synthesis and study of polyaniline/mwcnt composite for

Synthesis and study of polyaniline/MWCNT compositefor optoelectronic application

ATUL KUMAR SHARMA1,*, ANUP KUMAR SHARMA2 and RITU SHARMA1

1 ECE Department of Malviya National Institute of Technology, Jaipur 302017, India2 EIE Department, National Institute of Technology, Silchar 788010, India

*Author for correspondence ([email protected])

MS received 30 July 2020; accepted 23 November 2020

Abstract. In this study, the effect of multi-walled carbon nanotubes (MWCNTs) addition on optical bandgap of

polyaniline (PANI) was reported. Pure PANI and 5 mg MWCNTs/PANI, 10 mg MWCNTs/PANI and 15 mg MWCNTs/

PANI composite samples were synthesized by in-situ polymerization process. Synthesized composite sample and pure

PANI sample were characterized by X-ray diffraction, scanning electron microscopy, Fourier transform-infrared and UV–

visible spectroscopy. Optical bandgap, molar absorptivity coefficient are estimated for pristine MWCNTs sample, pure

PANI sample and composite samples using UV–visible spectroscopy data and Tauc plot. It is observed that optical

bandgap of PANI decreases on increasing the concentration of MWCNTs, while keeping the concentration of PANI

constant. Lowest bandgap of 3.55 eV is obtained for 15 mg MWCNTs/PANI composite sample. Strong interaction

between p-bonded surface of MWCNTs and quinoid rings of PANI is found, as indicated by the obtained results of FTIR

and UV–visible spectroscopy. The information on the optical bandgap of the composite samples is of great importance for

the development of optical antenna and other optoelectronic devices.

Keywords. Carbon nanotubes; UV–visible spectroscopy; optical bandgap; absorption; conducting polymer.

1. Introduction

Carbon nanotubes having excellent electrical, optical and

mechanical properties stimulated multi and interdisciplinary

research in various fields of science [1]. Many researchers

have done research by incorporating carbon nanotubes in

optoelectronic and electronic applications, such as solar

cells [2], supercapacitors [3], Schottky diodes [4], etc.

Recent studies have shown that introduction of carbon

nanotubes in conducting polymer matrix will enhance

comprehensive properties of polymer. Conducting polymers

have lots of interesting properties, such as light weight,

corrosion resistance, flexibility and variable conductivity,

which can be used in various electronic applications,

biomedical engineering [5,6], supercapacitors [7,8], energy

storage devices [9,10], and so on [11].

Among the several conducting polymers, polyaniline

(PANI) have shown remarkable research interest due to its

high electrical conductivity, abundant raw materials, ease of

synthesis, good environmental stability, cost effectiveness

and simple redox chemistry [12,13]. The CNT/PANI com-

posites have studied for gas sensors [14,15], biosensors

[16–18], supercapacitor [19–21], solar cell [4], fuel cell

[22,23] and so on. It was shown that a strong site selective

interaction between PANI and CNTs takes place by

introduction of nanotubes during in-situ chemical poly-

merization process [3].

In this research publication, we explore the synthesis and

characterization of CNT/PANI composite material by

introduction of MWCNTs during the in-situ chemical

polymerization process. It reports that the optical bandgap

of PANI changed with the amount of MWCNTs introduced

during the in-situ chemical polymerization process. This

research can be further used for development of optical

antenna and other optoelectronic devices by varying the

optical bandgap of conducting polymers.

2. Materials and methods

2.1 Materials

There were several methods to synthesize MWCNT/PANI

composite. In this study, in-situ chemical polymerization of

aniline with several wt% of functionalized MWCNT was

used. Three samples were prepared for the reported work

containing 5, 10 and 15 mg MWCNT with 0.5 ml aniline.

Pure PANI sample and functionalized CNT samples were

also prepared for reference purpose. Synthesized MWCNT

obtained from chemical vapour deposition method after

Bull. Mater. Sci. (2021) 44:121 � Indian Academy of Scienceshttps://doi.org/10.1007/s12034-021-02388-4Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)

functionalization and double-distilled aniline monomer

(C6H5NH2; 99% purity, Merck) has been used as composite

materials, in which polymerized aniline was used as matrix

material and CNT was used as filler material. To start the

in-situ chemical polymerization, strong oxidant catalyst

ammonium peroxodisulphate [(NH4)2S2O8 or APS; 98%

purity, Merck] was used. Deionized water was used as

solvent, and for acidic dopant, sulphuric acid (H2SO4;

purity 98%, Merck make), nitric acid (HNO3, Fisher Sci-

entific; Qualigens make) and hydrochloric acid (HCl; 37%

purity, Merck) were used.

2.2 Functionalization of MWCNT

MWCNTs were synthesized using chemical vapour depo-

sition method, in which acetylene was used as carbon

containing gas and cobalt thin film was used as catalyst

[24–27]. In order to reduce the strong Vander-Wall force

and to increase the dispersion of MWCNT in composite

solution, functionalization of MWCNTs has been done

using acid solution. The functionalized MWCNT

(COOH-MWCNTs) has been obtained by treatment of as-

synthesized MWCNTs with 3:1 ratio of H2SO4 and HNO3.

The dispersion was kept for stirring for 2 h maintaining

temperature at 80�C. Then the suspension was cooled to

room temperature and washed several times using deionized

water to remove acid and neutralize the pH of material.

Washing and filtering have been done using centrifuge

process. Samples were collected in clean glass beaker, and

dried first in hot air oven for 6 h and then kept in vacuum

oven for 24 h. The dried f-MWCNT powder was collected

in clean vials.

2.3 Synthesis of MWCNT/PANI composite

In-situ chemical polymerization of aniline along with dis-

persed functionalized MWCNT was done for preparation of

MWCNT and PANI composite [28]. Three samples with

different weight concentrations of MWCNTs 5, 10 and 15

mg with 0.5 ml aniline were prepared in the reported study.

Functionalized MWCNT were dispersed with known con-

centration in 10 ml volume of 1 N HCl and kept for

Figure 1. SEM images of (a) pristine MWCNT, (b) PANI, (c) 5 mg

MWCNT/PANI, (d) 10 mg MWCNT/PANI and (e) 15 mg MWCNT/PANI

samples.

121 of 8 Bull. Mater. Sci. (2021) 44:121

ultrasonication for 1 h to get homogeneous dispersion. After

that the suspension was kept in ice bath to maintain the

temperature at 0�C. Aniline monomer, 0.5 ml, was added

slowly into CNT suspension with continuous stirring in the

next step. A freshly prepared solution of 1.14 g strong

oxidant ammonium persulphate (APS) (NH4)2S2O8 in 10 ml

water was added into the suspension. After few minutes,

colour of suspension turned into green from dark colour,

which indicates the good initiation of polymerization

reaction. The reaction was kept in continuation for 6 h by

maintained stirring and ice bath and after that dark green

material was washed several times with deionized water and

acetone to remove any unreacted monomers and oxidants.

After filtering, the material was kept for vacuum drying at

80�C for overnight to get dry PANI–CNT composite.

2.4 Synthesis of PANI

In-situ chemical polymerization has been done for prepar-

ing pure PANI sample for reference purpose. The same

procedure was followed as explained in the section 2.3

without addition of MWCNT.

After the process was completed, in total five samples

were prepared which includes functionalized CNT, 5 mg

MWCNT/PANI, 10 mg MWCNT/PANI, 15 mg MWCNT/

PANI and pure PANI. All were collected in clean vials.

2.5 Characterization

All the as-prepared five samples were characterized to obtain

details of the composition, morphology, optical and electrical

properties. Morphological properties were examined by

scanning electron microscopy (SEM) image of the samples.

SEM measurements were carried out under field-emission

scanning electron microscope. The cross-sectional images of

the PANI and f-MWCNTs filled PANI nanocomposites were

taken at 5000-X, 20000-X and 50000-X. Fourier transform

infrared (FTIR) spectroscopy gives the structural properties

of PANI and MWCNT/PANI composite. To investigate the

optical properties, UV–visible spectroscopy technique is

used for all samples and bandgap was calculated by absorp-

tion spectra of sample using Tauc Plot method. The absorp-

tion spectra of matrix material were recorded using

ultraviolet–visible spectroscopy (200–800 nm). Powder

X-ray diffraction (PXRD) patterns of MWCNTs, PANI and

MWCNT/PANI samples were collected with the help of high-

resolution X-ray diffractometry (XRD; PaNalytical X’Pert

Pro, Cu Ka radiation, wavelength = 1.54 A). The XRD data

were collected in the 2h range from 10� to 90�; step size 0.02�with a scan rate of 0.7 s.

3. Results

3.1 Morphological analysis

Morphological details of samples can be examined from

SEM image of samples as shown in figure 1.

Figure 2 shows the morphology of pristine MWCNT.

Average diameter of MWCNTs was in the range 15–55 nm.

As per reported morphological study of single-wall nan-

otubes in various literatures, the diameter of single-wall

nanotube is around 1–3 nm and diameter of MWCNT varies

between 3 and 60 nm. The estimation of diameter of

MWCNTs was done using ImageJ analysis tool as shown in

figure 2.

Figure 1b shows large and thick flakes of pure PANI,

while figure 1c shows the morphology of 5 mg MWCNT/

PANI sample, in which CNTs were overlapped by the

polymer that ensures the good chemical polymerization on

the surface of MWCNTs. Figure 1d and e shows the mor-

phology of 10 and 15 mg MWCNT/PANI sample, in which

one particular CNT was in focus. A long single CNT can be

examined in both SEM images (10 and 15 mg MWCNT/

PANI samples) having smooth surface and enhanced

thickness due to polymerization. SEM images show wrap-

ping of PANI over MWCNTs. The polymer composites

Figure 2. SEM image of pristine MWCNT.

Figure 3. Digital images of as-prepared samples.

Bull. Mater. Sci. (2021) 44:121 of 8 121

synthesized by grafting PANI on outer wall of MWCNTs

producing thick layer PANI on MWCNT resulted in elon-

gated granular form. Figure 3 shows the digital image of as-

prepared samples.

3.2 XRD analysis

XRD was a very powerful tool to analyse the crystal

structure and the inter-atomic spacing. Information related

to structural and crystallinity of PANI, f-MWCNT and 15

mg MWCNT/PANI composite can be analysed using XRD

pattern, as shown in figure 4a–c. Figure 4a shows XRD

pattern of PANI, in which occurrence of crystalline peaks at

2h = 15.3�, 20.41� and 25.5� corresponding to (011), (020)

and (200) plane are shown, which indicate the formation of

conductive PANI [29]. XRD spectra of CNTs show a

diffraction peak at 26.4�, corresponding to the characteristic

peak (002) of the MWCNTs having d-spacing 0.34 nm

(figure 4b) [30,31].

On analysing the XRD patterns, it was observed that the

MWCNT/PANI composites have similar XRD pattern as of

pure PANI XRD pattern. On comparing the XRD pattern of

pure PANI and MWCNT/PANI composites, it was observed

that the sharpness of the peaks increased with an increase in

MWCNT content. It indicates that an additional crystalline

order has been introduced into the composites as aniline

polymerizes along the axis of the CNT [32]. In addition, the

intensity of amorphous peak of PANI at angle 20.4� reduces

for MWCNT/PANI composites.

3.3 FTIR spectroscopy

FTIR spectroscopy of samples 5 mg MWCNT/PANI, 10 mg

MWCNT/PANI, 15 mg MWCNT/PANI and functionalized

MWCNT are shown in figure 5, which shows the trans-

mission spectra of all samples. The peak at around 3441

cm–1 in all the spectra’s was corresponding to N–H

stretching and/or O–H stretching modes. IR spectra of

MWCNTs have band at around 2922 cm–1, which was

associated to asymmetric and symmetric CH2 stretching. IR

spectra of MWCNTs have peak at 1722 cm–1 corresponding

to the C=O stretching vibrations [33]. During oxidation and

purification processes, functional groups may be introduced

on the sidewalls of MWCNTs. All bands in the 1050–1300

cm-1 region, corresponds to C–O group.

The peak position at 1563 and 1476 refers to the C=C

stretching vibration bands of quinoid ring and C=C

stretching deformation of benzenoid ring, respectively (Red

Spectra) [33,34]. The shoulder position at 1323 cm-1 and

peak at 1294 cm-1 were associated with asymmetric C–N

stretching of aromatic amine and C–N stretching of sec-

ondary aromatic amine, respectively [35]. The shoulder

Figure 4. XRD spectra of (a) pure PANI, (b) MWCNT and (c) XRD

pattern of MWCNT/PANI composites.

121 of 8 Bull. Mater. Sci. (2021) 44:121

position at 1235 cm–1 refers to C–N? stretching vibration in

the polaron structure of PANI, which indicates doped form

of PANI [36–38], and the dominating absorption peak at

wavelength 1121 cm–1 associated with in-plane bending

vibration of aromatic C–H [39,40]. The comparison of the

occurred peak with data provided in related references

confirms the successful formation of MWCNT/PANI

composite.

3.4 UV–visible spectroscopy

UV–visible spectroscopy of functionalized MWCNT, pure

PANI and 5 mg MWCNT/PANI, 10 mg MWCNT/PANI

and 15 mg MWCNT/PANI samples are shown in figure 6.

The peak obtained at 350 nm in UV–visible spectrum of

pure PANI corresponds to p–p* transition [41]. Effect of

MWCNT concentration can be observed in UV–visible

spectroscopy by analysing sample with different concen-

trations of MWCNT in constant concentration of PANI. A

red shift can be observed from 350 nm to towards 400 nm

with increasing p–p* transition bond intensity along with

concentration of CNT, which reveal that new excitation

energy levels were formed by addition of MWCNTs near

the bandgap of the material. Another characteristic peak of

doped PANI and pure PANI was observed at 430 nm, which

corresponds to polaron–p* transition [33]. It is well-known

fact that pure MWCNTs have the characteristics absorption

peak at 260 nm corresponding to the 1D van Hove singu-

larities [42], and other composite samples show the red peak

shift around 280, which verify the presence of CNT in

composition. UV–visible spectrum of CNT does not possess

any other absorption peak in region 350–1000 nm and the

peaks found in the region 350–1000 nm was due to the

presence of PANI [39]. There was red shift of band as

concentration of CNT increases in composite. The satis-

factory results obtained from UV–visible spectrum validate

the strong interaction between the polymer and CNT.

3.5 Optical bandgap calculation

Optical bandgap was calculated using Tauc relation, which

is plot of energy hv (eV) against square of absorption (ahv)2

[39]. The Tauc relation was given by expression:

ahm ¼ B hm� Eg

� �c ð1Þ

where Eg is the optical bandgap, m the frequency of incident

photon, c the index value that is equal to � for direct

allowed transition, B is the band tailing parameter, a con-

stant and a the absorption coefficient [43]. Bandgap of

samples were calculated directly from spectrum of UV–

visible by extrapolating a straight line over absorption peak

to horizontal energy (hv) axis, which are shown in fig-

ure 7a–e).

Comparison analysis of different samples can be done

by bandgap values given in table 1. It can be noted that

as amount of MWCNTs is increased in polymer sample,

the value of bandgap is decreased. This reduction is due

to the new excitation energy levels created below the

regular bandgap because of charge transfer from MWNT

to PANI [44–46]. It can be observed that the bandgap

shift depends on the ratio of material in composite and

desired bandgap can be obtained by selecting proper ratio

of composition.

In figure 8, it was clearly observed that plot has

decreasing nature with the increase in the weight concen-

tration of CNT in composite sample. A steep decrement of

bandgap is shown in plot from no concentration (pure

PANI) to 10 mg concentration in composite sample and

Figure 5. IR spectra of MWCNTs, 5 mg MWCNT/PANI, 10 mg

MWCNT/PANI and 15 mg MWCNT/PANI.

Figure 6. UV–visible absorbance spectra of MWCNT, pure

PANI, 5, 10 and 15 mg MWCNT/PANI samples.

Bull. Mater. Sci. (2021) 44:121 of 8 121

after that slop moves approximately flat from 10 mg sample

to pure CNT sample. These data points can be easily fitted

using exponential decaying regression function curve

with goodness of fit (R2) value 0.95, which is shown in

figure 8b.

4. Conclusion

Composites of MWCNTs and PANI have been prepared by

introduction of MWCNTs during the in-situ chemical poly-

merization process and their structural, UV absorbance, and

optical bandgap studies were carried out. The UV–visible

Figure 7. Calculation of the bandgaps for pristine MWCNT, pure PANI sample, 5 mg MWCNT/PANI, 10 mg

MWCNT/PANI and 15 mg MWCNT/PANI samples, obtained from UV–visible spectra by plotting (ahm)2 (eV-cm-1)2

vs. hm (eV). The energy gap was obtained by the intercept on the abscissa of the best fitting of equation (1).

Table 1. Bandgap values of samples.

Samples PANI CNT 5 mg 10 mg 15 mg

Bandgap (eV) 4.17 3.49 4 3.6 3.55

121 of 8 Bull. Mater. Sci. (2021) 44:121

spectra and FTIR spectra of different composites revealed

that strong interaction between MWCNTs and PANI takes

place. The optical bandgap for PANI and different MWCNT/

PANI composites sample have been measured by using UV–

visible results and Tauc plot. It is observed that the optical

bandgap decreases with the increase in concentration of

MWCNTs in composites, while keeping the concentration of

PANI constant during the in-situ polymerization process.

These material constants were of great importance for the

engineering of optical antenna and other optoelectronic

devices based on composites of MWCNTs and a conducting

polymer. The change in electronic properties of conducting

polymer by introducing MWCNTs provides a pathway for

applications of composites of nanotubes and conducting

polymer in optical antenna and other optoelectronic devices.

Acknowledgements

The MNIT, Jaipur, is acknowledged for the MRC support

for the synthesis of CNTs using chemical vapour deposition

method. Composite sample preparation, characterization

using SEM and XRD, spectroscopic study using FTIR and

UV–visible were performed at OEMD Laboratory, Depart-

ment of MEMS, IIT Bombay. We are also very thankful to

Dr M P Gururajan (Department of MEMS, IITB, India) and

Ashwini Yella (Department of MEMS, IITB, India) for their

valuable guidance during the whole work for this article.

We acknowledge the AICTE-CRS sanctioned project

(1-5748447161) under TEQIP-III grant by NPIU, MHRD,

India, for funding of this research work.

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