ORIGINAL PAPER

Electrochemical copolymerization of carbazole and 2,20:50-200

terthiophene: characterization and micro-capacitor application

Murat Ates • Nuri Eren

Received: 24 November 2013 / Accepted: 4 May 2014

� Iran Polymer and Petrochemical Institute 2014

Abstract In this paper, a copolymer of carbazole (Cz)

and 2,20:50,200-terthiophene (TTh) was electropolymerized

in 0.1 M sodium perchlorate (NaClO4)/acetonitrile

(CH3CN) on glassy carbon electrode. The optimum con-

ditions of resulting homopolymers of Cz, TTh and

copolymer of Cz and TTh in the initial feed ratio of [Cz]0/

[TTh]0 = 1/10 were characterized by cyclic voltammetry,

Fourier-transform infrared-attenuated total reflectance,

scanning electron microscopy, energy dispersive X-ray

analysis, and electrochemical impedance spectroscopy.

Morphological analysis of copolymer shows that a micro-

spherical and web-like morphology was formed for

copolymer at different initial feed ratios of [Cz]0/

[TTh]0 = 1/2, 1/5 and 1/10. The capacitive behavior of the

modified electrodes was defined via Nyquist, Bode-mag-

nitude, and Bode-phase plots. The highest low-frequency

capacitance (CLF) was obtained as 4.11 mFcm-2 in the

initial feed ratio of [Cz]0/[TTh]0 = 1/10. Double-layer

capacitance (Cdl) and phase angles (h) were obtained for

homopolymer and copolymer systems. The highest Cdl was

obtained as 2.01 mFcm-2 for the copolymer in the initial

feed ratio of [Cz]0/[TTh]0 = 1/2. The highest phase angle

of copolymer was obtained as h = *75� in the initial feed

ratio of [Cz]0/[TTh]0 = 1/1. These capacitance results

confirmed that films of copolymer Cz/TTh are promising

materials for micro-capacitor applications.

Keywords Carbazole � Terthiophene � Electrochemical

impedance spectroscopy � Conducting polymers �Copolymerization

Introduction

Electrochemical copolymerization has recently been

widely applied to the synthesis of conducting polymers. It

is one strategy for developing new materials by combining

the individual properties of polymers through linking two

different monomers into a polymer chain [1]. Thus, the

copolymer is expected to gain better conductivity and good

thermal, capacitive and mechanical properties than either

of its components [2].

Polythiophenes have better solubility properties

amongst conducting polymers. Polymerization of the

thiophene ring can result in linkage at the a- or b-

position [3, 4]. Polymer conductivity and stability

values reduce when a–b or b–b binding occurs. It is

difficult to observe of terthiophenes [5] during poly-

merization process due to the steric effects. The

bindings reduce the oxidation potential of polymer

which increases the easy polymerization and revers-

ibility [6].

Polycarbazole is one of the most used one amongst the

conducting polymers due to its good electrochemical

behaviors [7–10]. The conductive form of polycarbazole

can easily be obtained by the electrochemical method [11].

The functional carbazole polymers were used in many

applications due to its interesting thermal, electrical, pho-

toelectrical ion-exchange and other physicochemical

properties [12–14].

M. Ates (&) � N. Eren

Department of Chemistry, Faculty of Arts and Sciences,

Namik Kemal University, Degirmenalti Campus,

59030 Tekirdag, Turkey

e-mail: mates@nku.edu.tr

URL: http://www.atespolymer.org

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Petrochemical Institute 123

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DOI 10.1007/s13726-014-0252-9

There are many copolymer formations in previous

studies [15–17]. For example, electrocopolymerization of

3,4-ethylenedioxythiophene and 30-octylthiophene was

synthesized by Buvat and Hourquebie [18]. Terthiophene

and carbazole monomers were electropolymerized as

molecularly imprinted polymer and used in SPR sensing of

drug molecules [19]. p-conjugated dendrimer-protected

gold nanoparticles have been successfully prepared in

stable colloidal form via simultaneous reduction of AuCl3with oxidative polymerization of terthiophene and carba-

zole [20]. A comparative study of the copolymerization

behavior between terthiophene and carbazole conjugated

polymer precursor was investigated using electrochemical

methods [21]. Conjugated block copolymers represent a

class of materials with potential applications such as

electronics and optoelectronics. Metal compounds based on

poly(p-phenylenevinyleneborane) and terthiophene were

investigated in literature [22].

Electrochemical impedance spectroscopy (EIS) is an

effective and reliable method to extract information about

electrochemical characteristics of the electrochemical sys-

tems [23–25]. Double-layer capacitance, diffusion imped-

ance, the rate of charge transfer and charge transport

processes, and solution resistance of the electrochemical

systems can be achieved by EIS technique [26]. EIS is also

considered one of the best techniques for analyzing the

properties of conducting polymer electrodes [27, 28].

There are two principal approaches to modeling the

impedance of such systems: a uniform, homogeneous film

[29, 30] and a porous membrane [31, 32].

To the best of our knowledge, no systematic work has

yet been reported on the electrochemical random copoly-

merization of carbazole (Cz) and TTh in 0.1 M NaClO4/

CH3CN. Homopolymers and copolymers were character-

ized by cyclic voltammetry (CV), Fourier transform

infrared-attenuated total reflectance (FTIR-ATR), scanning

electron microscopy–energy dispersive X-ray analysis

(SEM–EDX), and EIS analyses. The low frequency

capacitance and double-layer capacitances of the copoly-

mers were comparatively investigated in detail. Thus, a full

paper on this aspect is the need of the time.

Experimental

Materials

The monomers of Cz and 2,20:50,200-terthiophene (TTh)

were provided by Sigma-Aldrich (USA). All other reagents

included acetonitrile (Aldrich, 99.93 %), and sodium per-

chlorate (Aldrich, [98.0 %) were purchased from Merck

(Germany) and used without any treatment or further

purification.

Instrumentation

Cyclic voltammetry was performed using IviumStat (Ivium

Technologies, the Netherland) (software: Iviumsoft and

Faraday cage: BASI Cell Stand C3) in a three-electrode

configuration, which employed a glassy carbon electrode

(GCE) (area: 0.07 cm2) as the working electrode, platinum

wire as the counter electrode and Ag|AgCl|0.3 M KCl as

the reference electrode. The working electrode was care-

fully polished with alumina slurry and cleaned in an

ultrasonic bath before each experiment.

Fig. 1 Cyclic voltammetries of (a) Cz, potential range: 0–1.6 V,

[Cz]0 = 1 mM (b) 2,20:50,200-TTh, potential range: 0–1.4 V,

[TTh]0 = 1 mM (c) P(Cz-co-TTh), potential range: 0–1.4 V, [Cz]0/

[TTh]0 = 1/10 on glassy carbon electrode (GCE) in 0.1 M NaClO4/

CH3CN, 8 cycle, and scan rate: 100 mV s-1

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A modified carbon fiber microelectrode (CFME) was

characterized by FTIR-ATR spectroscopy (Spectrum

One B, PerkinElmer, USA, with a universal ATR

attachment with a diamond and ZnSe crystal). UV–Vis

measurements were performed with a T80 UV–Vis (PG

Instruments, UK) spectrophotometer. Morphological

investigations were performed with scanning electron

microscopy (SEM) and energy dispersive X-ray analysis

(EDX) using a Leo 1430 VP Carl Zeiss (Germany)

instrument.

Scheme 1 Mechanism of electrochemical formation of copolymer TTh/Cz on GCE

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Electrochemical impedance spectroscopic (EIS) mea-

surements were performed in 0.1 M NaClO4/CH3CN

solution. The EIS measurements were done in a monomer-

free electrolyte solution with perturbation amplitude of

10 mV rms over a frequency range of 0.01 Hz to

100,000 Hz with a IviumStat model potentiostat/galvano-

stat (Ivium Technologies, The Netherlands).

Results and discussion

Electropolymerization of Cz and TTh on GCE

The CV of TTh has higher oxidation potential (?1.05 V)

and lower reduction peak potential (?0.47 V) than P(Cz)

(Fig. 1a). The CV of Cz showed one oxidation peak at

?0.88 V and a reduction peak at ?0.84 V which shows the

electro-activity of the monomer (Fig. 1b). An increase in

the peak intensities was observed upon eight cycles. After

increasing the terthiophene concentration in the copolymer

composition, the electro-active formed copolymer had an

oxidation peak at ?1.08 V and one reduction peak at

?0.48 V (Fig. 1c). The monomer concentration of TTh is

10 times higher than Cz in the copolymer composition. The

redox behavior of copolymer is completely different from

its components. The oxidation and reduction peak poten-

tials of copolymer exist between the peak potentials of its

homopolymers.

The electrosynthesis route and possible copolymer for-

mation mechanism of TTh and Cz on GCE is given in

Scheme 1. The electrosynthesis of the TTh (A) and Cz

(B) monomers show in radical cation formation by electron

transfer from monomers to GCE. Dimer formation of TTh

(A) and Cz (B) gives radical cation coupling, deprotona-

tion, and neutral dimer and oligomer formations occur. As

a result by adding of dimers with TTh and Cz shows a

copolymer formation (C).

Effect of scan rate in a monomer-free solution

The scan rate dependence of the anodic and cathodic peak

current densities shows a linear dependence on scan rates

from 25 to 1,000 mV s-1 (R1an = 0.89644 and R1cat =

-0.90209) for P(Cz-co-TTh)/GCE (Fig. 2a). The peak

current density was determined to be proportional with m1/2

in the range of the square root of scan rates from 2 to

20.01 mV s-1 (regression coefficients (R2an) = 0.95850,

and R2cat = -0.97036) as shown in Fig. 2b where diffu-

sion control applies [33]. The scan rate dependence of the

electro-active film was investigated on the reversible sys-

tem of the oxidation peak potential at *1.14 V and

reduction peak potential at *0.21 V (Fig. 2c). The peak

current density (ip) for a reversible voltammogram at room

temperature is given by the following equation:

ip ¼ 2:69� 105� �

� A� D1=2 � C0 � m1=2

where, m is the scan rate, A is the electrode area, D is the

diffusion coefficient of the electro-active species in the

solution. This demonstrates that the electrochemical pro-

cess has diffusion controlled process [34–37].

Fig. 2 Plots of a scan rate vs. current density for P(Cz-co-TTh)/GCE

in 0.1 M NaClO4/CH3CN, b square root of scan rate vs. current

density for P(Cz-co-TTh)/GCE, [Cz]0/[TTh]0 = 1/10, c CV of P(Cz-

co-TTh)/GCE in monomer-free solution, 8 cycle, scan rate:

25–1,000 mV s-1 and potential range: 0–1.4 V

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FTIR-ATR and UV–Vis analyses

The FTIR-ATR spectra of P(Cz), P(TTh) and P(Cz-co-

TTh) were obtained from the surface of the electrocoated

carbon fiber microelectrodes (CFMEs) by reflectance

measurements. The absorption bands of each spectrum are

given in Fig. 3. P(Cz) has the following characteristic

peaks: 1,544 cm-1 (C–N), 1,060 cm-1 (ClO4- from the

electrolyte of NaClO4). The characteristic peak of C–N

from copolymer was given at 1,325 cm-1 [38]. P(TTh) has

the following characteristic peaks: 1,429 cm-1 (aromatic

C=C), 1,068 cm-1 (ClO4- from the electrolyte of Na-

ClO4). The characteristic peak at 790 cm-1 refers to the C–

S bond for P(TTh). C–S bond does not exist in P(Cz).

However, it exists at 786 cm-1 in the copolymer spectrum.

In the literature, C–S stretching appeared at 793 and

632 cm-1 [39, 40]. These two important peaks are incor-

porated into the copolymer structure. This is strong evidence

of copolymer formation. In addition, the peak at 1,030 cm-1

refers to the dopant ion of ClO4- in the copolymer spectrum.

The UV–Vis spectra of P(Cz), P(TTh) and P(Cz-co-

TTh) are shown in Fig. 4a–c. The peaks of PCz were

obtained as 345 and 377 nm. However, there are no certain

peaks for P(TTh) and copolymer.

SEM analysis

The surface morphology of the electrochemically formed

carbazole, 2,20:50,200-terthiophene, and copolymer of TTh and

Cz/CFME interphases varied in the initial feed ratio of [Cz]0/

[TTh]0 = 1/1, 1/2, 1/5 and 1/10 as shown in Fig. 5a–d. The

coatings with micro-spherical and web-like morphology were

formed using the solution of 0.1 M NaClO4/CH3CN. In pre-

vious studies, Diaz et al. reported that the physical structure

differences of the surfaces of thin films formed by electro-

chemical polymerization did not affect the surface energies of

the films. Many process parameters, such as the monomer

concentration, type of electrolyte, applied voltage, and the

polymerization time affect the surface energy of the coated

fibers. The low frequency capacitances (CLF) were obtained

during the electrogrowth process as 1.78 and 4.11 mFcm-2

for P(Cz-co-TTh) in the feed ratios of [Cz]0/[TTh]0 = 1/5 and

1/10, respectively. The highest low frequency capacitance

(CLF = 4.11 mFcm-2) was obtained for the copolymer in the

initial feed ratio of [Cz]0/[TTh]0 = 1/10.

Fig. 3 FTIR-ATR spectra of P(Cz), P(TTh) and P(Cz-co-TTh)/

CFME, [Cz]0 = 1 mM, potential range: 0 to ?1.6 V, scan rate:

100 mV s-1, 0.1 M NaClO4/CH3CN, 30 Cycle, [TTh]0 = 1 mM

potential range: 0 to ?1.4 V, scan rate: 100 mV s-1, 0.1 M NaClO4/

CH3CN, 30 Cycle, and [Cz]0/[TTh]0 = 1/10, potential range: 0 to

?1.4 V, scan rate: 100 mV s-1, 0.1 M NaClO4/CH3CN, and 30 cycle

Fig. 4 UV–Vis spectra of a P(Cz), b P(TTh) and c P(Cz-co-TTh).

The spectrums were obtained in the oligomeric forms of polymers in

0.1 NaClO4/CH3CN

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EDX analysis

Energy dispersive X-ray analysis provides important evi-

dence of polymer film formation, as shown in Fig. 6a–c.

There is no sulfur element in the structure of P(Cz) as

shown in Fig. 6a. The nitrogen element is not included in

the P(TTh) as given in Fig. 6b. However, these two ele-

ments exist in the copolymer structure as shown in Fig. 6c.

The highest percentage of sulfur and nitrogen elements

exist in the initial feed ratio of [Cz]0/[TTh]0 = 1/10. The

sodium and chlorine elements prove the dopant ion, which

exist in the polymer structure obtained from the electrolyte

of NaClO4.

Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) is an

important technique used to obtain electrochemical infor-

mation and capacitive behavior of a polymer/electrolyte

system, such as electrolyte resistance, charge transfer

resistance, and Faradaic capacitance [41, 42]. The low-

frequency capacitance (CLF) values can be obtained from a

Nyquist plot as the following equation:

CLF ¼ 1=2p� f � Z 00

The results are the following values for CLF : 0.53 mFcm-2

for P(Cz), 3.48 mFcm-2 for P(TTh), and 1.44 mFcm-2 for

Fig. 5 SEM micrographs of a P(TTh) on a single CFME,

[TTh]0 = 1 mM, potential range: 0 to ?1.4 V, scan rate:

100 mV s-1, 0.1 M NaClO4/CH3CN, 30 cycle, (magnification

910,000), b P(TTh) on a single CFME, [TTh]0 = 1 mM, potential

range: 0 to ?1.4 V, scan rate: 100 mV s-1, 0.1 M NaClO4/CH3CN,

30 cycle (magnification 925,000), c P(Cz-co-TTh) on a single

CFME, [Cz]0/[TTh]0 = 1/10, Potential range: 0 to ?1.4 V, scan rate:

100 mV s-1, 0.1 M NaClO4/CH3CN, 30 Cycle, (magnification

910,000), and d P(Cz-co-TTh) on a single CFME, [Cz]0/

[TTh]0 = 1/10, potential range: 0 to ?1.4 V, scan rate:

100 mV s-1, 0.1 M NaClO4/CH3CN, 30 cycle, (magnification

950,000)

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P(Cz-co-TTh) in the feed ratio of [Cz]0/[TTh]0 = 1/1, and

0.66 mFcm-2 for [Cz]0/[TTh]0 = 1/2, 1.78 mFcm-2 for

[Cz]0/[TTh]0 = 1/5, and 4.11 mFcm-2 for [Cz]0/

[TTh]0 = 1/10 (Fig. 7). The highest low frequency

capacitance was obtained as 4.11 mFcm-2 for the

copolymer in the initial feed ratio of [Cz]0/[TTh]0 = 1/10.

If the initial monomer concentration of terthiophene

increases in the copolymer structure, the low frequency

capacitance value increases from 0.66 to 4.11 mFcm-2.

Double-layer capacitance (Cdl) can be calculated from

the Bode-magnitude plot of the equation IZI = 1/Cdl by

extrapolating the linear section to the value of x = 1 (log

x = 0). Double-layer capacitance of the system was

obtained as 1.10 mFcm-2 for P(Cz), 0.75 mFcm-2 for

P(TTh), and 1.27, 2.01, 1.26 and 0.89 mFcm-2 for the

copolymer samples with the initial feed ratios of [Cz]0/

[TTh]0 = 1/1, 1/2, 1/5 and 1/10, respectively (Fig. 8). The

Cdl values of the copolymer especially for initial feed ratios

of [Cz]0/[TTh]0 = 1/1, 1/2 and 1/5 are higher values than

its homopolymer counterparts.

The phase angles were determined from the Bode-phase

plot, as shown in Fig. 9. The phase angle was obtained at

*77� at the frequency of 15.85 Hz for P(TTh). The highest

Fig. 6 EDX point analyses of a P(Cz) on a single CFME,

[Cz]0 = 1 mM, potential range: 0 to ?1.6 V, and b P(TTh) on a

single CFME, [TTh]0 = 1 mM, potential range: 0 to ?1.4 V, and

c P(Cz-co-TTh) on a single CFME, [Cz]0/[TTh]0 = 1/10, potential

range: 0 to ?1.4 V (All experiments were performed at the scan rate

of 100 mV s-1, 0.1 M NaClO4/CH3CN, and 30 cycle)

Fig. 7 Nyquist plots of P(Cz) at [Cz]0 = 1 mM, P(TTh) at

[TTh]0 = 1 mM, and P(Cz-co-TTh), at [Cz]0/[TTh]0 = 1/10 (Elec-

trodeposition was done on GCE in 0.1 M NaClO4/CH3CN at a scan

rate of 100 mV s-1, using multiple cycles (8 cycles) and potential

range: 0–1.4 V)

Fig. 8 Bode-magnitude plots of P(Cz) at [Cz]0 = 1 mM, P(TTh) at

[TTh]0 = 1 mM, and P(Cz-co-TTh) at [Cz]0/[TTh]0 = 1/10 (Elec-

trodeposition was done on GCE in 0.1 M NaClO4/CH3CN at a scan

rate of 100 mV s-1, using multiple cycles (8 cycles) and potential

range: 0–1.4 V)

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phase angle (h = *81�) was obtained at the frequency of

25.12 Hz for P(Cz). The phase angle (h = *75�) was

obtained for P(Cz-co-TTh) in the initial feed ratio of [Cz]0/

[TTh]0 = 1/1 at the frequency of 10 Hz. The minimum

phase angles were obtained as *53� at 10 Hz, *53� at

10 Hz and *35� at 10 Hz for P(Cz-co-TTh) in the initial

feed ratios of [Cz]0/[TTh]0 = 1/2, 1/5, 1/10, respectively.

As we can see that phase angles of copolymers depends

strongly on the initial monomer concentration of TTh in the

copolymer structure as given in Fig. 10.

Conclusion

In this work, the copolymers of carbazole and 2,20:50,200-terthiophene were electropolymerized on a GCE with

various initial feed ratios [Cz]0/[TTh]0 = 1/1, 1/2, 1/5 and

1/10. The synthesis of Cz and TTh copolymers was pro-

posed as a capacitor electrode and discussed with charac-

terizations of CV, FTIR-ATR, SEM–EDX, and EIS

analyses. Monomer feed ratio strongly affects the capaci-

tance value of the copolymer. As a result, the highest low-

frequency capacitance (CLF = 4.11 mFcm-2) was

obtained for P(Cz-co-TTh) at [Cz]0/[TTh]0 = 1/10 ratio.

The highest double-layer capacitance was obtained as

Cdl = 2.01 mFcm-2 for the copolymer with the initial feed

ratio of [Cz]0/[TTh]0 = 1/2. The phase angle of copolymer

was obtained as h = ~75� for copolymer with the initial

feed ratio of [Cz]0/[TTh]0 = 1/1. This phase angle value

h = *75� (f = 10 Hz) was between the homopolymers of

Cz (h = ~81�, f = 25.12 Hz) and h = ~77�(f = 15.85 Hz) for TTh.

Acknowledgments Partial financial support for this work by the

Research Foundation of Namik Kemal University (Turkey) Project

number: NKUBAP.00.10.YL.12.02) is gratefully acknowledged.

Authors thank to Dr. A. Gokceoren (ITU, Turkey) for FTIR-ATR and

S. Tıkız (TUAM, Turkey) for SEM–EDX measurements.

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