Materials Sciences and Applicatio ns, 2011, 2, 1491-1498
doi:10.4236/msa.2011.210201 Published Online October 2011 (http://www.SciRP.org/journal/msa)
1491
Synthesis of Polypyrrole Using Ammonium Peroxy
Disulfate (APS) as Oxidant Together with Some
Dopants for Use in Gas Sensors
Hemant K. Chitte1, Narendra V. Bhat2, Ajit V. Gore2, Ganesh N. Shind3
1Dnyansadhana College, Near Eternity Mall, Thane, India; 2Bombay Textile Research Association, LBS Marg, Mumbai, India;
3Indira Gandhi College, Cidco Colony, New Nanded, India.
Received June 3rd, 2011; revised June 24th, 2011; accepted July 25th, 2011.
ABSTRACT
Polypyrrole (Ppy) was synthesized using Ammonium Peroxy Disulfate (APS) as oxidant in a standard ratio of monomer
to oxidants at 5˚C. Attempts were made to increase the electrical conductivity by using various dopants viz. Lithium per
Chlorate (LiClO4), para-Toluene Sulfonate (p-TS) and Napthalene Sulfonic acid (NSA). The materials were character-
ized using FTIR, X Ray diffraction and SEM. The electrical conductivity was measured by two probe method and was
found to be in the range of 10−3 S/cm. Thin films of these preparations were casted on the interdigited electrodes to
study the detection of gas such as ammonia. It was found that for the pure Ppy when ammonia gas was allowed to flow
in, there was a sudden increase in the current, which decreased rapidly when gas was stopped. However when Ppy
doped with p-TS, NSA and LiClO4, the trend was reversed.
Keywords: Polypyrrole, Structure, Doping, Gas Sensor, Ammonia
1. Introduction
Over the last few decades, conducting polymers have
emerged as a new class of materials with interesting
electron-transport behavior and a material with immense
potential in technological applications. Their ease of
processing together with their chemically tunable proper-
ties makes them especially useful in electronic, optoelec-
tronic and electromechanical devices [1-5]. One such
area where the conducting polymers have shown great
promise is in sensory applications. Delocalized electronic
states combined with the restriction on the extent of de-
localization makes most of the conductive polymers be-
have like p-type semiconductors. As these polymers are
Redox-active, their conductivity can be changed by
means of doping/dedoping. A great number of sensing
applications are designed by exploiting the very nature of
conducting polymers [6].
Conducting polymers, such as polypyrrole (Ppy), poly-
aniline (Pani), polythiophene (PTh) and their derivatives,
have been used as the active layers of gas sensors since
early 1980s [7]. In comparison with most of the com-
mercially available sensors, based usually on metal ox-
ides and operated at high temperatures, the sensors made
of conducting polymers have many improved character-
istics. They have high sensitivities and short response
time; especially, these features are ensured at room tem-
perature. Conducting polymers are easy to be synthesized
through chemical or electrochemical processes, and their
molecular chain structure can be modified conveniently
by copolymerization or structural derivations [8]. Poly-
pyrrole, in the form of films, has been used for sensors
for detection of various gases and volatile organic com-
pounds [9-13]. Polypyrrole sensors are sensitive with
good response to these gases, showing larger response to
polar than non polar compounds.
Ammonia is a toxic gas even at low concentrations.
Environmental and health safety restrictions or recom-
mendations determine a very low allowable concentration,
which has not been reliably detectable with the commer-
cially available sensors. These are suffering from the lack
of high sensitivity and selectivity. Generally they also
need heating for their operation. All industries where
ammonia is produced or used or is a by-product, as well as
agricultural plants where ammonia is present, its early
detection is highly needed [14].

Synthesis of Polypyrrole Using Ammonium Peroxy Disulfate (APS) as Oxidant Together with 1492
Some Dopants for Use in Gas Sensors
3
3
The physical properties of conducting polymers
strongly depend on the type of dopants and the doping
levels. The doping levels can be easily changed by
chemical reactions at room temperatures and this pro-
vides a simple technique to facilitate detection of several
gases. Most of the conducting polymers are doped/un-
doped by redox reactions. When such conducting poly-
meric material is brought in contact with either a gas or a
liquid, transfer of electrons from or to the analyte takes
place. Electron transferring can cause the change in re-
sistance and work function of the sensing material. When
Ppy is exposed to some gases, redox reaction can take
place. Gases such as NO2 and I2 which are electron ac-
ceptors can remove electrons from the aromatic ring of
polypyrrole. When this occurs, for a p-type conductive
polymer, the doping level as well as electric conductance
of the conductive polymers is enhanced. On the other
hand an electron donating gas, such as ammonia, reacts
with the Ppy, the electrical conductance falls down
sharply. However when desorption of the gas occurs the
process can be reversed. The following reaction is invol-
ved in the process [15].
2
3
Ppy + NH Ppy + NH  absorption
+
3
PpyNH Ppy+ NH
 desorption
In spite of such advances, the results obtained for
many gases may differ from this generalized postulate of
detection mechanism. For example work reported in thin
polycarboazole film sensors shows increase in conduc-
tance when the electron donating gas has been used [16].
Conducting polymers usually can be synthesized by
chemical or electrochemical techniques, oxidizing the
corresponding monomers. Chemical oxidation involves
mixing monomer and oxidant in solution. The most
widely used oxidants are Ammonium Peroxy Disulfate,
Ferric Chloride, Hydrogen peroxide, Potassium dichro-
mate, Cerium sulfate, and so on. Both aqueous and or-
ganic media are used.
In the present paper we report preparation of polypyr-
role in pure form and by using dopants such as LiClO4,
p-TS and NSA. The structure and morphology of these
materials has been investigated using the methods of
FTIR, X-Ray diffraction and SEM. The electrical con-
ductivity has been measured by two probe method. The
response of these materials, when exposed to ammonia is
being reported.
2. Experimental Methods and Preparations
Pyrrole (Sisco Research laboratory, 99% pure) was dis-
tilled before use. All other reagents and solvents obtained
from SDL were of reagent grade and were used as re-
ceived. All solutions were prepared using distilled water.
All reactions were conducted at a temperature of 5˚C
[17]. The solution of the oxidizing agent, APS, was pre-
pared using distilled water and was used in the ratio of
1:2.4 (monomer:oxidant) [12]. Dopants were mixed with
Pyrrole solution (10% w/w) and stirred for 30 minutes
for proper mixing and then the oxidant solution was
added slowly.
The Polypyrrole was prepared by chemical polymeri-
zation method. 1 M Pyrrole solution was prepared using
distilled water and then mixed with an oxidizing agents
in the ratio mentioned above, slowly under constant stir-
ring for 30 minutes. Then the polymerization was con-
ducted for 4 hours under constant stirring. This prepara-
tion was kept unagitated for 24 hours so that Ppy powder
settled down. The Polypyrrole powder was filtered out
under vacuum and washed with distilled water several
times to remove any impurities present. The Polypyrrole
was dried for 2 days at room temperature.
The chemical bonding was analyzed using FTIR spec-
troscopy by FTIR spectrometer model Perkin-Elmer Ltd.
system 2000 using KBr pellets. The Polypyrrole was
characterized by XRD using PAnalytical (Philips), model
XpertPro. The XRD patterns were recorded between 2

= 10˚ to 40˚ The X-ray diffractometer uses CuKα radia-
tion of
 = 1.5418 Å generated at 40 kV/20 ma.
The morphology of Polypyrrole was examined using
Scanning Electron Microscope (SEM) JEOL make JSM-
5400 model. The Polypyrrole was tested for conductivity
by preparing its pellets of area of cross section of 1 sq·cm.
and 1 mm. thickness. The conductivity was tested by two
probe apparatus fabricated in our laboratory. It consists
of a copper plate which serves as one electrode, heated
from the bottom to raise the temperature. The second
electrode was a copper rod which was spring loaded to
give enough pressure on the sample pallet so that electri-
cal contact is maintained The conductivity was measured
at room temperature and various temperatures up to 70
˚C in the voltage range from 0 to 12 V.
In order to measure the gas response, interdigited elec-
trodes, separated by 1 mm. from all sides, were prepared
on a printed circuit board (PCB) and the slurry of
Polypyrrole powder prepared with distilled water was
spread over it uniformly and dried for 24 hours under
vacuum at room temperature. A specially prepared gas
chamber was used in which the PCB was fitted firmly.
Various gases were passed through the chamber at room
temperature with Nitrogen gas as a carrier. The current
was recorded, at a constant voltage, for every 15 seconds
for the total time of 3 minutes. The flow of the gas was
stopped and the desorption was also recorded for interval
of every 10 seconds till current recovered up to 90% of
its original value. The gas response was recorded con-

Synthesis of Polypyrrole Using Ammonium Peroxy Disulfate (APS) as Oxidant Together with
Some Dopants for Use in Gas Sensors
1493
with p-TS (24.6˚). This indicates that the inter planer
spacing increases with the addition of dopants. This
would make the chains get apart from each other due to
the large sizes of the dopant molecules. In addition it was
noted that the diffraction profile is highly asymmetrical
and there is considerable broadening towards lower an-
gles from the peak positions.
tinuously for 3 cycles.
3. Result and Discussions
3.1. Characterization of Ppy
3.1.1. FTIR Spectroscopy
The Polypyrrole powders prepared in different ways
were analyzed by FTIR. FTIR spectra showed the main
characteristic peaks at 1558 cm−1, 1542 cm−1 and 1471
cm−1 corresponding to the fundamental vibrations of
polypyrrole ring. The band at 1294 cm−1 corresponds C-H
deformation. Other low intensity peaks are observed at
around 2927 cm−1 - 2814 cm−1 which can be attributed to
aromatic C-H stretching vibrations. The peaks at 1685
m−1 and 801 cm−1 represents C=N and C-N bonds, the
bond of C-H in plane deformation vibration is situated at
985 cm−1 and of the C-C out of plane ring deformation
vibrations or C-H rocking is at 681 cm−1 which occurs at
695 cm−1 in our spectrum [18]. These peaks were ob-
served in the present work for preparations using APS as
oxidants and various dopants such LiClO4, p-TS and
NSA (Figure 1). This agrees well with the ones available
the literature, confirming the formation of Polypyrrole
[19,20].
3.1.3. Morphology Using SE M
The morphological features of polypyrrole synthesized
Figure 1. FTIR of polypyrrole using APS (2), doped with
P-TS (1); NSA (3); LiClO4 (4).
3.1.2. X-Ray Diffrac ti on Anal ysis
A typical X ray diffraction pattern for polypyrrole pre-
pared using APS as oxidant is shown in Figure 2.The
XRD patterns for samples using the dopants also show
broad peaks in the region 15˚ < 2θ < 30˚ revealing that
the resulting polypyrrole powders are amorphous in na-
ture. This agrees well with the structure reported in lit-
erature [14]. Such broad peak usually indicates short
range arrangement of chains. The half width of this peak
was measured for all the four samples and their values
are given in Table 1. The half widths were measured
from the diffraction curve from the angles higher than the
peak positions and assuming Gaussian distribution for
the low angle part of the diffraction curve.
However the diffraction peak centered at around 25.4˚
for pure pyrrole shows some displacement when we go to
the doped ones. In general the doping leads to shift the
peaks toward the lower angle, being lowest for Ppy doped Figure 2. X ray diffraction pattern for polypyrrole prepar- ed using APS as oxidant.
Table 1. X ray Peak Positions, Half widths, Globular sizes and conductivities of Ppy and with dopants.
Sample X ray peak position X ray half width SEM size of globule Conductivity at room temp.
over a linear portion of curve
Pure Polypyrrole 25.4˚ 8˚ 0.59 μm 1.70 × 10−3
Polypyrrole with LiClO4 25.27˚ 3.24˚ 0.89 μm 1.02 × 10−3
Polypyrrole with p-TS 24.6˚ 6.8˚ 0.27 μm 4.60 × 10−3
Polypyrrole with NSA 25.3˚ 6.6˚ 0.63 μm 2.84 × 10−3

Synthesis of Polypyrrole Using Ammonium Peroxy Disulfate (APS) as Oxidant Together with 1494
Some Dopants for Use in Gas Sensors
chemically and electrochemically has revealed that mostly
the growth is in the globular form but changes some time
due to dopant molecules [21]. Typical SEM images of
different Polypyrrole preparations are shown in Figures
3 (a)-(d). All the photographs show a globular structure.
It can be seen from Figure 3(a) that when polymeriza-
tion was done with APS the average size of globules was
found to be 0.59 μm. The individual granules observed
were nearly spherical and have a close packing. It seems
that such spherulites are growing one over the other and
forming a continuous structure. The sizes of these spher-
ulites are varying from 0.2 μm to 0.7 μm. When dopants
were used during the polymerization with APS, the sizes
of the granules were found to be different. With LiClO4
the average size was found to be 0.89 μm, with variation
between 0.3 μm to 0.9 μm, which is rather large in com-
parison to other preparations. When p-TS were used as a
dopant, there was a considerable reduction in the size
amounting to 0.27 μm. More over the morphological fea-
ture was spongy in nature and there is considerable dif-
ficulty in distinguishing the granules from each other.
This shows that a much closed packed structure is form-
ed and this fact supports our earlier conclusion on the
basis of X ray diffraction. When NSA was used as do-
pant it was observed that average globular size is 0.63
μm. Such morphological features are considered to be
good for gas sensing applications.
3.2. I-V Characteristics
Typical plots of I vs. V for polypyrrole prepared using
APS as oxidant and LiClO4, p-TS and NSA as dopants are
given in Figures 4(a)-(d). It was observed that the electri-
cal conductivity of polypyrrole increased when dopants
such as LiClO4, p-TS and NSA were used. Nearly linear
relationship of the graph of I vs V curve was noted upto 8
volts for samples prepared using only APS and APS +
(a) (b)
(c) (d)
Figure 3. (a) SEM of Ppy prepared by our method using APS as an oxidant; (b) SEM of Ppy using APS as a oxidant and Li-
ClO4 as dopant prepared by our method; (c) SEM of Ppy using APS as an oxidant and p-Ts as dopant prepared by our
method; (d) SEM of polypyrrole using APS as a oxidant and NSA as dopant prepared by our method.

Synthesis of Polypyrrole Using Ammonium Peroxy Disulfate (APS) as Oxidant Together with 1495
Some Dopants for Use in Gas Sensors
(a) (b)
(c) (d)
Figure 4. I vs V characteristic of Polypyrrole prepared using (a) APS; (b) LiClO4; (c) NSA; (d) p-TS.
LiClO4. When other dopants like NSA and p-TS were
used the initial part of rising current peaked at 6 and 4
volts respectively. After this peaks when voltage was
increased further, a decrease in current was observed.
The reason for such departure from ohm’s law is either
that the current is mainly contributed by ions or degrada-
tion of the sample at higher voltage. However the latter
reason can be ruled out as the temperature variation of
the I vs V plot does not show such behavior of decreas-
ing current at temperatures 40˚C to 70˚C. Additional evi-
dences for the contribution to the current by ions are be-
ing explored. The conductivities were found in the range
of 1.0 × 10−3 to 4.6 × 10−3 S/cm.
The temperature dependence of the electrical conduc-
tivity of the polypyrrole, prepared with APS and their
doped varieties were studied at various temperatures
from 30˚C up to 70˚C. It was observed that the electrical
conductivity decreased gradually for all the samples.
This behavior resembles the metallic conductors. It may
be mentioned that pure pyrrole is more like insulator but
behaves like semiconductor in the doped form. The
mechanism of conduction is supposed to be by polarons
and bipolarons formation due to the dopant molecules
[22].
The mechanism of polaron formation is illustrated in
Figure 5. In case of polypyrrole the absence of electron
in the chain leads to formation of holes i.e. p-type con-
duction. Thus addition of dopants leads to the modifica-
tion of energy levels as shown in Figure 5.
Additional energy bands are formed above the valence
band and just below the conduction band. This reduces
the energy gap and the doping leads to semiconduction.
Thus it is expected that increasing temperature will lead
to increase of energy for an electron in the valance band
which jumps to conduction band and hence the conduc-
tivity should increase with the temperature. However the

Synthesis of Polypyrrole Using Ammonium Peroxy Disulfate (APS) as Oxidant Together with 1496
Some Dopants for Use in Gas Sensors
Figure 5. Conduction mechanism in polypyrrole.
present studies revealed that the materials do not behave
like semiconductor but like metals where the conductiv-
ity is decreasing with temperature. Such behavior can be
caused due to the large number of intermediate energy
states in the energy gap region. In the present studies the
doping level was 10% which is comparatively high and
leads to overlap of a large number of energy states. In
addition the size of the dopants ions ClO4 and 3 SO

is
quite large and overlap of energy states can occur. Thus
the temperature dependence can be understood. The
conductivities change in the order p-TS > NSA > LiClO4
> pure.
Of all the samples studied in the present investigation
polypyrrole doped with p-TS was found to have maxi-
mum conductivity. In addition it was noted that the con-
ductivity changes with temperature to a small extent for
NSA and p-TS doped samples.
3.3. Gas Sensor
All the polypyrrole samples were studied for detection of
ammonia gas. A typical plot of current vs time for
polypyrrole prepared using APS as oxidant and exposed
to ammonia gas is given in Figure 6. All samples were
studied for 3 cycles to check their reproducibility and
absorption and desorption process. It may be seen from
the Figures 6(a)-(d) that the i vs t plot for 2nd and 3rd
cycles somewhat differ from the first cycle. This may be
because desorption may be not completed within the
given time.
The Sensitivity factor is calculated using the equation
Rg Ro
SRo


where Rg and Ro are resistances with gas and without
gas (in air) respectively [23,24]. The values calculated
during the present investigation for sensors fabricated
using differently doped polypyrrole and for different
gases are given in Table 2.
The response of different materials towards ammonia
gas was seen to be different. When pure Ppy was ex-
posed to ammonia gas there was an increase in current.
This behavior is exactly opposite to that observed for Ppy
prepared by electrochemical method [25]. This is mainly
because in chemical preparation ion is incorpo-
rated as counter ion to maintain the electronutrality. Usu-
ally partial oxidation state (upto + 0.33) is produced in
the process. Thus there is a possibility that higher oxida-
tion state can be induced in the interaction with ammonia.
In such a situation the higher charge on cations can lead
to increase in current.
2
28
SO
It was noted during these investigations that when Ppy
doped with LiClO4, p-TS and NSA were used, a decrease
in current was observed when exposed to ammonia gas.
The electrical conductivity of these products showed

Synthesis of Polypyrrole Using Ammonium Peroxy Disulfate (APS) as Oxidant Together with 1497
Some Dopants for Use in Gas Sensors
(a) (b)
(c) (d)
Figure 6. Response for ammonia of polypyrrole prepared using (a) APS; (b) LiClO4; (c) NSA; (d) p-TS.
Table 2. Sensitivity of pure and doped Ppy for ammonia
gas.
Sample Sensitivity for Ammonia
Pure Polypyrrole 0.54
Polypyrrole with LiClO4 1.28
Polypyrrole with p-TS 2.08
Polypyrrole with NSA 2.86
higher conductivity than pure Ppy which means that the
doping levels and the number of charge carriers produced
in the process are quite high. This is probably due to the
presence of sulfonic acid group inducing more charge on
pyrrole ring. Thus there already exists a higher density of
charge and therefore the presence of ammonia cannot
produce more charges, but rather decreases the effective
charge. In such a situation therefore the conductivity of
Ppy will decrease, which has indeed been observed in
our studies. It may be noted from the curves b to d that
the variations in the current in the second and third cy-
cles decreases, which is probably due to the fact that, the
desorption of the gas is not complete in the time interval
that was allowed in the present investigation.
4. Conclusions
Polypyrrole was synthesized in the pure form and doped
with LiClO4, p-TS and NSA. All these four verities were
characterized using FTIR, X ray diffraction and SEM. It
was seen that there are structural and morphological dif-
ferences which affect their electrical properties. The elec-
trical conductivity was highest for p-TS doped polypyr-
role (4.6 × 10−3 S/cm). These materials were used as gas
sensors for the detection of ammonia. It was found that
ammonia could be detected efficiently.
5. Acknowledgements
The authors wish to thank Director, BTRA for interest
and support during this work. Thank are also due to Mr.

Synthesis of Polypyrrole Using Ammonium Peroxy Disulfate (APS) as Oxidant Together with 1498
Some Dopants for Use in Gas Sensors
V. E. Walunj for help during the experimental work. H.
K. C. is grateful to the Principal Dnyanasadhana College
for granting the necessary leave.
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