Synthesis and Characterization of Monodisperse Magnetite Hollow Microspheres

Monodisperse hollow magnetite microspheres were successfully synthesized by an one-step process through a template-free hydrothermal approach employing simultaneously a mixture of FeCl3·6H2O and ferrocene as precursor and a propylene glycol-isopropanol mixture as solvent. The morphologies and properties of the magnetite microspheres were characterized by X-ray diffraction (XRD), Raman and infrared spectroscopy, field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), adsorption of nitrogen (BET method) and SQUID magnetometry. The results show that the microspheres consist of a single crystalline phase of magnetite. On the other hand, the size and the internal hollow of the microspheres depend on the temperature of growth and can be tuned by modifying the synthesis conditions.


Introduction
During the last decade an increasing attention has been paid to the design and fabrication of nanomaterials with controlled shapes and morphologies.Recently, this research has been focused on the synthesis of functional materials with novel structures including nanorods, nanotubes, nanospheres or microspheres in a variety of sizes, ranging from a few tenths of nanometers to several hundreds of nanometers [1][2][3][4].Among these materials, magnetic nanostructures [5][6][7] have attracted a great deal of attention due to their interesting advantages such as magnetic properties [8] or low toxicity [9,10] and potential applications in different fields such as catalysis [11,12], information storage [13], ferrofluids [14], optoelectronics [15], spintronics [16], or even in biomedical fields as magnetic resonance imaging (MRI) [17,18] or drugs delivery [19,20].Magnetite (Fe 3 O 4 ) nanoparticles are among the most important magnetic materials and they have been prepared by different approaches such as sol-gel techniques [21], coprecipitation [22], ultrasound irradiation [23], hydrothermal methods [24,25], thermal decomposition of metalorganic compounds [26], and so on.Among these magnetic materials, magnetite nanoparticles with an interior void, represent an interesting family of nanostructures with paramount relevance in biological applications.These materials are characterized by having a large surface area, a very low density and also a strong magnetic response that make them candidates for a number of potential applications.Thus, the internal hollow spaces may potentially be used as hosts for the encapsulation of guest molecules and sensitive compounds such as specific drugs, catalysts, highly toxic compounds or any other group of substances that in the living environment can be directed towards the desired objective by applying external magnetic fields [27,28].Different strategies have been reported on the synthesis of monodisperse magnetite hollow particles such as the Kirkendall effect [29], molten salts [30], Ostwald ripening [31,32], using micelles as a template [33], layer by layer [34], and so on.These methods have in common the use of templates, including hard-templates (i.e.silica nanoparticles, polymer beads or carbon nanospheres) [35][36][37] and soft-templates (i.e.supramolecular micelles or vesicles, emulsion droplets and even gas bubbles) [38,39].However, due to the difficulties in controlling the size when soft-templates are used, hollow spheres are normally synthesized by the hard-template procedure.After synthesis, the template should be selectively removed by different methods (i.e.thermal decomposition, dissolution or etching) to finally obtain the hollow nanospheres.Although strategies that use templates have provided interesting results, the synthetic procedures are often very complex and tedious, resulting in low yields, high costs and even the by-products obtained after removal of the templates can contaminate the synthesized materials.In this regard, the development of new synthetic strategies necessarily involves the use of novel template-free routes to produce hollow magnetite nanoparticles with a total control of the dimensions of the internal void and the thickness of the walls.
Here, we report an one-step hydrothermal procedure without the use of surfactants or templates.In our case, we have used a mixture of FeCl 3 •6H 2 O and ferrocene as precursors and a propylene glycol-isopropanol mixture as solvent.One of the most interesting advantages of this method is the relatively low synthesis temperature used in some cases (120˚C) and also the possibility of controlling efficiently the dimensionality of the magnetite microspheres, including the thickness of the walls and the volume of the internal void.Thus, we have obtained hollow microspheres ranging from ca. 300 to 600 nm with diameters that can be adjusted by varying the synthesis temperature.

Synthesis of Hollow Microspheres
All reagents used in the present investigation were of analytical grade.FeCl 3 •6H 2 O, propylene glycol and isopropanol were provided by Sigma-Aldrich and used as received, without further purification.Ferrocene (98%) was purchased from Sigma-Aldrich and purified by double vacuum sublimation before use.MilliQ water (18.2MΩ•cm at 25˚C) was used for all experiments.
In a typical synthesis of hollow magnetite microspheres, 2 mmol of ferrocene were added to 50 mL of isopropanol.The mixture was stirred magnetically at room temperature for 1 hour.Next, 1 mmol of FeCl 3 •H 2 O was added to this solution, followed by dropwise addition of 10 mL of propylene glycol.After addition was complete, the mixture was refluxed for 2 hours.Next, the solution was allowed to cool and centrifuged at 1000 rpm for 10 min.The small sediment was discarded and the supernatant was incorporated into Teflon-lined stainless steel autoclaves of 50 mL capacity and maintained at different temperatures (ranging from 120 to 200˚C) for 10 hours.After cooling to room temperature, the black sediment is collected and washed from the impurities of the supernatant by adding isopropanol and centrifuged at least 5 times.Next, the black powder was dried overnight at 70˚C and maintained in sealed containers before characterization.
With the aim of modifying the microspheres size (including the wall thickness and void diameter) and size distribution of the nanoparticles, the influence of the reaction time, growth temperature and the initial reaction mixture was investigated.

Characterization Methods
Field emission scanning electron microscopy (FESEM) images were obtained using a JEOL JM-6400 microscope.Raman spectra were recorded using a Confocal Raman Microscope (Renishaw RM2000) equipped with laser sources at 514 nm and 633 nm and an electrically refrigerated CCD camera.The spectral resolution was set at 4 cm −1 , laser power employed was less 15 mW and acquisition time of each spectrum was around 200 sec.
XPS measurements were performed on an ESCALAB 220I-XL spectrometer, by using the non-monochromated Mg K α (1253.6 eV) radiation of a twin-anode, operating at 20 mA and 12 kV in the constant analyzer energy mode with a pass energy of 50 eV.Before spectra acquisition, the samples were degassed for 24 hours.The vacuum during spectra acquisition was better than 5 × 10 −9 mbar.In order to remove charging shifts and deal with Fermi edge coupling problems, binding energies were corrected using the peak of the C-(C,H) component coming from contamination carbon (set to 284.6 eV).
The specific surface area, the pore volume and the pore size distribution (PSD) of the hollow magnetite microspheres, were measured using an Autosorb-1.With the help of the 2 adsorption isotherms, the micropore volume, W MP [cm 2 /g] was measured using the t-plot method and the specific surface area, S [m 2 /g], applying the BET method [40].The beginning of the adsorption isotherm is used for this purpose, i.e.,: 0.00001 < P/P o < 0.02 for the t-plot and 0.05 < P/P o < 0.3, for the BET plot, where, P, is the equilibrium adsorption pressure, and P 0 , is the vapor pressure of the adsorbate at the temperature of the adsorption experiment.For the calculation of the pore size distribution (PSD), the Saito and Foley (S-F) method was employed [40].

N
Fourier transform-infrared (FT-IR) spectra were recorded in transmission mode on a Thermo Scientific Nicolet iS10 FTIR spectrometer using KBr wafers.The spectra were collected with a resolution of 4 cm −1 and were made 200 scans per sample.
XRD diffractograms were obtained using a Bruker D8 Advance system in Bragg-Brentano vertical goniometer configuration.The angular measurements (θ -2θ) were made with reproducibility of: 0.0001 degree, applying steps of 0.05 degrees from 5 to 110 degrees.The X-ray radiation source was a ceramic X-ray diffraction Cu anode tube type KFL C 2K of 2.2 kW, with long fine focus.A Ni filter was placed to eliminate the Cu-K ß radiation.
The magnetic properties (M-H curves) of the magnetite samples were measured by using a Quantum Design MPMS XL-7 superconducting quantum interference device (SQUID) magnetometer at room temperature.

Morphological Characterization
Typical synthesis of hollow magnetite nanoparticles have been carried out by using a hydrothermal procedure.Different experimental parameters, including temperature and reaction time, were tested during the synthesis to determine the effects on the morphology and size of the nanoparticles.Apparently, only the reaction temperature could be correlated with relevant changes in the nanoparticles.The effect of different reaction times (ranging from 2 h to 20 h) was studied and it was found that after 4 hours of reaction, the particles were almost formed.The average particle size does not seem to depend on the time of growth but nevertheless, the sphericity of these improved from 8 hours of reaction.For this reason, we fixed the reaction time to 10 h.Another parameter studied was the growth temperature.It was found that the average size and the hole and wall dimensions of hollow magnetite particles clearly depended on the reaction temperature.Results are summarized on Table 1.
The SEM images of the as-synthesized samples are shown in Figure 1.As can be seen there, magnetite particles show high sphericity.In all cases, the particles have an inner hole whose dimensions vary with the reaction temperature.Sample a was obtained at low temperature (120˚C).In this case, the thickness of the wall, as determined by SEM, was apparently of ca.50 nm with an average external diameter of ca.295 nm.The higher the synthesis temperature (samples b and c) the average particle diameter increases to ca. 396 and 500 nm respectively.The sample obtained at 200˚C d is characterized by having the best homogeneity.In this case, we could not observe any broken particle.In similar synthesis conditions (at temperatures ranging from 180 to 210˚C and without ferrocene as precursor) we could obtain magnetite microspheres with unchanged wall thickness and for this reason we can presumably admit that the wall thickness of sample d should be similar to the sample c.

XRD Characterization
Figure 2 shows the XRD patterns of samples synthesized at different temperatures.As can be seen there, all pat- terns indicate the crystalline nature of the samples.All reflections have been indexed and their observed intensities are in very good agreement with the inverse cubic spinel structure of Fe 3 O 4 (JCPDS 85-1436).However, the γ-Fe 2 O 3 and Fe 3 O 4 phases are very similar and for this reason, the possible presence of the γ-Fe 2 O 3 phase cannot be excluded only from the results obtained by XRD.Nevertheless, the unambiguously characterization of the phase can be assigned by XPS.

XPS Characterization
With the aim to distinguish between γ-Fe 2 O 3 and Fe 3 O 4 phases and clearly indentify the chemical composition, the surface structure was further characterized by XPS.
Figure 3 shows the Fe2p and O1s regions, corresponding to nanoparticles synthesized at different temperatures.
As can be seen in Figure 3(A), in all analyzed samples the main peaks appear at ca. 711.1 and 724.6 eV indicating that the Fe is present as Fe 3 O 4 .As shown in Figure 3(B), the XPS profile corresponding to the O1s region is characterized by only one peak at ca. 531.1 eV that can be attributed to O-Fe in magnetite phase.These assignations agree with the literature [41,42] and unambiguously evidence that the only phase present in the samples is Fe 3 O 4 .Apparently, the synthesis temperature affects only to the dimensions of the magnetite nanoparticles and not to the chemical composition and crystalline structure.
It is worth noting that the Fe/O atomic ratio (as determined by XPS) for samples grown at different temperatures, ranged from 0.71 to 0.82.The expected value for magnetite is 0.75, being these results in agreement with the previous phase assignation.

Infrared and Raman Characterizations
The FT-IR spectra of the magnetite nanoparticles are shown in Figure 4(A).In all cases, infrared spectra are characterized by a strong absorption band in the range of 583 cm −1 (synthesized at 120˚C) to 570 cm -1 (synthesized at 200˚C).The observed shift is gradual and appears to    the Raman signal was very low and it was necessary to increase the acquisition time, and subsequently the irradiation.This effect has been previously described in the literature [46] being even responsible for structural transformations from magnetite to maghemite.

Magnetic Characterization
Magnetic characterization of the hollow magnetite nanoparticles measured at 25˚C is shown in The Ms values can be justified by the amount of magnetite in the sample that depends on the particle diameter and on the internal hollow.In fact, the Ms of the bulk magnetite is 92 emu/g [47] and, in our case, the highest value is observed for magnetite nanoparticles synthesized at 200˚C, with larger diameters (600 nm) and greater amount of magnetite.On the other hand, another possible effect that could be the reason for these M s values is the possible incorporation of metallic Fe.In fact, the Fe/O atomic ratio measured by XPS was higher in magnetite nanoparticles synthesized at 200˚C (0.82) than in those synthesized at 120˚C (0.70) being Copyright © 2011 SciRes.SNL this result in agreement with the possible presence of Fe 0 in the magnetite nanoparticles with larger diameters.

Adsorption Isotherm
The Brunauer-Emmett-Teller (BET) surface area and pore parameters of the sample synthesized at 120˚C was determined by nitrogen adsorption-desorption isotherm measurement at 77 K.As determined by SEM, the nanoparticle diameter of the samples synthesized at low temperature is ca.300 nm.In this case nanoparticles have a surface area of 60 m 2 g −1 and an average pore diameter of 4 nm.An explanation for these values could be found in the fact that nanoparticles are composed of much smaller primary nanoparticles that are organized to form hollow spheres allowing the existence of pores.With the aim to clarify this possibility, further adsorption studies on the magnetite particles synthesized in the range of 130 to 200˚C are in progress.

Conclusions
In this study, we have explored the synthesis of hollow magnetite nanoparticles via a simple hydrothermal process.Fe 3 O 4 magnetic nanoparticles were synthesized at different temperatures using FeCl 3 •6H 2 O and ferrocene as precursors and a mixture of propylene glycol-isopropanol as solvent.As a result we have obtained hollow microspheres whose internal void space and wall thickness depend on the synthesis temperature.Additional work is currently being undertaken in order to understand more fully the growth mechanism.Considering the magnetic properties and the presence of an internal void space, these magnetite nanoparticles may have potential applications in catalysis, bioengineering and biological fields.

Figure 4 (
B) illustrates the Raman spectra measured for the samples synthesized at different temperatures.The smaller particles synthesized at 120˚C (Figure4(B)-a) exhibit a main band centered at 668 cm −1 and peaks at ca. 540 cm −1 and 300 cm −1 that have been assigned to A 1g , T 2g and E g vibrations of magnetite.As can be seen in Figure4(B), two peaks (marked as arrows) are observed in samples a and b.In samples c and d these small peaks almost disappeared.This effect could be justified as possibly due to structural transformations induced by laser irra iation.In samples a and b

Figure 5 .
As can be seen there, the analyzed samples exhibit ferromagnetic property at room temperature, with low coercivity values at 28.67 Oe (Figure5(a)) and 60.50 Oe (Figure 5(b)).The saturation magnetization (M s ) of sample synthesized at 200˚C (Figure 5(a)) is highest at 25.65 emu/g.This value is clearly higher than that observed for the sample synthesized at 120˚C (Figure 5(b)) whose maximum M s is observed at 5.50 emu/g.As determined by XPS, XRD and IR-Raman, in both cases, nanoparticles are composed by Fe 3 O 4 .

Figure 5 .
Figure 5. Magnetization curves obtained at 25˚C of hollow magnetite nanoparticles synthesized at 200˚C (a) and 120˚C (b).Insets in (a) and (b) correspond to magnified images of the origin region.