The potential of graphene membranes: ion transport through graphene nanopores

Abstract

This thesis is about ion transport through a very thin nanoporous membrane called graphene. Even though the pore sizes are bigger than the ion sizes, this single atomic nanoporous layer can selectively pass positive ions and block negative ions. This can be followed from the cover page of this thesis. The front cover has equal number of cations and anions which are trying to pass through the nanopores of the graphene. But as the membrane is cation selective, the back cover has mostly cations indicating selective transport. The thesis explains the transport mechanism. In this thesis I have tried to understand the ion transport at varied external factors such as type of ions, concentration, pH or external bias and used theoretical models to explain our experimental observations. CHAPTER 1 Introduction 1.1 Background on separation techniques Membrane filtration and separation are very important processes especially in water, food, dairy and chemical industries. These processes typically require ambient temperature, avoid harsh chemical reactions and are thus considered a green technique. Di↵erent industries and daily life applications such as water treatment to produce drinking water, industrial waste water treatment and food processes require the selectivity of membranes [1-3]. Ion separation and mineral recovery is an increasingly important aspect related to water treatment. Ion separation processes involving membranes concern reverse osmosis, nanofiltration, and (electro)dialysis. The first two processes typically involve a pressure gradient as driving force, whereas the latter is driven by concentration and potential gradients. The transport of ions through these membranes is governed by their interaction with the membrane phase, based on size and charge. New generations of membranes are continuously researched specifically for smaller scale application, including bio-sensing and DNA translocation. With the steeply growing interest in 2D materials like graphene, studies regarding transport of ions through and between these atomically thin sheets are jointly increasing. The research objective described in this thesis is to engineer such a nano-porous membrane (based on 2D graphene) and investigate the corresponding ion transport through these pores. Before going into detail of this particular type of membrane, the essential concepts relevant for this area of research will be summarized. V T heory = RT zF ln c 1 c 2 (1.2) where z is the ion valence, F is the Faraday constant (C/mol), R is the gas constant (J/mol K), T is the temperature (K), and c i is the ion concentration (mol/m 3). Now according to Donnan equilibrium, species present in contacting phases will have the same electrochemical potential once equilibrium is reached. This implies that for phases with concentrations c 1 and c 2 , their electrochemical potential will be equal: µ o + RT ln c 1 + zF 1 = µ o + RT ln c 2 + zF 2 (1.4) The bulk aqueous electrolyte solutions as well as the membrane phase are electrically neutral. For a 1:1 salt, there are posi...