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Zeolitic imidazolate frameworks (ZIFs) are a subset of metal-organic frameworks (MOFs) with more than 200 characterized crystalline and amorphous networks made of divalent transition metal centers (e.g., Zn2+ and Co2+) linked by imidazolate linkers. ZIF thin films have been pursued intensively motivated by the desire to prepare membranes for selective gas and liquid separations. To achieve membranes with high throughput, as in Å-scale biological channels with nanometer-scale pathlengths, ZIF films with the minimum possible thickness, down to just one unit cell, are highly desired. Control of ZIF film thickness at the 10-nm-scale may also enable emerging, MOF-inspired, applications including patterned crystalline MOF films, and amorphous organic-inorganic resists for high-resolution electron-beam (e-beam) and extreme UV (EUV) lithography. However, the state-of-the-art methods yield ZIF films with thicknesses exceeding 40 nanometers. Here, we report a deposition method from ultra-dilute precursor mixtures that within minutes yields uniform ZIF deposits with nm-scale thickness control. On crystalline substrate such as graphene, two-dimensional crystalline ZIF (2DZIF) film with thickness of a unit-cell could be achieved, which composed of a six-membered zincimidazolate coordination ring enabling record-high H2 permselective separation performance. Deposition under identical conditions on amorphous substrates yields macroscopically smooth amorphous ZIF (aZIF) films, which can be used as negative- and positive-tone resists yielding pattern features down to 20 nm. The method reported here will likely accelerate the development of 2D crystalline and ultrathin amorphous MOF films for applications ranging from separation membranes to sensors and patterning for microelectronic applications.

Poly(triazine imide) or PTI is a promising material for molecular sieving membranes, thanks to its atom-thick ordered lattice with an extremely high density (1.6 × 1014pores per cm2) of triangular-shaped nanopores of ∼0.34 nm diameter. Here, we investigate the application of PTI nanopores in the purification of Kr from Xe to reduce the storage volume of the mixture of85Kr/Xe. Using van der Waals density-functional theory (vdW-DFT) calculations, benchmarked against the random phase approximation (RPA), we calculate the potential energy profiles for Kr and Xe across the nanopores. For each gas, starting from the RPA potential-energy profile, the force-field parameters to be used in the classical restrained molecular dynamics framework are trained to calculate the Helmholtz free energy barrier as a function of temperature, and therefore, the corresponding entropic loss. Overall, due to the much higher activation energy from the adsorbed state in Xe (17.61 and 42.10 kJ per mole for Kr and Xe, respectively), a large Kr/Xe separation selectivity is postulated from the PTI membrane. Furthermore, the combination of the atom-thick PTI lattice and high pore density leads to extremely large yet selective permeances for Kr. For example, a Kr permeance of 1000 gas permeation units (GPU) accompanying a large Kr/Xe selectivity (>10 000) is calculated at 25 °C, which is significantly better than that of the state-of-the-art membranes for Kr/Xe separation, making PTI-based membranes a leading candidate for processing the hazardous waste of85Kr/Xe mixture.

Oxidation of graphitic materials has been studied for more than a century to synthesize materials such as graphene oxide, nanoporous graphene, and to cut or unzip carbon nanotubes. However, the understanding of the early stages of oxidation is limited to theoretical studies, and experimental validation has been elusive. This is due to (i) challenging sample preparation for characterization because of the presence of highly mobile and reactive epoxy groups formed during oxidation, and (ii) gasification of the functional groups during imaging with atomic resolution, e.g., by transmission electron microscopy. Herein, we utilize a low-temperature scanning tunneling microscope (LT-STM) operating at 4 K to solve the structure of epoxy clusters form upon oxidation. Three distinct nanostructures corresponding to three stages of evolution of vacancy defects are found by quantitatively verifying the experimental data by the van der Waals density functional theory. The smallest cluster is a cyclic epoxy trimer. Their observation validates the theoretical prediction that epoxy trimers minimize the energy in the cyclic structure. The trimers grow into honeycomb superstructures to form larger clusters (1-3 nm). Vacancy defects evolve only in the larger clusters (2-3 nm) in the middle of the cluster, highlighting the role of lattice strain in the generation of vacancies. Semiquinone groups are also present and are assigned at the carbon edge in the vacancy defects. Upon heating to 800 °C, we observe cluster-free vacancy defects resulting from the loss of the entire epoxy population, indicating a reversible functionalization of epoxy groups.

Generating pores in graphene by decoupled nucleation and expansion is desired to achieve a fine control over the porosity, and is desired to advance several applications. Herein, epoxidation is introduced, which is the formation of nanosized epoxy clusters on the graphitic lattice as nucleation sites without forming pores. In situ gasification of clusters inside a transmission electron microscope shows that pores are generated precisely at the site of the clusters by surpassing an energy barrier of 1.3 eV. Binding energy predictions using ab initio calculations combined with the cluster nucleation theory reveal the structure of the epoxy clusters and indicate that the critical cluster is an epoxy dimer. Finally, it is shown that the cluster gasification can be manipulated to form Å-scale pores which then effectively sieve gas molecules based on their size. This decoupled cluster nucleation and pore formation will likely pave the way for an independent control of pore size and density.

A robust synthesis methodology for crystallizing nanoporous single-layer graphene hosting a high density of size-selective nanopores is urgently needed to realize the true potential of two-dimensional membranes for gas separation. Currently, there are no controllable etching techniques for single-layer graphene that are self-limiting, and that can generate size-selective nanopores at a high pore-density. In this work, we simulate a unique chemical vapor deposition based crystallization of graphene on Cu(111), in the presence of an etchant, to generate a high density (>1013 cm−2) of sub-nanometer-sized, elongated nanopores in graphene. An equilibrium between the growth rate and the etching rate is obtained, and beyond a critical time, the total number of the carbon atoms and the edge carbon atoms do not change. Using an optimal first-order etching chemistry, a log-mean pore-size of 5.0 ± 1.7 (number of missing carbon atoms), and a pore-density of 3 × 1013 cm−2 was achieved. A high throughput calculation route for estimating gas selectivity from ensembles of thousands of nanopores was developed. The optimized result yielded H2/CO2, H2/N2 and H2/CH4 selectivities larger than 200, attributing to elongated pores generated by the competitive etching and growth. The approach of competitive etching during the crystal growth is quite generic and can be applied to a number of two-dimensional materials.

C2N is an ordered two-dimensional carbon nitride with a high density (1.7 × 1014 cm-2) of 3.1 Å-sized nanopores, making it promising for high-flux gas sieving for energy-efficient He and H2 purification. Herein, we discuss the accurate calculation of potential energy surfaces for He, H2, N2, and CO2 across C2N nanopores, to characterize the gas-sieving potential of C2N. We compare the potential energy surface derived from density-functional theory calculations using five commonly used van der Waals (vdW) approximations. While all five functionals point that the C2N nanopore yields He/N2 and H2/N2 selectivities over 1000, adsorption energies and energy barriers vary remarkably depending on the approximation chosen. To make progress, we compare the calculations against the results from the adiabatic connection fluctuation dissipation theory, with random-phase approximation, known to be accurate in capturing vdW interactions. The comparison indicates that the interaction energy is less accurate with vdW density functional theory. On the other hand, more empirical corrections work reasonably well, a finding that we also confirm for another carbon nitride lattice, poly(triazine imide). Overall, we recommend these for screening carbon nitride materials for gas separation, but also comparing functionals with higher-order approaches when dealing with different materials.

Tight control of pore size distribution at high pore density achieved by millisecond gasification., Etching single-layer graphene to incorporate a high pore density with sub-angstrom precision in molecular differentiation is critical to realize the promising high-flux separation of similar-sized gas molecules, e.g., CO2 from N2. However, rapid etching kinetics needed to achieve the high pore density is challenging to control for such precision. Here, we report a millisecond carbon gasification chemistry incorporating high density (>1012 cm−2) of functional oxygen clusters that then evolve in CO2-sieving vacancy defects under controlled and predictable gasification conditions. A statistical distribution of nanopore lattice isomers is observed, in good agreement with the theoretical solution to the isomer cataloging problem. The gasification technique is scalable, and a centimeter-scale membrane is demonstrated. Last, molecular cutoff could be adjusted by 0.1 Å by in situ expansion of the vacancy defects in an O2 atmosphere. Large CO2 and O2 permeances (>10,000 and 1000 GPU, respectively) are demonstrated accompanying attractive CO2/N2 and O2/N2 selectivities.

Metal soaps or metal alkanoates are metal-organic complexes held together with metal cations and the functional groups of hydrocarbon chains. They can be synthesized at a high yield by simply mixing the metal and organic sources, forming crystalline frameworks with diverse topology, and have been studied in the past because of their rich polymorphism-like liquid crystals. Their ability to melt while retaining the crystalline properties upon cooling is unique among nanoporous materials and is especially attractive for membrane fabrication. Herein, metal soaps as a new class of material for molecular separation are reported. Three metal soaps, Ca(SO4C12H25)(2), Zn(COOC6H13)(2), and Cu(COOC9H19)(2), hosting lamellar structure with molecular-sized channels are synthesized. They are processed in thin, intergrown, polycrystalline films on porous substrates by two scalable methods, interfacial crystallization and melting with an extremely small processing time (a minute to an hour). The resulting crystalline films are oriented with the alkyl chains perpendicular to the porous substrate which favors molecular transport. The prepared membranes demonstrate attractive gas separation behavior, e.g., 300-nm-thick Ca(SO4C12H25)(2)membrane prepared in a minute using interfacial crystallization yields H(2)permeance of 6.1 x 10(-7)mol m(-2)s(-1)Pa(-1)with H-2/CO(2)selectivity of 10.5.

High-flux nanoporous single-layer graphene membranes are highly promising for energy-efficient gas separation. Herein, in the context of carbon capture, a remarkable enhancement in the CO(2)selectivity is demonstrated by uniquely masking nanoporous single-layer graphene with polymer with intrinsic microporosity (PIM-1). In the process, a major bottleneck of the state-of-the-art pore-incorporation techniques in graphene has been overcome, where in addition to the molecular sieving nanopores, larger nonselective nanopores are also incorporated, which so far, has restricted the realization of CO2-sieving from graphene membranes. Overall, much higher CO2/N(2)selectivity (33) is achieved from the composite film than that from the standalone nanoporous graphene (NG) (10) and the PIM-1 membranes (15), crossing the selectivity target (20) for postcombustion carbon capture. The selectivity enhancement is explained by an analytical gas transport model for NG, which shows that the transport of the stronger-adsorbing CO(2)is dominated by the adsorbed phase transport pathway whereas the transport of N(2)benefits significantly from the direct gas-phase transport pathway. Further, slow positron annihilation Doppler broadening spectroscopy reveals that the interactions with graphene reduce the free volume of interfacial PIM-1 chains which is expected to contribute to the selectivity. Overall, this approach brings graphene membrane a step closer to industrial deployment.