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Effect of f-SWCNT on the structure, electrical and optical properties of PANI thin films

Abstract

Polyaniline (PANI) and PANI/f-SWCNT thin films have been synthesized by a chemical oxidative polymerization method. Ammonium persulfate (NH4)2S2O8 and hydrochloric acid (HCL) were used as an oxidizing agent and protonic acid dopant respectively. The results of XRD pattern showed that the neat PANI and PANI/f-SWCNT thin films have an amorphous structure with a broad diffraction peak around 2θ = 25 o and 2θ = 23 o respectively. FESEM images showed that both neat PANI and PANI/f-SWCNT thin films have nanorod structures and the diameter of PANI nanorod is about 95.8 nm. Also it is revealed that the aniline monomer was polymerized on the surfaces of f-SWCNT that leads to increase the nanorod diameters which become in the range of 100-195 nm. The conductivity measurements revealed that the value of percolation threshold (PT) was about 0.39 W% and the conductivity value of neat PANI is increased after incorporating f-SWCNT in PANI matrix. The value of direct allowed transition energy gap of neat PANI was 2.56 eV and it is decreased significantly after adding f-SWCNT to it. 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.
OPTOELECTRONICS AND ADVANCED MATERIALS RAPID COMMUNICATIONS Vol. 14, No. 9-10, September-October 2020, p. 421 - 426
Effect of f-SWCNT on the structure, electrical and
optical properties of PANI thin films
TARIQ J. ALWAN*, ZAIN A. MUHAMMAD
Physics Department, College of Education, Al-Mustansiriyah University, Baghdad, Iraq
Polyaniline (PANI) and PANI/f-SWCNT thin films have been synthesized by a chemical oxidative polymerization method.
Ammonium persulfate (NH4)2S2O8 and hydrochloric acid (HCL) were used as an oxidizing agent and protonic acid dopant
respectively. The results of XRD pattern showed that the neat PANI and PANI/f-SWCNT thin films have an amorphous
structure with a broad diffraction peak around 2θ = 25o and = 23o respectively. FESEM images showed that both neat
PANI and PANI/f-SWCNT thin films have nanorod structures and the diameter of PANI nanorod is about 95.8 nm. Also it is
revealed that the aniline monomer was polymerized on the surfaces of f-SWCNT that leads to increase the nanorod
diameters which become in the range of 100-195 nm. The conductivity measurements revealed that the value of percolation
threshold (PT) was about 0.39 W% and the conductivity value of neat PANI is increased after incorporating f-SWCNT in
PANI matrix. The value of direct allowed transition energy gap of neat PANI was 2.56 eV and it is decreased significantly
after adding f-SWCNT to it. 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.
(Received January 30, 2020; accepted October 21, 2020)
Keywords: Conducting polymer, PANI/f-SWCNT thin films, In-situ polymarization method
1. Introduction
Polymers which are able to conducting electricity are
called conjugated polymers that are inherently conducting
ones. They can be used like the metallic conductors or
semiconductors. The carrier mobility is a biggest
difference between inorganic semiconductors and
conducting polymers. Many years ago, carrier mobility
was lower in conductive polymers due to disorder in the
polymeric system [1]. For time being, this difference is
reduced as a new conducting polymers are invented and
modern processing methods are developed. Organic
polymers often consist of two types of bonds, σ–bond and
π–bond. The electrons that belong to σbond are not able
to move along the backbone of molecular since the carbon
atoms are bounded by covalent bonds, whilst The
conjugated polymers contains π–electrons, which are
formed by alternating single and double bonds, are
delocalized and hence have ability to conduct electricity
[2]. In general, conjugated polymers are insulators, or
semiconducting when they are in the pristine state such as
polyaniline, polyacetylenes and polythiophenes. The
energy gap of which can be greater than 2 eV and their
electrical conductivity is 10−10 to 10−8 S/cm. The doping
process greatly increases the electric conductivities of such
polymers even for very low level of doping (<1%),
reaching several orders of magnitude up to ~0.1 S/cm. The
conductivity of various polymers increases to ~0.110
kS/cm after successive doping of conducting polymers [3].
Among many conducting polymers the PANI is
considered as the most promising one due to its unique
electrical and optical properties, in addition, its preparation
is relatively simple and has excellent environmental
stability. It is widely accepted that there are three redox
forms of PANI: leucoemeraldine, emeraldine and
pernigraniline, only one of PANIs' forms whose ability to
posses electric conductivity which is a protonated
emeraldine form. It is known that polaronic structure of
PANI-ES determines its electric conductivity. A wide
variety of PANI applications has been explored, including
solar cells, display devices, anticorrosion coatings and
chemical sensors [6]. Singlewalled carbon nanotube
(SWCNT) comprises a one graphite sheet in the form of a
tubular cylinder and has outstanding mechanical and
electrical properties. SWCNT are used with many
polyemers including polyanline to form nanocomposites
materials. CNT need to be distributed homogeneously in
the polymer matrix in order to fabricate a nanocomposites
of high-quality polymer-carbon with optimal performance
[7]. The dispersion and alignment of CNT in the matrix is
considered as a critical challenge in preparing a polymer
CNT composite with good processability characteristics,
this challenge arises from the fact that the van der waal
attraction among CNT often causes nanotubes to
agglomerate. Hence, they can not readily dispersed in the
polymer matrix. To overcome this problem, the
Functionalisation by chemical reactions with extended
molecular chains should be used [8]. The functionalized
CNT should be readily dispersible in organic solvents and
be more compatible with the polymer. The composites
with chemical CNT bonds are much better than that
mechanically mixed and have superior chemical and
electrical properties [9]. Many of the previous studies
prepared PANI/CNT thin films in different methods,
Shalini Nagabooshanam et. al., 2020 [10] fabrication
PANI/f-MWCNT thin films by electropolymerized
422 Tariq J. Alwan, Zain A. Muhammad
depostion and used for enzymatic detection of
organophosphates. Weiyu Zhang et, al,. 2020 [11]
fabrication PANI/MWCNT film by coating the ceramic
substrate with a layer of PANI/MWCNT paste using a thin
brush to form gases sensing film. Rawat Jaisutti et, al,.
2015 [12] prepared of PANi/MWCNT thin films by spin
coating technique and used it for alcohol sensors. Samir
Abdul Almohsin et, al,. 2012 [13] also used
electropolymerized methed to depostion PANI/MWCNT
thin films and investment in solar cell.
This paper presents the synthesis and characterization
of PANI (ES) and PANI/f-SWCNT thin films prepared by
in situ polymerization method. For enhancing SWCNT
and PANI interface, chemical functionalization of
SWCNT was used. The influence of f-SWCNT dopant
concentration on the structural, optical and electrical
properties of PANI was investigated.
2. Experimental
2.1. Functionalization of SWCNT
SWCNT was used as received. The concentrated
H2SO4 and HNO3 acids at ratio 3:1 were first utilized to
treat ultrasonically one gram of SWCNT at 50 oC for 4 h.
These acids enhance the solubility of SWCNT in HCl
solution and add to the defect sites some carboxylic acid
groups. When the mixture has reached to room
temperature by cooling process, it was centrifuged at 4000
rev/min in order to separate the treated SWCNT from
mixture, then a filter paper with 0.22 µm porous was used
for filtration. Deionized water was used to wash
thoroughly the filtrated solid until the acid was removed
completely. The filtered sample was placed in a furnace at
80 oC for 6 h to dry. The final product was named as f-
SWCNT.
2.2. Prepare PANI and PANI/f-SWCNT thin
films
Aniline monomer, distilled water, ammonium
persulfate (APS) and hydrochloric acid (HCL) were used
as received. PANI was chemically synthesized by in-situ
method by using aniline monomer and ammonium
persulfate ((NH4)2S2O8) as oxidant agent. 1 M of HCL
was placed in a 250 mL beaker, and during the
polymerization process the beaker was kept inside a vessel
containing an ice at a temperature of (0-5) °C. Separately,
50 mL of 1 M HCL was used to dissolve 15 g of APS
which is added as dropwise into the beaker that contains
the aniline acid solution for 5 min and under constant
stirring. Acouple of seconds later, the growth of
polyaniline with a green color was observed and the color
gradually spread into the aqueous solution which is
changed its color into dark green within 5 min. The
temperature of the reaction medium was maintained
throughout this whole process at the range (0-5) oC .
PANI thin films were produced on glass slides. Prior
to use the glasses, they washed with acetone and distilled
water via ultrasonic. To cover only one side of the glass
with PANI, an adhesive tape was adhered to the other side.
Using plastic clamps, the glasses was placed in the 250 ml
beaker after mixing the aniline monomer and oxidant
solutions, then all glasses taken out from the medium after
30 min from the start of reaction. It is observed that this is
the best time to obtain uniform thin films. To confirm that
the prepared thin films are all in the the emeraldine
oxidation state (EOS), it was necessary to place the
glasses again in a beaker which contains a solution of 2 ml
aniline in 100 ml of 1 M HCL in order to reduce any
polymer's oxidized pernigraniline form in EOS, at (0-5)
°C[14]. Afterwards, the precipitate of the adhering PANI
was removed by washing the glasses with 1 M HCl,
removed from the tape, washed with acetone ,and dried up
at ambient temperature. For preparing PANI/f-SWCNT
thin films, a different weight ratios of f-SWCNTs (0, 0.39,
0.5, 1 , 1.5 and 2) W% were ultrasonicated in 50 ml of 1 M
HCL for 30 min, then 5 ml of aniline monomer added to
the solution under constant stirring for 30 min. At a
reaction temperature of (05) oC and constant stirring the
APS was gradually poured dropwise into aniline/f-
SWCNT solution for 3 min. The rest of procedure of
obtained PANI/f-SWCNT thin films is similar to that of
PANI thin films. The measured thicknesses of the films
were about (200±20) nm. At normal incidence, spectrum
of transmittance and absorbance in the range (300900)
nm were utilized to carry out the optical measurements of
thin films prepared on glass slides, the type of UV-VIS
spectrophotometer used is (SHIMADZU)(UV-1600/1700
series). FTIR analysis was carried out by using SIDCO
England series FT-IR spectrometer over the range 650 to
2500 cm-1. X-ray diffraction (XRD) of type (SHIMADZU-
6000) was used to investigate the crystal structure of thin
films , the Bragg's angle 2θ was recorded in the range (10o
- 80o) with the CuKα source of wavelength λ = 1.5406Ǻ.
The field emission scanning electron microscopy
(TESCAN- MIRA3 -FESEM) was used to observe surface
morphology of prepared thin films.
3. Results and discussion
XRD was used to examine the crystallinity properties
of the neat PANI and PANI/f-SWCNT thin films as shown
in Fig. 1. The results show that the neat PANI sample
Fig. 1. XRD analysis of the neat PANI and
PANI/f-SWCNT thin films
10 20 30 40 50 60 70 80
Intensity (a.u.)
2
neat PANI
0.39 W %
0.5 W %
1 W %
1.5 W%
2 W %
Effect of f-SWCNT on the structure, electrical and optical properties of PANI thin films 423
PANI
0.34 W%
0.5 W%
1 W%
1.5 W%
2 W%
Fig. 2. FESEM images for the neat PANI and PANI/f-SWCNT thin films
424 Tariq J. Alwan, Zain A. Muhammad
is highly disordered with a broad diffraction peak around
2θ = 25o. Aslo it can be seen that the observed XRD
patterns of PANI/f-SWCNT thin films display an abroad
peak around = 23o. The latter observation suggest that
the amorphous structure of the neat PANI are not altered
by employing f-SWCNT in PANI matrix. The morphology
of prepared samples was examined by an excellent
technique which is FESEM. Fig. 2 shows FESEM images
for the neat PANI and PANI/f-SWCNT thin films
deposited on glass substrates at different weight ratios
(W%) of f-SWCNTs and there are two scale bars 500 nm
and 1µm for each image. It is clear from the images that
the method of in situ oxidative polymerization is a suitable
method to achieve a more homogeneous dispersion of f-
SWCNT in polymer matrix. Closer inspection of the
image of neat PANI, as shown in the image A, reveals that
it contains nanorod structure and the surfaces of these
nanorods are almost smooth, with the diameter is about
95.8 nm. After adding carbon nanotubes to PANI matrix,
as shown in the images B, C, D, E and F, it is noted that
the shapes of the nanorods begin to become clearer and
separated from each other and their dispersion in the
sample increase. In addition, The length and diameter of
the nanaorods start to increase, which become in the range
of 100-195 nm. The surfaces of these nanorods become
more roughness. This roughness is attributed to the aniline
monomer that polymerized uniformly on the surface of the
f-SWCNT and forms a tubular shell of PANI/f-SWCNT.
These results are a good agreement with a literature [15].
Conductivity measurements were carried out on the neat
PANI and PANI/f-SWCNT thin films using a two-point
probe method. The values of the conductivity are given by
the following equation:
=1/ (1)
where electrical conductivity and resistivity SWCNT
has a low critical concentration which is called as
percolation threshold (PT). PT is defined as a minimum
concentration required to make a conducting network
within the host material [16]. The parameters on which PT
depends are polymer type, nanotube type, tube size,
synthesis method and dispersion method [17]. Fig. 3
shows that the obtained results indicate that as the W% of
the f-SWCNT in the samples increases, the conductivity
increases and reaches roughly a constant value after the
concentration 0.5 W%, where the MWNTs dominate the
electrical transport. PT is about 0.39 W% which agrees
with a paper [16]. The values of the conductivity range
from 2.95 S/cm for neat PANI to 7.31 S/cm for 0.5 W% as
shown in the Table 1, this an increase provides a good
indication that the incorporation of f-SWCNT into PANI
was taken place. The increase of the free carrier density
cause an increase in the conductivity. f-SWCNT may
generate those free carries in PANI matrix and forms a
conducting network that leads to an increase in the current
[18].
Table 1. The conductivity and optical
energy gap values at different W% of f-SWCNT %
f-SWCNTs
(W %)
(300K)
S/cm
Eg (eV)
0
2.95
2.58
0.39
5.40
2.58
0.5
7.31
2.58
1
7.26
2.58
1.5
7.19
2.56
2
7.30
2.49
Optical absorption spectra are considered one of the
most important tools to compute the optical energy gap Eg
of organic and inorganic semiconductors. The electrical
conductivity and optical properties of PANI are specified
by optical energy gap, so it is of fundamental importance.
In many amorphous materials , it is found that the photon
absorption can be calculated by Tauc relation [19], which
is of the form:
αhν = B (hν − Eg)r
where α stands for the absorption coefficient, hv represents
photon energy, Eg is optical energy gap, B is a constant
depending on the material's properties and r is a constant
which can take various values that depends on the kind 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 r = 1/2. Fig. 4
illustrates the (αhv)2 as a function of photon energy (hv)
for direct allowed transition occuring in the neat PANI and
PANI/f-SWCNT thin films. The optical energy gap are
determined from this figure. It is observed that the optical
energy gap of neat PANI is 2.58 eV and remains nearly
the same at (0.39, 0.5 and 1 ) W% of f-SWCNT. However,
it is decreased significantly to 2.56 eV and 2.49 eV at (1.5
and 2) W% of f-SWCNT respectively, as shown in the
table 1. The decrease of optical band gap may be attribute
to the interaction of f-SWCNT with PANI that gives rise
Fig. 3. The Conductivity of PANI
and PANI/f-SWCNT thin films
Conductivity (S /cm )
f-SWCNT (W %)
Effect of f-SWCNT on the structure, electrical and optical properties of PANI thin films 425
to the modification of the polymer structure [20]. To
examine the molecules structure and their bonding, FTIR
spectroscopy which is a suitable selection for both organic
and inorganic materials was utilized. The unidentified
elements present in the sample can also be identified.
FTIR spectra of neat PANI and PANI/f-SWCNT thin films
in the region of (6502500) cm−1 were illustrated in Fig. 5.
The characteristic band peaks for neat PANI occur 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 [21, 22]. The stretching of C
N+• polaron structure appears at 1235 cm−1 which
confirms the presence of conducting form of doped PANI
[23]. The band 832 cm−1 corresponding to aromatic ring
out of plane deformation vibrations which belongs to C
H deformation in the para disubstituted ring [24]. The
band near 1450 cm-1 may be obscured by the aliphatic C
H deformation vibration, finally the band peak 1280 cm−1
is attributed to CH plane bending, The characteristic
peaks at 3422 cm-1 assigned to asymmetric N-H stretching
vibration [25,26]. In comparison neat PANI spectra, the
characteristic vibrational peaks of PANI are also appeared
in the PANI/f-SWCNT thin films. FTIR spectra and the
transmission peak intensities of PANI/f-SWCNT thin
films are lower than that of pure PANI, indicating that the
interaction between the f-SWCNT and the PANI matrix
was taken place [27].
Fig. 4. The variation of (αhv)2 vs. (hv) for a neat PANI and PANI /f-SWCNT thin films
Fig. 5. FTIR spectra of PANI and PANI/f-SWCNT thin films
0
2E+10
4E+10
6E+10
8E+10
1E+11
1.2E+11
0
5E+10
1E+11
1.5E+11
2E+11
2.5E+11
3E+11
3.5E+11
2.3 2.4 2.5 2.6 2.7 2.8 2.9 3
Photon Energy (eV)
PURE 0.5 W %
2 W % 0.39 W%
1.5 W% 1 W %
(αhv)2 (cm-1 eV)2
2 W%
1.5 W%
1 W%
0.5 W%
0.39 W%
Neat PANI
Wavenumber (cm-1)
T %
692
830
1235
1280
1450
1556
3422
426 Tariq J. Alwan, Zain A. Muhammad
4. Conclusion
The neat PANI and PANI/f-SWCNT thin films are
successfully deposited on a glass substrates by chemical
oxidation polymerization technique. The XRD pattern
revealed that the structure of neat PANI is an amorphous
and not altered by adding f-SWCNT to its matrix. The
FESEM images for neat PANI and PANI/f-SWCNT thin
films showed that both have nanorod structures and the
nanorod diameters of PANI/f-SWCNT thin films increased
as result of polymerization PANI on the surface of f-
SWCNT. The obtained value of PANI conductivity was
2.95 S/cm and turned out it is depend on the increase in
W% of f-SWCNT that changed its conductivity to the
roughly constant value of about 7.3 S/cm after 0.5 W%.
The optical energy gap is effected by the addition of the f-
SWCNT to PANI matrix, it was 2.58 eV for pure PANI
sample and reduced to 2.49 eV for the 2 W% of f-
SWCNT. 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-SWCNT, indicating that the
interaction between the f-SWCNT and the PANI matrix
was taken place. The optical and electrical results
indicated that neat PANI and PANI/f-SWCNT thin films
are a semiconductor and can be used to fabricate
optoelectronic devices.
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__________________
*Corresponding author: tariqjaffer2000@yahoo.com
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Conducting Polymers with Micro or Nanometer Structure describes a topic discovered by three winners of the Nobel Prize in Chemistry in 2000: Alan J. Heeger, University of California at Santa Barbara, Alan G. MacDiarmid at the University of Pennsylvania, and Hideki Shirakawa at the University of Tsukuba. Since then, the unique properties of conducting polymers have led to promising applications in functional materials and technologies. The book first briefly summarizes the main concepts of conducting polymers before introducing micro/nanostructured conducting polymers dealing with their synthesis, structural characterizations, formation mechanisms, physical and chemical properties, and potential applications in nanomaterials and nanotechnology. The book is intended for researchers in the related fields of chemistry, physics, materials, nanomaterials and nanodevices. Meixiang Wan is a professor at the Institute of Chemistry, Chinese Academy of Sciences, Beijing. © 2008 Tsinghua University Press, Beijing and Springer-Verlag GmbH Berlin Heidelberg.
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Raman spectroelectrochemical study has been done with electrochemically prepared films of polyaniline and a copolymer of polyaniline and metanilic acid using a green laser excitation (532nm). The experimental variables included solutions of different pH between 0.5 and 9.0, and varying electrode potential between 0.0 and 0.8V versus Ag/AgCl. Raman bands within the wavenumber limits of 500–1700cm−1 have been analysed, and their changes, proceeding with varying of electrode potential and solution pH, have been interpreted. It has been stressed that the spectral changes of polymer films proceed continuously rather than stepwise by changing the electrode potential. Considering leucoemeraldine and pernigraniline forms of polyaniline as fully reduced and oxidised structures, respectively, it could be concluded that many different redox forms can exist between these two limiting forms, rather than the only possible emeraldine form.
Article
An ‘in situ’ deposition method is presented where combination of solutions of aniline monomer and an oxidizing agent leads to the growth of uniform polyaniline (PAn) conducting thin films on submerged substrates. The PAn films can be deposited on a variety of substrates including glass and organic materials. Film thickness can be controlled by varying the duration of the substrate dipping time; about 300 Å was produced during a dipping time of 5 min. Atomic force microscopy (AFM) was used to analyze the PAn film surface morphology and roughness. AFM images of the PAn surfaces revealed very smooth surfaces having a surface roughness (mean and root mean square) of about 30 Å. Measurements on the PAn films using UV, visible and near IR were consistent with the thickness measurements obtained with AFM. The PAn film surface morphology, as determined from AFM images, was found to be characterized by particle-like features of about 50 to 100 nm in size which were packed tightly to produce a high density structure. Using a four-probe measurement approach, the conductivity of the doped PAn films was determined to be 2–6 S/cm.
Materials and Device Engineering for Efficient and Stable Polymer Solar Cells
  • R Hansson
R. Hansson, Materials and Device Engineering for Efficient and Stable Polymer Solar Cells, Ph.D. Thesis, Karlstad Univ., Sweden, 2017.
  • M T Stejskal
  • Irina Sapurina
M. T. Jaroslav Stejskal, Irina Sapurina, Progress in Polymer Science 35(1), 1420 (2010).
  • G X Chaoqing Bian
  • Yijun Yu
G. X. Chaoqing Bian, Yijun Yu, Journal of Applied Polymer Science 116(5), 2658 (2006).