Institute of Fundamental Technological Research

This work presents a synthesis of iron nanochains through magnetic-field-induced reduction reaction performed with sodium borohydride in water, ethanol and isopropanol. After their preparation, the nanomaterials obtained in three different processes are washed several times in ethanol and acetone to remove side-products. The performed cleaning step is very sufficient for water-based synthesis of iron nanochains. In contrary, the nanostructures obtained in ethanol and isopropanol contain a significant amount of sodium chlorides which is hard to dispose. Moreover, the use of ethanol and isopropanol solvents causes the reduction of nanochains’ diameters. Both the presence of sodium chlorides and the reduction of diameter size result in the decrease of saturation magnetization of iron nanochains and the increase of their coercivities.

One-dimensional nanostructures, Iron nanochains, Magnetic materials, Magnetic-field-induced synthesis

This work presents a growth of Fe3O4 nanoparticle films on silicon substrate. The iron oxide is deposited applying a pulsed laser deposition technique. The process is performed in open air in the absence and presence of external magnetic field. In fact, the morphologies of the obtained Fe3O4–Si samples are similar. The Fe3O4 nanoparticles are spherical with average diameters of 30 nm and are densely agglomerated on the Si substrate. The Fe3O4–Si material prepared in the absence of magnetic field has revealed more intense signals during X-ray diffraction and Raman measurements. The magnetic investigations indicate that the Fe3O4 nanoparticles are significantly coupled with the Si substrate and do not exhibit superparamagnetic behavior. Moreover, the Verwey transition is 98 K for both investigated Fe3O4–Si samples.

Fe3O4 nanoparticles,magnetic materials,pulsed laser deposition

This work presents the evolution of morphological, structural and magnetic properties of amorphous Fe-Ni wire-like nanochains caused by thermal oxidation. The initial Fe1−xNix samples (x = 0.75; 0.50; 0.25) were prepared through the magnetic-field-induced synthesis, and then they were heated in dry air at 400 °C and 500 °C. These treatments led to two competing simultaneous processes occurring in the investigated samples, i.e., (i) a conversion of amorphous material into crystalline material, and (ii) their oxidation. Both of them strictly affected the morphological and structural properties of the Fe-Ni nanochains which, in turn, were associated with the amount of iron in material. It was found that the Fe0.75Ni0.25 and Fe0.50Ni0.50 nanochains were covered during thermal treatment by the nanoparticle oxides. This coverage did not constitute a good barrier against oxidation, and these samples became more oxidized than the Fe0.25Ni0.75 sample which was covered by oxide nanosheets and contained additional Ni3B phase. The specific morphological evolutions of the Fe-Ni nanochains also influenced their saturation magnetizations, whereas their coercivities did not vary significantly. The obtained results constitute an important source of information for future application of the thermally treated Fe-Ni nanochains which could be applied in the energy storage devices or catalysis.

Thermal treatment is a post-synthesis treatment that aims to improve the crystallinity and interrelated physical properties of as-prepared materials. This process may also cause some unwanted changes in materials like their oxidation or contamination. In this work, we present the post-synthesis annealing treatments of the amorphous Fe1−xCox (x = 0.25; 0.50; 0.75) Wire-like nanochains performed at 400 °C in two different atmospheres, i.e., a mixture of 80% nitrogen and 20% hydrogen and argon. These processes caused significantly different changes of structural and magnetic properties of the initially-formed Fe-Co nanostructures. All of them crystallized and their cores were composed of body-centered cubic Fe-Co phase, whereas their oxide shells comprised of a mixture of CoFe2O4 and Fe3O4 phases. However, the annealing carried out in hydrogen-containing atmosphere caused a decomposition of the initial oxide shell layer, whereas a similar process in argon led to its slight thickening. Moreover, it was found that the cores of thermally-treated Fe0.25Co0.75 nanochains contained the hexagonal closest packed (hcp) Co phase and were covered by the nanosheet-like shell layer in the case of annealing performed in argon. Considering the evolution of magnetic properties induced by structural changes, it was observed that the coercivities of annealed Fe-Co nanochains increased in comparison with their non-annealed counterparts. The saturation magnetization (MS) of the Fe0.25Co0.75 nanomaterial annealed in both atmospheres was higher than that for the non-annealed sample. In turn, the MS of the Fe0.75Co0.25 and Fe0.50Co0.50 nanochains annealed in argon were lower than those recorded for non-annealed samples due to their partial oxidation during thermal processing.

annealing, amorphous materials, Fe-Co nanochains, magnetic-field-induced synthesis, wire-like nanostructure

So far, it has been proven that the magnetic-field-induced (MFI) synthesis is a process which mainly leads to the formation of magnetic metallic one-dimensional nanostructures. Taking advantage of this method, the new procedures which allow manufacture of the magnetic bimetallic iron–cobalt wire-like nanochains with Fe0.75 Co0.25, Fe0.50 Co0.50, and Fe0.25 Co0.75 compositions are demonstrated in this work. They were produced through a simple one-step magnetic-field-induced (MFI) chemical co-reduction of three different mixtures containing a proper amount of Fe2+ and Co2+ ions with aqueous sodium borohydride solution as the reducing agent. The synthesis process was carried out at room temperature without the use of templates, surfactants, complexing agents, and organic solvents. The morphological and structural studies indicated that all as-prepared materials were amorphous, and they were composed of nanoparticles aligned in almost straight chains. Moreover, they revealed the core–shell structures with bimetallic alloy cores containing desired iron-to-cobalt ratios and very thin oxide shells. Furthermore, the obtained nanostructures behaved as ferromagnetic materials. Their magnetic properties were correlated with their structural properties and chemical compositions. It was observed that their saturation magnetization decreased significantly with increasing content of cobalt in the chains, whereas the variation of their coercivity was less pronounced.

Manufacturing process of wire-like binary or ternary metal nanoalloys applying the magnetic-field-induced (MFI) synthesis is still a challenging task. Hence, this work demonstrates for the first time how to produce the iron-nickel wire-like nanostruc-tures with Fe0.75Ni0.25, Fe0.5Ni0.5 and Fe0.25Ni0.75 compositions. In a contrary to the previously reported synthesis of the Fe-Ni wire-like nanomaterials, this process has been carried out at room temperature without employment of templates, surfactants, organic solvents, and other chemical additives. The as-prepared samples exhibit specific structures with the amorphous bimetallic alloy cores covered by thin amorphous oxide shells. Moreover, they are composed of nanoparticles which are aligned in nearly linear chains. The Fe-Ni samples are ferromagnetic materials. Their coercivity values and saturation magnetizations depend on chemical compositions and dimensions of the investigated chains. The highest saturation magnetization and the lowest coercivity is found for the material with the lowest content of nickel and vice versa.

amorphous materials, Fe–Ni chains, magnetic-field-induced synthesis, magnetic materials, wire-like nanostructure

The magnetically-assisted growth of the amorphous bimetallic iron–nickel wire-like nanostructures is presented in this work. The applied process is based on a simple reduction reaction of aqueous solutions containing Fe2+ and Ni2+ ions with NaBH4 in the presence of an external magnetic field of about 0.05 T. The morphology, chemical composition, and magnetic properties of as-prepared Fe–Ni nanostructures have been determined by means of scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffractometry, and vibrating sample magnetometry. The obtained experimental data indicate that the as-prepared samples exhibit quite complex architectures i.e., they comprise of nanoparticles aligned in almost straight lines. In addition, they reveal the typical core-shell structures where the amorphous bimetallic alloy cores are covered by thin amorphous oxide shells. In turn, the magnetic measurements show that the Fe–Ni wire-like nanostructures behave as typical ferromagnetic nanomaterials and their magnetic parameters like saturation magnetizations and coercivities are strictly dependent on their sizes and chemical compositions.

amorphous materials, bimetallic nanostructures, magnetic-field-induced synthesis, magneticmaterials, wire-like nanostructures

The thermal treatment of chains composed of amorphous Fe 1–x Co x nanoparticles in two different oxygen atmospheres was studied. The nanostructures were manufactured using a magnetic-field-induced coreduction reaction, in which the precursor solutions containing 1:3 and 3:1 proportions of Fe 2+ and Co 2+ ions were reduced with sodium borohydride. The as-prepared nanochains were then heated for 30 min at 400 and 500 °C in dry air or argon containing about 1% oxygen. These processes led to their oxidation, and, as a result, the thermally treated Fe 1–x Co x nanochains were transformed into cobalt ferrite. Heating at 500 °C in the air-containing atmosphere caused the nanomaterials to lose their nanochain structures. In accordance to room-temperature magnetic measurements, the as-prepared and thermally treated Fe 1–x Co x nanochains were ferromagnetic. The highest saturation magnetization (Ms) was measured for the Fe 1–x Co x nanochains treated at 400 °C in dry air (105 A*m^2/kg and 154 A*m^2/kg for Fe 0.25 Co 0.75 and Fe 0.75 Co 0.25, respectively), whereas the lowest Ms was found for the Fe 0.25 Co 0.75 heated at 500 °C in dry air (17 A*m^2/kg) and the Fe 0.75 Co 0.25 heated at 500 °C in argon (16 A*m^2/kg).

nanomagnetics, FeCo nanochains, magnetic-field-induced synthesis, thermal treatment

The main objective of this work is to study the influence of thermal oxidation on the chemical composition and magnetic properties of iron nanoparticles which were manufactured in a simple chemical reduction of Fe3+ ions coming from iron salt with sodium borohydride. The annealing processing was performed in an argon atmosphere containing the traces of oxygen to avoid spontaneous oxidation of iron at temperatures ranging from 200 °C to 800 °C. The chemical composition and magnetic properties of as-prepared and thermally-treated nanoparticles were determined by means of X-ray diffractometry, Raman spectroscopy, Mössbauer spectroscopy and vibrating sample magnetometry. Due to the magnetic interactions, the investigated iron nanoparticles tended to create the dense aggregates which were difficult to split even at low temperatures. This caused that there was no empty space between them, which led to their partial sintering at elevated temperatures. These features hindered their precise morphological observations using the electron microscopy techniques. The obtained results show that the annealing process up to 800 °C resulted in a progressive change in the chemical composition of as-prepared iron nanoparticles which was associated with their oxidation. As a consequence, their magnetic properties also depended on the annealing temperature. For instance, considering the values of saturation magnetization, its highest value was recorded for the as-prepared nanoparticles at 1 T and it equals 149 emu/g, while the saturation point for nanoparticles treated at 600 °C and higher temperatures was not reached even at the magnetic field of about 5 T. Moreover, a significant enhancement of coercivity was observed for the iron nanoparticles annealed over 600 °C.

Chemical composition, Chemical reduction, Iron nanoparticle, Magnetic properties, Oxidation

The main goal of this work is to study the structural and magnetic properties of iron nanowires and iron nanoparticles, which have been fabricated in almost the same processes. The only difference in the synthesis is an application of an external magnetic field in order to form the iron nanowires. Both nanomaterials have been examined by means of transmission electron microscopy, energy dispersive X-ray spectrometry, X-ray diffractometry and Mössbauer spectrometry to determine their structures. Structural investigations confirm that obtained iron nanowires as well as nanoparticles reveal core–shell structures and they are composed of crystalline iron cores that are covered by amorphous or highly defected phases of iron and iron oxides. Magnetic properties have been measured using a vibrating sample magnetometer. The obtained values of coercivity, remanent magnetization, saturation magnetization as well as Curie temperature differ for both studied nanostructures. Higher values of magnetizations are observed for iron nanowires. At the same time, coercivity and Curie temperature are higher for iron nanoparticles.

iron nanoparticles, iron nanostructures, iron nanowires, magnetic properties, structural properties

This work presents the results of high temperature annealing of iron nanowires at five different temperatures (ranging 200–800 °C) in the slightly oxidative atmosphere. Investigated nanomaterial was prepared in simple chemical reduction process from aqueous solution of iron trichloride placed in external magnetic field. Experimental results allowed determining how magnetic properties of as-prepared as well as annealed iron nanowires change in respect to their structures. They also delivered information about phase transitions occurred in as-prepared sample under thermal treatment.

annealing, iron, magnetic properties, nanowires

The aim of this work is to study the preparation and characterization of a new nanocomposite which consists of chemically-modified multiwall carbon nanotubes covered by randomly-deposited nanoparticles of hematite. The morphology, structural and physical properties of the investigated nanomaterial were determined by means of transmission electron microscopy, X-ray diffraction and vibrating sample magnetometry at ambient conditions. The presence of residual catalyst nanospheres inside multiwall carbon nanotubes was confirmed by transmission electron microscopy. The signal coming from this contamination was under the detection limit of X-ray diffractometer, therefore it was not registered.

Hematite, Multiwall carbon nanotubes, CVD, TEM, XRD, VSM

magnetic-field-induced process, magnetic material, nanoalloy, wire-like nanostructure