Iron Oxide Systems Research Articles

When Density Functional Approximations Meet Iron Oxides.

Three density functional approximations (DFAs), PBE, PBE+U, and Heyd-Scuseria-Ernzerhof screened hybrid functional (HSE), were employed to investigate the geometric, electronic, magnetic, and thermodynamic properties of four iron oxides, namely, α-FeOOH, α-Fe2O3, Fe3O4, and FeO. Comparing our calculated results with available experimental data, we found that HSE (a = 0.15) (containing 15% "screened" Hartree-Fock exchange) can provide reliable values of lattice constants, Fe magnetic moments, band gaps, and formation energies of all four iron oxides, while standard HSE (a = 0.25) seriously overestimates the band gaps and formation energies. For PBE+U, a suitable U value can give quite good results for the electronic properties of each iron oxide, but it is challenging to accurately get other properties of the four iron oxides using the same U value. Subsequently, we calculated the Gibbs free energies of transformation reactions among iron oxides using the HSE (a = 0.15) functional and plotted the equilibrium phase diagrams of the iron oxide system under various conditions, which provide reliable theoretical insight into the phase transformations of iron oxides.

Comparative proteomic analysis of sulfur-oxidizing Acidithiobacillus ferrooxidans CCM 4253 cultures having lost the ability to couple anaerobic elemental sulfur oxidation with ferric iron reduction

In extremely acidic environments, ferric iron can be a thermodynamically favorable electron acceptor during elemental sulfur oxidation by some Acidithiobacillus spp. under anoxic conditions. Quantitative 2D-PAGE proteomic analysis of a resting cell suspension of a sulfur-grown Acidithiobacillus ferrooxidans CCM 4253 subculture that had lost its iron-reducing activity revealed 147 protein spots that were downregulated relative to an iron-reducing resting cell suspension of the antecedent sulfur-oxidizing culture and 111 that were upregulated. Tandem mass spectrometric analysis of strongly downregulated spots identified several physiologically important proteins that apparently play roles in ferrous iron oxidation, including the outer membrane cytochrome Cyc2 and rusticyanin. Other strongly repressed proteins were associated with sulfur metabolism, including heterodisulfide reductase, thiosulfate:quinone oxidoreductase and sulfide:quinone reductase. Transcript-level analyses revealed additional downregulation of other respiratory genes. Components of the iron-oxidizing system thus apparently play central roles in anaerobic sulfur oxidation coupled with ferric iron reduction in the studied microbial strain.

(Invited) Toward the Formation of Ordered Nanoparticle Films By Electrophoretic Deposition and the Subsequent Assessment of Order Via Voronoi Tessellation Analysis

Electrophoretic deposition (EPD) is a liquid phase, materials casting approach whose technological advantages range from ease of application to size scalability, cost effectiveness, and rapid performance. Material coatings, casts, and films assembled by EPD can be fabricated in just a few minutes and can easily be produced in nanometer to centimeter thicknesses across square nanometer to square meter surfaces. EPD and other liquid-phase casting techniques can yield various degrees of order among the fabricated supercrystals. Recently, we observed the evolution of order among nanoparticles (NPs) of cobalt ferrite. We uncovered evidence that the NPs not only arranged into hexagonal close packing arrays when deposited onto silicon substrates by EPD but also appeared to have their crystallographic axes to be oriented collinearly. This discovery could have far reaching ramifications in the rapid assembly of nanomaterials for applications, such as magnetic recording media and catalysis, among others. Further, recently developed statistical measurement tools, specifically Voronoi tessellation analysis, that can be used to quantify the degree of ordering in monolayers of nanoparticles have been applied to the cobalt ferrite and the iron oxide systems. The evolution of order in cobalt ferrite NP monolayers are juxtaposed with that of iron oxide NPs to explore the underlying causes of the differences in ordering based and the physical properties of the NP films.

Liquid phase combustion of iron in an oxygen atmosphere

In this article, we report an investigation of laser-initiated ignition of pure iron rods, using optical pyrometry, video observations, and analysis of metallographic cross section of quenched burning liquid on copper plates. When ignition occurs, caused by the melting of metal, the combustion takes place in the liquid. Two distinct superposed phases (L1 and L2) are identified in the liquid, according to the known phase diagram of the iron oxide system. Our observations show that the L1 and L2 phases can be either distinct and immiscible or mixing together. The temperature of the transition at which the mixing occurs is around 2350 K. Two mechanisms are proposed to explain the mixing occurring at high temperature: the spontaneous emulsification resulting from a strong decrease of the interfacial tension between L1 and L2 and the reduction of the miscibility gap between them at high temperature. Based on the experimental data of the evolution of the temperature and the video observation of the melt for different ignition conditions, we provide a complete description of the combustion process of iron induced by laser. Eventually, an extrapolation of the iron–oxygen phase diagram, to temperatures higher than 2000 K, is proposed.

Magnetite-hematite transformation: correlation between natural and synthetic features

The iron-oxide system can be used as a marker of oxidized and reduced conditions in closed systems. However, natural rocks with iron oxide minerals also exhibit such reactions, although the natural system is typically open. To understand the behaviour of this natural system, some similarities were investigated, in terms of crystallographic textures, between the microstructures of two systems: natural open system and synthetic closed system of iron oxide phase transformation. Particular cases of phase transformation in iron oxide minerals, described as natural reactions of magnetite to hematite and synthetic reactions of hematite to magnetite, were chosen. It is observed, in both scenarios, that the transformation obeys the topotaxial and epitaxial relationship, which are well described for the iron oxide system. However, in natural open systems, the precipitation of a new phase during the in situ transformation modifies the microstructures and must be taken into account as an important factor to describe them.

Fe2O3/C nanocomposites having distinctive antioxidant activity and hemolysis prevention efficiency

Tailored magnetic iron oxide nanoparticles hold the prospect for nouveau applications in the field of biomedical technology. Herein, we report novel functionalities of this iron oxide system by developing a hybrid of Fe2O3/C to make it a multifunctional biomedical agent. A detailed magnetic study carried out at varying temperatures confirms the intrinsic superparamagnetic character of these iron oxide–carbon composites. The potential of the nanocomposite for biomedical applications has been evaluated by its ability to scavenge free radical by 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) assay. Moreover, the nanocomposite was monitored for successful hemolysis inhibition of mammalian erythrocytes. The nanocomposite showed promising compatibility with the peripheral blood mononuclear cells (PBMC) which was visualized from trypan blue dye exclusion assay. Biocompatible carbon coating over the iron oxide nanoparticles with these functionalities has transformed it to a multifunctional nanoparticulate biomedical agent potential for future clinical translation.

Single sheet iron oxide based films: electrochemical properties with in situ UV-vis measurement

Stable single sheet iron(III) oxide (SSI) colloidal suspensions can be obtained via delamination of the corresponding layered iron(III) oxides in sodium hydroxide followed by redispersion in tetrabutylammonium hydroxide. Multilayer films with alternate layers of poly(diallyldimethylammonium) (PDDA) ions and SSI ((PDDA/SSI)n) can be fabricated via layer-by-layer (LBL) electrostatic assembly using this stable suspension. For a single PDDA/SSI film, atomic force microscopy (AFM) shows high surface coverage and an average thickness of 14 nm. Each successive PDDA/SSI film has the same iron content which is confirmed by linear enhancement of the UV-vis absorption versus the number of layers (n). Films of (PDDA/SSI)n deposited on indium tin oxide-coated quartz electrodes are in electrical contact and the film layers are electrochemically identical to each other. The redox properties of (PDDA/SSI)n films depend on the cathodic switching potential (Ec). Ec values less negative than −0.5 to −0.7 V (vs. Ag/AgCl) result in highly reversible scans between fully oxidized SSI and partially reduced SSI. Ec values more negative than −0.9 to −1.1 V (vs. Ag/AgCl) result in irreversible cycles and deterioration of the (PDDA/SSI)n films. The half-wave potential of the (PDDA/SSI)n films in the latter case is −0.12 V (vs. Ag/AgCl) at pH 7 and −0.47 V (vs. Ag/AgCl) at pH 9 similar to other iron oxide systems. UV-vis spectroelectrochemical measurements reveal that the charge transfer into the (PDDA/SSI)n films causes a reversible in situ UV-vis spectral change when the potential is cycled between −0.7 V and +0.6 V (vs. Ag/AgCl) at pH 12. The (PDDA/SSI)n films become bleached in the UV spectral region (200–400 nm) and colored in the visible spectral region (400–900 nm) when exposed to a cathodic potential. The high mobility of protons results in fast charge transfer during reduction and oxidation of (PDDA/SSI)n films.

Computational Modeling of Octahedral Iron Oxide Clusters: Hexaaquairon(III) and Its Dimers

Octahedral monomeric and dimeric iron oxide clusters represent the basic units in many iron oxide and oxide-hydroxide minerals. In this paper, we provide a detailed theoretical analysis of the structural and optical properties of the most important of these clusters in a vacuum and in an aqueous environment. An evaluation of various computational methods was performed on the experimentally well-known monomer [Fe(H2O)6]3+, and it is found that all methods provide similar and reliable structures. Most density functional theory (DFT) methods reasonably reproduce the spin-forbidden sextet–quartet d–d transition energy, which also resembles the lowest transition energies in many infinite octahedral iron oxide systems. On the other hand, Hartree–Fock (HF) and MP2 methods significantly overestimate this energy. The ligand-to-metal charge transfer (LMCT) energy is highly sensitive to the method employed, with the closest agreement with experiment provided by the BHandHLYP functional. Thermodynamic property calculations suggest that dimerization reactions starting from [Fe(H2O)6]3+ are highly exothermic in a vacuum. In contrast, these reactions have insignificant energy changes in solution, though the singly μ-oxo bridged dimer is slightly favored. The electrostatic repulsion between two charged monomers hinders their close contact. The singly μ-oxo bridged dimer suffers less from this because of its maximal Fe–Fe distance, which is consistent with the existence of stable crystal structures for this dimer. A comparison between the calculated structures and experimental results suggests that several dimer species coexist in solution. The calculated ferromagnetic and antiferromagnetic states of the dimers are found to have comparable energies and structures. While the singly μ-oxo and doubly μ-hydroxo bridged dimers have spin states that are well separated in energy, the spin states in the triply μ-hydroxo bridged dimer pack closely. The single excitation d–d transition in the dimer structure is comparable in energy to the d–d transition in the monomer, while the double excitation d–d transition, i.e., simultaneous excitation of two iron centers, has a higher excitation energy that is 1.6–2.6 times the single excitation energy but below the LMCT energy. This means that doubly excited states can be populated during the non-radiative relaxation of iron oxide clusters following initial photoexcitation of the LMCT state.

Phosphate aggravates iron chlorosis in sensitive plants grown on model calcium carbonate−iron oxide systems

Background and aims The possible influence of phosphorus (P) on iron (Fe) deficiency chlorosis in susceptible plants needs elucidation. In this work, we tested the hypothesis that Fe chlorosis can be aggravated at high levels of P in the substrate.

Natural and synthetic iron oxides for hydrogen storage and purification

In this paper, the hydrogen storage capacity of some synthetic and natural iron oxides is presented. The results of the activity tests and characterization techniques of natural and synthetic iron oxides (N2 adsorption–desorption isotherms, temperature-programmed reduction, X-ray diffraction, and plasma atomic emission spectroscopy) suggest that the use of chromium on iron oxide systems improved their hydrogen storage capacity. This is related to the capacity of chromium to modify the iron oxide reduction profile when Cr was incorporated. A direct reduction from Fe3O4 to Fe was observed as the mechanism for H2 storage. In addition, natural oxides as commercial Superfine and Densinox-L oxides are proved to be suitable materials to store and purify H2 due to their high stability during different cycles of reduction and oxidation. The best results among the natural ones were Densinox-L and among the synthetic ones Fe–10Cr.

Solar thermal decoupled water electrolysis process I: Proof of concept

A new concept for a solar thermal electrolytic process was developed for the production of H2 from water. A metal oxide is reduced to a lower oxidation state in air with concentrated solar energy. The reduced oxide is then used either as an anode or solute for the electrolytic production of H2 in either an aqueous acid or base solution. The presence of the reduced metal oxide as part of the electrolytic cell decreases the potential required for water electrolysis below the ideal 1.23 V required when H2 and O2 evolve at 1 bar and 298 K. During electrolysis, H2 evolves at the cathode at 1 bar while the reduced metal oxide is returned to its original oxidation state, thus completing the H2 production cycle. Ideal sunlight-to-hydrogen thermal efficiencies were established for three oxide systems: Fe2O3–Fe3O4, Co3O4–CoO, and Mn2O3–Mn3O4. The ideal efficiencies that include radiation heat loss are as high or higher than corresponding ideal values reported in the solar thermal chemistry literature. An exploratory experimental study for the iron oxide system confirmed that the electrolytic and thermal reduction steps occur in a laboratory scale environment.

Multiplexed Enrichment and Detection of Malarial Biomarkers Using a Stimuli-Responsive Iron Oxide and Gold Nanoparticle Reagent System

There is a need for simple yet robust biomarker and antigen purification and enrichment strategies that are compatible with current rapid diagnostic modalities. Here, a stimuli-responsive nanoparticle system is presented for multiplexed magneto-enrichment and non-instrumented lateral flow strip detection of model antigens from spiked pooled plasma. The integrated reagent system allows purification and enrichment of the gold-labeled biomarker half-sandwich that can be applied directly to lateral flow test strips. A linear diblock copolymer with a thermally responsive poly(N-isopropylacrylamide) (pNIPAm) segment and a gold-binding block composed of NIPAm-co-N,N-dimethylaminoethylacrylamide was prepared by reversible addition-fragmentation chain transfer polymerization. The diblock copolymer was used to functionalize gold nanoparticles (AuNPs), with subsequent bioconjugation to yield thermally responsive pNIPAm-AuNPs that were co-decorated with streptavidin. These AuNPs efficiently complexed biotinylated capture antibody reagents that were bound to picomolar quantities of pan-aldolase and Plasmodium falciparum histidine-rich protein 2 (PfHRP2) in spiked pooled plasma samples. The gold-labeled biomarker half-sandwich was then purified and enriched using 10 nm thermally responsive magnetic nanoparticles that were similarly decorated with pNIPAm. When a thermal stimulus was applied in conjunction with a magnetic field, coaggregation of the AuNP half-sandwiches with the pNIPAm-coated iron oxide nanoparticles created large aggregates that were efficiently magnetophoresed and separated from bulk serum. The purified biomarkers from a spiked pooled plasma sample could be concentrated 50-fold into a small volume and applied directly to a commercial multiplexed lateral flow strip to dramatically improve the signal-to-noise ratio and test sensitivity.

Photodegradation kinetics of 2,6-dimetylphenol by natural iron oxide and oxalate in aqueous solution

Abstract Heterogeneous photodegradation of 2,6-dimethylphenol (2,6-DMP) in the natural iron oxide (NIO) system with oxalic acid under UV irradiation was investigated. The NIO was characterized by X-ray diffraction (XRD), X-ray fluorescence and Brunauer–Emmett–Teller (BET) methods. The XRD pattern of NIO showed a mixed crystal structure. In order to show the effect of temperature on the structure of NIO, the last one was calcined at 800 °C and 1000 °C. The XRD patterns of calcined iron oxides at 800 °C (NIO/800) and 1000 °C (NIO/1000) depict the conversion from mixed crystalline hematite to completely pure crystalline hematite. The experiments showed that the most effective catalyst for degradation of 2,6-DMP is NIO in presence of oxalate. The dependence of 2,6-DMP degradation was attributable to the formation of the dissolved Fe–oxalate in the solution and the adsorbed Fe–oxalate on the surface of natural iron oxide. The photodegradation of 2,6-DMP depends also on the oxalate and pH. The optimal initial concentration of oxalic acid was found to be 5 × 10 −3 M. The results demonstrated that the optimal pH value should be in the range of 3–4. The use of 2.0% of isopropanol as a scavenger confirmed the intervention of hydroxyl radicals in the photodegradation of 2,6-DMP. Furthermore, an important effect of oxygen was observed.

Experimental investigation and thermodynamic simulation of the uranium oxide–zirconium oxide–iron oxide system in air

The behavior of melts and the phase composition of crystallization products of six compositions in the uranium oxide-zirconium oxide-iron oxide system in air have been investigated. It has been revealed that crystallized samples containing 20–50 wt % uranium oxide and 25–80 wt % iron oxide (the rest is zirconium oxide) consist of five crystalline phases and involve two types of eutectic structures. The possible factors responsible for this phenomenon have been considered.

SHS of metal-oxide systems in a DC magnetic field: Part 1. TRXRD and thermal imaging studies of the Fe-Fe2O3 system

The effect of a 0.2 T external magnetic field on the heterogeneous combustion of a mixture of iron metal and oxide (Fe2O3) system with solid oxidizer (NaClO4) was studied. Time-resolved X-ray diffraction (TRXRD) was used to study the kinetics of intermediate product formation and thermal imaging experiments to measure the reaction temperature and velocity. Metastable fcc iron phase formation was discovered as an intermediate predominantly in applied field reactions.

Iron-Modified Sepiolite for Ni2+ Sorption from Aqueous Solution: An Equilibrium, Kinetic, and Thermodynamic Study

Sepiolite was used for the synthesis of the sepiolite−iron oxide system to obtain a sorbent suitable for the removal of Ni2+ from aqueous solution. The modified material, Fe-sepiolite, was characterized by X-ray diffraction analysis, Fourier transform infrared spectroscopy, differential thermal/thermogravimetric analysis, and point of zero charge (pHpzc) determination. The sorption experiments were performed as a function of the initial metal concentration, equilibration time, and temperature using the batch method. The Fe-sepiolite had a higher sorption capacity than the natural sepiolite sample due to a higher specific sorption and a higher ion exchange with Mg2+ ions from the sepiolite surface. Thermodynamic parameters, such as enthalpy, Gibbs energy, and entropy, were calculated from the temperature-dependent sorption isotherms, and these values showed that the sorption of Ni2+ onto sepiolites was endothermic. Pseudo-second-order kinetic models provide the best correlation of the experimental kinetic data.

Ferrous iron oxidation in moderately thermophilic acidophileSulfobacillus sibiricusN1T

The iron-oxidizing system of a moderately thermophilic, extremely acidophilic, gram-positive mixotroph, Sulfobacillus sibiricus N1(T), was studied by spectroscopic, high-performance liquid chromatography and inhibitory analyses. Hemes B, A, and O were detected in membranes of S. sibiricus N1(T). It is proposed that the electron transport chain from Fe²(+) to O₂ is terminated by 2 physiological oxidases: aa₃-type cytochrome, which dominates in the early-exponential phase of growth, and bo₃-type cytochrome, whose role in iron oxidation becomes more prominent upon growth of the culture. Both oxidases were sensitive to cyanide and azide. Cytochrome aa₃ was more sensitive to cyanide and azide, with K(i) values of 4.1 and 2.5 µmol·L⁻¹, respectively, compared with K(i) values for cytochrome bo₃, which were 9.5 µmol·L⁻¹ for cyanide and 7.0 µmol·L⁻¹ for azide. This is the first evidence for the participation of a bo₃-type oxidase in ferrous iron oxidation. The respiratory chain of the mixotroph contains, in addition to the 2 terminal oxidases, a membrane-bound cytochrome b₅₇₃.

Elucidation of the role of hexamine and other precursors in the formation of magnetite nanorods and their stoichiometry

Hexamine is known to assist anisotropic growth of metal oxides and the same is also found to be true for magnetite nanosynthesis. In this work we elucidate the role of hexamine and other precursors in the formation of magnetite nanorods by the hydrothermal route and their stoichiometry. Various others hydrolyzing agents such as sodium hydroxide (NaOH), sodium hydroxide + hexamine, ammonia (NH(3)), ammonia + formaldehyde are also studied. The synthesized nanoparticles are characterized with the help of various techniques such as X-ray diffraction (XRD), FT-IR spectroscopy, UV-VIS-NIR spectroscopy, transmission electron microscopy (TEM), Mössbauer spectroscopy and SQUID magnetization measurements. It is found that only when ferric chloride, ferrous ammonium sulfate (FAS) and hexamine are used, well defined nanorods are formed. When sodium hydroxide and hexamine are used as a hydrolyzing system nearly spherical nanoparticles with small size (approximately 13 nm) are formed, as compared to the case of sodium hydroxide alone which leads to bigger cube like nanoparticles. Interestingly the decomposition products of hexamine do not lead to nanorod formation. Thus, slow decomposition of hexamine at elevated temperature and the consequent slow rise in pH is the key to the anisotropic growth of the iron oxide system.

Field gradient imaging of nanoparticle systems: analysis of geometry and surface coatingeffects

In this work we compare the standard imaging of various types of nanoparticlesdeposited on surfaces by atomic force microscopy (AFM) with a complementaryanalysis of the same samples by either electrostatic force microscopy (EFM) ormagnetic force microscopy (MFM). Experiments were carried out on goldnanoparticles (decahedrons and stars) and two different iron oxide systems: goethite(α-FeOOH)and hematite (α-Fe2O3). Regardless of the particular geometry, the EFM signal appears to be stronger on edges ortips of pure gold nanoparticles. Both EFM and MFM experiments were also carried out oniron oxide particles. Apart from the structural analysis, we analyzed the influence of a shelllayer deposited on the gold and iron oxide particles, the shell being amorphousSiO2. Although the silica layer was found to have an insulating effect around the particles, in allcases EFM/MFM measurements could still be performed by the proper choice of the scanlift height (with an eventual slight increase of the sample bias, where applicable).

Reconstitution of Iron Oxidase from Sulfur-Grown Acidithiobacillus ferrooxidans

The iron oxidation system from sulfur-grown Acidithiobacillus ferrooxidans ATCC 23270 cells was reconstituted in vitro. Purified rusticyanin, cytochrome c, and aa(3)-type cytochrome oxidase were essential for reconstitution. The iron-oxidizing activity of the reconstituted system was 3.3-fold higher than that of the cell extract from which these components were purified.