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Synthesis and Characterization of Magnetic Poly(divinyl benzene)/ Fe3O4, C/Fe3O4/Fe, and C/Fe Onionlike Fullerene Micrometer-Sized Particles with a Narrow Size Distribution Ron Snovski, Judith Grinblat, and Shlomo Margel* Institute of Nanotechnology & Advanced Materials, Department of Chemistry, Bar Ilan University, Ramat Gan 52900, Israel
bS Supporting Information ABSTRACT: Magnetic poly(divinyl benzene)/Fe3O4 microspheres with a narrow size distribution were produced by entrapping the iron pentacarbonyl precursor within the pores of uniform porous poly(divinyl benzene) microspheres prepared in our laboratory, followed by the decomposition in a sealed cell of the entrapped Fe(CO)5 particles at 300 °C under an inert atmosphere. Magnetic onionlike fullerene microspheres with a narrow size distribution were produced by annealing the obtained PDVB/Fe3O4 particles at 500, 600, 800, and 1100 °C, respectively, under an inert atmosphere. The formation of carbon graphitic layers at low temperatures such as 500 °C is unique and probably obtained because of the presence of the magnetic iron nanoparticles. The annealing temperature allowed control of the composition, size, size distribution, crystallinity, porosity, and magnetic properties of the produced magnetic microspheres.
’ INTRODUCTION Micrometer-sized particles with a narrow size distribution have attracted much attention in many applications such as adsorbents for high-pressure liquid chromatography, calibration standards, spacers for liquid crystals, inks, catalysis, and so forth.17 Dispersion polymerization is the common method of preparing uniform, nonporous, micrometer-sized particles in a single step. However, the particles formed by this method possess a relatively small surface area and their properties (e.g., porosity, surface morphology, and functionality) can hardly be manipulated.811 Furthermore, uniform particles with a diameter larger than approximately 6 μm usually cannot be prepared by dispersion polymerization. These limitations had been overcome by several swelling methods of uniform polystyrene (PS) template micrometer-sized particles with appropriate monomers and initiators (e.g., multistep swelling,1218 dynamic swelling,19,20 and a single-step swelling21) followed by the polymerization of the monomers within the swollen template particles. Porous particles are widely used as adsorbents, catalyst supports, and biocompatible materials.2224 For these applications, however, the separation of the porous material from the fluid is a crucial step. The use of porous magnetic particles allows separation by simply applying a magnetic field. This possibility significantly increases the interest in porous, micrometer-sized particles with magnetic properties. In addition to their use for separation, these magnetic particles may be useful for many other nonmedical and medical applications (e.g., magnetic storage, magnetic sealing, enzyme immobilization, nucleic acid purification, cell isolation and purification, molecular biology, diagnostics, drug targeting, radio immunoassay, etc2530). Significant efforts have been made r 2011 American Chemical Society
to prepare porous and nonporous magnetic carbon particles. One way to prepare magnetic carbon relies upon the ferromagnetic properties of pure carbon materials, as recently demonstrated.31,32 However, these materials possess a small magnetic moment; therefore, alternative methods should be investigated. Particles based on a composite of carbon with ferromagnetic metals seem to be more promising and can lead to the formation of magnetic carbon materials with a relatively high magnetic moment. The common magnetic micrometer-sized particles used for biomedical applications are based on magnetite (Fe3O4) and maghemite (γ-Fe2O3). The applications of these iron oxide particles rely upon the biocompatibility and biodegradability of the iron oxide.33 Elemental iron, however, has a significantly higher magnetic moment than its oxide. 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 other ferromagnetic elements such as nickel and cobalt. However, Fe particles are easily oxidized, leading to a rapid decrease in their magnetic moment. In order to prevent these phenomena, Fe particles should be protected by a protective layer (e.g., carbon,3436 silica,37 alumina,38 etc.). In previous work, Shpaisman et al.34,35 described the synthesis of magnetic Fe amorphous carbon microspheres by entrapping Fe(CO)5 within porous poly(divinyl benzene) (PDVB) uniform microspheres, Received: May 17, 2011 Revised: July 31, 2011 Published: August 01, 2011 11071
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Langmuir followed by the decomposition of the entrapped Fe(CO)5 particles at 600 °C under an inert atmosphere. However, the produced magnetic amorphous carbon microspheres had a relatively low magnetic moment (59 emu/g) because the decomposition of Fe(CO)5 entrapped within the PDVB particles was carried out in an open system so that a significant part of the entrapped Fe(CO)5 evaporated because of its high vapor pressure (40 mmHg at 30 °C), before its decomposition within the PDVB particles. This article describes a simple new approach to synthesizing magnetic Fe and iron oxide onionlike fullerene micrometer-sized particles with a narrow size distribution by annealing porous uniform PDVB/Fe3O4 microspheres at various temperatures. The annealing temperature allowed us to control the composition, size, size distribution, crystallinity, porosity, and magnetic properties of the produced uniform magnetic onionlike fullerene microspheres.
’ EXPERIMENTAL SECTION Materials. The following analytical-grade chemicals were purchased from Aldrich (Israel) and were used without further purification: Fe(CO)5 (>99%), benzoyl peroxide (BP, 98%), dimethylformamide (DMF), divinylbenzene (DVB, 99%), sodium dodecyl sulfate (SDS), poly(vinylpyrrolidone) (PVP, MW 360 000), ethanol (HPLC), 2-methoxy ethanol (HPLC), dibutyl phthalate (DBP), and methylene chloride (HPLC). Styrene (99% Aldrich) was passed through activated alumina (ICN) to remove inhibitors before use. Water was purified by passing deionized water through an Elgastat Spectrum reverse osmosis system (Elga, High Wycombe, U.K.). Synthesis of Uniform PS Template Microspheres. Uniform PS template microspheres of 2.20 ( 0.2 μm were prepared by the dispersion polymerization of styrene in 2-methoxy-ethanol according to the literature.8,10,39 Synthesis of Uniform PS/PDVB Micrometer-Sized Composite Particles. Uniform PS/PDVB micrometer-sized composite particles were formed by a single-step swelling process at room temperature of the PS template particles with DBP (a swelling solvent) droplets containing DVB and BP, followed by the polymerization of DVB within the swollen PS template particles at elevated temperature. In a typical experiment, the PS template microspheres of 2.20 ( 0.2 μm were swollen up to 6.18 ( 0.11 μm by adding to a 20 mL vial 10 mL of an SDS aqueous solution (0.75% w/v) and 1.5 mL of DBP containing 10 mg of BP and 1.5 mL of DVB. Emulsion droplets of the DBP solution were then formed by sonicating 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 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 PS/PDVB composite microspheres were then washed to remove undesired reagents by extensive centrifugation cycles with water, ethanol, and water again. The obtained particles were then dried by lyophilization.
Synthesis of Uniform PDVB Micrometer-Sized Particles. Uniform cross-linked micrometer-sized PDVB particles were prepared by dissolving the PS template part of the former PS/PDVB composite particles with DMF. Briefly, the PS/PDVB composite particles 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
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particles were washed twice with ethanol and water and then dried by lyophilization.
Synthesis of Uniform Magnetic PDVB/Fe3O4 MicrometerSized Composite Particles. The former dried PDVB microspheres (700 mg) were added to a two-necked round-bottomed flask equipped with a septum. The flask was then evacuated using an oil vacuum pump for 30 min, followed by the injection of 7 mL of Fe(CO)5. This quantity of Fe(CO)5 was used because when higher or lower quantities were used a small puddle or an aggregation of particles, respectively, was formed. All of the liquid Fe(CO)5 was entrapped within the PDVB pores, and no free liquid remained in the flask. PDVB/Fe3O4 microspheres were then formed by decomposition in a sealed cell (a letlock)40 containing Fe(CO)5-entrapped PDVB particles at 300 °C under an Ar atmosphere for 4 h.
Synthesis of Uniform Magnetic Onionlike Fullerenes C/Fe3O4/Fe and C/Fe Micrometer-Sized Composite Particles. C/Fe3O4/Fe composite microspheres were formed by annealing the PDVB/Fe3O4 microspheres at 500, 600, and 800 °C under an Ar atmosphere for 1 h. C/Fe composite microspheres were formed by annealing the PDVB/Fe3O4 microspheres at 1100 °C under an Ar atmosphere for 1 h. The furnace heating rate was 10 °C min1 from room temperature until the desired temperature was reached.
Characterization of the Magnetic and Nonmagnetic Particles. To investigate the features of the microspheres and the crystal structure of the FexOy nanoparticles, we conducted transmission electron microscopy (TEM) using an FEI Tecnai-12 and high-resolution transmission electron microscopy (HRTEM) by means of a JEOLJEM-2100 (LaB6) electron microscope operated at 200 kV that was integrated with a digital scanning transmission electron microscope (STEM) comprising annular dark-field and bright-field detectors and with a Noran System Six energy-dispersive X-ray spectrometer (EDS) system for elemental analysis. The TEM observations were made by taking bright-field (BF) images, HRTEM) images, and selected area electron diffraction (SAED) patterns. Elemental analysis, scanning, and mapping were performed in STEM mode and using the EDS system. Specimens for observation were powdered samples on TEM carbon support grids. The surface morphology was characterized with an FEI scanning electron microscope (SEM) model Inspect S. For this purpose, a drop of a dilute microsphere dispersion in water was spread on a glass surface that was attached by carbon tape and then dried at room temperature. The dried sample was coated with gold in vacuum before viewing under SEM. The particles’ average size and the size distribution of the electronic images were determined by measuring the diameter of more than 100 particles with AnalySIS Auto (Soft Imaging System GmbH, Germany) image analysis software. C, H, and O analyses of the various particles were performed using a model FlashEA1112 elemental analysis instrument from Thermoquast. Powder X-ray diffraction (XRD) patterns were recorded using an X-ray diffractometer (model D8 Advance, Bruker AXS) with Cu Ka radiation. The surface area of the various particles was measured by the BrunauerEmmetTeller (BET) method using Gemini III model 2375 from Micrometrics. Isothermal magnetization measurements at room temperature were performed in a commercial (Quantum Design) superconducting quantum interference device (SQUID) magnetometer. The thermal behavior of the particles was measured with a TC15 system equipped with TGA (thermal gravimetric analysis), model TG-50, and DSC (differential scanning calorimetry), model DSC-30 from Mettler Toledo. Emulsion droplets were generated by sonication with a Sonics and Materials model VCX-750 Ti horn at 20 kHz.
’ RESULTS AND DISCUSSION Figure 1 illustrates the preparation of the various uniform nonmagnetic and magnetic microspheres. First, the uniform 11072
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Figure 1. Scheme illustrating the synthesis of the various magnetic composite microspheres.
Figure 2. SEM images of the (a) PS and (b) PDVB microspheres.
PS/PDVB particles were prepared by the single-step swelling of the uniform PS template microspheres dispersed in an aqueous continuous phase with emulsion droplets of DBP containing the BP initiator and the DVB cross-linker monomer. Uniform PS/ PDVB composite particles were then formed by polymerizing the DVB within the swollen PS template microspheres at 73 °C. Uniform porous PDVB microspheres were then prepared by the dissolution of the PS template part of the former composite particles with DMF. Uniform magnetic PDVB composite microspheres were prepared by entrapping Fe(CO)5 within the pores of the PDVB microspheres by vacuum, followed by annealing these PDVB-containing Fe(CO)5 microspheres at 300 °C under an Ar atmosphere in a letlock. It is essential to decompose the Fe(CO)5 entrapped within the PDVB particles in a sealed cell in order to obtain PDVB/Fe3O4 particles with a relatively high magnetic moment (12 emu g1); otherwise, PDVB/Fe3O4 particles with significantly lower MS values are obtained (7 emu g1), as shown by Shpaisman et al.34,35 Uniform C/Fe3O4 and C/Fe composite microspheres were then formed by annealing under an Ar atmosphere at 500800 °C or at 1100 °C, respectively. Characterization of the PS and PDVB Microspheres. Figure 2 shows SEM pictures of the (a) PS and (b) PDVB microspheres. Figure 2a illustrates the uniformity, the smooth surface morphology, and the perfect spherical shape of the PS microspheres
of 2.35 ( 0.1 μm diameter. Figure 2b clearly demonstrates the increase in the size of the microspheres from 2.35 ( 0.1 to 6.1 ( 0.1 μm while retaining their spherical shape and narrow size distribution. Moreover, the produced PDVB microspheres possess bumpy and porous surfaces but the surface of the PS template particles is smooth, as mentioned above. This roughness and porosity are probably the main reasons for the significantly higher surface area of the PDVB particles compared to that of the PS, 594.2 and 3.5 m2 g1, respectively, as shown in Table 1. The measured surface area of the PS template microspheres of 2.35 ( 0.1 μm diameter is similar to the calculated surface area of spherically shaped particles with a density of 1.0 g mL1 and the same size (A = 4Πr2), indicating the nonporous structure of these PS microspheres. The relatively high surface area and porosity of the PDVB microspheres may be due to the following reasons. First, cross-linker monomer DVB leads to the formation of very small pores (micropores) as a result of DVB monomeric units tying together linear chains of styrene units at various points.41 Second, swelling solvent DBP serves as a porogen that forms macropores. Thus, pores are formed in the spaces where the porogen was extracted from the polymer particles.41 Third, the dissolution of the PS template part of the composite PS/PDVB microspheres also generates macropores within the PDVB particles.41 11073
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Table 1. Temperature, Size, Size Distribution, Surface Area, Pore Size, MS, and Coercivity Values of the PS, PDVB, PDVB/Fe3O4, C/Fe3O4/Fe, and C/Fe Particles particles
temperature (°C)
diameter (μm)
surface area (m2 g1)
pores size (Å)
MS (emu g1)
coercivity (Oe)
PS
2.35 ( 0.1
3.5
PDVB PDVB/Fe3O4
300
6.10 ( 0.1 6.03 ( 0.1
594.2 291.1
65.3 32.2
12.0
93.4
C/Fe3O4/Fe
500
5.87 ( 0.1
146.1
34.1
102.8
112.5
C/Fe3O4/Fe
600
5.21 ( 0.1
112.4
34.3
121
128.6
C/Fe3O4/Fe
800
4.17 ( 0.2
63.9
34.8
146.2
106.1
C/Fe
1100
2.67 ( 0.2
20.3
35.7
166
38.6
Figure 3. SEM images of thermally decomposed PDVB microspheres at (a) 400 and (b) 500 °C.
Figure 3a,b shows SEM images of the PDVB microspheres annealed at 400 and 500 °C, respectively. This figure shows that the PDVB microspheres produced at 400 °C are stable whereas those obtained at 500 °C collapse and lose their shape. This behavior can be understood by the TGA and DSC thermograms shown in Figure 4. The TGA thermogram (Figure 4a) exhibits a steep slope between 360 and 500 °C, indicating a 91% weight loss due to the PDVB decomposition, leaving residual carbon. This observation is in good agreement with the DSC curve that exhibits an endothermic peak at around 480 °C, which is probably related to the carbonization of the cross-linked PDVB microspheres. Characterization of the PDVB/Fe3O4 Microsphere. Figure 5 is the bright-field image of a cross section of a PDVB/Fe3O4 microsphere. The image illustrates that iron oxide nanoparticles of 3555 nm (represented by the dark areas) are caged within the PDVB particles. Point EDS taken from a 25 nm area of a dark spot shows that the nanoparticles contain Fe and O (Supporting Information Figure S1). The spectrum shows the presence of Fe, O, and C in the analyzed area .The Cu peak is from the specimen grid. This implies good penetration of Fe(CO)5 into the pores of the PDVB matrix. Also, a thin layer (130150 nm) of iron oxide film surrounds the surface of the microsphere. Figure 6 shows a TEM micrograph of a typical PDVB/Fe3O4 composite microsphere obtained at 300 °C. The insets in ac show the dark field (DF), SAED, and BF magnified images of the area marked by the white square, respectively. In the DF mode of
Figure 4. (a) TGA and (b) DSC thermograms of the PDVB microspheres. 11074
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Table 2. Elemental Analysis and Composition of PDVB, PDVB/Fe3O4, C/Fe3O4/Fe, and C/Fe Composite Microspheres at Different Temperatures weight % particles PDVB
temperature (°C)
C
H
O
Fea % Fe3O4 % Fe
room temperature 90.1 8.6 1.3
PDVB/Fe3O4 300
60.5 5.4 10.4 23.7
100
0
C/Fe3O4/Fe
500
31.9 1.1 9.1
57.9
41
59
C/Fe3O4/Fe
600
30.3 0.7 5.1
63.9
20.9
79.1
C/Fe3O4/Fe C/Fe
800 1100
25.4 0.3 2.4 25.8 0 0
71.9 74.2
8.8 0
91.2 100
a
The amount of Fe was calculated by subtracting the sum of the other elements from 100.
Figure 5. TEM cross section micrograph of the PDVB/Fe3O4 composite microspheres obtained at 300 °C.
Figure 6. Bright-field electron micrograph of a microsphere obtained by annealing the Fe(CO)5 entrapped within the PDVB particles at 300 °C. Insets a and c are the dark-field and bright-field enlarged electron micrographs taken in the (220 and 311) reflection for the area marked by the white square, respectively. Inset b is the SAED taken from the microsphere, showing the ring diffraction pattern that was indexed in terms of the FCC structure of the magnetite (Fe3O4, a = 8.39 Å, PDF no. 19-0629).
the imaging technique, only specific sets of reflection, caused by a specific crystalline element, make up the image, hence the diffracting areas will appear as bright spots in the dark-field image. In this case, the DF imaging technique (Figure 6a) was used to observe some crystalline iron oxide nanoparticles on the surface of a PDVB/Fe3O4 microsphere. Some individual crystalline particles (∼2030 nm) of Fe3O4 are clearly seen in the dark-field image taken in the (220 and 311) reflection. The ring diffraction pattern (b) is a result of polycrystalline particles present in the analyzed area. The reflection spots in the SAED (supported by the elemental analysis data presented in Table 2) were successfully indexed in terms of the cubic FCC unit cell that represents the structure of the magnetite (Fe3O4, a = 8.39 Å, SG:Fd3m PDF no. 19-0629). The distances measured from the SAED pattern
were 0.29, 0.25, and 0.20 nm, matching interplanar spacings d220, d311, and d400, respectively, in the above structure of Fe3O4. Characterization of the C/Fe3O4/Fe and the C/Fe Microspheres. Figure 7 illustrates the XRD pattern of the PDVB/ Fe3O4, C/Fe3O4/Fe and C/Fe composite microspheres. Pattern a refers to the particles obtained by annealing at 300 °C, and the peaks in this figure at 2θ = 35.4 (311), 43 (400), and 62.5 (440) correspond to the crystal plane of magnetite. As expected, the PDVB microspheres do not show any peaks in the XRD pattern because of their amorphous structure. The pattern of the particles obtained perfectly matches the spinel Fe3O4 crystal structure, where the oxygen atoms are closely packed in a cubic arrangement and the Fe ions occupy both octahedral and tetrahedral sites. It should be noted that the XRD patterns of magnetite (Fe3O4) and maghemite (γ-Fe2O3) are similar. Thus, elemental analysis and HRTEM were employed and showed that this fraction was indeed composed of the Fe3O4 phase, as described later in Table 2 and Figure 10. Figure 7bd refers to the particles that were annealed at 500, 600, and 800 °C, respectively. These images display both magnetite peaks, as shown before, and zerovalent iron peaks. The peaks demonstrate the formation of bodycentered cubic (bcc) Fe and are at 2θ = 44.6 (110) and 65 (200). As the annealing temperature rises, the intensity of the magnetite peaks decreases and the zero-valent iron peaks increase. This phenomenon demonstrates the transformation of the magnetite phase into the zero-valent iron phase. Figure 5e, which refers to the particles annealed at 1100 °C, demonstrates only zero-valent iron peaks, with all of the magnetite phase having been transformed completely into the zero-valent iron phase. This can be explained by the reduction of magnetite by carbon according to the following reaction:42 Fe3 O4 þ 2C f 3Fe þ 2CO2 Figure 8ae exhibits a surface morphology change for (a) the PDVB/Fe3O4 particles and the PDVB/Fe3O4 particles annealed at (b) 500, (c) 600, (d) 800, and 1100 (e) °C. This figure demonstrates that increasing the thermal decomposition temperature results in increased roughness while retaining the particles’ spherical shape. The morphology change can be explained by the carbon lost during thermal decomposition. It is interesting that the presence of the magnetic nanoparticles within the obtained microspheres preserves their structure and shape because in the absence of these nanoparticles the microspheres collapse and lose their spherical shape, as shown in Figure 3b. Table 1 shows 11075
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Figure 7. XRD patterns of the (a) PDVB/Fe3O4, (bd) C/Fe3O4/Fe, and (e) C/Fe composite microspheres obtained by annealing the Fe(CO)5entrapped PDVB particles at 300, 500, 600, 800, and 1100 °C, respectively.
that the particle size decreases as the temperature increases, with the most significant decrease between 800 to 1100 °C from
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4.17 ( 0.2 to 2.67 ( 0.2 μm. It is also interesting that, despite the extreme annealing temperature, the particles preserve their narrow size distribution. The influence of the thermal decomposition temperature on the surface area of the different composite microspheres is also summarized in Table 1. This table shows that increasing the thermal decomposition temperature results in particles with a significantly lower surface area. For example, increasing the decomposition temperature from 300 to 500, 800, and 1100 °C leads to the decrease in the composite particles’ surface area from 291.1 to 146.1, 63.9, and 20.3 m2 g1, respectively. This decrease is probably due to thermal decomposition and the resulting contraction of the particles as the temperature rises. The decomposition and contraction of the composite particles as the temperature increases leads, of course, to a decrease in the pore size and the diameter of the obtained particles, as shown in Table 1. Table 2 shows the elemental composition of the composite particles obtained at the various temperatures. This table illustrates that the PDVB microspheres contain 90.1, 8.6, and 1.3 wt % of C, H, and O, respectively. Holding these PDVB particles at 300 °C under an Ar atmosphere for 4 h preserved the same elemental composition of the particles. The unexpected O content of these PDVB particles is mainly due to the initiator (BP) fraction used for the polymerization of DVB, as shown in previous work.8,21,39 The oxygen content of the PDVB/Fe3O4 particles obtained at 300 °C is 10.4 wt % as shown in Table 2. This oxygen content is due to the initiator fraction (1.3%) and to the iron oxide (9.1%). Table 2 also shows that the Fe content of the PDVB/Fe3O4 particles is 23.7 wt %. A simple calculation indicates that the [Fe]/[O] mole ratio of these obtained particles is 0.75, which fits magnetite (Fe3O4) precisely. The O content of the particles annealed at 500800 °C was used to calculate the magnetite content of these particles. The elemental Fe content was then calculated by reducing the Fe content belonging to magnetite from the total Fe content of the particles. Table 2 indeed shows that the ratio of elemental Fe to magnetite ([Fe]/ Fe3O4]) in the C/Fe3O4/Fe particles increases as the annealing temperature rises. Table 2 also shows that the particles annealed at 1100 °C do not contain O and that all of the Fe content belongs to elemental Fe. Line scans were performed to evaluate the relative chemical composition of the microspheres annealed at different temperatures. Figure 9 is the line scan profile for elements C (red) and Fe (blue). In this figure, the changes in the elemental composition of the microspheres with temperature are displayed. (See the changes in the [C]/[Fe] ratio.) It can easily be observed that at 300 °C there is more carbon than iron and that the [C]/[Fe] ratio is inverted as the temperature is increased to 800 °C. As generally observed, there is uniformity in the Fe content but there are also agglomerated iron oxide particles on the surface of the PDVB/Fe3O4 microsphere (as seen in Figure 6) so that at those specific points the iron content will be higher. The line scan was performed with a probe size of ∼35 nm over the white line in Figure 9 and shows a locally higher concentration of the iron content at that particular point on the line. The identification of individual nanoparticles was accomplished using the Fourier transform analysis of high-resolution lattice images as illustrated in Figure 10. Inset a is a TEM micrograph displaying a few carbon-coated magnetic nanoparticles at the edge of a microsphere obtained at 600 °C. The central image (b) is the high-resolution electron micrograph showing an individual nanoparticle, a typical carbon-coated magnetic iron 11076
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Figure 8. SEM images of the (a) PDVB/Fe3O4, (bd) C/Fe3O4/Fe, and (e) C/Fe composite microspheres obtained by annealing the Fe(CO)5entrapped PDVB particles at 300, 500, 600, 800, and 1100 °C, respectively.
Figure 9. STEM bright-field images of the microspheres obtained at 300 and 800 °C. Elemental line scans are shown for Fe (blue) and C (red). These line scans show the changes in the C/Fe ratio as the annealing temperature increases. The white arrow marks the line on the microsphere where the line scan was performed.
oxide nanoparticle (the particle marked by the white arrow in a). In this micrograph, the crystalline nanoparticle displays a typical lattice-fringe contrast, and the graphitic carbon shell is also distinguishable (∼5 nm thick). In general, we observed graphitic carbon formation as a result of the thermal decomposition of the polymer at temperatures of 500 °C and above (as seen in Figures 12 and 13). The high-resolution image (Figure 10b) was taken by focusing on the core particle (the iron oxide, not the graphitic carbon shell) and presents an analysis of a partially carbon entrapped iron oxide nanoparticle. In general, the carbon layer protects the iron oxidation state. Inset c in the top right corner of Figure 10 represents the computed Fourier transform pattern of the portion of the image marked by the white square in b. An analysis of this pattern revealed sets of reflections that could be referred to as the cubic FCC structure of the magnetite (Fe3O4, a = 8.39 Å) and was successfully indexed. Inset d in the lower right corner of the image represents the filtered and magnified portion of the image outlined by the white square in b. The distances measured between lattice fringes were 0.48 and 0.3 nm,
which match interplanar spacings d111 and d220, respectively, in the cubic FCC structure of Fe3O4. Because other possible oxides, such as maghemite (γ-Fe2O3) and magnetite (Fe3O4), are structurally similar, they cannot be distinguished according to their electron diffraction patterns or the Fourier transform analysis of the lattice fringe high-resolution images. The supporting evidence that the resulting compounds are magnetite and not maghemite is presented in the elemental analysis section of Table 2. Hence, the iron oxide obtained in this work was assigned to the cubic FCC unit cell that represents the structure of the magnetite (Fe3O4, a = 8.39 Å, SG:Fd3m PDF no. 19-0629). Figure 11 represents the isothermal field dependence of the magnetization measured at 300 K of (a) the PDVB/Fe3O4 microspheres obtained at 300 °C, the C/Fe3O4/Fe microspheres obtained at (b) 500, (c) 600, and (d) 800 °C, and (e) the C/Fe microspheres obtained at 1100 °C. The particles show ferromagnetic-type curves due to the presence of the hysteresis loop. The magnetic susceptibility (MS) and coercivity values obtained at 300 K are summarized in Table 1. The MS bulk value for zero-valent iron is 222 emu g1 . In general, the MS value increases as the temperature of the thermal decomposition of the particles increases. For example, increasing the decomposition temperature from 300 to 500, 800, and 1100 °C leads to an increase in the MS value from 12.0 to 102.8, 146.2, and 166 emu g1, respectively. Magnetization curve a represents particles with only a magnetite phase. The MS value for bulk magnetite is 92 emu g1. However, the value that was measured was much lower than the bulk value because of the high percentage of carbon and low percentage of iron in these microspheres. The samples heated at 300 °C have a magnetic moment much smaller (12 emu g1) than the one expected from the magnetite percentage in the samples (21 emu g1). This is probably due to the relatively lower crystallinity of the magnetite phase obtained at 300 °C compared to that formed at higher temperatures, as shown in Figure 7a. Magnetization curves 11077
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Figure 10. (a) High-resolution micrograph of a small area at the surface of a microsphere obtained at 600 °C, displaying a few nanoparticles. Inset b is the high-resolution image of the area marked by the white arrow in part a displaying lattice-fringe contrast. A 5-nm-thick carbon coating is clearly seen. Inset c is the computed Fourier transform taken from the marked area, showing sets of reflections corresponding to the interplanar spacing, d111 and d220, belonging to the FCC structure of magnetite (Fe3O4, a = 8.39 Å). Inset d is the processed and magnified image of the marked area (white square) showing the lattice fringes of the d111 and d220 planes with the marked interplanar spacings.
Figure 11. Magnetization vs magnetic field at 300 K of (a) the PDVB/ Fe3O4 microspheres obtained at 300 °C, the C/Fe3O4/Fe microspheres obtained at (b) 500, (c) 600, and (d) 800 °C, and (e) the C/Fe microspheres obtained at 1100 °C .
bd represent particles containing magnetite (Fe3O4) and elemental iron (Fe). The MS values increase as a result of the magnetite phase gradually turning into the elemental iron phase. The microspheres that were thermally decomposed at 1100 °C (e) have an MS value of 166 emu g1. This value is equivalent to the bulk value of 222 emu g1 because these microspheres contain 74.2% Fe and 25.8% C as shown in Table 2.
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Figure 12. (a) Bright-field and (b) dark-field electron micrograph images of the C/Fe3O4/Fe composite microsphere obtained at 500 °C. In the DF image, the bright spots are the crystalline onionlike graphitic carbon obtained. (c) High-resolution image of the area marked by the white square in part a, exhibiting the fullerene onionlike coating of the magnetic nanoparticle. (d) SAED taken from the microsphere that was successfully indexed in terms of the hexagonal graphitic carbon with cell parameters of a = 2.47 and c = 6.724. (e) High-resolution image of the carbon coating displaying lattice-fringe image d = 0.34 nm of the (00.2) carbon plane.
Onionlike fullerene coatings were observed in this study on the surface of the magnetic C/Fe3O4/Fe and C/Fe particles. TEM combined with HRTEM studies have proven that the coating is a layer of crystalline graphitic carbon of varying thickness. Figures 12 and 13 exhibit TEM micrographs of the composite microspheres obtained at 500 and 1100 °C, respectively, displaying the carbon graphitic coating. The bright field (BF) and the dark field (DF) images are shown in a and b, respectively. The bright spots displayed in b are crystalline graphitic carbon, the onionlike fullerenes that coat the magnetic nanoparticles in the microsphere. Part c represents the magnified portion of the image outlined by the white square (Figure 12a) and the area marked by the arrow in Figure 12b. Part d is the SAED of the microsphere showing the (00.2), (01.1), and (00.4) reflections of the hexagonal graphitic carbon with cell parameters of a = 2.47 and c = 6.724 (SG: P63/mmc) (PDF no. 41-1487). Part e is the high-resolution image of the carbon coating, displaying latticefringe image d = 0.34 nm of the (00.2) carbon plane. In Figure 13, c and e are magnified images of a nanoparticle at the surface of the microsphere obtained at 1100 °C. This amazing phenomenon of fullerene formation can be observed at both 500 and 1100 °C and all temperatures in between. It can be seen that at both temperatures presented the structure of the fullerene and the electron diffraction are the same, which indicates that despite the extreme increase in the annealing temperature the structure remains the same and the microspheres do not collapse and lose their structure. Additional evidence that the composite microspheres annealed at 800 and 1100 °C contain graphitic carbon can be seen in the Raman spectra of these particles, as shown in Supporting Information Figure S2. 11078
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel: 972-3-5317743. Fax: 972-3-7384053.
’ ACKNOWLEDGMENT This study was partially supported by a Minerva grant (microscale and nanoscale particles and thin films for medical applications). ’ REFERENCES
Figure 13. (a) Bright-field and (b) dark-field electron micrograph images of the C/Fe composite microsphere obtained at 1100 °C. In the DF image, the bright spots are the crystalline onionlike graphitic carbon obtained. Inset c is the high-resolution image of the area marked by the white square in a, exhibiting the fullerene onionlike coating of the magnetic nanoparticle. Inset d is the SAED taken from the microsphere that was successfully indexed in terms of hexagonal graphitic carbon with cell parameters of a = 2.47 and c = 6.724. Inset e is a high-resolution image of the carbon coating displaying lattice-fringe image d = 0.34 nm of the (00.2) carbon plane.
’ SUMMARY This work demonstrates a simple method for the preparation of magnetic PDVB/Fe3O4, C/Fe3O4/Fe, and C/Fe composite microspheres with a narrow size distribution. The particles’ size and size distribution, composition, crystallinity, magnetic properties, and surface area can be controlled by the annealing temperature. All of the particles can be dispersed in ethanol and water following a short period of sonication. The particles annealed at temperatures of 5001100 °C are protected from iron oxidation by the graphitic onionlike fullerene layers. The formation of graphitic onionlike fullerene layers reported in the literature occurs at significantly higher temperatures of at least 750 °C.44,45 Obtaining the carbon graphitic layers at such a low temperature of 500 °C is a unique phenomenon and probably occurs because of the presence of iron and iron oxide. This unique behavior, of course, requires further investigation. The particles preserve their magnetic properties for at least 1 year. In the future, further research will be performed to obtain C/Fe3O4 and C/Fe microspheres with significantly higher surface areas by activation of the particles with carbon dioxide.46 The high-surface-area magnetic carbon microspheres produced may then be used for separation processes and for various biomedical applications.2630,47,48 ’ ASSOCIATED CONTENT
bS
Supporting Information. EDS spectrum recorded from nanoparticles contained in a microsphere annealed at 300 °C showing characteristic peaks of elements C, O, and Fe. Raman spectra of the PDVB/Fe3O4, C/Fe3O4/Fe, and C/Fe composite microspheres. This material is available free of charge via the Internet at http://pubs.acs.org.
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