Microwave enabled synthesis of carbon based materials with controlled structures
Creators: name: Patel, Mehulkumar
Publisher: name: No Publisher Supplied
Year: 2016

description: Graphene is a single-layered sheet of sp2- bonded carbon atoms arranged in a honeycomb structure, whose discovery won the 2010 Nobel Prize in physics. Due to its excellent electronic, optical, thermal and mechanical properties, and its large surface area and low mass, graphene holds great potential for a broad range of applications. It seems that the research in graphene has now proceeded from the initial phase of developing myriad strategies for the synthesis of graphene sheets to the use of graphene in various research fields. However, it is still challenging to controllably produce solution processable highly conductive graphene sheets in large quantity, at low cost, and energy saving process, with optimal sheet size, layer thickness, defects (vacancies and holes) and molecular structures (oxygen-containing groups and non-defective graphene domains). All these structural parameters determine their electronic, thermal and mechanical properties of graphene, which are key warrants for their practical application in various devices. As examples, fundamental studies and high-frequency electronics require pristine graphene. However, "bulk"  applications such as flexible macro-electronics, and mechanically and electronically reinforced composites, require large quantities of solution-processable highly conductive large graphene sheets manufactured at low cost. On the other hand, holey graphene, referring to graphene with nanoholes in their basal plane, demonstrates much better performance in their application as metal-free catalysts and in energy storage. Finally, there is a surge of interests in nanosized graphene sheets for various biological applications due to their unique size effects, edge effects, and even quantum confinement effects.   As one part of this thesis, we have demonstrated that by understanding the oxidation mechanism of nitronium ions and KMnO4, which were both used in the widely used Hummers method for fabrication of graphene oxides, we developed various microwave chemistries for rapid (30-40 seconds) and controllable fabrication of graphene with controlled lateral sizes, holey structures, and oxidation levels. As examples, by intentionally excluding KMnO4 in the reaction system while controlling the concentration of nitronium ions and microwave irradiation power and time, we can rapidly and directly fabricate graphene nanosheets with uniform lateral sizes. The as-fabricated graphene nanosheets largely retain the intrinsic properties of graphene. These nanosheets exhibit strong and wavelength-independent absorption in NIR regions, which ensures their applications in Near-Infrared (NIR) photoacoustic imaging, photothermal treatment, and multifunctional drug delivery. On the other hand, by including KMnO4 in the recipe and still taking advantage of the unique thermal and kinetic effects of microwave heating, we developed approaches to directly fabricate micrometer sized graphene oxide with controlled holey structures. Taking one step further; we have also developed microwave chemistry to dope these graphene oxide sheets with/without holes in their basal planes with N controllable bond configurations. We have shown that the N-doping and holey structure of graphene is important for their excellent electrochemical catalytic performance in oxygen reduction reaction (ORR).   In the drive towards green and sustainable chemistry, there is an ever-increasing interest in developing the heteroatom-doped carbon-based catalysts to replace the metal-based catalysts for organic reactions. Compared to ORR, studies that use doped and/or co-doped carbon materials as catalysts for selective organic synthesis is in the early stages of development. This might be due to lack of systematic studies about how the electronic and geometrical structures, surface functionalities, and therefore, the interface properties of graphene-based materials determine their catalytic performance. Also lacking is the inability to synthesize these doped carbon catalysts in bulk quantity with simple and cost effective approaches. In the second part of this dissertation, we have reported extremely simple and rapid (seconds) approaches to directly synthesize gram quantities of single or multiple heteroatom-doped graphitic porous carbon materials from abundant and cheap biomass molecules (inositol or phytic acid) with controlled doping configuration. The porous structure of the catalyst is beneficial for efficient mass transport and dramatically increases edges and surface area, and therefore creates more accessible catalytic centers.  Furthermore, we have also explored the catalytic center of these heteroatom-doped carbon catalysts (especially phosphorus-doped and phosphorus, sulfur-codoped) to gain a fundamental understanding of how the heteroatom (P and S) configuration affect the catalytic properties of carbon material in ORR and industry oxidation reactions, such as benzyl alcohol oxidation. This fundamental understanding will help us to design more efficient heteroatom-doped carbon catalysts.