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]:

Figure 1

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

Figure 2

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

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).

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 T_b 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 (V_p) of a magnetic particle can be calculated by the following Eq [46]:

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.

Figure 5

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

where V_p is the upper superparamagnetic volume of a magnetic particle, k_B is the Boltzmann’s constant, T is the temperature, and K_u 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.

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.

Figure 7

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

Figure 8

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

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.

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).

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 M_s 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 M_s 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 (T_b) of the superparamagnetic particles. The measured T_b are summarized in Table 5. As expected, the T_b values depend directly on the size of the magnetic nanoparticles entrapped in the polymeric particles. Indeed, the PDVB/Fe₃O₄ possess the lowest T_b value of 85 K, whereas the T_b 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 T_b 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.

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.

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).

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.

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 (M_S), 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 M_S 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, d_311, 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.

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).

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 M_s 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 M_s bulk value of Fe₃C in the literature was found to be 140 emu/g [59]. The relatively lower M_s 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.

Figure 15

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

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).

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.

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).

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).