CoFe2O4 Nanocrystals Research Articles

Microwave-edge-thermal effect induced growth of coral-like self-supporting CoFe2O4/NF electrocatalysts for efficient water splitting at high-current–density

High-efficiency and long-term water-splitting to produce hydrogen under the high-current–density (HCD) environment is a severe challenge because the swift-consumption of electrolyte and large-production of bubbles require better charge/mass-transfer capability and durability of electrocatalysts. Rationally design and effectively construct of three-dimensional (3D) hierarchical electrocatalysts is the promising solutions to accelerate the charge/mass transfer rates during water splitting process. Herein, a 3D self-supporting coral-like oxygen evolution reaction (OER) electrode was synthesized using a microwave-edge-thermal (MET) effect induced heterogeneous-nucleation/oriented-attachment growth process by in-situ growing CoFe2O4 nanocrystals on nickel-foams (CoFe2O4/NF). SEM, TEM, electrolyte/catalyst contact angles, in-situ observation of O2 bubbles generation-evolution process, and three-electrode configuration (TEC) and anion-exchange-membrane (AEM) electrocatalytic OER tests at the HCD were used to investigated the structure–activity relationship of the CoFe2O4/NF electrode. The CoFe2O4/NF electrode offered super hydrophilic/aerophobic surfaces and firmly bonded interfaces, resulting in quicker charge/mass-transfer rates and long-term durability under the HCD conditions. The typical electrode exhibited a low OER overpotential of 0.24 V at 10 mA cm−2 and a Tafel slope of 35.2 mV dec−1; its alkaline AEM electrolyzer (Pt || CoFe2O4/NF) yielded a HCD of 1200 mA cm−2 at only 2.36 V and 60˚C and kept stable over 120 h. This work provides a new insight into the design and fabrication of active and stable non-noble metal-based OER electrocatalysts for cost-effective water-electrolysis applications.

Mesoporous CoFe2O4 nanocrystals: Rapid microwave-hydrothermal synthesis and effect of synthesis temperature on properties

Cobalt ferrite (CoFe2O4) nanoparticles of the size of 7–20 nm have been easily synthesized by microwave-hydrothermal route with rapid heating at 100, 150, and 200 °C. The XRD and TEM studies establish that the pure spinel CoFe2O4 phase is obtained at all the synthesis temperatures. FTIR examination presents the bands representative of cubic CoFe2O4 phase and triton X-100 surfactant. Thermal analysis shows nominal weight loss between 25 and 1000 °C demonstrating high thermal stability of the samples. Electron microscopy examination reveals spherical morphology with the particle size increasing as the synthesis temperature increases. The EDX spectrum reveals the stoichiometric proportion to be nearly 1:2 for Co: Fe as anticipated, and the elemental mapping results show the extreme distribution of elements Co, Fe, and O. The SAED patterns show bright and distinct diffraction rings associated with the different lattice planes of cubic CoFe2O4 phase. The clear lattice fringes in HRTEM images endorse the single-crystalline nature of the nanoparticles. N2 sorption studies demonstrate mesoporous structure with specific surface area decreasing with rising synthesis temperature. These nanoparticles exhibit hysteretic magnetization at room temperature, characteristic of the ferrimagnetic material, with the room temperature saturation magnetization close to that of bulk CoFe2O4. The MS value increases, but Mr and Hc values decrease as the synthesis temperature/particle size increases. It is anticipated that the findings of this study will offer more insight into the effective use of microwave-hydrothermal synthesis to tailor the properties of CoFe2O4 nanoparticles for applications.

Unveiling the Coercivity-Induced Electrocatalytic Oxygen Evolution Activity of Single-Domain CoFe2O4 Nanocrystals under a Magnetic Field.

Spin polarization modulation in ferromagnetic materials has become an effective way to promote the electrocatalytic oxygen evolution reaction (OER). Herein, to reveal the coercivity-related OER performance, single-domain ferromagnetic CoFe2O4 (CFO) nanocrystals with different coercivities are synthesized and subjected to OER under an in situ tunable magnetic field. As the more ordered spin polarization state of CFO with a higher coercivity can afford a facilitated electron transfer process, the magnetic field-assisted OER activity can be more improved with an increase in coercivity. Specifically, the decreased magnitudes of the overpotential, Tafel slope, and charge transfer resistance increase on the samples with higher coercivity. The CFO with the largest coercivity (7500 Oe) shows the best OER performance with an overpotential of 350 mV at a current density of 10 mA cm-2 under a magnetic field of 14000 G. In addition, a hysteresis effect that maintains enhanced OER current density after the magnetic field has been withdrawn is observed, where higher coercivity affords a longer hysteresis period. The exploration of coercivity-related OER enhancement may provide new insights into the design and synthesis of promising "magnetic effect" catalysts.

Assembling g-C3N4 nanosheets on rod-like CoFe2O4 nanocrystals to boost photocatalytic degradation of ciprofloxacin with peroxymonosulfate activation

Water pollution caused by organic pollutants seriously threatens the ecological balance. Photocatalytic oxidation technology with green, environment-friendly characteristics can effectively dispose of organic pollutants in water, is the potential replacement of the traditional sewage treatment technology. In this work, CoFe2O4/g-C3N4 heterojunction with nanosheets-on microrods structure was designed and prepared by a simple solvothermal-calcination synthetic method, which was applied for the degradation of cyprofloxacin (CIP) through the combination of peroxymonosulfate (PMS) and photocatalysis. The results reveal that the optimal photocatalytic degradation efficiency of CIP (20 mg/L) in the prepared CoFe2O4/g-C3N4-10/PMS system reached 97% within 60 min, and the degradation rate at a high level during the four cycles. The effects of different environmental factors (catalyst dose, PMS dose and initial CIP concentration) on the degradation of the system were studied. The enhancement of photocatalytic activity in the CoFe2O4/g-C3N4 composite can be mainly attributed to accelerated separation and transfer of electrons and holes through the formation of heterojunctions, which is beneficial to stimulate PMS to produce more SO4•- and •OH.

Oxygen-vacancy-rich spinel CoFe2O4 nanocrystals anchored on cage-like carbon for high-performance oxygen electrocatalysis

Oxygen-vacancy-rich spinel CoFe2O4 nanocrystals anchored on cage-like carbon for high-performance oxygen electrocatalysis

CoFe2O4 nanocrystals for interface engineering to enhance performance of perovskite solar cells

Interface engineering has been widely applied to perovskite solar cells (PSCs) for solving the problems caused by high density of parasitic traps and the defects that exist at grain boundaries and surfaces. Various inorganic metal oxides play roles in PSCs, helping to achieve higher efficiency and better long-term stability. In this work, two-phase hydrothermal method is used for CoFe2O4 nanocrystals (NCs) synthesis and the obtained NCs were used to modify the surface between perovskite (PVK) and hole transport layer (HTL). Results show that CoFe2O4 modified devices have enhanced hole extraction, reduced trap density, and hydrophobic ligand connected to CoFe2O4 NCs provide better moisture resistance to PVK layer. Devices with CoFe2O4 layer exhibit a best PCE of 19.65% with improved open circuit voltage (VOC), fill factor (FF) and reproducibility. At steady-state, CoFe2O4 modified devices show a current density of 20.16 mA/cm2 and PCE of 18.97%.

Harnessing the Synergistic Interplay of Fischer‐Tropsch Synthesis (Fe‐Co) Bimetallic Oxides in Na‐FeMnCo/HZSM‐5 Composite Catalyst for Syngas Conversion to Aromatic Hydrocarbons

AbstractThe efficient conversion of syngas to aromatic hydrocarbons (STA) has attracted attention in recent years for its extensive utilization in the energy and defense sectors. In current study, the alloying of two active FT synthesis metal components (Fe−Co) was employed in Na‐FeMnCo/HZSM‐5 composite catalyst for STA process. Different calcination temperatures and Fe/Mn/Co molar ratios were modulated for harnessing the different Fe2O3/CoFe2O4/MnCo2O4 structures that possessed divergent structural, reduction, carburization and catalytic behaviors to give Fe3O4/FexC/CoxC reactive species. Herein, the effective inclusion of Co and Mn into Fe brought about the expedient geometric and electronic modulations for providing the homogenously distributed CoFe2O4 nanocrystals. This particular substitution of Fe played a vital role for tuning the density of oxygen vacancies, tailoring the reduction behavior of Fe‐Ox, giving the Fe−Co alloy structure, and adjusting the adsorption capabilities of catalytic surface that mainly dictate the concentration of active Fe5C2 phase. An escalated selectivity to olefinic intermediates and subsequent higher aromatics fractions (∼55 %) thus was imparted with an inhibited CO2 (21 %) generation by meliorating the different Fischer‐Tropsch/Aromatization reactions in stable CO conversion of ∼98 %.

Magnetic CoFe2O4 nanocrystals derived from MIL-101 (Fe/Co) for peroxymonosulfate activation toward degradation of chloramphenicol

In this study, porous magnetic CoFe2O4 nanocrystals (NCs) were successfully synthesized by using bimetal-organic framework (MOF) as a precursor, and used as catalysts to activate peroxymonosulfate (PMS) for the removal of chloramphenicol (CAP) in the solution. The structure and physicochemical properties of CoFe2O4 NCs were thoroughly examined by a series of characterization techniques. The results revealed as-synthesized CoFe2O4 had a nanorod-shaped structure with high specific surface area (83.00 m2 g−1) and pore volume (0.31 cm3 g−1). Furthermore, the degradation efficiency (100%) and the removal of total organic carbon (68.09%) were achieved after 120 min with 0.1 g/L CoFe2O4 NCs, 2 mM PMS and 10 mg/L CAP at pH of 8.20. In addition, effects of catalyst dosage, PMS dosage, initial pH values, CAP concentration and co-existing anions as well as natural organic matters in the solution on the degradation efficiencies were studied and all the removal can be well fitted with pseudo-first-order kinetic model (R2 > 0.96). Sulfate radicals (SO4•−) and hydroxyl radicals (HO•) were proved to be two main reactive species for CAP removal in CoFe2O4/PMS system based on quenching experiments. CAP was degraded by the main pathways of dichlorination, denitration, decarboxylation, hydroxylation, ring cleavage and chain cleavage on CoFe2O4/PMS system through high performance liquid chromatograph-mass spectrometry analysis. We believe that this study would be very meaningful to promote the applications of MOFs-derived catalysts on the SO4•− based advanced oxidation processes (SR-AOPs) for the environmental remediation.

Size-effect induced cation redistribution on the magnetic properties of well-dispersed CoFe2O4 nanocrystals

Abstract This work reports the synthesis of size-controlled and well-dispersed CoFe2O4 nanocrystals through a facile thermal decomposition method, of which size can be simply tuned via the change of reaction time. The fcc Co or hcp Co impurities can be precipitated out in the CoFe2O4 nanocrystals by further prolonging the reaction time. Magnetic measurements show that the pure CoFe2O4 nanocrystals show size-dependent ferrimagnetism at room temperature, in which the saturation magnetization increases and the coercivity anomalously decreases as the particle size increases. The variation of magnetic properties is attributed to the surface-randomized spin and cations redistribution caused by surface effect and size effect. Besides, the precipitated Co impurities can greatly improve the saturation magnetization and coercivity of the CoFe2O4 nanocrystals. This paper provides a new path to tailor the magnetic properties of spinel ferrite nanostructures by tuning the cations distribution or adding magnetic impurities.

In situ synthesis of CoFe2O4 nanocrystals decorated in mesoporous carbon nanofibers with enhanced electromagnetic performance

CoFe2O4 nanocrystals decorated in mesoporous carbon nanofibers (CoFe2O4/C NFs) are prepared by electrospinning and subsequent calcination. The CoFe2O4/C NFs exhibit optimal electromagnetic waves absorption performance. The minimum reflection loss (RL) of CoFe2O4/C NFs reaches −14.0 dB at a frequency of 9.0 GHz and a thickness of 3.5 mm with low doping mass of 20 wt%. In addition, the maximum effective absorption bandwidth range (RL ≤ -10 dB) is 3.6 GHz with a thickness of 2.5 mm. The interfacial polarization effect of the material itself and the synergistic effect between the binary components are promoted by combining one dimensional (1D) carbon nanofibers with magnetic CoFe2O4 spinel nanocrystals. Therefore, this strategy can achieve good impedance matching and effectively improve the absorbing properties of our materials. This work shows that enriching the diversity of material loss mechanisms, constructing hierarchical nanostructures of fibers, the simple and mature preparation methods can make CoFe2O4/C NFs composites a potentially new-type of microwave absorbing material which can be easily put into production.

Enhanced Activation of Persulfate by Meso-CoFe2O4/SiO2 with Ultrasonic Treatment for Degradation of Chlorpyrifos.

Magnetic mesoporous CoFe2O4/SiO2 (Meso-CoFe2O4/SiO2) composites were simply synthesized. On the basis of previous studies, optimum preparation conditions of their structure and physical properties can be readily determined. CoFe2O4 nanocrystals and their mesoporous structure were authenticated by low-angle and wide-angle X-ray diffraction, high-resolution transmission electron microscopy, scanning electron microscopy, element mapping, X-ray photoelectron spectroscopy, nitrogen adsorption isotherms, and so on. They were applied to degrade chlorpyrifos where Meso-CoFe2O4/SiO2 composites provide a mesoporous microenvironment and combined with ultrasonic treatment can enhance heterogeneous activation of persulfate. Research findings showed that the system can be conducive to remove quickly chlorpyrifos and the removal ratios reached 99.99%. The results provided a strategy for the chlorpyrifos degradation and, similarly, pollution control of pesticide wastewater.

Magnetic and Electrochemical Properties Study of CoFe2O4 Nanocrystals Synthesized by a Facile Hydrothermal Route

Magnetic CoFe2O4@carbon (CFO@C) nanoparticles were synthesized by employing glucose as carbon source via hydrothermal process, and their magnetic and electrochemical properties of CFO@C are both studied in this work. The Ms and Mr values of CFO@C nanoparticles are lower than those of pure CFO samples. The changed magnetic properties may be related to the carbon layer extinguishing the surface magnetic moment with spin canting. Benefiting from the amorphous structure and good electronic conductivity of carbon shells, the CFO@C 20 wt % electrode exhibited the capacity of 201 mA h g–1 at the current density of 500 mA g–1 and high reversible capacity up to 353 mA h g–1 after 100 cycles at the current density of 50 mA g–1, respectively.

Magnetic Traits in CoO‐Core@CoFe2O4‐shell Like Nanoparticles

AbstractAntiferromagnetic (AF) CoO pseudo‐single crystals 125±20 nm in diameter were prepared by forced hydrolysis in polyol and used as seeds to grow a poly‐ and nano‐crystalline (5 nm in diameter) ferrimagnetic (F) CoFe2O4 shell. Transmission electron microscopy (TEM) showed a certain epitaxy between the face‐centered cubic lattices of the AF rock‐salt core and the F spinel shell. Magnetometry measurements do not evidence any exchange bias feature between the two phases in direct contact. The hysteresis loops recorded between 7 and −7 T after cooling the composite particles from 400 K (between the Néel and the Curie temperatures of the AF and F phases, respectively) to 5 K, with and without a cooling magnetic field (μ0HFC=+7 T), are the same, meaning that the AF core does not allow any spin pinning on the CoFe2O4 nanocrystals. Nevertheless, differences exist between these magnetic data and those collected on two CoFe2O4 reference particles: the low temperature coercive field of the composite is between those of 5 nm sized single‐crystals and their assembly as submicrometer polycrystals. This order was also observed for the blocking temperature, although the constituting CoFe2O4 nanocrystals of all these particles, the composite and the two references, were the same. Advanced analysis by X‐ray diffraction and TEM established that the coherent crystallographic domain length of the ferrite phase and the magnetic behavior of the three systems are interdependent. Depending on the growth conditions of the CoFe2O4 phase, a relative epitaxy between the nanocrystals may exist. It is much greater in the polycrystalline CoFe2O4 particles than in their CoO−CoFe2O4 core‐shell counterparts, and does not occur in free single crystal CoFe2O4 particles.

Synthesis, structure and magnetic properties of multipod-shaped cobalt ferrite nanocrystals

A non-aqueous sol–gel route followed by oriented attachment to make multi-pod CoFe2O4 nanocrystals showing large room temperature saturation magnetization.

2D Magnetic Mesocrystals for Bit Patterned Media

AbstractBit patterned media exhibits great potential in satisfying the increasing demand for higher information storage density, but the present media fabrication technique is complicated. Here, a 2D magnetic mesocrystal with monodisperse CoFe2O4 (CFO) nanocrystals is fabricated through a combination of nanoseeds layer growth and oxides self‐assembly method. The monodisperse CFO nanomagnets protrude from the substrate surface with perfect crystal facets, specific crystallographic orientation, and are fully relaxed. Systematic magnetic characterizations, such as dynamic cantilever magnetometry for single CFO nanocrystal, magnetic force microscopy, and superconducting quantum interference device magnetometry for large area, reveal the novel tilted magnetic anisotropy and the switchable single domain state. This work provides a new way to fabricate bit patterned media with controlled material structure simply by a nanoseeds‐mediated self‐assembly method.

MOF-templated synthesis of CoFe2O4 nanocrystals and its coupling with peroxymonosulfate for degradation of bisphenol A

Porous magnetic cobalt ferrite nanocrystals (CoFe2O4 NC) are synthesized via a one-step process by using bimetal-organic frameworks (Co/Fe bi-MOFs) as a template for the catalytic degradation of bisphenol A (BPA). The properties of the prepared catalyst are evidenced by a series of characterization techniques. Overall, the unique ferromagnetic nature (34.73 emu/g) make its efficient separation from the liquid phase possible. CoFe2O4 NC activates peroxymonosulfate (PMS) to degrade BPA more efficiently than hydrothermally fabricated CoFe2O4 nanoparticles. The difference in catalytic capacity is attributable to the larger specific surface area (60.4 m2 g−1) and well developed mesoporous structure (0.64 cm3 g−1) of the CoFe2O4 NC. EPR analysis demonstrate the production of HO and SO4− radicals in the CoFe2O4 NC/PMS system. The degradation process positively correlates with the increase of initial solution pH, catalysts dosage, and PMS dosage. The degradation rate of 0.112 min−1 is achieved at [PMS]/[BPA] of 10, catalyst dosage of 0.1 g L−1, temperature of 25 °C, and initial pH of 10.2 in deionized water. The existence of Cl- and HCO3−/CO32− show significant positive synergistic effects on the catalytic process. Moreover, simple thermal treatment at 400 °C for 15 min in open air fully regenerates the catalytic capacity of CoFe2O4 NC for reuse. Findings from this work shed light on the rational design of bimetallic oxide catalysts and provide new insight into the development of high-performance magnetic separable catalysts.

CoFe2O4 Nanocrystals Mediated Crystallization Strategy for Magnetic Functioned ZSM‐5 Catalysts

AbstractZeolites have many applications in the petrochemical and fine chemical industry and their functionalization does expand the spectrum of potentials. However, the integration of functional nanocrystals into zeolite frameworks with controlled size, dispersion, and crystallization behavior still remains a significant challenge. Here, a new synthesis of magnetic functioned ZSM‐5 zeolite catalysts via a CoFe2O4 nanocrystal mediated crystallization strategy is reported. It is found that high crystallinity of CoFe2O4 nanocrystals results in a well‐dispersed encapsulation of them into a single‐crystal of ZSM‐5 due to non‐further‐grown nanocrystals during the fast ZSM‐5 growth. On the contrary, low crystallinity of CoFe2O4 nanocrystals leads to the polycrystalline zeolite growth due to the secondary growth of nanocrystals accompanied by the zeolite crystallization and large lattice mismatch between them. The successful encapsulation of small CoFe2O4 nanocrystals (≈4 nm) into single crystals lies on the preattachment of them into solid silica gel. During the growth of ZSM‐5 crystals, no secondary growth of nanocrystals happens and its motion is restricted. The encapsulation of magnetic CoFe2O4 nanocrystals not only endows magnetic function into zeolites for the first time, but also does not impact catalytic performance of ZSM‐5 in acetalization of cyclohexanone with methanol, which is highly promising in catalytic industries.

Synthesis, structure and magnetic properties of CoFe2O4 ferrite nanoparticles

CoFe2O4 nanoparticles with different morphologies were successfully synthesized by co-precipitation method, followed by heat treatment. XRD patterns showed that a single phase ferrite was obtained. By the method of heating, the effect on the morphology and properties of the CoFe2O4 nanoparticles is studied. The synthesis conditions (oxidation process and temperature) dominated the formation of crystals with different morphologies. The oxidation and growth mechanisms behind the formation of CoFe2O4 nanocrystals were investigated. The saturation magnetization of the CoFe2O4 samples was lower than the bulk value, which could be explained by the reduction of the particle size. The highest value of coercivity of all the samples is 155.5 kA m−1 for the CoFe2O4 nanorods. Compared with the Hc value of cobalt ferrite nanoflakes, the rod shaped cobalt ferrite has a higher coercivity. The remanence ratio of the CoFe2O4/FeCo is 0.56, which indicates a strong exchange coupling between hard and soft phase.

Cyclic Voltammetry, Impedance and Thermal Properties of CoFe2O4 Obtained from Waste Li-Ion Batteries

In the present study, CoFe2O4 nanocrystals were synthesized from waste lithium ion battery powder using citric acid as eco- friendly material by hydrothermal method. The prepared materials were characterized by X-ray powder diffraction (XRD) to study particle size and crystallinity, the morphology of the nano composite are analysed by using Scanning Electron Microscopy (SEM), functional group were observed using Fourier Transform Infrared Spectrometer (FTIR). The thermal properties of CoFe2O4 nanocrystals were analysed using Thermo Gravimetric Analysis (TGA), Differential Scanning Calometry (DSC) and Differential Thermal Analysis. The synthesized nano particle was studied for the electrochemical studies such as Cyclic Voltammetry (CV), electrochemical impedance spectra, capacitance and electron transfer resistance

Role of Composition and Size of Cobalt Ferrite Nanocrystals in the Oxygen Evolution Reaction

AbstractSub‐10 nm CoFe2O4 nanoparticles with different sizes and various compositions obtained by (partial) substitution of Co with Ni cations have been synthesized by using a one‐pot method from organic solutions by the decomposition of metal acetylacetonates in the presence of oleylamine. The electrocatalytic activity of CoFe2O4 towards the oxygen evolution reaction (OER) is clearly enhanced with a smaller size (3.1 nm) of the CoFe2O4 nanoparticles (compared with 4.5 and 5.9 nm). In addition, the catalytic activity is improved by partial substitution of Co with Ni, which also leads to a higher degree of inversion of the spinel structure. Theoretical calculations attribute the positive catalytic effect of Ni owing to the lower binding energy differences between adsorbed O and OH compared with pure cobalt or nickel ferrites, resulting in higher OER activity. Co0.5Ni0.5Fe2O4 exhibited a low overpotential of approximately 340 mV at 10 mA cm−2, a smaller Tafel slope of 51 mV dec−1, and stability over 30 h. The unique tunability of these CoFe2O4 nanocrystals provides great potential for their application as an efficient and competitive anode material in the field of electrochemical water splitting as well as for systematic fundamental studies aiming at understanding the correlation of composition and structure with performance in electrocatalysis.