Enhanced Sensing Ability of Brush-Like Fe2O3-ZnO Nanostructures towards NO2 Gas via Manipulating Material Synergistic Effect

Brush-like α-Fe₂O₃–ZnO heterostructures were synthesized through a sputtering ZnO seed-assisted hydrothermal growth method. The resulting heterostructures consisted of α-Fe₂O₃ rod templates and ZnO branched crystals with an average diameter of approximately 12 nm and length of 25 nm. The gas-sensing results demonstrated that the α-Fe₂O₃–ZnO heterostructure-based sensor exhibited excellent sensitivity, selectivity, and stability toward low-concentration NO₂ gas at an optimal temperature of 300 °C. The α-Fe₂O₃–ZnO sensor, in particular, demonstrated substantially higher sensitivity compared with pristine α-Fe₂O₃, along with faster response and recovery speeds under similar test conditions. An appropriate material synergic effect accounts for the considerable enhancement in the NO₂ gas-sensing performance of the α-Fe₂O₃–ZnO heterostructures.     

Progress toward Room-Temperature Synthesis and Functionalization of Iron-Oxide Nanoparticles

Novel magnetic nanohybrids composed of nanomaghemite covered by organic molecules were successfully synthesized at room temperature with different functionalization agents (sodium polystyrene sulfonate, oxalic acid, and cetyltrimethylammonium bromide) in low and high concentrations. Structural, vibrational, morphological, electron energy-loss spectroscopy, magnetic, and Mössbauer characterizations unraveled the presence of mainly cubic inverse spinel maghemite (γ-Fe₂O₃), whilst X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy showed that most samples contain a minor amount of goethite phase (α-FeOOH). Raman analysis at different laser power revealed a threshold value of 0.83 mW for all samples, for which the γ-Fe₂O₃ to α-Fe₂O₃ phase transition was observed. Imaging microscopy revealed controlled-size morphologies of nanoparticles, with sizes in the range from 8 to 12 nm. Organic functionalization of the magnetic nanoparticles was demonstrated by vibrational and thermogravimetric measurements. For some samples, Raman, magnetic, and Mössbauer measurements suggested an even more complex core-shell-like configuration, with a thin shell containing magnetite (Fe₃O₄) covering the γ-Fe₂O₃ surface, thus causing an increase in the saturation magnetization of approximately 11% against nanomaghemite. Field cooling hysteresis curves at 5 K did not evidence an exchange bias effect, confirming that the goethite phase is not directly interacting magnetically with the functionalized maghemite nanoparticles. These magnetic nanohybrids may be suitable for applications in effluent remediation and biomedicine.     

Hematites Precipitated in Alkaline Precursors: Comparison of Structural and Textural Properties for Methane Oxidation

Hematite (α-Fe₂O₃) catalysts prepared using the precipitation methods was found to be highly effective, and therefore, it was studied with methane (CH₄), showing an excellent stable performance below 500 °C. This study investigates hematite nanoparticles (NPs) obtained by precipitation in water from the precursor of ferric chloride hexahydrate using precipitating agents NaOH or NH₄OH at maintained pH 11 and calcined up to 500 °C for the catalytic oxidation of low concentrations of CH₄ (5% by volume in air) at 500 °C to compare their structural state in a CH₄ reducing environment. The conversion (%) of CH₄ values decreasing with time was discussed according to the course of different transformation of goethite and hydrohematites NPs precursors to magnetite and the structural state of the calcined hydrohematites. The phase composition, the size and morphology of nanocrystallites, thermal transformation of precipitates and the specific surface area of the NPs were characterized in detail by X-ray powder diffraction, transmission electron microscopy, infrared spectroscopy, thermal TG/DTA analysis and nitrogen physisorption measurements. The results support the finding that after goethite dehydration, transformation to hydrohematite due to structurally incorporated water and vacancies is different from hydrohematite α-Fe₂O₃. The surface area SBET of Fe₂O₃_NH-70 precipitate composed of protohematite was larger by about 53 m²/g in comparison with Fe₂O₃_Na-70 precipitate composed of goethite. The oxidation of methane was positively influenced by the hydrohematites of the smaller particle size and the largest lattice volume containing structurally incorporated water and vacancies.     

One-pot synthesis of α-Fe2O3 nanospheres by solvothermal method

We have successfully prepared α-Fe₂O₃ nanospheres by solvothermal method using 2-butanone and water mixture solvent for the first time, which were about 100 nm in diameter and composed of very small nanoparticles. The as-prepared samples were characterized using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results showed that the product was α-Fe₂O₃ nanosphere, and the temperature was an important factor on the formation of α-Fe₂O₃ nanospheres.     

Study on an iron–manganese Fischer–Tropsch synthesis catalyst prepared from ferrous sulfate

A systematic study was undertaken to investigate the effects of the initial oxidation degree of iron on the bulk phase composition and reduction/carburization behaviors of a Fe–Mn–K/SiO₂ catalyst prepared from ferrous sulfate. The catalyst samples were characterized by powder X-ray diffraction (XRD), Mössbauer spectroscopy, X-ray photoelectron spectroscopy (XPS) and H₂ (or CO) temperature-programmed reduction (TPR). The Fischer–Tropsch synthesis (FTS) performance of the catalysts was studied in a slurry-phase continuously stirred tank reactor (CSTR). The characterization results indicated that the fresh catalysts are mainly composed of α-Fe₂O₃ and Fe₃O₄, and the crystallite size of iron oxides is decreased with the increase of the initial oxidation degree of iron. The catalyst with high content of α-Fe₂O₃ in its as-prepared state has high content of iron carbides after being reduced in syngas. However, the catalyst with high content of Fe₃O₄ in its as-prepared state cannot be easily carburized in CO and syngas. FTS reaction study indicates that Fe-05 (Fe³⁺/Fe_total = 1.0) has the highest CO conversion, whereas Fe-03 (Fe³⁺/Fe_total = 0.55) has the lowest activity. The catalyst with high CO conversion has a high selectivity to gaseous hydrocarbons (C₁–C₄) and low selectivity to heavy hydrocarbons (C₅₊).     

Iron oxide synthesis using a continuous hydrothermal and solvothermal system

Iron oxide synthesis via a continuous hydrothermal and solvothermal reaction were studied. In the hydrothermal synthesis, fine α-Fe₂O₃ (hematite) particles were obtained at 250–420 °C and 30 MPa. The α-Fe₂O₃ crystals were grown in sub-critical water via a dissolution and precipitation process. The growth of α-Fe₂O₃ crystals in supercritical water was suppressed due to the rather low solvent power of water. Crystalline Fe₃O₄ was obtained as the temperature was raised above the supercritical state in the solvothermal preparation. Isopropanol (IPA) was oxidized in acetone which provided a reducing atmosphere. Acetone molecule adsorption onto the Fe₃O₄ particle surface inhibited crystallite growth.     

Synthesis of new type of Au-magnetic nanocomposite and application for protein separation thereof

We present a different strategy for synthesizing the Au-γ-Fe₂O₃ bifunctional nanoparticle by using a larger (50 nm) Au nanoparticle as the core surrounded by smaller (10 nm) γ-Fe₂O₃ nanoparticles. The synthesis of the composite nanoparticles is quite facile based on a simple redox process whereby Fe²⁺ is used to reduce Au³⁺. The morphology and composition of the product is measured by transmission electron microscopy, X-ray powder diffraction and UV–vis spectroscopy. We demonstrate the utility of these as-prepared Au-γ-Fe₂O₃ nanoparticles by showing they can be used to separate proteins in solution. For example, bovine serum is efficiently removed from an aqueous solution with the simple addition of the NPs and application of a small magnet. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis is performed to evaluate the fidelity and efficiency of the protein separation procedure.     

Magnetic properties of fluffy Fe@α-Fe2O3 core-shell nanowires

Novel fluffy Fe@α-Fe₂O₃ core-shell nanowires have been synthesized using the chemical reaction of ferrous sulfate and sodium borohydride, as well as the post-annealing process in air. The coercivity of the as-synthesized nanowires is above 684 Oe in the temperature range of 5 to 300 K, which is significantly higher than that of the bulk Fe (approximately 0.9 Oe). Through the annealing process in air, the coercivity and the exchange field are evidently improved. Both the coercivity and the exchange field increase with increasing annealing time (T_A) and reach their maximum values of 1,042 and 78 Oe, respectively, at T_A = 4 h. The magnetic measurements show that the effective anisotropy is increased with increasing the thickness of theα-Fe₂O₃ by annealing. The large values of coercivity and exchange field, as well as the high surface area to volume ratio, may make the fluffy Fe@α-Fe₂O₃ core-shell nanowire a promising candidate for the applications of the magnetic drug delivery, electrochemical energy storage, gas sensors, photocatalysis, and so forth.     

Laser Shock Fabrication of Nitrogen Doped Inverse Spinel Fe3O4/Carbon Nanosheet Film Electrodes towards Hydrogen Evolution Reactions in Alkaline Media

The reliable and cost-effective production of high-performance film electrodes for hydrogen evolution reactions remains a challenge for the laser surface modification community. In this study, prior to a thermal imidization reaction, a small number of Fe₃O₄ nanoparticles were vortexed into a poly(amic acid) (PAA) prepolymer, and the achieved flat composite film was then ablated by a 1064 nm fiber laser. After laser irradiation, the hierarchical architectures of carbon nanosheets decorated with Fe₃O₄ nanoparticles were generated. Although pure polyimide (PI) film and laser carbonized PI film, as well as bare Fe₃O₄, showcase poor intrinsic catalytic activity toward alkaline hydrogen evolution reactions, our laser-derived Fe₃O₄/carbon nanosheet hybrid film demonstrated enhanced electrocatalytic activity and stability in 1 M KOH electrolyte; the overpotential(η₁₀) reached 247 mV when the current density was 10 mA cm⁻² with a slight current decay in the chronoamperometric examination of 12 h. Finally, we proposed that the substitution of N to O in Fe−O sites of trans spinel structured magnetite would be able to modulate the free energy of hydrogen adsorption (ΔG_H*) and accelerate water dissociation.     

Phase transformations of a spray-dried iron catalyst for slurry Fischer–Tropsch synthesis during activation and reaction

The consecutive phase transformations of a precipitated spray-dried iron-based catalyst for slurry Fischer–Tropsch synthesis (FTS) during activation and reaction process were investigated using Mössbauer effect spectroscopy (MES). It was found that the fresh iron catalyst activation in situ using syngas resulted in the formation of a mixture of iron carbides and superparamagnetic (spm) phases. The relatively small size of fresh iron crystallites was an important factor in the formation of ε′-Fe_2.2C. During the reduction process, Fe³⁺ (spm) phase was easier to be reduced than α-Fe₂O₃ phase. Fe₃O₄ was not an active phase for FTS. The transformation of α-Fe₂O₃ into Fe₃O₄ before carbides formation was necessary to obtain FTS activity of the iron catalyst. There was a correlation between the content of CH₄ in tail gas and the amount of iron carbides during activation. It was found that carbonization was the dominating phase transformation when the FTS reaction temperature increased from 250 °C to 270 °C. However, the oxidization was more remarkably at higher FTS reaction temperature. χ-Fe₅C₂ was the main iron phase at lower reaction temperature. The changes in the bulk compositions resulted in the variation in catalyst activity during FTS. The results of this study showed that the active phase for FTS was a mixture of carbides and corresponding amounts of superparamagnetic phase.     

The investigation of frequency response for the magnetic nanoparticulate assembly induced by time-varied magnetic field

The field-induced assembly of γ-Fe₂O₃ nanoparticles under alternating magnetic field of different frequency was investigated. It was found that the assembly was dependent upon the difference between colloidal relaxation time and field period. The same experiments on DMSA-coated γ-Fe₂O₃ nanoparticles exhibited that the relaxation time may be mainly determined by the magnetic size rather than the physical size. Our results may be valuable for the knowledge of dynamic assembly of colloidal particles.     

Effect of Na, K and Fe on the formation of α- and β-Ca2SiO4

The contribution of Fe to C₂S polymorphs is effectively revealed for β-C₂S formation and is not for α′- or α-C₂S. However in co-existing of Na or K with Fe, the α-C₂S is easily stabilized, though accompanied with small amount of crystalline Ca₂Fe₂O₅ as the interstitial material. This effect of Fe on α- and β-C₂S synthesis was investigated by XRD, chemical analysis and Mössbauer spectra observation. Comparing calculated Mössbauer parameters of α- and β-C₂S with those of other minerals, it was confirmed that they included only Fe³⁺ at octahedral and tetrahedral sites with the ratio 30:70 in Na---Fe substituted α-form and with 63:37 in K---Fe substituted α-form. In β-C₂S, Fe³⁺ was mostly situated at tetrahedral site. The formulas of α- and β-C₂S were decided and shown as, (Ca_1.88Fe_0.05Na_0.24)(Si_0.88Fe_0.11)O₄, (Ca_1.94 Fe_0.09K_0.18) (Si_0.88Fe_0.05)O₄ for α-form and (Ca_1.93Fe_0.003Na_0.04) (Si_0.99Fe_0.05)O₄, (Ca_1.94Fe_0.01K_0.04) (Si_0.92Fe_0.13)O₄ for -form.     

One-pot hydrothermal synthesis of Mn3O4 nanorods grown on Ni foam for high performance supercapacitor applications

Mn₃O₄/Ni foam composites were synthesized by a one-step hydrothermal method in an aqueous solution containing only Mn(NO₃)₂ and C₆H₁₂N₄. It was found that Mn₃O₄ nanorods with lengths of 2 to 3 μm and diameters of 100 nm distributed on Ni foam homogeneously. Detailed reaction time-dependent morphological and component evolution was studied to understand the growth process of Mn₃O₄ nanorods. As cathode material for supercapacitors, Mn₃O₄ nanorods/composite exhibited superior supercapacitor performances with high specific capacitance (263 F · g^-1 at 1A · g^-1), which was more than 10 times higher than that of the Mn₃O₄/Ni plate. The enhanced supercapacitor performance was due to the porous architecture of the Ni foam which provides fast ion and electron transfer, large reaction surface area, and good conductivity.     

Photocatalytic degradation of organics in water in the presence of iron oxides: Influence of carboxylic acids

The effects of four carboxylic acids: malic, citric, tartaric and oxalic acids on the leaching of iron from two commercial iron oxides (hematite, α-Fe₂O₃, and magnetite, Fe₃O₄) have been investigated. The variables studied were the doses of iron oxides and carboxylic acids used as well as aqueous pH, temperature and the presence of hydrogen peroxide and/or UV-A radiation. On the whole, Fe₃O₄ led to higher amounts of leached iron than α-Fe₂O₃, and oxalic acid was the most effective carboxylic acid used. The importance of iron leaching has been considered to explain the photodegradation of bisphenol A (BPA) by UV-A/iron oxides systems. The influence of the presence of hydrogen peroxide and/or titania on the efficiency of these oxidation systems was also investigated. At the conditions tested, advanced oxidation with the UV-A/iron oxide/oxalic acid/H₂O₂/TiO₂ system led to the lowest BPA half life (<15 min) among those processes studied.     

Chemoselective transfer hydrogenation reactions over nanosized γ-Fe2O3 catalyst prepared by novel combustion route

γ-Fe₂O₃ catalyst was prepared by the novel combustion route. The as synthesized catalyst was characterized by several analytical techniques such as XRD, SEM, TG-DTA, etc. Chemoselective reduction of nitro compounds was studied over γ-Fe₂O₃ using propan-2-ol as a hydrogen donor and KOH promotor in liquid phase reaction. The catalyst used for this synthetically useful transformation showed good activity.     

Ti addition effect on α-Al2O3 flakes synthesis using a mixture of boehmite and potassium sulfate

Single-crystal α-Al₂O₃ hexagonal flakes with a diameter of about 200 nm and 20 nm in thickness were obtained by mixing different molar ratios of potassium sulfate to boehmite and heating at 1000 °C. Co-doping 1 mol% TiO₂ can increase the shape anisotropy of α-Al₂O₃ hexagonal flakes, increasing the diameter to 400 nm. The effects of potassium sulfate, Fe₂O₃ and TiO₂ on the phase transformation and morphology development of alumina were investigated using X-ray diffraction analysis (XRD), differential thermal analysis (DTA) and transmission electron microscopy (TEM). The results indicate that co-doping potassium sulfate, Fe³⁺ and Ti⁴⁺ can promote γ → α-Al₂O₃ phase transformation and change the morphology from a vermicular structure into hexagonal platelets. The shape anisotropy of α-Al₂O₃ hexagonal flakes can be increased by adding TiO₂ due to the segregation of Ti⁴⁺ ions onto the surfaces of basal planes of α-Al₂O₃ single crystal particle.     

Mechanism and kinetic control of the oxyprecipitation of sulphuric liquors from steel pickling

Steel pickling liquors are one of the main environmental problems of steel making. Currently, there are several processes for the treatment of sulphuric liquors, although most recover only acid and haematite (α-Fe₂O₃). We propose an oxyprecipitation process for this recovery, allowing several kinds of iron oxide or oxyhydroxide to be obtained. The aim of this paper is to determine the kinetic control and reaction mechanism. The influence of different variables is evaluated and two experimental ranges for the synthesis of goethite (α-FeOOH) and magnetite (Fe₃O₄) are defined. A systematic study is carried out in these experimental intervals, and the results are analysed in order to determine the reaction order and the type of kinetic control. Finally, from these data, morphological and crystallinity studies and new experiments, the reaction mechanism is proposed.     

Facile synthesis of morphology and size-controlled α-Fe2O3 and Fe3O4 nano-and microstructures by hydrothermal/solvothermal process: The roles of reaction medium and urea dose

This paper reports a systematic study of the influences on the synthesis of α-Fe₂O₃ and Fe₃O₄ via a hydro/solvothermal process at 200 °C. Both the reaction medium and urea dose have been investigated. The products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM). Results showed that the reaction mediums, such as water and ethylene glycol, played important roles in forming different types of iron oxides. Pure crystalline α-Fe₂O₃ was formed via the hydrothermal process, and Fe₃O₄ was obtained through a solvothermal route with ethylene glycol as reaction medium. Increasing urea dose tuned the particle sizes of α-Fe₂O₃ and Fe₃O₄ from a few hundreds to several tens of nanometers. With addition of urea, the morphology of α-Fe₂O₃ evolved from olive-like to rhomb-like, and Fe₃O₄ evolved from hollow sphere, to pinecone-like, and finally into cracked nanostructures. The variations of the surface area of products were mainly dependent on the microstructure and intrinsic features of the iron oxide particles. Results of the mechanistic studies indicated that the generation of CO₂ and NH₃ via in situ thermal decomposition of urea was crucial for the formation of α-Fe₂O₃ and Fe₃O₄ nano-and microstructures. The as-synthesized α-Fe₂O₃ and Fe₃O₄ were used as catalysts for methylene blue degradation in the presence of H₂O₂, and α-Fe₂O₃ showed a higher degradation efficiency. Our findings demonstrated a promising strategy for the developments of rationally designed iron oxides.     

Electrochemically deposited gallium oxide nanostructures on silicon substrates

We report a synthesis of β-Ga₂O₃ nanostructures on Si substrate by electrochemical deposition using a mixture of Ga₂O₃, HCl, NH₄OH, and H₂O. The presence of Ga³⁺ ions contributed to the deposition of Ga₂O₃ nanostructures on the Si surface with the assistance of applied potentials. The morphologies of the grown structures strongly depended on the molarity of Ga₂O₃ and pH level of electrolyte. β-Ga₂O₃ nanodot-like structures were grown on Si substrate at a condition with low molarity of Ga₂O₃. However, Ga₂O₃ nanodot structures covered with nanorods on top of their surfaces were obtained at higher molarity, and the densities of nanorods seem to increase with the decrease of pH level. High concentration of Ga³⁺ and OH^- ions may promote the reaction of each other to produce Ga₂O₃ nanorods in the electrolyte. Such similar nature of Ga₂O₃ nanorods was also obtained by using hydrothermal process. The grown structures seem to be interesting for application in electronic and optoelectronic devices as well as to be used as a seed structure for subsequent chemical synthesis of GaN by thermal transformation method.