Fe3O4 Films Research Articles

One-step in-situ formation of TiO2 nanosheets interconnected hematite photoanode for enhanced water oxidation

In this study, by using a facile one-pot hydrothermal method, a Ti3C2 MXene derived TiO2 nanosheets/hematite photoanode with interconnected structures is fabricated via the in-situ chemical etching and exfoliation of bulk Ti3AlC2 powder. Compared to the pristine Fe2O3 film, the as-obtained Fe2O3/TiO2 photoanode exhibits an increased photocurrent density of 1.72 mA cm−2 at 1.23 V vs. RHE, and a further improved performance of 2.03 mA cm−2 at 1.23 V vs. RHE accompanied by a cathodic shift in the onset potential are also achieved after the loading of FeNiOOH cocatalyst. Further characterizations reveal that the enhanced PEC performance of Fe2O3/TiO2 photoanode can be ascribed to the facilitated charge separation and transport as well as the accelerated OER kinetics owing to the decoration of TiO2 nanosheets and the formation of interconnected structures. This study provides a favorable route for effectively improving the PEC activity of hematite photoanode, and also promotes the application of Ti3C2 MXene in solar water splitting.

Fast Atomic Diffusion: Bulk and Short‐Circuit Anion Diffusion in Epitaxial Fe2O3 Films Quantified Using Buried Isotopic Tracer Layers (Adv. Mater. Interfaces 9/2021)

Fast atomic diffusion through structural defects in Fe2O3 is directly imaged for the first time at the sub-nanometer level by atom probe tomography. In article number 2001768 by Tiffany C. Kaspar and co-workers, model epitaxial and defect-rich thin films of Fe2O3 are deposited with molecular beam epitaxy, incorporating a tracer layer of 18O within the film itself. Atomic-level self-diffusion is then visualized and quantified in 3D by APT, revealing ≈104 times faster oxygen diffusion along structural flaws than through the pristine lattice.

Polarization switching in Bi0.8La0.2FeO3 films with ferroelectric/semiconductor heterojunctions

Bi0.8La0.2FeO3 (BLFO) films are deposited directly on low-resistance Si (100) substrates by the pulsed laser deposition method. Compared with the typical hysteresis loops of ferroelectric films, those of the BLFO/Si samples, with almost horizontal upper and lower ends, severely shrink in the center part, and the coercive voltage is extremely small. A systematic analysis of the dielectric and ferroelectric characteristics reveals that a p–n junction forms between BLFO and Si, with oxygen vacancies accumulating near the interface. The polarization switching is accompanied by the change of the interface barrier and the migration of charged defects. This work provides a reference for the direct growth of ferroelectric films on silicon substrates and is of great significance for the development of the silicon-based electronic devices.

Comprehending the role of the S-phase on the corrosion behavior of austenitic stainless steel exposed to H2S/CO2-saturated liquid and vapor environments

The corrosion behavior of austenitic stainless steel after low-temperature liquid oxy-nitriding (LON) was investigated by exposing in H2S/CO2-saturated liquid and vapor environments up to 720 h at 60 °C. The corrosion rates before and after LON were compared by the weightlessness method, and the microstructure as well as the corrosion scales were characterized using surface analysis methods. The results indicated that the composite S-phase layer with the outer Fe3O4 layer and the inner nitrogen-rich sublayer could improve the corrosion performance in H2S/CO2-saturated environment. The base material (BM) suffered local corrosion first, which then transformed into uniform corrosion. As a comparison, The LON sample, covered with a thin corrosion product layer, indicated slight local corrosion. The excellent corrosion resistance of the S-phase should be attributed to the blocking effect of the continuous Fe3O4 film as well as the suppression of the atomic mobility by the nitrogen-containing supersaturated solid solution.

Rational construction of porous N-doped Fe2O3 films on porous graphene foams by molecular layer deposition for tunable microwave absorption

Graphene-based materials with porous microstructure have attracted immense attentions due to their wide application in microwave absorption. However, constructing magnetic film with both porous microstructure and uniform pore size by using traditional methods still remains a challenge. To overcome this problem, we reported a facile strategy of molecular layer deposition (MLD) for successfully fabrication of the hybrid-architecture of porous graphene foams and nitrogen-doped porous Fe2O3 films. The surfaces of porous graphene foams are uniformly covered by porous Fe2O3 films without aggregation and the pore structures are widely distributed. The porous graphene-based composites exhibit remarkably enhanced microwave absorption performance compared to the pristine graphene foams. The minimum reflection loss value is increased by approximately 8 times, reaching −64.36 dB with a thickness of only 2.18 mm. More importantly, the absorption property can be precisely modulated by tuning the MLD cycle numbers and effective absorption bandwidth covers 3.04–18.0 GHz by adjusting the thickness from 1.0 to 5.0 mm. This work provides new insights for exploring novel and high-performance graphene-based microwave absorbents and offers a new idea to rationally design three-dimensional composites with porous magnetic films.

Magnetic Texture in Insulating Single Crystal High Entropy Oxide Spinel Films.

Magnetic insulators are important materials for a range of next-generation memory and spintronic applications. Structural constraints in this class of devices generally require a clean heterointerface that allows effective magnetic coupling between the insulating layer and the conducting layer. However, there are relatively few examples of magnetic insulators that can be synthesized with surface qualities that would allow these smooth interfaces and precisely tuned interfacial magnetic exchange coupling, which might be applicable at room temperature. In this work, we demonstrate an example of how the configurational complexity in the magnetic insulator layer can be used to realize these properties. The entropy-assisted synthesis is used to create single-crystal (Mg0.2Ni0.2Fe0.2Co0.2Cu0.2)Fe2O4 films on substrates spanning a range of strain states. These films show smooth surfaces, high resistivity, and strong magnetic responses at room temperature. Local and global magnetic measurements further demonstrate how strain can be used to manipulate the magnetic texture and anisotropy. These findings provide insight into how precise magnetic responses can be designed using compositionally complex materials that may find application in next-generation magnetic devices.

Atomic-Scale Mechanism of Grain Boundary Effects on the Magnetic and Transport Properties of Fe3O4 Bicrystal Films.

In strongly correlated materials, change of the local lattice configuration is expected to tune or even generate new properties otherwise in the ideal bulk materials. For highly spin-polarized materials, the spin-dependent transport is sensitive to the local magnetic structure. Here, the artificial grain boundaries (GBs) with different tilt angles are produced in Fe3O4 films using SrTiO3 bicrystal substrates. The saturation magnetization of Fe3O4 bicrystal films is enhanced. The detailed atomic structural results combining with density functional theory calculations reveal that the elongated FeA-O bond length at GBs resulting in the reduction of charge transfer reduces the FeA magnetic moments, which enhances the total magnetic moments of Fe3O4. The in-plane rotation of the Fe3O4 lattice on bicrystal substrates alters the magnetization processes. Especially, the Fe3O4 bicrystal film with a tilt angle of 22.6° shows strong in-plane magnetic anisotropy due to the zigzag GBs. The altered magnetic anisotropy of Fe3O4 bicrystal films enhances the anisotropic magnetoresistance. The findings reveal the mechanism of GBs on the magnetic and transport properties and manifest that the strategy of utilizing GBs can tune the physical properties in highly spin-polarized materials.

Large spin-dependent tunneling magnetoresistance in Fe3O4/PET heterostructures developed at room temperature: A promising candidate for flexible and wearable spintronics

Half-metallic nanocrystalline magnetite (Fe3O4) thin films, with different thicknesses were developed on polyethylene-terephthalate (PET) substrates, by reactive sputtering at room temperature. Fe3O4 film (200-nm thick)/PET heterostructures possess superior electrical and magnetic characteristics, with a Verwey transition temperature (Tv) of ~122 K and a saturation magnetization (Ms) ~ 361 emu/cm3. Furthermore, the antiferromagnetic (AFM)-coupled antiphase boundaries (APBs) controlled the transport properties of the Fe3O4 thin films, due to the tunneling of spin-polarized electrons through the films. Very-high magnetoresistance (MR) value (−8.9%) were observed for H‖Film plane, constructed from Fe3O4 (200-nm thick)/PET when H values were below 60 kOe at 300 K. In addition, flexibility tests, to examine resistivity, M-H and MR, were performed using with 90° and 45° bent angles and cyclability experiments were implemented to validate the reproducibility of these characteristics. These outcomes demonstrated that Fe3O4/PET heterostructures may represent a promising candidate for flexible/wearable spintronics.

Reversible metamorphosis from Fe3O4 to FeO of epitaxial iron oxide films grown on the Fe-p(1 × 1)O surface

The reduction and oxidation of epitaxial Fe3O4 films grown by reactive deposition on a Fe-p(1 × 1)O surface have been investigated by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and scanning tunneling microcopy (STM). The as-grown iron oxide samples display a square LEED pattern with a lattice constant compatible with a p(1 × 1) bulk terminated Fe3O4(001) surface. STM topographic images of Fe3O4 are characterized by atomically flat terraces separated by highly oriented steps running along the (010) and (100) crystallographic directions of the substrate. Upon annealing at 800 K in an ultra-high vacuum, AES reveals that magnetite transforms to FeO. The sample exposes the (001) surface of the rock salt structure, with a lattice parameter close to that of bulk wüstite. The Fe3O4 phase can be recovered by oxidation at 10−6 mbar of molecular oxygen.

Facet-dependent magnon-polarons in epitaxial ferrimagnetic Fe3O4 thin films

Magnon-polarons are coherently mixed quasiparticles that originate from the strong magnetoelastic coupling of lattice vibrations and spin waves in magnetic-ordered materials. Recently, magnon-polarons have attracted a lot of attention since they provide a powerful tool to manipulate magnons, which is essential for magnon-based spintronic devices. In this work, we report the experimental observation of facet-dependent magnon-polarons in epitaxial ferrimagnetic Fe3O4 thin films via spin Seebeck effect measurement. The critical magnetic fields for the magnon-polarons in the (110)- and (100)-oriented Fe3O4 films are 1.5 T and 1.8 T, respectively, which arises from the different phonon velocities along the [110] and [100] directions. As the temperature decreases, the magnon-polarons-enhanced spin Seebeck voltage decreases in both (110)- and (100)-oriented Fe3O4 films, which could be attributed to the enhanced magnon-polarons scattering at elevated temperatures. This work demonstrates the crystal structure engineering in epitaxial magnetic films as a promising route to manipulate the magnon-polarons for future magnon spintronic applications.

Effect of liquid oxy-nitriding at various temperatures on wear and molten aluminum corrosion behaviors of AISI H13 steel

In this study, liquid oxy-nitriding (LON) at different temperatures (430 °C, 450 °C, 470 °C) was employed on the AISI H13 steel. Effects of LON on wear and molten aluminum corrosion of AISI H13 steel were investigated. LON treatments significantly improved wear resistance due to synergy between high-hardness nitriding layer and lubricating oxide film. Molten aluminum corrosion resistance was also improved, especially by LON 470 treatment. Fe3O4 film and nitriding layer on LON 470 transformed the corrosion mechanisms from dissolution into oxidation. Therefore, LON process may be a choice for combating the dissolution corrosion of liquid metals.

Structural and Optical Characterization of Sprayed nanostructured Indium Doped Fe2O3 Thin Films

Nanostructured pure and Indium doped iron oxide thin films were deposited via spray pyrolysis technique (SPT). The effects of Indium (2 and % 4) concentration was studied. X-ray diffraction patterns disclosed that Fe2O3 films have a rhombohedral crystalline of α-Fe2O3 phase and their crystallite size was vary from (12.13 – 13.84) nm with Indium content. The strain(%) parameter decrease from 28.57 to 25.04, AFM images of films show changes in morphology with decreased in surface roughness from 2.75 nm to 1.7 nm with Indium 4% doping, The 3-D images and grain size distribution are illustrated that they exhibit spherical nano-grains ranged from 72.72 nm for pure Indium to 51.22 nm for 4% Indium doped Fe2O3. The transmittance decreases with increasing Indium concentration. The bandgap energy of Fe2O3 thin film was 2.75 eV and it decreases to 2.55 eV for Fe2O3:4% In.

Coupling effects of indium oxide layer on hematite enabling efficient photoelectrochemical water splitting

Hematite (α-Fe2O3) based nanomaterials are considered as the most promising photoelectrodes for solar water splitting. However, some inherent shortcomings like high density of surface states is still the critically challengeable issue. This paper reports the integration of In2O3 nanolayers (NLs) on the surface of worm-like Fe2O3 films to significantly improve the photoelectrochemical (PEC) water oxidation activity. The In2O3 NLs on the modified α-Fe2O3 photoelectrodes exhibit dual positive effects on PEC performance: i) acting as the passivation material to decrease surface electron-hole recombination losses; ii) constructing heterojunctions with Fe2O3 to facilitate efficient separation of surface charges and spatial transfer with extended lifetimes. The resulting treated Fe2O3 photoanode yields a current density of 3.4 mA cm―2 at 1.23 V vs. reversible hydrogen electrode and high incident photon-to-current efficiency of 80 % at 400 nm. These results may bring inspiration to introduce multifunctional modifiers on photoelectrodes that offer practically high-performance PEC applications.

N-doped two-dimensional ultrathin NiO nanosheets for electrochromic supercapacitor

The performance of electrochromic supercapacitors (ESCs) can be improved by the use of electrodes with high electronic conductivity and ion transport ability. Herein, two-dimensional (2D) N-doped NiO ultrathin nanosheets (N-NiO UTNSs) synthesized in situ on fluorine-doped tin oxide glass were used as bifunctional ESC electrodes. These electrodes exhibited an elevated specific capacitance (540 F g−1 at 1 A g−1), superior cycling stability (85% capacitance retention after 10,000 cycles), and good electrochromic properties (color change from colorless to black with a transmittance modulation of 73% and a coloration efficiency of up to 83.47 cm2 C−1 at 550 nm), which was ascribed to the high electronic conductivity of NiO (due to N doping) and the short ion diffusion path (due to the ultralow nanosheet thickness). To further explore the practical applications of N-NiO UTNSs, we constructed asymmetric ESCs by integrating NiO films with Fe2O3 films and revealed that these devices displayed applied voltage–dependent colors. Two charged asymmetric ESCs connected in series could light up a red light-emitting diode and changed color synchronously with charge state alteration. Thus, our results contribute to the development of Ni-based electrochromic supercapacitors exhibiting charge state-dependent color changes.

Strong spin-lattice coupling in tetragonal-like BiFeO3 films with thermal expansion anomalies

The evidence for a spin-lattice coupling in a tetragonal-like BiFeO3 (BFO) film is derived from thermal expansion measurements. Taking the advantage of La doping, the Néel temperature (TN) of the BFO film can be further adjusted over a broad range of temperatures. The lattice parameters exhibit anomalies near the Néel temperatures. The Bi0.8La0.2FeO3 film has a spin order-to-disorder transition at the TN without a structural phase transition, suggesting that a spin-lattice coupling drives the thermal expansion anomalies. The spin-lattice coupling can be enhanced by the coexistence of phases in the thicker BFO films. The density functional theory calculations show a smaller lattice constant of the c axis in the spin-order state than that in the spin-disorder state for the same tetragonal-like phase, which supports that the thermal expansion anomalies are a consequence of the spin-lattice coupling. Our findings may have application prospects for functional materials with controllable thermal expansions.

Magnetic coupling modulation of flexible Fe3O4/CoFe2O4 heterojunction grown on mica

Recently, flexible wearable devices have attracted enough attention due to their outstanding potential in practical applications. As two classic ferrite materials, Fe3O4 and CoFe2O4 (CFO) have been introduced in many applications such as catalyst, magnetic information and communication technologies. In this paper, a layer of CFO is epitaxially grown on a mica substrate deposited by pulsed laser deposition (PLD), and then a Fe3O4 films is further deposited, using sintered CFO and Fe3O4 poly crystalline targets. The as-deposited film material and surface roughness were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). By measuring the hysteresis loop of Fe3O4/CFO film, the magnetic properties of films with different curvatures were studied. The relationship between the thickness ratio of the CFO layer to the Fe3O4 layer and the magnetic properties of the thin film crystals was studied, and the tunability of the function was proved.

High-temperature oxidation behavior of the Crofer 22 APU steel coated with (La,Sr)(Co,Fe)O3 film prepared by pulsed laser deposition

The La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF48) film was deposited on Crofer 22 APU ferritic stainless steel using the pulsed laser deposition (PLD) technique. The LSCF48 film with a thickness of about 1.1 μm was dense and homogeneous, with good adhesion to the metallic substrate. Transmission electron microscopy combined with the energy-dispersive X-ray spectroscopy studies showed that the stoichiometry of the target was precisely transferred to the film by pulsed laser deposition. Oxidation studies of bare Crofer 22 APU steel, and steel with the LSCF48 film were carried out in laboratory air at 1073 K under both isothermal and cyclic oxidation conditions. The calculated parabolic rate constant kp after oxidation for 200 h for the steel with LSCF48 film was approximately three times lower than the oxidation rate of the steel without surface modification. The oxide scale formed on Crofer 22 APU steel consisted of two layers: an inner layer built of chromia and an outer layer consisting of the Mn1.5Cr1.5O4spinel. In the case of the steel with LSCF48 film, the oxide scale was two times thinner and had a similar phase composition. In both cases, the presence of an internal oxidation zone with SiO2 and Al2O3 precipitates was observed. In addition, small amounts of spinel crystals were observed on the film surface. After 2400 h of oxidation at 1073 K under thermal cyclic conditions, the total mass gain was 1.04 mg · cm−2 and 0.67 mg · cm−2 for the respectively bare and coated substrate. The results indicated that the spallation of the oxide scale and/or the film did not occur. In both cases, the composition of the scales was similar to the scales formed on the samples after isothermal oxidation. Studies of the electrical properties show, that Crofer 22 APU steel with LSCF48 film deposited by PLD method meets criteria for the interconnector materials for intermediate-temperature solid oxide fuel cells applications.

All-electrochemically synthesized tin and nickel oxide-modified hematite as photo-electrocatalyst anodes for solar-driven water splitting

Sn- and NiOx-modified Fe2O3 (hematite) films were entirely synthesized via electrodeposition procedure, followed by thermal treatment, and tested as photo-electrocatalyst for water splitting, in alkaline electrolyte. The optimization of the Sn and NiOx electrodeposition loadings on Fe2O3 produced a maximum photocurrent density of 1.35 mA cm−2 at 1.23 V (vs. RHE), with the highest shift on the reaction onset potential, representing a large improvement in relation to bare hematite. Long-term photoelectrolysis studies of the Fe2O3/Sn-10/NiOx at 1.23 V showed that the photocurrent density remained stable for at least 24 h. The assessment of the results from the calculations of charge separation (ƞsep) and charge injection (ƞcat) efficiencies revealed an improvement in both parameters, with the addition of tin into hematite. This was ascribed to its beneficial effect on lowering the charge recombination of the photogenerated charge carriers on grain-boundaries and on the hematite/electrolyte interface. Additionally, it was evidenced that NiOx was more important on the increase in the charge injection, since it is a hole collector, acting as co-catalyst, which improves charge transfer rate at the interface with the electrolyte. The outcome of the Mott-Schottky analysis displayed a negative shift of the flat band potentials (Efb) for both, Sn- and NiOx-modified photo-electrocatalysts, due to the passivation of the surface states, and this was in agreement with the mathematical fitting of the curves obtained via photoelectrochemical impedance spectroscopy (PEIS) measurements. The present findings clearly showed that, by allowing a fine-tuning of the amount of the metal modifiers and with an efficient deposition at functional sites of the hematite, this electrodeposition method is a promising strategy to boost hematite performance as photo-electrocatalyst.

α-Fe2O3 nanoarrays photoanodes decorated with Ni-MOFs for enhancing photoelectrochemical water oxidation

Owing to severe recombination of photogenerated charges, sluggish kinetics of oxygen evolution reaction (OER) and high overpotential, the efficiency of photoelectrocatalytic (PEC) water splitting is severely restricted currently. Herein, a metal-organic framework (Ni-MOF) as cocatalyst has been introduced onto Fe2O3 nanoarrays for PEC water oxidation. The new Ni-MOFs/Fe2O3 photoanode obviously improves the PEC water oxidation performance with respect to the Fe2O3. Specifically, a high photocurrent density is achieved on the Ni-MOF/Fe2O3 film, which corresponds to two-fold over the pristine Fe2O3 film at 1.23 V vs. RHE. Moreover, the photoanode also exhibits a significant cathodic shift of the onset potential (~240 mV) relative to the bare Fe2O3. The enhanced PEC performance is attributed to effective utilization the surface-reaching holes and reduction of the surface charge recombination, which are confirmed by electrochemical impedance spectroscopy and the derived Bode analysis. This study brings new insight into the development of MOF-based materials in the field of PEC water splitting.

The effect of different turbulent flow on failure behavior in secondary loop of the pressurized water reactor

To investigate the effect of different two-phase turbulent flow on the failure behavior, two areas with different distances from the orifice, which suffered from different degrees of two-phase turbulent flow, were studied by transmission electron microscope (TEM) and computational fluid dynamic (CFD). The results indicated that the two-phase turbulent flow affected the wall thinning by accelerating increase of the mass transfer rate. The Erosion at pipe head adjacent to the orifice was found to form cavitation, forming a single layer of nanocrystalline Fe3O4 film on the ferrite matrix; while a moderate Flow accelerated corrosion (FAC) was found in the middle of the pipe, producing a four-layer Fe3O4 film, which consisted of an inner thin nanocrystalline oxide layer, a second dense blocky-shaped oxide layer, a third loose particle layer, an outermost layer deposited large particle layer. Obviously, the four-layer oxide film structure had better protection than the single layer oxide film due to low mass transfer rate.