Materials prepared using graphite oxides : properties and applications

Sammanfattning: Graphite oxides are hydrophilic materials, which have attracted a lot of attention due to unique properties and possible applications. The current thesis includes studies of fundamental properties and applications of graphite oxides as well as materials prepared from graphite oxides such as graphene oxide membranes, defect-rich graphite oxide, reduced graphene oxide and activated reduced graphene oxide. We have studied Hummers and Brodie graphite oxides swelling in a set of normal alcohols from methanol to 1-nonanol using XRD, TGA, DSC, vapour and liquid sorption. Swollen structures with one to five parallel layers of intercalated alcohol molecules were found for Brodie graphite oxide immersed in liquid alcohols. Phase transitions between some of these phases were observed at low temperatures. Brodie graphite oxide was also studied for swelling in molten sugar alcohols. We have demonstrated the formation of an expanded structure of graphite oxide intercalated by molten xylitol and sorbitol. The structure remains stable at ambient conditions after the melt solidification. The swelling of graphite oxide was also investigated in tetrafluoroborate solution in acetonitrile, the solution used as a common electrolyte in energy storage applications. Graphite oxide in these experiments served as a model system due to flexible "pores" provided by temperature-dependent swelling in acetonitrile. Intercalation of electrolyte ions into GO structure resulted in a formation of distinct phase with expanded inter-layer distance. Results of our experiments allow to evaluate minimal “pore size” required to accommodate electrolyte ions in solvate and desolvated states. Swelling pressure is one of the fundamental properties of graphite oxide, which has not been reported previously but important in applications involving swelling under confinement conditions. Significant pressures up to 220bar were measured for bulk graphite oxide due to swelling in water and ethanol. Swelling pressure in the range 3-25 bar was also measured for μm thick membranes. Both powder and membrane samples showed a significant difference in kinetics of building pressure in water and ethanol.Ageing effects were studied using graphene oxide membranes stored on air over prolonged periods starting from weeks and up to five years. The pronounced effects of ageing on chemical composition and swelling of graphene oxide membranes in a set of alcohols (methanol to 1-nonanol) were found. These effects were assigned to chemical modification during air storage rather than to intrinsic metastability of graphene oxide. The structural changes related to the chemical modification of graphene oxide on air are crucial for membrane applications because they significantly affect the size of “permeation channels” provided by swelling. Extremely defective graphene oxide was demonstrated as a promising material for sorbent applications. The defect-rich graphene oxide was prepared using Hummers oxidation of strongly defective reduced graphite oxide instead of well crystalline graphite. The defect-rich graphene oxide showed significant improvement in sorption of radionuclides (U(VI), Am(III), and Eu(III)) compared to standard graphite oxides. High abundance of defects was demonstrated to correlate with the maximal sorption capacity of graphite oxide towards radionuclides and methylene blue. Finally, we have studied activated reduced graphene oxide for application in supercapacitors. This study aimed to reveal correlations between structural parameters of electrode material and supercapacitor performance. We have prepared two sets of materials with a broad range of N2 BET surface area 1000-3000m2g-1 , significant variation of pore size distribution and oxygen content using change of activation temperature and post-synthesis mechanical treatment. Analysis of nitrogen and water sorption isotherms showed that despite negligibly small H2O BET surface area, the total pore volume is very similar for nitrogen and water sorption. The best supercapacitor performance was found to depend on combination of several parameters in a delicate balance of specific surface area, oxygen content, micropore volume and conductivity