Graphene is currently investigated as a promising membrane material in which selective pores can be created depending on the requirements of the application. However, to handle large-area nanoporous graphene a stable support material is needed. Here, we report on composite membranes consisting of large-area single layer nanoporous graphene supported by a porous polymer. The fabrication is based on ion-track nanotechnology with swift heavy ions directly creating atomic pores in the graphene lattice and damaged tracks in the polymer support. Subsequent chemical etching converts the latent ion tracks in the supporting polymer foil, here polyethylene terephthalate (PET), into open microchannels while the perfectly aligned pores in the graphene top layer remain unaffected. To avoid unintentional damage creation and delamination of the graphene layer from the substrate, the graphene is encapsulated by a protecting poly(methyl methacrylate) (PMMA) layer. By this procedure a stable composite membrane is obtained consisting of nanoporous graphene (coverage close to 100%) suspended across selfaligned track-etched microchannels in a polymer support film. Our method presents a facile way to create high quality suspended graphene of tunable pore size supported on a flexible porous polymeric support, thus enabling the development of membranes for fast and selective ultrafiltration separation processes.

We investigated the dependence of ion transport through perforated graphene on the concentrations of the working ionic solutions. We performed our measurements using three salt solutions, namely, KCl, LiCl, and K2SO4. At low concentrations, we observed a high membrane potential for each solution while for higher concentrations we found three different potentials corresponding to the respective diffusion potentials. We demonstrate that our graphene membrane, which has only a single layer of atoms, showed a very similar trend in membrane potential as compared to dense ion-exchange membranes with finite width. The behavior is well explained by Teorell, Meyer, and Sievers (TMS) theory, which is based on the Nernst–Planck equation and electroneutrality in the membrane. The slight overprediction of the theoretical Donnan potential can arise due to possible nonidealities and surface charge regulation effects.

The confinement of water in quasi two-dimensional layers is intriguing because its physical properties can be significantly different when compared to those of the bulk fluid. This work describes spectroscopic ellipsometry study of confined water layers trapped between sheets of graphene oxide at varied thermal annealing temperatures. The wavelength-dependent refractive index of graphene oxide changes abruptly with annealing temperatures for Tann ≈ 125–160 °C, and we demonstrate that these changes are primarily governed by the expulsion of trapped water. This expulsion is associated with the decrease of interlayer separation of graphene oxide sheets from 7.8 Å to 3.4 Å. Graphene oxide annealed at high temperatures lacks trapped water layers and robust estimates of refractive index can be obtained within a Lorentz oscillator model. The trends in oscillator parameters are extended to lower annealing temperatures, where trapped water is present, in order to estimate the refractive index of confined water, whose value is found to be enhanced as compared to that of bulk. Temperature-dependent ellipsometry data show anomalous changes in ellipsometric parameters over a wide temperature interval (−10 to 10 °C) about the ice-point and these may be attributed to possible phase transition(s) of confined water.

Nanoporous graphene displays salt-dependent ion permeation. In this work, we investigate the differences in Donnan potentials arising between reservoirs, separated by a perforated graphene membrane, containing different cations. We compare the case of monovalent cations interacting with nanoporous graphene with the case of bivalent cations. This is accomplished through both measurements of membrane potential arising between two salt reservoirs at different concentrations involving a single cation (ionic potential) and between two reservoirs containing different cations at the same concentration (bi-ionic potential). In our present study, Donnan dialysis experiments involve bivalent MgCl 2 , CaCl 2 , and CuCl 2 as well as monovalent KCl and NH 4 Cl salts. For all salts, except CuCl 2 , clear Donnan and diffusion potential plateaus were observed at low and high salt concentrations, respectively. Our observations show that the membrane potential scaled to the Nernst potential for bivalent cations has a lower value (≈50%) than for monovalent cations (≈72%) in the Donnan exclusion regime. This is likely due to the adsorption of these bivalent cations on monolayer graphene. For bivalent cations, the diffusion regime is reached at a lower ionic strength compared to the monovalent cations. For Mg 2+ and Ca 2+ , the membrane potential does not seem to depend upon the type of ions in the entire ionic strength range. A similar behavior is observed for the KCl and NH 4 Cl membrane potential curves. For CuCl 2 , the membrane potential curve is shifted toward lower ionic strength compared to the other two bivalent salts and the Donnan plateau is not observed at the lowest ionic strength. Bi-ionic potential measurements give further insight into the strength of specific interactions, allowing for the estimation of the relative ionic selectivities of different cations based on comparing their bi-ionic potentials. This effect of possible ion adsorption on graphene can be removed through ion exchange with monovalent salts.

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In this work, we have studied the pH-dependent surface charge nature of nanoporous graphene. This has been investigated by membrane potential and by streaming current measurements, both with varying pH. We observed a lowering of the membrane potential with decreasing pH for a fixed concentration gradient of potassium chloride (KCl) in the Donnan dominated regime. Interestingly, the potential reverses its sign close to pH 4. The fitted value of effective fixed ion concentration (C̅ R) in the membrane also follows the same trend. The streaming current measurements show a similar trend with sign reversal around pH 4.2. The zeta potential data from the streaming current measurement is further analyzed using a 1-pK model. The model is used to determine a representative pK (acid–base equilibrium constant) of 4.2 for the surface of these perforated graphene membranes. In addition, we have also theoretically investigated the effect of the PET support in our membrane potential measurement using numerical simulations. Our results indicate that the concentration drop inside the PET support can be a major contributor (up to 85%) for a significant deviation of the membrane potential from the ideal Nernst potential.

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...