Optical and Electrical Properties ofPolyaniline-Cadmium Sulfide Nanocomposite

M. Ghoswami,1 R. Ghosh,2 G. Chakraborty,1 K. Gupta,1 A.K. Meikap1

1Department of Physics, National Institute of Technology, Durgapur, Mahatma Gandhi Avenue,Durgapur 713 209, West Bengal, India

2Centre for Advanced Material Processing, CSIR, Central Mechanical Engineering Research Institute,Durgapur, Mahatma Gandhi Avenue, Durgapur 713 209, West Bengal, India

Polyaniline-cadmium sulfide nanocomposite has beensynthesized by the chemical oxidative polymerization ofaniline with ammonium peroxodisulfate as an initiator inpresence of cadmium sulfide nanoparticles. TEM, XRD,FTIR, TGA, UV–vis spectroscopy, and photolumines-cence studies were done for the structural, thermal andoptical characterization of the samples. The particlesize of nanocomposites lies in between 7 and 10 nm.XRD spectrum shows that polyaniline is amorphous, butpeaks present in the spectrum of polymer nanocompo-sites are for cadmium sulfide nanoparticles. TGA resultshows that nanocomposite is more thermally stable.The band gap of nanocomposite decreases withincreasing content of cadmium sulfide nanoparticles. Anenhancement in photoluminescence has been observedin the nanocomposite than that in pure polyaniline. Thedc and ac electronic transport property of polyanilinecadmium sulfide composites has been investigatedwithin a temperature range 77 ≤ T ≤ 300 K and in thefrequency range 20 Hz–1 MHz. The dc conductivityfollows variable range hopping (VRH) model. The acconductivity follows a power law whereas the tempera-ture dependence of frequency exponent s can beexplained by correlated barrier hopping (CBH) model.The dielectric behavior of the samples has beenexplained in terms of the grain and grain boundaryresistance and capacitance. POLYM. COMPOS., 32:2017–2027, 2011. ª 2011 Society of Plastics Engineers

INTRODUCTION

Development in nanoscience and nanotechnology has

allowed people to create nanosized materials with inter-

esting electronic and optical properties different from

those of their bulk state [1]. Many research groups were

trying hard to know the collective properties of nanopar-

ticles [2–4]. Recently, the interest in the development of

inorganic–organic nanocomposite has grown rapidly for

their wide range of potential use in devices. Among these

materials, one important class is that in which the organic

part is conducting polymers like polyaniline, polypyrrole

etc [5]. Now-a-days conducting polymers are emerging as

one of the thrust areas of experimental research [6]. One

of the remarkable features of conducting polymers is that

it is possible to control the electrical conductivity of these

polymers from insulating to metallic by doping [7, 8].

They have unique electrical, optical, magnetic and chemi-

cal properties leading to the wide range of technological

applications in various fields like electromagnetic interfer-

ence shielding (EMI), sensors, corrosion protection

coatings, microwave absorption, light emitting diodes

etc. [9–14] We have selected polyaniline because of its

simple synthetic procedure, good environmental and

thermal stability, and low cost price of monomer, high

conductivity and for its good electrical, optical, magnetic,

and chemical properties [15–20].

Khiew et al [21] had synthesized and characterized

polyaniline coated cadmium sulfide nanocomposites from

reverse microemulsion. They were characterized by UV–

vis absorption spectroscopy, energy filter transmission

electron microscopy, FTIR spectroscopy, and TGA analy-

sis. The UV–vis spectrum reveals the enhancement of

doping level for the nanocomposites, which is assigned to

the existence of greater number of charges on the polymer

backbone. The FTIR spectra indicate that the polymers

are in highly doped and existed in conducting emeraldine

salt form. Average size of the composite is �17.8 nm.

They were thermally more stable than pure polyaniline.

Khanna et al. [22] had prepared Polyaniline–CdS nano-

composite from organometallic cadmium precursor and

investigated optical properties. The absorption band at

�440–445 nm is identified due to nanosized CdS particles

indicating a blue shift of about 70 nm with respect to

Correspondence to: Ajit Kumar Meikap; e-mail: meikapnitd@yahoo.com

Contract grant sponsor: Human Resource Development, Government of

India.

DOI 10.1002/pc.21235

Published online in Wiley Online Library (wileyonlinelibrary.com).

VVC 2011 Society of Plastics Engineers

POLYMER COMPOSITES—-2011

bulk CdS. Chandrakanthi et al. [23] had prepared thin films

of composites consisting of conducting polymers and nano-

sized CdS. They also reported that these composites were

scientifically and technologically attractive because of their

unique electronic and optical properties. However most of

them deal with the study of structural, thermal, and optical

characterization. Very few works have been done to inves-

tigate the electrical properties of these composites above

room temperature. But there is still dearth of systematic

study on the resistivity and dielectric properties of

PANI-CdS nanocomposites below room temperature. This

was the inspiration behind this study of electron transport

properties of PANI-CdS nanocomposites below room tem-

perature. In this investigation we have prepared polyaniline

cadmium sulfide nanocomposite by a simple chemical

route. TEM, XRD, FTIR, TGA, UV–vis spectroscopy and

Photoluminescence studies were done for the structural,

thermal, and optical characterization of the polyaniline

nanocomposite. To find the changes in the electrical proper-

ties of PANI made by CdS, an extensive study is done

to investigate the dc resistivity, ac resistivity and dielectric

properties of the PANI-CdS nanocomposites in the

temperature range 77–300 K and in the frequency range

20 Hz to 1 MHz.

SAMPLE PREPARATION AND EXPERIMENTALTECHNIQUES

Materials

Aniline C6H5NH2, cadmium acetate (CH3COO)2Cd,

acetic acid CH3COOH, sodium sulfide (Na2S), ammonium

peroxodisulfate (NH4)2S2O8 (APS), acetone, ethanol were

used as received from the market and purified as required

for the investigation. Double distilled water was used in

this investigation.

Synthesis

Polyaniline–cadmium sulfide nanocomposite was pre-

pared in-situ as follows. One gram of cadmium acetate is

mixed with 30 ml distilled water and 0.5 g sodium sulfide

is mixed with 15 ml distilled water. Three gram of ammo-

nium peroxodisulfate (APS) is mixed with 50 ml distilled

water and kept this solution in refrigerator. Na2S is mixed

drop wise in the cadmium acetate solution; a yellow pre-

cipitate of cadmium sulfide (CdS) is obtained. Then 2 ml

of double distilled aniline and 2 ml of acetic acid is

mixed to this solution with magnetic stirring. Ice cooled

APS solution is added to this solution taken in an ice

bath. A green colored solution is obtained after two hours

of stirring. The solution was kept in refrigerator at rest

for 24 h to complete the polymerization process and then

this solution is centrifuged at 10,000 rpm for 30 min. The

solid mass obtained was washed with acetone, ethanol

and double distilled water to remove monomer, oligomer,

and excess of oxidant until the filtrate turned colorless.

For comparison purposes we have prepared another three

samples taking 1, 1.5, 2 g of cadmium acetate and one

without using cadmium acetate. Samples are marked as

PC0, PC1, PC1.5, PC2 where PC0, PC1, P1.5, PC2 indicates

pure polyaniline and polyaniline containing 1, 1.5, 2 g of

cadmium acetate respectively.

Characterization

Morphology and particle sizes of composites were

noticed using transmission electron microscope JEOL-

2010 (TEM). Elemental analysis is done by Energy

Dispersive Spectroscopy (EDX) attached with scanning

electron microscope (SEM, Hitachi S3000N). The phase

identification of the fine powdered composite and polymer

was performed using X ‘Pert pro X-ray diffractometer

with nickel filter Cu ka radiation (k ¼ 1.5414 A) in 2yrange from 20 to 708. Thermo gravimetric analyses

(TGA) of the samples were carried out on STA 449 F1TGA instrument at a heating rate of 108C per minute in

N2 atmosphere over a temperature range of 25 to 7008C.Fourier transform infrared spectrums (FTIR) were

recorded with an IRPrestige-21 Shimadzu, instrument in

the region of 500 to 2,000 cm21. The UV–vis spectrum

of the samples was taken by a double beam spectropho-

tometer (U-3010) using dimethylsulphoxide (DMSO) as a

solvent. Photoluminescence spectra of the samples were

obtained out using F-2500 FL spectrophotometer using

dimethylsulphoxide (DMSO) as a solvent. The excitation

wavelength for the photoluminescence study of the

sample was 270 nm. DC resistivity of the samples is

measured in the temperature range 77 � T � 300 K by

standard four probe methods magnetic field. The magne-

toconductivity is measured by varying the transverse

magnetic field (B � 1T) using an electromagnet. AC

measurement is done by a 4284A Agilent Impedance

analyzer up to the frequency 1 MHz in the temperature

range 77 � T � 300 K. Fine copper wires are used for

connecting the wire and silver paint is used for coating.

The capacitance (CP) and the dissipation factor (D) are

measured at various frequencies and temperatures. The

real part of ac conductivity and real and imaginary part of

dielectric permittivity have been calculated using the rela-

tions r0(f) ¼ 2pfeoe//(f), e0(f) ¼ CPd/e0A and e//(f) ¼ e0(f)D

respectively, where e0 ¼ 8.854 3 10212 F/m, A and d are

the area and thickness of the sample respectively. CP is

the capacitance; f is the frequency in Hz.

RESULTS AND DISCUSSION

Morphology

The TEM micrograph and electron diffraction of the

polyaniline-CdS nanocomposite are given in Fig. 1a and b.

TEM image shows granular morphology with particle size

2018 POLYMER COMPOSITES—-2011 DOI 10.1002/pc

of 2–10 nm. Selected area electron diffraction pattern

presented in Fig. 1b and it shows different circular rings,

which indicates the plane (002), (110), (112) of hexagonal

phase. Interplanar d-spacing between the planes is

0.213 nm. Thus PANI-CdS nanocomposite is in polycrys-

talline nature. Figure 2 shows the EDX spectrum of PC2.

It shows the presence of Cd and S in the composite. C

element is obtained from both the graphite paste used for

holding the sample and polymer unit.

Structural Characterization

X-ray diffraction pattern (XRD) of polyaniline (PC0)

and polyaniline-cadmium sulfide nanocomposite (PC1,

P1..5, PC2) is presented in Fig. 3. In XRD diffractogram

polyaniline showed broad peak at 2y angle 25.38. This

peak in polyaniline may arise due to regular repetition of

monomer unit aniline. The XRD pattern of polyaniline-

cadmium sulfide nanocomposite shows characteristic

peaks at 2y ¼ 26.328, 44.628, 52.128 representing Bragg’s

reflections from (002), (110), (112) planes of the hexago-

nal phase (JCPDS no-00-006-0314). These extra peaks in

XRD study of nanocomposites confirm that cadmium sul-

fide is present in the polyaniline matrix. Average particle

size (D) in these cases is given by the Scherrer formula:

D ¼ kk/beff cosy, where k is particle shape factor (gener-

ally taken as 0.9), k is the wave length of Cu ka radiation

(k ¼ 1.5414 A), y is the diffraction angle of the most

intense peak, and beff is defined as beff2 ¼ bm

2 2 bs2,

where bm and bs are the experimental FWHM of the pres-

ent sample and the FWHM of a standard silicon sample

respectively. Particle sizes obtained using this formula lie

in between 7 and 10 nm.

Fourier transform infra-red (FTIR) spectrum of polya-

niline (PC0) and polyaniline-cadmium sulfide nanocompo-

site (PC2) is presented in Fig. 4. Pure polyaniline has

characteristic peaks at 1,580, 1,490, 1,310, 1,130, and

815 cm21 . The band at 1,580, 1,490 cm21 may be attrib-

uted to C¼¼C and C¼¼N stretching modes of vibration for

FIG. 1. HRTEM micrograph and electron diffraction of the polyaniline-CdS nanocomposite (PC2). [Color

figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

DOI 10.1002/pc POLYMER COMPOSITES—-2011 2019

the quinoid (��N¼¼Q¼¼N�� where Q¼¼Quinoid ring) and

benzenoid units, while band at 1310 cm21 is assigned to

the C��N stretching of mode of benzenoid unit. The band

at 1,130 cm21 is due to the quinoid unit of polyaniline

(��N¼¼Q¼¼N��where Q¼¼Quinoid ring). The band at 815

cm21 may be attributed to C¼¼C and C¼¼H stretching for

benzenoid unit of polyaniline and band at 681.92 cm21

may be assigned to out of plane C��H vibration. Assign-

ment of peak reveals that the synthesized product is

polyaniline [24]. Incorporation of CdS nanoparticles in

polyaniline matrix leads to small shift of the peaks to the

lower wavelengths and also decreases in the intensity of

peaks, which indicates that the structural change of poly-

mer occurs with doping. The band at 1,580, 1,490, 1,310,

1,130, 815 cm21 were shifted to 1,560, 1,485, 1,290,

1,120, and 795 cm21 respectively in the nanocomposite

and it indicates the interaction of CdS nanoparticles with

nitrogen and other reaction sites of polyaniline. Shifting

of the band at 1,580 cm21 to 1560 cm21 indicates that

CdS nanoparticles may have interaction with nitrogen site

of polyaniline.

Thermal Stability

Thermogravimetric analysis (TGA) of polyaniline

(PC0) and polyaniline-cadmium sulfide nanocomposite

(PC1 and PC2) was done in the temperature range 258C to

7008C and the thermograms are presented in Fig. 5. Ther-

mogram of pure polyaniline shows that the mass loss

began around 508C and continued upto 1008C. The mass

loss remained steady upto 2008C and then rapid mass loss

FIG. 2. EDX spectrum of polyaniline-CdS nanocomposite (PC2).

[Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

FIG. 3. XRD pattern of polyaniline and its nanocomposite with CdS.

FIG. 4. FTIR spectrum of polyaniline and its nanocomposite with CdS.

FIG. 5. TGA spectrum of polyaniline and its nanocomposite with CdS.

2020 POLYMER COMPOSITES—-2011 DOI 10.1002/pc

occurred till 6008C. The initial mass loss was due to the

loss of water molecules, the very next mass loss may be

attributed to the loss of oligomers and the subsequent

rapid mass loss occurred due to the degradation of the

polymer chain. On the other hand, in case of nanocompo-

site rate of loss of mass is very small in comparison to

polyaniline. This thermogravimetric analysis indicates

better thermal stability of the composite than that of pure

polyaniline. The better thermal stability of polyaniline

composite could be explained by the dominancy of the

benzenoid structure. Alternatively, the inferior thermal

stability of pure undoped polyaniline was due to the

presence of a quinoid ring in its structure [25].

Optical Characterization

UV–vis absorption spectrum of polyaniline (PC0) and

polyaniline-cadmium sulfide nanocomposite (PC1, PC2) is

presented in Fig. 6. There were two main absorption bands

at 280–300 nm and 600–620 nm in polyaniline. First band

is attributed to p-p* transition in the benzenoid rings and

the second band is due to exciton absorption of the quinoid

rings (n-p*) [26, 27]. In case of nanocomposite there

occurs a red shift of the absorption bands. Absorption band

due to p-p* transition in the benzenoid rings shifts from

280 nm (PC0) to 315 and 323 nm for PC1 and PC2, respec-

tively. This also confirms the dominancy of the benzenoid

ring as is evident in TGA analysis. An additional absorp-

tion peak is obtained at around 345 nm for the nanocompo-

site and it may be due to the presence of cadmium sulfide

nanoparticles in the composite.

The optical absorption is calculated using the equation

ahm ¼ A(hm 2 Eg)n, where Eg, a, m, A are the band gap,

absorption coefficient, frequency, constant respectively

and n can take values of 0.5, 1.5, 2, and 3 depending on

the mode of transition [28]. Here n ¼ 0.5 offers the best

fit for the optical absorption data of polyaniline and poly-

aniline2cadmium sulfide nanocomposites, lending support

to the allowed direct band transition of the materials. To

get the idea about band gap, a plot of (ahm)2 versus hmhas been done (given Fig. 7). Then the band gap has been

extracted by extrapolating the straight portion of the graph

on hm axis at a ¼ 0 and those are 3.9, 3.7, 3.4, 2.7 eV for

PC0, PC1, PC1.5, and PC2 respectively. The band gap of

polyaniline–CdS reduces than pure polyaniline due to the

presence of CdS nanoparticles having lower band gap

2.57 eV [29]. Gupta et al. [30, 31] has also reported the

similar behavior in polyaniline-rare earth chloride and

polyaniline–zirconium nanoparticles respectively. This

decrease in band gap with increase in CdS nanoparticles

suggests an increase in conductivity in polyaniline-CdS

nanocomposite, which has also been observed from

electrical conductivity study.

Photoluminescence (PL) spectrum of polyaniline (PC0)

and polyaniline–cadmium sulfide nanocomposite (PC0,

PC1, PC1.5, and PC2) is presented in Fig. 8. It is observed

from the spectrum that the intensity of photoluminescence

FIG. 6. UV–vis spectrum of polyaniline and its nanocomposite with

CdS.

FIG. 7. Band gap determination of polyaniline and its nanocomposite

with CdS. FIG. 8. PL spectrum of polyaniline and its nanocomposite with CdS.

DOI 10.1002/pc POLYMER COMPOSITES—-2011 2021

increases with increase in CdS nanoparticles content and

the emission peak is centered at 350, 355, 358, and 395

nm for PC0, PC1, PC1.5, and PC2 respectively. From this

study a red shift of the peak has been observed in PANI-

CdS nanocomposites. In this investigation exciton wave-

length was taken as 270 nm to excite CdS nanoparticles.

Photoluminescence is obtained due to the p-p* transition

of the benzenoid unit of polyaniline [32]. Quinonoid unit

quenches the photoluminescence emission because of the

intrachain energy dissipation [32]. Since polyaniline-CdS

nanocomposite is more crystalline than pure polyaniline

as evident from its XRD spectrum, hence the benzenoid

and quinonoid units in it are more orderly arranged with-

out any unfavorable clustering of quinonoid units [33].

The higher extent of p conjugation coupled with the more

orderly arrangement of the benzenoid and quinonoid units

observed in polyaniline-CdS nanocomposite favors the

formation of singlet excitons. The singlet exciton thus

formed decays to the ground state with the emission of

light. It is also observed that singlet exciton formation

increases with increase in the conjugation length of the

polymer chain [34]. This may be attributed to the fact

that the delocalization length of singlet exciton in conju-

gated polymers is comparable with its conjugation length

[34]. The triplet exciton of the conjugated polymer is con-

fined. In conjugated polymers, singlet excitons are mostly

responsible for photoluminescence emission because con-

jugated polymers cannot produce the spin flip which is

necessary for an optical transition [35]. Hence, one should

expect higher photoluminescence emission from polyani-

line–cadmium sulfide nanocomposite, which has a higher

extent of p conjugation. In support of this view we are

including the results of dc electrical conductivity of these

samples. Many authors reported the optical properties

of different polymer nanocomposites in the literature.

Preparation and enhancement of optical properties of CdS

nanoparticles embedded in liquid crystal monomers have

been reported by Lee et al. [36] They obtained an

increase in PL intensity and also red shift in the spectrum

of composites. Gupta et al. [37] have reported a blue shift

in polyaniline–silver and polyaniline–zirconium nanocom-

posites; however, they observed a red shift in polyaniline-

rare earth chloride nanocomposites. Lu et al. [38] have

also shown a blue shift in polyaniline microwires-CdS

nanoparticles. It was reported by Veinot et al. [39], the

blue shift in the luminescence of CdS nanoparticles might

be caused by the different chemical environment.

Although most of the studies shows blue shift, the

observed red shift in our samples is interesting, which

may be due to the formation of defect states in the polya-

niline-CdS nanocomposites [40].

Electrical Properties

To have an idea regarding the electronic transport

properties of the PANI-CdS composites, the direct current

conductivity of all the samples have been measured in the

temperature range 77–300 K. Figure 9 shows the variation

of dc resistivity with temperature and the room tempera-

ture conductivity (r300K) of all the samples with increas-

ing content of cadmium sulfide (CdS) is shown in the

inset of Fig. 9. With the increasing content of cadmium

sulfide, there is an increase in the conductivity of the

samples by a significant amount. The resistivity ratio qr(¼q77/q300) of different samples, mentioned in Table 1,

also increases with increasing cadmium sulfide content in

different samples. Thus, the increase in room temperature

conductivity and resistivity ratio may be attributed to the

incorporation of CdS into the insulating PANI matrix for

the better change transfer process between them. The tem-

perature variation of all the samples shows semi conduct-

ing behavior i.e., their resistivity decrease with rise in

temperature. Similar behavior has been observed in CdS

nanorod-polyaniline composites [41] and CdS/polyaniline

heterojunction [42]. Such variation of resistivity with

FIG. 9. Temperature dependence of the dc conductivity of polyaniline

and its nanocomposite with CdS. The solid lines are fitted to Eq. 1. Inset

shows the variation of room temperature conductivity of polyaniline-CdS

nanocomposites.

TABLE 1. Different physical parameters of polyaniline-cadmium

sulfide composites.

Parameters PC0 PC1 PC1.5 PC2

Conc (wt%) 0 1.0 1.5 2.0

q (300 K) (O-m) 5.58 3 106 2.98 3 106 1.59 3 105 2.56 3 104

qr (¼ q77/q300) 19.023 27.23 297.77 669.67

c 0.25 0.25 0.25 0.25

TMott, low (K) 7.64 3 103 1.20 3 104 4.5 3 105 6.8 3 105

TMott,high (K) 5.0 3 106 9.64 3 106 2.8 3 107 2.21 3 108

WH (eV) 2.9076 2.1280 1.7196 0.9097

s0 (sec) 3.93 3 10223 3.82 3 10219 4.94 3 10219 2.0 3 10215

Concentration of the samples in weight %, Resistivity at room temp-

erature (q(300K)), Resistivity ratio (qr), VRH exponent (c), Mott charac-

teristic temperature (TMott) in lower temperature range, Mott characteristic

temperature (TMott) in higher temperature range, effective barrier height

(WH), characteristic relaxation time (s0).

2022 POLYMER COMPOSITES—-2011 DOI 10.1002/pc

temperature can be explained in terms of Mott’s variable

range hopping (VRH) model [43].

qðTÞ ¼ q0 expTMott

T

8>: 9>;c� �ð1Þ

TMott ¼ 24= pKBL3locNðEFÞ

� � ð2Þwhere qo is the resistivity at infinite temperature, TMott is

the Mott characteristic temperature depending on the hop-

ping barrier, electronic structure and energy distribution

of the localized states, kB is the Boltzman constant, Lloc isthe localization length and N(EF) is the density of states

at the Fermi level. The dimensionality (d) can be obtained

from the VRH exponent c by the relation c ¼ 1=1þ d.For three, two and one dimensional system, the possible

values of c are 1/4, 1/3, and 1/2, respectively. The graph

of ln[qdc(T)] vs. T21/4 gives a nonlinear variation of resis-

tivity with temperature for all the samples where two dif-

ferent slopes can be obtained. At lower temperature,

smaller but linear change in resistivity for all the samples

has been obtained but a rapid and linear decrease in resis-

tivity at higher temperature with different slope is

observed. In our previous work we have observed such

behavior in polyaniline–zirconium nanocomposites [31].

Thus, it may be concluded that the three-dimensional

(3D) charge transport is the dominating charge transport

mechanism in the present investigation. The electronic

wave functions extend in three-dimension as the conduct-

ing islands formed by CdS are present in between the

insulating polymer matrix. As a result, three dimensional

hopping of electrons occurs in the investigated samples.

The values of TMott for both the temperature ranges have

been calculated from the slopes of the graph and are

indicated in Table 1. It is noticed from the table that the

values of TMott and qr increases with increasing CdS con-

tents. It is also observed that the values of TMott increases

with increasing the value qr of the PVA-CdS composites.

As the increase of qr represents the more disorder present

in the composite, the values of TMott strongly depend on

the disorder present in the sample. For higher disorder in

the samples the electronic wave functions are localized

into smaller regions resulting in a smaller localization

length. So the localization length has an inverse relation-

ship with resistivity ratio as well as the extent of disorder

present in the sample. Thus, the localization length is

reduced by increasing the disorder present in the sample,

as a result the values of TMott increases (Eq. 2) with

increasing the CdS contents. Therefore, increase in disor-

der may be the reason for high values of TMott for our

samples.

The alternating current (a.c.) conductivity of the PANI-

CdS composites has been measured in the temperature

range 77 � T � 300 K and in the frequency range

20 Hz–1 MHz. At lower frequency, the conductivity is

almost frequency independent but becomes predominant

at higher frequency for a particular temperature. In

general many amorphous semiconductors or disordered sys-

tems have dc conductivity contribution (rdc) besides the acconductivity. This may be the reason behind the frequency

independence of conductivity at lower frequency region.

The total conductivity at a particular temperature over a

wide range of frequency obeys a power law with frequency,

which can be expressed as [43–46].

r0ðf Þ ¼ rdc þ rdcðf Þ ¼ rdc þ a f s ð3Þwhere rdc is the dc conductivity, a are the temperature

dependent constant and the frequency exponent s � 1. The

frequency dependent contribution can be calculated by

subtracting the dc contribution from the total conductivity.

Figure 10 shows the linear variation of ln[rac(f)] with ln[f]at different constant temperature for the sample PC2.

Similar behavior can be observed for all the other samples.

This linear variation of ln[rac(f)] with ln[f] shows that the

FIG. 10. Frequency dependence of ac conductivity of the sample PC2

at different temperatures.

FIG. 11. The temperature variation of the frequency exponents ‘‘s’’ for

different PANI-CdS composites. The solid lines are fitted to Eq. 4.

DOI 10.1002/pc POLYMER COMPOSITES—-2011 2023

frequency exponent ‘‘s’’ in Eq. 4, is independent of fre-

quency. The variation of ‘‘s’’ with temperature for different

samples is shown in Fig. 11. A gradual decrease of ‘‘s’’with increasing temperature can be observed in the figure.

In general, the nature of conduction process of disordered

system is governed by two physical processes such as

correlated barrier hopping (CBH) [45] and quantum

mechanical tunneling (electron tunneling [46], small

polaron tunneling [45] and large polaron tunneling [44].

Different variation of ‘‘s’’ with temperature for different

conduction process are observed from which the exact

nature of charge transport mechanism can be obtained. The

frequency exponent ‘‘s’’ becomes independent of tempera-

ture in electron tunneling theory whereas it increases with

increasing temperature in small polaron theory and

increases at first and then decreases with decreasing tem-

perature according to large polaron theory. According to

CBH model, the value of ‘‘s’’ only decrease gradually with

temperature. The nature of variation of ‘‘s’’ in the present

investigated samples suggests that the CBH model is suita-

ble for explaining the experimental data. According to this

model, the charge carriers hop between the sites over

the potential barrier separating them and the frequency

exponent ‘‘s’’ can be expressed as [45]

s ¼ 1� 6kBT

WH � kBT ln 1xs0

8: 9; ð4Þ

where kB, WH, x, and so are Boltzmann constant, effective

barrier height, angular frequency, and characteristic relaxa-

tion time respectively. Thus, the experimental data has

been analyzed with Eq. 4 as function of temperature keep-

ing WH and xso as a fitting parameter in Fig. 11. The points

indicate the experimental data and solid lines give the theo-

retical best fit obtained from Eq. 4 for different samples.

The values of WH and so have been calculated at a fixed

frequency of 10 KHz and are enlisted in Table 1. Thus, the

trend of variation of ‘‘s’’ with temperature suggests that the

charge transport mechanism of the investigated samples

can be explained by the CBH model. Polyaniline–

zirconium nanoparticles have also followed the CBH model

in the same temperature range [31].

Figure 12 represents the temperature dependence of ac

conductivity for the sample PC1.5 at different yet constant

frequencies. At lower temperature, a weak variation of ac

conductivity with temperature can be noticed whereas this

variation becomes larger at higher temperature. The real

part of complex ac conductivity is found to follow a power

law r0(f) ! Tn. The points in Fig. 12 represent the

experimental data whereas the solid line represents the best

fit obtained by the above equation. The value of n has been

obtained as a fitting parameter and is shown in Fig. 12. The

value of ‘‘n’’ depends strongly on the frequency. For the

sample PC1.5, the value of n varies from 12.69 to 8.28 with

a frequency variation from 1 KHz to 1 MHz. Similar

behavior has been observed in polyvinyl alcohol-multiwall

carbon nanotubes [47]. According to the CBH model [45]

the ac resistivity r0(f) is expressed as r0(f) ! TnRx6 [%Tn

with n ¼ 2 þ (1 2 s)ln(1/xso)] for broad band limit and

r0(f) ! Rx6 % Tn with n ¼ (1 2 s)ln(1/xso) for narrow

band limit, where Rx ¼ e2/{peeo[WH 2 kBT ln(1/xso)]}.The theoretical values of n, has been calculated using the

values of s and so for the sample PC1.5. The variation in the

calculated values of n are in the range 8.0–6.8 for 300 K

and 3.4–3.3 for 77 K for broad band limit and 6.0–4.8 for

300 K and 1.4 to 1.3 for 77 K for narrow band limit with

frequency variation from 1 KHz to 1 MHz. The experimen-

tal values are not close to the theoretical values obtained

from broad band limit and narrow band limit, i.e., there is a

discrepancy between theoretical and experimental result.

Anyway, more studies are necessary to formulate the true

mechanisms.

Figure 13 shows the temperature dependence of real

part of dielectric permittivity of e0(f) for the sample PC1.5

for different yet constant frequencies. At different con-

FIG. 12. AC conductivity as a function of temperature of the sample

PC1.5 at different frequencies.

FIG. 13. Temperature variation of real part of permittivity of the sam-

ple PC1.5 at different frequencies.

2024 POLYMER COMPOSITES—-2011 DOI 10.1002/pc

stant frequencies, the real part of dielectric permittivity

increases with temperature following a power law

e0(f) ! Tp. In the figure the points represents the experi-

mental data and the solid lines give the theoretical best

fitting in accordance with the power law. The value of the

temperature exponent p is found to depend on the fre-

quency and its value decreases from 10.0 to 4.85 with

increasing frequency from 1 KHz to 1 MHz. Similar

behavior has been observed for all other samples. Thus, a

larger variation in the e0(f) with temperature is observed

at lower frequency in comparison to the higher frequency.

In general, structural inhomogenities and existence of free

charges in disordered semiconductors exhibit interfacial

polarization. At low frequencies, the hopping electron

may be trapped by the inhomogenities. Due to the

decrease in the resistance of the composites with increas-

ing temperature, e0(f) increases with temperature at

constant frequency. Electron hopping increase for low

resistance and hence a larger polarizibility or larger e0(f)results in. The variation of e0(f) with frequency are shown

in Fig. 14 for the sample PC1 at different yet constant

temperatures. All the samples show the similar variation.

At a fixed temperature, a sharp increase in e0(f) at lower

frequency can be observed. This may occur due to the

presence of large degree of dispersion caused by the

charge transfer within the interfacial diffusion layer pres-

ent between the electrodes. The magnitude of dielectric

dispersion depends on the temperature. At lower temp-

erature, the electric dipoles freeze easily through the

relaxation process due to which there exists decay in

polarization with respect to the applied electric field. As a

result, a sharp decrease in e0(f) at lower frequency region

can be observed. At higher temperature there is a quick

rate of polarization and hence relaxation occurs at higher

frequency. Thus the inhomogeneous nature of the samples

containing different permittivity and conductivity regions

governs the frequency behavior of e0(f) where the poorly

conducting region blocks the charge carriers. The effec-

tive dielectric of such inhomogeneous system can be

explained by Maxwell-Wagner capacitor model [48–50].

The complex impedance of inhomogeneous system is

compared with an ideal equivalent circuit having resist-

ance and capacitance due to grain and interfacial grain

boundary contribution, according to which

Z ¼ 1

ixC0e xð Þ ¼ Z0 � iZ00 ð5Þ

Z0 ¼ Rg

1þ xRgCg

� �2 þ Rgb

1þ xRgbCgb

� �2 ð6Þ

Z00 ¼ xR2gCg

1þ xRgCg

� �2 þ xR2gbCgb

1þ xRgbCgb

� �2 ð7Þ

where the sub indexes ‘‘g’’ and ‘‘gb’’ represents the grain

and interfacial grain boundary respectively, R, resistance;C, capacitance; x, 2pf; Co, free space capacitance. The

real part of the complex impedance for all the samples

have been calculated by the relation

Z0ðf Þ ¼ e00ðf ÞxC0 e0ðf Þ2 þ e00ðf Þ2

� �h i ð8Þ

where e0(f) and e//(f) are the real and imaginary part of

dielectric permittivity respectively. The real part of com-

plex impedance has been analyzed by Eq. 6. The fre-

quency variation of real part of the complex impedance

of PC2 is shown in Fig. 14 at different temperatures. The

points in Fig. 15 represent the experimental data whereas

the solid lines represent the theoretical best fit obtained

from Eq. 6. The grain and grain boundary resistance

and capacitance have been evaluated from the fitting. TheFIG. 14. Variation of dielectric constant as function of frequency at

different temperatures of the sample PC1.

FIG. 15. The real part of the complex impedance versus frequency at

different constant temperatures of the sample PC2. The solid lines are

fitted to Eq. 6.

DOI 10.1002/pc POLYMER COMPOSITES—-2011 2025

values lie in the range 0.08–29.34 MO for Rg, 0.10–0.7 nF

for Cg, 0.2–61.36 MO for Rgb 0.15 nF to 1.25 nF for Cgb

for different samples. The figure shows that the experi-

mental data is well fitted with the theory. The grain

boundary resistance is much greater than that of grain

resistance. Thus it may be concluded that, the grain bound-

ary contribution is larger than the grain contribution.

CONCLUSION

In summary, we have synthesized polyaniline and

polyaniline-cadmium sulfide nanocomposite in chemical

oxidative method. Polyaniline composite is more ther-

mally stable than pure polyaniline. Confirmation of the

presence of CdS nanoparticles in polyaniline is obtained

by TEM, XRD, and FTIR analysis. Band gap polyaniline

decreases with increasing CdS nanoparticle content and

hence increases the conductivity. This composite can be

used in different optoelectronic purposes and it is a prom-

ising material with prospect of application in polymer

light emitting diodes (PLED). So, this is a simple way by

which optical and electrical properties of other conducting

polymers may be enhanced by using different nanopar-

ticles. The dc conductivity of all the samples follows a

simple hopping type of charge conduction mechanism. At

lower temperature, there is a very weak variation of dc

conductivity with temperature but the variation becomes

larger beyond T [ 150 K. The real part of ac conductiv-

ity follows a power law given by r0(f) ! fS The tempera-

ture dependence of universal dielectric responses is found

to follow correlated barrier hopping charge transfer mech-

anism. The variation of ac conductivity and dielectric

permittivity with temperature for all samples follow the

equations r0(f) ! Tn and e0(f) ! Tp where the value of

n and p are found to be strongly frequency dependent.

The real part of complex permittivity shows a large

degree of dispersion at lower frequency which is inter-

preted in terms of Maxwell-Wagner capacitor model. The

contribution due to grain resistance is smaller than that of

grain boundary resistance.

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