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Application of Metallocenes for the Synthesis of Multiwalled Carbon Nanotubes

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Application of Metallocenes for the Synthesis of Multiwalled Carbon Nanotubes APPLICATION OF METALLOCENES FOR THE SYNTHESIS OF MULTIWALLED CARBON NANOTUBES by Nonjabulo Philile Dominica Ngidi Submitted in fulfilment of the academic requirement for the degree of Master of Science in the School of Chemistry and Physics University of KwaZulu-Natal, Durban, Westville campus February 2016 ABSTRACT Multiwalled carbon nanotubes (MWCNTs) are carbon materials which have one-dimensional structure. They possess unique properties such as semi-conductor and high tensile strength that allow them to be widely used in many applications. MWCNTs and other shaped carbon nanomaterials (SCNMs) were synthesized by chemical vapour deposition (CVD) method. Three factors that affect the morphology, thermal, chemical, mechanical and electrical properties of SCNMs were investigated. The parameters are: carbon source, catalyst (metallocenes), and growth temperature. Two different carbon sources were studied for the synthesis of MWCNTs i.e., toluene and acetonitrile (also used as a nitrogen source). The metallocenes: nickelocene, cobaltocene and ruthenocene were used as catalysts (2.5 wt.%) while ferrocene was employed as control. These metallocenes were investigated because they have similar structure as ferrocene, a well-known catalyst for the synthesis of SCNMs. The synthesis was carried out at five different growth temperatures, 800, 850, 900, 950 and 1000 °C. As-synthesized MWCNTs and other SCNMs were further purified, in order to remove amorphous carbon and this was performed by testing different methods of purification. The effective method for purification of MWCNTs was method 2 which involved refluxing for 24 hours. It was chosen to be the best method because it produced purer MWCNTs as compared to other methods which caused damage to the MWCNTs. The SCNMs obtained were characterized using various techniques namely, transmission electron microscopy, scanning electron microscopy, electron dispersive X-ray spectroscopy, Raman spectroscopy, thermogravimetric analysis, elemental analysis, Branuaer-Emmet-Teller surface area and porosity analysis. It was observed that growth temperature has an impact on the yield and SCNMs distribution. At higher growth temperatures of 900, 950 and 1000 °C, carbon spheres were mainly formed while lower growth temperature (800 °C) favoured the formation of amorphous carbon. The best growth temperatures, in terms of high yield of MWCNTs, thermal stability and i crystallinity, was observed to be 850 °C. Apart from growth temperature, the morphology of the SCNMs was also found to be carbon source dependent. Acetonitrile as a carbon source, was found to form nitrogen-doped MWCNTs with bamboo compartments while toluene produced pristine-MWCNTs. In all cases other SCNMs were obtained. The length of the bamboo compartments in the nitrogen-doped MWCNTs decreased at lower growth temperatures. Smaller bamboo compartment lengths correlate with lower thermal stability and crystallinity. This suggests a larger incorporation of nitrogen in the MWCNT framework. The highest nitrogen-doping level was obtained at a growth temperature of 850 °C in all catalysts used. MWCNTs formed from toluene were more thermally stable as compared to those synthesized from acetonitrile. The catalyst had a great influence in determining the morphology of the N-MWCNTs. Ferrocene and nickelocene produced well- aligned N-MWCNTs while cobaltocene and ruthenocene yielded ‘spaghetti-like’ structures. This study has shown that the synthesis of MWCNTs is highly dependent on the catalyst, carbon source and growth temperature of reaction used. ii PREFACE The experimental work described in this thesis was carried out in the School of Chemistry and Physics, University of KwaZulu-Natal, Durban, from February 2014 to February 2016, under the supervision of Prof Vincent O. Nyamori and co-supervised by Dr Bernard O. Owaga. These studies represent original work by the author and have not otherwise been submitted in any form for any degree or diploma to any tertiary institution. Where use has been made of the work of others it is duly acknowledged in the text. Signed: _________________ iii DECLARATION 1 – PLAGIARISM I, Nonjabulo Philile Dominica Ngidi declare that; 1. The research reported in thesis, except where otherwise indicated, is my original research. 2. This thesis has not been submitted for any degree or examination at any other University. 3. This thesis does not contain other person’s data, pictures, graphs or other information, unless specifically acknowledged as being sourced from the persons. 4. This thesis does not contain other person’s writing, unless specifically acknowledged as being sourced from other researchers. Where other written sources have been quoted, then: a. Their words have been re-written but, the general information attributed to them has been referenced. b. Where their exact words have been used, then their writing has been placed in italics and inside quotation marks, and referenced. 5 This thesis does not contain text, graphics or tables copied and pasted from the internet, unless specifically acknowledged, and the source being detailed in the thesis and in the references sections. Signed: _________________ iv DECLARATION 2 – PRESENTATIONS AND PUBLICATION Presentations 1. Nonjabulo P. D. Ngidi, Vincent O. Nyamori and Bernard O. Owaga. Application of metallocenes for the synthesis of multiwalled carbon nanotubes. UKZN, College of Agriculture, Engineering and Science Postgraduate Research Day. Pietermaritzburg, South Africa, 22nd September 2015. 2. Nonjabulo P. D. Ngidi, Vincent O. Nyamori and Bernard O. Owaga. Application of metallocenes for the synthesis of nitrogen-doped multiwalled carbon nanotube. 42nd National Convention of the South African Chemical Institute. Durban, South Africa, 30th November – 4th December 2015. Publication A manuscript is being prepared for submission under the title “Effect of carbon source, metallocene and temperature on carbon nanotubes”. Signed: _________________ v ACKNOWLEGDEMENTS It gives me great pleasure in expressing my gratitude to all those people who have supported me and had their contributions in making this study possible. First and foremost, I would like to acknowledge and thank the Almighty God, for blessing, protecting and guiding me throughout this period. I would like to offer my sincerest gratitude to my supervisors, Prof V.O. Nyamori and Dr B.O. Owaga, who have supported me throughout my project with their patience and knowledge, whilst allowing me the room to work independently. I attribute the level of my Masters degree to their encouragement and without them this project would not have been completed in time. I also wish to thank the other academic staff, technical staff in the School of Chemistry and Physics and as well as the members of the Microscopy Units for their valuable contribution and assistance with the instruments. Special gratitude goes to Mr Siphesihle Zulu for introducing me to my best friend ever, thin layer chromatography. After all, it is the fastest, cheapest and easiest way to obtain meaningful information about what is going on in your reaction. My deepest gratitude also goes to Dr Moses Ollengo, Dr Godfrey Keru, Mr Eric Njogu and Mr Bheki Gumbi for their critical comment and proofreading of my thesis. Many thanks go to my fellow labmates in Nanochemistry research group: Ntokozo, Samantha, Josiana, Tonde, Kudzai, Ayomide and close friends who constantly motivated me even when times were hard and I appreciate you all. Through all the ups and downs, for the many hours that we spent together, I would not replace any of you I have worked with in the laboratory. I can’t imagine my current position without the love and support from my family. I thank my parents, Mr Nqaba and Mrs Hlengiwe Ngidi for striving hard to provide a good education for me and my siblings. I always fall short of words to adequately describe their support. Thank you for giving me strength to reach for the stars and chase my dreams. My sisters: Nolwazi vi and Minenhle, brothers: Philani and Mvelo, cousins: Kholiwe and Sphamandla, grandfather: Mr Vusimuzi Ngidi, aunties and uncles deserve my wholehearted thanks as well. I would like to remember and thank my late paternal grandmother, Mrs Nokwethuka Ngidi (may her soul rest in peace). She might not be physically around but her prayers are. This work would not have been possible without the financial contribution of National Research Foundation (NRF) of South Africa. Last but not least, although there may be many who remain unacknowledged in this humble note of gratitude there is none who remain unappreciated. vii DEDICATION My dissertation work is dedicated to my parents who introduced me to the joy of reading from birth, enabling such a study to take place today. viii TABLE OF CONTENTS Title Page ABSTRACT i PREFACE iii DECLARATION 1 – PLAGIARISM iv DECLARATION 2 – PRESENTATIONS AND PUBLICATION v ACKNOWLEGDEMENTS vi DEDICATION viii TABLE OF CONTENTS ix LIST OF FIGURES xiii LIST OF TABLES xvii LIST OF SCHEMES xviii LIST OF ABBREVIATION, FORMULAE AND SYMBOLS xix CHAPTER 1: INTRODUCTION 1 1.1 Background 1 1.2 Motivation 3 1.3 Aims and objectives 4 1.3.1 The specific objectives were as follows: 4 1.3.2 Research questions 4 1.4 Research approach 5 1.5 Outline of this dissertation 6 1.6 References 7 CHAPTER 2: LITERATURE REVIEW 11 2.1 Metallocenes 11 2.2 Carbon-based nanomaterial 13 2.2.1 Carbon nanotubes 17 2.2.2 Discovery of CNTs 17 2.2.3 Structural features and classification of CNTs 18 2.2.4 Properties of CNTs 22 . 2.2.4.1
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