Subnanometer Pores Research Articles

Facile Size-Selective Defect Sealing in Large-Area Atomically Thin Graphene Membranes for Sub-Nanometer Scale Separations.

Atomically thin graphene with a high-density of precise subnanometer pores represents the ideal membrane for ionic and molecular separations. However, a single large-nanopore can severely compromise membrane performance and differential etching between pre-existing defects/grain boundaries in graphene and pristine regions presents fundamental limitations. Here, we show for the first time that size-selective interfacial polymerization after high-density nanopore formation in graphene not only seals larger defects (>0.5 nm) and macroscopic tears but also successfully preserves the smaller subnanometer pores. Low-temperature growth followed by mild UV/ozone oxidation allows for facile and scalable formation of high-density (4-5.5 × 1012 cm-2) useful subnanometer pores in the graphene lattice. We demonstrate scalable synthesis of fully functional centimeter-scale nanoporous atomically thin membranes (NATMs) with water (∼0.28 nm) permeance ∼23× higher than commercially available membranes and excellent rejection to salt ions (∼0.66 nm, >97% rejection) as well as small organic molecules (∼0.7-1.5 nm, ∼100% rejection) under forward osmosis.

Three-Dimensional SiC/Holey-Graphene/Holey-MnO2 Architectures for Flexible Energy Storage with Superior Power and Energy Densities.

Although nanostructured materials have recently enabled a dramatic improvement of the current energy-storage units in portable electronics with enhanced functionality, it is still challenging to provide a cost-efficient solution to attain the ultrahigh energy and power densities of supercapacitors (SCs) since nearly arbitrary electrodes are limited to the thinner porous structure with de facto rather low mass loading (∼1 mg cm-2) because of the huge limitations of pronounced impaired ion transport in subnanometer pores in thicker compact electrodes. In this contribution, we report the fabrication of a macro/mesoporous hybrid hierarchical nanocomposite SiC/holey-graphene/holey-MnO2 (SiC/HG/h-MnO2) with tailored porosity by knitting together the quasi-aligned single-crystalline doped 3C-SiC nanowire array and in situ surface-reduced holey graphene framework into a three-dimensional quasi-ordered structure, which enables the mass growth of ultrathin h-MnO2 nanosheets at approximately practical levels of mass loading. The produced synergistically favorable interconnected porous architecture allows for the highly efficient electron transfer and rapid ion transport up to interior surfaces of the network. Remarkably, the all-solid-state flexible asymmetric supercapacitors (ASCs) made with SiC/HG/h-MnO2 and SiC/graphitic carbon (GC) nanoarrays are mechanically robust and show a high areal capacity (0.32 mWh cm-2) and a high rate capability (280 mW cm-2) at ultrahigh mass loading (6.5 mg cm-2), much higher than most of previous superior SCs in aqueous or gelled electrolytes and thus offer an entirely new prototype of textile-based ASCs, which represents a critical step toward practical applications for various portable electronics.

Effect of hydrophilicity on water transport through sub-nanometer pores

Pore hydrophilicity is crucial parameter for water transport through nanopores. However, the effect of pore hydrophilicity on water permeance at the sub-nanometer scale remains unclear because it cannot be described by the conventional fluid mechanics. Herein, to determine the separate relationship between water permeance and pore hydrophilicity at the sub-nanometer scale, an equation derived from interfacial friction is obtained to exclude the effects of other variables. Using non-equilibrium molecular dynamics, we investigate water transport through a series of nanopores which is constructed from 12 types of functionalized covalent organic frameworks (COFs) with various degrees of hydrophilicity but similar skeletons and sub-nanometer pore sizes. The variable-excluded permeance has a linear relationship with contact angle. However, this linear relationship is not applicable to the hydrophobic and extremely hydrophilic nanopores under experimental pressure drops. For experimental predictions, the final relationship is corrected by considering the combined effect within all-range degrees of hydrophilicity. Therefore, a range of moderate pore hydrophilicity is suggested for the application of sub-nanometer pores in water transport. This study reveals the quantitative effect of pore hydrophilicity on water permeance at the sub-nanometer scale, which can be useful for the screening of nanomaterials for various applications.

Towards single-species selectivity of membranes with subnanometre pores.

Synthetic membranes with pores at the subnanometre scale are at the core of processes for separating solutes from water, such as water purification and desalination. While these membrane processes have achieved substantial industrial success, the capability of state-of-the-art membranes to selectively separate a single solute from a mixture of solutes is limited. Such high-precision separation would enable fit-for-purpose treatment, improving the sustainability of current water-treatment processes and opening doors for new applications of membrane technologies. Herein, we introduce the challenges of state-of-the-art membranes with subnanometre pores to achieve high selectivity between solutes. We then analyse experimental and theoretical literature to discuss the molecular-level mechanisms that contribute to energy barriers for solute transport through subnanometre pores. We conclude by providing principles and guidelines for designing next-generation single-species selective membranes that are inspired by ion-selective biological channels.

Polyamide nanofiltration membrane with highly uniform sub-nanometre pores for sub-1\u2009\xc5 precision separation

Separating molecules or ions with sub-Angstrom scale precision is important but technically challenging. Achieving such a precise separation using membranes requires Angstrom scale pores with a high level of pore size uniformity. Herein, we demonstrate that precise solute-solute separation can be achieved using polyamide membranes formed via surfactant-assembly regulated interfacial polymerization (SARIP). The dynamic, self-assembled network of surfactants facilitates faster and more homogeneous diffusion of amine monomers across the water/hexane interface during interfacial polymerization, thereby forming a polyamide active layer with more uniform sub-nanometre pores compared to those formed via conventional interfacial polymerization. The polyamide membrane formed by SARIP exhibits highly size-dependent sieving of solutes, yielding a step-wise transition from low rejection to near-perfect rejection over a solute size range smaller than half Angstrom. SARIP represents an approach for the scalable fabrication of ultra-selective membranes with uniform nanopores for precise separation of ions and small solutes.

Single-Crystal Polycationic Polymers Obtained by Single-Crystal-to-Single-Crystal Photopolymerization.

The efficient preparation of single-crystalline ionic polymers and fundamental understanding of their structure-property relationships at the molecular level remains a challenge in chemistry and materials science. Here, we describe the single-crystal structure of a highly ordered polycationic polymer (polyelectrolyte) and its proton conductivity. The polyelectrolyte single crystals can be prepared on a gram-scale in quantitative yield, by taking advantage of an ultraviolet/sunlight-induced topochemical polymerization, from a tricationic monomer-a self-complementary building block possessing a preorganized conformation. A single-crystal-to-single-crystal photopolymerization was revealed unambiguously by in situ single-crystal X-ray diffraction analysis, which was also employed to follow the progression of molecular structure from the monomer, to a partially polymerized intermediate, and, finally, to the polymer itself. Collinear polymer chains are held together tightly by multiple Coulombic interactions involving counterions to form two-dimensional lamellar sheets (1 nm in height) with sub-nanometer pores (5 Å). The polymer is extremely stable under 254 nm light irradiation and high temperature (above 500 K). The extraordinary mechanical strength and environmental stability-in combination with its impressive proton conductivity (∼3 × 10-4 S cm-1)-endow the polymer with potential applications as a robust proton-conducting material. By marrying supramolecular chemistry with macromolecular science, the outcome represents a major step toward the controlled synthesis of single-crystalline polyelectrolyte materials with perfect tacticity.

Ion Structure Transition Enhances Charging Dynamics in Subnanometer Pores.

Using electrodes with subnanometer pores and ionic liquid electrolytes can improve the charge storage capacity at the expense of the charging rate. The fundamental understanding of the charging dynamics of nanoporous electrodes can help to avoid compromising the power density. In this work, we performed molecular dynamics simulations to reveal the charging mechanism of subnanometer pores in ionic liquids. Different from the traditional view that a smaller pore results in slower charging, a non-monotonic relation is found between the charging rate and pore size, in which the charging process is accelerated in some subnanometer pores. Our analysis uncovers that the mechanism of the charging enhancement can be attributed to the transition of in-pore ion structure.

Permselective ion electrosorption of subnanometer pores at high molar strength enables capacitive deionization of saline water

Small carbon pores below 1 nm become increasingly ionophobic which enables the more and more permselective charge storage and perspectives for capacitive deionization with porous carbons even at high molar strength.

Controlled Pore Generation in Single-Layer Graphene Oxide for Membrane Desalination.

Nanoporous graphene was proposed as an efficient material for reverse osmosis water desalination membranes because it allows water molecules to pass at high flux while rejecting hydrated salt ions. Nevertheless, from an experimental point of view it is still difficult to control the pore size. A scalable method to generate pores is urgently required for the diffusion of this technology. We propose, by theoretical calculations, an innovative and scalable strategy to better control the dimension of the pores in graphene-based membranes by reduction of single-layer graphene oxide (GO). The latter is first annealed at a controlled mild temperature to induce the aggregation of its randomly distributed oxygen-containing functional groups into small nanometric clusters. The layer then undergoes a high-temperature reducing treatment that causes the desorption of the functional groups along with carbon removal only in the oxidized areas, producing subnanometric pores while leaving unchanged the remaining pristine graphene areas.

Transport properties of H2 confined in carbide-derived carbons with different pore shapes and sizes

Hydrogen adsorption in highly porous carbon with well-defined pores, with three different shapes, and different sizes ranging from sub-to nanometers is investigated. Using a combined approach of volumetric gas adsorption method and in-situ quasi-elastic neutron scattering method the relationship between final macroscopic intake properties, details of the local adsorbent structure and the molecular behaviour of confined hydrogen are established. It is shown that sub-nanometer pores of spherical and cylindrical shape strongly limit the diffusion of H2, and thus, enhance the H2 storage capability of carbons with well-tailored pore structure. In mesoporous carbide-derived carbon, the formation of a hydrogen layer with reduced mobility close to the pore walls is observed. With the increase in the amount of confined hydrogen and the occupation of the centre pore area, the mobility of confined hydrogen increases in a jump–like fashion. Surprisingly, the increase of hydrogen diffusion is also observed at higher hydrogen loadings, indicating that cooperative H2–H2 interactions might play a role.

Copper-Halide Polymer Nanowires as Versatile Supports for Single-Atom Catalysts.

Single-atom catalysts are heterogeneous catalysts with atomistically dispersed atoms acting as a catalytically active center, and have recently attracted much attention owing to the minimal use of noble metals. However, a scalable and inexpensive support that can stably anchor isolated atoms remains a challenge due to high surface energy. Here, copper-halide polymer nanowires with sub-nanometer pores are proposed as a versatile support for single-atom catalysts. The synthesis of the nanowires is straightforward and completed in a few minutes. Well-defined sub-nanometer pores and a large free volume of the nanowires are advantageous over any other support material. The nanowires can anchor various atomistically dispersed metal atoms into the sub-nanometer pores up to ≈3 at% via a simple solution process, and this value is at least twice as big as previously reported data. The hydrogen evolution reaction activity of -18.0 A mgPt -1 at -0.2 V overpotential shows its potential for single-atom catalysts support.

Theoretical Prediction of Mechanical Strength and Desalination Performance of One-Atom-Thick Hydrocarbon Polymer in Pressure-Driven Separation.

One-atom-thick materials hold promise for the future of membrane-based gas purification and water filtration applications. However, there are a few investigations on the mechanical properties of these materials under pressure-driven condition. Here, by employing molecular simulation techniques and continuum mechanics simulation, we investigate the mechanical strength of two-dimensional hydrocarbon polymers containing sub-nanometer pores with various topologies. We demonstrate that the mechanical strengths of the membranes are correlated with their pore sizes and geometries. In addition, when the pore size of substrates is controlled within a reasonable range, all of the membrane candidates can withstand the practical hydraulic pressure of few megapascal. The studied materials also exhibit better seawater desalination performance as compared to the traditional polymeric reverse osmosis membrane. This work presents a new route to design new separation membrane, and also propose a simulation method to evaluate the mechanical strength and desalination performance.

Ionic liquid gated 2D-CAP membrane for highly efficient CO2/N2 and CO2/CH4 separation

Recently, a two-dimensional conjugated aromatic polymer (2D-CAP) membrane has been synthesized, and it shows promising applications in the selected gas separation. However, the sub-nanometer pores of the 2D-CAP membrane are relatively large compared with small gas molecules like CO2, N2, and CH4, which impedes the efficiency of the gas separation. Here, we report a strategy to improve the gas separation performance of 2D-CAP membranes. Specifically, coating ultra-thin ionic liquid (IL) onto the 2D-CAP membrane was found to well harmonize the pore size of 2D-CAP membranes. In this work, the gas separation performance of this 2D-CAP supported IL membrane (2D-CAP SILM) was investigated using molecular dynamics simulations. An ultrahigh CO2 permeance of ~105 GPU, which is larger than many reported theoretically predicted results, was exhibited. Meanwhile, an excellent selectivity of CO2/N2 and CO2/CH4 beyond 40 was obtained. The ultrathin membrane and the high-density pore are responsible for the high gas permeance, and the selectivity could be ascribed to IL adsorption selectivity of CO2 over N2/CH4 and a fascinating gating effect that anion of IL ([BF4]−) suspending upside the pore center allows CO2 passage while prohibits N2/CH4 passage. The present work suggests a promising membrane for practical highly efficient CO2 separation.

Effect of fine-tuning pore structures on the dynamics of confined water

Confinement of water in sub-nanometer pores strongly alters its vibrational dynamics from that of bulk water. The effect of confinement can, furthermore, be finely tuned by small changes in the size and symmetry of the confining pore. Using inelastic neutron scattering (INS), we recently studied the dynamics of water confined in the channels of beryl and cordierite in which, at low temperatures, water shows similar behavior, indicating an absence of hydrogen bonds acting on the water molecule and a shallow water potential in the direction perpendicular to the channels. In addition, we observed multiple tunneling modes (between 0.66 and 14.7 meV) in the INS spectra of beryl due to transitions between the split ground-state of the water protons. Here, we present a study of (i) the effect of pressure on the dynamics of water in beryl, (ii) the dynamics of water in beryl containing alkali metals (which results in changing the orientation of the water molecule in the crystal), and (iii) the dynamics of water in cordierite at low energies. We found a shift in the tunneling and vibrational modes of water in beryl to higher energies at 22 kbar relative to 1 bar. No tunneling modes were observed for water in cordierite and type-II water in beryl. Therefore, we conclude that very small differences in the size and structure of the pores and the orientation of the water molecule in these minerals result in changes in the potential of the water protons and drastic changes in the confined water dynamics.

(Invited) Fundamental Understanding of Faradaic and Non-Faradaic Processes in Carbon-Based Supercapacitors

Nanostructured carbon materials, such as graphene and nanoporous carbon, have become increasingly prominent for use as electrodes in supercapacitors, owing to their high specific and volumetric surface area and good electrical conductivity. A conventional understanding of supercapacitors relates the high power to fast ion accumulation at the polarized electrode interface, forming the so-called electric double layer, and the low energy to limited electrode surface area (SA). As such, low-dimensional carbon nanostructures have been extensively explored. However, anomalous and nonlinear relationships between observed capacitances and SAs have prompted a need to revise our mechanistic understanding of charge storage. In this talk, we present our recent findings regarding charge storage mechanisms, along with possible faradaic reactions, in carbon-based supercapacitors from combined quantum mechanical and classical molecular dynamics simulations. Our first-principles studies suggest that the total interfacial capacitance (CT ) of graphene-based supercapacitors is determined from both electrode (CQ ) and electric double layer (CD ) capacitances which are in series. It has also been demonstrated that the CQ , and thereby CT , can be significantly enhanced as a result of changes to the electronic structure through chemical functionalization, doping, and/or structural deformation. Furthermore, our voltammetric molecular dynamics simulations have revealed the complex non-equilibrium processes associated with extreme confinement of ionic liquid electrolytes within subnanometer pores. Based on these results, we will also discuss a mechanistic perspective centered around ion reorganization kinetics, particularly the dependence of ion migration efficiency, and relatedly, the capacitance, on pore morphology. This talk will also briefly touch on possible faradaic reactions that may cause self-discharge.

Graphene Utilization for Water Desalination Process

Though above 70% of the Earth is covered by water, most of the seas and oceans are unusable for drinking. Freshwater lakes, rivers and underground aquifers imply 2.5 percent of the global’s whole freshwater supply. Unfortunately, in addition to being scarce, fresh water is dreadfully unevenly spread. Enhanced demand for freshwater is a global concern. In many countries demanding is further than regular reserves. Sensible use of water, reducing spreading losses and upgraded treatment of recycled water to mitigate the concern, though, water scarcity is still presented consequently desalination of seawater is highly required. Graphene, a single sheet of carbon atoms, possibly will deliver the principal for a novel category of extremely permeable membranes for water purification and desalination. Though, a one atom thickness graphene reveals both brilliant mechanical strength and impermeability to atoms as small as helium. High-density, subnanometer pores within graphene have the potential for ultra-fast water permeance and high solute rejection as the atomic thinness makes slight resistance to stream which deters the transfer of solutes bigger than the pores. The two-dimensional, nanoporous membrane is expected to display orders-of-magnitude permeability and selectivity enhancement over current separation membranes for processes such as brackish water, water softening, or nanofiltration. This study is aimed that the existing desalination methods are not adequate to upgrade water sources unless the desalination technologies are improved significantly. Nanotechnology and utilizing graphene will deliver desalination technology to meet the requirements in the near future. Lately, novel procedures have been technologically progressed by means of nanotechnology and applying graphene for water desalination. This research will emphasize the concept of water desalination for the near futures.

A Pentacene-based Nanotube Displaying Enriched Electrochemical and Photochemical Activities.

Unlike previously well-studied, acyclic pentacene oligomers, the first synthesis of a cyclic pentacene trimer with a fixed tubular conformation is reported. A short-step synthesis starting from common pentacenequinone yielded the target molecule with a 1.5 nanometer length and a subnanometer pore. Steady-state spectroscopic analyses revealed that the close proximity of the non-conjugated, three pentacene chromophores allows the nanotube to display stepwise electrochemical/chemical oxidation characteristics. Furthermore, time-resolved transient absorption measurements elucidated the generation of an excited triplet state of the nanotube, with high quantum yield reaching about 180 % through intramolecular singlet fission and a very long triplet lifetime.

A Pentacene‐based Nanotube Displaying Enriched Electrochemical and Photochemical Activities

AbstractUnlike previously well‐studied, acyclic pentacene oligomers, the first synthesis of a cyclic pentacene trimer with a fixed tubular conformation is reported. A short‐step synthesis starting from common pentacenequinone yielded the target molecule with a 1.5 nanometer length and a subnanometer pore. Steady‐state spectroscopic analyses revealed that the close proximity of the non‐conjugated, three pentacene chromophores allows the nanotube to display stepwise electrochemical/chemical oxidation characteristics. Furthermore, time‐resolved transient absorption measurements elucidated the generation of an excited triplet state of the nanotube, with high quantum yield reaching about 180 % through intramolecular singlet fission and a very long triplet lifetime.

Water transport through subnanopores in the ultimate size limit: Mechanism from molecular dynamics

Ab initio and classical molecular dynamics simulations show that water can flow through graphdiyne—an experimentally fabricated graphene-like membrane with highly dense (2.4 × 1018 pores/m2), uniformly ordered, subnanometer pores (incircle diameter 0.57 nm and van der Waals area 0.06 nm2). Water transports through subnanopores via a chemical-reaction-like activated process. The activated water flow can be precisely controlled through fine adjustment of working temperature and pressure. In contrast to a linear dependence on pressure for conventional membranes, here pressure directly modulates the activation energy, leading to a nonlinear water flow as a function of pressure. Consequently, high flux (1.6 L/Day/cm2/MPa) with 100% salt rejection efficiency is achieved at reasonable temperatures and pressures, suggesting graphdiyne can serve as an excellent membrane for water desalination. We further show that to get through subnanopores water molecule must break redundant hydrogen bonds to form a two-hydrogen-bond transient structure. Our study unveils the principles and atomistic mechanism for water transport through pores in ultimate size limit, and offers new insights on water permeation through nanochannels, design of molecule sieving and nanofluidic manipulation.

The effect of additional off-diagonal disorder of interionic interaction on charge-storage in sub-nanometer pores of supramolecular carbon supercapacitors

The effect of additional off-diagonal disorder of interionic interaction on charge-storage in sub-nanometer pores of supramolecular carbon supercapacitors