Size and morphology controlled NiSe nanoparticles as efficient catalyst for the reduction reactions

Mechanochemical Synthesis of Nickel Mono- and Diselenide: Characterization and Electrical and Optical Properties

Nickel mono- (NiSe) and diselenide (NiSe₂) were produced from stoichiometric mixtures of powdered Ni and Se precursors by the one-step, undemanding mechanochemical reactions. The process was carried out by high-energy milling for 30 and 120 min in a planetary ball mill. The kinetics of the reactions were documented, and the products were studied in terms of their crystal structure, morphology, electrical, and optical properties. X-ray powder diffraction confirmed that NiSe has hexagonal and NiSe₂ cubic crystal structure with an average crystallite size of 10.5 nm for NiSe and 13.3 nm for NiSe₂. Their physical properties were characterized by the specific surface area measurements and particle size distribution analysis. Transmission electron microscopy showed that the prepared materials contain nanoparticles of irregular shape, which are agglomerated into clusters of about 1-2 μm in diameter. The first original values of electrical conductivity, resistivity, and sheet resistance of nickel selenides synthesized by milling were measured. The obtained bandgap energy values determined using UV-Vis spectroscopy confirmed their potential use in photovoltaics. Photoluminescence spectroscopy revealed weak luminescence activity of the materials. Such synthesis of nickel selenides can easily be carried out on a large scale by milling in an industrial mill, as was verified earlier for copper selenide synthesis.

Highly efficient degradation of metronidazole drug using CaFe2O4/PNA nanohybrids as metal-organic catalysts under microwave irradiation

Catalytic degradation based on microwave irradiation is an emerging technique which promises prompt and efficient catalytic degradation of organic pollutants. Calcium ferrite (CaFe₂O₄), poly(1-napththylamine) (PNA), and PNA/CaFe₂O₄ nanohybrids were synthesized via microwave-assisted technique. The properties of the as-prepared CaFe₂O₄, PNA, and PNA/CaFe₂O₄ nanohybrids were characterized by the thermogravimetric analysis (TGA), FTIR, XRD, SEM, and ultraviolet-visible spectrophotometry (UV-vis) analyses. The formation of inorganic-organic hybrids was confirmed by the FTIR and XRD studies. Loading of PNA was confirmed to be 8%, 16%, 32%, and 40% in CaFe₂O₄ which was established by TGA studies and the thermal stability was found to follow the order: CaFe₂O₄ > 8-PNA/CaFe₂O₄ > 16-PNA/CaFe₂O₄ > 32-PNA/CaFe₂O₄ > 40-PNA/CaFe₂O₄ > PNA. CaFe₂O₄ and PNA revealed band gap values of 3.42 eV and 2.60 eV respectively while for the PNA/CaFe₂O₄ nanohybrids, the values were found to be ranging between 2.46 and 3.00 eV. The PNA modified CaFe₂O₄ nanohybrids showed higher degradation efficiency towards metronidazole (MTZ) drug as compared with PNA and pure CaFe₂O₄. MTZ drug showed around 94% degradation within 21 min of microwave irradiation using 40-PNA/CaFe₂O₄ as catalyst. The enhanced catalytic activity was attributed to the high surface area of the nanohybrid catalyst as well as improved microwave catalytic activity of PNA. The reactive species responsible for degradation were confirmed by scavenger studies which formation of ^·OH and O₂^·- radicals. Recyclability tests showed that the 40-PNA/CaFe₂O₄ nanohybrid exhibited 86% degradation of MTZ (90 mg/l) even after the third cycle, which reflected higher reusability of the catalyst. The MTZ fragments were identified using liquid chromatography-mass spectrometry (LC-MS).

Controllable Synthesis of Ni xSe (0.5 ≤ x ≤ 1) Nanocrystals for Efficient Rechargeable Zinc-Air Batteries and Water Splitting

The development of earth-abundant, highly active, and corrosion-resistant electrocatalysts to promote the oxygen reduction reaction (ORR) and oxygen and hydrogen evolution reactions (OER/HER) for rechargeable metal-air batteries and water-splitting devices is urgently needed. In this work, Ni _xSe (0.5 ≤ x ≤ 1) nanocrystals with different crystal structures and compositions have been controllably synthesized and investigated as potential electrocatalysts for multifunctional ORR, OER, and HER in alkaline conditions. A novel hot-injection process at ambient pressure was developed to control the phase and composition of a series of Ni _xSe by simply adjusting the added molar ratio of the nickel resource to triethylenetetramine. Electrochemical analysis reveals that Ni_0.5Se nanocrystalline exhibits superior OER activity compared to its counterparts and is comparable to RuO₂ in terms of the low overpotential required to reach a current density of 10 mA cm^-2 (330 mV), which may benefit from the pyrite-type crystal structure and Se enrichment in Ni_0.5Se. For the ORR and HER, Ni_0.75Se nanoparticles achieve the best performance including lower overpotentials and larger apparent current densities. Further investigations demonstrate that Ni_0.75Se could not only provide an enhanced electrochemical active area but also facilitate electron transfer during the electrocatalytic process, thus contributing to the remarkable catalytic activity. As a practical application, the Ni_0.75Se electrode enables rechargeable Zn-air battery with a considerable performance including a long cycling lifetime (200 cycles), high specific capacity (609 mA h g^-1 based on the consumed Zn), and low overpotential (0.75 V) at 10 mA cm^-2. Meanwhile, the water-splitting cell setup with an anode of Ni_0.5Se for the HER and a cathode of Ni_0.75Se for the OER exhibits a considerable performance with low decay in activity of 12.9% under continuous polarization for 10 h. These results suggest the promising potential of nickel selenide nanocrystals as earth-abundant and high-performance electrocatalysts for metal-air batteries and alkaline water splitting.