One-step hydrothermal process to prepare highly crystalline Fe3O4 nanoparticles with improved magnetic properties
Introduction
Over the past several years, the preparation and characterization of nanoscale magnetic materials have attracted much attention as the materials in this size range would allow investigating the fundamental aspects of magnetic-ordering phenomena in magnetic materials with reduced dimensions and could also lead to new potential applications such as high-density recording based on arrays of nanosized magnetic particles.
Various methods have been reported in the literature for the preparation of nanoscale Fe3O4 particles, such as reduction of hematite Fe2O3 by CO [1] or H2 [2], co-precipitation of a solution of ferrous/ferric mixed with NH3·H2O solution [3], oxidation of the ferrous hydroxide gels using KNO3 [4], γ-ray radiation [5], microwave plasma synthesis [6], oil-in-water emulsion route using a small amount of cyclohexane as the oil phase [7]. However, the reported magnetic properties such as saturation magnetization are much lower than that of buck materials, and the values are also various from method to method. It is suggested that nanoparticles possess a large surface-to-volume ratio, the surface disorder phase and non-magnetic layer would reduce the magnetization behavior of the material. It is found that the crystallinity of nanosized particles is largely dependent on formation conditions. Hydrothermal process is one of the successful ways to grow crystals of many different materials such as quartz, malachite, etc. This technique has also been used to grow dislocation free single crystal particles, and grains formed in this process could have a better crystallinity than those from other processes. It is the purpose of this work to prepare nanoscale Fe3O4 grains with increased crystallinity by a hydrothermal process and to evaluate the correlation between their magnetic properties and crystallinity.
It is reported that ultrafine magnetite powder could be prepared by a hydrothermal reaction of ferric chloride and iron powder in an alkaline medium [8]. However, the amount of iron powder must be carefully controlled to avoid the trace residue of metal iron in final product. In this work, we developed a one-step hydrothermal process to prepare nanoscale Fe3O4 powder at low temperatures, in which the particles were well crystallized. Their magnetic properties were evaluated by vibrating sample magnetometer.
Section snippets
Experimental
The chemical reagents used in the work were ferrous chloride (FeCl2·4H2O), sodium hydroxide (NaOH), and diamine hydrate (H4N2·H2O, 50% water). All the chemicals were of analytical grade. Distilled water, degassed with N2 gas for half an hour, was used for the preparation of an aqueous solution of Fe2+ ions (0.2 mol/l). 30.0 ml Fe2+ solution was put into a Teflon-lined stainless autoclave, sodium hydroxide (0.6 g) was dissolved into 5.0 ml H4N2·H2O and then the mixture was slowly dropped into the
Results and discussion
The reaction of hydrothermal synthesis of Fe3O4 from ferrous chloride (FeCl2) and diamine hydrate (H4N2·H2O) can be expressed as follows:
In alkalescence medium, N2H4 can serve as either an oxidant or a reducer [9].
As an oxidant:
As a reducer:where Φ0 is criterion electromotance.
Electrode potential of iron in alkalescence medium is given as follows:N2H4, therefore, is able to oxidize Fe2+ into Fe3+ in
Conclusions
We have successfully prepared nanoscale Fe3O4 powder by a one-step hydrothermal process. It is found that the particle size and crystallinity are temperature and time dependent. XRD and TEM studies reveal that higher temperature and longer time of the process tend to result in formation of larger grains with better crystallinity. The magnetic measurements show that the powder obtained by a hydrothermal process at 140 °C possesses excellent magnetic properties with saturation magnetization of 85.8
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