The Thinnest Molecular Separation Sheet by Graphene Gates of Single-Walled Carbon Nanohorns

Graphene is possibly the thinnest membrane that could be used as a molecular separation gate. Several techniques including absorption, cryogenic distillation, adsorption, and membrane separation have been adopted for constructing separation systems. Molecular separation using graphene as the membrane has been studied because large area synthesis of graphene is possible by chemical vapor deposition. Control of the gate sizes is necessary to achieve high separation performances in graphene membranes. The separation of molecules and ions using graphene and graphene oxide layers could be achieved by the intrinsic defects and defect donation of graphene. However, the controllability of the graphene gates is still under debate because gate size control at the picometer level is inevitable for the fabrication of the thinnest graphene membranes. In this paper, the controlled gate size in the graphene sheets in single-walled carbon nanohorns (NHs) is studied and the molecular separation ability of the graphene sheets is assessed by molecular probing with CO₂, O₂, N₂, CH₄, and SF₆. Graphene sheets in NHs with different sized gates of 310, 370, and >500 pm were prepared and assessed by molecular probing. The 310 pm-gates in the graphene sheets could separate the molecules tested, whereas weak separation properties were observed for 370 pm-gates. The amount of CO₂ that penetrated the 310 pm-gates was more than 35 times larger than that of CH₄. These results were supported by molecular dynamics simulations of the penetration of molecules through 300, 400, and 700 pm-gates in graphene sheets. Therefore, a gas separation membrane using a 340-pm-thick graphene sheet has high potential. These findings provide unambiguous evidence of the importance of graphene gates on the picometer level. Control of the gates is the primary challenge for high-performance separation membranes made of graphene

Water Assistance in Ion Transfer during Charge and Discharge Cycles

Knowledge of the dynamic properties of electrolyte solutions during charge and discharge cycles is crucial for understanding and developing electric energy devices. Molecular dynamics simulations of aqueous NaCl solution in nanopores between charged graphene layers were performed to assess the dynamical mechanism of ion transfer. Ions moved to the oppositely charged graphene layer according to the strength of their partial charges. The ion hydration numbers increased during ion transfer, suggesting quick rearrangement of water molecules around the ions to form a hydration shell. The extent of hydrogen bonding also increased during ion transfer. Water molecules participating in ion transfer hydrated the ion and simultaneously maintained hydrogen bonding, supporting a quick ion transfer mechanism during charge and discharge cycles

Intensive Edge Effects of Nanographenes in Molecular Adsorptions

Graphene has become a primary material in nanotechnology and has a wide range of potential applications in electronics. Fabricated graphenes are generally nanosized and composed of stacked graphene layers. The edges of nanographenes predominantly influence the chemical and physical properties because nanographene layers have a large number of edges. We demonstrated the edge effects of nanographenes and discrimination against basal planes in molecular adsorption using grand canonical Monte Carlo simulations. The edge sites of nanographene layers have relatively strong Coulombic interactions as a result of the partial charges at the edges, but the basal planes rarely have Coulombic interactions. CO₂ and N₂ prefer to be adsorbed on the edge sites and basal planes, respectively. As a result of these different preferences, the separation ability of CO₂ is higher than that of N₂ in the low-pressure region, thereby offering selective adsorptions, reactions, and separations on nanographene edges

Competition of Desolvation and Stabilization of Organic Electrolytes in Extremely Narrow Nanopores

Organic electrolytes are widely used for electric double-layer capacitors. However, the molecular mechanism involved is far from being understood. We demonstrate the structures and stabilities of tetraethylammonium and tetrafluoroborate ions in propylene carbonate solution in carbon nanopores using Monte Carlo simulations. These ions were significantly desolvated at nanopore widths below 1.0 nm. The nanopore potential compensated for the loss of stability of the ions as a result of desolvation for nanopore widths of 0.7–1.2 nm for Et₄N⁺ and 0.6–0.9 nm for BF₄^–. High-capacitance electrodes can therefore be obtained using such nanoporous carbons

A Highly Viscous Imidazolium Ionic Liquid inside Carbon Nanotubes

We report a combined experimental (X-ray diffraction) and theoretical (molecular dynamics, hybrid density functional theory) study of 1-ethyl-3-methylimidazolium chloride, [C₂C₁MIM]­[Cl], inside carbon nanotubes (CNTs). We show that despite its huge viscosity [C₂C₁MIM]­[Cl] readily penetrates into 1–3 nm wide CNTs at slightly elevated temperatures (323–363 K). Molecular simulations were used to assign atom–atom peaks. Experimental and simulated structures of RTIL inside CNT and in bulk phase are in good agreement. We emphasize a special role of the CNT–chloride interactions in the successful adsorption of [C₂C₁MIM]­[Cl] on the inner sidewalls of 1–3 nm carbon nanotubes

Interruption of Hydrogen Bonding Networks of Water in Carbon Nanotubes Due to Strong Hydration Shell Formation

We present the structures of NaCl aqueous solution in carbon nanotubes with diameters of 1, 2, and 3 nm based on an analysis performed using X-ray diffraction and canonical ensemble Monte Carlo simulations. Anomalously longer nearest-neighbor distances were observed in the electrolyte for the 1-nm-diameter carbon nanotubes; in contrast, in the 2 and 3 nm carbon nanotubes, the nearest-neighbor distances were shorter than those in the bulk electrolyte. We also observed similar properties for water in carbon nanotubes, which was expected because the main component of the electrolyte was water. However, the nearest-neighbor distances of the electrolyte were longer than those of water in all of the carbon nanotubes; the difference was especially pronounced in the 2-nm-diameter carbon nanotubes. Thus, small numbers of ions affected the entire structure of the electrolyte in the nanopores of the carbon nanotubes. The formation of strong hydration shells between ions and water molecules considerably interrupted the hydrogen bonding between water molecules in the nanopores of the carbon nanotubes. The hydration shell had a diameter of approximately 1 nm, and hydration shells were thus adopted for the nanopores of the 2-nm-diameter carbon nanotubes, providing an explanation for the large difference in the nearest-neighbor distances between the electrolyte and water in these nanopores

Significant Hydration Shell Formation Instead of Hydrogen Bonds in Nanoconfined Aqueous Electrolyte Solutions

Nanoscale confined electrolyte solutions are frequently observed, specifically in electrochemistry and biochemistry. However, the mechanism and structure of such electrolyte solutions are not well understood. We investigated the structure of aqueous electrolyte solutions in the internal nanospaces of single-walled carbon nanotubes, using synchrotron X-ray diffraction. The intermolecular distance between the water molecules in the electrolyte solution was increased because of anomalously strong hydration shell formation. Water correlation was further weakened at second-neighbor or longer distances. The anomalous hydrogen-bonding structure improves our understanding of electrolyte solutions in nanoenvironments

Facilitation of Water Penetration through Zero-Dimensional Gates on Rolled-up Graphene by Cluster–Chain–Cluster Transformations

We demonstrate a water penetration mechanism through zero-dimensional nanogates of a single-walled carbon nanohorn. Water vapor adsorption via the nanogates is delayed in the initial adsorption stage but then proceeds at a certain rate. The mechanism is proposed to be a water cluster–chain–cluster transformation via the nanogates. The growth of water clusters in internal nanospaces facilitates water penetration into these nanospaces, providing an intrinsic mechanism for zero-dimensional water

Nanocrystallization of Imidazolium Ionic Liquid in Carbon Nanotubes

Room-temperature ionic liquids (ILs) have been of considerable worldwide interest as universal solvents, reaction media, gas scavengers, and electrolytes, particularly in supercapacitors. The behavior of ILs confined in nanoscale cavities is essential for high-performance capacitors, yet it is not well understood. Here, the structural properties of the 1-ethyl-3-methyl­imidazolium chloride IL confined in small diameter carbon nanotubes (CNTs) are characterized by experimental structural and vibrational analyses, complemented by molecular simulations. The IL poorly fills the 1 nm CNTs and the included IL possesses a similar local structure to that seen in the bulk. For 2 and 3 nm diameter CNTs, highly ordered cationic networks associated with restricted vibrational motion are observed. Anions are relatively mobile in the ordered cationic network within the CNTs. Rigid crystalline cationic networks and mobile chloride anions distinguish the unique properties of the confined IL