Fabrication of Carboxylate-Functionalized 2D MOF Nanosheet with Caged Cavity for Efficient and Selective Extraction of Uranium from Aqueous Solution

The efficient removal of radioactive uranium from aqueous solution is of great significance for the safe and sustainable development of nuclear power. An ultrathin 2D metal-organic framework (MOF) nanosheet with cavity structures was elaborately fabricated based on a calix[4]arene ligand. Incorporating the permanent cavity structures on MOF nanosheet can fully utilize its structural characteristics of largely exposed surface area and accessible adsorption sites in pollutant removal, achieving ultrafast adsorption kinetics, and the functionalized cavity structure would endow the MOF nanosheets with the ability to achieve preconcentration and extraction of uranium from aqueous solution, affording ultrahigh removal efficiency even in ultra-low concentrations. Thus, more than 97% uranium can be removed from the concentration range of 50-500 µg L-1 within 5 min. Moreover, the 2D nano-material exhibits ultra-high anti-interference ability, which can efficiently remove uranium from groundwater and seawater. The adsorption mechanism was investigated by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) analysis, and density functional theory (DFT) calculations, which revealed that the cavity structure plays an important role in uranium capture. This study not only realizes highly efficient uranium removal from aqueous solution but also opens the door to achieving ultrathin MOF nanosheets with cavity structures, which will greatly expand the applications of MOF nanosheets.

Fabrication of Protonated Two-Dimensional Metal-Organic Framework Nanosheets for Highly Efficient Iodine Capture from Water

Radioactive iodine (¹²⁹I and ¹³¹I), produced or released from nuclear-related activities, posed severe effects on both human health and environment. The efficient removal of radioiodine from aqueous medium and vapor phase is of paramount importance for the sustainable development of nuclear energy. Herein, a metal-organic framework (MOF) nanosheet with a positive charge was constructed for the capture of iodine for the first time. The as-synthesized ultrathin nanosheets, with a thickness of 4.4 ± 0.1 nm, showed a record-high iodine adsorption capacity (3704.08 mg g^-1) from aqueous solution, which is even higher than that from the vapor phase (3510.05 mg g^-1). It can be ascribed to the fully interactions between the extensive accessible active sites on the largely exposed surface of 2D MOF nanosheets and the target pollutants, which also gave rise to fast adsorption kinetics with relative high removal efficiencies in the low concentrations, even in seawater. Moreover, a facile recyclability with fast desorption kinetics can also be achieved for the MOF nanosheets. The excellent iodine removal performance in aqueous solution demonstrated that the electrostatic attraction between MOF nanosheets with a positive charge and the negatively charged triiodide (I₃^-, the dominant form of iodine in aqueous solution) is the driving force in adsorption, which endows the adsorbents with the characteristics of fast adsorption and desorption kinetics. The adsorption mechanism was systematically verified by the studies of ζ potential, Fourier transform infrared, X-ray photoelectron spectroscopy, and Raman spectra.

Layered Uranyl Phosphonates Encapsulating Co(II)/Mn(II)/Zn(II) Ions: Exfoliation into Nanosheets and Its Impact on Magnetic and Luminescent Properties

Layered heterometallic 5f-3d uranyl phosphonates can exhibit unique luminescent and/or magnetic properties, but the fabrication and properties of their 2D counterparts have not been investigated. Herein we report three heterobimetallic uranyl phosphonates, namely, [(UO₂ )₃ M(2-pmbH)₄ (H₂ O)₄ ] ⋅ 2H₂ O [MU, M=Co(II), CoU; Mn(II), MnU; Zn(II), ZnU; 2-pmbH₃ =2-(phosphonomethyl)benzoic acid]. They are isostructural and display two-dimensional layered structures where the M(II) centers are encapsulated inside the windows generated by the diamagnetic uranyl phosphonate layer. Each M(II) has an octahedral geometry filled with four water molecules in the equatorial positions and two phosphonate oxygen atoms in the axial positions. The uranium atoms adopt UO₇ pentagonal bipyramidal and UO₆ square bipyramidal geometries. The lattice and coordination water molecules can be released by thermal treatment and reabsorbed in a reversible manner, accompanied with changes of magnetic dynamics. Interestingly, the bulk samples of MU can be exfoliated in acetone via freezing and thawing processes forming nanosheets with single-layer or two-layer thickness (MU-ns). Magnetic studies revealed that the CoU and MnU systems exhibited field-induced slow magnetization relaxation at low temperature. Compared with crystalline CoU, the magnetic relaxation of the CoU-ns aggregates is significantly accelerated. Moreover, photoluminescence measured at 77 K showed slight red-shift of the five characteristic uranyl emission bands for ZnU-ns in comparison with those of the crystalline ZnU. This work gives the first examples of 2D materials based on 5f-3d heterometallic uranyl phosphonates and illustrates the impact of dimension reduction on their magnetic/optical properties.

Alkali Metal Di-tert-butyl Phosphates: Single-Source Precursors for Homo- and Heterometallic Inorganic Phosphate Materials

The reaction of alkali metal acetates, M(OAc)·nH₂O (M = Li, Na, K), with thermally and hydrolytically unstable di-tert-butylphosphate ((^tBuO)₂PO₂H, dtbp-H) in a 1:1 molar ratio in MeOH at room temperature leads to clean formation of group 1 metal phosphates [Li(μ-dtbp)]_n (1), [Na(μ-dtbp)]_n (2), and [K₄(μ-dtbp)₄(μ-H₂O)₃]_n (3). All three compounds are essentially M/L 1:1 complexes. Owing to the presence of larger potassium ions, additional coordinated water molecules are found in 3, which has been further employed as a precursor for the synthesis of a mixed-metal phosphate polymer [CaK(μ-H₂O)₃(μ-dtbp)₃]_n (4) by reacting 3 with Ca(OAc)₂. Compounds 1-4 have been characterized by various analytical and spectroscopic techniques. Molecular structures of 1-4 have been established in the solid state by single-crystal X-ray diffraction studies, which reveal them to be one-dimensional polymers, where the adjacent metal centers are connected through -O-P-O- bridges formed by the dtbp ligand. These complexes are rare examples of analytically pure alkali metal alkyl phosphates bearing no additional N-donor ligands (other than dtbp ligands, only water molecules are coordinated to the metal centers). Therefore, these compounds can be employed as single-source precursors (SSPs) for nano-sized ceramic phosphates. The thermogravimetric analysis of 1-4 reveals the loss of thermally labile tert-butyl substituents of the organophosphate ligands to form organic-free phosphate materials in the temperature range 300-500 °C. Solvothermal decomposition of 1-3 in boiling toluene leads to the formation of corresponding dihydrogen phosphates M(H₂PO₄) (M = Li, Na, and K). The thermal decomposition of heterometallic 4 in the temperature range 400-800 °C leads to the formation of phase-pure mixed-metal calcium potassium metaphosphate CaK(PO₃)₃.

Emerging Two-Dimensional Covalent and Coordination Polymers for Stable Lithium Metal Batteries: From Liquid to Solid

Lithium metal anodes (LMAs) have attracted much attention in recent years because of their high theoretical capacity (3860 mAh g^-1) and low electrochemical potential (-3.040 V vs standard hydrogen electrode). Lithium metal can be coupled with various cathodes to construct high-energy-density lithium metal batteries (LMBs) which hold great promise for next-generation batteries. However, the unstable solid electrolyte interphases (SEIs) and the uncontrollable lithium dendrite growth severely hinder the commercial development of LMAs. The emerging 2D polymers (2DPs), which possess high mechanical flexibility, high specific surface area, abundant surface chemistry, and rich chemical modification characteristics, have shown great advantages in addressing the inherent issues of LMAs. Herein, the current progress of 2DPs for stable and dendrite-free LMAs in liquid- and solid-based batteries is comprehensively reviewed. Some perspectives for the application of 2DPs in LMBs are also discussed. It is believed that the emerging 2DPs will provide insights into developing high-energy-density LMBs and beyond.

Uranyl phosphonates: crystalline materials and nanosheets for temperature sensing

Ultrathin nanosheets of luminescent metal-organic frameworks or coordination polymers have been widely used for sensing ions, solvents and biomolecules but, as far as we are aware, not yet used for temperature sensing. Herein we report two luminescent uranyl phosphonates based on 2-(phosphonomethyl)benzoic acid (2-pmbH₃), namely (UO₂)(2-pmbH₂)₂ (1) and (H₃O)[(UO₂)₂(2-pmb)(2-pmbH)] (2). The former has a supramolecular layer structure, composed of chains of corner-sharing {UO₆} octahedra and {PO₃C} tetrahedra which are connected by hydrogen bonds between phosphonate and carboxylic groups. Compound 2 possesses a unique 2D anionic framework structure, where the inorganic uranyl phosphonate chains made up of {UO₇} and {PO₃C} polyhedra are cross-linked by 2-pmb^3- ligands. The carboxylic groups of 2-pmbH^2- ligands are pendant on the two sides of the layers and form hydrogen bonds between the layers. Both compounds can be exfoliated in acetone via a top-down freeze-thaw method, resulting in nanosheets of two-layer thickness. Interestingly, the photoluminescence (PL) of 1 and 2 is highly temperature sensitive. Variable temperature PL studies revealed that compounds 1 and 2 can be used as thermometers in the temperature ranges 120-300 K and 100-280 K, respectively. By doping the nanosheets into polymer matrix, 1-ns@PMMA and 2-ns@PMMA were prepared. The PL intensity of 1-ns@PMMA is insensitive to temperature, unlike that of the bulk sample. While 2-ns@PMMA exhibits similar temperature-dependent luminescence behaviour to its bulk counterpart, thereby enabling its potential application as a thermometer in the temperature range 100-280 K.

Cobalt(II)-dianthracene Frameworks: Assembly, Exfoliation and Properties

Metal-organic frameworks containing responsive organic linkers are attractive for potential applications in sensors and molecular devices. Herein we report three cobalt(II) phosphonates incorporating responsive dianthracene linkers, namely, Co₂ (amp₂ H₂ )₂ (H₂ O)₄  ⋅ 6H₂ O (MDAF-1), Co₂ (amp₂ )(H₂ O)₄  ⋅ 2H₂ O (MDAF-2) and Co(amp₂ H₂ ) ⋅ 2H₂ O ⋅ 0.5DMF (MDAF-3), where amp₂ H₄ is pre-photodimerized 9-anthrylmethylphosphonic acid. MDAF-1 shows a layer structure in which dinuclear Co₂ (PO₃ H)₂ units are inter-connected by dianthracene ligands. In MDAF-2 and MDAF-3, inorganic chains of corner-sharing {CoO₄ } (or {CoO₆ }) and {PO₃ C} are cross-linked by dianthracene ligands into 3D frameworks. All compounds underwent thermo-induced phase transitions, first the de-solvation and then the de-dimerization of dianthracene (as well as the release of the remaining solvent molecules for MDAF-2 and -3), associated with magnetic changes. MDAF-1 can be exfoliated into single-layer nanosheets in water which show light-triggered luminescent changes.

A Multifunctional Dysprosium-Carboxylato 2D Metall-Organic Framework

We report the microwave assisted synthesis of a bidimensional (2D) MOF of formula [Dy(MeCOO)(PhCOO)₂ ]_n (1) and its magnetically diluted analogue [La_0.9 Dy_0.1 (MeCOO)(PhCOO)₂ ] (1 d). 1 is a 2D material with single-ion-magnet (SIM) behaviour and 1 d is a multifunctional, magnetic and luminescent 2D material. 1 can be exfoliated into stable nanosheets by sonication.

Lamellar Nanosheets of Water-Insoluble Amphiphiles via Aqueous Solution and Air/Liquid Interface Self-Assembly

Two-dimensional (2D) lamellar nanostructures have attracted much interest due to their unique structure and properties. Various fabrication methods have been developed in recent years, including solution self-assembly, exfoliation, and Langmuir monolayer and Langmuir-Blodgett (LB) deposition. In this work, two kinds of facile methods were applied to fabricate lamellar structures of amphiphilic molecules, such as 10,12-pentacosadiynoic acid (PCDA). In method I, the amphiphilic molecules were introduced into aqueous solutions with dimethylformamide (DMF), a solvent miscible with water, through a mass transfer process across a planar liquid/liquid interface; in method II, the DMF solution of the amphiphilic molecules was added directly onto the aqueous solution surface. With the spread and diffusion of DMF, nanosheets with lamellar structures formed in the aqueous solution and at the air/liquid interface, respectively. It is very interesting that the nanosheets obtained through these two methods consist of an even number and odd number of PCDA monolayers, respectively, reflecting different fabrication mechanisms. Method I provides an approach to gently mix organic solutions with aqueous solutions, while method II can be regarded as an extension of the Langmuir monolayer technique, which combines the interfacial assembly with that in solution. These methods have been extended to a series of amphiphilic molecules, and ordered layered structures have been obtained successfully.

Tuning the electronic energy level of covalent organic frameworks for crafting high-rate Na-ion battery anode

Crystalline Covalent Organic Frameworks (COFs) possess ordered accessible nano-channels. When these channels are decorated with redox-active functional groups, they can serve as the anode in metal ion batteries (LIB and SIB). Though sodium's superior relative abundance makes it a better choice over lithium, the energetically unfavourable intercalation of the larger sodium ion makes it incompatible with the commercial graphite anodes used in Li-ion batteries. Also, their sluggish movement inside the electrodes restricts the fast sodiation of SIB. Creating an electronic driving force at the electrodes via chemical manipulation can be a versatile approach to overcome this issue. Herein, we present anodes for SIB drawn on three isostructural COFs with nearly the same Highest Occupied Molecular Orbitals (HOMO) levels but with varying Lowest Unoccupied Molecular Orbitals (LUMO) energy levels. This variation in the LUMO levels has been deliberately obtained by the inclusion of electron-deficient centers (phenyl vs. tetrazine vs. bispyridine-tetrazine) substituents into the modules that make up the COF. With the reduction in the cell-potential, the electrons accumulate in the anti-bonding LUMO. Now, these electron-dosed LUMO levels become efficient anodes for attracting the otherwise sluggish sodium ions from the electrolyte. Also, the intrinsic porosity of the COF favors the lodging and diffusion of the Na⁺ ions. Cells made with these COFs achieve a high specific capacity (energy density) and rate performance (rapid charging-discharging), something that is not as easy for Na⁺ compared to the much smaller sized Li⁺. The bispyridine-tetrazine COF with the lowest LUMO energy shows a specific capacity of 340 mA h g^-1 at 1 A g^-1 and 128 mA h g^-1 at a high current density of 15 A g^-1. Only a 24% drop appears on increasing the current density from 0.1 to 1 A g^-1, which is the lowest among all the top-performing COF derived Na-ion battery anodes.

Spin-crossover in iron(II)-phenylene ethynylene-2,6-di(pyrazol-1-yl) pyridine hybrids: toward switchable molecular wire-like architectures

Luminescent oligo(p-phenylene ethynylene) (OPE) and spin-crossover (SCO) active Fe(II)-2,6-di(pyrazol-1-yl) pyridine (BPP) systems are prominent examples proposed to develop functional materials such as molecular wires/memories. A marriage between OPE and Fe(II)-BPP systems is a strategy to obtain supramolecular luminescent ligands capable of metal coordination useful to produce novel spin-switchable hybrids with synergistic coupling between spin-state of Fe(II) and a physical property associated with the OPE skeleton, for example, electronic conductivity or luminescence. To begin in this direction, two novel ditopic ligands, namely L¹ and L², featuring OPE-type backbone end-capped with metal coordinating BPP were designed and synthetized. The ligand L² tailored with 2-ethylhexyloxy chains at the 2 and 5 positions of the OPE skeleton shows modulated optical properties and improved solubility in common organic solvents relative to the parent ligand L¹. Solution phase complexation of L¹ and L² with Fe(BF₄)₂·6H₂O resulted in the formation of insoluble materials of the composition [Fe(L¹)] _n (BF₄)_2n and [Fe(L²)] _n (BF₄)_2n as inferred from elemental analyses. Complex [Fe(L¹)] _n (BF₄)_2n underwent thermal SCO centred at T _1/2  =  275 K as well as photoinduced low-spin to high-spin transition with the existence of the metastable high-spin state up to 52 K. On the other hand, complex [Fe(L²)] _n (BF₄)_2n , tethered with 2-ethylhexyloxy groups, showed gradual and half-complete SCO with 50% of the Fe(II)-centres permanently blocked in the high-spin state due to intermolecular steric interactions. The small angle x-ray scattering (SAXS) pattern of the as-prepared solid complex [Fe(L¹)] _n (BF₄)_2n revealed the presence of nm-sized crystallites implying a possible methodology towards the template-free synthesis of functional-SCO nanostructures.