Effect of High-Anisotropic Co2+ Substitution for Ni2+ on the Structural, Magnetic, and Magnetostrictive Properties of NiFe2O4: Implications for Sensor Applications

Enhanced Electrocatalytic Activity of Ecofriendly and Earth-Abundant (Zn,Cu)Fe2O4 + CuO Nanocomposites for Water Splitting

The projection of sustainable, low-cost, and environmentally friendly energy technologies demands innovation of electrocatalysts utilizing earth-abundant materials. The current study aims to improve the catalytic activity of spinel zinc ferrite (ZF), which is an earth-abundant and economically viable material, via a doping strategy. The spinel ZF shows a weak catalytic activity for water splitting, whereas the substitution of Cu ions at octahedral sites results in improving the catalytic performance in both acidic and basic electrolytes. Structural characterization using high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction demonstrates that, depending on the Cu concentration, Cu ions either incorporate into spinel Zn-ferrite oxide as doping agents or form CuO nanocomposites, where Cu-induced construction of a composite containing ZCF nanoparticles and CuO nanophase coexists. Substituting Zn with Cu in the octahedral sites of the ZF crystal structure leads to a decrease in the unit cell lattice parameter, and the crystal symmetry is impacted, including the creation of strain and dislocation density. HRTEM analyses provide evidence that the ZF particles nucleate and grow randomly due to the asymmetric reaction dynamics of spinel oxide and the lack of surfactant, while the ZCF nanoparticles are elongated in preferential orientation, forming oriented nanoparticles with a greater surface-to-volume ratio. To attain the current density of 10 mA cm-2, the nanocomposite of the ZCF-50 electrode shows the lowest overpotential of 280 mV for oxygen evolution reaction (OER) among other electrodes. The Tafel slope also decreases significantly in which the nanocomposite of ZCF-50 shows the lowest value of 80 mV dec-1. The measured double-layer capacitance (Cdl) for the nanocomposite structure of ZCF-50 offers the highest value of 27 mF cm-2, which indicates that the nanocomposite contains the largest electrochemically active surface area (ECSA). The catalytic activity of Cu-doped spinel ZCF for hydrogen evolution reaction is also evaluated. The nanocomposite of ZCF-50 shows the lowest onset overpotential of 60 mV compared to 200 mV for the ZF electrode. The obtained Cdl over cathodic potentials for the ZCF-50 electrode shows the highest value of 11.3 mF cm-2 compared with other electrodes. These results confirm that ZCF-50 contains the largest ECSA and highest electrochemical activity. Electrochemical impedance spectroscopy studies also demonstrate that the ZCF-50 electrode shows the lowest charge-transfer resistance, indicating that the catalytic OER is improved significantly at its interfaces. We realize that Cu doping into the ferrite structure and the formation of the CuO semishells synergistically can improve interparticle and transparticle charge transfer.

Highly sensitive magnetostrictive NiZnCo ferrites for low magnetic field sensor applications

Ferrites with enhanced magnetostrictive strain and sensitivity possess significant potential for various multifunctional devices. Herein, the magnetostrictive ferrites (Ni0.8Zn0.2)1-xCoxFe2O4 (0 ≤ x ≤ 1) are prepared by a facile sintering method. The X-ray diffraction and scanning electron microscope results demonstrate that the lattice constant and mean grain size increase with x while the porosity decreases. Through magnetic measurements, it is found that the saturation magnetization, coercivity and magnetocrystalline anisotropy constant, all increase with x. However, the permeability decreases due to the inherent hard magnetic nature of cobalt ferrite. Magnetostrictive strain measurements reveal that with the increasing x, the saturation magnetostrictive strain is elevated, owing to the strong magnetocrystalline anisotropy of the octahedrally coordinated Co2+. The strain sensitivity variation of the samples with 0.2 ≤ x ≤ 0.8 is consistent with the theoretical parameter. The maximum strain sensitivity is achieved in the sample with x = 0.4 (0.229 ppm Oe-1), and the magnitude of the external magnetic field is also the smallest at this time, which indicates that this material may be applied in the field of low magnetic field magnetoelectric sensors and devices. Finally, in order to further optimise the properties of magnetostrictive materials, an innovative approach is introduced.

Removal of Germanium from a Solution by a Magnetic Iron-Based Precipitant

This study presents a method for the precipitation of germanium from a solution using magnetic iron-based precipitants and contrasts this method with the commonly employed neutralization-precipitation technique in industrial production, analyzing and comparing their reaction conditions and the properties of their precipitates. This study analyzes the influence of varying experimental conditions (reaction time, reaction temperature, iron:germanium molar ratio, Fe3+:Fe2+ molar ratio, and reaction pH) on the germanium precipitation efficiency. With a precipitation time of 30 min, a precipitation temperature of 30 °C, an iron:germanium molar ratio of 30:1, an Fe3+:Fe2+ molar ratio of 3:1, and a reaction pH of 5.0, the optimal germanium precipitation efficiency achieved was 99.5%. Furthermore, this study employed X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and vibrating sample magnetometry to analyze the properties and composition of the precipitate, providing support for the conclusion regarding germanium precipitation using magnetic iron-based precipitants. Through theoretical analysis and instrumental testing, it was determined that the precipitation of germanium from a solution using magnetic iron-based precipitants significantly reduces the reaction time compared to those of neutralization-precipitation methods. Moreover, a magnetic iron-based precipitant substantially reduces the amount of precipitate, allows for magnetic separation of the precipitate, and effectively alleviates the problem of the presence of other valuable metals in the precipitate.

3D Hierarchical MOF-Derived Defect-Rich NiFe Spinel Ferrite as a Highly Efficient Electrocatalyst for Oxygen Redox Reactions in Zinc-Air Batteries

The strategy of defect engineering is increasingly recognized for its pivotal role in modulating the electronic structure, thereby significantly improving the electrocatalytic performance of materials. In this study, we present defect-enriched nickel and iron oxides as highly active and cost-effective electrocatalysts, denoted as Ni0.6Fe2.4O4@NC, derived from NiFe-based metal-organic frameworks (MOFs) for oxygen reduction reactions (ORR) and oxygen evolution reactions (OER). XANES and EXAFS confirm that the crystals have a distorted structure and metal vacancies. The cation defect-rich Ni0.6Fe2.4O4@NC electrocatalyst exhibits exceptional ORR and OER activities (ΔE = 0.68 V). Mechanistic pathways of electrochemical reactions are studied by DFT calculations. Furthermore, a rechargeable zinc-air battery (RZAB) using the Ni0.6Fe2.4O4@NC catalyst demonstrates a peak power density of 187 mW cm-2 and remarkable long-term cycling stability. The flexible solid-state ZAB using the Ni0.6Fe2.4O4@NC catalyst exhibits a power density of 66 mW cm-2. The proposed structural design strategy allows for the rational design of electronic delocalization of cation defect-rich NiFe spinel ferrite attached to ultrathin N-doped graphitic carbon sheets in order to enhance active site availability and facilitate mass and electron transport.