Synthesis,structure, and magnetic properties of iron and nickel nanoparticles encapsulated into carbon

Nanocomposites based on iron and nickel particles encapsulated into carbon (Fe@C and Ni@C), with an average size of the metal core in the range from 5 to 20 nm and a carbon shell thickness of approximately 2 nm, have been prepared by the gas-phase synthesis method in a mixture of argon and butane. It has been found using X-ray diffraction, transmission electron microscopy, and Mössbauer spectroscopy that iron nanocomposites prepared in butane, apart from the carbon shell, contain the following phases: iron carbide (cementite), α-Fe, and γ-Fe. The phase composition of the Fe@C nanocomposite correlates with the magnetization of approximately 100 emu/g at room temperature. The replacement of butane by methane as a carbon source leads to another state of nanoparticles: no carbon coating is formed, and upon subsequent contact with air, the Fe₃O₄ oxide shell is formed on the surface of nanoparticles. Nickel-based nanocomposites prepared in butane, apart from pure nickel in the metal core, contain the supersaturated metastable solid solution Ni(C) and carbon coating. The Ni(C) solid solution can decompose both during the synthesis and upon the subsequent annealing. The completeness and degree of decomposition depend on the synthesis regime and the size of nickel nanoparticles: the smaller is the size of nanoparticles, the higher is the degree of decomposition into pure nickel and carbon. The magnetization of the Ni@C nanocomposites is determined by several contributions, for example, the contribution of the magnetic solid solution Ni(C) and the contribution of the nonmagnetic carbon coating; moreover, some contribution to the magnetization can be caused by the superparamagnetic behavior of nanoparticles.     

Mössbauer effect phase determination in iron oxide–polyaniline nanocomposites

Mössbauer effect spectroscopy and thermal analysis techniques were applied to characterize polyaniline composites successfully synthesized by embedding Fe oxide nanoparticles (about 10–13 nm) in a polymeric matrix in the presence of dodecyl benzene sulfonic acid and HCl (dopant). Thermal techniques provided quantitative information on iron oxide content and on polyaniline stability and transformations. Mössbauer results indicated that for the whole studied composition range, 3.4 to 100 iron oxide wt.%, composites hold maghemite particles. A preliminary study of the conductivity of the nanocomposites was performed. The largest conductivity was observed for a 8 wt.% maghemite composite where all particles are magnetically unblocked at room temperature within the Mössbauer time window.     

Thermal effects in highly dispersed iron catalysts

The Mössbauer spectra of three Fe/SiO₂ catalysts with 5 wt% iron content show the presence of several Fe species and display different magnetic behaviours when the precursors are subjected to various thermal treatments. Based on the Mössbauer parameters and CO chemisorption measurements, the average crystal sizes of the catalysts are estimated and discussed in connection with the thermal pretreatment severity and magnetic properties of the samples.     

The effect of carboxylic acids on the oxidation of coated iron oxide nanoparticles

⁵⁷Fe Mössbauer spectroscopy, XRD, and TEM were used to investigate the effect of mandelic- and salicylic acid coatings on the iron oxide nanoparticles. These two carboxylic acids have similar molecules size and stoichiometry, but different structure and acidity. Significant differences were observed between the Mössbauer spectra of samples coated with mandelic acid and salicylic acid. These results indicate that the occurrence of iron microenvironments in the mandelic- and salicylic acid-coated iron oxide nanoparticles is different. The results can be interpreted in terms of the influence of the acidity of carboxylic acids on the formation, core/shell structure, and oxidation of coated iron oxide nanocomposites.     

Mössbauer study of Mg–Ni(Fe) alloys processed as materials for solid state hydrogen storage

Mg–Ni–Fe magnesium-rich intermetallic compounds were prepared following two distinct routes. A Mg₈₈Ni₁₁Fe₁ sample (A) was prepared by melt spinning Mg–Ni–Fe pellets and then by high-energy ball milling for 6 h the obtained ribbons. A (MgH₂)₈₈Ni₁₁Fe₁ sample (B) was obtained by high-energy ball milling for 20 h a mixture of Ni, Fe and MgH₂ powders in the due proportions. A SPEX8000 shaker mill with a 10:1 ball to powder ratio was used for milling in argon atmosphere. The samples were submitted to repeated hydrogen absorption/desorption cycles in a Sievert type gas–solid reaction controller at temperatures in the range 520?÷?590 K and a maximum pressure of 2.5 MPa during absorption. The samples were analysed before and after the hydrogen absorption/desorption cycles by X-ray diffraction and Mössbauer spectroscopy. The results concerning the hydrogen storage properties of the studied compounds are discussed in connection with the micro-structural characteristics found by means of the used analytical techniques. The improved kinetics of hydrogen desorption for sample A, in comparison to sample B, has been ascribed to the different behaviour of iron atoms in the two cases, as proved by Mössbauer spectroscopy. In fact, iron results homogeneously distributed in sample A, partly at the Mg₂Ni grain boundaries, with catalytic effect on the gas–solid reaction; in sample B, instead, iron is dispersed inside the hydride powder as metallic iron or superparamagnetic iron.     

Chemical transformations of iron compounds during the process of hard coal pyrolysis

The coal with a very high sulphure content was used as a charge of a reactor for hydropyrolysis processes in the 820–1120 K range and for pyrolysis processes in an argon atmosphere. The results of Mössbauer measurements for the semi-cokes, obtained during hydropyrolysis, showed that the pyrite was transformed to pyrrhotites in the 820–870 K range when it was transformed to metallic iron in the 1000–1120 K range. The pyrolysis in an argon atmosphere leads to decomposition of pyrite to pyrrhotites but the metallic iron was not evidenced in these cokes.     

Hematite reaction with tar to produce carbon/iron composites for the reduction of Cr(VI) contaminant

In this work, highly reactive carbon–iron composites were prepared using a waste, i.e. tar, as carbon precursor and a simple iron oxide, i.e. hematite. Tar was impregnated on Fe₂O₃ with different tar/hematite weight ratios of 1:1; 2:1 and 4:1, and thermally treated under N₂ atmosphere (400°C, 600°C and 800°C). Mössbauer, XRD and magnetization measurements suggested that treatment at 400°C and 600°C produces Fe₃O₄ but treatment at 800°C produced mainly Fe°. Raman and TG analyses of the different composites suggested the formation of carbon contents of 18, 24 and 32 wt.% as amorphous and graphitic highly dispersed on the Fe surface. The composites obtained at 800°C showed high efficiency to reduce Cr(VI) as CrO $_{4}^{2-}$ in aqueous medium with much better results compared to finely ground commercial Fe°.     

Iron-zirconium oxide catalysts for the hydrogenation of carbon monoxide: In situ studies by iron-57 Mössbauer spectroscopy

Some unsupported iron-zirconium oxide catalysts have been prepared by the calcination in air of precipitates containing 15 mole % iron. The catalyst formed at 500°C was shown by powder X-ray diffraction to consist of a non-equilibriated solid solution of iron(III) in a tetragonal or cubic zirconium dioxide structure whereas the catalyst formed at 1000°C was found to contain a zirconium-doped α-iron(III) oxide, or a magnetically ordered iron-zirconium oxide, in combination with an iron-containing monoclinic polymorph of zirconium dioxide. The⁵⁷Fe Mössbauer spectra recorded in situ following the pretreatment of the solids in nitrogen, carbon monoxide and hydrogen showed that little change is induced in the catalysts under such conditions. The⁵⁷Fe Mössbauer spectra also showed that the pretreated catalysts were unchanged by exposure to a 1:1 mixture of carbon monoxide and hydrogen at 270°C and 1 atmosphere pressure but were partially converted to iron carbide when used for the hydrogenation of carbon monoxide at 330°C and at 20 atmospheres pressure. The hydrocarbon product distribution showed Schulz-Flory α-values of 0.73 to 0.76 which were higher than the α-values obtained from pure iron catalysts which had been prepared and pretreated in a similar fashion. The⁵⁷Fe Mössbauer spectra and the results of the catalytic evaluation may be associated with an interaction between zirconium(IV) and the electron-rich atoms of the reactant carbon monoxide.     

The biogeochemical iron cycle and astrobiology

Biogeochemistry investigates chemical cycles which influence or are influenced by biological activity. Astrobiology studies the origin, evolution and distribution of life in the universe. The biogeochemical Fe cycle has controlled major nutrient cycles such as the C cycle throughout geological time. Iron sulfide minerals may have provided energy and surfaces for the first pioneer organisms on Earth. Banded iron formations document the evolution of oxygenic photosynthesis. To assess the potential habitability of planets other than Earth one looks for water, an energy source and a C source. On Mars, for example, Fe minerals have provided evidence for the past presence of liquid water on its surface and would provide a viable energy source. Here we present Mössbauer spectroscopy investigations of Fe and C cycle interactions in both ancient and modern environments. Experiments to simulate the diagenesis of banded iron formations indicate that the formation of ferrous minerals depends on the amount of biomass buried with ferric precursors rather than on the atmospheric composition at the time of deposition. Mössbauer spectra further reveal the mutual stabilisation of Fe-organic matter complexes against mineral transformation and decay of organic matter into CO₂. This corresponds to observations of a ‘rusty carbon sink’ in modern sediments. The stabilisation of Fe-organic matter complexes may also aid transport of particulate Fe in the water column while having an adverse effect on the bioavailability of Fe. In the modern oxic ocean, Fe is insoluble and particulate Fe represents an important source. Collecting that particulate Fe yields small sample sizes that would pose a challenge for conventional Mössbauer experiments. We demonstrate that the unique properties of the beam used in synchrotron-based Mössbauer applications can be utilized for studying such samples effectively. Reactive Fe species often occur in amorphous or nanoparticulate form in the environment and are therefore difficult to study with standard mineralogical tools. Sequential extraction techniques are commonly used as proxies. We provide an example where Mössbauer spectroscopy can replace sequential extraction techniques where mineralogical information is sought. Where mineral separation is needed, for example in the investigation of Fe or S isotope fractionation, Mössbauer spectroscopy can help to optimize sequential extraction procedures. This can be employed in a large number of investigations of soils and sediments, potentially even for mineral separation to study Fe and S isotope fractionation in samples returned from Mars, which might reveal signatures of biological activity. When looking for the possibility of life outside Earth, Jupiter’s icy moon Europa is one of the most exciting places. It may be just in reach for a Mössbauer spectrometer deployed by a future lander to study the red streak mineral deposits on its surface to look for clues about the composition of the ocean hidden under the moon’s icy surface.     

Size determination of metallic iron particles by temperature dependent Mössbauer and ferromagnetic resonance studies

The thermal decomposition products of NaX·Fe(CO)₅ after various steps of heat treatment, in vacuum as well as under gas atmosphere (Ar, H₂), have been investigated by temperature-dependent Mössbauer and ferromagnetic resonance measurements. The measured spectra of both techniques consistently are analyzed using superparamagnetic relaxation behavior of magnetic particles. From this analysis information concerning composition and size of magnetic iron particles is derived.     

Mössbauer study of SnO<Subscript>2</Subscript> powders doped with dilute <Superscript>57</Superscript>Fe prepared by a sol-gel method

SnO₂ powders, doped with various ⁵⁷Fe contents were prepared by a sol-gel method, and annealed finally at 500 °C and 650 °C. These samples were characterized by Mössbauer spectroscopy, vibrating sample magnetometer (VSM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) to investigate the relationship of magnetic properties, grain sizes, annealing temperatures and Mössbauer parameters. The particle sizes of SnO₂ powders reduced to less than 100 nm with the increase of Fe contents up to 5%. Rutile SnO₂ was the only phase obtained for all samples. Room temperature Mössbauer spectra suggest the presence of two different paramagnetic iron sites for all samples and one magnetically relaxed species for those samples with the lowest iron concentrations. The magnetization increased with the Fe content, but was reduced for the samples annealed at 650 °C perhaps due to a segregation of α-Fe₂O₃ doped with tin.     

Effect of particle size and alloying with different metals on 57Fe Mössbauer spectra

Iron nanoparticles of various sizes have been synthesized using the chemical route which involves the preparation of iron bipyridine complexes in presence of different capping agents followed by thermal decomposition at 450°C in inert atmosphere. The bimetallic nanoalloys of Fe with Mg and Pd have also been prepared by following the same route. The resulting nanoparticles have been characterized by EDX-RF, XRD, AFM and ⁵⁷Fe Mössbauer spectroscopy. The appearance of quadrupole doublets in the Mössbauer spectra of Fe nanoparticles indicates the absence of magnetic interaction and variation in parameters is due to the varying particle size. The Mössbauer spectrum of Fe–Mg₂ bimetallic nanoalloy shows two doublets indicating the presence of superparamagnetism. The two doublets can be attributed to change in s-electron density of iron resulting from its two neighboring magnesium atoms. Fe–Pd nanoalloy Mössbauer spectrum is characterized by having a superparamagnetic doublet and a ferromagnetic sextet.     

Mössbauer and XRD study of Al-Sn linished steel bimetal alloy

Aluminium alloy free CS1 type steel (0.06 wt% C, 0.45 wt% Mn) and samples of cold roll bonded steel bimetal alloys (MAS15 and MAS16) were fabricated and investigated by X-ray diffraction (XRD), ⁵⁷Fe conversion electron Mössbauer spectroscopy (CEMS) at room temperature. XRD has revealed only the existence of the alpha iron solid solution (steel) phase in the steel only sample, while identified steel and metallic Al and Sn constituent phases in the bimetallic alloys. ⁵⁷Fe Mössbauer spectroscopy revealed the presence of 4 % secondary iron-bearing phase attributed mainly to iron oxide/ oxyhydroxides (ferrihydrite) besides the steel matrix on the surface of the steel sample. A significant difference between the occurrences of the secondary phase of differently prepared bimetal alloys found in their ⁵⁷Fe CEM spectra allowed to identify the main phase of debris as different iron oxide/ oxyhydroxides.     

A Mössbauer investigation of zirconium distribution and diffusion in ball milled nanocrystalline iron powders

Ball milled nanocrystalline iron with minor zirconium additions was examined using ⁵⁷Fe Mössbauer spectroscopy and X-ray diffraction. Powder samples were synthesized using 0, 5, and 10 wt.% zirconium additions and milled at room temperature for periods up to 24 h. Progressive decrease in grain size as determined by X-ray diffraction was observed as a function of milling time. Mössbauer spectroscopy indicates increased iron-zirconium coordination with increased milling time. After milling, the powder samples were then heat treated in an inert atmosphere of argon at up to 925 K for various times up to 25 min. Analysis of X-ray peak line width (FWHM) was used to characterize grain size and grain growth kinetics as a function of heat treatment, milling time, and alloy content and reveal an increasingly finer post-heated structure in the alloy samples containing more zirconium. Mössbauer measurements were made and suggest Zr is steadily distributed into the Fe lattice with milling and rapidly diffuses to the grain boundaries with heat treatment. The impurity-rich grain boundaries appear to considerably stabilize the refined structure.     

Integral low-energy electron Mössbauer spectroscopic studies of the surfaces of carbon nanotube-nanocomposite powders

The surface state of CNTs-Fe-Al₂O₃ and CNTs-Fe-MgAl₂O₄ nanocomposite powders was studied by integral low-energy electron Mössbauer spectroscopy (ILEEMS). Several samples, prepared by reduction of α-(Al,Fe)₂O₃ or (Mg,Fe)Al₂O₄ precursors in a H₂-CH₄ atmosphere, were investigated, demonstrating that ILEEMS is a promising tool completing transmission Mössbauer spectroscopy for the investigation of the metal Fe and iron-carbide particles in the different carbon nanotube systems.     

Mössbauer, X-ray, electrical and magnetic properties of lithium borosilicate glasses containing iron and manganese Part I. Mössbauer measurements

The effect of the replacement of MnO₂ by Fe₂O₃ on the oxidation states of iron in some lithium borosilicate glasses were studied using Mössbauer spectroscopy. DTA and infrared measurements showed the presence of two glassy phases, one of them containing iron and the other manganese. The Mössbauer spectra were measured at room temperature for all the glass samples. The spectra revealed the presence of Fe ions only in ferric state in both tetrahedral and octahedral coordination. Also, it was observed that the ratio between the number of iron ions in both coordination states has not changed with the increase of MnO₂ content.     

Mössbauer spectroscopy study of iron oxide nanoparticles obtained by spray pyrolysis

Mössbauer spectroscopy was used in this study to investigate magnetite nanoparticles, obtained by spray pyrolysis and thermal treatment under H₂ reduction atmosphere. Room temperature XRD data indicate the formation of magnetite phase and a second phase (metallic iron) which amount increases as the time of reduction under H₂ is increased. While room temperature Mössbauer data confirm the formation of the cubic phase of magnetite and the occurrence of metallic iron phase, the more complex features of 77 K-Mössbauer spectra suggest the occurrence of electronic localization favored by the different crystalline phase of magnetite at low temperatures which transition to the lower symmetry structure should occur at T ～120 K (Verwey transition).     

Studies of oxidation of iron nanowires encased in porous aluminium oxide template

The transformations of phase composition of iron nanowires deposited into porous alumina template when annealing in the air were studied. The samples of iron nanowires of different diameter (8, 13, 15, 30 nm) were annealed for 1.5 h at temperature up to 600°C. In addition, for nanowires of 15 nm diameter the dependence of phase composition on annealing time was investigated. The phases were determined by applying Mössbauer spectroscopy. New Fe(II) and Fe(III) contributions to Mössbauer spectra were found and those were indentified as caused by the formation of hercynite FeAl₂O₄ and (Fe _x Al_1???x )₂O₃ with small x values (x?≤?0.15). It has been found that though initially the Fe(II) compound forms rapidly, afterwards its formation rate becomes lower than that of Fe(III) and after longer annealing time the Fe(III) content exceeds Fe(II) one.     

Structural studies of iron in vitrified toxic wastes

The formation of iron carbides by reactive milling of α-Fe and C powders is reported. The products formed were analyzed by Mössbauer spectroscopy and X-ray diffraction. It was found that iron carbide phases start forming after an incubation period of about 3 h depending on the ball-to-powder weight ratio (BPR). Carbide amounts increased with increasing milling time while α-Fe content decreased. Energy transfer increased with increasing BPR and high BPR resulted in an increase in the reaction rate. Although it was not possible to selectively synthesise a specific Fe _x C phase, samples containing predominantly one type of carbide phase, either Hägg carbide or cementite, were successfully prepared. The formation of the different iron carbide phases is discussed within the context of the Fe–C phase diagram for non-equilibrium processes.     

Phase composition of nanocrystalline iron films deposited in a nitrogen atmosphere

The phase composition of iron films prepared by pulsed-plasma deposition in a controlled nitrogen atmosphere is investigated by Mössbauer spectroscopy. The observed changes in the phase composition are dictated by the nanocrystalline structure of the samples and the dynamics of the substrate temperature during film deposition.