Elucidating the Variable-Temperature Mechanical Properties of a Negative Thermal Expansion Metal-Organic Framework

From Ductile to Brittle: Defect-Engineered Mechanical Properties of Metal-Organic Frameworks

The introduction of defects in metal-organic frameworks (MOFs) is an effective method to improve the performance of MOFs in many applications, but it also compromises the mechanical properties of MOFs. Thus, a comprehensive understanding of the mechanical properties of defective MOFs becomes important for the defect engineering in MOFs. Herein, using the in situ compression tests, we directly observe very different mechanical responses in HKUST-1 MOFs with various defect concentrations. The elastic-plastic deformation followed by a ductile flattened failure is found in the defective HKUST-1 with a small defect concentration, while a hyperelastic-plastic behavior accompanied by the brittle fracture failure could occur in the HKUST-1 with a large defect concentration. The strong dependence of deformation and failure behaviors of defective HKUST-1 crystals on the defect concentration is ascribed to the change in their local deformation mechanism and stress distribution with varying defect concentration, according to the analysis by finite element and molecular dynamics (MD) simulations. Our compression experiments and MD simulations also indicate a significant reduction in both Young's modulus and yield strength of HKUST-1 with growing defect concentration, which agrees well with the theoretical predictions of micromechanics theory. This study is expected to provide a more precise understanding of the mechanical properties of defective MOFs.

Plasticity of Metal-Organic Framework Crystals: Thermally Activated Collapse of Nanopores

Unrecoverable deformation or plasticity can be generated in crystalline metal-organic frameworks (MOFs) by compressive loading with different rates in various applications. Herein, plastic behaviors of MOF HKUST-1 crystals are investigated by a series of in situ strain-rate-dependent compression tests. The yield strength is found to significantly increase with increasing strain rate, following a logarithmic dependence. Our reactive molecular dynamics simulations illustrate that the yielding of crystalline HKUST-1 is induced by the irreversible collapse of its nanopores, which can be accelerated by thermal activation at finite temperatures. Based on this mechanism together with the reaction rate theory, we derive an analytical expression relating the yield strength of MOFs and strain rate, which fits experimental findings well. Overall, this work can expand our current understanding of MOF plasticity, which is of importance for the mechanical shaping and various applications of MOF crystals.

Indentation Depth-Dependent Hardness of Metal-Organic Framework Crystals: The Effect of Local Amorphization Induced by Indentation

The hardness of metal-organic frameworks (MOFs) is an important mechanical property metric measuring their resistance to the permanent plastic deformation. The hardness of most MOFs measured from nanoindentation experiments usually exhibits the similar unique indentation depth dependence feature, the mechanism of which still remains unclear. In order to explain the effect of the indentation depth on the hardness of MOFs, we conducted nanoindentation simulations on HKUST-1 by using reactive molecular dynamics simulations. Our simulations reveal that the HKUST-1 material near the indenter can transform from the parent crystalline phase to a new amorphous phase due to the high pressure generated, while its counterpart far from the indenter remains in the crystalline phase. By considering the crystalline-amorphous interface in the energy analysis of MOFs, we derived an analytical expression of the hardness at different indentation depths. It is found that the interface effect can greatly increase the hardness of MOFs, as observed in nanoindentation simulations. Moreover, the proposed analytical expression can well explain the indentation depth-dependent hardness of many MOF crystals measured in nanoindentation experiments. Overall, this work can provide a better understanding of the indentation depth dependence of the hardness of MOFs.

Current Advancement and Future Prospects: Biomedical Nanoengineering

Recent advancements in biomedicine have seen a significant reliance on nanoengineering, as traditional methods often fall short in harnessing the unique attributes of biomaterials. Nanoengineering has emerged as a valuable approach to enhance and enrich the performance and functionalities of biomaterials, driving research and development in the field. This review emphasizes the most prevalent biomaterials used in biomedicine, including polymers, nanocomposites, and metallic materials, and explores the pivotal role of nanoengineering in developing biomedical treatments and processes. Particularly, the review highlights research focused on gaining an in-depth understanding of material properties and effectively enhancing material performance through molecular dynamics simulations, all from a nanoengineering perspective.

Plastic Avalanches in Metal-Organic Framework Crystals Due to the Dynamic Phase Mixing

The compressive properties of metal-organic framework (MOF) crystals are not only crucial for their densification process but also key in determining their performance in many applications. We herein investigated the mechanical responses of a classic crystalline MOF, HKUST-1, using in situ compression tests. A serrated flow accompanied by the unique strain avalanches was found in individual and contacting crystals before their final flattening or fracture with splitting cracks. The plastic flow with serrations is ascribed to the dynamic phase mixing due to the progressive and irreversible local phase transition in HKUST-1 crystals, as revealed by molecular dynamics and finite element simulations. Such pressure-induced phase coexistence in HKUST-1 crystals also induces a significant loading-history dependence of their Young's modulus. The observation of plastic avalanches in HKUST-1 crystals here not only expands our current understanding of the plasticity of MOF crystals but also unveils a novel mechanism for the avalanches and plastic flow in crystal plasticity.

Anomalous Loading Rate Dependence of the Mechanical Properties of Metal-Organic Framework Crystals: Latent Heat Effects of the Pressure-Induced Local Phase Transition

The loading rate dependence of the mechanical properties of metal-organic framework (MOF) crystals is key in determining their performance in many engineering applications, which, however, remains almost unexplored. Here, in situ nanoindentation experiments were conducted to investigate the impact of loading rate on mechanical properties of HKUST-1, a classic MOF. The Young's modulus and hardness of crystalline HKUST-1 are found to stay stable or decline with decreasing loading rate by creeping when the loading rate is below a particular speed, but they significantly decrease as the loading rate grows when it has higher magnitudes. Our molecular dynamics simulations indicate that the anomalous loading rate dependence of mechanical properties is attributed to the competition between the release and transfer of latent heat from the pressure-induced amorphous HKUST-1 because the increase in local temperature at large loading rates could induce the softening of HKUST-1 and the increase in the volume of transformed materials.

Temperature-Dependent Mechanical Properties of a Metal-Organic Framework: Creep Behavior of a Zeolitic Imidazolate Framework-8 Single Crystal

Zeolite Imidazole Framework-8 (ZIF-8) with a robust structure and high thermal stability is a strong candidate to act as the catalyst matrix for various chemical applications, especially for those at higher temperatures, like hydrogenation. In this study, the time-dependent plasticity of a ZIF-8 single crystal was explored by a dynamic indentation technique to explore its mechanical stability at higher temperatures. The thermal dynamic parameters for the creep behaviors, like activation volume and activation energy, were determined, and possible mechanisms for the creep of ZIF-8 were then discussed. A small activation volume implies the localization of the thermo-activated events, while high activation energy, high stress exponent n, and weak dependence of the creep rate on temperature all favor pore collapse over volumetric diffusion as the creep mechanism.

Controlling Thermal Expansion in Supramolecular Halogen-Bonded Mixed Cocrystals through Synthetic Feed and Dynamic Motion

Control over thermal expansion (TE) behaviors in solid materials is often accomplished by modifying the molecules or intermolecular interactions within the solid. Here, we use a mixed cocrystal approach and incorporate molecules with similar chemical structures, but distinct functionalities. Development of mixed cocrystals is at a nascent stage, and here we describe the first mixed cocrystals sustained by one-dimensional halogen bonds. Within each mixed cocrystal, the halogen-bond donor is fixed, while the halogen-bond acceptor site contains two molecules in a variable ratio. X-ray diffraction demonstrates isostructurality across the series, and SEM-EDS shows equal distribution of heavy atoms and similar atomic compositions across all mixed cocrystals. The acceptor molecules differ in their ability to undergo dynamic motion in the solid state. The synthetic equivalents of motion capable and incapable molecules were systematically varied to yield direct tunabililty in TE behavior.

HKUST-1-derived highly ordered Cu nanosheets with enriched edge sites, stepped (211) surfaces and (200) facets for effective electrochemical CO2 reduction

A novel electrode composed of Cu nanosheets constructed from nanoparticles was synthesized by in situ electrochemical derivation from the metal-organic framework (MOF) HKUST-1. The prepared derivative electrode (HE-Cu) exhibited higher Faradaic efficiency (FE, 56.0%) of electrochemical CO₂ reduction (CO₂R) compared with that of pristine Cu foil (p-Cu, 32.3%) at an overpotential of -1.03 V vs. a reversible hydrogen electrode (RHE). HE-Cu also exhibited lower onset potential of CO₂R as well as inhibiting the H₂ evolution reaction. Electrochemical measurements revealed that HE-Cu exhibited higher CO₂ adsorption (1.58-fold) and a larger electrochemical active surface area (1.24-fold) compared with p-Cu. Physicochemical characterization and Tafel analysis showed that stepped Cu (211) surfaces, (200) facets and Cu edge atoms on HE-Cu contributed significantly to the enhanced CO₂R activity and/or HCOOH and/or C2 product selectivity. The FEs of HCOOH and C2 products for HE-Cu increased 1.57-fold and 10.6-fold at an overpotential of -1.19 V vs. RHE compared with p-Cu. Although CH₄ was produced on p-Cu, its formation was totally suppressed on HE-Cu due to the increase of edge sites and (200) facets. Our study demonstrates that electroreduction of MOFs is a promising method to prepare novel and stable electrochemical catalysts with unique surface structures. The fabricated derivative electrode not only promoted electrochemical CO₂R activity but also exhibited high C2 product selectivity.

Self-supporting covalent organic framework membranes synthesized through two different processes: solvothermal annealing and solvent vapor annealing

Rigid, freestanding covalent organic framework (COF-1) membranes have been synthesized from 1,4-benzenediboronic acid (BDBA) precursors using two different approaches: room temperature solvent-vapour annealing (SVA) and solvothermal annealing (SA). Characterization of films using Fourier-transform infrared (FTIR) spectroscopy, x-ray diffraction (XRD), and various microscopies shows that the films obtained through the two different routes vary in their retained BDBA proportion, crystal size and macroscale morphology. Gas adsorption measurements give specific surface areas of 579 ± 7 m² g^-1 and 739 ± 11 m² g^-1 respectively, suggesting that the average porosity of these films is competitive with bulk-synthesized COF-1 particles. The films have a stratified structure, with a dense, thin top layer and a thicker, sponge-like base layer. Using nanoindentation, we measured the Young's modulus at the top surface of the SVA and SA films to be 3.64 ± 1.20 GPa and 3.33 ± 0.12 GPa respectively, with the smaller uncertainty for the SA film attributed to a more uniform morphology. These measurements provide useful experimental data pertaining to COF-1 mechanical properties, furnishing information relevant to the use of these free-standing membranes in applications such as gas filtration or storage.

Mechanical properties of metal-organic frameworks

As the field of metal–organic frameworks (MOFs) continues to grow, the physical stability and mechanical properties of these porous materials has become a topic of great interest.

As the field of metal–organic frameworks (MOFs) continues to grow, the physical stability and mechanical properties of these porous materials has become a topic of great interest. While strategies for synthesizing MOFs with desirable chemical functionalities or pore sizes have been established over the past twenty years, design principles to modulate the response of MOFs to mechanical stress are still underdeveloped. The inherent porosity of these frameworks results in many interesting and sometimes unexpected phenomena upon exposure to elevated pressures and other physical stimuli. Beyond its fundamental importance, an understanding of mechanical properties (e.g. bulk modulus, shear modulus, Young's modulus, linear compressibility, and Poisson's ratio) plays an essential role in the post-synthetic processing of MOFs, which has implications in the successful transition of these materials from academic interest to industrial relevance. This perspective provides a concise overview of the efforts to understand the mechanical properties of MOFs through experimental and computational methods. Additionally, current limitations and possible future directions for the field are also discussed briefly.

Metal-Organic Frameworks in Modern Physics: Highlights and Perspectives

Owing to the synergistic combination of a hybrid organic-inorganic nature and a chemically active porous structure, metal-organic frameworks have emerged as a new class of crystalline materials. The current trend in the chemical industry is to utilize such crystals as flexible hosting elements for applications as diverse as gas and energy storage, filtration, catalysis, and sensing. From the physical point of view, metal-organic frameworks are considered molecular crystals with hierarchical structures providing the structure-related physical properties crucial for future applications of energy transfer, data processing and storage, high-energy physics, and light manipulation. Here, the perspectives of metal-organic frameworks as a new family of functional materials in modern physics are discussed: from porous metals and superconductors, topological insulators, and classical and quantum memory elements, to optical superstructures, materials for particle physics, and even molecular scale mechanical metamaterials. Based on complementary properties of crystallinity, softness, organic-inorganic nature, and complex hierarchy, a description of how such artificial materials have extended their impact on applied physics to become the mainstream in material science is offered.