Excellent ethanol sensing properties based on Er_2O_3-Fe_2O_3 nanotubes

In this work, pure α-Fe2O3 and Er2O3-Fe2O3 nanotubes were synthesized by a simple single-capillary electrospinning technology followed by calcination treatment. The morphologies and crystal structures of the as-prepared samples were characterized by scanning electron microscopy and x-ray diffraction, respectively. The gas-sensing properties of the as-prepared samples have been researched, and the result shows that the Er2O3-Fe2O3 nanotubes exhibit much better sensitivity to ethanol. The response value of Er2O3-Fe2O3 nanotubes to 10 ppm ethanol is 21 at the operating temperature240?, which is 14 times larger than that of pure α-Fe2O3 nanotubes(response value is 1.5). The ethanol sensing properties of α-Fe2O3 nanotubes are remarkably enhanced by doping Er, and the lowest detection limit of Er2O3-Fe2O3 nanotubes is300 ppb, to which the response value is about 2. The response and recovery times are about 4 s and 70 s to 10 ppm ethanol,respectively. In addition, the Er2O3-Fe2O3 nanotubes possess good selectivity and long-term stability.     

Canted antiferromagnetic and optical properties of nanostructures of Mn2O3 prepared by hydrothermal synthesis

We have reported new magnetic and optical properties of Mn2O3 nanostructures.The nanostructures have been synthesized by the hydrothermal method combined with the adjustment of pH values in the reaction system.The particular characteristics of the nanostructures have been analyzed by employing X-Ray diffraction(XRD),scanning electron microscopy(SEM),energy dispersive X-ray(EDX) analysis,transmission electron microscopy(TEM),high resolution transmission electron microscopy(HRTEM),Raman spectroscopy(RS),UV-visible spectroscopy,and the vibrating sample magnetometer(VSM).Structural investigation manifests that the synthesized Mn2O3 nanostructures are orthorhombic crystal.Magnetic investigation indicates that the Mn2O3 nanostructures are antiferromagnetic and the antiferromagnetic transition temperature is at TN=83 K.Furthermore,the Mn2O3 nanostructures possess canted antiferromagnetic order below the Neel temperature due to spin frustration,resulting in hysteresis with large coercivity(1580 Oe) and remnant magnetization(1.52 emu/g).The UV-visible spectrophotometry was used to determine the transmittance behaviour of Mn2O3 nanostructures.A direct optical band gap of 1.2 eV was acquired by using the Davis-Mott model.The UV-visible spectrum indicates that the absorption is prominent in the visible region,and transparency is more than 80% in the UV region.     

Enhancement of microwave absorption of nanocomposite BaFe_(12)O_(19)/α-Fe microfibers

Nanocomposite BaFe12O19/α-Fe microfibers with diameters of about 1-5 μm are prepared by the organic gelthermal selective reduction process. The binary phase of BaFe12O19 and α-Fe is formed after reduction of the precursor BaFe12O19/α-Fe2O3 microfibers at 350℃ for 1 h. These nanocomposite microfibers are fabricated from α-Fe (16-22 nm in diameter) and BaFe12O19 particles (36-42 nm in diameter) and basically exhibit a single-phase-like magnetization behavior, with a high saturation magnetization and coercive force arising from the exchange-coupling interactions of soft α-Fe and hard BaFe12O19 . The microwave absorption characteristics in a 2-18 GHz frequency range of the nanocomposite BaFe12O19/α-Fe microfibers are mainly influenced by their mass ratio of α-Fe/BaFe12O19 and specimen thickness. It is found that the nanocomposite BaFe12O19/α-Fe microfibers with a mass ratio of 1:6 and specimen thickness of 2.5 mm show an optimal reflection loss (RL) of 29.7 dB at 13.5 GHz and the bandwidth with RL exceeding 10 dB covers the whole Ku-band (12.4-18.0 GHz). This enhancement of microwave absorption can be attributed to the heterostructure of soft, nano, conducting α-Fe particles embedded in hard, nano, semiconducting barium ferrite, which improves the dipolar polarization, interfacial polarization, exchange-coupling interaction, and anisotropic energy in the nanocomposite BaFe12O19/α-Fe microfibers.     

One-dimensional iron oxides nanostructures

Iron oxides,including α-Fe2O3,γ-Fe2O3,Fe3O4,etc.are one of the most widely investigated materials for their fundamental properties and potential applications.One-dimensional(1-D) iron oxides nanostructures are the focus of recent research activities because of their wide applications in magnetic refrigeration,information storage,electronics,catalysts,Li-ion battery,pigment,gas sensors,etc.This review covers the recent progress in the synthesis,properties and applications of 1-D iron oxides nanostructures.Th...     

Growth mechanism and photoluminescence of the SnO2 nanotwists on thin film and the SnO2 short nanowires on nanorods

SnO2 nanotwists on thin film and SnO2 short nanowires on nanorods have been grown on single silicon substrates by using Au-Ag alloying catalyst assisted carbothermal evaporation of SnO2 and active carbon powders.The morphology and the structure of the prepared nanostructures are determined on the basis of field-emission scanning electron microscopy(FESEM),transmission electron microscopy(TEM),selected area electronic diffraction(SAED),high-resolution transmission electron microscopy(HRTEM),x-ray diffraction(XRD),Raman and photoluminescence(PL) spectra analysis.The new peaks at 356,450,and 489 nm in the measured PL spectra of two kinds of SnO2 nanostructures are observed,implying that more luminescence centres exist in these SnO2 nanostructures due to nanocrystals and defects.The growth mechanism of these nanostructures belongs to the vapour-liquid-solid(VLS) mechanism.     

A new modulated structure in α-Fe_2O_3 nanowires

A new modulated structure consisting of periodic （1120） stacking faults （SFs） in the α-Fe2O3 nanowires （NWs） formed by the thermal oxidation of Fe foils is reported, using a combination of high-resolution transmission electron microscopy （HRTEM） observations and HRTEM image simulations. The periodicity of the modulated structure is 1.53 nm, which is ten times （3500） interplanar spacing and can be described by a shift of every ten （3500） planes with 1/2 the interplanar spacing of the （1120） plane. An atomic model for the Fe203 structure is proposed to simulate the modulated structure. HRTEM simulation results confirm that the modulated structure in α-Fe2O3 NWs is caused by SFs.     

Gigahertz longitudinal acoustic phonons originating from ultrafast ligand field transitions in hematite thin films

The creation and propagation of longitudinal acoustic phonons(LAPs) in high quality hematite thin films(α-Fe2O3)epitaxially grown on different substrates(BaTiO3, SrTiO3, and LaAlO3) are investigated using the femtosecond pump–probe technique. Transient reflection measurements(?R/R) indicate the photo-excited electron dynamics, and the initial decay less than 1 ps and the slow decay of ～500 ps are attributed to the electron-LO phonon coupling and electron–hole nonradiative recombination, respectively. LAPs in α-Fe2O3 film can be created by ultrafast excitation of the ligand field state, such as the ligand field transitions under 800-nm excitation as well as the ligand to metal charge-transfer with 400-nm excitation. The strain modulations of the sound velocity and the out-of-plane elastic properties are demonstrated inα-Fe2O3 film on different substrates.     

Growth of β-Ga2O3 nanorods by ammoniating Ga2O3/V thin films on Si substrate

This paper reports that/3-Ga2O3 nanorods have been synthesized by ammoniating Ga2O3 films on a V middle layer deposited on Si（111） substrates. The synthesized nanorods were confirmed as monoclinic Ga2O3 by x-ray diffraction,Fourier transform infrared spectra. Scanning electron microscopy and transmission electron microscopy reveal that the grown β-Ga2O3 nanorods have a smooth and clean surface with diameters ranging from 100 nm to 200 nm and lengths typically up to 2μm. High resolution TEM and selected-area electron diffraction shows that the nanorods are pure monoclinic Ga2O3 single crystal. The photoluminescence spectrum indicates that the Ga2O3 nanorods have a good emission property. The growth mechanism is discussed briefly.     

Growth of Indium Nanorods by Magnetron Sputtering

Indium nanorods are grown on silicon substrates by using magnetron-sputtering technique. Film morphologies and nanorod microstructure are investigated by using scanning electron microscopy, high-resolution transmission electron microscopy （HRTEM）, and x-ray diffraction. It is found that the mean diameter of the nanorods ranges from 30nm to 100nm and the height ranges from 30nm to 200nm. The HRTEM investigations show that the indium nanorods are single crystals and grow along the [100] axis. The nanorods grow from the facets near the surface undulation that is caused by compressive stress in the indium grains generated during grain coalescence process. For low melting point and high diffusivity metal, such as bismuth and indium, this spontaneous nanorod growth mechanism can be used to fabricate nanostructures.     

Selective Area Growth and Characterization of GaN Nanorods Fabricated by Adjusting the Hydrogen Flow Rate and Growth Temperature with Metal Organic Chemical Vapor Deposition

GaN nanorods are successfully fabricated by adjusting the how rate ratio of hydrogen(H_2)/nitrogen(N_2) and growth temperature of the selective area growth(SAG) method with metal organic chemical vapor deposition(MOCVD).The SAG template is obtained by nanospherical-lens photolithography.It is found that increasing the flow rate of H_2 will change the GaN crystal shape from pyramid to vertical rod,while increasing the growth temperature will reduce the diameters of GaN rods to nanometer scale.Finally the GaN nanorods with smooth lateral surface and relatively good quality are obtained under the condition that the H_2:N_2 ratio is 1:1 and the growth temperature is 1030℃.The good crystal quality and orientation of GaN nanorods are confirmed by high resolution transmission electron microscopy.The cathodoluminescence spectrum suggests that the crystal and optical quality is also improved with increasing the temperature.     

Synthesis of flower-shape clustering GaN nanorods by ammoniating Ga2O3 films

Flower-shape clustering GaN nanorods are successfully synthesized on Si（111） substrates through ammoniating Ga2O3/ZnO films at 950℃. The as-grown products are characterized by x-ray diffraction （XRD）, scanning electron microscope （SEM）, field-emission transmission electron microscope （FETEM）, Fourier transform infrared spectrum （FTIR） and fluorescence spectrophotometer. The SEM images demonstrate that the products consist of flower-shape clustering GaN nanorods. The XRD indicates that the reflections of the samples can be indexed to the hexagonal GaN phase and HRTEM shows that the nanorods are of pure hexagonal GaN single crystal. The photoluminescence （PL） spectrum indicates that the GaN nanorods have a good emission property. The growth mechanism is also briefly discussed.     

Thermodynamical Phase Equilibrium of Iron Nitrides at Low Temperatures

The presence of α-Fe(N),γ‘-Fe4N and ε-Fe2N1-x phases and the phase equilibrium between αFe(N) and γ′-Fe4N,and γ′-Fe4N and ε-Fe2N1-x have been studied at low temperatures below 350℃ ,according to the binary Fe-N phase diagram calculated using the sublattice-compound energy model,The α-Fe(N),γ′-Fe4N and ε-Fe2N1-x phases would be thermodynamically stable at the low temperatures.Thermodynamic phase equilibrium of the iron nitrides is supproted by the experimental results,explaining the phase formation and its stability in the Fe-N system at low temperatures.     

Controlling the orientation of ZnO nanorod arrays using TiO<Subscript>2</Subscript> thin film templates dip-coated by sol–gel

The oriented ZnO nanorod arrays have been synthesized on a silicon wafer that coated with TiO₂ films by aqueous chemical method. The morphologies, phase structure and the photoluminescence (PL) properties of the as-obtained product were investigated by field-emission scanning electron microscopy (FE-SEM), X-ray diffractometer (XRD), transmission electron microscope (TEM) and PL spectrum. The nanorods were about 100 nm in diameter and more than 1 μm in length, which possessed wurtzite structure with a c axis growth direction. The room-temperature PL measurement of the nanorod arrays showed strong ultraviolet emission. The effect of the crystal structure and the thickness of TiO₂ films on the morphologies of ZnO nanostructures were investigated. It was found that the rutile TiO₂ films were appropriate to the oriented growth of ZnO nanorod arrays in comparison with anatase TiO₂ films. Moreover, flakelike ZnO nanostructures were obtained with increasing the thickness of anatase TiO₂ films.     

Microwave absorption properties of a double-layer absorber based on nanocomposite BaFe12O19/α-Fe and nanocrystalline α-Fe microfibers

The nanocomposite BaFe12O19/α-Fe and nanocrystalline α-Fe microfibers with diameters of 1–5 μm, high aspect ratios and large specific areas are prepared by the citrate gel transformation and reduction process. The nanocomposite BaFe12O19/α-Fe microfibers show some exchange–coupling interactions largely arising from the magnetization hard(BaFe12O19) and soft(α-Fe) nanoparticles. For the microwave absorptions, the double-layer structures consisting of the nanocomposite BaFe12O19/α-Fe and α-Fe microfibers each exhibit a wide band and strong absorption behavior. When the nanocomposite BaFe12O19/α-Fe microfibers are used as a matching layer of 2.3 mm in thickness and α-Fe microfibers as an absorbing layer of 1.2 mm in thickness, the optimal reflection loss(RL) achieves-47 dB at 15.6 GHz, the absorption bandwidth is about 12.7 GHz ranging from 5.3 to 18 GHz, exceeding-20 dB, which covers 72.5% C-band(4.2–8.2 GHz)and whole X-band(8.2–12.4 GHz) and Ku-band(12.4–18 GHz). The enhanced absorption properties of these double-layer absorbers are mainly ascribed to the improvement in impedance matching ability and microwave multi-reflection largely resulting from the dipolar polarization, interfacial polarization, exchange–coupling interaction, and small size effect.     

Three-dimensional noble-metal nanostructure: A new kind of substrate for sensitive,uniform, and reproducible surface-enhanced Raman scattering

Surface-enhanced Raman spectroscopy(SERS) is a powerful vibrational spectroscopy technique for highly sensitive structural detection of low concentration analyte. The SERS activities largely depend on the topography of the substrate.In this review, we summarize the recent progress in SERS substrate, especially focusing on the three-dimensional(3D)noble-metal substrate with hierarchical nanostructure. Firstly, we introduce the background and general mechanism of3 D hierarchical SERS nanostructures. Then, a systematic overview on the fabrication, growth mechanism, and SERS property of various noble-metal substrates with 3D hierarchical nanostructures is presented. Finally, the applications of 3D hierarchical nanostructures as SERS substrates in many fields are discussed.     

Co-adsorption of O_2 and H_2O on α-uranium(110) surface:A density functional theory study

First-principles calculations based on density functional theory corrected by Hubbard parameter U(DFT+U) are applied to the study on the co-adsorption of O2 and H_2O molecules to α-U(110) surface. The calculation results show that DFT+U method with Ueff= 1.5 e V can yield the experimental results of lattice constant and elastic modulus of α-uranium bulk well. Of all 7 low index surfaces of α-uranium, the(001) surface is the most stable with lowest surface energy while the(110) surface possesses the strongest activity with the highest surface energy. The adsorptions of O_2 and H_2O molecules are investigated separated. The O_2 dissociates spontaneously in all initial configurations. For the adsorption of H_2O molecule,both molecular and dissociative adsorptionsoccur. Through calculations of co-adsorption, it can be confirmed that the inhibition effect of O_2 on the corrosion of uranium by water vapor originates from the preferential adsorption mechanism,while the consumption of H atoms by O atoms exerted little influence on the corrosion of uranium.     

C,igahertz longitudinal acoustic phonons originating from ultrafast ligand field transitions in hematite thin films

The creation and propagation of longitudinal acoustic phonons （LAPs） in high quality hematite thin films （α-Fe203） epitaxially grown on different substrates （BaTiO3, SrTiO3, and LaAlO3） are investigated using the femtosecond pump- probe technique. Transient reflection measurements （AR/R） indicate the photo-excited electron dynamics, and the initial decay less than 1 ps and the slow decay of -500 ps are attributed to the electron-LO phonon coupling and electron-hole nonradiative recombination, respectively. LAPs in α-Fe2O3 film can be created by ultrafast excitation of the ligand field state, such as the ligand field transitions under 800-nm excitation as well as the ligand to metal charge-transfer with 400- nm excitation. The strain modulations of the sound velocity and the out-of-plane elastic properties are demonstrated in α-Fe2O3 film on different substrates.     

Room temperature NO_2-sensing properties of hexagonal tungsten oxide nanorods

Hexagonal WO_3 nanorods were synthesized through a facile hydrothermal method. The nanorods properties were investigated by scanning electron microscope(SEM), transmission electron microscope(TEM), energy dispersive spectroscopy(EDS), and x-ray diffraction(XRD). The NO_2-sensing performances in terms of sensor response, response/recovery times and repeatability at room temperature were optimized by varying the heat treatment temperature of WO_3 nanorods. The optimized NO_2sensor(400-℃-annealed WO_3 nanorods) showed an ultra-high sensor response of 3.2 and short response time of 1 s to 5-ppm NO_2. In addition, the 400-℃-annealed sample exhibited more stable repeatability.Furthermore, dynamic responses measurements of annealed samples showed that all the annealed WO_3 nanorods sensors presented p-type behaviors. We suppose the p-type behavior of the WO_3 nanorods sensor to be that an inversion layer is formed in the space charge layer when the sensor is exposed to NO_2 at room temperature.Therefore, the 400-℃-annealed WO_3 nanorods sensor is one of the most energy conservation candidates to detect NO_2 at room temperature.     

Ab initio molecular dynamics simulations of nano-crystallization of Fe-based amorphous alloys with early transition metals

The addition of early transition metals(ETMs) into Fe-based amorphous alloys is practically found to be effective in reducing the α-Fe grain size in crystallization process. In this paper, by using ab initio molecular dynamics simulations, the mechanism of the effect of two typical ETMs(Nb and W) on nano-crystallization is studied. It is found that the diffusion ability in amorphous alloy is mainly determined by the bonding energy of the atom rather than the size or weight of the atom. The alloying of B dramatically reduces the diffusion ability of the ETM atoms, which prevents the supply of Fe near the grain surface and consequently suppresses the growth of α-Fe grains. Moreover, the difference in grain refining effectiveness between Nb and W could be attributed to the larger bonding energy between Nb and B than that between W and B.     

Dielectric properties of electrospun titanium compound/polymer composite nanofibres

Poly(vinylpyrrolidone)/tetrabutyl titanate (PVP/ [CH3(CH2)3O]4Ti) composite nanofibres are prepared by elec- trospinning. After calcining parts of composite nanofibres in air at 700 C, petal-like TiO2 nanostructures are obtained. The characterizations of composite nanofibres and TiO2 nanostructures are carried out by a scanning electron micro- scope, an x-ray diffractometer, and an infrared spectrometer. Electrospun nanofibres are pressed into pellets under different pressures in order to explore their dielectric properties. It is found that the dielectric constants decrease with frequency increasing. The dielectric constant of the composite nanofibre pellet increases whereas its dielectric loss tangent decreases due to the doped titanium ions compared with those of pure PVP nanofibre pellets. In addition, it is observed that the dielectric constant of the composite nanofibre pellet decreases with the increase of the pressure applied in pelletization.