The β‐to‐γ Transition in BiFeO3: A Powder Neutron Diffraction Study

High‐temperature powder neutron diffraction experiments are conducted around the reported β–γ phase transition (～930 °C) in BiFeO₃. The results demonstrate that while a small volume contraction is observed at the transition temperature, consistent with an insulator–metal transition, both the β‐ and γ‐phase of BiFeO₃ exhibit orthorhombic symmetry; i.e., no further increase of symmetry occurs during this transition. The γ‐orthorhombic phase is observed to persist up to a temperature of approximately 950 °C before complete decomposition into Bi₂Fe₄O₉ (and liquid Bi₂O₃), which subsequently begins to decompose at approximately 960 °C.     

Low‐Symmetry Monoclinic Phases and Polarization Rotation Path Mediated by Epitaxial Strain in Multiferroic BiFeO3 Thin Films

A morphotropic phase boundary driven by epitaxial strain has been observed in lead‐free multiferroic BiFeO₃ thin films and the strain‐driven phase transitions have been widely reported as iso‐symmetric Cc‐Cc by recent works. In this paper, it is suggested that the tetragonal‐like BiFeO₃ phase identified in epitaxial films on (001) LaAlO₃ single crystal substrates is monoclinic M_C. This M_C phase is different from the M_A type monoclinic phase reported in BiFeO₃ films grown on low mismatch substrates, such as SrTiO₃. This is confirmed not only by synchrotron X‐ray studies but also by piezoresponse force microscopy measurements. The polarization vectors of the tetragonal‐like phase lie in the (100) plane, not the (11 0) plane as previously reported. A phenomenological analysis is proposed to explain the formation of M_C Phase. Such a low‐symmetry M_C phase, with its linkage to M_A phase and the multiphase coexistence open an avenue for large piezoelectric response in BiFeO₃ films and shed light on a complete understanding of possible polarization rotation paths and enhanced multiferroicity in BiFeO₃ films mediated by epitaxial strain. This work may also aid the understanding of developing new lead‐free strain‐driven morphotropic phase boundary in other ferroic systems.     

Systematic Study of the Growth of Aligned Arrays of α‐Fe2O3 and Fe3O4 Nanowires by a Vapor–Solid Process

Growth of aligned and uniform α‐Fe₂O₃ nanowire (NW) arrays has been achieved by a vapor–solid process. The experimental conditions, such as type of substrate, local growth and geometrical environment, gas‐flow rate, and growth temperature, under which the high density α‐Fe₂O₃ NW arrays can be grown by a vapor–solid route via the tip‐growth mechanism have been systematically investigated. The density of the α‐Fe₂O₃ NWs can be enhanced by increasing the concentration of Ni atoms inside the alloy substrate. The synthesized temperature can be as low as 400 °C. Fe₃O₄ NWs can be produced by converting α‐Fe₂O₃ NWs in a reducing atmosphere at 450 °C. The transformation of phase and structure have been observed by in situ transmission electron microscopy. The magnetic and field‐emission properties of the NWs indicate their potential applications in nanodevices.     

“Naked” Magnetically Recyclable Mesoporous Au–γ‐Fe2O3 Nanocrystal Clusters: A Highly Integrated Catalyst System

Naked magnetically recyclable mesoporous Au–γ‐Fe₂O₃ clusters, combining the inherent magnetic properties of γ‐Fe₂O₃ and the high catalytic activity of Au nanoparticles (NPs), are successfully synthesized. Hydrophobic Au–Fe₃O₄ dimers are first self‐assembled to form sub‐micrometer‐sized Au–Fe₃O₄ clusters. The Au–Fe₃O₄ clusters are then coated with silica, calcined at 550 °C, and finally alkali treated to dissolve the silica shell, yielding naked‐Au–γ‐Fe₂O₃ clusters containing Au NPs of size 5–8 nm. The silica protection strategy serves to preserve the mesoporous structure of the clusters, inhibit the phase transformation from γ‐Fe₂O₃ to α‐Fe₂O₃, and prevent cluster aggregation during the synthesis. For the reduction of p‐nitrophenol by NaBH₄, the activity of the naked‐Au–γ‐Fe₂O₃ clusters is ≈22 times higher than that of self‐assembled Au–Fe₃O₄ clusters. Moreover, the naked‐Au–γ‐Fe₂O₃ clusters display vastly superior activity for CO oxidation compared with carbon‐supported Au–γ‐Fe₂O₃ dimers, due to the intimate interfacial contact between Au and γ‐Fe₂O₃ in the clusters. Following reaction, the naked‐Au–γ‐Fe₂O₃ clusters can easily be recovered magnetically and reused in different applications, adding to their versatility. Results suggest that naked‐Au–γ‐Fe₂O₃ clusters are a very promising catalytic platform affording high activity. The strategy developed here can easily be adapted to other metal NP–iron oxide systems.     

Integration of Self‐Assembled Epitaxial BiFeO3–CoFe2O4 Multiferroic Nanocomposites on Silicon Substrates

Perovskite‐spinel epitaxial nanocomposite thin films are commonly grown on single crystal perovskite substrates, but integration onto a Si substrate can greatly increase their usefulness in devices. Epitaxial BiFeO₃–CoFe₂O₄ nanocomposites consisting of CoFe₂O₄ pillars in a BiFeO₃ matrix are grown on (001) Si with two types of buffer layers: molecular beam epitaxy (MBE)‐grown SrTiO₃‐coated Si and pulsed‐laser‐deposited (PLD) Sr(Ti_0.65Fe_0.35)O₃/CeO₂/yttria‐stabilized ZrO₂/Si. The nanocomposite grows with the same crystallographic orientation and morphology as that observed on single crystal SrTiO₃ when the buffered Si substrates are smooth, but roughness of the Sr(Ti_0.65Fe_0.35)O₃ promoted additional CoFe₂O₄ pillar orientations with 45° rotation. The nanocomposites on MBE‐buffered Si show very high magnetic anisotropy resulting from magnetoelastic effects, whereas the hysteresis of nanocomposites on PLD‐buffered Si can be understood as a combination of the hysteresis of the Sr(Ti_0.65Fe_0.35)O₃ film and the CoFe₂O₄ pillars.     

Origin of a Tetragonal BiFeO3 Phase with a Giant c/a Ratio on SrTiO3 Substrates

A tetragonal BiFeO₃ phase with giant c/a of approximately 1.25 has been of great interest recently as it potentially possesses a giant polarization and much enhanced electromechanical response. This super‐tetragonal phase is known to be a stable phase only under high compressive strains of above approximately 4.5%, according to first principle calculations. However, in previous work, this super‐tetragonal BiFeO₃ phase was obtained in films deposited at high growth rate on SrTiO₃ substrates with compressive strain of only around 1.5%. By detailed structure analysis using high resolution synchrotron X‐ray diffraction, atomic force microscopy, and transmission electron microscopy, the parasitic β‐Bi₂O₃ phase is identified as the origin inducing the formation of super‐tetragonal BiFeO₃ phase on SrTiO₃ substrates. In addition, ab initio calculations also confirm that this super‐tetragonal phase is more stable than monoclinic phase when Bi₂O₃ is present. Using Bi₂O₃ as a buffer layer, an alternative route, not involving strain engineering, is proposed to stabilize this promising super‐tetragonal BiFeO₃ phase at low growth rates.     

Study on the multiferroic properties of 2-2 type of BiFeO3-CuFe2O4 thin films

The 2-2 type BiFeO₃-CuFe₂O₄(BFO-CFO) bilayer thin films were deposited on FTO/glass substrates by sol-gel method, using Bi(NO₃)₃·5H₂O, Fe(NO₃)₃·9H₂O and Cu(NO₃)₃·3H₂O as the raw materials. The structure, surface morphologies and electric properties of the thin films were investigated. The XRD patterns show that the structure of BFO is the distorted rhombohedral perovskite with R3c:H (161) space lattice, CFO is the tetragonal spinel with type I41/amd (141) space lattice. The interface is obvious between the CFO and BFO films, which shows that the BFO and CFO have no inter-diffusion phenomena. When the frequency is 10 kHz, the dielectric loss peak is consistent with the dielectric relaxation of Wagner Maxwell. The leakage current density of BFO-CFO under positive and negative bias voltages is asymmetric, and there is a hysteresis phenomenon in the positive bias voltage. The results of XPS show that the Fe²⁺ of the BFO-CFO is decreased and the leakage current of the film is improved. The saturation magnetization (M_s) of BFO-CFO is 25.8 emu/cm³ at room temperature, which is about 40 times of the pure BFO.     

Novel Composites of α‐Fe2O3 Tetrakaidecahedron and Graphene Oxide as an Effective Photoelectrode with Enhanced Photocurrent Performances

Novel composites composed of α‐Fe₂O₃ tetrakaidecahedrons and graphene oxide have been easily fabricated and demonstrated to be efficient photoelectrodes for photoelectrochemical water splitting reaction with superior photocurrent response. α‐Fe₂O₃ tetrakaidecahedrons are facilely synthesized in a green manner without any organic additives and then modified with graphene oxide. The morphological and structural properties of α‐Fe₂O₃/graphene composite are intensively investigated by several means, such as X‐ray diffraction, field‐emission scanning electron microscope, transmission electron microscope, X‐ray photoelectron spectroscopy, Fourier Transform infrared spectroscopy, and Raman spectroscopy. The tetrakaidecahedronal hematite particles have been indicated to be successfully coupled with graphene oxide. Systematical photoelectrochemical and impedance spectroscopy measurements have been carried out to investigate the favorable performance of α‐Fe₂O₃/graphene composites, which are found to be effective photoanodes with rapid, steady, and reproducible feature. The coupling of graphene with α‐Fe₂O₃ particles has greatly enhanced the photoelectrochemical performance, resulting in higher photocurrent and lower onset potential than that of pure α‐Fe₂O₃. This investigation has provided a feasible method to synthesize α‐Fe₂O₃ tetrakaidecahedron and fabricate an efficient α‐Fe₂O₃/graphene photoelectrode for photoelectrochemical water oxidation, suggesting a promising route to design noble metal free semiconductor/graphene photocatalysts.     

A Three Component Self‐Assembled Epitaxial Nanocomposite Thin Film

A self‐assembled three phase epitaxial nanocomposite film is grown consisting of ≈3 nm diameter fcc metallic Cu nanorods within square prismatic SrO rocksalt nanopillars in a Sr(Ti,Cu)O_3‐δ perovskite matrix. Each phase has an epitaxial relation to the others. The core–shell‐matrix structures are grown on SrTiO₃ substrates and can also be integrated onto Si using a thin SrTiO₃ buffer. The structure is made by pulsed laser deposition in vacuum from a SrTi_0.75Cu_0.25O₃ target, and formed as a result of the limited solubility of Cu in the perovskite matrix. Wet etching removes the 3 nm diameter Cu nanowires leaving porous SrO pillars. The three‐phase nanocomposite film is used as a substrate for growing a second epitaxial nanocomposite consisting of CoFe₂O₄ spinel pillars in a BiFeO₃ perovskite matrix, producing dramatic effects on the structure and magnetic properties of the CoFe₂O₄. This three‐phase vertical nanocomposite provides a complement to the well‐known two‐phase nanocomposites, and may offer a combination of properties of three different materials as well as additional avenues for strain‐mediated coupling within a single film.     

Effect of Fe doping on structural and magnetic properties of La2/3Ca1/3Mn1−yFeyO3 (y=0–0.03) thin films

We have characterized the magnetic and structural properties of pure and ⁵⁷Fe-doped La_2/3Ca_1/3MnO₃ thin films and targets, substituted with 1% and 3% of ⁵⁷Fe on the Mn site. The films were prepared via high O₂-pressure (500 mTorr) by DC magnetron sputtering on (1 0 0) SrTiO₃ and (1 0 0) LaAlO₃ single-crystal substrates. Mössbauer spectra measured at room temperature confirm the presence of Fe³⁺ with octahedral coordination, thus indicating that Fe is incorporated into the structure by substituting Mn. Structural analysis by X-ray diffraction (XRD) shows that the films are single phase and c-axis oriented and that the Fe doping gives rise to a relaxation of the epitaxial strain. Interestingly, the Curie temperature and the magnetoresistance (MR) show a non-monotonic behavior with Fe doping. This indicates that initially the strain relaxation induced by the Fe doping is more important than the reduction of ferromagnetic coupling due to the Fe incorporation.     

Room Temperature Ferrimagnetism and Ferroelectricity in Strained,Thin Films of BiFe0.5Mn0.5O3

Highly strained films of BiFe_0.5Mn_0.5O₃ (BFMO) grown at very low rates by pulsed laser deposition were demonstrated to exhibit both ferrimagnetism and ferroelectricity at room temperature and above. Magnetisation measurements demonstrated ferrimagnetism (T_C ～ 600K), with a room temperature saturation moment (M_S) of up to 90 emu/cc (～ 0.58 μ_B/f.u) on high quality (001) SrTiO₃. X‐ray magnetic circular dichroism showed that the ferrimagnetism arose from antiferromagnetically coupled Fe³⁺ and Mn³⁺. While scanning transmission electron microscope studies showed there was no long range ordering of Fe and Mn, the magnetic properties were found to be strongly dependent on the strain state in the films. The magnetism is explained to arise from one of three possible mechanisms with Bi polarization playing a key role. A signature of room temperature ferroelectricity in the films was measured by piezoresponse force microscopy and was confirmed using angular dark field scanning transmission electron microscopy. The demonstration of strain induced, high temperature multiferroism is a promising development for future spintronic and memory applications at room temperature and above.     

Nanowire Structural Evolution from Fe3O4 to ϵ‐Fe2O3

The ?‐Fe₂O₃ phase is commonly considered an intermediate phase during thermal treatment of maghemite (γ‐Fe₂O₃) to hematite (α‐Fe₂O₃). The routine method of synthesis for ?‐Fe₂O₃ crystals uses γ‐Fe₂O₃ as the source material and requires dispersion of γ‐Fe₂O₃ into silica, and the obtained ?‐Fe₂O₃ particle size is rather limited, typically under 200 nm. In this paper, by using a pulsed laser deposition method and Fe₃O₄ powder as a source material, the synthesis of not only one‐dimensional Fe₃O₄ nanowires but also high‐yield ?‐Fe₂O₃ nanowires is reported for the first time. A detailed transmission electron microscopy (TEM) study shows that the nanowires of pure magnetite grow along [111] and <211> directions, although some stacking faults and twins exist. However, magnetite nanowires growing along the <110> direction are found in every instance to accompany a new phase, ?‐Fe₂O₃, with some micrometer‐sized wires even fully transferring to ?‐Fe₂O₃ along the fixed structural orientation relationship, (001) ∥ (111), [010] ∥ <110>. Contrary to generally accepted ideas regarding epsilon phase formation, there is no indication of γ‐Fe₂O₃ formation during the synthesis process; the phase transition may be described as being from Fe₃O₄ to ?‐Fe₂O₃, then to α‐Fe₂O₃. The detailed structural evolution process has been revealed by using TEM. 120° rotation domain boundaries and antiphase boundaries are also frequently observed in the ?‐Fe₂O₃ nanowires. The observed ?‐Fe₂O₃ is fundamentally important for understanding the magnetic properties of the nanowires.     

One‐Step Hydrothermal Synthesis of 2D Hexagonal Nanoplates of α‐Fe2O3/Graphene Composites with Enhanced Photocatalytic Activity

There has been significant progress in the field of semiconductor photocatalysis, but it is still a challenge to fabricate low‐cost and high‐activity photocatalysts because of safety issues and non‐secondary pollution to the environment. Here, 2D hexagonal nanoplates of α‐Fe₂O₃/graphene composites with relatively good distribution are synthesized for the first time using a simple, one‐step, template‐free, hydrothermal method that achieves the effective reduction of the graphene oxide (GO) to graphene and intimate and large contact interfaces of the α‐Fe₂O₃ nanoplates with graphene. The α‐Fe₂O₃/graphene composites showed significantly enhancement in the photocatalytic activity compared with the pure α‐Fe₂O₃ nanoplates. At an optimal ratio of 5 wt% graphene, 98% of Rhodamine (RhB) is decomposed with 20 min of irradiation, and the rate constant of the composites is almost four times higher than that of pure α‐Fe₂O₃ nanoplates. The decisive factors in improving the photocatalytic performance are the intimate and large contact interfaces between 2D hexagonal α‐Fe₂O₃ nanoplates and graphene, in addition to the high electron withdrawing/storing ability and the highconductivity of reduced graphene oxide (RGO) formed during the hydrothermal reaction. The effective charge transfer from α‐Fe₂O₃ nanoplates to graphene sheets is demonstrated by the significant weakening of photoluminescence in α‐Fe₂O₃/graphene composites.     

Structure of organometallic chemical vapor deposited BaTiO3 thin films on LaAIO3

BaTiO₃ thin films grown on LaA1O₃ by organometallic chemical vapor deposition were characterized with cross-sectional high resolution transmission electron microscopy. Epitaxy was confirmed for the films grown on (100) oriented substrates. The films displayed an aaxis orientation. The interface between the film and substrate was nearly atomically abrupt. Partial relaxation of the films was observed as a result of misfit dislocation formation.     

Structure and Properties of Nanostructured YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7–δ</Subscript>, BiFeO<Subscript>3</Subscript>, and Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>

The development of new nanostructured materials based on YBa₂Cu₃O_7–δ, BiFeO₃, and Fe₃O₄ compounds is considered. The structure, morphology, and properties of these materials are studied. The possibilities of fabricating YBa₂Cu₃O_7–δ ceramics with given densities from nanopowders in a single stage by an energy efficient method and growing superconducting films of the same composition on a silicon substrate (on a SiO₂ layer) are demonstrated. The technique for fabricating BiFeO₃ nanopowder, making it possible to obtain nanostructured ceramics without additional accompanied phases commonly forming during BiFeO₃ synthesis is developed. Two methods of the single-stage synthesis of Fe₃O₄ nanopowder are presented: burning of nitrate-organic precursors and the electrochemical three-electrode method in which one of the electrodes, i.e., an anode containing scrap iron and slurry, is used as an expendable material.     

Controlled Growth of Porous α‐Fe2O3 Branches on β‐MnO2 Nanorods for Excellent Performance in Lithium‐Ion Batteries

Hierarchical nanocomposites rationally designed in component and structure, are highly desirable for the development of lithium‐ion batteries, because they can take full advantages of different components and various structures to achieve superior electrochemical properties. Here, the branched nanocomposite with β‐MnO₂ nanorods as the back‐bone and porous α‐Fe₂O₃ nanorods as the branches are synthesized by a high‐temperature annealing of FeOOH epitaxially grown on the β‐MnO₂ nanorods. Since the β‐MnO₂ nanorods grow along the four‐fold axis, the as‐produced branches of FeOOH and α‐Fe₂O₃ are aligned on their side in a nearly four‐fold symmetry. This synthetic process for the branched nanorods built by β‐MnO₂/α‐Fe₂O₃ is characterized. The branched nanorods of β‐MnO₂/α‐Fe₂O₃ present an excellent lithium‐storage performance. They exhibit a reversible specific capacity of 1028 mAh g^?1 at a current density of 1000 mA g^?1 up to 200 cycles, much higher than the building blocks alone. Even at 4000 mA g^?1, the reversible capacity of the branched nanorods could be kept at 881 mAh g^?1. The outstanding performances of the branched nanorods are attributed to the synergistic effect of different components and the hierarchical structure of the composite. The disclosure of the correlation between the electrochemical properties and the structure/component of the nanocomposites, would greatly benefit the rational design of the high‐performance nanocomposites for lithium ion batteries, in the future.     

Fe3O4‐Decorated Co9S8 Nanoparticles In Situ Grown on Reduced Graphene Oxide: A New and Efficient Electrocatalyst for Oxygen Evolution Reaction

Cobalt sulfide materials have attracted enormous interest as low‐cost alternatives to noble‐metal catalysts capable of catalyzing both oxygen reduction and oxygen evolution reactions. Although recent advances have been achieved in the development of various cobalt sulfide composites to expedite their oxygen reduction reaction properties, to improve their poor oxygen evolution reaction (OER) activity is still challenging, which significantly limits their utilization. Here, the synthesis of Fe₃O₄‐decorated Co₉S₈ nanoparticles in situ grown on a reduced graphene oxide surface (Fe₃O₄@Co₉S₈/rGO) and the use of it as a remarkably active and stable OER catalyst are first reported. Loading of Fe₃O₄ on cobalt sulfide induces the formation of pure phase Co₉S₈ and highly improves the catalytic activity for OER. The composite exhibits superior OER performance with a small overpotential of 0.34 V at the current density of 10 mA cm^?2 and high stability. It is believed that the electron transfer trend from Fe species to Co₉S₈ promotes the breaking of the Co–O bond in the stable configuration (Co–O–O superoxo group), attributing to the excellent catalytic activity. This development offers a new and effective cobalt sulfide‐based oxygen evolution electrocatalysts to replace the expensive commercial catalysts such as RuO₂ or IrO₂.     

Emerging Nanoclay Composite for Effective Hemostasis

Uncontrolled bleeding following trauma is associated with a high risk of death. Here, an emerging kaolinite nanoclay composite (iron oxide‐kaolinite, α‐Fe₂O₃‐kaolin_KAc) is developed, based on the natural hemostat hematitum used in traditional Chinese medicine to effectively control hemorrhage. α‐Fe₂O₃‐kaolin_KAc stops bleeding in ≈183 ± 16 s and exhibits higher hemostatic activity than the related compounds FeOOH‐kaolin_KAc (298 ± 14 s), γ‐Fe₂O₃‐kaolin_KAc (212 ± 11 s), and Fe₃O₄‐kaolin_KAc (218 ± 15 s). This rapid effect is attributed to efficient absorption of the fluid in blood, activation of blood platelets, and induction of the coagulation cascade by kaolinite and the aggregation of red blood cells induced by α‐Fe₂O₃. α‐Fe₂O₃‐kaolin_KAc shows slight hemolysis (<0.11%) as compared to kaolinite (30%), which accelerates wound healing. The biocompatibility, hemostatic activity, and low cost of α‐Fe₂O₃‐kaolin_KAc make it a safe and effective agent for preventing massive blood loss after traumatic injury.     

Compositionally Modulated Magnetic Epitaxial Spinel/Perovskite Nanocomposite Thin Films

There is great interest in self‐assembled oxide vertical nanocomposite films consisting of epitaxial spinel pillars in a single crystal perovskite matrix, due to their tunable electronic, magnetic, and multiferroic properties. Varying the composition or geometry of the pillars in the out‐of‐plane direction has not been previously reported but can provide new routes to tailoring their properties in three dimensions. In this work, ferrimagnetic epitaxial CoFe₂O₄, MgFe₂O₄, or NiFe₂O₄ spinel nanopillars with an out‐of‐plane modulation in their composition and shape are grown in a BiFeO₃ matrix on a (001) SrTiO₃ substrate using pulsed laser deposition. Changing the pillar composition during growth produces a homogeneous pillar composition due to cation interdiffusion, but this can be suppressed using a sufficiently thick blocking layer of BiFeO₃ to produce bi‐pillar films containing for example a layer of magnetically hard CoFe₂O₄ pillars and a layer of magnetically soft MgFe₂O₄ pillars, which form in different locations. A thinner blocking layer enables contact between the top of the CoFe₂O₄ and the bottom of the MgFe₂O₄ which leads to correlated growth of the MgFe₂O₄ pillars directly above the CoFe₂O₄ pillars and provides a path for interdiffusion. The magnetic hysteresis of the nanocomposites is related to the pillar structure.     

Neutron Diffraction Investigations of Magnetism in BiFeO3 Epitaxial Films

The recovery of a modulated magnetic structure in epitaxial BiFeO₃ thin films as revealed by neutron diffraction is reported. The magnetic structure in thin films is found to strongly depend on substrate orientation. The substrate orientation causes different strain–relaxation processes resulting in different thin‐film crystal structures. The (110) oriented film with a monoclinic structural phase has a single‐domain modulated magnetic structure where the magnetic moment lies in the HHL plane. For the (111) oriented film that has a rhombohedral structure, a modulated structure superimposed on the G‐type antiferromagnetic order is found. These results indicate that slight structural modifications in the BiFeO₃ thin film cause drastic changes in the magnetic structure.