Institute of Fundamental Technological Research

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

Magnetic nanoparticles based on iron or iron oxides represent an excellent nanomaterial in respect of their fundamental research as well as numerous applications. This work presents a Mössbauer study of the influence of thermal treatment on the structure and phase composition of iron nanoparticles fabricated via a simple reduction reaction. Identification of iron-containing phases has been carried out for the series of samples annealed at different temperatures ranging between 200 C and 800 C. Both crystalline and amorphous iron as well as variety of iron oxides have been detected. It has been proved that the nanoparticles annealed at 500 C show superparamagnetic behavior as a result of ultrafine sizes of crystallites of iron oxides that arise in the oxidation processes.

This work describes an oxidation process of iron–iron oxide core–shell nanowires at temperatures between 100°C and 800°C. The studied nanomaterial was synthesized through a simple chemical reduction of iron trichloride in an external magnetic field under a constant flow of argon. The electron microscopy investigations allowed determining that the as-prepared nanowires were composed of self-assembled iron nanoparticles which were covered by a 3 nm thick oxide shell and separated from each other by a thin interface layer. Both these layers exhibited an amorphous or highly-disordered character which was traced by means of transmission electron microscopy and Mössbauer spectroscopy. The thermal oxidation was carried out under a constant flow of argon which contained the traces of oxygen. The first stage of process was related to slow transformations of amorphous Fe and amorphous iron oxides into crystalline phases and disappearance of interfaces between iron nanoparticles forming the studied nanomaterial (range: 25–300°C). After that, the crystalline iron core and iron oxide shell became oxidized and signals for different compositions of iron oxide sheath were observed (range: 300–800°C) using X-ray diffraction, Raman spectroscopy and Mössbauer spectroscopy. According to the thermal gravimetric analysis, the nanowires heated up to 800°C under argon atmosphere gained 37% of mass with respect to their initial weight. The structure of the studied nanomaterial oxidized at 800°C was mainly composed of α-Fe2O3 (∼93%). Moreover, iron nanowires treated above 600°C lost their wire-like shape due to their shrinkage and collapse caused by the void coalescence.

annealing, core-shell nanostructure, iron nanoparticles, iron nanowires, oxidation, thermal treatment

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

Raman spectroscopy as well as Mössbauer spectroscopy were applied in order to study the phase composition of iron nanowires and its changes, caused by annealing in a neutral atmosphere at several temperatures ranging from 200°C to 800°C. As-prepared nanowires were manufactured via a simple chemical reduction in an external magnetic field. Both experimental techniques proved formation of the surface layer covered by crystalline iron oxides, with phase composition dependent on the annealing temperature (Ta). At higher Ta, hematite was the dominant phase in the nanowires.

amorphous iron and iron oxides, iron nanowires, Mössbauer spectroscopy, Raman spectroscopy, thermal annealing