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Optical properties and FT-IR spectra of PANI/f-MWCNT thin films

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Abstract

Polyaniline (PANI) and PANI/f-MWCNT thin films have been synthesized by the in-situ chemical polymerization method. Ammonium persulfate (NH4)2S2O8 and hydrochloric acid (HCL) were used as an oxidizing agent and protonic acid dopant respectively. The f-MWCNT was added to PANI matrix to enhance and modify its optical properties. The films were deposited on glass substrates by dip coating method and its characterizations were done by UV-Vis (Ultra-Violet Visible Spectrophotometer) and FTIR (Fourier transform Spectroscopy) in the region of (650-2500) cm −1. The optical energy gap and optical constants such as the reflective index, the extinction coefficient and others were carried out from the optical measurements in the wavelength range (300-900) nm. The optical results indicate that the prepared films have allowed direct transition and the optical energy gap depends on the different weight percentages of f-MWCNT which was used as a dopant. FTIR spectrum shows several absorption peaks centred at around 1556, 692, 1235, 830, 1450 and 1280 cm −1 which consider the characteristic band peaks of polyaniline.
Exp. Theo. NANOTECHOLOGY 5 (2021) 47 55
Optical properties and FT-IR spectra of PANI/f-MWCNT
thin films
Zain A. Muhammad, Tariq J. Alwan*
Physics Department, College of Education, Al-Mustansiriyah University, Baghdad, Iraq
E-mail: tariqjaffer2000@yahoo.com
Received: 22/2/2020 / Accepted: 25/7/2020 / Published: 1/1/2021
Polyaniline (PANI) and PANI/f-MWCNT thin films have been synthesized by the in-situ
chemical polymerization method. Ammonium persulfate (NH4)2S2O8 and hydrochloric acid
(HCL) were used as an oxidizing agent and protonic acid dopant respectively. The f-MWCNT
was added to PANI matrix to enhance and modify its optical properties. The films were
deposited on glass substrates by dip coating method and its characterizations were done by
UV-Vis (Ultra-Violet Visible Spectrophotometer) and FTIR (Fourier transform Spectroscopy)
in the region of (6502500) cm−1. The optical energy gap and optical constants such as the
reflective index, the extinction coefficient and others were carried out from the optical
measurements in the wavelength range (300-900) nm. The optical results indicate that the
prepared films have allowed direct transition and the optical energy gap depends on the
different weight percentages of f-MWCNT which was used as a dopant. FTIR spectrum shows
several absorption peaks centred at around 1556, 692, 1235, 830, 1450 and 1280 cm−1 which
consider the characteristic band peaks of polyaniline.
Keywords: PANI/f-MWCNT thin films, In-situ polymarization method, Optical properties.
1. INTRODUCTION
Due to the possibility of their commercial application in electronic devices, intrinsically
conducting polymers have drawn excellent attention in latest years. For centuries, polymeric
materials have been regarded as insulators; however, a redox and oxidation reaction called
doping has shown that these polymers can display conductive characteristics [1]. The
distinction between a semi-conductive polymer and a non-conductive polymer is due to the
nature of the chemical bonds along the backbone of the molecule. The σ-bonds or hybrid
orbitals which are formed by head-on overlap are usually responsible for the formation of single
bonds. The σ-bonds are consisting of electrons that are localized and have no ability to pass
along the backbone of molecular; hence a polymer will be an electrical insulator if only contains
single bonds. If instead, the polymer has a conjugated system which are formed by alternating
single and double bonds then each carbon atom which lies along the backbone of that system
will form one unhybridized p-orbitals and three hybridized orbitals. When sp2 orbitals overlap,
Theo. Exp. NANOTECHOLOGY 15 (2019) 47 55
48
σ-bonds will be formed, also the overlapping of p-orbitals results in π-bonds. No longer,
electrons, take part in π-bonds, belong to a certain atom but will be delocalized. The combining
of two p-orbitals leads to two different orbitals, one is the bonding π-molecular orbital which
has a lower
energy and the other is the antibonding π*-molecular orbital that has a higher energy. The
energy gap of polymer is determined by taking the energy difference between the highest
occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).
The energy gap of polymer lies usually in the range (1-4) eV, this range is similar to that of
inorganic semiconductors [3]. Because of its easy synthesis technique, low cost monomer,
tunable characteristics, and better stability, polyaniline is a promising material as a conductive
polymer. There are three distinct types of PAni: leucoemeraldine base (LEB), emeraldine base
(EB, partly oxidized) and perennigraniline base (PNB, fully oxidized) polyaniline has been
widely researched since the study of an insulator-to-metal shift to the emeraldine salt (ES-
conductive) shape after protonation of the emeraldine base(EB) [4]. In applications such as
screens, photoelectrode semiconductor coatings, solar cell and chemical sensors, the
conductive type of PANI was used [5]. Most composites that composed of CNT and
polyaniline are widely studied. These studies suggested that, there is a π-π interaction between
graphitic structure of CNTs and aromatic rings of polyaniline. This interaction leads to the
electron delocalization that facilitate the motion of electrons and enhance the electrical and
optical properties of composites [6]. Multi-walled carbon nanotubes (MWCNTs) are
comparatively chemically inert, display outstanding electrical and thermal conductivity, and
demonstrate superior mechanical strength and nonlinear optical properties. Due to their
improved electronic and optical characteristics, functionalized MWCNTPANI
nanocomposite thin films have recently drawn considerable attention.
Because functionalized nanotubes are readily dispersed in organic solvents, the MWCNT dis
persion and homogeneity within the polymer composite are enhanced [7].
The aim of this paper is to prepare semiconducting polyaniline PANI and PANI/f-MWCNT
thin films using in-situ chemical method at low temperature, which allows us to get thin films
with a good quality and to deposit them over large area, and also to study the influence of
different weight percentages of f-MWCNT on the optical properties of PANI thin films.
2. EXPERIMENTAL
2.1.Functionalization of MWCNT
One gram of MWCNT was first immersed in 80 ml of a mixture of sulfuric acid and nitric acid
(in 3:1 ratio) and placed in the ultrasound bath for two hours after which the mixture was placed
under continuous stirring for a full day at 50o C. After that 10 ml of hydrochloric acid in the
form of drops was added to the mixture under continuous stirring. In order to neutralize the
mixture, ammonia hydroxide was also added to it in the form of drops until the solution is
neutralized and this was confirmed using the pH meter. Then the product is centrifuged at a
speed 4000 rpm for 15 minutes and filtered using filter paper, and the resulting powder is
washed several times in deionized water and then dried at 80 °C for 6 h. The sample was
abbreviated as f-MWCNT.
2.2. Prepare PANI and PANI/f-MWCNT Thin films
The PANI and PANI/f-MWCNT thin films was chemically synthesized by in-situ
polymerization method, where using aniline monomer and ammonium persulfate (APS) as
oxidant agent and hydrochloric acid (HCL) as protonic acid dopant, accordance to a method
Theo. Exp. NANOTECHOLOGY 15 (2019) 47 55
49
similar to the described by A. Jabbar 2018 [8]. The different weight percentage of f-MWCNTs
(0, 4, 6 and 8 w%) with respect to aniline were used. PANI and PANI/f-MWCNT films was
deposited on glass slides. The slides are dipped in aniline/HCl solution then the oxidant agent
(APS) was added under constant stirring to start the polymerization process, after 30 min, all
the slides are removed from a flask. The obtained films were again immersed in aniline/HCl
solution, then rinsed with 1 M HCl and acetone, finally left to dry in air at room temperature.
The optical measurements of the thin film deposited on glass substrate are calculated from the
transmittance and absorbance spectrum at normal incidence over the range (300900) nm, by
using UV-VIS spectrophotometer type (SHIMADZU)(UV-1600/1700 series). SIDCO
England series FT-IR spectrometer is used to carry out the infrared analysis of the wavenumber
that ranges 650 to 2500 cm-1.
3. RESULTS AND DISCUSSION
Figure 1 shows the UV-VIS spectra in the range of (300-900)nm for a pure PANI and PANI/f-
MWCNT thin films. From the spectra one can observe that there are three absorptions bands
for each sample. The bands of the pure PANI thin films are located at 350 nm, 444 nm and
788 nm. The absorption peak at around 350 nm is attributed to π – π* transitions, the bands at
444 nm and 788 nm correspond to transitions π to polaron and polaron to π* respectively [9].
As expected these three bands should appear in the UV-Vis spectrum of the conducting
emeraldine salt polymer. From the absorbance spectra it can be seen that the increasing of the
weight percentage of f-MWCNT causes the peaks to shift to higher wavelength region, the
reason for the band shifting is due to the new excitation energy levels created by f-MWCNT
near the band gap of material [10]. Also there is an increase in the absorption magnitude as the
weight percentage of the f-MWCNT increase. The obtained absorption bands are in good
agreement with that reported in the literature [11].
Figure. 1: The absorbance spectra for a pure PANI and PANI / f-MWCNT thin films
The absorption coefficient cm-1 is calculated in the fundamental absorption region using
Lambert law [12];
- α t
o
I=I e
(1)
Where I and I are the intensity of the incident and transmitted light respectively. If ( I / I ) =
T then α=Ln (1/ T )/t (2)
Where T is a transmittance and t is a thickness that has a value of about 200±20 nm for all
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
200 300 400 500 600 700 800 900 1000
Wavelength (nm)
4 % M
6% M
8 % M
PURE
Absorbance
350
444
Theo. Exp. NANOTECHOLOGY 15 (2019) 47 55
50
samples. The absorption coefficient is very important property of a material. The ability of
material to absorb light radiations is based on the value of absorption coefficient, aslo it is
represented a key parameter for designing many optoelectronic components like photovoltaic
cells, photodiodes and photo-detectors photo-detectors. Figure 2 shows the variation of the
calculated absorption coefficient of the pure PANI and PANI/f-MWCNT as a function of the
incident photon energy. It can be seen from Figure 2 the abosrption coefficient for all samples
is greater than 104 cm-1 and this confirms that the type of the transition is the direct allowed
transition[13]. Also from curves it is noticed that the abosrption coefficient increases with the
increase of the weight percentage of the f-MWCNT .
Figure. 2: The variation of α vs. photon energy for PANI and PANI / f-MWCNT thin films
Optical absorption spectra are considered one of the most important tools to compute the
optical energy gap (Eg) of organic and inorganic semiconductors. Energy gap is of fundamental
importance , since the energy gap specifies the electrical conductivity and optical absorption
character of the PANI. In many amorphous materials, photon absorption is found to obey the
Tauc relation [14], which is of the form:
n
g
αhv=A(hv-E )
(3)
where α is the absorption coefficient, hv is the photon energy, Eg is the optical band-gap, A is
a constant depending on the material's properties and n is a constant which can take various
values depending on the type of electronic transition, for the direct and indirect allowed
transition = 1/2 or 2, respectively. The best fit line is obtained for the direct allowed transition
n= 1/2 .
Figure 3 shows the variation of (αhυ)2 with photon energy (hυ) for direct allowed transition to
pure polyaniline and PANI/ f-MWCNT thin films. The optical energy gap are determined from
this fig. and listed in Table 1. It can be seen from the Table (1) as the f-MWCNT content
increases the optical band gap decreases, from 2.59 eV to 2.45 eV. The reduction in the optical
band gap is due to the modification of the polymer structure as result of the interaction of f-
MWCNT with polyaniline.[15]
0.00E+00
5.00E+04
1.00E+05
1.50E+05
2.00E+05
2.50E+05
3.00E+05
12345
Photon Energy (eV)
4% M
6 % M
8% M
Pure
Absorption Coefficient (cm)-1
Theo. Exp. NANOTECHOLOGY 15 (2019) 47 55
51
Figure. 3 : The variation of (αhv)2 vs. (hv) for a pure PANI and PANI / f-MWCNT thin
films
Table 1 :The optical energy gap values of pur polyaniline and PANI/ f-MWCNT thin films.
f-MWCNT
content (wt%)
Eg (eV)
Pure
2.59
4%
2.53
6%
2.46
8%
2.45
The extinction coefficient is a parameter that determines by how much light intensity will be
reduced as light moves a distance x through the medium. The Ko is calculated by the
following equation[16]:
0
α λ
K = 4π
(4)
where λ : is the wavelength of incident rays
Figure 4 illustrates the variation of extinction coefficient Ko with the photon energy. The
behavior of the extinction coefficients of a pure PANI and PANI/f-MWCNT thin films is
similar to that of the absorption coefficient since they are related each other by equation (4).
Figure. 4 : The variation of Ko vs. photon energy for PANI and PANI / f-MWCNT thin
0
5E+10
1E+11
1.5E+11
2E+11
2.5E+11
3E+11
3.5E+11
1 2 3
Photon Energy (eV)
4 % M
6% M
8 % M
PURE
(αhν)2
0
0.5
1
1.5
2
2.5
1 1.5 2 2.5 3 3.5 4 4.5
Photon Energy (ev)
4 % M
6 % M
8% M
Pure
Extinction Coefficient (K0)
Theo. Exp. NANOTECHOLOGY 15 (2019) 47 55
52
films
The refractive index is a ratio between the velocity of light in a vacuum to that of light in the
medium . its value can be computed by using the following equation [17] :
( )
( )
( )
( )
( )
1/2
2
2
o
2
1+R 1+R
n = - K +1 + 1-R
1-R




Where R is a reflectance. Figure 5 demonstrate the variation of the refractive index n with the
photon energy. The refractive index curve of the pure PANI is observed as a concave curve in
the range of photon energies (2.1 to 2.6) eV, also it is seen that in this range the higher the
weight percentage of the f-MWCNT content, the more flat the curve becomes. This means the
value of the refractive index becomes approximately constant in this region.
Figure. 5 : The variation of n vs. wavelength for PANI and PANI / f-MWCNT thin films
The complex dielectric constant ε is the material's ability to polarize, whose expression is giv
en by the following equation [18] :
ε= (n*)2= (n+iKo)2 (6)
But the complex dielectric constant has two parts as .
ε = ε1 +iε2 (7)
Where ε1 and ε2 represent the real and imaginary parts of complex dielectric constant and they
are illustrated by the following relations:
ε1 = ( n2 - Ko2 ) (8)
ε2 = 2 n Ko (9)
The two parts of the dielectric constant which are varied with photon energy are shown in
Figure 6 and 7 for pure PANI and f-MWCNT thin films. As the values of K2o is small
compared to that of n2, so the overall variation of ε1 will rely on the n2 values , while ε2 is
based on the K2o values that associated with the absorption coefficient variation.
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
1 2 3 4 5
Photon Energy (eV)
4 % M
6 % M
8 % M
pure
Refractive Index n
Theo. Exp. NANOTECHOLOGY 15 (2019) 47 55
53
Figure 6: The variation of ε1 vs. photon energy for PANI and PANI / f-MWCNT thin films
Figure. 7 : The variation of ε2 vs. Photon Energy for PANI and PANI / f-MWCNT thin films
3.1 FT-IR Studies
FTIR spectroscopy is a good selection for both organic and inorganic materials to study the m
olecular structures and their bonding.The unidentified elements present in the sample can also
be identified. FTIR spectra of pure PANI and PANI / f-MWCNT nanocomposites thin films
in the region of (6502500 ) cm−1 were illustrated in Fig.(8). The characteristic band peaks for
pure polyaniline occurs at 1556, 692, 1235, 830, 1450 and 1280 cm−1. The C=C stretch
absorption of aromatic compound was obtained at 1556 cm-1, The CCl stretching peak appears
in the band peak 692 cm-1 confirmed the Cl- doping of the synthesized polyaniline films in
HCL [19]. The peak around 1235 cm−1 is assigned to stretching of C–N+• polaron structure
that corresponds to the electrically conductive form of doped PANI [20]. The band 832 cm−1
corresponding to aromatic ring out of plane deformation vibrations which belongs to CH
deformation in the paradisubstituted ring [21]. The band near 1450 cm-1 may be obscured by
the aliphatic CH deformation vibration, finally the band peak 1280 cm−1 is attributed to CH
plane bending[22]. In comparison with PANI, the characteristic vibrational peaks of PANI are
0
5
10
15
20
25
30
12345
Photon Energy ( eV)
4 % M
6 % M
8 % M
pure
Real part of dielecric constant ε1
0
2
4
6
8
10
12
14
1 1.5 2 2.5 3 3.5 4 4.5
Photon Energy (eV)
4% M
6% M
8% M
pure
Theo. Exp. NANOTECHOLOGY 15 (2019) 47 55
54
also appeared in the PANI/f-MWCNT composites FTIR spectra and the transmission peak
intensities of PANI/f-MWCNT composite are lower than pure PANI, indicating the interaction
between the f-MWCNT and the polyaniline matrix was taken place [23].
Figure. 8 : FTIR spectra of PANI and PANI / f-MWCNT thin films
3. Conclusion
The PANI and PANI/f-MWCNT thin films are successfully deposited on a glass substrates by
in-situ chemical polymerization method. The optical energy gap is effected by the addition of
a f-MWCNT to PANI matrix, it was (2.59 eV) for pure PANI sample and reduced to (2.45 eV)
for the f-MWCNT of 8 w %. Also all the other optical constants are varied with the increase
of the f-MWCNT. The FTIR measurement revealed the formation of PANI by displaying the
characteristic band peaks belongs to it. The width and the intensity of the tansmittion peaks are
impacted by adding the f-MWCNT that indicated the interaction between the f-MWCNT and
the polyaniline matrix was taken place. The optical characteristics of all samples indicate that
they are an organic semiconductor. According to this, promising results in photovoltaic and
optoelectronic devices are expected .
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access article distributed under the terms and conditions of the Creative Commons
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Article
Carbon nanotube-silicon solar cells are a recently investigated photovoltaic architecture with demonstrated high efficiencies. Silicon solar-cell devices fabricated with a thin film of conductive polymer (polyaniline) have been reported, but these devices can suffer from poor performance due to the limited lateral current-carrying capacity of thin polymer films. Herein, hybrid solar-cell devices of a thin film of polyaniline deposited on silicon and covered by a single-walled carbon nanotube film are fabricated and characterized. These hybrid devices combine the conformal coverage given by the polymer and the excellent electrical properties of single-walled carbon nanotube films and significantly outperform either of their component counterparts. Treatment of the silicon base and carbon nanotubes with hydrofluoric acid and a strong oxidizer (thionyl chloride) leads to a significant improvement in performance.
  • Manawwer Alam
  • Anees A Ansari
  • Mohammed Rafi Shaik
  • Naser M Alandis
Manawwer Alam, Anees A. Ansari, Mohammed Rafi Shaik, Naser M. Alandis, Arabian Journal of Chemistry, 6 (2013) 341
  • Atanu Roy
  • Apurba Ray
  • Samik Saha
  • Sachindranath Das
Atanu Roy, Apurba Ray, Samik Saha, Sachindranath Das, international journal of hydrogen energy, (2018) 1-12
  • P Chutia
  • A Kumar
P. Chutia, A. Kumar, Physica B, 436 (2014) 200-207
  • Srilatha Koteswara
  • Ishwarya Punati
  • Avireni Srinivasulu
  • Exp Theo
Srilatha Koteswara, Ishwarya Punati and Avireni Srinivasulu and SM-IEEE, Exp. Theo. NANOTECHNOLOGY 2 (2018) 11-20.
  • Q Qingli Zhang
  • Weijie Wang
  • Jianlin Li
  • Lianjun Wang
  • Meifang Zhua
  • Wan Jiang
Q. Qingli Zhang, Weijie Wang, Jianlin Li, Lianjun Wang, Meifang Zhua and Wan Jiang, J. Mater. Chem. A, 1 (2013) 12109
Study the effect of carbon nanotubes doped on the physical properties of polyaniline blends
  • A S Jabbar
A. S. Jabbar "Study the effect of carbon nanotubes doped on the physical properties of polyaniline blends" M.Sc. Thesis, Al-Mustansiriyah University, College of Education, Physics Department, Iraq, (2018).
  • B Rahul
  • Aviraj A Patila
  • Rupesh S Jatratkarb
  • D Devanc
  • Yuan-Ron Mad
  • R K Purib
  • J B Vijaya Purie
  • Yadavb
Rahul B. Patila, Aviraj A. Jatratkarb, Rupesh S. Devanc,d, Yuan-Ron Mad, R.K. Purib,Vijaya Purie, J.B. Yadavb, Applied Surface Science, 327 (2015) 201
  • M J Almasi
  • T Sheikholeslami
  • M R Naghdi
M.J. Almasi, T. Fanaei Sheikholeslami, M.R. Naghdi, Composites Part B, 96 (2016) 63
  • Miroslava Trchova
  • Zuzana Moravkova
  • Ivana Šeděnkova
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Miroslava Trchova, Zuzana Moravkova, Ivana Šeděnkova, Jaroslav Stejskal, Chemical Papers, 66 (5) (2012) 415