Engineering Graphene Oxide for Membrane Applications

Abstract

This dissertation describes modified graphene oxide (GO) for membrane applications, such as gas separation and water treatment. In order to improve the limitation of GO especially used as a separation membrane in CO2 capture and water treatment, physical and chemical modification methods were used and transport behaviors of GO membranes was clarified and membrane performances were evaluated. This dissertation is organized into six chapters, containing introduction and conclusions, and the main topic of study can be classified as five primary areas: (1) preparation and characterization of GO and GO membranes, and drawbacks of GO membranes, (2) effect of surface porosity and interlayer distance of GO membrane on transport properties, (3) improving long-term stability of GO membrane at dry state, (4) tailoring sp2/sp3 ratio for understanding and improving transport performances, (5) creating pores and nanopockets for enhancing membrane properties, and (6) preparation of GO-incorporated mixed matrix membranes (MMMs) for applying to practical engineering. In chapter 1, the fundamental properties of GO and GO membranes were analyzed, and provide general description of preparation methods and application areas of GO-based membranes, as well as five challenging issues.ibes the background of polymeric membranes for gas separation, which is necessary to establish a strategy to overcome the challenging issues. Chapter 2 describes the modified GO with surface porosity and interlayer distanced increased by using sol-gel reaction of growing silica particles on the surface of GO sheet. Also, it was confirmed that the permeation performance of silica/GO membrane was improved. The surface porosity can be increased to increase the space for dissolving gas molecules, and the interlayer distance of the stacked GO can be widened so that the gas molecule can move rapidly inside GO membrane. Chapter 3 demonstrates preparation of Biosurfactant/GO composite membrane. The membrane was prepared by using sodium deoxycholate (SDC), which is one of Biosurfactant, to improve the long-term stability of permeation performance at dry state with structural stability of GO membrane. In the case of GO membranes, water molecules are contained between the laminated layers, which improves the separation performance by increasing the solubility of CO2. However, the permeation performance of the membrane decreases with time as the laminated structure is deformed because water molecules trapped in the interlayers are escaped under dry conditions. In order to improve this, it is possible to fabricate a membrane that maintains the permeation properties under non-humidified conditions by using a surfactant having an amphiphilic property such as GO and a high water sorption property. In chapter 4, sp2/sp3 ratio was controlled using the metastable property of GO, which changes its chemical binding structure without external stimuli due to unstable oxygen functional groups. GO was hydrothermally treated with different time and temperature, and it is confirmed through the basic characterization. Among them, controlled sp2/sp3 ratio was reconfirmed by conductivity measurement. Some of oxygen functional groups with large interactions with gas and water molecules are removed, and the remaining oxygen functional groups play a role for maintaining the interlayer distance. In addition, the sp2 region with relatively low interaction increases, thereby improving the permeability of gas and water molecules. Using this transport phenomenon, it is possible to study the transport mechanism according to the chemical structure of GO. Chapter 5 describes modified GO with improved transport behaviors and we explained it based on the solution-diffusion model. An additional transport path was created on individual GO sheets to improve the permeation properties in GO membrane. The GO sheets having pores were stacked, the empty void can be formed inside GO layers, and it was denoted as nanopockets in this study. It is possible to manufacture a GO membrane in which the molecules are rapidly diffused through the created pores on the surface of GO sheet and the solubility of the molecules is increased by the nanopockets formed inside the stacked structure, thereby greatly improving the permeation and separation performance. Chapter 6 demonstrates the fabrication and transport properties of GO-incorporated MMMs for practical engineering applications. In general, GO membrane shows decreased transport performance at high mole fraction and high pressure due to plasticization problem. Also, polymer membrane also shows similar phenomena as increasing time because of physical aging effect. To overcome these current limitations, GO was used as a nanofiller in this study. The effect of GO on the permeation and separation performance in the polymer matrix is explained by using solution-diffusion model. By comparison with the theoretical calculation values, the free volume of the mixed matrix membrane and the arrangement of GO in polymer matrix. Finally, we have fabricated MMMs with high transport performance over the trade-off relationship between permeability and selectivity. In this dissertation, GO was modified to improve the membrane performance. Also, the current issues on the physical, chemical, and structural limitations of GO membranes were discussed. Finally, future directions were represented for applying them in industrial level.