Jinxi Chen

Synthesis, morphology control, and properties of porous metal–organic coordination polymers

Abstract A “direct mixing” synthesis strategy has been demonstrated for the first time that allows fast (e.g., 0.5 h) synthesis of bulk quantity of thermally stable and highly porous metal–organic coordination polymers (MOCP) nanocrystals (30–150 nm diameter) at room temperature with high yield (∼90%). The MOCP materials constructed from Zn ( NO 3 ) 2 · 6 H 2 O and 1,4-benzenedicarboxylic acid (H 2 BDC) were characterized with scanning electron microscope, powder X-ray diffraction, thermal gravimetric analysis, FT-IR, and volumetric Ar adsorption/desorption. The direct mixing method also produced a highly porous nanometer-sized MOCP material, which is likely to be a new phase that has not been discovered by the more commonly used “diffusion” approach. “Soft” and “hard” template approaches were used to successfully manipulate the morphology of MOCP materials at nanometer scale. In addition, water molecules were shown to play an important role in the synthesis and eventual composition of MOCP materials. Exposure of MOCP materials to water resulted in dramatic drop in surface area and porosity because of possible hydrolysis of the framework. An acid hydrolysis process of MOCP materials was also revealed in which the crystals could be hydrolyzed back into metal salts and organic acid under acid treatment.

Nanochannels of Two Distinct Cross-Sections in a Porous Al-Based Coordination Polymer

A new aluminum naphthalenedicarboxylate Al(OH)(1,4-NDC) x 2 H2O compound has been synthesized. The crystal structure exhibits a three-dimensional framework composed of infinite chains of corner-sharing octahedral Al(OH)2O4 with 1,4-naphthanedicarboxylate ligands forming two types of channels with squared-shape cross-section. The large channels present a cross-section of 7.7 x 7.7 A(2), while the small channels are about 3.0 x 3.0 A(2). When water molecules are removed, no structural transformation occurs, generating a robust structure with permanent porosity and remarkable thermal stability. 2D (1)H-(13)C heteronuclear correlation NMR measurements, together with the application of Lee-Goldburg homonuclear decoupling, were applied, for the first time, to porous coordination polymers revealing the spatial relationships between the (1)H and (13)C spin-active nuclei of the framework. To demonstrate the open pore structure and the easy accessibility of the nanochannels to the gas phase, highly sensitive hyperpolarized (HP) xenon NMR, under extreme xenon dilution, has been applied. Xenon can diffuse selectively into the large nanochannels, while the small ones show no substantial uptake of xenon due to severe restrictions along the channels that prevent the diffusion. Two-dimensional exchange experiments showed the exchange time to be as short as 15 ms. Through variable-temperature HP (129)Xe NMR experiments we were able to achieve an unprecedented description of the large nanochannel space and surface, a physisorption energy of 10 kJ mol(-1), and the chemical shift value of xenon probing the internal surfaces. The large pore channels are straight, parallel, and independent, allowing one-dimensional anisotropic diffusion of gases and vapors. Their walls are composed of the naphthalene moieties that create an unique environment for selective sorption. These results prompted us to measure the storage capacity toward methanol, acetone, benzene, and carbon dioxide. The selective adsorption of methanol and acetone vs that of water, together with the permanent porosity and high thermal stability, makes this compound a suitable matrix for separation and purification.

Structure modulation of manganese coordination polymers consisting of 1,4-naphthalene dicarboxylate and 1,10-phenanthroline.

Three new manganese coordination polymers, {[Mn2(1,4-NDC)2(phen)2](H2O)}n (1), [Mn2(1,4-NDC)2(phen)(H2O)]n (2) and {[Mn4(1,4-NDC)4(phen)4](DMF)2}n (3) (1,4-H2NDC = 1,4-naphthalene dicarboxylic acid; phen = 1,10-phenanthroline; DMF = N,N-dimethylformamide), have been synthesized solvo/hydrothermally. 1,4-NDC(2-) ligands adopt different coordination modes under different solvents and concentrations which promotes different crystal structure formation. X-ray crystal structural data reveal that compounds 1, 2 and 3 crystallize in monoclinic space groups C2/c, P21/c and C2/c, respectively. Compound 1 has Mn2 dimers connected by 1,4-NDC(2-) linkers, packing into a 2D structure in a grid pattern. Compound 2 has a three-dimensional (3D) structure which is constructed by Mn2 dimers and 1,4-NDC(2-) linkers. Each MnO4N2 node of compound 3 is linked to another by 1,4-NDC(2-) ligands to form a two-dimensional (2D) structure. Variable-temperature magnetic susceptibilities of compounds 1-3 exhibit overall weak antiferromagnetic coupling between the adjacent Mn(II) ions.

Near-infrared light-induced dissociation of zeolitic imidazole framework-8 (ZIF-8) with encapsulated CuS nanoparticles and their application as a therapeutic nanoplatform

Incorporation of CuS nanoparticles into the framework of ZIF-8 provides a chance to integrate near-infrared (NIR) light/low pH triggered release and chemo-photothermal therapy into one system. For the first time, we observed that the framework of ZIF-8 could be disintegrated at pH 7.4 under NIR laser irradiation.

Surfactant-assisted synthesis of ZIF-8 nanocrystals in aqueous solution via microwave irradiation

Nanocrystals of ZIF-8 have been successfully prepared in the presence of non-ionic triblock copolymers P123 and F127 under microwave irradiation. Noticeably, the reaction time could be significantly reduced down to 1 min. The obtained ZIF-8 nanocrystals had sizes of sub-104 nm and exhibited a high BET surface area of 1599 m2 g−1. This approach is expected to be an efficient and environmentally friendly method for the preparation of ZIF-8.

Two 2-D layered coordination polymers based on 5-aminoisophthalate and 1,10-phenanthroline

Two new compounds, [Zn(aip)(phen)] n (1) and [Mn(aip)(phen)] n (2) (H2aip = 5-aminoisophthalic acid, phen = 1,10-phenanthroline), have been synthesized by solvothermal methods and structurally characterized. X-ray diffraction analyses indicate that 1 and 2 have a 2-D layer structure, with aip2− adopting two coordination motifs. The coordination configuration of the metal plays a crucial role in formation of different topological structures. Thermogravimetric analyses of 1 and 2 show considerable thermal stability. The fluorescence of 1 and 2 in the solid state has also been investigated.

Downsizing metal–organic frameworks with distinct morphologies as cathode materials for high-capacity Li–O2 batteries

Rechargeable nonaqueous Li–O2 batteries have been considered as one of the most promising candidate energy storage devices. In this work, metal–organic framework nanomaterials with distinct sizes and morphologies were successfully synthesized via a facile solvothermal method. By using modulators in a mixed solvent system, the dimension of Co-MOF-74 could be reduced down to several unit cell lengths. Furthermore, high specific capacities (11 350 mA h g−1 at 100 mA g−1) were achieved when they were directly employed as cathode materials for Li–O2 batteries. In addition to the high density of unsaturated active sites for electrochemistry, as provided by the MOF skeleton itself, the size confinement and inherent defects of the nanocrystals have further offered efficient diffusion paths with lowered transport barriers, contributing to their high performance in Li–O2 batteries.

Acid-induced Zn(II)-based metal–organic frameworks for encapsulation and sensitization of lanthanide cations

Two Zn(II)-based metal–organic frameworks, namely, [Zn3(Hbptc)2(DMF)2]·2DMF(1) and [Zn5(bptc)3(H2O)]((CH3)2NH2)2(2), (H4bptc = 3,3′,5,5′-biphenyltetracarboxylic acid, DMF = N,N′-dimethylformamide), have been synthesized using different amounts of nitric acid under mixed-solvothermal conditions. 1 and 2 display different coordination geometries and donor ligands for the Zn2+ ions. In 1, only three carboxylic acid groups of H4bptc take part in the construction of the three-dimensional (3-D) framework with Zn2+, and one remains uncoordinated that can bind to other metal ions. Postsynthetic exchange of 1 with Ln3+ cations demonstrates that 1 can effectively capture metal cations and sensitize the visible-emitting Ln3+.

Rod-shaped thiocyanate-induced abnormal band gap broadening in SCN− doped CsPbBr3 perovskite nanocrystals

In this work, the pseudohalide thiocyanate has been demonstrated as a promising alternative to the halide anion to engineer optoelectronic properties of inorganic/organic hybrid perovskites because it exhibits better chemical stability than the halide anion. Previous reports have suggested that the ionic radii and electronegativity of SCN− is close to that of I−; the SCN− doped CH3NH3PbI3 exhibited similar optical properties as pure CH3NH3PbI3. Consequently, it was expected that doping of CsPbBr3 perovskite with SCN− would result in band gap narrowing. Interestingly, the photoluminescent all-inorganic CsPbBr3 perovskite nanocrystals exhibit an abnormal blue shift in optical properties and improvement of the crystallinity when successfully doped by SCN−. Combined experimental and theoretical investigations revealed that doping of the CsPbBr3 perovskite with the rod-like SCN− anion introduced disorder in the crystal lattice, leading to its expansion, and impacted the electronic structure of the perovskite with band gap broadening.

A Stable Plasmonic Cu@Cu2O/ZnO Heterojunction for Enhanced Photocatalytic Hydrogen Generation

The localized surface plasmon resonance (LSPR) effect has been widely utilized in photocatalysis, but most reported LSPR materials are based on noble metals of gold or silver with high chemical stability. Plasmonic copper nanoparticles that exhibit an LSPR absorbance at 600 nm are promising for many applications, such as photocatalysis. Unfortunately, plasmonic copper nanoparticles are affected by serious surface oxidation in air. Herein, a novel lollipop-shaped Cu@Cu2 O/ZnO heterojunction nanostructure was designed, for the first time, to stabilize the plasmonic Cu core by decorating Cu@Cu2 O core-shell structures with ZnO nanorods. This Cu@Cu2 O/ZnO nanostructure exhibited significantly enhanced stability than that of regular Cu@Cu2 O, which accounted for the remarkably enhanced photocatalytic H2 evolution rate through water splitting, relative to pristine ZnO nanorods, over an extended wavelength range due to the plasmonic Cu core.