Air‐Free Synthesis of a Ferrous Metal‐Organic Framework Featuring HKUST‐1 Structure and its Mössbauer Spectrum

Single crystals of the Fe^II metal‐organic framework (MOF) with 1,3,5‐benzenetricarboxylate (BTC) as a linker were solvothermally obtained under air‐free conditions. X‐ray diffraction analysis of the crystals demonstrated a structure for Fe^II‐MOF analogous to that of [Cu₃(BTC)₂] (HKUST‐1). Unlike HKUST‐1, however, the Fe^II‐MOF did not retain permanent porosity after exchange of guest molecules. The Mössbauer spectrum of the Fe^II‐MOF was recorded at 80 K in zero field yielding an apparent quadrupole splitting of ΔE_Q = 2.43 mm · s^–1, and an isomer shift of δ = 1.20 mm · s^–1, consistent with high‐spin central iron(II) atoms. Air exposure of the Fe^II‐MOF was found to result in oxidation of the metal atoms to afford Fe^III. These results demonstrate that Fe^II‐based MOFs can be prepared in similar fashion to the [Cu₃(BTC)₂], but that they lack permanent porosity when degassed.     

Highly Facet‐Dependent Photocatalytic Properties of Cu2O Crystals Established through the Formation of Au‐Decorated Cu2O Heterostructures

This work confirms the presence of a large facet‐dependent photocatalytic activity of Cu₂O crystals through sparse deposition of gold particles on Cu₂O cubes, octahedra, and rhombic dodecahedra. Au‐decorated Cu₂O rhombic dodecahedra and octahedra showed greatly enhanced photodegradation rates of methyl orange resulting from a better separation of the photogenerated electrons and holes, with the rhombic dodecahedra giving the best efficiency. Au–Cu₂O core–shell rhombic dodecahedra also displayed a better photocatalytic activity than pristine rhombic dodecahedra. However, Au‐deposited Cu₂O cubes, pristine cubes, and Au‐deposited small nanocubes bound by entirely {100} facets are all photocatalytically inactive. X‐ray photoelectron spectra (XPS) showed identical copper peak positions for these Au‐decorated crystals. Remarkably, electron paramagnetic resonance (EPR) measurements indicated a higher production of hydroxyl radicals for the photoirradiated Cu₂O rhombic dodecahedra than for the octahedra, but no radicals were produced from photoirradiated Cu₂O cubes. The Cu₂O {100} face may present a high energy barrier through its large band edge bending and/or electrostatic repulsion, preventing charge carriers from reaching to this surface. The conventional photocatalysis model fails in this case. The facet‐dependent photocatalytic differences should be observable in other semiconductor systems whenever a photoinduced charge‐transfer process occurs across an interface.     

Copper–Organic Framework Fabricated with CuS Nanoparticles: Synthesis,Electrical Conductivity,and Electrocatalytic Activities for Oxygen Reduction Reaction

To apply electrically nonconductive metal–organic frameworks (MOFs) in an electrocatalytic oxygen reduction reaction (ORR), we have developed a new method for fabricating various amounts of CuS nanoparticles (nano‐CuS) in/on a 3D Cu–MOF, [Cu₃(BTC)₂⋅(H₂O)₃] (BTC=1,3,5‐benzenetricarboxylate). As the amount of nano‐CuS increases in the composite, the electrical conductivity increases exponentially by up to circa 10⁹‐fold, while porosity decreases, compared with that of the pristine Cu‐MOF. The composites, nano‐CuS(x wt %)@Cu‐BTC, exhibit significantly higher electrocatalytic ORR activities than Cu‐BTC or nano‐CuS in an alkaline solution. The onset potential, electron transfer number, and kinetic current density increase when the electrical conductivity of the material increases but decrease when the material has a poor porosity, which shows that the two factors should be finely tuned by the amount of nano‐CuS for ORR application. Of these materials, CuS(28 wt %)@Cu‐BTC exhibits the best activity, showing the onset potential of 0.91 V vs. RHE, quasi‐four‐electron transfer pathway, and a kinetic current density of 11.3 mA cm⁻² at 0.55 V vs. RHE.     

Copper–Organic Framework Fabricated with CuS Nanoparticles: Synthesis,Electrical Conductivity,and Electrocatalytic Activities for Oxygen Reduction Reaction

To apply electrically nonconductive metal–organic frameworks (MOFs) in an electrocatalytic oxygen reduction reaction (ORR), we have developed a new method for fabricating various amounts of CuS nanoparticles (nano‐CuS) in/on a 3D Cu–MOF, [Cu₃(BTC)₂?(H₂O)₃] (BTC=1,3,5‐benzenetricarboxylate). As the amount of nano‐CuS increases in the composite, the electrical conductivity increases exponentially by up to circa 10⁹‐fold, while porosity decreases, compared with that of the pristine Cu‐MOF. The composites, nano‐CuS(x wt %)@Cu‐BTC, exhibit significantly higher electrocatalytic ORR activities than Cu‐BTC or nano‐CuS in an alkaline solution. The onset potential, electron transfer number, and kinetic current density increase when the electrical conductivity of the material increases but decrease when the material has a poor porosity, which shows that the two factors should be finely tuned by the amount of nano‐CuS for ORR application. Of these materials, CuS(28 wt %)@Cu‐BTC exhibits the best activity, showing the onset potential of 0.91 V vs. RHE, quasi‐four‐electron transfer pathway, and a kinetic current density of 11.3 mA cm^?2 at 0.55 V vs. RHE.     

A Novel Electrochemical Sensor Based on Copper‐based Metal‐Organic Framework for the Determination of Dopamine

A glassy carbon electrode (GCE) modified with a copper‐based metal‐organic framework (MOF) [HKUST‐1, HKUST‐1 = Cu₃(BTC)₂ (BTC = 1,3,5‐benzenetricarboxylicacid)] was developed as a highly sensitive and simple electrochemical sensor for the determination of dopamine (DA). The MOF was prepared by a hydrothermal process, and the morphology and crystal phase of the MOF were characterized by scanning electron microscopy (SEM) and X‐ray diffraction (XRD), respectively. Meanwhile, the electrochemical performance was investigated using cyclic voltammetry (CV), differential pulse voltammetry (DPV), and electrochemical impedance spectroscopy (EIS). Under optimized conditions, the modified electrode showed excellent electrocatalytic activity and high selectivity toward DA. The linear response range was from 5.0 × 10⁻⁷ to 1.0 × 10⁻⁴ M and the detection limit was as low as 1.5 × 10⁻⁷ M. Moreover, the electrochemical sensor was used to detect DA in real samples with excellent results. MOF‐based sensors hold great promise for routine sensing applications in the field of electrochemical sensing.     

Direct Observation of Molecular‐Level Template Action Leading to Self‐Assembly of a Porous Framework

The molecular steps involved in the self‐assembly of Cu₃(BTC)₂ (BTC=1,3,5‐benzenetricarboxylic acid) metal–organic frameworks that enclose Keggin‐type H₃PW₁₂O₄₀ heteropolyacid molecules were unraveled by using solution ¹⁷O, ³¹P, and ¹⁸³W NMR spectroscopy, small‐angle X‐ray scattering, near‐IR spectroscopy, and dynamic light scattering. In aqueous solution, complexation of Cu²⁺ ions with Keggin‐type heteropolyacids was observed. Cu²⁺ ions are arranged around the Keggin structure so that linking through benzenetricarboxylate groups results in the formation of the Cu₃(BTC)₂ MOF structure HKUST‐1. This is a unique instance in which a templating mechanism that relies on specific molecular‐level matching and leads to explicit nanoscale building units can be observed in situ during formation of the synthetic nanoporous material.     

Crystal‐Plane‐Controlled Selectivity of Cu2O Catalysts in Propylene Oxidation with Molecular Oxygen

The selective oxidation of propylene with O₂ to propylene oxide and acrolein is of great interest and importance. We report the crystal‐plane‐controlled selectivity of uniform capping‐ligand‐free Cu₂O octahedra, cubes, and rhombic dodecahedra in catalyzing propylene oxidation with O₂: Cu₂O octahedra exposing {111} crystal planes are most selective for acrolein; Cu₂O cubes exposing {100} crystal planes are most selective for CO₂; Cu₂O rhombic dodecahedra exposing {110} crystal planes are most selective for propylene oxide. One‐coordinated Cu on Cu₂O(111), three‐coordinated O on Cu₂O(110), and two‐coordinated O on Cu₂O(100) were identified as the catalytically active sites for the production of acrolein, propylene oxide, and CO₂, respectively. These results reveal that crystal‐plane engineering of oxide catalysts could be a useful strategy for developing selective catalysts and for gaining fundamental understanding of complex heterogeneous catalytic reactions at the molecular level.     

Facet effect of single-crystalline Ag3PO4 sub-microcrystals on photocatalytic properties

We recently reported that Ag(3)PO(4) exhibits excellent photooxidative capabilities for O(2) evolution from water and organic dye decomposition under visible-light irradiation. However, very little is known about the shape and facet effects of Ag(3)PO(4) crystals on their photocatalytic properties. Herein we have developed a facile and general route for high-yield fabrication of single-crystalline Ag(3)PO(4) rhombic dodecahedrons with only {110} facets exposed and cubes bounded entirely by {100} facets. Moreover, studies of their photocatalytic performance have indicated that rhombic dodecahedrons exhibit much higher activities than cubes for the degradation of organic contaminants, which may be primarily ascribed to the higher surface energy of {110} facets (1.31 J/m(2)) than of {100} facets (1.12 J/m(2)).     

Copper‐based Metal‐organic Framework Applied in the Development of an Electrochemical Biomimetic Sensor for Catechol Determination

In this study, the synthesis and characterization of a Cu‐based metal‐organic framework (MOF) [Cu₃(BTC)₂(H₂O)₃]_n (where BTC=benzene‐1,3,5‐tricarboxylate), known as HKUST‐1, were performed. The Cu‐MOF was applied in the modification of a carbon paste to obtain a biomimetic sensor for the electrochemical determination of catechol. Kinetic assays confirmed that the Cu‐MOF acts as a catalyst for the oxidation of catechol and it can be considered as a catechol oxidase mimetic. Under optimized conditions, the calibration curve for catechol presented a linear range of 8.0×10⁻⁷ to 3.2×10⁻⁵ mol L⁻¹, with detection limit of=1.0×10⁻⁷ mol L⁻¹. The sensor demonstrated good intra‐day repeatability and inter‐electrode reproducibility (relative standard deviations of 3.8 % (n=10) and 4.3 % (n=6), respectively). In the selectivity study, an adequate peak‐to‐peak separation was observed for hydroquinone and uric acid in relation to catechol, demonstrating that this sensor has the potential for use in the simultaneous determination of these compounds. This sensor was successfully applied in the determination of catechol in water samples.     

POM‐Catalyzed In Situ Ligand Synthesis for the Construction of Metal Complexes and Their Use in the Formation of Coordination Polymers

Six organic–inorganic hybrid materials were synthesized by the in situ oxidation of neocuproine by using MoO₃/Na₂MoO₄ as the catalyst in the presence of Cu(NO₃)₂. The crystal structures of Mo8‐Cu4‐PHEN and Mo8‐Cu2‐5(2PIC) are composed of [Mo₈O₂₆]^4? polyoxometalate (POM) units, whereas the crystal structure of Mo6‐Cu‐COPHEN is composed of a [Mo₆O₁₉]^2? POM unit; both POM units could be considered as the active form of the catalyst. Reaction of the hybrid materials with 1,3,5‐benzenetricarboxylic acid (BTC) resulted in the formation of two different coordination polymers (CPs) under different reaction conditions. These CPs, depending on their structural attributes, exhibit distinct differences in the adsorption of H₂, CO₂, and water. The use of 2‐methylpyridine instead of neocuproine does not give any oxidation products under the same reaction conditions due to the incorrect positioning of the methyl group with respect to the Cu^II center.     

Controllable Syntheses of Porous Metal–Organic Frameworks: Encapsulation of LnIII Cations for Tunable Luminescence and Small Drug Molecules for Efficient Delivery

Two porous metal–organic frameworks (MOFs), [Zn₃(L)(H₂O)₂] ? 3 DMF ? 7 H₂O ( MOF‐1 ) and [(CH₃)₂NH₂]₆[Ni₃(L)₂(H₂O)₆] ? 3 DMF ? 15 H₂O ( MOF‐2 ), were synthesized solvothermally (H₆L=1,2,3,4,5,6‐hexakis(3‐carboxyphenyloxymethylene)benzene). In MOF ‐ 1 , neighboring Zn^II trimers are linked by the backbones of L ligands to form a fascinating 3D six‐connected framework with the point symbol (4¹².6³) (4¹².6³). In MOF‐2 , eight L ligands bridge six Ni^II atoms to generate a rhombic‐dodecahedral [Ni₆L₈] cage. Each cage is surrounded by eight adjacent ones through sharing of carboxylate groups to yield an unusual 3D porous framework. Encapsulation of Ln^III cations for tunable luminescence and small drug molecules for efficient delivery were investigated in detail for MOF‐1 .     

Calcium-based coordination polymers from a solvothermal synthesis of HKUST-1 in 3D printed autoclaves

A mixed-metal 1D coordination polymer [CaCu(HBTC)₂(H2O)₈]_n (where H₃BTC – benzene-1,3,5-tric arboxylic acid) was obtained in a solvothermal synthesis of a well-known copper-containing metal–organic framework [Cu₃(BTC)₂(H₂O)₃]_n (HKUST-1) in autoclaves 3D-printed from commercial polypropylene. This material was a source of calcium ions, apparently, leaking from a colorant (calcium carbonate) promoted by glacial acetic acid as a modulator used to produce large single crystals of HKUST-1. This finding was confirmed by elemental analysis and a model experiment that resulted in a new calcium-based 1D coordination polymer [Ca(H₂BTC)₂(H₂O)₅]_n under the same solvothermal conditions with no copper or calcium salts put into a 3D-printed autoclave.     

Catenation control in the two-dimensional coordination polymers based on tritopic carboxylate linkers and azamacrocyclic nickel(II) complexes

Four new coordination polymer frameworks, namely [(NiL(1))(3)(BTB)(2)]·6H(2)O, [(NiL(2))(3)(BTB)(2)]·6H(2)O, [(NiL(3))(3)(BTB)(2)]·6H(2)O and [(NiL(2))(3)(BTC)(2)]·10.25H(2)O (L(1) = 1,4,8,11-tetraazacyclotetradecane, L(2) = 3-methyl-1,3,5,8,12-pentaazacyclotetradecane, L(3) = 3,10-dimethyl-1,3,5,8,10,12-hexaazacyclotetradecane, BTC(3-) = benzene-1,3,5-tricarboxylate, BTB(3-) = 4,4',4'-benzene-1,3,5-triyl-tribenzoate) were prepared in water-N,N-dimethylformamide solutions. The molecular and crystal structures of these compounds are compared to the related coordination polymers formed by nickel(II) macrocyclic cations to examine the effect of carboxylate linker size on the framework architecture. Luminescent properties of the complexes based on the BTB(3-) bridging ligand are also discussed.     

Luminescent Metal–Organic Frameworks (MOFs) as a Chemopalette: Tuning the Thermochromic Behavior of Dual‐Emissive Phosphorescence by Adjusting the Supramolecular Microenvironments

Two classical copper(I)‐cluster‐based luminophores, namely, Cu₄I₄ and [Cu₃Pz₃]₂ (Pz=pyrazolate), are immobilized in a supramolecular system through the formation of metal–organic framework (MOF) materials. This series of luminescent MOF materials, namely, [Cu₄I₄(NH₃)Cu₃( L1 )₃]_n, [Cu₄I₄(NH₂CH₃)Cu₃( L1 )₃]_n, and [Cu₄I₄Cu₃( L2 )₃]_n ( L1 =3‐(4‐pyridyl)‐5‐(p‐tolyl)pyrazolate; L2 =3‐(4‐pyridyl)‐5‐(2,4‐dimethylphenyl)pyrazolate), exhibit diverse thermochromism attributed to the relative functioning efficacy of the two coordination luminophores. Such an intriguing chemopalette effect is regulated by the different supramolecular microenvironments between the two‐dimensional layers of these MOFs, and in particular, by the fine‐tuned Cu–Cu distances in the excimeric [Cu₃Pz₃]₂ luminophore. The structure–property elucidation of the thermochromic behavior allows one to understand these optical materials with unusual dual‐emissive properties.     

A Tale of Two Trimers from Two Different Worlds: A COF‐Inspired Synthetic Strategy for Pore‐Space Partitioning of MOFs

The introduction of a symmetry‐ and size‐matching pore‐partitioning agent in the form of either a molecular ligand, such as 2,4,6‐tri(4‐pyridinyl)‐1,3,5‐triazine ( tpt ), or a metal‐complex cluster, into the hexagonal channels of MIL‐88/MOF‐235‐type (the acs net) to create pacs ‐type (partitioned acs ) crystalline porous materials is an effective strategy to develop high‐performance gas adsorbents. We have developed an integrated COF–MOF coassembly strategy as a new method for pore‐space partitioning through the coassembly of [(M₃(OH)_1?x(O)_x(COO)₆] MOF‐type and [B₃O₃(py)₃] COF‐type trimers. With this strategy, the coordination‐driven assembly of the acs framework occurred concurrently and synergistically with the COF‐1‐type condensation of pyridine‐4‐boronic acid into a C₃‐symmetric trimeric boroxine molecule. The resulting boroxine‐based pacs materials exhibited dramatically enhanced gas‐sorption properties as compared to nonpartitioned acs ‐type materials and are among the most efficient NH₃‐sorption materials.     

Crystal‐Facet Engineering of Ferric Giniite by Using Ionic‐Liquid Precursors and Their Enhanced Photocatalytic Performances under Visible‐Light Irradiation

In the work presented here, well‐dispersed ferric giniite microcrystals with controlled sizes and shapes are solvothermally synthesized from ionic‐liquid precursors by using 1‐n‐butyl‐3‐methylimidazolium dihydrogenphosphate ([Bmim][H₂PO₄]) as phosphate source. The success of this synthesis relies on the concentration and composition of the ionic‐liquid precursors. By adjusting the molar ratios of Fe(NO₃)₃ ? 9H₂O to [Bmim][H₂PO₄] as well as the composition of ionic‐liquid precursors, we obtained uniform microstructures such as bipyramids exposing {111} facets, plates exposing {001} facets, hollow spheres, tetragonal hexadecahedron exposing {441} and {111} facets, and truncated bipyamids with carved {001} facets. The crystalline structure of the ferric giniite microcrystals is disclosed by various characterization techniques. It was revealed that [Bmim][H₂PO₄] played an important role in stabilizing the {111} facets of ferric giniite crystals, leading to the different morphologies in the presence of ionic‐liquid precursors with different compositions. Furthermore, since these ferric giniite crystals were characterized by different facets, they could serve as model Fenton‐like catalysts to uncover the correlation between the surface and the catalytic performance for the photodegradation of organic dyes under visible‐light irradiation. Our measurements indicate that the photocatalytic activity of as‐prepared Fenton‐like catalysts is highly dependent on the exposed facets, and the surface area has essentially no obvious effect on the photocatalytic degradation of organic dyes in the present study. It is highly expected that these findings are useful in understanding the photocatalytic activity of Fenton‐like catalysts with different morphologies, and suggest a promising new strategy for crystal‐facet engineering of photocatalysts for wastewater treatment based on heterogeneous Fenton‐like process.     

Semiconductor nanocrystals possessing broadly size‐ and facet‐dependent optical properties

Cu₂O cubes, octahedra, and rhombic dodecahedra have been shown to exhibit continuous light absorption and emission band shifts with increasing particle sizes from 10 nm to sub‐microcrystals. They also possess clear facet‐dependent optical properties. Ag₃PO₄, Ag₂O, SrTiO₃, and CeO₂ crystals show similar optical size and facet effects. Thus, spectral shifts over a broad size range far beyond the quantum‐size regime should be generally observable in many semiconductor materials. Facet‐dependent optical properties of a semiconductor can be understood to arise from the presence of an ultrathin surface layer with subtle bond and orbital level variations for different crystal faces. Although these optical features seem unexpected, they should be the general behaviors of semiconductor crystals. As more examples of these optical effects are available, we will find that these intrinsic properties of semiconductors have been ignored in the past. Furthermore, if valence and conduction band positions are broadly tunable by particle size, the knowledge should have tremendous impacts on the applications of semiconductors, where band energies are important to efficient interfacial charge transfer.     

Facet‐Dependent Catalytic Activity of Cu2O Nanocrystals in the One‐Pot Synthesis of 1,2,3‐Triazoles by Multicomponent Click Reactions

We report the highly facet‐dependent catalytic activity of Cu₂O nanocubes, octahedra, and rhombic dodecahedra for the multicomponent direct synthesis of 1,2,3‐triazoles from the reaction of alkynes, organic halides, and NaN₃. The catalytic activities of clean surfactant‐removed Cu₂O nanocrystals with the same total surface area were compared. Rhombic dodecahedral Cu₂O nanocrystals bounded by {110} facets were much more catalytically active than Cu₂O octahedra exposing {111} facets, whereas Cu₂O nanocubes displayed the slowest catalytic activity. The superior catalytic activity of Cu₂O rhombic dodecahedra is attributed to the fully exposed surface Cu atoms on the {110} facet. A large series of 1,4‐disubstituted 1,2,3‐triazoles have been synthesized in excellent yields with high regioselectivity under green conditions by using these rhombic dodecahedral Cu₂O catalysts, including the synthesis of rufinamide, an antiepileptic drug, demonstrating the potential of these nanocrystals as promising heterogeneous catalysts for other important coupling reactions.     

Self‐Assembly of Hybrid Oxidant POM@Cu‐BTC for Enhanced Efficiency and Long‐Term Stability of Perovskite Solar Cells

The controllable oxidation of spiro‐OMeTAD and improving the stability of hole‐transport materials (HTMs) layer are crucial for good performance and stability of perovskite solar cells (PSCs). Herein, we report an efficient hybrid polyoxometalate@metal–organic framework (POM@MOF) material, [Cu₂(BTC)_4/3(H₂O)₂]₆[H₃PMo₁₂O₄₀]₂ or POM@Cu‐BTC, for the oxidation of spiro‐OMeTAD with Li‐TFSI and TBP. When POM@Cu‐BTC is introduced to the HTM layer as a dopant, the PSCs achieve a superior fill factor of 0.80 and enhanced power conversion efficiency 21.44 %, as well as improved long‐term stability in an ambient atmosphere without encapsulation. The enhanced performance is attributed to the oxidation activity of POM anions and solid‐state nanoparticles. Therefore, this research presents a facile way by using hybrid porous materials to accelerate oxidation of spiro‐OMeTAD, further improving the efficiency and stability of PSCs.     

Simple Transformation of Hierarchical Hollow Structures by Reduction of Metal–Organic Frameworks and Their Catalytic Activity in the Oxidation of Benzyl Alcohol

Metal–organic framework (MOF)-based derivatives have been found to be promising heterogeneous catalysts for organic transformations. Herein, hollow-structure Cu-MOFs derived by reduction of Cu₃(BTC)₂ (BTC=1,3,5-benzenetricarboxylate; denoted as RCB) were prepared by using hydrazine hydrate as a reducing agent under various conditions. The influence of hydrazine hydrate induced the structure of Cu₃(BTC)₂ and led to dynamic variation in the interior and exterior as well as oxidation states of the Cu ion. The synthesized materials were characterized by SEM, TEM, N₂ sorption isotherms, XRD, and XPS. The product of the catalytic reaction was observed by GC-MS. In addition, the prepared RCBs were found to have excellent catalytic activity and selectivity for benzyl alcohol oxidation when assisted by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO).