Synthesis and Characterization of Superparamagnetic Fe 3 O 4 @ SiO 2 Core-Shell Composite Nanoparticles

The Fe3O4@SiO2 composite nanoparticles were obtained from as-synthesized magnetite (Fe3O4) nanoparticles through the modified Stöber method. Then, the Fe3O4 nanoparticles and Fe3O4@SiO2 composite nanoparticles were characterized by means of X-ray diffraction (XRD), Raman spectra, scanning electron microscope (SEM) and vibrating sample magnetometer (VSM). Recently, the studies focus on how to improve the dispersion of composite particle and achieve good magnetic performance. Hence effects of the volume ratio of tetraethyl orthosilicate (TEOS) and magnetite colloid on the structural, morphological and magnetic properties of the composite nanoparticles were systematically investigated. The results revealed that the Fe3O4@SiO2 had better thermal stability and dispersion than the magnetite nanoparticles. Furthermore, the particle size and magnetic property of the Fe3O4@SiO2 composite nanoparticles can be adjusted by changing the volume ratio of TEOS and magnetite colloid.


Introduction
Magnetite nanoparticles have attracted a great deal of attention because of their unique physicochemical properties and great potential use in various biomedical applications, such as contrast agents in magnetic resonance imaging (MRI), carriers for targeted drug delivery, the magnetic separation in microbiology, biochemical sensing [1][2][3][4], etc.
In this work, the silica-coated magnetite nanoparticles are synthesized through two steps.The magnetite nanoparticles are firstly prepared by coprecipitation method [5].Then the magnetite nanoparticles are used to synthesize the Fe 3 O 4 @SiO 2 composite nanoparticles through the modified Stöber method [12].The thermal stability and morphologies of Fe 3 O 4 and Fe 3 O 4 @SiO 2 are studied.Afterward, the effects of experimental parameters, such as the volume of TEOS and magnetite colloid on the properties of Fe 3 O 4 @SiO 2 composite nanoparticles are also systematically investigated.

Preparations of the Magnetite Nanoparticles
An aqueous solution of Fe ions with a molar ratio of Fe(II)/Fe(III) ~ 0.5 was prepared by dissolving 5.46 g FeCl 3 •6H 2 O, 2.00 g FeCl 2 •4H 2 O and 0.6 g PEG-2000 in 60 ml of aqueous acid of 50 ml distilled water with 10 ml of 1 M HCl, and then added dropwise into 100 ml of 1 M NaOH with 1.0 g PEG-2000 solution under vigorous stirring at 60˚C.The reaction was carried out in an inert atmosphere by purging the reactor with high purity argon (99.9%) all through.After all of the Fe ions solution was added, the mixture was stirred for a further 2 h.Then the colloid solution was washed by distilled water for several times until it is neutral.A part of colloid is dried in fridge.The remnant of the colloid was dispersed in distilled water by ultrasonic, and then was ready for coating process.

Preparations of the Fe 3 O 4 @SiO 2 Composite Nanoparticles
Placed a certain volume of Fe 3 O 4 colloid (2 wt.%) and distilled water (total volume is 19 ml) into a 250 ml three-neck flask, then added 80 ml ethanol, a certain amount of TEOS and ammonia under vigorous stirring (800 rpm) at room temperature for 12 h.The obtained Fe 3 O 4 @SiO 2 colloid was washed by repeated cycles of distilled water and ethanol.Then, the final products were dried in an oven at 60 °C for 24 h.

Characterization
The phase identification and crystalline structures of the nanoparticles was characterized by X-ray powder diffraction of the dried samples using a D/Max-2400 X-ray diffractometer equipped with a Cu K α monochromatic radiation source (λ = 1.54187Å).The Raman spectra of nanoparticles were collected using a Laser Confocal Raman Spectroscopy (LCRS).The morphologies were observed using a scanning electron microscope Hitachi S4800 operating at an accelerating voltage of 15 kV.Magnetic measurements were performed using a vibrating sample magnetometer Lake Shore 7304.

Thermal Stability of Fe 3 O 4 Nanoparticles and Fe 3 O 4 @SiO 2 Composite Nanoparticles
Phase identification is one of the most important uses of XRD.Firstly, we obtained XRD pattern of the materials, then, we compare data with known standards in the JCPDS file to preliminary identify the materials.Similarly, Raman spectra are also used to identify the phase of materials.To verify whether there is a phase transition from Fe 3 O 4 to γ-Fe 2 O 3 for the nanoparticles with and without 200˚C heat treatment, the corresponding Raman spectra were also obtained.Figure 2 illustrates the Raman spectra of magnetite nanoparticles before (Figure 2(a)) and after (Figure 2(b)) heat treatment at 200˚C.As we can see the peaks at 350 cm -1 , 550 cm -1 and 670 cm -1 can be observed (Figure 2(a)), which can be attributed to T 2g,3 , T 2g,2 , A 1g vibration mode of Fe 3 O 4 respectively.At higher wavenumber (~ 1378 cm -1 ), there are no apparent peak.Hence, this result gives the obvious evidence for the existence of Fe 3 O 4 other than γ-Fe 2 O 3 [14].

As shown in
In In general, the Fe 3 O 4 nanoparticles will partly transform into γ-Fe 2 O 3 under a 200˚C heat treatment for 3 h.On experiencing a 600˚C heat treatment for 3 h, the   content accumulates with the TEOS volume increasing.Figure 6, the SEM images of Fe 3 O 4 @SiO 2 composite nanoparticles with different ratio of TEOS, indicates that the increasing TEOS volume ratio leads to a bigger size of composite particle.This can be ascribed to the fact that when the dosage of TEOS increases, the quantity of SiO 2 increases.Therefore, the Fe 3 O 4 particles are adequately coated, and the size of composite nanoparticles increases with ratio of TEOS increasing.This result consisted with the XRD result.
Thermal stability of Fe 3 O 4 @SiO 2 composite nanoparticles is also investigated.In Figure 3, XRD patterns of Fe 3 O 4 @SiO 2 after heat treatments are shown.The peaks of Fe 3 O 4 and 2 amorphous hump at 23 ° can be observed for all samples.In addition, characteristic peaks of Fe 2 O 3 appear (Marked *) with the heat treatment temperature of 800 °C (Figure 3(e)).After 800 °C heat treatment for 3 h, the predominant phase of the Fe 3 O 4 @SiO 2 composite nanoparticles is still Fe 3 O 4 (Figure 3(e)).But the pure Fe 3 O 4 has completely transformed into Fe 2 O 3 at 600 °C for 3 h (see Figure 1(d)).This suggests that the Fe 3 O 4 @SiO 2 has a significantly higher thermal stability than the pure Fe 3 O 4 .
Room temperature magnetic properties of Fe 3 O 4 @ SiO 2 composite nanoparticles with different ratio of TEOS were measured using VSM, as shown in Figure 7.
It is manifest in Figure 7 that there is no hysteresis in the magnetization curve, the coercivity field and remnant magnetization cannot be found from the curve.It confirms that Fe 3 O 4 @SiO 2 composite nanoparticles are superparamagnetic.The magnetization does not saturate at 11000 Oe, and the values are 12.7 emu/g, 6.3 emu/g, 4.3 emu/g, 3.1 emu/g and 2.4 emu/g for various TEOS concentrations respectively, which shows a trend of gradual decrease.The reason is that SiO 2 coating grows thicker with increasing concentration of TEOS, thus reduces their magnetism.In a word, the magnetism of the composite particles can be controlled by adjusting the concentration of TEOS.

The improvement in Dispersion by SiO 2 Coating
The morphological characteristics of the Fe 3 O 4 nanoparticles and Fe 3 O 4 @SiO 2 composite nanoparticle are shown in Figure 4.The Fe 3 O 4 nanoparticles are agglomerated seriously and the size is very small (Figure 4(a)).
But the composite nanoparticles are almost monodisperse with uniform size (Figure 4(b)).It proves the dispersion of Fe 3 O 4 @SiO 2 composite nanoparticles is apparently improved.

The Effect of the Volume of Magnetite Colloid on the Structural, the Morphological and the Magnetic Properties
While increased the volume ratio of TEOS, other conditions, such as 4 ml Fe 3 O 4 colloid, 2 ml ammonia at room temperature, kept the same.
The influence of the concentration of magnetite colloid on structure, morphology and magnetism were also studied here.During this reaction, the ammonia and TEOS both were fixed to 2 ml, while the volume of magnetite colloid was different.finite coercivity field and remnant magnetization can be found from the curve.From the magnetization curve, we can also see that the magnetization does not saturate at 11000 Oe magnetic fields.The magnetization is 5.2 emu/ g, 8.3 emu/g, 16.7 emu/g, 20.2 emu/g and 23.8 emu/g for increasing volume of colloid.The results are well coincident with XRD spectrum and SEM results.This highlights proper concentration of magnetite colloid redounds to good despersion and magnetism.

Conclusions
In summary, the Fe 3 O 4 nanoparticles are seriously agglomerated after synthesis.Additionally, the Fe 3 O 4 nanoparticles completely transform into Fe 2 O 3 after a 600 °C heat treatment for 3 h.
What's more, the Fe 3 O 4 @SiO 2 composite nanoparticles are almost monodisperse.It proves the SiO 2 coating remarkably improves the dispersion of Fe 3 O 4 nanoparticles.When Fe 3 O 4 @SiO 2 composite particles experience an 800 °C heat treatment for 3 h, the major phase is still Fe 3 O 4 , which verifies that the composite particles exhibit better thermal stability than magnetite nanoparticles.Besides, silica surfaces are chemically stable, biocompatible and can be easily functionalized for bioconjugation purposes, Fe 3 O 4 @SiO 2 composite nanoparticles have great potential applications in various biomedical fields, such as DNA purification, protein-separation, targeted drug delivery, magnetic hyperthermia and magnetofection.In modified Stöber method, the experimental parameters such as volumes of TEOS and magnetite colloid are discussed.The particle size, dispersion and magnetic properties of the Fe 3 O 4 @SiO 2 composite particles can be controlled by changing the volume of TEOS and the magnetite colloid.

Figure 1 ,
XRD patterns of Fe 3 O 4 after heat treatments are shown.For the nanoparticles without (Figure 1(a)) and with heat treatment at 200˚C (Figure

Figure 1 .
Figure 1.XRD patterns of Fe 3 O 4 a nanoparticles under different temperature heat treatments for 3 h.(a: without treatment; b: 200˚C; c: 400˚C; d: 600˚C; e: 800˚C).1(b)); 400˚C (Figure 1(c)), the XRD patterns (marked ∆) are well indexed to the cubic spinel phase of magnetite (JCPDS No. 89-43191).No other significant peaks can be observed in Figures 1(a) and (b), however, because the diffraction peaks of γ-Fe 2 O 3 are similar with Fe 3 O 4 , we can only confirm that the nanoparticles with and without 200˚C heat treatment do not contain impurities except γ-Fe 2 O 3 .A further investigation will be obtained by Raman.What's more, the characteristic peaks of spinel structure Fe 2 O 3 (Marked *) can be simultaneously observed in Figure 1(c).We can infer that Fe 3 O 4 can be completely transformed into Fe 2 O 3 when Fe 3 O 4 experiences a heat treatment at 400˚C for enough time.Upon heating at 600˚C for 3 h (Figure 1(d)), all of Fe 3 O 4 nanoparticles transform into Fe 2 O 3 .Similarly, Raman spectra are also used to identify the phase of materials.To verify whether there is a phase transition from Fe 3 O 4 to γ-Fe 2 O 3 for the nanoparticles with and without 200˚C heat treatment, the corresponding Raman spectra were also obtained.Figure2illustrates the Raman spectra of magnetite nanoparticles before (Figure2(a)) and after (Figure2(b)) heat treatment at 200˚C.As we can see the peaks at 350 cm -1 , 550 cm -1 and 670 cm -1 can be observed (Figure2(a)), which can be attributed to T 2g,3 , T 2g,2 , A 1g vibration mode of Fe 3 O 4 respectively.At higher wavenumber (~ 1378 cm -1 ), there are no apparent peak.Hence, this result gives the obvious evidence for the existence of Fe 3 O 4 other than γ-Fe 2 O 3[14].In Figure 2(b), the peak at 1400 cm -1 as well as peaks at 350 cm -1 , 550 cm -1 and 670 cm -1 indicates part of Fe 3 O 4 transforms into γ-Fe 2 O 3 .We can confirm Fe 3 O 4 will completely transform into γ-Fe 2 O 3 under a 200˚C heat treatment for a long enough time.In general, the Fe 3 O 4 nanoparticles will partly transform into γ-Fe 2 O 3 under a 200˚C heat treatment for 3 h.On experiencing a 600˚C heat treatment for 3 h, the

Figure 2 (
b), the peak at 1400 cm -1 as well as peaks at 350 cm -1 , 550 cm -1 and 670 cm -1 indicates part of Fe 3 O 4 transforms into γ-Fe 2 O 3 .We can confirm Fe 3 O 4 will completely transform into γ-Fe 2 O 3 under a 200˚C heat treatment for a long enough time.

Figure 2 .
Figure 2. Raman spectra of Fe 3 O 4 nanoparticles before (a); and after heat treatment at 200˚C for 3 h (b).

Fe 3 O
4 nanoparticles will completely transform into Fe 2 O 3 .

Figure 5 Figure 8
Figure 5 summarizes the crystalline structure dependence on the ratio of TEOS.The amorphous hump of SiO 2 around 23 ° and the characteristic peaks of Fe 3 O 4 around 35.4 ° can be observed for all samples.The increase in intensity of amorphous hump suggests that SiO 2

Figure 6 .
Figure 6.SEM images of Fe 3 O 4 @SiO 2 nanoparticles synthesized at different TEOS volume (a: 1 ml TEOS; b: 2 ml TEOS; c: 3 ml TEOS; d: 5 ml TEOS; e: 8 ml colloid.The amorphous hump of SiO 2 and characteristic peaks of Fe 3 O 4 can be observed for all samples.But the width and the intensity of the amorphous hump decreases, opposing to the increasing volume of Fe 3 O 4 colloid.As we can see the morphologies of Fe 3 O 4 @SiO 2 composite nanoparticles with different ratio of TEOS in Figure 9, the size of composite nanoparticles tends to reduce gradually and the dispersion becomes worse with in-

Figure 8 .
Figure 8. XRD pattern of Fe 3 O 4 @SiO 2 nanoparticles with different magnetite colloid volume (a: 1 ml magnetite colloid; b: 2 ml magnetite colloid; c: 4 ml magnetite colloid; d: 6 ml magnetite colloid; e: 8 ml magnetite colloid).creasing amount of Fe 3 O 4 colloid.The reason is just that for increasing quantity of Fe 3 O 4 colloid, the concentration of the SiO 2 becomes diluted and consequently the SiO 2 is inadequate to coat Fe 3 O 4 colloid.Therefore composite particles become agglomerated, accompanied by reduced size of theirs.The magnetization curve of Fe 3 O 4 @SiO 2 nanoparticles with different volume of magnetite colloid are illustrated in Figure 10.It is also without hysteresis, and no sign of