Crystal, defect, and morphology engineering of porous two-dimensional materials for ionic separation

Ordered two-dimensional (2D) materials hosting Å-scale pores are highly promising for enabling challenging separation, thanks to well-defined pore geometry resulting in tight confinement of ions when hosted inside the pore. In addition, the 2D nature of these materials allows one to engineer thin films which are ideal for membrane-based separation while achieving a high ion flux. This dissertation investigates the structural characterization and ion-sieving applications of two classes of crystalline porous 2D materials, based on graphitic carbon nitride (g-C3N4) and metal-organic frameworks (MOFs) with well-defined pore architectures.The dissertation begins with g-C3N4 crystallized as poly(triazine imide) or PTI via a molten salt synthesis route. It is noted for its high pore density consisting of an array of 3.4-Å-sized pores which are ideal for size sieving of small molecules and ions. However, a significant challenge arises from the ions (primarily Li+ and Cl-) intercalated in as-synthesized PTI. In particular, Li+ positions itself inside the plane of the pore occluding the pore for transport. Addressing this, this research introduces acid treatment to modulate the ion depletion level in PTI, effectively substituting Li+ in the pore space with H+ and reducing Cl- concentration in the gallery space. A series of characterization tools is used to reveal, for the first time, the coexistence of AA' stacking of layers with the open pore channels aligned along c-axis, facilitating transport and AB stacking of layers, wherein adjacent PTI layers are shifted by a/3 and 2b/3 along the two crystallographic directions, resulting in closed channels. A notable increase in proton conductance through PTI layers is observed at higher depletion levels.Next, the dissertation investigates the synthesis of unit-cell-thick non-van der Waals (n-vdW) quasi 2D MOF films, particularly focusing on UiO-66-NH2, featuring well-defined pore structure of 6 Å, optimally positioned within the range for efficient ion-ion separation. Our study introduces an aqueous synthesis route, enabling the formation of ultrathin, crystalline n-vdW quasi 2D UiO-66-NH2 films under ambient conditions. This approach pinpointed an optimal condition with ultra-dilute precursor concentrations in a low pH regime to foster controlled nucleation and decelerate the reaction kinetics. In addition, the use of crystalline single-layer graphene as a substrate serves the crystallographic registry, promoting an exclusive in-plane growth. The thickness of these films can be controlled through synthesis time, ranging from half to two unit cell. The preferred orientation of UiO-66-NH2 along 200 lattice plane is revealed, shedding light on the structure correlation between graphene and UiO-66-NH2, where the lattice mismatch is minimal. The resulting UiO-66-NH2 films demonstrate a remarkable ion-ion separation performance and stability over 5 weeks.Overall, this dissertation highlights the correlation between structure and performance in nanoporous materials, paving the way for future breakthroughs in membrane technology. The research emphasizes the extraordinary potential of ultrathin 2D nanoporous materials for molecular separation, opening up new possibilities for the development of highly efficient, selective, and durable membranes for diverse applications.