Two-dimensional Framework Research Articles

Ionic Covalent Organic Framework-Based Membranes for Selective and Highly Permeable Molecular Sieving.

Two-dimensional covalent organic frameworks (COFs) with uniform pores and large surface areas are ideal candidates for constructing advanced molecular sieving membranes. However, a fabrication strategy to synthesize a free-standing COF membrane with a high permselectivity has not been fully explored yet. Herein, we prepared a free-standing TpPa-SO3H COF membrane with vertically aligned one-dimensional nanochannels. The introduction of the sulfonic acid groups on the COF membrane provides abundant negative charge sites in its pore wall, which achieve a high water flux and an excellent sieving performance toward water-soluble drugs and dyes with different charges and sizes. Furthermore, the COF membrane exhibited long-term stability, fouling resistance, and recyclability in rejection performance. We envisage that this work provides new insights into the effect of ionic ligands on the design of a broad range of COF membranes for advanced separation applications.

Transversely heterogeneous nonlocal Timoshenko beam theory: A reduced-order modeling via distributed-order fractional operators

Nonlocal effects are widely identified in micro/nano systems and have been studied extensively in recent decades. Nevertheless, existing studies mostly focus on the modeling of homogeneous nonlocal systems and as a result, cannot model micro/nanostructures with heterogeneously distributed nonlocal effects. This study introduces a reduced-order nonlocal Timoshenko beam model that facilitates precise modeling of micro-elastic beams with layer-wise heterogeneous nonlocal effects. First, a fully-resolved two-dimensional (2D) plane-strain fractional-order nonlocal elastic framework is developed to provide reference solutions for 2D nonlocal beam problems. Building upon this general nonlocal elasticity theory, an equivalent one-dimensional (1D) nonlocal Timoshenko model is then developed. The layer-wise heterogeneous nonlocal information is captured by using distributed-order (DO) operators, and the impacts arising from the nonlocal heterogeneity are further characterized by introducing two auxiliary parameters. Both 1D and 2D approaches are applied to simulate the mechanical responses of nonlocal beams. Direct comparisons of numerical simulations produced by either the 1D or the fully-resolved 2D model confirm that the DO Timoshenko beam formulation (together with the two auxiliary parameters) can capture not only the overall beam deflection but also the additional shear effect induced by the heterogeneous nonlocality. Computational cost assessment also indicates the proposed approach’s superior performance. The proposed DO Timoshenko model enables model-order reduction without compromising the heterogeneous nonlocal description of the material hence leading to an efficient and accurate reduced-order nonlocal modeling approach that addresses the limitations of prior nonlocal microstructural theories and extends applicability to a broader range of heterogeneous nano/microstructures.

A Two-dimensional Metal-Organic Framework as Promising Cathode for Advanced Lithium Storage.

Anthraquinone electrode materials are promising candidates for lithium-ion batteries (LIBs) due to the abundance of anthraquinone and the high theoretical capacity, and good reversibility of the anthraquinone electrodes. However, the active anthraquinone materials are soluble in organic electrolytes, resulting in a sharp decay of capacity during the charge and discharge processes. Herein, we report on a two-dimensional calcium anthraquinone 2,3-dicarboxy metal-organic framework (2D CaAQDC MOF) fabricated using a simple hydrothermal method. The 2D CaAQDC MOF not only effectively inhibits the dissolution of active electrode substances into the electrolyte, but also promotes the diffusion of lithium ion into the pores of the MOF. When used as a cathode for the LIBs, the resulting CaAQDC electrode delivers a high specific capacity of ~100 mAh g-1 at a current density of 50 mA g-1 after 200 cycles, demonstrating its good cycle stability. Even at a high current density of 200 mA g-1 , the CaAQDC electrode exhibits a specific capacity of ~60 mAh g-1 . The fabricated 2D coordination polymers effectively restrains the dissolution of anthraquinone into the organic electrolyte and enhances the structural stability, which greatly improves the electrochemical performance of anthraquinone. These research results offer a rational molecular design strategy to address the dissolution of this and other active organic electrode materials.

Enhancing understanding of moisture diffusion in wood: numerical approach and diffusion parameter optimization

Wood, an environmentally friendly construction material, is characterized by a critical attribute that significantly influences its mechanical properties and durability: its moisture absorption behavior. This study expands upon an existing one-dimensional model to develop an innovative two-dimensional numerical framework for simulating moisture propagation within wood. Utilizing the finite difference method, this approach offers a more detailed analysis of moisture behavior in wood structures. The research adopts an inverse modeling technique, integrating a simplex optimization algorithm programmed in VBA software. This algorithm is employed to deduce diffusion parameters from the evolution of moisture content over time. Focusing on the analysis of moisture diffusion parameters, the study examines both longitudinal and transverse directions in two temperate species (Beech and Fir) and two tropical species (Moabi and Ozigo). The findings provide insightful data on the hygroscopic behavior of these woods, revealing significant distinctions between temperate and tropical species. This research offers valuable information for the application of these wood species in construction and other fields, enhancing the understanding of their moisture-related properties.

Catalysis with Two-Dimensional Metal-Organic Frameworks: Synthesis, Characterization, and Modulation.

The demand for the exploration of highly active and durable electro/photocatalysts for renewable energy conversion has experienced a significant surge in recent years. Metal-organic frameworks (MOFs), by virtue of their high porosity, large surface area, and modifiable metal centers and ligands, have gained tremendous attention and demonstrated promising prospects in electro/photocatalytic energy conversion. However, the small pore sizes and limited active sites of 3D bulk MOFs hinder their wide applications. Developing 2D MOFs with tailored thickness and large aspect ratio has emerged as an effective approach to meet these challenges, offering a high density of exposed active sites, better mechanical stability, better assembly flexibility, and shorter charge and photoexcited state transfer distances compared to 3D bulk MOFs. In this review, synthesis methods for the most up-to-date 2D MOFs are first overviewed, highlighting their respective advantages and disadvantages. Subsequently, a systematic analysis is conducted on the identification and electronic structure modulation of catalytic active sites in 2D MOFs and their applications in renewable energy conversion, including electrocatalysis and photocatalysis (electro/photocatalysis). Lastly, the current challenges and future development of 2D MOFs toward highly efficient and practical electro/photocatalysis are proposed.

Water-Soluble, Self-Assembled Molecular Cage and Two-Dimensional Framework through Flexibility Adjustment of the Precursors

Water-Soluble, Self-Assembled Molecular Cage and Two-Dimensional Framework through Flexibility Adjustment of the Precursors

Ultra-selective uranium separation by in-situ formation of π-f conjugated 2D uranium-organic framework

With the rapid development of nuclear energy, problems with uranium supply chain and nuclear waste accumulation have motivated researchers to improve uranium separation methods. Here we show a paradigm for such goal based on the in-situ formation of π-f conjugated two-dimensional uranium-organic framework. After screening five π-conjugated organic ligands, we find that 1,3,5-triformylphloroglucinol would be the best one to construct uranium-organic framework, thus resulting in 100% uranium removal from both high and low concentration with the residual concentration far below the WHO drinking water standard (15 ppb), and 97% uranium capture from natural seawater (3.3 ppb) with a record uptake efficiency of 0.64 mg·g−1·d−1. We also find that 1,3,5-triformylphloroglucinol can overcome the ion-interference issue such as the presence of massive interference ions or a 21-ions mixed solution. Our finds confirm the superiority of our separation approach over established ones, and will provide a fundamental molecule design for separation upon metal-organic framework chemistry.

Controllable Switch of Thermodynamic and Kinetic Growing Paths in Two-Dimensional Covalent Organic Frameworks

Controllable Switch of Thermodynamic and Kinetic Growing Paths in Two-Dimensional Covalent Organic Frameworks

Donor-Acceptor Interactions Enhanced Colorimetric Sensors for Both Acid and Base Vapor Based on Two-Dimensional Covalent Organic Frameworks.

Due to the high surface area and uniform porosity of covalent organic frameworks (COFs), they exhibit superior properties in capturing and detecting even trace amounts of gases in the air. However, the COFs materials that possess dual detected functionality are still less reported. Here, an imine-based COF containing thiophene as a donor and triazine as an acceptor to form spatial-distribution-defined D-A structures was prepared. D-A system between thiophene and triazine facilitates the charge transfer process during the protonation process of the imine and the triazine units. The obtained COF exhibits simultaneous sensing ability toward both acidic and alkaline vapors with obvious colorimetric sensing functionality.

Toward Nanoscale Organic Tunnel Field-Effect Transistors with Small Subthreshold Swing and High On-State Current: A Computational Design Based on Two-Dimensional Covalent-Organic Frameworks

Toward Nanoscale Organic Tunnel Field-Effect Transistors with Small Subthreshold Swing and High On-State Current: A Computational Design Based on Two-Dimensional Covalent-Organic Frameworks

Efficient osmotic energy recovery from organic wastewater using a two-dimensional porphyrin-based metal-organic frameworks membrane

Harvesting osmotic energy plays a crucial role in addressing the global depletion crisis of fossil energy and achieving a structural transformation in energy. However, current research predominantly focuses on osmotic energy stored in natural seawater, with relatively little attention given to other saline sources such as industrial wastewater. In this study, a two-dimensional Cu-TCPP metal-organic framework (MOFs) nanosheet was prepared, and a nanofluidic membrane with high osmotic energy recovery efficiency from industrial organic saline wastewater was constructed. In a LiCl methanol solution system with a 100-fold salinity gradient, the Cu-TCPP membrane achieved a stable power density of 4.87 W/m2. Simulation analysis suggests that the membrane's efficient ion-selective transport in the organic system can be primarily attributed to abundant diffusion pathways, including intrinsic pores in the nanosheet and two-dimensional nanochannels between adjacent nanosheets. Furthermore, the physicochemical properties of the nanochannel membrane and the type of solvent and ions significantly impact osmotic energy recovery efficiency in organic solvents. This work provides a novel approach to the treatment and resource utilization of industrial organic wastewater in the chemical and pharmaceutical industries.

Covalent Organic Framework Ionomer Steering the CO2 Electroreduction Pathway on Cu at Industrial-Grade Current Density.

CO2 electroreduction holds great promise for addressing global energy and sustainability challenges. Copper (Cu) shows great potential for effective conversion of CO2 toward specific value-added and/or high-energy-density products. However, its limitation lies in relatively low product selectivity. Herein, we present that the CO2 reduction reaction (CO2RR) pathway on commercially available Cu can be rationally steered by modulating the microenvironment in the vicinity of the Cu surface with two-dimensional sulfonated covalent organic framework nanosheet (COF-NS)-based ionomers. Specifically, the selectivity toward methane (CH4) can be enhanced to more than 60% with the total current density up to 500 mA cm-2 in flow cells in both acidic (pH = 2) and alkaline (pH = 14) electrolytes. The COF-NS, characterized by abundant apertures, can promote the accumulation of CO2 and K+ near the catalyst surface, alter the adsorption energy and surface coverage of *CO, facilitate the dissociation of H2O, and finally modulate the reaction pathway for the CO2RR. Our approach demonstrates the rational modulation of reaction interfaces for the CO2RR utilizing porous open framework ionomers, showcasing their potential practical applications.

Reversible modulation of interlayer stacking in 2D copper-organic frameworks for tailoring porosity and photocatalytic activity

The properties of two-dimensional covalent organic frameworks (2D COFs), including porosity, catalytic activity as well as electronic and optical properties, are greatly affected by their interlayer stacking structures. However, the precise control of their interlayer stacking mode, especially in a reversible fashion, is a long-standing and challenging pursuit. Herein, we prepare three 2D copper-organic frameworks, namely JNM-n (n = 7, 8, and 9). Interestingly, the reversible interlayer sliding between eclipsed AA stacking (i.e., JNM-7-AA and JNM-8-AA) and staggered ABC stacking (i.e., JNM-7-ABC and JNM-8-ABC) can be achieved through environmental stimulation, which endows reversible encapsulation and release of lipase. Importantly, JNM-7-AA and JNM-8-AA exhibit a broader light absorption range, higher charge-separation efficiency, and higher photocatalytic activity for sensitizing O2 to 1O2 and O2•− than their ABC stacking isostructures. Consequently, JNM-8-AA deliver significantly enhanced photocatalytic activities for oxidative cross-coupling reactions compared to JNM-8-ABC and other reported homogeneous and heterogeneous catalysts.

Unravelling abnormal in-plane stretchability of two-dimensional metal-organic frameworks by machine learning potential molecular dynamics.

Two-dimensional (2D) metal-organic frameworks (MOFs) hold immense potential for various applications due to their distinctive intrinsic properties compared to their 3D analogues. Herein, we designed a highly stable NiF2(pyrazine)2 2D MOF in silico with a two-dimensional periodic wine-rack architecture. Extensive first-principles calculations and molecular dynamics (MD) simulations based on a newly developed machine learning potential (MLP) revealed that this 2D MOF exhibits huge in-plane Poisson's ratio anisotropy. This results in anomalous negative in-plane stretchability, as evidenced by an uncommon decrease in its in-plane area upon the application of uniaxial tensile strain, which makes this 2D MOF particularly attractive for flexible wearable electronics and ultra-thin sensor applications. We further demonstrated the unique capability of MLP to accurately predict the finite-temperature properties of MOFs on a large scale, exemplified by MLP-MD simulations with a dimension of 28.2 × 28.2 nm2, relevant to the length scale experimentally attainable for the fabrication of MOF films.

Construction of a novel pyrene-based two-dimensional supramolecular organic framework and the selective regulation of reactive oxygen species for photocatalysis

We report herein a novel pyrene-based two-dimensional supramolecular organic framework (SOF) in water via host–guest interactions, which exhibits regulation of ROS and high photocatalytic efficiency.

Two-dimensional metal organic frameworks in cancer treatment.

With large specific surface area, controllable pore size, increased active sites, and structural stability, two-dimensional metal organic frameworks (2D MOFs) have emerged as promising nanomedicines in cancer therapy. These distinctive features make 2D MOFs particularly advantageous in cancer treatment and the corresponding application has gained considerable popularity, signifying significant application potential. Herein, recent advances in various applications including drug delivery and chemotherapy, photodynamic therapy, sonodynamic therapy, chemodynamic therapy, catalytic therapy, and combined therapy were summarized, with emphasis on the latest progress of new materials and mechanisms for these processes. Moreover, the current challenges, potential solutions, and possible future directions are discussed as well.

Two-dimensional multifunctional metal-organic frameworks with large in-plane negative Poisson ratios and photocatalytic water splitting properties.

Auxetic materials with multifunctional properties are highly sought after for application in modern nano-devices. However, the majority of reported inorganic auxetic materials exhibit low negative Poisson's ratios (NPR), poor flexibility, and limited functionality. In this study, we employ density-functional-theory (DFT) first-principles simulations to design a series of two-dimensional (2D) metal-organic frameworks (MOFs) M2C4X4 (M = Cu, Ag, Au; X = O, S, NCN) that display intriguing auxetic behavior, superior flexibility and appropriate photocatalytic water-splitting properties. These M2C4X4 MOFs are assembled from carbon tetragon motifs and exist in both cis- and trans-isomer forms, with the NPR ranging from -0.17 to -0.90. Notably, trans-Cu2C4(NCN)4 exhibits a high NPR of -0.90, while cis-Cu2C4(NCN)4 achieves an NPR of -0.67. Both isomers demonstrate excellent flexibility, characterized by ultra-low Young's modulus and high fracture strengths. Furthermore, their direct band gaps, strong light-harvesting capabilities, and long excited-state lifetimes make them promising candidates for the photocatalytic oxygen evolution reaction in water. These results provide a viable strategy for the design and synthesis of novel optoelectronic multifunctional materials.

In-plane Polarity Boosts Photocatalytic Overall Water Splitting in Two-Dimensional Covalent Organic Frameworks with Polarized Linkages

Benefiting from the modular nature and tunable optoelectronic properties, two-dimensional (2D) covalent organic frameworks (COFs) emerge as promising candidates for photocatalytic overall water splitting (OWS). However, the rapid recombination of...

Immobilization of europium and terbium ions with tunable ratios on a dispersible two-dimensional metal-organic framework for ratiometric photoluminescence detection of D2O.

A two-dimensional zirconium-based metal-organic framework (2D Zr-MOF), ZrBTB (BTB = 1,3,5-tri(4-carboxyphenyl)benzene), is used as a platform to simultaneously immobilize terbium ions and europium ions with tunable ratios on its hexa-zirconium nodes by a post-synthetic modification. The crystallinity, morphology, porosity and photoluminescence (PL) properties of the obtained 2D Zr-MOFs with various europium-to-terbium ratios are investigated. With the energy transfer from the excited BTB linker to the installed terbium ions and the energy transfer from terbium ions to europium ions, a low loading of immobilized europium ions and a high loading of surrounding terbium ions in the 2D Zr-MOF result in the optimal PL emission intensities of europium; this phenomenon is not observable for the physical mixture of both terbium-installed ZrBTB and europium-installed ZrBTB. The role of installed terbium ions as efficient mediators for the energy transfer from the excited BTB linker to the installed europium ion is confirmed by quantifying PL quantum yields. As a demonstration, these materials with modulable PL characteristics are applied for the ratiometric detection of D2O in water, with the use of the stable emission from the BTB linker as the reference. With the strong emission of immobilized europium ions and the good dispersity in aqueous solutions, the optimal bimetal-installed ZrBTB, Eu-Tb-ZrBTB(1 : 10), can achieve the sensing performance outperforming those of the terbium-installed ZrBTB, europium-installed ZrBTB and the physical mixture of both.

Growth of two-dimensional covalent organic frameworks on substrates: insight from microsecond atomistic simulations.

While growing two-dimensional covalent organic frameworks (2D COFs) on substrates holds promise for producing functional monolayers, the presence of many defects in the resulting crystals often hinders their practical applications. Achieving structural order while suppressing defect formation necessitates a detailed atomic-level understanding. The key lies in understanding the polymerization process with high nano-scale accuracy, which presents significant challenges. Here, we perform microsecond atomistic molecular dynamics simulations to describe the deposition and polymerization of cyclohexa-m-phenylene on metal substrates, closely mimicking experimental conditions. Our improved approach highlights that 2D polymerization occurs through monomer addition and island coalescence, with a pre-bonding stage allowing monomers/oligomers to dynamically adjust their configurations to the expanding island structures. Our results elucidate the mechanisms underlying the formation of vacancy and dislocation defects during 2D polymerization as well as their healing processes. Overall, our findings underscore the significant roles that high surface mobility, effective monomer-substrate anchoring, high framework rigidity, moderate monomer coordination, and low bonding rate play in forming large, extended 2D crystals while suppressing vacancy and dislocation defects. We demonstrate how these factors can be tuned through substrate selection, deposition rate modulation, and temperature control, thereby offering valuable insight for strategically optimizing on-surface 2D polymerizations.