influence of dopant in the synthesis, characteristics and ammonia sensing behavior

In fluence of dopant in the synthesis, characteristics and ammonia sensing behaviorof processable polyaniline Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, I

In fluence of dopant in the synthesis, characteristics and ammonia sensing behavior

of processable polyaniline

Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India

a b s t r a c t

a r t i c l e i n f o

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 forfilm 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 coherentfilm 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 depositedfilms 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 PANIfilms 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

Polyaniline (PANI) salt synthesized by chemical route is insoluble

in organic solvents Therefore, it requires treatment with a base to

make it soluble in N-methyl-2-pyrrolidone (NMP) forfilm casting

Suchfilms are subsequently doped for obtaining better conductivity It

would be convenient if the PANI salt could be doped in situ during its

synthesis and made soluble in a solvent for directfilm casting But the

first technique generally suffers from poor processability and lower

conductivity Cao et al [1] and others [2–4] reported the use of

functionalized protonic acids, which doped PANI and simultaneously

resulting doped PANI completely soluble in common organic solvents

Palaniappan and Amarnath [5] synthesized polyaniline-dodecyl

hydrogen sulphate-acid salt by using aniline as the monomer, while

Yin and Ruckenstein[6]synthesized processable polyaniline co-doped

with dodecyl benzene sulfonic acid (DBSA) and hydrochloric acid

using aniline hydrochloride as the monomer Many researchers

mea-sured the conductivity using pellet of PANI salt powder[6–8] The

reported conductivity of PANI in pellet form is in the range 0.5 to 12 S/

cm Yin and Ruckenstein[6]although claimed to have good solubility

in organic solvents and good processability of PANI doped with DBSA,

but they measured the conductivity in the pellet form Lu and

coworkers[8]also measured the conductivity of PANI pellets doped

with DBSA and para toluene sulfonic acid (PTSA), although they

claimed their doped polymer to be soluble in organic solvents There

are only a few reports of direct doping and casting of PANIfilms in the salt form using functionalized dopants None of the above works have shown gas sensing performances of PANI For getting good response and reproducibility in gas sensing, a thinfilm would be better than a pellet Because in case offilm, absorption and desorption of gas would

be faster than pellet due to molecular order and compactness inside thefilm

Wu and coauthors[9]used a DBSA doped PANI thinfilm 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

150Ω 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 ammoammo-nia sensors possess

a wide range of recovery and response times, which might be due to different substrates used forfilm deposition, different film deposition techniques and differentfilm 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-PVAfilms 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 compositefilms

⁎ Corresponding author Tel.: +91 3222 283966; fax: +91 3222 255303.

E-mail address: ba@matsc.iitkgp.ernet.in (B Adhikari).

0040-6090/$ – see front matter © 2009 Elsevier B.V All rights reserved.

Contents lists available atScienceDirect Thin Solid Films

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / t s f

ammonium persulfate ((NH4)2S2O8) (Merck, India), hydrochloric

acid (Merck, India), polyvinyl alcohol (Fluka AG, Germany), and

maleic acid (Loba Chemie, India) were used without further

purification Tetrahydrofuran (THF) (Merck, India) was dried (by

using sodium metal) and used as solvent

2.2 Synthesis of processable polyaniline

Processable PANI_PTSA and PTSA_HCl_PTSA were synthesized by

chemical oxidative polymerization For synthesis of PANI_PTSA, aniline

(3.72 g) and PTSA (15.18 g) were dissolved in 170 ml THF in the mole ratio

of 1:2 in a round bottomflask and maintained at −5 °C To this solution

ammonium persulphate (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 PANI_PTSA using aniline

hydrochloride (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 PANI solution is unable to form afilm on a

glass surface, a MA crosslinked PVA surface was chosen for PANIfilm

deposition MA crosslinked PVA coated glass slides were immersed in

both the PANI reaction mixtures and taken out after 1 h to deposit thin

films of emeraldine salts on the slides The films deposited after

prolonged immersion beyond 1 h were found to develop cracks on

drying The deposited thinfilms were then dried at 40 °C for 2 h and then

washed several times with methanol and deionized water to remove any

unreacted APS and aniline or byproducts Finally the washedfilms were

dried in vacuum at 50 °C for 6 h A thin uniformfilm formation on

crosslinked PVA coated glass slide was observed which was used for

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 thinfilm on glass

surface On the contrary, when a non-solvent such as methanol was

added to the solutionfine particles of solid PANI were precipitated

slowly For the purpose of characterization of the precipitated

poly-2.4.2 Elemental analyses The elemental analyses of PANI_HCl_PTSA and PANI_PTSA were done in order to investigate the level of doping in both the polymer films The results are shown inTable 1 Carbon, hydrogen and nitrogen were analyzed by Perkin Elmer C H N S/O Analyzer 2400 while S and

Cl percentages were estimated by the Shöniger Flask combustion technique[18] The theoretical weight percentages of the elements shown inTable 1were calculated based onScheme 1

2.4.3 Viscosity shear-rate measurement The viscosity change with shear rate of the processable PANI_HCl_ PTSA and PANI_PTSA solutions in THF and water obtained after polymerization was studied in Advanced Rheometer, TA Instruments

AR 1000

2.4.4 Thermal characterizations Thermal stability of polymer samples was analyzed by thermo-gravimetric analysis (TGA) using Perkin Elmer Pyris Diamond ther-mogravimetric analyzer in N2atmosphere from 50 °C to 600 °C at a 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 afield emission SEM instrument (VEGA TESCAN) 2.4.6 Attenuated total reflectance Fourier transformed infrared spectroscopy (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 PANIfilm samples deposited on the MA crosslinked PVA coated glass slide was done in Philips Type PW 1710 using Cu Kα(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.

ratio Cl/

N ratio

Dopant (mol%) σ ×10 2

(S/cm)

PANI_HCl_PTSA 60.67 65.2 5.78 4.73 11.05 9.8 5.20 6.2 7.55 5.6 9.75 8.4 0.21 0.27 0.236 0.146 2.3

2.4.8 Electrical properties

A four-probe setup fabricated in our laboratory[19]was used to

measure the resistivity of the PANI films According to four-point

probe method the resistivity (ρ) was calculated using the relation

ρ = 2πS V = Ið Þ

Where S is the probe spacing (mm), which was kept constant, I is the

supplied current in mA and the corresponding voltage (V) was

measured in mV The conductivity (σ) was calculated using the

rela-tionship,σ=1/ρ

2.5 Gas sensing

In our gas sensor set up[19], the ammonia–air mixture was taken from

the headspace of a bottle containing ammonia solution The ammonia–air

mixture was continuously passed to a closed gas sensing chamber having

a small outlet The ammonia concentration in the mixture was estimated

by trapping a known volume of ammonia–air mixture in an ice-cold dilute

hydrochloric acid solution that was titrated with a standard sodium

hydroxide solution before and after ammonia trapping[11] The

polyani-linefilm specimen (1 cm×1 cm) was mounted on a bakelite sheet and

then four contacts were made on the specimen by copper wires and silver

paste with 2 mm distance between the probes A constant current source

(Keithley Model 224) and a multimeter (Keithley model 196) were used to

apply the current and to measure the voltage drop respectively R and R0

are the resistances measured in the presence of ammonia and air

respectively A known volume of ammonia gas was introduced

(con-centration calibrated to be at 100 ppm) and the change of resistance was

monitored at every 30 s interval allowing the reading to stabilize After

some time when the response (R/R0) became saturated the ammonia gas

flow was stopped and air was passed to allow the sensor material to

recover the original state This span covered one cycle of gas exposure The

sensitivity of the sensor was measured as the corresponding R/R0value

when the response curve reached the highest saturated level (when the

curve becomes parallel 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 value 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 convenpolymeriza-tional methods in the presence of aqueous hydrochloric acid[20] precipitates out due to polarization ofπ-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 same conditions of polymerization MA crosslinked PVA coated glass slide was inserted directly into the polymerization medium for polymer deposition (both single and co-doped) The thicknesses of both the polymerfilms were maintained within 0.08–0.09 mm The reaction schemes for both dopedfilms are shown inScheme 1 3.2 Elemental analysis

PANI_PTSA contains about 0.289 mol% dopant while PANI_HCl_ PTSA contains 0.382 mol% (0.236 mol% PTSA + 0.146 mol% HCl) dopant (Table 1) The amount of dopants in PANI_HCl_PTSA was calculated from the atomic S/N and Cl/N ratios This shows that the migration of HCl as a dopant is higher than that of PTSA because of the smaller size of the Clˉ ion Thus, co-doped PANI shows better conductivity than that of single doped PANI All other values for the elements are obtained as shown inTable 1

3.3 Viscosity-shear rate The change of viscosity of PANI solution with shear rate is related

to the intermolecular forces between polymer chains [21] and polymer chain length As chain length increases the intermolecular forces increase due to more dipole interaction or intermolecular

Scheme 1 Synthesis of polyaniline doped with PTSA (PANI_PTSA) and co-doped polyaniline with HCl and PTSA (PANI_HCl_PTSA).

hydrogen bonding with =N– atom.Fig 1describes the thixotropic

behavior of PANI_HCl_PTSA and PANI_PTSA solutions The shear

thinning of PANI_PTSA solution starts at a lower shear rate of 2.2 s− 1

compared to PANI_HCl_PTSA, which undergoes shear thinning at a

shear rate of 9.5 s− 1 The initial viscosity of PANI_HCl_PTSA is also

much higher Both these observations comply with the fact that

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) in both doped polymer is found

until 160 °C This occurred due to the release of bound water molecules

This is followed by a rapid 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

molecule with PTSA−, facilitates larger compactness of structure So, PANI_HCl_PTSA is better compatible with the matrix of MA cross-linked PVA than PANI _PTSA

3.6 ATR–FTIR analysis The ATR–FTIR spectra of PANI_HCl_PTSA and PANI_PTSA are shown

inFig 4 The N–H stretching, aromatic –CH2– stretching and –CH– stretching were observed at 3461 cm−1, 2931 cm−1and 2852 cm− 1 respectively for PANI_HCl_PTSA while for PANI_PTSA those bands were observed at higher wave numbers (Fig 4) showing lesser conjugation

[22] The quinoid (1644 cm−1) and benzenoid (1544 cm−1) stretching

Fig 1 The viscosity change with shear rate of the co-doped and single doped PANI

solutions obtained after polymerization reaction.

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.

were observed at higher wave numbers for PANI_PTSA than that of

PANI_HCl_PTSA (1609 cm− 1and 1517 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−1for PANI_PTSA and at 1737 cm−1for

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− 1for PANI_PTSA and PANI_HCl_PTSA respectively can be

assigned due to–NH– stretching of secondary amine The bands at

1158 cm−1and 1139 cm−1for PANI_HCl_PTSA and PANI_PTSA

respec-tively are due to the C–O stretching of the MA ester of the crosslinked

PVA The peaks at 1039 cm− 1and 1029 cm−1observed for PANI_PTSA

and PANI_HCl_PTSA respectively can be assigned as SfO stretching of

the sulphonic acid The peaks at 818 cm−1and 800 cm−1were observed

for PANI_PTSA and PANI_HCl_PTSA due to the C–H out of plane bending

vibration Therefore, the compliance of the vibrational bands of the

dopant ion and the characteristic bands of PANI indicates that the

deposited PANI on the PVA matrix is in the emeraldine salt form

3.7 XRD study

Since higher molecular order favors charge transport in conducting

polymer, we have done XRD analysis to assess such molecular order in

terms of crystallinity The X-ray diffractometer scans of the three samples, viz., PANI_HCl_PTSAfilm on MA crosslinked PVA matrix, PANI_PTSAfilm on the same substrate and the precoated glass slide with MA crosslinked PVA are shown inFig 5 For XRD analysis, both the PANIfilms having same thickness (0.08–0.09 mm) were taken The precoated PVA is essentially amorphous as indicated by the single broad scattering centered at the 2θ value of 22.7° The PANI_PTSA depositedfilm is crystalline as evidenced by existence of peaks at 15°, 20° and a sharp peak at 25.9° However, the PANI_HCl_PTSAfilm 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 inFig 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

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

On replacement of the ammonia atmosphere with fresh air, the

ammonium ions decompose into ammonia leaving the proton back on

the PANI chain The sensing mechanism is governed by the

protona-tion–deprotonation phenomenon[10,25,26] FromFig 7it is observed

that the response time of PANI_PTSA (4.5 min) is higher than that of

PANI_HCl_PTSA (4.0 min) The sensitivity (R/R0) is also higher for the

latter (2.5) than the former (2.0) This can be due to the presence of

the smaller dopant HCl in PANI_HCl_PTSA, which helps easy

accessibility by the nucleophile NH3 and also easy abstraction to

NH4+Cl−, while the larger dopant PTSA present in more abundance in

PANI_PTSA causes resistance to the adsorption of NH3 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 sites 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 NH3 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

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