Fe3O4 Nanowires Research Articles

Synthesis of Vertically Aligned Manganese-Doped Fe3O4 Nanowire Arrays and Their Excellent Room-Temperature Gas Sensing Ability

Vertically aligned Mn (10%)-doped Fe3O4 (Fe2.7Mn0.3O4) nanowire arrays were produced by the reduction/substitution of pregrown Fe2O3 nanowires. These nanowires were ferromagnetic with a Verwey temperature of 129 K. X-ray magnetic circular dichroism measurements revealed that the Mn2+ ions preferentially occupy the tetrahedral sites, substituting for the Fe3+ ions. We observed that the Mn substitution decreases the magnetization, but increases the electrical conductivity. We developed highly sensitive gas sensors using these nanowire arrays, operating at room temperature, whose sensitivity showed a correlation with their bond strength of diatomic/triatomic molecules.

Controlled-synthesis, characterization, and magnetic properties of Fe3O4 nanostructures

Fe3O4 nanostructures with different morphologies, including uniform nanoparticles, nanorods and nanowire bundles, have been successfully synthesized via a facile hydrothermal route. Based on the observation of TEM images, the growth mechanism of one-dimensional Fe3O4 nanostructures is in accordance with Ostwald ripening process. From the hysteresis loops of as-prepared Fe3O4 products, we found that the morphology has great influence on the magnetic properties. The uniform Fe3O4 nanoparticles have higher saturation magnetization and lower coercivity than that of Fe3O4 nanorods and nanowires bundles. These phenomena attribute to the high shape anisotropy of nanorods and nanowire bundles, which prevent them from magnetizing in directions other than along their easy magnetic axes.

Nanowire Structural Evolution from Fe3O4 to ϵ‐Fe2O3

AbstractThe ϵ‐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 ϵ‐Fe2O3 crystals uses γ‐Fe2O3 as the source material and requires dispersion of γ‐Fe2O3 into silica, and the obtained ϵ‐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 ϵ‐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, ϵ‐Fe2O3, with some micrometer‐sized wires even fully transferring to ϵ‐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 ϵ‐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 ϵ‐Fe2O3 nanowires. The observed ϵ‐Fe2O3 is fundamentally important for understanding the magnetic properties of the nanowires.

Reducing Reaction of Fe3O4 in Nanoscopic Reactors of a-CNTs

A reduction of Fe3O4 nanowires in nanoscopic reactors of amorphous C:H nanotubes (a-CNTs) was taken to understand features of the chemical reaction mechanism in nanoscale reactors. Fe3O4 nanowires encapsulated in a-CNTs were reduced into iron at a relatively low temperature of 570 degrees C, producing iron nanoparticles encapsulated in CNTs accompanied by the crystallization of the a-CNT shell. It was found that carbon in the a-CNT shell rather than hydrogen (5.5 wt % in it) reduced Fe3O4, showing features different from those in a macroscopic system. The possible mechanisms behind this phenomenon are discussed.

Vertically Aligned Mn-doped Fe3O4 Nanowire Arrays: Magnetic Properties and Gas Sensing at Room Temperature

AbstractVertically-aligned Mn (10%)-doped Fe3O4 (Fe2.7Mn0.3O4) nanowire arrays were produced by the reduction/substitution of pre-grown Fe2O3 nanowires. These nanowires were ferromagnetic with a Verwey temperature of 129 K. X-ray magnetic circular dichroism measurements revealed that the Mn2+ ions preferentially occupy the tetrahedral sites, substituting for the Fe3+ ions. We observed that the Mn substitution decreases the magnetization, but increases the electrical conductivity. We developed highly sensitive gas sensors using these nanowire arrays, operating at room temperature, whose sensitivity showed a correlation with their bond strength of diatomic/triatomic molecules. Based on the fact that the sensitivity was highest toward water vapor, an excellent-performance humidity sensor was fabricated.

Magnetic anisotropy of vertically aligned α-Fe2O3 nanowire array

Vertically aligned Fe2O3 nanowire arrays were synthesized by the thermal oxidation of Fe foil. They consist of single-crystalline rhombohedral α-Fe2O3 nanocrystals grown along the [112¯0] direction. Most of the nanowires have a sharp tip (diameter=50nm) and beltlike base. The magnetization measurement reveals a Morin temperature of 125K and unique magnetic anisotropy due to the easy axis of the magnetocrystalline anisotropy perpendicular to the nanowire axis.

PLD-Assisted VLS Growth of Aligned Ferrite Nanorods, Nanowires, and NanobeltsSynthesis, and Properties

We report here a systematic synthesis and characterization of aligned alpha-Fe2O3 (hematite), epsilon-Fe2O3, and Fe3O4 (magnetite) nanorods, nanobelts, and nanowires on alumina substrates using a pulsed laser deposition (PLD) method. The presence of spherical gold catalyst particles at the tips of the nanostructures indicates selective growth via the vapor-liquid-solid (VLS) mechanism. Through a series of experiments, we have produced a primitive "phase diagram" for growing these structures based on several designed pressure and temperature parameters. Transmission electron microscopy (TEM) analysis has shown that the rods, wires, and belts are single-crystalline and grow along <111>m or <110>h directions. X-ray diffraction (XRD) measurements confirm phase and structural analysis. Superconducting quantum interference device (SQUID) measurements show that the iron oxide structures exhibit interesting magnetic behavior, particularly at room temperature. This work is the first known report of magnetite 1D nanostructure growth via the vapor-liquid-solid (VLS) mechanism without using a template, as well as the first known synthesis of long epsilon-Fe2O3 nanobelts and nanowires.

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.

Visualization of Electric and Magnetic Fields of Nanomaterials by Electron Holography

Electron holography was carried out extensively to analyze the electric field around a single TaSi2 nanowire with applying the voltage and to visualize the lines of magnetic flux of Fe3O4 nanowires. In the electric field analysis, the electric potential distribution between the TaSi2 nanowire and anode was clearly visualized with an increase in the applied voltage. During the field electron emission condition, the equi-electric potential lines were blurred, which has been considered to result from the fluctuation of the electric potential between the nanowire and anode due to the field electron emission. In the magnetic field analysis, the lines of magnetic flux were basically parallel to the longitudinal direction of Fe3O4 nanowires, indicating that the shape anisotropy was significant in the magnetization distribution in these nanowires. In addition, the observation revealed the presence of a magnetic interaction between the neighboring nanowires.

Large scale growth and magnetic properties of Fe and Fe3O4 nanowires

Fe and Fe3O4 nanowires have been synthesized by thermal decomposition of Fe(CO)5, followed by heat treatments. The Fe wires are formed through the aggregation of nanoparticles generated by decomposition of Fe(CO)5. A core-shell structure with an iron oxide shell and Fe core is observed for the as-prepared Fe wires. Annealing in air leads to the formation of Fe2O3∕Fe3O4 wires, which after heat treatment in a N2/alcohol atmosphere form Fe3O4 wires with a sharp Verwey [Nature (London) 144, 327 (1939)] transition at 125K. The Fe3O4 wires have coercivities of 261 and 735Oe along the wire axis at RT and 5K, respectively. The large increase of coercivity at 5K as compared to RT is due to the increase of anisotropy resulting from the Verwey transition.

Effect of antiphase boundaries on electrical transport properties of Fe3O4 nanostructures

Fe 3 O 4 nanowires have been fabricated based on Fe3O4 thin films grown on α-Al2O3 (0001) substrates using the hard mask and ion milling technique. Compared with thin films, the Fe3O4 nanowire exhibits a slightly sharper Verwey transition but pronounced anisotropic magnetoresistance properties in the film plane at low magnetic field. Detailed bias-dependence study of both the conductance and magnetoresistance curves for both the thin films and nanowires suggests that the electrical conduction in magnetite near and above the Verwey transition temperature is dominated by a tunneling mechanism across antiphase boundaries.

Magnetic wires with DNA cores: A magnetic force microscopy study

Magnetic force microscopy (MFM) has been employed to study Fe3O4 nanowires containing DNA cores. The MFM experiments confirmed that long DNA molecules templated with Fe3O4 nanoparticles form a magnetic wire. The components of wires containing particles with sizes below 10 nm were recorded to behave as single domain particles with out-of-plane magnetization. The MFM study showed that one can change the magnetization states of the particles using a magnetic tip. The properties of the magnetic wires with DNA cores make them an attractive material for future magnetostatic devices.

Fe3O4 nanowire arrays synthesized in AAO templates

Fe3O4 nanowire arrays with an average diameter of about 120 nm and lengths up to 8 μm were synthesized in anodic aluminum oxide templates through electrodepositing and heat treating a precursor β-FeOOH. The nanowires have a polycrystalline spinel structure with a=8.31 A and each nanowire is composed of fine particles. Influences of the sintering and the reducing temperatures on the products have been demonstrated by Mossbauer spectra and X-ray diffraction. It was found that high-coercivity nanowires can be obtained when the precursor was sintered at 500 °C in air and then reduced at 325 °C in H2. Hysteresis loops measured at room temperature show a clear perpendicular magnetic anisotropy.

Controlled Growth of Mesostructured Crystalline Iron Oxide Nanowires and Fe-Filled Carbon Nanotube Arrays Templated by Mesoporous Silica SBA-16 Film

The three-dimensional (3D) accessible pore structures (Imm space groups) of continuous mesoporous silica SBA-16 thin films have been prepared by a dip-coating technique in nonaqueous media under acidic conditions on indium-tin oxide glass (ITO). The films are oriented with the (111) crystal plane perpendicular to the surface of the film. On one hand, deposition of iron metal into the mesopores of SBA-16 films was achieved by using an electrochemical method. The Fe2O3 nanowire arrays were synthesized. The crystalline structures of porous Fe2O3 nanowires and nanorods were studied via TEM, SEM, and XRD. On the other hand, a small amount of Fe was deposited into the pores of the SBA-16 thin film as a catalyst, and carbon nanotube arrays formed inside the pores of SBA-16 film were fabricated by catalytic decomposition of acetylene at 700 degrees C. The second-order template synthesis method for preparing the ordered array of carbon nanotubes filled with Fe has been used. The carbon nanotubes are very uniform in diameter and length and are aligned vertically with respect to the SBA-16 film.

Solution-phase synthesis of single-crystalline magnetic nanowires with high aspect ratio and uniformity

Single-crystalline Fe3O4 nanowires with uniform diameters and the largest aspect ratio (>500) were prepared by a one-step sol-gel process in the presence of ethylene glycol and poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol).

Formation, Characterization, and Magnetic Properties of Fe3O4 Nanowires Encapsulated in Carbon Microtubes

Iron oxide (Fe3O4) nanowires encapsulated in carbon microtubes (defined as “CIOs”) with a diameter range of 150−1500 nm, and lengths up to 6 um were obtained by pyrolyzing an ethanol/ferrocene mixture in an autoclave at 600 °C. The inner Fe3O4 nanowires (defined as “IOs”) are single crystalline with a diameter range of 55−750 nm. Magnetic hysteresis loop measurements show that the CIOs display ferromagnetic properties at room temperature. In addition, carbon microtubes (CMTs) were obtained after the inside IOs were dissolved by aqueous HCl acid.

Sol–gel electrophoretic deposition and optical properties of Fe2O3 nanowire arrays

Ordered Fe2O3 nanowire arrays embedded in anodic alumina membranes have been fabricated by Sol–gel electrophoretic deposition. After annealing at 600 °C, the Fe2O3 nanowire arrays were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and X-ray diffraction (XRD). SEM and TEM images show that these nanowires are dense, continuous and arranged roughly parallel to one another. XRD and SAED analysis together indicate that these Fe2O3 nanowires crystallize with a polycrystalline corundum structure. The optical absorption band edge of Fe2O3 nanowire arrays exhibits a blue shift with respect of that of the bulk Fe2O3 owing to the quantum size effect.

Synthesis, Mössbauer Spectra and Magnetic Properties ofQuasi-One-Dimensional Fe3O4 Nanowires

Quasi-one-dimensional Fe3O4 nanowires in diameter of about 200 nmwere assembled into anodic aluminium oxide templates viaelectrodepositing and heat-treating processes. The nanowires have apolycrystalline spinel structure with a = 8.317 Å, and each nanowireis composed of fine Fe3O4 crystallites with size of about 30 nm.The magnetic moments of Fe3O4 crystallites in nanowireshave a preferred orientation leaning to the wire axis but not parallelto the wire axis. Mössbauer spectra (MS) were used to verify thepresence of Fe3O4 further and the existence of superparamagneticparticles in nanowires. A perpendicular magnetic anisotropy was observedobviously. Temperature dependence of the magnetic moments (M–T) showsthat the Verwey transition for the Fe3O4 nanowires occurs at50 K, and the behaviour of the M–T curve is differentfrom that of other magnetite materials. These characteristic magneticproperties of Fe3O4 nanowires are mainly due to the reduceddimension and the configuration of preferred orientation in thenanowires caused by the shape anisotropy.

Synthesis and Microstructures of α-Fe2O3 Bicrystalline Nanowires

ABSTRACTNovel Fe2O3 nanowires have been successfully synthesized by a simple oxidation process of pure iron. The microstructure of the Fe2O3 nanowires have been systematically investigated by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM). The investigated materials are found to be stoichiometric rhombohedral α-Fe2O3 with typical diameters of 20–80 nm and lengths up to 20 μm. In addition to known single crystal Fe2O3 nanowires, a great amount of novel bicrystalline nanowires were found with ellipsoidal heads. Investigations indicate that most of the bicrystalline nanowires are twins and their orientation relationship is obtained to be (−1, 1, 10)M//(−1, 1, 10)T, [110]M//[-1-10]T. High resolution TEM with numerical reconstruction of the electron exit wave was used to investigated the atomic structure of the micro-twins. Their growth mechanism is briefly discussed on the basis of solid phase growth process.