Fe3O4 Thin Films Research Articles

Improved electrochemical properties of nanostructured Bi0.9Gd0.1FeO3 thin film as electrode material for supercapacitors

Improved electrochemical properties of nanostructured Bi0.9Gd0.1FeO3 thin film as electrode material for supercapacitors

Understanding the impact of Bi stoichiometry towards optimised BiFeO3 photocathodes: structure, morphology, defects and ferroelectricity.

BiFeO3 thin films have been widely studied for photoelectrochemical water splitting applications because of its narrow bandgap and good ferroelectricity which can promote the separation of photo-generated charges. Bismuth is well known as a volatile element and excess bismuth is usually added into the precursor to compensate the loss of bismuth during heat treatment, but the amount of excess bismuth required and how excess bismuth will affect PEC performance have not been clearly studied. Herein, self-doped Bi1+x FeO3 thin films are prepared via simple chemical solution deposition method with excess bismuth from 0-30% in the precursor. The loss of bismuth after annealing is confirmed by EDX and XPS. Multiple factors were investigated and it was found that non-stoichiometric Bi resulted in changes of structure, morphology, defects, electronic properties and PEC performance. An enhanced photocurrent is observed in bismuth-rich BiFeO3 films, which can be ascribed to the larger grain size, decreased oxygen vacancies, lattice distortion and supported charge separation. Moreover, the photocathodic performance can be further enhance by ferroelectric poling. Our work indicates that deficient bismuth should be carefully avoided during heat treatment and moreover, a slight excess of Bi is beneficial for PEC performance. Therefore, we offer a simple way to enhance PEC performance of BiFeO3-based ferroelectric materials through careful control of their stoichiometry.

Monitoring Reinforced Concrete Structures Using Iron Thin Film Coated Optical Fibre Sensors

Structural health monitoring (SHM) of reinforced concrete structures (RCS) is crucial for mitigating the consequences of their deterioration. By identifying and addressing the issues early, SHM helps reduce environmental impact, safeguard lives, and enhance economic resilience. Rebar corrosion is a leading cause of early RCS decay and optical fibre sensors (OFS) have been employed for its monitoring. Reflection optrodes using optical fibres where the tip is coated with iron (Fe) thin films offer a robust, longlasting and straightforward solution. This study investigates the tracking of spectral changes during the Fe thin film corrosion, which has been neglected in the literature, in favour of tracking reflection changes from thin film spalling. A multimode fibre tip, coated with a thin Fe layer embedded in concrete, allows spectral changes to be observed during corrosion. A 100 nm thick Fe film was deposited using radio frequency magnetron sputtering on polished fibre tips. Corrosion was induced by applying salted water drops and allowing the fibre tip to dry. Corrosion monitoring was successful for both air-exposed and cementembedded tips, with results compared to reflection simulations of Fe, Fe2O3, and Fe3O4 thin films. This study supports monitoring at different wavelengths, enhancing robustness, cost-effectiveness and earlier detection.

Photoelectrochemical activation of peroxymonosulfate using Sn-doped α-Fe2O3 thin film for degradation of anti-inflammatory pharmaceutical drug

Introduction of oxygen vacancies (OVs) has been investigated as a promising way to improve the electrical and catalytic characteristics of a hematite (α-Fe2O3) based photoelectrode. In this work, we develop a novel method for preparing porous Sn-doped α-Fe2O3 (Sn:Fe2O3) thin films with intrinsic OVs. The procedure included spin-coating an iron precursor onto a fluorine-doped tin oxide (FTO) substrate, followed by thermal treatment at elevated temperatures. The influence of Sn dopant on the optoelectronic properties of α-Fe2O3 was demonstrated by X-ray photoelectron spectroscopy and photoelectrochemical (PEC) measurements. The combined effect of OVs and Sn doping was found to play a synergistic role in reducing the charge recombination’s. The Sn:Fe2O3 photoanodes were used as a dual catalyst to oxidise water and break down an anti-inflammatory drug called 2-(4-isobutylphenyl)propanoic acid (IBPA). The Sn:Fe2O3 thin film with a 30-minute heat treatment time displayed the highest incident photon-to-current efficiency. For the first time, Sn:Fe2O3 thin films were utilised in the effective PEC degradation of IBPA employing peroxymonosulfate (PMS) under visible light illumination. The hydroxyl radicals (OH), singlet oxygen (1O2), photogenerated holes (h+), and sulfate radicals (SO4−) were discovered to be the main reactive species during PEC degradation. IBPA degradation and the formation of new compounds were verified using liquid chromatography-mass spectrometry. The Lepidium sativum L phytotoxicity test reveals that PEC-treated wastewater with IBPA exhibits decreased toxicity.

(Invited) Size, Composition Effects and Active State Formation of Nanostructured Cobalt and Iron-Cobalt Oxide-Based Catalysts during Oxygen Evolution Reaction

The efficient water electrolysis is a key technology to establish a CO2-neutral, electrochemical (green) hydrogen production. Nonetheless, the electrocatalysts’ near-surface structure during the anodic oxygen evolution reaction (OER) is still largely unknown, which hampers knowledge-driven optimization. Here, we provide quantitative near-surface structural insights into oxygen-evolving CoOX(OH)Y nanoparticles by tracking their size-dependent catalytic activity down to 1 nm and their structural adaptation to OER conditions combining operando X-ray absorption spectroscopy and density functional theory calculations. We uncover a superior intrinsic OER activity of sub-5 nm nanoparticles and a size-dependent oxidation leading to a near-surface Co-O bond contraction and charge redistribution during OER.I will also report on the role of Fe in the performance of Co-oxide-based electrocalysts. By comparing epitaxial Co3O4(111), Co1+δFe2-δO4(111) (|δ|<=0.2), and Fe3O4(111) thin film electrocatalysts combining quasi in-situ preparation and characterization in ultra-high vacuum (UHV) with electrochemistry experiments. Interestingly, Co1+δFe2-δ O4(111) was found to be up to five times more active than Co3O4(111) and Fe3O4(111) with the activity depending acutely on the Co/Fe concentration ratio. Under OER conditions, all three oxides are covered by oxyhydroxide but the characteristics of this overlayer, including its thickness, stability, structural and chemical evolution was found to depend on the Fe content.

Influence of in-situ annealing temperature upon structural, micro-structural, magnetic damping properties in sputtered Fe3O4 thin films

The influence of in-situ annealing temperature upon structural, micro-structural, magnetic properties and Gilbert damping parameters of sputtered Fe3O4 thin films (29–49 nm) upon SiO2/Si substrates have been systematically investigated. The oriented Fe3O4 structural phase was evaluated using grazing incidence X-ray diffraction, confocal Raman spectroscopy and X-ray photoelectron spectroscopy data. The atomic force and field-emission scanning electron microscope surface microstructural images reveal an enhanced grain growth with dominant anti-phase boundaries (APB) with in-situ annealing. The macroscopic spin dynamical response is evaluated using co-planar waveguide ferromagnetic resonance (CPW-FMR) spectroscopy from 15 to 35 GHz frequencies. The FMR data reveals a low Gilbert damping (α) ranging from 0.003 ± 0.001 to 0.009 ± 0.006 for all the in-situ annealed Fe3O4 thin films. Further, high inhomogeneity (ΔH0) parameter is correlated to the surface defects and APBs.

Comparative study of Co3O4(111), CoFe2O4(111), and Fe3O4(111) thin film electrocatalysts for the oxygen evolution reaction

Water electrolysis to produce ‘green H2’ with renewable energy is a promising option for the upcoming green economy. However, the slow and complex oxygen evolution reaction at the anode limits the efficiency. Co3O4 with added iron is a capable catalyst for this reaction, but the role of iron is presently unclear. To investigate this topic, we compare epitaxial Co3O4(111), CoFe2O4(111), and Fe3O4(111) thin film model electrocatalysts, combining quasi in-situ preparation and characterization in ultra-high vacuum with electrochemistry experiments. The well-defined composition and structure of the thin epitaxial films permits the obtention of quantitatively comparable results. CoFe2O4(111) is found to be up to about four times more active than Co3O4(111) and about nine times more than Fe3O4(111), with the activity depending acutely on the Co/Fe concentration ratio. Under reaction conditions, all three oxides are covered by oxyhydroxide. For CoFe2O4(111), the oxyhydroxide’s Fe/Co concentration ratio is stabilized by partial iron dissolution.

Enhanced magnetic properties and leakage current mechanism in bismuth lanthanum ferrite thin films coupled with impedance and magnetic studies

A highly crystalline and pure phase Bi0.993La0.007FeO3 (BLFO) thin films have been grown using a pulsed laser deposition technique. The structural analysis confirmed the thin film is well crystallized with a rhombohedral crystal structure with an R3c space group. The BLFO phase and composition are also confirmed by transmission electron microscopy and EDAX analysis. The surface chemistry of thin films, analysed by X-ray photon spectroscopy, reveals Fe's presence in Fe3+ and Fe2+ states. At RT, the deposited film exhibited high dielectric permittivity (∼4 × 102) and low dielectric loss (∼10−1). The isothermal magnetization on highly oriented (001)-MgO and LaAlO3 (LAO) substrates is investigated. The response is compared to the randomly oriented polycrystalline films on low-cost substrates (Pt/TiO2/SiO2/Si and n-type Si substrate). The enhanced saturation magnetization of 57 emu/cm3 and 36 emu/cm3 at 10 K and 300 K are observed at 2.5 kOe applied field for the film grown on the LAO substrate. Additionally, the low leakage current density of the film is obtained around ∼1 × 10−7 A/cm2 at a field of 100 kV/cm. Acquiring single-phase BLFO thin films with enhanced electrical and magnetic properties can be relevant in designing multiferroic materials for multiple-state memories, magnetic field sensors, and spintronics applications.

Crystal structure effects on the Co-sputtered p-type Fe2-xSnxO3 hydrogen gas sensors

Fe2-xSnxO3 structures grown by RF-DC magnetron co-sputtering. The band gap of Fe2O3, SnO2, Fe2-xSnxO3 structure was calculated as 2.86, 4.06, 2.77 eV, respectively. The Fe2O3 thin film has shown a tetragonal as well as the SnO2 structure, however, the Fe2-xSnxO3 (0 < x < 1) has shown rhombohedral structure. XPS measurements have shown change in binding energy which is due to the electron exchange of tin-iron with oxygen, and iron - tin oxide structure. This binding energy increases slightly with the chemical environment effect. The films were investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM). In AFM images was seen that the iron oxide structure was pyramidal and the tin oxide structure was spherical, similar to the SEM image. The response of the film to hydrogen gas was measured at flow values of 100, 500 and 1000 ppm at 300 °C. Fe2O3 thin films show 46%, SnO2 61% and, Fe2-xSnxO3 6.5% response to 1000 ppm H2 gas. It is seen that the gas sensor response was affected by the morphological and structural features of the grown film.

Charge transport and dielectric characteristics of Sm x Bi1−x FeO3 thin films from the perspective of grain and grain boundary properties

Samarium-substituted bismuth ferrite (Sm x Bi1−x FeO3) compositions comprise a system of important materials due to their combination of multiferroic properties. Several dielectric and charge transport reports in literature can be found in this system. However, as a typical polycrystalline electroceramic, their grains and grain boundaries (GBs) are expected to possess very different properties. To this date, these distinctions have not been determined for this system. In this work, through measurements via impedance spectroscopy on Sm x Bi1−x FeO3 thin films, we show that using a brick layer model allows the separation of the electrical properties of grains and GBs. Results indicate that grains have dielectric permittivity and electrical conductivity much higher than GBs. Their properties mostly control the characteristics observed in the studied thin films. The introduction of samarium reduces the electrical conductivity and increases the activation energies for charge transport in grains and GBs. In turn, dielectric permittivity is reduced in grains and subtly increased in GBs.

Observation of magnetodielectric effects in Nd0.5Dy0.5FeO3 thin film and its utilization in microstripline based resonator circuit

Nd0.5Dy0.5FeO3 epitaxial thin film grown on LaAlO3 (001) substrate by pulsed laser deposition technique shows a significant magnetodielectric effect. At 100 K, a variation of around 13% in relative permittivity is observed at frequencies higher than 2 MHz in the presence of 5 T. This variation in relative permittivity (ε΄) increases to about 50% at 2 K. To utilize the observed magnetodielectric effect, we have designed the microstripline based resonator circuit using High-Frequency Structure Simulator (HFSS). Simulations are performed for the two values of the real part of permittivity (in the absence and presence of an external magnetic field) to observe the magnetic field tunability of the resonance frequency. A shift of 27% is observed in the resonant frequency of the microstripline based resonator on Nd0.5Dy0.5FeO3 epitaxial thin film.

Hydrothermal synthesis of the CdS nanorods on electrochemically deposited Fe2O3 thin film for improving photoelectrochemical performance

In this study, a CdS nanorod layer is hydrothermally synthesized on an electrochemically deposited Fe2O3 thin film substrate. These two components are hierarchically stacked to form a type-II heterojunction for enhancing photoelectrochemical performance. The Fe2O3/CdS composite photoanode system shows enhanced photocurrent density (1724 μA·cm−2 at 1.23 VRHE), incident photon-to-current conversion efficiency (23.3% at 390 nm), applied bias photon-to-current conversion efficiency (0.48% at 0.68 VRHE), electron lifetime, reduced charge transfer resistance, and good stability in an alkaline solution. It also has enhanced optical absorbance compared to the Fe2O3 and CdS only samples. The optical band gap of the composite Fe2O3/CdS system is narrowed by adding the Fe2O3 component to the CdS nanorod layer. Therefore, the underlying Fe2O3 layer plays a significant role in transferring charges through the CdS nanorod layer in this photoelectrochemical cell system.

Improved photocarrier separation in Ga/Fe gradient (Ga, Fe)2O3 thin films

Metal oxides are promising photoanode materials for photoelectrochemical reactions due to their chemical stability and low-cost synthesis. However, their high photocarrier recombination rate limits their commercial applications. One strategy to promote photocarrier separation efficiency in metal oxides is building an inner electric field through band engineering techniques. In this study, a gradient band structure was prepared in (Ga, Fe)2O3 thin films on F:SnO2 (FTO) coated glass substrate by varying the Ga/Fe ratio. The performance of the gradient photoanode was evaluated by measuring its photocatalytic activity with photocurrent. The forward gradient photoanode, with increasing Ga/Fe ratio from bottom substrate to photoanode surface, exhibited a higher photocurrent density (0.44 mA/cm2 at 1.23 V vs reversible hydrogen electrode (RHE) under AM 1.5G illumination) compared to the reverse gradient photoanode with decreasing the Ga/Fe ratio in the same situation. The forward gradient band structure, which facilitates photocarrier separation and transportation, was believed to play a dominant role in improving the photocatalytic performance. This work demonstrates an effective strategy to improve photocarrier separation and photocatalytic performance of metal oxide photocatalysts through band engineering.

Photocatalytic thin films based on Au nanoparticles covered by iron oxyhydroxides by hydrothermal process

The use of ultrathin nanohybrid films in photocatalytic applications to decompose pollutants is a desirable and effective approach for industrial and practical purposes. In this study, ultrathin films composed of Fe2O3 oxyhydroxides with Au nanoparticles (NPs; Au–FeOOH) were prepared by Au evaporation deposition, a hydrothermal process, and thermal annealing. Transmission electron microscopy, elemental mapping, and X-ray photoelectron spectroscopy studies confirmed the attachment of the FeOOH matrix to the Au nanoparticles. Au–FeOOH thin films exhibited outstanding photodecomposition ability toward rhodamine 6G in a 0.5 µM aqueous solution under artificial light illumination (120 min; 1000 W m−2); such performance is the best reported thus far for Fe2O3 thin films. The superior photocatalytic activity of the Au–FeOOH nanohybrid thin films is attributed to their broad absorption encompassing the deep ultraviolet-to-near infrared regions and efficient charge transfer between the Au NPs and FeOOH NPs.

Synaptic behavior of Fe3O4-based artificial synapse by electrolyte gating for neuromorphic computing

Neuromorphic computing (NC) is a crucial step toward realizing power-efficient artificial intelligence systems. Hardware implementation of NC is expected to overcome the challenges associated with the conventional von Neumann computer architecture. Synaptic devices that can emulate the rich functionalities of biological synapses are emerging. Out of several approaches, electrolyte-gated synaptic transistors have attracted enormous scientific interest owing to their similar working mechanism. Here, we report a three-terminal electrolyte-gated synaptic transistor based on Fe3O4 thin films, a half-metallic spinel ferrite. We have realized gate-controllable multilevel, non-volatile, and rewritable states for analog computing. Furthermore, we have emulated essential synaptic functions by applying electrical stimulus to the gate terminal of the synaptic device. This work provides a new candidate and a platform for spinel ferrite-based devices for future NC applications.

Tellurium Nanosphere/Hematite Hybrid Thin Films with Enhanced Light Absorption for Efficient Photocatalysis

As a semimetal, tellurium (Te) shows metallic and dielectric properties in the UV and visible ranges, respectively, and has potential as a broad-band light absorber. In this paper, plasmonic–semiconductor hybrid nanoscale films consisting of Fe2O3 thin films and Te nanospheres are designed and demonstrated. Te clusters are proved to have an enhancement of the electric near-field in a broad UV–vis–NIR spectrum, which facilitates light absorption and charge separation in photoelectrocatalysis. The thickness of Fe2O3 films is adjusted from 22 to 137 nm. The photocurrent density enhancement factor of Te-modified films is 13 times higher than that of 22 nm Fe2O3 films. Particularly, a Te-decorated Fe2O3 ultrathin film exhibits photoelectrochemical activity comparable to that of bulk Fe2O3, suggesting significant near-field enhancement from plasmon modes of the Te clusters. It is found that the Te cluster decoration led to a significant decrease of the charge transfer resistance, demonstrating that the Te clusters could boost the formation rate and suppress the recombination rate of electron–hole pairs. The hybridization of Te nanoparticles with nanoscale semiconductor films represents an efficient way to enhance the solar energy conversion efficiency.

Sol-Gel-Derived Ordered Mesoporous High Entropy Spinel Ferrites and Assessment of Their Photoelectrochemical and Electrocatalytic Water Splitting Performance.

The novel material class of high entropy oxides with their unique and unexpected physicochemical properties is a candidate for energy applications. Herein, it is reported for the first time about the physico- and (photo-) electrochemical properties of ordered mesoporous (CoNiCuZnMg)Fe2 O4 thin films synthesized by a soft-templating and dip-coating approach. The A-site high entropy ferrites (HEF) are composed of periodically ordered mesopores building a highly accessible inorganic nanoarchitecture with large specific surface areas. The mesoporous spinel HEF thin films are found to be phase-pure and crack-free on the meso- and macroscale. The formation of the spinel structure hosting six distinct cations is verified by X-ray-based characterization techniques. Photoelectron spectroscopy gives insight into the chemical state of the implemented transition metals supporting the structural characterization data. Applied as photoanode for photoelectrochemical water splitting, the HEFs are photostable over several hours but show only low photoconductivity owing to fast surface recombination, as evidenced by intensity-modulated photocurrent spectroscopy. When applied as oxygen evolution reaction electrocatalyst, the HEF thin films possess overpotentials of 420mV at 10mA cm-2 in 1m KOH. The results imply that the increase of the compositional disorder enhances the electronic transport properties, which are beneficial for both energy applications.

Magneto-transport and magneto-dielectric anomalies in Co doped iron oxide thin films – Microwave assisted sol-gel route

Use of microwave (MW) energy as an alternate to high temperature treatment during material synthesis is gaining much attention as it offers the advantages of being faster and economical than other heating methods. A comprehensive study of structure, magnetization, and polarization behaviour of Cobalt (Co) doped Fe3O4 thin films, prepared by means of microwave assisted sol–gel method is performed. Co doped (0-10 wt%) sols are prepared after irradiating them at MW power of 1000 W. Phase analysis is confirmed using XRD where Fe3O4 phase persisted with Co concentrations of 0-6 wt%. Higher concentration results in the appearance of cobalt oxide phase. Magnetic and transport properties are strongly affected by the presence of Co ions. Co doped Fe3O4 films show anisotropic behaviour with high saturation magnetization obtained for 6 wt% Co doped Fe3O4 thin films, i.e. ∼ 125 emu/g. Verwey transition is observed for thin films with low dopant concentration while no sharp transition in magnetization behaviour is observed for higher dopant concentration. High frequency anomaly in dielectric analysis is observed at high frequencies for Co doped Fe3O4 thin films. Negative values of magneto-dielectric coefficient show coupling of magnetic and dielectric behaviour at room temperature.

Enhanced electrocatalytic oxygen redox reactions of iron oxide nanorod films by combining oxygen vacancy formation and cobalt doping.

A synergistic effect of Co-doping and vacuum-annealing on electrochemical redox reactions of iron oxide films is demonstrated in the present work. In this research, a series of defect-rich iron oxy/hydroxide nanorod arrays: α-FeOOH, Fe2O3, and FeOx nanorod thin film catalysts were synthesized via a hydrothermal approach followed by thermal and vacuum treatments. Besides, a cobalt doping process was employed to prepare the thin film of Co-doped FeOx nanorods. The morphology, crystallinity, and electrochemical activities of Co-doped oxygen-deficient FeOx (Co-FeOx/FTO) show strong correlations with metal concentration and thermal treatments. The electrochemical measurements demonstrated that the as-deposited Co-doped FeOx NR catalyst could achieve a maximum OER current of 30 mA cm-2, which was six times greater than that recorded by as-deposited Co-doped FeOOH NR catalysts (5.7 mA cm-2) at 1.65 V vs. RHE, confirming the superior electrocatalytic OER activity at the as-deposited Co-doped FeOx NR catalyst after cobalt doping. It is believed that these results are attributed to two factors: the synergistic effect of Co doping and the defect-rich nature of FeOx nanorod catalysts that are used in sustainable energy systems.

Reduced effective magnetization and damping by slowly relaxing impurities in strained γ-Fe2O3 thin films

Magnetically ordered insulators are of key interest for spintronics applications, but most of them have not yet been explored in depth regarding their magnetic properties, in particular with respect to their dynamic response. We study the static and dynamic magnetic properties of epitaxially strained γ-Fe2O3 (maghemite) thin films grown via pulsed-laser deposition on MgO substrates by SQUID magnetometry and cryogenic broadband ferromagnetic resonance experiments. SQUID magnetometry measurements reveal hysteretic magnetization curves for magnetic fields applied both in- and out of the sample plane. From the magnetization dynamics of our thin films, we find a small negative effective magnetization in agreement with a strain induced perpendicular magnetic anisotropy. Moreover, we observe a non-linear evolution of the ferromagnetic resonance-linewidth as a function of the microwave frequency and explain this finding with the so-called slow relaxor model. We investigate the magnetization dynamics and non-linear damping mechanisms present in our samples as a function of frequency and temperature and in particular, observe a sign change in the effective magnetization from the transition of the magnetic anisotropy from a perpendicular easy axis to an easy in-plane anisotropy for reduced temperatures. Its nonlinear damping properties and strain-induced perpendicular anisotropy render γ-Fe2O3 an interesting material platform for spintronics devices.