Journal of The Electrochemical Society, 149 共4兲 D51-D56 共2002兲
D51
0013-4651/2002/149共4兲/D51/6/$7.00 © The Electrochemical Society, Inc.
Electrochemical Polymerization of Aniline in Proton-Free
Nonaqueous Media
Dependence of Microstructure and Electrochemical Properties of
Polyaniline on Solvent and Dopant
Prem. C. Pandeyz and Govind Singh
Department of Chemistry, Banaras Hindu University, Varanasi-221 005, India
Electrochemical polymerization of aniline in few nonaqueous media containing varying supporting electrolytes is reported. Three
different nonaqueous solvents, i.e., dry acetonitrile, dichloromethane, and nitrobenzene containing three different electrolytes; 共i兲
sodium tetraphenylborate 共TPB兲, 共ii兲 tetraethylammonium tetrafluoroborate 共TETFB兲, and 共iii兲 tetramethylammonium perchlorate
共TMAP兲 are chosen for electrpopolymerization of aniline under potentiostatic mode 共2 V vs. Ag/AgCl兲 and potentiodynamic mode
共⫺0.7 to 2.0 V vs. Ag/AgCl at the scan rate of 100 mV/s兲. Polyaniline films with seven different microstructures 共polymer I to
polymer VII兲 are synthesized. Polymer I and polymer II are made under potentiostatic condition in acetonitrile and dichloromethane containing 0.1 M sodium tetraphenylborate 共TPB兲 and tetraethylammonium tetrafluoroborate, respectively, whereas
polymer III is made under similar conditions in acetonitrile containing 0.1 M tetraethylammonium tetrafluoroborate. Polyaniline
films of four other microstructures are made under potentiodynamic mode of electropolymerization. Polymer IV, polymer V, and
polymer VI are synthesized in acetonitrile, dichloromethane, and nitrobenzene, respectively, containing the same supporting
electrolyte, i.e., 0.1 M tetraethylammonium tetrafluoroborate 共TETFB兲, whereas polymer VII is made under similar conditions in
acetonitrile containing tetramethylammonium perchlorate 共TMAP兲. The microstructures of polyaniline films are characterized by
scanning electron microscopy and cyclic voltammetry. The results show varying microstructures of the films with change in the
nature of electroactivity from polymer I to polymer VII. The comparative results on pH sensitivity are reported.
© 2002 The Electrochemical Society. 关DOI: 10.1149/1.1455649兴 All rights reserved.
Manuscript submitted August 29, 2001; revised manuscript received October 28, 2001. Available electronically March 12, 2002.
The electropolymerization of aniline in aqueous acidic media has
been extensively studied.1-7 However, there are very few studies of
the electropolymerization of aniline in proton-free nonaqueous
media.8 Until recently, polyaniline appeared to be the most thoroughly investigated polymer for various applications. One such application of polyaniline-modified electrodes is for pH sensing in
aqueous media.9-13 Sometimes polyaniline of low molecular mass
can be solvent-extracted using a nonpolar solvent.14 Hence, studies
on the electropolymerization of aniline are still quite attractive, particularly if polymers with more practical applications and useful in
both aqueous and nonaqueous solvents can be synthesized. Furthermore, it is also suggested that the mechanism of electropolymerization of aniline is also a function of the nature of the solvent and
dopants. Therefore, studies of electropolymerization of aniline in
aprotic solvents with varying dopants are potentially important.
During last decade efficient efforts have been given on the developments of pH transducer based on polymer-modified electrode.
A number of organic materials have been tested as potential pH
transducers.15,16 Among these various materials polyaniline has been
found most suitable for pH sensing in aqueous medium.15-17 The
synthesis of this polymer was conducted in an aqueous acidic medium and again its use was limited to aqueous media. Recently an
advanced pH transducer based on processible polyaniline was reported, which was again limited to applications in aqueous medium
only, as the polymer was initially synthesized in aqueous acidic
medium followed by processing on a metal surface by dissolving the
polymeric material together with other additives in chloroform and
subsequently assembling the resulting solution on electrode
surface.18 We have also recently developed a solid-state poly 共3cyclohexyl兲 thiophene-treated electrode as pH sensor and subsequently urea sensor5 using purified urease as described earlier.19
There are still some major problems on the development of this pH
sensor on a commercial scale, these are 共i兲 the relatively complex
synthetic protocol of 3-cyclohexylthiophene, 共ii兲 lower stability of
3-cyclohexylthiophene under ambient conditions, 共iii兲 the solubility
of poly共3-cyclohexylthiophene兲 in several organic solvents, (i v )
poor electrochemical communication of the polymer-modified elec-
z
Email: [email protected]
trode prepared through self-evaporation of excess solvent after laying down a drop of polymer solution on the solid surface.5,18
The present research investigation was undertaken to synthesize
polyaniline films by electropolymerization of aniline in nonaqueous
media in the presence of varying electrolytes. Three nonaqueous
solvents, dry acetonitrile, dichloromethane, and nitrobenzene, were
chosen for electropolymerization of polyaniline films doped with
⫺
different anions, (Ph) 4 B⫺, ClO⫺
4 , and BF4 , using three different
supporting electrolytes, sodium tetraphenylborate 共TPB兲, tetraethylammonium tetrafluoroborate 共TETFB兲, and tetramethylammonium
perchlorate 共TMAP兲. Polymer-modified electrodes were made by
both potentiostatic and potentiodynamic modes of electropolymerization. The pH sensitivity of the polyaniline materials with varying
microstructure is reported.
Experimental
Reagents.—Dry acetonitrile 共HPLC grade兲, nitrobenzene, dichloromethane, and sodium tetraphenylborate were obtained from Merck
共Germany兲. Aniline and tetraethylammonium tetrafluorborate were
obtained from Aldrich Chemical 共USA兲. Tetramethylammonium
perchlorate was made from tetramethylammonium bromide and
HClO4 followed by recrystallization and drying the electrolyte.
Aniline was distilled under vacuum before use.
Electrochemical setup.—The electrochemical measurements
were performed using an electrochemical interface 共Solartron, 1287兲
connected to a PC through its serial port. The electrode body used
for the construction of electropolymerized polyaniline was made
from a Teflon cylinder. A platinum disk on a brass rod was screwed
into a hollow threaded Teflon cylinder resulting in an electrode body
with a recessed depth of 2 mm. The exposed diameter of the platinum disk to be used as the active surface was 2 mm. All electrochemical experiments were carried out in a three-electrode singlecompartment cell with a working solution volume of 5 mL equipped
with a working electrode, Ag/AgCl reference electrode, and a platinum foil auxiliary electrode. Seven systems as shown in Table I
were used for electropolymerization of polyaniline by both 10 min
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Journal of The Electrochemical Society, 149 共4兲 D51-D56 共2002兲
D52
Table I. Electropolymerization of aniline in three different nonaqueous solvents containing three different supporting electrolytes.
Polymer type
Aniline
共molar兲
Solvent
Polymer I
0.4
Acetonitrile
Polymer II
0.4
Acetonitrile
Polymer III
0.4
Dichloromethane
Polymer IV
0.4
Acetonitrile
Polymer V
0.4
Dichloromethane
Polymer VI
0.4
Nitrobenzene
Polymer VII
0.4
Acetonitirle
TPB ⫽ sodium tetraphenylborate.
TETFB ⫽ tetraethylammonium tetrafluoroborate.
TAMP ⫽ tetramethyammonium perchlorate.
PS ⫽ potentiostatic mode.
PD ⫽ potentiodynamic mode.
Electrolyte
共molar兲
0.5
0.1
0.1
0.1
0.1
0.1
0.1
M
M
M
M
M
M
M
TPB
TETFB
TETFB
TETFB
TETFB
TETFB
TMAP
PS/PD
PS
PS
PS
PD
PD
PD
PD
potentiostatic treatment 共2.0 V vs. Ag/AgCl with a reference solution
of same electrolyte兲 and by cycling the potential of the working
electrode between ⫺0.7 and 2.0 V vs. Ag/AgCl at a scan rate of 100
mV/s. Each polymer-modified electrode was washed ten times in the
nonaqueous solvent used in its preparation 共Table I兲 to remove
traces of excess oligomer, monomer, and supporting electrolyte.
Each polymer-modified electrode was characterized by cyclic
voltammetry in monomer-free nonaqueous solvent containing the
same concentration of supporting electrolyte used for electropolymerization.
Potentiometric observations.—All potentiometric measurements
were made under open-circuit mode using the polymer-modified
electrode as the working electrode and a Ag/AgCl reference electrode in aqueous medium with an electrochemical cell assembly as
follows
Polymer-modified electrode/0.1 M KCl/Ag/AgCl
共 containing 3 M NaCl solution saturated with AgCl兲
关1兴
The electrodes were inserted in a single-compartment electrochemical cell containing a suitable working solution. The working and
reference electrodes were connected to a Keithley multimeter
共model 2000兲 followed by interfacing to a PC through its serial port.
After a steady-state response had been attained, varying concentrations of a suitable acid or base were injected followed by data acquisition.
SEM observations.—Polyaniline films obtained from seven different systems 共Table I兲 were characterized by scanning electron
microscopy 共SEM兲. The polymer films were made on a Pt surface
and analyzed using a JEOL-JSM 840A scanning electron microscope 共SEM兲 at suitable magnifications for better presentation of the
polymeric microstructure.
Results and Discussion
Electropolymerization of aniline.—Since the growth of the polymer has been found to be a function of several factors including the
mode of electropolymerization, it was proposed to electropolymerize polyaniline both potentiostatically and potentiodynamically.
Polyaniline films in three nonaqueous media 共Table I兲 were prepared. The polymer-modified electrodes made from seven systems
共Table I兲 are subsequently referred to as polymer I, polymer II,
polymer III, polymer IV, polymer V, polymer VI, and polymer VII
for easy representation. Polymer I and polymer II were obtained by
electropolymerization of aniline in acetonitrile containing sodium
tetraphenylborate and TETFB, respectively, potentiostatically at 2 V
vs. Ag/AgCl. Polymer III was made in dichloromethane containing
TETFB under the same conditions used for making polymer I and
polymer II. Polymers IV to VII were made by cycling the potential
of the working electrode between ⫺0.7 and 2.0 V vs. Ag/AgCl at a
scan rate of 100 mV/s. Polymer IV was made in acetonitrile containing TETFB and polymer V was made in nitrobenzene containing
TETFB. Polymer VI was made in dichloromethane containing
TETFB and polymer VII was made in acetonitrile containing
TMAP. The results of potentiodynamic electropolymerization were
found to be quite interesting. Figure 1a shows the cyclic voltammograms of the growth of polymer IV in acetonitrile. The consecutive
cycling induces a rapid increase in the anodic peak current at 0.4 V
and a slow increase of the same at 1.25 V followed by a rapid
increase in the cathodic peak current at ⫺0.45 V. It was also found
that an increase in peak current is observed with step-wise cycling,
i.e., ten cycles at a time. Figure 1b shows the cyclic voltammograms
of the growth of polymer V in dichloromethane. The consecutive
cycling induces first an increase in the anodic peak current at 0.56 V
followed by a rapid decrease and finally a slow increase. A similar
nature of the change of cathodic peak current at 0.0 and 0.8 V was
recorded 共Fig. 1b兲. Similar observations were recorded for the variation in anodic and cathodic peak currents during the growth of polymer VI and polymer VII growth as shown in Fig. 1c and d, respectively. However, different redox peaks at different potentials 共single
anodic current peak at 0.54 V and cathodic current at 0.7 in nitrobenzene; two anodic peak currents at ⫺0.18 and 0.6 V with two
cathodic peak currents at ⫺0.14 and 0.16 V in acetonitrile containing TMAP兲 were observed in each case. The color of the films is
golden yellow for polymer I and polymer VII, light yellow for polymer V, dark brown for polymer II and polymer IV, and light brown
for polymer III.
Characterization of microstructures of polyaniline films by
SEM.—Polymers I to VII were characterized by scanning electron
microscopy. Figures 2a-g show the microstructures of polymers I to
VII, respectively. The microstructures of all these polyaniline films
show varying growth rates of the polymer with different polymeric
domains. Polymer IV appeared to be the most dense. Polymer II,
polymer III, polymer IV, and polymer VI show fibrous geometry, but
the domains are different in each case. Polymer I, polymer V, and
polymer VII all have different geometry with varying domains. As
already reported, the polymer film prepared in aqueous HClO4 solution has a fibrous structure, and the polymer chains grow favorably without oligomer flow, resulting in large-size grains from the
solutions of higher acid content.20,21 The polymer grown in acetonitrile containing TPB 共Fig. 2a兲 has an aggregate rod-shaped structure
with a smooth surface. Polymer VII has globular geometry and polymer V shows the highest porosity. These differences in the film
microstructure may be due to differences in the rate of film growth,
the nonpolarity of the solvents, and the nature of the dopants.
Electrochemistry of polymer-modified electrodes.—The electrochemistry of electrodes modified with polymers I to VII was studied
by cyclic voltammetry. Figure 3a shows the cyclic voltammetry of
an electrode modified with polymer I between ⫺0.45 and ⫹0.5 V
vs. Ag/AgCl at various scan rates. The electrochemistry of polymer
I is entirely different from that reported for conventional polyaniline. There is a large anodic and cathodic current plateau instead of
the reversible redox electrochemistry reported for conventional
polyaniline synthesized in an aqueous medium. The reason for such
behavior may be the large lipophilic dopant, (Ph) 4 B⫺, that restricts
doping and undoping of the (Ph) 4 B⫺ anion within the polymeric
interstices that is necessary for reversible electroactivity of the
polymer-modified electrode. The upper part of Fig. 3a shows a voltammogram at a single scan rate 共5 mV/s兲. We also studied the
cyclic voltammograms at scan rates as low as 1 mV/s and as high as
400 mV/s. The redox electrochemistry of the film is similar at all
scan rates as shown in Fig. 3a. Figure 3b shows the voltammogram
of BF⫺
4 -doped polymer-modified electrode 共polymer II兲 in acetonitrile containing 0.1 M TETFB as supporting electrolyte at a scan rate
of 2.5 mV/s. The upper part of Fig. 3b shows the voltammograms at
scan rate of 5 and 10 mV/s, respectively. There is a single anodic
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Journal of The Electrochemical Society, 149 共4兲 D51-D56 共2002兲
D53
Figure 1. Electrochemical polymerization of 0.4 M aniline by sweeping the potential between ⫺0.7 and 2.0 V vs. Ag/AgCl at a scan rate of 100 mV/s in seven
systems: 共a, top left兲 acetonitrile containing 0.1 M tetraethylammonium tetrafluoroborate; 共b, top right兲 dichloromethane containing 0.1 M tetraethylammonium
tetrafluoroborate; 共c, bottom left兲 nitrobenzene containing 0.1 M tetraethylammonium tetrafluoroborate, and 共d, bottom right兲 acetonitrile containing 0.1 M
tetramethylammonium perchlorate. The voltammograms show recordings of consecutive cycling.
peak followed by a single cathodic peak. Figure 3c shows the voltammograms of BF⫺
4 -doped polyaniline-modified electrode 共polymer III兲 in dichloromethane containing 0.1 M TETFB as supporting
electrolyte at 2.5 mV/s. There are some remarkable observations on
the redox behavior of the BF⫺
4 -doped polyaniline film grown in
acetonitrile and dichloromethane containing the same supporting
electrolyte: 共i兲 two anodic peaks are recorded for the polyanilinemodified electrode made in dichloromethane 共polymer III兲 as compared to same made in acetonitrile 共polymer II兲 under similar conditions; 共ii兲 both films show a single cathodic peak at nearly same
peak potential; 共iii兲 the difference between the anodic and cathodic
peak currents is much greater for the film grown in acetonitrile
共1200 ␮A; Fig. 3b兲 as compared to that of grown in dichloromethane 共50 ␮A; Fig. 3c兲. These observations and the yields of
polymer II and polymer III suggest that a thicker polyaniline film is
formed with continuous growth in acetonitrile and that it has a relatively regular and relatively porous polymeric microstructure 共Fig.
2b兲. By comparison, the film grown in dichloromethane is compact
and thin 共Fig. 2c兲.
Figure 3d shows the cyclic voltammetry of an electrode modified
with polymer IV in acetonitrile containing 0.1 M TETFB between
⫺0.7 and 21.8 V vs. Ag/AgCl at scan rates of 10, 20, 50, and 100
mV/s. The upper part of Fig. 3d shows a voltammogram at 5 mV/s.
Two anodic peaks at 0.4 and 1.2 V and two cathodic peaks at 0.25
and 1.4 V are notable 共upper part of Fig. 3d兲. Figure 3e shows the
cyclic voltammetry of an electrode modified with polymer V in
dichloromethane containing 0.1 M TETFB between ⫺0.7 and 1.8 V
vs. Ag/AgCl at scan rates of 10, 20, 50, and 100 mV/s. The upper
part of Fig. 3e shows a voltammogram at 5 mV/s. At lower scan
rates two anodic peak at 0.96 and 1.3 V and three cathodic peaks at
⫺0.05, 0.7, and 1.14 V are notable 共upper part of Fig. 3e兲. At higher
scan rates a single anodic peak at 0.8 V and a single cathodic peak
at 0.37 V are recorded 共Fig. 3e兲. Figure 3f shows the cyclic voltammetry of a polymer VI-modified electrode in nitrobenzene containing 0.1 M TETFB between ⫺0.7 and 1.8 V vs. Ag/AgCl at scan
rates of 10, 20, 50, and 100 mV/s. The upper part of Fig. 3f shows
a voltammogram at 5 mV/s. Two anodic peaks at 0.9 and 1.32 V and
a single cathodic peak at 0.3 V are notable 共upper part of Fig. 3f兲. At
higher scan rates the anodic peak is converted into a plateau followed by a single cathodic peak at 0.2 V 共Fig. 3f兲. Figure 3g shows
the cyclic voltammetry of a polymer VII-modified electrode in acetonitrile containing 0.1 M TMAP between ⫺0.7 and 1.8 V vs.
Ag/AgCl at scan rates of 10, 20, 50, and 100 mV/s. The upper part
of Fig. 3g shows a voltammogram at 5 mV/s. A single anodic peak
at 1.2 V and a single cathodic peak at ⫺0.24 V are notable 共upper
part of Fig. 3g兲. The difference between the anodic and cathodic
peak currents is 450 ␮A for polymer IV, 100 ␮A for polymer V, 150
␮A for polymer VI, and 70 ␮A for polymer VII suggesting that
thicker film growth of polyaniline film occurs in acetonitrile containing TETFB.
The pH sensing behavior of polymer-modified electrodes.—The
typical proton-sensing behavior of electrodes modified with polymers I to VII was studied in 0.1 M KCl with the electrochemical cell
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D54
Journal of The Electrochemical Society, 149 共4兲 D51-D56 共2002兲
Figure 2. Scanning electron micrographs of polyaniline films: 共a, top left兲 polymer I; 共b, top right兲 polymer II, 共c, 2nd row left兲 polymer III, 共d, 2nd row right兲
polymer IV, 共e, 3rd row left兲 polymer V, 共f, 3rd row right兲 polymer VI, and 共g, bottom兲 polymer VII.
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Journal of The Electrochemical Society, 149 共4兲 D51-D56 共2002兲
D55
Figure 3. Cyclic voltammograms of polyaniline-modified electrodes at different scan rates:; 共a, top left兲 polymer I; 共b, top right兲 polymer II; 共c, 2nd row left兲
polymer III; 共d, 2nd row right兲 polymer IV; 共e, 3rd row left兲 polymer V; 共f, 3rd row right兲 polymer VI, and 共g, bottom兲 polymer VII. The voltammograms were
recorded in the respective solvent and supporting electrolyte used for electropolymerization.
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D56
Journal of The Electrochemical Society, 149 共4兲 D51-D56 共2002兲
sodium tetraphenylborate 共TPB兲, tetraethylammonium tetrafluoroborate 共TBFB兲, and tetramethylammonium perchlorate. The
polyaniline films of seven different microstructures were made by
electropolymerization of aniline both potentiostatically and potentiodynamically. A remarkable dependence of the polymer microstructure on solvents and dopants was observed. The polyaniline film
made in dry acetonitrile containing sodium tetraphenylborate as supporting electrolyte is highly sensitive for proton sensing. The electrochemistry of seven polymer-modified electrodes was studied and
the results reveal variable redox behavior of each film that may be
due to variations in polymeric domains and chain length that subsequently affect the doping and undoping reactions.
Acknowledgments
The authors are thankful to U.G.C., New Delhi, for financial
assistance. G.S. is thankful to CSIR for the award of a senior research fellowship.
References
Figure 4. Relative pH sensing behavior of seven polymer-modified electrodes.
assembly shown in Eq. 1. The relative variation in pH sensitivity of
electrodes modified with polymers I to VII is qualitatively shown in
Fig. 4. The electrode modified with polymer I, doped with (Ph) 4 B⫺
anions, shows excellent pH sensitivity as compared to that of the
other polymer-modified electrodes. The reasons for such variations
in pH sensing of polyaniline films might be related to the growth of
polymer I in specific solvent and dopant which subsequently constitutes a different geometry of the polymer network that ultimately
generates the presence of free sites around nitrogen for protonation
and deprotonation.
We have also used the (Ph) 4 B⫺-doped polyaniline-modified
electrode for designing a urea biosensor.22 The urea biosensor
responded nicely with better sensitivity and improved storage and
operational stability. The TPB-doped polyaniline-modified electrode
has also been used for pH sensing in a nonaqueous medium.22 The
results reported22 were quite interesting.
Conclusions
In summary, we report herein the electropolymerization of
aniline in proton-free solvents 共dry acetonitrile, dichloromethane,
and nitrobenzene兲 and in the presence of three different electrolytes,
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