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A metal organic framework (MOF), synthesized from cobalt salt, melamine (mela), and 1,4-dicarboxybezene (BDC), was used as precursor to prepare Co/CoN/N-CNT/C electrocatalyst via heat treatment at different temperature (700-900 °C) under nitrogen atmosphere. Crystallites size and microstrain in the 800 °C heat-treated sample (MOFs-800) were the lowest, whereas the stacking fault value was the highest among the rest of the homemade samples, as attested to by the Williamson-Hall analysis, hence assessing that the structural or/and surface modification of Co nanoparticles (NPs), found in MOFs-800, was different from that in other samples. CNTs in MOFs-800, interacting with Co NPs, were formed on the surface of the support, keeping the hexagonal shape of the initial MOF. Among the three homemade samples, the MOF-800 sample, with the best electrocatalytic performance toward oxygen reduction reaction (ORR) in 0.1 M KOH solution, showed the highest density of CNTs skin on the support, the lowest I/I ratio, and the largest N atomic content in form of pyridinic-N, CoN, pyrrolic-N, graphitic-N, and oxidized-N species. Based on the binding energy shift toward lower energies, a strong interaction between the active site and the support was identified for MOFs-800 sample. The number of electron transfer was 3.8 on MOFs-800, close to the value of 4.0 determined on the Pt/C benchmark, thus implying a fast and efficient multielectron reduction of molecular oxygen on CoN active sites. In addition, the chronoamperometric response within 24 000 s showed a more stable current density at 0.69 V/RHE on MOFs-800 as compared with that of Pt/C.

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The development of highly active, sensitive and durable gas sensing materials for the detection of volatile organic compounds (VOCs) is extremely desirable in gas sensors. Herein, a series of mesoporous hierarchical Co3O4-TiO2 p-n heterojunctions have been prepared for the first time via the facile thermal conversion of hierarchical CoTi layered double hydroxides (CoTi-LDH) precursors at 300-400 ºC. The resulting Co3O4-TiO2 nanocomposites showed superior sensing performance towards toluene and xylene in comparison with Co3O4 and TiO2 at low temperature, and the sample with a Co/Ti molar ratio of 4 shows optimal response (Rg/Ra = 113, Rg and Ra denote the sensor resistance in a target gas and in air, respectively) to 50 ppm xylene at 115 ºC. The ultrahigh sensing activity of theses Co3O4-TiO2 p-n heterojunctions originates from their hierarchical structure, high specific surface area (>120 m 2 g −1 ), and the formation of numerous p-n heterojunctions, which results in full exposure of active sites, easy adsorption of oxygen and target gases, and large modulation of resistance. Importantly, hierarchical Co3O4-TiO2 heterojunctions possess advantages of simple preparation, structural stability, good selectivity and long-term durability. Therefore, this work provides a facile approach for the preparation of hierarchical Co3O4-TiO2 p-n heterojunctions with excellent activity, sensitivity and durability, which can be used as a promising material for the development of high-performance gas sensors.

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Layered double hydroxides (LDH) has wide applications as non-toxic and halogen-free flame retardant for various resins and highly efficient thermal stabilizer for halogen-containing polymers. This review will discuss some public patents and relevant papers on the flame retardancy and the thermal stability of LDH/polymer composites when the LDHs with different chemical compositions are used as the additive in the polymer matrix. We have summarized these related LDHs in two tables: one for flame retardant and the other for thermal stabilizer.

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