Mechanical Properties Of Metal-organic Frameworks Research Articles

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.

Tuning Electrical and Mechanical Properties of Metal-Organic Frameworks by Metal Substitution.

Metal-organic frameworks (MOFs), synthesized by the self-assembly of organic ligands and metal centers, are structurally designable materials. In the current study, first-principles calculation based on density functional theory (DFT) was performed to investigate the intrinsic mechanical and electrical properties and mechanical-electrical coupling behavior of MOF-5. To improve the conductivity of MOF-5, homologous elements of Cu, Ag, and Au were adopted to replace the Zn atom in MOF-5, reducing the band gap and improving its electrical performance. Cu-MOF-5 and Au-MOF-5, with stable structures, exhibit better conductivity. The intrinsic mechanical properties such as independent elastic constants of MOF-5 and M-MOF-5 (M = Cu, Ag, Au) were obtained. MOF-5 and Cu-MOF-5 were experimentally synthesized to demonstrate the reduction in the band gap after metal substitution. The study of the strain effect of MOF-5 and Cu-MOF-5 proves that strain engineering is an effective method to regulate the band gap and this modulation is repeatable. This study clarifies the tunability of the band gap of MOF-5 with metal substituents and provides an efficient strategy for the development of new types of MOFs with desired physical properties using the combination of theoretical prediction and experimental synthesis and validation.

Mechanobiology of Dental Pulp Stem Cells at the Interface of Aqueous-Based Fabricated ZIF8 Thin Film.

The limited knowledge on how biological systems sense and respond to the mechanical properties of metal-organic framework (MOF) thin films is a critical restriction factor for their extensive usage. To bridge this gap, we performed an in vitro study for defining and linking surface characteristics at the interface of the zeolitic imidazolate framework-8 (ZIF8) thin layer to stem cell behavior. First, the physio-mechanical properties of the ZIF8 layer grown on polydopamine (PDA) and tannic acid (TA) layers have been studied. The response of dental pulp stem cells (DPSCs) to different surface states was examined. The results showed that the uniform crystalline microstructure of the ZIF8 on PDA and TA effectively led to the 61- and 388-fold increased surface roughness, 3- and 2.5-fold moderated elastic modulus, almost 3-fold elevated surface free energy, and highly charged surfaces (ζ = -60 mV for TA/ZIF8), respectively. Beyond the inherent bioactivity of the ZIF8 layer, these substrate cues promoted advanced cell adhesion (∼two times) and high proliferation rate. Furthermore, we found a substantial increment in the differentiation efficiency of DPSCs on the ZIF8 layer, in a way that the expression of functional adipocyte (PPARG) and osteoblast (SPP1) markers was, respectively, elevated around 30 000- and 10 000-fold on the TA/ZIF8-coated silicon wafer (SW). Our findings support the impact of fabrication strategy on the biointerface properties of the ZIF8 layer and bring SW/TA/ZIF8 as a robust platform for managing stem cells for biomedical applications.

Isolating the Role of the Node-Linker Bond in the Compression of UiO-66 Metal–Organic Frameworks

Understanding the mechanical properties of metal–organic frameworks (MOFs) is essential to the fundamental advancement and practical implementations of porous materials. Recent computational and experimental efforts have revealed correlations between mechanical properties and pore size, topology, and defect density. These results demonstrate the important role of the organic linker in the response of these materials to physical stresses. However, the impact of the coordination bond between the inorganic node and organic linker on the mechanical stability of MOFs has not been thoroughly studied. Here, we isolate the role of this node–linker coordination bond to systematically study the effect it plays in the compression of a series of isostructural MOFs, M-UiO-66 (M = Zr, Hf, or Ce). The bulk modulus (i.e., the resistance to compression under hydrostatic pressure) of each MOF is determined by in situ diamond anvil cell (DAC) powder X-ray diffraction measurements and density functional theory (DFT) simulations. These experiments reveal the distinctive behavior of Ce-UiO-66 in response to pressures under 1 GPa. In situ DAC Raman spectroscopy and DFT calculations support the observed differences in compressibility between Zr-UiO-66 and the Ce analogue. Monitoring changes in bond lengths as a function of pressure through DFT simulations provides a clear picture of those which shorten more drastically under pressure and those which resist compression. We hypothesize that the presence of ∼10% Ce3+ in the nodes of Ce-UiO-66 may contribute to the weakening of the node–linker coordination, manifesting in the distinct behavior under pressure. This study demonstrates that changes to the node–linker bond can have significant ramifications on the mechanical properties of MOFs.

Improving the Mechanical Stability of Metal-Organic Frameworks Using Chemical Caryatids.

Metal–organic frameworks (MOFs) have emerged as versatile materials for applications ranging from gas separation and storage, catalysis, and sensing. The attractive feature of MOFs is that, by changing the ligand and/or metal, they can be chemically tuned to perform optimally for a given application. In most, if not all, of these applications one also needs a material that has a sufficient mechanical stability, but our understanding of how changes in the chemical structure influence mechanical stability is limited. In this work, we rationalize how the mechanical properties of MOFs are related to framework bonding topology and ligand structure. We illustrate that the functional groups on the organic ligands can either enhance the mechanical stability through formation of a secondary network of nonbonded interactions or soften the material by destabilizing the bonded network of a MOF. In addition, we show that synergistic effect of the bonding network of the material and the secondary network is required to achieve optimal mechanical stability of a MOF. The developed molecular insights in this work can be used for systematic improvement of the mechanical stability of the materials by careful selection of the functional groups.

Research Update: Mechanical properties of metal-organic frameworks – Influence of structure and chemical bonding

Metal-organic frameworks (MOFs), a young family of functional materials, have been attracting considerable attention from the chemistry, materials science, and physics communities. In the light of their potential applications in industry and technology, the fundamental mechanical properties of MOFs, which are of critical importance for manufacturing, processing, and performance, need to be addressed and understood. It has been widely accepted that the framework topology, which describes the overall connectivity pattern of the MOF building units, is of vital importance for the mechanical properties. However, recent advances in the area of MOF mechanics reveal that chemistry plays a major role as well. From the viewpoint of materials science, a deep understanding of the influence of chemical effects on MOF mechanics is not only highly desirable for the development of novel functional materials with targeted mechanical response, but also for a better understanding of important properties such as structural flexibility and framework breathing. The present work discusses the intrinsic connection between chemical effects and the mechanical behavior of MOFs through a number of prototypical examples.

Transferable Force Field for Metal-Organic Frameworks from First-Principles: BTW-FF.

We present an ab-initio derived force field to describe the structural and mechanical properties of metal–organic frameworks (or coordination polymers). The aim is a transferable interatomic potential that can be applied to MOFs regardless of metal or ligand identity. The initial parametrization set includes MOF-5, IRMOF-10, IRMOF-14, UiO-66, UiO-67, and HKUST-1. The force field describes the periodic crystal and considers effective atomic charges based on topological analysis of the Bloch states of the extended materials. Transferable potentials were developed for the four organic ligands comprising the test set and for the associated Cu, Zn, and Zr metal nodes. The predicted materials properties, including bulk moduli and vibrational frequencies, are in agreement with explicit density functional theory calculations. The modal heat capacity and lattice thermal expansion are also predicted.