Seeded growth of large single-crystal copper foils with high-index facets.

The production of large single-crystal metal foils with various facet indices has long been a pursuit in materials science owing to their potential applications in crystal epitaxy, catalysis, electronics and thermal engineering1-5. For a given metal, there are only three sets of low-index facets ({100}, {110} and {111}). In comparison, high-index facets are in principle infinite and could afford richer surface structures and properties. However, the controlled preparation of single-crystal foils with high-index facets is challenging, because they are neither thermodynamically6,7 nor kinetically3 favourable compared to low-index facets6-18. Here we report a seeded growth technique for building a library of single-crystal copper foils with sizes of about 30 × 20 square centimetres and more than 30 kinds of facet. A mild pre-oxidation of polycrystalline copper foils, followed by annealing in a reducing atmosphere, leads to the growth of high-index copper facets that cover almost the entire foil and have the potential of growing to lengths of several metres. The creation of oxide surface layers on our foils means that surface energy minimization is not a key determinant of facet selection for growth, as is usually the case. Instead, facet selection is dictated randomly by the facet of the largest grain (irrespective of its surface energy), which consumes smaller grains and eliminates grain boundaries. Our high-index foils can be used as seeds for the growth of other Cu foils along either the in-plane or the out-of-plane direction. We show that this technique is also applicable to the growth of high-index single-crystal nickel foils, and we explore the possibility of using our high-index copper foils as substrates for the epitaxial growth of two-dimensional materials. Other applications are expected in selective catalysis, low-impedance electrical conduction and heat dissipation.

Gas-Triggered Gate-Opening in a Flexible Three-Dimensional Covalent Organic Framework.

The development of novel soft porous crystals (SPCs) that can be transformed from nonporous to porous crystals is significant because of their promising applications in gas storage and separation. Herein, we systematically investigated for the first time the gas-triggered gate-opening behavior of three-dimensional covalent organic frameworks (3D COFs) with flexible building blocks. FCOF-5, a 3D COF containing C-O single bonds in the backbone, exhibits a unique "S-shaped" isotherm for various gases, such as CO2, C2, and C3 hydrocarbons. According to in situ characterization, FCOF-5 undergoes a pressure-induced closed-to-open structural transition due to the rotation of flexible C-O single bonds in the framework. Furthermore, the gated hysteretic sorption property of FCOF-5 can enable its use as an absorbent for the efficient removal of C3H4 from C3H4/C3H6 mixtures. Therefore, 3D COFs synthesized from flexible building blocks represent a new type of SPC with gate-opening characteristics. This study will strongly inspire us to design other 3D COF-based SPCs for interesting applications in the future.

Linker-Guided Growth of Single-Crystal Covalent Organic Frameworks.

The core features of covalent organic frameworks (COFs) are crystallinity and porosity. However, the synthesis of single-crystal COFs with monomers of diverse reactivity and adjustment of their pore structures remain challenging. Here, we show that linkers that can react with a node to form single-crystal COFs can guide other linkers that form either COFs or amorphous polymers with the node to gain single-crystal COFs with mixed components, which are homogeneous on the unit cell scale with controlled ratios. With the linker-guided crystal growth method, we created nine types of single-crystal COFs with up to nine different components, which are more complex than any known crystal. The structure of the crystal adapted approximately to that of the main component, and its pore volume could be expanded up to 8.8%. Different components lead to complex and diverse pore structures and offer the possibilities to gain positive synergies, as exemplified by a bicomponent COF with 2200 and 733% SO2 uptake capacity of that of the two pure-component counterparts at 298 K and 0.002 bar. The selectivity for separation of SO2/CO2 ranges from 1230 to 4247 for flue gas based on ideal adsorbed solution theory, recording porous crystals. The bicomponent COF also exhibits a 1300% retention time of its pure-component counterparts for SO2 in a dynamic column breakthrough experiment for deep desulfurization.

A Fluorescent Three-Dimensional Covalent Organic Framework Formed by the Entanglement of Two-Dimensional Sheets.

Herein, we report the introduction of steric hindrance in molecular building blocks to prevent π···π stacking, thus allowing two-dimensional (2D) covalent organic sheets to form three-dimensional (3D) covalent organic frameworks (COFs) through entanglement. Starting from the rationally designed precursors containing a bulky anthracene unit in the vertical direction, a highly crystalline COF (3D-An-COF) was successfully synthesized. Very interestingly, 3D-An-COF was determined as an entangled 2D square net (sql) structure, and the high-resolution data (1.1 Å) obtained by the continuous rotation electron diffraction technique allowed us to directly locate all non-hydrogen atoms. Structurally, the presence of an anthracene group outside the C2h symmetry plane strongly reduces the π···π interactions and promotes the formation of square entanglements. In addition, 3D-An-COF is fluorescent and can be used as a sensor to detect the trace amount of antibiotics in water. This study provides a new strategy for the structural diversification of 3D COFs and will certainly motivate us to construct more entangled COFs for interesting applications in the future.

Single-Crystal Three-Dimensional Covalent Organic Framework Constructed from 6-Connected Triangular Prism Node.

The limited structural diversity of three-dimensional covalent organic frameworks (3D COFs) greatly restricts their application exploration. Therefore, there is an urgent need to expand their library of molecular building blocks, such as the development of highly connected (>4 reaction sites) polyhedral nodes. Herein, by precisely controlling the precursor conformation, we rationally designed a new 6-connected triangular prism node derived from the triphenylbenzene molecule and further used it to construct a novel 3D COF (3D-TMTAPB-COF) via imine condensation reaction. Surprisingly, without the addition of competing reagents, 3D-TMTAPB-COF crystallized directly into single crystals of ∼15 µm in size and was determined to adopt a rare 6-fold interpenetrated (Class IIIa interpenetration) acs topology. In addition, 3D-TMTAPB-COF showed a high SF6 adsorption capacity (60.9 cm3 g-1) and good SF6/N2 selectivity (335) at 298 K and 1 bar, superior to those of most crystalline porous materials. This work not only confirms the possibility of growing large-size single-crystal 3D COFs formed with strong covalent bonds by a solvothermal method in the absence of modulators, but also reports a novel triangular prism node for future construction of 3D COFs with interesting applications.

Crystallization of Dimensional Isomers in Covalent Organic Frameworks.

Dimensional isomers, defined in reticular chemistry as frameworks consisting of identical molecular building blocks but extended in two or three dimensions (2D or 3D), are an important type of framework isomers that have never been isolated. Herein, we report the crystallization of dimensional isomers in covalent organic frameworks (COFs) for the first time. By polymerization of the same molecular building blocks at different temperatures, both 2D and 3D COFs were successfully constructed due to the temperature-induced conformational changes of precursors from planar to tetrahedral. In addition, the non-fluorescent 2D COF can be gradually converted into the fluorescent 3D COF by increasing the temperature under solvothermal conditions. Therefore, it is reasonable to crystallize the dimensional isomers of reticular materials by controlling the conformation of molecular building blocks, and more examples can be expected. Since the obtained dimensional isomers show different properties and functions, this work will definitely motivate us to design reticular materials for target applications in the future.

Growth and Local Structures of Single Crystalline Flakes of Three-Dimensional Covalent Organic Frameworks in Water.

Due to the enormous chemical and structural diversities and designable properties and functionalities, covalent organic frameworks (COFs) hold great promise as tailored materials for industrial applications in electronics, biology, and energy technologies. They were typically obtained as partially crystalline materials, although a few single-crystal three-dimensional (3D) COFs have been obtained recently with structures probed by diffraction techniques. However, it remains challenging to grow single-crystal COFs with controlled morphology and to elucidate the local structures of 3D COFs, imposing severe limitations on the applications and understanding of the local structure-property correlations. Herein, we develop a method for designed growth of five types of single crystalline flakes of 3D COFs with controlled morphology, front crystal facets, and defined edge structures as well as surface chemistry using surfactants that can be self-assembled into layered structures to confine crystal growth in water. The flakes enable direct observation of local structures including monomer units, pore structure, edge structure, grain boundary, and lattice distortion of 3D COFs as well as gradually curved surfaces in kinked but single crystalline 3D COFs with a resolution of up to ∼1.7 Å. In comparison with flakes of two-dimensional crystals, the synthesized flakes show much higher chemical, mechanical, and thermal stability.

Under-Coordinated CoFe Layered Double Hydroxide Nanocages Derived from Nanoconfined Hydrolysis of Bimetal Organic Compounds for Efficient Electrocatalytic Water Oxidation.

Hierarchically structured bimetal hydroxides are promising for electrocatalytic oxygen evolution reaction (OER), yet synthetically challenging. Here, the nanoconfined hydrolysis of a hitherto unknown CoFe-bimetal-organic compound (b-MOC) is reported for the controllable synthesis of highly OER active nanostructures of CoFe layered double hydroxide (LDH). The nanoporous structures trigger the nanoconfined hydrolysis in the sacrificial b-MOC template, producing CoFe LDH core-shell octahedrons, nanoporous octahedrons, and hollow nanocages with abundant under-coordinated metal sites. The hollow nanocages of CoFe LDH demonstrate a remarkable turnover frequency (TOF) of 0.0505 s-1 for OER catalysis at an overpotential of 300 mV. It is durable in up to 50 h of electrolysis at step current densities of 10-100 mA cm-2 . Ex situ and in situ X-ray absorption spectroscopic analysis combined with theoretical calculations suggests that under-coordinated Co cations can bind with deprotonated Fe-OH motifs to form OER active Fe-O-Co dimmers in the electrochemical oxidation process, thereby contributing to the good catalytic activity. This work presents an efficient strategy for the synthesis of highly under-coordinated bimetal hydroxide nanostructures. The mechanistic understanding underscores the power of maximizing the amount of bimetal-dimer sites for efficient OER catalysis.

Apple polyphenol biotransformation using probiotics in vitro and dynamic simulated digestion by bionic rats.

BACKGROUND: The present study investigated the effects of fermentation by Lactobacillus rhamnosus zrx01 (LR-zrx01), Lactobacillus acidophilus zrx02 (LA-zrx02), and Lactobacillus plantarum zrx03 (LP-zrx03), as well as dynamic simulated digestion by bionic rats, on the biotransformation and antioxidant potential of apple polyphenols. Polyphenols were determined by ultra-high-performance liquid chromatography-mass spectrometry, the dynamic simulated digestion of fermented apple pulp was determined by bionic rats, and the antibacterial and antioxidant activities were analyzed. RESULTS: The polyphenol content of apple pulp fermented using the three strains was respectively 1.41, 1.38, and 1.36 times that of non-fermented pulp. The antibacterial activity of apple pulp improved dramatically after fermentation. Moreover, the antioxidant potential of apple pulp increased after fermentation and digestion. After dynamic simulated digestion by bionic rats, the polyphenol content in unfermented and the three fermented groups increased significantly by 1.19, 1.23, 1.20, and 1.19 times compared to that before digestion, respectively. The major polyphenols in each group with obvious changes were epicatechin, rutin, kaempferol, quercetin-3 galactoside, p-coumaric acid, and two unknown substances, 1 and 2. CONCLUSION: Fermented and digested apple polyphenols showed better biotransformation effects and mostly existed in the form of small molecules, which was conducive to the improvement of polyphenol bioavailability and beneficial to the absorption of active substances by the human body. These findings build a foundation for the development of functional food beverages. © 2023 Society of Chemical Industry.

Locating Hydrogen Positions for COF-300 by Cryo-3D Electron Diffraction.

Covalent organic frameworks (COFs) have wide-ranging applications, and their host-guest interactions play an essential role in the achievement of COF functions. To investigate these host-guest interactions, it is necessary to locate all atoms, especially hydrogen atoms. However, it is difficult to determine the hydrogen atomic positions in COFs because of the complexities in synthesizing high-quality large single crystals. Three-dimensional electron diffraction (3D ED) has unique advantages for the structural determination of nanocrystals and identification of light atoms. In this study, it was demonstrated for the first time that the hydrogen atoms of a COF, not only on the framework but also on the guest molecule, can be located by 3D ED using continuous precession electron diffraction tomography (cPEDT) under cryogenic conditions. The host-guest interactions were clarified with the location of the hydrogen atoms. These findings provide novel insights into the investigation of COFs.

Co-Co Dinuclear Active Sites Dispersed on Zirconium-doped Heterostructured Co9 S8 /Co3 O4 for High-current-density and Durable Acidic Oxygen Evolution.

Developing cost-effective and sustainable acidic water oxidation catalysts requires significant advances in material design and in-depth mechanism understanding for proton exchange membrane water electrolysis. Herein, we developed a single atom regulatory strategy to construct Co-Co dinuclear active sites (DASs) catalysts that atomically dispersed zirconium doped Co9 S8 /Co3 O4 heterostructure. The X-ray absorption fine structure elucidated the incorporation of Zr greatly facilitated the generation of Co-Co DASs layer with stretching of cobalt oxygen bond and S-Co-O heterogeneous grain boundaries interfaces, engineering attractive activity of significantly reduced overpotential of 75 mV at 10 mA cm-2 , a breakthrough of 500 mA cm-2 high current density, and water splitting stability of 500 hours in acid, making it one of the best-performing acid-stable OER non-noble metal materials. The optimized catalyst with interatomic Co-Co distance (ca. 2.80 Å) followed oxo-oxo coupling mechanism that involved obvious oxygen bridges on dinuclear Co sites (1,090 cm-1 ), confirmed by in situ SR-FTIR, XAFS and theoretical simulations. Furthermore, a major breakthrough of 120,000 mA g-1 high mass current density using the first reported noble metal-free cobalt anode catalyst of Co-Co DASs/ZCC in PEM-WE at 2.14 V was recorded.

Single-Crystalline Imine-Linked Two-Dimensional Covalent Organic Frameworks Separate Benzene and Cyclohexane Efficiently.

Two-dimensional (2D) covalent organic frameworks (COFs) are composed of structurally precise, permanently porous, layered macromolecular sheets, which are traditionally synthesized as polycrystalline solids with crystalline domain lengths smaller than 100 nm. Here, we polymerize imine-linked 2D COFs as suspensions of faceted single crystals in as little as 5 min at moderate temperature and ambient pressure. Single crystals of two imine-linked 2D COFs were prepared, consisting of a rhombic 2D COF (TAPPy-PDA) and a hexagonal 2D COF (TAPB-DMPDA). The sizes of TAPPy-PDA and TAPB-DMPDA crystals were tuned from 720 nm to 4 µm and 450 nm to 20 µm in width, respectively. High-resolution transmission electron microscopy revealed that the COF crystals consist of layered, 2D polymers comprising single-crystalline domains. Continuous rotation electron diffraction resolved the unit cell and crystal structure of both COFs, which are single-crystalline in the a-b plane but disordered in the stacking c dimension. Single crystals of both COFs were incorporated into gas chromatography separation columns and exhibited unusual selective retention of cyclohexane over benzene, with single-crystalline TAPPy-PDA significantly outperforming single-crystalline TAPB-DMPDA. Polycrystalline TAPPy-PDA exhibited no separation, while polycrystalline TAPB-DMPDA exhibited poor separation and the opposite order of elution, retaining benzene more than cyclohexane, indicating the importance of improved material quality for COFs to exhibit properties that derive from their precise, crystalline structures. This work represents the first example of synthesizing imine-linked 2D COF single crystals at ambient pressure and short reaction times and demonstrates the promise of high-quality COFs for molecular separations.

Atomically precise single-crystal structures of electrically conducting 2D metal-organic frameworks.

Electrically conducting 2D metal-organic frameworks (MOFs) have attracted considerable interest, as their hexagonal 2D lattices mimic graphite and other 2D van der Waals stacked materials. However, understanding their intrinsic properties remains a challenge because their crystals are too small or of too poor quality for crystal structure determination. Here, we report atomically precise structures of a family of 2D π-conjugated MOFs derived from large single crystals of sizes up to 200 µm, allowing atomic-resolution analysis by a battery of high-resolution diffraction techniques. A designed ligand core rebalances the in-plane and out-of-plane interactions that define anisotropic crystal growth. We report two crystal structure types exhibiting analogous 2D honeycomb-like sheets but distinct packing modes and pore contents. Single-crystal electrical transport measurements distinctively demonstrate anisotropic transport normal and parallel to the π-conjugated sheets, revealing a clear correlation between absolute conductivity and the nature of the metal cation and 2D sheet packing motif.

Stabilization of Extra-Large-Pore Zeolite by Boron Substitution for the Production of Commercially Applicable Catalysts.

Stable extra-large-pore zeolites are desirable for industrial purposes due to their ability to accommodate bulky reactants and diffusion through channels. Although there are several extra-large pore zeolites reported, stable ones are rare. Thus, their stabilization is a feasible strategy for industrial applications. Here, an extra-large-pore zeolite EWT with boron substitution is presented, and the resulting zeolite B-RZM-3 increased the thermal stability from 600 °C in its silica form to 850 °C. The crystal structure, determined by combining continuous rotation electron diffraction (cRED) and powder X-ray diffraction (PXRD), shows that B atoms preferentially substitute the interrupted (HO)T(OT)3 (Q3 ) sites and are partially converted into 3-coordination to relax framework deformation upon heating. After Al-reinsertion post-treatment, Al-B-RZM-3 shows higher ethylbenzene selectivity and ethylene conversion rate per mol acid site than commercial ZSM-5 and Beta zeolite in benzene alkylation reaction. Synthesizing extra-large-pore zeolite in borosilicate form is a potential approach to stabilize interrupted zeolites for commercial applications.

SCM-25: A Zeolite with Ordered Meso-cavities Interconnected by 12 × 12 × 10-Ring Channels Determined by 3D Electron Diffraction.

Zeolites with large cavities that are accessible via wide pore windows are desirable but very rare. They have been dominantly used as catalysts in industry. Here we report a novel porous germanosilicate SCM-25, the zeolite structure containing ordered meso-cavities (29.9 × 7.6 × 6.0 Å3) interconnected by 10- and 12-ring channels. SCM-25 was synthesized as nanosized crystals by using a simple organic structure-directing agent (OSDA). Three-dimensional (3D) electron diffraction shows that SCM-25 crystallizes in the orthorhombic space group Cmmm with a = 14.62 Å, b = 51.82 Å, c = 13.11 Å, which is one of the zeolites with the largest unit cell dimensions. We demonstrate that 3D electron diffraction is a powerful technique for determining the complex structure of SCM-25, including the disorders and distributions of framework atoms silicon and germanium. SCM-25 has a high surface area (510 m2/g) and high thermal stability (700 °C). Furthermore, we propose a potential postsynthetic strategy for the preparation of zeolites with ordered meso-cavities by applying the ADOR (assembly-disassembly-organization-reassembly) approach.

Pressure-induced semiconductor-to-metal phase transition of a charge-ordered indium halide perovskite.

Phase transitions in halide perovskites triggered by external stimuli generate significantly different material properties, providing a great opportunity for broad applications. Here, we demonstrate an In-based, charge-ordered (In+/In3+) inorganic halide perovskite with the composition of Cs2In(I)In(III)Cl6 in which a pressure-driven semiconductor-to-metal phase transition exists. The single crystals, synthesized via a solid-state reaction method, crystallize in a distorted perovskite structure with space group I4/m with a = 17.2604(12) Å, c = 11.0113(16) Å if both the strong reflections and superstructures are considered. The supercell was further confirmed by rotation electron diffraction measurement. The pressure-induced semiconductor-to-metal phase transition was demonstrated by high-pressure Raman and absorbance spectroscopies and was consistent with theoretical modeling. This type of charge-ordered inorganic halide perovskite with a pressure-induced semiconductor-to-metal phase transition may inspire a range of potential applications.

Tailoring the Pore Surface of 3D Covalent Organic Frameworks via Post-Synthetic Click Chemistry.

Three-dimensional covalent organic frameworks (3D COFs) have gained increasing attention for their attractive features. However, the development of 3D COFs is strongly restricted, mainly due to their synthetic difficulty and complicated structure determination. Post-synthetic modification, which can avoid these problems by incorporating functional moieties into a predetermined framework, provides an alternative way to construct 3D COFs with specific functions. Herein, we report the designed synthesis and characterization of a series of highly crystalline 3D COFs with different loadings of ethynyl groups. Notably, these alkyne-tagged 3D COFs provide a platform for targeted anchoring various specific groups onto the pore walls via click reactions. Moreover, the pore surface engineering can accordingly change their properties, for example, the obtained click products exhibited higher CO2 /N2 selectivity. We describe a simple but powerful strategy to build functional 3D COFs, which will certainly advance them for a ranging of interesting applications in the future.

Pyrogallate-Based Metal-Organic Framework with a Two-Dimensional Secondary Building Unit.

We report a metal-organic framework (MOF) with a rare two-dimensional (2D) secondary building unit (SBU). The SBU comprises mixed-valent Fe2+ and Fe3+ metal ions bridged by oxygen atoms pertaining to the polytopic ligand 3,3',4,4',5,5'-hexahydroxybiphenyl, which also define the iron-oxide 2D layers. Overall, the anionic framework exhibits rare topology and evidences strong electronic communication between the mixed-valence iron sites. These results highlight the importance of dimensionality control of MOF SBUs for discovering new topologies in reticular chemistry, and especially for improving electronic communication within the MOF skeleton.

Design and Synthesis of a Zeolitic Organic Framework.

The development of novel zeolite-like materials with large channel windows and high stability is of importance but remains a tremendous challenge. Herein, we report the first example of a 3D covalent organic framework with zeolitic network, namely the zeolitic organic framework (ZOF). By combining two kinds of tetrahedral building blocks with fixed or relatively free bond angles, ZOF-1 with the zeolitic crb net has been successfully synthesized. Its structure was determined by the single-crystal 3D electron diffraction technique. Remarkably, ZOF-1 shows high chemical stability, large pore size (up to 16 Å), and excellent specific surface area (≈2785 m2 g-1 ), which is superior to its analogues with the same network, including traditional aluminosilicate zeolites and zeolitic imidazole frameworks. This study thus opens a new avenue to construct zeolite-like materials with pure organic frameworks and will promote their potential applications in adsorption and catalysis for macromolecules.

Tuning the Topology of Three-Dimensional Covalent Organic Frameworks via Steric Control: From pts to Unprecedented ljh.

Whether or not the topology of three-dimensional covalent organic frameworks (3D COFs) can be tuned via steric control remains a big question and has never been reported. Herein, we describe the designed synthesis of two highly crystalline 3D COFs (3D-TPB-COF-OMe and 3D-TPB-COF-Ph), through the polycondensation of tetra(p-aminophenyl)methane and methoxy- or phenyl- substituted 1,2,4,5-tetrakis(4-formylphenyl)benzene on the 3- and 6-positions. Amazingly, by using the continuous rotation electron diffraction technique, 3D-TPB-COF-OMe is determined to have a 5-fold interpenetrated structure with a reported pts net, while 3D-TPB-COF-Ph adopts an unprecedented self-penetrated ljh topology (ljh = Luojia Hill) that does not exist in the database of ToposPro. Therefore, by altering the substituents from methoxy to phenyl groups, the topology of designed 3D COFs changes accordingly, and a rare net is now available. This result clearly demonstrates that such COF structures need to be carefully determined due to its complexity, and moreover, it is promising to design 3D COFs with new topology for interesting application by increasing the steric hindrance of molecular building blocks.