The influence of thermal annealing on structure and oxidation of iron nanowires

Abstract

Abstract. 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 fi eld. Both experimental techniques proved formation of the surface layer covered by crystalline iron oxides, with phase composition dependent on the annealing temperature (T a ). At higher T a , hematite was the dominant phase in the nanowires.

Structural and magnetic properties of iron nanowires and iron nanoparticles fabricated through a reduction reaction

“…The differences between them are hardly discernible, which reflects a similarity of the studied nanomaterials. It is also worth noting that they are quite similar to the spectrum of as-prepared iron nanowires presented in our previous studies [15]. Nevertheless, both spectra are composed of several overlapping components.…”

“…The second strongest signal corresponds to the presence of crystalline α-Fe with a magnetic hyperfine field of 33.05 T and an isomer shift of 0.00 mm/s. The last easily distinguishable subspectrum characterized by a high magnetic hyperfine field of over 34 T is seen in the form of side slight “wings” [15]. This component can be attributed to iron ions situated in the surface oxide layer of the nanowires.…”

“…It is well-known that iron nanomaterials in the presence of even small quantities of oxygen tend to be oxidized immediately. The increasing temperature leads to progressive oxidation of iron to Fe 3 O 4 (large nanostructures) or γ-Fe 2 O 3 (small nanostructures) and following transformation to α-Fe 2 O 3 [12,15]. Above 550 °C, it seems that the studied nanowires as well as nanoparticles consist of crystalline α-Fe 2 O 3 located on the surface of the investigated nanomaterials and an α-Fe core, which do not change the structural properties during further heating.…”

“…The estimated Curie temperatures of Fe NWs and Fe NPs are slightly lower than the Curie temperature of the bulk crystalline iron (770 °C [9]) and they could be different than the real T C of studied nanostructures due to the application of a quite simple model fitted for the cooling curves as well as the calculation error. Additionally, these differences may be caused by certain inhomogeneities, admixtures and proximity effects of the iron core with respect to paramagnetic iron oxide phases that have been formed during the oxidation reaction [12,15] and have become paramagnetic at temperatures lower than ferromagnetic-paramagnetic transition of α-Fe. On the other hand, it is worth noting that the measurements of magnetization as a function of the temperature confirm that even though both fabricated nanomaterials change their structures during the heating, they still contain the crystalline iron, because none of iron-based oxides or hydroxides exhibit such high Curie temperature as pristine iron.…”

“…Fabrication of Fe NWs and Fe NPs: Iron nanowires as well as iron nanoparticles have been prepared in a similar manner as described in [3,12,15,35–36]. Therefore, the fabrication procedures are described briefly here.…”