INTRODUCTION
Iron oxide is one of the oldest materials used even in archeological founds, e.g., in Ajanta or in Altamira, as red pigments. Even today, its significance in the pigmental materials is not lost. Since the advent of magnetic recording systems, however, ferromagnetic iron oxides gained its importance quite rapidly. These two important application fields are coupled to gain magnetic or intelligent inks.
There are many chemical species categorized as iron oxides. While hematite (a-Fe2O3), maghemite (g-Fe2O3), and magnetite (Fe3O4) are, by far, the most important species of pure iron oxides, many hydroxides, such as ferric hydroxide (Fe(OH)3), and oxyhydroxides, such as goethite (a-Fe(O)OH), are also of industrial significance, particularly as precursors of pure or complex oxides. As for the complex iron oxides, ferrites are of particular importance as ferromagnetic materials.
Like many other nanoparticles, those of iron oxides and ferrites are prepared either via wet chemical routes such as colloid chemical or sol-gel methods or by dry processes such as vapor deposition techniques. Usually, nanoparti-cles are not shaped by conventional granulometrical methods. Instead, they are most frequently arranged to films, so that many preparation methods of thin films are regarded as those for nanoparticles as well. It is often desired to assemble or pattern iron oxide nanoparticles to give magnetic or optomagnetic functions.
This review summarizes the preparation, magnetic properties, and some representative examples for application of the pure and complex iron oxides.
PREPARATION OF PARTICULATES
Concepts
Morphological states of nanoparticles are generally divided into two, i.e., matrix-supported and self-supported. It is also important to select whether the produced primary particles should be well dispersed or aligned. While self-supported or free particles are prepared either by wet chemical or by vaporization-condensation processes, supported particles are prepared by the methods usually used for making thin films, as mentioned in ”Introduction.”
Preparation of Pure Oxides via Colloid Chemical Routes
Preparation of iron oxide ultrafine particles has been developed in view of obtaining magnetic fluids, whereby iron oxide nanoparticles are usually produced via a surface modification after the formation of magnetic nanoparticles.[1,2] A typical preparation method of magnetite nanosol is to wash magnetite precipitates by very diluted HCl and put into a soap solution, e.g., 1% aqueous solution of sodium oleate.[3] It is possible, however, to peptize a nanosol without using any organic species. The method is quite simple: A mixed aqueous solution of ferric and ferrous chlorides (e.g., 40 ml of 1 M FeCl2 and 10 ml of 2 M FeCl3 in 2 M HCl) was added to an ammonia solution (e.g., 500 ml 0.7 M). When the precipitate was stirred with 2 M aqueous solution of 2 M perchloric acid, peptization was achieved by merely diluting with water.[3]
On the other hand, we have wonderful models of preparing iron oxide nanoparticles in nature. Many wandering animals have inborn abilities to orient themselves by magnetic particles, mostly magnetite. They are formed generally from soluble precursors within a soft-tissue matrix.[4] The difference from common preparation methods of magnetic fluids by using soaps or similar surfactants and biological formation is the growth of the magnetite nanoparticles in the presence of polymers in the latter. It is therefore reasonable to prepare magnetic nanoparticles in some aqueous polymer dispersion. One of the examples is to precipitate magnetite particles by adding ferrous and ferric chlorides in an aqueous solution of poly(vinyl chloride) containing NaOH to set the initial pH value as high as 13.8. Well-dispersed, phase pure magnetite particles of ca. 8 nm were obtained.[5,6]
When 0.1% of OH groups of poly(vinyl alcohol) (PVA) were converted to COOH groups, a polymer layer was observed on the particles; forming a cluster, they are similar to magnetotactic bacteria, which contain chains of magnetic nanoparticles.[7]
Fig. 1 XRD patterns of the prepared powder for various ultrasonic irradiation times; •: NiFe2O4.
Ferrites and Composites
Usually, ferrite nanocrystals are available only after heating at elevated temperatures. Immediate products from colloid chemical processing, most commonly by coprecipitation, are quite often either amorphous or poorly crystallized, so that they are regarded as precursors and subject to subsequent heating. It is therefore important to promote nucleation in the precursor and decrease the temperature needed for crystallization. Ultrasonication, for instance, was revealed to promote crystallization. Shin et al.[8] obtained fairly well crystallized Ni-Cu ferrite nanoparticles by ultrasonication. After ultrasonication for 5 hr, distinct crystalline ferrite powders were obtained, as shown in Fig. 1. By prolonged irradiation (for 25 hr), its crystallinity became higher than those heated at 500°C for 2 hr.
Reactant solutions are often confined into a microspace to guarantee limited growth of the particles to obtain well-dispersed nanoparticles. Inverse micelle or microemulsion methods are typical examples under these concepts. Barium ferrite nanoparticles were prepared in a micro-emulsion dispersed in n-octane by using cetyl trimethyl ammonium bromide (CTAB) as a surfactant and 1-butanol as a cosurfactant.[9] Two identical emulsions were prepared containing a mixed aqueous solution of barium and ferric nitrates in the first one and an aqueous solution of ammonium carbonate in the second. Well-dispersed precipitates of the precursor comprising particles between 3 and 8 nm were calcined at 950°C for 12 hr to obtain phase pure magnetoplumbite. Particle size did not exceed 100 nm despite fairly severe heating condition.
An alternative preparation of ferrite nanoparticles is autocombustion. Mg-Zn ferrites doped with Cu were prepared from a mixed solution comprising nitrates of respective metallic species and citric acid.[10] Dried gel obtained by heating the solution at 135°C under constant stirring was subsequently ignited to commence self-propagating combustion, which ended up with the formation of fairly well crystallized loose powder. Average particle size was 56 nm determined from a XRD profile by Scherrer method.[11]
Iron oxide-metal nanocomposites were obtained from ethanolic solution of ferric nitrate by slow addition of propylene oxide to occur rapid gelation.[12] When other fine-grained metal powders (e.g., Al) were added to the stirred Fe(III)/epoxide solution just before gelation, rigid composite gel was obtained. Slow evaporation or supercritical extraction with CO2 brought about xerogel. As shown in Fig. 2, the microstructure of the monolithic xerogel comprises interconnected clusters of hematite (3-10 nm) with well-dispersed Al powers of ca. 30 nm.
Fig. 2 (a) HRTEM of Fe2O3/ultrafine-grained Al powers xerogel nanocomposite and (b) SAED pattern of the labeled Al particle in (a).
ORIENTATION AND PATTERNING
Concepts
One of the most important properties of the ensemble of magnetic particles is their coherency or chain-like clustering. Anisotropic aggregation, in turn, inevitably results in the anisotropy of the magnetic properties. Patterning and two-dimensional assemblage, on the other hand, are not exclusive for magnetic particles. Many functional materials could be prepared, however, from these techniques on iron oxides as well. Some typical examples are given below.
Thin Films with Aligned Magnetic Nanoparticles
Starting from a commercially available water-based magnetic fluid comprising Fe3O4 particles of ca. 10 nm, PVA was added and the viscosity was adjusted by water evaporation.[13] Spin-coated films were then formed in the magnetic field up to 5 kOe. Orientation of the chain clusters with increasing the magnetic field is visible in Fig. 3. The separation of the chain cluster was estimated to be 0.27 mm regardless of the magnetic field.
Fig. 3 Photographs of films for three different alignment field strengths Hd: (a) 5.0 kOe; (b) 0.1 kOe; and (c) 0 kOe.
Fig. 4 Change in the susceptibility, w, of films with Hd.
Magnetic susceptibility was measured in the field parallel (||) and perpendicular (.) to that during the film preparation. As shown in Fig. 4, their deviation becomes larger with increasing the applied field during film formation.
The film serves as an optical grating because of the angle dependence of the transmittance measured by using 628 nm linearly polarized light. Optical anisotropy was revealed to be proportional to the magnetic anisotropy. This kind of materials can serve as a light-controlling device utilizing magneto-optical effects.
Two-Dimensional Array
Not only one-dimensional assembly, as mentioned in the previous section, but also two-dimensional array of nanoparticles gives rise to many new functions. Use of self-organizing properties of organic molecules is a rational method to realize such an assemblage. Ferritin, being a popular iron-containing protein, is appropriate for such a purpose.[14] Ferritin was obtained from a horse spleen by adding CdSO4. On the surface of dilute ferritin solution containing NaCl and phosphate buffer, poly-1-benzyl-L-histidine was spread after solubilization in chloroform. After a heat treatment, well-ordered two-dimensional array of ferritin molecules was obtained at the air-water interface. The array was easily transferred to different substrates, i.e., Si single crystals. By subsequent heat treatment up to 700°C in nitrogen, protein shell of the ferritin molecules was eliminated to obtain FeO nanopar-ticles.[14] Closer examination by FT-IR and thermogravi-metry, it turned out that FeO was obtained after heating at above 450°C. Heating at higher temperatures tends to break assemblage, as shown in Fig. 5.
Fig. 5 H-SEM images of arrays of the ferritin molecules (a) before and (b-d) after heat treatment under nitrogen at 300°C, 500°C, and 700°C for 1 h, respectively.
The array of iron oxide cores can be observed after heating up to 500°C. As SEM cannot visualize the protein shell of the ferritin molecule, AFM study in contact mode is necessary to examine the existence of the shell and subunits of the ferritin molecule.[14]
In Situ Patterning
Microscopic patterning by ion oxides was made by using the hydrophilic self-assembled organic monolayers and subsequent microcontact printing (p.CP).[15] Stamps with patterned relief structure were prepared by mixing poly(dimethyl siloxane) (PDMS) with a curing agent at room temperature and subsequently cured at 100°C. Chrome and gold layers were sputtered. For inking, the stamps were dipped into a dilute ethanolic solution of hexadekanethiol. After curing the inked stamps, they were conformably contacted to the gold surface to create hydrophobic self-assembled monolayers (SAMs). The samples were dipped into hydrophilic SAMs solutions to deposit dithiothreitol SAMs for the area, where hexade-kanethiol was not stamped. Selective deposition of iron oxide films was then carried out from an iron nitrate aqueous solution. By annealing under different conditions, patterned a- or g-Fe2O3 or Fe3O4 nanoparticles were obtained with a line width as narrow as 1 p.m.[15]
MAGNETIC PROPERTIES
Concepts
Downsizing of magnetic particles is accompanied by the decrease in the saturation magnetization. The mechanisms of such magnetic degradation are manifold but are somehow associated with increases in the specific surface and lattice defects. Some data with this context are given with possible strategy of preventing size-dependent degradation.
Mossbauer spectroscopy is a versatile tool for iron-containing materials. The second half of this article deals with the use of Mossbauer spectra in the analyses of iron oxide nanoparticles.
