Carbon nanotube based nanocomposite for photocatalytic, photoelectrochemical water splitting and resistive switching devices

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CARBON NANOTUBE BASED NANOCOMPOSITES FOR PHOTOCATALYTIC, PHOTOELECTROCHEMICAL WATER

SPLITTING AND RESISTIVE SWITCHING DEVICES

DEEPTI CHAUDHARY

DEPARTMENT OF PHYSICS

INDIAN INSTITUTE OF TECHNOLOGY DELHI

OCTOBER 2017

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© Indian Institute of Technology Delhi, New Delhi, 2017

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CARBON NANOTUBE BASED NANOCOMPOSITES FOR PHOTOCATALYTIC, PHOTOELECTROCHEMICAL WATER

SPLITTING AND RESISTIVE SWITCHING DEVICES

by

DEEPTI CHAUDHARY

Department of Physics

Submitted

in fulfillment of the requirements of the degree of Doctor of Philosophy to the

INDIAN INSTITUTE OF TECHNOLOGY DELHI

OCTOBER 2017

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CERTIFICATE

This is to certify that the thesis entitled

“Carbon nanotube based nanocomposites for Photocatalytic, Photoelectrochemical water splitting and Resistive switching devices” being

submitted by Deepti Chaudhary to the Department of Physics, Indian Institute of Technology Delhi for the award of the degree of Doctor of Philosophy is a record of bonafide work carried by her. She has worked under our supervision and guidance and has fulfilled the requirement for the submission of this thesis, which in our opinion has reached the requisite standard. The results contained in it have not been submitted in part or full to any other university or institute for the awards of any degree or diploma.

Prof. Neeraj Khare Prof. V. D. Vankar Department of Physics Department of Physics

Indian Institute of Technology Delhi Indian Institute of Technology Delhi New Delhi-110016 New Delhi-110016

India India

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Acknowledgements

A thesis work can never be completed with single person’s effort. It requires the combined effort of several persons and this thesis is not an exception to that.

First of all, I express my deep sense of gratitude and indebtedness to my respected supervisors Prof. Neeraj Khare and Prof. V. D. Vankar for guiding me at every step. I am extremely indebted to them for their critical evaluation of my work and most importantly for their encouragement through the thick and thin phases of my work. Their continuous support and motivation has brought this work to its fruition. Their spirited dedication to work has throughout been an everlasting source of inspiration and indeed fortunate to have them as my supervisors. I am deeply obliged to both of them for supporting me at every stage of my work.

I would like to acknowledge and thank Dr. Sangeeta Khare for proof reading my thesis and valuable suggestions. I am greatly indebted to her fruitful advices and encouragement at various occasions.

I would like to express my profound respect to all Head of the Physics Department at IIT Delhi during my stay here, Prof. H. C. Gupta, Prof. B. R. Mehta and Prof. Anurag Sharma for their support and providing the basic infrastructural facilities needed to carry out the research work. I am very much thankful to my SRC members Prof. Pankaj Srivastava, Prof. Viresh Dutta and Prof. M.C. Bhatnagar for their valuable comments and suggestions.

I would like to acknowledge the Nanoscale Research Facility (NRF) IIT Delhi for providing the basic infrastructural facilities needed to carry out the research work.

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I wish to acknowledge the kind help and care received from my seniors Dr.

Himani Sharma, Dr. Vishakha Kaushik, Dr. Bharti Singh at various stages during the course of my Ph. D. work. They spent their valuable time to help me getting acquainted with various experimental techniques.

I express my thanks to my colleagues, Mr. Faraz Ahmad, Mr. Zubair Ansari, Mr.

Deepanshu Sharma, Mr. Simrjit Singh, Mr. Sandeep Munjal, Mr. Nikhil Agarwal, Mr.

Sunil Kumar, Ms. Surbhi Sharma, Ms. Pratisha Gangwar, Mr. Hudiram Hemojit Singh, Mr. Dheeraj Kumar, Ms. Mamta for their support and help in various stages of my work.

I owe my thanks to Nanostech Laboratory people for helping me. I would like to express my gratitude to Dr. Pushpsen Satyarthi and Mr. Shreekant Madaka for their timely help. I take the opportunity to express my grateful thanks to Mr. D.C. Sharma and Mr. Kuldeep Singh in conducting SEM measurements.

My heartest thanks to my friends Dr. Ashutosh Kumar, Dr. Intu Sharma, Dr.

Sudheer Kumar, for all the cheerful moments that I shared with them.

I am extremely grateful to my family and in-laws for their love, understanding and support. I owe my deep gratitude to my parents: Mr. Gajendra Singh and Mrs.

Kavita Rani for their love, encouragement and unconditional support in all my pursuits. I also thank my supportive and caring siblings Ms. Pooja Chaudhary and Mr. Abhinav Chaudhary for their love and encouragement to keep my moral high throughout the period of my doctoral work.

I am extremely happy to express my sincere appreciation to my husband Mr.

Vivek Chaudhary for his understanding, care, support and encouragement for all these years. He is always there where I need him. I am very lucky to have him in my life.

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Finally, I gratefully acknowledge the financial assistance which I received as Junior Research Fellow and Senior Research Fellow from Council of Scientific and Industrial Research, New Delhi during the course of my Ph.D. work. The financial assistance from IIT Delhi is highly acknowledged.

Above all, I express my indebtedness to the “Almighty” for all his blessings and kindness.

Deepti Chaudhary

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ABSTRACT

The right combination of properties, nanometer size diameter, structural integrity, high electrical conductivity, and chemical stability makes carbon nanotube (CNT) appropriate for variety of applications. CNT has been incorporated in a variety of materials with even more attractive properties and possible applications in different areas of nanotechnology. Recent applications of CNT based nanocomposites have been investigated in memory devices, supercapacitor, photocatalysis and solar hydrogen generation. Of specific interest in this thesis are the photocatalytic degradation of organic dyes, photoelectrochemical (PEC) water splitting and resistive switching device applications of CNT based nanocomposites.

As a potential solution to the global energy and environmental pollution, design and synthesis of artificial photocatalysts with high activities have attracted increasing scientific interest worldwide. Existing photocatalysts suffer from poor activity or no activity in visible light irradiation which restricts them from solar light utilization.

Therefore, it is essential to develop an alternative visible-light-driven photocatalyst that should be stable and efficient. Owing to the excellent mechanical, electrical and optical properties of CNT, it can serve as an ideal building block in hybrid catalysts and improve the performance of photocatalysts. The present study is related to the synthesis of multi-walled carbon nanotubes (MWCNT) and their nanocomposites with metal oxide/sulfide semiconductors for the photocatalytic and photoelectrochemical water splitting.

To work in this direction, a thermal chemical vapor deposition system has been used for the synthesis of CNT. Structural, morphological and microstructure studies have been carried out of the synthesized CNTs. Nanocomposites of CNT with metal oxides/sulfide semiconductors (TiO2, ZnO and CdS) have been synthesized using chemical method. The semiconductor nanoparticles are grown on the surface of CNT.

Enhanced photocatalytic activity of the CNT-based nanocomposites is obtained as compared to the metal oxide/sulfide. TiO2/CNT nanocomposite degrades 98.5%

methylene blue (MB) dye from the solution within 40 min, under UV irradiation, which is higher than ZnO/CNT and CdS/CNT nanocomposites.

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Noble metal (Ag) nanoparticles have been loaded onto TiO2/CNT nanocomposite, for the fabrication of efficient visible light active photocatalyst. It is observed that Ag nanoparticles affects the optical properties of TiO2 and greatly shift the photo-response of Ag/TiO2/CNT nanocomposite to visible light range. The Ag/TiO2/CNT ternary nanocomposite shows ~9 times higher photocatalytic activity compared to TiO2/CNT binary nanocomposite under visible light irradiation. The ternary Ag/TiO2/CNT photoanode also shows good PEC water splitting performance.

Ag/TiO2/CNT ternary nanocomposite photoanode exhibits four times higher photocurrent density (0.91 mA/cm²) as compared to the binary TiO2/CNT photoanode (0.23 mA/cm²). With the synergistic effect of Ag nanoparticles and CNT the Ag/TiO2/CNT nanocomposite photoanode shows low recombination rate of charge carriers and high charge carrier density than that of the binary TiO2/CNT and bare TiO2 photoanode.

A metal-free g-C3N4/TiO2/CNT ternary hybrid photocatalyst has been synthesized via a facile hydrothermal method. The g-C3N4/TiO2/CNT ternary nanocomposite shows enhanced PEC water splitting and photocatalytic dye degra-

dation performance under visible light irradiation as compared to bare g-C3N4 or g-C3N4/TiO2 nanocomposite. The g-C3N4/TiO2/CNT ternary nanocomposite

photoanode, for photoelectrochemical water splitting exhibits a six times higher photocurrent density (2.94 mA/cm²) as compared to the pure g-C3N4 photoanode (0.46 mA/cm²). A stepwise transfer of electrons in g-C3N4/TiO2/CNT ternary system

efficiently suppresses the charge carrier recombination compared to binary (g-C3N4/TiO2) and pure (g-C3N4) photocatalysts, resulting in enhanced photocatalytic

activity.

Finally, Resistive Switching phenomenon has been studied in the thin film of P3HT-CNT nanocomposite, using FTO/P3HT-CNT/Al sandwich structure. The study has been performed using with different doping amount of MWCNT in the P3HT- MWCNT nanocomposite films. The device with 4% CNT exhibited a typical bipolar resistive switching with set voltage of ∼1.8 V and a high ON/OFF ratio of >10² and good retention properties for >10³sec. It is observed that, ON/OFF state current ratio of the resistive switching device increases and turn-on voltage decreases with increasing the MWCNT content in the nanocomposite film.

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साराांश

नैनोमीटर आकार व्यास, सांरचनात्मक अखांडता, उच्च विद्युत चालकता, और रासायननक स्थिरता आदि

गुणों का सही सांयोजन कार्बन नैनोट्यूर् (CNT) को विविध अनुप्रयोगों के ललए उपयुक्त र्नाता है । CNT को

नैनोटेक्नोलॉजी के विलिन्न क्षेत्रों में विलिन्न प्रकार की सामग्रियों में और अग्रधक आकर्बक गुणों और सांिावित अनुप्रयोगों के ललए सस्ममललत ककया गया है । CNT आधाररत नैनोसांलमग्रितों के निीन अनुप्रयोगों में थमृनत उपकरण, उत्तम-सांधाररत्र, प्रकाशउत्प्रेरण और सौर ऊजाब द्िारा हाइड्रोजन उत्पािन आदि का समािेशन ककया जा

सकता है। इस शोध प्रर्ांध की विलशष्ट रुग्रच में कार्बननक रांजकों की सौरिैधुतरासायननक अिननत, सौरिैधुतरासायननक जल विपाटन और CNT आधाररत नैनोसांलमग्रितों के प्रनतरोध पररितबन युस्क्त में अनुप्रयोग सस्ममललत हैं ।

उच्च सकियता िाले कृत्रत्रम प्रकाश-उत्प्रेरकों की रचना और सांश्लेर्ण ने िैस्श्िक ऊजाब और पयाबिरण प्रिूर्ण के सांिावित समाधान के रूप में िुननया िर के िैज्ञाननकों को आकवर्बत ककया है । ितबमान प्रकाश-उत्प्रेरक या तो तुच्छ सकियता से िथत हैं या दृश्यमान प्रकाश विककरण में कोई सकियता प्रिलशबत नहीां करते है जो उन्हें

सौर प्रकाश के उपयोग से रोकता है । इसललए, िैकस्पपक दृश्य-प्रकाश- सांचाललत प्रकाश-उत्प्रेरक विकलसत करना

अत्यािश्यक है जो कक थिायी और सक्षम होना चादहए । CNT के उत्कृष्ट याांत्रत्रक, विद्युत और प्रकालशक गुणों

के कारण, यह सांकररत उत्प्रेरक के आिशब खंड के रूप में काम कर सकता है और प्रकाश-उत्प्रेरक के प्रिशबन में

सुधार कर सकता है। ितबमान अध्ययन मपटी-िापड कार्बन नैनोट्यूर्स (CNT) के सांश्लेर्ण और प्रकाश-उत्प्रेरक तिा प्रकाश-िैधुतरासायननक जल विपाटन (PEC) के ललए धातु ऑक्साइड/सपफाइड अधबचालक के साि उनके

नैनोसांलमग्रितों के र्ारे में है।

इस दिशा में कायब करने के ललए, एक ताप रासायननक िाष्प जमाि विधी का CNT के सांश्लेर्ण में

उपयोग ककया गया है । सांश्लेवर्त CNT का सांरचनात्मक, रूपात्मक और सूक्ष्म-सांरचना अध्ययन ककया गया है । CNT के साि धातु-आक्साइड/सपफाइड अधबचालक (TiO2, स्जांक और कैडलमयम सपफाइड ) के नैनोसांलमग्रितों

का रासायननक विग्रध से सांश्लेर्ण ककया गया है। अधबचालक नैनोकणों को CNT की सतह पर सांश्लेवर्त ककया

गया है । CNT-आधाररत नैनोसांलमग्रितों की प्रकाश-उत्प्रेरण सकियता को धातु-ऑक्साइड/सपफाइड के तुलनात्मक रूप में ज्ञात ककया गया है। TiO2/CNT नैनोसांलमग्रित परार्ैंगनी विककरण की उपस्थिनत में विलयन में उपस्थित 98.5% लमिायीलीन ब्लू (एमर्ी) को 40 लमनट में अिननतत कर िेता है, जो स्जांक/CNT और सीडीएस/CNT नैनोसांलमग्रित से अग्रधक है ।

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एक सक्षम दृश्य प्रकाश सकिय प्रकाश-उत्प्रेरक के ननमाबण के ललए नोर्ल धातु (Ag) नैनोकणों को

TiO2/CNT नैनोसांलमग्रित पर लोड ककया गया है । यह िेखा गया है कक Ag नैनोकणों की उपस्थिनत TiO2 के

प्रकालशक गुणों को प्रिावित करती है और Ag/TiO²/CNT नैनोसांलमग्रित के प्रकाश के प्रनत प्रनतकिया को दृश्य प्रकाश क्षेत्र में ले जाती है । दृश्य प्रकाश की उपस्थिनत में Ag/TiO2/CNT नैनोसांलमग्रित की प्रकाश-उत्प्रेरण सकियता TiO2/CNT नैनोसांलमग्रित की तुलना में ~ 9 गुना अग्रधक है। प्रकाश िैधुतरासायननक जल विपाटन के

ललए त्रीचर Ag/TiO2/CNT नैनोसांलमग्रित प्रकाश एनोड द्विचर TiO2/CNT प्रकाश एनोड (0.23 mA/cm²) की

तुलना में चार गुना उच्च प्रकाश धारा घनत्ि (0.91 mA/cm²) प्रिलशबत करता है । Ag नैनोकणों और CNT के

सहकियाशील प्रिाि के साि Ag/TiO2/CNT नैनोसांलमग्रित प्रकाश-एनोड द्विचर TiO2/CNT और एकल TiO2

प्रकाश-एनोड की तुलना में िग्रधबत दृश्य प्रकाश अिशोर्ण, आिेश िाहकों का कम पुनसंयोजन और उच्च आिेश

िाहक घनत्ि प्रिलशबत करता है ।

एक धातु मुक्त त्रीचर g-C3N4/TiO2/CNT प्रकाश उत्प्रेरक एक सरल जलतापीय विग्रध के माध्यम से

सांश्लेवर्त ककया गया है। दृश्य प्रकाश विककरण के अांतगबत, अनािृत g-C3N⁴ या g-C3N4/TiO2 नैनोसांलमग्रित की

तुलना में त्रीचर g-C³N4/TiO2/CNT नैनोसांलमग्रित PEC जल विपाटन और रांजक की प्रकाश उत्प्रेररत अिननत का र्ढा हुआ प्रिशबन पता चलता है । त्रीचर g-C³N4/TiO2/CNT नैनोसांलमग्रित, प्रकाश-एनोड, प्रकाश िैधुत रासायननक जल विपाटन के ललए के रूप में शुद्ध g-C3N4 प्रकाश एनोड (0.46 mA/cm²) की तुलना में एक छह गुना ज्यािा प्रकाश धारा घनत्ि (2.94 mA/cm²) िशाबती है। त्रीचर g-C3N4/TiO2/CNT में इलेक्रॉनों की एक चरणर्द्ध हथताांतरण सक्षमता से द्िीचर (g-C3N4/TiO2) और शुद्ध (g-C3N4) प्रकाश उत्प्रेरक की तुलना में

आिेश िाहक पुनसंयोजन का िमन करता है, स्जसके पररणामथिरूप प्रकाश उत्प्रेरण गनतविग्रध में िृद्ग्रध होती है।

अांततः, FTO/P3HT-CNT/Al सैंडविच सांरचना का उपयोग प्रनतरोधी पररितबन युस्क्त का अध्ययन (P3HT-CNT नैनोसांलमग्रित की पतली कफपम में) ककया गया है। P3HT-CNT नैनोसांलमग्रित कफपमों में CNT की अलग-अलग डोवपांग मात्रा के साि अध्ययन का प्रिशबन ककया गया है। 4% CNT डोवपांग डडिाइस ने ~1.8 V के सेट िोपटेज और 10³ सेकांड के ललए एक उच्च ओन/ऑफ अनुपात और 10³ सेकांड के ललए अच्छी प्रनतधारण गुणों के साि एक द्विध्रुिी प्रनतरोधी स्थिग्रचांग का प्रिशबन ककया। यह िेखा गया है कक, नैनोसांलमग्रित कफपम में

CNT मात्रा को र्ढाने के साि प्रनतरोधक स्थिग्रचांग डडिाइस की ओन/ऑफ विधुत धारा अनुपात र्ढ जाता है टनब- ओन िोपटेज घट जाती है ।

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Acknowledgements

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Abstract

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Table of Contents vii

List of Figures xiii

List of Tables xviii

CHAPTER 1: Introduction 1-37

1.1 Carbon nanotubes 2

1.2 Properties of carbon nanotubes 4

1.3 Synthesis of carbon nanotubes 6

1.3.1 Chemical vapour deposition 7

1.4 Growth mechanism of CNT 9

1.5 Processing of carbon nanotube composites 11

1.6 Applications of CNT 11

1.6.1 Photoelectrochemical water splitting 12

1.6.1.1 Basic principle 13

1.6.1.2 Semiconductor/Electrolyte Interface 15

1.6.2 Photocatalytic dye degradation 17

1.6.2.1 Basic principle 17

1.6.3 Basic requirements of the semiconductor 19 1.6.3.1 CNT-Semiconductor composite system 22 1.6.3.2 Noble Metal-Semiconductor composite system 22

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1.6.3.3 Semiconductor-Semiconductor composite system 23

1.6.4 Resistive switching 24

1.6.4.1 Conduction mechanism 26

1.6.5.1.1 Bulk limited conduction 27 1.6.4.2 Resistive switching in CNT 28

1.7 Objectives of the present thesis 29

1.8 Thesis organization 30

References

CHAPTER 2: Experimental and characterization techniques 38-60

2.1 Thermal chemical vapour deposition system 39

2.1.1 Synthesis of MWCNTs 39

2.1.2 Purification of MWCNTs 40

2.2 Hydrothermal technique 41

2.3 Spin coating technique 42

2.4 Characterization techniques 43

2.4.1 X-ray diffraction 43

2.4.2 Scanning electron microscopy 45

2.4.3 Transmission electron microscopy 47

2.4.4 Raman spectroscopy 49

2.4.5 Fourier transforms infrared spectroscopy 51 2.4.6 UV-VIS absorption spectroscopy 51 2.4.7 Photoluminescence spectroscopy 53 2.5 Photocatalytic dye degradation measurements 54

2.6 Photoelectrochemical measurements 56

2.7 Electrical measurements 58

References

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CHAPTER 3: CNT-Semiconductor hybrid nanocomposites for photocatalytic dye degradation 61-85

3.1 Introduction 62

3.2 Carbon nanotubes: Synthesis, structural, morphological and 63 optical characterizations

3.2.1 Synthesis of CNT 63

3.2.2 Results and Discussion

3.2.2.1 Structural analysis 64

3.2.2.2 Surface morphology of CNT 65

3.2.2.3 Microstructure of CNT 66

3.2.2.4 Raman spectroscopy studies of CNT 67 3.3 Synthesis of carbon nanotubes modified metal oxides/sulphides 68 nanocomposites and their comparative photocatalytic activity

3.3.1 Experimental details

3.3.1.1 Preparation of TiO2/CNT nanocomposite 68 3.3.1.2 Preparation of ZnO/CNT nanocomposite 69 3.3.1.3 Preparation of CdS/CNT nanocomposite 69 3.3.1.4 Photocatalytic experiment 70 3.3.2 Results and discussion

3.3.2.1 Structure and morphology analysis of CNT 70 nanocomposites

3.3.2.2 Raman analysis 74

3.3.2.3 Photocatalytic activity 76

3.3.2.4 Photocatalysis mechanism 78

3.4 Conclusions 83

References

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CHAPTER 4: Ag/TiO

2

/CNT ternary nanocomposite for visible-light- driven photocatalytic and photoelectrochemical water

splitting 86-112

4.1 Introduction 87

4.2 Experimental details 89

4.2.1 Preparation of Ag/TiO2/CNT nanocomposite 89

4.2.2 Photocatalytic experiment 89

4.2.3 Photoelectrochemical measurements 90 4.3 Result and Discussion

4.3.1 Structural analysis 91

4.3.2 Morphological and Elemental analysis 92

4.3.3 Raman analysis 94

4.3.4 Optical absorption analysis 95

4.3.5 Photocatalytic activity 97

4.3.6 Photocatalytic mechanism under UV light irradiation 99 4.3.7 Photocatalytic mechanism under visible light irradiation 101 4.3.8 Identification of Reactive species generation in Photocatalysis 102

4.3.9 Stability test 104

4.4 Photoelectrochemical measurements

4.4.1 Linear Sweep Voltammetry (LSV) results 105 4.4.2 Electrochemical impedance spectra 106

4.4.3 Photoluminescence spectra 107

4.4.4 Mott-Schottky analysis 108

4.4.5 PEC water splitting mechanism 110

4.5 Conclusions 112

References

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CHAPTER 5: g-C

3

N

4

/TiO

2

/CNT ternary nanocomposite for visible- light-driven photocatalytic and photoelectrochemical water splitting 115-140

5.1 Introduction 116

5.2 Experimental details

5.2.1 Synthesis of g-C3N4 117

5.2.3 Preparation of g-C3N4/TiO2/CNT nanocomposite 118

5.2.4 Photocatalysis experiment 118

5.2.5 Photoelectrochemical measurements 119 5.3 Result and Discussion

5.3.1 Structure and morphology analysis 120

5.3.2 Elemental analysis 122

5.3.3 FTIR spectra 123

5.3.4 Raman spectra 124

5.3.5 UV-Vis DRS results 125

5.3.6 Photocatalytic activity 126

5.3.6.1 Identification of reactive species by trapping experiments 128 5.3.6.2 Photocatalytic mechanism 129 5.3.6.3 Stability of photocatalyst 132 5.3.7 Photoelectrochemical measurements

5.3.7.1 Linear sweep voltammetry studies 133 5.3.7.2 Electrochemical impedance spectroscopy studies 135

5.3.7.3 Mott−Schottky analysis 136

5.3.7.4 PEC water splitting mechanism 137

5.4 Conclusions 138

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CHAPTER 6: Study of resistive switching effect in Poly (3-hexyl- thiophene) –Carbon Nanotube composite

films 141-155

6.1 Introduction 142

6.2 Experimental details 143

6.2.2 Device fabrication and characterization 144

6.3 Results and discussion 145

6.3.1 SEM results 145

6.3.2 I-V measurements 145

6.3.3 Retention characteristics 148

6.3.4 Endurance characteristics 149

6.3.5 Resistance-Temperature (R-T) characteristics 149

6.3.6 Conduction mechanism 150

6.3.7 Resistive switching mechanism 152

6.4 Conclusions 153

References

CHAPTER 7: Conclusions and future scope of the work

156-161

7.1 Conclusions of the present study 156

7.2 Future scope of the present study 161

List of publications

162

Author Biodata

165

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List of Figures

Figure 1.1 Different types of CNTs, SWCNT, DWCNT and MWCNT. 3 Figure 1.2 Growth model for CNT showing tip and base growth mechanism. 10 Figure 1.3 Reaction process of water splitting on a heterogeneous

photocatalyst. 13

Figure 1.4 Schematic representation of a three electrode PEC cell. 14 Figure 1.5 Band-bending at n-type semiconductor/electrolyte interface in

aqueous solution. 16

Figure 1.6 Shematic diagram showing the photocatalysis mechanism. 18 Figure 1.7 Solar water splitting mechanism for the H2 and O2 generation. 19 Figure 1.8 Band-edge positions of various semiconductors on a potential scale

(V) versus the normal hydrogen electrode (NHE). 20 Figure 1.9 Schematic diagram of different types of semiconductor

heterostructure. 24

Figure 1.10 Schematic diagram of a ReRAM memory cell. The device is a two terminal device where the metal/insulator layer is sandwiched by two metal electrodes.

25

Figure 1.11 Schematic diagram showing (a) Unipolar and (b) Bipolar resistive switching behavior. Current compliance (CC) is used to prevent permanent breakdown in the sample.

26

Figure 2.1 Schematic of thermal chemical vapour deposition system used for

the growth of carbon nanotubes. 40

Figure 2.2 Schematic diagram shows the film fabrication process by using

spin coating technique. 42

Figure 2.3 Basic principle of X-ray diffraction. 44 Figure 2.4 Schematic diagram of scanning electron microscope and electron

beam interaction with the sample. 46

Figure 2.5 Schematic diagram of transmission electron microscope. 48 Figure 2.6 Different scattering process in Raman spectroscopy. 50

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Figure 2.7 Schematic diagram of photocatalytic dye degradation experi-

mental setup. 55

Figure 2.8 (a) Spray gun, (b) film preparation by spray coating, and (c) PEC

cell. 57

Figure 2.9 Photoelectrochemical measurement experimental setup. 57 Figure 2.10 Schematic of experimental setup used for measuring I-V and

resistive switching characteristics. 58

Figure 3.1 XRD pattern of MWCNT power sample. 64

Figure 3.2 SEM micrograph (a) entangled CNT (b) aligned growth of CNT over the substrate (c) vertically aligned growth of CNT and (d) aligned CNT.

65

Figure 3.3 TEM micrograph of (a) as-synthesized and (b) purified CNT

sample, (c) HRTEM image of CNT. 66

Figure 3.4 Raman spectrum of MWCNT. 67

Figure 3.5 XRD patterns of TiO2/CNT, ZnO/CNT and CdS/CNT nanocomposites, pure TiO2, ZnO, CdS nanostructures and of MWCNTs. 71 Figure 3.6 HRTEM and TEM images of (a, b) TiO2/CNT nanocomposite, (c, d)

CdS/CNT nanocomposite and (e, f) ZnO/CNT nanocomposite. 73 Figure 3.7 Raman spectra of (a) TiO2/CNT, (b) ZnO/CNT, (c)CdS/CNT

nanocomposites, and (d) D and G−band position of MWCNT in the nanocomposites.

75

Figure 3.8 Photocatalytic degradation of MB under UV irradiation in the presence of TiO2/CNT, ZnO/CNT and CdS/CNT nanocomposites and pure TiO2, ZnO and CdS nanostructures.

76

Figure 3.9 Plot of the variation ln( c₀/c) with irradiation time in the presence of TiO2/CNT, ZnO/CNT or CdS/CNT nanocomposites and pure TiO2, ZnO or CdS.

77

Figure 3.10 Photocatalytic degradation efficiency graph showing a relative comparison of the efficiency of each photocatalyst after 40 min exposure to light.

78

Figure 3.11 Schematic diagram shows the reaction mechanism for dye degradation in the presence of Semiconductor/MWCNT nanocomposite.

79

Figure 3.12 Photoluminescence spectra of (a) TiO2/CNT and pure TiO2, (b) ZnO/CNT and pure ZnO, (c) CdS/CNT and pure CdS samples. 81

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Figure 3.13 Position of energy bands of TiO2, ZnO and CdS semiconductors and O₂/O₂^•−, OH⁻/OH^• with respect to normal hydrogen electrode scale (NHE).

82

Figure 4.1 XRD patterns of pure TiO2 nanoparticles and TiO2/CNT, Ag/TiO2,

Ag/TiO2/CNT nanocomposites. 91

Figure 4.2 TEM and HRTEM images of (a,b) TiO2/CNT and (c,d)

Ag/TiO2/CNT nanocomposites. 92

Figure 4.3 EDX spectra and elemental mapping of Ag/TiO2/CNT

nanocomposite. 93

Figure 4.4 (a) Raman spectra of TiO2, TiO2/CNT, Ag/TiO2 and Ag/TiO2/CNT samples, (b) D and G-band positions of CNT in the TiO2/CNT and Ag/TiO2/CNT nanocomposites.

94

Figure 4.5 Kubelka-Munk plot for the band-gap calculation of pure TiO2, Ag/TiO2, TiO2/CNT and Ag/TiO2/CNT nanocomposites. 96

Figure 4.6 (a) Absorption spectra of MB aqueous solution in the presence of Ag/TiO2/CNT nanocomposite as a function of irradiation time under visible light irradiation, (b) Photocatalytic degradation of MB dye (C/C0) in the presence of TiO2, Ag/TiO2, TiO2/CNT and Ag/TiO2/CNT nanocomposites under visible light irradiation, and (c) under UV irradiation. (d) Rate constant (k) values for TiO2, Ag/TiO2, TiO2/CNT and Ag/TiO2/CNT nanocomposites under UV and visible light irradiation.

98

Figure 4.7 Proposed mechanism for the photocatalytic degradation of MB over Ag/TiO2/CNT nanocomposite under UV light irradiation. 99 Figure 4.8 Proposed mechanism for the photocatalytic degradation of MB

under visible light irradiation. 100

Figure 4.9 Effects of different scavengers on the photodegradation efficiency of Ag/TiO2/CNT ternary nanocomposite under, (a) UV and (b) visible irradiation.

102

Figure 4.10 XRD spectra of the fresh and after several cycling runs of the

Ag/TiO2/CNT photocatalyst. 103

Figure 4.11 LSV curves of TiO2, TiO2/CNT and Ag/TiO2/CNT electrodes under dark (D) and visible illumination (L), shown by hollow sphere and solid sphere plot, respectively, using 0.5 M Na2SO4 electrolyte solution.

105

Figure 4.12 Nyquist plots of TiO2, TiO2/CNT and Ag/TiO2/CNT samples under visible-light illumination. In Nyquist plots solid sphere denotes the experimental data points and solid lines represent fitted data.

106

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Inset shows the equivalent circuit model.

Figure 4.13 PL spectra of TiO2, TiO2/CNT and Ag/TiO2/CNT samples. 107 Figure 4.14 Mott–Schottky plots for TiO2, TiO2/CNT and Ag/TiO2/CNT photo-

anode taken under dark conditions in a 0.5 M Na2SO4 solution.

Inset shows the Mott Schottky plots for Ag/TiO2/CNT.

108

Figure 4.15 Schematic representation of the mechanism of PEC water splitting under visible light irradiation. Band structure of the Ag/TiO2/CNT photoanode is also highlighted which shows photogenerated charge transfer process.

110

Figure 5.1 XRD patterns of pure g-C3N4, TiO2 and of g-C3N4/TiO2,

g-C3N4/TiO2/CNT nanocomposites. 119

Figure 5.2 SEM and TEM images of (a, c) g-C3N4, and (b,d) g-C3N4/TiO2/CNT

nanocomposite. 120

Figure 5.3 Elemental mapping of g-C3N4/TiO2/CNT nanocomposite showing

the distribution of C, N, Ti, and O. 121

Figure 5.4 FTIR spectra of (a) pure TiO2, (b) g-C3N4/TiO2 nanocomposite, (c) g-C3N4/TiO2/CNT and (d) pure g-C3N4 sample. 122

Figure 5.5 (a) Raman spectra of pure TiO2 and g-C3N4/TiO2, g-C3N4/TiO2/CNT nanocomposites. (b) Raman spectra of

g-C3N4/TiO2/CNT ternary nanocomposite and pure g-C3N4.

124

Figure 5.6 Kubelka–Munk transformed reflectance spectra and estimated bandgap of g-C3N4/TiO2, g-C3N4/TiO2/CNT, g-C3N4 and TiO2. 125 Figure 5.7 (a) MB degradation under visible light illumination for 90 min in

the presence of pure g-C3N4, TiO2, g-C3N4/TiO2 and g-C3N4/TiO2/CNT-X (X=0.4, 0.8 and 1.2 wt%) ternary

nanocomposite photocatalysts. (b) Degradation rate constant k (min⁻¹) of g-C3N4, TiO2, g-C3N4/TiO2 and g-C3N4/TiO2/CNT ternary nanocomposites. The error bars shown in figure represent the standard deviations of three independent measurements for each data point.

126

Figure 5.8 The effect of reactive species on the photocatalytic degradation of methylene blue dye in the presence of g-C3N4/TiO2/CNT ternary nanocomposite under visible light irradiation. The error bars shown in figure represent the standard deviations of three independent measurements for each data point.

128

Figure 5.9 Schematic diagram showing the separation and transfer of photogenerated charge carriers in g-C3N4/TiO2/CNT ternary nanocomposite as well as the possible reaction mechanism.

130

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Figure 5.10 (a) Stability test of the g-C3N4/TiO2/CNT-0.8 ternary nanocomposite for the degradation of MB under visible light irradiation. (b) XRD pattern of g-C3N4/TiO2/CNT sample before and after several uses for the photodegradation of MB.

132

Figure 5.11 J–V characteristics of g-C3N4, g-C3N4/TiO2 and g-C3N4/TiO2/CNT photoelectrodes in dark and light illumination. 133 Figure 5.12 Nyquist plots for the g-C3N4, g-C3N4/TiO2 and g-C3N4/TiO2/CNT

photoanodes in 0.5 M Na2SO4 solution under visible light. In Nyquist plots solid sphere denotes the experimental data points and dotted lines represent fitting results of these data.

134

Figure 5.13 Mott–Schottky plots for g-C3N4, g-C3N4/TiO2 and g-C3N4/TiO2/CNT photoanode taken under dark conditions in a

0.5 M Na2SO4 solution.

136

Figure 5.14 Schematic representation of the mechanism of PEC water splitting of g-C3N4/TiO2/CNT nanocomposite under visible light irradiation. 137 Figure 6.1 Schematic diagram of the resistive switching device of

FTO/P3HT−CNT/Al structure. 144

Figure 6.2 Cross sectional SEM image of P3HT−CNT composite film of

FTO/P3HT−CNT/Al device structure. 145

Figure 6.3 Current–Voltage characteristics of the FTO/P3HT−CNT/Al devices containing (a) 0%, (b) 2%, (c) 4%, and (d) 8% CNT. 146 Figure 6.4 Retention performance of the FTO/P3HT-CNT/Al device doped

with (a) 2% and (b) 4% CNT, under 0.1 V read voltage. 148 Figure 6.5 Endurance performance of the FTO/P3HT-CNT/Al device doped

with (a) 2% and (b) 4% CNT, under 0.1 V read voltage. 149 Figure 6.6 Temperature dependence of the resistances in (a) HRS, and (b)

LRS of the FTO/P3HT-2%CNT/Al memory device. 150 Figure 6.7 Experimental and fitted log I–log V curves of the memory devices

in a double logarithmic scale under SET process. FTO/P3HT- 2%CNT/Al process.

151

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List of Tables

Table 1.1 Different properties of carbon nanotubes.

Table 1.2 Comparison of various growth techniques to form carbon