Controlling Microstructure of Cu for Catalytic Properties of Graphene Synthesis and CO2 Reduction

Abstract

Cu is a metal that has various excellent properties, such as high electrical and thermal conductivities, low thermal expansion, corrosion resistance, tensile strength, and ductility. Cu has been applied in various electrical devices due to its desirable electrical properties. Additionally, Cu has notable catalytic characteristics, and recently, Cu has received much attention as a catalyst for the synthesis of novel materials and for the production of energy resources. For the effective activity of Cu as a catalyst, the characteristics of surface and microstructure is important. Many researchers have attempted to modify the surface morphology and control the composition of Cu based alloys. Because catalytic reactions occur on the surface of a catalyst, an increase of Cu surface area and a modified Cu surface morphology are critical factors. The surface structure of a Cu catalyst has a significant effect on the reaction rate or products of a reaction. Defects of the surface, surface morphology, grain boundaries and surface orientation are also important factors. In addition, Cu can be easily combined with other metals to form alloys, and the addition of small amounts of impurities could modulate the catalytic properties of Cu. Recently, the synthesis of high quality, large area graphene film has been achieved using Cu catalyst. Following this achievement, many researchers have studied the improvement of Cu catalytic properties and analyzed the effects of Cu on graphene synthesis. In other field, reduction of CO2 gases to hydrocarbon materials using Cu catalysts for resolving the environmental problems has received much attention because Cu is an effective catalyst for the production of hydrocarbon materials. In this thesis, microstructural control of Cu to improve its catalytic properties in graphene synthesis and CO2 reduction is investigated. For the improvement of the quality of synthesized graphene films, a Cu-Ag alloy was applied with Ag plating on the surface of a Cu foil. Ag plating and post annealing formed a dilute Cu alloy, and highly uniform graphene films were synthesized on the Cu-Ag alloy catalyst. The plated Ag diffused into the Cu, and the formation of a uniform Cu-Ag alloy was demonstrated by various analyses. The synthesis of full-grown graphene films was achieved at 900 ℃, which is lower than the conventional synthesis temperature. Further, the synthesis of highly uniform graphene monolayers was confirmed using Raman spectroscopy mapping analysis and TEM analysis. Graphene synthesis was enhanced with the increasing of Ag plating thickness for the thin Ag plating, while non-uniformity of graphene also increased by the thick Ag plating. Optimization of the amount of Ag plating was investigated to obtain a highly uniform monolayer film of graphene without defects. This optimized Cu-Ag alloy controlled the formation of multilayer nucleation, which allowed a lower synthesis temperature with enhanced monolayer coverage. In addition, the unusual grain growth of Cu into a cube texture with giant grain sizes on the scale of mm2 was investigated on the Cu-Ag alloy with graphene synthesis. This unusual grain growth was observed only after graphene synthesis on the Cu-Ag alloy with various type of Cu foils. The giant grain growth of Cu into a cube texture with reduced grain boundaries and the ratio of (100) exceeded more than 90 % of the Cu-Ag alloy with graphene synthesis, and grain size was more than 1.05 mm2. A small addition of Ag atoms and a one-atom thick graphene film induced the remarkable microstructural evolution of a μm-thick Cu foil. Finally, a novel structure on the surface of Cu was developed to improve its catalytic activity for CO2 reduction. Extrusion and intrusion structures were fabricated on the surface of a Cu film on a flexible substrate with cyclic bending deformation. Previously, many researchers have focused on the surface roughness or orientation of Cu. However, the extrusion/intrusion structures of fatigued Cu were induced by the dislocation motion through the (111) slip plane of Cu, and this is the first time this structure has been fabricated for CO2 reduction. Fatigued Cu with extrusion/intrusion structures exhibited higher efficiency for total current density and for an increase in the production of hydrocarbon materials. The efficiency of CO2 reduction was improved by approximately 58 %, and the various hydrocarbon products, such as CO, CH4, C2H4, CH3OH, C2H5OH, and HCOOH, were generated at significantly increased production rates. The microstructure of Cu and catalytic performance of the fatigued Cu were analyzed for demonstration of the effects of the extrusion/intrusion structures. This study investigated the improvement of Cu catalytic properties using Cu based alloy formation and a mechanical metallurgy method. A simple fabrication method was introduced for the control of Cu microstructure, i.e., Ag plating and post annealing for the formation of a Cu-based alloy and cyclic bending for the evolution of extrusion/intrusion structures on the Cu surface. These novel ideas for improving the catalytic properties of Cu for graphene synthesis and CO2 reduction extend the possibilities for Cu application as an advanced catalyst for various systems.