Direct Visualization of Atomic Structure in Multivariate Metal-Organic Frameworks (MOFs) for Guiding Electrocatalysts Design

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.

Accelerated Dynamic Reconstruction in Metal-Organic Frameworks with Ligand Defects for Selective Electrooxidation of Amines to Azos Coupling with Hydrogen Production

Electrosynthesis coupled hydrogen production (ESHP) mostly involves catalyst reconstruction in aqueous phase, but accurately identifying and controlling the process is still a challenge. Herein, we modulated the electronic structure and exposed unsaturated sites of metal-organic frameworks (MOFs) via ligand defect to promote the reconstruction of catalyst for azo electrosynthesis (ESA) coupled with hydrogen production overall reaction. The monolayer Ni-MOFs achieved 89.8 % Faraday efficiency and 90.8 % selectivity for the electrooxidation of 1-methyl-1H-pyrazol-3-amine (Pyr-NH2) to azo, and an 18.5-fold increase in H2 production compared to overall water splitting. Operando X-ray absorption fine spectroscopy (XAFS) and various in situ spectroscopy confirm that the ligand defect promotes the potential dependent dynamic reconstruction of Ni(OH)2 and NiOOH, and the reabsorption of ligand significantly lowers the energy barrier of rate-determining step (*Pyr-NH to *Pyr-N). This work provides theoretical guidance for modulation of electrocatalyst reconstruction to achieve highly selective ESHP.

A NiMOF integrated with conductive materials for efficient bifunctional electrocatalysis of urea oxidation and oxygen evolution reactions

The development of urea oxidation reaction (UOR) and oxygen evolution reaction (OER) bifunctional electrocatalysts has dual significance in promoting hydrogen energy production and urea-rich wastewater treatment. Herein, a carboxylated multi-walled carbon nanotube (MWCNT-COOH)-ferrocene carboxylic acid (Fc-COOH) modulated NiMOF hybrid material (MWCNT-NiMOF(Fc)) has been synthesized for dual electrocatalysis of the UOR and OER. The material characterization results indicated that MWCNT-COOH and Fc-COOH were integrated into the framework structure of the NiMOF. The direct interaction between the NiMOF and MWCNT/Fc facilitated electron transfer in the hybrid material and led to lattice strain, which improved the charge transfer kinetics, promoted the exposure of more unsaturated Ni sites, and increased the electrochemically active surface area. These factors together enhanced the electrocatalytic activity of MWCNT-NiMOF(Fc) towards the UOR and OER. Using a glassy carbon electrode as the substrate, MWCNT-NiMOF(Fc) exhibited low potential requirements, low Tafel slopes, and high stability. In overall urea and water splitting electrolysis cells, the excellent UOR and OER dual functional catalytic ability and enormous practical application potential of the MWCNT-NiMOF(Fc) modified foam nickel electrode were further demonstrated. On the basis of the above research, the influence of a KOH environment on urea electrolysis was further studied, and the urea electrolysis products were analyzed, promoting a more comprehensive understanding of the catalytic performance of MWCNT-NiMOF(Fc) for urea oxidation. This study provides a new approach for developing high-performance NiMOF-based electrocatalysts for challenging bifunctional UOR/OER applications, and has potential application value in hydrogen production from urea-containing wastewater electrolysis.

Revolutionizing the structural design and determination of covalent-organic frameworks: principles, methods, and techniques

Covalent organic frameworks (COFs) represent an important class of crystalline porous materials with designable structures and functions. The interconnected organic monomers, featuring pre-designed symmetries and connectivities, dictate the structures of COFs, endowing them with high thermal and chemical stability, large surface area, and tunable micropores. Furthermore, by utilizing pre-functionalization or post-synthetic functionalization strategies, COFs can acquire multifunctionalities, leading to their versatile applications in gas separation/storage, catalysis, and optoelectronic devices. Our review provides a comprehensive account of the latest advancements in the principles, methods, and techniques for structural design and determination of COFs. These cutting-edge approaches enable the rational design and precise elucidation of COF structures, addressing fundamental physicochemical challenges associated with host-guest interactions, topological transformations, network interpenetration, and defect-mediated catalysis.

Accelerating structure reconstruction to form NiOOH in metal-organic frameworks (MOFs) for boosting the oxygen evolution reaction

Structural reconstruction of electrocatalysts to generate metal hydroxide/oxyhydroxide species is critical for an efficient oxygen evolution reaction (OER), but the controllable regulation of the reconstruction process still remains a challenge. Given the designable nature of metal-organic frameworks (MOFs), herein, we have reported a localized structure disordering strategy to accelerate the structural reconstruction of Ni-BDC to generate NiOOH for boosting the OER. The Ni-BDC nanosheets were modified by Fe3+ and urea to form cracks, which could promote the accessibility of the Ni sites by the electrolyte and thus promote the reconstruction to form NiOOH. In addition, the interaction between Ni2+ and Fe3+ allows the electron flow from Ni2+ to Fe3+, further enhancing the NiOOH generation. As a result, the optimized sample exhibits excellent OER activity with a small overpotential of 251 mV at 10 mA cm-2, which is superior to most of the MOF-based OER catalysts reported previously. This work provides a controllable strategy to regulate the structural reconstruction for promoting the OER, which could provide important guidance for the development of more efficient OER electrocatalysts.

Lewis acidic Fe3+-driven catalytic active Ni3+ formation in Fe-free metal-organic framework for enhanced electrochemical glucose sensing

Manipulating metal valence states and porosity in the metal-organic framework (MOF) by alloying has been a unique tool for creating high-valent metal sites and pore environments in a structure that are inaccessible by other methods, favorable for accelerating the catalytic activity towards sensing applications. Herein, we report Fe3+-driven formation of catalytic active Ni3+ species in the amine-crafted benzene-dicarboxylate (BDC-NH2)-based MOF as a high-performance electrocatalyst for glucose sensing. This work took the benefit of different bonding stability between BDC-NH2 ligand, and Fe3+ and Ni2+ metal precursor ions in the heterometallic NixFe(1-x)-BDC-NH2 MOF. The FeCl3 that interacts weakly with ligand, oxidizes the Ni2+ precursor to Ni3+-based MOF owing to its Lewis acidic behavior and was subsequently removed from the structure supported by Ni atoms, during solvothermal synthesis. This enables to create mesopores within a highly stable Ni-MOF structure with optimal feed composition of Ni0.7Fe0.3-BDC-NH2. The Ni3+-based Ni0.7Fe0.3-BDC-NH2 demonstrates superior catalytic properties towards glucose sensing with a high sensitivity of 13,435 µA mM-1 cm-2 compared to the parent Ni2+-based Ni-BDC-NH2 (10897 μA mM-1cm-2), along with low detection limit (0.9 μM), short response time (≤5 s), excellent selectivity, and higher stability. This presented approach for fabricating high-valent nickel species, with a controlled quantity of Fe3+ integrated into the structure allowing pore engineering of MOFs, opens new avenues for designing high-performing MOF catalysts with porous framework for sensing applications.

A Self-Templated Design Approach toward Multivariate Metal-Organic Frameworks for Enhanced Oxygen Evolution

Multivariate metal-organic framework (MOF) is an ideal electrocatalytic material due to the synergistic effect of multiple metal active sites. In this study, a series of ternary M-NiMOF (M = Co, Cu) through a simple self-templated strategy that the Co/Cu MOF isomorphically grows in situ on the surface of NiMOF is designed. Owing to the electron rearrange of adjacent metals, the ternary CoCu-NiMOFs demonstrate the improved intrinsic electrocatalytic activity. At optimized conditions, the ternary Co3 Cu-Ni2 MOFs nanosheets give the excellent oxygen evolution reaction (OER) performance of current density of 10 mA cm-2 at low overpotential of 288 mV with a Tafel slope of 87 mV dec-1 , which is superior to that of bimetallic nanosheet and ternary microflowers. The low free energy change of potential-determining step identifies that the OER process is favorable at Cu-Co concerted sites along with strong synergistic effect of Ni nodes. Partially oxidized metal sites also reduce the electron density, thus accelerating the OER catalytic rate. The self-templated strategy provides a universal tool to design multivariate MOF electrocatalysts for highly efficient energy transduction.

Applications of Transmission Electron Microscopy in Phase Engineering of Nanomaterials

Phase engineering of nanomaterials (PEN) is an emerging field that aims to tailor the physicochemical properties of nanomaterials by precisely manipulating their crystal phases. To advance PEN effectively, it is vital to possess the capability of characterizing the structures and compositions of nanomaterials with precision. Transmission electron microscopy (TEM) is a versatile tool that combines reciprocal-space diffraction, real-space imaging, and spectroscopic techniques, allowing for comprehensive characterization with exceptional resolution in the domains of time, space, momentum, and, increasingly, even energy. In this Review, we first introduce the fundamental mechanisms behind various TEM-related techniques, along with their respective application scopes and limitations. Subsequently, we review notable applications of TEM in PEN research, including applications in fields such as metallic nanostructures, carbon allotropes, low-dimensional materials, and nanoporous materials. Specifically, we underscore its efficacy in phase identification, composition and chemical state analysis, in situ observations of phase evolution, as well as the challenges encountered when dealing with beam-sensitive materials. Furthermore, we discuss the potential generation of artifacts during TEM imaging, particularly in scanning modes, and propose methods to minimize their occurrence. Finally, we offer our insights into the present state and future trends of this field, discussing emerging technologies including four-dimensional scanning TEM, three-dimensional atomic-resolution imaging, and electron microscopy automation while highlighting the significance and feasibility of these advancements.

Enlarging the Ni-O Bond Polarizability in a Phosphorene-Hosted Metal-Organic Framework for Boosted Water Oxidation Electrocatalysis

The emerging lattice-oxygen oxidation mechanism (LOM) presents attractive opportunities for breaking the scaling relationship to boost oxygen evolution reaction (OER) with the direct OLattice-*O interaction. However, currently the LOM-triggering rationales are still debated, and a streamlined physicochemical paradigm is extremely desirable for the design of LOM-defined OER catalysts. Herein, a Ni metal-organic framework/black phosphorene (NiMOF/BP) heterostructure is theoretically profiled and constructed as a catalytic platform for the LOM-derived OER studies. It is found that the p-type BP host can enlarge the Ni-O bond polarizability of NiMOF through the Ni-O bond stretching and Ni valence declining synergically. Such an enlarged bond polarizability will in principle alleviate the lattice oxygen confinement to benefit the LOM pathway and OER performance. As a result, the optimized NiMOF/BP catalyst exhibits promising OER performance with a low overpotential of 260 mV at 10 mA cm-2 and long-term stability in 1 M KOH electrolyte. Both experiment and calculation results suggest the activated LOM pathway with a more balanced step barrier in the NiMOF/BP OER catalyst. This research puts forward Ni-O bond polarizability as the criterion to design LOM-scaled electrocatalysts for water oxidation.

Imaging the dynamic influence of functional groups on metal-organic frameworks

Metal-organic frameworks (MOFs) with different functional groups have wide applications, while the understanding of functionalization influences remains insufficient. Previous researches focused on the static changes in electronic structure or chemical environment, while it is unclear in the aspect of dynamic influence, especially in the direct imaging of dynamic changes after functionalization. Here we use integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) to directly 'see' the rotation properties of benzene rings in the linkers of UiO-66, and observe the high correlation between local rigidity and the functional groups on the organic linkers. The rigidity is then correlated to the macroscopic properties of CO2 uptake, indicating that functionalization can change the capability through not only static electronic effects, but also dynamic rotation properties. To the best of our knowledge this is the first example of a technique to directly image the rotation properties of linkers in MOFs, which provides an approach to study the local flexibility and paves the way for potential applications in capturing, separation and molecular machine.