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Surface Functionalization of Graphene-Based Materials By ABSTRACT Surface Functionalization of Graphene-based Materials by Akshay Mathkar Graphene-based materials have generated tremendous interest in the past decade. Manipulating their characteristics using wet-chemistry methods holds distinctive value, as it provides a means towards scaling up, while not being limited by yield. The majority of this thesis focuses on the surface functionalization of graphene oxide (GO), which has drawn tremendous attention as a tunable precursor due to its readily chemically manipulable surface and richly functionalized basal plane. Firstly, a room-temperature based method is presented to reduce GO stepwise, with each organic moiety being removed sequentially. Characterization confirms the carbonyl group to be reduced first, while the tertiary alcohol is reduced last, as the optical gap decrease from 3.5 eV down to 1 eV. This provides greater control over GO, which is an inhomogeneous system, and is the first study to elucidate the order of removal of each functional group. In addition to organically manipulating GO, this thesis also reports a chemical methodology to inorganically functionalize GO and tune its wetting characteristics. A chemical method to covalently attach fluorine atoms in the form of tertiary alkyl fluorides is reported, and confirmed by MAS 13C NMR, as two forms of fluorinated graphene oxide (FGO) with varying C/F and C/O ratios are synthesized. Introducing C-F bonds decreases the overall surface free energy, which drastically reduces GO’s wetting behavior, especially in its highly fluorinated form. Ease of solution processing leads to iii development of sprayable inks that are deposited on a range of porous and non- porous surfaces to impart amphiphobicity. This is the first report that tunes the wetting characteristics of GO. Lastly as a part of a collaboration with ConocoPhillips, another class of carbon nanomaterials - carbon nanotubes (CNTs), have been inorganically functionalized to repel 30 wt% MEA, a critical solvent in CO2 recovery. In addition to improving the solution processability of CNTs, composite, homogeneous solutions are created with polysulfones and polyimides to fabricate CNT-polymer nanocomposites that display contact angles greater than 150o with 30 wt% MEA. This yields materials that are inherently supersolvophobic, instead of simply surface treating polymeric films, while the low density of fluorinated CNTs makes them a better alternative to superhydrophobic polymer materials. iv Acknowledgments While it is my name that appears on the front page of this thesis, there are a number of people who have greatly contributed to getting it to where it is. I can confidently say that the last 5 years have been the best of my life, and the people I’ve interacted with, on both a professional and personal level, have had a tremendously positive impact on me. I would like to express my sincere gratitude towards my advisor, Dr. P.M. Ajayan. Working under his guidance and being around his refreshing approach towards research has been eye-opening for me. From my very first days as a graduate student, he has been a great mentor and every day I look forward to emulating his actions as a leader. I am also grateful to the NSF-Eager Fund (Award# CMMI-1153648) and ConocoPhillips for their financial support. I would also like to thank Dr. Robert Vajtai. The impromptu coffee breaks and conversations in his office have been greatly enriching, and will be dearly missed. I also appreciate Dr. Angel Marti and Dr. Enrique V. Barrera, who agreed to serve on my defense committee at such short notice, and provided me with due guidance. I would like to thank Dr. Lawrence Alemany for his expertise in dealing with fluorine-based 13C-NMR as well as giving the work done in Chapter 3 a much clearer direction and focus. Every member of the Ajayan group at Rice University has been a contributor to this thesis in one form or another, and I am honored to acknowledge them. Amongst those, Dr. T.N. Narayanan, Dr. Kaushik Balakrishnan, Charudatta Galande, Neelam Singh, Sanketh Gowda, Hemtej Gullapalli and Daniel v Paul Hashim have been peers that I have approached in moments of confusion, and at times desperation! My experience with ConocoPhillips was another turning point during my Ph. D., and the time spent specifically with Dr. Clint Aichele and Dr. Imona Omole gave me a look at research in industry, which is something that not a lot of graduate students have the opportunity to do, and for that, I am thankful. I would also like to thank Carl Deppisch at Intel Corporation, who I worked with during the summer of 2012. Over the past 5 years, I have also been an active member of Rice Club Tennis, which has been a fantastic way to remain in touch with my one of my long-standing hobbies and also provided a continuous influx of talented and hard-working undergraduate interns. Dylan Tozier, Patricia Chang and Patrick Nguyen were instrumental in each of the three first-author papers I published, and for the long weekends and late weeknights, I am extremely grateful. It wouldn’t be fair for me to not mention Dr. Raul Caretta, at the University of Minnesota, who has been a great mentor, and teachers from Gandhi Memorial International School in Jakarta, Indonesia who have stressed on the value of continuing to pursue learning in me from the get go. Lastly, it would be safe to say that I would not have gotten through this degree without the support of my family members. My parents, Uday and Savita Mathkar and my sister Ankita, have both been a great source of emotional stability, always reminding me that life is more about going through the journey, rather than getting to the final destination. Contents Acknowledgments ..................................................................................................... iv Contents ................................................................................................................... vi List of Figures ............................................................................................................ ix List of Tables ............................................................................................................ xvi List of Equations ...................................................................................................... xvii Nomenclature ........................................................................................................ xviii Introduction ............................................................................................................... 1 1.1. Introduction to Carbon Materials ............................................................................ 1 1.2. A Brief Introduction to Graphene Oxide .................................................................. 5 1.3. Proposed Synthetic Schemes ................................................................................... 7 1.3.1. First Studies on Graphite & the Brodie Method ................................................ 7 1.3.2. Origins of the Hummer’s Method ...................................................................... 8 1.3.3. An Improved Method to synthesize GO .......................................................... 10 1.4. Proposed Structures of GO..................................................................................... 13 1.4.1. Initial Models ................................................................................................... 13 1.4.2. Lerf-Klinowski Model & Recent Advances ....................................................... 15 1.5. Reduction of GO ..................................................................................................... 16 1.5.1. Chemical Reduction of GO ............................................................................... 16 1.5.2. Thermal Reduction of GO ................................................................................ 23 1.6. Surface Manipulation of GO ................................................................................... 27 1.6.1. Organic Manipulation of GO ............................................................................ 27 1.6.2. Inorganic Manipulation of GO ......................................................................... 29 1.6.2.1. N-Doping of Graphene Oxide .................................................................... 29 1.6.2.2. Covalent Bulk Functionalization of graphene ........................................... 30 1.7. Motivation for Existing Work ................................................................................. 32 Organic Manipulation of GO: Stepwise Reduction ..................................................... 34 2.1. General Introduction & Motivation ....................................................................... 35 2.2. Experimental Methods ........................................................................................... 38 vii 2.2.1. Synthesis of GO ................................................................................................ 38 2.2.2. Reduction of GO ............................................................................................... 38 2.2.3. Analytical Techniques ...................................................................................... 40 2.2.3.1. Band gap Measurement – Tauc’s Analysis ................................................ 40 2.3. Results & Discussion ............................................................................................... 42 2.4. Concluding
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