One-Step Synthesis of Hollow CoS2 Spheres Derived from Polyoxometalate-Based Metal-Organic Frameworks with Peroxidase-like Activity.

In this work, hollow CoS2 particles were prepared by a one-step sulfurization strategy using polyoxometalate-based metal-organic frameworks as the precursor. The morphology and structure of CoS2 have been monitored by scanning electron microscopy, X-ray photoelectron spectroscopy, and X-ray powder diffraction. The mechanism for the formation of CoS2 is discussed. The reaction time and sulfur content are found to be important factors that affect the morphology and pure phase formation of CoS2, and a hollow semioctahedral morphology of CoS2 with open voids was obtained when the sulfur source was twice as large as the precursor and the reaction time was 24 h. The CoS2 (24 h) particles show an excellent peroxidase-like activity for the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to blue oxidized (oxTMB) by hydrogen peroxide. The polyoxometalate used as a precursor helps to stabilize oxTMB during catalytic oxidation, forming a stable curve platform for at least 8 min. Additionally, the colorimetric detection of hydroquinone is developed with a low detection limit of 0.42 µM. This research provides a new strategy to design hollow materials with high peroxidase-mimicking activity.

Synthesis of a Polyoxometalate-Encapsulated Metal-Organic Framework via In Situ Ligand Transformation Showing Highly Catalytic Activity in Both Hydrogen Evolution and Dye Degradation.

In situ molecular transformation under hydrothermal conditions is a feasible method to introduce distinct organic ligands and suppress competitive reactions between different synthons. However, this strategy has not yet been explored for the preparation of polyoxometalate (POM)-encapsulated metal-organic frameworks (MOFs). In this work, we designed and prepared a new compound, [Co2(3,3'-bpy)(3,5'-bpp)(4,3'-bpy)](H2O)3[SiW12O40] (1) (4,3'-bpy = 4,3'-dipyridine, 3,5'-bpp = 3,5'-bis(pyrid-4-yl)pyridine, and 3,3'-bpy = 3,3'-bis(pyrid-4-yl) dipyridine), via an in situ ligand synthesis route. The compound shows a novel POM-encapsulated MOF structure with two pairs of left- and right-handed double helixes. These left- and right-handed helical chains further lead to triangular and rhombus-like channels, respectively. Moreover, the as-synthesized title compound shows superior electrocatalytic activity toward the hydrogen evolution reaction (HER) in 1 M KOH aqueous solution with a low overpotential and Tafel slope of 92 mV and 92.1 mV dec-1, respectively, under a current density of 10 cm-2. Also, the compound exhibits a high activity for the photocatalytic degradation of the dye rhodamine B. The excellent performance of the compound may be attributed to the synergistic effect between W and Co elements and the presence of encapsulated POMs. The title compound proves that it is possible to prepare multifunctional MOFs with POMs and transition metals showing HER activity and dye degradation activity.

Two Polyoxometalate-Encapsulated Two-Fold Interpenetrating dia Metal-Organic Frameworks for the Detection, Discrimination, and Degradation of Phenolic Pollutants.

Phenols are widely used for commercial production, while they pose a hazard to the environment and human health. Thus, investigation of convenient and efficient methods for the detection, discrimination, and degradation of phenols becomes particularly important. Herein, two new polyoxometalate (POM)-based compounds, [Co2(btap)4(H2O)4][SiW12O40] (Co-POM) and [Ni2(btap)4(H2O)4][SiW12O40] (Ni-POM) (btap = 3,5-bis(triazol-1-yl)pyridine), are prepared via a hydrothermal synthesis method. The compounds show a fascinating structural feature of a POM-encapsulated twofold interpenetrating dia metal-organic framework. More importantly, besides the novel structures, the compound Co-POM realizes three functions, namely, the simultaneous detection, discrimination, and degradation of phenols. Specifically, Co-POM shows an excellent colorimetric detection performance toward phenol with a detection limit (LOD) ca. 1.32 µM, which is lower than most reported colorimetric detectors for phenol. Also, a new colorimetric sensor system based on Co-POM can discriminate phenol, 4-chlorophenol, and o-cresol with ease. Further, Co-POM exhibits a photocatalytic degradation property for 4-chlorophenol under irradiation of visible light with the highest degradation rate at 62% after irradiation for 5 h. Therefore, this work provides the first example of a POMs-based multifunctional material for achieving the detection, discrimination, and degradation of phenolic pollutants.

pH-Controlled Assembly of Five New Organophosphorus Strandberg-Type Cluster-Based Coordination Polymers for Enhanced Electrochemical Capacitor Performance.

Cluster-based coordination polymers (CCPs) have shown promise as capacitors. To investigate the relativity between capacitor performance and crystal structure, herein, five new CCPs based on organophosphorus Strandberg-type clusters were synthesized via in situ hydrothermal reactions at different pH's, namely, (H2bipy)2[(C6H5PO3)2Mo5O15]·2H2O (1), (H2bipy)1.5[CuI(bipy)(C6H5PO3)2Mo5O15]·H2O (2), H2[CuI2(bipy)2.5(C6H5PO3)2Mo5O15]·2H2O (3), Na2[CuI4CuII(bipy)4(C6H5PO3)2(Mo5O15)2]·15H2O (4), [CuII2(bipy)(H2O)4(C6H5PO3)2Mo5O15] (5) (bipy = 4,4'-bipyridine). Compound 1 is a zero-dimensional monomer, in which the protonated bipy ligands as countercations combine Strandberg-type clusters by hydrogen bonding and π-π interaction forming a supramolecular layer. Compound 2 represents a unique one-dimensional (1D) channel chain structure linked by intermolecular hydrogen bonding and π-π interaction. Compounds 3 and 4 exhibit the first example of an interdigitated architecture based on organophosphorus Strandberg-type clusters [1D + 1D → two-dimensional (2D) for 3 and 2D + 2D → three-dimensional (3D) for 4]. Compound 5 displays a novel (3,4)-connected 3D microporous framework with (81·62) (83·63) topology. Notably, the more complicated structures of compounds 1-5 were obtained with an increase in pH. The isolation of five compounds is beneficial for our systematic understanding of the effect of pH on the assembly of CCPs. Organophosphorus Strandberg-type polyoxometalate clusters were explored as supercapacitor electrode materials for the first time. Compared with other CCPs in this work, compound 5 shows the highest specific capacitance, 160.9 F g-1, at a current density of 2 A g-1, and for favorable cycling stability, after 1000 cycles, the retention rate of the capacitance is 95.6% at 10 A g-1.

A non-enzymatic voltammetric xanthine sensor based on the use of platinum nanoparticles loaded with a metal-organic framework of type MIL-101(Cr). Application to simultaneous detection of dopamine, uric acid, xanthine and hypoxanthine.

A Cr-based metal-organic framework MIL-101(Cr) was used to load platinum nanoparticles (PtNPs) that were placed on a glassy carbon electrode (GCE). The modified GCE was used as a non-enzymatic xanthine sensor. Compared to bare GCE, it requires a strongly decreased working potential and an increased signal current for xanthine oxidation. This is due to the crystalline ordered structure and large specific surface of the MIL-101(Cr), and to the high conductivity of the Pt NPs. Differential pulse voltammetry (DPV) shows the sensor to have a wide linear range (0.5 - 162 µM), a low detection limit (0.42 µM), and high selectivity. It was applied to the simultaneous determination of dopamine, uric acid, xanthine and hypoxanthine at working potentials of 0.13, 0.28, 0.68 and 1.05 V, respectively (vs. Ag/AgCl) and to quantify xanthine in spiked serum samples. Graphical abstract This is the first report of non-enzymatic xanthine electrochemical sensor based on metal-organic framework loaded with nanoparticles.

Nitrogen doped 1 T/2H mixed phase MoS2/CuS heterostructure nanosheets for enhanced peroxidase activity.

Heteroatom doping and phase engineering are effective ways to promote the catalytic activity of nanoenzymes. Nitrogen-doped 1 T/2H mixed phase MoS2/CuS heterostructure nanosheets N-1 T/2H-MoS2/CuS are prepared by a simple hydrothermal approach using polyoxometalate (POM)-based metal-organic frameworks (MOFs) (NENU-5) as a precursor and urea as nitrogen doping reagent. The XPS spectroscopy (XPS) and Raman spectrum of N-1 T/2H-MoS2/CuS prove the successful N-doping. NENU-5 was used as the template to prepare 1 T/2H-MoS2/CuS with high content of 1 T phase by optimizing the reaction time. The use of urea as nitrogen dopant added to 1 T/2H-MoS2/CuS, resulted in N-1 T/2H-MoS2/CuS with an increase in the content of the 1 T phase from 80 % to 84 % and higher number of defects. N-1 T/2H-MoS2/CuS shows higher peroxidase activity than 1 T/2H-MoS2/CuS and a catalytic efficiency (Kcat/Km) for H2O2 twice as high as that of 1 T/2H-MoS2/CuS. The enhanced catalytic activity has probably been attributed to several reasons: (i) the insertion of urea during the hydrothermal process in the S-Mo-S layer of MoS2, causing an increase in the interlayer spacing and in 1 T phase content, (ii) the replacement of S atoms in MoS2 by N atoms from the urea decomposition, resulting in more defects and more active sites. As far as we know, N-1 T/2H-MoS2/CuS nanosheets have the lowest detection limit (0.16 µm) for the colorimetric detection of hydroquinone among molybdenum disulfide-based catalysts. This study affords a new approach for the fabrication of high-performance nanoenzyme catalysts.

In situ hierarchical encapsulation of bimetallic selenides into honeycomb-like nitrogen doped porous carbon nanosheets for highly sensitive and selective guanosine detection.

An innovative electrochemical nanocomposite for the detection of guanosine (Gua) was proposed by in situ encapsulation of nickel-iron bimetallic selenides confined into honeycomb-like nitrogen doped porous carbon nanosheets, denoted as (Ni,Fe)Se2/N-PCNs. The porous carbon nanosheets were prepared by utilizing nickel-iron layered double hydroxide (Ni-Fe LDH) as the substrate and zeolitic imidazolate frameworks (ZIF-67) nanocrystals as the sacrificial templates via hydrothermal synthesis, followed by a process of acid etching and pyrolysis selenylation. Interestingly, the nickel-ferric bimetallic selenides material (Ni,Fe)Se2, is rarely fabricated successfully using selenylation treatment, which is a highly conductive and robust support to promote the electron transport. Meanwhile, the obtained (Ni,Fe)Se2/N-PCNs have the favorable architectural features of both unique three-dimensional (3D) porous structural and hierarchical connectivity, which are expected to provide more active sites for electrochemical reactions and ease of electron, ion, and biomolecule penetration. Benefiting from the inherent virtues of its composition, together with unique structural advantages, the (Ni,Fe)Se2/N-PCNs possess ideal sensing properties for guanosine detection with a low detection limit of 1.20 × 10-8 M, a wide linear range of 5.30 × 10-8 ~ 2.27 × 10-4 M and a good stability. Superb selectivity for potential interfering species and superb recoveries in serum suggests its feasibility for practical applications.

Structure-designed synthesis of hollow/porous cobalt sulfide/phosphide based materials for optimizing supercapacitor storage properties and hydrogen evolution reaction.

Cobalt-based transition metal phosphides/sulfides have been viewed as promising candidates for supercapacitor (SCs) and hydrogen evolution reaction (HER) featured with their intrinsic merits. Nevertheless, the sluggish reaction kinetics and drastic volume expansion upon electrochemical process hinder their commercial application. In this work, the hollow/porous cobalt sulfide/phosphide based nanocuboids (C-CoP4 and CoS2 HNs) with superior specific surface area are achieved by employing a novel chemical etching-phosphatization/sulfuration strategy. The hollow/porous structure could offer rich active sites and shorten electrons/ions diffusion length. In virtue of their structural advantage, the obtained C-CoP4 and CoS2 HNs perform superior specific capacitance, fast charge/discharge rate and beneficial cycling stability. The advanced asymmetrical supercapacitors assembled by C-CoP4 and CoS2 HNs deliver exceptional energy density, respectively. Furthermore, when employed as hydrogen evolution reaction electrocatalysts, C-CoP4 and CoS2 HNs yield favorable electrocatalytic activity. These findings shed fundamental insight on the design of dual-functional transition metal phosphide/sulfide based materials for optimizing hydrogen evolution reaction and supercapacitor storage properties.

Bimetallic FeNi-MIL-88-derived NiFe2O4@Ni-Mn LDH composite electrode material for a high performance asymmetric supercapacitor.

A simple continuous hydrothermal method was used to synthesize a NiFe2O4@Ni-Mn LDH/NF composite. The layered structure provides a large void to transfer the electron effectively, and the composite materials exhibit remarkable electrochemical performance including excellent specific capacitance (1265 F g-1 at 1 A g-1) and remarkable cycling stability (the specific capacitance remains at 80.9% after 5000 cycles). In addition, the asymmetric supercapacitor exhibits a high energy density of 96.2 W h kg-1 at a power density of 700 W kg-1, and there is an extraordinarily good cycling stability with a capacity retention rate of 92.5% after 4000 cycles. The outcomes indicate that the NiFe2O4@Ni-Mn LDH/NF composite electrode has potential application as a high-performance supercapacitor.

Rational regulation of transition metals in polyoxometalate hybrids without noble metal assistance for efficient light-driven H2 production.

Four new rationally designed polyoxometalate (POM)-based hybrids are reported with a maximum photocatalytic hydrogen evolution rate of 12245.59 µmol g-1 h-1, which outperform all known POM-based metal-organic photocatalysts.

Mo-Based crystal POMOFs with a high electrochemical capacitor performance.

Mo-Based crystalline polyoxometalate-based metal-organic frameworks (POMOFs), namely, [CuIH2(C12H12N6)(PMo12O40)]·[(C6H15N)(H2O)2] (1) and [Cu(C12H12N6)4(PMoMoO39)] (2) (C12H12N6, 1,4-bis(triazol-1-ylmethyl) benzene, abbreviation btx) as promising capacitor electrode materials were synthesized by a hydrothermal reaction. Compound 1 consisted of two-dimensional (2D) lattice structures with free triethylamine (abbreviation, TEA) molecules and H2O molecules, and compound 2 showed a 3D host-guest structure, in which 1D polyoxometalate (POM) chains were encapsulated into a 3D Cu(ii)-btx metal-organic framework (MOF). The compound 1-based electrode showed much higher specific capacitance (249.0 F g-1 at 3 A g-1) than the 2-based one (154.5 F g-1 at 3 A g-1). Moreover, the specific capacitance of the 1-based electrode was not only higher than those of the majority of the reported POMOF materials as supercapacitors, but also higher than those of most state-of-the-art MOF-based and POM-based supercapacitor electrode materials. This superior capacitance performance of the 1-based electrode could be attributed to the high redox capacity and excellent electronic conductivity. More importantly, this work may open a new avenue for optimizing the performance of POMOF-based capacitor electrode materials.

Enhanced visible-light photocatalytic activity and photostability of Ag3PO4/Bi2WO6 heterostructures toward organic pollutant degradation and plasmonic Z-scheme mechanism.

Novel Ag3PO4/Bi2WO6 heterostructured materials with enhanced visible-light catalytic performance were successfully synthesized by assembly combined with a hydrothermal treatment. The microstructures, morphologies, and optical properties of the prepared samples were characterized by multiple techniques. The irregular Ag3PO4 nanospheres dispersed on the surface of Bi2WO6 nanoflakes, and their catalytic performances were evaluated via the degradation of organic pollutants including rhodamine B (RB), methylene blue (MB), crystal violet (CV), methyl orange (MO), and phenol (Phen) under visible-light irradiation. The resulting Ag3PO4/Bi2WO6 heterostructured materials displayed higher photocatalytic activity than that of either pure Bi2WO6 or Ag3PO4. The enhanced photocatalytic activity was due to the good formation of heterostructures, which could not only broaden the spectral response range to visible light but also effectively promoted the charge separation. Meanwhile, the reasonable photoreactive plasmonic Z-scheme mechanism was carefully investigated on the basic of the reactive species scavenging tests, photoelectrochemical experiments, and photoluminescence (PL) spectrum. In addition, the excellent photostability of Ag3PO4/Bi2WO6 was obtained, which Ag formed at the early photocatalytic reaction acted as the charge transmission-bridge to restrain the further photoreduction of Ag3PO4.