Facile synthesis of magnetite iron oxide nanoparticles ...

Facile synthesis of magnetite iron oxide nanoparticles via precipitation method at different reaction temperatures M. F. Tai, C. W. Lai, S. B. A. Hamid*, D. D. Suppiah, K. S. Lau, W. A. Yehya, N. M. Julkapli, W. H. Lee and Y. S. Lim The nano-scale magnetite iron oxide particles have been synthesised by a facile precipitation method. Magnetite iron oxide nanoparticles were synthesised in a bath with electrolytes composed of 0·10 M of iron (II) chloride with 0·45 M of sodium hydroxide at different reaction temperatures under oxidising environment. In the present study, the influence of reaction temperatures (30, 45 and 80°C) on the morphology, particle size and crystallinity of the iron oxide particles were investigated in detail. Based on the Malvern Zetasizer analysis, the iron oxide particles with variable size from ∼250 to ∼70 nm could be achieved when increasing the reaction temperature up to 80°C. The magnetite phase of iron oxide particles was determined by using X-ray diffraction analysis. In addition, field emission scanning electron microscopy micrographs were further affirmed that our synthesised iron oxide particles were in nano-scale with a spherical shape. It was found that the high reaction temperature is helpful in controlling the formation of uniform magnetite iron oxide nanoparticles. Keywords: Magnetite, Iron oxide particles, Precipitation, Particle size, Reaction temperature

Introduction Today, development and design of nano-scale materials have gained a lot of attention because mechanical, chemical, electrical, optical, magnetic, electro-optical and magneto-optical properties of these materials are different from their bulk properties and depend on the particle size.1 In this manner, magnetite iron oxide has exhibited great potential for their applications as catalytic materials,2,3 wastewater treatment adsorbents,4,5 pigments,6 flocculants, coatings,7 gas sensors, magnetic recording devices, magnetic data storage devices,8 magnetic resonance imaging9 and medicine applications.10 Iron oxide nanoparticles have been comprehensively studied because of their non-toxicity, biocompatibility,11 superparamagnetism, high coercivity and low Curie temperature properties. Several synthesis procedures have been developed for preparing magnetite nanoparticles, such as chemical co-precipitation,12 sol–gel13 and hydrothermal. The precipitation technique is the simplest and most efficient chemical route to obtain iron oxide particles. However, it has difficulty in the control of particle size, size distribution and sometimes simultaneous the

Nanotechnology & Catalysis Research Centre (NANOCAT), 3rd Floor, Block A, Institute of Postgraduate Studies (IPS), University of Malaya, 50603 Kuala Lumpur, Malaysia *Corresponding author, email [email protected]

presence of different iron oxide phases such as magnetite and maghemite.14,15 A wide variety of experimental factors such as pH, temperature, Fe2+/Fe3+ molar ratio and stirring velocity affect the synthesis of iron oxide nanoparticles and their properties. In this study, a simple and efficient one-step precipitation method is reported to study the influence of reaction temperatures (30, 45 and 80°C) on the morphology, particle size and crystallinity of the iron oxide particles synthesised.

Experimental procedure Ferrous chloride tetrahydrate (FeCl2·4H2O, Sigma ≥99%) and sodium hydroxide (NaOH, Sigma ≥97·0%) were used for the synthesis of iron oxide nanoparticles. All chemicals used were of reagent grade and without further purification. The aqueous solution was prepared by dissolving 2·00 wt-% of FeCl2 in 100 mL deionised water. Subsequently, 0·45 M of NaOH solution was added at a speed of 1·0 mL min−1 into the solution under vigorous stirring to reach a final pH 12. These reactions were carried in air medium for three different temperatures, i.e. 30, 45, and 80°C. It was observed that the solution became black because of the formation of Fe3O4 particles. The resultant black precipitate was isolated by centrifugation at 7000 rev min−1 and then removed by decantation. The precipitate was rinsed three times with deionised water and then dried in an oven at 50°C for 12 hours. Microstructure properties of

© W. S. Maney & Son Ltd 2014

S6-470 DOI 10.1179/1432891714Z.0000000001000

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nanoparticles were observed using a field emission scanning electron microscope (FESEM). Particles sizes of the magnetite Fe3O4 were measured by the dynamic light-scattering method (DLS) by a Malvern Zetasizer Nano-ZS. The crystalline phase of magnetite Fe3O4 nanoparticles was studied by X-ray diffraction (XRD) using CuKα radiation (λ = 1·5406 Å, FK60-40 X-ray diffractometer). The samples were also characterised by Fourier transform infrared spectroscopy (FTIR, Perkin Elmer-1600 Series).

Results and discussion In this part of the experimental study, the effect of reaction temperatures on the morphology of magnetite Fe3O4 is discussed. The surface morphology of prepared nanoparticles at 30, 45 and 80°C was determined by FESEM (Fig. 1). From Fig. 1, it is shown that increasing reaction temperatures during precipitation could modify the particles size and morphology significantly.16 Referring to Tao et al.14 the process of Ostwald ripening would accelerate with the increase in reaction temperatures, which means smaller nanoparticles would decrease in their size more easily and bigger particles would keep growing further, therefore leading to the widening of the size distributions. In contrast, the resultant particle size of magnetite Fe3O4 became more uniform as reaction temperature slowly rose to 80°C. As shown in these FESEM micrographs, the particle size of Fe3O4 was dependent on the reaction temperatures. It could be noted that the Fe3O4 particles synthesised at 30 and 45°C were spherical in shape with an average diameter of 180 and 80 nm as exhibited in Fig. 1a and b, respectively. Interestingly, smaller Fe3O4 particles with an average diameter of 60 nm were obtained when the reaction temperature slowly rose to 80°C. The reason might be attributed to the activation energy of the nucleus formation, which was inversely proportional to the square of temperature. The nucleation rate and number of nucleus would increase with the increasing temperature and, therefore, nanoparticles would reduce in their size slightly. According to the FESEM images, there was agglomeration of Fe3O4 particles occurred in the samples. The agglomeration happened because of van der Waals force between Fe3O4 particles. The volume-based particle size of the sample obtained by DLS measurement is shown in Table 1. The particles size of magnetite synthesised was decreased as temperature elevated. Karaagac et al. 16 and Babes et al.17 reported that synthesis temperature exhibited insignificant influence on the Fe3O4 particles size, while Liu et al. 18 reported that Fe3O4 particles size increased with increasing temperature. However, our results showed that the particle size of magnetite Fe3O4 was decreased significantly when increasing the reaction temperatures. It was found that the magnetite Fe3O4 synthesised at 30°C was approximately 257·20 nm while the particle size dropped down to 93·82 and 67·57 nm for the magnetite Fe3O4 synthesised at 45 and 80°C, respectively. At higher temperature, the reaction condition set to be completed, and samples with reduced crystal defects and smaller particle size were produced.19 Nevertheless, the particles size obtained by DLS was larger as compared to the average diameter measured by FESEM. This

1

Field emission scanning electron microscope images of magnetite Fe3O4 synthesised at reaction temperatures of a 30°C, b 45°C and c 80°C

Table 1

Particles size for magnetite Fe3O4 synthesised at different reaction temperatures

Sample

Reaction temperatures (°C)

Particles size (d.nm)

A B C

30 45 80

257·20 93·82 67·57


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X-ray diffraction patterns of magnetite Fe3O4 synthesised at reaction temperatures of a 30°C, b 45°C and c 80°C

phenomenon may be attributed to the aggregation of Fe3O4 nanoparticles.20 As shown in Fig. 2, all of the resultant samples have a face-centred cubic spinel structure, which indicates a relative phase purity of Fe3O4. It has the characteristic (220), (311), (400), (511) and (440) peaks of magnetite at around 2θ ≈ 30, 35, 43, 57 and 63°, respectively. However, the maghemite phase also has the same crystal as the magnetite phase. Figure 3 shows that the intensity of peaks increased from Sample A, B to Sample C. Karaagac et al. 16 reported that Fe3O4 synthesised at higher temperature showed intensified peaks, which indicated that the crystallinity of Fe3O4 became stronger. It was observed that the peaks of Sample C at 35° 2θ (311) were broadened because of the smaller particle size.21 The average crystallite size was calculated from the fullwidth at half-maximum (FWHM) of the Fe3O4 (311) diffraction peak at 2θ = 35·5° using Scherrer’s equation. From this equation, crystallite size of samples synthesised at 30, 45 and 80°C was determined 23·60, 17·70 and

14·10 nm, respectively. Based on the result, it could be assumed that some of the samples might contain maghemite as impurity but no other non-magnetic impurities. The black colour of the samples inferred that these maghemite impurities were low in content since maghemite Fe3O4 was brown in colour.22 The FTIR spectra of magnetite Fe3O4 synthesised at 30, 45 and 80°C are presented in Fig. 3a–c, respectively. In the present study, FTIR analysis was performed to identify the structure of the samples within the 500–1500 cm−1 region. Based on the results obtained, all three samples have a transmittance peak at 584 cm−1 which is corresponding to δ(Fe–O) stretching vibration in the tetrahedral site.23 This finding was conﬁrmed that the presence of magnetite Fe3O4 within the sample. In addition, another obvious peak could be observed at around 631 cm−1 from the FTIR spectra. The resultant peak indicated the existence of maghemite Fe3O4.24 This analysis shows the destructive influence of the atmospheric oxygen on the structural stability of magnetite nanoparticles.

Conclusion In summary, reaction temperature during the precipitation played a critical role in controlling the particles size and crystallinity of magnetite Fe3O4 nanoparticles. Based on our results, it can be concluded that higher reaction temperature of 80°C resulted in higher crystallinity of the magnetite phase with uniform nano-scale particles size (∼70 nm in diameter). The main reasons might be attributed to the loss of surface defect, nucleation rate and number of nucleus increased with temperature increased.

Acknowledgements The authors would like to thank University of Malaya for funding this research work under National Nanotechnology Directorate (NND-53-02-03-1090) and High Impact Research (H21001-F0032).

References

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Fourier transform infrared spectroscopy spectra of magnetite Fe3O4 synthesised at reaction temperatures of a 30°C, b 45°C and c 80°C

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