Chapter 4

SYNTHESIS AND CHARACTERIZATION

STUDIES OF Fe3O4 MNPS

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4.1. Introduction

Distinct physico-chemical properties of nanomaterials have made them

interesting candidates for their use in various fields in recent years. These

properties differ from those in bulk and are strongly influenced by size and shape

of the material. Due to the unique optical, electrical, magnetic and catalytic

properties of transition metal oxides, they have been of great interest [1]. Among

these, magnetite (Fe3O4) is an important material and has been mostly studied

nanomaterial. Magnetite nanocrystals have been widely used in fields like multi-

ferroics [2], ultrahigh density magnetic storage media [3], ferrofluids [4], in vivo

and in-vitro biomedical applications [5, 6]. Due to better biocompatibility,

injectibility, chemical stability over physiological circumstances and substantial

accumulation at the diseased site, magnetite nanocrystals have been widely used

for biomedical applications. These magnetic nanoparticles (MNPs) can be used

either as a diagnostic tool or in therapeutic applications. Their potential as a

contrasting agent makes them suitable for diagnosing cancer in magnetic

resonance imaging technique [7], while their heating ability allows them to have

therapeutic applications as well which include hyperthermia [8].

4.2. Synthesis methods

Several methods are reported in the literature to synthesize Fe3O4 NPs. It can

be synthesized using various chemical as well as biological routes, as already

stated earlier. Chemical methods used to synthesize magnetite include co-

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precipitation reaction, solvothermal method, combustion synthesis, sol-gel

method, microwave assisted synthesis etc. Biological route involves the use of

biological materials for magnetite synthesis, e.g. magnetotactic bacteria.

4.3. Co-precipitation method

4.3.1. Experimental

The co-precipitation technique is being mostly used chemical synthesis

method for MNPs. FeCl3 and FeCl2 were used as precursors for the reaction in 1:2

proportions. In this process the salt solution of the required metallic elements is

reduced by NaOH solution. The reactants when mixed are at temperatures of 90ºC.

After the mixing the reaction is continued for 40 minutes along with heating at

90ºC. The reaction taking place is already explained in chapter 1 (eq. 1.1).

2 Fe3+

+ Fe2+

+ 8 OH− → Fe3O4 + 4 H2O

4.3.2. Results and Discussions

4.3.2.1. Structural and phase analysis

Fig. 4.1 shows the powder XRD patterns for bare Fe3O4 NPs synthesized using

co-precipitation method. The main characteristic peaks were obtained with the

(hkl) values of (220), (311), (400), (422) and (511) which correspond to Fe3O4

phase. The NPs show inverse spinel structure. The crystallite size of NPs was

calculated from FWHM of the most intense peaks using the Debye-Scherrer

formula. The crystallite size obtained was 25.8 nm for bare MNPs.

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Fig. 4.1: XRD patterns obtained from bare Fe3O4 MNPs synthesized using co-

precipitation method.

4.3.2.2. Morphological study

The TEM image of Fe3O4 synthesized using co-precipitation method is

shown in Fig. 4.2. Bare Fe3O4 NPs are highly agglomerated with particle size 23.8

± 4.1 nm. These results are comparable with the XRD results. Interparticle

agglomeration occurs due to a strong magnetic dipole-dipole interaction.

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Fig. 4.2: TEM images of bare Fe3O4 MNPs synthesized using co-precipitation

method.

4.3.2.3. Magnetic properties

Fig 4.3 shows MH curve of bare Fe3O4 synthesized using co-precipitation

method at 300K. The graph clearly shows superparamagnetic nature of the NPs at

300K as coercivity and remenance values are very negligible. Saturation

magnetization (Ms) of bare NPs is observed to be 57.88 emu/g is obtained for

300K which is small compared to that of theoretical value of bulk Fe3O4 (Ms= 92

emu/g).

50 nm

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Fig. 4.3: MH curve of bare Fe3O4 at 300K synthesized using co-precipitation

method.

4.4. Alkaline precipitation method

Alkaline precipitation is a simpler form of co-precipitation method. Unlike

the co-precipitation, alkaline precipitation uses only ferrous chloride as the sole

precursor for the synthesis of Fe3O4 MNPs. No data is available on synthesis of

Fe3O4 NPs using FeCl2 as the sole precursor. But the reported syntheses using

FeCl2 as the sole source were carried out in presence of an oxidant like nitrous

oxide, which is not the case in the present work. This type of synthesis procedure

is not reported in earlier literature. Conventional co-precipitation method is more

simplified and made cost-effective in this work.

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4.4.1. Experimental

Fe3O4 nanoparticles were synthesized via alkaline precipitation method. An

evaluation with respect to particles size, morphology and magnetic properties of

the as-formed magnetite particles were performed. The brief procedure for

preparation of MNPs is as follows:

2g FeCl2.4H2O was dissolved in 50 mL 1M HCL by heating upto 70 oC. 50

mL 3M NaOH was added to it at 60oC drop by drop with constant stirring. A black

precipitate was formed which was nothing but magnetite (Fe3O4) NPs. The

possible reaction taking place is showed below:

3 FeCl2.4 H2O + 6 NaOH + ½ O2 Fe3O4 + 6 NaCl + 15 H2O

……. (4.1)

Fig. 4.4: Flow-chart of the procedure for synthesis of Fe3O4 NPs using alkaline

precipitation method.

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The precipitate was allowed to settle down by applying external magnetic

field. The precipitate was then separated, washed with distilled water till the

neutral pH. The precipitate was then dried at RT and used for further studies. The

Fig. 4.4 represents the steps followed during the procedure.

4.4.2. Results and Discussions

4.4.2.1. Structural and phase analysis

Fig. 4.5 shows the powder XRD patterns for bare Fe3O4 NPs. The main

characteristic peaks were obtained with the (hkl) values of (220), (311), (400),

(422) and (511). These were then matched with the JCPDS file number 82-1533,

which corresponds to Fe3O4 phase.

Fig. 4.5: XRD patterns obtained from bare Fe3O4 MNPs synthesized using alkaline

precipitation method.

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The NPs show inverse spinel structure. The crystallite size of NPs was

calculated from FWHM of the most intense peaks using the Debye-Scherrer

formula. The crystallite size obtained was 20.3 nm for bare MNPs.

The Selected Area Electron Diffraction (SAED) patterns for bare NPs are

shown in Fig. 4.6. It shows bright ring patterns indicating polycrystalline nature of

the MNPs, as indicated by XRD patterns. The ring pattern corresponds to (220),

(311), (400), (422) and (511) planes which can be clearly seen in XRD results.

Fig. 4.6: Selected Area Electron Diffraction (SAED) patterns of bare Fe3O4 MNPs

synthesized using alkaline precipitation method.

The EDAX spectrum was used as a quantitative elemental analysis of bare

Fe3O4 NPs, which is shown in Fig. 4.7. The corresponding peaks in bare NPs are

due to Fe and O only. The spectra do not show any additional impurity peak

implying purity of the samples.

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Fig. 4.7: EDAX spectra of bare Fe3O4 MNPs synthesized using alkaline

precipitation method.

4.4.2.2. Morphological study

SEM image of Fe3O4 is shown in Fig. 4.8. The bare particles show a high

degree of agglomeration due to dipole – dipole interaction. MNPs are spherical in

shape.

Fig. 4.8: SEM images of bare Fe3O4 MNPs synthesized using alkaline

precipitation method.

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Fig. 4.9: TEM images of bare Fe3O4 MNPs synthesized using alkaline

precipitation method.

The TEM image of Fe3O4 is shown in Fig. 4.9. Bare Fe3O4 NPs are highly

agglomerated with particle size 21.8 ± 5.3 nm. These results are comparable with

the XRD results. The particle size obtained using FeCl2 only is similar to that of

earlier reports [9-11].

4.4.2.3. Magnetic properties

Generally Fe3O4 NPs show superparamagnetic behavior below size less than

20nm which is characterized by zero coercivity and zero remenance [12-15]. Fig

4.10 shows MH curve of bare Fe3O4 at 300K. The graph clearly shows

superparamagnetic nature of the NPs at 300K as coercivity and remenance values

are very negligible. Saturation magnetization (Ms) of bare NPs is observed to be

51.68 emu/g is obtained for 300K which is small compared to that of theoretical

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value of bulk Fe3O4 (Ms= 92 emu/g). Saturation magnetization has been reported

to decrease as the particles size of magnetite decreases below 30 or 20 nm, due to

finite size effect [16].

Fig. 4.11 shows the variation of magnetization M as a function of

temperature of bare MNPs in the range 5 to 350 K in an external magnetic field of

100 Oe recorded in zero-field-cooled (ZFC) and field-cooled (FC). From the

curves it is clearly observed the superimposition of the ZFC and FC curves take

place at 300 K. The superimposition of ZFC and FC curves is one of the

characteristic features of a superparamagnetic system. The superparamagnetism is

induced in the system when the system comes from multidomain to single and

uniformly magnetized domains. Then the overall system is in a state of uniform

magnetization and its phase transition occurs from ferromagnetic to

superparamagnetic and the system behaves like a small permanent magnet.

Fig. 4.10: MH curve of bare Fe3O4 at 300K synthesized using alkaline precipitation

method.

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Fig. 4.11: FC/ZFC curve of bare MNPs at applied magnetic field 100 Oe.

4.5. Conclusions

The study confirmed that the pure phase Fe3O4 MNPs can be synthesized

using both co-precipitation as well as alkaline precipitation methods. Synthesized

MNPs are superparamagnetic in nature with zero coercivity and remenance values.

The particle size 23.8 ± 4.1 nm and 21.8 ± 5.3 nm were obtained for both the

methods, respectively, which are suitable for their application in biomedical field.

Alkaline precipitation is a simpler and cost effective alternative for co-

precipitation technique. Also, the particle size obtained for alkaline precipitation is

smaller compared to co-precipitation method. Hence the MNPs synthesized with

alkaline precipitation method are used for the further proposed work.

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