Ferromagnetism in Fe-doped tetra-needle like ZnO whiskers

The Fe-doped tetra-needle like ZnO whiskers (T-ZnOw) were prepared with different loadings of Fe³⁺ by a solid solution process. The structures and magnetic properties of the samples were characterized by using X-ray diffraction (XRD) and superconducting quantum interference device (SQUID) magnetometer, respectively. The results indicate that Fe³⁺ ions are successfully doped into the crystal lattice of T-ZnOw matrix. The second phase of ZnFe₂O₄ is formed by heavy doping of Fe³⁺ ions, and the solid solubility limit of Fe³⁺ in T-ZnOw crystal is about 0.7 mol%. The magnetic properties reveal that some of the Fe-doped T-ZnOw exhibits room-temperature ferromagnetic behaviors. The room-temperature ferromagnetism in the Fe-doped T-ZnOw system is ascribed to be originated from the change of the electronic states of ZnOw caused by irons doping.     

Microstructures using RF sputtered PSG film as a sacrificial layer in surface micromachining

In this paper, we explore RF magnetron sputtered Phosphor-silicate-glass (PSG) film as a sacrificial layer in surface micromachining technology. For this purpose, a 76 mm diameter target of phosphorus-doped silicon dioxide was prepared by conventional solid-state reaction route using P₂O₅ and SiO₂ powders. The PSG films were prepared in a RF (13·56 MHz) magnetron sputtering system at 300 watt RF power, 20 mTorr pressure and 45 mm target-to-substrate spacing without external substrate heating. Microstructures of sputtered silicon dioxide film were fabricated using sputtered PSG film as sacrificial layer in surface micromachining process.     

Preparation, characterization and photocatalytic activity of in situ Fe-doped TiO2 thin films

Fe-doped TiO₂ thin films were prepared in situ on stainless steel substrates by liquid phase deposition, followed by calcination at various temperatures. It was found that some Fe³⁺ ions were in situ doped into the TiO₂ thin films. At 400 °C, the film became photoactive due to the formation of anatase phase. At 500 °C, the film showed the highest photocatalytic activity due to an optimal Fe³⁺ ion concentration in the film. At 900 °C, the photocatalytic activity of the films decreased significantly due to the further increase of Fe³⁺ ion concentration, the formation of rutile phase and the sintering and growth of TiO₂ crystallites.     

Studies of radiofrequency plasma deposition of hexamethyldisiloxane films and their thermal stability and corrosion resistance behavior

Hexamethyldisiloxane (HMDSO) films (thickness: 282-929 nm) are prepared by the radiofrequency plasma assisted chemical vapour deposition (RF-PACVD) method using an Ar/HMDSO/O₂ gas mixture. The deposition process is carried out in an RF reactor at a working pressure of 1.2 × 10⁻¹ mbar and an RF power range of 20-100 W. From the studies on Ar/O₂ and Ar/HMDSO/O₂ discharge characteristics using a self-compensated emissive probe, it is revealed that electrons play an important role in the plasma polymerization of HMDSO monomers. Optical emission spectroscopy (OES) and Fourier transform infrared (FT-IR) spectroscopy show that the plasma deposited HMDSO films tend to be more inorganic in nature at higher RF powers. A film prepared at an RF power of 100 W exhibits more thermal stability and corrosion resistance behavior in comparison to films deposited at lower powers (20-80 W). A correlation of the results obtained from OES and FT-IR analyses with the thermal stability and corrosion resistance behavior of the films has been attempted.     

Thermodynamic phase stability of three nano-oxides

The phase stability and phase transition temperature of three nano-oxides Al₂O₃, TiO₂ and Fe₂O₃ are thermodynamically determined by considering effects of surface energy and internal pressure induced by surface stress on the total Gibbs free energy. The established model corresponds to available experimental and simulation results of Al₂O₃, TiO₂ and Fe₂O₃ nanocrystals.     

Effect of oxygen flow rate and radio-frequency power on the photoconductivity of highly ultraviolet sensitive ZnO thin films grown by magnetron sputtering

We have investigated the effect of oxygen flow rate and radio-frequency (RF) power on the photoconductivity properties of ZnO thin films grown by magnetron sputtering and correlated the changes to the structural qualities. The electrical measurements show that the carrier concentration decreases with increase in the oxygen flow rate which is attributed to the probable increase in the oxygen vacancy (V_O)-related defects. The photocurrent spectra show that as the oxygen content increases, the films become lesser and more sensitive to the visible and ultraviolet (UV) lights respectively. As a result, the photo-to-dark current ratio (gain) increases to a value of 1.10 × 10⁶. As the RF power increases from 50 W to 150 W, the films become more conducting. The photoconductivity results show that as the RF power increases, the UV gain decreases slowly indicating that highly UV sensitive films can be grown at lower RF power.     

Growth morphology study of cathodically electrodeposited Fe3O4 thin films at elevated temperatures

We investigated the electrodeposition of Fe₃O₄ thin films in the Fe³⁺-triethanolamine system by galvanostatic deposition at elevated temperatures. It was found that with a fixed current density, the concentration of Fe³⁺ ions had a significant impact on the electrodeposition rate, while the deposition temperature and time strongly affected the morphology of the Fe₃O₄ thin films. Fe₃O₄ thin films deposited in electrolyte with high Fe³⁺ concentration and at high temperature (>80 °C) exhibited a dense and uniform morphology and were composed of globular or polyhedral crystallites, while the Fe₃O₄ thin films deposited at low temperatures (<70 °C) were loose and flake-like. Based on the empirical observations, a hypothetical growth model for the electrodeposition of Fe₃O₄ at elevated temperatures was proposed.     

The d.c. magnetron sputtering behavior of TiO2−x targets with added Fe2O3 or Nd2O3

Targets of 6 mm thickness were prepared from TiO₂ powder with different amounts of Fe₂O₃ and Nd₂O₃ added. The targets were sintered in a vacuum furnace to obtain sub-stoichiometric rutile TiO_2−x. For both added metal oxides, it was found that they were present as titanates forming a second phase. The sputtering behavior of these mixed-oxide targets was investigated. First, the dependence of the discharge voltage and the deposition speed on the argon pressure was measured. Second, the effect of the oxygen addition on the same two parameters was investigated. Both were compared with the dependencies measured for a pure TiO_2−x target. All experiments were performed in constant power mode. The results show that Fe₂O₃ and Nd₂O₃ exhibit different effects on the sputtering behavior of TiO_2−x targets.     

Photoluminescence of Fe2O3 nanoparticles prepared by laser oxidation of Fe catalysts in carbon nanotubes

Iron oxide nanoparticles have been produced on the top surface of aligned multi-walled carbon nanotubes by CO₂ laser processing. They were characterized to be Fe₂O₃ nanoparticles by X-ray photoelectron spectroscopy, X-ray diffraction and high resolution scanning electronic microscopy. Absorption bands in the visible region were found to be redshifted compared with the absorption of Fe₂O₃ nanoparticles prepared by traditional chemical methods. Photoluminescence from these Fe₂O₃ nanoparticles shows a broad emission band in the near infrared region for both excitations at 514 and 633 nm. Particle size is considered to be responsible for the unique optical properties of the Fe₂O₃ nanoparticles.     

Heterostructured Fe3O4/Bi2O2CO3 photocatalyst: Synthesis,characterization and application in recyclable photodegradation of organic dyes under visible light irradiation

Heterostructured Fe₃O₄/Bi₂O₂CO₃ photocatalyst was synthesized by a two-step method. First, Fe₃O₄ nanoparticles with the size of ca. 10 nm were synthesized by chemical method at room temperature and then heterostructured Fe₃O₄/Bi₂O₂CO₃ photocatalyst was synthesized by hydrothermal method at 180 °C for 24 h with the addition of 10 wt% Fe₃O₄ nanoparticles into the precursor suspension of Bi₂O₂CO₃. The pH value of synthesis suspension was adjusted to 4 and 6 with the addition of 2 M NaOH aqueous solution. By controlling the pH of synthesis suspension at 4 and 6, sphere- and flower-like Fe₃O₄/Bi₂O₂CO₃ photocatalysts were obtained, respectively. Both photocatalysts demonstrate superparamagnetic behavior at room temperature. The UV–vis diffuse reflectance spectra of the photocatalysts confirm that all the heterostructured photocatalysts are responsive to visible light. The photocatalytic activity of the heterostructured photocatalysts was evaluated for the degradation of methylene blue (MB) and methyl orange (MO) in aqueous solution over the photocatalysts under visible light irradiation. The heterostructured photocatalysts prepared in this study exhibit highly efficient visible-light-driven photocatalytic activity for the degradation of MB and MO, and they can be easily recovered by applying an external magnetic field.     

Fe dopants enhancing ethanol sensitivity of ZnO thin film deposited by RF magnetron sputtering

Pure and 5 % Fe-doped ZnO thin films (TFs) have been successfully deposited on Al₂O₃ substrate from pre-doped target material by RF magnetron sputtering technique. X-ray diffraction (XRD) patterns confirm the formation of both films in single phase wurtzite structure without any extra impurity peak. The calculated average crystallite sizes are found to be 22 and 17 nm for pure and Fe-doped ZnO TFs, respectively. The broadening in XRD peaks of Fe-doped ZnO TF occurs due to decrease in crystallite size and increase in lattice strain. Field emission scanning electron microscopy images exhibit that the particles growth in Fe-doped ZnO TF is more uniform and smaller than pure ZnO. Energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy results confirm the existence of Fe dopants into ZnO matrix. The doping effect enhances the sensitivity of ZnO sensor almost three times for ethanol gas sensing, the improvement in the response time and recovery time is noticeable as the size reduction effect increases the surface to volume ratio, and resulting more numbers of ethanol gas molecules are adsorbed to produce a higher concentration of oxygen ions which leads a larger deviation in capacitance.     

Preparation and characterization of porous ultrafine Fe2O3 particles

Porous ultrafine Fe₂O₃ particles were prepared by homogeneous precipitation method. Fe³⁺ and urea were chosen as starting materials and anionic surfactant as the template. It is shown that the reaction results in the precipitation of a gelatinous hydrous iron oxide/surfactant mixture, which gives ultrafine Fe₂O₃ particles after drying and calcinations. The products were characterized by XRD, TEM, TG/DTA and BET. Conventional XRD patterns show that the products are mixture of γ-Fe₂O₃ and α-Fe₂O₃ phase after being sintered at 350 °C, and γ-Fe₂O₃ transforms entirely to α-Fe₂O₃ when sintered at 650 °C. The low-angle XRD patterns indicate that the mesostructure can only exist between 350 and 400 °C. TEM results show that the Fe₂O₃ particles have diameters of about 30 nm and lengths ranging from 100 to 120 nm; in each particle, there are several vermiculate-like mesopores with diameter of about 20-25 nm. The BET surface areas in excess of 50 m²/g are obtained after calcinations at 350 °C. The BJH desorption average pore width is around 22 nm, which is in agreement with the TEM results. The results show that anionic surfactant and sintering temperature are important to obtain this special morphology.     

Ion conducting properties of hydrogen-containing Ta2O5 thin films prepared by reactive sputtering

Hydrogen-containing Ta₂O₅ (Ta₂O₅:H) thin films are considered to be a candidate for a proton-conducting solid-oxide electrolyte. In this study, Ta₂O₅:H thin films were prepared by reactively sputtering a Ta metal target in an O₂ + H₂O mixed gas. The effects of sputtering power and post-deposition heat treatment on the ion conducting properties of the Ta₂O₅:H thin films were studied. The ionic conductivity of the films was improved by decreasing the RF power and a maximum conductivity of 2 × 10⁻⁹ S/cm was obtained at an RF power of 20 W. The ionic conductivity decreased by heat-treatment in air, and no ion-conduction was observed after treatment at 300 °C due to the decrease in hydrogen content in the films.     

Orthogonal analysis for perovskite structure microwave dielectric ceramic thin films fabricated by the RF magnetron-sputtering method

The perovskite structure microwave dielectric ceramic thin films have been deposited by radio frequency (RF) magnetron sputtering on SiO₂(110) substrates. Subsequently, orthogonal analysis has been adopted to optimize the process parameters. The experimental results indicate that sputtering pressure has the greatest impact on comprehensive evaluation indicators such as the film quality, whereas sputtering power has a lower effect; the ratio of O₂/Ar and substrate temperature have the least impact on the process. Thus, the optimal process parameters to prepare perovskite structure dielectric thin films by RF magnetron sputtering are as follows: 200 W of sputtering power, 0.25 Pa of sputtering pressure, Ar as working gas, and substrate temperature of 610 °C.     

Synthesis and characterization of Bi2Fe4O9 powders

Bi₂Fe₄O₉ have been successfully prepared using ethylenediaminetetraacetic (EDTA) acid as a chelating agent and ethylene glycol as an esterification agent. Heating of a mixed solution of EDTA, ethylene glycol, and nitrates of iron and bismuth at 140 °C produced a transparent polymeric resin without any precipitation, which after pyrolysis at 250 °C was converted to a powder precursor for Bi₂Fe₄O₉. The precursors were heated at 400–800 °C in air to obtain Bi₂Fe₄O₉ powder and differential scanning calorimetry (DSC), thermogravimetric (TG), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) techniques were used to characterize the precursors and the derived oxide powders. XRD analysis showed that well-crystallized and single-phase Bi₂Fe₄O₉ with orthorhombic symmetry was obtained at 700 °C for 2 h and BiFeO₃ and Fe₂O₃/FeCO₃ were intermediate phases before the formation of Bi₂Fe₄O₉. Bi₂Fe₄O₉ powders show weak ferromagnetism at room temperature.     

Self-rising approach to synthesize hierarchically porous metal oxides

Hierarchically porous metal oxides are prepared using a novel self-rising approach. The method is ‘hard-template’ free and relatively easy to perform, utilizes inexpensive precursors and processing conditions, and is versatile, thus offering tremendous opportunities with a wide variety of conditions to explore to synthesize single or mixed metal oxides with hierarchically porous structures. Fe₂O₃, Sm_0.2Ce_0.8O_1.9, LaFeO₃, LaCoO₃, and La_0.5Sr_0.5Co_0.5Fe_0.5O₃ have been successfully synthesized while Fe₂O₃ and La_0.5Sr_0.5Co_0.5Fe_0.5O₃ as examples for single and mixed metal oxides have been studied in detail. Sm_0.2Ce_0.8O_1.9 has been applied as electrolyte for solid oxide fuel cells, showing good sinterability and high conductivity. A tentative scheme is provided to illustrate the pore formation mechanism using the self-rising approach.     

Structural and optical characterization of c-axis textured BaTiO<Subscript>3</Subscript> thin films on MgO fabricated by RF magnetron sputtering

In this paper, BaTiO₃ thin films were prepared by RF magnetron sputtering on MgO substrates and their properties such as the crystal structure and optical waveguide properties were investigated. The optimum deposition parameters, such as substrate temperature, deposition pressure, gas flow ratio, the RF power and the after annealing temperature, were obtained in order to get the best BaTiO₃ film quality. The XRD results show that highly c-axis textured BaTiO₃ thin films were successfully grown on MgO substrate. Films obtained under the optimum deposition parameters, substrate temperature of 650°C, RF power of 50 W, deposition pressure 18 mTorr and gas flow ratio O₂/(Ar+ O₂) of 15% namely, reaches a full width at half maximum intensity (FWHM) of BaTiO₃ (002) XRD peak of 0.25°. The FWHM of BaTiO₃ (002) XRD peak was further reduced to 0.24° via post-treatment with furnace annealing (at 800°C for 2 h) which indicates the film crystal quality is further improved. The bright and sharp TE modes measured by m-line spectroscopy of the BaTiO₃ film were observed indicating its possible application in optical waveguide.     

Transport properties and Mössbauer spectra of Fe-substituted La10−x(Si,Al)6O26 apatites

Increasing iron content in apatite-type La_9.83Si_4.5Al_1.5−yFe_yO_26+δ (y=0.5-1.5) leads to increasing unit cell volume, fraction of Fe⁴⁺, partial oxygen ionic and p-type electronic conductivities, and ceramics sinterability. The oxygen ion transference numbers, determined by Faradaic efficiency (FE) measurements at 973-1223 K in air, are in the range 0.986-0.994. Data on total conductivity and Seebeck coefficient as functions of the oxygen partial pressure, varying in the range 10⁻² Pa to 70 kPa, confirm that under oxidizing conditions the ionic conduction in Fe-substituted La_9.83(Si,Al)₆O_26+δ apatites is dominant. Due to stabilization of Fe³⁺, substantially worse transport properties are observed for A-site stoichiometric La₁₀Si₄Fe₂O₂₆, having activation energy for ionic conductivity of 107 kJ/mol and electron transference numbers close to 0.03. The correlation between partial ionic and electron-hole conductivities suggests a significant role of Fe⁴⁺ formation compensated by extra oxygen incorporation into the vacant sites, which are formed due to Frenkel-type disorder induced by La vacancies. The average thermal expansion coefficients of Fe-doped La_10−x(Si,Al)₆O_26+δ ceramics, calculated from dilatometric data in air, are 8.9×10⁻⁶ to 9.9×10⁻⁶ K⁻¹ at 300-1250 K.     

Fabrication and characterization of Fe3O4 thin films deposited by reactive magnetron sputtering

Fe-O thin films with different atomic ratio of iron to oxygen were deposited on glass and thermally oxidized silicon substrates at temperatures of 300, 473 and 593 K, by reactive magnetron sputtering in Ar+O₂ atmosphere. The composition and structure of the thin films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electrical resistivity. It was found from XRD that with increasing the oxygen partial pressure in the working gas, the crystalline structure of the Fe-O films deposited at the substrate temperature of 473 K gradually changed from α-Fe, amorphous Fe-O, Fe₃O₄, γ-Fe₂O₃ to Fe_21.34O₃₂. The structure and chemical valence of the Fe₃O₄ films were analyzed by electron microscopy and XPS, respectively.     

Synthesis of nanocrystalline iron oxide ultrathin films by thermal decomposition of iron nitropruside: Structural and optical properties

Ultrathin films of nanocrystalline α-Fe₂O₃ have been deposited on glass substrates from an inorganic precursor, iron nitropruside. This is a novel route of synthesis for iron oxide thin films on glass substrates, by annealing the precursor thin film in air at 650 °C for 15 min. The films were characterized using TG-DTA analysis, X-ray diffraction, UV-visible, FESEM, AFM and Raman measurements. X-ray diffraction and Raman analyses revealed that the deposited films contain α-phase of Fe₂O₃ (hematite). The synthetic route described here provides a very simple and cost-effective method to deposit α-Fe₂O₃ thin films on glass substrates with band gap energy of about 2.75 eV. The deposited films were found to show catalytic effect for the photo-degradation of phenol.