Mass Transport via In-Plane Nanopores in Graphene Oxide Membranes.

Angstrom-confined solvents in 2D laminates can travel through interlayer spacings, through gaps between adjacent sheets, and via in-plane pores. Among these, experimental access to investigate the mass transport through in-plane pores is lacking. Our experiments allow an understanding of this mass transport via the controlled variation of oxygen functionalities, size and density of in-plane pores in graphene oxide membranes. Contrary to expectations, our transport experiments show that higher in-plane pore densities may not necessarily lead to higher water permeability. We observed that membranes with a high in-plane pore density but a low amount of oxygen functionalities exhibit a complete blockage of water. However, when water-ethanol mixtures with a weaker hydrogen network are used, these membranes show an enhanced permeation. Our combined experimental and computational results suggest that the transport mechanism is governed by the attraction of the solvents toward the pores with functional groups and hindered by the strong hydrogen network of water formed under angstrom confinement.

Understanding Mono- and Bivalent Ion Selectivities of Nanoporous Graphene Using Ionic and Bi-ionic Potentials.

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 MgCl2, CaCl2, and CuCl2 as well as monovalent KCl and NH4Cl salts. For all salts, except CuCl2, 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 Mg2+ and Ca2+, 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 NH4Cl membrane potential curves. For CuCl2, 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.

Charge Regulation at a Nanoporous Two-Dimensional Interface.

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.

Superoleophobic behavior induced by nanofeatures on oleophilic surfaces.

The control of surface wetting properties to produce robust and strong hydrophobic and oleophobic effects on intrinsically oleophilic surfaces is at the heart of many technological applications. In this paper, we explore the conditions to observe such effects when the roughness of the substrate is of fractal nature and consists of nanofeatures obtained by the ion track etching technique. The wetting properties were investigated using eight different liquids with surface tensions gamma varying from 18 to 72 mN m(-1). While it is observed that all the tested oils readily wet the flat substrates, it is found that the contact angles are systematically exalted on the rough surfaces even for the liquids with very low surface tension. For liquids with gamma > or = 25 mN m(-1) an oleophobic behavior is clearly induced by the nanostructuration. For liquids with gamma < 25 mN m(-1), although the contact angle is enhanced on the nanorough surfaces, it conserves its oleophilic character (theta* lower than 90 degrees). Moreover, our experiments show that even in the case of hexane, liquid having the lowest surface tension, the homogeneous wetting (Wenzel state) is never reached. This high resistance to liquid impregnation is discussed within the framework of recent approaches explaining the wetting properties of superoleophobic surfaces.

High resolution AFM studies of irradiated mica-following the traces of swift heavy ions under grazing incidence.

High resolution AFM imaging of swift heavy ion irradiated muscovite mica under grazing incidence provides detailed insight into the created nanostructure features. Swift heavy ions under grazing incidence form a complex track structure along the surface, which consists of a double track of nanohillocks at the impact site accompanied by a single, several 100 nm long protrusion. Detailed track studies by varying the irradiation parameters, i.e. the angle of incidence (0.2°-2°) and the kinetic energy of the impinging ions (23, 55, 75, 95 MeV) are presented. Moreover, the track formation in dependence of the sample temperature (between room temperature and 600 °C) and of the chemical composition (muscovite mica and fluorphlogopite mica) is studied.

Highly active single-layer MoS2 catalysts synthesized by swift heavy ion irradiation.

Two-dimensional molybdenum-disulfide (MoS2) catalysts can achieve high catalytic activity for the hydrogen evolution reaction upon appropriate modification of their surface. The intrinsic inertness of the compound's basal planes can be overcome by either increasing the number of catalytically active edge sites or by enhancing the activity of the basal planes via a controlled creation of sulfur vacancies. Here, we report a novel method of activating the MoS2 surface using swift heavy ion irradiation. The creation of nanometer-scale structures by an ion beam, in combination with the partial sulfur depletion of the basal planes, leads to a large increase of the number of low-coordinated Mo atoms, which can form bonds with adsorbing species. This results in a decreased onset potential for hydrogen evolution, as well as in a significant enhancement of the electrochemical current density by over 160% as compared to an identical but non-irradiated MoS2 surface.

Swift heavy ion irradiation of CaF2 - from grooves to hillocks in a single ion track.

A novel form of ion-tracks, namely nanogrooves and hillocks, are observed on CaF2 after irradiation with xenon and lead ions of about 100 MeV kinetic energy. The irradiation is performed under grazing incidence (0.3°-3°) which forces the track to a region in close vicinity to the surface. Atomic force microscopy imaging of the impact sites with high spatial resolution reveals that the surface track consists in fact of three distinct parts: each swift heavy ion impacting on the CaF2 surface first opens a several 100 nm long groove bordered by a series of nanohillocks on both sides. The end of the groove is marked by a huge single hillock and the further penetration of the swift projectile into deeper layers of the target is accompanied by a single protrusion of several 100 nm in length slowly fading until the track vanishes. By comparing experimental data for various impact angles with results of a simulation, based on a three-dimensional version of the two-temperature-model (TTM), we are able to link the crater and hillock formation to sublimation and melting processes of CaF2 due to the local energy deposition by swift heavy ions.