Effect of Annealing on Structural and Optical Propertiesof Polypyrrole Doped With Different Acids

Rashmi Saxena, Kananbala Sharma, N.S. Saxena, T.P. SharmaSemi-Conductor and Polymer Science Laboratory, University of Rajasthan, Jaipur 302004, India

Polypyrrole (pure) and polypyrrole doped with differentprotonic acids (HCl and H3PO4) have been synthesizedusing NaOH as reducing agent in aqueous HCl mediumand ammonium persulphate (APS) as an oxidant,respectively. These pure and doped samples werenamed as S1, S2, and S3 for further investigations.These pure and doped samples were annealed at200oC for 4 hr. The amorphous nature of theseannealed and as-prepared polypyrrole samples wasconfirmed by XRD. FTIR spectra and SEM images of allthe samples were taken at room temperature, whichindicate: significant change in annealed samples. Tocalculate the energy band gap of all the samples, theabsorption spectra were recorded by USB-2000 spec-trophotometer at room temperature in the wavelengthrange 300–800 nm. From the analysis of absorptionspectra, optical band gap of as-prepared samples (S1–S3) was determined to be 2.64, 2.43, and 2.29 eV,respectively and for annealed samples A1–A3 valuesobtained were 2.30, 2.25, and 2.17 eV. POLYM. COMPOS.,30:820–826, 2009. ª 2009 Society of Plastics Engineers

INTRODUCTION

Conducting polymers like polyaniline, polythiophene,

polyacetylene, polybenzadene, polypyrrole, etc. have

attracted much attention not only from fundamental scien-

tific interest but also from practical view point for various

functional applications such as manufacturing of printed

circuit boards, corrosion protection, electro chromic dis-

plays, electrolytic capacitors, rechargeable batteries, and

conductive coating for textile, sensors, light-emitting

diodes electromagnetic shielding, and so on [1–4]. Among

these conducting polymers, polypyrrole is becoming

increasingly important because of its various applications,

good environmental stability, easy synthesis [5, 6], and

also it has good electrical, thermal, mechanical, and opti-

cal properties than many other conducting polymers [7–

9]. Polypyrrole is one of the most widely studied conduct-

ing polymers and has been well characterized. Polypyrrole

powder was amorphous in nature and known as pyrrole

black. It is electro active and acts as an anion exchanger.

Polypyrrole is a non-degenerate ground state polymer

with different types of chain configuration containing

repeating units of aromatic as well as quinoid units as

shown in Fig. 1. During oxidative polymerization, pyrrole

(PPy) typically polymerizes by linkage at a position,

along with the loss of a proton at each of these positions.

The process of polymerization is expressed in Fig. 2 [10].

The reaction for the formation of a polymeric unit [11]

is:

nPPyþ Cl� �! ðPPyÞþCl� þ 2nHþ þ ð2nþ 1Þe�

ðchlorine-doped polypyrroleÞ ð1Þ

ðPPyÞþCl� þ NaOH �! ðPPyÞ0 þ NaClþ H2O

ðpure polypyrroleÞ ð2Þ

In an oxidative doping of polypyrrole, an electron is

removed from the p-system of the backbone producing

free radical and spinless positive charge. The radical ion

and cation are coupled to each other via local resonance

of the charge and the radical. This combination of a

charge site and a radical is called as polaron as shown in

Fig. 3.

Upon further oxidation, free radical of the polarons is

removed, creating a new spinless entity called as bipo-

laron, which is of lower energy than that of two distinct

polarons. At higher doping levels, it becomes possible

that two polarons combine to form bipolaron as shown in

Fig. 3c [12]. It is inferred that doping affects the packing

dimensions, and pyrrole rings are linked by a double

bond (p��r) in a quinoid form rather than the normal

aromatic form (Fig. 1).

At low dopant concentration, the dopant molecules

occupy random positions between the chains. The effect

on the electronic properties by their coulomb potential or

by their hybridization with the p-orbital results into the

formation of polarons that have long lifetime, which are

treated as quasi particles. The polarons have low mobility,

which results in obtaining moderate conductivity at low

doping concentration. As the doping level is increased,

the concentration of polarons goes up and they become

Correspondence to: Rashmi Saxena; e-mail: rashmi_saxena29@yahoo.co.in

DOI 10.1002/pc.20778

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2009 Society of Plastics Engineers

POLYMER COMPOSITES—-2009

crowded together, close enough to form bipolarons.

Because of the doping process, conductivity undergoes a

marked increase. Once the radical component of the po-

laron has combined to form bonds, the remaining charges

achieve high mobility along the chain. Thus, for a highly

doped polymer, it is conceivable that the upper and lower

bipolaron bands will merge with the conduction and va-

lence band respectively to produce partially filled states

as shown in Fig. 4 [12].

During oxidative polymerization, conjugated organic

polymers are either electrical insulator or semiconductors.

As indicated earlier, the doping process results in dra-

matic changes in the electronic, electrical, magnetic, opti-

cal, and structural properties of the polymers. Electrically

conducting polymers are semiconductors with a filled va-

lence band and an empty conduction band; these bands

are separated by an energy gap. Doping of these polymers

creates a new band in the energy gap, making it possible

for the electrons to move to these new bands and increas-

ing the conductivity of the materials [13]. In the reduced

(undoped) form, PPy conducting polymers are insulators.

Bipolarons (radical-di-ions) play a major role in the elec-

tronic and transport properties.

A good deal of work [14–18] has been done on poly-

pyrrole to determine its various properties such as ther-

mal, electrical, and optical properties. Work has also been

done to observe the effect of annealing on polypyrrole.

Some reported works are as follows.

Kassim et al. [19] investigated the effect of tempera-

ture on the preparation of conducting polypyrrole. Liu

et al. [20] studied the enhancement in conductivity of

electro-polymerized polypyrrole. DC electrical conductiv-

ity of some poly(pyrrole-metal) complexes have been

reported by Shekeil and Aghbari [21]. Uladzimir et al.

[22] have reported the effect of annealing of poly(3-hexy

lthiophene) fullerene bulk heterojunction composites on

structural and optical properties. Effect of thermal treat-

ment on electrical conductivities of polypyrrole has been

studied by Khalkhali [23]. Liu et al. [24] investigated the

effect of thermal annealing on the performance of poly-

mer light emitting diodes whereas Chen et al. [25]

reported the stability of polypyrrole.

However, no studies have been undertaken for chemi-

cally prepared samples and to observe the effect of

annealing on optical properties. In the present study, the

samples have been prepared by chemical synthesis and

the effect of annealing on the optical properties of the

so-prepared samples has been observed.

Annealing is a time/temperature process. The term

‘‘annealing’’ means the thermal heating of a material for

a particular interval of time at a fixed temperature. Heat

treatment plays a very important role in fabricating

devices. In many researches, it is found that polypyrrole

is very sensitive to moisture because this leads to leach-

ing of the counter ion and thus to a decrease in conduc-

tivity [26]. Annealing makes the chain mobile to acquire

a stable morphology and also to enhance the properties

by elimination of moisture content. It also improves the

local ordering of the material and the removal of nonpo-

lymeric impurities trapped into the polymer matrix dur-

ing growth. So the primary aim of this study is to deter-

mine the optical band gap of as-prepared and annealed

samples.

FIG. 2. Oxidative polymerization

FIG. 1. Structural of polypyrrole.

DOI 10.1002/pc POLYMER COMPOSITES—-2009 821

MATERIAL PREPARATION AND EXPERIMENTALTECHNIQUE

Pure and doped polypyrrole samples can be prepared

via chemical polymerization. In this reaction, a conju-

gated monomer (pyrrole) is polymerized and charge car-

riers are generated through the doping process. In the

chemical process, the HCl doped polypyrrole sample (S2)

was prepared by oxidative polymerization of double dis-

tilled pyrrole using ammonium persulphate, (NH4)2S2O8

as an oxidant. A calculated amount of ammonium persul-

phate was dissolved in HCl (1M) solution. The polymer-

ization was performed at 08C temperatures with oxidant/

monomer molar ratio 0.2. The black precipitate resulting

from the reactions is washed with distilled water and

methanol and then dried under vacuum for 6–8 hr [10].

Similarly, polypyrrole doped with phosphoric acid (S3)

was prepared. For pure polypyrrole (S1) sample, the pre-

cipitate of polypyrrole in its salt form is reduced by

NaOH with distilled water and dried under vacuum. Now

these pure and doped samples were annealed at 2008C for

4 hr. For optical measurement polypyrrole powder of both

types (as prepared and annealed) of the samples dissolved

in DMSO (dimethyl sulfoxide) solution and stirred for 6

hr at room temperature to obtain homogeneous solutions,

then absorption spectra was taken for the band gap deter-

mination.

X-ray diffraction studies were carried out for these as-

prepared (S1–S3) and annealed samples (A1–A3) using

Philips model no. 1840. The Fe target with wavelength

1.937 A was used as a source of radiation. XRD patterns

of all the samples were taken at room temperature. The

diffractograms were recorded in terms of 2y in the range

20–808.The IR spectra of these samples were recorded on a

model no. 84005 Shimadzu photometer in KBr medium at

room temperature. For recording IR spectra sample pow-

der was mixed with KBr in the ratio 1:6 by weight. This

powder was then pressed in a small cylindrical die to

obtain clean spectra. The characterization of polypyrrole

by spectroscopic methods is important and gives informa-

FIG. 3. Formation of polaron and bipolaron.

FIG. 4. Bands in conducting polymers.

822 POLYMER COMPOSITES—-2009 DOI 10.1002/pc

tion not only about various molecular-level interactions

but also on the type of charge carriers.

Scanning electron micrograph (SEM) results give the

valuable information of the material morphology of the

samples before and after annealing. In our study, the

SEM of all the samples was carried out using a SEM

(Quanta Fe-200 model). Gold coating was applied prior to

recording the images.

The optical absorption of as-prepared samples (S1–S3)

and annealed samples (A1–A3) was recorded at room

temperature in the wavelength range from 300 to 800 nm

using Ocean Optics USB2000 spectrophotometer. In this

equipment, a tungsten light bulb or other light source

‘‘white light’’ is used. The light is focused through the

sample and a diffraction grating then disperses the wave-

lengths from the lamp’s continuous spectrum. The light

that has passed through the sample and has been dis-

persed, strikes an array of detectors: one for each wave-

length. These detectors record the amount of transmitted

light at each wavelength. The signal given by each detec-

tor is used to calculate the absorbance for each wave-

length. The computer displays the signal as a plot of ab-

sorbance versus wavelength and this graph is called the

spectrum of the sample. Absorption spectra are a powerful

tool, which is used for measuring the energy band gap

(Eg) of polycrystalline and polymeric material.

According to Tauc relation [27] the absorption coeffi-

cient for direct band gap material is given by

ahm ¼ A hm� Eg

� �1=2 ð3Þ

where hm is a photon energy, Eg the band gap and A is

constant which is different for different transitions, A plot

of (a hm)2 versus photon energy (hm), when extrapolated

to zero absorption provides the value of energy band gap

as depicted in Figs. 8 and 9.

RESULTS AND DISCUSSION

Environmental stability is related to the reactivity of

the charged polymer backbone toward oxygen or water.

PPy is more sensitive to moisture, this leads to extraction

of counter ion and thus a decrease in conductivity, which

further decreases its possibility of application in device

fabrication. This problem can be overcome by using

annealing process. So in the present study the annealing

method was used.

X-ray diffraction pattern of as-prepared samples (S1–

S3) and annealed samples (A1–A3) show no sharp peak

in Fig. 5. This confirms the amorphous nature of the ma-

terial. However, hump is observed at low angle scattering

of all the samples. In annealed samples, hump is shifted

to low angle, which indicates that particle size of these

samples increases as compared with the as-prepared sam-

ples because of growth of particle due to annealing.

IR band is the most conventional and powerful tool for

the determination of organic and inorganic compounds.

The physical properties like thermal, electrical, and opti-

cal of polypyrrole depend on the electronic structure the

chemical nature of repeated unit. Annealing process

results in change in all the above-mentioned properties

[28]. The FTIR spectra for these as-prepared and annealed

samples are presented in Fig. 6 whereas the attributions

of the observed bands are indicated in Table 1. The spec-

tra exhibit similar feature for the main polymer chain and

the observed differences between them are attributed to

the doping with protonic acid.

The C��Cl stretching peak arises in the range of 590–

700 cm21 and the peak at 650 cm21 in the HCl doped

PPy is due to the C��Cl stretching and the peak at 1008

cm21 in H3PO4 doped (PPy) is due to the stretching of

phosphoric group [12, 29]. The IR spectrum of annealed

sample shows a drastic change in the range of 3550–2950

cm21, 1400–1050 cm21, and 600–500 cm21. This range

FIG. 5. XRD pattern of as-prepared and annealed samples.

FIG. 6. FTIR spectra of as-prepared and annealed samples.

TABLE 1. Structural information by FTIR.

Band assignment Wave number (cm21)

N��H stretch 3,850–2,350

Pyrrole ring vibration 1,700

C¼¼C stretch 1,500–1,650

Presence of PPy 750–780

N��H bending 900–650

C��H stretch 3,000–2,500

C��N stretch vibration 1,300–1,380

O��H bending 3,600–3,200, 1,050

As-prepared and annealed samples.

DOI 10.1002/pc POLYMER COMPOSITES—-2009 823

indicates the moisture present in the as-prepared samples

and samples show the peak around these ranges which is

assignable to the OH bending vibration of H2O, and it is

absent in the annealed samples which indicates the re-

moval of moisture. The result in annealed samples indi-

cates that the heat-treatment changes the structure of the

annealed sample, leading to a decrease in value of optical

band gap.

We have studied the effect of thermal annealing of all

the as-prepared and annealed samples on their surface

structures. SEM images of all the samples are shown in

Figs. 7 and 8. Figure 7 (pure and doped PPy, as-prepared

samples) show globular clusters, which are big in size,

have void space in between them and the surface of these

samples shows roughness. However, in Fig. 8 annealed

samples show well-defined structure with lesser distance

between clusters. So the surface becomes more and more

compact, which gives the smaller value of band gap

(Table 2). Doping of pure polypyrrole affects the packing

dimensions and pyrrole rings are linked by a double bond

(p��r) in a quinoid form rather than the normal aromatic

form. The optical band gap results of all samples are

given in the Table 2 and Figs. 9 and 10 show the plot of

optical band gap. This leads to the formation of polaronic

and bipolaronic sublevels with in the gap, which enhances

the conductivity and decreases the optical band gap.

These sublevels are absent in case of pure polypyrrole. In

the optical study, it is found that the band gap values of

as-prepared samples are 2.60, 2.43, 2.29 eV and annealed

samples values are 2.30, 2.25, and 2.17 eV. The band gap

of Cl2, & PO432 doped samples is almost same which is

due to fact that proton doping exists in all the samples.

The HCl doped PPy shows lower value of band gap, i.e.

2.17 eV as compared with H3PO4, i.e. 2.25 eV. This dif-

ference in the band gap of two acid doped polypyrrole

can be explained on the basis of the fact that HCl is very

strong acid and is a smaller molecule in size whereas

phosphoric acid is weak acid and has bigger size. In the

doping process the proton (Hþ) makes a bond with the

imine (��NH¼¼) group but the anions (M2) is just

attached to the polymer backbone as counter ion. When

PPy base is protonated by HCl, then (��N¼¼) sites become

{(��N��)þ} {Cl}—sites with þ and 2 center, which

may produce dipole structure. Because of creation of the

dipole structure the conductivity of HCl doped PPy

becomes higher [30], and band gap value is lowest in all

the samples. The H3PO4 acid doped polypyrrole also fol-

lows similar mechanism but since H3PO4 has lesser acidic

strength, the content of doping in this case is lesser thus

results in a higher value of band gap as compared with

HCl doped PPy. Polypyrrole shows rougher and more po-

rous surface morphology. However, after annealing pro-

FIG. 7. SEM images of as-prepared samples.

FIG. 8. SEM images of annealed samples.

824 POLYMER COMPOSITES—-2009 DOI 10.1002/pc

cess the surface becomes very smooth which indicates

that a complete ordering of the polymeric chain has

occurred. Also annealing must have modified the mor-

phology of the samples as we see in Fig. 8, annealing

improves the local ordering within the sample and also

removes the nonpolymeric impurities trapped in the poly-

mer matrix during growth, so this improves either the

charge injection or the charge conduction throughout the

devices like solar cells, LED, etc. and increases the effi-

ciency of polymeric devices. From the present study, it is

observed that annealing process improves the sample

structure, which results in decrement of the optical band

gap.

CONCLUSION

Structural characterization suggests the amorphous na-

ture of all the samples and the annealing effect as traced

by FTIR and SEM analyses. The energy band gap of pure

and polypyrrole doped with HCl, H3PO4, and annealed

samples was found in increasing order (Pure\H3PO4\HCl). Alteration of donor groups is a successful

methodology for inducing a decrease of band gap in these

conjugated polymers. The optical properties of these

annealed samples indicate the effect of annealing, and sug-

gest that the annealing process brings about an improve-

ment of the sample structure, which is important for possi-

ble usage of the material for the device fabrication.

ACKNOWLEDGMENTS

One of the authors (Rashmi Saxena) is thankful to Ms.

Vinodini Shaktawat, Mr. Dinesh Patidar, Ms. Mansavi,

and Ms. Deepika who have helped in various ways during

the course of this work.

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TABLE 2. Optical band gap values of as-prepared and annealed

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Samples

Optical band gap values (eV)

As-prepared Annealed

Pure PPy 2.64 2.30

H3PO4 doped 2.43 2.25

HCl doped 2.29 2.17

DOI 10.1002/pc POLYMER COMPOSITES—-2009 825

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