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A visual ratiometric fluorescent sensor based on blue carbon dot/Rhodamine B is used to selectively detect NO2− in the environment.

Poly(m-phenylenediamine) is an excellent adsorbent material. Nevertheless, it is difficult to recover and can even generate secondary pollution due to its powder-like form. Aerogels, monolithic three-dimensional structured materials, that are ultralight and have porous properties are efficient at adsorbing contaminants from water and can solve these problems. Here, we synthesized an aerogel based on molecular-modified poly(m-phenylenediamine) (mPmPD) crosslinking with polyvinyl alcohol (PVA) and graphene oxide (GO) (GO/mPmPD/PVA). Of note is that 3-aminophenylboronic acid was introduced into the polymer structure to induce a crosslinking reaction between boric acid units and PVA to constrain poly(m-phenylenediamine) in the aerogel. The GO/mPmPD/PVA aerogel shows stable mechanical properties in aqueous solution and an effective adsorption capacity for Ag(I); the maximum Ag(I) adsorption capacity is 917.41 mg g−1. The mechanism of Ag(I) adsorption and reduction was clarified in that Ag(I) chelated with imine units, and a redox reaction occured between Ag(I) and the benzenoid amine units. Furthermore, the GO/mPmPD/PVA aerogel also shows excellent adsorption ability toward methyl orange and Congo red dyes. This GO/mPmPD/PVA aerogel shows enormous potential for application to silver recovery and dye removal due to its low cost, effective adsorption capacity and facile separation with aqueous solution.

Hexavalent chromium Cr(VI), one of the most toxic contaminants, is released in the environment due to various anthropogenic activities. This study presents a novel sandwiched nanocomposite synthesized using graphene oxide (GO), manganese dioxide (MnO2) nanowires, iron oxide (Fe3O4) nanoparticles and polypyrrole (PPy) to remove hexavalent chromium ion Cr(VI) from water by an adsorption–reduction mechanism. In the sandwiched nanocomposites, GO provided enough surface area, functional groups, and hydrophilic surface for efficient absorption. Fe3O4 nanoparticles with excellent magnetic properties make it easy to separate and recover from water. Under acidic conditions, MnO2 nanowires act as both template and oxidant to initiate the polymerization of pyrrole monomers on its freshly activated surface to obtain GO/MnO2/Fe3O4/PPy (designated as GMFP) nanocomposite. GMFP could effectively adsorb Cr(VI) through electrostatic attraction, and the adsorbed Cr(VI) ions were partly reduced to trivalent chromium Cr(III) (62%), resulting in the efficient adsorption and high removal of Cr(VI) from water. Hexavalent chromium adsorption by GMFP is strongly pH dependent and the adsorption kinetics followed the pseudo-second-order model. The Langmuir isothermal model described the adsorption isotherm data well and the maximum adsorption capacity was up to 374.53 mg g−1 at pH 2.0. These experimental results suggested that GMFP had great potential as an economic and efficient adsorbent of hexavalent chromium from wastewater, which has huge application potential.

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Non-precious metal-based catalysts with low cost and rich reserves are emerging as promising alternatives for Pt-based catalysts for the hydrogen evolution reaction (HER). Herein, we reported a facile and large-scale strategy for Co nanocrystals embedded in a nitrogen-doped carbon matrix through direct carbonization of Co-based metal–organic frameworks (MOFs) in N2. After carbonization, the ultrafine Co nanocrystals are well dispersed in the nitrogen-doped carbon matrix. We further studied their electrocatalytic properties toward the HER in acidic media and found that the as-prepared catalyst exhibited enhanced catalytic activity and stability.

Highly porous Co3O4 nanobelts were successfully synthesized by using a hydrothermal technique, followed by calcination of the Co(OH)2 precursor. The as-prepared Co3O4 nanobelts were analyzed by scanning electron microscopy, X-ray power diffraction, transmission electron microscopy, and Brunauer–Emmett–Teller methods. The electrochemical properties of porous Co3O4 nanobelts were examined by cyclic voltammetry and galvanostatic charge–discharge studies. Owning to the unique 2D structural features, the Co3O4 nanobelts exhibited a high specific capacity of 857 after 60 cycles at a current density of 100 mA g−1, and good cycling stability. These exceptional electrochemical performances could be attributed to the remarkable structural feature with a high surface area and large amounts of nanopores within the surface of nanobelts, which can provide large contact areas between electrolyte and active materials for electrolyte diffusion, improve structural stability and buffer volume expansion during the Li+ insertion/extraction processes.

In this article, we report a facile and scalable route for the fabrication of Mn2O3 hollow microspheres by direct pyrolysis of Mn-based metal–organic frameworks at 450 °C with a heating rate of 10 °C min−1 in air. The effect of the heating rate on the morphology of the final samples has been also investigated. When evaluated as an anode material for LIBs, these Mn2O3 microspheres exhibited a reversible and stable capacity of 582 mA h g−1 after 60 cycles at a current density of 100 mA g−1. The improved capacity and excellent cycling stability of the as-prepared Mn2O3 microspheres could be attributed to the porous hollow structures, which can reduce diffusion length for lithium ions and electrons, and also can enhance structural integrity for buffering the volume expansion during the discharge/charge processes.