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Nanotechnology Reviews

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Volume 2, Issue 3

Issues

Synthesis and characterization of elemental iron and iron oxide nano/microcomposite particles by thermal decomposition of ferrocene

Daniel Amara
  • Institute of Nanotechnology and Advanced Materials, Department of Chemistry, Bar Ilan University, 52900 Ramat Gan, Israel
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Shlomo Margel
  • Corresponding author
  • Institute of Nanotechnology and Advanced Materials, Department of Chemistry, Bar Ilan University, 52900 Ramat Gan, Israel
  • Email
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
Published Online: 2013-05-01 | DOI: https://doi.org/10.1515/ntrev-2012-0061

Abstract

The unique chemical and physical properties of the nano and microscale materials have led to important roles in the several scientific and technological fields. The magnetic nano/microparticles are of great interest because of its potential applications in, e.g., hyperthermia, magnetic resonance imaging (MRI), catalytic applications, etc. The decomposition of iron pentacarbonyl is one of the most common methods for the preparation of magnetic iron oxide and iron nanoparticles. However, Fe(CO)5 is severely toxic and alternative precursors should be used. Here, we describe the recent advances in the synthesis and characterization of the elemental iron and iron oxide nano/microcomposite particles by the thermal decomposition of ferrocene. The described synthesis process is based on simple nontoxic approaches including, for example, a solventless process. The particle size and size distribution as well as their composition, crystallinity, shape, and magnetic properties can be controlled via the synthesis conditions.

Keywords: Fe nanoparticles; iron oxide nanoparticles; magnetic nanocomposite; thermal decomposition of ferrocene

1 Introduction

The magnetic nanoparticles have been studied for many years. Theories were devised to describe the expected magnetic properties of the magnetic nanoparticles, but were difficult to test without an independent measure of the particle sizes and shapes. With the application of the electron microscopy to these systems in the 1940s and 1950s, the particle sizes, shapes, and distribution information could be readily determined. There was a renewed interest in the finely divided magnetic iron and iron oxides, as the properties could now be correlated with the sizes and shapes of the particles. By the early 1960s, the theory describing the magnetism of the iron nanoparticles was fully formed and had been largely confirmed by experiments [1, 2]. The research on iron and iron oxide nanoparticles has continued since then, but has experienced a surge in interest in the recent two decades. This is likely due to the new synthetic techniques as well as the interest in the new applications of iron and iron oxide nanoparticles [3–6]. The reason for the great interest in the zero-valent iron nanoparticles arise from its special magnetic properties [7]. Table 1 shows some of the properties of the ferromagnetic elements; the table clearly demonstrates that iron is the most practical among the various magnetic elements. Although gadolinium has a higher saturation magnetization magnetic saturation moment (Ms) at 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 ferromagnetic elements; it has the highest magnetic moment at room temperature, and a curie temperature that is high enough for the vast majority of practical applications. In addition, iron is a widespread element and, therefore, significantly cheaper than the other ferromagnetic elements 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 magnetocrystalline anisotropy [8, 9], which is part of what makes the iron nanoparticles such an attractive material to work with. The sufficiently small magnetic nanoparticles show a superparamagnetic behavior, and the maximum volume particle that can be superparamagnetic at a given temperature varies directly with the magnetocrystalline anisotropy. This means that much larger iron nanoparticles are superparamagnetic than is the case with cobalt [7, 9]. The moment that a superparamagnetic iron particle can exhibit (which is the product of the moment per atom and the number of atoms) is, therefore, much larger than is possible in the case of any other metal. However, the main shortcoming of iron is its reactivity [10–12], especially at ambient atmosphere (water and oxygen). This general weakness is greatly multiplied in the case of the iron nanoparticles, where the iron rapidly and completely oxidizes in air [10]. Maintaining the iron nanoparticles in its zero-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-oxidizing environment. There are already a number of examples 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 and breaking the carbon-carbon bonds [18]. The formation or cleavage of the carbon-carbon bonds is critical for an enormous number of industrially important chemical transformations [19], from the production of clean fuels to the production of carbon nanotubes (CNTs) [20]. In order to prevent the iron tendency to be oxidized, the Fe particles should be protected by a protective layer, e.g., carbon [21–23], silica [24, 25], alumina [26], etc. Recently, a few publications describe the synthesis of iron oxide magnetic silica particles by entrapment of iron nitrate within the mesopores of the silica particles, followed by impregnation with ethylene glycol and then annealing at 450°C [27]. Although iron oxide posses poor magnetic properties compared to the zero-valent iron [23], magnetite (Fe3O4) and maghemite (γ–Fe2O3) particles are useful for biomedical applications [28, 29]. The applications of these iron oxide particles rely upon its biocompatibility [30]. The size, shape, composition, and structure of the magnetic particles are the key factors that determine their magnetic properties (ferromagnetic, superparamagnetic, etc.) [4, 31–33]. Thus, there is a great interest in the development of simple and economical synthesis methods for the preparation of iron and iron oxides nanoparticles. Iron and iron oxide nanoparticles are typically prepared by the decomposition of the soluble iron precursors in solution containing a stabilizer [34]. The decomposition of the iron precursors is accomplished by means such as sonochemistry [35–37], thermal decomposition [38], electrochemical [39], and laser decomposition [40]. Among the iron precursors, iron carbonyl compounds are very useful, as they can easily be dissociated, and CO is a labile ligand that can be removed from the reaction mixture [23, 41]. One of the most common method for the preparation of iron oxide and iron nanoparticles is based on the decomposition of iron pentacarbonyl in the organic continuous phase [34, 42]. However, Fe(CO)5 is severely toxic, which is of concern because of its volatility [43]. This review demonstrates the synthesis of magnetic iron and iron oxides nano/microcomposite particles with various shapes, by the thermal decomposition of ferrocene. The described synthesis processes posses simple nontoxic approaches, among them a solventless process, which is a highly environmental and economical process.

Table 1

Properties of the ferromagnetic elements.

2 Synthesis of magnetic microspheres

2.1 Synthesis of porous superparamagnetic and ferromagnetic composite micrometer-sized particles of narrow size distribution prepared by solventless thermal decomposition of ferrocene [22]

The uniform micrometer-sized PS/PDVB (PS/polydivinyl benzene) composite particles were formed by a single-step swelling process at room temperature of the PS template particles of 2.35±0.1 μm (prepared as described in the literature) [44] with dibutyl phthalate (DBP) (a swelling solvent), droplets containing divinyl benzene (DVB) and benzoyl peroxide (BP) (DVB as a crosslinker monomer and BP as an initiator), followed by the polymerization of the DVB within the swollen PS template particles at elevated temperature. In a typical experiment, the PS template microspheres of 2.35±0.1 μm were swollen up to 7.6±0.6 μm by adding to a 20-ml vial containing 10 ml of a sodium dodecyl sulfate (SDS) aqueous solution (0.75% w/v) and 1.5 ml of DBP containing 10 mg of BP and 1.5 ml of DVB. The emulsion droplets of the swelling solvent were then formed by sonication (Sonics and Materials, model VCX-750, Ti-horn 20 kHz) of the mixture for 1 min. An aqueous dispersion (3.5 ml) of the PS template microspheres (7% w/v) was then added to the stirred DBP emulsion. After the swelling of the PS particles was completed, and the mixture did not contain any small droplets of the emulsified swelling solvent, as verified by optical microscopy, the diameter of the swollen microspheres was measured. For the polymerization of the monomers within the swollen particles, the temperature of the shaken vial containing the swollen particles was raised to 73°C for 24 h. The produced composite microspheres were then washed from undesired reagents by the extensive centrifugation cycles with water, ethanol, and water again. The obtained particles were then dried by lyophilization.

The uniform crosslinked micrometer-sized PDVB particles were prepared by dissolving the PS template part of the PS/PDVB composite particles with Dimethylformamide (DMF). Briefly, the PS/PDVB particles prepared as described in the previous paragraph were dispersed in 50 ml of DMF and then shaken at room temperature for ca. 15 min. The dispersed particles were then centrifuged, and the supernatant containing the dissolved PS template polymer was discarded. This procedure was repeated five times with DMF. The obtained PDVB particles were washed twice with ethanol and water and then dried by lyophilization. The PDVB/α-Fe2O3 composite particles were prepared as follows: 100 mg of the dried PDVB microspheres was added to a two-neck round bottom flask equipped with a septum. The flask was then evacuated using an oil vacuum pump for 1 h, after which 1 ml of a 0.25-m solution of FeCl2·4H2O THF was injected into the flask. The microspheres containing the iron salts were then dried. The PDVB/α-Fe2O3 microspheres were then produced by annealing these microspheres at 250°C under ambient atmosphere. C/Fe3O4 and the C/Fe microspheres were formed by annealing the PDVB/α-Fe2O3 microspheres at 500°C and 800°C, respectively, under argon atmosphere. Figure 1 summarizes the preparation scheme of the uniform superparamagnetic PDVB/iron oxide, C/iron oxide, and C/Fe3O4/Fe composite microspheres. First, PS/PDVB composite particles were prepared by a single-step swelling of the uniform PS template microspheres dispersed in an aqueous continuous phase with the emulsion droplets of DBP containing the initiator BP and the crosslinker monomer DVB. The PS/PDVB composite particles of narrow size distribution were then formed by polymerizing the DVB within the swollen PS template microspheres at 73°C. The uniform porous PDVB microspheres were then prepared by the dissolution of the PS template part of the former composite particles. The uniform superparamagnetic PDVB/iron oxide composite microspheres were then prepared by entrapping by vacuum sodium acetate and then ferrocene within these microspheres. At this stage, the solid mixture was annealed in a sealed cell at 300°C for 2 h under ambient atmosphere. The produced superparamagnetic PDVB/iron oxide composite microspheres were then washed of the excess reagents, e.g., sodium acetate, with an excess mixture of water and THF (2:1 v/v). The ferromagnatic C/Fe3O4 and C/Fe3O4/Fe composite microspheres were formed by annealing the PDVB/iron oxide composite particles at 500°C or 700°C, respectively, for 2 h under inert atmosphere. Figure 2 shows, by the low and high magnification of the SEM pictures, the perfect spherical shape and narrow size distribution of the PS (A and B) and the PDVB (C and D) microspheres. The size and size distribution of these particles are 2.35±0.1 and 5.5±0.1 μm, respectively. Figure 2 also illustrates that the surface of the PS template microspheres has a smooth nonporous morphology (B), while that of the PDVB microspheres is rough and porous (D). The increased roughness and porosity of the PDVB particles are expressed by an increased surface area from 2.7 m2/g, for the PS template microspheres, to 657 m2/g for the PDVB microspheres (see Table 2). The measured surface area of the PS template microspheres with a 2.45-μm average diameter is similar to the calculated surface area of the spherical-shaped particles with a density of 1.0 g/ml and the same size (A=4πr2), indicating the nonporous structure of these PS microspheres. The high surface area and porosity of the PDVB microspheres are probably due to two main reasons. First, the swelling solvent DBP serves as a porogen that forms macropores. Thus, the pores are formed in the spaces where the porogen was extracted from the polymer particles [21]. Second, the dissolution of the PS template part of the composite PS/PDVB microspheres also generates the macropores within the PDVB particles. Figure 3 presents the low and high magnification SEM images of the composite microspheres annealed at 300°C (A and B), 500°C (C and D), and 700°C (E and F). Table 2 summarizes the size, surface area, and elemental composition, including the [Fe[/[O] mole ratio of the PDVB and these composite microspheres. Table 2 shows that the annealing of the PDVB particles containing the ferrocene at 300°C did not change the size and size distribution of the particles (5.5±0.1 and 5.5±0.2 μm, respectively), while the surface area decreased (657 and 512 m2/g, respectively). These results may indicate that the ferrocene mainly penetrated into the PDVB particles’ pores, so that the iron oxide formed at 300°C was generated there, thus leading to the same diameter and decreased surface area. Figure 3 and Table 2 also illustrate the spherical shape and the decrease in the diameter and the surface area of the composite microspheres as a function of the increased annealing temperature, e.g., the size of the particles annealed at 300°C, 500°C, and 700°C is 5.5±0.2, 2.7±0.4, and 1.7±0.3 μm, respectively, and the surface area is 512, 342, and 145 m2/g, respectively. The decrease in the size is due to the increased thermal degradation of the microspheres as the annealing temperature increases, as illustrated in Table 2 by the significant decrease in the C and H contents, e.g., the C content for the particles annealed at 300°C decreased from 69.7% to 56.3% and 26.3% for the particles annealed at 500°C and 700°C, respectively. This decomposition of the particles as the annealing temperature increases leads to the collapse of the inner structure of the particles, resulting, thereby, in the decrease in the surface area of these composite particles. The XRD patterns and Mossbauer spectroscopy measurements of the composite microspheres were demonstrated in the iron oxide phases, which are not well crystallized for the particles obtained at 300°C and 500°C. However, it was impossible to determine whether it was the magnetite (Fe3O4) or maghemite (γ-Fe2O3), due to the similarity in their unit cell parameters, which leads to a similarity in the XRD patterns of these two oxides. On the other hand, the XRD pattern of the microspheres annealed at 700°C demonstrated the crystalline materials that match the spinel Fe3O4 crystal structure and the body centered cubic (bcc) Fe structure, which was also confirmed by the Mossbauer spectroscopy. Table 2, indeed, exhibits that part of the iron oxide phase obtained for the composite particles annealed at 300°C and 500°C were partially reduced to a zero-valent iron at 700°C. This can be explained by the following reaction [45]:

The schematic scheme illustrating the synthesis of the various magnetic composite microspheres prepared by the solventless thermal decomposition of ferrocene.
Figure 1

The schematic scheme illustrating the synthesis of the various magnetic composite microspheres prepared by the solventless thermal decomposition of ferrocene.

The low- and high-magnification SEM photomicrographs of the PS (A, B) and PDVB (C, D) microspheres.
Figure 2

The low- and high-magnification SEM photomicrographs of the PS (A, B) and PDVB (C, D) microspheres.

The low and high magnification SEM images of the composite microspheres annealed at 300°C (A and B), 500°C (C and D), and 700°C (E and F).
Figure 3

The low and high magnification SEM images of the composite microspheres annealed at 300°C (A and B), 500°C (C and D), and 700°C (E and F).

Table 2

Elemental analysis, size and size distribution, and BET surface area of the PDVB, PDVB/iron oxide, C/iron oxide, and C/Fe3O4/Fe composite microspheres.a

Fe3O4+2C→3Fe+2CO2.

Part of the carbon formed by the carbonization of the PDVB microspheres (see Table 2) served as the reducing agent of the iron oxide to form the elemental Fe and CO2, as illustrated in the above equation. This formed carbon also protects the Fe, as well as the Fe3O4 particles, against the oxidation by air, thus preserving the particles’ magnetic properties. Moreover, Table 2 indicates, as previously mentioned, that the [Fe]/[0] mole ratios of the PDVB/iron oxide, C/iron oxide, and C/Fe3O4/Fe composite microspheres, are 0.5, 0.7, and 1.1, respectively. The increased [Fe]/[0] mole ratio of the C/iron oxide particles obtained by annealing the PDVB/iron oxide particles at 500°C is due to the reduction of the iron oxide at the higher oxidation state, e.g., Fe2O3, obtained at 300°C to magnetite, according to the following reaction: [45]

6Fe2O3+C→4Fe3O4+CO2.

The increased [Fe]/[0] mole ratio of the C/Fe3O4/Fe composite microspheres obtained at 700°C is due to the reduction of part of the magnetite phase to a zero-valent Fe phase, as shown in the above equation.

Figure 4A–C and Table 3 represent the magnetization properties measured at 300 and 5 K of the PDVB/iron oxide (A), C/iron oxide (B), and C/Fe3O4/Fe (C) composite microspheres obtained at 300°C, 500°C, and 700°C, respectively. Generally, increasing the annealing temperature resulted in the increase in the magnetic saturation moment (Ms) and coercivity at both 5 and 300 K. For example, at 5 K, the magnetic moments and the coercivities of the composite particles annealed at 300°C, 500°C, and 700°C were 9.8, 22.1, and 58 emu/g and 190, 370, and 340 Oe, respectively. Similarly, the hysteresis increases slightly as the annealing temperature increases, as shown in Figure 4. Table 3 and Figure 4 also show that the composite particles annealed at 300°C, 500°C, and 700°C (PDVB/iron oxide and C/iron oxide, C/Fe3O4/Fe, respectively) exhibit hysteresis when measured at 5 K. Similarly, when measured at 300 K, the particles annealed at 700°C (C/Fe3O4/Fe) also exhibit hysteresis. On the other hand, no hysteresis was observed for the particles annealed at 300°C and 500°C (PDVB/iron oxide and C/iron oxide, respectively) when measured at 300 K. These results indicate that the PDVB/iron oxide and the C/iron oxide composite particles obtained at 300°C and 500°C, respectively, may be superparamagnetic. In order to examine this possibility, temperature-dependent magnetic-moment measurements of these composite particles were made. Figure 5 shows the field cooled (FC) and zero field cooled (ZFC) curves of the PDVB/iron oxide composite microspheres measured from 5 to 300 K at 100 Oe. The two curves merge at Tb of 150 K and overlap completely at higher temperatures, indicating the superparamagnetic behavior of these microspheres. On the other hand, the ZFC and FC curves of the C/iron oxide and the C/Fe3O4/Fe composite microspheres merge above 300 K, indicating the ferromagnetic behavior of these particles (not shown here). Further evidence for the superparamagnetic nature of the PDVB/iron oxide particles and the ferromagnetic nature of the C/iron oxide and C/ Fe3O4/Fe particles may be the size of the nanoparticles entrapped within these composite microspheres. The superparamagnetic behavior is typical for the nanosized magnetic particles. The upper superparamagnetic volume (Vp) of a magnetic particle can be calculated by the following Eq [46]:

Table 3

Magnetic properties of the PDVB/iron oxide, C/iron oxide, and C/Fe3O4/Fe composite microspheres.

The magnetization (M) vs. magnetic field (H) at 300 and 5 K of the PDVB/iron oxide (A), C/iron oxide (B), and C/Fe3O4/Fe (C) composite microspheres.
Figure 4

The magnetization (M) vs. magnetic field (H) at 300 and 5 K of the PDVB/iron oxide (A), C/iron oxide (B), and C/Fe3O4/Fe (C) composite microspheres.

The zero field-cooled (ZFC) and field-cooled (Fc) temperature-dependent magnetization curves of the PDVB/iron oxide composite microspheres.
Figure 5

The zero field-cooled (ZFC) and field-cooled (Fc) temperature-dependent magnetization curves of the PDVB/iron oxide composite microspheres.

where Vp is the upper superparamagnetic volume of a magnetic particle, kB is the Boltzmann’s constant, T is the temperature, and Ku is the anisotropy constant [-1.1×105, 1.35×105, and 4.8×105 for maghemite, magnetite, and zero-valent iron, respectively]. The upper diameters of the superparamagnetic maghemite, magnetite, and zero-valent iron nanoparticles are 26, 24, and 18 nm, respectively [46]. Indeed, Figure 6 demonstrates by cross-sectional typical TEM pictures that for the PDVB/iron oxide composite particles, the iron oxide nanoparticles entrapped within the PDVB matrix are of 15–22 nm (Figure 6A–B), while for the C/iron oxide and the C/Fe3O4/Fe particles, the iron oxide nanoparticles entrapped within the C matrix are 50–70 nm (Figure 6C) and 80–120 nm (Figure 6D), respectively. The structure of the iron oxide nanoparticles was also investigated in the high-resolution electron microscope (HRTEM) mode. Figure 7 shows a cross-sectional HRTEM image of a typical single superparamagnetic nanoparticle entrapped within the PDVB matrix. This micrograph clearly demonstrates the lattice fringe of the iron oxide nanoparticle. The measured size of this nanoparticle is about 20 nm, which is in good agreement with the cross-sectional typical TEM pictures (Figure 8) that may explain the superparamagnetic behavior of the PDVB/iron oxide composite microspheres. The sodium acetate was used as a separating media between the formed iron oxide nanoparticles embedded within the PDVB matrix, thereby enabling the production of these individual supeparamagnetic particles. The sodium acetate was then washed from the magnetic particles by extensive centrifugation cycles with a mixture of THF and water. The ferromagnetic C/iron oxide and C/Fe3O4/Fe composite microspheres were then formed by annealing the superparamagnetic PDVB/iron oxide microspheres at 500°C and 700°C, respectively, under argon atmosphere. This thermal decomposition leads to the decomposition of the PDVB/iron oxide composite microspheres, thus, causing the formation of the ferromagnetic particles by the partial agglomeration of the iron oxide and Fe nanoparticles within the C matrix.

The cross-sectional TEM pictures of the PDVB/iron oxide (A and B), C/iron oxide (C), and C/Fe3O4/Fe (D) composite microspheres.
Figure 6

The cross-sectional TEM pictures of the PDVB/iron oxide (A and B), C/iron oxide (C), and C/Fe3O4/Fe (D) composite microspheres.

The cross-sectional HRTEM of a typical single superparamagnetic particle entrapped within the PDVB matrix.
Figure 7

The cross-sectional HRTEM of a typical single superparamagnetic particle entrapped within the PDVB matrix.

The schematic scheme illustrating the synthesis of the various magnetic composite microspheres prepared by the swelling process.
Figure 8

The schematic scheme illustrating the synthesis of the various magnetic composite microspheres prepared by the swelling process.

2.2 Synthesis of superparamagnetic core-shell micrometer-sized particles of narrow size distribution prepared by a swelling process [47]

The uniform micrometer-sized core-shell PS/(PDVB/ ferrocene) and PS/(P(S-DVB)/ferrocene) composite particles were formed by a room temperature single-step swelling process of the PS template particles with toluene (a swelling solvent) containing S (styrene) and DVB as monomers, AIBN as initiator, and ferrocene, followed by the polymerization of the S and/or DVB at the elevated temperature within the swollen particles. In a typical experiment, the PS template microspheres of 2.35±0.1 μm were swollen up to 7.6±0.6 μm (as was observed by light microscopy) by adding to a 20-ml vial containing 10 ml of SDS aqueous solution (0.75% w/v), 2 ml of toluene containing 10 mg AIBN, 400 mg ferrocene, and 1.5 ml DVB or 1.5 ml of a mixture of DVB and S, wherein the [DVB]/[S] volume ratio was altered: 1/1, 2/1, or 1/2. The emulsion droplets of the swelling solvent were then formed by the sonication of the mixture for 1 min. An aqueous dispersion (3.5 ml) of the PS template microspheres (7% w/v) was then added to the stirred toluene emulsion. After the swelling of the template particles was completed, and the mixture did not contain any small droplets of the emulsified swelling solvent, as verified by light microscopy, the diameter of the swollen microspheres was measured. For the polymerization of the monomers within the swollen particles, the temperature of the shaken vial containing the swollen particles was raised to 73°C for 24 h. The composite microspheres produced were then washed from the undesired reagents by the extensive centrifugation cycles with water, mixture of water and ethanol, and again water. The obtained composite particles were then dried by lyophilization.

The uniform superparamagnetic micrometer-sized PDVB/Fe3O4 oxide and P(S-DVB)/Fe3O4 composite particles were formed by annealing of the PS/(PDVB/ferrocene) and PS/(P(S-DVB)/ferrocene) composite microspheres prepared at [DVB]/[S] volume ratios of 2:1 and 1:1 at 330°C for 2h in a stainless steel sealed cell. The various microspheres were then dispersed in 50 ml of DMF and shaken at room temperature for 15 min. The dispersed particles were then centrifuged and the supernatant containing the dissolved PS template polymer was discarded. This procedure was repeated five times with DMF. The obtained PDVB/Fe3O4 and P(S-DVB)/Fe3O4 composite particles were then washed twice with ethanol and water and then dried by lyophilization. The uniform ferromagnetic micrometer-P(S-DVB)/Fe3O4 composite particles were formed similarly, substituting the volume ratio [DVB]/[S] from 2:1 and 1:1 to 1:2. The uniform C/Fe micrometer-sized particles were formed by annealing the PDVB/Fe3O4 particles at 450°C under hydrogen atmosphere for 2 h. Figure 8 summarizes the synthesis scheme through which the various uniform magnetic micrometer-sized particles were prepared. First, the uniform micrometer-sized PS/(PDVB/ferrocene) and PS/(P(S-DVB)/ferrocene) composite particles were prepared by swelling of the uniform PS template microspheres dispersed in an aqueous continuous phase with the emulsion droplets of toluene containing the initiator AIBN and the monomers S and/or DVB stabilized by SDS, followed by the polymerization of the monomer/s within the swollen template PS microspheres. The magnetic micrometer-sized PDVB/Fe3O4 and P(S-DVB)/Fe3O4 composite particles of narrow-size distribution were then formed by the decomposition of ferrocene entrapped within the various composite microspheres at 330°C in a stainless steel sealed cell, followed by the dissolution of the PS part with DMF. The magnetic uniform C/Fe micrometer-sized particles of narrow size distribution were formed by the reduction of the Fe3O4 nanoparticles entrapped within the PDVB/Fe3O4 particles with hydrogen at 450°C. Figure 9 demonstrates, by the low- and high-magnification SEM pictures, the spherical shape and narrow size distribution of the PDVB/Fe3O4 (A, B) and P(S-DVB)/Fe3O4 micrometer-sized particles prepared at [DVB]/[S] volume ratio of 1:1 (C, D). Figure 2 shows that the size of the PS particles is 2.35±0.1 μm, while that of the PDVB/Fe3O4 and P(S-DVB)/Fe3O4 particles is significantly higher and the same: 6.0±0.1 μm. It should be noted that the measured size and size distribution of the P(S-DVB)/Fe3O4 composite microspheres prepared at [DVB]/[S] volume ratios of 2:1 and 1:2 was also 6.0±0.1 μm (see Table 4). It can, therefore, be concluded that neither the size nor the size distribution of the various composite microspheres is significantly changed by the adjustment of the [DVB]/[S] volume ratios. The surface of the PS template microspheres has a smooth nonporous morphology (as showed in the previous section), while that of the PDVB/Fe3O4 and PDVB/P(S-DVB)/Fe3O4 particles are rough and highly porous (B, D). The increased roughness and porosity of the PDVB-derived composite particles is expressed by the increased surface area (see Table 4) from 2.7 m2/g for the PS template microspheres to 450, 157, 96 and 84 m2/g for the PDVB/Fe3O4 and the P(S-DVB)/Fe3O4 composite particles prepared at [DVB]/[S] volume ratios of 2:1, 1:1, and 1:2, respectively. The reasons for the increase in the surface area are explained in the previous section. Table 4 demonstrates the mass % of C, H, O, and Fe, as well as the size and size distribution of the different composite microspheres. A simple calculation indicates that the weight ratio of [Fe]/[O] in magnetite is 2.6/1. Thus, the % magnetite content of the PDVB/Fe3O4 and the P(S-DVB)/Fe3O4 composite particles obtained at [DVB]/[S] volume ratios of 2:1, 1:1, and 1:2 are 12.9%, 12.3%, 14.1%, and 16.1%, respectively. The [Fe]/[O] weight ratios values for the various composite microspheres are 2.6, 2.6, 2.7, and 2.5, respectively. Thus, the elemental analysis supplies further evidence that the iron oxide phase of the entrapped nanoparticles within the various composite microspheres is mainly magnetite, as also demonstrated by the FTIR and XRD measurements. The weight ratio of [C]/[H] for these samples are 12.2, 11.9, 11.6, and 11.9, respectively. These ratios are almost the same as those calculated for the pure PS and PDVB ((C10H10)n and (C8H8)n, respectively). Figure 10 demonstrates the typical low- and high-magnification cross-sectional TEM pictures of the PDVB/Fe3O4 (A, B) and P(S-DVB)/Fe3O4 composite microspheres prepared at [DVB]/[S] volume ratio of 1:1 (C, D). These pictures clearly demonstrate the presence of the Fe3O4 nanoparticles of 16.4±2.1 and 23±3.2 nm diameter, respectively, caged in the entire PDVB matrix. The measured size and size distribution of the nanoparticles entrapped within the P(S-DVB)/Fe3O4 composite microspheres prepared at the [DVB]/[S] volume ratios of 2:1 and 1:2 were 19±3.5 and 34±2.9 as shown in Table 5. It can be deduced from the size measurements of the entrapped magnetic nanoparticles within the various composite microspheres that the size of the entrapped Fe3O4 nanoparticles within the composite particles decreases as the DVB content increases. This can be explained by the entrapment of the ferrocene molecules by the PDVB or P(S-DVB) matrices. These rigid matrices may limit the growth of the Fe3O4 nanoparticles. Thus, the particle size is influenced by the DVB content, so that the Fe3O4 nanoparticle size can be controlled by adjusting the [DVB]/[S] volume ratio during the PS particles swelling process.

Table 4

Size and size distribution and BET surface area of the C/Fe and the PDVB/Fe3O4 and P(S-DVB)/Fe3O4 composite microspheres prepared in the presence of different [DVB]/[S] volume ratios.a

Table 5

Magnetic properties of the PDVB/Fe3O4, P(S-DVB)/Fe3O4 prepared in the presence of the different [DVB]/[S] volume ratios and the C/Fe composite microspheres.a

The low and high magnification SEM pictures of the PDVB/Fe3O4 (A, B), P(S-DVB)/Fe3O4 micrometer-sized particles prepared at [DVB]/[S] volume ratio of 1:1 (C, D) and of the C/Fe micrometer-sized composite particles (E, F).
Figure 9

The low and high magnification SEM pictures of the PDVB/Fe3O4 (A, B), P(S-DVB)/Fe3O4 micrometer-sized particles prepared at [DVB]/[S] volume ratio of 1:1 (C, D) and of the C/Fe micrometer-sized composite particles (E, F).

The cross-sectional TEM pictures of the PDVB/Fe3O4 (A–B) and the P(S-DVB)/Fe3O4 composite microspheres prepared at the [DVB]/[S] volume ratio of 1:1 (C, D) and of the C/Fe micrometer-sized composite particles (E, F).
Figure 10

The cross-sectional TEM pictures of the PDVB/Fe3O4 (A–B) and the P(S-DVB)/Fe3O4 composite microspheres prepared at the [DVB]/[S] volume ratio of 1:1 (C, D) and of the C/Fe micrometer-sized composite particles (E, F).

The air-stable C/Fe composite micrometer-sized particles of narrow size distribution were formed by annealing the PDVB/Fe3O4 particles at 450°C under a H2 atmosphere. The typical low- and higher-magnification SEM pictures of the C/Fe composite particles are presented in Figure 2E–F. These figures clearly show the rough porous structure of the C/Fe composite microspheres. Table 4 indicates that the formation of these C/Fe composite particles from the PDVB/Fe3O4 particles leads to the decrease in their size from 6.0±0.1 to 4.2±1.4 μm and in the surface area from 450 to 147 m2/g, respectively. This change in the size, roughness, and surface area can be attributed to the thermal degradation of the PDVB part in the composite particles, which leads to the collapse of the rough porous structure of the cross-linked microspheres. The pattern matches the crystal structure of bcc Fe. The XRD pattern demonstrated that the Fe3O4 nanoparticles entrapped within the PDVB/Fe3O4 microspheres were reduced to the zero-valent iron due to the hydrogen treatment at an elevated temperature. This can be explained by the following reaction [48]:

Fe3O4+4H2→3Fe+4H2O.

Figure 10E–F demonstrate the typical low (E) and high (F) magnification cross-sectional TEM pictures of the C/Fe composite microspheres. These pictures clearly show the presence of the iron nanoparticles of 17.9±3.9 nm diameter caged in the entire carbon matrix (see Table 5). As previously discussed, the C/Fe composite particles were prepared by reducing the Fe3O4 entrapped within the PDVB/Fe3O4 composite particles at 450°C. The comparison of Figure 10E–F with Figure 10A–B clearly illustrates that the size of the nanoparticles within the composite particles’ matrix was relatively retained, and the hydrogen reduction at 450°C did not lead to damage or aggregation of these nanoparticles. We believe that the rigid cage of the PDVB surrounding the entrapped Fe3O4 nanoparticles retained their size even at elevated temperatures such as 450°C in a hydrogen atmosphere. The Ms as well as the coercive fields obtained are summarized in Table 6. At 300 K, the superparamagnetic behavior was observed for the PDVB/Fe3O4, P(S-DVB)/Fe3O4 prepared at [DVB]/[S] volume ratios of 2:1 and 1:1 due to the small size of the nanoparticles entrapped within the various composites. However, for the P(S-DVB)/Fe3O4 prepared at the [DVB]/[S] volume ratio of 1:2, the ferromagnetic behavior was observed. The measured size and size distribution of the nanoparticles entrapped within this composite microspheres is 34±2.9 nm, which is higher from the upper diameter of a spherical superparamagnetic magnetite particles (24 nm as demonstrated previously). The Ms values obtained at 5 K for the PDVB/Fe3O4, P(S-DVB)/Fe3O4 prepared at the [DVB]/[S] volume ratios of 2:1, 1:1, and 1:2 and for the C/Fe micrometer-sized composite particles are 8.6, 11.2, 13.4, 14, and 37.3 emu/g, respectively (see Table 5). The merging temperature of the two ZFC/FC branches is defined as the blocking temperature (Tb) of the superparamagnetic particles. The measured Tb are summarized in Table 5. As expected, the Tb values depend directly on the size of the magnetic nanoparticles entrapped in the polymeric particles. Indeed, the PDVB/Fe3O4 possess the lowest Tb value of 85 K, whereas the Tb values for the P(S-DVB)/Fe3O4 composite microspheres prepared at the [DVB]/[S] volume ratios of 2:1 and 1:1 are 104 and 110 K. These values are very close due to the similarity in the size of these entrapped Fe3O4 nanoparticles. On the other hand, the C/Fe composite particles possess a relatively high Tb value (220 K), which can be attributed to the fact that the size of the entrapped Fe particles is very close to the upper diameter of the superparamagnetic zero-valent iron. The FC and the ZFC curves of the P(S-DVB)/Fe3O4 composite microspheres prepared at the [DVB]/[S] volume ratio of 1:2 tend to merge above the room temperature as expected for the ferromagnetic particles.

Table 6

Elemental analysis, size, and size distribution of the nanocubes/spheres obtained by annealing the different ratios of ferrocene and PVP mixtures for different time periods.a

3 Synthesis of magnetic nanoparticles

3.1 Synthesis of magnetite nanocubes and nanospheres prepared by solventless thermal decomposition of ferrocene [49]

The Fe3O4 nanocubes were formed by grinding the mixtures of ferrocene and polyvinylpyrrolidone (PVP) (of mw of 360,000) of various weight ratios (1:1, 1:2, and 1:5). Of the solid mixtures, 300 mg was then introduced into a 1-ml stainless steel sealed cell. The solid mixtures were then introduced into a tube furnace preheated to 350°C for 2 h in an ambient atmosphere. The sealed cell was then cooled to room temperature, and the resulting black powder was collected. The obtained magnetite nanocubes were then washed from the excess reagents by the extensive centrifugation cycles with ethanol. The Fe3O4 nanospheres were obtained by a similar solventless process by increasing the annealing time to 4 h at a [PVP] [ferrocene] weight ratio of 5:1. The PVP was used as a separating media and stabilizer of the formed iron oxide nanocubes/spheres. The TEM images of the nanocubes obtained by thermal decomposition at 350°C for 2 h of the solid mixtures of ferrocene and PVP of weight ratios of 1:1, 1:2, and 1:5 are presented in Figure 11A–C, respectively. The images demonstrate the cubic morphology of the obtained nano-iron oxides. Moreover, the images clearly demonstrate that the size of the nanocubes depends directly on the [ferrocene]/[PVP] weight ratio. The nanocubes’ size, as measured by the diagonal length of the cubes, decreased from 49±4 to 41±5.2 and 29±3.4 nm as the [ferrocene]/[PVP] weight ratio decreased from 1:1 to 1:2 and 1:5, respectively (see Table 6). It should be noted that the ferrocene has a boiling point of 249°C, and the various nanocubes were formed at 350°C. Thus, the decomposition of the ferrocene was accomplished in the gas phase, resulting in the formation of the nanocubes in the PVP domain. This process is actually a chemical vapor deposition (CVD) reaction in which the ferrocene is the volatile precursor, and the PVP is the solid substrate. Moreover, the TEM images clearly demonstrate the individual nanocubes for the various samples. This may suggest that the solid PVP matrix is used in this process as a separating media during the decomposition of the ferrocene to form the iron oxide nanocubes. It is remarkable that the decomposition temperature of ferrocene is above 450°C, and annealing the ferrocene at 350°C for 2 h in a sealed cell in the absence of PVP did not lead to the decomposition of the organometallic compounds. However, the thermal decomposition of ferrocene in the presence of the PVP leads to its decomposition to iron oxide nanocubes/spheres. This may imply that the PVP catalyzes the thermal decomposition of the ferrocene. Figure 12A–B show by the low- and high-magnification TEM pictures the perfect spherical shape of the nanospheres of 32±5.4 nm obtained by annealing the solid mixture of ferrocene and PVP of a 1:5 weight ratio for 4 h. On the other hand, to our surprise, the annealing of the other solid mixtures of the ferrocene and PVP of 1:1 and 1:2 weight ratios for 4 h did not alter their cubic shape to spheres. Figure 12B demonstrates the core-shell architecture of the iron oxide spherical particles. The core is composed of the iron oxide phase, while the shell is composed of the PVP. Table 6 also exhibits that the [C]:[H]:[N]:[O] (oxygen belonging to the PVP only) weight ratios of these nanocubes/spheres are 8.2:1.0:1.7:2.0, 7.9:1.0:1.5:1.8 and 8.0:1.0:1.5:1.8, respectively. These ratios are almost the same as those calculated for the pure PVP (C6H9NO). The % magnetite content of the nanocubes obtained by the thermal decomposition for 2 h of the solid mixtures of ferrocene and PVP of the weight ratios of 1:5 cannot be calculated due to the presence of the small FeO phase impurity as identified by the XRD measurements. It should be noted that the mixtures obtained after the decomposition of the various ferrocene/PVP mixtures, before ethanol washing, did not indicate the presence of ferrocene traces, as verified by the FTIR spectra. The yield of the ferrocene decomposition to iron oxide was almost 100%, whereas the excess PVP was removed by the ethanol washing. Thus, the [PVP]:[Fe] weight ratios are lower than their original ratios in the reagents as shown in Table 6. The effect of the PVP concentration and reaction time on the particle size can be explained as follows: the previous studies have demonstrated, by the FTIR spectrometry, that the PVP molecules may coordinate with the metal ions to form a stable metal-PVP complex [50, 51]. The PVP used in this study probably influences the nucleation, growth, and aggregation of the obtained magnetite crystallites, by forming the iron-PVP complex molecules. This complex formation may explain the effect of the PVP concentration on the size and morphology of the formed magnetite nanocubes/spheres. The generation of this complex inevitably increases the time for the iron atoms to reach supersaturation and to their final size. This means that the growth rate of the magnetite crystallites will decrease as its face adsorbed the PVP molecules because the crystal growth rate is generally lowered with the adsorbed polymer [52]. Moreover, the number of ‘free sites’ on the PVP surface that can be served as a bounding site to form the iron-PVP complex increases with increasing PVP concentration, thereby, resulting in the nanocubes/spheres of decreasing size. The PVP concentration and the annealing time are also probably the key factors in explaining the shape alteration of the magnetite crystallites, by effecting the crystal growth in different directions. It is known that the crystal growth rate generally decreases with the adsorbed polymer, and the crystallite morphology can be altered by the presence of the polymer specifically interacting with the crystal faces [53]. The magnetite was formed as cubes when the growth in some direction was restricted by the adsorbed PVP molecules, while one direction was free to allow growth. This led to the formation of the nanocubes. Contrarily, the magnetite nanospheres were formed when the annealing time increased, allowing the growth of the crystallites in different directions. The previous studies reported that a shift of the carbonyl band was observed in the IR spectra of the PVP in the presence of the various metal ions. According to these previous studies, this band shift is due to the interaction between the carbonyl oxygen of the PVP and the metal ions [54–56]. Indeed, the pure PVP spectrum demonstrates the C=O band at 1650 cm-1 (Figure 13A), while the various nanocubes/spheres demonstrated this carbonyl peak at 1644 cm-1, a shift of 6 cm-1 (Figure 13B). Please note that Figure 13B illustrates the carbonyl PVP peak of the magnetite nanocubes obtained by thermal decomposition at 350°C for 2 h of a solid mixture of ferrocene and PVP of a weight ratio of 1:5. However, the same carbonyl peak was also observed for the other nanocubes/spheres.

The TEM micrographs of the iron oxide nanocubes obtained by the thermal decomposition at 350°C for 2 h of solid mixtures of ferrocene and PVP of the weight ratios of 1:1 (A), 1:2 (B), and 1:5 (C).
Figure 11

The TEM micrographs of the iron oxide nanocubes obtained by the thermal decomposition at 350°C for 2 h of solid mixtures of ferrocene and PVP of the weight ratios of 1:1 (A), 1:2 (B), and 1:5 (C).

The low- (A) and high- (B) magnification TEM micrographs of the iron oxide nanospheres obtained by thermal decomposition at 350°C for 4 h of a solid mixture of ferrocene and PVP of a weight ratio of 1:5.
Figure 12

The low- (A) and high- (B) magnification TEM micrographs of the iron oxide nanospheres obtained by thermal decomposition at 350°C for 4 h of a solid mixture of ferrocene and PVP of a weight ratio of 1:5.

The FTIR spectra of the carbonyl peaks of pure PVP (A) and of the magnetite nanocubes obtained by the thermal decomposition at 350°C for 2 h of a solid mixture of ferrocene and PVP of a weight ratio of 1:5.
Figure 13

The FTIR spectra of the carbonyl peaks of pure PVP (A) and of the magnetite nanocubes obtained by the thermal decomposition at 350°C for 2 h of a solid mixture of ferrocene and PVP of a weight ratio of 1:5.

The carbonyl peak shift in the nanocubes/spheres samples probably implies the formation of the Fe-PVP complex, which directly relates to the previously suggested mechanism. The magnetic saturation moments (MS), as well as the coercive fields of these particles, are summarized in Table 7. The Ms values obtained at 300 K are 29.7, 17.3, and 9.4 emu/g, for the magnetite nanocubes obtained by the thermal decomposition of the mixtures of ferrocene and PVP of 1:1, 1:2, and 1:5 weight ratios, respectively, and 8.2 emu/g for the magnetite nanospheres. By subtracting the PVP content, the calculated MS values in terms of emu/(g of Fe3O4) are 58.2 and 61.7 for the nanocubes obtained by the thermal decomposition of the mixtures of ferrocene and PVP of 1:1 and 1:2, respectively, and 67.8 for the nanospheres (the value for the nanocubes obtained by the thermal decomposition for 2 h of a solid mixture of ferrocene and PVP of a weight ratio of 1:5 cannot be calculated due to the presence of the FeO impurity). It should be noted that the Ms bulk value of magnetite is 92 emu/g [46]. The relatively lower Ms values of the magnetic nanocubes/spheres compared to the bulk values arise from the nonmagnetic PVP content, which leads to the decrease in the magnetization per weight. Another explanation for the relatively low magnetization values is the surface effect that can occur in the case of the magnetic core and the nonmagnetic shell structures. This effect leads to the reduction in the magnetic moment by a different mechanism, e.g., the existence of a magnetically dead layer on the cubes/spheres’ surface, the existence of canted spins, or the existence of a spin glass-like behavior of the surface spins [57]. The structure of the obtained nanocubes/spheres was also investigated in HRTEM mode using either the conventional selected area electron diffraction (SAED) and nanobeam (NBD) diffraction technique or the Fourier transform analysis (FFT) of the high-resolution images, depending on the size and the orientation of the materials. The two possible oxides maghemite (γ-Fe2O3) and magnetite (Fe3O4) are structurally similar; hence, they cannot be distinguished according to their electron diffraction patterns. All our electron diffraction patterns could be indexed in terms of the FCC structure of both the maghemite and magnetite, a=8.34 Å and a=8.39 Å (PDF#000391346 and PDF#010890950), respectively. The Mössbauer spectra results provided the supporting evidence that the resulting compounds are, indeed, magnetite. Figure 14A is a HRTEM of a typical single crystalline nanocube obtained by the thermal decomposition of 1:1 weight ratio of ferrocene and PVP mixture. This figure is displaying a lattice-fringe contrast of the d022 family of planes (0.3 nm). The nanocube was identified and characterized using the SAED pattern shown in Figure 14B. This electron diffraction pattern was taken from an area of 300 nm comprising several nanocubes, and it shows a typical ring diffraction pattern as expected from the polycrystalline materials. The marked reflections correspond to the interplanar spacings, d220, d311, and d400 in the FCC structure of the magnetite Fe3O4 a=8.39 Å, and the pattern was indexed as magnetite. Figure 14C and E are the HRTEM micrographs of the individual Fe3O4 nanocubes obtained by the thermal decomposition of 1:2 and 1:5 weight ratios of the ferrocene and PVP mixtures for 2 h, respectively. Both the nanocubes display a well-resolved lattice-fringe contrast as displayed in the respective Figures (14C and D), and their identification was based on the analysis of these high-resolution images. The inset on the top right in Figure 14C is the computed Fourier transform of the portion of the image outlined by the white square which, like a diffraction pattern, was indexed on the basis of the unit cell of the magnetite. Marked are the d022 and d113 family of planes. The inset on the bottom right represents the filtered and magnified portion of the image outlined by the square. The distances measured between the lattice fringes were 0.3 nm (d022) and 0.25 nm (d113) of the cubic FCC structure of Fe3O4 (a=8.35 Å). Figure 14E shows the lattice fringe d113 (0.25 nm) plane of the nanocubes obtained by the 1:5 weight ratios of ferrocene and PVP for 2 h. Figures 14D and F are the NBD patterns taken from the nanocubes obtained by the thermal decomposition of 1:2 and 1:5 weight ratios of ferrocene and PVP for 2 h, respectively. All the NBD patterns were taken from a nano-area of 4–7 nm of the nanocubes. The NBD pattern (14D) shows the sets of reflections of the d022 planes and <d113> family of planes, and the NBD pattern (14F) shows the sets of reflections for the d222, d133, and d115 planes. These patterns were also indexed according to the FCC cubic structure of Fe3O4. Figure 14G is the HRTEM micrograph of the crystalline Fe3O4 nanosphere coated with a thin amorphous layer of PVP. The inset represents the magnified portion of the image outlined by the white square. The distances measured between the lattice fringes were 0.24 and 0.25 nm matching the interplanar spacings for the d222 and d113 family of planes, respectively. Figure 14H is the SAED pattern taken from the several nanospheres showing reflections that correspond to the interplanar spacing, d220, d311, d222, and d400 and was indexed on the basis of the FCC structure of magnetite. The utilization of these advanced nanotechniques, together with the Mössbauer spectra results, provided unambiguous evidence that the resulting compounds are the FCC structure magnetite with the unit cell parameter a=8.39 Å.

Table 7

Magnetic properties of the nanocubes/spheres obtained by annealing the different ratios of ferrocene and PVP mixtures for different time periods.a

The high-resolution electron micrograph of the typical magnetite nanocubes/spheres obtained by the thermal decomposition at 350°C for 2 or 4 h of solid mixtures of ferrocene and PVP of the weight ratios of 1:1 (A), 1:2 (C), and 1:5 (E and G) and the corresponding ED pattern (B, D, F, and H, respectively). The nanocubes (A–F) were formed by the thermal decomposition of the ferrocene for 2 h and 4 h for the nanospheres (G, H). The inset marked by the white square (C and G) is the magnified image and Fourier transform taken from the area (C).
Figure 14

The high-resolution electron micrograph of the typical magnetite nanocubes/spheres obtained by the thermal decomposition at 350°C for 2 or 4 h of solid mixtures of ferrocene and PVP of the weight ratios of 1:1 (A), 1:2 (C), and 1:5 (E and G) and the corresponding ED pattern (B, D, F, and H, respectively). The nanocubes (A–F) were formed by the thermal decomposition of the ferrocene for 2 h and 4 h for the nanospheres (G, H). The inset marked by the white square (C and G) is the magnified image and Fourier transform taken from the area (C).

3.2 Synthesis of ferromagnetic Fe3C/C composite nanoparticles as a catalyst for carbon nanotube growth

The uniform micrometer-sized PS/ferrocene composite microspheres were formed by a room temperature swelling process of the PS template microspheres with methylene chloride (a swelling solvent) containing ferrocene. In a typical experiment, the PS template microspheres of 2.4±0.1 μm were swollen up to 4.9±0.1 μm (as observed by a light microscope) by adding to a 20-ml vial containing 10 ml SDS aqueous solution (0.75% w/v), 1 ml of methylene chloride containing 100 mg of dissolved ferrocene. The emulsion droplets of the swelling solvent in the aqueous continuous phase were then formed by the sonication of the mixture for 1 min. An aqueous dispersion (3.5 ml) of the PS template microspheres (7% w/v) was then added to the stirred methylene chloride emulsion. After the swelling was completed, and the mixture did not contain any small emulsion droplets of the swelling solvent, as verified by optical microscopy, the diameter of the swollen microspheres was measured. The swelling extent of the PS template microspheres by CH2Cl2 and ferrocene was studied with the different volumes of CH2Cl2 (1, 2, and 4 ml) in the absence of ferrocene and in the presence of 10%, 20%, and 30% of ferrocene (w/v) dissolved in the CH2Cl2. The methylene chloride, after the completion of the swelling process, was removed by nitrogen flow for 4 h through a shaken open vial containing the swollen particle aqueous mixture at room temperature. The obtained PS/ferrocene microspheres were then washed from the excess reagents by several centrifugation cycles with water, ethanol, and again water, and then dried by nitrogen flow for l0 h. The air-stable Fe3C nanoparticles and CNTs were formed by heating the dried PS/ferrocene obtained by swelling the PS template microspheres with 2 ml of CH2Cl2 containing 20% (w/v) ferrocene at 500°C and 600, 700°C and 1000°C for 2 h in a sealed cell. The sealed cell was then cooled to room temperature and opened, then, to release the gases formed during the formation of the various particles. Figure 15 illustrates the synthetic scheme through which the magnetic materials were prepared. First, the uniform micrometer-sized methylene chloride swollen PS/ferrocene composite microspheres were prepared by a swelling process of the uniform PS template microspheres dispersed in an aqueous continuous phase with the emulsion droplets of methylene chloride containing the ferrocene. The PS/ferrocene composite microspheres were then formed by the removal of the methylene chloride by nitrogen flow at room temperature. The magnetic air-stable Fe3C nanoparticles embedded in the amorphous carbon matrix have been prepared by the thermal decomposition of the PS/ferrocene particles at 500°C in a sealed cell. The heating of these ferrocene-containing template particles at higher temperatures, e.g., 600°C, 700°C, and 1000°C, led to the formation of the CNTs in addition to the Fe3C/C composite nanoparticles. Figure 16 shows the light microscope pictures that allow one to compare the swelling ability of the template PS particles by 2 ml of CH2Cl2 and by 2 ml of CH2Cl2 containing 20% (w/v) ferrocene. The diameter of the PS microspheres before swelling is 2.4±0.1 μm (Figure 16A). As a consequence of their swelling with the 2 ml of CH2Cl2, their diameter increased from 2.4±0.1 to 5.4±0.2 μm (Figure 16B), a 225% increase in the average diameter. On the other hand, a similar swelling process with 2 ml methylene chloride containing 20% ferrocene (w/v) led to an increase in the particle size from 2.4±0.1 to 6.0±0.2 μm (Figure 16C), a 255% increase in the average diameter. These results may indicate that both CH2Cl2 and ferrocene have the ability to swell the PS template microspheres and preserve the low size distribution of these template particles. As the goal of this study was to fill the swollen PS particles with ferrocene while retaining their narrow size distribution, the trials using the different mixtures of methylene chloride and ferrocene have been performed. Figure 17 demonstrates the influence of the different volumes of the swelling solvents on the diameter and size distribution of the template PS particles. For each volume, four types of swelling solvents have been tested: methylene chloride alone and three mixtures of methylene chloride containing 10%, 20%, and 30% (w/v) ferrocene. Figure 17 illustrates that increasing the volume of all the types of the swelling solvents resulted, as expected, in an increased average diameter of the swollen particles. For example, in the absence of methylene chloride, and in the presence of 1, 2, and 4 ml of methylene chloride, the diameter of the swollen particles increased from 2.4±0.1 to 4.7±0.1, 5.4±0.2, and 6.5±0.3 μm, respectively. A further increase in the volume of methylene chloride significantly damaged the uniformity of the swollen particles. The addition of 7 ml of methylene chloride resulted in the dissolution of the PS particles by methylene chloride dispersed in the aqueous phase. Figure 17 also shows that increasing the amount of ferrocene dissolved in methylene chloride resulted in an increase in the size of the swollen particles. For example, in the absence and in the presence of 4 ml of the different swelling solvents: methylene chloride alone and methylene chloride containing 10%, 20%, and 30% of ferrocene, the size of the swollen particles increased from 2.4±0.1 to 6.5±0.3, 6.7±0.2, 6.9±0.3, and 8.0±0.3 μm, respectively. This observation again indicates that the ferrocene, in addition to the CH2Cl2, has a substantial ability to swell the PS template particles. The kinetics studies of the swelling of the PS template microspheres by 4 ml methylene chloride containing various concentrations of the dissolved ferrocene indicated that under the experimental conditions, the swelling process is completed within ca. 40 min. It should also be noted, as shown in Figure 17, that the increase in the diameter (and volume) of the swollen particles was not linearly proportional to the volume of the added swelling solvent. For example, the addition of 1.0 or 4.0 ml of methylene chloride led to an increase in the average diameter of the template particles of 195% and 275%, respectively. The first 1 ml of methylene chloride increased the diameter of the PS particles significantly more than the additional 3 ml. This nonlinear behavior is probably due to the packing arrangement of the PS chains within the template particles. The degree of entanglement of these chains determines the size (and volume) of the particles. The swelling solvents swell the template particles by penetrating within the PS chains of the particles, decreasing their degree of entanglement, and thereby increasing the counter length of the PS polymeric chains. As a consequence, the particles are less compact, and their size and volume increase according to their degree of swelling. The magnetic air-stable nanoparticles embedded in an amorphous carbon matrix have been prepared by the thermal decomposition of the PS/ferrocene obtained by swelling the PS template microspheres with 2 ml CH2Cl2 containing 20% (w/v) ferrocene at 500°C in a sealed cell. On the other hand, heating these PS/ferrocene particles at 600°C, 700°C, and 1000°C led to the formation of the magnetic CNTs in addition to the Fe3C/C composite nanoparticles. The TEM image of the nanoparticles obtained by thermal decomposition at 500°C is presented in Figure 18A. This image demonstrates the hemispherical morphology of the obtained nanoparticles. The measured particle size is 42±7 nm. Moreover, the image clearly demonstrates that these nanoparticles are embedded in a protective carbon matrix. Interestingly, the annealing product at temperatures higher than 500°C led to the formation of the magnetic CNTs in addition to the Fe3C/C composite nanoparticles. Figure 18B–D illustrates the TEM images of the nanoparticles and the CNTs obtained at 600°C, 700°C, and 1000°C, respectively. These images demonstrate that the yield of the CNT formation increases significantly as the annealing temperature is increased. The formation of the CNTs is a catalytic process in which the Fe3C nanoparticles serve as the catalyst for their formation from the amorphous carbon, which is formed by the decomposition of the PS template particles. It should be noted here that the annealing of the PS template microspheres in the absence of the entrapped ferrocene under the same conditions did not lead to the formation of the CNTs. This, of course, illustrates the importance of the magnetic nanoparticles for the catalytic process. The measured diameters of the CNTs and the Fe3C nanoparticles obtained at 600°C, 700°C, and 1000°C are 95±12, 36±10, and 33±6 nm and 94±17, 64±22, and 38±6 nm, respectively. The decrease in the Fe3C nanoparticle size can be explained as follows: the thermal decomposition of the PS/ferrocene composite particles was performed in a sealed cell. Upon heating, both the PS and the ferrocene decomposed to give gaseous hydrocarbon products. In addition, the sealed cell contained air (mainly oxygen and nitrogen), which was introduced into the cell during the sample preparation. As the reaction temperature increased, the pressure within the cell increased, and the resulting particles were smaller in size. It is suggested that the particle growth is limited by the pressure within the cell, which allows the formation of the smaller particles at higher temperatures. The above results demonstrate that the nanoparticle size as well as the CNT diameter can be controlled by adjusting the annealing temperature: an increase in the annealing temperature from 600°C to 1000°C led to a decrease in the CNT diameter. This can be explained by two main points: first, the Fe3C catalyst particle size decreased with increasing reaction temperature, and the CNT diameter was directly influenced by the catalyst size. Second, at higher temperatures, the growth rate of the CNTs increases dramatically due to the enhanced diffusion and reaction rates of the carbons, which led to the fast growth of the CNTs to their final size with smaller diameters. Moreover, the CNTs formed at 1000°C are longer compared to those obtained at 700°C. This can also imply that there are kinetic effects on the CNT growth; the mechanism of the formation of the CNTs is not yet clear. However, recently Futaba et al. showed that the CNTs can be synthesized by a “growth-enhancer-containing oxygen’’ (e.g., water, acetone, methyl-benzoate, etc.) and a “carbon source that does not contain oxygen” (e.g., ethylene, acetylene) [58]. In this study, the PS template microspheres serve as the “carbon source,” and the “growth enhancer” is the oxygen that is present in the cell or water and CO2 that are formed by the combustion of PS. Further study is necessary in order to confirm the mechanism. The XRD patterns of the nanoparticles obtained at 500°C, 600°C, 700°C, and 1000°C, perfectly match the orthorhombic structure of Fe3C. The isothermal field dependence of the magnetization measured at 300 K for the nanoparticles obtained at 500°C, 600°C, 700°C, and 1000°C was characterized by the ferromagnetic-type curves, which show hysteresis loops. The Ms as well as the coercive fields obtained are 39, 54, 58, and 61 emu/g and 410, 490, 345, and 500 Oe, respectively. It should be noted that the Ms bulk value of Fe3C in the literature was found to be 140 emu/g [59]. The relatively lower Ms values of the magnetic nanoparticles compared to the bulk value arise from the nonmagnetic carbon content, which leads to a decrease in the magnetization per weight. Another explanation for the relatively low magnetization values is the surface effect that can occur in the case of the magnetic core and nonmagnetic shell structures. This effect leads to a reduction in the magnetic moment by a different mechanism, such as the existence of a magnetically dead layer on the particles’ surface, the existence of canted spins, or the existence of a spin glass-like behavior of the surface spins [57]. In order to study the stability of the Fe3C nanoparticles obtained at 500°C, 600°C, 700°C, and 1000°C TGA measurements in the air atmosphere have been performed. Figure 19A is a HRTEM of a typical single crystalline Fe3C nanoparticle obtained at 500°C. The image demonstrates a Fe3C core of 47 nm coated by a thin layer of 5 nm amorphous carbon. The inset in Figure 19A represents the magnified portion of the image outlined by the white square. The distances measured between the lattice fringes was 0.2008 nm matching the interplanar spacings for the d031 of Fe3C. The TEM and HRTEM images of the CNTs obtained at 1000°C are presented in Figure 19B–C, respectively. The images clearly demonstrate the cylindrical structure and the curvature of the sidewall of the CNTs. The walls of the presented CNTs are composed of carbon shells with a spacing of 0.34 nm, which is consistent with that of graphite. However, the images demonstrated disordered MWCNTs in which graphite layers are misaligned with the primary MWCNT axis.

A schematic scheme illustrating the synthesis of the magnetic CNTs and the Fe3C/C composite nanoparticles.
Figure 15

A schematic scheme illustrating the synthesis of the magnetic CNTs and the Fe3C/C composite nanoparticles.

The light microscopy pictures of the PS template microspheres before (A) and after swelling with 2 ml of methylene chloride (B) and with 2 ml of methylene chloride containing 20% (w/v) ferrocene (C).
Figure 16

The light microscopy pictures of the PS template microspheres before (A) and after swelling with 2 ml of methylene chloride (B) and with 2 ml of methylene chloride containing 20% (w/v) ferrocene (C).

The influence of the CH2Cl2 volume and the weight ratio [CH2Cl2]/[ferrocene] on the diameter and size distribution of the template PS particles.
Figure 17

The influence of the CH2Cl2 volume and the weight ratio [CH2Cl2]/[ferrocene] on the diameter and size distribution of the template PS particles.

The TEM pictures of the magnetic CNTs and the Fe3C/C composite nanoparticles obtained at 500°C (A), 600°C (B), 700°C (C), and 1000 (D).
Figure 18

The TEM pictures of the magnetic CNTs and the Fe3C/C composite nanoparticles obtained at 500°C (A), 600°C (B), 700°C (C), and 1000 (D).

(A) The HRTEM image of the typical Fe3C/C composite nanoparticles obtained at 500°C; (B) TEM, and (C) HRTEM of the CNTs obtained at 1000°C. The inset is the magnified image showing the lattice-fringes (A).
Figure 19

(A) The HRTEM image of the typical Fe3C/C composite nanoparticles obtained at 500°C; (B) TEM, and (C) HRTEM of the CNTs obtained at 1000°C. The inset is the magnified image showing the lattice-fringes (A).

4 Summary and conclusions

The decomposition of iron pentacarbonyl is one of the most common methods for the preparation of magnetic iron oxide and the elemental iron nano/microparticles. However, Fe(CO)5 is severely toxic, and alternative precursors should be used. This review demonstrates a new simple nontoxic approach for the synthesis of magnetic iron and the iron oxide nano/microcomposite particles with various shapes, properties, and sizes, by thermal decomposition in various ways of the iron precursor ferrocene. In all processes, the particles’ size, composition, shape, crystallinity, magnetic properties, and surface area can be controlled by the adjustment of the entrapment and decomposition conditions of ferrocene.

The present review describes the four different ways to prepare the above magnetic nano/microparticles, as follows:

  1. The superparamagnetic PDVB/iron oxide composite micrometer-sized particles of narrow size distribution were prepared by entrapping ferrocene and a separating media within the pores of the uniform porous PDVB microspheres, followed by solventless thermal decomposition at 300°C in ambient atmosphere in a sealed cell. The uniform ferromagnetic C/iron oxide and C/Fe3O4/Fe composite microspheres were then formed by annealing the superparamagnetic PDVB/iron oxide particles at 500°C and 700°C, respectively, under argon atmosphere.

  2. The core-shell PS/PDVB/ferrocene and PS/P(DVB-S)/ferrocene micrometer-sized particles of narrow size distribution were prepared by a single-step swelling of the uniform PS template microspheres dispersed in an aqueous continuous phase with the emulsion droplets of a swelling solvent such as toluene containing the monomers S and/or DVB, the initiator AIBN, and ferrocene. The monomer/s within the swollen uniform PS template microspheres were then polymerized at an elevated temperature. The superparamagnetic and ferromagnetic PDVB/Fe3O4 and P(S-DVB)/Fe3O4 composite microspheres of narrow size distribution were then formed by annealing the previous micrometer-sized composite particles containing ferrocene at 330°C for 2 h under ambient atmosphere in a sealed cell, followed by the dissolution of the PS template part with DMF. The superparamagnetic C/Fe micrometer-sized particles of narrow size distribution were then formed by annealing the PDVB/Fe3O4 particles at 450°C under hydrogen atmospheres for 2 h.

  3. The Fe3O4 nanocubes and nanospheres were synthesized by the solventless thermal decomposition of the various mixtures of ferrocene and PVP. The magnetite nanocubes were prepared by grinding and mixing the solid mixtures of ferrocene and PVP. The mixtures were then annealed at 350°C for 2 h in a sealed cell. The nanocubes’ size was controlled by adjusting the [ferrocene]/[PVP] weight ratio. Increasing the annealing time to 4 h when the [ferrocene]/[PVP] weight ratio was 1:5 led to the formation of the magnetite nanospheres. The formed nanocubes/spheres exhibit a ferromagnetic behavior at room temperature. These magnetite nanocubes/spheres were actually formed by a CVD reaction through which the ferrocene molecules, which are in the gas phase at the reaction conditions, decomposed to the magnetite nanocubes/spheres dispersed in the solid PVP matrix.

  4. The air-stable Fe3C nanoparticles embedded in the amorphous carbon matrix (C/Fe3C) have been prepared by thermal decomposition of the ferrocene swollen template PS particles at 500°C for 2 h in a sealed cell. The decomposition of these swollen template particles for 2 h at higher temperatures led to the formation of the CNTs in addition to the C/Fe3C composite nanoparticles. The yield of the CNTs increased as the annealing temperature was raised. The opposite behavior was observed for the diameter of the formed CNTs. This method for the formation of the magnetic CNTs is relatively new and promising. Further studies related to this CNT formation and its applications, particularly for catalysis, are ongoing in our laboratory. In addition, for future work, we plan to extend this study for the other applications, particularly for the biomedical uses, e.g., hyperthermia, cell labeling, and separation, drug delivery, etc.

This study was partially supported by a Minerva Grant (Micro and Nano Scale Particles and Thin Films for Biomedical Applications).

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About the article

Daniel Amara

Dr. Daniel Amara received his PhD in Materials Science in 2012 from the Institute of Nanotechnology and Advanced Materials, Bar Ilan University, Israel, under the supervision of Prof. Shlomo Margel. As part of his PhD research, he developed and published novel methods for the synthesis of magnetic nano/microcomposite particles. These methods focused on the environmentally friendly synthesis. He is currently conducting research on the solventless synthesis of magnetic materials.

Shlomo Margel

Shlomo Margel received his PhD from the Weizmann institute of Science, Department of Material Science, Rehovot, Israel in 1976 and obtained a Post-Doctorate from the California Institute of Technology, Department of Chemistry, Pasadena, CA in 1977. He worked as a senior Scientist, at the Califorina Institute of Techno logy, Jet Propulsion Lab., Pasadena, CA from 1978 to 1979. From 1980 to 1984 he worked as a Senior Scientist at the Weizmann Institute of Science, Department of Materials Science, Rehovot, Israel and became an Associate Professor of this department in 1985. From 1986 to 1987 and in July to November 1990 he was a Visiting Scientist at Du Pont, Central R&D, Wilmington, DE. He was a Visiting Scientist at the University of Ulm, Department of Chemisty from July to September 1992. Polymer Section, Ulm, Germany. From 1988 to 1994 he was an Associate Professor at Bar-Ilan University, Ramat Gan, and became a Full Professor there in 1994. In 1997 he was a Visiting Professor at theDepartment of Physical Electronics at theTokyo Institute of Electronics (TIT). From 1999 to 2001 he was Head of the Chemistry Department at Bar-Ilan University and from 2000 to 2003 he was the Head of the National Committe for Chemisrty in High School Education. From 2002 to 2003 he was the Dean of the Faculty of Exact Sciences and in 2005 hewas a Visiting Scientist at MIT’a Institute from Soldier’s Nanotechnologies in Cambridge, MA. From 2006 to 2009 he wasthe President of the Israel Chemical Society and from 2010 to 2012 he was the Chairman of the National Committee of Chemistry towards IUPAC (he was nominated by the Israel Academy of Science and Humanities). The major research interests of Prof. Margel’s group are in the fields of polymers and biopolymers, encapsulation, synthesis of functional particles of narrow size distribution, water purification, surface chemistry, immobilization techniques, colloidal chemistry, functional thin films, and biological and medical applications of nano/microparticles. Prof. Margel’s research group includes about 15 PhD and MSc students and four postdocs. He is the author of more than 00 publications and 29 patents and patent applications.


Corresponding author: Shlomo Margel, Institute of Nanotechnology and Advanced Materials, Department of Chemistry, Bar Ilan University, 52900 Ramat Gan, Israel


Received: 2013-01-01

Accepted: 2013-03-28

Published Online: 2013-05-01

Published in Print: 2013-06-01


Citation Information: Nanotechnology Reviews, Volume 2, Issue 3, Pages 333–357, ISSN (Online) 2191-9097, ISSN (Print) 2191-9089, DOI: https://doi.org/10.1515/ntrev-2012-0061.

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