indra bahadur thesis.pdf
TRANSCRIPT
SYNTHESIS AND CHARACTERIZATION
OF PURE AND DOPED CONDUCTING AND
SEMICONDUCTING MATERIALS
THESIS
SUBMITTED TO THE
UNIVERSITY OF LUCKNOW
FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
IN
PHYSICS
UNDER THE SUPERVISION OF
Prof. R. K. Shukla
BY
INDRA BAHADUR M.Sc. (Physics)
DEPARTMENT OF PHYSICS
UNIVERSITY OF LUCKNOW
LUCKNOW 226007 – INDIA
JANUARY-2015
CONTENTS
CERTIFICATE
Page No.
ACKNOWLEDGEMENTS i
SUMMARY iii-vi
Chapter 1 Introduction 1-50
1.1 Materials 1
1.1.1 Classification of materials 2
1.1.2 Conducting and Semiconducting materials 5
1.2 Materials used as dopant (Al2O3, CuO, TiO2) 9
1.3 Conducting Polymers (Polythiophene,
Polyaniline, Polypyrrole)
13
1.3.1 Historical developments 15
1.3.2 Doping in conducting polymers 16
1.3.3 Metal-insulator transition in doped conducting
polymers
21
1.3.4 Importance of Conducting Polymers as
Sensors
26
1.3.5 Ionization in Conducting Polymers 27
1.3.6 Bulk and nanopolymer 28
1.3.7 Applications of Polymer 29
1.4 Synthesis of Polymers and Polymer
Composites
29
1.4.1 Preparation of Thin films 30
1.4.2 Sol-gel Process 32
1.4.3 Sol-gel spin coating 34
1.4.4 Spray pyrolysis 35
1.4.5 Dip Coating film 37
1.5 Characterization Techniques 38
1.5.1 X-Ray Diffraction (XRD) 39
1.5.2 Scanning Electron Microscopy (SEM) 40
1.5.3 Fourier Transform Infrared Spectroscopy 43
(FTIR)
1.5.4 UV-visible Spectroscopy (UV-vis) 44
1.5.5 Photoluminescence Spectroscopy(PL) 45
1.6 Objective of present work 48
1.7 Organization of present work
49
Chapter 2 Synthesis and Characterization of Undoped
and Al2O3 Doped Polythiophene
Nanocomposites
51-65
2.1 Introduction 51
2.2 Experimental 53
2.2.1 Chemicals 53
2.2.2 Sample preparation 53
2.2.3 Characterizations 54
2.3 Results and discussion 54
2.3.1 Structural study 54
2.3.2 Morphological study 55
2.3.3 Optical properties 60
2.4 Conclusion
65
Chapter 3
Synthesis and Characterization of
Chemically Synthesized Undoped and CuO
Doped Polyaniline Nanocomposites
66-80
3.1 Introduction 66
3.2 Experimental 68
3.2.1 Pellet preparation 68
3.2.1 Characterizations 68
3.3 Results and discussion 69
3.3.1 X-ray Diffraction 69
3.3.2 Scanning Electron Microscopy 72
3.3.3 Optical Properties 74
3.4 Conclusion
80
Chapter 4
Synthesis and Characterization of Undoped
and Al2O3 Doped Polypyrrole
Nanocomposites
81-93
4.1 Introduction 81
4.2 Experimental 83
4.2.1 Synthesis of Polypyrrole 83
4.2.2 Characterizations 83
4.3 Results and discussion 84
4.3.1 X-Ray Diffraction (XRD) 84
4.3.2 Scanning Electron Microscopy (SEM) 85
4.3.3 Optical Properties 88
4.4 Conclusion
93
Chapter 5
Structural, Morphological and Optical
Studies of Undoped and Al2O3 Doped
Polyaniline Thin Films
94-108
5.1 Introduction 94
5.2 Experimental 96
5.2.1 Synthesis of Polyaniline 96
5.2.2 Characterizations 97
5.3 Results and discussion 98
5.3.1 X-ray diffraction (XRD) 98
5.3.2 Scanning Electron Microscopy (SEM) 100
5.3.3 Optical Properties 102
5.4 Conclusion
108
Chapter 6
Structural, Morphological and Optical
Studies of Undoped and TiO2 Doped
Polypyrrole Thin Films
109-124
6.1 Introduction 109
6.2 Experimental 112
6.2.1 Sample preparation 112
6.2,2 Characterizations 113
6.3 Results and discussion 114
6.3.1 Structural Analysis 114
6.3.2 Scanning Electron Microscopy (SEM) 115
6.3.3 Optical Properties 118
6.4 Conclusion
124
Chapter 7
Conclusion 125-130
References 131-152
Laboratory Instruments 153-161
i
ACKNOWLEDGEMENTS
“I am the only one, But still I am one.
I cannot do everything, But still I can do something.
I will not refuse to do, The something I can do.”
During the entire course of time, I was assisted by many people
and it is indeed my pleasure to express my gratitude for all of them.
At this fortuitous moment, I take this opportunity to express my
profound gratitude to my esteemed supervisor Prof. R. K. Shukla,
Department of Physics for his exemplary guidance, monitoring and
constant encouragement throughout the course of this thesis. His patience
and support rescued me from despair on countless occasions. The help,
advice, guidance and blessing given by him time to time shall carry me
long way in the journey of life.
I gratefully acknowledge the advice and guidance of Prof. Kirti
Sinha, Head, Department of Physics, Prof U. D. Misra, University of
Lucknow, without which this assignment would not have been possible.
I also take this opportunity to express enormous thanks to Prof.
Anchal Srivastava, Department of Physics for her cordial support,
providing characterizations facility, valuable information and guidance
which helped me in completing this task through various stages.
I also take this opportunity to express enormous thanks to Sri
Tarun Gauba (IPS), ADC to Governor UP, Sri Indra Jeet Singh Rawat
(C.S.O. Rajbhawan) and all my senior’s and well wishers from
Rajbhawan, Lucknow for their cordial support.
I express my sincere thanks to my colleague Dr. Akhilesh Tripathi,
Mr. Nishant Pandey, Dr. Sheo kumar Mishra, Mr. Susheel Singh and Mr.
Arvind Verma, Ms. Divyanshi and Mr. Vijendra for their cooperation and
ii
willingness to share their knowledge to understand and complete this
task.
I would like to thank the members of Departments of Physics,
University of Lucknow. Thanks to all of my friends, especially
Dharmendra Pratap Singh, Anoop, Abhishek, Dr. Puneet Pandey and Mr.
Vrijesh Pandey for their encouragement.
Last, but certainly not least, my deepest thanks go to my mother
Smt. Jadawati Devi, my wife Mrs Saroja Yadav and my children, younger
brother Mahendra Yadav and my heartiest friends Dr. R. K. Mishra, Dr.
M. L. Pal and Sri C. J. Yadav for supporting and encouraging me to
achieve this goal and be successful. I am forever grateful to them for
showing me the value of education.
Lastly, and most importantly, I am grateful to the almighty GOD
without whom this assignment would not be possible.
In the end of my acknowledgement, I would like to convey my
message to the next generation that I have learned during the Ph.D.
program, communicating by following lines of Confucius :-
“To be able under all circumstances to practice five things
constitutes perfect virtue; these five things are gravity, generosity of soul,
sincerity, earnestness and kindness.”
(Indra Bahadur)
iii
SUMMARY
Chapter 1 deals with the brief introduction of materials, their types and
applications. In this chapter, we have discussed about basics of materials
and three conducting polymers including polythiophene (PTh),
polyaniline (PAni) and polypyrrole (PPy) and their metal
nanocomposites. The metal dopants such as Al2O3, CuO and TiO2 have
been discussed in the present work. It also deals the discussion of
deposition techniques of the thin films as well as characterization
techniques. It also contains the organization and objective of thesis.
In Chapter 2 undoped and Al2O3/polythiophene nanocomposites have
been synthesized by chemical oxidation method. All the samples are
characterized by X-ray diffraction (XRD), Scanning electron microscopy
(SEM), Ultraviolet visible Spectroscopy (UV-vis), Photoluminescence
(PL) spectra and Fourier transform infra red (FTIR) spectroscopy. XRD
spectra show the polycrystalline nature of all the samples. SEM images
are indicating formation of spherical shape of nanostructures. As-
synthesized samples of undoped polythiophene and Al2O3/PTh
nanocomposites exhibit many pores on the surface of nanostructures.
Synthesis of Al2O3 polythiophene composite material is confirmed by
FTIR spectroscopy. UV-visible absorption spectra show absorption peak
at around 300 nm which is due to π- π* inter-band-transition of PTh rings.
A small change in optical absorption spectra is observed which can be
associated with the degree of oxidation. PL spectra exhibit mainly three
visible emission peaks at around 462 nm, 490 nm and 522 nm. The two
emission peaks 462 nm and 490 nm in the Soret band region whereas
single peak at 522 nm in the Q band emission. The intensity and peak
position of polythiophene have been randomly changed with amount of
Al2O3 dopant.
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Chapter 3 deals with synthesis of undoped and CuO/PAni
nanocomposite by the chemical oxidation method at room temperature
and their characterizations. The prepared samples have been
characterized by XRD, SEM, FTIR, UV- visible and photoluminescence
spectroscopy. XRD spectra show weak crystalline quality of all the
samples, whereas the PAni synthesized is amorphous in nature. The
scanning electron microscopy images of all the samples show granular
coral like structure. The study of FTIR spectra confirm the formation of
conducting PAni and also suggests that doping of CuO in PAni does not
affect the structures. The UV–visible absorption spectra of the solutions
of all the samples contain some peak at 305 nm. The observed
bathochromic shift at the intense absorption band 305 nm is due to the π-
π* transition of benzenoid ring. The PL spectra of 0, 2, 4, 6 and 8 wt%
CuO/PAni samples show peaks in visible emission region which at
around 362 nm, 405 nm in violet region 459 nm, 486 nm in blue region
and 528 nm in green region.
In Chapter 4, we have synthesized undoped and Al2O3 doped PPy
samples by chemical oxidation method. The prepared samples have been
characterized by XRD, SEM, FTIR, UV-Vis and PL. X-ray diffraction
patterns of PPy/Al2O3 nanocomposites result show several broad peaks
while undoped sample shows only one single peak indicating poor
crystalline phase of PPy. In the SEM images, the results were found
granular coral like structures. As a characteristic of Polypyrrole,
secondary nucleation also takes place because of which the granular coral
like particles come together to form aggregates. We noticed that as the
amount of Al2O3 was increased; the number of pores and the size of pores
were also increased, which is very important for sensing. The study of
FTIR spectra confirms the formation of PPy and also suggests that doping
v
of Al2O3 in PPy does not affect its structure. The UV absorption can
significantly determine the interaction between the Al2O3 and PPy.
Solutions of all the samples show peak, which oriented around 306 nm.
The peak at 306 nm is associated with the exciton transition of π–π*. PL
shows the main emission band of the nanocomposites, located at 365 nm
with two shoulders at 473 and 533 nm. The direct band gap energies of
the PPy/Al2O3 nanocomposite of different ratios are found as 3.09 and
2.19 eV. The band gap gets decreased due to increased content of Al2O3
nano particles.
In chapter 5, we have synthesized undoped and Al2O3 doped PAni
samples by the chemical oxidation method. The prepared samples have
been characterized by XRD, SEM, FTIR, UV-Vis and PL. The XRD
spectra shows a peak around 25o which confirm the synthesis of PAni and
another peak at 55.08o
for 8 wt% Al2O3 doped PAni which as the
confirmations of successful doping in PAni. The study of FTIR spectra
confirms the formation of PAni and also suggests that doping of Al2O3 in
PAni does not affect its structure. The SEM images of all the samples
show coral like structure. UV spectra show single broad peak at around
305 nm and small peak at around 450 nm. The peak 305 nm is associated
with the exciton transition of π-π*. The longer wavelength peak at around
450 nm can be associated to the transition between benzenoid to quinoid
rings. PL spectra recorded with excitation wavelength 325 nm show a
strong UV peak at 384 nm with weak visible peak at 484 nm and 527 nm.
In Chapter 6, we have prepared undoped PPy and TiO2 doped PPy thin
films by sol-gel spin coating method. The prepared samples have been
characterized by XRD, SEM, FTIR, UV-Vis and PL. XRD spectra show
the weak crystalline quality of all the samples. SEM images show the
sphere shape of nanostructures. The amount of TiO2 doping increases the
vi
number of pores as well as size of the pores that play a very important
role in sensing of gas. The study of FTIR spectra confirms the formation
of conducting PPy which suggests that doping of TiO2 in PPy does not
affect its structures. All the samples of PPy and PPy/TiO2 nanocomposites
thin films show the peak at 309 nm which is assigned to the π-π*
transition or the excitation transition. The PL spectra of PPy and TiO2
doped PPy show three main peaks, first is in UV region around at 368
nm, second broad peak in visible region around 480 nm and another sharp
peak at around 530 nm in green region.
Chapter 7 deals with the conclusion of all the work and the
recommendations for further work.
1
Chapter 1
Introduction
The entire work, presented here, for the doctoral degree in physics is
focused on material science. Material science is an interdisciplinary field
which deals with the discovery and design of new materials. It is also
known as material science and engineering because it involves the study
of materials through the material paradigm (synthesis, structure,
properties and performance), its intellectual origins reach back to the
emerging fields of chemistry, mineralogy and engineering during the
enlightenment. In recent years, material science has become more widely
known as a specific field of science and engineering dedicated for the
synthesis, characterization and applications of new materials.
1.1 Materials
A material is defined as a substance (most often a solid, but other
condensed phases can be included) that is intended to be used for certain
applications. Raw materials are the pristine state of materials which are
metallurgically refined for specific use or application. Materials Science
involves the study of the relationships between the synthesis, processing,
structure, properties, and performance of materials that enable an
engineering function. It incorporates elements of physics and chemistry
and is at the forefront of nanoscience and nanotechnology research.
There are several types of materials around us as they can be found in
anything from buildings to spacecrafts. Materials can generally be
divided into two classes: crystalline and non-crystalline materials. The
traditional examples of materials are metals, ceramics and polymers. New
and advanced materials that are being developed include semiconductors,
nanomaterials and biomaterials etc [1].
2
Basically the materials science involves in studying the structure of
materials and relating them to their properties including electrical, optical
and electronic properties. Once material scientists know about their
structural property correlation, they can go onto study the relative
performance of a material in a certain applications. The major
determinants of the structure of a material and thus of its properties are its
constituent chemical elements and the way in which it has been processed
into its final form. These characteristics, taken together and related
through the laws of thermodynamics and kinetics, govern a
material‟s structure and thus its properties. The formation of materials for
a specific use from its raw state is shown in Fig. 1.1.
1.1.1 Classification of Materials
Materials are classified into two classes:
(a) Crystalline materials
(b) Non-crystalline materials
Crystalline materials are the materials which contain long range order
between structural units whereas in the non-crystalline materials, the long
range order is found to be absent. A crystal or crystalline solid is
a solid material whose constituents, such as atoms, molecules or ions, are
arranged in a highly ordered microscopic structure, forming a crystal
lattice that extends in all directions. In addition, macroscopic single
crystals are usually identifiable by their geometrical shape, consisting of
flat faces with specific, characteristic orientations. Examples of large
crystals include snowflakes, diamonds and tablesalt. Most inorganic
solids are not crystals but polycrystals, i.e. many microscopic crystals
fused together into a single solid. Examples of polycrystals include most
metals, rocks, ceramics and ice.
3
Fig. 1.1: The formation of material for a specific use from its raw state.
A third category of solids is amorphous solids, where the atoms have no
periodic structure whatsoever. Examples of amorphous solids
include glass, wax and many plastics.
Polycrystalline materials are solids that are composed of
many crystallites of varying size and orientation. Crystallites are also
referred to as grains. They are small or even microscopic crystals and
form during the cooling of many materials. Their orientation can be
random with no preferred direction, called random texture, or directed,
possibly due to growth and processing conditions. Fibre texture is an
example of the latter. The areas where crystallite grains meet are known
as grain boundaries.
Most inorganic solids are polycrystalline, including all common metals,
many ceramics, rocks and ice. The extent to which a solid is crystalline
(crystallinity) has important effects on its physical properties. Sulfur,
4
while usually polycrystalline, may also occur in other allotropic forms
with completely different properties. Although crystallites are referred to
as grains, powder grains are different, as they can be composed of smaller
polycrystalline grains themselves. While the structure of a
(monocrystalline) crystal is highly ordered and its lattice is continuous
and unbroken, amorphous materials, such as glass and polymers, are non-
crystalline and do not display any structures as their constituents are not
arranged in an ordered manner. Polycrystalline structures
and polycrystalline phases are in between these two extremes.
In condensed matter physics and material science, an amorphous or non-
crystalline solid is a solid that lacks the long-range order characteristic of
a crystal. In some older books, the term has been used synonymously
with glass. Now days, amorphous solid is considered to be the over-
arching concept and glass the more special case. A glass is an amorphous
solid that exhibits a glass transition [2]. Polymers are often amorphous.
Other types of amorphous solids include gels, thin films, and
nanostructured materials. Fig. 1.2 differentiates between crystalline,
polycrystalline and amorphous materials. Some of the crystal is
composed of many small grains, if the arrangements between the grains
are no rules, this is called polycrystalline crystal, such as copper and iron.
But there are also the crystal itself is a complete large grains, the crystal
is called single crystal, crystal and crystal diamond. A metallic system
can be made amorphous by decreasing the chance of crystallization:–
Allow less time for crystallization during solidification – Rapid
solidification processing (RSP).
5
Fig.1.2: Differentiation between crystalline, polycrystalline and
amorphous materials.
Generally, materials are inorganic including metals, ceramics as well as
organic materials include polymers etc. Some advance and important
organic and inorganic materials that are synthesized in the present time in
form of bulk, nanomaterials and biomaterials, etc for new industrial
applications.
1.1.2 Conducting and semiconducting materials
Conductor is an object or type of material that allows the flow
of electrical current in one or more directions. For example, a wire is an
electrical conductor that can carry electricity along its length. In metals
such as copper or aluminium, the movable charge particles are electrons.
Positive charges may also be mobile, such as the cationic electrolyte(s) of
a battery, or the mobile protons of the proton conductor of a fuel
cell. Insulators are non-conducting materials such as diamonds, glass, etc.
Copper has a high conductivity. Annealed copper is the international
6
standard to which all other electrical conductors are compared. The main
grade of copper used for electrical applications, such as building
wire, motor windings, cables and busbars. Silver is more conductive than
copper, but due to cost it is not practical in most cases. However, it is
used in specialized equipment, such as satellites, and as a thin plating to
mitigate skin effect losses at high frequencies.
Aluminium wire, which has 61% of the conductivity of copper, has been
used in building wiring for its lower cost. By weight, aluminum has
higher conductivity than copper, but it has properties that cause problems
when used for building wiring. It can form a resistive oxide within
connections that makes wiring terminals heat. Aluminum can "creep,"
slowly deforming under load, eventually causing device connections to
loosen, and also has a different coefficient of thermal
expansion compared to materials used for connections. This accelerates
the loosening of connections. These effects can be avoided by using
wiring devices approved for use with aluminium. Aluminium wires used
for low voltage distribution, such as buried cables and service drops,
require use of compatible connectors and installation methods to prevent
heating at joints. Aluminum is also the most common metal used in high-
voltage transmission lines, in combination with steel as structural
reinforcement. Anodized aluminum surfaces are not conductive. This
affects the design of electrical enclosures that require the enclosure to be
electrically connected.
Organic compounds such as octane, which has 8 carbon atoms and 18
hydrogen atoms, cannot conduct electricity. Oils are hydrocarbons, since
carbon has the property of tetracovalency and forms covalent bonds with
other elements such as hydrogen, since it does not lose or gain electrons,
thus does not form ions. Covalent bonds are simply the sharing of
7
electrons. Hence, there is no separation of ions when electricity is passed
through it. So the liquid (oil or any organic compound) cannot conduct
electricity. While pure water is not an electrical conductor, even a small
portion of impurities, such as salt, can rapidly transform it into a
conductor.
A semiconductor material has an electrical conductivity value between
a conductor such as copper and an insulator such as glass.
Semiconductors are the foundation of modern electronics. The modern
understanding of the properties of a semiconductor relies on quantum
physics to explain the movement of electrons and holes in a crystal
lattice. An increased knowledge of semiconductor materials and
fabrication processes has made possible continuing increases in the
complexity and speed of microprocessors and memory devices.
The electrical conductivity of a semiconductor material increases with
increasing temperature, which behaviour is opposite to that of a
metal. Semiconductor devices can display a range of useful properties
such as passing current more easily in one direction than the other,
showing variable resistance, and sensitivity to light or heat. Because the
electrical properties of a semiconductor material can be modified by
controlled addition of impurities or by the application of electrical fields
or light, devices made from semiconductors can be used for
amplification, switching, and energy conversion [3]. Current conduction
in a semiconductor occurs through the movement of free electrons and
holes, collectively known as charge carriers. Adding impurity atoms to a
semiconducting material, known as doping, greatly increases the number
of charge carriers within it. When a doped semiconductor contains mostly
free holes it is called p-type, and when it contains mostly free electrons it
is known as n-type. The semiconductor materials used in electronic
8
devices are doped under precise conditions to control the location and
concentration of p-type and n-type dopants. A single semiconductor
crystal can have many p-type and n-type regions; the p–n
junctions between these regions are responsible for the useful electronic
behaviour. Some of the properties of semiconductor materials were
observed throughout the mid 19th
and first decades of the 20th century.
Development of quantum physics in turn allowed the development of
the transistor in 1948. Although some pure elements and many
compounds display semiconductor properties, silicon, germanium, and
compounds of gallium are the most widely used in electronic devices.
Semiconductors are defined by their unique electric conductive behavior,
somewhere between that of a metal and an insulator. The differences
between these materials can be understood in terms of the quantum
states for electrons, each of which may contain zero or one electron (by
the Pauli Exclusion Principle). These states are associated with
the electronic band structure of the material. Electrical conductivity arises
due to the presence of electrons in states that are delocalized (extending
through the material), however in order to transport electrons a state must
be partially filled, containing an electron only part of the time [4]. If the
state is always occupied with an electron, then it is inert, blocking the
passage of other electrons via that state. The energies of these quantum
states are critical, since a state is partially filled only if its energy is near
the Fermi level.
High conductivity in a material comes from it having many partially
filled states and much state delocalization. Metals are good electrical
conductors and have many partially filled states with energies near their
Fermi level. Insulators, by contrast, have few partially filled states, their
Fermi levels sit within band gaps with few energy states to occupy.
9
Importantly, an insulator can be made to conduct by increasing its
temperature: heating provides energy to promote some electrons across
the band gap, inducing partially filled states in both the band of states
beneath the band gap (valence band) and the band of states above the
band gap (conduction band). An intrinsic semiconductor has a band gap
that is smaller than that of an insulator and at room temperature
significant numbers of electrons can be excited to cross the band gap.
A pure semiconductor, however, is not very useful, as it is neither a very
good insulator nor a very good conductor. However, one important
feature of semiconductors (and some insulators, known as semi-
insulators) is that their conductivity can be increased and controlled
by doping with impurities and gating with electric fields. Doping and
gating move either the conduction or valence band much closer to the
Fermi level, and greatly increase the number of partially filled states.
Some wider-band gap semiconductor materials are sometimes referred to
as semi-insulators. When undoped, these have electrical conductivity
nearer to that of electrical insulators; however they can be doped (making
them as useful as semiconductors). Semi-insulators find suitable
applications in micro-electronics. An example of a common semi-
insulator is gallium arsenide [5]. Some materials, such as titanium
dioxide, can even be used as insulating materials for some applications,
while being treated as wide band-gap semiconductors for other
applications.
1.2 Materials used as dopant:
In the present work several oxide materials has been incorporated in the
host materials such as Al2O3, CuO and TiO2. Aluminium oxide is
a chemical compound of aluminium and oxygen with the chemical
formula Al2O3. It has a wide band-gap of ~8.8 eV for bulk material.
10
Al2O3 has been extensively investigated dopants to serve as catalysts, fire
redundant, absorbents and fillers for structural materials [6]. It is stable in
acidic and oxidative mediums and well known for reactivity with
aromatic organic materials.
It is the most commonly occurring of several aluminium oxides, and
specifically identified as aluminium (III) oxide. It is commonly
called alumina, and may also be called aloxide or alundum depending on
particular forms or applications. It commonly occurs in its crystalline
polymorphic phase α-Al2O3, in which it comprises the mineral corundum,
varieties of which form the precious gemstones ruby and sapphire.
Al2O3 is significant in its use to produce aluminium metal, as
an abrasive owing to its hardness, and as a refractory material owing to its
high melting point.
Al2O3 is an electrical insulator but has a relatively
high thermal conductivity around 30 Wm−1
K−1
[6] for a ceramic material.
Aluminium oxide is insoluble in water. It is usually found in crystalline
form, called corundum or α-aluminium oxide, its hardness makes it
suitable for use as an abrasive and as a component in cutting tools.
Aluminium oxide is responsible for the resistance of metallic aluminium
to weathering. Metallic aluminium is very reactive with atmospheric
oxygen, and a thin passivation layer of aluminium oxide (4 nm thickness)
forms on any exposed aluminium surface [6]. This layer protects the
metal from further oxidation. The thickness and properties of this oxide
layer can be enhanced using a process called anodising. A number
of alloys, such as aluminium bronzes, exploit this property by including a
proportion of aluminium in the alloy to enhance corrosion resistance. The
aluminium oxide generated by anodising is typically amorphous, but
discharge assisted oxidation processes such as plasma electrolytic
11
oxidation result in a significant proportion of crystalline aluminium oxide
in the coating, enhancing its hardness.
Copper (II) oxide or cupric oxide (CuO) is the higher oxide of copper. As
a mineral, it is known as tenorite. It is a black solid with an ionic structure
which melts above 1200 °C with some loss of oxygen. It can be formed
by heating copper in air:
2 Cu + O2 → 2 CuO
Copper (II) oxide belongs to the monoclinic crystal system, with
a crystallographic point group of 2/m or C2h. The space group of its unit
cell is C2/c, and its lattice parameters are a = 4.6837, b = 3.4226, c =
5.1288, α = 90°, β = 99.54 °, γ = 90°. The copper atom is coordinated by
four oxygen atoms in an approximately square planar configuration [7].
Cupric oxide is used as a pigment in ceramics to produce blue, red, and
green (sometimes gray, pink, or black) glazes. It is also used to
produce cuprammonium hydroxide solutions, used to make rayon. It is
also occasionally used as a dietary supplement in animals, against copper
deficiency. Copper (II) oxide has application as a p-type semiconductor,
because it has a narrow band gap of 1.2-1.8 eV. It is an abrasive used to
polish optical equipment. Cupric oxide can be used to produce dry cell
batteries. It has been used in wet cell batteries as the cathode, with
lithium as an anode, and dioxalane mixed with lithium perchlorate as the
electrolyte. Copper (II) oxide can be used to produce other copper salts. It
is used in welding with copper alloys.
Titanium dioxide is known as titanium (IV) oxide or titania. It is the
naturally occurring oxide of titanium and its chemical formula is TiO2. It
has a wide bandgap of ~3.2 eV for bulk material. Generally it is sourced
from ilmenite, rutile and anatase. It has a wide range of applications, from
12
paint to sunscreen to food colouring. When used as a food colouring. The
most important application areas are paints and varnishes as well as paper
and plastics, which account for about 80% of the world's titanium dioxide
consumption. Other pigment applications such as printing inks, fibers,
rubber, cosmetic products and foodstuffs account for another 8%. The
rest is used in other applications, for instance the production of technical
pure titanium, glass and glass ceramics, electrical ceramics, catalysts,
electric conductors and chemical intermediates. It is in most red-coloured
candy.
Titanium dioxide is the most widely used as white pigment because of its
brightness and very high refractive index, in which it is surpassed only by
a few other materials. Approximately 4.6 million tons of pigmentary,
TiO2 are used annually worldwide and this number is expected to
increase as utilization continues to rise. When deposited as a thin films,
its refractive index and colour make it an excellent reflective optical
coating for dielectric mirrors and some gemstones like mystic fire topaz.
In paint application, it is often referred to off handedly as the perfect
white, the whitest white or other similar terms. Opacity is improved by
optimal sizing of the titanium dioxide particles. Some grades of titanium
based pigments as used in sparkly paints, plastics, finishes
and pearlescent cosmetics are man-made pigments whose particles have
two or more layers of various oxides–often titanium dioxide, iron
oxide or alumina in order to have glittering, iridescent and or pearlescent
effects similar to crushed mica or guanine-based products. In addition to
these effects a limited colour change is possible in certain formulations
depending on how and at which angle the finished product is illuminated
and the thickness of the oxide layer in the pigment particle; one or more
colours appear by reflection while the other tones appear due to
13
interference of the transparent titanium dioxide layers [8].
In some
products, the layer of titanium dioxide is grown in conjunction with iron
oxide by calcination of titanium salts (sulphates, chlorates) around 800 °C
[8] or other industrial deposition methods such as chemical vapour
deposition on substrates such as mica platelets or even silicon dioxide
crystal platelets of no more than 50 µm in diameter. The iridescent effect
in these titanium oxide particles (which are only partly natural) is unlike
the opaque effect obtained with usual ground titanium oxide pigment
obtained by mining, in which case only a certain diameter of the particle
is considered and the effect is only due to scattering. In ceramic
glazes titanium dioxide acts as an opacifier and seeds crystal formation.
Titanium dioxide has been shown statistically to increase skimmed milk's
whiteness, increasing skimmed milk's sensory acceptance score [9].
Titanium dioxide is used to mark the white lines of some tennis courts.
In the present work, various conducting and semiconducting materials
such as Al2O3, CuO and TiO2 have been used as dopant in the host
conducting polymers such as polythiophene (PTh), polyaniline (PAni)
and polypyrrole (PPy) for the synthesis of conducting polymer
composites.
1.3 Conducting Polymer
The word “polymer” is derived from ancient Greek word (poly means
"many" and mer means "parts"). Polymers are large molecules, or
macromolecules, composed of many repeated subunits. Because of their
broad range of properties, both synthetic and natural polymers play an
essential and ubiquitous role in everyday life. Polymers range from
familiar synthetic plastics such as polystyrene to natural biopolymers
such as DNA and proteins that are fundamental to biological structure
14
and function. Polymers, both natural and synthetic, are created via
polymerization of many small molecules, known as monomers. Their
consequently large molecular mass relative to small molecule compounds
produces unique physical properties, including toughness, visco-
elasticity, and a tendency to form glasses and semi-crystalline structures
rather than crystals.
Materials are generally classified into three types as insulators,
semiconductors and conductors based on their electrical properties. A
material with conductivity less than 10-7
S/cm is regarded as an insulator.
Metals have conductivity larger than 103 S/cm whereas the conductivity
of a semiconductor varies from 10-4
to 10 S/cm depending upon the
degree of doping. It is generally believed that plastics (polymers) and
electronic conductivity are mutually exclusive and the inability of
polymers to carry electricity notable them from metals and
semiconductors. Polymers are traditionally used as inert, insulating and
structural materials in many applications such as packaging, electrical
insulations and textiles.
Intrinsically conducting polymers (CPs) are different from other
conducting polymers in which a conducting material including
metal/carbon powder is dispersed in a non-conductive polymer [10].
These polymers are referred as conjugated polymers belong to a totally
different class of polymeric materials with alternate single-double or
single-triple bonds in their main chain and are capable of conducting
electricity when it is doped. Intrinsically CPs, similar to other organic
polymers, usually described by sigma (σ) and pi (π) bonds. While the σ
electrons are fixed and immobile due to the formation of covalent bonds
between the carbon atoms, the remaining π -electrons which can be easily
delocalized upon doping. Fig. 1.3 shows the molecular structures of
popular intrinsic conducting polymers such as polythiophene (PTh),
15
polypyrrole (PPy) and polyaniline (PAni) that have been used as a host
material in the present thesis work.
Polyaniline (PAni)
Fig. 1.3: Molecular structures of conjugated polymers such as
polythiophene (PTh), polypyrrole (PPy) and polyaniline (PAni).
1.3.1 Historical developments
Although polymeric materials have been used by mankind since
prehistoric times in the form of wood, bone, skin, and fibers, the
existence of macromolecules was accepted only after Hermann
Staudinger developed the concept of macro-molecules during the 1920s,
which got him the Nobel Prize in Chemistry in 1953 „„for his discoveries
in the field of macro-molecular chemistry‟‟ [11]. The research field of
conjugated (conducting) polymers came into spotlight with the
preparation of polyacetylene by Shirakawa and coworkers along with the
subsequent discovery of enhancement in its conductivity after doping
[12-15]. Some of the most important representatives in the family of
conjugated polymers in non-conducting as well as conducting forms viz.,
16
polythiophene (PTh), polyaniline (PAni) and polypyrrole (PPy) were
already being prepared chemically or electrochemically in the nineteenth
century. The existence of Polyaniline (PAni) in four oxidation states was
also recognized. A reaction scheme for the electro-oxidation of aniline at
a carbon electrode was suggested by Yasui in 1935 [16]. It was almost a
century after Letheby‟s observations that Mohilner and coworkers
reinvestigated the mechanism of the electro-oxidation of aniline in
aqueous sulphuric acid solution at a platinum electrode and characterized
polyaniline (PAni) [17]. The first real breakthrough came in 1967, when
Buvet and his group established that polyanilines are redox active
electronic conductors and PAni pellets can be used as electrodes for
conductivity measurements [18-19].
Polypyrrole (PPy), on the other hand, was known as pyrrole black and
was formed due to the oxidation of pyrrole in air. PPy is an inherently
conducting polymer with interesting electrical properties first discovered
and reported in the early 1960s [20]. It was followed by the preparation of
coherent and free standing polypyrrole films by electrochemical
polymerization by Diaz and his co-workers [21].
1.3.2 Doping in conducting polymers
Doping in conjugated organic polymers is responsible for the great
scientific and technological importance achieved by these materials since
their discovery in 1977. The concept of doping is the unique and main
issue that unites all the conducting polymers and differentiates them from
all other types of polymers [13, 14]. During the doping process, an
organic polymer such as an insulating or semi-conducting polymer could
be converted into electronic polymers exhibiting metallic conductivity
(1–105 S/cm). The concept of doping in conducting polymers is much
17
different than that in case of inorganic semiconductors. Fig. 1.4 shows a
schematic illustration to explicit the difference between the doping
mechanisms in inorganic semiconductors and conjugated polymers. In
semiconductor physics, doping describes a process where dopant content
present in small quantities occupy positions within the lattice of the host
material, resulting in a large-scale change in the conductivity of the doped
material compared to the undoped one.
The doping process in conjugated polymers is, however, essentially a
charge transfer reaction, resulting in the partial oxidation (or less
frequently reduction) of the polymer. Unlike inorganic semi-conductors,
doping in conjugated polymers is reversible in a way that upon de-doping
the original polymer can be retained with almost no degradation of the
polymer backbone. Another very important difference between the
doping in conjugated polymers and that in inorganic semiconductors is
that doping in conjugated polymers is interstitial whereas in inorganic
semiconductors the doping is substitutional.
One can easily obtain a conductivity anywhere between that of the
undoped (insulating or semiconducting) and that of the fully doped
(highly conducting) form of the polymer by simply adjusting the doping
level. During doping and de-doping processes a stabilized doped state of
the conducting polymer may be obtained using dopant counter ions by
chemical or electrochemical processes [22]. Conducting polymers can be
p or n doped chemically and electrochemically to obtain a metallic state
[13, 14]. Doping of conjugated polymers can also be carried out by
methods that introduce no dopant ions such as field induced charging
[23]. In the doped state, the backbone of a conducting polymer consists of
highly delocalized π-electrons. Fig. 1.5 represents a chart showing the
different methods that have been used usually for doping in conducting
polymers.
18
Fig. 1.4: Schematic illustration indicating the difference between the
doping mechanisms in inorganic semiconductors and conjugated
polymers.
19
Fig 1.5: Different methods for doping in conducting polymers.
Doping of conjugated polymers either by oxidation or by reduction in
which the number of electrons in the polymeric backbone gets changed is
generally referred to as redox doping [22]. The charge neutrality of the
conducting polymer is maintained by the incorporation of the counter
ions.
Redox doping can be further subdivided into three main classes: p type
doping, n type doping and doping involving no dopant ions viz., photo-
doping and charge injection doping [24, 25]. Both chemical and
20
electrochemical redox doping techniques can be employed to dope
conjugated polymers either by removal of electrons from the polymer
back-bone chain (p-doping) or by the addition of electrons (n-doping) to
the chain. In chemical doping the polymer is exposed to an oxidizing
agent such as iodine vapours or a reducing agents viz., alkali metal
vapours, whereas in electrochemical doping process a polymer coated,
working electrode is suspended in an electrolyte solution in which the
polymer is insoluble, along with separate counter and reference
electrodes.
Photo-doping is a process where conducting polymers can be doped
without the insertion of cations or anions simply by irradiating the
polymer with photons of energy higher than the band gap of the
conducting polymer. This leads to the promotion of electrons to higher
energy levels in the band gap. Charge injection doping is another type of
redox doping that can also be used to doped an undoped conducting
polymer [23, 26, 27]. In this method, thin film of conducting polymer is
deposited over a metallic sheet separated by a high dielectric strength
insulator.
The non-redox doping of conducting polymer is a process of doping
conducting polymers in which the number of electrons associated with
the polymer chain is kept constant. In fact it is the energy level in the
conducting polymer that gets rearranged in the non-redox doping process
[28]. The best example of non-redox doping is the conversion of
emeraldine base form of polyaniline to protonated emeraldine base
(polysemiquinone radical cation) when treated with protic acids [19]. It
has been observed that the conductivity of polyaniline is increased by
approximately 10 orders of magnitude by non-redox doping. Ion
implantation and heat treatment methods have also been used to dope
21
conducting polymers, however, it has rarely been used for doping
conducting polymers.
1.3.3 Metal-Insulator transition in doped conducting
polymers
Metal-Insulator (M-I) transition is one of the most interesting physical
aspects of conducting polymers. When the mean free path becomes less
than the inter-atomic spacing due to increase in disorder in a metallic
system, coherent metallic transport is not possible [29]. When the
disorder is sufficiently large, the metal exhibits a transition from the
metallic to insulating behaviour. As a result of this transition which is
also known as the Anderson transition all the states in a conductor
become localized and it converts into a Fermi glass [30] with a
continuous density of localized states occupied according to Fermi
statistics. Although there is no energy gap in a Fermi glass but due to the
spatially localized energy states a Fermi glass behaves as an insulator
[30]. It has been found that electrical conductivity of a material near the
critical regime of Anderson transition obeys power law temperature
dependence [31]. This type of M-I transition has been observed for
different conducting polymers viz., polyacetyle, polyaniline, polypyrrole
and poly(p-phenylene vinylene) etc. and is particularly interesting
because the critical behaviour has been observed over a relatively wide
temperature range [32]. In conducting polymers, the critical regime is
easily tunable by varying the extent of disorder by means of doping or by
applying external pressure and/or magnetic fields [32]. In the metallic
regime, the zero temperature conductivity remains finite, and σ(T) remains
constant as T approaches zero [32]. Although disorder is generally
recognized to play an important role in the physics of metallic polymers,
22
the effective length scale of the disorder and the nature of the M-I
transition [33-34].
In the present thesis, we have taken three most popular host conducting
polymers such as polythiophene (PTh), polyaniline (PAni) and
polypyrrole (PPy) for the synthesis of conducting polymer composites.
Polythiophene (PTh) is one of the most valuable types of conducting
polymers that may be easily modified to afford a variety of useful
electrical and physical properties such as solubility, electrical
conductivity, mobility and others. Polythiophenes are the polymerization
of thiophene (a sulphur heterocycle), i.e. a linear chain of thiophene
monomers. It possesses lower band gap and better electronic properties. It
may give rise some very useful properties such as increased ionization
potential and stability. Polythiophenes usually do not possess metallic
type conductivity even in a doped state. Therefore, they are much more
commonly used as organic semiconductors. Many of them possess also
good luminescent, nonlinear-optical, and other useful optoelectronic
properties [14]. It is polymerized thiophene of a sulfur heterocycle. They
can become conducting when electrons are added or removed from
the conjugated π-orbitals via doping. The study of polythiophenes has
intensified over the last three decades. The maturation of the field of
conducting polymers was confirmed by the awarding of the 2000 Nobel
Prize in Chemistry to Alan J. Heeger, Alan Mac Diarmid, and Hideki
Shirakawa for the discovery and development of conductive polymers.
The most notable property of these materials, electrical conductivity,
results from the delocalization of electrons along the polymer backbone
hence the term synthetic metals. However, conductivity is not the only
interesting property resulting from electron delocalization. The optical
properties of these materials respond to environmental stimuli, with
23
dramatic colour shifts in response to changes in temperature, applied
potential, solvent and binding to other molecules. Both colour changes
and conductivity changes are induced by the same mechanism. The
twisting of the polymer backbone and disrupting conjugation makes
conjugated polymers attractive as sensors that can provide a range of
optical and electronic responses.
Polythiophene (PTh)
Polypyrrole (PPy) is a type of organic polymer formed by polymerization
of pyrrole. It is also known as conducting polymers. The Nobel Prize in
Chemistry was awarded in 2000 for work on conductive polymers
including polypyrrole [15].
24
Most commonly PPy is prepared by oxidation of pyrrole, which can be
achieved using ferric chloride in methanol:
n C4H4NH + 2 FeCl3 → (C4H2NH)n + 2 FeCl2 + 2 HCl
Polymerization is thought to occur via the formation of the pi-radical
cation C4H4NH+. This electrophile attacks the C-2 carbon of an un-
oxidized molecule of pyrrole to give a dimeric cation (C4H4NH)2]++
. The
process repeats itself many times. Conductive forms of PPy are prepared
by oxidation (p-doping) of the polymer:
(C4H2NH)n + x FeCl3 → (C4H2NH)nClx + x FeCl2
The polymerization and p-doping can also be affected electrochemically.
The resulting conductive polymers are peeled off of the anode.
Polypyrrole is also being investigated in low temperature fuel cell
technology to increase the catalyst dispersion in the carbon support
layers [16-19] and to sensitize cathode electro-catalysts, as it has been
inferred that the metal electro-catalysts (Pt, Co, etc.) when coordinated
with the nitrogen in the pyrrole monomers show enhanced oxygen
reduction activity. Polypyrrole (together with other conjugated polymers
such as polyaniline, poly(ethylenedioxythiophene) etc. has been actively
studied as a material for artificial muscles, a technology that would offer
numerous advantages over traditional motor actuating elements [20].
Polypyrrole was used to coat silica and reverse phase silica to yield a
material capable of anion exchange and exhibiting hydrophobic
interactions. Polypyrrole was used in the microwave fabrication of multi-
walled carbon nanotubes, a new method that allows obtaining CNTs in a
matter of seconds [21]. Chemical and Engineering News reported in June
2013 that Chinese research has produced a water-resistant polyurethane
sponge coated with a thin layer of polypyrrole that absorbs 20 times its
weight in oil and is reusable.
25
Polyaniline (PAni) is a conducting polymer of the semi-flexible rod
polymer family. Although the compound itself was discovered over 150
years ago, only since the early 1980s has polyaniline captured the intense
attention of the scientific community. This interest is due to the
rediscovery of high electrical conductivity. Amongst the family of
conducting polymers and organic semiconductors, polyaniline has many
attractive processing properties. Because of its rich chemistry, polyaniline
is one of the most studied conducting polymers of the past 50 years [22].
Polymerized from the inexpensive aniline monomer, polyaniline can be
found in one of three idealized oxidation states:
Polyaniline (PAni)
In above Figure, x equals half the degree of polymerization (DP).
Leucoemeraldine with n=1, m=0 is the fully reduced state. Pernigraniline
is the fully oxidized state (n=0, m=1) with imine links instead
of amine links [23]. Studies have shown that most forms of polyaniline
are one of the three states or physical mixtures of these components. The
emeraldine (n=m=0.5) form of polyaniline, often referred to as
emeraldine base (EB), is neutral, if doped (protonated) it is called
emeraldine salt (ES), with the imine nitrogens protonated by an acid.
Protonation helps to delocalize the otherwise trapped diiminoquinone-
diaminobenzene state. Emeraldine base is regarded as the most useful
form of polyaniline due to its high stability at room temperature and the
fact that, upon doping with acid, the resulting emeraldine salt form of
26
polyaniline is highly electrically conducting. Leucoemeraldine and
pernigraniline are poor conductors, even when doped with an acid [24].
The colour change associated with polyaniline in different
oxidation states can be used in sensors and electrochromic
devices. Although colour is useful, the best method for making a
polyaniline sensor is arguably to take advantage of the dramatic changes
in electrical conductivity between the different oxidation states or doping
levels [25]. Treatment of emeraldine with acids increases the electrical
conductivity by ten orders of magnitude [26]. Undoped polyaniline has a
conductivity of 6.28×10−9
S/m, while conductivities of 4.60×10−5
S/m can
be achieved by doping to 4% HBr. The same material can be prepared by
oxidation of leucoemeraldine. Polyaniline is more noble than copper and
slightly less noble than silver which is the basis for its broad use in
printed circuit board manufacturing and in corrosion protection [27].
1.3.4 Importance of Conducting Polymers as Sensors
Conducting polymers are basically plastics which are made out of small
building blocks called monomers just like ordinary plastics but they can
conduct electricity. A common feature of conducting polymers is the
alteration of single and double bonds, at least in the backbone of the
polymer structure [28]. Polymers constitute another class of materials
which are also very promising for application in chemical sensors. In
general, polymers could be used in all types of sensors as long as they can
function at room temperature. The great advantage of conducting polymer
based sensors over other available technologies is that the conducting
polymers have the potential for improved response properties and are
sensitive to small perturbations. A large number of chemical sensors use
polymers because they offer great design flexibility [29]. The flexibility
of polymers properties, however, is attained at the expense of doping and
27
the introduction of functional additives. The primary dopants (anions)
introduced during chemical or electrochemical polymerization maintains
charge neutrality and generally increases the electrical conductivity.
Doping generates charge carriers in the polymer chain through chemical
modification of the polymer structure and involves charge exchange
between the polymer and the dopant species. The nature of the anion also
strongly influences the morphology of the polymer. In addition, anions
can serve as specific binding sites for interaction of the conducting
polymer with the analyte gas. They are relatively open materials that
allow ingress of gases into their interior. Common classes of organic
conducting polymers are acceptable for conductometric gas sensor
application.
Conducting polymers display a wide variety of properties, ensuring a vast
number of potential applications in a large number of technologies.
According to Persaud (2005), polymer based sensors have the following
advantages [30]:
i-The sensors have rapid adsorption and desorption kinetics at room
temperature.
ii-The sensor elements feature low power consumption (of the order of
microwatts), because no heater element is required.
iii-The polymer structure can be correlated to specificity toward
particular classes of chemical compounds.
iv-The sensors are resilient to poisoning by compounds that would
normally inactivate some inorganic semiconductor type sensors.
1.3.5 Ionization in Conducting Polymers
Ionization in conducting polymer is due to doping. The introduction of
charge during the doping process leads to a structural distortion of a
28
polymeric structure in the region of the charge, giving an energetically
favourable conformation. These structural distortions are intrinsic to the
development of ionization states called polarons and bipolarons.
1.3.6 Bulk and nano polymer
Polymer nanocomposites (PNC‟s) consist of a polymer or copolymer
having nanoparticles or nanofillers dispersed in the polymer matrix.
These may be of different shape (e.g. platelets, fibres, spheroids), but at
least one dimension must be in the range of 1–50 nm. These PNC‟s
belong to the category of multi-phase systems (MPS, viz. blends,
composites, and foams) that consume nearly 95% of plastics production.
These systems require controlled mixing/compounding, stabilization of
the achieved dispersion, orientation of the dispersed phase, and the
compounding strategies for all MPS, including PNC, are similar. Polymer
nanoscience is the study and application of nanoscience to polymer-
nanoparticle matrices, where nanoparticles are those with at least one
dimension of less than 100 nm. The transition from micro to nano-
particles, lead to change in its physical as well as chemical properties.
Two of the major factors in this are the increase in the ratio of the surface
area to volume, and the size of the particle. The increase in surface area-
to-volume ratio, which increases as the particles get smaller, leads to an
increasing dominance of the behaviour of atoms on the surface area of
particle over that of those interior of the particle. This affects the
properties of the particles when they are reacting with other particles.
Because of the higher surface area of the nano-particles, the interaction
with the other particles within the mixture is more and this increases the
strength, heat resistance, etc. and many factors do change for the mixture.
An example of a nanopolymer is silicon nanospheres which show quite
29
different characteristics; their size is 40–100 nm and they are much
harder than silicon, their hardness being between that of sapphire and
diamond.
1.3.7 Applications of Polymer
Due to their poor processability, conductive polymers have few large-
scale applications. They have promise in antistatic materials and they
have been incorporated into commercial displays and batteries, but there
have had limitations due to the manufacturing costs, material
inconsistencies, toxicity, poor solubility in solvents, and inability to
directly melt process. Literature suggests they are also promising
in organic solar cells, printing electronic circuits, organic light-emitting
diodes, actuators, electrochromism, super capacitors, chemical sensors
and biosensors [31], flexible transparent displays, electromagnetic
shielding and possibly replacement for the popular transparent conductor
indium tin oxide. Another use is for microwave-absorbent coatings,
particularly radar-absorptive coatings on stealth aircraft. Conducting
polymers are rapidly gaining attraction in new applications with
increasingly processable materials with better electrical and physical
properties and lower costs. The new nanostructured forms of conducting
polymers particularly augment this field with their higher surface area
and better dispersability.
1.4 Synthesis of Polymers and their Polymer Composites
Electrochemical polymerization (ECP) is performed in a single-
compartment cell containing electrochemical bath which includes a
monomer and a supporting electrolyte dissolved in appropriate solvent. It
also includes three different electrode such as working electrode
(cathode), reference electrode and counter electrode (anode). Film
30
deposited on the counter electrode (anode). Usually ECP is carried out
either Potentiostatically (i.e. constant voltage condition) or
Galvanostatically (i.e. constant current condition) by using a suitable
power supply. Potentiostatic conditions are recommended to obtain thin
films while galvanostatic conditions are recommended to obtain thick
films [32].
Chemical polymerization is the process in which relatively small
molecules, called monomers, combine chemically to produce a very large
chainlike or network molecule. The monomer molecules may be all alike,
or they may represent two, three, or more different compounds. Usually
at least 100 monomer molecules must be combined to make a product
that has certain unique physical properties such as elasticity, high tensile
strength, or the ability to form fibers that differentiate polymers from
substances composed of smaller and simpler molecules; often, many
thousands of monomer units are incorporated in a single molecule of a
polymer. The formation of stable covalent chemical bonds between the
monomers sets polymerization apart from other processes, such as
crystallization, in which large numbers of molecules aggregate under the
influence of weak intermolecular forces.
1.4.1 Preparation of Thin films
Obtaining a thin film of any material on a substrate surface with proper
adherence is thin film deposition. Deposition techniques can be broadly
classified as physical deposition methods and chemical deposition
methods. The deposition process of a film can be divided into three basic
phases:
(a) Preparation of the film forming particles (atoms, molecules, cluster)
(b) Transport of the particles from the source to the substrate
31
(c) Adsorption of the particles on the substrate and film growth
There are two type of deposition method has been involved in the
preparation of thin-films such as physical deposition method and
chemical deposition method.
Physical Deposition Methods
Physical deposition methods include several methods such as thermal
evaporation, sputtering, pulsed laser deposition and cathodic arc
deposition etc. All these methods require maintenance of vacuum of 10-6
Torr or more. Besides these deposition methods, other methods like
reactive sputtering, molecular beam epitaxy, liquid phase epitaxy etc. are
also used for obtaining improved quality of films by precisely controlling
the deposition parameters. Rate of deposition, substrate temperature,
ambient conditions, residual gas pressure in the system, purity of the
material to be deposited, in homogeneity of the films, structural or
compositional varieties of the films in the localized or wider areas
determine the electrical, optical, magnetic and surface properties of the
deposited films.
Chemical Deposition Method
Chemical deposition methods include electroplating, chemical vapour
deposition, plasma enhanced chemical vapour deposition,
organometallic solutions etc. A non vacuum technique, for producing thin
films, is the use of organometallic solutions that are applied to a substrate
surface either by dipping or by spinning or by spraying. The film is then
dried and baked in the furnace. Thermal deposition takes place and the
film is converted to pure oxide while the organic constituents evaporate.
Thickness control is achieved by adjusting the temperature, viscosity and
other properties of the solutions. This technique besides being
continuously used in materials related research has been used
32
successfully to coat high power laser optical components with anti
reflection design that have extremely high damage thresholds. In the
present thesis thin films prepared by sol-gel spin coating and spray
pyrolysis methods using organometallic solutions have been used. The
thickness of the films obtained from sol-gel spin coating and spray
pyrolysis methods lie in the range of few hundreds of nanometers.
1.4.2 Sol-gel process
Sol-gel process is a chemical deposition process which is very widely
used for the deposition of thin films, fibers, rods etc. The sol-gel
preparation of any metal oxide involves combining reactants and the
subsequent solidification of the resultant solution into an amorphous
oxide gel [33]. The porous oxide is then heated to give densified glasses
and polycrystalline solids. Incorporation of impurities and formation of
composites is easy [34-35].
The advantages of the process are ultra homogenation due to atomic scale
mixing, high degree of uniformity control on thickness, possibility of
multilayer coating and no restriction on shape and size of the substrate.
The sol-gel method is an alternative to vacuum deposition techniques.
Sol-gel process consists essentially of three steps [36-37].
(a) Formation of low viscosity solutions of suitable precursors i.e. metal
derivatives (organic/inorganic) which could finally yield the oxides or
metal oxides themselves. Low viscosity insures homogenation.
(b) Formation of a uniform sol and causing it to gel to endow chemical
homogeneity on the ceramic product during desiccation.
(c) Shaping during or after gelation into the final form fibers, surface coating
etc. before annealing.
The temperatures, pH of the medium and anions/complexing agents
present in the solution are the main factors affecting the sol-gel process.
33
The sol-gel process is used in developing thin film transistors, anti-
reflection coatings, real view mirrors, solar reflecting glass, excitonic
devices working at high temperature, transparent conducting films,
magnetic films, protective coatings, films for sensors etc [38-42].
Sol and Precursor
Precursors are the starting chemicals which are compounds of relevant
components acting as solutes in sol-gel process and they should be able to
form reactive inorganic monomers. Precursors should be soluble in the
reaction media and should be reactive enough to participate in the sol gel
forming process. The reactivity of precursor depends on its chemical
nature as well as on applied reaction conditions. Metal oxides or metal
hydroxides usually remain in the solution rendering it directly coatable on
the substrates. Variation is the viscosity or concentration of the solution
can be used to adjust the thickness of the film. When metal is not
available in a soluble form, its organic compound can be taken as a
precursor in a suitable solvent and then is made to undergo slow and
controlled hydrolysis till it provides a metal-oxygen metal network in a
sol state maintaining most of the time the solubility and transparency of
the solution. In case of metal oxide precursors the product can be easily
freed from the carbonaceous residue. However the homogeneity is
limited. Alkoxides of group I and II metals are non volatile solids and
often depict low solubility in organic solvents. As an alternative, metal
salts, which are soluble in organic solvents and can be converted easily to
the oxide by thermal or oxidative decomposition, are used.
Gelation
The transformation of fluid sols to solidified gel depends on the rheology
of the sols gelation reduces the distance between the colloidal particles
34
and bonds are formed. The transformation of the solution into a semi-
rigid wet gel can be done by-
(a) By water evaporation or extraction using tray drying, spray drying or
dispersion in an immiscible fluid
(b) By removal or neutralization at anions or
(c) By polymerization of organometallic compounds
The network forming components, monomers or colloids, react into
active forms after the preparation of a solution or a homogeneous
colloidal sol. For stable colloids the neutralization of surface charges,
aggregation and further condensation by reactive surface groups leads to
gelation. To deposit thin film sol or precursor state is used while for
fabricating fibers gel-state is used [43].
1.4.3 Sol-gel spin coating
In the sol-gel spin coating process the substrate spins around an axis
perpendicular to the coating area. The following are four distinct stages to
the spin coating process. In the first step a controlled amount of the
precursor is poured over the substrate which wets substrate uniformly. If
needed, sub-micron filter is also used to eliminate larger particles from
the precursor.
The second step involves spinning of substrate with desired rotation
speed to remove the excess fluid. The top of the fluid layer exerts inertia
while the substrate rotates at a faster speed. These two forces result in
twisting motion which may lead to formation of spiral vortices. But in
normal cases the precursor is thin enough so that it keeps co-rotating with
the substrate and any evidence of thickness difference is absent.
Ultimately the substrate reaches its desired speed and the viscous shear
drag is exactly balanced by the rotational acceleration.
35
In the third step viscous forces dominate the thinning behavior of the
fluid. The fluid thinning tends to formation of uniform films. However in
few cases edge effect are also seen. The fluid which is rotating flows
uniformly outside. If the fluid is in excess then the drops must be formed
at the edges so that they may fling off. Thus the thickness at the ends may
be slightly greater than that at the central portion of the substrate leading
to edge effect.
In the fourth and final stage fluid starts evaporating and dominates its
own thinning behavior. It is the stage in which the solvent phase gets
removed and the sol is converted to dense ceramic. As the fluid rotates at
a high speed, its temperature rises and leads to evaporation of the fluid.
Thus the viscosity of the remaining solution increases leading to the „gel-
state‟ of the coating. The resulting film usually has an amorphous
structure. Although the third and fourth stages i.e. viscous flow and
evaporation occur simultaneously, the viscous flow effect dominates
initially and evaporation dominates later.
The sol should be kept in an air tight flask to maintain its viscosity
otherwise it gets converted into gel and cannot be used for the deposition
of films [44]. The factors that affect thickness of the film are viscosity of
the coating solution, rotation rate, annealing temperature and time
duration. Gel coated films are porous and sintered when they are heated.
1.4.4 Spray pyrolysis
The chemical spray pyrolysis technique is one of the major techniques to
deposit a wide variety of materials in thin film from over a considerably
large area. In spray deposition process, a precursor solution is pulverized
by means of a neutral gas so that it arrives at the substrate in the form of
very fine droplets. The substrate is generally held at high temperature
36
where the constituents of the precursor solution react to form the desired
compound while other reaction products leave as volatile components.
The optimization of the chemical solution into a spray of fine droplets is
effected by spray nozzle with the help of a carrier gas which may not be
involved in the pyrolytic reaction. Large area uniform coverage of the
substrate is achieved by scanning either or both the spray head and the
substrate employing electromechanical arrangements. The chemicals used
for spray pyrolysis have to satisfy the following conditions:
I. The desired thin film material must be obtained as a result of thermally
activated reaction between the various species/complexes dissolved in
spray solutions.
II. The remainder of the constituents of the chemicals, including the
carrier liquid should be volatile at the pyrolysis temperature.
The spray pyrolysis technique had been used for the production of thin
films of simple oxides, mixed oxides, metal spinal type oxides, binary
and ternary chalcogenides, copper compounds and also super conducting
oxide films because they have satisfied both the above conditions. Thin
film deposition by spray pyrolysis has a number of advantages which are
enumerated below:
(a) The film properties can be easily changed by changing the generated
droplet sizes which can be done by employing different external pressures
during atomization.
(b) It offers an extremely easy way for doping and thus the cation to anion
ratio in compound materials can be easily varied by adding complexing
agents while electronic properties of the deposited thin films.
(c) Viscosity of the precursor solution and the spray rate/flow rate can be
changed for obtaining desired film thickness.
(d) Unlike closed vapour deposition methods, spray pyrolysis does not
require vacuum at any state.
37
(e) Above all this, the method offers deposition of thin films over a large area
and therefore the technique can be scaled up easily for industrial
applications.
However, the disadvantage with spray pyrolysis technique is that only a
few percent of the material supplied is deposited onto the substrate. After
atomization a large fraction of the droplets does the deposition efficiency
is low. Also there is a significant variation in the generated droplet size in
the aerosol. The aerosols generated by atomizers differ in droplet size,
rate of atomization and droplet velocity.
In spray pyrolysis decomposition, the impinging aerosols on the substrate
undergo endothermic reaction with substrate surface resulting in a
heterogeneous type of growth. Hence, there does not occur an oriented
type of growth in the first few layers. As the thickness increases the
arrangements to atoms in further layers get gradually modified by the
proceeding layers. As a result, the growth takes place in a more definite
way resulting in a film of preferred orientation.
1.4.5 Dip-coating
Sol-gel dip coating was invented in Europe in 1960. This is one of the
best and simpler coating process in which first of all the substrate to be
coated is put into the pot containing the sol of desired material suitably
tuned for gelation property. Now the substrate is taken out from this pot
at constant speed may be under controlled environmental conditions if
desired so. Special arrangement for lifting the substrate is required which
can ensure constant speed without jerks or vibrations. Generally
microprocessor controlled equipment is used these days for this purpose.
Coated film thickness depends on lifting speed, angle of substrate lifting
38
with the liquid surface and environmental conditions under which coating
procedure is carried out [45].
1.5 Characterization Techniques
The standard methods of measurement and characterizations are
constantly employed for the investigation of nanostructures.
Structural/morphological and optical properties determination and
understanding are an important and integral part of nanomaterials
research.There are a number of powerful experimental techniques that
can be used to characterize structural/morphological, surface and optical
properties of nanomaterials either directly or indirectly, e.g. XRD (X-ray
diffraction), STM (scanning tunneling microscopy), AFM (atomic force
microscopy), SEM (scanning electron microscopy), TEM (transmission
electron microscopy), IR (infrared). Some of these techniques are more
surface sensitive than others. The choice of characterization technique
depends strongly on the information being sought about the material.
Optical spectroscopy such as IR and Raman provide more direct
information about structure while UV-visible absorption spectroscopy
and photoluminescence (PL) spectroscopy provide indirect structural
information. In general optical spectroscopy is sensitive to structural
properties but cannot provide a direct probe of the structural details.
Optical properties are commonly characterized using spectroscopic
techniques including UV-visible and photoluminescence spectroscopy, as
both yield information about the electronic structure of nanomaterials.
Other characterization techniques such as TG-DTA (Thermal
Gravimetric-Differential Thermal Analysis) and FTIR (Fourier-transform
infrared) spectroscopy are also useful for polymers. In this section, the
39
characterization techniques used in the present works are discussed as
follows:
1.5.1 X-Ray Diffraction (XRD)
X-ray diffraction technique is a powerful tool for material
characterization. This technique is applied not only for structure
determination of solids but also to some other problems, such as chemical
analysis, stress measurement, study of phase equilibrium, determination
of particle size, determination of orientation of crystal. We know that the
physical properties of solids (e.g. electrical, optical, magnetic etc) depend
on atomic arrangements of materials, so the determination of the crystal
structure is an indispensable part of the characterization of materials. If a
crystalline specimen is visualized as being made up of tiny fragments of
completely random arrangement, it is called a fine crystalline powder. X-
rays are used to establish the atomic arrangement or structure of the
materials because the interplanar spacing (d) of the diffracting planes is
of the order of X-ray wavelength. For a crystal with a given d-spacing
and for a given wavelength λ, the various orders n of reflection occur
only at the precise values of angle θ, which satisfy the Bragg equation
nd sin2
The powder profile of the substance, even without further interpretation
can be used for identification of materials. The simplicity and advantage
of X-Ray powder diffraction method can be given as follows:
a) The powder diffraction pattern is the characteristic of the substance,
b) Each substance in a mixture produces its pattern independent to others,
c) It describes the state of chemical combination of elements in the
materials,
d) The method is capable to develop quantitative analysis of substances.
40
1.5.2 Scanning Electron Microscopy (SEM)
Scanning electron microscopy is used primarily for the study of surface
morphology of solid materials. An electron beam passing through an
evacuated column is focused by electromagnetic lenses onto the specimen
surface; the beam is then rastered over the specimen in synchronism with
the beam of a cathode ray tube display screen. In elastically scattered
secondary electrons are emitted from the sample surface and collected by
a scintillator, the signal from which is used to modulate the brightness of
the cathode ray tube. In this way the secondary electron emission from
the sample is used to form an image on CRT display screen.
Sample preparation for obtaining SEM
For conventional imaging in the SEM, specimens must be electrically
conductive at least at the surface and electrically grounded to prevent the
accumulation of electrostatic charge at the surface. Metal objects require
little special preparation for SEM except for cleaning and mounting on a
specimen stub. Non conductive specimens tend to have charge
accumulated on them when scanned by the electron beam which causes
scanning faults and other image artifacts. They are therefore usually
coated with an ultrathin coating of electrically conducting material,
commonly gold. It is deposited on the sample either by low vacuum
sputter coating or by high vacuum evaporation. Conductive materials in
current use for specimen coating include gold, gold-palladium alloy,
platinum, osmium, iridium, tungsten, chromium and graphite etc. Gold-
palladium alloy has been used as the coating material for the SEM
recording of the samples in the present thesis.
41
Fig. 1.6: Schematic representation of X-ray diffraction.
42
Fig. 1.7: Schematic diagram of SEM.
43
1.5.3 Fourier Transformed Infrared Spectroscopy (FTIR)
The FTIR spectroscopy is a powerful modern technique in which
spectrum is first produced as an interferogram which is processed and
computed in real time through a dedicated computer to provide high
resolution information. Infrared spectroscopic studies are carried out.
Polyaniline samples with different dopant ions are analysed by using
Fourier transform infra-red spectrometer (Model-430, LEO Cambridge,
England). Infrared spectroscopy provides information about the
concentration of the impurities, and their bonding with the host material.
In FTIR, the infrared radiation is split into two beams, out of which one is
kept static and the other moving. These are combined to give a modulated
beam which is passed through the sample. It is then digitized and Fourier-
transformed by the computer to give the infrared spectrum. For analysis
of the data, standard spectra of bulk powder and used evaporation source,
which are mixed with pure and dry KBr powder processed into thin
pellets, are recorded. The schematic representation of FTIR is shown in
Fig. 1.8.
Atmospheric moisture and carbon dioxide can cause problem in infrared
spectra. Water absorbs around 4000 to 3500 cm-1
and 2000 to 1300 cm-1
,
while carbon dioxide absorbs at 2350 and 688 cm-1
. This absorbance
often mask weak feature that are of interest for a particular investigation.
Purging with dry air or nitrogen and annealing treatments are performed
prior to loading the film for FTIR spectra investigation. FTIR
spectroscopy has following applications:
(a) Useful spectral information can be obtained from a sample of a
microgram or less. Utilizing special techniques, as little as 50 picograms
may be analysed.
44
(b) A spectrum can be obtained in much shorter time than is possible with
a dispersive spectrometer. Thus, spectra can be obtained to transient
species or phenomenon down to as short a time as 1/30th
of a second.
(c) A combination of both those advantages make it possible to obtain
infrared spectra of gas chromatographic and high pressure liquid
chromatographic cuts as they emerge from the chromatograph, thus
providing considerable structural about the material giving rise to each
peak in the chromatograms.
(d) The spectrum can be obtained at very high resolution, which has
certain advantages in studying small molecules in vapour phase.
(e) Utilizing microscope accessories, spectra of individual particles or
inclusions of the order of 5µm in size can be obtained.
1.5.4 UV-Visible Spectroscopy
UV-visible absorption spectroscopy is the widely used method of optical
characterization such as studying the band gap of semiconductor
materials; identifying some functional group, assaying and determination
of content as well as strength of substance. In UV-visible absorption
spectroscopy, absorption spectra are recorded with UV-vis double beam
spectrophotometer in a 1 cm path length quartz cuvette after
ultrasonification of colloidal solution to disperse the particles. The
spectrophotometer is equipped with two continuous light sources (i) a
hydrogen or deuterium lamp for measurement in ultraviolet range and (ii)
a tungsten or halogen lamp for the measurement in visible range (1000
nm to 400 nm). In this way, radiation across the whole range is scanned
by the spectrophotometer. The schematic representation of UV-visible
absorption spectroscopy is shown in Fig. 1.9 (A). In brief, a beam of light
from a visible and/or UV light source is separated into its component
wavelengths by a prism or diffraction grating. Each monochromatic
45
(single wavelength) beam in turn is split into two equal intensity beams
by a half-mirrored device. The sample beam passes through a sample
cuvette containing a solution of the compound being studied in a
transparent solvent. The reference beam passes through an identical
reference cuvette containing only the solvent. The intensities of these
light beams are then measured by electronic detectors and compared.
Over a short period of time, the spectrometer automatically scans all the
component wavelengths in the manner described. The ultraviolet (UV)
region scanned is normally from 200 to 400 nm, and the visible portion is
from 400 to 800 nm. Generally, Photomultiplier tube (PMT) or
Semiconductor Photodiodes or Charge Coupled Devices (CCD) can be
used as a detector.
1.5.5 Photoluminescence (PL) Spectroscopy
It is a powerful technique for extracting information about the electronic
structure of the material from the spectrum of light emitted. At room
temperature (RT) there is a non-zero occupancy of electron states near the
bottom of the conduction band and hole states near the top of the valence.
Hence, electron-hole pairs can recombine to emit photons over a range of
energies, producing a broad band rather than a sharp peak. Donor,
acceptor and defects states are generally fully ionized at room
temperature and do not contribute significantly to the observed spectrum.
These problems can be avoided by recording photoluminescence spectra
at low temperature in which the sample is enclosed in a cryostat and
illuminated with above band gap source. The most basic use of low
temperature PL spectra is as an indicator of overall crystal quality. The
crystal quality is indicated by two factors: first the ratio of excitonic
luminescence to donor-acceptor and deep level luminescence, second the
sharpness of the spectrum i.e. the extent to which adjacent lines can be
46
resolved. Apart from indicating overall crystal quality, the low
temperature PL can sometimes be helpful in identifying specific
impurities. The schematic representation of photoluminescence
spectroscopy is shown in Fig. 1.9 (B).
Fig. 1.8: The schematic representation of FTIR spectroscopy.
47
Fig. 1.9: Schematic representations of (A) UV-visible absorption
spectroscopy and (B) photoluminescence Spectroscopy.
48
1.6 Objective of present work
The main objective of present work is to synthesize different conducting
polymer and their polymer composites for study the structural,
morphological and optical properties for various device applications. In
whole work, polythiophene, polyaniline and polypyrrole have been used
as host materials in form of pellets and thin films. The different dopants
such as Al2O3, CuO and TiO2 have been used to improve the properties of
synthesized conducting polymer. The study focuses on the following
objectives:
(a) To synthesize undoped and Al2O3 doped polythiophene by chemical
method and their pelletization. The structural, surface morphological and
optical characterizations are done so as synthesized materials of optimum
quality
(b) To synthesize undoped and CuO doped polyaniline by chemical
method and their pelletization. The structural, surface morphological and
optical characterizations are done so as synthesized materials of optimum
quality
(c) To synthesize undoped and Al2O3 doped polypyrrole by chemical
method and their pelletization. The structural, surface morphological and
optical characterizations are done so as synthesized materials of optimum
quality
(d) To synthesize undoped and Al2O3 doped polyaniline by chemical
method and sol-gel dip coating films. The structural, surface
morphological and optical characterizations are done so as synthesized
materials of optimum quality
49
(e) To synthesize undoped and TiO2 doped polypyrrole by chemical
method and sol-gel spin coating thin films. The structural, surface
morphological and optical characterizations are done so as synthesized
materials of optimum quality
(f) To perform the structural, surface morphological and optical studies
so as to obtain thin films of optimum quality.
1.7 Organization of present work
The content of the present thesis has been divided into seven chapters.
Chapter 1: It gives the introduction to the subject. Deposition and various
characterization methods of pellets and thin films are described.
Chapter 2: The chapter 2 presents the synthesis and characterization of
Al2O3 doped polythiophene by chemical oxidation method and their
pelletization and their characterization using X-ray study, SEM, FTIR
spectroscopy and PL spectroscopy.
Chapter 3: This chapter deals with the synthesis and characterizations of
CuO doped polyanilne by chemical oxidation method and their
pelletization. The structural, morphological and optical characterizations
are also present.
Chapter 4: This chapter deals also with the synthesis and characterization
of undoped and Al2O3 doped polypyrrole by chemical oxidation method
and their pelletization. Their characterizations have been done using
XRD, SEM, FTIR, UV-Vis and PL.
Chapter 5: The chapter 5 presents structural, morphological and optical
study of undoped and Al2O3 doped polyaniline by chemical oxidation
50
method and sol-gel dip coating thin films. Their characterizations have
been done using XRD, SEM, FTIR, PL and transmission.
Chapter 6: Structural, morphological and optical study of undoped and
TiO2 doped polypyrrole by chemical oxidation method and sol-gel spin
coating thin films is presented in Chapter 6. The structural,
morphological, FTIR and optical characterizations are also present.
Chapter 7: Conclusions of the present work are presented in chapter 7
along with the recommendation for further work.
51
Chapter 2
Synthesis and Characterization of Undoped and Al2O3
Doped Polythiophene Nanocomposites
2.1 Introduction
Currently, conducting polymers (CPs) such as polypyrrole (PPy),
polyaniline (PAni), polythiophene (PTh) and their derivatives are
promising materials for the synthesis of nanocomposites (NCs) material
and their device applications. Among conducting polymers composites,
the polythiophene– metal oxide composites have received much more
attention from scientific communities and researchers in recent years
because of their unique electrical, electrochromic and electronic
properties with high environmental and thermal stabilities [1-3]. The
polythiophene–metal interfacial structure is very important issue for
various scientific and technological points of view. It is one of the
important and most frequently used CPs in the industries with wide range
of potential applications including chemical and optical sensors, light-
emitting diodes (LEDs), display devices, photovoltaic solar cells and
transistors [4-5]. However, characterization, processing and applications
of this class of materials have been limited by the poor solubility in
organic solvents. Initially, the synthesized CPs was insoluble and
infusible because of their strong inter-chain interactions. These two
features are extremely disadvantageous for basic research and
technological applications [6-7]. This inherent insolubility also greatly
hinders the understanding of their molecular properties. In-spite of above
problems with PTh, efforts are being made to improve its solubility with
organic acids by concerning protonation. The soluble CPs is synthesized
by chemical oxidative polymerizations, electro-polymerizations and
52
metal-catalyzed coupling reactions [8]. Currently a new class of materials
emerged, known as composites, synthesized by mixing suitable organic
and inorganic base materials in proper proportions. The composite
materials have their unique properties but in some of the cases, it can also
have other desirable properties of both the parent organic and inorganic
materials. As a result, there are increasing interests in combing both
organic and inorganic materials for device applications. The research on
polymer based nanocomposites (NCs) are one of the most flourishing
field in material science owing to their potential applications such as
electrochemical, mechanical, magnetic and dielectric properties [9].
Aluminium oxide is a chemical compound of aluminium and oxygen with
the chemical formula Al2O3. It is also called alumina. It has a wide
bandgap of ~8.8 eV for bulk material. It is an electrical insulator but has a
relatively high thermal conductivity around 30 Wm−1
K−1
for a ceramic
material. Aluminium oxide is insoluble in water. Al2O3 has been
extensively investigated dopants to serve as catalysts, fire redundant,
absorbents and fillers for structural materials [10]. It is stable in acidic
and oxidative mediums and well known for reactivity with aromatic
organic materials.
In this chapter an attempt is made to synthesize undoped PTh and
Al2O3/polythiophene NCs via chemical oxidation method and their
optical properties are studied. The doping of Al2O3 enhances the optical
properties of the polythiophene and therefore, UV-vis absorption
spectroscopy, FTIR and PL measurements have been investigated in
details.
53
2.2 Experimental
2.2.1 Chemicals
The cetyltrimethylammonium bromide (CTAB, 1.64 gm),
triethanolamine (TEA, 13.18 gm), thiophene and ammonium persulfate
(APS, 8.26 gm) were obtained from Aldrich. These chemical were of
high purity (99.999%) with analytical grade and were used directly
without special treatment.
2.2.2 Sample preparation
Cetyltrimethylammonium bromide (CTAB, 1.64 gm), triethanolamine
(TEA, 13.18 gm) and thiophene are dissolved in deionized water in a
flask (Sol-A). The mixture solution (designated as Sol-A) was placed in
ultrasonic bath for 30 min. Ammonium persulfate (APS, 8.26 gm) was
dissolved in 20 ml de-ionized water and formed solution (Sol-B). The
Sol-B was added drop-wise into Sol-A. The mixture of Sol-A and Sol-B
was heated without stirring for 24 h at 70 0C. The resulting precipitate
was collected by filtration and then washed several times by de-ionized
water and methanol. Finally obtained precipitate was dried in air for 30
min and then placed in oven for 3 h at 60 0C. The resulted product of
polythiophene was in form of powder with brown colours. The
synthesized polythiophene was doped with Al2O3 in the proper proportion
of 2, 4, 6 and 8 wt%. The undoped and doped polythiophene was ground
into make fine powders. Pellets are prepared by compressing the powder
under a pressure of 10 tons with the help of a hydraulic press machine.
All the pellets were annealed at 150 0C for 1 hour. The diameter of the
pellets was found to be 13 mm.
54
2.2.3 Characterizations
The XRD spectra of all the samples recorded by Phillips X’pert PW3020
diffractometer using CuKα radiation (λ=1.54056 Ao) were presented for
structural analysis of the samples. The SEM images of all the pellets were
taken by scanning electron microscope (Model-430, LEO Cambridge,
England). FTIR spectra of all the samples in the form of powder were
recorded on the Bruker Alpha spectrometer to determine the formation of
polythiophene. To record absorbance spectra, 0.01 gm of each sample is
dissolved in 5 ml of dimethyl formamide (DMF). Then the absorption
spectra of the solutions thus formed were recorded with UV-vis
spectrophotometer (Model No.V-670 Jasco). PL spectra of all the
samples were recorded using LS-55, Perkin Elmer fluorescence
spectrometer at room temperature with excitation wavelength (λexc.) 325
nm.
2.3 Results and discussion
2.3.1 Structural study
Fig. 2.1 shows the X-ray diffraction (XRD) patterns of undoped PTh and
Al2O3/ PTh nanocomposites. For undoped sample ‘a’ exhibits only single
peak at 2θ = 20.910 corresponding to (001) plane and four peaks along
(001), (120), (111) and (110) planes are clearly observed in doped
samples which indicate that all the samples are polycrystalline in nature
and confirmation of synthesis of polythiophene. Some additional peaks of
PTh are also seen in doped samples indicates doping effect of low
percentage of Al2O3 dopants on the lattice structure of PTh. After doping
these peaks are found to have shifted, particularly the peak appearing at
20.910 in undoped polythiophene sample which may be attributed to the
modification of crystal structures after doping [11-12]. The diffused
diffraction maxima and line broadening confirms the formations of
55
nanometre-sized particles. It is well established that during doping
processes of metal-oxides in polythiophene, it undergoes interfacial
interaction with metal crystallites and losses their original morphology.
The sample doped with 8 wt.% Al2O3 tends to change the polycrystalline
nature into single crystalline nature. XRD results indicate the weak
crystalline quality. It is evidenced from XRD results that with increase of
Al2O3 percentage, the crystallinity of prepared PTh samples are also
improved. None of the XRD patterns show any evidence of Al2O3 or
other impurity phase. The crystallite sizes of all the samples are
calculated using Debye–Scherrer’s (DS) formula:
cos
ktDS ,
Where tDS is the crystallite size, λ is the wavelength of radiation used, θ
is the Bragg’s angle and β is the full-width at half-maximum (FWHM)
measured in radian. The average crystallite size of all the samples lies in
the range between 22 to 34 nm.
2.3.2 Morphological study
The SEM is used to study the surface morphology of as-synthesized
samples. The SEM micrographs of as-synthesized samples of pure
polythiophene and Al2O3/PTh nanocomposites are shown in Fig. 2.2.
Figure shows the formation of spherical shape of nanostructures. As-
synthesized samples of undoped polythiophene and Al2O3/PTh
nanocomposites exhibit many pores on the surface of nanostructures. The
formations of spherical shaped nanostructures during polymerization
process are investigated [13] with schematic representation. In these
schematic, during occurrence of polymerization, the surfactant CTAB
forms micelles in the starting of reaction. When ammonium persulfate is
added to the solution of CTAB and triethanolamine (TEA), some white
56
precipitate appears immediately. It is found that the colour of white
precipitate gradually changed into black after few minutes. The
precipitate (CTA)2S2O8 forms lamellar meso-structure and provides
templates for polymerization that plays an important role in the
morphology transformation of PTh [14]. Furthermore, the monomer is
polymerized by the anion of (CTA)2S2O8 and form a sheet like structures.
The lamellar inorganic/organic meso-structure based templates form
during polymerization of surfactant cations with oxidizing anions which
further degraded automatically after polymerization process done. During
polymerizations, some enlarge holes are appeared on the surface which
forms a unique ‘sphere-fibre-transition’ like structures. Such type of
structure formation during polymerization is concentrated and then
broken into the spherical shaped nanoparticles during secondary growth
[15]. These transformations are outlined as shown in Scheme 1.
The number and size of pores increases with increasing doping
percentage of Al2O3.These pores are very useful in sensing properties.
The change in morphology can be explained by the adsorption and
intercalation of polythiophene on the surface of Al2O3. As a result of this
absorption process, Al2O3 are finely coated with polythiophene particles
by polymerization of thiophene monomers. Thus, it is aptly believed that
adsorption probability of thiophene monomer on the whole surface of
Al2O3 is equipotent, resulting in the formation of continuous
polythiophene coating on the surface of Al2O3 [16]. Therefore, due to the
change in surface morphology, porosity of the PTh increases with the
addition of Al2O3.
57
Scheme 1: Schematic representations of the proposed mechanism of
forming spherical polythiophene.
58
Fig. 2.1: X-ray diffraction patterns of (a) undoped PTh and Al2O3-doped
with (b) 2 wt%, (c) 4 wt%, (d) 6 wt% and (e) 8 wt% PTh nanocomposites.
59
Fig. 2.2: SEM images of the (a) undoped PTh and, Al2O3-doped with (b)
2 wt%, (c) 4 wt%, (d) 6 wt% and (e) 8 wt% PTh nanocomposites.
60
2.3.3 Optical properties
Fourier Transform Infrared Spectroscopy
In order to observe the nature of bonding in the prepared samples, we
investigated FTIR transmission spectra of pure polythiophene and Al2O3/
PTh nanocomposites are shown in Fig. 2.3. The FTIR spectra of all the
samples show strong absorption band in the range of 500-3200 cm-1
,
which correspond to the characteristics of polythiophene [17]. It is similar
to the reported standard FTIR results of polythiophene and quite different
from that of monomer based thiophene. This result further confirms the
successful polymerization of thiophene monomer and the formation of
polythiophene [17], which is well evidenced in XRD and SEM results.
The absorption bands appearing in the range of 600-800 cm-1
indicate
singly substituted benzene ring [18]. The peak located at around 1064 cm-
1 that can be assigned to the C-H in plane bending vibrations and its
intensity increases with incorporation of Al2O3. In addition, the shift in
peak position towards lower wave number up to 1053 cm-1
is also
observed after incorporation of 4 wt.% Al2O3 [1]. The peak appeared at
1433 cm-1
in pure polythiophene is attributed to υ cycles [1] which
changes after incorporation of Al2O3.
The broad peaks in FTIR spectra present at 1635 cm-1
and 3208 cm-1
are
associated with aromatic C=C stretching and C-H stretching, respectively
[1]. Additional peak at 2919 cm-1
is observed in the samples doped after 4
wt.% Al2O3 which may be attributed to C-H inplane-bending. The large
descending base line in the spectral region of 4000-2000 cm-1
is attributed
to the free-electron conduction in the doped polymer [19]. The observed
bands shifts in undoped polythiophene and appearance of extra bands in
Al2O3/PTh composites samples indicate formation of complexes between
Al2O3 molecule and polythiophene [1].
61
Fig. 2.3: FT-IR spectra of (a) undoped PTh, and Al2O3-doped with (b) 2
wt%, (c) 4 wt%, (d) 6 wt% and (e) 8 wt% PTh nanocomposites.
62
UV-visible absorbance spectroscopy
Fig. 2.4 shows the UV-visible absorption spectra of undoped
polythiophene and Al2O3/PTh nanocomposites at room temperature. The
obtained spectra are recorded with base line correction by
spectrophotometer in the wavelength ranges 200-900 nm. The absorption
spectra of all the samples exhibit absorption peak at around 300 nm
which is due to π- π* inter-band-transition of PTh rings. In this study,
small change in optical absorption spectra is observed which can be
associated with the degree of oxidation [20-21]. The change in optical
absorption spectra after doping of Al2O3 in polythiophene indicates
interaction between the Al2O3 and polythiophene [22].
Photoluminescence study
Photoluminescence is one of the suitable techniques to determine the
crystalline quality and the presence of impurities/defect states lies inside
the materials as well as exciton fine structures [11]. Generally,
polythiophene based sample exhibits optical emission in the half oxidized
states under solid form [23]. Fig. 2.5 illustrates the photoluminescence
spectra of undoped as well as Al2O3/PTh nanocomposites. In the PL
spectra of undoped and Al2O3/PTh nanocomposites, we observed mainly
three visible emission peaks centred at around 462 nm, 490 nm and 522
nm. The two emission peaks 462 nm and 490 nm in the Soret band region
where as single peak at 522 nm in the Q band emission [24]. The blue-
green emission peak at 490 nm may be originated from the side-chains.
PL intensity of polythiophene nanocomposites are rarely affected by self-
absorption due to the presence of thin shell layer of polythiophene. The
PL spectra of the polymers were almost the same, which indicates that
there is intramolecular energy transfer of the excitons from the
conjugated side chain to the main chain [25].
63
Fig. 2.4: UV-visible absorbance spectra of (a) undoped PTh, and Al2O3-
doped with (b) 2 wt%, (c) 4 wt%, (d) 6 wt% and (e) 8 wt% PTh
nanocomposites.
64
Fig. 2.5: Photoluminescence (PL) spectra of (a) undoped PTh, and Al2O3-
doped with (b) 2 wt%, (c) 4 wt%, (d) 6 wt% and (e) 8 wt% PTh
nanocomposites.
65
2.4 Conclusion
Undoped and Al2O3/polythiophene nanocomposites have been
synthesized by chemical oxidation method. The samples are characterized
by XRD, SEM, UV-vis, PL and FTIR spectroscopy. XRD spectra show
the polycrystalline nature of all the samples. SEM images are indicating
formation of spherical shape of nanostructures. As-synthesized samples
of undoped polythiophene and Al2O3/PTh nanocomposites exhibit many
pores on the surface of nanostructures. Synthesis of Al2O3/polythiophene
composite material is confirmed by FTIR spectroscopy. UV-visible
absorption spectra show absorption peak at around 300 nm which is due
to π- π* inter-band-transition of PTh rings. A small change in optical
absorption spectra is observed which can be associated with the degree of
oxidation. PL spectra exhibit mainly three visible emission peaks at
around 462 nm, 490 nm and 522 nm. The two emission peaks 462 nm and
490 nm in the Soret band region where as single peak at 522 nm in the Q
band emission. The intensity and peak position of polythiophene have
been randomly changed with amount of Al2O3 dopant.
66
Chapter 3
Synthesis and Characterization of Chemically Synthesized
Undoped and CuO Doped Polyaniline Nanocomposites
3.1 Introduction
Polyaniline (PAni) is one of the most studied conducting polymers known
to date because of it is relatively cheap, easy to synthesise and very stable
under a wide variety of experimental conditions. Like other classes of
conducting polymers, polyaniline is also easy to handle and can be
readily processed into polymeric blends [1]. Intrinsically conducting
polymer nanocomposites have been used in various kinds of electronic
devices.Conducting polymers are combined with metal oxides because of
their enhanced physical and electronic properties and find useful
applications in various devices such as sensors, electrodes, batteries and
photovoltaics [2]. Polymer-metal composites are attracting considerable
attention of the researchers due to their striking advantageous properties
such as easy processing flexibility and light weight [3] etc. Metal oxides
such as ZnO, CdS, Al2O3, TiO2 and CuO have attracted much attention to
researchers and scientific communities due to their potential applications
in electrical, optical and optoelectronics devices. Among these metal
oxides, cupric oxide (CuO), a versatile semiconductor material, has been
attracting attention because of the commercial demand for optoelectronic
devices operating at blue and ultraviolet regions [4]. Cupric oxide (CuO)
has been a hot topic among the studies on transition metal-oxides (MOs)
because of its interesting properties as a p-type semiconductor with
narrow band gap energy of 1.5–1.8 eV [5-6]. It has very large excitation
binding energy (60 meV) at room temperature [5-6]. CuO has recently
due to its exotic properties and wide applications from
67
heterogeneouscatalysts, gas sensors [7, 8], field-emission emitters to high
temperature superconductors [9], magnetic storage media [10], solar
cells, lithium ion electrode materials, catalysis [11,12] and field emitters
[13] and etc. [14, 15] because it has good mechanical flexibility and
environmental stability as well as its resistivity could be controlled with
acid/base (doping/undoping), it has application in various areas, such as
light weight battery electrode, electromagnetic shielding device,
anticorrosion coatings, solar cells, photodetectros and sensors [16–19].
As one of the important metal oxides, cupric oxide (CuO) is frequently
used as anode materials due to its high capacity, environmental friendly,
safety and low-cost [20-22]. Therefore, the preparation of composite of
polyaniline and metal oxide becomes a novel challenge for people. CuO
is stable in the acidic and oxidative environments while polymerizing
aniline together with low cost. CuO could therefore be a good candidate
as seed to fabricate different PAni/CuO nanostructures targeting at
different physiochemical properties. In the present chapter, PAni and its
composite with CuO have been synthesized successfully by chemical
oxidation method. X-ray diffraction (XRD) results show the
polycrystalline nature of prepared samples. The crystallinity of composite
materials is found to increase with increasing doping percentage of CuO.
The shifting of some bands and appearance of some extra bands in FTIR
results of doped samples with respect to undoped PAni [23-26] have been
discussed in the present chapter.
68
3.2 Experimental
3.2.1 Pellet Preparation
Aniline hydrochloride (2.59 gm) was dissolved in distilled water in a
volumetric flask to make 50 ml solution. Ammonium peroxydisulfate
(5.71 gm) was dissolved in water also to make 50 ml of solution. Both the
solutions were kept for 1 hour at room temperature. They were then
mixed with a brief stirring and left at rest to polymerize. The solution
turned to dark green within few minutes. Next day Polyaniline (PAni)
precipitate was collected on a filter paper, washed three times with 100
ml portions of 0.2M HCl to remove the unreacted aniline and its
oligomers from the precipitate. After this process, precipitate was washed
three times with 100 ml portions of acetone to absorb the water molecules
and for the removal of any residual organic impurities. PAni, synthesized
by this method, is formed in its protonated state. The precipitate was
firstly dried in air for 30 min and then in oven for 3 hours at 60°C [27].
The synthesized PAni has been doped by CuO in ratio of 2, 4, 6 and 8
wt%. The undoped and CuO doped PAni composite was ground in form
of fine powder. Pellets were prepared by compressing the powder under a
pressure of 10 tons with the help of a hydraulic press machine. All the
pellets were annealed at 100°C for one hour. The thickness of the pellets
of composite samples was found to be 0.65 mm.The diameter of the
pellets was also found to be 13 mm. The 0, 2, 4, 6 and 8 wt% CuO doped
PAni composite are denoted as samples a, b, c, d and e respectively.
3.2.2 Characterizations
X-ray diffraction (XRD) spectra of all the samples recorded by Phillips
X’pert PW3020 diffractometer using Cu Kα radiation (λ=1.54056 Ao)
were presented for structural analysis of the samples. The scanning
69
electron microscopy (SEM) images of all the sample pellets were taken
by scanning electron microscope (Model-430, LEO Cambridge,
England). FTIR spectra of all the samples in the form of powder were
recorded on the Bruker Alpha spectrometer to determine the formation of
Polyaniline. To record absorbance spectra, 0.01 gm of each sample is
dissolved in 5 ml of dimethyl formamide (DMF). The UV-visible
absorption spectra of the samples were recorded with UV-vis
spectrophotometer (Model No.V-670 Jasco) and PL spectra of all the
samples were recorded using LS-55, Perkin Elmer fluorescence
spectrometer at room temperature with excitation wavelength (λexc.) 325
nm.
3.3 Results and discussion
3.3.1 X-ray Diffraction
XRD spectra of the pure PAni and the CuO mixed PAni composite
samples a, b, c, d, and e (Fig. 3.1) show the weak crystalline quality of all
the samples. There is a main peak around at 2θ = 260 in sample a while at
around 290 in samples b, c, d and e, which correspond to (100) and (110)
planes respectively [28]. There is no peak for the cupric oxide in the
composite samples, which indicates that the low percentage of CuO does
not affect the lattice structure of PAni, similar type of result has been
reported in literature [27]. Thus the XRD spectra suggest that during the
doping of metal oxides in PAni, it undergoes interfacial interactions with
metal crystallites and losses its own morphology. The crystallite size can
be estimated with the help of full width at half maximum (FWHM) of the
X-ray diffraction data. The broadening of the FWHM is inversely
proportional to the average crystallite size, D, as predicted by the well-
70
known Scherer's formula. The crystallite size, D, is calculated from the
following relation [29]:
D = kλ/β cosθ
where, λ is the X-ray wavelength; k, the shape factor; D, the average
diameter of the crystals in angstroms; θ, the Bragg angle in degree; and β
is the line broadening measured by half-height in radian. The value of k
depends on several factors including the miller index of the reflection
plane and the shape of crystal. If the shape is unknown, k is often
considered to be 0.89. The crystallite size of PAni/CuO nano-composites
is formed to lies between 20 to 50 nm.
71
Fig 3.1: X-ray diffraction spectra for the samples a, b, c, d and e, curves
correspond to 0, 2, 4, 6 and 8wt% CuO doped PAni nanocomposites
respectively.
72
3.3.2 Scanning Electron Microscopy
Scanning electron microscope (SEM) is used to study the surface
morphology of the prepared samples. SEM images of the samples show
the formation of spongy structures as shown in Fig. 3.2, which are almost
the same for all the undoped and PAni/CuO nanocomposite samples.
Figure 3.2 (a–e) corresponds to 0, 2, 4, 6, and 8 wt % CuO and PAni/CuO
nanocomposites respectively. The spongy structure formation in the
polyaniline takes place by heterogeneous nucleation. As a result, granular
coral like structures are formed. As a characteristic of polyaniline,
secondary nucleation also takes place because of which the granular coral
like particles come together to form aggregates [29]. We noticed that as
the amount of CuO was increased; the number of pores and the size of
pores were also increased, which is very important for sensing properties.
The change in morphology can be explained by the adsorption and
intercalation of PAni on the surface of CuO. There is another possibility
that the CuO is sandwiched between the PAni layers or CuO uniformly
mixed into the PAni matrix [30-31]. The aniline monomer is likely to be
absorbed onto the surface of CuO through electrostatic attraction and by
the formation of weak charge-transfer complexes between aniline
monomer and the structure of CuO [29]. As a result of this absorption
process, CuO are finely coated by PAni particles by the polymerization of
aniline monomer. Thus, it is suitably believed that adsorption probability
of aniline monomer on the whole surface of CuO is equipotent, which
results in the formation of continuous PAni coating on the surface of
cupric-oxide. Therefore, the change in surface morphology causes the
porosity of the PAni which increases with the addition of the CuO.
73
Fig. 3.2: Scanning electron microscope (SEM) images of samples a, b, c,
d and e images a, b, c , d and e correspond to 0, 2, 4, 6 and 8 wt% CuO
doped PAni nanocomposite respectively.
74
3.3.3 Optical properties
Fourier Transform Infrared (FTIR) spectroscopy
Fourier transform infrared (FTIR) spectra of all the samples of undoped
PAni and PAni/CuO nanocomposite at different wt% are obtained in the
absorbance range 500–4000 cm-1
which are shown in Fig 3.3. The
vibrational bands observed for undoped PAni and PAni/CuO
nanocomposite are explained on the basis of normal modes. FTIR Spectra
of all the samples of undoped PAni and PAni/CuO nanocomposite show
strong absorption bands in the region 805–1591cm-1
which correspond to
the characteristics of polyaniline. The absorbance band at around 805 cm-
1 observed for undoped PAni and PAni/CuO nanocomposite samples
show characteristics peaks of the C-H out-of plane bending vibration of
the 1, 4-distibuted benzene ring. The observed peak around 1143 cm-1
for
undoped PAni and PAni/CuO nanocomposite C-H bending vibration and
observed peaks around 1309 cm-1
C-N stretching vibration for undoped
PAni and PAni/CuO nanocomposite [32]. The observed peak around
1475 cm-1
is attributed to stretching vibration of C=C of the benzenoid
ring. The absorption peaks observed around 1591 cm-1
is attributed to
C=C stretching vibration of the quinoid ring form doped PAni and
PAni/CuO nanocomposite [29].The observed peaks around at 2338 cm-1
,
3740 cm-1
for the undoped PAni and different weight percentage of
PAni/CuO nanocomposite can be probably related to the valence
oscillation of the C-H and N-H bond stretching within the benzene rings,
which have been associated with electrical conductivity and high degree
of electron delocalization in PAni [33]. The splitting and intensity of
absorption band on increasing the CuO weight percentage [Fig. 3.3 (b-e)]
suggest the presence of higher extent of protonation in these samples.
75
UV-visible Absorption Spectra
The UV–visible absorption spectra of the undoped PAni and the
PAni/CuO nanocomposite are recorded at room temperature by using a
spectrophotometer between the wavelength range 200–800 nm as shown
in Fig.3.4. Optical spectroscopy is an important technique to understand
the conducting states corresponding to the absorption bands of inter and
intra gap states of conducting polymers. Fig.3.4 illustrates the major
absorption peaks at around 305 nm. The observed bathochromic shift at
the intense band 305 is due to the π-π* transition of benzenoid ring which
is related to the extent of conjugation between the adjacent phenylene
rings in the polymeric chain and the forced planarization of π-system
induced by aggregation.It leads to increased conjugation and thus lowers
the band gap which is well agreed with the band gap result obtained in the
Polyaniline. The transition of π-π* of benzenoid ring and the formation of
polaron band in the nanocomposites are responsible for increase of the
electrical conductivity of the nanocomposite [28, 33]. Fig 3.4
demonstrates the high intense blue shift of absorption peaks of PAni from
its actual position in PAni/CuO nanocomposite which indicates that the
addition of CuO filler particles in the polyaniline matrix has large
influence on absorption spectra in the PAni/CuO nanocomposite [35, 36].
As seen in Fig 3.4 a, b, c, d and e as the dopant percentage increases the
absorbance peaks is decreases due to hypochomic effect [37].The
difference between the two spectra is due to the presence of an electron
with drawing sulfonic group in the complex and therefore the transition
band is observed at a lower wavelength.
76
Fig. 3.3: Fourier transform infrared (FTIR) spectra for the sample a, b, c,
d and e curves a, b, c, d and e correspond to 0, 2, 4, 6, and 8wt% CuO
doped PAni composite respectively.
77
Fig. 3.4: UV–visible absorption spectra for the samples a, b, c, d and e at
room temperature. Curves a, b, c, d and e correspond to 0, 2, 4, 6 and 8 wt
% CuO and PAni/CuO composites respectively.
78
Photoluminescence Studies
Photoluminescent organic molecules are a new class of compounds with
interesting properties. They undergo emission over a wide range from the
violet to the red. They can also be combined in several different forms to
produce white light. One category of organic material with
photoluminescence properties is conjugated organic polymers. PL spectra
were measured for all the four samples in the range of 300-650 nm and
the wavelength of excitation chosen for all the samples is 325 nm. The
photoluminescence spectroscopy (PL) of CuO doped PAni has been
performed and spectra is shown in Fig. 3.5. The PL spectra of 0, 2, 4, 6
and 8 wt% CuO doped PAni samples show peaks in visible region around
at 362 nm, 405 nm, 459 nm, 486 nm and 528 nm. The relative heights of
the emission peaks alter with different dopant concentrations and nature
of solvents is due to polarity. In addition, this peak becomes sharp and
intense which may be due to inter-chain species that plays an important
role in the emission process of conjugated polymers [38]. The intensity of
peaks depends on factors such as polymer coil size the nature of polymer-
solvent, polymer-dopant interactions, and the degree of chain overlapping
[38]. The PL spectra of all the samples have the same shape, which
indicates that it is an efficient way to tune the intensities of the peak by
employing specific dopant with different wt% at different concentration
levels. Overall, it is clear that the nature of conjugated polymer
aggregation depends upon many factors, including the polymer coil size,
the nature of polymer–solvent and polymer–dopant interactions and the
degree of chain overlapping [39].
79
Fig. 3.5: Photoluminescence (PL) spectra for the samples a, b, c, d and e
at room temperature. Curves a, b, c, d and e correspond to 0, 2, 4, 6 and 8
wt % CuO and PAni/CuO composites respectively.
80
3.4 Conclusion
In this chapter, we have synthesized undoped and CuO/PAni
nanocomposites by the chemical oxidation method at room temperature.
The prepared samples have been characterized by XRD, SEM,UV-vis,PL
and FTIR. XRD spectra show weak crystalline quality of all the samples,
whereas the PAni synthesized is amorphous in nature. The scanning
electron microscopy (SEM) images of all the samples show granular coral
like structure. The study of FTIR spectra confirm the formation of
conducting PAni and also suggests that doped of CuO in PAni does not
affect the structures. The UV–visible absorption spectra of the solutions
of all the samples contain some peak at 305 nm.The observed
bathochromic shift at the intense absorption band 305nm is due to the π-
π* transition of benzenoid ring.The PL spectra of 0, 2, 4, 6 and 8 wt%
CuO doped PAni samples show peaks in visible emission peaks which at
around 362 nm, 405 nm in violet region 459 nm, 486 nm in blue region
and 528 nm in green region.
81
Chapter 4
Synthesis and Characterization of Undoped and Al2O3
Doped Polypyrrole Nanocomposites
4.1 Introduction
Conducting polymers (CPs) such as polythiophene (PTh), polyaniline
(PAni) and polypyrrole (PPy) however, arouse an immense interest
among researchers because of their curious electronic, magnetic and
optical properties [1-5]. Among the CPs, Polypyrrole (PPy) is one of the
most investigated polymers due to their environmental stability, relative
ease of synthesis, good electro-optical and mechanical properties [6].
Potential technological applications such as in electronic and
electrochromic devices, sensors, counter electrode in electronic
capacitors, chromatographic stationary phases, light-weight batteries, and
membrane separation consequently, have attracted great deal of attentions
recently [7-16]. Long term stability of PPy is a key factorfor application
of new polymeric material in future applicationsand seems to be a good
candidate [17]. Polymer-metal composites are attracting considerable
attention of the researchers due to their striking advantageous properties
such as easy processing flexibility and light weight [18-22]. Polypyrrole
is one of the most stable conducting polymers and also one of the easiest
to synthesize. It displays a good conductivity in combination with high
stability in its oxidized form. A polypyrrole that is polymerized
electrochemically or chemically is known to be insoluble.
Electrochemically polymerization on a metal electrode results in good
quality films [23], while chemical polymerization yields fine conducting
powders [24]. PPy is most frequently used in commercial application
such as batteries, super capacitors, sensors and corrosion protection.
Polymer–inorganic nano particle hybrids have attracted great
82
attention, since they have interesting physical properties and potential
applications [25]. Electronically conducting polymers have been great
interest to chemist and physicists in recent years because of large number
of possible applications of these materials in various electronic devices
such as electrochromic displays (ECD), light emitting diodes (LEDs),
field effect transistors (FETs), chemical sensors etc. [26]. Among those
conducting polymers, polypyrrole (PPy) is especially promising for
commercial applications because of its good environ-mental stability,
facile synthesis and higher conductivity than many other conducting
polymers. PPy can often be used as biosensors, gas sensors wires, micro
actuators, anti-electrostatic coatings, solid electrolytic capacitor,
electrochromic windows and displays, and packaging, polymeric
batteries, electronic devices and functional membranes, etc [27-35].
Aluminium oxide is a chemical compound of aluminium and oxygen with
the chemical formula Al2O3. It is also called alumina.It has a wide band-
gap of ~8.8 eV for bulk material.It is an electrical insulator but has a
relatively high thermal conductivity around 30 Wm−1
K−1
for a ceramic
material.Aluminium oxide is insoluble in water.Al2O3 has been
extensively investigated dopants to serve as catalysts, fire redundant,
absorbents and fillers for structural materials [36]. It is stable in acidic
and oxidative mediums and well known for reactivity with aromatic
organic materials.
Several methods have been used in the preparation of polypyrrole (PPy)
and a wide range of its derivatives by simple chemical or electrochemical
methods [13-16, 37-39]. In the present chapter, chemical polymerization
method is used in the preparation of PPy. It is a simple and fast process
with no need for special instruments. Bulk quantities of polypyrrole (PPy)
can be obtained as fine powders using oxidative polymerization of the
monomer by chemical oxidants in aqueous or non-aqueous solvents [15-
83
16, 37, 40] or by chemical vapour deposition [38]. However, the use of
chemical polymerization limits the range of conducting polymers that can
be produced since only a limited number of counterions can be
incorporated. The chemical polymerization of Pyrrole appears to be a
general and useful tool for the preparation of conductive composites
[41,42] and dispersed particles in aqueous media [43, 44].
4.2 Experimental
4.2.1 Synthesis of Polypyrrole
The polypyrrole was prepared by chemical polymerization method. In
this approach, 1M pyrrole solution was prepared using distillation and
then mixed with an oxidizing agent ammonium persulphate slowly added
under constant stirring for 30 minutes. Then the polymerization was
conducted for 4 hours under constant stirring. This preparation was kept
un-agitated for 24 hours so that polypyrrole powder settled down. The
polypyrrole powder was filtered out and washed with distilled water
several times to remove any impurities present in the samples. The
synthesized polypyrrole has been doped by Al2O3 in ratio of 2, 4 and 6
wt%. The undoped and doped polypyrrole was ground in form of fine
powder. Pellets were prepared by compressing the powder under a
pressure of 10 tons with the help of a hydraulic press machine. All the
pellets are annealed at 1000C for 1hour. The diameter of the pellets was
found to be 13 mm. The 0, 2, 4 and 6wt% Al2O3 doped polypyrrole
samples was denoted as samples a, b, c, and d respectively.
4.2.2 Characterizations
The XRD spectra of all the samples recorded by Phillips X’pert PW3020
diffractometer using CuKα radiation (λ=1.54056 Ao) were presented for
structural analysis of the samples. The SEM images of all the pellets were
taken by scanning electron microscope (Model-430, LEO Cambridge,
84
England). FTIR spectra of all the samples in the form of powder were
recorded on the Bruker Alpha spectrometer to determine the formation of
polythiophene. To record absorbance spectra, 0.01 gm of each sample is
dissolved in 5 ml of dimethyl formamide (DMF). Then the absorption
spectra of the solutions thus formed were recorded with UV-vis
spectrophotometer (Model No.V-670 Jasco). PL spectra of all the
samples were recorded using LS-55, Perkin Elmer fluorescence
spectrometer at room temperature with excitation wavelength (λexc.) 325
nm.
4.3 Results and discussion
4.3.1 X-Ray Diffraction (XRD)
XRD spectra of undoped PPy and PPy/Al2O3 nanocomposites are shown
in Fig. 4.1. Undoped sample shows only one broad peak at 260 which
shows poor crystallinity phase of PPy (curve a) and corresponds to (101)
plane. Samples b, c and d shows two main peaks one at 25.850 and other
at 30.890 which corresponding to (101), (004) plane of PPy/Al2O3
nanocomposites. XRD patterns of PPy/Al2O3 nanocomposites show that
the broad weak diffraction peak of PPy still exists, but its intensity has
been decreased. It implies that when pyrrole is polymerized on Al2O3,
each phase maintains his initial structure [45-46].The crystallite size, D,
is calculated for undoped PPy and PPy/Al2O3 nanocomposites using
relation [47]:
D = kλ/β cosθ,
where, λ, is the X-ray wavelength; k, the shape factor; D, the average
diameter of the crystals in angstroms; θ, the Bragg angle in degree; and β
is the line broadening measured by half-height in radian. The value of k
depends on several factors including the miller index of the reflection
plane and the shape of crystal. If the shape is unknown, k is often
85
considered to be 0.89. The crystallite sizes of PPy/Al2O3 nano-composites
are formed to lies between 10 nm to 20 nm.
4.3.2 Scanning Electron Microscopy (SEM)
Scanning electron microscopy (SEM) is used here to study the surface
morphology of the samples. SEM images of the samples show the
formation of spongy structures (Fig.4.2), which are almost the same for
undoped and PPy/Al2O3 nanocomposite samples. Figure 4.2(a–d)
corresponds to 0, 2, 4 and 6 wt% PPy/Al2O3 nanocomposites
respectively. The spongy structure formation in the polypyrrole takes
place by heterogeneous nucleation. As a result granular coral like
structures are formed. As a characteristic of polypyrrole, secondary
nucleation also takes place because of which the granular coral like
particles come together to form aggregates. We noticed that as the
amount of Al2O3 is increased, the number of pores and the size of pores
are also increased, which is very important parameter for sensing
properties. Small change in the morphology has been observed in SEM
images after doping. The change in morphology can be explained by the
adsorption and intercalation of PPy on the surface of Al2O3. There is
another possibility that the Al2O3 is sandwiched between the PPy layers
or Al2O3 uniformly mixed into the PPy matrix. The pyrrole monomer is
likely to be absorbed onto the surface of Al2O3 through electrostatic
attraction and by the formation of weak charge-transfer complexes
between pyrrole monomer and the structure of Al2O3. As a result of this
absorption process Al2O3 are finely coated by PPy particles by the
polymerization of pyrrole monomer. Thus, it is appropriately believed
that adsorption probability of pyrrole monomer on the whole surface of
Al2O3 is equipotent, resulting in the formation of continuous PPy coating
on the surface of Al2O3. Therefore, because of the change in surface
morphology, porosity of the PPy increases with the addition of the Al2O3.
86
Fig. 4.1: XRD spectra of the samples a, b, c and d i.e.0, 2, 4 and 6 wt%
doping of Al2O3 in PPy Curves a, b, c and d correspond to samples a, b ,c
and d respectively.
87
Fig. 4.2: SEM images of the samples a, b, c and d i.e. 0, 2, 4 and 6 wt%
doping of Al2O3 in PPy/Al2O3 nanocomposites. Curves a, b, c and d
correspond to samples a, b, c and d respectively.
88
4.3.3 Optical Properties
Fourier Transform Infrared (FTIR) Spectroscopy
Fourier transforms infrared (FTIR) spectra of all the samples are obtained
in the range 500–4000 cm-1
and are shown in Fig. 4.3. The peak around at
936cm-1
is observed in all samples that are because of N–H out-of-plane
bending [48]. The C–N stretching vibrations mode in the polymer chain
gives rise to peak at 1109 cm-1
. The peak around at1400cm-1
may be
attributed to the in-plane deformation of the N–H bonds. The peaks
around at1580 cm-1
may be attributed to N-H bending vibration [49-50].
The absorption peaks are present in all the samples from (a – d) and no
significant shift is observed in any of the samples.
UV-visible absorption spectroscopy
The UV-visible absorption spectra of the polypyrrole and the PPy/Al2O3
nanocomposites are recorded at room temperature by using a
spectrophotometer between the wavelength range 275-650 nm and are
shown in Fig. 4.4. The UV-vis absorption can significantly determine the
interaction between the Al2O3 and PPy. Solutions of all the samples show
peak at 306 nm. The peak at 306 nm is associated with the exciton
transition of π–π* [51-54]. Intensity of the peak is randomly varied as the
dopant concentration increased and there is no shift in the peak at 306
nm.
89
Fig.4.3: FT-IR spectra of the samples a, b, c and d i.e. 0, 2, 4 and 6 wt%
doping of Al2O3 in PPy/Al2O3 nanocomposite. Curves a, b, c and d
correspond to samples a, b, c and d respectively.
90
Fig. 4.4: UV-vis absorption spectra of the samples a, b, c and d i.e. 0, 2, 4
and 6 wt% doping of Al2O3 in PPy/Al2O3 nanocomposites. Curves a, b, c
and d correspond to samples a, b, c and d respectively.
91
Photoluminescence (PL) spectroscopy
The photoluminescence (PL) properties of the polypyrrole/Al2O3
nanocomposites are studied using LS-55, Perkin Elmer fluorescence
spectrometer at room temperature with excitation wavelength (λexc.)
325nm. Fig.4.5 shows the main emission band of the nanocomposites is
located at 365 nm with two shoulders at 473 nm and 533 nm. The
observed reduced height of the photoluminescence emission intensity
peaks with increased wt% Al2O3 doped PPy might due to the possibility
of atoms/molecules of dopant (Al2O3) forming aggregation in the polymer
chains [55-57].The direct band gap is calculated by using this formula,
(Eg= hc/λ) where, ‘h’ is a constant, ‘c’ is velocity of light, ‘λ' is emission
wavelength in photoluminescence spectrum. The direct band gap energies
of the PPy/Al2O3 composite of different ratios are found as 3.09 and 2.19
eV. The band gap gets decreased due to increased content of Al2O3
nanoparticles. As the luminescence of these oxide/polymer
nanocomposites is proportional to the surface features, it is possible to
tailor the wavelength and the intensity of the luminescence by varying the
particle size.
92
250 300 350 400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
Inte
ns
ity
(a
.u.)
Wavelength (nm)
Pure PPy
PPy: 2wt% Al2O3
PPy: 4wt% Al2O3
PPy: 6wt% Al2O3
Fig. 4.5: Photoluminescence of the samples a, b, c and d i.e. 0, 2, 4 and 6
wt% doping of Al2O3 in PPy/Al2O3 nanocomposites. Curves a, b, c and d
correspond to samples a, b, c and d respectively.
93
4.4 Conclusion
In this chapter, we have synthesised undoped and Al2O3 doped PPy
samples by the chemical oxidation method. The prepared samples have
been characterized by XRD, SEM, FTIR, UV-Vis absorption and PL
spectroscopy. X-ray diffraction patterns of PPy/Al2O3 nanocomposites
result show several broad peaks while undoped sample shows only one
single peak indicating poor crystalline phase of PPy. In the SEM images,
the results were found granular coral like structures. As a characteristic of
Polypyrrole, secondary nucleation also takes place because of which the
granular coral like particles come together to form aggregates. We
noticed that as the amount of Al2O3 was increased; the number of pores
and the size of pores were also increased, which is very important for
sensing. The study of FTIR spectra confirms the formation of PPY and
also suggests that doping of Al2O3 in PPY does not affect its structure.
The UV absorption can significantly determine the interaction between
the Al2O3 and PPy. Solutions of all the samples show peak, which
oriented around 306 nm. The peak at 306 nm is associated with the
exciton transition of π–π*. PL shows the main emission band of the
nanocomposites is located at 365 nm with two shoulders at 473 nm and
533 nm. The direct band gap energies of the PPy/Al2O3 nanocomposite of
different ratios are found as 3.09 and 2.19 eV. The band gap gets
decreased due to increased content of Al2O3 nanoparticles.
94
Chapter 5
Structural, Morphological and Optical Studies of Undoped
and Al2O3 Doped Polyaniline Thin Films
5.1 Introduction
Polyaniline is most attractive conducting polymer because of its low cost,
high environmental stability, good electrical conductivity and potential
applications in molecular electronics [1]. The electrical properties of
polymers can be modified by addition of inorganic fillers. Nanoscale
particles are more attractive due to intringuing properties arising from the
nanosize and large surface area [2-3]. Polyaniline is one of the most
studied conducting polymers known to date because of it is relatively
cheap, easy to synthesise and very stable under a wide variety of
experimental conditions. Like other classes of conducting polymers,
polyaniline is also easy to handle and can be readily processed into
polymeric blends [4]. Intrinsically conducting polymer thin films have
been used in various kinds of electronic devices [5-8]. Conducting
polymers are combined with metal oxides because of their enhanced
physical and electronic properties and find useful applications in various
devices such as sensors, electrodes, batteries and photovoltaics [9-14].
Polymer metal composites are attracting considerable attention of the
researchers due to their striking advantageous properties such as easy
processing flexibility and light weight [15-17]. However it is still difficult
to fabricate PAni with different nanostructures through a simple chemical
process. Polymer–metal composites are the potential candidates for the
researchers and scientific communities due to their remarkable
advantageous properties such as easy processing and light weight etc.
[18–22]. All conducting polymers exhibit reversible redox behavior with
95
a distinguished chemical memory and hence have been considered as a
most important class of new materials for the fabrication of biological
and chemical sensors. The adsorption and desorption of volatile species
cause a measurable change in the resistance of conducting polymers.
Conducting polymeric sensors have advantages over metal oxide sensors.
First a wide variety of polymers are available. Secondly, they are easily
grown by chemical polymerization of monomer and also sense at room
temperature [23–25]. Among the conducting polymers, polyaniline
(PAni) is advantageous because of comparatively high conductivity that
can be achieved by doping [25-28]. Fabrication of PAni with different
nanostructures without using any foreign material is difficult.
Conducting polymers such as polyaniline, polypyrrole, polythiophene etc,
have attracted considerable attention for their unique properties and their
potential application in a number of growing technologies. Their
supremacy is demonstrated by wide range of dependent applications such
as electrodes for batteries, energy storage, electrochemical display
devices, electromagnetic interference (EMI) shielding [29], corrosion
protection [30], organic light emitting diodes, plastic solar cells,
optoelectronic devices [31 ] etc. Most of the above applications are based
on thin film technology. In case of conducting polymers, such films can
be easily prepared by chemical/electrochemical polymerisation or by
solution processing techniques [34-36]. In the chemical polymerization
process, monomers are oxidized by oxidizing agents or catalysts to
prepare conducting polymers. The advantage of chemical synthesis is that
it offers mass production at reasonable cost. On the other hand
electrochemical method involves the direct formation of conducting
polymers with better control of polymer film thickness, internal
morphology and minimizing the contamination due to oxidant residues
[32-33,37-39]. Solution processing techniques such as spin coating are
96
used to prepare thin films of those polymers which are soluble in a
solvent or mixture of solvents. In spin coating process, film thickness and
morphology is controlled by spin rate and time of rotation, which make
them suitable for use in sensing, electro chromic and other electronic
applications.
Aluminium oxide is a chemical compound of aluminium and oxygen with
the chemical formula Al2O3. It is also called alumina.It has a wide band
gap of ~8.8 eV for bulk material.It is an electrical insulator but has a
relatively high thermal conductivity around 30 Wm−1
K−1
for a ceramic
material. Aluminium oxide is insoluble in water.Al2O3 has been
extensively investigated dopants to serve as catalysts, fire redundant,
absorbents and fillers for structural materials [40]. It is stable in acidic
and oxidative mediums and well known for reactivity with aromatic
organic materials. The use of inorganic component such as Al2O3 can
assist the process. It can serve as a catalyst, absorbent, fire retardant, as
well as filler for structural materials. In addition, Al2O3 is stable in both
acidic and oxidative environments during polymerization of aniline. Thus
Al2O3 is a good candidate as seed to fabricate different PAni/Al2O3
nanostructures. Several device applications of PAni include electrodes of
rechargeable batteries, sensors, electrochromic displays, and photovoltaic
devices [28].
5.2 Experimental
5.2.1 Synthesis of Polyaniline
Aniline hydrochloride (2.59 g) was dissolved in distilled water in a
volumetric flask to make 50 mL solution. Ammonium peroxydisulfate
(5.71 g) was dissolved in water of 50 mL of solution. Both the solutions
were kept for 1 hour at room temperature. They were then mixed with a
brief stirring and left at rest to polymerization. The solution turned to
97
dark green within few minutes. Now, we were prepared precursor
solutions for undoped PAni and PAni doped with Al2O3 as 2, 4, 6 and 8
wt %. The films were prepared on glass substrate by dip coating method
for all solutions. We take all the solutions in different beakers of 200 ml
and dipped one glass slide in each solution and rest it for 24 hours for the
deposition of thin films. After deposition of thin films on the glass slide,
we removed it from the solution one by one and washed several times
with 100 ml portions of 0.2 M HCl to remove the unreacted aniline and
its oligomers from the precipitate. After this process, thin films were
washed again several times with 100 ml portions of acetone to absorb the
water molecules and for the removal of any residual organic impurities.
Finally all thin films are annealed at 100 oC in furnace. The 0, 2, 4, 6 and
8 wt% Al2O3 doped PAni thin films are denoted as samples a, b, c, d and
e respectively.
5.2.2 Characterizations
The XRD spectra of all the sample of thin films recorded by Phillips
X’pert PW3020 diffractometer using CuKα radiation (λ=1.54056 Ao)
were presented for structural analysis of the samples. The SEM images of
all the thin films were taken by scanning electron microscope (Model-
430, LEO Cambridge, England). FTIR spectra of all the samples in the
form of thin films were recorded on the Bruker Alpha spectrometer to
determine the formation of PAni. The absorption spectra of the thin films
were recorded with UV-vis spectrophotometer (Model No.V-670 Jasco).
PL spectra of all the samples were recorded using LS-55, Perkin Elmer
fluorescence spectrometer at room temperature with excitation
wavelength (λexc.) 325nm.
98
5.3 Results and discussion
5.3.1 X-ray diffraction (XRD)
XRD spectra of the undoped and Al2O3 doped PAni samples a, b, c, d and
e (Fig. 5.1) show weak crystalline quality of all the samples. There is a
main peak around 2θ =24.39o
in sample a, b, c and around 24.55o in
samples d and e, which correspond to (110) plane. The same peaks of
PAni are earlier reported by Zhu et al. and Krishna et al [41, 42]. There is
no peak for the aluminium oxide in the composite sample or other
impurities level. In addition, the new peak at 2θ = 55.08o
in 8 wt% Al2O3
doped sample is found to have appeared that shows the higher doping
percentage of Al2O3, affects the lattice structure of PAni. The XRD
spectra suggest that during the doping of metal-oxides in PAni, it
undergoes interfacial interactions with metal crystallites and losses its
own morphology. The particle size for all the samples is estimated from
Debye–Scherrer’s (DS) formula:
cos
ktDS
where tDS is the crystallite size, λ is the wavelength of radiation used, θ is
the Bragg’s angle and β is the full-width at half-maximum (FWHM)
measured in radian. The calculated average crystallite size of all the
samples lies in the range between 30 to 55 nm.
99
Fig. 5.1: X-ray diffraction (XRD) spectra of the samples a, b, c, d and e
i.e. 0, 2, 4, 6 and 8 wt% doping of Al2O3 in PAni thin films. Curves a, b,
c, d and e correspond to a, b, c, d and e respectively.
100
5.3.2 Scanning Electron Microscopy (SEM)
Scanning electron microscopy (SEM) is used here to study the surface
morphology of the samples. SEM images of the samples show the
formation of spongy shaped spherical structures. In Fig. 5.2, samples a, b,
c, d and e correspond to 0, 2, 4, 6 and 8 wt% Al2O3 doped PAni
nanocomposites thin films respectively. The spongy structure formation
in the polyaniline takes place by heterogeneous nucleation. As a result,
granular coral like structures are formed. As a characteristic of
polyaniline, secondary nucleation also takes place because of which the
granular coral like particles come together to form aggregates. We have
noticed that as amount of Al2O3 is increased, the granular coral like
structures changes into spongy shaped spherical structures which are
clearly evidenced in 8 wt% Al2O3 doped PAni sample. The change in
morphology can be explained by the adsorption and intercalation of PAni
on the surface of Al2O3. There is another possibility that the Al2O3 is
sandwiched between the PAni layers or Al2O3 uniformly doped into the
PAni matrix as reported in literature [43]. The aniline monomer is likely
to be absorbed onto the surface of Al2O3 through electrostatic attraction
and by the formation of weak charge-transfer complexes between aniline
monomer and the structure of Al2O3. As a result of this absorption
process, Al2O3 are finely coated by PAni particles by the polymerization
of aniline monomer [43].
101
Fig. 5.2: SEM images of the samples a, b, c, d and e i.e. 0, 2, 4, 6 and 8
wt% doping of Al2O3 in PAni thin films. Curves a, b, c, d and e
correspond to samples a, b, c, d and e respectively.
102
5.3.3 Optical properties
Fourier transform infrared (FTIR) Studies
Fourier transform infra-red (FTIR) spectra of undoped PAni and
PAni/Al2O3 thin film samples are recorded in the transmission range 400
to 4000 cm-1
are shown in Fig. 5.3. In a spectrum the band observed at
3739 cm-1
is due to N-H stretching. The polymer shows the broad peak at
2346 cm-1
is associated with NH + unsaturated amine [43-45]. The
absorption peaks observed at 1698 cm-1
is attributed to C=C stretching in
aromatic nuclei. The bands obtained at 1600-1500 cm-1
corresponds to C-
H stretching in aromatic compounds [46]. The absorption peaks at around
1525 cm-1
is assigned to the quinoide ring structure of PAni [47]. FTIR
spectra of all the samples show strong absorption band in the region 750-
1500 cm-1
, which correspond to the characteristics of PAni. The
absorption bands lies below 1000 cm-1
are the characteristics of mono
substituted benzene [48,49].The out-of-plane bending of C–H in the
substituted benzene ring is reflected in the 867 cm-1
peak. These results
are in good agreement with the previous spectroscopic characterization of
polyaniline [50,51]. Therefore, it can be concluded that PAni co-exists
with inorganic Al2O3 in the PAni/Al2O3 thin films on the basis of the
FTIR spectral similarity between the PAni/Al2O3 thin films and the
undoped PAni thin film as well as the FTIR spectral features of Al2O3.
103
Fig. 5.3: FTIR transmission spectra of the samples a, b, c, d and e i.e. 0,
2, 4, 6 and 8 wt% doping of Al2O3 in PAni thin films. Curves a, b, c, d
and e correspond to samples a, b, c, d and e respectively.
104
UV-visible absorption spectroscopy
The UV-visible absorption spectra of undoped PAni and PAni/Al2O3 thin
film samples are recorded at room temperature by using a
spectrophotometer between the wavelength range 200-900 nm and are
shown in Fig. 5.4. The UV absorption spectra have been recorded with
base line. The UV absorption can significantly determine the
interaction between the Al2O3 and PAni [52]. All the samples show
single broad peak at around 305 nm and small peak at around 450 nm.
The peak 305nm is associated with the exciton transition of π-π*. The
longer wavelength peak at around 450 nm can be associated to the
transition between benzenoid to quinoid rings [55]. Intensity of the peak
is randomly varied as the dopant concentration increases [54]. Normally,
the change in first peak is because of the degree of oxidization and the
change in other peak is because of the change in polymerization [53,55-
56].
Photoluminescence (PL) spectroscopy
Most of the electronic polymers exhibit photoluminescence (PL) only in
their reduced state. Much like other polymers, PAni also exhibits visible
emission in the solid state and in the solution. In the present work, the
photoluminescence studies of undoped PAni and Al2O3 doped PAni thin
films have been carried out with excitation wavelength of 325 nm and are
given in Fig. 5.5. The PL spectra show a strong peak in UV region at
around 384 nm and several weak visible emission peaks located at around
450 nm in blue region, 484 nm in blue-green and 525 nm in green
emission region. The peak in UV region is reported in literature [57]. The
overall intensity of UV region peak in doped samples is found to decrease
as compared to undoped sample. The intensity decreases for sample b, c,
with respect to undoped sample a, it again increases PL intensity for
105
sample d and e samples. The visible emission peaks don’t show any
significant changes in the intensity of PL. The decrease in intensities of
PL with the increase of dopant concentration in PAni indicates the change
in the oxidation state of the doped PAni. It has been reported that PAni
exhibits visible emission in the half oxidized state under solid state [1].
The relative heights of the emission peaks alter with different dopant
concentrations and nature of solvents may be due to polarity. In addition,
this peak becomes sharp and intense. This may be due to interchain
species which plays an important role in the emission process of
conjugated polymers. The intensity of peaks depends on factors such as
polymer coil size, the nature of polymer-solvent, polymer-dopant
interactions, and the degree of chain overlapping [58]. The PL spectra of
samples have the same shape, which indicates that it is an efficient way to
tune the intensities of the peak by employing specific dopant with
different wt%. The observed reduced height of the photoluminescence
emission intensity peaks with increased wt% Al2O3 doped PAni might
due to the possibility of atoms/molecules of dopant forming aggregation
in the polymer chain [59-60].
106
Fig. 5.4: UV-visible absorption spectra of the samples a, b, c, d and e i.e.
0, 2, 4, 6 and 8 wt% doping of Al2O3 in PAni thin films. Curves a, b, c, d
and e correspond to samples a, b, c, d and e respectively.
107
Fig. 5.5: Photoluminescence (PL) spectra of the samples a, b, c, d and e
i.e. 0, 2, 4, 6 and 8 wt% doping of Al2O3 in PAni thin films. Curves a, b,
c, d and e correspond to samples a, b, c, d and e respectively.
108
5.4 Conclusion
In this chapter, we have synthesized undoped and Al2O3 doped PAni thin
films by the chemical oxidation method. The prepared thin films have
been characterized by XRD, SEM,UV-vis, FTIR and PL. The XRD
spectra shows a peak around 25o which confirm the synthesis of PAni and
another peak at 55.08o
for 8 wt% Al2O3 doped PAni which is
confirmations of successful doping in PAni. The study of FTIR spectra
again confirms the formation of PAni. SEM images show the granular
coral like structure which converted into spherical shaped structures for
higher doping percentage. UV spectra show single broad peak at around
305 nm and small peak at around 450 nm. The peak 305nm is associated
with the exciton transition of π-π*. The longer wavelength peak at around
450 nm can be associated to the transition between benzenoid to quinoid
rings. PL spectra recorded with excitation wavelength 325 nm show a
strong UV peak at 384 nm with weak visible peak at 484 nm and 527 nm.
109
Chapter-6
Structural, Morphological and Optical Studies of Undoped
and TiO2 Doped Polypyrrole Thin Films
6.1 Introduction
Polypyrrole is one of the most stable conducting polymers and also one of
the easiest to synthesize. It displays a good conductivity in combination
with high stability in its oxidized form. Electrochemically polymerization
on a metal electrode results in good quality film [1]. In recent years,
electro-active polymers, particularly aromatic conducting polymers, have
received much research attention for use as advance materials due to their
remarkable physical attributes [2–4]. Conducting polymers (CP),
however, arouse an immense interest among researchers because of their
curious electronic, magnetic and optical properties. Conducting polymers
can be prepared by chemical or electrochemical polymerization. In the
chemical polymerization process, monomers are oxidized by oxidizing
agents or catalysts to produce conducting polymers. The advantage of
chemical synthesis is that it offers mass production at reasonable cost. On
the other hand, the electrochemical method involves the direct formation
of conducting polymers with better control of polymer film thickness and
morphology, which makes them suitable for use in electronic devices [5-
6]. Therefore, the physical and chemical properties of conducting
polymers are considerably dependent upon the dopant and polymerization
conditions. In terms of CP, Polypyrrole (PPy) is one of the most studied
polymers due to its environmental stability, relative ease of synthesis and
good electrical conductivity. Long term stability of PPy is a key factor for
application of new polymeric material in future applications and seems to
be a good candidate [7]. PPy is most frequently used in commercial
application such as batteries, super capacitors, sensors and corrosion
110
protection. Polymer–inorganic nano-particle hybrids have attracted great
attention, since they have interesting physical properties and potential
applications. These particles not only combine the advantageous
properties of metals and polymers but also exhibit many new characters
that single phase materials do not have [8]. The incorporation of the
conducting polymer as the shell in the core–shell structure can increase
the surface area of the conducting polymers over that of the bulk
polymer. This structure can be obtained from an in-situ chemical
oxidative polymerization in the presence of nanoparticles [9]. The
inorganic core can be a metal or a metal oxide, and the organic shell can
be a conducting polymer. Moreover, recent investigations on PPy/TiO2
nanocomposites (NCs) for use as pigments indicate that a TiO2
nanoparticle (NP) core can increase the charge resistance of the coatings
[10-12].
Titanium dioxide also known as titanium (IV) oxide or titania, is the
naturally occurring oxide of titanium, chemical formula TiO2.It has a
wide band gap of ˜ 3.2 eV.Generally it is sourced from ilmenite,
rutile and anatase. It has a wide range of applications, from paint
to sunscreen to food colouring. When used as a food colouring. The most
important application areas are paints and varnishes as well as paper and
plastics, which account for about 80% of the world's titanium dioxide
consumption. Other pigment applications such as printing inks, fibers,
rubber, cosmetic products and foodstuffs account for another 8%. The
rest is used in other applications, for instance the production of technical
pure titanium, glass and glass ceramics, electrical ceramics, catalysts,
electric conductors and chemical intermediates. It also used in most red-
coloured candy.
111
Titanium dioxide is the most widely used white pigment because of its
brightness and very high refractive index, in which it is surpassed only by
a few other materials. Approximately 4.6 million tons of pigmentary
TiO2 are used annually worldwide, and this number is expected to
increase as utilization continues to rise. When deposited as a thin films,
its refractive index and colour make it an excellent reflective optical
coating for dielectric mirrors and some gemstones like mystic fire topaz .
In paint application, it is often referred to offhandedly as the perfect
white, the whitest white, or other similar terms. Opacity is improved by
optimal sizing of the titanium dioxide particles. Some grades of titanium
based pigments as used in sparkly paints, plastics, finishes
and pearlescent cosmetics are man-made pigments whose particles have
two or more layers of various oxides–often titanium dioxide, iron
oxide or alumina in order to have glittering, iridescent and or pearlescent
effects similar to crushed mica or guanine-based products. In addition to
these effects a limited colour change is possible in certain formulations
depending on how and at which angle the finished product is illuminated
and the thickness of the oxide layer in the pigment particle; one or more
colours appear by reflection while the other tones appear due to
interference of the transparent titanium dioxide layers [13].
In some
products, the layer of titanium dioxide is grown in conjunction with iron
oxide by calcination of titanium salts (sulfates, chlorates) around 800 °C
[14] or other industrial deposition methods such as chemical vapour
deposition on substrates such as mica platelets or even silicon dioxide
crystal platelets of no more than 50 µm in diameter. The iridescent effect
in these titanium oxide particles (which are only partly natural) is unlike
the opaque effect obtained with usual ground titanium oxide pigment
obtained by mining, in which case only a certain diameter of the particle
is considered and the effect is due only to scattering. In ceramic
112
glazes titanium dioxide acts as an opacifier and seeds crystal formation.
Titanium dioxide has been shown statistically to increase skimmed milk's
whiteness, increasing skimmed milk's sensory acceptance score [15].
Titanium dioxide is used to mark the white lines of some tennis courts.
Several methods have been used in the preparation of polypyrrole (PPy)
and a wide range of its derivatives by simple chemical or electrochemical
methods [16-22]. In the present chapter, chemical polymerization method
is used in the preparation of PPy. It is a simple and fast process with no
need for special instruments. Bulk quantities of polypyrrole (PPy) can be
obtained as fine powders using oxidative polymerization of the monomer
by chemical oxidants in aqueous or non-aqueous solvents [18-23] or by
chemical vapor deposition [21]. However, the use of chemical
polymerization limits the range of conducting polymers that can be
produced since only a limited number of counter ions can be
incorporated. The chemical polymerization of pyrrole appears to be a
general and useful tool for the preparation of conductive composites
[24,25] and dispersed particles in aqueous media [26, 27].
6.2 Experimental
6.2.1 Sample preparation
The Polypyrrole was prepared by chemical polymerization method. 1 M
Pyrrole solution was prepared using distillation and then mixed with an
oxidizing agent ammonium persulfate slowly under constant stirring for
30 minutes. Then the polymerization was conducted for 4 hours under
constant stirring. This preparation was kept unagitated for 24 hours so
that polypyrrole powder settled down. The Polypyrrole powder was
filtered out and washed with distilled water several times to remove any
impurities present. The precursor solution has been prepared by
113
dissolving predetermined amount of polypyrrole in m-cresol. The mixture
is then magnetically stirred at 400C for half an hour to get a homogeneous
solution. To this solution appropriate weight of TiO2 is added in
polypyrrole solution to obtain TiO2 doping of 0, 10, 20, 30 and 40 wt.% .
All solutions are again stirred for 24 hours at same temperature. All the
solutions are aged for 15 days to achieve proper viscosity and stability.
The precursor solutions thus obtained are spin coated on glass slides.
Prior to film deposition the substrate of glass slides are properly cleaned
in an ultrasonic cleaner using methanol, acetone and de-ionized water.
Spinning speed is kept at 3000 rpm while the spinning time is 30 seconds.
The films thus obtained are dried at 1000C at the rate of 10
0 C/min for
two hours and then cooled back to room temperature. The process is
repeated twelve times to obtain appreciable thickness. After getting
appreciable thickness, the finally prepared the precursor films were
annealed at 1000C for 4 hours. The 0, 10, 20,30 and 40 wt% TiO2 doped
PPy thin films are denoted as samples a ,b ,c ,d and e respectively
6.2.2 Characterizations
The XRD spectra of all the samples recorded by PANalytical, X’pert
PRO diffractometer using CuKα radiation (λ=1.54056 Ao) were presented
for structural analysis of the samples. The SEM images of all the thin film
samples were taken by scanning electron microscope (Model-430, LEO
Cambridge, England). FTIR spectra of all the samples in the form of thin
films were recorded on the Bruker Alpha spectrometer to determine the
formation of polyaniline. The Photoluminescence (PL) spectra were
recorded for all samples in the form of thin films by Perkier Elmer
photoluminescence spectrophotometer (LS-55, Perkin Elmer fluorescence
spectrometer) at excitation (λexc) wavelength 325 nm. The incident
excitation density was controlled by using calibrated neutral density
114
filters in front of the spectrometer slit. The slit widths for emission
spectra recording have been chosen as 10nm. The excitation source was a
20 kW Xenon discharge lamp. The light beam used for excitation is
focused on the film surface in circular area of diameter ~5nm.
6.3 Results and discussion
6.3.1 Structural Analysis
Fig 6.1 shows the X-ray diffraction (XRD) patterns of undoped PPy and
TiO2 doped PPy thin films. The broad peak in the region of 2θ = 20–800
in XRD pattern of undoped PPy and TiO2 doped PPy films show weak
crystalline quality of all the samples. The XRD spectra show the
appearance of peaks at 24.66o, 29.39
o, 37.95
o, 42.47
o, 44.05
o, 47.19
o,
53.28o and 64.37
o match with JCPDS data No: 21-1276 [28] which
confirms the existence of PPy/TiO2. However, it is seen due to the PPy
deposition on the surface structure of TiO2 nanostructures, the diffractions
of TiO2 gets slightly shifted from their positions [29]. The crystallite
sizes of all the samples are calculated using Debye–Scherrer’s (DS)
formula [30]:
cos
ktDS ,
where tDS is the crystallite size, λ is the wavelength of radiation used, θ is
the Bragg’s angle and β is the full-width at half-maximum (FWHM)
measured in radian. The average crystallite size of all the samples lies in
the range between 30 to 50 nm.
115
6.3.2 Scanning Electron Microscopy (SEM)
Scanning electron microscope (SEM) is used to study the surface
morphology of the pure polypyrrole and polypyrrole doped with various
dopant percentages of TiO2 thin films as shown in Fig. 6.2(a-e), which
show the smooth surface morphology. SEM images shown in Fig.6.2 (b-
d) for the PPy/TiO2 thin films have large nanospheres are due to the
introduction of the relatively higher content of TiO2. The particle size of
all the samples is found in the range of 40-50 nm. It is seen that the
particle size decreases with increasing doping concentration as well as
number of pores and size of the pores also increases with dopant. The
increase of number of pores shows the suitable application in the sensing
properties. It is the initiator molecules act as a stabilizer for the as-formed
nano micelles. These nano micelles are acting as templates to encapsulate
pyrrole and oxidant leading to the formations of nanospheres during the
polymerization [28, 31].
.
116
Fig 6.1: XRD spectra for the samples a, b, c and d. Curves a, b, c and d
corresponds to 0, 10, 30 and 40 wt% TiO2 doped PPy thin films
respectively.
.
117
Fig 6.2: SEM images of samples a, b, c, d and e. Images a, b, c, d and e
are correspond to 0, 10, 20, 30 and 40 wt% TiO2 doped PPy thin films
respectively.
118
6.3.3 Optical properties
Fourier Transform Infrared (FTIR) Spectroscopy
The Fourier transform infrared (FTIR) transmittance spectra of undoped
PPy and TiO2 doped PPy thin films are recorded in the range of 600 -
4000 cm-1
, which is shown in Fig.6.3 (a-e). The FTIR spectra of undoped
PPy and TiO2 doped PPy thin films are found the different absorbance
band and it conform the synthesis and chemical structure of PPy. The
peaks at 739 cm-1
and 881 cm-1
can be attributed to C-H out-of-plane
vibrations [32]. The absorbance band observed in the range of 1400 cm−1
to 1600 cm−1
in the FTIR spectra of the synthesized polymers can be
attributed to the fundamental vibrations of the pyrrole rings [15]. The
absorbance band at 1690 cm-1
corresponds to C-N=C bond, the peaks at
3732 cm−1
and 3848 cm−1
are attributed to N–H bond [33-34]. The FTIR
spectrum of TiO2 doped PPy thin films in Fig.6.3 (b-e) are similar to the
undoped PPy spectrum, which shows that PPy chains have been formed
in the doped thin films. However, the incorporation of TiO2 leads to the
obvious small shift of some FTIR bands of PPy. TiO2 probably led to the
reduction of the conjugation length of PPy in TiO2 doped PPy thin films
[35-36].
119
Fig. 6.3 FTIR spectra for the sample a, b, c, d and e. Curves a, b, c, d and
e correspond to 0, 10, 20, 30 and 40 wt% TiO2 doped PPy thin films
respectively.
120
UV-visible absorption spectroscopy
The UV–vis absorption spectra of undoped PPy and TiO2 doped PPy thin
films are recorded at room temperature by using a spectrophotometer
between the wavelength range 300–1500 nm as shown in Fig. 6.4. Optical
spectroscopy is an important technique to understand the conducting
states corresponding to the absorption bands of inter and intra gap states
of conducting polymers [37].The thin films of undoped PPy and TiO2
doped PPy show the observed peak at 309 nm as seen in the Fig. 6.4
which may be due to the π-π* transition or the excitation transition [38].
As the dopant percentage of TiO2 increases, the intensity of TiO2 doped
ppy thin films increased and the polaron band appears to be sharp peak.
This indicates that an increase in the dopant percentage leads to the
formation of a chain which forms the best sharp peaks [39]. The
differences between the spectra are due to the presence of an electron-
withdrawing sulfonic group in the complex and therefore the transition
band is observed at a lower wavelength. The absorption of the polaron
band is strongly dependent on the molecular weight of the polymer [40].
121
Fig.6.4: UV–vis absorbance spectra for the sample a, b, c, d and e.
Curves a, b, c, d and e correspond to 0, 10, 20, 30 and 40 wt % TiO2 and
PPy/TiO2 thin films.
122
Photoluminescence (PL) Studies
Photoluminescent of organic materials have a new class with interesting
properties. They undergo emission over a wide range from the violet to
the red. They can also be combined in several different forms to produce
white light. One category of organic material with photoluminescence
properties is conjugated organic polymers. PL spectra were measured for
all the four samples in the range of 325-650 nm and the wavelength of
excitation chosen for all the samples is 325 nm. The photoluminescence
(PL) of TiO2 doped PAni has been performed and is shown in Fig. 6.5.
The PL spectra of 0, 10, 20, 30 and 40 wt% TiO2 doped PPy samples
show a broad peak in visible emission around at 385 nm. One of the
similar related peaks are reported for the PL spectra of TiO2 doped PPy
observed at the 362 nm [37]. The direct band gap was calculated by using
this formula,
(Eg= hc/λ)
Where ‘h’ is a constant ‘c’ is velocity of light, ‘λ’ is emission wavelength
in Photoluminescence spectrum. The direct band gap energies of undoped
PPy and TiO2 doped ppy thin films of different wt% are found to be the
corresponding main visible emission peaks as 3.65 eV. As the
luminescence of this oxide/polymer nanocomposite is proportional to the
surface features, it is possible to tailor the wavelength and the intensity of
the luminescence by varying the particle size [41, 42]. In addition, PL
spectra of all the samples show the small visible emission peaks located
at around 480 nm and 530 nm in green region. Similar peaks have been
observed in our earlier reported chapter 4.
123
Fig. 6.5: Photoluminescence spectra for the samples a, b, c, d and e.
Curves a, b, c, d and e correspond to 0, 10, 20, 30 and 40 wt% TiO2 and
PPy/TiO2 thin films respectively.
124
6.4 Conclusion
In the present chapter, we have synthesized undoped PPy and TiO2 doped
PPy nanocoposite thin films by the chemical oxidation method at room
temperature. The prepared samples have been characterized by XRD,
SEM,UV-vis, PL and FTIR. XRD spectra show the weak crystalline
quality of all the samples. SEM images show the sphere shape of
nanostructures. The amount of TiO2 doping increases the number of pores
as well as size of the pores that play a very important role in sensing of
gas. The study of FTIR spectra confirms the formation of conducting PPy
which suggests that doping of TiO2 in PPy does not affect its structures.
All the samples of PPy and PPy/TiO2 nanocomposites thin films show the
peak at 309 nm which is assigned to the π-π* transition or the excitation
transition. The PL spectra of PPy and TiO2 doped PPy show three main
peaks, first is in UV region around at 368 nm, second broad peak in
visible region around 480 nm and another sharp peak at around 530 nm in
green region.
125
Chapter 7
Conclusion
The present thesis is an effort to synthesize the polymer and poymer
nanocomposite for various important applications. Ploythiophene (PTh),
polyaniline (PAni) and polypyrrole (PPy) have been used as a host
materials in pellets and thin films form. The different metal oxide dopants
such as Al2O3, CuO and TiO2 have been used to improve the structural,
morphological and optical properties of synthesized samples. Sol-gel
spin coating, dip coating has been used to deposit the films for
present investigations. Undoped and Al2O3 doped polythiophene and
polypyrrole, undoped and CuO doped polyaniline pellets are also
prepared. The synthesized samples are characterized by various
characterizations techniques including XRD, SEM, FTIR, UV-vis and
PL. Thesis contains following chapters.
[1] introduction
[2] undoped and Al2O3 doped polythiophene pellets
[3] undoped and CuO doped polyaniline pellets
[4] undoped and Al2O3 doped polypyrrole pellets
[5] undoped and Al2O3 doped polyaniline sol-gel dip coating thin
films
[6] undoped and TiO2 doped polypyrrole sol-gel spin coating thin films
[1] Chapter 1 deals with the brief introduction of materials, their types
and applications. In this chapter, we have discussed about basics of
materials and three conducting polymers including polythiophene (PTh),
polyaniline (PAni) and polypyrrole (PPy) and their metal
126
nanocomposites. The metal dopants such as Al2O3, CuO and TiO2 have
been discussed in the present work. It also deals the discussion of
deposition techniques of the thin films as well as characterization
techniques. It also contains the organization and objective of thesis.
[2] in this chapter Undoped and Al2O3/polythiophene nanocomposites
have been synthesized by chemical oxidation method. The samples are
characterized by XRD, SEM, UV-vis, PL and FTIR spectroscopy. XRD
spectra show the polycrystalline nature of all the samples. SEM images
are indicating formation of spherical shape of nanostructures. As-
synthesized samples of undoped polythiophene and Al2O3/PTh
nanocomposites exhibit many pores on the surface of nanostructures.
Synthesis of Al2O3 polythiophene composite material is confirmed by
FTIR spectroscopy. UV-visible absorption spectra show absorption peak
at around 300 nm which is due to π- π* inter-band-transition of PTh
rings. A small change in optical absorption spectra is observed which can
be associated with the degree of oxidation. PL spectra exhibit mainly
three visible emission peaks at around 462 nm, 490 nm and 522 nm. The
two emission peaks 462 nm and 490 nm in the Soret band region where
as single peak at 522 nm in the Q band emission. The intensity and peak
position of polythiophene have been randomly changed with amount of
Al2O3 dopant.
[3] In this chapter, we have synthesized undoped and CuO doped PAni
nanocomposites by the chemical oxidation method at room temperature.
The prepared samples have been characterized by XRD, SEM, UV-vis,
PL and FTIR. XRD spectra show weak crystalline quality of all the
samples, whereas the PAni synthesized is amorphous in nature. The
scanning electron microscopy images of all the samples show granular
127
coral like structure. The study of FTIR spectra confirm the formation of
conducting PAni and also suggests that doped of CuO in PAni does not
affect the structures. The UV–visible absorption spectra of the solutions
of all the samples contain some peak at 300 nm.The observed
bathochromic shift at the intense absorption band 305nm is due to the π-
π* transition of benzenoid ring.The PL spectra of 0, 2, 4, 6 and 8 wt%
CuO doped PAni samples show in visible emission peaks which is at
around 362 nm, 405 nm in violet region 459 nm, 486 nm in blue region
and 528 nm in green region.
[4] In this chapter, we have synthesized undoped and Al2O3 doped PPy
nanocomposite samples by the chemical oxidation method. The prepared
samples have been characterized by XRD, SEM, FTIR, UV-Vis
absorption and PL spectroscopy. X-ray diffraction patterns of PPy/Al2O3
nanocomposites result show several broad peaks while undoped sample
shows only one single peak indicating poor crystalline phase of PPy. In
the SEM images, the results were found granular coral like structures. As
a characteristic of polypyrrole, secondary nucleation also takes place
because of which the granular coral like particles come together to form
aggregates. We noticed that as the amount of Al2O3 was increased; the
number of pores and the size of pores were also increased, which is very
important for sensing. The study of FTIR spectra confirms the formation
of PPY and also suggests that doping of Al2O3 in PPY does not affect its
structure. The UV absorption can significantly determine the interaction
between the Al2O3 and PPy. Solutions of all the samples show peak,
which oriented around 306 nm. The peak at 306 nm is associated with the
exciton transition of π–π*. PL shows the main emission band of the nano
composites is located at 365 nm with two shoulders at 473 nm and 533
nm. The direct band gap energies of the PPy/ Al2O3 nanocomposite of
128
different ratios are found as 3.09 and 2.19 eV. The band gap gets
decreased due to increased content of Al2O3 nano particles.
[5] In this chapter, we have synthesized undoped and Al2O3 doped PAni
thin films by the chemical oxidation method. The prepared thin films
have been characterized byXRD, SEM, UV-vis, PL and FTIR. The XRD
spectra shows a peak around 25o which confirm the synthesis of PAni and
another peak at 55.08o
for 8 wt% Al2O3 doped PAni which as the
confirmations of successful doping in PAni. The study of FTIR spectra
again confirms the formation of PAni. SEM images show the granular
coral like structure which converted into spherical shaped structures for
higher doping percentage. UV-vis spectra show single broad peak at
around 305 nm and small peak at around 450 nm. The peak 305 nm is
associated with the exciton transition of π-π*. The longer wavelength
peak at around 450 nm can be associated to the transition between
benzenoid to quinoid rings. PL spectra recorded with excitation
wavelength 325 nm show a strong UV peak at 384 nm with weak visible
peak at 484 and 527 nm.
[6] In this chapter, XRD spectra show the crystalline quality of all the
samples, whereas the PPy synthesized is amorphous in nature. The study
of FTIR spectra confirms the formation of conducting PPy and also
suggests that doping of TiO2 in PPy does not affect its structure. Optical
spectroscopy is an important technique to understand the conducting
states corresponding to the absorption bands of inter and intra gap states
of conducting polymers. Solution of all PPy and PPy/TiO2 sol-gel spin
coating films shows the observed peak at 308 nm was assigned to the π-
π* transition or the excitation transition. Pure PPy and PPy/TiO2 sol-gel
spin coating films as the dopant percentage of TiO2 increases, the
129
intensity of PPy/TiO2 sol-gel spin coating films increased and the polaron
band appears to be sharp peak. This indicates that an increase in the
dopant percentage leads to the formation of a chain which forms the best
sharp peaks. The PL spectra show one main peaks in visible emission
around at 384 nm and broad peak around 386 nm in blue region. SEM
images shows PPy/TiO2 have large nanospheres are due to the
introduction of the relatively higher content of TiO2. We noticed that as
the amount of TiO2 increased the number of pores and size of the pores
was also increased, which play very important role for conductivity.
In the chapter no.[2],[3] and [4], Al2O3 is doped in polythiophene (PTh)
and polypyrrole (PPy) while CuO is doped in polyaniline (PAni) in the
form of pellets. It is found that the crystallite size of PPy/Al2O3
nanocomposites is smaller than other two PTh/Al2O3 and PAni/CuO
nanocomposites. The porosity is found to increase significantly as
compared to other two nanocomposites of PTh/Al2O3 and PAni/CuO,
which can be very useful in gas sensing applications. PPy/Al2O3
nanocomposites exhibit larger PL intensity as compared to other two
nanocomposites of PTh/Al2O3 and PAni/CuO, which can be useful for
OLEDs.
In the last two Chapter [5] and Chapter [ 6], we have prepared thin films
using sol-gel dip coating and sol-gel spin coating thin films of Pani/
Al2O3 and PPy/TiO2 respectively. The overall structural, morphological
and optical properties of PAni/ Al2O3 thin films are found better as
compared to PPy/TiO2 thin films.
130
7.1 Recommendations for further work
a) Patterning of electrodes on sample pellets and films for further
observations.
b) Gas sensing and humidity sensing using above samples.
c) Study of effects of higher annealing duration and higher annealing
temperatures on structural, optical and the Gas sensing and humidity
sensitivity.
d) Further optimization of dopant concentration in Polymers like-
polypyrrole, polythiophene and polyaniline.
e) Study regarding selectivity and sensitivity to different gases like CO2,
LPG and NH3 etc..
131
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Fig. (a): Spin coating Unit
154
Fig. (b): Magnetic Stirrer with Hot Plate
155
Fig. (c): Weighing Unit
156
Fig. (d): Motorized Furnace Unit
157
Fig. (e): Hot Air Oven Unit
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Fig. (f): Photoluminescence Unit
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Fig. (g): UV-visible absorption Unit
160
Fig. (h): FTIR Spectroscopy Unit
161
Fig. (i): Hydraulic Press Machine Unit