Although polycrystalline metal‐organic framework (MOF) membranes offer several advantages over other nanoporous membranes, thus far they have not yielded good CO2 separation performance, crucial for energy‐efficient carbon capture. ZIF‐8, one of the most popular MOFs, has a crystallographically determined pore aperture of 0.34 nm, ideal for CO2/N2 and CO2/CH4 separation; however, its flexible lattice restricts the corresponding separation selectivities to below 5. A novel postsynthetic rapid heat treatment (RHT), implemented in a few seconds at 360 °C, which drastically improves the carbon capture performance of the ZIF‐8 membranes, is reported. Lattice stiffening is confirmed by the appearance of a temperature‐activated transport, attributed to a stronger interaction of gas molecules with the pore aperture, with activation energy increasing with the molecular size (CH4 > CO2 > H2). Unprecedented CO2/CH4, CO2/N2, and H2/CH4 selectivities exceeding 30, 30, and 175, respectively, and complete blockage of C3H6, are achieved. Spectroscopic and X‐ray diffraction studies confirm that while the coordination environment and crystallinity are unaffected, lattice distortion and strain are incorporated in the ZIF‐8 lattice, increasing the lattice stiffness. Overall, RHT treatment is a facile and versatile technique that can vastly improve the gas‐separation performance of the MOF membranes.

Three-dimensional (3D) architectures obtained by the structural assembly of 1D nanomaterials are regarded as the next generation building blocks for sensors, electronics, photonics, and bioelectronic applications. Purification and functionalization of such 3D ordered structures are crucial for realizing their full potential. Plasma functionalization, compared to any solution based process, is favorable in retaining the alignment while functionalizing such structures. However, the commonly employed plasma processes like O 2 or Ar plasma can be highly detrimental to well-aligned ordered nanostructures and thus might affect the properties intimately associated with their 3D structure. Here, for the first time, we investigate the mild nature of a radio frequency CO 2 gas plasma as an effective source for purification and functionalization of vertically aligned CNT structures and study the effects of this functionalization onto the purification and functionalization by physical and chemical techniques (HRTEM, XPS, Raman). We found that CO 2 plasma selectively etches the amorphous carbon present in the vertically aligned CNT structure. Moreover, it is as effective as the widely used but more aggressive O 2 plasma in functionalizing the CNT. Unlike an O 2 or Ar plasma, CO 2 plasma has the tremendous advantage of retaining the structural integrity of the CNT structures.

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Grand-canonical Monte Carlo simulations and adsorption experiments are conducted to understand the adsorption of CO 2 onto bundles of 3D aligned double-walled carbon-nanotubes of diameter 5 nm at 303 K. The simulation of partial adsorption isotherms, i.e., only inner tube volume, only interstices between tubes, and unrestricted, allows a breakdown of the experimental adsorption isotherms into contributions of different regions. The results are compatible with microscopic observations of the majority of the inner tube volumes being accessible for CO 2 . Further, the unrestricted adsorption isotherm is quantitatively equivalent to the sum of inner and outer adsorption for the pressure range considered in this work, p < 40 bar, indicating no significant interference between inner and outer regions. The intertube distance, which is varied from 0 to 15 nm, dramatically affects the isosteric heat of adsorption and adsorption capacity. Excess adsorption is found to display a nonlinear behavior with d, for unrestricted and outer cases. For low pressures (p ≤ 14 bar), maximum adsorption occurs at d = 0.5 nm. However, for higher pressures, 14 < p < 40 bar, the adsorption peaks at d = 1 nm. The Freundlich isotherm is found to fit the experimental and simulation data. The adsorption sequence changes with the intertube distance for the unrestricted case. At d ≤ 0.5 nm, adsorption proceeds with increasing loading in the following order: grooves and inner surface adsorption → fill interstitial region → fill inner region. However, at higher distances, d > 0.5 nm, the sequence changes the following: inner surface adsorption + partial outer surface adsorption → complete outer surface adsorption → fill interstitial, groove, inner adsorption. The change in mechanism of adsorption is clearly reflected in the behavior of the heat of adsorption, where we observed a crossover behavior at around d = 0.5 nm.

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.