Size-Controlled Synthesis of Fe 3 O 4 Magnetic Nanoparticles in the Layers of Montmorillonite

Iron oxide nanoparticles (Fe 3 O 4 -NPs) were synthesized using chemical coprecipitation method. Fe 3 O 4 -NPs are located in interlamellar space and external surfaces ofmontmorillonite (MMT) as a solid supported at room temperature.The size ofmagnetite nanoparticles could be controlled by varying the amount of NaOH as reducing agent in the medium. The interlamellar space changed from 1.24 nm to 2.85 nm and average diameter of Fe 3 O 4 nanoparticles was from 12.88 nm to 8.24 nm. The synthesized nanoparticles were characterized using some instruments such as transmission electron microscopy, powder X-ray diffraction, energy dispersive X-ray spectroscopy, field emission scanning electron microscopy, vibrating sample magnetometer, and Fourier transform infrared spectroscopy.


Introduction
In recent years, considerable attention has been paid to iron oxides, especially on magnetic (Fe 3 O 4 ) nanoparticles due to their potential applications such as pigment, magnetic resonance imaging, magnetic drug delivery, Ferro fluids, recording material, and data storage media [1,2].There are many reports on the synthesis and characterization of magnetic nanoparticles; this is due to its significance in various fields, especially, when this material is made at the nanoscale.Magnetic nanoparticles have been prepared by various methods such as arc discharge, mechanical grinding, laser ablation, microemulsion, and high temperature decomposition of organic precursors [3].However, research is still going on to obtain well-dispersed magnetic nanoparticles.
Several methods have been developed recently for preparing magnetic nanoparticles, including coprecipitation [4], sol-gel method [5], hydrothermal process [6], and the solvothermal method [7].Among these methods, coprecipitation is considered as the simplest, most efficient, and most economic method.However, there are three major challenges that these nanoparticles present.One is related to easy oxidation/dissolution of magnetic nanoparticles (NPs).The other two drawbacks are the difficulty in recycling such small sized NPs and coaggregation of nanoparticles in which case, leads to decrease the effective surface area of nanoparticles and thus reduce their reaction activities.In order to protect the magnetic NPs against oxidation, a shell structure is often introduced, such as a silica shell or polymer shell [8].Several methods have been accordingly developed to minimize the coaggregation of nanoparticles, obtain the soft sediment, and improve their manipulation, such as supporting of magnetic nanoparticles on polymers or inorganic matter, like porous silica [9,10].
Clay minerals, which are abundant and environmental friendly, are used as the supporting materials of magnetic nanoparticles.Sodium montmorillonite (Na-MMT) (Na 0.7 (Al 3.3 Mg 0.7 ) Si 8 O 20 (OH) 4 .nH 2 O), a kind of 2 : 1 type layered silicates in which each layer comprises an alumina octahedral sheet sandwiched between two silica tetrahedral sheets, is known as a polymer modifier due to its high specific surface area [11].

Journal of Nanomaterials
Such layers are stacked by weak dipolar or van der Waals forces, leading to the interaction of charge compensating cations into the interlayer space.Therefore, not only adsorption on the external surface but also intercalation into the interlayer space can occur [12].
Here we present our results obtained from synthesized magnetic nanoparticles well dispersed on montmorillonite using sodium hydroxide as a reducing agent.

Materials and Methods
In this work, all reagents were of analytical grade.Ferric chloride hexahydrate (FeCl 3 .6H 2 O) and ferrous chloride tetrahydrate (FeCl 2 .4H 2 O) of 96 wt.% were used as the iron precursors and also montmorillonite powder was obtained from (Kunipia-F, Tokyo, Japan).NaOH of 99 wt.% was obtained from Merck KGaA (Darmstadt, Germany).All solutions were freshly prepared using deionized water.
2.1.Fe 3 O 4 /Montmorillonite NCs Preparation.The Fe 3 O 4 nanoparticles were synthesized using chemical coprecipitation method.Prior to use, all glassware used in experimental procedures were cleaned in a fresh solution of HNO 3 /HCl (3 : 1, v/v), washed thoroughly with double distilled water, and dried.For the synthesis of Fe 3 O 4 /montmorillonite nanocomposites (Fe 3 O 4 /MMT NCs), different volumes of deionized water were bubbled by N 2 gas for 15 min and then 2.0 g of montmorillonite and measured amount of Fe 3+ and Fe 2+ with a molar ratio of 1 : 2 were successively dissolved in ultrapure water with vigorous mechanical stirring for 2 hours.Under the protection of nitrogen gas, 1.50, 3.0, 5.0, 7.50, 9.50, and 12.50 mL of 2 M NaOH were added dropwise into the above solutions.Eventually the Fe 3 O 4 -NPs content of the MMT matrix was 1.0, 3.0, 5.0, 7.0, 9.0, and 12.0 wt.%, respectively.After that, the mechanical stirrer was switched off and Fe 3 O 4 -NPs settled gradually.The suspensions were finally centrifuged, washed twice with ethanol and distilled water, and kept in a vacuum stove at 100 ∘ C.

Characterization.
The structures of the synthesized Fe 3 O 4 -NPs in MMT were examined using Philips X'pert PXRD (copper K radiation; PANalytical, Almelo, The Netherlands).The changes in the interlamellar spacing of MMT were also studied by using PXRD in the smallangle range of 2 (5-15 degrees).The scan speed of 2 degrees/minutes was applied to PXRD patterns recording.TEM images of the samples were recorded with an H-7100 electron microscope (Hitachi Ltd., Tokyo, Japan).The samples were prepared by dispersing about 0.1 g powder with 8 mL ethanol by ultrasound for 20 min.A drop of the dispersion was applied to a holey carbon TEM support grid.Excess solution was blotted off using a filter paper.The analysis of the surface morphologies of the samples was performed with an (XL 30; Philips) environmental scanning electron microscope (SEM).The cleaned and dried samples were first coated with gold using sputter coater.For elemental analysis of the nanoparticles, energy dispersion X-ray spectroscopy was carried out on a Shimadzu EDX-700HS spectrometer attached to the SEM.The FTIR spectrum was used to identify the functional groups present in the synthesized compound.FTIR spectra were recorded over the range of 200-4000 cm −1 utilizing the Series 100 FTIR 1650 spectrophotometer (PerkinElmer, Waltham, MA, USA).Magnetization measurements were carried out with a Lakeshore (model 7407) vibrating sample magnetometer (VSM) to study magnetic properties of the Fe 3 O 4 nanoparticles under magnetic field up to 10 kG at room temperature.

Results and Discussion
The customized shape and size of prepared magnetic nanoparticles have been a challenge in scientific and technological issues.The wet chemical methods to magnetic nanoparticles are more effective and simpler with good control over composition and size [13].Fe 3 O 4 -NPs can be prepared through the coprecipitation of Fe 3+ and Fe 2+ aqueous salts solution by addition of base as reducing agent.The chemical reaction may be presented as follows [14]: The surface of MMT is assisting the Fe 3 O 4 -NPs nucleation during the reduction process.The schematic illustration of the synthesis of Fe 3 O 4 /MMT NCs from MMT/Fe 3+ -Fe 2+ suspension produced by using sodium hydroxide is shown in Figure 1.Meanwhile, as shown in Figure 2, the MMT suspension was gray (a) and the addition of Fe 3+ -Fe 2+ ions to the MMT change color to the light brown (b), brown (c), and dark brown (d) for 1.0, 7.0, and 12.0 wt.%, respectively, but after the addition of NaOH to the suspensions they turned to black color (e), (f), and (g) that indicate the abundance of Fe 3 O 4 -NPs in the MMT suspensions.

Powder X-Ray Diffraction.
The basal spacing of MMT (Figure 3(a)) was 1.24 nm at 2 ∘ , 8.83 ∘ .Combination with Fe 3 O 4 NPs expanded the basal spacing to 1.47, 1.68, 1.87, 2.10, 2.53, and 2.85 nm at the 2 ∘ , 8.75 ∘ , 8.66 ∘ , 8.51 ∘ , 8.21 ∘ , 7.72 ∘ , and 7.46 ∘ in 1.0, 3.0, 5.0, 7.0, 9.0, and 12.0 wt.%, respectively.It may be assumed that the iron ions penetrated into the interlayer space of montmorillonite via ion-exchange and were reduced by NaOH to Fe 3 O 4 NPs.In this case, the interlayer space would have acted as a size controller and microreactor [15].Moreover the intensities of the reflections were significantly decreased, and the highly ordered parallel lamellar structure of the MMT was disrupted by the Fe 3 O 4 -NPs formation [16].
Mixing at room temperature    reducing agent promotes a fast rate and favors the formation of smaller NPs.Meanwhile, we have found that the amount of NaOH has a significant effect on the size of Fe 3 O 4 -NPs [18] and increasing the NaOH amount will lead up to the decrease of Fe 3 O 4 -NPs size gradually.The possible reason is that repulsive force between hydroxide ions hinders the growth of crystal grains when the excess hydroxide ions produced from NaOH are adsorbed on the surface of crystal nuclei [19].Additionally, as shown in Figure 3  to the high density of ion-exchange sites on MMT, the highly charged Fe 3 O 4 -NPs are strongly bound via electrostatic interaction to the surface.The higher abundance of NPs aggregates in the MMT composite is in direct correlation with the smaller primary NPs dimensions [20].

Scanning Electron Microscopy (SEM)
. SEM images show that the Fe 3 O 4 -NPs are in spherical shape (Figures 5(a) and 5(b)).Fe 3 O 4 -NPs have very high surface free energy, which lead to instability in their dispersions thermodynamic system; stability is restored upon Ostwald ripening [21].The isotopic nucleation rate per unit area at the interface between the Fe 3 O 4 -NPs leads to forming the spherical shape NPs due to equivalent growth rate along every direction of the nucleation as the sphere has the smallest surface area per unit volume compared to other shapes [22].Figure 6 clearly shows that the size of Fe 3 O 4 -NPs in 1.0 wt.% sample is smaller than 12.0 wt.% which is in line with the TEM and XRD results previously mentioned.
A general view of the MMT with a typical sheet-like structure with large flakes can be observed in Figure 6

Energy Dispersive X-Ray Spectroscopy (EDX).
The oxygen and iron peaks in EDX spectrum reveal the existence of Fe 3 O 4 -NPs (Figure 7).In Figure 7(a), the peaks around 1.7, 2.7, 2.9, 3.  The FTIR spectra demonstrated the inflexibility of silicate layers and nonbond chemical interface between the silicate layers and Fe 3 O 4 -NPs in Fe 3 O 4 /MMT NCs.These results confirmed that, with an increase amount of Fe 3 O 4 -NPs in the Fe 3 O 4 /MMT NCs due to the existence of van der Waals interactions between the oxygen groups of MMT and Fe 3 O 4 -NPs, peak areas shifted to low wave numbers and the intensity of peaks decreased [16].

Conclusion
In this work, we synthesized successfully Fe 3 O 4 /MMT NCs through coprecipitation method which is a cheap, facile, and environmental friendly method.Sodium hydroxide was used as reducing agent and ferric chloride and ferrous chloride were used as the iron precursors.These synthesized Fe 3 O 4 -NPs with high magnetization and good dispersion are on the external surface and between the layers of montmorillonite.Our results show that the amount of NaOH (reducing agent) has significant effect on the size of nanoparticles.Smaller sized nanoparticles were obtained due to more reducing agent amount.Moreover, these results indicate that montmorillonite used here can act as an effective support for the synthesis of magnetic nanoparticles.
The specific surface areas of Fe 3 O 4 /MMT NCs were larger than pure montmorillonite.The comparison between the PXRD patterns of MMT and the prepared Fe 3 O 4 /MMT NCs indicated the immobilized formation of Fe 3 O 4 -NPs in the interlamellar space of the MMT layers.The TEM images showed that the mean diameter of the nanoparticles ranged from 8.24 nm to 12.88 nm.
(b), Fe 3 O 4 /MMT NCs, 12.0 wt.%, completely absorbed when exposed to an external magnetic field, and the response decreased with the decreasing in Fe 3 O 4 percentage.
7, 4.0, 4.5, 6.4, and 7.1 keV are related to the binding energies of MMT.The peaks around 0.2, 2.2, 6.4, and 7.0 keV are related to Fe 3 O 4 -NPs elements, respectively, in Figures 7(b)-7(d) [20].Moreover, with an increase amount of iron oxide, the iron peaks height increases in the Fe 3 O 4 /MMT NCs (1.0, 5.0, and 12.0 wt.wt.%).Additionally, the EDXRF spectra for the MMT and for Fe 3 O 4 /MMT NCs confirm the presence of elemental compounds in the MMT and Fe 3 O 4 -NPs without any impurity peaks.The results indicate that the synthesized Fe 3 O 4 -NPs are in high purity.
The vibrating sample magnetometer (VSM) was applied to test the magnetic properties of Fe 3 O 4 /MMT NCs.The magnetization curves for Fe 3 O 4 /MMT NCs in 1.0 and 12.0 wt.% are shown in Figure 9.It can be seen from the magnetization curves that the saturation magnetization (Ms) of the Fe 3 O 4 /MMT NCs increased from 12.10 to 32.40 emu.g −1 when Fe 3 O 4 content increased from 1.0 to 12.0 wt.% of composite.It indicated more Fe 3 O 4 being trapped in the MMT layers.