DOI 10.1515/ntrev-2012-0061 Nanotechnol Rev 2013; 2(3): 333–357 Review Daniel Amara and Shlomo Margel * Synthesis and characterization of elemental iron and iron oxide nano/microcomposite particles by thermal decomposition of ferrocene Abstract: The unique chemical and physical properties difficult to test without an independent measure of the of the nano and microscale materials have led to impor- particle sizes and shapes. With the application of the elec- tant roles in the several scientific and technological fields. tron microscopy to these systems in the 1940s and 1950s, The magnetic nano/microparticles are of great interest the particle sizes, shapes, and distribution information because of its potential applications in, e.g., hyperther- could be readily determined. There was a renewed inter- mia, magnetic resonance imaging (MRI), catalytic appli- est in the finely divided magnetic iron and iron oxides, cations, etc. The decomposition of iron pentacarbonyl as the properties could now be correlated with the sizes is one of the most common methods for the preparation and shapes of the particles. By the early 1960s, the theory of magnetic iron oxide and iron nanoparticles. However, describing the magnetism of the iron nanoparticles was Fe(CO)5 is severely toxic and alternative precursors should fully formed and had been largely confirmed by experi- be used. Here, we describe the recent advances in the ments [1, 2] . The research on iron and iron oxide nano- synthesis and characterization of the elemental iron and particles has continued since then, but has experienced iron oxide nano/microcomposite particles by the thermal a surge in interest in the recent two decades. This is likely decomposition of ferrocene. The described synthesis pro- due to the new synthetic techniques as well as the interest cess is based on simple nontoxic approaches including, in the new applications of iron and iron oxide nanopar- for example, a solventless process. The particle size and ticles [3 – 6] . The reason for the great interest in the zero- size distribution as well as their composition, crystallin- valent iron nanoparticles arise from its special magnetic ity, shape, and magnetic properties can be controlled via properties [7] . Table 1 shows some of the properties of the the synthesis conditions. ferromagnetic elements; the table clearly demonstrates that iron is the most practical among the various mag- Keywords: Fe nanoparticles; iron oxide nanoparticles; netic elements. Although gadolinium has a higher satura- magnetic nanocomposite; thermal decomposition of tion magnetization magnetic saturation moment (Ms) at ferrocene. 0 K, it has a Curie temperature (Tc) just below the room temperature, making it impractical for use in the majority of the applications. Iron is the most useful among the fer- *Corresponding author: Shlomo Margel, Institute of romagnetic elements; it has the highest magnetic moment Nanotechnology and Advanced Materials, Department of Chemistry, Bar Ilan University, 52900 Ramat Gan, Israel , at room temperature, and a curie temperature that is high e-mail: [email protected] enough for the vast majority of practical applications. In Daniel Amara: Institute of Nanotechnology and Advanced Materials, addition, iron is a widespread element and, therefore, sig- Department of Chemistry, Bar Ilan University, 52900 Ramat Gan, nificantly cheaper than the other ferromagnetic elements Israel such as nickel and cobalt. In addition to this, iron is a very soft magnetic material, in particular, in comparison to cobalt, which has the second-highest room temperature Ms value. In addition, iron also has a low magnetocrys- 1 Introduction talline anisotropy [8, 9] , which is part of what makes the iron nanoparticles such an attractive material to work The magnetic nanoparticles have been studied for many with. The sufficiently small magnetic nanoparticles years. Theories were devised to describe the expected mag- show a superparamagnetic behavior, and the maximum netic properties of the magnetic nanoparticles, but were volume particle that can be superparamagnetic at a given 334 D. Amara and S. Margel: Synthesis and characterization of elemental iron Table 1 Properties of the ferromagnetic elements. and iron oxide nanoparticles are typically prepared by the decomposition of the soluble iron precursors in solution Element M (emu/g) T (K) S c containing a stabilizer [34] . The decomposition of the iron Fe 218 1043 precursors is accomplished by means such as sonochem- Co 161 1388 istry [35 – 37] , thermal decomposition [38] , electrochemical Ni 54 627 [39] , and laser decomposition [40] . Among the iron pre- cursors, iron carbonyl compounds are very useful, as they can easily be dissociated, and CO is a labile ligand that can temperature varies directly with the magnetocrystalline be removed from the reaction mixture [23, 41] . One of the anisotropy. This means that much larger iron nanoparti- most common method for the preparation of iron oxide cles are superparamagnetic than is the case with cobalt and iron nanoparticles is based on the decomposition of [7, 9] . The moment that a superparamagnetic iron particle iron pentacarbonyl in the organic continuous phase [34, can exhibit (which is the product of the moment per atom 42] . However, Fe(CO) is severely toxic, which is of concern and the number of atoms) is, therefore, much larger than is 5 because of its volatility [43] . This review demonstrates the possible in the case of any other metal. However, the main synthesis of magnetic iron and iron oxides nano/micro- shortcoming of iron is its reactivity [10 – 12] , especially at composite particles with various shapes, by the thermal ambient atmosphere (water and oxygen). This general decomposition of ferrocene. The described synthesis pro- weakness is greatly multiplied in the case of the iron nano- cesses posses simple nontoxic approaches, among them a particles, where the iron rapidly and completely oxidizes solventless process, which is a highly environmental and in air [10] . Maintaining the iron nanoparticles in its zero- economical process. valent state generally limits it to the applications where water and oxygen are largely excluded, or it is maintained in a reducing atmosphere. However, the extreme reactivity of the iron nanoparticles can be beneficial in a non-oxidiz- 2 Synthesis of magnetic ing environment. There are already a number of examples microspheres of iron nanoparticles as catalysts [13 – 17] , and certainly, more will be developed in the near future. In particular, the use of iron as a catalyst primarily involves making 2.1 Synthesis of porous superparamagnetic and breaking the carbon-carbon bonds [18] . The forma- and ferromagnetic composite micrometer- tion or cleavage of the carbon-carbon bonds is critical for sized particles of narrow size distribution an enormous number of industrially important chemical prepared by solventless thermal transformations [19] , from the production of clean fuels decomposition of ferrocene [22] to the production of carbon nanotubes (CNTs) [20] . In order to prevent the iron tendency to be oxidized, the Fe The uniform micrometer-sized PS/PDVB (PS/polydivinyl particles should be protected by a protective layer, e.g., benzene) composite particles were formed by a single-step carbon [21 – 23] , silica [24, 25] , alumina [26] , etc. Recently, swelling process at room temperature of the PS template a few publications describe the synthesis of iron oxide particles of 2.35 ± 0.1 μ m (prepared as described in the magnetic silica particles by entrapment of iron nitrate literature) [44] with dibutyl phthalate (DBP) (a swelling within the mesopores of the silica particles, followed by solvent), droplets containing divinyl benzene (DVB) and impregnation with ethylene glycol and then annealing benzoyl peroxide (BP) (DVB as a crosslinker monomer and at 450° C [27] . Although iron oxide posses poor magnetic BP as an initiator), followed by the polymerization of the properties compared to the zero-valent iron [23] , magnet- DVB within the swollen PS template particles at elevated γ ite (Fe3 O4 ) and maghemite ( – Fe2 O3 ) particles are useful temperature. In a typical experiment, the PS template for biomedical applications [28, 29] . The applications of microspheres of 2.35 ± 0.1 μ m were swollen up to 7.6 ± 0.6 μ m these iron oxide particles rely upon its biocompatibility by adding to a 20-ml vial containing 10 ml of a sodium [30] . The size, shape, composition, and structure of the dodecyl sulfate (SDS) aqueous solution (0.75% w/v) and magnetic particles are the key factors that determine their 1.5 ml of DBP containing 10 mg of BP and 1.5 ml of DVB. The magnetic properties (ferromagnetic, superparamagnetic, emulsion droplets of the swelling solvent were then formed etc.) [4, 31 – 33] . Thus, there is a great interest in the devel- by sonication (Sonics and Materials, model VCX-750, opment of simple and economical synthesis methods for Ti-horn 20 kHz) of the mixture for 1 min. An aqueous dis- the preparation of iron and iron oxides nanoparticles. Iron persion (3.5 ml) of the PS template microspheres (7% w/v) D. Amara and S. Margel: Synthesis and characterization of elemental iron 335 α ° ° was then added to the stirred DBP emulsion. After the PDVB/ -Fe2 O3 microspheres at 500 C and 800 C, respec- swelling of the PS particles was completed, and the tively, under argon atmosphere. Figure 1 summarizes the mixture did not contain any small droplets of the emul- preparation scheme of the uniform superparamagnetic sified swelling solvent, as verified by optical microscopy, PDVB/iron oxide, C/iron oxide, and C/Fe3 O4 /Fe compo site the diameter of the swollen microspheres was meas- microspheres. First, PS/PDVB composite particles were ured. For the polymerization of the monomers within the prepared by a single-step swelling of the uniform PS tem- swollen particles, the temperature of the shaken vial con- plate microspheres dispersed in an aqueous continuous taining the swollen particles was raised to 73° C for 24 h.