ir.amu.ac.inir.amu.ac.in/10736/1/t10005.pdf1 department of applied chemistry faculty of engineering...

Studies on Conducting Composites of Polyaniline

Thesis

Submitted for the award of degree of

Doctor of Philosophy

In

Applied Chemistry

By

Sharique Ahmad

Under the Supervision of

Prof. Faiz Mohammad

Department of Applied Chemstry Faculty of Engineering and Technology

Aligarh Muslim University, Aligarh 2017

1

Department of Applied Chemistry

Faculty of Engineering and Technology

Aligarh Muslim University

Aligarh-202002 (India)

STUDIES ON CONDUCTING COMPOSITES OF POLYANILINE

ABSTRACT

In chapter one, electrically conducting polymer, polyaniline and polyaniline

composites have been discussed. Conducting polymer composites are the future

materials for emerging technologies as they possess a combination of unique

properties of their constituents. The properties become more interesting when one of

the components is in the nanorange. Due to the synergistic effect of the polymer and

the fillers, the resulting materials are expected to display desirable properties with

enhanced performance.

One of the most promising composites systems would be the hybrids based on

organic polymer such as polyaniline, nanoparticles and natural polymers such as SiC,

NiO, BN, silk fibroin and graphene. These components combined with polyaniline

can give rise a variety of polymer composites of interesting properties such as

physical, chemical, electrical, electrochemical and so on and thus may become the

potential materials for newer and novel applications. Sufficient literature review has

been done on the important aspects of these materials to get the idea of the work done

in this field and to formulate the plan of work for this thesis.

In chapter two, the electrical properties and photocatalytic activity of HCl

doped polyaniline (Pani) and polyaniline/boron nitride (Pani/BN) nanocomposite

prepared by in-situ polymerization of aniline using potassium persulphate (K2S2O8) in

the presence of hexagonal boron nitride (h-BN) were studied. The prepared Pani and

Pani/BN nanocomposite were characterized by FTIR, XRD, TGA, SEM and TEM.

2

The stability of the Pani/BN nanocomposite in comparison of Pani in terms of the DC

electrical conductivity retention was investigated under isothermal and cyclic aging

conditions. The Pani/BN nanocomposite in terms of DC electrical conductivity was

observed to be comparatively more thermally stable than Pani. The degradation of

Methylene blue (MB) and Rhodamine B (RhB) under UV-light irradiation were 50%

and 56.4% respectively over Pani and 65.7 and 71.6% respectively over Pani/BN. The

results indicated that the extent of degradation of MB and RhB was greater over

nanocomposite material than Pani, which may result due to high electron–hole pairs

charge separation under UV light.

In chapter three, the electrical properties and photocatalytic activity of

polyaniline (Pani) and polyaniline/silk fibroin (Pani/SF) composite were studied. The

Pani/SF composite has been synthesized first time by chemical oxidative in-situ

polymerization method by using potassium persulphate (K2S2O8) in an acidic

medium. Thus prepared Pani and Pani/SF composite were characterized by using

Fourier transform infrared spectroscopy, x-ray diffraction, thermogravimetric analysis

and scanning electron microscopy. The stability in terms of the DC electrical

conductivity of Pani/SF composite and Pani was investigated under isothermal and

cyclic aging conditions. Results indicated that the electrical properties of the

composite were significantly influenced by the loading of SF to Pani and observed to

be comparatively more thermally stable than Pani. The as prepared Pani and Pani/SF

composite were studied for the degradation of Methylene blue (MB) and Rhodamine

B (RhB) under UV-light irradiations. The results showed that 91.4% & 85.4%

degradation of RhB and MB takes place respectively after 90 min over Pani/SF

composite. The decomposition rate of composite was 1.8-2.2 times greater than that

of Pani. The improvement of photocatalytic activity of composite may be attributed to

the electron-sink function of silk fibroin which increases the optical absorption

property and separation of photogenerated charge carriers than Pani alone.

In chapter four, an electrical conductivity based rapid response and excellent

recovery cigarette smoke sensor based on polyaniline/silicon carbide (Pani/SiC)

nanocomposite prepared by in-situ chemical oxidative polymerization technique in an

acidic medium. The Pani/SiC nanocomposites and polyaniline (Pani) were

characterized by using Fourier transform infrared spectroscopy, x-ray diffraction,

scanning electron microscopy and transmission electron microscopy. Results

3

indicated that the well ordered nanocomposites were successfully prepared. The

morphology and electrical properties of the nanocomposites were influenced by the

extent of loading of SiC nanoparticles. The as-prepared materials were studied for the

change in their electrical conductivity on exposure to cigarette smoke followed by

ambient air at room temperature. It was observed that Pani/SiC-3 nanocomposite

shows eight times higher amplitude of conductivity change than Pani on exposure

cigarette smoke followed by ambient air. The conductivity response in the carbon

monoxide, ammonia and methanol was also measured. It was observed that the carbon

dioxide (CO2) shows low response while in the case of methanol and ammonia the

significant responses are observed, suggesting the possibility of contribution of above

gases towards response against cigarette smoke. The cigarette smoke also contains

many other volatile chemicals such as polyaromatic hydrocarbon (PAH), nicotine,

hydrogen cyanide (HCN) and formaldehyde which may also interacted with

polarons/bipolarons of polyaniline leading to decreasing in conductivity.

In chapter five, camphor sulphonic acid doped conducting polyaniline/silk

fibroin (Pani/SF) composites were prepared by in situ polymerization method. The

Pani/SF composite fibers were characterized by using Attenuated total reflectance-

Fourier transform infrared spectroscopy, thermogravimetric analysis, scanning

electron microscopy and digital photographs. Results indicated that the Pani was

successfully coated on silk fibroin. The electrical properties of Pani/SF composite

fibers were influenced by the extent of loading of aniline monomers. The as-prepared

material was studied as sensing material by observing change in their electrical

conductivity response on exposure to ammonia and acetaldehyde followed by ambient

air at room temperature. It was found that the composite fibers showed good

reversibility and conductivity decreased on exposure to higher concentration of

ammonia and acetaldehyde vapours at room temperature.

In chapter six, polyaniline@graphene/nickel oxide (Pani@GN/NiO),

polyaniline/graphene (Pani/GN), polyaniline/nickel oxide (Pani/NiO) nanocomposites

and polyaniline (Pani) were successfully synthesized and tested for ammonia sensing.

Pani@GN/NiO, Pani/NiO, Pani/GN and Pani were characterized by using x-ray

diffraction, UV-vis spectroscopy, Raman spectroscopy, scanning electron microscopy

and transmission electron microscopy. The as-prepared materials were studied for

comparative DC electrical conductivity and the change in their electrical conductivity

4

on exposure to ammonia vapours followed by ambient air at room temperature. It was

observed that the Pani@GN/NiO nanocomposite showed about 99 times greater

amplitude of conductivity change than pure Pani on exposure to ammonia vapours

followed by ambient air. The fast response and excellent recovery time could be

probably ascribed to relatively high surface area of Pani@GN/NiO nanocomposite,

proper sensing channels and efficaciously available active sites. Pani@GN/NiO was

observed to be good in selectivity towards ammonia because of comparatively high

basic nature of ammonia than other volatile organic compounds (VOCs) tested. The

sensing mechanism explained on the simple acid base chemistry of polyaniline.

In chapter seven, the overview and directions towards the future work were

discussed.

CANDIDATE’S DECLARATION

I, Sharique Ahmad, Department of Applied Chemistry certify that the work

embodied in this Ph.D. thesis is my own bonafide work carried out by me under the

supervision of Professor Faiz Mohammad at Aligarh Muslim University, Aligarh.

The matter embodied in this Ph.D. thesis has not been submitted for the award of any

other degree.

I declare that I have faithfully acknowledged, given credit to and referred to

the research workers wherever their works have been cited in the text and the body of

the thesis. I further certify that I have not willfully lifted up some other’s work, para,

text, data, result, etc. reported in the journals, books, magazines, reports, dissertations,

thesis, etc., or available at web-sites and included them in this Ph.D. thesis and cited

as my own work.

Date: (Signature of the candidate)

SHARIQUE AHMAD (Name of the candidate)

Certificate from the Supervisor

This is to certify that the above statement made by the candidate is correct to the best of our knowledge.

Signature of the Supervisor Name & Designation: Professor (Dr.) Faiz Mohammad Department: Applied Chemistry

(Signature of the Chairperson of the Department with Seal)

COURSE/COMPREHENSIVE EXAMINATION/PRE-SUBMISSION SEMINAR COMPLETION CERTIFICATE

This is to certify that Mr. Sharique Ahmad, Department of Applied

Chemistry has satisfactorily completed the course work/comprehensive

examination and pre-submission seminar requirement which is part of his Ph.D.

programme.

Date: ……………. (Signature of the Chairperson of the Department)

COPYRIGHT TRANSFER CERTIFICATE

Title of the Thesis: STUDIES ON CONDUCTING COMPOSITES OF

POLYANILINE

Candidate’s Name: SHARIQUE AHMAD

Copyright Transfer

The undersigned hereby assigns to the Aligarh Muslim University, Aligarh,

copyright that may exist in and for the above thesis submitted for the award of

Ph.D. degree.

(Signature of the Candidate)

Note: However, the author may reproduce or authorize others to reproduce material

extracted verbatim from the thesis or derivative of the thesis for author’s

personal use provided that the source and the university’s copyright notice are

indicated

V

Dedicated to

My Parents

Mr. Aftab Ahmad Mrs. Sayeeda

Khatoon

i

ACKNOWLEDGEMENTS

I am highly grateful to “Almighty Allah”, the sustainer and the supreme, “who

taught the use of pen” and “taught Man which he knew not” for his diverse favours

and blessings, without which it would have not been possible for me to advance in

life and reach to this point in my career.

I would like to express my heartfelt gratitude to my supervisor Dr. Faiz

Mohammad, Professor, Department of Applied Chemistry, Aligarh Muslim

University, Aligarh, for his guidance, necessary suggestions, constant

encouragement and generous support and it is only because of his vast knowledge,

understanding, patience and valuable advice that this work has reached its

completion.

I am grateful to Prof. R.A.K. Rao, Chairperson, Department of Applied

Chemistry, for all the research facilities provided to me during the tenure of my

research work. I appreciate and thank to all the faculty members and the non-

teaching staff of the department for their cooperation and assistance.

Special thanks are due to Mr. Javaid Shabir, Dr. Mohammad Aslam O.P.,

Mr. Mohammad Shoeb, Mr. Md. Samim Hassan, Dr. Waseem Raza and Dr.

Mohammad Omaish Ansari who were always there for me and provided constant

support during this period. My stay at A.M.U. during all these years has been

particularly fascinating and it gives me immense pleasure to recognize the love of all

those who made this stay a memorable one. The list is long but I wish to

acknowledge the excellent and sincere support specially from Dr. Shahid Pervez

Ansari, Dr. Tarique Anwer, Mr. Adil Sultan, Mr. Mahfooz ur Rahman Khan, Mr.

Syed Md. Humayun Akhter and Mr.Ahmad Husain as my laboratory colleagues. I

would also like to thank to my friends Mr. Nazish Ali, Dr. Ajahar Khan, Mr.

Firdous Ahmad Butt, Ms. Azra Aftab, Ms. Adeela Asim, Ms. Saman Zehra, Dr.

ii

Ms. Shakeeba Shaheen, Mr. Mohammad Salih P.P., Mr. Syed Mohammad Adnan,

Mr. Syed Nasar Ata, Mr. Mohammad Zeeshan, Mr. Gulam Nabi Naz, Mr.

Mudasir Ahmad Yatto, Dr. Nayeem Ahmad, Mr. Wajid Ali and Mr. Sheth Zulfiqar

for their support during the tenure of my work.

Of course no acknowledgements would be complete without giving thanks to

my family. I would like to thank my brothers Rais Ahmad, Danish Aftab, Jalees

Ahmad, Shan Qamar, Arshad Nawab, Aqeel Ahmad, Asad Nawab and my sisters

Nilofer, Tabassum Parveen and Late Samreen Iqbal for their warmth,

encouragements and cooperation. They have been a constant source of inspiration

and motivation for me and have always spurred me towards excellence. I would also

like to extend my thanks to my uncle Janab Ayyub Ahmad Sahib for his constant

supports when the spirits were low during this journey.

Finally, I don’t find sufficient words to thank my father Janab Aftab

Ahmad and my mother Mohtarma Sayeeda Khatoon for their unconditional love,

support and guidance at every stage of my life. They have been there for me even

when I didn’t know anything about the world and myself. They’ve taught me hard

work, self-respect, about persistence and how to be independent. Without their

sincere love, prayers and encouragement, it would have not been possible for me to be

successful in my endeavours.

Sharique Ahmad

Page No.

Certificate

Candidates Declaration

Course Work Certificate

Copyright Transfer Certificate

Dedication

Acknowledgement i-ii

Contents

Abstract iii-vi

CHAPTER 1

GENERAL INTRODUCTION 1-26

1.1. ELECTRICALLY CONDUCTING POLYMERS 1

1.2. DOPING OF CONDUCTING POLYMERS 2

1.2.1. Protonic Acid Doping of Conducting Polymers 3

1.2.2. Redox Doping 3

1.2.3. Radiation Doping 4

1.2.4. Self Doping 4

1.3. ELECTRICAL CONDUCTION 4

1.3.1. Band Theory 5

1.3.2. Hopping and Tunnelling 6

1.4. POLYANILINE 7

1.4.1. Synthesis of Polyaniline 8

1.4.2. Polyaniline Composites 9

1.4.2.1. Composites of polyaniline with boron nitride 10

1.4.2.2. Composites of polyaniline with silk fibroin 12

1.4.2.3. Composites of polyaniline with silicon carbide 14

1.4.2.4. Nanocomposites of polyaniline with grapheme 15

1.4.2.5. Composites of polyaniline with graphene/NiO 18

1.5. CONCLUDING REMARKS 19

1.6. OBJECTIVE OF THE RESEARCH 20

References 22

CHAPTER 2 27-47

BORON NITRIDE BASED POLYANILINE

NANOCOMPOSITE: PREPARATION, PROPERTY AND

APPLICATION

2.1. INTRODUCTION 27

2.2. EXPERIMENTAL 28

2.2.1. Materials 28

2.2.2. Preparation of Pani and Pani/BN Nanocomposite 28

2.2.3. Photodegradation Experiment 29

2.3. CHARACTERIZATION 30

2.4. RESULT AND DISCUSION 30

2.4.1. Mechanism of Preparation of Pani and Pani/BN Nanocomposite 30

2.4.2. FTIR Spectroscopic Study 31

2.4.3. X-rays Diffraction (XRD) Studies 32

2.4.6. Thermogravimetric Analysis (TGA) 33

2.4.4. Scanning Electron micrograph (SEM) Studies 34

2.4.5. Transmission Electron Micrograph (TEM) Studies 35

2.5. PHOTOCATALYTIC STUDIES 36

2.6. POSSIBLE MECHANISM 39

2.7. ELECTRICAL CONDUCTIVITY 40

2.7.1. Stability under Isothermal Ageing Conditions 41

2.7.2. Stability under Cyclic Ageing Conditions 42

CONCLUSIONS 44

References 45

(J. APPL. POLYM. SCI. 2016, DOI: 10.1002/APP.43989)

CHAPTER 3 48-66

PREPARATION, CHARACTERIZATION AND

APPLICATION OF POLYANILINE/SILK FIBROIN

COMPOSITE



3.2.1. Materials 49

3.2.2. Preparation of Silk Fibroin 49

3.2.3. Preparation of Pani and Pani/SF Composite 49

3.2.4. Photodegradation Experiment 50


3.4. RESULT AND DISCUSSION 51

3.4.1. Preparation of Pani/SF Composite 51

3.4.2. FTIR Analysis 52



3.4.5. Scanning Electron Micrographic (SEM) Studies 55

3.5. PHOTOCATALYTIC STUDIES 56




CONCLUSIONS 63

References 64

(Polymers & Polymer Composites, Vol. 24, No. 8, 2016)

CHAPTER 4 67-87

HIGH RESPONSE AND EXCELLENT RECOVERY OF

POLYANILINE/SILICON CARBIDE NANOCOMPOSITE

IN CIGARETTE SMOKE SENSING WITH ENHANCED

THERMALLY STABLE DC ELECTRICAL

CONDUCTIVITY



4.2.1. Materials 68

4.2.2. Synthesis of Pani and Pani/SiC Nanocomposites 68



4.4.1. Possible Interaction between Pani and SiC Nanoparticles 70

4.4.2. FTIR Analysis 70


4.4.4. Scanning Electron Micrograph (SEM) Studies 72





4.6. SENSING 78

4.6.1. Proposed Mechanism for Sensing 83

CONCLUSIONS 84

References 85

(RSC Adv., 2016, 6, 59728–59736)

CHAPTER 5 88-105

ELECTRICALLY CONDUCTIVE POLYANILINE/SILK

FIBROIN COMPOSITE FOR AMMONIA AND

ACETALDEHYDE SENSING



5.2.1. Materials 89

5.2.2. Preparation of Silk Fibroin 89

5.2.3. Preparation of Pani and Pani/SF Composites 89


5.4. RESULTS AND DISCUSSION 91

5.4.1. Mechanism of Preparation of Pani/SF Composite Fibers 91

5.4.2. ATR-FTIR Analytical Studies 92



5.4.5. Digital Photographic Studies 94

5.5. ELECTRICAL CONDCUTIVITY STUDIES 95



5.6. SENSING STUDIES 98


CONCLUSIONS 103

References 104

(Polymers & Polymer Composites, ACCEPTED FOR PUBLICATION)

CHAPTER 6 106-125

DC ELECTRICAL CONDUCTIVITY BASED

POLYANILINE@GRAPHENE/NICKEL OXIDE

NANOCOMPOSITE AS AMMONIA SENSOR



6.2.1. Materials 107

6.2.2. Preparation of Pani, Pani/NiO, Pani/GN, Pani@GN/NiO 107






6.4.4. Optical Studies 112

6.4.5. Raman Spectroscopy 113




6.6. SENSING 118


CONCLUSIONS 123

References 124

(Sensors and Actuators B: Chemical, UNDER REVIEW, R-1)

CHAPTER 7 126-127

CONCLUSIONS

7.1. OVERVIEW 126

7.2. DIRECTIONS TOWARDS FUTURE WORK 126

PAPERS PUBLISHED

BORON NITRIDE BASED POLYANILINE NANOCOMPOSITE:

PREPARATION, PROPERTY AND APPLICATION (J. APPL.

POLYM. SCI. 2016, DOI: 10.1002/APP.43989)

PREPARATION, CHARACTERIZATION AND APPLICATION OF

POLYANILINE/SILK FIBROIN COMPOSITE (Polymers & Polymer

Composites, Vol. 24, No. 8, 2016)

HIGH RESPONSE AND EXCELLENT RECOVERY OF

POLYANILINE/SILICON CARBIDE NANOCOMPOSITE IN

CIGARETTE SMOKE SENSING WITH ENHANCED THERMALLY

STABLE DC ELECTRICAL CONDUCTIVITY (RSC Adv., 2016, 6,

59728–59736)

iii

Department of Applied Chemistry

Faculty of Engineering and Technology

Aligarh Muslim University

Aligarh-202002 (India)

STUDIES ON CONDUCTING COMPOSITES OF POLYANILINE

ABSTRACT

In chapter one, electrically conducting polymer, polyaniline and polyaniline

composites have been discussed. Conducting polymer composites are the future

materials for emerging technologies as they possess a combination of unique

properties of their constituents. The properties become more interesting when one of

the components is in the nanorange. Due to the synergistic effect of the polymer and

the fillers, the resulting materials are expected to display desirable properties with

enhanced performance.

One of the most promising composites systems would be the hybrids based on

organic polymer such as polyaniline, nanoparticles and natural polymers such as SiC,

NiO, BN, silk fibroin and graphene. These components combined with polyaniline

can give rise a variety of polymer composites of interesting properties such as

physical, chemical, electrical, electrochemical and so on and thus may become the

potential materials for newer and novel applications. Sufficient literature review has

been done on the important aspects of these materials to get the idea of the work done

in this field and to formulate the plan of work for this thesis.

In chapter two, the electrical properties and photocatalytic activity of HCl

doped polyaniline (Pani) and polyaniline/boron nitride (Pani/BN) nanocomposite

prepared by in-situ polymerization of aniline using potassium persulphate (K2S2O8) in

the presence of hexagonal boron nitride (h-BN) were studied. The prepared Pani and

Pani/BN nanocomposite were characterized by FTIR, XRD, TGA, SEM and TEM.

iv

The stability of the Pani/BN nanocomposite in comparison of Pani in terms of the DC

electrical conductivity retention was investigated under isothermal and cyclic aging

conditions. The Pani/BN nanocomposite in terms of DC electrical conductivity was

observed to be comparatively more thermally stable than Pani. The degradation of

Methylene blue (MB) and Rhodamine B (RhB) under UV-light irradiation were 50%

and 56.4% respectively over Pani and 65.7 and 71.6% respectively over Pani/BN. The

results indicated that the extent of degradation of MB and RhB was greater over

nanocomposite material than Pani, which may result due to high electron–hole pairs

charge separation under UV light.

In chapter three, the electrical properties and photocatalytic activity of

polyaniline (Pani) and polyaniline/silk fibroin (Pani/SF) composite were studied. The

Pani/SF composite has been synthesized first time by chemical oxidative in-situ

polymerization method by using potassium persulphate (K2S2O8) in an acidic

medium. Thus prepared Pani and Pani/SF composite were characterized by using

Fourier transform infrared spectroscopy, x-ray diffraction, thermogravimetric analysis

and scanning electron microscopy. The stability in terms of the DC electrical

conductivity of Pani/SF composite and Pani was investigated under isothermal and

cyclic aging conditions. Results indicated that the electrical properties of the

composite were significantly influenced by the loading of SF to Pani and observed to

be comparatively more thermally stable than Pani. The as prepared Pani and Pani/SF

composite were studied for the degradation of Methylene blue (MB) and Rhodamine

B (RhB) under UV-light irradiations. The results showed that 91.4% & 85.4%

degradation of RhB and MB takes place respectively after 90 min over Pani/SF

composite. The decomposition rate of composite was 1.8-2.2 times greater than that

of Pani. The improvement of photocatalytic activity of composite may be attributed to

the electron-sink function of silk fibroin which increases the optical absorption

property and separation of photogenerated charge carriers than Pani alone.

In chapter four, an electrical conductivity based rapid response and excellent

recovery cigarette smoke sensor based on polyaniline/silicon carbide (Pani/SiC)

nanocomposite prepared by in-situ chemical oxidative polymerization technique in an

acidic medium. The Pani/SiC nanocomposites and polyaniline (Pani) were

characterized by using Fourier transform infrared spectroscopy, x-ray diffraction,

scanning electron microscopy and transmission electron microscopy. Results

v

indicated that the well ordered nanocomposites were successfully prepared. The

morphology and electrical properties of the nanocomposites were influenced by the

extent of loading of SiC nanoparticles. The as-prepared materials were studied for the

change in their electrical conductivity on exposure to cigarette smoke followed by

ambient air at room temperature. It was observed that Pani/SiC-3 nanocomposite

shows eight times higher amplitude of conductivity change than Pani on exposure

cigarette smoke followed by ambient air. The conductivity response in the carbon

monoxide, ammonia and methanol was also measured. It was observed that the carbon

dioxide (CO2) shows low response while in the case of methanol and ammonia the

significant responses are observed, suggesting the possibility of contribution of above

gases towards response against cigarette smoke. The cigarette smoke also contains

many other volatile chemicals such as polyaromatic hydrocarbon (PAH), nicotine,

hydrogen cyanide (HCN) and formaldehyde which may also interacted with

polarons/bipolarons of polyaniline leading to decreasing in conductivity.

In chapter five, camphor sulphonic acid doped conducting polyaniline/silk

fibroin (Pani/SF) composites were prepared by in situ polymerization method. The

Pani/SF composite fibers were characterized by using Attenuated total reflectance-

Fourier transform infrared spectroscopy, thermogravimetric analysis, scanning

electron microscopy and digital photographs. Results indicated that the Pani was

successfully coated on silk fibroin. The electrical properties of Pani/SF composite

fibers were influenced by the extent of loading of aniline monomers. The as-prepared

material was studied as sensing material by observing change in their electrical

conductivity response on exposure to ammonia and acetaldehyde followed by ambient

air at room temperature. It was found that the composite fibers showed good

reversibility and conductivity decreased on exposure to higher concentration of

ammonia and acetaldehyde vapours at room temperature.

In chapter six, polyaniline@graphene/nickel oxide (Pani@GN/NiO),

polyaniline/graphene (Pani/GN), polyaniline/nickel oxide (Pani/NiO) nanocomposites

and polyaniline (Pani) were successfully synthesized and tested for ammonia sensing.

Pani@GN/NiO, Pani/NiO, Pani/GN and Pani were characterized by using x-ray

diffraction, UV-vis spectroscopy, Raman spectroscopy, scanning electron microscopy

and transmission electron microscopy. The as-prepared materials were studied for

comparative DC electrical conductivity and the change in their electrical conductivity

vi

on exposure to ammonia vapours followed by ambient air at room temperature. It was

observed that the Pani@GN/NiO nanocomposite showed about 99 times greater

amplitude of conductivity change than pure Pani on exposure to ammonia vapours

followed by ambient air. The fast response and excellent recovery time could be

probably ascribed to relatively high surface area of Pani@GN/NiO nanocomposite,

proper sensing channels and efficaciously available active sites. Pani@GN/NiO was

observed to be good in selectivity towards ammonia because of comparatively high

basic nature of ammonia than other volatile organic compounds (VOCs) tested. The

sensing mechanism explained on the simple acid base chemistry of polyaniline.

In chapter seven, the overview and directions towards the future work were

discussed.

Chapter 1

1

1.1. ELECTRICALLY CONDUCTING POLYMERS

Few decades earlier, it was believed that insulating polymers that are plastics provide

significant advantages for many applications [1]. However, this concept was first

challenged when reports of conducting or semiconducting polymeric materials such as

polyacetylene and polyaniline began to appear [2].

It was only when the doping of polyacetylene had succeeded that new

concepts of physics began to be realized and a new class of organic polymers with the

remarkable ability to conduct electrical current began to promote tremendous

technological and scientific interest.

In semiconductor physics, undoped semiconductors are intrinsically

conducting while doped semiconductors are extrinsically conducting in nature. In

contrast, doped conducting polymers are often referred to as “intrinsically conducting

polymers”. This is to distinguish them from the polymers which acquire electrical

conductivity by loading with conducting particles such as carbon black, metal flakes

or fibers of stainless steel. They may be referred to as polymer based “conducting

composites” which are finding use in a variety of applications due to their light weight

and fairly high electrical conductivity.

The intrinsically conducting polymers (ICPs), more commonly known as

“synthetic metals”, refer to a class of organic polymers which possess unique

electrical, electronic, magnetic and optical properties of metals besides the fairly good

mechanical properties and processability of conventional polymers [3]. The unique

property of this class of polymers is the intrinsic ability of the conjugated polymer

backbone to support the movement of charge carriers (electrons and holes) for

electrical conduction.

Since the initial discovery in 1977 [2] by Alan G. MacDiarmid and his co-

workers that polyacetylene, (CH)x, now commonly known as the prototype

conducting polymer, could either be chemically or electrochemically p- or n-doped to

become an electrical conductor.

The development of the conducting polymer field has continued to accelerate

at an unexpectedly rapid rate. The rapid growth rate has been stimulated not only by

the field’s fundamental synthetic novelty and importance to a cross-disciplinary

Chapter 1

2

section of investigators - chemists, electrochemists, experimental and theoretical

physicists and electronic and electrical engineers - but also to its actual potential

technological applications.

Thus conducting polymers may be perceived as conjugated polymer

macromolecules having the fully conjugated sequence of covalent bonds along the

polymer backbones which acquire the positive or negative charges by oxidation or

reduction process. With the many more discoveries in the field of electrically

conducting polymers, today we have vast variety of polymers which have shown to

exhibit electrical conductivity. The structures of some the electrically conducting

polymers are given in Fig. 1.1.

Fig. 1.1 Chemical structures of some important conjugated polymers [4].

1.2. DOPING OF CONDUCTING POLYMERS

To conduct, a polymer must have a conjugated backbone for easy movement of

charge carriers. However, the conjugated polymers do not contain any intrinsic

charge-carriers, charge-carriers have to be provided extrinsically, typically by a

charge transfer process, generally known as ‘doping’ in a very poor analogy to silicon

technology. Polymers have the electronic profiles of either insulator or

semiconductors. Thus, the band gap in a fully saturated chain such as polyethylene is

~5 eV and decreases to about ~1.5 eV in the fully conjugated system of

polyacetylene. The respective intrinsic conductivities are ~10-17

Scm-1

and ~10-8

Scm-

1, both of them are very low. Conjugated polymers can be made conductive either by

oxidizing or reducing the polymer using a suitable reagent i.e. the dopant.

Chapter 1

3

1.2.1. Protonic Acid Doping of Conducting Polymers

The polymers having π-conjugated system may also be doped by no-redox means in

certain situations. In such cases, the number of electrons that are associated with the

conducting polymer backbone does not change during doping process. However, the

energy levels are rearranged during doping. One way to dope without involving redox

chemistry is to use protonic acids.

Protonic acid doping has been observed in polyacetylene in which the

conductivity increased by some four orders of magnitude [5]. Polyaniline is the first

example of the doping of an organic polymer to highly conducting regime by this

process. Protonic acid doping is extensively used when converting polyaniline from

the non-conducting to conducting form [6].

Acid-Base protonation in chemistry leads to an internal redox reaction and

converts the non-conducting emeraldine base into highly conducting emeraldine salt.

The chemical structure of the emeraldine base is somewhat similar to an alternating

block copolymer. Upon protonation of the emeraldine base leading to the formation of

emeraldine salt, the proton induced spin unpairing mechanism leads to a structural

change with one unpaired spin per repeat unit without any change in the number of

the electrons.

This remarkable conversion of polyaniline from non-conductor to conductor

causes a 9-10 orders of magnitude increase in the electrical conductivity. This has

very well been described in the literature but it is not well understood from the point

of basic theories and concepts [7, 8].

1.2.2. Redox Doping

The commonly used oxidants (p-type dopants) may include HClO4, FeCl3, AsF5, I2,

NH4BF4, SO3CF3, NOPF6, p-toluene sulfonate, HCl, HNO3, H2SO4, H3COOH, H3PO4

etc. whereas reductants (n-type dopants) may include Li, K, Na etc. A general

equation for doping of a conjugated polymer may be given by the following chemical

reactions.

Chapter 1

4

+ FeCl2(-P-) + FeCl4(-P-) + 2 FeCl3

2+ FeCl2(-P-) + FeCl4(-P-) + 2 FeCl3

(-P-) + Na(-P-) + Na

2(-P-) + Na(-P-) + Na

1.2.3. Radiation Doping

Semi-conducting polymer chain is locally oxidized and nearby chain is reduced by

photo-absorption and charge separation i.e. electron-hole pair creation and separation

into free charge-carriers:

π-polymer (π-polymer)+x

+ (π-polymer)-x

(1.5)

where x is the number of electron-hole pairs. In case of the photo-excitation, the

photoconductivity is transient and lasts only until excitation are either trapped or

decay back to ground state. In contrast, the induced electrical conductivity is

permanent in case of chemical or electrochemical doping until the charge carriers are

purposely removed/destroyed by undoping.

High energy radiations such as γ-rays, electron beam and neutrons are used for

doping of polymer by neutral dopants. For example, γ -rays irradiation in the presence

of SF6 gas or neutron radiation in presence of I2 has successfully been used to dope

polythiophene [9, 10].

1.2.4. Self Doping

It does not require any external doping agent. The ionizable groups attached to the

polymer chain act as dopant for the polymers [11].

1.3. ELECTRICAL CONDUCTION

Conjugated polymers can undergo redox reaction more easily than conventional

polymers. It is because of the presence of π-conjugated structure and the electrical

conduction is obtained through “doping” leading to the generation of charge-carriers

in the form of free electrons or holes. The charge carriers are usually delocalized over

the conjugated polymer chain [12].

(1.1)

(1.2)

(1.3)

(1.4)

Chapter 1

5

The transport of the charge-carriers along a conjugated backbone can be

described by the Band Model as has been done for metals and semiconductors.

Besides this intrachain conduction incorporating very high intrinsic conductivity to

the doped conjugated polymers, several hopping and tunnelling processes are also in

operation for non-intrinsic (interchain and interfiber) conduction processes.

1.3.1. Band Theory

Electrical conductivity is a function of the number of charges carriers (n), the

electronic charge on each carrier (ε) and the mobility of the charge carriers (μ) as

given under:

σ = n ε μ (1.6)

Conduction in solids is usually explained in terms of the Band Theory. This

theory postulates that when atoms or molecules are aggregated in the solid state, the

outer atomic orbitals containing the valence electrons split into bonding and

antibonding orbitals and mix to form two series of closely-spaced energy levels i.e.

bands.

These bands are usually called the valence band and conduction bands

respectively. If the valence band is only partly filled by the available electrons or the

two bands overlap so that no energy gap exists between them. Then application of

potential in

such materials will raise some of the electrons into empty levels where they will be

free to move throughout the solid thereby producing a current. This is the description

of a conductor.

On the other hand, the valence band is full and is separated from the empty

conduction band by an energy gap, there can be no net flow of electrons under the

influence of an external field unless electrons are elevated into the empty conduction

band. This will require a considerable expenditure of energy. Such materials are either

semiconductors or insulators depending on how large the energy gap is? The majority

of polymers are insulators due to the large band gap. The Band model thus assumes

that the electrons are delocalized and can extend over the lattice as depicted in Fig.

1.2.

Chapter 1

6

Fig. 1.2 Schematic presentation of the band structures of insulators, semiconductors

and metals [13].

When we come to consider electronic conduction in polymers, Band Theory is

not totally suitable because the atoms are covalently bonded to one another forming

polymeric chains that experience weak intermolecular interactions. Thus, the

macroscopic electronic conduction will require the explanation of theories and

mechanism for electron movement not only along the chains but also from one chain

to another as described in the forth coming section [13].

1.3.2. Hopping and Tunnelling

The precise mechanism of charge transport in conducting polymers is still not very

well understood. Conjugated polymers possess a complex morphological distribution

of crystalline and amorphous regions. Therefore, this complex situation poses a great

hindrance to trace the movement of charge-carriers in various domains [14]. Solitons

and bipolarons constitute the majority of charge-carriers. There is an intermediate

range of electronic energy state in which mobility is very low. Conduction is possible

only if electrons are excited to higher state with greater mobility.

Large Band Gap Small Band Gap Band Overlap VB Band Half Filled

Insulators SemiconductorsMetals

Intr

insi

c E

nergy

Energy Levels in Conduction Band

Energy Levels in Valance Band

Chapter 1

7

Conduction via localized electrons implies direct jumps across or tunnel

through an energy barrier from one site to the next. Several mechanisms for

conduction and charge transport have been reported, however, the relative

contribution of these mechanisms depends upon the shape of the barrier and

availability of the thermal energy [15].

The movement of charge along a specific chain as well as the movement from

one chain to another chain and from crystalline domain to amorphous domain or vice

versa have to be considered. Inter-soliton hopping mechanism has been proposed to

explain such conduction. The solitons move around by exchanging electrons with the

nearby charged solitons [16].

In hopping and tunnelling mechanisms, the charge carriers hop/tunnel from

one localized state to another within energy band gap [17-19]. The energy for hopping

to take place is provided by the phonon at non-zero temperature. Electrons hop from

one energy state to next by absorbing phonons. At zero temperature, the conductivity

is zero in those conducting polymers where the hopping mechanism is dominant. As

the temperature increases, more and more electrons absorb phonons and start hopping

[20].

1.4. POLYANILINE

Polyaniline came into notice around 150 years ago as “aniline black”, a term used for

any product obtained by the oxidation of aniline. It has only recently caught the

attention of the scientific community due to its high electrical conductivity on

treatment with protonic acids and became the most investigated conducting polymer

since its discovery about three decades ago.

Fritzche [21] carried out the tentative analysis of the products obtained by the

chemical oxidation of this aromatic amine. Subsequent investigators [21, 22] have

verified these results and similar observations have been made during the oxidation of

aqueous hydrochloride acid solutions of aniline. The interest in polyaniline as an

important conducting polymer has increased significantly over the past decade

resulting into a number of review articles published a few years ago [23-25]. At the

same time, a number of groups looked at the reaction conditions necessary to produce

optimum quality polyaniline [26-28].

Chapter 1

8

Recently, reports of high molecular weight polyaniline synthesized at

temperatures of between -30°C and -40°C appeared in which lithium chloride was

used as an inert solute to keep the reaction mixture mobile [29-32]. They used either a

large molar deficit of ammonium persulfate oxidant to aniline, which gave a low yield

of polymer, or electrochemical polymerization; but no attempts were made to assess

the structural quality of the polyaniline. It has been found that polyaniline can exist in

three different, isolable oxidation states at the molecular level [33]. They are the

emeraldine, leucoemeraldine and pernigraniline as shown in Fig. 1.3.

Fig. 1.3 Schematic illustration of three important oxidation states of polyaniline [34].

1.4.1. Synthesis of Polyaniline

The emeraldine salt form of polyaniline can be synthesized by the oxidative

polymerization of aniline monomer in the presence of aqueous acid by chemically or

electrochemically methods. A typical chemical synthesis of polyaniline by chemical

oxidative route involves the use of acids in the presence of oxidizing agent such as

ammonium persulfate (NH4)2S2O8, potassium persulfate K2S2O8 etc. in aqueous

medium. The principal function of the oxidant is to withdraw a proton from aniline

molecule without forming a strong co-ordination bond either with the

substrate/intermediate or with the final product. However, optimum quantity of

oxidant is used to avoid any oxidative degradation of the polymer formed.

Gospodinova et al. [35] reported that the propagation step of polymerization

of aniline proceeds via a redox process between the growing chain (an oxidant) and

Chapter 1

9

aniline (a reductant) along with the addition of monomer units to the growing chain

end. The high concentration of strong oxidant, (NH4)2S2O8, at the initial stage of the

polymerization enables the fast oxidation of oligomers and polyaniline as well as their

existence in the oxidized form.

The electrochemical preparation of conducting polymer dates back to early

attempts of Dall’olio and co-workers [36] who obtained “pyrrole black” as it was

called at that time on electrochemical oxidation of pyrrole in aqueous sulphuric acid

in the form of highly insoluble powder on platinum electrode. Electrochemical

polymerization is regarded as a simple and novel method for synthesis of conducting

polymers. The beauty of this method is that the polymerization in suitable electrolytic

medium directly gives the doped polymer as a flexible freestanding film. In this

method, films are produced on the electrode surface by oxidative coupling. In this

respect, this method is somewhat similar to the electrochemical deposition of metal.

The first electrochemical synthesis of emeraldine salt was reported by Letheby

[37] in 1862. In 1962, Mohilner et al. [38] reported the mechanistic aspects of aniline

oxidation. Major interest in the electrochemistry of Pani was generated only after the

discovery that aromatic amine, pyrrole, thiophene, furan, indole and benzene can be

polymerized anodically to electrically conducting films. Electrochemically prepared

polyaniline is the preferred method to obtain clean and ordered polymer thin films.

1.4.2. Polyaniline Composites

Composites of conducting polymers with insulating polymers and other materials are

often used to impart synergistic effects such as mechanical strength, environmental

stability, and processability to thus produced materials. The word 'composite' consists

of the Latin prefix 'com' meaning 'together' and 'posit' meaning 'to put or place', which

means 'put together or made up of separate parts'. Polyaniline composites can be

made by dispersing of particles such as graphene, natural polymers, carbon nanotubes,

metals and metal oxides into the polymer matrices.

The composite materials possess combination of each individual’s

characteristics with enhancement in required properties due to the synergism between

the constituents. Recently, demand of composites of polyaniline with metal oxides as

well as graphitic materials has been observed to be exponentially increasing. When

Chapter 1

10

one of the components is polyaniline and the infusing materials are in the nano range,

thus prepared composite materials are termed as “Polyaniline Nanocomposites”.

Classically, they are also synthesized via same method as discussed earlier for the

preparation of polyaniline but with additional supplementation of nanoparticles like

metallic oxides or graphitic nanomaterials.

Out of several polyaniline nanocomposites, the interest of researchers in

polyaniline combined with metallic oxides could possibly be linked to the numerous

applications that exist for the electronic conducting polymers. Aniline also combines

with them very easily resulting into an economical and very stable material with

improved properties.

On the other hand, the nanocomposites of polyaniline with these inorganic

semiconducting nanomaterials show the improved mechanical, electrical and thermal

properties due to the synergistic effects of both the combining components. In

particular, polyaniline/metal oxides nanocomposites display applications on a large

scale for various electrochemical, electrorheological and in the electronic fields such

as batteries, sensors, controlling systems and organic displays [39-42].

1.4.2.1. Composites of polyaniline with boron nitride

Boron Nitride (BN) exists in various crystalline forms that are isoelectronic similar to

the graphene structured carbon lattice. The hexagonal form corresponding to graphite

is the most stable and soft among boron nitride polymorphs and is therefore used as a

lubricant and an additive to cosmetic products (Fig. 1.4).

Because of excellent thermal and chemical stability, boron nitride ceramics are

traditionally used as parts of high-temperature equipments. The nanocomposites of

polyaniline with boron nitride have been anticipated promising materials for many

electrical, photocatalytic and electrochromic devices.

In 2005, Chunyi Zhi et al. [43] reported a composite film which is indicative

of the strong interactions between boron nitride nanotubes (BNNTs) and polyaniline.

They observed that polyaniline becomes more ordered in the composite with respect

to its pure form. The studies revealed that mechanically tough BNNTs may find

application as an effective composite additive in emeraldine base (EB) in order to

Chapter 1

11

improve the polymer processing and handling due to the strong BNNT–polymer

interactions. In addition, the optical properties of the EB–BNNT composites may be

of significant interest.

Later in 2008, Chunyi Zhi et al. [44] discovered a new method to trigger an

ordered phase appearance in polyaniline via strong π-interactions between multi-

walled BNNTs and polyaniline. They found that the embedment of BNNTs in

polyaniline induces the notable variations in vibrational spectra and wettability of

polyaniline due to modified functional groups and/or surface morphology.

In 2011, Jianmin Wu and Longwei Yin [45] produced BNNTs-Pani-Pt hybrids

in which Pt nanoparticles decorated on polyaniline wrapped boron nitride nanotubes

(BNNTs). They found that π-interactions take place between BNNTs and polyaniline

located at N atoms of BNNTs and aromatic hexagonal component of polyaniline.

They also observed that the glucose oxidase can successfully be immobilized on

BNNTs-Pani-Pt hybrids. The resulting amperometric glucose biosensor based on the

BNNTs- Pani-Pt hybrids exhibited a fast response time (within 3 s) and a linear

calibration range from 0.01 to 5.5 mM with a high sensitivity and low detection limit

of 19.02 mA M-1

cm-2

and 0.18 μM glucose (S/N = 3). The developed biosensor

exhibited excellent acid stability and heat resistance.

In 2013, Emrah Çakmakçı and Seyfullah Madakbaş [46] prepared

polyaniline/h-BN composite materials. They observed that dielectric measurements of

revealed dipolar polarization behaviour of the Pani/h-BN composites. The glass

transition temperatures of the composites were found to increase with the addition of

h-BN due to the decrease in the segmental motion of the polymer chains.

Chapter 1

12

Fig. 1.4 Structure of hexagonal boron nitride.

1.4.2.2. Composites of polyaniline with silk fibroin

Fibroin is an insoluble protein present in silk created by spiders, the larvae of Bombyx

mori and other moth genera. Silk in its raw state consists of two main proteins, sericin

and fibroin as glue-like layer of sericin coating on two singular filaments of fibroin

called brins (Fig. 1.5) [47].

Since silk fibroin is not intrinsically conductive, the incorporation of

conductive fillers into or onto the fiber is necessary for obtaining electrically

conducting silk fibroin based composite materials. Comparing with the usual method

(electrospinning of silk fibroin solution blended with conductive inorganic materials),

the incorporation of conductive conjugated polymers such as polyaniline (Pani),

polypyrrole (Ppy) and poly3,4-ethylenedioxythiophene (PEDOT) on silk fibroin

fibers provides a useful tool for producing new silk fibroin based electrically

conducting materials because such conducting polymers have been proven to be

promising alternatives for developing new biodegradable scaffolds [48-50].

In 2011, Youyi Xia et al. [51] synthesized silk fibroin fibers supported with

high density of Au nanoparticles by modifying silk fibroin with sulfonated polyaniline

(S-Pani) and subsequently in situ reduction of AuCl4 on the fibers surface. They

Chapter 1

13

observed that the as prepared composite fibers exhibited good catalytic activity and

may have other potential applications in a range of fields such as biosensors and

textiles.

Later in 2013, Youyi Xia et al. [52] reported the silk fibroin/polyaniline

(core/shell) coaxial fibers scaffold fabricated by an in situ polymerization method.

The thickness of uniform polyaniline layer was estimated about 500 nm by electron

microscopy resulting into the as-prepared coaxial fibers of high electrical conductivity

(σ = 2.5–4.8 x 10-1

Scm-1

) and preventing the silk fibroin core from being destroyed

during heat treatment. Their experimental data demonstrated that the coaxial fibers are

biocompatible, supporting attachment and proliferation of murine fibroblast L929

cells.

Later in 2014, Yahya A. Ismail et al. [53] fabricated silk fibroin/polyaniline

hybrid microfibrous mat as reactive sensor. They observed that the reverse oxidation

of polyaniline in methane sulfonic acid aqueous solution giving closed

coulovoltammetric responses. These responses sense the reaction variables:

electrolyte concentration, pH, temperature and driving current. The potential of the

materials or the consumed electrical energy for a constant reaction rate follow those

linear or semilogarithmic relationships with each of the experimental variables

predicted by the electrochemical kinetics. The charge of the closed coulovoltammetric

loop also senses the chemical or thermal energetic conditions of the reaction acting on

the conformational movements in the polymer getting deeper oxidation states for

rising energetic working conditions.

Fig. 1.5 (a) Natural silk cocoon and (b) Primary structure of silk fibroin.

Chapter 1

14

1.4.2.3. Composites of polyaniline with silicon carbide

Silicon carbide (SiC) is a semiconducting material with tuneable band gap (Fig. 1.6).

Electronic applications of silicon carbide such as light-emitting diodes (LEDs) and

detectors in early radios were first demonstrated around 1907. The nanocomposites of

polyaniline and SiC have been found potential materials for different applications

[54].

A. Kassiba et al. [55] prepared hybrid core-shell nanocomposites based on

silicon carbide nanoparticles functionalized by electrically conducting polyaniline for

the investigation of electron paramagnetic resonance. They carried a consistent EPR

investigations on original systems based on inorganic nanoparticles whose surfaces

are functionalized by electrically conducting polyaniline. EPR results showed that the

nanoparticles overcame the interchain bipolaron formation and the polaron conversion

process. They concluded that the performed experiments indicate that the main role of

the SiC nanoparticles on the nanocomposite performance includes improved thermal

and structural stability associated with similar electrical responses.

Later in 2011, J F Felix et al.[56] fabricated high-quality Pani/SiC

heterojunctions by spin coating of Pani films onto n-type 6H-SiC and 4H-SiC

substrates. They obtained linear G/I–G plots indicating good device quality. Also the

surface state density at the Pani/SiC interface was evaluated by modelling the devices

as Schottky diode with series resistance and an oxide interfacial oxide layer. Using the

forward bias I–V characteristics method, they found that interface trap density for

Pani/4H-SiC heterojunctions is approximately one order of magnitude higher than that

for Pani/6H-SiC heterojunctions. The average value of interface trap densities for 6H-

SiC device was 8.4×1011

eV−1

cm−2

and for 4H-SiC it was 2.7×1013

eV−1

cm−2

. From

the results obtained, they suggested that these heterojunctions present high potential to

be used as platforms for future development of devices such as gas, radiation sensors

and surface acoustic wave (SAW) devices exploiting the properties of both the

materials.

In 2013, Sha Li et al. [57] prepared Pani/SiC composites by emulsion

polymerization for the study of microwave absorbing properties. According to the

transmission line theory, the RL values showed that the reflection coefficient (RL)

Chapter 1

15

maximum of Pani/SiC composites increased to 27.1 dB with a thickness of 0.9 mm

and the bandwidth under 10 dB reached 6.2 GHz compared to single SiC absorber.

J. F. Felix et al. [58] later in 2014 reported the effect of gamma radiations on

electrical conductivity of Pani/SiC heterojunctions from polyaniline/silicon carbide

composite. The responsiveness of the Pani/SiC devices was observed through shifts of

the impedance and capacitance curves. From the results, they observed that these

devices are very good candidates for applications in low dose radiation dosimetry and

that the impedance spectroscopy seems to be a good tool to characterize the electrical

devices subjected to gamma radiation. Also, the Pani/SiC rectifier heterojunctions are

almost insensitive to radiation doses under 100Gy. However, this behavior requires

further investigations in order to propose these kinds of heterojunctions as

semiconductor devices to operate under gamma radiation exposure.

Fig. 1.6 Structure of silicon carbide.

1.4.2.4. Nanocomposites of polyaniline with graphene

Graphene, “The Wonderful Material”, is rapidly rising on the frontier line of material

science and condensed matter physics creating interdisciplinary passion in concerned

ones. Just after Nobel Prize winning work of Novoselov et al. [59], graphene research

has developed truly with a relentless pace. This sp2 hybridized C-flatland arrayed in a

honeycomb pattern is the building block for graphitic materials of all other

dimensionalities [60]. From the application view point, it is no surprise to consider it

Chapter 1

16

even more promising than other nanostructured carbon allotropes. The most explored

and really interesting aspect of graphene physics and chemistry is its electronic

properties which are explained through its electronic spectrum.

Here in, whenever electrons propagate through honeycomb lattice of graphene

they effectively lose their mass producing quasi-particles (Fig. 1.7). Their results are

described by a 2D analogue of the Dirac equation rather than the Schrödinger

equation for spin-½ particles [61]. The credit for escalating attraction towards

graphene goes to their superior characteristics of electronic, mechanical, optical and

transport nature. To make operative the above mentioned novel properties of both the

constituents i.e. polyaniline and graphene, preparation of nanocomposites are

emphasized to conserve as well as develop extrafunctional properties.

Since both the polyaniline and graphene have conjugated π-electrons, it seems

to be interesting to observe any after effects of this π-π stacking on electronic and

transport properties of as-prepared polyaniline/graphene nanocomposites from both

the fundamental and the application point of view. Thus prepared

polyaniline/graphene nanocomposites with improved mechanical, electrical, thermal

and related properties because of the synergism between the constituents make them

potentially ideal for use in modern applications.

But an unavoidable problem is that vast amount of literature is appearing

every day making it really difficult to keep up with the emerging trends and opening

opportunities. To combat this curse of success and “gold rush” we are concerned in

making a mini review of polyaniline/graphene nanocomposites studied so far in

following aspects only methods of preparation, the electrical properties and the

sensing properties.

Up till now various methods have been proposed by different scientists to

fabricate graphene-based polyaniline nanocomposites such as in situ chemical

oxidative polymerization, in situ electropolymerization, solution mixing, self-

assembly etc. But most of the polyaniline/graphene nanocomposites were prepared

using in situ chemical oxidative polymerization [62-64] and in situ

electropolymerization [65, 66]. Other conventional methods such as interfacial

http://en.wikipedia.org/wiki/Quasi-particle

http://en.wikipedia.org/wiki/Dirac_equation

http://en.wikipedia.org/wiki/Schr%C3%B6dinger_equation

http://en.wikipedia.org/wiki/Schr%C3%B6dinger_equation

Chapter 1

17

polymerization [67], solution mixing [68-70], self assembly approach [71, 72] and

soon have also been reported to prepare graphene-based polyaniline nanocomposites.

In fact, these preparation methods direct the morphology and consequent

properties of thus prepared nanohybrids. For example, polyaniline/graphene

nanocomposites with ordered structure could provide both a larger contact surface

area for the intercalation/de-intercalation of protons into/out of polyaniline and a

highly porous structure for the fast access of ions to and from electrolytes. It was also

observed that preparation via in situ mixing gave better result compared with

corresponding ex situ and other conventional methods [73]. Thus, further efforts are

still being devoted to prepare the graphene-based polyaniline nanocomposites with

ordered structures and significant enhancement in desirable properties.

The underpinning of graphene in polyaniline matrix has shown significant step

up in electrical conductivity of in situ prepared polyaniline/graphene nanocomposites.

This enhancement was due to the large surface area of graphene.

Later in 2009, H. Bai et al. [74] coincidentally observed that the electrical

conductivity of polyaniline/sulfanilic acid sulfonated graphene was 25.6 Scm-1

which

was 20 times higher than that of pristine polyaniline (1.42 Scm-1

). The improvement

in conductivity in sulfonated graphene-doped polyaniline can be attributed to the

graphene sheets acting as a bridge for hopping of electrons between polyaniline

chains.

Another kind of sulfonated polyaniline modified reduced graphene (SPani/r-

G) showed enhanced electrocatalytic activity and improved electrochemical stability

[75]. The better performance may be attributed to the strong interactions between

SPani chains and the basal planes of graphene combined with the aggregation of

graphene sheets which resulted into the formation of a cross linked 3D network [75].

The specific capacitance was also enhanced in graphene-based polyaniline

nanocomposites [76] and it may be due to the synergistic effect between graphene

nanosheets (GNS) and polyaniline in which the GNS provides a highly conductive

path as well as high surface area for the deposition of nanometer-sized polyaniline.

The thermoelectric properties have also been enhanced in graphene-based

polyaniline nanocomposites [77, 78]. The thermoelectric performance of the

Chapter 1

18

polyaniline/exfoliated graphene nanoplatelets (GNP) nanocomposites is 2-folds

higher than either of the constituents exhibiting a significant synergistic effect [77].

Herein, the polyaniline nanofibril coating would make the polarons mobility easier

and the presence of graphene nanoplatelets (GNP) also bridge the carrier transport

between chains through the tunnelling mechanism and lead to improved electrical

conductivity for the resulting polyaniline/GNP composites [77]. The much improved

electrical conductivity, excellent thermoelectric properties, long-term electrochemical

stability and high specific capacitance ensure their potential applications in super

capacitors, sensing platforms, solar cells, lithium ion batteries etc.

Fig. 1.7 Honey comb structure of graphene.

1.4.2.5. Composites of polyaniline with graphene/NiO

Graphene is the most ideal electrode material among carbon materials such as active

carbon [78-80], carbon fiber [81] and [82], glassy carbon [83], graphite [84] and [85],

carbon black [86], carbon aerogels [87], carbon nanotubes [88]. Nevertheless,

graphene usually suffers from restacking and agglomeration which lead to great loss

of effective area and exhibit lower capacitance than expected. Simultaneously,

graphene-based supercapacitors work as EDLCs (Electric Double Layer Capacitor)

can't meet the high power density request in high-performance devices. Therefore, it

is crucial to improve the two-dimensional structure and surface modification of

graphene in order to reinforce the capacity and power density.

Chapter 1

19

To address this issue, graphene has been introduced to prepare graphene/metal

oxide composites. RuO2 [89], Co3O4 [90], SnO2 [91-93], Fe3O4 [94] and NiO [95, 96]

are the main modification materials for graphene. Theses graphene-based metal

oxides uniformly distributed on graphene sheets can eliminate restacking and

agglomeration of graphene during synthesis and stabilize the volume changes of metal

oxide in cycling process. Among the metal oxides, NiO has caught more attention due

to its high theoretical capacity (2584 F g−1

), low price and environmental friendliness.

Recently in 2016, Xinming Wu et al. [97] designed a nano nickel oxide coated

graphene/polyaniline (NiO-GN/Pani) nanostructure and synthesized as electrode

material for flexible supercapacitor application via hydrothermal and in situ chemical

oxidative polymerization methods. They found that this nanostructure exhibits

outstanding electrochemical performance such as high specific capacitance (1409 Fg-1

at 1 Ag-1

) and good cycling stability (92% capacity retention after 2500 cycles at 1

Ag-1

). The enhanced electrochemical performance of this ternary composite is due to

high surface area, low internal resistance, reduction of diffusion length and synergistic

effect between individual constituents.

1.5. CONCLUDING REMARKS

In summary, sustained research on polyaniline and their composites has enthralled the

work interest due to its surprising acid-base doping/de-doping chemistry especially

when combined either with metallic oxides or graphitic materials. This synergistic

combination greatly improves the performance of as-prepared nanocomposites. As a

consequence, such polyaniline nanocomposites can widely be employed in various

applications.

To date, various preparation methods have been developed to synthesize

nanoparticles loaded polyaniline composites such as in situ chemical oxidative

polymerization, in situ electropolymerization, interfacial polymerization, solution

mixing, self-assembly and so on. In fact, the properties of polyaniline and its

composites are highly governed by its preparation method and thus morphologies.

In addition, non-stop progress has been made on the understanding of the

mechanisms of electronic conduction and various theories have been proposed so far.

Step-wise mechanistic understanding of aniline polymerization has also been a matter

Chapter 1

20

of great enthusiasm but the major drawback is the lack of experimental confirmation.

It is still a challenge to develop new methods to realize the mechanisms.

Among various fillers studied so far, SiC and graphene-based polyaniline

composites have shown better electrical, mechanical, magnetic, thermal properties

etc. This enhancement can be ascribed to the large specific surface area and unique

properties of SiC and graphene along with electrocatalytic activity of polyaniline as

well as their cooperative effects.

Much attention has been focused on the enhanced electrical and

electrochemical properties but there are few reports on mechanical, optical or other

properties. The electrical and electrochemical properties of graphene based

polyaniline nanocomposites could further be improved by optimizing the methods of

preparation to obtain ordered structures. With these enhanced properties, graphene

based polyaniline nanocomposite materials are widely employed in applications such

as supercapacitors, sensing platforms, electrochromic devices, lithium ion batteries,

photocatalysis, anticorrosion coatings and so on.

However, they suffer from problems regarding solubility and limited energy

density. More attention to overcome these problems is still needed. A potential

alternative is functionalization of polyaniline and their nanocomposites to furnish

them with desired characteristics which are rarely reported at present. Therefore,

greater efforts should be made to improve the performance of polyaniline

nanocomposites, explore their applications and realize their commercialization in the

near future.

1.6. OBJECTIVES OF THE RESEARCH

The present work was concentrated on in situ preparation of conducting polyaniline

nanocomposites in order to improve the electrical and electronic properties besides

overcoming the problems faced due to poor processibility of polyaniline. Among the

variety of property improving nanoscale materials, we have tried to make use of a

silicon carbide, h-boron nitride, silk fibroin and graphene/nickel oxide.

Graphene/nickel oxide nanomaterial was incorporated to form the polyaniline

nanocomposite with the assumption that they will not only maintain rather enhance

the electrical properties because many applications of polyaniline are based on their

Chapter 1

21

electroactive switching properties. The main objectives of these studies are presented

as below.

Synthesis of a series of polyaniline composites and nanocomposites using h-boron

nitride, silk fibroin, silicon carbide and graphene/nickel oxide.

Characterization of composites and nanocomposites using various material

analytical techniques such as scanning electron microscopy (SEM), transmission

electron microscopy (TEM), fourier transform infra-red (FT-IR), x-ray diffraction

(XRD), thermogravimetric analysis (TGA) etc.

Test sample preparation of composites and nanocomposites and characterization

of the electrical properties of composites and nanocomposites by using four-in-

line probe device.


of the photocatalytic properties of some composites and nanocomposites by using

photocatalytic reactor.


of the chemical sensing properties based on DC electrical conductivity of some

composites and nanocomposites by four-in-line probe device for ammonia,

acetaldehyde, carbon dioxide, formaldehyde, isopropanol, methanol, ethanol,

acetone and cigarette smoke.

Development of the suitable sensing mechanisms for the sensing processes of

different sensing systems studied.

Chapter 1

22

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Chapter 2

27

2.1. INTRODUCTION

Since the discovery of the first conducting polymer (polythiazyl) [1], such polymers

have generated humongous interest due to their unique properties such as thermal

stability, photocatalytic activity, electrical conductivity etc [2]. These properties of

conducting polymers furnished excellent potential for their applications in various

fields such as in the manufacturing of cheap electronic devices [3-5], television and

other audio-video equipments [6, 7].

Polyaniline (Pani) is the most studied polymer among the different types of

conducting polymers because of its easy synthesis, low cost of monomer, exclusive

chemical properties and excellent environmental stability [8-10]. Moreover, it exists

in three oxidation states leucoemeraldine, pernigraniline and emeraldine. Among

these, emeraldine salt form has highest conductivity [11] in the order of 1 Scm

-1 and

can be used in various applications due to reversible doping (conductive) and

dedoping (non-conductive) mechanism such as in chemical and biological sensors

[12-15] and actuators [16]

etc.

Hexagonal boron nitride (h-BN) is a structural analogue of graphite having

good thermal transport properties [17, 18]. Among all other BNs, h-BN is the most

preferred polymorph in which boron and nitrogen atoms form planar conjugated

layers [19]. BNs have various unique properties such as hardness, thermal stability,

high melting point, good corrosion resistance, good dielectric breakdown strength,

resistance to oxidation and chemical inertness.

There are many applications e.g. optical storage, medical treatment,

photocatalytic reactions and electrical insulation [20, 21] in which BNs used. Wang et.

al. [22] Reported that h-BN/SnO2 material showed good photocatalytic activity was

mainly due to its suitable band gap energy, strong adsorption ability for methyl orange

and effective charge separation at the h-BN/SnO2 photocatalyst interface. Sadia et. al.

[23] synthesized the polyaniline/graphene nanocomposite that delivered a significant

degradation of RB (rose Bengal) dye by ~56% within 3 h under UV-VIS light. They

observed that the heterojunctions are formed in the space charge separation region at

the polyaniline/graphene interfaces and suggested that graphene may accept the

Chapter 2

28

photoexcited electrons from polyaniline. Similarly to graphene, h-BN may be a

promising material of photocatalytic nanocomposite with polyaniline.

To the best of our knowledge, the photocatalytic degradation of RhB and MB

dyes over the surface of Pani/BN nanocomposite as photocatalyst has not been

reported elsewhere. Herein, we have tried to look forward our effort for enhancement

of photocatalytic activity of polyaniline by using boron nitride under UV- light. The

h-BN nanosheets have large surface area which may have significantly increased the

adsorption of dyes and photo-induced charge transfer along the h-BN sheets in

nanocomposite.

A conducting nanocomposite of Pani with h-BN (Pani/BN) was prepared via

In situ polymerization method. The photocatalytic degradation of MB and RhB dyes

have been studied over the surface of the prepared Pani/BN nanocomposite which

efficiently degraded both of the dyes by 65.7 % (MB) and 71.6 % (RhB). The

structure and surface morphology were investigated. The thermal stability was also

investigated in terms of DC electrical conductivity retention under isothermal and

cyclic accelerated aging conditions.

2.2. EXPERIMENTAL

2.2.1. Materials

In the preparation of Pani and Pani/BN nanocomposite the main chemicals used were:

aniline, 99% (E.Merck, India), hydrochloric acid, 35% (E. Merck, India), h-BN (MK

Nano, Canada), potassium persulphate (E.Merck, India) and methanol. Double

distilled water was used throughout the experiments.

2.2.2. Preparation of Pani and Pani/BN Nanocomposite

The nanocomposite of Pani/BN was prepared by in situ oxidative polymerization of

aniline in the presence of h-BN using potassium persulphate as an oxidizing agent. At

first 10 wt% h-BN was ultrasonicated for 3 hrs in 100 mL of 1M HCl then it was

added to aniline solution dropwise under constant stirring for 1 h to enable the proper

dispersion of h-BN in aniline. Oxidant solution was then added dropwise in the above

dispersed solution of h-BN and aniline to polymerize the aniline. As the oxidant, was

added the colour of the reaction mixture starts changing from light purple to dark

Chapter 2

29

green within 20 mins of stirring. The stirring was kept continuous for 20 h. The final

dark green coloured reaction mixture was then filtered, washed with double distilled

water and methanol to remove excess acid, potassium persulphate and Pani oligomers

until filtrate became colourless and neutral.

Thus prepared Pani/BN nanocomposite was dried at 80oC for 12 h in an air

oven, crushed into fine powder and was stored in desiccator for further investigations.

Pani was also prepared using the same method as described above in absence of h-

BN. The as prepared materials were designated as Pani and Pani/BN.

2.2.3. Photodegradation Experiment

The photocatalytic oxidation of Rhodamine B (RhB) and Methylene Blue (MB)

solution was done under UV-light illumination to evaluate the photocatalytic activity

of the Pani and Pani/h-BN nanocomposite under constant stirring and bubbling of

atmospheric oxygen. An immersion well photochemical reactor made of Pyrex glass

with a 250 mL working volume of dyes equipped with magnetic stirring bar and an

opening for supply of molecular oxygen was used for all the experiments. A 125-W

medium pressure mercury lamp was used as the UV-light source.

During all the experiments, the temperature of aqueous solutions of dyes was

maintain ~ 20 ± 0.5oC by refrigerated water circulation to prevent the heating of the

solutions by radiations emitted by the UV-lamp (IR and short wave length). The

radiation intensity falling on the solutions was measured using UV light intensity

detector (Lutron UV-340) and found to be 3.22 mW/cm2. Typically, 250 mg Pani and

Pani/h-BN nanocomposite was added into 250 ml aqueous solution of RhB (8 ppm)

and MB (10 ppm) respectively.

The solution was continuously stirred in the dark for 30 min to ensure the

establishment of adsorption−desorption equilibrium between the photocatalyst and the

dyes prior to irradiation [24]. Then the solution was exposed to UV light. During

irradiation, sample was withdrawn from reactor using a syringe at regular intervals

and centrifuged before the measurement to remove the powder photocatalyst. The

decolourization of dye was measured by using UV-Vis spectrophotometer (Perkin

Elmer ) at the max = 553 and 663 respectively based on Beer Lambert Law

[25]. The degradation efficiency of dyes was calculated by using equation (2.1).

Chapter 2

30

Degradation % = A0 AtA0 X 100% (2.1)

Where A0 and At are the concentrations of sample at time‘0’ and time‘t’

2.3. CHARACTERIZATIONS

The Fourier transform infrared spectroscopy (FTIR) spectra were recorded using

Perkin-Elmer 1725 instrument. X-ray diffraction (XRD) pattern were recorded by

Bruker D8 diffractometer with Cu Kα radiation at 1.540 A˚. Scanning electron

microscope (SEM) studies were carried out by JEOL, JSM, 6510-LV (Japan).

Transmission electron microscope (TEM) studies were carried out by using JEM

2100, JEOL (Japan). Thermogravimetric analysis (TGA) was done by using a Perkin

Elmer instrument in the temperature range from 35 to 800oC.

The thermal stability of Pani and Pani/BN nanocomposite under isothermal

and cyclic ageing conditions was studied in terms of DC electrical conductivity

retention. For this study a four-in-line probe with a temperature controller, PID-200

(Scientific Equipments, Roorkee, India) was used to measure the DC electrical

conductivity and its temperature dependence. The equation used in calculation of DC

electrical conductivity was

σ = [ln2(2S/W)]/[2πS(V/I)] (2.2)

where I, V, W and S are the current (A), voltage (V), thickness of the pellet (cm) and

probe spacing (cm) respectively and σ is the conductivity (S/cm) [2]. In testing of

isothermal stability, the pellets were heated at 50oC, 70

oC, 90

oC, 110

oC and 130

oC in

an air oven and the DC electrical conductivity was measured at particular temperature

at an interval of 10 min in the accelerated ageing experiments. In testing of the

stability under cyclic ageing condition, DC conductivity measurements were taken 5

times at an interval of about 90 min within the temperature range of 50-150oC.

2.4. RESULT AND DISCUSION

2.4.1. Mechanism of Preparation of Pani and Pani/BN Nanocomposite:

The mechanistic view of the polymerization process seems to involve the anilinium

cations (phenyl-NH3+) getting hooked up by Coulombic attraction between anilinium

cations and the surface of h-BN. Thus the h-BN gets completely surrounded by

Chapter 2

31

anilinium cations. This arrangement comes in contact with K2S2O8, the anilinium

cations get polymerized on the surface of H-BN forming Pani (emeraldine salt). h-BN

has partially polar bonds [26] which may cause interaction with polarons and lone

pairs of Pani, as mentioned in our previous work with polypyrrole/BN nanocomposite

[27]. The Pani (emeraldine salt) formed on the surface of h-BN get attached by

Coulombic attraction between the positive charge on nitrogen of Pani and negative

charge on boron of h-BN and also the interaction between lone pair of nitrogen of

Pani (emeraldine salt) and positive charge on nitrogen of h-BN. Schematic

presentation of coulombic attraction between the +ve and –ve charges is given in Fig.

2.1.

Fig. 2.1 Schematic presentation of Coulombic attraction between Pani and h-BN in

Pani/BN nanocomposite.

2.4.2. FTIR Spectroscopic Study

FTIR spectra of the Pani/BN nanocomposite and Pani are shown in Fig. 2.2. The

characteristic peak of Pani at 3434 cm-1

corresponds to N-H stretching vibration. The

peak at 1571 cm-1

and 1489 cm-1

is due to C=C stretching mode of the quinoid and

benzenoid rings respectively and the peak at about 1287 cm-1

can be assigned to C-N

stretching. The peak at 790 cm-1

is usually assigned to an out-of-plane bending

vibration of C-H which confirmed the formation of Pani [28].

In the Pani/BN nanocomposite, there are the two more peaks situated at 1375

cm-1

and 792 cm-1

respectively. The absorption band at 1375 cm-1

can be attributed to

the in-plane B-N stretching, and the absorption band at 792 cm-1

belongs to the B–N–

B out-of-plane bending vibration which is assumed to overlap with C-H out of plane

bending vibration.29

While the remaining peaks are similar to that of Pani but slightly

Chapter 2

32

shifted to higher wavenumber with reduced intensity indicative of strong interaction

between Pani and h-BN.

Fig. 2.2 FTIR spectra of: (a) Pani/BN nanocomposite and (b) Pani.

2.4.3. X-rays Diffraction (XRD) Studies

The XRD patterns of Pani and Pani/BN nanocomposite are shown in Fig. 2.3. Fig.

2.3b shows the broad peak at 2θ = 25.29o may be attributed to Pani. In the Fig. 2.3a

shows XRD pattern of Pani/H-BN nanocomposite, a sharp peak observed at 2θ =

25.78o which suggested that the interaction between h-BN and Pani.

In Pani/BN nanocomposite, highly crystalline nature of h-BN and amorphous

nature of polyaniline has been merged and shifted from 25.29°. The other peaks

around at 2θ = 40.23o, 42.83

o and 49.25

o in the pattern of Pani/BN may be attributed

to h-BN [30].

Chapter 2

33

Fig. 2.3 XRD patterns of: (a) Pani/BN nanocomposite and (b) Pani.

2.4.6. Thermogravimetric Analysis (TGA)

Fig. 2.4 represents the TGA thermograms of Pani and Pani/BN nanocomposite, which

shows the three step weight loss process. In the case of Pani, there are three major

stages of weight loss; first around 120oC, second in range from 130

oC to 400

oC and

third in range from 400oC to 550

oC due to the loss of water, removal lower oligomers

of Pani and thermo-oxidative decomposition of Pani respectively [31]. It is found that

the degradation of nanocomposite is quite similar to that of Pani.

The observable difference is the higher thermal stability of Pani/BN

nanocomposite. The decomposition of Pani/BN started at around 492oC which is at

much higher temperature than in Pani (~475oC). This enhanced stability may be due

to high thermal stability of h-BN that has strong Coulombic attraction with Pani.

20 30 40 50 60 70 80

(b)

(a)

Inte

nsi

ty (

a.u

)

2Theta

(a) Pani/BN

(b) Pani

Chapter 2

34

Fig. 2.4 TGA thermograms of: (a) Pani and (b) Pani/BN nanocomposite.

2.4.4. Scanning Electron Micrograph (SEM) Studies

The SEM of Pani and Pani/BN nanocomposite are shown in Fig. 2.5 at different

magnifications. The SEM micrograph in Fig. 2.5a shows the sheet like structure of h-

BN. In Fig. 2.5b SEM image of Pani shows short tubes along with flakes like

structure. The SEM images in Fig. 2.5c & 2.5d represent the Pani/BN, in which the

polymer matrix is well enwrapped on h-BN with uniform dispersion and some sheet

like morphology may also be seen. This suggests that h-BN acted as sheet on which

polymerization took place and facilitated in the formation of some sheet like

structures.

Chapter 2

35

Fig. 2.5 SEM micrographs of: (a) h-BN, (b) Pani and (c & d) Pani/BN nanocomposite

at different magnifications.

2.4.5. Transmission Electron Micrograph (TEM) Studies

TEM micrograph of Pani/BN nanocomposite is shown in Fig. 2.6. From the figure it

is observed that the light grey sheet type structure seem to boron nitride and dark grey

seem to polyaniline which is enwrapped on the boron nitride nanosheets. Thus it may

be said that aniline underwent polymerization on the surface of h-BN giving the sheet

type structure.

Fig. 2.6 TEM micrograph of Pani/BN nanocomposite.

Chapter 2

36

2.5. PHOTOCATALYTIC STUDIES

The photocatalytic activity of Pani and Pani/BN nanocomposite was investigated by

the decolourization of RhB & MB in aqueous solution which is discarded by textile

industries under UV-light illumination. The controlled experiment indicates that RhB

and MB were resistant towards the degradation under UV-light irradiation without a

photocatalyst. However, little decomposition of dyes takes place in the presence of

prepared photocatalyst in dark due to adsorption of dyes on the surface of catalysts.

The RhB showed good adsorption on the surface of synthesized photocatalyst.

The result indicates that both light and catalyst were required for efficient

photocatalytic degradation. The decomposition of the dyes was monitored by

measuring the change in the absorbance at their (max 663 and 553) as a function of

irradiation time. Fig. 2.7 (a and b) showed that 50% and 56.4% decolourization of

MB and RhB dyes take place respectively, after 90 min of irradiation time over Pani.

Fig. 2.8 (a and b) indicates the 65.7% and 71.1% degradation of MB and RhB

respectively, after 90 min as a function of irradiation time in the presence of Pani/BN

nanocomposite material.

The results of Fig. 2.7 and Fig. 2.8 illustrate that the main peaks of both the

dyes (553 nm and 663 nm) decreases gradually as irradiation time increase. The

colour of the dyes solution became lighter as the irradiation time increased due to

gradually degradation of chromophoric groups present in the dyes [32, 33]. The Fig.

2.7 & Fig. 2.8 displayed the decolourization of MB was found to be lower than RhB

due to presence of stable and bulky aromatic rings which suppress the interaction

between catalysts and dye [34]. The adsorption of MB was lower than RhB on the

surface of catalyst which decrease photocatalytic degradation of MB. Fig. 2.9 showed

the percentage degradation of dyes under UV-light illumination over Pani and

Pani/BN nanocomposite. It was found that the degradation of RhB was more than that

of MB by hydroxyl radicals. It can be attributed due to absorption of less UV-light by

RhB than MB [35].

Hence more photons were available to photocatalyst which raised the

formation of hydroxyl radicals. The degradation of both dyes was found to be higher

over Pani/BN nanocomposite material than Pani. The Pani interacted with h-BN

nanosheets that substantially increased the surface area of the nanocomposite,

Chapter 2

37

therefore more degradation of both dyes over the surface of Pani/BN nanocomposite

than Pani. It may be also due to the electron transfer from excited polyaniline to h-BN

and further across nanocomposite interface which leads to formation of trapping sites

by h-BN which increase the charge separation by splitting the arrival time of

photogenerated electron and hole to reach the surface of photocatalyst and thus

decrease the electron-hole recombination rate [36-38].

The photocatalytic activity of Pani/BN nanocomposite was also investigated

for waste water treatment by taking sewage from department of chemistry, Aligarh

Muslim University, India. Fig. 2.10 indicates 53 % degradation of waste water after

90 min as a function of irradiation time in the presence of Pani/BN nanocomposite.

Thus Pani/BN nanocomposite found to be a good photocatalyst also for waste water

treatment.

Fig. 2.7 (a and b). Photocatalytic degradation of MB and RhB dyes respectively at

different time intervals in the presence of Pani.

Chapter 2

38

Fig. 2.8 (a and b) Photocatalytic degradation of MB and RhB dyes respectively at

different time intervals in the presence of Pani/BN nanocomposite.

Fig. 2.9 (a) Percentage degradation of MB & RhB dyes in aqueous solution as a

function of time in the presence and absence of Pani and presence and absence of UV-

light, (b) Percentage decomposition of both dyes in aqueous solution as a function of

time in the presence and absence of Pani/BN and presence and absence of UV-light.

Fig. 2.10 Photocatalytic treatment of waste water at different time intervals in the

presence of Pani/BN nanocomposite.

Chapter 2

39

2.6. POSSIBLE MECHANISM

The possible mechanism for the degradation of dye using Pani/BN nanocomposite can

schematically be represented in Fig. 2.11. Pani is a typical semiconducting polymer

with an extended π-electron conjugation system. Pani serves as a good photocatalyst

for degradation of pollutants under UV light irradiation due to its electronic structure

characterized by a filled valence band (HOMO) and an empty conduction band

(LUMO). On absorption of photons that match or exceed the band gap energy of Pani,

an electron may be promoted from the valence band to the conduction band leaving

behind an electron vacancy or “hole” in the valence band [39]. In Pani/BN

nanocomposite the charge separation is maintained by transferring of electrons from

Pani to h-BN through nanocomposite interface which decreased the electrons and

holes recombination.

Thus electrons and holes can migrate to the catalyst surface where they

participate in redox reactions with adsorbed dyes. Specially, the holes generated in the

valence band (h+

VB) can react with surface bound H2O molecules to produce hydroxyl

radicals and the electrons present in the conduction band (e−

CB) are picked up by

oxygen to generate superoxide radical anions [25, 39].

Pani/BN e

CB + h+

VB (2.3)

O2 e

CB O2

(2.4)

H2O + h+

VB H+ +

•OH (2.5)

O2 + 2(e

CB) + 2H+

H2O2 (2.6)

e

CB + O2•−

+ 2H+

•OH +

−OH (2.7)

The superoxide radical anions act as strong reducing agent and hydroxyl radical act as

strong oxidizing agent and degrade the pollutant dyes to the mineral end products.

h+

VB + Dye Degraded products + H2O + CO2 (2.8)

•OH + Dye Degraded products + H2O + CO2 (2.9)

(UV- radiations)

Chapter 2

40

Fig. 2.11 Schematic presentation of probable degradation process of MB and RhB

over Pani/BN nanocomposite.

2.7. ELECTRICAL CONDUCTIVITY

The electrical conductivities of Pani and Pani/BN nanocomposite were measured by

standard four-in-line probe method. From the measured electrical conductivity it may

be inferred that both the as prepared materials are semiconducting in nature and the

addition of h-BN to the Pani has a significant effect on its electrical conductivity. The

measured electrical conductivities of Pani and Pani/BN nanocomposite were 0.0622

S/cm and 0.045 S/cm respectively as shown in Fig. 2.12.

Thus it is observed that the electrical conductivity decreased after loading of

boron nitride. The boron nitride, having insulating behaviour may induce the

formation of less efficient network for charge transport in the polyaniline chains

leading to the less electrical conductivity than Pani. The reason for decrease in

electrical conductivity of Pani/BN nanocomposite may also be due to interaction of

boron (-ve charge) and nitrogen (+ve charge) atoms of h-BN with polarons/bipolarons

of polyaniline as presented in Fig. 2.1. This causes the loss of mobility of charge

carriers leading to reduced electrical conductivity in Pani/BN nanocomposite.

Chapter 2

41

Fig. 2.12 Initial DC electrical conductivity of (a) Pani and (b) Pani/BN.

2.7.1. Stability under Isothermal Ageing Conditions

The stability of Pani and Pani/BN nanocomposite in terms of DC electrical

conductivity retention was studied under isothermal ageing conditions as shown in

Fig. 2.13. The representation of relative electrical conductivity was calculated by the

equation:

(2.10)

where σ is the change in relative electrical conductivity/minute, σ f is the final relative

electrical conductivity at temperature T, σi is the initial relative electrical conductivity

at temperature T and t is the duration of the experiment (40 min). The electrical

conductivity was measured for each temperature (50, 70, 90, 110, 130oC) versus time.

From the Fig. 2.13a, it can be understood that the stability of Pani is very fair at 50,

70 and 90oC while the electrical conductivity becomes unstable at 110 and 130

oC. The

Pani/BN nanocomposite is quite stable at 50oC and also significantly stable at 70 and

90oC as shown in Fig. 2.13b. The nanocomposite shows more instability as compared

to Pani at 110 and 130oC.

Chapter 2

42

Fig. 2.13 DC electrical conductivity retention under isothermal conditions at 50, 70,

90, 110 and 130°C of (a) Pani and (b) Pani/BN.

2.7.2. Stability under Cyclic Ageing Conditions

The stability of Pani and Pani/BN nanocomposite in terms of DC electrical

conductivity was also studied by cyclic ageing technique. From the Fig. 2.14, it may

be observed that the initial DC electrical conductivity at the start of each cycle

decreases with increase in cycle number for both Pani and Pani/BN nanocomposite.

The difference in electrical conductivity of Pani and Pani/BN nanocomposite from

cycle 1 to cycle 5 was observed 0.0221 S/cm and 0.0158 S/cm respectively. Therefore

Pani/BN nanocomposite is shown more stable than Pani.

Fig. 2.14 DC electrical conductivity at the start of each cycle of Pani and Pani/BN

nanocomposite under cyclic ageing conditions.

Chapter 2

43

The stability in the term DC electrical conductivity retention of Pani and Pani/BN

nanocomposite was studied by cyclic ageing technique within the temperature range

of 50 to 150 oC as shown in Fig. 2.15. As the number of cycle increases, the DC

electrical conductivity gain for Pani decreases but the change in electrical

conductivity becomes less from cycle 3rd to 5th. In case of Pani/BN nanocomposite,

it is observed that there is only gain in the electrical conductivity from 1st cycle to 5th

cycle. After 2nd

cycle, the gain in electrical conductivity of Pani/BN nanocomposite

seems stable.

From this observations it may attributed that the Pani/BN has more stable

electrical conductivity under cyclic ageing condition. The different pattern of

conductivities in different cycles for Pani and Pani/BN nanocomposite may be

attributed to the removal of moisture, excess of HCl or low molecular weight

oligomers of aniline [31].

Fig. 2.15 DC electrical conductivity retention under cyclic ageing conditions of Pani

and Pani/BN nanocomposite.

Chapter 2

44

CONCLUSIONS

Pani and Pani/BN nanocomposite were synthesized by in-situ polymerization method

and characterized by different instrumental techniques. The DC electrical conductivity

retention under isothermal and cyclic ageing conditions has also been presented. It

was observed that Pani showed greater electrical conductivity as well as isothermal

stability than that of Pani/BN nanocomposite but in terms of cyclic stability Pani/BN

showed good stability than that of Pani. The as prepared materials possessed the

excellent photodecomposition of model dyes under UV-light irradiation. It was

observed that photogenetrated hydroxyl radical and hole (h+) and superoxide ions

were the main active species responsible for degradation of both the dyes. The results

highlighted that the extent of degradation of MB and RhB was greater over Pani/BN

nanocomposite than Pani. This indicates that the as prepared Pani/BN nanocomposite

may be used as a photocatalyst even at high thermal conditions (less than or equal to

90oC) for waste water treatment.

Chapter 2

45

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Chapter 3

48

3.1. INTRODUCTION

Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa discovered that a

polymer can be made conductive almost like a metal [1]. This discovery generated

humongous interest in conducting polymers. Out of the different types of conducting

polymers explored, polyaniline (Pani) draws more attention because it possesses

excellent properties such as low cost, great environmental stability, reversible and

tailorable electrical properties by controlled charge-transfer processes [2-4]. This

makes polyaniline a versatile material for potential applications as electrodes in

primary and secondary batteries [5,6], microelectronics [7], photocatalyst [8], sensors

and actuators etc [9].

Silk fibroin (SF) is a natural polymer produced by a variety of insects and

spiders. Bombyx mori silk consists of two main proteins, sericin (glue-like coating of

a nonfilamentous protein) and fibroin (a filament core protein). The natural silk

fibroin (SF) fibres obtained by removing the outer sericin from silk fibres with

anhydrous sodium carbonate solution at appropriate temperature [10].

The primary structure of Bombyx mori SF protein is identified by the presence

of three amino acids glycine, alanine and sericin with dominated sequence as [Gly-

Ser-Gly-Ala-Gly-Ala]n. The silk fibroin from the silkworm has widely been used in

textile production, clinical sutures, as a scaffold for tissue regeneration and various

biomedical applications because of its mechanical properties, biocompatibility and

biodegradability [11]. The SF can also be used in photocatalytic activity by making its

composite with inorganic or organic semiconductors. Janjira et al reported the TiO2

coated SF (silk fibroin) filter for the photocatalytic degradation of formaldehyde gas

by the highest removal efficiency (54.72 ± 1.75%) at the initial formaldehyde

concentration ~5.00 ± 0.50ppm [12].

Herein, we have tried to explore our effort for enhancement of photocatalytic

activity of polyaniline by using silk fibroin under UV- light. In SF, it may be assumed

that the amide bonds get partially polarized by the conjugation presents in peptide

linkage [13] which may cause electrostatic interaction with polyaniline (emeraldine

salt) and hence significantly increased the photo-induced charge transfer to SF

resulting in increased photogenerated charge separation in Pani/SF composite than

that of Pani.

https://en.wikipedia.org/wiki/Sericin

Chapter 3

49

A conducting composite of Pani with SF (Pani/SF) was prepared via in-situ

polymerization method. The photocatalytic degradation of MB and RhB dyes have

been studied over the surface of the prepared Pani/SF composite which efficiently

degraded both the dyes by 85.4 % (MB) and 91.4 % (RhB). The structure and surface

morphology were investigated. The thermal stability was also investigated in terms of

DC electrical conductivity retention under isothermal and cyclic accelerated aging

conditions.

3.2. EXPERIMENTAL

3.2.1. Materials

In the preparation of Pani and Pani/SF composite, the chemicals used were: aniline,

99% (E.Merck, India), hydrochloric acid, 35% (E. Merck, India), Cocoons for Silk

(Banaras, India), potassium persulphate (E.Merck, India) and methanol. Double


3.2.2. Preparation of Silk Fibroin

Cocoons obtained from Banaras (India), were degummed twice for 1 hr in aqueous

solution 0.02 M Na2CO3 and then rinsed thoroughly with distilled water to extract the

Silk fibroin (SF). The extracted SF was then dried at 40°C for 24 h.

3.2.3. Preparation of Pani and Pani/SF Composite

The Pani/SF composite was prepared by in situ oxidative polymerization of aniline in

the presence of SF using potassium persulphate as an oxidizing agent. Firstly, the

extracted silk fibroin (200 mg) was dissolved in hydrochloric acid (11.6 M) to prepare

its solution. Then, it was added to aniline solution under constant stirring for 1 h to

enable the proper dispersion of SF in aniline. K2S2O8 solution was then added

dropwise in the dispersed solution of SF and aniline to polymerize the aniline. The

colour of the reaction mixture started changing from light yellow to dark green within

15 min of adding oxidant solution under constant stirring. The stirring was kept

continuous for 20 h. The final dark green coloured reaction mixture was then filtered,

washed with double distilled water and methanol to remove excess acid, potassium

persulphate and Pani oligomers until filtrate became colourless and neutral.

Chapter 3

50

Thus prepared Pani/SF composite was dried at 60oC for 12 h in an air oven,

crushed into fine powder and was stored in desiccators for further investigations. Pani

was also prepared using the same method as described above.

3.2.4. Photodegradation Experiment

The photocatalytic activity experiment was performed in an immersion well

photochemical reactor made of Pyrex glass using 125-W medium pressure mercury

lamp as UV light source.

The photocatalytic performance of the as prepared materials was evaluated by

the degradation of RhB & MB in aqueous solution under UV light illumination. In

order to maintain the temperature (20 ± 0.5 °C) of dye solution, the photoreactor

surrounded by refrigerated water circulation. Light intensity falling on the solution

was measured using UV light intensity detector (Lutron UV-340) and found to be

2.56mW/cm2. The radiation emitted by the UV-lamp (IR & short wave length) was

eliminated by a water circulating jacket. In the photocatalytic experiment 250 mg of

prepared composite was added into the 250 ml of RhB and MB with a concentration

of (10 mg/L) for RhB (10 mg/L) for MB respectively.

Prior to illumination, the aqueous suspension was magnetically stirred in the

dark for 30 min to obtain the saturated adsorption of dyes onto the surface of catalyst

[14]. During irradiation at regular time intervals, the suspension was collected and

then centrifuged (4000 rpm, 20 min) to remove the powder material. The

concentrations of the dyes were monitored by using UV-Vis spectrophotometer

max = 553 and 663 respectively based on Beer

Lambert Law [15]. The degradation efficiency of dyes was calculated by using

equation-3.1.

Degradation % = (3.1)

where Ao is initial concentration and At is concentration at particular time (t)

3.3. CHARACTERIZATION

In order to investigate the morphology, structure and chemical composition of Pani

and Pani/SF composite, a variety of techniques were used including the Fourier

transform infrared spectroscopy (FTIR) spectra were recorded using Perkin-Elmer

Chapter 3

51

1725 instrument. X-ray diffraction (XRD) pattern were recorded by Bruker D8

diffractometer with Cu Kα radiation at 1.540 A˚. Scanning electron microscope

(SEM) studies were carried out by JEOL, JSM, 6510-LV (Japan).

The thermal stability in terms of DC electrical conductivity of Pani and

Pani/SF composite under isothermal and cyclic ageing conditions was studied. For

this study, a four-in-line probe with a temperature controller, PID-200 (Scientific

Equipments, Roorkee, India) was used to measure the DC electrical conductivity and

its temperature dependence. The equation used in calculation of DC electrical

conductivity was

σ = [ln2(2S/W)]/[2πS(V/I)] (3.2)




oC, 90

oC, 110

oC and 150

oC in




times at an interval of about 90 min within the temperature range of 50-150oC.

3.4. RESULT AND DISCUSSION

3.4.1. Preparation of Pani/SF Composite

In the mechanistic view of the polymerization process, the aniline molecules in HCl

form anilinium cations (phenyl-NH3+) and get attached with the SF. The attachment

may be due to Coulombic attraction between anilinium cations and the partial

negative charge on the oxygen of partially polarized amide group of SF.

Thus it is expected that the SF gets completely surrounded by anilinium

cations. When this arrangement comes in contact with K2S2O8, the anilinium cations

get polymerized on the surface of SF forming Pani (emeraldine salt). The polyaniline

(emeraldine salt) formed on the surface of SF get hooked by Coulombic attraction

between the positive charge on nitrogen of Pani and partial negative charge on oxygen

and also the interaction between lone pair of nitrogen of Pani (emeraldine salt) and

positive charge on nitrogen of partially polarized amide groups of silk fibroin.

Chapter 3

52

Schematic presentation of Coulombic attraction between the +ve and –ve charges is

given in Fig. 3.1.

Fig. 3.1 Schematic representation of Coulombic attraction between Pani and peptide

bonds of SF in Pani/SF composite.

3.4.2. FTIR Analysis

FTIR spectra of the Pani/SF composite and Pani are shown in Fig. 3.2. In the spectra

of Pani the five major peaks were observed. The peak observed at 3438 cm−1

corresponds to N-H stretching vibration. The peak at 1572 cm−1

is due to C=C

stretching mode of the quininoid rings and 1487 cm−1

is due to C=C stretching mode

of benzenoid rings. The peak at about 1294 cm−1

can be attributed to C-N stretching.

The peak at around 798 cm−1

may assigned to an out-of-plane bending vibration of C-

H which confirmed the formation of Pani [17].

In the Pani/SF composite, the peaks also observed at 1636 cm

−1and 1532

cm−1

which correspond to amide I and amide II vibrations and attributed to the β sheet

structure [18]. The remaining peaks in the spectra of Pani/SF composite are similar

but slightly shifted than in Pani due to interaction of peptide bond in silk fibroin with

Pani.

Chapter 3

53

Fig. 3.2 FTIR spectra of: (a) Pani and (b) Pani/SF.


The XRD patterns of Pani and Pani/SF composite are shown in Fig. 3.3. Fig. 3.3a

shows the two peaks at 2θ = 20.16o and 25.31

o which are attributed to HCl doped Pani

[19]. In Fig. 3.3b it is observed that the intensity of peaks increased due to semi

crystalline nature of silk fibroin [20], also the peak of silk fibroin (2θ = 20o) [21] has

been merged and shifted to 2θ = 20.40o in Pani/SF composite. The peak observed at

2θ = 25.01o is due to existence of Pani in Pani/SF composite but slightly shifted which

suggest the formation of Pani/SF composite.

0 500 1000 1500 2000 2500 3000 3500 4000 4500

(b)

(a)

Wavenumber [cm-1]

Tra

nsm

itta

nce [

%]

(a) Pani

(b) Pani/SF

Chapter 3

54

Fig. 3.3 XRD patterns of: (a) Pani and (b) Pani/SF.


The thermal stability of as-prepared Pani and Pani/SF composite was investigated and

shown in Fig. 3. 4. The weight loss occurred in three step process, in case of Pani, the

initial weight loss at around 98oC may correspond to removal of adsorbed water in

Pani. Later, the weight loss is observed at about 189oC, which is due to removal of

lower oligomers of Pani and third in the range from 450oC to 600

oC due to thermo-

oxidative decomposition of Pani [22].

Silk fibroin is remarkably stable under thermal conditions upto 250oC [23], so

in case of Pani/SF the second step degradation occurs at around 206oC which is higher

than that of Pani (~189oC). The thermal stability of silk fibroin (upto ~250

oC) is

making difference in the composite and causing Pani/SF to degrade comparatively at

higher temperature.

20 30 40 50 60 70 80 90

0

50

100

150

200

250

Inte

nsi

ty (

a.u

.)

(a) Pani

(b) Pani/SF

Chapter 3

55

Fig. 3.4 TGA thermograms of: (a) Pani and (b) Pani/SF.

3.4.5. Scanning Electron Micrographic (SEM) Studies

The FESEM of Pani and Pani/SF composite are shown in Fig. 3.5 at different

magnifications. Fig. 3.5a and 3.5c shows the FESEM images of Pani, having uniform

interconnected short tubes along with flakes like structure. The SEM images in Fig.

3.5b and 3.5d represent the Pani/SF composite which revealed similar morphology

but with slightly more agglomeration compared to Pani that forming lumps. The silk

fibroin in aqueous salt systems mainly exists in the forms of lumps and random

globules [24]. So the formation of lump in Pani/SF composite may be attributed to the

presence of silk fibroin.

100 200 300 400 500 600 700 800

40

50

60

70

80

90

100

(b)

(a)

Wei

ght

(%)

Temperature (0C)

(a) (Pani)

(b) (Pani/SF)

Chapter 3

56

Fig. 3.5 SEM micrographs of: (a and c) Pani, (b and d) Pani/SF composite at

different magnifications.

3.5. PHOTOCATALYTIC STUDIES

The as-prepared samples were tested for their photocatalytic activity by choosing two

different chromophoric model dyes, the common contaminants in waste water, under

UV light illumination. On the basis of a blank experiment, the self-photodegradtion of

both the dyes could be neglected. Fig. 3.6a and Fig. 3.6b displayed the 50% & 56.4%

photodegradation efficiency of MB & RhB respectively in the presence of Pani under

UV light illumination after 90 min.

Chapter 3

57


different time intervals in the presence of Pani under UV light.

Fig. 3.7a and Fig. 3.7b indicates the 85.4% & 91.4% photodegradation of MB &

RhB respectively in the presence of Pani/SF composite after 90 min under UV light

irradiation. The results revealed that the degradation of the RhB was found to higher

than MB. This may be due to the higher adsorption of RhB than that of MB on the

surface of catalyst which increased the photocatalytic degradation of RhB.


different time intervals in the presence of Pani/SF composite under UV light.

Fig. 3.8 showed the percentage degradation of both the dyes over Pani and Pani/SF

under UV-light illumination. The interaction between RhB and prepared photocatalyst

was found to be excellent. Therefore RhB showed good degradation over Pani and

Chapter 3

58

Pani/SF [25, 26]. This selective adsorption of both the dyes on the surface of Pani and

Pani/SF promises a green method for the degradation of both dyes from waste water.

The absorbed dyes then degraded by active species.

On the basis of above experimental data, the enhanced degradation of both

dyes in the presence of Pani/SF composite was mainly ascribed to the high charge

separation efficiency of photoinduced electron hole pair. This may be due to the

electron-sink function of silk fibroin which inhibits the recombination rate of electron

and hole leading to fast degradation of dyes compared with Pani [27, 28].

Fig. 3.8 Percentage degradation of MB & RhB dyes in aqueous solution as a function

of time in the presence Pani and Pani/SF composite under UV light.

The possible mechanism for the degradation of dye using Pani and Pani/SF can be

represented as given in Fig. 3.9. Pani is a typical conducting polymer with extended

π-electron conjugation system. Pani serve as a good photocatalyst for degradation of

pollutants under UV light irradiation due to its electronic structure characterized by a

filled valence band (HOMO) and an empty conduction band (LUMO). On absorption

of photons of higher energy than the band gap energy of Pani, electrons may be

promoted from the valence band to the conduction band leaving behind electron

vacancies or “holes” in the valence band [29]. The charge separation is maintained in

Pani/SF composite through interface, attached by Coulombic attraction. The electron

and hole may migrate to the catalyst surface where they participate in redox reactions

Chapter 3

59

with adsorbed dyes on the surface. Specially, the holes generated in the valence band

(h+

VB) may react with surface bound H2O to produce the hydroxyl radical and electron

present at conduction band (e¯CB) is picked up by oxygen to generate superoxide

radical anion [30,31].

Pani/SF + UV-rays e¯

CB + h+

VB (3.3)

O2 + e¯CB O2• (3.4)

H2O + h+

VB H+ +

•OH (3.5)

O2 + 2(e¯CB) + 2H

+ H2O2 (3.6)

e¯CB + H2O2 2

•OH (3.7)

The superoxide radical anion act as strong reducing agent and hydroxyl radical act as

strong oxidizing agent and degrade the pollutant dyes to the end products.

O2●−

+ Dye Degraded product + (H2O + CO2) (3.8)

•OH + Dye Degraded product + (H2O + CO2) (3.9)

Fig. 3.9 Probable degradation process of MB and RhB over Pani/SF composite.

Chapter 3

60


The electrical conductivities of Pani and Pani/SF composite were measured by

standard four-in-line probe method. From the measured electrical conductivity, it may

be observed that both the as-prepared materials are semiconducting in nature and the

electrical conductivity of Pani decreased slightly after loading of silk fibroin. The

measured electrical conductivities of Pani and Pani/SF composite were 0.087 S/cm

and 0.072 S/cm respectively as shown in Fig. 3.10.

In silk fibroin due to conjugation present in amide groups thus the partial

charge developed on the oxygen and nitrogen that partially polarized the amide

groups [13]. he partially polar amide groups in silk fibroin bind the polarons of the

ani and the counterion l . This causes the reduction of mobility in polarons leading

to decline of electrical conductivity in Pani/SF composite. Also the silk fibroin,

having insulating behaviour may induce the formation of less efficient network for

charge transport in the polyaniline chains leading to the slightly lower electrical

conductivity than Pani.

Fig. 3.10 Initial DC electrical conductivity of: (a) Pani and (b) Pani/SF.

Chapter 3

61

3.6.1. Stability under Isothermal Ageing

The stability of Pani and Pani/SF composite in terms of DC electrical conductivity

retention was studied under isothermal ageing conditions as shown in Fig. 3.11. The

relative electrical conductivity was calculated by the equation:

(3.10)

where σr,t = relative electrical conductivity at time t, σt = electrical conductivity at

time t, σo = electrical conductivity at time zero. The stability in terms of DC electrical

conductivity retention of the prepared materials is used for the comparative study of

relative electrical conductivity with respect to time at different temperatures. The

electrical conductivity of the samples was measured for the temperature 50, 70, 90,

110 and 130°C versus time at an interval of 10 min for 40 min. It may be observed

from the Fig. 3.11b, that the stability of relative DC electrical conductivity of Pani is

very fair at 50 and 70°C while becomes unstable at 90, 110 and 130°C.

The relative DC electrical conductivity of Pani/SF composite is seems to be

comparatively more stable than Pani even at higher temperature range as shown in

Fig. 3.11a. Thus Pani/SF composite is observed to be more stable than Pani in the

term of electrical conductivity under isothermal aging condition.

Fig. 3.11 DC electrical conductivity retention under isothermal conditions at 50, 70,

90, 110 and 130°C of: (a) Pani/SF and (b) Pani.

Chapter 3

62


The stability in the term DC electrical conductivity retention of Pani and Pani/SF

composite was studied by cyclic ageing technique within the temperature range of 50

to 150°C as shown in Fig. 3.12. The electrical conductivity was recorded for

consecutive cycles and it was observed that the conductivity increased gradually for

each of the cycle showing a regular trend in all the cases. The relative electrical

conductivity was calculated using the following equation:

(3.11)

where σr is relative electrical conductivity, σT is electrical conductivity at temperature

T (°C) and σ50 is electrical conductivity at 50°C.

Fig. 3.12 DC electrical conductivity retention under cyclic ageing conditions of (a)

Pani and (b) Pani/SF composite.

From Fig. 3.12, the relative electrical conductivity of Pani and Pani/SF may be

observed within temperature range from 50 to 150°C. As temperature increases from

50 to 150°C the relative electrical conductivity of both the samples (Pani and Pani/SF)

increase may be due to formation of charge carriers, which can be connected with the

delocalization effect of doping and formation of the polarons or bipolarons. The gain

Chapter 3

63

in conductivity in Pani/SF is comparatively lower than Pani may be due to insulating

behaviour of SF. It may also be observed from Fig. 3.12a that electrical conductivity

of Pani at C-2, 4 and 5 (cycle-2, 4 and 5) are very similar but different at C-1 (cycle-

1) and C-3 (cycle-3) and observed to be deviated from the cycles-2, 4 and 5.

While in case of Pani/SF the relative electrical conductivity for each cycle are

at same trend and very less deviation is observed in conductivity as shown in Fig.

3.12b. The cycle C-1 is slightly deviated may be due to the instrumental deviation.

The more deviation in electrical conductivity represents more instability in the

electrical conductivity of the sample. Thus, it may be inferred that the Pani/SF

composite are more stable semiconductor than Pani. The difference may be attributed

to the removal of moisture, excess HCl or low molecular weight oligomers of aniline

[32].

CONCLUSIONS

Pani and Pani/SF composite were successfully synthesized by in-situ polymerization

method and characterized by different instrumental techniques. Pani and Pani/SF

composite were studied for the electrical properties in terms of DC electrical

conductivity under isothermal and cyclic ageing conditions and found in

semiconductor range. The Pani/SF composite showed low conductivity than Pani but

better stability in terms of DC electrical conductivity under isothermal and cyclic

ageing conditions. The as prepared materials displayed the excellent

photodecomposition of model dyes under UV-light irradiation. It was observed that

photogenetrated hydroxyl radical and hole (h+) and superoxide anion radical were the

main active species responsible for decolourization of model dyes pollutant. The

results highlighted that the extent of degradation of MB and RhB was greater over

Pani/SF composite than Pani. Thus, Pani/SF composite may find applications in

various electrical, electronic and UV light photocatalyst devices in view of their fairly

good photocatalytic activity and thermal stability of electrical properties.

Chapter 3

64

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Chapter 4

67

4.1. INTRODUCTION

Since the discovery of first conducting polymer the various types of conducting

polymers explored out of which polyaniline (Pani) has inspired a great deal of

interests due to its low cost, easy preparation, high environmental stability, reversible

and tuneable electrical properties by controlled charge-transfer processes [1-3]. This

makes polyaniline a versatile material for potential applications [4] as electrodes in

primary and secondary batteries [5, 6], microelectronics [7], photocatalyst [8], sensors

and actuators [9] etc.

The conjugated π-bonds present in Pani can experience changes on adsorption

of chemical species onto its surface due to acid-base type interaction between the

polymer and the chemical species. Also because of its high sensitivity, quick response

and working at room temperature [10, 11], Pani and its composite materials have been

demonstrated as sensing material for humidity, NO2, NH3, toxic solvents [12-15],

organic vapours such as methanol, ethanol, chloroform, dichloromethane, and hexane

[16, 17].

A number of nanocomposites of polyaniline have been reported with ferrite

(MFe2O4) [18], manganese dioxide (MnO2) [19], silver [20] and clay [21] etc. Here

silicon carbide (SiC) nanoparticles (˂ 50nm) is selected in this work due to its unique

properties such as wide and tuneable band gap, resistive towards high temperatures,

chemical inertness, high tensile strength and hardness [22-27].

It is a well admitted fact that smoking hosts dangerous diseases like cancer,

heart disease, lung diseases etc. Cigarette smoke contains tar, polynuclear aromatic

hydrocarbons (PAHs), toxic gases like carbon monoxide and about thousands of

chemicals produced when cigarette burns.

Devasish Chowdhury did an excellent work on cigarette smoke sensing by Ni

coated polyaniline nanowire material and observed a good response about four order

changes in the impedance response on exposure to cigarette smoke [28]. Also Yuan

Liu et. al. synthesized polyaniline films in detection of secondhand cigarette smoke

via nicotine. They observed that the polyaniline film demonstrated fare recovery

between exposures, and was functional in the presence of a wide range of exposures

to nicotine and tobacco smoke [29]. Besides, polyaniline composites have also been

Chapter 4

68

demonstrated potential applications in gamma radiations detection [31] and Electron

paramagnetic resonance (EPR) investigations [30]. Also carbon nanotubes [32],

graphene [33] and carbon nanotubides [34] and there composites have also been

widely used in gas sensing applications. In our previous work we have synthesised

polypyrrole/boron nitride nanocomposite for the detection of LPG leaks [35].

In this chapter, we present a simple strategy for the preparation of Pani/SiC

nanocomposites by oxidative polymerization of aniline. The morphology, thermal

stability and electrical conductivity of the prepared materials were investigated. Also,

we have tried to explore the polyaniline-silicon carbide nanocomposite as chemical

sensor by examining the dynamic response of electrical conductivity towards cigarette

smoke using a simple 4-in line probe electrical conductivity measurement set up.

4.2. EXPERIMENTAL

4.2.1. Materials

Aniline, 99% (E. Merck, India), hydrochloric acid, 35% (E. Merck, India), SiC (SRL,

India), potassium persulphate (E. Merck, India) and methanol. Double distilled water

was used throughout the experiments.

4.2.2. Synthesis of Pani and Pani/SiC Nanocomposites

Aniline (2.5 mL) was mixed with 100 mL of 1 M Hydrochloric acid (HCl) under

stirring for 30 min. A solution of potassium persulphate (PPS) was added dropwise

and the mixture was allowed to react for 20 h. The final dark green coloured reaction

mixture was then filtered, washed several times with double distilled water and

methanol and then dried in an air oven at 70°C for 10 hrs.

Pani/SiC nanocomposites were prepared by in-situ oxidative polymerization of

aniline in the presence of different amounts SiC nanoparticles. Typically 2.5 mL of

aniline was added to 100 mL of 1M HCl under constant stirring for 30 min. Different

amounts of SiC nanoparticles were ultrasonicated in 1 M HCl for 2 h and added into

the mixture. A solution of K2S2O8 in 100 mL of 1M HCl was then poured dropwise

into the mixture at room temperature with constant stirring. The colour of the solution

changed from light indigo to greenish black indicating the polymerization of aniline.

The reaction mixture was then stirred for further 20 hours. The reaction mixture was

Chapter 4

69

then filtered, washed thoroughly with distilled water and methanol to remove excess

acid, potassium persulphate and Pani oligomers until filtrate became colourless.

Thus prepared nanocomposites were dried at 70°C for 10 h in an air oven,

converted into fine powders and were stored in desiccators for further experiments.

For electrical conductivity measurements, 0.20 gm material from each sample was

pelletized at room temperature with the help of a hydraulic pressure machine at 80 kN

load for 15 min. The details of the synthesis are given in Table 4.1.

Table 4.1 The details of the synthesis of Pani/SiC nanocomposites.

Sample I.D Aniline (mL) K2S2O8 (gm) SiC nanoparticles (wt %)

Pani 2.5 6 0

Pani/SiC-1 2.5 6 10

Pani/SiC-2 2.5 6 15

Pani/SiC-3 2.5 6 20


In order to interrogate the morphology, structure and chemical composition of Pani

and Pani/SiC nanocomposites, a variety of methods were used including the Fourier

transform infrared spectroscopy (FTIR) spectra were recorded using Perkin-Elmer

1725 instrument. X-ray diffraction (XRD) pattern were recorded by Bruker D8

diffractometer with Cu Kα radiation at 1.540 A˚. Scanning electron microscope

(SEM) studies were carried out by JEOL, JSM, 6510-LV (Japan).


Pani/SiC composite under isothermal and cyclic ageing conditions was also studied.

For this study, a four-in-line probe with a temperature controller, PID-200 (Scientific

Equipments, Roorkee, India) was used to measure the DC electrical conductivity and

its temperature dependence. The equation used in calculation of DC electrical

conductivity was:

σ = [ln2(2S/W)]/[2πS(V/I)] (4.1)

Chapter 4

70




oC, 90

oC, 110

oC and 130

oC in


at an interval of 5 min in the accelerated ageing experiments. In testing of the stability

under cyclic ageing condition, DC conductivity measurements were taken 5 times at

an interval of about 90 min within the temperature range of 40-130oC.


4.4.1. Possible Interactions between Pani and SiC Nanoparticles

Polyaniline and polyaniline/silicon carbide nanocomposites were prepared by

oxidation of aniline in aqueous medium using K2S2O8 as an oxidant in presence of

HCl. It is expected that lone pair present on the nitrogen atom of polyaniline get

interacted with silicon carbide and forms an efficient network and may be responsible

for enhanced conductivity as shown in Fig. 4.1.

Fig. 4.1 Schematic representation of possible interactions between Pani and SiC

nanoparticles in Pani/SiC nanocomposite.

4.4.2. FTIR Analysis

FTIR spectra of the Pani and Pani/SiC nanocomposites are shown in Fig. 4.2. The

five major peaks were observed in all of the spectra. The peak observed at 3411 cm−1

of Pani may due to N-H stretching vibrations. The peaks at 1573 cm−1

and 1482 cm−1

are due to C=C stretching mode of the quininoid and benzenoid rings respectively.

Chapter 4

71

The peak at 1298 cm−1

may be attributed to C-N stretching. The peak at around 810

cm−1

may assigned to an out-of-plane bending vibration of C-H which confirmed the

formation of Pani [37].

In the spectra of nanocomposites, it is observed that the peaks at 1573 cm-1

,

1482 cm-1

and 1298 cm-1

for the quininoid, benzenoid and C-N stretching of Pani

respectively shift to 1568 cm-1

, 1475 cm-1

and 1289 cm-1

after the loading of

nanoparticles in the polyaniline matrix, indicating a strong interaction between

polyaniline and the SiC nanoparticles [38]. The peak observed at 810 cm-1

in Pani

due to C-H out of plane has been merged with Si-C stretching vibration [39] (800 cm-

1) and shifted to 804 cm

-1 in Pani/SiC nanocomposites.

Fig. 4.2 FT-IR spectra of: (a) Pani, (b) Pani/SiC-1, (c) Pani/SiC-2 and (d) Pani/SiC-3.


Fig. 4.3 shows the x-ray diffraction patterns of Pani and Pani/SiC nanocomposites.

Fig. 4.3a shows the characteristic peak of Pani at 2θ = 25.50o. The presence of

polyaniline in Pani/SiC nanocomposites is confirmed by the observation of Pani

bands in all of the samples which shifts to 25.29o with loading of silicon carbide

nanoparticles in polyaniline. The Four major peaks observed at 2θ =35.37o, 41.34

o,

59.91o, 71.70

o confirms the presence of silicon carbide in nanocomposites [39]. The

Chapter 4

72

diffraction peak of Pani in Pani/SiC nanocomposites become weaker and the

characteristic peaks of SiC appear to be more prominent as the percentage of silicon

carbide increases due to the interaction between Pani and SiC nanoparticles.

Fig. 4.3 XRD patterns of: (a) Pani, (b) Pani/SiC-1, (c) Pani/SiC-2 and (d) Pani/SiC-3.


The morphology of Pani and Pani/SiC-3 nanocomposite were studied by SEM as

presented in Fig. 4.4. Fig. 4.4a and Fig. 4.4b show the SEM images of Pani which

seems to be flaky, flat surfaced and agglomerated. The differences in the structure of

the nanocomposite and Pani samples are visible at lower magnification indicating

sharp edges in Pani/SiC-3 nanocomposite rather flat surfaces in Pani [39]. It may be

seen that from Fig. 4.4c and 4.4d the SiC nanoparticles were successfully decorated

by Pani and no free SiC nanoparticles were observed which indicate that the SiC

nanoparticles have a nucleating effect on the aniline polymerization and caused a

homogeneous Pani shell around them. Also the SiC nanoparticles bind the surface of

polyaniline granules and their surface area seems to increase as the SiC nanoparticles

are well dispersed in Pani/SiC-3 nanocomposite. Thus the study reveals

transformation of morphology from flat Pani to the core-shell Pani/SiC-3

nanocomposite with sharp edges.

10 20 30 40 50 60 70 80 90

0

500

1000

1500

(d)

(c)

(b)

(a)

Inte

nsi

ty (

a.u

)

2 Theta

Chapter 4

73

Fig. 4.4 SEM micrographs of: (a and b) Pani, (c and d) Pani/SiC-3 nanocomposite at

different magnifications.


Fig. 4.5 represents the TEM micrograph of Pani/SiC-3 nanocomposite. From the

TEM image of nanocomposite, it may be observed that dark parts with diameter about

50 nm coated by light shaded outer shell of polymer matrix. This observation

confirms the SiC nanoparticles are fully covered by polyaniline by successful

deposition of Pani on SiC nanoparticles forming the Pani/SiC nanocomposites with

core-shell structure.

Chapter 4

74

Fig. 4.5 TEM micrograph of Pani/SiC-3 nanocomposite.


The initial DC electrical conductivity of Pani and Pani/SiC nanocomposite containing

10, 15 and 20% of SiC was measured by standard four-in-line probe method and

found to be 0.54, 1.30, 1.49 and 1.52 S/cm for Pani, Pani-1, Pani-2 and Pani-3

respectively. From the measured electrical conductivity, it may be inferred that all the

as prepared materials possess electrical conductivity in semiconducting range. The

electrical conductivity of the Pani/SiC nanocomposites increases with increase in the

SiC nanoparticles content in the nanocomposites as shown in Fig. 4.6. It may be

assumed that the electrical conductivity of Pani was improved after the loading of SiC

nanoparticles may be due to forming an efficient network in Pani chains and also due

to interaction of lone pair of nitrogen of polyaniline with silicon carbide which

increase the mobility of polarons in polyaniline leading to increase in electrical

conductivity.

It is also observed from Fig. 4.6 that the conductivity increases sharply with

loading of 10 % of SiC and after that shows little increment with further loading of 15

and 20 % of SiC. Pallavi et. al. studied polypyrrole/silicon carbide nanocomposites

reported a similar increase in conductivity of the nanocomposites with an initial

particle loading upto 10 wt %, and then decreases at higher particle loadings [39]. By

these observations, it may concluded that the loading of SiC into Pani forms an

Chapter 4

75

efficient network in Pani resulting in increase in electrical conductivity. Above 10 %

loading of SiC into Pani does not show any significant increase in conductivity may

be forming a percolative pathway.

Fig. 4.6 Initial DC electrical conductivities of Pani and Pani/SiC nanocomposites.


The stability of Pani and Pani/SiC nanocomposites in terms of DC electrical

conductivity was studied under isothermal ageing conditions as shown in Fig. 4.7.

The representation of relative electrical conductivity was calculated by using the

equation:

(4.2)

Where σr,t is the relative electrical conductivity at time t, σt = electrical conductivity

at time t, σo = electrical conductivity at time zero. The comparative study on relative

electrical conductivity with respect to time at different temperatures was done for

analysing the stability of prepared materials in terms of DC electrical conductivity.

The electrical conductivity of each of the samples was measured for

temperature (50, 70, 90, 110, 130°C) versus time at an interval of 05 min upto 20 min.

Chapter 4

76

It may be observed from the Fig. 4.7a, that the stability of relative DC electrical

conductivity of Pani is very fair at 50 and 70°C. In Fig. 4.7b, the relative DC

electrical conductivity of Pani/SiC-1 nanocomposite is seems to be very stable at 50,

70, 90 and 110°C. While the relative DC electrical conductivity of Pani/SiC-2 and

Pani/SiC-3 nanocomposite are seems to be fairly stable at 50°C and 70°C as shown in

Fig. 4.7c and Fig. 4.7d. Thus it is inferred that Pani/SiC-1 nanocomposite showed

greatest stability than Pani in terms of DC electrical conductivity under isothermal

ageing condition.

Fig. 4.7 Relative electrical conductivity of: (a) Pani, (b) Pani/SiC-1, (c) Pani/SiC-2 (d)

Pani/SiC-3 under isothermal ageing conditions.


The stability in the term DC electrical conductivity retention of Pani and Pani/SiC

nanocomposites was also studied by cyclic ageing method within the temperature

range of 50 to 130°C as shown in Fig. 4.8. The electrical conductivity was recorded

for successive cycles and observed to be increased gradually for each of the cycle

with a regular trend in all these cases. The relative electrical conductivity was

calculated using the following equation:

(4.3)

Chapter 4

77

Where σr is relative electrical conductivity, σT is electrical conductivity at

temperature T (°C) and σ50 is electrical conductivity at 50°C.

Fig. 4.8 Relative electrical conductivity of: (a) Pani, (b) Pani/SiC-1, (c) Pani/SiC-2

and (d) Pani/SiC-3 under cyclic ageing conditions.

As temperature increases from 50 to 130°C the relative electrical conductivity of all

of the samples increases which may be due to the increment of number of charge

carriers as polarons or bipolarons at elevated temperature. From Fig. 4.8a, it may be

observed that the electrical conductivity of Pani for all of the cycles is similar and

following the same trend with highest gain in the conductivity (lowest stability)

among all of the samples. In Pani/SiC nanocomposites, the relative DC electrical

conductivity also follows the similar trend with less deviation as shown in Fig. 4.8b,

Fig. 4.8c and Fig. 4.8d.

In all of these nanocomposites, the Pani/SiC-3 nanocomposite showed greater

stability by very less deviation in electrical conductivity. Increment and decrement of

relative electrical conductivity represents instability in the electrical conductivity of

the sample. The more deviation in electrical conductivity represents more instability

in the electrical conductivity of the sample. Thus it may be understood that the

Pani/SiC nanocomposites are more stable semiconductor than Pani and among these

Pani/SiC nanocomposites, the Pani/SiC-3 has greatest stability in terms of DC

electrical conductivity under cyclic ageing condition. The difference observed may be

attributed to the removal of moisture, excess HCl or low molecular weight oligomers

of aniline [41].

Chapter 4

78

4.6. SENSING

The cigarette smoke is very well known health hazardous and known cause of cancer,

lung diseases, heart diseases and responsible for other related problems. About 4000

chemical constituents are produced by burning a cigarette but main constituents are

tar, carbon dioxide, carbon monoxide, nitrogen oxides, hydrogen cyanide, ammonia,

polycyclic aromatic hydrocarbons, chlorinated dioxins, furans etc [28]. It is very

difficult to evaluate that what constituent in cigarette smoke is responsible for

conductivity change of the material.

The experiment was also carried out to know the conductivity response in the

presence of carbon dioxide (CO2), ammonia gas and methanol. Devasish Chowdhury

reported a four order change in the impedance response of Ni-coated PANI-CSA on

exposure to cigarette smoke [28]. Also Yuan Liu et. al. reported conductive

polyaniline films in detection of secondhand cigarette smoke via nicotine with very

fare recovery between exposures [29]. Herein we have tried to look into the effect of

cigarette smoke on electrical conductivity of Pani/SiC nanocomposite.

The cigarette smoke sensitivity of Pani/SiC-3 was analysed by measuring the

changes in the electrical conductivity at 25°C as shown in Fig. 4.9. The sensitivity of

Pani/SiC-3 was investigated on the two parameters; response time and sensing

intensity. The pellet fabricated was attached with four probes and placed in a closed

chamber with an inlet for introducing smoke. Cigarette smoke was introduced to the

inlet chamber for 60 seconds and after that the pellet was removed from chamber and

exposed to air for next 60 seconds. When Pani/SiC-3 came in contact with cigarette

smoke, the electrical conductivity decreased with increase in time. This decrease in

conductivity may be due to neutralization of some Pani by interaction of lone pair of

electrons of nitrogen atoms of ammonia [42] or may be due interaction with lone pair

of oxygen atom of CO2 gas [43] or also may be due to interaction of lone pair of

electrons of oxygen of methanol with Pani [44].

Cigarette smoke is composed of all of these and many other gases which

hindered the mobility of charge carriers leading to decrease in electrical conductivity.

When the pellet was exposed to air, the electrical conductivity started to increase with

time and reverted to its maximum value after 60 seconds reason being desorption of

these gases from the surface of Pani/SiC-3.

Chapter 4

79

Fig. 4.9 Effect on the DC electrical conductivity of Pani/SiC-3 on exposure to

cigarette smoke followed by ambient air with respect to time.

The reversibility response of Pani/SiC-3 was measured in terms of the DC

electrical conductivity. The reversibility was determined by first keeping sample in

smoke environment for 30 sec followed by 30 sec in open air for a total duration of

180 seconds. From Fig. 4.10, it may be observed that the as prepared material showed

excellent reversibility by variation of conductivity in the range from 4.9 S/cm to 4.2

S/cm in absence and presence of smoke.

Fig. 4.10 Variation in electrical conductivity of Pani/SiC-3 on alternate exposure to

cigarette smoke and ambient air.

Chapter 4

80

The cigarette smoke sensitivity of Pani was also analysed by measuring the

changes in the electrical conductivity at 25°C. The similar experiment was done for

Pani in absence and presence of smoke for 120 seconds. On exposing by smoke for 60

sec the electrical conductivity decreased with increase in time and by exposing air for

next 60 sec the electrical conductivity increased with time the reasons mentioned in

previous paragraph.

Fig. 4.11 Effect on the DC electrical conductivity of Pani on exposure to cigarette

smoke followed by ambient air with respect to time.

The reversibility response of Pani was also measured in terms of the DC

electrical conductivity as it was done in Pani/SiC-3. The similar procedure was

adopted for Pani in absence and presence of smoke for 180 sec by first keeping the

sample in smoke for 30 sec followed by 30 sec in air. From Fig. 4.12, it may be

observed that the Pani also showed reversibility but with less variation range than

Pani/SiC-3 which is from 1.15 S/cm to 1.06 S/cm in the absence and presence of

smoke respectively. In this case, too, there is observed decrease in conductivity by

0.09 S/cm and it seems to be less efficient than Pani/SiC-3 in terms of reversibility

which has excellent variation range in between 4.9 S/cm and 4.2 S/cm with

conductivity observed to be decreased by 0.7 S/cm.

Thus there was around eight times greater variation range observed in

conductivity of Pani/SiC-3 nanocomposite than that of Pani. The greater sensing

Chapter 4

81

response and excellent reversibility of the Pani/SiC-3 nanocomposite may be

attributed to an increase in the surface area of Pani coated over the SiC nanoparticles,

leading to the exposure of more active sites and more adsorption and desorption,

which leads to increase in sensitivity and reversibility.

Fig. 4.12 Variation in electrical conductivity of Pani on alternate exposure to cigarette

smoke and ambient air.

The sensing response of the Pani/SiC-3 in the presence of carbon dioxide

(CO2), ammonia (NH3) and methanol (CH3OH) was carried out to know the

conductivity response in the presence of these gases. The Fig. 4.13 shows that when

the sample get exposed of CO2 gas there was decrease in conductivity observed due to

interaction of lone pairs of oxygen atom of CO2 gas [40]. In case of methanol and

ammonia the significant loss in conductivity is observed in the environment of these

gases and the conductivity starts to increase as the sample exposed with air due to

desorption of gases. The significant loss in conductivity may be attributed to more

availability of loosely bind electrons pair of nitrogen atom of ammonia and oxygen

atom of methanol which interacts and neutralize some Pani with each exposure.

Cigarette smoke contains CO2, NH3 and CH3OH, so they may be contributing gases

for sensing the cigarette smoke.

Ansari et. al. have defined the sensing response of nanocomposite of Pani with

multiwall carbon nanotubes towards ammonia mentioned good response and quick

recovery and also the resistance of composite and Pain decreased in presence of

Chapter 4

82

ammonia [42]. Konwer et. al. reported the sensing response of composite of Pani with

graphene oxide towards methanol and mentioned the resistance of both, composite

and Pani decreased in presence of methanol and the sensitivity of Pani/GO composite

increases towards methanol as compared with the pure Pani [44].

Habib Ullah et. al. mentioned the interaction of CO2, CO and NH3 gas with

Pani. They observed that the carbon dioxide interact with polyaniline toward oxygen

atom and conductance of Pani (ES) decreased on interaction with ammonia and

carbon dioxide. They also mentioned that CO gas show neutral effect on conductance

of polyaniline [43]. We also consider that the polycyclic aromatic hydrocarbons

(PAHs), nicotine, hydrogen cyanide (HCN) and formaldehyde also adsorb on the

surface of Pani/SiC-3 nanocomposite and also responsible for change in conductivity

by occupying some active sites temporarily of polyaniline networks coated on SiC

nanoparticles. Thus results in decrease in the mobile charge carrier leading to

decrease in conductivity.

Fig. 4.13 Effect on the DC electrical conductivity of Pani/SiC-3 on exposure to

ammonia, methanol and carbon dioxide with respect to time.

http://pubs.acs.org/author/Ullah%2C+Habib

Chapter 4

83

4.6.1. Proposed Mechanism for Sensing

The sensing mechanism of cigarette smoke was explained through DC electrical

conductivity response by simply adsorption and desorption mechanism of smoke at

room temperature. The DC electrical conductivity varies with the exposure of

cigarette smoke. In Pani/SiC-3 nanocomposite the DC electrical conductivity

decreases after exposure to cigarette smoke which may be attributed to decrease in

mobility of charge carrier as smoke get interacted with nanocomposite. When the

nanocomposite is exposed to air, the electrical conductivity reverted to its previous

value.

Cigarette smoke is composed of about ~ 4000 chemicals and it is expected that

the carbon dioxide, ammonia and methanol present in the smoke may be responsible

for electrical conductivity response. Individual conductivity experiment was also done

for carbon dioxide, ammonia and methanol and they also showed the same response

by decreasing in electrical conductivity on interaction with nanocomposite.

In Pani/SiC-3 nanocomposite the lone pair of electrons of nitrogen in

polyaniline gets interacted with positively charged silicon of silicon carbide. The lone

pairs of oxygen of carbon dioxide, nitrogen of ammonia and oxygen of methanol get

interacted with polarons of polyaniline as shown in scheme 4.1 [42-44].

This

interaction of lone pairs with polarons of polyaniline leads to decrease in mobility of

charge carriers thus decreasing electrical conductivity.

Chapter 4

84

Scheme-4.1 Proposed interaction of Pani/SiC-3 nanocomposite with methanol,

ammonia, carbon dioxide and cigarette smoke.

CONCLUSIONS

The Pani and Pani/SiC nanocomposites were successfully synthesized by in-situ

polymerization method and confirmed by FTIR and XRD analysis. SEM and TEM

analysis confirmed the uniformly deposition of Pani on SiC nanoparticles. Pani/SiC

nanocomposites showed greater thermal stability in terms of DC electrical

conductivity under isothermal and cyclic ageing conditions than pristine Pani. The

results highlighted that the Pani/SiC-3 nanocomposite found to be very effective

material for cigarette smoke sensing as there was around eight times greater variation

range observed in conductivity of Pani/SiC-3 nanocomposite than that of Pani.

Therefore, the sensor based on Pani/SiC may be useful as an efficient material for

cigarette smoke sensing.

Chapter 4

85

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http://pubs.acs.org/author/Ullah%2C+Habib

http://pubs.acs.org/author/Shah%2C+Anwar-ul-Haq+Ali

http://pubs.acs.org/author/Bilal%2C+Salma

http://pubs.acs.org/author/Ayub%2C+Khurshid

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88

5.1. INTRODUCTION

Conducting polymers are very versatile materials with potential applications in

various evolving technologies such as electrodes in primary and secondary batteries

[1, 2], energy storage devices [3, 4],P

Pmicroelectronics [5], photocatalysts [6], opto-

electronic devices [7, 8], sensors and actuators [9] etc.

Among all conducting polymers, polyaniline (Pani) drew widespread attention

because it possesses unique properties such as low cost, high environmental stability,

reversible and tailorable electrical properties by controlled charge-transfer processes

[10-12]. Polyaniline can experience changes on adsorption and desorption of chemical

species onto its surface, making it a potential sensing material for humidity, R Rtoxic

solvents [13-16],P

Porganic vapours P

Psuch as methanol, ethanol, chloroform,

dichloromethane and hexane [17, 18].

Silk fibroin (SF) is an attractive scaffold biomaterial because of its excellent

mechanical properties, biocompatibility, biodegradability, tissue regeneration and

various other biomedical applications [19]. Natural silk fibroin fibers (a filament core

protein) are obtained by removing the outer sericin (glue-like coating of a

nonfilamentous protein) from silk fibres with anhydrous sodium carbonate solution at

appropriate temperature [20].

A number of composites of silk fibroin have been reported for various

potential applications. Yahya A. Ismail et. al. [21] reported Fibroin/Polyaniline

composite for electrochemical characterization as reactive sensor. Youyi Xia et. al.P

P[22] reported silk fibroin/polyaniline (core/shell) coaxial fiber for the application in

cell proliferation. Developments of gas sensors by conducting composite materials

have drawn more attention over the last few years [23].

Among various toxic compounds, acetaldehydes and ammonia are classified

as potentially dangerous to the environment even at low concentration. Thus, the

development of new materials for the detection and determination of these toxic

volatile compounds in the atmosphere are of the immediate concern to environmental

researchers. Herein we have tried to explore the ammonia and acetaldehyde sensing

capabilities of Pani by modifying the non conducting silk fibroin into Pani coated

conducting composite fibers (Pani/SF). To the best of our knowledge, the sensing

Chapter 5

89

application with Pani modified SF fibers (Pani/SF) has not been yet reported

elsewhere.

5.2. EXPERIMENTAL

5.2.1. Materials

In the preparation of Pani and Pani/SF composite fibers, the chemicals used were:

aniline 99% (E.Merck, India), CSA (camphor sulphonic acid) ≥98% (TCI, Tokyo),

Cocoons for silk (Banaras, India), potassium persulphate (E.Merck, India) and

methanol. Double distilled water was used throughout the experiments.

5.2.2. Preparation of Silk Fibroin

Cocoons were degummed twice for 1.5 h in aqueous solution of 0.02 M NaR2RCOR3R and

then rinsed thoroughly with double distilled water to extract the silk fibroin (SF). The

extracted SF was then dried at 40 P

°PC for 24 h.

5.2.3. Preparation of Pani and Pani/SF Composites

Aniline (2.0 mL) was mixed with 100 mL of 0.1 M camphor sulphonic acid (CSA)

under constant stirring for 1 hr. A solution of potassium persulphate (PPS) was added

dropwise and the mixture was allowed to react for 24 h. The final dark green coloured

reaction mixture was then filtered, washed several times with double distilled water

and methanol and then dried in an air oven at 70°C for 10 h.

The Pani/SF composite fibers were prepared by in situ oxidative

polymerization of aniline in the presence of CSA modified SF fibers using potassium

persulphate as an oxidizing agent. The extracted silk fibroin fibers were dipped in 100

mL of 0.1 M CSA aqueous solution and stirred for 1 h. The resultant SF fibers were

filtered and dried at 40°C. Thus the CSA modified SF fibers (300 mg) each were

added to different amounts of aniline solutions prepared in 0.1 M CSA and stirred for

1 hr. A solution of KR2RSR2ROR8 R(6 g) in 100 mL of 0.1 M CSA was then poured dropwise

into the mixture at room temperature with constant stirring. The colour of the SF

fibers changed from light yellow to greenish black indicating the polymerization of

aniline on silk fibroin fibers. The fibrous reaction mixture was then stirred for further

24 h.

Chapter 5

90

The final Pani/SF composite fibers was then filtered, washed thoroughly with

double distilled water and methanol to remove excess acid, potassium persulphate and

Pani oligomers. Thus prepared Pani/SF composite fibers were dried at 50°C for 20 h

in an air oven and were stored in desiccators for further experiments. For electrical

conductivity measurements, 250 mg material from each sample was pelletized at

room temperature with the help of a hydraulic pressure machine at 80 kN load for 15

min. The preparation details of are given in Table 5.1.

Table 5.1 Preparation details of Pani/SF fibers.

Sample I.D. Aniline (mL) KR2RSR2ROR8R (g) SF Fibers (mg)

Pani 2.0 6 0

Pani/SF-1 1.0 6 300

Pani/SF-2 2.0 6 300

Pani/SF-3

Pani/SF-4

3.0

5.0

6

6

300

300


In order to investigate the morphology, structure and chemical composition of

Pani/SF fibers, a variety of methods were used including the Attenuated total

reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). The spectra were

recorded using Perkin-Elmer 1725 instrument. Scanning electron microscope (SEM)

studies were carried out by JEOL, JSM, 6510-LV (Japan). Thermogravimetric

analysis (TGA) was done by using a Perkin Elmer instrument in the temperature

range from 35 to 700P

oPC.


Pani/SF composite fibers under isothermal and cyclic ageing conditions was also

studied. For this study, a four-in-line probe with a temperature controller, PID-200

(Scientific Equipments, Roorkee, India) was used to measure the DC electrical

Chapter 5

91

conductivity and its temperature dependence. The equation used in calculation of DC

electrical conductivity was:

σ = [ln2(2S/W)]/[2πS(V/I)] (5.1)



isothermal stability, the pellets were heated at 50P

oPC, 70 P

oPC, 90 P

oPC, 110P

oPC and 130 P

oPC in




times within the temperature range of 50-130P

oPC.

5.4. RESULTS AND DISCUSSION

5.4.1. Mechanism of Preparation of Pani/SF Composite Fibers:

The mechanistic view of the polymerization process seems to involve the anilinium

cations (phenyl-NHR3RP

+P) getting hooked up by Coulombic attraction between anilinium

cations and the sulphonate group of camphor sulphonic acid adsorbed on the surface

of silk fibroin. Thus the SF fibers get uniformly surrounded by anilinium cations. This

arrangement comes in contact with KR2RSR2ROR8R, the anilinium cations get polymerized on

the surface of SF fibers forming uniform smooth layer of Pani (emeraldine salt). The

uniform deposition of Pani on the CSA modified surface of silk fibroin may be

supported by the interaction between negatively charged sulphonate groups and

polarons of Pani.

The similar mechanism is mentioned by Youyi Xia et. al. in which SF fibers

were modified by methyl orange (MO) and then aniline was polymerized on the

surface of MO modified SF fibers [22]. The schematic presentation of coulombic

attraction is given in Fig. 5.1.

Chapter 5

92

Fig. 5.1 Schematic representation of possible interaction between Pani and CSA

modified SF fibers in Pani/SF composite fibers.

5.4.2. ATR-FTIR Analytical Studies

ATR-FTIR spectra of the SF and Pani/SF-4 composite fibers are shown in Fig. 5.2.

Fig. 5.2a shows the spectra of pure SF fibers in which the peaks observed at 1636

cmP

−1Pand 1540 cmP

−1P correspond to amide I and amide II vibrations which may be

attributed to the β sheet structure of silk fibroin [25].

In the spectra of Pani/SF-4 composite fibers, the four major peaks are

observed. The peaks observed at 1569 cm P

-1 Pand 1488 cmP

-1P may be due to C=C

stretching mode of quininoid and benzenoid rings respectively. The peaks at around

1292 cmP

-1P and 800 cmP

-1P may be attributed to C-N bond stretching and C-H bond out

of plane bending respectively as shown in Fig. 5.2b [26]. There are some extra peaks

observed in Pani/SF-4 spectra which may be related to presence of camphor sulphonic

acid in Pani. The peak at around 1726 cm P

-1P may be attributed to the characteristic

peak of CSA due to stretching of C=O bond, also the peak at 620 cm P

-1P may be related

to S-O bond stretching of sulphonate group [27]. All these results indicated and

supported the presence of CSA doped Pani coated on the SF fibers.

Chapter 5

93

Fig. 5.2 ATR-FTIR spectra of: (a) SF fibers and (b) Pani/SF-4 fibers.


The thermal stability of the Pani/SF-4 composite fibers was investigated and shown in

Fig. 5.3. The initial weight loss may be due to removal of low weight oligomers of

Pani and absorbed water on the surface of Pani. Afterward, the sharp degradation of

the Pani/SF-4 composite fibers is observed at about 295.4°C [22], which is higher

than that of pristine SF fibers (281.3°C) [28]. This degradation pattern of Pani/SF-4

fibers suggesting that Pani layer resists the degradation thermo oxidative conditions.

Fig. 5.3 TGA thermograms of Pani/SF-4 composite fibers.

Chapter 5

94


The morphology of SF fibers and Pani/SF-4 composite fibers were studied by SEM at

different magnifications as presented in Fig. 5.4. Fig. 5.4a and Fig. 5.4b represent the

SEM image of SF fibers which seems to be very rough surfaced. The change in rough

surfaced SF fibers to smooth surfaced Pani/SF-4 composite fibers observed in Fig.

5.4c and Fig. 5.4d indicating the growth of smooth Pani layer on SF fibers. P

PThe SF

fibers were modified by CSA helping in uniformly adsorption of anilinium molecules

on the fibers providing uniform coating on SF fibers. Thus the Pani/SF-4 composite

fibers exhibit smooth surfaced morphology.

Fig. 5.4 SEM micrographs of: (a and b) SF fibers, (c and d) Pani/SF-4 composite

fibers at different magnifications.

5.4.5. Digital Photographic Studies

Fig. 5.5a and Fig. 5.5b show the digital photographs of SF and Pani/SF-4 composite

fibers respectively. Evidently, it may be seen that the light yellow colour of SF fibers

changed into deep green suggesting that the SF fibers has been successfully covered

with Pani.

Chapter 5

95

Fig. 5.5 Digital photograph of: (a) SF fibers and (b) Pani/SF-4 composite fibers.


Fig. 5.6 Initial DC electrical conductivities of Pani and Pani/SF composite fibers.

For the electrical conductivity measurements and sensing experiments, 250 mg

material from each sample was pelletized at room temperature with the help of a

hydraulic pressure machine at 80 kN force for 15 min. The initial DC electrical

conductivity Pani and Pani/SF composite fibers were measured by standard four-in-

line probe method. The variations in DC electrical conductivity in Pani/SF composites

with loading of different amounts of aniline monomer are shown in Fig. 5.6.

Chapter 5

96

In Pani/SF-4 composite fibers, the highest electrical conductivity was found

with highest loading (5mL) of aniline monomer [24]. The electrical conductivity of

CSA doped Pani/SF composite fibers increases with increase in aniline monomer

content in the composites materials varied from 10 P

-6 Pto 10 P

-2P S/cm. Therefore, Pani/SF-

4 was selected for temperature dependence of electrical conductivity studies and as

sensing material.

The DC electrical conductivity in CSA doped Pani/SF composite fibers was

found to be lower than that of pristine CSA doped Pani. In all Pani/SF composite

fibers, the silk fibroin was functionalised by CSA having negatively charged

functional sulphonate group (SOP

-3P) which interacts strongly with positively charged

nitrogen of polyaniline which induce the formation of conductive polymer. Based on

this mechanism involved in the formation, the SO P

-3P group interrupt the polarons of the

Pani thus decreasing the mobility of polarons resulting into decrease in electrical

conductivity in Pani/SF composite fibers.


Fig. 5.7 Relative electrical conductivity of: (a) Pani, (b) Pani/SF-4 composite fibers

under isothermal ageing conditions.

The stability of Pani and Pani/SF-4 composite fibers in terms of DC electrical

conductivity was studied under isothermal ageing conditions as shown in Fig. 5.7.

The representation of relative electrical conductivity was calculated by the equation:

Chapter 5

97

(5.2)

where σRr,tR is the relative electrical conductivity, σRtR = electrical conductivity at

time t, σRoR = electrical conductivity at time zero. The comparative study of relative

electrical conductivity with respect to time at different temperatures was done by

analysing the stability of prepared materials in terms of DC electrical conductivity.

The electrical conductivity of each of the samples was measured for temperature (50,

70, 90, 110, 130°C) versus time at an interval of 10 min upto 40 min.

It can be understood From the Fig. 5.7a that the stability of relative DC

electrical conductivity of Pani is very fair at 50 and 70°C and from 90 to 130°C seems

to be unstable. In Fig. 5.7b the relative DC electrical conductivity of Pani/SF-4

composite fibers is seems to be very stable at 50, 70, and 90°C. Thus it is inferred that

the fibrous Pani/SF-4 material is sufficiently stable under ambient conditions in terms

of DC electrical conductivity retention upto 90°C under isothermal ageing condition.


The stability in the term DC electrical conductivity retention of Pani and Pani/SF-4

composite fibers was also studied by cyclic ageing method within the temperature

range of 50 to 130°C as shown in Fig. 5.8. The electrical conductivity was recorded

for successive cycles and observed to be gradually increased in Pani while decreased

in Pani/SF-4 composite fibers for each of the cycle. The relative electrical

conductivity was calculated using the following equation:

(5.3)

where σRrR is relative electrical conductivity, σRTR is electrical conductivity at temperature

T (°C) and σR50R is electrical conductivity at 50°C.

Chapter 5

98

Fig. 5.8 Relative electrical conductivity of: (a) Pani, (b) Pani/SF-4 composite fibers

under cyclic ageing conditions.

As cycle runs from temperature 50 to 130°C the relative electrical

conductivity of Pani seems to be increased may be due to the increment of number of

charge carriers which may be attributed to the formation of greater number of

polarons/bipolarons. From Fig. 5.8a it may observed that the electrical conductivity of

Pani for all of the cycles is similar and following the same trend with gain in the

conductivity.

In Pani/SF-4 composite fibers just opposite trend of Pani relative electrical

conductivity is observed. The relative DC electrical conductivity found to decrease as

temperature increases as shown in Fig. 5.8b. The decrease in electrical conductivity

may be attributed to the removal of moisture, loss of low molecular weight oligomers

of aniline and earlier degradation of coated polyaniline on silk fibroin under cyclic

ageing conditions.

5.6. SENSING STUDIES

Acetaldehyde is a highly reactive volatile organic compound which may be

responsible for dizziness, headache and even cancer if someone exposed for

prolonged periods. Although, ammonia solutions does not usually cause problems for

humans and other mammals but it is highly toxic to aquatic animals even at very low

concentration.

Chapter 5

99

Thus the both compounds are classified as dangerous for the environment and

the detection of acetaldehyde and ammonia vapours are important in environmental

monitoring and chemical control. The acetaldehyde and ammonia vapours sensitivity

of Pani/SF-4 fibers was monitored by measuring the changes in the electrical

conductivity at room temperature by using a four-in-line probe electrical conductivity

device as shown in Fig. 5.9.

Fig. 5.9 Schematic representation of acetaldehyde and ammonia vapours sensor unit

by four-in-line probe technique.

The electrical conductivity of 250 mg pelletized Pani/SF-4 composite fibers

showed significant response on exposure to different concentrations (0.1, 0.2, 0.3, and

0.5 M) of aqueous acetaldehyde at room temperature as shown in Fig. 5.10.

The sensitivity of material was investigated on the two parameters; response

time and sensing intensity. The pellet fabricated was attached with four probes and

placed in a closed chamber of acetaldehyde vapours for 60 seconds and after that the

pellet was removed from chamber and exposed to air for next 60 seconds. It can be

seen that the electrical conductivity decreased for first 60 seconds for all of the

different concentration of solutions of acetaldehyde and recovered in next 60 seconds

on exposing with fresh air. Relatively good changes in electrical conductivity and fast

recovery observed in the atmosphere of 0.5 M aqueous acetaldehyde vapours.

Chapter 5

100

Fig. 5.10 Effect on the DC electrical conductivity of Pani/SF-4 on exposure to

different concentrations (0.1 M, 0.2 M, 0.3 M, 0.5 M) of acetaldehyde with respect to

time.

The reversibility response of Pani/SF-4 composite fibers towards 0.5 M

aqueous acetaldehyde was investigated as shown in Fig. 5.11. The reversibility was

determined by first keeping sample in acetaldehyde vapours environment for 10 sec

followed by 10 sec in open air for a total duration of 120 seconds. The conductivity

after each cycle never recovered upon exposure to air due to two process occurring

simultaneously, i.e. electrical neutralization of Pani backbone and the desorption of

acetaldehyde.

Fig. 5.11 Reversible electrical conductivity response of Pani/SF-4 composite fibers

for 0.5 M acetaldehyde solution.

Chapter 5

101

The similar experiment was done in presence of different concentrations (0.1,

0.2, 0.3, and 0.5 M) of ammonia solution at room temperature as shown in Fig. 5.12.

It may be seen that the electrical conductivity decreased on exposure to ammonia

solutions (0.1, 0.2, 0.3, and 0.5 M) for first 60 seconds and regained in next 60

seconds on exposing with fresh air. The electrical conductivity response of Pani/SF-4

in 0.5 M ammonia solution is found to be better than other ammonia solutions.

Fig. 5.12 Effect on the DC electrical conductivity of Pani/SF-4 composite fibers on

exposure to different concentrations (0.1 M, 0.2 M, 0.3 M, 0.5 M) of ammonia with

respect to time.

The reversibility response of Pani/SF-4 composite fibers towards 0.5 M

ammonia solution was also investigated as shown in Fig. 5.13. The similar

experiment was done in absence and presence of ammonia vapours for total 120 sec

by first keeping the sample in ammonia vapours for 10 sec followed by 10 sec in fresh

air. It is observed that the conductivity after each cycle never reverted upon exposure

to air. The reason for not recovering electrical conductivity of Pani/SF-4 composite

fibers is same as was in case of acetaldehyde. If the air exposing time is increased, the

conductivity may regain due to complete desorption of ammonia from the polymer

surface.

Chapter 5

102

Fig. 5.13 Reversible electrical conductivity response of Pani/SF-4 composite fibers

for 0.5 M ammonia solution.


The sensing mechanism of ammonia and acetaldehyde vapours was explained through

DC electrical conductivity response by simply adsorption and desorption mechanism

of vapours at room temperature. In Pani/SF-4 composite fibers, the DC electrical

conductivity decreases after exposure to acetaldehyde and ammonia vapours which

may be attributed to decrease in mobility of charge carriers. The decrease in mobility

of charge carriers may be due to interaction of lone pairs of oxygen and nitrogen of

acetaldehyde and ammonia respectively with positive charged polarons at nitrogens of

polyaniline [24, 29].

Thus the mobility of polarons decreases resulting in the decrease in electrical

conductivity. The adsorbed vapours of ammonia and acetaldehyde on the surface of

polyaniline get loosen from the surface under ambient conditions leads to increase in

electrical conductivity.

Chapter 5

103

Scheme-5.1 Proposed interactions of Pani/SF-4 composite fibers with: (a) ammonia

and (b) acetaldehyde.

CONCLUSIONS

The Pani/SF composite fibers were successfully prepared by in-situ polymerization

method and confirmed by FTIR and TGA analysis. SEM analysis confirmed the

uniformly deposition of Pani on CSA modified SF fibers. Pani/SF composite fibers

showed lower conductivity but greater thermal stability in terms of DC electrical


results highlighted the excellent reversible acetaldehyde and ammonia sensing

indicating that the Pani/SF-4 composite fibers could be utilized as a sensor material.

Chapter 5

104

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Chapter 6

106

6.1. INTRODUCTION

A variety of materials such as carbon nanomaterials, inorganic semiconductors and

conjugated polymers have been explored to fabricate gas sensors [1-7]. Conducting

polymer based gas sensors have high sensitivity due to its large surface area, fast

response, lower power consumption, small size and lightweight.

Among them, polyaniline (Pani) is considered to be the most promising and

widely applied sensing material because of its low cost, easy preparation, high

environmental stability, tuneable electrical properties and its unique functions by

controlled charge-transfer processes [8-10]. However, its low sensitivity and

inadequate thermal stability still restrict its application in operable sensors.

In order to improve the sensing performance of the polyaniline, many efforts

have been made by loading of noble metals, semiconductors and other components to

construct composite materials [11-13]. Recently nickel oxide (NiO) nanoparticles

have drawn considerable attention due to its low price, acceptable sensing properties

with good environmental stability. Researchers developed the gas sensors based on

NiO films which have demonstrated significant sensing properties [14]. Because of its

high resistivity, there is still drawback using NiO in composite materials for practical

sensing applications. To overcome this problem, many carbonaceous materials with

utmost electrical conductivity have been introduced to prepare carbon/NiO

nanocomposites [15].

Herein, we have decorated NiO nanoparticles on graphene sheets (GN) which

increase the resultant surface area. The material obtained GN/NiO nanocomposite is

also conductive with enhanced surface area which may be a suitable material for

making conducting nanocomposite with polyaniline for the purpose of gas sensing

studies.

A number of composites of polyaniline have been reported with metal oxides,

metal nanoparticles, graphene and carbon nanotubes widely used in gas sensing

applications [16]. In this work, polyaniline@graphene/nickel oxide (Pani@GN/NiO)

nanocomposite was studied as ammonia sensor by examining the dynamic response of

electrical conductivity using a simple 4-in line probe DC electrical conductivity

measurement set up.

Chapter 6

107

6.2. EXPERIMENTAL

6.2.1. Materials

Aniline, 99%, (E. Merck, India) and commercially available natural graphite powder

(Sigma Aldrich, USA) was used to prepare graphene oxide (GO). Sodium dodecyl

sulfate (SDS), ammonium persulphate (APS), sulphuric acid (H2SO4), hydrated

nickel nitrate (Ni(NO3)26H2O), sodium hydroxide (NaOH) and hydrazine hydrate

85% were obtained from the local suppliers.

6.2.2. Preparation of Pani, Pani/NiO, Pani/GN, Pani@GN/NiO

Polyaniline nanofibers were prepared through the interfacial polymerization reaction

in which aniline monomers (10 mmol) were dissolved in hexane (10 mL) under

stirring for 30 min. Ammonium persulphate (30 mmol) was gently added to 1M

sulphuric acid solution (10 mL) under constant magnetic stirring. The solution of

ammonium persulphate was added dropwise into aniline solution and the mixture was

allowed to react for 1 h. The final greenish reaction product was then filtered, washed

with double distilled water and then dried in an air oven at 70°C for 10 h.

In a typical synthesis procedure of Pani/NiO nanocomposite, aniline (2 mL)

was dissolved in 1 M H2SO4 (2 mL) in 90 mL water. Solution of 0.5 g of nickel oxide

(NiO) was prepared in accordance with existing literature [17] which was then added

to aniline solution. APS (5 g) dissolved in 1 M H2SO4 (10 mL) was added to the

aniline solution and the reaction mixture was stirred for 24 h at room temperature.

The final reaction product was then filtered, washed thoroughly with distilled water

and acetone until filtrate became colourless. The resulting product was dried at 70°C

for 10 h.

To prepare the Pani/GN nanocomposite, the graphene oxide (20 mg) was

prepared in accordance with existing literature [18] which was then dissolved in 1 M

H2SO4 (10 mL) with constant magnetic stirring for 1 h. Subsequently, aniline solution

(10 mmol) prepared in 1 M H2SO4 was mixed to the graphene oxide solution and

stirred for another 1 h. The APS (30 mmol) solution prepared in 1 M H2SO4 was then

added to the above solution. The final reaction mixture was kept under vigorous

stirring followed by conventional microwave treatment at 300 W for 30 min. The

Chapter 6

108

resulting suspension was separated by centrifugation and washed with double distilled

water and ethanol. The final product was dried at 70°C under vacuum for 12 h.

In synthesis of GN/NiO, as prepared GO (25 mg) was ultrasonically dispersed

in 10 mL of double distilled water for 1 h and 25 mL of Ni(NO3)26H2O (0.5 g)

aqueous solution was added into the above dispersion. After constant magnetic

stirring for 6 h, 1M NaOH (10 mL) aqueous solution was added into the above mixed

solution drop by drop. The pH value was adjusted to 10 by adding 10% ammonia

solution. After that, 10 mL of hydrazine hydrate (85%) was added to it. The reaction

mixture was stirred for several minutes before being treated with microwave for 30

min at 300 watt. The product was separated by centrifuge and washed with double

distilled water and ethanol. The product was dried in a vacuum oven at 50°C for 24 h

to evaporate the liquid. The dried sample was heated in N2 atmosphere at 300°C for 3

h to obtain GN/NiO.

The Pani@GN/NiO nanocomposite was prepared by mixing NiO/GN to

aniline monomer by weight ratio 5:1 in the precursors. The NiO/GN was dispersed in

ethanol and water (1:1 weight ratio) by stirring for 60 min and then added to aniline

solution prepared in 1 M H2SO4. An aqueous solution of ammonium persulphate (10

mL) prepared in 1 M H2SO4 was added dropwise to the mixture of aniline and

GN/NiO at 5-10°C and kept under constant stirring for 24 h. The resultant product

was washed several times by vacuum filtration using ethanol and double distilled

water. Final product was dried at 50°C for 12 h.


The morphology, structure and chemical composition of Pani, Pani/NiO, Pani/GN and

Pani@GN/NiO were investigated by a variety of methods. X-ray diffraction (XRD)

pattern were recorded by Bruker D8 diffractometer with Cu Kα radiation at 1.540 A˚.

Scanning electron microscope (SEM) studies were carried out by JEOL, JSM, 6510-

LV (Japan). Transmission electron microscope (TEM) studies were carried out by

using JEM 2100, JEOL (Japan). The Raman spectrum was taken using Varian FT-

Raman spectrometer. UV–Vis spectrum was recorded at room temperature using

Shimadzu UV–Vis spectrophotometer (Model 1601).

Chapter 6

109

DC electrical conductivity measurements were performed by using a 4-in-line

probe electrical conductivity measuring instrument with a PID controlled oven

(Scientific Equipment, Roorkee, India). The thermal stability of all as prepared

materials in terms of DC electrical conductivity under isothermal and cyclic ageing

conditions was also studied. The equation used in calculation of DC electrical

conductivity was:

σ = [ln2(2S/W)]/[2πS(V/I)] (6.1)




oC, 90

oC, 110

oC and 130

oC in


at an interval of 5 min in the accelerated ageing experiments. In testing of the stability

under cyclic ageing condition, DC conductivity measurements were taken 5 times

within the temperature range of 50-130oC.



X-ray diffraction (XRD) spectra of Pani, Pani/NiO, Pani/GN and Pani@GN/NiO

nanocomposites are shown in Fig 6.1. Fig. 6.1a exhibits the XRD spectrum of Pani

showed two characteristics peaks at 2θ=20.31° and 25.23°. The XRD spectrum of

Pani/GN has similar peaks as those for Pani slightly shifted to 2θ=20.45° and 25.27°

due to incorporation of graphene in Pani/GN nanocomposite as shown in Fig. 6.1b. In

Fig. 6.1c, there are five extra peaks along with characteristic peaks of Pani (at

2θ=20.25° and 25.34°) which are observed at 2θ=37.42°,43.35°, 62.86°, 75.26° and

79.42° due to presence of NiO nanoparticles in Pani/NiO nanocomposite.

The XRD spectrum of Pani@GN/NiO nanocomposite is similar to that of

Pani/NiO nanocomposite as shown in Fig. 6.1d. The peaks due to polyaniline at

2θ=20.05° and 25.69° respectively shifted from 2θ=20.31° and 25.23° that may be

attributed due to interaction of polyaniline with NiO embedded on graphene sheets.

The five intense peaks observed in Pani@GN/NiO nanocomposite at

2θ=37.13°,43.48°, 63.12°, 75.39° and 79.56° due to incorporation NiO deposited on

Chapter 6

110

graphene sheets with slight shift than those in Pani/NiO nanocomposite indicate the

successful synthesis of Pani@GN/NiO nanocomposite [19, 20].

Fig. 6.1 XRD patterns of: (a) Pani, (b) Pani/GN, (c) Pani/NiO and (d) Pani@GN/NiO.


Fig. 6.2 presents the SEM images of Pani and Pani@GN/NiO nanocomposite. The

scanning electron microscopy (SEM) images of the Pani@GN/NiO nanocomposite

demonstrated the sheets type morphology due to the presence of GN/NiO as shown in

Fig. 6.2a and 6.2b. However, the NiO nanoparticles deposited on GN sheets are not

visible here which indicates that the NiO nanoparticles deposited on GN sheets have a

nucleating effect on the polymerization of aniline and completely covered by

polyaniline shell. Fig 6.2c and 6.2d presents the SEM images of Pani which seems to

possess nanotubes like morphology.

mailto:Pani@GN/NiO%20nanocomposite.1

Chapter 6

111

Fig. 6.2 SEM micrographs of: Pani@GN/NiO nanocomposite (a and b) and Pani and

(c and d) at different magnifications.


The morphology of Pani@GN/NiO, Pani/NiO, Pani/GN and Pani was further

analyzed by transmission electron microscopy (TEM) as shown in Fig 6.3. In this

synthesis, as we deposited NiO nanoparticles on graphene sheet and then aniline

monomers were polymerized on the surface of GN/NiO. Fig. 6.3a showed the NiO

nanoparticles were posited on GN sheets upon which polyaniline nanotubes are

deposited signifying the successful incorporation of NiO nanoparticles onto graphene

sheets for a nanocomposite with polyaniline nanotubes. Fig. 6.3b shows that the NiO

particles are uniformly distributed in the outer shell of polymer matrix. In Fig. 6.3c it

may be observed that the short tubes of polyaniline are precisely deposited on the

graphene sheets. We also synthesized Pani using same procedure in the absence of the

graphene and NiO, short tubes of Pani were obtained as shown in Fig. 6.3d. In the

light of above observation of TEM analysis, it can be inferred that the NiO

nanoparticles were precisely deposited on graphene sheets and covered by polyaniline

nanotubes.

Chapter 6

112

Fig. 6.3 TEM micrograph of: (a) Pani@GN/NiO nanocomposite, (b) Pani/NiO

nanocomposite, (c) Pani/GN nanocomposite and (d) Pani.

6.4.4. Optical Studies

UV–vis absorption spectra of Pani@GN/NiO, Pani/NiO, Pani/GN and Pani are given

in Fig. 6.4. In Fig. 6.4a the band observed at 327 nm for pure Pani may be attributed

to * electronic transition of benzenoid rings [21]. In case of Pani/GN

nanocomposite, the band red shifted to 332 nm form 327 nm may be attributed to

interaction π-electrons of benzenoid rings with π-bonds of graphene as shown in Fig.

6.4b [22]. In Fig. 6.4c, Pani/NiO nanocomposite illustrate the two bands at 281 nm

and 323 nm which may be related to the electronic transition from the valence band to

the conduction band in the NiO and due to the * transition of the benzenoid

rings respectively [23, 24]. In case of Pani/NiO, the blue shift is observed in contrast

to that in Pani which may be attributed to decrease in conjugation of Pani by loading

of NiO in Pani, reducing the path of polarons in Pani.

From Fig. 6.4d it is illustrated that the Pani@GN/NiO nanocomposite

reflected the two maxima at 276 nm blue shifted in comparison with the Pani/NiO and

the other at 375 nm red shifted in comparison of Pani from 327 nm which may be

attributed to increase in extent of conjugation of Pani by forming efficient network by

electronic transition from NiO to graphene [25]. The significant red shift in

Chapter 6

113

Pani@GN/NiO nanocomposite supports the enhanced DC electrical conductivity due

to ease in movement of polarons by extended conjugation in Pani@GN/NiO.

Fig. 6.4 UV-Vis spectra of: (a) Pani, (b) Pani/GN nanocomposite, (c) Pani/NiO

nanocomposite and (d) Pani@GN/NiO nanocomposite.

6.4.5. Raman Spectroscopy

Fig. 6.5 shows the Raman spectra of Pani@GN/NiO, Pani/NiO, Pani/GN and Pani. In

Fig. 6.5a, the different modes of vibration are observed. The sharp peak at 1410 cm-1

may be associated with presence of C-N+ polarons [26]. The peaks observed at 1330

and 1570 cm-1

may be due to the presence of graphene sheets bands (D & G) and the

bands at 1370 and 1592 cm-1

may be due to 2M and 2P bands of NiO.

In Fig. 6.5b, the Raman spectrum of Pani/NiO displays two vibrational bands

at 1363 cm-1

and 1574 cm-1

may be due to presence of NiO nanoparticles that

correspond to the two-magnon (2M) and two-phonon (2P) models [27-29]. The Fig.

6.5c displays the Raman spectrum of Pani/GN nanocomposite showing the two bands

at 1345(D) cm-1

due to disordered structures in the graphene layers and 1576(G) cm-1

due to sp2 electronic configuration of graphitic carbon phase in Pani/GN

nanocomposite [30]. In Raman spectra of Pani, the bands at 1350 cm-1

and 1565 cm-1

Chapter 6

114

may be attributed to C-N+ and C=C mode of vibration respectively as shown in Fig.

6.5d.

The significant shifts of Raman bands of Pani@GN/NiO are observed in

comparison with Pani/GN and Pani/NiO nanocomposites. The D and G bands of

graphene in Pani/GN nanocomposite blue shifted from 1345 cm-1

and 1576 cm-1

to

1330 cm-1

and 1570 cm-1

respectively in Pani@GN/NiO nanocomposite while the 2M

and 2P bands of NiO in Pani/NiO are red shifted from 1363 cm-1

and 1574 cm-1

to

1370 cm-1

and 1592 cm-1

respectively. The substantial band shifts of graphene and

nickel oxide constituent in Pani/GN and Pani/NiO nanocomposites indicate strong

electrical interaction between NiO and GN sheets, favouring the charge transfer from

NiO to graphene sheets [25] and forming more holes in polyaniline. Thus increment

in electrical conductivity of Pani@GN/NiO also strongly support the interaction and

charge transportation between NiO and graphene.

Fig. 6.5 Raman spectra of: (a) Pani@GN/NiO nanocomposite, (b) Pani/NiO

nanocomposite, (c) Pani/GN nanocomposite and (d) Pani.

Chapter 6

115


The initial DC electrical conductivity of Pani, Pani/NiO, Pani/GN and Pani@GN/NiO

nanocomposites were measured by standard four-in-line probe method as shown in

Fig. 6.6a. The electrical conductivity of Pani was observed to be 1.832 S/cm. The

electrical conductivity of Pani/NiO nanocomposite prepared by the method mention

above was observed to be 0.853 S/cm which is slightly less than that of Pani. It may

be proposed that the movement of polarons of Pani get retarded due to the Coulombic

interaction between the polarons of Pani and lone pairs of oxygen of NiO as shown in

Fig. 6.6b. Although, the d-orbitals of Ni may increase the number of polarons in the

Pani chains by accommodating electrons from Pani, the Coulombic interaction seems

to be dominating factor. Shambharkar and Umare [20] also reported that the electrical

conductivity of Pani/NiO nanocomposite decreases with low NiO content in the

nanocomposite.

The initial electrical conductivity at room temperature of as-prepared Pani/GN

nanocomposite was observed to be 9.2 S/cm which was much higher than that of Pani

(1.2 S/cm). It may be proposed that the charge carriers hop from Pani to GN where

they gain high mobility along the π-conjugated system of GN as shown in Fig. 6.6c.

The overall mobility of charge carriers thus enhanced due to unobstructed movement

along the graphene nanosheets leading to increase in electrical conductivity of

Pani/GN nanocomposite. Ansari et. al [31] also reported that sulphonated-Pani/GN

nanocomposite exhibited high electrical conductivity due to π-π interaction between

sulphonated-Pani and GN.

In case of Pani@GN/NiO, the electrical conductivity was observed to be 11.34

S/cm which was the highest among all of the samples prepared. It seems that the NiO

nanocomposites act as a bridge material between Pani and GN which helps in

movement of charge carrier from Pani to GN and thus increase electrical conductivity

due to greater mobility of charge carriers along the π-conjugated system of GN

nanosheets as shown in Fig. 6.6d. Thus, NiO bridges additionally facilitate hopping

movement of charge carriers between Pani and GN in Pani@GN/NiO

nanocomposites.

Chapter 6

116

Fig. 6.6 (a) Initial DC electrical conductivities of Pani, Pani/NiO, Pani/GN and

Pani@GN/NiO nanocomposites. The mechanism of conductivity behaviour of:

(b) Pani/NiO, (c) Pani/GN and (d) Pani@GN/NiO.


The stability of Pani, Pani/NiO, Pani/GN and Pani@GN/NiO nanocomposites in

terms of DC electrical conductivity was studied under isothermal ageing conditions as

shown in Fig. 6.7. The relative electrical conductivity at particular time was

calculated by using the equation:

(6.2)

where σt = electrical conductivity at time t and σo = electrical conductivity at time

zero at experimental temperature. The as-prepared materials were examined for the

stability in terms of DC electrical conductivity with respect to time at different

temperatures. The electrical conductivity of each of the samples was measured at the

temperatures (50, 70, 90, 110, 130°C) versus time at an interval of 5 min upto 20 min.

From Fig. 6.7a and Fig. 6.7b, it may be observed that Pani and Pani/NiO are fairly

Chapter 6

117

stable upto 90°C. While the Pani/GN and Pani@GN/NiO nanocomposites are stable

upto 110°C in terms DC electrical conductivity as shown in Fig. 6.7c and Fig. 6.7d.

Thus by the above observation, it is inferred that relative electrical conductivity of

Pani@GN/NiO and Pani/GN nanocomposites showed greater stability than Pani and

Pani/NiO nanocomposite under isothermal ageing conditions.

Fig. 6.7 Relative electrical conductivity of: (a) Pani, (b) Pani/NiO, (c) Pani/GN and

(d) Pani@GN/NiO under isothermal ageing conditions.


The stability in the term DC electrical conductivity retention of Pani, Pani/NiO,

Pani/GN and Pani@GN/NiO nanocomposites was also studied by cyclic ageing

conditions within the temperature range of 50 to 130°C as shown in Fig. 6.8. The

relative electrical conductivity (σr) was calculated using the following equation:

(6.3)

where σT is electrical conductivity at temperature T (°C) and σ50 is electrical

conductivity at 50°C i.e. at the beginning of each cycle.

Chapter 6

118

Fig. 6.8 Relative electrical conductivity of: (a) Pani, (b) Pani/NiO, (c) Pani/GN and

(d) Pani@GN/NiO under cyclic ageing conditions.

The electrical conductivity was recorded for successive cycles and observed to

be increased gradually for each of the cycle with a regular trend in all these cases may

be due to the increment of number of charge carriers as polarons or bipolarons at

elevated temperatures. From Fig. 6.8a, it can be shown that the relative electrical

conductivity of Pani following the same trend for each of the cycle with gain in the

conductivity as temperature increases. Also, the relative electrical conductivity of

Pani/NiO, Pani/GN and Pani@GN/NiO nanocomposites increased as temperature

start to increase from 50 to 130°C as shown in Fig. 6.8b, Fig. 6.8c and Fig. 6.8d.

In all of these nanocomposites, the Pani@GN/NiO nanocomposite showed

lowest gain in conductivity with less deviation. Thus, it may be inferred that the

Pani@GN/NiO nanocomposite is more stable semiconductor than Pani and among all

these nanocomposites under cyclic ageing conditions. The difference observed may

be attributed to the removal of moisture, excess HCl or low molecular weight

oligomers of aniline [32].

6.6. SENSING

The DC electrical conductivity responses of the sensors based on Pani, Pani/NiO,

Pani/GN and Pani@GN/NiO nanocomposites were measured on exposure to

ammonia (NH3) at room temperature (~25°C) as shown in Fig. 6.9a. The pellet

Chapter 6

119

fabricated was attached with four probes and placed in a closed chamber of ammonia

vapours. The electrical conductivity immediately decreases on exposure to ammonia

for 60 s and rapidly reverts back in ambient air in next 60 s in all of the as-prepared

materials. However, the greatest change in electrical conductivity was observed for

Pani@GN/NiO nanocomposite. The conductivity change in 60 s was observed to be

highest for Pani@GN/NiO nanocomposite. Therefore the Pani@GN/NiO

nanocomposite was also tested for different concentration of ammonia as shown in

Fig. 6.9b.

The highest change in electrical conductivity was observed for 0.1 M

ammonia. As the concentration of ammonia increases from 0.01 M to 0.1 M, the more

polarons of polyaniline get neutralized by the lone pair of electrons of ammonia

molecules thus the more electrical conductivity decrease. The electrical conductivity

change of the Pani@GN/NiO nanocomposite could be observed on exposure to

ammonia as low as 0.01M concentration.

Fig. 6.9 (a) Effect on the DC electrical conductivity of Pani, Pani/NiO, Pani/GN and

Pani@GN/NiO nanocomposites on exposure to ammonia vapours followed by

exposure to ambient air with respect to time, (b) Effect on the DC electrical

conductivity of Pani@GN/NiO nanocomposite on exposure to ammonia vapours at

different concentrations.

The sensitivity and reversibility are the two important parameters for a gas

sensor which were defined as the time to revert to the final value after exposure to

vapours and ambient air respectively. The reversibility of Pani and Pani@GN/NiO

Chapter 6

120

nanocomposite were measured in terms of the DC electrical conductivity. The

reversibility of both the samples was determined by first keeping sample in 0.1 M

ammonia vapours for 30 sec followed by 30 sec in air for a total duration of 150

seconds. Fig. 6.10a represents the reversibility response of Pani in terms of the DC

electrical conductivity with variation range from 1.849 to 1.812 S/cm in the absence

and presence of ammonia vapours respectively. The change in conductivity observed

was 0.037 S/cm.

While in case of Pani@GN/NiO nanocomposite the reversibility showed with

excellent variation range than Pani which is from 11.146 S/cm to 7.489 S/cm in the

absence and presence of ammonia vapours respectively as shown in Fig. 6.10b. In this

case, there is observed decrease in conductivity by 3.657 S/cm and it seems to be very

much efficient than Pani in terms of reversibility. There was around about 99 times

greater variation range observed in conductivity of Pani@GN/NiO nanocomposite

than that of Pani.

Fig. 6.10 Variation in electrical conductivity of (a) Pani and (b) Pani@GN/NiO on

alternate exposure to ammonia vapours and ambient air.

For a worthy and applicative gas sensor, high selectivity is also an important

requirement. The conductivity response of the Pani@GN/NiO nanocomposite to

ammonia, isopropanol, methanol, ethanol, acetone, acetaldehyde and formaldehyde

(VOCs) at room temperature (26°C) are shown in Fig. 6.11. It was observed that the

mailto:Pani@GN/NiO%20nanocomposite%20to%200.1

Chapter 6

121

conductivity response of Pani@GN/NiO nanocomposite to 0.1 M ammonia was 9.8,

6.2, 6.6, 7.9, 6.5 and 6.1 times higher than that for isopropanol, methanol, ethanol,

acetone, acetaldehyde and formaldehyde respectively.

Such selectivity results from the difference of their sensing mechanism i.e. the

neutralization of polarons of Pani that plays an important role in changing electrical

conductivity on exposure to VOCs. The more lone pairs of gas are available, the

greater conductivity change is observed. As ammonia has more electron donating

nature and thus greater neutralize of more number of polarons of Pani. While in other

VOCs used in sensing process, there are interactions from oxygen atom which has

more –I effect than nitrogen atom. So the electron donating tendency decreased thus

leading to low conductive change response. This indicates the high selectivity of

Pani@GN/NiO nanocomposite toward NH3.

Fig. 6.11 Selectivity of Pani@GN/NiO nanocomposite to 0.1 M of different VOCs in

water.

The advancement of gas sensing properties of Pani@GN/NiO may be

attributed to its large surface area, which anchored the more adsorption and

desorption of gaseous molecules improving their sensing performance and high

polarons mobility of polyaniline at room temperature, which is very essential for rapid

response of any sensor [13].



Chapter 6

122

It is expected that the NiO embedded on graphene sheets interacts with lone

pairs of nitrogen atoms of polyaniline thus increasing the more number of polarons in

polyaniline, creating the more number of active sites with large surface area for the

interaction of lone pairs ammonia molecules to polarons of polyaniline. However,

detailed understanding for the role of NiO and graphene sheets in the sensing

mechanism of Pani@GN/NiO nanocomposite is still lacking.


The sensing by Pani@GN/NiO nanocomposite is based on the decrease in electrical

conductivity on exposure to analyte vapours and to revert on exposure to ambient air.

Therefore the different aspects of emergence of high electrical conductivity in the

nanocomposite have been discussed in above paragraph (Paragraph 5) in order to

understand the sensing behaviour of the Pani@GN/NiO nanocomposite. The sensing

mechanism of Pani@GN/NiO nanocomposite was explained through DC electrical

conductivity response by simply adsorption and desorption mechanism of ammonia

vapours at room temperature as shown in Scheme 6.1.

In Pani@GN/NiO nanocomposite, NiO nanoparticles are embedded on

graphene sheets gets interacted with lone pairs of nitrogen atoms of polyaniline thus

the number of polarons and bipolarons increased in polyaniline chain. In presence of

ammonia vapours, the lone pairs of ammonia molecules gets interacted with polarons

of Pani@GN/NiO nanocomposite which inpede the mobility of polarons leading to

decrease in electrical conductivity. When Pani@GN/NiO exposed to ambient air the

ammonia molecules are desorbed from the surface and thus electrical conductivity

reverted to its initial value. Thus the simple adsorption and desorption of ammonia

vapours on the Pani@GN/NiO nanocomposite governs the mobility of polarons.

The selective response of Pani@GN/NiO nanocomposite towards ammonia

may be due to its highly basic nature which readily gets interacted with polarons of

polyaniline. While all the others VOCs tested are much less basic than ammonia

because they contain oxygen atom which is more electronegative than nitrogen so the

electronic interaction with polyaniline in these compounds is comparatively poorer.

Chapter 6

123

Scheme-6.1 Proposed mechanism of interaction of ammonia with Pani@GN/NiO

nanocomposite.

CONCLUSIONS

The Pani, Pani/NiO, Pani/GN, Pani@GN/NiO were successfully prepared and

characterized by XRD, SEM, TEM, UV-vis and XRD analysis. Pani@GN/NiO

nanocomposite showed highest thermal stability in terms of DC electrical


results highlighted that the Pani@GN/NiO nanocomposite found to be excellent

material for ammonia sensing with excellent selectivity against VOCs and rapid

response. There was about ninety-nine times greater variation in electrical

conductivity of Pani@GN/NiO nanocomposite during sensing than that of Pani. The

enhancement of sensing properties in Pani@GN/NiO nanocomposite may be

attributed to the synergistic effect between NiO, GN and Pani. Therefore, the sensor

based on Pani@GN/NiO may be useful, efficient and promising material for ammonia

vapours sensing.

Chapter 6

124

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http://dx.doi.org/10.1039/1364-5501/1991

http://dx.doi.org/10.1039/1364-5501/1991

http://www.sciencedirect.com/science/article/pii/S0167577X13013426






Chapter 7

126

7.1. OVERVIEW

In summary, the nanocomposite and composites of polyaniline were synthesized and

studied with the aim of their relevance and applications in the field of nanoscience

and technology. Further, these prepared products were characterized by FTIR, XRD,

SEM, TEM, TGA etc. along with the investigation of their electrical properties. The

results of these instrumental techniques and surface morphology approved the

formation Pani and their composites depending upon type of fillers applied in

different cases.

From the outcome of the measured electrical conductivity, the as prepared

composites and nanocomposites of Pani with different materials have shown

noticeable enhancement in their electrical conductivity and photocatalytic activity as

compared with polyaniline. The very simple 4-probe instruments were applied to

examine their DC electrical conductivity. It may be noted that the significant

enhancement in electrical conductivity and thermal stability will be useful for the

manufacturing of low cost, light weight electronic/electrical devices. It was also

observed that there is slight shift in FTIR and XRD peaks in composites and

nanocomposites of Pani. This shifting may be attributed to the interaction of Pani with

filler material. This strong interaction function at molecular level also contributes to

significant enhancement in electrical conductivity of the resultant hybrids. High

electrical conductivity, photocatalytic activity and gas sensing properties in the as

prepared composites and nanocomposites with Pani were also observed.

At the end, the embodied work in the present thesis has high value and greater

scope for the fabrication of electrically conducting sensing devices and photocatalyst.

7.2. DIRECTIONS TOWARDS FUTURE WORK

The most encouraging and promising task is the modification of inorganic/organic

nanomaterials and fabrication of devices with polyaniline. For this purpose, the fillers

like SF, BN, SiC and graphene with some functional parts attached with them are

being embedded into polyaniline matrix. This will attract the future work on the

incorporation newer nanomaterials for increasing electrical, magnetic and adsorption

capacities of the conducting polymers.

Chapter 7

127

In the age of global warming, there is an urgent need for the fabrication of the

sensing devices which can detect the minute concentration of harmful gases to

minimize the risk of health hazards. Further, the functionalized nanocomposite

materials with their distinctive electrochemical behaviour and versatile chemical

reactivity are highly expected for the emergence of new technologies and applications

in the range of batteries, sensors, microelectronics and photocatalyst. Finally, it would

be exciting to assume that the most conspicuous applications may cause the

commercialization of these novel polyaniline nanocomposite materials in the near

future.

Boron nitride based polyaniline nanocomposite: Preparation,property, and application

Sharique Ahmad,1 Adil Sultan,1 Waseem Raza,2 M. Muneer,2 Faiz Mohammad1

1Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 202002, India2Department of Chemistry, Faculty of Science, Aligarh Muslim University, Aligarh 202002, IndiaCorrespondence to: F. Mohammad (E - mail: [email protected])

ABSTRACT: In this study, we report first time the electrical properties and photocatalytic activity of HCl doped polyaniline (Pani) and

Pani/boron nitride (Pani/BN) nanocomposite prepared by in-situ polymerization of aniline using potassium persulfate (K2S2O8) in

the presence of hexagonal boron nitride (h-BN). The prepared Pani and Pani/BN nanocomposite were characterized by Fourier trans-

form infrared, X-ray diffraction, Thermogravimetric analysis, Scanning electron microscope, and Transmission electron microscope.

The stability of the Pani/BN nanocomposite in comparison of Pani in terms of the DC electrical conductivity retention was investi-

gated under isothermal and cyclic aging conditions. The Pani/BN nanocomposite in terms of DC electrical conductivity was observed

to be comparatively more thermally stable than Pani. The degradation of Methylene blue (MB) and Rhodamine B (RhB) under UV-

light irradiation were 50 and 56.4%, respectively, over Pani and 65.7 and 71.6%, respectively, over Pani/BN. The results indicated that

the extent of degradation of MB and RhB was greater over nanocomposite material than Pani, which may result due to high elec-

tron–hole pairs charge separation under UV light. VC 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43989.

KEYWORDS: catalysts; composites; conducting polymers

Received 4 February 2016; accepted 25 May 2016DOI: 10.1002/app.43989

INTRODUCTION

Since the discovery of the first conducting polymer (polythia-

zyl),1 such polymers have generated humongous interest due to

their unique properties such as thermal stability, photocatalytic

activity, electrical conductivity, etc.2 These properties of con-

ducting polymers furnished excellent potential for their applica-

tions in various fields such as in the manufacturing of cheap

electronic devices,3–5 television and other audio-video equip-

ments6,7 Polyaniline (Pani) is the most studied polymer among

the different types of conducting polymers because of its easy

synthesis, low cost of monomer, exclusive chemical properties

and excellent environmental stability.8–10 Moreover, it exists in

three oxidation states leucoemeraldine, pernigraniline, and

emeraldine. Among these, emeraldine salt form has highest con-

ductivity11 in the order of 1 Scm21 and can be used in various

applications due to reversible doping (conductive) and dedop-

ing (nonconductive) mechanism such as in chemical and bio-

logical sensors12–15 and actuators16 etc. Hexagonal boron nitride

(h-BN), a structural analogue of graphite having good thermal

transport properties.17,18 Among all other BNs, h-BN is the

most preferred polymorph in which boron and nitrogen atoms

form planar conjugated layers.19 BNs have various unique prop-

erties such as hardness, thermal stability, high melting point,

good corrosion resistance, good dielectric breakdown strength,

resistance to oxidation, and chemical inertness. There are many

applications, for example, optical storage, medical treatment,

photocatalytic reactions, and electrical insulation20,21 in which

BNs used. Wang et al. reported22 that h-BN/SnO2 material

showed good photocatalytic activity was mainly due to its suita-

ble band gap energy, strong adsorption ability for methyl

orange, and effective charge separation at the h-BN/SnO2 pho-

tocatalyst interface. Sadia et al.23 synthesized the Pani/graphene

nanocomposite that delivered a significant degradation of RB

(rose Bengal) dye by �56% within 3 h under UV–vis light.

They observed that the heterojunctions are formed in the space

charge separation region at the Pani/graphene interfaces and

suggested that graphene may accept the photoexcited electrons

from Pani. Similarly to graphene, h-BN may be a promising

material of photocatalytic nanocomposite with Pani. To the best

of our knowledge, the photocatalytic degradation of RhB and

MB dyes over the surface of Pani/BN nanocomposite as photo-

catalyst has not been reported elsewhere. Herein, we have tried

to look forward our effort for enhancement of photocatalytic

activity of Pani by using BN under UV-light. The h-BN nano-

sheets have large surface area, which may have significantly

increased the adsorption of dyes and photoinduced charge

VC 2016 Wiley Periodicals, Inc.

WWW.MATERIALSVIEWS.COM J. APPL. POLYM. SCI. 2016, DOI: 10.1002/APP.4398943989 (1 of 9)

transfer along the h-BN sheets in nanocomposite. A conducting

nanocomposite of Pani with h-BN (Pani/BN) was prepared via

In-situ polymerization method. The photocatalytic degradation

of MB and RhB dyes have been studied over the surface of the

prepared Pani/BN nanocomposite which efficiently degraded

both of the dyes by 65.7% (MB) and 71.6% (RhB). The struc-

ture and surface morphology were investigated. The thermal sta-

bility was also investigated in terms of DC electrical

conductivity retention under isothermal and cyclic accelerated

aging conditions.

EXPERIMENTAL

Materials

In the preparation of Pani and Pani/BN nanocomposite the

main chemicals used were aniline, 99% (E.Merck, India), hydro-

chloric acid, 35% (E. Merck, India), h-BN (MK Nano, Canada),

potassium persulfate (E.Merck, India) and methanol. Double


Preparation of Pani and Pani/BN Nanocomposite

The nanocomposite of Pani/BN was prepared by in situ oxida-

tive polymerization of aniline in the presence of h-BN using

potassium persulfate as an oxidizing agent. At first 10 wt % h-

BN was ultrasonicated for 3 h in 100 mL of 1 M HCl then it

was added to aniline solution dropwise under constant stirring

for 1 h to enable the proper dispersion of h-BN in aniline. Oxi-

dant solution was then added dropwise in the above dispersed

solution of h-BN and aniline to polymerize the aniline. As the

oxidant, was added the color of the reaction mixture starts

changing from light purple to dark green within 20 min of stir-

ring. The stirring was kept continuous for 20 h. The final dark

green colored reaction mixture was then filtered, washed with

double distilled water and methanol to remove excess acid,

potassium persulfate, and Pani oligomers until filtrate became

colorless and neutral. Thus prepared Pani/BN nanocomposite

was dried at 80 8C for 12 h in an air oven, crushed into fine

powder and was stored in desiccator for further investigations.

Pani was also prepared using the same method as described

above in absence of h-BN. The as prepared materials were des-

ignated as Pani and Pani/BN.

Photodegradation Experiment

The photocatalytic oxidation of Rhodamine B (RhB) and Methyl-

ene Blue (MB) solution was done under UV-light illumination to

evaluate the photocatalytic activity of the Pani and Pani/h-BN

nanocomposite under constant stirring and bubbling of atmos-

pheric oxygen. An immersion well photochemical reactor made

of Pyrex glass with a 250 mL working volume of dyes equipped

with magnetic stirring bar and an opening for supply of molecu-

lar oxygen was used for all the experiments. A 125-W medium

pressure mercury lamp was used as the UV-light source. During

all the experiments, the temperature of aqueous solutions of dyes

was maintain �20 6 0.5 8C by refrigerated water circulation to

prevent the heating of the solutions by radiations emitted by the

UV-lamp (IR and short wave length). The radiation intensity fall-

ing on the solutions was measured using UV light intensity

detector (Lutron UV-340) and found to be 3.22 mW/cm2. Typi-

cally, 250 mg Pani and Pani/h-BN nanocomposite was added

into 250 mL aqueous solution of RhB (8 ppm) and MB (10

ppm), respectively. The solution was continuously stirred in the

dark for 30 min to ensure the establishment of adsorp-

tion2desorption equilibrium between the photocatalyst and the

dyes prior to irradiation.24 Then the solution was exposed to UV

light. During irradiation, sample was withdrawn from reactor

using a syringe at regular intervals and centrifuged before the

measurement to remove the powder photocatalyst. The decolor-

ization of dye was measured by using UV–vis spectrophotometer

(Perkin Elmer k 5 35) at the kmax 5 553 and 663, respectively,

based on Beer Lambert Law.25 The degradation efficiency of dyes

was calculated by using eq. (1).

Degradation % 5 A02At=A0 3 100% (1)

where A0 and At are the concentrations of sample at time “0”

and time “t.”

CHARACTERIZATIONS

The Fourier transform infrared spectroscopy (FTIR) spectra

were recorded using Perkin-Elmer 1725 instrument. X-ray dif-

fraction (XRD) pattern were recorded by Bruker D8 diffractom-

eter with Cu Ka radiation at 1.540 A˚. Scanning electron

microscope (SEM) studies were carried out by JEOL, JSM,

6510-LV (Japan). Transmission electron microscope (TEM)

studies were carried out by using JEM 2100, JEOL (Japan).

Thermogravimetric analysis (TGA) was done by using a Perkin

Elmer instrument in the temperature range from 35 to 800 8C.

The thermal stability of Pani and Pani/BN nanocomposite

under isothermal and cyclic ageing conditions was studied in

terms of DC electrical conductivity retention. For this study a

four-in-line probe with a temperature controller, PID-200 (Sci-

entific Equipments, Roorkee, India) was used to measure the

DC electrical conductivity and its temperature dependence. The

equation used in calculation of DC electrical conductivity was

r 5 ln2 2S=Wð Þ½ �= 2pS V=Ið Þ½ � (2)

where I, V, W, and S are the current (A), voltage (V), thickness

of the pellet (cm), and probe spacing (cm) respectively and r is

the conductivity (S/cm).2 In testing of isothermal stability, the

pellets were heated at 50, 70, 90, 110, and 130 8C in an air oven

and the DC electrical conductivity was measured at particular

temperature at an interval of 10 min in the accelerated ageing

experiments. In testing of the stability under cyclic ageing con-

dition, DC conductivity measurements were taken five times at

an interval of about 90 min within the temperature range of

50–150 8C.

RESULT AND DISCUSSION

Mechanism of Preparation of Pani and Pani/BN

Nanocomposite

The mechanistic view of the polymerization process seems to

involve the anilinium cations (phenyl-NH13 ) getting hooked up

by Coulombic attraction between anilinium cations and the sur-

face of h-BN. Thus the h-BN gets completely surrounded by

anilinium cations. This arrangement comes in contact with

K2S2O8, the anilinium cations get polymerized on the surface of

h-BN forming Pani (emeraldine salt). h-BN has partially polar

bonds,26 which may cause interaction with polarons and lone

pairs of Pani, as mentioned in our previous work with

ARTICLE WILEYONLINELIBRARY.COM/APP


polypyrrole/BN nanocomposite.27 The Pani (emeraldine salt)

formed on the surface of h-BN get attached by Coulombic

attraction between the positive charge on nitrogen of Pani and

negative charge on boron of h-BN and also the interaction

between lone pair of nitrogen of Pani (emeraldine salt) and pos-

itive charge on nitrogen of h-BN. Schematic presentation of

coulombic attraction between the 1ve and 2ve charges is given

in Figure 1.

FTIR Spectroscopic Study

FTIR spectra of the Pani/BN nanocomposite and Pani are

shown in Figure 2. The characteristic peak of Pani at

3434 cm21 corresponds to NAH stretching vibration. The peak

at 1571 and 1489 cm21 is due to C@C stretching mode of the

quinoid and benzenoid rings, respectively, and the peak at

about 1287 cm21 can be assigned to CAN stretching. The peak

at 790 cm21 is usually assigned to an out-of-plane bending

vibration of CAH, which confirmed the formation of Pani.28 In

the Pani/BN nanocomposite, there are the two more peaks situ-

ated at 1375 and 792 cm21, respectively. The absorption band

at 1375 cm21 can be attributed to the in-plane B-N stretching,

and the absorption band at 792 cm21 belongs to the B–N–B

out-of-plane bending vibration which is assumed to overlap

with CAH out of plane bending vibration.29 While the remain-

ing peaks are similar to that of Pani but slightly shifted to

higher wavenumber with reduced intensity indicative of strong

interaction between Pani and h-BN.

XRD Studies

The XRD patterns of Pani and Pani/BN nanocomposite are

shown in Figure 3. Figure 3(b) shows the broad peak at

2u 5 25.298 may be attributed to Pani. In the Figure 3(a) shows

XRD pattern of Pani/H-BN nanocomposite, a sharp peak

observed at 2u 5 25.788, which suggested that the interaction

between h-BN and Pani. In Pani/BN nanocomposite, highly

crystalline nature of h-BN and amorphous nature of Pani has

been merged and shifted from 25.298. The other peaks around

at 2u 5 40.238, 42.838, and 49.258 in the pattern of Pani/BN

may be attributed to h-BN.30

Thermogravimetric Analysis

Figure 4 represents the TGA thermograms of Pani and Pani/BN

nanocomposite, which shows the three step weight loss process.

In the case of Pani, there are three major stages of weight loss;

first around 120 8C, second in range from 130 to 400 8C and

third in range from 400 to 550 8C due to the loss of water,

removal lower oligomers of Pani and thermo-oxidative decom-

position of Pani, respectively.31 It is found that the degradation

of nanocomposite is quite similar to that of Pani. The observ-

able difference is the higher thermal stability of Pani/BN nano-

composite. The decomposition of Pani/BN started at around

Figure 1. Schematic presentation of Coulombic attraction between Pani and h-BN in Pani/BN nanocomposite. [Color figure can be viewed in the online

issue, which is available at wileyonlinelibrary.com.]

Figure 2. FTIR spectra of: (a) Pani/BN nanocomposite and (b) Pani.

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

wileyonlinelibrary.com.]

Figure 3. XRD patterns of: (a) Pani/BN nanocomposite and (b) Pani.





492 8C, which is at much higher temperature than in Pani

(�475 8C). This enhanced stability may be due to high thermal

stability of h-BN that has strong Coulombic attraction with

Pani.

SEM Studies

The SEM of Pani and Pani/BN nanocomposite are shown in

Figure 5 at different magnifications. The SEM micrograph in

Figure 5(a) shows the sheet like structure of h-BN. In Figure

5(b) SEM image of Pani shows short tubes along with flakes

like structure. The SEM images in Figure 5(c,d) represent the

Pani/BN, in which the polymer matrix is well enwrapped on h-

BN with uniform dispersion and some sheet like morphology

may also be seen. This suggests that h-BN acted as sheet on

which polymerization took place and facilitated in the forma-

tion of some sheet like structures.

TEM Studies

TEM micrograph of Pani/BN nanocomposite is shown in Figure

6. From the Figure, it is observed that the light gray sheet type

structure seem to BN and dark gray seem to Pani, which is

enwrapped on the BN nanosheets. Thus, it may be said that

aniline underwent polymerization on the surface of h-BN giving

the sheet type structure.

Figure 4. TGA thermograms of: (a) Pani and (b) Pani/BN nanocomposite.



Figure 5. SEM micrographs of: (a) h-BN, (b) Pani, and (c, d) Pani/BN nanocomposite at different magnification. [Color figure can be viewed in the

online issue, which is available at wileyonlinelibrary.com.]

Figure 6. TEM micrograph of Pani/BN nanocomposite. [Color figure can

be viewed in the online issue, which is available at wileyonlinelibrary.

com.]



PHOTOCATALYTIC STUDIES

The photocatalytic activity of Pani and Pani/BN nanocomposite

was investigated by the decolorization of RhB and MB in aque-

ous solution which is discarded by textile industries under UV-

light illumination. The controlled experiment indicates that

RhB and MB were resistant towards the degradation under UV-

light irradiation without a photocatalyst. However, little decom-

position of dyes takes place in the presence of prepared photo-

catalyst in dark due to adsorption of dyes on the surface of

catalysts. The RhB showed good adsorption on the surface of

synthesized photocatalyst. The result indicates that both light

and catalyst were required for efficient photocatalytic degrada-

tion. The decomposition of the dyes was monitored by meas-

uring the change in the absorbance at their (kmax 663 and 553)

as a function of irradiation time. Figure 7(a,b) showed that 50

and 56.4% decolorization of MB and RhB dyes take place,

respectively, after 90 min of irradiation time over Pani. Figure

8(a,b) indicates the 65.7 and 71.1% degradation of MB and

RhB, respectively, after 90 min as a function of irradiation time

in the presence of Pani/BN nanocomposite material. The results

of Figures 7 and 8 illustrate that the main peaks of both the

dyes (553 and 663 nm) decreases gradually as irradiation time

increase. The color of the dyes solution became lighter as the

irradiation time increased due to gradually degradation of chro-

mophoric groups present in the dyes.32,33 The Figures 7 and 8

displayed the decolorization of MB was found to be lower than

RhB due to presence of stable and bulky aromatic rings, which

suppress the interaction between catalysts and dye.34 The

adsorption of MB was lower than RhB on the surface of catalyst

which decrease photocatalytic degradation of MB. Figure 9

showed the percentage degradation of dyes under UV-light illu-

mination over Pani and Pani/BN nanocomposite. It was found

that the degradation of RhB was more than that of MB by

hydroxyl radicals. It can be attributed due to absorption of less

UV-light by RhB than MB.35 Hence more photons were avail-

able to photocatalyst, which raised the formation of hydroxyl

radicals. The degradation of both dyes was found to be higher

over Pani/BN nanocomposite material than Pani. The Pani

Figure 7. (a, b). Photocatalytic degradation of MB and RhB dyes, respectively, at different time intervals in the presence of Pani. [Color figure can be

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

Figure 8. (a, b) Photocatalytic degradation of MB and RhB dyes, respectively, at different time intervals in the presence of Pani/BN nanocomposite.

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



interacted with h-BN nanosheets that substantially increased the

surface area of the nanocomposite, therefore more degradation

of both dyes over the surface of Pani/BN nanocomposite than

Pani. It may be also due to the electron transfer from excited

Pani to h-BN and further across nanocomposite interface, which

leads to formation of trapping sites by h-BN, which increase the

charge separation by splitting the arrival time of photogenerated

electron and hole to reach the surface of photocatalyst and thus

decrease the electron-hole recombination rate.36–38 The photo-

catalytic activity of Pani/BN nanocomposite was also investi-

gated for waste water treatment by taking sewage from

department of chemistry, Aligarh Muslim University, India.

Figure 10 indicates 53% degradation of waste water after 90

min as a function of irradiation time in the presence of Pani/

BN nanocomposite. Thus Pani/BN nanocomposite found to be

a good photocatalyst also for waste water treatment.

Possible Mechanism

The possible mechanism for the degradation of dye using Pani/

BN nanocomposite can schematically be represented in Figure

11. Pani is a typical semiconducting polymer with an extended

p-electron conjugation system. Pani serves as a good photocata-

lyst for degradation of pollutants under UV light irradiation

due to its electronic structure characterized by a filled valence

band (HOMO) and an empty conduction band (LUMO). On

absorption of photons that match or exceed the band gap

energy of Pani, an electron may be promoted from the valence

band to the conduction band leaving behind an electron

vacancy or “hole” in the valence band.39 In Pani/BN nanocom-

posite the charge separation is maintained by transferring of

electrons from Pani to h-BN through nanocomposite interface

which decreased the electrons and holes recombination. Thus

electrons and holes can migrate to the catalyst surface where

they participate in redox reactions with adsorbed dyes. Specially,

the holes generated in the valence band (h1VB) can react with

surface bound H2O molecules to produce hydroxyl radicals and

Figure 10. Photocatalytic treatment of waste water at different time inter-

vals in the presence of Pani/BN nanocomposite. [Color figure can be

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

Figure 11. Schematic presentation of probable degradation process of MB

and RhB over Pani/BN nanocomposite. [Color figure can be viewed in

the online issue, which is available at wileyonlinelibrary.com.]

Figure 9. (a) Percentage degradation of MB and RhB dyes in aqueous solution as a function of time in the presence and absence of Pani and presence

and absence of UV-light, (b) Percentage decomposition of both dyes in aqueous solution as a function of time in the presence and absence of Pani/BN

and presence and absence of UV-light. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]



the electrons present in the conduction band (e2CB) are picked

up by oxygen to generate superoxide radical anions.25,39

Pani=BN��!ðUV-radiationsÞ

e2CB1 h1

VB (3.1)

O21e2CB ��! O2

:2 (3.2)

H2O 1 h1VB ��! H11•OH (3.3)

O212ðe2CBÞ 1 2H1 ��! H2O2 (3.4)

e2CB1 O2

•21 2H1 ��! •OH 12OH (3.5)

The superoxide radical anions act as strong reducing agent and

hydroxyl radical act as strong oxidizing agent and degrade the

pollutant dyes to the mineral end products.

h1VB 1 Dye ��! Degraded products 1 H2O 1 CO2

(3.6)

•OH 1 Dye ��! Degraded products 1 H2O 1 CO2

(3.7)

ELECTRICAL CONDUCTIVITY

The electrical conductivities of Pani and Pani/BN nanocompo-

site were measured by standard four-in-line probe method.

From the measured electrical conductivity it may be inferred

that both the as prepared materials are semiconducting in

nature and the addition of h-BN to the Pani has a significant

effect on its electrical conductivity. The measured electrical con-

ductivities of Pani and Pani/BN nanocomposite were 0.0622

and 0.045 S/cm, respectively, as shown in Figure 12. Thus it is

observed that the electrical conductivity decreased after loading

of BN. The BN, having insulating behaviour may induce the

formation of less efficient network for charge transport in the

Pani chains leading to the less electrical conductivity than Pani.

The reason for decrease in electrical conductivity of Pani/BN

nanocomposite may also be due to interaction of boron (2ve

charge) and nitrogen (1ve charge) atoms of h-BN with polar-

ons/bipolarons of Pani as presented in Figure 1. This causes the

loss of mobility of charge carriers leading to reduced electrical

conductivity in Pani/BN nanocomposite.

Stability under Isothermal Ageing

The stability of Pani and Pani/BN nanocomposite in terms of

DC electrical conductivity retention was studied under isother-

mal ageing conditions as shown in Figure 13. The representa-

tion of relative electrical conductivity was calculated by the

equation:

r5rf 2ri

t(4)

where r is the change in relative electrical conductivity/minute,

rf is the final relative electrical conductivity at temperature

T, ri is the initial relative electrical conductivity at temperature

T, and t is the duration of the experiment (40 min). The electri-

cal conductivity was measured for each temperature (50, 70, 90,

110, and 130 8C) versus time. From the Figure 13(a), it can be

Figure 13. DC electrical conductivity retention under isothermal conditions at 50, 70, 90, 110, and 130 8C of (a) Pani and (b) Pani/BN. [Color figure

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

Figure 12. Initial DC electrical conductivity of (a) Pani and (b) Pani/BN.





understood that the stability of Pani is very fair at 50, 70, and

90 8C while the electrical conductivity becomes unstable at 110

and 130 8C. The Pani/BN nanocomposite is quite stable at 50 8C

and also significantly stable at 70 and 90 8C as shown in Figure

13(b). The nanocomposite shows more instability as compared

to Pani at 110 and 130 8C.

Stability under Cyclic Ageing Conditions

The stability of Pani and Pani/BN nanocomposite in terms of

DC electrical conductivity was also studied by cyclic ageing

technique. From the Figure 14, it may be observed that the ini-

tial DC electrical conductivity at the start of each cycle

decreases with increase in cycle number for both Pani and Pani/

BN nanocomposite. The difference in electrical conductivity of

Pani and Pani/BN nanocomposite from cycle 1 to cycle 5 was

observed 0.0221 and 0.0158 S/cm, respectively. Therefore Pani/

BN nanocomposite is shown more stable than Pani.

The stability in the term DC electrical conductivity retention of

Pani and Pani/BN nanocomposite was studied by cyclic ageing

technique within the temperature range of 50 to 150 8C as

shown in Figure 15. As the number of cycle increases, the DC

electrical conductivity gain for Pani decreases but the change in

electrical conductivity becomes less from cycle third to fifth. In

case of Pani/BN nanocomposite, it is observed that there is only

gain in the electrical conductivity from first cycle to fifth cycle.

After second cycle, the gain in electrical conductivity of Pani/

BN nanocomposite seems stable. From these observations, it

may attributed that the Pani/BN has more stable electrical con-

ductivity under cyclic ageing condition. The different pattern of

conductivities in different cycles for Pani and Pani/BN nano-

composite may be attributed to the removal of moisture, excess

of HCl or low molecular weight oligomers of aniline.31

CONCLUSION

Pani and Pani/BN nanocomposite were synthesized by in-situ

polymerization method and characterized by different instru-

mental techniques. The DC electrical conductivity retention

under isothermal and cyclic ageing conditions has also been

presented. It was observed that Pani showed greater electrical

conductivity as well as isothermal stability than that of Pani/BN

nanocomposite but in terms of cyclic stability Pani/BN showed

good stability than that of Pani. The as prepared materials pos-

sessed the excellent photodecomposition of model dyes under

UV-light irradiation. It was observed that photogenetrated

hydroxyl radical and hole (h1) and superoxide ions were the

main active species responsible for degradation of both the

dyes. The results highlighted that the extent of degradation of

MB and RhB was greater over Pani/BN nanocomposite than

Pani. This indicates that the as prepared Pani/BN nanocompo-

site may be used as a photocatalyst even at high thermal condi-

tions (�90 8C) for waste water treatment.

ACKNOWLEDGMENTS

Sharique Ahmad gratefully acknowledges the Maulana Azad

National Fellowship granted by University Grant Commission.

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©Smithers Information Ltd., 2016

633Polymers & Polymer Composites, Vol. 24, No. 8, 2016

Preparation, Characterization and Application of Polyaniline/silk Fibroin Composite

1. IntroductIon

Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa discovered that a polymer can be made conductive almost like a metal1. This discovery generated enormous interest in conducting polymers. Out of the different types of conducting polymers explored, polyaniline (Pani) draws much attention because it possesses excellent properties such as low cost, great environmental stability, reversible and tailorable electrical properties by controlled charge-transfer processes2-4. This makes polyaniline a versatile material for potential applications as electrodes

in primary and secondary batteries5,6, microelectronics7, photocatalysis8, sensors and actuators etc.9. Silk fibroin (SF) is a natural polymer produced by a variety of insects and spiders. Bombyx mori silk consists of two main proteins, sericin (glue-like coating of a nonfilamentous protein) and fibroin (a filament core protein). The natural silk fibroin (SF) fibres obtained by removing the outer sericin from silk fibres with anhydrous sodium carbonate solution at an appropriate temperature10. The primary structure of Bombyx mori SF protein is identified by the presence of three amino acids glycine, alanine and sericin with dominated sequence

as [Gly-Ser-Gly-Ala-Gly-Ala]n. The silk fibroin from the silkworm has widely been used in textile production, clinical sutures, as a scaffold for tissue regeneration and various biomedical applications because of its mechanical properties, biocompatibility and biodegradability11. The SF can also be used in photocatalytic activity by making its composite with inorganic or organic semiconductors. Janjira et al. reported the TiO2 coated SF (silk fibroin) filter for the photocatalytic degradation of formaldehyde gas by the highest removal efficiency (54.72 ± 1.75%) at the initial formaldehyde concentration ~5.00 ± 0.50 ppm12. Herein, we have tried to explore the potential for enhancement of photocatalytic activity of polyaniline by using silk fibroin under UV light. In SF, it may be assumed that the amide bonds get partially polarized by the conjugation

Preparation, characterization and Application of Polyaniline/silk Fibroin composite

Sharique Ahmad1, Adil Sultan1, Waseem raza2, M. Muneer2, and Faiz Mohammad1*1Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 202002, India2Department of Chemistry, Faculty of Science, Aligarh Muslim University, Aligarh 202002, India

received 23 March 2016, Accepted 21 April 2016

SuMMArYWe report the electrical properties and photocatalytic activity of polyaniline (Pani) and polyaniline/silk fibroin (Pani/SF) composite. The Pani/SF composite has been synthesized first time by chemical oxidative in-situ polymerization method by using potassium persulphate (K2S2O8) in an acidic medium. Thus prepared Pani and Pani/SF composite were characterized by using Fourier-transform infrared spectroscopy, x-ray diffraction, thermogravimetric analysis and scanning electron microscopy. The stability in terms of the DC electrical conductivity of Pani/SF composite and Pani was investigated under isothermal and cyclic aging conditions. Results indicated that the electrical properties of the composite were significantly influenced by the loading of SF to Pani and observed to be more thermally stable than those of Pani. As-prepared Pani and Pani/SF composites were studied for the degradation of methylene blue (MB) and Rhodamine B (RhB) under UV-light irradiations. The results showed that 91.4% & 85.4% degradation of RhB and MB takes place respectively after 90 min over Pani/SF composite. The decomposition rate of composite was 1.8-2.2 times greater than that of Pani. The improvement of photocatalytic activity of composite may be attributed to the electron-sink function of silk fibroin which increases the optical absorption property and separation of photogenerated charge carriers than Pani alone.

Keywords: Polyaniline/silk fibroin composite, Photocatalysis and electrical conductivity

*Corresponding author.

634 Polymers & Polymer Composites, Vol. 24, No. 8, 2016

Sharique Ahmad, Adil Sultan, Waseem Raza, M. Muneer, and Faiz Mohammad

present in peptide linkage13 which may cause electrostatic interaction with polyaniline (emeraldine salt) and hence significantly increased the photo-induced charge transfer to SF resulting in increased photogenerated charge separation in Pani/SF composite than that of Pani. A conducting composite of Pani with SF (Pani/SF) was prepared via an in-situ polymerization method. The photocatalytic degradation of MB and RhB dyes have been studied over the surface of the prepared Pani/SF composite which efficiently degraded both the dyes by 85.4% (MB) and 91.4% (RhB). The structure and surface morphology were investigated. The thermal stability was also investigated in terms of DC electrical conductivity retention under isothermal and cyclic accelerated aging conditions.

2. ExPErIMEntAl

2.1 Materials In the preparation of Pani and Pani/SF composite, the chemicals used were: aniline, 99% (E. Merck, India), hydrochloric acid, 35% (E. Merck, India), cocoons for silk (Banaras, India), potassium persulphate (E. Merck, India) and methanol. Doubly-distilled water was used throughout the experiments.

2.2 Preparation of Silk Fibroin Cocoons were degummed twice for 1 h in aqueous solution 0.02 M Na2CO3 and then rinsed thoroughly with distilled water to extract the silk fibroin (SF). The extracted SF was then dried at 40 °C for 24 h.

2.3 Preparation of Pani and Pani/SF compositeThe Pani/SF composite was prepared by in situ oxidative polymerization of aniline in the presence of SF using potassium persulphate as an oxidizing agent. Firstly, the extracted silk fibroin (200 mg) was dissolved in hydrochloric acid (11.6 M). Then, it was added to a 3% aniline solution under constant

stirring for 1 h to enable the proper dispersion of SF in aniline. K2S2O8 solution was then added dropwise in the dispersed solution of SF and aniline to polymerize the aniline. The colour of the reaction mixture started changing from light yellow to dark green within 15 min of adding oxidant solution under constant stirring. The stirring was kept continuous for 20 h. The final dark green reaction mixture was then filtered, washed with doubly-distilled water and methanol to remove excess acid, potassium persulphate and Pani oligomers until filtrate became colourless and neutral. Thus-prepared Pani/SF composite was dried at 60 oC for 12 h in an air oven, crushed into fine powder and was stored in desiccators for further investigations. Pani was also prepared using the same method as described above.

2.4 Photodegradation Experiment The photocatalytic activity experiment was performed in an immersion well photochemical reactor made of Pyrex glass using a 125 W medium pressure mercury lamp as UV light source. The photocatalytic performance of the as-prepared materials was evaluated by the degradation of RhB & MB in aqueous solution under UV light illumination. In order to maintain the temperature (20 ± 0.5 °C) of dye solution, the photoreactor surrounded by refrigerated water circulation. Light intensity falling on the solution was measured using UV light intensity detector (Lutron UV-340) and found to be 2.56 mW/cm2. The radiation emitted by the UV-lamp (IR and short wavelength) was eliminated by a water circulating jacket. In the photocatalytic experiment 250 mg of prepared composite was added into the 250 ml of RhB and MB with a concentration of (10 mg/L) for RhB (10 mg/L) for MB respectively. Prior to illumination, the aqueous suspension was magnetically stirred in the dark for 30 min to obtain the saturated adsorption of dyes onto the surface

of catalyst14. During irradiation at regular time intervals, the suspension was collected and then centrifuged (4000 rpm, 20 min) to remove the powder material. The concentrations of the dyes were monitored by using a UV-VIS spectrophotometer (Perkin Elmer λ-35) at their λmax = 553 and 663 respectively, based on the Beer-Lambert Law15. The degradation efficiency of dyes was calculated from Equation (1).

Degradation % = [( Ao– At) ÷ Ao] x 100% (1)

where Ao is the initial concentration and At is the concentration at a particular time (t).

3. chArActErIzAtIon

In order to investigate the morphology, structure and chemical composition of Pani and Pani/SF composite, a variety of techniques were used including Fourier-transform infrared spectroscopy (FTIR), using a Perkin-Elmer 1725 instrument. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 diffractometer with Cu Ka radiation at 1.540 A˚. Scanning electron microscopy (SEM) studies were carried out by JEOL, JSM, 6510-LV (Japan). The thermal stability in terms of DC electrical conductivity of Pani and Pani/SF composite under isothermal and cyclic ageing conditions was studied. For this study, a four-in-line probe with a temperature controller, PID-200 (Scientific Equipments, Roorkee, India) was used to measure the DC electrical conductivity and its temperature dependence. The equation used in calculation of DC electrical conductivity was:

σ = [ln2(2S/W)]/[2πS(V/I)] (2)

where I, V, W and S are the current (A), voltage (V), thickness of the pellet (cm) and probe spacing (cm) respectively and σ is the conductivity (S/cm)16. In testing of isothermal stability, the pellets were heated at 50, 70, 90, 110



and 150 oC in an air oven and the DC electrical conductivity was measured at particular temperature at an interval of 10 min in the accelerated ageing experiments. In testing the stability under cyclic ageing condition, DC conductivity measurements were taken 5 times at intervals of about 90 min within the temperature range of 50-150 oC.

4. rESultS And dIScuSSIon

4.1 Preparation of Pani/SF compositeIn the mechanistic view of the polymerization process, the aniline molecules in HCl form anilinium cations (phenyl-NH3

+) and become attached to the SF. The attachment may be due to Coulombic attraction between anilinium cations and the partial negative charge on the oxygen of partially polarized amide group of SF. Thus it is expected that the SF gets completely surrounded by anilinium cations. When this arrangement comes in contact with K2S2O8, the anilinium cations get polymerized on the surface of SF forming Pani (emeraldine salt). The polyaniline (emeraldine salt) formed on the surface of SF get hooked by Coulombic attraction between the positive charge on nitrogen of Pani and partial negative charge on oxygen and also the interaction between lone pair on the nitrogen of Pani (emeraldine salt) and the positive charge on the nitrogen of partially-polarized amide groups of silk fibroin. A schematic presentation of Coulombic attraction between the positive and negative charges is given in Figure 1.

4.2 FtIr Analysis FTIR spectra of the Pani/SF composite and Pani are shown in Figure 2. In the spectra of Pani the five major peaks were observed. The peak observed at 3438 cm−1 corresponds to N-H stretching vibration. The peak at 1572 cm−1 is due to C=C stretching mode of the quinonoid rings and

1487 cm−1 is due to C=C stretching mode of benzenoid rings. The peak at about 1294 cm−1 can be attributed to C-N stretching. The peak at around 798 cm−1 may assigned to an out-of-plane bending vibration of C-H which confirmed the formation of Pani17. In the Pani/SF composite, the peaks also observed at 1636 cm−1and 1532 cm−1which correspond to amide I and amide II vibrations and attributed to the β sheet structure18. The remaining peaks in the spectra of Pani/SF composite are similar but slightly shifted relative to Pani due

to interaction of peptide bond in silk fibroin with Pani.

4.3 x-ray diffraction (xrd) StudiesThe XRD patterns of Pani and Pani/SF composite are shown in Figure 3. Figure 3a shows the two peaks at 2θ = 20.16o and 25.31o which are attributed to HCl-doped Pani19. In Figure 3b it is observed that the intensity of peaks increased due to the semicrystalline nature of silk fibroin20, also the peak of silk fibroin (2θ = 20o)21 has been

Figure 1. Schematic representation of coulombic attraction between Pani and peptide bonds of SF in Pani/SF composite

Figure 2. FtIr spectra of: (a) Pani and (b) Pani/SF



merged and shifted to 2θ = 20.40o in Pani/SF composite. The peak observed at 2θ = 25.01o is due to existence of Pani in Pani/SF composite but slightly shifted, which suggests the formation of Pani/SF composite.

4.4 thermogravimetric Analysis (tGA)The thermal stability of as-prepared Pani and Pani/SF composite was investigated and shown in Figure 4. The weight loss occurred in three step process, in case of Pani, the initial weight loss at around 98 oC may correspond to removal of adsorbed water from the Pani. Later, a weight loss is observed at about 189 oC, which is due to removal of lower oligomers of Pani and third in the range from 450 to 600 oC due to thermo-oxidative decomposition of Pani22. Silk fibroin is remarkably stable under thermal conditions up to 250 oC23, so in the case of Pani/SF the second degradation step occurs at around 206 oC which is higher than that of Pani (~189 oC). The thermal stability of silk fibroin (up to ~250 oC) makes a difference in the composite and causing Pani/SF to degrade comparatively at higher temperature.

4.5 Scanning Electron Micrographic (SEM) StudiesThe FESEM of Pani and Pani/SF composite are shown in Figure 5 at different magnifications. Figure 5 (a and c) shows the FESEM images of Pani, having uniform, interconnected short tubes along with a flake-like structure. The SEM images in Figure 5 (b and d) represent the Pani/SF composite which reveals a similar morphology but with slightly more agglomeration compared to Pani that forms lumps. The silk fibroin in aqueous salt systems mainly exists in the forms of lumps and random globules24. So the formation of lumps in Pani/SF composite may be attributed to the presence of silk fibroin.

Figure 3. xrd patterns of: (a) Pani and (b) Pani/SF

Figure 4. tGA thermograms of: (a) Pani and (b) Pani/SF

Figure 5. SEM micrographs of: (a and c) Pani, (b and d) Pani/SF composite at different magnifications



5. PhotocAtAlYtIc StudIES The as-prepared samples were tested for their photocatalytic activity by choosing two different chromophoric model dyes, the common contaminants in wastewater, under UV illumination. On the basis of a blank experiment, the self-photodegradation of both the dyes could be neglected. Figure 6 (a and b) display the 50% and 56.4% photodegradation efficiency of MB and RhB respectively in the presence of Pani under UV illumination after 90 min.

Figure 7 (a and b) indicates the 85.4% and 91.4% photodegradation of MB and RhB respectively in the presence of Pani/SF composite after 90 min under UV irradiation. The degradation of RhB was found to be higher than that of MB. This may be due to the higher adsorption of RhB than that of MB on the surface of the catalyst, which increased the photocatalytic degradation of RhB.

Figure 8 shows the percentage degradation of both the dyes over Pani and Pani/SF under UV illumination.

The interaction between RhB and the prepared photocatalyst was found to be excellent. Therefore RhB showed good degradation over Pani and Pani/SF25,26. This selective adsorption of both the dyes on the surface of Pani and Pani/SF promises a green method for the degradation of both dyes from waste water. The absorbed dyes were then degraded by active species. On the basis of above experimental data, the enhanced degradation of both dyes in the presence of Pani/SF composite was mainly ascribed to the high charge separation efficiency of photoinduced

Figure 6a, b. Photocatalytic degradation of MB and rhB dyes respectively at different time intervals in the presence of Pani under uV light.

Figure 7a, b. Photocatalytic degradation of MB and rhB dyes respectively at different time intervals in the presence of Pani/SF composite under uV light



electron hole pair. This may be due to the electron-sink function of silk fibroin which inhibits the recombination rate of electrons and holes, leading to fast degradation of the dyes compared with Pani27,28.

The possible mechanism for the degradation of dye using Pani and Pani/SF can be represented as given in Figure 9. Pani is a typical conducting polymer with extended π-electron conjugation system. Pani serve as a good photocatalyst for degradation of pollutants under UV irradiation due to its electronic structure characterized by a filled valence band (HOMO) and an

empty conduction band (LUMO). On absorption of photons of higher energy than the band gap energy of Pani, electrons may be promoted from the valence band to the conduction band leaving behind electron vacancies or “holes” in the valence band29. The charge separation is maintained in Pani/SF composite through interface, attached by Coulombic attraction. The electron and hole may migrate to the catalyst surface where they participate in redox reactions with adsorbed dyes on the surface. Specially, the holes generated in the valence band (h+

VB) may react with surface-bound H2O to produce the hydroxyl radical and

the electron present in the conduction band (e¯CB) is picked up by oxygen to generate a superoxide radical anion30,31.

Pani/SF + UV-rays → e¯CB + h+

VB

O2 + e¯CB → O2•–

H2O + h+VB → H+ + •OH

O2 + 2(e¯CB) + 2H+ → H2O2

e¯CB + H2O2→ 2 •OH

The superoxide radical anion acts as a strong oxidizing agent and hydroxyl radical acts as a strong reducing agent, and they degrade the pollutant dyes to the end products.

O2•– + Dye→ Degraded product +

(H2O + CO2)

•OH + Dye → Degraded product + (H2O + CO2)

6. ElEctrIcAl conductIVItY

The electrical conductivities of Pani and Pani/SF composite were measured by the standard four-in-line probe method. From the measured electrical conductivity, it may be observed that both the as-prepared materials are semiconducting in nature and the electrical conductivity of Pani decreased slightly after loading of silk fibroin. The measured electrical conductivities of Pani and Pani/SF composite were 0.087 S/cm and 0.072 S/cm respectively, as shown in Figure 10. In silk fibroin, due to the conjugation present in the amide groups, the partial charge developed on the oxygen and nitrogen partially polarize the amide groups13. The partially polar amide groups in silk fibroin bind the polarons of the Pani and the counterion Cl–. This causes a reduction in the mobility of the polarons, leading to a decline of electrical conductivity in Pani/SF composite. Also the silk fibroin, having insulating behaviour, may induce the formation of a less efficient network for charge transport between

Figure 8. Percentage degradation of MB and rhB dyes in aqueous solution as a function of time in the presence of Pani and Pani/SF composite under uV light

Figure 9. Probable degradation process of MB and rhB over Pani/SF composite



the polyaniline chains, leading to the slightly lower electrical conductivity than that of Pani.

6.1 Stability under Isothermal AgeingThe stabilities of Pani and Pani/SF composite in terms of DC electrical conductivity retention was studied under isothermal ageing conditions, as shown in Figure 11. The relative electrical conductivity was calculated by the equation:

(3)

where σ r,t = relative electrical conductivity at time t, σt = electrical conductivity at time t, σo = electrical conductivity at time zero. The stability in terms of DC electrical conductivity retention of the prepared materials is used for the comparative study of relative electrical conductivity with respect to time at different temperatures. The electrical conductivities of the samples were measured for the temperature 50, 70, 90, 110 and 130 °C versus time at intervals of 10 min for 40 min. It may be observed from Figure 11b, the stability of relative DC electrical conductivity of Pani is quite good at 50 and 70 °C, while it becomes unstable at 90, 110 and 130 °C. The relative DC electrical conductivity of Pani/SF composite seems to be more stable than Pani even in the higher temperature range, as shown in Figure 11a. Thus Pani/SF composite is observed to be more stable than Pani in terms of electrical conductivity under isothermal aging conditions.

6.2 Stability under cyclic Ageing conditionsThe stability in terms of DC electrical conductivity retention of Pani and Pani/SF composite was studied by a cyclic ageing technique within the temperature range 50 to 150 °C, as shown in Figure 12. The electrical conductivity was recorded for consecutive cycles and it was observed

Figure 10. Initial dc electrical conductivity of: (a) Pani and (b) Pani/SF

Figure 11. dc electrical conductivity retention under isothermal conditions at 50, 70, 90, 110 and 130 °c of: (a) Pani/SF and (b) Pani

Figure 12. dc electrical conductivity retention under cyclic ageing conditions of (a) Pani and (b) Pani/SF composite



that the conductivity increased gradually for each cycle, showing a regular trend in all cases. The relative electrical conductivity was calculated using the following Equation:

(4)

where σ r is relative electrical conductivity, σT is electrical conductivity at temperature T (°C) and σ50 is electrical conductivity at 50 °C.

From Figure 12, the relative electrical conductivity of Pani and Pani/SF may be observed within temperature range from 50 to 150 °C. As the temperature increases from 50 to 150 °C, the relative electrical conductivity of both Pani and Pani/SF increase, which may be due to the increased delocalization of the polarons or bipolarons. The gain in conductivity in Pani/SF is comparatively lower than Pani, which may be due to the insulating behaviour of SF. It may also be observed from Figure 12a that the electrical conductivities of Pani in C-2, 4 and 5 (cycles 2, 4 and 5) are very similar, but different in C-1 (cycle 1) and C-3 (cycle 3) and observed to deviate from cycles 2, 4 and 5. In the case of Pani/SF, the relative electrical conductivities for each cycle are in the same trend, and much less deviation is observed, as shown in Figure 12b. The cycle C-1 slightly deviates, which may be due to instrumental effects. It may be inferred that the Pani/SF composites are more stable conductors than Pani. The difference may be partly attributable to the removal of moisture, excess HCl or low molecular weight oligomers of aniline32.

7. concluSIonS

Pani and Pani/SF composite were successfully synthesized by an in-situ polymerization method, and characterized by several instrumental techniques. The electrical properties of Pani and Pani/SF composite were studied in terms of DC conductivity under

isothermal and cyclic ageing conditions and found to be in the semiconductor range. The Pani/SF composite showed lower conductivity than Pani, but it had better stability in terms of DC electrical conductivity under isothermal and cyclic ageing conditions. The as-prepared materials displayed excellent photodecomposition of model dyes under UV irradiation. It was observed that photogenerated hydroxyl radical and hole (h+) and superoxide anion radical were the main active species responsible for decolorization of model dyes pollutant. The results demonstrated that the extent of degradation of MB and RhB was greater over the Pani/SF composite than Pani. Thus, Pani/SF composite may find applications in various electrical, electronic and UV light photocatalytic devices in view of their fairly good photocatalytic activity and the thermal stability of their electrical properties.

AcKnoWlEdGEMEntS

S h a r i q u e A h m a d g r a t e f u l l y acknowledges the Maulana Azad National Fellowship granted by University Grant Commission.

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Rapid response and excellent recovery ofa polyaniline/silicon carbide nanocomposite forcigarette smoke sensing with enhanced thermallystable DC electrical conductivity

Sharique Ahmad, Adil Sultan and Faiz Mohammad*

In this paper, we present an electrical conductivity based rapid response cigarette smoke sensor with

excellent recovery based on a polyaniline/silicon carbide (Pani/SiC) nanocomposite prepared by an in

situ chemical oxidative polymerization technique in an acidic medium. The Pani/SiC nanocomposites

and polyaniline (Pani) were characterized using Fourier transform infrared spectroscopy, X-ray

diffraction, scanning electron microscopy and transmission electron microscopy. Results indicated that

well ordered nanocomposites were successfully prepared. The morphology and electrical properties of

the nanocomposites were influenced by the extent of loading of SiC nanoparticles. The as-prepared

materials were studied for the change in their electrical conductivity on exposure to cigarette smoke

followed by ambient air at room temperature. It was observed that the Pani/SiC-3 nanocomposite shows

an eight times higher amplitude conductivity change than Pani on exposure to cigarette smoke followed

by ambient air. The conductivity response in carbon dioxide, ammonia and methanol was also measured.

It was observed that carbon dioxide (CO2) shows a low response while in the case of methanol and

ammonia significant responses are observed, suggesting the possibility of a contribution from the above

gases towards the response against cigarette smoke. Cigarette smoke also contains many other volatile

chemicals such as polyaromatic hydrocarbons (PAHs), nicotine, hydrogen cyanide (HCN) and

formaldehyde which may also interact with the polarons/bipolarons of polyaniline leading to a decrease

in conductivity.

1. Introduction

Since the discovery of the rst conducting polymer various typesof conducting polymers have been explored, of which polyani-line (Pani) has inspired a great deal of interest due to its lowcost, easy preparation, high environmental stability, and itsreversible and tunable electrical properties by controlledcharge-transfer processes.1–3 This makes polyaniline a versatilematerial for potential applications4 such as electrodes inprimary and secondary batteries,5,6 microelectronics,7 photo-catalysts,8 sensors and actuators9 etc. The conjugated p-bondspresent in Pani can experience changes upon adsorption ofchemical species onto its surface due to an acid–base typeinteraction between the polymer and the chemical species. Alsobecause of its high sensitivity, quick response and ability towork at room temperature,10,11 Pani and its composite materialshave been demonstrated as sensing materials for humidity,NO2, NH3, toxic solvents,12–15 and organic vapours such as

methanol, ethanol, chloroform, dichloromethane, andhexane.16,17 A number of nanocomposites of polyaniline havebeen reported with ferrite (MFe2O4),18 manganese dioxide(MnO2),19 silver,20 clay21 etc. Here silicon carbide (SiC) nano-particles (<50 nm) are selected in this work due to their uniqueproperties such as a wide and tunable band gap, resistancetowards high temperatures, chemical inertness, high tensilestrength and hardness.22–27

It is a well acknowledged fact that smoking causesdangerous diseases like cancer, heart disease, lung diseases etc.Cigarette smoke contains tar, polynuclear aromatic hydrocar-bons (PAHs), and toxic gases like carbon monoxide, and thou-sands of chemicals are produced when a cigarette burns.Devasish Chowdhury did excellent work on cigarette smokesensing by a Ni coated polyaniline nanowire material andobserved a good response of about a four order change in theimpedance response on exposure to cigarette smoke.28 AlsoYuan Liu et al. synthesized polyaniline lms for the detection ofsecondhand cigarette smoke via nicotine. They observed thatthe polyaniline lm demonstrated fair recovery between expo-sure periods, and was functional over a number of periods ofexposure to nicotine and tobacco smoke.29 Besides, polyaniline

Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh

Muslim University, Aligarh-202002, India. E-mail: faizmohammad54@rediffmail.

com; Tel: +91 9412623533

Cite this: RSC Adv., 2016, 6, 59728

Received 16th May 2016Accepted 10th June 2016

DOI: 10.1039/c6ra12655c

www.rsc.org/advances

59728 | RSC Adv., 2016, 6, 59728–59736 This journal is © The Royal Society of Chemistry 2016

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composites have also been demonstrated to have potentialapplications in gamma radiation detection31 and electronparamagnetic resonance (EPR) investigations.30 Also carbonnanotubes,32 graphene33 and carbon nanotubides34 and theircomposites have also been widely used in gas sensing applica-tions. In our previous work we synthesised a polypyrrole/boronnitride nanocomposite for the detection of LPG leaks.35

In this paper, we present a simple strategy for the prepara-tion of Pani/SiC nanocomposites by oxidative polymerization ofaniline. The morphology, thermal stability and electricalconductivity of the prepared materials were investigated. Also,we have tried to explore the potential of a polyaniline–siliconcarbide nanocomposite as a chemical sensor by examining thedynamic response of electrical conductivity towards cigarettesmoke using a simple 4-in line probe electrical conductivitymeasurement set up.

2. Experimental2.1 Materials

Aniline, 99% (E. Merck, India), hydrochloric acid, 35%(E. Merck, India), SiC (SRL, India), potassium persulphate(E. Merck, India) and methanol were used. Double distilledwater was used throughout the experiments.

2.2 Synthesis of Pani and Pani/SiC nanocomposites

Aniline (2.5 mL) was mixed with 100 mL of 1 M hydrochloricacid (HCl) with stirring for 30 min. A solution of potassiumpersulphate (PPS) was added dropwise and the mixture wasallowed to react for 20 h. The nal dark green coloured reactionmixture was then ltered, washed several times with doubledistilled water and methanol and then dried in an air oven at70 �C for 10 h. Pani/SiC nanocomposites were prepared by insitu oxidative polymerization of aniline in the presence ofdifferent amounts of SiC nanoparticles. Typically 2.5 mL ofaniline was added to 100 mL of 1 M HCl under constant stirringfor 30 min. Different amounts of SiC nanoparticles were ultra-sonicated in 1 M HCl for 2 h and added to the mixture. Asolution of K2S2O8 in 100 mL of 1 M HCl was then poureddropwise into the mixture at room temperature with constantstirring. The colour of the solution changed from light indigo togreenish black indicating the polymerization of aniline. Thereaction mixture was then stirred for a further 20 hours. Thereaction mixture was then ltered, washed thoroughly withdistilled water and methanol to remove the excess acid, potas-sium persulphate and Pani oligomers until the ltrate becamecolourless. The thus prepared nanocomposites were dried at

70 �C for 10 h in an air oven, converted into ne powders andwere stored in desiccators for further experiments. For theelectrical conductivity measurements, 0.20 g of material fromeach sample was pelletized at room temperature with the helpof a hydraulic pressure machine at 80 kN load for 15 min. Thedetails of the synthesis are given in Table 1.

3. Characterization

In order to investigate the morphology, structure and chemicalcomposition of Pani and the Pani/SiC nanocomposites, a varietyof methods were used. Fourier transform infrared spectroscopy(FTIR) spectra were recorded using a Perkin–Elmer 1725instrument. X-ray diffraction (XRD) patterns were recorded bya Bruker D8 diffractometer with Cu Ka radiation at 1.540 A.Scanning electron microscope (SEM) studies were carried out bya JEOL, JSM, 6510-LV (Japan). The thermal stability in terms ofthe DC electrical conductivity of Pani and the Pani/SiCcomposite under isothermal and cyclic ageing conditions wasalso studied. For this study, a four-in-line probe with a temper-ature controller, a PID-200 (Scientic Equipments, Roorkee,India) was used to measure the DC electrical conductivity andits temperature dependence. The equation used in the calcu-lation of the DC electrical conductivity was:

s ¼ [ln 2(2S/W)]/[2pS(V/I)] (1)

where I, V, W and S are the current (A), voltage (V), thickness ofthe pellet (cm) and probe spacing (cm) respectively and s is theconductivity (S cm�1).36 In order to test the isothermal stability,the pellets were heated at 50 �C, 70 �C, 90 �C, 110 �C and 130 �Cin an air oven and the DC electrical conductivity was measuredat a particular temperature at an interval of 5 min in theaccelerated ageing experiments. When testing the stabilityunder cyclic ageing conditions, the DC conductivity measure-ments were taken 5 times at intervals of about 90min within thetemperature range of 40–130 �C.

4. Results and discussion4.1 Possible interaction between Pani and SiC nanoparticles

Polyaniline and the polyaniline/silicon carbide nanocompositeswere prepared by oxidation of aniline in aqueous medium usingK2S2O8 as an oxidant in the presence of HCl. It is expected that

Table 1 Preparation details of Pani and Pani/SiC nanocomposites

Sample ID Aniline (mL) K2S2O8 (gm) SiC nanoparticles (wt%)

Pani 2.5 6 0Pani/SiC-1 2.5 6 10Pani/SiC-2 2.5 6 15Pani/SiC-3 2.5 6 20 Fig. 1 Schematic representation of the possible interaction between

Pani and SiC nanoparticles in the Pani/SiC nanocomposite.

This journal is © The Royal Society of Chemistry 2016 RSC Adv., 2016, 6, 59728–59736 | 59729

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the lone pair present on the nitrogen atom of polyanilineinteracts with silicon carbide and forms an efficient networkandmay be responsible for the enhanced conductivity as shownin Fig. 1.

4.2 FTIR analysis

FTIR spectra of Pani and the Pani/SiC nanocomposites areshown in Fig. 2. The ve major peaks were observed in all of thespectra. The peak observed at 3411 cm�1 for Pani may due to N–H stretching vibrations. The peaks at 1573 cm�1 and 1482 cm�1

are due to the C]C stretching mode of the quininoid andbenzenoid rings respectively. The peak at 1298 cm�1 may beattributed to C–N stretching. The peak at around 810 cm�1 maybe assigned to an out-of-plane bending vibration of C–H whichconrmed the formation of Pani.37 In the spectra of the nano-composites, it is observed that the peaks at 1573 cm�1, 1482cm�1 and 1298 cm�1 for the quininoid, benzenoid and C–Nstretching of Pani respectively shi to 1568 cm�1, 1475 cm�1

and 1289 cm�1 aer loading of the nanoparticles in the poly-aniline matrix, indicating a strong interaction between poly-aniline and the SiC nanoparticles.38 The peak observed at 810cm�1 in Pani due to the C–H out of plane bending vibration hasmerged with the Si–C stretching vibration39 (800 cm�1) andshied to 804 cm�1 in the Pani/SiC nanocomposites.

4.3 X-ray diffraction (XRD) studies

Fig. 3 shows the X-ray diffraction patterns of Pani and the Pani/SiC nanocomposites. Fig. 3a shows the characteristic peak ofPani at 2q ¼ 25.50�. The presence of polyaniline in the Pani/SiCnanocomposites is conrmed by the observation of the Panibands in all of the samples which shis to 25.29� with loadingof the silicon carbide nanoparticles in polyaniline. The fourmajor peaks observed at 2q ¼ 35.37�, 41.34�, 59.91�, and 71.70�

conrm the presence of silicon carbide in the nano-composites.39 The diffraction peak of Pani in the Pani/SiCnanocomposites becomes weaker and the characteristic peaks

of SiC appear to be more prominent as the percentage of siliconcarbide increases due to the interaction between Pani and theSiC nanoparticles.

4.4 Scanning electron micrograph (SEM) studies

The morphology of Pani and the Pani/SiC-3 nanocomposite wasstudied by SEM as presented in Fig. 4. Fig. 4(a) and (b) show theSEM images of Pani which seems to be aky, at surfaced andagglomerated. The differences in the structure of the nano-composite and Pani samples are visible at lower magnicationindicating sharp edges in the Pani/SiC-3 nanocomposite ratherthan the at surfaces in Pani.39 It may be seen that from Fig. 4(c)and (d) the SiC nanoparticles were successfully decorated byPani and no free SiC nanoparticles were observed which indi-cates that the SiC nanoparticles have a nucleating effect on

Fig. 2 FT-IR spectra of: (a) Pani, (b) Pani/SiC-1, (c) Pani/SiC-2 and (d)Pani/SiC-3.

Fig. 3 XRD patterns of: (a) Pani, (b) Pani/SiC-1, (c) Pani/SiC-2 and (d)Pani/SiC-3.

Fig. 4 SEM micrographs of: (a and b) Pani, (c and d) the Pani/SiC-3nanocomposite at different magnifications.


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aniline polymerization and caused a homogeneous Pani shellaround them. Also the SiC nanoparticles bind to the surface ofthe polyaniline granules and their surface area seems toincrease as the SiC nanoparticles are well dispersed in the Pani/SiC-3 nanocomposite. Thus the study reveals a transformationof morphology from at Pani to the core–shell Pani/SiC-3nanocomposite with sharp edges.

4.5 Transmission electron micrograph (TEM) studies

Fig. 5 represents the TEM micrograph of the Pani/SiC-3 nano-composite. From the TEM image of the nanocomposite, darkparts with a diameter of about 50 nm coated by a light shadedouter shell of the polymer matrix can be observed. This obser-vation conrms that the SiC nanoparticles are fully covered bypolyaniline by the successful deposition of Pani on SiC nano-particles forming the Pani/SiC nanocomposites with a core–shell structure.

5. Electrical conductivity

The initial DC electrical conductivity of Pani and the Pani/SiCnanocomposite containing 10, 15 and 20% of SiC was measuredby a standard four-in-line probe method and found to be 0.54,1.30, 1.49 and 1.52 S cm�1 for Pani, Pani-1, Pani-2 and Pani-3respectively. From the measured electrical conductivity, it maybe inferred that all the as prepared materials possess electricalconductivity in the semiconducting range. The electricalconductivity of the Pani/SiC nanocomposites increases with anincrease in the SiC nanoparticle content in the nanocompositesas shown in Fig. 6. It may be assumed that the improvement inthe electrical conductivity of Pani aer loading of the SiCnanoparticles may be due to the formation of an efficientnetwork in the Pani chains and also due to the interaction of thelone pair on the nitrogen of polyaniline with silicon carbidewhich increases the mobility of the polarons in polyanilineleading to an increase in electrical conductivity. It is alsoobserved from Fig. 6 that the conductivity increases sharplywith a loading of 10% of SiC and aer that shows little increasewith further loadings of 15 and 20% of SiC. Mavinakuli et al.

studied polypyrrole/silicon carbide nanocomposites and re-ported a similar increase in the conductivity of the nano-composites with an initial particle loading of up to 10 wt%, andthen decreases at higher particle loadings.39 From these obser-vations, it may be concluded that loading SiC into Pani forms anefficient network in Pani resulting in an increase in the elec-trical conductivity. Loadings above 10% of SiC into Pani do notcause any signicant increase in conductivity and may forma percolative pathway.

5.1 Stability under isothermal ageing

The stability of Pani and the Pani/SiC nanocomposites in termsof their DC electrical conductivity was studied under isothermalageing conditions as shown in Fig. 7. Representation of theirrelative electrical conductivity was calculated by using theequation:

sr;t ¼st

s0

(2)

where sr,t is the relative electrical conductivity at time t, st ¼electrical conductivity at time t, and s0 ¼ electrical conductivityat time zero. A comparative study on the relative electricalconductivity with respect to time at different temperatures was

Fig. 5 TEM micrograph of the Pani/SiC-3 nanocomposite.Fig. 6 Initial DC electrical conductivities of Pani and the Pani/SiCnanocomposites.

Fig. 7 Relative electrical conductivity of: (a) Pani, (b) Pani/SiC-1, (c)Pani/SiC-2 and (d) Pani/SiC-3 under isothermal ageing conditions.


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done for analysis of the stability of the prepared materials interms of their DC electrical conductivity. The electricalconductivity of each of the samples was measured for temper-ature (50, 70, 90, 110, and 130 �C) versus time at intervals of 5min for up to 20 min. It may be observed from Fig. 7(a), that thestability of the relative DC electrical conductivity of Pani is veryfair at 50 and 70 �C. In Fig. 7(b), the relative DC electricalconductivity of the Pani/SiC-1 nanocomposite seems to be verystable at 50, 70, 90 and 110 �C. While the relative DC electricalconductivity of the Pani/SiC-2 and Pani/SiC-3 nanocompositesseems to be fairly stable at 50 �C and 70 �C as shown in Fig. 7(c)and (d). Thus it is inferred that the Pani/SiC-1 nanocompositeshowed greater stability than Pani in terms of DC electricalconductivity under isothermal ageing conditions.

5.2 Stability under cyclic ageing conditions

The stability in terms of the DC electrical conductivity retentionof Pani and the Pani/SiC nanocomposites was also studied bya cyclic ageing method within the temperature range of 50 to130 �C as shown in Fig. 8. The electrical conductivity wasrecorded for successive cycles and observed to increase gradu-ally for each of the cycles with a regular trend in all these cases.The relative electrical conductivity was calculated using thefollowing equation:

sr ¼sT

s50

(3)

where sr is the relative electrical conductivity, sT is the electricalconductivity at temperature T (�C) and s50 is the electricalconductivity at 50 �C.

As the temperature increases from 50 to 130 �C the relativeelectrical conductivity of all of the samples increases which maybe due to the increasing number of charge carriers such aspolarons or bipolarons at elevated temperatures. From Fig. 8(a),it may be observed that the electrical conductivity of Pani for allof the cycles is similar and follows the same trend with thehighest gain in conductivity (the lowest stability) among all ofthe samples. In the Pani/SiC nanocomposites, the relative DCelectrical conductivity also follows a similar trend with lessdeviation as shown in Fig. 8(b)–(d). In all of these nano-composites, the Pani/SiC-3 nanocomposite showed the greaterstability by having much less deviation in the electrical

conductivity. Increases and decreases of the relative electricalconductivity represents instability in the electrical conductivityof the sample. The more deviation in electrical conductivityrepresents more instability in the electrical conductivity of thesample. Thus it may be understood that the Pani/SiC nano-composites are more stable semiconductors than Pani andamong these Pani/SiC nanocomposites, Pani/SiC-3 has greateststability in terms of DC electrical conductivity under cyclicageing conditions. The difference observed may be attributed tothe removal of moisture, excess HCl or low molecular weightoligomers of aniline.41

6. Sensing

Cigarette smoke is a very well known health hazard and knowncause of cancer, lung diseases, heart diseases and responsiblefor other related problems. About 4000 chemical constituentsare produced by burning a cigarette but the main constituentsare tar, carbon dioxide, carbon monoxide, nitrogen oxides,hydrogen cyanide, ammonia, polycyclic aromatic hydrocarbons,chlorinated dioxins, furans etc.28 It is very difficult to evaluatewhich constituent in cigarette smoke is responsible for theconductivity change of the material. An experiment was alsocarried out to study the conductivity response in the presence ofcarbon dioxide (CO2), ammonia gas and methanol. DevasishChowdhury reported a four order change in the impedanceresponse of Ni-coated PANI-CSA on exposure to cigarettesmoke.28 Also Yuan Liu et al. reported conductive polyanilinelms for the detection of secondhand cigarette smoke vianicotine with very fair recovery between exposure periods.29

Herein we have tried to look into the effect of cigarette smoke onthe electrical conductivity of the Pani/SiC nanocomposite.

The cigarette smoke sensitivity of Pani/SiC-3 was analysed bymeasuring changes in the electrical conductivity at 25 �C asshown in Fig. 9. The sensitivity of Pani/SiC-3 was investigatedbased on two parameters; response time and sensing intensity.

Fig. 9 Effect on the DC electrical conductivity of Pani/SiC-3 onexposure to cigarette smoke followed by ambient air with respect totime.

Fig. 8 Relative electrical conductivity of: (a) Pani, (b) Pani/SiC-1, (c)Pani/SiC-2 and (d) Pani/SiC-3 under cyclic ageing conditions.


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The pellet fabricated was attached to four probes and placed ina closed chamber with an inlet for introducing smoke. Cigarettesmoke was introduced to the inlet chamber for 60 seconds andaer that the pellet was removed from the chamber and exposedto air for the next 60 seconds. When Pani/SiC-3 came in contactwith cigarette smoke, the electrical conductivity decreased withan increase in time. This decrease in conductivity may be due toneutralization of some Pani by an interaction of the lone pair ofelectrons on the nitrogen atoms of ammonia42 or may be due toan interaction with the lone pair on the oxygen atoms of CO2

gas43 or also may be due to the interaction of the lone pair ofelectrons on the oxygen of methanol with Pani.44 Cigarettesmoke is composed of all of these and many other gases whichhinder the mobility of charge carriers leading to a decrease inelectrical conductivity. When the pellet was exposed to air, theelectrical conductivity started to increase with time and revertedto its maximum value aer 60 seconds, the reason for this beingdesorption of these gases from the surface of Pani/SiC-3.

The reversibility response of Pani/SiC-3 was measured interms of the DC electrical conductivity. The reversibility wasdetermined by rst keeping a sample in the smoke environmentfor 30 s followed by 30 s in open air for a total duration of 180seconds. From Fig. 10, it may be observed that the as preparedmaterial showed excellent reversibility by the variation inconductivity in the range of 4.9 S cm�1 to 4.2 S cm�1 in theabsence and presence of smoke.

The cigarette smoke sensitivity of Pani was also analysed bymeasuring the changes in the electrical conductivity at 25 �C(Fig. 11). A similar experiment was done for Pani in the absenceand presence of smoke for 120 seconds. On exposure to smokefor 60 s the electrical conductivity decreased with an increase intime and by exposing to air for the next 60 s, the electricalconductivity increased with time for the reasons mentioned inthe previous paragraph.

The reversibility response of Pani was also measured interms of the DC electrical conductivity as was done for Pani/SiC-3. A similar procedure was adopted for Pani in the absence andpresence of smoke for 180 s by rst keeping the sample insmoke for 30 s followed by 30 s in air. From Fig. 12, it may beobserved that Pani also showed reversibility but with a lesspronounced variation range from 1.15 S cm�1 to 1.06 S cm�1 in

the absence and presence of smoke respectively, compared withPani/SiC-3. In this case, too, there is an observed decrease inconductivity by 0.09 S cm�1 and Pani seems to be less efficientthan Pani/SiC-3 in terms of reversibility which has an excellentvariation range between 4.9 S cm�1 and 4.2 S cm�1 with anobserved decrease in conductivity by 0.7 S cm�1. Thus there wasaround an eight times greater variation range observed in theconductivity of the Pani/SiC-3 nanocomposite than that of Pani.The greater sensing response and excellent reversibility of thePani/SiC-3 nanocomposite may be attributed to an increase inthe surface area of Pani coated over the SiC nanoparticles,leading to the exposure of more active sites and more adsorp-tion and desorption, which leads to an increase in sensitivityand reversibility.

The sensing response of Pani/SiC-3 in the presence of carbondioxide (CO2), ammonia (NH3) and methanol (CH3OH) wasmeasured to know the conductivity response in the presence ofthese gases. Fig. 13 shows that when the sample is exposed toCO2 gas there was decrease in the conductivity observed due tothe interaction of the lone pairs on the oxygen atom of CO2

gas.40 In the case of methanol and ammonia, a signicant lossin conductivity is observed in the environment of these gasesand the conductivity starts to increase as the sample is exposedto air due to desorption of the gases. The signicant loss in

Fig. 10 Variation in the electrical conductivity of Pani/SiC-3 onalternating exposure to cigarette smoke and ambient air.

Fig. 11 Effect on the DC electrical conductivity of Pani on exposure tocigarette smoke followed by ambient air with respect to time.

Fig. 12 Variation in the electrical conductivity of Pani on alternatingexposure to cigarette smoke and ambient air.


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conductivity may be attributed to the greater availability ofa loosely bound electron pair on the nitrogen atom of ammoniaand the oxygen atom of methanol which interacts andneutralizes some Pani with each exposure. Cigarette smokecontains CO2, NH3 and CH3OH, so these may be contributinggases towards sensing cigarette smoke. Ansari et al. havedened the sensing response of nanocomposites of Pani withmultiwalled carbon nanotubes towards ammonia andmentioned a good response and quick recovery and also theresistance of the composite and Pani decreased in the presenceof ammonia.42 Konwer et al. reported the sensing response ofcomposites of Pani with graphene oxide towards methanol andmentioned that the resistance of both the composite and Panidecreased in the presence of methanol and that the sensitivityof the Pani/GO composite increases towards methanol

compared with that of pure Pani.44Habib Ullah et al. studied theinteractions of CO2, CO and NH3 gases with Pani. They observedthat carbon dioxide interacts with polyaniline towards theoxygen atom and that the conductance of Pani (ES) decreasedupon interaction with ammonia and carbon dioxide. They alsomentioned that CO gas shows a neutral effect on the conduc-tance of polyaniline.43 We also consider that polycyclic aromatichydrocarbons (PAHs), nicotine, hydrogen cyanide (HCN) andformaldehyde also adsorb on the surface of the Pani/SiC-3nanocomposite and are also responsible for the change inconductivity by temporarily occupying some active sites of thepolyaniline networks coated on the SiC nanoparticles. Thisresults in a decrease in the mobile charge carrier leading toa decrease in conductivity.

6.1 Proposed mechanism for sensing

The sensing mechanism of the Pani/SiC-3 nanocompositetowards cigarette smoke was explained through the DC elec-trical conductivity response by a simple adsorption anddesorption mechanism of smoke at room temperature. The DCelectrical conductivity varies with exposure to cigarette smoke.In the Pani/SiC-3 nanocomposite the DC electrical conductivitydecreases aer exposure to cigarette smoke which may beattributed to a decrease in the mobility of charge carriers assmoke interacts with the nanocomposite. When the nano-composite is exposed to air, the electrical conductivity revertedto its previous value. Cigarette smoke is composed of about�4000 chemicals and it is expected that carbon dioxide,ammonia and methanol present in the smoke may be respon-sible for the electrical conductivity response. Individualconductivity experiments were also done for carbon dioxide,ammonia and methanol and they also showed the same

Fig. 13 Effect on the DC electrical conductivity of Pani/SiC-3 onexposure to ammonia, methanol and carbon dioxide with respect totime.

Scheme 1 Proposed interaction of the Pani/SiC-3 nanocomposite with methanol, ammonia, carbon dioxide and cigarette smoke.


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response of a decrease in the electrical conductivity uponinteraction with the nanocomposite. In the Pani/SiC-3 nano-composite the lone pair of electrons on the nitrogen in poly-aniline interacts with the positively charged silicon of siliconcarbide. The lone pairs on the oxygen of carbon dioxide, thenitrogen of ammonia and the oxygen of methanol interact withthe polarons of polyaniline as shown in Scheme 1.42–44 Thisinteraction of lone pairs with the polarons of polyaniline leadsto a decrease in the mobility of the charge carriers thusdecreasing the electrical conductivity.

7. Conclusions

Pani and the Pani/SiC nanocomposites were successfullysynthesized by an in situ polymerization method and charac-terised by FTIR and XRD analysis. SEM and TEM analysisconrmed the uniform deposition of Pani on SiC nanoparticles.The Pani/SiC nanocomposites showed greater thermal stability,in terms of their DC electrical conductivity under isothermaland cyclic ageing conditions, than pristine Pani. These resultshighlighted that the Pani/SiC-3 nanocomposite was found to bea very effective material for cigarette smoke sensing as there wasaround an eight times greater variation range observed in theconductivity of the Pani/SiC-3 nanocomposite than that of Pani.Therefore, the sensor based on Pani/SiC may be useful as anefficient material for cigarette smoke sensing.

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ir.amu.ac.inir.amu.ac.in/10736/1/t10005.pdf1 department of applied chemistry faculty of engineering...

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