The Potential of Graphene Membranes: Ion Transport through Graphene Nanopores

In this thesis, ion transport through nanoporous graphene membrane has been studied. Graphene is completely impermeable in its pristine state. To make it permeable, nanopores are created by heavy ion beam bombardment. To give robustness to the membrane, PET support is used. The pore size in the PET support layer is bigger compared to graphene pores so that there is no influence of PET layer on the ion transport through graphene. The ion transport is initiated by creating a concentration gradient across the membrane. The membrane being selective to cations, a potential is generated across the membrane. We measure this potential by varying the concentrations of both the reservoirs and keeping the same ratio between them. The membrane potential is scaled to the theoretical Nernst potential (potential for 100% selective membrane). The scaled membrane potential versus concentration plot shows two distinct Donnan and diffusion dominated regimes which is described by a modified version of Teorell-Meyer-Sievers (TMS) theory. Our ion transport study for bivalent cations shows that the scaled membrane potential for bivalent cations are lower compared to the monovalent case. This is due to ions getting adsorbed on to the surface which we have experimentally verified. We have also conducted bi-ionic potential measurements which quantifies the affinity of different ions towards the membrane. We have found that the ions having the similar affinity towards the membrane have overlapping Donnan plateaus and the ions having different affinity towards the membrane have different Donnan plateaus. The membrane potential is also measured by varying pH to understand the nature of the surface charge in graphene. We have found that with decrease in pH, the membrane potential decreases and reverses its sign at the lowest pH. Streaming current measurements shows similar trend with zeta potential data at varied pH. Finally, we have done numerical simulation to describe our experimental results obtained from membrane potential measurements. The dependence of membrane potential on the surface/zeta potential, pore radius and stern layer thickness have been studied.