Preparation and photocatalytic property of α-Fe2O3 hollow core/shell hierarchical nanostructures

We report the preparation of a novel kind of α-Fe₂O₃ hollow core/shell hierarchical nanostructures self-assembled by nanosheets. A green precursor powder is first prepared using nontoxic and inexpensive FeCl₃ and urea in ethylene glycol by a surfactant-free solvothermal method at 160 °C for 15 h. The α-Fe₂O₃ hollow core/shell hierarchical nanostructures are obtained by the thermal treatment of the green precursor powder. The as-prepared α-Fe₂O₃ hollow core/shell hierarchical nanostructures are porous, and exhibit a good photocatalytic activity for the degradation of phenol. The samples are characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).     

Synthesis and characterization of hybrid nanocomposites of polypyrrole filled with iron oxide nanoparticles

Hybrid polypyrrole (PPy)/α-Fe₂O₃ nanocomposite films were fabricated by spin coating on a glass substrate. X-Ray diffraction analysis revealed the crystalline structure of α-Fe₂O₃ nanostructures and the nanocomposites. The broad PPy peak weakened in intensity as the α-Fe₂O₃ content increased in PPy/α-Fe₂O₃ nanocomposites. Characteristic Fourier-transform IR peaks for pure PPy shifted to higher wavenumbers on addition of α-Fe₂O₃ to PPy/α-Fe₂O₃ nanocomposites. This can be attributed to better conjugation and interactions between PPy and α-Fe₂O₃ nanoparticles. Field-emission scanning electron microscopy, transmission electron microscopy, and atomic force microscopy images of the nanocomposites reveal a uniform distribution of α-Fe₂O₃ nanoparticles in the PPy matrix. UV-vis absorption spectroscopy revealed a blue shift from λ_max= 441 nm for PPy to λ_max= 392 nm for PPy/α-Fe₂O₃, reflecting strong interactions between PPy and α-Fe₂O₃ nanoparticles. The room-temperature dc electrical conductivity increased from 4.33×10⁻⁹ to 1.81×10⁻⁸ S/cm as the α-Fe₂O₃ nanoparticle content increased from 10 to 50 wt.% in PPy/α-Fe₂O₃ nanocomposites.     

Formation of γ-Fe2O3 hierarchical nanostructures at 500 °C in a high magnetic field

Three-dimensional (3-D) hierarchical nanostructures of γ-Fe₂O₃ are prepared by a solvothermal process combined with subsequent thermal treatment in air at 500 or 350 °C with the aid of a high magnetic field. The experimental results indicate that γ-Fe₂O₃ instead of α-Fe₂O₃ forms in air at 500 °C in a 12 T field. The products are characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The magnetization curves measured at room temperature exhibit superparamagnetic characteristics.     

Morphology and orientation of iron oxide precipitates in epitaxial BiFeO3 thin films grown under two non-optimized oxygen pressures

Microstructures of multiferroic BiFeO₃ thin films epitaxially grown on SrRuO₃-buffered SrTiO₃ (001) substrates by laser molecular-beam epitaxy under two non-optimized oxygen pressures were characterized by means of transmission electron microscopy. The results showed that the films grown under oxygen pressures of 1 Pa and 0.3 Pa contain a secondary phase embedded in the BiFeO₃ matrix. High-angle annular dark-field imaging, elemental mapping and composition analysis in combination with selected area electron diffraction revealed that the parasitic phase is mainly antiferromagnetic α-Fe₂O₃. The α-Fe₂O₃ particles are semi-coherently embedded in the BiFeO₃ films, as confirmed by high-resolution transmission electron microscopy. In addition to the α-Fe₂O₃ phase, ferromagnetic Fe₃O₄ precipitates were found in the BiFeO₃ films grown under 0.3 Pa and shown to accumulate in areas near the film/substrate interfaces. In our heteroepitaxy systems, very low density misfit dislocations were observed at the interfaces between the BiFeO₃ and SrRuO₃ layers implying that their misfit strains may be relieved by the formation of the secondary phases. Using X-ray photoelectron spectroscopy it was found that Fe exists in the +3 oxidation state in these films. The possible formation mechanisms of the secondary phases are discussed in terms of film growth conditions.     

One-step solution synthesis of Fe2O3 nanoparticles at low temperature

A new synthesis method of α-Fe₂O₃ nanoparticles was developed, in which the ferrous and ferric salts as well as polyaniline acted as the precursor and dispersant, respectively. From the investigation of X-ray diffraction and FT-IR spectra, the α-Fe₂O₃ nanoparticles can be directly prepared by the co-precipitation method without high-temperature calcining. Transmission electron microscope (TEM) and scanning electron microscope (SEM) observation revealed that the α-Fe₂O₃ nanoparticles had average diameters ranging from 30.0 to 75.0 nm. Compared with previous methods, this present method shows an easy processing and can be applied on the large-scale produce of α-Fe₂O₃ nanoparticles in one step.     

High coercivity in α-Fe2O3 nanostructures synthesized by surfactant-free microwave-assisted solvothermal method

In this report, the cube-like shaped α-Fe₂O₃ nanostructures were prepared by the simple microwave-assisted solvothermal method without using any surfactants. The as-prepared samples were characterized by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy and field emission scanning electron microscopy. The well dispersed and size-controlled cubic-like shaped α-Fe₂O₃ nanostructures were obtained by systematic variation of solvents, reaction temperature and time. The magnetic studies manifest that the magnetic properties of α-Fe₂O₃ samples are strongly dependent on the shape and size of the nanostructures. The maximum coercivity (Hc) ∼5.6 kOe is observed for Fe-160-30 sample, which is originating from the varying synthesis conditions, oriented sub-particle structures, surface spin disorder, surface/interface anisotropy and interactions of the nanoparticles at the surface/interface of the nanostructures. Represented synthesis approach facilitates the preparation of nanostructured materials with controlled morphology and properties.     

Solvothermal synthesis and characterization of α-Fe2O3 nanodiscs and Mn3O4 nanoparticles with 1,10-phenanthroline

α-Fe₂O₃ nanodiscs and Mn₃O₄ nanoparticles have been prepared by the 1,10-phenanthroline as complexing agent in the presence of sodium hydroxide under hydrothermal conditions. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR) spectra. The average diameter of α-Fe₂O₃ nanodiscs is of 2 μm. In the case of Mn₃O₄ sample, the Mn₃O₄ crystallites are nanoparticles with an average size of 34 nm. A formation mechanism for the α-Fe₂O₃ and Mn₃O₄ nanomaterials was proposed.     

Fabrication and characterization of ZnO micro and nanostructures prepared by thermal evaporation

A simple method of thermal evaporation to fabricate micro and nanostructures of zinc oxide was presented. ZnO micro and nanostructures, prepared under different quantity of O₂, were characterized by techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy and analytical transmission electron Microscope. The SEM images indicated that the products prepared under the condition of sufficient O₂ were needle-like microrods and the samples synthesized under the condition of deficient O₂ were nanorods and nanowires with very high aspect ratio. The results of XRD and Raman shifts revealed that the ZnO micro and nanostructures synthesized under different quantity of O₂ were both single crystalline with the hexagonal wurtzite structure. The HRTEM images indicated that the ZnO nanowire prepared under the condition of deficient O₂ was single crystalline and grown along the direction of [0 0 1]. Photoluminescence measurement was carried out and it showed that the spectra of ZnO micro and nanostructures prepared under different quantity of O₂ exhibited similar emission features. In addition, the growth mechanism of ZnO micro and nanostructures was preliminarily discussed.     

Interparticle interaction effects on magnetic behaviors of hematite (α-Fe2O3) nanoparticles

The interparticle magnetic interactions of hematite (α-Fe₂O₃) nanoparticles were investigated by temperature and magnetic field dependent magnetization curves. The synthesis were done in two steps; milling metallic iron (Fe) powders in pure water (H₂O), known as mechanical milling technique, and annealing at 600 °C. The crystal and molecular structure of prepared samples were determined by X-ray powder diffraction (XRD) spectra and Fourier transform infrared (FTIR) spectra results. The average particle sizes and the size distributions were figured out using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The magnetic behaviors of α-Fe₂O₃ nanoparticles were analyzed with a vibrating sample magnetometer (VSM). As a result of the analysis, it was observed that the prepared α-Fe₂O₃ nanoparticles did not perform a sharp Morin transition (the characteristic transition of α-Fe₂O₃) due to lack of unique particle size distribution. However, the transition can be observed in the wide temperature range as “a continuously transition”. Additionally, the effect of interparticle interaction on magnetic behavior was determined from the magnetization versus applied field (σ(M)) curves for 26±2 nm particles, dispersed in sodium oxalate matrix under ratios of 200:1, 300:1, 500:1 and 1000:1. The interparticle interaction fields, recorded at 5 K to avoid the thermal interactions, were found as ∼1082 Oe for 26±2 nm particles.     

Cu-doped flower-like hematite nanostructures for efficient water splitting applications

This study reports the successful preparation of Cu-doped hematite (α-Fe₂O₃) flower-like nanostructures with different Cu concentrations on FTO glass substrates using a facile hydrothermal method. The Cu-doped α-Fe₂O₃ flower-like nanostructure combines the advantage of p-type doping with the feature of a flower-like architecture. The prepared nanostructure film was applied as a photocathode in a photoelectrochemical (PEC) water splitting experiment and achieved a significantly improved photocurrent density of −5.34 mA cm⁻² at −0.6 V vs. reversible hydrogen electrode (RHE) for 1 mol% Cu doping. The obtained photocurrent is about 4.85 times higher than that of the pure α-Fe₂O₃ based photoelectrode. The incorporation of Cu into α-Fe₂O₃ results in a dramatic enhancement in the water splitting performance. The enhancement is gained through an improvement in light harvesting and charge carrier separation. The copper-modified α-Fe₂O₃ sample also exhibited an up shift in the conduction band edge potential, which is energetically favorable for the water reduction reaction. This result demonstrated high performance PEC water splitting as a potential route for the production of hydrogen gas using a single Cu-doped α-Fe₂O₃ photoelectrode without the need for other catalysts and hybrid structures.     

Magneto-optical properties of α-Fe2O3@ZnO nanocomposites prepared by the high energy ball-milling technique

Magnetic–fluorescent nanocomposites (NCs) with 10 wt% of α-Fe₂O₃ in ZnO have been prepared by the high energy ball-milling. The crystallite sizes of α-Fe₂O₃ and ZnO in the NCs are found to vary from 65 nm to 20 nm and 47 nm to 15 nm respectively as milling time is increased from 2 to 30 h. XRD analysis confirms presence of α-Fe₂O₃ and ZnO in pure form in all the NCs. UV–vis study of the NCs shows a continuous blue-shift of the absorption peak and a steady increase of band gap of ZnO with increasing milling duration that are assigned to decreasing particle size of ZnO in the NCs. Photoluminescence (PL) spectra of the NCs reveal three weak emission bands in the visible region at 421, 445 and 485 nm along with the strong near band edge emission at 391 nm. These weak emission bands are attributed to different defect – related energy levels e.g. Zn-vacancy, Zn interstitial and oxygen vacancy. Dc and ac magnetization measurements show presence of weakly interacting superparamagnetic (SPM) α-Fe₂O₃ particles in the NCs. ⁵⁷Fe-Mössbauer study confirms presence of SPM hematite in the sample milled for 30 h. Positron annihilation lifetime measurements indicate presence of cation vacancies in ZnO nanostructures confirming results of PL studies.     

Study of microstructured indium oxide by cathodoluminescence and XPS microscopy

In this work sintered thick microcrystalline films as well as micro and nanostructures of In₂O₃ have been studied. The results obtained by XPS microscopy show that the boundary regions of the microcrystalline films present a higher amount of oxygen, as well as a different O (1s) core level XPS spectrum with respect to the grains. CL images recorded at room temperature show that the emission is preferentially associated with the grain boundaries and the main emission band appeared at 1.9 eV in the recorded CL spectra.Core level and valence band spectromicroscopy measurements of the indium oxide arrows grown at the surface of the sintered InN revealed the incorporation of nitrogen, coming from the starting material. In these structures the N (1s) core level splits into two components, showing a higher amount of nitrogen in the pyramid surface than in the columns of the structures which correlates with an increase of CL intensity.     

Cathodoluminescence study of isoelectronic doping of gallium oxide nanowires

Isoelectronic (In, Al) doped gallium oxide nanowires have been grown by a vapour solidification process. XRD and TEM were used for their structural characterization. The morphology and optical properties of the In(Al)-doped Ga₂O₃ nanowires have been investigated by means of the secondary electrons and cathodoluminescence (CL) techniques in the SEM. Red and blue-UV emission bands appear as complex bands and their components are influenced by the presence of In or Al, leading to a blue-shift of the blue-UV band usually observed in undoped gallium oxide. These In and Al related changes in the luminescence features of doped Ga₂O₃ nanostructures are discussed.     

Characterization of the Calcination Products of the Precipitates Obtained from the Bio-Oxidation with Thiobacillus Ferrooxidans of Sulphuric Water Pickling Liquors

The characterization of the calcination products of the precipitates obtained from the bio-oxidation with Thiobacillus ferrooxidans of sulphuric water pickling liquors has been carried out by means of Mössbauer spectroscopy, x-ray powder diffraction, infrared spectroscopy and transmission electron microscopy. The results show that a full transformation of the precipitates into α-Fe₂O₃ is achieved at temperatures higher than 850°C. Calcination at 700°C during two hours results in the formation of α-Fe₂O₃, ζ-Fe₂O₃ and Fe₁₂O₃(SO₄)₁₅. The Mössbauer parameters of ζ-Fe₂O₃ and Fe₁₂O₃(SO₄)₁₅ at 298 and 17K are reported.     

Ultra-sharp α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoflakes: growth mechanism and field-emission

We report the synthesis of single-crystalline α-Fe₂O₃ nanoflakes from a simple Fe–air reaction within the temperatures range of 260–400 °C. The nanoflakes synthesized at the lowest temperature (260 °C) in this work show an ultra-sharp morphology: 5–10 nm in thickness, 1–2 μm in length, 20 nm in base-width and around 5 nm at the tips; successfully demonstrate the promising electron field emission properties of a large-scaled α-Fe₂O₃ nanostructure film and exhibit the potential applications as future field-emission (FE) electron sources and displays (FEDs). The formation and growth of α-Fe₂O₃ nanostructures were discussed based on the surface diffusion mechanism. PACS 79.60.Jv; 79.70.+q; 77.84.Bw     

Morphology and optical investigations of ZnO pyramids and nanoflakes for optoelectronic applications

Zinc-oxide (ZnO) pyramidal and nanoflakes were grown by electrochemical deposition of Zn(NO₃)₂·6H₂O on n-type Si substrate with different crystallographic orientations and on indium tin oxide (ITO)-coated glass. Various morphological shapes of deposited ZnO nanostructures were observed, which were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The bulk modulus was calculated to determine the material stiffness. Two peaks were observed at room temperature photoluminescence spectrum, i.e., a near-band-edge (NBE) emission in the UV region and a broad deep-level emission (DLE) in the green emission region. The optical properties were calculated to confirm the specific models validity of ZnO nanostructures for optoelectronics. The measured and calculated values show good agreement with other data.     

Dye-sensitized solar cells with 3D flower-like α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-decorated reduced graphenes oxide as photoanodes

For the first time, we report a one-step fabrication of an environment-friendly approach to synthesize flower-like α-Fe₂O₃ hierarchical nanoparticles (NPs)/reduced graphene oxide (RGO) hybrids by combining the graphene oxide (GO) with the growth of α-Fe₂O₃ NPs. The GO sheet which possesses the functional group, such as hydroxyl (–OH) and carbonyl groups (–OOH), can be easily incorporated with the petal of the flower-like α-Fe₂O₃ in ethanol and water solution through a solvothermal process, during which GO is reduced to RGO without the addition of any strong reducing agent or requiring any post-high-temperature annealing process. The as-prepared samples are loose and porous with flower-like structure, and the RGO hybrids were wrapped up uniformly on the sheet of α-Fe₂O₃ NPs. To demonstrate the potential applications, we have fabricated dye-sensitized solar cells (DSSCs) from the as-synthesized hierarchical flower-like α-Fe₂O₃/RGO and investigated it for the photoanode of DSSCs. Results show that the hierarchical α-Fe₂O₃/RGO solar cell exhibits improved performances in comparison with the free α-Fe₂O₃ NPs. The enhancement of photovoltaic properties is attributed to the unique porous nature and good conductivity which allow more efficient diffusion of I^? ions and facilitate the transfer of electron in the network.     

Enhanced photoelectrochemical performance of Ti-doped hematite thin films prepared by the sol-gel method

Ti-doped α-Fe₂O₃ thin films were successfully prepared on FTO substrates by the sol-gel route. Hematite film was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy dispersive spectrometer (EDS). The XRD data showed α-Fe₂O₃ had a preferred (1 1 0) orientation which belonged to the rhombohedral system. Interestingly, the grains turned into worm-like shape after annealed at high temperature. The IPCE could reach 32.6% at 400 nm without any additional potential vs. SCE. Titanium in the lattice can affect the photo electro chemical performance positively by increasing the conductivity of the thin film. So the excited electrons and holes could live longer, rather than recombining with each other rapidly as undoped hematite. And the efficient carrier density on the Ti-doped anode surface was higher than the undoped anode, which contribute to the well PEC performance.     

Synthesis of single crystalline GaN nanoribbons on sapphire (0001) substrates

Single crystalline GaN nanoribbons were synthesized through nitriding Ga₂O₃ thin films deposited on sapphire (0001) substrates by radio frequency magnetron sputtering. The component and structure of nanoribbons were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The flat and smooth ribbon-like nanostructures are high quality single crystalline hexagonal wurtzite GaN. The thickness and width-to-thickness ratio of the grown GaN nanoribbons are in the range of 8-15 nm and ∼5-10, respectively.     

The effect of different gas atmospheres on luminescent properties of pulsed laser ablated SrAl2O4:Eu,Dy thinfilms

SrAl₂O₄:Eu²⁺,Dy³⁺ thin films were grown on Si (1 0 0) substrates using the pulsed laser deposition (PLD) technique to investigate the effect of vacuum, oxygen (O₂) and argon (Ar) deposition atmospheres on the structural, morphological, photoluminescence (PL) and cathodoluminescence (CL) properties of the films. The films were ablated using a 248 nm KrF excimer laser. Atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and fluorescence spectrophotometry were used to characterize the thin films. Auger electron spectroscopy (AES) combined with CL spectroscopy were employed for the surface characterization and electron-beam induced degradation of the films. Better PL intensities were obtained from the unannealed films prepared in Ar and O₂ atmospheres with respect to those prepared in vacuum. A stable green emission peak at 515 nm, attributed to 4f⁶5d¹→4f⁷ Eu²⁺ transitions were obtained with less intense peaks at 619 nm, which were attributed to transitions in Eu³⁺. After annealing the films prepared in vacuum at 800 °C for 2 h, the intensity of the green emission (520 nm) of the thin film increased considerably. The amorphous thin film was crystalline after the annealing process. The CL intensity increased under prolonged electron bombardment during the removal of C due to electron stimulated surface chemical reactions (ESSCRs) on the surface of the SrAl₂O₄:Eu²⁺, Dy³⁺ thin films. The CL stabilized and stayed constant thereafter.