Li-Ming Yang

Theoretical Investigations on the Chemical Bonding, Electronic Structure, And Optical Properties of the Metal-Organic Framework MOF-5

The chemical bonding, electronic structure, and optical properties of metal-organic framework-5 (MOF-5) were systematically investigated using ab initio density functional calculations. The unit cell volume and atomic positions were optimized with the Perdew-Burke-Ernzerhof (PBE) functional leading to a good agreement between the experimental and the theoretical equilibrium structural parameters. The calculated bulk modulus indicates that MOF-5 is a soft material. The estimated band gap from a density of state (DOS) calculation for MOF-5 is about 3.4 eV, indicating a nonmetallic character. As MOFs are considered as potential materials for photocatalysts, active components in hybrid solar cells, and electroluminescence cells, the optical properties of this material were investigated. The detailed analysis of chemical bonding in MOF-5 reveals the nature of the Zn-O, O-C, H-C, and C-C bonds, that is, Zn-O having mainly ionic interaction whereas O-C, H-C, and C-C exhibit mainly covalent interactions. The findings in this paper may contribute to a comprehensive understanding about this kind of material and shed insight into the synthesis and application of novel and stable MOFs.

Two-dimensional Cu2Si monolayer with planar hexacoordinate copper and silicon bonding.

Two-dimensional (2D) materials with planar hypercoordinate motifs are extremely rare due to the difficulty in stabilizing the planar hypercoordinate configurations in extended systems. Furthermore, such exotic motifs are often unstable. We predict a novel Cu2Si 2D monolayer featuring planar hexacoordinate copper and planar hexacoordinate silicon. This is a global minimum in 2D space which displays reduced dimensionality and rule-breaking chemical bonding. This system has been studied with density functional theory, including molecular dynamics simulations and electronic structure calculations. Bond order analysis and partitioning reveals 4c-2e σ bonds that stabilize the two-dimensional structure. We find that the system is quite stable during short annealing simulations up to 900 K, and predict that it is a nonmagnetic metal. This work opens up a new branch of hypercoordinate two-dimensional materials for study.

Four Decades of the Chemistry of Planar Hypercoordinate Compounds.

The idea of planar tetracoordinate carbon (ptC) was considered implausible for a hundred years after 1874. Examples of ptC were then predicted computationally and realized experimentally. Both electronic and mechanical (e.g., small rings and cages) effects stabilize these unusual bonding arrangements. Concepts based on the bonding motifs of planar methane and the planar methane dication can be extended to give planar hypercoordinate structures of other chemical elements. Numerous planar configurations of various central atoms (main-group and transition-metal elements) with coordination numbers up to ten are discussed herein. The evolution of such planar configurations from small molecules to clusters, to nanospecies and to bulk solids is delineated. Some experimentally fabricated planar materials have been shown to possess unusual electrical and magnetic properties. A fundamental understanding of planar hypercoordinate chemistry and its potential will help guide its future development.

Ab initio investigations on the crystal structure, formation enthalpy, electronic structure, chemical bonding, and optical properties of experimentally synthesized isoreticular metal–organic framework-10 and its analogues: M-IRMOF-10 (M = Zn, Cd, Be, Mg, Ca, Sr and Ba)

The equilibrium solid-state structure, electronic structure, formation enthalpy, chemical bonding, and optical properties of IRMOF-10 and its alkaline earth metal analogues M-IRMOF-10 (M = Cd, Be, Mg, Ca, Sr, Ba) have been investigated with density functional calculations. The unit cell volume and atomic positions were fully optimized with the GGA functional. This supplements the incomplete experimental structural parameters available for Zn-IRMOF-10. The calculated bulk moduli decrease monotonically from Zn to Cd, and from Be to Ba, and indicate that Zn-IRMOF-10 and its analogues are relatively soft materials. The estimated bandgap values are in the range 2.9 to 3.0 eV, indicating nonmetallic character. Importantly, the bandgaps within the M-IRMOF-10 series (containing a rather long 4,4′-biphenyldicarboxylate linker) are smaller than those within the M-IRMOF-1 series (shorter benzene dicarboxylate linker). The optical properties (dielectric function e(ω), refractive index n(ω), absorption coefficient α(ω), optical conductivity σ(ω), reflectivity R(ω), and electron energy-loss spectrum L(ω)) of the M-IRMOF-10 series were computed. The observation of very small reflectivities over a wide energy range suggests possible uses in hybrid solar cell applications. The main characteristics of the optical properties are similar for the whole series although differences are seen in the details. An analysis of chemical bonding in the M-IRMOF-10 series reveals as might be anticipated that M–O bonds are largely ionic whereas C–O, C–H and C–C exhibit mainly covalent interactions. The BOP values of M–O decrease through the series when going from Zn to Cd, and from Be to Ba, i.e. the ionicity increases and the covalency decreases for the M–O bonds.

Computational exploration of newly synthesized zirconium metal–organic frameworks UiO-66, -67, -68 and analogues

One of the major weaknesses of metal–organic framework (MOF) materials is their rather low thermal, hydrothermal, and chemical stabilities. Identification of stable and solvent resistant MOF materials will be key to their real world utilization. Recently, Lillerud and coworkers reported the synthesis of a new class of Zr MOF materials. These materials have very high surface area and exceptional thermal stability, are resistant to water and some solvents, acids, bases, and remain crystalline at high pressure. The newly synthesized Zr metal–organic frameworks (UiO-66, -67, and -68) as well as analogues substituting Ti and Hf for Zr, are explored using density functional theory calculations. The crystal structure, phase stability, bulk modulus, electronic structure, formation enthalpies, powder X-ray diffraction, chemical bonding, and optical properties are studied. We find bulk moduli of 36.6, 22.1, and 14.8 GPa for UiO-66, -67, and -68 respectively. As the linkers are extended, the bulk modulus drops. The highest occupied crystal orbital to lowest unoccupied crystal orbital gaps range from 2.9 to 4.1 eV. The compounds have similar electronic structure properties. Experimental powder X-ray diffraction patterns compare well with simulation. The large formation enthalpies (−40 to −90 kJ mol−1) for the series indicate high stability. This is consistent with the fact that these materials have very high decomposition temperatures. A detailed analysis of chemical bonding is carried out. Potential applications for these new materials include organic semiconducting devices such as field-effect transistors, solar cells, and organic light-emitting devices. We hope that the present study will stimulate research on UiO-based photocatalysis and will open new perspectives for the development of photocatalysts for water splitting and CO2 reduction. The large surface areas also make these materials good candidates for gas adsorption, storage, and separation.

A Fundamental Understanding of Catechol and Water Adsorption on a Hydrophilic Silica Surface: Exploring the Underwater Adhesion Mechanism of Mussels on an Atomic Scale

Mussels have a remarkable ability to bond to solid surfaces under water. From a microscopic perspective, the first step of this process is the adsorption of dopa molecules to the solid surface. In fact, it is the catechol part of the dopa molecule that is interacting with the surface. These molecules are able to make reversible bonds to a wide range of materials, even underwater. Previous experimental and theoretical efforts have produced only a limited understanding of the mechanism and quantitative details of the competitive adsorption of catechol and water on hydrophilic silica surfaces. In this work, we uncover the nature of this competitive absorption by atomic scale modeling of water and catechol adsorbed at the geminal (001) silica surface using density functional theory calculations. We find that catechol molecules displace preadsorbed water molecules and bond directly on the silica surface. Using molecular dynamics simulations, we observe this process in detail. We also calculate the interaction force as a function of distance, and observe a maximum of 0.5 nN of attraction. The catechol has a binding energy of 23 kcal/mol onto the silica surface with adsorbed water molecules.

Formation of an intermediate band in isoreticular metal–organic framework-993 (IRMOF-993) and metal-substituted analogues M-IRMOF-993

Intermediate band (IB) materials are attractive for multiple photon harvesting in solar cells thus increasing their efficiency beyond the Shockley–Quassier limit. However, it has so far been demonstrated that this can only be achieved in a few inorganic solids by appropriate doping. Here we demonstrate that it may be possible to achieve intermediate band materials with the isoreticular metal–organic framework IRMOF-993 and metal-substituted analogues. The equilibrium crystal structures, electronic structures, formation enthalpies, chemical bonding, and optical properties of M-IRMOF-993 (M = Zn, Cd, Be, Mg, Ca, Sr, Ba) were systematically investigated using density functional theory methods. The unit cell volume and atomic positions were optimized with the Perdew–Burke–Ernzerhof (PBE) functional; there was good agreement between the current theoretical equilibrium structural parameters and previously reported structural data for Zn-IRMOF-993. The calculated bulk moduli indicate that Zn-IRMOF-993 and its analogues are soft materials. The estimated fundamental bandgap values from the electronic structure studies for the whole series are ca. 3.5–3.6 eV, indicating a semiconducting character. The bandgap values estimated from the bottom of the IB to the top of VB are ca. 1.5–1.6 eV, and those from the top of IB to the bottom of CB are ca. 2.0 eV, suggesting that these materials may be suitable for enhancing the efficiency of solar cells. As MOFs are considered as potential materials for photocatalysts, active components in hybrid solar cells, electroluminescence cells, organic semiconducting devices such as field-effect transistors, and organic light-emitting devices, the optical properties and chemical bonding of M-IRMOF-993 were also systematically investigated.

Coexistence of Three Ferroic Orders in the Multiferroic Compound [(CH3)4N][Mn(N3)3] with Perovskite‐Like Structure

The perovskite azido compound [(CH3 )4 N][Mn(N3 )3 ], which undergoes a first-order phase change at Tt =310 K with an associated magnetic bistability, was revisited in the search for additional ferroic orders. The driving force for such structural transition is multifold and involves a peculiar cooperative rotation of the [MnN6 ] octahedral as well as order/disorder and off-center shifts of the [(CH3 )4 N](+) cations and bridging azide ligands, which also bend and change their coordination mode. According to DFT calculations the latter two give rise to the appearance of electric dipoles in the low-temperature (LT) polymorph, the polarization of which nevertheless cancels out due to their antiparallel alignment in the crystal. The conversion of this antiferroelectric phase to the paraelectric phase could be responsible for the experimental dielectric anomaly detected at 310 K. Additionally, the structural change involves a ferroelastic phase transition, whereby the LT polymorph exhibits an unusual and anisotropic thermal behavior. Hence, [(CH3 )4 N][Mn(N3 )3 ] is a singular material in which three ferroic orders coexist even above room temperature.

Covalent Triazine Frameworks via a Low-Temperature Polycondensation Approach

Abstract Covalent triazine frameworks (CTFs) are normally synthesized by ionothermal methods. The harsh synthetic conditions and associated limited structural diversity do not benefit for further development and practical large‐scale synthesis of CTFs. Herein we report a new strategy to construct CTFs (CTF‐HUSTs) via a polycondensation approach, which allows the synthesis of CTFs under mild conditions from a wide array of building blocks. Interestingly, these CTFs display a layered structure. The CTFs synthesized were also readily scaled up to gram quantities. The CTFs are potential candidates for separations, photocatalysis and for energy storage applications. In particular, CTF‐HUSTs are found to be promising photocatalysts for sacrificial photocatalytic hydrogen evolution with a maximum rate of 2647 μmol h−1 g−1 under visible light. We also applied a pyrolyzed form of CTF‐HUST‐4 as an anode material in a sodium‐ion battery achieving an excellent discharge capacity of 467 mAh g−1.

Solid-state structure and calculated electronic structure, formation energy, chemical bonding, and optical properties of Zn4O(FMA)3 and its heavier congener Cd4O(FMA)3.

The equilibrium solid-state structure of the experimentally synthesized but incompletely characterized Zn4O(FMA)3 is revised with the help of density functional theory computational methods. The electronic structure, formation energy, chemical bonding, and optical properties of Zn4O(FMA)3 and its heavier congener Cd4O(FMA)3 have been systematically investigated. The calculated bulk moduli for Zn4O(FMA)3 and Cd4O(FMA)3 are similarly small (and slightly smaller than the previously reported values for MOF-5), indicative of relatively soft materials. Their estimated band-gap values are ca. 3.2 eV (somewhat lower than that of MOF-5, 3.4-3.5 eV), indicating semiconducting character. The optical properties including dielectric function ε(ω), refractive index n(ω), absorption coefficient α(ω), optical conductivity σ(ω), reflectivity R(ω), and electron energy-loss spectrum L(ω) of M4O(FMA)3 (M = Zn, Cd) were systematically studied. Analysis of chemical bonding reveals that the M-O bonds are largely ionic, with an increase in ionicity from Zn to Cd. The total energy calculations establish that compounds M4O(FMA)3 have large negative formation energies, ca. -80 and -70 kJ·mol(-1) for Zn and Cd, respectively. Whereas Zn4O(FMA)3 has already been synthesized, the results suggest that the heavier congener Cd4O(FMA)3 might be experimentally accessible.