Unit-cell-thick zeolitic imidazolate framework films for membrane application

Atomic-level imaging of beam-sensitive COFs and MOFs by low-dose electron microscopy

Electron microscopy, an important technique that allows for the precise determination of structural information with high spatiotemporal resolution, has become indispensable in unravelling the complex relationships between material structure and properties ranging from mesoscale morphology to atomic arrangement. However, beam-sensitive materials, particularly those comprising organic components such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), would suffer catastrophic damage from the high energy electrons, hindering the determination of atomic structures. A low-dose approach has arisen as a possible solution to this problem based on the integration of advancements in several aspects: electron optical system, detector, image processing, and specimen preservation. This article summarizes the transmission electron microscopy characterization of MOFs and COFs, including local structures, host-guest interactions, and interfaces at the atomic level. Revolutions in advanced direct electron detectors, algorithms in image acquisition and processing, and emerging methodology for high quality low-dose imaging are also reviewed. Finally, perspectives on the future development of electron microscopy methodology with the support of computer science are presented.

Advances in zeolitic-imidazolate-framework-based catalysts for photo-/electrocatalytic water splitting, CO2 reduction and N2 reduction applications

Harnessing electrical or solar energy for the renewable production of value-added fuels and chemicals through catalytic processes (such as photocatalysis and electrocatalysis) is promising to achieve the goal of carbon neutrality. Owing to the large number of highly accessible active sites, highly porous structure, and charge separation/transfer ability, as well as excellent stability against chemical and electrochemical corrosion, zeolite imidazolate framework (ZIF)-based catalysts have attracted significant attention. Strategic construction of heterojunctions, and alteration of the metal node and the organic ligand of the ZIFs effectively regulate the binding energy of intermediates and the reaction energy barriers that allow tunable catalytic activity and selectivity of a product during reaction. Focusing on the currently existing critical issues of insufficient kinetics for electron transport and selective generation of ideal products, this review starts from the characteristics and physiochemical advantages of ZIFs in catalytic applications, then introduces promising regulatory approaches for advancing the kinetic process in emerging CO2 reduction, water splitting and N2 reduction applications, before proposing perspective modification directions.

Facet engineering of a two-dimensional metal-organic framework with uniquely oriented layered-structure for electrocatalytic oxygen reduction reaction

The properties of metal-organic framework (MOF) nanocrystals are highly dependent on their sizes, morphologies, and exposed facets. Facet engineering of MOFs offers an efficient strategy to tailor the active sites and optimize the catalytic activity of both MOFs and their derivatives. In this study, we prepared 1D zeolitic imidazolate framework-nanorod (ZIF-NR) through facet engineering of the parental 2D ZIF-L. The introduction of cetyltrimethylammonium bromide (CTABr) surfactant into the synthesis solution hindered the crystal growth along the c-axis of leaf-like ZIF-L, resulting in the formation of 1D ZIF-NR. The derived Co nanoparticle encapsulated N doped carbon nanorod (denoted as Co-NCR) exhibited high activity and stability for electrocatalytic oxygen reduction reactions and Zn-air batteries. Facet engineering of a 2D MOF with a uniquely oriented layered structure demonstrates the possibility of designing novel electrocatalysts.

Porous organic framework membranes based on interface-induced polymerisation: design, synthesis and applications

Porous organic frameworks (POFs) are novel porous materials that have attracted much attention due to their extraordinary properties, such as high specific surface area, tunable pore size, high stability and ease of functionalisation. However, conventional synthesised POFs are mostly large-sized particles or insoluble powders, which are difficult to recycle and have low mass transfer efficiencies, limiting the development of their cutting-edge applications. Therefore, processing POF materials into membrane structures is of great significance. In recent years, interface engineering strategies have proved to be efficient methods for the formation of POF membranes. In this perspective, recent advances in the use of interfaces to prepare POF membranes are reviewed. The challenges of this strategy and the potential applications of the formed POF membranes are discussed.

Advancing Molecular Sieving via Å-Scale Pore Tuning in Bottom-Up Graphene Synthesis

Porous graphene films are attractive as a gas separation membrane given that the selective layer can be just one atom thick, allowing high-flux separation. A favorable aspect of porous graphene is that the pore size, essentially gaps created by lattice defects, can be tuned. While this has been demonstrated for postsynthetic, top-down pore etching in graphene, it does not exist in the more scalable, bottom-up synthesis of porous graphene. Inspired by the mechanism of precipitation-based synthesis of porous graphene over catalytic nickel foil, we herein conceive an extremely simple way to tune the pore size. This is implemented by increasing the cooling rate by over 100-fold from -1 °C min-1 to over -5 °C s-1. Rapid cooling restricts carbon diffusion, resulting in a higher availability of dissolved carbon for precipitation, as evidenced by quantitative carbon-diffusion simulation, measurement of carbon concentration as a function of nickel depth, and imaging of the graphene nanostructure. The resulting enhanced grain (inter)growth reduces the effective pore size which leads to an increase of the H2/CH4 separation factor from 6.2 up to 53.3.