Growth of single-crystal magnetite nanowires from Fe3O4 nanoparticles in a surfactant-free hydrothermal process

Single-crystal magnetite nanowires with average diameter of ca. 20 nm and length of up to several micrometers were prepared by a simple alkaline surfactant-free hydrothermal process. The crystallinity, purity, morphology, and structural features of the as-prepared magnetite nanowires were investigated by powder X-ray diffraction, transmission electron microscopy (TEM) and selected area electron diffraction. The composition and length of nanowires depends on the pH, with higher pH favoring longer nanowires composed entirely of Fe₃O₄. A mechanism for nanowire growth is proposed.     

Synthesis and magnetic properties of spinel CoFe2O4 nanowire arrays

Spinel CoFe₂O₄ nanowire arrays were synthesized in nanopores of anodic aluminum oxide (AAO) template using aqueous solution of cobalt and iron nitrates as precursor. The precursor was filled into the nanopores by vacuum impregnation. After heat treatment, it transformed to spinel CoFe₂O₄ nanowires. The structure, morphology and magnetic properties of the sample were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The results indicate that the nanowire arrays are compact. And the individual nanowires have a high aspect ratio, which are about 80 nm in diameter and 10 μm in length. The nanowires are polycrystalline spinel phase. Magnetic measurements indicate that the nanowire arrays are nearly magnetic isotropic. The reason is briefly discussed. Moreover, the temperature dependence of the coercive force of the nanowire arrays was studied.     

Raman spectroscopic and photoluminescence study of single-crystalline SnO2 nanowires

Single-crystalline SnO₂ nanowires with sizes of 4-14 nm in diameter and 100-500 nm in length were produced in a molten salt approach by using hydrothermal synthesized precursor. Structural characters of the nanowires were examined by X-ray diffraction and high-resolution electron transmission microscopy. Raman, photoluminescence and X-ray photoelectron spectra of the samples were examined under heat treatments. Three new Raman modes at 691, 514 and 358 cm⁻¹ were recorded and assigned. The former two are attributed to activation of original Raman-forbidden A_2uLO mode and the third is attributed to defects in small-sized nanowires. A strong photoluminescence is observed at about 600 nm, the temperature effects is examined and the origin of the PL process is discussed via X-ray photoelectron spectra.     

Synthesis of In-doped Ga2O3 zigzag-shaped nanowires and optical properties

In-doped Ga₂O₃ zigzag-shaped nanowires and undoped Ga₂O₃ nanowires have been synthesized on Si substrate by thermal evaporation of mixed powders of Ga, In₂O₃ and graphite at 1000 °C without using any catalyst via a vapor-solid growth mechanism. The morphologies and microstructures of the products were characterized by field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and photoluminescence spectroscopy (PL). The nanowires range from 100 nm to several hundreds of nanometers in diameter and several tens of micrometers in length. A broad emission band from 400 to 700 nm is obtained in the PL spectrum of these nanowires at room temperature. There are two blue-emission peaks centering at 450 and 500 nm, which originate from the oxygen vacancies, gallium vacancies and gallium-oxygen vacancy pairs.     

Mesoporous TiO2-B nanowires synthesized from tetrabutyl titanate

Ultralong mesoporous TiO₂-B nanowires were synthesized via a hybrid hydrothermal-ion exchanging-thermal treatment using tetrabutyl titanate (TBOT) as a raw material. The phase transformations and porous structures of TiO₂-B nanowires were characterized and studied by X-ray diffraction (XRD), transmission electron microscopy (TEM) and N₂ adsorption-desorption measurement. Mesoporous TiO₂-B nanowires showed a length of several micrometers and diameter of about 25 nm. The porous structures of obtained TiO₂-B nanowires were demonstrated by BJH pore distribution measurement. The wirelike morphologies and porous structures of monodisperse nanowires calcined at 600 °C showed little change, which indicated that such nanowires possessed high thermal stability. The formation mechanism of TiO₂-B nanowires with mesoporous structures were also discussed based on our experimental results.     

Synthesis of anatase TiO2 nanowires by modifying TiO2 nanoparticles using the microwave heating method

Anatase TiO₂ nanowires with a diameter of 5-10 nm and length of 500 nm to 2 μm have been successfully synthesized by modifying TiO₂ nanoparticles (P25) using the microwave heating method. The microwave power, reaction pressure, and reaction time for the synthesis of TiO₂ nanowires were 500 W, 0.5-3.0 MPa (corresponding to a temperature range of 175-260), and 40-70 min, respectively. X-ray diffraction (XRD), field-emission scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and the BET techniques were used to investigate the phase structures, morphologies, and specific surface areas of the TiO₂ nanowires. The effects of reaction time, pressure, and different post-treatment processes on the microstructures of TiO₂ nanowires were discussed. It has been shown that the microwave heating method is efficient in transforming TiO₂ nanoparticles to anatase TiO₂ nanowires.     

Nanostructured Fe50Ni50 alloy formed by chemical reduction

A high purity Fe₅₀Ni₅₀ nanometric alloy was synthesized by ultra rapid autocatalytic chemical reduction of the corresponding transition metal ions in an aqueous solution. The ratio of metal concentration in solution is preserved in the precipitated powder alloy and no metal segregation has been detected. The alloy was characterized as a nanostructured chemically disordered taenite phase by X-ray diffraction (XRD) and Mössbauer spectroscopy (MS). Transmission electron microscopy (TEM) showed that the as prepared alloy contained spherical particles with 96 nm mean diameter size. The particles are composed of crystallites (of ∼15 nm size) and a predominant disordered interfacial region. A thermal treatment of 673 K/2 h produced a structural relaxation with a significant narrowing in the XRD and Mössbauer lines with a exothermic flow in the DSC signal and an increase in the crystallite size to 30 nm.     

Synthesis and characterization of single-crystalline MnFe2O4 nanorods via a surfactant-free hydrothermal route

Uniform, high-quality, single-crystalline MnFe₂O₄ nanorods with diameter around 25 nm and length up to 500 nm, have been reproducibly synthesized via a surfactant-free hydrothermal route. The growth direction of the obtained nanowires was determined to be its [1 1 1] direction, resulting in the increase of saturation magnetization. Mn²⁺ is responsible for one-dimensional growth of the nanorods, and the effects of reaction time and solution concentration on the morphology and crystallization of the MnFe₂O₄ nanorods were investigated. Saturation magnetization of the nanorods is 74.0 emu/g, which is among the best value reported so far.     

Magnetic coupling in Co1−xPtx (x>0.5) nanowires electrodeposited on an AAO template

Thirty nanometer diameter Co-Pt nanowires of different composition were fabricated by electrodepositing the Co and Pt atoms to nanoporous anodized aluminium oxide (AAO) templates. The structure and magnetic properties are studied by transmission electron microscopy (TEM), induction-coupled plasma spectrometer (ICP), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). The as deposited nanowires with Pt content about 50 at.% present a single ferromagnetic phase of fcc CoPt. When the Pt content of the nanowires varies from about 55 to about 75 at.%, the nanowires include a soft phase of fcc CoPt₃ and a relatively hard phase of fcc CoPt and the two phases are separate as seen from the hysteresis loops. After annealing to 600 °C, the two phases coupled completely and the coupled phase has the same coercivity as the original hard one.     

Synthesis of crystalline SrMoO4 nanowires from polyoxometalates

Crystalline SrMoO₄ nanowires were synthesized via a facile hydrothermal process at 180 °C for 10 h. α-(NH₄)₆-P₂Mo₁₈O₆₂·nH₂O, one of polyoxometalates with Dawson structure, was employed as the source of molybdates. The diameter and length of the obtained SrMoO₄ nanowires are about 20 nm and 5-10 μm, respectively. HRTEM results show that the SrMoO₄ nanowires are of high crystallinity with rough surface. However, when Na₂MoO₄·2H₂O was used, there are only SrMoO₄ nanorods with smaller aspect ratio (200/70 nm) in the similar hydrothermal process. The probable growth mechanism was discussed.     

Fabrication of coaxial p-Cu2O/n-ZnO nanowire photodiodes

The authors report the deposition of Cu₂O onto vertically well aligned ZnO nanowires by DC sputtering. The average length, average diameter and density of these VLS-synthesized ZnO nanowires were 1 μm, 100 nm and 23 wires/μm², respectively. With proper sputtering parameters, the deposited Cu₂O could fill the gaps between the ZnO nanowires with good step coverage to form coaxial p-Cu₂O/n-ZnO nanowires with a rectifying current–voltage characteristic. Furthermore, the fabricated coaxial p-Cu₂O/n-ZnO nanowire photodiodes exhibit reasonably large photocurrent-to-dark-current contrast ratio and the fast responses.     

Synthesis and characterization of GaN nanowires by ammoniating Ga2O3/Cr thin films deposited on Si(1 1 1) substrates

GaN nanowires have been successfully synthesized on Si(1 1 1) substrates by magnetron sputtering through ammoniating Ga₂O₃/Cr thin films at 950 °C. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), FT-IR spectrophotometer, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM), and photoluminescence (PL) spectrum were carried out to characterize the microstructure, morphology, and optical properties of GaN samples. The results demonstrate that the nanowires are single-crystal GaN with hexagonal wurtzite structure and high-quality crystalline, have the size of 30-80 nm in diameter and several tens of microns in length with good emission properties. The growth direction of GaN nanowires is perpendicular to the fringe of (1 0 1) plane. The growth mechanism of GaN nanowires is also discussed in detail.     

Electromagnetic wave absorption properties of Fe3C/carbon nanocomposites prepared by a CVD method

Dendrite-shaped iron nanowires with 50-200 nm in diameter and 10-20 μm in length were prepared by the CVD method from Fe(CO)₅ as a starting source. Ethanol was cracked on the surface of the resultant iron nanowires to form the Fe₃C/carbon nanocomposites, in which nanosized carbon beads covered the surface of Fe₃C. Resin compact of the resultant Fe₃C/carbon nanocomposites had excellent electromagnetic wave absorption ability in the range of 0.9-9.0 GHz, and such available absorption range more enhanced compared to that observed on the resin compact prepared from the original iron nanowires by the hybridization of magnetic (Fe₃C) and dielectric (carbon) materials.     

Synthesis of β-Ga2O3 nanowires through microwave plasma chemical vapor deposition

In this study, we demonstrate the large-scale synthesis of beta gallium oxide (β-Ga₂O₃) nanowires through microwave plasma chemical vapor deposition (MPCVD) of a Ga droplet in the H₂O and Ar atmosphere at 600 W. Unlike the commonly used MPCVD method, the H₂O, not mixture of gas, was employed to synthesize the nanowires. The ultra-long β-Ga₂O₃ nanowires with diameters of about 20-30 nm were several tens of micrometers long. The morphology and structure of products were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscope (HRTEM). The growth of β-Ga₂O₃ nanowires was controlled by vapor-solid (VS) crystal growth mechanism.     

From crystalline V2O5 to nanostructured vanadium oxides using aromatic amines as templates

Nanoneedles, nanorods of B-VO₂, and vanadium oxide nanotubes with high crystallinity were synthesized via a one-step hydrothermal treatment using crystalline V₂O₅ as a precursor and aromatic amines (C₆H₅-(CH₂)_n-NH₂ with n=0, 1, 3) as structure-directing templates. Samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), thermal analysis, nitrogen adsorption/desorption isotherms and infrared spectroscopy. Nanoneedles, 0.5-5 μm in length and about 50 nm in average diameter and VO₂(B) nanorods about 20-100 nm wide and up to 2.5 μm long, have been obtained. The inner and the outer diameters of the vanadium oxide nanotubes vary, respectively, between 15-25 and 70-100 nm with a length up to 4 μm.     

Superparamagnetic nanocomposites based on poly(styrene-b-ethylene/butylene-b-styrene)/cobalt ferrite compositions

Magnetic nanoparticles were created in or around the sulfonated (s) polystyrene domains in a phase separated poly[styrene-b-(ethylene-co-butylene)-b-styrene)] block copolymer (BCP) using an in situ inorganic precipitation procedure. The sBCP was neutralized with a mixed iron/cobalt chloride electrolyte and the doped samples were converted to their oxides by reaction with sodium hydroxide and further washing with water. Transmission electron microscopy indicated the presence of nanoparticles in the 5–25 nm size range. The metal oxide particle structures were studied using select area electron diffraction, which revealed that they are of the cobalt iron oxide composition (CoFe₂O₄). These nanocomposites were shown, using a superconducting quantum interference device magnetometer, to be superparamagnetic at 300 K and ferrimagnetic at 5 K. Nanocomposites consisting of smaller particles have a blocking temperature of 70 K, whereas it was 140 K for larger particles.     

Synthesis and materials properties of transparent conducting In2O3 films prepared by sol-gel-spin coating technique

Transparent conducting indium oxide (In₂O₃) thin films have been prepared on glass substrates by the simple sol-gel-spin coating technique. These films have been characterized by X-ray diffraction, resistivity and Hall effect measurements, optical transmission, scanning electron microscopy and atomic force microscopy for their structural, electrical, optical and morphological properties. The influence of spin parameters, number of coating, process temperature on the quality of In₂O₃ films are studied. In the operating range of deposition, 400-475 °C, all the films showed predominant (2 2 2) orientation. Films deposited at optimum process conditions exhibited a resistivity of 2×10⁻² Ω cm along with the average transmittance of about 80% in the visible spectral range (400-700 nm).     

Supercapacitive cobalt oxide (Co3O4) thin films by spray pyrolysis

Uniform and adherent cobalt oxide thin films have been deposited on glass substrates from aqueous cobalt chloride solution, using the solution spray pyrolysis technique. Their structural, optical and electrical properties were investigated by means of X-ray diffraction (XRD), scanning electron micrograph (SEM), optical absorption and electrical resistivity measurements. Along with this, to propose Co₃O₄ for possible application in energy storage devices, its electrochemical supercapacitor properties have been studied in aqueous KOH electrolyte. The structural analysis from XRD pattern showed the oriented growth of Co₃O₄ of cubic structure. The surface morphological studies from scanning electron micrographs revealed the nanocrystalline grains alongwith some overgrown clusters of cobalt oxide. The optical studies showed direct and indirect band gaps of 2.10 and 1.60 eV, respectively. The electrical resistivity measurement of cobalt oxide films depicted a semiconducting behavior with the room temperature electrical resistivity of the order of 1.5 × 10³ Ω cm. The supercapacitor properties depicted that spray-deposited Co₃O₄ film is capable of exhibiting specific capacitance of 74 F/g.     

Cobalt nanoparticles synthesis from Co(CH3COO)2 by thermal decomposition

Nontoxic cobalt acetate tetrahydrate was used as a precursor to prepare cobalt nanoparticles of 8–200 nm in average diameter by thermal decomposition. The different combinations of trioctylphosphine, oleylamine and oleic acid were added as surfactants to control the particle size. These combinations resulted in the particles with saturation magnetization and coercive force ranging from 55.0 to 100.1 emu/g and from 0 to 459.3 Oe, respectively.     

Fabrication of tungsten oxide microfibers with photocatalytic activity by electrospunning from PVA/H3PW12O40 gel

Regarding gel poly (vinyl alcohol)/H₃PW₁₂O₄₀ as precursor, the ultra-fine fibers tungsten oxide (WO₃) was prepared by using electrospinning and calcinating techniques. Scanning electron microscope (SEM) shows that the average diameter of fibrous WO₃ were changed from 200 nm to 600 nm after calcined PVA/H₃PW₁₂O₄₀ fibers at 600 and 800 °C, respectively. X-ray diffraction (XRD) and Raman spectroscope revealed that the fibrous WO₃ was monoclinic phase, and the band-gap energies were observed by UV-vis diffuse reflectance spectra. The small size WO₃ exhibits excellent photocatalytcic activity in degradation of Rhodamine B at 365 nm wavelength.