Article pubs.acs.org/Langmuir
Engineered Magnetic Core−Shell SiO2/Fe Microspheres and “Medusa-like” Microspheres of SiO2/Iron Oxide/Carbon Nanofibers or Nanotubes On Mero,† Moulay-Tahar Sougrati,‡ Jean-Claude Jumas,‡ and Shlomo Margel*,† †
Institute of Nanotechnology and Advanced Materials, Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel Institute Charles Gerhardt (UMR 5253 CNRS), Université Montpellier 2, CC 1502, Place E. Bataillon, 34095 Montpellier, Cedex 5, France
‡
S Supporting Information *
ABSTRACT: Iron oxide (IO) thin coatings of controlled thickness on SiO2 microspheres of narrow size distribution were prepared by decomposition at 160 °C of triiron dodecacarbonyl onto silica microspheres dispersed in diethylene glycol diethyl ether free of surfactant or stabilizer. The dried washed SiO2/IO core−shell microspheres were annealed at different temperatures and time periods under inert (Ar) or reducing (H2) atmosphere. The effect of temperature on the chemical composition, morphology, crystallinity, and magnetic properties of the IO and the elemental Fe nanoparticles type coatings onto the SiO2 core microspheres has been elucidated. “Medusa-like” SiO2/IO/carbon nanofibers and tubes particles were prepared by CVD of ethylene on the surface of the SiO2/IO microspheres at different temperatures. The morphology change of the grafted carbon nanofibers and tubes as a function of the CVD temperature was also elucidated.
1. INTRODUCTION The preparation of core−shell particles has attracted considerable interest in the past decade since the properties of these core−shell composite particles can be tailored to fit a variety of applications depending on the characteristics of the core and the shell.1,2 Materials with shells consisting of magnetic nanoparticles (e.g., Fe or IO) have been intensively studied in recent years due to the chemical and physical properties of the magnetic nanoparticles which differ significantly from bulk materials. These nanoparticles have been used in a broad range of applications such as hyperthermia,3,4 magnetic resonance imaging (MRI),5 biomedical applications,6 catalysis,7−9 cell separation,10 and cancer identification and therapy.11 Core−shell SiO2/IO particles, where the IO shell is composed of maghemite (γ-Fe2O3) or magnetite (Fe3O4), are known to be useful as catalysts for several processes.8,9 Organic polymers such as polystyrene and poly(methyl methacrylate) are good candidates as core compounds.12,13 Nevertheless, inorganic cores possess superior thermal stability for the core−shell composites and enable the modulation of their magnetic properties by a simple thermal treatment.14 Nanocomposites of Fe/SiO2 possess these dual characteristics15,16 and are suitable for this purpose. Indeed, different procedures were applied to deposit uniform silica shells on IO nanoparticles.17,18 The purpose of this article, however, is to coat SiO2 micrometer-sized particles with a thin coating of IO or Fe. Some attempts to deposit iron compounds on SiO2 microspheres can be found in the literature, but most did not result in homogeneous coatings.19,20 Clusters and © 2014 American Chemical Society
nonadherent coatings have been reported by sonication of Fe(CO) 5 solutions in decalin in the presence of SiO2 particles.17 Also, low loadings of magnetite nanoparticles were coated on SiO2 particles by charging the silica surface with a polyelectrolyte cationic film.20 IO layers on SiO2 particles were also prepared via hydrolysis of an iron(III) acetylacetonate precursor in water/ethanol solutions containing SiO2 core particles and sodium dodecyl sulfate (SDS) at 60−85 °C.21 However, the obtained coating was of hematite with a poor magnetic saturation value, and even after reduction of the hematite to elemental Fe the magnetic saturation remained fairly low. Core−shell magnetic SiO2 particles of high magnetic moment may be obtained by decomposition of a zerovalent Fe precursor onto SiO2 particles as already described for the synthesis of Fe thin films onto Si wafers.22,23 For this purpose iron carbonyl compounds are very useful, since they can easily be dissociated, and CO is a labile ligand that can easily be removed from the reaction mixture. A common method for preparation of IO and Fe nanoparticles is based on the decomposition of iron pentacarbonyl (Fe(CO)5) in an organic continuous phase.24,25 However, Fe(CO)5 is a severely toxic volatile liquid according to the MSDS of this material.26 In contrast, the Fe precursor triiron dodecacarbonyl (Fe3(CO)12) is a solid powder that decomposes at 160 °C without a melting point.27 Moreover, Fe3(CO)12 is at least 100 times less toxic Received: June 6, 2014 Revised: August 4, 2014 Published: August 4, 2014 9850
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than Fe(CO)526 and has already been proved as a good precursor for the preparation of magnetite nanoparticles of narrow size distribution.26 The present article describes a simple new procedure to coat SiO2 microspheres with homogeneous layers of IO nanoparticles by thermal decomposition of (Fe3(CO)12) at 160 °C onto uniform SiO2 microspheres dispersed in diethylene glycol diethyl ether as a continuous phase. The dried SiO2/IO particles prepared at 160 °C were annealed at 450 and 750 °C under Ar flow for 2 h. In addition, the IO coating of the dried SiO2/IO particles prepared at 160 °C was reduced to elemental Fe by H2 flow at 450 °C for 2 and 6 h. The chemical composition, morphology, crystallinity, and magnetic properties of the IO and the elemental Fe coatings were found to be dependent on the annealing temperature and the inert gas atmosphere. “Medusa-like” SiO2/IO/carbon nanofibers and tubes particles were prepared by CVD of ethylene at 450, 500, and 600 °C on the surface of the SiO2/IO microspheres prepared at 160 °C.
controllers (model number 247D). For the preparation of the SiO2/ IO/carbon nanofibers and tubes particles, droplets (8.0 μL) taken from the prepared core−shell SiO2/IO microspheres dispersion in isopropanol (0.5 mg/mL) were placed on alumina-coated Si wafers. These samples were then placed in a crucible in the heating zone at the bottom of the quartz tube. The preheating temperature in the first furnace was set at 770 °C, and the growth temperature of the carbon nanofibers and tubes particles in the second furnace was set at various temperatures: 450, 500, and 600 °C. The process consisted of a 5 min annealing step with flows of Ar (100 sccm) and H2 (400 sccm), followed by 15 min growth step with the additional flows of C2H4 (200 sccm) and Ar−O2 (100 sccm) which contains a mixture of 99% Ar and 1% oxygen and when combined with H2 forms a control amount of vapor. At the end of the growth step, the Ar−O2 and C2H4 flows were stopped, 1 min later the H2 flow was stopped, and after another minute the quartz tube was pushed out from the furnace, and the samples were cooled to room temperature under Ar flow. 2.3. Characterization. A high resolution transmission electron microscope, HRTEM (JEOL JEM- 2100 (LaB6) at 200 kV), was used for investigating the structure of the SiO2/IO, SiO2/Fe, and SiO2/IO/ carbon nanofibers and tubes composite particles. The HRTEM was integrated with a digital scanning device (STEM) comprised of annular dark-field (DF) and bright-field (BF) detectors and a Thermo Scientific energy-dispersive X-ray spectrometer (EDS) system for elemental analysis. TEM observations were made by taking BF and DF images, HRTEM images, and selected area electron diffraction (SAED) patterns. Fast Fourier transform analysis of high resolution images technique was used for structural analysis of the IO and Fe thin coatings. Elemental analysis, scanning, and mapping were performed in the STEM mode, and the elemental analysis was performed using the EDS system equipped with the Noran System Six software. Specimens for observation in the HRTEM were prepared via an ultramicrotome device (PT-PC PowerTome Ultramicrotome, Boeckeler Instruments Inc., Tucson, AZ): powders were immersed in an epoxy resin (part A) and mixed with a slow curing hardener (part B) (PELCO Epoxy Resin, TED PELLA, Inc.). The mixture was left to dry for 24 h, and then the hard matrix was fixed on a stage and vertically sliced to 70 nm films by a diamond knife advancing via a rotary system. The slices were moved by a ring manipulator into the carbon TEM grid for characterization. Surface morphology was accomplished with a FEI scanning electron microscope (SEM) Model Inspect S. For this purpose, a drop of dilute microsphere dispersion in ethanol was spread on a silicon wafer surface that was attached by carbon tape and then dried at room temperature. The dried sample was coated with iridium in a vacuum before viewing under SEM. The average size and size distribution of the electronic images of the dry nanoparticles taken by TEM and SEM were determined by measuring the diameter of approximately 200 particles with AnalySIS Auto (Soft Imaging System GmbH, Germany) image analysis software. The Fe content of the composite particles was determined by inductive coupled plasma (ICP) analysis (ULTIMA JY2501). Isothermal magnetization measurements at room temperature were performed in a commercial (Quantum Design) superconducting quantum interference device (SQUID) magnetometer. Mössbauer spectroscopy (MS) studies were performed using a conventional constant acceleration drive and a 25 mCi 57Co:Rh source. The velocity calibration was performed using a room temperature α-Fe absorber, and the isomer shift (IS) values reported are relative to that of Fe. The observed spectra were least-squares fitted to theoretical spectra, using Lorentzian lines. The absorbers are made by mixing 20−50 mg of samples with 50−80 mg boron nitride as a binder.
2. EXPERIMENTAL PART 2.1. Materials. The following analytical-grade chemicals were purchased from Aldrich and used without further purification: ethanol, triiron dodecacarbonyl, Fe3(CO)12, powder protected by 5−10% (w/ w) ethanol, diethylene glycol diethyl ether (>98%), ammonium hydroxide (28%), and tetraethyl orthosilicate (TEOS, >98%). Ultrahigh purity precursor gases ethylene (C2H4), Ar, H2, and a mixture of Ar with 1% O2 (Ar−O2) were purchased from Maxima Air Separation Center Ltd., Israel. Alumina (Al2O3)-coated Si wafers were prepared at the Hebrew University, Jerusalem, Israel, by e-beam evaporation of 10 nm interlayer of alumina on Si wafers. Water was purified by passing deionized water through an Elgastat Spectrum reverse osmosis system (Elga Ltd., High Wycombe, UK). 2.2. Synthetic Route. 2.2.1. Synthesis of SiO2 Microspheres of Narrow Size Distribution. SiO2 microspheres were synthesized according to the Stöber−Fink−Bohn method.28 Briefly, SiO2 microspheres of 630 ± 26 nm were prepared by adding ethanol (73 mL), water (10.8 mL), and NH4OH (28%) (9.8 mL) to a flat-bottomed flask equipped with a magnetic stirrer. Tetraethyl orthosilicate (5.6 mL) was added all at once to the flask, which was stirred for 12 h. The obtained SiO2 microspheres were then washed from excess reagents by extensive centrifugation cycles with ethanol and water, in turn. 2.2.2. Synthesis of SiO2/IO and SiO2/Fe Core−Shell Microspheres. The 630 ± 26 nm silica particles (2.5 g) were dispersed in diethylene glycol diethyl ether (50 mL) and then added to a three-neck roundbottom flask immersed in a silicone oil bath and equipped with a condenser corked with a drying tube and a mechanical stirrer. The temperature in the flask was raised to 70 °C, the system was stirred for 1 h, then Fe3(CO)12 (0.4 mmol) was added to the reaction flask, and the solution was mixed for an additional 1 h at 70 °C. The temperature in the flask was then raised to 160 °C for 2 h and then cooled to 70 °C. This coating process of IO onto the SiO2 microspheres was then repeated an additional three times. The obtained SiO2/IO core−shell microspheres were then washed by extensive centrifugation cycles with ethanol and water, in turn, and then dried by lyophilization. Portions of the obtained dry powder were annealed at 450 and 750 °C under Ar flow for 2 h. Additional portions of the dry powder were annealed at 450 °C for 2 and 6 h under a H2 atmosphere to obtain the SiO2/Fe core−shell microspheres. The rest of the dry powder was used for the synthesis of the “Medusa-like” SiO2/IO/carbon nanofibers and tubes composite microspheres, as described in the next paragraph. 2.2.3. Synthesis of “Medusa-like” SiO2/IO/Carbon Nanofibers and Tubes Composite Microspheres. “Medusa-like” SiO2/IO/carbon nanofibers and tubes particles were prepared by a CVD coating process, in a similar way to that published in our previous papers.20,21 Briefly, the CVD system used was composed of two one-zone tube furnaces (Lindberg Blue M, model number TF55035C-1) with a quartz tube with an external diameter of 25 mm and MKS mass flow
3. RESULTS AND DISCUSSION Uniform SiO2 microspheres of 630 ± 26 nm size and smooth surface morphology were prepared by the Stöber method.28 Figure S1 (see Supporting Information) illustrates a typical SEM micrograph of these particles. Brunauer−Emmett−Teller (BET) measurements of these particles gave a surface area of 6.3 m2/g, similar to the theoretically calculated surface area of 9851
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nonporous silica microspheres of 630 nm diameter (5.3 m2/g). This insignificant difference in the surface area is indicative of the nonporous nature of the prepared SiO2 microspheres. Uniform magnetic SiO2/IO core−shell composite microspheres were then prepared by decomposition at 160 °C of Fe3(CO)12 on the silica particles dispersed in diethylene glycol diethyl ether, according to the description in the Experimental Part. The thickness of the IO coating on the SiO2 particles obtained in this process can be varied by the number of the coating process repetitions. In a one-cycle coating process (using 0.4 mmol of Fe3(CO)12) a nonhomogenous coating composed of IO nanoparticles of 5 ± 1 nm diameter surrounding the silica microspheres was produced, as shown in Figure S2. ICP measurements showed that the formed SiO2/ IO core−shell particles obtained after one coating cycle contains approximately 2% Fe of the total particles’ weight. In an effort to maintain the IO coating on the SiO2 particles, four cycles of the coating process (1.6 mmol) of Fe3(CO)12 were performed, so that all reported results from now on are related to the SiO2/IO particles produced after four cycles of the IO coating process. The first layer of IO on the surface of the SiO2 particles was probably formed as a consequence of the decomposition of absorbed Fe3(CO)12 on the surface of the silica, followed by the deposition and decomposition of more of the soluble precursor on the new surface to create these core− shell particles. The core−shell structure of these particles is illustrated in Figure S3, by cross-sectional TEM elemental mapping of these particles. These coated dry particles prepared at 160 °C were annealed at various temperatures (450 and 750 °C) under an inert (Ar) atmosphere, In addition, the IO coatings of these dried SiO2/IO particles were reduced to elemental Fe by H2 atmosphere at 450 °C for 2 and 6 h. The core−shell particles of all samples, as measured by ICP, indicated 7% Fe content of the total weight of the particles. The chemical composition, morphology, crystallinity, and magnetic properties of the IO and the elemental Fe coatings have been elucidated by various methods such as HRTEM, XRD, Mössbauer spectroscopy, and magnetic measurements. Figures 1A and 2A show SEM and cross-section TEM images, respectively, of the noncoated SiO2 particles (630 ± 26 nm). Figures 1B, 2B, and 2E illustrate the core−shell structure of the SiO2/IO particles obtained at 160 °C, as described in the Experimental Part. It is clearly observed in Figure 2E that the IO shell is composed of sintered nanoparticles, each with a diameter of 5 ± 1 nm. The measured thickness of the IO coating of these SiO2/IO particles was 40 ± 8 nm. Annealing of these dried SiO2/IO particles under Ar atmosphere at 450 °C for 2 h led to an increase in nanoparticle size from 5 ± 1 to 9 ± 2 nm, as demonstrated in Figures 1C and 2C,F. Annealing of the dried SiO2/IO particles obtained at 160 °C under an Ar atmosphere at 750 °C for 2 h led to aggregation of the IO nanoparticles on the SiO2 microspheres and thereby formation of a thin coating (Figure 2D) composed of nanoparticles of 10.5 ± 4 nm (Figure 1D). It should also be noted that the boundary between these SiO2 particles and the IO coating annealed at 750 °C is not clear, probably due to the formation of hybrids between Fe and SiO2 as demonstrated later in XRD and Mössbauer spectroscopy. Efforts to obtain zerovalent Fe coating on the SiO2 microspheres were made by reducing the IO coating on the SiO2 particles with H2 at 450 °C for 2 and 6 h. The X-ray diffraction (XRD) pattern and Mössbauer spectroscopy (see
Figure 1. SEM images of the SiO2 (A) and the SiO2/IO (B−D) core− shell particles prepared as described in the Experimental Part. Image B demonstrates the SiO2/IO core−shell particles prepared at 160 °C; images C and D demonstrate the SiO2/IO core−shell particles annealed under an Ar atmosphere for 2 h at 450 and 750 °C, respectively.
Figure 2. Typical cross-section HRTEM images of the SiO2/IO core− shell particles. Image A represents noncoated SiO2 microspheres, image B demonstrates the SiO2/IO core−shell particles prepared at 160 °C, images C and D demonstrate the SiO2/IO core−shell particles annealed under an Ar atmosphere for 2 h at 450 and 750 °C, respectively, and images E and F illustrate magnification of the insets marked in images B and C, respectively, showing the iron oxide coating structure.
below) after the 2 h reducing period indicated that indeed most of the IO reduced to elemental Fe, although quite a significant percentage of IO still remained in the coating. Efforts to increase the elemental Fe phase and to decrease that of the IO phase were made by increasing the reduction time at 450 °C from 2 to 6 h. Indeed, the XRD pattern after the 6 h annealing period indicates a slight increase in the elemental Fe phase and decrease in the IO. Further efforts to increase the weight percent ratio [Fe]/[IO] by a reduction process at 450 °C for 14 h did not lead to any significant change in this ratio. Figures 3 and 4 reveal, by HRTEM micrographs, the nanostructural differences between the IO and the elemental Fe coatings on 9852
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“reflection spots” on the SAED were successfully indexed in terms of the cubic FCC unit cell that represents the structure of the IO. The distances measured from the SAED pattern were 0.29, 0.25, and 0.20 nm, matching the interplanar spacings d220, d311, and d400, respectively, in the above structure of IO. Image D, which represents a magnification of the square insert on the particle shell shown in image A, indicates that this IO phase may belong to Fe3O4, by showing distances between the lattice fringes of 0.48 and 0.3 nm which fit to the interplanar spacings d111 and d220, respectively, of Fe3O4 cubic FCC structure. Figures 4A−C (image B is a magnification of the inset shown in image A) exhibit typical HRTEM images of a SiO2/Fe core− shell particle obtained by annealing the SiO2/IO core−shell microspheres at 450 °C under a H2 atmosphere for 2 h. These HRTEM micrographs clearly represent the significant surface morphology change of the iron phase on the SiO2 microsphere obtained by the H2 reduction process. The Fe phase is now composed of 40 ± 7 nm core−shell nanoparticles with an axial length of 20 ± 4 nm height, probably as a result of sintering of the IO nanoparticles which occurred as a consequence of the reduction process. This new Fe phase nanostructure appears to be composed of a nanocrystal core coated by a thin IO layer (7 ± 2 nm) as shown by the lattice fringes. It should be noted that the composition of the nanocrystal core could not be determined by this method due to the interruption of the IO layer. Image C shows another particle from the same sample for which images D and E represent the crystal diffraction pattern and DF image, respectively. The distances measured from the SAED pattern in image D were 0.2, 0.14, and 0.17 nm, fitting the interplanar spacings d110, d200, and d211, respectively, matching the structure of zerovalent Fe. The zerovalent Fe fringes of these nanoparticles, however, are not visible since they are coated by thin IO layers as observed in image B. Crystalline phase structures and oxidation states of all the obtained particles were measured by XRD (Figure 5) and Mö ssbauer spectroscopy (Figure 6) which are used as complementary tools to distinguish between different iron
Figure 3. Typical HRTEM images of a single SiO2/IO core−shell particle annealed at 450 °C under an Ar atmosphere for 2 h. Image B illustrates the crystal diffraction of the IO coating of image A, image C represents the dark field of image A, and image D exhibits higher magnification of the inset shown in image A.
Figure 4. Typical HRTEM images (A−C) of a SiO2/Fe core−shell particle obtained by annealing the SiO2/IO core−shell microspheres at 450 °C under a H2 atmosphere for 2 h. Image B represents higher magnification of the Fe coating marked by the inset shown in image A. Image C represents another particle from the same sample, image D illustrates the X-ray crystal diffraction of the zerovalent Fe coating shown in image C, and image E represents the dark field of image C taken from the X-ray crystal diffraction of image D.
the shell of the SiO2 particles, annealed at 450 °C for 2 h under an Ar or H2 atmosphere, respectively. It should be noted that the HRTEM pictures of the samples annealed at 450 °C under a H2 atmosphere for 6 h appears to be the same as that annealed for 2 h. Figure 3A exhibits a typical cross-section HRTEM micrograph of a single SiO2/IO core−shell particle annealed at 450 °C under an Ar atmosphere for 2 h. This image clearly demonstrates that the coating is composed of defined nanoparticles of 9 ± 2 nm diameter. Figures 3B and 3C illustrate by the selected area electron diffraction (SAED) and DF image, respectively, that these nanoparticles are of IO (magnetite or maghemite). The ring diffraction pattern shown in image B exhibits the crystalline structure of the IO nanoparticles present in the analyzed area. Individual crystalline nanoparticles of IO are clearly seen in the DF image (image C) taken in the (220 and 311) reflection. The
Figure 5. Image A demonstrates typical XRD pattern of the SiO2/IO core−shell particles prepared at 160 °C as described in the Experimental Part. Images B and C demonstrate XRD patterns of the SiO2/IO core−shell particles annealed under an Ar atmosphere for 2 h at 450 and 750 °C, respectively. Images D and E demonstrate XRD patterns of the SiO2/IO core−shell particles annealed under a H2 atmosphere at 450 °C for 2 and 6 h, respectively. 9853
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nanoparticles (IO NP a) and well-ordered magnetite nanoparticles, (Fe3O4 (a,b)). The ratio between IO NP (b)/IO NP (a) is related to the size of the IO (Fe3O4, γ-Fe2O3 or a mixture of them) nanoparticles.30 Increasing this ratio leads to an increase in the diameter of the obtained IO nanoparticles.32−34 This ratio was found to be temperature related. For example, SiO2/IO core/ shell particles prepared at 160 °C whose Mössbauer spectrum indicates only a central doublet curve (IO NP (a)) (Figure 6A) is composed of IO nanoparticles with a diameter of 5 ± 1 nm (Figure 4E). On the other hand, SiO2/IO core/shell particles annealed at 450 °C under an Ar atmosphere for 2 h whose Mössbauer spectrum (Figure 6B) indicates a ratio between the broad peak/doublet peak of 3.13/1 (as calculated from the ratio of the area below the curves) is composed of IO nanoparticles with a diameter of 9 ± 1 nm (Figure 2F), while particles annealed at 750 °C under an Ar atmosphere for 2 h whose Mössbauer spectrum (Figure 6C) indicates a ratio between the broad peak/doublet peak of 5.14/1 is composed of IO nanoparticles with a diameter of 10.5 ± 4 nm (Figure 2D). A summary of the weight percent of the three IO phases obtained by annealing the SiO2/IO microspheres obtained at 160 and 450 °C under an Ar atmosphere for 2 h can be found in Table S1: Fe3O4 38%, IO NP (a) 15% and IO NP (b) 47%. The XRD pattern of the SiO2/IO core−shell particles annealed at 750 °C for 2 h exhibits peaks of Fe3O4, α-Fe2O3, and Fe2SiO4 crystalline structures, as shown in Figure 5C. The most prominent structure (marked in black) corresponds to the cubic bcc crystal planes of Fe3O4 (MW = 55.85 g/mol, SG: Fd3m, Z = 8, a = 8.4 Å) at 2θ = 30.06 (2,2,0), 2θ = 35.41 (3,1,1), 2θ = 43.04 (4,0,0), 2θ = 53.39 (4,2,2), 2θ = 56.91 (5,1,1) and 2θ = 62.49 (4,4,0). The second prominent structure (marked in blue) corresponds to the orthorhombic crystal planes of olivine (iron silicate-Fe2SiO4) at 2θ = 25.02 (1,1,1), 2θ = 31.65 (1,3,0), 2θ = 51.36 (2,2,2), and 2θ = 51.53 (2,4,0). The third structure (marked in red) corresponds to the rhombo H crystal planes of hematite (α-Fe2O3) at 2θ = 33.12 (1,0,4), 2θ = 49.53 (2,2,0), and 2θ = 54.15 (2,3,1). The Mössbauer spectrum of the same particles confirmed the XRD results, as shown in Figure 6C. This spectrum exhibits two additional IO phases to those obtained by annealing at 450 °C: hematite (marked by an orange line) and 2% high spin Fe2+ (marked by a blue line). This last component is characteristic of olivine iron silicate35 that probably was obtained as a result of an interface reaction between magnetite and silica, which is characteristic at this annealing temperature. Table S1 summarizes the weight percent of the six IO phases obtained by annealing the SiO2/ IO microspheres at 750 °C under an Ar atmosphere for 2 h: Fe3O4 52%, IO NP (a) 7%, IO NP (b) 36%, Fe2SiO4 2%, and α-Fe2O3 3%. XRD patterns (Figures 5D,E) and Mö ssbauer spectra (Figures 6D,E) were also taken for the SiO2/IO particles annealed at 450 °C under a H2 atmosphere for 2 h (Figure 5D) and 6 h (Figure 5E). The XRD pattern observed after a 2 h reduction period indicates, in addition to the IO peaks, new peaks at 2θ = 45.5 (1,1,0) and 2θ = 66 (2,2,0) corresponding to the bcc cubic structure of zerovalent Fe. Efforts to increase the elemental Fe phase and to decrease that of the IO phase were made by increasing the reduction time at 450 °C from 2 to 6 h. Indeed, the XRD pattern (Figure 5E) indicates a slight increase in the elemental Fe phase and decrease in the IO phase. A further trial to increase the weight percent ratio [Fe]/[Fe3O4] by a reduction process at 450 °C for 14 h did not lead to any
Figure 6. Room temperature Mössbauer spectra of the SiO2/IO core− shell particles prepared at 160 °C (A). SiO2/IO core−shell particles annealed under an Ar atmosphere for 2 h at 450 and 750 °C (B and C, respectively). Spectra D and E are of SiO2/IO core−shell particles annealed at 450 °C under a H2 atmosphere for 2 and 6 h, respectively.
oxides, iron silicates, and zerovalent Fe. The information gathered from the XRD patterns of the core−shell particles prepared at 160 °C indicated lack of crystallinity, as can be observed from Figure 5A. The Mössbauer spectrum of the same core−shell particles exhibits only a central doublet curve (marked by a red line, Figure 6A) that can be assigned either to Fe3O4 nanoparticles or to γ-Fe2O3 nanoparticles29 or to a mixture of them. For simplicity, this IO phase is marked in Table S1 (in the Supporting Information) as “IO NP (a)”. It should however be noted that a recent report by Goya et al.30 claims that this doublet curve corresponds to a superparamagnetic Fe3O4 NP of approximately 4 nm size. Table S1 indicates that 100% of the IO coating on the SiO2 microspheres was composed of IO NP (a). The XRD pattern of the these core−shell particles annealed at 450 °C under an Ar atmosphere for 2 h exhibits, as observed in Figure 5B, peaks at 2θ = 30.06 (2,2,0), 35.41 (3,1,1), 43.04 (4,0,0), 53.39 (4,2,2), 56.91 (5,1,1), and 62.49 (4,4,0) corresponding to the cubic bcc crystal planes of Fe3O4 (MW = 55.85 g/mol, SG: Fd-3m, Z = 8, a = 8.4 Å). This crystalline evolution demonstrates the transition of the IO coating from mainly amorphous to crystalline magnetite. This transition affects the Mössbauer spectrum as well as the particles magnetic properties. The Mössbauer spectrum of the same particles annealed at 450 °C under an Ar atmosphere for 2 h (Figure 6 B) indicates that the IO coating under these conditions is composed of three IO phases. The first phase is composed of two sextets (marked by green (curve a) and brown (curve b) lines) which are characteristic of Fe3O4 nanoparticles with diameter size larger than 10 nm29 and are marked in Table S1 as Fe3O4 (a, b). The sextet marked by the green line (curve a) (higher field) is assigned to Fe3+ ions in both tetrahedral and octahedral sites, whereas the sextet marked by the brown line (curve b)) is assigned to iron ions with an effective valence of 2.5+ (i.e., paired Fe2+ and Fe3+ ions) at the octahedral sites.31 The second IO phase (marked by the red line) exhibits the same central doublet curve which was previously reported for the core−shell particles prepared at 160 °C and marked as “IO NP (a)”. The third IO phase is represented by a broad peak (marked by the purple line) that appears only in the presence of the central doublet curve (IO NP (a)) and marked in Table S1 as “IO NP (b)”. The broad peak signature is observed for superparamagnetic particles and suggests an intermediate particle size between the small 9854
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Figure 7. Magnetization vs magnetic field curves of the SiO2/IO core−shell particles prepared at 160 °C (curve A-a) and annealed under an Ar atmosphere for 2 h at 450 °C (curve A-b) and 750 °C (curve A-c) and of the SiO2/IO core−shell particles annealed at 450 °C under an Ar atmosphere for 2 h (curve B-a) or under a H2 atmosphere for 2 h (curve B-b) and 6 h (curve B-c). Magnetization vs temperature (ZFC/FC curve) of the SiO2/IO core−shell particles prepared at 160 °C (image C) and annealed at 450 °C under an Ar atmosphere for 2 h (image D).
(curve a), which represents the M vs H value of the SiO2/IO core−shell microspheres prepared at 160 °C, exhibits Ms of 0.42 emu/g and apparent paramagnetic behavior with no hysteresis loop. However, a closer look at the access origin of Figure 7A (curve a), presented by the inset at the bottom-right side of this figure, exhibits superparamagnetic behavior since the curve becomes steeper near the accesses origin. Therefore, all together, Figure 7A (curve a) indicates that the IO coating is composed of both amorphous and superparamagnetic nanoparticles. Figure 7C demonstrates the zero field cool/field cool (ZFC/FC) behavior of these coated IO nanoparticles, illustrating blocking temperatures (TB) at two different temperatures: TB‑a at 25.5 °C and TB‑b at 120 °C. These two different blocking temperatures indicate that these IO-coated nanoparticles are composed of two populations of superparamagnetic nanoparticles of different sizes. The lower TB (25.5 °C) is consistent with the measured 5 ± 1 nm nanoparticles’ size, and the higher TB (120 °C) according to Goya et al.30 is related to nanoparticles of approximately 12 nm size. These larger nanoparticles may have been created as a result of 5 ± 1 nm nanoparticles that underwent a sintering process to form larger particles. It should be noted that the Mössbauer spectroscopy measurements (Table S1) of these particles did not reveal the two populations and indicated only one population (IO NP (a)) of 5 ± 1 nm diameter (Figure 2E)). Curves b and c in Figure 7A and Table S2 reveal that after annealing at 450 and 750 °C under an Ar atmosphere for 2 h, these particles show a ferromagnetic graph structure with a hysteresis loop (coercivity of 26 Oe for the particles annealed at
significant change in the XRD pattern. Mössbauer spectra of the same samples, as shown in Figures 5D,E, demonstrate the formation of three phases where two of the phases, characteristic of metallic iron, are marked in Table S1 as α-Fe (a) (green line in Figures 6D,E) and α-Fe (b) (light blue line in Figures 6D,E), and a third phase corresponds to IO NP (a). The Mössbauer spectra of the two metallic Fe phases are fitted using two sextets of iron that differ only by the values of the hyperfine field (H), as shown in Table S1. The main phase α-Fe (a) is characteristic of well-crystalline Fe nanoparticles with a hyperfine magnetic field of about 33 T. The second metallic phase α-Fe (b) is characteristic of amorphous Fe nanoparticles with broader peaks and a hyperfine magnetic field of about 26 T. The latter can be assigned to superparamagnetic zerovalent Fe nanoparticles with a diameter lower than 10 nm which are known to have an amorphous Fe phase surface.36 Table S1 indicates that after a reduction period of 2 h the weight percents of the three phases were as follows: α-Fe (a) 41%, αFe (b) 20%, and IO NP (a) 39%. Increasing the reduction time to 6 h resulted in a significant rise in the α-Fe (a) phase (from 41% to 49%), a significant decrease in the α-Fe (b) phase (from 20% to 14%), and a slight decrease in the IO NP (a) phase (from 39% to 37%). The magnetic properties (magnetic susceptibility (Ms), coercivity, and hysteresis) of the various core−shell particles demonstrate a temperature dependence, as shown in Figure 7 and summarized in Table S2 (in the Supporting Information). Figure 7A (curves a, b, and c) shows the magnetization of the sample through a variable magnetic field (M vs H). Figure 7A 9855
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450 °C and 60 Oe for those annealed at 750 °C). In addition, ZFC/FC measurements of the sample annealed at 450 °C under Ar (shown in Figure 7D) show no blocking temperature. Similar behavior (no blocking temperature) was also observed for samples annealed at 750 °C under Ar and at 450 °C under H2. These results indicate loss of the superparamagnetic nature of the coated IO nanoparticles, probably due to slight aggregation and increase in the diameter of the IO nanoparticles (from 5 ± 1 to 9 ± 2 and 10.5 ± 4 nm at 450 and 750 °C, respectively) due to the temperature annealing process, as already shown by the TEM photomicrographs of Figures 3C,D and 5C,D,F. Figure 7A, curves b and c, indicates that the magnetic susceptibility and coercivity were enhanced with temperature elevation from 0.42 emu/g and no coercivity at 160 °C up to 8.65 emu/g and 26 Oe at 450 °C, followed by a slight decrease of Ms to 5.53 emu/g and significant increase of the coercivity (60 Oe) at 750 °C. ICP measurements indicate that the Fe weight percent content of the SiO2/IO core−shell microspheres is 7%, which results by calculation in a 9.71% magnetite content. The Ms value of these particles annealed at 450 °C (8.65 emu/g) is very close to the value of bulk magnetite (92 emu/g, or 8.9 emu/g for 9.71%).37 At higher annealing temperature, e.g., 750 °C, the particles’ magnetic susceptibility decreased from 8.65 to 5.53 emu/g as shown in Table S2. The decrease in Ms is probably related to the formation of iron silicate and α-Fe2O3 compounds (see Table S1) which generally possess a low magnetic moment. Figure 7 B (curves a−c) shows the M vs H of particles annealed at 450 °C under an Ar atmosphere for 2 h (curve a) and those annealed at 450 °C under H2 for 2 h (curve b) and 6 h (curve c). H2 reduced particles show increasing magnetic susceptibility: 14.36 and 14.84 emu/g after reduction at 450 °C for 2 and 6 h, respectively, as shown in Figure 7B and Table S2. These measured Ms values are lower than those expected from bulk elemental Fe (222 emu/g, or 15.54 emu/g for 7% iron phase).38 This can easily be explained by the fact that these core−shell particles comprise, in addition to the elemental Fe phase, also approximately 38% of IO NP (a) phase as shown in Table S1. Another interesting result is the considerable increase in the coercivity with increasing H2 reduction time (from 275 Oe in 2 h to 400 Oe in 6 h), as shown in Table S2. This phenomenon is probably related to the decrease in the content of the superparamagnetic nanoparticles and formation of a more continuous thin crystalline coating with the increase in reduction time, as demonstrated by the Mössbauer results (Figure 6 and Table S2). The stability against oxidation of the 2 and 6 h reduced SiO2/Fe particles was tested by keeping them dry for a year or dispersing them in water for a 2 week period and indicated no change in the XRD patterns, Mössbauer spectra, and magnetic properties. These results may indicate that the elemental Fe nanostructures are entrapped within iron oxide nanostructures, thereby protecting the elemental Fe from oxidation. It is also possible that in addition to the IO protection layer a thin carbon layer (obtained by the decomposition of the Fe3(CO)12 precursor as already shown by our previous publications20,25) adds to the stability against oxidation of these core−shell particles. The next step was to check the possibility to obtain carbon nanotubes emerging from the SiO2/IO core−shell microspheres, following our previous publications.22,23 The mechanisms of CNT synthesis are multiple and complex and include catalysts,39 substrates,40 gases,41 and their mutual interactions.42 These carbon nanotube coatings may then be used for various
applications such as high-capacity hydrogen storage media,43 biofuel production,44 removal of metal oxides and organic contaminants,45 etc. For this purpose, the core−shell particles prepared at 160 °C were annealed at 450, 500, and 600 °C under a mixture of precursor gases (Ar, H2, C2H4, and Ar−O2) as described in the Experimental Part. Annealing the core−shell particles at 450 °C produced particles that appeared 60 nm thicker and more bulgy than particles annealed at 450 °C under Ar, as demonstrated in Figures 8A and 4C, respectively. This
Figure 8. Typical SEM micrograph of SiO2/Fe microspheres prepared by annealing of the SiO2/IO microspheres in Ar atmosphere at 450 °C (A). SEM micrographs of the SiO2/Fe/carbon nanofibers and tubes “Medusa-like” microspheres prepared by annealing of the SiO2/IO microspheres in ethylene atmosphere at 450, 500, and 600 °C (B, C, and D, respectively).
bulgy surface is composed of short carbon nanofibers with a 25 ± 7 nm diameter fish-bone structure35 with a 3.4 Å fringes spacing generated by the (002) basal plane spacing of a graphite lattice. These graphite fringes appear to be perpendicular to the growth direction as demonstrated by HRTEM micrographs in Figures 9A,D. Annealing the core− shell particles at 500 °C created irregular multiwall carbon nanotubes (MWCNT) with 17 ± 7 nm diameter and the same 0.34 nm graphite lattice fringe spacing bursting out of the particles to form a “Medusa-like” particle structure, as demonstrated in Figure 8B. A closer look at these nanotubes with HRTEM revealed that these nanotubes are indeed hollow, but their wall fringes are still not parallel to the growth direction. These fringes are angular, and their behavior is responsible for the bamboo structure of these nanotubes, as illustrated in Figures 9B,E. Annealing the core−shell particles at 600 °C created longer entangled MWCNTs with a diameter of 17 ± 7 nm, with the same graphite lattice fringes spacing (0.34 nm) and the “Medusa-like” particle structure as demonstrated in Figure 8C. A closer look at these nanotubes with HRTEM revealed that these nanotubes are indeed hollow with fringes parallel to the growth direction. These nanotubes are multiwall and entangled as demonstrated in Figures 9C,F. Annealing the core−shell particles at higher temperatures such as 700 °C caused most of the carbon nanotubes to bundle together while a few of them detached from the particle surface. Based on the results at hand, it can be observed that at 450 °C the system has 9856
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images of SiO2/IO core−shell particles prepared after four coating cycles with elemental mapping of the IO and SiO2 phases (Figure S3), a table presenting the properties and concentrations of the various Fe phases of the SiO2/IO and SiO2/Fe core−shell particles measured from the Mossbauer spectroscopy results (Table S1), and a table presenting the magnetic parameters of the SiO2/IO and SiO2/Fe core−shell particles as measured by SQUID (Table S2). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (S.M.). Notes
The authors declare no competing financial interest.
Figure 9. Typical TEM micrographs of the SiO2/IO/carbon nanofibers and tubes “Medusa-like” microspheres prepared by annealing of the SiO2/IO microspheres in ethylene atmosphere at 450, 500, and 600 °C (A, B, and C, respectively). HRTEM images of the carbon nanofibers and tubes showed in images A, B, and C (D, E, and F, respectively). The insets in images D−F present higher magnifications of the carbon lattice fringes taken from the adjacent marked rectangles.
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enough energy to produce carbon nanofibers and that the transformation mechanism from carbon nanofibers to carbon nanotubes is energy dependent and the parallel walls of the carbon nanotubes are more energy consuming than the stacked grapheme of the carbon nanofibers.
4. SUMMARY AND CONCLUSIONS The present article describes a simple synthesis of highly magnetic core−shell SiO2/IO and SiO2/Fe microspheres of narrow size distribution, by thermal decomposition of an iron precursor onto uniform SiO2 microspheres dispersed in an organic solvent with no need of a surfactant or stabilizer. This study also demonstrated that thermal and reduction treatments of the above core−shell microspheres significantly affect the chemical and physical properties (e.g., chemical composition, surface morphology, magnetic properties, and crystalline structure) of the core−shell microspheres. The stability of the reduced core−shell SiO2/Fe particles against oxidation in air and water was proven. Carbon nanofibers and tubes were also produced on the iron surface of the above core−shell microspheres through CVD of ethylene gas at different temperatures for the formation of “Medusa-like” microspheres. The structural temperature dependence of the formed carbon nanofibers and tubes and fringes directionality has been illustrated. In future work, we plan to broaden the present study by substituting the SiO 2 microspheres for SiO 2 nanoparticles. Efforts to study the use of the obtained core− shell microspheres and nanoparticles for catalysis, environmental remediation, and various biomedical applications, e.g., protein immobilization, cell labeling and separation, drug delivery, and cancer therapy by hyperthermia, will then be carried out.
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REFERENCES
ASSOCIATED CONTENT
S Supporting Information *
SEM micrograph of SiO2 core particles prepared via Stöber method images (Figure S1), cross-section TEM micrograph images of SiO2/IO core−shell particles prepared after a single coating cycle (Figure S2), cross-section TEM micrograph 9857
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