Td Sites Research Articles

Unveiling the Geometric Site Dependence of Co‐Based Spinel Oxides in the Halogen Evolution Reaction

AbstractCobalt‐based spinel oxides, such as Co3O4, have emerged as promising electrocatalysts for chlorine and bromine evolution reactions (CER and BrER) in recent years. However, the role of Co valence in determining the exceptional performance of Co3O4 for both CER and BrER remains ambiguous due to the coexistence of both octahedrally coordinated Co3+ (Co3+Oh) and tetrahedrally coordinated Co2+ (Co2+Td) sites, despite their high catalytic activity and stability. Herein, combining experiment results and electrochemical data analysis, the Co3+Oh site functions as the primary active site for CER is demonstrated. In contrast, for BrER, both Co3+Oh and Co2+Td sites exhibit good catalytic activity, with Co3+Oh sites displaying better BrER catalytic performance than Co2+Td sites. To further enhance the CER catalytic activity of the Co3+Oh site, inert Co2+Td is replaced with Cu2+ cations. As expected, CuCo2O4 featuring an optimized Co3+Oh site demonstrates an overpotential of 24 mV at a current density of 10 mA cm−2 while exhibiting exceptional stability for ≈60 h, surpassing the performance of the majority of non‐noble and even noble metal‐based electrocatalysts reported to date. Therefore, the study elucidates the significance of geometric configuration‐dependent activity in electrocatalytic halogen evolution reactions.

Modulating charge transfer and constructing synergistic bimetallic active sites for CO-SCR reactions in inverse spinel

The selective catalytic reduction of NO by CO (CO-SCR) are frequently catalyzed by spinel due to its exceptional stability and redox capabilities. In this research, a ternary metal-based inverse spinel catalyst Cu0.67Mn0.33Fe2O4 was synthesized utilizing the glycerol self-templating technique. Some Fe3+ (Fe3+Td) sites are transferred to vacant octahedral sites through the occupancy of tetrahedral sites by Mn species. Mn3+ (Mn3+Td) sites synergistically interact with Cu2+ and Fe3+ ions on the octahedral sites to promote charge transfer, producing numerous oxygen-defective and highly mobile oxygen species and constructing Cu+-Ov-Fe2+ active sites on the surface. Various characterizations demonstrated that Cu+-Ov-Fe2+ has greater catalytic activity, resistance to alkali metals, and water toxicity than the Cu2+-Ov-Fe2+ active site of CuFe2O4. In situ DRIFTS further revealed the reaction mechanism of the ternary metal-based inverse spinel catalyst, and this study provides novel perspectives on the development of Cu-based catalysts for CO-SCR.

Solution and solubility of H atoms at the W/Cu interface.

Metallic interfaces are locations where hydrogen (H) is expected to segregate and lead to the formation and stabilization of defects. This work focuses on the tungsten/copper (W/Cu) interface built according to theWbcc(001)/Cuhcp(112-0)orientation. H behavior is subsequently determined at the interface and in its vicinity with electronic structure calculations based on the density functional theory (DFT). The electronic and vibrational properties determined in this way followed a thermodynamic treatment to deliver the solubility of H as a function of the temperature and chemical potential. The 96 interstitial positions we investigated reveal that H predominantly occupies the octahedral (Oh) sites in the copper network. Reversely, H exclusively occupies the tetrahedral (Td) sites in the tungsten network. The solubility of H is higher in the interface plane where both octahedral and tetrahedral sites are occupied. Despite this work is a first step toward kinetic modeling of hydrogen transport across the W/Cu interface, we conclude that theWbcc(001)/Cuhcp(112-0)would behave like a sink where hydrogen isotopes could accumulate.

Enhancing Electrochemical Sensor Performance: Studies of Electrodes Tailored with ZnO/ZnFe2O4 Nanoparticles

Spinel metal oxides possess excellent magnetic, electrical, and optical properties. They take AB2O4 (face centered cubic) form with oxygen anions providing tetrahedral (Td) and octahedral (Oh) sites for A+2 and B+3 cations. Spinel can have a normal, inverse, or mixed form based on the occupancy of different cations in Td and Oh sites. The peculiarity of the spinel crystal structure is that its composition can be easily modified without affecting the crystal structure based on the type of cation. The type of cation in the composition defines if the spinel has a normal or inverse form [1]. Based on these premises, we have already synthesized ZnxNi1-xFe2O4 (x =0, 0.2, 0.4, 0.6, 0.8, 1) nanomaterials achieving a clear gradual transition from inverse (x=0) to normal (x=1) spinel. Synthesized nanomaterials were employed as mediators in electron transfer between the screen-printed carbon working electrode and paracetamol to understand the effect of chemical composition and crystal structure on the electron transfer at the electrochemical interface [1]. Normal spinel ZnFe2O4 was found to be the best nanomaterial in terms of sensitivity and kinetic rate constant. In further works, with the aim to understand the effect of ionic radii on sensitivity and electron transfer rate constant in electrochemical sensing of paracetamol, we have focused on the normal spinel structure and modified the composition by varying the concentration of Fe3+ with Cr3+ and Bi3+ [2,3]. This study also proved that the normal spinel ZnFe2O4 has the highest sensitivity and electron transfer rate constant towards paracetamol sensing.In this work, we will present the synergic effect that can be obtained by interfacing ZnFe2O4 with ZnO nanomaterials by tuning the band gap of the heterogeneous structure. The aim is to understand the effect of band gap on sensitivity and electron transfer rate constant in electrochemical sensing. ZnFe2O4, ZnO, and ZnO/ZnFe2O4 nanomaterials are synthesized by a simple, single step auto combustion technique using the respective metal nitrates as precursors. Nanomaterial morphology and particles size are investigated by scanning electron microscopy. X-ray diffraction technique is employed to analyze the crystal structure and identify different phases in the newly synthesized materials. Then, commercially available screen-printed carbon electrodes with carbon working electrode and carbon counter electrodes are used for the electrochemical measurements in combination with an external double junction Ag/AgCl as a reference electrode. The synthesized nanomaterials are mixed with 1-butanol and a 5 μL solution is used to modify the surface of the carbon working electrode to mediate the redox reactions between the carbon surface and the molecule of interest. Primarily the sensors are characterized using cyclic voltammetry with ferri/ferrocyanide redox couple as a probe molecule. Improvement in sensitivity is observed for ZnFe2O4 (8.85 ± 0.50 μA/mM), ZnO (8.50 ± 0.30 μA/mM), and ZnO/ZnFe2O4 (8.22 ± 0.16 μA/mM) sensors compared to the bare carbon one (6.30 ± 0.40 μA/mM). By performing cyclic voltammetry at different scan rates (ν) from 25 to 125 mV/s, a good linearity of redox currents with respect to v0.5 is observed and redox peak positions are varying linearly with ln(ν). Peak-to-peak separation (ΔEp) is reduced for ZnO/ZnFe2O4 sensors compared to the carbon one. All these results suggest a faster electron transfer at the interface when the modified electrodes are used. Laviron model is employed to calculate the electron transfer rate coefficient and constant. The rate constant for ZnFe2O4 (41.8 ± 2.6 ms-1), ZnO (46.0 ± 4.0 ms-1), and ZnO/ZnFe2O4 (33.1 ± 4.5 ms-1) sensors is 3 to 5 times higher as compared to the bare carbon one (9.97 ± 0.78 ms-1). We are currently studying the potential application of ZnO/ZnFe2O4 nanomaterials in electrochemical sensing of small molecules relevant in biomedical field (dissolved oxygen, pH) and pharmaceutical drugs (paracetamol) to assess the potential for their use in different clinical settings.

Spinel cobalt-based binary metal oxides as emerging materials for energy harvesting devices: synthesis, characterization and synchrotron radiation-enabled investigation.

The synthesis and characterization of spinel cobalt-based metal oxides (MCo2O4) with varying 3d-transition metal ions (Ni, Fe, Cu, and Zn) were explored using a hydrothermal process (140 °C for two hours) to be used as alternative counter electrodes for Pt-free dye-sensitized solar cells (DSSCs). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) revealed distinct morphologies for each metal oxide, such as NiCo2O4 nanosheets, Cu Co2O4 nanoleaves, Fe Co2O4 diamond-like, and Zn Co2O4 hexagonal-like structures. The X-ray diffraction analysis confirmed the cubic spinel structure for the prepared MCo2O4 films. The functional groups of MCo2O4 materials were recognized in metal oxides throughout Fourier transform infrared (FTIR) analysis. The local structure analysis using X-ray absorption fine structure (XAFS) at Fe and Co K-edge identified octahedral (Oh) Co3+ and tetrahedral (Td) Co2+ coordination, with Zn2+ and Cu2+ favoring Td sites, while Ni3+ and Fe3+ preferred Oh active sites. Further investigations utilizing the Fourier transformation (FT) analysis showed comparable coordination numbers and interatomic distances ranked as Co-Cu > Co-Fe > Zn-Co > Co-Ni. Furthermore, the utilization of MCo2O4 thin films as counter electrodes in DSSC fabrication showed promising results. Notably, solar cells based on CuCo2O4 and ZnCo2O4 counter electrodes showed 1.9% and 1.13% power conversion efficiency, respectively. These findings indicate the potential of employing these binary metal oxides for efficient and cost-effective photovoltaic device production.

Effect of Gd3+ ion doping on structural, optical, magnetic and dielectric properties on superparamagnetic Mg0.5Zn0.5Fe2−xGdxO4 (0 ≤ x ≤ 1) nanoparticles synthesized via oleic acid assisted chemical co-precipitation method

In the present study, different compositions having general formula Mg0.5Zn0.5GdxFe2−xO4, (x = 0, 0.025, 0.050, 0.075, 0.1) were synthesized using chemical co-precipitation method. The substitution of Gd3+ ions have exhibited profound impacts on the structural, morphological, magnetic, and optical parameters of Mg-Zn ferrites. Single-phase cubic spinel structure has been substantiated by X-ray diffractometer (X-ray diffraction, XRD) analysis for all compositions without any contamination peak. The values of the lattice constant and X-ray density both show an increasing trend when the Gd3+ ion concentration increases. (Fourier transformed infrared spectroscopy) FTIR spectra recorded in the mid-infrared region (4000–400 cm−1) exhibit two main absorption bands associated with the Oh and Td sites within the frequency range of 400–600 cm−1. Morphological studies using (High-resolution transmission electron microscopy) HR-TEM analysis reveal agglomeration of nanoparticles and the average particle size lies between 8 and 9 nm. Maximum magnetization displays an erratic trend with increasing Gd3+ ion concentration, although both retentivity and coercivity decreases. The hysteresis curve for samples of differing ratios of Gd3+ ions demonstrated the superparamagnetic nature. Tauc’s plot was employed to calculate the energy bandgap (Eg) of synthesized nanoparticles and Eg value were observed to decline from 3.563 eV to 3.521 eV with an increasing Gd3+ ion concentration. Impedance analyzer was used in the frequency range of 100 Hz–100 MHz to study dielectric properties at room temperature. The dielectric parameters declined as the frequency values increased. Due to lower values of dielectric parameters in Gd3+ ion doped Mg–Zn nano-ferrite, these materials are suitable in the fabrication of devices that need to operate at high frequencies such as switching memory storage devices. Moreover, these materials are appropriate for application in ferrofluids and medicinal treatments such as magnetic hyperthermia because of their low coercive field value.

Regulating the distribution of iron active sites on γ-Fe2O3via Mn-modified α-Fe2O3 for NH3-SCR

Developing below 150 °C highly activity and broad reaction window catalysts has been challenging by using Fe-based catalysts for NH3-SCR. The octahedral Fe3+ (Fe3+Oh) sites exist in hematite (α-Fe2O3) and maghemite (γ-Fe2O3) are inactive for NH3-SCR due to the redox circle of Fe2+→Fe3+ reliance on the distribution of iron sites in crystal structure. Here we modified the α-Fe2O3 crystal structure by substituting part inactive Fe3+Oh sites with catalytically active Mn3+Oh sites, which promoted the formation of γ-Fe2O3 to generate the active tetrahedral Fe3+ (Fe3+Td) sites and enhanced the magnetism of the Fe1−yMnyOx. The strong Fe-O-Mn interaction established by crystal coordinative configurations not only boosted the formation of NO2 but also facilitated the Brønsted acid circle. Surprisingly, the Fe0.85Mn0.15Ox exhibited the superior low temperature NH3-SCR activity, with NOx conversion above 100% at 100–275 °C under a GHSV of 60000 h−1.

Regulating Co-O covalency to manipulate mechanistic transformation for enhancing activity/durability in acidic water oxidation.

Developing earth-abundant electrocatalysts with high activity and durability for acidic oxygen evolution reaction is essential for H2 production, yet it remains greatly challenging. Here, guided by theoretical calculations, the challenge of overcoming the balance between catalytic activity and dynamic durability for acidic OER in Co3O4 was effectively addressed via the preferential substitution of Ru for the Co2+ (Td) site of Co3O4. In situ characterization and DFT calculations show that the enhanced Co-O covalency after the introduction of Ru SAs facilitates the generation of OH* species and mitigates the unstable structure transformation via direct O-O coupling. The designed Ru SAs-CoO x catalyst (5.16 wt% Ru) exhibits enhanced OER activity (188 mV overpotential at 10 mA cm-2) and durability, outperforming most reported Co3O4-based and Ru-based electrocatalysts in acidic media.

A Facilely Synthesized NiCo2S4 with Two Mutually Reinforcing Active Sites for High-Performance Lithium-Sulfur Batteries.

The shuttle effect and slow conversion kinetics of soluble polysulfides hinder the commercial application of lithium-sulfur batteries (LSBs). In this context, we propose a three-dimensional lamellar-stacked nanostructure of nickel cobalt sulfide (D-NiCo2S4) enriched with lattice defects by manipulating the cations in spinel sulfides. It has an obvious synergistic promotion mechanism for the adsorption and catalysis of lithium sulfides. Specifically, Ni3+ on tetrahedral (Td) sites with strong Ni-S covalency anchors LiPSs, whereas Co3+ on octahedral (Oh) sites promotes a highly efficient catalytic conversion of LiPSs, which is confirmed by experimental results and density functional theory (DFT) calculations. Besides, the crystal defects and distortions in the lamellar region could expose more active sites and enhance the redox reaction kinetics of polysulfides. Hence, Li-S batteries with D-NiCo2S4@S as the cathode show outstanding cycle stability; upon cycling at 1 A/g, the battery achieves a high initial specific capacity of 1001.12 and 655.31 mAh g-1 after 1000 cycles (decay rate as low as 0.05% per cycle), as well as a high initial areal capacity of 3.15 mAh cm-2 under high S loading (4.2 mg cm-2). This work provides a viable scheme for designing efficient bimetal sulfide catalysts and furnishes a rational strategy for constructing LSB cathodes with high specific capacity and high area capacity.

Cyclohexane Oxidative Dehydrogenation on Graphene-Oxide-Supported Cobalt Ferrite Nanohybrids: Effect of Dynamic Nature of Active Sites on Reaction Selectivity.

In this work, we investigated cyclohexane oxidative dehydrogenation (ODH) catalyzed by cobalt ferrite nanoparticles supported on reduced graphene oxide (RGO). We aim to identify the active sites that are specifically responsible for full and partial dehydrogenation using advanced spectroscopic techniques such as X-ray photoelectron emission microscopy (XPEEM) and X-ray photoelectron spectroscopy (XPS) along with kinetic analysis. Spectroscopically, we propose that Fe3+/Td sites could exclusively produce benzene through full cyclohexane dehydrogenation, while kinetic analysis shows that oxygen-derived species (O*) are responsible for partial dehydrogenation to form cyclohexene in a single catalytic sojourn. We unravel the dynamic cooperativity between octahedral and tetrahedral sites and the unique role of the support in masking undesired active (Fe3+/Td) sites. This phenomenon was strategically used to control the abundance of these species on the catalyst surface by varying the particle size and the wt % content of the nanoparticles on the RGO support in order to control the reaction selectivity without compromising reaction rates which are otherwise extremely challenging due to the much favorable thermodynamics for complete dehydrogenation and complete combustion under oxidative conditions.

Tunable inversion degree of MnIn2S4 thiospinels prepared by high-pressure synthesis, and its implication in the optical and magnetic properties

Indium thiospinels (AIn2S4, A= transition metals) have attracted significant attention due to their potential for manipulating magnetic and optoelectronic properties through changes in chemical composition. Defined in the cubic space group Fd-3 m (No. 227), the crystal structure consists of a framework of edge-sharing InS6 octahedra (Oh symmetry), with A cations coordinated tetrahedrally (Td symmetry) to sulfur. In this study, we describe a rapid high-pressure synthetic method (3.5 GPa) to prepare MnIn2S4 polycrystalline samples, with tunable degree of inversion (λ) or Mn-In antisite, meaning that some Mn occupy the octahedral sites and some In atoms are located at the tetrahedral positions. We found that λ closely depends on the synthesis temperature. Notably, the refinement of the crystal structure from X-ray diffraction data reveals positive correlations between the inversion degree and different properties such as the band gap, expanding its spectral coverage, and the magnetic behavior, depending on the occupancy rate of the octahedral or tetrahedral sites by the strongly magnetic Mn2+ ions. Whereas the paramagnetic behavior is not affected by the inversion degree, for higher λ values (30%<λ < 38%), achieved at synthesis temperatures as high of 800ºC, the random distribution of Mn2+ ions between Td and Oh sites accounts for a spin-glass state with a clear cusp in the magnetic susceptibility (ac and dc), based on the strong short-range antiferromagnetic interactions.

Engineered tenorite structure of barium-enriched copper oxide for on-site monitoring of cytotoxic methotrexate in environmental samples

Emerging pharmaceutical pollutants pose a threat to both human and environmental health. The removal and monitoring of such pollutants necessitate the use of practical on-site monitoring devices; however, the designs of such devices are underdeveloped. This study involves the fabrication of a low-cost sensor based on barium-incorporated copper oxide (Ba-CuO) for the on-site monitoring of the cytotoxic drug methotrexate (MTRX) in water and sediment samples. The tenorite structure of CuO was slightly enriched with Ba ions at the td sites, distorting the tetrahedron and enhancing its electrochemical properties. Ba-CuO was obtained from Cu(NO3)2 and Ba(OH)2 by a ligand exchange protocol and was characterized using X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, transmission electron microscopy, and energy dispersive X-ray analysis. In addition, the Ba-CuO sensor was tested under various conditions, and it could detect MTRX at concentrations as low as 0.4 nM, with a high sensitivity of 1.3567 µA µM−1 cm−2. On-site monitoring yielded recoveries of greater than 93 % from spiked samples, thus exhibiting excellent reproducibility and stability. Therefore, the developed method is practical and has no matrix effect on the MTRX sensor.

Structure, microstructure, magnetic and low temperature Mössbauer spectroscopy studies of Bismuth substituted zinc ferrite composite

We address the modifications made to Bismuth(3 + )-substituted ZnFe2O4 composites in order to comprehend their structural, microstructural, and magnetic properties. Bi@ZnFe2O4 is produced using the solution combustion method and sintered at 500 °C for two hours. The X-ray diffraction (XRD) patterns of the generated samples exhibit cubic symmetry with spinel structure. The computation of structural parameters utilised fundamental formulas. The crystallite diameters of each sample were determined to be between 20 and 22 nm. The scanning electron micrographs of the samples clearly demonstrated the porous character of the specimens. HRTEM verifies that particles are significantly agglomerated. The TEM image of nanoparticles reveals that their sizes range between 26 and 28 nm.. At room temperature and temperature (low & high) dependently, 57Co in Rh matrix was employed as a -quanta source to demonstrate the Superparamagnetic nature of the samples. Bi3+ occupied the Td site, Zn the Oh site, and Co the Td and Oh sites in a ratio of 2:3. Using the cation occupancies, the observed magnetic properties of our Bi @ ZnFe2O4 composite may be described by Néel's two sublattice model.

Mn Interdiffusion Mobility Controlled By Simple Drying Process for Cobalt Free Core-Shell Ni Rich Cathode Material in Lithium Ion Batteries

Lithium-ion batteries (LIBs) become an essential part of many portable devices and even electric vehicles than ever before. Among the various cathode materials, Ni rich layered cathode material has big attention because it has higher specific capacity and energy density. However, at a delithiated state, unstable Ni4+ leads to oxygen release and structural degradation and as a result, irreversible phase transition from R3m to electrochemical inactive Fm3m is observed. So, various strategy is applying to overcome this limitation like three-component system (NCM) or surface coating. However, NCM still cannot solve that problem perfectly and surface coating makes a problem like reducing specific capacity caused by insulating coating material (Al2O3, MnO2...etc) And also, this day, Co free structure of Ni rich cathode material has received more great attention as an alternative to NCM because of its hazardous toxicity and increasing price of Co. At this perspective, many studies have demonstrated Core-Shell or FCG (Full Concentration Gradient) structure can make better structural stability without Co metal than surface coating, which has insulated coating materials. In addition, core shell structure is easier to control the composition than FCG structure. However, in Core-Shell structure, interdiffusion of shell metal to core deteriorate the stability of Core-Shell material, so it needs very delicate heat treatment. And generally, to prevent interdiffusion from traditional core-shell structure, lower synthesis temperature or high valence metal dopant would be needed and that leads lower initial specific capacity or additional doped metal. So, we prevent the Mn interdiffusion by changing valence state of Mn in precursor through simple convection drying process not vacuum drying without decreasing synthesis temperature and additional dopant. Atomic interdiffusion in layered metal oxide structures follows the atomic migration through octahedral and tetrahedral sites. In case of various stated Mn, higher valence state Mn has higher energy barrier to migrate between each Oh and Td sites. Based on this theory, surface Mn rich shell can be oxidized easily under convection oven drying and highly oxidized Mn will be remained better than lower state Mn during high temperature calcination. These more remained Mn in shell can protect the particle surface from electrolyte attack and also higher valence state Mn makes slightly more Ni2+ due to thermodynamic stability and charge balance of Mn on the surface. And that Ni2+ in Li slab (Cation mixing) acts as a pillar to suppress irreversible phase transition of Ni rich materials. Through this surface passivation, phase transition (layered to rock salt ) propagation surface to bulk can be blocked. Furthermore, mechanical pulverization of secondary particle is also prevented because of less permeating electrolyte into bulk structure. Therefore, Li ion diffusion will not be sluggish after cycling as observed by GITT. In addition, more clear Ni rich core assure high specific capacity and faster electrochemical reaction kinetic without severe capacity fading. And also, in Full cell test (using graphite as anode), convection oven dried sample has better structure stability and that means our strategy for core-shell LiNi0.97Mn0.03O2 sample can be good candidate to alternate conventional cathode active material for Lithium ion battery practically. As prepared materials, atomic distribution was observed by EDS and XPS analysis. This study suggests useless of vacuum drying and more cost effective way to prepare Co free Core Shell LiNi0.97Mn0.03O2 cathode material for Lithium Ion Batteries.

Morphology, structural and magnetic study of superparamagnetic Mg0.5Zn0.5Fe2-xLaxO4[formula omitted] ferrite nanoparticles synthesized by chemical coprecipitation method

Lanthanum doped magnesium-zinc spinel ferrite samples were synthesized by the simple co-precipitation method with composition Mg0.5Zn0.5Fe2−xLaxO4(x=0,0.025,0.05,0.075and0.1). The synthesized ferrite samples were investigated using by powder X-ray diffractometer (XRD), Fourier transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FE-SEM) coupled with energy dispersive spectroscopy (EDS), high resolution transmission electron microscopy (HR-TEM), electron paramagnetic resonance spectroscopy (EPR) and vibrating sample magnetometer (VSM). The effect of the La3+ ion doping on the crystallite size and lattice parameter of the material has been discussed. The crystallite size for all the samples were found to be in the range of 7–12 nm. The percentage porosity and percentage crystallinity of all the samples were found to be in the range of 49–50% and 80–85% respectively. In the frequency region of 400–600 cm−1 two absorption bands attributed to the Td and Oh sites of the spinel ferrite have been observed. FTIR band spectra shift from lower to higher region with La3+ ion doping. Agglomeration of the particles is observed in the FE-SEM and HR-TEM images. Additional impurities except carbon are not present in the sample which is confirmed by EDS analysis. EPR analysis shows a decrease in the g-value from 2.18 to 2.12 for the synthesized samples with La3+ ion doping. The resonance field for synthesized sample shifts to higher magnetic field which may be due to ferromagnetic interaction caused by La3+ ion doping. The maximum spin concentration (1.508 × 1021 spin/g) and the relaxation time (4.132 × 10−11 s−1) was observed for the MZL2 composition. Magnetization curve shows the typical superparamagnetic nature of synthesized samples. The calculated squareness value of the synthesized sample refers to the magnetostatic interaction among particles. The obtained coercive field value is low which makes these materials suitable for ferrofluids and hyperthermia treatment applications.

Magnetic Field Manipulation of Tetrahedral Units in Spinel Oxides for Boosting Water Oxidation

Magnetic field enhanced electrocatalysis has recently emerged as a promising strategy for the development of a viable and sustainable hydrogen economy via water oxidation. Generally, the effects of magnetic field enhanced electrocatalysis are complex including magnetothermal, magnetohydrodynamic and spin selectivity effects. However, the exploration of magnetic field effect on the structure regulation of electrocatalyst is still unclear whereas is also essential for underpinning the mechanism of magnetic enhancement on the electrocatalytic oxygen evolution reaction (OER) process. Here, it is identified that in a mixed NiFe2 O4 (NFO), a large magnetic field can force the Ni2+ cations to migrate from the octahedral (Oh ) sites to tetrahedral (Td ) sites. As a result, the magnetized NFO electrocatalyst (NFO-M) shows a two-fold higher current density than that of the pristine NFO in alkaline electrolytes. The OER enhancement of NFO is also observed at 1 T (NFO@1T) under an operando magnetic field. Our first-principles calculations further confirm the mechanism of magnetic field driven structure regulation and resultant OER enhancement. These findings provide a strategy of manipulating tetrahedral units of spinel oxides by a magnetic field on boosting OER performance.

In Situ/Operando Soft X-ray Spectroscopic Identification of a Co4+ Intermediate in the Oxygen Evolution Reaction of Defective Co3O4 Nanosheets.

Defect engineering is an important means of improving the electrochemical performance of the Co3O4 electrocatalyst in the oxygen evolution reaction (OER). In this study, operando soft X-ray absorption spectroscopy (SXAS) is used to explore the electronic structure of Co3O4 under OER for the first time. The defect-rich Co3O4 (D-Co3O4) has a Co2.45+ state with Co2+ at both octahedral (Oh) and tetrahedral (Td) sites and Co3+ at Oh, whereas Co3O4 has Co2.6+ with Co2+ and Co3+ at Td and Oh sites, respectively. SXAS reveals that upon increasing the voltage, the Co2+ in D-Co3O4 is converted to low-spin Co3+, some of which is further converted to low-spin Co4+; most Co2+ in Co3O4 is converted to Co3+ but rarely to Co4+. When the voltage is switched off, Co4+ intermediates quickly disappear. These findings reveal Co(Oh) in D-Co3O4 can be rapidly converted to active low-spin Co4+ under operando conditions, which cannot be observed by ex situ XAS.

The impact of indium ion on structural, magnetic, and electrodynamic traits of Co-Ni nanospinel ferrites

Indium doped cobalt–nickel nano-spinel ferrites, Co0.5Ni0.5Fe2-xInxO4 (x ≤ 0.10) (NSFs), have been fabricated through solution-combustion (SC) approach and characterized for structural, magnetic morphological behavior. XRD (X-ray powder diffraction) affirmed that all NSFs have a single phase without any impurities. SEM (Scanning Electron Microscope) along with EDX (Energy dispersive X-ray), TEM (Transmission Electron Microscope), HR-TEM (High-Resolution Transmission Electron Microscope) analyses, and elemental mappings confirmed the morphology and chemical composition of all products, respectively. The crystallite size has decreased from ∼50 nm to ∼19 nm, while the lattice constant has been seen to vary because of the replacement of Fe3+ ions by In3+ ions. The fitting of room temperature Mössbauer spectra provided the hyperfine parameters, and it was found that the In3+ ions occupy the A site. The isomer shift values are associated with the characteristics of the high spin Fe3+ charge state. The magnetic behaviors of produced nanospinel ferrites (NSFs) were assessed through M vs. H loops registered at 10 and 300 K. It was found that the pristine as well as In3+ ions substituted Co-Ni NSFs exhibit ferrimagnetic nature at both temperatures. In addition, considerable variations in the magnetic parameters were noticed with respect to the level of the substituent element. The values of Ms at both 10 and 300 K increased up to x = 0.03 and then decreased with a further increase in the indium content. Unlike the variations in Ms and Mr, the Hc values at both 10 and 300 K are found to reduce almost linearly with the indium concentration (x). The variations in different magnetic parameters were explained based on the cation distribution, magnetic moments, ionic radii, magnetocrystalline anisotropy, structural and morphological changes, etc. It was predicted that In3+ ions tend to substitute Fe3+ ions at the Td (A) sites for x ≤ 0.03 and then start to partially substitute Fe3+ ions at the Oh (B) sites as x reaches above 0.03. Such a prediction in combination with the variations in the entire magnetic moment could explain the observed variations for Ms values. The reduction in Hc values was mainly ascribed to a drop in the anisotropy constant. Electrodynamic properties were investigated as S-parameters (S11-S21) in the 33–50 GHz frequency range. Main parameters such as real and imaginary parts of permittivity and permeability vs. frequency were calculated based on the standard model. A good correlation between the indium content and high-frequency properties was determined. It was observed the decrease of the electrical and magnetic losses with the increase of the indium concentration due to the diamagnetic origin and differences between Fe3+/In3+ ionic radii of the substituent.

Active coordination sites of Co spinel oxides for NO reduction by CO

Cobalt spinel oxides, containing octahedral (Oh) and tetrahedral (Td) sites, exhibit high activity for NO reduction by CO. The active sites of cobalt spinel oxides for NO–CO reaction was identified using various cobalt spinel oxides having different CoOh/CoTd ratio. Comparing the CoOh/CoTd ratio determined by EXAFS fitting analysis and the rate of NO reduction, it was clarified that octahedral site is responsible for NO reduction by CO. Furthermore, synchrotron-based in-situ XAFS measurements unveiled the contribution of CoOh sites: The cobalt spinel oxides with high CoOh fraction were more easily reduced by CO to form oxygen vacancies than those with low CoOh fraction, and resulted in the higher activity for NO reduction by CO.

Experimental and optical studies of the new organic inorganic bromide: [(C3H7)4N]2CoBr4

The alkylammonium halogenocobalate families are subjected to diverse studies according to their wide field application. In this work, the [(C3H7)4N]2CoBr4 compound was synthesized using a slow evaporation solution growth method at room temperature and characterized by X-ray powder diffraction (XRPD) and UV-VIS-IR absorption spectrometry. The X-ray powder diffraction (XRPD) data confirmed the formation of a single-phase with monoclinic system (C2/c space group). The optical absorption and reflectance spectra of the named compound were measured at the range [240 nm–1900 nm] at room temperature. Using three methods, we have determined the direct optical band gap estimated at 3.8 eV and the Urbach energy. Different models were used to calculate the refractive index, which was found to be in the range of 2.12–2.58. This spectrum consists also of electronic transition d-d of Co2+ (3 d7) ions. This work examines a detailed crystal-field analysis of the UV–Vis–NIR lines associated to the transitions between the Stark levels of Co2+, occupying a Td site symmetry in [CoBr4]2- (organic part). The theoretical study is based on the Racah tensor algebra methods. This analysis permits us to determine the electronic structure of the cobalt ion Co2+ based on the crystal field theory. As a result, Racah and crystal-field parameters have been reliably obtained. A good agreement between the theoretical and the experimental energy levels are obtained. This theoretical study confirms the Td site symmetry of Co2+ in [CoBr4]2- and leads to a satisfactory correlation between experimental and calculated energy levels. In addition to internal transitions, the absorption spectrum shows vibrational peaks witness of anharmonic behavior.