Photocatalytic investigation of textile dyes and E. coli bacteria from wastewater using Fe3O4@MnO2 heterojunction and investigation for hydrogen generation on NaBH4 hydrolysis

Various impurities found nowadays in water can be detrimental to human health. This work focused on utilizing Fe₃O₄@MnO₂ nanocomposite for cleaning organic contaminants from water, including rhodamine B (RhB) and Escherichia coli (E. coli). Analysis methods such as XRD, UV-vis, TEM, and FTIR were used to describe the nanocomposite. The results showed that the developed nanocomposite has good photocatalytic activity against pollutants in wastewater. The E. coli was destroyed after 90 min, and the RhB photodegradation rate was 75%. Moreover, the Fe₃O₄@MnO₂ efficiency as a catalyst for producing hydrogen as an alternative energy source was tested. According to the calculations, the nanomaterial's turnover frequency, activation energy, enthalpy, and entropy are 1061.3 h^-1, 28.93 kJ/mol, 26.38 kJ/mol, and -128.41 J/mol.K, respectively. Four reusability tests were completed, and the average reusability was 78%. The obtained data indicated the excellent potential for the developed Fe₃O₄@MnO₂ nanomaterial to act as an adsorbent, thus representing an alternative to the classical depollution methods. This study showed that nanoparticles have a photocatalytic effect against pathogenic bacteria and RhB azo dye in polluted waters and offer an effective catalytic activity to produce hydrogen as an alternative energy source.

Fe3+ in a tetrahedral position determined the electrocatalytic properties in FeMn2O4

As an electrocatalyst for the oxygen evolution reaction (OER) for water decomposition purposes, spinel ferrite materials have gained a lot of attention from many researchers. Herein, we document a co-precipitation synthesis of antitypical spinel nanoparticles (FeMn₂O₄) by post-annealing at different temperatures to enable modulation of the cationic oxidation state and tuning of the conversion degree for efficient and good OER performance. The electrocatalytic activity test shows that the sample annealed at 500 °C has the most optimal catalytic activity with an overpotential of 360 mV at a current density of 10 mA cm⁻² and a Tafel slope as low as 105.32 mV dec⁻¹. The formation of FeOOH during in situ OER promotes the catalytic activity of the catalysts. More importantly, according to the results of Brunauer–Emmett–Teller normalization, we demonstrate that the activity of the catalyst is also inseparable from the internal crystal structure. This work broadens the field of research on the electrocatalysis of spinel manganese ferrites.

Inspired by the flexibility of cation exchange and valence state variation in spinel ferrite, a high activity OER catalyst FeMn₂O₄ has been observed without inducing the change of composition, morphology, and atomic doping.

(De)lithiation of spinel ferrites Fe3O4, MgFe2O4, and ZnFe2O4: a combined spectroscopic, diffraction and theory study

Iron based materials hold promise as next generation battery electrode materials for Li ion batteries due to their earth abundance, low cost, and low environmental impact. The iron oxide, magnetite Fe3O4, adopts the spinel (AB2O4) structure. Other 2+ cation transition metal centers can also occupy both tetrahedral and/or octahedral sites in the spinel structure including MgFe2O4, a partially inverse spinel, and ZnFe2O4, a normal spinel. Though structurally similar to Fe3O4 in the pristine state, previous studies suggest significant differences in structural evolution depending on the 2+ cation in the structure. This investigation involves X-ray absorption spectroscopy and X-ray diffraction affirmed by density functional theory (DFT) to elucidate the role of the 2+ cation on the structural evolution and phase transformations during (de)lithiation of the spinel ferrites Fe3O4, MgFe2O4, and ZnFe2O4. The cation in the inverse, normal and partially inverse spinel structures located in the tetrahedral (8a) site migrates to the previously unoccupied octahedral 16c site by 2 electron equivalents of lithiation, resulting in a disordered [A]16c[B2]16dO4 structure. DFT calculations support the experimental results, predicting full displacement of the 8a cation to the 16c site at 2 electron equivalents. Substitution of the 2+ cation results in segregation of oxidized phases in the charged state. This report provides significant structural insight into the (de)lithiation mechanisms for an intriguing class of iron oxide materials.

Tunable Metallogels Based on Bifunctional Ligands: Precursor Metallogels, Spinel Oxides, Dye and CO2 Adsorption

A semisolid gel material is a gift of serendipity via various chemical interactions, and metal incorporation (metallogels) imparts diverse functional properties. In this work, we have synthesized four metallogels from tetrapodal and hexapodal carboxylic acid/amide-based low-molecular-weight gelators with Ni(II) and Cu(II) salts. These metallogels can be tuned to be a low-temperature precursor of porous spinel oxides. These xerogels exhibit impressive water soluble dye and carbon dioxide adsorption, which coupled with the tunability and facile synthesis of porous spinel oxides underscores their potential in environmental remediation and energy applications.

Unveiling the Reaction Mechanism during Li Uptake and Release of Nanosized "NiFeMnO4": Operando X-ray Absorption, X-ray Diffraction, and Pair Distribution Function Investigations

Here, we report that the trimetallic nanosized oxide NiFeMnO₄ consists of a mixture of NiO and a strained cubic spinel phase, which is clearly demonstrated by analysis of the pair distribution function (PDF) and synchrotron X-ray data. Such a finding can easily be overlooked by using only inhouse X-ray powder diffraction, leading to inaccurate assumption of the stoichiometry and oxidation states. Such advanced characterization is essential because a homogeneous distribution of the elements is observed in energy-dispersive X-ray spectroscopy maps, giving no hints for a phase separation. Cycling of the sample against Li delivers a high reversible capacity of ≈840 mAh/g in the 50th cycle. Operando X-ray absorption spectroscopy (XAS) experiments indicate that ≈0.8 Li/fu is consumed without detectable changes of the electronic structure. Increasing amounts of Li, Mn³⁺, and Fe³⁺ are simultaneously reduced. The disappearance of the pre-edge features in X-ray absorption near-edge spectroscopy indicates movement of these cations from tetrahedral sites to octahedral sites. PDF analysis of the pattern after an uptake of 2 Li/fu evidences that the principal structure can be sufficiently well modeled assuming coexisting NiO, a mixed monoxide, and a small amount of residual spinel phase. Thus, the majority of cations is located on octahedral sites. Furthermore, an improvement of the PDF model is achieved taking into account small amounts of LiOH. The ⁷Li MAS NMR spectrum of this sample clearly shows the signal of Li in a diamagnetic environment, excluding Li-O-TM bonds. A further increase of the Li content leads to a successive conversion of the cations to nanosized metal particles embedded in a LiOH/Li₂O matrix. Ex situ XAS results indicate that Fe can be reversibly reoxidized to Fe³⁺ during charge whereas Mn does not reach the oxidation state observed in the pristine material. After excessive cycling, reoxidation of metallic Ni is suppressed and contributes to a capacity loss compared with the early discharge/charge cycles.

Transition metal cations on the move: simultaneous operando X-ray absorption spectroscopy and X-ray diffraction investigations during Li uptake and release of a NiFe2O4/CNT composite

We report on results of a comprehensive investigation on reaction mechanisms occurring during Li uptake and release of the composite NiFe2O4/CNT. Operando X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) data collected simultaneously using one in situ cell allowed thorough elucidation of structural and electronic alterations happening during Li uptake. From the beginning of Li uptake, the Bragg intensity of the spinel reflections decreases which can be explained by reduction of Fe3+ ions and simultaneous movement of the Fe2+ cations from tetrahedral 8a to empty octahedral 16c sites. The reduction of Fe3+ is clearly evidenced by XAS. The occupation of tetrahedral sites by Li+ can be excluded based on results of density functional theory calculations. Increasing the Li content leads to formation of a new crystalline phase resembling a monoxide with a NaCl-like structure. The appearance of the new phase is accompanied by a steady decrease of the sizes of coherently scattering domains of the spinel and a growth of the domains of the monoxide phase. After uptake of about 2.5 Li per NiFe2O4, all Fe3+ cations are reduced to Fe2+ and the tetrahedral 8a sites are empty (XAS spectra). Careful Rietveld refinements of X-ray powder patterns demonstrate that the tetrahedral 8a site is successively depleted with increasing Li content. Interestingly, the occupancy of the octahedral 16d site is also slightly reduced. Increasing the Li content beyond 2.5 Li/NiFe2O4 leads to successive reduction of the cations to very small metal particles embedded in a Li2O matrix (as evidenced by 7Li MAS NMR investigations). During Li release metallic Ni and Fe are reoxidized to Ni2+ resp. Fe3+. The cycling stability of NiFe2O4/CNT is significantly improved compared to pure NiFe2O4 or a mechanical mixture of NiFe2O4 and CNTs.

High electrochemical performance of 3D highly porous Zn0.2Ni0.8Co2O4 microspheres as an electrode material for electrochemical energy storage

3D highly porous Zn_0.2Ni_0.8Co₂O₄ microspheres unveil superior electrochemical energy storage properties.

Reversible control of the magnetization of spinel ferrites based electrodes by lithium-ion migration

Lithium-ion (Li-ion) batteries based on spinel transition-metal oxide electrodes have exhibited excellent electrochemical performance. The reversible intercalation/deintercalation of Li-ions in spinel materials enables not only energy storage but also nondestructive control of the electrodes’ physical properties. This feature will benefit the fabrication of novel Li-ion controlled electronic devices. In this work, reversible control of ferromagnetism was realized by the guided motion of Li-ions in MnFe₂O₄ and γ-Fe₂O₃ utilizing miniature lithium-battery devices. The in-situ characterization of magnetization during the Li-ion intercalation/deintercalation process was conducted, and a reversible variation of saturation magnetization over 10% was observed in both these materials. The experimental conditions and material parameters for the control of the ferromagnetism are investigated, and the mechanism related to the magnetic ions’ migration and the exchange coupling evolution during this process was proposed. The different valence states of tetrahedral metal ions were suggested to be responsible for the different performance of these two spinel materials.

Pseudocapacitance of Mesoporous Spinel-Type MCo2O4 (M = Co, Zn, and Ni) Rods Fabricated by a Facile Solvothermal Route

We present the structural properties and electrochemical capacitance of mesoporous MCo₂O₄ (M = Co, Zn, and Ni) rods synthesized by a facile solvothermal route without necessity to use templates. The Brunauer-Emmett-Teller specific surface areas of these mesoporous rods are found to be about 24, 54, and 62 m² g^-1 with major pore diameters of about 31, 15, and 9 nm for MCo₂O₄, M = Co, Zn, and Ni, respectively. X-ray photoelectron spectroscopy and X-ray diffraction studies reveal the phase purity of the samples with a predominant spinel-type crystal structure. The spinel crystal structure with lattice parameters of 8.118, 8.106, and 8.125 Å is obtained for MCo₂O₄, M = Co, Zn, and Ni, respectively. The transmission electron microscopy study reveals that the mesoporous rods are built by self-assembled aggregates of nanoparticles which are well-interconnected to form stable mesoporous rods. The electrochemical capacitor performance was investigated by means of cyclic voltammetry, galvanostatic charge/discharge cycling, and impedance spectroscopy in a three-electrode configuration. As a result, the spinel-type MCo₂O₄ rods exhibit high specific capacitances of 1846 F g^-1 (CoCo₂O₄), 1983 F g^-1 (ZnCo₂O₄), and 2118 F g^-1 (NiCo₂O₄) at a scan rate of 2 mV/s. Furthermore, the mesoporous spinel-type metal oxides show desirable stability in alkaline electrolyte during long-term cycling with excellent cycling efficiency.

Elucidation of the Conversion Reaction of CoMnFeO4 Nanoparticles in Lithium Ion Battery Anode via Operando Studies

Conversion reactions deliver much higher capacities than intercalation/deintercalation reactions of commercial Li ion batteries. However, the complex reaction pathways of conversion reactions occurring during Li uptake and release are not entirely understood, especially the irreversible capacity loss of Mn(III)-containing oxidic spinels. Here, we report for the first time on the electrochemical Li uptake and release of Co(II)Mn(III)Fe(III)O4 spinel nanoparticles and the conversion reaction mechanisms elucidated by combined operando X-ray diffraction, operando and ex-situ X-ray absorption spectroscopy, transmission electron microscopy, (7)Li NMR, and molecular dynamics simulation. The combination of these techniques enabled uncovering the pronounced electronic changes and structural alterations on different length scales in a unique way. The spinel nanoparticles undergo a successive phase transition into a mixed monoxide caused by a movement of the reduced cations from tetrahedral to octahedral positions. While the redox reactions Fe(3+) ↔ Fe(0) and Co(2+) ↔ Co(0) occur for many charge/discharge cycles, metallic Mn nanoparticles formed during the first discharge can only be oxidized to Mn(2+) during charge. This finding explains the partial capacity loss reported for Mn(III)-based spinels. Furthermore, the results of the investigations evidence that the reaction mechanisms on the nanoscale are very different from pathways of microcrystalline materials.

Sol–gel synthesis and electrochemical properties of c-axis oriented LiCoO2 for lithium-ion batteries

This paper expounds upon the relationship between the electrochemical performance and the degree of c-axis orientation of LiCoO₂.