Progress in Crystal Growth and Characterization of Materials
Synthesis, properties, and applications of magnetic iron oxide nanoparticles
Introduction
Iron oxides exist in many forms in nature, with magnetite (Fe3O4), maghemite (γ-Fe2O3), and hematite (α-Fe2O3) being probably the most common [1]. These three oxides are also very important technologically, and they are therefore the subject of this review. Some of their physical and magnetic properties are summarized in Table 1.
Hematite is the oldest known of the iron oxides and is widespread in rocks and soils. It is also known as ferric oxide, iron sesquioxide, red ochre, specularite, specular iron ore, kidney ore, or martite. Hematite is blood-red in color if finely divided, and black or grey if coarsely crystalline. It is extremely stable at ambient conditions, and often is the end product of the transformation of other iron oxides. Magnetite is also known as black iron oxide, magnetic iron ore, loadstone, ferrous ferrite, or Hercules stone. It exhibits the strongest magnetism of any transition metal oxide [1], [2]. Maghemite occurs in soils as a weathering product of magnetite, or as a product of heating of other iron oxides. It is metastable with respect to hematite, and forms continuous solid solutions with magnetite [3].
The crystal structure of the three iron oxides can be described in terms of close-packed planes of oxygen anions with iron cations in octahedral or tetrahedral interstitial sites. In hematite, oxygen ions are in a hexagonal close-packed arrangement, with Fe(III) ions occupying octahedral sites (Fig. 1a). In magnetite and maghemite, the oxygen ions are in a cubic close-packed arrangement (Fig. 1b). Magnetite has an inverse spinel structure with Fe(III) ions distributed randomly between octahedral and tetrahedral sites, and Fe(II) ions in octahedral sites [4]. Maghemite has a spinel structure that is similar to that of magnetite but with vacancies in the cation sublattice. Two-thirds of the sites are filled with Fe(III) ions arranged regularly, with two filled sites being followed by one vacant site [1]. Haneda and Morrish [5] found that the degree of vacancy ordering decreases with decreasing particle size, with no vacancy ordering in maghemite smaller than about 20 nm.
Section snippets
Magnetic behavior of iron oxides
The iron atom has a strong magnetic moment due to four unpaired electrons in its 3d orbitals. When crystals are formed from iron atoms, different magnetic states can arise as shown in Fig. 2. In the paramagnetic state, the individual atomic magnetic moments are randomly aligned with respect to each other, and the crystal has a zero net magnetic moment. If this crystal is subjected to an external magnetic field, some of these moments will align, and the crystal will attain a small net magnetic
Applications
The magnetic properties of iron oxides have been exploited in a broad range of applications including magnetic seals and inks, magnetic recording media, catalysts, and ferrofluids [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], as well as in contrast agents for magnetic resonance imaging and therapeutic agents for cancer treatment [39], [42], [48], [50], [51], [52], [53], [54], [55]. These applications demand nanomaterials of specific sizes, shapes, surface characteristics,
Gas phase methods
Gas phase methods for preparing nanomaterials depend on thermal decomposition (pyrolysis), reduction, hydrolysis, disproportionation, oxidation, or other reactions to precipitate solid products from the gas phase [116]. In the chemical vapor deposition (CVD) process, a carrier gas stream with precursors is delivered continuously by a gas delivery system to a reaction chamber maintained under vacuum at high temperature (>900 °C) [114], [117]. The CVD reactions take place in the heated reaction
Summary
Substantial progress has been made in the synthesis of monodisperse magnetic nanoparticles for applications in nanotechnology and biotechnology. Methods have been developed that offer control over the size, size distribution, shape, crystal structure, defect distribution and surface structure of nanoparticles and their magnetic properties. Among the methods reviewed, continuous supercritical hydrothermal synthesis probably offers the most promise for process control and scalability.
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