Influence of dopant in the synthesis, characteristics and ammonia sensing behavior
of processable polyaniline
Partha Pratim Sengupta, Pradip Kar, Basudam Adhikari
⁎
Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India
abstractarticle info
Article history:
Received 6 September 2007
Received in revised form 20 December 2008
Accepted 22 December 2008
Available online 11 January 2009
Keywords:
Processable polyaniline
Thin film sensor
Organic semiconductors
Ammonia sensing
Polymers
Polyaniline (PANI) was synthesized by oxidative polymerization of aniline as well as aniline hydrochloride by
ammonium persulfate in the presence of para-toluene sulfonic acid (PTSA). This helped in direct usage of the
conducting PANI solution for film casting and use as a device for ammonia gas sensing. Viscosity change with
applied shear rate was measured for both the polymers. Solid PANI powder was isolated from its tetrahydrofuran
solution by using methanol as non-solvent. Thermogravimetric analysis investigated the thermal properties of
the solid PANI salts. Elemental analysis of both PANI synthesized in presence of PTSA and PANI synthesized in
presence of HCl and PTSA was investigated. A thin coherent film of both the conducting PANI were deposited on
glass slides precoated with poly vinyl alcohol (PVA) crosslinked with maleic acid (MA) and was directly used in
the sensor device. The morphology of the deposited films was analyzed by scanning electron micrograph. The
films were further characterized by Attenuated total reflectance Fourier transformed infrared spectroscopy, ultra
violet-visible spectroscopy and X-ray diffraction analyses. Finally, both the doped PANI films on MA crosslinked
PVA coated glass slides were used to measure the conductivity and ammonia gas-sensing characteristics.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction

Wu and coauthors [9] used a DBSA doped PANI thin film as an
ammonia sensor and found response and recovery time of 2 and 5 min
respectively for 100 ppm ammonia gas. They found a resistance of
150 Ω for such films. Other researchers [10,11] used a templated PANI
film as an ammonia gas sensor for different concentrations of ammo-
nia gas. These works reveal that PANI based ammonia sensors possess
a wide range of recovery and response times, which might be due to
different substrates used for film deposition, different film deposition
techniques and different film thicknesses. A useful approach for the
improvement of the processability of conducting polymers involves
blending with suitable matrix polymers such as poly (vinyl alcohol)
(PVA) [12–14]. An ammonia sensor based on conducting polypyrrole
was one of the early practical realizations of conducting polymer
based sensors. Its sensitivity, however, was relatively low and the
response was not very reversible
[15]. However, Ojio and Miyata [16]
prepared polypyrrole-poly (vinyl alcohol) (PPy-PVA) films by electro-
chemical polymerization. Linsey and Street [17] studied gas sensing
behavior of PPy-PVA films prepared by electrochemical polymeriza-
tion on to a precoated PVA matrix. These studies contrasted the
advantages of mechanical properties of the host polymer with the
electrical properties of PPy-PVA composite films.
Thin Solid Films 517 (2009) 3770–3775
⁎ Corresponding author. Tel.: +91 3222 283966; fax: +91 3222 255303.
E-mail address: [email protected] (B. Adhikari).
0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2008.12.049
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Thin Solid Films
journal homepage: www.elsevier.com/locate/tsf

(3.72g)andPTSA(15.18g)weredissolvedin170mlTHFinthemoleratio
of 1:2 in a round bottom flask and maintained at − 5 ° C. T o this s olutio n
ammonium persulphat e (APS) (11.42 g) dissolved in 30 ml deionized
water was added drop wise. To synthesize PANI_HCl_PTSA the
same procedure was followed as that of PA NI_PTSA using ani line
hydr ochloride (5 .18 g) instead of aniline. Two reaction mixtures were
gently stirred and polymerizations were carried out for 48 h. A green
solution of PANI salts were formed in both the cases instead of
precipitation. Since this P ANI solution is unable to form a film on a
glass surface, a MA cr osslinked PVA surface was chosen for PANI film
deposition. MA crosslinked PVA coated glass slides were immersed in
boththePANIreactionmixturesandtakenoutafter1htodepositthin
films of emer ald ine salts on the sl ides. The films deposi t ed aft er
prolonged immersion beyond 1 h wer e found to dev elop crac ks on
drying. The deposited thin films were then dried at 40 °C for 2 h and then
washed several times with metha nol and d eionized water to remov e any
unreacted APS and a niline or bypr oducts. Finally the washed films wer e
dried in v a cuum at 50 °C for 6 h. A th in unifo rm film formati on on
crosslinked PVA coated glass slide was observed which was used fo r
further study.
2.3. Precipitation of doped polyaniline from green solution
It is mentioned in the previous section that the synthesized PANI
salt solution in aqueous THF could not form a thin film on glass
surface. On the contrary, when a non-solvent such as methanol was
added to the solution fine particles of solid PANI were precipitated
slowly. For the purpose of characterization of the precipitated poly-
mer, it was filtered and washed several times with methanol and
deionized water. The solid polymer was then converted to polyaniline
base by treating with 1 M ammonia solution for 48 h. The PANI base
was washed several times with deionized water and dried under

heating rate of 10 °C/min.
2.4.5. Scanning electron microscopy (SEM)
For the surface morphology study, the SEM images of the polymer
films deposited on the glass slides (coated with MA crosslinked PVA)
were taken in a field emission SEM instrument (VEGA TESCAN).
2.4.6. Attenuate d total reflectance Fourier transf ormed infr ared sp ectr oscopy
(ATR–FTIR)
The ATR–FTIR spectra of the doped polyaniline films deposited on
precoated glass slide were taken on a Thermo Nicolet Nexus 870
spectrophotometer between 400 and 4000 cm
− 1
.
2.4.7. X-ray diffraction (XRD) analysis
The XRD analysis of the PANI film samples deposited on the MA
crosslinked PVA coated glass slide was done in Philips Type PW 1710
using Cu K
α
(λ =1.542 A°). The XRD analysis of MA crosslinked PVA
coated glass slide was also done in the same instrument.
Table 1
Elemental analysis of the polyaniline co doped with HCl and PTSA and polyaniline doped with only PTSA.
Sample % elements S/N
ratio
Cl/
N
ratio
Dopant (mol%) σ ×10
2
(S/cm)
C H N Cl S O PTSA HCl

monitored at every 30 s int erval allowing the reading to stabiliz e. Aft er
some time when the r esponse (R/R
0
) became saturat ed the a mmonia gas
flo w w a s st opped and air was passed to allow the s ensor m ateri al t o
recover the original state. This span covered one cycle of gas exposure. The
sensitivity of the sensor was measured as the corresponding R/R
0
value
when the response curv e reached the highest satur at ed level (when the
curve becomes par allel to X-axis). The response time was measured as
the time between the entry of ammonia gas to the polyaniline sensor
and the saturation of its response. The recovery time is the time in be-
tween the saturation response to the initial v alue in presence of air [11].
3. Results and discussion
3.1. Synthesis of processable PANI_ PTSA and PANI_HCl_PTSA
Due to the presence of methyl group in the functionalized protonic
acid PTSA, in-situ doped PANI became soluble in organic polymeriza-
tion medium (THF). PANI synthesized by conventional methods in the
presence of aqueous hydrochloric acid [20] precipitates out due to
polarization of π-electron cloud of the growing polymer. Once the
polymer phases out from the polymerization medium, it becomes
very difficult to solvate. However, the methyl group of PTSA imparts a
non-polar interaction with the surrounding THF medium and resists
phasing out of PANI. The para-toluene sulphonate anion (PTSA
−
)
dopant is likely to decrease the conductivity of PANI. Therefore, we
opted to use both HCl and PTSA as dopant and found good processable
polymer with better conductivity than only PTSA doped PANI at the

PANI_HCl_PTSA has higher polymer chain length and inter-chain
forces compared to PANI_PTSA. During protonation to the amine
group of aniline a competition occurs between HCl and PTSA. The
smaller size of the Cl
−
counterion present in the PANI_HCl_PTSA
actually favors better polymer chain growth than only PTSA
−
anion
from PANI_PTSA.
3.4. Thermal studies
An initial 2–3% weight loss (Fig. 2)inbothdopedpolymerisfound
until 160 °C. This occurred due to the release of bound water molecules.
This is followed by a rap id loss in weight in the PANI_HCl_PTSA due to
the release of HCl molecules followed by a slow release of PTSA until the
500 °C (Fig. 2) when the third stage of degradation starts. In case of
PANI_PTSA only single stage degradation starts at 180 °C and at about
400 °C the weight loss is about 30% due to the dedoping of PTSA (Fig. 2).
Finally, a third stage degradation occurs for both the doped PANI due to
complete degradation and decomposition of polymer backbone.
However, for the PANI_HCl_PTSA the third stage of decomposition
starts at a much higher temperature (530 °C) and about 55% mass is
retained at 600 °C while at that temperature only 35% mass is retained in
PANI_PTSA (Fig. 2). Therefore, it can be said that PANI_HCl_PTSA is
thermally less stable than that of PANI_PTSA due to the release of HCl
molecule at low temperature. Otherwise, the co-doped PANI is more
stable than that of single doped PANI.
3.5. SEM analysis
The SEM image of co-doped PANI deposited on MA crosslinked
PVA coated glass slide shows a uniform distribution while that of

) stretching
Fig. 1. The viscosity change with shear rate of the co-doped and single doped PANI
solutions obtained after polymerization reaction.
Fig. 2. Thermogravimetric analysis of co-doped and single doped PANI powder.
Fig. 3. SEM images of (a) single doped PANI_PTSA and (b) co-doped PANI_HCl_PTSA
films deposited on crosslinked PVA coated glass slide.
3773P.P. Sengupta et al. / Thin Solid Films 517 (2009) 3770–3775
were observed at higher wave numbers for PANI_PTSA than that of
PANI_HCl_PTSA (1609 cm
− 1
and 151 7 cm
− 1
). The overall shift of the
benzenoid and quinoid stretching might be due to the structural
modification of the deposited PANI on the MA crosslinked PVA matrix
[23]. A shoulder peak at 1692 cm
− 1
for PANI_PTSA and at 1737 cm
− 1
for
PANI_HCl_PTSA were observed due to the CfO stretching of the acetate
group of the PVA substrate. The shift to lower wave number can also be
ascribed to PANI–PVA structural modification. The peaks at 1324 cm
− 1
and 1296 cm
− 1
for PANI_PTSA and PANI_HCl_PTSA respectively can be
assigned due to –NH– stretching of secondary amine. The bands at
1158 cm
− 1

20° and a sharp peak at 25.9°. However, the PANI_HCl_PTSA film
shows more crystallinity than PANI_PTSA with more sharp peaks at 2θ
value of 20° and 25.7°. The above peaks are characteristic of the
crystalline phase of the emeraldine salt [18]. The higher crystallinity in
PANI_HCl_PTSA can be attributed to the smaller dopant HCl helping in
closer chain arrangement while the presence of bulky PTSA in larger
amounts causes ring distortion in PANI_PTSA.
3.8. DC conductivity
I–V characteristics of the PANI_HCl_PTSA and PANI_PTSA are
shown in Fig. 6. The PANI_HCl_PTSA shows a lower resistance com-
pared to PANI_PTSA. The linear I–V characteristics for both the doped
PANI have shown some ohmic behavior. The more compact alignment
Fig. 4. FTIR–ATR spectra of the co-doped and single doped PANI films deposited in
crosslinked PVA coated glass slide.
Fig. 5. XRD patterns of the crosslinked PVA coated glass slide and co-doped and single
doped PANI films deposited on crosslinked PVA coated glass slide.
Fig. 6. I–V characteristics of the co-doped and single doped PANI films deposited of
crosslinked PVA coated glass slide.
Fig. 7. Ammonia sensing study of the co-doped and single doped PANI films deposited
on crosslinked PVA coated glass slide for 3 cycles with an ammonia concentration of
100 ppm.
3774 P.P. Sengupta et al. / Thin Solid Films 517 (2009) 3770–3775
of chains in the PANI_HCl_PTSA compared to PANI_PTSA leads to more
localized sites of polaron and bipolaron states and better charge
mobility [24]. This leads to lower resistance of the former. In
PANI_PTSA the presence of the dopant PTSA in higher amounts causes
adjacent ring distortion due to its bulky size and hence hinders
conjugation than that of PANI_HCl_PTSA.
3.9. Ammonia sensing
The time dependence of R/R

by its bulky
methyl group. Another reason for better sensitivity and response time
for PANI_HCl_PTSA than that of PANI_PTSA, is the more compact
structure and smooth surface (confirmed from SEM image) of the co-
doped PANI, which leads to more potential si tes of ammonia
adsorption and higher sensitivity. However, the recovery time of
PANI_PTSA is lower (3.5 min) than that of PANI_HCl_PTSA (4.0 min).
The reason might be the presence of bulky dopant ion PTSA in more
amounts in PANI_PTSA causing weaker chemisorptions of the NH
3
. So,
PANI_HCl_PTSA appears to be better ammonia sensor than the
PANI_PTSA in terms of response time and sensitivity although the
former shows slightly lower recovery time.
4. Conclusion
Functionalized protonic acids like PTSA when used as an in situ
dopant in the polymerization of aniline, a green solution of PANI_PTSA
in THF and water mixture was obtained. This can be directly used as a
sensor element by depositing the polymer film on MA crosslinked PVA
coated glass slide. Using HCl (present in aniline hydrochloride) as a
co-dopant with PTSA, a processable PANI_HCl_PTSA was synthesized.
PANI_HCl_PTSA has higher intermolecular force and closer packing of
molecular chains than in PANI_PTSA. This higher chain ordering and
good processability of PANI_HCl_PTSA leads to higher conductivity
and better ammonia gas sensing performance.
Acknowledgements
We gratefully acknowledge the University Grants Commission,
Government of India for providing fellowship to authors Partha Pratim
Sengupta and Pradip Kar.
References

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