Investigation into the active sites for two dimensional electrocatalysts
Creators: name: Fullon, Raymond Raphael
Publisher: name: No Publisher Supplied
Year: 2018

description: Two dimensional (2D) materials are an exceptional class of materials that have many remarkable optoelectronic, mechanical, and chemical properties. In combination with the layered structure of 2D materials, they allow us many opportunities and scientific pathways to explore. The most well-known 2D material, graphene, has been widely studied and researched and found to be a single atomically thin nanosheet of carbon in the lateral dimension. Due to the high cost of synthesizing graphene, alternatives have been investigated that allow for solution processing such as graphene oxide suspensions in water. Recently, a class of 2D materials beyond graphene have been of significant interest, transition metal dichalcogenides (TMDs) due to their varied ability to be semiconducting, insulating or metallic. 2D materials have found applications in electronics, electrochemistry, superconducting, photovoltaics and many more.As global demand for energy increases, it is clear that depending on fossil fuels is not sustainable. In order to combat our fossil fuel dependence, cost effective and efficient alternatives must be found such as hydrogen powered fuel cells. However, the high cost of producing hydrogen and catalysts found in fuel cells have prevents its commercialization. Developing cost-effective and high performing catalysts that can electrocatalytically produce hydrogen and can function within fuel cells are imperative for realizing this alternative fuel.2D materials such as MoS2 and graphene oxide have recently been shown to be active for the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR), respectively. In any electrocatalytic reaction, the active site is a location on the catalyst where the reaction takes place. Determining the active site and its location is challenging, as electrocatalytic reactions occur on such a small scale and it can be difficult to isolate from external influences. Insight into the active site is crucial for using these materials in electrocatalysis, as a fundamental understanding of the active site yields the best routes to developing high performing catalysts. In this thesis, we developed a method of fabrication electrochemical microcells on individual monolayers of 2D materials using electron beam lithography. These microcells allow us to electrochemically test 2D materials for electrocatalytic reactions. We first developed this method on 2D MoS2 grown by chemical vapor deposition. Using this method, we measured the electrocatalytic performance of the edge and, previously thought to be inactive, basal plane of 2D MoS2, and found no relationship between edges and HER activity. Investigation into the contact resistance of our microcell devices yielded important insight into its importance in HER measurements. When the contact resistance is decreased, the electrocatalytic performance was enhanced. If sufficiently decreased, the basal plane of MoS2 was found to have high performance for the HER. In addition, we have recently developed a microwave reduction process to achieve high quality graphene oxide with high graphene character. Microwave reduced graphene oxide (MWrGO) was doped with nitrogen by annealing in ammonia (NH3) gas at elevated temperature. Defects were found to be highly important for nitrogen incorporation and once nitrogen had been incorporated into the MWrGO lattice we determined the configuration of the doped nitrogen. Our electrocatalytic results showed that when the pyridinic nitrogen bonding configuration dominates the nitrogen content, the ORR activity in acidic conditions is improved. In order to further investigate this behavior, we sought to fabricate electrochemical microcells on graphene grown by chemical vapor deposition and nitrogen doped graphene (N-graphene) doped by NH3 annealing. A method for patterning the graphene was developed using negative electron beam lithography, but was found to damage N-graphene. Attempts were made to maintain the nitrogen content throughout the device processing but were not successful. Our electrocatalytic results did demonstrate that electrocatalytic measurements for the ORR could be made on CVD graphene, and is a promising technique moving forward.