledgements
A grateful acknowledgement is made to the Korea
Science and Engineering Foundation ŽKOSEF. for the
support of this research by grant aNK97258.
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